Observation of higher-order Raman modes in C60 films

Vibrational anatomy of C90, C96, and C100 fullertubes: probing Frankenstein's skeletal structures of fullerene head endcaps and nanotube belt midsection

Fullertubes are tubular fullerenes with nanotube-like middle section and fullerene-like endcaps. To understand how this intermediate form between spherical fullerenes and nanotubes is reflected in the vibrational modes, we performed comprehensive studies of IR and Raman spectra of fullertubes C₉₀-D_5h, C₉₆-D_3d, and C₁₀₀-D_5d. An excellent agreement between experimental and DFT-computed spectra enabled a detailed vibrational assignment and allowed an analysis of the localization degree of the vibrational modes in different parts of fullertubes. Projection analysis was performed to establish an exact numerical correspondence between vibrations of the belt midsection and fullerene headcaps to the modes of nanotubes and fullerene C₆₀-I_h. As a result, we could not only identify fullerene-like and CNT-like vibrations of fullertubes, but also trace their origin in specific vibrational modes of CNT and C₆₀-I_h. IR spectra were found to be dominated by vibrations of fullerene-like caps resembling IR-active modes of C₆₀-I_h, whereas in Raman spectra both caps and belt vibrations are found to be equally active. Unlike the resonance Raman spectra of CNTs, in which only two single-phonon bands are detected, the Raman spectra of fullertubes exhibit several CNT-like vibrations and thus provide additional information on nanotube phonons.

Guest-Host Raman Under liquid Nitrogen Spectroscopy for the acquisition of improved vibrational spectra of solids

Guest-Host Raman under liquid nitrogen spectroscopy (GHRUNS) is introduced whereby solid state guest molecules are isolated inside cage-like host environments for the facile acquisition of their Raman spectra. This convenient method features reduced fluorescence, the analysis of populations in their ground states, and increased signal to noise ratios. Samples are also preserved through the reduction of thermal degradation and oxidation. To demonstrate the benefits of this new method, Raman spectra of the ubiquitous molecule C 60 inside a cage of water ice are presented. Using this technique, a new normal mode of C 60 is elucidated. The GHRUNS methodology is of interest to those seeking to acquire and characterize the vibrational spectra, structure, and properties of emissive, air-sensitive molecules.

Charge Transfer-Mediated Dramatic Enhancement of Raman Scattering upon Molecular Point Contact Formation

Charge-transfer enhancement of Raman scattering plays a crucial role in current-carrying molecular junctions. However, the microscopic mechanism of light scattering in such nonequilibrium systems is still imperfectly understood. Here, using low-temperature tip-enhanced Raman spectroscopy (TERS), we investigate how Raman scattering evolves as a function of the gap distance in the single C₆₀-molecule junction consisting of an Ag tip and various metal surfaces. Precise gap-distance control allows the examination of two distinct transport regimes, namely tunneling regime and molecular point contact (MPC). Simultaneous measurement of TERS and the electric current in scanning tunneling microscopy shows that the MPC formation results in dramatic Raman enhancement that enables one to observe the vibrations undetectable in the tunneling regime. This enhancement is found to commonly occur not only for coinage but also transition metal substrates. We suggest that the characteristic enhancement upon the MPC formation is rationalized by charge-transfer excitation.

Ab Initio Investigation of Nonlinear Mode Coupling in C60

Strong light fields may be used to control molecular structure, thus providing a route to new, light-induced phases of matter. In this context, we present an ab initio molecular dynamics investigation of nonlinear mode coupling in C₆₀ and show that, under suitable conditions, resonant infrared excitation induces significant structural changes in the system. Surprisingly, exciting the highest-frequency infrared mode at field strengths employed in a recent experiment [Nature 2016, 530, 461-464], we observe no significant structural change. However, when targeting the first two infrared modes using stronger fields, all Raman modes gain energy through nonlinear coupling to the infrared modes, leading to large-amplitude vibrational excitations. We find a strong response of the H_g(5) mode, which is symmetric with respect to the equilibrium structure. For sufficiently strong field, it is of the same order as the infrared modes' excitation. This encourages further investigations into the light-induced superconducting properties of alkali-doped fullerenes.

Assignment of the Internal Vibrational Modes of C70 by Inelastic Neutron Scattering Spectroscopy and Periodic-DFT

The fullerene C70 may be considered as the shortest possible nanotube capped by a hemisphere of C60 at each end. Vibrational spectroscopy is a key tool in characterising fullerenes, and C70 has been studied several times and spectral assignments proposed. Unfortunately, many of the modes are either forbidden or have very low infrared or Raman intensity, even if allowed. Inelastic neutron scattering (INS) spectroscopy is not subject to selection rules, and all the modes are allowed. We have obtained a new INS spectrum from a large sample recorded at the highest resolution available. An advantage of INS spectroscopy is that it is straightforward to calculate the spectral intensity from a model. We demonstrate that all previous assignments are incorrect in at least some respects and propose a new assignment based on periodic density functional theory (DFT) that successfully reproduces the INS, infrared, and Raman spectra.

Mapping intermolecular bonding in C₆₀

The formation of intermolecular bonds in C₆₀ has been investigated in detail at pressures below 2.2 GPa and up to 750 K. Fullerene samples were heated in a temperature gradient to obtain data on the formation of dimers and low-dimensional polymers along isobars. Intermolecular bonding was analyzed ex situ by Raman scattering, using both intramolecular modes and intermolecular stretching modes. Semi-quantitative reaction maps are given for the formation of dimers and chains. The activation energy for dimer formation decreases by 0.2 meV pm(-1) when intermolecular distances decrease and dimer formation is noticeably affected by the rotational state of molecules. Above 400-450 K larger oligomers are formed; below 1.4 GPa most of these are disordered, with small domains of linear chains, but above this the appearance of stretching modes indicates the existence of ordered one-dimensional polymers. At the highest pressures and temperatures two-dimensional polymers are also observed.

Deciphering the metal-C60 interface in optoelectronic devices: evidence for C60 reduction by vapor deposited Al

The formation of interfacial midgap states due to the reduction of buckminsterfullerene (C60) to amorphous carbon upon subsequent vapor deposition of Al is confirmed using Raman spectroscopy and X-ray, ultraviolet, and inverse photoemission spectroscopies. We demonstrate that vapor deposition of Al results in n-type doping of C60 due to an electron transfer from Al to the LUMO of C60, resulting in the formation of midgap states near the C60 Fermi level. Raman spectroscopy in ultrahigh vacuum clearly identifies the presence of the C60 anion radical (C60(•-)) as well as amorphous carbon created by further degradation of C60(•-). In contrast, the interface formed by vapor deposition of Ag shows only a slight Ag/C60 interfacial charge displacement with no evidence for complete metal-to-C60 electron transfer to form the anion radical or its further degradation products. These results confirm previous speculations of metal-induced chemical damage of C60 films after Al deposition, which is widely suspected of decreasing charge collection efficiency in OPVs, and provide key insight into charge collection at metal/organic interfaces in such devices.

Control of giant breathing motion in c60 with temporally shaped laser pulses

Femtosecond laser pulses tailored with closed-loop, optimal control feedback were used to excite oscillations in C60 with large amplitude by coherent heating of nuclear motion. A characteristic pulse sequence results in significant enhancement of C2 evaporation, a typical energy loss channel of vibrationally hot C60. The separation between subsequent pulses in combination with complementary two-color pump-probe data and time-dependent density functional theory calculations give direct information on the multielectron excitation via the t(1g) resonance followed by efficient coupling to the radial symmetric a(g)(1) breathing mode.

Investigation of the mechanism of influence of colloidal gold/silver substrates in nonaqueous liquids on the surface enhanced Raman spectroscopy (SERS) of fullerenes C60 (C70)

Surface enhanced Raman scattering (SERS) spectra of C60 (C70) were obtained in nonaqueous colloidal systems with newly developed methods. The enhancement factor was estimated to be over 10(5). This paper aims at the investigation of a fine influence mechanism for SERS in the nonliquid phase, which may help bring forth the perfect SERS mechanism. The detailed investigation is based on abundant comparative experiments where we found that the SERS effect is sensitive not only to the character of colloidal particles, dielectric constants, and polarizability, but also to the substrates, the solvent intermediate, and even the coating techniques. These detailed comparisons enrich the proofs of the SERS mechanism and provide a new way to optimize SERS systems.

A complete nearest-neighbor force field model for C60

A force field model is developed for C(60) that features 13 force constants representing all interactions between nearest-neighboring atoms. The model is compared with, and tested against, other force field models in the literature. Force constants for C(60) are then deduced by fitting the model to the 14 known optically accessible vibrational frequencies of the molecule. Finally, the model is fitted to two existing theoretical calculations of the complete vibrational spectrum of C(60). Fair agreement is obtained with the theoretical calculations, implying that interactions with atoms other than nearest neighbors are small.

End-Cap Effects on Vibrational Structures of Finite-Length Carbon Nanotubes

Vibrational structures of C60-related finite-length nanotubes, C(40+20n) and C(42+18n) (1 < or = n < or = 4), in which n is, respectively, the number of cyclic cis- and trans-polyene chains inserted between fullerene hemispheres, are analyzed from density functional theory (DFT) calculations. To illuminate the end-cap effects on their vibrational structures, the corresponding tubes terminated by H atoms C(20n)H20 and C(18n)H18 (1 < or = n < or = 5) are also investigated. DFT calculations show a broad range of vibrational frequencies for the finite-size nanotubes: high-frequency modes (1100-1600 cm(-1)) containing oscillations along tangential directions (tangential modes), medium-frequency modes (700-850 cm(-1)) whose oscillations are located on the edges or end caps, and low-frequency modes (300-600 cm(-1)) involving oscillations along the radial directions (radial modes). Broadening of the calculated frequencies is due to the number of nodes in the standing waves of normal modes in the finite-size tubes. In the capped tubes, calculated vibrational frequencies are insensitive to the number of chains (n), whereas in the uncapped tubes, most vibrational frequencies change significantly with an increase in tube length. The discrepancy in the size dependency is reasonably understood by their C-C bonding networks; the capped tubes have similar bond-length alternation patterns within the polyene chains irrespective of n, whereas the uncapped tubes have various bond-deformation patterns. Thus, DFT calculations illuminate that the edge effects have strong impacts on the vibrational frequencies in the finite-size nanotubes.