Xingxing Zhang

Solution‐processed lateral photodetectors have many advantages in preparation, operation, and application. From active materials to device architectures, the developed strategies toward high performance for devices based on different kinds of active materials are reviewed. The strategies are discussed in detail, and the common physical rules behind all these strategies are generalized.

Abstract

Due to their low cost and ease of integration, solution‐processed lateral photodetectors (PDs) are becoming an important device type among the PD family. In recent years, enormous effort has been devoted to improving their performances, and great achievements have been made. A summary of the core progress, especially from the perspective of design principles and device physics, is necessary to further the development of the field, but is currently lacking. Here, to address this need, first, the working mechanism of PDs and the device figures‐of‐merit are introduced. Second, by classifying the active materials into four categories, including inorganic, organic, hybrid, and perovskite, the developed strategies toward high performance are discussed respectively. To close, the common physical rules behind all these strategies are generalized, and suggestions for future development are given accordingly.

Ultrathin CoSe nanoplates with tunable structure phases are synthesized by a chemical vapor deposition route. Electrical transport studies reveal that both types of CoSe nanoplates show strong thickness‐tunable electrical properties, excellent breakdown current density, yet distinct conductance trends with the decreasing temperature. The tetragonal CoSe nanoplates show angle‐dependent magnetoresistance and weak antilocalization at lower field.

Abstract

Multiple structural phases in transition metal dichalcogenides have attracted considerable recent interest for their tunable chemical and electronic properties. Herein, a chemical vapor deposition route to ultrathin CoSe nanoplates with tunable structure phases is reported. By precisely tailoring the growth temperature, ultrathin 2D layered tetragonal CoSe nanoplates and nonlayered hexagonal CoSe nanoplates can be selectively prepared as square or hexagonal geometries, with thickness as thin as 2.3 and 3.7 nm, respectively. X‐ray diffraction, transmission electron microscopy, and selected area electron diffraction studies show that both types of nanoplates are high‐quality single crystals. Electrical transport studies reveal that both the tetragonal and hexagonal CoSe nanoplates show strong thickness‐tunable electrical properties and excellent breakdown current density. The 2D hexagonal CoSe nanoplates display metallic behavior with an excellent conductivity up to 6.6 × 10⁵ S m⁻¹ and an extraordinary breakdown current density up to 3.9 × 10⁷ A cm⁻², while the square tetragonal nanoplates show considerably lower conductivity up to 8.2 × 10⁴ S m⁻¹ with angle‐dependent magnetoresistance and weak antilocalization effect at lower field. This study offers a tunable material system for exploring multiphase 2D materials and their potential applications for electronic and magnetoelectronic devices.

Plasmonic photodetection in Mo₂CT _x MXene flexible thin films is demonstrated. The photocurrent generation in Mo₂CT _x is principally controlled by surface plasmon‐assisted hot electrons. The distribution of various surface plasmon modes over individual Mo₂CT _x nanosheets is visualized by the combination of scanning transmission electron microscopy and ultrahigh‐resolution electron energy‐loss spectroscopy.

Abstract

MXenes have recently shown impressive optical and plasmonic properties associated with their ultrathin‐atomic‐layer structure. However, their potential use in photonic and plasmonic devices has been only marginally explored. Photodetectors made of five different MXenes are fabricated, among which molybdenum carbide MXene (Mo₂CT _x ) exhibits the best performance. Mo₂CT _x MXene thin films deposited on paper substrates exhibit broad photoresponse in the range of 400–800 nm with high responsivity (up to 9 A W⁻¹), detectivity (≈5 × 10¹¹ Jones), and reliable photoswitching characteristics at a wavelength of 660 nm. Spatially resolved electron energy‐loss spectroscopy and ultrafast femtosecond transient absorption spectroscopy of the MXene nanosheets reveal that the photoresponse of Mo₂CT _x is strongly dependent on its surface plasmon‐assisted hot carriers. Additionally, Mo₂CT _x thin‐film devices are shown to be relatively stable under ambient conditions, continuous illumination and mechanical stresses, illustrating their durable photodetection operation in the visible spectral range. Micro‐Raman spectroscopy conducted on bare Mo₂CT _x film and on gold electrodes allowing for surface‐enhanced Raman scattering demonstrates surface chemistry and a specific low‐frequency band that is related to the vibrational modes of the single nanosheets. The specific ability to detect and excite individual surface plasmon modes provides a viable platform for various MXene‐based optoelectronic applications.

Energy transport across metal–2D material interfaces is systematically investigated and shows a highly anisotropic thermal boundary resistance (TBR). Experimental measurements and atomistic calculations of the interface thermal transport reveal detailed fundamental understanding of TBR–structure relationships. This study may open up new opportunities in the rational design and control of novel interface materials for advanced thermal management technologies.

Abstract

Interfacial thermal boundary resistance (TBR) plays a critical role in near‐junction thermal management of modern electronics. In particular, TBR can dominate heat dissipation and has become increasingly important due to the continuous emergence of novel nanomaterials with promising electronic and thermal applications. A highly anisotropic TBR across a prototype 2D material, i.e., black phosphorus, is reported through a crystal‐orientation‐dependent interfacial transport study. The measurements show that the metal–semiconductor TBR of the cross‐plane interfaces is 241% and 327% as high as that of the armchair and zigzag direction‐oriented interfaces, respectively. Atomistic ab initio calculations are conducted to analyze the anisotropic and temperature‐dependent TBR using density functional theory (DFT)‐derived full phonon dispersion relation and molecular dynamics simulation. The measurement and modeling work reveals that such a highly anisotropic TBR can be attributed to the intrinsic band structure and phonon spectral transmission. Furthermore, it is shown that phonon hopping between different branches is important to modulate the interfacial transport process but with directional preferences. A critical fundamental understanding of interfacial thermal transport and TBR–structure relationships is provided, which may open up new opportunities in developing advanced thermal management technology through the rational control over nanostructures and interfaces.

High‐crystalline transition metal dichalcogenides nanosheets (including MoS₂, WS₂, MoSe₂, and WSe₂) can be mass produced with a reaction time of only several minutes through a molten salt method.

Abstract

2D transition metal dichalcogenides (TMDs) are well suited for energy storage and field–effect transistors because of their thickness‐dependent chemical and physical properties. However, as current synthetic methods for 2D TMDs cannot integrate both advantages of liquid‐phase syntheses (i.e., massive production and homogeneity) and chemical vapor deposition (i.e., high quality and large lateral size), it still remains a great challenge for mass production of high‐quality 2D TMDs. Here, a molten salt method to massively synthesize various high‐crystalline TMDs nanosheets (MoS₂, WS₂, MoSe₂, and WSe₂) with the thicknesses less than 5 nm is reported, with the production yield over 68% with the reaction time of only several minutes. Additionally, the thickness and size of the as‐synthesized nanosheets can be readily controlled through adjusting reaction time and temperature. The as‐synthesized MoSe₂ nanosheets exhibit good electrochemical performance as pseudocapacitive materials. It is further anticipates that this work will provide a promising strategy for rapid mass production of high‐quality nonoxides nanosheets for energy‐related applications and beyond.

Nature Communications, Published online: 21 June 2019; doi:10.1038/s41467-019-10632-z

For the first time, a 2D material–based photodetector is reported using ionic glass as the electrostatic gating method, choosing MoSe₂ over LaF₃ ionic glass as an archetypal system. The wider possibilities offered by this architecture are unveiled, and a careful analysis of its unique optoelectronic properties is provided.

Abstract

Modulating the carrier density of 2D materials is pivotal to tailor their electrical properties, with novel physical phenomena expected to occur at a higher doping level. Here, the use of ionic glass as a high capacitance gate is explored to develop a 2D material–based phototransistor operated with a higher carrier concentration up to 5 × 10¹³ cm⁻², using MoSe₂ over LaF₃ as an archetypal system. Ion glass gating reveals to be a powerful technique combining the high carrier density of electrolyte gating methods while enabling direct optical addressability impeded with usual electrolyte technology. The phototransistor demonstrates I _ON/I _OFF ratio exceeding five decades and photoresponse times down to 200 µs, up to two decades faster than MoSe₂ phototransistors reported so far. Careful phototransport analysis reveals that ionic glass gating of 2D materials allows tuning the nature of the carrier recombination processes, while annihilating the traps' contribution in the electron injection regime. This remarkable property results in a photoresponse that can be modulated electrostatically by more than two orders of magnitude, while at the same time increasing the gain bandwidth product. This study demonstrates the potential of ionic glass gating to explore novel photoconduction processes and alternative architectures of devices.

To achieve enhanced hydrogen evolution reaction (HER) performance, few‐layer 1T'‐MoTe₂ films are precisely patterned with a focused ion beam to create active sites. Electrochemical measurements indicate that the HER performance, although inconspicuous in pristine 1T'‐MoTe₂ ultrathin films prepared through the chemical vapor deposition method, can be greatly enhanced after patterning and precisely controlled by the morphologies as well as the amounts of the defects.

Abstract

2D transition metal dichalcogenides (TMDs) have presented outstanding potential for efficient hydrogen evolution reaction (HER) to replace traditional noble metal catalysts. Here, to achieve enhanced HER performance, specific areas of the few‐layer 1T'‐MoTe₂ film are precisely controlled with a focused ion beam to create particular active sites. Electrochemical measurements indicate that the HER performance, although inconspicuous in pristine 1T'‐MoTe₂ ultrathin films prepared through the chemical vapor deposition method, can be greatly enhanced after patterning and precisely controlled by the morphologies as well as the amounts of the defects, reaching a small onset potential and a record‐low Tafel slope of 44 mV per decade for few‐layer TMDs. Conductivity tests, visualized copper electrodeposition, and density functional theory calculations also confirm that the enhancement of HER performance comes from the exposed edges by patterning. In this pioneering work, not only is the catalysis mechanism of the edge active sites of 1T'‐MoTe₂ unveiled, but also a universal route to study the properties of 2D materials is demonstrated.

Nature Nanotechnology, Published online: 17 June 2019; doi:10.1038/s41565-019-0467-1

A 1.5 nm 2D layered tantalum sulfide diffusion barrier/liner for Cu interconnects is developed by converting tantalum at 400 °C. Superior diffusion barrier and liner properties are demonstrated as compared to standard barrier/liner. A bottleneck of interconnect scaling can be overcome by replacing conventional barrier/liner bilayer with a single‐stack of tantalum sulfide.

Abstract

The interconnect half‐pitch size will reach ≈20 nm in the coming sub‐5 nm technology node. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be >4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross‐section and they are much more resistive than Cu, the effective conductance of an ultrascaled interconnect will be compromised by the thick bilayer. Therefore, 2D layered materials have been explored as diffusion barrier alternatives. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back‐end‐of‐line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued. Here, a 2D layered tantalum sulfide (TaS _x ) with ≈1.5 nm thickness is developed to replace the conventional TaN/Ta bilayer. The TaS _x ultrathin film is industry‐friendly, BEOL‐compatible, and can be directly prepared on dielectrics. The results show superior barrier/liner properties of TaS _x compared to the TaN/Ta bilayer. This single‐stack material, serving as both a liner and a barrier, will enable continued scaling of interconnects beyond 5 nm node.

Nitrogen vacancies on 2D layered W₂N₃ reveal stable and efficient nitrogen reduction performance. The activity and selectivity of the unique active sites are confirmed by mutually corroborating electrochemical experiments and theoretical computation. The nitrogen vacancies on W₂N₃ have an electron deficient environment for the acceptance of the lone‐pair electrons of N₂, which can facilitate dinitrogen molecule adsorption and activation.

Abstract

Electrochemical nitrogen reduction reaction (NRR) under ambient conditions provides an avenue to produce carbon‐free hydrogen carriers. However, the selectivity and activity of NRR are still hindered by the sluggish reaction kinetics. Nitrogen Vacancies on transition metal nitrides are considered as one of the most ideal active sites for NRR by virtue of their unique vacancy properties such as appropriate adsorption energy to dinitrogen molecule. However, their catalytic performance is usually limited by the unstable feature. Herein, a new 2D layered W₂N₃ nanosheet is prepared and the nitrogen vacancies are demonstrated to be active for electrochemical NRR with a steady ammonia production rate of 11.66 ± 0.98 µg h⁻¹ mg_cata ⁻¹ (3.80 ± 0.32 × 10⁻¹¹ mol cm⁻² s⁻¹) and Faradaic efficiency of 11.67 ± 0.93% at −0.2 V versus reversible hydrogen electrode for 12 cycles (24 h). A series of ex situ synchrotron‐based characterizations prove that the nitrogen vacancies on 2D W₂N₃ are stable by virtue of the high valence state of tungsten atoms and 2D confinement effect. Density function theory calculations suggest that nitrogen vacancies on W₂N₃ can provide an electron‐deficient environment which not only facilitates nitrogen adsorption, but also lowers the thermodynamic limiting potential of NRR.

Two‐inch atomically flat 2H‐MoTe₂ monolayers with tunable coverage from 40 to 100% are directly grown on inert SiO₂ dielectrics by molecular beam epitaxy. A single‐step nucleation and growth strategy is developed to promote lateral growth under low Mo‐flux. The effective layer‐by‐layer growth is controlled solely by the molecular beam dynamics rather than the assistance of catalysts, seeding promoters, and epitaxial templates.

Abstract

Monolayer MoTe₂, with the narrowest direct bandgap of ≈1.1 eV among Mo‐ and W‐based transition metal dichalcogenides, has attracted increasing attention as a promising candidate for applications in novel near‐infrared electronics and optoelectronics. Realizing 2D lateral growth is an essential prerequisite for uniform thickness and property control over the large scale, while it is not successful yet. Here, layer‐by‐layer growth of 2 in. wafer‐scale continuous monolayer 2H‐MoTe₂ films on inert SiO₂ dielectrics by molecular beam epitaxy is reported. A single‐step Mo‐flux controlled nucleation and growth process is developed to suppress island growth. Atomically flat 2H‐MoTe₂ with 100% monolayer coverage is successfully grown on inert 2 in. SiO₂/Si wafer, which exhibits highly uniform in‐plane structural continuity and excellent phonon‐limited carrier transport behavior. The dynamics‐controlled growth recipe is also extended to fabricate continuous monolayer 2H‐MoTe₂ on atomic‐layer‐deposited Al₂O₃ dielectric. With the breakthrough in growth of wafer‐scale continuous 2H‐MoTe₂ monolayers on device compatible dielectrics, batch fabrication of high‐mobility monolayer 2H‐MoTe₂ field‐effect transistors and the three‐level integration of vertically stacked monolayer 2H‐MoTe₂ transistor arrays for 3D circuitry are successfully demonstrated. This work provides novel insights into the scalable synthesis of monolayer 2H‐MoTe₂ films on universal substrates and paves the way for the ultimate miniaturization of electronics.

2D metal chalcogenides, including two‐dimensional metal mono‐, di‐, and tri‐chalcogenides, are a large family of van der Waals layered materials. They possess different crystal structures contributing to distinct physical characteristics. Numerous studies have focused on their electronic and optoelectronic applications. Two‐dimensional metal chalcogenides have been considered promising candidates for future practical applications.

Abstract

Emerging 2D metal chalcogenides present excellent performance for electronic and optoelectronic applications. In contrast to graphene and other 2D materials, 2D metal chalcogenides possess intrinsic bandgaps, versatile band structures, and superior atmospheric stability. The many categories of 2D metal chalcogenides ensure that they can be applied to various practical scenarios. 2D metal monochalcogenides, dichalcogenides, and trichalcogenides are the three main categories of these materials. They have distinct crystal structures resulting in different characteristics. Some basic device characteristics, such as the charge carrier characteristics, scattering mechanisms, interfacial contacts, and band alignments of heterojunctions, are vital factors for practical device applications that ensure that the desired properties can be achieved. Various electronic, optoelectronic, and photonic applications based on 2D metal chalcogenides have been extensively investigated. 2D metal chalcogenides are considered as competitive candidates for future electronic and optoelectronic applications.