We present the photodetection properties of graphene/Si heterojunctions both in the photocurrent and photovoltage modes. Monolayer graphene/Si junctions were found to be excellent weak-signal detectors with photovoltage responsivity exceeding 10(7) V/W and with noise-equivalent-power reaching ∼1 pW/Hz(1/2), potentially capable of distinguishing materials with transmittance, T = 0.9995 in a 0.5 s integration time. In the photocurrent mode, the response was found to remain linear over at least six decades of incident power (P), with tunable responsivity up to 435 mA/W (corresponding to incident photon conversion efficiency (IPCE) > 65%) obtained by layer thickening and doping. With millisecond-scale responses and ON/OFF ratios exceeding 10(4), these photodiodes are highly suitable for tunable and scalable broadband (400 < λ < 900 nm) photodetectors, photometers, and millisecond-response switching, spectroscopic and imaging devices, and further, and are architecturally compatible with on-chip low-power optoelectronics.

Nanoscale structures need to be arranged into well-defined configurations in order to build integrated systems. Here we use a chemical-vapour deposition method with gas-phase catalyst delivery to direct the assembly of carbon nanotubes in a variety of predetermined orientations onto silicon/silica substrates, building them into one-, two- and three-dimensional arrangements. The preference of nanotubes to grow selectively on and normal to silica surfaces forces them to inherit the lithographically machined template topography of their substrates, allowing the sites of nucleation and the direction of growth to be controlled.

Here, we report a generic synthetic approach to rationally design multiply connected and hierarchically branched nanopores inside anodic aluminum oxide templates. By using these nanochannels, we controllably fabricate a large variety of branched nanostructures, far more complex than what exists today. These nanostructures include carbon nanotubes and metallic nanowires having several hierarchical levels of multiple branching. The number and frequency of branching, dimensions, and the overall architecture are controlled precisely through pore design and templated assembly. The technique provides a powerful approach to produce nanostructures of greater morphological complexity, which could have far-reaching implications in the design of future nanoscale systems.T he design and controlled synthesis of complex nanowire (1-5) and carbon nanotubes (CNTs) (6-8) will impact developments in nanotechnology applications. The present synthesis approaches, however, limit the degree of complexity that can be controllably configured into these structures. Fabrication inside rationally designed porous templates [such as anodic aluminum oxide (AAO) templates] is ideal to produce nanowire morphologies, but this feat has been accomplished controllably only for linear (9-15) and Y-shaped (6, 7) architectures. The creation of controlled pore structure, with various levels of complexity, inside these templates provides a powerful way to produce predesigned multiply connected and branched nanowires and nanotubes. We have developed a rational approach for creating hierarchically branched nanoporous AAO templates and have fabricated a whole generation of branched nanowires and nanotubes inside these templates. As a suitable example, we detail the case of CNT structures in this work, but other material systems can also be made into similar architectures, as highlighted for the case of metallic nanowires in Materials and Methods. Materials and MethodsPreparation of AAO Templates. AAO templates were prepared by using a modified two-step anodization process (6, 16). The first-step anodization was the same for all templates. High-purity Al foils were anodized in 0.3 M oxalic acid solution at 8-10°C under a constant voltage (in the range of 40-72 V dc ) for 8 h. Then, the formed anodic aluminum layer was removed. In the second-step anodization, templates with different pore architectures underwent different processes of anodization as follows. AAO Templates with Multiple Generations of Y-Branched Pores.We reduced the anodizing voltage multiple times in the second-step anodization. Initially, the anodization was performed under the same conditions as those in the first step to create the primary stem pores; then, the anodizing voltage was reduced by a factor of 1͞ ͌ 2 to form Y-branched pores. Two-, three-, and fourgeneration Y-branched pores can be obtained by further sequential reduction of anodizing voltages. It is noted that if a subsequent anodizing voltage is Յ25 V, after any prior anodization, the samples should be washed in deionized w...

We present the fabrication and electrical characterization of a flexible hybrid composite structure using aligned multiwall carbon nanotube arrays in a poly(dimethylsiloxane) (PDMS) matrix. Using lithographically patterned nanotube arrays, one can make these structures at any length scale from submicrometer levels to bulk quantities. The PDMS matrix undergoes excellent conformal filling within the dense nanotube network, giving rise to extremely flexible conducting structures with unique electromechanical properties. We demonstrate its robustness against high stress conditions, under which the composite is found to retain its conducting nature. We also demonstrate that these structures can be utilized directly as flexible field-emission devices. Our devices show some of the best field-enhancement factors and turn-on electric fields reported so far.