Phase Transition Of Nanocrystals Research Articles

Size-induced phase transitions

ABSTRACT This paper presents a holistic view of the structural phase transitions in nanocrystals. The databases for size-induced phase transitions is a main part of this paper. The database on transitions with temperature contains 603 items on 415 crystals, and database on transitions under high pressure contains 209 items in 124 crystals. Moreover, some new aspects of such transitions are presented like island diagram and new analysis of anomalous compressibility observed in some nanocrystals.

BaAl2Si2O8 polymorphs and a novel reversible transition of BaAlF5 in supercooled oxyfluoride aluminosilicate liquids

Crystallization plays a critical role in realizing the functionalization of glass materials. However, it is imperative to understand the microscopic mechanism of both the formation and the phase transitions of nanocrystals during heat processes. Here, in the as-prepared SiO2-Al2O3-BaF2-Na2O glass, we studied the polymorphic phase transition of BaAl2Si2O8 during heating-cooling circles, and a novel reversible transformation is found by serendipity. Using differential scanning calorimetry (DSC), we observed an endothermic event around 1047 K during heating at 20 K/min to 1173 K, and an exotherm event at 1024 K during the subsequent cooling. Based on the X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), the reversible process was attributed to the transition between two morphs of BaAlF5 nano-crystal. Na2O introduced in the studied glass system promotes the transformation between hexacelsian (Hexagonal) and celsian (Monoclinic). The results also show the formation of different types of crystals, such as, BaF2, Na5Al3F14 and BaAlF5 etc.

The features of CoFeZr alloy nanocrystals formation in film composites of (CoFeZr)x(MgF2)100-x

Abstract The influence of relative content of the metal component on the phase composition and substructure of (CoFeZr)x(MgF2)100−x film nanocomposites between x = 7÷51 (at%) has been established by X-ray diffraction (XRD), X-ray electron spectroscopy (XPS) and infrared spectroscopy (IR). Ion-beam sputtering of a composite target from Co45Fe45Zr10 amorphous alloy plate with MgF2 dielectric inserts allow prepared micron-thick nanocomposite layers in the argon atmosphere. The results show that at a relatively low content х 25, the MgF2 matrix becomes X-ray amorphous, and CoFeZr alloy nanocrystals of the hexagonal syngony based on the α-Co structure with dimensions of about 10 nm are first formed; they are predominantly oriented in the plane of the hexagonal lattice (001). With a further increase of the metal alloy content up to х~40 in the region nearer to the percolation threshold, the crystal structure of nanocrystals of the CoFeZr alloy becomes a cubic phase based on α-Fe with a preferred orientation of (110). XPS studies confirm the presence of a predominant type of metal bonds in clusters and nanocrystals of CoFeZr, despite the oxidation of the thinnest surface layers, as well as ionic bonds in the MgF2 dielectric matrix, preserved because of self-organization even in non-equilibrium conditions of ion-plasma sputtering. Two features on the curve of the dependence of the resistivity of nanocomposites on the alloy concentration are due to: the first one (x~25) - the formation of nanocrystals of hexagonal symmetry from clusters of the amorphous CoFeZr alloy, and the second one (x~40) nearer the percolation threshold - coincides with the phase transition of nanocrystals from hexagonal to cubic structure.

Reversible formation-melting of nano-crystals in supercooled oxyfluoride germanate liquids

The nanocrystals play a critical role in generating and affecting functionalities of glass materials. Therefore, scientists have made considerable efforts in clarifying microscopic mechanisms of nanocrystal formation in glass to obtain the desired type of nanocrystals. However, the phase transitions of nanocrystals during heating have not been well understood. Here we report on a discovery of the reversible melting-formation of nanocrystals in an oxyfluoride germanate glass during heating-cooling circles. Using a differential scanning calorimetry (DSC), we detected a striking endothermic event at 925 K during heating, after the glass underwent a DSC upscan to a temperature between 925–986 K and subsequent cooling. Based on Raman spectroscopy, X-ray diffraction and transmission electron microscopy, the endotherm is attributed to the melting of nano-crystal BaGeF6 (˜20 nm). An exothermal response was observed at 890 K during the DSC downscan, implying the re-formation of BaGeF6 nano-crystals. This suggests that the melting-formation of BaGeF6 nano-crystals is a typical first-order transition.

Polymer-Mediated Synthesis of Iron Oxide (Fe_2O_3) Nanorods

Ferric oxide (Fe_2O_3) nanoparticles were synthesized by iron chloride hexahydrate (FeCl_3．6H_2O) as precursor and polyvinylpyrrolydon (PVP) as surfactant and polymer agent. The samples were characterized by high resolution transmission electron microscopy (HRTEM), field effect scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV-Vis spectrophotometer and vibration sampling magnetometer (VSM). XRD pattern showed that the iron oxide nanoparticles exhibited gamma- Fe_2O_3 (maghemite) to alpha-Fe_2O_3 (hematite) structural phase transition in nanocrystals. The particle size of as-prepared sample was around 20 nm and annealed sample was around 48 nm in diameter as estimated by XRD technique and direct HRTEM observation. The surface morphological studies from SEM depicted sphere-like shaped to rod-like shaped particles with formation of clusters by increasing annealing temperature. The sharp peaks in FTIR spectrum determined the purity of Fe_2O_3 nanoparticles and absorbance peak of UV-Vis spectrum showed the bandgap energy of 2.58 ev. The result of magnetic measurements showed a good coercive field and saturation magnetism around 85 G and 9.83 emu/g, respectively.

Optical study of the effect of the impurity content on the ferroelectric properties of Er3+ doped SBN glass-ceramic samples

Photoluminescence measurements have been performed on glass ceramic samples of strontium barium niobate doped with Er3+. The glass-ceramic samples were obtained from the precursor glass by thermal treatment and were doped with different contents of Er3+ ions. The average size of the nanocrystals in the samples was about 50 nm. The optical properties of the Er3+ ions were studied upon 532 nm excitation and the luminescence changes were measured as a function of temperature. The study of the luminescence spectra with increasing temperature has allowed us to ascertain the occurrence of the ferroelectric-paraelectric phase transition in the strontium barium niobate nanocrystals embedded in the glass matrix. We have particularly focused on the temperature range around the Curie temperature, when the system undergoes the transition from the ferroelectric phase to the paraelectric phase. Doping of the strontium barium niobate samples results in a change in the measured value of the Curie temperature which induces a change in the ferroelectric properties. It is interesting to note that this phase transition in the nanocrystals immersed in the glass matrix had previously not been detected.

Size Dependence of the Pressure-Induced Phase Transition in Nanocrystals

Extending the recently developed bond-order−length-strength (BOLS) correlation mechanism [Sun, C. Q. Prog. Solid-State Chem. 2007, 35, 1−159] to the pressure domain has led to atomistic insight into the phase stability of nanostructures under the varied stimuli of pressure and solid size. It turns out that the competition between the pressure-induced overheating (TC elevation) and the size-induced undercooling (TC depression) dominates the measured size trends of the pressure-induced phase transition. Reproduction of the measured size and pressure dependence of the phase stability for CdSe, Fe2O3, and SnO2 nanocrystals evidence the validity of the solution derived from the perspective of atomic cohesive energy and its response to the external stimulus.

Size dependence of the pressure-inducedγ toα structural phase transition in iron oxide nanocrystals

The size trend for the pressure-inducedγ-Fe2O3 (maghemite)to α-Fe2O3 (haematite) structural phase transition in nanocrystals has been observed. The transition pressurewas found to increase with decreasing nanocrystal size: 7 nm nanocrystals transformed at27 ± 2 GPa, 5 nm ones at34 ± 3 GPa and 3 nm ones at37 ± 2 GPa. Annealing of abulk sample of γ-Fe2O3 was found to reduce the transition pressure from35 ± 2 to24 ± 2 GPa. The bulk moduluswas determined as 262 ± 6 GPa for7 nm nanocrystals of γ-Fe2O3, which is significantly higher than the value of190 ± 6 GPa that we measured for bulk samples. Forα-Fe2O3, the bulk modulifor 7 nm nanocrystals (336 ± 5) and the bulk (300 ± 30) were found to be almost the same within error. The bulk modulus for theγ phase was found to decrease with decreasing particle size between 10 and 3.2 nm particlesize. Values for the ambient pressure molar volume were found within 1% to be:33.0 cm3 mol−1 forbulk γ-Fe2O3;32.8 cm3 mol−1 for 7 nmdiameter γ-Fe2O3 nanocrystals; 30.7 cm3 mol−1 for bulk α-Fe2O3; and 30.6 cm3 mol−1 for α-Fe2O3 nanocrystals.

Monte Carlo analysis of the surface and size effects in ferroelectric nanocrystals

Computer Monte Carlo simulations have been carried out to study the ferroelectric phase transition in nanocrystals. The microscopic description of the system includes up to four pseudospins interactions describing first order phase transitions in bulk systems. Of main importance is the analysis of the critical temperature (Tc) shifts due to size effects in the nanostructure. The average polarization for temperatures ranging from below to above Tc have been obtained for particles of different sizes. The results show that Tc shifts down as the size of the particle is reduced. This shift is followed by the modification of the transition from first order to second order type. However, the simulations show that surface effects can drive up Tc, and if the magnitude of the interaction parameters at the surface differ significantly from those at the inside, the particle can spontaneously polarize its surface and its interior at different temperatures.

Size dependence of dielectric properties and structural metastability in ferroelectrics

The size effect of the dielectric properties and the barrier height was investigated in the ferroelectric solid solution Ba x Sr1−x TiO3 system. The decrease of the grain size causes the suppression of the ferroelectricity, and the increase of the relaxation frequency. Barrier heights increase with increasing grain size. The result is analogous to magnetic phase transitions in nanocrystals and other solid-solid phase transitions in nanocrystals. It suggests a general rule that may be of use in the discovery of new metastable phases. An explanation of this phenomenon was given by an electric potential model that agrees well with the experimental results. For Ba x Sr1−x TiO3 system, the decrease of x causes the decrease of the barrier height.

A Relation Between Energy Barrier and Ferroelectricity in BaxSr1-xTiO3

In the BaxSr1-xTiO3 system, the decrease of the grain size causes the suppression of the ferroelectricity and the increase of the relaxation frequency. Barrier heights increase with the increasing grain size. The result is analogous to magnetic phase transitions in nanocrystals and the solid-solid phase transitions in nanocrystals.

Scaling law for structural metastability in semiconductor nanocrystals

AbstractIn contrast to extended solids, nanometer size crystals convert from one structure to another by single nucleation events, and the transformation obey simple kinetics. Barrier heights are observed to increase in nanocrystals of larger size, but also depend on the nature of the nanocrystal surface. These results are analogous to magnetic phase transitions in nanocrystals and suggest some general rules which may be of use in the discovery of new metastable phases.

Size Dependence of Structural Metastability in Semiconductor Nanocrystals

The kinetics of a first-order, solid-solid phase transition were investigated in the prototypical nanocrystal system CdSe as a function of crystallite size. In contrast to extended solids, nanocrystals convert from one structure to another by single nucleation events, and the transformations obey simple unimolecular kinetics. Barrier heights were observed to increase with increasing nanocrystal size, although they also depend on the nature of the nanocrystal surface. These results are analogous to magnetic phase transitions in nanocrystals and suggest general rules that may be of use in the discovery of new metastable phases.

Semiconductor Nanocrystals: Exciton Quantum Mechanics, Single Nanocrsytal Luminescence, and Metastable High Pressure Phases

AbstractWe review three areas where significant progress has recently occurred in our understanding of semiconductor nanocrystals. The first two involve luminescence properties of single and ensembles of Cadmium Selenide (CdSe) nanocrystallites (Quantum Dots) between 10 and 50 Å in radius. The size, magnetic field, and temporal dependence of emission from ensembles of nanocrystallites at cryogenic temperatures uncovers the fundamental mechanism of radiative recombination in these nanocrystals. Effective mass models that take into account the electron-hole exchange interaction can quantitatively account for observed luminescence Stokes shifts. Furthermore, the magnetic field dependence of luminescence lifetimes and longitudinal-optical (LO) phonon ratios demonstrate that the exciton ground state in these nanocrystals is optically passive (“dark exciton”) with spin projection ±2. Picosecond time resolved measurements probe exciton relaxation into this level. Recent results on the spectroscopy of single CdSe nanocrystals at room temperature are also presented. Remarkably, emission from a single CdSe nanocrystal under C.W illumination is observed to turn on and off discretely (fluorescence intermittency) on a ∼0.5s timescale. The excitation intensity dependence, and the influence of a passivating high band gap shell of Zinc Sulfide (ZnS) encapsulating the CdSe nanocrystal on the on/off times, suggest that this phenomenon is caused by photoionization. Finally, the third area originates in diamond anvil studies of the solid-solid phase transitions of nanocrystals under pressure. These studies show that a single nucleation event occurs per nanocrystal, and that as a consequence the nanocrystals change shape. The kinetic activation barrier increases with increasing size. Under suitable conditions nanocrystals in dense, six-coordinate high pressure phases may be metastable at STP.