Recent Advances in Optoelectronic Devices Based on

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Recent Advances in Optoelectronic Devices Based on 2D Materials and Their Heterostructures Jinbing Cheng, Chunlan Wang, Xuming Zou,* and Lei Liao* material films can be used for minimizing equipment.[17] Xu et al. fabricated flexible tactile sensor based on graphene films for a smart panel. The device was assembled on a polyethylene terephthalate substrate and displayed a high sensitivity of 0.23 mm−1.[18] Yao and Yang designed an all-2D Bi2Te3–SnS–Bi2Te3 photodetector for a wearable device, which was fabricated on polyimide substrate.[19] A good bending durability with unaltered photoresponse was achieved after 100 bending cycles. 2D material can also be stacked to form various kinds of heterostructures with desired functionalities without lattice matching constraints.[20] Their applications in nanoelectronics and optoelectronics are focused with wide interests.[21–27] Compared with traditional bulk materials, 2D materials have many natural advantages in the field of photodetectors. Among them, graphene was first discovered and studied as early as 2004.[2] Because of the reduced feature dimensions, quantum confinement effects in 2D systems are particularly significant.[28] Graphene and graphene-based materials show great potential application in the field of new energy and optoelectronics due to their excellent characteristics, such as high electron mobility, high transmittance, and large specific surface area.[29] Different from graphene, TMDs can range from semiconductors to semimetals/metals due to the presence of unsaturated d-orbital transition metals.[30] The bandgap of TMDs can be tuned by changing the number of layers in the crystal, which is especially suitable for photodetectors.[31] The emergence of 2D BP has greatly enriched the 2D material family. Since its successful exfoliation in 2014, it has attracted worldwide attention.[32,33] Additionally, as an emerging semiconductor material, BP has a unique structure similar to that of graphene. In the monoatomic layer, each phosphorus atom is covalently attached to the three adjacent phosphorus atoms to form a wrinkled honeycomb structure, and the layers are stacked together by van der Waals forces.[34] Compared with the zero bandgap of graphene and the indirect bandgap of TMDs (multilayers), BP is a direct bandgap semiconductor. This means that BP has a good photoelectric conversion efficiency, and it can well achieve the “on” and “off” states of the current. The unique properties in BP and TMDs make them the core material for a new generation of semiconductor industry. At present, as the increasing requirements for the integration and functionalization of optoelectronic devices, 2D material

2D materials, such as graphene, transition metal dichalcogenides, and black phosphorus, have become the most potential semiconductor materials in the field of optoelectronic devices due to their extraordinary properties. Owing to the layer-dependent and appropriately sized bandgaps, photodetectors based on various 2D materials are designed and manufactured rationally. Utilizing the unique properties of 2D materials, many surprising physical phenomena of junctions based on 2D materials can be obtained after different 2D materials are stacked together. This makes heterojunctions more popular than 2D materials themselves, and the design of 2D materials for human beings is easier than ever. In this review, recent progress in optoelectronic applications based on 2D materials and their heterojunctions is summarized and discussed.

1. Introduction In 1959, Richard Feynman first proposed the view of layered materials in one question: “What could we do with layered structures with controlled layers?”[1] After years of relentless effort, Novoselov et al. discovered graphene[2,3] and its many unique properties,[4–8] attracting considerable attention for layered materials.[9–12] We are now approaching the answer with different categories of 2D materials.[13] To date, the family of 2D materials has extended to graphene, transition metal dichalcogenides (TMDs), black phosphorus (BP), and so on. These materials possess unique structural and physical properties due to their atomic thickness, and their properties can be extensively tuned by doping and introducing an external field.[14–16] Based on their mechanical flexibility, 2D Dr. J. Cheng, Prof. L. Liao School of Physics and Technology Wuhan University Wuhan 430072, China E-mail: [email protected] Prof. C. Wang School of Science Xi’an Polytechnic University Xi’an 710048, China Prof. X. Zou, Prof. L. Liao Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education School of Physics and Electronics Hunan University Changsha 410082, China E-mail: [email protected]

DOI: 10.1002/adom.201800441

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heterojunctions with excellent optoelectronic properties have received more and more attention by researchers. Not only can 2D material heterojunctions realize the special carrier transfer behavior, the interlayer quantum coupling effect in the heterojunctions can also lead to novel physical properties. The electrical and optical properties of the devices can be modulated by adjusting the heterostructure interfaces. Realizing effective control of 2D material heterojunction structure is the premise of constructing high-performance and highly integrated devices. Here, we will introduce the synthesis methods of 2D materials and the structure, working principle, and performance of heterojunction-based optoelectronic devices.

2. Basic Properties of 2D Materials 2D materials usually refer to crystal materials consisting of a single or several atomic layers, of which the thickness ranges from one atomic layer to 10 nm. To date, many 2D materials have been successfully manufactured and researched. In this section, we will focus on those materials used for optoelectronic performance, including graphene, TMDs, and BP, as shown in Figure 1.

2.1. Graphene Graphene is a 2D allotrope of carbon consisting of a six-membered cyclic arrangement of carbon atoms (Figure 1a).[35] Several layers of graphene are stable in air,[2] and their polarity is changed by graphene oxide doping.[36] Furthermore, the linear electronic structure of graphene allows the work function to be highly tuneable via an external electric field,[37] chemical modification,[38–40] metal configuration,[41] or thermal annealing.[42] The carrier mobility of graphene reaches 106 cm2 V−1 s−1 at 1.8 K and 105 cm2 V−1 s−1 at room temperature,[43–46] exhibiting broad application prospects. Graphene offers tremendous opportunities and flexibility for infrared and visible light manipulation (Figure 1d).[47–50] As a series of unique crystal lattice structures, monolayer graphene theoretically absorbs 2.3% of incident white light, and the opacity increases by 2.3% after adding a layer.[51]

Jinbing Cheng is currently a Ph.D. student in School of Physics and Technology at Wuhan University under the supervision of Prof. Lei Liao. His research interest is mainly focused on the 2D materials for nanoelectronic device application.

Chunlan Wang is currently a Professor at School of Science, Xi’an Polytechnic University. She received her B.S. and Ph.D. degrees under the direction of Prof. Lei Liao at Department of Physics, Wuhan University. Her research is focused on the high-performance thin film transistor based on nanomaterials, and high-speed radio frequency devices, e.g., high-speed graphene transistors. Xuming Zou received his B.S. and Ph.D. degrees from Wuhan University in 2006 and 2016, respectively. He is currently an Associate Professor in School of Physics and Electronics at Hunan University since 2016. His research interests focus on high-performance optoelectronic devices based on 2D materials.

2.2. Transition Metal Dichalcogenides There are more than 40 types of TMDs; the basic chemical formula is MX2, in which M is a transition metal element and X is a chalcogen element, such as MoS2, WS2, and MoSe2. Similar to graphene, TMDs also have a lamellar structure, and the hexagonal symmetric unit cells are stacked together (Figure 1b). The thickness of each layer is ≈6–7 Å. TMDs have shown a direct bandgap (1–2.5 eV) with the ability to replenish graphene. Due to the quantum confinement effect and surface effect, monolayer and few-layer TMDs exhibit properties different from bulk materials. In addition, the bandgaps in TMDs have also been shown to be tuneable through an external electric field or mechanical strain from a semiconductor to a near metal, and their photoelectronic responsivity is easy to modulate.[52–56] In particular, TMDs can be applied to photovoltaic applications and the design of powerful ultrathin field-effect transistor (FET)

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structures, which can attain subthreshold swings of ≈60 mV dec−1 and Ion/Ioff ratios of up to 108.[57] TMDs are inherently highly flexible and capable of carrier transport at the atomic level. Therefore, films made from these materials are ideally suited for use as flexible substrates, which have great prospects as portable and flexible electronic products.[58]

2.3. Black Phosphorus In general, bulk layered BP shows a wrinkled structure (Figure 1c). BP is an allotrope of the most stable phosphorous at atmospheric pressure. BP has a narrow bandgap (0.3–2 eV), which can

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results in an adhesive residue on the BP surface. Therefore, an improved method was used, and BP was repeatedly thinned by a tape and pressed to a polydimethylsiloxane (PDMS) substrate. They obtained two-layer BP nanosheets by this means.[63] The surface of the sample prepared by this method is clean, and the sample is of high quality, which is suitable for studying the basic characteristics of the device. However, the efficiency of obtaining the samples is relatively low. The size of the prepared sample is small, and the thickness of the sample layer cannot be controlled. Therefore, this method has a great limitation in large-scale industrialization studies. Figure 1. a–c) 2D materials as functional layers for optoelectronic devices: schematic diagrams of a) graphene structure, b) TMD monolayer, and c) BP. d) Electronic bandgaps of typical 2D materials. Intrinsic single-layer graphene is a zero-bandgap semimetallic material, while its Fermi level can be tuned up to 1 eV under an external electric field, covering the range from terahertz to visible wavelength. The transition metal dichalcogenides have layer numberdependent bandgaps ranging from 1.0 to 2.5 eV. Direct-bandgap black phosphorus also has layer number-dependent bandgaps ranging from 0.3 to 2 eV.

be modulated by its layer number, and can absorb light from visible to near-infrared (NIR), as shown in Figure 1d. The carrier mobility of few-layered BP can reach 10 000 cm2 V−1 s−1.[33] In addition, it also has a high on–off ratio (105) and exhibits ambipolar behavior, opening the possibility of complementary 2D semiconductor electronics. Engel et al. implemented a multilayer BP detector with stable performance.[59] There is no obvious signal fluctuation during the data acquisition. In addition, if the gain of the detector is increased, the image quality is further improved.

3. Synthesis Techniques for 2D Materials 3.1. Micromechanical Exfoliation Micromechanical exfoliation method is the first physical method for preparing graphene. Novoselov et al.[2] performed dry plasma etching on 1 mm thick highly oriented pyrolytic graphite surface, which was then bonded to a glass substrate, placed on a photoresist surface, baked, repeatedly peeled off, and washed in acetone solution. Finally, the remaining graphite on the glass substrate was treated with propanol to obtain single-layer graphene. In addition to graphene, NbSe2,[4] MoS2,[60] and WS2[61] monolayers were also obtained by using this procedure. Most wafers have a diameter of less than 20 µm. Except for the single layer, several layers can also be realized. Zhang and co-workers used adhesive tape to dissociate BP to a thin layer by using the exfoliation method of graphene.[62] In this way, BP nanosheets (5 nm) were obtained, and the resulting FET Ion/Ioff ratios were as high as 105. They also found that transistor carrier mobility was related to the thickness of BP, which can be maximized when the thickness was 10 nm. The conventional mechanical exfoliation method

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3.2. Liquid Exfoliation

Liquid-phase exfoliation is capable of producing a large number of samples. Crystal powder is mainly dispersed by a liquid dispersion medium, and an external force is provided by means of ultrasound and centrifugation. 2D materials are dispersed in the solution; the subsequent modification and coating are also more convenient. Layered materials, such as TaSe2, MoTe2, boron nitride (BN), WS2, and MoS2, can be easily obtained by this method.[64–67] Organolithium compounds are common intercalating agents and have been used for the intercalation of TMDs. Yang and Frindt used butyllithium as an intercalating agent of WS2 crystals at room temperature.[68] It was found that the lithium-ion concentration between the WS2 layers was not significantly affected as the intercalation time was prolonged. However, the interlaminar lithium-ion content was improved with the increased temperature. When the temperature was 100 °C, the x value in the LixWS2 could reach 1.45. Then, LixWS2 can be completely exfoliated in the aqueous solution to attain WS2 nanosheets. Fan et al. found that ultrasound can assist in the complete intercalation of butyllithium in hexane solution, and the intercalated MoS2 can be exfoliated after being sonicated in an aqueous solvent for 5 min.[69] The ion-exchange method is based on the ion exchange between the external ions and interlayer ions of a layered crystal material and introduces ions with a stronger hydration capacity and larger hydration radius into the interlayers, thereby increasing the lamellar spacing of the layered crystal and weakening the interaction force of the interlamella to achieve the liquid layered crystal and the corresponding 2D nanomaterials. In the ion-exchange method, the driving force for realizing the ions into the material layer mainly comes from the ion osmotic pressure balance.[70] Another strategy for obtaining 2D materials is to use ultrasonic waves in the solvents. The sonic-assisted liquid-phase exfoliation method disperses the bulk material into a solvent and degrades it. The formation of bubbles in the process of cracking produces microjets and vibration waves, and the concentrated tensile stress that occurs in the bulk material assists

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in the exfoliation.[71] Then, the crystals are separated by centrifugation. Figure 2a–c shows a schematic view of the above liquid exfoliation. Due to the uneven distribution and poor quality of the obtained samples, mechanical exfoliation is still the first choice for cleaving layered 2D nanomaterials.[9]

3.3. Chemical Vapor Deposition The chemical vapor deposition (CVD) method is a common method for preparing semiconductor film crystals due to the large area, high quality, and uniformity of the products obtained. The principle is based on the redox reaction between

the reactants to generate solid precipitate films. Compared with other methods, CVD technology has the advantages of high repeatability, convenient operation, and strong flexibility. The CVD method has been applied to synthesize large-area graphene at the centimeter level.[72,73] 2D TMDs have also been synthesized this way. Usually, MoO3 or WO3 and S or Se powders are used as the sources in the vapor phase to form the TMDs. This approach has shown inspiring diversity and has been used to synthesize W- and Mo-based dichalcogenides as well as for doping,[74] alloying,[75] and forming heterojunctions.[76] Lee et al.[77] demonstrated the CVD method to synthesize large-scale monolayer MoS2 films. They reported a CVD method that used a very small amount of MoO3 (0.01 mg),

Figure 2.  Mechanism diagram of the liquid exfoliation process: a) intercalation, b) ion exchange, and c) ultrasonication. Reproduced with permission.[71] Copyright 2013, American Association for the Advancement of Science. d) Large-area MoS2 film synthesized via the CVD process. Reproduced with permission.[77] Copyright 2017, Wiley-VCH. e) Two-step vapor epitaxy growth of WSe2/SnS2 heterostructure. Reproduced with permission.[87] Copyright 2017, Springer Nature.

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which induced low supersaturation and reduced nucleation density dramatically over the whole substrate surface (Figure 2d). Sulfur powder, pure argon, and 300 nm SiO2/Si were used as the precursor material, carrier gas, and receiving substrate, respectively. By using solution-processed precursor deposition and the evaporation of very thin precursor layers, low supersaturation levels were induced, which significantly reduced the nucleation density in the thermodynamically stable environment, resulting in uniform and clean monolayers and large crystal sizes of up to 500 µm. Based on the same method, the authors also synthesized large single-crystal WS2. The crystal size was increased up to 360 µm with a low nucleation density of 27 mm−2. Zhang and co-workers used a three-zone tube furnace to grow large-area single-crystal MoS2.[78] Argon/oxygen gas and sapphire (c-face) were used as the carrier gas and receiving substrate, respectively. Different from other growth methods, this method introduced a small amount of oxygen to reduce nucleation density. In addition, sulfur powder and MoO3 were installed in two small tubes to avoid cross-contamination. A large (350 µm) single-layer MoS2 was obtained through this method. The device exhibited excellent performance with a high mobility of 90 cm2 V−1 s−1. During the CVD reaction, the selection of the growth substrate is also extremely important. Loh and co-workers reported the rapid growth of high-quality millimeter-sized single-layer MoSe2 crystals on molten glass.[79] As a substrate, glass has few defects and high homogeneity, which provides favorable conditions for large-area growth. Monolayer MoSe2 with a large dimension of 2.5 mm was achieved, which is the largest reported to date. In addition, the growth of polycrystalline graphene disks with average diameters of 0.8–1.1 µm has also been attained using the same substrate at 1000 °C.[80] The glass substrate provides a smooth surface and reduces the nucleation point, thus facilitating the growth of large single crystals and adding a new substrate selection for the growth of other materials.

reported the direct epitaxial growth of WSe2/SnS2 vertical junctions on the SiO2/Si substrate, with lateral dimensions on the millimeter scale (Figure 2e).[87] The heterostructure was synthesized via a two-step strategy. First, large-scale WSe2 monolayers (≈350 µm) were grown on the SiO2/Si substrate through a physical vapor deposition (PVD) process at 1100 °C. Second, the prepared WSe2 monolayers were used as templates for subsequent epitaxial growth of SnS2 monolayers to achieve the vertical two-layer heterostructures. Compared with the mechanical stacking approach, the direct growth strategy has the advantages of easy size control, good interface contact, and great application potential.

3.5. Hydrothermal Synthesis Hydrothermal synthesis or solvothermal synthesis is a growth method that utilizes water or another solvent as the reaction medium to dissolve and recrystallize commonly insoluble substances in a high-temperature and high-pressure environment. The reaction temperature is generally 100–1000 °C, and the pressure range is from 1 MPa to 1 GPa. With features such as simple operation, low pollution, low cost, and high purity, this synthesis method is full of potential. In 2001, Qian and coworkers synthesized single molecular-layer MoS2 and MoSe2 by a hydrothermal method at 150–180 °C.[88] Furthermore, Chen and co-workers reported the use of the one-pot solvothermal route to produce 4–8-layered WSe2 nanosheets.[89] Zhang et al. dissolved selenium powder and sodium molybdate in hydrazine hydrate and then transferred the solution to a closed reaction vessel at 200 °C to obtain nanoflower-like MoSe2.[90] In addition to the synthesis of TMDs, it is also possible to synthesize 2D metal oxide nanosheets, such as TiO2 nanobelts, which can be prepared in alkaline- and acid-assisted hydrothermal processes and are expected to be used as a photocatalytic material.[91] Characterization methods of X-ray diffraction and transmission electron microscopy have shown the good crystal structure of 2D nanomaterials by this technique.

3.4. Van der Waals Epitaxial Growth on a Substrate The recent resurgence of interest in TMDs has given rise to a dramatic expansion of the number of materials and heterostructures grown by van der Waals epitaxy, such as WSe2, WTe2, MoSe2, SnSe2, PtSe2, and ReSe2.[81–86] Monolayer PtSe2 film was achieved by this method. The molecular beam epitaxy (MBE) technology was used to evaporate high-purity Pt and Se in an ultrahigh vacuum to grow PtSe2 films on the surface of graphene.[85] MBE provides good control of the growth kinetics and enables large, high-quality single-crystal films with controlled film thicknesses. The growth conditions are selected for practical purposes, such as preserving ultrahigh vapor pressure and maintaining reasonable growth times and compatibility with the thermal budget constraints of the substrate. Substrate temperature drives much of the growth because the activated barrier for desorption and surface diffusion is governed by this parameter. Another important role of epitaxial growth is the synthesis of 2D material heterostructures. Recently, Pan and co-workers

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4. Photodetectors Based on 2D Materials 4.1. Mechanisms of Photodetection in 2D Materials Photocurrent generation mechanisms are generally divided into photovoltaic effect, photo-thermoelectric effect, and photo­ bolometric effect. In the photovoltaic effect, electron–hole separation is caused by a built-in electric field. Schottky barriers at the metal/semiconductor interface may generate the built-in electric field. Photodetectors under this photovoltaic effect mechanism are often referred to as photo­ diodes. The photo-thermoelectric effect means that when the semiconductor is affected by nonuniform light-induced heating, a temperature gradient will be generated in the channel, and carriers migrate from the high-temperature zone to the low-temperature zone under the action of a temperature gradient. These carriers accumulate at the end of the low temperature, producing the potential. The photobolometric effect

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Table 1.  Key parameters of merit for photodetectors. Parameter Responsivity

Response time

Expression

R=

Unit

Symbol definition

−1

Ip is the photocurrent. P is the light power.

A  W

Ip P

τr/τf

s

The time required for the photocurrent to change from 10% to 90% of the excitation light modulation.

Dark current

ID

A

Dark current

Noise current

iN

A Hz−1/2

Noise current

iN R

W Hz−1/2

The minimum detectable light power of a detector. It is equal to the light power when the signal-to-noise ratio is 1.

EQE =

hcIP eλP



The external quantum efficiency refers to the ratio of the number of carriers that contribute to the photocurrent and the number of incident photons, where h is the Planck’s constant, c is the speed of light, λ is the wavelength of the incident laser, and e is the unit charge.

D* =

A

cm Hz1/2 W−1 (Jones)

A is the effective area of the detector in units of cm2, and iN is the noise current.

Noise equivalent power

External quantum efficiency

Specific detectivity

NEP =

iN

R

is based on the change of the resistivity of the material caused by uniform heating under illumination. The magnitude of this effect is directly proportional to the conductivity variation of the material and the temperature increment caused by light irradiation heating. Unlike the photo-thermoelectric effect, the photothermal radiation effect does not drive the current, but only changes the current intensity under external bias and illumination. Widely used photodetector performance indicators include responsivity (R), time response (τ), specific detection rate (D*), and spectral selectivity. These parameters are given in Table 1. However, it is not possible to achieve both ultrahigh responsivity and ultrashort response time in practice. Figure 3 shows the photoresponsivities and response times of photodetectors based on various 2D materials.[92–107]

Figure 3.  The responsivity and response time of photodetectors based on various 2D materials. Data from refs. [92–107].

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4.1.1. Photodetectors Based on Graphene Graphene has many good regulatable properties, and its Fermi level can be modulated in various ways. Therefore, graphene’s superior controllability enables it to be widely used in a large spectral range. In 2009, Mueller et al. used scanning photoelectric imaging technology to investigate the photodetector mechanism of graphene FETs.[108] Due to the unique zero bandgap structure of graphene, the metal electrode and the carrier injection effect can modulate the Fermi level under a bias voltage, which leads to the bending of the interface energy band in which the metal electrode and graphene form contacts, thereby forming a built-in electric field to drive the separation and transmission of the photo­generated carriers. It can be clearly seen that the photocurrent signal is the strongest at the metal–graphene interface and becomes weaker at a distance away from the electrode. Later, Xia et al. designed one of the ultrafast photodetector transistors based on single- and few-layer graphene (Figure 4a).[109] They achieved an ultrafast photodetector with a bandwidth of 40 GHz. According to their predictions, the bandwidth of the graphene photodetector could be further increased to 500 GHz, which had a huge impact on the high-speed detection field. However, the responsivity was extremely low due to the symmetry of the electric field. Here, an asymmetric metallization scheme is used to break the mirror symmetry of the internal electric-field distribution. In 2013, Pospischil et al. designed a graphene electro-optical modulator.[110] The asymmetric arrangement of the electrical contacts with respect to the waveguide also helped to improve responsivity; the device exhibited a flat response over a wide range of wavelength. Interfacial gating effect is also used to enhance the performance of photodetectors. Guo et al. reported an ultrafast and ultrahigh sensitivity photodetector by using a graphene/SiO2/Si composite architecture (Figure 4b).[111] The hybrid structure can separate the photoexcited electron–hole pairs with an intrinsic self-built electric field at the SiO2/Si interface and accumulate charges at the graphene/gate interface. The electron–hole pairs in silicon are separated, and the holes diffuse into Si while

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Figure 4.  a) High-bandwidth graphene photodetector. Reproduced with permission.[109] Copyright 2009, Springer Nature. b) High-sensitivity graphene photodetector. Reproduced with permission.[111] Copyright 2016, Optical Society of America. c) Plasmon resonance-enhanced graphene photodetector. Reproduced with permission.[114] Copyright 2011, Springer Nature. d) Graphene–silicon waveguide optical modulator. Reproduced with permission.[116] Copyright 2011, Springer Nature.

the electrons are retained at the SiO2/Si interface. A negative voltage is formed at the graphene/gate interface. Due to the gating effect, the hole density of graphene is increased, thus generating a high photocurrent. The device showed superior performance under a very weak signal (