Application of quantum dots in organic memory devices: A brief overview

Quantum dots (QDs) are nanosized regions capable of restricting a single electron, or a few electrons, to the region in three dimensions and in which the electrons no longer occupy band-like energy states, but rather discrete energy states just as they would in an atom. Quantum mechanical phenomena result from this, hence the term quantum confinement. Originally, QDs were grown from semiconductors such as cadmium selenide or cadmium telluride. Since then, however, the synthesis of QDs from nearly every semiconductor and from many metals and insulators has been reported. Quantum dots of semiconductors and metals are currently the focus of intense research. Their electrical, optical, and magnetic properties are different from those of the bulk systems being more like those from molecular-like clusters in which a large number of atoms are on or near the surface. Apart from unique physical properties, QDs also exhibit interesting applications. With their advantage of size, they are ideal for data storage or memory applications to provide high-density memory elements. Potential applications of nonvolatile flash memory devices utilizing QDs have resulted in extensive efforts being made to form QDs, acting as both charging and discharging islands, by a variety of methods. Semiconductor or metallic QDs incorporated within organic or polymeric materials have demonstrated a memory effect when subjected to an electrical bias voltage. Memory phenomenon in QDs arise from their electrical bistability, which is triggered by charge confinement via a suitable voltage pulse. These materials have shown potential applications in digital information storage because of their good stability, flexibility and fast response speed. Organic electrical bistable materials are those that exhibit two kinds of different stable conductive states by applying appropriate voltages. The materials can be switched from low conductive state ("0" or OFF state) to high conductive state ("1" or ON states) by applying an activation voltage. This process is called 'write'. The high conductive state can remain stable without a bias voltage, and can be read back at a lower voltage. The reverse process is realized by applying a reverse bias when the conductive status changes from a high conductive state to a low conductive state, this being termed 'erase'. Materials functionalized with 'erase' and 'write' can be used as RAM (random access memory) and Flash memory. Some materials are write-once-read-many times (WORM), which can be used as ROM (read-only-memory) devices. For commercial use of data storage, devices should satisfy a number of requirements, such as, room temperature operation, low activation voltage to save energy, high ON/OFF ratio, short response time, long retention time and durability. This chapter reviews the recent progress of memory devices exhibiting electric bistability, such devices being based on composites containing quantum dots of semiconductors or metals embedded in organic macromolecular materials.