Density-Functional Studies Of The Surface Effect For Low-dimensional Silicon Materials

As one of the most important semiconductors, crystalline silicon that is regarded as an electrical and photovoltaic material has made a great success. However, bulk silicon has found limited optical applications due to its indirect-bandgap character. With the rapid development in nanoscale science and technology since the early1980s, low-dimensional silicon materials have been showing excellent optoelectronic properties and extending silicon technology into other fields.Silicon nanocrystals (Si NCs), the zero-dimensional silicon material, have found applications in a wide range of applications such as microelectronics, photovoltaics and bioimaging. It is well known that surface chemistry may significantly impact the optical behavior of Si NCs together with quantum confinement. Hydrosilylation, silanization, alkylation, alkoxylation and amination have been carried out experimentally. The effect of surface modification on the optical properties of Si NCs has been studied. However, the explains on the experimental phenomena are short of theoretical supports, the effect of surface modification on the electronic and optical properties of Si NCs needs further illumination. Therefore, theoretical simulation for surface modification of Si NCs is highly desired.Silicene, the silicon counterpart of graphene, is a two-dimensional silicon material with electrons that behave like massless Dirac fermions around the K point. There is an immediate possibility of application of silicene based nano-devices in existing Si-microelectronics. Given the vulnerability of silicene to oxidation, surface modification should be first carried out in most applications of silicene. Due to its single-layer structure, silicene is sensitive to the surface effect induced by oxidation and surface modification. This leads to the imperative need to use quantum-mechanical methods to study the effect of oxidation and surface modification on the electronic and optical properties of silicene.On the basis of density functional theory (DFT), we have investigated the surface effect of Si NCs and silicene. The main findings are as follows:(1) Ab initio methods based on DFT are employed to investigate the effect of surface modification (hydrosilylation) on the electronic and optical properties of hydrogen-passivated Si NCs, the oxidation of hydrosilylated Si NCs. We clearly demonstrate the thermodynamically favored surface bonding for hydrosilylation of Si NCs and the relative reactivity of alkenes and alkynes. Hydrosilylation is found to enhance the light emission from Si NCs. The effect of chain length and surface coverage of alkyl/alkenyl ligands on the excitation energy and emission energy of Si NCs is negligible. Alkenes with-NH2and-C4H3S at the distal end decrease the excitation energy of the1.4nm Si NC, while introduce negligible changes to the emission energy of the1.4nm Si NC. Hydrosilylation of conjugated alkynes decreases the excitation energy and emission energy of Si NCs. For the hydrosilylated Si NCs in the size range from0.8to1.6nm, quantum confinement effect is dominant for all the alkene-hydrosilylated Si NCs at the ground state. At the excited state, the prevailing effect of surface chemistry only occurs to the0.8nm Si NCs hydrosilylated with alkenes containing-NH2and-C4H3S. Quantum confinement effect is dominant for alkyne-hydrosilylated Si NCs at the ground state. However, at the excited state the effect of surface chemistry induced by the hydrosilylation with conjugated alkynes is strong enough to prevail over that of quantum confinement. As to the oxidation of hydrosilylated Si NCs, we find that a hydrosilylated Si NC is less prone to oxidation than a fully H-passivated Si NC in the point of view of thermodynamics. The formation energy of an oxidized hydrosilylated Si NC increases with the variation of oxygen configurations in the order of back bonded O (BBO), hydroxyl (OH) and doubly bonded O (DBO).(2) The silanization, alkylation, alkoxylation and aminization of Cl-passivated Si NCs are studied in the framework of DFT. We have found that aminization most significantly affects the electronic structures of Si NCs. The effect of aminization depends on the substituents of amines, rather than the coverage of amine-derived ligands at the NC surface. For aminized Si NCs, the LUMO is more sensitive to the NC size than the HOMO. Only the HOMO is sensitive to surface modification. It is found that all the aminization schemes lead to the decrease of the HOMO-LUMO gap despite that the dominant role of quantum confinement effect is maintained in most cases. The only exception appears when the NC size changes from1.4to1.2nm for aniline aminization, where the effect of surface chemistry is strong enough to counter that of the quantum confinement.(3) By means of first-principles study in the framework of DFT, we begin with silicene, which is composed of a single layer of silicon atoms. Oxidizing agents such as atomic oxygen (O) and hydroxyl (OH) in a variety of bonding configurations are incorporated into the lattice of silicene to form SOs. The formation of all the SOs is evaluated in the point of view of thermodynamics. It turns out that the charge state of Si in partially oxidized silicene ranges from+lto+3, while that of Si in fully oxidized silicene is+4. We find that the electronic properties of SOs significantly depend on the bonding of O and OH. Metallic, semimetallic, semiconducting and insulating SOs can all be obtained. The carrier mobilities of the semiconducting oxidized silicene are in the order of magnitude of10-102cm2V-1s-1.(4) Based on the DFT, we investigated the effect of hydrosilylation, alkoxylation, aminization and phenylation on the electronic and optical properties of hydrogen-passivated silicene (H-silicene). It is found that surface modification increases the buckling distance of H-silicene, while introduces negligible effect on the geometrical parameters of the hexagonal ring. H-silicene is an indirect-bandgap semiconductor with bandgap of2.3eV. After surface modification, the bandgap is decreased by0.3-0.6eV. Meanwhile, alkoxylation or amination changes H-silicene to a direct-bandgap semiconductor. The effect of surface modification on the absorption spectra of H-silicene is negligible.