In situ study on atomic mechanism of melting and freezing of single bismuth nanoparticles

Abstract

Experimental study of the atomic mechanism in melting and freezing processes remains a formidable challenge. We report herein on a unique material system that allows for in situ growth of bismuth nanoparticles from the precursor compound SrBi₂Ta₂O₉ under an electron beam within a high-resolution transmission electron microscope (HRTEM). Simultaneously, the melting and freezing processes within the nanoparticles are triggered and imaged in real time by the HRTEM. The images show atomic-scale evidence for point defect induced melting, and a freezing mechanism mediated by crystallization of an intermediate ordered liquid. During the melting and freezing, the formation of nucleation precursors, nucleation and growth, and the relaxation of the system, are directly observed. Based on these observations, an interaction-relaxation model is developed towards understanding the microscopic mechanism of the phase transitions, highlighting the importance of cooperative multiscale processes.

Figure 1. The metastable state and crystallization of a Bi nanodroplet.

(a,b) Sequential snapshots of HRTEM imaging showing the metastable state of the Bi nanodroplet. The blue arrows in b indicate the domains still retained in liquid phase. (c,d) Sequential snapshots of HRTEM imaging showing fast crystallization of the nanodroplet after the induction period. The insets show the corresponding FFT patterns of the nanoparticle. (e) Schematic illustration of the reversible freezing/melting of the Bi nanoparticle. The scale bar in a is 5 nm, which applies to a–d. The time labels correspond to when the video snaps were taken.

Figure 2. Point defect induced melting of Bi nanocrystal.

(a–e,k–n) Sequential snapshots of HRTEM imaging showing the microscopic structural details of melting process. (f–j) The enlarged images corresponding to the regions as marked by the yellow squares in a–e. The lattice fringes in f,g are highlighted by red lines. The dashed blue circle in h indicates a vacancy. The blue arrow in g indicates the atomic column before transforming to the vacancy in h. The area marked in dashed blue line in i indicates the gap formed by the coalescence of the defects. The areas marked in dashed blue lines in j indicate the domains in liquid-like structure with an irregular shape. The insets of a–e and k–n show the corresponding FFT patterns of the nanoparticle. The scale bar in a is 5 nm, which applies to a–e and k–n. The scale bar in f is 1 nm, which applies to f–j.

Figure 3. Crystallization of a Bi nanodroplet by an intermediate ordered liquid.

(a–d,f–h) Sequential snapshots of HRTEM imaging showing a crystallization through transformation of ordered liquid structure. The insets show the corresponding FFT patterns of the nanoparticle. The spots indicated by yellow circles in the FFT patterns in b–d reflect the formation of the periodic structure. The areas marked in dashed blue lines in f,g indicate the domains in disordered structure. The enlarged images of the regions as marked by the yellow squares in d,h are shown in e,i, respectively. The part that cannot transform into the well crystallized phase is indicated by the dashed orange line. The scale bar in a is 5 nm, which applies to a–d,f–h. The scale bar in e is 1 nm, which applies to e,i.

Figure 4. Quantifying the phase transitions of the nanoparticle.

The amount of crystalline order was measured from the integrated profiles taken from the 2D FFT images. The 2D FFT images in Figs 2 and 3 were transformed to FFT spectra by the Fit2D program (see details in Supplementary Fig. 9). The distinctive peaks on the integrated profiles were produced by pixel intensities of the spots in the FFT patterns. (a,b) The intensity of the left-most peak in the integrated profile, I, corresponds to the time labels in Figs 2 and 3, respectively.