Lead Phthalocyanine Films Research Articles

Impact of molecular distribution on unoccupied image potential states of PbPc/HOPG surfaces

Two-photon photoemission (2PPE) spectroscopy revealed modified image potential states (IPSs) of sub-monolayer lead phthalocyanine (PbPc) films on graphite (HOPG). Angle-resolved (AR) 2PPE for room temperature films showed unique energy/momentum maps in which the inside of the parabolic dispersion curve of IPS on HOPG area is filled up to a specific energy. For cooled films, AR maps showed two parabolic curves originating from IPSs on HOPG areas and PbPc islands. The changes are explained with simple models of electron scattering by molecular islands. IPS feature is an indicator of nano-scale superstructures, which are not easily assessed by other experiments.

Towards high-performance near-infrared photodetectors based on SnS nanowires

Compared with bulk structures, semiconductor nanowires exhibit a higher surface-to-volume ratio, as well as unique electrical and optical properties. Due to its narrow band gap, tin (ii) sulfide (SnS) nano wire is a promising candidate for constructing near-infrared (NIR) photodetectors. Uniformly distributed and well-aligned SnS nanowires were grown on a mica substrate by chemical vapor deposition, and NIR photodetectors with Au (Au-device) and Al (Al-device) as the electrode were fabricated and characterized. Compared to the Au-device, the Al-device achieved higher photodetectivity due to reduced dark current. More importantly by incorporating a photosensitive lead phthalocyanine (PbPc) film into the Al-device, both responsivity and detectivity could be apparently improved, especially at weak light intensities. Under a weak light intensity of 0.79 mW/cm 2the photoresponsivity and specific detectivity were improved from and Jones to 0.96 A/W and Jones, respectively.

Metastable phase of lead phthalocyanine films on graphite: Correlation between geometrical and electronic structures

The geometrical and electronic structures of a metastable phase of lead phthalocyanine (PbPc) films on graphite have been studied by combined use of low energy electron diffraction (LEED) and two-photon photoemission (2PPE) spectroscopy. In submonolayer (sub-ML) PbPc films on graphite, islands in a metastable phase are formed just after deposition, as we reported previously by use of photoelectron emission microscopy (PEEM) [I. Yamamoto, N. Matsuura, M. Mikamori, R. Yamamoto, T. Yamada, K. Miyakubo, N. Ueno, and T. Munakata, Surf. Sci. 602, 2232 (2008)]. On single crystalline graphite substrates, the metastable islands produce clearly discernible LEED spots. By comparing the unit cell with that of annealed 1 ML films, molecules in the metastable islands are standing upright with a molecular density 1.8 times higher than that in the well-ordered 1 ML films. The LEED spots for the sub-ML films disappear after annealing. The islands in the metastable phase are surrounded by areas of a two-dimensional (2D) gaslike phase composed of flat-lying molecules. The metastable islands melt into the 2D gas phase, consistent with the PEEM results. In 2PPE spectroscopy, the lowest unoccupied molecular orbital (LUMO) derived level of the metastable phase is clearly distinguishable from that of flat-lying molecules. By tracking the thermal annealing process of the films by 2PPE spectroscopy, we clarify the decay of the LUMO derived peak intensity, the work function shift, and the energy shifts of molecular states associated with the transition from the metastable phase to the 2D gas phase. With this, we demonstrate the complementary capabilities of LEED and 2PPE spectroscopy to probe phase transitions of organic films in a nondestructive manner.

Lead phthalocyanine films deposited by ECR plasma-induced sublimation

Lead phthalocyanine (PbPc) thin films deposited by vacuum sublimation with Ar low-temperature electron cyclotron resonance (ECR) plasma treatment are investigated by means of reflection high-energy electron diffraction, atomic force microscopy, infrared spectroscopy, x-ray photoelectron spectroscopy and spectral ellipsometry. The intensity of plasma treatment is varied by the radio-frequency power input and the value of electrical bias applied to the sample. The films grown with and without plasma assistance are compared. The plasma intensity is considered to significantly influence the crystalline structure, surface morphology and film composition. At a low intensity the plasma assistance is analogous to vacuum annealing of films. At a medium intensity, a porous polymer matrix filled with PbPc in crystalline phases (monoclinic and triclinic) is formed in the films. At a high intensity, all the films consist of a compact polymer matrix, similar to a glassy solid, with small inclusions of PbPc molecules. The succession of PbPc molecule destructions in the growing film layer with plasma intensity increase is established by IR spectroscopy. The mechanism of polymer matrix formation is suggested.

High performance lead phthalocyanine films and its effect on the field-effect transistors

The film morphology, structure, and electrical properties of lead phthalocyanine (PbPc) epitaxially grown on 5,5″-bis(3′-fluoro-biphenyl-4-yl)-2,2′:5′,2″-terthiophene (m-F2BP3T) inducing layer substrates were systematic investigated. The morphologies of PbPc films sensitively depend on the thickness of the inducing layer and substrate temperature. All the epitaxial PbPc films with high quality presented the triclinic form with a variation of the out-of-plane orientation. The field-effect mobility of the epitaxial PbPc films was 0.05–0.31cm2/Vs, which was significantly improved by 1–2 orders of magnitude compared to the traditional films. The evolution of the device performance is the synergistic effect of the morphology and out-of-plane orientation of the triclinic form of PbPc. The higher quality of the films and the smaller ratio of (100)/(001), the higher device performance is. A clear relationship between the morphology, structure, and the performance of epitaxial PbPc-based organic field-effect transistors was reported.

Dispersions of image potential states on surfaces of clean graphite and lead phthalocyanine film

Dispersions of image potential states on a graphite surface (denoted IPS1) and on 1 monolayer (ML) film (denoted IPS2) of lead phthalocyanine (PbPc) are investigated by the micro-spot angle-resolved two-photon photoemission (micro-AR-2PPE) spectroscopy. On the graphite surface, whole dispersions of the two members of IPS1 (n = 1 and 2) are observed. The n = 1 IPS1 peak is weakly visible at energy higher than the vacuum level. The effective mass of an electron in the n = 1 IPS1 becomes slightly light at the high momentum region, suggesting the interaction between the IPS1 and the unoccupied σ-band of graphite. On the PbPc film, the IPS2 band forms a band gap and back-folds at the boundary of the Brillouin zone. A 1-dimensional Kronig-Penny model is used to reproduce the effective mass and the shift of binding energy.

Angle- and Time-Resolved Two-Photon Photoemission Spectroscopy for Unoccupied Levels of Lead Phthalocyanine Film

We have measured angle- and time-resolved two-photon photoemission (2PPE) from unoccupied levels of graphite (HOPG) and lead phthalocyanine (PbPc) films. Our angle-resolved 2PPE microspot spectroscopy with a lateral resolution of 0.4 μm can effectively select flat and homogeneous regions of the film, allowing measurements of the dispersion of the image potential state (IPS) on the PbPc film. The effective mass of the IPS on the film is 2.2 ± 0.2, which is heavier than that for the bare HOPG, 1.2 ± 0.1. The lifetimes of the states at 30 K are 30 fs for both the HOPG and the PbPc film. These results suggest a hybridization of the IPS with a molecule-derived unoccupied level. The lifetimes of the lowest unoccupied molecular level (LUMO)-derived levels are about 80 fs. A possible decay process to lower states is suggested.

Lateral inhomogeneity of unoccupied states for PbPc films

Micro-spot two-photon photoemission (micro-2PPE) spectroscopy is used to probe unoccupied electronic levels at sub-monolayer (ML) films of lead phthalocyanine (PbPc) on graphite (HOPG). The high-energy resolution (30 meV) 2PPE spectra with high-lateral resolution (0.4 μm) show well-resolved features due to molecule-derived occupied/unoccupied levels and the image potential state (IPS). The surface images based on photoemission from the highest occupied molecular orbital (HOMO) become laterally uniform after an annealing procedure. By contrast, the images based on the peak due to the lowest unoccupied molecular orbital (LUMO) and the next LUMO (LUMO + 1) are laterally inhomogeneous even after the annealing. The IPS peak is broadened to higher energy by 0.3 eV for sub-ML films and becomes sharp when a 1 ML film is formed. The broadening indicates that the electron in the IPS is scattered by molecules within the mean free path in the range from 1 to 10 nm. PbPc molecules are randomly distributed including nm-clusters. The LUMO and LUMO + 1 levels are stabilized as the cluster size increases. The inhomogeneity of the surface image due to the LUMO + 1 peak arises from the size distribution of the nm-clusters. The present results demonstrate that the unoccupied levels are more sensitive to the environment than the occupied levels.

Structural Evolution of Evaporated Lead Phthalocyanine Thin Films for Near-Infrared Sensitive Solar Cells

In this work, we focus on the deposition conditions as a means to control the structural evolution of lead phthalocyanine (PbPc) films in order to promote the triclinic structure, thereby inducing a shift in the absorption spectrum toward the near-infrared (NIR). Absorption spectra of PbPc films exhibit an enhanced NIR absorption peak at a wavelength of λ = 900 nm upon (i) increasing film thickness, (ii) increasing substrate temperature, or (iii) decreasing evaporation rate. X-ray diffraction measurements correlate the enhancement of the NIR absorption peak with an improved crystallinity and increased average volume of triclinic domains in the mixed monoclinic−triclinic films. As the surface structure of 10 and 60 nm thick films differ, this implies an asymmetric layer structure with a semicrystalline monoclinic film close to the substrate, evolving to a predominantly triclinic structure in the upper part of the film. We have demonstrated the use of structural control of the PbPc layer in a planar heterojunction solar cell with NIR-sensitivity. Decreasing the evaporation rate results in solar cells with significantly enhanced short-circuit current density (JSC), because of the change in absorption in combination with the longer exciton diffusion length that was estimated for donor layers exhibiting a predominantly triclinic structure. Overall, an optimized solar cell yields a power conversion efficiency of 2.6% (2.1% when correcting for the solar spectrum mismatch), and has external quantum efficiencies above 11% from λ = 320−990 nm with a peak value of 34% at λ = 900 nm.

Resonant effects on two-photon photoemission spectroscopy: Linewidths and intensities of occupied and unoccupied features for lead phthalocyanine films on graphite

Two-photon photoemission (2PPE) spectroscopy has been performed for lead phthalocyanine films on graphite surface. Fully resolved occupied and unoccupied levels and resonant optical transitions between them are characteristics of the film surface [Phys. Rev. B 77, 115404 (2008)]. 2PPE peaks due to photoemission from normally unoccupied levels ($1\ensuremath{\omega}$ peaks) and those due to coherent two-photon process from occupied levels ($2\ensuremath{\omega}$ peaks) show unexpected variations in intensities and widths when the pump photon energy crosses the resonances. At around a resonance between molecule-derived levels, we find an intensity switching in which only the $2\ensuremath{\omega}$ peak appears at photon energy below the resonance, and at above the resonance, the $1\ensuremath{\omega}$ peak becomes prominent and the $2\ensuremath{\omega}$ peak becomes very weak. The $1\ensuremath{\omega}$ peak is broadened with increasing photon energy. These results cannot be interpreted by a simple energy level scheme, and point to further understanding of 2PPE process. Temperature dependence reveals the origin of indirect transitions involving the unoccupied substrate $\ensuremath{\pi}$ band. We also demonstrate that resonant 2PPE spectroscopy is effective to probe deeper occupied levels which are not well resolved in one-photon photoemission.

Vibrationally resolved two-photon photoemission spectroscopy for lead phthalocyanine film on graphite

Vibronic structure has been resolved in two-photon photoemission (2PPE) from the highest occupied molecular orbital (HOMO)-derived level of lead phthalocyanine film formed on graphite. The 2PPE from the HOMO-derived level was enhanced by the resonance with the third lowest unoccupied molecular orbital $(\text{LUMO}+2)$-derived level. At photon energies below and well above the resonance, the intensity of the vibronic structure was similar to that of one-photon photoemission. On the other hand, the vibronic structure was significantly weak at photon energies slightly above $(<0.4\text{ }\text{eV})$ the resonance. The decrease in the vibronic structure was attributed to the nuclear motion of molecule in the intermediate state associated with $\text{LUMO}+2$.

High-precision sample stage for photoemission microscopy of organic films

A high-precision sample stage for photoemission microscopy has been constructed to translate the sample by ±3 mm with accuracy better than 100 nm. The stage is actuated by step motors settled outside the vacuum. The accuracies of the translations were measured by observing a standard patterned sample with a photoemission electron microscope (PEEM) of 50 nm resolution. The accuracy was nearly independent of the distance of each translation step and the error was not accumulated by repeated steps. After round-trip translations up to 0.2 mm, the sample came back to the original position with accuracy of ±50 nm. The performance of the stage was demonstrated by observing growth processes of lead phthalocyanine (PbPc) films formed on graphite.

Schottky barrier characterization of lead phthalocyanine/aluminium interfaces

Thin sandwich film structure consisting of several cells is fabricated by successive thermal sublimation of aluminium, lead phthalocyanine and aluminium under high vacuum conditions (10 − 4 Pa). The dark current density–voltage ( J– V) characteristics indicate rectifying junctions exists at PbPc/Al interface. Devices exposed to oxygen were found to exhibit an enhanced Schottky type behaviour. Measurements on the dependence of capacitance and conductance on frequency and temperature are also investigated. These are quantitatively interpreted using an equivalent circuit model. Structural properties of lead phthalocyanine film were studied using X-ray diffraction techniques.

Multisensor arrays for gas analysis based on photosensitive organic materials: An increase in the discriminating capacity under selective illumination conditions

The discriminating capacity of multisensor arrays for gas analysis (an electronic nose) with respect to multicomponent chemical atmospheres containing predominant components can be increased by the spectrally selective illumination of photosensitive sensor coatings. The classification of mixtures with ethanol as a predominant component was studied with the use of an array of three sensors based on piezoelectric quartz resonators modified with hydrogen, copper, and lead phthalocyanine films. Illumination in the red region of the spectrum several times decreased the adsorption capacity of the phthalocyanine films for ethanol. This allowed us to classify the following five analytes: triethylamine, propylamine, ethanol, and ethanol-triethylamine and ethanol-propylamine mixtures. The proposed method also allowed us to improve the reproducibility of the measurement results and to shorten the duration of analysis.

Imaging of electronic structure of lead phthalocyanine films studied by combined use of PEEM and Micro-UPS

The thin film growth of lead phthalocyanine (PbPc) on graphite substrate has been studied by photoemission electron microscopy (PEEM) operated with tunable UV laser radiation. Origin of the contrast of PEEM images was elucidated by comparison with microspot ultraviolet photoemission spectroscopy (Micro-UPS). The photoelectron yield curves measured with PEEM for annealed films of different coverages were in agreement with the shifts of the vacuum levels determined by Micro-UPS. Below 1 monolayer (ML) coverage, PbPc molecules did not form islands/domains but were sparsely distributed and formed uniform films. Above 1 ML, PbPc molecules formed stable islands of bilayer in a well-ordered 1 ML film. On the other hand, for as-deposited films with thickness equivalent to annealed 0.3 ML coverage, PbPc molecules formed metastable bilayer islands of a few μm-size in the sparsely packed uniform film. These results demonstrate the capability of combined use of the complementary methods PEEM and Micro-UPS to resolve inhomogeneity of electronic structure.

Resonant two-photon photoemission study of electronically excited states at the lead phthalocyanine/graphite interface

Occupied and unoccupied electronic states of lead phthalocyanine (PbPc) films of 1 ML thickness formed on graphite substrates have been measured by two-photon photoemission (2PPE) spectroscopy. Highly reproducible and well-resolved 2PPE spectra were obtained by selecting sample positions with submicrometer light spot. We have identified the molecule-derived states due to the $\mathrm{HOMO}\ensuremath{-}1$, HOMO, LUMO, $\mathrm{LUMO}+1$, $\mathrm{LUMO}+2$ as well as the first image-potential state (IS). The occupied states are in good agreement with one-photon photoemission results. Resonant excitations between the HOMO and $\mathrm{LUMO}+2$-related levels as well as those between the HOMO level and IS were observed in consistency with energies of relevant levels determined from off-resonant conditions.

Lead Phthalocyanine Films by Near Edge X-ray Absorption Fine Structure Spectroscopy

We measured NEXAFS spectra on both C and N K-edges of an ordered monolayer of lead phthalocyanine adsorbed onto an InSb(001) 4 × 2/c(8 × 2) substrate. We compared the spectra with the calculated density of unoccupied states of an isolated molecule. We demonstrate that molecules lay flat on the surface, determine the energy position of the lowest excited state, and show that it is site dependent. Finally, we compare the core exciton structure with the valence exciton structure previously measured by two different valence spectroscopic techniques and discuss their differences in terms of electron−nuclear and electron−hole Coulomb interactions.

Alternating current measurements of thermally evaporated triclinic lead phthalocyanine thin films

Electronic properties of thermally evaporated thin films of triclinic lead phthalocyanine were investigated employing symmetric gold ohmic electrodes in the frequency range of 100–20 kHz. At low temperatures, ac conductivity was identified to be via a hopping type mechanism whereas at higher temperatures, the free-band model is considered to be the major cause for electronic transitions. Measurements on the dependence of capacitance on frequency (100–20 kHz) indicates that the capacitance is strongly frequency dependent, decreasing with increasing frequency at low frequency and less rapidly at higher frequencies. This is quantitatively interpreted using an equivalent circuit model. Oxygen doping was found to increase the capacitance of PbPc. The phenomenon was understood in terms of a reduction in the value of device internal resistance induced by oxygen absorption.

Measurements and modeling of the DC temperature dependence of electrical conductivity in thin films of lead phthalocyanine

Many of the phthalocyanines exhibit p-type conductivity, and electrical conductivity through thin films of these materials having ohmic contacts show space-charge-limited conductivity (SCLC) dominated by trap levels located within the band gap. In the present work evaporated thin films of lead phthalocyanine with ohmic gold electrodes were prepared, which showed two distinct regions in the dependence of current density J on applied voltage V. At low voltages sample conductivity was ohmic, changing at higher voltages to a square-law dependence of J on V, which is indicative of SCLC dominated by trap levels located at a single discrete energy level. The results of temperature measurements indicate three distinct regions, in each of which the hole concentrations are controlled by different activation energies. A simple model is proposed in which a single trap level is located at the same energy spacing E t from the valence band edge as a single acceptor level. This predicts three different temperature ranges, two of which correspond to those covered by the experimental results. The experimental results indicate a trap level located at an energy E t = 0.36 eV above the valence band edge and a thermal band gap E g = 1.51 eV. Using the proposed model together with data from the experimental J–V characteristics, an acceptor concentration of 4.85 × 10 19 m − 3 and a trap concentration of 5.18 × 10 25 m − 3 are indicated. Measurements of mobility based on this model yield a value of 2.6 × 10 − 4 m 2 V − 1 s − 1 , which is in close agreement with previous work.

Conductivity measurements on phthalocyanine films: evaporated vs. pressure contacts

Conductivity measurements on thick films of lead phthalocyanine in a sandwich cell with one pressure contact are reported. The results obtained at different loads are discussed and compared with those for a cell with both evaporated contacts.