Shashi Paul

Memory effect in thin films of insulating polymer and C60 nanocomposites

We describe the use of C60 fullerene molecules as the charge storage medium in an insulating poly-vinyl-phenol (PVP) polymer. The simple metal?organic?metal (MOM) sandwich structure devices deposited from solution exhibit distinct high and low conduction states, which can be used to program read, write and erase memory operations. The charge transfer and retention in C60 molecules at room temperature has been confirmed by capacitance?voltage and Raman spectroscopy measurements. Conducting atomic force microscopy has been used to demonstrate that high and low conductance states persist even at the nanoscale.

Overview of organic memory devices.

The demand for more efficient and faster memory structures is greater today than ever before. The efficiency of memory structures is measured in terms of storage capacity and the speed of functioning. However, the production cost of such configurations is the natural constraint on how much can be achieved. Organic memory devices (OMDs) provide an ideal solution, in being inexpensive, and at the same time promising high performance. However, all OMDs reported so far suffer from multiple drawbacks that render their industrial implementation premature. This article introduces the different types of OMDs, discusses the progress in this field over the last 9 years and invokes conundrums that scholars of this field are currently faced with, such as questions about the charging mechanism and stability of devices, contradictions in the published work and some future directions.

Realization of Nonvolatile Memory Devices Using Small Organic Molecules and Polymer

Intensive research is currently underway to exploit the highly interesting properties of nano-sized particles and organic molecules, for applications in the optical and electronic sectors. Recently, it has been shown that nano-sized particles and small molecules can be used in polymer matrices to realize memory devices. Such memory devices are simple to fabricate, relying typically on the spin on technique. In this work, an attempt is made to utilize small organic molecules embedded in an organic polymer, in the making of the presented memory devices. The ever elusive charging mechanism in such configurations, is also discussed

Growth of low temperature silicon nano-structures for electronic and electrical energy generation applications

This paper represents the lowest growth temperature for silicon nano-wires (SiNWs) via a vapour-liquid–solid method, which has ever been reported in the literature. The nano-wires were grown using plasma-enhanced chemical vapour deposition technique at temperatures as low as 150°C using gallium as the catalyst. This study investigates the structure and the size of the grown silicon nano-structure as functions of growth temperature and catalyst layer thickness. Moreover, the choice of the growth temperature determines the thickness of the catalyst layer to be used.The electrical and optical characteristics of the nano-wires were tested by incorporating them in photovoltaic solar cells, two terminal bistable memory devices and Schottky diode. With further optimisation of the growth parameters, SiNWs, grown by our method, have promising future for incorporation into high performance electronic and optical devices.

Gold Nanoparticle Based Electrically Rewritable Polymer Memory Devices

Organic and polymer based electronic devices are currently the subject of a great deal of scientific investigation and development. This interest can be attributed to the low cost, easy processing steps and simple device structures of organic electronics when compared to conventional silicon and inorganic electronics. In the field of organic electronic memories, non-volatile, rewritable polymer memory devices (PMDs) have shown promise as a future technology where cost and compatibility with flexible substrates are important factors. In this paper PMDs based on active layers containing an admixture of polystyrene, gold nanoparticles and 8-hydroxyquinoline will be presented, showing the devices’ electrical characteristics and memory performance attributes, and where possible discussing possible mechanisms of operation.

Electrical bistability in a composite of polymer and barium titanate nanoparticles

Growth in the use of organic materials in the fabrication of electronic devices is on the rise. Recently, some attempts have been undertaken to manufacture polymer memory devices. Such devices are fabricated by depositing a blend (an admixture of organic polymer, small organic molecules and nanoparticles) between two metal electrodes. These devices show two electrical conductivity states (‘high’ and ‘low’) when a voltage is applied, thus rendering the structures suitable for data retention. In this paper, we describe an attempt to fabricate memory devices using ferroelectric nanoparticles embedded in an organic polymer. This paper also discusses issues related to the observed memory effect.

Electrical properties of nanometre thin film polystyrene for organic electronic applications

Polystyrene is a promising organic insulator for use in organic electronic devices owing to its excellent electrical and mechanical characteristics. However, much of the work that has been done in characterising polystyrene has been done on films several micrometres thick, and as such is difficult to relate to the properties that need to be considered for electronic devices, where films thicknesses are generally in the nanometre range. In this report, we focus on the electrical characteristics of nanometre thin films, both in terms of current-voltage behavior, and capacitance-voltage characteristics and how the properties of the polystyrene films are altered by differing annealing temperatures.

Making Plastic Remember: Electrically Rewritable Polymer Memory Devices

The first author would like to thank the Engineering and Physical Sciences Research Council EPSRC and the National Physical Laboratory NPL for the financial support received for his studentship. The second author would like to thank EPSRC for the financial support Grant No. EP/ E047785/1.

Substrate Sensitivity of the Adhesion and Material Properties of RF-PECVD Amorphous Carbon

Hydrogenated amorphous carbon (a-C:H), deposited by the if-plasma enhanced chemical vapour deposition (rf-PECVD) technique, is a promising material for large area electronic and interlayer dielectric applications. The structural and electronic properties of rf-PECVD a-C:H, deposited at room temperature from CH 4 /He and CH 4 /Ar gas mixtures, are shown to be sensitive to the substrate on which the thin film is deposited. The choice of substrate (c-Si or C7059 glass), and the existence and geometrical dimensions of any metallic pattern on the substrate surface, can result in significant spatial variations in the a-C:H adhesion and material properties. The observed effects are attributed to potential variations across the metal patterned substrates which influence the ‘local’ dc self-bias. This leads to spatial variations in the growth conditions and hence material properties. For electronic device and dielectric isolation applications this effect can result in significant variations in operating performance. The nature of the substrate and any overlying metallisation pattern are therefore important considerations.

Statistical analysis of multiple access interference in Rayleigh fading environment for MIMO CDMA systems

A major limiting factor in the performance of MIMO-CDMA systems is multiple access interference (MAI) which can reduce the system capacity as well as bit error rate (BER). Thus, statistical characterization of MAI is vital in analyzing the performance of such systems. Since, the statistical analysis of MAI in MIMO-CDMA is quite involved especially in the presence of fading channels, existing works in literature employ suboptimal approaches to detect the subscriber without involving the need for MAI statistics such as successive interference cancellation (SIC) and parallel interference cancellation (PIC). To date, the exact characterization of multiple access interference in MIMO-CDMA is an unsolved problem. In this paper, we derive the expressions for the probability density function of MAI and MAI plus noise in MIMO-CDMA systems in the presence of both Rayleigh fading channels and additive Gaussian noise. Simulation results show that the theoretical predictions are very well substantiated.