Surface dipoles and work functions of alkylthiolates and fluorinated alkylthiolates on Au(111)

We study the dipole formation at the surface formed by -CH(3) and -CF(3) terminated short-chain alkylthiolate monolayers on Au(111). In particular, we monitor the change in work function upon chemisorption using density functional theory calculations. We separate the surface dipole into two contributions, resulting from the gold-adsorbate interaction and the intrinsic dipole of the adsorbate layer, respectively. The two contributions turn out to be approximately additive. Adsorbate dipoles are defined by calculating dipole densities of free-standing molecular monolayers. The gold-adsorbate interaction is, to a good degree, determined by the Au-S bond only. This bond is nearly apolar and its contribution to the surface dipole is relatively small. The surface dipole of the self-assembled monolayer is then dominated by the intrinsic dipole of the thiolate molecules. Alkylthiolates increase the work function of Au(111), whereas fluorinated alkylthiolates decrease it.     

Adsorption of benzene on coinage metals: a theoretical analysis using wavefunction-based methods

The interaction of benzene with a Ag(111) surface has been determined using reliable ab initio electronic structure calculations. The results are compared to a recent detailed analysis of the interaction of benzene with copper and gold surfaces, thus making it possible to derive a consistent picture for the electronic structure changes encountered when benzene is brought into contact with the densely packed coinage metal surfaces. To avoid the problems encountered when the presently most frequently employed computational approach, density functional theory (DFT), is applied to adsorbate systems where dispersion (or van der Waals) forces contribute substantially, we use a wavefunction-based approach. In this approach, the weak van der Waals interactions, which are dominated by correlation effects, are described using second-order perturbation theory. The surface dipole moment and the work function changes induced upon adsorption are also discussed.     

Adsorbate-adsorbate interactions and chemisorption at different coverages studied by accurate ab initio calculations: CO on transition metal surfaces

We use density functional theory (DFT) with the generalized gradient approximation (GGA) and our first-principles extrapolation method for accurate chemisorption energies (Mason et al. Phys. Rev. B 2004, 69, 161401R) to calculate the chemisorption energy for CO on a variety of transition metal surfaces for various adsorbate densities and patterns. We identify adsorbate through-space repulsion, bonding competition, and substrate-mediated electron delocalization as key factors determining the preferred chemisorption patterns for different metal surfaces and adsorbate coverages. We discuss how the balance of these interactions, along with the inherent adsorption site preference on each metal surface, can explain the observed CO adsorbate patterns at different coverages.     

Surface modification of III–V semiconductors: chemical processes and electronic properties

The possibilities of controlling the surface electronic properties of III–V semiconductors by varying the adsorption chemistry are analyzed. Variations of the adsorption process parameters and the adsorbate reactivity are able to affect the surface atomic and electronic structure of the semiconductor. The adsorbate reactivity is considered within the framework of the density functional theory using the reactivity indices. The easiest way to affect the reactivity of a particular adsorbate is to create a solvation shell around it, as is possible in both liquid solutions and the gas phase (microsolvation). Solvation of ions before their adsorption by different solvents affects considerably the relative nucleophility of the central atom in the ion, which results in a different charge transfer mechanism from the surface states on adsorption, and thus, in a different modification of the surface electronic structure of the semiconductor. The effect of halogen, sulfur and metal atoms reactivity on the electronic structure of the resulting adsorbate-covered surface of III–V semiconductor is discussed.     

Origin of the photoinduced optical second harmonic generation arising in N-phenyl microcrystalline films

Photoinduced second-order nonlinear optical effects, particularly optical second harmonic generation (SHG) of N-phenyls with different numbers of aromatic rings deposited on glass substrates were studied. As a fundamental beam, a 5-ps pulsed Nd:YAG laser was used. Quantum chemical time-dependent density functional theory (TDDFT) simulations of the nonlinear optical properties were performed. The first-order hyperpolarizabilities of isolated molecules were calculated, under the influence of a polarized pumping beam, to evaluate the role played by the nanointerfaces separating the microcrystallites and the amorphous environment. Consideration was performed within a framework of steady-state Langevin order parameters for amorphous-like films. A strong dependence of the photoinduced SHG versus the number of aromatic rings determining the degree of film crystallinity was shown. A comparison of experimental data and theoretically evaluated results shows that for the photoinduced first-order nonlinear optical effect the dominant contribution is an amorphous-like structural component, unlike the transport properties, where the crucial role is played by the nanointerface region. This may reflect a specific feature of the multiphoton processes in such types of nanointerfaces because of nanoconfined effects.     

Characterization of the interface dipole at organic/ metal interfaces

In organics-based (opto)electronic devices, the interface dipoles formed at the organic/metal interfaces play a key role in determining the barrier for charge (hole or electron) injection between the metal electrodes and the active organic layers. The origin of this dipole is rationalized here from the results of a joint experimental and theoretical study based on the interaction between acrylonitrile, a pi-conjugated molecule, and transition metal surfaces (Cu, Ni, and Fe). The adsorption of acrylonitrile on these surfaces is investigated experimentally by photoelectron spectroscopies, while quantum mechanical methods based on density functional theory are used to study the systems theoretically. It appears that the interface dipole formed at an organic/metal interface can be divided into two contributions: (i) the first corresponds to the "chemical" dipole induced by a partial charge transfer between the organic layers and the metal upon chemisorption of the organic molecules on the metal surface, and (ii) the second relates to the change in metal surface dipole because of the modification of the metal electron density tail that is induced by the presence of the adsorbed organic molecules. Our analysis shows that the charge injection barrier in devices can be tuned by modulating various parameters: the chemical potential of the bare metal (given by its work function), the metal surface dipole, and the ionization potential and electron affinity of the organic layer.     

酞菁铜/金属薄膜界面电位与光二次谐波特性分析

Using the Kelvin probe and the optical second harmonic generation (SHG) measurement, the Space charged phenomenon and the non-linear optical effects at the interfaces of the coppertetra-tert-butyl phthalocyanine (CuttbPc) Langmuir-Blodgett (LB) film deposited on a metal (AlorAu) coated glass slide substrate were in vestigated. The surface potentials decreaseas the film thickness increases and eventually approaches a saturated value. The SHG has been detected although there is a centro-symmetric systemin the Cuttb Pcmolecular, and avery strong SH signal can be investigated at 1260 nm band for Cuttb Pc/Al samples. According to a proposed physical model for Cuttb PcLB film/metal, the nonlinear mechanism were analyzed by using electromagnetic wave theory. It is considered that the enhanced SH peak of CuttbPc/Al is attributed to the strong surface potential aroused by SCIEF at the interface, and shows that the production of SH signal is correlated closely with the electrostatic phenomena at the interface.     

用激光第二谐振光研究阴离子在多晶铜电极表面上的吸附

首次用激光产生的第二谐振光(SHG)检测到金属/水溶液界面上阴离子在多晶铜电极表面上的吸附,阴离子吸附特性对SHG强度影响明显,由多晶铜电极在(0.5-x)mol/L NaClO_4+xmol/L NaBr溶液中的SHG强度-电位曲线表明铜电极表面对ClO_4~-的吸附非常弱,对Br~-有特定的吸附,SHG强度随Br~-浓度增加而增强,结果表明SHG是定量研究电化学界面区吸附特性的灵敏有效的探针,可揭示金属与吸附质间相互作用的本质。     

A model for electron-transfer reactions via adsorbed intermediates

A theory is presented for electron-transfer reactions between a strongly coupled metal electrode/adsorbate system and a simple redox system in solution. A model Hamiltonian is set up in which the adsorbate is described by the Anderson model; the current is then calculated within the transfer Hamiltonian formalism. An important factor in the equation for the current is the adsorbate density of states evaluated at the saddle point of the reaction hypersurface; an explicit expression for this quantity is derived. The relation of this work to experimental results and to other theories of electron-transfer reactions is discussed.     

黄铜矿型铜基硫属半导体材料的电子结构和光学性质

采用基于赝势平面波基组的密度泛函理论方法, 对具有黄铜矿结构的6种CuXY₂(X=Ga, In; Y=S, Se, Te)晶体的构型、 电子结构、 线性及二阶非线性光学性质进行了研究. 结果表明, 6种CuXY₂均为直接带隙半导体, 具有相似的能带结构. 当X原子相同时, 随着Y原子按S→Se→Te依次改变时, 体系的静态介电常数、 静态折射率和静态倍频系数(d₃₆)依次递增. 在占据带中, 位于价带顶附近的能带对体系倍频效应影响最为显著, 该系列化合物的能带主要成分为Cu的3d轨道和Y原子价层p轨道; 对于空能带, 对倍频系数影响较大的是以X原子价层p轨道为主要成分的能带. 6种晶体中, CuInSe₂晶体具有较高的光电导率并对太阳光具有较好的吸收性能. 综合考虑体系的双折射率和倍频效应等因素, CuGaS₂和CuGaSe₂ 2种晶体在二阶非线性光学领域具有潜在的应用价值.     

Electrodeposition of flattened Cu nanoclusters on a p-GaAs(001) electrode monitored by in situ optical second harmonic generation

In situ optical second harmonic generation (SHG) technique was employed to investigate the shape and density of Cu nanoclusters, which were electrochemically formed on p-GaAs(001) electrode surfaces. Since GaAs is not a centrosymmetric medium, a significant portion of SHG signal arises from the bulk dipole susceptibility, but it was possible to separate a surface-induced signal from a bulk-induced signal by choosing an appropriate experimental geometry and appropriate data processing. The rotational anisotropy (RA) pattern of the SHG signal from a p-GaAs(001) electrode changed in both shape and magnitude during potential cycling in an electrolyte solution containing Cu2+. The surface plasmon-induced SHG signal from Cu nanoclusters deposited on GaAs was attributed to the modulation source for the RA-SHG pattern. More detailed study was carried out with both in situ SHG and ex situ AFM measurements for Cu nanoclusters deposited by potential step. The results showed that the SHG signal at the present optical geometry was sensitive to the number of oblate or flattened Cu nanoclusters with lateral diameter larger than 30 nm and that the SHG enhancement occurred because of resonant coupling between the surface plasmon induced in the flattened Cu nanoclusters and the near-infrared fundamental light.     

Giant Enhancement of Second Harmonic Generation Accompanied by the Structural Transformation of 7-Fold to 8-Fold Interpenetrated Metal–Organic Frameworks (MOFs)

Interpenetration in metal–organic frameworks (MOFs) is an intriguing phenomenon with significant impacts on their properties, and functional applications. Herein, we show that a 7-fold interpenetrated MOF ( 1 ) is transformed into an 8-fold interpenetrated MOF by the loss of DMF in a single-crystal-to-single-crystal manner. This is accompanied by a giant enhancement of the second harmonic generation (SHG ca. 125 times) and two-photon photoluminescence (ca. 14 times). The strengthened π–π interaction between the individual diamondoid networks and intensified oscillator strength of the molecules aid the augment of dipole moments and boost the nonlinear optical conversion efficiency. Large positive and negative thermal expansions of 1 occur at 30–150 °C before the loss of DMF. These results offer an avenue to manipulate the NLO properties of MOFs using interpenetration and provide access to tunable single-crystal NLO devices.     

Bivalent ability for charge transfer in process of hydrogen interaction with surfaces of transition metals

The ability for bivalent charge transfer (CT) during hydrogen adsorption on transition metal surfaces and in the course of transition metal hydride formation is discussed. The change of the dipole moment of hydrogen adatoms, caused by transition from a strongly bound adsorption state to a weakly bound state, is demonstrated. The possibility of the CT reversion between the hydrogen adsorbate and the transition metal adsorbent, caused by a change of the surface structure, is described. The CT within the adsorbate, corresponding to the distinguished steps of the process of transition metal hydride formation, is shown.     

SHG active crystals of a remote functionalized achiral NLO-phore assembled through zinc(II) complexation

An achiral nonlinear optical chromophore with a "remote functionality" that can act as a ligand is developed on the basis of 4-nitroaniline derivatized with pyridine. The molecules are assembled through complexation with simple achiral zinc(II) salts and the H-bond network mediated by the counterions, to generate noncentrosymmetric materials exhibiting optical second harmonic generation (SHG). The crystal structures of the new complexes are determined; the counterion strongly influences the ligand orientations and lattice structure. SHG of the microcrystalline materials is investigated. Correlation between the structure and SHG is rationalized using semiempirical quantum chemical estimation of the hyperpolarizabilities of molecules and molecular clusters. The metal complexation plays a significant role in molecular assembly but affects the SHG very little, enabling simplified analysis of the bulk property in terms of molecular responses. Organization of remote functionalized molecules by metal ion complexation thus offers a convenient approach to the rational design of quadratic NLO materials.     

First-principles study of K and Cs adsorbed on Pd(111)

The adsorptions of K and Cs on Pd(111) were studied by the density functional calculations within the generalized gradient approximation. The site preference, bonding character, work function, and electron structure of the system were analyzed. For K and Cs adsorption, the hcp hollow site was found to be preferred for all the coverages investigated. The calculated adsorption geometries for (2 x 2) and (square root 3 x square root 3)R30 degrees phases are both in reasonable agreement with the observed results. The decrease of the work function upon the adsorption of K and Cs can be attributed to a dipole moment associated with the polarized adsorbate atom, which is characterized by depletion of the electron charge in the alkali metal layer and a charge accumulation in the interface region. Our results indicate that the bonding of alkali metal with the Pd(111) surface has a mixed ionic and metallic bond character at low coverage and a metallic bond of covalent character at high coverage.     

Mechanism for large first hyperpolarizabilities of phosphonic acid stilbene derivatives

This paper presents calculations of dipole moments (mu), static polarizabilities (alpha), and first hyperpolarizabilities (beta) of phosphonic acid stilbene derivatives calculated in the framework of density functional theory. These calculations were performed using a finite field approach implemented in the density functional program ALLCHEM and were of an all-electron type using local exchange-correlation functional and specially designed basis sets. The molecular structures have been fully optimized using the semiempirical program MSINDO. Some of the investigated stilbenes have been synthesized very recently while others are described for the first time. Donor and acceptor groups of these analogues have been modified and the influence of these changes on the first hyperpolarizabilities has been investigated. This work demonstrates that the nonlinear optical response beta of these compounds increases dramatically when the acceptor moiety is displaced by analogues containing alkali metal groups. A general mechanism for the design of novel nonlinear optical materials with large first hyperpolarizabilities is described.     

Giant Enhancement of Second Harmonic Generation Accompanied by the Structural Transformation of 7‐Fold to 8‐Fold Interpenetrated Metal–Organic Frameworks (MOFs)

Interpenetration in metal–organic frameworks (MOFs) is an intriguing phenomenon with significant impacts on their properties, and functional applications. Herein, we show that a 7‐fold interpenetrated MOF ( 1 ) is transformed into an 8‐fold interpenetrated MOF by the loss of DMF in a single‐crystal‐to‐single‐crystal manner. This is accompanied by a giant enhancement of the second harmonic generation (SHG ca. 125 times) and two‐photon photoluminescence (ca. 14 times). The strengthened π–π interaction between the individual diamondoid networks and intensified oscillator strength of the molecules aid the augment of dipole moments and boost the nonlinear optical conversion efficiency. Large positive and negative thermal expansions of 1 occur at 30–150 °C before the loss of DMF. These results offer an avenue to manipulate the NLO properties of MOFs using interpenetration and provide access to tunable single‐crystal NLO devices.     

Characterization of the interface dipole at the paraphenylenediamine-nickel interface: a joint theoretical and experimental study

In organic-based (opto)electronic devices, charge injection into conjugated materials is governed to a large extent by the metal-organic interface dipole. Controlling the injection of charges requires a better understanding of the fundamental origin of the interface dipole. In this context, photoelectron spectroscopies and density functional theory calculations are used to investigate the interaction between para-phenylenediamine (PPDA), an electron donor, and a polycrystalline nickel surface. The interface dipole formed upon chemisorption of one PPDA monolayer strongly modifies the work function of the nickel surface from 5.10 to 3.55 eV. The work function decrease of 1.55 eV is explained by the electron-donor character of PPDA and the modification of the electronic density at the metal surface. PPDA monolayers are composed of tilted molecules interacting via the nitrogen lone-pair and PPDA molecules chemisorbed parallel to the surface via their pi-electron density. Annealing the monolayer leads to dehydrogenation of PPDA activated by the nickel surface, as found for other amines.     

Electronic and structural effects on the nonlinear optical behavior in push-pull TTF/tricarbonyl chromiun arene complexes

A novel D-pi-A system in which tetrathiafulvalene (TTF) and pi-extended TTFs as strong electron donors are covalently connected to a tricarbonyl (eta(6)-arene)chromium complex as the acceptor moiety through a systematically increased conjugated bridge of vinylene units (12a-c, 16a-c) have been synthesized by Wittig-Horner olefination reaction. The electronic spectra as well as the electrochemical data reveal a different behavior of TTF derivatives (12a-c) and of exTTF derivatives (16a-c). Cyclic voltammetry shows the influence of the tricarbonylchromium arene on the oxidation potentials in compounds 12a-c, and no remarkable effect is observed for exTTFs (16a-c). The nonlinear optical properties of 12a-c and 16a-c have been calculated by using the ab initio CPHF/6-31G//B3P86/6-31G model, and the time-dependent density functional theory (TD-DFT) method has been used for the calculation of the electronic transitions. The calculations reveal that an intraligand charge-transfer transition (ILCT) and the metal to ligand charge-transfer transition (MLCT) are responsible for the nonlinear response. In addition, the large angles formed by the ground-state dipole moment and the vectorial hyperpolarizability are responsible for the mubeta values determined experimentally by the EFISH technique.