Resonantly hybridized excitons in moiré superlattices in van der Waals heterostructures.

Atomically thin layers of two-dimensional materials can be assembled in vertical stacks that are held together by relatively weak van der Waals forces, enabling coupling between monolayer crystals with incommensurate lattices and arbitrary mutual rotation1,2. Consequently, an overarching periodicity emerges in the local atomic registry of the constituent crystal structures, which is known as a moiré superlattice3. In graphene/hexagonal boron nitride structures4, the presence of a moiré superlattice can lead to the observation of electronic minibands5-7, whereas in twisted graphene bilayers its effects are enhanced by interlayer resonant conditions, resulting in a superconductor-insulator transition at magic twist angles8. Here, using semiconducting heterostructures assembled from incommensurate molybdenum diselenide (MoSe2) and tungsten disulfide (WS2) monolayers, we demonstrate that excitonic bands can hybridize, resulting in a resonant enhancement of moiré superlattice effects. MoSe2 and WS2 were chosen for the near-degeneracy of their conduction-band edges, in order to promote the hybridization of intra- and interlayer excitons. Hybridization manifests through a pronounced exciton energy shift as a periodic function of the interlayer rotation angle, which occurs as hybridized excitons are formed by holes that reside in MoSe2 binding to a twist-dependent superposition of electron states in the adjacent monolayers. For heterostructures in which the monolayer pairs are nearly aligned, resonant mixing of the electron states leads to pronounced effects of the geometrical moiré pattern of the heterostructure on the dispersion and optical spectra of the hybridized excitons. Our findings underpin strategies for band-structure engineering in semiconductor devices based on van der Waals heterostructures9.

Author Correction: Resonantly hybridized excitons in moiré superlattices in van der Waals heterostructures.

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

Highly Sustainable h-BN Encapsulated MoS2 Hydrogen Evolution Catalysts.

Despite the importance of the stability of the 2D catalysts in harsh electrolyte solutions, most studies have focused on improving the catalytic performance of molybdenum disulfide (MoS2) catalysts rather than the sustainability of hydrogen evolution. In previous studies, the vulnerability of MoS2 crystals is reported that the moisture and oxygen molecules can cause the oxidation of MoS2 crystals, accelerating the degradation of crystal structure. Therefore, optimization of catalytic stability is crucial for approaching practical applications in 2D catalysts. Here, it is proposed that monolayered MoS2 catalysts passivated with an atomically thin hexagonal boron nitride (h-BN) layer can effectively sustain hydrogen evolution reaction (HER) and demonstrate the ultra-high current density (500 mA cm⁻2 over 11 h) and super stable (64 h at 150 mA cm⁻2) catalytic performance. It is further confirmed with density functional theory (DFT) calculations that the atomically thin h-BN layer effectively prevents direct adsorption of water/acid molecules while allowing the protons to be adsorbed/penetrated. The selective penetration of protons and prevention of crystal structure degradation lead to maintained catalytic activity and maximized catalytic stability in the h-BN covered MoS2 catalysts. These findings propose a promising opportunity for approaching the practical application of 2D MoS2 catalysts having long-term stability at high-current operation.

Unusual Raman Enhancement Effect of Ultrathin Copper Sulfide.

In surface-enhanced Raman spectroscopy (SERS), 2D materials are explored as substrates owing to their chemical stability and reproducibility. However, they exhibit lower enhancement factors (EFs) compared to noble metal-based SERS substrates. This study demonstrates the application of ultrathin covellite copper sulfide (CuS) as a cost-effective SERS substrate with a high EF value of 7.2 × 104 . The CuS substrate is readily synthesized by sulfurizing a Cu thin film at room temperature, exhibiting a Raman signal enhancement comparable to that of an Au noble metal substrate of similar thickness. Furthermore, computational simulations using the density functional theory are employed and time-resolved photoluminescence measurements are performed to investigate the enhancement mechanisms. The results indicate that polar covalent bonds (Cu─S) and strong interlayer interactions in the ultrathin CuS substrate increase the probability of charge transfer between the analyte molecules and the CuS surface, thereby producing enhanced SERS signals. The CuS SERS substrate demonstrates the selective detection of various dye molecules, including rhodamine 6G, methylene blue, and safranine O. Furthermore, the simplicity of CuS synthesis facilitates large-scale production of SERS substrates with high spatial uniformity, exhibiting a signal variation of less than 5% on a 4-inch wafer.

Evolution of Local Structural Motifs in Colloidal Quantum Dot Semiconductor Nanocrystals Leading to Nanofaceting.

Colloidal nanocrystals (NCs) have shown remarkable promise for optoelectronics, energy harvesting, photonics, and biomedical imaging. In addition to optimizing quantum confinement, the current challenge is to obtain a better understanding of the critical processing steps and their influence on the evolution of structural motifs. Computational simulations and electron microscopy presented in this work show that nanofaceting can occur during nanocrystal synthesis from a Pb-poor environment in a polar solvent. This could explain the curved interfaces and the olivelike-shaped NCs observed experimentally when these conditions are employed. Furthermore, the wettability of the PbS NCs solid film can be further modified via stoichiometry control, which impacts the interface band bending and, therefore, processes such as multiple junction deposition and interparticle epitaxial growth. Our results suggest that nanofaceting in NCs can become an inherent advantage when used to modulate band structures beyond what is traditionally possible in bulk crystals.

Shift of switching threshold in low-dimensional semiconductor-based complementary inverters via inkjet printing.

MoS2crystals grown by chemical vapor deposition are suited for realization of practical 2D semiconductor-based electronics. In order to construct complementary circuits with n-type MoS2, another p-type semiconductor, whose performance can be adjusted corresponding to that of MoS2in the limited chip area, has to be sought. Herein, we present a method for tuning switching threshold voltages of complementary inverters simply via inkjet printing without changing their channel dimensions. Random networks of inkjet printed single-walled carbon nanotubes are formed as p-channels beside MoS2, and their density and thickness are controlled by varying the number of printed layers. As a result, p-type transistor characteristics as well as inverter characteristics are facilely tuned only by varying the number of printed layers.

Facile one-pot iodine gas phase doping on 2D MoS2/CuS FET at room temperature.

Electronic devices composed of semiconducting two-dimensional (2D) materials and ultrathin 2D metallic electrode materials, accompanying synergistic interactions and extraordinary properties, are becoming highly promising for future flexible and transparent electronic and optoelectronic device applications. Unlike devices with bulk metal electrode and 2D channel materials, devices with ultrathin 2D electrode and 2D channel are susceptible to chemical reactions in both channel and electrode surface due to the high surface to volume ratio of the 2D structures. However, so far, the effect of doping was primary concerned on the channel component, and there is lack of understanding in terms of how to modulate electrical properties of devices by engineering electrical properties of both the metallic electrode and the semiconducting channel. Here, we propose the novel, one-pot doping of the field-effect transistor (FET) based on 2D molybdenum disulfide (MoS2) channel and ultrathin copper sulfide (CuS) electrodes under mild iodine gas environment at room temperature, which simultaneously modulates electrical properties of the 2D MoS2channel and 2D CuS electrode in a facile and cost-effective way. After one-pot iodine doping, effective p-type doping of the channel and electrode was observed, which was shown through decreased off current level, improvedIon/Ioffratio and subthreshold swing value. Our results open up possibility for effectively and conveniently modulating electrical properties of FETs made of various 2D semiconductors and ultrathin contact materials without causing any detrimental damage.

Field effect transistors and phototransistors based upon p-type solution-processed PbS nanowires.

We demonstrate the fabrication of solution processed highly crystalline p-type PbS nanowires via the oriented attachment of nanoparticles. The analysis of single nanowire field effect transistor (FET) devices revealed a hole conduction behaviour with average mobilities greater than 30 cm2 V-1 s-1, which is an order of magnitude higher than that reported to date for p-type PbS colloidal nanowires. We have investigated the response of the FETs to near-infrared light excitation and show herein that the nanowires exhibited gate-dependent photo-conductivities, enabling us to tune the device performances. The responsivity was found to be greater than 104 A W-1 together with a detectivity of 1013 Jones, which benefits from a photogating effect occurring at negative gate voltages. These encouraging detection parameters are accompanied by relatively short switching times of 15 ms at positive gate voltages, resulting from a combination of the standard photoconduction and the high crystallinity of the nanowires. Collectively, these results indicate that solution-processed PbS nanowires are promising nanomaterials for infrared photodetectors as well as p-type nanowire FETs.

Imaging of Interlayer Coupling in van der Waals Heterostructures Using a Bright-Field Optical Microscope.

Vertically stacked atomic layers from different layered crystals can be held together by van der Waals forces, which can be used for building novel heterostructures, offering a platform for developing a new generation of atomically thin, transparent, and flexible devices. The performance of these devices is critically dependent on the layer thickness and the interlayer electronic coupling, influencing the hybridization of the electronic states as well as charge and energy transfer between the layers. The electronic coupling is affected by the relative orientation of the layers as well as by the cleanliness of their interfaces. Here, we demonstrate an efficient method for monitoring interlayer coupling in heterostructures made from transition metal dichalcogenides using photoluminescence imaging in a bright-field optical microscope. The color and brightness in such images are used here to identify mono- and few-layer crystals and to track changes in the interlayer coupling and the emergence of interlayer excitons after thermal annealing in heterobilayers composed of mechanically exfoliated flakes and as a function of the twist angle in atomic layers grown by chemical vapor deposition. Material and crystal thickness sensitivity of the presented imaging technique makes it a powerful tool for characterization of van der Waals heterostructures assembled by a wide variety of methods, using combinations of materials obtained through mechanical or chemical exfoliation and crystal growth.

Strain-Mediated Interlayer Coupling Effects on the Excitonic Behaviors in an Epitaxially Grown MoS2/WS2 van der Waals Heterobilayer.

van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS2/WS2 vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS2 to MoS2 in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices.

Toward Ideal Low-Frequency Noise in Monolayer CVD MoS2 FETs: Influence of van der Waals Junctions and Sulfur Vacancy Management.

The pursuit of sub-1-nm field-effect transistor (FET) channels within 3D semiconducting crystals faces challenges due to diminished gate electrostatics and increased charge carrier scattering. 2D semiconductors, exemplified by transition metal dichalcogenides, provide a promising alternative. However, the non-idealities, such as excess low-frequency noise (LFN) in 2D FETs, present substantial hurdles to their realization and commercialization. In this study, ideal LFN characteristics in monolayer MoS2 FETs are attained by engineering the metal-2D semiconductor contact and the subgap density of states (DOS). By probing non-ideal contact resistance effects using CuS and Au electrodes, it is uncovered that excess contact noise in the high drain current (ID) region can be substantially reduced by forming a van der Waals junction with CuS electrodes. Furthermore, thermal annealing effectively mitigates sulfur vacancy-induced subgap density of states (DOS), diminishing excess noise in the low ID region. Through meticulous optimization of metal-2D semiconductor contacts and subgap DOS, alignment of 1/f noise with the pure carrier number fluctuation model is achieved, ultimately achieving the sought-after ideal LFN behavior in monolayer MoS2 FETs. This study underscores the necessity of refining excess noise, heralding improved performance and reliability of 2D electronic devices.

2D Nanostructures for Optoelectronic and Green Energy Devices.

Two-dimensional (2D) materials and nanostructures have gathered significant attention due to their excellent mechanical properties [...].

Engineering electrode interfaces for telecom-band photodetection in MoS2/Au heterostructures via sub-band light absorption.

Transition metal dichalcogenide (TMD) layered semiconductors possess immense potential in the design of photonic, electronic, optoelectronic, and sensor devices. However, the sub-bandgap light absorption of TMD in the range from near-infrared (NIR) to short-wavelength infrared (SWIR) is insufficient for applications beyond the bandgap limit. Herein, we report that the sub-bandgap photoresponse of MoS2/Au heterostructures can be robustly modulated by the electrode fabrication method employed. We observed up to 60% sub-bandgap absorption in the MoS2/Au heterostructure, which includes the hybridized interface, where the Au layer was applied via sputter deposition. The greatly enhanced absorption of sub-bandgap light is due to the planar cavity formed by MoS2 and Au; as such, the absorption spectrum can be tuned by altering the thickness of the MoS2 layer. Photocurrent in the SWIR wavelength range increases due to increased absorption, which means that broad wavelength detection from visible toward SWIR is possible. We also achieved rapid photoresponse (~150 µs) and high responsivity (17 mA W-1) at an excitation wavelength of 1550 nm. Our findings demonstrate a facile method for optical property modulation using metal electrode engineering and for realizing SWIR photodetection in wide-bandgap 2D materials.

Controlled p-Type Doping of MoS2 Monolayer by Copper Chloride.

Electronic devices based on two-dimensional (2D) MoS2 show great promise as future building blocks in electronic circuits due to their outstanding electrical, optical, and mechanical properties. Despite the high importance of doping of these 2D materials for designing field-effect transistors (FETs) and logic circuits, a simple and controllable doping methodology still needs to be developed in order to tailor their device properties. Here, we found a simple and effective chemical doping strategy for MoS2 monolayers using CuCl2 solution. The CuCl2 solution was simply spin-coated on MoS2 with different concentrations under ambient conditions for effectively p-doping the MoS2 monolayers. This was systematically analyzed using various spectroscopic measurements using Raman, photoluminescence, and X-ray photoelectron and electrical measurements by observing the change in transfer and output characteristics of MoS2 FETs before and after CuCl2 doping, showing effective p-type doping behaviors as observed through the shift of threshold voltages (Vth) and reducing the ON and OFF current level. Our results open the possibility of providing effective and simple doping strategies for 2D materials and other nanomaterials without causing any detrimental damage.

Observation of Strong Interlayer Couplings in WS2/MoS2 Heterostructures via Low-Frequency Raman Spectroscopy.

Van der Waals (vdW) heterostructures based on two-dimensional (2D) transition metal dichalcogenides (TMDCs), particularly WS2/MoS2 heterostructures with type-II band alignments, are considered as ideal candidates for future functional optoelectronic applications owing to their efficient exciton dissociation and fast charge transfers. These physical properties of vdW heterostructures are mainly influenced by the interlayer coupling occurring at the interface. However, a comprehensive understanding of the interlayer coupling in vdW heterostructures is still lacking. Here, we present a detailed analysis of the low-frequency (LF) Raman modes, which are sensitive to interlayer coupling, in bilayers of MoS2, WS2, and WS2/MoS2 heterostructures directly grown using chemical vapor deposition to avoid undesirable interfacial contamination and stacking mismatch effects between the monolayers. We clearly observe two distinguishable LF Raman modes, the interlayer in-plane shear and out-of-plane layer-breathing modes, which are dependent on the twisting angles and interface quality between the monolayers, in all the 2D bilayered structures, including the vdW heterostructure. In contrast, LF modes are not observed in the MoS2 and WS2 monolayers. These results indicate that our directly grown 2D bilayered TMDCs with a favorable stacking configuration and high-quality interface can induce strong interlayer couplings, leading to LF Raman modes.

Electromagnetic Interference Shielding with 2D Copper Sulfide.

Electronic devices in highly integrated and miniaturized systems demand electromagnetic interference shielding within nanoscale dimensions. Although several ultrathin materials have been proposed, satisfying various requirements such as ultrathin thickness, optical transparency, flexibility, and proper shielding efficiency remains a challenge. Herein, we report an ultrahigh electromagnetic interference (EMI) SSE/t value (>106 dB cm2/g) using a conductive CuS nanosheet with thickness less than 20 nm, which was synthesized at room temperature. We found that the EMI shielding efficiency (EMI SE) of the CuS nanosheet exceeds that of the traditional Cu film in the nanoscale thickness, which is due to high conductivity and the presence of internal dipole structures of the CuS nanosheet that contribute to absorption due to the damping of dipole oscillation. In addition, the CuS nanosheet exhibited high mechanical stability (104 cycles at 3 mm bending radius) and air stability (25 °C, 1 atm), which far exceeded the performance of the Cu nanosheet film. This remarkable performance of nanometer-thick CuS proposes an important pathway toward designing EMI shielding materials for wearable, flexible, and next-generation electronic applications.

Balanced Charge Carrier Transport Mediated by Quantum Dot Film Post-organization for Light-Emitting Diode Applications.

In light-emitting diodes (LEDs), balanced electron and hole transport is of particular importance to achieve high rates of radiative recombination. Most quantum dot (QD)-based LEDs, however, employ infinitesimal core-shell QDs which inherently have different electron and hole mobilities. As QDs are the core building blocks of QD-LEDs, the inherent mobility difference in the core-shell QDs causes significantly unbalanced charge carrier transport, resulting in detrimental effects on performances of QD-LEDs. Herein, we introduce a post-chemical treatment to reconstruct the QD films through the solvent-mediated self-organization process. The treatment using various poly-alkyl alcohol groups enables QD ensembles to transform from disordered solid dispersion into an ordered superlattice and effectively modulate electron and hole mobilities, which leads to the balanced charge carrier transport. In particular, ethanol-treated QD films exhibit enhanced charge carrier lifetime and reduced hysteresis due to the balanced charge carrier transport, which is attributed to the preferential-facet-oriented QD post-organization. As a result, 63, 78, and 54% enhancements in the external quantum efficiency were observed in red, green, and blue QD-LEDs, respectively. These results are of fundamental importance to understand both solvent-mediated QD film reconstruction and the effect of balanced electron and hole transport in QD-LEDs.

Electrode-Induced Self-Healed Monolayer MoS2 for High Performance Transistors and Phototransistors.

Contact engineering for monolayered transition metal dichalcogenides (TMDCs) is considered to be of fundamental challenge for realizing high-performance TMDCs-based (opto) electronic devices. Here, an innovative concept is established for a device configuration with metallic copper monosulfide (CuS) electrodes that induces sulfur vacancy healing in the monolayer molybdenum disulfide (MoS2 ) channel. Excess sulfur adatoms from the metallic CuS electrodes are donated to heal sulfur vacancy defects in MoS2 that surprisingly improve the overall performance of its devices. The electrode-induced self-healing mechanism is demonstrated and analyzed systematically using various spectroscopic analyses, density functional theory (DFT) calculations, and electrical measurements. Without any passivation layers, the self-healed MoS2 (photo)transistor with the CuS contact electrodes show outstanding room temperature field effect mobility of 97.6 cm2 (Vs)-1 , On/Off ratio > 108 , low subthreshold swing of 120 mV per decade, high photoresponsivity of 1 × 104  A W-1 , and detectivity of 1013 jones, which are the best among back-gated transistors that employ 1L MoS2 . Using ultrathin and flexible 2D CuS and MoS2 , mechanically flexible photosensor is also demonstrated, which shows excellent durability under mechanical strain. These findings demonstrate a promising strategy in TMDCs or other 2D material for the development of high performance and functional devices including self-healable sulfide electrodes.

Room Temperature Wafer-Scale Synthesis of Highly Transparent, Conductive CuS Nanosheet Films via a Simple Sulfur Adsorption-Corrosion Method.

The development of highly conductive electrodes with robust mechanical durability and clear transmittance in the visible to IR spectral range is of great importance for future wearable/flexible electronic applications. In particular, low resistivity, robust flexibility, and wide spectral transparency have a significant impact on optoelectronic performance. Herein, we introduce a new class of covellite copper monosulfide (CuS) nanosheet films as a promising candidate for soft transparent conductive electrodes (TCEs). An atmospheric sulfur adsorption-corrosion phenomenon represents a key approach in our work for the achievement of wafer-scale CuS nanosheet films through systematic control of the neat Cu layer thickness ranging from 2 to 10 nm multilayers at room temperature. These nanosheet films provide outstanding conductivity (∼25 Ω sq-1) and high transparency (> 80%) in the visible to infrared region as well as distinct flexibility and long stability under air exposure, yielding a high figure-of-merit (∼60) that is comparable to that of conventional rigid metal oxide material-based TCEs. Our unique room temperature synthesis process delivers high quality CuS nanosheets on any arbitrary substrates in a short time (< 1 min) scale, thus guaranteeing the widespread use of highly producible and scalable device fabrication.

Surface functionalization-induced photoresponse characteristics of monolayer MoS2 for fast flexible photodetectors.

Monolayered, semiconducting molybdenum disulfide (MoS2) is of considerable interest for its potential applications in next-generation flexible, wearable, and transparent photodetectors because it has outstanding physical properties coupled with unique atomically thin dimensions. However, there is still a lack of understanding in terms of the underlying mechanisms responsible for the photoresponse dynamics, which makes it difficult to identify the appropriate device design strategy for achieving a fast photoresponse time in MoS2 photodetectors. In this study, we investigate the importance of surface functionalization on controlling the charge carrier densities in a MoS2 monolayer and in turn the corresponding behavior of the photoresponse in relation to the position of the Fermi-level and the energy band structure. We find that the p-doping and n-doping, which is achieved through the surface functionalization of the MoS2 monolayer, leads to devices with different photoresponse behavior. Specifically, the MoS2 devices with surface functional groups contributing to p-doping exhibited a faster response time as well as higher sensitivity compared to that observed for the MoS2 devices with surface functional groups contributing to n-doping. We attribute this difference to the degree of bending in the energy bands at the metal-semiconductor junction as a result of shifting in the Fermi-level position, which influences the optoelectronic transport properties as well as the recombination dynamics leading to a low dark and thus high detectivity and fast decay time. Based upon these findings, we have also demonstrated the broad applicability of surface functionalization by fabricating a flexible MoS2 photodetector that shows an outstanding decay time of 0.7 s, which is the fastest response time observed in flexible MoS2 detectors ever reported.