1. Growth of Individual 2D material (MoS2 and WS2)
2. The Growth of the WS2/MoS2 Vertical Single Hetero-structure
NOTE: This section is used to create a single hetero-structure consisting of a sapphire layer with 5 layers of MoS2 and 4 layers of WS2.
3. The Film Transferring and Device Fabrication Procedures
The Raman spectrum and the cross-sectional HRTEM images of individual MoS2 and WS2 fabricated using the sulfurization of pre-deposited transition metals are shown in Figure 1a-b17, respectively. Two characteristic Raman peaks are observed for both MoS2 and WS2, which correspond to in-plane and out-of-plane A1g phonon vibration modes in the 2D crystals. The frequency difference Δk of two Raman peaks for the MoS2 sample is 24.1 cm-1, which suggests that 4 – 5 layers of MoS2 was obtained19. However, it is difficult to determine the possible layer number directly from the large Δk value 63.4 cm-1 of WS217. The cross-sectional HRTEM images of the two samples, shown in Figure 1c-d, have revealed that 5- and 4-layer MoS2 and WS2 are obtained for the two samples, respectively. The results have demonstrated that through sulfurizing the transition metals, large-area and uniform MoS2 and WS2 films can be obtained.
The cross-sectional HRTEM image of the 1.0 nm Mo film sulfurized under the sulfur deficient condition is shown in Figure 2a, wherein clusters covered with few-layer 2D crystals were observed. These results indicated that two growth mechanisms were observed during the sulfurization procedure15. Under the sulfur sufficient condition, sulfur-for-oxygen reactions took place quickly such that planar MoS2 covered the whole sample in a short time. This planar MoS2 film on the sample surface could prevent further material migration such that a uniform and layer-number-controllable MoS2 film could be obtained after the sulfurization procedure. However, when the sample was sulfurized under the sulfur deficient condition, the background sulfur was not sufficient to form a complete MoS2 film such that the Mo oxide segregation and coalescence was the dominant mechanism at the early growth stage. In this case, a sample with Mo oxide clusters covered by few-layer MoS2 would be obtained after the sulfurization procedure15. The schematic diagram describing the model of the transition metal sulfurization is shown in Figure 2b15. Since two growth mechanisms were observed during the sulfurization procedure, there was an upper limit for the MoS2 layer number with one-time growth.
Using sequential metal deposition with sulfurization procedures discussed above, a WS2/MoS2 single hetero-structure was prepared after two procedures of transition metal deposition/sulfurization. The Raman spectra and the cross-sectional HRTEM image of the sample are shown in Figure 3a-b17. Besides the characteristic Raman peaks corresponding to MoS2 and WS2, respectively, the identical layer number 9 with the summation of the individual 5- and 4- layer MoS2 and WS2 suggests that the sample was a WS2/MoS2 single hetero-structure. Following similar growth procedures, a WS2/MoS2/WS2 double hetero-structure was prepared after three procedures of transition metal deposition/sulfurization. The Raman spectra and the cross-sectional HRTEM image of the sample are shown in Figure 3c-d. With a similar observation of MoS2 and WS2 characteristic Raman peaks discussed above, only three layers of 2D crystals were observed for this sample. These results have revealed that (a) good layer number controllability down to a single layer was obtained for this growth technique and (b) a vertical 2D crystal double hetero-structure can be established in three atomic layer thickness16.
Another sample with half-covering transition metal deposition was prepared to show the possibility of selective growth using the growth technique discussed in this report. By shielding half of the sapphire substrate during the 1.0 nm Mo deposition, half of the substrate could be covered with MoS2 after sulfurization. After that, the sample was rotated 90° to deposit W to cover half of the sapphire substrate. The same sulfurization procedure was conducted again. In this case, four regions with (a) blank sapphire substrate, (b) standalone MoS2, (c) WS2/MoS2 hetero-structure, and (d) standalone WS2 were obtained within a single sapphire substrate17. The picture and the Raman spectra of the four different regions on the sample are shown in Figure 4. As shown in the figure, large-area and uniform WS2 and MoS2 films and their vertical hetero-structures were selectively grown on the same sapphire substrate. These results have indicated that besides the establishment of vertical hetero-structures, the growth method of transition metal sulfurization selectively grew 2D crystals on substrates. This flexibility may give more room to practical device fabrications based on 2D materials and their hetero-structures.
To compare the device performance of the transistors with the MoS2 and the WS2/MoS2 vertical hetero-structure as device channels, two transistors were fabricated following the fabrication procedure described in step 3 of the protocol. The schematic diagram showing the fabrication procedure is also shown in Figure 5a. The ID-VGS curves of the devices at VDS = 10 V are shown in Figure 5b. Compared with the MoS2 transistor, a substantial drain current increase was observed for the hetero-structure device. The field-effect mobility values of the two devices with MoS2 and WS2/MoS2 hetero-structure as the channels extracted from the curves are 0.27 and 0.69 cm2/V·s, respectively. Our previous prediction of electron injection from WS2 to MoS2 and from higher electron concentration channels under thermal equilibrium could be responsible for this phenomenon.
After the thin Mo film deposition, the sample was moved out of the sputtering chamber and exposed to air. Since the Mo film is very thin, it was oxidized and formed Mo oxides quickly under ambient conditions. The XPS curve (X-ray photoelectron spectroscopy) of the sample before the sulfurization procedure is shown in Figure 6a. As shown in the figure, the film was composed of MoO2 and MoO3 before the sulfurization procedure. These results suggest that the Mo film was oxidized during the transferring procedure from the sputtering chamber to the hot furnace. The other supporting evidence for the formation of the 2D crystal hetero-structure may have come from the equivalent selective etching of the 2D crystal hetero-structure. For this purpose, we have demonstrated that atomic etching can be achieved for both MoS2 and WS2 using low-power oxygen plasma treatment20. We can achieve equivalent selective etching to the vertical hetero-structure by repeating the atomic layer etching procedure. The Raman spectrum of the etched and un-etched 4-layer WS2/3-layer MoS2 vertical hetero-structure are shown in Figure 6b. The atomic layer etching times were consistent with the layer number of WS2 (4 times). The observations of both MoS2 and WS2 Raman peaks on the un-etched region, and MoS2 signals only on the etched region, suggest that a vertical hetero-structure was established using the growth technique discussed in this paper.
Figure 1: Individual 2D crystals of MoS2 and WS2. (a, b) The Raman spectra and (c, d) the cross-sectional HRTEM images of standalone MoS2 and WS2, respectively17. The samples are obtained by sulfurizing 1.0 nm Mo and W films prepared by a sputtering system. As shown in the Raman spectra, two characteristic Raman peaks were observed for both MoS2 and WS2, which corresponded to in-plane and out-of-plane
phonon vibration modes in the 2D crystals. The layer number numbers of MoS2 and WS2 grown using the method discussed in the current manuscript were proportional to the sputtering times of the pre-deposited Mo and W films. The determination of sputtering times to obtain MoS2 and WS2 with required layer numbers is based on the cross-sectional HRTEM images for samples with different sputtering times. However, if the pre-deposited Mo and W films are too thick, Mo and W oxide segregation will become the dominant growth mechanism instead of the planar MoS2 and WS2 film growth. Therefore, the proportionality of layer numbers with the sputtering times was limited to few-layer TMDs. With the growth conditions of MoS2 in the current manuscript, the layer numbers will be proportional to the sputtering times when the MoS2 film is fewer than 10 layers. The sputtering time is 30 s for the growth of 5-layer MoS2. This figure has been modified from Wu et al.17 Please click here to view a larger version of this figure.
Figure 2: The growth model of transition metal sulfurization. (a) The cross-sectional HRTEM image of the 1.0 nm Mo film sulfurized under the sulfur deficient condition and (b) The schematic diagram describing the model for the transition metal sulfurization15. The growth conditions of the sample prepared with no sulfur powder placed in the furnace is referred to as the sulfur deficient condition. Since there is always residue sulfur accumulation near the downstream of the growth chamber after repeating growth cycles, it is expected that a small amount of sulfur will still diffuse to the sample surface and result in MoS2 growth. However, under such sulfur deficient conditions, not all the pre-deposited Mo will be transformed into MoS2. This figure has been modified from Wu et al15. Please click here to view a larger version of this figure.
Figure 3: MoS2/WS2 single- and double-hetero-structures. The Raman spectra and the cross-sectional HRTEM images of WS2/MoS2 (a, b) single- and (c, d) hetero-structures16,17. As shown in the Raman spectrum, the in-plane and out-of-plane
phonon vibration modes of both MoS2 and WS2 are observed for the 2D crystal hetero-structures. The panels have been modified from Chen et al. and Wu et al.16,17 Please click here to view a larger version of this figure.
Figure 4: Selective growth of 2D crystals. The picture and the Raman spectra of four regions of the sample prepared with half-covering transition metal depositions on a single sapphire substrate17. Raman spectra in the (a) blank sapphire substrate, (b) standalone MoS2, (c) WS2/MoS2 hetero-structure and (d) standalone WS2 regions of the sample revealed characteristic Raman peaks. This figure has been modified from Wu et al.17 Please click here to view a larger version of this figure.
Figure 5: The device performance of MoS2 and WS2/MoS2 vertical hetero-structure transistors. (a) The fabrication procedure of the transistors with the MoS2 and the WS2/MoS2 vertical hetero-structure as the channels and (b) the ID-VGS curves of the two devices at VDS = 10 V17. The thicknesses of 1.0 nm for Mo and W films were obtained from the readings of the quartz crystal resonator. The sputtering times were 30 s for both materials. This figure has been modified from Wu et al.17 Please click here to view a larger version of this figure.
Figure 6: The oxidation of pre-deposited Mo films and the equivalent selective etching of WS2/MoS2 vertical hetero-structures. (a) The XPS curve of the sample with the pre-deposited Mo film before the sulfurization procedure. The film is composed of MoO2 and MoO3 before the sulfurization procedure. These results suggest that the Mo film was oxidized during the transferring procedure from the sputtering chamber to the hot furnace. (b) The Raman spectra of the etched and un-etched 4-layer WS2/3-layer MoS2 vertical hetero-structure. After four times of atomic layer etchings, only MoS2 peaks were observed on the etched region; Panel B has been modified from Chen et al.20 Please click here to view a larger version of this figure.
RF sputtering system | Kao Duen Technology | N/A | |
Furnace for sulfurization | Creating Nano Technologies | N/A | |
Polymethyl methacrylate (PMMA) | Microchem | 8110788 | Flammable |
KOH, > 85% | Sigma-Aldrich | 30603 | |
Acetone, 99.5% | Echo Chemical | CMOS110 | |
Sulfur (S), 99.5% | Sigma-Aldrich | 13803 | |
Molybdenum (Mo), 99.95% | Summit-Tech | N/A | |
Tungsten (W), 99.95% | Summit-Tech | N/A | |
C-plane Sapphire substrate | Summit-Tech | X171999 | (0001) ± 0.2 ° one side polished |
300 nm SiO2/Si substrate | Summit-Tech | 2YCDDM | P-type Si substrate, resistivity: 1-10 Ω · cm. |
Sample holder (sputtering system) | Kao Duen Technology | N/A | Ceramic material |
Mechanical pump (sputtering system) | Ulvac | D-330DK | |
Diffusion pump (sputtering system) | Ulvac | ULK-06A | |
Mass flow controller | Brooks | 5850E | The maximum Argon flow is 400 mL/min |
Manual wheel Angle poppet valve | King Lai | N/A | Vacuum range from 2500 ~1 × 10-8 torr |
Raman measurement system | Horiba | Jobin Yvon LabRAM HR800 | |
Transmission electron microscopy | Fei | Tecnai G2 F20 | |
Petri dish | Kwo Yi | N/A | |
Tweezer | Venus | 2A | |
Digital dry cabinet | Jwo Ruey Technical | DRY-60 | |
Dual-channel system sourcemeter | Keithley | 2636B |
We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition metal dichalcogenides (TMDs) MoS2 and WS2 can be prepared on sapphire substrates. By controlling the metal film thicknesses, good layer number controllability, down to a single layer of TMDs, can be obtained using this growth technique. Based on the results obtained from the Mo film sulfurized under the sulfur deficient condition, there are two mechanisms of (a) planar MoS2 growth and (b) Mo oxide segregation observed during the sulfurization procedure. When the background sulfur is sufficient, planar TMD growth is the dominant growth mechanism, which will result in a uniform MoS2 film after the sulfurization procedure. If the background sulfur is deficient, Mo oxide segregation will be the dominant growth mechanism at the initial stage of the sulfurization procedure. In this case, the sample with Mo oxide clusters covered with few-layer MoS2 will be obtained. After sequential Mo deposition/sulfurization and W deposition/sulfurization procedures, vertical WS2/MoS2 hetero-structures are established using this growth technique. Raman peaks corresponding to WS2 and MoS2, respectively, and the identical layer number of the hetero-structure with the summation of individual 2D materials have confirmed the successful establishment of the vertical 2D crystal hetero-structure. After transferring the WS2/MoS2 film onto a SiO2/Si substrate with pre-patterned source/drain electrodes, a bottom-gate transistor is fabricated. Compared with the transistor with only MoS2 channels, the higher drain currents of the device with the WS2/MoS2 hetero-structure have exhibited that with the introduction of 2D crystal hetero-structures, superior device performance can be obtained. The results have revealed the potential of this growth technique for the practical application of 2D crystals.
We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition metal dichalcogenides (TMDs) MoS2 and WS2 can be prepared on sapphire substrates. By controlling the metal film thicknesses, good layer number controllability, down to a single layer of TMDs, can be obtained using this growth technique. Based on the results obtained from the Mo film sulfurized under the sulfur deficient condition, there are two mechanisms of (a) planar MoS2 growth and (b) Mo oxide segregation observed during the sulfurization procedure. When the background sulfur is sufficient, planar TMD growth is the dominant growth mechanism, which will result in a uniform MoS2 film after the sulfurization procedure. If the background sulfur is deficient, Mo oxide segregation will be the dominant growth mechanism at the initial stage of the sulfurization procedure. In this case, the sample with Mo oxide clusters covered with few-layer MoS2 will be obtained. After sequential Mo deposition/sulfurization and W deposition/sulfurization procedures, vertical WS2/MoS2 hetero-structures are established using this growth technique. Raman peaks corresponding to WS2 and MoS2, respectively, and the identical layer number of the hetero-structure with the summation of individual 2D materials have confirmed the successful establishment of the vertical 2D crystal hetero-structure. After transferring the WS2/MoS2 film onto a SiO2/Si substrate with pre-patterned source/drain electrodes, a bottom-gate transistor is fabricated. Compared with the transistor with only MoS2 channels, the higher drain currents of the device with the WS2/MoS2 hetero-structure have exhibited that with the introduction of 2D crystal hetero-structures, superior device performance can be obtained. The results have revealed the potential of this growth technique for the practical application of 2D crystals.
We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition metal dichalcogenides (TMDs) MoS2 and WS2 can be prepared on sapphire substrates. By controlling the metal film thicknesses, good layer number controllability, down to a single layer of TMDs, can be obtained using this growth technique. Based on the results obtained from the Mo film sulfurized under the sulfur deficient condition, there are two mechanisms of (a) planar MoS2 growth and (b) Mo oxide segregation observed during the sulfurization procedure. When the background sulfur is sufficient, planar TMD growth is the dominant growth mechanism, which will result in a uniform MoS2 film after the sulfurization procedure. If the background sulfur is deficient, Mo oxide segregation will be the dominant growth mechanism at the initial stage of the sulfurization procedure. In this case, the sample with Mo oxide clusters covered with few-layer MoS2 will be obtained. After sequential Mo deposition/sulfurization and W deposition/sulfurization procedures, vertical WS2/MoS2 hetero-structures are established using this growth technique. Raman peaks corresponding to WS2 and MoS2, respectively, and the identical layer number of the hetero-structure with the summation of individual 2D materials have confirmed the successful establishment of the vertical 2D crystal hetero-structure. After transferring the WS2/MoS2 film onto a SiO2/Si substrate with pre-patterned source/drain electrodes, a bottom-gate transistor is fabricated. Compared with the transistor with only MoS2 channels, the higher drain currents of the device with the WS2/MoS2 hetero-structure have exhibited that with the introduction of 2D crystal hetero-structures, superior device performance can be obtained. The results have revealed the potential of this growth technique for the practical application of 2D crystals.