Study Of Halogens For Silicon Nanocrystals

The remarkable optical properties of silicon nanocrystals (Si NCs) hold great promise for applications in microelectronics, optoelectronics, photovoltaics, thermoelectrics and bioimaging. Quantum confinement and surface chemistry significantly affects the electronic structures and optical properties of Si NCs. Halogen are a possible unintentionally introduced impurity for Si NCs. On the basis of density functional theory, we have calculated the electronic and optical properties of halogen-incorporated Si NCs to investigate the effects of quantum confinement and surface chemistry. The main findings of our work are as follows.(1) We have investigated the electronica structures and optical properties of Si NCs with chlorine incorporated in different locations. The chlorine atom is most likely one-coordinated at the surface of Si NCs, passivating Si atoms. It is rather hard for chlorine atom to be incorporated inside Si NCs. In a very Cl-poor environment the surface of Si NCs is covered by H. As the environment becomes less Cl-poor, however, the surface may be covered by H and Cl. The chlorine coverage increases with the increase of the chemical potential of chlorine. The problem of rather high chlorine coverage is the distortion of NC structure, facilitating the decomposition of Si NCs. For chlorine-incorporated Si NCs without deep energy levels introduced in the bandgap, the band-edge recombination rates are larger than that of completely H-passivated Si NCs. Both the excitation energy and emission energy of Si NCs decrease with the increase of the chlorine coverage.(2) Based on the density functional theory, we calculated the electronica structure and optical properties of fluorine-passivated Si NCs. We have found that the effect of surface chemistry on Si NC becomes more significant as fluorine coverage increases. For a Si NC whose size changes from 1.4 to 1.7 nm high fluorine coverage (> 50%) increasingly enables the effect of surface chemistry to prevail over that of quantum confinement. For a very small Si NC (< 1.4 nm) at ground state, the quantum confinement effect is dominant no matter how large fluorine coverage is. It is found that medium fluorine coverage leads to enhanced radiative recombination in most cases. But 100%fluorine coverage causes the radiative recombination to decrease. The theoretical insights gained in this work not only encourage experimental investigation on F-passivated Si NCs, but also urge close attention paid to the interplay between quantum confinement and surface chemistry in the study of Si NCs.(3) By means of first-principles study we have found that the distortion of NC structure becomes more significant as halogen coverage or atom size increases. The NC with rather high coverage cannot converge in the calculation. The formation energy of Si NCs for different coverage and halogen atoms decreases variously. As the increase of halogen coverage or the decrease of electronegativity, the HOMO, LUMO and HOMO-LUMO gap of Si NCs decrease apparently. The halogen atom will introduce band-edge energy level in the excited state. Medium halogen coverage leads to enhanced radiative recombination in most cases while high coverage leads to decrease.