At present, two-dimensional graphene-like materials are considered as promising materials for creating fundamentally new devices for nano- and optoelectronics. Monolayers of transition metal dichalcogenides (TMDCs) with the chemical formula MX2, where M = Mo, W, Ti; X = S, Se, are direct-band semiconductors with a band gap of 1.2-2 eV in contrast to the graphene possessing semimetallic conductivity. There are a number of works that give examples of the use of TMDCs in nano- and optoelectronic devices (phototransistors, gas sensors, solar cells, memory devices, light-emitting sources of electroluminescence, sources of electron emission, etc.).  To create optoelectronics devices with a wide spectral range of operation [1], or devices that have spectral selectivity, it is necessary to vary the value of the band gap of the material used. With a controlled change of chemical elements concentration in the TMDCs alloys flakes, a smooth rearrangement of their band gap structure is possible, which allows to vary their spectral properties in a wide range [2].  Mo(1−x)W(x)S(1−y)Sey monolayers with concentrations x,y from 0 to 1 were obtained. A complex evaluation of the optical properties of the resulting two-dimensional (monolayer) crystallites was carried out. Based on these materials, photodetectors were created.  In summary, the possibility of obtaining flakes of Mo(1−x)W(x)S(1−y)Sey alloys possessing essentially different concentrations of the initial elements was shown. Monolayer flakes were synthesized from a single bulk crystal obtained by the chemical gas transport reactions technique with the additional creation of a temperature gradient during crystallization. These results differ from presented earlier, where the concentration of the initial elements was specified directly in the process of growth of the bulk material which had its own composition [3]. Thus, a simple mechanism for obtaining two-dimensional semiconductor heterogeneous structures possessing a wide range of the luminescence spectrum (from 1.56 eV to 1.87 eV), which is promising for use in photovoltaics, is presented [4]. Volt-current characteristics of the photodetectors, created on the basis of TMDC monolayers are also presented.