Correlation between in Situ Raman Scattering and Electrical Conductance for an Individual Double-Walled Carbon Nanotube Shaoning Yuan, † Qing Zhang,* ,† Yumeng You, ‡ Ze-Xiang Shen, ‡ Daisuke Shimamoto, § and Morinobu Endo § Microelectronics Center, School of Electrical and Electronic Engineering, Nanyang Technological UniVersity, S1-B2c-20, 639798, Singapore, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, 21 Nanyang Link, 637616, Singapore, and Faculty of Engineering, Shinshu UniVersity, 4-17-1 Wakasato, Nagano-shi 380-8553, Japan Received October 21, 2008; Revised Manuscript Received November 25, 2008 ABSTRACT In situ Raman scattering is performed on an individual semiconducting double-walled carbon nanotube (DWNT) in a field-effect transistor (FET) geometry, while the transfer characteristics of the DWNT-FET are measured. Through studying the Raman spectra with response to forward and backward gate voltage (V gs ) sweeping, respectively, we observe hysteresis loops in the curves of G - peak frequency and the intensity ratio of G - to G + (I G - /I G + ) as a function of V gs . These loops correlate very well with the hysteretic transfer characteristics of the device. The clear correlations suggest that G - peak line width and I G - /I G + increase with the carrier concentration in the DWNT induced by V gs . In addition, unique G - peak line width variations with V gs can be attributed to interband electron transitions between the energy bands of two concentric shells of the DWNT excited by G phonons. In situ Raman scattering is a nondestructive optical technique which can be used to study electron transport in a carbon nanotube (CNT) on a field-effect transistor (FET) platform while the CNT-FET is under real-time operation. 1,2 The Raman scattering from an individual single-walled carbon nanotube (SWNT) with diameter ∼2.4 nm on an FET platform has been reported by Tsang et al. in 2007. 1 For a metallic (m-) SWNT, both frequency (f) and line width (Γ) of its G - are dependent on gate voltage (V gs ). 1 In contrast, for a semiconducting (s-) SWNT, the frequency of its G + is a function of V gs . 1 The frequency shift has been interpreted as the change of charge density (F) in the SWNT channel while the line width change is associated with excitations of electron-hole pairs by G phonons. 1 When electrons are excited by G phonons from a lower to a higher energy band, the line width of G - is broadened. Since the energy of G phonons is ∼0.2 eV, the transition of electrons excited by G phonons are only allowed when the energy difference between two energy bands is ∼0.2 eV. The line width broadening caused by the transition is easily observed in a m-SWNT, i.e., when the Fermi level ε f is located around the center of the band diagram (intrinsic m-SWNT), electrons could be excited by G phonons and a broadened line width is observed. Moreover, when ε f is shifted by V gs to a higher (lower) position, the transition is suppressed due to electronic states for the transition are filled (evacuated), resulting in a line width narrowing. 1 Simultaneously, the blue shift of G - frequency is observed because F is increased by V gs . 1 However, for a s-SWNT with a diameter less than 3 nm, the energy bandgap is larger than 0.3 eV. Thus, the line width broadening is not observed. Interestingly, for an individual semiconducting double-walled carbon nanotube (DWNT), it is found that both the frequency shift and line width change of the G - band are functions of V gs . 2 Analogous to the interpretation for an s-SWNT, the frequency shift for a DWNT-FET has been attributed to the change in F induced by V gs . 2 In contrast, the unique line width change is interpreted as an interband electron-phonon coupling be- tween the valence bands of two concentric shells of the DWNT. 2 In addition, the intensity ratio of G - to G + * Corresponding author: tel, +65 67905061; fax, +65 67920415; e-mail, eqzhang@ntu.edu.sg. † School of Electrical and Electronic Engineering, Nanyang Technological University. ‡ School of Physical and Mathematical Sciences, Nanyang Technological University. § Faculty of Engineering, Shinshu University. NANO LETTERS 2009 Vol. 9, No. 1 383-387 10.1021/nl803188g CCC: $40.75  2009 American Chemical Society Published on Web 12/12/2008