Environment-insensitive and gate-controllable photocurrent enabled by bandgap engineering of MoS2 junctions

Single-Gate MoS2 Tunnel FET with a Thickness-Modulated Homojunction

Two-dimensional (2D) materials stand as a promising platform for tunnel field-effect transistors (TFETs) in the pursuit of low-power electronics for the Internet of Things era. This promise arises from their dangling bond-free van der Waals heterointerface. Nevertheless, the attainment of high device performance is markedly impeded by the requirement of precise control over the 2D assembly with multiple stacks of different layers. In this study, we addressed a thickness-modulated n/p+-homojunction prepared from Nb-doped p+-MoS2 crystal, where the issue on interface traps can be neglected without any external interface control due to the homojunction. Notably, our observations reveal the existence of a negative differential resistance, even at room temperature (RT). This signifies the successful realization of TFET operation under type III band alignment conditions by a single gate at RT, suggesting that the dominant current mechanism is band-to-band tunneling due to the ideal interface.

Retinomorphic optoelectronic devices for intelligent machine vision

Biological visual system can efficiently handle optical information within the retina and visual cortex of the brain, which suggests an alternative approach for the upgrading of the current low-intelligence, large energy consumption, and complex circuitry of the artificial vision system for high-performance edge computing applications. In recent years, retinomorphic machine vision based on the integration of optoelectronic image sensors and processors has been regarded as a promising candidate to improve this phenomenon. This novel intelligent machine vision technology can perform information preprocessing near or even within the sensor in the front end, thereby reducing the transmission of redundant raw data and improving the efficiency of the back-end processor for high-level computing tasks. In this contribution, we try to present a comprehensive review on the recent progress achieved in this emergent field.

Visualizing Band Profiles of Gate-Tunable Junctions in MoS2/WSe2 Heterostructure Transistors

Heterostructure devices based on two-dimensional materials have been under intensive study due to their intriguing electrical and optical properties. One key factor in understanding these devices is their nanometer-scale band profiles, which is challenging to obtain in devices. Here, we use a technique named contact-mode scanning tunneling spectroscopy to directly visualize the band profiles of MoS₂/WSe₂ heterostructure devices at different gate voltages with nanometer resolution. The long-held view of a conventional p-n junction in the MoS₂/WSe₂ heterostructure is reexamined. Due to strong inter- and intralayer charge transfer, the MoS₂ layer in contact with WSe₂ is found to convert from n-type to p-type, and a series of gate-tunable p-n and p-p⁺ junctions are developed in the devices. Highly conductive edges are also discovered which could strongly affect the device properties.

Acoustically Driven Stark Effect in Transition Metal Dichalcogenide Monolayers

The Stark effect is one of the most efficient mechanisms to manipulate many-body states in nanostructured systems. In mono- and few-layer transition metal dichalcogenides, it has been successfully induced by optical and electric field means. Here, we tune the optical emission energies and dissociate excitonic states in MoSe₂ monolayers employing the 220 MHz in-plane piezoelectric field carried by surface acoustic waves. We transfer the monolayers to high dielectric constant piezoelectric substrates, where the neutral exciton binding energy is reduced, allowing us to efficiently quench (above 90%) and red-shift the excitonic optical emissions. A model for the acoustically induced Stark effect yields neutral exciton and trion in-plane polarizabilities of 530 and 630 × 10^-5 meV/(kV/cm)², respectively, which are considerably larger than those reported for monolayers encapsulated in hexagonal boron nitride. Large in-plane polarizabilities are an attractive ingredient to manipulate and modulate multiexciton interactions in two-dimensional semiconductor nanostructures for optoelectronic applications.

Aggregation induced non-emissive-to-emissive switching of molecular platinum clusters

We show here for the first time the Aggregation Induced Emission (AIE) mechanism and solvatochromic impact on Pt-SG (SG-deprotonated glutathione) nanoclusters. In this work, the AIE properties of Pt-SG clusters were investigated through computational and spectroscopic investigations. Computational data established that aggregation triggers a distinct change in the frontier molecular orbitals (FMOs) from metal d-orbital centered FMOs in the monomer to metal-thiolate and thiolate centered FMOs in the dimer improving the radiative decay process. Solvent dependent photoluminescence studies proved that a Lewis-acidic environment can significantly perturb the metal-thiolate and thiolate centered FMOs that are involved in the electronic transitions as predicted by our computational work. These semiconducting clusters exhibit a large Stokes shift and zero spectral overlap between absorption and emission which makes this Pt-SG cluster an excellent material for solar concentrators and solid-state light emitters. This AIE-OFF-ON emission was utilized to delineate a proof-of-concept sensor device that is sensitive to temperature and an acid/base.

All MoS2-Based Large Area, Skin-Attachable Active-Matrix Tactile Sensor

Large-area, ultrathin flexible tactile sensors with conformal adherence are becoming crucial for advances in wearable electronics, electronic skins and biorobotics. However, normal passive tactile sensors suffer from high crosstalk, resulting in inaccurate sensing, which consequently limits their use in such advanced applications. Active-matrix-driven tactile sensors could potentially overcome such hurdles, but it demands the high performance and reliable operations of the thin-film-transistor array that could efficiently control integrated pressure gauges. Herein, we utilized the benefit of the semiconducting and mechanical excellence of MoS₂ and placed it between high- k Al₂O₃ dielectric sandwich layers to achieve the high and reliable performance of MoS₂-based back-plane circuitry and strain sensor. This strategical combination reduces the fabrication complexity and enables the demonstration of an all MoS₂-based large area (8 × 8 array) active-matrix tactile sensor offering a wide sensing range (1-120 kPa), sensitivity value (Δ R/ R₀: 0.011 kPa^-1), and a response time (180 ms) with excellent linearity. In addition, it showed potential in sensing multitouch accurately, tracking a stylus trajectory, and detecting the shape of an external object by grasping it using the palm of the human hand.

Flexible Vertical p-n Diode Photodetectors with Thin N-type MoSe2 Films Solution-Processed on Water Surfaces

Two-dimensional (2D) nanosheets of transition metal dichalcogenides (TMDs) are of significant interest for potential photoelectronic applications. However, the fabrication of solution-processed arrays of mechanically flexible thin TMD films-based vertical type p-n junction photodetectors over a large area is a great challenge. Our method is based on controlled solvent evaporation of MoSe₂ suspension spread on water surface. Single or few-layered MoSe₂ nanosheets modified with the dispersant amine-terminated poly(styrene) (PS-NH₂) were homogeneously deposited and stacked on water upon solvent evaporation, giving rise to uniform MoSe₂/PS-NH₂ composite films that can be readily transferred onto other substrates. A p-n junction vertical diode of Al/p-type Si/p-type poly(9,9-di- n-octylfluorenyl-2,7-diyl)/n-type MoSe₂ composite/Au stacked from bottom to top exhibited characteristic rectifying current behavior upon voltage sweep with a rectification ratio of 10³. Subsequent illumination of near-infrared light on the device resulted in a substantially enhanced dark current of approximately 10³ times greater than that of the nonexposed device. The photodetection performance, that is, switching time, responsivity, and detectivity, were 100.0 ms, 2.5 AW^-1, and 2.34 × 10¹⁴, respectively. Furthermore, the performance of mechanically flexible photodetectors devices was comparable with that of the devices fabricated on the hard Si substrate even after 1000 bending cycles at a bending diameter of 7.2 mm.

Hyaluronan, Cancer-Associated Fibroblasts and the Tumor Microenvironment in Malignant Progression

This review summarizes the roles of CAFs in forming a “cancerized” fibrotic stroma favorable to tumor initiation and dissemination, in particular highlighting the functions of the extracellular matrix component hyaluronan (HA) in these processes. The structural complexity of the tumor and its host microenvironment is now well appreciated to be an important contributing factor to malignant progression and resistance-to-therapy. There are multiple components of this complexity, which include an extensive remodeling of the extracellular matrix (ECM) and associated biomechanical changes in tumor stroma. Tumor stroma is often fibrotic and rich in fibrillar type I collagen and hyaluronan (HA). Cancer-associated fibroblasts (CAFs) are a major source of this fibrotic ECM. CAFs organize collagen fibrils and these biomechanical alterations provide highways for invading carcinoma cells either under the guidance of CAFs or following their epithelial to mesenchymal transition (EMT). The increased HA metabolism of a tumor microenvironment instructs carcinoma initiation and dissemination by performing multiple functions. The key effects of HA reviewed here are its role in activating CAFs in pre-malignant and malignant stroma, and facilitating invasion by promoting motility of both CAFs and tumor cells, thus facilitating their invasion. Circulating CAFs (cCAFs) also form heterotypic clusters with circulating tumor cells (CTC), which are considered to be pre-cursors of metastatic colonies. cCAFs are likely required for extravasation of tumors cells and to form a metastatic niche suitable for new tumor colony growth. Therapeutic interventions designed to target both HA and CAFs in order to limit tumor spread and increase response to current therapies are discussed.

Role of Hole Trap Sites in MoS2 for Inconsistency in Optical and Electrical Phenomena

Because of strong Coulomb interaction in two-dimensional van der Waals-layered materials, the trap charges at the interface strongly influence the scattering of the majority carriers and thus often degrade their electrical properties. However, the photogenerated minority carriers can be trapped at the interface, modulate the electron-hole recombination, and eventually influence the optical properties. In this study, we report the role of the hole trap sites on the inconsistency in the electrical and optical phenomena between two systems with different interfacial trap densities, which are monolayer MoS₂-based field-effect transistors (FETs) on hexagonal boron nitride (h-BN) and SiO₂ substrates. Electronic transport measurements indicate that the use of h-BN as a gate insulator can induce a higher n-doping concentration of the monolayer MoS₂ by suppressing the free-electron transfer from the intrinsically n-doped MoS₂ to the SiO₂ gate insulator. Nevertheless, optical measurements show that the electron concentration in MoS₂/SiO₂ is heavier than that in MoS₂/h-BN, manifested by the relative red shift of the A_1g Raman peak. The inconsistency in the evaluation of the electron concentration in MoS₂ by electrical and optical measurements is explained by the trapping of the photogenerated holes in the spatially modulated valence band edge of the monolayer MoS₂ caused by the local strain from the SiO₂/Si substrate. This photoinduced electron doping in MoS₂/SiO₂ is further confirmed by the development of the trion component in the power-dependent photoluminescence spectra and negative shift of the threshold voltage of the FET after illumination.