The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

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A hybrid chip is described which combines a microfluidic network fabricated in a silicone elastomer (PDMS) with planar microelectrodes. It was used to measure extracellular potentials from single adult murine cardiac myocytes in a restricted extracellular space. The recorded variations in the extracellular potentials were caused by transmembrane currents associated with spontaneously initiated intracellular calcium waves. Single cells were trapped inside the 100 pl microchamber by pressure gradients and maintained for several hours by continuous perfusion. In addition, the localized delivery of drugs to a portion of the cell was demonstrated. The impedance of the electrodes was reduced by a factor of 10 to 20 after the electrodeposition of platinum black. Biopotentials recorded from single cells with platinum black electrodes showed a three-fold decrease in the noise, resulting in a maximum signal-to-noise ratio of 15:1. Characteristic variations in the frequency and shape of the extracellular potentials were observed among different cells which are most likely due to differences in the sarcoplasmic reticulum Ca(2+) load. Our device architecture permits the integration of electrochemical and optical sensors for multiparameter recordings.

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We report on the first successful operation of a field-emitter-array cathode in a conventional Lband radio-frequency electron source. The cathode consisted of an array of ∼ 10 6 diamond diamond tips on pyramids. Maximum current on the order of 15 mA were reached and the cathode did not show appreciable signs of fatigue after weeks of operation. The measured Fowler-Nordheim characteristics, transverse beam density, and current stability are discussed. Numerical simulations of the beam dynamics are also presented.PACS numbers: 41.75.Fr Over the past years, field-emission (FE) electron sources have been the subject of intense investigations due to several advantages they offer over photoemission and thermionic sources. The main advantages of FE sources stem from their ability to produce very lowemittance bunched beams, their capability to generate high-average current beams, and the absence of requirement for an auxiliary laser system. A single-tip FE cathode emits electrons from a very small transverse area and can therefore produce beams with extremely small, near quantum-degenerate, transverse emittances [1,2]. When arranged as large arrays, field-emission-array (FEA) cathodes can provide substantial average currents [3] to the detriment of emittance which then scales linearly with the FEA macroscopic radius [4,5].Pulsed field-emission occurs when a FE cathode experiences a time-dependent field, e.g., when located in a resonant radiofrequency (RF) cavity. Taking the example of a cylindrical-symmetric resonant pillbox cavity operating on the TM 010 mode with axial electric field E z (r = 0, z, t) = E 0 cos(2πf t), where f and E 0 are respectively the field frequency and peak amplitude, field-emitted bunches have a root-mean-square (rms) duration σ t ≃ ω −1 [β e E 0 /B(φ)] 1/2 where ω ≡ 2πf . The latter pulse duration is obtained by taking the current density to follow the Fowler-Nordheim's (F-N) law [6] j(t) = A(φ)β 2 e E(t) 2 exp[−B(φ)/(β e E(t))] where A(φ) and B(φ) are functions of the work function φ of the cathode material and β e is a field-enhancement factor [7,8]. Nominally, the bunch rms duration is a significant fraction of the RF field period typically resulting in beams with large energy spread. This limitation can however be circumvented by exposing the FE cathode to superimposed electromagnetic fields operating at harmonic frequencies with properly tuned relative phases and amplitudes. A practical implementation of this technique consists in a RF gun supporting two harmonic modes with axial electric fields [9].In this letter we report on the first operation of a diamond FEA (DFEA) cathode in a conventional L-band RF gun nominally operated with a Cesium Telluride (Cs 2 Te) photocathode. The DFEA is composed of ungated diamond pyramids which have proven to be rugged. Depending on the size and pitch of the pyramids, tests under DC voltages have showed field emission to begin at macroscopic fields E 0 ≃ 5 MV/m, and peak currents per tip as high as 15 µA has been obtained [10].The geometry of the DFE...

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Prior work with free-electron lasers (FELs) showed that wavelengths in the 6- to 7-µm range could ablate soft tissues efficiently with little collateral damage; however, FELs proved too costly and too complex for widespread surgical use. Several alternative 6- to 7-µm laser systems have demonstrated the ability to cut soft tissues cleanly, but at rates that were much too low for surgical applications. Here, we present initial results with a Raman-shifted, pulsed alexandrite laser that is tunable from 6 to 7 µm and cuts soft tissues cleanly—approximately 15 µm of thermal damage surrounding ablation craters in cornea—and does so with volumetric ablation rates of 2–5 × 10−3 mm3/s. These rates are comparable to those attained in prior successful surgical trials using the FEL for optic nerve sheath fenestration.

The interface between photoactive biological materials with two distinct semiconducting electrodes is challenging both to develop and analyze. Building off of our previous work using films of photosystem I (PSI) on p-doped silicon, we have deposited a crystalline zinc oxide (ZnO) anode using confined-plume chemical deposition (CPCD). We demonstrate the ability of CPCD to deposit crystalline ZnO without damage to the PSI biomaterial. Using electrochemical techniques, we were able to probe this complex semiconductor-biological interface. Finally, as a proof of concept, a solid-state photovoltaic device consisting of p-doped silicon, PSI, ZnO, and ITO was constructed and evaluated.