Transition metal dichalcogenides (TMDCs) heterostructures: Optoelectric properties

Abstract

Transition metal dichalcogenides (TMDCs) have suitable and adjustable band gaps, high carrier mobility and yield. Layered TMDCs have attracted great attention due to the structure diversity, stable existence in normal temperature environment and the band gap corresponding to wavelength between infrared and visible region. The ultra-thin, flat, almost defect-free surface, excellent mechanical flexibility and chemical stability provide convenient conditions for the construction of different types of TMDCs heterojunctions. The optoelectric properties of heterojunctions based on TMDCs materials are summarized in this review. Special electronic band structures of TMDCs heterojunctions lead to excellent optoelectric properties. The emitter, p-n diodes, photodetectors and photosensitive devices based on TMDCs heterojunction materials show excellent performance. These devices provide a prototype for the design and development of future high-performance optoelectric devices.

Graphical abstract

Keywords

transition metal dichalcogenides (TMDCs) / heterostructures / optoelectric properties

1 Introduction

Graphene has sparked a wave of research into two-dimensional (2D) materials since its discovery in 2004 [1]. The discovery of graphene has changed the understanding of traditional nanomaterials and opened the door of novel 2D nanomaterials. Subsequently, 2D materials such as black phosphorus [2, 3], transition metal dichalcogenides (TMDCs) [4], borophene [5,6], and others [7] aroused the interest of researchers. The structure formed by the upper and lower layers of chalcogen elements sandwiching a layer of transition metal elements is called transition metal dichalcogenides (TMDCs) [8], whose chemical formula is MX₂, M stands for transition metal element, which is sixfold coordination, and X stands for chalcogen, which is threefold coordination [9]. And some of the structures belong to the lattice structure of the hexagonal system, and some belong to the lattice structure of the triangular system. The TMDCs layers are connected by weak van der Waals force (vdW), while the M and X atoms within the layer are connected by covalent bonds (M-X bonds) [10]. The layered transition metal dichalcogenides, such as MoSe₂, MoS₂, WSe₂ and WS₂, are semiconductors and the wavelengths corresponding to the band gaps of TMDCs are between the infrared region and the visible light region [11,12]. TMDCs show excellent electrical [13,14], optical [15-18] and catalytic [19] properties. Due to these excellent properties, TMDCs can serve as candidate materials for optoelectronic devices [16, 20], photovoltaic devices [21], energy storage devices [10, 22, 23], catalysis [24], etc.

Van der Waals heterojunctions composed of TDMCs have not only ultrathin, flat, and almost defect-free surfaces, but also excellent mechanical flexibility [25] and chemical stability [26-29]. More importantly, van der Waals heterojunctions combine the excellent properties of two or more constituent materials into one, and can exhibit better electrical, optical [4, 30-32] and catalytic properties [33]. TMDCs materials can form vertical heterojunctions and horizontal heterojunctions. TDMCs heterojunctions provide new physical properties and improved device performance, which can be synthesized by Chemical Vapor Deposition (CVD) [34], dry transfer [35] and epitaxial growth [36] methods. As one of the most competitive materials in the design of optoelectrical devices, TMDCs materials have the advantages of wide detection wavelength range, high photoresponsivity, and sensitivity to polarization. TMDCs heterojunctions can be used in optoelectronic devices [37, 38], particularly in light-emitting diodes (LEDs) [39], fabricating field-effect transistor (FET) [40], p-n diodes [41], etc.

2 Transition metal dichalcogenides (TMDCs) heterostructures

Appropriate band gap of 2D materials is an important parameter for their application in optoelectronic devices. Fig.1 is the band gap of several common 2D materials. Among them, TMDCs materials have attracted the attention of researchers due to their tunable band gaps [42]. The band gap of TMDCs can be adjusted by changing thickness or forming heterojunctions with other materials. The semiconducting TMDCs are mainly the 2H phase [43]. For example, the graphene/MoS₂ heterojunction is a direct bandgap semiconductor with a bandgap of 2 meV at K point [44]. MoS₂/h-BN shows the property of indirect band gap with a CBM at the K point and a VBM at the \mathrm{\Gamma } point and increasing the number of h-BN layers will cause the band gap to decrease [45, 46]. The band gap of MoTe₂ is about 1 eV, which can be used as infrared photodetector, but the small mobility of MoTe₂ limits its application in optoelectronic devices. The combination of MoTe₂ and graphene to form a heterojunction can solve this problem [47]. TMDCs/metal/TMDCs/graphene heterostructure materials can be used to design high-sensitivity angular and phase SPR sensors [48]. TMDCs-based 2D heterostructures can be formed by two types of stacking, horizontal and vertical stacking. The former is usually synthesized by “bottom-up” method, while the latter can be synthesized by “bottom-up” and “top-down” methods [49].

According to the different arrangement characteristics of valence and conduction bands of heterojunction materials at the interface, the heterojunctions can be divided into three types, straddling, staggered and broken heterojunctions as shown in Fig.2. Each type can be applied to different kinds of equipment. Straddling heterojunctions are widely used in optical devices, such as light-emitting diodes (LEDs) [50], due to electrons and holes can be spatially confined, allowing for effective restructuring. Staggered heterojunctions have the potential to be used as unipolar electronic devices because a higher offset (conduction or valence band) on one side could occur in this type of heterojunction, and results in extremely strong carrier limitations [30]. Staggered and broken heterojunctions are widely used in tunneling field effect transistors (TFETs) [52] and wavelength photodetectors [53]. The layered transition metal dichalcogenides (MoS₂, WS₂, and WSe₂) can form staggered heterojunctions. The VBM and CBM of staggered heterojunction are distributed in different transition metal dichalcogenides materials. The position of the conduction band minimum depends on the material with a narrower bandgap, and the position of valence band maximum is related to the material with a wider bandgap. The staggered heterojunction has important application value in the fields of photovoltaic, photoelectric detection and solar cells. All in all, heterojunctions formed by different transition metal chalcogenides can be used to control the generation, transport and transition properties of free carriers, excitons, phonons and photons at the interface of atomic layers. Therefore, the construction of van der Waals heterojunctions by using transition metal chalcogenides provides a new way to utilize materials, which has played a huge role in the research and development of special optoelectronic functional devices [30].

3 First-principles studies of TMDCs heterojunctions

First-principles calculations also play an important role in the study of TMDCs heterojunctions. For example, the calculations of IVB-VIA group TMDCs show that after absorbing photons, electronic transitions mainly occur between the first, second, and third valence band and the first conduction band. The parallel band effect occurs in monolayer structures means that there is strong optics-matter interaction in these TMDCs materials [54]. Calculations for VIIB-VIA group TMDCs show that the band gap is between 1.70 eV and 2.12 eV. And the positions of the highest occupied state in the valence band and the lowest occupied state in the conduction band are mainly determined by the d electron states of the transition metal atoms [11]. The calculations of the IVB-VIA transition metal trisulfide (MX₃) show that it is suitable for photocatalytic water splitting [55].

Mu et al. [56] investigated the interfacial charge transfer in vertical and lateral MoS₂/WS₂ heterojunctions by first-principles calculations. The energy bands of the two structures are shown in Fig.3(a). The dielectric function of the vertical MoS₂/WS₂ heterojunction shows a strong negative real part near 465 nm, which indicates that there is a plasmon effect and manifested as a strong optics-matter interaction. There is a strong charge transfer (CT) exciton peak at 800 nm corresponding to a direct transition and an indirect transition marked in the band, the electrons are located on MoS₂ and the holes are located on WS₂ as shown in Fig.3(b). For the lateral heterojunction, the imaginary part of the dielectric constant does not show a negative value, so there is no plasmon properties. And there is a weak absorption peak at 700 nm corresponding to the intralayer CT excitons corresponding to the direct transition of \mathrm{\Gamma } point.

By adjusting the radius of the Ag disk, the surface plasmon resonance (SPR) peak of the Ag disk can matched with the SPR peak and CT exciton peak of the heterojunctions, respectively. When the radius of the Ag disk is 75 nm, it matches the SPR peak of the vdW heterojunction, and a strong coupling occurs between them, which makes the SPR peak shift by 124.75 meV, and Rabi splitting occurs. The SPR peak of Ag disk (the radius is 145 nm) matches the CT exciton peak of the vdW heterojunction, and then they are weakly coupled to produce the Purcell effect as shown in Fig.4(a). Since there is no SPR peak in the lateral MoS₂/WS₂ heterojunction, the SPR peak of Ag is coupled with the CT exciton peak of the heterojunction. When its radius is 40 nm, the SPR peak of Ag is coupled with the strong absorption peak, and Rabi splitting occurs. When the radius of the Ag disk is 88 nm, the SPR of Ag is weakly coupled with CT excitons, resulting in a 90.4 nm shift of the SPR peak as shown in Fig.4(b).

Fan et al. [57] analyzed pressure-dependent charge-transfer excitons in WSe₂−MoSe₂ heterostructures in 2021. As the pressure increases, the conduction band of WSe₂ near the Fermi level gradually decreases, and the valence band of MoSe₂ near the Fermi level gradually increases, which results in the characteristics of direct and indirect band gaps varying with pressure, and the trend is different, as shown in Fig.5(a). The effective mass of electrons and holes also changes with pressure, the effective mass of electrons decreases with the increasing pressure, while that of holes first decreases and then increases [Fig.5(b)]. The change of energy band and effective mass causes the change of intrinsic carrier concentration. When the pressure increases, the intrinsic carrier concentration of direct band gap first decreases and then increased slightly with a 4.16 GPa pressure, and that of indirect band gap gradually increases as shown in Fig.5(c). The pressure-dependent absorption spectra show that the CT exciton absorption peak at 825 nm red-shifts with the increasing pressure, which is consistent with the change of energy band. And when the pressure increases to 1.6 GPa, the direction of charge transfer changes. When the pressure continues increasing to 4.16 GPa, the direction of charge transfers no longer changes, but the degree deepens. The increase in pressure reduces the spacing of the heterojunctions and changes the interfacial state of WSe₂−MoSe₂ heterojunctions.

4 The optoelectric properties of TMDCs heterostructures

4.1 WSe₂/MoS₂ heterojunction-based p−n diode

The most basic component of most optoelectronic devices is the p−n diode [41]. Due to the difficulty in selectively doping TMDCs into p-type or n-type semiconductors, it is very difficult to manufacture p−n diodes in layer TMDCs. Chenget al. [41] synthesized monolayer WSe₂ with 300 nm thick Si/SiO₂ as the substrate, and the triangular region in the center was bilayer [Fig.6(a)], the exfoliated MoS₂ sheets were transferred to the WSe₂ structure to assemble into a vertical WSe₂−MoS₂ heterojunction [Fig.6(b)]. Photoluminescence (PL) mapping in Fig.6(c) can clearly distinguish MoS₂ and WSe₂ layers. The electrodes were established by electron beam lithography and evaporation [Fig.6(d)].

The PL spectra of the WSe₂ samples exhibit a strong layer-dependent performance [Fig.7(a)], where the PL intensity of monolayer WSe₂ is 10 times stronger than that of bilayer WSe₂. It can be seen that monolayer WSe₂ shows a PL peak at the wavelength of 785 nm. The PL of bilayer WSe₂ also shows the 785 nm exciton peak and a broad peak at the wavelength of 877 nm, which can be attributed to the indirect band gap emission. There is a peak at 677 nm of the MoS₂ PL spectrum. Fig.7(b) and (c) show a linear relationship between I_ds and V_ds of MoS₂ and WSe₂, which indicates that both materials achieve Ohmic contacts, so the heterojunction could demonstrate excellent electronic and optoelectronic properties. The current of MoS₂ increases as the positive gate voltage increasing, which is typical n-type semiconductor. On the contrary, the current of the WSe₂ FET increases as the negative gate voltage increasing, and it is the p-type. Once Ohmic contact is formed, the I_ds−V_ds of the WSe₂/MoS₂ heterojunction shows obvious current rectification behavior. Only when the WSe₂ semiconductor is positively biased, the current can pass through the heterojunction device, suggesting that the formation of a p−n diode in the WSe₂/MoS₂ heterojunction [Fig.7(d)]. Moreover, due to WSe₂ layer, the current is inversely proportional to the magnitude of the positive V_g. If V_g is 0, the ideality factor of the heterojunction device is 1.2, and when the gate voltage is −20 V, the ideality factor is 1.3, which indicating that the device shows good diode performance.

The zero-bias photocurrent mapping with 514 nm laser excitation shows that the WSe₂ and MoS₂ overlap region has obvious light response, indicating that p−n junction is formed in the overlap region. Due to the generation of photocurrent depends on the direct bandgap, the ML-WSe₂/MoS₂ area shows larger photocurrent than the BL-WSe₂/MoS₂ region [Fig.8(a)]. The output characteristic curve of the p−n diode shows that the V_open-circuit is 0.27 V and the I_{short-circuit} is 0.22 µA, which indicates obvious photovoltaic effect. And the rapid photo response indicates that it is caused by the generation of photocarrier. Fig.8(c) and (d) show the Electroluminescence (EL) spectra of monolayer WSe₂/MoS₂ heterojunction and bilayer WSe₂/MoS₂ heterojunction, respectively. Their strength increases with the current. Fig.8(e) shows that there are both threshold currents in two types of heterojunctions, and the threshold current of monolayer is lower than that of bilayer heterojunction. Fig.8(f) shows the explanation of the physical mechanism. The potential barrier of hole transport is smaller than that of electron. As the positive bias increases, the holes of WSe₂ transfer to MoS₂, and the electrons cannot overcome the barrier. Because few layers MoS₂ is indirect band gap semiconductor, the yield of radiation composite is relatively low at this time, the EL spectrum is very weak. As V_bias increases, the energy of the conduction band minimum of MoS₂ increases, both electrons and holes could overcome the barrier, the radiation composite in WSe₂ is dominant in EL, and the intensity increases linearly with current.

4.2 MoTe₂/graphene heterostructure photodetectors

TMDCs materials show excellent performance in photoelectric devices [58], especially in photodetectors [59]. MoTe₂ is a special TMDCs material, whose absorption range is located in the near infrared region, which is very suitable for near-infrared photodetectors. The high mobility of graphene [60] can be combined with the high yield of optical carriers in TMDCs materials by forming a heterojunction with graphene [19]. Unlike common TMDCs materials, the band gap of MoTe₂ is almost independent of thickness, which is about 1 eV and is suitable for near-infrared Photodetectors. Combined with low resistance and high carrier mobility properties of graphene, multilayer MoTe₂, which shows high optical absorption intensity and appropriate band gap, can be assembled into photodetectors with excellent performance. The device can achieve 970.82 A·W⁻¹ photoelectric response at 1064 nm, 4.69 × 10⁸ photoconductive gain and 1.55 × 10¹¹ Hz^1/2·W⁻¹ detectivity. Graphene/MoTe₂ heterostructure is placed on a Si/SiO₂ substrate, the drain/source electrodes are in contact with graphene, and V_G is connected with Si as shown in Fig.9(a). The SEM image of photoelectric device is shown in Fig.9(b). The surface potential profile from graphene to MoTe₂ shows that the V_CPD of MoTe₂ is 58 mV larger than that of graphene. Large difference in work function between probe and material results in large V_CPD. Therefore, the work function of MoTe₂ is lower than that of graphene, which causes the energy band to tilt towards MoTe₂ when the two materials come into contact and create an electric field from MoTe₂ to graphene [Fig.9(d)]. The I_D−V_G characteristic of the heterojunction device was measured with dark and 980 nm illumination, respectively, showing the characteristics of p-type semiconductor. The voltage of charge neutral point in dark condition is smaller than that with light, so there is photogating effect. The scanning photocurrent mapping shows that there is large photocurrent when V_ds is 0.1 V. The entire region between the source and drain electrodes shows strong optical response (as shown in Fig.9(f)) because of the built-in potential. The photocurrent of graphene/MoTe₂ heterojunction is 120 times higher than that of MoTe₂ at 980 nm incident light and heterojunction shows good repeatability. V_G-dependent photocurrent with different power shows the photocurrent is larger at high power. With a V_G of −3 V, the Fermi level of graphene is located at a suitable location of the photoexcitation hole and it can be effectively transferred from MoTe ₂ to graphene. When V_G is 16 V, the Fermi level of graphene is located at the Dirac point, with a small slope in the transport and separation of photoexcited carriers, so the photocurrent is very small. Fig.9(j) shows the variation of photocurrent and photo response with incident light power. The photocurrent increases linearly at low power and subsequently saturates due to the increased rate of photoexcited carrier recombination.

4.3 ReS₂/graphene heterostructure ultrahigh photoresponsive device

ReS₂, which exhibits a 1.5 V direct band gap, is a novel TMDCs material. Superimposed TMDCs materials on graphene can improve the response speed and photoresponsivity [47,61]. In addition, the responsivity of ReS₂ photodetector is 16.14 A·W⁻¹, which is very suitable to form heterojunction with graphene as ultra-high photoresponsive device. Fig.10(a) and (b) show the ReS₂/Graphene heterostructure device and the STM image, respectively. Heterojunction exhibits certain metallic properties. Raman spectra clearly show two peaks representing the in-plane and out-of-plane vibration of ReS₂ at 161 cm⁻¹ and 217 cm⁻¹, respectively. There is a strong peak at 1.5 eV in PL spectra of both the heterojunction and ReS₂.

When V_G is −30 V, the 550 nm laser with high power can cause the device to produce a larger photocurrent as Fig.11(a). Responsivity can be expressed by the ratio of photocurrent to laser power [63-65]. Fig.11(b) shows that large laser power corresponds to small responsivity. The calculated responsivity of the heterojunction device is 10³ times higher than that of ReS₂, which indicates that high carrier transport of graphene greatly improves the performance of heterojunction devices. The drain voltage dependent photocurrent reveals that the photocurrent of the hybrid device is 10³ times than that generated by ReS₂, as shown in Fig.11(c).

Kelvin probe force microscopy (KPFM) shows that the work functions of graphene and ReS₂ are similar. The electronic band structure is shown in Fig.12(b). Since the close work function, the positions of the charge neutral point (CNP) are almost identical under dark and light conditions as shown in Fig.12(c). The CNP displacement caused by the difference of work function ReS₂/graphene heterojunction is special in TMDCs/ graphene heterojunction.

The ReS₂ photodetector shows a long response time because of low carrier mobility. For ReS₂/graphene heterojunction, when the photoexcited holes in the ReS₂ layer transfer to the graphene layer, the graphene immediately transfers carriers due to the high carrier mobility, which can improve response time as shown in Fig.13(a). The heterojunction demonstrates fast rise time and decay time (30 ms). However, the rise and decay times of ReS₂ devices are both several seconds as shown in Fig.13(c). When the temperature drops to 110 K, the responsivity increases slightly, and the photocurrent in Fig.13(d) increases due to the decrease in the contact resistance of graphene and the graphene-metal.

4.4 The optoelectronic applications of perovskite/WS₂ heterostructures

Organic−inorganic hybrid perovskites shows outstanding optoelectrical properties, such as strong optical absorption ability, long carrier spread range, and adjustable band gap [66, 67], so it is widely used in photodetectors [37,38], lasers [68], and LED [69,70]. Chemical vapor deposition (CVD) method is used to obtain the monolayer WS₂ and 2D PbI₂ was deposited on WS₂ by vapor-phase selective deposition, and then converted into organic−inorganic Perovskite. 2D Perovskite/WS₂ heterostructures can be obtained by this method. Fig.14(a) and (b) show the STEM images of epitaxially ranged PbI₂/WS₂ heterostructure and the MAPbI₃/WS₂ heterostructure generated through inserting MAI. PL spectra of the heterojunction, WS₂ and perovskite show the quenching of PL peaks at 1.65 eV and 2 eV in the heterojunction, due to the charge transfer of electrons to WS₂ and holes to perovskite layers as shown in Fig.14(d). And the heterojunction shows type II (staggered) energy band.

Fig.15(a) shows the optical micrograph taken from perovskite/WS₂ heterojunction with Au/Ni electrodes and the schematic, perovskite is formed between the source and drain. The I−V characteristics of the heterostructure shows that when the laser wavelength is 532 nm and V_g = 60 V, high photocurrent generated by high laser power and the photocurrent value can reach 32.8 nA. When the optical power is 0.04 mW·cm⁻², the photoresponsivity is as high as 43.6 A·W⁻¹. As V_g increasing from 10 V to 20 V and 60 V, the photocurrent increases significantly with a 4.4 mW·cm⁻² laser power and a V_d of 10 V, as shown in Fig.15(c). Fig.15(d) demonstrates that perovskite/WS₂ heterojunction device demonstrates good stability and repeatability and is suitable for stable and reliable optoelectrical device.

4.5 Electrically driven graphene/h-BN/WSe₂ heterostructure-based emitter

The optoelectrical characteristics of TMDCs could be effectively controlled by strain [72-76], and TMDC-based emitters can be modulated by electricity. The energy band structures also change with the electric field [39,77]. Graphene, several layers of h-BN, and WSe₂ were placed on the SiO₂/Si substrate in sequence. Finally, top graphene layer was placed on the WSe₂ to form the composite heterojunction as shown in Fig.16(a). The strain is applied to the heterojunction material through atomic force microscope (AFM) tip. Tunneling effect can inject current into WSe₂ uniformly, which can be achieved by applying bias voltage to the graphene at top and bottom. When the bias is 0, no charge can pass through several layers of h-BN. However, when the bias is applied, the Fermi level rises to near conduction band of WSe₂, and electrons tunnel through the h-BN layers, which results in hole−electron radiative recombination [Fig.16(b)]. Fig.16(c) and (d) show the optical microscope and AFM image of the heterostructure, respectively.

Obvious bright spots can be observed at 7 indentations from the EL spectrum with 2 V bias voltages [Fig.17(a)], which is due to the strain that reduces the opening voltage of the tunneling current from 4 V to 1.5 V as shown in Fig.17(b). The electroluminescence spectra at the seven indentation sites all have narrow single emission peaks from localized exciton states between 735 nm and 785 nm. The photon correlation functions g⁽²⁾(0) of 1−3 indentation sites are all less than the threshold line (0.5) and single-photon emission exhibit photon antibunching behavior. Substantially no background EL emission is observed, indicating the high purity of single-photon emission. The biased-dependent electroluminescence spectra showed a strong peak at 739.8 nm when the bias voltage was 1.5 V. As the bias increasing to 4 V, there are some small side peaks, which are consistent withFig.17(b), and the appearance of additional peaks leads to additional inflection points in the I−V curve. The peak strength increases rapidly from 1.5 V bias voltage and is saturated at 2.5 V bias voltage as shown in Fig.17(f). The higher-resolution EL spectrum shows there is a fine structure splitting [Fig.17(g)], which caused by the anisotropic electron−hole exchange interactions due to the strain-induced low symmetry.

5 Conclusion

As an important branch of layered nanomaterials, TMDCs materials not only show the excellent physical properties like graphene, but also make up the deficiency of graphene in photoelectric applications due to their zero bandgap and semi-metallic properties. First, TMDCs provide the basis for the formation of heterojunctions with excellent optoelectric properties due to their outstanding electronic band structure and operability. The rich interface structure provides convenient conditions for the construction of different types of heterojunctions. Second, the adjustable band gap of TMDCs material greatly expands its application in photoelectric sensor and other fields. TMDCs heterojunction materials have broad application prospects in photoelectric devices, flexible electronic devices, solar photovoltaic and photocatalysis. Through the design of the electronic band structure of the two materials that make up the heterojunction, the TMDCs heterojunctions can combine the advantages of the two or more materials to exhibit unique optoelectronic properties.

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