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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

Two-dimensional semiconductors have a range of electronic and optical properties that can be used in the development of advanced electronic devices. However, unlike conventional silicon semiconductors, simple doping methods to monolithically assemble n- and p-type channels on a single two-dimensional semiconductor are lacking, which makes the fabrication of integrated circuitry challenging. Here we report the reversible photo-induced doping of few-layer molybdenum ditelluride and tungsten diselenide, where the channel polarity can be reconfigured from n-type to p-type, and vice versa, with laser light at different frequencies. This reconfigurable doping is attributed to selective light–lattice interactions, such as the formation of tellurium self-interstitial defects under ultraviolet illumination and the incorporation of substitutional oxygen in tellurium and molybdenum vacancies under visible illumination. Using this approach, we create a complementary metal–oxide–semiconductor (CMOS) device on a single channel, where the circuit functions can be dynamically reset from a CMOS inverter to a CMOS switch using pulses of different light frequencies. Few-layer molybdenum ditelluride and tungsten diselenide field-effect transistors can be reversibly doped with different carrier types and concentrations using pulses of ultraviolet and visible light, allowing reconfigurable complementary metal–oxide–semiconductor circuits to be created.

Summary Poor solubility of π-conjugated two-dimensional polymers (C2Ps) has been a significant roadblock in incorporating these emergent materials into electronic devices by simple solution processing. Here, we report a rational design of triphenylene-based building blocks that condense via a series of reversible reactions for self-error-correction and a final irreversible fixation to form a pyrazine motif, which leads to the formation of crystalline C2Ps in solution without any aid of template or preorganization. Unlike typical covalent organic frameworks, remarkably, these materials remain dispersed as single- to few-layer films in solution, presumably because of protonation of pyrazine motifs by the strong acid employed in the synthesis. The C2Ps can be easily drop-cast onto solid surfaces for characterization and fabrication of electronic devices. The thin polymer films display a long-range internal order and exhibit high hole mobility up to 4 cm2V−1s−1, the highest among C2Ps, which paves the way to design high-performance optoelectronic devices.

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). We found that hot electrons, optically excited in Au nanorod (NR) arrays at sub-Eg radiations, can be injected into 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 using spectrally and light-polarization resolved measurements, we showed that these sub-Eg excitations of hot electrons can be modulated by tuning the plasmon resonance to the shape-controlled AuNRs.

Programmed alloying within a van der Waals (vdW) semiconductor monolayer (ML) is achieved by metalorganic vapor-phase epitaxy. Specifically, two versions of on-ML alloying are demonstrated by preci...

We propose evidence that the multilayer Td-WTe2 is a higher-order topological insulator. By measuring the Kerr effect using the graphene/WTe2 heterostructure, we revealed the existence of spinful topological hinge states protected by the time-reversal symmetry.

Resolving the complex interplay between surface and bulk response is a long-standing issue in the topological insulators (TIs). Some studies have reported surface-dominated metallic responses, yet ...

Coherent light–matter interaction can transiently modulate the quantum states of matter under nonresonant laser excitation. This phenomenon, called the optical Stark effect, is one of the promising...

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.

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.

A broad range of transition metal dichalcogenide (TMDC) semiconductors are available as monolayer (ML) crystals, so the precise integration of each kind into van der Waals (vdW) superlattices (SLs) could enable the realization of novel structures with previously unexplored functionalities. Here we report the atomic layer-by-layer epitaxial growth of vdW SLs with programmable stacking periodicities, composed of more than two kinds of dissimilar TMDC MLs, such as MoS2, WS2 and WSe2. Using kinetics-controlled vdW epitaxy in the near-equilibrium limit by metal-organic chemical vapour depositions, we achieved precise ML-by-ML stacking, free of interlayer atomic mixing, which resulted in tunable two-dimensional vdW electronic systems. As an example, by exploiting the series of type II band alignments at coherent two-dimensional vdW heterointerfaces, we demonstrated valley-polarized carrier excitations-one of the most distinctive electronic features in vdW ML semiconductors-which scale with the stack numbers n in our (MoS2/WS2)n SLs on optical excitations.

Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with the linear-dichroic light-matter interaction and the widely tunable direct band gap in the infrared frequency range. Despite recent theoretical predictions of strongly bound excitons with unique properties, it remains experimentally challenging to probe excitonic quasiparticles due to the severe oxidation that occurs during device fabrication. In this study, we report the observation of strongly bound excitons and trions with highly anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN) in a field-effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic optical spectra from anisotropic excitons and trions in the hBN-encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) \ensuremath{\sim}30 meV below the optical band gap of the charge-neutral system. Our work shows exciting possibilities for monolayer and few-layer phosphorene as a platform to explore many-body physics and photonics and optoelectronics based on strongly bound excitons with twofold anisotropy.

The valley Hall effect (VHE) in two-dimensional (2D) van der Waals (vdW) crystals is a promising approach to study the valley pseudospin. Most experiments so far have used bound electron-hole pairs (excitons) through local photoexcitation. However, the valley depolarization of such excitons is fast, so that several challenges remain to be resolved. We address this issue by exploiting a unipolar VHE using a heterobilayer made of monolayer MoS2/WTe2 to exhibit a long valley-polarized lifetime due to the absence of electron-hole exchange interaction. The unipolar VHE is manifested by reduced photoluminescence at the MoS2 A exciton energy. Furthermore, we provide quantitative information on the time-dependent valley Hall dynamics by performing the spatially-resolved ultrafast Kerr-rotation microscopy; we find that the valley-polarized electrons persist for more than 4 nanoseconds and the valley Hall mobility exceeds 4.49 × 103 cm2/Vs, which is orders of magnitude larger than previous reports., The valley Hall effect in 2D materials is a promising approach for future valleytronic applications, but it is usually based on excitons with short lifetimes. Here, spin polarized electrons are injected from WTe2 into MoS2, leading to a unipolar valley Hall effect with enhanced lifetimes and mobility.

Strong interactions between excitons are a characteristic feature of two-dimensional (2D) semiconductors, determining important excitonic properties, such as exciton lifetime, coherence, and photon-emission efficiency. Rhenium disulfide ($\mathrm{Re}{\mathrm{S}}_{2}$), a member of the 2D transition-metal dichalcogenide (TMD) family, has recently attracted great attention due to its unique excitons that exhibit excellent polarization selectivity and coherence features. However, an in-depth understanding of exciton-exciton interactions in $\mathrm{Re}{\mathrm{S}}_{2}$ is still lacking. Here we used ultrafast pump-probe spectroscopy to study exciton-exciton interactions in monolayer (1L), bilayer (2L), and triple layer $\mathrm{Re}{\mathrm{S}}_{2}$. We directly measure the rate of exciton-exciton annihilation, a representative Auger-type interaction between excitons. It decreases with increasing layer number, as observed in other 2D TMDs. However, while other TMDs exhibit a sharp weakening of exciton-exciton annihilation between 1L and 2L, such behavior was not observed in $\mathrm{Re}{\mathrm{S}}_{2}$. We attribute this distinct feature in $\mathrm{Re}{\mathrm{S}}_{2}$ to the relatively weak interlayer coupling, which prohibits a substantial change in the electronic structure when the thickness varies. This work not only highlights the unique excitonic properties of $\mathrm{Re}{\mathrm{S}}_{2}$ but also provides novel insight into the thickness dependence of exciton-exciton interactions in 2D systems.

The development of complex electronics based on two-dimensional (2D) materials will require the integration of a large number of 2D devices into circuits. However, a practical method of assembling such devices into integrated circuits remains elusive. Here we show that a scanning visible light probe can be used to directly write electrical circuitry onto the 2D semiconductor molybdenum ditelluride (2H-MoTe2). Laser light illumination over metal patterns deposited onto 2D channels of 2H-MoTe2 can convert the channels from an n-type semiconductor to a p-type semiconductor, by creating adatom–vacancy clusters in the host lattice. With this process, diffusive doping profiles can be controlled at the submicrometre scale and doping concentrations can be tuned, allowing the channel sheet resistance to be varied over four orders of magnitudes. Our doping method can be used to assemble both n- and p-doped channels within the same atomic plane, which allows us to fabricate 2D device arrays of n–p–n (p–n–p) bipolar junction transistor amplifiers and radial p–n photovoltaic cells. A laser can be used to locally dope two-dimensional molybdenum ditelluride channels, allowing both n- and p-doped channels to be assembled within the same atomic plane and for device arrays of n–p–n bipolar junction transistor amplifiers and radial p–n photovoltaic cells to be fabricated.

Quantum optoelectronic devices capable of isolating a target degree of freedom (DoF) from other DoFs have allowed for new applications in modern information technology. Many works on solid-state spintronics have focused on methods to disentangle the spin DoF from the charge DoF1, yet many related issues remain unresolved. Although the recent advent of atomically thin transition metal dichalcogenides (TMDs) has enabled the use of valley pseudospin as an alternative DoF2,3, it is nontrivial to separate the spin DoF from the valley DoF since the time-reversal valley DoF is intrinsically locked with the spin DoF4. Here, we demonstrate lateral TMD–graphene–topological insulator hetero-devices with the possibility of such a DoF-selective measurement. We generate the valley-locked spin DoF via a circular photogalvanic effect in an electric-double-layer WSe2 transistor. The valley-locked spin photocarriers then diffuse in a submicrometre-long graphene layer, and the spin DoF is measured separately in the topological insulator via non-local electrical detection using the characteristic spin–momentum locking. Operating at room temperature, our integrated devices exhibit a non-local spin polarization degree of higher than 0.5, providing the potential for coupled opto-spin–valleytronic applications that independently exploit the valley and spin DoFs. Lateral TMD–graphene–topological insulator hetero-devices enable room-temperature optoelectronic transport of the valley-locked spin polarization degree of freedom.

Understanding the mutual interaction between electronic excitations and lattice vibrations is key for understanding electronic transport and optoelectronic phenomena. Dynamic manipulation of such interaction is elusive because it requires varying the material composition on the atomic level. In turn, recent studies on topological insulators (TIs) have revealed the coexistence of a strong phonon resonance and topologically protected Dirac plasmon, both in the terahertz (THz) frequency range. Here, using these intrinsic characteristics of TIs, we demonstrate a new methodology for controlling electron–phonon interaction by lithographically engineered Dirac surface plasmons in the Bi2Se3 TI. Through a series of time-domain and time-resolved ultrafast THz measurements, we show that, when the Dirac plasmon energy is less than the TI phonon energy, the electron–phonon coupling is trivial, exhibiting phonon broadening associated with Landau damping. In contrast, when the Dirac plasmon energy exceeds that of the p...

Crystal polymorphism selectively stabilizes the electronic phase of atomically thin transition-metal dichalcogenides (TMDCs) as metallic or semiconducting, suggesting the potential to integrate these polymorphs as circuit components in two-dimensional electronic circuitry. Developing a selective and sequential growth strategy for such two-dimensional polymorphs in the vapour phase is a critical step in this endeavour. Here, we report on the polymorphic integration of distinct metallic (1T′) and semiconducting (2H) MoTe2 crystals within the same atomic planes by heteroepitaxy. The realized polymorphic coplanar contact is atomically coherent, and its barrier potential is spatially tight-confined over a length of only a few nanometres, with a lowest contact barrier height of ∼25 meV. We also demonstrate the generality of our synthetic integration approach for other TMDC polymorph films with large areas. Sequential heteroepitaxy of transition-metal dichalcogenide polymorphs with different electronic properties is used to build coplanar ultrathin circuitry with atomic-scale precision.

A theoretically designed van der Waals ferromagnet Fe4GeTe2 is synthesized and shows the nearly room temperature ferromagnetism., In spintronics, two-dimensional van der Waals crystals constitute a most promising material class for long-distance spin transport or effective spin manipulation at room temperature. To realize all-vdW-material–based spintronic devices, however, vdW materials with itinerant ferromagnetism at room temperature are needed for spin current generation and thereby serve as an effective spin source. We report theoretical design and experimental realization of a iron-based vdW material, Fe4GeTe2, showing a nearly room temperature ferromagnetic order, together with a large magnetization and high conductivity. These properties are well retained even in cleaved crystals down to seven layers, with notable improvement in perpendicular magnetic anisotropy. Our findings highlight Fe4GeTe2 and its nanometer-thick crystals as a promising candidate for spin source operation at nearly room temperature and hold promise to further increase Tc in vdW ferromagnets by theory-guided material discovery.

Many two-dimensional (2D) semiconductors represented by transition metal dichalcogenides have tunable optical bandgaps in the visible or near IR-range standing as a promising candidate for optoelectronic devices. Despite this potential, however, their photoreactions are not well understood or controversial in the mechanistic details. In this work, we report a unique thickness-dependent photoreaction sensitivity and a switchover between two competing reaction mechanisms in atomically thin chromium thiophosphate (CrPS4), a two-dimensional antiferromagnetic semiconductor. CrPS4 showed a threshold power density 2 orders of magnitude smaller than that for MoS2 obeying a photothermal reaction route. In addition, reaction cross section quantified with Raman spectroscopy revealed distinctive power dependences in the low and high power regimes. On the basis of optical in situ thermometric measurements and control experiments against O2, water, and photon energy, we proposed a photochemical oxidation mechanism involving singlet O2 in the low power regime with a photothermal route for the other. We also demonstrated a highly effective encapsulation with Al2O3 as a protection against the destructive photoinduced and ambient oxidations., Comment: 24 pages, 5 figures

Two-dimensional (2-D) hexagonal boron nitride (h-BN) has attracted considerable attention for deep ultraviolet optoelectronics and visible single photon sources, however, realization of an electrically-driven light emitter remains challenging due to its wide bandgap nature. Here, we report electrically-driven visible light emission with a red-shift under increasing electric field from a few layer h-BN by employing a five-period Al

Coherent light-matter interaction can transiently modulate the quantum states of matter under nonresonant laser excitation. This phenomenon, called the optical Stark effect, is one of the promising candidates for realizing ultrafast optical switches. However, the ultrafast modulations induced by the coherent light-matter interactions usually involve unwanted incoherent responses, significantly reducing the overall operation speed. Here, by using ultrafast pump-probe spectroscopy, we suppress the incoherent response and modulate the coherent-to-incoherent ratio in the two-dimensional semiconductor ReS

Atomically thin van der Waals semiconductor films were synthesized in a three-dimensional membrane form with monolayer precision., We report wafer-scale growth of atomically thin, three-dimensional (3D) van der Waals (vdW) semiconductor membranes. By controlling the growth kinetics in the near-equilibrium limit during metal-organic chemical vapor depositions of MoS2 and WS2 monolayer (ML) crystals, we have achieved conformal ML coverage on diverse 3D texture substrates, such as periodic arrays of nanoscale needles and trenches on quartz and SiO2/Si substrates. The ML semiconductor properties, such as channel resistivity and photoluminescence, are verified to be seamlessly uniform over the 3D textures and are scalable to wafer scale. In addition, we demonstrated that these 3D films can be easily delaminated from the growth substrates to form suspended 3D semiconductor membranes. Our work suggests that vdW ML semiconductor films can be useful platforms for patchable membrane electronics with atomic precision, yet large areas, on arbitrary substrates.

Many two-dimensional (2D) semiconductors represented by transition metal dichalcogenides have tunable optical bandgaps in the visible or near IR-range standing as a promising candidate for optoelectronic devices. Despite this potential, however, their photoreactions are not well understood or controversial in the mechanistic details. In this work, we report a unique thickness-dependent photoreaction sensitivity and a switchover between two competing reaction mechanisms in atomically thin chromium thiophosphate (CrPS

Strong interlayer photoresponses in monolayer (ML) semiconductor stacks, such as substantial light absorption and charge separation across interlayer band alignments, suggest potentials for two-dimensional photovoltaics (PVs). Here, we report an interlayer PV conversion in a type-I ML heterojunction by ultrafast interlayer transfer of photoexcited hot carriers in the broad spectral ranges. Specifically, low-energy photoexcitation on a stack of a narrow-band-gap (Eg) Bi2Te3 few-layer and a large-Eg MoS2 ML permits interlayer transfer of transient hot carriers from the Bi2Te3 layer to the excitonic states of the neighboring MoS2 ML within a time scale of ∼70 fs, producing interlayer charge separation. Thereby we achieve substantial conversion efficiency from a MoS2 ML with visible to infrared light illumination.

Broadband detection of mid-infrared (IR) photons extends to advanced optoelectronic applications such as imaging, sensing, and telecommunications. While graphene offers an attractive platform for broadband visible/IR photodetection, previous efforts to improve its responsivity, for example, by integrating light-absorbing colloids or waveguide or antenna fabrication, were achieved at the cost of reduced photon detection bandwidth. In this work, we demonstrate room-temperature operation of a novel mid-IR photodetector based on a graphene–Bi2Se3 heterostructure showing broadband detection and high responsivity (1.97 and 8.18 A/W at mid- and near-IR, respectively), in which simultaneous improvement of the spectral range and responsivity is achieved via exploiting broadband absorption of mid-IR and IR photons in a small-band-gap Bi2Se3 topological insulator and efficient hot carrier separation and strong photogating across the Bi2Se3/graphene interface. With sufficient room for further improvement by interface...

Plasmonics is a technology aiming at light modulation via collective charge oscillations. Topological insulators, where Dirac-like metallic surfaces coexist with normal insulating bulk, have recently attracted great attention in plasmonics due to their topology-originated outstanding properties. Here, we introduce a new methodology for controlling the interaction of a plasmon with a phonon in topological insulators, which is a key for utilizing the unique spectral profiles for photonic applications. By using both static and ultrafast terahertz spectroscopy, we show that the interaction can be tuned by controlling the chemical composition of (Bi1–xInx)2Se3 microribbon arrays. The topological quantum-phase transition induced by varying the composition drives a dramatic change in the strength of the plasmon–phonon interaction. This was possible due to the availability of manipulating the spatial overlap between topological surface plasmonic states and underlying bulk phonons. Especially, we control the laser...

We have achieved heteroepitaxial stacking of a van der Waals ( vdW) monolayer metal, 1T'-WTe2, and a semiconductor, 2H-WSe2, in which a distinctively low contact barrier was established across a clean epitaxial vdW gap. Our epitaxial 1T'-WTe2 films were identified as a semimetal by low temperature transport and showed the robust breakdown current density of 5.0 × 107 A/cm2. In comparison with a series of planar metal contacts, our epitaxial vdW contact was identified to possess intrinsic Schottky barrier heights below 100 meV for both electron and hole injections, contributing to superior ambipolar field-effect transistor (FET) characteristics, i.e., higher FET mobilities and higher on-off current ratios at smaller threshold gate voltages. We discuss our observations around the critical roles of the epitaxial vdW heterointerfaces, such as incommensurate stacking sequences and absence of extrinsic interfacial defects that are inaccessible by other contact methods.

Isolated monolayer (ML) crystals of hexagonal transition-metal dichalcogenides ( h -TMDCs) can be manually stacked to form atomic 2D heterostructures, showing the inter-ML coupled electronic and photonic responses. Nevertheless, from the materials science’ point of views, there remain two fundamental issues to be resolved. First, controlled large-area growth of the ML heterostructures is not available and second the precise control over the crystallographic rotations of each h -ML in such 2D superstructures is lacking. Here, we show the heteroepitaxial 2D stacking and stitching of MoS 2 and WS 2 MLs, by manipulation of 2D nucleation kinetics during the sequential vaporphase growth. It enables to create the hexagon-on-hexagon unit cell stacking and the hexagon-by-hexagon stitching without interlayer rotation misfi ts. The scalable 2D hetero-integrations of ML crystals into such coherent stacking and stitching units may provide an innovative design rule of the 2D h -TMDC heterostructures as the novel electronic and optical semiconductor platforms in large-areas, yet precisely controlled at the ultimate size limit. h -TMDCs MLs are recognized as an emerging class of atomic 2D materials with the novel electronic, photonic, and phonon processes. [ 1‐8 ] Therein the central scientifi c issues quickly converge into the realization of ML heterostructures and the associated interlayer electronic and optical transitions. In this regard, the artifi cial 2D superlattices based on such h -TMDCs MLs in both vertical stacking and lateral stitching manners can serve as novel device platforms for the 2D band engineering. [ 9‐17 ]

van der Waals layered materials have large crystal anisotropy and crystallize spontaneously into two-dimensional (2D) morphologies. Two-dimensional materials with hexagonal lattices are emerging 2D confined electronic systems at the limit of one or three atom thickness. Often these 2D lattices also form orthorhombic symmetries, but these materials have not been extensively investigated, mainly due to thermodynamic instability during crystal growth. Here, we show controlled polymorphic growth of 2D tin-sulfide crystals of either hexagonal SnS2 or orthorhombic SnS. Addition of H2 during the growth reaction enables selective determination of either n-type SnS2 or p-type SnS 2D crystal of dissimilar energy band gap of 2.77 eV (SnS2) or 1.26 eV (SnS) as a final product. Based on this synthetic 2D polymorphism of p–n crystals, we also demonstrate p–n heterojunctions for rectifiers and photovoltaic cells, and complementary inverters.

Quantum optoelectronic devices capable of isolating a target degree of freedom (DoF) from other DoFs have allowed for new applications in modern information technology. Many works on solid-state spintronics have focused on methods to disentangle the spin DoF from the charge DoF

Understanding the mutual interaction between electronic excitations and lattice vibrations is key for understanding electronic transport and optoelectronic phenomena. Dynamic manipulation of such interaction is elusive because it requires varying the material composition on the atomic level. In turn, recent studies on topological insulators (TIs) have revealed the coexistence of a strong phonon resonance and topologically protected Dirac plasmon, both in the terahertz (THz) frequency range. Here, using these intrinsic characteristics of TIs, we demonstrate a new methodology for controlling electron-phonon interaction by lithographically engineered Dirac surface plasmons in the Bi

We present the photoinduced terahertz responses of topological insulators Bi2Se3. The photoconductance sign is determined by the competition between the topological surface state and the bulk response in n-type, p-type and bulk-insulating Bi 2 Se 3 .