2D material based photodetectors have attracted many research projects due to their unique structures and excellent electronic and optoelectronic properties. These 2D materials, including semimetallic graphene, semiconducting black phosphorus, transition metal dichalcogenides, insulating hexagonal boron nitride, and their various heterostructures, show a wide distribution in bandgap values. To date, hundreds of photodetectors based on 2D materials have been reported. Here, a review of photodetectors based on 2D materials covering the detection spectrum from ultraviolet to infrared is presented. First, a brief insight into the detection mechanisms of 2D material photodetectors as well as introducing the figure-of-merits which are key factors for a reasonable comparison between different photodetectors is provided. Then, the recent progress on 2D material based photodetectors is reviewed. Particularly, the excellent performances such as broadband spectrum detection, ultrahigh photoresponsivity and sensitivity, fast response speed and high bandwidth, polarization-sensitive detection are pointed out on the basis of the state-of-the-art 2D photodetectors. Initial applications based on 2D material photodetectors are mentioned. Finally, an outlook is delivered, the challenges and future directions are discussed, and general advice for designing and realizing novel high-performance photodetectors is given to provide a guideline for the future development of this fast-developing field.

Progress, Challenges, and Opportunities for 2D Material Based Photodetectors

Inspired by rich physics and functionalities of graphenes, scientists have taken an intensive interest in two-dimensional (2D) crystals of h-BN (analogue of graphite, so-called "white" graphite). Recent calculations have predicted the exciting potentials of BN nanoribbons in spintronics due to tunable magnetic and electrical properties; however no experimental evidence has been provided since fabrication of such ribbons remains a challenge. Here, we show that few- and single-layered BN nanoribbons, mostly terminated with zigzag edges, can be produced under unwrapping multiwalled BN nanotubes through plasma etching. The interesting stepwise unwrapping and intermediate states were observed and analyzed. Opposed to insulating primal tubes, the nanoribbons become semiconducting due to doping-like conducting edge states and vacancy defects, as revealed by structural analyses and ab initio simulations. This study paves the way for BN nanoribbon production and usage as functional semiconductors with a wide range of applications in optoelectronics and spintronics.

"White graphenes": boron nitride nanoribbons via boron nitride nanotube unwrapping

Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated high-performance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.

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Graphene-MoS2 Hybrid Technology for Large-Scale Two-Dimensional Electronics

2D Black Phosphorus–Based Biomedical Applications

Rapid digital technology advancement has resulted in a tremendous increase in computing tasks imposing stringent energy efficiency and area efficiency requirements on next-generation computing. To meet the growing data-driven demand, in-memory computing and transistor-based computing have emerged as potent technologies for the implementation of matrix and logic computing. However, to fulfil the future computing requirements new materials are urgently needed to complement the existing Si complementary metal–oxide–semiconductor technology and new technologies must be developed to enable further diversification of electronics and their applications. The abundance and rich variety of electronic properties of two-dimensional materials have endowed them with the potential to enhance computing energy efficiency while enabling continued device downscaling to a feature size below 5 nm. In this Review, from the perspective of matrix and logic computing, we discuss the opportunities, progress and challenges of integrating two-dimensional materials with in-memory computing and transistor-based computing technologies. This Review discusses the recent progress and future prospects of two-dimensional materials for next-generation nanoelectronics.

Two-dimensional materials for next-generation computing technologies.

Recently, both lateral and vertical p–n junctions have been realized in 2D materials using various strategies, with a number of works on exploring the potential of lateral heterojunctions resulting from thickness-modulated bandgaps at the interface. Here, electrically tunable all-black-phosphorus (BP) lateral heterojunction diodes, without the need of split-gating or selective chemical doping or transfer-based vertical stacking, are experimentally demonstrated. The BP heterojunction diode, which exhibits an ultralow off-state current density of 8 pA µm−1 at a mere Vd of 100 mV and a significant gate-tunable current-rectifying behavior with the highest rectification ratio exceeding 600, is able to harvest solar energy at both visible and near-infrared wavelengths beyond the bandgap limitation of transition metal dichalcogenides. Specifically, at 660 nm, the device achieves an open-circuit voltage (Voc) of 210 mV and a short-circuit current (Isc) of 1.5 nA at 3.6 W cm−2 power density, resulting in an external quantum efficiency of 7.4% which outperforms both split-gating and chemically doped homojunctions. This work paves the way for the exploitation of BP lateral heterojunction for broadband energy harvesting towards future optoelectronic applications.

Pronounced Photovoltaic Effect in Electrically Tunable Lateral Black-Phosphorus Heterojunction Diode

Fabry-Perot cavity enhanced light-matter interactions in two-dimensional van der Waals heterostructure

Many, black phosphorus (BP) based field-effect transistors, homojunctions, and vertical van der Waals structures have been developed for optoelectronic applications, with few studies being conducted on exploring the potential of their naturally formed heterojunctions. Here, we report a novel thickness-modulated, gate-tunable BP heterojunction phototransistor for multiple purposes and high performance optoelectronics. Despite its thickness of less than 5 nm, the device, whose fabrication spares the need for split-gate or chemical doping or vertical stacking requirements, achieves an excellent photoresponsivity of 383 A W−1 at 1550 nm under zero gate bias, which is among the best photoresponse performance of all-BP-based photodetectors in this spectral range. Furthermore, it exhibits a shot-noise-limited noise equivalent power (NEPshot) of less than 10−2 pW Hz−1/2, making it very promising for ultra-low power detection. Additionally, owing to the heterojunction-induced built-in electric field, the device can be readily used for infrared photovoltaic devices in the absence of source–drain bias (Vd), a feature that is distinctively superior to traditional phototransistors. The multifunctionality demonstrated in our BP heterojunction transistor paves the way towards realizing tunable improved performance optoelectronics based on 2D materials platform.

Tunable black phosphorus heterojunction transistors for multifunctional optoelectronics

DOI: 10.1002/aelm.201800274 properties.[1–3] The utilization of 2DLMs for nanoelectronic devices offers many benefits for overcoming the scaling limits governed by Moore’s Law.[4,5] Graphene, which is attractive for its high mobility, is unsuitable for circuit application due to high off-state leakage current arising from its zero-bandgap property.[6,7] To overcome the shortcoming of graphene circuits with low on/off ratio, TMDs and more recently black phosphorus (BP), have been explored as the new channel materials for digital circuits because of their finite bandgap and superior transport properties. BP is becoming one of the promising 2DLMs for electronic applications due to its superior properties.[8] First, the unique property of BP is that it has a high mobility compared with other 2D materials, such as TMDs, thus ensuring a higher transistor current density. Besides, it shows a tunable bandgap ranging from 0.3 (bulk) to 2 eV (single layer), which enables a wide range of applications in electronics and optoelectronics. On the contrary, the main disadvantage of BP is that it is unstable and the material properties degrade quickly under ambient condition, thus an encapsulation layer, such as Al2O3 and poly (methyl methacrylate) (PMMA),[10] is necessary to protect BP from ambient degradation. Numerous field-effect transistors (FETs) based on BP[11–13] have been reported recently, in which the on/off ratio of the BP FETs is up to ≈105. In addition, BP is also demonstrated to be a superior candidate for nonvolatile memories applications.[8] However, to date, complementary 2D inverters are mostly fabricated by hybrid integration approach where two different types of 2D semiconductor are used as the channel materials. Such hybrid inverters are typically made of n-type FET, such as MoS2 n-FET,[12,14–21] and MoSe2 n-FET,[22] while typical p-type FET comprise of BP p-FET,[12,21] MoTe2 p-FET,[17] and WSe2 p-FET (Table S1, Supporting Information).[18,22] These hybrid inverters are usually achieved by either a transfer or external wiring process, which is complicated and expensive to implement for large scale manufacturing. Thus, monolithically integrated complementary inverter circuits are highly desirable and actively sought after for their simple fabrication process. Arising from the feasibility to achieve complementary doping, BP emerges as a promising material for realizing complementary integrated circuits. For instance, metal atoms have been effective in transforming pristine p-type BP into n-type conductivity,[23–26] thus allowing complementary inverters to be demonstrated on a single BP flake.[28–30] In a recent Black phosphorus (BP) has attracted enormous interest for logic applications due to its unique electronic properties. However, pristine BP exhibits predominant p-type channel conductance, which limits the realization of complementary circuits unless an effective n-type doping is found. Here, a practical approach to transform the conductivity of BP from p-type to n-type via a spatially controlled aluminum (Al) doping is proposed. Symmetrical threshold voltage for the pair of p-type and n-type BP field-effect transistors can be achieved by tuning the Al doping concentration. The complementary inverter circuit shows a clear logic inversion with a high voltage gain of up to ≈11 at a supply voltage (VDD) of 1.5 V. Simultaneously, a high noise margin of 0.27 × VDD is achieved for both low (NML) and high (NMH) input voltages, indicating excellent noise immunity. Moreover, a three-stage ring oscillator with a theoretical frequency above 1.8 GHz and microwatt level power dissipation is modeled, which shows a low propagation delay per stage. This study demonstrates a practical approach to fabricate high performance complementary integrated circuits on a homogenous BP channel material, paving the way toward complex cascaded circuits and sensor interface applications.

Gigahertz Integrated Circuits Based on Complementary Black Phosphorus Transistors

The miniaturization of ferroelectric devices in non-volatile memories requires the device to maintain stable switching behavior as the thickness scales down to nanometer scale, which requires the coercive field to be sufficiently large. Recently discovered metal-free perovskites exhibit advantages such as structural tunability and solution-processability, but they are disadvantaged by a lower coercive field compared to inorganic perovskites. Herein, we demonstrate that the coercive field (110 kV/cm) in metal-free ferroelectric perovskite MDABCO-NH4-(PF6)3 (MDABCO = N-methyl-N'-diazabicyclo[2.2.2]octonium) is one order larger than MDABCO-NH4-I3 (12 kV/cm) owing to the stronger intermolecular hydrogen bonding in the former. Using isotope experiments, the ferroelectric-to-paraelectric phase transition temperature and coercive field are verified to be strongly influenced by hydrogen bonds. Our work highlights that the coercive field of organic ferroelectrics can be tailored by tuning the strength of hydrogen bonding. 

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Lin Wang

Papers

Pronounced Photovoltaic Effect in Electrically Tunable Lateral Black-Phosphorus Heterojunction Diode

Fabry-Perot cavity enhanced light-matter interactions in two-dimensional van der Waals heterostructure

Tunable black phosphorus heterojunction transistors for multifunctional optoelectronics

Gigahertz Integrated Circuits Based on Complementary Black Phosphorus Transistors

Tailoring the coercive field in ferroelectric metal-free perovskites by hydrogen bonding