Jiuxiang Dai

Nature Communications, Published online: 07 September 2023; doi:10.1038/s41467-023-41313-7

Donor defect states in MoS₂ are efficiently healed by depositing an Al₂O₃ protection barrier for oxygen plasma. The proposed method improves the subthreshold swing, mobility, and on-current along with the p-type doping effect due to the healing of sulfur vacancies. Owing to significant channel quality improvement, metal–insulator transition is observed at a carrier concentration of 5.7 × 10¹² cm⁻².

Abstract

Molybdenum disulfide (MoS₂), a metal dichalcogenide, is a promising channel material for highly integrated scalable transistors. However, intrinsic donor defect states, such as sulfur vacancies (V_s), can degrade the channel properties and lead to undesired n-doping. A method for healing the donor defect states in monolayer MoS₂ is proposed using oxygen plasma, with an aluminum oxide (Al₂O₃) barrier layer that protects the MoS₂ channel from damage by plasma treatment. Successful healing of donor defect states in MoS₂ by oxygen atoms, even in the presence of an Al₂O₃ barrier layer, is confirmed by X-ray photoelectron spectroscopy, photoluminescence, and Raman spectroscopy. Despite the decrease in 2D sheet carrier concentration (Δn_2D = −3.82×10¹² cm⁻²), the proposed approach increases the on-current and mobility by 18% and 44% under optimal conditions, respectively. Metal–insulator transition occurs at electron concentrations of 5.7×10¹² cm⁻² and reflects improved channel quality. Finally, the activation energy (E _a) reduces at all the gate voltages (V _G) owing to a decrease in V_s, which act as a localized state after the oxygen plasma treatment. This study demonstrates the feasibility of plasma-assisted healing of defects in 2D materials and electrical property enhancement and paves the way for the development of next-generation electronic devices.

The mechanism of high-mobility of In₂O₃:H films is deeply explored. Hydrogen doping can decrease the concentration of V_O and reduce the local distortion of the film, resulting in a decrease in carrier scattering probability. The decline in mobility observed is primarily attributed to hydrogen diffusion and crystal corrosion. This work contributes to a deeper understanding of high-mobility In₂O₃:H films.

Abstract

Wide bandgap semiconductors, particularly In₂O₃:Sn (ITO), are widely used as transparent conductive electrodes in optoelectronic devices. Nevertheless, due to the strohave beenng scattering probability of high-concentration oxygen vacancy (V_O) defects, the mobility of ITO is always lower than 40 cm² V⁻¹ s⁻¹. Recently, hydrogen-doped In₂O₃ (In₂O₃:H) films have been proven to have high mobility (>100 cm² V⁻¹ s⁻¹), but the origin of this high mobility is still unclear. Herein, a high-resolution electron microscope and theoretical calculations are employed to investigate the atomic-scale mechanisms behind the high carrier mobility in In₂O₃:H films. It is found that V_O can cause strong lattice distortion and large carrier scattering probability, resulting in low carrier mobility. Furthermore, hydrogen doping can simultaneously reduce the concentration of V_O, which accounts for high carrier mobility. The thermal stability and acid–base corrosion mechanism of the In₂O₃:H film are investigated and found that hydrogen overflows from the film at high temperatures (>250 °C), while acidic or alkaline environments can cause damage to the In₂O₃ grains themselves. Overall, this work provides insights into the essential reasons for high carrier mobility in In₂O₃:H and presents a new research approach to the doping and stability mechanisms of transparent conductive oxides.

Nature, Published online: 06 September 2023; doi:10.1038/s41586-023-06404-x

Covalent nanowires (NWs) are reshaped postsynthesis by exploiting an unexpected plasticity. Single-crystal CuO nanowires subjected to external mechanical loading can form double kinks, mediated by deformation-induced twinning along the (1¯10)$( {\bar{1}10} )$ plane. Moreover, this plastic deformation process is possibly reversible, as found both in experiment and simulation.

Abstract

Nanowires (NWs) are among the most studied nanostructures as they have numerous promising applications thanks to their various unique properties. Furthermore, the properties of NWs can be tailored during synthesis by introducing structural defects such as nano-twins, periodic polytypes, and kinks, i.e., abrupt changes in their axial direction. Here, this work reports for the first time the postsynthesis formation of such defects, achieved by exploiting a peculiar plasticity that may occur in nanosized covalent materials. Specifically, in this work the authors found that single-crystal CuO NWs can form double kinks when subjected to external mechanical loading. Both the microscopy and atomistic modeling suggest that deformation-induced twinning along the (1¯10)$( {\bar{1}10} )$ plane is the mechanism behind this effect. In a single case the authors are able to unkink a NW back to its initial straight profile, indicating the possibility of reversible plasticity in CuO NWs, which is supported by the atomistic simulations. The phenomenon reported here provides novel insights into the mechanisms of plastic deformation in covalent NWs and offers potential avenues for developing techniques to customize the shape of NWs postsynthesis and introduce new functionalities.

A blue AIEgen of DPDPB-AC was designed by fusing a hot exciton AIEgen of TPB-AC with a triplet-triplet annihilation (TTA) up-conversion luminogen of DMPPP. When it was used as non-doped EML of organic light-emitting diode, an outstanding maximum external quantum efficiency (EQE_max) of 10.3 % and an exceptional maximum brightness (L_max) of 69311 cd m⁻² were achieved.

Abstract

Aggregation-induced emission (AIE) luminogens (AIEgens) are attractive for the construction of non-doped blue organic light-emitting diodes (OLEDs) owning to their high emission efficiency in the film state. However, the large internal inversion rate (k _IC ^(Tn)) between high-lying triplet levels (T_n) and T_n-1 causes a huge loss of triplet excitons, resulting in dissatisfied device performance of these AIEgens-based non-doped OLEDs. Herein, we designed and synthesized a blue luminogen of DPDPB-AC by fusing an AIEgen of TPB-AC and a DMPPP, which feature hot exciton and triplet-triplet annihilation (TTA) up-conversion process, respectively. DPDPB-AC successfully inherits the AIE feature and excellent horizontal dipole orientation of TPB-AC. Furthermore, it owes smaller k _IC ^(Tn) than TPB-AC. When DPDPB-AC was applied in OLED as non-doped emitting layer, an outstanding external quantum efficiency of 10.3 % and an exceptional brightness of 69311 cd m⁻² were achieved. The transient electroluminescent measurements and steady-state dynamic analysis confirm that both TTA and hot exciton processes contribute to such excellent device performance. This work provides a new insight into the design of efficient organic fluorophores by managing high-lying triplet excitons.

Ferromagnetic amorphous gallium oxide (a-GMO) films are fabricated with a tunable two-level system of Mn dopants, i.e., Mn²⁺ and Mn³⁺ ions. The Pt/a-GMO/Pt memristors show a high R _off/R _on ratio of 10³, at least one order of magnitude higher than those of previously reported purely gallium oxide-based memristors, thanks to the abundant oxygen vacancies induced low resistance state and Mn²⁺-enhanced high resistance state.

Abstract

Purely gallium oxide-based memristors (GOMRs) show great potentials in resistive random-access-memory (RRAM) due to their chemical stability and resistive switching characteristics with R _off/R _on ratios up to 10²; indeed, GOMRs with higher R _off/R _on ratios and more functionalities are more expected. In this study, ferromagnetic amorphous gallium oxide (a-GMO) films with a tunable two-level system of Mn dopants, i.e., Mn²⁺ and Mn³⁺ ions, are prepared by scalable polymer assisted deposition. The Pt/a-GMO/Pt memristors show a high R _off/R _on ratio of 10³, at least one order of magnitude higher than those of previously reported purely GOMRs, thanks to the abundant oxygen vacancies (V_Os)-induced low resistance state and Mn²⁺-enhanced high resistance state. Meanwhile, magnetic modulation (MM) is realized electrically in the a-GOMRs during the RS, through the tuning of bound magnetopolarons (BMPs) by bias voltage-induced V_Os variations, which may be useful for quaternary information coding. Notably, the transition between Mn³⁺ and Mn²⁺ions is observed in the GOMRs, which is closely related to the variations of V_O concentration and BMP amount, providing an in situ tool to probe the V_O-induced RS and BMP-dependent MM. The results give insights to Mn-doped GOMRs and may be useful for design, fabrication, and testing of multifunctional high-performance RRAMs.

Devices based on 2DM van der Waals heterostructures always compose of multiple contacts and are instable during operation. Indium selenide (InSe) and black phosphorus (BP) on Au electrodes exhibit variation of I–V characteristics due to the evolution and rearrangement of junctions. This research emphasizes the importance of dealing with heterogeneous contacts and junction directions in designing 2DM heterostructure photodetectors.

Abstract

Devices based on 2DMs van der Waals (vdW) heterostructures always compose of multiple contacts. Due to the instability of nanoscale 2DMs and interfaces, these contacts can be affected by the operation-induced photo or thermal effect. They can trigger the evolution of junctions and rearrange the junctions across a device, which are detrimental for applications. Herein, vdW heterostructure of indium selenide (InSe) and black phosphorus (BP) on Au electrodes are investigated to reveal the contact evolution and its relation to device performance. During operation, light irradiation changes the I–V characteristics from symmetry to strong rectification. Photocurrent mapping and Kelvin-probe force microscopy (KPFM) reveal triple junctions in this heterostructure, i.e., Au-InSe junction, InSe homojunction, and InSe-BP heterojunction. The variation of I–V characteristics of vdW heterostructure is ascribed to the evolution of Au-InSe junction from quasi-ohmic junction with a near-zero work function difference (Δφ) to a strong Schottky junction (Δφ = ≈0.27 eV). The stabilized device demonstrates distinguished time-domain response at individual junctions and overall device, indicating the evolution of contacts and the consequent opposite junction directions degrade the overall device performance. This research emphasizes the importance of dealing with heterogeneous contacts and junction directions in designing vdW heterostructure photodetectors.

Optoelectronic synapses based on 3D graphene/MoS₂ heterostructure field-effect transistors are developed. The device demonstrates remarkable light response with photoresponsivity up to 10⁵ A W⁻¹ at 590 nm. Furthermore, multiple synaptic neuromorphic functions are successfully emulated in ultraviolet, visible, near-infrared, and different polarized light. A new concept is provided here for designing high-performance optoelectronic synapses with polarization sensitivity.

Abstract

Optoelectronic synapses with information sensing, processing, and memory function are promoting the development of artificial visual perception systems. However, optoelectronic synapses' relatively inferior optoelectronic performance impedes their application in complex neuromorphic computing. Herein, optoelectronic synapses based on 3D graphene/molybdenum disulfide (MoS₂) heterostructure field-effect transistors are developed by using the double stress layer self-rolled-up method. The graphene, with excellent electrical properties, enhances the carrier transport capacity of the device. The unique continuous photoconductivity of MoS₂ is suitable for simulating various synaptic nerve morphological functions. Meanwhile, the 3D resonant microcavity is added to enhance the optical field and make the device polarization sensitive, which reveals more intangible features of objects. The device demonstrates room-temperature photodetection at ultraviolet, visible, near-infrared, and mid-infrared regions, with photoresponsivity up to 10⁵ A W⁻¹ at 590 nm. Furthermore, multiple synaptic neuromorphic functions, such as inhibitory postsynaptic current, paired-pulse facilitation, short-term depression, and long-term depression, are successfully emulated. Here a new concept is provided for designing high-performance optoelectronic synapses with polarization sensitivity and excellent potential in artificial intelligence is shown.

Nature Protocols, Published online: 05 September 2023; doi:10.1038/s41596-023-00870-3

Nature Communications, Published online: 05 September 2023; doi:10.1038/s41467-023-41230-9

npj 2D Materials and Applications, Published online: 05 September 2023; doi:10.1038/s41699-023-00424-x

A chemical-scissor-mediated protocol enables metal intercalate into transition metal chalcogenides (TMDCs) via electron solvation in molten salt. Molten salts with solvated electrons can serve as a bridge to connect adducts to atomic layer with empty orbitals, transforming the surface characteristics of the 2D materials without altering their lattice structure and topology.

Abstract

Van der Waals (vdW)-layered materials have drawn tremendous interests due to their unique properties. Atom intercalation in the vdW gap of layered materials can tune their electronic structure and generate unexpected properties. Here a chemical-scissor-mediated method that enables metal intercalation into transition metal dichalcogenides (TMDCs) in molten salts is reported. By using this approach, various guest metal atoms (Mn, Fe, Co, Ni, Cu, and Ag) are intercalated into various TMDC hosts (such as TiS₂, NbS₂, TaS₂, TiSe₂, NbSe₂, TaSe₂, and Ti_0.5V_0.5S₂). The structure of the intercalated compound and intercalation mechanism are investigated. The results indicate that the vdW gap and valence state of TMDCs can be modified through metal intercalation, and the intercalation behavior is dictated by the electron work function. The adjustable charge transfer and intercalation endow a channel for rapid mass transfer to enhance the electrochemical performances. Such a chemical-scissor-mediated intercalation provides an approach to tune the physical and chemical properties of TMDCs, which may open an avenue in functional application ranging from energy conversion to electronics.

Atomic layer and valley controlled generation of THz photocurrents in PtSe2—spin-resolved band structure (red = spin up, blue = spin down) of ML (left) and multilayer (right) PtSe2 on a substrate under circular femtosecond excitation pulse. For the ML semiconductor case, indirect interband excitations around Γ point (orange) are possible, while by engineering the number of multilayers, direct interband transitions in the vicinity of KK′ under circular left (right) polarized light can be excited, leading to a new class of circular dichroism 2D materials beyond the ML limit.

Abstract

Platinum diselenide (PtSe2) is a promising two-dimensional (2D) material for the terahertz (THz) range as, unlike other transition metal dichalcogenides (TMDs), its bandgap can be uniquely tuned from a semiconductor in the near-infrared to a semimetal with the number of atomic layers. This gives the material unique THz photonic properties that can be layer-engineered. Here, we demonstrate that a controlled THz nonlinearity—tuned from monolayer to bulk PtSe2—can be realized in wafer size polycrystalline PtSe2 through the generation of ultrafast photocurrents and the engineering of the bandstructure valleys. This is combined with the PtSe2 layer interaction with the substrate for a broken material centrosymmetry, permitting a second order nonlinearity. Further, we show layer dependent circular dichroism, where the sign of the ultrafast currents and hence the phase of the emitted THz pulse can be controlled through the excitation of different bandstructure valleys. In particular, we show that a semimetal has a strong dichroism that is absent in the monolayer and few layer semiconducting limit. The microscopic origins of this TMD bandstructure engineering are highlighted through detailed DFT simulations, and shows the circular dichroism can be controlled when PtSe2 becomes a semimetal and when the K-valleys can be excited. As well as showing that PtSe2 is a promising material for THz generation through layer controlled optical nonlinearities, this work opens up a new class of circular dichroism materials beyond the monolayer limit that has been the case of traditional TMDs, and impacting a range of domains from THz valleytronics, THz spintronics to harmonic generation.

The Tauc plot is probably the most commonly used method to determine the size of bandgaps. This perspective shows that, despite the establishment of this method, the Tauc plot should be used with caution, since various peculiarities in the electronic structure can prevent its application.

Abstract

The Tauc plot is a method originally developed to derive the optical gap of amorphous semiconductors such as amorphous germanium or silicon. By measuring the absorption coefficient α(hν) and plotting (αhv)12$(\alpha {hv})^{\frac{1}{2}}$ versus photon energy hν, a value for the optical gap (Tauc gap) is determined. In this way non-direct optical transitions between approximately parabolic bands can be examined. In the last decades, a modification of this method for (poly-) crystalline semiconductors has become popular to study direct and indirect interband transitions. For this purpose, (ahν) ⁿ (n = 12$\frac{1}{2}$, 2) is plotted against hν to determine a value of the electronic bandgap. Due to the ease of performing UV–vis measurements, this method has nowadays become a standard to analyze various (poly-) crystalline solids, regardless of their different electronic structure. Although this leads partially to widely varying values of the respective bandgap of nominally identical materials, there is still no study that critically questions which peculiarities in the electronic structure prevent a use of the Tauc plot for (poly-) crystalline solids and to which material classes this applies. This study aims to close this gap by discussing the Tauc plot and its limiting factors for exemplary (poly-) crystalline solids with different electronic structures.

This study implements the first integrated photonic and acousto-optic devices in novel thin-film boron-doped gallium phosphide, indicating its potential as a highly scalable hybrid photonics platform for integration with color centers in high-index materials. Optical, acoustic, and materials characterization is performed, revealing both strong intrinsic properties and clear pathways toward improvement.

Abstract

The compact size, scalability, and strongly confined fields in integrated photonic devices enable new functionalities in photonic networking and information processing, both classical and quantum. Gallium phosphide (GaP) is a promising material for active integrated photonics due to its high refractive index, wide bandgap, strong nonlinear properties, and large acousto-optic figure of merit. This study demonstrates that silicon-lattice-matched boron-doped GaP (BGaP), grown at the 12-inch wafer scale, provides similar functionalities as GaP. BGaP optical resonators exhibit intrinsic quality factors exceeding 25,000 and 200,000 at visible and telecom wavelengths, respectively. It further demonstrates the electromechanical generation of low-loss acoustic waves and an integrated acousto-optic (AO) modulator. High-resolution spatial and compositional mapping, combined with ab initio calculations, indicate two candidates for the excess optical loss in the visible band: the silicon-GaP interface and boron dimers. These results demonstrate the promise of the BGaP material platform for the development of scalable AO technologies at telecom and provide potential pathways toward higher performance at shorter wavelengths.

Here, a new strategy for controllable growth of large-scale monolayer, 2H-stacking, and 3R-stacking bilayer transition metal dichalcogenides is developed, by modulating the resolidified chalcogen precursors supply kinetics. Noteworthy, the 3R-stacking bilayer tungsten disulfide (WS₂) exhibits better electrical performance compared to 2H-stacking bilayer WS₂, due to the difference of stacking-order-dependent surface potentials.

Abstract

Bilayer semiconductors have attracted much attention due to their stacking-order-dependent properties. However, as both 3R- and 2H-stacking are energetically stable at high temperatures, most of the high-temperature grown bilayer materials have random 3R- or 2H-stacking orders, leading to non-uniformity in optical and electrical properties. Here, a chemical vapor deposition method is developed to grow bilayer semiconductors with controlled stacking order by modulating the resolidified chalcogen precursors supply kinetics. Taking tungsten disulfide (WS₂) as an example, pure 3R-stacking (100%) and 2H-stacking dominated (87.6%) bilayer WS₂ are grown by using this method and both show high structural and optical quality and good uniformity. Importantly, the bilayer 3R-stacking WS₂ shows higher field effect mobility than 2H-stacking samples, due to the difference in stacking order-dependent surface potentials. This method is universal for growing other bilayer semiconductors with controlled stacking orders including molybdenum disulfide and tungsten diselenide, paving the way to exploit stacking-order-dependent properties of these family of emerging bilayer materials.

Domain wall formation and manipulation are known to greatly impact nanoscale ferroelectricity and device functionalities. Here, the authors experimentally demonstrate the ferroelectric thickness size effects on domain wall static, dynamic, and dimensional scaling behavior along with the interdimensional crossover from two- to quasi-one-dimension, which is most likely to be driven by compressive strain in coherence with long-range strain–dipolar interactions.

Abstract

Domain walls separating differently oriented polarization regions of ferroelectric materials are known to greatly impact nanoscale materials and device functionalities. Though the understanding of size effects in ferroelectric nanostructures has progressed, the effect of thickness downsizing on domain wall scaling behavior has remained unexplored. Using piezoresponse force microscopy, epitaxial BaTiO₃ film thickness size (2–90 nm) effects on the critical scaling universality of the domain wall dynamical creep and static roughness exponents including dimensionality is demonstrated. Independently estimated static roughness exponents ranging between 0.34 and 0.28 and dynamical creep exponents transition from 0.54 to 0.22 elucidate the domain wall dimensionality transition from two- to quasi-one-dimension in the thickness range of 10–25 nm, which is later validated by evaluating effective dimensionality within the paradigm of random-bond universality. The observed interdimensional transition is further credenced to the compressive strain and long-range strain–dipolar interactions, as revealed by the structural analyses and additional measurements with modified substrate-induced strain. These results provide new insights into the understanding of size effects in nanoscale ferroelectricity, paving the way toward future nanodevices.

This study explores plasmonic photodetection using graphene, achieving non-scattering near-field detection of surface plasmon polaritons. The maximum photoresponsivity is 29.2 mA W⁻¹, and the polarization state of input light produces a 400% contrast. This has potential applications in on-chip optoelectronic circuits.

Abstract

2D materials have manifested themselves as key components toward compact integrated circuits. Because of their capability to circumvent the diffraction limit, light manipulation using surface plasmon polaritons (SPPs) is highly-valued. In this study, plasmonic photodetection using graphene as a 2D material is investigated. Non-scattering near-field detection of SPPs is implemented via monolayer graphene stacked under an SPP waveguide with a symmetric antenna. Energy conversion between radiation power and electrical signals is utilized for the photovoltaic and photoconductive processes of the gold-graphene interface and biased electrodes, measuring a maximum photoresponsivity of 29.2 mA W⁻¹. The generated photocurrent is altered under the polarization state of the input light, producing a 400% contrast between the maximum and minimum signals. This result is universally applicable to all on-chip optoelectronic circuits.

We demonstrate an artificial optoelectronic synapse array of 64 pixels based on two indium gallium zinc oxide (IGZO) synaptic phototransistors that respectively correspond to excitatory and inhibitory synapses. This synapse can be modulated by the spatiotemporal irradiation of light pulses to each phototransistor, enabling fully optically-triggered potentiation and depression. Finally, the synapse array is used to classify three digits with a high recognition rate (98.3%) and perform image preprocessing.

Abstract

To overcome the intrinsic inefficiency of the von Neumann architecture, neuromorphic devices that perform analog vector–matrix multiplication have been highlighted for achieving power- and time-efficient data processing. In particular, artificial synapses, of which conductance should be programmed to represent the synaptic weights of the artificial neural network, have been intensively researched to realize neuromorphic devices. Here, inspired by excitatory and inhibitory synapses, we develop an artificial optoelectronic synapse that shows both potentiation and depression characteristics triggered only by optical inputs. The design of the artificial optoelectronic synapse, in which excitatory and inhibitory synaptic phototransistors are serially connected, enables these characteristics by spatiotemporally irradiating the phototransistor channels with optical pulses. Furthermore, a negative synaptic weight can be realized without the need for electronic components such as comparators. With such attributes, the artificial optoelectronic synapse is demonstrated to classify three digits with a high recognition rate (98.3%) and perform image preprocessing via analog vector–matrix multiplication.

Nature Nanotechnology, Published online: 04 September 2023; doi:10.1038/s41565-023-01497-x

Nature Nanotechnology, Published online: 04 September 2023; doi:10.1038/s41565-023-01484-2