Soumen Samanta

Enhanced Cl2 sensitivity of cobalt-phthalocyanine film by utilizing a porous nanostructured surface fabricated on glass

In this paper, we demonstrate a very simple and effective approach to improve the sensitivity and the low detection limit of cobalt phthalocyanine (CoPc) films towards the detection of chlorine by creating a porous nanostructured surface on a glass substrate via a vapor phase etching process. CoPc films grown on etched glass (CoPc-etched films) exhibited entirely different morphology as compared to CoPc films grown on plain glass (CoPc-plain films). For 60 nm thickness, randomly distributed CoPc nanostructures were grown on the etched surface, whereas the CoPc-plain film showed an elongated granular structure. For 250 ppb Cl2 exposure, the CoPc-etched film showed a response of ∼105%, which is ∼5 times higher than the CoPc-plain film (20%). In addition, it can detect Cl2 down to 100 ppb concentration; this low detection limit is superior to CoPc-plain film (250 ppb). The improved gas sensing property of CoPc-etched film is ascribed to the presence of more interaction sites for gas adsorption, which is confirmed by charge transport, X-ray photoelectron spectroscopy and Kelvin probe measurement. This novel approach of improving the sensitivity and low detection limit paves a new way for the application of surface etching in the gas sensing field of organic semiconductors.

Trap Free Space Charge Limited Conduction and High Mobility in Cobalt Phthalocyanine - Iron Phthalocyanine Composite Thin Films

Charge transport of pure and composite thin films (20 nm thickness) of cobalt phthalocyanine (CoPc) and iron phthalocyanine (FePc) grown by molecular beam epitaxy has been investigated in the temperature range of 300 K-25 K. Composite films with optimum composition showed very high mobility of 110 cm2V-1s-1 at room temperature. X-ray diffraction and UV-Vis studies showed that the films were well oriented with planar co-facial structure. The current density-voltage (J-V) characteristics of composite films showed trap free space charge limited conduction (SCLC) while individual phthalocyanine films showed SCLC with exponential distribution of traps. Several factors such as effect of substrate, very small thickness and effect of mixing two phthalocyanines are responsible for the very high mobility observed in present study.

Molecular Beam Epitaxy Growth of Iron Phthalocyanine Nanostructures

FePc films of different thickness have been deposited by molecular beam epitaxy (MBE) as a function of substrate temperature (25–300°u2009C) and deposition rate (0.02–0.07 nm/s). The morphology of a 60 nm alpha‐phase film has been tuned from nanobrush (nearly parallel nanorods aligned normal to the substrate plane) to nanoweb (nanowires forming a web‐like structure in the plane of the substrate) by changing the deposition rate from 0.02 to 0.07 nm/s. We propose growth mechanisms of nanoweb and nanobrush morphology based on the van der Waals (vdW) epitaxy. For air exposed FePc films I‐V hysteresis was observed at 300 K and it is attributed to surface traps created by chemisorbed oxygen.

Growth and Electrical Transport Properties of Organic Semiconductor Thin Films

Hexadecafluorophthalocyanine (F16CuPc) and Cobalt phthalocyanone (CoPc) thin films of different thickness (20-200nm) have been grown by Molecular Beam Epitaxy (MBE) using different deposition rate (0.2 – 1.0 Å/s). For nanowire type growth lower deposition rate and for films of smooth surface higher deposition rate are found suitable. Charge transport (J~V) of CoPc and F16CuPc films is governed by bulk-limited processes with a bias dependent crossover from Ohmic to trap-free space-charge-limited conduction. The mobility (μ) values at 300 K were found 4.5 and 5.5 cm2 V−1 s−1 for CoPc and F16CuPc films respectively. Mechanism of reverse rectification behavior of an organic heterojunction comprising of CoPc and F16CuPc is explained by Kelvin Probe measurement.

Influence of adsorbed oxygen on charge transport and chlorine gas-sensing characteristics of thin cobalt phthalocyanine films

We have investigated the morphology, charge transport, and gas-sensing characteristics of thin films of cobalt phthalocyanine (CoPc) deposited on glass and sapphire substrates, using molecular beam epitaxy (MBE). CoPc films deposited on glass were found to be highly disordered. The ambient oxygen was found to be chemisorbed and created deep trap states, which led not only to hysteretic current-voltage (J-V) characteristics but also reduce the charge mobility. These properties render them unsuitable for gas-sensing. On the other hand, films deposited on sapphire were polycrystalline, which was attributed to an improved molecule-substrate interaction. The physically sorbed oxygen only created shallow traps, and the J-V characteristics were non-hysteretic, rendering them suitable for gas-sensing applications. It was demonstrated that the ultrathin (20 nm) CoPc films deposited on sapphire acted as highly sensitive and selective sensors for chlorine present in the wCl concentration range of 5 × 10−9−2 × 10−6 (5–2000 ppb).

Chemi-resistive gas sensing properties of cobalt-phthalocyanine / iron-phthalocyanine composite films

Thin film of cobalt phthalocyanine (CoPc), iron phthalocyanine (FePc) and CoPc-FePc composite were grown on sapphire substrate by molecular beam epitaxy under identical experimental conditions. Composite films showed very high mobility ∼ 110 cm²V⁻¹s⁻¹ at room temperature, which is nearly two orders of magnitude higher as compared to the pure CoPc and FePc films. We demonstrate that due to high mobility of composite phthalocyanine films, it showed enhanced sensitivity for chlorine (Cl<inf>2</inf>) gas as compared to pure CoPc and FePc films