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Silicon nanowires were selectively grown at temperatures below 400 °C by plasma enhanced chemical vapor deposition using silane as the Si source and gold as the catalyst. A detailed growth study is presented using electron microscopy, focused ion beam preparation, and Raman spectroscopy. A radio-frequency plasma significantly increased the growth rate. The Si nanowires show an uncontaminated, crystalline silicon core surrounded by a 2-nm-thick oxide sheath. The as-grown diameters are small enough for the observation of quantum confinement effects. Plasma activation could allow a further decrease in deposition temperature. A growth model for plasma enhanced nanowire growth is discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Gold was used as the catalyst and silane as the Si source to selectively grow silicon nanowires at temperatures below 400 degrees C by plasma enhanced chemical vapor deposition. Electron microscopy, focused ion beam preparation and Raman spectroscopy were used for a detailed growth study.

The recent discovery of graphene has led to many advances in two-dimensional physics and devices. The graphene devices fabricated so far have relied on SiO2 back gating. Electrochemical top gating is widely used for polymer transistors, and has also been successfully applied to carbon nanotubes. Here we demonstrate a top-gated graphene transistor that is able to reach doping levels of up to 5×1013 cm−2, which is much higher than those previously reported. Such high doping levels are possible because the nanometre-thick Debye layer in the solid polymer electrolyte gate provides a much higher gate capacitance than the commonly used SiO2 back gate, which is usually about 300 nm thick. In situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, but the 2D peak shows a different response to holes and electrons. The ratio of the intensities of the G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor the doping. [ABSTRACT FROM AUTHOR]

Abstract: Raman spectroscopy is a standard tool for the characterisation of carbon materials, from graphite to diamond-like carbon and carbon nanotubes. An important factor is the dependence of the Raman spectra on excitation energy, which is due to resonant processes. Here, we calculate the resonant Raman spectra of tetrahedral amorphous carbon. This is done by a tight-binding method, using an approach different from Placzek''s approximation, which allows calculation of Raman intensities also in resonant conditions. The calculated spectra confirm that the G peak arises from chains of sp2 bonded atoms and that it correlates with the atomic and electronic structure of the samples. The calculated dispersion of the G peak position with excitation energy follows the experimental observations. Our ab initio calculations also show that the sp3 phase can only be seen by using UV excitation above 4 eV, confirming the assignment of the T peak at ∼1060 cm−1, seen only in UV Raman measurements, to C–C sp3 vibrations. [Copyright &y& Elsevier]

We measure the effects of phonon confinement on the Raman spectra of silicon nanowires (SiNWs). We show how previous reports of phonon confinement in SiNWs and nanostructures are actually inconsistent with phonon confinement, but are due to the intense local heating caused by the laser power used for Raman measurements. This is peculiar to nanostructures, and would require orders of magnitude higher power in bulk Si. By varying the temperature, power and excitation energy, we identify the contributions of pure confinement, heating and carrier photo-excitation. After eliminating laser-related effects, the Raman spectra show confinement signatures typical of quantum wires. [Copyright &y& Elsevier]

Graphene edges are of particular interest since their orientation determines the electronic properties. Here we present a detailed Raman investigation of graphene flakes with edges oriented at different crystallographic directions. We also develop a real space theory for Raman scattering to analyze the general case of disordered edges. The position, width, and intensity of G and D peaks are studied as a function of the incident light polarization. The D-band is strongest for polarization parallel to the edge and minimum for perpendicular. Raman mapping shows that the D peak is localized in proximity of the edge. For ideal edges, the D peak is zero for zigzag orientation and large for armchair, allowing in principle the use of Raman spectroscopy as a sensitive tool for edge orientation. However, for real samples, the D to G ratio does not always show a significant dependence on edge orientation. Thus, even though edges can appear macroscopically smooth and oriented at well-defined angles, they are not necessarily microscopically ordered.

We demonstrate n- and p-type field-effect transistors based on Si nanowires (SiNWs) implanted with P and B at fluences as high as 10(15) cm (-2). Contrary to what would happen in bulk Si for similar fluences, in SiNWs this only induces a limited amount of amorphization and structural disorder, as shown by electrical transport and Raman measurements. We demonstrate that a fully crystalline structure can be recovered by thermal annealing at 800 degrees C. For not-annealed, as-implanted NWs, we correlate the onset of amorphization with an increase of phonon confinement in the NW core. This is ion-dependent and detectable for P-implantation only. Hysteresis is observed following both P and B implantation.

Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality. We show that its electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers. The D peak second order changes in shape, width, and position for an increasing number of layers, reflecting the change in the electron bands via a double resonant Raman process. The G peak slightly down-shifts. This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area.

We demonstrate the key role of phonon occupation in limiting the high-field ballistic transport in metallic carbon nanotubes. In particular, we provide a simple analytic formula for the electron transport scattering length, which we validate by accurate first principles calculations on (6, 6) and (11, 11) nanotubes. The comparison of our results with the scattering lengths fitted from experimental I-V curves indicates the presence of a nonequilibrium optical phonon heating induced by electron transport. We predict an effective temperature for optical phonons of thousands Kelvin.

We demonstrate that graphite phonon dispersions have two Kohn anomalies at the Gamma-E(2g) and K-A'1 modes. The anomalies are revealed by two sharp kinks. By an exact analytic derivation, we show that the slope of these kinks is proportional to the square of the electron-phonon coupling (EPC). Thus, we can directly measure the EPC from the experimental dispersions. The Gamma-E(2g) and K-A'1 EPCs are particularly large, while they are negligible for all the other modes at Gamma and K.