Real-time visualization of nanocrystal solid-solid transformation pathways

Pressure-Induced Modulation of Tin Selenide Properties: A Review

Tin selenide (SnSe) holds great potential for abundant future applications, due to its exceptional properties and distinctive layered structure, which can be modified using a variety of techniques. One of the many tuning techniques is pressure manipulating using the diamond anvil cell (DAC), which is a very efficient in situ and reversible approach for modulating the structure and physical properties of SnSe. We briefly summarize the advantages and challenges of experimental study using DAC in this review, then introduce the recent progress and achievements of the pressure-induced structure and performance of SnSe, especially including the influence of pressure on its crystal structure and optical, electronic, and thermoelectric properties. The overall goal of the review is to better understand the mechanics underlying pressure-induced phase transitions and to offer suggestions for properly designing a structural pattern to achieve or enhanced novel properties.

Dynamics of Ultrafast Phase Transitions in (001) Si on the Shock-Wave Front

We demonstrate an ultrafast (<0.1 ps) reversible phase transition in silicon (Si) under ultrafast pressure loading using molecular dynamics. Si changes its structure from cubic diamond to β-Sn on the shock-wave front. The phase transition occurs when the shock-wave pressure exceeds 11 GPa. Atomic volume, centrosymmetry, and the X-ray-diffraction spectrum were revealed as effective indicators of phase-transition dynamics. The latter, being registered in actual experimental conditions, constitutes a breakthrough in the path towards simple X-ray optical cross-correlation and pump-probe experiments.

Anisotropic Transient Disordering of Colloidal, Two-Dimensional CdSe Nanoplatelets upon Optical Excitation

Nanoplatelets (NPLs)-colloidally synthesized, spatially anisotropic, two-dimensional semiconductor quantum wells-are of intense interest owing to exceptionally narrow transition line widths, coupled with solution processability and bandgap tunability. However, given large surface areas and undercoordinated bonding at facet corners and edges, excitation under sufficient intensities may induce anisotropic structural instabilities that impact desired properties. We employ time-resolved X-ray diffraction to study the crystal structure of CdSe NPLs in response to optical excitation. Photoexcitation induces greater out-of-plane than in-plane disordering in 4 and 5 monolayer (ML) NPLs, while 3 ML NPLs display the opposite behavior. Recovery dynamics suggest that out-of-plane cooling slightly outpaces in-plane cooling in 5 ML NPLs with recrystallization occurring on indistinguishable time scales. In comparison, for zero-dimensional CdSe nanocrystals, disordering is isotropic and recovery is faster. These results favor the use of NPLs in optoelectronic applications, where they are likely to exhibit superior performance over traditional, zero-dimensional nanocrystals.

Interplay between Chemical Transformations and Atomic Structure in Nanocrystals and Nanoclusters

ConspectusChemically induced transformations are postsynthetic processing reactions applied to first generation (as-synthesized) nanomaterials to modify property-defining factors such as atomic structure, chemical composition, surface chemistry, and/or morphology. Compared with conditions for direct synthesis of colloidal nanocrystals, postsynthetic chemical transformations can be conducted in relatively mild conditions with a more controllable process, which makes them suitable for the precise manipulation of nanomaterials and for trapping metastable phases that are typically inaccessible from the conventional synthetic routes. Each of the chemically induced transformations methods can result in substantial restructuring of the atomic structure, but their transformation pathways can be very different. And the converse is also true: the atomic structure of the parent material plays a large role in the pathway toward and the resulting chemically transformed product. Additionally, the characteristic length of the parent material greatly affects the structure, which affects the outcome of the reaction.In this Account, we show how the atomic structure and nanoscale size directs the product formation into materials that are inaccessible from analogous chemically transformations in bulk materials. Through examples from the three chemical transformation processes (cation/anion exchange, redox reactions, and ligand exchange and ligand etching), the effect of the atomic structure on chemical transformations is made apparent, and vice versa. For cation exchange, an anisotropic atomic lattice results in a unidirectional exchange boundary. And because the interface can extend through the full crystal, a substantial strain field can form, influencing the phase of the material. In the redox reaction that leads to the nanoscale Kirkendall effect, the atomic structure is the key to inverting the diffusion rates in a diffusion couple to form the hollow cores. And for ligand etching, if one of the materials in a heterostructure has a defected and\or defect-tolerant atomic structure, it can be preferentially etched and its atomic structure can undergo phase transformations while the other composition remains intact. For length scales, we show how the chemically induced transformations greatly differ between bulk, nanocrystal, and nanocluster characteristic sizes. For instance, the structural transformation on relatively large nanocrystals (2-100 nm) can be a continuous process when the activation volume is smaller than the nanocrystal, while for smaller nanoclusters (<2 nm) the transformation kinetics could be swift resulting in only discrete thermodynamic states. Comparing the two nanosystems (nanocrystals to small nanoclusters), we address how their atomic structural differences can direct the divergent transformation phenomena and the corresponding mechanisms. Understanding the nanoscale mechanisms of chemically induced transformations and how they differ from bulk processes is key to unlocking new science and for implementing this processing for functional materials.

Optical and Physical Probing of Thermal Processes in Semiconductor and Plasmonic Nanocrystals

This article reviews thermal properties of semiconductor and emergent plasmonic nanomaterials, focusing on mechanisms through which hot carriers and phonons are produced and dissipated as well as the related impacts on optoelectronic properties. Elevated equilibrium temperatures, of particular relevance for implementation of nanomaterials in devices, affect absorptive and radiative transitions as well as emission efficiency that can present reversible and irreversible changes with temperature. In noble metal or doped semiconductor/insulator nanomaterials, hot carriers and lattice heating can substantially influence localized surface plasmon resonances and yield large ultrafast changes in transmission or strongly oscillatory coherences. Transient optical and diffraction characterizations enable nonequilibrium investigations of phonon dynamics and cooling such as lattice expansion and crystal phase stability. Timescales of nanoparticle thermalization with surroundings and transport of heat within films of such materials are also discussed.

Pressure Induced Nanoparticle Phase Behavior, Property, and Applications

Nanoparticle (NP) high pressure behavior has been extensively studied over the years. In this review, we summarize recent progress on the studies of pressure induced NP phase behavior, property, and applications. This review starts with a brief overview of high pressure characterization techniques, coupled with synchrotron X-ray scattering, Raman, fluorescence, and absorption. Then, we survey the pressure induced phase transition of NP atomic crystal structure including size dependent phase transition, amorphization, and threshold pressures using several typical NP material systems as examples. Next, we discuss the pressure induced phase transition of NP mesoscale structures including topics on pressure induced interparticle separation distance, NP coupling, and NP coalescence. Pressure induced new properties and applications in different NP systems are highlighted. Finally, outlooks with future directions are discussed.

Transient Melting and Recrystallization of Semiconductor Nanocrystals Under Multiple Electron-Hole Pair Excitation

Ultrafast optical pump, X-ray diffraction probe experiments were performed on CdSe nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump fluence. Bragg peak shifts related to heating and peak amplitude reduction associated with lattice disordering are observed. For smaller NCs, melting initiates upon absorption of as few as ∼15 electron-hole pair excitations per NC on average (0.89 excitations/nm³ for a 1.5 nm radius) with roughly the same excitation density inducing melting for all examined NCs. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. Zincblende and wurtzite NCs revert to initial structures following intense photoexcitation suggesting melting occurs primarily at the surface, as supported by simulations. Electronic structure calculations relate significant band gap narrowing with decreased crystallinity. These findings reflect the need to consider the physical stability of nanomaterials and related electronic impacts in high intensity excitation applications such as lasing and solid-state lighting.

Solution Synthesis of Nonequilibrium Zincblende MnS Nanowires

Uniform four-coordinate nonequilibrium MnS nanowires mainly in zincblende structure, other than the stable rock-salt phase, are reported for the first time. The MnS nanowires are grown via a solution-solid-solid model from the reaction of a Mn(II) source with dibenzyl disulfide in oleylamine at 180-200 °C catalyzed by Ag₂S nanocrystals in a body-centered cubic (bcc) fast-ionic phase transformed from their low-temperature monoclinic form. Investigations show that most of the zincblende MnS nanowires are grown along the ⟨112⟩ zone axis but a small proportion grow along the ⟨111⟩_ZB/⟨0001⟩_Wur axis with zincblende/defect-section and/or wurtzite/defect-section superlattices connected with the stems along the ⟨112⟩ direction. The nanowires have a tendency to grow straight at relatively low reaction temperature for short reaction times but twist at high temperature for long reaction times. Meanwhile, relatively high temperatures and long times favor the transition of the MnS nanowires in the zincblende phase to the corresponding thermodynamic ones in rock-salt form. Interestingly, even small increases in reaction pressure (1-2 atm) sensitively influence the growth of the MnS nanowires from zincblende to wurtzite form in the present catalytic system although low-pressure changes commonly do not have an obvious effect on condensed matter. In addition, the optical and magnetic properties of the zincblende MnS nanowires were studied, and they are varied largely from the bulk.

Atomic-scale observation of pressure-dependent reduction dynamics of W18O49 nanowires using environmental TEM

The real-time observation of structural evolution of materials can provide critical information for understanding their reduction mechanisms under different environments. Herein, we report the atomic-scale observation of the reduction dynamics of W₁₈O₄₉ nanowires (NWs) using environmental transmission electron microscopy. Intriguingly, the reduction pathway is found to be affected by oxygen pressure. Under high oxygen pressure (∼0.095 Pa), a W₁₈O₄₉ NW epitaxially transforms into a WO₂ NW via mass transport across the interface between (010)_W18O49 and (101)_WO2. While under low oxygen pressure (∼0.0004 Pa), the transformation follows the sequence of W₁₈O₄₉(NW) → WO₂(NW) → β-W(nanoparticles), which is identified as a new reduction pathway. These findings reveal the pressure-dependent reduction and a new transformation pathway, and extend our current understanding of the reduction dynamics of metal oxides.