We fabricated flexible transparent conducting electrodes by printing films of single-walled carbon nanotube ͑SWNT͒ networks on plastic and have demonstrated their use as transparent electrodes for efficient, flexible polymer-fullerene bulk-heterojunction solar cells. The printing method produces relatively smooth, homogeneous films with a transmittance of 85% at 550 nm and a sheet resistance ͑R s ͒ of 200 ⍀ / ᮀ. Cells were fabricated on the SWNT/plastic anodes identically to a process optimized for ITO/glass. Efficiencies, 2.5% ͑AM1.5G͒, are close to ITO/glass and are affected primarily by R s. Bending test comparisons with ITO/plastic show the SWNT/plastic electrodes to be far more flexible.

Images of electron flow from the quantum point contact (QPC) are obtained by raster scanning a negatively charged SPM tip above the surface of the device and simultaneously measuring the position-dependent conductance of the device. The negatively charged tip capacitively couples to the 2DEG, creating a depletion region that backscatters electron waves. When the tip is positioned over areas with high electron flow from the QPC the conductance is decreased, whereas when the tip is over areas of relatively low electron flow the conductance is unmodified. By raster scanning the tip over the sample and simultaneously recording the effect the tip has on device conductance, a two dimensional image of electron flow can be obtained.The quantum point contact sample is mounted in an atomic force microscope and cooled to liquid He temperatures. The QPC is formed in the 2DEG inside a GaAs/AlGaAs heterostructure by negatively biasing two gates on the surface -a negative potential on these gates creates two depletion regions that define a variable width channel between them as shown in Fig. 1a. The conductance of the QPC is measured using an ac lock-in amplifier at 11kHz. The heterostructure for the devices used in this experiment was grown by molecular beam epitaxy on an n-type GaAs substrate.The 2DEG resides 57 nm below the surface with mobility µ = 1.0x10 6 cm 2 /Vs and density n = 4.5x10 11 /cm 2 . These values of mobility and density correspond to a mean free path l = 11 µm, Fermi wavelength λ F = 37 nm, and Fermi energy E F = 16 meV. The root mean square voltage across the QPC was chosen so as to not heat electrons -0.2 mV for 1.7K scans. The conductance of the quantum point contact, shown in Fig. 1b, increases as the width of the channel is increased (by changing the gate voltage V g ) and shows well defined conductance plateaus at integer multiples of the conductance quantum 2e 2 /h 1,2 . When probing the electron flow, the SPM tip was held at a negative potential relative to the 2DEG and was scanned at 10nm above the surface of the heterostructure. Figures 2a and 2b show images of electron flow from two different quantum point contacts at the temperature 1.7K; both QPCs are biased on the G = 2e 2 /h conductance plateau. Figure 2b shows the flow patterns on each side of a quantum point contact (the gated region at the center was not scanned), and Figure 2a shows a higher-resolution image of flow from one side of a different QPC.In both these images, the current exits the point contact in a central lobe, as would be expected from an exact quantum-mechanical calculation of electron flow through an ideal QPC without impurities or non-uniform distributions of dopant atoms. Rather than continuing out as a smoothly widening fan, it quickly forks into several different paths and continues to branch off into ever smaller rivulets for the full width of the scan. This branching behavior was observed in all of the 13 QPC exit patterns observed so far. Previously, there have been suggestions of an unexpected narrowness in observe...

Scanning a charged tip above the two-dimensional electron gas inside a gallium arsenide/aluminum gallium arsenide nanostructure allows the coherent electron flow from the lowest quantized modes of a quantum point contact at liquid helium temperatures to be imaged. As the width of the quantum point contact is increased, its electrical conductance increases in quantized steps of 2 e(2)/h, where e is the electron charge and h is Planck's constant. The angular dependence of the electron flow on each step agrees with theory, and fringes separated by half the electron wavelength are observed. Placing the tip so that it interrupts the flow from particular modes of the quantum point contact causes a reduction in the conductance of those particular conduction channels below 2 e(2)/h without affecting other channels.

Carbon nanotube network field effect transistors (CNTN-FETs) are promising candidates for low cost macroelectronics. We investigate the microscopic transport in these devices using electric force microscopy and simulations. We find that in many CNTN-FETs the voltage drops abruptly at a point in the channel where the current is constricted to just one tube. We also model the effect of varying the semiconducting/ metallic tube ratio. The effect of Schottky barriers on both conductance within semiconducting tubes and conductance between semiconducting and metallic tubes results in three possible types of CNTN-FETs with fundamentally different gating mechanisms. We describe this with an electronic phase diagram.

GaAs-based two-dimensional electron gases (2DEGs) show a wealth of remarkable electronic states [1][2][3] , and serve as the basis for fast transistors, research on electrons in nanostructures 4,5 , and prototypes of quantum-computing schemes 6 . All these uses depend on the extremely low levels of disorder in GaAs 2DEGs, with low-temperature mean free paths ranging from microns to hundreds of microns 7 . Here we study how disorder affects the spatial structure of electron transport by imaging electron flow in three different GaAs/AlGaAs 2DEGs, whose mobilities range over an order of magnitude. As expected, electrons flow along narrow branches that we find remain straight over a distance roughly proportional to the mean free path. We also observe two unanticipated phenomena in high-mobility samples. In our highest-mobility sample we observe an almost complete absence of sharp impurity or defect scattering, indicated by the complete suppression of quantum coherent interference fringes.Also, branched flow through the chaotic potential of a high-mobility sample remains stable to significant changes to the initial conditions of injected electrons.Scanning gate microscopy (SGM) images of electron flow in two-dimensional electron gases (2DEGs) [8][9][10][11][12][13][14][15][16][17][18] provide direct spatial information not available in conventional electrical transport measurements. Our SGM studies show how varying disorder affects electron flow, and enable us to infer information about the disorder potential in our different samples. Achieving a detailed picture of the disorder potential 19, 20 may help to understand why exotic electron organization emerges in some 2DEGs and not others, and to aim for ever weaker disorder or even tailored disorder 21 .By analyzing the differences between images of flow in our samples, we find that the highermobility samples are increasingly dominated by small-angle scattering instead of hard-scattering 22 .2 Finally, we investigate an unusual property of electron flow through such a small-angle scattering disorder potential: though the disorder potential is classically chaotic, branches of flow are stable to significant changes in initial conditions.On each of three 2DEG samples defined in GaAs/AlGaAs heterostructures (Table 1), we use a home-built scanning gate microscope to image the flow of electrons emanating from a split-gate quantum point contact (QPC) 23, 24 at 4.2 K, as schematically shown in Fig. 1d. Using a recently established technique 9-15 , we measure the conductance across the QPC while scanning a sharp conducting tip ∼20 nm above the surface of the sample. We negatively bias the tip to create a depletion region in the 2DEG below. When the tip is above a region of high electron flow from the QPC, it backscatters electrons through the QPC, reducing the measured conductance. By scanning the tip and recording the drop in conductance ∆G for each tip location, we thus image electron flow. Images of flow gathered in this way have been found to accurately reproduce the under...

We demonstrate the use of a scanned probe microscope (SPM) at 4 Kelvin to study electron transport through a ballistic point contact in the two-dimensional electron gas inside a GaAs/AlGaAs heterostructure. The electron gas density profile is locally perturbed by the charged SPM tip providing information about the electron flow through the point contact. As the tip is scanned, one obtains a spatial image of the ballistic electron flux as well as the topographic profile of the structure. Calculations indicate the spatial resolution is comparable to the electron gas depth.

We show an electron interferometer between a quantum point contact (QPC) and a scanning gate microscope (SGM) tip in a two-dimensional electron gas. The QPC and SGM tip act as reflective barriers of a lossy cavity; the conductance through the system thus varies as a function of the distance between the QPC and SGM tip. We characterize how temperature, electron wavelength, cavity length, and reflectivity of the QPC barrier affect the interferometer. We report checkerboard interference patterns near the QPC and, when injecting electrons above or below the Fermi energy, effects of dephasing. PACS numbers: 85.35.Ds, 73.23.Ad When electronic device dimensions become smaller than the electron coherence length, the wavelike nature of electrons becomes critical to understanding device operation and provides opportunities to build devices taking advantage of quantum properties. Interferometers have been used to study electron interference in systems such as carbon nanotubes [1] and GaAs two-dimensional electron gases (2DEGs) [2]. Direct spatial visualizations of interference effects appear as fringes in images of electron flow in 2DEGs taken by scanning gate microscopy (SGM) [3,4] and can give information about the local potential [5,6].Fringes appear in SGM images when different reflected electron paths interfere. Previously observed fringes were due to an interferometer similar to a Michelson interferometer. One path of the interferometer is created by an SGM tip and the other path by impurities [3][4][5] or a reflector gate [7]. However, fringes were not seen in highmobility samples with a low density of impurities [6]. In this Rapid Communication we report interference fringes in SGM images taken in one of the same high-mobility samples at lower temperatures. Now multiple interfering paths are created by the SGM tip, similar to the movable reflector of a Fabry-Pérot interferometer. We report spatial interference patterns different from those previously observed. The recovery of fringes in clean samples at lower temperatures allows us to spatially probe phase coherent properties, such as local phase and electron wavelength, and we demonstrate how these may be useful for studying dephasing. Furthermore, the understanding of interference fringes which we achieve is necessary before using fringes of a different origin to measure electron interactions in nanostructures [8].Thermal averaging limits how far from a coherent source interference effects can be observed, even if each individual electron is still coherent [9]. Electrons around the Fermi energy with a spread in energies comparable to the thermal energy are involved in transport and FIG. 1: Mechanisms for interference fringe formation and appearance of fringes at low temperature. (a) Schematic of imaging technique showing the 2DEG (green), as well as surface gates and metallic tip (orange) creating depletion regions (black) in the 2DEG below. When above an area of high electron flow, the tip scatters electrons back through the QPC (blue path) and reduces the mea...

New scanning probe techniques provide fascinating glimpses into the detailed behavior of semiconductor devices in the quantum regime.