Distortions of C-60(4-) studied by infrared spectroscopy

The Jahn-Teller effect plays a crucial role in the explanation of the insulating character of A(4)C(60) (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 A(4)C(60) 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 Na(4)C(60) 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 Cs(4)C(60) 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

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

Study of charge dynamics in transparent single-walled carbon nanotube films

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.Comment: 6 pages, 4 figures, submitted to Phys. Rev. B v3: Fig. 2 replaced, changes in caption of Table II, minor changes in tex

Broadband electromagnetic response and ultrafast dynamics of few-layer epitaxial graphene

We study the broadband optical conductivity and ultrafast carrier dynamics of epitaxial graphene in the few-layer limit. Equilibrium spectra of nominally buffer, monolayer, and multilayer graphene exhibit significant terahertz and near-infrared absorption, consistent with a model of intra- and interband transitions in a dense Dirac electron plasma. Non-equilibrium terahertz transmission changes after photoexcitation are shown to be dominated by excess hole carriers, with a 1.2-ps mono-exponential decay that reflects the minority-carrier recombination time.Comment: 4 pages, 3 figures, final versio

Topokémiai reakciók kristályos fullerénekben és rokon szerkezetű anyagokban = Topochemical reactions in crystalline fullerens and related materials

A pályázat keretén belül kifejlesztettük a molekulakristályok újszerű családját, amelyet szokatlan dinamikai sajátosságai alapján rotor-sztátor fázisoknak neveztünk. A nagy szimmetriájú kristályok forgó fullerén és álló kubán molekulák alternáló hálózatából épülnek fel. Szerkezetüket a komplementer molekuláris felületek tökéletes illeszkedése stabilizálja. A fulleréneket molekuláris csapágy módjára körülvevő kubánok szokatlanul alacsony hőmérsékletekig könnyed forgást biztosítanak. Magas hőmérsékleten a kristályok egyfázisú topokémiai reakció révén nagy hőstabilitású kopolimerekké alakulnak. Az átalakulás után az eredeti kristályok külső jegyei megmaradnak. Különféle cikloaddíciós oligomereket izoláltunk C60 fotopolimerekből. Topokémiai modell segítségével értelmeztük a polimerizáció folyamatát és a fotopolimer szerkezetét. Tanulmányoztuk fullerid sók és polimerek fizikai tulajdonságait. Új nanocső származékokat állítottunk elő reduktív funkcionalizálással. | In the framework of the project we have developed a novel family of molecular crystals that we called rotor-stator phases, on the basis of their unusual dynamics. The high-symmetry crystals consist of alternating arrays of rotating fullerene and static cubane molecules. The structures are stabilized by the perfect match of the complementary molecular surfaces. The cubanes, surrounding the fullerenes like a molecular bearing, make possible a smooth rotation down to unusually low temperatures. At elevated temperatures the crystals transform to high-thermal-stability copolymers via single-phase topochemical reactions. The transformed materials preserve the macroscopic crystalline characteristics of the parent phases. We have isolated various cycloadduct oligomers from the photopolymers of C60. We explained the polymerization and the structure of the photopolymer in terms of a topochemical model. We have studied the physical properties of fulleride salts and polymers. We have prepared new derivatives of nanotubes via reductive functionalization

BiTeCl and BiTeBr: a comparative high-pressure optical study

We here report a detailed high-pressure infrared transmission study of BiTeCl and BiTeBr. We follow the evolution of two band transitions: the optical excitation $\beta$ between two Rashba-split conduction bands, and the absorption $\gamma$ across the band gap. In the low pressure range, $p< 4$~GPa, for both compounds $\beta$ is approximately constant with pressure and $\gamma$ decreases, in agreement with band structure calculations. In BiTeCl, a clear pressure-induced phase transition at 6~GPa leads to a different ground state. For BiTeBr, the pressure evolution is more subtle, and we discuss the possibility of closing and reopening of the band gap. Our data is consistent with a Weyl phase in BiTeBr at 5$-$6~GPa, followed by the onset of a structural phase transition at 7~GPa.Comment: are welcom

Structure and properties of the stable two-dimensional conducting polymer Mg5C60

We present a study on the structural, spectroscopic, conducting, and magnetic properties of Mg5C60, which is a two-dimensional (2D) fulleride polymer. The polymer phase is stable up to the exceptionally high temperature of 823 K. The infrared and Raman studies suggest the formation of single bonds between the fulleride ions and possibly Mg-C-60 covalent bonds. Mg5C60 is a metal at ambient temperature, as shown by electron spin resonance and microwave conductivity measurements. The smooth transition from a metallic to a paramagnetic insulator state below 200 K is attributed to Anderson localization driven by structural disorder

Molekuláris szénszerkezetek optikai spektroszkópiája = Optical spectroscopy of molecular carbon structures

Több módszerrel vizsgáltuk az alkáli fulleridsók Jahn-Teller torzulását és kimutattuk, hogy az A4C60 alkáli fulleridekben és a Na2C60 magas hőmérsékletú fázisában a torzulás szimmetriája D3d/D5d, ez folyamatos pszeudorotáció során, dinamikusan valósul meg. Hasonlóan bizonyítottuk be, hogy a Na2C60 szobahőmérsékleten szegregálódik nanoméretű fémes Na3C60 és szigetelő C60 tartományokra. Fullerénekből és kubánból álló rotor-sztator rendszerekben spektroszkópiai módszerekkel kimutattuk a fullerénmolekulák forgását szobahőmérsékleten, és a rezgési spektrumok szimmetria-analíziséből következtettünk a C60-C8H8 polimer szerkezetére. Szén nanocsövekből álló átlátszó, önhordó vékonyrétegek széles sávú optikai spektroszkópiai vizsgálatával meghatároztuk azok optikai állandóit és áteresztési tartományát. Fotovezetés és optikai abszorpció együttes mérése alapján arra következtettünk, hogy a fotoválasz bolometrikus. Mágneses térben orientált nanocsőminták optikai reflexiójában jelentős anizotrópiát találtunk. Az oldalfalon funkcionalizált nanocsövek spektruma alapján megállapítottuk, hogy a kisebb átmérőjű csövek reaktivitása nagyobb. Eredményeinket nemzetközi folyóiratokban publikáltuk, ezek közül legfontosabbak négy Physical Review B, két Journal of Physical Chemistry, egy Science és egy Nature Materials folyóiratcikk, továbbá öt meghívott előadás nemzetközi konferenciákon. | We investigated the Jahn-Teller distortion in alkali fulleride salts by several methods and showed that the symmetry of the distortion is D3d/D5d in both the alkali fullerides A4C60 (A=K,Rb,Cs) and in the high-temperature phase of Na2C60, and it is realized during continous pseudorotation in a dynamic way. Likewise, we proved that Na2C60 is segregated at room temperature into nanometer size regions of metallic Na3C60 and insulating C60. In rotor-stator systems of fullerenes and cubane, we found spectroscopic proof of the rotation of the fullerene molecules at room temperature, and deducted the structure of the C60-C8H8 polymer from the symmetry analysis of the vibrational spectra. We determined the wide-range optical spectrum of transparent self-supporting nanotube films and calculated the optical constants and transmission windows of several types of nanotubes. From the measurement of photoconductivity and optical absorption on the same nanotube film we concluded that the photoresponse of the tubes is bolometric. We found optical anisotropy in magnetically oriented nanotubes. From the spectra of sidewall-functionalized nanotubes we found the smaller diameter ones to be more reactive. Our results were published in international journals, of which the most important are four papers in Physical Review B, two in Journal of Physical Chemistry, one in Science and one in Nature Materials, in addition to five invited lectures at international conferences