Non-epitaxial single-crystal 2D material growth by geometric confinement.

Two-dimensional (2D) materials and their heterostructures show a promising path for next-generation electronics1-3. Nevertheless, 2D-based electronics have not been commercialized, owing mainly to three critical challenges: i) precise kinetic control of layer-by-layer 2D material growth, ii) maintaining a single domain during the growth, and iii) wafer-scale controllability of layer numbers and crystallinity. Here we introduce a deterministic, confined-growth technique that can tackle these three issues simultaneously, thus obtaining wafer-scale single-domain 2D monolayer arrays and their heterostructures on arbitrary substrates. We geometrically confine the growth of the first set of nuclei by defining a selective growth area via patterning SiO2 masks on two-inch substrates. Owing to substantial reduction of the growth duration at the micrometre-scale SiO2 trenches, we obtain wafer-scale single-domain monolayer WSe2 arrays on the arbitrary substrates by filling the trenches via short growth of the first set of nuclei, before the second set of nuclei is introduced, thus without requiring epitaxial seeding. Further growth of transition metal dichalcogenides with the same principle yields the formation of single-domain MoS2/WSe2 heterostructures. Our achievement will lay a strong foundation for 2D materials to fit into industrial settings.

Dephasing Dynamics Accessed by High Harmonic Generation: Determination of Electron-Hole Decoherence of Dirac Fermions.

We reveal the critical effect of ultrashort dephasing on the polarization of high harmonic generation in Dirac fermions. As the elliptically polarized laser pulse falls in or slightly beyond the multiphoton regime, the elliptically polarized high harmonic generation is produced and exhibits a characteristic polarimetry of the polarization ellipse, which is found to depend on the decoherence time T2. T2 could then be determined to be a few femtoseconds directly from the experimentally observed polarimetry of high harmonics. This shows a sharp contrast with the semimetal regime of higher pump intensity, where the polarimetry is irrelevant to T2. An access to the dephasing dynamics would extend the prospect of high harmonic generation into the metrology of a femtosecond dynamic process in the coherent quantum control.

Thermodynamically Driven Tilt Grain Boundaries of Monolayer Crystals Using Catalytic Liquid Alloys.

We report a method to precisely control the atomic defects at grain boundaries (GBs) of monolayer MoS2 by vapor-liquid-solid (VLS) growth using sodium molybdate liquid alloys, which serve as growth catalysts to guide the formations of the thermodynamically most stable GB structure. The Mo-rich chemical environment of the alloys results in Mo-polar 5|7 defects with a yield exceeding 95%. The photoluminescence (PL) intensity of VLS-grown polycrystalline MoS2 films markedly exceeds that of the films, exhibiting abundant S 5|7 defects, which are kinetically driven by vapor-solid-solid growths. Density functional theory calculations indicate that the enhanced PL intensity is due to the suppression of nonradiative recombination of charged excitons with donor-type defects of adsorbed Na elements on S 5|7 defects. Catalytic liquid alloys can aid in determining a type of atomic defect even in various polycrystalline 2D films, which accordingly provides a technical clue to engineer their properties.

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.

Crossover between Photochemical and Photothermal Oxidations of Atomically Thin Magnetic Semiconductor CrPS4.

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.

Light Polarization-Controlled Conversion of Ultrafast Coherent-Incoherent Exciton Dynamics in Few-Layer ReS2.

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 ReS2. We selectively convert the coherent and incoherent responses of an anisotropic exciton state by solely using photon polarizations, improving the control ratio by 3 orders of magnitude. The efficient modulation was enabled by transient superpositions of differential spectra from two nondegenerate exciton states due to the light polarization dependencies. This work provides a valuable contribution toward realizing ideal ultrafast optical switches.

Epitaxial van der Waals Contacts between Transition-Metal Dichalcogenide Monolayer Polymorphs.

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.

Electroluminescence from h-BN by using Al2O3/h-BN multiple heterostructure.

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 Al2O3/h-BN multiple heterostructure and a graphene top electrode. Investigation of electrical properties reveals that the Al2O3 layers act as potential barriers confining injected carriers within the h-BN wells, while suppressing the electrostatic breakdown by trap-assisted tunneling, to increase the probability of radiative recombination. The result highlights a promising potential of such multiple heterostructure as a practical and efficient platform for electrically-driven light emitters based on wide bandgap two-dimensional materials.

Control over Electron-Phonon Interaction by Dirac Plasmon Engineering in the Bi2Se3 Topological Insulator.

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 phonon resonance, we observe suppressed electron-phonon interaction leading to unexpected phonon stiffening. Time-dependent analysis of the Dirac plasmon behavior, phonon broadening, and phonon stiffening reveals a transition between the distinct dynamics corresponding to the two regimes as the Dirac plasmon resonance moves across the TI phonon resonance, which demonstrates the capability of Dirac plasmon control. Our results suggest that the engineering of Dirac plasmons provides a new alternative for controlling the dynamic interaction between Dirac carriers and phonons.

Deterministic Two-Dimensional Polymorphism Growth of Hexagonal n-Type SnS2 and Orthorhombic p-Type SnS Crystals.

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.

Atomic layer-by-layer thermoelectric conversion in topological insulator bismuth/antimony tellurides.

Material design for direct heat-to-electricity conversion with substantial efficiency essentially requires cooperative control of electrical and thermal transport. Bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3), displaying the highest thermoelectric power at room temperature, are also known as topological insulators (TIs) whose electronic structures are modified by electronic confinements and strong spin-orbit interaction in a-few-monolayers thickness regime, thus possibly providing another degree of freedom for electron and phonon transport at surfaces. Here, we explore novel thermoelectric conversion in the atomic monolayer steps of a-few-layer topological insulating Bi2Te3 (n-type) and Sb2Te3 (p-type). Specifically, by scanning photoinduced thermoelectric current imaging at the monolayer steps, we show that efficient thermoelectric conversion is accomplished by optothermal motion of hot electrons (Bi2Te3) and holes (Sb2Te3) through 2D subbands and topologically protected surface states in a geometrically deterministic manner. Our discovery suggests that the thermoelectric conversion can be interiorly achieved at the atomic steps of a homogeneous medium by direct exploiting of quantum nature of TIs, thus providing a new design rule for the compact thermoelectric circuitry at the ultimate size limit.

Efficient room-temperature near-infrared detection with solution-processed networked single wall carbon nanotube field effect transistors.

Efficient room temperature NIR detection with sufficient current gain is made with a solution-processed networked SWNT FET. The high performance NIR-FET with significantly enhanced photocurrent by more than two orders of magnitude compared to dark current in the depleted state is attributed to multiple Schottky barriers in the network, each of which absorb NIR and effectively separate photocarriers to corresponding electrodes.

Growth of straight one-dimensional Ge/ZnSe heterojunctions with atomically sharp interfaces by catalytic residue controls.

One-dimensional (1D) heteroepitaxy with an abrupt interface is essential to construct the 1D heterojunctions required for photonic and electronic devices. During catalytic 1D heteroepitaxial growth, however, the heterojunctions are generically kinked and composition-diffused across the interfaces. Here, we report a simple synthetic route for straight 1D heteroepitaxy with atomically sharp interfaces of group IV(Ge)/group II-VI(ZnSe) nanowires (NWs) during Au-catalytic growth. Specifically, it is discovered that eliminating residues in Au catalysts by Se vapour treatments lowers the energy barrier for the Ge NW axial heteroepitaxy on ZnSe NWs, and forms atomically abrupt heterointerfaces. We verified such 1D variation in the local electronic band structure of the grown Ge/ZnSe NW heterojunctions with spatially resolved photocurrent measurements.

Quantum confinement effects in transferrable silicon nanomembranes and their applications on unusual substrates.

Two dimensional (2D) semiconductors have attracted attention for a range of electronic applications, such as transparent, flexible field effect transistors and sensors owing to their good optical transparency and mechanical flexibility. Efforts to exploit 2D semiconductors in electronics are hampered, however, by the lack of efficient methods for their synthesis at levels of quality, uniformity, and reliability needed for practical applications. Here, as an alternative 2D semiconductor, we study single crystal Si nanomembranes (NMs), formed in large area sheets with precisely defined thicknesses ranging from 1.4 to 10 nm. These Si NMs exhibit electronic properties of two-dimensional quantum wells and offer exceptionally high optical transparency and low flexural rigidity. Deterministic assembly techniques allow integration of these materials into unusual device architectures, including field effect transistors with total thicknesses of less than 12 nm, for potential use in transparent, flexible, and stretchable forms of electronics.

Correlation-driven nonequilibrium exciton site transition in a WSe2/WS2 moiré supercell.

Moiré superlattices of transition metal dichalcogenides offer a unique platform to explore correlated exciton physics with optical spectroscopy. Whereas the spatially modulated potentials evoke that the exciton resonances are distinct depending on a site in a moiré supercell, there have been no clear demonstration how the moiré excitons trapped in different sites dynamically interact with the doped carriers; so far the exciton-electron dynamic interactions were presumed to be site-dependent. Thus, the transient emergence of nonequilibrium correlations are open questions, but existing studies are limited to steady-state optical measurements. Here we report experimental fingerprints of site-dependent exciton correlations under continuous-wave as well as ultrashort optical excitations. In near-zero angle-aligned WSe2/WS2 heterobilayers, we observe intriguing polarization switching and strongly enhanced Pauli blocking near the Mott insulating state, dictating the dominant correlation-driven effects. When the twist angle is near 60°, no such correlations are observed, suggesting the strong dependence of atomic registry in moiré supercell configuration. Our studies open the door to largely unexplored nonequilibrium correlations of excitons in moiré superlattices.

Full automation of point defect detection in transition metal dichalcogenides through a dual mode deep learning algorithm.

Point defects often appear in two-dimensional (2D) materials and are mostly correlated with physical phenomena. The direct visualisation of point defects, followed by statistical inspection, is the most promising way to harness structure-modulated 2D materials. Here, we introduce a deep learning-based platform to identify the point defects in 2H-MoTe2: synergy of unit cell detection and defect classification. These processes demonstrate that segmenting the detected hexagonal cell into two unit cells elaborately cropped the unit cells: further separating a unit cell input into the Te2/Mo column part remarkably increased the defect classification accuracies. The concentrations of identified point defects were 7.16 × 1020 cm2 of Te monovacancies, 4.38 × 1019 cm2 of Te divacancies and 1.46 × 1019 cm2 of Mo monovacancies generated during an exfoliation process for TEM sample-preparation. These revealed defects correspond to the n-type character mainly originating from Te monovacancies, statistically. Our deep learning-oriented platform combined with atomic structural imaging provides the most intuitive and precise way to analyse point defects and, consequently, insight into the defect-property correlation based on deep learning in 2D materials.

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.

Integrated 1D epitaxial mirror twin boundaries for ultrascaled 2D MoS2 field-effect transistors.

In atomically thin van der Waals materials, grain boundaries-the line defects between adjacent crystal grains with tilted in-plane rotations-are omnipresent. When the tilting angles are arbitrary, the grain boundaries form inhomogeneous sublattices, giving rise to local electronic states that are not controlled. Here we report on epitaxial realizations of deterministic MoS2 mirror twin boundaries (MTBs) at which two adjoining crystals are reflection mirroring by an exactly 60° rotation by position-controlled epitaxy. We showed that these epitaxial MTBs are one-dimensionally metallic to a circuit length scale. By utilizing the ultimate one-dimensional (1D) feature (width ~0.4 nm and length up to a few tens of micrometres), we incorporated the epitaxial MTBs as a 1D gate to build integrated two-dimensional field-effect transistors (FETs). The critical role of the 1D MTB gate was verified to scale the depletion channel length down to 3.9 nm, resulting in a substantially lowered channel off-current at lower gate voltages. With that, in both individual and array FETs, we demonstrated state-of-the-art performances for low-power logics. The 1D epitaxial MTB gates in this work suggest a novel synthetic pathway for the integration of two-dimensional FETs-that are immune to high gate capacitance-towards ultimate scaling.

Large electroabsorption susceptibility mediated by internal photoconductive gain in Ge nanowires.

Large spectral modulation in the photon-to-electron conversion near the absorption band-edge of a semiconductor by an applied electrical field can be a basis for efficient electro-optical modulators. This electro-absorption effect in Group IV semiconductors is, however, inherently weak, and this poses the technological challenges for their electro-photonic integration. Here we report unprecedentedly large electro-absorption susceptibility at the direct band-edge of intrinsic Ge nanowire (NW) photodetectors, which is strongly diameter-dependent. We provide evidence that the large spectral shift at the 1.55 µm wavelength, enhanced up to 20 times larger than Ge bulk crystals, is attributed to the internal Franz-Keldysh effect across the NW surface field of ~10(5) V/cm, mediated by the strong photoconductive gain. This classical size-effect operating at the nanometer scale is universal, regardless of the choice of materials, and thus suggests general implications for the monolithic integration of Group IV photonic circuits.

Tunable catalytic alloying eliminates stacking faults in compound semiconductor nanowires.

Planar defects in compound (III-V and II-VI) semiconductor nanowires (NWs), such as twin and stacking faults, are universally formed during the catalytic NW growth, and they detrimentally act as static disorders against coherent electron transport and light emissions. Here we report a simple synthetic route for planar-defect free II-VI NWs by tunable alloying, i.e. Cd(1-x)Zn(x)Te NWs (0 ≤ x ≤ 1). It is discovered that the eutectic alloying of Cd and Zn in Au catalysts immediately alleviates interfacial instability during the catalytic growth by the surface energy minimization and forms homogeneous zinc blende crystals as opposed to unwanted zinc blende/wurtzite mixtures. As a direct consequence of the tunable alloying, we demonstrated that intrinsic energy band gap modulation in Cd(1-x)Zn(x)Te NWs can exploit the tunable spectral and temporal responses in light detection and emission in the full visible range.