Correlated Orientations in Nanocrystal Fluctuations

Abstract

The dynamical behavior of small particles has been observed by high resolution electron microscopy. Spontaneous structural fluctuations, between various orientations, were observed and quantified. Clear transitions involving appearance and vanishing of twins were detected. Measurements of the angle change between succeeding configurations of the ͗111͘ atomic plane reveal the existence of several preferred angular changes. Twin related transformations give a good fit to the observed angular correlation, which excludes complete particle melting during the transition between successive configurations. It was also found that the particles rotate by a few degrees during this transformation.[S0031-9007(97)02816-0] PACS numbers: 61.46.+w, 61.16.Bg, 61.72.Mm, 63.90.+t The structure of atomic clusters is interesting because they present a state of matter that is intermediate between single atoms and the bulk state. In ultrafine particles ͑R , 10 nm͒, new phenomena may be expected, especially due to the high ratio between surface area and volume.Previous studies performed on small particles revealed a structure which is characterized by multiple twinned domains in many fcc metals [1-3].High resolution electron microscopy (HREM) studies in ultrafine particles revealed instability in the particles structure [4] which was defined as quasimelting phenomenon. A comprehensive review on the structure and instability of small clusters has been presented by Marks [5]. Several models were proposed to explain the dynamical evolution in the cluster structure. These models can be divided into two categories. The first one relates the restructuring of the particle to a short but complete melting period, followed by recrystallization of a new structure [6][7][8]. The second approach suggests that due to the small energy barrier between the various particle configurations, there can be random fluctuations between the different structures without melting [9].Previous study of lead particles reveals a definite range of particle size in which the structure is unstable, while in the solid state [10]. In this Letter, we report the first quantitative analysis of the fluctuations which occur in lead particles. This analysis reveals a definite angular correlation between successive structures.The samples are lead particles embedded in an amorphous SiO matrix. The samples were prepared by successive evaporation of 10 nm of SiO, 5 nm of Pb, 10 nm of SiO on a copper grid covered by a thin film of carbon. The embedded case ensures better thermal coupling, which defines good thermodynamical variables and prevents oxidation, migration, and evaporation [11]. During deposition, the substrate was held at a temperature of 200 ± C, causing the particles to coalesce in the liquid state, and resulting in an equilibrium spherical shape. The specimens were observed at room temperature in a HREM (JEOL 4000-EX) at 400 kV. The structure evolution of the particles was video recorded via a low level video CCD camera with a time resolution of 25 frames͞sec. The re...

Unraveling the Atomic Structure of Ultrafine Iron Clusters

“…The starting point to explore these properties is to unravel the atomic structures of the ultrafine clusters. Clusters with diameters larger than 3 nm have been extensively studied by atomic resolution transmission electron microscopy (TEM) over the past few decades345678910111213. However, unambiguous determination of the three-dimensional (3D) atomic structure of ultrafine clusters remains a challenge.…”

Surface‐Charge‐Induced Reversible Phase Transitions of Bi Nanoparticles

“…A similar instability of nanoparticles observed in electron microscopy has been reported previously for Bi, [9] Au, [10]- [12] Pd and In, [13] and Pb. [14] Under more extreme conditions (increased electron current density) the Bi particles melt. However, for very high current densities the Bi particles solidify and high-frequency fluctuations similar to the case of lower current density are observed.…”

Electronic growth of Pb on the vicinal Si surface

“…Twin structures are mainly single and multiple twins -up to 5 twins. At room temperature a twin boundary structure of particles is unstable [21] for particle sizes in the range of <10nm while a stacking faults are quite stable under the same conditions due to its higher energy. The particle structure fluctuates with time between the above configurations.…”

“…Previous studies performed on small Pb nanoparticles by electron microscopy [21] revealed an unstable structure which is characterized by multiple twinned domains and stacking faults defects. Twinning is a predominant deformation mechanism in fcc metals and especially in Pb [20][21][22][23]. Twin structures are mainly single and multiple twins -up to 5 twins.…”