Tailored growth of MoS2 few-layer films
Few-layer MoS2 films are promising candidates for applications in many areas, such as photovoltaics, photocatalysis, nanotribology, lithium batteries, hydrogenation desulfurization catalysis and dry lubricants, mainly due to their characteristic electronic, optical and catalytic properties. In general, two orientation options of the MoS2 layers are possible – horizontal (with the c-axis perpendicular to the plane of the substrate) and vertical (where the c-axis is parallel to the substrate), having different physicochemical properties. In the preparation of these materials by the sulfurization of molybdenum layers, the initial thickness of the molybdenum proved to be a critical parameter affecting the final orientation of the MoS2 layers. Unlike standard CVD chambers, where the reaction takes place in a two-zone furnace, we use a single zone furnace where we have a substrate and sulfur together at a high temperature in the center of the furnace. The aim of the work was to study the influence of other annealing parameters on the orientation of layers. The heating rate has been shown to be a critical parameter for the growth mechanism where rapid sulfurization leads to the growth of vertical MoS2 layers and slow sulfur evaporation leads to horizontal growth even for thicker initial molybdenum layers.
GIWAXS reciprocal space maps of MoS2 films on the c-plane sapphire substrate prepared from 3 nm thick Mo layers at 800 °C during 30 min with the heating rate of (a) 25 °C / min, (b) 5 °C / min and (c) 0.5 °C / min. The peaks at ~ 1 Å-1 originate from the (002) diffraction planes.
In addition, the single-zone sulfurization method used allowed the growth of MoS2 on the surface of the CVD microcrystalline diamond layers. This experimental design results in a sulfur-rich environment during the process, and diffusion of sulfur into molybdenum at temperatures below that required for formation of molybdenum carbide prevents the formation of the latter at the Mo-diamond interface. This finding may open a way for growing MoS2 layers on substrates which are otherwise susceptible to a chemical reaction with molybdenum. We have also shown that horizontal and vertical growth of the MoS2 layers is possible, depending on the thickness of the Mo layer as in the case of unstructured substrates. The combination of unique diamond properties and ultra-thin MoS2 layers with tunable crystallographic orientation can offer material properties relevant to a wide range of applications.
SEM images of MoS2 layers grown from (a) 1 nm, (b) 3 nm and (c,d) 6 nm thick Mo films deposited on the microcrystalline CVD diamond substrate. In (d), standing MoS2 flakes are seen on the edge of a diamond crystallite.
- Sojková, M., Végso, K., Mrkývkova, N., Hagara, J., Hutár, P., Rosová, A., Čaplovičová, M., Ludacka, U., Skákalová, V., Majková, E., Šiffalovič, P., and Hulman, M.: Tuning the orientation of few-layer MoS2 films using one-zone sulfurization, RSC Adv. 9 (2019) 29645-29651. IF 3.049, Q SJR 1
- Sojková, M., Šiffalovič, P., Babchenko, O., Vanko, G., Dobročka, E., Hagara, J., Mrkývková, N., Majková, E., Ižák, T., Kromka, A., and Hulman, M.: Carbide-free one-zone sulfurization method grows thin MoS2 layers on polycrystalline CVD diamond, Sci Rep. 9 (2019) 2001. IF 4.011, Q SJR 1
Study of semiconductor detectors of ionizing radiation based on 4H-SiC epitaxial layer
Semiconductor detectors of ionizing radiation are important area of research activity because of their utilization in many areas of human activities like monitoring radiation in nuclear energetics, medicine, cosmic research and so on. Because detectors operate at harsh environment by definition, there is much effort to search for base semiconductor material with high radiation and temperature resistance. Silicon carbide is very promissing material. Our research is also concentrated on radiation resistance characteristics of prepared detector samples and their comparision with standartly used Silicon detectors. Picture below shows comparison of Si and SiC detectors and their energy resolution worsering after 5 MeV electrons irradition. You can see that Si detector lost its energy resolution of testing X-ray peaks generated by 241Am radioisotope after dose of 1 kGy but SiC is still relatively good after dose of 30 kGy.
Zaťko, B., Hrubčín, L., Šagátová, A., Osvald, J., Boháček, P., Zápražný, Z., Sedlačková, K., Sekáčová, M., Dubecký, F., Skuratov, V.A., Korytár, D., and Nečas, V.: Schottky barrier detectors based on high quality 4H-SIC semiconductor: electrical and detection properties, Applied Surface Sci 461 (2018) 276-280.
Hrubčín, L., Gurov, J.B., Zaťko, B., Mitrofanov, S.V. Rozov, S.V., Sedlačková, K., Sandukovskij, V.G. Semin, V.A., Nečas, V., and Skuratov, V.A.: Characteristics of Si and SiC detectors at registration of Xe ions, J. Instrument. 13 (2018) P11005.