Highest Occupied Molecular Orbital Band Research Articles

Hole-injection barrier across the intermolecular interaction mediated interfacial DNTT layer

The electronic structures and morphology of dinaphthothienothiophene (DNTT) thin films on highly oriented pyrolytic graphite (HOPG) and polycrystalline Au (pc-Au) substrates have been investigated using photoelectron spectroscopy and atomic force microscopy techniques to better correlate the charge-injection barrier with the interfacial structure. DNTT molecules follow Stranski–Krastanov-type growth mode, i.e., highly-dewetted islands/fibers above a relatively-wetted interfacial layer, on both the substrates, but distinctively different electronic structures. On the HOPG surface, the as-grown and thermal-annealed films show a broad and a sharp highest occupied molecular orbital (HOMO) bands, respectively. On the pc-Au surface, a significantly split HOMO band and a reduced hole-injection barrier are observed in a monolayer thick film, for the first time, which are absent in other thick films. Such splitting indicates a presence of a strong intermolecular interaction in the interfacial layer, presumably due to the frontier π-orbital overlapping arising from partially-covered densely-packed herringbone-like arranged molecules. The dewetted-fibers seem to form with up-right oriented molecules having negligible interaction, which desorb easily by thermal annealing to retain the highly-stable interfacial layer. The reduction of the hole-injection barrier at the DNTT/pc-Au interface, due to the formation of such interfacial layer having strong intermolecular interaction, has an immense importance in increasing the charge-injection efficiency from a practical electrode.

Surface plasmon-induced photodegradation of methylene blue with single layer graphene quantum dots/Au nanospheres under visible-light irradiation

Single-layer graphene quantum dots and gold (SLGQDs/Au) nanospheres composite was synthesized by hydrothermal and photochemical methods. Single-layer graphene quantum dot was synthesized by glucose precursor via hydrothermal method and SLGQDs/Au nanocomposite was prepared by adding HAuCl4 under UV irradiation. Characterization of sample was performed using XRD, Raman, FESEM, UV–vis spectroscopy and PL analysis. The quantum yield of SLGQDs/Au composite was calculated. The role of surface plasmon resonance (SPR) of gold nanoparticles in quantum yield reduction and the effect of quantum yield on photodegradation was discussed. Using the optimal sample, a photocatalyst test was performed on methylene blue, which near 90% photocatalytic degradation was achieved under visible light irradiation in 120 min. Compared to bare GQD the high photocatalytic efficiency of composite could be due to the presence of SPR level of Au nanoparticles between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of SLGQDs. Under light irradiation, SLGQDs and Au nanoparticles absorb visible light energy. By absorbing the photons, the electrons of SLGQDS could be excited to LUMO band and leave holes in HOMO band. Spontaneously the emitted photons excite the Fermi level electrons of Au nanoparticles to the SPR state. Created SPR level can suppress electron-holes recombination. Then the photo-exited electrons react with dissolved oxygen to generate ° O2− and ° OH radicals meanwhile, the holes could oxide OH− into OH°. Created radicals which are strong oxidants degrade the pollutants.

Influence of interfacial trap states on injecting and extracting of charges across a metal–organic interface

The processes of injecting and extracting holes across a metal–organic interface were examined. The height and the width of the corresponding Schottky barriers existing at the metal–organic interfaces were determined from the current–voltage characteristics using the Simmons approximation. The measured temperature dependence of the I–V characteristics suggest that the injection process occurred in two steps. Charge carriers were initially injected into interfacial trap states, from where they eventually hopped to the transport states of the highest occupied molecular orbital (HOMO) band of the organic p-semiconductor. Such a two-step injection process was dominant at low voltages. For large voltages, where the Fermi level of the metal electrode was located near the HOMO level, injection is dominated by direct tunneling of holes from metal to the HOMO level. The voltage ranges, for which either a two-step process or direct tunneling dominated, depended on the height and width of the Schottky barrier, but also on the depth and distribution of the interfacial trap states. For the extraction of holes from organic to metal, an increase of the extracting resistance with increasing voltage could be observed at low voltages, which is identified as a consequence of the filling of the interfacial trap states with holes from the HOMO band. For large voltages, holes were extracted from the HOMO to the Fermi level of gold in two ways: (a) in a direct transition and (b) in a two-step transition involving trap states at the interface.

Charge Transport through Methylated DNA Strand

ABSTRACTCharge transport through an eight-base pair methylated DNA strand and its native counterpart have been investigated. We focus on three factors, contact coupling, decoherence and temperature, which can contribute to DNA charge transport. Our results show that with the same choice of contact coupling, in the phase-coherent limit the transmission of the methylated strand is smaller in the bandgap at energies close to the highest occupied molecular orbital (HOMO), while inside the HOMO band, the transmission is oscillatory and the methylated DNA may have a larger transmission in certain energy windows. The trend in transmission also holds in the presence of the decoherence though there is a crossover in the transmission of the native and methylated strands away from the HOMO level. We also find that the transport depends on the strength of contact coupling and the measurement temperature.

Low-density band-gap states in pentacene thin films probed with ultrahigh-sensitivity ultraviolet photoelectron spectroscopy

We detected a very low density of electronic states in the band gap of a nondoped pentacene thin film by using ultraviolet photoelectron spectroscopy with ultrahigh sensitivity and ultralow background. The gap states, which may originate from the highest occupied molecular orbital (HOMO) state in imperfect molecular packing regions, are distributed exponentially up to the Fermi level and control the Fermi level relative to the HOMO band.

HOMO band structure and anisotropic effective hole mass in thin crystalline pentacene films

The band dispersion of the two highest occupied molecular orbital (HOMO)-derived bands in thin crystalline pentacene films grown on Bi(001) was determined by photoemission spectroscopy. Compared to first-principles calculations, our data show a significantly smaller bandwidth and a much larger band separation, indicating that the molecular interactions are weaker than predicted by theory---a direct contradiction to previous reports by H. Kakuta et al. [Phys. Rev. Lett. 98, 247601 (2007)]. The effective hole mass ${\mathbit{m}}^{\ensuremath{\ast}}$ at $\overline{\text{M}}$ is found to be anisotropic and larger than theoretically predicted. Comparison of ${\mathbit{m}}^{\ensuremath{\ast}}$ to field effect mobility measurements shows that the band structure has a strong influence on the mobility even at room temperature.

The electronic structure of pentacene revisited

Recently, there have been reports of the valence band photoemission of pentacene films grown on various substrates with particular emphasis on the highest occupied molecular orbital (HOMO) and its dispersion. In various works, evidence for HOMO band dispersion as high as 0.5 eV, even for polycrystalline films, has been presented. In apparent contradiction to these results, we have previously reported a band dispersion of only 50 meV, measured on a well characterised film with a single polymorph and single crystalline orientation, 5A(0 2 2). Here, we first present the two-dimensional momentum distribution of the HOMO of a 5A(0 2 2) film. Then the development of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of the two-dimensional momentum distribution of the HOMO, we show that a reasonably large dispersion (0.25 eV) does indeed exist in 5A(0 2 2).

Defect-induced states in the electronic structure of a Cu(100)-benzenethiolate-pentacene heterostructure

We report on the influence of defects in the electronic properties of a pentacene single-layer grown on a benzenethiolate self-assembled-monolayer (Bt-SAM) adsorbed on Cu(100) at 300 K, using high-resolution ultraviolet photoelectron spectroscopy. The weak interaction between pentacene and the underlying Bt-SAM is deduced by the lineshape and evolution of the highest-occupied molecular orbital (HOMO). Closer inspection of the pentacene HOMO band reveals a skewed feature, where the contribution in the low binding energy side is found originating from states associated to structural defects that can be reduced by controlled mild-temperature annealing (355 K). Presence of such defects influences also the pentacene HOMO onset energy and hole-injection barrier that can be reduced to typical value of this organic heterostructure after the annealing.

Fine structure of the highest occupied band in OTi-phthalocyanine monolayer

Ultraviolet photoelectron spectra were measured for ultrathin films of OTi-phthalocyanine (OTiPc), which has an electric dipole perpendicular to the molecular plane, prepared on highly oriented pyrolytic graphite in order to study effects of the molecular orientation and the electric dipole layer on the organic electronic states. For the as-grown films, the observed highest occupied molecular orbital (HOMO) band consists of two prominent peaks that can be assigned to different molecular orientations. For the oriented monolayer obtained by annealing the as-grown film, we detected a very sharp HOMO band at 290 K. The binding-energy ( E B) difference between the HOMO bands of the as-grown and annealed films was found to agree with the shift in the vacuum level. For the oriented monolayer, the observed sharp HOMO band involves at least four components that are ascribed to the coupling between the HOMO hole and the molecular vibration. Upon cooling the sample to 95 K, the HOMO bandwidth became sharper than that at 290 K. From the peak fitting using Voigt function, additional components are expected in the HOMO band at 95 K. Moreover, we detected the E B shift in the HOMO band for the oriented monolayer upon cooling, which can be originated from decrease in the HOMO-hole screening due to the change in the film structure and/or the molecule-substrate interaction.

Energy band dispersion in well ordered N,N′-dimethyl-3,4,9,10-perylenetetracarboxylic diimide films

The electronic properties of well ordered N,N′-dimethyl-3,4,9,10-perylenetetracarboxylic dimide (DiMe-PTCDI) films prepared on sulfur passivated GaAs(001) substrates were studied by means of photoemission spectroscopy. From the photon energy dependence of normal emission spectra an intermolecular energy band dispersion of about 0.2eV was determined for the highest occupied molecular orbital (HOMO). Simulation of the density of states reveals that the HOMO band has a single π -character. The observed energy band dispersion thus originates from the intermolecular π-π interaction and is modeled using the tight binding model. The analysis provides a value of 0.04eV for the transfer integral. The inner potential was treated as a fitting parameter such that the expected periodicity of the dispersion in the reciprocal space was obtained.

Structural, Electrical, and Magnetic Properties of α-(ET)7[MnCl4]2·(1,1,2-C2H3Cl3)2 (ET = Bis(ethylenedithio)tetrathiafulvalene)

Abstract A new charge-transfer salt of ET with a chloromanganate(II) complex anion has been synthesized and characterized by X-ray structural analysis, resistivity measurements, magnetic susceptibility, electron spin resonance (ESR), and an extended Hückel tight biding (EHTB) band calculation. The crystal has a sheet structure comprised of an α-type two-dimensional (2D) donor arrangement in the bc-plane and insulating sheets of discrete [MnCl4]2− anions and 1,1,2-C2H3Cl3 (TCE) molecules. Its conducting property exhibits considerable anisotropy, which is of effectively metallic along the b-axis down to 1.2 K under 2.9 kbar and higher pressure. The magnetic susceptibility is approximately reproduced by the Curie–Weiss law with a Weiss temperature of θ = −(1.35 ± 0.07) K from 2–300 K. ESR measurements revealed that the π-electron system in this salt exhibits Pauli paramagnetism at least at 3.6 to ∼50 K. The band calculation suggests that the HOMO (the highest occupied molecular orbital) band has very small dispersion, almost solely along the b*-axis.

HOMO-band fine structure of OTi- and Pb-phthalocyanine ultrathin films: effects of the electric dipole layer

Ultraviolet photoelectron spectra were measured for titanyl- and lead-phthalocyanine ultrathin films prepared on graphite in order to study effects of the electric dipole layer on the organic energy levels. Each of these molecules has an electric dipole perpendicular to the molecular plane, and hence a well-defined electric dipole layer could be intentionally prepared by using oriented monolayer of these molecules. For as-grown films, the observed highest occupied molecular orbital (HOMO) band consists of many peaks that could be assigned to different molecular orientations/aggregations. For well-oriented monolayer films obtained by annealing the as-grown films, we observed a very sharp HOMO band with two satellites for both molecules as for copper phthalocyanine. Difference of binding energy of HOMO bands between the oriented monolayer and the doublelayer in which molecular electric dipoles are cancelled was found to agree with the vacuum level shift for both molecules, leading to important conclusions that (1) the molecular energy level with respect to the substrate Fermi level is changed when the molecule is in the dipole layer and (2) the binding-energy shift corresponds with the vacuum level shift.

Very narrow photoemission bandwidth of the highest occupied state in a copper-phthalocyanine monolayer

We observed a very narrow bandwidth of the highest occupied molecular orbital (HOMO) state in ultraviolet photoemission spectra (UPS) of copper-phthalocyanine monolayer deposited on graphite. The HOMO band in UPS consists of three components which may originate from the vibrational coupling. The full width at half maximum of each component was found to be ∼150 meV at 295 K. This HOMO-bandwidth leads to an estimation that the lifetime of the HOMO hole should be at least longer than 2.2 fs, which may be dominated by the electron transfer rate from the substrate to the molecule.

Pressure induced structural change in PbPc studied by infrared and UV–visible spectroscopy and theoretical calculation

Lead phthalocyanine (PbPc) has a non-planar ‘shuttle-cock’ structure with a C 4 v molecular symmetry and forms a one-dimensional column in the crystal. We measured infrared and UV–visible spectra for the PbPc crystal under high hydrostatic pressure by using a diamond anvil cell. The IR spectrum of PbPc shows three strong peaks in the 1000–1200 cm −1 region. With increasing pressure, the intensity ratio of the middle peak to the other two peaks increased. This result suggests a structural transformation of the PbPc molecule from the shuttle-cock structure toward the planar structure with increasing pressure. In the UV–visible spectra, two remarkable changes were observed under high pressure: the peak intensity of the band at 2.7 eV was decreased, and the band at 1.5 eV was shifted to lower energy and broadened. The former feature suggests that the highest occupied molecular orbital (HOMO) band is not filled perfectly in the solid-state of PbPc under ambient pressure, and that the filling of the HOMO band occurs with increasing pressure. The change on the low energy band at 1.5 eV due to increasing pressure can be attributed to an increase in the intermolecular interaction.

Photoemission study of K- and Sc-doped C84

The electronic structure of solid C84 with doping of K or Sc metal was studied by ultraviolet photoemission spectroscopy. Successive filling of the LUMO (lowest unoccupied molecular orbital), LUMO+1, and LUMO+2 bands by electrons donated from doped K atoms was clearly observed in K-doped C84. In the case of Sc-doped one, no clear successive filling of unoccupied bands was observed although new electronic states appear between the HOMO (highest occupied molecular orbital) band and the Fermi level with doping. In either of K- and Sc-doped solid C84, no clear Fermi-edge cutoff indicative of its metallic nature was observed in the spectrum.

Photoemission study of K- and Sc-doped C 84

The electronic structure of solid C 84 with doping of K or Sc metal was studied by ultraviolet photoemission spectroscopy. Successive filling of the LUMO (lowest unoccupied molecular orbital), LUMO+1, and LUMO+2 bands by electrons donated from doped K atoms was clearly observed in K-doped C 84. In the case of Sc-doped one, no clear successive filling of unoccupied bands was observed although new electronic states appear between the HOMO (highest occupied molecular orbital) band and the Fermi level with doping. In either of K- and Sc-doped solid C 84, no clear Fermi-edge cutoff indicative of its metallic nature was observed in the spectrum.

Threshold Lowering and by Intensity and Efficiency Enhancement by Dopants in Polymer Emitting Diodes

A new method to increase the luminance and quantum efficiency of polymer light emitting diodes with a lower threshold voltage has been reported. The threshold voltage, luminance and quantum efficiency have been significantly improved by doping certain dopants with a lower highest occupied molecular orbital (HOMO) level into the hole transporting layer. A high performance device has been achieved by addition of the perylene and triphenylamine as a dopant into poly(N-vinylcarbazole). The luminance and quantum efficiency increase by 2-3 times in comparison with the undoped device, reaching 10000 cd/m2 in luminance and 0.58% in quantum efficiency, while threshold voltage is reduced to one half value. The energy diagram has been obtained by measuring the HOMO levels and band gap values. Based on this, the carriers injection and balance between electrons and holes as well as the action of dopant are discussed.