Size-dependent Phase Transition Research Articles

Amorphous Sb2Te3 nanowires: Synthesis, characterization and size-dependent phase transition behavior

Understanding the phase transition behavior of phase-change (PC) material with respect to the scaling effect is essential for the application of PC materials. Among all the PC materials, SbTe binary plays a significant role in the well-known Ge-Sb-Te system. Here we have used an unconventional and cost-effective method to fabricate a wide range of prototypical Sb2Te3amorphous nanowires (18–220 nm in diameter) using templated electrodeposition. Compositional, morphological, and structural characterization of the amorphous nanowires were performed, and the crystallization temperature of in-template nanowires was measured using a four-probe resistivity meter. We report that the crystallization temperature of amorphous Sb2Te3 nanowires can be tuned with respect to the diameter of the nanowires and a significant increase was observed for the nanowires with diameters ≤35 nm. Our study sheds light on the size-dependent phase transition behavior of PC NWs with implications for further advancement of the PC memory technology.

Crown‐Ether‐Ring Size Dependent Crystal Structures, Phase Transition and Dielectric Properties of [M(crown)]BF4⋅xH2O (M+=Na+, K+; crown=15‐crown‐5, 18‐crown‐6; x=0 or 1)

AbstractCrown ethers demonstrate significant conformational flexibility and rotational symmetry, rendering them invaluable in the realms of supramolecular chemistry and crystal engineering. These unique natures facilitate the construction of supramolecular crystals of crown ethers, characterized by disorder‐order phase transitions, imparts unique properties that hold promise for diverse applications across multiple fields. In this study, four supramolecular compounds, namely [Na(15‐crown‐5)]BF4 (1), [Na(18‐crown‐6)]BF4⋅H2O (2), [K(15‐crown‐5)]BF4 (3) and [K(18‐crown‐6)]BF4⋅H2O (4) were synthesized and characterized by microanalysis, thermogravimetric analysis, differential scanning calorimetry and powder X‐ray diffraction techniques. Herein, 15‐crown‐5 and 18‐crown‐6 correspond to 1,4,7,10,13‐pentaoxacyclopentadecane and 1,4,7,10,13,16‐hexaoxacyclooctadecane, respectively. It was observed that the crystal structure, phase transition, and dielectric properties of these supramolecular compounds are significantly influenced by the size of the crown‐ether rings. The research extensively discussed the correlation between the coordination mode of metal ions of K+ or Na+ with crown ethers, the compatibility between metal ions and crown‐ether rings in terms of size, and the effects of crown‐ether disorder on dielectric permittivity during phase transitions. Our discoveries hold significant implications for the design and development of crown‐ether supramolecular functional materials.

Size-Dependent Phase Transition in Ultrathin Ga2O3 Nanowires.

Gallium oxide (Ga2O3) has attracted extensive attention as a potential candidate for low-dimensional metal-oxide-semiconductor field-effect transistors (MOSFETs) due to its wide bandgap, controllable doping, and low cost. The structural stability of nanoscale Ga2O3 is the key parameter for designing and constructing a MOSFET, which however remains unexplored. Using in situ transmission electron microscopy, we reveal the size-dependent phase transition of sub-2 nm Ga2O3 nanowires. Based on theoretical calculations, the transformation pathways from the initial monoclinic β-phase to an intermediate cubic γ-phase and then back to the β-phase have been mapped and identified as a sequence of Ga cation migrations. Our results provide fundamental insights into the Ga2O3 phase stability within the nanoscale, which is crucial for advancing the miniaturization, light weight, and integration of wide-bandgap semiconductor devices.

Direct Observation of Size-Dependent Phase Transition in Methylammonium Lead Bromide Perovskite Microcrystals and Nanocrystals.

Methylammonium (MA) lead halide perovskites have been widely studied as active materials for advanced optoelectronics. As crystalline semiconductor materials, their properties are strongly affected by their crystal structure. Depending on their applications, the size of MA lead halide perovskite crystals varies by several orders of magnitude. The particle size can lead to different structural phase transitions and optoelectronic properties. Herein, we investigate the size effect for phase transition of MA lead bromide (MAPbBr3) by comparing the temperature-dependent neutron powder diffraction patterns of microcrystals and nanocrystals. The orthorhombic-to-tetragonal phase transition occurs in MAPbBr3 microcrystals within the temperature range from 100 to 310 K. However, the phase transition is absent in nanocrystals in this temperature range. In this work, we offer a persuasive and direct evidence of the relationship between the particle size and the phase transition in perovskite crystals.

Size-dependent phase transitions boost catalytic activity of sub-nanometer gold clusters.

The characterization and identification of the dynamics of cluster catalysis are crucial to unraveling the origin of catalytic activity. However, the dynamical catalytic effects during the reaction process remain unclear. Herein, we investigate the dynamic coupling effect of elementary reactions with the structural fluctuations of sub-nanometer Au clusters with different sizes using ab initio molecular dynamics and the free energy calculation method. It was found that the adsorption-induced solid-to-liquid phase transitions of the cluster catalysts give rise to abnormal entropy increase, facilitating the proceeding of reaction, and this phase transition catalysis exists in a range of clusters with different sizes. Moreover, clusters with different sizes show different transition temperatures, resulting in a non-trivial size effect. These results unveil the dynamic effect of catalysts and help understand cluster catalysis to design better catalysts rationally.

Surface depletion field in 2D perovskite microplates: Structural phase transition, quantum confinement and Stark effect

Surface depletion field would introduce the depletion region near surface and thus could significantly alter the optical, electronic and optoelectronic properties of the materials, especially low-dimensional materials. Two-dimensional (2D) organic—inorganic hybrid perovskites with van der Waals bonds in the out-of-plane direction are expected to have less influence from the surface depletion field; nevertheless, studies on this remain elusive. Here we report on how the surface depletion field affects the structural phase transition, quantum confinement and Stark effect in 2D (BA)2PbI4 perovskite microplates by the thickness-, temperature- and power-dependent photoluminescence (PL) spectroscopy. Power dependent PL studies suggest that high-temperature phase (HTP) and low-temperature phase (LTP) can coexist in a wider temperature range depending on the thickness of the 2D perovskite microplates. With the decrease of the microplate thickness, the structural phase transition temperature first gradually decreases and then increases below 25 nm, in striking contrast to the conventional size dependent structural phase transition. Based on the thickness evolution of the emission peaks for both high-temperature phase and low-temperature phase, the anomalous size dependent phase transition could probably be ascribed to the surface depletion field and the surface energy difference between polymorphs. This explanation was further supported by the temperature dependent PL studies of the suspended microplates and encapsulated microplates with graphene and boron nitride flakes. Along with the thickness dependent phase transition, the emission energies of free excitons for both HTP and LTP with thickness can be ascribed to the surface depletion induced confinement and Stark effect.

Size-dependent solid-solid phase transition process of Ag2S nanoparticles

It is well known that for a nanoparticle the solid-solid phase transition begins with the appearance of a high temperature disordered phase at the surface and the phase interface moves inward gradually with the increase of temperature. However, the size-dependent phase transition behavior remains unclear. Here we report an in-situ TEM study of the phase transition process of different-sized Ag2S nanoparticles at atomic resolution. The onset temperature of disordered phase of the small nanoparticle is found to be lower than that of the big nanoparticle. And, the disordered phase thickness of small nanoparticle is always thicker than that of big nanoparticle. By considering surface and interface free-energy, a phenomenological model based on the minimization of system free-energy is established, which could well explain our experimental results. These discoveries extend our understanding of size dependent phase transition mechanism.

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.

Size-dependent structural phase transitions and their correlation with photoluminescence and optical absorption behavior of annealed Zn0.45Cd0.55S quantum dots

In this paper, we investigate the effect of thermally induced structural phase transitions on the photoluminescence (PL) and optical absorption behaviour of Zn0.45Cd0.55S nanoparticles (NPs). Analysis of X-ray diffraction (XRD) patterns and high-resolution electron microscope (HRTEM) images reveal that the as-synthesized sample possesses zinc-blende-type cubic structure. In addition, at annealing temperature (Ta) 400 °C, the cubic structure transforms completely into the wurtzite-type hexagonal structure. Furthermore, the second phase transition of the as-synthesized sample has observed at 700 °C, where the cubic structure has transformed into mixed polycrystalline phases of hexagonal ZnO, cubic CdO, monoclinic CdSO3, and orthorhombic ZnSO4 structures. These new phases have also confirmed from the analysis of Raman and FTIR spectra. Analysis of UV–visible optical absorption spectra demonstrates that Increasing Ta results in the decrease of optical band gap due the improvement in crystallinity accompanied by the increase in the particle size. The PL emission bands at an excitation energy of 3.818 eV exhibit redshift and a decrease in the intensity with increasing Ta up to 500 °C. Meanwhile, further increase in Ta up to 700 °C results in the enhancement of green emission intensity. On the other hand, PL emission spectra at 3.354 eV and Ta 700 °C, reveal a dramatic increase in the emission intensity nearly by one-order of magnitude with respect to its value of the as-synthesized sample. This behaviour is ascribed to the incorporation of oxygen-related defects via thermal annealing in air, which act as additive radiative centers. Also, we have interpreted the observed spectral blue shift of PL emission spectrum with increasing excitation energy.

Size-dependent phase transition of Er2O3 under high pressure

The size effect on the structural and optical properties of cubic Er2O3 was investigated under pressure by in-situ angular dispersive synchrotron x-ray diffraction (AD-XRD), Raman scattering, photoluminescence (PL), and impedance spectroscopy. Contrary to the phase transition sequence of cubic→monoclinic→hexagonal in bulk Er2O3, a transformation from cubic directly to hexagonal was observed in Er2O3 nanoparticals. Compared with bulk Er2O3, nano-Er2O3 showed an obvious elevation of phase transition pressure and larger bulk module. A third-order Birch-Murnaghan fitting yields zero pressure bulk moduli (B0) of 181(5), and 226(4) GPa and their pressure derivatives (B′0) of 4.0(7), 1.9(5) for the cubic and hexagonal phases, respectively. The multiple PL lines of 4S3/2→4I15/2 originating from the cubic phase are also altered due to phase transformation. The impedance spectroscopy indicated that the nano-Er2O3 is an insulator up to 30 GPa. These findings give a fresh understanding of size influence on the phase transition sequences and sheds light on the applications of nano-Er2O3.

Time-resolved light scattering by photoexcited V2O3

ABSTRACTUsing ultrafast angle-resolved light scattering technique, we were able to trigger photoinduced phase transition processes in V2O3 film grown on a glass substrate. The phase transition is caused by photoacoustic wave in the film and appears as coherent oscillations of scattering signal at various time scales. These processes strongly depend on the size of microstructures constituting the V2O3 film. One of the key findings of our study is the presence of a size dependent phase transition threshold for V2O3 microstructures, where small size structures ($\tilde <$ 0.5μm) have lowest contribution to the phase transition. The presence of this threshold can be well described by considering uneven internal strain in the films which is one of the key parameters controlling phase transition dynamics in various vanadium oxides.

The influence of structural size on thermal stability in single crystalline hematite uniform nano/micro-cubes

We have successfully synthesized monodisperse α-Fe2O3 nano/micro-cubes with sizes ranging from 40 to 840 nm. High-temperature magnetic measurements under a high vacuum were used to systematically study the size dependence of phase transition from α-Fe2O3 to Fe3O4. Through a series of characterizations, we concluded that the different sizes of particles have a significant impact on their thermal stability. The data shows a clear correlation between the onset temperature of the phase transition and the size of particles. This phenomenon can be well explained in terms of the dependence of enthalpy on surface area. Moreover, Morin transition of the α-Fe2O3 phase also have been measured for our products at the nanoscale. The result reflects that the Morin transition temperature decreases with decreasing the size of the particle, which supports finite-size scaling law according to previous findings. Our conclusions provide an important insight into many other size-dependent structural phase transitions.

Grain size-dependent dielectric, piezoelectric and ferroelectric properties of Sr2Bi4Ti5O18 ceramics

Fine powders comprising nano-crystallites of Sr2Bi4Ti5O18 (SBT) were synthesized via citrate-assisted sol–gel route. Different ceramics of SBT with different grain sizes (93 nm–1.42 μm) were fabricated by varying sintering temperatures and durations. The usage of nano-crystalline powders for fabricating ceramics facilitated lower sintering temperatures. The grain growth in these powder compacts was found to be controlled by the grain boundary curvature mechanism, associated with the activation energy of 181.9 kJ/mol. Interestingly, with a decrease in grain size there was an increase in structural distortion which resulted in a shift of Curie temperature (phase transition) toward higher temperatures than that of conventional bulk ceramics. Extended Landau phenomenological theory for the spherical ferroelectric particles was invoked to explain experimentally observed size-dependent phase transition temperature, and the critical size for SBT is predicted to be 11.3 nm. Grain size-dependent dielectric, ferroelectric and piezoelectric properties of the SBT ceramics were studied and the samples comprising average grain size of 645 nm exhibited superior physical properties that include remnant polarization (2Pr) = 16.4 μC cm−2, coercive field (Ec) = 38 kV cm−1, piezoelectric coefficient (d33) = 22 pC N−1 and planar electromechanical coupling coefficient (kp) = 14.8 %.

Solid-solid phase transition of (1-C14H29NH3)2ZnCl4 in nanopores of silica gel for thermal energy storage

Latent heat storage performance of a layered perovskite-type compound, 1-C14H29NH3)2ZnCl4 (C14Zn), embedded in a series of silica gel (SG) with pore sizes of d=15–200nm is investigated using differential scanning calorimetry (DSC), and powder X-ray diffractions (XRD). C14Zn in the nanopores of silica gel shows size-dependent phase transition temperature, enthalpy change and supercooling. They have a stable transition temperature and heat capacity at each size in a short-term thermal cycling. Similar X-ray diffraction patterns are observed for the nano-sized and the bulk C14Zn. The encapsulation of a phase change material in nanopores is a new way of tuning its thermal energy storage properties for a wider range of temperature regulation.

Nanosize effects assisted synthesis of the high pressure metastable phase in ZrO2.

The size effects on the high pressure behaviors of monoclinic (MI) ZrO2 nanoparticles were studied using in situ high pressure synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). A size-dependent phase transition behavior under high pressure was found in nanoscale ZrO2. The normal phase transition sequence of MI-orthorhombic I (OI)-orthorhombic II (OII) occurs in 100-300 nm ZrO2 nanoparticles, while only the transition of MI-OI exists in ultrafine ∼5 nm ZrO2 nanoparticles up to the highest experimental pressure of ∼52 GPa. This indicates that the size effects preclude the transition from the OI to the OII phase in ∼5 nm nanoparticles. Upon decompression, the OII and OI phases are retained down to ambient pressure, respectively. This is the first observation of the pure OI phase ZrO2 under ambient conditions. The bulk moduli of the MI ZrO2 nanoparticles were determined to be B0 = 192 (7) GPa for the 100-300 nm nanoparticles and B0 = 218 (12) GPa for the ∼5 nm nanoparticles. We suggest that the significant high surface energy precludes the transition from the OI to the OII phase and the nanosize effects enhance the incompressibility in the ultrafine ZrO2 nanoparticles (∼5 nm). Our study indicates that this is a potential way of preparing novel nanomaterials with high pressure structures using nanosize effects.

Grain size dependent phase transition and superparaelectric behavior of ferroelectric BST

In this article, we investigate the grain size dependent phase transition and polarization behavior of ferroelectric Ba0.9Sr0.1TiO3 (BST). Starting with nanocrystalline powders (crystalline size≈22nm), various grain sizes were obtained in nano–submicron domain (202–745nm) by regulating the sintering temperatures (Tsin) in the range of 1050–1300°C. All samples were found to possess diffuse phase transition characteristics with frequency independent broad dielectric maxima near transition temperature due to the lattice strain contribution. Dielectric stiffness showed tarnished step-like anomalous behavior in the paraelectric state for Tsin≥1200°C due to the existence of polar (superparaelectric) nano-regions generated by local polarization by off-centered Ti4+ ions exhibiting an exceptionally rare delicate polarization hysteresis loop.

In Situ Triggering and Dynamically Tracking the Phase Transition in Vanadium Dioxide

As the most widely studied thermochromic material, monoclinic vanadium dioxide (VO2 (M)) shows promising applications in the energy-saving field. Solid-state transformation, especially fast annealing, plays an important role in the production of VO2 nanomaterials. On the other hand, the fast process makes it impossible to real-time monitor the phase transition in VO2. In this paper, a differential scanning calorimetry technique is proposed to in situ trigger and dynamically track the phase transition of VO2 nanoparticles, which gives a distinguished method to identify the underlying size-dependent and defect-mediated structure phase transition.

Heusler nanoparticles for spintronics and ferromagnetic shape memory alloys

Heusler nanoparticles emerge as a new class of multifunctional materials. In this critical review, the latest progress in studies on Heusler nanoparticles is summarized. The authors discuss their structural and physical properties interesting for research fields such as spintronics and ferromagnetic shape memory alloys. As a young research field, the majority of studies on Heusler nanoparticles focus on their synthesis, structure, and magnetic characterizations. Important issues such as size dependent structure, phase transition, magnetic, and spin-related properties are still open. Further investigations are needed to verify the technical significance of Heusler nanoparticles for practical applications such as data storage, magnetic sensors, and microactuators.

Order–Disorder Transition Involving the A-Site Cations in Ln3+Mn3V4O12 Perovskites

A crossover from the A-site-ordered double-perovskite structure with Im3̅ cubic symmetry to the simple-perovskite structure with Pnma orthorhombic symmetry is found in LnMn3V4O12 (Ln = La, Nd, Gd, Y, Lu) synthesized under high-pressure conditions. Relatively large Ln(3+) ions (La(3+), Nd(3+), and Gd(3+)) induce the a(+)a(+)a(+) in-phase cooperative tilting of the VO6 octahedra, resulting in the A-site-ordered double-perovskite structure with chemical composition Ln(3+)Mn(2+)3V(3.75+)4O12. Compounds with small Ln(3+) ions like Y(3+) and Lu(3+), on the other hand, crystallize with the Pnma simple-perovskite structure with chemical composition (Ln(3+)1/4Mn(2+)3/4)V(3.75+)O3, where the Ln(3+) and Mn(2+) ions are disordered at the A site. The random distribution of the small A-site cation induces the a(-)b(+)a(-) tilting distortion of the VO6 octahedra. The observed phase crossover is well explained by the structural stability calculation based on the bond-valence-sum model, and the most stable crystal structure gives the smallest unit-cell volume. This A-site-cation size-dependent phase transition between the A-site-ordered double-perovskite and A-site-disordered simple-perovskite structures in LnMn3V4O12 is thus a result of the structural stability due to the cooperative tilting of the VO6 octahedra. The Mn(2+) ions at the A'(A) site contribute local magnetic moments, whereas the V(3.75+) ions at the B site play a role in metallic conduction. The observed magnetic behaviors are consistent with the order-disorder distribution of the Mn(2+) ions at the A site, antiferromagnetism in the A-site-ordered double perovskites, and magnetic spin glass in the A-site-disordered simple perovskites.

Phase-controlled preparation of TiO2 films and micro(nano)spheres by low-temperature chemical bath deposition

• Simultaneously preparation of TiO 2 films and sphere-like precipitates by low-temperature chemical bath deposition. • The same building blocks observed in the films and precipitates. • Cluster-by-cluster attachment is succeeded by the ion-by-ion growth. • Phase-controlled growth of TiO 2 by the effects of solvent or fluorine ion. TiO 2 films and micro(nano)spheres have been simultaneously prepared by means of low-temperature chemical bath deposition (CBD). The films and precipitates are found to be constructed of the same building blocks regardless of the bath conditions. Thin nanosheets of ca. 20 nm in thickness serve as the building blocks of rutile TiO 2 grown in an acidic bath. They aggregate to form spheres to reduce the surface energy of the precipitates, while in the films they grow into discrete crystallites on the seeded substrate. Fine control over the crystalline phase of TiO 2 is achieved by replacing some water with ethanol or the addition of NH 4 F. A gradual transition from rutile to anatase is observed as the ethanol and NH 4 F contents are increased. Moreover, the crystallite size of TiO 2 is significantly reduced and there exists monodisperse nanoparticles and aggregate microspheres in the products modified with ethanol and NH 4 F, respectively. This size-dependent phase transition is consistent with previous theoretical and thermodynamic studies. The facile tune of the TiO 2 phase by low-temperature CBD method may be useful to improve the performance of TiO 2 for its various applications.