Nanowires, Nanorods and Nanotubes

• Optical anisotropy in single-walled carbon nanotube thin films: Implications for transparent and conducting electrodes in organic photovoltaics
  Authors: Fanchini G, Miller S, Parekh LB, Chhowalla M
  NANO LETTERS 8 (8) 2176-2179 (2008)
  DOI: 10.1021/nl080563p
  Abstract: Optical anisotropy in single-walled carbon nanotube thin film networks is reported. We obtain the real and imaginary parts of the in- (II) and out-of-plane (perpendicular to) complex dielectric functions of the single-walled carbon nanotube (SWNT) thin films by combining transmission measurements at several incidence angles with spectroscopic ellipsometry data on different substrates. In sparse networks, the two components of the real part of the complex dielectric constant (epsilon(1II) and epsilon(1 perpendicular to) were found to differ by 1.5 at 2.25 eV photon energy. The resulting angular dependence (from 0 to 70 degrees incidence angles) of transmittance is reflected in the conversion efficiency of organic solar cells utilizing SWNT thin films as the hole conducting electrodes. Our results indicate that, in addition to the transparency and sheet resistance, factors such as the optical anisotropy must be considered for optical devices incorporating SWNT networks.



• In situ ellipsometric analysis of growth processes of anodic TiO2 nanotube films
  Authors: Joo S, Muto I, Hara N
  JOURNAL OF THE ELECTROCHEMICAL SOCIETY 155 C154-C161 (2008)
  DOI: 10.1149/1.2837859
  Abstract: The growth process of TiO2 nanotube films formed on Ti by anodization in NH4H2PO4-NH4F electrolytes has been studied by electrochemical measurements, field emission-scanning electron microscopy (FE-SEM) observations, and ellipsometric analyses. The results of electrochemical measurements and FE-SEM observations showed that the change in current density during anodization partly reflects the formation process of nanotube films but does not give detailed information on the film growth process. Ellipsometry could be successfully applied to monitor the growth of nanotube films in situ. Ellipsometric data (Delta and Psi) obtained during anodization in 0.5 wt % NH4F revealed that the growth process of TiO2 nanotubes can be divided into four stages, that is, the barrier layer growth (1st), the outer nanoporous layer formation and inner nanotube layer growth (2nd), the pure nanotube growth (3rd), and the tube mouth dissolution (4th) stages. The outer nanoporous layer in the 2nd stage is formed by F-- induced breakdown of the initially grown barrier layer. The final 4th stage is characterized by an inward spiral of the Delta - Psi plot and a significant reduction of the amplitude of a cyclic change in Delta. On the basis of these characteristics of ellipsometric data, one can control the end point of anodization for preparing ordered TiO2 nanotubes.
  Email: haran@material.tohoku.ac.jp



• Characterization of Si nanorods by spectroscopic ellipsometry with efficient theoretical modeling
  Authors: Hsu SH, Liu ES, Chang YC, Hilfiker JN, Kim YD, Kim TJ, Lin CJ, Lin GR
  PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE 205 876-879 (2008)
  DOI: 10.1002/pssa.200777832
  Abstract: Spectroscopic ellipsometry (SE) is applied to characterize Si columnar nanostructures. By employing effective medium approximation (EMA) theory, Si nanorods are treated as a graded layer with each sub-layer modeled as a mixture of Si and voids with varying porosity fraction. In addition, the rigorous coupled-wave analysis and finite-element Green's function method were used in modeling Si nanorods as a stack of disks with varying diameters and thicknesses, and the calculations are in satisfactory agreement with the measurement results.
  Email: shhsu@gate.sinica.edu.tw



• MOCVD template approach to the fabrication of free-standing nickel(II) oxide nanotube arrays: Structural, morphological, and optical properties characterization
  Authors: Malandrino G Perdicaro LMS, Fragala IL, Lo Nigro R, Losurdo M, Bruno G
  JOURNAL OF PHYSICAL CHEMISTRY C 111 (8) 3211-3215 (2007)
  DOI: 10.1021/jp067696o
  Abstract: Ordered homogeneous arrays of nickel (II) oxide nanotubes have been fabricated through an MOCVD template route using Ni(tta)(2)tmeda (Htta = 2 thenoyl-trifluoroacetone, tmeda = tetramethylendiamine) as a source. The X-ray diffraction pattern shows the formation of the NiO cubic phase. Scanning electron microscopy (SEM) images of the NiO nanotubes, after removing the template, indicate the formation of well ordered nanotube arrays. There is evidence, through transmission electron microscopy studies, that nanotubes are opened on both sides. The optical properties of NiO nanotubes, determined using spectroscopic ellipsometry, strongly depend on the polarization direction of the light electric field, indicating optical anisotropy that relates to the vertical alignment of nanotubes.
  Email: maria.losurdo@ba.imip.cnr.it