Changes in the geometric structure and hydrogen-termination modify the electronic and optical properties of porous silicon

•Pores in the bulk of Si induce gap states (states in the energy gap).•Hydrogen termination of the Si atoms in the pores eliminates the gap states.•Amount of dangling bonds and the number of SiSi bonds affect optical properties.•The refractive indexes of porous Si systems are determined in this study. Here, we show that the electronic and optical properties of silicon (Si) can be controlled by varying the amount of dangling bonds and the number of SiSi bonds in the system. By introducing pores in the bulk (in the form of Si vacancies), we can induce the appearance of gap states (states in the energy gap/forbidden region) in porous Si (pSi), as compared to pure Si. Terminating the Si atoms in the pores with hydrogen atoms (H) induces the disappearance of these induced gap states (IGS). As a result, we can continuously decrease (increase) the corresponding refractive indices of both pSi and H-terminated pSi by increasing (decreasing) the porosity.