Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W31: Nano-tubes, Wires, and Dots: Optical and Raman Experiments |
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Sponsoring Units: DCMP Chair: Walter Smith, Haverford College Room: Colorado Convention Center 401 |
Thursday, March 8, 2007 2:30PM - 2:42PM |
W31.00001: Raman Spectroscopy of Heat-Treated Boron Doped Double Wall Carbon Nanotubes F. Villalpando, H.B. Son, G.G. Samsonidze, S.G. Chou, Y.A. Kim, H. Muramatsu, T. Hayashi, M. Endo, M. Terrones, M.S. Dresselhaus We performed Raman spectroscopy experiments on undoped and boron-doped double walled carbon nanotubes that exhibit the ``coalescence inducing mode'' as they are heat treated at temperatures between 1200C and 2000C. From the radial breathing mode spectra we find that the smaller diameter tubes disappear before the larger diameter tubes as the sample is heat treated at higher temperatures. By using different laser excitation energies ranging from 1.57eV to 2.41eV, we observe in agreement with prior work that the outer tubes shield the inner tubes and give the G band its characteristic lineshape. We also find that the G' feature contains contributions from the inner and outer layers of a DWNT and its frequency shifts are related to the change in the diameter distribution of the DWNT sample caused by increasing the heat treatment temperatures. Finally, we report on the observation of a four fold splitting of the G' Raman feature and analyze the similarities with recent studies on 2-layer graphene. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W31.00002: Optical Transmittance and Sheet Resistance of B-doped Single-Walled Carbon Nanotubes XiaoMing Liu, Hugo Romero, Humberto Gutierrez, Kofi Adu, Peter Eklund Thin films of carbon nanotubes have been reported to be a replacement for transparent conducting films of Indium-Tin-Oxide (ITO). Nanotube films can be deposited on flexible plastic and are predicted as a new technology for touch screens, solar cells, etc. Here we report results on thin films of boron-doped single-walled carbon nanotubes (B-SWNTs) obtained from CarboLex, Inc. Boron-doping is expected to raise the conductance of semiconducting nanotubes while not lowering significantly that of the metallic tubes. At room temperature, we have measured the four-probe sheet resistance and the optical transmission in the NIR-UV range to evaluate the performance of these chemically enhanced SWNT films. The structure in the optical spectrum is essentially the same as in pristine tubes, although the positions of optical absorption bands are slightly upshifted ($\sim $ 50 meV) relative to pristine SWNTs. The B-loading, microstructure, bonding and defects of the B-doped SWNTs were characterized, respectively, by inelastic neutron scattering, transmission electron microscopy, electron energy loss spectroscopy and Raman spectroscopy. Our preliminary results on B-SWNTs show that the visible optical transmittance is higher and the sheet resistance is much lower than that of similar thickness SWNT films. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W31.00003: Raman Spectroscopy Studies of Oxygen and Hydrogen RF-Plasma Treated Single Wall Carbon Nanotubes Chaminda Jayasinghe, David B. Mast We present results from resonant Raman spectroscopy, x-ray photoelectron spectroscopy (XPS) and thermoelectric power (TEP) measurements on hydrogen and oxygen plasma treated single-wall carbon nanotubes (SWNTs). For oxygen treated SWNTs, Raman spectroscopy of the BWF band ($\sim $1522cm-1) show a dramatic up shift for both the $\sim $1522cm-1 band and the $\sim $1578cm-1 band by nearly 20 cm-1. In addition, there is a considerable change in the appearance of the D' mode ($\sim $1620cm-1) with oxygen plasma treated SWNTs. These results show that oxygen plasma treatment affects the metallic nanotubes in our sample more than semiconducting ones. The Raman spectra of hydrogen treated SWNTs show a much smaller D band peak than the oxygen treated SWNTs. The G bands of the hydrogen treated SWNTs are also up shifted compared to untreated nanotubes. The surface of the oxygen plasma treated SWNTs was also analyzed by XPS. These results show that the O1s to C1s intensity ratio is considerably higher in oxygen treated SWNTs compared to untreated nanotubes. The TEP measurements confirm that these plasma treatments induce defects as well as add side wall functionalization to the SWNTs. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W31.00004: Persistent Photoconductivity and Photo-induced Morphology Changes of Porphyrin Nanorods B.E. Feldman, E.A. Muller, V.H. Joines, W.F. Smith, A.D. Schwab, J.C. de Paula, D.E. Johnston, A.T. Johnson Tetrakis(4-sulfonatophenyl) porphine self assembles into well-defined nanorods with intriguing photoelectronic properties.$^{1}$ Recently, we have found that, over long time scales, they undergo a transition from non-persistent photoconductivity (NPPC) to a new mode, in which part of the conductivity persists after the light is blocked, decaying over hundreds of seconds. NPPC initially dominates, but its growth asymptotes within 2-3 hours of illumination, while the persistent current continues to grow, even after 8 hours of light exposure. The decay of persistent current after the light is blocked can be roughly modeled by a single exponential; a double exponential fits much better. The morphology of some nanorods changes as a result of long-term illumination---they become shorter, thinner and less well-formed. Other rods, however, appear unchanged. This and other structural changes may be related to the slow growth of persistent current. $^{1}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W31.00005: Finite Size Effect in ZnO Nanowires Chung-Jen Chien, Pai-Chun Chang, Zhiyong Fan, Jia Grace Lu, Daniel Stichtenoth, Carsten Ronning In this talk, we present electrical and optical measurements on ZnO nanowires whose sizes do not yet reach quantum confinement region. Thin ZnO nanowires were synthesized via carbon thermal chemical vapor deposition method under low growth temperature using tin as catalyst. Electron microscopy reveals that the as-grown nanowires are of high crystalline quality with an average diameter around 12 nm. Electrical transport measurements show significant increase in conductivity with a lack of gate modulation and a reduction in mobility. This phenomenon is attributed to the enrichment of surface states owing to the larger surface-to-volume ratio. This enhanced surface effect in thinner nanowires is confirmed by the temperature dependent photoluminescence measurements. In addition, the photoluminescence spectra clarify the apparent blue shift observed at room temperature with respect to the nanowires with larger diameters. These results provide a fundamental insight into nanowires of smaller diameters, and show that their surface states are extremely important and should be properly tailored or controlled for future device applications. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W31.00006: Electrical and Photoconductive Properties of Vertical ZnO Nanowires in High Density Arrays Zhiyong Fan, Deepanshu Dutta, Chung-Jen Chien, Evan C. Brown, Pai-chun Chang, Jia Grace Lu High density vertical zinc oxide nanowire arrays were synthesized using highly ordered channels in anodic alumina membranes via catalytic chemical vapor deposition assisted by electrochemical deposition methods. The anodic alumina membranes were fabricated using a two-step anodization method. High resolution transmission electron microscopy and energy dispersive x-ray microanalysis studies revealed that the nanowire growth was governed by a vapor-liquid-solid mechanism, and the nanowires are single crystalline grown along the [001] direction. Using conductive atomic force microscopy (AFM), the electrical transport and photoconduction of individual vertical nanowires were investigated. The role of the AFM probe coating on the $I-V$ characteristics will be presented. A negative photoconductivity was first observed as a result of electron trapping in the alumina membrane due to its duplex oxide layered structure. In contrast, positive photoconductivity was observed using a thermally annealed anodic alumina membrane as the nanowire growth template. These studies render a pathway for constructing high density nanoscale electronic and optoelectronic circuits. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W31.00007: Imaging the response of individual carbon nanotubes to polarized light in aqueous environments Bryant Walker, Todd Brintlinger, Michael S. Fuhrer, John Cumings, Erik Hobbie Individual carbon nanotubes are grown using chemical vapor deposition (methane-ethylene carrier gas and iron nitrate catalyst), freely suspended in an aqueous solution using a surfactant (sodium dodecyl sulfate), and imaged in an optical microscope using either fluorescent dye (PKH67 and PKH23) or intrinsic near-infrared fluorescence. Freely suspended, individual carbon nanotubes of length 1-8 micrometers show an increasing response to illuminating light as the polarization becomes parallel to tube axis. More intriguingly, some of the carbon nanotubes are found to collapse and fold under 10-30 seconds of illumination, with increasing tube length showing longer time-to-collapse. Unperturbed persistence lengths in these nanotubes are estimated to be 200-300 micrometers. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W31.00008: Raman spectroscopic investigation of the confined optical phonon modes in the aligned CdSe nanorod arrays Concetta Nobile, Luigi Carbone, Stefan Kudera, Liberato Manna, Roberto Cingolani, Roman Krahne, Vladimir A. Fonoberov, Alexander A. Balandin, Gerwin Chilla, Tobias Kipp, Detlef Heitmann Nanocrystal rods have emerged as promising nanostructured material for both fundamental studies of nanoscale effects and for optical and electronic device applications. We investigated the optical phonon excitations in laterally aligned CdSe nanocrystal rod arrays using resonant Raman scattering. Electric-field mediated alignment between interdigitated electrodes has been used to prepare the samples. We report Raman experiments that probe the optical lattice vibrations in ordered arrays of CdSe nanorods with respect to the nanorod orientation. The packing of nanorods into dense arrays leads to the suppression of the surface optical phonon modes. In the longitudinal-optical phonon peak we observe a fine structure that depends on the relative orientation of the nanorods with respect to the incident light polarization. Detailed comparison of the experimental data with the first-principle calculations for corresponding nanostructures, which reveal the symmetry of the phonon potentials for the Raman active modes, provides a qualitative explanation of the experimentally observed phonon modes. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W31.00009: Raman Scattering from Si$_{1-x}$Ge$_{x}$ Alloy Nanowires Qiujie Lu, Kofi Adu, Xi Zhang, Kok-Keong Lew, Pramod Nimmatoori, Elizabeth Dickey, Joan Redwing, Peter Eklund Bulk Si$_{1-x}$Ge$_{x}$ crystals can be prepared over a wide composition range 0$<$x$<$1. These materials are of interest because alloying can be used to vary the bandgap of the system. Here we present Raman scattering results on Si$_{1-x}$Ge$_{x}$ nanowires (0$<$x$<$1) grown by the vapor-liquid-solid growth mechanism using a Chemical Vapor Deposition (CVD) approach. TEM and XRD were used to characterize the morphology growth axis and lattice constant of these materials. Typical wire diameters were observed to be in the range 80-130 nm. Based on Raman scattering studies of the bulk, three Raman bands are expected that can be identified as a perturbed Si-Si ($\sim $500 cm$^{-1})$ mode, a Ge-Ge ($\sim $280 cm$^{-1})$ mode or a new mode at ($\sim $390 cm-1) assigned to Si-Ge or Ge-Si clusters. Peaks in this region are also observed in the case of our nanowires, although the frequencies are a few cm$^{-1}$ lower than observed in the bulk. We also observe that the compositional (x) dependence of the Si-Ge band in nanowires is somewhat different than in the bulk. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W31.00010: Transport and optical properties of electrochemically fabricated Bi nanowires Hong Zhang, Haidong Liu, Zuxin Ye, Wenhao Wu, Serguei Jerebtsov, Alexandre Kolomenski, Hans Schuessler We present transport and optical studies of Bi nanowires electrochemically deposited into porous aluminum oxide (AAO) membranes. The Bi wires have nominal diameters of 20$\sim $100 nm and length of 60 $\mu $m. For transport measurements, electric contacts with negligible contact resistance were formed on single Bi nanowires \textit{in-situ} during electrochemical deposition. The temperature dependence of the resistance of Bi single nanowires in AAO showed a semiconductor-semimetal transition when the samples were cooled~below 50 K. The transverse magnetoresistance increases monotonically up to the highest available field of 8 T, while the longitudinal magnetoresistance tends to flatten at high field values. For optical measurements we first etched away the membranes and suspended Bi nanowires in water and then collected Bi nanowire on glass substrates. Coherent optical phonons were studied in Bi nanowires using a femtosecond pump-probe technique. The frequency of the excited phonon oscillations was found to be 2.35 THz at a pump fluency of 10 mJ/cm$^{2}$. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W31.00011: Suspension of SWNTs in water: a dynamic optical study Goki Eda, Giovanni Fanchini, Manish Chhowalla Dispersion of single-wall carbon nanotubes (SWNTs) in water is critical for processing. Dispersions are often unstable and the dynamics associated with SWNT bundle formation are poorly understood. We have devised a simple method to examine the dispersion properties based on changes in transmittance at three different levels of the suspension using 650 nm lasers. Our results show that the suspension is stable up to 1 hr with the transmittance remaining constant. Above 1 hr, anomalous transmittance behavior is observed. It initially increases, as expected, but then decreases for a short amount of time before constantly increasing. These fluctuations suggest Mie scattering from changes in the SWNT bundle size. The dynamic nature of the suspension and the fact that the SWNTs and their bundles are significantly smaller than the wavelength of the lasers means that the interpretation of the transmittance data requires careful analysis in terms of the effective medium theory. Our findings provide an understanding of the SWNT suspension behavior which has important implications for the emerging applications of SWNTs based on solution processing. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W31.00012: Optical Properties of Nanostructured of Ce-doped Y$_{2}$SiO$_{5}$ Luiz G. Jacobsohn, Bryan L. Bennett, Ross E. Muenchausen, James F. Smith, Stephanie C. Sitarz, Michael W. Blair, D. Wayne Cooke Nanophosphors correspond to nanostructured inorganic insulator materials that emit light under particle or electromagnetic radiation excitation. In this work, we present structural and optical characterization of Ce-doped Y$_{2}$SiO$_{5}$ nanophosphor prepared by the solution combustion method. Characterization by TEM and x-ray diffraction shows that nanopowders are composed of 30-70 nm nanocrystals agglomerated into micron-sized particles. The Ce content was varied up to 10 at.{\%}. Photoluminescence excitation and emission spectra are composed of two major bands centered at 360 and 430 nm, respectively. These results revealed larger Stokes shift for the nanophosphors when compared to bulk. Ce content was also found to affect photoluminescence emission intensity and lifetime. Concentration quenching curve presents a broad maximum centered at 1 at.{\%}. Lifetime measurements showed a continuous decrease from 34 to 21 ns for higher Ce contents. These results confirm the uniqueness nature and properties of nanophosphors, and show that nanophosphors are promising materials for new basic science and technological applications. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W31.00013: Optical Properties of Molecular Dots Timothy Russin, Gugang Chen, James Adair, Peter Eklund A ``Molecular Dot'' (or ``M-dot'') refers to a mixed organic/inorganic phase of nanomatter where a small number of organic molecules are encapsulated in an inorganic nanoparticle. Particular interest has been initiated in these systems when the molecules exhibit photoluminescence (PL) and the nanoparticle provides a transparent medium allowing easy entrance and exit of photons. They show promise for medical applications. In preliminary experimental studies, the encapsulation has been found to enhance the PL and suppress the photo-degradation of organic dye molecules such as Rhodamine B encapsulated in SiO$_{2}$ or CaPO$_{4}$. In this paper, we present the results of an optical model to predict the optical properties of M-dots. Using the discrete dipole approximation, we take into account the effects of Mie scattering and the effective dielectric function of the dye molecules encapsulated in an inorganic host of known refractive index. The results of the modeling will be compared to recent experimental results on M-dots in dilute solution, i.e., optical absorption and dispersion in the NIR-Vis-UV regions. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W31.00014: Synthesis and Optical Properties of Boron Nitride Nanotubes, Nanowires and Nanorods Ying Chen, Hua Chen, Jun Yu, Hongzhou Zhang Quasi-one-dimensional boron nitride nanomaterials such as BN nanotubes, BN nanowires, BN nanorods, and BN whiskers have different nanostructures but uniform electronic band gaps independent of their diameters and chiralities. Their quantum confinement effects in these low dimensional materials can enhance their optical emission substantially by inducing an indirect-to-direct conversion of the optical transition. Different nanostructures and dimensions have different emission behaviors. Therefore, one-dimensional BN nanomaterials are likely to find further applications in optoelectronics. We have achieved controlled and patterned growth of the BN nanotubes [1,2], BN nanowires [3,4] and conical boron nitride nanorods [5, 6] and investigated their optical properties including photoluminescence, cathodoluminescence and Raman spectroscopy. Relationships between the different nanostructures and corresponding properties will be discussed. [1] J. Yu, Y. Chen, et al., Chemistry of Materials, 17 (2005) 5172. [2] H. Chen, Y. Chen, et al. Chemical Physics Letters 42(2006) 315. [3] YJ Chen, H. Zhang, Y. Chen, Nanotechnology, 17 (2006) 786. [4] YJ Chen, et al., Nanotechnology, 17 (2006) 2942 [5] H. Zhang, et al., Physical Review B, 74 (2006) 045407 [6] H. Zhang, et al., Applied Physics letters, 88 (2006)093117. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W31.00015: Second-Harmonic Generation of Aligned Single-Walled 0.4nm Carbon Nanotubes Kam Sing Wong, Huimin Su, Jianting Ye, Zikang Tang The second-harmonic generation (SHG) is measured for the first time from monosized and well-aligned single-walled carbon nanotubes (SWCNT) in the channel of aluminophosphate AlPO4-5 (AFI) zeolite. The SHG yield scales as quadratic function of the pump laser intensity. Due to the different polarization preference, we are able to discriminate the SHG contribution from the chiral (4,2) CNTs and those from the AFI template. The polarization direction and the anisotropic dependence of the SHG intensity on the excitation polarizations are investigated in the transmission geometry. In the case of normal incidence, the intensity of SHG is maximized when the excitation polarization is 45 degree against the tube axis and the SH radiation is linear-polarized on the plane perpendicular to the tube axis. The experiment results are in excellent agreement with the theoretical prediction of the second-order nonlinear optical process in chiral carbon nanotubes. [Preview Abstract] |
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