Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G27: Focus Session: Chemical Physics of Clusters: Bridging from Angstrom-scale Clusters to Micron-scale Aerosol Particles II |
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Sponsoring Units: DCP Chair: Gerald Wilemski, Missouri University of Science and Technology Room: 204B |
Tuesday, March 3, 2015 11:15AM - 11:51AM |
G27.00001: Low temperature thermodynamics of water clusters studied by nanocalorimetry Invited Speaker: Bernd von Issendorff Water exhibits many unusual properties, which mirror the complexity of its hydrogen network structure. It is therefore not surprising that water clusters and nanoparticles are special in many aspects as well. We have recently shown that negatively charged water clusters exhibit a melting-like transition at surprisingly low temperatures (at about 120 K for H$_{2}$0$_{118}^{-}$) [1]. Further studies have shown that this behavior depends only weakly on the charge state of the cluster or on the type of impurity incorporated [2]. Furthermore the size dependence indicates that the transition does not extrapolate to the melting transition of normal ice, but rather to the glass transition of amorphous ice, which occurs at about 136 K. This can be rationalized by the fact that water clusters with few hundred molecules do not form a crystalline network like bulk ice, but exhibit structures much closer to that of the amorphous forms of solid water. \\[4pt] [1] C. Hock, M. Schmidt, R. Kuhnen, C. Bartels, L. Ma, H. Haberland, and B. v.Issendorff, Phys. Rev. Lett. 103, 073401 (2009)\\[0pt] [2] M. Schmidt and B. v. Issendorff, J. Chem. Phys. 136, 164307 (2012) [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:03PM |
G27.00002: Melting behavior of metallic clusters: An order parameter by instantaneous normal modes Ten-Ming Wu In this paper, we investigated the melting behaviors of Ag$_{17}$Cu$_{2\, }$and$_{\, }$Ag$_{14\, }$metallic clusters, which were generated by isothermal Brownian-type molecular dynamics simulation with the empirical many-body Gupta potentials, from low to high temperatures. For the two clusters, the temperature variation in the specific heat exhibited a maximum at a temperature, which was defined as the cluster melting temperature. However, an additional prepeak at a lower temperature was found in the specificheat variation of Ag$_{14\, }$but no prepeak was found in that of Ag$_{17}$Cu$_{2}$. The instantaneous normal mode (INM) analysis was used to dissect dynamics of the two clusters. A new order parameter associated with a cluster was proposed to describe the melting behaviors of the cluster; the order parameter can be defined using either the INM vibrational density of states or three orthogonal eigenvectors describing the rotational motions of the cluster by considering it as a rigid body. For the two metallic clusters studied, our results showed the mutual agreement for the order parameter defined by the two methods. The interpretation by the order parameter for the melting transition of a cluster was consistent with the temperature variation in the specific heat of the cluster. Furthermore, the new order parameter provided a connection between the melting of clusters and the concept of broken symmetry, which was successfully applied for understanding the melting transition of bulk systems. [Preview Abstract] |
Tuesday, March 3, 2015 12:03PM - 12:15PM |
G27.00003: Structural determination of metal nanoparticles from their vibrational (phonon) density of states Ignacio Garzon, Huziel Sauceda The vibrational (phonon) density of states of metal nanoparticles with size between 2-6 nm can be measured using nuclear resonant inelastic x-ray or plasmon resonance Raman scattering. In this work, we present atomistic calculations, based on a semiempirical tight-binding many-body Gupta potential, of the vibrational density of states (VDOS) for FCC, decahedral, and icosahedral (ICO) gold and silver nanoparticles with sizes $\sim $ 4 nm ($\sim $ 2000 atoms). The calculated VDOS are compared with experimental data, recently published for gold and silver nanoparticles of similar size, obtained through plasmon resonance Raman scattering. The best agreement between the calculated and measured VDOS's is obtained for the ICO morphology for both metal nanoparticles. These results indicate that most of the nanoparticles in the experimental samples should have icosahedral structures. The present study also shows that, as in the case of molecular systems and small clusters, vibrational spectroscopy of metal nanoparticles with few nanometers in size, together with theoretical calculations, are powerful tools for their structure determination. with your abstract body. [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:51PM |
G27.00004: Simulation studies of glassy nanoclusters Invited Speaker: Richard Bowles Glassy materials are amorphous solids usually formed by rapidly cooling a liquid below its equilibrium freezing temperature, trapping the particles in a liquid-like structure at the glass transition temperature. While appearing throughout nature and industry, these systems continue to challenge the way we think about the dynamics and thermodynamics of condensed matter and a fundamental understanding of the glass state remains elusive. This talk describes molecular simulation studies of glassy behaviour in binary Lennard--Jones nanoclusters. We show that the relaxation dynamics of the clusters is nonuniform and the core of the cluster goes through a glass transition at higher temperatures than at the surface. As the nanoclusters are cooled, they also exhibit a fragile--strong crossover in their dynamics and we explore how this phenomena is linked to the potential energy landscape of the clusters. Finally, we compare the properties of nanoclusters formed through vapour condensation, directly to the glassy state, with those of glassy clusters formed through traditional supercooling. The condensation clusters are shown to form ultra-stable glassy states analogous to the ultra-stable glasses formed by thin film vapour deposition onto a cold substrate. In all, our work suggests that nanoscale clusters exhibit some unique glassy features, while also offering potential insights into the fundamental nature of the glass transition. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G27.00005: Effects of Thermal Annealing and X-ray Exposure on Local Structures Surrounding Co in (Y. Co)-codoped CeO$_{2}$ Nanocrystals Tai-Sing Wu, Horng-Tay Jeng, Shih-Lin Chang, Yun-Liang Soo Codoping of Y and Co in CeO$_{2}$ has been found to incur an unconventional bandgap narrowing effect in the host nanocrystal material. The dormant bandgap-tuning ability of Y appears to be turned on by the Co codopant in the oxygen-vacancy-rich CeO$_{2}$ host. The physical mechanism underlying such effect has also been revealed by DFT calculations. Synchrotron-Radiation-based X-ray absorption analysis has further demonstrated that the Co codopant atoms can be located in either of two different local bonding environments in the CeO$_{2}$ host. Thermal annealing is capable of driving Co atoms from one bonding environment to the other. On the other hand, when exposed to x-rays of sufficient photon energy, Co can resume the original local structure as in the as-made sample. A simple model is proposed to explain such intriguing interplay between the effects of annealing and x-ray exposure in these codoped nanoceria systems. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:39PM |
G27.00006: The correspondence between the infrared vibrational spectra and the underlying hydrogen bonding network in aqueous clusters: caveats and tactics Invited Speaker: Sotiris Xantheas The structural -- spectral correspondence relates the observed infrared (IR) vibrational spectra to the underlying molecular structure. In the case of hydrogen bonded clusters the IR ``fingerprint'' region in the 3,000 -- 4,000 cm$^{-1}$ spectral range provides a direct probe of the connectivity and dynamics of the cluster's hydrogen bonding network. For medium size ($n $ 20) aqueous neutral and ionic clusters, the presence of several closely lying isomers that differ substantially in the oxygen atom network complicates both the sampling of the respective potential energy surfaces as well as the accurate determination of their energy order. Traditionally, a hierarchical procedure based on initial sampling with classical potentials and subsequent refinement with electronic structure methods has been used. We will highlight representative examples for the (H$_{2}$O)$_{20}$, (H$_{2}$O)$_{25}$ and H$_{3}$O$^{+}$(H$_{2}$O)$_{20}$ clusters [1-3] where sampling with classical potentials fails to produce the most stable minima and outline approaches and strategies that are based on a combination of enhanced sampling of configurations in conjunction with electronic structure theory to obtain realistic cluster configurations that are consistent with the measured IR spectra. \\[4pt] [1] S. S. Xantheas, \textit{Can. J. Chem. Eng.} \textbf{90}, 843 (2012).\\[0pt] [2] N. Sahu, S. R. Gadre, P. Bandyopadhyay, E. Miliordos and S. S. Xantheas, \textit{J. Chem. Phys.} \textbf{141}, 164304 (2014).\\[0pt] [3] C. C. Pradzynski, C. W. Dierking, F. Zurheide, R. M. Forck, T. Zeuch, U. Buck and S. S. Xantheas, \textit{Phys. Chem. Chem. Phys.} \textbf{16}, 26691 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G27.00007: Studies of energetics and spectroscopy of water clusters: size evolution and connections to the spectrum of liquid water Anne McCoy In this talk, I will focus on recent work in our group on water clusters. The first part will focus on interpreting the origins of the band at 2100 cm$^{-1}$ in the absorption spectrum of liquid water. Based on its frequency, the band has long been assigned to a combination band involving intramolecular bend vibration and intermolecular librations. The intensity arises from changes in the transition moment for the HOH bend as the low-frequency librations form and break intermolecular hydrogen bonds. In the second part we investigate approaches for using Diffusion Monte Carlo for studying the zero-point averaged vibrational structure of water clusters. The weak coupling between the inter- and intra-molecular degrees of freedom makes these challenging systems for DMC. Approaches taken and insights gained from this work will be described. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G27.00008: Infrared spectral challenges of individual, respirable, micron-sized dust particles: Strong phonons and their distorted lineshapes James Coe Consideration of cluster properties as they grow through the nanosize regime and into the micron-sized regime, leads to expectations of bulk-like trends which are well understood. However, individual micron-sized particles are of comparable size to the wavelength of probing infrared (IR) light, so vibrational spectra will be dominated by scattering effects and lineshapes will have dispersion and saturation distortions. Airborne dust particles of $\sim$ 4 micron widths are of particular health interest because they get past the nose, throat, and thorax and can be inhaled into people's lungs. This talk will describe the use of plasmonic metal mesh to obtain scatter-free, IR absorption spectra of single, $\sim$ 4 micron respirable particles. A dust library of single particle IR spectra is being compiled to chemically characterize respirable dust and a Mie-Bruggeman model has been created to predict the IR spectra of collections of mixed-composition dust particles. Having dealt with scattering effects, the remaining difficulty involves the effect of strong phonons. Many of the most common mineral components of dust have strong phonons with intensity cross sections comparable to the size of the particle which leads to severe and interesting lineshape distortions. [Preview Abstract] |
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