Large enhancement of thermoelectric performance in MoS2/h-BN heterostructure due to vacancy-induced band hybridization.

Local impurity states arising from atomic vacancies in two-dimensional (2D) nanosheets are predicted to have a profound effect on charge transport due to resonant scattering and can be used to manipulate thermoelectric properties. However, the effects of these impurities are often masked by external fluctuations and turbostratic interfaces; therefore, it is challenging to probe the correlation between vacancy impurities and thermoelectric parameters experimentally. In this work, we demonstrate that n-type molybdenum disulfide (MoS2) supported on hexagonal boron nitride (h-BN) substrate reveals a large anomalous positive Seebeck coefficient with strong band hybridization. The presence of vacancies on MoS2 with a large conduction subband splitting of 50.0 ± 5.0 meV may contribute to Kondo insulator-like properties. Furthermore, by tuning the chemical potential, the thermoelectric power factor can be enhanced by up to two orders of magnitude to 50 mW m-1 K-2 Our work shows that defect engineering in 2D materials provides an effective strategy for controlling band structure and tuning thermoelectric transport.

Suspended MoS2 Photodetector Using Patterned Sapphire Substrate.

The introduction of patterned sapphire substrates (PSS) has been regarded as an effective method to improve the photoelectric performance of 2D layered materials in recent years. Molybdenum disulfide (MoS2 ), an intriguing transition metal 2D materials with splendid photoresponse owing to a direct-indirect bandgap transition at monolayer, shows promising optoelectronics applications. Here, a large-scale, continuous multilayer MoS2 film is prepared on a SiO2 /Si substrate and transferred to flat sapphire substrate and PSS, respectively. An enhanced dynamic distribution of local electric field and concentrated photon excitons across the interface between MoS2 and patterned sapphire substrates are revealed by the finite-difference time-domain simulation. The photoelectric performance of the MoS2 /PSS photodetector is improved under the three lasers of 365, 460, and 660 nm. Under the 365 nm laser, the photocurrent increased by 3 times, noise equivalent power (NEP) decreases to 1.77 × 10-14 W/Hz1/2 and specific detectivity (D*) increases to 1.2 × 1010 Jones. Meanwhile, the responsivity is increased by 7 times at 460 nm, and the response time of the MoS2 /PSS photodetector is also shortened under three wavelengths. The work demonstrates an effective method for enhancing the optical properties of photodetectors and enabling simultaneous detection of broad-spectrum emissions.

Exciton-Enabled Meta-Optics in Two-Dimensional Transition Metal Dichalcogenides.

Optical wavefront engineering has been rapidly developing in fundamentals from phase accumulation in the optical path to the electromagnetic resonances of confined nanomodes in optical metasurfaces. However, the amplitude modulation of light has limited approaches that usually originate from the ohmic loss and absorptive dissipation of materials. Here, an atomically thin photon-sieve platform made of MoS2 multilayers is demonstrated for high-quality optical nanodevices, assisted fundamentally by strong excitonic resonances at the band-nesting region of MoS2. The atomic thin MoS2 significantly facilitates high transmission of the sieved photons and high-fidelity nanofabrication. A proof-of-concept two-dimensional (2D) nanosieve hologram exhibits 10-fold enhanced efficiency compared with its non-2D counterparts. Furthermore, a supercritical 2D lens with its focal spot breaking diffraction limit is developed to exhibit experimentally far-field label-free aberrationless imaging with a resolution of ∼0.44λ at λ = 450 nm in air. This transition-metal-dichalcogenide (TMDC) photonic platform opens new opportunities toward future 2D meta-optics and nanophotonics.

The organic-2D transition metal dichalcogenide heterointerface.

Since the first isolation of graphene, new classes of two-dimensional (2D) materials have offered fascinating platforms for fundamental science and technology explorations at the nanometer scale. In particular, 2D transition metal dichalcogenides (TMD) such as MoS2 and WSe2 have been intensely investigated due to their unique electronic and optical properties, including tunable optical bandgaps, direct-indirect bandgap crossover, strong spin-orbit coupling, etc., for next-generation flexible nanoelectronics and nanophotonics applications. On the other hand, organics have always been excellent materials for flexible electronics. A plethora of organic molecules, including donors, acceptors, and photosensitive molecules, can be synthesized using low cost and scalable procedures. Marrying the fields of organics and 2D TMDs will bring benefits that are not present in either material alone, enabling even better, multifunctional flexible devices. Central to the realization of such devices is a fundamental understanding of the organic-2D TMD interface. Here, we review the organic-2D TMD interface from both chemical and physical perspectives. We discuss the current understanding of the interfacial interactions between the organic layers and the TMDs, as well as the energy level alignment at the interface, focusing in particular on surface charge transfer and electronic screening effects. Applications from the literature are discussed, especially in optoelectronics and p-n hetero- and homo-junctions. We conclude with an outlook on future scientific and device developments based on organic-2D TMD heterointerfaces.

New Insights into Planar Defects in Layered α-MoO3 Crystals.

The observation of regular ( h0 l) planar defects in α-MoO3 crystals can be traced back to over 60 years ago. Two mechanisms have been proposed to interpret the formation of the planar defects. One is related to the diffusion of oxygen vacancies because of thermal-driven release of oxygen atoms in vacuum and the consequent crystallographic shear of α-MoO3. The other is associated with redox reactions of moisture and/or hydrocarbons that give rise to H xMoO3 precipitates. Here, we report that regularly spaced (302) planar defects can be introduced into α-MoO3 belt crystals by heating in liquid sulfur at 300 °C. These defects are undetectable by both atomic force microscopy and scanning electron microscopy at the crystal surface. Raman scattering enhancement and weakening have been observed for different phonon modes of α-MoO3 at the (302) planar defects as probed from the (010) surface. Their comparisons with the Raman scattering enhancements at the edges and the argon-plasma-induced Raman spectral evolutions of the as-grown α-MoO3 belt crystals provide new insights into the planar defects with regard to their formation and characteristics.

Gap States at Low-Angle Grain Boundaries in Monolayer Tungsten Diselenide.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties of interest to future device applications. In particular, the presence of grain boundaries (GBs) can significantly influence the material properties of 2D TMDs. However, direct characterization of the electronic properties of the GB defects at the atomic scale remains extremely challenging. In this study, we employ scanning tunneling microscopy and spectroscopy to investigate the atomic and electronic structure of low-angle GBs of monolayer tungsten diselenide (WSe2) with misorientation angles of 3-6°. Butterfly features are observed along the GBs, with the periodicity depending on the misorientation angle. Density functional theory calculations show that these butterfly features correspond to gap states that arise in tetragonal dislocation cores and extend to distorted six-membered rings around the dislocation core. Understanding the nature of GB defects and their influence on transport and other device properties highlights the importance of defect engineering in future 2D device fabrication.

The effect of crystallinity on photocatalytic performance of Co3O4 water-splitting cocatalysts.

Cocatalysts, when loaded onto a water splitting photocatalyst, accelerate the gas evolution reaction and improve the efficiency of the photocatalyst. In this paper, we report that the efficiency of the photocatalyst is enhanced using an amorphous cobalt oxide cocatalyst. The WO3 film, when loaded with amorphous or nanocrystalline Co3O4, shows an improvement of up to 40% in photocurrent generation and 34% in hydrogen gas evolution. The effect of cocatalyst crystallinity on performance was systematically studied, and we found that the photocurrent deteriorates with the conversion of the cocatalyst to a highly crystalline phase at an annealing temperature of 500 °C. The mechanism of this effect was studied in detail using electrochemical impedance spectroscopy, and the enhancement effect produced by the amorphous cocatalyst is attributed to the large density of unsaturated catalytically active sites in the amorphous material.

Enhancing multiphoton upconversion through energy clustering at sublattice level.

The applications of lanthanide-doped upconversion nanocrystals in biological imaging, photonics, photovoltaics and therapeutics have fuelled a growing demand for rational control over the emission profiles of the nanocrystals. A common strategy for tuning upconversion luminescence is to control the doping concentration of lanthanide ions. However, the phenomenon of concentration quenching of the excited state at high doping levels poses a significant constraint. Thus, the lanthanide ions have to be stringently kept at relatively low concentrations to minimize luminescence quenching. Here we describe a new class of upconversion nanocrystals adopting an orthorhombic crystallographic structure in which the lanthanide ions are distributed in arrays of tetrad clusters. Importantly, this unique arrangement enables the preservation of excitation energy within the sublattice domain and effectively minimizes the migration of excitation energy to defects, even in stoichiometric compounds with a high Yb(3+) content (calculated as 98 mol%). This allows us to generate an unusual four-photon-promoted violet upconversion emission from Er(3+) with an intensity that is more than eight times higher than previously reported. Our results highlight that the approach to enhancing upconversion through energy clustering at the sublattice level may provide new opportunities for light-triggered biological reactions and photodynamic therapy.

Amorphous ruthenium nanoparticles for enhanced electrochemical water splitting.

This paper demonstrates an optimized fabrication of amorphous Ru nanoparticles through annealing at various temperatures ranging from 150 to 700 °C, which are used as water oxidation catalyst for effective electrochemical water splitting under a low overpotential of less than 300 mV. The amorphous Ru nanoparticles with short-range ordered structure exhibit an optimal and stable electrocatalytic activity after annealing at 250 °C. Interestingly, a small quantity of such Ru nanoparticles in a thin film on fluorine-doped tin oxide glass is also effectively driven by a conventional crystalline silicon solar cell that has excellent capability for harvesting visible light. Remarkably, it achieves an overall solar-to-hydrogen efficiency of 11.3% in acidic electrolyte.

Lanthanide-doped upconversion materials: emerging applications for photovoltaics and photocatalysis.

Photovoltaics and photocatalysis are two significant applications of clean and sustainable solar energy, albeit constrained by their inability to harvest the infrared spectrum of solar radiation. Lanthanide-doped materials are particularly promising in this regard, with tunable absorption in the infrared region and the ability to convert the long-wavelength excitation into shorter-wavelength light output through an upconversion process. In this review, we highlight the emerging applications of lanthanide-doped upconversion materials in the areas of photovoltaics and photocatalysis. We attempt to elucidate the fundamental physical principles that govern the energy conversion by the upconversion materials. In addition, we intend to draw attention to recent technologies in upconversion nanomaterials integrated with photovoltaic and photocatalytic devices. This review also provides a useful guide to materials synthesis and optoelectronic device fabrication based on lanthanide-doped upconversion materials.

Towards large area and continuous MoS2 atomic layers via vapor-phase growth: thermal vapor sulfurization.

We report on the effects of substrate, starting material, and temperature on the growth of MoS(2) atomic layers by thermal vapor sulfurization in a tube-furnace system. With Mo as the starting material, atomic layers of MoS(2) flakes are obtained on sapphire substrates while a bell-shaped MoS(2) layer, sandwiched by amorphous SiO(2), is obtained on native-SiO(2)/Si substrates under the same sulfurization conditions. An anomalous thickness-dependent Raman shift (A(1g)) of the MoS(2) atomic layers is observed in Mo-sulfurizations on sapphire substrates, which can be attributed to the competition between the effects of thickness and the surface/interface. Both effects vary with the sulfurizing temperatures for a certain initial Mo thickness. The anomalous frequency trend of A(1g) is missing when using MoO(3) instead of Mo as the starting material. In this case, the lateral growth of MoS(2) on sapphire is also largely improved. Furthermore, the area density of the resultant MoS(2) atomic layers is significantly increased by increasing the deposition temperature of the starting MoO(3) to 700 °C; the adjacent ultrathin MoS(2) grains coalesce in one or other direction, forming connected chains in wafer scale. The thickness of the so-obtained MoS(2) is generally controlled by the thickness of the starting material; however, the structural and morphological properties of MoS(2) grains, towards large area and continuous atomic layers, are strongly dependent on the temperature of the initial material deposition, and on the temperature of sulfurization, because of the competition between surface mobility and atom evaporation.

Sol-gel deposited Cu2O and CuO thin films for photocatalytic water splitting.

Cu2O and CuO are attractive photocatalytic materials for water splitting due to their earth abundance and low cost. In this paper, we report the deposition of Cu2O and CuO thin films by a sol-gel spin-coating process. Sol-gel deposition has distinctive advantages such as low-cost solution processing and uniform film formation over large areas with a precise stoichiometry and thickness control. Pure-phase Cu2O and CuO films were obtained by thermal annealing at 500 °C in nitrogen and ambient air, respectively. The films were successfully incorporated as photocathodes in a photoelectrochemical (PEC) cell, achieving photocurrents of -0.28 mA cm(-2) and -0.35 mA cm(-2) (for Cu2O and CuO, respectively) at 0.05 V vs. a reversible hydrogen electrode (RHE). The Cu2O photocurrent was enhanced to -0.47 mA cm(-2) upon incorporation of a thin layer of a NiOx co-catalyst. Preliminary stability studies indicate that CuO may be more stable than Cu2O as a photocathode for PEC water-splitting.

Van der Waals Layer Transfer of 2D Materials for Monolithic 3D Electronic System Integration: Review and Outlook.

Two-dimensional materials (2DMs) have attracted a great deal of interest due to their immense potential for scientific breakthroughs and technological innovations. While some 2D transition metal dichalcogenides (TMDC) such as MoS2 and WS2 are considered as the ultimate channel materials in unltrascaled transistors as replacements for Si, there has also been increasing interest in the monolithic 3D integration of 2DMs on the Si CMOS platform or in flexible electronics as back-end-of-line transistors, memory devices/selectors, and sensors, taking advantage of 2DM properties such as a high current driving capability with low leakage current, nonvolatile switching characteristics, a large surface-to-volume ratio, and a tunable bandgap. However, the realization of both of these scenarios critically depends on the development of manufacturing-viable high-yield 2DM layers transfer from the growth substrate to the Si, since the growth of high-quality 2DM layers often requires a high-temperature growth process on template substrates. Motivated by this, extensive efforts have been made by the 2DM research community to develop various 2DM layer transfer methods, leveraging the van der Waals transfer capability of the layer-structured 2DMs. These efforts have led to a number of successful demonstrations of wafer-scale 2D TMDC layer transfer, while 2DM-enabled template growth/transfer of some functional bulk materials such as III-V, Ge, and AlN has also been demonstrated. This review surveys and compares different 2DM transfer methods developed recently, with a focus on large-area 2D TMDC film transfer along with an introduction of 2DM template-assisted van der Waals growth/transfer of non-2D thin films. We will also briefly present an outlook of our envisioned multifunctionalities in 3D integrated electronic systems enabled by monolithic 3D integration of 2DMs and III-V via van der Waals transfer and discuss possible technology options for overcoming remaining challenges.

Homogeneous in-plane WSe2 P-N junctions for advanced optoelectronic devices.

Conventional doping schemes of silicon (Si) microelectronics are incompatible with atomically thick two-dimensional (2D) transition metal dichalcogenides (TMDCs), which makes it challenging to construct high-quality 2D homogeneous p-n junctions. Herein, we adopt a simple yet effective plasma-treated doping method to seamlessly construct a lateral 2D WSe2 p-n homojunction. WSe2 with ambipolar transport properties was exposed to O2 plasma to form WOx on the surface in a self-limiting process that induces hole doping in the underlying WSe2via electron transfer. Different electrical behaviors were observed between the as-exfoliated (ambipolar) region and the O2 plasma-treated (p-doped) region under electrostatic modulation of the back-gate bias (VBG), which produces a p-n in-plane homojunction. More importantly, a small contact resistance of 710 Ω µm with a p-doped region transistor mobility of ∼157 cm2 V-1 s-1 was achieved due to the transformation of Schottky contact into Ohmic contact after plasma treatment. This effectively avoids Fermi-level pinning and significantly improves the performance of photodetectors. The resultant WSe2 p-n junction device thus exhibits a high photoresponsivity of ∼7.1 × 104 mA W-1 and a superior external quantum efficiency of ∼228%. Also, the physical mechanism of charge transfer in the WSe2 p-n homojunction was analyzed. Our proposed strategy offers a powerful route to realize low contact resistance and high photoresponsivity in 2D TMDC-based optoelectronic devices, paving the way for next-generation atomic-thickness optoelectronics.

In-Sensor Computing Realization Using Fully CMOS-Compatible TiN/HfOx-Based Neuristor Array.

With the evolution of artificial intelligence, the explosive growth of data from sensory terminals gives rise to severe energy-efficiency bottleneck issues due to cumbersome data interactions among sensory, memory, and computing modules. Heterogeneous integration methods such as chiplet technology can significantly reduce unnecessary data movement; however, they fail to address the fundamental issue of the substantial time and energy overheads resulting from the physical separation of computing and sensory components. Brain-inspired in-sensor neuromorphic computing (ISNC) has plenty of room for such data-intensive applications. However, one key obstacle in developing ISNC systems is the lack of compatibility between material systems and manufacturing processes deployed in sensors and computing units. This study successfully addresses this challenge by implementing fully CMOS-compatible TiN/HfOx-based neuristor array. The developed ISNC system demonstrates several advantageous features, including multilevel analogue modulation, minimal dispersion, and no significant degradation in conductance (@125 °C). These characteristics enable stable and reproducible neuromorphic computing. Additionally, the device exhibits modulatable sensory and multi-store memory processes. Furthermore, the system achieves information recognition with a high accuracy rate of 93%, along with frequency selectivity and notable activity-dependent plasticity. This work provides a promising route to affordable and highly efficient sensory neuromorphic systems.

Sublimation-based wafer-scale monolayer WS2 formation via self-limited thinning of few-layer WS2.

Atomically-thin monolayer WS2 is a promising channel material for next-generation Moore's nanoelectronics owing to its high theoretical room temperature electron mobility and immunity to short channel effect. The high photoluminescence (PL) quantum yield of the monolayer WS2 also makes it highly promising for future high-performance optoelectronics. However, the difficulty in strictly growing monolayer WS2, due to its non-self-limiting growth mechanism, may hinder its industrial development because of the uncontrollable growth kinetics in attaining the high uniformity in thickness and property on the wafer-scale. In this study, we report a scalable process to achieve a 4 inch wafer-scale fully-covered strictly monolayer WS2 by applying the in situ self-limited thinning of multilayer WS2 formed by sulfurization of WOx films. Through a pulsed supply of sulfur precursor vapor under a continuous H2 flow, the self-limited thinning process can effectively trim down the overgrown multilayer WS2 to the monolayer limit without damaging the remaining bottom WS2 monolayer. Density functional theory (DFT) calculations reveal that the self-limited thinning arises from the thermodynamic instability of the WS2 top layers as opposed to a stable bottom monolayer WS2 on sapphire above a vacuum sublimation temperature of WS2. The self-limited thinning approach overcomes the intrinsic limitation of conventional vapor-based growth methods in preventing the 2nd layer WS2 domain nucleation/growth. It also offers additional advantages, such as scalability, simplicity, and possibility for batch processing, thus opening up a new avenue to develop a manufacturing-viable growth technology for the preparation of a strictly-monolayer WS2 on the wafer-scale.

Integration of Single-Atom Catalyst with Z-Scheme Heterojunction for Cascade Charge Transfer Enabling Highly Efficient Piezo-Photocatalysis.

Piezo-assisted photocatalysis (namely, piezo-photocatalysis), which utilizes mechanical energy to modulate spatial and energy distribution of photogenerated charge carriers, presents a promising strategy for molecule activation and reactive oxygen species (ROS) generation toward applications such as environmental remediation. However, similarly to photocatalysis, piezo-photocatalysis also suffers from inferior charge separation and utilization efficiency. Herein, a Z-scheme heterojunction composed of single Ag atoms-anchored polymeric carbon nitride (Ag-PCN) and SnO2- x is developed for efficient charge carrier transfer/separation both within the catalyst and between the catalyst and surface oxygen molecules (O2 ). As revealed by charge dynamics analysis and theoretical simulations, the synergy between the single Ag atoms and the Z-scheme heterojunction initiates a cascade electron transfer from SnO2- x to Ag-PCN and then to O2 adsorbed on Ag. With ultrasound irradiation, the polarization field generated within the piezoelectric hybrid further accelerates charge transfer and regulates the O2 activation pathway. As a result, the Ag-PCN/SnO2- x catalyst efficiently activates O2 into ·O2 - , ·OH, and H2 O2 under co-excitation of visible light and ultrasound, which are consequently utilized to trigger aerobic degradation of refractory antibiotic pollutants. This work provides a promising strategy to maneuver charge transfer dynamics for efficient piezo-photocatalysis by integrating single-atom catalysts (SACs) with Z-scheme heterojunction.

Celebrating 25 Years of IMRE: Research Highlights on Nanomaterials and Nanotechnologies.

The Institute of Materials Research and Engineering (IMRE) is a research institute of the Science and Engineering Research Council (SERC), Agency for Science, Technology and Research (A*STAR). IMRE was established in September 1997. Over the past 25 years, IMRE has developed core competencies and interdisciplinary teams for material development from fundamental discoveries to industrial translation. Currently, with over 400 researchers and state-of-the-art research facilities, IMRE conducts world class research in important material and material technology fields, including polymer composites, optical materials, electronic materials, soft materials, structural materials, energy materials, biomaterials, quantum technologies, as well as advanced characterization. As a material-centered research institute in Singapore, IMRE has played important roles in pushing science boundaries and developing cutting-edge technologies. One of the key strategies is to partner international organizations, research institutes, and industry to fulfill its vision to be a leading research institute to accelerate materials research, moving from "Made in Singapore" toward "Created in Singapore".

Wafer-Scale 2D Hafnium Diselenide Based Memristor Crossbar Array for Energy-Efficient Neural Network Hardware.

Memristor crossbar with programmable conductance could overcome the energy consumption and speed limitations of neural networks when executing core computing tasks in image processing. However, the implementation of crossbar array (CBA) based on ultrathin 2D materials is hindered by challenges associated with large-scale material synthesis and device integration. Here, a memristor CBA is demonstrated using wafer-scale (2-inch) polycrystalline hafnium diselenide (HfSe2 ) grown by molecular beam epitaxy, and a metal-assisted van der Waals transfer technique. The memristor exhibits small switching voltage (0.6 V), low switching energy (0.82 pJ), and simultaneously achieves emulation of synaptic weight plasticity. Furthermore, the CBA enables artificial neural network with a high recognition accuracy of 93.34%. Hardware multiply-and-accumulate (MAC) operation with a narrow error distribution of 0.29% is also demonstrated, and a high power efficiency of greater than 8-trillion operations per second per Watt is achieved. Based on the MAC results, hardware convolution image processing can be performed using programmable kernels (i.e., soft, horizontal, and vertical edge enhancement), which constitutes a vital function for neural network hardware.

Anthracene Single-Crystal Scintillators for Computer Tomography Scanning.

X-ray imaging and computed tomography (CT) technology, as the important non-destructive measurements, can observe internal structures without destroying the detected sample, which are always used in biological diagnosis to detect tumors, pathologies, and bone damages. It is always a challenge to find materials with a low detection limit, a short exposure time, and high resolution to reduce X-ray damage and acquire high-contrast images. Here, we described a low-cost and high-efficient method to prepare centimeter-sized anthracene crystals, which exhibited intense X-ray radioluminescence with a detection limit of ∼0.108 µGy s-1, which is only one-fifth of the dose typically used for X-ray diagnostics. Additionally, the low absorption reduced the damage in radiation and ensured superior cycle performance. X-ray detectors based on anthracene crystals also exhibited an extremely high resolution of 40 lp mm-1. The CT scanning and reconstruction of a foam sample were then achieved, and the detailed internal structure could be clearly observed. These indicated that organic crystals are expecting to be leading candidate low-cost materials for low-dose and highly sensitive X-ray detection and CT scanning.