QDIN · Quantum Dots incorported into Nanowires

Final Activity Report Summary - QDIN (Quantum Dots Incorported into Nanowires)

Technological advancements by the microelectronics industry have been phenomenal; it is however widely accepted that the next generation of nanoscale devices and sensors will have to be fabricated using alternative techniques and approaches. The current top-down methodology is limited, firstly due to resolution, and, secondly, because of cost. Clearly, a bottom-up approach will have great advantages over the current technology; nevertheless controlling and understanding the conditions for ‘self-assembly’ remains a challenge. A viable route towards success is to mimic the assembly processes that occur naturally and the best examples are found in biology. Biological molecules tend to have a particular shape, size, charge, and chemical functionality (chemical groups) that enable them to assemble into larger structures that have a function. The aim of this project was to assemble nanowires and incorporate quantum dots into them. Quantum dots are tiny semiconductor crystals, which are so small that the electronic states within them are confined, rather like the electrons within an atom. Quantum dots are believed by many scientists to have properties that will enable us to fabricate solar cells with much higher efficiencies than the currently available ones. Other potential applications include light emitting diodes, single electron transistors, fluorescent tags and components of in quantum computers. Hence, they are of great interest to scientists and technologists alike. Nanowires also demonstrate interesting properties, which are being studied to develop smaller transistors, biological sensors, components of photovoltaic cells, interconnects and much more. It was hence an important step to be able to fabricate and study nanowires that had quantum dots incorporated. In this project, a methodology for the fabrication of quantum dots incorporated into nanowires was developed. The latter was achieved using gold nanowires that were electrochemically deposited into nanoporous membrane templates producing large numbers of uniform structures. A segment of semiconductor material could then be deposited onto the gold within the template and, given that the layer was thin enough, it could quantum confine the electronic states within the semiconductor material. A further segment of gold was then deposited on top of the semiconductor to produce nanowires in a template with a metal-semiconductor-metal junction. The template was subsequently dissolved to release millions of nanowires with semiconductor junctions. Cadmium telluride (CdTe) was selected as the semiconductor in this project because its properties were identified as being appropriate for photovoltaic cells. The slow electrochemical growth rate of CdTe allowed for good control over the length of the semiconductor sections within the nanowires. If the semiconductor section was small enough it would confine the electronic states and become known as a quantum dot. Electronic properties were measured using a unique multi-probe scanning tunnelling microscope that had a scanning electron microscope positioned directly above. Apart from that, the fellow demonstrated that CdTe quantum dots could be synthesised in water-based solution. Careful tuning of the environment in which the quantum dots were made and stored enabled them to self-assemble into nanowires and nanoribbons. Exposing the nanoribbons to ambient light caused stress in the structure, giving them a twist. This proved to be an elegant method of generating helical nanowires with quantum dots incorporated into the structure and the relevant work was published in an article in Science.

Data: CORDIS, © European Union