Lattice Thermal Expansion Research Articles

Structural and Physical Properties of DyCu, NdAg, LaCd, YIn, ErCu, ErAg, and ErAu Rare-Earth Intermetallic Compounds: Ab initio Investigations Analyzed by Data Mining Technique

In this work, the structural, elastic, mechanical, thermodynamic, and electronic properties of ErCu, ErAg, ErAu, DyCu, NdAg, LaCd, and YIn compounds have been investigated systematically using full-potential linearized augmented plane wave (FP-LAPW) within the density functional theory (DFT). In this approach, the exchange–correlation energy was calculated with generalized gradient approximation (GGA), by Perdew–Burke–Ernzerhof (PBE). We have firstly calculated lattice constants, bulk modulus, elastic constants, and mechanical properties such as Young’s and shear moduli, Pugh factor (B/G), anisotropy constant, and Cauchy pressure, as well as Poisson coefficient. The elastic constants and their related properties verify the criteria of mechanical stability, signifying that these compounds are stable in B2 structure. The ductility of the intermetallics investigated here is evaluated. The obtained values show that ErCu is the most rigid while NdAg the most ductile. The total and partial electron density of states (DOS) were evaluated to offer a detailed explanation of the contribution of atomic orbitals in the energy bands. The thermodynamic properties of ErCu, ErAg, ErAu, NdAg, LaCd, YIn, and DyCu are calculated via the quasi-harmonic Debye model, in which the lattice vibrations are considered. So temperature-dependent parameters such as lattice constant, thermal expansion, and specific heat at constant volume are analyzed. Furthermore, the relationship between several thermo-physical and mechanical properties was discussed and analyzed with data mining techniques. The obtained results confirm that this B2-type of rare-earth intermetallics has very interesting mechanical and thermal properties for structural applications.

Multichannel thermal transport in crystalline Tl3VSe4

The validity of the particlelike phonon picture in describing the thermal transport physics in strongly anharmonic crystalline materials is a subject of recent debate. On the one hand, the Peierls-Boltzmann transport equation-based particlelike treatment of phonons was found to be insufficient in explaining the experimentally observed ultralow thermal conductivity of ${\text{Tl}}_{3}\text{V}{\text{Se}}_{4}$, and subsequently another thermal transport theory based on an additional thermal transport channel was proposed. On the other hand, the validity of this multichannel transport was questioned through a higher-level theory accounting for higher-order terms. Here, using the highest level of thermal transport theory with contributions from a temperature-dependent potential energy surface, lattice thermal expansion, force constant renormalization, long-range Coulombic interactions, and higher-order quartic phonon scattering processes, the validity of multichannel thermal transport is tested for ${\text{Tl}}_{3}\text{V}{\text{Se}}_{4}$. The particlelike transport channel is found to severely underpredict the experimentally measured thermal conductivity by a factor of 3 at a temperature of 300 K. The contribution of the wavelike coherent transport channel is found to be twice that of the particlelike channel, and the total thermal conductivity from multichannel transport theory is found to be in agreement with experimental measurements.

Role of defects and grain boundaries in the thermal response of wafer-scale hBN films

With more widespread applications of nanotechnology, heat dissipation in nanoscale devices is becoming a critical issue. We study the thermal response of wafer-scale hexagonal boron nitride (hBN) layers, which find potential applications as ideal substrates in two dimensional devices. Sapphire-supported thin hBN films, 2′′ in size and of different thicknesses, were grown using metalorganic vapour phase epitaxy. These large-scale films exhibit wrinkles defects and grain boundaries over their entire area. The shift of phonon mode with temperature is analysed by considering the cumulative contribution of anharmonic phonon decay along with lattice thermal expansion, defect, and strain modulation. The study demonstrates that during heat treatment the strain evolution plays a dominating role in governing the characteristics of the wrinkled thinner films. Interestingly we find that both defects and strain determine the spectral line-width of these wafer-scale films. To the end, from Raman line-width, the changes in phonon lifetime in delaminated and as-grown films is estimated. The results suggest the possibility of a reduction in thermal transport in these wafer-scale films compared to their bulk counterpart.

(Invited) Heterogeneous Integration of GaN and Ga2O3 with High Thermal Conductivity Substrates Via Surface Activated Bonding

The high-power high-frequency devices of both GaN and Ga2O3 needs careful thermal management to migrate the self-heating effect during operation. One promising strategy is to integrate GaN and Ga2O3 with a high thermal conductivity substrate such as SiC and diamond. Epitaxial growth, usually at high temperature, met many challenges because of the lattice mismatch and thermal expansion mismatch. Recently, we realized room-temperature bonding of GaN and Ga2O3 with high thermal conductivity substrates via surface activated bonding methods, which can avoid the transition layer necessary for growth method and make the GaN or Ga2O3 device close to the high thermal conductivity substrates as much as possible.

On the Dependence of Ionic Transport on Crystal Orientation in Nasicon-Type Solid Electrolytes

The dependence of ionic transport on crystal orientations in NaSICON-type solid electroytes is studied on flux-grown M3Sc2(PO4)3 (M = Na, Ag) single crystals with well-defined facets. Herein, we provide the first impedance spectroscopy study to characterize ion conduction along different crystallographic orientations in this important class of materials for electrochemical energy storage systems. Moreover, we used single crystal X-ray diffraction, differential scanning calorimetry, 23Na NMR spin-lattice relaxation measurements, and ab initio molecular dynamics simulations to study the interplay of structure and ion transport taking place at different length scales. We conclude that the phase behavior in NaSICON-type materials is strongly linked to ion diffusion. At room temperature, ionic conductivity is slightly anisotropic along the crystallographic orientations [001] and [100]. The slightly different activation energies are related to diffusion bottlenecks solely changing along [001]. This change is caused by anisotropic thermal lattice expansion. With increasing temperature, ion transport increasingly becomes isotropic finally resulting in an order-disorder phase transition from C2/c to R−3c. This phase transition is associated with a clear change in activation energy solely along [001]; it can be traced back to the increasing jump distance along this crystal orientation with temperature. Astonishingly, changing the ionic charge carrier, i.e., when going from Na+ to Ag+, shifts the phase transition temperature by 140 K toward lower temperature. The Arrhenius behaviour remains, however, similar. This finding is related to the higher mobility of Ag+ in the NaSICON framework leading to isotropic ion diffusion at much lower temperatures. Overall, flux-grown M3Sc2(PO4)3 allowed us to show that ionic transport parameters and phase stability sensitively depend on crystal chemistry.

Performance Estimation of Hybrid Plasmonic Waveguide in Presence of Stress

In this paper, a detailed numerical investigation has been carried out to find the effect of stress on the optical performances of hybrid plasmonic waveguides (HPWs) operating at 1550 nm wavelength. In the HPWs, the stress is induced due to mismatch in lattice constant and thermal expansion coefficient (TEC) between its different layers. In this paper, optical performances of two HPWs having rectangular and triangular cross-section have been investigated using the finite-element method (FEM) in the presence of stress. For HPW with triangular cross-sections, 25° and 35° ridge angles (α) have been chosen categorically, as they are feasible to fabricate. The effect of stress on optical performance parameters, namely effective refractive index, propagation loss, propagation length, and temperature sensitivity for the HPWs, has been investigated. In this study, different metals (Au and Ag) and dielectric materials (SiO2 and Si3N4) have been introduced in HPWs to realize different magnitude of stresses, which alter the optical performances of the waveguide. Our simulation studies reveal that for all metal and dielectric materials the triangular HPW with 35° ridge angle introduces the highest magnitude of stress as compared to the triangular waveguide with 25° ridge angle and rectangular waveguide. Further, the results show that the presence of stress in the HPW reduces propagation loss and helps in achieving larger propagation length (LP). The work also shows that the largest magnitude of stress is exerted by the triangular HPW (α = 35°) having Si3N4 and Ag materials as dielectric and metal, respectively; and the stress-induced changes in effective index, propagation loss, propagation length and temperature sensitivity of effective index, and propagation length are 0.35%, 42%, 42%, 0.141 × 10− 4/°C (1.45 × 10− 4/°C to 1.309 × 10− 4/°C), and 15.6 nm/°C (5.5 nm/°C to 21.1 nm/°C), respectively. Significant performance variation in HPW in the presence of stress emphasizes the need for the consideration of stress-optic effect while designing HPWs, which can then be employed in photonic integrated circuits.

Thermophysical properties of high-density, sintered monoliths of yttrium dihydride in the range 373–773 K

Yttrium dihydride is a promising nuclear reactor moderator for high-temperature, thermal micro-reactors due to its ability to retain hydrogen to high temperatures while having a relatively low impact on neutron economy. However, it is difficult to fabricate yttrium dihydride in high-density, near net-shape monoliths for moderator applications due to challenges associated with the hydrogen absorption process. In this study, high-density monoliths of yttrium dihydride were prepared by powder metallurgical methods. The lattice strain and thermal expansion coefficient were measured using neutron diffraction of powders of directly-hydrided yttrium, while the molar heat capacity and thermal diffusivity of sintered yttrium dihydride were measured using differential scanning calorimetry (DSC) and laser flash analysis (LFA), respectively. The molar heat capacity of yttrium dihydride was also calculated using density functional theory (DFT) for comparison. These parameters were used to then calculate the thermal conductivity and resistivity of yttrium dihydride as a function of temperature. The thermophysical properties of materials produced by both methods were observed to be consistent with the values from literature for yttrium dihydride. Due to the novelty of producing yttrium dihydride by powder metallurgical methods, this result indicated that the sintered monoliths were of high quality and that powder metallurgy is a viable method for large-scale production of yttrium dihydride monoliths for nuclear reactor moderator applications.

Effects of triaxial strain on carrier activity, carrier lifetime, and absorption spectrum of (S, Se, Te) and 2 N Co-doped ZnO

The acceptor (N)-isovalent (S, Se, Te) elements that separately co-dope ZnO is a novel doping technology. However, the effects of triaxial strain on the photocatalytic performance and mechanism of doped systems are often neglected in experimental studies. Moreover, the influence of interstitial H on the doped ZnO system is not considered. This paper adopted first-principles and selected the appropriate strain range and step length. Assuming the equivalent effect of several strain points of tensile strain or compressive strain, we used simulation calculation to fit the complex strain problem in the actual process (such as doping, the strain problem caused by the difference of the lattice constant and thermal expansion coefficient of the substrate and the doping system), and then studied physical properties of the doped system. The planar wave ultrasoft pseudopotential + U method based on the first-principles generalized gradient approximation under the framework of density functional theory was used to calculate (S, Se, Te) and 2 N co-doped ZnO. The interstitial H-doped Zn36SO33N2 was studied as a representative ZnO system. Results showed that the binding energy of all doping systems under unstrained conditions was negative with a relatively high stability. The strain increased under tensile strain and compressive strain. As the formation energy of the doping systems increased, system stability decreased. Although the band gap of all compressive strain systems widened, impurities formed in the band gap, and valence band electrons absorbed photons of low energy to transition to the energy level of the impurities and then to the energy level of conduction band. The photoexcited electron-graded transitions induced the absorption spectrum to red-shift. The band gap of the system widened, whereas the band gaps of other tensile strain systems narrowed, except for when the tensile strain was 1%. However, regardless of whether the band gap of the tensile strain system became narrow or wide, the photoexcited electrons were still able to transition in stages. Hence, the absorption spectrum of the tensile strain system still red-shifted. The stability, activity, carrier separation, and lifetime of Zn36SHiO33N2 were relatively better than those of Zn36SO33N2 at 5% tensile strain. These properties make Zn36SHiO33N2 an ideal photocatalyst. This study provides a theoretical reference for the experimental design and preparation of novel ZnO photocatalysts.

Fabrication of FeNi intermetallic using the wire-arc additive manufacturing process: A feasibility and neutron diffraction phase characterization study

Abstract FeNi intermetallic has been continuously attractive due to the appreciable magneto-crystalline anisotropy and high saturation magnetization in the nominally-equiatomic chemically ordered L10 structure. In the present research, the bulk FeNi alloy is successfully fabricated using an innovative wire-arca additive manufacturing process, which is also an in-situ alloying method. In the meantime, to characterize the thermal lattice evolution of the FeNi, neutron diffraction is conducted to the as-fabricated sample in-situ during a heat treatment process. According to the neutron results, an anisotropic fcc-FeNi lattice evolution has been detected, which indicates the existence of FeNi superstructure in the WAAM fabricated FeNi bulk material. In addition, due to the featured advantage of the neutron data, the fcc-FeNi volumetric lattice thermal expansion coefficients during heating and cooling processes are calculated.

First-principles calculations of the structural, electronic, mechanical and thermodynamic properties of MAX phase Mon+1GeCn (n = 1–3) compounds

A novel class of nitrides and carbides called MAX phase materials offers an exceptional blend of metal and ceramic properties together with new guidelines to structure modifications for a variety of technological and engineering applications. In this study, we predict the structural, electronic, elastic, mechanical, and thermodynamic properties of the Mon+1GeCn (n = 1, 2, and 3) MAX phase materials via first-principles (density functional theory (DFT)) calculations. The lattice parameters and formation energies are calculated for the simulated MAX phases under consideration. Five independent elastic constants (C11, C12, C13, C33, and C44 = C55) are determined and we have used them to calculate the bulk modulus (B), Young’s modulus (E), shear modulus (G), Poisson’s ratio (υ) and anisotropy index (A). From our calculations, the predicted electronic band structure and density of states plots show that for the investigated MAX phases, a major contribution in the electronic density of states around the Fermi energy originates from the Mo-4d states. The investigated compounds are metallic with electrical conductivity arising primarily due to the Mo-4d electrons. Thermal effects on some macroscopic properties (lattice constants, Debye temperature, heat capacity, and thermal expansion coefficient) of Mon+1GeCn (n = 1, 2, and 3) are investigated by employing the quasi-harmonic Debye model in a temperature range from 0 K to 1000 K and in the pressure range from 0 GPa to 40 GPa as well.

Temperature and pressure dependence on structural, electronic and thermal properties of ZnO wurtzite phase – first principle investigation

ABSTRACT The ground-state properties of ZnO compound in the Wurtzite-type structure are investigated using the optimization of total-energy calculations based on the full-potential augmented plane-wave (FP-LAPW) method within the density functional theory (DFT). The exchange–correlation potential for structural properties is calculated by the generalized gradient approximation (GGA), while for electronic properties, the Engel and Vosko GGA (EV-GGA) as well the modified Becke–Johnson (mBJ) schemes are applied. The equilibrium lattice constants (a, c and μ) and bulk modulus are found in good agreement with the available reported data. On the other hand, the estimated mBJ energy band gap value is found in good agreement with the literature. In addition, the influence of pressure and temperature on the evolution of lattice constants (a, c and μ), bulk modulus, energy gap, heat capacity, and thermal expansion coefficient are investigated as function of pressure (0–10 GPa) and temperature (0–1000 K).

On the dependence of ionic transport on crystal orientation in NaSICON-type solid electrolytes

The dependence of ionic transport on crystal orientations in NaSICON-type solid electrolytes is studied on flux-grown M3Sc2(PO4)3 (M = Na, Ag) single crystals with well-defined facets. Herein, we provide the first impedance spectroscopy study to characterize ion conduction along different crystallographic orientations in this important class of materials for electrochemical energy storage systems. Moreover, we used single crystal x-ray diffraction, differential scanning calorimetry, 23Na NMR spin-lattice relaxation measurements, and ab initio molecular dynamics simulations to study the interplay of structure and ion transport taking place at different length scales. We conclude that the phase behavior in NaSICON-type materials is strongly linked to ion diffusion. At room temperature, ionic conductivity is slightly anisotropic along the crystallographic orientations [001] and [100]. The slightly different activation energies are related to diffusion bottlenecks solely changing along [001]. This change is caused by anisotropic thermal lattice expansion. With increasing temperature, ion transport increasingly becomes isotropic finally resulting in an order-disorder phase transition from C2/c to R−3c. This phase transition is associated with a clear change in activation energy solely along [001]; it can be traced back to the increasing jump distance along this crystal orientation with temperature. Astonishingly, changing the ionic charge carrier, i.e. when going from Na+ to Ag+, shifts the phase transition temperature by 140 K towards lower temperature. The Arrhenius behavior remains, however, similar. This finding is related to the higher mobility of Ag+ in the NaSICON framework leading to isotropic ion diffusion at much lower temperatures. Overall, flux-grown M3Sc2(PO4)3 allowed us to show that ionic transport parameters and phase stability sensitively depend on crystal chemistry.

Rationalization of Lattice Thermal Expansion for Beta-Blocker Organic Crystals

Anisotropic lattice expansion could be a source of misunderstanding in powder pattern recognitions, especially in the case of organic crystals where for the interpretation of room temperature patterns single crystal data at low temperature are usually used. Trying to rationalize the thermal lattice expansion, we studied two close related β-blocker molecules with similar packing in the solid state but with different thermal behavior. Solid state calculations, using the fast and accurate HF-3c method and the quasi harmonic approximation for the simulation of the lattice expansion, were able to reproduce the experimental trends with good accuracy. The complete analysis of the calculated thermal expansion of the two structures, as well as of other structures with similar packing found in a database survey, revealed the primary role of the hydrogen bonds. Secondary non-covalent interactions in the plane perpendicular to the hydrogen bond system could also play a role.

Realization of high-quality InGaAs/GaAs quantum dot growth on Ge substrate and improvement of optical property through ex-situ ion implantation

Epitaxial growth of III-V heterostructures on non-native substrates such as Silicon (Si) or Germanium (Ge) is one of the promising research topics for the last two decades. The interface between polar III-V semiconductors and non-polar substrates (Si or Ge) plays a crucial role in monolithic integration. However, there is an anticipation of epitaxial GaAs growth on Ge substrate because of the lower mismatch of lattice constants and thermal expansion coefficients between them. Therefore, the high-quality growth of III-V semiconductor heterostructures on Ge substrates would overcome the impediment to Si-photonics, where the monolithic integration of optoelectronic device structures can be done using a Ge graded layer on Si. Here, we have explored the epitaxial growth of multi-layer InGaAs/GaAs quantum dot heterostructures on Ge substrates and compared the optical and structural properties with the QDs grown on GaAs substrate. The optical properties of all samples are investigated with the help of photoluminescence (PL) and time-resolved photoluminescence (TRPL), whereas the morphology of the QDs is observed through cross-sectional transmission electron microscopy (XTEM) images. An enhancement in the optical characteristics (PL peak wavelength, activation energy, carrier lifetime) is found for the QDs grown on Ge substrate with the super-lattice buffer (SLB) layer. The minimization of defects and dislocations in the heterostructure is also realized for the structure with the SLB layer. Furthermore, a two-fold enhancement in PL intensity and 24 meV increment in activation energy is achieved through ex-situ H− ion-implantation, which approached the values obtained for the QD heterostructure grown on GaAs substrate.

Simultaneous Enhancement and Modulation of Upconversion by Thermal Stimulation in Sc2Mo3O12 Crystals.

Rational control of photoluminescence against a change in temperature is important for fundamental research and technological applications. Herein, we report an anomalous temperature dependence of upconversion luminescence in Yb/Ho co-doped Sc2Mo3O12 crystals. By leveraging negative thermal expansion of the crystal lattice, energy transfer between the lanthanide dopants is promoted as the temperature is increased from 303 to 573 K, resulting in an ∼5-fold enhancement of the emission. Meanwhile, the emission profile is also substantially altered due to the concurrent thermal quenching of selective energy states, corresponding to a clear shift in color from green to red. Via correlation of the red-to-green emission intensity ratio of Ho3+ dopant ions with temperature, a ratiometric luminescence thermometer is constructed with a maximum sensitivity of 2.75% K-1 at 543 K. As the Sc2Mo3O12 crystals are thermally stable and nonhygroscopic, our findings highlight a general approach for highly reversible control of upconversion by temperature in ambient air.

Hot carrier dynamics and phonon anharmonicity of ZrTe5 revealed with femtosecond transient optical spectroscopy

We present a time-resolved ultrafast optical spectroscopy study on thermoelectric material ${\mathrm{ZrTe}}_{5}$, regarding its carrier decay dynamics and anharmonic decay of coherent optical phonons at 5--280 K. A reflectivity time series consists of a single exponential decay due to the relaxation of carriers via electron-phonon coupling and decaying oscillations due to coherent phonon vibration of the ${A}_{g}$ mode. The carrier decay characteristics follow the temperature-induced electronic transitions of ${\mathrm{ZrTe}}_{5}$. The coherent optical phonon vibrations can be well described with the anharmonicity model including lattice thermal expansion and phonon-phonon coupling. The phonon vibration frequency and dephasing time are obtained as a function of temperature, and decrease with increasing temperature. The effects of pump fluence are also explored.

Thermal transport properties of GaN with biaxial strain and electron-phonon coupling

Strain inevitably exists in practical GaN-based devices due to the mismatch of lattice structure and thermal expansion brought by heteroepitaxial growth and band engineering, and it significantly influences the thermal properties of GaN. In this work, thermal transport properties of GaN considering the effects from biaxial strain and electron-phonon coupling (EPC) are investigated using the first principles calculation and phonon Boltzmann transport equation. The thermal conductivity of free GaN is 263 and 257 W/mK for in-plane and cross-plane directions, respectively, which are consistent better with the experimental values in the literature than previous theoretical reports and show a nearly negligible anisotropy. Under the strain state, thermal conductivity changes remarkably. In detail, under +5% tensile strain state, average thermal conductivity at room temperature decreases by 63%, while it increases by 53% under the −5% compressive strain, which is mostly attributed to the changes in phonon relaxation time. Besides, the anisotropy of thermal conductivity changes under different strain values, which may result from the weakening effect from strain induced piezoelectric polarization. EPC is also calculated from the first principles method, and it is found to decrease the lattice thermal conductivity significantly. Specifically, the decrease shows significant dependence on the strain state, which is due to the relative changes between phonon-phonon and electron-phonon scattering rates. Under a compressive strain state, the decreases of lattice thermal conductivity are 19% and 23% for in-plane and cross-plane conditions, respectively, comparable with those under a free state. However, the decreases are small under the tensile strain state, because of the decreased electron-phonon scattering rates and increased phonon anharmonicity.

Low-temperature dependence on the THz spectrum of CL-20/TNT energetic cocrystal by molecular dynamics simulations.

Based on the unique advantages of terahertz (THz) spectrum on the detection of energetic cocrystals, the low-temperature dependent THz spectra of CL-20/TNT cocrystal were investigated by using molecular dynamics (MD) simulations from 5 to 296K, as well as three different crystal faces, (001), (120), and (010). When the temperature decreases below 95K, we have observed two new peaks for CL-20/TNT cocrystal, at 4.58 and 5.99THz, respectively. Also, the THz peaks below 1.5THz gradually disappear under cooling from 296 to 5K, and they should originate from the lattice thermal vibrations. THz absorption peaks of CL-20/TNT cocrystal reveal frequency shifting, linearly dependent on temperature. Four of them are red shift and other two are blue shift of THz vibrational peaks of CL-20/TNT cocrystal with the temperature increase. The frequency shifts can be attributed to the effects of lattice thermal expansion on inter-/intramolecular vibrational modes as well as their coupling. From the temperature-dependent THz spectra of different crystal faces, we further confirm the response of different kinds of intermolecular interactions on the THz spectrum of CL-20/TNT cocrystal. Graphical abstractThe intermolecular interactions and peak positions of THz spectra of CL-20/TNT cocrystal in the range of 0-6 THz versus temperature.

Analytical solutions of electroelastic fields in piezoelectric thin-film multilayer: applications to piezoelectric sensors and actuators

Analytical expressions are derived for the stresses and electric fields induced in a piezoelectric multilayer deposited on a substrate with lattice misfit and thermal expansion coefficient mismatch. The piezoelectric multilayer can be subjected to an externally applied force, moment and electric potential. The derived formulations can model any number of layers using recursive relations that minimize the computation time. A proper rotation matrix is utilized to generalize the derived expressions to accommodate various orientations allowing each layer to have hexagonal crystal symmetry. The influence of lattice misfit and thermal expansion coefficient mismatch in the presence of externally applied force, moment and electric potential on the state of the electroelastic fields in each layer is evaluated for various applications. Comparison with finite element analysis results shows excellent agreement. The analytical expressions developed here can be useful in designing electromechanical sensors, actuators and optoelectronic devices made from piezoelectric multilayers.

Photoluminescence signatures of thermal expansion, electron-phonon coupling and phase transitions in cesium lead bromide perovskite nanosheets.

In this article, photoluminescence (PL) behaviors of inorganic cesium lead bromide (CsPbBr3) nanosheets are investigated in a broad temperature range from 5 to 500 K. As can be seen from the temperature evolution of the PL peak position, the bandgap blueshift induced by thermal lattice expansion is found to be gradually compensated by the bandgap redshift caused by electron-phonon coupling for the temperature variation from 5 K to 360 K. As the temperature is further increased, the nearly completely compensated PL peak position turns to exhibit a rapid blueshift, again at ∼360 K. Such turning behavior is consistent with an orthorhombic-tetragonal (γ-β) phase transition at this critical temperature. For the PL linewidth, it shows a continuous broadening at temperatures beyond 40 K, suggesting the dominant role of phonon scattering, especially at high temperatures. These findings of temperature dependent photophysical properties of CsPbBr3 nanosheets may provide useful information of their further applications in the emerging nano photo-electronic devices.