Xingxing Zhang

Recent progress on polaritons in low‐dimensional materials, particularly the scaling and reflection behaviors of polaritons, is reviewed. On the one hand, the strongly confined polaritons exhibit a large tunability of wavelength following different scaling behaviors in different systems. On the other hand, they show novel reflection behaviors when they encounter topographic boundaries or electronic discontinuities.

Abstract

Polaritons in low‐dimensional materials have shown their unique capabilities to concentrate light at the deep subwavelength scale. They behave quite differently from those in conventional metals. On the one hand, the strongly confined polaritons exhibit a large tunability in wavelength following different scaling behaviors in different systems. On the other hand, they show novel reflection behaviors when they encounter topographic boundaries or electronic discontinuities. Here, recent progress on the scaling and reflection behaviors of strongly confined polaritons in three representative low‐dimensional material systems is reviewed. It is shown that the polariton wavelength changes sensitively with carrier density, thickness, and the number of conducting channels for graphene, hexagonal boron nitride, and carbon nanotubes. Also, it is demonstrated how polaritons reflect in low‐dimensional materials when encountering edges, corrugations, domain walls, strain regions, etc.

Interfacial charge transfer is a crucial process in photoelectric conversion. Plasmon‐induced hot‐electron transfer is demonstrated as a sufficiently fast charge‐transfer process to realize high‐speed photoelectric conversion. A near‐infrared photodetector with fast detection speed and extended spectral response to the communication band (1550 nm) is achieved built on a tungsten suboxide nanocrystal arrays–graphene heterostructure.

Abstract

Interfacial charge transfer is a fundamental and crucial process in photoelectric conversion. If charge transfer is not fast enough, carrier harvesting can compromise with competitive relaxation pathways, e.g., cooling, trapping, and recombination. Some of these processes can strongly affect the speed and efficiency of photoelectric conversion. In this work, it is elaborated that plasmon‐induced hot‐electron transfer (HET) from tungsten suboxide to graphene is a sufficiently fast process to prevent carrier cooling and trapping processes. A fast near‐infrared detector empowered by HET is demonstrated, and the response time is three orders of magnitude faster than that based on common band‐edge electron transfer. Moreover, HET can overcome the spectral limit of the bandgap of tungsten suboxide (≈2.8 eV) to extent the photoresponse to the communication band of 1550 nm (≈0.8 eV). These results indicate that plasmon‐induced HET is a new strategy for implementation of efficient and high‐speed photoelectric devices.

A new approach to clean the surface of graphene is reported by using a force‐engineered “lint roller”, which is enabled by selectively removing intrinsic surface contaminants on graphene. The as‐obtained superclean graphene can be transferred to dielectric substrates with significantly reduced polymer residues and exhibits superior electronic and optical properties such as ultrahigh carrier mobility and low contact resistance.

Abstract

Contamination is a major concern in surface and interface technologies. Given that graphene is a 2D monolayer material with an extremely large surface area, surface contamination may seriously degrade its intrinsic properties and strongly hinder its applicability in surface and interfacial regions. However, large‐scale and facile treatment methods for producing clean graphene films that preserve its excellent properties have not yet been achieved. Herein, an efficient postgrowth treatment method for selectively removing surface contamination to achieve a large‐area superclean graphene surface is reported. The as‐obtained superclean graphene, with surface cleanness exceeding 99%, can be transferred to dielectric substrates with significantly reduced polymer residues, yielding ultrahigh carrier mobility of 500 000 cm² V⁻¹ s⁻¹ and low contact resistance of 118 Ω µm. The successful removal of contamination is enabled by the strong adhesive force of the activated‐carbon‐based lint roller on graphene contaminants.

In article number https://doi.org/10.1002/adma.2019010771901077, Lih‐Juann Chen, Yi‐Hsien Lee, and co‐workers demonstrate ultraclean transfer of synthesized monolayers for artificially stacked monolayers with tunable twisting and a heterointerface. Diverse Moiré electronic superlattices are directly visualized by scanning tunneling microscopy, which opens a new avenue toward correlated properties in artificial 2D lattices.

A WSe₂/MoS₂‐based p–n heterostructure array is realized by a solution‐based direct growth method. WSe₂ wires are selectively stacked over the MoS₂ wires at the desired angle to form parallel‐ or cross‐aligned heterostructures over a large area. The p–n heterojunction array has a clean interface, resulting in outstanding rectification. Additionally, a prototype photosensing device with good photoresponsivity and response time is demonstrated.

Abstract

Functional van der Waals heterojunctions of transition metal dichalcogenides are emerging as a potential candidate for the basis of next‐generation logic devices and optoelectronics. However, the complexity of synthesis processes so far has delayed the successful integration of the heterostructure device array within a large scale, which is necessary for practical applications. Here, a direct synthesis method is introduced to fabricate an array of self‐assembled WSe₂/MoS₂ heterostructures through facile solution‐based directional precipitation. By manipulating the internal convection flow (i.e., Marangoni flow) of the solution, the WSe₂ wires are selectively stacked over the MoS₂ wires at a specific angle, which enables the formation of parallel‐ and cross‐aligned heterostructures. The realized WSe₂/MoS₂‐based p–n heterojunction shows not only high rectification (ideality factor: 1.18) but also promising optoelectrical properties with a high responsivity of 5.39 A W⁻¹ and response speed of 16 µs. As a feasible application, a WSe₂/MoS₂‐based photodiode array (10 × 10) is demonstrated, which proves that the photosensing system can detect the position and intensity of an external light source. The solution‐based growth of hierarchical structures with various alignments could offer a method for the further development of large‐area electronic and optoelectronic applications.

Fast gelation of Ti₃C₂T _x MXenes is initiated by divalent metal ions in aquesous solution. Typically, Fe²⁺ ions eliminate the electrostatic repulsion, networking MXene nanosheets into a 3D structured hydrogel. The wet hydrogel avoids nanosheet restacking and is ideal for applications highlighting the surface utilization, especially as freestanding electrodes for high‐rate supercapacitors.

Abstract

Gelation is an effective way to realize the self‐assembly of nanomaterials into different macrostructures, and in a typical use, the gelation of graphene oxide (GO) produces various graphene‐based carbon materials with different applications. However, the gelation of MXenes, another important type of 2D materials that have different surface chemistry from GO, is difficult to achieve. Here, the first gelation of MXenes in an aqueous dispersion that is initiated by divalent metal ions is reported, where the strong interaction between these ions and OH groups on the MXene surface plays a key role. Typically, Fe²⁺ ions are introduced in the MXene dispersion which destroys the electrostatic repulsion force between the MXene nanosheets in the dispersion and acts as linkers to bond the nanosheets together, forming a 3D MXene network. The obtained hydrogel effectively avoids the restacking of the MXene nanosheets and greatly improves their surface utilization, resulting in a high rate performance when used as a supercapacitor electrode (≈226 F g⁻¹ at 1 V s⁻¹). It is believed that the gelation of MXenes indicates a new way to build various tunable MXene‐based structures and develop different applications.

Exciton polaritons (EPs) in group‐VI transition‐metal dichalcogenides (TMDs) have attracted a lot of research interest in the 2D materials and photonics communities in the past few years. Here, recent studies of EPs in TMDs, highlighting their key properties and functionalities are reviewed, and the potential directions for future research are discussed.

Abstract

Exciton polaritons (EPs) are half‐light, half‐matter quasiparticles formed due to the coupling between photons and excitons in semiconductors. Their uniqueness lies at the strong light–matter interactions and long‐distance transport, thus promising for many novel applications in photonics, information, and quantum technologies. Recently, EPs in group‐VI transition‐metal dichalcogenides (TMDs) have attracted a lot of research interest due to their room‐temperature stability, long‐distance propagation, and controllability through electric gating, valley‐selective optical pumping, and precise thickness control. In this progress report, recent studies of EPs in TMDs are reviewed, highlighting their key properties and functionalities, and then discussing the potential directions for future research.

Van der Waals epitaxy (vdWE) is presently the most promising strategy to obtain flexible III‐nitride (opto)electronic devices based on sp²‐bonded two‐dimensional (2D) layered materials. Recent progress in the fabrication of 2D materials, vdWE, and transfer printing of III‐nitride films based on graphene, hexagonal boron nitride, and transition metal dichalcogenides is reviewed, and key points and future perspectives are discussed.

Abstract

III‐nitride semiconductors have attracted considerable attention in recent years owing to their excellent physical properties and wide applications in solid‐state lighting, flat‐panel displays, and solar energy and power electronics. Generally, GaN‐based devices are heteroepitaxially grown on c‐plane sapphire, Si (111), or 6H‐SiC substrates. However, it is very difficult to release the GaN‐based films from such single‐crystalline substrates and transfer them onto other foreign substrates. Consequently, it is difficult to meet the ever‐increasing demand for wearable and foldable applications. On the other hand, sp²‐bonded two‐dimensional (2D) materials, which exhibit hexagonal in‐plane lattice arrangements and weakly bonded layers, can be transferred onto flexible substrates with ease. Hence, flexible III‐nitride devices can be implemented through such 2D release layers. In this progress report, the recent advances in the different strategies for the growth of III‐nitrides based on 2D materials are reviewed, with a focus on van der Waals epitaxy and transfer printing. Various attempts are presented and discussed herein, including the different kinds of 2D materials (graphene, hexagonal boron nitride, and transition metal dichalcogenides) used as release layers. Finally, current challenges and future perspectives regarding the development of flexible III‐nitride devices are discussed.

In article number https://doi.org/10.1002/adma.2019024311902431, Bo Tian, Weiwei Cai, Xixiang Zhang, and co‐workers reveal the existence of a fractal‐growth‐based mechanism in 2D‐material chemical vapor deposition (CVD) syntheses. Based on a 2D diffusion‐limited aggregation model, perfect correlations between theoretically simulated data and CVD experimental results are obtained. Eventually, the precise control of 2D‐material shapes and qualities is achieved by adjusting single‐domain net growth rates in the CVD‐growth process.

Selective tuning of ambipolar WSe₂ monolayer to p‐ and n‐type semiconductors by chemical doping is demonstrated. The chemical doping not only allows to control over the main charge carriers, but also increases the carrier mobility of the WSe₂ significantly. Furthermore, a complementary metal‐oxide‐semiconductor inverter and an in‐plane p–n junction with superior performance are successfully fabricated by integrating the chemically doped WSe₂.

Abstract

Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post‐silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom‐thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p‐ and n‐type semiconductors is essential for various device applications, such as complementary metal‐oxide‐semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe₂ is demonstrated by chemical doping. Two different molecules, 4‐nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe₂ field‐effect transistors (FETs) to p‐ and n‐type, respectively. Moreover, the chemically doped WSe₂ show increased effective carrier mobilities of 82 and 25 cm² V⁻¹s⁻¹ for holes and electrons, respectively, which are much higher than those of the pristine WSe₂. The doping effects are studied by photoluminescence, Raman, X‐ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe₂ FETs are integrated into CMOS inverters, exhibiting extremely low power consumption (≈0.17 nW). Furthermore, a p‐n junction within single WSe₂ grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe₂ will contribute to the development of TMDC‐based advanced electronics.