Visualization of nanocrystal breathing modes at extreme strains

Abstract

Nanoscale dimensions in materials lead to unique electronic and structural properties with applications ranging from site-specific drug delivery to anodes for lithium-ion batteries. These functional properties often involve large-amplitude strains and structural modifications, and thus require an understanding of the dynamics of these processes. Here we use femtosecond X-ray scattering techniques to visualize, in real time and with atomic-scale resolution, light-induced anisotropic strains in nanocrystal spheres and rods. Strains at the percent level are observed in CdS and CdSe samples, associated with a rapid expansion followed by contraction along the nanosphere or nanorod radial direction driven by a transient carrierinduced stress. These morphological changes occur simultaneously with the first steps in the melting transition on hundreds of femtosecond timescales. This work represents the first direct real-time probe of the dynamics of these large-amplitude strains and shape changes in few-nanometre-scale particles.

Beyond a phenomenological description of magnetostriction

“…[23][24][25] Our use of unconstrained FePt single-crystal nanoparticles allows us to study the full 3dimensional lattice motion on the natural timescale of the strain propagation through the nanoparticles. 26,27 This approach separates the individual contributions from electrons, spins, and phonons to lattice stress in FePt particles via their different symmetry properties and temporal onsets, as illustrated in Fig.1b. By employing ab initio calculations to capture the non-equilibrium stresses of electrons and phonons, we show that a large magnetoelastic stress term related to magnetostrictive spin-lattice coupling must contribute to the observed strain.…”

A deeper insight into strain for the sila‐bi[6]prismane () cluster with its endohedrally trapped silicon atom,

“…[ ] that any nanocrystal strain is necessarily released to isomerization, fragmentation, or autoionization processes: While we can immediately rule out the first two mechanisms for the nanocrystal based on our AIMD simulations, we like to supply published experimental evidence for the third process. Since it is not possible to measure internal strain directly, we like to refer to two recent experimental studies where nanocrystals have been exposed to extremely high external strains, but no indication of any possible SE release to autoionization or fragmentation processes was observed . Only at the most extreme strain conditions, nanocrystals show breathing modes eventually being followed by ultrafast disordering and melting.…”

Recent Advances in Ultrafast Structural Techniques

“…Since the very first ultrafast structural experiments at SPPS [43,103,104], the brightness of XFELs has increased by about one million fold, allowing for the study of more complex systems and photoinduced phenomena such as protein dynamics in crystalline [105][106][107][108][109][110][111][112] and liquid phases, [113] bond formation in the [Au(CN) 2 -] 3 trimer in solution, [114,115] lattice dynamics in individual gold nanoparticles, [116] ultrafast melting of charge and orbital order in PCMO, [117] non-linear lattice dynamics [118] and CWD order in YBCO, [119] visualization of breathing modes in nanocrystals at extreme photoexcitation conditions, [120] photoinduced insulator-to-metal (IM) transitions, [121] warm dense matter physics, [122] and ultrafast crystallization [123] and melting [124] in shock-compressed crystals. Figure 9a-d shows time-resolved X-ray diffraction results obtained by Barends et al [107] at LCLS in the investigation of the ultrafast structural dynamics that follows CO photodissociation in Myoglobin-CO (MbCO).…”