Melting Behavior of Noble-Metal-Based Bimetallic Clusters

Abstract

The isothermal Brownian-type molecular dynamics simulation was applied to study the melting scenario of noblemetal-based bimetallic clusters. The failure of the simulation results to portend a compatible melting temperature, T melt , which is defined in the specific heat CV at its principal peak and in the Lindermann-like parameter at the temperature which it exhibits drastic change, has prompted us to calculate the velocity autocorrelation function VACF or its Fourier-transform, the power spectrum as another useful variable for describing cluster melting. Two bimetallic clusters, namely Ag13Au1 and Ag13Cu1, were selected for illustration. We effected comparative studies of the thermal and dynamic properties of the Ag13Au1, Ag13Cu1 and Ag14 and explored isothermally the VACF and spectral density of individual atoms in each of these clusters as well as the corresponding whole cluster. We examined, in particular, the influence of the impurity atoms Au and Cu. It is observed that the CV of Ag14 displays a prepeak which is absent in Ag13Au1 and Ag13Cu1. The physical origin of this prepeak feature was studied and its presence is ascribed to the migrational relocation of the adatom in the cluster. From the temperature change of VACF and spectral density, we deduced T melt . It is found that the calculated T melt inferred from VACF and power spectrum agrees quite well with that determined from the main peak position of CV .

Melting behavior of Ag14 cluster: An order parameter by instantaneous normal modes

“…Up to 100 K, the D j (ω) spectra do not contain any imaginary-frequency INM and are rather similar to the power spectra obtained by the Fourier-transform of the velocity autocorrelation functions of individual atoms presented in Ref. 30. The D j (ω) of the surface atoms are generally indiscernible at 150 K, and they are seen to merge with the spectrum of the F atom at 200 K into an almost single spectrum, with the appearance of some imaginary-frequency modes and weak structures in the realfrequency lobe.…”

“…1 of Ref. 30. As shown there, the main maximum of C V, which is defined as the melting temperature T m , is located at approximately 920 K. Besides the main peak, one notices a prepeak structure near 300 K. In the temperature range 0 < T ≤ 500 K, we present also the cluster's excited states by an energy histogram.…”

“…As shown there, the main maximum of C V, which is defined as the melting temperature T m , is located at approximately 920 K. Besides the main peak, one notices a prepeak structure near 300 K. In the temperature range 0 < T ≤ 500 K, we present also the cluster's excited states by an energy histogram. 30 For T < 140 K, the cluster is only found in the LES. The first excited state of the cluster first appears near 150 K and by 200 K the second excited state emerges.…”

“…10, 11, and 24, to which the interested readers are referred. As in one of our previous works for Ag-based cluster, 30 we employ the empirical n-body Gupta potential 31 which reads…”

“…29 This order parameter interprets pretty well the melting behavior of Ag 17 Cu 2 and the predicted T m is in fairly good agreement with that inferred from C V . 30 In this paper, we study the melting transition of metallic cluster Ag 14 based on a generalization of the INM analysis which will be described in Sec. III.…”