Study On Synthesis Methodology Of Metal Selenide Semiconductor Nanomaterials Based On The Acitivation Of Elemental Selenium

Metal selenides are a type of important semiconductor materials, and have been widely applied in the field of solar cells, gas sensors, photodetector, acousto-optic devices and phase change random access memory due to their desirable optoelectronic and structure properties. Metal selenides synthesized by vacuum-based methods are complicated and need exorbitant capital investment, thereby hindering their large-scale application. For the synthesis of selenide nanocrystals, non-vacuum methods, such as hot-injection and solvothermal synthesis, are capable of adjusting component for ternary and quaternary selenides, like Cu InSe2 and Cu2ZnSnSe4. Furthermore, these non-vacuum methods are cost-efficient, especially when elemental selenium is used as the Se source. Elemental Se, however, has limited solubility in most conventional solvents, which leads to a restricted reactivity and thus relatively slow reaction kinetics towards the formation of selenides.This thesis describes a convenient route to activate elemental selenium that is suitable to be used in the synthesis of metal selenide nanocrystals through solvothermal method. Elemental selenium is reduced to highly activated Se2- by Na BH4 in ethylenediamine at solvothermal conditions. The main achievements are as follows:(1) Elemental selenium is reduced to highly activated Se2- by NaBH4 in ethylenediamine at solvothermal conditions(200 °С, 2 h).(2) Cu2-xSe, SnSe and ZnSe have been synthesized with activated Se, compared with reactions using normal elemental selenium, the result indicates that activated Se play an important role in improve the formation kinetics of metal selenide and promote the generation of pure phase. Meanwhile, activated Se can lower reaction temperature and shorten the reaction time. Synthesis of MoSe2, In2Se3, ZnSe, Ga2Se3 and Sb2Se3 binary selenides with activated Se also has been studied. Although the pure target products have not been obtained, the study indicates the direction of improvement.(3) To demonstrate the reactivity of the resulting selenization reagent, Cu InSe2 nanocrystals have been synthesized with significantly promoted reaction kinetics through a solvothermal process, compared with reactions using normal elemental selenium. The promoted reaction rate associated with the activiated selenium can be attributed to the homogeneous reaction between Se2- and metal ions and higher activity of Se2- than Se(0). The CIS nanocrystals synthesized with activated Se show single phase, homogeneous particle size and morphology, and reliable photoelectric properties, which are suitable for photovoltaic applications. Impact of PVP on the morphology of CuInSe2 synthesized with activated Se are investigated, the results indicate that PVP with molecular weight of 3×104 can lead to the formation of flower-like morphology consisted of nanosheets. However, PVP with molecular weight of 1.3×106 can result in a spherical morphology composed of sheets. The formation pathway of CIS have also been investigated, we propose that Cu-Se compounds are the intermediate for the formation of CIS. We did not observe any In-Se phase in our experiments. This implies that the generation of In2Se3 or(InSe2)ˉ is the rate determine step in the formation of CIS, or the formation of CIS relies on the diffusion of In3+ ions into Cu-Se compounds.(4) Pure phase of Cu2SnSe3 are obtained with activated Se through solvothermal process at 200 °С and reaction 3 h. However, the synthesis reaction using normal Se requires a prolonged time of 12 h to go to completion; this is probably due to the difficult formation pathway of SnSe which act as an intermediate in the formation mechanism of Cu2SnSe3.(5) CuGaSe2 with single phase constituent can not be obtained by using activated Se through the solvothermal method. The possible reason is the lower reaction activity of Ga(acac)3 in ethylenediamine system.