P.H.M. van Loosdrecht

Exciton Spectra and the Microscopic Structure of Self-Assembled Porphyrin Nanotubes

The optical properties of tubular aggregates formed by self-assembly of zwitterionic meso-tetra(4-sulfonatophenyl)porphyrin (TPPS4) molecules are studied through a combination of experimental and theoretical techniques. The interest in these systems, with diameters of 18 nm and lengths extending up to micrometers, derives from their strong interaction with light and their similarity to natural light-harvesting systems for photosynthesis. The absorption and linear dichroism spectra are obtained in the spectral region from 300 to 750 nm, which includes the exciton bands deriving from the molecular B (Soret) as well as the Q transitions. We demonstrate that a Frenkel exciton model which takes into account the four dominant molecular excited states (Bx, By, Qx, and Qy) provides a good global fit to the experimental spectra. From comparison between theory and experiment, we propose a detailed molecular structure within the nanotube.

Raman scattering in single crystal C60

Raman spectra (using 514 and 740 nm excitation) of high-purity single crystal C60 are presented for the high- and low-temperature phases, showing activity of all “gerade” modes. Within the experimental accuracy, the spectra are found to be consistent with the selection rules for Raman scattering in both the high-temperature fcc and low-temperature 2a0-fcc phases of solid C60. A complete assignment of the observed peaks is proposed.

Evidence for differentiation in the iron-helicoidal chain in GdFe3(BO3)4.

A single-crystal X-ray structure study of gadolinium triiron tetraborate, GdFe3(BO3)4, at room temperature and at 90 K is reported. At room temperature GdFe3(BO3)4 crystallizes in a trigonal space group, R32 (No. 155), the same as found for other members of the iron borate family RFe3(BO3)4. At 90 K the structure of GdFe3(BO3)4 transforms to the space group P3(1)21 (No. 152). The low-temperature structure determination gives new insight into the weakly first-order structural phase transition at 156 K and into the related Raman phonon anomalies. The presence of two inequivalent iron chains in the low-temperature structure provides a new perspective on the interpretation of the low-temperature magnetic properties.

A road to hydrogenating graphene by a reactive ion etching plasma

We report the hydrogenation of single and bilayer graphene by an argon-hydrogen plasma produced in a reactive ion etching (RIE) system. Electronic transport measurements in combination with Raman spectroscopy are used to link the electric mean free path to the optically extracted defect concentration. We emphasize the role of the self-bias of the graphene in suppressing the erosion of the flakes during plasma processing. We show that under the chosen plasma conditions the process does not introduce considerable damage to the graphene sheet and that hydrogenation occurs primarily due to the hydrogen ions from the plasma and not due to fragmentation of water adsorbates on the graphene surface by highly accelerated plasma electrons. For this reason the hydrogenation level can be precisely controlled. The hydrogenation process presented here can be easily implemented in any RIE plasma system.

Phonon and crystal field excitations in geometrically frustrated rare earth titanates

The phonon and crystal field excitations in several rare earth titanate pyrochlores are investigated. Magnetic measurements on single crystals of Gd(2)Ti(2)O(7), Tb(2)Ti(2)O(7), Dy(2)Ti(2)O(7), and Ho(2)Ti(2)O(7) are used for characterization, while Raman spectroscopy and terahertz time domain spectroscopy are employed to probe the excitations in the materials. The lattice excitations are found to be analogous across the compounds over the whole temperature range investigated (295-4 K). The resulting full phononic characterization of the R(2)Ti(2)O(7) pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in Tb(2)Ti(2)O(7) in Raman spectroscopy, among which all of the previously reported excitations. The presence of additional crystal field excitations, however, suggests the presence of two inequivalent Tb(3+) sites in the low-temperature structure. Furthermore, the crystal field level at approximately 13 cm(-1) is found to be both Raman and dipole active, indicating broken inversion symmetry in the system and thus undermining its current symmetry interpretation. In addition, evidence is found for a significant crystal field-phonon coupling in Tb(2)Ti(2)O(7). The additional crystal field information on Tb(2)Ti(2)O(7) adds to the recent discussion on the low temperature symmetry of this system and may serve to improve its theoretical understanding.

Raman scattering in electronically excited C60

Abstract The low-temperature Raman spectrum of 2a 0 -fcc single crystal C 60 recorded using low-irradiance 514 nm excitation shows only a single peak in the region of the 1468 cm −1 out-of-phase ring mode. For higher irradiances a new broad peak appears in the spectrum at a somewhat lower frequency. Simultaneously, an increase of the luminescence is observed. As the irradiance increases the new peak gains intensity and shifts to lower frequencies, whereas the orginal peak slowly disappears. It is argued that this peak originates from electronically excited C 60 . A simple electronic four-level model is proposed to explain the observed effects.

Cascade of phase transitions in GdFe3(BO3)4

Cascade of phase transitions in GdFe3(BO3)4 at 156, 37, and 9 K has been detected by specific heat measurements and further studied by Raman scattering and Nd3+ spectroscopic probe method. A weakly first-order structural phase transition at 156 K is followed by a second-order antiferromagnetic ordering phase transition at 37 K and a first-order spin-reorientational phase transition at 9 K.

Isolation and Monitoring of the Endohedral Metallofullerenes Y@C82 and Sc3@C82 : On-Line Chromatographic Separation with EPR Detection

The direct coupling of high-performance liquid chromatography (HPLC) with on-line electron paramagnetic resonance (EPR) detection is demonstrated for monitoring separations of endohedral metallofullerenes (MC{sub 2a}). The HPLC-EPR approach readily permits detection of the paramagnetic species, such as YC{sub 82} and Sc{sub 3}C{sub 82}, in the presence of the dominant empty-cage fullerenes (C{sub 60}, C{sub 70}) and diamagnetic metallofullerenes (e.g., M{sub 2}C{sub 2n}). The results indicate that on-line EPR provides a noninvasive, selective detector for HPLC metallofullerene separations that is readily adaptable to air-sensitive and/or labile compounds. Specifically, the `EPR-active` metallofullerenes, YC{sub 82} and Sc{sub 3}C{sub 82}, are selectively monitored on-line for an initial separation of the metallofullerene fraction from the dominant empty-cage fullerenes utilizing a combination of polystyrene columns. This preparative `cleanup` procedure is followed by HPLC-EPR separation and monitoring of YC{sub 82} and Sc{sub 3}C{sub 82} species using a selective tripodal {pi}-acidic-phase column (Trident-Tri-DNP) for the final stages of isolation. 30 refs., 8 figs.

Magnetodielectric and magnetoelastic coupling in TbFe3(BO3)(4)

We have studied the magnetodielectric and magnetoelastic coupling in TbFe3(BO3)(4) single crystals by means of capacitance, magnetostriction, and Raman spectroscopy measurements. The data reveal strong magnetic field effects on the dielectric constant and on the macroscopic sample length which are associated to long-range magnetic ordering and a field-driven metamagnetic transition. We discuss the coupling of the dielectric, structural, and magnetic order parameters and attribute the origin of the magnetodielectric coupling to phonon mode shifts according to the Lyddane-Sachs-Teller relation.

Suppressed spin dephasing for two-dimensional and bulk electrons in GaAs wires due to engineered cancellation of spin-orbit interaction terms

We report a study of suppressed spin dephasing for quasi-one-dimensional electron ensembles in wires etched into a GaAs/AlGaAs heterojunction system. Time-resolved Kerr-rotation measurements show a suppression that is most pronounced for wires along the [110] crystal direction. This is the fingerprint of a suppression that is enhanced due to a strong anisotropy in spin-orbit fields that can occur when the Rashba and Dresselhaus contributions are engineered to cancel each other. A surprising observation is that this mechanism for suppressing spin dephasing is not only effective for electrons in the heterojunction quantum well but also for electrons in a deeper bulk layer.