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[research background]
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The new two-dimensional semiconductor atomic crystal is the core material for the construction of high-performance nano optoelectronic devices in the future because of its atomic level thickness, nano level layered structure and high carrier mobility. Band gap is one of the most important basic parameters in two-dimensional semiconductor electronic devices and optoelectronic devices. It is also one of the important factors that affect the switching ratio and photocurrent response of two-dimensional semiconductor electronic devices. Therefore, it is an important method to precisely control the band structure of two-dimensional semiconductor atomic crystal to improve the device performance. For example, graphene is a zero band gap semiconductor, and its band gap can be opened by doping, modifying and patterning, but the band gap regulation range is limited; 1.0 EV); the band gap of g-c3n4 can be up to 2.7 EV, which can be partially reduced by doping (~ 1.9 EV); the transition metal chalcogenides represented by MoS2 and WS2 only have direct band gap structure, while the double-layer and multi-layer are indirect semiconductors, whose band gap can be changed by element doping, but the regulation range is limited by their own structure (& lt; 2.1 EV). It can be seen that controlling and optimizing the structure of two-dimensional semiconductor atomic crystals to achieve band gap regulation is an important research direction of nano semiconductor materials in the future.
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[achievement introduction]
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Recently, through theoretical calculation and structural design, Professor Feng Wei's team of Tianjin University synthesized - H / - Oh capped binary germanium, which was named gersiloxane. By controlling the content of germanium and silicon, germanium silanes with different chemical and crystal structures were obtained. The band gap control of germanium and silicon-based binary two-dimensional materials was realized for the first time. The two-dimensional germanium silanes with suitable band gap structure, high specific surface area and surface chemical activity can be used as photocatalysts to achieve high-efficiency hydrogen production at room temperature and significantly improve the capacity of photocatalytic CO2 reduction.
The lack of layered materials is an important difficulty in the preparation of two-dimensional germanium based and silicon-based semiconductor materials. To solve this problem, the team prepared germanane (GEH) and silane (SIH) with two-dimensional layered structure by directly hydrogenating Zintl phase cage2 and casi2 combined with topological chemical reaction. On this basis, by controlling the stoichiometric ratio of calcium (CA), germanium (GE) and silicon (SI), the precursor Ca (ge1-xsix) 2 alloy was prepared by high temperature sintering, and then the precursor Ca (ge1-xsix) 2 alloy was intercalated with concentrated hydrochloric acid at low temperature (- 30 ℃), finally a series of two-dimensional germanosilane with different chemical structures were obtained. The structure characterization shows that the two-dimensional germanosilane is a honeycomb network structure two-dimensional material composed of hydrogen capped Ge atom and hydrogen (- H) or hydroxyl (- OH) capped Si atom in the form of binary alloy. At the same time, the chemical structure of two-dimensional germanosilane is closely related to the ratio of Ge and Si. When X & lt; 0.5, the chemical bonds of ge-h and Si Oh are formed in the material, respectively. When x ≥ 0.5, the chemical bonds of Si-H appear in the material, so the structure of germanosilane is (GEH) 1-xsix (OH) 0.5hx-0.5.
On this basis, the theoretical model of germanosilane is constructed. The first principle calculation results based on the density functional theory show that the two-dimensional germanosilane and bulk materials are direct band gap semiconductors. Different from the transition metal chalcogenides, the band gap type does not depend on the number of layers of germanosilane, but also has nothing to do with the proportion of Ge and Si elements in germanosilane. The band gap structure increases with the increase of X Increase. The optical band gap test results show that when x increases from 0.1 to 0.9, the band gap of two-dimensional germanosilane increases from 1.8 ev to 2.57 EV, which is consistent with the theoretical calculation results.
At the same time, its band structure is suitable for photocatalytic hydrogen production and CO2 reduction under different complex conditions. When x = 0.5, the two-dimensional germanium silane (hgesioh) shows the best photocatalytic performance. In the photocatalytic water reduction, it can be used as 1.58 mmol g-1 H-1 can also catalyze the reduction of CO2, and generate CO at the rate of 6.91 mmol g-1 H-1, which is higher than the reported photocatalyst. These results show that the two-dimensional germanium silane with band gap adjustable performance has great potential in the production of hydrogen and the reduction of CO2.
Two dimensional germanosilane is one of the ideal materials for the preparation of nano energy converters and nano optoelectronic devices in the future. This research is the first time to realize the doping precise regulation of band structure of germanium silicon type IVA family two-dimensional atomic crystal semiconductor, which will provide important material basis and technical support for the future synthesis, design, electronic structure regulation and photoelectric performance improvement of new semiconductor two-dimensional atomic crystal materials.
This work was recently published on the journal Nature communications (DOI: 10.1038 / s41467-020-15262-4) under the title of "two dimensional gersiloxenes with tunable bandgap for photographic H2 evolution and CO2 photoreducation to co". The first author of this paper is Zhao Fulai, Ph.D. student, the corresponding author is Professor Feng Wei, and the co corresponding author is Professor Feng Yiyu.
Relevant work was supported by national key R & D projects (2016yfa0202302) and National Outstanding Youth Fund (51425306).
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[text guide]
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Figure 1 crystal structure characterization of germanosilane
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Figure 2 chemical structure characterization of germanosilane
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Fig. 3 TEM image of germanosilane
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Fig. 4 Calculation of electronic properties density functional theory of germanosilane with single layer structure
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Fig. 5 calculation of electronic properties density functional theory of germanosilane with bilayer structure
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Figure 6 optical properties and band gap of germanosilane
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Figure 7 band structure of germanosilane
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Figure 8 characterization of photocatalytic performance
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