Symposium NM5 : Nanomembrane Materials—From Fabrication to Application

Recent advances in soft electronics have attracted great attention due in large to the potential applications in personalized, bio-integrated healthcare devices. The mechanical mismatch between conventional electronic/optoelectronic devices and soft human tissues causes many challenges, such as the low signal to noise ratio of biosensors because of the incomplete integration of rigid devices with the body, inflammations and excessive immune responses of implanted stiff devices originated from frictions and foreign nature to biotic systems, and the huge discomfort and consequent stress to users in wearing/implanting these devices. Ultraflexible and stretchable electronic devices utilize the low system modulus and the intrinsic system-level softness to solve these issues. Here, we describe our unique strategies in the synthesis and functionalization of nanoscale two dimensional materials, their seamless assembly and integration, and corresponding device designs toward wearable devices. These wearable systems combine recent breakthroughs in unconventional soft electronics to address unsolved issues in the clinical medicine.

Single-crystal silicon (c-Si) has been considered as one of the most important materials in modern industry. Its extraordinary properties such as high carrier mobility, reliability, stability and reproducibility have played critical role in amazing advancement of electronic industries. In addition, via intense research for half-century, the infrastructure in Si industry has been well-established and the advanced techniques, such as doping process using ion-implantation have been developed, which are not available for the other advanced electronic materials such as organic-, oxide-, carbon- and two-dimensional materials.[1-4] However, its mechanical and optical drawbacks, such as brittleness and opacity keep the c-Si away from the novel electronics such as flexible and transparent electronics.
Here, we report the ultra-thin c-Si (UT-Si) whose thickness varies from 1.3 nm to 100 nm. As the brittle and opaque graphite becomes flexible and transparent graphene, the UT-Si with thickness of 7 nm becomes extremely flexible with bending radius of 500 nm, easy to deform with 10⁵ lower bending stiffness than that of wafer and transparent with transmittance of ~80%. In addition, in this regime of thickness from 1.3 nm to 100 nm, we observed a significant change in the intrinsic properties of c-Si, such as band gap and piezoresistivity. Such extraordinary mechanical and optical properties enable the UT-Si to be integrated in the flexible and transparent electronics while we can take advantages of the c-Si, such as good electrical mobility, stability, reproducibility, doping controllability as well as well-established infrastructure. We believe that the UT-Si can offer novel way to breakthrough in flexible electronics where the research has been focused on the advanced materials such as oxide-, polymer-, carbon and 2D layered materials.

[1] T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi and T. Sakurai, Proc. Natl Acad. Sci. USA, 101(27), p.9966-9970 (2004).
[2] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano and H. Hosono, Nature 432, p.488-492 (2004).
[3] S. Jang, H. Jang, Y. Lee, D. Suh, S. Baik, B. H. Hong and J. –H. Ahn, Nanotechnology 21, p.425201 (2010).
[4] H. –Y. Chang, S. Yang, J. Lee, L. Tao, W. –S. Hwang, D. Jena, N. Lu and D. Akinwande, ACS Nano, 7(6), p.5446-5452 (2013).

Assembly of 3D micro/nanostructures in advanced functional materials has important implications across broad areas of technology. Existing approaches are compatible, however, only with narrow classes of materials and/or 3D geometries. This paper introduces ideas for a form of Kirigami that allows precise, mechanically driven assembly of 3D mesostructures of diverse materials from 2D micro/nanomembranes with strategically designed geometries and patterns of cuts. Theoretical and experimental studies demonstrate applicability of the methods across length scales from macro to nano, in materials ranging from monocrystalline silicon to plastic, with levels of topographical complexity that significantly exceed those that can be achieved using other approaches. A broad set of examples includes 3D silicon mesostructures and hybrid nanomembrane–nanoribbon systems, including heterogeneous combinations with polymers and metals, with critical dimensions that range from 100 nm to 30 mm. A 3D mechanically tunable optical transmission window provides an application example of this Kirigami process, enabled by theoretically guided design.

Membrane proteins (MPs) are the biologically derived high-performance nanomaterials that act as gatekeepers for biomembranes. They mediate matter transport, information processing, and energy conversion across the nanoscale membrane boundaries, and show great potential for bio-nanoengineering in synthetic systems. However, a critical challenge exists on how to design and control the properties of synthetic membranes to reconstitute and support MP functions in a robust and scalable manner. Our studies on three different MPs with different structural and functional complexity, i.e., proteorhodopsin (a light-driven proton pump), bacterial reaction center (a light-driven electron charge generator), and bovine rhodopsin (a canonical G-protein coupled receptor), respectively, reveal a broadly applicable charge-interaction-directed reconstitution (CIDR) paradigm that induces spontaneous reconstitution of MPs into a series of well-defined amphiphilic block copolymer membranes. I will discuss our structural and functional assays of the reconstituted MPs, which suggest that proteorhodopsin-mediated proton pumping kinetics depends critically on the membrane moduli and the structural flexibility at the protein-membrane interfaces (ACS Nano, 8: 537-545 (2014)), while reaction center-mediated electron charge separation appears insensitive to that (JPC Lett, 5: 787-791 (2014)). Membrane surface chemistry, on the other hand, plays a key role on activating bovine rhodopsin for its interaction with the G protein transducin (Angewandte Chemie 55(2), 588-592 (2016)). I will also discuss our recent progress on controlling the 3D and 2D assembly of proteopolymer membrane arrays amenable for various biotechnological assays and engineering applications, including the development of polymer nanodiscs (PNDs) that support functional MPs. These results suggest that versatile synthetic nanomembrane designs exist to optimize the stability and performance of MPs in synthetic systems.

Silicon nanomembrane based nanophotonic devices provide novel applications not only on silicon but also on a myriad of unconventional substrates such as glass, III-V compounds and PC boards. It will greatly enhance applications in communications and various sensing applications in rigid and conformable surfaces of various military and civilian platforms. In this presentation, we will present intra-chip and inter-chip optical interconnects using silicon subwavelength gratings. Unlike electrical interconnects, optical interconnects provides the possibility of having three dimensional interconnection layers with two dimensional geometry with very low crossing loss (0.02 dB/node experimentally confirmed). 2D optical beam steering with very large steering angle is demonstrated. Further applications using defect engineered photonic crystal waveguide (PCW) based slow light devices provide us with an ultra-sensitive biosensing platform for any biomarker detection as shown in Fig.1 . Early lung and breast cancer detection results will be presented also with high sensitivity without sacrificing specificity.

Free-standing ultra-thin silicon membranes have been instrumental understand confinement effects, modifications in dispersion relations and phonon lifetime impacting thermal processes [1-5]. We will present a comprehensive study of thermal transport in free-standing Si membrane-based two-dimensional phononic crystals . We will compare holey membranes with membranes having a periodic array of pillars on it and their respective characteristics from a phononics perspective. We will discuss the mechanisms at play presented in the works of other authors and compared them to ours towards a better understanding of nano scale thermal transport and possible information processing phononic components.

[1] A. Shchepetov et al., Appl. Phys. Lett. 102 192108 (2013).
[2] J. Cuffe et al., Nano Lett., 12 3569 (2012).
[3] J. Cuffe et al., Phys. Rev. Lett. 110 095503 (2013).
[4] J. A. Johnson et al., Phys. Rev. Letts. 110 025901 (2013).
[5] S. Neogi et al., under review, ACS Nano 9 3820 (2015).
[6] B. Graczykowski et al., Phys. Rev. B 91 075414 (2015) and to be published

Nanoporous materials have attracted a great deal of interest in research fields such as energy conversion, drug delivery, and medical diagnostics due to their large internal surface area and tunable pore size distributions. Porous silicon (PSi), a nanostructured material formed by electrochemical etching of silicon substrates, has been considered as a favorable material for constructing low-cost optical biosensors due to the easy manipulation of its pore sizes, optical properties, and surface chemistries. One of the challenges facing PSi biosensors is the infiltration difficulty of target analytes through nanoscale pore openings. Due to the high aspect ratio of the nanopores in these structures, the diffusive transport into each individual pore can be as slow as a few molecules per second for molecules whose size approaches that of the pore opening. In order to overcome inefficient mass transport, we demonstrate an open-ended PSi membrane that allows analytes to flow through the pores in microfluidic-based assays and interact more favorably with the inner pore surfaces. Our experimental results agree with finite element method simulations and show that flow-through biosensing using the PSi membranes enables a 6-fold improvement in sensor response time compared to closed-ended, flow-over PSi sensors when detecting high molecular weight analyte (e.g., streptavidin 52.8 kDa). For small analytes, such as linker molecules, little to no sensor performance improvement is observed as the closed-ended PSi films do not suffer significant mass transport challenges with these molecules. We further quantitatively show how control over the flow velocity of the analyte delivered to PSi membrane sensors affects the sensor response time and total volume of analtye consumed. The experimental and simulation results indicate that the flow-through scheme enables more reasonable response times (i.e., several minutes) for the detection of dilute analytes (i.e., nM to μM) and reduces the volume of solution required for analysis (i.e., μL). The PSi membrane fabrication is compatible with integration in on-chip sensor arrays such that multiplexed detection is enabled. We will conclude our presentation by discussing design modifications that support simple colorimetric detection using a smart phone camera, leading to a portable, low-cost, and highly effective diagnostic solution.

Light emission from germanium (Ge), as a CMOS compatible material, would make it a powerful addition to Group IV electronics and optoelectronics. The introduction of sufficient tensile strain in very thin sheets (nanomembranes) of single-crystal Ge has recently allowed enhancement of light emission and wavelength tuning via modification of the band structure to achieve a direct gap. [1]

It is, however, difficult to observe a large amount of luminescence via the recombination of charges in the conduction band and the highest-energy-level valence subband (cΓ-LH), due to polarization effects. We have developed a special pressure cell that can stretch the Ge nanomembrane (GeNM) biaxially while collecting the emission from cΓ-LH recombination efficiently. Strong emission in the mid-infrared is observed, in good agreement with simulations.

A 2D photonic crystal is integrated with a GeNM in oder to overcome the polarization effect and enhance the luminescence from it. Using electron beam lithography an array of holes, which is embedded in Poly(methyl methacrylate) (PMMA), is fabricated on the Ge template layer of Ge-on-insulator (GOI), then silicon (Si) pillars are formed on this GeNM through a deposition and lift-off procedure. The Si pillars array is released along with the GeNM and transferred to a polyimide (PI) film that can be clamped onto the top of the pressure cell, which can then be expanded to provide biaxial stress. Emission features in the near-infrared and mid-infrared can be observed; these signals are strongly depended on the design of the pillar array, as expected.

The design of the new pressure cell together with the 2D photonic crystal provides a unique capability for investigating the light emission behavior of strained Ge and exploring potential optoelectronic applications.

[1] J.R. Sanchez-Perez et al. Proc. Natl. Acad. Sci. U. S. A., 108 (2011) 18893-18898.

Supported by NSF and AFOSR.

Abstract: Microwave flexible electronics that can perform over 10 GHz of operation frequency, based on various transferrable semiconductor nanomembranes will be reviewed. In addition, highly sensitive flexible phototransistor enabled by the flip transferring of the finished devices and organic/inorganic heterostructured photosensors will be discussed. The demonstrations showed the great potential to further develop advanced flexible components and systems.

Single-crystal semiconductor nanomembranes that can be released from various source wafers such as Si, SiGe, and III-V are mechanically very flexible and exhibit outstanding electronic properties that are equivalent to those of their bulk counterparts [1]-[2]. These thin, flexible single-crystal materials can furthermore be placed, via transfer printing techniques, onto flexible polymer substrate, thus creating the opportunity to realize high performance flexible electronics and optoelectronics. In this talk, we will review some of the research accomplishments we made over the last few years on this emerging field, including SiNM, strained SiNM, III-V NM based microwave flexible transistors [3]-[6]. Moreover, the advanced design and fabrication process for highly sensitive flexible phototransistors and photosensors will be discussed [7]-[8]. Finally, some of preliminary advanced flexible system and process will also be reviewed.
The work was supported by AFOSR under grant FA9550-09-1-0482.

[1] J. A. Rogers et al., Nature, vol. 477, pp. 45-53, 2011.
[2] K. Zhang et al., Journal of Physics D: Applied Physics, vol. 45, pp. 143001, 2012.
[3] H. Zhou et al., Scientific reports, vol. 3, pp. 1291. 2013.
[4] G. Qin et al., Applied Physics Letters, vol. 106, pp. 043504, 2015.
[5] Y. Jung et al., Nature Communication, vol. 6, pp. 7170, 2015.
[6] J.-H. Seo et al., Scientific Reports, vol. 6, pp. 24771, 2016.
[7] J.-H. Seo et al., Advanced Functional Materials, vol. 23, pp. 3365-3365, 2013.
[8] J.-H. Seo et al., Advanced Optical Materials, vol. 4, pp. 120-125, 2016.

We review surface-normal Fano resonance photonic crystal membrane photonic devices based on heterogeneously integrated silicon and InP crystalline semiconductor nanomembranes. Devices to be reviewed include two types of photonic crystal surface emitting membrane lasers on SOI and on bulk silicon substrates, with potentials for on-chip integrated 3D silicon photonics and flexible optoelectronics.

Light emitters that are wavelength-tunable over the 2.1-2.5-µm spectral region have applications in chemical and biological sensing, as well as spectroscopy and secure free-space optical communications. The introduction of biaxial strain to the Ge crystal lattice modifies the band structure from indirect-band-gap to direct,[1] offering the possibility of wavelength tunable light emitting diodes in a CMOS compatible material. Precise control over the strain state would allow tuning of the emission wavelength.
Although germanium-on-insulator (GOI) has been commercially available, there are distinct advantages in using III-V substrates for creating GOI structures. These advantages are based both on the low quality of commercial GOI and on the possibility of generating many new NM-based materials architectures and electronic/optical properties by combining Ge with Group III-V materials. In particular, the near perfect lattice match between GaAs and Ge allows the direct growth of low-defect-density heterostructures. Furthermore, the availability of equally well-lattice-matched sacrificial buried alloy layers and selective wet chemical etches can allow fabrication of high-quality and freestanding crystalline composite NMs with unexpected new properties. Growth on III-V materials also allows a wide range of Ge thicknesses, as critical-thickness constraints are avoided because strain is externally introduced after growth and transfer to a new host.
We present here initial results on MOCVD growth of Ge films on GaAs, and release/ transfer methods to generate Ge NM’s, which we transfer to new host substrates. The ability to stretch these NMs mechanically, after transfer to a flexible substrate, combined with the integration of Ge with III-V materials, offers both the opportunity to introduce strain in more complex NM heterostructures, and a route to generating hybrid III-V NM materials.

[1] J.R. Sanchez-Perez et al. Proc. Natl. Acad. Sci. U. S. A., 108 (2011) 18893-18898.

Supported by MRSEC and DOE

Novel semiconducting laser sources are always of interest for the budding field of integrated photonics especially those emitting in the near-IR (~1.5μm) and geared towards telecommunications applications. Our membrane laser devices are crafted from InGaAsP epitaxially grown multiple quantum wells on top of an InP substrate. These lasers operate on the principle of bound states in the continuum (BIC). The BIC lasers are fabricated using standard nanofabrication techniques. The structure is fabricated using electron-beam lithography and reactive ion etching (RIE) to define the cylindrical resonators followed by a wet etching step to remove the substrate in order to create the membrane. The release of the membrane has been optimized for best performance. These devices are vertical emitting lasers that have low lasing thresholds (i.e. power efficient).

Due to their highly favorable materials properties such as direct bandgap, appropriate bandgap energy against solar spectrum, and ability to form multiple junctions, epitaxially grown III-V compound semiconductors such as gallium arsenide have provided unmatched performance over silicon in solar energy harvesting. However, their large-scale deployment in terrestrial photovoltaics remains as a daunting challenge mainly due to the high cost of growing device-quality epitaxial materials. In this regard, releasable multilayer epitaxial growth in conjunction with printing-based deterministic materials assemblies represents a promising approach that can overcome this challenge but also create novel engineering designs and device functionalities, each with significant practical values in photovoltaic technologies. This talk will provide an overview of recent advances in materials design, fabrication concept, and nanophotonic light management of multilayer-grown nanomembrane-based GaAs solar cells aiming for high performance, cost-efficient platforms of III-V photovoltaics.

III-nitride nanomembranes (NMs) represent a new embodiment of the nitride compound semiconductors having identical crystalline perfection and optoelectronic efficacy. The III-nitride NMs are naturally compatible with flexible hosts due to the reduced flexural rigidity and can be incorporated into layered stacks with other two-dimensional materials. A major reason to hinder the nitride NM from realizing devices is the ceramic-like chemical inertness of nitride compound semiconductor, making it difficult to etch or to undercut which resulted in the formation of freestanding nanomembrane.
Based on a recent discovery of conductivity selective electrochemical (EC) etching of GaN, we demonstrated GaN NMs with a freestanding thickness from 50 to 500 nm produced from state-of-the-art epitaxial structures. We confirmed that the microstructural, morphological, and optical properties of the separated NM layers were not affected by the membrane fabrication process. The performance of devices including LEDs, MOS transistors, AlGaN heterostructure transistors, and novel photonic devices will be discussed.

Deep understanding of the phonon transport properties in nanoscale systems as well as nanostructured materials is of fundamental importance in realizing high-performance thermoelectric devices for both energy harvesting and solid-state refrigeration. As the modern electronic device dimensions continue to shrink, the effect of nanomembrane structure and interfacial scattering is increasingly dominating the electron and phonon transport. Here we present temperature dependent thermal conductivity measurements of self-assembled and free-standing hybrid multilayer nanomembrane systems: radial and planar Si/SiO₂ superlattices, which are fabricated by a straightforward “roll-up” and “compression” technique ^[1]. Different from conventional nanomembrane transferring techniques, which are used to fabricate single-crystalline nanomembranes layer-by-layer, tubular radial superlattices consisting of mechanically bonded single-crystalline Si nanomembranes are self-assembled by the intrinsic build-in strain in the thin Si film. Ultrathin native SiO₂ layers form on each side of the Si nanomembranes in-situ during the rolling process, resulting in a Si/SiO₂ hybrid multilayer system with well-defined interfaces. The in-plane thermal conductivity of as-rolled Si/SiO₂ tubes with five windings (containing five coaxial nanomembranes in the tubes) is 4 WK^-1m^-1 at 300K, being one fourth of that of single-crystalline Si nanomembranes with an equivalent thickness ^[2] and only slightly higher than that of nano-patterned Si thin-film phononic crystals ^[3]. Detailed structural characterization of the as-rolled and compressed interfaces by high-resolution transmission electron microscopy will be presented, as well as a theoretical model calculating the phonon dispersion relations within the framework of Born-von Karman lattice dynamics. The theoretical results demonstrate that the thermal conductivity of the fabricated Si/SiO₂ hybrid nanomembrane superlattices is to a great extent determined by the phonon processes in the amorphous SiO₂ layers, which is important for any thermoelectric application of Si-based nanomaterials.

Titanates form an interesting family of materials that can be ferroelectric, ferromagnetic, insulating or highly conductive, and can be used in various applications, such as varactors and memory components. Titanates are difficult to grow and usually pulsed laser deposition (PLD) is used to grow the films. Very promising thermoelectric properties and very high figure of merit have been reported on PLD grown thin films [1], and, consequently, titanates are considered as promising materials for high temperature thermoelectrics. Using PLD epitaxial films can be deposited but typically only on small areas. In this work we have deposited 25-150 nm thick Nb doped strontium titanate (STO) films on oxidized 150 mm Si wafers by magnetron sputtering and released the membranes by deep etching through the wafer from the back side. The different thermal expansion coefficients of Si and STO tend to create very high tensile stress in the STO films, preventing the release without breaking the membranes. By optimizing the sputtering parameters, it is possible to decrease the stress to about 300 MPa enabling the release and fabrication of relatively large area free-standing membranes. Fabrication of corrugated membranes using the low stress process is also possible. The resistivity of the Nb doped films with Nb content of about 10 % is in the range of a few mOhmcm. The grain size of the polycrystalline low stress membranes is very small, leading to low thermal conductivity and the thermal conductivity measured by 3w method is only 2.5 W/mK. These values are very promising for thermoelectric applications. In this talk we will report on the fabrication and properties of the thin free-standing STO:Nb membranes deposited by magnetron sputtering and discuss the potential applications.
[1] Ohta et al., Nature Materials 6 (2007) 129; C. Yu et al., Appl. Phys. Lett. 92 (2008) 092118.

Mesoporous zeolitic thin films (MZTFs), containing vertical mesochannels and high thermal stability, are fabricated via decane-induced self-assemblies of beta-zeolite seeds onto flat substrates, including silicon wafer and conducting ceramics. With different ratios in surfactants, the dimensions of the mesochannels are ranged from 2 to 10 nm and their shapes can be tuned from polygons (n=3-8) to hexagons (n=6) only. With intermittent rates of injecting beta-zeolite seeds, bilayered MZTFs are obtained. To realize periodicity of MZTFs on substrates, we use grazing-incidence small-angle scattering (GISAXS) analysis that explains hexagonal packing and superlattice structures of the mesochannels. With complementary X-ray reflectivity (XRR) analysis, in-plain signals suggest the bilayer structures, directly evidenced in cross-sectional TEM and SEM images. MZTFs maintain highly ordered morphology even after high temperature calcination (700 ^oC) and hydrothermal (100 ^oC) treatments, suggesting their enhanced thermal and hydrothermal stabilities. The MZTFs are employed to confine growths (5-20 nm) of metallic nanoparticles (e.g. Ag, Au, Cu) for plasmonic applications, as well as to inhibit biological infection (e.g. E.coli) at room temperature. Our works reveal a facile method of fabricating robust on-substrate thin film materials that demonstrate functionalities for multiple interfacial applications.

Polysulphone (PSF) is one of the most popular thermoplastic materials used in the manufacturing of various types of microfiltration and ultrafiltration membranes. Despite the high structural and chemical stability of PS membranes, the flux produced from such membranes is relatively low. Therefore, an increase in the porosity and flux of PSF membranes is highly desirable to increase the productivity and efficiency of these membranes. In this work, the effect of addition of Arabic gum as a new pore-forming into the casting solution on the properties of the fabricated PS membranes has been studied. PS membranes were cast via phase inversion at different concentrations of Arabic gum (0, 1, 3, 5 and 7%) in PSF and dimethyacetamide (DMAc) solvent. Philos casting system was used for membrane preparation. The effect of Arabic gum on the pore size and water flux of the prepared membranes has been studied. The porosity, pore size and morphology of the developed membranes have been characterized with SEM. Moreover, the oil removal capability of polysulfonic membranes (with Arabic gum) has also been performed when oil-water solution (100 mg/l) have been tested with the new membrane. The use of Arabic gum as a surfactant has been found to successfully remove oil from water (up to 97%) and increase the porosity and flux of the prepared membranes.

It is well-known that only the high-quality graphene possesses its unique physical, chemical and mechanical properties, when it is used in practical applications. However, it is still a challenge to obtain the high-quality graphene in large-scale and low-cost, which hinders its broad applications. In this paper, we introduce an electrochemical exfoliation for producing graphene in large scale, and then follow a spark plasma sintering (SPS) treatment for obtaining the mass high-quality graphene.
Recently, electrochemical exfoliation has drawn great attention as a promising method for producing graphene on an industrial scale with high efficiency, low cost, and non-pollution. In comparison to cationic intercalation, anionic intercalation (primarily in aqueous electrolytes) is more effective and less time demanding, which has predominated the literatures concerning the electrochemical exfoliation of graphite. Sulfuric acid is the most commonly used electrolyte because the interlayer spacing of graphite (0.335 nm) is comparable to the size of sulfate ion (0.46 nm), which facilitates the intercalation of sulfate ions. In addition, the sulfate ions can also act as a surfactant to prevent the re-stacking of graphene product in aqueous solution. However, the induced oxidation and chemical functionalization is unavoidable for the anodic electrochemical exfoliation of graphite in acidic electrolytes.
Herein we proposed a simple and effective route to eliminate the oxygen functional groups by using a spark plasma sintering (SPS) system. SPS is a newly developed sintering method applying high temperature spark plasma generated momentarily. The spark plasma, produced by the large pulse current, has an effect of cleansing impurities of samples and enhancing the heat transfer effect that produces better bonding. During the sintering, the pristine graphene could be effectively converted to high-quality graphene by the elimination of oxygen-containing groups and restoration of its intrinsic structures and properties. Moreover, the output could be readily scaled up due to the rapid sintering process. According to the experimental results, the produced graphene exhibited very low defects density, extremely high carbon to oxygen (C/O) ratios and good processability in various solvents. Free-standing graphene paper (G-paper) was further fabricated from the high-quality graphene, and a high conductivity of 38460 S/m could be attained. The G-paper without any binders, which was used as supercapacitor electrodes, delivered a specific capacitance of 129.0 F/g at 1 A/g, retaining 97% capacitance even after 1000 cycles.

There is great interest in intracellular delivery, recording, and sensing, and significant progress has been made with mammalian cells using various micro/nano-needle platforms^1-3. Additionally, cell transfection has been achieved by applying an electroporation voltage across a nanomembrane pore⁴. These techniques, however, are still in their infancy especially with respect to their application to the plant system, where their rigid cell wall poses additional challenges and complexity. In particular, there is great need to deliver molecules (especially DNA) to plant organisms such as microalgae. Microalgae are single-celled plants that are attractive for applications in biofuel and biochemical production, but the lack of genetic engineering tools has been a major impediment to the research progress⁵. In order to non-destructively pierce the cells, which are 5-10 μm in diameter, a needle must necessarily have sub-micron-scale dimensions and a hollow needle must therefore have precise nano-scale wall thickness.

In this work, we discuss the fabrication of a unique nanomembrane consisting of vertically-protruding hollow needles and its application in interfacing with microalgae cells. The needles are conical, several microns tall, and have tips that are ~100-200 nm in diameter. The wall thickness of the hollow needles is defined by the nanomembrane thickness and is a few tens of nanometers. This platform is capable of penetrating the cell wall of microalgae via a forced piercing event, allowing direct fluidic access to the cytosol. Using fluorescence microscopy and electron microscopy techniques, we demonstrate the ability to deliver reporter molecules to the cell interior, and investigate how the delivery efficiency is affected by such variables as the protruding needle dimensions and the force of the piercing event.

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Wearable electronics, as an important branch of modern electronics, demonstrates huge potentials to change our lifestyles in the near future. However, it remains challenging to develop matchable energy harvesting and storage devices to meet the flexible and wearable requirements of wearable devices. Here we have designed a new family of high-performance fiber-shaped energy harvesting and storage devices, and integrated multifarious functions, including high output voltage, magnetic response, self-healing and energy conversion/storage to meet a broader range of applications. Three main systems are described below.

The invention of fiber-shaped supercapacitors with high output voltages. The output voltages of fiber-shaped supercapacitor based on aqueous electrolytes are too low for practical applications, which requires efficient strategies for in-series connection. We have invented fiber-shaped supercapaitors with high and controllable output voltages in mimicking electric eel, the strongest bioelectricity producer in nature. High output voltages up to 1000 V have been achieved in a single fiber, which breaks the voltage limit and maintains the high integration of the resulting devices.

The invention of self-healable fiber-shaped supercapacitors. One concern for fiber-shaped devices lies in the breaking of delicate fibers under deformation, which may cause the failure of the entire module. We have invented a novel family of fiber-shaped supercapacitors with self-healing capacity to extend their lifetime. Even completely cut off, they can quickly self-heal within 10 s at room temperature, and 92% of the capacitance can be recovered.

The invention of integrated fiber-shaped energy harvesting and storage devices. The integration of energy harvesting and storage into one single device is of great importance for practical applications. We have developed a series of fiber-shaped integrated devices with high energy storage capacity and integration, which represent promising candidates in a variety of application fields.

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Global healthcare in its current reactionary format is irrepressibly overwhelmed by a surplus of patients and tremendous medical expenses; associated treatment strategies are progressively insufficient. A paradigm shift centered on nanotechnology is anticipated to enable the gradual transition from reactionary to predictive medicine needed to enhance global wellness. In particular, it is hypothesized that the integration of synthetic immune cell mimics together with modern medical intervention will substantiate the concept of a smart vaccine capable of coordinating aggressive phagocyte and B-cell activities. The objective of the current study is to fabricate synthetic immune cells by exploiting tunable polymersome (Ps) membrane properties. Ps are artificial, biocompatible vesicles that self-assemble via the hydrophobicity interactions of admixed aqueous and organic substances. For the current application, chemical viability studies have shown that a Poly(D,L-lactide)-b-poly(ethylene glycol)-carboxylic acid (PDLLA-PEG-COOH) diblock copolymer ideally self-assembles to form Ps. Strategies were devised to supply a polymeric vesicle affixed by IgG antibodies along the peripheral hydrophilic PEG regime and isolated subcellular material within the aqueous core. The utility of EDC and Sulfo-NHS crosslinking chemistry facilitated the adherence of fluorescein isothiocyanate (FITC)-labeled antibodies to Ps surfaces. Transmission electron microscopy (TEM) and fluorescence microscopy images provide a validation of bilayer formation in addition to an optical quantification of the degree of IgG surface conjugation. As part of a complementary experiment, mitochondria were isolated from human dermal fibroblasts via a rigorous centrifugation and homogenization routine. Structural integrity was ensured through TEM characterization prior to Ps conjugation, and chemical viability was quantified by an assessment of the enzymatic activity of isolated organelles. Associated bacterial and cytotoxicity assays measured the viability of Ps that were functionalized to include antibodies and/or subcellular material. Clinical injection of the composite Ps solution is hypothesized to activate an aggressive B-cell response in which antibodies disperse and mark antigens for destruction. Associated mitochondrial interactions are expected to drive controlled antibody release mechanisms. Accelerated morphological considerations assume the generation of a fully functional immune cell appended by several classes of antibodies. Postliminary studies will follow closely the development of an in-situ sensing device that would enable clinicians to externally steer nanocarriers within the vasculature. Automated design is anticipated to provide a nanodevice with capabilities that far exceed the healing properties shown by innate immune cells. The shift from reactionary to predictive intervention will prompt a new era in which early disease detection and treatment are clinically significant.

A sub-nanoporous carbon thin film has been considered as a promising candidate for a next-generation robust reverse osmosis (RO) membrane. We focused on fabrication of carbon thin films using plasma-enhanced chemical vapor deposition (PECVD) method. The method has many industrial advantages such as high-throughput and large-area production, defect elimination, precise control of film thickness, and flexible design of a multi-layer structure. Although some researchers have reported carbon-based RO membranes in the 1980's, the rational design on making sub-nanopores in a PECVD-made carbon thin film has not been established to date. Here we present how we can introduce sub-nanometer pores in a carbon thin film and control its size and porosity by tuning plasma parameters.

Using PECVD method, a sub-nanometer porous carbon thin film was directly deposited on a porous polymer support at room temperature. Although conventional hydrocarbon monomers did not give water-permeable thin films due to hydrophobicity, a hydrophilic carbon thin film that can quickly permeate water molecules was obtainable using a nitrogen-containing amine monomer. Eighty percent of nitrogen atoms in the monomer was successfully transferred in the carbon film, while atomic ratio of hydrogen decreased largely due to generation of radicals by detaching a hydrogen atom in a monomer, which results in the film deposition by formation of carbon-carbon bonds. A transmission electron microscope revealed amorphous structure of the carbon thin film. Positron annihilation lifetime spectroscopy clearly detected large number of sub-nanometer pores in the film. Since the hydrophilic film can absorb water molecules (typically 5 to 20%), Young's modulus of 8.0 GPa in dry state decreased to 2.0 GPa when the film is wet with water.

Among various plasma parameters, we investigate in detail the effect of input power, monomer pressure, and monomer flow rate. A diagram of deposition rate clearly exhibits different regimes of low-temperature plasma state. By examining wide range of plasma parameter combinations, we found the complicated effects of these plasma parameters could be rationalized using a simple parameter, electron temperature (i.e. electron energy in plasma state). As a result, we can optimize film density by tuning electron temperature. In our experiments, a refractive index value measured with ellispometry was used as an index for a relative evaluation of a film density because the atomic composition does not changed significantly. The refractive index has direct correlation with water permeability. While high-refractive-index carbon film does not permeate water even under high pressure of 4.0 MPa, a carbon film of medium refractive index shows both water permeability and high salt rejection. By optimizing an electron temperature, twenty-nanometer-thick sub-nanoporous carbon thin film realized high water flux and salt rejection (more than 90% for NaCl).

The emergence of multi-resistant pathogens has encouraged the investigation of new strategies to cope
with this ever-increasing threat to public health. In this context, silver nanoparticles (AgNPs)
were combined with doxycycline (DO) to evaluate the potentiality of this
hybrid as a bactericidal agent against E. coli. Polyvinylpyrrolidone (PVP) was used as a stabilizer to
prevent the excessive growth and agglomeration of AgNPs. With a 2² full-factorial design and with the help of UV-vis and TEM, we found out that the highest concentrations of silver ions and PVP delivered the smallest nanopartilces with narrow size distribution. Interestingly, DO bound directly to PVP and had its concentration increased around the particle as a consequence of this interaction, as evidenced by time-resolved fluoresnce and FTIR measurements. As a result, the AgNPs/DO conjugates presented enhanced bactericidal properties compared to the individual components. Stabilizing agents are generally undesirable on the surfaces of nanoparticles because they block adsorption surface sites. However, we have shown that PVP played a paramount role in concentrating DO around the particle, which culminated in an increased bactericidal activity towards E. coli.

The broad applications of self-rolling nanomembranes in nano-elecromechanical/micro-electromechanical systems (NEMS/MEMS), biomedical devices and optoelectronics have driven extensive research efforts, aiming to control the size and shape of the rollup structure. To achieve this objective, accurate knowledge of the mechanics underlying the rollup process is necessary. In the present study, we formulated a comprehensive analytical model for bidirectional rollup of nanomembranes, in which the contributions of both longitudinal and transverse mismatch strains to the rollup curvature are accounted for. Our model predictions show the transverse mismatch strain can have significant influence on the final rollup curvature, validated through finite element (FE) simulations. Our results explain the considerable discrepancy between experimentally measured and the previous theoretically predicted curvatures. Furthermore, the new model we formulated is shown to be capable to predict potential bistable curvature configurations, providing new guidance in manipulating the rollup direction and curvature. Guided by our model, a novel design strategy based on corner geometry engineering has been proposed to realise the unidirectional rollup, validated by experimental results and FE simulations

Graphene is a two-dimensional carbon material with excellent properties in mechanical strength, atomic thickness and flexibility. Because of these remarkable properties, Graphene has been a prominent candidate as structural material in various fields since its first isolation in 2004. Graphene Oxide (GO) is a derivative of Graphene with oxygen-containing functional groups. Recently, climate change has been emerged as a serious problem, and a lot of effort has been tried to solve this problem in the respects of CO₂ capture, water purification and energy harvesting. Most of all, membrane-based technology has been regarded as a promising method due to simple and economical fabrication process, safety and environmental friendliness. Among them, GO membrane of layered carbon structure is a novel candidate in this field with its exceptional characteristics compared with traditional materials such as polymers and zeolites. It has been reported that GO membrane has fast and selective permeability for the specific matters like water and gases depending on the morphology, thickness, forms of containing groups and so on.
Here, we focus on the effect of the morphology of GO membrane in terms of three different fabrication methods. We used GO suspension with same concentration to produce GO laminates on the Anodisc Aluminum Oxide (AAO) as supporting membrane through vacuum-filtration, spray-coating and spin-coating methods. After every deposition process, the membranes were dried at 60°C for 24 hours. Raman, XRD, SEM and AFM were conducted to analyze the condition and morphology of the structure. Finally we confirmed that the surface of the GO layered structure deposited by vacuum filtration was the most uniform. We attribute this to vacuum pressure which stacks the GO flakes perpendicularly to the membrane. However, further research is needed for understanding and explaining the main reasons in detail.

Electrospun nanofibrous mats have been shown to be useful as membranes for separation and filtration applications, including in the biomedical field for white blood cell filtration, because of their high surface area and interconnected non-woven structure. This study is focused on structure-property relationships of electrospun fibers of poly(butylene terephthalate) (PBT, viscosity averaged molecular weight, M_v = 45,000 g/mol.) with potential use as a filtration membranes. Nanofibers were obtained from a 15% w/v solution of PBT in trifluoroacetic acid and dichloromethane (TFA/DCM = 1/1 by volume) as solvents. Electrospinning was done using a flow rate of 0.04 ml/min and a distance of 12 cm between the needle tip and the counter-electrode with an applied voltage of 20 kV between them. Scanning electron microscopy was used to study the morphology of the fibers. Nanofibers free of defects such as beads were obtained which had an average diameter of 1.9 microns. The crystal structure of the nanofibers was studied using X-ray diffraction, and thermal properties were evaluated using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). X-ray diffraction showed the as-spun fibers were slightly crystalline. TGA analysis showed ~0.5% of bound solvent was removed at around 60 °C, and major degradation of the fibers occurred at around 390 °C. Highly stretched PBT fibers tend to shrink during heating through the glass transition temperature (T_g) region. However, a well-resolved glass transition (T_g) step was observed in the TMDSC reversing heat flow, showing T_g occurs at ~52 °C. Future work will be reported on pore size measurements and contact angle, quasi-isothermal TMDSC studies and dielectric relaxation analysis of PBT nanofibers.

The field of flame retardancy for furniture (i.e., foams and textiles) is currently facing a few changes and challenges because the halogenated or phosphorus-based flame retardants have proven to be bioaccumulative and toxic in mammals. Therefore, the study for highly efficient eco-friendly flame retardant products, which are developed by using simple techniques (i.e., layer-by-layer), is lead the researchers towards the development of creditable alternatives. In the past two decades, layer-by-layer (LbL) assembly has been widely used as a convenient and versatile method to fabricate functional thin films. Thin films of cationic starch (CS) and sodium montmorillonite (MMT) clay, prepared via LbL assembly, are eco-friendly and bio-based flame retardant. In order to obtain a better understanding of flame retardant behavior of three different coatings, 5, 10 and 20 bilayers samples were fabricated and evaluated. UV-vis absorbance was used to measure the linear growth of this film as a function of bilayers deposited. Thermal and flame retardant properties were measured by thermogravimetric analysis (TGA) and vertical flame test (VFT), indicating that the char residue at temperatures from 400 to 800 °C and burning time were significantly enhanced as compared with the control sample. SEM analysis of cotton samples before and after VTF revealed the increasing char forming ability by LbL coating layers, showing that the structure of cotton fabric in all coated fabrics were preserved by CS and ceramic surface layer char. This work provided a simple but effective method of enhanced flame retardancy of cotton fabric and can be applied to other polymeric materials.

As the global populations grow, water demand and pollution of water resources will increase. As a consequence, water borne disease outbreaks are on the rise and current disinfection methods have been shown to be ineffective in inactivating all pathogens during water treatment. Aluminum oxide nanoparticles (Al₂O₃ NPs) have been shown to poses antimicrobial properties due to oxidative stress and particle interaction with cell walls that lead to rupture and cell death. Also, Al₂O₃ has high thermal and chemical stability, which makes these NPs an excellent candidate for water treatment applications. Thus, the objective of this work is to asses the bactericidal properties of Al₂O₃ NPs synthesized using a polyol-based process in presence of polyvinylpyrrolidone (PVP) as a growth inhibitor reagent. For practical applications nanoparticles must be immobilized in a medium to ensure that particles are not dispersed into the treated water. For this reason, synthesized nanoparticles were dispersed in electrospun polyacrylonitrile (PAN) membranes to also evaluate the bactericide capacity. X-Ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FT-IR) analysis suggests that synthesized nanoparticles are γ-Al₂O₃ after annealing at 800°C for 6 hours. Scanning Electron Microscopy (SEM) characterization was used to determine the morphology and size of synthesized nanoparticles. Composite electrospun membranes where also characterized by XRD, FT-IR, and SEM. The bactericide activity of the synthesized γ-Al₂O₃ NPs, commercially available Al₂O₃ particles and Al₂O₃-PAN composite electrospun membranes against E. coli bacteria was evaluated by the disc diffusion method. Both synthesized and industrially produced particles exhibited antibacterial activity against E. coli, but polyol-based synthesized nanoparticles demonstrated better bactericide properties. Al₂O₃ Nanoparticles embedded in electrospun PAN also exhibited antibacterial properties. Results suggest that the polyol-synthesized γ-Al₂O₃ embedded in electrospun membranes have the potential to be used in alternative water disinfection processes.

Particulate matter (PM) pollution and bacteria transmission have raised serious concerns for public health. We know outdoor individual protection could be achieved by facial masks. In this research, various materials nanofiber membranes can be used to purify the air pollution and bacteriostatic. We use the surface effect and active energy of the nanofibers surface to remove micro-particle and eliminate bacteria. This work creates another field for nanomaterial application.

A novel nanofiltration membrane was produced by reacting polyethersulfone (PES) functionalized with acyl chloride (PES-COCl) and m-phenylenediamine (MPDA). Membranes composed of PES and PES-COCl were fabricated using a non-solvent-induced phase separation process, and then MPDA was reacted with acyl chloride in the PES-COCl on the membrane surface. The PES-COCl was synthesized by reacting the aminated PES with trimesoyl chloride (TMC), and the formation of PES-COCl and reactions between PES-COCl and MPDA was confirmed using FT-IR, XPS, and FE-SEM analyses. Contents of acyl chloride groups in the membranes was controlled by changing mixing ratio of PES and PES-COCl. Membranes composed of PES and PES-COCl exhibited salt rejection lower than 10% against MgSO₄, while those fabricated by reacting MPDA with acyl chloride in the PES-COCl exhibited salt rejection higher than 50% against MgSO₄. Salt rejection increased by increasing acyl chloride content in the membrane.

Ultrafiltration membranes composed of polyethersulfone (PES) and multi-walled carbon nanotube grafted with carboxylated polyethersulfone (MWCNT-PES-COOH) were fabricated to provide hydrophilic and antibiotic properties to PES membranes. Carboxylated PES (CPES), which was prepared by reacting PES, N-butyllithium, and carbondioxide, was reacted with trimesoyl chloride (TMC) to functionalize with acyl chloride. Aminated MWCNT (MWCNT-NH₂), was prepared by reacting carboxylated MWCNTs with (3-aminopropyl)triethoxysilane (APTES). The formation of MWCNT-NH₂ and MWCNT-PES-COOH was confirmed by FT-IR, NMR, XPS, and FE-SEM analyses. The hydrophilicity and water flux of the PES/ MWCNT-PES-COOH membranes increased with increasing MWCNT-PES-COOH content. No antibacterial activity was observed for PES membranes, while PES/MWCNT-PES-COOH membranes containing at least 5 wt% MWCNT-PES-COOH displayed antibacterial activity (=6.1). Membranes having antibacterial activity, high water flux, and improved fouling resistance without a loss in solute rejection could be fabricated by blending PES with MWCNT-PES-COOH.

Transient electronics has attracted great attention due to its unique capability to disappear instantly and/or after programmed time by external triggering. This key advantage generates new opportunities on the field of information security. The data stored in nonvolatile memory can be electrically erased for security purpose, however, they can be easily restored. The information in missed or stolen memory devices cannot be protected. In these cases, chemical/physical destruction of the data storage module can be a potential solution for the permanent data erasure and reliable information security. Here, we demonstrate new information security technology by developing a novel class of nanomembrane-based transient resistive random access memory (RRAM) integrated with photo-acid generators (PAGs) and upconverting nanoparticles (UCNPs). The ultrathin transient RRAM is composed of 5-nm-thick zinc oxide nanomembrane and has advantage of low-power consumption and fast switching. The device is coated with an acid-generating matrix which is composed of ultraviolet (UV)-responsive PAGs and the multi-dye sensitized UCNP in polyethylene oxide. UCNPs are synthesized and integrated to use wide wavelength range photons for triggering the physical disappearance of the device. Therefore, illumination of NIR and/or visible, UV light on UCNPs triggers the emission of UV, resulting in the generation of photo-acid. Due to the ultrathin nature of RRAM, fast and complete chemical destruction of stored data in the memory device could be achieved. This integrated system provides a novel concept of information security in nonvolatile data storage devices as well as new opportunities in mobile electronics and defense applications.

As a strategy to save the cost of expensive substrates in semiconductor processing, the technique called “layer-transfer” has been developed. In order to achieve real cost-reduction via the “layer-transfer”, the followings need to be insured: (1) Reusability of the expensive substrate, (2) Minimal substrate refurbishment step after the layer release, (3) Fast release rate, and (4) Precise control of a released interface. Although a number of layer transfer methods have been developed including chemical lift-off, optical lift-off, and mechanical lift-off, none of those three methods fully satisfy conditions listed above. In this talk, we will discuss our recent development in a “graphene-based layer-transfer” process that could completely fullfill the above requirements, where epitaxial graphene can serve as a universal seed layer to grow single-crystalline III-N, III-V, II-VI and IV semiconductor films and a release layer that allows precise and repeatable release at the graphene surface.

Graphene, a monolayer of carbon atoms in honeycomb configuration, has become a subject of active study since its discovery in 2004. Many potential applications that exploits its mechanical properties and gas impermeability [1] have been proposed such as pressure sensing [2]. Moreover, suspending graphene on circular cavities or trenches prevents substrate effects and enables electro- or opto-mechanical actuation [3].

Single-layer graphene drums have been extensively studied, and several groups have reported the statistical variations of their properties by measuring few drums with laser interferometry, Raman spectroscopy, and atomic force microscopy. However, a parallel, non-invasive, and affordable characterization technique is necessary for any attempt to commercialize graphene mechanical sensors. Furthermore, CVD SLG usually contains gas permeable lattice defects and nanoscale pores due to its growth on imperfect substrates, which blocks its application in gas pressure sensing devices that require impermeable membranes. A possible route to overcome this difficulty is to stack several CVD layers to reduce the probability of having nanopores from different layers aligned on the same spot [4].

In this work, we introduce a new non-invasive optical technique to characterize the mechanical properties and the permeability of large arrays of suspended graphene membranes. We observe Newton's rings on a suspended CVD double-layer graphene (DLG) drumhead when applying a pressure step between inside and outside of the cavity, which allows us to study the deformation of this mechanical system. Based on these observations, the permeability of the DLG membrane is determined and found to be similar to that of pristine graphene.

REFERENCES:
[1] Bunch, J. S., & Verbridge, S. S. (2008). Impermeable atomic membranes from graphene sheets. Nano Letters, 8(8), 2458–2462.
[2] Dolleman, R. J., Davidovikj, D., Cartamil-Bueno, S. J., Van Der Zant, H. S. J., & Steeneken, P. G. (2016). Graphene Squeeze-Film Pressure Sensors. Nano Letters, 16(1), 568–571.
[3] Ferrari, A. C., Bonaccorso, F., Falko, V., Novoselov, K. S., Roche, S., Bøggild, P., … Kinaret, J. (2014). Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems. Nanoscale, 7(11), 4598–4810.
[4] Celebi, K., Buchheim, J., Wyss, R. M., Droudian, A., Gasser, P., Shorubalko, I., … Park, H. G. (2014). Ultimate Permeation Across Atomically Thin Porous Graphene. Science, 344(6181), 289–292.

Determining the chemical state of a catalyst under realistic reaction conditions is of crucial importance in designing catalytic systems with improved activity and selectivity towards sought after products, and a key step in developing or improving existing industrial processes. Ambient pressure X-ray photoelectron spectroscopy (APXPS), based on analyzers that incorporate a differentially pumped lens system, has proved a powerful technique for providing quantitative, surface sensitive information on the chemical composition of surfaces/interfaces under reaction conditions.[1] Whilst this approach proves practical up to the tens of mbar regime, significant gas phase scattering of photoelectrons at higher pressures makes measurement impractical. However numerous reactions of interest occur at atmospheric pressures and above, and thus the behavior observed in existing APXPS systems may not be truly representative of such reactions.
Here we demonstrate atmospheric pressure XPS using single-layer graphene membranes as photoelectron-transparent barriers that sustain pressure differences in excess of 6 orders of magnitude.[2] The graphene-based membranes are produced by transferring graphene grown by chemical vapour deposition (CVD)[3,4] onto metal coated(Au or Al), perforated silicon nitride grids using a polymer-free transfer technique. The suspended graphene then serves as a support for catalyst nanoparticles under atmospheric pressure reaction conditions (up to 1.5 bar), where XPS allows the oxidation state of Cu nanoparticles and gas phase species to be simultaneously probed. We thereby observe that the Cu²⁺ oxidation state is stable in O₂ (1 bar) but is spontaneously reduced under vacuum. We further demonstrate the detection of various gas-phase species (Ar, CO, CO₂, N₂, O₂) in the pressure range 10-1500 mbar including species with low photoionization cross-sections (He, H₂). Pressure-dependent changes in the apparent binding energies of gas-phase species are observed, attributable to changes in work function of the metal-coated grids supporting the graphene. We expect this graphene membrane approach to be a valuable tool for studying nanoparticle catalysis under atmospheric pressure reaction conditions, as well as a promising technique for studying solid-liquid interfaces during electrochemical reactions.[5]

(1) Eren et al. J. Am. Chem. Soc. 2016 (ASAP)
(2) Weatherup et al. J. Phys. Chem. Lett. 2016, 7, 1622–1627
(3) Weatherup et al. J. Am. Chem. Soc. 2015, 137, 14358–14366.
(4) Weatherup et al. J. Am. Chem. Soc. 2014, 136, 13698-13708
(5) Velasco-Velez et al. Angew. Chemie Int. Ed. 2015, 54, 14554–14558.

Novel single-component ultrathin nanocomposites were fabricated via non-conventional layer-by-layer (LbL) assembly of graphene oxide (GO) flakes using hydrophobic solvent binder in order to establish the uniform layered growth and furnish strong complementary interactions. Ultrastrong freestanding graphene oxide (rGO) LbL nanomembranes having low thickness of 3 nm (3 monolayers of GO), which can be transferred over a large surface area across tens of square centimeters by using a facile surface tension-assisted release technique, were concocted by eliminating organic and regular polymeric binders from assembly process or needs for the intermediate surface chemical modification. These ultra-smooth and highly uniform rGO nanomembranes demonstrate outstanding elastic modulus of 120 GPa and mechanical strength of 0.5 GPa, which are several times stronger than other reported regular rGO films, with high electrical conductivity (up to 3000 S/m after additional chemical reduction) and high transparency (up to 93% at 550 nm before reduction). Furthermore, a flexible freestanding protected noble metal monolayers having surface enhanced Raman scattering (SERS) properties can be constructed by sandwiching up to 94 wt% of silver nanoplates between 5 nm GO layers. With the superior mechanical properties, high optical transmittance and high conductivity, these transferrable flexible rGO/Ag/rGO nanomembranes can be valuable for potential applications in the protective molecular coatings, flexible electronic devices, SERS sensing elements, energy harvesting, optical sensors, and ion separators.

Graphene and CNTs have been studied for gas sensing applications due to their outstanding physical, electronic, mechanical and optical properties. The high surface-to-volume ratio (giving a large exposed area) and mono-atomic thickness of graphene sheets makes it suitable for sub-ppm gas sensing as graphene membranes are easily deflected under small pressure, resulting in change in its electronic properties. In this work, graphene sheets are used as piezoresistive materials to detect the change in the electrical resistance due to strain caused by a pressure load. The graphene used was grown by CVD machine using Cu as catalyst. First step was the integration of Graphene in the desired sensor architecture, the membrane structure. This sensor was tested under atmospheric pressure in order to be tested at a later stage in vacuum. A vacuum setup was put in place for the testing, the integrity of this setup was achieved for a base pressure of 2.5 mTorr. The device was fabricated using a silicon wafer with 1µm PECVD SiO₂. Photolithography was used to pattern the oxide and then etched into cavities using reactive ion etching (RIE). The cavities were of the size of 5µm, 10µm, 25µm, 35µm and 50µm and etched 723 nm deep using CHF₃ gas in the reactive ion etching (RIE). The graphene was transferred over the substrate and on top Metal contacts of Titanium/Aluminum were deposited. The optical images showed that the graphene sheet transferred to the substrate was not continuous nor completely flat due to the exfoliation process used to transfer the graphene from Cu to the substrate. Electrical characterization in a kelvin like 4-points probe was carried out on the graphene membrane to ascertain conductive path within the metal contact. The resistance was found to be about 1.3kΩ for a voltage sweep of 0-1 V. This is a result of the good electrical conductivity of graphene. A sweep of the temperature from 21.5C to 47C was applied on the graphene to check the stability in accordance to the temperature. The resistance of the graphene increase slightly with increasing temperature and become unstable for a temperature higher than 45C. The graphene was cooled down and brought back to the initial room temperature of 21.5C and the resistance was found to be 1.3kΩ. Which leads us to the conclusion that the graphene is able to reset to the initial resistance value after increase in temperature. Thus the temperature do not affect the physical properties of the graphene. Raman spectroscopy was performed on Graphene on substrate and Graphene on top of cavity, the intensity ratio of the peak I(2D)/I(G) decrease from 4 to 1.7.We have been able to successfully fabricate a membrane structure to serve as a sensor for gas. The resulting electrical characterization shows the material can be used as a sensing material. Ongoing work involves the integration of the membrane structure and testing it in vacuum using the setup already installed and test with different gases.

Two-dimensional (2-D) semiconductors exhibit excellent device characteristics, as well as novel optical, electrical, and optoelectronic characteristics. In this talk, I will present our recent advancements in defect passivation, contact engineering, surface charge transfer doping, and heterostructure devices of layered chalcogenides. We have developed a defect repair/passivation technique that allows for observation of near-unity quantum yield in monolayer MoS₂. The work presents the first demonstration of an optoelectronically perfect monolayer. Forming Ohmic contacts for both electrons and holes is necessary in order to exploit the performance limits of enabled devices while shedding light on the intrinsic properties of a material system. In this regard, we have developed different strategies, including the use of surface charge transfer doping at the contacts to thin down the Schottky barriers, thereby, enabling efficient injection of electrons or holes. We have been able to show high performance n- and p-FETs with various 2D materials. Additionally, I will discuss the use of layered chalcogenides for various heterostructure device applications, exploiting charge transfer at the van der Waals heterointerfaces. I will also present progress towards achieving tunnel transistors using layered semiconductors.

Deterministic self-assembly, demanded in nanotechnology, needs a highly precise control of driving forces and energy minimization at the nanoscale, which has been applied in the macro-level, for example, capillary origami and 3D devices. We utilize dry-releasing approach via rapid thermal annealing and manipulate the surface tension of nanodroplets to assist the rolling of strained nanomembranes, and thus push the downscale limitation of rolled-up tubes. The capillary torque generated by the nanoparticles on the nanomembrane surface can efficiently reduce the diameter of rolled-up tubes and thus overcome the downscale limitation (below 100 nm), which is consistent with our theoretical predication. Furthermore, such small tubes embedded with Pt nanoparticles can work as nanoengines and exhibit a dramatic acceleration in speed compared to those with smooth Pt surface due to enhanced mass transfer and large surface area.
We also demonstrated that the unique geometry of the whispering-gallery plasmonic nanotubular cavities, which are fabricated by a strain-engineered self-rolling approach, can significantly enhance the surface plasmon resonance as a result of highly concentrated optical fields. Such synchronous and coherent coupling of the plasmonic and whispering-gallery resonance greatly enhances Raman signals, which could potentially be a simple and robust method towards single-molecular detection with good optimization. The tunability of the coupling effect could open up novel ways to develop new device concepts for high performance deep-sub-wavelength optoelectronic devices such as plasmonic waveguides, single cell photonic nanoprobes, and hyperbolic metamaterials. Our methodology offers a great opportunity for mechanical deformation, such as folding, bending, buckling, and zipping, in nanoscale self-assembly, and may enable solid nanomembranes becoming an essential building blocks in flexible electronics, and lab-on-a-chip micro/nano-electromechanical systems (MEMS/NEMS).

Stronger and more robust nacre-like laminated GO (graphene oxide)/SF (silk fibroin) nanocomposite membranes are obtained by enhancing the interfacial interaction of “bricks”-GO sheets and “mortar”-silk interlayers via controlled water vapor annealing. This annealing has been utilized to relax the confined silk backbones that resulted in significant increase in all mechanical properties measured by bulging test: ultimate strength (by up to 41%), Young’s modulus (up to 75%) and toughness (up to 45%). The highest value achieved for the nanocomposite membrane after annealing shows ultimate stress of 460 ± 65 Mpa, Young’s modulus of 105 ± 10 GPa and toughness of 2.1 ± 0.3 MJ/m³, all characteristics among the highest values for GO based nacre-mimic nanocomposites. Reinforcement mechanism is further investigated. We suggest that though water vapor annealing, SF molecules recrystallization by using hydrophobic surface regions of GO as nucleation sites for beta-sheet formation and directly assembled into nanofibrils that strongly cling on GO layer in situ. Well-ordered packing of GO layers and SF nanofibrils with aligned b-sheet domains results in stronger interfacial interactions with enhanced shear strength across SF layer. The work presented here not only gives the better understanding of SF and GO interfacial interactions, but also provides insight on how to enhance the mechanical properties for the nacre-mimic composites by focusing on adjusting the delicate interactions of heterogeneous “bricks” and adaptive “mortar”.

Single layer graphene is a promising system as a separation membrane. We consider mechanisms and methods of analysis for gas transport through graphene. We also present a detailed analysis of experimental gas permeation data through single layer graphene membranes under batch depletion conditions parametric in starting pressure for He, H₂, Ne, and CO₂ between 100 and 670 kPa. We show mathematically that the observed intersections of the membrane deflection curves parametric in starting pressure are indicative of a time dependent membrane permeance (pressure normalized molecular flow). Analyzing these time dependent permeance data for He, Ne, H₂, and CO₂ shows remarkably that the latter three gases exhibit discretized permeance values that are temporally repeated. Such quantized fluctuations (called “gating” for liquid phase nanopore and ion channel systems) are a hallmark of isolated nanopores, since small, but rapid changes in the transport pathway necessarily influence a single detectable flux. We analyze the fluctuations using a Hidden Markov model to fit to discrete states and estimate the activation barrier for switching at 1.0 eV. This barrier is and the relative fluxes are consistent with a chemical bond rearrangement of an 8–10 atom vacancy pore. Furthermore, we use the relations between the states given by the Markov network for few pores to determine that three pores, each exhibiting two state switching, are responsible for the observed fluctuations; and we compare simulated control data sets with and without the Markov network for comparison and to establish confidence in our evaluation of the limited experimental data set.

Membrane separations play an important role in the mitigation of global problems, such as water shortage, or air pollution. Nanoporous graphene membranes have significant potential to advance membrane technologies for gas separation, water desalination, chemical separation and nanofiltration. Understanding the mechanical strength of porous graphene is critical because membrane separations often involve high pressures. We studied the burst strength of CVD graphene membranes placed on porous supports at applied pressures up to 100 bar by monitoring the gas flow rate across the membrane as a function of pressure. Increase of gas flow rate with pressure allowed for measurement of the fraction of graphene that failed under increasing pressure. SEM and AFM images acquired before and after the burst test were in good agreement with the gas flow rate measurements, and revealed that wrinkles in graphene were prone to failure whereas non-wrinkled areas could sustain high pressure. Graphene membranes consisting of graphene on support membranes with smaller pore sizes tend to have better overall quality, since the extent of damage due to wrinkles is limited. As an essential aspect of the graphene membranes for separations, the effect of created defects on mechanical strength is also studied. We find that the porous graphene membranes with created defects are still ultra-strong, even though finite decrease of the strength occurs. Our study shows that polycrystalline CVD graphene has ultra-high burst strength under applied pressure, suggesting the possibility for its use in high-pressure membrane separations.

Strain engineering has long been recognized as an effective approach for tuning electronic and transport properties of electronic devices as well as for fabrication of nanostructures. In this talk, I will discuss a novel concept of strain engineering to tune spin transport via mechanical bending of a 2D topological insulator (TI). It works by the physical principle that the spin orientations of helical edge states of a 2D TI nanoribbon change continuously when the nanoribbon is bent into a curved shape, as shown by the solution of Kane-Mele model in a curved graphene. Using TI nanofilm of Bi/Cl/Si (111) as a material example, we further demonstrate by first-principles calculations that the relative spin orientations between the two edge states of a Bi/Cl/Si(111) nanoribbon change gradually from antiparallel to parallel when it self-bends from a planar structure to a half cylinder driven by inherent strain. This novel approach of strain engineering of spins via “topological nanomechanical architecture” affords a promising route towards realization of robust spin injectors with 100% spin polarization and large spin current density.

In this study, we report the abnormal and significant thickness-dependent optical property modulation of Methlyammonium Lead Bromide (MAPbBr₃) up to 100 nm grown from Van der Waals (VDW) epitaxy on mica substrates. Single crystalline perovskite square films with thickness ranging from four to several hundreds of nanometers have been obtained which display a blue shift of the photoluminescence peak - 150 meV. The XRD study and TEM characterization reveal clearly the VDW epitaxial relation between MAPbBr₃ and mica. Multiple explanations including the epitaxial strain, band-filling, size-dependent Stokes shift and photon recycling effects have been explored after which the strain factor turns out to play a significant role. Our study is the first attempt to reveal how significant VDW strain could be in such materials system. It sheds light on how we should design halides materials growth via vacuum technology.

Graphene-based materials have attracted considerable interest due to their potential use in a broad range of applications including membrane separations, electrical devices and biomaterials. Their performance in many of these applications relies on the ability to precisely control the layer number and film density of the materials produced. Chemical vapor deposition (CVD) is one of the most promising methods to produce single layer graphene. However, the high cost, limited choice of substrates and error-prone transfer processes limit its application in large-scale roll-to-roll fabrication. On the other hand, laboratory-scale film processing approaches like spin-coating, filtration and drop-casting are incapable of producing films with nanometer-scale control of film thickness and uniformity and/or over the large length scales required for many applications. Recently, the Langmuir-Blodgett (LB) deposition technique has been applied to graphene and graphene oxide with some success. In this method, the material is dispersed at the air-water interface, the floating material, held up by surface tension, and is densified by the compression of two floating barriers. The resulting film is transferred onto substrates by dip coating. While fine control over layer number and film density has been achieved, the technique is currently limited to small area films and suffers from the challenge of losing up to 99% of the material used for transfer. In our work, we developed a directed assembly process for producing graphene oxide monolayers at the air-water interface which assemble into densely tiled films without the need for adjustable barriers. By simply changing the spreading solvent, we obtain high-yield transfer and are able to measure Langmuir surface areas as high as 800 m²/g. Furthermore, we use both in situ Brewster angle microscopy and a custom-made Langmuir-Adam balance to study the mechanism of film formation. The process has also been successfully extended to other 2D nanomaterials such as MoS₂. We believe that the directional film growth mechanism and its applicability to a variety of 2D materials make it a promising process for continuous roll-to-roll deposition for a wide range of applications.

Despite their widely recognized potential in gas storage, catalysis, separation methods, etc., metal-organic frameworks (MOFs) have not yet found widespread industrial applications, mostly on account on the inability to upscale these unique materials. Typically, MOFs come as small disjoint crystallites with the largest reported dimensions in millimeters¹. Recently, there has been significant effort to develop methods that would deposit MOFs over large surfaces but they have not, so far, yielded freestanding or formable/moldable structures. Here, we report synthesis of freestanding, porphyrin-based MOF films at 6-inch-wafer scales, with preferred crystalline orientation, and with the ability to form such structures over arbitrary surface topographies, including micropatterns. Other unique features of this approach are that (1) the formation of the pre-complex promoting MOF growth and the growth itself take place simultaneously, in one pot, and without any need for prior surface activation/functionalization; (2) the orientation of the crystallites in the film can be controlled by the substrate and (3) the method is easily extended to various porphyrin-based systems. Demonstrations in controlling surface wettability (including both water-pinning Wenzel² and water-repellant^3,4 Cassie states), in the recovery of oil spilled over seawater and Volatile Organic Compounds (VOCs), and in large–area chemo or chemo-resistive sensors illustrate the practical potential of truly macroscopic MOF materials.

Over the last years substrate-supported composite films of ligand-stabilized or cross-linked gold nanoparticles (GNPs) attracted significant interest due to their intrinsic, tunneling-based charge transport mechanism, which is highly sensitive to external stimuli. For example, such films were used for the fabrication of highly sensitive strain gauges or resistive sensors for chemical vapors and gases.
In a recent publication we demonstrated the lift-off of alkanedithiol cross-linked GNP thin films and their transfer to 3D electrode microstructures, producing freestanding, conductive nanomembranes with thicknesses in the 20 to 100 nm range and lateral dimensions of several hundreds of micrometers.[1] The tunable elasticity as well as their unique charge transport properties make these membranes highly interesting for the application as functional materials in nano-/microelectromechanical systems (NEMS/MEMS), such as sensors or actuators.

Here, we present the focus of our current research, which is the exploration of novel NEMS/MEMS devices, exploiting the unique properties of cross-linked GNP nanomembranes.
In a recent paper[2] we reported the fabrication of a novel resistive pressure sensor, employing a 1,6-hexanedithiol (6DT) cross-linked GNP nanomembrane as both, diaphragm and strain sensitive transducer. The nanomembrane was deposited over a microfabricated cavity featuring proximal electrodes, suitable for monitoring its resistance. An applied pressure difference resulted in deflection and strain of the nanomembrane and a corresponding resistance change. The elasticity as well as good strain sensitivity of the membrane material enabled a pressure sensitivity which outranges values reported for resistive graphene and silicon based sensors.
Furthermore, we were able to show that freely suspended GNP nanomembranes, placed closely above a counter electrode, can be deflected by electrostatic forces.[3] Electrostatic actuation is a versatile principle, which is prevalently used in NEMS/MEMS. Applying AC voltages, we utilized this actuation mechanism to excite oscillations of the GNP nanomembranes, which were characterized using interferometry. By variation of the excitation frequency, different vibrational modes of the nanomembranes could be observed and imaged by mapping experiments. For circular nanomembrane resonators (diameters of 50 and 100 µm) resonance frequencies in the high kHz to low MHz range and quality factors of up to ~2000 were observed.[4] We present current investigations, exploring the applicability of these devices as microgravimetric sensors.

[1] H. Schlicke, J. H. Schröder, M. Trebbin, A. Petrov, M. Ijeh, H. Weller, T. Vossmeyer, Nanotechnology 2011, 22, 305303.
[2] H. Schlicke, M. Rebber, S. Kunze, T. Vossmeyer, Nanoscale 2016, 8, 183-186.
[3] H. Schlicke, D. Battista, S. Kunze, C. J. Schröter, M. Eich, T. Vossmeyer, ACS Appl. Mater. Interfaces 2015, 7, 15123–15128.
[4] H. Schlicke, C. J. Schröter, T. Vossmeyer, submitted.

As always in physics the response of a material is strongly altered once the dimensionality is reduced. This is especially true when electronic and thermal transport processes are considered. Consequently, the dimensionality can be gauged by the mean free path of e.g. an electron or a phonon as compared to the size of the system. This translates into the condition of two-dimensionality for a nanomembrane when the membrane is made to be thinner than the phonon mean free path L_ph at room temperature, i.e. 300 nm for silicon. Hence, such nanomechanical membranes offer novel applications, such as mass sensing of large bio-molecules with extraordinary sensitivity. The general idea is that perfect mechanical mass sensors should be of extremely small size to achieve zepto- or yocto-gram sensitivity in weighing single molecules similar to a classical scale. However, the small effective size and long response time for weighing biomolecules with a cantilever restricts their usefulness as a high-throughput method. Commercial mass spectrometry (MS) on the other hand, such as electro-spray ionization (ESI)-MS and matrix-assisted laser desorption/ionization (MALDI)-time of flight (TOF)-MS and their charge amplifying detectors are the gold standards to which nanomechanical resonators have to live up to. These two methods rely on the ionization and acceleration of biomolecules and the following ion detection after a mass selection step, such as time-of-flight (TOF). The principle we are describing here for ion detection is based on conversion of kinetic energy of the biomolecules into thermal excitation of CVD diamond nanomembranes via phonons, followed by phonon-mediated detection via field emission of thermally emitted electrons. We fabricated ultrathin diamond membranes with large lateral dimensions for MALDI-TOF MS of high mass proteins. These diamond membranes are realized by straightforward etching methods based on semiconductor processing. With a minimal thickness of 100 nm and cross sections of up to 400 x 400 µm² the membranes offer extreme aspect ratios. Ion detection is demonstrated in MALDI-TOF analysis over a broad range from Insulin to BSA. The resulting data in detection shows much enhanced resolution as compared to existing detectors, which can offer better sensitivity and overall performance in resolving protein masses.

Biological lipid bilayer membrane is an ideal example for precise and efficient molecular separation. One of its characteristics is a free-standing property with molecular thickness, and molecular scale phenomena become dominant in the direction of the membrane thickness. Thus, artificial membrane with a free standing properties and nanometer thickness would be a unique property different from conventional thicker membrane. Based on this idea, we have developed functional free-standing nanomembranes with a centimeter-scale of lateral size. These membrane are manipulable macroscopically, event its thickness is a few tens nanometers.
We have succeeded to prepare a free-standing and ultrathin membrane with precise molecular filtration ability by designing nanochannels structures across a membrane. Our next target is to separate further small molecules, including CO₂ and gaseous molecules, because membrane separation of CO₂ is one of promising CO₂ capture technologies. In this scope, we have developed membranes composed of polymer and inorganic materials.
In polymeric nanomembranes, we have investigated cross linkable materials, such as an epoxy resin, urea and melamine derivatives, for the preparation of nanomembrane. In all case, we have succeeded to prepare free-standing membrane with a few tens nanometer thick, and the gas permeance of each membrane was investigated.
In inorganic membrane, we employed the composite materials composed of titanium alkoxide carboxylic derivatives, such as phthalic acid, to control the gas selectivity of the membrane. Based on a spin-coating process, titania composite membrane with the thickness of 100 nm or less was prepared on a PDMS support. Some composite membrane, show preferential CO₂ permeation over nitrogen.
In membrane separation, the thickness plays an important role for the efficient separation. Further thinning to reach the thickness of a biological lipid membrane is our challenge to create ideal membrane separation based on molecular dynamics.

Addressing water pollution arising from oil spillage and chemical leakage is still challenging and much progress has been made currently toward separating oil-water mixture with high flux by super-wetting membranes [1]. As permeation theory predicts that filtration flux is inversely proportional to the membrane thickness, membrane with thickness at nanometer scale has been a trend to obtain high performance in both selectivity and permeability [2, 3]. However, durability of ultrathin films is a major tough challenge, especially for the ability to undergo external mechanical forces or damages. Moreover, biofouling resulted from growth of unwanted marine organisms cannot be effectively avoided and causes detrimental effects on membrane separation systems [4]. Therefore, a new approach for fabricating a durable micro- or nano-membrane that can withstand a series of harsh conditions, while ensuring high-flux molecular transport, has been presented.

Herein, an extremely robust carbon nanofiber-polydimethylsiloxane (CNFs-PDMS) network inlay-gated stainless steel mesh (SSM) that shows superhydrophobic property is subtly designed and prepared by improved vacuum-based suction. Carbon nanofibers with 100 nm diameter were deposited into SSM pores to form network membrane. The resulting SSM/CNFs-PDMS membrane exhibits excellent resistance to harsh environments such as acid, salt, organic, biofouling, and mechanical abrasion treatments. Particularly, mechanical damage to the CNFs-PDMS network can be avoided using the protective support of SSM to ensure super-wetting performance. Compared to previous superhydrophobic membranes, the thickness significantly decreases, leading to enhanced oil-in-water emulsion separation flux. The membrane shows a gravity-driven water-in-oil emulsion separation with flux up to 2970 L/m²h¹. Even if the SSM/CNFs-PDMS membranes are eroded and abraded, high separation performance was still maintained accordingly. Moreover, this technique has been applied for preparing highly durable ultra-thin carbon nanotubes (CNTs) nano-membrane with high flux in present research. Thus, this work can provide a brand new route for creating durable and high-flux separation systems by combining nano-membranes with protective mesh with inlay-gated structure.
References
[1] B. Wang et al. Chem. Soc. Rev. 336-361(2015) 44
[2] S. Karan et al. Science 1347-1351 (2015) 348
[3] Z. Shi et al. Adv. Mater. 2422-2427 (2013) 25
[4] J. A. Callow et al. Nat. Commun. 244 (2011) 2

A process that is of particular interest in bio-sensing is that of a single molecule passing through a membrane. The capability to fabricate precise solid-state nanopores provides an exciting tool to study the passage and detection of bio-molecules. Some molecules develop a surface charge when mixed into a salt solution. When these molecules pass through the solid-state nanopore, they reduce or increase the ionic current. The drop or rise in the current is dependent on the size and shape of the molecule, and consequently unique for each bio-molecule.


At Ingenuity Lab, we conduct both COMSOL Multiphysics simulations and ionic experiments to characterize solid-state nanopores. 50 nm thick free standing silicon nitride (SiN_x) membranes, 80x80 µm² in dimension are fabricated on a silicon base using chemical vapor deposition, photo-lithography, reactive ion and chemical etch. Hour glass shaped nanopores, with a half cone angle of 20°, varying between 2 nm to 8 nm in size are sputtered out on the SiN_x membrane using JOEL 2200FS TEM. Their structure is studied by tomography. The pore current is then measured in a specialized cell, with 1M KCl acting as the buffer solution for all experiments. Bias is applied using Ag/AgCl electrodes interfaced to Axon-patch 200B, with the bias ranging between ±0.2 V. COMSOL Multiphysics 5.2 is used to setup Electrokinetic simulations, for pore conductance measurements. Poisson-Nernst-Planck equations coupled with Navier-Stokes system of equations are utilized to setup the models. The simulations were conducted in 1M KCl solution for a ±0.2 V bias, and a surface charge of -0.02 C/m² was used for the SiN_x membrane. These were studied for pore sizes from 2.2 nm to 10 nm and demonstrated the conductance ranges from 2.35 nS to 44.22 nS. The geometry of the pore for simulation was modeled by duplicating the structure obtained from tomography. The simulations results matched the experimental measurements within 10 to 30 percent error.


Molecular sensing is one of the predominant application of solid-state nanopores. The desired molecule is added to trans side of the device; the charged molecule is then electrophoretically pulled through the pore onto the electrode oncis side. Here, we observe the translocation of the following molecules, Lambda DNA translocation through a 5 nm conical pore, sulfated glycosaminoglycan polysaccharide translocation through a 8 nm glass hour shaped pore, in both the cases folded and unfolded molecular translocation is observed . The results from this study will be presented.

We seek to mimic the folded molecular architectures found in nature with synthetic polymers. In order to achieve this goal, we must be able to efficiently produce polymer chains that contain chemically dissimilar monomer units arranged in a precise sequence. We achieve this by using the automated solid-phase submonomer synthesis method to generate sequence-defined peptoid polymers up to 50 monomers in length. The method uses readily available primary amine synthons, allowing hundreds of chemically diverse sidechains to be cheaply introduced.

Here we examine the design of peptoid sequences that can form highly ordered supramolecular nanosheets and nanotubes, and compare their molecular structures to protein structures found in biology. Peptoid nanosheets are biocompatible, can be readily functionalized, and produced in high yield by solution assembly, making them ideal for applications in molecular recognition and separations.

Membrane gas separation process is more energy efficient when compared to the conventional cryogenic distillation, pressure swing and amine based sorption processes. Thus, it has gained enormous interest due to the increasing industrial demand for pure gas species and the urgency for carbon capture. The ultimate challenge of membrane gas separation is to simultaneously achieve both high flux and high selectivity. Based on the inverse relation between the flux and the membrane thickness, a typical gas separation membrane has an asymmetric structure with ultrathin gas selective layer on top of the ultrapermeable porous support, and is usually fabricated by either phase inversion or interfacial polymerization. However, these two state-of-the-art techniques have difficulty producing pinhole-free selective layers below 100 nm. In this talk, we report the employment of chemical vapor deposition (CVD) as an up-scalable technique to deposit ultrathin gas selective polymer films on top of ultrapermeable poly[1-(trimethylsilyl)-propyne] (PTMSP) cast membranes. Gas selective membranes as large as 175 cm² were successfully fabricated. Depending on the choice of starting monomer, dramatic difference in the gas permeation properties of such ultrathin films was observed. For instance, supported ultrathin polymer films obtained from the robust phorphyrin based monomer lead to high CO₂/N₂ selectivity and negligible H₂/CO₂ selectivity. In contrast, the polymer films derived from ionic monomer acrylic acid (AA) resulted in significant H₂/CO₂ selectivity but small CO₂/N₂ selectivity. Additional ultrathin films based on different vinyl monomers are also being investigated for their gas separation properties.

Reactive electrochemical membrane (REM) based on Magnéli phase titanium oxides (Ti_nO_2n−1, n = 4 to 10), are attractive, because of their high conductivities, chemical stability, and low cost with unique functionality to produce OH radical via water oxidation. Most of the present Magnéli phases based REM are limited to the formation of high-quality, the most conductive and single magnéli phase, i.e., Ti₄O₇. This study reports the synthesis and electro-oxidation capabilities of Magnéli Ti₄O₇ REM using probe molecules and organic compounds through both direct oxidation (oxalic acid) and interaction with OH radical (terephthalic acid). High membrane fluxes of (1736 LMH bar^-1) resulted in a convection-enhanced rate constant for Fe(CN)₆ ^4- oxidation of 8.13 × 10^-4 m s^-1 and that for Fe(CN)₆ ^3- reduction of 10.84 × 10⁴ m s^-1 that approaches the kinetic limit. Batch experiments were also performed for the electrochemical destruction of N-nitrosodimethylamine (NDMA) in a flow-through electrode for single-pass sequential reduction–oxidation.

Graphene oxide (GO) and other carbon nanoarchitectures could represent a valid alternative to traditional polymeric membranes in environmental applications, such as water filtration. In this work, we individuated and tackled the key challenges for the realization of a polymer-free and fully carbon nanomembrane (CNM). Some of these challenges include an understanding of water transport inside CNM, the influence of the substrate on CNM performance, the analysis of CNM stability when in operation, the investigation of possible cleaning procedures and the study of the contaminants removal properties of CNM. In particular, CNM could be used for the treatment of contaminated water, in which the removal properties are dictated by size exclusion and electrochemical processes. The CNM is composed of a thin (<100 nm) hybrid structure made of GO and carbon nanotubes (CNT). The effective area of the CNM is circa 10 cm², but it could be easily scaled-up. The selectivity of the nanomembrane can be tuned by varying the GO:CNT ratio and by chemically modifying the CNM with easy-to-implement techniques such as UV irradiation and hydriodic acid treatment Moreover, we provide fluid dynamics mesoscopic simulations for the water flow in this CNM, which allowed us to include 10⁵ water molecules. This represents a two order of magnitude improvement compared to traditional molecular dynamics simulations. The fluid dynamics simulations and the challenges tackled in this work can be extended to other 2D materials (e.g. MoS₂) and pave the road for understanding how the nanomembrane could be used for water treatment and other engineering applications.

In this paper, the graphene test paper (GTP) is fabricated by ion-assisted filtration, to detect different types of liquid in daily life. The layer-by-layer structure of GTP is obtained through preparation parameters control, to enhance the liquid sensing sensitivity. The droplet could impact the graphene flakes' contact state and sheet carrier density, and therefore cause varieties in the total resistance. The droplet's parameters, such as volume, polarity, viscosity and volatilization rate, influence the GTP relative resistance waveform. We extract the waveform feature parameters from the performance data by applying waveform analysis, reduce the dimension in the data structure by applying the principle component analysis (PCA) method, and finally demonstrate a classification on the data to identify different types of liquid. We further demonstrate quality test applications of GTP by successfully identifying milk, genuine liquor and their fake version. The GTP also has promising applications in human health monitoring, such as urine detection for diabetes mellitus.

I discuss recent results from our research group on designing flexible and strong responsive polymer and biopolymer nanocomposite materials for advanced flexible electronic applications.[1] Ultrathin silk fibroin proteins and cellulose nanocrystals are assembled in order to conduct surface modification and control assembly with graphene oxide sheets with controlled chemical composition. We demonstrate and discuss fexible laminated bionanocomposites with developed interphase morphology with extremely high elastic modulus and toughness, and conductive patterning capability with high local electrical conductivity.[2,3] Some peculiar features of these flexible nanocomposites for prospective localized tactile sensing are demonstrated and discussed.[4]

[1] K. Hu, et al., Graphene–Polymer Nanocomposites for Structural and Functional Applications, Prog. Polym. Sci., 2014, 39, 1934.
[2] C. Ye, et al., Cellulose Nanocrystal Microcapsules as Tunable Cages for Nano- and Microparticles, ACS Nano, 2015, 9, 10887.
[3] R. Xiong, et al., Combining Strength, Toughness and Stretchability in Cellulose Nanocrystal-Graphene Oxide Nanomaterials, Adv. Mater., 2016, 28, 1501.
[4] K. Hu, et al., Self-Powered Electronic Skin with Bio-Tactile Sensitivity, Adv. Mater. 2016, 28, 3549

Self-assembly of nanoparticles at liquid surfaces or interfaces has emerged as a simple way to create two-dimensional membranes with tunable properties. In these membranes, inorganic nanoparticles are coated with a shell of organic ligands that interlock as spacers and provide tensile strength for the monolayer. In this talk, I will show that in situ grazing incidence small angle X-ray scattering, couple with molecular dynamics simulation effort, can offer many detailed insight into understanding the self-assembly process. I will describe our study on two different systems: thiolated Au nanoparticles in an evaporating toluene droplet and on the surface of a water droplet. These studies elucidate the importance of self-assembly kinetics, the crystallinity of the membrane and the molecular distribution of ligand around the nanoparticle, which all contribute to the mechanical property of final structure.

Nanofiltration membrane technology will be highly attractive typically in pharmaceutical, petrochemical and water-purification industries because of its low energy-consumption attribute. In pursuit of this paramount application potential, our study develops a thin carbon nanocomposite membrane on a porous kaolinite disc through co-pyrolyzing a polymer coating layer comprising a special matrix and poly(furfuryl alcohol) (PFA) nanospheres (NS) distributed in the matrix. This polymer coating is fabricated by casting a copolymer solution containing a small amount of PFA-NS on the kaolinite disc. We synthesize this copolymer that has a comb-like chain structure formed of lauryl methacrylate (LMA) and the adduct of glycidyl methacrylate and dodecylamine (GMA-DDA). Such a structure allows it to be converted to a uniform and dense carbon matrix strongly adhered to the kaolinite disc through pyrolysis at 400 ^oC in Ar. To introduce nanofiltration channels into the resulting dense carbon matrix, a low dose of PFA-NS, ca. 10 wt% of the P(LMA-co-GMA-DDA), is incorporated into the copolymer coating matrix, which is then subjected to pyrolysis. It is found that this co-pyrolysis design results in a carbon composite membrane where the carbonized PFA-NS are embedded in a carbon matrix consisting of even smaller carbon grains compared with the C-NS. Clearly, the co-pyrolysis process causes interfacial induction that translates the porosity from the embedded C-NS to adjacent carbon matrix generated at the same time. This alternative carbon nanocomposite membrane demonstrates unique ultrafiltration performance for the separation of different types of organic solvents from designated solutes because of its nano-grain packing structure. The presentation will also include structural characterization data to support the membrane synthesis and separation assessment.

Membrane process for water treatment has been increasingly used as an effective solution of water scarcity problem. Among many kinds of water treatment membranes, nanofiltration (NF) membrane with nano-thickness active layer shows outstanding performance due to its short penetration path length. With expansion of application area of the NF membrane, membrane damage at harsh condition is newly issued. However, investigation on membrane damage is scarcely conducted. Thus, we developed two types of nanofiltration membrane with nano-thickness active layer composed of branched-poly(ethyleneimine) (b-PEI) derivatives which had acid stability and mendable property. Firstly, we fabricated a positively charged membrane with acid stability and selective ion permeation via introduction of b-PEI nano-thickness active layer on the top of support layer, which was prepared by cross-linking b-PEI at the surface of support. Furthermore, we prepared mendable membrane by introducing stimuli-responsive dynamic bonding to formation of nano-thickness b-PEI active layer, which can repair damaged membrane by connecting new b-PEI chains to damaged membrane surface. Formation of positively charged b-PEI active layer was confirmed by chemical and zeta-potential analysis. The fabricated membrane selectively permeated acid substance, and rejected metal ions under strong acidic condition. Through the selective permeation, the nanomembrane showed high acid recovery performance above 80%, and the performance was well preserved after exposure to highly concentrated sulfuric acid solution over 30 days. In case of the mendable membrane, the mendable nano-thickness active layer was immobilized on the support layer by stimuli-responsive covalent bond. The resulting membrane was repeatedly repaired through stimuli-induced mending process. As a result, the mendable membrane recovered membrane performance. This property could improve the membrane lifetime by restoration of damaged membrane surface. Therefore, we verified that the nanomembranes based on b-PEI derivatives were successfully fabricated with desired properties such as acid resistance, selective ion permeation and mendable property.

Two-dimensional (2D) materials have attracted intense interest due to their novel properties and potential for applications in molecular separation, electronics, catalysis, optics, energy storage, and biomedicine. An ability to develop sequence-defined synthetic polymers that mimic lipid amphiphilicity for self-assembly of highly stable membrane-mimetic 2D materials and exhibit protein-like, sequence-specific molecular recognition would significantly advance the development of functional 2D materials including artificial membranes.

Here I will report my group’s recent discovery in assembly of lipid-like peptoids into extremely stable, crystalline, free-standing and self-repairable 2D membrane materials. They were formed through solvent-induced crystallization process, in which inter-peptoid hydrophobic interactions drove the anisotropic packing of peptoids to form a bilayer-like membrane structure containing strips of hydrophilic domains. Molecular dynamics simulations confirm that the membrane structure deduced from experimental data is energetically favored; moreover, they provide more detailed (atomic level) structural information to show the anisotropic packing of peptoids within the membrane structure.

These peptoid membranes exhibit a number of properties associated with cell membranes, including thicknesses in the 3.5 - 5.6 nm range, spontaneous assembly at interfaces, thickness variations in response to external stimuli, and the ability to self-repair. The quantified repair rates revealed by in situ AFM imaging show that peptoid membranes exhibit much faster repair along x-direction (parallel to the straight edges of membranes) than along y-direction, which is consistent with results observed in the time-dependent membrane crystallization and molecular dynamics simulations.

We further demonstrated that these membranes are superior to lipid bilayers and other assembled 2D materials because: 1) they are free-standing, atomically ordered, and highly stable in pure organic solvents (e.g. acetonitrile and ethanol) as well as high temperature; and 2) a broad range of functional objects can be incorporated and patterned within membranes through large side-chain diversity and/or co-crystallization approaches. Given that peptoids are sequence-specific, highly stable, biocompatible, and exhibit protein-like molecular recognition, we anticipate this new class of 2D materials will provide a robust platform for development of biomimetic membranes tailored to specific applications.

Thin anodic porous alumina (APA), engineered by simple and scalable electrochemical anodization of aluminum and post-fabrication etching, formerly allowed to obtain surfaces with SERS activity, after over coating with a thin gold film [1]. On the other hand, the APA nanoporous surface, which is biocompatible and presents specific roughness, has been extensively investigated for biomedical applications such living cell culture substrate [2] and drug delivery systems [3]. We are interested in combining the above properties, in a SERS-based biosensor of living cells endowed with drug delivery capabilities. In this work we did not improve the SERS functionality, but focused rather on the drug loading/elution of a test drug, the nonsteroidal anti-inflammatory and analgesic molecule Diclofenac. We first fabricated thin APA directly on the aluminium film working as an electrode of a quartz crystal microbalance (QCM) sensor. Then, we carried out pore loading of differently concentrated aqueous solutions of the test drug inside the QCM cell. This allowed using the ultrasonic agitation of the device itself also for enabling improved loading, as well as for continuous time monitoring of the process. After loading with the test drug molecules, an appropriate change of solution, containing phospholipid vesicles of different composition, allowed the formation of supported lipid bilayers (SLB) on the thin APA quartz, which process was also monitored by the QCM for successful performance. Finally, a change in the QCM cell environment was investigated, by means of variations in the parameters of temperature and pH. This allowed driving the opening of pores in the SLB membrane overcoating the thin APA, which in turn allowed for release of the previously loaded drug. All these events are monitored in real time, and we try to model them, according to the respective kinetics. Throughout the proposed activity, a significant improvement in understanding all the mentioned mechanisms is foreseen, by working on a QCM sensor surface, which will then hopefully drive towards the development of a possible biosensor/bioassay, when culturing live cells on the substrate (after preliminary gold coating) and carrying out additional sensing by SERS.
[1] C. Toccafondi, R. La Rocca, A. Scarpellini, M. Salerno, G. Das, and S. Dante, “Thin nanoporous alumina-based SERS platform for single cell sensing,” Appl. Surf. Sci., vol. 351, pp. 738–745, 2015.
[2] “InRedox cell culture substrates of APA,” 2015. [Online]. Available: https://www.inredox.com/applications/cell-culturing-on-nanosubstrates/#AAOcellcult. [Accessed: 14-Jun-2016].
[3] E. Gultepe, D. Nagesha, S. Sridhar, and M. Amiji, “Nanoporous inorganic membranes or coatings for sustained drug delivery in implantable devices,” Adv. Drug Deliv. Rev., vol. 62, no. 3, pp. 305–315, 2010.

Ultra-thin nanoparticle (NP) membranes can be fabricated via the self-assembly of gold NPs at a liquid-air interface. Dried NP monolayers have high mechanical strength with a surprisingly large Young’s modulus on the order of several GPa and can suspend over micron-size holes. NP membranes also have a filtration coefficient (volume flux per ∼82 kPa applied pressure) for aqueous solutions that is ∼100 times larger than typical polymer-based nanofiltration systems. In order to reliably control the large-scale structure and morphology of NP membranes for practical applications such as high-throughput filtration, it is necessary to understand the underlying principles governing the behavior of NP membranes, which often relates to nanoscale phenomena originating from the NP ligands. In our experimental-theoretical study, we looked into the mechanical properties of NP membranes (~5 nm NPs) and observed a drastic (>50%) change in the Young’s modulus during a heating and cooling cycle (a temperature range of 10 – 90 degrees Celsius). The Young’s modulus v.s. temperature curves show two distinct regions with strong dependence on temperature, humidity, and degree of ligand crosslinking. Using large-scale coarse-grained molecular dynamics (CGMD) based on the well-known MARTINI force field, we obtained simulation results that are in excellent qualitative agreement with the experimental observations. The simulation trajectories revealed ligand reorganization/dynamics, quantified by the changes in lattice spacing, number of interdigitating ligands, and level of ligand interdigitation, which explained the hysteresis in the Young’s modulus curves. The simulations also showed a drop in the Young’s modulus when water is present in the membrane, which is consistent with experimental data. Water molecules effectively screen the interactions between NP ligands and also induce their re-organization. Since all the above described phenomena depend heavily on the amount of NP ligand coverage and the mobility of ligands on the NP surface (typically not modeled in MD simulations), this study highlighted the importance of developing new modeling techniques for the realistic simulation of experimental systems.

Atomically thin (2D) membranes have generated a lot of interest in filtration and separation applications. 2D materials like graphene offer the minimum theoretical membrane resistance along with the opportunity to tune pore sizes at the nanometer scale in complete contrast to solution-diffusion of molecules through conventional membranes.
While several methods of synthesis for 2D materials exist, chemical vapor deposition has emerged as the preferable route towards scalable, cost effective synthesis. Here, selective molecular transport through precise and controllably engineered, high-density, sub nanometer diameter pores in graphene grown via chemical vapor deposition processes is demonstrated. A combination of pressure and diffusive transport measurements shows clear evidence for size selective transport behavior. Specifically, we show the ability to achieve selective transport of small molecules across centimeter-scale single-layer porous graphene membranes through facile creation of pores by oxygen plasma treatment of after sealing of leakage. Second, we show that, by tuning the parameters of the CVD process, it is possible to directly synthesize graphene with selective pores that permit passage of salt, but block the transport of small molecules.
The possibility of tuning the selectivity in atomically thin membranes through controlled creation of sub nanometer pores addresses a significant challenge in the development of advanced nano-porous membranes for nanofiltration, desalination, gas and chemical separation and several biological applications. These membranes hence offer opportunities to study and elucidate the complex dynamics and transport phenomena at sub-nanometer length scales.

References:
Kidambi et al. (manuscript in preparation)
O’Hern et al. Nano Letters (2015).
Kidambi et al. Chemistry of Materials (2014).
Boutilier et al. ACS Nano (2014).
Kidambi et al. Nano Letters (2013).
O’Hern et al. Nano Letters (2013).
O’Hern et al. ACS Nano (2012).

We report on the characterization of films containing vertically aligned silicon nanowires (SiNWs) deposited from a solution onto a solid substrate. Because of the inherently high anisotropy of the nanowires, vertically and horizontally aligned SiNWs exhibited spectral and structural differences that could be probed by X-ray diffraction, UV-Vis or Raman spectroscopy, and scanning electron microscopy (SEM), although SEM was used as the primary tool to confirm the orientation of the NW assemblies. Raman scattering results for two forms of tailored nanowires revealed distinct heat dissipation processes upon variation of the orientation of the SiNWs, as predicted recently by theoretical calculations.

Recently our group demonstrated the fabrication of freely suspended nanomembranes of organically cross-linked gold nanoparticles (GNPs) with thicknesses in the 20-100 nm range and lateral dimensions of several hundreds of micrometers.[1] The membranes enable the tunability of their electronic and mechanical properties, e.g. by varying the size of the GNPs or the length and chemical nature of the cross-linker molecules. This tunability as well as their unique intrinsic charge transport mechanism make these materials highly interesting for the application in nano-/microelectromechanical systems (NEMS/MEMS), as we demonstrated by the fabrication of a highly sensitive GNP nanomembrane based pressure sensor[2] and electrostatically driven drumhead resonators.[3] To understand and model the behavior of cross-linked GNP nanomembranes in NEMS and MEMS, the fundamental knowledge of their mechanical and electronic properties is crucial.

In this contribution we report on the mechanical characterization of GNP nanomembranes by micro-bulge tests.[4] In these experiments GNP membranes were placed onto circular orifices and bulged by the application of nitrogen overpressure to their back sides. The resulting deflection of the membranes was recorded using atomic force microscopy (AFM). Using a geometric model, the biaxial stress and strain of the membranes could be computed using the topographic data measured at varying applied overpressure and biaxial moduli, which are closely related to the membranes' Young's moduli, could be derived. We observed that the elasticity of the membranes can be tuned by variation of the alkanedithiol cross-linker's chain length and the GNP diameter.
Further, we investigated the charge transport in the membrane material by low-temperature conductance measurements. Due to its thermally activated tunneling-based mechanism, the conductivity of the composites strongly depends on the interparticle spacing. We show that the conductivity of the membrane materials can be tuned over several orders of magnitude by variation of the alkanedithiol cross-linker's alkylene chain length and the GNP size.

[1] H. Schlicke, J. H. Schröder, M. Trebbin, A. Petrov, M. Ijeh, H. Weller, T. Vossmeyer, Nanotechnology 2011, 22, 305303.
[2] H. Schlicke, M. Rebber, S. Kunze, T. Vossmeyer, Nanoscale 2016, 8, 183-186.
[3] H. Schlicke, C. J. Schröter, T. Vossmeyer, submitted.
[4] H. Schlicke, E. W. Leib, A. Petrov, J. H. Schröder, T. Vossmeyer, J. Phys. Chem. C 2014, 118, 4386-4395.

Determination of chirality and enantiomeric purity is of critical importance in the asymmetric synthesis of chiral compounds such as pharmaceuticals. Chiral sensing components in combination with spectroscopic techniques have been instrumental in obtaining chiral information. High throughput screening of products from combinatorial asymmetric synthesis requires further development of simple and inexpensive tools for enantioselective discrimination. Here we show that a chiral alanine-appended benzene-tricarboxamides gelator as host can initiate heterochiral discrimination toward amines for visual detection of guest chirality by virtue of induced supramolecular nanofiber assembly resulting in gelation. Implementing the chiral gelator in electrospun fibers resulted in solid state films with enantioselective surface wetting properties for determining chirality through contact angle measurements. Chiral amplification in the supramolecular nanofiber assembly enhanced the induced circular dichroism signal allowing precise quantification of enantiomeric purity. Additional confirmation by NMR spectroscopy was realized by distinct chemical shifts afforded by distinct homochiral and heterochiral interactions.

The need for clean water and air is increasing explosively and ceramic membranes received considerable attention for alternative material to use in eco-friendly engineering tools. The advantages of ceramic membranes include good chemical stability, high mechanical resistance, long life and good defouling properties. Their superior chemical, thermal, and mechanical properties mean that they can be backwashed, cleaned with harsh cleaning agents and sterilised at high temperatures, extending their lifetime, cutting down on operating costs. The hollow fiber configuration provided by the much higher membrane packing density can make them more competitive, compared to the tubular and monolith architectures. In general, the packing densities for tubes and multi-channel monolithics are about 30-250 m²/m³ and 130-400 m²/m³, respectively. For ceramic hollow fiber membranes, packing density as high as 1000 m²/m³ can be easily obtained. Therefore, many efforts have been made in the preparation of ceramic hollow fiber membranes. The phase inversion-extrusion technique is a new method for producing ceramic hollow fiber membranes with significantly improved mechanical strength and spinning speed. A suspension of ceramic particles in solvent with a polymer binder is first prepared and then through the phase inversion-extrusion of the polymer binder via exchange with a nonsolvent. Then calcination is required to remove all the polymer binder from the membrane precursor. The ceramic hollow fiber membranes with relatively narrow pore size distribution and strong strength in this work have been successfully prepared by the phase inversion-extrusion technique. The ultrafiltration membranes with small pore sizes below 100nm required for highly treatment. Pore size reduction occurred to 10nm by the spray coating of alumina nano sol and highly uniform layers on ceramic hollow fiber membranes were observed. Water flux and mechanical strength of these membranes were above 200 LMH and 150 MPa, respectively. The ceramic hollow fiber membranes prepared in this study are shown as promising candidate membrane in water treatment applications involving harsh environments.

With the discovery of graphene [1], several research groups have begun to look for new ways to take advantage of the excellent physical properties of graphene [2,3]. Despite the excellent properties of graphene, this one presents difficulties in its use for the development of semiconductor devices [2]. However, small pieces of graphene like nanoribbons exhibit semiconducting properties that allow its use in opto-electronic devices [4]. In this work, we report a capacitive coupling architecture through membranes (like oscillators chain) formed by graphene nanoribbons. For this system we consider only vibrations in the bending axis which is normal to the axis of each membrane. Thus, based on this system we simulate the phonon coherent transport via Monte Carlo trajectories within the jump quantum formalism implemented in QuTip code [5], where the number of thermal occupation is zero. Our results show the conditions of generate or not coherent transport, it may used in developing quantum states engineering protocols similar to the tunneling effect of quasi-particles in the chain Josephson Junction or simulation of Anderson localizations.


[1] K. S. Novoselov et. al, Science, 306, 666 (2004).
[2] J. –C. Chalier, P. C. Eklund, J. Zhu, A. C. Ferrari, Topics in Applied Physics, v 11, pp 673-709 (2007).
[3] A. H. C. Neto et. Al, Rev. Modern Physics, 81, 109 (2009).
[4] G. D Sanders et. al, Journal of Physics: Condensed Matter, 25, 144201 (2013).
[5] J. R. Johansson, P. D. Nation, and F. Nori, Comp. Phys. Comm., 183, 1760–1772 (2012).

Artificial skins based on the flexible, stretchable, conformal, and multi-functional sensor have recently attracted a lot of attentions for monitoring the living body signal, imitating the functionalities of human skin, and applying to wearable electronics. There are two features, which are important issues for such classes of artificial skins; 1) conformal contact on a rough surface of skin and 2) detection of normal force, shear force and vibrations at the same time. There have been a few reports of the bio-mimic tactile sensors that mimic the fingerprint to realize the higher sensitivity as well as the sensing capability of vibrations.[1] However, the high mechanical stiffness of thick sensors not only hinders the detection of vibrational response, but also induces the delamination of devices limiting its application in artificial skins.[2-4]
Herein, we report the novel approach with conformally transferred all-graphene-based pressure sensors onto a human fingertip for human tactile-sensing. We fabricated the ultrathin conformal sensors, which is formed with graphene, on the SU-8 substrate with thickness of 500 nm. Including passivation layer, the total thickness of the resulted sensor is less than 800 nm offering extraordinary mechanical flexibility and enabling conformal contact on a rough surface of fingerprint. Due to the extremely low stiffness of sensor, the correlated vibration of fingerprint easily transfer to the graphene pressure sensors and can be detected with the broad range capability of vibration. In addition, the protruding structure of fingertip induces the anisotropic deformation over the fingertip to enable the detection of direction of shear force. Finally, using the graphene sensor formed on the human fingertip in the measuring system, we successfully 1) distinguished the different surface morphology of various texture samples by vibration response, 2) demonstrated the skin-attachable human-machine interface by the applied shear force, and 3) included the temperature sensor for practically compensating the thermal effect of the human body temperature on pressure sensor. We expect that our novel approach of conformal sensor can offer a breakthrough toward the quantitative analysis and communication of human touch via electronics.

Hydrogen is considered as a green energy carrier of the future. Steam reforming is the most popular process for producing hydrogen from a hydrocarbon feedstock. In order to get pure hydrogen, it is necessary to remove CO2 from the reformer gas by a suitable technique. The present work is concerned with the separation of hydrogen from a mixture with CO₂ by indigenously developed inorganic membranes. Inorganic membranes when compared to organic membranes possess more mechanical, thermal, and chemical stability. In the present study silica membranes with alumina supports have been developed to separate hydrogen and CO₂.The basic pressing technique was used to achieve Alumina supports for membranes. Then the silica membranes have been fabricated by sol gel dip coating method. Characterization of membranes was done under scanning electron microscope and surface area, pore size distribution, pore volume and the average pore size were measured using BET surface area analyzer. The membranes were found to be porous having good mechanical strength as well as high hydrogen permeance and selectivity. The feed mixtures of H₂ and CO₂ were passed separately across the alumina supported silica membranes to study the membrane permanence and selectivity. The effect of pressure and temperature were studied and the process conditions were optimized. The membranes were found to withstand pressure up to 6 bar and a temperature of 600^oC.

The use of graphene derivatives in direct methanol fuel cells (DMFCs) shows a reduction in the methanol crossover, with different methodologies applied to modify the Nafion® membrane in order to decrease the methanol passage. Graphene monolayers present essential requirements for DMFCs as they are natural barriers to atoms and molecules, however, with high proton conduction and stability. Due to natural difficulties to cover large areas, the layer-by-layer (LbL) technique is an attractive methodology to assemble graphene monolayers onto Nafion®. Here, using molecular dynamics simulations with the Assisted Model Building with Energy Refinement (AMBER) force field, we study the structure of reduced graphene oxide (rGO) wrapped by poly (styrene sulfonate) (PSS), namely GPSS nanoplatelet. The water molecules were simulated with the Transferable Intermolecular Potential 3P (TIP3P). The simulations were performed at temperature and pressure of 300 K and 1 atm, respectively, so that the structure and behaviour of the system can be observed at experimental conditions. GPSS forms stable suspensions in water, suitable to fabricate interlocked multi-layered LbL films onto Nafion®. Therefore, the rGO structures considered here were built with several vacancies, hydroxyl, epoxy, carboxyl and carbonyl groups in order to resemble chemically synthesized GPSS nanoplatelets. In addition, it was studied the structure itself of rGO surrounded by PSS molecules and water molecules, in order to verify the similarity of the system with experimental conditions. We found that the “wrapping” of rGO by PSS happened mainly by electrostatic attraction between PSS oxygen atoms and hydrogen atoms present at hydroxyl and carbonyl groups in rGO. These observations are in good agreement with experimental results.

Polyethylene glycol (PEG) in various forms has been widely used in medicine, industry, and nanotechnology. By virtue of its marginal solubility, it exhibits amphiphilic properties that can be readily tuned by adding various salts to its solutions that bring about an intriguing phase separation to a PEG-rich phase at low salt concentrations, and to a nearly PEG-free solution of high salt. Despite the fact that this property has been exploited in various technologies, it is still not fully understood. Here, we present systematic experimental and simulation results on the interfacial behavior of PEG (varying in length) in the presence of a series of alkaline carbonates (A₂CO₃; A = Li, Na, K, and Cs). Using surface tension measurements, we determine the critical PEG concentration (CPC) at which a significant decrease in surface tension occurs in the presence and absence of the aforementioned salts. Unlike the typical critical micelle concentration found in charged surfactants and polyelectrolytes, we find that the change in concentration of K₂CO₃ barely changes the CPC within experimental error, rather it significantly affects the extent to which surface tension decreases at CPC. We have also used X-ray reflectivity and X-ray fluorescence near total reflection to determine the nature of the film formed by PEG at the interface in the presence of salts. Theoretical and computational studies provide an insight on the basis of this effect. The implications of our results to the properties of PEG-functionalized nanoparticles are also discussed.

Nowadays, nanoscale semiconductors constitute one of the most rapid growing research subjects as consequence of their use in microelectronics, which is mainly based on silicon due to its abundance in the earth's crust and its energy gap of 1.1 eV suitable for integrated circuits operated at room temperature. In particular, porous silicon (PSi) has an extensive surface area of about 300 m²/g, allowing important applications such as gas and biological molecule sensors, as well as drug dispensers. In this work, an ab-initio molecular dynamics (MD) study of the adsorption of atoms and molecules on the surface of PSi nanomembrane has been carried out within the Density Functional Theory (DFT) framework. Specifically, we analyze the adsorption at different temperatures of atoms as hydrogen, oxygen, nitrogen and fluorine. This analysis has been performed by using the CASTEP code [1] within the Materials Studio software. On the experimental side, PSi samples were fabricated by means of an anodic electrochemical dissolution of boron-doped p⁺-type (100)-oriented crystalline Si wafers in an electrolyte of hydrofluoric acid and ethanol [2]. During the etching process, an electric direct current (DC) was applied between the platinum electrode and the wafer backside contact. To remove the PSi nanomembrane from the wafer, an additional larger DC was following applied. The thickness of membrane can be controlled by the current intensity and etching duration. Finally, we measure the X-ray diffraction and infrared absorption spectra of samples with and without added atoms or molecules, and compare them with the theoretical prediction, as done for the oxygen case [3].

This work has been partially supported by UNAM-IN113714 and CONACyT-252943. Computations were performed at Miztli of DGTIC, UNAM.

[1] M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark and M.C. Payne, J. Phys. Condens. Matter 14, 2717 (2002).
[2] R. Cisneros, H. Pfeiffer and C. Wang, Nanoscale Res. Lett. 5, 686 (2010).
[3] P. Alfaro, A. Palavicini and C. Wang, Thin Solid Films 571, 206 (2014).

Microfluidic devices provide an excellent way to homogeneously mix and precisely control rate of fluids at the length scales of the structures being made. Further, it gives rise to the mono-dispersed emulsion droplets of functional polymer with electro-optical property and/or bio-medical activity, which leads to the particle array or films by controlling the types of surfactant.
Herein, the porous particles of block copolymer were fabricated with glass capillary fluidics. Specifically, the block copolymers consisting of mechanically and chemically robust polystyrene (PS) and polylactic acid (PLA) suitable for easily selective chemical-etching were employed. The morphology of particle can be tuned depending on the types of surfactants. The monodisperse emulsion droplets of block copolymer with a diameter of ~ 40 µm were generated with surfactant via microfluidic devices. The porous particles (~ 8 µm) embedded with nanopores inside were successfully generated after evaporation of solvent. Consequently, after chemical etching of porous particle containing sacrificial block by reducing agent, the pores in a molecular scale within the porous particles were induced, which can be confirmed via fluorescence spectrometer using fluorescent adsorbents.

The purpose of this study is to develop HMFC (hydrogen membrane fuel cell) using a non-Pd hydrogen permeable alloy membranes, a new concept fuel cell different from the existing thin film SOFCs. By utilizing the high-degree of mechanical stability of metal complex electrolyte which is mainly composed of metal hydrogen separation membrane and thin proton-conducting electrolyte, HMFCs have high potential in the field of thin-film fuel cells. The alloy membranes mainly used for hydrogen separation excessively dependent upon Pd with effective hydrogen storage and transmission performances and an alternative material has not been found. Accordingly, there is a need for studies on non-Pd system hydrogen permeable membrane that can be used in high temperatures with hydrogen permeable rate similar to existing Pd. For decades, Ni-Zr based alloys have been studied as a strong candidate of Pd substitute. In this study, selectivity and permeability of hydrogen of the fabricated Ni-Zr alloy membranes are characterized and compared to those of Pd membrane. A prototype HMFC was build using the fabricated Ni-Zr alloy hydrogen permeable membranes and its performance was tested.

Recently, Ion transport membrane (ITM) or solid oxide membrane (SOM) technologies have been developed for new metal reduction technology because traditional extraction process emitted massive CO and CO2 gas and consumed large amount of electricity. The membrane based extraction process is now a promising candidate for resolving previous problems. Using high oxide ion conductivity such as yttria-stabilized zirconia (YSZ) or gadolinium-doped cerium oxide (GDC) for solid electrolytes between the anode and liquid electrolyte.
Here we successfully were synthesized gadolinium doped ceria (GDC) below 100oC by urea-mediated routes in two steps. Gadolinium nitrate hydrates and Cerium nitrate hydrates were used as reagents. Hydrolysis reactions were induced by various concentration of Urea. The Collected powders were calcined to transform from cerium carbonate hydrates to cerium oxide at 700oC for 2 hours in air.
The GDC powders were analyzed by X-ray diffractometer (XRD) for reveal its phases. The amount of Gadolinium dopants were measured by X-ray fluorescence (XRF) and Energy dispersive X-ray spec troscopy (EDX). Scanning Electron Microscopy (SEM) and Transmission electron microscopy (TEM) were exhibits its size and morphology. Finally, its ion conductivity were measured by electrochemical impedance spectroscopy in various temperature for comparison with commercial YSZ.

The detection of biological toxins in the food is essential for eliminating the risks arising from the consumption of food infected with the toxins [1].The prevention of these diseases through on-field screening of the food for the presence of disease causing toxins remains a global challenge. The conventional methods of detection lack the scope for rapid and on-field detection. Hence, there is a significant demand for the development of biosensor for rapid detection of toxins at the threshold concentration of toxicity.
Our goal is focused towards the development of a nanomembrane based electrochemical biosensor for the detection of marine toxins. We have investigated the use of nanoporous flexible polymer membranes as the substrate for the biosensor. The use of a nanomembrane as a substrate offers nano-confinement of biomolecule thereby enabling an enhanced signal response resulting due to the binding of biomolecules. The nanomembrane provides size matching between the porous nanostructures and the target analytes of interest. The pore size is of the order of few hundreds of nm and is comparable to the size of the target biomolecule The nanostructures of the nanomembrane eliminates the charge screening effect arising from non-specific molecules at the diffuse region of the electrical double layer[2]. The polymeric nanomembrane aids the building of immunoassay to detect the target biomolecule through an affinity based biosensing approach. The use of a nanomembrane for biosensing applications enhances the electrical signal response obtained as a result of biomolecular binding. The flexible nature of the nanomembrane offers distinct advantage of varying the shape of the biosensor for efficient signal acquisition from the test samples. Also, the flexible substrate based biosensors holds scope for large scale manufacturing.
The experimental approach consists of material characterization of the sensor using SEM in order to evaluate the conformal deposition of electrode on the pores of the membrane. The electrical characterization is performed with Electrochemical Impedance Spectroscopy (EIS) technique. The biosensor has been designed as an electronic swab for the detection of presence of toxins. The signal response of the biosensor has been compared between the planar and conformal substrates in order to experimentally validate the benefits of the use of nanomembranes. Thus, the benefits of nanomembranes have been leveraged towards the design of a novel electrochemical biosensor for detection of marine toxins.
1. Palchetti, I. and M. Mascini, Electroanalytical biosensors and their potential for food pathogen and toxin detection. Analytical and Bioanalytical Chemistry, 2008. 391(2): p. 455-471.
2. Panneer Selvam, A., et al., Electrical nanowell diagnostics sensors for rapid and ultrasensitive detection of prostate-specific antigen. Nanomedicine, 2015. 10(16): p. 2527-2536.

Atomically thin (2D) materials such as graphene have recently attracted significant research interest as an ultrathin barrier for mass transport. While several methods of synthesis of 2D materials exist, chemical vapor deposition has emerged as one of the most preferable routes for scalable, cost effective, high quality material synthesis. However, to this date the quality of graphene and other 2D materials have largely been optimized for electronics applications. The quality requirement for mass transport barrier applications tend to be significantly more stringent - for example, a 5 nm defect would make a gas separation membrane completely leaky.
Here we report on the development of a simple, cost effective and fast characterization technique to assess the quality of membrane grade 2D materials directly on the catalyst after synthesis by CVD. Using a combination of diffusion driven liquid transport and pressure driven gas transport measurements across atomically thin membranes, we report on the origin, aggregation and subsequent manifestation of the defects, specifically in relation to barrier applications. These observations allow for the development of a fundamental understanding to tailor the quality of 2D materials for gas/liquid separation and barrier applications and demonstrate centimeter-scale gas barrier membranes.

Kidambi et al. (manuscript in preparation)
O’Hern et al. Nano Letters (2015).
Kidambi et al. Chemistry of Materials (2014).
Boutilier et al. ACS Nano (2014).
Kidambi et al. Nano Letters (2013).
O’Hern et al. Nano Letters (2013).
O’Hern et al. ACS Nano (2012).

As the most abundant renewable organic substance on earth, cellulose is one of the most popular polysaccharide used by industry. Crystalline cellulose has strong mechanical strength and can be used as reinforcement in composite materials. Recently, a nano-composite membrane made by nano-cellulose fibers and single layer graphene oxide has shown a great hydrophilic properties with the losing hydrophobic peak (200) through the X-ray diffraction analysis. We adopt molecular dynamics simulation to investigate the fundamental physics behind this phenomena, especially the interaction between graphene-oxide functional groups and cellulose nano-fibers with different orientations ((110), (1-10) and (200)). Through molecular dynamics simulations, we will identify the main factor to lead to this phenomena and provide the design optimization for further experimental synthesis. The objective is to design high performance cellulose graphene-oxide based nano-composite membranes for hydrophilic applications such as membrane filters and etc.

Effective H₂ sensors that can quickly and sensitively respond to H₂ gas are crucial for the safe deployment of all hydrogen-based applications. Most metallic hydrogen sensors use palladium or its alloys as they are dependent on the high hydrogen sensitivity of palladium. However, since palladium is a novel metal, which is expensive, active research has been conducted on non-palladium hydrogen sensors as an alternative.
Accordingly, this study was conducted on a hydrogen sensor made of a thin film of Ni₆₄Zr₃₆ alloy by using a flexible hydrogen permeation membrane and Ni₆₄Zr₃₆ characteristics. The sensor was made with a polymer/metal/polymer (PMMA/Ni₆₄Zr₃₆/PDMS) structure by depositing a thin film of the Ni₆₄Zr₃₆ alloy between the PDMS of high gas permeation and the PMMA of high hydrogen selectivity by the sputtering process. The study confirmed the hydrogen-sensing property of the sensor by verifying the change in the electronic characteristics as a result of the reaction between Ni₆₄Zr₃₆ and hydrogen. FE-SEM was used to observe the surface and side of the hydrogen sensor, XRD was used to verify the crystal structure, and XPS depth profiling was used to study the formation and thickness of the Ni₆₄Zr₃₆ film of the hydrogen sensor. The hydrogen permeation rate was also measured with respect to change in pressure.

One of the two main processes of engineering nanostructures is the top down method, which is a direct engineering method for Si-type materials using photolithography or e-beam lithography. The other method is the bottom-up method, using nano-imprinting. However, these methods are very dependent on the equipment used, and have a high process cost⁽¹⁾. They are also relatively inefficient methods in terms of processing time and energy. Therefore, some researchers have studied the replication of nano-scale patterns via the soft lithographic concept, which is more efficient and requires a lower processing cost. In this study, accurate nanostructures with various aspect ratios are created on several types of materials. A silicon (Si) nanomold is preserved using the method described, and target nanostructures are replicated reversibly and unlimitedly to or from various hard and soft materials. The optimum method of transferring nanostructures on polymeric materials to metallic materials using electroplating technology was also described. Optimal replication and demolding processes for nanostructures with high aspect ratios, which proved the most difficult, were suggested by controlling the surface energy between the functional materials. Relevant numerical studies and analysis were also performed. Our results showed that it was possible to realize accurate nanostructures with high depth aspect ratios of up to 1:18 on lines with widths from 100~400 nm.


In addition, we were able to expand the applicability of the nano structured mold by adopting various backing materials, including a rounded substrate. The application scope was extended further by transferring the nanostructures between different types of materials with reversible way, as well as an identical species of material.

Single-crystal materials in sheet form provide a platform for fascinating new science and numerous technology breakthroughs. The key enablers of this platform are thinness (compliancy/ flexibility, novel mechanical properties, ready strain management), a large surface-area-to-volume ratio, and, in semiconductors, the ability to manipulate electronic/optoelectronic properties. Especially visible progress has been made in the Group-IV materials. This talk describes efforts, not all ours but all based on sheets, that build on this progress to provide opportunity for new science and for improved materials that may enable better performance in device structures of dizzying diversity. In the first category are thermal transport measurements across nanomembrane (NM) interfaces; the behavior of cells on SiNMs with a compliancy near that of biological material; graphene as a protective barrier, and studies of superfast dynamics in very thin SiNMs, using electron diffraction. In the category of improved materials are increasing biaxial tensile strain in Ge to obtain better light emission and a tuneable source of 2-2.5 um IR light, fabrication of SiGe single crystal NMs and NM stacks to enable eventual SiGe quantum cascade lasers and improved strained-Si qubits, and a new approach to fabricate reduced-defect Ge NMs, using III-V film growth technology, for use in extending the degree of possible tensile strain.

Research supported by DOE, AFOSR, or NSF, collaborating with R. Paiella, Boston Univ.; M.A. Arnold, UW-Madison; F. Cavallo, Univ. New Mexico, J. Faure, Univ. Paris-Saclay.

Carbon Nanomembranes (CNMs) are extremely thin (~1nm), synthetic two-dimensional (2D) layers or sheets with tailored physical, chemical or biological function. A universal scheme for the fabrication of functional CNMs is presented [1,2]. Its first step is the formation of a monolayer of aromatic molecules on a solid surface. This precursor layer is exposed to electrons or UV, which leads to a dehydrogenation, followed by a 2D cross-linking between neighboring molecules. The cross-linked monolayer is then released from the surface, forming a self-supporting CNM with properties that are determined by the precursor molecules. CNMs can then be further processed, for example perforated or surfaces functionalized. It will be shown that CNMs can be engineered with a controlled thickness, elasticity, conductivity and permeability. CNMs are also tested as ballistic membranes for the separation of gases and liquids [3]. Helium ion microscopy, spectroscopic methods and functional tests are applied to investigate the structure and composition as well as permeation properties of the CNMs.
[1] A. Turchanin, A. Gölzhäuser, Adv. Mater., DOI: 10.1002/adma.201506058.
[2] P. Angelova, H. Vieker, N. Weber, D. Matei, O. Reimer, I. Meier, S. Kurasch, J. Biskupek, D. Lorbach, K. Wunderlich, L. Chen, A. Terfort, M. Klapper, K. Müllen, U. Kaiser, A. Gölzhäuser, A. Turchanin, ACS Nano 7, 6489 (2013)
[3] M. Ai, S. Shishatskiy, J. Wind, X. Zhang, C.T. Nottbohm, N. Mellech, A. Winter, H. Vieker, J. Qui, K.J. Dietz, A. Gölzhäuser, A. Beyer, Adv. Mater. 26, 3421 (2014).

We have developed and established a reliable fabrication process for large-area (≥ 6×6 mm²) and ultra-thin (≥ 150 nm) boron-doped nanocrystalline diamond (B:NCD) membranes. Currently, such membranes are successfully employed at the synchrotrons of ESRF, in Grenoble, and SOLEIL, in Paris (both in France) for monitoring the intensity of soft X-ray beams. In this work, we explain the sophisticated fabrication process and present the results of electromechanical characterization of such membranes. The thickness of investigated membranes ranges from 150 nm to 3 µm. The investigation includes van der Pauw transport measurements under varying differential pressures to bulge the membranes, and theoretical finite-element calculations of the strain distribution in the inflated membranes. At various differential pressures (0 – 18 mbar) the resistivity of the membranes was measured and 3D images were made with an optical surface profilometer. The calculated strain maps reveal patterns that indicate strain accumulation at the edges of the bulged membranes. The resistivity and surface area of the membranes both scale linearly with the differential pressure, indicating that the change of resistivity is driven by stretching of the membrane, which is believed to be accommodated by the grain boundaries containing sp²-bonded carbon.

Diamond is a very robust, chemically inert, biocompatible material with an extremely high Young’s modulus and thermal conductivity. In addition, due to its wide band gap it can be doped efficiently with boron, phosphorus or nitrogen, which leads to (semi) conductive properties. With their outstanding properties diamond membranes are a promising candidate for future use as sensitive pressure detectors [1,2]. Nanocrystalline diamond (NCD) membranes are able to withstand harsh conditions; allowing a straightforward fabrication procedure on different types of glass. The thinnest achievable closed NCD layers are ~50 nm, grown with resonance cavity microwave enhanced chemical vapour deposition technology. In this work a fabrication procedure for ultra-thin (150 nm) circular boron-doped NCD membranes, with diameters between 130 and 860 micrometers, in the middle of Hall bar structures on glass substrates, is presented [1].
Piezoresistive effect measurements were performed on these B-NCD membranes by bulging them with various differential pressures. Pressures up to 0.8 bar were reached without causing damage to the membranes.
The underlying substrate material and more specifically its thermal expansion coefficient leads to thermal stress in the diamond film. We show that this residual stress results in different sensitivity of fabricated pressure detectors dependent on the substrate. For B-NCD membranes on glass, the residual compressive stress leads to wrinkle formation when the membrane is formed upon removal of the substrate.
We evaluate the sensitivity of heavily B-doped NCD sensors with respect to different types of glass used as substrates. A direct dependence of the sensor sensitivity on the substrate material due to the amount of stress created in the film during the diamond growth process, is demonstrated. After membrane fabrication this stress level is directly observed by the amount of wrinkles formed in the membrane. To confirm that the difference in thermal expansion coefficient between the different glass types and the NCD film is responsible for this difference in stress and thus sensitivity, we show an increase in sensitivity ( [mV/V bar]) of 53% for membranes on Corning Eagle 2000 glass compared to those on Schott AF45 glass [3].

REFERENCES

[1] S. D. Janssens, S. Drijkoningen, K. Haenen, Applied Physics Letters 104 (7), 073107 (2014)
[2] S. D. Janssens, S. Drijkoningen, K. Haenen, Applied Physics Letters 105 (10), 101601 (2014)
[3] S. Drijkoningen, S.D. Janssens, P. Pobedinskas, S. Koizumi, M.K. Van Bael, K. Haenen, “Tuning of hierarchical wrinkling in nanocrystalline diamond membranes”, under review (2016).

ACKNOWLEDGEMENTS

P. Pobedinskas and S.D. Janssens are Postdoctoral Fellows of respectively the Research Foundation - Flanders (FWO) and the Japan Society for the Promotion of Science (JSPS).

Graphene is a type of two-dimensional nanostructure with extraordinary physical properties, and is promising for a number of applications. Due to its ultra-thin characteristics, graphene can easily fold under external stimuli such as mechanical forces. The substantial local deformation in folded graphene has a prominent influence on its electrical properties. Understanding and ultimately having a good command of the mechanical deformation in folded graphene is of importance to the design and manufacturing of graphene origami and its functional mechanical and electrical properties. The study focuses on investigating the local folding and unfolding behaviors of few-layer graphene sheets by using atomic force microscopy (AFM) techniques. The bending rigidity of few-layer graphene and the interlayer shear interaction during the graphene folding process are studied. The results reveal that the bending stiffness of two to six layers graphene follows a square-power relationship with its thickness. The study demonstrates that it is a plausible venue to qualify the pure bending stiffness of graphene through measuring its self-folding conformation on flat substrates. The nanomechanical measurements also reveal that individual graphene sheets can be mechanically folded in a buckling delamination mode, which leads to accordion-shape self-folded graphene on flat substrates. This work is useful to better understand the structural and mechanical properties of graphene, and in the pursuit of its applications, in particular, as programmable nanoscale origami structures.

The interplay between bending rigidity and out-of-plane stresses, capillary forces or swelling in thin films can be manipulated so as to cause patterned 2D films to curve, bend and fold into 3D materials and devices. In this talk, the design, fabrication and characterization of such materials and devices will be described. The emphasis of our approach has been on ensuring mass-production of micro and nanodevices in a high-throughput manner with diverse materials such as 2D layered materials (e.g. graphene), device grade silicon and related materials and hydrogels. By leveraging the precision of planar lithography approaches such as photo, e-beam and nanoimprint methodologies, a range of functional patterns can be incorporated into these thin film self-assembling systems so as to provide value for optics, electronics and medicine. These include metamaterials, flexible devices, curved microfluidics, drug-delivery capsules, anatomically realistic models for tissue engineering, antennas, e-blocks, sensors, soft-robotic actuators and surgical tools.

Recent reviews:

1. J. Rogers, Y. Huang, O. G. Schmidt, D. H. Gracias, Origami MEMS and NEMS, MRS Bulletin, 41, 2, 123-129 (2016).
2. D. H. Gracias, Stimuli responsive self-folding using thin polymer films, Current Opinion in Chemical Engineering 2, 112-119 (2013).
3. V. Shenoy, D. H. Gracias, Self-folding thin film materials: From nanopolyhedra to graphene origami MRS Bulletin 37, 12, 847-854 (2012).
4. R. Fernandes, D. H. Gracias, Self-folding polymeric containers for encapsulation and delivery of drugs, Advanced Drug Delivery Reviews 64, 1579-1589 (2012)
5. C. L. Randall, E. Gultepe, D. H. Gracias, Self-folding materials and devices for biomedical applications, Trends in Biotechnology 30, 3, 138-146 (2012)

A wide variety of materials can be isolated into large-area, thin sheets. These new structural elements range from graphene and transition-metal dichalcogenides, i.e., true 2D materials, to inorganic nanomembranes (NMs), including single-crystal semiconductor sheets as thin as several nanometers. As nanosheets are readily transferrable and bondable to other hosts, they open new avenues to integrating materials which are dramatically different, when it comes to their crystal structure, geometry, and chemical and mechanical properties. Such materials combinations cannot be obtained by direct growth. For instance, nanomembrane release and transfer enable fabrication of device-grade semiconductors (hard materials) on soft substrates, integration of III-V and group IV semiconductors, fabrication of superlattices alternating amorphous and single crystalline layers, and combinations of 3D structures of different geometries and sizes on the same substrate.
I have recently demonstrated several examples, in this rapidly expanding and fertile research field, that illustrate the range of technological value of the hybrid materials enabled by NM technology. I present here three different types of hybrid integration based on NM release and transfer processes. First, I will show how single-crystalline silicon NMs are combined with soft materials (with elastic modulus in the range of ~0.01-1 MPa) to integrate and interface devices and biological cells, including 3T3 fibroblasts and neuronal cells.
Next, I will demonstrate how to harness epitaxial lift-off of Sb-based SL membranes and their transfer on Si substrates, to fabricate high yield and low-cost focal-plane arrays for IR detection. Finally, I will illustrate how release and transfer of single-layer graphene and MoS₂, produces locally strained films with unique 3D geometry, such as nanoscale wigglers and arrays of ultra-sharp tips, respectively. I will discuss potential application of nanoscale serpentine conductors as THz sources, and elucidate the potential of textured MoS₂ as building block for photovoltaic devices, and exciton lasers.

Foldable and reconfigurable electronics can be realized through origami concepts. Advanced nano and micro metamaterials are being conceived for “synthetic cells”, which are folded structural membranes encapsulating nano circuits. Such mimicry of biological cells can increase their mechanical robustness for many potential applications in neuromorphic engineering and bio-related materials. A folding and locking mechanism for reversible and autonomous folding is proposed, imparting dynamically adaptable geometric features with tunable electronic and mechanical properties to 2D materials such as graphene. Molecular dynamics simulations demonstrate that hydrogenating a narrow strip on a single side of graphene induces folding with relatively sharp hinges due to sp3 hybridization of carbon and van der Waals repulsion. Adaptive biasing forces are applied to determine the free energy minima at different extents of hydrogenation of zigzag and armchair graphene. These minimas correspond to the equilibrium folding angle which allows the optimization of folding angles in the origami structure. The folded structures can be stabilized by functionalizing the edges of the graphene with nucleotides (cytosine and guanine base pairs) or amino acids (arginine and glutamic acid positive/negative charged pairs). “T”-shaped graphene monolayers are patterned with hydrogen to allow folding into a cubic geometry in either vacuum or water. Graphene functionalized with varying numbers of amino acid pairs folded into cubes within 100ps in vacuum and are stabilized by charge interactions between the amino acid pairs. These same structures are unable to fold well in water due to charge screening of the amino acid pairs which inhibits their binding. In contrast, nucleotide functionalization cannot reliably stabilize folded graphene in vacuum due to strong van der Waals interactions, whereas reasonably folded cubes were obtained in water. This study show that simple strategies can be employed to create spontaneously folded 3D architecture in 2D materials. These structures could be unfolded through simple chemical processes such as enzymatic degradation and thermal annealing to remove functionalized groups and hydrogens respectively.

Nanoporous silicon (NPSi) membranes have recently received significant attention for their utility in a wide variety of applications, from filtration of water and other media to energy generation and storage. Though a plethora of fabrication techniques developed by the semiconductor industry have been applied for the fabrication of NPSi, a lack of processes which are both capable of reaching the nanometer feature sizes desired while still being scalable is preventing the implementation of NPSi in the aforementioned sectors.

Here we explore the scalable and economical fabrication of NPSi via two modified metal-assisted chemical etching (MACE) techniques. Standard MACE involves the use of noble metal features deposited on a silicon substrate to electrochemically catalyze the reduction of hydrogen peroxide in solution, resulting in the injection of a hole into the substrate, which is oxidized and etched by hydrofluoric acid. Catalysts are drawn via van der Waals forces in the direction of etching, allowing for a continuous and anisotropic process. Despite a recent focus on the use of nanoparticle catalysts for the etching of nanopores in silicon, particle aggregation and lack of directional control in the etching process have hampered the production of NPSi with well-spaced, homogeneously sized nanoscale pores. The modified MACE processes presented here are introduced with the goal of overcoming these challenges to yield viable nanoporous membranes.

In one approach, a silica shell is employed as a sacrificial spacer layer surrounding a gold core. The composite nanoparticles are allowed to self-assemble from solution into close-packed monolayers on the sample surface. Immersion in the MACE etchant results in the consumption of the silica shell followed seamlessly by the etching of individual, well-spaced sub-10 nm pores. The fine degree of control over gold core diameter and silica shell thickness in the particle synthesis process translate to a novel degree of pore size and spacing control. The second approach utilizes sputtering of noble metals to nucleate sub-5 nm catalyst islands with homogeneous morphology on the macroscale. The planar interface and close spacing of the sputtered islands facilitate directional hole injection and uniform etching normal to the surface, resulting in pore aspect ratios of over 400:1. Characterization of the resulting nanopore morphology in each case is performed via focused ion beam cross-sectional milling, electron microscopy, and elemental analysis. Both presented techniques surmount the typical challenges of particle-catalyzed MACE, and can be differentiated by their individual advantages.

The portable consumer electronics necessitates functional elements to be lightweight, flexible, and wearable [1-4]. The unique possibility to adjust the shape of the devices offered by this alternative formulation of the electronics provides vast advantages over the conventional rigid devices particularly in medicine and consumer electronics. There is already a remarkable number of available flexible devices starting from interconnects, sensing elements towards complex platforms consisting of communication and diagnostic components.
We developed shapeable magnetoelectronics [5] – namely, flexible [6-8], printable [9,10], stretchable [11,12] and even imperceptible [13] magnetosensitive large area elements, which were completely missing in the family of flexible electronics, e.g. for smart skin applications. On the other hand, we realized self-assembled compact tubular microchannels based on strain engineering [14] with integrated passive sensory elements [15-17] and communication antenna devices [18] for on-chip and bio-medical applications, e.g. smart implants [19,20].
Combining these two research directions carried out at different length scales into a single truly interdisciplinary topic opens up the novel field of smart biomimetics [20]. In this respect, we demonstrated mechanically and electrically active compact biomimetic microelectronics, which can serve as a base for realization of novel regenerative neuronal cuff implants with unmatched functionalities. The biomimetic microelectronics can mechanically adapt to and impact the environment possessing the possibility to assess, adopt and communicate the environmental changes and even stimulate the environment electrically.
In my talk, these recent developments will be covered.

[1] M. G. Lagally, MRS Bull. 32, 57 (2007).
[2] J. A. Rogers et al., Nature 477, 45 (2011).
[3] S. Bauer et al., Adv. Mater. 26, 149 (2014).
[4] M. Kaltenbrunner et al., Nature 499, 458 (2013).
[5] D. Makarov et al., Appl. Phys. Rev. 3, 011101 (2016).
[6] G. Lin, D. Makarov et al., Lab Chip 14, 4050 (2014).
[7] M. Melzer, D. Makarov et al., Adv. Mater. 27, 1274 (2015).
[8] N. Münzenrieder, D. Makarov et al., Adv. Electron. Mater. (2016), 10.1002/aelm.201600188.
[9] D. Karnaushenko, D. Makarov et al., Adv. Mater. 27, 880 (2015).
[10] D. Karnaushenko, D. Makarov et al., Adv. Mater. 24, 4518 (2012).
[11] M. Melzer, D. Makarov et al., Adv. Mater. 27, 1333 (2015).
[12] M. Melzer, D. Makarov et al., Nano Lett. 11, 2522 (2011).
[13] M. Melzer, D. Makarov et al., Nat. Commun. 6, 6080 (2015).
[14] O. G. Schmidt et al., Nature 410, 168 (2001).
[15] I. Mönch, D. Makarov et al., ACS Nano 5, 7436 (2011).
[16] C. Müller, D. Makarov et al., Appl. Phys. Lett. 100, 022409 (2012).
[17] E. J. Smith, D. Makarov et al., Lab Chip 12, 1917 (2012).
[18] D. D. Karnaushenko, D. Makarov et al., NPG Asia Materials 7, e188 (2015).
[19] D. Karnaushenko, D. Makarov et al., Adv. Mater. 27, 6582 (2015).
[20] D. Karnaushenko, D. Makarov et al., Adv. Mater. 27, 6797 (2015).

Some of the major trends in RF/microwave communications include miniaturization, high frequency operation, and wearability. Applications include wireless communication beyond 4G, wireless body area network (WBAN) for health care and security, wearable gadgets for commercial as well as defense related high-bandwidth communications, etc. All of them require improvements of the current widely used lumped passive components in monolithic integrated circuits (ICs). Their drawbacks include large on-wafer footprint (especially planar inductors), low frequency operation, and sensitivity to substrate doping levels and deformation. In monolithic ICs, limited by traditional 2D fabrication method for a long time, passive components in IC have never been successfully minimized for practical commercialization. To this day, they occupy as much as 90% of the IC size, which becomes the bulk of the IC production cost and the bottle of neck of IC performance evolution.
The solution to solve these problems is to go 3D. Hierarchical 3D structures enable efficient use of materials and lead to advanced functionalities that are otherwise out of reach. However, if conventional fabrication technologies used to process 3D structures, issues such as mechanical stability, conformity, alignment, process complexity and cost are difficult to address.
The self-rolled-up membrane (S-RuM) nanotechnology provides a revolutionary platform for realizing on-chip passive components in 3D. This 3D platform promises to make IC chips that are extremely small and light, with dramatically enhanced functionalities and, at the same time, a significant reduction of the fabrication cost per chip. In this talk, we will present experimental demonstration and modeling results on S-RuM inductors, filters, transformers, and power converters with ultra-small footprint for high inductance, high frequency, and high efficiency energy storage and conversion.

Reducing the thermal conductivity by engineering the phonon transport is critical for the advancement of efficient thermal energy convertors. Here we propose a new approach to decrease the in-plane thermal conductivity of single crystalline silicon as the heart of virtually all high-tech devices by altering the phonon band structure through the formation of rolled-up nano-architectures. Multishell tubular structures of 20 nm thick single crystalline silicon were produced through Molecular Beam Epitaxy (MBE) growth and strain-driven rolled-up procedure. The temperature-dependent intrinsic thermal conductivity of rolled-up nanotubes with multiple windings was measured using a suspended microdevice. Significant reduction in the in-plane thermal conductivity compared to a single nanomembrane layer was observed. To investigate the origin of this reduction, we calculated the acoustic phonon dispersion spectra and corresponding phonon group velocity in the rolled-up nano-architectures by solving the equations of elastodynamics for silicon and native amorphous oxide shells. The phonon group velocity is remarkably altered by number of windings and its trend agrees well with the experimental thermal conductivity measurements. The obtained results prove that single crystalline silicon rolled-up nano-architecture are a good candidate for phononics, nanoscale thermal management and thermoelectric application owning to their remarkable reduction of thermal conductivity without degradation of the electrical transport.

Rolled-up quantum dot micro- and nano-scale tubes have emerged as a highly promising candidate for a broad range of nanophotonic devices, including low threshold nanoscale lasers, high efficiency photodetectors, and high speed modulators. They are formed when coherently strained semiconductor bilayers, with the incorporation of self-organized quantum dot heterostructures are selectively released from the host substrates, due to the relaxation of strain. In this context, we have performed a detailed investigation of the design, fabrication, and perform characteristics of InAs/InGaAsP quantum dot tubes and have demonstrated, for the first time, high performance rolled-up quantum dot tube lasers. These devices operate in the wavelength range of 1.3 to 1.5 µm, and exhibit a very low threshold current ~ 1 mA. We have further demonstrated that these devices can be directly integrated on a Si platform without any performance degradation.
The InAs quantum dot heterostructures were grown on either GaAs or InP substrate by molecular beam epitaxy. Multiple quantum dots were incorporated as the gain medium. The InGaAsP barrier layers were doped n-type using silicon. A Si-doped, tensile-strained InGaAsP capping layer was also grown. The fabrication of tube lasers involves the following steps. First, a single step ion-implantation of Be was performed to selectively p-dope part of the tube. The implanted dopant was activated by annealing the sample at elevated temperatures. Pd/Ti/Pd/Au and Ni/Ge/Au were deposited as the p- and n-metal contacts, respectively. Subsequently, a U-shaped mesa was defined by using standard photolithography and wet etching. Rolled-up quantum dot tubes were then formed by selectively etching the underlying InP. The resulting quantum dot tubes exhibit diameters of ~ 5 µm and wall thicknesses of 50 nm, or larger. Three-dimensional optical confinement was achieved by fabricating free-standing quantum dot tubes and by introducing parabolic corrugations on the tube surface. Such devices exhibit strong coherent emission in the wavelength range of 1.3 to 1.5 µm. The measured lasing threshold and the estimated Purcell factor are ~ 1.05 mA and ~ 4.31, respectively.
We have further demonstrated the direct integration of rolled-up tube nanophotonic devices with Si waveguides and achieved optical-optical modulation on a single microtube. The near-ideal performance of rolled-up tube devices in the wavelength range of 1.31 to 1.65 µm will be reported, and their emerging applications in integrated photonics will be discussed.

Prestrained nanomembranes can shape into various photonic structures via rolled-up nanotech [1]. In such geometries, light can be confined and propagate along the rolled-up dielectric nanomembrane, leading to novel phenomena of photon-plasmon coupling in opto-plasmonic cavities [2-4] and optical spin-orbit coupling in asymmetric cavities [5]. Here, the opto-plasmonic cavities were fabricated by depositing a gold nanocap onto rolled-up microtube cavities [2]. Due to the sub-wavelength thin cavity wall, the optical evanescent field considerably penetrates out of the cavity surface allowing for an enhanced coupling with surface plasmons. By considering both the cavity wall and the metal coating layer thicknesses, the formation mechanism of weakly, moderately and strongly hybridized photon-plasmon modes were revealed [3]. More interestingly, the coupling of photon and localized surface plasmon was realized in a novel type of opto-plasmonic cavity by creating a vertical gold nanogap on the cavity surface [4]. Both finite element method calculations and potential well models were used to understand our experimental observations. In addition, the light polarization, i.e. the transverse electric field vector, can be defined and regulated by curved nanomembranes, which opens a way to realize optical spin-orbit coupling in asymmetric microtube cavities. Recently, we have demonstrated optical spin-orbit coupling in cone-shaped microtube cavities [5]. The optical polarization state of light is found to change both in orientation and eccentricity due to the occurrence of geometrical phase together with a mode conversion, generating a non-cyclic geometrical phase in a non-Abelian evolution. The design and investigation of nanomembrane shaped photonic devices constitutes a versatile platform for the study of enhanced light-matter interactions and quantum optics.

[1] O. G. Schmidt and K. Eberl, Nanotechnology: Thin solid films roll up into nanotubes. Nature 410, 168 (2001).
[2] Y. Yin, S. L. Li^*, S. Giudicatti, C. Y. Jiang, L. B. Ma^* and O. G. Schmidt, Strongly hybridized plasmon-photon modes in optoplasmonic microtubular cavities. Phys. Rev. B 92, 241403(R) (2015).
[3] Y. Yin^*, S. L. Li, V. Engemaier, S. Giudicatti, E. Saei Ghareh Naz, L. B. Ma^* and O. G. Schmidt, Hybridization of photon-plasmon modes in metal-coated microtubular cavities. Phys. Rev. A, in press.
[4] Y. Yin, S. L. Li^*, S. Böttner, F. Yuan, S. Giudicatti, E. Saei Ghareh Naz, L. B. Ma^* and O. G. Schmidt, Localized surface plasmons selectively coupled to resonant light in tubular microcavities. Phys. Rev. Lett., in press.
[5] L. B. Ma^*, S. L. Li, V. Fomin, M. Hentschel, J. Götte, Y. Yin, M. Jorgensen and O. Schmidt, Spin-orbit coupling of light in asymmetric microcavities. Nat. Commun. 7 (2016).

By combining top-down (through the design of strained dielectric nanomembranes) and bottom-up (through the self-assembly of rolled-up structures) approaches, rolled-up optical microcavities are produced in a well-controlled manner, which have gained much attention in nanophotonics owing to the customizability of their geometry and material. Unfortunately, the quality (Q) factor of rolled-up microcavities is low compared with that of other dielectric microcavities. It is due to the intrinsically structural defects, i.e. notches formed by the edges of the rolled-up nanomembrane. However, in this talk, we will show that the spoiling of Q factors in a rolled-up microcavity can be healed when this open optical system is tuned to approach exceptional points (EPs) in parameter space, which can be simply done by tailoring the structural notches.

EPs are singularities in parameter space at which eigenvectors and eigenvalues of a non-Hermitian system (e.g. an open optical system) coalesce simultaneously. The topological structure of EPs is nontrivial and leads to richer and broader physics ranging from the Berry phase, quantum chaos to the non-reciprocity of light in a parity-time related system. Recently, optical detection of single-particle using EPs in dielectric microdisk microcavities has been proposed, and a sevenfold enhanced sensitivity compared to that using diabolic point (DPs) has been proved. It is worth mentioning that rolled-up microcavities have been demonstrated as optical sensors with a high sensitivity because of their subwavelength-thin wall thickness. Therefore, we have explored the possibility of using EPs in rolled-up microcavities for optical sensing applications, and the results will be given in this talk.

As a fundamental "top down" nanofabrication approach, patterning through photolithography, combined with mechanical self-assembly of the nanomembrane, has spurred considerable interests due to its great potential in various technological applications including nano-elecromechanical/micro-electromechanical systems (NEMS/MEMS) and sensors to microrobotics, active materials, drug delivery, and optoelectronics. However, to-date no model exists, rendering quantitative evaluation of the patterning effects on the self-rolling not possible. In this study, we develop an analytical model on base of Von-Karman nonlinear theory and Ritz method, where the effects of patterning on self-rolling are effectively accounted for through local film thickness variation induced by patterning. The dependence of rollup direction and curvature of a nanomembrane on the size and density of patterns has been systematically studied. The accuracy of our theoretical model has been demonstrated by its application to helix-shape structures to showcase a quantitative route to achieve controllable helical rolling of nanomembranes through strip patterning, showing excellent agreement with both experimental observations and finite-element simulations. Our results provide critical insights for the design of complex 3D structures of self-assembly of nanomembranes through controllable surface patterning.