准一维纳米管的结构设计与电子结构调控
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摘要
纳米材料科学已经成为当今材料科学中最活跃的领域之一,正深刻地影响着生产和生活的方方面面。当器件的尺度进入纳米级别,许多新奇的现象将展现在人们面前,并表现出潜在的应用价值。准一维纳米管是一类被广泛研究的纳米材料。独特的结构和量子限制效应使得纳米管表现出奇特的电学、光学、力学和磁学现象,比如激子束缚能的提升、光吸收蓝移、化学反应活性增强、力学性能提升、磁耦合行为改变,等等。同时,纳米管中空的管状结构使得它在分子存储和输运、受约束化学反应、光和气体传感等方面有着其它材料无法比拟的优势。自从1991年碳纳米管被发现以来,各种理论和实验工作开始针对碳纳米管超强的力学性能和丰富的电子结构展开。同时,以BN,WS2为代表的无机纳米管也在实验上成功地被制备出来。纳米管材料可以用于构造诸如：固体润滑剂、扫描探针针尖、锂电池和场发射器件等,在许多领域有着广阔的应用前景。自从碳纳米管被发现之日起,人们便开始关注准一维纳米管丰富的成分和结构特点以及结构与性质的相互联系,并将目光放在更多新型纳米管的探索和合成上,以实现特定功能的需要。对纳米管的研究仍然是今天纳米材料领域理论和实验的研究重点之一。
在材料的微观设计和性能预测方面,基于密度泛函理论的第一性原理方法(First-Principles Theory)被证明是一种行之有效的理论方法,与分子动力学模拟相结合,人们可以对纳米材料的结构和性质,包括：几何结构和电子结构、光学吸收和激发、磁性、力学性能、动态化学反应过程、有限温度下的原子分子碰撞等等,作出有效的判断和预测。将实验研究与第一性原理方法相结合,已经成为当前纳米材料研究的有效手段之一。人们已经将第一性原理的计算方法成功地应用于包括材料缺陷设计与修复、掺杂改性、材料自组装、气体感应与储存、纳米电路设计等领域,将以往只有在传统实验平台才能探知的有效信息迅速而较为完整地通过计算机动态呈现。这对传统的材料设计过程是一种创造性的改进,可以有效地提高工作效率,缩短开发周期。
本论文针对四种典型的纳米管状材料,即：SiC、CoSi2、SiN和TiO2纳米管,采用量子力学第一性原理和分子动力学模拟相结合的理论方法,对纳米管的构造规律、稳定性和电子结构的调控规律进行了系统地研究,揭示了纳米管状材料的结构和功能的关系,为新型纳米管的合成的应用提供了理论依据。主要的研究内容和研究结果如下：
●与碳纳米管不同,所有的SiC纳米管都呈现半导体特性,因此,如何有效地调控其电子结构成为应用SiC纳米管的关键。本论文系统地模拟研究了N原子、NH和NH2基团对SiC纳米管的电子结构的调控方式和调控规律。从理论上预言：修饰所形成的不同结构可以有效地改变SiC纳米管的电子结构,使修饰后的SiC纳米管表现出p型或n型半导体乃至半金属(semimetal)的特性：当N原子吸附于(5,5)SiC纳米管壁形成四面体构型时,以及NH2基团吸附于(5,5)和(8,0)SiC纳米管的Si原子位时,体系为p型半导体；当NH2基团吸附于(5,5)和(8,0)SiC纳米管的C原子位时,以及N原子替代(5,5)SiC纳米管的任意原子时,体系为n型半导体。当N原子替代(8,0)SiC纳米管任意原子时,体系为半金属。
●Si纳米管是与C同族的纳米管材料,但是理论与实验结果均表明单壁Si纳米管在室温下不稳定。为此,我们从理论上提出可以通过掺杂Co原子的方法稳定单壁Si纳米管,同时对硅纳米管的电学特性进行调控。理论研究表明：Co原子可以扩散进入硅纳米管的管壁中,形成具有Si-Co-Si三明治管壁结构的CoSi2复合纳米管。掺杂Co原子可以使硅纳米管的形成能降低66%以上,在室温(300K)和高温(1000 K)中连续加热4 ps仍然保持完整的结构。CoSi2复合纳米管表现出与手性和管径无关的统一的金属导电性。
●SiN纳米管在实验上已经能够大面积的合成,但是超薄SiN纳米管是否存在一直悬而未决。本论文在第一性原理计算的基础上,提出一种氢化的SiN (H-SiN)纳米单层和单壁纳米管状结构,SiN纳米单层中的Si和N交替分布在二维六边形网格上,Si原子的悬挂键被H原子钝化,H可分布于纳米层的两侧,呈现多种氢化模式。通过SiN纳米单层卷曲则可形成单壁SiN纳米管。分子动力学模拟显示这种纳米单层和纳米管可以在室温下稳定存在。H-SiN纳米管中的氢化模式以及内外壁氢化比例会随着管径增大而发生改变,使得不同管径的H-SiN纳米管具有不同的特征构型。最稳定的H-SiN纳米层和锯齿型H-SiN纳米管是直接带隙半导体。
●Ti02纳米管因其在水环境中的氧化还原活性和较快的电子传导速率而在太阳能利用方面有广泛的应用。实验制备的TiO2纳米管的微观结构至今仍然不清楚。本论文提出了一种具有很高表体比的超薄TiO2纳米层(Nanosheet)和纳米管模型。TiO2纳米层由二维的三角原子晶格构成,其原子具有与块材料相同的配位数,因此具有很高的稳定性。超薄TiO2纳米层按照不同的卷曲矢量卷曲可形成不同手性的超薄TiO2纳米管。扶手椅型TiO2纳米管是间接带隙半导体,而锯齿型TiO2纳米管是直接带隙半导体,带隙随着管径的增大而减小。非金属掺杂可以调控超薄TiO2纳米管对太阳能吸收的频率范围,不仅可以做到减小带隙(NH掺杂),而且能产生金属性(C掺杂)。
●单层石墨纳米带是当前一个研究热点,锯齿型石墨纳米带具有自旋极化的边界,在自旋电子学上有广阔的应用前景。本论文系统地研究了在超薄TiO2纳米层的基础上形成的TiO2纳米带(Nanoribbon)的电子结构变化规律。计算表明所有的TiO2纳米带都具有O边界,且都是宽带隙(大于2.93 eV)半导体。锯齿型TiO2纳米带比相同宽度的扶手椅型TiO2纳米带稳定。其带隙随纳米带宽度的变化规律可分为两类(二元性),即：含有偶数Ti原子列的TiO2纳米带的带隙比含有奇数Ti原子列的TiO2纳米带的带隙宽,前者随着纳米带宽度增加而减少,而后者随着纳米带宽度增加而增加。这种二元的变化规律与已发现的其他纳米带的带隙变化规律(锯齿型的单调性以及扶手椅型的三元性)截然不同,是一种新的变化规律,可以通过边界间的库伦作用加以解释。在缺氧的氛围下还可能形成非TiO2配比的TiO2-x纳米带,并且边界从自旋非极化状态转变为自旋极化状态。当TiO2-x。纳米带只有一个边界形成O空位时,自旋向上能带的带隙为0.06 eV,而自旋向下能带的带隙为2.31 eV；当两个边界都形成O空位时,TiO2-x纳米带表现出优秀的半金属(half-metal)性。
Quasi-one-dimensional nanotubes have been extensively studied both theoretically and experimentally. The unique geometries and quantum-confinement-effects endow the nanotubes distinctive nanoelectronic, photonic, mechanic, and magnetic properties, such as the upgrade of the binding energy of excitons, the blue-shift of absorption peak, the promotion of chemical reactivity, the promotion of mechanical property, the change of magnetical correlation, and so forth. The hollow space in nanotubes makes it an ideal candidate material in wide-range fields, such as gas storage and delivery, confined chemical reaction, and light and gas sensor. Ever since the discovery of carbon nanotubes (CNTs) in 1991, abundant theoretical and experimental researches have been paid on revealing its novel mechanical and electronic properties. Additionally, other inorganic nanotubes such as WS2 and BN nanotubes have been synthesized successfully, which have applications in lubrication, composing the scanning probe tips, lithium cells, field-emission devices, etc. The relationship between the structures of nanomaterials and their functions is therefore quite crucial and becomes the goal of research. Searching for nanotubular materials with novel geometries and fascinating properties is highly desirable, which is the aim of present thesis.
The first-principles caculatiosn on the basis of density-functional-theory (DFT) combined with molecular dynamics simulations (MDSs) have been proved to be a useful theoretical method in revealing the structures and properties of nanomaterials. The geometric configutaions, electronic structures, optoelectronic absorption and excitation, magnetism, mechanic properties, dynamics of chemical reaction, the procedure of atomic and molecular collisions, etc., can be predicted using this theoretical scheme. The first-principles calculations have also been successfully employed to deal with the problems of defects devising and restoring, doping and functionalization, self-assembly, gas sensor and storage, and nanoelectric circuit device, which increases the efficiency and shortens the development period.
In this thesis, we performed first-principles calculations in conjunction with molecular dynamics simulations to reveal the structures and electronic properties of four typical nanotubes, SiC, CoSi2, SiN and TiO2. These nanotubes possess different structural features and tuning mechanisms of electronic properties, which are quire crucial for their applications in nanoscaled devices. The roles of defects in modulating the electronic structures are predicted. The main conclusions are summarized as follows.
·SiC nanotubes possess high surface-to-bulk, which makes them sensitive to foreign decoration. Using first-principles method, we found that N and NHx (x=1,2) groups can be chemically incorporate into the network of SiC nanotubes in different ways, accompanied with the formation of N-C and N-Si bonds. The adsorbing energy of N and NHx (x=1,2) groups on (5,5) and (8,0) SiC nanotubes ranges from -1.82 to -7.19 eV. The electronic structures of SiC nanotubes can be effectively modified by these groups and display diverse characters ranging from semiconducting to semimetallic, depending on the chirality of SiC nanotubes as well as the way of the incorporation of these functional groups. The N-adsorbed (5,5) SiC nanotube which has tetrahedral adsorbing configuration, and silicon-site NH2-adsorbed (5,5) and (8,0) SiC nanotubes, are p-type semiconductors. The carbon-site NH2-adsorbed (5,5) and (8,0) SiC nanotubes, and N-substituted (5,5) SiC nanotubes, are n-type semiconductors. The N-substituted (8,0) SiC nanotube is semimetallic.
·Spin-polarized DFT calculations showed that the incorporation of Co atoms into Si nanotubes can not only stabilize these tubes but also tune their electronic properties. The stable configurations have the adsorbed Co atoms locating on an intra-layer between the outer layer and the inner layer of the tube wall. There is no energy barrier for a Co atom to enter the Si nanotubes through the center of the Si hexagon. The formation energies of (5,5) and (8,0) CoSi2 nanotubes are much lower than those of corresponding prinstine Si nanotubes by about 67% and 66%, respectively. The stabilities of Si nanotubes with Co and other TM atoms (TM= Cr, V and Mo) adsorbed were examined by either CG optimization or NVT dynamic simulations at 300 and 1000 K with a Nose thermostat for 2 ps, and only the CoSi2 nanotubes were found to be stable at these temperatures. The potential energy profiles indicate that with the increase of Co concentration, Co atoms favor the formation of a Co intra-layer in the walls of the Si nanotubes. Isolated CoSi2 nanotubes favor gathering together and forming bundles with lower energy. It was found that electrons transfer from 7r-orbital of Si nanotubes and atomic Co to the interspace between the Si hexagon and Co. The electronic structures of these CoSi2 nanotubes exhibit the characters of metals with high electron density states at Fermi level.
·The hydrogen-stabilized silicon nitride nanosheets and nanotubes with the stoichiometry of HSiN were studied by first-principles calculations. The stable H-SiN nanosheet has a two-dimensional hexagonal grid of Si and N atoms with the Si dangling bonds being passivated by H atoms, which are placed alternatively on the two sides of the sheet, whereas H-SiN nanotubes can be built from rolling up the nanosheet. The hydrogen arrangement strongly affects the energetic favorability of the H-SiN nanotubes and makes some distinctively favorable configurations possible. The stable H-SiN nanosheet and zigzag tubes have a direct band gap at theΓpoint, which are crucial for building nanoscale optical and photonic devices.
·We propose stable layered structures and ultrathin tubular configurations of titanium oxide (TiO2) nanomaterials on the basis of first-principles calculations within density functional theory. The layered structures are stable for the TiO2 nanosheets containing less than four (001) rutile bilayers. When TiO2 nanosheet is composed of a single (001) bilayer, the energetically most favorable configuration has a two-dimensional triangular structure where each atom is fully coordinated. Ultrathin TiO2 nanotubes can be modeled by rolling up this triangular sheet with different rollup vectors. The strain energies of these nanotubes decrease with the increase of tube diameters. Armchair tubes are indirect-band gap semiconductors, while zigzag tubes are direct-band gap semiconductors. The band gap values of the tubes increase with increasing tube diameter. NH doping reduces the band gap of the (7,7) tube by about 0.66 eV and that of the (12,0) tube by only 0.16 eV. The incorporation of C atoms into (7,7) ultrathin TiO2 nanotube induces metallization of the tube.
·First-principles calculations indicate that TiO2 nanoribbons can be formed from the already-synthesized ultra-thin TiO2 nanosheet. Zigzag TiO2 nanoribbons terminated by oxygen atoms are energetically more preferable than the armchair ones with the same ribbon width. Both zigzag and armchair TiO2 nanoribbons are semiconductors with band gap larger than 2.93 eV in our L(S)DA+U procedure. The band gaps as a function of ribbon width are separated into two different categories with a hierarchy of gap size given by Eg,2m＞Eg,2m+1.For the TiO2 nanoribbons containing even number of Ti lines (NZ or NA= 2m), the band gap (Eg,2m) decrease with the increase of width, whereas for the TiO2 nanoribbons with NZ (or NA)= 2m+1, the band gap (Eg,2m+1) increase with increasing width. The spin-polarization of edge states can be achieved in the nonstoichiometric TiO2-x nanoribbons containing threefold-coordinated Ti atoms at the edge which may be formed under poor oxygen conditions. When Vo defects are formed at one edge of TiO2-x nanoribbons, the spin-up branch has a band gap of 0.06 eV, while the band gap of spin-down branch is 2.31 eV. When Vo defects are formed at both edges, the TiO2-x nanoribbon becomes a half-metal. The hydrogenation of Vo defects quenches the magnetic moments. The tunable band gaps, half-metallicity, and the sensitivity to H atoms of ultra-thin TiO2 nanoribbons containing Vo defects imply their potential applications in solar cells, spintronics-based devices, and sensors.
在材料的微观设计和性能预测方面,基于密度泛函理论的第一性原理方法(First-Principles Theory)被证明是一种行之有效的理论方法,与分子动力学模拟相结合,人们可以对纳米材料的结构和性质,包括：几何结构和电子结构、光学吸收和激发、磁性、力学性能、动态化学反应过程、有限温度下的原子分子碰撞等等,作出有效的判断和预测。将实验研究与第一性原理方法相结合,已经成为当前纳米材料研究的有效手段之一。人们已经将第一性原理的计算方法成功地应用于包括材料缺陷设计与修复、掺杂改性、材料自组装、气体感应与储存、纳米电路设计等领域,将以往只有在传统实验平台才能探知的有效信息迅速而较为完整地通过计算机动态呈现。这对传统的材料设计过程是一种创造性的改进,可以有效地提高工作效率,缩短开发周期。
本论文针对四种典型的纳米管状材料,即：SiC、CoSi2、SiN和TiO2纳米管,采用量子力学第一性原理和分子动力学模拟相结合的理论方法,对纳米管的构造规律、稳定性和电子结构的调控规律进行了系统地研究,揭示了纳米管状材料的结构和功能的关系,为新型纳米管的合成的应用提供了理论依据。主要的研究内容和研究结果如下：
●与碳纳米管不同,所有的SiC纳米管都呈现半导体特性,因此,如何有效地调控其电子结构成为应用SiC纳米管的关键。本论文系统地模拟研究了N原子、NH和NH2基团对SiC纳米管的电子结构的调控方式和调控规律。从理论上预言：修饰所形成的不同结构可以有效地改变SiC纳米管的电子结构,使修饰后的SiC纳米管表现出p型或n型半导体乃至半金属(semimetal)的特性：当N原子吸附于(5,5)SiC纳米管壁形成四面体构型时,以及NH2基团吸附于(5,5)和(8,0)SiC纳米管的Si原子位时,体系为p型半导体；当NH2基团吸附于(5,5)和(8,0)SiC纳米管的C原子位时,以及N原子替代(5,5)SiC纳米管的任意原子时,体系为n型半导体。当N原子替代(8,0)SiC纳米管任意原子时,体系为半金属。
●Si纳米管是与C同族的纳米管材料,但是理论与实验结果均表明单壁Si纳米管在室温下不稳定。为此,我们从理论上提出可以通过掺杂Co原子的方法稳定单壁Si纳米管,同时对硅纳米管的电学特性进行调控。理论研究表明：Co原子可以扩散进入硅纳米管的管壁中,形成具有Si-Co-Si三明治管壁结构的CoSi2复合纳米管。掺杂Co原子可以使硅纳米管的形成能降低66%以上,在室温(300K)和高温(1000 K)中连续加热4 ps仍然保持完整的结构。CoSi2复合纳米管表现出与手性和管径无关的统一的金属导电性。
●SiN纳米管在实验上已经能够大面积的合成,但是超薄SiN纳米管是否存在一直悬而未决。本论文在第一性原理计算的基础上,提出一种氢化的SiN (H-SiN)纳米单层和单壁纳米管状结构,SiN纳米单层中的Si和N交替分布在二维六边形网格上,Si原子的悬挂键被H原子钝化,H可分布于纳米层的两侧,呈现多种氢化模式。通过SiN纳米单层卷曲则可形成单壁SiN纳米管。分子动力学模拟显示这种纳米单层和纳米管可以在室温下稳定存在。H-SiN纳米管中的氢化模式以及内外壁氢化比例会随着管径增大而发生改变,使得不同管径的H-SiN纳米管具有不同的特征构型。最稳定的H-SiN纳米层和锯齿型H-SiN纳米管是直接带隙半导体。
●Ti02纳米管因其在水环境中的氧化还原活性和较快的电子传导速率而在太阳能利用方面有广泛的应用。实验制备的TiO2纳米管的微观结构至今仍然不清楚。本论文提出了一种具有很高表体比的超薄TiO2纳米层(Nanosheet)和纳米管模型。TiO2纳米层由二维的三角原子晶格构成,其原子具有与块材料相同的配位数,因此具有很高的稳定性。超薄TiO2纳米层按照不同的卷曲矢量卷曲可形成不同手性的超薄TiO2纳米管。扶手椅型TiO2纳米管是间接带隙半导体,而锯齿型TiO2纳米管是直接带隙半导体,带隙随着管径的增大而减小。非金属掺杂可以调控超薄TiO2纳米管对太阳能吸收的频率范围,不仅可以做到减小带隙(NH掺杂),而且能产生金属性(C掺杂)。
●单层石墨纳米带是当前一个研究热点,锯齿型石墨纳米带具有自旋极化的边界,在自旋电子学上有广阔的应用前景。本论文系统地研究了在超薄TiO2纳米层的基础上形成的TiO2纳米带(Nanoribbon)的电子结构变化规律。计算表明所有的TiO2纳米带都具有O边界,且都是宽带隙(大于2.93 eV)半导体。锯齿型TiO2纳米带比相同宽度的扶手椅型TiO2纳米带稳定。其带隙随纳米带宽度的变化规律可分为两类(二元性),即：含有偶数Ti原子列的TiO2纳米带的带隙比含有奇数Ti原子列的TiO2纳米带的带隙宽,前者随着纳米带宽度增加而减少,而后者随着纳米带宽度增加而增加。这种二元的变化规律与已发现的其他纳米带的带隙变化规律(锯齿型的单调性以及扶手椅型的三元性)截然不同,是一种新的变化规律,可以通过边界间的库伦作用加以解释。在缺氧的氛围下还可能形成非TiO2配比的TiO2-x纳米带,并且边界从自旋非极化状态转变为自旋极化状态。当TiO2-x。纳米带只有一个边界形成O空位时,自旋向上能带的带隙为0.06 eV,而自旋向下能带的带隙为2.31 eV；当两个边界都形成O空位时,TiO2-x纳米带表现出优秀的半金属(half-metal)性。
Quasi-one-dimensional nanotubes have been extensively studied both theoretically and experimentally. The unique geometries and quantum-confinement-effects endow the nanotubes distinctive nanoelectronic, photonic, mechanic, and magnetic properties, such as the upgrade of the binding energy of excitons, the blue-shift of absorption peak, the promotion of chemical reactivity, the promotion of mechanical property, the change of magnetical correlation, and so forth. The hollow space in nanotubes makes it an ideal candidate material in wide-range fields, such as gas storage and delivery, confined chemical reaction, and light and gas sensor. Ever since the discovery of carbon nanotubes (CNTs) in 1991, abundant theoretical and experimental researches have been paid on revealing its novel mechanical and electronic properties. Additionally, other inorganic nanotubes such as WS2 and BN nanotubes have been synthesized successfully, which have applications in lubrication, composing the scanning probe tips, lithium cells, field-emission devices, etc. The relationship between the structures of nanomaterials and their functions is therefore quite crucial and becomes the goal of research. Searching for nanotubular materials with novel geometries and fascinating properties is highly desirable, which is the aim of present thesis.
The first-principles caculatiosn on the basis of density-functional-theory (DFT) combined with molecular dynamics simulations (MDSs) have been proved to be a useful theoretical method in revealing the structures and properties of nanomaterials. The geometric configutaions, electronic structures, optoelectronic absorption and excitation, magnetism, mechanic properties, dynamics of chemical reaction, the procedure of atomic and molecular collisions, etc., can be predicted using this theoretical scheme. The first-principles calculations have also been successfully employed to deal with the problems of defects devising and restoring, doping and functionalization, self-assembly, gas sensor and storage, and nanoelectric circuit device, which increases the efficiency and shortens the development period.
In this thesis, we performed first-principles calculations in conjunction with molecular dynamics simulations to reveal the structures and electronic properties of four typical nanotubes, SiC, CoSi2, SiN and TiO2. These nanotubes possess different structural features and tuning mechanisms of electronic properties, which are quire crucial for their applications in nanoscaled devices. The roles of defects in modulating the electronic structures are predicted. The main conclusions are summarized as follows.
·SiC nanotubes possess high surface-to-bulk, which makes them sensitive to foreign decoration. Using first-principles method, we found that N and NHx (x=1,2) groups can be chemically incorporate into the network of SiC nanotubes in different ways, accompanied with the formation of N-C and N-Si bonds. The adsorbing energy of N and NHx (x=1,2) groups on (5,5) and (8,0) SiC nanotubes ranges from -1.82 to -7.19 eV. The electronic structures of SiC nanotubes can be effectively modified by these groups and display diverse characters ranging from semiconducting to semimetallic, depending on the chirality of SiC nanotubes as well as the way of the incorporation of these functional groups. The N-adsorbed (5,5) SiC nanotube which has tetrahedral adsorbing configuration, and silicon-site NH2-adsorbed (5,5) and (8,0) SiC nanotubes, are p-type semiconductors. The carbon-site NH2-adsorbed (5,5) and (8,0) SiC nanotubes, and N-substituted (5,5) SiC nanotubes, are n-type semiconductors. The N-substituted (8,0) SiC nanotube is semimetallic.
·Spin-polarized DFT calculations showed that the incorporation of Co atoms into Si nanotubes can not only stabilize these tubes but also tune their electronic properties. The stable configurations have the adsorbed Co atoms locating on an intra-layer between the outer layer and the inner layer of the tube wall. There is no energy barrier for a Co atom to enter the Si nanotubes through the center of the Si hexagon. The formation energies of (5,5) and (8,0) CoSi2 nanotubes are much lower than those of corresponding prinstine Si nanotubes by about 67% and 66%, respectively. The stabilities of Si nanotubes with Co and other TM atoms (TM= Cr, V and Mo) adsorbed were examined by either CG optimization or NVT dynamic simulations at 300 and 1000 K with a Nose thermostat for 2 ps, and only the CoSi2 nanotubes were found to be stable at these temperatures. The potential energy profiles indicate that with the increase of Co concentration, Co atoms favor the formation of a Co intra-layer in the walls of the Si nanotubes. Isolated CoSi2 nanotubes favor gathering together and forming bundles with lower energy. It was found that electrons transfer from 7r-orbital of Si nanotubes and atomic Co to the interspace between the Si hexagon and Co. The electronic structures of these CoSi2 nanotubes exhibit the characters of metals with high electron density states at Fermi level.
·The hydrogen-stabilized silicon nitride nanosheets and nanotubes with the stoichiometry of HSiN were studied by first-principles calculations. The stable H-SiN nanosheet has a two-dimensional hexagonal grid of Si and N atoms with the Si dangling bonds being passivated by H atoms, which are placed alternatively on the two sides of the sheet, whereas H-SiN nanotubes can be built from rolling up the nanosheet. The hydrogen arrangement strongly affects the energetic favorability of the H-SiN nanotubes and makes some distinctively favorable configurations possible. The stable H-SiN nanosheet and zigzag tubes have a direct band gap at theΓpoint, which are crucial for building nanoscale optical and photonic devices.
·We propose stable layered structures and ultrathin tubular configurations of titanium oxide (TiO2) nanomaterials on the basis of first-principles calculations within density functional theory. The layered structures are stable for the TiO2 nanosheets containing less than four (001) rutile bilayers. When TiO2 nanosheet is composed of a single (001) bilayer, the energetically most favorable configuration has a two-dimensional triangular structure where each atom is fully coordinated. Ultrathin TiO2 nanotubes can be modeled by rolling up this triangular sheet with different rollup vectors. The strain energies of these nanotubes decrease with the increase of tube diameters. Armchair tubes are indirect-band gap semiconductors, while zigzag tubes are direct-band gap semiconductors. The band gap values of the tubes increase with increasing tube diameter. NH doping reduces the band gap of the (7,7) tube by about 0.66 eV and that of the (12,0) tube by only 0.16 eV. The incorporation of C atoms into (7,7) ultrathin TiO2 nanotube induces metallization of the tube.
·First-principles calculations indicate that TiO2 nanoribbons can be formed from the already-synthesized ultra-thin TiO2 nanosheet. Zigzag TiO2 nanoribbons terminated by oxygen atoms are energetically more preferable than the armchair ones with the same ribbon width. Both zigzag and armchair TiO2 nanoribbons are semiconductors with band gap larger than 2.93 eV in our L(S)DA+U procedure. The band gaps as a function of ribbon width are separated into two different categories with a hierarchy of gap size given by Eg,2m＞Eg,2m+1.For the TiO2 nanoribbons containing even number of Ti lines (NZ or NA= 2m), the band gap (Eg,2m) decrease with the increase of width, whereas for the TiO2 nanoribbons with NZ (or NA)= 2m+1, the band gap (Eg,2m+1) increase with increasing width. The spin-polarization of edge states can be achieved in the nonstoichiometric TiO2-x nanoribbons containing threefold-coordinated Ti atoms at the edge which may be formed under poor oxygen conditions. When Vo defects are formed at one edge of TiO2-x nanoribbons, the spin-up branch has a band gap of 0.06 eV, while the band gap of spin-down branch is 2.31 eV. When Vo defects are formed at both edges, the TiO2-x nanoribbon becomes a half-metal. The hydrogenation of Vo defects quenches the magnetic moments. The tunable band gaps, half-metallicity, and the sensitivity to H atoms of ultra-thin TiO2 nanoribbons containing Vo defects imply their potential applications in solar cells, spintronics-based devices, and sensors.
引文
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