Single-wall carbon nanotube interactions with copper-oxamato building block of molecule-based magnets probed by resonance Raman spectroscopy

The electronic interactions between the [Cu(opba)]2- anions (where opba is orthophenylenebis (oxamato)) and single-wall carbon nanotubes (SWCNTs) were investigated by resonance Raman spectroscopy. The opba can form molecular magnets, and the interactions of opba with SWCNTs can produce materials with very different magnetic/electronic properties. It is observed that the electronic interaction shows a dependence on the SWCNT diameter independent of whether they are metallic or semiconducting, although the interaction is stronger for metallic tubes. The interaction also is dependent on the amount of complex that is probably adsorbed on the carbon surface of the SWCNTs. Some charge transfer can be also occurring between the metallic complex and the SWCNTs. Copyright (c) 2012 John Wiley & Sons, Ltd.

Dependence of brittle-to-ductile transition on crystallographic direction in diamond turning of single-crystal silicon

The objective of this paper is to show the dependence relationship between the crystallographic orientations upon brittle-to-ductile transition during diamond turning of monocrystalline silicon. Cutting tests were performed using a -5 degrees rake angle round nose diamond tool at different machining scales. At the micrometre level, the feedrate was kept constant at 2.5 micrometres per revolution (mu m/r), and the depth of cut was varied from 1 to 5 mu m. At the submicrometre level, the depth of cut was kept constant at 500 nm and the feedrate varied from 5 to 10 mu m/r. At the micrometre level, the uncut shoulder generated with an interrupted cutting test procedure provided a quantitative measurement of the ductile-to-brittle transition. Results show that the critical chip thickness in silicon for ductile material removal reaches a maximum of 285 nm in the [100] direction and a minimum of 115 nm in the [110] direction, when the depth of cut was 5 mu m. It was found that when a submicrometre depth of cut was applied, microcracks were revealed in the [110] direction, which is the softer direction in silicon. Micro Raman spectroscopy was used to estimate surface residual stress after machining. Compressive residual stress in the range 142 MPa and smooth damage free surface finish was probed in the [100] direction for a depth of cut of 5 mu m, whereas residual stresses in the range 350 MPa and brittle damage was probed in the [110] direction for a depth of cut of 500 nm.

Use of Carr-Purcell pulse sequence with low refocusing flip angle to measure T-1 and T-2 in a single experiment

The Carr-Purcell pulse sequence, with low refocusing flip angle, produces echoes midway between refocusing pulses that decay to a minimum value dependent on T*(2). When the refocusing flip angle was pi/2 (CP90) and tau > T*(2), the signal after the minimum value, increased to reach a steady-state free precession regime (SSFP), composed of a free induction decay signal after each pulse and an echo, before the next pulse. When tau < T*(2), the signal increased from the minimum value to the steady-state regime with a time constant (T*) = 2T(1)T(2)/(T-1 + T-2). identical to the time constant observed in the SSFP sequence, known as the continuous wave free precession (CWFP). The steady-state amplitude obtained with M-cp90 = M0T2/(T-1+T-2) was identical to CWFP. Therefore, this sequence was named CP-CWFP because it is a Carr-Purcell sequence that produces results similar to the CWFP. However, CP-CWFP is a better sequence for measuring the longitudinal and transverse relaxation times in single scan, when the sample exhibits T-1 similar to T-2. Therefore, this sequence can be a useful method in time domain NMR and can be widely used in the agriculture, food and petrochemical industries because those samples tend to have similar relaxation times in low magnetic fields. (C) 2011 Elsevier Inc. All rights reserved.

Polymorphic-induced transformations in CaTa2O6 single-crystal fibers obtained by laser-heated pedestal growth

Calcium tantalite (CaTa2O6) single crystal fibers were obtained by the laser-heated pedestal growth method (LHPG). At room temperature, this material can present three polymorphic modifications. The rapid crystallization inherent to the LHPG method produced samples within the Pm3 space group, with some chemical disorder. In order to check for polymorphic-induced transformations, the CaTa2O6 fibers have been submitted to different thermal treatments and investigated by micro-Raman spectroscopy. For short annealing times (15 min) at 1200 °C, the cubic modification was maintained, though with an improved crystalline quality, as evidenced by the enhanced inelastic scattered intensity (by ca. 250%) and narrowing of Raman bands. The polarized Raman spectra respected very well the predicted symmetries and the selection rules for this cubic modification. On the other hand, long annealing times (24 h) at 1200 °C led to a complete (irreversible) polymorphic transformation. The Raman bands became still more intense (ca. 15 times larger than for the as-grown fibers), narrower, and several new modes appeared. Also, the spectra became unpolarized, demonstrating a polycrystalline nature of the transformed crystals. The observed Raman modes could be fully assigned to an orthorhombic modification of CaTa2O6 belonging to the Pnma space group.

Effect of resonant tunneling on exciton dynamics in coupled dot-well nanostructures

Excitonic dynamics in a hybrid dot-well system composed of InAs quantum dots (QDs) and an InGaAs quantum well (QW) is studied by means of femtosecond pump-probe reflection and continuous wave (cw) photoluminescence (PL) spectroscopy. The system is engineered to bring the QW ground exciton state into resonance with the third QD excited state. The resonant tunneling rate is varied by changing the effective barrier thickness between the QD and QW layers. This strongly affects the exciton dynamics in these hybrid structures as compared to isolated QW or QD systems. Optically measured decay times of the coupled system demonstrate dramatically different response to temperature change depending on the strength of the resonant tunneling or coupling strength. This reflects a competition between purely quantum mechanical and thermodynamical processes.

ScPdZn and ScPtZn with YAlGe type structure: group-subgroup relation and 45Sc solid state NMR spectroscopy

The intermetallic compounds ScPdZn and ScPtZn were prepared from the elements by high-frequency melting in sealed tantalum ampoules. Both structures were refined from single crystal X-ray diffractometer data: YAlGe type, Cmcm, a = 429.53(8), b = 907.7(1), c = 527.86(1) pm, wR2 = 0.0375, 231 F2 values, for ScPdZn and a = 425.3(1), b = 918.4(2), c = 523.3(1) pm, wR2 = 0.0399, 213 F2 values for ScPtZn with 14 variables per refinement. The structures are orthorhombically distorted variants of the AlB2 type. The scandium and palladium (platinum atoms) build up ordered networks Sc3Pd3 and Sc3Pt3 (boron networks) which are slightly shifted with respect to each other. These networks are penetrated by chains of zinc atoms (262 pm in ScPtZn) which correspond to the aluminum positions, i.e. Zn(ScPd) and Zn(ScPt). The corresponding group-subgroup scheme and the differences in chemical bonding with respect to other AlB2-derived REPdZn and REPtZn compounds are discussed. 45Sc solid state NMR spectra confirm the single crystallographic scandium sites. From electronic band structure calculations the two compounds are found metallic with free electron like behavior at the Fermi level. A larger cohesive energy for ScPtZn suggests a more strongly bonded intermetallic than ScPdZn. Electron localization and overlap population analyses identify the largest bonding for scandium with the transition metal (Pd, Pt).