CoPc Films Research Articles

Annealing temperature effect on structural, electronic features and current transport process of Au/CoPc/undoped-InP MPS-type Schottky structure

The influence of annealing on structural, electronic features and the current transport process of the Au/CoPc/undoped-InP (MPS)-type structure is explored. Raman measurements of as-deposited CoPc thin films reveal significant vibrational bands at 593 cm−1, 682 cm−1 and 1537 cm−1 assigned to C-C and C-N band stretching, showing the structural confirmation of CoPc films. The surface topology of CoPc is assessed by AFM at various annealing temperatures. The roughness of CoPc films falls with rising annealing temperature up to 200 °C and then slightly rises after annealing at 300 °C and 400 °C. The electrical features of the MPS structure are explored by the I-V method and observed good rectification nature regardless of annealing temperatures. The mean barrier heights are estimated to be 0.76 eV, 0.89 eV, 0.91 eV, 0.86 eV and 0.85 eV for as-deposited, 100 °C, 200 °C, 300 °C and 400 °C by statistical distribution analysis. Results showed that the BHs increase as annealing rises to 200 °C and decrease after annealing at 300 °C and 400 °C. The derived series resistance of the MPS structure reduces with rising annealing up to 200 °C and then rises after annealing at 300 °C and 400 °C. The acquired density of states (NSS) of the MPS structure varied with annealing temperature may be the reduction in the recombination center and the prevalence of an interfacial layer between the CoPc and InP layers. Findings suggest that the PFE governs reverse current in the lower bias region, while, the SE ruled reverse current in the higher bias region of the MPS structure irrespective of annealing temperatures.

Heterostructures Based on Cobalt Phthalocyanine Films Decorated with Gold Nanoparticles for the Detection of Low Concentrations of Ammonia and Nitric Oxide.

This work is aimed at the development of new heterostructures based on cobalt phthalocyanines (CoPc) and gold nanoparticles (AuNPs), and the evaluation of the prospects of their use to determine low concentrations of ammonia and nitric oxide. For this purpose, CoPc films were decorated with AuNPs by gas-phase methods (MOCVD and PVD) and drop-casting (DC), and their chemiresistive sensor response to low concentrations of NO (10–50 ppb) and NH3 (1–10 ppm) was investigated. A comparative analysis of the characteristics of heterostructures depending on the preparation methods was carried out. The composition, structure, and morphology of the resulting hybrid films were studied by X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma atomic emission (ICP-AES) spectroscopy, as well as electron microscopy methods to discuss the effect of these parameters on the sensor response of hybrid films to ammonia and nitric oxide. It was shown that regardless of the fabrication method, the response of Au/CoPc heterostructures to NH3 and NO gases increased with an increase in the concentration of gold. The sensor response of Au/CoPc heterostructures to NH3 increased 2–3.3 times compared to CoPc film, whereas in the case of NO it increased up to 16 times. The detection limits of the Au/CoPc heterostructure with a gold content of ca. 2.1 µg/cm2 for NH3 and NO were 0.1 ppm and 4 ppb, respectively. It was shown that Au/CoPc heterostructures can be used for the detection of NH3 in a gas mixture simulating exhaled air (N2—74%, O2—16%, H2O—6%, CO2—4%).

Growth of ordered two-dimensional cobalt phthalocyanine films on a one-dimensional substrate

The growth of Co phthalocyanine (CoPc) films on the one-dimensional Si(111)(4 × 1)-In surface is studied using scanning tunneling microscopy (STM). At low CoPc coverages, the molecules are highly mobile on the surface. Only when the coverage approaches the saturation coverage, the molecules stabilize each other and a highly ordered two-dimensional CoPc monolayer forms. Based on the STM images, a structure model for this film could be developed, with the molecules selectively adsorbing on the In chains with a (4 × 4) periodicity.

Heterostructures based on Pd–Au nanoparticles and cobalt phthalocyanine for hydrogen chemiresistive sensors

In this work, the effect of Pd, Au and PdAu nanoparticles on sensor response of cobalt phthalocyanine films to hydrogen was studied. For this purpose, novel heterostructures based on cobalt phthalocyanine and PdAu nanoalloys were obtained by a combination of vacuum thermal evaporation and pulsed metalorganic chemical vapor deposition (MOCVD) and investigated as active layers for hydrogen detection. The structural features and phase composition of the prepared heterostructures were studied by the techniques of X-ray diffraction, transmission electron microscopy and electron diffraction. The concentration of metal nanoparticles in the samples was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The chemiresistive sensor response of CoPc/M (M = Pd, Au, Pd0.2Au0.8 and Pd0.8Au0.2) to hydrogen (100–400 ppm, room temperature) was compared with that of bare CoPc films. It was shown that the sensor response of the investigated heterostructures to hydrogen (300 ppm) increased in the order CoPc (0.2%) < CoPc/Pd0.2Au0.8 (1.9%) ~ CoPc/Au (2.2%) < CoPc/Pd (2.7%) < CoPc/Pd0.8Au0.2 (5.6%).

Sol-Jel Yöntemiyle Hazırlanan Kobalt Ftalosiyanin Katkılı TiO2 Filmlerin Optik ve Yapısal Özelliklerinin İncelenmesi

Bu çalışmada, sol-jel spin kaplama yöntemi kullanılarak mikroskop cam altlıklar üzerine TiO2, TiO2/CoPc ve CoPc filmleri hazırlanmış ve CoPc katkı konsantrasyonunun filmlerin optik ve yapısal özellikleri üzerine etkisi araştırılmıştır. İnce filmlerin optik geçirgenliğini ölçmek için 300-1100 nm dalgaboyu aralığında bir spektrofotometre kullanılmıştır. Uv-Vis çalışmaları, B ve Q bandı absorpsiyon bölgelerine karşılık gelen aralıklarda artan CoPc konsantrasyonu ile filmlerin geçirgenliğinin azaldığını göstermiştir. İnce filmlerin yüzey morfolojisini karakterize etmek için taramalı elektron mikroskobu (SEM) kullanılmış ve homojen olarak kaplandıkları görülmüştür. Tavlanmış filmlerin XRD ölçümlerinde, TiO2 filmlerde anataz-brookit karışık faz görülürken TiO2/CoPc filmlerin anataz olduğu tespit edilmiştir. Filmlerin dislokasyon yoğunluğu, kristal boyutu gibi önemli parametreleri hesaplanmıştır.

Spectro-microscopy characterization of CoPc-Au(111) interface

Abstract Co-phthalocyanine molecules have wide range of optoelectronics and sensor applications. Motivated by the importance of this material, we studied the adsorption of CoPc thin to thick films on Au(111) surface by using ultraviolet photoelectron spectroscopy (UPS) coupled with angle resolved UPS (AR-UPS) and scanning tunnelling microscopy (STM). Molecular spectral emissions are found to be at 5.13, 7.94, and 10.32 eV below the Fermi level. Observed UPS and AR-UPS measurements show both changes and shifting in valence band features. The work functions of clean Au(111) surface and thick CoPc film are found circa 5.32 eV and 5.25 eV, respectively. Both the work function variation and shifting of molecular features dictate strong adsorbate-substrate interaction. AR-UPS measurements further illustrates that CoPc molecules lie flat on the Au(111) surface. Our STM results disclose that at sub-monolayer (Sub-ML, 0.4–0.8 ML) coverage and room temperature, CoPc molecules tend to adsorb in random fashions; which reveal a strong interaction at the CoPc-Au(111) interface. However, at the ML coverage and elevated temperature (~520K), long range pattern with the oblique structure is observed. Beyond ML coverage, existence of mixed phases consisting of well arranged patterns and molecular clusters have been observed. Overall, our STM results show that depending on the molecular thickness, CoPc molecules form three types of organized oblique, rectangular and square geometries which is consistent with the findings of CuPc and CoPc on Au(111) surface. Our findings may seek potential use in the organic molecular based electronic devices.

Effect of Pyridine on the Structure and Vibrations of Cobalt Phthalocyanine

Abstract Effect of pyridine on cobalt phthalocyanine (CoPc) has been studied by the density functional theory and Raman spectroscopy. Thermal evaporation method has been used for the preparation of a thin film of CoPc and then Raman spectra of CoPc film have been recorded with and without exposure of pyridine. The molecular structure of CoPc and its coordinated complexes with pyridine have been computed using B3LYP/6-31(d,p) level of density functional theory calculations by employing Gaussian program package. Calculations show some in-plane and out-of-plane distortions in the structure of CoPc. Some noticeable changes have been observed in the Raman spectrum of CoPc thin film after its exposure with pyridine. Changes in wavenumbers and relative intensities of some of the bands have been observed. Most of the observed bands show downward shifts in the wavenumbers. These changes, are attributed to the weakening of some bonds and change of symmetry of CoPc, and confirm the coordination of pyridine molecule(s) with CoPc molecule

High-Performance Self-Powered Ultraviolet Photodetector Based on Nano-Porous GaN and CoPc p-n Vertical Heterojunction.

Gallium nitride (GaN) is a superior candidate material for fabricating ultraviolet (UV) photodetectors (PDs) by taking advantage of its attractive wide bandgap (3.4 eV) and stable chemical and physical properties. However, the performance of available GaN-based UV PDs (e.g., in terms of detectivity and sensitivity) still require improvement. Fabricating nanoporous GaN (porous-GaN) structures and constructing organic/inorganic hybrids are two effective ways to improve the performance of PDs. In this study, a novel self-powered UV PD was developed by using p-type cobalt phthalocyanine (CoPc) and n-type porous-GaN (CoPc/porous-GaN) to construct a p–n vertical heterojunction via a thermal vapor deposition method. Under 365 nm 0.009 mWcm−2 light illumination, our device showed a photoresponsivity of 588 mA/W, a detectivity of 4.8 × 1012 Jones, and a linear dynamic range of 79.5 dB, which are better than CoPc- and flat-GaN (CoPc/flat-GaN)-based PDs. The high performance was mainly attributed to the built-in electric field (BEF) generated at the interface of the CoPc film and the nanoporous-GaN, as well as the nanoporous structure of GaN, which allows for a higher absorptivity of light. Furthermore, the device showed excellent stability, as its photoelectrical property and on/off switching behavior remained the same, even after 3 months.

Charge transfer and electromagnetic enhancement processes revealed in the SERS and TERS of a CoPc thin film

Abstract Phthalocyanines are frequently used as probing molecules in the field of single-molecule surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). In this work, we systematically compare the SERS and TERS spectra from a thin cobalt phthalocyanine (CoPc) film that is deposited on a Au film. The contributions from electromagnetic (EM), resonance, and charge-transfer enhancements are discussed. Radially and azimuthally polarized vector beams are used to investigate the influences of molecular orientation and the localized surface plasmon resonance (SPR). Furthermore, two different excitation wavelengths (636 and 532 nm) are used to study the resonant excitation effect as well as the involvement of the charge-transfer processes between CoPc and the Au substrate. It is shown that the Raman peaks of CoPc are mostly enhanced by 636 nm excitation through a combination of resonant excitation, high EM enhancement, and chemical enhancement via charge transfer from the metal to the molecule. At 532 nm excitation, however, the SERS and TERS spectra are dominated by photoluminescence, which originates from a photo-induced charge-transfer process from the optically excited molecule to the metal. The contributions of the different enhancement mechanisms explain the optical contrasts seen in the TERS images of Au nanodisks covered by the CoPc film. The insight achieved in this work will help to understand the optical contrast in sub- or single-molecule TERS imaging and apply SERS or TERS in the field of photocatalysis.

Ammonia sensing performance of thin films of cobalt(II) phthalocyanine bearing fluorinated substituents

In this work, a new cobalt phthalocyanine derivative bearing 5-(trifluoromethyl)-2-mercaptopyridine substituents (CoPc) was synthesized and characterized. The redox properties of CoPc were studied by cyclic voltammetry (CV) and square wave voltammetry (SWV) methods. The sensing performance of CoPc thin films deposited by spin coating of its solution in ethylacetate was studied against low concentrations of ammonia (0.3 to 50 ppm) by measuring changes in the films’ conductivity at different humidity. It has been shown that the CoPc-based sensor displays an improved ammonia sensing performance in terms of sensitivity, response and recovery times. Ammonia can be detected in the presence of volatile organic vapors, and its detection limit was found to be 0.3 ppm. At the same time, low concentrations of CO2 comparable with the concentrations of ammonia do not influence on the sensitivity of CoPc film to ammonia, however the sensor response noticeably distorts in the presence of high CO2 concentrations in the gas mixture.

Direct Imaging of Space-Charge Accumulation and Work Function Characteristics of Functional Organic Interfaces.

The tailoring of organic systems is crucial to further extend the efficiency of charge transfer mechanisms and represents a cornerstone for molecular device technologies. However, this demands control of electrical properties and understanding of the physics behind organic interfaces. Here, a quantitative spatial overview of work function characteristics for phthalocyanine architectures on Au substrates is provided via kelvin probe microscopy. While macroscopic investigations are very informative, the current approach offers a nanoscale spatial rendering of electrical characteristics which is not possible to attain via conventional techniques. Interface dipole is observed due to the formation of charge accumulation layers in thin F16 CuPc, F16 CoPc, and MnPc films, displaying work functions of 5.7, 6.1, and 5.0 eV, respectively. The imaging and quantification of interface locations with significant surface potential and work function response (<0.33 eV for material thickness <1 nm) show also a dependency on the crystalline state of the organic systems. The work function mapping suggests space-charge carrier regions of about 4 nm at the organic interface. This reveals rich spatial electric parameters and ambipolar characteristics that may drive electrical performance at device scales, opening a realm of possibilities toward the development of functional organic architectures and its applications.

π-Electron-Assisted Relaxation of Spin Excited States in Cobalt Phthalocyanine Molecules on Au(111) Surface

We present our investigation on the spin relaxation of cobalt phthalocyanine (CoPc) films on Au(111) (CoPc/Au(111)) surface using scanning tunneling microscopy and spectroscopy. The spin relaxation time derived from the linewidth of spin-flip inelastic electron tunneling spectroscopy is quantitatively analyzed according to the Korringa-like formula. We find that although this regime of the spin relaxation time calculation by just considering the exchange interaction between itinerant conduction electrons and localized d-shells (s-d exchange interaction) can successfully reproduce the experimental value of the adsorbed magnetic atom, it fails in our case of CoPc/Au(111). Instead, we can obtain the relaxation time that is in good agreement with the experimental result by considering the fact that the π electrons in CoPc molecules are spin polarized, where the spin polarized π electrons extended at the Pc macrocycle may also scatter the conduction electrons in addition to the localized d spins. Our analyses indicate that the scattering by the π electrons provides an efficient spin relaxation channel in addition to the s-d interaction and thus leads to much short relaxation time in such a kind of molecular system on a metal substrate.

Sensing Properties of Cobalt-Phthalocyanine-Based Multipurpose Sensor

Cobalt phthalocyanine (CoPc), an organic semiconductor, has been introduced as an active sensing layer in a surface-type multipurpose sensor owing to its stability, low fabrication cost, and multifunctional sensitivity. The capacitance of the sensor was recorded to increase 26.7-fold for a change in relative humidity (RH) from 0% to 92.3%, 12.6-fold for a change in illumination from 11.5 lx to 23,000 lx, and 5.2-fold for a change in temperature from 27°C to 187°C. The morphology of the active thin film of the sensor was analyzed by atomic force microscopy, revealing a rough surface favorable for moisture absorption and light harvesting. The CoPc film was amorphous in nature according to x-ray diffraction analysis. By virtue of its response to humidity, light, and temperature, this represents an attractive approach for cost-effective environmental sensing applications.

Generation of Transparent Oxygen Evolution Electrode Consisting of Regularly Ordered Nanoparticles from Self-Assembly Cobalt Phthalocyanine as a Template.

The decoration of (photo)electrodes for efficient photoresponse requires the use of electrocatalysts with good dispersion and high transparency for efficient light absorption by the photoelectrode. As a result of the ease of thermal evaporation and particulate self-assembly growth, the phthalocyanine molecular species can be uniformly deposited layer-by-layer on the surface of substrates. This structure can be used as a template to achieve a tunable amount of catalysts, high dispersion of the nanoparticles, and transparency of the catalysts. In this study, we present a systematic study of the structural and optical properties, surface morphologies, and electrochemical oxygen evolution reaction (OER) performance of cobalt oxide prepared from a phthalocyanine metal precursor. Cobalt phthalocyanine (CoPc) films with different thicknesses were deposited by thermal evaporation on different substrates. The films were annealed at 400 °C in air to form a material with the cobalt oxide phase. The final Co oxide catalysts exhibit high transparency after thermal treatment. Their OER measurements demonstrate well expected mass activity for OER. Thermally evaporated and treated transition metal oxide nanoparticles are attractive for the functionalization of (photo)anodes for water oxidation.

Effect of x-ray irradiation on Co-phthalocyanine thin films studied by surface plasmon resonance

We explore here the effect of x-ray irradiation (7.7 keV) on Co-phthalocyanine (CoPc) thin films using surface plasmon resonance (SPR). We have found a small and partially reversible modification of the SPR spectra of CoPc in CoPc/Au bilayers upon irradiation. For a beam intensity of ~1011 photons · s−1 · mm−2 and an irradiation time of the order of 1 h, we estimate small variations of the order of ~1–5% in the refractive index associated with the scattering of the x-rays with the soft matter. A Raman study of the irradiated regions did not show structural modifications, suggesting that the observed variations in the optical properties of the CoPc films are related to modifications of their electronic configuration.

Development of stable nuclear radiation detectors based on n-silicon/cobalt-phthalocyanine heterojunctions

Abstract n-silicon/cobalt-phthalocyanine (CoPc) heterojunction based nuclear detectors have been fabricated using thermally evaporated CoPc films. Two different thicknesses of CoPc film (viz. 100 nm and 200 nm) were tried out to make detectors by depositing on chemically polished n-Si wafers. Gold film on CoPc was used as electrical contact. The detectors were characterized by measuring their current–voltage ( I–V ) and leakage current–time ( I–t ) characteristics, followed by alpha energy spectra obtained on exposure to α-particles. Variation of alpha energy resolution with applied reverse bias voltage for each of the detectors was also studied. The detectors showed very low leakage current and high breakdown voltage as compared to conventional Au/n-Si surface barrier detectors. Finally, the durability of the detectors was established by measuring their I–V characteristics and energy resolution for nearly 15 months.

Correlation between Charge Transfer and Adsorption Site in CoPc Overlayers Adsorbed on Ag(100)

In this paper we investigate the local adsorption geometry of CoPc films adsorbed on a Ag(100) surface and its effect on the molecule–substrate interactions. Using high-resolution electron energy loss spectroscopy we demonstrate that the charge-transfer mechanism at the organic–metal interface depends on the structural properties of the CoPc film. We discuss these findings in terms of the molecular adsorption sites, as determined by X-ray photoelectron diffraction measurements and multiple scattering simulations. We show that the distance between the central Co atom and the topmost Ag layer is of the order of 0.3 nm. In addition, we demonstrate that the CoPc molecules adsorb on different atomic sites depending on the superlattice symmetry, as varied by changing the coverage. This is further confirmed by low-energy electron diffraction and scanning tunneling microscopy measurements, emphasizing that, depending on the molecular superstructure adopted by the molecules, there are one or two molecules per unit...

Characteristics of CoPc/CdS hybrid diode device

CdS/CoPc hybrid heterojunctions were fabricated and characterized. CdS films were deposited by the spray pyrolysis technique on indium tin oxide (ITO)-coated glass substrates and CoPc films coated on CdS by chemical precipitation. Ag contact metal deposited on CoPc by e-beam evaporation and glass/ITO/CdS/ CoPc/Ag structures were fabricated. Rectification ratio, ideality factor, barrier height and junction parameters of the devices were determined. It is shown that device has diode characteristics with the ideality factor (n) of 4.8, rectification ratio of 4.5 and the built-in voltage (V b) of 0.48 V. Absorption energy for CoPc was found as 1.57 eV. The results encourage utilizing CoPc as absorber organic material for solar cells.

Capacitive-resistive measurements of cobalt-phthalocyanine organic humidity sensors

In this study, the fabrication and characterization of capacitive humidity sensors using cobalt-phthalocyanine (CoPc) as the active material were presented. Thin films of CoPc were deposited by drop casting on glass substrates with pre-deposited aluminum electrodes to form Al/CoPc/Al surface-type humidity sensors. The effect of humidity on the electrical properties of the CoPc film was investigated by measuring capacitance and resistance of the samples at four different frequencies of the applied voltage. It was observed that the capacitance of the sensor increased while the resistance decreased with raising the relative humidity. It was also found that the values of capacitance and resistance decreased with increasing frequency. The optical absorption spectra and optical band gap energy of CoPc films were measured. The structure of CoPc powder and thin films has been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Results of XRD studies show that the film structure is polycrystalline with the monoclinic structure while thin films have a peak for annealing temperatures with (100) orientation. Also, the surface morphology (grain size and roughness) for CoPc films have been studied by AFM.

Site-specific intermolecular valence-band dispersion in α-phase crystalline films of cobalt phthalocyanine studied by angle-resolved photoemission spectroscopy.

The valence band structure of α-phase crystalline films of cobalt phthalocyanine (CoPc) grown on Au(111) is investigated by using angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation. The photo-induced change in the ARPES peaks is noticed in shape and energy of the highest occupied molecular orbital (HOMO, C 2p) and HOMO-1 (Co 3d) of CoPc, and is misleading the interpretation of the electronic properties of CoPc films. From the damage-free normal-emission ARPES measurement, the clear valence-band dispersion has been first observed, showing that orbital-specific behaviors are attributable to the interplay of the intermolecular π-π and π-d interactions. The HOMO band dispersion of 0.1 eV gives the lower limit of the hole mobility for α-CoPc of 28.9 cm(2)V(-1)s(-1) at 15 K. The non-dispersive character of the split HOMO-1 bands indicates that the localization of the spin state is a possible origin of the antiferromagnetism.