BACKGROUND Materials by design is an appealing idea that is very hard to realize in practice. Combining the best of different ingredients in one ultimate material is a task for which we currently have no general solution. However, we do have some successful examples to draw upon: Composite materials and III-V heterostructures have revolutionized many aspects of our lives. Still, we need a general strategy to solve the problem of mixing and matching crystals with different properties, creating combinations with predetermined attributes and functionalities. ADVANCES Two-dimensional (2D) materials offer a platform that allows creation of heterostructures with a variety of properties. One-atom-thick crystals now comprise a large family of these materials, collectively covering a very broad range of properties. The first material to be included was graphene, a zero-overlap semimetal. The family of 2D crystals has grown to includes metals (e.g., NbSe 2 ), semiconductors (e.g., MoS 2 ), and insulators [e.g., hexagonal boron nitride (hBN)]. Many of these materials are stable at ambient conditions, and we have come up with strategies for handling those that are not. Surprisingly, the properties of such 2D materials are often very different from those of their 3D counterparts. Furthermore, even the study of familiar phenomena (like superconductivity or ferromagnetism) in the 2D case, where there is no long-range order, raises many thought-provoking questions. A plethora of opportunities appear when we start to combine several 2D crystals in one vertical stack. Held together by van der Waals forces (the same forces that hold layered materials together), such heterostructures allow a far greater number of combinations than any traditional growth method. As the family of 2D crystals is expanding day by day, so too is the complexity of the heterostructures that could be created with atomic precision. When stacking different crystals together, the synergetic effects become very important. In the first-order approximation, charge redistribution might occur between the neighboring (and even more distant) crystals in the stack. Neighboring crystals can also induce structural changes in each other. Furthermore, such changes can be controlled by adjusting the relative orientation between the individual elements. Such heterostructures have already led to the observation of numerous exciting physical phenomena. Thus, spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system. The possibility of positioning crystals in very close (but controlled) proximity to one another allows for the study of tunneling and drag effects. The use of semiconducting monolayers leads to the creation of optically active heterostructures. The extended range of functionalities of such heterostructures yields a range of possible applications. Now the highest-mobility graphene transistors are achieved by encapsulating graphene with hBN. Photovoltaic and light-emitting devices have been demonstrated by combining optically active semiconducting layers and graphene as transparent electrodes. OUTLOOK Currently, most 2D heterostructures are composed by direct stacking of individual monolayer flakes of different materials. Although this method allows ultimate flexibility, it is slow and cumbersome. Thus, techniques involving transfer of large-area crystals grown by chemical vapor deposition (CVD), direct growth of heterostructures by CVD or physical epitaxy, or one-step growth in solution are being developed. Currently, we are at the same level as we were with graphene 10 years ago: plenty of interesting science and unclear prospects for mass production. Given the fast progress of graphene technology over the past few years, we can expect similar advances in the production of the heterostructures, making the science and applications more achievable.

2D materials and van der Waals heterostructures

Encapsulated few-layer InSe exhibits a remarkably high electronic quality, which is promising for the development of ultrathin-body high-mobility nanoelectronics. A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound1,2. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes3,4. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103 cm2 V−1 s−1 and 104 cm2 V−1 s−1 at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides5,6,7 and black phosphorus8,9,10,11.

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High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit remarkable electronic and optical properties. The 2D features, sizable bandgaps and recent advances in the synthesis, characterization and device fabrication of the representative MoS2, WS2, WSe2 and MoSe2 TMDs make TMDs very attractive in nanoelectronics and optoelectronics. Similar to graphite and graphene, the atoms within each layer in 2D TMDs are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the physical and chemical properties of 2D TMDs layer-dependent. In this review, we discuss the basic lattice vibrations of 2D TMDs from monolayer, multilayer to bulk material, including high-frequency optical phonons, interlayer shear and layer breathing phonons, the Raman selection rule, layer-number evolution of phonons, multiple phonon replica and phonons at the edge of the Brillouin zone. The extensive capabilities of Raman spectroscopy in investigating the properties of TMDs are discussed, such as interlayer coupling, spin–orbit splitting and external perturbations. The interlayer vibrational modes are used in rapid and substrate-free characterization of the layer number of multilayer TMDs and in probing interface coupling in TMD heterostructures. The success of Raman spectroscopy in investigating TMD nanosheets paves the way for experiments on other 2D crystals and related van der Waals heterostructures.

Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material

Two-dimensional (2D) materials have attracted a great deal of interest in recent years. This family of materials allows for the realization of versatile electronic devices and holds promise for next-generation (opto)electronics. Their electronic properties strongly depend on the number of layers, making them interesting from a fundamental standpoint. For electronic applications, semiconducting 2D materials benefit from sizable mobilities and large on/off ratios, due to the large modulation achievable via the gate field-effect. Moreover, being mechanically strong and flexible, these materials can withstand large strain (>10%) before rupture, making them interesting for strain engineering and flexible devices. Even in their single layer form, semiconducting 2D materials have demonstrated efficient light absorption, enabling large responsivity in photodetectors. Therefore, semiconducting layered 2D materials are strong candidates for optoelectronic applications, especially for photodetection. Here, we review the state-of-the-art in photodetectors based on semiconducting 2D materials, focusing on the transition metal dichalcogenides, novel van der Waals materials, black phosphorus, and heterostructures.

Photocurrent generation with two-dimensional van der Waals semiconductors.

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

Two-dimensional crystals with a wealth of exotic dimensional-dependent properties are promising candidates for next-generation ultrathin and flexible optoelectronic devices. For the first time, we demonstrate that few-layered InSe photodetectors, fabricated on both a rigid SiO2/Si substrate and a flexible polyethylene terephthalate (PET) film, are capable of conducting broadband photodetection from the visible to near-infrared region (450–785 nm) with high photoresponsivities of up to 12.3 AW–1 at 450 nm (on SiO2/Si) and 3.9 AW–1 at 633 nm (on PET). These photoresponsivities are superior to those of other recently reported two-dimensional (2D) crystal-based (graphene, MoS2, GaS, and GaSe) photodetectors. The InSe devices fabricated on rigid SiO2/Si substrates possess a response time of ∼50 ms and exhibit long-term stability in photoswitching. These InSe devices can also operate on a flexible substrate with or without bending and reveal comparable performance to those devices on SiO2/Si. With these excelle...

High Performance and Bendable Few-Layered InSe Photodetectors with Broad Spectral Response

We experimentally demonstrate the visualization of exciton energy transport in a non-uniformly strained WSe2 monolayer by monitoring the propagation of exciton densities via spectrally, temporally, and spatially resolved photoluminescence measurements at room temperature. Our measurements indicate that excitons in the WSe2 monolayer exhibit anomalous diffusion due to disorder in the system, which leads to a time-varying diffusion coefficient. In addition, we show that the sensitivity of monolayer WSe2's bandgap to strain gives rise to a built-in excitonic potential that results in the funneling of excitons towards high tensile strain points. The observed drift and diffusion agree reasonably with our proposed model that takes into account the strain field on the monolayer to describe the exciton dynamics.

Exciton transport in strained monolayer WSe2

Combining organic and inorganic semiconductors enables us to integrate complementary advantages of each material system into a single hybrid material platform. Here, we report a study on the energy transport across a hybrid interface consisting of j-aggregates of organic dye and monolayer molybdenum disulfide (MoS2). The excellent overlap between the photoluminescence spectra of j-aggregates and the absorption of MoS2 B-exciton enables the material system to be used to study Forster resonance energy transfer (FRET) across the hybrid interface. We report a short Forster radius of 1.88 nm for the hybrid system. We achieve this by fabricating photodetectors based on the hybrid organic–inorganic interface that combine the high absorption of organics with the high-charge mobility of inorganics. Concomitantly, the hybrid photodetectors show nearly 93 ± 5% enhancement of photoresponsivity in the excitonic spectral overlap regime due to efficient energy transfer (ET) from j-aggregate to MoS2. This work not only p...

Efficient Energy Transfer across Organic-2D Inorganic Heterointerfaces.

We investigate the energy transport in an organic-inorganic hybrid platform formed between semiconductors that support stable room temperature excitons. We find that following photoexcitation, fast...

Energy Transport of Hybrid Charge-Transfer Excitons.

The enhanced transmission of higher order plasmon modes with random gold nanoparticles embedded in periodic hole arrays using asymmetric pair aperture as a unit is investigated in the midinfrared regime. Different thicknesses of gold film were deposited inside holes and then annealed to form randomly sized and distributed nanoparticles. The holes deposited with thin gold film exhibit significantly enhanced transmission in higher order modes after thermal annealing. The enormous local electric field around the nanoparticles enhances the scattering effect that contributes to the enhanced infrared transmission. This unique design, which integrates localized and propagating surface plasmons, provides an easy way for midinfrared applications in need of enhanced transmission in higher order modes.

Che-Hsuan Cheng

Papers

High Performance and Bendable Few-Layered InSe Photodetectors with Broad Spectral Response

Exciton transport in strained monolayer WSe2

Efficient Energy Transfer across Organic-2D Inorganic Heterointerfaces.

Energy Transport of Hybrid Charge-Transfer Excitons.

Enhanced Transmission of Higher Order Plasmon Modes With Random Au Nanoparticles in Periodic Hole Arrays