【作者】 王蓉；

【导师】 杨德仁； 皮孝东；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2014， 博士

【摘要】 晶体硅作为当前最重要的半导体材料,在微电子领域和太阳电池领域有着不可替代的应用。然而,由于其间接禁带半导体特性,晶体硅不能高效地发光从而限制了其在光学领域的应用。自20世纪80年代以来,随着纳米科学技术的飞速发展,低维(零维、一维、二维)硅材料正逐渐显示出优异的光电性能,极大地丰富了硅材料在各个领域的应用。其中,硅纳米晶体(零维硅材料)由于量子限域效应,可以在可见光范围内进行有效的光吸收和光发射,从而将晶体硅材料的应用扩展到了生物成像、光敏材料等光学领域。由于量子限域效应,硅纳米晶体的尺寸对其光学性能的影响非常大。另一方面,由于硅纳米晶体比表面积较大,表面效应也会显著影响其光学性能。在实验中,研究者们已经成功对硅纳米晶体进行了氢化硅烷化、硅烷化、烷基化、烷氧基化、胺化等表面改性,并研究了表面改性带来的表面效应对硅纳米晶体光学性能的影响。但是,研究者们对于实验现象的解释缺乏理论支持,表面效应对硅纳米晶体光学性能影响的微观机制却尚不明晰。因此,通过理论计算,从微观机理上讨论表面改性前后硅纳米晶体的电子结构和光学性质,并理解量子限域效应和表面效应对硅纳米晶体性质的影响就变得尤为迫切。硅烯(二维硅材料)是石墨烯的硅对应物,具备与石墨烯相似的狄拉克型电子结构,其布里渊区有六个线性色散的狄拉克锥。因此,硅烯在纳米电子器件中有很大的应用潜力。与传统的硅材料一样,硅烯在制备和应用的过程中会不可避免的氧化,因此必须对其进行表面改性。硅烯由于是单原子层结构,氧化和表面改性带来的表面效应会显著影响其电子、光学性质。因此,通过量子力学计算方法研究氧化及表面改性对其电子结构和光学性能的影响,就成为理解与优化硅烯的氧化及表面改性最迫切的需求。本论文通过密度泛函理论研究了硅纳米晶体和硅烯表面效应对其电子结构和光学性能的影响,取得了以下一系列创新成果：(1)研究了氢钝化的硅纳米晶体表面改性(氢化硅烷化)时的成键状况、碳链的长度、钝化量及所含官能团对硅纳米晶体电子结构和光学性质的影响,并进一步研究了氢化硅烷化硅纳米晶体的氧化行为。研究发现：氢化硅烷化可以增强硅纳米晶体的发光强度；烯烃和炔烃的链长和钝化量对硅纳米晶体激发能和发射能的影响非常小；含-NH2和-C4H3S的烯烃氢化硅烷化会显著降低1.4nm硅纳米晶体的激发能,但对其发射能的影响非常小。共轭炔烃氢化硅烷化会显著降低硅纳米晶体的激发能和发射能,并提高其发光效率。对于0.8-1.6nm的烯烃氢化硅烷化的硅纳米晶体,其电子性能主要由量子限域效应决定。对于炔烃氢化硅烷化的硅纳米晶体,在基态时,硅纳米晶体的电子性能仍由量子限域效应决定；而在激发态时,共轭炔烃氢化硅烷化引起的表面化学效应代替量子限域效应决定着硅纳米晶体的电子结构。关于氢化硅烷化硅纳米晶体的氧化行为,我们发现,从热力学角度讲,氢钝化硅纳米晶体比氢化硅烷化硅纳米晶体更容易氧化。对于不同成键类型的氧,形成能以背键氧(BBO)、羟基(OH)、双键氧(DBO)的顺序依次增加。(2)研究了硅烷化、烷化、烷氧基化及胺化这四种表面改性对氯钝化硅纳米晶体电子结构和光学性能的影响。研究发现：这四种表面改性方法中,只有胺化反应显著降低了硅纳米晶体的激发能和发射能。在胺化反应中,胺化物的取代基显著影响硅纳米晶体的的激发能和发射能,而胺化物在硅纳米晶体表面的钝化量对硅纳米晶体的影响非常小。胺化反应带来的表面化学效应显著影响硅纳米晶体的最高占据分子轨道(HOMO),而硅纳米晶体的最低未占分子轨道(LUMO)能级的变化则取决于其尺寸。对于0.8-1.6nm的硅纳米晶体,尺寸变化引起的量子限域效应对硅纳米晶体电子结构的影响占主导作用；只有与苯胺反应的硅纳米晶体的尺寸在1.2-1.4nm范围内变化时,表面化学效应才对其电子结构的影响占主导作用。(3)研究了硅烯氧化物的几何结构、电子性能。研究发现：所有的硅烯氧化物在热力学上都容易形成,并且不会分解。部分氧化的硅烯中硅的价态都是+1,+2或+3价,完全氧化硅烯中硅的价态为+4价。不同氧的成键类型导致硅烯的蜂窝状二维单原子层结构得以保存、扭曲、或者消失。硅烯本来是一种半金属,而硅烯氧化物的电子结构取决于O及OH的成键方式而表现为金属、半金属、半导体或绝缘体。这些结果表明,氧化能够极大地丰富硅烯的电子性能。但呈半导体性的硅烯氧化物中电子和空穴的有效质量增加,载流子迁移率有所下降。(4)研究了氢化硅烷化、烷氧基化、胺化及苯基化这四种表面改性对氢钝化硅烯几何结构、电子和光学性能的影响。结果表明,表面改性会增大氢钝化硅烯的翘曲间距,但对其平面内六元环的几何结构影响较小。表面改性会降低氢钝化硅烯的禁带宽度,其中,烷氧基化和胺化反应使氢钝化硅烯变为直接禁带半导体,而氢化硅烷化和苯基化保持了其间接禁带半导体的特征。更多还原

【Abstract】 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.更多还原