Monolayers of transition-metal dichalcogenides (TMDs) exhibit numerous crystal phases with distinct structures, symmetries and physical properties. Exploring the physics of transitions between these different structural phases in two dimensions may provide a means of switching material properties, with implications for potential applications. Structural phase transitions in TMDs have so far been induced by thermal or chemical means; purely electrostatic control over crystal phases through electrostatic doping was recently proposed as a theoretical possibility, but has not yet been realized. Here we report the experimental demonstration of an electrostatic-doping-driven phase transition between the hexagonal and monoclinic phases of monolayer molybdenum ditelluride (MoTe). We find that the phase transition shows a hysteretic loop in Raman spectra, and can be reversed by increasing or decreasing the gate voltage. We also combine second-harmonic generation spectroscopy with polarization-resolved Raman spectroscopy to show that the induced monoclinic phase preserves the crystal orientation of the original hexagonal phase. Moreover, this structural phase transition occurs simultaneously across the whole sample. This electrostatic-doping control of structural phase transition opens up new possibilities for developing phase-change devices based on atomically thin membranes.

Bottom-up synthesized graphene nanoribbons and graphene nanoribbon heterostructures have promising electronic properties for high-performance field-effect transistors and ultra-low power devices such as tunneling field-effect transistors. However, the short length and wide band gap of these graphene nanoribbons have prevented the fabrication of devices with the desired performance and switching behavior. Here, by fabricating short channel (L ch ~ 20 nm) devices with a thin, high-κ gate dielectric and a 9-atom wide (0.95 nm) armchair graphene nanoribbon as the channel material, we demonstrate field-effect transistors with high on-current (I on > 1 μA at V d = −1 V) and high I on /I off ~ 105 at room temperature. We find that the performance of these devices is limited by tunneling through the Schottky barrier at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high-performance short-channel field-effect transistors with bottom-up synthesized armchair graphene nanoribbons.

Functionalities of two-dimensional (2D) crystals based on semiconducting transition metal dichalcogenides (TMDs) have now stemmed from simple field effect transistors (FETs) to a variety of electronic and opto-valleytronic devices, and even to superconductivity. Among them, superconductivity is the least studied property in TMDs due to methodological difficulty accessing it in different TMD species. Here, we report the systematic study of superconductivity in MoSe2, MoTe2 and WS2 by ionic gating in different regimes. Electrostatic gating using ionic liquid was able to induce superconductivity in MoSe2 but not in MoTe2 because of inefficient electron accumulation limited by electronic band alignment. Alternative gating using KClO4/polyethylene glycol enabled a crossover from surface doping to bulk doping, which induced superconductivities in MoTe2 and WS2 electrochemically. These new varieties greatly enriched the TMD superconductor families and unveiled critical methodology to expand the capability of ionic gating to other materials.

Chirality of materials are known to affect optical, magnetic and electric properties, causing a variety of nontrivial phenomena such as circular dichiroism for chiral molecules, magnetic Skyrmions in chiral magnets and nonreciprocal carrier transport in chiral conductors. On the other hand, effect of chirality on superconducting transport has not been known. Here we report the nonreciprocity of superconductivity—unambiguous evidence of superconductivity reflecting chiral structure in which the forward and backward supercurrent flows are not equivalent because of inversion symmetry breaking. Such superconductivity is realized via ionic gating in individual chiral nanotubes of tungsten disulfide. The nonreciprocal signal is significantly enhanced in the superconducting state, being associated with unprecedented quantum Little-Parks oscillations originating from the interference of supercurrent along the circumference of the nanotube. The present results indicate that the nonreciprocity is a viable approach toward the superconductors with chiral or noncentrosymmetric structures.

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We report superconducting resistive transition characteristics for array͑s͒ of coupled 4 Å single wall carbon nanotubes embedded in aluminophosphate-five zeolite. The transition was observed to initiate at 15 K with a slow resistance decrease switching to a sharp, order of magnitude drop at ϳ7.5 K. The latter exhibits anisotropic magnetic field dependence, and the differential resistance versus current ͑voltage͒ measurements show a rich variety of details that are consistent with the establishment of coherence, in stages, as the temperature is lowered below 15 K. In particular, the resistance drop that starts at 7.5 K exhibits attributes that are consistent with the manifestations of a Berezinskii-Kosterlitz-Thouless transition that establishes quasi-long-range order in the plane transverse to the c axis of the aligned nanotubes.

Fig. 4. Opposing effects of TCDD and EGF on differentiation of NHEKs. (A)NHEKs were grown to confluence, and basal medium, or medium with ␣-naphthoflavone (NF, 1 M) was added 24 h before treatment. CEs were isolated after treatment with either 0.1% DMSO or TCDD (10 nM) for 5 days. (B) NHEKs were grown to confluence, and basal medium with or without EGF (10 ng/mL) was added 24 h before treatment. CEs were isolated after treatment with either 0.1% DMSO or TCDD (10 nM) for 5 days. (C) NHEKs were grown to confluence, and basal medium, or medium with EGF (10 ng/mL), was added 24 h before treatment. CEs were isolated after treatment with either 0.1% DMSO or TCDD (10 nM) in the presence or absence of PD153035 (300 nM) for 5 days. (A-C) The values for CEs are a mean of triplicate samples Ϯ SD. (D-G) NHEKs were grown to a cell density of either 50% or 100% confluence before basal medium, or medium with EGF (10 ng/mL), was added for 24 h before treatment. Total mRNA was isolated after treatment with either control vehicle (0.1% DMSO) or TCDD (10 nM) for 24 h. Real-time PCR was used to determine the relative expression of each indicated gene (y axis). Levels of mRNA [mean (n ϭ 3) Ϯ SD] are expressed in units relative to the minimum, given a value of 1. (A-G) The a indicates that the value from treatment with TCDD is significantly different from the DMSO control; the b indicates that the value from cotreatment with TCDD and NF is significantly different from TCDD alone; the c indicates that the value from cotreatment with TCDD and EGF is significantly different from with TCDD alone; the d indicates that the value from treatment with TCDD, EGF, and PD153035 is significantly different from treatment with TCDD and EGF; and the e indicates that treatment with EGF is significantly different from DMSO control treatment. For each of these comparisons, P Յ 0.01 by Student's t test. Comparisons made in D-G are within the group grown to a cell density of 100% confluence.

A key feature of two-dimensional materials is that the sign and concentration of 15 their carriers can be externally controlled with techniques such as electrostatic 16 gating. However, conventional electrostatic gating has limitations, including a 17 maximum carrier density set by the dielectric breakdown, and ionic liquid gating 18 and direct chemical doping also suffer from drawbacks. Here, we show that an 19 electron-beam doping technique can be used to reversibly write high-resolution 20 doping patterns in hexagonal boron nitride encapsulated graphene and 21 molybdenum disulfide (MoS 2 ) van der Waals heterostructures. The MoS 2 device 22 exhibits an order of magnitude decrease of subthreshold swing after doping, 23