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2011

Purdue University

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Articles 1 - 30 of 74

Full-Text Articles in Nanoscience and Nanotechnology

Geometrical Frustration In Nanowire Growth, K. W. Schwarz, J. Tersoff, S. Kodambaka, Y.C. Chou, F. M. Ross Dec 2011

Geometrical Frustration In Nanowire Growth, K. W. Schwarz, J. Tersoff, S. Kodambaka, Y.C. Chou, F. M. Ross

Birck and NCN Publications

Idealized nanowire geometries assume stable sidewalls at right angles to the growth front. Here we report growth simulations that include a mix of nonorthogonal facet orientations, as for Au-catalyzed Si. We compare these with in situ microscopy observations, finding striking correspondences. In both experiments and simulations, there are distinct growth modes that accommodate the lack of right angles in different ways-one through sawtooth-textured sidewalls, the other through a growth front at an angle to the growth axis. Small changes in conditions can reversibly switch the growth between modes. The fundamental differences between these modes have important implications for control of …

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On The Accuracy Of Classical And Long Wavelength Approximations For Phonon Transport In Graphene, Dhruv Singh, Jayathi Murthy, Timothy S. Fisher Dec 2011

On The Accuracy Of Classical And Long Wavelength Approximations For Phonon Transport In Graphene, Dhruv Singh, Jayathi Murthy, Timothy S. Fisher

Birck and NCN Publications

This paper presents a critical evaluation of the approximations usually made in thermal conductivity modeling applied to graphene. The baseline for comparison is thermal conductivity computations performed using a rigorous calculation of three-phonon scattering events and accounting for the anharmonicity of interatomic forces. Three central assumptions that underlie published theories are evaluated and shown to compromise the accuracy of thermal conductivity predictions. It is shown that the use of classical phonon occupation statistics in place of the Bose-Einstein distribution causes the overprediction of specific heat and the underprediction of phonon relaxation time; for ZA phonons, the classical approximation can underpredict …

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Electronic Structure Of Realistically Extended Atomistically Resolved Disordered Si:P Delta-Doped Layers, Sunhee Lee, Hoon Ryu, Lloyd Hollenberg, Michelle Simmons, Gerhard Klimeck Nov 2011

Electronic Structure Of Realistically Extended Atomistically Resolved Disordered Si:P Delta-Doped Layers, Sunhee Lee, Hoon Ryu, Lloyd Hollenberg, Michelle Simmons, Gerhard Klimeck

Birck and NCN Publications

The emergence of scanning tunneling microscope (STM) lithography and low temperature molecular beam epitaxy (MBE) opens the possibility of creating scalable donor based quantum computing architectures. In particular, atomically precise Si:P monolayer structures (delta-doped layers) serve as crucial contact regions and in-plane gates in single impurity devices. In this paper we study highly confined delta-doped layers to explain the disorder in the P dopant placements in realistically extended systems. The band structure is computed using the tight-binding formalism and charge-potential self-consistency. The exchange-correlation corrected impurity potential pulls down subbands below the silicon valley minima to create impurity bands. Our methodology …

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Effects Of Carbon Nanotube-Tethered Nanosphere Density On Amperometric Biosensing: Simulation And Experiment, Jonathan Caussen, James Hengenius, Monique Wickner, Timothy Fisher, David Umulis, Marshall Porterfield Nov 2011

Effects Of Carbon Nanotube-Tethered Nanosphere Density On Amperometric Biosensing: Simulation And Experiment, Jonathan Caussen, James Hengenius, Monique Wickner, Timothy Fisher, David Umulis, Marshall Porterfield

Birck and NCN Publications

Nascent nanofabrication approaches are being applied to reduce electrode feature dimensions from the microscale to the nanoscale, creating biosensors that are capable of working more efficiently at the biomolecular level. The development of nanoscale biosensors has been driven largely by experimental empiricism to date. Consequently, the precise positioning of nanoscale electrode elements is typically neglected, and its impact on biosensor performance is subsequently overlooked. Herein, we present a bottom-up nanoelectrode array fabrication approach that utilizes low-density and horizontally oriented single-walled carbon nanotubes (SWCNTs) as a template for the growth and precise positioning of Pt nanospheres. We further develop a computational …

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Spectral Phonon Conduction And Dominant Scattering Pathways In Graphene, Dhruv Singh, Jayathi Murthy, Timothy Fisher Nov 2011

Spectral Phonon Conduction And Dominant Scattering Pathways In Graphene, Dhruv Singh, Jayathi Murthy, Timothy Fisher

Birck and NCN Publications

In this paper, we examine the lattice thermal conductivity and dominant phonon scattering mechanisms of graphene. The interatomic interactions are modeled using the Tersoff interatomic potential and perturbation theory is applied to calculate the transition probabilities for three-phonon scattering. The matrix elements of the perturbing Hamiltonian are calculated using the anharmonic interatomic force constants obtained from the interatomic potential as well. The linearized Boltzmann transport equation is applied to compute the thermal conductivity of graphene for a wide range of parameters giving spectral and polarization-resolved information. The complete spectral detail of selection rules, important phonon scattering pathways, and phonon relaxation …

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Enhanced Valence Force Field Model For The Lattice Properties Of Gallium Arsenide, Sebastian Steiger, Mehdi Salmani-Jelodar, Denis Areshkin, Abhijeet Paul, Tillmann Kubis, Michael Povolotskyi, Hong-Hyun Park, Gerhard Klimeck Oct 2011

Enhanced Valence Force Field Model For The Lattice Properties Of Gallium Arsenide, Sebastian Steiger, Mehdi Salmani-Jelodar, Denis Areshkin, Abhijeet Paul, Tillmann Kubis, Michael Povolotskyi, Hong-Hyun Park, Gerhard Klimeck

Birck and NCN Publications

An enhanced valence force field model for zinc-blende crystals is developed to provide a unified description of the isothermal static and dynamical lattice properties of gallium arsenide. The expression for the lattice energy includes a second-nearest-neighbor coplanar interaction term, the Coulomb interaction between partially charged ions, and anharmonicity corrections. General relations are derived between the microscopic force constants and the macroscopic elastic constants in zinc-blende crystals. Applying the model to gallium arsenide, parameter sets are presented that yield quantitative agreement with experimental results for the phonon dispersion, elastic constants, sound velocities, and Gru ̈neisen mode parameters.

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Effects Of (Nh(4))(2)S Passivation On The Off-State Performance Of 3-Dimensional Ingaas Metal-Oxide-Semiconductor Field-Effect Transistors, J J Gu, A T. Neal, P D. Ye Oct 2011

Effects Of (Nh(4))(2)S Passivation On The Off-State Performance Of 3-Dimensional Ingaas Metal-Oxide-Semiconductor Field-Effect Transistors, J J Gu, A T. Neal, P D. Ye

Birck and NCN Publications

Planar and 3-dimensional (3D) buried-channel InGaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) have been experimentally demonstrated at deep-submicron gate lengths. The effect of (NH(4))(2)S passivation with different concentrations (20%, 10%, or 5%) on the off-state performance of these devices has been systematically studied. 10% (Na(4))(2)S treatment is found to yield the optimized high-k/InP harrier layer interface property, resulting in a minimum subthreshold swing (SS) lower than 100 mV/dec. Moreover, the 3D device structure greatly improves the off-state performance and facilitates enhancement-mode operation. A scaling metrics study has been carried out for 10% (NH(4))(2)S treated 3D devices with gate lengths down to 100 …

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Graphene Formation On Step-Free 4h-Sic(0001), M. L. Bolen, R. Colby, E A. Stach, Michael A. Capano Oct 2011

Graphene Formation On Step-Free 4h-Sic(0001), M. L. Bolen, R. Colby, E A. Stach, Michael A. Capano

Birck and NCN Publications

Step-free SiC was thermally decomposed in vacuum to better understand graphene formation in the absence of step fronts. Atomic force microscopy revealed graphene nucleating at surface pits that preferentially form along SiC{1 $(1) over bar $ 00} planes. The density of these pits is 1 x 10(8)cm(-2), which is three orders of magnitude greater than the measured density of SiC threading dislocations. Additionally, Raman spectroscopy demonstrated that graphene on step-free regions have a redshifted 2D peak position and a smaller peak width than does graphene grown on stepped regions. This difference is attributed to film thickness, which is confirmed by …

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The Significance Of In Situ Conditions In The Characterization Of Gasb Nanopatterned Surfaces Via Ion Beam Sputtering, Osman El-Atwani, Jean Paul, Alex Cimaroli, Anastassiya Suslova, Sami Ortoleva Oct 2011

The Significance Of In Situ Conditions In The Characterization Of Gasb Nanopatterned Surfaces Via Ion Beam Sputtering, Osman El-Atwani, Jean Paul, Alex Cimaroli, Anastassiya Suslova, Sami Ortoleva

Birck and NCN Publications

A systematic study is conducted in order to elucidate the underlying mechanism(s) for nanopatterning with low-energy irradiation of GaSb (100) under normal incidence. Ion energies between 50 and 1000 eV of Ar+ and ion fluences of up to 10(18) cm(-2) were employed. Characterization of the shallow (e.g., 1 to 6 nm) amorphous phase region induced by irradiation and the subsurface crystalline phase region is accomplished with low-energy ion scattering spectroscopy and x-ray photoelectron spectroscopy, respectively. In situ studies are conducted due to the strong chemical affinity for oxygen of GaSb. The studies conclude that at energies below 200 eV, the …

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Laser Direct Write Of Silicon Nanowires, James Mitchell, Se Jun Park, C Adam Watson, Pornsak Srisungsitthisunti, Chookiat Tansarawiput, Minghao Qi, Eric Stach, Chen Yang, Xianfan Xu Oct 2011

Laser Direct Write Of Silicon Nanowires, James Mitchell, Se Jun Park, C Adam Watson, Pornsak Srisungsitthisunti, Chookiat Tansarawiput, Minghao Qi, Eric Stach, Chen Yang, Xianfan Xu

Birck and NCN Publications

Using laser direct writing in combination with chemical vapor deposition to produce nanometer scale electronics holds several advantages over current large scale photolithography methods. These include single step electrical interconnect deposition, mask-less patterning, and parallel processing. When taken together they make quick production of individualized electronic circuits possible. This work demonstrates the ability of combining laser direct write and chemical vapor deposition to produce silicon wires a few hundred nanometers wide. Optimized parameters will be discussed, with a particular emphasis paid to the laser-material interactions. The feasibility for electronic applications will be shown by examining the deposition formation on a …

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Ridge Aperture Antenna Array As A High Efficiency Coupler For Photovoltaic Applications, Edward C. Kinzel, Pornsak Srisungsitthisunti, Xianfan Xu Sep 2011

Ridge Aperture Antenna Array As A High Efficiency Coupler For Photovoltaic Applications, Edward C. Kinzel, Pornsak Srisungsitthisunti, Xianfan Xu

Birck and NCN Publications

Weak absorption of light near the absorption band edge of a photovoltaic material is one limiting factor on the efficiency of photovoltaics. This is particularly true for silicon thin-film solar cells because of the short optical path lengths and limited options for texturing the front and back surfaces. Directing light laterally is one way to increase the optical path length and absorption. We investigate the use of a periodic array of apertures originated from bowtie aperture antennas to couple incident light into guided modes supported within a thin silicon film. We show the presence of the aperture array can increase …

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Isostaticity Of Constraints In Amorphous Jammed Systems Of Soft Frictionless Platonic Solids, Kyle C. Smith, Timothy Fisher, Meheboob Alam Sep 2011

Isostaticity Of Constraints In Amorphous Jammed Systems Of Soft Frictionless Platonic Solids, Kyle C. Smith, Timothy Fisher, Meheboob Alam

Birck and NCN Publications

The average number of constraints per particle < C(total)> in mechanically stable amorphous systems of Platonic solids approaches the isostatic limit at the jamming point (< C(total)> -> 12), though average number of contacts are hypostatic. By introducing angular alignment metrics to classify the degree of constraint imposed by each contact, constraints are shown to arise as a direct result of local orientational order reflected in edge-face and face-face alignment angle distributions. With approximately one face-face contact per particle at jamming, chainlike face-face clusters form with finite extent-a signature of amorphous jammed systems.

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Lifetime Enhanced Transport In Silicon Due To Spin And Valley Blockade, Gabriel Lansbergen, Rajib Rahman, J. Verdujin, Giuseppe Tettamanzi, Nadine Collaert, Serge Biesemans, Gerhard Klimeck, Lloyd Hollenberg, Sven Rogge Sep 2011

Lifetime Enhanced Transport In Silicon Due To Spin And Valley Blockade, Gabriel Lansbergen, Rajib Rahman, J. Verdujin, Giuseppe Tettamanzi, Nadine Collaert, Serge Biesemans, Gerhard Klimeck, Lloyd Hollenberg, Sven Rogge

Birck and NCN Publications

We report the observation of Lifetime Enhanced Transport (LET) based on perpendicular valleys in silicon by transport spectroscopy measurements of a two-electron system in a silicon transistor. The LET is manifested as a peculiar current step in the stability diagram due to a forbidden transition between an excited state and any of the lower energy states due perpendicular valley (and spin) configurations, offering an additional current path. By employing a detailed temperature dependence study in combination with a rate equation model, we estimate the lifetime of this particular state to exceed 48 ns. The two-electron spin-valley configurations of all relevant …

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Experimental And Atomistic Theoretical Study Of Degree Of Polarization From Multilayer Inas/Gaas Quantum Dot Stacks, Muhammad Usman, Tomoya Inoue, Yukihiro Harda, Gerhard Klimeck, Takashi Kita Sep 2011

Experimental And Atomistic Theoretical Study Of Degree Of Polarization From Multilayer Inas/Gaas Quantum Dot Stacks, Muhammad Usman, Tomoya Inoue, Yukihiro Harda, Gerhard Klimeck, Takashi Kita

Birck and NCN Publications

Recent experimental measurements, without any theoretical guidance, showed that isotropic po- larization response can be achieved by increasing the number of QD layers in a QD stack. Here we analyse the polarization response of multi-layer quantum dot stacks containing up to nine quan- tum dot layers by linearly polarized PL measurements and by carrying out a systematic set of multi-million atom simulations. The atomistic modeling and simulations allow us to include cor- rect symmetry properties in the calculations of the optical spectra: a factor critical to explain the experimental evidence. The values of the degree of polarization (DOP) calculated from …

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Quantitative Analysis Of The Disorder Broadening And The Intrinsic Gap For The V=5/2 Fractional Quantum Hall State, N. Samkharadze, J. D. Watson, G. Gardner, Michael J. Manfra, L. N. Pfeiffer, K. W. West, G. A. Csathy Sep 2011

Quantitative Analysis Of The Disorder Broadening And The Intrinsic Gap For The V=5/2 Fractional Quantum Hall State, N. Samkharadze, J. D. Watson, G. Gardner, Michael J. Manfra, L. N. Pfeiffer, K. W. West, G. A. Csathy

Birck and NCN Publications

We report a reliable method to estimate the disorder broadening parameter from the scaling of the gaps of the even and major odd denominator fractional quantum Hall states of the second Landau level. We apply this technique to several samples of vastly different densities and grown in different molecular beam epitaxy chambers. Excellent agreement is found between the estimated intrinsic and numerically obtained energy gaps for the v = 5/2 fractional quantum Hall state. Furthermore, we quantify the dependence of the intrinsic gap at v = 5/2 on Landau-level mixing.

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Nonlinear Thermal Transport And Negative Differential Thermal Conductance In Graphene Nanoribbons, Jiuning Hu, Yan Wang, Ajit Vallabhaneni, Xiulin Ruan, Yong P. Chen Sep 2011

Nonlinear Thermal Transport And Negative Differential Thermal Conductance In Graphene Nanoribbons, Jiuning Hu, Yan Wang, Ajit Vallabhaneni, Xiulin Ruan, Yong P. Chen

Birck and NCN Publications

We employ classical molecular dynamics to study the nonlinear thermal transport in graphene nanoribbons (GNRs). For GNRs under large temperature biases beyond linear response regime, we have observed the onset of negative differential thermal conductance (NDTC). NDTC is tunable by varying the manner of applying the temperature biases. NDTC is reduced and eventually disappears when the length of the GNR increases. We have also observed NDTC in triangular GNRs, where NDTC exists only when the heat current is from the narrower to the wider end. These effects may be useful in nanoscale thermal managements and thermal signal processing utilizing GNRs. …

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Strain Engineering Via Amorphization And Recrystallization In Si/Ge Heterostructures, Yumi Park, Winnie Tan, Alejandro Strachan Sep 2011

Strain Engineering Via Amorphization And Recrystallization In Si/Ge Heterostructures, Yumi Park, Winnie Tan, Alejandro Strachan

Birck and NCN Publications

We use molecular dynamics with the reactive potential ReaxFF to study strain relaxation during the amorphization and recrystallization of silicon-germanium epitaxial nanolaminates. Starting with a coherent heterostructure with (010) Si/Ge interfaces and 3D periodic boundary conditions, we use local heating and quenching to amorphize half of the simulation cell with crystal-amorphous interfaces normal to [001]. We find strain relaxation along [001] as the crystalline Ge section expands into the amorphous material and crystalline Si contracts from it. The amount of strain relaxation correlates with the atomic transport from the amorphized Ge to Si section and increases as the periodic width …

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Amorphous Interface Layer In Thin Graphite Films Grown On The Carbon Face Of Sic, R. Colby, M. L. Bolen, Michael A. Capano, E. A. Stach Sep 2011

Amorphous Interface Layer In Thin Graphite Films Grown On The Carbon Face Of Sic, R. Colby, M. L. Bolen, Michael A. Capano, E. A. Stach

Birck and NCN Publications

Cross-sectional transmission electron microscopy (TEM) is used to characterize an amorphous layer observed at the interface in graphite and graphene films grown via thermal decomposition of C-face 4H-SiC. The amorphous layer does not cover the entire interface, but uniform contiguous regions span microns of cross-sectional interface. Scanning transmission electron microscopy (STEM) images and electron energy loss spectroscopy (EELS) demonstrate that the amorphous layer is a carbon-rich composition of Si/C. The amorphous layer is clearly observed in samples grown at 1600 degrees C for a range of growth pressures in argon, but not at 1500 degrees C, suggesting a temperature-dependent formation …

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Design Of A Gainp/Gaas Tandem Solar Cell For Maximum Daily, Monthly, And Yearly Energy Output, Alexander W. Haas, John R. Wilcox, Jeffery L. Gray, Richard J. Schwartz Sep 2011

Design Of A Gainp/Gaas Tandem Solar Cell For Maximum Daily, Monthly, And Yearly Energy Output, Alexander W. Haas, John R. Wilcox, Jeffery L. Gray, Richard J. Schwartz

Birck and NCN Publications

Solar concentrator cells are typically designed for maximum efficiency under the AM1.5d standard spectrum. While this methodology does allow for a direct comparison of cells produced by various laboratories, it does not guarantee maximum daily, monthly, or yearly energy production, as the relative distribution of spectral energy changes throughout the day and year. It has been suggested that achieving this goal requires designing under a nonstandard spectrum. In this work, a GaInP/GaAs tandem solar cell is designed for maximum energy production by optimizing for a set of geographically-dependent solar spectra using detailed numerical models. The optimization procedure focuses on finding …

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Role Of Surface Orientation On Atomic Layer Deposited Al2o3/Gaas Interface Structure And Fermi Level Pinning: A Density Functional Theory Study, Ganesh Hegde, Gerhard Klimeck, Alejandro Strachan Sep 2011

Role Of Surface Orientation On Atomic Layer Deposited Al2o3/Gaas Interface Structure And Fermi Level Pinning: A Density Functional Theory Study, Ganesh Hegde, Gerhard Klimeck, Alejandro Strachan

Birck and NCN Publications

We investigate the initial surface reaction pathways in the atomic layer deposition (ALD) of Al2O3 on GaAs (111)A and (111)B substrates using precursors trimethylaluminum (TMA) and water to ascertain the effect of surface orientation on device performance. We find that the condition of the respective substrates prior to deposition of TMA and water has a major impact on the surface reactions that follow and on the resulting interface structure. The simulations explain the atomistic mechanism of the interfacial self-cleaning effect in ALD that preferentially removes As oxides. The electronic structure of the resulting atomic configurations indicates states throughout the bandgap …

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Strain Effects On The Phonon Thermal Properties Of Ultra-Scaled Si Nanowires, Abhijeet Paul, Gerhard Klimeck Aug 2011

Strain Effects On The Phonon Thermal Properties Of Ultra-Scaled Si Nanowires, Abhijeet Paul, Gerhard Klimeck

Birck and NCN Publications

The impact of uniaxial and hydrostatic stress on the ballistic thermal conductance (jl) and the specific heat (Cv) of [100] and [110] Si nanowires are explored using a Modified Valence Force Field phonon model. An anisotropic behavior of jl and isotropic nature of Cv under strain are predicted for the two wire orientations. Compressive (tensile) strain decreases (increases) Cv. The Cv trend with strain is controlled by the high energy phonon sub-bands. Dominant contribution of the low/mid (low/high) energy bands in [100] ([110]) wire and their variation under strain governs thebehaviorofjl.VC 2011AmericanInstituteofPhysics.

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Mechanism Of Thermal Conductivity Reduction In Few-Layer Graphene, Druv Singh, Jayathi Y. Murthy, Timothy S. Fisher Aug 2011

Mechanism Of Thermal Conductivity Reduction In Few-Layer Graphene, Druv Singh, Jayathi Y. Murthy, Timothy S. Fisher

Birck and NCN Publications

Using the linearized Boltzmann transport equation and perturbation theory, we analyze the reduction in the intrinsic thermal conductivity of few-layer graphene sheets accounting for all possible three-phonon scattering events. Even with weak coupling between layers, a significant reduction in the thermal conductivity of the out-of-plane acoustic modes is apparent. The main effect of this weak coupling is to open many new three-phonon scattering channels that are otherwise absent in graphene. However, reflection symmetry is only weakly broken with the addition of multiple layers, and out-of-plane acoustic phonons still dominate thermal conductivity. We also find that reduction in thermal conductivity is …

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Relaxation Of Optically Excited Carriers In Graphene, Raseong Kim, Vasili Perebeinos, Phaedon Avouris Aug 2011

Relaxation Of Optically Excited Carriers In Graphene, Raseong Kim, Vasili Perebeinos, Phaedon Avouris

Birck and NCN Publications

We explore the relaxation of photoexcited graphene by solving a transient Boltzmann transport equation with electron-phonon (e-ph) and electron-electron (e-e) scattering. Simulations show that when the excited carriers are relaxed by e-ph scattering only, a population inversion can be achieved at energies determined by the photon energy. However, e-e scattering quickly thermalizes the carrier energy distributions washing out the negative optical conductivity peaks. The relaxation rates and carrier multiplication effects are presented as a function of photon energy, graphene doping, and dielectric constant.

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Entanglement Dynamics Of One-Dimensional Driven Spin Systems In Time-Varying Magnetic Fields, Bedoor Alkurtass, Gehad Sadiek, Sabre Kais Aug 2011

Entanglement Dynamics Of One-Dimensional Driven Spin Systems In Time-Varying Magnetic Fields, Bedoor Alkurtass, Gehad Sadiek, Sabre Kais

Birck and NCN Publications

We study the dynamics of nearest-neighbor entanglement for a one-dimensional spin chain with a nearest-neighbor time-dependent Heisenberg coupling J(t) between the spins in the presence of a time-dependent external magnetic field h(t) at zero and finite temperatures. We consider different forms of time dependence for the coupling and magnetic field: exponential, hyperbolic, and periodic. Solving the system numerically, we examined the system-size effect on the entanglement asymptotic value. It was found that, for a small system size, the entanglement starts to fluctuate within a short period of time after applying the time-dependent coupling. The period of time increases as the …

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Computational Study Of The Seebeck Coefficient Of One-Dimensional Composite Nano-Structures, Raseong Kim, Mark S. Lundstrom Aug 2011

Computational Study Of The Seebeck Coefficient Of One-Dimensional Composite Nano-Structures, Raseong Kim, Mark S. Lundstrom

Birck and NCN Publications

The Seebeck coefficient (S) of composite nano-structures is theoretically explored within a self-consistent electro-thermal transport simulation framework using the non-equilibrium Green's function method and a heat diffusion equation. Seebeck coefficients are determined using numerical techniques that mimic experimental measurements. Simulation results show that, without energy relaxing scattering, the overall S of a composite structure is determined by the highest barrier within the device. For a diffusive, composite structure with energy relaxation due to electron-phonon scattering, however, the measured S is an average of the position-dependent values with the weighting factor being the lattice temperature gradient. The results stress the importance …

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Anti-Stokes Fluorescence Imaging Of Microscale Thermal Fields In Thin Films, A. N. Kuzmin, A. Baev, A. V. Kachynski, Timothy S. Fisher, Ali Shakouri, P. N. Prasad Aug 2011

Anti-Stokes Fluorescence Imaging Of Microscale Thermal Fields In Thin Films, A. N. Kuzmin, A. Baev, A. V. Kachynski, Timothy S. Fisher, Ali Shakouri, P. N. Prasad

Birck and NCN Publications

We demonstrate that the in-plane thin film heat transport can be accurately mapped via confocal anti-Stokes fluorescence imaging using fluorescent dye probes and nanoheaters. We employ 3D finite elements analysis to evaluate the thermal conductivity of a control sample and to assess the validity of a point heat source approximation. We have found that this approximation is adequate with use of a tightly focused laser beam, providing a robust means for determining the thermal conductivity of a sample under investigation. (C) 2011 American Institute of Physics. [doi:10.1063/1.3606429]

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Atomic-Layer-Deposited Al2o3 On Bi2te3 For Topological Insulator Field-Effect Transistors, Han Liu, Peide D. Ye Aug 2011

Atomic-Layer-Deposited Al2o3 On Bi2te3 For Topological Insulator Field-Effect Transistors, Han Liu, Peide D. Ye

Birck and NCN Publications

We report dual-gate modulation of topological insulator field-effect transistors (TI FETs) made on Bi2Te3 thin flakes with integration of atomic-layer-deposited (ALD) Al2O3 high-k dielectric. Atomic force microscopy study shows that ALD Al2O3 is uniformly grown on this layer-structured channel material. Electrical characterization reveals that the right selection of ALD precursors and the related surface chemistry play a critical role in device performance of Bi2Te3 based TI FETs. We realize both top-gate and bottom-gate control on these devices, and the highest modulation rate of 76.1% is achieved by using simultaneous dual gate control. (C) 2011 American Institute of Physics. [doi:10.1063/1.3622306]

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Observation Of Nonclassical Scaling Laws In The Quality Factors Of Cantilevered Carbon Nanotube Resonators, Ajit Vallabhaneni, Jeff Rhoads, Jayathi Y. Murthy, Xiulin Ruan Aug 2011

Observation Of Nonclassical Scaling Laws In The Quality Factors Of Cantilevered Carbon Nanotube Resonators, Ajit Vallabhaneni, Jeff Rhoads, Jayathi Y. Murthy, Xiulin Ruan

Birck and NCN Publications

This work examines the quality factors (Q factors) of resonance associated with the axial and transverse vibrations of single-wall carbon nanotube (SWCNT) resonators through the use of molecular dynamics (MD) simulation. Specifically, the work investigates the effect of device length, diameter, and chirality, as well as temperature, on the resonant frequency and quality factor of these devices and benchmarks the results of MD simulations against classical theories of energy dissipation. The quality factor (Q) associated with transverse vibration is found to increase with increasing device length (Q similar to L(theta), where 0.8 < theta < 1.4) and decrease with increasing device diameter (Q similar to D(-mu), where 1.4 < mu < 1.6), while the Q associated with axial vibration is almost independent of length and diameter. We show that to accurately predict temperature dependence of Q, the external and internal energies need to be properly decomposed, and temperature quantum correction should be performed. For both vibrational modes, Q shows a temperature dependence Q similar to T(-alpha), where alpha > 1 when below Debye temperature due to quantum …

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Dislocation Pinning Effects Induced By Nano-Precipitates During Warm Laser Shock Peening: Dislocation Dynamic Simulation And Experiments, Yiliang Liao, Chang Ye, Huang Gao, Bong-Joong Kim, Sergey Suslov, Eric A. Stach, Gary J. Cheng Jul 2011

Dislocation Pinning Effects Induced By Nano-Precipitates During Warm Laser Shock Peening: Dislocation Dynamic Simulation And Experiments, Yiliang Liao, Chang Ye, Huang Gao, Bong-Joong Kim, Sergey Suslov, Eric A. Stach, Gary J. Cheng

Birck and NCN Publications

Warm laser shock peening (WLSP) is a new high strain rate surface strengthening process that has been demonstrated to significantly improve the fatigue performance of metallic components. This improvement is mainly due to the interaction of dislocations with highly dense nanoscale precipitates, which are generated by dynamic precipitation during the WLSP process. In this paper, the dislocation pinning effects induced by the nanoscale precipitates during WLSP are systematically studied. Aluminum alloy 6061 and AISI 4140 steel are selected as the materials with which to conduct WLSP experiments. Multiscale discrete dislocation dynamics (MDDD) simulation is conducted in order to investigate the …

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Periodically Changing Morphology Of The Growth Interface In Si, Ge, And Gap Nanowires, C-Y Wen, J. Tersoff, K. Hillerich, M.C. Reuter, J. H. Park, S. Kodambaka, Eric Stach, F.M. Ross Jul 2011

Periodically Changing Morphology Of The Growth Interface In Si, Ge, And Gap Nanowires, C-Y Wen, J. Tersoff, K. Hillerich, M.C. Reuter, J. H. Park, S. Kodambaka, Eric Stach, F.M. Ross

Birck and NCN Publications

Nanowire growth in the standard < 111 > direction is assumed to occur at a planar catalyst-nanowire interface, but recent reports contradict this picture. Here we show that a nonplanar growth interface is, in fact, a general phenomenon. Both III-V and group IV nanowires show a distinct region at the trijunction with a different orientation whose size oscillates during growth, synchronized with step flow. We develop an explicit model for this structure that agrees well with experiment and shows that the oscillations provide a direct visualization of catalyst supersaturation. We discuss the implications for wire growth and structure.

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