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Seminar khoa hoc

9.00 AM - 10.00 AM: MSc. Vo Duy Dat presents the topic: "Strain effect on Electronic and Optical Properties of 2D Janus HfSeO and HfSO" 

Abstract: A first-principles calculation was performed to investigate the photocatalytic behavior of 2D Janus monolayer HfSO and HfSeO at equilibrium and under the influence of strains and external electric fields. A thorough investigation of the structural, electrical, and optical properties of HfS2, and HfO2 was also provided. In the 1T configuration, the four 2D Janus monolayers were found to be dynamically and thermally stable. Young's modulus and Poisson's ratio of 1T HfSO, and HfSeO reveal isotropic elastic characteristics on the x-y plane as well as the tendency to expand transversally under compressive strains. The three monolayers HfS2, HfO2, and HfSO have appropriate band gaps for redox reaction activation, with corresponding band gaps of 2.48 eV, 6.50 eV, and 2.98 eV. Furthermore, the extremely anisotropic mobility of electrons and holes on the x-y plane benefits the electron-hole separation process. The equilibrium HfSO has a moderate absorption rate of 5x105 cm-1 in the visible range, with intensity increasing by around 20% with a 6% strain. Consequently, the absorption range is moved to a lower energy level, covering the majority of visible regions. The absorption rate α(ω) of compressed HfSeO is higher in both infrared, visible, and ultra-violet regions, while the intensity of α(ω) is also increased. Both HfSO and HfSeO are expected to be very promising materials for application in optoelectronic devices, as well as photovoltaic and water-splitting catalysts, which can be greatly enhanced by appropriate stresses. 

10.00 AM – 11.00 AM: Dr. Nguyen Minh Tam presents the topic: "Structures, growth mechanism, and ionization energies of lithium doped silicon clusters: toward silicon baby crystals with alkali linkers" 

Abstract: The geometric structures of neutral and cationic states of lithium doped silicon clusters are investigated using combined experimental and computational methods. The adiabatic and vertical ionization energies of SinLim clusters are determined using quantum chemical methods whereas experimental values are derived from threshold photoionization experiments in the 4.68–6.24 eV range. The ionization threshold and vertical ionization energy obtained from the experimental photoionization efficiency curves agree well with the computed values. Based on the optimized structures, the growth mechanism of the lithium doped silicon clusters were also established. Some frameworks can be regarded as possible starting blocks for silicon assemblies, giving rise to potential baby crystal materials.