Sizing Up The Electronic Structure of Atomically Precise Semiconductor Nanoparticles

Empirically the effect of reducing the characteristic size of semiconductor materials on their optical and by extension electronic properties is well known. Similarly, the fact that this change with size can at least be qualitatively understood in terms of spatially constraining the excited-electron and hole pair to a volume smaller than what it would occupy in the bulk. However, when reducing the size of a semiconductor material more changes than just simply the characteristic size. The role of the surface and the nature of its termination become more important, the dielectric screening changes often reduce as more field lines travel around rather than through the semiconductor, as well as the (electronic) structure can become different to that of the bulk. This is especially true for atomically precise nanoparticles,¹ which typically are on the smaller side of the size spectrum.<br/><br/>To properly understand the effect of reducing the size of semiconductor materials and predict the properties of atomically precise nanoparticles of such materials one needs to perform electronic structure calculations on true nanoparticles. Such calculations should ideally consider the possible surface chemistry of the nanostructures and not assume that any of their properties, other than perhaps their internal structure, is the same as the bulk. Luckily such calculations have become tractable now, both using time-dependent density functional theory (TD-DFT) and the many-body perturbation theory combination of GW and solving the Bethe-Salpeter Equation (GW-BSE).^2-4<br/><br/>In our contribution we will compare and contrast the results of calculations on ligand-capped cadmium sulfide nanoparticles, perhaps the archetypical atomically precise semiconductor nanoparticles, and their hydrogen terminated silicon counterparts, the c. elegans of computational nanoparticle research. Both ligand-capped binary semiconductor nanoparticles and hydrogen terminated silicon nanocrystals are interesting from an application point of view,^5,6 but also are derived from bulk materials that are both structurally similar but electronically different. We will discuss not only how the optical and electronic properties differ between the two classes of particles and how they change with particle size but also how these changes are linked to the particles’ surface chemistry and changes in the dielectric screening and the fundamental electronic structure of the particles with particle size. An example of the latter being the onset of band-like behaviour.<br/><br/>1. Z. Hens and J. De Roo, <i>Journal of the American Chemical Society</i>, 2020, <b>142</b>, 15627–15637.<br/>2. D. Rocca, M. Vörös, A. Gali and G. Galli, <i>Journal of Chemical Theory and Computation</i>, 2014, <b>10</b>, 3290–3298.<br/>3. M. A. Zwijnenburg, <i>Physical Chemistry Chemical Physics</i>, 2021, <b>23</b>, 21579–21590.<br/>4. M. A. Zwijnenburg, <i>Physical Chemistry Chemical Physics</i>, 2022, <b>24</b>, 21954–21965.<br/>5. M. A. Cotta, <i>ACS Applied Nano Materials</i>, 2020, <b>3</b>, 4920–4924.<br/>6. B. F. McVey and R. D. Tilley, <i>Accounts of Chemical Research</i>, 2014, <b>47</b>, 3045–3051.