P-type laser-doped WSe2/MoTe2 van der Waals heterostructure photodetector

Laser-assisted synthesis and modification of 2D materials

Two-dimensional (2D) materials with unique physical, electronic, and optical properties have been intensively studied to be utilized for the next-generation electronic and optical devices, and the use of laser energy in the synthesis and modification of 2D materials is advantageous due to its convenient and fast fabrication processes as well as selective, controllable, and cost-effective characteristics allowing the precise control in materials properties. This paper summarizes the recent progress in utilizations of laser technology in synthesizing, doping, etching, transfer and strain engineering of 2D materials, which is expected to provide an insight for the future applications across diverse research areas.

Performance Limits and Advancements in Single 2D Transition Metal Dichalcogenide Transistor

Two-dimensional (2D) transition metal dichalcogenides (TMDs) allow for atomic-scale manipulation, challenging the conventional limitations of semiconductor materials. This capability may overcome the short-channel effect, sparking significant advancements in electronic devices that utilize 2D TMDs. Exploring the dimension and performance limits of transistors based on 2D TMDs has gained substantial importance. This review provides a comprehensive investigation into these limits of the single 2D-TMD transistor. It delves into the impacts of miniaturization, including the reduction of channel length, gate length, source/drain contact length, and dielectric thickness on transistor operation and performance. In addition, this review provides a detailed analysis of performance parameters such as source/drain contact resistance, subthreshold swing, hysteresis loop, carrier mobility, on/off ratio, and the development of p-type and single logic transistors. This review details the two logical expressions of the single 2D-TMD logic transistor, including current and voltage. It also emphasizes the role of 2D TMD-based transistors as memory devices, focusing on enhancing memory operation speed, endurance, data retention, and extinction ratio, as well as reducing energy consumption in memory devices functioning as artificial synapses. This review demonstrates the two calculating methods for dynamic energy consumption of 2D synaptic devices. This review not only summarizes the current state of the art in this field but also highlights potential future research directions and applications. It underscores the anticipated challenges, opportunities, and potential solutions in navigating the dimension and performance boundaries of 2D transistors.

Atomistic Probing of Defect-Engineered 2H-MoTe2 Monolayers

Point defects dictate various physical, chemical, and optoelectronic properties of two-dimensional (2D) materials, and therefore, a rudimentary understanding of the formation and spatial distribution of point defects is a key to advancement in 2D material-based nanotechnology. In this work, we performed the demonstration to directly probe the point defects in 2H-MoTe2 monolayers that are tactically exposed to (i) 200 °C-vacuum-annealing and (ii) 532 nm-laser-illumination; and accordingly, we utilize a deep learning algorithm to classify and quantify the generated point defects. We discovered that tellurium-related defects are mainly generated in both 2H-MoTe2 samples; but interestingly, 200 °C-vacuum-annealing and 532 nm-laser-illumination modulate a strong n-type and strong p-type 2H-MoTe2, respectively. While 200 °C-vacuum-annealing generates tellurium vacancies or tellurium adatoms, 532 nm-laser-illumination prompts oxygen atoms to be adsorbed/chemisorbed at tellurium vacancies, giving rise to the p-type characteristic. This work significantly advances the current understanding of point defect engineering in 2H-MoTe2 monolayers and other 2D materials, which is critical for developing nanoscale devices with desired functionality.

Emerging Two-Dimensional Tellurene and Tellurides for Broadband Photodetectors

As with all stylish 2D functional materials, tellurene and tellurides possessing excellent physical and chemical properties such as high environmental stability, tunable narrow bandgap, and lower thermal conductivity, have aroused the great interest of the researchers. These properties of such materials also form the basis for relatively newfangled scholarly fields involving advanced topics, especially for broadband photodetectors. Integrating the excellent properties of many 2D materials, tellurene/telluride-based photodetectors show great flexibility, higher frequency response or faster time response, high signal-to-noise ratio, and so on, which make them leading the frontier of photodetector research. To fully understand the excellent properties of tellurene/tellurides and their optoelectronic applications, the recent advances in tellurene/telluride-based photodetectors are maximally summarized. Benefiting from the solid research in this field, the challenges and opportunities of tellurene/tellurides for future optoelectronic applications are also discussed in this review, which might provide possibilities for the realization of state-of-the-art high-performance tellurene/telluride-based devices.

Atomistic Observation of the Local Phase Transition in MoTe2 for Application in Homojunction Photodetectors

Direct atomic-scale observation of the local phase transition in transition metal dichalcogenides (TMDCs) is critically required to carry out in-depth studies of their atomic structures and electronic features. However, the structural aspects including crystal symmetries tend to be unclear and unintuitive in real-time monitoring of the phase transition process. Herein, by using in situ transmission electron microscopy, information about the phase transition mechanism of MoTe₂ from hexagonal structure (2H phase) to monoclinic structure (1T' phase) driven by sublimation of Te atoms after a spike annealing is obtained directly. Furthermore, with the control of Te atom sublimation by modulating the hexagonal boron nitride (h-BN) coverage in the desired area, the lateral 1T'-enriched MoTe₂ /2H MoTe₂ homojunction can be one-step constructed via an annealing treatment. Owing to the gradient bandgap provided by 1T'-enriched MoTe₂ and 2H MoTe₂ , the photodetector composed of the 1T'-enriched MoTe₂ /2H MoTe₂ homojunction shows fast photoresponse and ten times larger photocurrents than that consisting of a pure 2H MoTe₂ channel. The study reveals a route to improve the performance of optoelectronic and electronic devices based on TMDCs with both semiconducting and semimetallic phases.

Recent Advances on Tuning the Interlayer Coupling and Properties in van der Waals Heterostructures

2D van der Waals (vdW) heterostructures are receiving increasing research attention due to the theoretically amazing properties and unprecedented application potential. However, the as-synthesized heterostructures are generally underperforming due to the weak interlayer coupling, which inspires the researchers to find ways to modulate the interlayer coupling and properties, realizing the tailored performance for actual applications. There have been a lot of publications regarding the controllable regulation of the structures and properties of 2D vdW heterostructures in the past few years, while a review work summarizing the current advances is not yet available, though it is significant. This paper conducts a state-of-the-art review regarding the current research progress of performance modulation of vdW heterostructures by different techniques. First, the general synthesis methods of vdW heterostructures are summarized. Then, different performance modulation techniques, that is, mechanical-based, external fields-assisted, and particle beam irradiation-based methods, are discussed and compared in detail. Some of the newly proposed concepts are described. Thereafter, applications of vdW heterostructures with tailored properties are reviewed for the application prospects of the topic around this area. Moreover, the future research challenges and prospects are discussed, aiming at triggering more research interest and device applications around this topic.

Polarity-Tunable Photocurrent through Band Alignment Engineering in a High-Speed WSe2/SnSe2 Diode with Large Negative Responsivity

Excellent light-matter interaction and a wide range of thickness-tunable bandgaps in layered vdW materials coupled by the facile fabrication of heterostructures have enabled several avenues for optoelectronic applications. Realization of high photoresponsivity at fast switching speeds is a critical challenge for 2D optoelectronics to enable high-performance photodetection for optical communication. Moving away from conventional type-II heterostructure pn junctions towards a WSe₂/SnSe₂ type-III configuration, we leverage the steep change in tunneling current along with a light-induced heterointerface band shift to achieve high negative photoresponsivity, while the fast carrier transport under tunneling results in high speed. In addition, the photocurrent can be controllably switched from positive to negative values, with ∼10⁴× enhancement in responsivity, by engineering the band alignment from type-II to type-III using either the drain or the gate bias. This is further reinforced by electric-field dependent interlayer band structure calculations using density functional theory. The high negative responsivity of 2 × 10⁴ A/W and fast response time of ∼1 μs coupled with a polarity-tunable photocurrent can lead to the development of next-generation multifunctional optoelectronic devices.

Artificially created interfacial states enabled van der Waals heterostructure memory device

Two-dimensional (2D) interface plays a predominate role in determining the performance of a device that is configured as a van der Waals heterostructure (vdWH). Intensive efforts have been devoted to suppressing the emergence of interfacial states during vdWH stacking process, which facilitates the charge interaction and transfer between the heterostructure layers. However, the effective generation and modulation of the vdWH interfacial states could give rise to a new design and architecture of 2D functional devices. Here, we report a 2D non-volatile vdWH memory device enabled by the artificially created interfacial states between hexagonal boron nitride (hBN) and molybdenum ditelluride (MoTe₂). The memory originates from the microscopically coupled optical and electrical responses of the vdWH, with the high reliability reflected by its long data retention time over 10⁴s and large write-erase cyclic number exceeding 100. Moreover, the storage currents in the memory can be precisely controlled by the writing and erasing gates, demonstrating the tunability of its storage states. The vdWH memory also exhibits excellent robustness with wide temperature endurance window from 100 K to 380 K, illustrating its potential application in harsh environment. Our findings promise interfacial-states engineering as a powerful approach to realize high performance vdWH memory device, which opens up new opportunities for its application in 2D electronics and optoelectronics.