We investigate the origin of the hysteresis observed in the transfer characteristics of back-gated field-effect transistors with an exfoliated MoS2 channel. We find that the hysteresis is strongly enhanced by increasing either gate voltage, pressure, temperature or light intensity. Our measurements reveal a step-like behavior of the hysteresis around room temperature, which we explain as water-facilitated charge trapping at the MoS2/SiO2 interface. We conclude that intrinsic defects in MoS2, such as S vacancies, which result in effective positive charge trapping, play an important role, besides H2O and O2 adsorbates on the unpassivated device surface. We show that the bistability associated to the hysteresis can be exploited in memory devices.

We study electrical transport properties in exfoliated molybdenum disulfide (MoS) back-gated field effect transistors at low drain bias and under different illumination intensities. It is found that photoconductive and photogating effect as well as space charge limited conduction can simultaneously occur. We point out that the photoconductivity increases logarithmically with the light intensity and can persist with a decay time longer than 10 s, due to photo-charge trapping at the MoS/SiO interface and in MoS defects. The transfer characteristics present hysteresis that is enhanced by illumination. At low drain bias, the devices feature low contact resistance of [Formula: see text] ON current as high as [Formula: see text] 10 ON-OFF ratio, mobility of ∼1 cm V s and photoresponsivity [Formula: see text].

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.

We study the effect of electric stress, gas pressure and gas type on the hysteresis in the transfer characteristics of monolayer molybdenum disulfide (MoS 2 ) field effect transistors. The presence of defects and point vacancies in the MoS 2 crystal structure facilitates the adsorption of oxygen, nitrogen, hydrogen or methane, which strongly affect the transistor electrical characteristics. Although the gas adsorption does not modify the conduction type, we demonstrate a correlation between hysteresis width and adsorption energy onto the MoS 2 surface. We show that hysteresis is controllable by pressure and/or gas type. Hysteresis features two well-separated current levels, especially when gases are stably adsorbed on the channel, which can be exploited in memory devices.

Permeance indicates how fast the gas molecule transports through the membrane, usually expressed in gas permeation unit (GPU, 1 GPU = 10 −6 cm 3 (STP) cm −2 scmHg −1 ), while selectivity represents how efficiently the gas is separated from the mixture during the transportation. The nature of molecular transportation in membranes depends on the porosity and type of membrane materials, which can be understood by using mechanisms based on solution-diffusion, size-dependent molecular sieving, Knudsen diffusion, and Poiseuille flow. [3] Materials such as zeolites, [4][5][6] metalorganic frameworks (MOFs), [7][8][9] and carbon-based constituents [10][11][12][13][14] in the form of membranes have been intensively studied as gas barriers in the past. These materials are featured by tuneable nanosized pores, which provide sites for gas