Molybdenum disulfide is considered as one of the most promising two-dimensional semiconductors for electronic and optoelectronic device applications. So far, the charge transport in monolayer molybdenum disulfide is dominated by extrinsic factors such as charged impurities, structural defects and traps, leading to much lower mobility than the intrinsic limit. Here we develop a facile low-temperature thiol chemistry route to repair the sulfur vacancies and improve the interface, resulting in significant reduction of the charged impurities and traps. High mobility 480 cm 2 V À 1 s À 1 is achieved in backgated monolayer molybdenum disulfide field-effect transistors at room temperature. Furthermore, we develop a theoretical model to quantitatively extract the key microscopic quantities that control the transistor performances, including the density of charged impurities, short-range defects and traps. Our combined experimental and theoretical study provides a clear path towards intrinsic charge transport in two-dimensional dichalcogenides for future high-performance device applications.

2D transition metal dichalcogenides are emerging with tremendous potential in many optoelectronic applications due to their strong light-matter interactions. To fully explore their potential in photoconductive detectors, high responsivity is required. Here, high responsivity phototransistors based on few-layer rhenium disulfi de (ReS 2 ) are presented. Depending on the back gate voltage, source drain bias and incident optical light intensity, the maximum attainable photoresponsivity can reach as high as 88 600 A W −1 , which is a record value compared to other individual 2D materials with similar device structures and two orders of magnitude higher than that of monolayer MoS 2 . Such high photoresponsivity is attributed to the increased light absorption as well as the gain enhancement due to the existence of trap states in the fewlayer ReS 2 fl akes. It further enables the detection of weak signals, as successfully demonstrated with weak light sources including a lighter and limited fl uorescent lighting. Our studies underscore ReS 2 as a promising material for future sensitive optoelectronic applications.

Research into the physical properties of MoS 2 and other semiconducting transition metal dichalcogenides [1] (TMDs) has increased considerably in recent years, owing to their potential applications in post-CMOS electronics [2][3][4] , optoelectronics [5][6][7] and valleytronics [8][9][10] . Some of the properties of monolayer MoS 2 that are advantageous for electronic applications include a direct band gap of 1.8 eV [11,12] as well as a film thickness of less than 1 nm which gives superior electrostatic control of the charge density and current even at the transistor scaling limit [3,13] . In spite of these favorable properties, the widely reported low electron mobility in monolayer MoS 2 poses a serious obstacle to its integration into post-CMOS nanoelectronics. The nature of charge transport in MoS 2 , especially at room temperature, remains poorly understood despite considerable amount of theoretical and experimental researches.For example, the theoretically predicted intrinsic phonon-limited mobility at room temperature is in the range of 200-410 cm 2 /Vs [14.15] while most experimentally reported values are much smaller [16][17][18][19][20][21][22][23] . Before any semiconducting material can become useful for potential nanoelectronic device applications, a critical assessment of its intrinsic charge transport properties at room temperature is needed, requiring the realization of high-quality samples with carrier mobility in the phonon-limited regime.The phonon-limited transport regime was demonstrated for graphene [24] and carbon nanotubes [25] . However, despite many recent efforts to improve carrier mobility by means of topgate [17] , chemical functionalization [21] and BN gate dielectrics [22] , phonon-limited transport regime has not been explicitly demonstrated in monolayer TMDs including MoS 2 .The possible reasons for the discrepancy between the theoretical upper limit and experimental data include Coulomb impurities (CI), traps and defects in low-quality samples [19][20][21][22][23] . These extrinsic sources of scattering have so far precluded any rigorous examination of the intrinsic scattering mechanisms that affect electron mobility. A particularly important source of scattering is from CI at the semiconductor-dielectric interface, which is believed to be the most important limiting factor in current MoS 2 devices [26] . Recently, it has been demonstrated that by sandwiching the monolayer MoS 2 channel between BN layers, CI scattering can be significantly suppressed, leading to a record-high mobility of over 1000 cm 2 /Vs at low temperatures [22][23] . The technologically relevant room-temperature mobility, however, still lags the best devices on SiO 2 for reasons not well understood. Nonetheless, significant recent progress in reducing the deleterious effects of CI, traps and defects on the mobility [20][21][22][23] begin to set the stage for the realization of room-temperature charge transport in the phonon-limited regime.It has been shown experimentally that the deposition of a high-κ top ...

The combination of high-quality Al2 O3 dielectric and thiol chemistry passivation can effectively reduce the density of interface traps and Coulomb impurities, leading to a significant improvement of the mobility and a transition of the charge transport from the insulating to the metallic regime. A record high mobility of 83 cm(2) V(-1) s(-1) (337 cm(2) V(-1) s(-1) ) is reached at room temperature (low temperature) for monolayer WS2 . A theoretical model for electron transport is also developed.

Recently it was demonstrated that Sr intercalation provides a new route to induce superconductivity in the topological insulator Bi2Se3. Topological superconductors are predicted to be unconventional with an odd-parity pairing symmetry. An adequate probe to test for unconventional superconductivity is the upper critical field, Bc2. For a standard BCS layered superconductor Bc2 shows an anisotropy when the magnetic field is applied parallel and perpendicular to the layers, but is isotropic when the field is rotated in the plane of the layers. Here we report measurements of the upper critical field of superconducting SrxBi2Se3 crystals (Tc = 3.0 K). Surprisingly, field-angle dependent magnetotransport measurements reveal a large anisotropy of Bc2 when the magnet field is rotated in the basal plane. The large two-fold anisotropy, while six-fold is anticipated, cannot be explained with the Ginzburg-Landau anisotropic effective mass model or flux flow induced by the Lorentz force. The rotational symmetry breaking of Bc2 indicates unconventional superconductivity with odd-parity spin-triplet Cooper pairs (Δ4-pairing) recently proposed for rhombohedral topological superconductors, or might have a structural nature, such as self-organized stripe ordering of Sr atoms.

The progress in exploiting new electronic materials has been a major driving force in solid-state physics. As a new state of matter, a Weyl semimetal (WSM), in particular a type-II WSM, hosts Weyl fermions as emergent quasiparticles and may harbour novel electrical transport properties. Nevertheless, such a type-II WSM material has not been experimentally observed. In this work, by performing systematic magneto-transport studies on thin films of a predicted material candidate WTe2, we observe notable negative longitudinal magnetoresistance, which can be attributed to the chiral anomaly in WSM. This phenomenon also exhibits strong planar orientation dependence with the absence along the tungsten chains, consistent with the distinctive feature of a type-II WSM. By applying a gate voltage, we demonstrate that the Fermi energy can be in-situ tuned through the Weyl points via the electric field effect. Our results may open opportunities for implementing new electronic applications, such as field-effect chiral devices.

van der Waals (vdW) heterojunctions formed by two-dimensional (2D) materials have attracted tremendous attention due to their excellent electrical/opticalproperties and device applications. However, current 2D heterojunctions are largely limited to atomic crystals, and hybrid organic/inorganic structures are rarely explored. Here, we fabricate hybrid 2D heterostructures with p-type dioctylbenzothienobenzothiophene (C 8 -BTBT) and n-type MoS 2 . We find that

In the foundation of quantum mechanics, the spatial dimensions of electron wavepacket are understood only in terms of an expectation valuethe probability distribution of the particle location. One can still inquire how the quantum electron wavepacket size affects a physical process. Here we address the fundamental physics problem of particle-wave duality and the measurability of a free electron quantum wavepacket. Our analysis of stimulated radiative interaction of an electron wavepacket, accompanied by numerical computations, reveals two limits. In the quantum regime of long wavepacket size relative to radiation wavelength, one obtains only quantum-recoil multiphoton sidebands in the electron energy spectrum. In the opposite regime, the wavepacket interaction approaches the limit of classical point-particle acceleration. The wavepacket features can be revealed in experiments carried out in the intermediate regime of wavepacket size commensurate with the radiation wavelength.      , where   0ˆz Ee z iq z z E e  represents the dominant slow component of the radiation wave, and transverse field components, as well astransverse variation of the field are neglected. We examplify our modeling here for a case of Smith-Purcell radiation (see Fig. 1), for which the radiation wave is a Floquent mode: 4   zm iq z m m ze   EEwith 0 2 zm z G q q m   , G  is the grating period, 0 Θ, / z q qcos q c   and Θ is the incidence angle of the radiation wave relative to the axial interaction dimension. The radiation wave number z zm qq  represents one of the space harmonics m that satisfies synchronizm condition with the electron [6]: 0 zm vq   .We note that the analysis would be similar for the Cerenkov interaction scheme with   Θ z q n cos   , and () n  the index of refraction of the medium. Furthermore, the analysis can be extended to the case of FEL and other interaction schemes [13,14].The solution of Schrodinger equation to zero order (i.e. free-space propagation) is well known. Assuming that the initial wavepacket, which is emitted at some point