Structural evolution and mechanical behaviour of Pt nanoparticle superlattices at high pressure

High-pressure behavior of hydrophobically coated gold nanoparticle supercrystals: role of the structure

We report here an extensive high pressure small-angle X-ray scattering study on 3D supercrystals self-assembled from colloidal spherical gold crystalline nanoparticule (NPs). We used a large variety of NPs with different gold core diameter, from 2 to 10 nm, grafted with different ligands: alkane-thiols or oleylamine. The self assembly of these various NPs leads to supercrystals of different structures: face centered cubic (FCC), body centered cubic (BCC), as well as the C14 Frank and Kasper phase. Using a Diamond Anvil Cell to apply pressure on these wide range of samples, we provide a unique overview on the mechanical properties of gold NPs supercrystals. In particular, bulk modulii have been determined from low pressure regime and the different behavior between FCC and BCC structures has been interpreted as due to an easier restructuring of the ligand conformation in the FCC structure compared to the BCC structure. At higher pressure, a fingerprint of irreversible structural transition has been observed. We have ascribed this irreversibility to the sintering of nanoparticles and confirmed this interpretation by transmission electron microscopy.

Pressure-stimulus-responsive behaviors of core-shell InP/ZnSe nanocrystals: remarkable piezochromic luminescence and structural assembly

Piezochromic luminescence materials with optical properties can be adjusted (the colors most sensitive to the human eye range from red to green) to provide powerful means for information acquisition in various applications. Inorganic quantum dots, typically based on heavy metals such as cadmium and lead, have congenital advantages as luminescence materials, including strong inoxidizability and excellent photoelectric properties. However, small band-gap shifts under pressure have hindered the development of inorganic-based piezochromic materials. Herein, we combined in situ high-pressure photoluminescence (PL) and absorption measurements with synchrotron X-ray scattering spectra to elucidate the remarkable modulation of optical properties and morphologies by pressure, particularly that of the piezochromic luminescence, in all-inorganic core-shell InP/ZnSe nanocrystals (NCs). We observed a stepwise PL color change from red to green, and an ultrabroad bandgap tunability of 0.46 eV was observed from 1.99 to 2.45 eV in the pressure range of 14.2 GPa for InP/ZnSe NCs. Moreover, two-dimensional (2D) InP/ZnSe nanosheets were synthesized by the stress-driven attachment of nanoparticles. These results demonstrate the ability of the pressure-stimulus response to trigger remarkable piezochromic luminescence and 2D nanosheet assembly in InP/ZnSe NCs, which paves the way for new applications of all-inorganic InP-based semiconductor NCs.

Mechanics under pressure of gold nanoparticle supracrystals: the role of the soft matrix

We report on High Pressure Small Angle X-ray Scattering (HP-SAXS) measurements on 3D face-centered cubic (FCC) supracrystals (SCs) built from spherical gold nanoparticles (NPs). Dodecane-thiol ligands are grafted on the surface and ensure the stability of the gold NPs by forming a protective soft layer. Under a hydrostatic pressure of up to 12 GPa, the SC showed a high structural stability. The bulk elastic modulus of the SC was derived from the HP-SAXS measurements. The compression of the SC undergoes two stages: the first one related to the collapse of the voids between the NPs followed by the second one related to the compression of the soft matrix which gives a major contribution to the mechanical behavior. By comparing the bulk modulus of the SC to that of dodecane, the soft matrix appears to be less compressible than the crystalline dodecane. This effect is attributed to a less optimized chain packing under pressure compared to the free chains, as the chains are constrained by both grafting and confinement within the soft matrix. We conclude that these constraints on chain packing within the soft matrix enhance the stability of SCs under pressure.

The bulk modulus of 3D FCC supracrystals of spherical gold nanoparticles is determined using high pressure-SAXS measurements. The organic ligand shell is found to be less compressible than pure dodecane with the same chain length.

Tension-compression behavior in gold nanoparticle arrays: a molecular dynamics study

The mechanical properties of Au nanoparticle arrays are studied by tensile and compressive deformation, using large-scale molecular dynamics simulations which include up to 16 million atoms. Our results show that mechanical response is dominated by nanoparticle size. For compression, strength versus particle size shows similar trends in strength than full-density nanocrystals. For diameters (d) below 10 nm there is an inverse Hall-Petch (HP) regime. Beyond a maximum at 10 nm, strength decreases following a HP d ^-1/2 dependence. In both regimes, interparticle sliding and dislocation activity play a role. The array with 10 nm nanoparticles showed the same mechanical properties than a polycrystalline bulk with the same grain size. This enhanced strength, for a material nearly 20% lighter, is attributed to the absence of grain boundary junctions, and to the array geometry, which leads to constant flow stress by means of densification, nanoparticle rotation, and dislocation activity. For tension, there is something akin to brittle fracture for large grain sizes, with NPs debonding perpendicular to the traction direction. The Johnson-Kendall-Roberts contact theory was successfully applied to describe the superlattice porosity, predicting also the array strength within 10% of molecular dynamics values. Although this study is focused on Au nanoparticles, our findings could be helpful in future studies of similar arrays with NPs of different kinds of materials.

Size and morphology effects on the high pressure behaviors of Mn3O4 nanorods

The high-pressure behaviors of Mn₃O₄ nanorods were studied by high pressure powder synchrotron X-ray diffraction and Raman spectroscopy. We found that the initial hausmannite phase transforms into the orthorhombic CaTi₂O₄-type structure, and then to the marokite-like phase upon compression. Upon decompression, the marokite-like phase is retained at the ambient pressure. Compared with Mn₃O₄ bulk and nanoparticles, Mn₃O₄ nanorods show obviously different phase transition behaviors. Upon compression, the phase transition sequence of Mn₃O₄ nanorods is similar with the nanoparticles, while the decompression behavior is consistent with the bulk counterparts. The hausmannite phase shows higher stability and smaller bulk modulus in Mn₃O₄ nanorods than those of the corresponding bulk and nanoparticles. We proposed that the higher phase stability and compressibility of the nanorods are concerned with their nanosize effects and the rod morphology. Both the growth orientation and the suppressed Jahn-Teller distortion of the Mn₃O₄ nanorods are crucial factors for their high pressure behaviors.

Site-Specific Pressure-Driven Spin-Crossover in Lu1-xScxFeO3

Pressure-driven spin-crossover (PSCO) is a collective quantum phenomenon frequently observed in transition-metal-based systems. According to the crystal-field theory, PSCO highly depends on the surrounding coordination environment of a given magnetic ion; nevertheless, it has never been verified experimentally up to now. Herein, we report the observation of a site-specific PSCO phenomenon in Lu_1-xSc_xFeO₃, in which octahedrally coordinated Fe³⁺ in orthorhombic LuFeO₃ and trigonal-bipyramidally coordinated Fe³⁺ in hexagonal Lu_0.5Sc_0.5FeO₃ show distinct PSCO response to external pressure. X-ray emission spectra and DFT calculations reveal the key role of coordination environment in a PSCO process and predict the occurrence of PSCO for trigonal-bipyramidally coordinated Fe³⁺ above 100 GPa, far beyond that of 50 GPa for octahedrally coordinated Fe³⁺ in LuFeO₃. The demonstration of site-specific PSCO sheds light on the state-of-the-art design of PSCO materials for directional applications.

Tunable photoluminescence and an enhanced photoelectric response of Mn2+-doped CsPbCl3 perovskite nanocrystals via pressure-induced structure evolution

Mn²⁺:CsPbCl₃ nanocrystals (NCs) were synthesized using a modified one-pot injection method, which exhibits significantly improved thermal stability. For the first time, the pressure-treated optical and structural properties of synthetic Mn²⁺:CsPbCl₃ NCs were further investigated, and their associated intriguing electrical and photoelectric properties were revealed from impedance spectra and photocurrent measurements under compression. The pressure-dependent photoluminescence experienced an initial redshift before 1.7 GPa followed by a continuous blueshift, as evidenced by the bandgap shifts. High-pressure XRD spectra uncovered a cubic-to-orthorhombic structural transition at about 1.1 GPa and subsequent amorphization upon further compression, which was fully reversible. Furthermore, the sample annealing from 340 K drove grain growth and decreased grain boundary resistance at ambient pressure. The compression further decreased the grain boundary barrier and improved the electrical conductivity (up to ∼10^-2Ω^-1 cm^-1) of the thermally annealed Mn²⁺:CsPbCl₃ NC surface. Simultaneous photocurrent enhancement of thermally annealed NCs was also achieved as expected, and reached optimal performance at 0.7 GPa. Strikingly, after the pressure cycling (loading-releasing), the results show that thermally annealed Mn²⁺:CsPbCl₃ NCs gained preservable higher electrical conductivity (∼10 times increase) and an improved photoelectric response compared to the ambient state before compression. This work proves that high pressure is useful for opening the versatility in the structure and properties of metal-halide perovskite nanocrystals leading to a promising way for superior optoelectronic materials-by-design.

Ligand dynamics control structure, elasticity, and high-pressure behavior of nanoparticle superlattices

Precise engineering of nanoparticle superlattices (NPSLs) for energy applications requires a molecular-level understanding of the physical factors governing their morphology, periodicity, mechanics, and response to external stimuli. Such knowledge, particularly the impact of ligand dynamics on physical behavior of NPSLs, is still in its infancy. Here, we combine coarse-grained molecular dynamics simulations, and small angle X-ray scattering experiments in a diamond anvil cell to demonstrate that coverage density of capping ligands (i.e., number of ligands per unit area of a nanoparticle's surface), strongly influences the structure, elasticity, and high-pressure behavior of NPSLs using face-centered cubic PbS-NPSLs as a representative example. We demonstrate that ligand coverage density dictates (a) the extent of diffusion of ligands over NP surfaces, (b) spatial distribution of the ligands in the interstitial spaces between neighboring NPs, and (c) the fraction of ligands that interdigitate across different nanoparticles. We find that below a critical coverage density (1.8 nm-2 for 7 nm PbS NPs capped with oleic acid), NPSLs collapse to form disordered aggregates via sintering, even under ambient conditions. Above the threshold ligand coverage density, NPSLs surprisingly preserve their crystalline order even under high applied pressures (∼40-55 GPa), and show a completely reversible pressure behavior. This opens the possibility of reversibly manipulating lattice spacing of NPSLs, and in turn, finely tuning their collective electronic, optical, thermo-mechanical, and magnetic properties.

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.

Pressure-Induced Emission (PIE) of One-Dimensional Organic Tin Bromide Perovskites

Low-dimensional halide perovskites easily suffer from the structural distortion related to significant quantum confinement effects. Organic tin bromide perovskite C₄N₂H₁₄SnBr₄ is a unique one-dimensional (1D) structure in which the edge sharing octahedral tin bromide chains [SnBr₄^2-]_∞ are embraced by the organic cations C₄N₂H₁₄²⁺ to form the bulk assembly of core-shell quantum wires. Some unusual phenomena under high pressure are accordingly expected. Here, an intriguing pressure-induced emission (PIE) in C₄N₂H₁₄SnBr₄ was successfully achieved by means of a diamond anvil cell. The observed PIE is greatly associated with the large distortion of [SnBr₆]^4- octahedral motifs resulting from a structural phase transition, which can be corroborated by in situ high-pressure photoluminescence, absorption, and angle-dispersive X-ray diffraction spectra. The distorted [SnBr₆]^4- octahedra would accordingly facilitate the radiative recombination of self-trapped excitons (STEs) by lifting the activation energy of detrapping of self-trapped states. First-principles calculations indicate that the enhanced transition dipole moment and the increased binding energy of STEs are highly responsible for the remarkable PIE. This work will improve the potential applications in the fields of pressure sensors, trademark security, and information storage.

Pressure-induced emission of cesium lead halide perovskite nanocrystals

Metal halide perovskites (MHPs) are of great interest for optoelectronics because of their high quantum efficiency in solar cells and light-emitting devices. However, exploring an effective strategy to further improve their optical activities remains a considerable challenge. Here, we report that nanocrystals (NCs) of the initially nonfluorescent zero-dimensional (0D) cesium lead halide perovskite Cs₄PbBr₆ exhibit a distinct emission under a high pressure of 3.01 GPa. Subsequently, the emission intensity of Cs₄PbBr₆ NCs experiences a significant increase upon further compression. Joint experimental and theoretical analyses indicate that such pressure-induced emission (PIE) may be ascribed to the enhanced optical activity and the increased binding energy of self-trapped excitons upon compression. This phenomenon is a result of the large distortion of [PbBr₆]^4- octahedral motifs resulting from a structural phase transition. Our findings demonstrate that high pressure can be a robust tool to boost the photoluminescence efficiency and provide insights into the relationship between the structure and optical properties of 0D MHPs under extreme conditions.

High-Pressure-Induced Comminution and Recrystallization of CH3 NH3 PbBr3 Nanocrystals as Large Thin Nanoplates

High pressure (HP) can drive the direct sintering of nanoparticle assemblies for Ag/Au, CdSe/PbS nanocrystals (NCs). Instead of direct sintering for the conventional nanocrystals, this study experimentally observes for the first time high-pressure-induced comminution and recrystallization of organic-inorganic hybrid perovskite nanocrystals into highly luminescent nanoplates with a shorter carrier lifetime. Such novel pressure response is attributed to the unique structural nature of hybrid perovskites under high pressure: during the drastic cubic-orthorhombic structural transformation at ≈2 GPa, (301) the crystal plane fully occupied by organic molecules possesses a higher surface energy, triggering the comminution of nanocrystals into nanoslices along such crystal plane. Beyond bulk perovskites, in which pressure-induced modifications on crystal structures and functional properties will disappear after pressure release, the pressure-formed variants, i.e., large (≈100 nm) and thin (<10 nm) perovskite nanoplates, are retained and these exhibit simultaneous photoluminescence emission enhancing (a 15-fold enhancement in the photoluminescence) and carrier lifetime shortening (from ≈18.3 ± 0.8 to ≈7.6 ± 0.5 ns) after releasing of pressure from 11 GPa. This pressure-induced comminution of hybrid perovskite NCs and a subsequent amorphization-recrystallization treatment offer the possibilities of engineering the advanced hybrid perovskites with specific properties.

Nanoparticle Superlattices: The Roles of Soft Ligands

Nanoparticle superlattices are periodic arrays of nanoscale inorganic building blocks including metal nanoparticles, quantum dots and magnetic nanoparticles. Such assemblies can exhibit exciting new collective properties different from those of individual nanoparticle or corresponding bulk materials. However, fabrication of nanoparticle superlattices is nontrivial because nanoparticles are notoriously difficult to manipulate due to complex nanoscale forces among them. An effective way to manipulate these nanoscale forces is to use soft ligands, which can prevent nanoparticles from disordered aggregation, fine-tune the interparticle potential as well as program lattice structures and interparticle distances - the two key parameters governing superlattice properties. This article aims to review the up-to-date advances of superlattices from the viewpoint of soft ligands. We first describe the theories and design principles of soft-ligand-based approach and then thoroughly cover experimental techniques developed from soft ligands such as molecules, polymer and DNA. Finally, we discuss the remaining challenges and future perspectives in nanoparticle superlattices.

Nanocube Superlattices of Cesium Lead Bromide Perovskites and Pressure-Induced Phase Transformations at Atomic and Mesoscale Levels

Lead halide perovskites are promising materials for a range of applications owing to their unique crystal structure and optoelectronic properties. Understanding the relationship between the atomic/mesostructures and the associated properties of perovskite materials is crucial to their application performances. Herein, the detailed pressure processing of CsPbBr₃ perovskite nanocube superlattices (NC-SLs) is reported for the first time. By using in situ synchrotron-based small/wide angle X-ray scattering and photoluminescence (PL) probes, the NC-SL structural transformations are correlated at both atomic and mesoscale levels with the band-gap evolution through a pressure cycle of 0 ↔ 17.5 GPa. After the pressurization, the individual CsPbBr₃ NCs fuse into 2D nanoplatelets (NPLs) with a uniform thickness. The pressure-synthesized perovskite NPLs exhibit a single cubic crystal structure, a 1.6-fold enhanced photoluminescence quantum yield, and a longer emission lifetime than the starting NCs. This study demonstrates that pressure processing can serve as a novel approach for the rapid conversion of lead halide perovskites into structures with enhanced properties.