C. Gaire

Tunable Bandgap in Graphene by the Controlled Adsorption of Water Molecules

Graphene, a single-atom-thick layer of sp 2 -hybridized carbon atoms, has generated considerable excitement in the scientifi c community due to its peculiar electronic band structure, which leads to unusual phenomena such as the anomalous quantum Hall effect, [ 1,2 ] spin-resolved quantum interference, [ 3 ] ballistic electron transport, [ 4 ] and bipolar supercurrent. [ 5 ] However, pristine graphene is a semimetal with zero bandgap; the local density of states at the Fermi level is zero and conduction can only occur by the thermal excitation of electrons. [ 2 ] This lack of an electronic bandgap is the major obstacle limiting the utilization of graphene in nano-electronic and -photonic devices, [ 6,7 ] such as p–n junctions, transistors, photodiodes, and lasers. The graphene band structure is sensitive to lattice symmetry and several methods have been developed to break this symmetry and open an energy gap. These methods are based on a variety of techniques, such as defect generation, [ 8 ] doping (e.g., with potassium [ 9 ] ), applied bias, [ 10–12 ] and interaction with gases [ 13 ] (e.g., nitrogen dioxide). For instance, in reference [ 12 ] a tunable bandgap of up to 0.25 eV was achieved for electrically gated bilayer graphene by a variable external electric fi eld. Similarly, an internal electric fi eld produced by an imbalance of doped charge between two graphene layers has been shown to open a bandgap. [ 9 ] It has been demonstrated that a gap of ≈ 0.26 eV can be produced by growing graphene epitaxially on silicon carbide substrates. [ 14 ] This gap originated from the breaking of sublattice symmetry due to the graphene–substrate interaction. Patterned adsorption of atomic hydrogen onto the Moire superlattice positions of graphene [ 15 ] has resulted in a bandgap of ≈ 0.73 eV opening, while half-hydrogenated graphene [ 16 ] resulted in a bandgap of ≈ 0.43 eV. A graphene nanomesh structure [ 17 ] has also been shown to exhibit a bandgap. In this graphene structure, lateral quantum confi nement and localization effects due to

Morphology and texture evolution of nanostructured CaF2 films on amorphous substrates under oblique incidence flux

The morphology and biaxial texture of vacuum evaporated CaF(2) films on amorphous substrates as a function of vapour incident angle, substrate temperature and film thickness were investigated by scanning electron microscopy, x-ray pole figure and reflection high energy electron diffraction surface pole figure analyses. Results show that an anomalous [220] out-of-plane texture was preferred in CaF(2) films deposited on Si substrates at < 200 °C with normal vapour incidence. With an increase of the vapour incident angle, the out-of-plane orientation changed from [220] to [111] at a substrate temperature of 100 °C. In films deposited with normal vapour incidence, the out-of-plane orientation changed from [220] at 100 °C to [111] at 400 °C. In films deposited with an oblique vapour incidence at 100 °C, the texture changed from random at small thickness (5 nm) to biaxial at larger thickness (20 nm or more). Using first principles density functional theory calculation, it was shown that [220] texture formation is a consequence of energetically favourable adsorption of CaF(2) molecules onto the CaF(2)(110) facet.

Texture evolution of vertically aligned biaxial tungsten nanorods using RHEED surface pole figure technique

Vertically aligned biaxial tungsten nanorods with cubic A15 crystal structure were deposited by DC magnetron sputtering on native oxide covered Si(100) substrates with glancing angle flux incidence (theta approximately 85 degrees) and a two-step substrate rotation mode at room temperature. These vertical nanorods were grown to different thicknesses (10, 25, 50 and 100 nm) and analyzed for biaxial texture evolution using a highly surface sensitive reflection high-energy electron diffraction (RHEED) pole figure technique. The initial polycrystalline film begins to show the inception of biaxial texture with a fiber background between 10 and 25 nm. Biaxial texture development is eventually completed between 50 and 100 nm thicknesses of the film. The out-of-plane crystallographic direction is [002] and the in-plane texture is selected so as to obtain maximum capture area. In a comparison with 100 nm thick inclined tungsten nanorods deposited at 85 degrees without substrate rotation, it is found that the selection of in-plane texture does not maintain maximum in-plane capture area. This anomalous behavior is observed when the [002] texture axis is tilted approximately 17 degrees from the substrate normal in the direction towards the glancing incident flux.

Deformation of amorphous silicon nanostructures subjected to monotonic and cyclic loading

An atomic force microscope (AFM) was used to characterize the deformation behavior of amorphous Si (a-Si) nanostructures subjected to monotonic and cyclic loading. The sample geometry was specially designed (in the form of elbow) using finite element modeling for the purpose of these tests, and the samples were grown by glancing angle deposition. When deformed monotonically at room temperature, the a-Si specimens exhibited a nonlinear force–displacement response at forces larger than a critical force, a phenomenon not observed in bulk silicon. A fatigue testing methodology based on the use of the AFM was established. The fatigue life of the a-Si specimens was observed to increase by five orders of magnitude with a 50% reduction in the applied force amplitude. It was verified that this delayed failure is caused by progressive damage accumulation during cyclic loading. These results are compared with literature data obtained from micron-size specimens.

Quasi-single Crystal Semiconductors on Glass Substrates Through Biaxially Oriented Buffer Layers

High efficiency photovoltaic devices are normally fabricated on single crystalline substrates. These single crystalline substrates are expensive and volume production for widespread usage has not been realistic. To date, large volume production of solar cells is on less expensive noncrystalline substrates such as glass. Typically the films grown on glass are polycrystalline with less than ideal efficiency. It was proposed that a dramatic gain in the efficiency may be achieved if one uses a biaxially oriented buffer layer on glass to grow biaxial semiconductor films to fabricate solar devices compared to that of films grown directly on glass. Biaxial films are not exactly single crystal but have strongly preferred crystallographic orientations in both the out-ofplane and in-plane directions. Typically the misorientation between grains can be small (within a few degrees) and may possess low carrier recombination rate. In this paper we shall discuss growth techniques that would allow one to produce biaxial buffer layers on glass. A specific strategy using an atomic shadowing mechanism in an oblique angle deposition configuration that allows one to grow biaxial buffer layers such as CaF2 on glass substrate will be discussed in detail. Results of heteroepitaxy of semiconductor materials such as CdTe and Ge on these biaxial buffer/glass substrates characterized by x-ray pole figure, reflection high energy electron diffraction (RHEED) pole figure and transmission electron microscopy (TEM) will be presented.