Since the discovery of the photoluminescence (PL) of porous silicon , and the possible usage of silicon nanocrystals (NC) in light emiting devices, the study of the PL properties of the Si NCs has been an active field of research. However, the PL is greatly influenced by the impurities on the surface of the Si NCs. Thus, we focus our study on the surface impurities and their effect on the electronic and optical properties of Si NCs.
For the purpose of our study we considered spherical Si NCs with three different diameters: 1.0, 1.5 and 1.9 nm. We tested the influence of different impurities on the surface, as: OH, O, CH3, CH2, SiH3, SiH2, as well as the small molecule C2H5NH2. All of the systems are analyzed using the pseudopotential DFT package SIESTA, with SZP basis set for Si, and DZ basis set for O, C, N and H. The atom positions are relaxed until all the forces acting on the atoms were lower than 0.04 eV/Å. For each of the relaxed structures we calculated the density of states (DOS) and the absorption indices.
We analized the influence of the impurities on the DOS and the absorption indices in terms of the induced changes in the region around the gap. We see that the single bonded impurities (-OH, -CH3, -O-, -SiH3) induce much smaller changes compare to the ones influenced by the double bonded impurities (=O, =CH2, =SiH2). The results are compared with the previous DFT calculations [2-4]. Moreover, neighboring impurities induce bigger changes in the DOS and the absorption indices, compared to spatially separated impurities. This study is the first step into understanding of the PL properties of Si NCs passivated with different atom or molecules. One of those systems are Si NCs passivated with C2H5NH2 molecules, which has been shown to exibit interesting PL properties . Therefore we passivated the surface of the Si NCs with only C2H5NH2 molecules, and we calculated the DOS and the absorption indices for the systems. We also looked into the layer resolved projected DOS and the spatial distribution of the wavefunctions of the states around the gap and their overlap integrals.
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