Tailoring functional interfaces with computer simulations Materials interfaces play an important role in electronic devices, from solar cells to transistors, yet controlling their physics is a significant challenge. Researchers in the Dandelion project are developing methods to predict the electronic properties of functional interfaces, as Professor Silvana Botti explains. The ability to
control the flow of electrons is central to the functionality of electronic devices, from transistors, to solar cells, to light-emitting diodes. In a solar cell for example light energy is converted in an electron-hole pair, which then needs to be separated – eg by an interface - to prevent recombination. “If there is an electric field or a specific alignment of the energy levels at the interface, charge carriers with opposite sign will be driven apart, generating an electric current,” explains Silvana Botti, Professor of Physics at the University of Jena. The ability to manipulate and shape potential gradients at the interface opens up the unique possibility to control electrons and develop new technology. Much remains to be learned about the physics of interfaces however, a topic at the heart of Professor Botti’s work as the leader of the Dandelion project, in which she is developing methods to predict the electronic properties of different functional interfaces. “The idea came about because we have been working for some years on the establishment of a new theory to accurately predict the electronic energy levels at an interface,” she outlines.
Functional interfaces There is a transfer of electrons from within the two materials when the system is put in contact, leading to an alignment of the energy levels between them, the result of which is impossible to predict with simple empirical rules and difficult to calculate from first principles. The existing methods for predicting energy levels in a single material are largely based on density-functional theory, yet material junctions present a more complex challenge. “When we try to predict what happens at a complex materials interface the calculations are much more involved, because simulating the interface or making a model for it requires many more atoms. We cannot so easily use the translational symmetry as we can in a perfect crystal, when we have a simple unit that is repeated periodically,”
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This can provide an insight into excitation processes involving electrons, essentially defining where they want to go and how they will move within a material. The properties of an inhomogenous material may vary in different regions, which typically has a negative impact on charge transport within a device. “Describing the effects of structural and chemical disorder at interfaces is the next task on our to-do list,” outlines Professor Botti.
Database of interface properties
says Professor Botti. It is possible to perform these heavier calculations for functional interfaces if there is sufficient computing power, yet Professor Botti says that if one resorts to using ‘cheaper’ approximations they are not completely sound. “Many simple approximations that are used for bulk crystals no longer work effectively when it comes to describing an interface,” she explains. A number of methods are available to obtain accurate band alignments at interfaces, but the most accurate ones are too expensive to be used in high-throughput calculations. At the same time, the simplest approximations of density functional theory are efficient enough for large scale calculations but give poor band alignments. “The problem is that we want to both describe realistic interfaces with unit cells large enough to accommodate inhomogeneities, defects, doping - and also to do calculations for as many interfaces as possible, covering a variety of different types,” explains Professor Botti. “This includes for example 2D heterostructures, interfaces between a topological material and a trivial semiconductor, or electrical junctions between a metal and a semiconductor.”
The aim is to strike a balance between the accuracy of the calculations and the efficiency with which they can be performed, and progress has been made in this respect. Researchers in Professor Botti’s group have developed two different density functionals in the framework of density functional theory, which can be used in describing band energy diagrams across interfaces. “Electrons are quantum objects that interact with other electrons and the positive nuclei through the Coulomb interaction. At an interface it is particularly important to capture how the neighbouring electron cloud screen charges of the nuclei,” she says. In the Dandelion project, Professor Botti and her colleagues are building further on these foundations to develop effective quantum many-body methods of calculating the properties of electrons at interfaces. “We have developed specific functionals for density-functional theory, approximations that we can use to get accurate descriptions of energy band alignment at any type of interface. So we can look at how the energy levels change going from one material to another and at charge dynamics at the interface,” she continues.
EU Research
The aim in this research is to find interfaces with the ideal characteristics for new technology. A large number of calculations on different types of interfaces are being performed in the project and the resulting data will then be brought together in an open-access materials database, providing an invaluable resource to interested parties. “When we have enough data, we can ask artificial intelligence to go through this data, extract some important features and try to build with them predictive models. So if we want a specific functionality at the interface, maybe it will be possible to identify what structural/chemical characteristics we will need at the interface,” explains Professor Botti. The database itself will be part of the NOMAD repository, with Professor Botti working to deliver it by the conclusion of the project. “We are collecting calculations on interfaces, while we are also developing machine-learning models to predict the properties of even more interfaces,” she continues. “We want to have predictive calculations of the electronic properties of as many interfaces as possible, so that machine learning can be trained to find the proverbial needle in the haystack.” This could then help guide the future development of interfaces. There is not enough experimental data available to train a machine-learning model however, so Professor Botti says theoretical data is required. “We have already seen for crystalline materials that if we have enough data, then the machine is going to discover unexpected things. This is a very active area of research, with some very big projects working on data infrastructure initiatives. I am involved in the FAIRmat consortium, an initiative supported by Germany’s National Research Data Initiative (NFDI),” she explains. Researchers in Dandelion are still making calculations and putting together the
www.euresearcher.com
Developing an e-lab for interfaces on demand Developing an e-lab for interfaces on demand - dandelion Project Objectives
Upper panel: Atomic model of an interface between silicon (blue) and silicon oxide (oxygen in red). Lower panel: Band diagram showing the density of available energy levels (lDOS) for electrons along the axis perpendicular to the interface. Blue: No levels available. Yellow: Max density of states.
database, with thousands of calculations required to train a neural network. “We expect to identify tens of candidate interfaces, for which we will perform more advanced calculations, which we will then pass on to our experimental collaborators. Our objective is to promote sustainable research, as we can save time and money by guiding our experimental colleagues towards the most promising systems,”says Professor Botti. The idea here is to make the search for innovation and progress more efficient, and open up new possibilities in the design of interfaces. “Instead of always using the same materials because we know how to control them, and trying to make relatively small changes to make improvements, we could really find new solutions,” outlines Professor Botti. The project’s research is not targeted at one single application, with Professor Botti adopting a general perspective rather than focusing more narrowly on a specific area, although she is very much aware of the wider picture. There is a lot of interest in improving solar cells so that thinner material layers are capable of absorbing more of the available light, for which Professor Botti says effective interfaces will be required. “Instead of only having one active interface you will have several. In this way you can have interfaces in successive layers, absorb light of different frequencies and so cover more of the solar spectrum,” she outlines. With more interfaces there are more things to control, yet this also provides a route to improved solar cell efficiency, underlining the wider relevance of the project’s research. “One can think about properties that we may like to have at the interface, and then go and search for interfaces that fit with those expectations,” says Professor Botti.
Interfaces are at the heart of electronic devices, from transistors, to sensors, to lasers. However, a deep theoretical understanding and reliable mastery of the physics of interfaces has not yet been achieved. Professor Botti and her colleagues have taken up this challenge and are working on enabling predictive highthroughput calculations of the spectral and transport properties of a variety of functional interfaces.
Project Funding
Dandelion is funded by the Volkswagen Foundation, through the programme Momentum - Funding for Recently Tenured Professors.
Contact Details
Project Coordinator, Prof. Dr. Silvana Botti Institute for Solid State Theory and Optics Friedrich Schiller University Jena Max-Wien-Platz 1 07743 Jena T: +49-(0)3641-947150 E: sylvia.hennig@uni-jena.de W: http://www.ico.uni-jena.de Prof. Dr. Silvana Botti
After receiving her PhD in Physics in 2002 from the University of Pavia, Italy, Silvana Botti was Marie-Curie Fellow at the Ecole Polytechnique, Paris-Saclay University, where she was appointed CNRS Research Scientist in 2004. In 2008 she moved to the University of Lyon, before joining the Friedrich-Schiller University Jena as full professor in 2014. Her research activities focus on computational materials design, as well as on the development and application of many-body treatments for theoretical spectroscopy.
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