Two-dimensional semiconductors such as MoS 2 are promising for future electrical devices. The interface to metals is a crucial and critical aspect for these devices because undesirably high resistances due to Fermi level pinning are present, resulting in unwanted energy losses. To date, experimental information on such junctions has been obtained mainly indirectly by evaluating transistor characteristics. The fact that the metal− semiconductor interface is typically embedded, further complicates the investigation of the underlying physical mechanisms at the interface. Here, we present a method to provide access to a realistic metal−semiconductor interface by large-area exfoliation of single-layer MoS 2 on clean polycrystalline gold surfaces. This approach allows us to measure the relative charge neutrality level at the MoS 2 −gold interface and its spatial variation almost directly using Kelvin probe force microscopy even under ambient conditions. By bringing together hitherto unconnected findings about the MoS 2 −gold interface, we can explain the anomalous Raman signature of MoS 2 in contact to metals [ACS Nano. 7, 2013, 11350] which has been the subject of intense recent discussions. In detail, we identify the unusual Raman mode as the A 1g mode with a reduced Raman shift (397 cm −1 ) due to the weakening of the Mo−S bond. Combined with our X-ray photoelectron spectroscopy data and the measured charge neutrality level, this is in good agreement with a previously predicted mechanism for Fermi level pinning at the MoS 2 −gold interface [Nano Lett. 14, 2014, 1714. As a consequence, the strength of the MoS 2 −gold contact can be determined from the intensity ratio between the reduced A 1g reduced mode and the unperturbed A 1g mode.

The prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and post-ionization techniques. The largest factor affecting secondary ion efficiency is believed to be the poor ionization probability (α) of sputtered material, a value rarely measured directly, but estimated to be in some cases as low as 10. Our lab has developed a method for the direct determination of α in a SIMS experiment using laser post-ionization (LPI) to detect neutral molecular species in the sputtered plume for an organic compound. Here, we apply this method to coronene (CH), a polyaromatic hydrocarbon that exhibits strong molecular signal during gas-phase photoionization. A two-dimensional spatial distribution of sputtered neutral molecules is measured and presented. It is shown that the ionization probability of molecular coronene desorbed from a clean film under bombardment with 40 keV C cluster projectiles is of the order of 10, with some remaining uncertainty arising from laser-induced fragmentation and possible differences in the emission velocity distributions of neutral and ionized molecules. In general, this work establishes a method to estimate the ionization efficiency of molecular species sputtered during a single bombardment event. Graphical Abstract .

An optimized microelectromechanical systems Pirani sensor with increased sensitivity for pressure measurements in the fine and high vacuum regime (from 100 to 10−6 Torr) is presented. Theoretical calculations of the signal voltage as a function of pressure are in good agreement with the measured voltage–pressure response. Fabrication technologies, design optimization using thermal modeling and finite element method simulations, and measurement results are presented.

The prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and postionization techniques. An important factor affecting the detection sensitivity in molecular SIMS and other desorption techniques as well is believed to be the poor ionization probability of a sputtered molecule, a value which is often assumed to be as low as 10 −5 but at present is basically unknown. In order to estimate how much headroom there is for future developments toward strategies aimed at enhancing the ionization probability, we study the protonation efficiency of sputtered guanine molecules for formation of [M + H] + secondary ions using strong field laser postionization (LPI) to detect the corresponding neutral molecules. In order to allow a quantitative comparison of secondary ion and neutral yields, the postionization signal is corrected for undersampling of the principally detectable plume of sputtered neutral particles by the focused laser beam. It is shown that the protonation probability of molecular guanine desorbed from a clean film under bombardment with 20 keV C 60 cluster projectiles is of the order of 1−2 × 10 −3 , with some remaining uncertainty arising from laser-induced fragmentation and possible differences in the emission velocity distributions of neutral and ionized molecules. Moreover, we find that the postionization signal can in principle be boosted by 2 orders of magnitude if a more powerful ionization laser is employed.

241-6009-53235 y Both the authors contributed equally to this work.Two types of microvalves based on temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) and pH-responsive poly(sodium acrylate) (PSA) hydrogel films have been developed and tested. The PNIPAAm and PSA hydrogel films were prepared by means of in situ photopolymerization directly inside the fluidic channel of a microfluidic chip fabricated by combining Si and SU-8 technologies. The swelling/shrinking properties and height changes of the PNIPAAm and PSA films inside the fluidic channel were studied at temperatures of deionized water from 14 to 36 8C and different pH values (pH 3-12) of Titrisol buffer, respectively. Additionally, in separate experiments, the lower critical solution temperature (LCST) of the PNIPAAm hydrogel was investigated by means of a differential scanning calorimetry (DSC) and a surface plasmon resonance (SPR) method. Mass-flow measurements have shown the feasibility of the prepared hydrogel films to work as an on-chip integrated temperature-or pH-responsive microvalve capable to switch the flow channel on/off.