Electrodeposited submicron thermocouples with microsecond response times.

We describe a procedure for preparing submicron scale silver-nickel thermocouples (TCs) using electrochemical step edge decoration on graphite surfaces. Each fabrication operation produced ensembles of 2-20 TCs with diameters in the 1.0 microm to 500 nm range. These "sub-mum TCs" (SMTCs) produced linear voltage versus temperature output over the range from 20 to 100 degrees C characterized by a Seebeck coefficient of 20 +/- 1 microV/degrees C, equal to the 21 microV/degrees C that is theoretically expected for a junction between these two metals. The time response of SMTCs was evaluated using two different laser-heating methods and compared with the smallest mechanically robust commercially available type J TCs. Electrochemical etching of the silver wire introduced constrictions at grain boundaries that reduced the thermal mass of the junction without altering its integrity or its overall diameter, producing a decrease of the measured rise time for SMTCs up to 96%.

Molybdenum disulfide nanowires and nanoribbons by electrochemical/chemical synthesis.

Molybdenum disulfide nanowires and nanoribbons have been synthesized by a two-step, electrochemical/chemical synthetic method. In the first step, MoO(x) wires (a mixture of MoO(2) and MoO(3)) were electrodeposited size-selectively by electrochemical step-edge decoration on a highly oriented pyrolytic graphite (HOPG) surface. Then, MoO(x) precursor wires were converted to MoS(2) by exposure to H(2)S either at 500-700 degrees C, producing "low-temperature" or LT MoS(2) nanowires that were predominantly 2H phase, or above 800 degrees C producing "high-temperature" or HT MoS(2) ribbons that were predominantly 3R phase. The majority of these MoS(2) wires and ribbons were more than 50 microm in length and were organized into parallel arrays containing hundreds of wires or ribbons. MoS(2) nanostructures were characterized by X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, selected area electron diffraction, X-ray diffraction, UV-visible absorption spectrometry, and Raman spectroscopy. HT and LT MoS(2) nanowires were structurally distinct: LT MoS(2) wires were hemicylindrical in shape and nearly identical in diameter to the MoO(x) precursor wires from which they were derived. LT MoS(2) wires were polycrystalline, and the internal structure consisted of many interwoven, multilayer strands of MoS(2); HT MoS(2) ribbons were 50-800 nm in width and 3-100 nm thick, composed of planar crystallites of 3R-MoS(2). These layers grew in van der Waals contact with the HOPG surface so that the c-axis of the 3R-MoS(2) unit cell was oriented perpendicular to the plane of the graphite surface. Arrays of MoS(2) wires and ribbons could be cleanly separated from the HOPG surface and transferred to glass for electrical and optical characterization. Optical absorption measurements of HT MoS(2) nanoribbons reveal a direct gap near 1.95 eV and two exciton peaks, A1 and B1, characteristic of 3R-MoS(2). These exciton peaks shifted to higher energy by up to 80 meV as the wire thickness was decreased to 7 nm (eleven MoS(2) layers). The energy shifts were proportional to 1/ L( parallel)(2), and the effective masses were calculated. Current versus voltage curves for both LT and HT MoS(2) nanostructures were probed as a function of temperature from -33 degrees C to 47 degrees C. Conduction was ohmic and mainly governed by the grain boundaries residing along the wires. The thermal activation barrier was found to be related to the degree of order of the crystallites and can be tuned from 126 meV for LT nanowires to 26 meV for HT nanoribbons.

Selective ultra-trace detection of NO and NO2 in complex gas mixtures using broad-bandwidth REMPI mass spectrometry.

In this paper we demonstrate the feasibility of ultra-trace resonance enhanced multiphoton ionization (REMPI) detection employing a small broad-bandwidth solid state laser system. The results reported here are compared with measurements carried out with a conventional excimer pumped dye laser combination. Mass selected broad-bandwidth REMPI spectra for the environmentally relevant nitrogen oxides NO and NO2 are presented. Tunable broad-bandwidth laser radiation with a spectral resolution of > 10 cm(-1) in the wavelength range 560-400 nm was employed for the detection of NO2. For NO detection, the range 230-224 nm was covered. Laser radiation was generated using an optical parametric oscillator pumped by an unseeded Nd:YAG laser. A mobile time-of-flight mass spectrometer equipped with an atmospheric pressure laser ionization source allowed for mass selective parent ion detection at m/z 30 for NO and m/z 46 for NO2. The limit of detection was 10 pptV for NO and 20 pptV for NO2. A selectivity of > 2000 for both compounds with respect to N2O5, organic nitrates and NO2 in the case of NO is reported. An improved laser system currently under construction is expected to provide detection limits below pptv mixing ratios for both nitrogen oxides in a 20 s integration interval.

Metallocyclic receptors with Re(I)/Os(II)-based moieties: molecular photophysics and selective molecular sensing.

New metallocyclic Re(I) and Os(II) complexes with polyphosphane/polyyne spacers, including dimers [(Re(CO)3Cl(C2nP2))2] (n = 1, 1; 2, 2) and tetramers [(Re(CO)3Cl(C2nP2))4] (n = 1, 3; 2, 4, C2P2 = Ph2P-C...C-PPh2, C4P2 = Ph2P-C...C-C...C-PPh2), as well as the mixed-metal [(Re(CO)3Cl)2(Os(bpy)2)2(C2P2)4](PF6)4 (6, bpy = 2,2'-bipyridine) and its precursor [Os(bpy)2(C2P2)2](PF6)2 (5) have been synthesized. Characterization has been carried out using 31P(1H) NMR, FAB/MS, ESI/MS, IR spectroscopy, elemental analysis (EA), and X-ray single crystal structure determination. These new metallocyclic complexes are found to be emissive, with a characteristic ReI-based emission at 505-525 nm (lifetimes of 3.4-6.8 ns) and an Os(II)-based emission at 600-605 nm (lifetimes of 650-675 ns). High quantum yields of 0.25 and 0.17 were observed for 5 and 6, which were representative of the few most emissive species reported with Os(II) centers. Efficient energy transfer from the Re(I) donor to the Os(II) acceptor was also found. In addition, a host-guest study was performed using emissive metallocycle 6, and host-guest binding constants of 775M(-1), 1580M(-1), and 1680M(-1) were obtained for the guests anisole, 1,4-dimethoxybenzene, and 1,3,5-trimethoxybenzene, respectively. The correlation between the guest molecule size, cavity dimension, and the host-guest binding constant is discussed. Furthermore, the relationship between the pi-acceptor ability of the nonchromophoric phosphanes, the energy gap between the ground and excited state, and the nonradiative decay rate constant (knr) is also explored.

Structural aspects, IR study, and molecular photophysics of the cumulene-bridged complexes with rhenium(I), ruthenium(II), and osmium(II) centers.

The synthesis of a series of ReI, RuII, and OsII complexes that contain rigid polyphosphine/cumulene spacers is reported here. These cumulenic ligands, namely, 1,1',3,3'-tetrakis(diphenylphosphino)allene (C3P4) and 1,1',4,4'-tetrakis(diphenylphosphino)cumulene (C4P4), utilize diphenylphosphino linkage components to coordinate to the metal-polypyridyl or metal-carbonyl units. Characterization of all mono-, homo-, and heterobimetallic complexes is achieved using 31P(1H) NMR, IR, and fast atom bombardment mass spectroscopy (FAB/MS) and elemental analysis. The two ReI homobimetallic complexes were also characterized by single-crystal X-ray structure determination, which provided the structural evidence of a 90 degrees rotation between the C3 and C4 adducts causing a change in the electrochemical behavior. The ground-state electronic absorption and redox interactions, along with the excited-state photophysical characteristics, are also explored. Electrochemical studies showed that an increase in the carbon chain length resulted in a greater amount of sigma-donation from the ligand to the metal centers, as well as a greater amount of electronic communication between the metal termini of the bimetallic species. The electronic absorption and emission spectra of the new complexes were also determined and characterized. The lifetimes of the excited-state luminescence of the ReI mono- and homobimetallic complexes were found to be an order of magnitude shorter than the lifetimes of the heterobimetallic complexes containing the RuII and OsII moieties. Excited-state energy transfer was observed from the higher MLCT excited state of the ReI centers to the lower energy MLCT excited state of the RuII and OsII centers on the following basis: no ReI-based emission was detected in the steady-state emission measurements, the time-resolved decay traces were fitted to only single-exponential decays, and the quantum yields were identical for each compound at two different excitation wavelengths where different percentages of the metal-based chromophores were excited.

Highly efficient single-longitudinal-mode beta-BaB(2)O(4) optical parametric oscillator with a new cavity design.

A new coupled-cavity design for single-longitudinal-mode operation of an optical parametric oscillator (OPO) is presented. The OPO is based on a beta-BaB(2)O(4) crystal and is pumped by the third harmonic of a Nd:YAG laser. With this design, we achieved single-longitudinal-mode operation of the OPO with a decrease in the threshold and an increase in external efficiency compared with those of a conventional grazing-incidence OPO. A mathematical model that describes the mode spacings for this cavity is given.