Session Th-A2

New Nano-materials: Growth and Characterization II

Chair: Asim Ray, Brunel University

Th-A2.1 10:30–10:50

Structures of Amorphous Te and Te Nanoparticles Deposited at Liquid Nitrogen Temperature

Hiroyuki Ikemoto (1), Satoru Fujita (1), Taku Watanabe (1), and Takafumi Miyanaga (2)

1. Department of Physics, Univ. of Toyama, Toyama 930-8555, Japan

2. Department of Advanced Physics, Hirosaki Univ., Hirosaki 036-8561, Japan

Tellurium is the representative of the elements that have a hierarchic structure. Trigonal tellurium (t-Te) has a chain structure with two-fold coordinated covalent bonds. The overlapping between lone-pair orbital and antibonding orbital in adjacent chain causes interchain interactions. Covalent bonds of the Te nanoparticles are stronger than those of trigonal Te as shown by a shorter bond length and higher Einstein temperature[1,2]. We report structures of amorphous Te (a-Te) and Te nanoparticles (n-Te) deposited at liquid nitrogen temperature.

Layers of Te and NaCl were deposited alternately onto graphite substrates cooled with liquid nitrogen. We made two types samples; one is thick Te film whose Te thickness was 300 nm, others are Te nanoparticles whose Te thickness were thinner than 10 nm. The thin Te films were discontinuous with isolated island formation, so n-Te isolated in NaCl matrix were obtained. X-ray absorption fine structure (XAFS) measurements for Te K-edge were carried out at the NW10A of PF-AR.

The Te thick film deposited at liquid nitrogen temperature is amorphous [3]. In the hierarchic element, there are two sides' viewpoints, that is, the primary and the secondary structures. In a-Te the first neighbor (1NN) bond length is shortened compared to that of t-Te, but the coordination number of 1NN is 2.0 which is the same number of that of t-Te. This suggests that the chain structure is preserved in a-Te, but the covalent bond is strengthened. About the secondary structure, the coordination number of the interchain nearest neighbor is reduced by one third. These results indicate that the primary structure is preserved while the second structure is collapsed.

In n-Te the primary structure is similar situations with a-Te, that is, the chain structure is preserved and the covalent bonds are strengthened. In the Fourier transforms of XAFS functions, two peaks are observed at 3.5 and 3.7Å. The former peak locates near the peak position of the 1NN interchain peak of t-Te. There is a possibility that the former and later peaks are originated from the core and the surface of n-Te.

[1] H. Ikemoto and T. Miyanaga, Phys. Rev. Lett. , 99 (2007) 165503

[2] H. Ikemoto, A. Goyo, and T. Miyanaga, J. Phys. Chem. C, 115 (2011) 2931

[3] T. Ichikawa, Japan. J. Appl. Phys. Suppl., 2 (1974) 785

Keywords: tellurium, amorphous, nanoparticle, XAFS

Th-A2.2 10:50–11:10

Electronic Properties of Hybrid Cu2S/Ru Semiconductor/Metallic-Cage Nanoparticles

Oded Millo (1), Yehonadav Bekenstein (1,2), Kathy Vinokurov (2), and Uri Banin (2)

1. Racah Institute of Physics and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

2. Institute of Chemistry, and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

Hybrid metal-semiconducting nanoparticles exhibit an inherent enigma. Two materials with different characteristic energy scales, the semiconductor bandgap and the single-electron charging energy of the metallic part, are connected and confined within a minute space, of dimensions much smaller than the Schottky depletion width which conventionally characterizes the interface between and metallic materials. Such architecture holds promise for emergent synergetic electronic properties of the hybrid nano-system, reflecting, in a modified and non-trivial way, those of the two separate components. Here we present a scanning tunneling spectroscopy (STS) study of hybrid inorganic nanoparticles, comprising a semiconducting Cu2S nanocrystal (NC) core encapsulated by a metallic Ru cage-like shell, and of each of their individual components, along with transport measurements on Cu2S-NC arrays. The tunneling spectra acquired from the bare Cu2S NCs were nearly independent of the tip position over the NC, showing semiconducting-like I-V characteristics, while the empty Ru cages exhibit single electron tunneling effects—the Coulomb blockade and staircase. Surprisingly, in some cases negative differential conductance features with periodicity that correlates to the Coulomb staircase were observed. The tunneling spectra measured on the hybrid nanoparticles largely vary along a single particle, manifesting synergetic electrical properties that originate from this unique semiconducting-metallic interface. Transport and photo-transport measurements of Cu2S NC arrays exhibit unique temperature dependence, in particular a sharp increase in the conductance, by more than three orders of magnitude, around 350K. This behavior can be associated with phase- and stoichiometry transitions Cu2S undergoes at this temperature range. The stoichiometry change gives rise to free carriers (holes) that are manifested in the tunneling spectra via the appearance of in-gap sates and a shift of the Fermi level toward the valence-band edge, as well as by the emergence an IR plasmon resonance.

Keywords: semiconductor nanocrystals, scanning tunneling microscopy, single electron tunneling

Th-A2.3 11:10–11:30 (new)

Tunnel Optical radiation in InxGa1–xN

Dimiter Alexandrov, Rositsa Gergova, Peter Binsted

Semiconductor Research Laboratory, Lakehead University Thunder Bay, Ontario, Canada

Theoretical and experimental results about electro-luminescence observed in n-InxGa1–xN/p-GaN hetero-junctions are reported in this paper. The theoretical conclusions are made on the basis of previous author's contributions [1, 2]. The hetero-junctions are designed and technologically produced in the Semiconductor Research Laboratory at Lakehead University by original technology that is developed in this lab. In fact this type hetero-junction is further development of this technology in a way of design of electronic devices. Technological development of hetero-structure n-InxGa1–xN/p-GaN was made by relatively low temperature epitaxial growth ~540°C that is one of main advantages of this novel technology. The hetero-structures were investigated experimentally in term of their electro-luminescence properties and the experimental results are explained by author's theory that was previously developed. Experimental results of electro-luminescence spectra for n-InxGa1–xN/p-GaN hetero-junctions were obtained and they show two well expressed optical bands, one in range 500–540 nm and other in range 550–610 nm. Interesting thing is that each band begins and ends by sharp drops of the radiation, which are almost zeros. A theoretical investigation of this unusual behavior of these spectra was done using LCAO electron band structure calculations. The optical ranges of these bands show that the radiation occurs in InxGa1–xN region. In fact substitutions of In atoms on Ga sites create defects in the structure of InxGa1–xN and the corresponding LCAO matrix elements are found on this basis. The LCAO electron band structures are calculated in consideration of interactions between nearest-neighbor orbitals. Electron energy pockets are found in both the conduction and the valence bands at the Γ point of the electron band structures. Also it is found that these pockets are separated by distances, for which there is overlapping between the electron wave functions describing localized states belonging to the pockets, and as a result tunnel optical radiation can take place. This type of electron transition—between pocket in the conduction band and pocket in valence band—occurs in InxGa1–xN determining the above described optical bands. This conclusion concurs with the fact that the shapes of these bands change with change of the applied voltage. Qualitative correspondence between the experimental data and the theoretical results was found.

[1] D. Alexandrov, Journal of Crystal Growth, 246, 325 (2002)

[2] D. Alexandrov, S. Butcher, T. Tansley, Physica Status Solidi (a), 203, 25 (2006)

Keywords: nitride semiconductors, defects, quantum tunneling

Th-A2.4 11:30–11:50

Chemical Aspects Driving Silicon Nanowire Growth on Sn Nanotemplates below the Eutectic Temperature

Nagsen Meshram, Alka Kumbhar, and R. O. Dusane

Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India

The fabrication of Si nanowires (SiNWs) by the vapor-liquid-solid (VLS) technique is attractive for the development of nanowire-based electronic, photovoltaic and charge storage devices. In the conventional VLS process the Si nanowires are grown at temperatures slightly higher than the eutectic temperature of Si- metal combination. Among the various catalysts tin (Sn) appears to be favorable because of the relatively low eutectic temperature (232°C) of Sn–Si alloy. The vapor liquid solid (VLS) mechanism of silicon nanowire (SiNW) growth using different metal catalyst nanoparticles is usually carried out at temperatures above the silicon-metal eutectic temperature and has been treated as a purely thermodynamic phenomenon. We demonstrate here that by employing the hot wire chemical vapor process (HWCVP) it is possible to fabricate Sn nano-templates and also grow silicon nanowires at temperatures as low as 200°C. This temperature is below the eutectic point of tin with silicon (232°C). This low temperature nanowire growth is attributed to the peculiar Si containing but hydrogen rich gas phase environment created by the hot-wire chemical vapor process (HWCVP) subsequent to silane (SiH4) dissociation coupled with high catalytic reactivity of the Sn nano-templates due to their nano dimension. Raman spectra and high resolution transmission electron microscope (HRTEM) images reveal the crystalline nature of the nanowires. Moreover the dimensions of the particles of the tin nano-templates and the nanowires can be precisely controlled by tuning the hot wire process parameters.

Keywords: Si nanowires, vapor-liquid-solid, hot wire CVD, nano-template