Other Nanotechnology Publications

Universality of non-Ohmic shunt leakage in thin-film solar cells

Sourabh Dongaonkar et al. — Wed, 06 Jul 2011 12:11:30 PDT

We compare the dark current-voltage (IV) characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon (a-Si:H) p-i-n cells, organic bulk heterojunction (BHJ) cells, and Cu(In, Ga)Se-2 (CIGS) cells. All three device types exhibit a significant shunt leakage current at low forward bias (V < similar to 0.4) and reverse bias, which cannot be explained by the classical solar cell diode model. This parasitic shunt current exhibits non-Ohmic behavior, as opposed to the traditional constant shunt resistance model for photovoltaics. We show here that this shunt leakage (I-sh), across all three solar cell types considered, is characterized by the following common phenomenological features: (a) voltage symmetry about V = 0, (b) nonlinear (power law) voltage dependence, and (c) extremely weak temperature dependence. Based on this analysis, we provide a simple method of subtracting this shunt current component from the measured data and discuss its implications on dark IV parameter extraction. We propose a space charge limited (SCL) current model for capturing all these features of the shunt leakage in a consistent framework and discuss possible physical origin of the parasitic paths responsible for this shunt current mechanism. (C) 2010 American Institute of Physics. [doi:10.1063/1.3518509]

nanoHUB.org Serving Over 120,000 Users Worldwide: It's First Cyber-Environment Assessment

Krishna PL.C. Madhavan et al. — Wed, 06 Jul 2011 11:40:19 PDT

nanoHUB.org is a major engineering cyber- environment that annually supports over 120,000 users with online simulation and more. Over 8,500 nanoscale engineering and science researchers, educators, and learners run over 340,000 simulations with over 170 simulation tools annually. These tools allows them to transparently and interactively leverage a range of computational resources ranging from small jobs to massive simulations that execute on the Teragrid or the Open Science Grid (OSG). In this paper, we provide some background into the working of nanoHUB as a virtual organization and a cyber- environment and describe its growth pattern focusing on the mechanisms that allow the formation of a community around it.

Subband Engineering for p-type Silicon Ultra-Thin Layers for Increased Carrier Velocities: An Atomistic Analysis

Neophytos Neophytou et al. — Wed, 06 Jul 2011 11:40:13 PDT

Ultra-thin-body (UTB) channel materials of a few nanometers in thickness are currently considered as candidates for future electronic, thermoelectric, and optoelectronic applications. Among the features that they possess, which make them attractive for such applications, their confinement length scale, transport direction, and confining surface orientation serve as degrees of freedom for engineering their electronic properties. This work presents a comprehensive study of hole velocities in p-type UTB films of widths from 15 nm down to 3 nm. Various transport and surface orientations are considered. The atomistic sp3d5s*-spin-orbit-coupled tight-binding model is used for the electronic structure, and a semiclassical ballistic model for the carrier velocity calculation. We find that the carrier velocity is a strong function of orientation and layer thickness. The (110) and (112) surfaces provide the highest hole velocities, whereas the (100) surfaces the lowest velocities, almost 30% lower than the best performers. Additionally, up to 35% velocity enhancements can be achieved as the thickness of the (110) or (112) surface channels is scaled down to 3 nm. This originates from strong increase in the curvature of the p-type UTB film subbands with confinement, unlike the case of n-type UTB channels. The velocity behavior directly translates to ballistic on-current trends, and correlates with trends in experimental mobility measurements. VC 2011 American Institute of Physics. [doi:10.1063/1.3556435]

Strain energy and lateral friction force distributions of carbon nanotubes manipulated into shapes by atomic force microscopy

Mark C. Strus et al. — Tue, 12 Oct 2010 10:50:28 PDT

The interplay between local mechanical strain energy and lateral frictional forces determines the shape of carbon nanotubes on substrates. In turn, because of its nanometer-size diameter, the shape of a carbon nanotube strongly influences its local electronic, chemical, and mechanical properties. Few, if any, methods exist for resolving the strain energy and static frictional forces along the length of a deformed nanotube supported on a substrate. We present a method using nonlinear elastic rod theory in which we compute the flexural strain energy and static frictional forces along the length of single walled carbon nanotubes (SWCNTs) manipulated into various shapes on a clean SiO2 substrate. Using only high resolution atomic force microscopy images of curved single walled nanotubes, we estimate flexural strain energy distributions on the order of attojoules per nanometer and the static frictional forces between a SWCNT and SiO2 surface to be a minimum of 230 pN nm−1

Performance comparison between p-i-n tunneling transistors and conventional MOSFETs

Siyuranga O. Koswatta et al. — Tue, 12 Oct 2010 10:50:25 PDT

We present a detailed performance comparison between conventional n-i-n MOSFET transistors, and tunneling field-effect transistors (TFETs) based on the p-i-n geometry, using semiconducting carbon nanotubes as the model channel material. Quantum transport simulations are performed using the nonequilibrium Green's function formalism considering realistic phonon scattering and band-to-band tunneling mechanisms. Simulations show that TFETs have a smaller quantum capacitance at most gate biases. Despite lower on current, they can switch faster in a range of on/off current ratios. Switching energy for TFETs is observed to be fundamentally smaller than that for MOSFETs, leading to lower dynamic power dissipation. Furthermore, the beneficial features of TFETs are retained with different bandgap materials. These reasons suggest that the p-i-n TFET is well-suited for low power applications.

A domain adaptive stochastic collocation approach for analysis of MEMS under uncertainties

Nitin Agarwal et al. — Tue, 12 Oct 2010 10:50:24 PDT

This work proposes a domain adaptive stochastic collocation approach for uncertainty quantification, suitable for effective handling of discontinuities or sharp variations in the random domain. The basic idea of the proposed methodology is to adaptively decompose the random domain into subdomains. Within each subdomain, a sparse grid interpolant is constructed using the classical Smolyak construction [S. Smolyak, Quadrature and interpo- lation formulas for tensor products of certain classes of functions, Soviet Math. Dokl. 4 (1963) 240–243], to approximate the stochastic solution locally. The adaptive strategy is governed by the hierarchical surpluses, which are computed as part of the interpolation procedure. These hierarchical surpluses then serve as an error indicator for each subdo- main, and lead to subdivision whenever it becomes greater than a threshold value. The hierarchical surpluses also provide information about the more important dimensions, and accordingly the random elements can be split along those dimensions. The proposed adaptive approach is employed to quantify the effect of uncertainty in input parameters on the performance of micro-electromechanical systems (MEMS). Specifically, we study the effect of uncertain material properties and geometrical parameters on the pull-in behavior and actuation properties of a MEMS switch. Using the adaptive approach, we resolve the pull-in instability in MEMS switches. The results from the proposed approach are verified using Monte Carlo simulations and it is demonstrated that it computes the required statistics effectively.

Toward Nanowire Electronics

Joerg Appenzeller et al. — Thu, 13 Aug 2009 11:13:51 PDT

This paper discusses the electronic transport properties of nanowire field-effect transistors (NW-FETs). Four different device concepts are studied in detail: Schottky-barrier NW-FETs with metallic source and drain contacts, conventional-type NW-FETs with doped NW segments as source and drain electrodes, and, finally, two new concepts that enable steep turn-on characteristics, namely, NW impact ionization FETs and tunnel NW-FETs. As it turns out, NW-FETs are, to a large extent, determined by the device geometry, the dimensionality of the electronic transport, and the way of making contacts to the NW. Analytical as well as simulation results are compared with experimental data to explain the various factors impacting the electronic transport in NW-FETs.

Tunneling phenomena in carbon nanotube field-effect transistors

Joachim Knoch et al. — Thu, 13 Aug 2009 11:13:50 PDT

In the present article we will discuss the electronic transport properties of carbon nanotube field-effect transistors (CNFETs). Three different device concepts will be studied in more detail: Schottky-barrier CNFETs with metallic source and drain contacts, conventional-type CNFETs with doped nanotube segments as source and drain electrodes and finally a new concept, the tunneling CNFET. As it turns out, tunneling phenomena play a prominent role in all three CNFET designs and determine their electrical behavior to a large extend. In addition, the one-dimensionality of the electronic transport makes them ideally suited for novel device architecture such as the tunneling CNFET. Analytical as well as simulation results will be given and compared with each other and with experimental data in order to explain the different influences on the electronic transport in CNFETs and thus on the device behavior.

Outperforming the conventional scaling rules in the quantum-capacitance limit

Joachim Knoch et al. — Thu, 13 Aug 2009 11:13:49 PDT

We present a study on the scaling behavior of field-effect transistors in the quantum-capacitance limit (QCL). It will be shown that a significant performance improvement in terms of the power delay product can be obtained in devices scaled toward the QCL. As a result, nanowires or nanotubes exhibiting a 1-D transport are a premier choice as active channel materials for transistor devices since the QCL can be attained in such systems.

Carbon nanotubes for high-performance electronics - Progress and prospect

Joerg Appenzeller — Thu, 13 Aug 2009 11:13:48 PDT

Carbon nanotube devices offer intrinsic advantages for high-performance logic device applications. The ultrasmall body of a carbon nanotube-the tube diameter-is the key feature that should allow aggressive channel length scaling, while the intrinsic transport properties of the nanotube ensure at the same time high on-currents. in addition, the narrowness of the tube is critical to implementation of novel device concepts like the tunneling transistor. By understanding the unique capabilities of carbon nanotubes and using them in unconventional designs, novel nanoelectronic applications may become feasible. However, much better control of materials quality must be obtained, and new fabrication processes must be developed before such applications can be realized.

Externally assembled gate-all-around carbon nanotube field-effect transistor

Zhihong Chen et al. — Thu, 13 Aug 2009 11:13:46 PDT

In this letter, we demonstrate a gate-all-around single-wall carbon nanotube field-effect transistor. This is the first successful experimental implementation of an off-chip gate and gate-dielectric assembly with subsequent deposition on a suitable substrate. The fabrication process and device measurements are discussed in the letter. We also argue in how far charges in the gate oxide are responsible for the observed nonideal device performance.

Understanding Coulomb effects in nanoscale Schottky-barrier-FETs

Klaus M. Indlekofer et al. — Thu, 13 Aug 2009 11:13:44 PDT

We employ a novel multiconfigurational self-consistent Green's function approach (MCSCG) for the simulation of nanoscale Schottky-barrier-field-effect transistors (SB-FETs). This approach allows the calculation of electronic transport with a seamless transition from the single-electron regime to room-temperature FET operation. The particular improvement of the MCSCG stems from a self-consistent division of the channel system into a small subsystem of resonantly trapped states for which a many-body Fock space approach becomes numerically feasible and the rest of the system which can be treated adequately on a conventional mean-field level. The Fock space description allows for the calculation of few-electron Coulomb charging effects beyond the mean-field. We compare a conventional Hartree nonequilibrium Green's function calculation with the results of the MCSCG approach. Using the MCSCG method, Coulomb blockade effects are demonstrated at low temperatures while, under strong nonequilibrium and high-temperature conditions, the Hartree approximation is retained. Finally, the visibility of quantum and single-electron effects in scaled transistor structures is discussed.

1/f noise in carbon nanotube devices - On the impact of contacts and device geometry

Joerg Appenzeller et al. — Thu, 13 Aug 2009 11:13:43 PDT

We report on the 1/f noise in various ballistic carbon nanotube devices. A common means to characterize the quality of a transistor in terms of noise is to evaluate the ratio of the noise amplitude A and the sample resistance R. By contacting semiconducting tubes with different metal electrodes we are able to show that a small A/R value by itself is no indication of a suitable metal/tube combination for logic applications. We discuss how current in a nanotube transistor is determined by the injection of carriers at the electrode/nanotube interface, while at the same time excess noise is related to the number of carriers inside the nanotube channel. In addition, we demonstrate a substantial reduction in noise amplitude for a tube transistor with multiple carbon nanotubes in parallel.

Electrical transport and 1/f noise in semiconducting carbon nanotubes

Yu-Ming Lin et al. — Thu, 13 Aug 2009 11:13:42 PDT

We investigate electrical transport,and noise m semiconducting carbon nanotubes. By studying carbon nanotube devices with various,diameters, and contact metals, we show that the,ON-currents of CNFETs are governed by the heights of the Schottky barriers at the metal/nanotube interfaces. The current fluctuations are dominated by 1/f noise at low-frequencies and correlate with the number of transport carriers in the device regardless of contact metal.

Improved carrier injection in ultrathin-body SOI Schottky-barrier MOSFETs

M Zhang et al. — Thu, 13 Aug 2009 11:13:40 PDT

The impact of the gate oxide and the silicon-on-insulator (SOI) body thickness on the electrical performance of SOI Schottky-barrier (SB) MOSFETs with fully nickel silicided source and drain contacts is experimentally investigated. The subthreshold swing S is extracted from the experimental data and serves as a measure for the carrier injection through the Sills. It is shown that decreasing the gate oxide and body thickness allows to strongly increase the carrier injection and hence, a significantly improved ON-state of SB-MOSFETs can be obtained.

Physics of ultrathin-body silicon-on-insulator Schottky-barrier field-effect transistors

Joachim Knoch et al. — Thu, 13 Aug 2009 11:13:39 PDT

In this article we give an overview over the physical mechanisms involved in the electronic transport in ultrathin-body SOI Schottky-barrier MOSFETs. A strong impact of the SOI and gate oxide thickness on the transistor characteristics is found and explained using experimental as well as simulated data. We elaborate on the influence of scattering in the channel and show that for a significant barrier the on-state current is insensitive to scattering once the mean free path for scattering is larger than a characteristic length scale. In addition, recent efforts to lower the Schottky barrier at the source/drain channel interfaces are presented. Using dopant segregation during silicidation significantly lower effective Schottky barriers can be realized that allow for high performance SB-MOSFET devices.

Interfacial energy between carbon nanotubes and polymers measured from nanoscale peel tests in the atomic force microscope

Mark C. Strus et al. — Wed, 29 Jul 2009 08:47:36 PDT

The future development of polymer composite materials with nanotubes or nanoscale fibers requires the ability to understand and improve the interfacial bonding at the nanotube-polymer matrix interface. In recent work [Strus MC, Zalamea L, Raman A, Pipes RB, Nguyen CV, Stach EA. Peeling force spectroscopy: exposing the adhesive nanomechanics of one-dimensional nanostructures. Nano Lett 2008;8(2):544–50], it has been shown that a new mode in the Atomic Force Microscope (AFM), peeling force spectroscopy, can be used to understand the adhesive mechanics of carbon nanotubes peeled from a surface. In the present work, we demonstrate how AFM peeling force spectroscopy can be used to distinguish between elastic and interfacial components during a nanoscale peel test, thus enabling the direct measurement of interfacial energy between an individual nanotube or nanofiber and a given material surface. The proposed method provides a convenient experimental framework to quickly screen different combinations of polymers and functionalized nanotubes for optimal interfacial strength.

Gold nanorod-mediated photothermolysis induces apoptosis of macrophages via damage of mitochondria

Ling Tong et al. — Wed, 29 Jul 2009 08:47:35 PDT

Aims: Induction of apoptosis or necrosis in activated macrophages by gold nanorod-mediated photothermolysis is demonstrated and the mechanisms underlying the processes are investigated. Materials & methods: Gold nanorods were functionalized with cysteine-octaarginine peptides (R8-NRs). Uptake of R8-NRs by activated macrophages was monitored by two-photon luminescence imaging. The laser irradiation conditions were controlled to induce apoptosis or necrosis to R8-NR-internalized macrophages. Mitochondrial damage and reactive oxygen species overproduction during photothermolysis was investigated by confocal fluorescence microscopy and transmission-electron microscopy. Results: Activated macrophages efficiently uptake R8-NRs both in vitro and in live animals. Laser irradiation of internalized nanorods with controlled power density induces apoptosis of macrophages via intracellular perturbation and subsequent injury of mitochondria. Conclusions: Gold nanorod-mediated photothermolysis provides one promising way to eliminate activated macrophages in autoimmune and inflammatory diseases.

Imaging Gold Nanorods by Plasmon-Resonance-Enhanced Four Wave Mixing

Yookyung Jung et al. — Wed, 29 Jul 2009 08:47:33 PDT

The current work investigates the four-wave mixing (FWM) signal from gold nanorods (NRs) using two synchronized lasers and its potential applications in bioimaging. Using the lightning rod model, we show that the strongest FWM occurs when the pump laser wavelength is tuned to be resonant with the longitudinal plasmon resonance wavelength of NR. The calculation is experimentally demonstrated by comparing the intensities of FWM from NRs with different plasmon resonance wavelengths. The FWM signal is further found to be enhanced by aggregation of NRs and is strongly dependent on pulse width. The FWM intensity from NRs is similar to 39 times stronger than the coherent anti-Stokes Raman scattering intensity from melamine beads. This plasmon-resonance-enhanced FWM signal enables NRs to be used as a nonlinear optical (NLO) imaging probe.

Gold Nanorods as Contrast Agents for Biological Imaging: Optical Properties, Surface Conjugation and Photothermal Effects

Ling Tong et al. — Wed, 29 Jul 2009 08:47:32 PDT

Gold nanorods (NRs) have plasmon-resonant absorption and scattering in the near-infrared (NIR) region, making them attractive probes for in vitro and in vivo imaging. In the cellular environment, NRs can provide scattering contrast for darkfield microscopy, or emit a strong two-photon luminescence due to plasmon-enhanced two-photon absorption. NRs have also been employed in biomedical imaging modalities such as optical coherence tomography or photoacoustic tomography. Careful control over surface chemistry enhances the capacity of NRs as biological imaging agents by enabling cell-specific targeting, and by increasing their dispersion stability and circulation lifetimes. NRs can also efficiently convert optical energy into heat, and inflict localized damage to tumor cells. Laser-induced heating of NRs can disrupt cell membrane integrity and homeostasis, resulting in Ca2+ influx and the depolymerization of the intracellular actin network. The combination of plasmon-resonant optical properties, intense local photothermal effects and robust surface chemistry render gold NRs as promising theragnostic agents.