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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. Here, the authors report the realization of light-emitting field-effect transistors based on van der Waals heterostructures with conduction and valence band edges at the Γ-point of the Brillouin zone, showing electrically tunable and material-dependent electroluminescence spectra at room temperature. [ABSTRACT FROM AUTHOR]

We explore solid electrolytes for electrostatic gating using field-effect transistors (FETs) in which thin WSe2 crystals are exfoliated and transferred onto a lithium-ion conducting glass ceramic substrate. For negative gate voltages (VG < 0), the devices work equally well as ionic liquid gated FETs while offering specific advantages, whereas no transistor action is seen for VG > 0. For VG < 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 WSe2 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 the existing liquid gates, as they enable experiments that are not possible when the semiconductor is buried under the liquid itself. [ABSTRACT FROM AUTHOR]

We perform scanning photocurrent microscopy on WS2 ionic liquid-gated field effect transistors exhibiting high-quality ambipolar transport. By properly biasing the gate electrode, we can invert the sign of the photocurrent showing that the minority photocarriers are either electrons or holes. Both in the electron- and hole-doping regimes the photocurrent decays exponentially as a function of the distance between the illumination spot and the nearest contact, in agreement with a two-terminal Schottky-barrier device model. This allows us to compare the value and the doping dependence of the diffusion length of the minority electrons and holes on a same sample. Interestingly, the diffusion length of the minority carriers is several times larger in the hole accumulation regime than in the electron accumulation regime, pointing out an electron-hole asymmetry in WS2. [ABSTRACT FROM AUTHOR]

Research on organic thin-film transistors tends to focus on improvements in device performance, but very little is understood about the ultimate limits of these devices, the microscopic physical mechanisms responsible for their limitations, and, more generally, the intrinsic transport properties of organic semiconductors. These topics are now being investigated through the study of transport in organic transistors realized using molecular single crystals of unprecedented chemical purity and structural quality. These studies are elucidating detailed microscopic aspects of the physics of organic semiconductors and corresponding devices and have also led to unforeseen high values for carrier mobility in these materials. Here, we discuss developments in this area and present a brief outlook on future goals that have come into experimental reach. [ABSTRACT FROM PUBLISHER]

The authors report a systematic study of the bias-dependent contact resistance in rubrene single-crystal field-effect transistors with Ni, Co, Cu, Au, and Pt electrodes. They show that the reproducibility in the values of contact resistance strongly depends on the metal, ranging from a factor of 2 for Ni to more than three orders of magnitude for Au. Surprisingly, field-effect transistors with Ni, Co, and Cu contacts exhibit an unexpected reproducibility of the bias-dependent differential conductance of the contacts once this has been normalized to the value measured at zero bias. This reproducibility may enable the study of microscopic carrier injection processes into organic semiconductors. [ABSTRACT FROM AUTHOR]

We have investigated the contact resistance of rubrene single-crystal field-effect transistors (FETs) with nickel electrodes by performing scaling experiments on devices with channel length ranging from 200 nm up to 300 μm. We find that the contact resistance can be as low as 100 Ω cm with narrowly spread fluctuations. For comparison, we have also performed scaling experiments on similar gold-contacted devices, and found that the reproducibility of FETs with nickel electrodes is largely superior. These results indicate that nickel is a very promising electrode material for the reproducible fabrication of low resistance contacts in organic FETs. [ABSTRACT FROM AUTHOR]

We realize and investigate ionic liquid gated field-effect transistors (FETs) on large-area MoS2monolayers grown by chemical vapor deposition (CVD). Under electron accumulation, the performance of these devices is comparable to that of FETs based on exfoliated flakes. FETs on CVD-grown material, however, exhibit clear ambipolar transport, which for MoS2 monolayers had not been reported previously. We exploit this property to estimate the bandgap Δ of monolayer MoS2 directly from the device transfer curves and find Δ ≈ 2.4-2.7 eV. In the ambipolar injection regime, we observe electroluminescence due to exciton recombination in MoS2, originating from the region close to the hole-injecting contact. Both the observed transport properties and the behavior of the electroluminescence can be consistently understood as due to the presence of defect states at an energy of 250-300 meV above the top of the valence band, acting as deep traps for holes. Our results are of technological relevance, as they show that devices with useful optoelectronic functionality can be realized on large-area MoS2monolayers produced by controllable and scalable techniques. [ABSTRACT FROM AUTHOR]

We investigate transport throughionic liquid gated field effect transistors (FETs) based on exfoliatedcrystals of semiconducting WS2. Upon electron accumulation,at surface densities close to, or just larger than, 1014cm–2, transport exhibits metallic behavior withthe surface resistivity decreasing pronouncedly upon cooling. A detailedcharacterization as a function of temperature and magnetic field clearlyshows the occurrence of a gate-induced superconducting transitionbelow a critical temperature Tc≈4 K, a finding that represents the first demonstration of superconductivityin tungsten-based semiconducting transition metal dichalcogenides.We investigate the nature of superconductivity and find significantinhomogeneity, originating from the local detaching of the frozenionic liquid from the WS2surface. Despite the inhomogeneity,we find that in all cases where a fully developed zero resistancestate is observed, different properties of the devices exhibit a behaviorcharacteristic of a Berezinskii–Kosterlitz–Thoulesstransition, as it could be expected in view of the two-dimensionalnature of the electrostatically accumulated electron system. [ABSTRACT FROM AUTHOR]

We have realized ambipolar ionicliquid gated field-effect transistorsbased on WS2mono- and bilayers, and investigated theiropto-electronic response. A thorough characterization of the transportproperties demonstrates the high quality of these devices for bothelectron and hole accumulation, which enables the quantitative determinationof the band gap (Δ1L= 2.14 eV for monolayers andΔ2L= 1.82 eV for bilayers). It also enables theoperation of the transistors in the ambipolar injection regime withelectrons and holes injected simultaneously at the two opposite contactsof the devices in which we observe light emission from the FET channel.A quantitative analysis of the spectral properties of the emittedlight, together with a comparison with the band gap values obtainedfrom transport, show the internal consistency of our results and allowa quantitative estimate of the excitonic binding energies to be made.Our results demonstrate the power of ionic liquid gating in combinationwith nanoelectronic systems, as well as the compatibility of thistechnique with optical measurements on semiconducting transition metaldichalcogenides. These findings further open the way to the investigationof the optical properties of these systems in a carrier density rangemuch broader than that explored until now. [ABSTRACT FROM AUTHOR]

The article discusses the high electric mobility and ambient for n-channel single-crystal (SC) field-effect transistors (FETs) or SCFETs. It says that SCFETs exhibit field-effect mobilities approaching those of the best p-channel FETs when fabricated with dielectric materials. It cites that future studies should focus on how the SC semi-conductor functions with specialized dielectric materials and device architectures.