Molecular Orbital States Research Articles

Effectiveness of organic molecules for spin filtering in an organic spin valve: Reaction-induced spin polarization for Co atopAlq3

The spin polarization of organic-ferromagnetic interfaces in an organic spin valve critically affects the efficiency of spin injection or detection. We examined the chemical and electronic properties of ferromagnetic Co deposited on organic $\mathrm{Al}{\mathrm{q}}_{3}$ and the interfacial spin-polarized capability of the electronic states. Our x-ray photoemission spectra and calculations with density-functional theory indicate a sequential and unequal distribution of charge from Co clusters to N and then to O atoms in $\mathrm{Al}{\mathrm{q}}_{3}$. The preferential orbital hybridization at specific functional sites produces efficient spin polarization of organic molecules. Element-specific measurements of x-ray magnetic circular dichroism demonstrate the preferential spin polarization in the lowest unoccupied molecular orbital state of N atoms at the complex interface for Co atop $\mathrm{Al}{\mathrm{q}}_{3}$, which agrees satisfactorily with calculation. Our results indicate that an induced interfacial spin polarization on engineering the dominant reaction of Co with mainly N and O atoms in $\mathrm{Al}{\mathrm{q}}_{3}$ might pave a way for effective spin filtering in organic spintronics.

Molecular electronic level alignment at weakly coupled organic film/metal interfaces.

Electronic level alignment at interfaces of molecular materials with inorganic semiconductors and metals controls many interfacial phenomena. How the intrinsic properties of the interacting systems define the electronic structure of their interface remains one of the most important problems in molecular electronics and nanotechnology that can be solved through a combination of surface science experimental techniques and theoretical modeling. In this article, we address this fundamental problem through experimental and computational studies of molecular electronic level alignment of thin films of C(6)F(6) on noble metal surfaces. The unoccupied electronic structure of C(6)F(6) is characterized with single molecule resolution using low-temperature scanning tunneling microscopy-based constant-current distance-voltage spectroscopy. The experiments are performed on several noble metal surfaces with different work functions and distinct surface-normal projected band structures. In parallel, the electronic structures of the quantum wells (QWs) formed by the lowest unoccupied molecular orbital state of the C(6)F(6) monolayer and multilayer films and their alignment with respect to the vacuum level of the metallic substrates are calculated by solving the Schrödinger equation for a semiempirical one-dimensional (1D) potential of the combined system using input from density functional theory. Our analysis shows that the level alignment for C(6)F(6) molecules bound through weak van der Waals interactions to noble metal surfaces is primarily defined by the image potential of metal, the electron affinity of the molecule, and the molecule surface distance. We expect the same factors to determine the interfacial electronic structure for a broad range of molecule/metal interfaces.

Comparison between organic sensitizers containing stilbene and azo group as bridging unit for dye sensitized solar cells.

While azo dyes have been widely used in dye industry, the azo dyes have been seldom applied as sensitizers to dye sensitized solar cells. In this study, new metal-free organic sensitizers, ST and AZ, which are same structures except bridging units, were synthesized and evaluated. ST containing stilbene as bridging unit gave higher energy conversion efficiency than AZ containing azo group as bridging unit. As a result of density functional theory (DFT) calculations, ST displayed more localized frontier molecular orbitals at lowest unoccupied molecular orbital (LUMO) states than AZ.

Solution-processible brilliantly luminescent Eu(III) complexes with host-featured phosphine oxide ligands for monochromic red-light-emitting diodes.

A series of solution-processible electroluminescent (EL) Eu(3+) complexes were constructed with a self-host strategy, in which neutral ligands were employed as functionalized bidentate phosphine oxide (PO) ligands named DPEPOArn (DPEPO = bis(2-(diphenylphosphino)phenyl) ether oxide). The solubility of these complexes was dramatically improved owing to the increased ratios of organic components. This further enhanced the antenna effect of these ligands in both singlet and triplet energy-transfer processes to support high photoluminescent quantum yields (PLQYs) up to 86 % for their Eu(3+) complexes, which is outstanding among conjugated Eu(3+) complexes. Density function theory (DFT) simulations and electrochemical analysis further verified the contributions of DPEPOArn to the carrier injecting/transporting ability of the complexes. In this sense, these functionalized PO ligands served as hosts in optoelectronic processes, which rendered the self-host feature of their Eu(3+) complexes. With the enhanced electrical properties, the spin-coated single-layer organic light-emitting diodes (OLEDs) of these complexes achieved improved low driving voltages, such as onset voltages about 6 V, compared to their Eu(3+)-contained red-emitting polymeric analogues. [Eu(DBM)3DPEPODPNA2] (DBM = 1,3-diphenylpropane-1,3-dione, DPNA = diphenylnaphthylamine) with the most enhanced electrical properties and suitable frontier molecular orbital (FMO) and triplet state locations endowed its devices with the biggest maximum luminance of >90 cd m(-2) and the highest EL efficiencies. This work verified the potential of small molecular EL Eu(3+) complexes for solution-processed OLEDs through rational function integrations.

Effect of Dye Structure on Optical Properties and Photocatalytic Behaviors of Squaraine-Sensitized TiO2 Nanocomposites

TiO2 nanoparticles obtained by an improved polymerization-induced colloid aggregation (Im-PICA) method were sensitized by three squaraine dyes to construct dye/TiO2 nanocomposite photocatalysts (1:3 mass ratios) with visible-light responses by a facile hydrothermal method. Scanning electron microscopy (SEM) and transmission electron miscroscopy (TEM) were employed to visually observe the surface morphologies of the samples. We studied the visible-light response using UV–vis diffuse reflectance spectroscopy and optical band gap calculations using the Tauc plot equation. The dyes exhibit excellent thermal stability, which is ascribed to the decomposition temperature reaching 300 °C based on thermogravimetric–differential thermogravimetric (TG–DTG) analysis. The photocatalytic activities of the as-prepared SQ/TiO2 composites were assessed in terms of the degradation rate of methylene blue (MB) under visible-light irradiation. The ISQ/TiO2 composites achieved best enhancement of photocatalytic activity under 150 min of visible-light irradiation. When ISQ/TiO2 nanocomposites were applied for different photocatalytic degradation cycling times, the degradation rate still reached 68%, which reflects the fact that SQ/TiO2 composites have a high photostability in the photocatalytic process. When taking the degradation rate and photoresponse into consideration, the combination mode between the ISQ dye and TiO2 that obtained the optimal degradation rate tended to be a hydrogen bond, which exhibited a stronger binding force than the simple physical absorption of m-ISQ/TiO2 and c-BSQ/TiO2 based on the FT-IR analysis. Using total organic carbon (TOC) test and liquid chromatography (LC) results, we verified the degree of mineralization of the MB molecule. Based on the fluorescence spectra, the highest fluorescence quantum yield of ISQ dye (0.124) was far greater than those of c-BSQ (0.036) and m-ISQ (0.02). The molecular structures of the squaraine dyes and the electron distributions of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states were given by geometrical optimization with Gaussian 98 at the B3LYP/6-311+G(d,p) level. Compared with the optical band gap calculated using the Tauc plot equation, we suggest that its energy was slightly less than the band gap due to the absorbed photon. This might be related to the localized central electronic, phonon, and local energy levels between the conduction band (CB) and the valence band (VB). The value of the optical band gap calculated from UV diffuse reflection spectroscopy (DRS) becomes less than the theoretical calculated energy band gap.

New D-π-A type indole based chromogens for DSSC: Design, synthesis and performance studies

Three new Donor-π-Acceptor type dyes D1–3 carrying 3-(1-hexyl-1H-indol-3-yl)-2-(thiophen-2-yl)acrylonitrile as backbone with three different acceptor units were designed and synthesized as promising sensitizers for solar cell application. The new dyes were characterized using various spectral and elemental analyses. Their optical and electrochemical properties were investigated using spectrophotometry and cyclic voltammetry respectively, while their photovoltaic performance was evaluated by a device fabrication study. The devices were subjected to electrochemical impedance spectroscopy to gain an insight into the interfacial charge transfer and recombination process while in use. Further, density functional theory study was carried out to investigate their Frontier Molecular Orbital energy states. The study reveals that the dye carrying 4-aminobenzoic acid as an acceptor showed the highest photovoltaic efficiency among the three dyes. This can be attributed to the longer electron lifetime and lower recombination rates. Additionally, a Single crystal X-ray diffraction study confirmed the structure of a key intermediate.

Strain dependence of the nonlinear optical properties of strained Si nanoparticles.

We report on the lattice strain dependence of the nonlinear optical (NLO) parameters of strained Si nanoparticles (NPs), which are prepared in a controlled way by a mechanical ball milling process. X-ray diffraction analysis shows that the nature of strain is compressive and is primarily caused by milling-induced lattice dislocations, which is further supported by high-resolution transmission electron microscopy imaging. It is found that the nonlinear refractive index (n₂) and nonlinear absorption coefficient (β) are strongly influenced by the associated lattice strain present in Si NPs. With the increase of lattice strain, the β gradually decreases while n₂ increases slowly. The strain-dependent observed changes in the NLO parameters of Si NPs are found to be advantageous for application purpose, and it is explained on the basis of strain-induced modification in the electronic structure of the highest occupied molecular orbital and lowest unoccupied molecular orbital states of Si NPs. These results demonstrate the potential of strain-dependent enhancement of nonlinearities for silicon photonics applications.

Variation of Kondo temperature induced by molecule-substrate decoupling in film formation of bis(phthalocyaninato)terbium(III) molecules on Au(111).

We demonstrate that the lattice formation of an adsorbed molecule decouples the molecule-substrate interaction to change the Kondo resonance, which occurs due to interactions between conduction electrons and the molecule's unpaired spin. The double-decker bis(phthalocyaninato)terbium(III) complex, which is single-molecule magnet and forms a Kondo resonance on a Au(111) surface through an unpaired π-radical spin, is studied using scanning tunneling microscopy/spectroscopy (STM/STS). In the STS spectra, an unusual sharp, strong peak (peak A) is found only for the molecule in a film. The peak position of peak A (εA) cyclically shifts by several hundred millivolts as the STS tip position shifts along the outer circle of the molecule, reflecting the tilting of the upper phthalocyanine (Pc) ligand from the flat-lying lower Pc ligand. The Kondo resonance, which is detected as a sharp peak at the Fermi level, also shows cyclic variations of the peak width and intensity. As εA approaches EF, the Kondo temperature (TK) increases. We propose a model that peak A originates from the singly occupied molecular orbital state whose energy is shifted by an unscreened final state effect due to a decrease in the molecule-substrate chemisorptive interaction. We further examine this model using density functional theory calculations, confirming a decreased molecule-substrate interaction for molecules in the film compared to that of isolated molecules. Further calculations of a tilted upper Pc ligand configuration show a site-dependent, cyclic variation of the molecule-substrate interaction within a molecule.

Molecular electronic states in charge transfer complex studied by x-ray absorption spectroscopy

The electronic states of tetrathiafulvalene (TTF: TTF = C6H4S4) molecule in organic ferroelectric TTF-p-bromanil (TTF-BA: BA = C6Br4O2) and TTF crystals have been investigated by x-ray absorption spectroscopy (XAS) measurement at S K-edge. We elucidated that the peak structure at 2470.5 eV directly reflects the existence of hole in the highest occupied molecular orbital (HOMO) state of the TTF molecule in TTF-BA; that is consistent with the ionic TTF molecule (TTF+). The XAS of TTF-BA was evaluated on the basis of first-principles calculations, and the calculated spectra reproduce well the shape of experimental spectrum and the peak energy of the HOMO state.

Measuring Quantum Capacitance in Energetically Addressable Molecular Layers

The Fermi level or electrochemical signature of a molecular film containing accessible orbital states is ultimately governed by two measurable series energetic components, an energy loss term related to the charging of appropriately addressable molecular orbitals (resonant or charge transfer resistance), and an energy storage or electrochemical capacitance component. The latter conservative term is further divisible into two series contributions, one being a classic electrostatic term and the other arising from the involvement and charging of quantized molecular orbital states. These can be tuned in and out of resonance with underlying electrode states with an efficiency that governs electron transfer kinetics and an energetic spread dependent on solution dielectric. These features are experimentally resolved by an impedance derived capacitance analysis, a methodology which ultimately enables a convenient spectroscopic mapping of electron transfer efficacy, and of density of states within molecular films.

A polarization rule on atomic arrangements of graphite-like boron carbonitride

B and N K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra of boron carbonitride (B–C–N) films prepared by ion beam deposition are interpreted by molecular orbital calculations with the core–hole effect. Model clusters with different atomic arrangements are compared in terms of photoabsorption cross section (PACS) and net charge, and they are classified into two groups, i.e., polarization and non-polarization types. PACS of π∗ peaks near the lowest unoccupied molecular orbital (LUMO) state increase at B K-edge and decrease at N K-edge for polarization type and vice versa for non-polarization type. Based on a comparison between experimental and theoretical results, we propose a rule for atomic arrangements of boron, carbon, and nitrogen atoms in graphite-like B–C–N. Finally, the relationship between the rule and structural stability is discussed.

Tuning the Electronic Structure of Tin Sulfides Grown by Atomic Layer Deposition

In this study, tin sulfide thin films were obtained by atomic layer deposition (ALD) using Tetrakis(dimethylamino)tin (TDMASn, [(CH3)2N]4Sn) and hydrogen sulfide (H2S). The growth rate of the tin sulfides (SnSx) was shown to be highly dependent on the deposition temperature, and reaction times of 1 second for the TDMASn and H2S were required to reach the saturation regime. Surface morphologies were smooth or rectangular with rounded corners as observed by a field emission scanning electron microscope (FE-SEM) and were dependent on temperature. X-ray diffraction results confirmed that the crystal structure of SnSx can be tuned by changing the ALD temperature. Below 120 °C, SnSx films appeared to be amorphous. In addition, SnSx films were SnS2 hexagonal at 140 and 150 °C and SnS orthorhombic above 160 °C. Similarly, the values of the optical band gap and binding energy showed significant differences between 150 and 160 °C. The electronic structures of SnSx were extracted by UPS and absorption spectroscopy, and the unsaturated Sn 3d molecular orbital (MO) states in the band edge were found to be responsible for the great improvement in electrical conductivity. This study shows that TDMASn-H2S ALD is an effective deposition method for SnSx films, offering a simple approach to tune the physical properties.

X-Ray Spectroscopic Study of the Conduction Band of K3:Anthracene and K3:Phenanthrene

We study anthracene and phenanthrene doped with potassium using X-ray absorption spectroscopy and electronic structure calculations. In addition, a comparison of molecular orbital calculations and solid state density functional theory calculations are presented. We find that potassium-doping partially populates the LUMO level of anthrancene and phenanthrene and that both the measured and calculated electronic structures of the doped systems are quite different from that of the pristine molecular systems. This suggests that the extra charge carriers in the doped system are responsible for the increased conductivity and greater intermolecular interaction. Finally, our calculations suggest that both K3:phenanthrene and K3:anthracene have reduced or nonexistent band gaps as compared to their pure counterparts, further supporting the conclusion that doping is responsible for increased conductivity.

Experimental Reorganization Energies of Pentacene and Perfluoropentacene: Effects of Perfluorination

Electron–phonon coupling of the highest occupied molecular orbital (HOMO) state is studied by high-resolution ultraviolet photoelectron spectroscopy (UPS) for pentacene (PEN) and perfluoropentacene (PFP) monolayers on graphite. The reorganization energy and related coupling constants associated with the interaction between holes and molecular vibrations are obtained experimentally using a single mode analysis (SMA) of the observed vibronic-satellite intensities of the monolayers. The results are compared with those estimated by multimode analyses of UPS spectra and those derived by means of theoretical approaches, indicating that the purely experimental method with SMA is useful for studying the reorganization energy and the hopping mobility of organic systems. Furthermore, we found that the reorganization energy of PFP is significantly greater than that of PEN, which is ascribed to the extended HOMO distribution of PFP by perfluorination of PEN. The comparison with the results derived from gas-phase UPS measurements is also discussed.

Structural and electronic properties ofα-tin nanocrystals from first principles

The $\ensuremath{\alpha}$ phase of tin is a zero-gap semiconductor with an inverted band structure with respect to other group-IV elements like Ge. The ${\ensuremath{\Gamma}}_{6c}$ states lie energetically below the ${\ensuremath{\Gamma}}_{8v}$ levels. How these unique electronic properties transform in nanostructures with spatial confinement has not been studied. We apply density-functional theory within the local density approximation to investigate the energetic, structural, and electronic properties of bulk $\ensuremath{\alpha}$-Sn and its nanocrystals (NCs) up to a size of 363 Sn atoms. For NCs with larger diameters up to 14 nm the tight-binding method is applied for the electronic states. Spin-orbit coupling is taken into account. The clusters are modeled in such a way that the ${T}_{d}$ symmetry of the bulk system is conserved. Their surfaces are passivated with hydrogen. We show that the spatial confinement causes not only a decrease of the fundamental gap for increased NC size but also a topological transition where the ordering of $s$- and $p$-like highest-occupied molecular orbital and lowest-unoccupied molecular orbital states is interchanged. The influence of quasiparticle and excitonic effects on the lowest pair excitation energies is investigated within approximations based on the hybrid exchange-correlation functional by J. Heyd, G. E. Scuseria, and M. Ernzerhof [J. Chem. Phys. 118, 8207 (2003)] (HSE) and the $\ensuremath{\Delta}$SCF method.

Quasiparticle energies and lifetimes in a metallic chain model of a tunnel junction.

As electronics devices scale to sub-10 nm lengths, the distinction between "device" and "electrodes" becomes blurred. Here, we study a simple model of a molecular tunnel junction, consisting of an atomic gold chain partitioned into left and right electrodes, and a central "molecule." Using a complex absorbing potential, we are able to reproduce the single-particle energy levels of the device region including a description of the effects of the semi-infinite electrodes. We then use the method of configuration interaction to explore the effect of correlations on the system's quasiparticle peaks. We find that when excitations on the leads are excluded, the device's highest occupied molecular orbital and lowest unoccupied molecular orbital quasiparticle states when including correlation are bracketed by their respective values in the Hartree-Fock (Koopmans) and ΔSCF approximations. In contrast, when excitations on the leads are included, the bracketing property no longer holds, and both the positions and the lifetimes of the quasiparticle levels change considerably, indicating that the combined effect of coupling and correlation is to alter the quasiparticle spectrum significantly relative to an isolated molecule.

The electronic transport properties for a single-wall ZnO nanotube with different coupling interfaces

The transport properties of a single-wall ZnO nanotube contacted with two Au (Al or Cu ) electrodes are investigated by a theoretical approach. Our results suggest the contact resistance for ZnO nanotube connected with Au electrodes is the largest one as compared with Al and Cu acting as electrodes. The local density of states (LDOS) near the ZnO nanotube/Cu(Al) interface shows the strong electronic interaction. Also shown is that for Au–ZnO system, we can observe a best rectifying performance, the next is the Al–ZnO system, and the third is Cu–ZnO system. This rectification is also fully rationalized by the calculated transmission spectra, the spatial distribution of the lowest unoccupied molecular orbital and highest occupied molecular orbital states, and the electrostatic potential distribution.

Impact of molecule-dipole orientation on energy level alignment at the submolecular scale

The molecular orientation--dependent electronic properties of monolayer dipolar molecule chloroaluminum phthalocyanine (ClAlPc) on Au(111) are investigated by ultraviolet photoemission spectroscopy and scanning tunneling microscopy. The relation between geometrical and electronic structures has been revealed in the binding energies of the highest occupied molecular orbital states and vacuum level (VL) shifts. Two molecular orientations, Cl-up- and Cl-down-oriented molecules, coexist in the as-grown monolayer ClAlPc films on Au(111) without the formation of staggered molecular pairs to cancel the dipoles and phase separation, as is the case on graphite. After annealing, only the Cl-up-oriented molecules remain on Au(111), as on graphite. Interestingly, an extraordinarily large VL shift of \ensuremath{-}0.89 eV is observed in the annealed monolayer ClAlPc film on Au(111), which is opposite to that of +0.46 eV on graphite even though the molecular dipoles are oriented similarly.

Excited States in Cycloparaphenylenes: Dependence of Optical Properties on Ring Length

Hoop-shaped conjugated molecules, cycloparaphenylenes (CPPs), are simple strings of benzene rings with para linkages that have an ideal quasi-one-dimensional structure without edges. Here, we report optical properties of [n]CPPs (n = 9, 12, 14, 15, 16) clarified by one- and two-photon excitation spectroscopy. We showed that in this system the lowest unoccupied molecular orbital (LUMO) state has the same symmetry as the highest occupied molecular orbital (HOMO) state, and determined the transition energy of the optically forbidden HOMO-LUMO gap. It is found that the ring-length dependence of the HOMO-LUMO transition energy is identical to that of the photoluminescence (PL) energy, and that phonon-assisted transition causes efficient PL.

Reactivity of Borylenes toward Ethyne, Ethene, and Methane

The electronic and geometric structure of various substituted borylenes BR (where R = H, F, Cl, Br, CH(3), Ph, NH(2), NHMe, and NMe(2)) in their lowest singlet and triplet electronic states was investigated by computational means using hybrid density functional (B3LYP) and second-order Møller-Plesset perturbation theories combined with 6-311+G** and cc-pVTZ basis sets. The reactivity of singlet borylenes towards prototypical saturated and unsaturated hydrocarbons was examined by the MP2 method in conjugation with the cc-pVTZ basis set and also by coupled cluster [CCSD(T)] computations in combination with the aug-cc-pVTZ basis set. To study the energetics and the mechanism of the addition reaction of borylenes to unsaturated CC bonds, ethyne and ethene are chosen as model compounds. The insertion reaction of borylene into a C-H bond of methane was also investigated. The addition reactions of borylenes to multiple C-C bonds are strongly exothermic. In case of the BH molecule the reactions proceed without barrier and are the most exothermic. For the insertion reaction of borylenes into methane, two approaches could be identified. Again, the smallest reaction barriers and highest reaction energies were computed for the BH insertion, while the highest barriers and the smallest exothermicities were obtained for the BF molecule. On the basis of frontier molecular orbital energies, barrier heights, reaction energies, and transition state geometries BH is the most electrophilic borylene, followed by BPh, while aminoborylenes and BF are the most nucleophilic ones among the investigated derivatives. Accordingly, reactions of BH have the smallest barriers (if there is one at all) and the largest reaction energies, while the reactions of BF have the highest barriers and the smallest reaction energies.