Baopeng Cao

Indiana University Bloomington, Bloomington, IN, United States

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Publications (17)57.08 Total impact

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    ABSTRACT: Kinetic energy thresholds have been measured for the chemisorption of N2 onto Al114 (+), Al115 (+), and Al117 (+) as a function of the cluster's initial temperature, from around 200 K up to around 900 K. For all three clusters there is a sharp drop in the kinetic energy threshold of 0.5-0.6 eV at around 450 K, that is correlated with the structural transition identified in heat capacity measurements. The decrease in the thresholds corresponds to an increase in the reaction rate constant, k(T) at 450 K, of around 10(6)-fold. No significant change in the thresholds occurs when the clusters melt at around 600 K. This contrasts with behavior previously reported for smaller clusters where a substantial drop in the kinetic energy thresholds is correlated with the melting transition.
    The Journal of chemical physics. 11/2014; 141(20):204304.
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    ABSTRACT: Cross sections for chemisorption of N2 onto Al44(+/-) cluster ions have been measured as a function of relative kinetic energy and the temperature of the metal cluster. There is a kinetic energy threshold for chemisorption, indicating that it is an activated process. The threshold energies are around 3.5 eV when the clusters are in their solid phase and drop to around 2.5 eV when the clusters melt, indicating that the liquid clusters are much more reactive than the solid. Below the melting temperature the threshold for Al44(-) is smaller than for Al44(+), but for the liquid clusters the anion and cation have similar thresholds. At high cluster temperatures and high collision energies the Al44N2(+/-) chemisorption product dissociates through several channels, including loss of Al, N2, and Al3N. Density functional calculations are employed to understand the thermodynamics and the dynamics of the reaction. The theoretical results suggest that the lowest energy pathway for activation of dinitrogen is not dynamically accessible under the experimental conditions, so that an explicit account of dynamical effects, via molecular dynamics simulations, is necessary in order to interpret the experimental measurements. The calculations reproduce all of the main features of the experimental results, including the kinetic energy thresholds of the anion and cation and the dissociation energies of the liquid Al44N2(+/-) product. The strong increase in reactivity on melting appears to be due to the volume change of melting and to atomic disorder.
    Journal of the American Chemical Society 09/2010; 132(37):12906-18. · 10.68 Impact Factor
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    ABSTRACT: Heat capacities have been measured as a function of temperature for isolated aluminum nanoclusters with 84-128 atoms. Most clusters show a single sharp peak in the heat capacity which is attributed to a melting transition. However, there are several size regimes where additional features are observed; for clusters with 84-89 atoms the peak in the heat capacity is either broad or bimodal. For Al(115) (+), Al(116) (+), and Al(117) (+) there are two well-defined peaks, and for Al(126) (+), Al(127) (+), and Al(128) (+) there is a dip in the heat capacity at lower temperature than the peak. The broad or bimodal peaks for clusters with 84-89 atoms are not significantly changed by annealing to 823 K (above the melting temperature), but the dips for Al(126) (+), Al(127) (+), and Al(128) (+) disappear when these clusters are annealed to 523 K (above the temperature of the dip but below the melting temperature). Both the melting temperatures and the latent heats change fairly smoothly with the cluster size in the size regime examined here. There are steps in the melting temperatures for clusters with around 100 and 117 atoms. The step at Al(100) (+) is correlated with a substantial peak in the latent heats but the step at Al(117) (+) correlates with a minimum. Since the latent heats are correlated with the cluster cohesive energies, the substantial peak in the latent heats at Al(100) (+) indicates this cluster is particularly strongly bound.
    The Journal of Chemical Physics 01/2010; 132(3):034302. · 3.12 Impact Factor
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    ABSTRACT: Heat capacities measured as a function of temperature for Al(115)(+), Al(116)(+), and Al(117)(+) show two well-resolved peaks, at around 450 and 600 K. After being annealed to 523 K (a temperature between the two peaks) or to 773 K (well above both peaks), the high temperature peak remains unchanged but the low temperature peak disappears. After considering the possible explanations, the low temperature peak is attributed to a structural transition and the high temperature peak to the melting of the higher enthalpy structure generated by the structural transition. The annealing results show that the liquid clusters freeze exclusively into the higher enthalpy structure and that the lower enthalpy structure is not accessible from the higher enthalpy one on the timescale of the experiments. We suggest that the low enthalpy structure observed before annealing results from epitaxy, where the smaller clusters act as a nucleus and follow a growth pattern that provides access to the low enthalpy structure. The solid-to-solid transition that leads to the low temperature peak in the heat capacity does not occur under equilibrium but requires a superheated solid.
    The Journal of Chemical Physics 09/2009; 131(12):124305. · 3.12 Impact Factor
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    ABSTRACT: Heat capacities have been measured as a function of temperature for aluminum cluster anions with 35–70 atoms. Melting temperatures and latent heats are determined from peaks in the heat capacities; cohesive energies are obtained for solid clusters from the latent heats and dissociation energies determined for liquid clusters. The melting temperatures, latent heats, and cohesive energies for the aluminum cluster anions are compared to previous measurements for the corresponding cations. Density functional theory calculations have been performed to identify the global minimum energy geometries for the cluster anions. The lowest energy geometries fall into four main families: distorted decahedral fragments, fcc fragments, fcc fragments with stacking faults, and “disordered” roughly spherical structures. The comparison of the cohesive energies for the lowest energy geometries with the measured values allows us to interpret the size variation in the latent heats. Both geometric and electronic shell closings contribute to the variations in the cohesive energies (and latent heats), but structural changes appear to be mainly responsible for the large variations in the melting temperatures with cluster size. The significant charge dependence of the latent heats found for some cluster sizes indicates that the electronic structure can change substantially when the cluster melts.
    The Journal of Chemical Physics 07/2009; 131(4):044307-044307-11. · 3.12 Impact Factor
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    ABSTRACT: The internal energy distributions for melting aluminum cluster cations with 100, 101, 126, and 127 atoms have been investigated using multicollision induced dissociation. The experimental results can be best fit with a statistical thermodynamic model that incorporates only fully solidlike and fully liquidlike clusters so that the internal energy distributions become bimodal during melting. This result is consistent with computer simulations of small clusters, where rapid fluctuations between entirely solidlike and entirely liquidlike states occur during the phase change. To establish a bimodal internal energy distribution, the time between the melting and freezing transitions must be longer than the time required for equilibration of the energy distribution (which is estimated to be around 1-2 micros under our conditions). For Al(100)(+) and Al(101)(+), the results indicate that this criterion is largely met. However, for Al(126)(+) and Al(127)(+), it appears that the bimodal energy distributions are partly filled in, suggesting that either the time between the melting and freezing transitions is comparable to the equilibration time or that the system starts to switch to macroscopic behavior where the phase change occurs with the two phases in contact.
    The Journal of Chemical Physics 06/2009; 130(20):204303. · 3.12 Impact Factor
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    ABSTRACT: The kinetic energy threshold for chemisorption of N(2) on Al(100)(+) has been measured as a function of the nanocluster's temperature from 440 to 790 K. When the Al(100)(+) cluster melts at 620-660 K, the threshold drops by approximately 1 eV (approximately 96 kJ/mol). A decrease in the activation energy of this magnitude causes a 10(8)-fold increase in the reaction rate at the melting temperature. The decrease in the activation energy may result from the mobility of the surface atoms on the liquid cluster, which allows them to move to a lower energy arrangement as the N(2) approaches.
    Journal of the American Chemical Society 03/2009; 131(7):2446-7. · 10.68 Impact Factor
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    ABSTRACT: Clusters of certain elements are known to undergo phase transitions from solid-like to liquid-like states. Aluminum clusters have emerged as a model system for metal cluster phase transitions [1]. We report here the measurement of heat capacities of cationic clusters containing 84 to 127 Al atoms using a multi-collision induced dissociation mass spectrometry method [2]. We find two major changes in the heat capacities with increasing cluster size: (1) the fluctuations in the temperature of the phase transition vary more smoothly and (2) the peaks in heat capacity become sharper. Furthermore, we have found a range of cluster sizes (115-117 atoms) that contain two distinct peaks, separated by baseline, in their heat capacities. The origin of the extra peaks in the heat capacity, which is suspected to be due either to a pre-melting transition or to a solid-to-solid transition prior to the melting transition, will be further investigated by means of annealing experiments. The current work extends prior work on singly charged Al cluster cations having 16-83 atoms [2, 3]. [1] Breaux, G. A.; Neal, C. M.; Cao, B.; Jarrold, M. F. Physical Review Letters 2005, 94. [2] Neal, C. M.; Starace, A. K.; Jarrold, M. F. Journal of the American Society for Mass Spectrometry 2007, 18, 74-81. [3] Neal, C. M.; Starace, A. K.; Jarrold, M. F. Physical Review B 2007, 76. [4] This work is supported by NSF.
    03/2009;
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    ABSTRACT: Dissociation energies have been determined for Al(n)(+) clusters (n=25-83) using a new experimental approach that takes into account the latent heat of melting. According to the arguments presented here, the cohesive energies of the solidlike clusters are made up of contributions from the dissociation energies of the liquidlike clusters and the latent heats for melting. The size-dependent variations in the measured dissociation energies of the liquidlike clusters are small and the variations in the cohesive energies of solidlike clusters result almost entirely from variations in the latent heats for melting. To compare with the measured cohesive energies, density-functional theory has been used to search for the global minimum energy structures. Four groups of low energy structures were found: Distorted decahedral fragments, fcc fragments, fcc fragments with stacking faults, and "disordered." For most cluster sizes, the measured and calculated cohesive energies are strongly correlated. The calculations show that the variations in the cohesive energies (and the latent heats) result from a combination of geometric and electronic shell effects. For some clusters an electronic shell closing is responsible for the enhanced cohesive energy and latent heat (e.g., n=37), while for others (e.g., n=44) a structural shell closing is the cause.
    The Journal of Chemical Physics 11/2008; 129(14):144702. · 3.12 Impact Factor
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    ABSTRACT: Heat capacities have been measured for Al(n-1)Cu(-) clusters (n=49-62) and compared with results for pure Al(n) (+) clusters. Al(n-1)Cu(-) and Al(n) (+) have the same number of atoms and the same number of valence electrons (excluding the copper d electrons). Both clusters show peaks in their heat capacities that can be attributed to melting transitions; however, substitution of an aluminum atom by a copper atom causes significant changes in the melting behavior. The sharp drop in the melting temperature that occurs between n=55 and 56 for pure aluminum clusters does not occur for the Al(n-1)Cu(-) analogs. First-principles density-functional theory has been used to locate the global minimum energy structures of the doped clusters. The results show that the copper atom substitutes for an interior aluminum atom, preferably one with a local face-centered-cubic environment. Substitution does not substantially change the electronic or geometric structures of the host cluster unless there are several Al(n) (+) isomers close to the ground state. The main structural effect is a contraction of the bond lengths around the copper impurity, which induces both a contraction of the whole cluster and a stress redistribution between the Al-Al bonds. The size dependence of the substitution energy is correlated with the change in the latent heat of melting on substitution.
    The Journal of Chemical Physics 10/2008; 129(12):124709. · 3.12 Impact Factor
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    ABSTRACT: Heat capacities measured for isolated aluminum clusters show peaks due to melting. For some clusters with around 60 and 80 atoms there is a dip in the heat capacities at a slightly lower temperature than the peak. The dips have been attributed to structural transitions. Here we report studies where the clusters are annealed before the heat capacity is measured. The dips disappear for some clusters, but in many cases they persist, even when the clusters are annealed to well above their melting temperature. This indicates that the dips do not result from badly formed clusters generated during cluster growth, as originally suggested. We develop a simple kinetic model of melting and freezing in a system consisting of one liquidlike and two solidlike states with different melting temperatures and latent heats. Using this model we are able to reproduce the experimental results including the dependence on the annealing conditions. The dips result from freezing into a high energy geometry and then annealing into the thermodynamically preferred solid. The thermodynamically preferred solid has the higher freezing temperature. However, the liquid can bypass freezing into the thermodynamically preferred solid (at high cooling rates) if the higher energy geometry has a larger freezing rate.
    The Journal of Chemical Physics 08/2008; 129(1):014503. · 3.12 Impact Factor
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    ABSTRACT: The melting of alloyclusters is currently of great interest and emerging as an important research area. In this talk, we report the synthesis and melting transition of CuAln-1^- nanoalloy clusters (n = 49 -- 62). Heat capacities and melting behaviors have been determined for CuAln-1^- nanoalloy clusters using a novel collision induced dissociation method and are compared with those of pure aluminum cluster Aln^+. All these nanoalloys present a first order melting transition at temperatures well-below the melting temperature of the bulk aluminum and the eutectic temperature of their bulk alloys. No eutectic characteristic is detected for these nanoalloyclusters. Upon substitution of Al with a single copper atom, the melting of pure aluminum clusters has been altered considerably. Size and charge effects of the doping atom on the melting of host nanoclusters are discussed.
    03/2008;
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    ABSTRACT: Heat capacities measured for Al 45 - and Al 47 - nanoclusters have reproducible jumps at ~ 200 K. These jumps are consistent with theoretical predictions that some clusters with highly degenerate electronic states near the Fermi level will undergo a transition into a high T c superconducting state. An analysis based on a theoretical treatment of pairing in Al 45 - and Al 47 - agrees well with the experimental data in both the value of the critical temperature and in the size and width of the jumps in the heat capacity. Superconductivity, the flow of electric current without resistance, occurs in a wide variety of materials when the temperature drops below a critical temperature, T c . The transition to the superconducting state is accompanied by diamagnetism and by a jump in the heat capacity. The superconducting transitions observed to date are all below 135 K under atmospheric pressure and
    Journal of Superconductivity and Novel Magnetism 01/2008; 21(3):163-166. · 0.93 Impact Factor
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    ABSTRACT: The translating and rotating rod or disk of a conventional laser vaporization cluster source is replaced by a liquid metal target. The self-regenerating liquid surface prevents cavities from being bored into the sample by laser ablation. The laser beam strikes a near pristine surface with each pulse, resulting in signals with much better short and long term stabilities. While this approach cannot be used for refractory metals such as tungsten and molybdenum, it is ideal for studies of bimetallic clusters, which can easily be prepared by laser vaporization of a liquid metal alloy.
    Review of Scientific Instruments 08/2007; 78(7):075108. · 1.60 Impact Factor
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    ABSTRACT: Ion mobility and calorimetry measurements have been used to probe the nature of the phase transitions in gallium clusters with 29-55 atoms. While most clusters appear to undergo a first-order transition between solidlike and liquidlike phases, a few show the signature of melting without a significant latent heat. These transitions appear to be the finite size analogue of a second-order phase transition, and they presumably occur for some cluster sizes because their solidlike phase is amorphous.
    The Journal of Physical Chemistry B 10/2005; 109(35):16575-8. · 3.61 Impact Factor
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    ABSTRACT: Heat capacities have been determined for unsupported aluminum clusters, Al49(+) - Al63(+), from 150 to 1050 K. Peaks in the heat capacities due to melting occur between 450 and 650 K (well below the bulk melting point of 933 K). The peaks for Al+51 and Al+52 are bimodal, suggesting the presence of a premelting transition where the surface of the clusters melts around 100 K before the core. For clusters with n > 55 the melting temperatures suddenly drop, and there is a dip in the heat capacities due to a transition between two solid forms before the clusters melt.
    Physical Review Letters 05/2005; 94(17):173401. · 7.73 Impact Factor
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    ABSTRACT: The study focuses on metallic nanoclusters containing N˜10^2-10^3 free carriers (e.g., Ga56, Al45^-). The delocalized cluster electrons form energy shells similar to those in atoms or nuclei. Under special conditions, superconducting pairing in such nanoclusters can become very strong, and they form a new family of high temperature superconductors. For realistic sets of parameters one can expect a high value of Tc (150 K) as well as strong modification of the energy spectrum. In principle, it is possible to raise Tc up to room temperature. Specific experiments aimed at detecting the phenomenon of pair correlation in nanoclusters can be proposed: spectroscopy, magnetic, and thermodynamic properties.Transition to the superconducting state of the cluster is accompanied by the peak in its heat capacity. The phenomenon is promising for the creation of high Tc superconducting tunneling networks, and hence macroscopic superconductivity.