Vanadyl Phthalocyanine Films Research Articles

Assembly of multilayer phthalocyanine vanadium oxide molecules on Ag<sub>2</sub>Sb alloy monolayer

Fabricating ordered molecular films and further tuning their assembly behavior is important for constructing organic devices with diverse performances. By using high-resolution scanning tunneling microscopy, in this work, we demonstrate that well-organized vanadyl phthalocyanine (VOPc) films can be formed <i>via</i> ‘bottom-up’ molecular self-assembly on a binary alloy Ag<sub>2</sub>Sb/Ag(111). The Ag<sub>2</sub>Sb monolayer is prepared by evaporating Sb atoms on clean Ag(111) and followed by annealing. The VOPc molecules are deposited on the Ag<sub>2</sub>Sb layer <i>via</i> thermal evaporation. The molecular configuration, structural and orbital characteristics of VOPc are clearly clarified at a submolecular level. It is found that initially the ordered VOPc membrane only exhibits the O-up adsorption configuration. Its square-shaped unit cell consists of five VOPc molecules where two adsorption orientations coexist with the horizontal axis of VOPc which is rotated by about 11° or 21° relative to the side of the unit cell. Due to the molecular dipole-dipole interaction, further-deposited molecules result in the assembly of the second-layer VOPc films with the O-down configuration and the square-shaped unit cell that contains only one VOPc molecule. Subsequently, due to the dipole-dipole interaction between layered molecules, following VOPc molecular layers adopt alternating O-up and O-down configurations as well as the square-shaped unit cell, similar to the case of the second layer. In addition, we find that the molecular orbitals overlap in each assembled molecular layer due to the π-π interaction which could facilitate the charge transport along the π stacking direction of VOPc. This research provides possibility to regulate the adsorption configuration and assembly behavior of functional organic molecules on metal surfaces by forming surface alloys.

Highly-sensitive organic field effect transistor sensors for dual detection of humidity and NO2

Sensors based on organic field effect transistors have attracted extensive interests, but most sensors are applied to a single analyte detection. Herein, a facile strategy was proposed to optimize the growth of active layers by regulating the thickness of vanadyl-phthalocyanine (VOPc) films and carrying out the thermal annealing. Thanks to the improved film morphology and crystal properties, the carrier mobility of transistors reached 0.15 cm2V−1s−1, humidity and nitrogen dioxide (NO2) induced multi-parameter response of transistors could be considered as an efficient sensing approach. The sensitivity of threshold voltage reached 0.59 V/% RH ranging from 42%−82% RH. In addition, the gas sensing performance was systematically studied. Sensors based on 10 nm VOPc films achieved a 4710% response to 10 ppm NO2 gas, with a theoretical limit of detection down to 125.2 ppb, and the response current was improved by two orders of magnitude under the amplification effect of transistor. The obtained gas sensors also exhibited high repeatability after being stored at 90% ambient humidity. It was concluded that different charge carrier transfer mechanism in moisture and NO2 gas provided a simple idea for tunable applications of phthalocyanine-based transistors.

Vanadyl Phthalocyanine Films and Their Hybrid Structures with Pd Nanoparticles: Structure and Sensing Properties.

In this work, thin films of vanadyl phthalocyanines (VOPc and VOPcF4) are studied as active layers for the detection of gaseous ammonia and hydrogen. The effect of F-substituents on the structural features of vanadyl phthalocyanine films and their sensor response toward ammonia (10–50 ppm) and hydrogen (100–500 ppm) is investigated by X-ray diffraction (XRD) and chemiresistive methods, respectively. It is shown that the sensor response of VOPcF4 films to ammonia is 2–3 times higher than that of VOPc films. By contrast, the sensor response to hydrogen is higher in the case of VOPc films. Apart from this, the hybrid structures of vanadyl phthalocyanine films with Pd nanoparticles deposited on their surface by a chemical vapor deposition method are also tested to reveal the effect of Pd nanoparticles on the sensitivity of VOPc films to hydrogen. Deposition of Pd nanoparticles on the surface of VOPc films leads to the noticeable increase of their sensitivity to hydrogen.

Thin films of chlorosubstituted vanadyl phthalocyanine: charge transport properties and optical spectroscopy study of structure

Electrophysics and structure of thin films of chlorosubstituted vanadyl phthalocyanine (VOPcCl16) were studied using complementary experimental and theoretical techniques. To study charge transport properties of the latter films, organic field-effect transistors were fabricated by physical vapor deposition. The device exhibited good air stability without any extent of degradation after a storage in air for two months. The charge carrier mobility was measured to be (2.0 ± 0.1) × 10−3 cm2 V−1 s−1. This value was rationalized by poor ordering of the VOPcCl16 films revealed with the use of polarization dependent Raman and UV–Vis spectroscopies as well as by X-ray diffraction. Apart from this, we performed a detailed assignment of all intense bands in the vibrational spectra of VOPcCl16. To this end, the experimental IR and Raman data were complemented by quantum chemical calculations at the B3LYP/6-311++G(2df,p) level of theory.

Polymorphism and electronic properties of vanadyl-phthalocyanine films

Polymorphs of vanadyl-phthalocyanine (VOPc) films were realized on the polymer-modified substrates. Balanced ambipolar transport was observed in the Phase II of VOPc film on the PMMA modified substrate, while only electron transport was obtained on PVA layer. The different transport behavior is believed to be original from their crystalline structures, which may lead to their different energy-band structures.

Optical Spectroscopy and XRD Study of Molecular Orientation, Polymorphism, and Phase Transitions in Fluorinated Vanadyl Phthalocyanine Thin Films

The molecular arrangement and phase transitions in the vanadyl hexadecafluorophthalocyanine (VOPcF16) thin films grown by physical vapor deposition have been studied using in situ X-ray diffraction, atomic force microscopy, and optical spectroscopy techniques (UV, IR, and Raman). The complete transition from the low-temperature linear cofacial structure to the slipped dimeric one occurs in the temperature range 160–220 °C. This conversion was found to be irreversible upon cooling the VOPcF16 film back to 20 °C. The structural transformation leads to decrease of the in-plane conductivity of the film by 2 orders of magnitude. According to the polarized Raman spectroscopy measurements, the mean tilt angles between the VOPcF16 species and the substrate surface were 59 ± 5° and 30 ± 5° in the as-deposited and annealed films, respectively. For the sake of comparison, the structure of the thin films of vanadyl phthalocyanine (VOPc) was also studied. The mean tilt angle between the VOPc species and the substrate surface was found to be 77 ± 5°, in good agreement with existing experimental data (∼70°). All intense bands in the experimental IR and Raman spectra of VOPcF16 and VOPc were assigned using DFT calculations (B3LYP) and the 15N isotopic shifts in the vibrational spectra of VOPc.

Room temperature nitrogen dioxide chemresistor using ultrathin vanadyl-phthalocyanine film as active layer

A room temperature nitrogen dioxide (NO 2) chemresistor using 1.8 nm ultrathin vanadyl-phthalocyanine (VOPc) film as active layer has been prepared by Weak Epitaxy Growth (WEG) method. Its gas sensing properties to NO 2 have been compared with a 7 nm polycrystalline VOPc chemresistor. The ultrathin film device shows stronger response and better recovery character. Further results reveal that the two-dimensional transport film provides an effective charge transfer system after NO 2 adsorption and it gets rid of bulk diffusion at the same time. Kelvin probe force microscopy (KPFM) results suggest that grain boundaries are easily doped by analyte but is slow in desorption compared with crystal grains. The ultrathin film device shows advanced sensing mechanism compared with the polycrystalline film.

Structural phase transition in Langmuir films of vanadyl phthalocyanine

The Langmuir-Blodgett films based on vanadyl phthalocyanine (NHSO2C18H37)4 were studied. By the method of piezoresonance quartz balance, the isotherms of adsorption of water, pyridine, and hexane on film surfaces were obtained. A structural phase transition in the Langmuir films of vanadyl phthalocyanine at about 313 K was revealed. The phase transition was observed in ultrathin films and disappeared as the number of layers increased.

Spectral characterization of thin films of vanadyl hexadecafluorophthalocyanine VOPcF 16

In this work, structural features and optical properties of hexadecafluorosubstituted vanadyl phthalocyanine (VOPcF 16) films deposited by Organic Molecular Beam Deposition (OMBD) technique have been investigated. Thin films of VOPcF 16 were characterised by ellipsometry, optical absorption and vibrational (IR and Raman) spectroscopy. VOPcF 16 films deposited onto silicon and quartz substrates held at room temperature are well organized and characterised by a predominantly co-facial parallel arrangement of molecules vertical to the surface. Thermal treatment of these films at 220 °C leads to the transition from one phase to another and formation of the polycrystalline VOPcF 16 films with randomly distributed crystallites on the substrate surface.

Temperature dependence of the optical anisotropy of vanadyl phthalocyanine films

The optical anisotropy of vanadyl phthalocyanine films is investigated in the temperature range from 300 K to 10 K by reflection anisotropy spectroscopy in the energy range from 1.5 eV to 5.5 eV. With changing temperature all spectral features show reversible changes in their relative intensity. The most pronounced changes are observed in the sub-structure of the Q band related feature when cooling down from 300 K to 100 K. All changes are recovered upon annealing back to room temperature, indicating a reversible structural modification of the films assisted by the illumination.

High mobility vanadyl-phthalocyanine polycrystalline films for organic field-effect transistors

The organic films of vanadyl-phthalocyanine (VOPc) compounds showed weak epitaxy growth (WEG) behavior on thin ordered para-sexiphenyl (p-6P) layer with high substrate temperature. The WEG of VOPc molecules standing up on the p-6P layer leaded to high in-plane orientation and their layer-by-layer growth behavior. In consequence, high quality VOPc films were obtained, which were consisted of lamellar crystals. Organic field-effect transistors with VOPc∕p-6P films as active layers realized high mobility of above 1cm2∕Vs. This result indicated that nonplanar compounds can obtain a device performance better than planar compounds, therefore, it may provide a rule to find disklike organic semiconductor materials.

Effect of gas phase adsorption on the conductivity of Langmuir-Blodgett films of vanadyl phthalocyanine

Frequency dependences are obtained of the conductivity in Langmuir films of modified vanadyl phthalocyanine in vacuum and under the conditions of adsorption of donor and acceptor molecules. It is found that, upon the adsorption of NO2 and CH4O molecules, the film conductivity at f < 10 kHz increases by several orders of magnitude. It is shown that, in vacuum and upon adsorption of acceptor molecules, the dominant charge transfer mechanism is the hopping of holes via localized states. In the case of adsorption of donor molecules, the ionic conductivity mechanism appears to prevail.

Electronic structure of the organic semiconductor vanadyl phthalocyanine (VO-Pc)

The electronic structure of thin films of the organic semiconductor vanadyl phthalocyanine has been studied using synchrotron radiation-excited resonant soft X-ray emission spectroscopy, resonant inelastic soft X-ray scattering, X-ray absorption spectroscopy, and X-ray photoemission spectroscopy. The C, N, V, and O partial densities of states for both the valence and conduction bands have been determined, as well as the core level electronic structure. Good agreement was found between the experimental measurements of the valence and conduction bands, and the results of a density functional calculation.

Magnetic field influence on the molecular alignment of vanadyl phthalocyanine thin films

Thin vanadyl phthalocyanine (VOPc) films with thicknesses ranging between 10 and 100 nm were grown by organic molecular beam deposition (OMBD) in high vacuum on H–Si(1 1 1) and quartz substrates. The influence of a magnetic field (1.3 T) applied during the film growth was investigated by means of variable angle spectroscopic ellipsometry (VASE), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction and atomic force microscopy (AFM). The dielectric function was determined by means of VASE. The VOPc layers were found to be uniaxially anisotropic, i.e. exhibit the same optical constants along two of the Cartesian axes, and different optical constants along the third axis. The values of the dielectric function with in-plane of the substrate are larger compared to those out-of-plane. The in-plane and out-of-plane values of the imaginary part of the dielectric function in the region of the Q-band were used to determine the tilt angle of the molecular plane with respect to the substrate plane. Films deposited without magnetic field have the molecular planes tilted with respect to the substrate plane by about 24°, while those deposited in a magnetic field perpendicular to the substrate, the molecules tend to align their molecular planes with respect to the substrate at ca. 3°. All the samples were found to have a triclinic crystallographic structure. From the comparison between the FTIR spectra of VOPc layers and pellets dispersed in KBr, it can be also deduced that the molecules are lying parallel to the substrate in the case of samples grown in the presence of a magnetic field and slightly tilted in those grown without magnetic field.

Improved photoreceptor decay characteristics of vanadyl-phthalocyanine films annealed under magnetic field

Magnetic field was applied during the annealing of vanadyl-phthalocyanine films prepared by vacuum vapor deposition. It was observed that the Q-band absorption peak position in the UV–vis absorption spectra of the films shifted to longer wavelength, and the X-ray diffraction intensity of the films was enhanced for the film annealed under magnetic field. Remarkable improvements in the photoreceptor decay characteristics of the films were also found for the films annealed under magnetic field. It is suggested that molecular orientation during annealing under magnetic field is responsible for the above phenomena.

Transparent Organic Thin‐Film Transistor with a Laterally Grown Non‐Planar Phthalocyanine Channel

An improved‐performance transparent organic thin‐film transistor (OTFT) is described, which has a vanadyl‐phthalocyanine (VOPc) film as an active p‐channel, lattice‐matched (Sc0.7Y0.3)2O3 film as a high‐k gate dielectric, and atomically flat indium tin oxide (ITO) film as the bottom contact (see Figure). These features—lattice matching and atomic flatness—are expected to lead to further improvements in transparent OTFTs.

Exciplex emission in mixed films of vanadylphthalocyanine and N, N′-bis(neopentyl)-3,4,9,10-perylenebis(dicarboximide)

The absorption and emission properties and film structure of co-evaporated mixed thin solid films of vanadyl-phthalocyanine (VOPc) and N, N ′-bis(neopentyl)-3,4,9,10-perylenebis(dicarboximide) (BNPTCD) are reported. Exciplex emission from mixed films is clearly identified and the intensity of new near-infrared light source depends on the degree of mixing and molecular orientation in the film that control the exciplex formation. A clear correlation between molecular organization and emission properties is established. The IR spectroscopic data and AFM topographical analysis show that thermal annealing of the mixed films induces phase separation enhancing energy transfer as main deactivation pathway of the absorbing BNPTCD molecules.

有機ガスによるバナジルフタロシアニン (VOPc) 膜の分子配向制御

有機ガスによるバナジルフタロシアニン (VOPc) 膜の分子配向制御

有機ガス処理されたＫＢｒ基板上のバナジルフタロシアニン膜の相遷移

有機ガス処理されたＫＢｒ基板上のバナジルフタロシアニン膜の相遷移

エピタキシー成長したバナジルフタロシアニン膜の非線形光学特性

Vanadyl-phthalocyanine (VOPc) films were prepared on KBr substrate by MBE. They were characterized with RHEED, optical absorption spectra in visible and infrared regions (UV/VIS) and SEM. The thickness of prepared films is above 100 nm. The second and third harmonic generations (SHG and THG) of films were also measured by a Maker fringe method using a Nd·YAG laser. From these results, it was concluded that films are epitaxy films. We also estimated that the SH intensity of VOPc films are about 20 times higher than that of a Y cut quartz crystal and the third-order susceptibility χ(3) of VOPc film is about 3 times greater than the value reported recently by Fang et al.