Supplementary Materials for Terahertz Conductivity and Infrared Transmission of Hybrid Tellurium Nanowire/PEDOT:PSS Materials:

Vibrational spectra.

J. N. Heyman, B. Alebachew*, Z. Kaminski, M. Nguyen*

Physics Department, Macalester College, St. Paul, MN 55105

N. E. Coates, J. J. Urban

The Molecular Foundry, Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720

We have used IR and Raman spectroscopy to investigate the vibrational modes of the tellurium nanowire (TeNW), PEDOT:PSS (PD) and hybrid (TeNW/PD) materials used in this study. Far-infrared absorption probed tellurium phonon modes in the nanowires. Mid-infrared absorption and Raman scattering were used to examine the vibrational modes of PEDOT:PSS in the PD and TeNW/PD materials. The goal of these measurements was to look for shifts in vibrational mode frequencies as markers for possible structural changes in the PEDOT due to the introduction of the nanowires. Structural changes associated with increases in DC conductivity have been reported in PEDOT:PSS doped with solvents1-3. However, while we did see spectroscopic evidence for a decrease in free-carrier density, we did not find evidence of any structural change accompanying the introduction of the nanowires.

We observe optical phonons in the tellurium nanowires in far-infrared absorption (Figure S1). Three IR-active TO phonon modes are observed in bulk tellurium4: the A2 mode couples to light polarized along the crystallographic c-axis, while the E’ and E’’ modes couple to light polarized perpendicular to c. In the TeNW material, an asymmetric peak is observed at 2.80 THz in all samples. Two additional modes are observed at 4.45 THz and 5.19 THz in Fig. 4, and the frequencies of these modes are sample dependent. In the TeNW/PD hybrid material, a single, narrow symmetric absorption line is observed at 2.76 THz and a weak, broad feature is observed at ~4.5THz.

Mid-infrared absorption spectra show a series of vibrational modes associated with PEDOT (Figure S2). We do not observe significant shifts in mode frequencies between the PD and TeNW/PEDOT samples. Instead we observe a shift in the free-carrier absorption background and a change in the relative strengths of low and high frequency vibrational modes, with bands in the spectral range 50meV – 125 meV weaker relative to bands in the range 150-200meV in the TeNW/PD material than in the PD material. Previous studies5, 6 have identified C-C, C-O-C and C-S stretching bands in infrared absorption in PEDOT, and Kvarnstrom, et. al.,5 have reported an increase in the relative strength of low-frequency C-S vibrational modes to higher frequency C-C modes with increasing hole concentration. Therefore the shift in line-strengths we observe is consistent with a lower hole concentration in the TeNW/PD material relative to the PD material.

Raman spectra of our samples (Figure S3) were obtained with a Sierra IM-52 Raman Spectrometer with laser excitation at l = 785nm, using 40mW power and a 40X objective. Raman-active PEDOT vibrational modes are observed on a large fluorescence background. Ouyang, et. al.,1, 2 reported modes at 176 meV and 179 meV in PEDOT:PSS which they associated with C-C bands in quinoid and benzoid PEDOT isomers and reported an increase in the relative strength of quinoid mode that correlated with an increase in DC conductivity in solvent-doped PEDOT:PSS. We see no clear change in the relative strength of the benziod and quinod features between our samples. This indicates that the structural change that Ouyang, et. al., associate with increased in conductivity of their solvent-doped PEDOT samples does not occur in our TeNW/PD hybrid material.


FIGURE S1. Far-infrared absorbance of the TeNW and TeNW/PD materials, measured at T = 4.2K. Markers indicate the frequencies of the IR-active single phonon absorption lines in bulk tellurium4.


FIGURE S2. Mid-IR absorption spectra of the PD and TeNW/PD materials showing the PEDOT vibrational modes. Spectra have been separately scaled for comparison. Labeled markers show mode assignments from Taggart, et. al.6.

FIGURE S3. Raman Scattering in PD and TeNW/PD samples. Markers show mode frequencies identified as Raman active C-C stretching bands in quinoid and benzoid PEDOT isomers by Ouyang, et. al.1

References

1. J. Ouyang, C. W. Chu, F. C. Chen, Q. Xu and Y. Yang, Advanced Functional Materials 15 (2), 203-208 (2005).

2. J. Ouyang, C. W. Chu, F. C. Chen, Q. F. Xu and Y. Yang, J Macromol Sci Pure A41 (12), 1497-1511 (2004).

3. M. Yamashita, C. Otani, M. Shimizu and H. Okuzaki, Appl Phys Lett 99 (14) (2011).

4. in Landolt-Bornstein - Group III Condensed Matter, edited by O. Madelung (Springer, 1998), Vol. 41C, pp. 1-5.

5. C. Kvarnstrom, H. Neugebauer, A. Ivaska and N. S. Sariciftci, J Mol Struct 521, 271-277 (2000).

6. D. K. Taggart, Y. G. Yang, S. C. Kung, T. M. McIntire and R. M. Penner, Nano Lett 11 (5), 2192-2193 (2011).

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12-Jan-14