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.