Theoretical investigation of toxic-metal-free nanocrystals for technological applications

Lead Research Organisation: University of Leeds
Department Name: Electronic and Electrical Engineering


Context of research:
Nanotechnology centers around the ability to engineer materials at the nanoscale (that is, to manipulate structures with typical dimensions of the order of 1/100,000 of the thickness of a human hair), for technological applications. Colloidal quantum dots (QDs, aka nanocrystals, NCs) are chemically synthesised single-crystalline spherical semiconductor nanostructures with typical dimensions of a few nanometers which exhibit several characteristics that make them attractive for this field: (i) their size-tunable optical properties enable their application in very different fields such as optoelectronics (in lasers and LEDs), as well as biology and medicine (as organic molecule markers), and PV, allowing their absorption energies to be tailored to maximise solar photon absorption; (ii) their colloidal (i.e., chemical) nature enables low cost and large scale production; (iii) the very high degree of size monodispersity (less than 5%) achievable in their synthesis provides reproducibility and growth control. These properties make them also compatible with existing fibre-optic technologies and useful as building blocks for bottom-up assembly of various optical and electronic devices, including optical amplifiers, lasers and single-electron transistors. However, most of the nanocrystals exploited in such applications are made of Cd- and Pb-based materials (i.e., of CdX and PbX, where X=S, Se, Te), which are highly toxic for humans and the environment. It is therefore paramount to find non-toxic alternatives that can represent viable substitutes to such well characterised and well performing materials.
This is the aim of this project.

Aims and objectives:
To apply the atomistic semiempirical pseudopotential method (SEPM) developed in the Solid State Theory group at the National Renewable Energy Lab, Golden (CO) U.S.A., to theoretically screen different ("novel" and more conventional) Cd- and Pb-free colloidal materials and alternative topological structures for technological applications at the nanoscale, ranging from PV to nanoelectronics. Possible materials include Ga- and In- based ones (i.e., GaX and InX, where X=As, Sb and P), whereas possible structures include rods and tetrapods (potentially better suited for transport in films), as well as spherical nanocrystals.

Potential applications and benefits:
One of the many possible implications of the study on tetrapods of different materiasl could be the enhancement of miniband formation and transport in QD films, with applications ranging from PV to nanoelectronics (transistors). Others could involve the design of novel biosensors exploiting the long carrier lifetimes in Ga-based materials.
Furthermore, the student will develop a deep knowledge and competence in the use of a state-of-the-art theoretical modelling method and, through collaborations with national and international experimental groups and attendance to conferences, will start building a network of collaborators.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509681/1 01/10/2016 30/09/2021
2037499 Studentship EP/N509681/1 01/09/2017 31/05/2021 Panagiotis Rodosthenous
Description Throughout our investigation so far, we have performed scientific calculations on a new material, GaSb, that has never been synthesized before. With no experimental data and limited literature reviews, we published our findings on this material, presenting a theoretical approach on its unique electronic and optical properties focusing on possible applications and specifically in the area of photocatalysis. Another interesting material whose properties were investigated was InP. InP is been studied for the past 20 years however there is lack of satisfactory explanation of its properties when it comes to the effect of chemistry of the surface of nanocrystals (stoicheometry) and how this affects the optical properties. For this regard, we performed an optimization of the passivation parameters so to have a nanocrystal without any trap states for carriers. Interestingly, we predict a change in the radiative lifetimes (raditative recombination) by changing the number of Phosphorus atoms at the surface. By this way, we illustrate the importance of P atoms at the surface of the NC. Additionally, we performed calculations for investigating the transport properties of InP QDs, which is again a new area of research. Our results will now be compared with relative studies on other In-based and P-based materials to see whether InP is a unique case. Furthermore, we find GaP interestingly promising, since it is another green material that has not been synthesized and studied in depth yet. We have started working on the calculation of the electronic structure of quantum dots and the plan is to make a full theoretical investigation on electronic and optical properties for this material, comparing with other Ga-based an In-based materials.
Exploitation Route The outcomes of this project could be used by other theoreticians for further studying the materials mentioned above. In addition, the results could be used to be compared with other similar materials. Most importantly, this research could stimulate the experimentalists to synthesis these materials and consequently improve the quality of many products in terms of commercialization and performance.
Sectors Chemicals,Electronics,Energy,Environment,Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

Description Investigation of transport properties in promising "green" materials. 
Organisation University of Granada
Country Spain 
Sector Academic/University 
PI Contribution Our contribution was to perform the calculations on the electronic structure of different materials and make sure we have the required input files and format for the tight-binding calculations. What is more, the further analysis of results is essential and needs to be carried out in such a way of combining all the data, comparing different cases and identifying key aspects and unique properties of materials as well as possible application that can benefit from those properties.
Collaborator Contribution This collaboration gave us the opportunity to investigate the transport properties of various "green" materials, as explained before, that we believe to be promising for future technological applications. By following the tight-binding approach and the theory behind the algorithms used, we were able to reproduce closely spaced quantum dots and observe the transport behavior of electrons from one dot to the other. In addition, the mini-bands formed were also observed and from these results we were able to extract the mobility of the carriers. The results are also compared with previously studied materials in a relative manner as well as with other materials using similar elements.
Impact The transport properties of InP QDs are being investigated. At the moment, calculations are taking place for GaP, GaSb and InP QDs with similar number of atoms for the periodic and defect QDs so to have a common comparison base. The mobility and the mini-bands formed will be compared with CdSe and InSb to see whether we have similar behaviors.
Start Year 2019