Introduction

Monolayer transition-metal dichalcogenides (TMDs), consisting of a transition-metal atomic layer (e.g., W and Mo) sandwiched by two chalcogen atomic layers (S, Se and Te), have been attracting considerable attention because of their desirable physical properties for semiconductor devices such as a high optical absorption coefficient1,2,3, efficient photoluminescence4,5 and a valley structure6,7,8. Different combinations of transition metals and chalcogen atoms can provide band gaps varying over a wide range (1–3 eV) and a wide variety of pn junctions, which are essential building blocks for electronic and optoelectronic devices, have been realized using such atomically thin materials9,10,11,12,13,14,15. The engineering of the electronic/optical properties of semiconductors by using such heterojunctions has been a central concept in semiconductor science and technology and from a fundamental standpoint, heterostructures formed by two-dimensional materials provide a new platform for exploring new physics.

Recently, tunable band alignment has been realized using Mo1−xWxS2 alloy16,17,18,19, showing the high potential of this material for future device technology. However, the microscopic view of the electronic structures of Mo1−xWxS2-based heterojunctions, which is the basis for the band engineering, has not yet been clarified. Here, we report the first scanning tunneling microscopy/spectroscopy (STM/STS) study on the electronic structures of a monolayer WS2/Mo1−xWxS2 lateral heterojunction. The atomic-scale analysis by STM/STS allowed the verification of the basic semiconductor physics of this material, providing a microscopic basis for the band engineering.

Results

Atomically modulated electronic structures and macroscopic band structure of Mo1−xWxS2 alloy

Monolayer WS2/Mo1−xWxS2 lateral heterojunctions for the STM measurements were prepared by chemical vapor deposition (CVD) on a graphite substrate (see Method). All STM/STS measurements were carried out using an Omicron low temperature-STM and a W tip at 87 K.

Figure 1a,b show a typical STM image of Mo1−xWxS2 monolayer alloy with a triangular shape and the cross section along the blue line in the STM image, respectively. The low resolution of the STM image in Fig. 1a is due to the fact that it was difficult to obtain a high-quality image at a low temperature when the observed area included an island edge. As shown in the cross section, the height of the layer is ~0.7 nm over the triangular structure. Close inspection of Fig. 1a and the cross section reveals a slightly smaller triangular area inside the triangular shape, with the interface between the inner and outer triangles shown by two dark blue triangles. The two triangular structures are schematically shown below the cross section in Fig. 1b, in which the three red triangles indicate the positional relationship with Fig. 1a.

Figure 1
figure 1

STM/STS on the WS2/Mo1−xWxS2 heterostructure.

(a) STM image of a typical Mo1−xWxS2 structure grown by CVD on a graphite substrate (Vs = +2.5 V, It = 0.25 A). (b) Cross section along the blue line in a and schematic structure of the STM image in (a). (c) Magnification of the part of the interface indicated by two red triangles at the top and bottom in a. (d,e) Schematic models of the STM images in (c,a). (f,g) STM images of Mo-rich Mo1−xWxS2 area obtained at sample bias voltages of Vs = +1.6 and +1.3 V, respectively. (h) dI/dV spectra obtained in the Mo and W areas indicated by the white and blue squares in (g). The gray rectangle indicates the noise level. (i) Magnification of the spectra in (h) near the conduction band edge ECBM. (j) High-resolution STM image of Mo-rich Mo1−xWxS2 area.

Full size image

Figure 1c shows a magnification of a part of the interface between the smaller inner triangle and the larger outer triangle along the line joining the two red triangles at the top and bottom of Fig. 1a. The interface in the STM image in Fig. 1c is also indicated by two red triangles. To obtain the Mo and W distributions, the bias voltage was set at +1.35 V and the Mo and W atoms were imaged with a suitable contrast. As shown in Fig. 1c, small bright islands are distributed in the outer area (left side of the image), while dark lines form a netlike structure in the inner area (right side of the image), as previously observed for Mo1−xWxS2 alloy exfoliated from its bulk structure17. Namely, the inner triangular area in Fig. 1a (right side of Fig. 1c) is not a second layer but a Mo-rich Mo1−xWxS2 alloy area surrounded by the W-rich alloy area corresponding to the larger triangle (left side of Fig. 1c), i.e., a heterostructure of W-rich and Mo-rich Mo1−xWxS2 alloys was successfully formed, as observed for Mo1−xWxS2 grown on a SiO2 or sapphire substrate11,12,13,14,15,18. Structural models for Fig. 1c,a are schematically shown in Fig. 1d,e, respectively.

The heterojunction interface was formed parallel to the edge of the triangular area. An atomically sharp heterojunction interface is clearly visible in the close-up view of the heterojunction interface shown in Fig. 1c. Such a sharp in-plane compositional variation is considered to be a result of the CVD growth sequence. At the initial growth stage, Mo-rich Mo1−xWxS2 alloy was formed in the inner triangle by the Mo rich atmosphere around the substrate owing to the high vapor pressure of MoO3 compared with that of WO3. Subsequently, the W-rich structure was epitaxially grown from the edge of the Mo-rich structure because of the shortage of Mo and the reduced diffusion of Mo atoms. Similar sequential atomic growth has been observed in previous studies11,18. Although local fluctuation was observed, the compositional distributions of Mo and W atoms, i.e., the ratio between them, were almost the same over the island and among the islands formed in this sample.

Figure 1f,g show the bias dependence of the Mo1−xWxS2 structure. The STM image is almost flat at a sample bias voltage of Vs = +1.6 V, while darker W atoms marked by red circles appeared at Vs = +1.3 V, as observed in Fig. 1c. This is in good agreement with the theoretical result that the local density of states near the conduction band edge ECBM is localized at the Mo sites17. To observe the electronic structures in more detail, STS measurements were carried out over the W and Mo areas indicated by the white (upper left) and blue squares in Fig. 1g, respectively and the results are shown in Fig. 1h. Figure 1i shows a magnification of the spectra in Fig. 1h near ECBM. The spectrum denoted by ‘All’ was obtained by averaging over the surface in Fig. 1g. Although the valence band edges EVBM of the three spectra were located close to each other, ECBM in the Mo area was about 75 meV lower than that in the W area. From the spectrum ‘All’, the band gap in the area was estimated to be ~2.5 eV, which is between those for pure MoS2 (2.40 eV)20 and pure WS2 (2.73 eV)21. Here, ECBM and EVBM were determined using the bias voltages at which the signal became higher than the noise level. The shift of 75 meV was estimated from the shift between the spectra obtained in the Mo and W areas, respectively, at higher bias voltages of 0.7–1.0 V.

In the same manner as in the estimation of composition-dependent photoluminescence (PL) peak energies16, the band gap Eg of the Mo1−xWxS2 alloy was estimated using the following equation, where b is the bowing factor;

Figure 1j shows a high-resolution STM image of the larger Mo-rich area, in which the compositional ratio of W was estimated to be x = 0.3. The bowing factor of b = 0.14 eV given in ref. 16 was employed. Then, Eg (Mo1−xWxS2) was macroscopically evaluated to be 2.47 eV, which is in good agreement with the experimental value of 2.5 eV obtained by averaging the STS local spectra over the surface.

Localized electronic states of the conduction band

Next, we analyzed the localized electronic states of the conduction band in more detail using the Mo-derived electronic structure. To obtain an area of isolated Mo atoms easily, we carried out CVD growth using a lower Mo content (see Method). Figure 2a shows a high-resolution STM image of isolated Mo atoms in a W-rich Mo1−xWxS2 area obtained at Vs = +1.5 V. Mo atoms are brightly imaged, similarly to in Fig. 1. Figure 2b shows dI/dV spectra obtained above the Mo atom (blue) labeled by A in Fig. 2a and in the WS2 area (red). Figure 2c shows a magnification of the spectra near ECBM. Since the sample is a monolayer, the measurement is free from tip-induced band bending22,23. In the WS2 area, ECBM was located 0.77 V above the Fermi level EF and EVBM was located 1.95 eV below EF. Therefore, the band gap of this Mo1−xWxS2 monolayer was estimated to be 2.72 eV, which is comparable to that of a pure WS2 monolayer obtained from a two-photon absorption spectrum (2.73 eV)21. Since the compositional ratio of Mo atoms on this surface estimated from the STM image was very low (x = 0.025), the band gap of the WS2 area of this monolayer is considered to be almost identical to that of the pure WS2 monolayer. EVBM for the spectrum obtained above the Mo atom was located at an almost identical point to that obtained in the WS2 area. ECBM for the Mo spectrum is about 50 meV lower than that for the W spectrum.

Figure 2
figure 2

STM/STS on single Mo atoms.

(a) STM image of an area with isolated single Mo atoms in a WS2 area (Vs = +1.5 V, It = 0.1 nA). (b) dI/dV spectra obtained above Mo atom labeled A in (a) and in WS2 area. The gray rectangle indicates the noise level. (c) Magnification of the spectra in b near ECBM. (d) STM image of the area shown in a obtained at Vs = +1.2 V. (e) Map of the variation in ECBM near the Mo atoms A, B and C in (d). (f) Cross section along the dashed line in (e). The circles in e indicate the area of influence of the Mo-derived state determined from the cross section in (f). The red dots in e indicate the positions of Mo atoms.

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It is known that the contribution of the metal elements to the valence band electronic state is identical for MoS2 and WS2, namely, both are dxy and EquationSource math mrow msub mid mtextx msub mrow msup mrow mn2 mrow mtext-y msup mrow mn2 orbitals, while the main contribution of the metal to the conduction band electronic states is the EquationSource math mrow msub mid mtextz msup mrow mn2 orbital in MoS2 but the dxy, EquationSource math mrow msub mid mtextx msub mrow msup mrow mn2 mrow mtext-y msup mrow mn2 and EquationSource math mrow msub mid mtextz msup mrow mn2 orbitals in WS216,24. Because of the identical orbital contributions of W and Mo atoms to the valence band, these states are strongly coupled with each other to delocalize the EVBM state, whereas Mo-derived states are localized at ECBM because the contribution of the coupling between the Mo d orbital and the W d orbitals to the conduction band is small. Our experimental results clearly show these characteristics.

To what extent does the Mo-derived localized electronic state affect the neighboring WS2 area? Figure 2d shows an STM image of the area shown in Fig. 2a obtained at Vs = +1.2 V to highlight the Mo-derived electronic state. As expected, some W atoms near the Mo atoms are imaged brighter than those in the WS2 area. To show this effect more clearly, the variation in ECBM near the Mo atoms labeled A, B and C in Fig. 2d is mapped in Fig. 2e. The value at each of the 75 × 75 pixel was derived from the onset bias voltage (dI/dVonset = 0.01 nA/V) of each dI/dV spectrum. Figure 2f shows the cross section along the dashed line in Fig. 2e. A gradual decrease in ECBM toward the Mo atom can be observed in Fig. 2f. The variation in ECBM around Mo atoms A, B and C are indicated by dotted circles (1.5 nm diameter) in Fig. 2e. This is the first demonstration of the atomically resolved spatial variation in the localized electronic structure of Mo in Mo1−xWxS2 alloy.

Spatial variation in the band structure of WS2/Mo1−xWxS2 heterojunction

Finally, the band structure of the heterojunction interface was analyzed. Figure 3a shows an STM image of a Mo1−xWxS2-based heterojunction interface similar to that in Fig. 1c, where the left and right regions correspond to W- and Mo-rich Mo1−xWxS2 monolayers, respectively. To investigate the inner potential of the heterojunction, STS was carried out over the surface. The inset in Fig. 3a shows the STM image simultaneously obtained with the STS measurement.

Figure 3
figure 3

Potential landscape of WS2/Mo1−xWxS2 interface.

(a) STM image of WS2/MoxW1−xS2 interface (Vs = +1.6 V, It = 0.3 nA). The inset shows the STM image simultaneously obtained with spectroscopy. (b) Map of color scale dI/dV curves calculated from the spatially resolved STS spectra measured along the white dashed line in the inset of (a). (c) Map of ECBM obtained over the area shown in (a). (d) Cross sections of ECBM (top) and EVBM (middle), respectively corresponding to the upper and lower edges in (b) and electric field Efield (bottom) obtained by differentiating the cross section of ECBM (top). (e) dI/dV spectra obtained at the positions indicated by red and blue arrows in (b), which were averaged over the left and right edge lines in the inset of (a), respectively. The gray rectangle indicates the noise level. (f) Schematic image of the band structure of the WS2/Mo1−xWxS2 heterojunction. The values in (f) are those in (e). WSCR indicates the width of space charge region.

Full size image

Figure 3b shows a map of color scale dI/dV curves calculated from the spatially resolved STS spectra measured along the white dashed line in the inset of Fig. 3a. The upper and lower edges of the band gap region, corresponding respectively to ECBM and EVBM, continuously shifted as a function of the distance across the interface, whose position was determined from the STM image (Fig. 3a inset) and is indicated by the dashed black line in Fig. 3b. Figure 3b clearly demonstrates that a type-II staggered gap heterojunction with a nanoscale built-in potential distribution was formed at the interface9,14,25,26. This is the first observation of the electronic structure of a TMD heterojunction. The spatial variation of ECBM over the surface in Fig. 3a was mapped in Fig. 3c to visualize the electrostatic potential landscape at the interface in more detail, which is almost flat along the direction of the interface compared with the change along the direction crossing the interface (from pink to dark blue).

To better understand the positional relationship between the interface and the electrostatic potential variation, the cross section profile of the ECBM map along the dashed line in Fig. 3c (white dotted line in Fig. 3a inset) was plotted (Fig. 3d top) along with the profile of EVBM (Fig. 3d middle) along the line at the same position. The comparison of the topographic image with the cross sections of ECBM and EVBM reveals that the variation in the potential was greater on the Mo-rich side. Namely, ECBM is almost constant in the W-rich area, while it gradually changes over the Mo-rich area, possibly reflecting the asymmetric carrier screening length, which may be due to the difference in the doping characteristics and/or dielectric constant between the W-rich and Mo-rich areas. In addition, the profile of the electric field Efield was obtained from the derivative of the ECBM profile with respect to the lateral distance (Fig. 3d bottom). As expected from classical semiconductor theory, the electric field reached its maximum value at the interface position indicated by the dashed line in Fig. 3b,d. The strong electric field of as high as 80 × 106 Vm−1 observed at the interface is consistent with the observed charge separation efficiency at the interface11.

The red and blue spectra shown in Fig. 3e are the dI/dV spectra obtained at the positions indicated by red and blue arrows in Fig. 3b, respectively, where neither ECBM nor EVBM is affected by the built-in potential at the interface. The spectra were averaged over the left and right edges of the inset of Fig. 3c, respectively. From these spectra, the band offsets of ECBM and EVBM between the W-rich and Mo-rich areas were determined to be 0.30 eV and 0.17 eV and the band gaps of these areas were also determined to be 2.71 eV and 2.58 eV, respectively. Figure 3f shows the first ever schematic image of the band profile of a WS2/Mo1−xWxSx obtained from the experimental results.

In conclusion, we carried out atomically resolved analysis by low-temperature STM/STS on the electronic structures of a monolayer WS2/Mo1−xWxS2 alloy heterojunction that provides a tunable band alignment. The formation of a WS2/Mo1−xWxS2 heterojunction on a graphite substrate was confirmed for the first time. Then the atomically modulated spatial variation in the electronic structures, which is the basis for the macroscopic band structure of the WS2/Mo1−xWxS2 heterojunction, was directly observed. The macroscopic band structure of Mo1−xWxS2 alloy was reproduced by the STS spectra averaged over the surface. An electric field of as high as 80 × 106 Vm−1 was observed at the interface for the alloy with x = 0.3, verifying the efficient separation of photoexcited carriers at the interface. The atomic-scale analysis of TMD heterostructures by STM/STS allows the verification of basic semiconductor physics and is expected to play an essential role in the further advancement of various applications.

Method

Sample preparation

Monolayer WS2/Mo1−xWxS2 lateral heterojunctions were formed on Kish graphite (Covalent Materials Co.) by high-temperature chemical vapor deposition28. The graphite was mechanically exfoliated onto a quartz substrate using Nitto tape (SPV-224). The substrate was placed in a quartz tube (3 cm diameter, 100 cm long) with WO3 powder (Aldrich, 99% purity, 100 mg), MoO3 powder (Aldrich, 99% purity, 0.2 mg, reduced to 0.1 mg for the measurement of the effect of a single Mo atom shown in Fig. 2) and sulfur flakes (Aldrich, 99.99% purity, 2 g). The quartz tube was then filled with Ar gas at a flow rate of 100 cm3/min. The temperature of the substrate and the WO3 and MoO3 was gradually increased to the growth temperature (1100 °C) over 60 min using an electrical furnace. When the substrate temperature reached the set value, the sulfur was heated at 200 °C for 15–30 min to supply sulfur vapor to the substrate using another electrical furnace. After the growth, the quartz tube was immediately cooled using an electric fan.

Additional Information

How to cite this article: Yoshida, S. et al. Microscopic basis for the band engineering of Mo1−xWxS2-based heterojunction. Sci. Rep. 5, 14808; doi: 10.1038/srep14808 (2015).