Light Induced Local Reversible Doping of 2D van der Waals Semiconductors

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Light-Induced Local Reconfigurable Doping of 2D van der Waals Semiconductor

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Atomic Healing of Chalcogen Vacancies in Monolayer WSe2

Two-dimensional (2D) transition-metal dichalcogenide (TMDC) semiconductors are the atomically thin platforms for a new type of photonics. Therein, atomic-scale deformations such as point defects and grain boundaries, often generate novel 2D physical properties. For example, single point defects in WSe2 and WS2 monolayers (MLs) serve as sources for single-photon emissions or electronic dopants, and mirror twin boundaries in MoSe2 MLs provide topologically protected edge-states. In order to exploit these local physical properties into practical device platforms, such atomic-scale deformations must be deterministically controlled in the 2D host lattices of TMDC MLs. Here, we report a simple process for the healing of chalcogen point vacancies on the synthetic ML WSe2 by metal-organic selenium passivation. We verified such atomic healing process from substantial reduction of the localized exciton states by low temperature photoluminescence. Our work suggests steps to realize the higher quality photonics with atomic precision.1

Sub Band-Gap Photoresponse by Hot Electron Injections in AU-Nanorod Decorated Van Der Waals Semiconductor Monolayers

Hot electrons in metal nanostructures can be exploited in a wide range of optical functions, including photocatalysis, surface-enhanced Raman scattering, photodetectors and photovoltaics. Here, we report the sub band-gap (Eg) photoresponse in Au-nanorod decorated van der Waals (vdW) semiconductor, MoS2 and WSe2, monolayers (MLs). It was found that hot electrons, optically excited in Au nanorod (NR) arrays at the sub band-gap (Eg) radiations, can be injected to vdW ML semiconductors over Schottky barriers, producing substantial photocurrents in n-type MoS2 and p-type WSe2 ML photodetectors, as well as photovoltages in n-MoS2/p-WSe2 ML stack junctions. Moreover, by spectrally and light-polarization resolved measurements, we showed that these sub band-gap (Eg) excitations of hot electrons can be modulated by tuning the plasmon resonance at the shape controlled Au NRs.1

Atomically Thin Synapse Networks on Van Der Waals Photo-Memtransistors

A new type of atomically thin synaptic network on van der Waals (vdW) heterostructures is reported, where each ultrasmall cell (approximate to 2 nm thick) built with trilayer WS2 semiconductor acts as a gate-tunable photoactive synapse, i.e., a photo-memtransistor. A train of UV pulses onto the WS2 memristor generates dopants in atomic-level precision by direct light-lattice interactions, which, along with the gate tunability, leads to the accurate modulation of the channel conductance for potentiation and depression of the synaptic cells. Such synaptic dynamics can be explained by a parallel atomistic resistor network model. In addition, it is shown that such a device scheme can generally be realized in other 2D vdW semiconductors, such as MoS2, MoSe2, MoTe2, and WSe2. Demonstration of these atomically thin photo-memtransistor arrays, where the synaptic weights can be tuned for the atomistic defect density, provides implications for a new type of artificial neural networks for parallel matrix computations with an ultrahigh integration density.11Nsciescopu

Identification of Point Defects in Atomically Thin Transition-Metal Dichalcogenide Semiconductors as Active Dopants

Selective doping in semiconductors is essential not only for monolithic integrated circuity fabrications but also for tailoring their properties including electronic, optical, and catalytic activities. Such active dopants are essentially point defects in the host lattice. In atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDCs), the roles of such point defects are particularly critical in addition to their large surface-to-volume ratio, because their bond dissociation energy is relatively weaker, compared to elemental semiconductors. In this Mini Review, we review recent advances in the identifications of diverse point defects in 2D TMDC semiconductors, as active dopants, toward the tunable doping processes, along with the doping methods and mechanisms in literature. In particular, we discuss key issues in identifying such dopants both at the atomic scales and the device scales with selective examples. Fundamental understanding of these point defects can hold promise for tunability doping of atomically thin 2D semiconductor platforms.11Nsciescopu

Reconfigurable doping of atomically thin van der Waals semiconductors by light colours

We report reversible photo-induced doping on atomically thin van der Waals (vdW) semiconductors, whose channel polarities can be repeatedly reconfigured from n-type to p-type and vice versa with light colours. Evidently, this reconfigurable doping is explained by selective light-lattice interactions, such as the characteristic point defect generations and the charged ion incorporation. The precise doping tunability in a reconfigurable manner of this work may provide a key technological solution toward realisation of new types of monolithic integrated circuitry on atomically thin vdW semiconductors. We demonstrate such a proof-of-concept device, a reconfigurable complementary metal-oxide-semiconductor (CMOS) device on a single vdW semiconductor channel, in which the circuit functions can be dynamically reset from a CMOS inverter to a CMOS switch with the choice of light colours.2

Gate-tunable quantum pathways of high harmonic generation in graphene

AbstractUnder strong laser fields, electrons in solids radiate high-harmonic fields by travelling through quantum pathways in Bloch bands in the sub-laser-cycle timescales. Understanding these pathways in the momentum space through the high-harmonic radiation can enable an all-optical ultrafast probe to observe coherent lightwave-driven processes and measure electronic structures as recently demonstrated for semiconductors. However, such demonstration has been largely limited for semimetals because the absence of the bandgap hinders an experimental characterization of the exact pathways. In this study, by combining electrostatic control of chemical potentials with HHG measurement, we resolve quantum pathways of massless Dirac fermions in graphene under strong laser fields. Electrical modulation of HHG reveals quantum interference between the multi-photon interband excitation channels. As the light-matter interaction deviates beyond the perturbative regime, elliptically polarized laser fields efficiently drive massless Dirac fermions via an intricate coupling between the interband and intraband transitions, which is corroborated by our theoretical calculations. Our findings pave the way for strong-laser-field tomography of Dirac electrons in various quantum semimetals and their ultrafast electronics with a gate control.11Ysciescopu

Atomistic Probing of Defect-Engineered 2H-MoTe₂ Monolayers

Point defects dictate various physical, chemical, and optoelectronic properties of two-dimensional (2D) materials, and therefore, a rudimentary understanding of the formation and spatial distribution of point defects is a key to advancement in 2D material-based nanotechnology. In this work, we performed the demonstration to directly probe the point defects in 2H-MoTe2 monolayers that are tactically exposed to (i) 200 °C-vacuum-annealing and (ii) 532 nm-laser-illumination; and accordingly, we utilize a deep learning algorithm to classify and quantify the generated point defects. We discovered that tellurium-related defects are mainly generated in both 2H-MoTe2 samples; but interestingly, 200 °C-vacuum-annealing and 532 nm-laser-illumination modulate a strong n-type and strong p-type 2H-MoTe2, respectively. While 200 °C-vacuum-annealing generates tellurium vacancies or tellurium adatoms, 532 nm-laser-illumination prompts oxygen atoms to be adsorbed/chemisorbed at tellurium vacancies, giving rise to the p-type characteristic. This work significantly advances the current understanding of point defect engineering in 2H-MoTe2 monolayers and other 2D materials, which is critical for developing nanoscale devices with desired functionality