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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.

mechanisms of the synaptic devices are inherent to the nature of each memristive channel. Atomically thin 2D van der Waals (vdW) semiconductors [14][15][16] and their heterostructures have recently emerged as a promising platform for neuromorphic applications, [17,18] benefitting from their electrical tunability, [19] low-power switching capability, [20] and strong lightmatter interactions. [21] Synaptic devices based on 2D vdW materials can be categorized into two types in terms of the device geometry: three-terminal synaptic transistors in a lateral geometry [22][23][24][25][26][27][28][29] and vertical two-terminal memristors. [30][31][32][33][34][35][36][37] In lateral synaptic transistors, [1] the specific synaptic functions are included in a transistor by using gate dielectrics as a charge trapping layer [22,23,26] or a tunnel barrier for electron injection [27,28] to modify the conductivity of the channel. Thus, the terminals for reading and writing are decoupled, enabling nondestructive device operations. However, the lateral geometry of the synaptic transistors is inherently unfavorable for the high-density integration. An alternative type of synaptic devices is a vertical two-terminal memristor, of which the resistive switching is realized by hysteresis engineering in an active layer between top and bottom electrodes. [7] The simple structure of the vertical memristors enables them to easily extend to a crossbar array with a high integration density, [38,39] which is advantageous to perform parallel matrix computations for ANNs. [40] However, the number of feasible conductance states is limited due to the filamentary nature of the conduction channels in the vertical memristors. [9] Here, we report an ultrasmall synaptic device (1 µm × 1 µm × 2 nm), configured in a crossbar array circuitry, based on a 2D vdW heterostructure, [41][42][43] where the 2D semiconductor WS 2 is vertically sandwiched by a pair of single-layer graphene (SLG) electrodes on a gate stack. Such artificial synapse cells were triggered by light for their synaptic weight variations, enabling individual cell addressing in the crossbar array, i.e., photo-memtransistors. By exploiting direct light-lattice interactions, [44][45][46] a train of UV pulses onto the small volume memristor generates the step-wise modulation of lattice dopants achieves the accurate operation of the synaptic dynamics of the memristor. We also explain such synaptic dynamics in a parallel resistor network model. In addition, we show that the gatevoltage dynamically reconfigures the individual conductance states in a wider range, enabling more complex heterosynaptic A new type of atomically thin synaptic network on van der Waals (vdW) heterostructures is reported, where each ultrasmall cell (≈2 nm thick) built with trilayer WS 2 semiconductor acts as a gate-tunable photoactive synapse, i.e., a photo-memtransistor. A train of UV pulses onto the WS 2 memristor generates dopants in atomic-level precision by direct light-lattice interactions, which, along with the gate ...

Under 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.

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We report atomic layer-by-layer epitaxial growth of van der Waals (vdW) semiconductor superlattices (SLs) with programmable stacking periodicities, composed of more than two kinds of dissimilar transition-metal dichalcogenide monolayers (MLs), such as MoS2, WS2 and WSe2. The kinetics-controlled vdW epitaxy in the near equilibrium limit by metalorganic chemical vapour depositions enables to achieve accurate ML-by-ML stacking, free of interlayer atomic mixing, resulting in the tunable two-dimensional (2D) vdW electronic systems. We identified coherent atomic stacking orders at the vdW heterointerfaces, and present scaling valley polarized optical excitations that only pertain to a series of 2D type II band alignments.