Nimmakayala V. V. Subbarao

Vapor phase sensing of ammonia at the sub-ppm level using a perylene diimide thin film device

The fabrication of a two terminal sensor device based on a histidine substituted perylene diimide (PDI-HIS) thin film for the sensitive detection and quantification of ammonia (NH3) vapors by monitoring the changes in its current intensity is reported at room temperature under ambient conditions. The thin film morphological variations of the drop cast PDI-HIS films before and after exposure to NH3 vapors are characterized by FESEM and TEM confirming the diffusion/adsorption of the NH3 vapors. The solution cast PDI-HIS thin film gas sensor device exhibited rapid, highly sensitive and selective vapor phase response towards NH3 with a detection limit as low as 0.56 ppm which is much lower than the maximum permissible limit set for NH3 (25 ppm) for prolonged exposure. Furthermore, control sensing experiments performed using alkyl substituted PDI (PDI-n-octyl) demonstrated that the presence of histidine groups at the imide position of PDI-HIS drastically affects the solid-state aggregation mode as well as redox potential that ultimately enhances the sensing response of the device. The key performance parameters of the device such as sensitivity, response/recovery time, selectivity, recyclability, stability and detection limit demonstrated the protocol as simple, reliable, cost-effective and most efficient in performing NH3 detection under very realistic conditions.

Enhanced Environmental Stability Induced by Effective Polarization of a Polar Dielectric Layer in a Trilayer Dielectric System of Organic Field-Effect Transistors: A Quantitative Study

We report a concept fabrication method that helps to improve the performance and stability of copper phthalocyanine (CuPc) based organic field-effect transistors (OFETs) in ambient. The devices were fabricated using a trilayer dielectric system that contains a bilayer polymer dielectrics consisting of a hydrophobic thin layer of poly(methyl methacrylate) (PMMA) on poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) or polystyrene (PS) with Al2O3 as a third layer. We have explored the peculiarities in the device performance (i.e., superior performance under ambient humidity), which are caused due to the polarization of dipoles residing in the polar dielectric material. The anomalous behavior of the bias-stress measured under vacuum has been explained successfully by a stretched exponential function modified by adding a time dependent dipole polarization term. The OFET with a dielectric layer of PVA or PVP containing hydroxyl groups has shown enhanced characteristics and remains highly stable without any degradation even after 300 days in ambient with three times enhancement in carrier mobility (0.015 cm(2)·V(-1)·s(-1)) compared to vacuum. This has been attributed to the enhanced polarization of hydroxyl groups in the presence of absorbed water molecules at the CuPc/PMMA interface. In addition, a model has been proposed based on the polarization of hydroxyl groups to explain the enhanced stability in these devices. We believe that this general method using a trilayer dielectric system can be extended to fabricate other OFETs with materials that are known to show high performances under vacuum but degrade under ambient conditions.

High carrier mobility of CoPc wires based field-effect transistors using bi-layer gate dielectric

Polyvinyl alcohol (PVA) and anodized Al2O3 layers were used as bi-layer gate for the fabrication of cobalt phthalocyanine (CoPc) wire base field-effect transistors (OFETs). CoPc wires were grown on SiO2 surfaces by organic vapor phase deposition method. These devices exhibit a field-effect carrier mobility (μEF) value of 1.11 cm2/Vs. The high carrier mobility for CoPc molecules is attributed to the better capacitive coupling between the channel of CoPc wires and the gate through organic-inorganic dielectric layer. Our measurements also demonstrated the way to determine the thicknesses of the dielectric layers for a better process condition of OFETs.

Low voltage, low cost, flexible and balanced ambipolar OFETs based on Br2PTCDI-C18/CuPc fabricated on an Al foil gate substrate with good ambient stability

Low cost, flexible, ambient stable, low operating voltage and balanced ambipolar organic field-effect transistors were fabricated with plastic over-head projector (OHP) sheets as transparent substrates using laminated Al foil as a gate electrode without any mechanical or electrochemical polishing, using Br2PTCDI-C18 (n-channel) and copper phthalocyanine (CuPc) (p-channel) as the active materials and PMMA/PVA or C-PVA (crosslinked PVA) as the dielectric material. The optimized Br2PTCDI-C18/CuPc heterostructure OFET devices exhibited an ambipolar nature with operating voltages of ±10 V and ±3 V for the PMMA/PVA and C-PVA dielectric materials respectively. The electron and hole mobilities achieved are as high as 1.2 × 10−2 cm2 V−1 s−1 and 5.5 × 10−3 cm2 V−1 s−1 respectively. These ambipolar devices were also studied under different conditions (vacuum and ambient) and the results are reported here. The hole currents exhibited an anomalous bias stress effect whereas the electron currents decayed with time under vacuum. The anomalous bias stress was explained with the slow polarization of dipoles in the polar dielectric materials. However, under humidity, both currents decayed due to charge trapping at the interface by absorbed water molecules. The devices exhibited reproducible results in the ambient range even after several days of exposure. These results demonstrate the very easy fabrication of ambipolar devices on a low cost substrate and gate with good ambient and electrical stabilities, which are highly desirable for practical applications.

Local diffusion induced roughening in cobalt phthalocyanine thin film growth.

We have studied the kinetic roughening in the growth of cobalt phthalocyanine (CoPc) thin films grown on SiO2/Si(001) surfaces as a function of the deposition time and the growth temperature using atomic force microscopy (AFM). We have observed that the growth exhibits the formation of irregular islands, which grow laterally as well as vertically with coverage of CoPc molecules, resulting rough film formation. Our analysis further disclosed that such formation is due to an instability in the growth induced by local diffusion of the molecules following an anomalous scaling behavior. The instability relates the (ln(t))(1/2), with t as deposition time, dependence of the local surface slope as described in nonequilibrium film growth. The roughening has been characterized by calculating different scaling exponents α, β, and 1/z determined from the height fluctuations obtained from AFM images. We obtained an average roughness exponent α = 0.78 ± 0.04. The interface width (W) increases following a power law as W ∼ t(β), with growth exponent β = 0.37 ± 0.05 and lateral correlation length (ξ) grows as ξ ∼ t(1/z) with dynamic exponent 1/z = 0.23 ± 0.06. The exponents revealed that the growth belongs to a different class of universality.

Effects of Dielectric Material, HMDS Layer, and Channel Length on the Performance of the Perylenediimide-Based Organic Field-Effect Transistors

It is well-known that the improvement in the performance of organic field-effect transistors (OFETs) relies primarily on growth properties of organic molecules on gate dielectrics, their interface behavior, and on understanding the physical processes occurring during device operation. In this work, the relation of varying the dielectric materials in an n-type OFET device based on 1,7-dibromo-N,N′-dioctadecyl-3,4,9,10-perylenetetracarboxylic diimide (Br2PTCDI-C18) molecule on a low-cost glass substrate at different channel lengths is reported, which is conceptually very important and fundamental in the context of device performance. Anodized alumina (Al2O3) along with dielectric films of polyvinyl alcohol (PVA) or polymethylmethacrylate (PMMA) was used to fabricate the devices and study their influence on various transistor properties. In addition, the effects of a thin hexamethyldisilazane (HMDS) layer on the performance of OFETs including their contact resistances were studied with the channel length variations. The devices with PVA dielectric material exhibited the maximum mobility values of 0.012–0.025 cm2 V–1 s–1 irrespective of varying channel lengths from 25 to 190 μm. The bias-stress measurements were recorded to realize the effects of the channel length and HMDS layer on the stability of the devices. The on/off ratios and electrical stabilities of these devices were enhanced significantly by modifying the surface of the PVA dielectric layer using a thin layer of HMDS. Similarly, in the case of PMMA dielectric layer, a drastic enhancement in the on/off ratio and bias-stress stability was observed. Characterization of all devices at different channel lengths using different dielectric materials permitted us to identify the effects of contact resistance on OFET devices. The stability of the devices in relation to the bias-stress measurements of devices by varying channel lengths and surface modification was systematically investigated. A careful analysis of oxide gate dielectrics modified with polymer-based dielectric materials, contact resistance, influence of thin HMDS layer on the electrical properties, and other parameters on top-contact bottom-gated configured n-type OFET devices is presented herein.

Growth mechanism of Cobalt(II) Phthalocyanine(CoPc) thin films on SiO2 and muscovite substrates

Thin films of Cobalt(II) Phthalocyanine (CoPc) were grown by thermal evaporation technique on two different substrates namely SiO2 and atomically cleaned muscovite mica(001) at various substrate temperatures. Deposition rate has been maintained to 0.3A/sec during the growth of the films. The growth process is studied by means of atomic force microscopy (AFM). Films on SiO2 exhibit only three-dimensional islands and uniformity of these islands improved with substrate temperatures, whereas films on mica (001) consist of long oriented percolated structures. The results revealed that the growth mechanism of CoPc strongly depends on substrate temperatures as well as nature of substrate used. Optical properties were characterized by UV-Visible spectroscopy and structural properties were studied using X-ray diffraction.