Sizing up materials design Martijn A. Zwijnenburg Department of Chemistry, University College London, London, U.K.
Material design generally focusses on using composition and by extension doping to obtain materials with particular combinations of desired physical properties. However, besides composition, the properties of materials can also be tuned by changing the characteristic size of the material or domains of one material in the matrix of another material. This has been inadvertently exploited in blissful ignorance of the underlying physics since antiquity and knowingly since the work in the 1970s of Ekimov and co-workers on nanocrystals embedded in glass matrices and Brus and co-workers on colloidal nanocrystals in solution.1 The last 50 years has seen massive progress in the controlled growth of 0D and 1D nanostructures but progress on the theory side has been slower and does not yet match what can be predicted in terms of the link between material composition and material properties. This is due to a combination of much stronger excitonic effects in 0D and 1D nanostructures, requiring one to go beyond the standard single-particle picture and ground-state density functional theory (DFT), the fact that time-dependent DFT (TD-DFT) is even more sensitive than ground-state DFT on the exact density functional used, and the fact that explicit nanometre size nanostructures need to be studied and one cannot rely on periodic boundary conditions. Recently, we performed GW/BSE calculations on 1-2 nm nanoparticles of a range of materials, including insulators (MgO, CaO, SrO) and semiconductors (CdO, CdS).2-3 These calculations demonstrate that such calculations that go beyond the single-particle picture and which are inherently more accurate and functional independent than TD-DFT are now computationally tractable for true nanostructures and for size ranges such that one can explicitly study the evolution of properties with nanostructures size and thus truly predict the effect of nanostructuring materials. The calculations show that yes, semiconductor nanostructures show “quantum confinement” but that the excited states are not necessarily delocalised over the whole nanostructure volume but can be confined to just the surface of particles and that surface ligands in some cases can be completely electronically benign and in other cases control the nanostructure properties. Even in insulator nanostructures, for which there is no “quantum confinement” in the classical sense, nanostructuring is shown to be exploitable to change material properties by boosting the role of surface states relative to their bulk counterparts. In my contribution I’ll give an overview of this work, how this can now be taken forward to predict the effect of size more generally and how this links to material synthesis. References 1. A.L. Efros and L.E. Brus, ACS Nano 2021, 15, 4, 6192
2. M.A. Zwijnenburg, Phys. Chem. Chem. Phys., 2021, 23, 21579. 3. M.A. Zwijnenburg, Phys. Chem. Chem. Phys., 2022, 24, 21954.
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