Recent progress in synthesis, properties, and applications of hexagonal boron nitride-based heterostructures.

Featuring an absence of dangling bonds, large band gap, low dielectric constant, and excellent chemical inertness, atomically thin hexagonal boron nitride (h-BN) is considered an ideal candidate for integration with graphene and other 2D materials. During the past years, great efforts have been devoted to the research of h-BN-based heterostructures, from fundamental study to practical applications. In this review we summarize the recent progress in the synthesis, novel properties, and potential applications of h-BN-based heterostructures, especially the synthesis technique. Firstly, various approaches to the preparation of both in-plane and vertically stacked h-BN-based heterostructures are introduced in detail, including top-down strategies associated with exfoliation transfer processes and bottom-up strategies such as chemical vapor deposition (CVD)-based growth. Secondly, we discuss some novel properties arising in these heterostructures. Several promising applications in electronic and optoelectronic devices are also reviewed. Finally, we discuss the main challenges and possible research directions in this field.

Remote heteroepitaxy of atomic layered hafnium disulfide on sapphire through hexagonal boron nitride.

Two-dimensional (2D) heterostructures have attracted a great deal of attention due to their novel phenomena arising from the complementary properties of their constituent materials, and provide an ideal platform for exploring new fundamental research and realizing technological innovation. Here, for the first time, we report the formation of high quality HfS2/h-BN heterostructures by the remote heteroepitaxy technique, in which the large-area single-crystal HfS2 layers were epitaxially grown on c-plane sapphire through a polycrystalline h-BN layer via chemical vapor deposition. It is found that c-sapphire substrates can penetrate monolayer and bilayer h-BN to remotely handle the epitaxial growth of HfS2. Benefitting from the high crystal quality of HfS2 epilayers and the weak interface scattering of HfS2 on h-BN, the HfS2 photodetectors demonstrate excellent performance with a high on/off ratio exceeding 105, an excellent photoresponsivity up to 0.135 A W-1 and a high detectivity of over 1012 Jones. Furthermore, the HfS2/h-BN heterostructures prepared by the remote epitaxy can be rapidly released and transferred to a substrate of interest, which opens a new pathway for large-area advanced wearable electronics applications.

Two-dimensional hexagonal boron-carbon-nitrogen atomic layers.

Two-dimensional (2D) hexagonal boron-carbon-nitrogen (h-BCN) atomic layers are expected to possess interesting properties complementary to those of graphene and h-BN, enabling a rich variety of electronic structures, properties and applications. Herein, we demonstrate a novel method to synthesize 2D h-BCN atomic layers with a full range of compositions by ion beam sputtering deposition under a mixed Ar/CH4 atmosphere. The h-BCN layers have been thoroughly characterized by various techniques, aiming at the determination of their structure evolution and properties. We find that homogeneous h-BCN layers consisting of graphene and h-BN nanodomains can be obtained at an appropriate C content, whereas too high or too low C contents result in the segregation of large-sized graphene or h-BN islands. Furthermore, the band gap of h-BCN layers slightly decreases with the increasing C content, while their electric properties can be tuned from insulating to highly conducting. This work provides a novel approach for synthesizing 2D h-BCN atomic layers and paves the way for the development of h-BCN-based devices.

High-performance deep ultraviolet photodetectors based on few-layer hexagonal boron nitride.

Hexagonal boron nitride (h-BN), an isomorph of graphene, has attracted great attention owing to its potential applications as an ultra-flat substrate or gate dielectric layer in novel graphene-based devices. Besides, h-BN appears to be a promising material for deep ultraviolet (DUV) optoelectronic applications because of its extraordinary physical properties, such as wide band gap and high absorption coefficient. In this work, two-dimensional h-BN with controllable layers was synthesized on Cu foils by ion beam sputtering deposition, and DUV photodetectors were fabricated from the transferred h-BN layers on SiO2/Si substrates. The h-BN layers synthesized at the higher substrate temperature possess a lower density of domain boundaries and higher crystalline quality, and the photodetectors based on a 3 nm h-BN layer exhibited high performance with an on/off ratio of >103 under DUV light illumination at 212 nm and a cutoff wavelength at around 225 nm. This work demonstrates that two-dimensional h-BN layers are promising for the construction of high-performance solar-blind photodetectors.

Large-Area Synthesis of Layered HfS2(1- x )Se2 x Alloys with Fully Tunable Chemical Compositions and Bandgaps.

Alloying transition metal dichalcogenides (TMDs) with different compositions is demonstrated as an effective way to acquire 2D semiconductors with widely tunable bandgaps. Herein, for the first time, the large-area synthesis of layered HfS2(1- x )Se2 x alloys with fully tunable chemical compositions on sapphire by chemical vapor deposition is reported, greatly expanding and enriching the family of 2D TMDs semiconductors. The configuration and high quality of their crystal structure are confirmed by various characterization techniques, and the bandgap of these alloys can be continually modulated from 2.64 to 1.94 eV with composition variations. Furthermore, prototype HfS2(1- x )Se2 x photodetectors with different Se compositions are fabricated, and the HfSe2 photodetector manifests the best performance among all the tested HfS2(1- x )Se2 x devices. Remarkably, by introducing a hexagonal boron nitride layer, the performance of the HfSe2 photodetector is greatly improved, exhibiting a high on/off ratio exceeding 105, an ultrafast response time of about 190 µs, and a high detectivity of 1012 Jones. This simple and controllable approach opens up a new way to produce high-quality 2D HfS2(1- x )Se2 x layers, which are highly qualified candidates for the next-generation application in high-performance optoelectronics.