CVD synthesis of graphene and 2D Transition metal dichalcogenides

Transcription

1 CVD synthesis of graphene and 2D Transition metal dichalcogenides R E Beissenov 1,2, A T Galin 1, D A Muratov 1,2 and Z A Mansurov 1 1 Institute of Combustion Problems, Almaty, Kazakhstan 2 Institute of Physics and Technology, Almaty, Kazakhstan renat7787@mail.ru Abstract. Synthesis of graphene was carried out in a tubular vacuum CVD quartz reactor installed in a 3-zone high temperature furnace in atmospheric pressure, at 1050 O C temperature, and the ratio of H 2 : CH 4 : Ar - 0.2: 0.4: 3, for 20 minutes. After deposition of graphene it was carried out the transfer procedure from copper base to the FTO substrate by a method of applying a layer of PMMA. Raman spectra of transferred graphene layer shows a good quality. Synthesis of WS 2 performed by sulfurizing the WO 3 layer at 750 O C, which previously deposited on the FTO substrate by thermal evaporating. PL spectroscopy results of WS 2 crystal has band gap 1.96 ev. 1. Introduction Graphene is only the first member of a whole family of 2-D materials in the quantum limits. Germanene, silicene, boron nitride, phosphorene, oxides, chalcogenides and 2-D heterostructures based on two or more layers of these 2-D materials are also been investigated for their unique characteristics in the quantum limit in recent years. For example, a monolayer of boron nitride is an insulator and a monolayer of WS 2 is semiconductor with a direct bandgap of ~ 1.9 ev. Novel 2-D optoelectronic devices can be realized by engineering various 2-D materials into heterostructures to achieve the desirable characteristics. 2-D transition metal dichalcogenides (TMDs) have attracted a lot of attention. For example, the bulk MoS 2 is a semiconductor with an indirect bandgap of ~1.3 ev and the SL MoS 2 has a direct bandgap ~ 1.8 ev [1-3]. Therefore, MoS 2 could complement graphene for many electronic and photonic applications. The atomic structure of layered TMD of TX 2 type (T, transition metal; X, chalcogenide), e.g., MoS 2, MoSe 2, WS 2, and WSe 2 films. A single sheet of TX 2 consists of three atomic layers X T X, where T and X are covalently bonded. Weak interlayer bonds hold sheets together. Large-area of single and a few layer MoS 2 films have been synthesized by CVD, [4, 5] sulfurization of MoO 3, [1] or thermolysis of (NH 4 )MoS 4 [2]. Optoelectronic devices and sensors have been reported based on 2D MoS 2. However, devices fabricated from these polycrystalline MoS 2 films are still inferior to their exfoliated counterparts due to the detrimental effects of the grain boundary [6, 7]. However, the synthesis of graphene and related 2-D materials and heterostructures remains a challenge and, therefore, a hot research area. Chemical vapor deposition (CVD) has been recognized as the most viable method to synthesize graphene and many of these 2-D materials. However, these CVD films are polycrystalline. Electrical characteristics of devices located across grains suffer the detrimental effects of the grain boundary.

2 2. CVD synthesis of graphene Graphene segregation by cooling is a non-equilibrium phenomenon. Non-equilibrium segregation in general involves the transport of vacancy-impurity (vacancy-carbon in our case) complexes to sinks, such as grain boundaries and surfaces during cooling, and is closely related to the cooling rate [8]. The CVD grapheme has been epitaxial grown on copper foil. Synthesis of graphene was carried out in a tubular vacuum CVD quartz reactor installed in a 3-zone high temperature furnace (Figure 1). Methane gas (CH 4 ) has been used as the carbon source. Graphene have been synthesized in atmospheric pressure, at 1050 C temperature, and the ratio of H 2 : CH 4 : Ar - 0.2: 0.4: 3, for 20 minutes.samples were cooled down by mechanically pushing the sample holder to lower temperature zones in the range of 30~500 C in Ar atmosphere. Cooling rates were monitored by a thermocouple on the sample holder. Different cooling rates, corresponding to fast (20 C/s), medium (10 C/s) and slow (0.1 C/s), were employed. 1 - Ar, CH 4, H 2 ; 2-valves; 3- heating zones,4- thermocontrollers; 5-quartz tube; 6- heaters; 7- tube furnace; 8- foreline; 9- pump; 10- magnet sample holder. CVD setup Figure 1. Schematic of CVD synthesis of graphene and picture of CVD setup. a) b) Figure 2. a) PMMA assisted wet-transfer; b) graphene layer-coated PMMA.

3 After deposition of graphene it was carried out the transfer procedure from copper base to the FTO substrate by a method of applying a layer of PMMA (polymethyl methacrylate) (Figure 2). This process is very important, as it significantly affects the parameters of portable layers. Copper foil has etched in a solution of ironnitrate or chloride. The resulting graphene-coated PMMA layer was deposited on the surface of the FTO, after the polymer was washed out with acetone. Figure 3. Raman spectra of graphene during transfer. Raman spectra confirmed that the transferred graphenes maintain their high quality. In Figure 3, the blue curve is the Raman spectrum acquired from the grapheme segregated on Cu surface with 10 C/s cooling rate, the red curve is from the PMMA (with two peaks around 2900~3000 cm -1 ), and the black curve is from transferred graphene. Features in the spectrum of pre-transfer graphene are maintained in the spectrum of the post-transfer graphene. PMMA peaks can be seen from the transferred graphene on the glass plate, probably because the graphene layer is so thin that the signal from silicone rubber can pass through it and be detected. 3. Synthesis of 2D-WS 2 by CVD Synthesis of two dimension WS 2 layer were synthesized using sulfurization of thin WO 3 films deposited by thermal evaporating on the FTO (fluorine doped tin Oxide) substrate. 5nm thick WO 3 deposited on the FTO substrate in a system of vacuum thermal evaporation ZHD- 300M2 equipped with a resistive heater by using WO 3 powder. Synthesis of WS 2 performed in 3-zone tubular vacuum CVD furnace. Alumina boat with the sulfur (99,999%) installed at heating zone 1, and heated up to C for the sulfur sublimation. WO 3 / FTO substrate was installed into the reaction zone 2 and heated to C. Process was carried out under 500 sccm argon gas, where the argon molecule acted as carrier material for the sublimed sulfur. Figure 4 shows a micrograph of the 6-30 micron size WS 2 crystals on the FTO surface after synthesis in the CVD reactor by sulfurization. The microstructure has a similar structure of crystalline structures with geometric shapes in the form of triangular crystals at a temperature synthesis of C. Raman spectrum on figure 5 shows the formation of peaks with E 2g and A 1g modes that typical for the WS 2 crystals. The position of the peak makes possible to assume that the thickness of the grown 2D WS 2 more than 6 layers.

4 Obtained PL spectra of the synthesized WS 2 grain shows peak at 630 nm wavelength (Figure 6). The band gap of investigated grain spot is 1.96 ev that shows the thickness of the WS 2 is contains only bilayer. Figure 4. SEM micrograph of the synthesized 6-30 micron WS 2 crystals at 750 O C on FTO. Figure 5. Raman spectra of WS 2 grown on FTO. Figure 6. PL spectra of WS2 crystal. 4. Conclusion Epitaxial graphene layer and 2D WS 2 synthesized by CVD. Obtained results have a great interest in field of photocatalytic water splitting in the visible spectrum, as well as in many applications of microand nano-electronics. References [1] Lin Y C, Zhang W J, Huang J K, Liu K K, Lee Y H, Liang C T, Chu C W and Li L J 2012 Wafer-scale MoS 2 thin layers prepared by MoO 3 sulfurization Nanoscale [2] Liu K K, Zhang W J, Lee Y H, Lin Y C, Chang M T, Su C, Chang C S, Li H, Shi Y M, Zhang H, Lai C S and Li L J 2012 Growth of large-area and highly crystalline MoS 2 thin layers on insulating substrates Nano Letters [3] Radisavljevic B, Radenovic A, Brivio J, Giacometti V and Kis A 2011 Single-layer MoS 2 transistors Nature Nanotechnology [4] Zhan Y J, Liu Z, Najmaei S, Ajayan P M and Lou J 2012 Large-area vapor-phase growth and

5 characterization of MoS 2 atomic layers on a SiO 2 substrate Small [5] Lee Y H, Zhang X Q, Zhang W J, Chang M T, Lin C T, Chang K D, Yu Y C, Wang J T W, Chang C S, Li L J and Lin T W 2012 Synthesis of large-area MoS 2 atomic layers with chemical vapor deposition Advanced Materials [6] Radisavljevic B, Radenovic A, Brivio J, Giacometti V and Kis A 2011 Single-layer MoS 2 transistors Nature Nanotechnology [7] Kim S, Konar A, Hwang W S, Lee J H, Lee J, Yang J, Jung C, Kim H, Yoo J B, Choi J Y, Jin Y W, Lee S Y, Jena D, Choi W and Kim K 2012 High-mobility and low-power thin-film transistors based on multilayer MoS 2 crystals Nature Communications 3 [8] Thuvander M and Andrén H-O 2000 Mater. Charact