Since the discovery of graphene in 2004, two-dimensional (2D) layered materials have attracted considerable attention in the past decade.^[1–5] Atomically thin 2D materials, e.g., MoS , WS , ReS , and SnS , are burgeoning 2D semiconductor materials for their unique electronic and optical properties.^[6–9] Unlike graphene with a zero band gap, these atomically thin 2D materials possess direct band gaps and could give light emission at room temperature, which is important for both electronic and optoelectronic applications. These atomically thin 2D nanosheets derived from the layered materials share many interesting characteristics of the well-known graphene, such as excellent electronic properties, exceptional mechanical flexibility, and partial optical transparency. Furthermore, these 2D nanosheets typically have a well-defined crystalline structure with few surface dangling bonds that traditionally plague most semiconductor nanostructures, and therefore can exhibit excellent electronic properties that are not readily achievable in other semiconductor nanostructures.

Since the application of semiconductor materials is closely related to their band gaps, a vital task in atomically thin 2D semiconductor research is to realize the growth of new 2D materials with tunable band gaps for potential applications in functional electronic/optoelectronic devices.^[10–19] Since different atomically thin 2D materials have different band gaps, alloying semiconductors with different band gap variations can be synthesized by controlling the compositional proportions of two or more 2D materials. Thus, the range of band gaps of alloying semiconductors would vary between the minimum and maximum values of the band gaps of all the components. Alloying semiconductors with different band gaps have been widely used in the band gap engineering of bulk semiconductors. For applications in nanoelectronics and nanophotonics, it is very important to achieve semiconductor nanostructures with continuously tuned band gaps.^[20–26] Recent advances in 0D and 1D ternary semiconductor structures have shown that their band gaps and light emissions can be tuned gradually by changing their constituent stoichiometries,^[27–39] and based on these achieved composition modulated nanostructures, multi-wavelength lasers,^[31] wavelength converters,^[32] optical diodes,^[33] and nanoscale photodetectors^[36–38] were realized. For the broad applications in integrated devices and systems, direct growth of these band gap engineered 2D structures should be very important. In this review, we will summarize the recent progress on the band gap engineering of atomically thin two-dimensional semiconductor alloys. Considering from different 2D atomically thin alloys, various research progress will be reviewed systematically.

2. alloys

As two of the most important transition-metal dichalcogenides (TMDs), atomic layered MoS and MoSe have been extensively studied, due to their atomically thin geometry, unique electronic and optical properties, and potential application integrated nanosystems. For monolayer MoS and MoSe , the direct band gaps are 1.856 and 1.557 eV, respectively. To precisely control the band gap of these 2D TMDs is of central importance for creating optoelectronic devices with tunable spectral responses. Theoretical studies suggested that mixed Mo(S, Se, Te) alloys are thermodynamically stable, and thus it is possible to continuously tune their compositions between the constituent limits.^[40–43] A cartoon illustrating the atomic structure of a mixed 2D TMDs, along with the constituent parent materials is shown in Fig. 1.^[40] In practice, the alloys are modeled by 5×5 supercells. Biplab et al. have studied in detail the electronic structure of atomically thin uniform 2D MoS Se and nanosheets to have an understanding of composition dependent tunability of the electronic and optical properties of these nanosheets.^[41] Fatih et al. performed DFT calculations to examine the lithium adsorption and diffusion on the hexagonal MoS Se monolayers with variation of x for 0.00, 0.33, 0.50, 0.66, and 1.00.^[42] In this regard, band gap engineering of 2D alloy MoS Se has recently been achieved by using slightly different precursors with different band gaps: (i) mechanically exfoliated MoS Se bulk alloy,^[44] (ii) creating a composition of mixed alloys by controlling the ratio of the S and Se.^[45–62] For example, by using MoS and MoSe as precursors, Feng et al. have successfully synthesized MoS Se nanosheets (Fig. 1(b)).^[45] A typical scanning electron microscopy (SEM) image of a representative sample shows that the achieved structures are triangular nanosheets with the edge lengths of 30–80 μm (Fig. 1(c)).^[46] With Se dopants, some S2 sites become much brighter and display intensities close to or even higher than the Mo sites (Fig. 1(d)), while the intensities of the Mo sites remain unchanged.^[47] Figure 1(e) plots x-ray photo-electron spectra of all the synthesized MoS Se monolayers.^[45] Figure 1(f) shows the normalized Raman spectra of the asgrown MoS Se nanosheets with composition x decreasing gradually from 1 (down-most, pure MoS ) to 0 (upmost, pure MoSe ) excited with a 488 nm argon ion laser, respectively.^[46] From the curves a to 1, the intensity of the S−Mo related modes [E , A ] gradually decreases until they completely disappear, while the Se–Mo related modes [A , E ] are absent or very weak at the initial stage and gradually come into appearance with the corresponding intensity increased. Lateral composition graded atomic layered 2D MoS Se nanosheets have also been successfully synthesized using a simple moving source thermal evaporation method by an improved CVD route.^[51]

Fig. 1.

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Fig. 1. (color online) (a) Top and side views of the atomic structures of MoS2 (left), MoS Se alloy (middle), and MoSe (right) 2D TMDs. Note that the systems consist of three layers of atoms.[40] (b) Illustration of three-zone furnace for the growth of the MoS Se monolayer.[45] (c) Typical SEM morphology of the obtained ternary MoS Se nanosheets.[46] (d) ADF image of Se-doped MoS with ∼ 12% local Se concentration.[47] (e) XPS of all as-grown MoS Se monolayers.[45] (f) Raman spectrum of the MoS Se nanosheets excited with a 488 nm argon ion laser.[46]

Figure 2(a) shows the normalized photoluminescence (PL) spectra of the obtained composition modulated MoS Se nanosheets excited with a 488 nm argon ion laser.^[46] All the samples show single emission bands, with the spectral peaks continuously shifted from 668 nm (for pure MoS ) to 795 nm (for pure MoSe ), which is consistent with the band edge transitions of ternary MoS Se thin films. FETs were also fabricated on MoS Se monolayers.^[45] The MoS Se monolayer device exhibited n-type transport behavior with high on/off ratios, and the field-effect mobility was also calculated. These electrical measurements confirmed semiconductive behavior for all as-grown MoS Se monolayers. As a further characterization of the monolayer alloy, the temperature dependence of the PL was also measured. The MoS Se monolayer of Fig. 2(d) exhibits typical behavior, with the optical gap decreasing with temperature.^[56] Figure 2(e) gives the wavelength-selected PL emission mapping of an examined composition graded nanosheet alloy in the spectral regions of 680–690, 710–720, 750–760 nm, respectively.^[51] Obviously, the short wavelength region (680–690 nm) is mainly located at the center of the nanosheet, while the long wavelength region (750–760 nm) is mostly located around the edge of sheet.

Fig. 2.

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Fig. 2. (color online) (a) PL spectrum of the complete composition MoS Se nanosheets and a typical PL mapping of a single ternary nanosheet (the inset, scale bar, 7 μm) excited with a 488 argon ion laser.[46] (b), (c) Source–drain current to the gate voltage ( ) and the source–drain current to the source–drain voltage ( ) for FETs containing the MoS Se monolayer for x = 0.30.[45] The inset shows the SEM image of the device channel. Scale bar is 5 μm. (d) The exciton emission energy as a function of the temperature of the monolayer alloy.[56] (e) Wavelength-dependent PL mapping of a single composition graded nanosheet alloy in the spectral regions of 680–690, 710–720, 750–760 nm, respectively (scale bars, 5 μm).[51]

3. alloys

Except for MoS Se , atomically thin 2D WS Se alloy has also attracted wide range of interest. The schematic illustration in Fig. 3(a) shows the transformation from WO (monoclinic crystal) to WS Se (hexagonal crystal) through simultaneous sulfurization and selenization.^[62] Figure 3(b) illustrates a single-zone furnace utilized in the growth of the monolayer WSe S . Because of the much higher chemical activity of S atoms than Se atoms, it is possible to tune the contents in the monolayer WSe S by only varying the precursor weight of sulfur powder. The resulting alloy nanosheets are typically monolayers with a well-defined triangular shape, as identified by optical contrast and atomic force microscopy studies (Fig. 3(c)).^[64] HRSTEM was utilized to characterize the monolayer WS Se , which shows the Z-contrast (Z is the atomic number) as images with atomic-scale resolution, as observed in Fig. 3(d).^[62] The hexagonal rings of the alternative W and S/Se atoms in each unit. There are three levels of brightness in Fig. 3(d). The dimmest spots represent the S + S atoms, which are encircled in red; the relatively bright spots are the S + Se atoms, which are encircled in yellow; and the even brighter spots are the Se+Se atoms, which are encircled in blue. Figure 3(e) shows the spatially resolved EDS elemental mapping of the S, Se, and W elements of a typical WS Se alloy, all elements show relatively uniform distribution across a triangular domain, indicating composition uniformity throughout the entire domain.^[64] To further investigate the structural evolution in the WS Se alloy nanosheets, the Raman spectra from the same sample series were collected.^[64] The Raman spectra display five main modes for most WS Se nanosheets, which can be assigned to A mode (401.9−419.7 cm , A mode (251.6−264.7 cm ), A mode (379.5−385.2 cm ), E mode (∼ 354.7–355.9 cm ), and E −LA + A −LA mode (135.2−172.5 cm ).

Fig. 3.

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Fig. 3. (color online) (a) Schematic illustration of the transformation from WO (monoclinic crystal) to WS Se (hexagonal crystal) through simultaneous sulfurization and selenization processes.[62] (b) Optical microscopy image of typical WS Se nanosheets (x = 0.454; scale bar = 20 μm.[64] (c) HRSTEM image of the as-grown monolayer WS Se (x = 0.43).[42] (d) HAADF image of a small WS Se domain (x = 0.573; scale bar = 500 nm) and EDS mapping of the same triangular domain for S–K line, Se–K line, and W–L line, respectively.[64] (e) Evolution of Raman spectra in the WS Se monolayer nanosheets as a function of chemical composition: full-range Raman spectra, E –LA + A –LA mode, A of Se–W mode, and E of S–W mode, A of S–W–Se mode, and A of S–W mode.[64]

To investigate the band gap modulation in the resulting alloy nanosheets, μ-PL spectra were taken using a micro-Raman microscope and spectrometer excited by a 488 nm argon ion laser (power is 5 μW) in ambient condition. Photoluminescence studies demonstrate that all the monolayer samples display prominent emission with a single sharp peak with full width at half maximum (fwhm) about 25 nm. The PL spectral peak positions are continuously tunable from 626.6 nm (nearly pure WS ) to 751.9 nm (nearly pure WSe ) depending on the exact synthetic conditions (Fig. 4(a)).^[64] Furthermore, the spatially resolved mapping of the PL intensity also shows a highly uniform contrast (inset), demonstrating highly uniform optical properties and crystalline quality. To probe the electronic properties of the as-grown monolayer WSe S alloy, gated-FET devices based on the monolayer WSe S alloy by the EBL was fabricated by Yang et al.^[63] The typical current-voltage ( ) plots of the FET devices indicates the increase of the drain current by decreasing the back-gate voltage. The band gap tuning capability is ∼ 120 meV by varying the S content in the monolayer WSe S alloy. The top-gated FET configuration improves the carrier mobility with two orders larger than that in the back-gated FET device. The evolution of electrical properties and FET threshold voltages can be more clearly seen from an – plot under a source-drain bias of 3 V for the nanosheets with increasing sulfur atomic ratio from nearly pure WSe (brown curve) to nearly pure WS (black curve, Fig. 4(b)).^[64] For the WSe -rich alloys (∼ 0–0.55 S atomic ratio), the FETs predominantly display p-type semiconductor properties, whereas for the WS -rich alloys (∼ 0.55 –1 S atomic ratio), the n-type semiconductor properties are mainly observed. The polarization curves (Fig. 4(c)) after internal resistance (iR) compensation obtained at a scan rate of 5 mV show the normalized current density as a function of the voltage versus RHE in the monolayer WS Se (x = 0.43), WS , WSe , Pt, and bare GC electrode. Impressively, the monolayer WS Se electrode exhibits the significant HER catalytic activity with the lowest onset overpotential of ≈ 80 mV compared to WS (100 mV) and WSe (150 mV).

Fig. 4.

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Fig. 4. (color online) (a) Photoluminescence spectra of a series of composition tunable WS Se monolayer nanosheets. Inset: photoluminescence intensity mapping of the same WS Se nanosheets (x = 0.522; mapping peak is 687.5 nm; scale bar is 10 μm).[64] (b) Transfer characteristics ( – plot) of WS Se nanosheet transistors with different S atomic ratios from nearly pure WSe (brown curve) to nearly pure WS (black curve).[64] (c) Polarization curves after iR correction obtained at a voltage sweeping rate of 5 mV in the monolayer WS Se (x = 0.43), monolayer WS , monolayer WSe , Pt and GC electrode.[62]

4. WMoS alloys

2D MoS and WS can alloy to form atomically thin 2D WMoS alloy.^[65–75] Similar to MoS and WS , Mo W S alloy has a layered structure, and each layer consists of two hexagonal S atom layers and a sandwiched Mo/W atom layer. The Mo or W atom sits in the center of a trigonal prismatic cage formed by six S atoms (Fig. 5(a)).^[65] The two metal species form homo-nuclear chains along the edges of the triangular W Mo S alloys. Hydrostatic pressure up to 40 GPa was applied to Mo W S layered ternary compound to study the vibrational modes and understand the phonon and lattice distortion for high compressive forces.^[71] With increasing pressure, the in-plane E modes and out-of-plane A modes follow closely to that of pristine MoS or WS . The band gap of Mo W S monolayers could be tuned by MoS or WS composition, which was evidenced by PL measurements.^[65] As W composition x increases, the A exciton emission (∼ 1.8–2.0 eV) red shifts and then blue shifts (Fig. 5(b)). For Mo W S monolayer alloys, the PL spectra show one strong PL peak, i.e. A-exciton emission (low-energy one), with a weak shoulder B-exciton emission peak at the higher energy side.^[67] The PL intensities of the Mo W S monolayer alloys first decrease (due to electron–phonon interaction) and then increase with temperature increasing. Figure 5(c) presents a schematic diagram of the Mo W S photoresistor that was constructed from a PLD-grown film using the wire-masking technique.^[68] There are no sampling points at the rise and decay edges from the temporal switching cycle, indicating that the response time of the device is shorter than 150 ms, which is the sampling limit of the measurement system. While alloy scattering was found to be important for alloy compositions 0.5 < x < 0.7, the mobility is found to be limited by LO phonon scattering across the entire range of compositions except for the combination of Morich alloys with very low 2D carrier densities (10 cm ), where the impurity scattering dominates.^[69] The morphology, elemental composition, bonding information, and electrocatalytic properties toward hydrogen evolution reaction (HER) of 3D electrodes coated with Mo W S alloys with different Mo/W ratios were also studied.^[73]

Fig. 5.

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	Fig. 5. (color online) (a) Top view of Mo W S monolayer and a side view of a unit cell.[65] (b) PL spectra of Mo W S monolayers with different W compositions x.[65] (c) Schematic diagram of a Mo W S photoresistor and a temporal switching cycle of the Mo W S photodetector under a 635 nm illumination.[68]

5. alloys

Triangular Mo W Se monolayers^[76–79] with tens of microns in size were successfully grown on SiO /Si substrates at 780 °C by a low pressure CVD method. According to the simulation, the dimmed Se sites correspond to Se vacancies (VSe) where one Se atom at the site is missing (Fig. 6(a)).^[76] Figure 6(b) shows the atomic force micro-scopy (AFM) image of Mo W Se sheets, meaning the height of the monolayer alloy is about 1.3 nm.^[78] Figure 6(c) shows an HAADF STEM image of Mo W Se flakes. An atomically resolved image revealed its honeycomb atom arrangement, as shown in the inset of Fig. 6(c).^[78] PL measurements on Mo W Se monolayers with different W compositions showed tunable band gap exciton emissions (Fig. 6(d)).^[78] As W composition x increases, the PL emission shows a bowing effect, in which emission energy first red shifts to a minimum value at around x = 0.2–0.3 and then monotonically blue shifts. Therefore, the band gap emission of the Mo W Se monolayers can be continuously tuned from 1.56 eV (reached x = 0.21) to 1.65 eV (reached x = 1). Figure 6(e) shows unpolarized Raman spectra of Mo W Se monolayers with different W composition x and the corresponding composition-dependent Raman frequencies.^[78] The MoSe -like E shifts to lower frequency as the W composition increases. In addition, the second order Raman peaks also shift and showed two-mode behaviors as the W composition changes. Figure 6(f) shows the dynamics of excitons in MoSe , Mo W Se , and Mo W Se acquired by exciting the monolayers with a ≈ 300 fs pump pulse having a central wavelength of 620 nm and an energy fluence of ≈ 0.9 μJ⋅cm .^[76] The room temperature PL intensity from Mo W Se monolayers is ≈ 10 times stronger than that from MoSe , accompanied by a longer exciton lifetime, indicating the decrease of defect-mediated nonradiative recombination. The calculated mobility of Mo W Se monolayer FETs with x = 0.39 was about 7.8 × 10 cm .^[78]

Fig. 6.

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Fig. 6. (color online) (a) Top view of the atomic model corresponding to the Mo W Se monolayer.[76] (b) AFM image of Mo W Se sheets.[78] (c) HAADF STEM image of Mo W Se flakes. Inset in panel (c) is a high-resolution STEM image, and the scale bar is 3 nm.[78] (d) Composition-dependent PL spectra of Mo W Se monolayer alloys.[78] (e) Raman spectra of Mo W Se monolayers with different W composition x.[78] (f) Differential reflection signal (scattered dots) measured from MoSe , Mo W Se , and Mo W Se monolayers with a 620 nm pump and a 800 nm probe pulse.[76] Solid curves are biexponential decay fits.[76] (g) Output curves ( – ) under different bottom gate voltages of Mo W Se monolayer FETs for x = 0.39.[78]

Unlike other TMDs, MoTe and WTe are distinguished by the existence of a semimetallic distorted octahedral T′ phase with a very small energy barrier from the semiconducting H phase.

The calculations by Rajbanshi et al. showed that MoS Te has a direct band gap structure, both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are approaching toward the Fermi level with the increase of doping percentage.^[41] The formation energy diagram in Fig. 7(a) shows that the T′ phase of pristine monolayer WTe is thermodynamically more favorable, while monolayer MoTe is more stable in the H structure due to different contributions from ligand field splitting stabilization and CDW stabilization.^[80] It is found that in monolayer W Mo Te alloy, the energy difference between the T′ phase and the H phase is reduced when x approaches the transition point indicating the potential that a smaller amount of charge modulation than that in pristine MoTe can render the T′ phase more stable than the H phase of monolayer W Mo Te when x is closer to 0.333. At some chosen temperature T, the two phases’ Gibbs or Helmholtz free energies form a free energy landscape that allows determination of which phase or phase mixture minimizes total free energy as a function of W content. Panel (Fig. 7(b)) follows the thermodynamically stable convex hull: the monolayer minimizes its total free energy by existing in the H phase on the left-hand side of point 1, as single phase T′ on the right-hand side of point 3, and as a two-phase coexistence of a W-depleted H phase and a W-rich T′ phase between points 1 and 3. Panel (Fig. 7(c)) applies when long-ranged W diffusion across phases is quenched. Point 2 then marks the transition between metastable single-phase H and metastable single-phase T′.^[61] The converted critical voltages required to trigger the phase transition between the H and the T’ phase of monolayer W Mo Te alloy with different compositions are shown in Fig. 7(d).^[80] A fast switching device structure is proposed and reasonable critical gate voltages required to induce the phase transition are estimated by a simple capacitor model. The PL spectra of 2H WSe Te (x = 0–0.6) monolayer samples excited with a 532 nm laser are collected and shown in Fig. 7(e).^[82] All monolayer 2H WSe Te show the emission bands, and the spectral peak continuously shifts from 744 nm (pure WSe ) to 857 nm (near infrared). For 1Td sample, as expected, no PL signal is detected when x ≥ 0.6. Figure 7(f) plots the optical gap versus mole fraction of Te for monolayer WSe Te .^[82] The band gaps are continuously shifted from 1.67 eV (pure WSe ) to 1.44 eV.

Fig. 7.

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Fig. 7. (color online) (a) Formation energy of W Mo Te in both T′ structure (blue) and H structure (red) at various W concentration x.[80] (b), (c) Free energy landscape at fixed temperature.[81] (d) Positive (pink) and negative (blue) voltage required to switch the relative stability of H-MoTe and T′-MoTe .[80] (e), (f) The photoluminescence (PL) spectra of monolayer WSe Te (x = 0–1.0) alloys.[82]

The band gap for GaS, GaSe and and GaTe monolayers as 2.5, 2.1, and 1.7 eV (on average), respectively. In Fig. 8(a), wafer-scale GaTe Se films are successfully grown by MBE.^[83] Ga, Te, and Se elements distribute uniformly over a large range of 0.16 mm×0.16 mm. GaS Se multilayers with tunable compositions over the entire range have been successfully synthesized by CVT (Fig. 8(b)).^[84] Figure 8(c) displays the UV–visible diffuse reflectance spectrum measured for various compositions of GaS Se multilayered nanosheets.^[84] The onset of the absorption band, the indirect band gap obtained from the K–M plots, and the PL peak position (at 8 K) all show a linear dependence on the composition, with similar band gap ( ) values for any given value of x (Fig. 8(d)).^[84] Figure 8(e) shows a schematic view of the fabricated GaSe Te nanoflake device.^[85] The GaSe Te nanoflake photodetectors show extended light response wavelength compared to GaSe (Fig. 8(f)). Also, with increasing Te composition the photocurrent of GaTe Se alloys has a rising trend.^[83]

Fig. 8.

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Fig. 8. (color online) (a) EDX mapping of Se, Te, and Ga in a GaTe Se film.[83] (b) TEM images of GaS multilayered nanosheets.[84] (c), (d) UV visible diffuse reflectance spectrum (in absorption) and PL spectrum (at 8 K) of the GaS Se nanosheets with various values of x.[84] (e) Three-dimensional schematic view and the cross section view of the GaSe Te nanoflake device.[85] (f) Composition-dependent photocurrent of GaTe Se alloys under different laser intensities.[83]

Re dichalcogenides are promising candidates for the study of anisotropy. ReS forms a distorted 1T structure with triclinic symmetry (Fig. 9(a)).^[86] Figure 9(b) shows the absorption spectra of ReS , ReSSe and ReSe , respectively. Figure 9(c) shows a typical optical image of ReSSe flakes, with thicknesses ranging from monolayer to four layers. A field-effect transistor based on a ReSSe thin flake with a thickness of about 3 nm was fabricated and studied (Fig. 9(d)). The – data exhibit a slight super linear behaviour at low drain bias, which suggests that the electrons are injected through a Schottky barrier at the metal–semiconductor interface. Photosensitive devices based on few-layer ReSSe exhibit a very high photoresponsivity, up to 8 A⋅W .

Fig. 9.

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Fig. 9. (color online) (a) Crystal structure of the ReX compound.[86] (b) Absorption spectrum of ReS , ReSSe, and ReSe .[86] (c) Typical optical image of a ReSSe flake with regions of different thicknesses. Scale bar: 10 μm.[86] (d) – curve of a ReSSe thin-flake transistor with a thickness of about 3 nm.[86] (e) Band structures of Nb Re S systems (x = 0.5).[87] (f) Derived absorbance along in-plane directions.[87]

With , the concentrations of groups 5 and 7 transition metals are equal and thus Nb Re S is valence isoelectronic to MoS . Mirroring MoS , Nb Re S is a semiconductor albeit with a smaller band gap and has competing CB minima and VB maxima at the Γ and K points (Fig. 9(e)).^[87] Figure 9(f) shows the absorbance of the TMDs overlaid onto the solar spectrum.^[87] From first principle calculations, these alloy TMDs possess significantly smaller band gaps, which are close to the optimal value of 1.3 eV for efficient solar absorption and show enhanced solar absorption in the visible range of the solar spectrum.

Dopping is another effective method to tune the band gap of semiconductor materials. A number of experimental and theoretical studies have been committed to the atomically thin 2D alloys by dopping in recent years.^[88–96] Figure 10(a) is the hexagonal bilayer Co Mo S nanosheet synthesized by the CVD method.^[88] The A mode and E modes show one-mode and two-mode behaviors, respectively. The Co Mo S nanosheet devices demonstrated FET characteristics of a n-type semiconductor (Fig. 10(b)), with a mobility as high as 0.52 cm /(V⋅s), the on/off ratio (about 10) is lower than the reported bilayer MoS flakes.^[88] Meanwhile, good quality Sn Mn Se films with relatively high Mn concentrations have been successfully grown by MBE on GaAs (111)B substrates.^[89] The XRD patterns for the Sn Mn Se films are shown in Fig. 10(c). The samples show only (00l) peaks of the SnSe structure, illustrating excellent c-axis-oriented epitaxial growth. Figure 10(d) shows the Mn concentration as a function of Mn effusion cell temperature . One can see that the Mn concentration increases linearly from about 42% to 66% as increases from 670 °C to 760 °C. The electronic and magnetic properties of Mn-doped monolayer WS were also investigated by Zhao et al.^[90] According to their simulation calculations, a significant portion of the alloy’s spin density is localized on the Mn atom. For each considered Mn-doped WS configuration, the formation energy is lower under S-rich conditions, which indicates that it is energy favorable and relatively easier to incorporate Mn atom into WS nanosheet under S-rich experimental conditions. In addition, Deniz et al. investigated the electronic and magnetic properties of a ReS monolayer substitutionally doped with transition metal and nonmetal atoms.^[91] The mixed-metal fere crystal compounds, [(SnSe) ] [(Nb Mo )Se ] with x = 0, 0.26, 0.49, 0.83, and 1, were successfully produced via adaptation of the modulated elemental reactant method.^[93] Template-based sputtering method is used first to fabricate Au nanoantenna (NA)/MoS2 heterostructures with an enhanced exciton energy.^[95] The PL spectrum red-shifting and intensity attenuation happen in the n-type doping of GQD/MoS heterostructure configuration based on optical doping mechanism.^[96]

Fig. 10.

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	Fig. 10. (color online) (a) Optical image of the Co Mo S nanosheets on the substrate.[88] (b) Room-temperature characteristics of the Co Mo S nanosheet FET devices with 2 V applied bias voltage. The inset shows the optical image of a FET device.[88] (c) XRD spectra of Sn Mn Se films grown with different values of .[89] (d) Mn effusion cell temperature dependences of the Mn concentration.[89]

In conclusion, great progress has been made in recent years in search for the band gap engineering of atomically thin 2D semiconductor alloys. Up to now, almost all kinds of 2D semiconductor alloys have been obtained, and recent studies have shown exciting potential of these atomically thin semiconductor alloys, including the demonstration of atomically thin transistors, a new design of vertical transistors, as well as new types of optoelectronic devices such as tunable photovoltaic devices and light emitting devices. In addition to the growth and corresponding device applications, it is equally important to pursue the novel physical properties of the band gap engineered 2D nanosheets and use them in new 2D semiconductor-based functional device designs.

Although the progress achieved in band gap engineering of atomically thin 2D semiconductor nanosheets represents only an initial success in this new field of material science research. Many new opportunities and challenges remain in the coming years, such as the instability of 2D semiconductor alloys in the air, which is still the key problem when things come to related material applications. Is there any way to make the raw materials more harmless in order to fit with the requirements of the environment? We are looking for the answers.

Above all, through the tireless efforts of researchers and the development of research technology, alloys from other 2D materials, such as 2D metal halide, and 2D black phosphorus which attracts great attention lately will gradually enter the people’s vision in the near future.