Validated enhancement and temperature modulated absorbance of a WS2 monolayer based on a planar structure.

Transition-metal dichalcogenides (TMDCs), as emerging optoelectronic materials, necessitate the establishment of an experimentally viable system to study their interaction with light. In this study, we propose and analyze a WS2/PMMA/Ag planar Fabry-Perot (F-P) cavity, enabling the direct experimental measurement of WS2 absorbance. By optimizing the structure, the absorbance of A exciton of WS2 up to 0.546 can be experimentally achieved, which matches well with the theoretical calculations. Through temperature and thermal expansion strain induced by temperature, the absorbance of the A exciton can be tuned in situ. Furthermore, temperature-dependent photocurrent measurements confirmed the consistent absorbance of the A exciton under varying temperatures. This WS2/PMMA/Ag planar structure provides a straightforward and practical platform for investigating light interaction in TMDCs, laying a solid foundation for future developments of TMDC-based optoelectronic devices.

Phase-engineered synthesis of atomically thin te single crystals with high on-state currents.

Multiple structural phases of tellurium (Te) have opened up various opportunities for the development of two-dimensional (2D) electronics and optoelectronics. However, the phase-engineered synthesis of 2D Te at the atomic level remains a substantial challenge. Herein, we design an atomic cluster density and interface-guided multiple control strategy for phase- and thickness-controlled synthesis of α-Te nanosheets and ß-Te nanoribbons (from monolayer to tens of µm) on WS2 substrates. As the thickness decreases, the α-Te nanosheets exhibit a transition from metallic to n-type semiconducting properties. On the other hand, the ß-Te nanoribbons remain p-type semiconductors with an ON-state current density (ION) up to ~ 1527 µA µm-1 and a mobility as high as ~ 690.7 cm2 V-1 s-1 at room temperature. Both Te phases exhibit good air stability after several months. Furthermore, short-channel (down to 46 nm) ß-Te nanoribbon transistors exhibit remarkable electrical properties (ION = ~ 1270 µA µm-1 and ON-state resistance down to 0.63 kΩ µm) at Vds = 1 V.

Volume-Confined Fabrication of Large-Scale Single-Crystalline Molecular Ferroelectric Thin Films and Their Applications in 2D Materials.

With outstanding advantages of chemical synthesis, structural diversity, and mechanical flexibility, molecular ferroelectrics have attracted increasing attention, demonstrating themselves as promising candidates for next-generation wearable electronics and flexible devices in the film form. However, it remains a challenge to grow high-quality thin films of molecular ferroelectrics. To address the above issue, a volume-confined method is utilized to achieve ultrasmooth single-crystal molecular ferroelectric thin films at the sub-centimeter scale, with the thickness controlled in the range of 100-1000 nm. More importantly, the preparation method is applicable to most molecular ferroelectrics and has no dependency on substrates, showing excellent reproducibility and universality. To demonstrate the application potential, two-dimensional (2D) transitional metal dichalcogenide semiconductor/molecular ferroelectric heterostructures are prepared and investigated by optical spectroscopic method, proving the possibility of integrating molecular ferroelectrics with 2D layered materials. These results may unlock the potential for preparing and developing high-performance devices based on molecular ferroelectric thin films.

Interfacial Effect on the Transient Dielectric Function and Charge Transfer in a Monolayer WS2/Si Heterojunction.

Monolayer tungsten disulfide (WS2) is a highly promising material for silicon photonics. Thus, the WS2/Si interface plays a very important role due to the interfacial complex effects and abundant states. Among them, the effect of charge transfer on exciton dynamics and the optoelectronic property is determined by the dielectric function, which is very crucial for the performance of optoelectronic devices. However, research on the exciton dynamics or the transient dielectric function of WS2 in such WS2/Si junctions is still rare. In this work, both the transient dielectric function and charge transfer of WS2/Si heterojunctions are analyzed based on the transient reflectance spectra measured by the pump-probe spectrometer. The dynamic processes of the A exciton, affected by charge transfer within the WS2/Si heterojunction, are interpreted. Moreover, the transient dielectric function of WS2 is quantitatively analyzed. The dielectric function of WS2 exhibits a notable 19% change, persisting for more than 180 ps within the WS2/Si heterojunction. These findings can pave the way for the advancement of silicon photonic devices based on WS2.

Revealing the Polarizations of Molecular Ferroelectrics via SHG Polarimetry at the Nanoscale.

Multifarious molecular ferroelectrics with multipolar axial characteristics have emerged in recent years, enriching the scenarios for energy harvesting, sensing, and information processing. The increased polar axes have enhanced the urgency of distinguishing different polarization states in material design, mechanism exploration, etc. However, conventional methods hardly meet the requirements of in situ, fast, microscale, contactless, and nondestructive features due to their inherent limitations. Herein, SHG polarimetry is introduced to probe the multioriented polarizations on a nanosized multiaxial molecular ferroelectric, i.e., TMCM-CdCl3 nanoplates, as an example. Combined with the analysis of the second-order susceptibility tensor, SHG polarimetry could serve as an effective method to detect the polarization orders and domain distributions of molecular ferroelectrics. Profiting from the full-optical feature, SHG polarimetry can even be performed on samples covered by transparent mediums, 2D materials, or thin metal electrodes. Our research might spark further fundamental studies and expand the application boundaries of next-generation ferroelectric materials.

Silver nanoparticle-induced enhancement of light extraction in two-dimensional light-emitting diodes.

Monolayer transition metal dichalcogenides (TMDCs) with direct bandgaps are considered promising candidates for building light-emitting diodes (LEDs). One crucial indicator of their performance is the brightness of electroluminescence (EL). In this study, we fabricate WS2-based LEDs that make full use of the assistance of effective transient-mode charge injection. By introducing self-assembled silver nanoparticles (NPs) on top of the LED, the extraction efficiency is significantly improved, with a 2.9-fold EL enhancement observed in the experiment. Full-wave simulations further confirm that the improvement comes from the scattering capability of silver NPs, with results qualitatively fitting the experiment. This approach, with its compatibility with van der Waals heterostructures, can be further promoted to enhance the brightness of 2D monolayer TMDC-based LEDs.

Polarization-sensitive narrowband infrared photodetection triggered by optical Tamm state engineering.

Polarization-sensitive narrowband photodetection at near-infrared (NIR) has attracted significant interest in optical communication, environmental monitoring, and intelligent recognition system. However, the current narrowband spectroscopy heavily relies on the extra filter or bulk spectrometer, which deviates from the miniaturization of on-chip integration. Recently, topological phenomena, such as the optical Tamm state (OTS), provided a new solution for developing functional photodetection, and we experimentally realized the device based on 2D material (graphene) for the first time to the best of our knowledge. Here, we demonstrate polarization-sensitive narrowband infrared photodetection in OTS coupled graphene devices, which are designed with the aid of the finite-difference time-domain (FDTD) method. The devices show narrowband response at NIR wavelengths empowered by the tunable Tamm state. The full width at half maximum (FWHM) of the response peak reaches ∼100 nm, and it can potentially be improved to ultra-narrow of about 10 nm by increasing the periods of dielectric distributed Bragg reflector (DBR). The responsivity and response time of the device reaches 187 mA/W and ∼290 µs at 1550 nm, respectively. Furthermore, the prominent anisotropic features and high dichroic ratios of ∼4.6 at 1300 nm and ∼2.5 at 1500 nm are achieved by integrating gold metasurfaces.

Deciphering Adverse Detrapped Hole Transfer in Hot-Electron Photoelectric Conversion at Infrared Wavelengths.

Hot-carrier devices are promising alternatives for enabling path breaking photoelectric conversion. However, existing hot-carrier devices suffer from low efficiencies, particularly in the infrared region, and ambiguous physical mechanisms. In this work, the competitive interfacial transfer mechanisms of detrapped holes and hot electrons in hot-carrier devices are discovered. Through photocurrent polarity research and optical-pump-THz-probe (OPTP) spectroscopy, it is verified that detrapped hole transfer (DHT) and hot-electron transfer (HET) dominate the low- and high-density excitation responses, respectively. The photocurrent ratio assigned to DHT and HET increases from 6.6% to over 1133.3% as the illumination intensity decreases. DHT induces severe degeneration of the external quantum efficiency (EQE), especially at low illumination intensities. The EQE of a hot-electron device can theoretically increase by over two orders of magnitude at 10 mW cm-2 through DHT elimination. The OPTP results show that competitive transfer arises from the carrier oscillation type and carrier-density-related Coulomb screening. The screening intensity determines the excitation weight and hot-electron cooling scenes and thereby the transfer dynamics.

Approaching the quantum limit in two-dimensional semiconductor contacts.

The development of next-generation electronics requires scaling of channel material thickness down to the two-dimensional limit while maintaining ultralow contact resistance1,2. Transition-metal dichalcogenides can sustain transistor scaling to the end of roadmap, but despite a myriad of efforts, the device performance remains contact-limited3-12. In particular, the contact resistance has not surpassed that of covalently bonded metal-semiconductor junctions owing to the intrinsic van der Waals gap, and the best contact technologies are facing stability issues3,7. Here we push the electrical contact of monolayer molybdenum disulfide close to the quantum limit by hybridization of energy bands with semi-metallic antimony ([Formula: see text]) through strong van der Waals interactions. The contacts exhibit a low contact resistance of 42 ohm micrometres and excellent stability at 125 degrees Celsius. Owing to improved contacts, short-channel molybdenum disulfide transistors show current saturation under one-volt drain bias with an on-state current of 1.23 milliamperes per micrometre, an on/off ratio over 108 and an intrinsic delay of 74 femtoseconds. These performances outperformed equivalent silicon complementary metal-oxide-semiconductor technologies and satisfied the 2028 roadmap target. We further fabricate large-area device arrays and demonstrate low variability in contact resistance, threshold voltage, subthreshold swing, on/off ratio, on-state current and transconductance13. The excellent electrical performance, stability and variability make antimony ([Formula: see text]) a promising contact technology for transition-metal-dichalcogenide-based electronics beyond silicon.

One-Step Exfoliation Method for Plasmonic Activation of Large-Area 2D Crystals.

Advanced exfoliation techniques are crucial for exploring the intrinsic properties and applications of 2D materials. Though the recently discovered Au-enhanced exfoliation technique provides an effective strategy for the preparation of large-scale 2D crystals, the high cost of gold hinders this method from being widely adopted in industrial applications. In addition, direct Au contact could significantly quench photoluminescence (PL) emission in 2D semiconductors. It is therefore crucial to find alternative metals that can replace gold to achieve efficient exfoliation of 2D materials. Here, the authors present a one-step Ag-assisted method that can efficiently exfoliate many large-area 2D monolayers, where the yield ratio is comparable to Au-enhanced exfoliation method. Differing from Au film, however, the surface roughness of as-prepared Ag films on SiO2 /Si substrate is much higher, which facilitates the generation of surface plasmons resulting from the nanostructures formed on the rough Ag surface. More interestingly, the strong coupling between 2D semiconductor crystals (e.g., MoS2 , MoSe2 ) and Ag film leads to a unique PL enhancement that has not been observed in other mechanical exfoliation techniques, which can be mainly attributed to enhanced light-matter interaction as a result of extended propagation of surface plasmonic polariton (SPP). This work provides a lower-cost and universal Ag-assisted exfoliation method, while at the same time offering enhanced SPP-matter interactions.

A Time-Division Position-Sensitive Detector Image System for High-Speed Multitarget Trajectory Tracking.

High-speed trajectory tracking with real-time processing capability is particularly important in the fields of pilotless automobiles, guidance systems, robotics, and filmmaking. The conventional optical approach to high-speed trajectory tracking involves charge coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) image sensors, which suffer from trade-offs between resolution and framerates, complexity of the system, and enormous data-analysis processes. Here, a high-speed trajectory tracking system is designed by using a time-division position-sensitive detector (TD-PSD) based on a graphene-silicon Schottky heterojunction. Benefiting from the high-speed optoelectronic response and sub-micrometer positional accuracy of the TD-PSD, multitarget real-time trajectory tracking is realized, with a maximum image output framerate of up to 62 000 frames per second. Moreover, multichannel trajectory tracking and image-distortion correction functionalities are realized by TD-PSD systems through frequency-related image preprocessing, which significantly improves the capacity of real-time information processing and image quality in complicated light environments.

High output mode-locked laser empowered by defect regulation in 2D Bi2O2Se saturable absorber.

Atomically thin Bi2O2Se has emerged as a novel two-dimensional (2D) material with an ultrabroadband nonlinear optical response, high carrier mobility and excellent air stability, showing great potential for the realization of optical modulators. Here, we demonstrate a femtosecond solid-state laser at 1.0 µm with Bi2O2Se nanoplates as a saturable absorber (SA). Upon further defect regulation in 2D Bi2O2Se, the average power of the mode-locked laser is improved from 421 mW to 665 mW, while the pulse width is decreased from 587 fs to 266 fs. Moderate Ar+ plasma treatments are employed to precisely regulate the O and Se defect states in Bi2O2Se nanoplates. Nondegenerate pump-probe measurements show that defect engineering effectively accelerates the trapping rate and defect-assisted Auger recombination rate of photocarriers. The saturation intensity is improved from 3.6 ± 0.2 to 12.8 ± 0.6 MW cm-2 after the optimized defect regulation. The enhanced saturable absorption and ultrafast carrier lifetime endow the high-performance mode-locked laser with both large output power and short pulse duration.

Aggregation-Dependent Dielectric Permittivity in 2D Molecular Crystals.

The functionality of 2D molecular crystal-based devices crucially depends on their intrinsic properties, such as molecular energy levels, light absorption efficiency, and dielectric permittivity, which are highly sensitive to molecular aggregation. Here, it is demonstrated that the dielectric permittivity of the 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8 -BTBT) molecular crystals on monolayer WS2 substrates can be tuned from 4.62 in the wetting layer to 2.25 in the second layer. Its origin lies in the different molecular orientations in the wetting layer (lying-down) and in the subsequently stacked layers (standing-up), which lead to a positive Coulomb coupling (JCoup ) value (H-aggregation) and a negative JCoup value (J-aggregation), respectively. Polarized optical contrast spectroscopy reveals that the permittivity of C8 -BTBT is anisotropic, and its direction is related to the underlying substrate. The study offers guidelines for future manipulation of the permittivity of 2D molecular crystals, which may promote their applications toward various electronic and optoelectronic devices.

Potassium Iodide Doping Strategy for High-Efficiency Perovskite Solar Cells Revealed by Ultrafast Spectroscopy.

Organic-inorganic halide perovskites are promising materials for high-performance photovoltaics. The doping strategy is considered to be an effective method for regulating the performance of perovskite solar cells, yet its efficiency is still far below what has been anticipated. Here, we systematically investigate the regulatory mechanisms of the performance of perovskites by exploiting potassium iodide (KI) doping. We find that the surface states are passivated apart from the modified lattice structure. Most importantly, carrier recombination and transport are regulated by varying two different trap states when doping KI. The corresponding defect penalty can be effectively restrained at an optimal concentration of added KI (5%). A significant increase in the conductivity and radiative efficiency is achieved under such conditions. Our results provide fundamental insights into defect engineering through doping and a promising route toward highly efficient perovskite solar cells.

Spectroscopic Perception of Trap States on the Performance of Methylammonium and Formamidinium Lead Iodide Perovskite Solar Cells.

To enhance the efficiency and stability of the organic-inorganic hybrid perovskite (OIHP) solar cells, doping has been demonstrated as a straightforward method. Nevertheless, the perception of trap states regulated by doping and their effects on the performance of solar cells is not in-depth. Herein, typical OIHPs (CH3 NH3 PbI3 and Cs0.05 FA0.85 MA0.10 Pb(I0.97 Br0.03 )3 ) doped with RbI are employed to expound the doping mechanism in affecting the efficiency of devices. Systematic spectroscopic characterizations indicate that doping significantly influences the photocarrier dynamics via directly regulating the trap states. The results indicate that doping would reduce the trap density by passivating defects and induce extra trapping centers. This directly manipulates the transient transport of the photocarriers and finally influences the output of devices. The optimization of solar cell performance requires the tradeoff of competitive relation between the passivation and introduction of trapping centers. The results provide the spectroscopic perception on how doping concentration affects trap density, carrier dynamics, transport behavior, and ultimately the parameters of devices. It provides a straightforward guidance to the design and optimization of OIHP-based solar cells.

Multispectral photodetectors based on 2D material/Cs3Bi2I9heterostructures with high detectivity.

Group VA metal halide-based perovskites have emerged as intensively explored Pb-free perovskites, owing to their excellent environmental stability and low-toxicity. However, the relatively low carrier mobility and high photocarrier recombination rates restrict their applications in photodetectors. One promising approach to achieve higher performance is to integrate these Pb-free perovskites with 2D materials to form heterostructures. Here, we report on the high sensitivity photodetectors based on MoS2/Cs3Bi2I9and graphene/Cs3Bi2I9heterostructures for multispectral regions. The heterostructures combine the high carrier mobility of 2D materials with superior light-harvesting properties of perovskites, as well as the effective built-in electric filed at the junction area, leading to efficient photocarrier separation and extraction. The specific detectivity of MoS2/Cs3Bi2I9device reaches 1.15 × 1013Jones for the detection of ultraviolet (UV) light of 325 nm, which is four orders of magnitude higher than UV detectors built on GaN. As a result of the efficient dark current suppression, the specific detectivity of graphene/Cs3Bi2I9photodetector can be promoted to 5.24 × 1011Jones, 1.33 × 1011Jones, and 1.12 × 1011Jones for the detection of 325 nm, 447 nm, and 532 nm light, respectively.

Photoluminescence enhancement at a high generation rate induced by exciton localization.

The photoluminescence (PL) efficiency of two-dimensional (2D) transition metal dichalcogenides (TMDs) is extremely low under high power excitation, limiting its potential in display and light-emission application. This arises from the much shorter lifetime of non-radiative recombination than radiative recombination, wherein photo-carriers tend to decay through non-radiative processes. Herein, a "molecular state" near the valence band is successfully introduced into the ${{\rm MoS}_2}$ monolayer to increase the density of radiative states and speed up the exciton relaxation. This reduces the recombination lifetime of excitons by two orders of magnitude and forms vigorous competition with non-radiative decays. As a consequence, dozens of times enhancement of PL in ${{\rm MoS}_2}$ monolayers under high excitation power (${\rm G}\sim{{10}^{19}}\;{{\rm cm}^{- 2}}\cdot{{\rm S}^{- 1}}$) is realized. These results provide an effective method to improve PL efficiency under high injection levels for applications of 2D materials in light-emission industry.

Defect-related dynamics of photoexcited carriers in 2D transition metal dichalcogenides.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit enormous potential in the field of optoelectronics. The high performance of TMD materials and optoelectronic devices significantly depends on processes involved in photoelectric conversion, including photo-excitation, relaxation, transportation, and recombination. Remarkably, inevitable defects in materials prolong or shorten the characteristic time of these processes and even bring about new photoelectric conversion channels, namely, the defect-related relaxation pathways of photoexcited carriers tailor the performance of photoelectric applications. In recent years, there have been numerous investigations in exploring the variant transient signals caused by defects in TMDs utilizing ultrafast spectroscopies. They have the capability in providing an accurate and overall representation of ultrafast processes owing to the subtle temporal resolution. The defect-related mechanisms occurring in different time scales (from femtosecond (fs) to microsecond (µs)) play influential roles throughout the relaxation process of photoexcited species. Herein, we review the defect-related relaxation mechanisms of photoexcited species in TMDs according to the time scale utilizing ultrafast spectroscopy techniques. By interpreting and summarizing the defect-related transient signals, we furnish the direction in material design and performance optimization.

Electrochemically Exfoliated Platinum Dichalcogenide Atomic Layers for High-Performance Air-Stable Infrared Photodetectors.

Platinum dichalcogenide (PtX2), an emergent group-10 transition metal dichalcogenide (TMD) has shown great potential in infrared photonic and optoelectronic applications due to its layer-dependent electronic structure with potentially suitable bandgap. However, a scalable synthesis of PtSe2 and PtTe2 atomic layers with controlled thickness still represents a major challenge in this field because of the strong interlayer interactions. Herein, we develop a facile cathodic exfoliation approach for the synthesis of solution-processable high-quality PtSe2 and PtTe2 atomic layers for high-performance infrared (IR) photodetection. As-exfoliated PtSe2 and PtTe2 bilayer exhibit an excellent photoresponsivity of 72 and 1620 mA W-1 at zero gate voltage under a 1540 nm laser illumination, respectively, approximately several orders of magnitude higher than that of the majority of IR photodetectors based on graphene, TMDs, and black phosphorus. In addition, our PtSe2 and PtTe2 bilayer device also shows a decent specific detectivity of beyond 109 Jones with remarkable air-stability (>several months), outperforming the mechanically exfoliated counterparts under the laser illumination with a similar wavelength. Moreover, a high yield of PtSe2 and PtTe2 atomic layers dispersed in solution also allows for a facile fabrication of air-stable wafer-scale IR photodetector. This work demonstrates a new route for the synthesis of solution-processable layered materials with the narrow bandgap for the infrared optoelectronic applications.

Gate-Tunable Polar Optical Phonon to Piezoelectric Scattering in Few-Layer Bi2 O2 Se for High-Performance Thermoelectrics.

Atomically thin Bi2 O2 Se has emerged as a new member in 2D materials with ultrahigh carrier mobility and excellent air-stability, showing great potential for electronics and optoelectronics. In addition, its ferroelectric nature renders an ultralow thermal conductivity, making it a perfect candidate for thermoelectrics. In this work, the thermoelectric performance of 2D Bi2 O2 Se is investigated over a wide temperature range (20-300 K). A gate-tunable transition from polar optical phonon (POP) scattering to piezoelectric scattering is observed, which facilitates the capacity of drastic mobility engineering in 2D Bi2 O2 Se. Consequently, a high power factor of more than 400 µW m-1  K-2 over an unprecedented temperature range (80-200 K) is achieved, corresponding to the persistently high mobility arising from the highly gate-tunable scattering mechanism. This finding provides a new avenue for maximizing thermoelectric performance by changing the scattering mechanism and carrier mobility over a wide temperature range.