Symposium NT8 : Silicon Nanostructures---Doping, Interface Effects and Sensing

Whispering Gallery Modes (WGMs) have been widely studied for the past 20 years for various applications, including biological sensing. While the different WGM-based sensing approaches reported in the literature have shown tremendous performances, down to single molecule detection, at present this technology is still not yet mature, mainly for practical reasons. Our work has been focused on developing a simple, yet efficient, WGM-based sensing platform capable of being used as a dip sensor for in-vivo biosensing applications by combining WGMs in fluorescent microresonator with silica Microstructured Optical Fibers (MOFs).
We recently demonstrated that a dye-doped polymer microresonator, supporting WGMs, positioned onto the tip of a suspended core (MOFs) can be used as a dip sensor. In this architecture, the resonator is located on an air hole next to the fiber core at the fiber’s tip, enabling a significant portion of the sphere to overlap with the guided light emerging from the fiber tip. This architecture offers significant benefits that have never been reported in the literature in terms of radiation efficiency, compared to the standard freestanding resonators, which arise from breaking the symmetry of the resonator. In addition to providing the remote excitation and collection of the WGMs' signal, the fiber also allows easy manipulation of the microresonator and the use this sensor in a dip sensing architecture, alleviating the need for a complex microfluidic interface.
Here, we present our recent results on the microstructured fiber tip WGM-based sensor, including its lasing behavior and enhancement of the radiation efficiency, alleviating some fundamental limitation of fluorescent microresonators compared with passive ones. We also show that this platform can be used for clinical diagnostics and applying this technology to the detection of various clinically relevant biomarkers even in complex clinical samples. Approaches for limiting the impact of non-specific binding onto clinical results and multiplexed sensing are also shown.

Whispering-Gallery-Mode (WGM) microresonators have emerged as highly sensitive platforms for label-free detection of perturbations in their proximity due to their capability to significantly enhance light-matter interactions. They have been used for sensing biomarkers, DNA, and medium-size proteins at low concentrations, as well as for detecting viruses and nanoparticles at single-particle resolution. Any perturbation due to a chemical reaction, physical association, or a particle/molecule entering the mode volume or binding onto resonator surface induces a net change in the polarizability of the resonator-surrounding system and perturbs its optical properties. Traditionally, frequency shift or linewidth broadening of a resonance is monitored for detecting perturbations. We have observed that in addition to these, a resonance may exhibit splitting if the interaction between the WGM field and a nanomaterial is sufficiently strong. This method referred to as the "mode splitting" or "frequency splitting" allows single-shot measurement of the polarizability of each detected nanomaterial (e.g., single virion, dielectric particles, metallic particles).

The fundamental sensitivity limit of WGM microresonators is determined by the optical loss and the micro-scale mode volume of the WGM field. Thus, one should either compensate losses via providing optical gain or decrease the resonator size. We have developed techniques to fabricate rare-earth ion (e.g., erbium, ytterbium, thulium, etc) doped silica WGM microresonators on silicon chip in order to compensate material losses and to turn the resonators into WGM microlasers, thereby improving the detection limit and sensitivity of mode-splitting method beyond what can be achieved by the passive (no optical gain-providing mechanism). However, using rare-earth ions requires additional processing steps and costs and raises biocompatibility concerns. As an alternative to integrating rare-earth ions for loss compensation and for building microlasers, we have used intrinsic gain mechanisms such as Raman and parametric gain present in the materials from which resonators are fabricated. In the first implementation of Raman gain-induced loss compensation in silica WGM microresonators for improved detection and the first demonstration of frequency splitting in a WGM Raman microlaser, we have achieved detecting and counting single nanoparticles down to 10 nm with a polarizability sensitivity down to 3.82 × 10⁻⁶ μm³ without using plasmonic effects, passive or active stabilization, or frequency locking.

In this talk, we will briefly introduce our efforts on developing mode splitting in WGM microresonators and microlasers into highly sensitive detection platforms, and then discuss recent advancements in WGM microresonators that will enable investigating the properties and kinetic behaviors of nanomaterials, nanostructures, and nanoscale phenomena. In the end, I will discuss explorations with WGM resonators beyond sensing.

Silicon quantum dots (Si-QDs) are a well-known QD fluorophore that can emit throughout the visible spectrum depending on their interface structure and surface functional group. Detection of nitroaromatic compounds by monitoring the luminescence response of the sensor material (typically fluorescent polymers) currently forms the basis of new explosives sensing technologies. Freestanding silicon QDs may represent a benign alternative with more versatility as compared to sensors formed from the more widely-studied porous silicon. Here, we investigate dodecyl and amine-terminated S-QDs’ luminescence response to the presence of nitrobenzene and dinitrotoluene (DNT) in various solid, solution, and vapor forms. For dinitrotoluene vapor the detection limit approached the parts-per-trillion range. For nitroaromatics dissolved in toluene the detection limit was on the order of 300 nM, corresponding to ~50 pg of material distributed over ~1 cm² on the sensor surface. Solid traces of nitroaromatics were also easily detectable via a simple “touch test”. The samples showed minimal interference effects from common contaminants such as water, ethanol, and acetonitrile. The sensor can be as simple and inexpensive as a small circle of filter paper dipped into a QD solution, with a single vial of QDs able to make hundreds of these sensors. Additonally, a trial fiber-optic sensor device was tested by applying the QDs to one end of a 2x2 fiber coupler and exposing them to controlled DNT vapor. Finally, the quenching mechanism was explored via luminescence dynamics measurements and is proposed to be different for blue (amine) and red (dodecyl) fluorescent silicon QDs.

Silica nanoparticles have a wide variety of biomedical applications, including drug delivery, cancer therapies, biosensors, gene delivery, and biomedical imaging. Depending on the application, they may be doped with iron, and iron is also found as a common contaminant in manufactured nanomaterials due to its role as a catalyst in their synthesis. However, iron is known to contribute to lung inflammatory disease through Fenton chemistry. It is therefore important to study these effects using carefully controlled and characterized samples in order to determine specific chemistries and doses that may be of concern, and their mechanism of action. In this study, we examined the pro-inflammatory properties of commercially prepared 50 nm silica particles, undoped and iron doped, in vitro using human-derived THP-1 macrophages. Particles were characterized for size, size distribution, surface area, iron concentration, phase and iron oxidation state, and aggregation in cell culture media using SEM, HRTEM, BET, ICP-MS, XRD, XAS, and DLS. Bioassays were conducted to determine the influence of iron presence on nonlethal dose limit, superoxide production, lipid peroxidation, and inflammatory mediator production. It was found that at a nonlethal dose of 1 mg/ml, the presence of iron increased superoxide, lipid peroxidation, and cytokine production, while the addition of iron chelator, a superoxide dismutase/catalase mimic, and PC-PLC inhibitor were found to mitigate this effect. The results highlight the importance of iron in lung inflammation for engineered nanomaterials; and support the role of a superoxide, lipid peroxidation, and PC-PLC dependent mechanism for lung inflammation under low dose conditions. The results have important implications for application of iron-doped silica nanomaterials for biomedical applications.

Medicine as well as biology increasingly rely on the use of cutting edge physics and engineering, in order to pursue the next generation nanomedical applications and to address fundamental questions in the life sciences. Central to this task is the study of micro- and nano systems, focusing on how engineered systems combined with natural ones can advance sensing, medicine, and our understanding of how biological systems work. My research addresses these important questions with state of the art biosensor technologies, capable of detecting single molecules and their dynamics; and resolving the kinetics of complex molecular systems on timescales ranging from few nanoseconds to several hours.

Optofluidic lasers integrate microfluidics, gain medium in liquid environment, and microcavity. They hold great promise for miniaturized and reconfigurable coherent light sources on chip and unconventional biochemical detection and analysis. Here I will first introduce the principle of the optofluidic lasers and then review various types of designs of optofluidic lasers on chip, including ring resonator lasers, Fabry-Perot lasers, distributed-feedback lasers, and photonic crystal lasers, etc. Different types of gain media will be described, such as organic dyes, semiconductor quantum dots, enzyme-substrate reaction products, and fluorescent proteins, etc. Applications of the optofluidic laser will be discussed with emphasis on new optical functions that the optofluidic lasers can potentially offer.

Silicon nanowires (SiNWs) operating as field effect transistor (FET) are commonly considered as important building blocks for nanoscale sensors. While within the past decade substantial progress in the field of chemical and biochemical label-free sensing could be demonstrated for SiNW-FETs, silicon nanotubes (SiNTs) are almost unknown but offer the capability to be used as flow-through ion-sensitive FETs. Their geometry offers a superior surface charge sensitivity, probing of minute sample volumes as well as the possibility to interlink nanofluidics and electronics as required for instance in the exploration of living cells and tissues.
Here, we discuss electrical transport properties and ion diffusion through a single SiNTs based on both numerical calculations and experimental results. For the experiments, SiNT microfluidic devices were used, which allowed an evaluation of certain parameters such as SiNT length (10 µm to 40 µm), SiNT inner diameter (50 nm to 250 nm), ion size, and electrical potentials. SiNTs were synthesized from Ge|Si-core|shell nanowires grown bottom-up by a gold catalyzed vapor-liquid-solid growth. The inner diameter of the SiNTs is determined by the Ge nanowire diameter and was defined by the size of the gold colloid catalysts. After synthesis, the core|shell nanowires were transferred from their growth substrate to the device substrate and were functionally integrated by standard microfabrication technologies. The SiNT was finally obtained by selective H₂O₂ etching of the Ge core.

Silicon nanocrystals exhibit luminescence in the visible range of spectra. These nanomaterials can be used as labels in biology. In our previous work [1], we studied silicon nanocrystals with silicon oxide on the surface. For those nanoparticles we found out that they are biocompatible and nontoxic compared to, for example, nanodiamonds and cadmium selenide dots. In our current work, we study silicon nanocrystals strongly doped with P and B, which are forming sort of alloy-like silicon nanocrystals, where the doping atoms are on the surface of nanocrystal. This treatment enables their dispersity in aqueous solutions and thus usable for biological experiments. Surprisingly, the interaction with biological matter strongly depends on the annealing temperature at which the silicon nanocrystals were formed. Most importantly, the nanocrystals prepared at a temperature of 1100 Celsius enter the cell within few hours, unlike the nanocrystals prepared at 1050 Celsius those are entrapped in the cell exterior andwere found in cells much later. Also the nanoparticles interact differently according to used culture media (serum of various origin) probably forming distinct protein corona. In our presentation we will summarize physical and biological properties of doped silicon nanocrystals annealed at different temperature.


[1] Silicon nanocrystals and nanodiamonds in live cells: photoluminescence characteristics, cytotoxicity and interaction with cell cytoskeleton, A Fucikova, J Valenta, I Pelant, M Hubalek Kalbacova, A Broz, B Rezek, A Kromka, Z Bakaeva, RSC Advances 4 (20), 10334-10342, 2014

One of the most challenging issues in materials physics is to predict the properties of dopants in electronic materials. Dopants play an important role in functionalizing materials for use in electronic and optical devices. As the length scale for such devices approaches the nano-regime, the interplay of dimensionality, quantum confinement and dopant defects can be complex. In particular, the usual rules for describing defects in bulk may be inoperative, i.e., a shallow defect level in bulk may become a deep level at the nanoscale. The development of theoretical methods to describe the properties of nanoscale defects is a formidable challenge. Nanoscale systems may contain numerous electronic and nuclear degrees of freedom, and often possess little symmetry. My presentation will center on recent advances in this area based on new algorithms applied to Si nanostructures. I will illustrate applications to doped Si nanocrystals and nanowires. These applications will include predictions for dopant properties associated with core level shifts, Raman spectroscopy, spin resonance and diffusion.

The observation of localized surface plasmon resonance (LSPR) in P-doped [1,2] or B-doped [2,3] Si nanocrystals is very promising for the integration of plasmonics with Si technology. More generally, LSPR in doped semiconductor nanocrystals is very attractive because, in contrast to noble metal nanocrystals, the LSPR can be tuned by changing the doping level [4,5]. I will review recent theoretical progress in this field, including the Si-nanocrystal plasmonics. The theoretical description of LSPR in doped nanocrystals is usually based on the simple Drude model which is justified above certain thresholds in nanocrystal size and doping level, as shown by self-consistent tight-binding calculations [6]. In this talk, I will discuss several issues which are at the moment badly understood. These latter include the role of the quantum confinement and the lattice relaxation [7], the influence of the dopant potential and location, and the mechanisms determining the spectral width [8].

[1] D.J. Rowe, J.S. Jeong, K.A. Mkhoyan, and U.R. Kortshagen, Nano Lett. 13, 1317 (2013).
[2] N.J. Kramer, K.S. Schramke, and U.R. Kortshagen, Nano Lett. 15, 5597 (2015).
[3] S. Zhou, X. Pi, Z. Ni, Y. Ding, C. Delerue, D. Yang, and T. Nozaki, ACS Nano 9, 378 (2015).
[4] J.M. Luther, P.K. Jain, T. Ewers, A.P. Alivisatos, Nat. Mater. 10, 361 (2011).
[5] R. Buonsanti, A. Llordes, S. Aloni, B. A. Helms, D. J. Milliron, Nano Lett. 11, 4706(2011).
[6] X.D. Pi and C. Delerue, Phys. Rev. Lett. 111, 177402 (2013).
[7] X.D. Pi, Z.Y. Ni, D. Yang, and C. Delerue, J. Appl. Phys. 116, 194304 (2014).
[8] C. Delerue, unpublished.

Here we present investigations on boron and phosphorus doped size-controlled silicon nanocrystals that are embedded in a dielectric SiO2 matrix. The nanocrystals were prepared by annealing doped silicon-rich silicon oxide / silicon oxide multilayers. We thoroughly quantify the amount of introduced dopants and determine their quantititative influence on optical, structural and electrical properties. It is found that while phosphorus may enhance or quench the photoluminescence of silicon nanocrystals, the boron influence is rather neglible. From electrical measurements it is found however that neither boron nor phosphorus may introduce free carriers in those embedded silicon nanocrystals.

Doping semiconductor nanocrystals (NCs) is a promising way to tailor the optical and electronic behavior of these materials to enable their use in (opto)electronic applications [1]. Yet the practical exploitation of doping requires an understanding of its efficiency, of the dependence on external environment, and of the electronic localization of dopant states due to confinement effects. Here, using electron paramagnetic resonance (EPR) we experimentally assess the confinement energy of isolated donors in Si NCs, grown in amorphous SiO₂ by means of the phase segregation method, from the temperature dependence of their EPR spectra. From this, we provide experimental evidence for the confinement-induced increase of the ionization energy of dopants with decreasing NC size previously predicted by ab-initio calculations [2, 3]. Moreover, from quantitative EPR data we probe the efficiency of doping of Si NCs embedded in SiO₂. We estimate a phosphorus doping efficiency of these Si NCs of about 30 % and from this we infer that a considerable amount of P dopants are incorporated at substitutional sites of the NCs lattice and thus act as donors. We further show that the doping efficiency in Si NCs varies by several orders of magnitude depending on their external environment. Charge traps associated with air molecules adsorbed at the NCs surface give rise to a strong compensation of donors. We observe that this process can be reverted by desorbing the molecules from the NCs surface under vacuum. Our findings obtained for Si NCs produced by the phase segregation method will be thoroughly compared with the doping properties of freestanding Si NCs synthesized by means of plasma-induced decomposition of silane [4-6].

[1] D. J. Norris, A. L. Efros, S. C. Erwin, Science 319, 1776 (2008).
[2] G. Cantele, E. Degoli, E. Luppi, R. Magri, D. Ninno, G. Iadonisi, S. Ossicini, Phys. Rev. B 72, 113303 (2005).
[3] D. König, S. Gutsch, H. Gnaser, M. Wahl, M. Kopnarski, J. Gottlicher, R. Steininger, M. Zacharias, and D. Hiller, Sci. Rep. 5, 09702 (2015).
[4] A. R. Stegner, R. N. Pereira, K. Klein, R. Lechner, R. Dietmueller, M. S. Brandt, M. Stutzmann, H. Wiggers, Phys. Rev. Lett. 100, 026803 (2008).
[5] A. R. Stegner, R. N. Pereira, R. Lechner, K. Klein, H. Wiggers, M. Stutzmann, and M. S. Brandt, Phys. Rev. B 80, 165326 (2009).
[6] R. N. Pereira, A. J. Almeida, A. R. Stegner, M. S. Brandt, H. Wiggers, Phys. Rev. Lett. 108, 126806 (2012).

Nonthermal plasma synthesis of nanocrystals is particularly suited for covalently bonded materials that require high temperatures to be produced with good crystallinity. Recently, the capability of nonthermal plasmas to produce substitutionally doped nanocrystal materials has attracted attention. The substitutional doping of nanocrystals has presented a significant challenge both for liquid and gas phase synthesis due to effects such as self-purification, which lead to a segregation of dopant atoms at the nanocrystal surfaces.
In this presentation, we discuss the substitutional doping of silicon nanocrystals with boron and phosphorous using a nonthermal plasma technique. In the nonthermal plasma a silicon precursor such as silane and dopant precursors such as diborane and phosphine are dissociated through impact of energetic plasma electrons, leading to the formation of doped silicon nanocrystals through chemical clustering in the gas phase. The doping of the silicon nanocrystals is studied through surface analytical approaches and the plasmonic response of the nanocrystals. While the synthesis approach of boron and phosphorous doped nanocrystals is identical, the activation behavior of these two dopants is found to be dramatically different. Phosphorus-doped NCs exhibit a plasmon resonance immediately after synthesis, while boron-doped NCs require post-synthesis annealing or oxidation treatment.
We further discuss the electronic transport in films of phosphorous-doped silicon nanocrystals. The temperature dependent transport is consistent with Efros-Shklovskii variable range hopping at all temperatures and doping levels. With increasing doping level, the electron localization length increases up to three times the nanocrystal diameter.

This work was supported by the Army Office of Research under MURI Grant W911NF-12-1-0407 and by NSF under MRSEC grant DMR-1420013

Doping shallow impurities modifies optical and electrical transport properties of semiconductor nanocrystals (NCs). Furthermore, it affects the chemical properties of NCs significantly. We have recently developed silicon (Si) NCs with very high boron (B) and phosphorus (P) concentration shells. The heavily-doped shell induces negative potential on the surface and prevent agglomeration of Si-NCs in polar solvents. The co-doped Si-NCs are stable in water in a wide pH range without agglomeration for a long period and thus are potentially useful in bio-medical fields. In this presentation, we first discuss the structural and optical properties as well as energy state structures of co-doped Si-NCs. We show that the luminescence energy of co-doped Si-NCs can be controlled in a very wide range covering 0.85 to 1.85 eV. Because of perfect dispersion of Si-NCs in solution, very high quality NC films can be produced by spin-coating. We also demonstrate layer-by-layer growth of multilayer films of co-doped Si-NCs with the accuracy of a mono-layer. Finally, we show formation of different kinds of composite nanostructures consisting of metal nanostructures and co-doped Si-NCs and demonstrate the enhancement of light absorption and emission properties. J. Phys. Chem. C 116, 17969 (2012). J. Phys. Chem. C 117, 6807 (2013). J. Phys. Chem. C 117, 11850 (2013). Nanoscale 6, 122 (2014). J. Appl. Phys. 115, 084301 (2015). J. Mater. Chem. C, 2, 5644 (2014). Nanoscale 6, 12354 (2014). RSC Adv. 5, 8427 (2015). J. Phys. Chem. Letts. 6, 2761 (2015). Appl. Phys. Letts. 107, 041111 (2015). ACS Photo. 2, 1298 (2015).

Doping of silicon nanocrystals (Si-ncs) with quantum confinement properties allows for bandgap and Fermi energy tuning. While the size-dependent bandgap has been widely investigated, the position of the Fermi level and its dependence on doping, surface characteristics and quantum confinement has not been studied in details. Nonetheless, the Fermi level is of paramount importance for the development of optoelectronic devices based on p-/n-type Si-ncs. Furthermore, doping and surface properties are expected to interact so that control of suitable interfaces with the hosting organic/inorganic matrix is necessary. As a consequence, effective use of doping in Si-ncs can be prevented by a range of effects. For instance, dopant deactivation due to quantum confinement or band bending due to surface states are possible mechanisms of doping deactivation.
In recent years we have demonstrated that atmospheric pressure microplasma surface engineering of Si-ncs can be used as an efficient tool to control surface chemistry of surfactant-free Si-ncs. Therefore in this study, boron doped (p-doped) and phosphorous doped (n-doped) Si-ncs with quantum confinement size (~3 nm diameter) have been synthesized and the influence of surface engineering by atmospheric pressure DC/RF microplasma is investigated. We clearly demonstrate that this surface engineering technique induces different surface chemistry that depends on the Si-ncs dopants. We report that the absolute photoluminescence quantum yield is enhanced more than 3 times for n-type Si-ncs compared to boron doped p-type Si-ncs and the position of the Fermi level is also confirmed to be dependent on the doping type, as expected. We however also show that the position of the Fermi level can be tuned with atmospheric plasma surface engineering. Finally, we analyze the impact of the Si-ncs doping on hybrid solar cell performance. We report corresponding external quantum efficiency measurements, current voltage characteristics and the assessment of diffusion length and transport properties.

Research on silicon nanocrystals (Si NCs) has recently gained great momentum. This is largely due to the nontoxicity and abundance of Si and the compatibility of Si-NC technology with well-established bulk-Si technology. In our work we focus on the use of doping to achieve novel properties such as localized surface plasmon resonance (LSPR) from Si NCs.^1-4 It is well known that the size effect for intrinsic Si NCs has already been intensively investigated. However, rather limited work has been carried out on the size dependence of the properties of doped Si NCs. In this work we choose Si NCs doped with boron (B) to study the size-dependent structures, subbandgap absorption and LSPR absorption of doped Si NCs. It is found that the decrease of the NC size leads to the decrease of B-doping-induced lattice compression. B doping gives rise to less tensile strain for Si-Si bonds and less serious structural disorder in smaller Si NCs. For all the NC sizes the Urbach energy of Si NCs initially decreases and then increases with the increase of the concentration of B. With the decrease of the NC size, the corresponding decrease of the dielectric constant of Si NCs may give rise to the blueshift of the LSPR energy of B-doped Si NCs. For an extremely small NC size, the rather significant scattering of free carriers at the NC surface can totally damp the LSPR of B-doped Si NCs.

[1] S. Zhou, X. D. Pi, Z. Y. Ni, Y. Ding, Y. Y. Jiang, C. H. Jin, C. Delerue, D. Yang and T. Nozaki, ACS Nano 9, 378-386 (2015).
[2] Z. Y. Ni, X. D. Pi and D. Yang, Physical Review B 89, 035312 (2014).
[3] X. D. Pi, Z. Y. Ni, D. Yang and C. Derelue, Journal of Applied Physics 116, 194304 (2014).
[4] X. D. Pi and C. Delerue, Physical Review Letters 111, 177402 (2013).

Pushing the limits in IC-technology recent evolution towards more confined and even 3D-volumes (like Finfets) has created a demand for metrology suited for very small volumes and more atomic scale observations. Obviously the latter represents a serious metrology challenge (spatial resolution, signal intensity, statistical relevance,..) and seems to make macroscopic concepts like SIMS, RBS, Raman obsolete.
One obvious solution is to abandon 1D-metrology completely and to focus on metrology such as Atom probe tomography (APT) which is an extremely powerful method providing composition analysis within very small volumes (a few nm³) with high sensitivity and accuracy. Due to its excellent spatial and depth resolution (in many cases with the ability to resolve lattice planes) alloy composition analysis in small trenches, unexpected in diffusion in such volumes, dopant distribution and dopants decorating defects can in principle be identified. Nevertheless the presence of many materials with different evaporation fields does induce severe artefacts and trajectory aberrations which cannot always be corrected, causing severe measurement inaccuracies.
Complementary to the resolving power of APT, is the application of scanning probes (ssrm, c-afm) which enables to grasp the electrical activity of dopants or conduction paths within such volumes. As SPM is inherently a 2D-method, concepts for expanding into the depth dimension are explored cfr Scalpel SPM, ion beam sputtering icw SPM,..).
Despite their unique 3D-resolving power, APT and SSRM suffer from a poor productivity and a lack of statistical averaging over large areas as required in more production oriented metrology. We therefore present the concept of “self focusing SIMS” whereby we demonstrate that it is possible to determine the composition from trenches as small as 20 nm without having an ion beam with nm-resolution. In this case the spatial resolution is provided through the physics of the ion formation process creating a self focusing of the analytical signal to the feature of interest.
Somewhat similar is our approach to probe crystallinity of III-V growth in narrow trenches (< 50 nm) through channeling RBS whereby again we use a large beam but nevertheless probe the information from an array of very fine features. In all these cases, the averaging over a large array provides excellent statistics and improved productivity through the enhanced signal versus the case of a very focused probe beam. The latter is ultimately exemplified in Raman experiments on narrow SiGe-trenches where we demonstrate that the signal from very narrow features (20 nm) is dramatically enhanced (50-100x) as compared to it blanket counterpart enabling to probe composition and structural properties from a small volume.

Atom probe tomography (APT) constitutes a rather unique analytical technique for the 3D elemental characterization of solid materials with potentially atomic-scale spatial resolution and high analytical sensitivity [1-4]. In APT, ions are released via field evaporation from a tip with a very small radius of curvature (less than ~50 nm). The extracted ions are projected onto a position-sensitive detector, with the impact coordinates being directly correlated with their emission site at the sample. Because of the small tip radius, the overall magnification is very large (~10⁶). Due to the pulsed extraction of the ions (triggered either by a voltage or a laser pulse), the concurrent flight-time measurement yields the mass and hence the chemical identity of the ions. The removal of material from the tip releases atoms from continuously deeper layers. Eventually, the original 3D distribution of the atoms in the analyzed sample volume can thus be reconstructed.
APT is, therefore, very well suited for the analysis of nanostructured specimen such as matrix-embedded nanoparticles, ultra-thin films and junctions, grain boundaries, and others. The presentation will emphasize these capabilities, describing three methods of data mining that can be used to fully exploit APT: (i) The determination of iso-concentration surfaces and proximity histograms derived thereof, (ii) a cluster identification algorithm based on maximum-atom separations, (iii) and the visualization of atomic lattice planes in crystalline specimens. These approaches will be illustrated by means of different types of samples: Si nanocrystals embedded in a silicon oxide matrix, Mg clustering in GaN, various III/V semiconductor multilayer systems, and a crystalline metal specimen. The results demonstrate clearly that sub-nm sized structures can be characterized by APT.
[1] D. N. Seidman, Annu. Rev. Mater. Res. 37, 127 (2007).
[2] B. Gault, M. P. Moody, J. M. Cairney, S. P. Ringer, Atom Probe Microscopy, Springer, New York, 2012.
[3] T. F. Kelly, D. J. Larson, Annu. Rev. Mater. Res. 42, 1 (2012).
[4] D. J. Larson, T. J. Prosa, R. M. Ulfig, B. P. Geiser, T. F. Kelly, Local Electrode Atom Probe Tomography, Springer, New York, 2013.

Si Nanocrystals (Si NCs) embedded in a dielectric matrix have unique features such as a bandgap engineering by controlling the Si NCs size and the spacing between them. However, the behavior of dopant atoms in Si NCs is not well understood. Atom probe tomography (APT) enables to investigate the distribution of Si NCs as well as the location of dopant atoms. Boron and/or phosphorus doped samples were prepared by a sputtering or chemical vapor deposition and annealed at 1100–1150 ^○C to form Si NCs in Si-rich oxide. By analyzing APT results, it was found where dopant atoms preferred to position in the Si NCs, the matrix, or the interface between them. The difference in the distribution between boron and phosphorus will be discussed in detail.

Ge / Si core-shell nanowires (NWs) showing substantial potential in the application of biological detectors, solar cells, as well as one-dimension field-effect transistor have attracted considerable attention in recent years.[1] The dopant distribution in the Ge / Si core-shell NWs directly affects the NWs' electronic property. Therefore, it is important to get clear of the dopant distribution in individual Ge / Si core-shell NWs. In this study, the dopant, boron (B), distribution in Ge / Si core-shell NWs was investigated by atom probe tomography (APT), which enables to obtain three-dimensional atom mapping with nearly atomic-scale resolution.

The Ge / Si core-shell NWs were grown on a Si (111) substrate by chemical vapor deposition. The Ge core was grown at 320 ^oC for 30 min using 10 sccm of GeH₄ (100%) as the source gas with gold nano colloid particles of 3 nm in diameter as the catalyst. Then an additional 10 sccm of GeH₄ (100%) was introduced at 500 ^oC for 1 min to increase the diameter of Ge core to about 50 nm. The B doped Si shell was grown at 700 ^oC for 1 min using 19 sccm of SiH₄ (100%) and 1 sccm of B₂H₆ (1%) as gas sources. The total diameter of the NWs is about 100 nm.[2]

Individual Ge / Si core-shell NWs for APT analysis were prepared by gallium focused ion beam (FIB), combined with high resolution SEM system (Helios NanoLab600i, FEI). APT analysis was performed using a laser-assisted local electrode atom probe (LEAP4000X HR, AMETEK). A pulsed laser with a 355 nm wavelength was irradiated upon the NW with a repetition rate of 100 - 200 kHz and a laser-pulse energy of 10 - 50 pJ. The base temperature during the measurement was 50 K.

In the three-dimensional atom map, the Ge / Si core shell structure was observed where the Ge core was covered by the Si shell. Our APT results clearly show that the B atoms randomly distributed in the Si shell. Moreover, no boron atoms was found in the Ge core region. In this presentation, the details of sample preparation and more analysis data will be shown. The authors would thank Mr. N. Ebisawa for the technical support. This work was supported by JSPS KAKENHI Grant Number 15H05413.
[1] C. M. Lieber, MRS Bull. 36, 1052 (2011).
[2] N. Fukata et al., ACS Nano 6, 8887 (2012).

Conical intersections are points of degeneracy between potential energy surfaces known to facilitate non-radiative decay. Though often discussed in molecular photochemistry, conical intersections have rarely been invoked in explaining non-radiative recombination in materials. We have applied configuration interaction methods developed in our group to demonstrate that several defects on the surface of silicon nanocrystals introduce efficient pathways for non-radiative decay via conical intersection. These pathways are accessible upon excitation at visible energies, and thus relevant to visible light emission. Several experimentally observed features of silicon photoluminescence, including the apparent size-independence of the photoluminescence of oxidized silicon nanocrystals, will be discussed in the context of the theoretically predicted conical intersections.

Ultrasmall silicon (Si) nanoelectronic devices require an energy shift of electronic states for n- and p-conductivity. Nanocrystal (NC) self-purification and out-diffusion in field effect transistors cause doping to fail. Even if dopants manage to enter SiNCs, their ionization energy increases tremendously over values known from bulk Si; no free charge carriers can be provided [1]. We show that silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) create energy offsets of electronic states in embedded Si quantum dots (QDs) in analogy to doping [2]. Hybrid density functional theory (h-DFT), interface charge transfer (ICT), and experimental verifications arrive at the same size of QDs below which the dielectric dominates their electronic properties. Large positive energy offsets of electronic states and an energy gap increase exist for Si QDs in Si₃N₄ versa SiO₂. Using DFT results, the SiO₂/QD interface coverage is estimated with nitrogen (N) to be 0.1 to 0.5 monolayers (ML) for samples annealed in N₂ versus argon (Ar). The interface impact is described as nanoscopic field effect and propose the energy offset as robust and controllable alternative to impurity doping of Si nanostructures.

[1] D. König, S. Gutsch, H. Gnaser, et al., Sci. Rep. (Nature), 5, 09702 (2015), DOI: 10.1038/srep09702
[2] D. König, D. Hiller, S. Gutsch, M. Zacharias, Adv. Mater. Interfaces 1, 1400359 (2014)

The atomistic mechanism for dopant diffusion is not well understood yet, so that the entire process can not be well-controlled, particularly with regards to the contamination of the surface area during the shallow doping process and other important side effects. In this work, the complete modeling of the atomic reaction pathway, from methyl arsenate adsorption to self-decomposition and finally sub-surface diffusion, is unraveled for atomically flat, initially H-terminated, Si(111) surfaces. We have used a powerful combination of experimental techniques, such as Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS) and Low Energy Ion Scattering (LEIS) to accurately characterize the evolution of the surface adsorbates during annealing, as well as the doping depth profiles. The key to this work is the preparation of a model surface, consisting of exactly 2/3 monolayer (i.e. 1/3 monolayer remain H terminated) on atomically flat Si(111) surfaces. We experimentally show and theoretically confirm that Methylarsenic acid on Si(111) is more reactive than Methylphosphonic acid on Si(111). We further show that arsenic acids are chemically attached to the surface as mono-dentate via a Si-O-As bond, with the remaining groups (As=O and As-OH) forming an oriented 2D-network of hydrogen bonds that greatly stabilizes the surface. The perfection of the surface imparts its remarkable properties, highlighting the importance of oxide-free surfaces, and makes it possible to model the entire monolayer doping process using first principle calculations. The overall driving force is identified as a thermodynamic instability of As(V) in contact with silicon, which initiates a self-decomposition of chemisorbed methylarsenic molecules at 800 K. As the temperature is increased, the As-C bond breaks -- the weakest link of the adsorbed molecule -- with release of the organic ligand and a rearrangement from a monodentate to a bidentate bonding configuration. In this process, oxygen atoms evolve by partial desorption as H₂O and partial incorporation into the surface Si atom backbonds. At 1050 K, diffusion of As into the sub-surface region of silicon is observed. There is no evidence for As desorption and no remaining C contamination.

As a device has been scaled down, the external resistance increases while the channel resistance decreases. Thus, contact resistance at metal-to-semiconductor interface becomes a dominant factor among components to increase the parasitic series resistance in source and drain (S/D). It should be noted that contact resistivity between metal and semiconductor (MS) is primarily determined by the Schottky barrier height (SBH) and doping concentration in the semiconductor. But Fermi level pinning (FLP) at the metal to semiconductor interface contributes to increasing the SBH, thus the contact resistivity is unwantedly increased.
Many research groups have studied inserting various insulators between the MS to reduce the SBH as an attempt to find the most optimum insulator material. However, additional components of external resistance such as a tunneling resistance were also problematic even though insertion of the ultra-thin insulator is attractive to reduce the SBH by mitigating the FLP. TiO₂ is an attractive insertion insulator at MS interface, which has been used to mitigate the FLP due to the low conduction band offset between TiO₂ and Si.
In this study, the contact resistivity of a TiN/Ti/SiO₂/n-Si stack was comparable to that of a conventional TiN/TiO₂/n-Si structure with sputtered TiO₂. This result is attributed to a Ti-based oxide layer, which was formed by the reaction kinetics of a Ti film on SiO₂. In the TiN/Ti/SiO₂/n-Si stack, the TiO₂ layer is automatically formed as the Ti film absorbs oxygen from SiO₂ without additional process. Therefore, it is possible to make the Ohmic contact through alleviation of the Schottky barrier height by the aforementioned self-formation of TiO₂ without intentional TiO₂ insertion and the lowering of the SBH is experimentally demonstrated.

Semiconductor quantum dots (QDs) exhibit characteristic luminescence properties, such as size tunable luminescence wavelength and high chemical and photo stability in aqueous media. These properties make QDs promising for fluorescent probes in biomedical applications. Among many kinds of QDs, Si QDs have attracted significant attention because of the high biocompatibility and biodegradability. Recently, we have developed a new type of colloidal Si-QDs that can be dispersed in water in a wide pH range without functionalization by organic ligands [1] and exhibit size tunable luminescence in the 700–1400 nm range[2]. The unique structural feature of the Si QDs is the formation of a high B and P concentration shell, which makes the surface of QDs hydrophilic. A drawback of the Si QDs as a fluorescence marker is the small excitation cross section in the visible to near infrared range due to the indirect nature of the energy band structure. In this work, to enhance the excitation cross-section, we developed core-shell type composite nanoparticles consisting of a Au nanoparticle (NP) core and a shell of Si-QDs agglomerates by a simple method utilizing electrostatic attraction between positively charged Au-NPs and negatively charged Si-QDs. We characterized the composite NPs by transmission electron microscopy and confirmed the formation of the core-shell structure. We studied the photoluminescence properties of the composites in detail by changing the excitation wavelength in a wide range (400–600 nm) and demonstrated the enhancement of the luminescence excitation efficiency in the wavelength range of the localized surface plasmon resonance of Au-NPs. We also constructed a simple model to calculate the enhancement factor of the composites and confirmed the validity of the experimentally observed excitation efficiency enhancement.
[1] H. Sugimoto, et al. Nanoscale, 2014, 6, 122
[2] H. Sugimoto, et al. J. Phys. Chem. C 2013, 117, 11850-11857

The demand of alternative gate dielectric to SiO₂ has come to scene due to the aggressive scaling of the chip dimensions in recent years and high-k dielectrics have been showed as promising candidate to SiO₂. Among them, the rare earth oxides are quite important due to their large conduction band offset over 2 eV and large band gaps ~ 5.4 eV. In spite of the superior properties of the rare earth oxides, there is a little information about the radiation responses of them. The aim of this study is to investigate the use of Gd₂O₃ as gate dielectric in MOS based radiation sensor. For this purpose, Gd₂O₃ films were deposited on p-Si (100) by RF magnetron sputtering and annealed at 800 ^oC under N₂ ambient. The crystallite structure of films was confirmed by XRD. The film thickness was measured as 114 nm. Before the irradiation, the electrical characterizations of the capacitors were investigated at different frequencies by obtaining C-V and G/ω-V curves. After that, the capacitors were irradiated by ⁶⁰Co radioactive source in the dose range of 0.5–50 Gy. All of the C-V and G/ω-V measurements were performed at 1 MHz-frequency. The dielectric constant of the film was calculated as 12.96. Both capacitance and conductance values decreased with increasing voltage frequency. Two peaks related to depletion and inversion edge was observed at the conductance curve. The small peak located on the inversion edge may be attributed the interaction between minority carriers and donor-like interface states, whereas wide peaks located on depletion edge may be due to the interaction between majority carriers and acceptor- like interface states. After irradiation, the C-V curves shifted to less negative voltage values compared to non-irradiated one. The variation of oxide trapped charge density was determined in the range of 6.91×10¹¹ – 1.81×10¹² cm^-2 in the studied dose range. It was not observed a significant variation in the interface states densities, value of which always remained in order of 10¹⁰ eV^-1 cm^-2 in the studied dose range. In the light of these results, it can be said that gamma radiation doesn’t cause a significant degradation during the irradiation. The comparison of the interface and oxide trapped charge density shows that the oxide trapped charges are more responsible for the flat band voltage shift compared the interface states. The calibration curve of the capacitor was obtained by the flat band voltage shifts depending on gamma dose. The sensitivity of the capacitor was determined to be 36.6 mV/Gy. The electrical characteristics of the capacitor show that the Gd₂O₃ is promising dielectric for the MOS based technology. The irradiation results demonstrated that Gd₂O₃ MOS capacitor is more sensitive to the gamma radiation compared to the SiO₂ based capacitor. Therefore, more sensitive dosimetry is possible with thinner Gd₂O₃ film compared to SiO₂.This work is supported by the Ministry of Development of Turkey under Contract Number: 2012K120360.

Solution processing of silicon (Si) and germanium (Ge) semiconducting nanoparticles (NPs) enables low cost production of functional thin films on flexible substrates. The control of conductivity and carrier mobility within porous NP thin films from disperse systems is crucial for further device integration. As-deposited thin films from dispersions show poor conductivity and therefore need dedicated post treatment schemes [1], e.g. low-temperature annealing or laser sintering. In our previous work on Si-NPs [2] and Ge-NPs [3], respectively, the importance of the surface chemistry of NP surfaces was highligted and their effect on the device performance has been tested in TFT-like setups. Low-temperature (
The established approaches were extended and applied to different Ge-Si alloy NP systems with respect to the Ge/Si ratios. This work covers the entire range between pure Si and pure Ge. By varying the composition over large ranges, Si-like electrical behavior is expected to be combined with thermodynamic advantages of Ge-rich systems. GexSi1-x alloy NP production is described in detail elsewhere [4]. Thin films from solution processing of NPs are optimized with respect to suitable disperse media and layer morphology. The NP layers were electrically characterized in pre-structured TFT test structures. The whole experimental process chain is conducted under N2 atmosphere in a custom-built glovebox system in order to prevent surface oxidation. The conductivity of the layers is enhanced upon annealing and surface charge induced hysteresis of the i-v characteristics could be suppressed by encapsulation. The test devices show gateability with on/off ratios of 10-100. Temperature dependent conductivity measurements are applied for the evaluation of carrier transport mechanisms and indicate thermionic emission over inter-particle boundaries as predominant conduction mechanism. Additionally, we give an outlook for the application of the material in temperature sensing devices.

The technique used for particle production can be used to adjust compositions. Layers can be processed from solution which makes them attractive for low cost production. Additionally, low temperature processing enables the application on various substrates for e.g. future flexible electronics. As an outlook covering viable device integration schemes is given.

[1] Talapin et al., Science, 2005, 310, p. 86 [2] Weis et al., Small, 2011,7, p. 2853 [3] Meric et al., PCCP, 2015, 17, p. 22106 [4] Mehringer et al., Nanoscale, 2015, 12, p. 5186

Si Nanocrystals (Si NCs) have great potential to improve energy conversion efficiencies for all-Si tandem solar cells by exploiting the quantum confinement effect. High Si content Si rich oxide (SRO) has the advantage of increasing the conductivity and light absorption in the film. However, high Si content SRO also induces larger Si NCs and lowers the bandgap towards that of crystalline Si (1.12 eV). In this work, thin SRO and SiO₂ bilayers were alternatively deposited and the superlattice annealed at 1100 ^oC for 1 hour in N₂. Intrinsic (undoped), boron and phosphorus doped samples with high Si content SRO were prepared to study their structural, optical and electrical properties. The samples were analyzed by energy filtered transmission electron microscopy, atom probe tomography, photoluminescence spectroscopy and 4-point probe measurement.

Silicon heterojuction solar cells enable high energy conversion efficiencies. To obtain high-quality interface, an atomically sharp a-Si:H/c-Si interface is necessary. We are investigating the effect of atomic hydrogen by the hot wire method and studying the basic properties of the hetero-junction and the gap state density distribution at the n-i and p-i interfaces near the conduction and valence band edges. We report the effect of atomic hydrogen during deposition, hydrogenation after deposition and annealing.
Doped or undoped hydrogenated amorphous silicon was fabricated on the p/n crystalline silicon wafer by sputtering method at 200°C with supplying atomic hydrogen during the deposition. The atomic hydrogen was generated by the hot-wire method, which was decomposed near the tungsten wire heated to around 1100°C. The interface states were evaluated by Modulated Admittance Method applying voltages from 1 kHz to 10 MHz. The depth from the band edge was determined by the admittance signal dependence of the temperature from 20 to 100°C in air.
The state density at n-i interface was 5.46x10¹¹ eV^-1cm^-2 and the energy level is located at 74.85 meV from the conduction band edge in the case without supplying atomic hydrogen. The state density decreased to 3.84x10¹¹ eV^-1cm^-2 and the energy level becomes deeper to 108.5 meV from the conductions edge in the case with supplying atomic hydrogen. When it was annealed at 250°C in the vacuum, the interface state density become 9.29x10¹⁰ eV^-1cm^-2 and the energy level is located at 121.1meV. As in the case with the p-i interface, we have obtained the same tendency as n-i interface. From these results, atomic hydrogen terminates dangling bonds in the interface and provides structural flexibility between the hetero structures. The states appear to be deeper by annealing or hydrogenation, which is caused by the fact that the edge slope became steeper by the hydrogen termination and annealing and the energy from the edges become larger.

It is desirable to fabricate Si based nanostructures directly by a single application of laser pulses. We have observed formation of nanodots and nanowires on the Si(001) surfaces when they are exposed to interferential laser pulses. These dots and wires are observed from the Si surfaces when they are irradiated interferentially by high power laser pulses of 7 ns. The laser wavelength used are 532, 355, and 266 nm. These nanostructures form selectively as their placements can be controlled by controlling the interferential parameters. The morphologies of the nanostructures are studied by atomic force microscopy. Generally, the laser irradiated surfaces show nanowires but nanodots are also observed, depending on the interferential parameters. The nanowire width increases with interference period. The narrowest nanowires observed have the width smaller than 50 nm, which is four times smaller than the interference period while the nanodots have a base width of 43 nm and height of 8 nm. The formation mechanism of Si nanowires and nanodots will be discussed along with the comparison of nanowires and nanodots that are directly fabricated on III-V semiconductor surfaces.

The growth of Ge Quantum Dots on freestanding Silicon nanoribbon portrays the formation of anti-correlated pyramid-shaped nanocrystals (“hut” QDs) strained on the sides of the ribbons. This noticeable feature is anti-correlated in the order of certain range of the nanoribbons. While the biggest domain size of anti-correlated ordering is theoretically unknown, an experimental pursuit of the biggest domain size has no clear target. This report, therefore, seeks to present the theory that strain effect is the driving force of this anti-correlation.
In this study, we used finite element (FE) method (package ANSYS) to analyze the distributions of strain when the Ge QDs was deposited with different thickness (5 nm, 10 nm, 25 nm, respectively) on the freestanding nano-ribbon. The strained Ge island (nano-stressor) induced bending on the nano-ribbon. The lattice on the reverse side was compressed when the opposite side had QDs and relaxed at positions without dots on the other side. Our findings suggest that comparing the strain distribution and deformation of different thicknesses of the nano-ribbon, 10 nm is most suitable for the thickness of the stress transmission and practical manufacturing. FEM calculations suggested that the deformation induced by strain can be controlled by altering the thickness of the nano-ribbon. Quantum dots can transfer strain through the freestanding nano-ribbon, and affect the growth of quantum dots around the place, and form ordered quantum dots. This clearly indicates that a nano-ribbon bends by unbalanced induced strain. The anti-correlated domain size was also determined. Through summarizing and analyze the research result, we calculate the domain size of ordering growth is closely related with n_start. n_start belongs to the first layer quantum dots number within the whole orderly growth range (the ordering growth is not throughout the whole growth system, but appear clusters). In conclusion, it has been realized that ordering can be controlled by altering the thickness of the nanoribbon, and that the anti-correlated domain size depends on the distance of the dots deposited at a time (growth rate). While the other side needs to use a slow growth rate in the beginning and gradually increase the growth rate so that QDs nucleate alternately on the top and bottom surfaces a “shell” dot at a time.

We model fully OH- and NH₂-terminated silicon nanocrystals (SiNCs) - Si₃₅(OH)₃₆ and Si₃₅(NH2)₃₆ - by hybrid-DFT [1,2]. We substitute OH [NH₂] groups by double-bonded (=) O [=NH] segments and corner Si(OH)₂ [Si(NH₂)₂] segments for bridge-bonded (>) O [>NH] groups. Correlating ground state (GS) gaps and charge transfers from SiNCs to anion groups with atomic ratios of Si to O [N] atoms, we show the specific impact of = and > anion bonds. Excited state (ES) calculations yielded the three lowest singlet transition energies (ES gap, absorption) and oscillator strengths (optical transition probabilities P_ot). ES energies closely follow the trend of GS gaps for OH-terminated SiNCs with =O, while those for SiNCs with >O show an increasing deviation from GS gaps to lower energies with rising numbers of >O. An increased modulation of charge transfer occurs due to low negative ionization of >O atoms which appears to distort the exciton field and thus increases exciton binding energies. For SiNCs with =O atoms, stronger SiNC ionization seems to improve carrier separation, decreasing exciton binding energies and lifting ES gaps. Thus, P_ot of SiNCs rise hyperlinear with the number of =O atoms, while a saturation occurs for SiNCs with >O atoms. Similar results were found for NH₂-terminated SiNCs with =NH vs. >NH, with less influence on the electronic and optical SiNC behaviour due to lower SiNC ionization.

[1] D. König et al., Phys. Rev. B 78, 035339 (2008)
[2] D. König et al., Adv. Mater. Interf. 1, 201400359 (2014)

We report on phosphorous (P) doping of Si nanocrystal (SiNC)/SiO₂ systems [1]. Relevant P configurations within SiNCs, at SiNC surfaces, within the sub-oxide interface shell and in the SiO₂ matrix were evaluated by hybrid density functional theory (h-DFT). Atom probe tomography (APT) and its statistical evaluation provide detailed spatial P distributions. We obtain ionisation states of P atoms in SiNC/SiO₂ systems at room temperature using X-ray absorption near edge structure (XANES) spectroscopy in fluorescence yield mode. This non-destructive volume characterization technique leaves the material unchanged which is paramount to probe P in its original state. P K shell energies were confirmed by h-DFT. We found that P diffuses into SiNCs and resides virtually exclusive on interstitial sites. XANES and h-DFT delivered evidence that the ionization probability of P in the SiO₂/SiNC system is extremely low; free localized electrons to SiNCs are not provided.

[1] D. König, S. Gutsch, H. Gnaser, et al., Sci. Rep. (Nature), Vol. 5, 09702 (2015), DOI: 10.1038/srep09702

The doping of semiconductor nanocrystals (NCs) is vital to optimize the performance of semiconductor-NC-based structures and devices. Impressive progress has been recently made on the doping of compound semiconductor NCs and silicon NCs. However, the doping of germanium (Ge) NCs significantly lags behind. Here we show that Ge NCs can be successfully doped with phosphorus (P) during their synthesis in plasma. It is found that P-doped Ge NCs are less prone to oxidation in air than undoped Ge NCs. P-doped Ge NCs can be well dispersed in certain organic solvent to form stable solution, which may be readily cast to produce films for field-effect transistors (FETs). Our FET analysis shows that both the conductivity and electron mobility of a Ge-NC film increase with the increase of the doping level of P. The performance of the FETs based on P-doped Ge NCs may be improved by treating the Ge-NC films with atomic layer deposition.

The time evolution of the photoluminescence (PL) of porous silicon (π-Si) while being exposed to different chemical solutions is reported. The objective is to study the influence of hydroxyl radicals in π-Si surface and therefore in the active centres for light generation. The hydroxyl radicals are obtained from Fenton solution (sol): (NH₄)₂Fe(SO₄)₂●6(H₂O) (1mM), EDTA (1mM) acetate buffer (0.01M, pH 4.7), and H₂O₂ (10 mM) always freshly prepared [1]. H₂O₂ was added immediately before starting the experiments. For this reason the PL was studied under the influence of each of the constituents of the Fenton sol separately (1: water, 2: H₂O₂, 3: Fe²⁺ with EDTA and acetate buffer, 4: Fenton and also 5: in air, i.e. no sol). Porous silicon samples were prepared from n-type silicon through a galvanostatic technique using a Teflon electrochemical cell with a plastic optical window and a two electrode arrangement with a platinum wire as counter electrode. A starting current of 10 mA cm^-2 for 300 s under illumination conditions was applied in a 4% HF in a 50% absolute ethanol sol [2]. Either fresh samples (prepared just before the measurement) or aged ones (prepared a month before the experiment) were used. The PL was excited with CrystaLaser DL375-010-O and measured with an SEC2000 – VIS/NIR ALS spectrophotometer. An UQG U-340 bandpass optical filter was used to avoid spurious emission from the laser to influence the measurement. The PL was measured in situ, prior to the immersion in the sol and immediately after the immersion. In order to measure the evolution of the PL in the sol a spectrum was recorded every 30 s during 40 minutes.
As a general rule the PL spectra (in the red infrared region between 600 – 800 nm) decay with time. For this reason the normalized spectra PL_n(λ) were studied as well as the differential PL DPL defined as DPL = PL_n(λ)/PL_nm(λ) – 1, where PL_nm(λ) is the normalized spectrum corresponding to maximum PL (i.e. just after sample is put into contact with the sol). Except in the case samples are immersed in water (sol 1), a relative increase of the infrared emission is observed. These changes are more clearly observed for the Fenton sol (sol 4) and the same sol without H₂O₂ (sol 3) where a clear shift of the peak is observed. In these cases the shift in peak position of PL_n(λ) is as high as 60 nm. In air and H₂O₂ (sol 2), the spectra broadens (less than 50 nm). However, the changes are below 40 % in all cases, except for the Fenton sol (sol 4). In these cases the DPL reaches values higher than 100 % at 800 nm and – 80 % at 600 nm. These changes are expected to be originated in the oxidation of the π-Si surface which is highly increased in the presence of the hydroxyl radicals. These results may contribute in the potential applications of π-Si for designing of a semiconductor-based radical biosensor.

[1] – A. M. Nowicka, Angew. Chem. 122 (2010) 3070.
[2] – E. C. Muñoz, J. Chilean Chemical Society, 56 (2011) 781.

Macro porous silicon, prepared by an electrochemical process, has also gained interest in research for many applications which have a demand for mechanical and chemical stability as well as a high order of the pores and high aspect ratios (up to 250). The pore diameters can differ from 800 nm up to 100 µm using lithographic pre-structuring. The standard deviation of pore diameter and interpore distance is lower than 1 %. Because of the lithographic pre-structuring technique macro porous silicon with its high ordered structure represents an ideal 2-D photonic crystal (PC) exhibiting novel properties for the propagation of infrared light within the pores. Using this photo-electrochemical process the pore profile and size can be controlled by the light source intensity. Widening the pores leads to a so called lift-off process: the pores are widended at the bottom to such an extent that the pore wall to the neighboring pore dissolves and hence there is no connection to the bulk material as well; this results in a peak structure on the bottom surface in contrast to a smooth top surface of the membrane .
Those strucures can be used for sensong applications such as a humidity sensor or for ion-mobility spectrometry due to the field emission properties.

Silicon nanocrystals (SiNCs) have been attracting attention as active materials in a variety of proto-type devices including, solar cells, light-emitting diodes, and photodetectors. These, and other device structures require well-defined materials with predictable properties. Traditionally SiNC surfaces are rendered processable and stable toward oxidation by employing a variations of the general hydrosilylation reaction; they all involve the addition of a silicon-hydride bond in the SiNC surface across a carbon-carbon double (or triple) bond and affords a “monolayer” attached through a robust silicon-carbon linkage. Recently, it has come to light that typical hydrosilylation conditions can afford insulating multilayers that can render SiNCs electrically inactive. This finding led us to explore alternative functionalization protocols. In doing so, we discovered that surface chemistry provides another degree of freedom with which SiNC properties may be tailored. For example, it can be used to tailor the photoluminescent response throughout the visible spectrum, used to prepare SiNC/polymer hybrids, and even induce reactivity that provides polymerization of workhorse electronic polymers without the use of expensive transition metal catalysts. This presentation will include a discussion of our recent exploration into SiNC surface chemistry, new reactive platforms that have opened the door to new functional materials, and what our discoveries mean for the future of these fascinating materials.

Photo- and electro-luminescence quantum yield (QY) is a crucial parameter for any material to be used in light-emitting devices, fluorescent probes, wavelength converters etc. In case of materials composed of ensembles of nanocrystals (NCs) in a solid matrix or liquid medium we distinguish external and internal QY (EQY and IQY). EQY is defined as the total number of emitted photons divided by number of absorbed photons for the whole sample, while IQY concerns only the luminescing subensemble of NCs. In most of NC ensembles there is significant number of “dark” NCs that can absorb but not emit photons due to presence of non-radiative recombination centres (defects). EQY is conveniently measured using an integrating sphere [1, 2] but IQY is not easily measurable. The most feasible way to obtain IQY is variation of local density of optical states which affects radiative but not non-radiative lifetime, so enabling to decouple these two components and calculate IQY. We study both EQY and IQY of Si nanocrystals (SiNCs) using either special samples with variable distance between NCs and a high-n substrate or by approaching such substrates to a layer of NCs. We show that large number of SiNCs are dark and discuss the mechanisms limiting PL QY in ensembles of SiNCs. [1] J. Valenta, Nanoscience Methods 3 (2014) 11-27 (OA). [2] J. Valenta et al., Appl. Phys. Lett. 105 (2014) 243107.

In this work we present the absorption of single oxide-passivated silicon nanocrystals (Si-NCs) measured via photoluminescence excitation (PLE) between 2.0 and 3.5 eV. Two different types of samples with NCs of different geometries, emitting at around 1.7 eV, were probed at 300 and 70 K: spherical-like (nanodots) fabricated from a thin SOI wafer [1] and rod-like (nanorods) embedded in Si nanowalls [2]. Clear features in the absorption spectra of nanodots, barely detectable in nanorods, lead to the identification of four absorption peaks at average energies of 2.29 eV, 2.67 eV, 2.94 eV and 3.33 eV. In addition, the energy level structure of Si-NCs with similar size and shape calculated using the empirical pseudopotential method (EPM) allowed to assign corresponding level transitions to some of those peaks [3]. As expected, the increased asymmetry in nanorods leads to a degeneracy lifting of such transitions, “blurring” the absorption peaks. Moreover, for an absolute comparison, the PLE spectra of the NCs were normalized via direct measurement of their absorption cross-sections at 3.06 eV. The results show a 5-10 fold absorption enhancement in nanorods with respect to nanodots. This can be explained partly in terms of density of states but also resulting from local field effects. Large Stokes shift together with size- and shape- independent strong absorption at high energies favors the application of Si-NCs as biotags and phosphors.

References:
[1] I. Sychugov, J. Valenta, K. Mitsuishi, M. Fujii and J. Linnros “Photoluminescence Measurements of Zero-phonon Optical Transitions in Silicon Nanocrystals” Phys. Rev. B 84, 125326 (2011).
[2] B. Bruhn, J. Valenta and J. Linnros “Controlled Fabrication of Individual Silicon Quantum Rods Yielding High Intensity, Polarized Light Emission” Nanotechnology 20, 505301 (2009).
[3] J.-W. Luo, P. Stradins and A. Zunger “Matrix-embedded Silicon Quantum Dots for Photovoltaic Applications: a Theoretical Study of Critical Factors” Energy Environ. Sci., 4, 2546-2557 (2011).

We use a monosilane (SiH4) and oxygen (O2) based plasma-enhanced chemical vapor deposition process (PECVD) to fabricate Si nanocrystals in SiO2 matrix with controllable size and areal density.[1] The stoichiometry of the Si nanocrystal precursor layers can be tuned in a wide range which results in either completely separated quantum dots (QDs) or in an interconnected QD-network.
We present a detailed study based on electrical (C-V, I-V) measurements that reveal the transport properties at and around the percolation threshold.

[1] Laube et al. JAP, 116, 223501
[2] S. Gutsch et al., JAP 113, 2013

Metal-semiconductor hybrid nanoparticles (HNPs), which combine metal and semiconductor nano-domains into a single nanoparticle, are expected to exhibit novel optical properties useful for the biomedical applications, solar-energy conversion and photo-catalysts. Recently, there have been great advancements in the synthesis of HNPs based on Cd and Pb chalcogenide quantum dots (QD). On the other hand, the development of Si QD-based HNPs has scarcely been reported despite their high biocompatibility.
In this paper, we report synthesis of Si QD-based HNPs with different shapes and compositions. Si-QDs used in this work have very heavily boron (B) and phosphorus (P) doped Si shells on the surface.[1] The shell induced negative potential on the surface and due to the electrostatic repulsion between QDs, they form a perfect dispersion in water and alcohol without any organic surface ligands.[2]
In this work, for the formation of Si QD-based HNPs, we present two different synthesis processes utilizing the advantages of B and P codoped Si-QDs. The first process is to use electrostatic attraction between negatively-charged Si-QDs and positively-charged Au nanorods. We demonstrate the formation of water-dispersible Si-QDs-decorated Au nanorods with the polymer spacers.[3] Another method for the formation of HNPs is self-limiting growth of noble metal (Au, Ag, Pt) NPs by using codoped Si-QDs as catalysts and also as a protecting layer. Thanks to the absence of organic ligands on the surface of codoped Si-QDs, in the mixture solution of QDs and metal-salts, Au, Ag and Pt ions are reduced by QDs. This process results in the formation of very uniform HNPs with precise controllability of the metal NP size.
In the Si-QDs-decorated Au nanorods, we demonstrate the enhanced quantum efficiency and linearly-polarized luminescence of Si-QDs by optimizing the distance between Si-QDs and Au nanorods. In the latter type of HNPs, in which Si-QDs are directly attached to metal nanoparticles, we demonstrate efficient charge transfer from Si-QDs to metal NPs from the analysis of luminescence decay dynamics. From the results of optical properties of different sizes of Si-QDs and metal NPs, we will discuss the surface plasmon-driven photocatalytic properties of HNPs.
[1] H. Sugimoto, et al., J. Phys. Chem. C, 117, 11850 (2013), [2] H. Sugimoto, et al., Nanoscale, 6, 12354 (2014). [3] H. Sugimoto, et al., ACS Photonics, 2, 1298 (2015).

Group IV nanostructures are at the heart of modern-day electronic devices and systems. Due to their high refractive index, they also provide a myriad of opportunities to manipulate light. When properly sized and shaped, they can support strong optical resonances that boost light-matter interaction over bulk materials and enable their use in controlling the flow of light at the nanoscale. In this presentation, I will discuss the use of individual, resonant nanostructures and dense arrays thereof (metafilms) in a variety of optoelectronic devices and illustrate how the performance of these devices can be improved by engineering the size, shape, and spacing of the constituent nanostructures.

Approaching fundamental limits in scaling-down conventional silicon microelectronic devices forces the semiconductor industry to research and develop novel materials and device geometries. Semiconducting nanowires allow switching from a planar to a cylindrical transistor channel thus enabling increased electrostatic integrity required for further scaling of transistor dimensions. Reducing the operating voltage without loss in performance, however, demands further disruptive technologies, such as implementing high-mobility channel materials that can achieve improved performance-power trade-off.
In that regard bottom-up grown nanowires are very attractive materials for direct integration of III-V semiconductors on silicon thus opening up new possibilities for the design and fabrication of electronic and optoelectronic devices. The ability to epitaxially grow abrupt axial heterostructures with large lattice mismatch is unique for the nanowire geometry, and together with the possibility of in-situ doping and core-shell structures provides the basis for novel device concepts.
We have developed a novel Template-Assisted Selective Epitaxy (TASE) approach which allows for the monolithic integration of complex III-V nanostructures on silicon. Vertical as well as lateral nanowires and lateral complex nanostructures like constrictions along a nanostructure and nanowires cross junctions can be grown with this method. Based on this technique InAs nanostructures were grown including Hall-bar structures with Hall mobilities on the order of 5400 cm²/Vs which evidence the good material quality. Furthermore, InAs MOSFETs and InAs/Si nanowires heterostructure Tunnel FETs were demonstrated.

Silicon has been in the focus of research and development activities for several decades and doubtlessly is one of the best known semiconductors. However, when silicon is used to fabricate nanostructures several unexpected phenomena such as self-purification and dopant deactivation can be observed. Because of the small dimensions of nanostructures the influence of localized defect-states on electrical and optical properties of nanostructures and nanostructure containing devices such as hybrid solar cells become important.
This paper provides a comprehensive study on the influence of localized defects on the properties of S nanowires. For this purpose Si nanowires were grown by three different methods: (i) catalyst free pulsed laser deposition (PLD), (ii) vapor-liquid-solid (VLS) assisted PLD, and (iii) metal assisted chemical etching of single crystal silicon. The resulting nanostructures had a diameter of 20 to 50 nm and a length of up to 2 µm. The specimens were characterized using Raman backscattering, photoluminescence (PL) measurements, and photo-thermal deflection spectroscopy (PDS). The latter technique provides a direct measure of the sub band-gap absorption that is directly related to the density-of-states distribution.
For PLD grown nanowires the LO-TO phonon mode shifts from 511 to 517 cm^-1 indicating that the incorporated tensile stress decreases. When a gold catalyst is used for the PLD growth the resulting nanowires exhibit a phonon shift of -1.6 cm^-1. A similar shift of the LO-TO phonon mode is observed in Si nanostructures that were fabricated by metal induced chemical etching indicating the presence of tensile stress.
Photoluminescence spectra of nanowire samples were compared to reference spectra of single crystal silicon. While c-Si exhibits pronounced radiative band-band recombination the signal is quenched to zero for Si nanowires deposited with PLD or VLS assisted PLD. This is indicative of a high concentration of localized defects in the Si nanostructures. On the other hand, when the nanowires are etched into a wafer the band-band recombination decreases gradually with increasing etching time and is still detectable for nanostructures with a length of about 1 µm. This indicates that the defect concentration gradually increases but is still smaller than in PLD grown nanowires. The sub band-gap absorption measured with photo-thermal deflection spectroscopy is consistent with the PL data. PLD grown nanostructures exhibit an increase of the sub band-gap absorption by more than one order of magnitude in the energy range of 0.7 to 1.0 eV. This is accompanied by the appearance of an exponential band-tail. For nanowires produced with metal-induced chemical etching the increase of the sub band-gap absorption is less pronounced.

In recent years, organic/inorganic hybrid solar cells have attracted much attention owing to the synergetic advantages of the high carrier mobility and efficiency from the inorganic semiconductor and the simple process and low cost from the organic semiconductor. In this work, we prepared an array of silicon nanowires (SiNWs) with a controlled diameter and pitch interval using a metal-assisted chemical (MAC) etching and fabricated an organic/inorganic SiNWs/PEDOT:PSS solar cell. Due to large difference of the electron affinity of Si (4.0 eV) and PEDOT:PSS (5.0 eV), the heterointerface between n-type Si and PEDOT:PSS behaves like a Schottky diode. The MAC etching was carried out in aqueous HF/H₂O₂ solution using Au mesh patterns, which were prepared by the deposition of 20-nm-thick Au layer on self-assembled ITO nanodots, followed by subsequent removal of ITO nanodots by sonication. The reflectance of SiNWs surface was dramatically reduced below 5%, compared ~ 35% of planar Si wafer in the visible-to-NIR regions from 300 to 1100 nm, which is in good agreement with the simulation results using COMSOL Multiphysics. To fabricate an organic/inorganic hybrid solar cell, we spin-coated PEDOT:PSS on top of Si nanowires. We prepared two types of hybrid solar cells with different surface treatments on Si nanowires, one with BOE and the other with HNO₃ and we observed HNO₃ removed the etching damage, which improved the wettability of PEDOT : PSS. The HNO₃-treated solar cell represented the power conversion efficiency (PCE) of 15%, which is significantly higher than 13.1% of that with BOE treatment and even comparable to conventional Si solar cells. The enhancement of PCE was ascribed to the better conformal coating of PEDOT:PSS on HNO₃-treated Si surface because of the hydrophilic property. Our approaches propose a new way to fabricate highly efficient organic/inorganic hybrid solar cells.

In this work, we report on the development of doped microcrystalline silicon oxide (µc-SiOx:H) grown by plasma enhanced chemical vapor deposition (PECVD) and microcrystalline silicon carbide (µc-SiC:H) grown by hot wire chemical vapor deposition (HWCVD) for silicon heterojunction (SHJ) solar cells. The µc-SiOx:H consists of nanocrystalline silicon embedded in a silicon oxide tissue while the µc-SiC:H is based on a mixture of amorphous and nanocrystalline silicon carbide. These wide gap materials provide optical transparency and electrical conductivity superior to those of conventionally used doped amorphous silicon. This makes them promising candidates as window layer in SHJ solar cells to reduce the optical losses while maintaining the electrical performance. In particular, we report on the successes and obstacles we experienced during the implementation of these thin-film silicon alloys into SHJ solar cells, which give deeper insight into the process mechanisms during µc-SiOx:H and µc-SiC:H growth that not only affects the film properties but also the passivation quality of the underlying layers.

Hydrogenated amorphous silicon (a-Si:H) has been utilized in many material technologies such as
solar cells, thin film transistors, LCD, photo-sensors, and photoreceptors. The performance of all
these electronic devices is highly dependent on the shape of the Electronic Density of States
(EDOS) of a-Si:H inside the mobility gap. In fact, the purpose of using hydrogen in fabricating
amorphous Silicon (a-Si) is mainly due to tuning the EDOS in a desirable way and therefore
understanding the effect of hydrogen atoms on the EDOS is of fundamental importance and is the
key to control the functionality of any a-Si:H based electronic device. The purpose of our current
work is an investigation of the EDOS variation of a-Si:H material with hydrogen concentration (H
at% = 2-16) using Molecular Dynamics (MD) and Density Functional Theory (DFT) methods, in
connection with understanding the performance of HIT (Heterojunction with Intrinsic Thin layer)
solar cells.
In this study, the atomic structure of a-Si was obtained using a Molecular Dynamic (MD)
simulation combined by DFT method and the atomic structure of a-Si:H was obtained by adding
hydrogen atoms to the fully relaxed a-Si structure in a way that fully relax DFT calculation was
performed on all possible configuration after each hydrogen atom addition. Our electronic
structure calculations on the obtained structures clearly showed that adding hydrogen atoms to a-
Si structure does not significantly change the EDOS outside the mobility gap, while this addition
dramatically changes the EDOS inside the mobility gap. In agreement with previous experimental
studies, our results also show a monotanic increase in the band gap with increasing hydrogen
concentration. The EDOS variation inside the mobility gap (including the band gap) can strongly
affect the charge transport property of the materials, which can play a crucial rule in the efficiency
of HIT solar cells. In the presentation, we will consider the effect of this EDOS variation on elastic
and inelastic charge quantum transport through the ultra-thin layer of a-Si:H deposited on c-Si
(interfacial charge transport through a-Si:H/c-Si interface).

In this work, we present new insights into the microstructure of unintentionally n-type doped microcrystalline silicon carbide (µc-SiC:H) thin-films and the understanding of the doping mechanism responsible for their unique electrical behavior. Dark conductivities up to 10^-1 S/cm were achieved without the active use of any doping gas during µc-SiC:H deposition. To date, it is still under debate, if an incorporation of nitrogen or oxygen contaminants during growth is responsible for the n-type doping of the films. Using secondary ion mass spectroscopy and thermopower measurements, we show that nitrogen is the most likely candidate responsible for the unintentional n-type doping of SiC crystallites. However, the doping concentration is not determined by the nitrogen concentration but by the crystallite size, that elicits an exponential increase of the doping efficiency of the material by nitrogen impurities. In addition, we show that films with the highest dark conductivities seem to benefit from higher oxygen concentrations, giving rise to a strong increase in charge carrier mobility. Our temperature dependent thermopower data are in good agreement with the assumption that oxygen atoms might decrease the number of trap states in the band gap at the SiC crystallite boundaries, which leads to a lower barrier potential and finally might explain the increase of charge carrier mobility. This new understanding of the electrical mechanisms of n-type doped µc-SiC:H opens up the opportunity for further investigations of intentional oxygen incorporation

Solid state nanopores have recently attracted a large interest for detection of single molecules and as a possible method for DNA sequencing. The nanopores of a few nanometer dimensions are usually fabricated in a nitride membrane using electron beam drilling. The translocation of DNA strands (or proteins) is then detected by the blockade of the nanopore resulting in a drop of the ionic current between the two reservoirs on opposing sides of the membrane. For very small pore diameters in very thin membranes, the amplitude of the current drop may even be used to distinguish between different nucleotides in a DNA strand. While advances towards single nucleotide sensitivity have been impressive, the technique can be difficult to scale up from one pore to a large array of isolated nanopores thus being restricted by a relatively low throughput. As an alternative, optical techniques can be used in combination with arrays of nanopores for massive, parallel readout. This requires labelling of biomolecules, or even nucleotides, by fluorophores which can readily be read out by a CCD or CMOS camera.

We have developed a technique for fabricating arrays of nanopores in a silicon membrane reaching diameters down to 10 nm or below. The spacing between pores is a few microns to enable read out of individual pores using a CCD camera attached to an optical microscope. The fabrication scheme involves optical lithography and ICP etching from the backside of an SOI wafer to etch out membranes of a few hundred micrometer diameter. The silicon device layer at the front is then patterned using e-beam or optical lithography followed by anisotropic KOH etching to form inverted pyramids. These inverted pyramids serve as etch pits which are used to initiate pore formation in a subsequent electrochemical etching step. By carefully tuning the electrochemical etching parameters we have been able to demonstrate large arrays with nanopores of ~15 nm diameter fully extending through the silicon membrane.

In this presentation I will review the fabrication techniques to achieve nanopore arrays using electrochemical etching, but also other techniques such as shrinking by oxidation or by metal catalyst assisted etching. I will also review our initial DNA translocation experiments through nanopore arrays using fluorophore labelled DNA strands.

Silicon nanocrystals (Si NCs) are intensely investigated as active components in light emitting diodes, lasers and photovoltaics. The optical and electrical properties of ensembles of Si NCs are strongly determined by their size, size distribution, separation distance and crystallinity.

Here, we use state-of-the-art electron microscopy to quantitatively characterize Si NCs using various direct and analytical imaging techniques for free Si NCs as well as for Si NCs embedded in thin films close to the true device structure. In the first part, we will show how sample preparation and analysis may introduce imaging artefacts requiring low-dose conditions for the analysis. Then, we will focus on characterizing three different types of Si NC composites and devices covering various aspects of high-end structural characterization:
1) Si core/shell nanoparticles[1]: the core/shell structure is characterized by analytical TEM with a quantitative characterization of the Si NCs distribution and differences in crystallinity correlated with fluorescence quantum yield measurements.
2) Si NCs embedded in SiO₂ thin films prepared by thermal disproportionation from suboxide thin films on ultrathin TEM membranes [2]: the morphology and size distribution is imaged in plane-view and can be correlated with different growth conditions such as composition, layer thickness and annealing conditions. This technique uniquely allows us to obtain interparticle distances of embedded Si NCs and directly observe the percolation transition from isolated Si NCs to connected Si networks. The results are crucial to understand fluorescence quantum yield measurements and electrical properties of these embedded Si NCs. [3,4]
3) Hybrid LEDs (SiLEDs) with optically active Si NCs [5,6]: full devices are studied after different usage conditions by analytical STEM and EFTEM techniques to directly image the various organic layers and the active layer to identify degradation processes and device failure mechanisms.

This overview highlights the possibilities of high-end structural characterization for different types of Si NC composites and its correlation with the opto-electronical performance of Si quantum dot ensembles and devices.

[1] Mastronardi et al., Nano Lett., 12, 337 (2012)
[2] Gutsch et al., Beilstein J. Nano., 6, 964 (2015)
[3] Valenta et al., Appl. Phys. Lett., 105, 243107 (2014)
[4] Gutsch et al., J. Appl. Phys., 113, 133703 (2013)
[5] Maier-Flaig et al., Nano Lett., 13, 475 (2013)
[6] Maier-Flaig et al., Nano Lett., 13, 3539 (2013)

Nature has evolved a set of sophisticated biological machines for accomplishing molecular-level tasks including membrane receptors, channels, and pumps. Development in nanoscale engineering has enabled bioelectronics that can mimic and/or interact with these biological systems. Bio-functionalized Si nanowires are thought to be a promising candidate for the construction of electrochemical devices. We have developed and demonstrated assembly of 1-D phospholipid bilayers on a variety of nanomaterials, including silicon nanowires. These biomimetic lipid bilayers serve as a general host matrix for bio-functional components such as membrane proteins. Though meaningful technological advancement of these materials has been made, critical questions about the structural and chemical composition remain.

We present results from the first Scanning Transmission X-ray Microscopy (STXM) investigation of 1D lipid bilayers on silicon nanowires. STXM provides the high spatial resolution, chemical selectivity and the ability to probe a liquid system needed to investigate the structure of these bio-nanomaterials. In STXM a focused x-ray beam produced by a zone plate illuminates a sample and we then collect the subsequent transmitted x-rays. The transmitted intensity can be measured as a function of energy to give high spatial resolution element specific x-ray absorption spectra, or as a function of beam position to produce x-ray images. Si NWs in a liquid suspension can be clearly imaged at an x-ray energy near the Si K-absorption edge. We were also able to measure the C and P XAS associated with a phospholipid bilayer on the surface of a single Si nanowire. When coupled with small angle X-ray scattering measurements, the STXM data reveal structural motifs of the Si NWs that give rise to multi-bilayer formation and enables assignment of the orientation of specific bonds known to affect the order and rigidity of phospholipid bilayers.


This work was performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. The Advanced Light Source is supported by the Director, Office of Science, OBES, of the U.S. DoE under Contract No. DE-AC02-05CH11231.

Atomic-precision phosphorus doping of silicon by scanning tunneling microscope (STM) based hydrogen resist lithography is a promising fabrication platform for silicon based quantum computing. The doping is accomplished by selective depassivation of bound hydrogen with the STM tip and exposure to phosphine molecules. After low temperature incorporation of the phosphorus and capping with epitaxial silicon, high donor concentration nanostructures are buried 20-30 nm below the surface.

Locating and registering the buried nanostructured features for subsequent processing to make electrical contacts has been challenging. Recently, we have shown these buried structures can be imaged and registered to surface patterns using scanning capacitance microscopy (SCM). This technique is useful for registering the larger buried structures to deposited surface metal alignment marks, but it is not quantitative with respect to the measured signals, which precludes assessment of the buried features.

To address this limitation, we have imaged STM created donor nanostructures in silicon with scanning microwave impedance microscopy (sMIM). This scanning probe based technique uses shielded cantilever probes to deliver microwave signals at 3 GHz directly to the sample surface and measure the tip-sample impedance. We have created a series of buried donor structures via STM lithography to test the resolution and sensitivity of the sMIM. We find a monotonic response of the sMIM signal with different donor densities, and are able to resolve 10 nm-wide buried lines. Going forward, sMIM is an ideal technique for qualifying buried patterned devices and may allow for quantitative post fabrication characterization of donor structures, critical for realization of quantum computation schemes.