Search

A series of n-cycloparaphenylenes ([n]CPP, n = 8, 9, and 12) were studied by ultraviolet photoemission, inverse photoemission, ultraviolet-visible absorption, and X-ray photoemission spectroscopy to detect their unique electronic structures. [n]CPP has a cyclic structure in which both ends of n-poly(p-phenylene)s (nP) are connected. The molecular size dependence of the HOMO-LUMO gap of [n]CPP was investigated by direct observation and was found to increase as the molecular size increased. This trend is opposite to that of typical π-conjugated systems. Highly strained molecular structures, especially of small [n]CPPs, significantly impact their electronic structure. Insights into the electronic structure of [n]CPP obtained here will aid the design of electronic functionality of non-planar π-conjugation systems.

Room-temperature ionic liquids (RTILs) are being increasingly employed as novel solvents in several fields, including chemical engineering, electrochemistry, and synthetic chemistry. To further increase their usage potential, a better understanding of the structure of their surface layer is essential. Bi-layering at the surfaces of RTILs consisting of 1-alkyl-3-methylimidazolium ([Cnmim]+; n = 4, 6, 8, 10, and 12) cations and bis(trifluoromethanesulfonyl)amide ([TFSA]-) anions was demonstrated via infrared-visible sum-frequency generation (IV-SFG) vibrational spectroscopy and molecular dynamics (MD) simulations. It was found that the sum-frequency (SF) signal from the [TFSA]- anions decreases as the alkyl chain length increases, whereas the SF signal from the r+ mode (the terminal CH3 group) of the [Cnmim]+ cations is almost the same regardless of chain length. MD simulations show the formation of a bi-layered structure consisting of the outermost first layer and a submerged second layer in a "head-to-head" molecular arrangement. The decrease in the SF signals of the normal modes of the [TFSA]- anions is caused by destructive and out-of-phase interference of vibrations of corresponding molecular moieties oriented toward each other in the first and second layers. In contrast, the r+ mode of [Cnmim]+ does not experience destructive interference because the peak position of the r+ mode differs marginally at the surface and in the bulk. Our conclusions are not limited to the system presented here. Similar bi-layered structures can be expected for the surfaces of conventional RTILs, which necessitates the consideration of bi-layering in the design and application.

In our recent paper titled "Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- and molecular dynamics simulation-based study" co-authored by T. Iwahashi, T. Ishiyama, Y. Sakai, A. Morita, D. Kim, and Y. Ouchi, Phys. Chem. Chem. Phys., 2020, 22, 12565 (hereafter referred to as IW), the sum-frequency (SF) spectra for a homologous series of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Cnmim][TFSA] n = 4, 6, 8, 10, and 12) were reported. In particular, a clear decrease in the SF signals from the [TFSA]- anions with increasing chain length of the [Cnmim]+ cation (Fig. 5 of IW) was explained in terms of "head-to-head" bi-layer formation at the air/ionic liquid (IL) interface. A comment by M. Deutsch et al. (hereafter referred to as DE) questioned this report, claiming that our proposed structure is not consistent with a multilayered electron density (ED) profile obtained by X-ray reflectivity (XR).

The effect of Li+ addition at the interface of a 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide ([C2mim][FSA]) room-temperature ionic liquid (RTIL) and a Pt electrode is investigated by infrared-visible sum-frequency generation (IV-SFG) vibrational spectroscopy. Addition of Li+ to the Pt|[C2mim][FSA] system results in the extension of the electrochemical window (EW) by >1.0 V at its negative edge. The potential dependence of the SF signal reveals that the [FSA]− anion of neat [C2mim][FSA] is desorbed at −1.5 V while it remains in place even at −2.0 V when Li+ is added. The SFG spectra indicate that the [FSA]− anion at the Pt|[C2mim][FSA] interface interacts with Li+ at the interface with the negatively-charged Pt electrode. This [FSA]− anion layer anchored through Li+ suppresses [C2mim]+ cation adsorption on the negatively-charged Pt electrode, resulting in a wider electrochemical window. Keywords: IR-visible sum-frequency generation vibrational spectroscopy, Ionic liquid, RTIL, Electrical double layer

The potential-dependent interfacial structures of Pt/tetraglyme-lithium bis(trifluoromethylsulfonyl) amide ((Pt)/G4-Li[TFSA]) systems were assessed with and without 3-(1-butyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate ([Bimps]) zwitterions, to understand the impact of [Bimps] on the enhancement of the electrochemical stability of G4-Li[TFSA] electrolytes at Pt electrode surfaces. The aforementioned interfacial structures were investigated using in-situ infrared-visible sum frequency generation (IV-SFG) spectroscopy. The results of the linear sweep voltammetry (LSV) revealed that the oxidation limit of G4-Li[TFSA] at the Pt electrode shifted to a higher potential when [Bimps] was introduced, thus indicating that the electrochemical window is extended in the G4-Li[TFSA]-[Bimps] system. The SFG spectra of the G4-Li[TFSA] system confirmed that free G4 molecules and Li+-G4 complex cations are dominantly adsorbed on the Pt surface at 1 V (vs. Ag/Ag+). In contrast, the addition of [Bimps] to the G4-Li[TFSA] system resulted in almost full coverage of [Bimps] at the Pt surface by forming Li+-[Bimps] complex cations, even at 1 V (vs. Ag/Ag+), and no free G4 molecules were observed. The ab initio calculation indicated that the highest occupied molecular orbital (HOMO) energy level of free G4 was higher than that of the Li+-[Bimps] complex cation. This suggests that the Li+-[Bimps] complex was more stable for oxidation than the free G4, which accounts for the enhanced electrochemical stability of G4-Li[TFSA] system with [Bimps] additives.

IR-visible sum-frequency generation (IV-SFG) vibrational spectroscopy and a molecular dynamics (MD) simulation were used to study the local layering order at the interface of 1-butanol-d9 and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), a room-temperature ionic liquid (RTIL). The presence of a local non-polar layer at the interface of the two polar liquids was successfully demonstrated. In the SFG spectra of 1-butanol-d9, we observed significant reduction and enhancement in the strength of the CD3 symmetric stretching (r(+)) mode and the antisymmetric stretching (r(-)) mode peaks, respectively. The results can be well explained by the presence of an oppositely oriented quasi-bilayer structure of butanol molecules, where the bottom layer is strongly bound by hydrogen-bonding with the PF6(-) anion. MD simulations reveal that the hydrogen-bonding of butanol with the PF6(-) anion causes the preferential orientation of the butanols; the restriction on the rotational distribution of the terminal methyl group along their C3 axis enhances the r(-) mode. As for the [bmim](+) cations, the SFG spectra taken within the CH stretch region indicate that the butyl chain of [bmim](+) points away from the bulk RTIL phase to the butanol phase at the interface. Combining the SFG spectroscopy and MD simulation results, we propose an interfacial model structure of layering, in which the butyl chains of the butanol molecules form a non-polar interfacial layer with the butyl chains of the [bmim](+) cations at the interface.

In the present study, surface amino-functionalized silica nanofibers (f-SiO2NFs, average diameter = 400 and 1000 nm) are used as one-dimensional (1-D) fillers of ionic liquid (IL)-based quasisolid electrolytes. On adding f-SiO2NFs to an IL (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, EMITFSA) containing lithium bis(trifluoromethanesulfonyl)-amide (LiTFSA), the well-dispersed 1-D nanofillers easily form a three-dimensional network structure in the IL, function as physical cross-linkers, and increase the viscosity of the composites, consequently providing a quasisolid state at a 3.5 wt % fraction of the NFs. Rheological measurements demonstrate that the prepared composites exhibit “gel-like” characteristics at 40–150 °C. All prepared composites show high ionic conductivities, on the order of 10–3 S cm–1, around room temperature. To investigate the additive effect of f-SiO2NFs in the composites, the lithium transference numbers are also evaluated. It is found that thinner NFs enhance the transference numbers of the composites. In addition, quasisolid lithium-ion cells containing the prepared composites demonstrate relatively high rate characteristics and good cycling performance at high temperature (125 °C).

Nanosized antiferromagnets show anomalously large magnetization and superparamagnetism, having complicated magnetic properties due to a competition between core and surface spins in addition to the surface-induced effect, the finite size effect, and the magnetic moment distribution. A significant distribution of magnetic moments, which are responsible for the superparamagnetism, can also exist in real ensembles of nanoparticles, making the analysis of magnetization difficult. It has been a key issue for the understanding of the complicated magnetic property to correctly separate a superparamagnetic component from an antiferromagnetic component that linearly depends on magnetic fields in the observed magnetization. Here, we report the separation of these two components observed in the magnetization of antiferromagnetic NiO nanoparticles by applying the scaling law in the magnetization of superparamagnets. The distribution of the magnetic moment was taken into account without a priori assumption of the distribution function. The magnetic moments causing superparamagnetism had a bimodal distribution with different mean values. The contribution of the two different superparamagnetic components could thus be newly clarified and was ascribed to the uncompensated spins in the core and on the surface. The magnetic structure that leads to the bimodal distribution is discussed in light of the inhomogeneity of the surface structure.

Imidazolium-based ionic liquids having different anions 1-butyl-3-methylimidazolium ([BMIM]X: X = Cl(-), Br(-), I(-), and BF4(-)) and their aqueous mixtures were investigated by IR absorption and proton NMR spectroscopy. The IR spectra of these ionic liquids in the CHx stretching region differed substantially, especially for C-H bonds in the imidazolium ring, and the NMR chemical shifts of protons in the imidazolium ring also varied markedly for ILs having different anions. Upon the introduction of water to screen the electrostatic forces and separate the ions, both IR and NMR spectra of [BMIM]X (X = Cl(-), Br(-), I(-)) showed significant changes, while those of [BMIM]BF4 did not change appreciably. H-D isotopic exchange rates of C(2)-H in [BMIM]X-D2O mixtures exhibited an order: C(2)-HClC(2)-HBrC(2)-HI, while the C(2)-H of [BMIM]BF4 was not deuterated at all. These experimental findings, supported by DFT calculations, lead to the microscopic bulk configurations in which the anions and the protons of the cations in the halide ionic liquids have specific, hydrogen-bond type of interaction, while the BF4(-) anion does not participate in the specific interaction, but interacts less specifically by positioning itself more above the ring plane of the imidazolium cation. This structural change dictated by the anion type will work as a key element to build the structure-property relationship of ionic liquids.

Introduction Room temperature ionic liquids (RTILs) are liquid salts, which are generally composed of a pair of cation and anion. Due to their novel physicochemical properties, RTILs have attracted much interest over the past decade in basic and applied studies. Applications of RTILs in electrochemical devices, such as electrochemical cell, lithium ion battery, solar cell, have been reported, and improved performance as well as high safety upon the employment of RTILs is highly expected. Since electrochemical processes happen on interface, the interfacial properties of electrode/RTILs significantly affect the performance of the related devices. Thus, the knowledge of the electrode/RTILs interface is the key to improve their functional performance. In aqueous solutions the interfacial structure on electrode is generally explained by the Gouy-Chapman-Sterns (GCS) double layer model. However, since RTILs contain of ions only, the GCS model cannot explain the interfacial structure of electrode/RTILs. In recent years, various experimental techniques including differential capacitance measurement, surface vibrational spectroscopy, STM as well as theoretical simulations have been employed to study the electrode/RTILs interface. Herein, we report a combined study of in situ IRvisible sum frequency generation (SFG) spectroscopy with differential capacitance-electrode potential (C-E) curve measurements on Pt/RTILs interface. At electrode/RTILs interface, SFG signal is only generated in a noncentrosymmetric environment, i.e. the double-layer region because the symmetry is broken due to specific adsorption and electrostatic interaction, while bulk phase is SFG inactive. On the other hand, C-E curve significantly reflects the double layer structure in terms of bulk properties of ions layering. Potential dependent SFG signal of both the cation and the anion in the whole electrochemical window region have been observed. Significant hysteresis effect has been observed in both the SFG spectra and the C-E curve, indicating a unique property on the Pt/RTILs interface.

We investigated the electronic structure of spin-coated films of two soluble fullerenes; [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) and its bis-adduct (bis-PCBM) using ultraviolet photoelectron spectroscopy, inverse photoemission spectroscopy and molecular orbital calculations. The ionization energy and electron affinity of spin-coated films of bis-PCBM were determined to be 6.01 eV and 3.4 eV, respectively. Analysis of electron density suggested the stronger electron donation from the two side chains to fullerene-backbone in a bis-PCBM molecule, compared with PCBM. The electron donation raises the energies of the frontier orbitals of bis-PCBM, which mainly consist of π-orbitals of fullerene-backbone. As a result, the ionization energy and electron affinity of bis-PCBM are smaller than those of PCBM. Moreover, we also concluded that the larger open circuit voltage observed for bis-PCBM based organic photovoltaics was explained by the higher-lying unoccupied molecular orbital of bis-PCBM.

Room temperature ionic liquids having 1-butyl-3-methylimidazolium ([BMIM]) cations and three different anions ([BMIM][PF 6 ], [BMIM][BF 4 ], [BMIM]Br) were studied by using Optical Heterodyne Detected-Optical Kerr Effect (OHD-OKE) spectroscopy to find out the effect of the anion size on the intermolecular dynamics. Fourier transformation of the time-domain OHD-OKE signals yielded a broad band in the spectra below 150 cm − 1 . The modes with frequencies around 15, 80 and 100 cm − 1 used to fit the spectra all shifted to higher frequencies with the decrease in the anion size due to changes in the intermolecular force and the reduced mass.

This study examined the acoustic phonon mode of ionic liquids consisting of 1-alkyl-3-methyl-imidazolium family (CnMIM) cations with n values ranging from 2 to 10 and bis(trifluoromethylsulfonyl)amide (TFSA) anion in the temperature range from 300 K to 100 K. [CnMIM]+[TFSA]− showed depolarized (VH) components of Brillouin peaks at temperatures below the glass transition temperature when n is larger than 4. On the other hand, in the case of ionic liquids with different anions, such as [C4MIM]+[BF4]−, [C4MIM]+[PF6]− and [C8MIM]+[BF4]−, the VH component of Brillouin peaks was not observed in the temperature range investigated. The dielectric loss spectra showed that the temperature dependence of alkyl chain domain relaxation of all ionic liquids followed the Arrhenius law and showed an increase in activation energy at the temperature where the VH component of Brillouin peak appeared. These results suggest that the observed depolarized component of Brillouin peak might originate from uniquely induced polarization in the 2nd domain composed of head groups of cations and anions.

We present an investigation of the interface between organic semiconductor films and metal substrates (organic/metal interface) using photoelectron yield spectroscopy (PYS) as the probing technique. PYS studies were conducted on the pentacene/Au, copper phthalocyanine (CuPc)/Au, and perfluorinated zinc phthalocyanine (F16ZnPc)/Au, and the results were compared with literature results obtained using conventional ultraviolet photoemission spectroscopy (UPS). PYS is advantageous for probing the electronic structure of the organic/metal interface because of the relatively long mean free path of photoexcited electrons with very low kinetic energy in PYS, which enables the detection of the photoelectrons from the metal substrate buried deep in the organic film. We demonstrate herein that the use of PYS reduces the significance of the final state effect of the electronic density surrounding the photohole at the organic molecule generated after the photoemission; this effect is known as the electric polarization effect. Although this effect has a significant influence on the results obtained using conventional UPS, the reduced influence of the final state effect in PYS makes it possible to construct an energy level diagram at the organic/metal interface with greater accuracy than can be achieved with UPS. In addition, a novel mechanism of the photoelectron detection for PYS enables us to apply PYS to very thick organic films, and therefore, PYS provides a reliable value of ionization energy for organic films without the influence of the substrate. Because the interface electronic structure has a significant influence on the carrier injection properties of organic devices, the increased reliability of the information obtained by PYS will render it very useful for the improvement of device performance as well for understanding their operation principles.

The interfaces of water/room temperature ionic liquids (RTIL) (1-alkyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide ([C(n)mim]TFSA): n = 4, 8) are investigated by infrared-visible sum frequency generation (IV-SFG) vibrational spectroscopy and molecular dynamics (MD) simulation. SFG spectra taken within the SO stretch region drastically differ between air/RTIL and water/RTIL interfaces. When a RTIL surface is in contact with water, a broadened and blue-shifted SO2-ss mode peak is observed in the SFG spectra, indicating an inhomogeneous intermolecular interaction due to hydrogen bonding of the [TFSA]- anions and water molecules at the water/[C(n)mim]TFSA interface. MD simulations show the SO2 groups of the anion are preferentially orientated toward the water phase, which is consistent with the SFG spectral features. Polar orientation of the [TFSA] anion originates from the ordered structure of the alkyl chains of [C(n)mim]+ cations.

How annealing influences the morphology of a highly regioregular poly(3-hexylthiophene) (RR-P3HT) film at the substrate interface as well as the lateral inhomogeneity in the electronic structure of the film are elucidated. Whereas previous studies have reported that high-molecular-weight (MW) RR-P3HT films tend to show low crystallinity even after annealing, it is found that high-MW RR-P3HT does show high crystallinity after annealing at high temperature for a long time. Photoemission electron microscopy (PEEM), X-ray photoemission spectroscopy, and ultraviolet photoemission spectroscopy results clearly resolve a considerable lateral inhomogeneity in the morphology of RR-P3HT film, which results in a variation of the electronic structure depending on the local crystallinity. The PEEM results show how annealing facilitates crystal growth in a high-MW RR-P3HT film.

Molybdenum oxide (MoOx) is widely used as the hole-transport layer in bulk-heterojunction organic photovoltaics (BHJ-OPVs). During the fabrication of solution-processed BHJ-OPVs on vacuum-deposited MoOx film, the film must be exposed to N2 atmosphere in a glove box, where the donor/acceptor blends are spin-coated from a mixed solution. Employing photoelectron spectroscopy, we reveal that the exposure of the MoOx film to such atmosphere contaminates the MoOx surface. Annealing the contaminated MoOx film at 160 °C for 5 min, prior to spin-coating the blend film, can partially remove the carbon and oxygen adsorbed on the MoOx surface during the exposure of MoOx. However, the contamination layer on the MoOx surface does not affect the energy-level alignment at the interface between MoOx and the donor/acceptor blend. Hence, significant improvement in the performance of BHJ-OPVs by mildly annealing the MoOx layer, which was previously reported, can be explained by the reduction of undesired contamination.

The electrochemical interface between a polycrystalline Pt electrode and the ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf]) has been studied by in-situ IR-visible sum-frequency generation (SFG) spectroscopy. Potential dependent adsorption/desorption processes of OTf anions has been monitored within the electrochemical window. SFG results indicate that the ions form a double layer structure at the interface. Significant adsorption/desorption hysteresis has been observed for the anions on the Pt surface. Keywords: Ionic liquids, Pt, Adsorption hysteresis, SFG spectroscopy

The fullerene (C 60 )/copper phthalocyanine (CuPc) interface is one of the widely used donor/acceptor (DA) interfaces for organic photovoltaics (OPVs), and information on the electronic structure at the interface is essential for fully understanding the energetics ofexcitons and carriers in OPVs. Here, an investigation into the energy levels at the C 60 /CuPc interface is made using UV photoelectron, X-ray photoelectron, and inverse photoemission spectroscopies. The vacuum level and core levels rise with C 60 deposition on the CuPc film, which indicates that the interfacial dipole is formed with the negative charge on the C 60 side. The interfacial dipole can be formed by the electron transfer from CuPc to C 60 in the ground state at the interface, which is indicated by the analysis of the UV-vis-NIR absorption spectrum of the CuPc/C 60 blended film. On the other hand, the highest occupied and lowest unoccupied molecular orbitals of CuPc and C 60 shift in opposite directions at the interface. This is attributed to the changes of the polarization energies of CuPc and C 60 at the interface. The formation of the interfacial dipole and the change ofthe polarization energy result in the anomalous energy band offsets at the C 60 /CuPc interface, which are entirely different from those in inorganic p-n junctions.

The electronic structure at the α-NPD/MoO 3 /Au interfaces has been investigated with ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS) and inverse photoemission spectroscopy (IPES). It was found that the MoO 3 layer contains some number of oxygen vacancies prior to any treatment and gap states are induced by the partial filling of the unoccupied 4d orbitals of molybdenum atoms neighboring oxygen vacancies. The α-NPD thickness dependence of XPS spectra for the α-NPD/MoO 3 system clearly showed that molybdenum atoms at the surface of the MoO 3 film were reduced by α-NPD deposition through the charge-transfer interaction between the adsorbed α-NPD and the molybdenum atoms. This reduction at the α-NPD/MoO 3 interface formed a large interface dipole layer up to −1.79 eV. The deduced energy-level diagram for the α-NPD/MoO 3 /Au interfaces describes the energy-level matching that explains well the significant reduction in the hole-injection barrier due to the MoO 3 buffer layer.

The electrochemical interface between a polycrystalline Pt electrode and an ionic liquid 1-butyl-3-methylimidazolium trifluoromethane sulfonate ([bmim]OTf) has been studied by in situ sum frequency generation (SFG) spectroscopy. Potential dependent adsorption and desorption processes of OTf anion as well as [bmim] cation have been observed. A double-layer model of the interface structure has been suggested based on the behavior of both the anion and the cation in the electrochemical potential window. Hysteresis effect has been observed during the desorption and the re-adsorption processes of both the anion and the cation on the Pt surface.

We present the electronic structure of various pentacene thin films grown on Au(1 1 1), Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. A systematic variation of the metal surface such as the substrate metal and its surface symmetry allows a comprehensive discussion on the correlation between the electronic structure and the interface geometric structure. In the monolayer regime, we observed the evidence of the formation of the organic–metal interface state depending on the metal surface, i.e., the interface geometric structure. This evidence is explained by the different organic–metal and intermolecular interactions, which originate from the hybridization of the molecular orbitals with the metal wavefunction. These interface geometric and electronic phenomena can be a seed for the subsequent film growth and resultant films electronic structure.

The energy-level alignment at Cu-phthalocyanine/metal interfaces has been studied using the Kelvin probe method. The organic layer on a metal surface plays two important roles in the energy-level alignment: the formation of the interfacial dipole (ID) and the passivation of the metal surface. The ID layer determines the injection barrier between the metal and the organic semiconductor. In cases where the lowest unoccupied molecular orbital level of the organic layer on the ID layer is below the Fermi level of the passivated metal substrate, the spontaneous charge transfer from the passivated metal substrate to several organic layers leads to the Fermi-level pinning.

Electronic structures of ionic liquids formed by 1-buthyl-3-alkylimidazolium ion [Cnmim]+ (n = 4 and 8) with various inorganic and organic anions have been investigated by ultraviolet photoemission, X-ray photoemission, inverse photoemission and soft X-ray emission spectroscopies (SXES). The comparison of the calculated density of states with the observed spectra revealed that the molecular orbital energies of these ionic liquids are significantly affected by the electrostatic Madelung potential among the ions. The SXES results clearly show that the both highest occupied and lowest unoccupied states of [C4mim]+PF6− are derived from the cation as a result of strong Madelung potential. On the other hand, the SXES results show the valence electronic structures of ionic liquids with larger anion molecules, [Cnmim]+Tf2N− and [Cnmim]+OTf− are contributed from the both cation and anion.

The unoccupied π * states of the solid film of electron accepting organic molecules, 7,7,8,8-tetracyanoquinodimethane (TCNQ), fluorinated TCNQ derivatives, 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TNAP), C60, and 6,6-phenyl-C61-butyric acid methyl ester (PCBM) have been studied by inverse photoemission spectroscopy. The assignment of the π * affinity levels of these typical electron accepting molecules provides the basic information for the organic electronics and the new electronic functional molecular design. The comparison with density functional theory calculations enables understanding how the electron affinity evolves in terms of molecular orbitals. The correlation between the film morphology and the irradiation damage on the TCNQ derivative samples by electron impact during the inverse photoemission measurements is also discussed.

Modification of quasi two-dimensional surface electron system on Au(111) with adsorption of long-chain n -alkane molecule n -CH 3 (CH 2 ) 42 CH 3 (tetratetracontane: TTC) was investigated by angle-resolved photoemission spectroscopy. Significant modification of the energy band structure of the Shockley surface state was induced by the adsorption of TTC monolayer with “flat-on” structure. The energy shift of the Shockley surface state toward the Fermi level reaches about 160 meV at Γ point ( k = 0 A − 1 ). The possible origin of the stabilization of the surface electron system with TTC monolayer is discussed.

The adsorption of a phenalenyl-based singlet biradical, 8,16-diphenyl-s-indaceno[1,2,3-cd:5,6,7-c′d′]diphenalene (Ph2-IDPL), on a highly oriented pyrolytic graphite surface has been studied using scanning tunneling microscopy (STM), tunneling spectroscopy, and ultraviolet photoelectron spectroscopy (UPS) under ultra-high-vacuum conditions. Ph2-IDPL molecules form a highly ordered monolayer on the graphite surface. The STM images of Ph2-IDPL monolayers show a strong bias-dependence in the range from −1.8 to +1.6 V. The observed image contrast may be associated with the distortion of the adsorption configuration or structural deformation of the molecules. UPS and tunneling spectroscopy reveal the electronic structure of Ph2-IDPL. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was determined to be 2.7 eV from the measured peak-to-peak energy difference between the HOMO and LUMO in the tunneling spectrum.

We investigated the molecular orientation of organized molecular films with regard to solid-state structures for newly synthesized comb copolymers with 2-vinyl-4,6-diamino-1,3,5-triazine (VDAT) by surface pressure-area (pi-A) isotherms, in-plane and out-of-plane X-ray diffraction (XRD), atomic force microscopy (AFM), and polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Since VDAT has adsorption ability to an adenine-thymine base pair of a DNA molecule, control of orientation for VDAT units in monolayers is possible to form surface patterning of biomolecules and construct candidates of new biochip materials. In the bulk state, hydrogenated and fluorinated comb copolymers containing VDAT form side-chain crystals for a two-dimensional lattice spacing of 4.2 and 5.0 A, respectively. From the results of the differential scanning calorimetric (DSC) measurements, sharp-shaped melting peaks appear on the relatively lower temperature side of the thermograms. This result supports the formation of side-chain crystals in the synthesized comb copolymers. These monolayers of copolymers on the water surface were extremely condensed, except for the VDAT:OA = 5:1 copolymer. From the in-plane XRD measurement of multilayers on solids, changes in the two-dimensional lattice structure of fluorinated comb copolymer films containing VDAT units, as opposed to their bulk state, were confirmed. It seems that these structural changes are caused by the stronger pi-pi interaction between the s-triazine rings rather than the van der Waals interaction between fluorocarbons. Polarized NEXAFS spectroscopy showed highly ordered orientation of s-triazine groups in the films based on the incident angle dependency of C and N1s-pi*(CN) transitions with synchrotron radiation. These experimental findings relate to well-ordered arrangement of functional groups supporting the side-chain rearrangement caused by the pi-pi interaction between the s-triazine rings.

Surface sum-frequency generation spectroscopy and X-ray reflectivity were used to study the surface of [BMIM][X] ionic liquids (BMIM = 1-butyl-3-methylimidazolium cation, X = BF4−, PF6−, and I−). Sum-frequency signal strength from the terminal methyl groups of the cation at the surface indicates that the topmost surface of these ionic liquids is occupied by polar-oriented hydrophobic butyl chains having approximately 1/3 of the in-plane density of fully packed alkyl chains as observed by the same method for the hexadecanol Langmuir monolayer. X-ray reflectivity data reveal a layer with density larger than that of bulk. However, the reflectivity is not sufficiently sensitive to the exact location of this layer either at the vacuum interface or sandwiched between the bulk and the low-density alkyl chain, as observed in the sum-frequency measurements. Analysis of the reflectivity data in conjunction with the sum-frequency spectra strongly suggests the molecules forming the topmost layer are on average polar-...

We analyzed the vacuum level shift (Δ) induced by the dipole layer at the interfaces between perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and noble metals (Au, Ag and Cu). The variation of Δ observed by ultraviolet photoelectron spectroscopy does not show a simple dependence on the metal work function, which contradicts the prediction by the induced density of interface states (IDIS) model proposed by Vazquez et al. [H. Vazquez, F. Flores, R. Oszwaldowski, J. Ortega, R. Perez and A. Kahn, Appl. Surf. Sci 234 (2004) 107]. We found that two factors, (1) the energy separation between the lowest unoccupied molecular orbital (LUMO) of PTCDA and the metal d-band states, which results in the attractive effect due to the orbital hybridization, and (2) the coupling matrix element between the adsorbate states and the metal d-band states, which result in the repulsive effect due to the orbital orthogonalization between the adsorbate states and the metal d-band states, have a clear correlation with the Δ formation. Our results indicate that the interactions between the molecular orbitals of PTCDA and the metal d-band states play an important role in determining the interfacial electronic structure, which has not been taken into account within the framework of the IDIS model.

We report the electronic structure and the charge transport mechanism of highly ordered films of pentacene on Cu(1 10) surface studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. For a flat-lying monolayer film, we observed the evidences of (i) formation of the interface states and (ii) two-dimensional intermolecular band dispersion of the resultant interface states, which may originate from the hybridization between the molecular orbitals and the wave function of the substrate. For an upright-standing multilayer film, we observed the two-dimensional intermolecular band dispersion, which originates from the intermolecular π-π interaction. The observed effective masses of the hole for different azimuths demonstrate the presence of the anisotropy of the hole mobility in pentacene crystals also at higher temperatures.

Electronic structures of the ionic liquids formed by 1-buthyl-3-alkylimidazolium ion [Cnmim]+ (n = 4, 8, 10) with fluorine-containing anions (tetrafluoroboronate BF 4 - , hexafluorophosphate PF 6 - ) were studied by ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Comparison of the calculated density of states with the observed spectra has revealed that the energies of the molecular orbitals of these ionic liquids are significantly affected by the electrostatic Madelung potential among the ions, by which the top of the occupied states and the bottom of the unoccupied states are both derived from the cation.

The molecular arrangement in the monomolecular layer (1 ML) film of hexadecafluorozincphthalocyanine (F 16 ZnPc) vacuum-deposited on GeS(0 0 1) in ultrahigh vacuum were studied by low energy electron diffraction (LEED), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). From the LEED analysis, we found that the F 16 ZnPc film grows epitaxially on the substrate surface and that the lattice structure in the 1 ML region is commensurate with the lattice of the surface. The tilt angle between the molecular plane and the substrate surface is 12.5° ± 2.5° deduced from the NEXAFS and ARUPS results. We also determined the azimuthal angle of the adsorbed F 16 ZnPc molecule with respect to the surface orientation of GeS(0 0 1) by comparing the ARUPS spectra with the theoretical simulation. Finally, we suggested a possible full film structure.

The epitaxial growth of p -sexiphenyl (C 36 H 26 , 6P) on highly oriented pyrolytic graphite (HOPG) surface has been investigated by scanning tunneling microscopy (STM). 6P molecules prefer epitaxial growth with the long axis along the [110] direction (armchair direction) of the HOPG substrate, with the unit cell parameters b 1 = 0.67 ± 0.06 nm, b 2 = 5.97 ± 0.06 nm and angle of 88 ± 3° between them. The relation of the 6P overlayer lattice vectors with the HOPG substrate has also been deduced, i.e. the 5 × 1 supercell is in a point-on-point commensurate relation with respect to the HOPG substrate surface.

The fluorescence properties of 4-[4-(dimethylamino)styryl]-1-n-alkylpyridinium bromide (hemicyanine) dissolved in solvents of different polarities and viscosities (methanol, ethylene glycol, tetra-ethylene glycol, glycerol, benzyl alcohol, pyridine, and two ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM]BF4, and 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM]PF6) were investigated. Significant increase in the fluorescence quantum yield and the fluorescence decay lifetime was observed with the increase in the viscosity of the solvent medium. It is because the intramolecular rotational motion of the molecule becomes more difficult in viscous liquid, which leads to a decrease in the nonradiative decay processes. The fluorescence quantum yields for all of the solutions followed a semiempirical law that depends only on the solvent viscosity. The correlation function C(t) was obtained for each solution by joining fluorescence decay curves measured at different wavelengths. From the fitted results of C(t), we observed the distinctive feature unique to the ionic liquids, in which the correlation functions for ionic liquid solutions are fitted to be biphasic, while they are monophasic for other solvents. The fluorescence maximum of hemicyanine dissolved in these ionic liquids red-shifted following the increase in the excitation wavelength.

The adsorption of 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) on a gold surface has been studied using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) under ultrahigh vacuum conditions. TTTA was deposited onto the gold substrate by gas-phase dosing. XPS and UPS spectra show that TTTA molecules strongly adsorb on the gold surface. In addition, the UPS spectrum of the TTTA monolayer is in good agreement with the gas-phase UPS spectrum of TTTA, which indicates that the molecules adsorb as a radical without electron transfer from the gold substrate.

Oxygen effect on the electronic structure of the interface between C60 and highly-oriented pyrolytic graphite was investigated by ultraviolet photoelectron spectroscopy (UPS). The film deposited in ultrahigh vacuum showed downward band bending characteristic of n-type semiconductor, possibly caused by residual impurities working as unintentional n-type dopants. In contrast, the film deposited in O2 atmosphere (1.3 × 10−2 Pa) did not show such band bending. Possible origins of this difference are discussed in terms of (i) deactivation of an unidentified dopant, (ii) compensation by O2 working as an acceptor, and (iii) O2-related carrier-traps such as O2 itself, O 2 δ - C 60 δ + and/or C60Ox.

We report on the fabrication and characterization of the bottom contact organic thin film transistor and inverter based on a heterostructure of C60 on pentacene. The transistor shows ambipolar transport characteristics with high electron and hole mobilities of 0.23 cm2 V−1 s−1 and 0.14 cm2 V−1 s−1, respectively. Both the n-channel in C60 and the p-channel in pentacene are stable in N2 atmosphere. After exposure to air, the n-channel is completely degraded whereas the p-channel keeps working. The inverter exhibits typical transfer characteristics, which are interpreted by the distribution of the accumulated electrons and holes depending on the bias conditions. The combination of the high performance and the bottom configuration of our devices suggests a potential way to fabricate organic complementary circuits without patterning of organic materials.

The atmospheric effect on the ionization energy (I) of titanyl phthalocyanine (TiOPc) thin film was investigated by photoemission yield spectroscopy developed for the measurement of I for the same specimen both in vacuum and under gaseous atmosphere. The variation of I of TiOPc thin film induced by exposure to air was observed.

We report our study on the current-voltage characteristics of single layer devices employing pure and heavily tetrathianaphthacene (TTN)-doped tris(8-hydroxy-quinoline) aluminum (Alq3) films as active layers. Sandwich structures with magnesium (Mg) top and bottom electrodes were employed in the devices. In the undoped device, the current increases exponentially with the applied voltage, and is concluded to be limited by the carrier injection at the Mg/Alq3 interface with a small injection barrier. With the same device structure, both the bulk-doped and interface-doped devices show a bulk-limited current flow, which is much larger than that in the undoped device and behaves as trap-free space-charge-limited current (SCLC) with a field-independent mobility. The improvement of the current is ascribed to the large enhancement of carrier injection by the doping. An increase of the magnitude of the interfacial dipole has been proposed as the possible cause for the enhancement of carrier injection.

The effect of oxygen doping on titanyl phthalocyanine (TiOPc) films was investigated by ultraviolet photoelectron spectroscopy (UPS). The energy of the electronic levels of TiOPc films probed by UPS was clearly different between the film deposited in ultrahigh vacuum (UHV) and in O2 atmosphere. The film deposited in UHV showed downward band bending characteristic of n-type semiconductor. On the other hand, the film deposited under O2 atmosphere showed upward band bending characteristic of p-type semiconductor. Such trends are in excellent correspondence with reported field effect transistor characteristics.

Room-temperature ionic liquid (RIL) + H2O mixture system was investigated using surface tension measurement and surface specific IR–visible sum-frequency generation (SFG) spectroscopy. The surface tension measured as a function of a bulk mole fraction of the ionic liquid indicated that the liquid surface even at very low bulk concentration of the ionic liquid is covered mostly by the cations. A slight increase of surface tension from 0.016 to 0.05 bulk mole fraction suggested that the anions start to appear at the surface in this concentration range. After 0.05 bulk mole fraction, the cations and the anions both occupy the surface, and the chemical composition and the surface density are expected to change little with the further addition of the ionic liquid. The result from the SFG measured at different concentrations is in agreement with the above findings. This result and the similar studies on methanol + water mixtures suggested that SFG is mainly sensitive to the surfaces excess molecules at the air/mixture interface. © 2005 Elsevier B.V. All rights reserved.

The effect of oxygen doping on titanyl phthalocyanine (TiOPc) film was investigated by ultraviolet photoelectron spectroscopy (UPS). The electronic structure of the interface formed between TiOPc films deposited on highly oriented pyrolytic graphite (HOPG) was clearly different between the films prepared in ultrahigh vacuum (UHV) and under O 2 atmosphere (1.3 × 10 −2 Pa). The film deposited in UHV showed downward band bending characteristic of n -type semiconductor, possibly due to residual impurities working as unintentional n -type dopants. On the other hand, the film deposited under O 2 atmosphere showed upward band bending characteristic of p -type semiconductor. Such trends, including the conversion from n - to p -type, are in excellent correspondence with reported field effect transistor characteristics of TiOPc, and clearly demonstrates that bulk TiOPc film was p -doped with oxygen. In order to examine the Fermi level alignment between TiOPc film and the substrate, the energy of the highest occupied molecular orbital (HOMO) of TiOPc relative to the Fermi level of the conductive substrate was determined for various substrates. The alignment between the Fermi level of conductive substrate and Fermi level of TiOPc film at fixed energy in the bandgap was not observed for the TiOPc film prepared in UHV, possibly because of insufficient charge density in the TiOPc film. This situation was drastically changed when the TiOPc film exposed to O 2 , and clear alignment of the Fermi level fixed at 0.6 eV above the HOMO with the Fermi level of the conducting substrate was observed, probably by p -type doping effect of oxygen. These are the first direct and quantitative information about bulk oxygen doping from the viewpoint of the electronic structure. These results suggest that similar band bending with Fermi level alignment may be also achieved for other organic semiconductors under practical device conditions, and also call for caution at the comparison of experimental results obtained under UHV and ambient atmosphere.