Universal p-Type Doping via Lewis Acid for 2D Transition-Metal Dichalcogenides

Electrical Polarity Modulation in V-Doped Monolayer WS2 for Homogeneous CMOS Inverters

As demand for higher integration density and smaller devices grows, silicon-based complementary metal-oxide-semiconductor (CMOS) devices will soon reach their ultimate limits. 2D transition metal dichalcogenides (TMDs) semiconductors, known for excellent electrical performance and stable atomic structure, are seen as promising materials for future integrated circuits. However, controlled and reliable doping of 2D TMDs, a key step for creating homogeneous CMOS logic components, remains a challenge. In this study, a continuous electrical polarity modulation of monolayer WS2 from intrinsic n-type to ambipolar, then to p-type, and ultimately to a quasi-metallic state is achieved simply by introducing controllable amounts of vanadium (V) atoms into the WS2 lattice as p-type dopants during chemical vapor deposition (CVD). The achievement of purely p-type field-effect transistors (FETs) is particularly noteworthy based on the 4.7 at% V-doped monolayer WS2, demonstrating a remarkable on/off current ratio of 105. Expanding on this triumph, the first initial prototype of ultrathin homogeneous CMOS inverters based on monolayer WS2 is being constructed. These outcomes validate the feasibility of constructing homogeneous CMOS devices through the atomic doping process of 2D materials, marking a significant milestone for the future development of integrated circuits.

Spatially selective p-type doping for constructing lateral WS2 p-n homojunction via low-energy nitrogen ion implantation

The construction of lateral p-n junctions is very important and challenging in two-dimensional (2D) semiconductor manufacturing process. Previous researches have demonstrated that vertical p-n junction can be prepared simply by vertical stacking of 2D materials. However, interface pollution and large area scalability are challenges that are difficult to overcome with vertical stacking technology. Constructing 2D lateral p-n homojunction is an effective strategy to address these issues. Spatially selective p-type doping of 2D semiconductors is expected to construct lateral p-n homojunction. In this work, we have developed a low-energy ion implantation system that reduces the implanted energy to 300 eV. Low-energy implantation can form a shallow implantation depth, which is more suitable for modulating the electrical and optical properties of 2D materials. Hence, we utilize low-energy ion implantation to directly dope nitrogen ions into few-layer WS2 and successfully realize a precise regulation for WS2 with its conductivity type transforming from n-type to bipolar or even p-type conduction. Furthermore, the universality of this method is demonstrated by extending it to other 2D semiconductors, including WSe2, SnS2 and MoS2. Based on this method, a lateral WS2 p-n homojunction is fabricated, which exhibits significant rectification characteristics. A photodetector based on p-n junction with photovoltaic effect is also prepared, and the open circuit voltage can reach to 0.39 V. This work provides an effective way for controllable doping of 2D semiconductors.

Chemical passivation of 2D transition metal dichalcogenides: strategies, mechanisms, and prospects for optoelectronic applications

The interest in obtaining high-quality monolayer transition metal dichalcogenides (TMDs) for optoelectronic device applications has been growing dramatically. However, the prevalence of defects and unwanted doping in these materials remain challenges, as they both limit optical properties and device performance. Surface chemical treatments of monolayer TMDs have been effective in improving their photoluminescence yield and charge transport properties. In this scenario, a systematic understanding of the underlying mechanism of chemical treatments will lead to a rational design of passivation strategies in future research, ultimately taking a step toward practical optoelectronic applications. We will therefore describe in this mini-review the strategies, progress, mechanisms, and prospects of chemical treatments to passivate and improve the optoelectronic properties of TMDs.

Solution-Processable and Printable Two-Dimensional Transition Metal Dichalcogenide Inks

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with layered crystal structures have been attracting enormous research interest for their atomic thickness, mechanical flexibility, and excellent electronic/optoelectronic properties for applications in diverse technological areas. Solution-processable 2D TMD inks are promising for large-scale production of functional thin films at an affordable cost, using high-throughput solution-based processing techniques such as printing and roll-to-roll fabrications. This paper provides a comprehensive review of the chemical synthesis of solution-processable and printable 2D TMD ink materials and the subsequent assembly into thin films for diverse applications. We start with the chemical principles and protocols of various synthesis methods for 2D TMD nanosheet crystals in the solution phase. The solution-based techniques for depositing ink materials into solid-state thin films are discussed. Then, we review the applications of these solution-processable thin films in diverse technological areas including electronics, optoelectronics, and others. To conclude, a summary of the key scientific/technical challenges and future research opportunities of solution-processable TMD inks is provided.

Defect Engineering of 2D Semiconductors for Dual Control of Emission and Carrier Polarity

2D transition metal dichalcogenides (TMDCs) are considered as promising materials in post-Moore technology. However, the low photoluminescence quantum yields (PLQY) and single carrier polarity due to the inevitable defects during material preparation are great obstacles to their practical applications. Here, an extraordinary defect engineering strategy is reported based on first-principles calculations and realize it experimentally on WS2 monolayers by doping with IIIA atoms. The doped samples with large sizes possess both giant PLQY enhancement and effective carrier polarity modulation. Surprisingly, the high PL emission maintained even after one year under ambient environment. Moreover, the constructed p-n homojunctions shows high rectification ratio (≈2200), ultrafast response times and excellent stability. Meanwhile, the doping strategy is universally applicable to other TMDCs and dopants. This smart defect engineering strategy not only provides a general scheme to eliminate the negative influence of defects, but also utilize them to achieve desired optoelectronic properties for multifunctional applications.

Two-Dimensional Semiconductors and Transistors for Future Integrated Circuits

Silicon transistors are approaching their physical limit, calling for the emergence of a technological revolution. As the acknowledged ultimate version of transistor channels, 2D semiconductors are of interest for the development of post-Moore electronics due to their useful properties and all-in-one potentials. Here, the promise and current status of 2D semiconductors and transistors are reviewed, from materials and devices to integrated applications. First, we outline the evolution and challenges of silicon-based integrated circuits, followed by a detailed discussion on the properties and preparation strategies of 2D semiconductors and van der Waals heterostructures. Subsequently, the significant progress of 2D transistors, including device optimization, large-scale integration, and unconventional devices, are presented. We also examine 2D semiconductors for advanced heterogeneous and multifunctional integration beyond CMOS. Finally, the key technical challenges and potential strategies for 2D transistors and integrated circuits are also discussed. We envision that the field of 2D semiconductors and transistors could yield substantial progress in the upcoming years and hope this review will trigger the interest of scientists planning their next experiment.

Wavelength-Controlled Photoconductance Polarity Switching via Harnessing Defects in Doped PdSe2 for Artificial Synaptic Features

Optoelectronic synapses are currently drawing significant attention as fundamental building blocks of neuromorphic computing to mimic brain functions. In this study, a two-terminal synaptic device based on a doped PdSe2 flake is proposed to imitate the key neural functions in an optical pathway. Due to the wavelength-dependent desorption of oxygen clusters near the intrinsic selenide vacancy defects, the doped PdSe2 photodetector achieves a high negative photoresponsivity of -7.8 × 103 A W-1 at 473 nm and a positive photoresponsivity of 181 A W-1 at 1064 nm. This wavelength-selective bi-direction photoresponse endows an all-optical pathway to imitate the fundamental functions of artificial synapses on a device level, such as psychological learning and forgetting capability, as well as dynamic logic functions. The underpinning photoresponse is further demonstrated on a flexible platform, providing a viable technology for neuromorphic computing in wearable electronics. Furthermore, the p-type doping results in an effective increase of the channel's electrical conductivity and a significant reduction in power consumption. Such low-power-consuming optical synapses with simple device architecture and low-dimensional features demonstrate tremendous promise for building multifunctional artificial neuromorphic systems in the future.

Chemical Dopant-Free Controlled MoTe2/MoSe2 Heterostructure toward a Self-Driven Photodetector and Complementary Logic Circuits

Two-dimensional (2D) van der Waals heterostructures based on transition metal dichalcogenides are expected to be unique building blocks for next-generation nanoscale electronics and optoelectronics. The ability to control the properties of 2D heterostructures is the key for practical applications. Here, we report a simple way to fabricate a high-performance self-driven photodetector based on the MoTe₂/MoSe₂ p-n heterojunction, in which the hole-dominated transport polarity of MoTe₂ is easily achieved via a straightforward thermal annealing treatment in air without any chemical dopants or special gases needed. A high photoresponsivity of 0.72 A W^-1, an external quantum efficiency up to 41.3%, a detectivity of 7 × 10¹¹ Jones, and a response speed of 120 μs are obtained at zero bias voltage. Additionally, this doping method is also utilized to realize a complementary inverter with a voltage gain of 24. By configuring 2D p-MoTe₂ and n-MoSe₂ on demand, logic functions of NAND and NOR gates are also accomplished successfully. These results present a significant potential toward future larger-scale heterogeneously integrated 2D electronics and optoelectronics.

Chemical Vapor Deposition of Quaternary 2D BiCuSeO p-Type Semiconductor with Intrinsic Degeneracy

2D BiCuSeO is an intrinsic p-type degenerate semiconductor due to its self-doping effect, which possesses great potential to fabricate high-performance 2D-2D tunnel field-effect transistors (TFETs). However, the controllable synthesis of multinary 2D materials by chemical vapor deposition (CVD) is still a challenge due to the restriction of thermodynamics. Here, the CVD synthesis of quaternary 2D BiCuSeO nanosheets is realized. As-grown BiCuSeO nanosheets with thickness down to ≈6.1 nm (≈7 layers) and domain size of ≈277 µm show excellent ambient stability. Intrinsic p-type degeneracy of BiCuSeO, capable of maintaining even in a few layers, is comprehensively unveiled. By varying the thicknesses and temperatures, the carrier concentration of BiCuSeO nanosheets can be adjusted in the range of 10¹⁹ to 10²¹ cm^-3 , and the Hall mobility of BiCuSeO is ≈191 cm² V^-1 s^-1 (at 2 K). Furthermore, taking advantage of the p-type degeneracy of BiCuSeO, a prototypical BiCuSeO/MoS₂ TFET is fabricated. The emergence of the negative differential resistance trend and multifunctional diodes by modulating the gate voltage and temperature reveal the great practical implementation potential of BiCuSeO nanosheets. These results pave way for the CVD synthesis of multinary 2D materials and rational design of high-performance tunnel devices.

P-Type 2D Semiconductors for Future Electronics

2D semiconductors represent one of the best candidates to extend Moore's law for their superiorities, such as keeping high carrier mobility and remarkable gate-control capability at atomic thickness. Complementary transistors and van der Waals junctions are critical in realizing 2D semiconductors-based integrated circuits suitable for future electronics. N-type 2D semiconductors have been reported predominantly for the strong electron doping caused by interfacial charge impurities and internal structural defects. By contrast, superior and reliable p-type 2D semiconductors with holes as majority carriers are still scarce. Not only that, but some critical issues have not been adequately addressed, including their controlled synthesis in wafer size and high quality, defect and carrier modulation, optimization of interface and contact, and application in high-speed and low-power integrated devices. Here the material toolkit, synthesis strategies, device basics, and digital electronics closely related to p-type 2D semiconductors are reviewed. Their opportunities, challenges, and prospects for future electronic applications are also discussed, which would be promising or even shining in the post-Moore era.