Two-to-three dimensional transition in neutral gold clusters: The crucial role of van der Waals interactions and temperature

Abstract

We predict the structures of neutral gas-phase gold clusters (Aun, n = 5-13) at finite temperatures based on free-energy calculations obtained by replica-exchange ab initio molecular dynamics. The structures of neutral Au5-Au13 clusters are assigned at 100 K based on a comparison of experimental far-infrared multiple photon dissociation spectra performed on Kr-tagged gold clusters with theoretical anharmonic IR spectra and freeenergy calculations. The critical gold cluster size where the most stable isomer changes from planar to nonplanar is Au11 (capped-trigonal prism, D3h) at 100 K. However, at 300 K (i.e., room temperature), planar and nonplanar isomers may coexist even for Au8, Au9, and Au10 clusters. Density-functional theory exchangecorrelation functionals within the generalized gradient or hybrid approximation must be corrected for longrange van der Waals interactions to accurately predict relative gold cluster isomer stabilities. Our work gives insight into the stable structures of gas-phase gold clusters by highlighting the impact of temperature, and therefore the importance of free-energy over total energy studies, and long-range van der Waals interactions on gold cluster stability.Metal clusters in the gas phase are widely used as model systems to study fundamental properties of condensed matter (see, e.g., Refs. [1][2][3][4][5][6][7]). At the nanoscale, gold is not a fully noble, nonreactive material; thus gold clusters are of particular interest due to their possible applications in gas sensing, pollution reduction, and catalysis [8][9][10][11][12][13][14][15][16][17]. Gold clusters in the gas phase exhibit many structural isomers of similar energetics and can rapidly interconvert among them [18][19][20][21][22][23][24]. Geometry and size can impact the physicochemical properties of clusters, for example, HOMO-LUMO gap, polarizability, and catalytic activity [25][26][27][28][29][30][31]. Small gold clusters often adopt stable planar geometries [32][33][34][35][36]. The critical size where gold clusters begin favoring nonplanar (three-dimensional, 3D) structures over planar (two-dimensional, 2D) structures has attracted sustained interest [37][38][39][40][41][42][43][44]. Gasphase ion-mobility experiments at room temperature suggest a transition from a 2D to 3D ground state structure at size 12 [45] and 8 [46] for negatively and positively charged clusters, respectively. Computational and experimental evidence supports these ground-state structural assignments for the 2D to 3D transition size [47,48]. For neutral gold clusters, computational studies predict their critical transition size is between Au10 and Au14 at zero kelvin [26,38,44], but experimental evidence is lacking due to the difficulty in spectroscopically characterizing neutral gas-phase clusters relative to charged clusters.Both experimental and computational studies indicate that dynamic structural rearrangements are a common feature among clusters at finite temperatures [22,[49][50][51][52][53][54][55][56][57]. It is clear that small go...

Non‐covalent Interactions and Charge Transfer between Propene and Neutral Yttrium‐Doped and Pure Gold Clusters

“…It is well known that neutral Au n clusters are planar up to surprisingly large sizes. The energy differences between the most stable 2D and the 3D isomers are small in the range of n= 10–13, so the different computational methods may lead to a different transition size . According to our computations, the 2D structure of Au 10 is more stable than the 3D one, while for Au 11 the three‐dimensional structure is the energetically lowest lying isomer, in agreement with refs.…”

“…According to our computations, the 2D structure of Au 10 is more stable than the 3D one, while for Au 11 the three‐dimensional structure is the energetically lowest lying isomer, in agreement with refs. . The yttrium‐doped Au n −1 Y clusters prefer two‐dimensional structures only up to Au 8 Y .…”

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“…66,67 This method is selected because of its speed and reliability in calculating gold clusters, as shown in previous work. [68][69][70] A plane-wave basis set is utilized, including spin polarization. The plane wave cut-off energy is truncated at 400 eV.…”

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“…For instance, ReaxFF predicts the most stable isomer of Au 8 to be globular [26] in contrast to previous DFT calculations that show Au 8 to be planar. [66,52,14] Given these challenges, gold catalytic clusters represent an excellent system for testing the efficacy of our AL scheme. We show that our AL-NN is able to adequately represent the energy landscape for diverse sizes and geometries as well as the dynamical properties of both clusters and bulk by sampling minimal amount of reference data (�500 total reference data).…”