Switching mechanisms, electrical characterisation and fabrication of nanoparticle based non-volatile polymer memory devices.

Polymer and organic electronic memory devices offer the potential for cheap, simple memories that could compete across the whole spectrum of digital memories, from low cost, low performance applications, up to universal memories capable of replacing all current market leading technologies, such as hard disc drives, random access memories and Flash memories. Polymer memory devices (PMDs) are simple, two terminal metal-insulator-metal (MIM) bistable devices that can exist in two distinct conductivity states, with each state being induced by applying different voltages across the device terminals. Currently there are many unknowns and much ambiguity concerning the working mechanisms behind many of these PMDs, which is impeding their development. This research explores some of these many unanswered questions and presents new experimental data concerning their operation. One prevalent theory for the conductivity change is based on charging and charge trapping of nanoparticles and other species contained in the PMD. The work in this research experimentally shows that gold nanoparticle charging is possible in these devices and in certain cases offers an explanation of the working mechanism. However, experimental evidence presented in this research, shows that in many reported devices the switching mechanism is more likely to be related to electrode effects, or a breakdown mechanism in the polymer layer. Gold nanoparticle charging via electrostatic force microscopy (EFM) was demonstrated, using a novel device structure involving depositing gold nanoparticles between lateral electrodes. This allowed the gold nanoparticles themselves to be imaged, rather than the nanoparticle loaded insulating films, which have previously been investigated. This method offers the advantages of being able to see the charging effects of nanoparticles without any influence from the insulating matrix and also allows charging voltages to be applied via the electrodes, permitting EFM images to capture the charging information in near real-time. Device characteristics of gold nanoparticle based PMDs are presented, and assessed for use under different scenarios. Configurations of memory devices based on metal-insulator-semiconductor (MIS) structures have also been demonstrated. Simple interface circuitry is presented which is capable of performing read, write and erase functions to multiple memory cells on a substrate. Electrical properties of polystyrene thin films in the nanometre thickness range are reported for the first time, with insulator trapped charges found to be present in comparable levels to those in silicon dioxide insulating films. The dielectric breakdown strength of the films was found to be significantly higher than bulk material testing would suggest, with a maximum dielectric strength of 4.7 MV•cm-1 found, compared with the manufacturers bulk value of 0.2 – 0.8 MV•cm-1. Conduction mechanisms in polystyrene were investigated with the dominant conduction mechanism found to be Schottky emission.Engineering and Physical Sciences Research Council (EPSRC)The National Physical laboratory, UK (NPL

Effect of processing variables and bulk composition on the surface composition of spray dried powders of a model food system

Abstract The surface compositions of food powders created from spray drying solutions containing various ratios of sodium caseinate, maltodextrin and soya oil have been analysed by Electron Spectroscopy for Chemical Analysis. The results show significant enrichment of oil at the surface of particles compared to the bulk phase and, when the non-oil components only are considered, a significant surface enrichment of sodium caseinate also. The degree of surface enrichment of both oil and sodium caseinate was found to increase with decreasing bulk levels of the respective components. Surface enrichment of oil was also affected by processing conditions (emulsion drop size and drying temperature), but surface enrichment of sodium caseinate was relatively insensitive to these. The presence of "pock marks" on the particle surfaces strongly suggests that the surface oil was caused by rupturing of emulsion droplets at the surface as the surrounding matrix contracts and hardens

Electrical and morphological properties of polystyrene thin films for organic electronic applications.

This study investigates the electrical and morphological properties of polystyrene layers in the nanometre thickness range for organic and polymer based electronic applications. The paper aims to providing conduction data and information on trapped charges present in the polystyrene layer, as well as investigating how polystyrene properties change under differing annealing conditions. The maximum dielectric strength was found to be 4.0 MV cm−1, while fixed trapped charge and mobile trapped charge average densities were calculated to be 9.9 × 10^11 cm−2 and 2.6 × 10^12 cm−2 respectively. Optimum electrical characteristics were obtained at an anneal temperature of 90 °C, which is just below the glass transition temperature for polystyrene.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

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.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

Making plastic remember: electrically rewritable polymer memory devices.

Organic based electronic memory devices are currently receiving an unprecedented amount attention as possible alternatives for conventional semiconductor memories, due to their simple device structures, ease of fabrication, compatibility with flexible substrates and low cost. Polymer memory devices (PMDs) based on active layers containing gold nanoparticles (Q-Au) with 8-hydroxyquinoline (8HQ) will be presented in this report. Memory performance attributes will also be presented, which are less reported in the literature, but still essential for the realisation of viable organic memory technologies.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

First contact-charging of gold nanoparticles by electrostatic force.

The use of nanoparticle materials in the manufacture of electronic polymer memory devices is on the rise. Organic memory devices are fabricated by depositing a blend of organic polymer, small organic molecules, and nanoparticles between two metal electrodes. The primary aim is to produce devices that exhibit two distinct electrical conductance states when control voltages are applied. By retaining the states when power is removed can be viewed as the realization of nonvolatile memory. In this letter, an attempt is made to further understand the conundrums that scholars in this field are currently facing, with questions about the nanoparticle charging mechanism being investigate.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

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 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

Gold nanoparticle charge trapping and relation to organic polymer memory devices.

Nanoparticle-based polymer memory devices (PMDs) are a promising technology that could replace conventional silicon-based electronic memory, offering fast operating speeds, simple device structures and low costs. Here we report on the current state of nanoparticle PMDs and review some of the problems that are still present in the field. We also present new data regarding the charging of gold nanoparticles in metal–insulator–semiconductor capacitors, showing that charging is possible under the application of an electric field with a trapped charge density due to the nanoparticles of 3.3x10^12 cm−2.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