P.J.M. van Bentum

High-resolution liquid- and solid-state nuclear magnetic resonance of nanoliter sample volumes using microcoil detectors

The predominant means to detect nuclear magnetic resonance (NMR) is to monitor the voltage induced in a radiofrequency coil by the precessing magnetization. To address the sensitivity of NMR for mass-limited samples it is worthwhile to miniaturize this detector coil. Although making smaller coils seems a trivial step, the challenges in the design of microcoil probeheads are to get the highest possible sensitivity while maintaining high resolution and keeping the versatility to apply all known NMR experiments. This means that the coils have to be optimized for a given sample geometry, circuit losses should be avoided, susceptibility broadening due to probe materials has to be minimized, and finally the B(1)-fields generated by the rf coils should be homogeneous over the sample volume. This contribution compares three designs that have been miniaturized for NMR detection: solenoid coils, flat helical coils, and the novel stripline and microslot designs. So far most emphasis in microcoil research was in liquid-state NMR. This contribution gives an overview of the state of the art of microcoil solid-state NMR by reviewing literature data and showing the latest results in the development of static and micro magic angle spinning (microMAS) solenoid-based probeheads. Besides their mass sensitivity, microcoils can also generate tremendously high rf fields which are very useful in various solid-state NMR experiments. The benefits of the stripline geometry for studying thin films are shown. This geometry also proves to be a superior solution for microfluidic NMR implementations in terms of sensitivity and resolution.

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.

Infrared spectroscopic study of CuO: Signatures of strong spin-phonon interaction and structural distortion

~Received 24 January 2000; revised manuscript received 26 June 2000; published 2 February 2001! Optical properties of single-crystal monoclinic CuO in the range 70‐6000 cm 21 were studied at temperatures from 7 to 300 K. Normal reflection spectra were obtained from the ~001! and ~010! crystal faces thus giving separate data for the Au and Bu phonon modes excited in the purely transverse way ~TO modes!. Mode parameters, including polarizations of the Bu modes not determined by the crystal symmetry, were extracted by the dispersion analysis of reflectivity curves as a function of temperature. Spectra of all the components of the optical conductivity tensor were obtained using the Kramers-Kronig method recently extended to the case of the low-symmetry crystals. The number of strong phonon modes is in agreement with the factor-group analysis for the crystal structure currently accepted for the CuO. However, several ‘‘extra’’ modes of minor intensity are detected; some of them are observed in the whole studied temperature range, while existence of others becomes evident at low temperatures. Comparison of frequencies of ‘‘extra’’ modes with the available phonon dispersion curves points to possible ‘‘diagonal’’ doubling of the unit cell

Optimization of stripline-based microfluidic chips for high-resolution NMR

a,b,c%!

The role of Ni in increasing the reversibility of the hydrogen release from nanoconfined LiBH4

a1c,b,a-c% and formation of the superlattice. The previously reported softening of the Au mode (;400 cm 21 ) with cooling at TN is found to be ;10% for the TO mode. The mode is very broad at high temperatures and strongly narrows in the antiferromagnetic phase. We attribute this effect to strong resonance coupling of this mode to optical or

Towards nuclear magnetic resonance μ-spectroscopy and μ-imaging

We here report on the optimization, fabrication and experimental characterization of a stripline-based microfluidic NMR probe, realized in a silicon substrate. The stripline geometry was modelled in respect of rf-homogeneity, sensitivity and spectral resolution. Using these models, optimal dimensional ratios were found, which hold for every sample size. Based on the optimized parameters, a simple integrated stripline-based microfluidic chip was realized. The fabrication of this chip is described in detail. We achieved a sensitivity of 0.47 nmol/square root(Hz) and a resolution of 0.7 Hz. The rf-homogeneity (A(810 degrees)/A(90 degrees)) was 76% and was proved to be suitable for 2D-NMR analysis of glucose.

Raman scattering in electronically excited C60

Nanoconfinement and the use of catalysts are promising strategies to enhance the reversibility of hydrogen storage in light metal hydrides. We combined nanoconfinement of LiBH4 in nanoporous carbon with the addition of Ni. Samples were prepared by deposition of 5-6 nm Ni nanoparticles inside the porous carbon, followed by melt infiltration with LiBH4. The Ni addition has only a slight influence on the LiBH4 hydrogen desorption, but significantly enhances the subsequent uptake of hydrogen under mild conditions. Reversible, but limited, intercalation of Li is observed during hydrogen cycling. X-ray diffraction shows that the initial crystalline 5-6 nm Ni nanoparticles are not present anymore after melt infiltration with LiBH4. However, transmission electron microscopy showed Ni-containing nanoparticles in the samples. Extended X-ray absorption fine structure spectroscopy proved the presence of Ni(x)B phases with the Ni-B coordination numbers changing reversibly with dehydrogenation and rehydrogenation of the sample. Ni(x)B can act as a hydrogenation catalyst, but solid-state 11B NMR proved that the addition of Ni also enhanced the reversibility of the system by influencing the microstructure of the nanoconfined LiBH4 upon cycling.

Determination of the energy gap in a thin YBa2Cu3O7−x film by Andreev reflection and by tunneling

The first successful experiments demonstrating Nuclear Magnetic Resonance (NMR) were a spin-off from the development of electromagnetic technology and its introduction into civilian life in the late forties. It was soon discovered that NMR spectra held chemically relevant information making it useful as an analytical tool. By introducing a new way of detection, moving away from continuous wave spectroscopy, Fourier Transform NMR helped to overcome sensitivity problems and subsequently opened the way for multi-dimensional spectroscopy. As a result NMR has developed into one of the most powerful analysis techniques with widespread applications. Still sensitivity is a limiting factor in the applicability of NMR. Therefore we witness a renaissance of technique development in magnetic resonance striving to improve its receptiveness. This tutorial review introduces the efforts currently made in miniaturizing inductive detection by designing optimal radio-frequency microcoils. A second approach is to introduce a new way of detecting magnetic resonance signals by means of very sensitive micromechanical force detectors. This shows that the detection limits in terms of absolute sensitivity or imaging resolution are still open to significant improvements.

Morphology and surface topology of YBa2Cu3O7-x crystals; theory and STM observations

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.

Strategies for solid-state NMR in high-field Bitter and hybrid magnets

Abstract We have observed for the first time Andreev reflection at the normal metal-superconductor interface in a thin film Ag-YBa 2 Cu 3 O 7− x sample, which indicates a zero-momentum paired state in this high- T c superconductor. The energy-dependence of the reflection probability indicates a lower limit of the energy gap in the YBa 2 Cu 3 O 7− x equal to Δ = 12.5±2 meV. Tunneling measurements on the same thin-film samples are in reasonable agreement with this value and yield Δ = 14±2 meV. Both results are not incompatible with the weak-coupling BCS prediction 2 Δ / k B T c = 3.5.