Photocurrent spectroscopy of double wall carbon nanotubes

We describe spectroscopy measurements of double wall carbon nanotubes using a displacement photocurrent technique. Peaks in the photocurrent are observed at excitation energies corresponding to the spacing of van Hove singularities for both the inner and outer shells of the nanotube. The shell assignment for the peaks can be made by comparing the spectra before and after the formation of the inner shell. The level spacing observed for the inner shell suggests that the standard tight binding model is insufficient to describe the electron energy spectra for extremely small tubes. 2005 Elsevier B.V. All rights reserved. One of the fascinating aspects of single wall nanotube (SWNT) electronics is the clearly observable inter-relationship between nanotube structure and electronic spectra. The variation of the electron density of states with nanotube diameter and chirality has been well documented in both experiment and theory [1]. Multi wall nanotubes (MWNTs) have also received considerable experimental attention, in part because their large size makes them relatively easy to incorporate into electronic devices [2]. The MWNT electronic spectrum is difficult to model, however, since calculations must include a large number of individual shells, together with shell– shell interactions. Recently, techniques for synthesizing double wall nanotubes (DWNTs) have been developed [3–6]. Being the simplest form of multi wall tube, DWNTs provide a tractable system to explore the electron energy spectrum of a multi-shell system. Preliminary experiments on DWNTs have mainly concentrated on describing the vibrational spectra of the inner and outer shells using Raman techniques. There have been 0009-2614/$ see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.06.109 * Corresponding author. Fax: +1 502 852 1577. E-mail address: [email protected] (B.W. Alphenaar). few, if any, measurements of the DWNT electron energy spectrum. Here, we report on photocurrent spectroscopy measurements to determine the electron energy spectrum of DWNTs. We observe peaks in the photocurrent at excitation energies that we relate to the spacing of van Hove singularities in the electron density of states for both the inner and outer shells. The level spacing observed for the outer shell agrees with predictions of the standard zone folding approach applied to tight binding calculations for a graphene sheet. The level spacing associated with the inner shell, however, appears to be influenced by the effects of curvature on neighboring atomic orbitals. Overlap between the inner and outer shell features results in an apparent shift in the outer shell peaks following the formation of the inner shell. DWNTs are created using the peapod synthesis technique [3], which provides the possibility to study directly the transformation from a SWNT into a DWNT. First, SWNTs are synthesized using high-temperature pulsed laser vaporization of a carbon target containing approximately 1% Fe–Ni. Following acid purification, the SWNTs are heated at 420 C in dry air to remove A. Mohite et al. / Chemical Physics Letters 412 (2005) 190–194 191 residual amorphous carbon and to open the tube ends. Next, the SWNTs are combined with C60 molecules in a sealed ampoule and heated to 400 C. This results in the diffusion of the C60 vapor into the interior of the SWNTs forming peapods , or SWNTs containing chains of C60 molecules. Finally, the peapods are heated at 1200 C in vacuum, causing the C60 molecules to coalesce into small diameter tubes. The result is a double-shelled tube structure, with the inner shell diameter located around that of the C60 molecules and the outer shell diameter defined by the original SWNT. Transmission electron microscopy of the DWNTs synthesized in this way (inset to Fig. 1) shows that they are highly uniform with an average outer shell diameter of 1.4 nm and an average inner shell diameter of 0.7 nm. Raman scattering spectra were taken of our SWNT and DWNT samples at room temperature in the true backscattering configuration with an incident beam perpendicular to the sample surface using 1064-nm NdYAG-excitation. The Raman scattered laser light was analyzed using a FT-Raman spectrometer (BOMEM DA3+) equipped with a holographic notch filter to eliminate the elastically scattered light. Fig. 1 shows the Raman spectra of the SWNT (dashed line) and DWNT (solid line) samples. (The DWNT was formed using the peapod synthesis technique on the SWNT sample, so that the two may be compared directly.) Data are plotted for Raman shifts from 100 to 400 cm 1 in the range of the radial breathing modes. For the SWNTs, peaks are observed at x1(outer) = 162 cm 1 and x2(outer) = 178 cm . The diameter, d, of the outer shell can be estimated from d = A/xr, where A is a constant and xr is the radial breathing mode frequency. Using a value for A = 234 cm 1 (averaged from those available in the literature [7]) gives d1(outer) = 1.44 nm and d2(outer) = 1.32 nm. After the SWNTs are transformed into DWNTs, two additional peaks appear at x1(inner) = Fig. 1. Room temperature Raman spectra taken for the SWNT (dashed line) and DWNT (solid line) samples for an excitation wavelength of 1064 nm in the range of the radial breathing modes. Inset: TEM image of a typical DWNT. 337 cm 1 and x2(inner) = 360 cm . The inner diameter radii determined from these peaks are d1(inner) = 0.69 nm and d2(inner) = 0.65 nm. The inner and outer shell diameters obtained from the Raman spectra are in good agreement with those obtained from the TEM analysis. To probe the electron energy spectrum of the SWNT and DWNT samples, we use a displacement photocurrent spectroscopy technique – the details of which have been recently described [8]. Briefly, a nanotube film is deposited on a glass slide that is anchored to a grounded copper block within an optical access flow cryostat (see Fig. 2a). The nanotubes are coupled to ground capacitively, through the glass dielectric, and coupled to the high side of the circuit by a wire silver-painted to a corner of the film. Pulsed laser light incident on the nanotube film excites charge carriers, which redistribute to create an ac voltage across the nanotube/insulator/metal capacitor. This is measurable as a displacement current from the nanotube film to ground. We illuminate the sample using an optical parametric amplifier (OPA) excited by a pulsed Ti Sapphire regenerative amplifier. The pulse width is 120 fs with a repetition rate of 1 kHz and the photon energy is tunable between 0.4 and 4 eV. The laser power is kept constant at 25 mW. The resulting displacement current is amplified and measured using a lock-in detector. Fig. 2b shows the displacement photocurrent of a SWNT film measured at 300 K and with a dc bias of 10 V. A series of peaks are observed as a function of incident photon energy, the lowest three having energies of 0.62, 1.39, and 1.85 eV, along with at least two additional peaks at higher excitation energies. Similar peaks have been observed in absorbance measurements of SWNT films and have been attributed to excitation across van Hove singularities in the electron energy spectrum [9,10] (or more recently, to the formation of excitons [11,12]). The standard approach to determine the nanotube electron energy spectrum is zone-folding of the tight binding calculated dispersion for a graphene sheet (ZF-TB model) [13]. This method ignores the influence of curvature on the carbon–carbon bonds, but is thought to be accurate for large diameter tubes. Fig. 3a shows predictions of the ZF-TB model for the separation of the lowest three energy transitions (S11, S22, and M11) in the nanotube density of states, for all possible nanotubes with diameters ranging from 1.2 to 1.55 nm. (We do not consider the higher energy transitions, because there is considerable overlap in the transition energies in this regime, making mapping to the data difficult.) We also plot the positions of the three lowest energy SWNT photocurrent peaks (solid circle, square, and diamond) assuming a diameter of 1.48 nm (the average of the two diameters obtained from the Raman data). It is seen that the peaks in the SWNT displacement current spectrum are in good agreement with the ZF-TB model. Recent work has suggested that