Khoi T. Nguyen

University of Illinois, Urbana-Champaign, Urbana, IL, United States

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Publications (8)48.61 Total impact

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    ABSTRACT: Raman spectra of electrostatically gated single-layer graphene are measured from room temperature to 560 K to sort out doping and thermally induced effects. Repeated heating cycles under Ar led to convergent first-order temperature coefficients of the G-band (χ(G) = -0.03 cm(-1)/K) and the 2D-band (χ(2D) = -0.05 cm(-1)/K) frequencies, which are independent of doping level as long as the Fermi level does not shift with temperature. While the intrinsic behavior may be different (e.g., χ(G) ∼ -0.02 cm(-1)/K near room temperature), these values appear more appropriate in describing responses of most graphene samples on SiO(2) substrates. The more negative χ(G) value than theoretical expectations may be explained by interactions with the substrate reducing the lattice thermal expansion contribution to the temperature dependence of G-band frequency. Enhanced interactions with the substrate may also be responsible for zero-charge, room-temperature G-band line width increase and 2D-band frequency downshift.
    ACS Nano 05/2011; 5(6):5273-9. · 12.03 Impact Factor
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    ABSTRACT: How doping and defects alter linewidths and lifetimes of G-band optical phonons, in carbon nanotubes is examined. Optical phonon lifetimes, T1, in thin films of nanotubes are measured by time-resolved incoherent anti-Stokes Raman spectroscopy and considered along with Raman linewidths of isolated individual nanotubes. Within the doping range achievable in nanotube films in this study, T1 does not appear to change. A varying degree of doping in individual nanotubes by means of electrostatic gating reveals decreasing full-width at half-maximum Γ down to ∼4 cm-1 at the charge neutrality point. Increasing disorder, on the other hand, leads to a decrease in T1 along with an increase in Γ. We observe a decrease in T1 of ∼0.4 ps at an estimated effective crystallite size La ∼ 130 nm based on the D-band to G-band peak intensity ratio. In the limit of zero doping and zero defects, the measured Γ of single semiconducting nanotubes coincides with a lifetime broadening of ∼4 cm-1 based on a measured T1 of 1.2 ps. Samples displaying different degree of metallic or semiconducting contributions in their static Raman spectrum are also compared and are shown to exhibit similar values of T1.
    Physical Review B 05/2011; 83(20). · 3.66 Impact Factor
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    ABSTRACT: The relaxation of electronic excitations induced by high bias or photoexcitation occurs primarily through optical phonon emission. Optical phonon relaxation may be affected by metallic/semiconducting character of carbon nanotubes, defect concentration, as well as doping. Changes in carbon nanotube G-band optical phonon population and pure dephasing lifetimes with doping and defects are described. Time-resolved incoherent anti-Stokes Raman spectroscopy is used to directly measure phonon decay lifetime, T1, while total dephasing rate is inferred from static Raman linewidths. Defect concentration is varied by sample annealing and covalent functionalization showing increasing disorder reduces T1 as well as overall dephasing time, T2, with an even greater dependence. Samples with different metallic and semiconducting contribution have similar lifetimes, T1˜ 1.2 ± 0.1 ps in the no defect limit. Doping is shown to increase G-band linewidth, and therefore overall dephasing rate, for semiconducting nanotubes while leaving T1 unaffected.
    03/2011;
  • Khoi T Nguyen, Moonsub Shim
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    ABSTRACT: We have examined how electrical characteristics and charging dependent Raman G-band phonon softening in individual metallic carbon nanotubes are influenced by covalent defects. In addition to decreasing electrical conductance with increasing on/off current ratio eventually leading to semiconducting behavior, adding covalent defects reduces the degree of softening and broadening of longitudinal optical (LO) phonon mode of the G-band near the charge neutrality point where the bands cross. On the other hand, the transverse optical (TO) mode softening is enhanced by defects. Implications on the interpretation of Raman G-band phonon softening and on utilizing Raman spectroscopy to examine covalent functionalization are discussed.
    Journal of the American Chemical Society 06/2009; 131(20):7103-6. · 10.68 Impact Factor
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    ABSTRACT: Diversity in the Raman G-band phonon modes within individual metallic carbon nanotubes of the same chirality is examined. Comparisons between Raman spectra of as-synthesized nanotubes with those obtained under electrochemical gate potential are made. We show that most of the distribution in line width and peak position of the G-band modes within a single chirality type can be explained by variations in where the Fermi level lies with respect to the band crossing point (i.e., where the nanotube is at zero charge). Varying degree of charge transfer from adsorbed O2 is likely to be the main source of 2 eV or larger range of Fermi level positions. On average, the Fermi level of individual metallic nanotubes lies on the order of 1 eV below the band crossing point. Both charge transfer and physical disorder are evident upon O2 adsorption. Implications of these findings on electron−phonon coupling and charge transfer processes are discussed.
    The Journal of Physical Chemistry C 07/2008; · 4.84 Impact Factor
  • Daner Abdula, Khoi T. Nguyen, Moonsub Shim
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    ABSTRACT: Changes in the Raman spectra of individual metallic single-walled carbon nantoubes (SWNTs) upon sidewall covalent bond formation have been studied. In light of the Fermi level shift dependent G-band line shape and D-band intensity in metallic tubes, the validity of commonly used D to G intensity ratio as an assessment tool for covalent bond formation is first examined. The evolution of G-band and disorder Raman spectral features upon covalent reaction with 4-bromobenzene diazonium tetrafluoroborate is then presented. The initial degree of disorder has a strong influence on the covalent sidewall functionalization. Implications on developing electronically selective covalent chemistries and assessing their selectivity in separating metallic and semiconducting SWNTs are discussed.
    The Journal of Physical Chemistry C 05/2007; 111(48). · 4.84 Impact Factor
  • Khoi T Nguyen, Anshu Gaur, Moonsub Shim
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    ABSTRACT: Evolution of G-band modes of single metallic carbon nanotubes with the Fermi level shift is examined by simultaneous Raman and electron transport studies. Narrow Lorentzian line shape and upshifted frequencies are observed near the van Hove singularities. However, all G modes soften and broaden at the band crossing point. The concurrent appearance of an asymmetric Fano line shape at this point indicates that phonon-continuum coupling is intrinsic to single metallic tubes. The apparent Lorentzian line shapes of as-synthesized metallic tubes are induced by O2 adsorption causing the Fermi level shift.
    Physical Review Letters 05/2007; 98(14):145504. · 7.73 Impact Factor
  • D. Abdula, K. T. Nguyen, M. Shim
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    ABSTRACT: Changes in the Raman spectra of individual metallic single-walled carbon nantoubes (SWNTs) upon sidewall covalent bond formation have been studied. In light of the Fermi level shift dependent G-band line shape and D-band intensity in metallic tubes, the validity of commonly used D to G intensity ratio as an assessment tool for covalent bond formation is first examined. The evolution of G-band and disorder Raman spectral features upon covalent reaction with 4-bromobenzene diazonium tetrafluoroborate is then presented. The initial degree of disorder has a strong influence on the covalent sidewall functionalization. Implications on developing electronically selective covalent chemistries and assessing their selectivity in separating metallic and semiconducting SWNTs are discussed.
    The Journal of Physical Chemistry C 01/2007; 111:17755-17760. · 4.84 Impact Factor