Structures of CdSe and CdS Nanoclusters from Ab Initio Random Structure Searching

Abstract

The possible structures of (CdSe) n and (CdS) n (n up to 34) nanoclusters were systematically studied using the ab initio random structure searching (AIRSS) method. We have been able to find stable, symmetric nanoclusters not previously discovered; using the computed pair distribution functions, we have put forward a hypothesis for the increased stability of these clusters. We have also been able to investigate structural features of the transformation to the bulk: for (CdS) n with n < 16 and (CdSe) n with n < 21, structures based on rings and cages with coordination number up to 3 are favored, while for larger n, the minimum-energy structures are more bulklike with an average coordination number larger than 3. This indicates a gradual transformation from a cagelike structure to a bulklike structure as the number of atoms increases. One striking finding was that for any value of n, the range of structures formed a continuous distribution of cluster energies rather than there being significant energy differences between them. This work created an extensive database of benchmarks and candidate structures for these systems.

Analysis of the atomic structure of CdS magic-size clusters by X-ray absorption spectroscopy

“…The bulk CdS structure may be a misleading initial guess since for small clusters a variety of structures can be stable (or metastable) depending on particle size. 33 Hence, we carried out further refinement using a modelfree approach 44 (custom-written Python code) minimising the sum of squared residual 45 between the fit results and experiment, while making no assumptions about the structure of the sample. Such an approach ignores MS calculations, but the MS provides a negligible contribution to the overall signal in low-coordinated systems with large disorder.…”

“…However, the problem with MSCs is that their structure is generally unknown. The bulk CdS structure may be a misleading initial guess since for small clusters a variety of structures can be stable (or metastable) depending on particle size 33 .…”

“…Recent analysis 29 of MSCs' structure using the X-ray pair-distribution function suggests that their atomic arrangements are different from those found in the bulk CdS (zinc blende or wurtzite structure) and the regular QDs (cubic, zinc blende structure). This prompted a random structure search approach 33 that indicated a gradual transformation from a cage-like to a bulk-like structure as the number of atoms in a cluster increases. Despite these recent advances in structural characterisation, the atomic structure of the CdS MSCs is still under debate.…”

Ab initio random structure searching and catalytic properties of copper-based nanocluster with Earth-abundant metals for the electrocatalytic CO2-to-CO conversion

“…AIRSS is an unbiased method that has been widely applied to discovering structures of materials, including monometallic and bimetallic metallic clusters. 36,37 The starting point for each calculation is a set of atoms corresponding to the monometallic (Cu)n or bimetallic (Cu-M)m clusters placed randomly within the central volume of a large cube of edge length 20 Å to minimize the interactions between the periodic images. For each type of cluster, we generated 50 random structures.…”

Structure prediction of crystals, surfaces and nanoparticles

“…51 More recent implementations [51][52] of such an approach have preferred the use of quasirandom, low-discrepancy sequences in place of random numbers; these are deterministic sequences that maintain some of the qualities of random sampling, while ensuring a more uniform sampling of configuration space. The method has enjoyed success in several cases [53][54][55][56][57] and for small, less complex structures a random approach may provide a sufficient sampling of configurational space. It also has the advantage that there is no in-built bias, which can be beneficial in identifying metastable structures, where the application of structure prediction is not only interested in the lowest energy structures, and is perfectly parallelisable because all structures are generated independently.…”