We present transmission spectra from the far infrared through the ultraviolet region on

freestanding SWNT films at temperatures between 40 and 300 K. Several interesting features are

observed in the low-frequency part of the spectrum: the Drude-like frequency dependence of the

metallic tubes as well as a (sample-dependent) peak in the conductivity around 0.01 eV. We also

studied the accidental nitrate doping of the SWNT samples during purification by nitric acid. Asprepared

purified samples exhibit increased metallic absorption and decreased interband transitions;

these features disappear on heating in vacuum

Borondics, Ferenc

Chen, Z.

Kamarás, Katalin

Nikolou, M.

Rinzler, A. G.

Tanner, D. B.

English

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Wide Range Optical Studies on TransparentSWNT FilmsF. Borondics∗, K. Kamarás∗, Z. Chen†, A.G. Rinzler†, M. Nikolou† andD.B. Tanner†∗Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences,P. O. Box 49, Budapest, H-1525, Hungary†Department of Physics, University of Florida, Gainesville, FL 32611, U.S.A.Abstract. We present transmission spectra from the far infrared through the ultraviolet region onfreestanding SWNT films at temperatures between 40 and 300 K. Several interesting features areobserved in the low-frequency part of the spectrum: the Drude-like frequency dependence of themetallic tubes as well as a (sample-dependent) peak in the conductivity around 0.01 eV. We alsostudied the accidental nitrate doping of the SWNT samples during purification by nitric acid. As-prepared purified samples exhibit increased metallic absorption and decreased interband transitions;these features disappear on heating in vacuum.INTRODUCTIONFrequency-dependent optical measurements on carbon nanotubes can render a wealthof information about their electronic structure. Early optical evidence for metallic (orsemimetallic) carriers in single-wall carbon nanotubes came from reflectance spectraobtained in a wide temperature and frequency range [1]. Similar transmission studieson airbrushed nanotubes [2] and freestanding films [3] have not included temperaturedependence. Here we present temperature-dependent spectra and the effect of accidentalnitrate doping during purification, which influences the far-infrared part of the spectrumas well as the interband transitions.EXPERIMENTALSelf-standing SWNT films were prepared by filtration of a dilute solution [3] of laser-produced material [4]. The sample used for optical measurements was transferred to agraphite template with an approximately 4 mm hole. This sample geometry and the hightransparency of the film allowed us to measure transmission with high reproducibility.We used two different spectrometers with liquid helium cryostats: in the FIR throughMIR range a Bruker 113v spectrometer, in the NIR, visible and UV a modified Perkin-Elmer grating spectrometer. Spectra were obtained before and after heat treatment invacuum at 1000◦ C(to remove HNO3 that p-dopes the material; the procedure is hereafterreferred to as "baking"). We examined three different samples. Samples 1 and 2 were ofthe same batch, one baked and one unbaked; sample 3 was from a subsequent batch,CP723, Electronic Properties of Synthetic Nanostructures, edited by H. Kuzmany et al.© 2004 American Institute of Physics 0-7354-0204-3/04/$22.00137Downloaded 17 May 2007 to 128.227.127.61. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsphere data were taken on the same specimen before and after baking.FIGURE 1. Transmission of SWNT films before and after baking.RESULTS AND DISCUSSIONFigure 1 shows the transmission of the samples at room temperature. The films reachtransmission values of up to 80 per cent around 3000 cm−1 and are highly transparent(above 30 per cent) throughout the whole midinfrared range. Baking increases trans-parency in the far infrared and decreases it somewhat at higher frequencies. For appli-cation purposes, obviously the featureless midinfrared region is of importance, but theinformation about fundamental materials properties is carried by the highly absorbingstructures in the far and near infrared.The transmission must be consistent with Kramers-Kronig relations, just as is thereflection, and the phase shift on transmission may be estimated from a Kramers-Kronigintegral on lnT , just as with the reflectance. After computing the phase, one may extractthe complex refractive index (and all other optical constants) using equation (1).√T eiΘ =4N(N+1)2eiδ − (N−1)2eiδ , (1)where δ = ωNdc. The sample thickness d can be a fitting parameter as well, but in ourcase we used 350 nm for Sample 1 and 2 and 150 nm for Sample 3, obtained from AFMmeasurements.We fitted the transmission data by a Drude-Lorentz model. In the fits to the opticalconductivity σ1 of Sample 2 (totally removed HNO3) one can clearly see a low frequency138Downloaded 17 May 2007 to 128.227.127.61. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsposcillator (Figure 2.). This phenomenon has been predicted, attributed to a secondary gapcaused by the curvature of certain semimetallic tubes [5] or the symmetry breaking bythe neighboring tubes in metallic nanotube bundles [6]. Ouyang et al. [7] have measuredboth gaps on individual nanotubes by tunneling spectroscopy; based on their results, weattribute the 8 meV peak to a curvature-induced gap in low-gap semiconducting tubes.Consequently, we assign the broader Drude contribution to metallic (armchair) tubes.The sample dependence of the low-frequency behavior is probably caused by the factthat the small gaps are easily filled by extrinsic carriers even at minimal doping.FIGURE 2. Fits to room-temperature optical conductivity in Sample 2 and 3.Figure 3. summarizes the oscillator strength values for all measured temperatures inSample 3. It is apparent that observable changes on dedoping occur almost exclusivelyin the narrow Drude part. This change is accompanied by a sharpening and strengtheningof the high-frequency van Hove transitions, already shown by Hennrich et al. [8],as the valence bands of high-gap semiconducting tubes are filled by electrons. Theoscillator strength shows little temperature dependence. Such behavior is expected innormal metals, however, we see also very weak dependence of the scattering rates ontemperature. This is consistent with previous optical [1] and dc measurements and pointsto the possibility of exotic transport mechanisms in nanotubes.CONCLUSIONSWe measured the transmission spectra of ultrathin SWNT films in a wide temperatureand frequency range. The low-frequency part of the spectrum can be fitted with twocomponents, which we attribute to armchair and low-gap semiconducting tubes, respec-tively. Optical signatures of hole doping can be observed only in the semiconducting139Downloaded 17 May 2007 to 128.227.127.61. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jspFIGURE 3. Temperature dependence of low-frequency oscillator strength in Sample 3 before and afterbaking.tubes (both low- and high-gap). The temperature dependence of the optical functions ismuch weaker than what would be expected for conventional metals.ACKNOWLEDGMENTSThis work was supported by the following grants: OTKA 034198, ARO grant DAAD19-99-1-0316, NSF (ECS-0210574), US NSF-INT 9902050.REFERENCES1. Ugawa, A., Rinzler, A. G., and Tanner, D. B., Phys. Rev. B, 60, R11305 (1999).2. Itkis, M. E., Niyogi, S., Meng, M. E., Hamon, M. A., Hu, H., and Haddon, R. C., Nano Lett., 2, 155(2002).3. Hennrich, F., Lebedkin, S., Malik, S., Tracy, J., Barczewski, M., H. Rösner, and Kappes, M., Phys.Chem. Chem. Phys., 4, 2273 (2002).4. Rinzler, A. G., Liu, J., Dai, H., Nikolaev, P., Huffmann, C. B., F. J. Rodriguez-Macías, Boul, P. J., Lu,A. H., Heymann, D., Colbert, D. T., Lee, R. S., Fischer, J. E., Rao, A. M., Eklund, P. C., and Smalley,R. E., Appl. Phys. A, 67, 29 (1998).5. Kane, C. L., and Mele, E. J., Phys. Rev. Lett., 78, 1932 (1997).6. Delaney, P., Choi, H. J., Ihm, J., Louie, S. G., and Cohen, M. L., Nature, 391, 466 (1998).7. Ouyang, M., Huang, J.-L., Cheung, C. L., and Lieber, C. M., Science, 292, 702 (2001).8. Hennrich, F., Wellmann, R., Malik, S., Lebedkin, S., and Kappes, M. M., Phys. Chem. Chem. Phys.,5, 178 (2003).140Downloaded 17 May 2007 to 128.227.127.61. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

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Wide range optical studies on transparent SWNT films

Wide range optical studies on transparent SWNT films

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