Lowest Unoccupied Molecular Orbital States Research Articles

Electronic and Structural Properties of Core-Shell Amino-Silica Nanoparticles: DFT And SCC-DFTB Calculation

Introduction: Silica nanoparticles (SNP) are extremely promising tools in nanotechnology and nano medicine. In most of applications such as capture and release of bacteriophage viruses the nano-structures of silica are coated by bio-compatible groups such as amine compounds. The presence of amino groups on the surface of the biosensors enables the installation of analyte receptors and antifouling agents such as oligo (ethylene oxide). Therefore, in this study, the electronic and structural properties of Core-Shell amino- Silica Nanoparticles are investigated.Material and Methods: In this investigation, we aim at obtaining the optimized structures and evaluate the geometries of the ground state for (SiO2) n (n=16, 20) nanoclusters. The electronic properties computed by density functional theory with GGA approximation and SCC-DFTB with hybrid Slater-Koster files are investigated and the effect of functionalization on such properties is discussed.Results: Solvolysis of studied structures is examined and it is shown that the highest occupied and lowest unoccupied molecular orbital states shift to obviously higher energy levels, which lead to more stable hydrogenated nanoclusters. The stability of nanoclusters rises by functionalization with amino and methylamine groups. Charge analysis of functionalized systems indicates the reactivity of nanoclusters. The results obtained in this paper are useful for chemical and biochemical applications of silica nanostructures.Conclusion: Results show that the length of amine hydrocarbon chain can control the electronic and magnetic properties of studied silica nanocluster (SNP) with different number of SiO2 unit. Pure ultra-small nanocluster shows the impressive spin splitting around the Fermi level, which is due to the spin splitting of outer silicon atoms. This feature of silica nanoclusters may be notable for applications in electronics.

Benzyl Alcohol Photo-oxidation Based on Molecular Electronic Transitions in Metal Halide Perovskites

Vacancy ordered double perovskites Cs2Te(IV)X6 have been found to exhibit molecule-like electronic behavior when X is Cl– or Br– due to the zero-dimensional (0D) nature of their octahedral units. Electronically isolated building blocks, the [TeBr6]2– ionic octahedron, serve as the fundamental electronic unit of the Cs2TeBr6 solid. Herein, a detailed understanding of the Cs2TeBr6 electronic structure and its photoexcitation is presented with consideration of individual molecular orbitals from these isolated octahedral building blocks. Two optical absorption features correspond to two unique electronic transitions, (1) a highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition under 455 nm excitation and (2) mixed transitions including lower HOMO states to LUMO transition and HOMO to higher LUMO states transition under 365 nm excitation. With this in mind, we examined the excitation wavelength-dependent photo-oxidation of benzyl alcohol using Cs2TeBr6 as the photocatalyst. Significant differences in photocatalytic performance were observed, and different forms of activated alcohol radicals were detected under the two excitation wavelengths. As a case study, this work highlights the application of molecule-like halide perovskites in photocatalysis. The highly tunable energy band structures and catalytic centers in perovskites can offer a valuable platform for photocatalytic mechanistic studies and catalyst development in the foreseeable future.

Electronic properties of DNA-related molecules containing a bromine atom

Purpose To clarify the radiosensitization mechanism masking the Auger effect of the cells possessing brominated DNA, the electronic properties of DNA-related molecules containing Br were investigated by X-ray spectroscopy and specific heat measurement. Materials and Methods X-ray absorption near-edge structure (XANES) and X-ray photoemission spectroscopy (XPS) were used to measure the electronic properties of the nucleotides with and without Br. We determined the specific heat of 5-bromouracil crystals with thymine as a reference molecule at low temperatures of 3–48 K to calculate the microscopic state numbers. Results Obtained XANES and XPS spectra indicated that both the lowest unoccupied molecular orbital (LUMO) and the core-levels were not affected by the Br incorporation. The state numbers of 5-bromouracil calculated from the specific heats obtained around 25 K was about 1.5 times larger than that for thymine below 20 K, although the numbers were almost the same below 5 K. Discussion Our results suggest that the Br atom may not contribute substantially to the LUMO and core-level electronic states of the molecule, but rather to the microscopic states related to the excitation of lattice vibrations, which may be involved in valence electronic states.

Comparing study of picene thin films on SnSe and Au(1 1 1) surfaces

Picene, as one representative of polycyclic hydrocarbons, represents an appealing application in hydrocarbon superconductors. Here, using scanning tunneling microscopy and spectroscopy combined with density functional theory calculations, we investigated the structural and electronic properties of picene thin films on p-type SnSe and Au(1 1 1) surfaces. For both substrates, at low coverage, the molecules present a flat-lying geometry, while at high coverage, the molecules exhibit alternating flat-lying and tilt-standing configurations. The diffusion of the molecules on SnSe and the splitting of lowest unoccupied molecular orbital state on Au(1 1 1) indicate that SnSe surface has a weaker interaction with the adsorbed molecules compared to other metallic substrate. Therefore, based on our study on structural and electronic properties, we conclude that a semiconductor substrate can maintain the intrinsic properties of picene films. The study of picene on different substrates may pave a way to unveil the structural and electronic properties of hydrocarbon superconductors on substrates.

Enhanced visible-light photocatalytic decomposition of malachite green over squaraines sensitized SnIn4S8 composites and effect of dye structure on photocatalytic behaviors

A series of indole squaraines (SQs) sensitized SnIn4S8 particles with the mass ratio of 1:10 were fabricated by a facile ultrasonic method. The obtained samples have been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, diffuse reflection spectroscopy, Fourier transforms infrared spectroscopy and Raman spectroscopy. The as-prepared squaraines/SnIn4S8 composites exhibited the excellent thermal stability. The efficient charge-transfer ability of squaraines was calculated by Gaussian 98 at the B3YP/6-311 + G(d,p) level. The highest occupied molecular orbital and the lowest unoccupied molecular orbital states had been obtained to match the band gap of pure SnIn4S8. The malachite green (MG) was used as the degradation dye to measure the photocatalytic performance of different SQs sensitized SnIn4S8 composites. The results showed that the sensitized composites exhibited significantly enhanced photocatalytic activities. Among them, The BSQ/SnIn4S8 composites showed the best photocatalytic performance whose degradation rate reached 97.91% under 40 min visible-light irradiation. The kapp value of BSQ/SnIn4S8 composites (0.0962 min−1) was 4.20 times larger than that of bare SnIn4S8 (0.0229 min−1). The kapp value of ISQ/SnIn4S8, MSQ/SnIn4S8, FSQ/SnIn4S8 and CSQ/SnIn4S8 was evaluated to be 0.0328, 0.0358, 0.0526 and 0.0192 min−1, respectively. According to the fluorescence spectra, BSQ/SnIn4S8 possessed the highest fluorescence quantum yield of 0.4621 within all nanocomposites. Furthermore, the photocatalytic mechanism of squaraines/SnIn4S8 composites was also proposed.

Thickness of AVA+ Controls the Direction of Charge Transfer at TiO2/PbI2 Interfaces

Theoretical density functional theory analysis of 5-ammonium valeric acid (AVA+, HOOC–(CH2)4–NH3+) resulted in self-limitation of the length of AVA+ chains and in a redistribution of the electron density in the lowest unoccupied molecular orbital states of TiO2 and PbI6 clusters depending on the number of AVA+ ions between the clusters. This behavior had strong consequences for the direction of charge transfer at TiO2/PbI2 interfaces as demonstrated experimentally by a combined study of infrared spectroscopic ellipsometry and modulated surface photovoltage spectroscopy.

Growth Behavior of Pristine and Potassium Doped Coronene Thin Films on Substrates with Tuned Coupling Strength.

The growth of polycyclic aromatic hydrocarbon (PAH) molecular coronene film on various substrates and the subsequent doping of potassium under ultrahigh vacuum (UHV) conditions have been systematically investigated by low-temperature scanning tunneling microscopy and spectroscopy (STM/STS). The crystalline structures and molecular orientations of coronene thin films are both thickness-dependent and substrate-sensitive due to the competition between molecule-substrate interaction and intermolecular interaction. In mono- or bilayer films, coronene molecules are flat-lying on the surface with hexagonal lattice, whereas in multilayer films, the topmost molecules are in a standing-up but tilted configuration with rectangular lattice. In particular, a 2 × 1 superstructure with respect to that of bulk coronene is formed on thick KCl film. Furthermore, we have studied the potassium doped coronene monolayer and multilayer on Ag(100) and KCl/Ag(100) surface. For K-doped coronene monolayer, at certain doping ratio x = 3, the lowest unoccupied molecular orbital (LUMO) of coronene film moves to the Fermi level, and a splitting of the LUMO state is observed. Increased potassium doping would result in a filled LUMO state below the Fermi level. By contrast, no well-ordered structures are obtained in the K-doped coronene multilayers which are vulnerable to rather moderate annealing processes owing to their relatively weak bonding with the supporting substrates, implying a big challenge of growth of PAH thick films in vacuum. The differences in the crystal structures of coronene thin films compared with that in bulk crystals might shed insight on the controversies in the experimental results on the electronic properties of alkali-metal-doped PAHs.

Measurements of HOMO-LUMO levels of poly(3-hexylthiophene) thin films by a simple electrochemical method

Though poly(3-hexylthiophene) (P3HT) is one of the most commonly used polymers in organic solar cells, a broad range of values, derived from cyclic voltammetry (CV), has been reported for the lowest unoccupied molecular orbital (LUMO) position (from −3.53 to −2.70 eV); contrastingly, the highest occupied molecular orbital (HOMO) position is reported in a narrow range (from −4.92 to −5.20 eV). As a consequence of this wide distribution for the LUMO position, most researchers choose to use electrochemical techniques for determining only the HOMO position, and estimate the LUMO position by adding the experimental optical band-gap value. Here, three different electrochemical strategies (CV, potentiostatic EIS, and potentiodynamic EIS) for obtaining the HOMO and LUMO position for P3HT films formed under ambient conditions on transparent conductive substrates (indium tin oxide (ITO) glass) are compared. The results are discussed in the frame of limitations of each technique. The cyclic voltammetric response and the data derived from potentiostatic EIS using electric equivalent circuits include the response of all processes involved in the measurements, particularly masking the LUMO response due to the presence of energetic states. In contrast, potentiodynamic EIS measured in a wide frequency range results in a more reliable approach since it allows discerning between middle-frequency-dependent processes associated with energetic states in the gap of the P3HT/ITO films, and low-frequency-dependent processes associated with filling and emptying of LUMO and HOMO states, respectively. It is a powerful and simple method to analyze the electronic structure of semiconductor organic thin films.

Competition of edge effects on the electronic properties and excitonic effects in short graphene nanoribbons

We explore the electronic properties and exciton effects in short graphene nanoribbons (SGNRs), which have two armchair edges and two zigzag edges. Our results show that both of these two types of edges have profound effects on the electronic properties and exciton effects. Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states are alternatively changed between the bulk and the edge states as the lengths of the zigzag edges increase, due to the competition between the states of the two types of edges. The energy gaps, as a function of the lengths of the armchair edges, will then induce two kinds of trends. Furthermore, two kinds of exciton energies and exciton binding energies are found, which can be understood through the two kinds of HOMO and LUMO states in SGNRs. In addition, we find that the three triplet exciton states are not totally energy degenerate in SGNRs due to the spin-polarized states on the zigzag edges.

Band gap renormalization of diamondoids: vibrational coupling and excitonic effects

We perform ab initio density functional theory calculations along with the frozen phonon approach to study the renormalization of the electronic structure of diamondoids induced by coupling to nuclear vibrations. We obtain an energy gap reduction between 0.1 and 0.3 eV across nine different diamondoids. The energy renormalization of the highest occupied molecular orbital state varies with the sizes and the symmetry of the diamondoids while that of the lowest unoccupied molecular orbital state is nearly constant. This behavior is explained using the localization of the electronic states. The energy gap renormalization is shown to originate mainly from bending C-H and rotation/shear H-C-H modes with frequencies between 800 and 1400 cm−1. We calculate the band gap renormalization due to excitonic effects using a screened configuration interaction approach and find excitonic binding energies of around 2 eV in excellent agreement with experiment for our largest diamondoids.

Single-Molecule Rotational Switch on a Dangling Bond Dimer Bearing.

One of the key challenges in the construction of atomic-scale circuits and molecular machines is to design molecular rotors and switches by controlling the linear or rotational movement of a molecule while preserving its intrinsic electronic properties. Here, we demonstrate both the continuous rotational switching and the controlled step-by-step single switching of a trinaphthylene molecule adsorbed on a dangling bond dimer created on a hydrogen-passivated Ge(001):H surface. The molecular switch is on-surface assembled when the covalent bonds between the molecule and the dangling bond dimer are controllably broken, and the molecule is attached to the dimer by long-range van der Waals interactions. In this configuration, the molecule retains its intrinsic electronic properties, as confirmed by combined scanning tunneling microscopy/spectroscopy (STM/STS) measurements, density functional theory calculations, and advanced STM image calculations. Continuous switching of the molecule is initiated by vibronic excitations when the electrons are tunneling through the lowest unoccupied molecular orbital state of the molecule. The switching path is a combination of a sliding and rotation motion over the dangling bond dimer pivot. By carefully selecting the STM conditions, control over discrete single switching events is also achieved. Combined with the ability to create dangling bond dimers with atomic precision, the controlled rotational molecular switch is expected to be a crucial building block for more complex surface atomic-scale devices.

Interfacial chemical reaction and multiple gap state formation on three layer cathode in organic light-emitting diode: Ca/BaF2/Alq3

A three layer cathode is a promising stack structure for long lifetime and high efficiency in organic light-emitting diodes. The interfacial chemical reactions and their effects on electronic structures for alkaline-earth metal (Ca, Ba)/Alq3 [tris(8-hydroxyquinolinato)aluminum] and Ca/BaF2/Alq3 are investigated using in-situ X-ray and ultraviolet photoelectron spectroscopy, as well as molecular model calculation. The BaF2 interlayer initially prevents direct contact between Alq3 and the reactive Ca metal, but it is dissociated into Ba and CaF2 by the addition of Ca. As the Ca thickness increases, the Ca penetrates the interlayer to directly participate in the reaction with the underlying Alq3. This series of chemical reactions takes place irrespective of the BaF2 buffer layer thickness as long as the Ca overlayer thickness is sufficient. The interface reaction between the alkaline-earth metal and Alq3 generates two energetically separated gap states in a sequential manner. This phenomenon is explained by step-by-step charge transfer from the alkaline-earth metal to the lowest unoccupied molecular orbital states of Alq3, forming new occupied states below the Fermi level.

Excitonic optical properties of wurtzite ZnS quantum dots under pressure.

By means of atomistic empirical pseudopotentials combined with a configuration interaction approach, we have studied the optical properties of wurtzite ZnS quantum dots in the presence of strong quantum confinement effects as a function of pressure. We find the pressure coefficients of quantum dots to be highly size-dependent and reduced by as much as 23% in comparison to the bulk value of 63 meV/GPa obtained from density functional theory calculations. The many-body excitonic effects on the quantum dot pressure coefficients are found to be marginal. The absolute gap deformation potential of quantum dots originates mainly from the energy change of the lowest unoccupied molecular orbital state. Finally, we find that the exciton spin-splitting increases nearly linearly as a function of applied pressure.

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.

Site-dependent electronic structures of a single molecule on a metal surface studied by scanning tunneling microscopy and spectroscopy

Single-molecule observation of the electronic structures of para-cyanobenzoate (pCB) adsorbed on Cu(110) has been performed using scanning tunneling microscopy (STM) and spectroscopy (STS). We found that pCB has two types of the adsorption site on Cu(110); i.e., two oxygen atoms of pCB are bridged between adjacent Cu atoms at the short- or long-bridge sites. STS and STS mapping revealed that the pCB adsorbed at the short-bridge site has a resonant peak at 2.0V above the Fermi level, which is assigned to the lowest unoccupied molecular orbital (LUMO) of pCB. However, the LUMO state is shifted toward lower voltage (1.2V) when the pCB molecule is adsorbed at the long-bridge site. The energy levels of the LUMO state, depending on the adsorption site of pCB, can thus be ascribed to the degree of the electronic interaction between pCB and the Cu substrate. The site transformation of pCB induced by the injection of tunneling electrons from the STM tip has also been presented.

Electronic structure, stacking energy, partial charge, and hydrogen bonding in four periodic B-DNA models.

We present a theoretical study of the electronic structure of four periodic B-DNA models labeled (AT)(10), (GC)(10), (AT)(5)(GC)(5), and (AT-GC)(5) where A denotes adenine, T denotes thymine, G denotes guanine, and C denotes cytosine. Each model has ten base pairs with Na counterions to neutralize the negative phosphate group in the backbone. The (AT)(5)(GC)(5) and (AT-GC)(5) models contain two and five AT-GC bilayers, respectively. When compared against the average of the two pure models, we estimate the AT-GC bilayer interaction energy to be 19.015 Kcal/mol, which is comparable to the hydrogen bonding energy between base pairs obtained from the literature. Our investigation shows that the stacking of base pairs plays a vital role in the electronic structure, relative stability, bonding, and distribution of partial charges in the DNA models. All four models show a highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gap ranging from 2.14 to 3.12 eV with HOMO states residing on the PO(4) + Na functional group and LUMO states originating from the bases. Our calculation implies that the electrical conductance of a DNA molecule should increase with increased base-pair mixing. Interatomic bonding effects in these models are investigated in detail by analyzing the distributions of the calculated bond order values for every pair of atoms in the four models including hydrogen bonding. The counterions significantly affect the gap width, the conductivity, and the distribution of partial charge on the DNA backbone. We also evaluate quantitatively the surface partial charge density on each functional group of the DNA models.

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.

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.

Low-dimensional nanostructures and fullerene films on semiconductor surface

The morphology and atomic structures of C60 fullerene films on a Bi(0001)/Si(111)-7 × 7 surface and adsorption of fluorofullerene C60Fx molecules on a Si(111)-7 × 7 surface have been studied by scanning tunneling microscopy/spectroscopy and low-energy electron microscopy under ultra high-vacuum conditions. It has been shown that initial nucleation of C60 islands on the surface of an epitaxial Bi film occurs on double steps and domain boundaries, while tunnel spectra do not exhibit any significant charge transfer to the lowest unoccupied molecular orbital states. Fluorofullerene molecules allow local (at the nanoscale level) modification of Si surface through local etching.