12:45 PM - W4.11
Quantum Transport in Core/Shell GaAs-AlGaAs Nanowires.
Dominique Mailly 1 , Damien Lucot 1 , Fouzia Jabeen 1 , Giancarlo Faini 1 , Jean-Christophe Harmand 1 , Romain Giraud 1
1 , CNRS, Marcoussis France
Show AbstractRadial core-shell nanowires (NWs) attract increasing interest since they are models for clean one dimensional (1D) systems to probe physics in low dimension as well as potential new scheme for applications like sensors, light emitter or fast electronics. The peculiar geometry of core shell structures allows to be exempt from lithographic roughness usually found in etched or gate-defined 1D wires but also compare to standard nanowires to be insensitive to surface defect . With its well-established properties in 2D heterostructures and intrinsically high electron mobility [1], the GaAs/AlGaAs system is of particular interest to realize core-shell NW. Here, we report the controlled growth and electrical measurements of vertical GaAs/AlGaAs core/shell NW. Each NW is seeded from a electron beam patterned gold catalyst and grown via vapor-liquid-solid growth in a molecular beam epitaxy vessel. A silicium layer is inserted during the growth of the AlGaAs shell to dope the GaAs. NWs are then buried in an epitaxial undoped GaAs overgrowth [2]. This final step allows to protect the wires from oxidation of the AlGaAs shell and to facilitate the contact of individual wire at their top end.Transport measurements at ambient temperature show an ohmic behavior with two probe resistances of typically few hundred of ohm for a wire one micron long and a core 25nm diameter. At low temperature (1K
W5: Nanowire Based Field Effect Transistors
Session Chairs
Tuesday PM, November 30, 2010
Ballroom C, 3rd floor (Hynes)
2:30 PM - **W5.1
Nanowire Tunnel FETs - From All-silicon towards Heterostructures.
Mikael Bjork 1 , Heinz Schmid 1 , Kirsten Moselund 1 , Cedric Bessire 1 , Michelle Natera-Comte 1 , Hesham Ghoneim 1 , Siegfried Karg 1 , Emanuel Lortscher 1 , Heike Riel 1
1 , IBM Research Zurich, Ruschlikon Switzerland
Show AbstractThe continued miniaturization of field effect transistors (FETs) that goes along with an increase of device density and speed leads to power dissipation on the chip level that will severely limit the overall performance. To reduce power dissipation of future FETs, new architectures and materials that support steep inverse sub-threshold slopes and thereby low voltage operation are intensively investigated. The most prominent candidate to compete with the conventional FET is the tunnel FET, while on the material side, Ge and IIIV’s are again considered. Promising, but also most challenging, is the implementation of both concepts in the form of a hetero-junction tunnel FET. We report results on all-silicon based tunnel FETs, which are fabricated using vapor-liquid-solid growth and in-situ doping to create p-i-n structured nanowires. These devices unambiguously show tunnel FET characteristics with inverse sub-threshold slopes of around 100mV/dec over several orders of magnitude in current and even sub-60mV/dec slopes for the lowest currents. Furthermore, we review the options to build tunnel FETs by taking benefit of band engineering and the incorporation of a hetero-junction. The high lattice mismatch of most material combinations, stringent requirement on sharp doping profiles, as well as process limitations exclude traditional fabrication approaches. We detail on our fabrication process and results towards a hetero-junction tunnel FET using MOCVD and selective area epitaxial growth with the focus on the InAs-Si material system.
3:00 PM - W5.2
Semiconductor Nanowires for Ballistic Electronics and Spin Qubits.
Yongjie Hu 1 , Charles Marcus 2 , Charles Lieber 1 3
1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Physics, Harvard University, Cambridge, Massachusetts, United States, 3 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractSemiconductor nanowires represent a uniquely powerful platform for exploring a diverse range of physical phenomena at the nanoscale due to the demonstrated capabilities of rational design and precise control of diameter, composition, morphology, electronic properties during growth. Here we report ballistic electronic devices, including high performance field-effect transistors and coherent quantum devices, based on the prototypical Ge/Si core/shell heterostructure model system. First, we show that 100% of the ballistic limit is reached for Ge/Si nanowire transistors operating in the low bias voltage regime, even at room temperature. By modulating the contact transparency and cooling down the device to low temperature, we have also obtained detailed information about electronic subbands and spin degeneracy, including the splitting of heavy and light holes due to the radial quantum confinement in this 1D system. In addition, the assembly and detailed characterization of top-gate defined double quantum dot and charge sensor devices for spin qubit and coherent spin manipulation will be described. Specifically, finite bias transport spectroscopy demonstrates Pauli spin blockade for spin-to-charge conversion. Moreover, high-frequency pulsed-gate technique has been implemented to manipulate charge dynamics and elucidate the spin qubit lifetime. Notably, a remarkably long spin relaxation time, ~ 600 µs, has been identified. These new results provide significant implications and substantial promise for fundamental studies and quantum information processing.
3:15 PM - W5.3
InP-GaAs Nanowire Tunnel Field-effect Transistor.
Bahram Ganjipour 1 , Jesper Wallentin 1 , Magnus Borgstrom 1 , Lars Samuelson 1 , Claes Thelander 1
1 Solid State Physics, Lund University, Lund Sweden
Show AbstractThe tunnel field-effect transistor (TFET) is promising for low-power switching in electronics as the inverse sub-threshold swing is not fundamentally limited to 60 mV/dec. We will present recent results on TFET properties of InP-GaAs axial heterostructure nanowires. The nanowires were grown using metal organic vapor phase epitaxy (MOVPE) with heterostructure InP(n-i)-GaAs(p+) segments, with a short (150 nm) InP intrinsic region. The doping was carried out in situ, where hydrogen sulphide (H2S) was used for n-doping, and diethylzinc (DEZn) for p-doping [1]. Three-terminal device structures were processed to such nanowires, using HfO2 as gate dielectric, and omega-shaped top-gates aligned to the intrinsic InP segment. The transistors showed a band-to-band tunneling current in reverse bias regime with an on/off current ratio of 10E7 for a gate voltage swing of 1 V, and where both the on-current and sub-threshold slope are improved compared to previously reported nanowire TFETs. A minimum inverse sub-threshold slope of around 50 mV/dec was observed close to pinch-off, averaged over a gate voltage range of 100 mV.[1]Jesper Wallentin, Johan M. Persson, Jakob B. Wagner, Lars Samuelson, Knut Deppert and Magnus T. Borgström Nano Lett., 2010, 10 (3), 974–979
3:30 PM - W5.4
Engineering Device Interfaces for High Performance PbSe Nanowire Field-effect Transistors.
David Kim 1 , Soong Oh 1 , Tarun Vemulkar 1 , Weon-kyu Koh 2 , Christopher Murray 1 2 , Cherie Kagan 1 2 3
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 Electrical Systems and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractSemiconductor nanowires have motivated widespread interest because of their unique size and shape dependent electronic properties. In particular, PbSe is a high mobility, infrared absorbing semiconductor with large electron, hole and exciton Bohr radii, allowing us to study the physics of charge transport in 1D quantum confined materials by incorporating them into field-effect transistors (FETs). However, these devices suffer from very high contact resistance at low voltages and the I-V curves appear sigmoidal[1]. Forming Ohmic contacts to nanoscale materials is a general problem observed in a wide-range of materials systems and is critical to their device performance, scalability, and application. In this study, we use wet-chemical methods to synthesize single-crystalline PbSe nanowires and electric field-directed assembly to align nanowire arrays to form the semiconducting channel of FETs. We report bottom gold contact PbSe FETs with low-resistance Ohmic contacts for both p and n-type behaviors after hydrazine treatment, which was observed by the linear behavior in the ID-VDS characteristics at low voltage. Similar to carbon nanotubes [2, 3], we believe that the metal –nanowire interface contacts can be modified through charge transfer doping to improve device performance. In order to isolate the role of these contacts, hydrazine was used to “chemically dope” the nanowire FET only at the metal/semiconductor contact by fabricating a blocking layer over the channel of the FET. Despite selectively doping only the contacts with hydrazine and leaving the bulk of the channel p-type, we were able to convert the device to n-type, which signifies the dominating role the contact has in determining the carrier type in nanostructured materials. [1] D. V. Talapin, C. T. Black, C. R. Kagan, E. V. Shevchenko, A. Afzali, C. B. Murray, “Alignment, Electronic Properties, Doping, and On-Chip Growth of Colloidal PbSe Nanowires,” J. Phys. Chem. C, 111, 13244 (2007). [2] J. Chen, C. Klinke, A. Afzali, and P. Avouris, “Self-aligned carbon nanotube transistors with charge transfer doping,” Applied Physics Letters, 86, 123108 (2005). [3] C. Klinke, J. Chen, A. Afzali, and P. Avouris, “Charge Transfer Induced Polarity Switching in Carbon Nanotube Transistors,” Nano Letters, 3, 555 (2005).
3:45 PM - W5.5
Integration of III-V NW-based Vertical FETs on Si and Device Concept for Tunnel FET using III-V/Si Heterojunctions.
Katsuhiro Tomioka 1 2 , Tomotaka Tanaka 1 , Junichi Motohisa 1 , Shinjiroh Hara 1 , Kenji Hiruma 1 , Takashi Fukui 1
1 GS Information Science and Technology, and Research Center for Integrated Quantum Electronics (RCIQE), Hokkaido University, Sapporo Japan, 2 PRESTO, JST, Kawaguchi Japan
Show AbstractRapid progress in epitaxial growth technique such as Vapor-Liquid-Solid and selective-area growth has enabled us to integrate III-V nanowires (NWs) directly on Si substrate. These III-V NWs on Si, are expected for electrical and photonic devices on Si platforms because they can achieved high performance devices such as high-electron mobility transistors and avalence photodiodes with a vertical nano-architectures. Recently, we have reported on the growth of InAs NWs on Si (111) substrates by taking special care in preparing arsenic-terminated surface on Si [1], which has additional advantage for integration vertical III-V devices on Si platforms, and demonstrated InAs NW-based vertical surrounding-gate FETs (VSGFETs) on Si substrate []. In this report, we demonstrate fabrication of III-V NW-based VSGFETs with doping and propose a unique concept for steep-slope switches using such III-V NW-based VSGFETs on Si and III-V/Si heterojunctions. At first, we grew InAs NWs on n-Si (111) by low-pressure horizontal MOVPE system. Growth conditions were as follows; partial pressure of TMIn, [TMIn] = 4.87 x 10-7 atm, partial pressure of AsH3, [AsH3] = 1.25 x 10^-4 atm, growth temperature = 560 deg.C, growth time = 10 min. Next, same growth was continued for 10 min with introducing SiH4 doping to make axial n-n+ junctions inside the InAs NW. After these growths, hafnium alminate (HfAlO) was deposited by atomic layer deposition for high-k gate dielectric, followed by the deposition of tangsten (W) by plasma sputtering for gate metal. To remove the gate metal and high-k dielectric resides on the top portion of NWs, protecting layer for etching of low-k insulator resin was formed firstly by spin-coating and then by reactive ion etching (RIE) to etch back to the desired thickness, followed by dry and wet etching of W and HfAlO. After that, separating layer between gate and drain top contact was formed by spin-coating of low-k resin and then it was etched back by RIE to expose top region of NWs. Finally, drain and source metal was evaporated on the top of NWs and backside of the substrate, respectively. Fabricated VSGFET contained 50 NWs parallel in the channel. We observed n-type FET behavior in I_D-V_DS and I_D-V_G characteristics. The performance is summraized as follows; subthreshold slope, S = 320 mV/decade, threshold voltage ~ 0 V, peak transconductance, Gm,max = 0.26 mS (@VG = 0.4 V), and on-off ratio, Ion / Ioff = 10^6 in average. There exists non-liner characteristics in ID-VDS at around VDS = 0 V. We presume this effect was caused by band discontinuity at Si / InAs NWs hetero junction, so the electron carriers feel potential barriers at the heterojunction. Although such potential barrier results in the high access resistance at the source side of the channel and suppress the drive current, but will be used for steep-slope behavior based on tunneling inside NW-based FETs. The detailed concept will be explained on the day.
4:30 PM - **W5.6
Nanowire Tunnel FETs - Device Structure, Transistor Dimension and Material Choice.
Joachim Knoch 1
1 , TU Dortmund University, Dortmund Germany
Show AbstractTunnel FETs have recently attracted a great deal of interest due to their potentialabilit to provide a switching behavior superior to conventional MOSFETs. To date, very few experimental realizations showing an inverse subthreshold slope steeper than 60mV/dec have been reported in literature. However, a steep slope is only observed in a narrow gate voltagerange and in addition, the on-state performance is significantly lower compared to conventionalMOSFETs. In the presentation, the performance of tunnel FETs is investigated and the impact of device structure and dimension as well as the impact of the transistor materials will be studied. For instance, using nanowires with thin diameter providing one-dimensional transport together with a wrap-gate device structure strongly improves the tunnel FET performance. In addition, the use ofIII-V type II heterostructures is a further performance booster. However, the use of III-Vsemiconductors with low density of states can be problematic if the device is not designed properly. In the presentation we will give design guidelines and performance predictions of nanowire tunnel FETs based on non-equilibrium Greens functions formalism simulations.
5:00 PM - W5.7
Nano-electro-mechanical Field Effect Transistor Using Suspended Nanowire Channel.
Ji Hun Kim 1 , Zack Chen 1 , Soonshin Kwon 2 , Jie Xiang 1 2
1 Electrical and Computer Engineering, Univ. California, San Diego, La Jolla, California, United States, 2 Materials Science and Engineering, Univ. California, San Diego, La Jolla, California, United States
Show AbstractWith the shrinkage of feature size in VLSI circuit using CMOS technology, the static power consumption became one of the key limiting factors[1]. One major reason of static power consumption is the off-state subthreshold leakage current of the transistor. However, due to the fundamental thermal dynamical limit (kBT/q), the steepest transition is limited to 60 mV/dec (subthreshold swing, SS) at 300K. One approach for obtaining SS < 60 mV/dec is the use of nano-electro-mechanical-system (NEMS] technology. Previous studies have shown suspended-gate MOSFET (SG-MOSFET) can be used as logical switch[2,3], or suspended single-wall-carbon-nanotube channel as MEMS switch[4]. Here we report demonstration of nanowire NEMFET device using suspended nanowire (NW) channel. Coupled 3D simulation of mechanical stress/strain and electrical transport show that the electrostatic pull-in of the NW towards the gate stack enables abrupt switching to the off-state and zero SS with 1E15 on-off ratio and near 1 V pull-in voltage (Vpi) due to the enhanced 3D capacitive coupling. The NW beams’ extremely high aspect ratio and small dimensions allow scaled operation at very-high-frequency (VHF) and even ultra-high-frequency (UHF). Single silicon NW-NEMFET devices were fabricated using e-beam lithography technique and the DC/AC characteristics of the steep subthreshold device will be discussed. Coupled with our previously demonstrated sub-ps intrinsic delay and near-ballistic transport in semiconductor nanowires, the NEMFET provides a route towards future high speed, low power nanoelectronics. [1] D. A. Antoniadis, et al., Res. & Dev. Vol.50, No.4/5 (2006) [2] N. Abele, et al., IEEE IEDM, p.479-489 (2005) [3] K. Akarvardar, et al., Trans. Elect. Dev. 55, 48-59, (2008) [4] M. W. Jang, et al, IEEE Transducers, p.912-915 (2009)
5:15 PM - W5.8
Label-free, Electrical Biomarker Detection Based on Nanowire Biosensors Utilizing Antibody Mimics as Capture Probes.
Hsiao-Kang Chang 1 , Fumiaki Ishikawa 1 , Chongwu Zhou 1
1 Electrical Engineering, University of Southern California, Los Angeles, California, United States
Show Abstract Nanowire transistor based biosensors have shown their tremendous potential as highly selective, ultra sensitive devices capable of detecting specific proteins and DNA sequences. These devices utilize a capture agent on the sensor surface to selectively bind the target biomolecules and those commonly used include antibodies, oligonucleotides, and small ligands (e.g. biotin). Antibody mimic proteins (AMPs) are polypeptides that bind to their target analytes with high affinity and specificity like conventional antibodies, but are much smaller in size (2~5 nm, less than 10 kDa). In this report, we describe the first application of AMP in the field of nanobiosensors. In2O3 nanowire based biosensors have been configured with an AMP (Fibronectin, Fn) to detect nucleocapsid (N) protein, a biomarker for severe acute respiratory syndrome (SARS). Using these nanosensors, N protein was selectively detected at subnanomolar concentration in the presence of 44 uM bovine serum albumin as a background. Compared to current immunological detection methods, our sensing signals can be obtained in a relatively short time without the aid of any signal amplifier such as fluorescence labeled reagents. Furthermore, the binding constant of the AMP to Fn was determined from the concentration dependence of the response of our biosensors. Other alternatives to antibody as capture probes are also explored.
5:30 PM - W5.9
Detection of Spin Polarized Carrier in Silicon Nanodevices with Single Crystal MnSi as Magnetic Contact.
Yung-Chen Lin 1 , Yu Chen 1 , Yu Huang 1
1 MSE, UCLA, Los Angeles, California, United States
Show AbstractWe report the formation of single crystal MnSi nanowires, MnSi/Si/MnSi nanowire heterostructures, to study the spin transport in silicon nanowire devices. Scanning electron microscopy (SEM) studies show that silicon nanowires can be converted into single crystal MnSi nanowires through controlled solid state reaction. High-resolution transmission electron microscope (HRTEM) studies show that MnSi/Si/MnSi heterostructures have clean, atomically sharp interfaces with an epitaxial relationship of Si[3-1-1]//MnSi[1-20] and Si(345)//MnSi(-2-14) . Electrical transporting studies show that the single crystal MnSi nanowire exhibits metallic behavior with magnetic transition temperature of 29.7 K (paramagnet to ferromagnet) and a negative magnetoresistance (MR) up to 1.8% at low temperature. Furthermore, using single crystal MnSi/p-Si/MnSi nanowire heterostructures, we have studied carrier tunneling via the Schottky barrier and spin polarized carrier transport in the silicon nanodevices.
W6: Poster Session I
Session Chairs
Akram Boukai
Oliver Hayden
Wednesday AM, December 01, 2010
Exhibition Hall D (Hynes)
9:00 PM - W6.1
High Performance Field–effect Transistors Based on ZnO Nanowires and Nanobelts From a Hydrothermal Method.
Rui Zhang 1 , Yue Fu 2 , Mond Guo 3 , Chongwu Zhou 2 , Mark Thompson 1
1 Chemistry, University of Southern California, Los Angeles, California, United States, 2 Department of Electrical Engineering, University of Southern California, Los Angeles, California, United States, 3 Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California, United States
Show AbstractWe have demonstrated that high performance field-effect transistors (FETs) can be fabricated with ZnO nanomaterials (nanowires and nanobelts) prepared by a hydrothermal method. By applying a post-synthesis annealing step to the as-grown ZnO products prior to device fabrication, the so-obtained FETs show significantly enhanced device performance. ~20% of the devices exhibit excellent n-type semiconductor characteristics, with on/off ratio of ~105, on-current higher than 10-7 A, and the field-effect mobility of ~ 30 cm2 V-1 s-1. These devices are comparable to the FETs based on one dimensional (1D) ZnO nanostructures synthesized by gas phase approaches. This work suggests that 1D metal oxide nanomaterials from hydrothermal methods hold great potential in fabricating high performance nanoscale electronics.
9:00 PM - W6.10
Structural and Optical Properties of Sonochemically Synthesized ZnO Nanorods and Its Application as a Novel Gas Sensor.
Syamanta Kumar Goswami 1 , Byungwoo Lee 1 , Eunsoon Oh 1
1 Department of Physics, Chungnam National University, Daejeon Korea (the Republic of)
Show AbstractZinc Oxide nanorods hold a promising key to the world of device applications such as gas sensor, solar cell, light-emitting diode, photodetector, and optical modulator waveguide due to its wide band gap of 3.37 eV and large exciton binding energy of 60 meV. We synthesized vertically aligned ZnO nanorods at ambient temperature by sonochemical method under various mole concentrations of the precursors; HMT and Zn(NO3)2. These nanorods were characterized by x-ray diffraction, scanning electron microscopy, and photoluminescence. When the mole concentration of HMT alone was made decreased, the growth rate of ZnO nanorods became higher accompanied with vertically well-aligned formation of nanorods and vice versa. This is due to the large shielding effect of OH− ions at the interface of the (0001) face in case of high HMT concentrations; as HMT is the primary source of OH− ions. In the low temperature PL spectra, two distinct PL peaks are typically observed in the uv range, one arising from the donor-bound exciton and the other from the surface related defects. In our case, the deep level emission peak was found to be blue-shifted with increasing temperature, on the contrary to the red-shift of the near-bandedge emission peak associated with the bandgap shrinkage. The blue-shift is ascribed to the competition between the two transitions such as conduction band to VO, and Zni levels to VO levels respectively. By increasing the Zn(NO3)2 concentration, the average diameter of the nanorods was found to be increased and only the donor-bound exciton PL peak was observed in the uv range and the surface-related PL peak was diminished. For electrical measurements, ZnO nanorods were grown on pre-patterned alumina substrates and we measured the change in resistivity in presence of NO and H2 gas. The sensitivity for the sample prepared using Zn(NO3)2 as a precursor was found to be better as compared to the sensitivity of the sample prepared using ZnCl4 as a precursor.
9:00 PM - W6.11
Gallium Oxide Nanowires Arrays with Field Emission Properties.
Inaki Lopez 1 , Pedro Hidalgo 1 , Emilio Nogales 1 , Bianchi Mendez 1 , Javier Piqueras 1
1 Fisica de Materiales, Universidad Complutense de Madrid, Madrid Spain
Show AbstractMetal oxide nanowires are versatile materials due to their diverse properties and functionalities with applications in multidisciplinary fields. One of the potential applications of these nanowires is to use them as field emitters since their particular geometry contributes to a low threshold field for electron emission. Among others, zinc oxide or indium oxide nanowires have been demonstrated good field emission properties. Gallium oxide is a wide band gap semiconductor with good optical and electrical properties. Recently, the field emission properties of several arrangements involving gallium oxide nanowires, such as cactus-like nanostructures or aligned nanowires grown on brass wires have been reported.In this work, we have explored on the field emission properties of gallium oxide nanowires in different arrangements: single nanowires and arrays of aligned nanowires emerging from the lateral surfaces of micro plates. We have also studied the influence of the doping on the field emission properties of gallium oxide nanowires. The study of the structure, luminescence and field emission properties of all samples has been investigated by means of a tuned scanning electron microscope. A special setup for field emission measurements has been designed in our laboratory and incorporated into the SEM chamber. Our system enables to monitor the anode-cathode distance through the specimen holder remote control. Field emission current density versus an applied field curves were recorded for different nanostructures and different vacuum gaps. The results show that Sn doped Ga2O3 nanowires arrays exhibit the best performances.
9:00 PM - W6.12
Non-volatile Resistive Switching Effect in Limited Nanospace of a Single NiO Heterostructured Nanowire.
Keisuke Oka 1 , Takeshi Yanagida 1 2 , Kazuki Nagashima 1 , Jin-soo Kim 3 , Bae-Ho Park 3 , Tomoji Kawai 1 3
1 The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, Japan, 2 PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama, Japan, 3 WCU, Konkuk University, Seoul Korea (the Republic of)
Show AbstractThe mechanisms of non-volatile resistive switching (RS) phenomena have been intensively discussed in last decade. One of the difficulties to analyze these phenomena in detail is due to the occurrence of nanoscale RS events within bulk oxide materials. Individual core-shell type heterostructured nanowire offers extremely limited nano-space structures to investigate such nanoscale RS events. Here we demonstrate the presence of RS within NiO heterostructured nanowires and analyze the local RS events. MgO nanowires as a template were fabricated by pulsed laser deposition with metal catalysts (Au) via VLS mechanism on MgO(100) substrate, and NiO shell-layer for RS was deposited in-situ. A single nanowire was bridged between nano-electrodes using EB lithography, allowing the transport properties of a single nanowire. We found the presence of non-volatile RS effects within a single NiO heterostructured nanowire with the bipolar switching behavior. In addition, we have evaluated the atmosphere dependence on the RS by modulating the ambient gas atmosphere from oxidation to reduction. The reaction of gas atmosphere was the trend of p-type carrier on NiO at resistive switching. Thus above results as to RS events on single oxide nanowire highlight the importance of nanoscale events on the RS characteristics.
9:00 PM - W6.13
Direct Correlation of Structural and Optical Properties of ZnO Nanowires by Cathodoluminescence in the Scanning Transmission Electron Microscope.
Megan Brewster 1 , Sung Keun Lim 1 , Xiang Zhou 1 , Silvija Gradecak 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractZnO is a semiconductor with a wide direct-band gap (3.37 eV at 300 K) and a large exciton binding energy (60 meV) with promise for many optoelectronic devices, including single-nanowire light emitting diodes and lasers with UV light emission at room temperature. ZnO nanostructures have complex UV and visible light emission whose intensities and frequencies depend on surface-to-volume ratios and defect concentrations, among other factors. Controlled synthesis is essential for the development of ZnO nanostructures with predefined functionality; yet, the relationship between structural and optical properties in this system is not fully understood. Here, we study the correlation between structural and optical properties of individual ZnO nanowires by cathodoluminescence (CL) coupled with scanning transmission electron microscopy (STEM).ZnO nanowires were grown on Au-coated a-sapphire substrates in a horizontal tube furnace from graphite and ZnO powder sources and O2 and Ar gases at a variety of O2 partial pressures and system pressures. SEM, TEM, and x-ray diffraction (XRD) investigations reveal highly uniform, single crystalline wurtzite structures grown along the c-axis that are free of one- and two-dimensional dislocations. Room temperature photoluminescence measurements show strong UV emission, with no visible or near-IR emissions. Under standard growth conditions, panchromatic CL emission from individual ZnO nanowires is homogenous along the nanowire length. UV emission was observed from 100 K to 300 K, and the emission was resolved into three separate peaks identified as the bound exciton (3.342 eV), free exciton (3.307 eV), and LO phonon replica (3.229 eV) from their locations and temperature dependencies. The presence of free-exciton emission at 100 K, as well as the resolution of these two exciton peaks, indicates high crystalline quality. Spatial variations in CL emission due to the different oxygen partial pressures and system pressures were also studied to understand the impact of growth environment on optical properties. This work furthers the correlation between structural and optical properties of individual ZnO nanowires for the development and realization of nanowire optoelectronic devices.
9:00 PM - W6.14
ZnO Nanoplatelets Grown under Epitaxial Stress.
Jung-Il Hong 1 , Ee le Shim 1 , Yanling Chang 1 , Ji Il Choi 1 , Seung Soon Jang 1 , Zhong Lin Wang 1 , Robert Snyder 1
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractGrowth of various shapes of materials in nanometer scales has been demonstrated over the last two decades using various synthesis techniques. In contrast to the phenomenal development of graphene in recent years from carbon nanotubes, the majority of the efforts in the inorganic nanostructure research is still toward nanowires. While nanowire research is still very active, two dimensional platelet structures with nanometer scale thicknesses are just beginning to be considered with inorganic materials. Furthermore, thin platelets are theoretically predicted to show new physical properties that are not expected from bulk scale structures. Here, we present the controlled synthesis of free standing 2-dimensional platelet structures of ZnO with thickness in nanometer scale and the lateral size in micrometer scale, achieving the highest width/thickness ratio to our knowledge.The synthesis technique developed for the growth of ZnO nanowires can be employed for the synthesis of ZnO platelets, but the growth here is on GaN, AlN, or BN substrates, which have the wurtzite structure, the same as ZnO, but with different lattice parameters. As ZnO grows on these substrates, epitaxial stress is applied. The interfacial strain seems to suppress the growth of ZnO along the <001> direction, but promote the growth in the directions perpendicular to <001>, resulting in the formation of very thin platelets. In comparison, the growth of ZnO nanowires along the c-axis when they grow without applied stress on the surface ZnO particles was also confirmed. This is consistent with already well-known results. Detailed structures were studied by electron microscopy and x-ray diffraction, and the current efforts on the integration of these nanostructures into various devices will be discussed.
9:00 PM - W6.15
CO Assisted ZnO Nanowire Growth from Nanofilm Along Non-polar Direction.
Satyesh Yadav 1 , Paresh Shimpi 1 , Puxian Gao 1 , Ramamurthy Ramprasad 1
1 CMBE, University of Connecticut, Storrs, Connecticut, United States
Show AbstractNanostructure growth control has been an important topic in the past few decades for several reasons. Among nanostructures, ZnO nanowires have attracted special attention because of a wide range of possible applications in optoelectronic and sensor devices. Various methods have been developed to grow ZnO nanowires but reproducibility is a major issue, mainly because of a lack of fundamental understanding of the mechanism of growth. In this work we present a new C or CO assisted growth method of ZnO nanowires, with the nanowire axis being along a counter-intuitive and unexpected direction, namely, the non-polar [10-10] direction. Using first principles thermodynamics, this unusual growth direction is explained in terms of changes in the ordering of surface energies in the presence of CO. ZnO nanowires were grown by heating a ZnO nanofilm to 1200 oC, in the presence of graphite as a source and Argon as a carrier gas in a tube furnace. The nanowires grow along the [10-10] direction with the enclosing sidewall surfaces being {11-20} and {0001}. The presence of CO in the tube furnace gives an indication of its role in the nanowire growth. The surface energy of enclosing surfaces {11-20} and {0001} and growth direction surfaces {10-10} of the nanowires (both in the presence and absence of adsorbed CO) is calculated using density functional theory. Vibrational entropy contribution of adsorbed CO was considered within harmonic oscillator approximation. We find that the order of the stability of clean surfaces is: {10-10} (most stable), {11-20} and {0001} (least stable). However, upon CO adsorption, the {11-20} and {0001} surfaces become more stable compared to {10-10}. This gives a strong thermodynamic driving force for the growth of nanowires in [10-10] direction. This synergistic experimental-computational work has helped clarify the possible role played by CO in altering conventional growth tendencies in ZnO (and perhaps other wurtzitic systems).
9:00 PM - W6.17
Size-dependent Recombination Dynamics in ZnO Nanowires.
Frank Guell 1 , Juan Sebastian Reparaz 2 , Markus R. Wagner 2 , Axel Hoffmann 2 , Albert Cornet 1 , Joan Ramon Morante 1 3
1 Electrònica, Universitat de Barcelona, Barcelona, Catalunya, Spain, 2 Institut für Festkörperphysik, Universität Berlin, Berlin Germany, 3 , Institut de Recerca en Energia de Catalunya, Barcelona, Catalunya, Spain
Show AbstractOne-dimensional nanostructures such as nanowires (NWs) have attracted much attention due to their unique properties. The reduction in size leads to novel electrical, mechanical, chemical, and optical properties. NWs are also expected to be important functional units for optoelectronic nanoscale applications, when being integrated in nanodevices. Zinc oxide (ZnO) is a semiconductor material of great importance for optoelectronics due to its wide direct band-gap of 3.37 eV and its extremely large exciton binding energy of 60 meV. Therefore, ZnO NWs have great potential for an advantageous use in devices. In this respect, it is crucial to have detailed information about the recombination dynamics of the ZnO NWs and their dependence on wire dimensions. In this work, the influence of finite-size on the recombination dynamics of the neutral donor-bound exciton around 3.365 eV has been investigated for single-crystal ZnO NWs with different diameters grown on SiO2/Si substrates by the vapor transport method using Au as catalyst. We demonstrate that the lifetime of this excitonic transition decreases with increasing the surface-to-volume ratio due to a surface induced recombination process. Furthermore, we have observed two broad transitions around 3.341 (S1) and 3.314 eV (S2) whose intensity increases for the smaller NWs diameters. In order to study their origin we have investigated the temperature dependence of their photoluminescence intensity as well as their thermal activation energy. Comparing their intensities and recombination times to those of the main excitonic recombination around 3.365 eV we conclude that S1 and S2 might originate from surface states. These results are of great interest for a precise design of ZnO-based nanostructures, since they represent a step toward a deep understanding of its size-dependent recombination dynamics.
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Vertically Grown ZnO Nanostructures on Graphene.
Yong-Jin Kim 1 2 , Jae-Hyun Lee 1 , Gyu-Chul Yi 1
1 Physics and Astronomy, Seoul National University, Seoul Korea (the Republic of), 2 Materials Science and Engineering, POSTECH, Pohang, Gyeongbuk, Korea (the Republic of)
Show AbstractGraphene has a great potential for novel electronic devices based on “quasirelativistic” transport behavior due to its remarkable electrical and thermal properties. Meanwhile, semiconductor nanostructures grown on graphene layers are good candidates for device applications such as field emission emitters and light emitting diodes. Moreover, semiconductor nanostructures that have an extremely large surface-to-volume ratio and high porosity can also be exploited in novel electrical devices, including sensitive biological and chemical sensors and efficient energy conversion and storage devices. Accordingly, the growth of semiconductor nanostructures on graphene layers offers new functionality to graphene-based nanoelectronics. We report the vertical growth of ZnO nanostructures on graphene layers using catalyst-free metal-organic vapor-phase epitaxy.[1] Further, interesting growth behavior leading to the formation of aligned ZnO nanoneedles in a row and vertically aligned nanowalls was also observed and explained in terms of enhanced nucleation on graphene step edges and kinks. These direct band gap semiconductor nanostructures grown on graphene layers provides opportunity to exploit both characteristics of direct band gap semiconductor and graphene simultaneously.Reference[1] Y.-J. Kim, J.-H. Lee, G.-C. Yi, Appl. Phys. Lett. 2009, 95, 213101.
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Shape Controllability and Optical Properties of ZnO and CdO Nanostructures Grown by Atmospheric-pressure CVD Methods.
Tomoaki Terasako 1 , Tetsuro Fujiwara 1 , Yoshinori Ogura 2 , Toshihiro Takahashi 2 , Yukari Nakata 3 , Masakazu Yagi 3 , Sho Shirakata 1
1 , Graduate School of Science and Engineering, Ehime University, Matsuyama Japan, 2 , Faculty of Engineering, Ehime Univrsity, Matsuyama Japan, 3 , Kagawa National College of Technology, Mitoyo Japan
Show Abstract Nanostructures of ZnO and CdO have attracted much attention because of their potential for the future optoelectronics. The CVD methods utilizing vapor-liquid-solid (VLS) mechanism are suitable for positioning and size control of nanostructures. In this paper, growth mechanisms of ZnO and CdO nanostructures prepared by the atmospheric-pressure CVD method using Zn, Cd and H2O as source materials, and structural and optical properties of these nanostructures will be discussed in terms of substrate material, substrate temperature (TS), growth time and source supply condition. Under the simultaneous source supply (SSS) of metal powder (Zn or Cd) and H2O, various shapes of ZnO and CdO nanostructures, such as nanorods (NRs), nanobelts, nanowalls, nanotrees (NTs) and 3D networks of NTs, were successfully grown on the a- and c-plane sapphire substrates coated with Au nanocolloidal solution. For both the ZnO and CdO NRs, the tapering parameter σ proposed by Wang et al.[1] increased exponentially with decreasing 1000/TS. This tapering behavior is caused by the competition between the axial growth through the VLS mechanism and the radial growth due to the film growth on the NR’s side walls (vapor-solid (VS) mechanism) [2]. Photoacoustic measurements for the CdO NRs revealed that the increase in TS contributes to the increase in the absorption related to the structural defects.From a different angle, the catalyst particles can be regarded as the reservoirs of Zn atoms. In other words, even if the Zn supply is stopped, only the Zn atoms reserved in the catalyst particles are provided to the growth front for a while. Therefore, we have examined the possibility of selective VLS growth utilizing this reservoir effect under the alternate source supply (ASS) conditions. The TEM and SAED results revealed the successful VLS growth of single crystalline ZnO NR by the ASS technique. Moreover, the ASS-ZnO NRs exhibited the decrease in tapering parameter σ with decreasing 1000/TS. This tendency is fully opposite to that for the ZnO NRs grown under the SSS condition, suggesting that the ASS technique is effective in suppressing the radial growth. However, PL spectra of the ASS-ZnO NRs were dominated by the green band emission, indicating that these NRs were in the oxygen deficient condition. It is hard to explain the appearance of Y- and T-shaped CdO NTs without considering the splitting and migration of catalyst particles during the growth process. The TEM and SAED results indicate that both the trunk and branch of the T-shaped NT are single crystalline. Therefore, we believe that not only the competition between the VLS and VS mechanisms, but also the splitting and migration of catalytic particles is an important factor for shape control of nanostructures. This work is supported by Grants-in-Aid for Scientific Research C (No. 20510107).[1] Y. Wang et al.: Nat. Nanotechnol. 1 (2006) 186.[2] J. Johansson: Nat. Nanotechnol. 2 (2007) 534.
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Nanopipes in Thermally Grown Indium Oxide Nanowires.
David Maestre 1 , D. Haeussler 2 , Ana Cremades 1 , Javier Piqueras 1 , W. Jaeger 2
1 Materials Physics, Universidad Complutense de Madrid, Madrid Spain, 2 Institute of Materials Science, Christian-Albrechts-Universität zu Kiel, Kiel Germany
Show AbstractUndoped and Sn doped indium oxide nanowires and nanorods have been grown by a catalyst free evaporation-deposition method (1) with InN powders or a mixture of InN and SnO2 powders as precursors. The nanostructures have been characterized by XRD, energy dispersive spectroscopy and scanning and transmission electron microscopy techniques. TEM diffraction contrast of a high number of nanowires reveals the presence of nanopipes in the core, with diameter of about 18 nm. The nanopipe diameters are constant along the [100] growth axis and appear to be independent of the nanowire thickness which ranges between 150 and 600 nm. The presence of Sn as dopant generates longer and thinner nanowires, where small dislocation loops and nanoprecipitates associated with Sn, as confirmed by HAADF-STEM and EDS, are observed in the core region. The observation of nanopipes, almost always centered in the nanowire, suggests that the growth is connected with a screw dislocation-driven mechanism where the nanopipe represents an empty dislocation core. The presence of a screw dislocation in the axis of nanorods of other materials has been recently confirmed by TEM (2) (3). Extended thermal treatments lead to nanorods with more complex morphologies, roughened interfaces and voids, instead of well formed nanopipes, as observed by bright-field TEM imaging.1. D. A. Magdas, A. Cremades and J. Piqueras, Appl. Phys. Lett. 88, 113107 (2006)2. M. J. Bierman, Y. K. A. Lau, A. V. Kvit, A. L. Schmitt and S. Jin, Science 320, 1060 (2008)3. B. W. Jacobs, M. A. Crimp, K. McElroy and V. M. Ayres, Nano Letters 8, 4353 (2008)
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Wet Chemistry Based Copper Oxide and Zinc Oxide Nanowire Photovoltaic Cells.
Samuel MacNaughton 1 , Dante DeMeo 1 , Sameer Sonkusale 1 , Thomas Vandervelde 1
1 Electrical Engineering, Tufts University, Medford, Massachusetts, United States
Show AbstractWe propose a solar cell design that is cost-effective both in production and materials. Here, the junction is made of copper (I) oxide and zinc oxide, which are oxides of earth-abundant metals. Furthermore, we utilize a wet chemistry fabrication process, making the production of such cells inexpensive and easily scalable. The process involves growing copper nanowires, oxidizing, plating zinc oxide, and depositing a top contact. This results in coaxial core-shell nanowires. The nanowire geometry increases the conversion efficiency of the cell due to its higher effective absorption of photons and a possible decrease in hot carrier thermalisation. The electrochemically-grown transition metal oxides and junctions were characterized by photoluminescence and capacitance-voltage measurements. Current-voltage measurements under various levels of illumination demonstrate the photovoltaic function and efficiency of the nanowire cell. The ultimate figure of merit for solar cells is cost per watt: this decides whether a particular solar energy technology will become commercially viable. Use of almost exclusive wet chemistry methods and abundant materials has the potential for incredibly cheap and massively-scalable photovoltaics.
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Sensitization of Hydrothermally Grown Single Crystalline TiO2 Nanowire Array with CdSeS Nanocrystals for Photovoltaic Application.
Akshay Kumar 1 , Anuj Madaria 1 , Chongwu Zhou 1
1 Mork Family Department of Chemical and Mat. Sc., University of Southern California, Los Angeles , California, United States
Show AbstractAn oriented array of electron transporting nanowires, grown directly on a transparent conductor constitutes an optimal architecture for efficient photovoltaic applications. In addition, semiconductor nanocrystals quantum dots (QD) can work as efficient light absorbers because of their tunable optical properties. Here, we use an oriented array of TiO2 nanowire grown directly on transparent conductive electrode and subsequently sensitized with colloidally grown CdSeS nanocrystal quantum dots, using an efficient bi-linker assisted methodology, to demonstrate photovoltaic cells. Upon excitation with light, exciton dissociation takes place at the nanowire-nanocrystal interface, after which, electron is transported to the fluorine-doped tin oxide (FTO) electrode via single-crystalline TiO2 nanowire channels. We demonstrate that an ex-situ ligand exchange of QDs followed by sensitization on oxygen-plasma treated TiO2 nanowires results in enhanced loading of QDs, as compared to the in-situ ligand exchange approach. An array of 1 µm long TiO2 nanowire sensitized with CdSeS nanocrystals exhibits photovoltaic effects with short-circuit current of 2-3 mA/cm2, open circuit voltage of 0.6-0.7 V and a fill factor of 40-60 %.
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TiO2 Naotube Patterning by Combining ZnO Nanorod Template and Nanoimprint Lithography.
Mi-Hee Jung 1 , Man Gu Kang* 1
1 Thin Film Solar Cell Technology Research Team, Advanced Solar Technology Research Department,Convergence Components & Materials Research Laboratory, , Electronics and Telecommunications Research Institute (ETRI), Daejeon Korea (the Republic of)
Show AbstractIn the present study, we introduce a new approach to TiO2 nanotube patterning by combining Nanoimprint lithography and ZnO nanorod template. The polymer mask was made by NIL and was then subjected to a brief oxygen plasma step to remove the intermediate layer between the bumps and expose the silicon oxide surface. The silicon oxide surface was subsequently exposed to octadecyltrichlorosilane (OTS) vapors, followed by lift-off of the polymer mask and spin-coating of the ZnO seed layer. The seed layer coated on the surface was annealed at 300-500 oC for 30 min to make the crystalline ZnO nanoods by a decomposition of zinc acetate dehydrate and simultaneously to remove the OTS SAM. The removed OTS SAM surface was then treated by the oxygen plasma to inhibit ZnO growth; meanwhile, ZnO growth on the seed layer was accelerated. The surface coated with the seed layer was suspended with the wafer face-down in a mixed aqueous solution of zinc hydrate and hexamethylenetetramine to grow the ZnO nanorods by the hydrothermal method. The ZnO nanorods were grown only on the seed layer, providing selective patterning on the silicon surface. This ZnO patterned susbtrate was immersed in queous solution containing (NH4)2TiF6 and H3BO3. The (NH4)2TiF6 hydrolyzed to TiO2 on the surface of ZnO nanorods while ZnO dissolved simultaneously in the solution with acids produced by (NH4)2TiF6 hydrolysis. Therefore, TiO2 nanotube array was successfully formed on the silicon surface.
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Double Schottky Barrier UV Photodetectors Formed by ZnO Bascule Nanobridges.
Yanbo Li 1 , Alexander Paulsen 1 , Ichiro Yamada 1 , Jean-Jacques Delaunay 1
1 Engineering Synthesis, The University of Tokyo, Tokyo Japan
Show AbstractZnO nanowires have been widely studied for their possible application in UV photodetection. Photoconductive type and Schottky type photodetectors have been demonstrated by using ZnO nanowires as the sensing elements. However, the photoconductive detectors show a slow time response due to the surface depletion effects of ZnO nanowires which prolong the carrier lifetime. The Schottky type photodetectors exhibit much faster time response by employing high Schottky barriers in the device. Unfortunately, the photocurrent is very noisy due to the large fluctuation of the Schottky barrier height under illumination. In this report, we demonstrate a new type of nanowire photodetector by utilizing the surface depletion effects of ZnO nanowires. A bascule nanobridge structure consisted of two cross-bridged ZnO nanowires is self-assemble in a CVD process. A double Schottky barrier is created at the nanowire junction due to the surface depletion. The height of the double Schottky barrier is high in dark and low under UV. Therefore, the bascule nanobridge can be switched on and off using UV, which makes the structure an excellent UV photodetector. The fabricated device shows a high photocurrent to dark current ratio (> 4 orders of magnitude), a stable photocurrent, an extremely fast photocurrent decay time (<< 20 ms), and a clear cut-off wavelength at ~380 nm. The results are proved to be attributed to the tunable double Schottky barrier in the device. Further, the detailed band gap structure of the bascule nanobridge is studied, giving the zero-bias height of the double Schottky barrier (~ 0.77 eV).
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Morphological Evolution of Large-scale Vertical Aligned ZnO Nanowires and Their Light-extraction Photoluminescence Properties.
Miao Zhong 1 , Yanbo Li 1 , Takero Tokizono 1 , Ichiro Yamada 1 , Jean-Jacques Delaunay 1
1 Mechanical Department, The University of Tokyo, Tokyo Japan
Show AbstractTailoring the crystallographic orientation of semiconductor nanowires (NWs) is one of the key points for the growth control and for the performances of later applications. In this paper, large-scale vertical aligned ZnO NWs, with diameter around 50 nm and length around 500 nm, were dispersedly synthesized on A-plane sapphire by a chemical vapor deposition method. In addition to vertical aligned NWs, ZnO nanowall-like thin film was firstly formed on sapphire surface to support the subsequent growth of ZnO NWs. The XRD pattern of the product showed a strong intensity of (0002) peak and a weak intensity (0004) peak of ZnO, which suggested ZnO NWs were well oriented along (0001) direction, due to the small mismatch of lattice constant between ZnO a-axis and sapphire c-axis for epitaxial growth. The photoluminescence (PL) spectrum of as-prepared vertical aligned ZnO NWs was studied. A strong green emission peak centered at 520 nm and a weak near-band-edge emission peak centered at 380 nm were observed. After a hydrogen plasma treatment to the as-prepared sample, a significant increase of intensity of near-band edge emission and a strong decrease of green emission were found in PL spectrum, which indicated a combined effort of surface defects passivation of ZnO NWs by hydrogen atoms and emitting light extraction by the structure of vertical arrayed ZnO NWs. Such PL property also exhibited long term stability in ambient conditions. We expect the greatly enhanced efficiency of UV emission can be used to improve the performance of UV light emitting devices such as ZnO NW light-emitting diodes and nanolasers. Moreover, the morphological evolution of ZnO NWs was further investigated by altering the growth condition. Structure of hexagonal ZnO nano-plates was fabricated by under low oxygen pressure condition. With the enlargement of oxygen pressure gradually, separated ZnO nanodots on nanowalls, vertically aligned ZnO NWs on nanowalls, randomly oriented ultralong ZnO NWs (~80 nm in diameter and ~50 μm in length), were prepared individually. These results suggested that oxygen pressure and persistent time for chemical reaction is crucial for the control of morphology of ZnO NWs.
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Controlled Synthesis of ZnO Nanotubes and Nanotube-nanorod Hybrid Hexagonal Networks by Employing Hexagonal Polystyrene Colloidal Monolayers.
Dong Hyun Lee 1 , Yong Bum Pyun 1 , Jaeseok Yi 1 , Won woo Lee 1 , Kwang Soo Son 1 , Won Il Park 1
1 Material Science and Engineering, Hanyang Univ., Seoul Korea (the Republic of)
Show AbstractBottom-up synthesis of one-dimensional (1D) nanostructures with controlled structure and morphology represents a significant advance towards the understanding of the fundamental physics and chemistry of nanoscale materials as well as practical applications. In particular, tunning of their structures and morphologoes of hollow nanocrystals plays an important role to determine their properties and potential applications. Here we demonstrate a new synthetic approach for the fabrication of ZnO nanotubes and nanotube-nanorod hybrid structures via wet hydrothermal process with the use of polystyrene (PS) colloids. Contray to solid ZnO nanorods grown in the absence of PS colloids, ZnO tubular structures were formed on the substrates covered with PS colloids. In addition, ZnO nanorods enclosing the PS colloids are much longer than ZnO nanorods formed on the bare substrates. This unique growth mechanism was further exploited to modulate ZnO nanostructures from nanorods to either nanotube-nanorod hybrid hexagonal networks (NT-NR hNWs) or complete tubular forms by introducing a hexagonally closepacked PS colloidal monolayer. The synthetic approach that takes advantage of colloidal monolayers may readily be expanded to rational synthesis of tubular structures.
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Selective Growth of ZnO Nanorods on Graphene.
Won Mook Choi 1 , Jae-Young Choi 1
1 , Samsung Advanced Institute of Technology, Youngin Korea (the Republic of)
Show AbstractWe demonstrated one-dimensional zinc oxide (ZnO) nanorod growth on graphene layers using a low temperature solution process. High density of ZnO nanorods were grown over a large area, and most ZnO nanorods were vertically formed on graphene with having the good crystalline match. Further, we realized the selective growth of ZnO nanorods on graphene by applying the simple mechanical treatment, since ZnO nanorods formed on graphene are mechanically stable in atomic level. These results are confirmed with the first principle calculation study and show that the ZnO-graphene bindings have the low destabilized energy. The proposed method here for the nanostructure of ZnO-graphene would be promising for future flexible electronics and optical devices.
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Synthesis of ZnO-CdSe Core-shell Nanostructures and Their Photocatalytic Performances.
Young-Jin Choi 1 , Kyung-Soo Park 1 , Pirl-Joong Joo 1 , Jae-Gwan Park 1
1 , Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractHeterostructured ZnO-CdSe core-shell nanostructures were synthesized by a novel two-step process. The ZnO nanowires were grown by a carbothermal reduction process and CdSe shell layers were deposited on the ZnO nanowires using a pulsed laser deposition process. The morphological characteristics and structures of ZnO-CdSe nanostructures were examined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) analysis, high-resolution transmission electron microscopy (HR-TEM) associated with energy dispersive X-ray (EDX) spectroscopy and UV–vis spectra. The photocatalytic studies suggested that the ZnO-CdSe core-shell nanostructures showed enhanced photocatalytic efficiency of photodegradation of Methylene blue (MB) compared with the bare ZnO nanowires under the air mass (AM) 1.5 condition (100 mA/cm2) because of the improvement of the separation of photogenerated electrons and holes, as well as the change of amount of surface hydroxyl groups of the catalyst.
9:00 PM - W6.28
Improved Synthesis of α-Fe2O3 Nanowires and Investigation of Their Physical Properties.
Mark Lukowski 1 , Song Jin 1
1 Chemistry, University of Wisconsin - Madison, Madison, Wisconsin, United States
Show AbstractWe report an improved thermal oxidation method for producing α-Fe2O3 (hematite) nanowires grown directly on iron foils and their physical properties towards the goals of solar energy applications. α-Fe2O3 is an intrinsic n-type semiconductor with a 2.1 eV bandgap. Due to its stability, abundance, low cost, and negligible toxicity, hematite is an interesting alternative material for solar energy applications in photoelectrochemical cells. While bulk hematite suffers from poor materials properties such as low carrier concentrations, low mobility, and short carrier diffusion lengths, the one-dimensional nanowire morphology is expected to help to mitigate these problems. A systematic study of reaction parameters and their effect on resultant product morphology are to be presented. These nanostructures are comprehensively characterized using scanning electron microscopy and (scanning) transmission electron microscopy, while their phase and composition are determined using electron diffraction, x-ray diffraction, and energy dispersive spectroscopy. New insights into the relatively complicated growth mechanism of the hematite nanowires will also be discussed. Post- and co-growth treatments intended to dope the hematite nanowires are investigated and the electrical properties of these nanowires are thoroughly evaluated. The nanowires are also preliminarily characterized photoelectrochemically to explore their viability for use in solar energy conversion.
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Detection of Neurotransmitters with Titanium-oxide Modified Silicon Nanowire Sensors.
Steffen Strehle 1 , Charles Lieber 1
1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States
Show AbstractCatecholamines, such as dopamine and epinephrine, represent an important class of neurotransmitters. Imbalances in their concentrations can lead to neurological disorders like the Parkinson disorder [1], and are also linked to cardiovascular disease. Controlled therapeutic delivery of catecholamines as well as fundamental studies designed to understand neuronal response and signaling require rapid and accurate determination of neurotransmitter concentrations with high sensitivity. Label-free real-time measurement of catecholamines can be achieved electrochemically, yet limited in terms of bio-compatibility and selective detection limit. Nanowire-based electronic sensors have the potential to overcome these limitations, and thus could advance both fundamental studies and therapeutics related to these neurotransmitters. For example, semiconducting silicon nanowires operating as field effect transistors (FET) were successfully applied as electrical transducers for the label-free detection of disease marker proteins, nucleic acids and small molecules with high-sensitivity and selectivity when their surfaces are modified to present receptors for biomolecules of interest [2]. In our investigations, the ability to detect epinephrine and dopamine selectively versus salicylic and ascorbic acid of silicon nanowires with titanium-oxide modified surface is evaluated and compared to pure silicon nanowires. Silicon nanowires were grown by the gold catalyzed vapor-liquid-solid method with 30 nm in diameter. Titanium-oxide was coated subsequently by atomic layer deposition utilizing a Titanium(IV)-isopropoxide/water process. Titanium-oxide is bio-compatible and changes its electrical charge in reaction with catecholamines. Our approach is to sense the charge by using it as gate voltage of a nanowire FET. Stability against changes of the pH value or other potential determining ions in the measuring solution as well as selectivity for dopamine and epinephrine is supported by a phosphate modification of the titanium-oxide surface. References[1] R. Laverty, Drugs, 16 (1978) 418[2] F. Patolsky, et al., MRS Bulletin 32 (2007) 142AcknowledgmentThe support by the German Research Foundation (DFG) and the Lieber group is acknowledged gratefully.
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High Density Arrays of Nanowires Prepared by Lithographically Patterned Nanowire Electrodeposition.
Justin Hujdic 1 , Somnath Ghosh 1 , Alan Sargisian 1 , Erik Menke 1
1 , UC Merced, Merced, California, United States
Show AbstractThe diffraction limit of light has left modern lithography with obvious limitations. The pitch, or distance between materials, cannot be resolved past this diffraction limit.The need for a method that can create a pitch that beats the diffraction limit ranges from sensors to solar cells, and plasmonic lenses to memory devices. Electron beamlithography, nanoimprint lithography, superlattice nanowire transfer patterning, and a number of other methods can produce arrays with a pitch less than 100 nm. However, most of these methods are time intensive and expensive. I propose a modification to the conventional LPNE method that allows for the creation of a high density array of material. The conventional LPNE method is a multistep nanowire deposition process, developed by the Penner group that uses photolithography to create a template for nanowire deposition, and this method has been used to prepare gold, platinum, palladium, bismuth, lead telluride, and lead selenide nanowires. Briefly, the LPNE method, consists of seven-steps: 1) Evaporate nickel onto a piece of float glass; 2) Coat the nickel film with photoresist; 3) Pattern the photoresist; 4) Chemically etch away the exposed nickel, undercutting the photoresist to create the deposition template; 5) Electrodeposit desired material into the template; 6) Remove excess photoresist; and 7) Chemically etch away the excess nickel, leaving a freestanding nanowire. The conventional LPNE method has many benefits ranging from the ability to control the material of the wire via electrodeposition, the height of the wire by controlling the nickel deposition height, the width of the wire by varying the electrodeposition time, and the shape of the deposition by photolithography. There are however two main drawbacks to the conventional LPNE method- one being that the electrodeposited nanowires are nanocrystalline, and secondly, the density of arrays is limited by photolithography. I will not address the former drawback at this time, but instead the latter. The conventional LPNE method can be adapted to create a high density (HD-) array of material while still having the benefits of being fast and cheap. The HD-LPNE process begins in the same manner as the conventional LPNE method. The change in processes begins once an initial gold electrodeposition has taken place. The next step involves a subsequent nickel electrodeposition, upon which an additional gold layer can be electrodeposited. This varying electrodeposition process can be continued until the trench is full. The photoresist is then removed, the electrochemically deposited nickel is chemically etched, leaving the gold electrodeposition pitch, or distance between nanowires, to be defined by the width of the nickel electrodeposition. Preliminary results for the HD-LPNE method show that this project has legs. SEM images give hope that we can gain the ability to create arrays of nanowires that have a pitch, at sub 50 nm.
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Quasi-one Dimensional Nanostructures of Transition Metal Oxides for Smart Device Applications.
Padmanabha Chavvakula 1 , Haitao Zhang 1
1 Department of Mechanical Engineering and Engineering Science, University of North Carolina at Charlotte, Charlotte, North Carolina, United States
Show AbstractTransition metal oxides (TMOs) are a group of smart materials capable of changing their color reversibly in response to external stimuli, such as electric field (electrochromism) and light (photochromism), etc. Quasi-one dimensional (Quasi-1D) nanostructures of tungsten oxide, molybdenum oxide, and vanadium oxide has been synthesized using a vapor solid (VS) approach. Metal powders as source materials were heated at elevated temperatures in an environment with controlled oxygen partial pressures. Quasi-1D nanostructures of above TMOs with extremely high aspect ratios have been developed with shapes of wires or belts. The diameters or thicknesses of these TMO nanostructures are about 100-600 nanometers, and the lengths range from tens microns to several millimeters. The formation of these 1D nanostructures is explained by the vapor-solid mechanism. And the growth is sensitive to experimental conditions, such as source heating temperature, growth temperature, and gas flows. The as-synthesized samples were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and other techniques. The extremely high aspect ratios of these TMO nanostructures provide following advantages for smart devices based on electrochromism: (1) the small diameter which reduces the distance for ions diffusing inside the solid; (2) the large gap between NWs which provides channels for ions fast transporting outside the solid; and (3) the large surface to volume ratio which improves the usage of materials. The performance of electrochromic devices based on the quasi-1D nanostructures will be demonstrated.
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Dielectrophoretic Alignment of Metal-oxide Nanowires for Advanced Gas Nanosensor Fabrication.
Roman Jimenez-Diaz 1 , J. Daniel Prades 1 , Francisco Hernandez-Ramirez 2 3 , Albert Romano-Rodriguez 1
1 Department of Electronics, University of Barcelona, Barcelona Spain, 2 IREC, Catalonia Institute for Energy Research, Barcelona Spain, 3 , Electronic Nanosystems S.L., Barcelona Spain
Show AbstractIn this work, the use of dielectrophoretic techniques for the alignment of nanowires for the high throughput production of devices is presented.SnO2 nanowires were grown over an alumina substrate by chemical vapor deposition (CVD) of a molecular precursor [Sn(OtBu)4] as described elsewhere [2]. High-resolution TEM images of several nanowires showed them with radii in the range 20–200 nm and length over 15 microns, displaying interplanar spacings corresponding to the rutile structure of SnO2; no amorphous shell was observed [2].Some of these nanowires were dispersed on a solvent, i.e ethanol, until a homogeneous solution was obtained. Special Au microelectrodes were fabricated over a SiO2/Si wafer designed to confront two different electrodes by leaving a gap between them. An AC voltage was applied to this pair of electrodes and then a droplet of solution was spread over. Dielectrophoretic force tends to align the nanowires in the direction of the potential variation and positions them in the gap between the electrodes. Voltage was applied until the complete evaporation of the solvent. Afterwards, SEM inspection was used to evaluate the efficiency of the process.Finally, the sample was introduced in a Focused Ion Beam (FIB) system for the deposition of platinum obtained from the decomposition of a metalorganic precursor (trimethylcyclopentadienyl–platinum, (CH3)3CH3C5H4Pt), by means of Electron Beam Induced Deposition (EBID). Electrical contacts from the nanowire to the microelectrodes were fabricated with electron beam scanning of the sample to prevent modification of the sample by the ions [3]. After contacting the nanowires, two- and four-probe DC electrical measurements were performed using a Keithley 2602 Source Measure Unit. Ohmic and rectifying responses were obtained as commonly found by the use of the FIB method [4]. Some of these nanowires were tested as gas and UV sensors using well-controlled environmental conditions. The obtained results demonstrated the validity and huge potential of nanowires as building-blocks of a new generation of devices with improved performances. It is noteworthy that DEP-aligned nanowires did not exhibit any significant difference in their electrical response than those previously reported and based on the random dispersion of the nanowires on top of prepatterned substrates [5]. For this reason the DEP-based technologies as a promising approach for the fabrication of nanosensors in a scalable process will be discussed in this work.[1] Satyanarayana V.N.T. et al., Progress in Materials Science 52 2007 699.[2] S. Mathur, et al., Small 1 2005 713.[3] F. Hernandez-Ramirez, et al., Nanotechnology 17 2006 5577.[4] Z. Zhang, et al., Advanced Functional Materials 17 2007 2478.[5] F. Hernandez-Ramirez, et al., Sens. Actuators, B, Chem 118 2006 198.
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A Resistless Process for the Production of Patterned, Vertically Aligned ZnO Nanowires.
Mikhail Ladanov 1 2 3 , Kranthi Kumar Elineni 2 , Garrett Matthews 4 , Manoj Ram 2 3 , Nathan Gallant 2 , Ashok Kumar 2 3
1 Department of Electrical Engineering, University of South Florida, Tampa, Florida, United States, 2 Department of Mechanical Engineering, University of South Florida, Tampa, Florida, United States, 3 Nanotechnology Research and Education Center, University of South Florida, Tampa, Florida, United States, 4 Department of Physics, University of South Florida, Tampa, Florida, United States
Show AbstractZnO nanostructures have attracted a great deal of interest because of their biocompatibility and outstanding optical and piezoelectric properties. Their uses are widely varying, including incorporation in sensors, solar cells, and nanogenerators. One of the major complications in device development is how to grow ZnO nanowires in well aligned and patterned films with predefined geometrical shape and aspect ratio. Controlled growth is required to achieve optimal density of nanowires and to produce a defined geometric structure for incorporation in the device. In this work we have presented a method by which vertically aligned ZnO nanowires can be grown in defined patterns on surfaces without the use of resists. We used a hydrothermal method with growth modifiers to grow ZnO nanowires on a substrate that was pre-patterned with a seeding solution by means of microcontact lithography. This method produced vertically aligned ZnO nanowires of predefined size and shape with pattern resolution high enough for production of rows of single nanowires. The nanowires were characterized by SEM, TEM, and XRD techniques.
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Seeded Growth of ZnO Nanorods for Photovoltaic Applications.
Jon Downing 1 , Mary Ryan 1 , Natalie Stingelin 1 , Martyn McLachlan 1
1 Department of Materials Science, Imperial College London, London United Kingdom
Show AbstractThere is increasing interest in the development of low-cost, solution processable photovoltaic devices. Device heterostructures with nanostructured interfaces between inorganic and organic materials, so-called ‘hybrid cells’, have the potential to achieve improved efficiency whilst minimizing both material and processing costs. A hybrid cell is advantageous as it combines the desirable properties of each constituent. For example metal oxides, and in particular ZnO, have high electron mobilities and can be prepared with numerous morphologies. Furthermore, solution processing permits the formation of ZnO structures with feature sizes ranging from a few nanometres to tens of microns over large areas. For organic materials, and in particular poly(3-hexylthiophene) (P3HT), excellent hole transport properties are combined with strong light-absorption. Compatibility with solution processing is well documented for this material.We will report our initial studies, which focus on the identification of a reproducible route for the formation of a standard nanostructured ZnO template. Various solution processing methods have been considered with chemical bath deposition being seen to give the best results. It is shown that a well-defined seed layer controls the growth of orientated nanorods. Rod density and aspect ratios are governed by grain size of the seed layer, which can be varied to provide the optimum structure. These methods are investigated using substrates of ITO coated glass.The ZnO nanorods have been applied to photovoltaic devices in a study of the efficiency of P3HT filling. We characterised the viscosity of P3HT solutions over the molecular weight (Mw) range 10-440 Kg/mol. The use of a consistent and well-defined template has allowed the role of viscosity on filling efficiency to be studied, whilst the influence of device performance on the Mw of the organic phase has been studied in parallel. We will present SEM, AFM and XRD characterisation of the composite structures and correlate the macroscopic and interfacial structures with the measured device efficiencies.
9:00 PM - W6.35
Electrodeposited Iron Oxide Nanotubes.
Jin-Hee Lim 1 , Seong-Gi Min 2 , Leszek Malkinski 2 , John Wiley 1
1 Department of Chemistry, University of New Orleans, New Orleans, Louisiana, United States, 2 Department of Physics, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractIron oxide nanomaterials are important because of their potential applications in catalysis, sensor devices, magnetic data storage, etc.. Various iron oxide nanostructures have been studied including nanoparticles, nanowires, nanobelts and thin films. In contrast, little work has been carried out on iron oxide nanotubes. Here, we will report on the synthesis of iron oxide nanotube structures by a template-assisted electrodeposition method. This approach readily produces structures with varying lengths and wall-thicknesses. The morphology, structure, and magnetic properties of iron oxide nanotubes will be presented and the growth mechanism discussed.
9:00 PM - W6.5
Electrical Characteristics of IGZO/ZnO Hybrid Nanowire Network Field-effect Transistor.
Jaehyun Yang 1 , Myung Soo Lee 1 , Ju Li Song 1 , Young-Chul Byun 1 , Hoo-Jeong Lee 1 , Hyoungsub Kim 1
1 School of Advanced Materials Science and Engineering, Sungkyunkwan, Suwon Korea (the Republic of)
Show AbstractThese days, for a large-scale fabrication of the semiconducting nanowire-based transistors, the nanowire network (nanonet) device composed of multiply-aligned nanowires has been continuously explored [1]. In this nanowire-based nanonet transistor, the carriers are expected to travel along the multiple nanowires by jumping across the nanowire interfaces. However, because many disconnected interfaces may exist between the nanowires, there is a strong possibility of the degradation and/or the non-uniform distribution of the transistor parameters.In order to overcome these problems, in this presentation, we propose a hybrid-type transistor by combining the nanonet-structured ZnO nanowire transistor with a solution-deposited InGaZnO (IGZO) film. By filling the gaps between the ZnO nanowires with highly performing IGZO film, it was expected that more improved device performance and large-area device uniformity could be obtained.For the fabrication of the suggested hybrid transistor, first, the ZnO nanowires were separately synthesized by using a vapor-liquid-solid growth method on the thermally-annealed Au film acting as a catalyst. Then, the synthesized ZnO nanowires were transferred to a SiO2/highly-doped Si wafer by using a direct contact printing method, which uses a mechanical contact and a subsequent sliding of the nanowire-grown wafer on the receiving substrate [2]. After obtaining the reproducibly well-aligned nanonet structure, the sol-gel deposition of the IGZO film was carried out followed by a thermal densification process, and various transistor characteristics were measured after forming the source/drain electrodes. The electrical properties of this hybrid-type transistor will be compared with those of the typical nanonet-structured ZnO transistor and the detailed mechanism for the improved electrical characteristics will be discussed.[1] X. Duan, C. Niu, V. Sahi, J. Chen, J. W. Parce, S. Empedocles, and J. L.Goldman, Nature 425, 274 (2003).[2] Z. Y. Fan, J. C. Ho, Z. A. Jacobson, R. Yerushalmi, R. L. Alley, H. Razavi, and A. Javey, Nano Lett. 8, 20 (2008).
9:00 PM - W6.6
Resistive Switching in a Single Oxide Nanowire.
Takeshi Yanagida 1 2 , Kazuki Nagashima 1 , Keisuke Oka 1 , Masaki Kanai 1 , Jin-Soo Kim 3 , Bae Ho Park 3 , Tomoji Kawai 1 3
1 , Osaka University, Osaka Japan, 2 , JST-PRESTO, Saitama Japan, 3 , Konkuk University, Seoul Korea (the Republic of)
Show AbstractResistive switching (RS) memory effects of a metal/oxide/metal junction, frequently called “ReRAM” and/or “Memristors”, have attracted much attention due to the potential applications toward next generation non-volatile memories alternative to current flash memory technology but also for artificial neural computing systems beyond Boolean computing. Although the importance of nanoscale physical events on RS has been highlighted in previous studies based on thin film RS devices, investigating the occurrence of RS at nanoscale beyond the limitation of current lithographic length scales and extracting the exact nanoscale RS mechanisms have been difficult. However such knowledge as to nanoscale RS events is crucial to achieve reliable and high-density RS devices. Self-assembled oxide nanowire-based RS offers an alternative approach not only to reduce the size of the cells beyond the limitation of current lithographic length scales but also to extract the underlying nanoscale RS mechanisms. Here we report the fabrication of well-defined oxide nanowires via VLS mechanisms, the construction of heterostructured oxide nanowires and the nonvolatile resistive memory switching phenomena within a single oxide nanowire down to 10 nm scale. Single crystalline NiO and Co3O4 heterostructured nanowires were fabricated by newly developed in-situ formation technique. We constructed highly stable RS junctions with the endurance up to 10^8 by utilizing self-assembled nanowires and well-defined nano-gap electrodes. The importance of nanoscale redox events was clarified for the bipolar RS. The presented approaches by utilizing self-assembled oxide nanowire/metal junctions offer an important system and platform to investigate not only nanoscale RS mechanisms but also various nanoscale confined physical properties of transition metal oxides.References:[1]Appl. Phys. Lett., 90, 233103 (2007) [2] Appl. Phys. Lett., 91, 061502 (2007) [3] Appl. Phys. Lett., 93, 153103 (2008) [4] J. Am. Chem. Soc., 130, 5378 (2008) [5] Appl. Phys. Lett., 92, 173119 (2008) [6] Appl. Phys. Lett., 95, 133110 (2009) [7] Appl. Phys. Lett., 95, 053105 (2009) [8] Appl. Phys. Lett., 96, 073110 (2010) [9] J. Am. Chem. Soc., 131, 3434 (2009) [10] Nano Lett., 10, 1359 (2010) [11] J. Am. Chem. Soc., 132, 6634 (2010).
9:00 PM - W6.7
Betaluminescence of ZnO Nanowires.
Baojun Liu 1 , Fei Yan 1 , Usha Philipose 3 5 , Nazir Kherani 2 3 , Kevin Chen 1 , Walter Shmayda 4 , Harry Ruda 2
1 Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 3 Department of Material Science and Engineering, University of Toronto, Toronto, Ontario, Canada, 5 Department of Physics, University of North Texas, Denton, Texas, United States, 2 Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada, 4 Laboratory for Laser Energetics, University of Rochester, Rochester, New York, United States
Show AbstractSignificant research in the synthesis and optical characterization of nanostructured wide-band-gap semiconductors over the last decade has been motivated by their potential for lighting applications. Zinc Oxide (ZnO), a II-VI compound semiconductor with a wide and direct band gap of 3.37 eV, is considered to be a good candidate for blue and UV emission and hence an optical UV light source. Herein we report the beta-luminescence (BL) properties of ZnO nanowires (nW) and their potential as a tritium powered BL source.Zinc oxide nWs were grown on a silicon substrate using the standard vapor-liquid-solid (VLS) technique. The nanowires were tuned to emit in the green by modifying the oxygen content during the preparation. The morphology of the ZnO nanowires was studied using scanning electron microscopy (SEM). The nanowires have lengths of 8-10 um and diameters of 40-100 nm. Photoluminescence (PL) measurements at a temperature of 2 K, prior to beta ray exposure, show a weak band-edge luminescence at 380 nm and strong midgap emission at 525 nm. The BL of the ZnO NWs was studied using two forms of beta sources: the solid scandium tritide (ScTx) foil beta source and the gaseous tritium beta source. The scandium tritide film has a surface activity of 15 mCi/cm2, which is equivalent to a beta flux density of 90 pA/cm2. The luminescence of ZnO nWs under scandium tritide irradiation involved placing the ScTx source on the nWs while examining the BL laterally. The gaseous tritium irradiation experiment was performed with the nWs placed in an ultra-high vacuum stainless steel (SS) enclosure equipped with an optical view port. Tritium was introduced into the enclosure over a pressure range from hard vacuum to approximately 0.9 atmosphere. The tritium gas layer thickness above the nW sample is about 5 mm. The BL spectra of the nWs under beta illumination show a broad peak with a wavelength range of 400 to 600 nm. The BL peak is generally located in the same frequency range as the PL spectrum, suggesting similar mechanisms in light emission. Weak peaks in UV region and yellow band are also observed around 340-380 nm and 620 nm, respectively. The UV emission is attributed to direct exciton recombination while the yellow band emission is attributed to electron recombination with ionized oxygen vacancy or interstitial oxygen.While we do not expect degradation under beta fluences due to the relatively low energy (Eavg ~ 5.7 keV), ageing studies are needed to confirm the pretreatment needed (if any) (ie baking, etc) to ensure the stability of ZnO. As a relatively long-lived (t1/2 ~ 12 years) radioisotope, tritium has the potential of being an excellent source of carrier injection with high gain. With appropriate materials optimization and source integration, such beta-luminescent structures could be suitable for developing high efficiency self-powered light emitting devices, and potentially lead to a beta-luminescent laser.
9:00 PM - W6.8
Integrated Copper-tin Oxide Nanowires as Novel Chemical Sensors.
Xiaopeng Li 1 , JungHwan Cho 2 , Hongwei Sun 3 , Pradeep Kurup 2 , Zhiyong Gu 1
1 Chemical Engineering, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 2 Civil and Environmental Engineering, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 3 Mechanical Engineering, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractAmong various semiconducting metal oxide materials, p-type copper oxide and n-type tin oxide have been utilized as chemical sensors due to their favorable electrical and chemical properties and good sensing characteristics. Here we present a unique way of fabricating copper-tin mixed metal oxide nanowires with different compositions and their application as a new type of chemical sensors. Copper-tin (Cu-Sn) nanowires of three different compositions were synthesized using electrodeposition method in a nanoporous membrane. These metallic nanowires were dispersed on the top of interdigitated microelectrodes where a dielectrophoretic (DEP) field was applied to enable the directed assembly of the nanowires. An electric resistor was formed right after the DEP aligned nanowires established contacts and bridged the electrodes. Subsequent thermal oxidation process converted the metallic nanowires into their corresponding oxides, enabling the nanowires to respond to chemicals. We observed interesting morphology on the metal oxide nanowire surfaces at different compositions. Conductivity measurements change when exposed to different chemicals such as ethanol, acetone and ethyl acetate demonstrated significant enhancement in sensor performance as compared to tin oxide nanowire sensors fabricated using the same method. The temperature and composition effects of the oxide sensors were also studied towards the above analytes.
9:00 PM - W6.9
Fabrication of High Density Nanorod Arrays Using Electroplating through Block Copolymer Template Process.
Takenori Goda 1 2 , Tomokazu Iyoda 1
1 Chemical Resource Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan, 2 , Toppan Printing Co., Ltd, Chiyoda-ku, Tokyo, Japan
Show AbstractHighly ordered nanostructures arising from the microphase separation of block copolymers play a key role as templates to fabricate functional-nano materials as next-generation fundamental technology of nanoelectronics and photonoic materials. We developed highly ordered microphase separation with hydrophilic polyethylene oxide (PEO) cylindrical nanodomains perpendicularly oriented to thin films of a series of amphiphilic liquid crystalline block copolymer consisting of PEO and hydrophobic polymethacrylate bearing mesogen in side-chain (PMA(Az)). In the present report, we fabricated ultrahigh density and highly ordered metal nanorod arrays perpendicular to substrate (∼1011 /cm2) using a simple method, i.e., electroplating of gold through PEOm-b-PMA(Az)n block copolymer (BC) thin film coated on electrode surface as a template. Electroplating was conducted in a three electrode system equipped with a potentiostat. Pt plate, BC coated ITO and Ag/AgCl electrodes were adopted as the counter, working and reference electrodes, respectively. The surface of the bare ITO electrode was modified in advance by self-assembled monolayers (SAMs) of which structure is identical to the azobenzene-containing side chain of the BC so as to anchor the liquid crystalline PMA(Az) domains of BC template. Then we performed electroplating into the PEO domains using Au electroplating solution (K-24EA10, Koujundo Chemical Lab. Co, Ltd). After removal of BC template by immersing in toluene solution, AFM and FE-SEM observation gave nanorod arrays perpendicularly oriented to the ITO substrate. The metal nanorods with 15 nm of diameter, approximately 10 of aspect ratio and 3.5 x 1011 rods per square of density were arranged hexagonally over the surface of the ITO substrate, corresponding to the ordered nanostructure of the BC template. This BC templated electroplating was quite different from already reported processes by using physical porous BC and porous alumina. The hexagonally arranged cylinders filled with PEO, which work as nanoscaled electrolytic media without selective etching of one of the BC domains such as PEO.Such domain-selective electroplating process can be extended to a wide range of metal which has a affinity for PEO. These nanorod arrays have potential applications for field emission, nanostructured electrode, electrochemical sensing, and so on.
Symposium Organizers
Ritesh Agarwal University of Pennsylvania
Wei Lu University of Michigan
Oliver Hayden Siemens AG
Akram Boukai University of Michigan
W7: Novel Properties and Energy Applications
Session Chairs
Wednesday AM, December 01, 2010
Ballroom C, 3rd floor (Hynes)
9:30 AM - **W7.1
Piezotronic and Piezo-phototronic Effects and Applications.
Zhong Wang 1
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractOwing to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential (piezopotential) is created in the material by applying a stress. When a semiconductor crystal is strained, two typical effects are observed. One is the piezoresistance effect, which is introduced because of the change in bandgap and possibly density of states in the conduction band. This effect has no polarity so that it has equivalent effect on the source and drain of the FET. On the other hand, piezopotential is created in the crystal if the material is also piezoelectric. Since piezopotential has polarity, it can tune the effective heights of the Schottky barriers at the source and drain in opposite directions. This is a non-symmetric effect. Devices fabricated by using piezopotential as the gate voltage are called piezotronic devices with applications in strain/force/pressure triggered/controlled electronic devices, sensors and logic gates. Piezo-phototronic effect is a result of three-way coupling among piezoelectricity, photonic excitation and semiconductor transport, which allows tuning and controlling of electro-optical processes by strain induced piezopotential.[1] Z.L. Wang “Towards self-powered nanosystems: from nanogenerators to nanopiezotronics” (feature article), Advanced Functional Materials, 18 (2008) 3553 [2] Z.L. Wang “Nano-piezotronics”, Adv. Mater., 19, 889 (2007).[3] Z.L. Wang and J.H. Song “Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays”, Science, 312, 242 (2006).[4] Y.F. Hu, Y.L. Chang, P. Fei, R.L. Snyder, Z.L. Wang, “Designing the Electric Transport Characteristics of ZnO Micro/nanowire Devices by Coupling Piezoelectric and Photoexcitation Effects”. ACS Nano., 4 (2010) 1234–1240.[5] Z.L. Wang "Piezotronic and Piezo-phototronic Effects", The Journal of Physical Chemistry Letters, 1 (2010) 1388–1393.[6] Thanks to NSF, DOE, DARPA, NIH for support. for details: www.nanoscience.gatech.edu/zlwang
10:00 AM - W7.2
Enhancing Light-matter Coupling with Nanowire Waveguide Cavities.
Lambert van Vugt 1 , Brian Piccione 1 , Pavan Nukala 1 , Carlos Aspetti 1 , Ritesh Agarwal 1
1 Materials Science & Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractIn semiconductors, bound electron-hole pairs (excitons) can strongly couple to the light field, resulting in the formation of exciton-polaritons,1 which are of interest due to their application in Bose-Einstein condensation in the solid state, efficient light emitting diodes and low threshold polariton lasers, optical switching and slow light applications. This strong coupling regime is generally pursued by forcing excitons to interact with the low loss optical mode of micro cavities.2 However, exciton-polariton formation also occurs naturally in bulk semiconductors with high exciton transition strengths such as ZnO, CdS and GaN 3 providing a logical starting point for the observation and manipulation of strong-coupling phenomena. Using explicit exciton resonance levels obtained via spatially and spectrally resolved photoluminescence measurements on CdS nanowire waveguide cavities4 in the 80-250 nm radius range and a nanowire waveguide dispersion model incorporating polaritonic effects,5 we demonstrate the polaritonic nature of the excitations in these cavities as well as their manipulation. We show how the light-matter coupling strength can be manipulated by systematically reducing the lateral dimensions of the nanowire waveguide cavity.6 At large radii the bulk coupling strength is obtained, but as we decrease the waveguide width to at or below the optical diffraction limit, a significant enhancement of this coupling strength is observed. A clear transition from bulk polaritons to confined cavity polaritons with significantly higher oscillator strength is observed in the nanowire optical properties. We explain our results in terms of increased exciton transition strength due to the strong dielectric confinement of the optical field resulting in size-dependent formation of one-dimensional exciton-polaritons in nanowires. These results are significant for manipulating light-matter coupling strengths for various nanophotonic applications.References:1)Hopfield, J.J. Phys. Rev. 112 , 1555-1567 (1958).2)Vahala, K.J. Nature 424, 839-846 (2003).3)Hopfield, J.J. & Thomas, D.G. Phys. Rev. Lett. 15, 22-25 (1965).4)Piccione, B.,* Vugt, L.K.v.* & Agarwal, R. Nano Lett. 10, 2251 (2010).5)Vugt, L.K.v., Piccione, B. & Agarwal, R. Submitted (2010).6)Vugt, L.K.v., Piccione, B., Nukala, P. & Agarwal, R. Submitted (2010).
10:15 AM - W7.3
Diameter Dependence of the Modal Gain of ZnO-nanowires.
Jan-Peter Richters 1 , Joachim Kalden 1 , Juergen Gutowski 1 , Tobias Voss 1
1 Institute of Solid State Physics, University of Bremen, Bremen Germany
Show AbstractIn recent years, ZnO nanowires have attracted considerable interest as nanoscale building blocks for optoelectronic devices such as single-nanowire nanolasers which can be regarded as the smallest possible all-semiconductor-based lasers. Compared to conventional semiconductor lasers, nanowires provide a much stronger confinement of the optical mode in the active material due to the comparably large difference of the refractive indices of the active material (n>2) and the surrounding air (n=1). For this reason, theory has predicted modal gain values in the range of a few thousand cm-1 for wide-gap semiconductor nanowire nanolasers being significantly larger than for conventional edge-emitting semiconductor lasers [1].We present a systematic experimental study of the modal gain g_mod of single ZnO nano- and microwires under excitation with 100fs laser pulses at a wavelength of 266nm and a repetition rate of 1kHz. Using the variable-stripe-length method, we find clear indications of an electron-hole-plasma gain by analyzing the shape of the gain curves and quantitatively determine g_mod of wires with lengths l>20µm lying on glass substrates. The results show an excellent agreement with the theoretical predictions and yield g_mod>1000 cm-1 for wires with 150nm4µm much below the theoretically predicted values.Our results for the first time give experimentally determined values for the modal gain of ZnO nano- and microwires that are required for the design, simulation and optimization of single-nanowire nanolasers.[1]A. V. Maslov and C. Z. Ning, IEEE J. Quantum Electronics 40, 1389 (2004).[2]T. Voss, G. T. Svacha, S. Müller, C. Ronning, and E. Mazur, Nanotechnology 20, 095702 (2009).[3]T. Voss, G. T. Svacha, S. Müller, C. Ronning, D. Konjhodzic, F. Marlow, and E. Mazur, Nano Letters 7, 3675 (2007).
10:30 AM - W7.4
Investigating Structure Property Relations in Compound Semiconductors Using Single Nanowire Devices.
David Schoen 1 , Stefan Meister 1 , Hailin Peng 1 , Yi Cui 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractOne key challenge to the integration of bottom-up nanowires into optical and electronic devices is the common presence of structural heterogeneity, induced either during the initial growth process or during subsequent fabrication steps. The most famous example of such heterogeneity is that of semiconducting and metallic carbon nanotubes, but differences in crystallographic orientation, composition, and even phase have been observed in a large number of inorganic nanowire systems. Traditional device architectures integrating bottom-up nanowires with top-down lithography on wafer substrates offer little opportunity for careful high resolution structural characterization of the finished devices to be tested. A new approach to investigating structure property relationships combining in-situ transmission electron microscope (TEM) techniques with single nanowire devices and test structures fabricated on electron transparent membranes will be presented. This approach will enable not only a route to improved understanding of nanomaterial structural heterogeneity, but also dynamic observation of nanoscale structural transformations induced during processing and device operation. Insights gained from these studies can be used to design fabrication processes for more complicated device architectures. Example studies on several metal chalcogenide materials for use in nonvolatile memory and photovoltaics will be discussed, including Ag2Se and GeTe NWs for nonvolatile memory applications and CuInSe2 – CdS heterostructure nanowires for photovoltaics.
10:45 AM - W7.5
Understanding Charge Behavior in Si Nanowires for Solar Energy Conversion.
Guangbi Yuan 1 , Ken Aruda 1 , Jin Xie 1 , Dunwei Wang 1
1 Department of Chemistry, Boston College, Chestnut Hill, Massachusetts, United States
Show AbstractThe key challenge in present solar energy research is the inability to achieve competitive conversion efficiency on devices made of low-cost materials. One of the main reasons accounting for this challenge may be lacking knowledge of detailed charge behavior in these materials. For example, Si nanowires (SiNWs) are promising candidates for efficient photon-to-charge conversion. But only limited efficiency has been demonstrated in solar cells based on SiNWs made from cost-effective methods. Understanding SiNW charge behavior in a photovoltaic device is of paramount importance to solar energy research, but is currently not well studied. To fill the knowledge gap, we have done a systematic study to examine charge properties in vertically aligned silicon nanowire arrays based on a photoelectrochemical cell (PEC) by electrochemical impedance spectroscopy (EIS). As a powerful tool, EIS allowed us to obtain detailed information concerning the space charge region in the semiconductor and trap states at the semiconductor/electrolyte interface from both steady-state and transient measurements. Equivalent circuits were also used to fit the EIS experimental data. These electrical analogs allowed us to quantitatively characterize resistive and capacitive elements involved in the main charge transfer process in the PEC cell. Moreover, important properties of SiNWs including charge concentration, flat-band potential, energy level of surface states and other deep-level charge trap states can be understood by EIS data analysis. By systematically studying the pristine bare substrate, electrolysis etched SiNW (EESiNW) and SiNW grown by chemical vapor deposition (CVD) process, we have shown that ~10% energy conversion efficiency could be achieved for both bare substrate and EESiNW based PEC cells. Surface states originated from NW morphology did not have significant detrimental effects to the overall energy conversion process. Residue metal catalyst introduced from CVD grown process may account for the major efficiency loss for CVD grown SiNW devices. Our results shed new light on charge behavior in SiNW and these results are expected to pave the way for unprecedented cost-efficiency photovoltaic device fabrication.
11:30 AM - **W7.6
Nanotechnology-Enabled Flexible Energy Harvesting.
Yi Qi 1 , Thanh Nguyen 1 , Michael McAlpine 1
1 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractThe development of a method for integrating highly efficient energy conversion materials onto stretchable, biocompatible rubbers could yield breakthroughs in implantable or wearable energy harvesting systems. Being electromechanically coupled, piezoelectric crystals represent a particularly interesting subset of smart materials which function as sensors/actuators, bioMEMS devices, and energy converters. Yet, the crystallization of these materials generally requires high temperatures for maximally efficient performance, rendering them incompatible with temperature-sensitive plastics and rubbers. Here, we overcome these limitations by presenting a scalable and parallel process for transferring crystalline piezoelectric nanoribbons (NR) of lead zirconate titanate (PZT) from host substrates onto flexible rubbers over macroscopic areas. The nanoribbons are fabricated via the recently developed PENCiL approach, which allows for controllable, location-determinant PZT NR arrays hierarchically patterned over wafer scales. Fundamental characterization of the ribbons by piezo-force microscopy (PFM) indicates that their electromechanical energy conversion metrics are among the highest reported on a flexible medium. Finally, integration into energy harvesting devices reveals the ability to harvest energy from common body movements such as finger tapping. The excellent performance of the piezo-ribbon assemblies coupled with stretchable, biocompatible rubber may enable a host of exciting avenues in fundamental research and novel applications.
12:00 PM - W7.7
Tuning the Shape of Semiconductor Nanowires for Advanced Photovoltaics.
Jia Zhu 1 , Zongfu Yu 1 , IKang Ding 1 , Chingmei Hsu 1 , Mike McGehee 1 , Shanhui Fan 1 , Yi Cui 1
1 , Stanford University, Stanford, California, United States
Show AbstractTuning the shape of nanostructures can have a strong effect on photon management and charge carrier collection for photovoltaics. Here, I demonstrate two examples of nanowire shape designing: nanocones and branched nanowires.Photon management, involving both absorption enhancement and reflection reduction, is critical to all photovoltaic devices. It can improve the efficiency by minimizing optical and electrical losses, and cut cost by reducing material usage, process time and capital investment. Here I demonstrate a novel solar cell structure with an efficient photon management design. The centerpiece of the design is a novel nanocone structure, which is fabricated by a scalable low temperature process. With this design, devices with very thin active layer can achieve near perfect absorption because of both efficient antireflection and absorption enhancement over a broad spectral range and a wide range of angles of incidence. More strikingly, the design and process is not in principle limited to any specific material system, hence it opens up exciting opportunities for all classes of photovoltaic devices. I have used amorphous silicon and dye sensitized solar cells as two examples to demonstrate the concept. The device efficiencies of this design are significantly better compared to conventional devices. Moreover, I also have explored absorption enhancement on a sub-wavelength scale, compared to “classical” light trapping limits. PbSe nanocrystals have shown a greatly enhanced multi exciton generation (MEG) effect, one important step toward third generation solar cells. However, it is difficult to extract generated carriers from nanocrystals without good transport pathways. Three dimensional branched nanowire or nanotube networks, with strong quantum confinement within two dimensions, and the connected third dimension as an efficient charge carrier pathway, could be ideal for enhancing the MEG effect, light absorption, and carrier collection. I successfully demonstrate a large area growth of PbSe hyperbranced and chiral branched nanowires on a variety of substrates. More excitingly, Chiral branched nanowires reveal a new nanowire growth mechanism, dislocation driven growth, which can be applied to a variety of materials.
12:15 PM - W7.8
Effects of Surface Coating on the Electron Diffusion in 1-D Nanowired Based DSSCs.
Mengjin Yang 1 , Bo Ding 1 , Jung-Kun Lee 1
1 Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show Abstract Dye-sensitized solar cell (DSSC) has attracted tremendous attention due to its cost-effectiveness, easy-assembly and modest conversion efficiency. Photoanode of conventional DSSC employs TiO2 nanoparticles, which forms a porous network possessing considerable surface area suitable for anchoring dyes. The drawback of nanoparticle network is the introduction of numerous particle boundaries, which promotes traps of electrons. In the nanoparticle based DSSCs, therefore, the electron diffusion is a trap-limited process and the electron diffusion coefficient becomes several orders smaller than the expected one which was deduced from the physical properties of the single crystalline bulk TiO2. One dimensional (1-D) nanomaterials are promising alternatives to compromise surface area and diffusion coefficient, since the nanowire and nanotubes are believed to provide a ballistic pathway to the carriers. However, the recent studies on the nanowire based DSSCs have reported that the 1-D nanomaterials mainly increase the lifetime of the carriers and their effect on the electron mobility is not as much as expected. In this study, we systematically investigated the electron diffusion coefficient and life time of the nanowire based DSSCs with the emphasis on the role of the surface coating. Two methods, namely heterogeneous and homogeneous nucleation, were used to prepare the 1-D nanomaterials in a microwave hydrothermal reactor. Heterogeneously anisotropic nucleation of TiO2 on the top of fluorine-doped tin oxide (FTO) gave rise to the 1-D nanowire arrays, while homogeneously isotropic nucleation produced the chrysanthemum-like hierarchical assembly of the nanowires. Due to the volumetric heating of the microwave reaction technique, the uniform nanowires were grown fast and reliably. These two different types of the nanomaterials were employed to fabricate DSSCs. The energy conversion efficiency of the DSSCs ranges from 1.5% to 4%, depending on the surface coating states of the nanowires. We found that both electron mobility and carrier lifetime of the nanowire based DSSC are larger than those of the nanoparticle based DSSCs. In addition, it has been observed that only the electron diffusion coefficient is very sensitive to the surface states of the nanowires. When the porous layer is coated on the nanowires to increase the surface area to anchor the enough amount of the sensitizing dyes, the electron mobility of the nanowire based DSSCs significantly decreases. This indicates that the energy conversion efficiency of the nanowire based DSSCs is determined by a complex function of the electron mobility, the surface area and the carrier lifetime. In our presentation, we will report our experimental results and compare them with recent studies of other groups to explain the underlying physics of carrier diffusion in DSSCs.
12:30 PM - W7.9
Fabrication and Photovoltaic Measurements of Al Catalyzed Silicon Single-nanowire Radial Junction Solar Cell Devices.
Yue Ke 1 , Xin Wang 2 , Xiaojun Weng 1 , Chito Kendrick 1 , Yuwen Yu 2 , Sarah Eichfeld 1 , Heayoung Yoon 2 , Joan Redwing 1 , Theresa Mayer 2 , Youssef Habib 3
1 Department of Materials Science and Engineering, Pennsylvania State University, State College, Pennsylvania, United States, 2 Department of Electrical Engineering, Pennsylvania State University, State College, Pennsylvania, United States, 3 , Illuminex Corp., Lancaster, Pennsylvania, United States
Show AbstractRadial junction Si nanowire solar cells are of interest as a pathway to enhance charge collection efficiency and light absorption for cost reduction associated with high purity solar-grade Si material. Vapor-liquid-solid (VLS) growth has been pursued for the fabrication of the Si nanowires that form the core of these devices; however, the typical Au catalyst which is employed is problematic due to Au-related deep level states in Si that trap minority carriers. Recently, we demonstrated that Al can serve as a suitable replacement for Au in the VLS process yielding single crystal Si nanowires at high growth rates (>1 µm/min). Aluminum offers additional advantages for PV devices including reduced cost and p-type auto-doping which eliminates the need for additional dopant sources.In this study, we demonstrate single wire radial junction solar cells fabricated with Al-catalyzed Si nanowires. Both p-type/n-type (p-n) and p-type/intrinsic/n-type (p-i-n) radial junction single coaxial nanowire devices were fabricated. The Si nanowires were grown by low pressure chemical vapor deposition (LPCVD) on (111) Si substrates coated with a 10 nm layer of Al using SiH4 at 560 oC and 100 Torr total pressure. Prior to coating, the Al tips were removed from the nanowires by etching in 10:1 buffered oxide etch. The Si shell coating was then deposited by LPCVD at 650 oC using SiH4 and PH3 for n-type doping. The p-type and n-type doping levels of the Si nanowire core and n-type shell were estimated to be above 1x1019 cm-3 and ~1x1020 cm-3, respectively, for the growth conditions employed. The radial junction Si nanowires were removed from the Si substrate by mechanical agitation and were aligned onto pre-patterned substrates using field-assisted assembly. KOH was used to selectively etch the intrinsic and n-type shell layers from both ends of the nanowires and then photolithography was used to fabricate Au/Pd contacts to p-type core and n-type shell. The radial p-n and p-i-n devices exhibited current rectification with diode ideality factors on the order of 1.9. Under one sun AM 1.5G illumination, radial p-n devices exhibited a short circuit current density Jsc~3 mA/cm2, an open-circuit voltage Voc~ 0.14 V, fill-factor FF~44% and an overall efficiency of 0.7% while the radial p-i-n devices showed significantly improved cell characteristics (Jsc~34 mA/cm2, Voc~ 0.22 V FF~48% and efficiency of 5.3%). The higher efficiency of the p-i-n devices was caused by the improved carrier collection due to electric field introduced in the i-layer with p-i-n configuration, as well as the longer carrier diffusion length for the intrinsic material.
12:45 PM - W7.10
All Inorganic Core/Shell Nanowire Array with Type II Heterojunction for Three-dimensional Photovoltaic Device Fabrication.
Kai Wang 1 , Jiajun Chen 1 , Zhongming Zeng 1 , Weilie Zhou 1 , Josh Tarr 1 , Yong Zhang 2 , Yanfa Yan 3 , Chusheng Jiang 3 , John Pern 3 , Angelo Mascarenhas 3
1 Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, United States, 2 Department of Electrical and Computer Engineering, University of North Carolina at Charlotte, Charlotte, North Carolina, United States, 3 , National Energy Renewable Laboratory, Golden, Colorado, United States
Show AbstractVertically aligned ZnO/ZnSe and ZnO/ZnS core/shell nanowire array with type II band alignment were directly synthesized on indium-tin-oxides glass substrates by a two-step technique combining chemical vapor deposition and pulsed laser deposition. The morphologies, structures, optical properties and the photovoltaic effect of the core/shell nanowire array were systematically investigated. The unusual epitaxial relationships between wurtzite and zinc-blende structures were observed in both heterostructures. The coating is found to quench the photoluminescence of ZnO nanowires but enhance the photocurrent with faster response in the photovoltaic device, indicating improvement in charge separation and collection in the type II core/shell nanowires.
W8: Nanowire Growth II
Session Chairs
Wednesday PM, December 01, 2010
Ballroom C, 3rd floor (Hynes)
2:30 PM - **W8.1
Core-shell Synthesis, Orientation and Strain Control in Epitaxial Group IV Nanowire Arrays.
Paul McIntyre 1
1 , Stanford University, Stanford, California, United States
Show AbstractThis presentation will provide an overview of results both from our laboratory and from others on metal-catalyzed Ge and Si nanowire (NW) growth in which epitaxy is used to control the growth orientation and other key characteristics of NW arrays. Given the importance of growth temperature for integration of nanowires in Si-based circuitry, particular attention will be devoted to the topic of deep sub-eutectic vapor-liquid-solid (VLS) growth of Ge NWs. Recent results on strain control in epitaxial Si-shell/Ge-core nanowires will demonstrate the extent to which continuum elasticity models are helpful in understanding the complex interplay of stress-driven roughening and dislocation formation in coaxial nanostructures. Processes by which initially-vertical <111> nanowires kink onto inclined growth directions will also be discussed.
3:00 PM - W8.2
Electrochemically-controlled Crystallographic Orientationfor Optimized Spin Torque Transfer Switching of Multilayered Nanowires on Si.
Mazin Maqableh 1 , Liwen Tan 1 , Bethanie Stadler 1
1 Electrical Engineeing, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractCo/Cu multilayered nanowires were electrochemically deposited into anodic aluminum oxide (AAO) on Si, and the Co crystallographic orientation was carefully controlled. These arrays of magnetoresistive nanowires will be useful for applications such as current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) sensors, magnetic random access memory (MRAM) and microwave oscillator arrays. In this work, spin transfer torque (STT) switching was examined in arrays of nanowires that were composed of 50 bilayers of Co(5nm)/Cu(d) where d was varied between 2-10nm. The nanowires were 40nm in diameter and 600nm in length including long Cu leads on each end. A microprope station was used to contact patterned electrodes on top of the nanowires, and both giant magnetoresistance (GMR) and spin transfer torque (STT) were observed. We found that magnetoresistance as well as spin torque switching depended on how the successive Co layers were cystallographically aligned. Spin wave excitations at high applied fields were also observed. When the Co c-axes of the multilayers were kept in plane, the multilayers exhibited 8-10% MR for both perpendicular to the wires and parallel to the wires applied fields. Their switching current densities varied with the applied magnetic field with a minimum of 5 x 106 A/cm2 when the layers were switched from antiparallel to parallel aligned. On the other hand, alternating anisotropy led to a decrease in the MR% when the field was applied parallel to the wires. Moreover, for trilayered nanowires, the switching current density was on the order of 109 A/cm2. Therefore, switching current densities can be engineered to suit different applications, such as, spin torque random access memory (ST-RAM) which requires small critical currents, and magnetic sensors where low critical current is not desirable. Finally, understanding the GMR and STT behavior of these crystallographically optimized nanowires and enabling their integration on Si will lead to great potential for these nanostructures in future MRAM and microwave oscillator arrays with densities up to 2Tb/in2.
3:15 PM - W8.3
Insight on the Crystalline Matching Between Ti1-xVxO2 Shell and SnO2 Core in Coaxial Nanowires.
Reza Zamani 1 2 , Francisco Hernandez-Ramirez 2 3 , Sanjay Mathur 4 , Jordi Arbiol 1 5
1 , Catalonia Institute for Energy Research, Barcelona Spain, 2 , Institut de Ciència de Materials de Barcelona, CSIC, Barcelona Spain, 3 , University of Barcelona, Barcelona Spain, 4 , University of Cologne, Cologne Germany, 5 , University of Cologne, Cologne Germany
Show AbstractSince the first synthesis of 1D tin oxide nanostructures (nanobelts and nanowires) [1], many efforts have been made in order to improve their application in gas sensing [2]. Due to the excellent surface to volume ratio of these nanostructures, and their length, up to tenths of microns, they present perfect characteristics to implement nanodevices, some of them based on single nanowire gas detectors [3]. Enhancement of their catalytic properties can be obtained by doping their surface with noble metal [2], playing around with the different structural polytypes that tin oxide can offer when grown as a nanowire [4], or designing heterostructures with coaxial morphology. The later topic is the central study of the present work. When considering coaxial heterostructures, it is important to take into account the epitaxial relationship between the different materials. A high mismatch between them can induce undesirable effects to their physical properties, due to the creation of dislocations, grain boundaries and strain fields that could minimize the carrier transport and thus limiting the nanowire performances for electronic, optoelectronic or even sensing applications. In order to minimize the mismatch effects, it is important to control the growth processes. One important parameter to be considered is the possibility to play with binary compounds, trying to find the best stoichiometry to reduce the mismatch. In the present work we present the possibility to improve the Ti1-xVxO2 shell epitaxy on SnO2 core Nanowires by modulating the Ti content in this binary system. Theoretical simulations have been performed and corroborated by detailed HRTEM analysis of the core-shell heterostructures.References[1] Z. W. Pan et al., Science 291, 1947 (2001)[2] J. Arbiol et al., Appl. Phys. Lett., 80, 329 (2002)[3] F. Hernández-Ramírez, Sens. & Act. B, 121, 3 (2007)[4] J. Arbiol, J. Cryst. Growth, 310, 253 (2008)
4:00 PM - W8.4
Wafer-scale Vertically Aligned ZnO Nanowires Array Synthesized On Laser-interference-ablation Patterned GaN Substrate.
Wenzhuo Wu 1 , Yaguang Wei 1 , Yong Ding 1 , Zhong Lin Wang 1 , Dajun Yuan 2 , Rui Guo 2 , Suman Das 2
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractA novel and effective approach for patterned growth of vertically aligned ZnO nanowire (NW) arrays with high-throughput and low-cost at wafer-scale is demonstrated, by combining laser interference ablation and low temperature hydrothermal (HT) synthesis for the site-selective growth of ZnO NWs on Gallium Nitride (GaN) substrates. A nanosecond pulsed Nd:YAG laser with 266nm wavelength is used for generating ordered patterns directly on GaN by interference ablation of GaN substrates. The NWs, synthesized by a low temperature HT decomposition method on patterned substrates, follow the ablated pattern in size, dimensionality, and uniformity with high fidelity. Wafer-scale well patterned and vertically aligned ZnO NWs array can be achieved in a desired and controllable manner. This combined approach demonstrates a novel method of manufacturing large-scale patterned one-dimensional nanostructures on various substrates for applications in energy harvesting, sensing, optoelectronics and electronic devices. http://www.nanoscience.gatech.edu/zlwang/
4:15 PM - W8.5
Catalyst-free Growth of ZnO Nanowires Based on Topographical Confinement and Preferential Chemisorption and Their Use for Room Temperature CO Detection.
Margit Zacharias 1 , Seul Ki Youn 1 , Niranjan Ramgir 1 , Chunyu Wang 2 , Kittitat Subannajui 1 , Volker Cimalla 2
1 IMTEK, TF, Albert Ludwigs University, Freiburg Germany, 2 , Fraunhofer-Institute for Applied Solid-State Physics, Freiburg Germany
Show AbstractTwo novel approaches for patterned growth of ZnO nanowires (NWs) based on the selective deposition of zinc acetate (ZA) solution precursors are presented and compared [1]. The first is using the topographical confinement within a photoresist pattern on Si/SiO2 substrates (type I), and the second is using preferential chemisorption on self-assembled monolayer modified Au electrodes on Si/SiO2 substrates (type II). In both approaches, the ZnO seeds from the ZA solution form crystallites without severe defects over a large area via annealing at 350 C. These seed layers were used to grow ZnO NWs via a catalyst-free vapor-phase deposition method using temperatures of up to 900 C. The presented method is effective for realizing NW growth on both Si and Au electrodes by preserving the electrode configuration even at such high temperatures used for NW growth, which is a novelty and crucial for future sensor applications. As a result NWs connected through metal contact pads or electrodes have been realized in very simple and effective way. As a test of principle the resulting configurations were used to demonstrate a highly sensitive room temperature CO sensor. A CO concentration as low as 120 ppb was detected using both types of sensors. The type II sensor exhibited enhanced sensing properties compared to that of type I.[1] S. K. Youn, et al. J. Phys. Chem. C 2010, 114, 10092–10100.
4:30 PM - W8.6
Ordered Arrays of Luminescent Nanowire Heterostructures.
Tevye Kuykendall 1 , Shaul Aloni 1
1 The Molecular Foundry, LBNL, Berkeley, California, United States
Show AbstractControlling semiconductor heterostructure interfaces is the key for tuning their optical and electronic properties. As one of the leading materials for tunable light emitting devices, GaN has been widely investigated. Nanowire heterostructures offer a unique platform for the investigation of GaN-InGaN interfaces and for the development of novel device architectures. We report a growth strategy for the synthesis of ordered arrays of GaN nanowires with well-controlled crystallographic orientation and dimensions. Moreover, these arrays are used as a template for highly luminescent core/shell GaN/InGaN heterostructures. This is useful for fabricating devices and facilitating systematic characterization. Using this approach, ordered arrays of tunable nanoscale light emitters can be realized.
4:45 PM - W8.7
Strategies to Obtain Uniform Axial and Radial Doping in VLS-Grown Si Nanowires.
Yossi Rosenwaks 1 , Elad Koren 1 , Jerome K. Hyun 2 , Eric R. Hemesath 2 , Lincoln J. Lauhon 2
1 Physical Electronics, Tel Aviv University, Tel Aviv Israel, 2 Materials Science and Engineering, Northwestern university, Evanston, Illinois, United States
Show AbstractSemiconducting nanowires grown by the vapor-liquid-solid (VLS) method may develop non-uniform doping profiles along the growth direction due to unintentional surface doping during synthesis1,2. In addition, inhomogeneities in the radial distribution may be influenced by enhanced diffusion of the dopants from the nanowire surface to core during growth. We show that these non-uniformities can be mitigated by rapid thermal annealing following the growth, or suppressed by high H2 partial pressures during in situ doping; this paves the way to use Si nanowires as building blocks for a variety of electronic and optoelectronic devices.The radial doping profile was measured via successive chemical etching of a single nanowire and measuring its surface potential using Kelvin probe force microscopy (KPFM)3. We find that the radial active dopant distribution within a single n-type silicon nanowire decreases by almost two orders of magnitude from the wire surface to its core. Furthermore, the dopant profile is consistent with a very large diffusion coefficient of phosphorous in the silicon nanowires of D ~1x10-19 m2 s-1.Thermal annealing conducted at 460oC in forming gas decreased the difference between the dopant concentrations at the nanowires surface relative to the core. The dopant redistribution was confirmed also by current-voltage measurements: untreated nanowires showed a rectifying behavior after etching of ~ 20 nm from the nanowire surface, whereas the conductivity of the annealed etched nanowires was only weakly affected by the back gate voltage, as observed for highly doped nanowires.For CVD growth using hydride precursors, surface doping was suppressed by high H2 partial pressures during in situ doping, thereby improving the uniformity of the dopant distribution. Quantitative calculations of the electrostatic field and carrier concentration derived from scanning photocurrent microscopy measurements confirm suppression of phosphine surface doping for Si nanowires grown in H2 compared with those grown in He. Nanowires grown in He showed 100-fold increases in carrier concentration through surface doping, whereas nanowires grown in a large H2 partial pressure show only two-fold increase for similar growth times. 1.Koren, E.; Rosenwaks, Y.; Allen, J. E.; Hemesath, E. R.; Lauhon, L. J. Applied Physics Letters 2009, 95, (9), 092105. 2. Hyun, J. K.; Hemesath, E. R.; Lauhon, L. J. IEEE Nano 2010, submitted.3. Koren, E.; Berkovich, N.; Rosenwaks, Y. Nanoletters, 10, 1163-1167, (2010).
5:00 PM - W8.8
II-VI Radially Heterostructured Nanowires.
Keith Kahen 1 , Irene Goldthorpe 1 , Matthew Holland 1 , John Minter 1
1 Research Laboratories, Eastman Kodak Company, Rochester, New York, United States
Show AbstractZnSe and ZnMgSe have wide direct band gaps in the visible and near-UV range, and thus have been investigated extensively for their potential optoelectronic applications. However, limiting device development is the absence of inexpensive lattice-matched substrates for thin-film epitaxial growth. This hindrance can be overcome when II-VI materials are deposited as nanowires rather than as thin films, which allows single-crystalline, dislocation-free material to be grown on inexpensive, non-crystalline substrates, such as glass.Several groups have synthesized Zn-based II-VI nanowires by the vapor-liquid-solid (VLS) method, but photoluminescence (PL) measurements on the nanowires indicate weak band-edge and high sub-bandgap defect emission. The two main contributors to the non-optimal PL are nanowire growth at high temperatures (typically 500° C and above) and unpassivated surface states. As is well known, the optimal growth temperatures for II-VI materials with high optoelectronic quality are below 350° C, with higher growth temperatures leading to unwanted point and extended defects. The most effective method for passivating the surface states is to shell the core nanowires with higher bandgap material.We report the synthesis of II-VI core-shell nanowires, grown in the low 300° C range by the VLS method. The nanowires are formed on oxide surfaces by atmospheric pressure, metal organic vapor phase epitaxy using alternative metallic alloys as the catalysts. The low growth temperatures permit the synthesis of twin-free nanowires with superior optical and electrical properties. Results will be given for both ZnSe and ZnMgSe cores, shelled hetero-epitaxially with either ZnSeS or ZnMgSeS. For the ZnSe core nanowires, shelling them increases the band-edge luminescent intensity (at 77 K) of the nanowires by up to four orders of magnitude, and improves the band-edge to defect PL intensity ratio to 12,000:1. The corresponding FWHM of the band-edge exciton peaks can be as narrow as 2.5 nm, which is comparable to the results obtained for bulk ZnSe epitaxial films. The as-grown nanowires are intrinsic, as confirmed by electrical measurements taken on collections of nanowires. After doping n-type with chlorine, the resistivity of the nanowires decreases by over five orders of magnitude. Overall, these Zn-based core-shell nanowires show high promise as the operative materials in efficient and narrow bandwidth light-emitting diodes.
5:15 PM - W8.9
Fabrication of Aligned 1D Nanostructure for Sensing and Energy Harvesting.
Yang Liu 1 , David Pan 1 , Jennifer Lu 1
1 School of Engineering, UC Merced, Merced, California, United States
Show Abstract An active building block that contains a large number of suspended and electronically isolated 1D nanomaterials is required for advanced photonic and electronic devices. For example, high electrical output can be generated from a collective of horizontally aligned nanowires. A device with high-density aligned carbon nanotubes (CNTs) can capture an adequate amount of light for enabling CNT-based photovoltaic devices. Conversely, vertically aligned 1D nanomaterials can be used for field emission-based sensing. Herein, we present our work on the generation of highly engineered nanocatalysts using the micelle lithography approach. Using Zn-based binary alloys, the growth of vertically aligned ZnO nanowires has been achieved. Their field emission property and the potential for gas sensing will be discussed. Combining top-down lithography with bottom-up block copolymer self-assembly, monodispersed nanocatalysts deposited along the sidewalls of trenches have been generated. High-density aligned CNTs in suspension have been generated where the CNTs are distributed vertically along the trench depth with their tubes oriented orthogonally to the trench sidewalls. Raman analysis has shown that tube diameter is very uniform, around 1.2 nm. The lack of the Raman active D band associated with amorphous carbon and defects suggests that this direct synthesis method is able to produce high-quality CNTs. This unique hybrid process has been also used to grow high-density and high-quality ZnO nanowire with very small diameter. Photoluminescence measurements indicate that ZnO nanowires have very few defects. With enhanced piezoelectricity resulting from elongated grain structure, ZnO ensembles that contains high-quality and high-density nanowires are excellent candidates for sensing and energy harvesting.
Symposium Organizers
Ritesh Agarwal University of Pennsylvania
Wei Lu University of Michigan
Oliver Hayden Siemens AG
Akram Boukai University of Michigan
W10: Nanowire for Energy Applications
Session Chairs
Thursday AM, December 02, 2010
Ballroom C, 3rd floor (Hynes)
9:30 AM - **W10.1
Nanowire Energy Science and Engineering.
Yi Cui 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractNanowires are a family of highly engineered nanomaterials and afford the great opportunities for controlling electronic, photonic, mechanical and ionic processes, which are important for energy applications. Here I will present examples on high performance and/or low-cost nanowire energy applications including transparent electrode, photovoltaics, batteries, supercapacitors and large-scale energy storage devices.
10:00 AM - W10.2
Self-assembled GaAs Nanowire Bulk Heterojunction Solar Cell.
Shenqiang Ren 1 , Ni Zhao 2 , Samuel Crawford 1 , Vladimir Bulovic 2 , Silvija Gradecak 1
1 Materials Science and Engineering, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts, United States, 2 Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States
Show AbstractHybrid solar cells combine inorganic semiconductors and organic polymers and could offer the advantages of low production cost, facile solution processing and high efficiency. We demonstrate a novel architecture for hybrid nanowire-polymer bulk heterojunction solar cells composed of blends of GaAs nanowires and a conjugated polymer P3HT, in which the internal nanowire structure controls nanoscale blending and morphology of conjugated polymer in the active layer. Drying-mediated fabrication at a higher nanowire concentration yields self-aligned nanowires, which improves device performance. The enhanced photocurrent is consistent with improved electron transport perpendicular to the plane of the film. The radial shell of TiOx grown on the GaAs nanowires by sol-gel chemistry has a significant impact on the operating characteristics of our photovoltaic devices, and an eightfold increase in short circuit current was observed after the shell coating. We find that the nanowire cells attain solar power conversion efficiencies of 2.36% under 70 mW/cm2 white LED illumination for a device containing 50 wt% of TiOx-coated nanowires. With morphology addressed directly through thermal annealing, rather than templated array processing, the resulting devices offer significant practical advantages in fabrication and processing as compared to previous hybrid organic-inorganic composite solar cells.
10:15 AM - W10.3
Thermochromism as a Route to Engineering Interfaces in Conducting Polymer-semiconductor Nanowire Nanohybrid Systems.
Christopher Rodd 1 , Ritesh Agarwal 1
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractInterfaces in organic-inorganic systems have long been considered crucial to improving performance of bulk heterojunction devices such as photovoltaic cells. Research in this area has been hampered by an inability to create model single nanostructure materials in order to observe and probe this interface directly. Currently, single nanohybrid materials are generally made by grafting compatiblizing molecule to the generally incompatible organic and inorganic components. This technique allows easy formation of single nanohybrids but inherently changes the nature of the organic-inorganic interface. We believe we have found a method to circumvent this incompatibility in order to study the interface in a manner that can be directly applied to bulk heterojunction devices as they are traditionally made today. We have used bulk phase separation at the nanowire surface to create semicoductor-polymer nanohybrids with photovoltaic characteristics. Specifically, we have used poly (3-hexylthiophiene)-2,5-diyl (P3HT) in solution to create a shell of conducting polymer around cadmium sulfide (CdS) nanowire grown via the vapor-liquid-solid (VLS) method. Characterization via high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) shows an ~30nm amorphous P3HT shell around an ~60nm CdS wire can be produced. Additionally, creation of nanohybrids at different temperatures shows an increase in crystallinity at the nanowire-polymer interface. We attribute this to the thermochromic properties of P3HT. In the concentration regime understudy, at room temperature and above, the thiophene rings of the P3HT main chain are generally unaligned and the polymer takes on a more random coil conformation. Below room temperature, a greater proportion of thiophene rings align with respect to each other along the main chain of the polymer. Our HRTEM observations suggest that this rod-coil transistion by temperature (thermochromism) allows for more orientation of the polymer chain in the immediate vicinity of the nanowire surface. This thermochromism present in conducting polymers such as P3HT presents an interesting, new pathway to engineering nanohybrid interfaces that should be easily scalable to bulk heterojunction devices.
10:30 AM - W10.4
Iron Oxide-silicon Nanowires Heterojunction Photoelectrode for Water Splitting.
Kimin Jun 1 2 , Joseph Jacobson 2
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 Media Lab, MIT, Cambridge, Massachusetts, United States
Show AbstractSolar-water splitting is viewed as an efficient way to generate renewable chemical fuel. However, materials challenge arises since most popular photovoltaic materials cannot meet all the requirements at the same time: chemical stability and water redox level-semiconductor energy band matching. Therefore, heterojunction photoelectrodes have been tried based on possible materials sets. Silicon nanowires array is one of the preferred base materials due to its predicted high efficiency and high conduction bandedge position. As matching photoanode material, iron oxide(hematite) has several advantages. Its bandgap is narrow compared to other wide bandgap semiconductors, which provides higher light absorption possibility. And its oxide nature provides higher chemical stability than III-V counterparts. The downside of iron oxide lies in its low conductivity and short carrier lifetime. To overcome these problems, doping, thickness and crystallinity must be properly controlled. Also, the film should be conformally deposited for nanowire compatibility. In this research, for the first time, silicon nanowires array-iron oxide photoelectrode was fabricated in controlled manner. First, silicon wafer was patterned by optical lithography and gold film was deposited. Silicon nanowires array was prepared by metal assisted silicon etching. To improve carrier separation, this array was doped by spin-on-dopant to form core-shell n-p junction, and the annealing temperatures were adjusted to control junction depth. Then, iron oxide film were conformally deposited through atmospheric chemical vapor deposition(APCVD) method. During the APCVD process, titanium precursor was mixed with iron precursor to make controllable titanium doping. In our measurement, up to 5.5:1(Fe:Ti atomic %) were observed. Also, slow film growth allows thickness control on the order of tens of nanometers. Initially amorphous iron oxide film was crystallized in thermal annealing to decrease carrier scattering. All the thermal processing temperatures were designed to be low enough not to affect the previous conditions. Photocurrent measurement will be presented, and we believe our flexibility on parameters control enables to search optimal conditions for high efficiency.
10:45 AM - W10.5
The Fabrication and Characterization of ZnO/CuO Heterostructured Nanowire Solar Cells.
Kuo-ting Liao 1 , Paresh Shimpi 1 , Pu-Xian Gao 1
1 Chemical,Materials & Biomolecular Engineering and Institute of Material Science, University of Connecticut, Storrs, Connecticut, United States
Show AbstractZnO/CuO heterostructured nanowires have been synthesized on ZnO:ITO/glass substrates. A hydrothermal method has been used firstly to grow ZnO nanowires followed by the sputtering deposition of Cu nanofilm, which was annealed in various temperatures and atmospheres to form different quality CuO nanoshells. Using carbothermal vapor deposition, CuO nanowires are grown between those ZnO/CuO heterostructured nanowires. X-ray diffraction analysis and electron microscopy were systematically conducted to confirm the core-shell structure of these heterostructured nanowires and the existing CuO nanowires. The electrical characterization of ZnO/CuO heterostructured nanowires revealed a good rectifying current-voltage response, confirming the formation of p-n junction. Absorption spectrum shows that heterostructured nanowires has a higher absorption efficiency in visible region, whereas absorption efficiency decreases in UV region compared to pure ZnO nanowires as a result of the formation of core-shell heterostructured CuO/ZnO nanowires. Photovoltaic properties of fabricated CuO/ZnO solar cells have been characterized. The fabricated CuO/ZnO heterostructured nanowires could be useful nanoscale building blocks in photovoltaic devices.
11:30 AM - W10.6
Limiting Factors in the Photovoltaic Efficiency of Practical Core-shell Nanowire Solar Cells.
Ray LaPierre 1 , Josef Czaban 1
1 Engineering Physics, McMaster University, Hamilton, Ontario, Canada
Show AbstractA comprehensive numerical simulation of the current-voltage (J-V) characteristic curves of III-V nanowire core-shell p-n junction diodes is presented and compared to experimental J-V data. Surface defect states on the nanowire sidewalls and recombination in the thin film between the nanowires are found to play a dominant role in the dark and illuminated J-V characteristics. The core-shell structure is depleted by a pinch-off effect that acts in conjunction with carrier recombination in the film to create an anomalous kink in the J-V curves. This effect reduces the fill factor and open circuit voltage in the illuminated J-V characteristic and is responsible for limiting the energy conversion efficiency in practical nanowire-based photovoltaic devices. Potential methods of overcoming these limitations will be presented including nanowire geometry, orientation, and growth methods.
11:45 AM - W10.7
Core-shell ZnO/Fe2O3 Nanowires for Efficient Water Splitting.
Yongjing Lin 1 , Jin Xie 1 , Sa Zhou 1 , Staff Sheehan 1 , Dunwei Wang 1
1 Chemistry, Boston College, Chestnut Hill, Massachusetts, United States
Show AbstractThe ever-depleting reserves and the devastating environmental effects caused by burning fossil fuels – the dominating energy supply we rely on – has necessitated the development of new energy sources or carriers. Among energy forms that have been investigated, solar H2 from H2O splitting is particularly appealing as it utilizes the renewable solar energy and is environmentally friendly. Recently one dimensional nanostructures, such as nanowires or nanotubes, have showed promising results to improve the efficiency of solar water splitting due to the enhanced absorption and charge transfer process. Here we report our successful synthesis of ZnO/Fe2O3 core-shell nanowires as highly efficient photoanode for water splitting. Vertically aligned ZnO nanowires are synthesized by simple hydrothermal method. Ultrathin iron oxide is deposited by atomic layer deposition as active photoanode. Our preliminary result indicates that such core-shell nanowires are much more efficient than planar device due to efficient light absorption and reduced charge recombination, especially at longer wavelength where the penetration depth of photon is large. Detail studies on the length of ZnO nanowires, effective doping and thickness of iron oxide are performed in order to optimize the performance. Our results shall inspire new nanomaterials by design for energy-related applications.
12:00 PM - W10.8
SiGe Nanowires For Solar Cell Technology: A First Principles Study.
Rengin Pekoz 1 , Jean-Yves Raty 1
1 , Universite de Liege, Liege Belgium
Show AbstractVery recently, third generation solar cells based on quantum dots and nanowires (NWs) have been started to be investigated for the possibility of being replaced with the currently used Si- and thin-film- based photovoltaic cells because of their efficiency and cost problems [1,2]. Among the other possible candidates, NWs have an important advantage due to their carrier confinement in two dimensions and their very large aspect ratio which result in easiness of the radial charge collection and light absorption. Moreover, since the controlled growth of vertical NWs has been achieved, it is believed that this property can decrease the optical reflection by multi-bouncing which results in more absorption of light [3,4].In this work, two different geometries will be presented as promising models for the future solar cell technology. Triangularly shaped, layered SiGe NWs and cylindrical Si/Ge and Ge/Si core/shell NWs with different orientations and diameters have been investigated using first-principles methods. The electronic and optical properties of these systems will be discussed in detail. For both of the NWs, strong charge separation has been observed with valence band state localized on Ge atoms and conduction band states preferred to localize on Si atoms. The absorption spectrum have been analyzed and qualitatively compared to the bulk Si for quantum efficiency in order to predict their relative performances.[1] A. Shah, P. Torres, R. Tscharner, N. Wyrsch, and H. Keppner, Science 285, 692 (1999).[2] B. Tian, X. Zheng, T. J. Kempa, N. Yu, G. Yu, J. Huang, and C. M. Lieber, Nature 449, 885 (2007).[3] L. Hu and G. Chen, Nano Lett. 7, 3249 (2007).[4] K. Peng, X. Wang, and S.T. Lee, Appl. Phys. Lett. 92, 163103 (2008).
12:15 PM - W10.9
Planar Waveguide-nanowire Integrated Three-dimensional Dye-sensitized Solar Cells.
Yaguang Wei 1 , Chen Xu 1 , Sheng Xu 1 , Cheng Li 1 , Wenzhuo Wu 1 , Zhong Lin Wang 1
1 MSE, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractRenewable and green energy are the technological drivers of the future economy. Solar cells (SCs) are one of the most important sustainable energy technologies that have the potential to meet the world’s energy demands. Among the various SC approaches, excitonic SCs, including organic and dye sensitized solar cell (DSSC), appear to have significant potential as a low-cost alternative to conventional inorganic photovoltaic devices. The traditional DSSCs are based on a two-dimensional (2D) planar substrate, which has a relatively low surface area limiting the energy conversion efficiency of cells. Here we present an innovative hybrid structure integrating optical waveguide and nanowire (NW) arrays as three-dimensional (3D) DSSCs that have significantly enhanced energy conversion efficiency. The ZnO NWs are grown normal to the quartz slide. The 3D cell is constructed by alternatively stacking the slide and a planar electrode. The slide serves as a planar waveguide for light propagation. The 3D structure effectively increases the light absorbing surface area due to internal multiple reflections without increasing electron path length to the collecting electrode, resulting in a significant improvement in energy conversion efficiency by a factor of 5.8 in average compared to the planar illumination case; and the full sun efficiencies have been achieved up to 2.4%. It is possible to replace the quartz slide with highly transparent polymer substrates. The waveguide-NW 3D architecture provides a general approach for fabricating high efficiency, large scale excitonic SCs, such as dye sensitized and organic SCs. [1] Benjamin Weintraub, Yaguang Wei, and Zhong Lin Wang* ”Optical Fiber-Nanowire Hybrid Structures for Efficient Three-Dimensional Dye-Sensitized Solar Cells”, Angew Chem, 48 (2009) 8981-8985.[2] Yaguang Wei, Chen Xu, Sheng Xu, Cheng Li, Wenzhuo Wu and Zhong Lin Wang, Nano Letters, online, 2010[3] For more information: http://www.nanoscience.gatech.edu/zlwang/
12:30 PM - W10.10
Schottky Solar Cell with Embedded Ge Nanowires.
Juhyung Yun 1 , Joondong Kim 2 , Yun Chang Park 3 , Yong Jae Cho 4 , Jeunghee Park 4 , Chang-Soo Han 2 , Wayne Anderson 1
1 Dep. of Electrical Engineering, University at Buffalo, Buffalo, New York, United States, 2 Nano-Mechanical Systems Research Center, Korea Institute of Machinery and Materials (KIMM), Daejeon Korea (the Republic of), 3 Measurement and Analysis Division, National Nanofab Center (NNFC), Daejeon Korea (the Republic of), 4 Department of Chemistry, Korea University, Jochiwon Korea (the Republic of)
Show AbstractLarge area applicable Ge nanowire (GeNW)-embedded Schottky solar cells (SCs) were fabricated by a simple solution process. Electrical and optical characteristics were performed to single and multiple GeNW Schottky SCs consisted of GeNWs sitting on two different metal systems forming a Schottky contact between Al and Ge and an Ohmic contact between Pt and Ge. In a dark condition, the multiple GeNW Schottky SCs showed diode characteristics with averaging of the current transport characteristics of individual nanowires (NWs). Under one sun illumination, the GeNW Schottky SC gave a 19.2 nA short circuit current (Isc) corresponding to 6.69 mA/cm2 of current density, resulting from adding up the tiny photo-currents collected from parallel GeNW Schottky junctions showing 0.167V of open circuit voltage (Voc). The paper discusses fabrication schemes and the characteristics of multiple GeNWs-embedded Schottky SCs. Ultimately, GeNW Schottky SCs would provide the potential approach for highly efficient SC by harvesting infrared (IR) light simply mounted on higher bandgap material.
W11: Nanowire Large Scale Integration
Session Chairs
Thursday PM, December 02, 2010
Ballroom C, 3rd floor (Hynes)
3:00 PM - W11.2
Large-Scale Assembly of Nanowire-based Integrated Flexible Devices using Conventional Microfabrication Facilities.
Kwang Heo 1 , Jikang Jian 9 , Jee Woo Park 8 , Eunhee Cho 7 , Jee-Eun Yang 6 , Myoung-Ha Kim 5 , Hyungwoo Lee 2 , Byung Yang Lee 2 , Soon Gu Kwon 4 , Moon-Sook Lee 7 , Moon-Ho Jo 6 , Heon-Jin Choi 5 , Taeghwan Hyeon 4 , Seunghun Hong 1 2 3
1 Interdisciplinary Program in Nano-Science and Technology, Seoul National University, Seoul Korea (the Republic of), 9 College of Physical Science and Technology, Xinjiang University, Xinjiang China, 8 Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 7 Semiconductor R&D Center, Samsung Electronics Co. Ltd., Gyeonggi-Do Korea (the Republic of), 6 Department of Materials Science and Engineering, POSTECH, Pohang Korea (the Republic of), 5 Department of Materials Science and Engineering, Yonsei University, Seoul Korea (the Republic of), 2 Department of Physics and Astronomy, Seoul National University, Seoul Korea (the Republic of), 4 School of Chemical and Biological Engineering, Seoul National University, Seoul Korea (the Republic of), 3 Department Biophysics and Chemical Biology, Seoul National University, Seoul Korea (the Republic of)
Show AbstractThe directed-assembly process of nanowires is a key technology needed for the fabrication of nanowire-based integrated devices. Recently, diverse methods for assembling nanowires have been proposed. However, most of previous assembly methods can generate only specific-shape devices and require unconventional facilities, which has been a major hurdle for industrial applications. Herein, we propose a new method for assembling nanowires on flat and flexible substrate in virtually-general shape patterns using only conventional microfabrication facilities. In this method, single-crystalline nanowires were functionalized with self-assembled monolayer and dispersed in D.I. water. The functionalized nanowires selectively adsorbed onto polar surface regions on the substrates. As a proof of concepts, we demonstrated transistors based on individual Si-NWs and long networks of Si-NWs. And we also demonstrated the fabrication of highly-bendable top-gate transistors based on Si-NWs. These devices showed typical n-type semiconductor behaviors, and exhibited a much lower noise level compared to previous flexible devices based on organic conductors or other nanowires. In addition, the gating behaviors and low-noise characteristics of our devices were maintained, even under highly bent conditions.
3:15 PM - W11.3
Ni Catalyzed Growth of Si1-xGex Nanowire Heterostructure Arrays for Flexible Schottky Photodiode Applications.
Sang-Won Jee 1 , Joondong Kim 2 , Han-Don Um 1 , Kwang-Tae Park 1 , Jin-Young Jung 1 , Yoon-Ho Nam 1 , Min-Joon Park 1 2 , Chang-Soo Han 2 , Jung-Ho Lee 1
1 Department of Materials and Chemical Engineering, Hanyang university, Ansan Korea (the Republic of), 2 Nano-Mechanical Systems Research Center, Korea Institute of Machinery and Materials, Daejeon Korea (the Republic of)
Show AbstractWe report the vertical growth of Ni-catalyzed Si1-xGex nanowire heterostructures by vapor liquid solid mechanism utilizing SiCl4 and GeCl4 liquid precursors under atmospheric pressure. Self-assembled liquid germanosilicide (NiSiGe) particles seeded the growth of Si1-xGex nanowires and excess Ge simultaneously segregated along parallel axis upon cooling, which resulted morphologically in three parts, i.e., Ni(Si1-zGez) tip, Ge-rich Si1-yGey, and Si1-xGex nanowire. The composition of Si1-xGex nanowire could be readily modulated from x = 0.04 to x = 0.36. A Ge-rich Si1-yGey (0.2 < y < 0.9) region took place preferentially at the Ni(Si1-zGez)/Si1-xGex interface by Ge out diffusion from liquid NiSiGe during solidification. The length of Si1-yGey could be controlled from 20 nm to a several μm via controlling the GeCl4 partial pressure. Catalyst tips atop the nanowire remained thermodynamically as a form of mono-germanosilicide (Ni(Si1-zGez)) because the formation energy of NiSi was lower than that of NiGe so that the free energy could be decreased by having more NiSi and less NiGe, i.e., by expelling Ge from liquid NiSiGe. This resulting structure formed the low schottky barrier height (<0.5 eV) between Ni(Si1-zGez) tip and Si1-yGey, and could be modulated by varying the Ge concentration. For flexible applications, polyimide embedded Si1-xGex nanowire arrays were prepared for characterizing the Ni germanosilicided Schottky barrier diode. Current-voltage properties of the flexible nanowire photodiodes were also investigated using a solar simulator under AM 1.5G and 100 mA/cm2 after detaching the nanowire films from the substrate. Transparent conductive oxide was deposited as a top electrode onto a exposed tip of polyimide embedded nanowire arrays. Al was used as a back electrode for ohmic contact.
3:30 PM - W11.4
Electrical Characteristics of NOT Logic Circuits Based on Silicon Nanowire Arrays Constructed on Flexible Plastic Substrates.
Youngin Jeon 1 , Jeongmin Kang 1 , Sangsig Kim 1
1 Electrical Engineering, Korea University, Seoul Korea (the Republic of)
Show AbstractA NOT logic circuit made of arrayed silicon nanowires (SiNWs) on a plastic substrate is characterized in this study. Arrays of triangular shaped n-type SiNWs were obtained from an n-type Si wafer through photolithography and anisotropic wet etching processes. Each of the arrayed SiNWs had a diameter of 150 nm and a length of 300 μm. The arrays of the SiNWs were transferred onto a plastic substrate. A NOT logic circuit fabricated in this work with Al2O3 gate-dielectric layers is composed of two identical top-gate SiNWs field-effect transistors (FETs) in series. For the identical FETs, the Ion/Ioff characteristics were as high as ~104, and the electron mobility was estimated to be 37 cm2/Vs in average. And for the NOT logic circuit, the transition width was 4 V, and the logic swing between the high and low states was 40%.
3:45 PM - W11.5
Study and Comparison of Guided Silicon Nanostructures for MOS Devices: CVD versus Epitaxy.
Paul-Henry Morel 1 , Jean-Michel Hartmann 1 , Christine Morin 2 , Pascal Faucherand 2 , Simon Perraud 2 , Adeline Grenier 1 , Thierry Baron 3 , Bassem Salem 3 , Murielle Fayolle-Lecocq 1 , Thomas Ernst 1
1 , CEA, LETI, MINATEC, 17 rue des Martyrs, 38054 Grenoble, Cedex 9, France, 2 , CEA, LITEN, 17 rue des Martyrs, 38054 Grenoble, Cedex 9, France, 3 , LTM, UMR 5129, CEA-Grenoble, CNRS, 17 rue des Martyrs, 38054 Grenoble, Cedex 9, France
Show AbstractSilicon nanostructures have extensively been studied for the last decade due to their interest in nanoelectronics, biology, optics, energy storage and detectors. Silicon nanowires (NWs) can notably be used as vertical transistors thanks to their dimensions and crystalline properties. In this study the morphology of Chemical Vapour Deposition (CVD) grown NWs is compared to that of pillars fabricated thanks Selective Epitaxial Growth (SEG) in high aspect ratio holes. The impact of the two processes on electrical properties will otherwise be assessed thanks to the fabrication of Metal Oxide Semiconductor capacitors. The nanostructures are grown in similar conditions with the two techniques. A 1 µm thick SiO2 layer is deposited on a Si wafer then patterned thanks to deep-UV photolithography and etching. The template thus consists in holes through the oxide to the (100) p-type doped silicon substrate with diameters ranging from 250 nm to 1 µm. Prior to CVD, the holes are filled by 200 nm of gold thanks to a specially developed selective process. NWs and pillars are grown at temperatures ranging from 650°C to 950°C with HCl and either silane (CVD) or dichlorosilane (SEG) as the silicon gaseous precursor. CVD NWs are 1 to 2 µm long with diameters ranging from 250 nm up to 1 µm. They are perpendicular to the substrate only inside the holes. They have a common sawtooth faceting and polygonal cross-section. Electron Tomography analysis on the top of the nanostructures reveals gold clusters on some faces and not on others. This phenomenon is likely due to surface energy differences. In contrast, SEM characterizations show that SEG pillars have the same dimensions as the holes. Cross-sections reveal a parasitic growth of poly-silicon on SiO2 (removed by polishing), together with voids at the interface with the oxide template. Top views show circular and square pillar shapes. After gold removal on CVD nanowires, we have deposited the same capacitance stack on the two types of structures. Alumina is used as the dielectric and titanium nitride with aluminium as the top electrode. The capacitance stack is first characterized by SEM cross-sections showing from 7 to 20 nm of alumina and from 200 nm to 250 nm of metal top electrode. Capacitance voltage measurements at frequencies from 100 Hz to 1 MHz have been acquired for both types of NWs and for planar structures without NWs. We have obtained MOS capacitance curves showing differences due to NW morphologies and defects. In conclusion, we have identified the main morphological differences between NWs grown by CVD and SEG in an oxide template. We have studied the electrostatic control of a metal oxide stack on the nanostructures and the impact of structural and surface defects on the electrical properties.
4:30 PM - **W11.6
Electric Field-Assisted Deterministic Nanowire Assembly: Physics and Applications.
Theresa Mayer 1 2 , Boone Won 1 , Jaekyun Kim 1 , Thomas Morrow 3 , Kaige Sun 1 , Scott Levin 2 , Christine Keating 3 , Jeffrey Mayer 1
1 Electrical Engineering, Penn State University, University Park, Pennsylvania, United States, 2 Materials Science and Engineering, Penn State University, University Park, Pennsylvania, United States, 3 Chemistry, Penn State University, University Park, Pennsylvania, United States
Show AbstractDeterministic assembly of different types of batch-synthesized nanowires into complex 2D and 3D structures may lead to new electronic circuits and optical devices with novel properties that are difficult to obtain using conventional top-down fabrication methods. For example, integrating functionalized semiconductor nanowires with silicon CMOS circuits could be used to create advanced chips with chemical and biological sensing capabilities. Alternatively, the 3D assembly of multi-component metal nanowires would enable the design of sophisticated metamaterials with anisotropic optical properties. Realizing these goals requires new techniques to control the placement of individual wires at predefined locations on a substrate with high yield and reproducibility.This talk will discuss the use spatially-confined electric field forces to position individual nanowires in high density arrays with submicron registration accuracy to lithographic features on the chip. It will begin with an overview of an experimental and theoretical study that quantified the long-range dielectrophoretic forces and shorter-range electrostatic forces that govern the deterministic nanowire assembly process. Next several different lithographic structures that were designed to tailor the electric-field forces that control the deterministic nanowire assembly process will be described. The talk will conclude by discussing the application of this technique to the on-chip integration of multiplexed nanowire biosensor device arrays and the prospects of extending this approach to 3D nanowire assembly of electronic and optical devices.
5:00 PM - W11.7
Chemical Warfare Biosensor: Enzymes Immobilized on Polyethylenedioxythiophene Nanowires Grown on Doped Nanodiamond Films.
Pedro Villalba 3 4 , Manoj Ram 1 2 , Humberto Gomez 2 4 , Amrita Kumar 5 , Venkat Bhethanabotla 3 , Ashok Kumar 1 2
3 Chemical and Biomedical Engineering, University of South Florida, Tampa, Florida, United States, 4 Departamento de Medicina, Universidad del Norte, Barranquilla Colombia, 1 Nanotechnology Research & Education Center, University of South Florida, Tampa, Florida, United States, 2 Mechanical Engineering, University of South Florida, Tampa, Florida, United States, 5 Center for Cell and Molecular Signaling, Emory University , Atlanta, Georgia, United States
Show AbstractOne dimensional (1D) materials such as nanowires have been considered as very promising candidates for construction of high sensitive chemical and biological sensors. Amongst, 1D –conducting polymer nanowires holds the special properties of functionalization with wide range of chemical groups, therefore sensitive and specific sensor could be designed. Recently, we have developed polyethylenedioxythiophene (PEDOT) conducting polymer nanowires using in-situ self-assembled technique. Besides, doped nanodiamond (NND) possesses a unique combination of properties i.e., mechanical stability, high corrosion resistance, chemical inertness and biocompatibility which makes it highly suitable for applications in medicine and biosensors. Recently, we have also fabricated glucose sensor on nanodiamond film.In this study we are proposing an integrated solution for the detection and quantification of dimethyl methylphosphonate (DMMP), a stimulant to chemical warfare agent, to offer advantages of higher sensitivity, easy to use, ease of signal processing with better sensor-signal integrity and smaller in size,. Under this investigation nitrogen doped nanodiamond (NND) were deposited on N-type silicon, and later PEDOT nanowires was deposited using in-situ self assembly technique. The thickness of PEDOT film on NND/N-Si is tailored using our recently developed technique. The enzymes were functionalized on nanowires using self-assembly technique. The PEDOT/NND films deposited on N-doped silicon before and after enzyme immobilization were characterized by using Raman spectroscopy, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), FTIR and electrochemical techniques. The choline oxidase and cholinesterase enzymes co-immobilized enzymes on PEDOT nanowires/NND structures lead to the production of H2O2 with reaction to DMMP and oxygen molecules which could be amperometrically sensed. The aim of the present study was to investigate the effect of enzyme on surfaces of nanowires PEDOT on NND film on N-type silicon substrate. The calibration curve of ppm to ppb level has been studied using the novel electrode PEDOT nanowires/NND structured films. Further, the cell culture toxicity testing methods was applied to the develop PEDOT-based DMMP biosensor. The toxic effect of a multi-component was spatially visualized with mammalian cell monolayers. Further, freshly prepared electrode was also tested against long-term stored biosensor. Our study has opened the door to exploit nanowires growth and immobilization of enzymes on conducting polymer using in-situ self-assembly technique on ND for chemical warfare biosensor.
5:15 PM - W11.8
Silicon Nanowires Using CMOS Technology for Label-free Detection of Biomolecules.
Per Bjork 1 , Tommy Schoenberg 1 , Nima Jokilaakso 4 , Rodabeh Afrasiabi 3 , Si Chen 2 , Yong-Bin Wang 3 , Shi-Li Zhang 2 , Amelie Eriksson Karlstrom 4 , Christian Vieider 1 , Jan Linnros 3
1 Nanoelectronics, ACREO AB, Kista Stockholm Sweden, 4 School of Biotechnology, Royal Institute of Technology, Stockholm Sweden, 3 Materials Physics, ICT School, Royal Institute of Technology, Kista-Stockholm Sweden, 2 Department of Engineering Sciences, Uppsala University, Uppsala Sweden
Show AbstractThere is an increasing demand for portable sensor systems in health care that enable on-site analyses e.g. at local medicare centers, in ambulances or even at home. Silicon nanowires (SiNW) offer a highly sensitive and label free detection principle for biochemical applications that makes it possible to scale down the size of complete analysis systems. Thus SiNW devices show great potential in effectively providing rapid on-site detection to avoid costly laboratory analyses.The sensor function is similar to that of a MOS-transistor where the sense current is dependent on the gate voltage. For SiNW the current is dependent on the surface charges of the thin nanowires. When charged molecules are present at the surface, the conductance of the nanowire is affected which is detected as a change in channel current. Functionalization of the SiNW with antibodies activates the sensor for detection of specifically charged target molecules. With multiple SiNW functionalized with different antibodies several different target molecules can be detected simultaneously.The SiNW were fabricated using standard CMOS technology on SOI (Silicon On Insulator) wafers where the top silicon layer has been thinned down to less than 50 nm. The width of the nanowires is in the range 100 nm up to a few micrometers. By using a microfluidic delivery system that combines hard (silicon, resists and plastics) and soft (PDMS) materials, the sample volume can be minimized in order to take full advantage of the very high detection sensitivity. Our encapsulation process is compatible with the limitations in heat, process and solvent compatibility that are found in biochemically functionalized structures. The fabrication process also takes advantage of the high alignment accuracy provided by MEMS technology enabling well defined fluidic channels that can be connected to the macro world by using a standard tubing interface.We have so far characterized the sensor using buffer solutions at different pH concentrations and also using model systems such as the biotin-streptavidin interaction. For this we have previously [1] demonstrated a sensitivity in the picomolar regime, very much dependent on the thickness of the thinned top silicon layer. In the near future, we will address point of care health applications, such as detection of cancer markers in blood serum. However, the detection principle is very general and can easily be adapted to detecting contaminants, bacteria, drugs, DNA or different proteins.[1] N. Elfström, A. Eriksson Karlström, and J. Linnros, Nano Letters 8, 945 (2008).
5:30 PM - W11.9
Green Light Emission from Regularly Arranged Nanocolumn Array LEDs.
Kouji Yamano 1 3 , Katsumi Kishino 1 2 3 , Kazuya Nagashima 1 , Meiki Goto 1 , Akihiko Kikuchi 1 2 3
1 engineering and applied sciences, Sophia University, Tokyo-to Japan, 3 , CREST Japan Science and Technology Agency, Saitama-ken Japan, 2 , Sophia Nanotechnology Research Center, Tokyo-to Japan
Show AbstractIII-nitride nanocolumns [1] are very attractive for applications to LED, because they are dislocation-free having high optical property. We demonstrated visible nanocolumn LEDs on n-type (111) Si substrates, using self-assembled nanocolumns [2], [3]. Recently, selective area growth (SAG) technique of GaN has been developed with rf-plasma-assisted molecular beam epitaxy (rf-MBE) [4], fabricating regularly arranged GaN nanocolumn arrays. This achievement will contribute to the improved performance of InGaN-based nanocolumn LEDs.In this talk, we describe the demonstration of InGaN-based regularly arranged nanocolumn array LEDs, observing green light emission (520-540nm in wavelength) under DC current injection at room temperature. C-plane MOCVD-grown GaN templates were employed as substrates. Prior to the growth, a thin Titanium Oxide (TiO2) layer (thickness less than 10nm) was evaporated on the GaN surface, followed by the fabrication of triangular lattice nanohole patterns with various hole diameters and lattice constants through electron beam lithography and plasma etching process. Then the regularly arranged nanocolumn array LED crystals were grown by SAG of rf-MBE. The nanocolumn LEDs consisted of Si-doped n-type GaN nanocolumn, InGaN/GaN (25pairs) super-lattice layer, InGaN/GaN multi-quantum-well (MQW: 3 quantum wells) active layer, 20nm Mg-doped p-type AlGaN electron blocking layer, and finally 300nm-thick Mg-doped p-type GaN layer. The nanocolumn diameter was controlled from 230nm to 320nm at the height of 1.6 um. Then Ti/Al/Ti/Au metal electrode was formed on the etched off surface of underlying n-type GaN and ITO transparent circle electrode of 65um-diameter on p-type GaN layer. Under the DC current injection up to 15mA, pure green light single peak emission was observed from ITO window for devices with various nanocolumn diameters and lattice constants. The emission wavelengths were 525~545nm and the FWHM values were 30~45nm. When the pulse current was injected up to 250mA, the stable green emission peak spectra appeared. The emission peak shift under 10nm was observed under the high injection condition.Acknowledgement This study was supported by Grants-in-Aid for Scientific Research on Priority Areas No.18069010 and (B) #21310087 from the MEXT, Japan.Reference [1] M. Yoshizawa et al., Jpn. J. Appl. Phys. 36 (1997) L459. [2] A. Kikuchi et al., Jpn. J. Appl. Phys. 43 (2004) L1524. [3] K. Kishino et al., Proc. of SPIE, 6473 (2007) p.64730T-1. [4] K. Kishino et. al, J. Cryst. Growth 311 (2009) 2063.