Pranav Acharya

Quantum Dots

Excitonic complexes

Excitonic complexes are in a sense molecules formed from charge carriers binding together. Excitonic complexes can decay into more stable products, and the energy difference between a complex and their decay products is the binding energy. The complexes I am working on are an excitons(an electron-hole pair), biexcitons(2 electrons and 2 holes), negative trions(2 electrons and one hole), positive trions(1 electron and 2 holes), and a hole-hole bound pair. For example a biexciton could decay into 1 exciton, 1 free hole, and 1 free electron. In the process of this decay a photon will be emmited, which has the energy of an exciton subtracted by the binding energies of 1 biexciton (which decayed) and 1 exciton (which still exists as a product). Different binding energies would mean that photons of a different energy, and therefore different frequency and colour, would be emmited when an excitonic complex decays or would be absorbed when an excitonic complex is created. In such a way, the binding energies affect the optoelectronic properties of quantum dots. In turn my project is studing how the binding energies for different complexes are affected by the size of a quantum dot and the ratio between the effective masses of an electron and a hole.  

Simulating excitonic complexes in quantum dots

The aim of my Master of Physics project is to simulate excitonic complexes inside a quantum dot, while varying the two parameters of the hole/electron mass ratio and the radius of the quantum dot. Specifically, I will figure out how the binding energies of different excitonic complexes in type 2 quantum dots vary with the two paramaters. I've just gotten started with the bulk of the research, and this web page will cover basic concepts behind it.  

Electrons and Holes

Quantum dots are nano-metre scale semiconductor particles which confine charge carriers in all 3 dimensions. There are two main types of quantum dots, type 1 which confine both electrons and holes, and type 2 which only confine one category of charge carrier and exclude the other charge carrier category outside. For my project I am working on a spherical type 2 quantum dot, where holes are confined inside and electrons are kept outside. Quantum dots are also great for absorbing and emmiting light. This optoelectronic property means they're used in high efficiency solar panels and QLED TVs. They're also used in some quantum computers, where for example a single excess electron could be isolated in a quantum dot and be manipulated by light and magnetic fields to function as a quantum bit for computation. As you can see quantum dots are a versatile material, and research in will be useful and have an impact.

Electrons and Holes are both charge carriers. Electrons are very small and light-weight particles with the smallest observable unit of negative charge. They are central to electronics. Holes are gaps where there is an absence of an electron, which act as if they are electron-like particles moving around a conductor, as shown in the image above. They are in a sense an electron's opposite, carrying instead positive charge equal in magnitude to an electron charge. Both electrons and holes have something called an 'effective mass', or what their mass seems like as they move around. This has to do with the material that they are in, with different materials having different effective masses. I am interested in the ratio between effective masses of holes and electrons for this project.

So far, I have set up and have recently started simulations for all of the complexes. This has involved getting to grips with Quantum Monte Carlo simulation software, coding up appropriate trial wavefunctions to model electrons and holes, and figuring out a good base set of parameters for each complex to start further optimisation. My current phase of research is to find the binding energies of the excitonic complexes, while varying the dot radius and the ratio between effective masses of holes and electrons. With this I can plot graphs of binding energies against the two variables, and create fitting functions. Such functions can be used by researchers working on spherical type 2 quantum dots to quickly get an idea of reasonable binding energies, and hence better plan out further experiments using such quantum dots. After this, I will calculate charge densities and distributions. This research will have an impact in industry, by helping both future modelling and optimisation of quantum dots for applications. Thank you for taking the time to read this.

Project work

References: [1]Jacoby M. The future of low-cost solar cells. c&en [Internet]. 2016, May, 2nd [cited 28th Feb 2020]; 94(18). Available from: https://cen.acs.org/articles/94/i18/future-low-cost-solar-cells.html [2]2020 75" Q85T QLED 4K HDR Smart TV. [Internet]. [cited 28th Feb 2020]. Available from: https://images.samsung.com/is/image/samsung/uk-qledtv-q85t-qe75q85tatxxu-titanumsilver-235584816?$684_547_PNG$ [3]Kuphaldt TR. Vol. III - Semiconductors. [Internet]. All about circuits. Chapter 2-Solid state theory; Electrons and “holes”. Available from: https://www.allaboutcircuits.com/textbook/semiconductors/chpt-2/electrons-and-holes/

Acknowledgement:

Dr Neil Drummond is my supervisor for this project, and has taught me the skills necessary for this project and provided guidance throughout it.