R.A. Collins

Optical properties of lead phthalocyanine (PbPc) thin films

Abstract The optical properties of thin films of lead phthalocyanine in both monochlinic and triclinic forms have been studied. Optical parameters have been determined from both absorption and reflectivity data for wavelengths in the UV and visible regions. Monoclinic films show two absorption bands, identified as exciton absorption at low energy and fundamental absorption above 2.4 eV. A direct allowed transition at 2.42 eV was oserved. A doublet of energies in the absorption spectrum at 1.86 and 1.74 eV is in agreement with observations for other metal phthalocyanines. The spectrum for triclinic materials shows a strong absorption band at 1.61 eV and two shoulders at 1.72 and 1.95 eV. The present results support the view that a band model analysis of this material is valid despite the very low mobility values found for both monoclinic and triclinic films.

Electrical, structural and gas sensing properties of zinc phthalocyanine thin films

Abstract The dark conductivity of zinc phthalocyanine (ZnPc) has been studied as a function of material purity, crystal phase transformation and temperature with particular regard to gas sensitivity in air, O 2 , N 2 , argon, NH 3 and NH 3 -air mixtures. α-ZnPc was found to grow in the form of randomly oriented micro- crystallites but the β form showed oriented needle-like and whisker growth. The electrical properties were found to be dependent on material purity. Entrainer- sublimed ZnPc showed higher conductivity than impure material and displayed reproducible linear characteristics with less drift and hysteresis. The conductivity of both α- and β-ZnPc is found to be critically dependent on the presence of O 2 . The sensitivity to other gases differs for the α and β forms but in both cases NH 3 causes a large dark conductivity decrease, possibly owing to catalytic behaviour, effectively removing oxygen acceptors. Conductivity-temperature data indicate a transition from extrinsic to non-extrinsic conduction for most cases. The conductivity of β-ZnPc is found to be greater than that of α-ZnPc, in contrast with other phthalocyanines. The relative sensitivities to the various gases suggests that ZnPc may be a viable material for selective gas sensing devices.

A study of the interaction between nitrogen dioxide and lead phthalocyanine using electrical conduction and optical absorption

Abstract Electrical conductivity and optical absorption are studied simultaneously for lead phthalocyanine films exposed to nitrogen dioxide to assess their relative merits for the development of a PbPc-based thin film NO2 sensor. Additionally, frequency variations of an oscillating quartz crystal during NO2 adsorption together with Fourier Transform Infrared analysis have been used to investigate diffusion and bonding of NO2 at the surface and in the bulk of the films. The results indicate that sensors based on electrical conductivity offer a better response and recovery provided that exposure is limited. Long-term exposure prevents film recovery. This is related to the relatively slow in-diffusion of the NO2 into the bulk, leading to an NO2:PbPc molecular ratio of 4:1 in the bulk after extended exposure.

The effect of oxygen on the electrical characteristics of triclinic lead phthalocyanine

Abstract The electrical characteristics of triclinic PbPc have been studied for thin films exposed to oxygen and dry air at atmospheric pressure for periods up to 25 weeks. Conductivity is found to be critically dependent upon the presence of O2, prolonged exposure enhancing conductivity and decreasing the conduction activation energies. Both ohmic and space-charge-limited conduction (SCLC) are observed, together with a low temperature mechanism which may be variable range hopping. Oxygen exposure leads to more rapid oxygen doping of the films than exposure to dry air. SCLC is controlled by either an exponential trap distribution or a simple dominant level dependent upon the film exposure/annealing history. Oxygen impurities appear to play a dual role of both acceptors and trap levels.

The influence of chlorine on the electrical properties of lead phthalocyanine thin film gas sensors

Quantitative studies of changes in semiconductor behaviour of PbPc thin films as a function of exposure to chlorine, annealing in chlorine, and subsequently in vacuum, were made for a range of temperatures. It was found that Cl2 generates acceptor levels within the band gap of the PbPc. The acceptor level energy is a function of the concentration of chlorine incorporated into the bulk of the PbPc. The current-voltage characteristics at fixed temperatures and the variation of current as a function of temperature at constant bias voltage have been established. Both ohmic and space charge-limited conduction (SCLC) were identified and found to be controlled by the concentration of chlorine incorporated within the PbPc bulk. Optimized measuring conditions for the use of PbPc films as a viable chlorine sensor are also reported.

Electrical conduction mechanisms in thermally evaporated lead phthalocyanine thin films

The AC electrical behaviour of sandwich devices fabricated entirely in vacuum using thermally evaporated thin films of lead phthalocyanine and gold electrodes is observed over the frequency range to Hz and the temperature range 160 to 440 K. In the high-frequency region the AC conductivity is proportional to . Capacitance and dissipation factors decrease with increasing frequency and increase with increasing temperature. The results obtained show the relative importance of the hopping model and band theory in describing the film conduction with regard to the operating conditions. The former is dominant at low temperatures and high frequencies, whereas the latter is dominant at high temperatures and low frequencies.

Potential distribution and hot electron attenuation measurements in MIMIM thin film triodes

Abstract The electroforming and potential distribution in M-SiO x -M-SiO x -M thin film triodes have been studied using both Ag and Au electrodes. Asymmetric potential distributions were observed, in qualitative agreement with the observations of Hickmott, but no evidence was found to confirm the existence of a narrow high field region close to the cathode, as previously postulated. Hot electron attenuation was studied and attenuation lengths were determined to be in reasonable quantitative agreement with measurements using MIM diodes. Anomalous behaviour associated with the thickness of the central electrode appears to be a consequence of the self-healing dielectric breakdown during the electroforming process.

Spectral response of monoclinic and triclinic lead phthalocyanine to nitrogen dioxide

Abstract It has been found that the visible and near infrared spectra of both the monoclinic and triclinic phases of lead phthalocyanine (PbPc) thin films are affected significantly by the presence of nitrogen dioxide. However, due to chemisorption of the nitrogen dioxide the films do not recover completely on evacuation but require heating in vacuum to effect a partial recovery. The monoclinic phase gives the largest response at 725 nm but the interaction appears to be irreversible and the original state cannot be recovered, whereas the triclinic phase using 510 nm incident radiation can be recovered almost completely and thus offers a more viable prospect of development of an optical gas sensor. Values for the band gaps of both phases are obtained from the absorption data as follows: monoclinic PbPc, 2.48 eV (in vacuum), 2.44 eV (in NO 2 ); and triclinic PbPc, 2.49 eV (in vacuum), 2.46 eV (in NO 2 ).

Adsorption and bulk diffusion of chlorine in monoclinic lead phthalocyanine thin film gas sensors

Abstract The chlorine sensitivity of thin films of monoclonic lead phthalocyanine has been studied. The temporal behaviour of the dark conductivity during chlorine exposure has been interpreted on the basis of the creation of acceptor states both on the surface and in the bulk. Support for this model is provided by observations of bulk diffusion of the chlorine, and an approximate diffusion rate at room temperature has been determined. These observations suggest that bulk diffusion, which has received relatively little attention in previous studies of phthalocyanine gas sensors, may warrant a more detailed investigation.

Morphology and gas sensitivity of erbium di-phthalocyanine thin films

Studies have been made of the application of certain phthalocyanine films in the detection of toxic gases such as chlorine. Consideration has been given to preparation parameters such as deposition conditions (evaporation rate, ambient pressure, post deposition annealing) together with varying central metal atoms within the phthalocyanine molecule. Particular studies have been made concerning the relationship between elevated substrate temperature deposition, the molecular structure and the corresponding sensitivity of the films to gases. The present results are considered within the context of the development of an economically viable selective thin film gas sensor.