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1. B ₄₈ ⁻ : a bilayer boron cluster

   https://doi.org/10.1039/D0NR09214B  

   Chen, Wei-Jia; Ma, Yuan-Yuan; Chen, Teng-Teng; Ao, Mei-Zhen; Yuan, Dao-Fu; Chen, Qiang; Tian, Xin-Xin; Mu, Yue-Wen; Li, Si-Dian; Wang, Lai-Sheng (February 2021, Nanoscale)

   null (Ed.)

   Size-selected negatively-charged boron clusters (B n − ) have been found to be planar or quasi-planar in a wide size range. Even though cage structures emerged as the global minimum at B 39 − , the global minimum of B 40 − was in fact planar. Only in the neutral form did the B 40 borospherene become the global minimum. How the structures of larger boron clusters evolve is of immense interest. Here we report the observation of a bilayer B 48 − cluster using photoelectron spectroscopy and first-principles calculations. The photoelectron spectra of B 48 − exhibit two well-resolved features at low binding energies, which are used as electronic signatures to compare with theoretical calculations. Global minimum searches and theoretical calculations indicate that both the B 48 − anion and the B 48 neutral possess a bilayer-type structure with D 2h symmetry. The simulated spectrum of the D 2h B 48 − agrees well with the experimental spectral features, confirming the bilayer global minimum structure. The bilayer B 48 −/0 clusters are found to be highly stable with strong interlayer covalent bonding, revealing a new structural type for size-selected boron clusters. The current study shows the structural diversity of boron nanoclusters and provides experimental evidence for the viability of bilayer borophenes. 

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2. The effects of methanol clustering on methanol–water nucleation

   https://doi.org/10.1063/5.0120876  

   Sun, Tong; Wilemski, Gerald; Hale, Barbara N.; Wyslouzil, Barbara E. (November 2022, The Journal of Chemical Physics)

   The formation of subcritical methanol clusters in the vapor phase is known to complicate the analysis of nucleation measurements. Here, we investigate how this process affects the onset of binary nucleation as dilute water–methanol mixtures in nitrogen carrier gas expand in a supersonic nozzle. These are the first reported data for water–methanol nucleation in an expansion device. We start by extending an older monomer–dimer–tetramer equilibrium model to include larger clusters, relying on Helmholtz free energy differences derived from Monte Carlo simulations. The model is validated against the pressure/temperature measurements of Laksmono et al. [Phys. Chem. Chem. Phys. 13, 5855 (2011)] for dilute methanol–nitrogen mixtures expanding in a supersonic flow prior to the appearance of liquid droplets. These data are well fit when the maximum cluster size imax is 6–12. The extended equilibrium model is then used to analyze the current data. On the addition of small amounts of water, heat release prior to particle formation is essentially unchanged from that for pure methanol, but liquid formation proceeds at much higher temperatures. Once water comprises more than ∼24 mol % of the condensable vapor, droplet formation begins at temperatures too high for heat release from subcritical cluster formation to perturb the flow. Comparing the experimental results to binary nucleation theory is challenged by the need to extrapolate data to the subcooled region and by the inapplicability of explicit cluster models that require a minimum of 12 molecules in the critical cluster. 

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3. Growth of Uniquely Small Tin Clusters on Highly Oriented Pyrolytic Graphite

   https://doi.org/10.1021/acs.jpcc.3c08215  

   Beniwal, Sumit; Chai, Wenrui; Qiao, Mengxiong; Bajalan, Zahra; Ahsen, Ali S; Reddy, Kasala P; Chen, Yankun; Henkelman, Graeme; Chen, Donna A (February 2024, The Journal of Physical Chemistry C)

   Sn clusters have been grown on highly oriented pyrolytic graphite (HOPG) surfaces and investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. At low Sn coverages ranging from 0.02-0.25 ML, Sn grows as small clusters that nucleate uniformly on the terraces. This behavior is in contrast with the growth of transition metals such as Pd, Pt, and Re on HOPG, given that these metals form large clusters with preferential nucleation for Pd and Pt at the favored low-coordination step edges. XPS experiments show no evidence of Sn-HOPG interactions, and the activation energy barrier for diffusion calculated for Sn on HOPG (0.06 eV) is lower or comparable to those of Pd, Pt and Re (0.04, 0.22, and 0.61 eV, respectively), indicating that the growth of the Sn clusters is not kinetically limited by diffusion on the surface. DFT calculations of the binding energy/atom as a function of cluster size demonstrate that the energies of the Sn clusters on HOPG are similar to that of Sn atoms in the bulk for Sn clusters larger than 10 atoms, whereas the Pt, Pd, and Re clusters on HOPG have energies that are 1-2 eV higher than in the bulk. Thus, there is no thermodynamic driving force for Sn atoms to form clusters larger than 10 atoms on HOPG, unlike for Pd, Pt, and Re atoms, which minimize their energy by aggregating into larger, more bulk-like clusters. In addition, annealing the Sn/HOPG clusters to 800 K and 950 K does not increase the cluster size but instead removes the larger clusters, while Sn deposition at 810 K induces the appearance of protrusions that are believed to be from subsurface Sn. DFT studies indicate that it is energetically favorable for a Sn atom to exist in the subsurface layer only when the Sn atom is located at a subsurface vacancy. 

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4. Visualizing formation of high entropy alloy nanoparticles with liquid phase transmission electron microscopy

   https://doi.org/10.1039/D3NR01073B  

   Sun, Jiayue; Leff, Asher; Li, Yue; Woehl, Taylor J. (June 2023, Nanoscale)

   High entropy alloy (HEA) nanoparticles hold promise as active and durable (electro)catalysts. Understanding their formation mechanism will enable rational control over composition and atomic arrangement of multimetallic catalytic surface sites to maximize their activity. While prior reports have attributed HEA nanoparticle formation to nucleation and growth, there is a dearth of detailed mechanistic investigations. Here we utilize liquid phase transmission electron microscopy (LPTEM), systematic synthesis, and mass spectrometry (MS) to demonstrate that HEA nanoparticles form by aggregation of metal cluster intermediates. AuAgCuPtPd HEA nanoparticles are synthesized by aqueous co-reduction of metal salts with sodium borohydride in the presence of thiolated polymer ligands. Varying the metal : ligand ratio during synthesis showed that alloyed HEA nanoparticles formed only above a threshold ligand concentration. Interestingly, stable single metal atoms and sub-nanometer clusters are observed by TEM and MS in the final HEA nanoparticle solution, suggesting nucleation and growth is not the dominant mechanism. Increasing supersaturation ratio increased particle size, which together with observations of stable single metal atoms and clusters, supported an aggregative growth mechanism. Direct real-time observation with LPTEM imaging showed aggregation of HEA nanoparticles during synthesis. Quantitative analyses of the nanoparticle growth kinetics and particle size distribution from LPTEM movies were consistent with a theoretical model for aggregative growth. Taken together, these results are consistent with a reaction mechanism involving rapid reduction of metal ions into sub-nanometer clusters followed by cluster aggregation driven by borohydride ion induced thiol ligand desorption. This work demonstrates the importance of cluster species as potential synthetic handles for rational control over HEA nanoparticle atomic structure. 

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5. Nucleation pathways in barium silicate glasses

   https://doi.org/10.1038/s41598-020-79749-2  

   McKenzie, Matthew E.; Deng, Binghui; Van Hoesen, D. C.; Xia, Xinsheng; Baker, David E.; Rezikyan, Aram; Youngman, Randall E.; Kelton, K. F. (December 2021, Scientific Reports)

   null (Ed.)

   Abstract Nucleation is generally viewed as a structural fluctuation that passes a critical size to eventually become a stable emerging new phase. However, this concept leaves out many details, such as changes in cluster composition and competing pathways to the new phase. In this work, both experimental and computer modeling studies are used to understand the cluster composition and pathways. Monte Carlo and molecular dynamics approaches are used to analyze the thermodynamic and kinetic contributions to the nucleation landscape in barium silicate glasses. Experimental techniques examine the resulting polycrystals that form. Both the modeling and experimental data indicate that a silica rich core plays a dominant role in the nucleation process. 

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