应变调控单层氧化锌能带结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算对单层ZnO薄膜能带结构的应变调控进行了研究.计算结果表明:沿着之字形方向的压缩应变和扶椅形方向的拉伸应变对薄膜带隙的调控都是线性的,而且带隙调控的范围最大;相反地,在沿着之字形方向的拉伸应变和扶椅形方向的压缩应变的调控下,带隙则呈现出非线性的变化.对于双轴应变的拉伸与压缩,带隙的变化都是非线性的.这种通过不同的应变加载方式来实现对带隙不同程度的调控,对ZnO薄膜在光学和催化等领域的应用具有重要的指导意义.     

Electronic and magnetic properties of armchair and zigzag graphene nanoribbons

The electronic properties, band gap, and ionization potential of zigzag and armchair graphene nanoribbons are calculated as a function of the number of carbon atoms in the ribbon employing density functional theory at the B3LYP6-31G* level. In armchair ribbons, the ionization potential and band gap show a gradual decrease with length. For zigzag ribbons, the dependence of the band gap and ionization potential on ribbon length is different depending on whether the ribbon has an unpaired electron or not. It is also found that boron and nitrogen zigzag and armchair doped graphene nanoribbons have a triplet ground state and could be ferromagnetic.     

Tuning the Properties of Tetracene-Based Nanoribbons by Fluorination and N-Doping

The structural stabilities and electronic properties are studied for the recently synthesized one-dimensional (1-D) tetracene-based nanoribbons with four-membered rings by using first-principles calculation. All three configurations (named as straight, zigzag, and armchair) are stable and exhibit an indirect band gap of 1.46, 0.73, and 0.32 eV, respectively. The band gaps can be effectively tuned by substituting hydrogen with fluorine atoms and by doping with nitrogen atoms. Substituting hydrogen with fluorine atoms leads to gradual decrease of the electronic band gaps of all configurations. Nitrogen doping changes the band gap from indirect to direct, displaying flexibility of tuning the band structure.     

Structures and Electromagnetic Properties of Boron Nitride Nanoribbons Doped with Transition Metals

Inspired by the recent discovery of the Ti-doped BN nanocages, here we report the design of novel boron nitride (BN) nanoribbons (BNNRs) doped with fourth-row transition metals (Sc−Cu) and the prediction of their structural and electromagnetic properties. First-principles calculations and ab initio molecular dynamics simulations show that Ti-doped BNNR possesses both thermodynamic and kinetic stability at high temperatures for synthesis of BN materials. Metal doping may make the nonmagnetic pristine BNNR ferromagnetic or antiferromagnetic, depending on the metal. The doping with all considered metals reduces substantially the band gap of pristine BNNR. For example, Sc-doped BNNR is ferromagnetic with an indirect band gap of 1.18 eV, while V-doped nanoribbon is antiferromagnetic with a direct gap of 2.50 eV. Remarkably, the carrier mobility in both materials is significantly enhanced compared to the pristine BNNR. Our findings suggest that doping with different metals may endow BNNRs with versatile electronic and magnetic properties.     

Theory of nitrogen doping of carbon nanoribbons: edge effects

Nitrogen doping of a carbon nanoribbon is profoundly affected by its one-dimensional character, symmetry, and interaction with edge states. Using state-of-the-art ab initio calculations, including hybrid exact-exchange density functional theory, we find that, for N-doped zigzag ribbons, the electronic properties are strongly dependent upon sublattice effects due to the non-equivalence of the two sublattices. For armchair ribbons, N-doping effects are different depending upon the ribbon family: for families 2 and 0, the N-induced levels are in the conduction band, while for family 1 the N levels are in the gap. In zigzag nanoribbons, nitrogen close to the edge is a deep center, while in armchair nanoribbons its behavior is close to an effective-mass-like donor with the ionization energy dependent on the value of the band gap. In chiral nanoribbons, we find strong dependence of the impurity level and formation energy upon the edge position of the dopant, while such site-specificity is not manifested in the magnitude of the magnetization.     

Effects of electric and magnetic fields on the electronic properties of zigzag carbon and boron nitride nanotubes

We have investigated the electronic properties of zigzag CNTs and BNNTs under the external transverse electric field and axial magnetic field, using tight binding approximation. It was found that after switching on the electric and magnetic fields, the band modification such as distortion of the degeneracy, change in energy dispersion, subband spacing and band gap size reduction occurs. The band gap of zigzag BNNTs decreases linearly with increasing the electric field strength but the band gap variation for CNTs increases first and later decreases (Metallic) or first hold constant and then decreases (semiconductor). For type (II) CNTs, at a weak magnetic field, by increasing the electric field strength, the band gap remains constant first and then decreases and in a stronger magnetic field the band gap reduction becomes parabolic. For type (III) CNTs, in any magnetic field, the band gap increases slowly until reaches a maximum value and then decreases linearly. Unlike to CNTs, the magnetic field has less effects on the BNNTs band gap variation.     

Quenching of local magnetic moment in oxygen adsorbed graphene nanoribbons

The electronic and magnetic properties of oxidized zigzag and armchair graphene nanoribbons, with hydrogen passivated edges, have been investigated from ab initio pseudopotential calculations within the density functional scheme. The oxygen molecule in its triplet state is adsorbed most stably at the edge of a zigzag nanoribbon. The Stoner metallic behavior of the ferromagnetic nanoribbons and the Slater insulating (ground state) behavior of the antiferromagnetic nanoribbons remain intact upon oxygen adsorption. The formation of a spin-paired C-O bond drastically reduces the local atomic magnetic moment of carbon at the edge of the ferromagnetic zigzag ribbon.     

单壁AlAs(111)纳米管结构和电子性质的密度泛函理论研究

通过卷曲立方AlAs(111)单层片(sheets)构造了一系列(n,0)和(n,m)一维单壁纳米管。用周期性密度泛函理论(DFT)计算并比较了不同类型AlAs纳米管在几何结构、能量及电子性质等方面的差别。计算结果表明锯齿型和椅型纳米管应变能均为负值,并随着管径变大而逐渐变小。然而,它们的带隙相当不同:椅型纳米管为间接带隙,随着管径的增大而带隙减小;锯齿型纳米管为直接带隙,管径为1.87 nm时存在着一个极大带隙值(2.11 eV)。这种不同主要源于锯齿型纳米管铝原子间3p轨道的耦合贡献。     

由锐钛矿(101)片卷曲成单壁纳米管的紧束缚密度泛函理论研究

通过卷曲二维锐钛矿(101)周期性单层片(sheets)构造了一系列不同手性((n,0), (0,m), (n,m))的一维单壁TiO₂纳米管. 用周期性紧束缚密度泛函理论(DFTB)方法计算并比较了不同管径和手性的TiO₂纳米管在几何结构、电子性质等方面的差别. 结果表明: 除了(6,0)管, 其余纳米管随着管径的增大, 应变能和能隙减小. 而在管径相同的情况下, 不同手性的(n,m)纳米管的应变能随着n/m的增加呈现先增大后减小的趋势, 能隙变化不大.     

Achieving indirect-to-direct band gap transition and enhanced photocatalytic performance in blue phosphorene through doping and strain

Recently, blue phosphorene (BP) has demonstrated great potential in the field of photocatalytic water splitting due to the ultrahigh carrier mobility. However, the practical application of BP as an efficient photocatalyst is greatly limited by its indirect band gap. In this work, we investigate the synergistic effect of substitutional doping and biaxial strain on the electronic and photocatalytic properties of BP using hybrid density functional calculations. The results show that As/Sb doping not only reduces the band gap of BP without introducing any midgap states but also turns it into direct band gap semiconductor, which can be ascribed to the p states of the dopants appearing around the band edges. For these As/Sb-doped BP systems, the band gaps, band edge positions, and optical absorption abilities can be further tuned by applying a biaxial strain. In particular, we predict that compressive strains are more propitious for the doped systems than the tensile strains since the requirements for water splitting are satisfied, meanwhile preserving the direct band gap characteristics. Besides, our calculations also show that the band gap and the reducing and oxidizing power of multilayer BP are highly dependent on the layer thickness. These results suggest feasible modulation strategies for enabling BP to be a visible-light-driven photocatalyst for water splitting.     

Graphene nanoribbons from unzipped carbon nanotubes: atomic structures, Raman spectroscopy, and electrical properties

We investigated the atomic structures, Raman spectroscopic and electrical transport properties of individual graphene nanoribbons (GNRs, widths ~10-30 nm) derived from sonochemical unzipping of multiwalled carbon nanotubes (MWNTs). Aberration-corrected transmission electron microscopy (TEM) revealed a high percentage of two-layer (2 L) GNRs and some single-layer ribbons. The layer-layer stacking angles ranged from 0° to 30° including average chiral angles near 30° (armchair orientation) or 0° (zigzag orientation). A large fraction of GNRs with bent and smooth edges was observed, while the rest showed flat and less smooth edges (roughness ≤1 nm). Polarized Raman spectroscopy probed individual GNRs to reveal D/G ratios and ratios of D band intensities at parallel and perpendicular laser excitation polarization (D(∥)/D(⊥)). The observed spectroscopic trends were used to infer the average chiral angles and edge smoothness of GNRs. Electrical transport and Raman measurements were carried out for individual ribbons to correlate spectroscopic and electrical properties of GNRs.     

Giant Stark effect in double-stranded porphyrin ladder polymers

Using the first-principles calculations, we have investigated the stability and the electronic structure of two types of recently synthesized one-dimensional nanoribbons, i.e., double-stranded zinc(II) porphyrin ladder polymer (LADDER) arrays. First, electronic structure calculations were used to show that the LADDER is a semiconductor. Most importantly, the application of a transverse electric field significantly reduces the band gap of the LADDER, ultimately converting the LADDER to a metal at a field strength of 0.1 V∕A?. The giant Stark effect in this case is almost as strong as that in boron nitride nanotubes and nanoribbons. In the presence of an electric field, hole conduction and electronic conduction will occur entirely through spatially separated strands, rendering these materials useful for nanoelectronic devices. Second, the substitution of hydrogen atoms in the porphyrin units or that of zinc ions with other kinds of chemical species is found to increase the binding strength of the LADDER and reduce the band gap.     

First-Principles Study of Field Emission from Zigzag Graphene Nanoribbons Terminated with Ether Groups

Field emission properties of zigzag graphene nanoribbons terminated with C-O-C ether groups (including cyclic and alternative ether groups at edge, denoted as ZGNR-CE and ZGNR-AE) are studied by adopting a self-consistent method based on density functional theory calculation. The results show that the field emissions of these two nanoribbons are dominated by states around Brillouin zone center and close to Fermi level. Because of lower work function, the ZGNR-CE can produce much stronger emission current than recon-structed zigzag graphene nanoribbon. The ZGNR-AE has nearly completely spin-polarized emission current, although its emission current is not strong enough. It is also found that under the lower E-field, the uniaxial strain can effectively modulate their emission currents but the spin polarization of ZGNR-AE keeps unchanged with the varied strain. The under-lying mechanisms are revealed by combining the analyses of their work functions and bandstructures with edge dipole model.     

Computational analysis of geometric structures and edge-termination effects of boron-nitride and edge-termination boron-nitride nanoribbons

In this work, we examined geometry, electronic structure, and edged termination effect of boron-nitride nanoribbons (BNNRs) by employing localized Gaussian-type orbital, periodic-boundary condition, density functional theory (LGTO-PBC-DFT) calculations. Armchair (ABNNR) and zigzag (ZBNNR)-type BNNRs are obtained and then bond lengths variation in two types of BNNRs are analyzed. We find that the B-N bond length variation of ZBNNR is less diverse than that of ABNNR, and decreases with an increase in the ribbon width, monotonically. For those in ABNNR, it appears as an oscillatory convergence as a function of the ribbon width. The energy gap of edged termination ABNNR and ZBNNRs are also calculated using OH and SH as electron donating groups and CN-, and Cl- as electron-withdrawing groups. The introduction of SH- and Cl-terminator significantly reduce the energy gap to the semiconducting region for ZBNNR, but for ABNNR. These calculated results may be exploited for nano-electronic applications. To examine the surface behavior of the BNNRs, we also calculated their Raman shifts with the edge termination effects.     

Relationship Between Stress Modulated Metallicity and Plasmon in Graphene Nanoribbons

Nanoscale quantum plasmon is an important technology that restricts the application of optics, electricity, and graphene photoelectric devices. Establishing a structure–effect relationship between the structure of graphene nanoribbons (GNRs) under stress regulation and the properties of plasmons is a key scientific issue for promoting the application of plasmons in micro-nano photoelectric devices. In this study, zigzag graphene nanoribbon (Z-GNR) and armchair graphene nanoribbon (A-GNR) models of specific widths were constructed, and density functional theory (DFT) was used to study their lattice structure, energy band, absorption spectrum, and plasmon effects under different stresses. The results showed that the Z-GNR band gap decreased with increasing stress, and the A-GNR band gap changed periodically with increasing stress. The plasmon effects of the A-GNRs and Z-GNRs appeared in the visible region, whereas the absorption spectrum showed a redshift trend, indicating the range of the plasmon spectrum also underwent significant changes. This study provides a theoretical basis for the application of graphene nanoribbons in the field of optoelectronics under strain-engineering conditions.     

Magnetic and electronic properties of α-graphyne nanoribbons

Based on the first-principles calculations, we investigate the magnetic and electronic properties of α-graphyne nanoribbons (NRs). We show that all the armchair α-graphyne NRs are nonmagnetic semiconductors with band gaps as a function of ribbon widths. The zigzag α-graphyne NRs are found to have magnetic semiconducting ground state with ferromagnetic ordering at each edge and opposite spin orientation between the two edges. Under the application of transverse electric field, we further predict the existence of half-metallicity in the zigzag NRs which strongly depends on the width of the ribbon.     

Zigzag graphene nanoribbons: Flexible and robust transparent conductors

We report the effect of bending deformation on the optical and transport properties of zigzag graphene nanoribbons (ZGNRs) induced by the uniaxial strain using the first-principles method combined with non-equilibrium Green's function. The optical properties of ZGNRs in the region of visible light are almost unchanged under the uniaxial strain, whereas an absorption peak occurs at the infrared region for the bent ZGNRs under the transverses strain. The transport properties of ZGNRs under the transverses strain with the bending angle up to 65° remain almost the same as those of the flat one. The transmission coefficients around the Fermi level only slightly decrease when the bending angle further increases to 72.5°. Moreover, ZGNRs under the longitudinal strain show the same transmission conductance around the Fermi level as that of the flat one. The edge states of ZGNRs still behave as excellent ballistic transport channels under bending deformation, which makes them promising flexible and robust transparent conductors.     

Molybdenum disulfide nanowires and nanoribbons by electrochemical/chemical synthesis

Molybdenum disulfide nanowires and nanoribbons have been synthesized by a two-step, electrochemical/chemical synthetic method. In the first step, MoO(x) wires (a mixture of MoO(2) and MoO(3)) were electrodeposited size-selectively by electrochemical step-edge decoration on a highly oriented pyrolytic graphite (HOPG) surface. Then, MoO(x) precursor wires were converted to MoS(2) by exposure to H(2)S either at 500-700 degrees C, producing "low-temperature" or LT MoS(2) nanowires that were predominantly 2H phase, or above 800 degrees C producing "high-temperature" or HT MoS(2) ribbons that were predominantly 3R phase. The majority of these MoS(2) wires and ribbons were more than 50 microm in length and were organized into parallel arrays containing hundreds of wires or ribbons. MoS(2) nanostructures were characterized by X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, selected area electron diffraction, X-ray diffraction, UV-visible absorption spectrometry, and Raman spectroscopy. HT and LT MoS(2) nanowires were structurally distinct: LT MoS(2) wires were hemicylindrical in shape and nearly identical in diameter to the MoO(x) precursor wires from which they were derived. LT MoS(2) wires were polycrystalline, and the internal structure consisted of many interwoven, multilayer strands of MoS(2); HT MoS(2) ribbons were 50-800 nm in width and 3-100 nm thick, composed of planar crystallites of 3R-MoS(2). These layers grew in van der Waals contact with the HOPG surface so that the c-axis of the 3R-MoS(2) unit cell was oriented perpendicular to the plane of the graphite surface. Arrays of MoS(2) wires and ribbons could be cleanly separated from the HOPG surface and transferred to glass for electrical and optical characterization. Optical absorption measurements of HT MoS(2) nanoribbons reveal a direct gap near 1.95 eV and two exciton peaks, A1 and B1, characteristic of 3R-MoS(2). These exciton peaks shifted to higher energy by up to 80 meV as the wire thickness was decreased to 7 nm (eleven MoS(2) layers). The energy shifts were proportional to 1/ L( parallel)(2), and the effective masses were calculated. Current versus voltage curves for both LT and HT MoS(2) nanostructures were probed as a function of temperature from -33 degrees C to 47 degrees C. Conduction was ohmic and mainly governed by the grain boundaries residing along the wires. The thermal activation barrier was found to be related to the degree of order of the crystallites and can be tuned from 126 meV for LT nanowires to 26 meV for HT nanoribbons.     

Strain-tunable electronic properties and optical properties of Hf2CO2 MXene

Two-dimensional materials have been extensively applied because of their unusual electronic, mechanical, and optical properties. In this paper, the electronic structure and optical properties of Hf₂CO₂ MXene under biaxial and uniaxial strains are investigated by the Heys-Scuseria-Ernzerhof (HSE06) method. Monolayer Hf₂CO₂ can sustain stress up to 6.453 N/M for biaxial strain and 3.072 N/M for uniaxial strain. Monolayer Hf₂CO₂ undergoes the transition from semiconductor to metal under −12% strain whether it is under biaxial or uniaxial strain. With the increasing biaxial compressive strain, the blue shift of Hf-d, O-p, and C-p orbitals in valence band maximum results in the metallization of monolayer Hf₂CO₂, while the red shift of Hf-d and O-p orbitals in conduction band minimum results in the metallization of monolayer Hf₂CO₂ with increasing uniaxial compressive strain. The analysis of optical properties indicates that uniaxial strain weakens the reflectivity and refractive index of monolayer Hf₂CO₂ in the visible-light range. In addition, the effective mass and the charge distribution under biaxial and uniaxial strains are also explored.     

Assembly of [(eta5-C5Me5)MoS3Cu3]-supported one-dimensional chains with single, double, triple, and quadruple strands

The construction of a new set of [(eta5-C5Me5)MoS3Cu3]-based supramolecular compounds with different one-dimensional (1D) arrays from two preformed clusters [PPh4][(eta5-C5Me5)MoS3(CuX)3] (X = Br (1a), NCS (1b)) with 1,2-bis(4-pyridyl)ethane (bpe) and 1,3-bis(4-pyridyl)propane (bpp) is presented. Reactions of 1a with bpe in different molar ratios afforded ([((eta5-C5Me5)MoS3Cu3) 2(mu-bpe)3.5Br4].MeCN) n (2), ([((eta5-C5Me5)MoS3Cu3)2(mu-bpe)3Br4].Sol)n (3a: Sol = DMSO.3MeCN; 3b: Sol = 2aniline.3MeCN), ([((eta5-C 5Me5)MoS3Cu3)2(mu-bpe)3(bpe)Br4].0.35DMF)n (4), and ([((eta5-C5Me5)MoS3Cu3)2(mu-bpe)2(mu-Br)(mu3-Br)Br2].DMF.MeCN)n (5). On the other hand, treatment of 1a or 1b with bpp produced [(eta5-C5Me5)MoS3Cu3(mu-bpp)(mu-Br)Br]n (6) and ([((eta5-C5Me5)MoS3Cu3)2(mu-bpp)3(mu-NCS)2(NCS)](NCS))n (7). Compounds 2-7 have been characterized by elemental analysis, UV-vis spectroscopy, IR spectroscopy, 1H NMR, and X-ray analysis. In 2, each [(eta5-C5Me5)MoS3Cu3] core serves as an angular two-connecting node to link other equivalent cores by single and double bpe bridges to form a 1D "Great Wall"-like chain. In 3a and 3b, the [(eta5-C5Me5)MoS3Cu3] cores are linked alternatively by single and double bpe bridges to give a 1D zigzag chain. In 4, six cluster cores (two as a two-connecting node and four as a three-connecting node) are connected by four single bpe and two double bpe bridges to form a cyclohexane-shaped repeating unit, which is further fused with other units to generate a 1D double-stranded chain. Compound 5 has a simple 1D zigzag chain consisting of the cluster cores linked by single bpe bridges. In 6, the cluster cores are linked by single bpp bridges to give a 1D helical chain, which further holds two symmetry-related chains through C-H...Br hydrogen-bonding interactions, thereby forming a 1D H-bonded triple-stranded chain. Compound 7 has a rare 1D quadruple chain, in which the [(eta5-C5Me5)MoS3Cu3] cores work as planar four- and five-connecting nodes to interconnect other equivalent cores through single bpp bridges and single and double thiocyanate bridges. In addition, the third-order nonlinear optical properties of 1a, 2, 3a, and 4-7 in aniline were also investigated by using the Z-scan technique with a 4.5 ns pulse laser at 532 nm.