Search

Constructing three-dimensional (3D) metamaterials from functional nanoparticles endows them with emerging collective properties tailored by the packing geometries. Herein, we report 3D supercrystals self-assembled from upconversion nanorods (NaYF4:Yb,Er NRs), which exhibit both translational ordering of NRs and orientational ordering between constituent NRs in the superlattice (SL). The construction of 3D reciprocal space mappings (RSMs) based on synchrotron-based X-ray scattering measurements was developed to uncover the complex structure of such an assembly. That is, the two main orthogonal sets of hexagonal close-packing (hcp)-like SLs share the [110]SL axis, and NRs within the SL possess orientational relationships of [120]NR//[100]SL, [210]NR//[010]SL, and [001]NR//[001]SL. Notably, these supercrystals containing well-aligned NRs exhibit collectively anisotropic upconversion fluorescence in two perpendicular directions. This study not only demonstrates novel crystalline superstructures and functionality of NR-based 3D assemblies but also offers a unique tool for deciphering a wide range of complex nanoparticle supercrystals.

Nanocube (NC) assemblies display complex superlattice behaviors, which require a systematic understanding of their nucleation and growth as well transformation toward construction of a consistent superlattice phase diagram. This work made use of Fe3O4 NCs with controlled environments, and assembled NCs into three-dimensional (3D) superlattices of simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc), acute and obtuse rhombohedral (rh) polymorphs, and 2D superlattices of square and hexagon. Controlled experiments and computations of in situ and static small-angle X-ray scattering (SAXS) as well as electron microscopic imaging revealed that the fcc and bcc polymorphs preferred a primary nucleation at the early stage of NC assembly, which started from the high packing planes of fcc(111) and bcc(110), respectively, in both 3D and 2D cases. Upon continuous growth of superlattice grain (or domain), a confinement stress appeared and distorted fcc and bcc into acute and obtuse rh polymorphs, respectively. The variable magnitudes of competitive interactions between configurational and directional entropy determine the primary superlattice polymorph of either fcc or bcc, while emergent enhancement of confinement effect on enlarged grains attributes to late developed superlattice transformations. Differently, the formation of a sc polymorph requires a strong driving force that either emerges simultaneously or is applied externally so that one easy case of the sc formation can be achieved in 2D thin films. Unlike the traditional Bath deformation pathway that involves an intermediate body-centered tetragonal lattice, the observed superlattice transformations in NC assembly underwent a simple rhombohedral distortion, which was driven by a growth-induced in-plane compressive stress. Establishment of a consistent phase diagram of NC-based superlattices and reconstruction of their assembly pathways provide critical insight and a solid base for controlled design and scalable fabrication of nanocube-based functional materials with desired superlattices and collective properties for real-world applications.

In order to assess the suitability of mesoporous materials for applications in energy harvesting/storage processes occurring under extreme conditions, their mechanical, thermal and hydrothermal properties need to be fully investigated. In this study, the bulk mechanical and extreme hydrothermal properties of periodic mesoporous SBA-15 type silica and SBA-15 type aluminosilica (Al-SBA-15) were investigated using in situ small angle x-ray scattering (SAXS). In situ SAXS measurements were made on dry mesoporous SBA-15 silica and Al-SBA-15 aluminosilica samples as a function of pressure (at room temperature) to ∼ 12 GPa and on the same mesoporous materials under extreme hydrothermal conditions (to 255 °C and ∼114 MPa) using the diamond anvil cell (DAC). The analyses of the high-pressure SAXS data indicate that the mesoporous Al-SBA-15 aluminosilica has substantially greater bulk mechanical stability (isothermal bulk modulus κ = 34.7(4.5) GPa) than the mesoporous SBA-15 silica (κ = 12.0(3.0) GPa). Our molecular dynamics (MD) simulations are able to accurately model the bulk mechanical stability properties of mesoporous SBA-15 silica but underestimate the same properties of Al-SBA-15 aluminosilica. Analysis of the in situ SAXS data measured under extreme hydrothermal conditions indicates swelling of the pore walls due to water incorporation that is more significant in mesoporous Al-SBA-15 aluminosilica (∼2x) than in SBA-15 silica. In addition, the Al-SBA-15 aluminosilica clearly exhibits superior hydrothermal stability compared to SBA-15 silica under the extreme experimental temperature and pressure conditions.

Understanding the shape-dependent superlattices and resultant anisotropies of both structure and property allows for rational design of materials processing and engineering to fabricate transformative materials with useful properties for applications. This work shows the structural evolution from square lattice of two-dimensional (2D) thin film to rhombic lattice of large three-dimensional (3D) assembles of Pt3Co nanocubes (NCs). Synchrotron-based X-ray supercrystallography determines the superlattice of large 3D supercrystal into an obtuse rhombohedral (Rh) symmetry, which holds a long-range coherence of both NC translation and atomic crystallographic orientation. The Rh superlattice has a trigonal cell angle of 104°, and the constitute NCs orient their atomic Pt3Co(111) planes to the superlattice Rh[111] direction. The temperature-dependent in situ small and wide-angle X-ray scattering (SAXS/WAXS) measurements reveal a thermally induced superlattice contraction of supercrystal, which maintains translati...

Certain alumino-silicates display exotic properties enabled by their framework structure made of corner-sharing tetrahedral rigid units. Using in situ diamond-anvil cell x-ray diffraction (XRD), we study the pressure-induced transformation of β eucryptite, a prototypical alumino-silicate that undergoes a phase transformation at moderate pressures. The atomic structure and symmetry group of the new pressure-stabilized phase has not yet been reported. Based on density functional theory studies and Rietveld analysis of XRD patterns, we find that the new phase belongs to the Pna2 1 space group and report its atomic structure. Furthermore, we discover two other possible pressure-stabilized polymorphs, P1c1 and Pca2 1.

Periodic mesoporous materials possess high surface to volume ratio and nano-scale sized pores, making them potential candidates for heterogeneous catalysis, ion exchange, gas sensing and other applications. In this study, we use in situ small angle x-ray scattering (SAXS) and molecular dynamics (MD) simulations to investigate the mechanical and hydrothermal stability properties of periodic mesoporous SBA-15 silica and SBA-15 type aluminosilica (Al-SBA-15) to extreme conditions. The mesoporous SBA-15 silica and Al-SBA-15 aluminosilica possess amorphous frameworks and have similar pore size distribution (pore size ∼9-10 nm). The in situ SAXS measurements were made at the B1 beamline, at the Cornell High Energy Synchrotron Source (CHESS). The mesoporous SBA-15 silica and Al-SBA-15 aluminosilica specimens were loaded in a diamond anvil cell (DAC) for pressure measurements, and, separately, with water in the DAC for hydrothermal measurements to high P-T conditions (to 255 °C and ∼ 114 MPa). Analyses of the pressure-dependent SAXS data show that the mesoporous Al-SBA-15 aluminosilica is substantially more mechanically stable than the SBA-15 silica. Hydrothermal measurements show a small net swelling of the framework at elevated P-T conditions, due to dissolution of water into the pore walls. Under elevated P-T conditions, the Al-SBA-15 aluminosilica shows significantly greater hydrothermal stability than the SBA-15 silica. Our MD simulations show that the bulk modulus value of periodic mesoporous SBA-15 silica varies exponentially with percentage porosity. Molecular dynamics simulations are being made in order to better understand how the pore architecture and the chemical composition of the host structure govern the stability properties of the mesoporous materials.

Anisotropic nanocrystal assembled supercrystals with open superlattices (SLs) manifest novel and unique properties, but poor understanding of the nucleation/growth mechanisms limits their design and fabrication for practical applications. Using highly anisotropic Pt3Ni octahedral nanocrystals, we have grown large single supercrystals with an open body-centered cubic (bcc) superlattice that has a low filling factor of 26.8%. Synchrotron-based X-ray structural reconstruction fully revealed the coherence of translational and orientational orderings and determined that the constituent octahedra arrange themselves with the vertex-to-vertex and face-to-face configurations along the SL[100] and SL[111] directions, respectively. The large face-to-face separation and flexible vertex-to-vertex elastic contact provided the rattle space and supporting axis for local rotations of Pt3Ni octahedra within the bcc superlattice. Development of orientational disordering along with robust preservation of translational ordering during the heating process of a supercrystal in the oleic acid wetting environment confirmed the dominance of rotational entropy of hard octahedra in the formation of the open bcc superlattice. Ultimate achievement of dynamic equilibrium between the vertex-oriented elastic repulsions and the face-oriented attractions of surface-coating ligands governs the structural and mechanical stability of the supercrystal. This discovery provides significant insights into the design and control of geometrical shapes for the fabrication of highly anisotropic nanocrystals into desired open superlattices with tailored optical and electronic properties.

Pressure-induced crystallographic transitions and optical behavior of MAPbI3 (MA=methylammonium) were investigated using in-situ synchrotron X-ray diffraction and laser-excited photoluminescence spectroscopy. We observed that the tetragonal phase that presents under ambient pressure transformed to a ReO3-type cubic phase at 0.3 GPa, which further converted into a putative orthorhombic structure at 2.7 GPa. The sample was finally separated into crystalline and amorphous fractions beyond 4.7 GPa. During the decompression, the phase-mixed material restored the original structure in two distinct pathways depending on the peak pressures. Being monitored using a laser-excited photoluminescence technique under each applied pressure, it was determined that the bandgap reduced with an increase of the pressure till 0.3 GPa and then enlarged with an increase of the pressure up to 2.7 GPa. This work lays the foundation for understanding pressure-induced phase transitions and bandgap tuning of MAPbI3, enriching potentially the toolkit for engineering perovskites related photovoltaic devices. National Research Foundation (NRF) Accepted version This work was partially supported by NRF-CRP14-2014-03 and Custom Electronics, Inc. CHESS was supported by the NSF award DMR-1332208. Bandgap calculations were contributed by Hai Xiao, Jason Crowley and William A. Goddard III from Materials and Process Simulation Center (MSC) and Joint Center for Artificial Photosynthesis (JCAP), California Institute of Technology. S.J. acknowledges the support from Binghamton University.

In situ SAXS measurements were made on periodic mesoporous carbon and silica-based materials as a function of pressure and temperature and in water to supercritical conditions. Our data show that periodic mesoporous silica-based materials exhibit substantial mechanical strength. The pore structure, size and related mesoscale properties appear to directly impact the mechanical response of silica-based materials to high pressures and temperatures. Our results indicate that hydrolysis does not seem to be responsible for the mechanical collapse of the pore structure of silica-based mesoporous materials under high pressure conditions. SBA-15 type mesoporous carbon exhibits excellent hydrothermal stability under extreme conditions. The mesostructure of FDU-12 silica is irreversibly disordered after extreme hydrothermal exposure in a manner consistent with H 2 O dissociation reactions causing topological alteration of the silicate network.

The modification of dielectric surface with a self-assembled monolayer (SAM) such as octadecyltrichlorosilane (OTS) is a widely used method to tune the electrical property of diverse electronic devices based on organic semiconductors, graphene, transition metal dichalcogenides (TMDs), and so forth. The surface roughness of self-assembled OTS monolayer is a key factor in determining its effect on device performance, but the preparation of an ultrasmooth OTS monolayer is a technologically challenging task. In this work, an ultrasmooth OTS monolayer is prepared via a facile peeling method, which may serve as a postremedy strategy to remove the protuberant aggregates. Such a method has not been reported before. With organic semiconductors as a testing model, ultrasmooth OTS may significantly improve the charge mobility of organic field-effect transistors (OFETs). P-type dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) OFET with an ultrasmooth OTS monolayer yields good reproducibility and unprecendented maximum mobility of 8.16 cm(2) V(-1) s(-1), which is remarkably superior to that of the OFET with a pristine OTS monolayer. This work develops a simple method to resolve the common and significant problem of the quality of OTS modification, which would be highly promising for electronic applications as well as other fields such as surface and interface engineering.

Using an in situ synchrotron X-ray diffraction technique, a pressure-induced phase transformation of PbTe nanocrystals with sizes of 13 and 5 nm up to ∼20 GPa was studied. Upon an increase of pressure, we observed that the 13 nm PbTe nanocrystals start a phase transformation from rocksalt structure to an intermediate orthorhombic structure and finally CsCl-type structure at 8 GPa, which is 2 GPa higher than that in bulk PbTe. In contrast, the 5 nm PbTe nanocrystals do not display the same type of transition with a further increased transition pressure as expected. Instead of orthorhombic or CsCl-type structure, the 5 nm PbTe nanocrystals turn to amorphous phase under a similar pressure (8 GPa). Upon a release of pressure, the 13 nm PbTe nanocrystals transform from high pressure CsCl-type structure directly to rocksalt structure, whereas the 5 nm PbTe nanocrystals remain their amorphous phase to ambient conditions. The structure stability of rocksalt-type PbTe shows a significant reversal of Hall-Petch effect. On the basis of such an observation with a critical size determination of ∼9 nm, PbTe nanocrystals appear as the first class of material that demonstrates a pressure-induced structural change from order to disorder. By sharing the insight of this reversed Hall-Petch effect with associated transition types, we tuned our experimental protocol and successfully synthesized a sample with "high-pressure metastable structure", amorphous phase at ambient pressure. This integrative study provides a feasible pathway to understand nucleation mechanism as a function of particle size and to explore novel materials with high-pressure metastable structure and unique properties under lab-accessible conditions.

The isometric, pyrochlore structure type, A2B2O7, exhibits a wide variety of properties that find application in a large number of different technologies, from electrolytes in solid oxide fuel cells to actinide-bearing compositions that can be used as nuclear waste forms or inert matrix nuclear fuels. Swift xenon ions (1.43 GeV) have been used to systematically modify different compositions in the Gd2Zr2-xTixO7 binary at the nanoscale by radiation-induced phase transitions that include the crystalline-to-amorphous and order-disorder structural transformations. Synchrotron x-ray diffraction, Raman spectroscopy, and transmission electron microscopy provide a complete and consistent description of structural changes induced by the swift heavy ions and demonstrate that the response of pyrochlore depends strongly on chemical composition. The high and dense electronic energy deposition primarily results in amorphization of Ti-rich pyrochlore; whereas the formation of the fully disordered, defect-fluorite structure is the dominant process for Zr-rich pyrochlore.

Phase stability of titanium hydride (TiH2) was studied at high pressure-high temperature conditions using synchrotron radiation under non-hydrostatic conditions. Resistive heating method was used to heat the sample to a maximum temperature of 873 K in a diamond anvil cell (DAC) under pressure up to 12 GPa. Pressure–temperature behavior was studied by varying the temperature upto 823 K in steps of 50 K with pressure variations within 3 GPa. Structural phase transformation from tetragonal (I4/mmm) to cubic (Fm-3 m) was observed with increase in temperature. Tetragonal phase was found to be stabilized when the sample was subjected to pressure and temperature cycle.

The tetragonal orthophosphates, YbPO4 and LuPO4, were studied by in situ X-ray diffraction (XRD) at pressures up to 52 and 43 GPa, respectively. A reversible phase transition from the zircon structure-type to the scheelite structure-type was found at not, vert, similar22 GPa for YbPO4 and 19 GPa for LuPO4. Coinciding with the transition from the zircon structure-type to the scheelite structure-type, there is a not, vert, similar 10% reduction in volume and a significant increase in the bulk modulus for both compounds.

Pd nanocubes with an average side length of approximately 10 nm were compressed up to 24.8 GPa in a diamond-anvil cell (DAC). In situ synchrotron X-ray diffraction was used to monitor structural changes, and a face-centered cubic (fcc) to face-centered tetragonal (fct) distortion was observed for the first time. This novel discovery not only provides new insights into the pressure-induced behavior of faceted nanocrystals of palladium and other noble metals but also gives guidance for finding new phases in close-packed metals.

Sm{sub 2}O{sub 3} was compressed at room temperature up to 44.0 GPa and then decompressed back to ambient pressure. In situ X-ray diffraction was used to monitor the structural changes in the sample. A cubic to hexagonal phase transformation was observed in Sm{sub 2}O{sub 3} for the first time. After decompression back to ambient pressure, the hexagonal phase was not quenchable and transformed to a monoclinic phase. Ab initio Density-Functional-Theory (DFT) calculations were performed to obtain theoretical data for comparison with the experimental results and elucidation of the transformation mechanism. A possible phase transformation mechanism that is consistent with the experimental results and theoretical calculations is proposed.

We investigated the elastic compressibility of PbS nanocrystals (NCs) pressurized in a diamond anvil cell and simultaneously probed the structure using synchrotron-based X-ray diffraction. The compressibility of PbS NCs exhibits bimodal size dependence. The elastic modulus of small NCs increases with increasing diameter and peaks near a particle diameter of approximately 7 nm. For large NCs the elastic modulus decreases toward the bulk value with increasing NC diameter. We explain the bimodal size-dependence of the elastic modulus in terms of a core-shell model based on distinct elasticity of the crystal near the surface and in the core of the particle. We combined insights into the size-dependent elasticity and lattice spacing to determine the Debye temperature of PbS NCs as a function of particle diameter. Understanding the size-dependent elasticity of defect-free colloidal NCs provides new insights into their crystal structure and mechanical properties.

We grew large single three-dimensional supercrystals from colloidal Pt nanocubes (NCs) suspended in hexane. A synchrotron-based two circle diffractometer was used to obtain an unprecedented level of detail from full sets of small/wide-angle X-ray scattering (SAXS/WAXS) patterns. Automatic indexing and simulations of X-ray patterns enabled detailed reconstruction of NC translation and shape orientation within the supercrystals from atomic to mesometric levels. The supercrystal has an obtuse rhombohedral (Rh) superlattice with space group R3m and a trigonal cell angle of 106.2°. Individual NCs orient themselves in a manner of atomic Pt[111] parallel to superlattice Rh[111]. We analyzed the superlattice structure in context of three spatial relationships of proximate NCs including face-to-face, edge-to-edge, and corner-to-corner configurations. Detailed analysis of supercrystal structure reveals nearly direct corner-to-corner contacts and a tight interlocking NC structure. We employed the correlations between strain and lattice distortion and established the first structural correlating mechanism between five superlattice polymorphs to elucidate the superlattice transformations and associated developing pathways. Together, the experimental and modeling results provide comprehensive structural information toward controlling design and efficient materials-processing for large fabrication of nanobased functional materials with tailored structures and desired properties.

Nanometre-sized inorganic dots, wires and belts have a wide range of electrical and optical properties, and variable mechanical stability and phase-transition mechanisms that show a sensitive dependency on size, shape and structure. The optical properties of the semiconductor ZnS in wurtzite structures are considerably enhanced, but the lack of structural stability limits technological applications. Here, we demonstrate that morphology-tuned wurtzite ZnS nanobelts show a particular low-energy surface structure dominated by the +/-[210] surface facets. Experiments and calculations show that the morphology of ZnS nanobelts leads to a very high mechanical stability to approximately 6.8 GPa, and also results in an explosive mechanism for the wurtzite-to-sphalerite phase transformation together with in situ fracture of the nanobelts. ZnS wurtzite nanobelts provide a model that is useful not only for understanding the morphology-tuned stability and transformation mechanism, but also for improving synthesis of metastable nanobelts with quantum effects for electronic and optical devices.

An in-situ Raman spectroscopic study was conducted to explore the pressure induced phase transformation of spinel-type ferrite ZnFe2O4. Results indicate that ferrite ZnFe2O4 initially transforms to an orthorhombic structure phase (CaFe2O4-polymorph) at a pressure of 24.6 GPa. Such a phase transformation is complete at 34.2 GPa, and continuously remains stable to the peak pressure of 61.9 GPa. The coexistence of the two phases over a wide range of pressure implies a sluggish mechanism upon the spinel-to-orthorhombic phase transition. Upon release of pressure, the high pressure ZnFe2O4 polymorph is quenchable at ambient conditions.

An in situ Raman spectroscopic study was conducted to investigate the pressure induced phase transformation of spinel NiCr 2 O 4 to pressures of 57.1 GPa. Results indicate that NiCr 2 O 4 spinel dissociates to the mixture of Cr 2 O 3 and NiO at 13.1 GPa. Such a dissociation is complete at 25.1 GPa. The coexistence of Cr 2 O 3 and NiO remains up to the peak pressure of 57.1 GPa. Upon release of pressure to 33.3 GPa, the Cr 2 O 3 starts to react with NiO to form the spinel phase, and such a reaction is complete at 18.9 GPa. The recovered spinel phase may exhibit an inverse spinel structure, which differs from the initial normal spinel structure. Combined with the results obtained from other end member chromites, it is suggested that the variation of high-pressure behavior of chromites (ACr 2 O 4 ) significantly depends on the average A–O bond length of the A–O tetrahedron.

An in-situ Raman Spectroscopic study was conducted to explore the pressure-induced phase transformation of CaMn2O4 to pressures of 73.7 GPa . Group theory yields 24 Raman active modes for CaMn2O4, of which 20 are observed at ambient conditions. With the slight compression below 5 GPa , the pressure-induced contraction compensates the structural distortion induced by a Jahn–Teller (JT) effect, resulting in the occurrence of the zero pressure shifts of the JT-related Raman modes. Upon elevation of pressure to nearby 35 GPa , these Raman modes start to display a significant variation in pressure shift, implying the appearance of a pressure-induced phase transformation. Group factor analyses on all possible structure polymorphs indicate that the high-pressure phase is preferentially assigned to an orthorhombic structure, having the CaTi2O4 structure. The cooperative JT distortion is continuously reduced in the CaMn2O4 polymorph up to 35 GPa . Beyond 35 GPa , it is found that the JT effect was completely suppressed by pressure in the newly formed high-pressure phase. Upon release of pressure, this high-pressure phase transforms to the original CaMn2O4 phase, and continuously remains stable to ambient conditions.

An in situ Raman spectroscopic study was conducted to investigate the pressure induced phase transformation of MgCr2O4 spinel up to pressures of 76.4 GPa. Results indicate that MgCr2O4 spinel undergoes a phase transformation to the CaFe2O4 (or CaTi2O4) structure at 14.2 GPa, and this transition is complete at 30.1 GPa. The coexistence of two phases over a wide range of pressure implies a sluggish transition mechanism. No evidence was observed to support the pressure-induced dissociation of MgCr2O4 at 5.7–18.8 GPa, predicted by the theoretical simulation. This high pressure MgCr2O4 polymorphism remains stable upon release of pressure, but at ambient conditions, it transforms to the spinel phase.

Macro-crystalline γ-Fe 2 O 3 transforms to the α-Fe 2 O 3 structure at a pressure of 35 GPa. We performed a Raman spectroscopic study to explore the pressure induced phase transformation in nanocrystalline γ-Fe 2 O 3 to 57.5 GPa. We found that at a pressure of 26.6 GPa, there was a significant transformation of the γ-phase to α-structure. All Raman modes of the α-Fe 2 O 3 nanophase as extrapolated to 1 atm are higher than those observed in the bulk counterpart, and also exhibit lower pressure shifts, except one mode at 269.1 cm −1 . The α-Fe 2 O 3 phase transforms to a perovskite phase at ∼53.3 GPa, which is the same as that observed in the bulk material. Upon release of pressure to ambient conditions, the perovskite phase transforms to the α-Fe 2 O 3 phase by a slow distortional mechanism. The recovered α-Fe 2 O 3 phase exhibits the same Raman characteristics as those in the bulk sample. Particle size affects the transition pressure of the first phase transformation but does not affect the continuous post-phase transformations.

Compressibility of an oxide does not necessarily change due to decrease in particle size. This is found in nanocrystalline CuO, an important semiconductor. We studied nanometric CuO with high energy synchrotron radiation and Raman spectroscopic techniques to pressures of 47 GPa. Our results indicate that nanometric CuO has the same bulk modulus as observed in the macrometric CuO. Combination of results obtained from MgO, Ni, and e-Fe indicates that the contribution of the size-induced surface energy to total internal energy has very little effect on the high pressure behavior of this type of nanomaterials, in which their bulk counterparts exhibit the structural stability over a wide range of pressure. This result is contrary to some observations, showing that the reduction of particle size significantly leads to increased bulk modulus.

High-pressure compression of water contained in nanoporous silica allowed fabrication of novel porous ice phases as a function of pressure. The starting liquid nanoporous H2O transformed to ice VI and VII at 1.7 and 2.5 GPa, respectively, which are 0.6 and 0.4 GPa higher than commonly accepted pressures for bulk H2O. The continuous increase of pressure drives the formation of a tetragonally distorted VII structure with the space group I4mm, rather than a cubic Pn3m phase in bulk ice. The enhanced incompressibility of the tetragonal ice is related to the unique nanoporous configuration, and the distortion ratio c/a gradually increases with increasing pressure. The structural changes and enhanced thermodynamic stability may be interpreted by the two-dimensional distribution of silanol groups on the porous silica surfaces and the associated anisotropic interactions with H2O at the interfaces.

Ordered assemblies of inorganic nanocrystals coated with organic linkers present interesting scientific challenges in hard and soft matter physics. We demonstrate that a nanocrystal superlattice under compression serves as a nanoscopic pressure cell to enable studies of molecular linkers under uniaxial compression. We developed a method to uniaxially compress the bifunctional organic linker by attaching both ends of aliphatic chains to neighboring PbS nanocrystals in a superlattice. Pressurizing the nanocrystal superlattice in a diamond anvil cell thus results in compression of the molecular linkers along their chain direction. Small-angle and wide-angle X-ray scattering during the compression provide insights into the structure of the superlattice and nanocrystal cores under compression, respectively. We compare density functional theory calculations of the molecular linkers as basic Hookean springs to the experimental force–distance relationship. We determine the density of linkers on the nanocrystal surfaces. We demonstrate our method to probe the elastic force of single molecule as a function of chain length. The methodology introduced in this paper opens doors to investigate molecular interactions within organic molecules compressed within a nanocrystal superlattice.

An in situ x-ray diffraction study was conducted to study the pressure-induced phase transformation in nanocrystalline anatase/rutile (TiO2) to 35.1 GPa. The nano-anatase phase remains stable to approximately 16.4 GPa, and then transforms to an amorphous phase, which is returned upon release of pressure. The nano-rutile phase starts to transform to the baddeleyite (ZrO2) structure at ~8.7 GPa, and the transformation is complete at approximately 16.4 GPa. On release of pressure the ZrO2 structure transforms to the α-PbO2 structure. The results are compared to previous work on phase changes in TiO2 with different particle sizes.

Based on the relation between thermodynamic properties and temperature, one simple model has been developed to predict the melting of solids under high pressures. For this model, a critical tempera ...

A raman spectroscopic investigation was carried out to study the pressure-induced phase transformation in nanocrystalline anatase (TiO 2 ) up to 37 GPa. As compared to a macrocrystalline solid which transforms to the α-PbO 2 structure at 2.6–4.5 GPa, we found that the nano-anatase phase remains stable to pressures as high as ∼24 GPa, and then transforms to an amorphous phase. The new amorphous phase is quenchable upon release of pressure to ambient condition. An increased surface energy may stabilize the nano-anatase, suppressing the appearance of anatase-to-α-PbO 2 phase transformation (2.6–4.5 GPa in bulk anatase), and then leads to the amorphization of nano-anatase under strong compression.

High-pressure melting has been of general interest for our understanding of solid-liquid phase transition due to the importance in high-pressure physics and material science, and here a simple mode ...

Thermal pressure along with its temperature dependence has been used to calculate the melting temperatures of solids under high pressures. Compared with the reference volume (300 K), a critical mel ...

We have investigated the melting temperatures of MgSiO3 and CaSiO3 perovskites at high pressure conditions, using an empirical melting equation, combined with the assumption of a boundary phase exi ...

Pure andradite (Ca 3 Fe 2 Si 3 O 12 ) was synthesized at 1150°C and 3 GPa, and then compressed to pressure above 21 GPa, and heated to about 1000°C by a YAG laser in a Diamond Anvil Cell (DAC). After temperature quenching in situ high pressure X-ray diffraction studies were carried out. The results show that andradite has transformed to a cubic perovskite type polymorph with a =3.460(4) A. Upon decompression, in situ X-ray observations reveal that perovskite phase has transformed to an amorphous phase. The density of the perovskite polymorph of Ca 3 Fe 2 Si 3 O 12 is about 13.6% greater than that of an isochemical composition at 21 GPa, and ferric iron can replace Ca 2+ and Si 4+ in the perovskite structure (2Fe 3+ =Si 4+ +Ca 2+ ), giving a chemical formula: (Ca,Fe)(Si,Fe)O 3 . Combining with the electron hopping mechanism between Fe 2+ and Fe 3+ in semiconductivity materials, this new Fe 3+ -rich perovskite polymorph may provide new data to clarify the electrical conductivity of the lower mantle, based on the mineralogical model with magnesiuwustite (Mw) and perovskite (Pv).

Silver micro- and nanocrystals with sizes of ∼2−3.5 μm and ∼50−100 nm were uniaxially compressed under nonhydrostatic pressures (strong deviatoric stress) up to ∼30 GPa at room temperature in a symmetric diamond-anvil cell and studied in situ using angle-dispersive synchrotron X-ray diffraction. A cubic to trigonal structural distortion along a 3-fold rotational axis was discovered by careful and comprehensive analysis of the apparent lattice parameter and full width at half-maximum, which are strongly dependent upon the Miller index and crystal size.

The thermal pressure of perovskite (MgSiO3) and periclase (MgO) has been estimated in order to evaluate the accuracy of the pressure techniques as applied to obtain the pressure-volume-temperature ...

Combining in situ high-pressure X-ray scattering with transmission electron microscopy, we investigated the pressure-induced structural switches between the rock salt and amorphous phases as well as the associated mechanisms of their crystallization and growth in 6 nm PbTe nanocrystal. It was observed that rock salt PbTe nanocrystal started to become amorphous above 10 GPa and then underwent a low-to-high density amorphous phase transformation at pressures over 15 GPa. The low-density amorphous phase exhibited a structural memory of the rock salt phase, as manifested by a backward transformation to the rock salt phase via single nucleation inside each nanoparticle upon the release of pressure. In contrast, the high-density amorphous phase remained stable and could be preserved at ambient conditions. In addition, electron beam-induced heating could drive a recrystallization of the rock salt phase on the recovered amorphous nanoparticles. These studies provide significant insights into structural mechanisms for pressure-induced switching between amorphous and crystalline phases as well as their associated growth processes.

Assemblies of 3.5 nm PbS nanoparticles (NPs) nucleate in three dominant superlattice polymorphs: amorphous, body-centered-cubic (bcc) and face-centered-cubic (fcc) phase. This superlattice relationship can be controlled by the inter-NP distance without changing the NP size. Upon increase of inter-NP distance, the packing density decreases, and the capping molecules at NP surfaces change in structure and accordingly modify the surface energy. The driving force for NP assembly develops from an entropic maximization to a reduction of total free energy through multiple interactions between surface molecules and NPs and resulting variation of surface molecules. Upon long-term aging and additional thermal treatment, fcc undergoes a tetragonal distortion and subsequently transforms to bcc phase, and simultaneously, the NPs embedded in supercrystals reduce surface energy primarily in {200} facets. Linking molecule-NP interactions with a series of changes of packing density and surface lattice spacings of NPs allows for an interpretation of principles governing the nucleation, structure stability, and transformation of PbS NP-assembled supercrystals.

We demonstrate that the incompressibility of spinel Ge3N4 nanocrystals decreases when the pressure is elevated above ∼20 GPa. Ge3N4 nanocrystals initially exhibit a higher bulk modulus of 381(2) GPa. But, above 20 GPa, the bulk modulus is apparently reduced to 268(4) GPa, which is similar to the reported bulk modulus of 208−296 GPa for the bulk Ge3N4. Thus, a threshold pressure of ∼20 GPa was determined that signifies the onset of size-induced disappearance of elastic stiffness in nanocrystalline Ge3N4. Enhanced surface energy contributions to the shell layers of nanoparticles and resulting effect on the corresponding large d spacing planes are used to elucidate the observed phenomenon. This study provides a reasonable explanation for the different compressibility properties of numerous nanocrystals.

We demonstrate a direct fabrication of PbS nanocube supercrystals without size-selection pretreatment on the building blocks. Electron microscopic and synchrotron small angle X-ray scattering analyses confirm that nanocubes pack through a tilted face-centered-cubic (fcc) arrangement, that is, face-to-face along the110(super) direction, resulting in a real packing efficiency of as high as ∼83%. This new type of superstructure consisting of nanocubes as building blocks, reported here for the first time, is considered the most stable surfactant-capped nanocube superstructure determined by far.

We investigated the structural stability of colloidal PbS nanocrystals (NCs) self-assembled into superlattice (SL) allotropes of either face-centered cubic (fcc) or body-centered cubic (bcc) symmetry. Small-angle X-ray scattering analysis showed that the NC packing density is higher in the bcc than in the fcc SL; this is a manifestation of the cuboctahedral shape of the NC building block. Using the high-pressure rock-salt/orthorhombic phase transition as a stability indicator, we discovered that the transition pressure for NCs in a bcc SL occurs at 8.5 GPa, which is 1.5 GPa higher than the transition pressure (7.0 GPa) observed for a fcc SL. The higher structural stability in the bcc SL is attributed primarily to the effective absorption of loading force in specific SL symmetry and to a lesser extent to the surface energy of the NCs. The experimental results provide new insights into the fundamental relationship between the symmetry of the self-assembled SL and the structural stability of the constituent NCs.