Theoretical studies on the structure and electronic properties of cubic gold nanoclusters

A multi‐scale approach involving molecular mechanics, semi‐empirical and density functional techniques have been carried out on the cubic gold nanoclusters (Au_n, n = 63, 126, 252, 504, 756, 1008, 1260, 1512, 1764 and 2016) to monitor their structural and electronic properties. Definite correlations have been found to connect the ionisation potentials, conductance and absorption properties of such clusters with their shapes and sizes. Analytical equations are developed based on three‐dimensional particle in a box model to explain the validity of such correlations. © 2012 Canadian Society for Chemical Engineering     

Quantum monte carlo methods for electronic structure of nanosystems

We provide a brief review of recent applications of quantum Monte Carlo (QMC) methods to the electronic structure of nanosystems. We report on calculations of carbon rings with second-order Jahn-Teller effect, energy ordering of silicon clusters, dissociation enthalpies of protonated hydrogen clusters, and other interesting challenges. We point out the QMC accuracy and outline a few ideas that characterize the current position of QMC among the electronic structure methods and its future development.     

X-ray absorption studies of the electronic structure of nickel-copper catalysts

X-ray absorption threshold resonances associated with L_III and L_II absorption edges, which are attributed to excitation of electrons from 2p core levels to unoccupied d-states in the valence band, were investigated for a series of nickel-copper alloys of widely differing compositions. The catalysts were alloys in the form of metal powders with surface areas of the order of 1 m²/g. When the resonances for such samples are determined from measurements of the intensity of emission of electrons from the samples (i.e. by the so-called electron yield detection method), one can avoid experimental artifacts arising from the sample thickness effect encountered in X-ray transmission or fluorescence measurements. Edge resonances can then be determined reliably for metal powders. It was observed that copper had only a small attenuating effect on the resonances of nickel in the catalysts. The effect was smaller by an order of magnitude than that predicted by the rigid-band model of the electronic structure of nickel-copper alloys. The results provide strong support for earlier conclusions of others about the limitations of the rigid-band model.     

Emerging fabrication techniques for 3D nano-structuring in plasmonics and single molecule studies

The application of new methods and techniques to fields such as biology and medicine is becoming more and more demanding since the request of detailed information down to single molecules is a scientific necessity and a technical realistic possibility. In this effort a key role is played by emerging fabrication techniques. One of the hardest challenges is to incorporate the third dimension in the design and fabrication of novel devices. Significantly, this means that conventional nano-fabrication methods, intrinsically useful for planar structuring, have to be substituted or complemented with new approaches. In this paper we show how emerging techniques can be used for 3D structuring of noble metals down to nanoscale. In particular, the paper deals with electroless deposition of silver, ion and electron beam induced deposition, focused ion beam milling, and two-photon lithography. We exploited these techniques to fabricate different plasmonics nanolenses, nanoprobes and novel beads for optical tweezers. In the future these devices will be used for the manipulation and chemical investigation of single cells with sensitivity down to a few molecules in label free conditions and native environment. Although this paper is only devoted to nanofabrication, we foresee that the fields of biology and medicine will directly gain substantial advantages from this approach.     

Theoretical studies on the electronic and optical properties of two thiophene-fluorene based π-conjugated copolymers

The low PL quantum efficiency, typically 1-3%, in solid film, limits the application of polythiophene and derivatives (PTs) in PLEDs. The six-member aromatic rings polyfluorenes (PFs) with higher PL efficiencies have been introduced into the backbone of PTs, in an effort to develop highly efficient, desirable charge carrier transporting and low energy gap thiophene-fluorene based light-emitting polymers. In this contribution, quantum-chemical techniques are employed to study two fluorene-thiophene incorporated π-conjugated polymers, namely, poly((5,5-E-α-(2-thienyl)methylene)-2-thiopheneacetonitrile)-alt-2,7-(9,9-dimethylfluorene) (PFTCNVT) and poly((5,5″-(3′,4′-dimethyl-2,2′;5′,2″-terthiophene1′,1′-dioxide))-alt-2,7-(9,9-dimethylfluorene)) (PFTORT). Density functional theory (DFT) and time-dependent DFT approaches are employed to study the neutral molecules, positive and negative ions, the IPs and EAs, HOMO-LUMO gaps (Δ_H-L), as well as the lowest excitation energies (E_gs). It is interesting to note that the two copolymers PFTCNVT and PFTORT are superior to the properties of pristine polyfluorene (PF) and polythiophene (PT). In addition to the improved PL efficiency, they still presented lower energy-gap comparable to PTs. Furthermore, the LUMO energies lower about 1.4 eV and thus the EAs increase around 1.4 eV in PFTCNVT and PFTORT compared with PFs, suggesting the significant improved electron-accepting and transporting abilities in the two copolymers. These properties can be explained by the presence of more electron-accepting thiophene units in the repeated unit of the copolymers and the more planar conformations in the two copolymers under study.     

Theoretical investigation of electronic structure and field emission properties of carbon nanotube–ZnO nanocontacts

A density functional theory study is performed to understand electronic structures and field emission properties of carbon nanotube–ZnO nanocontacts. The carbon nanotube–ZnO nanocontacts have high energetic stabilities and small energy gaps. The energy gaps of the nanocontacts exhibit an oscillatory behavior as a function of the length of carbon nanotubes. The ionization potentials of all carbon nanotube–ZnO nanocontacts are smaller than 7.155 eV of the ZnO nanocage. The ionization potentials of carbon nanotube–ZnO nanocontacts with more 2 carbon layers exhibit odd–even oscillation in the absence and presence of an electric field. The carbon nanotube–ZnO nanocontact with 4 carbon layers has a smallest ionization potential of 3.625 eV under 0.2 eV/Å external electric field. These results indicate that the field-emission properties of simplex ZnO and carbon nanotube materials can be enhanced significantly by the formation of carbon nanotube–ZnO nanocontacts.     

Density functional studies of the electronic structure and adsorption at molybdenum oxide surfaces

The electronic structure and bonding at different oxygen sites of MoO₃(0 1 0) and (1 0 0) surfaces is reviewed on the basis of ab initio density functional theory (DFT-LCGTO) cluster calculations. The clusters are chosen as finite sections of the ideal MoO₃ surface where cluster embedding is achieved by bond saturation with hydrogen terminator atoms yielding clusters up to Mo₇O₃₀H₁₈. Resulting charge density distributions and binding properties are analyzed by populations, bond orders, and electrostatic potential maps. Interatomic binding at the surface is determined by both ionic and covalent contributions with a clear distinction between terminal oxygens and different bridging surface oxygens. Electronic differences between the MoO₃ (0 1 0) and (1 0 0) surfaces are found to be mainly due to the different atom arrangement while local atom charging and binding properties seem surface independent. The electronic surface parameters influence the behavior and reactions of adsorbed molecules as will be shown for H, OH, and C₃H₅ adsorbates.     

多孔陶瓷膜烟气水分回收理论与模型研究

化石燃料燃烧烟气中含有大量水分和潜热,高湿度烟气的直接排放造成极大的资源浪费和环境问题。多孔陶瓷膜是目前烟气水热回收最有前景的技术之一,其水分回收热力学和动力学的定量描述是该技术发展和装置设计的关键所在。分析了水分在多孔陶瓷膜表面及内部传质机理,基于Kelvin理论建立了水分在陶瓷膜内毛细凝聚热力学模型,并选取典型烟气温/湿度条件,得出不同工况下陶瓷膜发生毛细凝聚的临界孔径、凝聚水量及工作孔体积占比;进而基于毛细凝聚的表面传质和孔道输运Hagen-Poiseuille方程建立了陶瓷膜水分传质动力学模型,对典型温/湿度工况下回收水通量进行了计算,结果表明,多孔陶瓷膜的毛细凝聚效应对烟气水分回收的优越性十分明显,其表面回水通量远远大于冷凝法的水通量,孔径越小,表面水通量越高,但及时将孔道内的液态水输运到陶瓷膜另一侧需要的压差也越大,本文计算条件下,膜孔径为20.0 nm的陶瓷膜较为适宜。     

多孔陶瓷膜烟气水分回收理论与模型研究

化石燃料燃烧烟气中含有大量水分和潜热,高湿度烟气的直接排放造成极大的资源浪费和环境问题。多孔陶瓷膜是目前烟气水热回收最有前景的技术之一,其水分回收热力学和动力学的定量描述是该技术发展和装置设计的关键所在。分析了水分在多孔陶瓷膜表面及内部传质机理,基于Kelvin理论建立了水分在陶瓷膜内毛细凝聚热力学模型,并选取典型烟气温/湿度条件,得出不同工况下陶瓷膜发生毛细凝聚的临界孔径、凝聚水量及工作孔体积占比;进而基于毛细凝聚的表面传质和孔道输运Hagen-Poiseuille方程建立了陶瓷膜水分传质动力学模型,对典型温/湿度工况下回收水通量进行了计算,结果表明,多孔陶瓷膜的毛细凝聚效应对烟气水分回收的优越性十分明显,其表面回水通量远远大于冷凝法的水通量,孔径越小,表面水通量越高,但及时将孔道内的液态水输运到陶瓷膜另一侧需要的压差也越大,本文计算条件下,膜孔径为20.0 nm的陶瓷膜较为适宜。     

Theoretical studies on the electronic structures and optical properties of the oligomers involving bipyridyl,thiophenyl and ethynyl groups

Structural, electronic, and optical properties of a series of π-conjugated chain type oligomers composed of the bipyridyl, ethynyl and thiophenyl fragments are studied by advanced quantum chemical methods. The properties of the P-oligomers (P1–P4, with 1–4 repeating units, respectively) and the corresponding DP-oligomers (DP2–DP4, with 2–4 repeating units, respectively, but without the terminal thiophenyl fragments) are compared. The geometry optimization results at B3LYP/6-31G level show that all the fragments involved in both the P-oligomers and DP-oligomers in the ground state exhibit a zigzag chain within the molecular plane. The TD-DFT calculations reveal that the dipole-allowed lowest-lying X¹A → A¹B absorption band of each oligomer possesses the π → π¹ transition characters. The geometry structures of P1, P2 and DP2 in the excited state have been studied by the CIS method, the fluorescence of them revealed by TD-DFT method is originated from the ¹[ππ¹] excited state. Upon increasing the number of the repeating unit, there is a progressive lowering in HOMO–LUMO gaps, being consistent with the red shifts in the lowest-lying absorptions and fluorescence. The ionization potentials (IPs) and electron affinities (EAs) of the polymer are obtained by the extrapolation method. The calculated IPs of DP-polymer are lower than those of P-polymer and the EAs of DP-polymer are higher than those of the P-polymer, which indicates that the DP-polymer is more suitable for electron transferring and hole creating material.     

Computing MOSCED parameters of nonelectrolyte solids with electronic structure methods in SMD and SM8 continuum solvents

An efficient method to predict modified separation of cohesive energy density model (MOSCED) parameters for nonelectrolyte solids using electronic structure calculations in SMD and SM8 continuum solvents is proposed and applied to acetanilide, acetaminophen, and phenacetin. The resulting parameters are ultimately used to predict the equilibrium solubility in a range of solvents over a range of temperatures. By combining MOSCED with SMD and SM8, we are able to leverage the strengths of both methods while eliminating shortcomings that would prevent their use alone for solvent selection in design processes involving nonelectrolyte solid solutes. Comparing to 77 non‐aqueous experimental solubilities of acetaminophen over the range 10–30°C, the proposed method has an average absolute error of 0.03 and 0.04 mol fracs for SMD and SM8 regressed parameters, respectively. Aqueous solubilities of acetaminophen over this temperature range are predicted with an average error of 0.030 and 0.0023 mol fracs, respectively. © 2016 American Institute of Chemical Engineers AIChE J, 63: 781–791, 2017     

Multilayer structure of molybdena/γ-alumina catalysts prepared using different methods

The apparent surface coverage by molybdena in three series of$\text{MoO}{}_3\text{γ-Al}{}_2\text{O}{}_3$ catalysts prepared by different methods was determined by electrophoretic migration. These methods were designed to vary the extent of clustering (polymerization) in the solutions and on the wet catalysts. The combined use of electrophoretic migration and molybdenum extraction measurements showed that in alumina-supported molybdena catalysts with content below 14 wt%, molybdenum is present in both monolayer and multilayer forms over the entire range of molybdenum loading studied, irrespective of the method of catalyst preparation used.     

Theoretical studies of interstitials in graphite

CNDO (Complete neglect of differential overlap) calculations are reported for a model of interstitial C and C₂ species between graphite planes, modelled by two C₁₆H₁₀ graphite like molecules. For a single C atom interstitial the optimum position is about 0.82 Å above the centre of a ring. The interstitial is bonded to one plane with only a weak interaction with the plane above. For the C₂ interstitial, the optimum position is with each C atom 0.83 Å above the ring in adjacent layers. Agreement with experimental values for the migration energy of an interstitial is not found, and may be achieved if deformation of the graphite layers is also taken into account.     

橡胶增强的理论研究

修正的Einstein方程用于描述低填充橡胶的弹性模量,改进的Halpin—Tsai方程可较好预测较高填料体积分数下体系的弹性模量。描述了填料增强橡胶体系应力—应变曲线的4个重要模型：范德华网络模型、双网络模型、JGmC2L方程以及炭黑增强橡胶的链组理论。范德华网络模型以填料聚集体作为增强单元,且认为其可发生塑性变形和破坏;双网络模型强调了填料网络中橡胶分子链的完全伸展和取向对增强的贡献;JGmC2L方程体现了高伸长率时材料结构变化导致有限伸长特性的变化;链组理论以链组而不是单根链作为独立的形变单元。4种模型均有一定的合理性和局限性,有待建立可全面反应材料在整个大形变过程中黏弹性力学行为的新方程。     

Theoretical investigations of electronic structure and charge transport properties in polythiophene‐based organic field‐effect transistors

Regioregular poly(3‐hexylthiophene) (P3HT) is a hole transport polymer material used in organic field‐effect transistors (OFETs) and can reach mobilities as high as 0.1 cm² V^?1 s^?1. Factors that affect the charge mobility and the transport mechanisms of P3HT‐based OFET systems are therefore of great importance. We use quantum mechanical methods to interpret the charge mobility and the transport properties of self‐assembled P3HT molecules along the intra‐chain and inter‐chain directions. Our approach is illustrated by a hopping transport model, in which we examine the variation of charge mobility with torsional angle and the intermolecular distance between two adjacent thiophene segments. We also simulate packed P3HT structures via molecular dynamics (MD) simulations. The MD results indicate that the resultant mobility along the π?π inter‐chain direction is significantly less than that along the intra‐chain direction. Accordingly, the main charge‐transfer route within the P3HT ordered domains is an intra‐chain rather than an inter‐chain one. The calculation result for the inter‐chain hole mobility is around 10^?2 cm² V^?1 s^?1, which is consistent with experimental data from P3HT single fibril. Copyright © 2009 Society of Chemical Industry     

Study of the electronic structure of carbon materials near the bandgap using electron energy loss spectroscopy

The detailed understanding of the electronic properties of semiconducting materials requires the determination of their joint density of states (JDOS) and dielectric constant with high spatial resolution. Low electron energy loss spectroscopy (EELS) provides a continuous spectrum which represents all electronic excitations with energies ranging between zero and about 100 eV. Therefore EELS is potentially more powerful than conventional optical spectroscopy which has an intrinsic upper information limit of about 6 eV due to absorption of light from the optical components of the system or the ambient. However, when analysing EELS data, the extraction of the single scattered data needed for Kramers–Kronig calculations is subject to the deconvolution of the zero loss peak from the raw data. This procedure is particularly critical when attempting to study the near-bandgap region of materials with a bandgap below 1.5 eV. In this paper calculation of the electronic properties of some widely studied carbon materials; namely amorphous carbon (a-C), tetrahedral amorphous carbon (ta-C), C₆₀ fullerite crystal as well as synthetic and natural diamond, are presented. Extrapolation of a fit to the absorption data at the onset of an interband transition was used to get an accurate value for the bandgap energy and to determine whether the bandgap is direct or indirect in character. Particular problems relating to the extraction of the single scattered data for these materials are also discussed. The ta-C and C₆₀ fullerite materials are found to be direct bandgap-like semiconductors, having a bandgap of 2.63 and 1.59 eV, respectively. On the other hand, the electronic structure of a-C was unobtainable because it had such a small bandgap that most of the information is contained in the first 1.2 eV of the spectrum, which is a region removed during the zero loss peak deconvolution. The processing of the data obtained from the ta-C material revealed another benefit of using EELS: the electrons of the beam can transfer momentum and energy to the electrons of the material, thus enabling the probing of indirect interband transitions. Use of this extra feature of EELS coupled with the extremely high spatial resolution obtainable in a scanning electron microscope has indicated that tetrahedral carbon thin films might not be amorphous after all, implying the need for a fresh look at their use as electronic materials.