Ferenc Borondics

Charge dynamics in transparent single-walled carbon nanotube films from optical transmission measurements

We report the transmission over a wide frequency range (far infrared - visible) of pristine and hole-doped, free-standing carbon nanotube films at temperatures between 50 K and 300 K. Optical constants are estimated by Kramers-Kronig analysis of transmittance. We see evidence in the far infrared for a gap below 10 meV. Hole doping causes a shift of spectral weight from the first interband transition into the far infrared. Temperature dependence in both the doped and undoped samples is restricted to the far-infrared region.

Vibrational spectra of C60·C8H8 and C70·C8H8 in the rotor-stator and polymer phases

C(60).C(8)H(8) and C(70).C(8)H(8) are prototypes of rotor-stator cocrystals. We present infrared and Raman spectra of these materials and show how the rotor-stator nature is reflected in their vibrational properties. We measured the vibrational spectra of the polymer phases poly(C(60)C(8)H(8)) and poly(C(70)C(8)H(8)) resulting from a solid-state reaction occurring on heating. On the basis of the spectra, we propose a connection pattern for the fullerene in poly(C(60)C(8)H(8)), where the symmetry of the C(60) molecule is D(2h). On illuminating the C(60).C(8)H(8) cocrystal with green or blue light, a photochemical reaction was observed leading to a product similar to that of the thermal polymerization.

Breakdown of diameter selectivity in a reductive hydrogenation reaction of single-walled carbon nanotubes

Abstract Reductive hydrogenation was applied to two types of single-walled carbon nanotubes with different diameter range. Alkali metal intercalation, followed by reaction with methanol, led to hydrogenated products. Both yield and selectivity of this reaction showed strong dependence on diameter, contrary to expectation based on simple curvature effects. The observed yield, as detected by thermogravimetry–mass spectrometry and 1 H NMR, is drastically reduced in small-diameter tubes where the alkali dopant does not reach the inside of the bundles. Wide range optical transmission measurements were employed to determine the selectivity and indicate that besides higher yield, lower diameter selectivity occurs above a critical diameter.

Wide range optical studies on transparent SWNT films

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. As‐prepared purified samples exhibit increased metallic absorption and decreased interband transitions; these features disappear on heating in vacuum.

Distortions of C604− studied by infrared spectroscopy

The Jahn‐Teller effect plays a crucial role in the explanation of the insulating character of A4C60 (A = K, Rb, Cs). To detect possible phase transitions arising from the interplay between the molecular Jahn‐Teller distortion and the distorting potential field of the counterions, we measured the mid‐IR spectra of A4C60 compounds in the temperature range 90 – 300 K and found significant spectral changes with temperature in all three compounds. We also compare these spectra to that of Na4C60 in its room‐temperature polymeric phase, where the distortion is more pronounced and evident from the structure.

Jahn‐Teller distortion in Cs4C60 studied by vibrational spectroscopy

We have measured the infrared spectra of Cs4C60 in the temperature range 220 – 450 K. Two anomalies in the low‐frequency modes at 270 K and 400 K point to changes in molecular or crystal structure. The most probable explanation is a rotator phase above 400 K and a fully ordered phase below 220 K; the intermediate structure is one where molecular Jahn‐Teller distortions compete with crystal field effects.