The band structure of many semiconducting monolayer transition metal dichalcogenides (TMDs) possesses two degenerate valleys, with equal and opposite Berry curvature. It has been predicted that, when illuminated with circularly polarized light, interband transitions generate an unbalanced non-equilibrium population of electrons and holes in these valleys, resulting in a finite Hall voltage at zero magnetic field when a current flows through the system. This is the so-called valley Hall effect that has recently been observed experimentally. Here, we show that this effect is mediated by photo-generated neutral excitons and charged trions, and not by inter-band transitions generating independent electrons and holes. We further demonstrate an experimental 1 arXiv:1708.06914v2 [cond-mat.mes-hall]

Perpendicular electric fields can tune the electronic band structure of atomically thin semiconductors. In bilayer graphene, which is an intrinsic zero-gap semiconductor, a perpendicular electric field opens a finite band gap. So far, however, the same principle could not be applied to control the properties of a broader class of 2D materials, because the required electric fields are beyond reach in current devices. To overcome this limitation, we design double ionic gated transistors that enable the application of large electric fields up to 3 V/nm. Using such devices, we continuously supress the band gap of few-layer semiconducting transition metal dichalcogenides, bilayer to heptalayer WSe 2 , from 1.6 V to zero. Our results illustrate an unprecedented level of control on the band structure of 2D semiconductors.An electric field applied perpendicular to the surface of a bulk semiconductor is screened over a finite length, leaving the material interior unaffected. In atomically thin semiconductors [1], however, the small thickness prevents efficient screening, so that a perpendicular electric field uniformly influences the entire system, modifying its band structure [2][3][4][5][6][7][8][9][10][11].Indeed, a zero-gap semiconductor such as bilayer graphene can be turned into a gapped insulator using double-gated transistors to apply a perpendicular electric field [2][3][4]. Despite representing a breakthrough, continuous control of the band structure in transistors has not found widespread use because -as it has become apparent in multiple, recent experiments [7,10,12]-the limited maximum electric field that can be applied in common devices does not allow significant changes to be induced in most 2D materials. Indeed, whereas theory [5, 9], and our own calculations (see Supplementary Note S8), predict that sufficiently strong fields can quench the band gap of few layer semiconductors, earlier works [7,12] have only shown a 10 % gap reduction (or less) at fields reached in conventional double-gated transistors.

We explore solid electrolytes for electrostatic gating using field-effect transistors (FETs) in which thin WSe 2 crystals are exfoliated and transferred onto a lithium-ion conducting glass ceramic substrate. For negative gate voltages (V G < 0) the devices work equally well as ionic liquid gated FETs while offering specific advantages, whereas no transistor action is seen for V G > 0. For V G < 0 the devices can nevertheless be driven into the ambipolar injection regime by applying a large source-drain bias, and strong electroluminescence is observed when direct band-gap WSe 2 monolayers are used. Detecting and imaging the emitted light is much simpler in these FETs as compared to ionic liquid gated transistors, because the semiconductor surface is exposed (i.e., not covered by another material). Our results show that solid electrolytes are complementary to existing liquid gates, as they enable experiments not possible when the semiconductor is buried under the liquid itself.Modulating the charge carrier density at the surface of semiconductors or insulators is commonly done by means of electrostatic gating in field-effect transistors (FETs), employing conventional solid state dielectrics.

Ionic gating is a powerful technique to realize field‐effect transistors (FETs) enabling experiments not possible otherwise. So far, ionic gating has relied on the use of top electrolyte gates, which pose experimental constraints and make device fabrication complex. Promising results obtained recently in FETs based on solid‐state electrolytes remain plagued by spurious phenomena of unknown origin, preventing proper transistor operation, and causing limited control and reproducibility. Here, a class of solid‐state electrolytes for gating (Lithium‐ion conducting glass‐ceramics, LICGCs) is explored, the processes responsible for the spurious phenomena and irreproducible behavior are identified, and properly functioning transistors exhibiting high density ambipolar operation with gate capacitance of ≈20 − 50 µF cm−2\[20{\bm{ - }}50\;\mu F c{m^{{\bm{ - }}2}}\] (depending on the polarity of the accumulated charges) are demonstrated. Using 2D semiconducting transition‐metal dichalcogenides, the ability to implement ionic‐gate spectroscopy to determine the semiconducting bandgap, and to accumulate electron densities above 1014 cm−2 are demostrated, resulting in gate‐induced superconductivity in MoS2 multilayers. As LICGCs are implemented in a back‐gate configuration, they leave the surface of the material exposed, enabling the use of surface‐sensitive techniques (such as scanning tunneling microscopy and photoemission spectroscopy) impossible so far in ionic‐gated devices. They also allow double ionic gated devices providing independent control of charge density and electric field.

Light-emitting electronic devices are ubiquitous in key areas of current technology, such as data communications, solid-state lighting, displays, and optical interconnects. Controlling the spectrum of the emitted light electrically, by simply acting on the device bias conditions, is an important goal with potential technological repercussions. However, identifying a material platform enabling broad electrical tuning of the spectrum of electroluminescent devices remains challenging. Here, we propose light-emitting field-effect transistors based on van der Waals interfaces of atomically thin semiconductors as a promising class of devices to achieve this goal. We demonstrate that large spectral changes in room-temperature electroluminescence can be controlled both at the device assembly stage –by suitably selecting the material forming the interfaces– and on-chip, by changing the bias to modify the device operation point. Even though the precise relation between device bias and kinetics of the radiative transitions remains to be understood, our experiments show that the physical mechanism responsible for light emission is robust, making these devices compatible with simple large areas device production methods.