Showing papers by "Davide Donadio published in 2022"

Effect of crystallinity and thickness on thermal transport in layered PtSe2

Abstract: Abstract We present a comparative investigation of the influence of crystallinity and film thickness on the acoustic and thermal properties of layered PtSe 2 films of varying thickness (1–40 layers) using frequency-domain thermo-reflectance, low-frequency Raman, and pump-probe coherent phonon spectroscopy. We find ballistic cross-plane heat transport up to ~30 layers PtSe 2 and a 35% reduction in the cross-plane thermal conductivity of polycrystalline films with thickness larger than 20 layers compared to the crystalline films of the same thickness. First-principles calculations further reveal a high degree of thermal conductivity anisotropy and a remarkable large contribution of the optical phonons to the thermal conductivity in bulk (~20%) and thin PtSe 2 films (~30%). Moreover, we show strong interlayer interactions in PtSe 2 , short acoustic phonon lifetimes in the range of picoseconds, an out-of-plane elastic constant of 31.8 GPa, and a layer-dependent group velocity ranging from 1340 ms −1 in bilayer to 1873 ms −1 in eight layers of PtSe 2 . The potential of tuning the lattice thermal conductivity of layered materials with the level of crystallinity and the real-time observation of coherent phonon dynamics open a new playground for research in 2D thermoelectric devices and provides guidelines for thermal management in 2D electronics.

Non-Fourier heat transport in nanosystems

Abstract: Energy transfer in small nanosized systems can be very different from that in their macroscopic counterparts due to reduced dimensionality, interaction with surfaces, disorder, and large fluctuations. Those ingredients may induce non-diffusive heat transfer that requires to be taken into account on small scales. We provide an overview of the recent advances in this field from the points of view of nonequilibrium statistical mechanics and atomistic simulations. We summarize the underlying basic properties leading to violations of the standard diffusive picture of heat transport and its universal features, with some historical perspective. We complete this scenario by illustrating also the effects of long-range interaction and integrability on non-diffusive transport. Then we discuss how all of these features can be exploited for thermal management, rectification and to improve the efficiency of energy conversion. We conclude with a review on recent achievements in atomistic simulations of anomalous heat transport in single polymers, nanotubes and two-dimensional materials. A short account of the existing experimental literature is also given.}

Effect of sodium chloride adsorption on the surface premelting of ice

Abstract: We characterise the structural properties of the quasi-liquid layer (QLL) at two low-index ice surfaces in the presence of sodium chloride (Na$^+/$Cl$^-$) ions by molecular dynamics simulations. We find that the presence of a high surface density of Na$^+/$Cl$^-$ pairs changes the surface melting behaviour from step-wise to gradual melting. The ions lead to an overall increase of the thickness and the disorder of the QLL, and to a low-temperature roughening transition of the air-ice interface. The local molecular structure of the QLL is similar to that of liquid water, and the differences between the basal and primary prismatic surface are attenuated by the presence of Na$^+/$Cl$^-$ pairs. These changes modify the crystal growth rates of different facets and the solvation environment at the surface of sea-water ice with a potential impact on light scattering and environmental chemical reactions.

UV–Visible Absorption Spectra of Solvated Molecules by Quantum Chemical Machine Learning

Abstract: Predicting UV-visible absorption spectra is essential to understand photochemical processes and design energy materials. Quantum chemical methods can deliver accurate calculations of UV-visible absorption spectra, but they are computationally expensive, especially for large systems or when one computes line shapes from thermal averages. Here, we present an approach to predict UV-visible absorption spectra of solvated aromatic molecules by quantum chemistry (QC) and machine learning (ML). We show that a ML model, trained on the high-level QC calculation of the excitation energy of a set of aromatic molecules, can accurately predict the line shape of the lowest-energy UV-visible absorption band of several related molecules with less than 0.1 eV deviation with respect to reference experimental spectra. Applying linear decomposition analysis on the excitation energies, we unveil that our ML models probe vertical excitations of these aromatic molecules primarily by learning the atomic environment of their phenyl rings, which align with the physical origin of the π →π* electronic transition. Our study provides an effective workflow that combines ML with quantum chemical methods to accelerate the calculations of UV-visible absorption spectra for various molecular systems.

Barbalinardo et al. Reply.

Abstract: Powered by TCPDF (www.tcpdf.org) This material is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Barbalinardo, Giuseppe; Chen, Zekun; Dong, Haikuan; Fan, Zheyong; Donadio, Davide

PbTe/PbSe Thermoelectric Nanocomposites: The Impact of Length Modulations on Lowering Thermal Conductivity

Abstract: PbTe and PbSe are among the most promising thermoelectric materials used in the mid-temperature (400–900 K) power generation range. In these materials the efficiency increase in thermoelectric performance is critically related to the lowering of lattice thermal conductivity (κL), without compromising the electronic power factor. By means of state-of-the-art equilibrium molecular dynamics (EMD), we investigate heat transport in several nanostructured PbTe/PbSe models as a function of material morphology. Layered composites show a reduction of the average κL of about 35 % with respect to the bulk. The insertion of PbSe nanoparticles into a PbTe matrix, or viceversa PbTe into PbSe reduces κL by up to 45 % while in more anisotropic nanocomposites the reduction exceeds PbSe/PbTe alloys. Layered composites show the lowest lattice thermal conductivity in the direction of layer stacking, for which an optimal thickness is identified. Along this line we provide a full account of the impact of alloying and (sub)nanostructuring on heat transport for this important class of materials. Particularly anisotropic nano-dot morphologies and layered (sub)nanocomposites emerge as a paradigm for outstanding thermoelectric materials.

Effect of sodium chloride adsorption on the surface premelting of ice.

Abstract: We characterise the structural properties of the quasi-liquid layer (QLL) at two low-index ice surfaces in the presence of sodium chloride (Na+/Cl-) ions by molecular dynamics simulations. We find that the presence of a high surface density of Na+/Cl- pairs changes the surface melting behaviour from step-wise to gradual melting. The ions lead to an overall increase of the thickness and the disorder of the QLL, and to a low-temperature roughening transition of the air-ice interface. The local molecular structure of the QLL is similar to that of liquid water, and the differences between the basal and primary prismatic surface are attenuated by the presence of Na+/Cl- pairs. These changes modify the crystal growth rates of different facets and the solvation environment at the surface of sea-water ice with a potential impact on light scattering and environmental chemical reactions.

Evolution of structure and transport properties of the Ba8Cu16P30 clathrate-I framework with the introduction of Ga

Abstract: Two type-I clathrates were synthesized by introducing Ga into the framework of the Ba8Cu16P30 type-I clathrate. The introduction of minute amounts of Ga, 1.9% Ga/ Mtotal (where Mtotal = Cu + Ga), resulted in the disturbance of the completely ordered Pbcn superstructure of Ba8Cu16P30. Ba8Cu15.43(2)Ga0.3P30.26(3) crystallizes in a partially ordered orthorhombic Pmna clathrate-I superstructure with five out of 15 framework sites being jointly occupied by metal+phosphorus. Increasing the Ga content resulted in all framework sites being occupied by metal + phosphorus in the archetype cubic Pm[Formula: see text] n clathrate-I crystal structure of Ba8Cu14.5(3)Ga1.3P30.2(4) with 8.2% Ga/ Mtotal. A combination of energy dispersive x-ray spectroscopy, inductively coupled plasma mass spectroscopy, and single crystal x-ray diffraction was used to determine the structures alongside the compositions. The positional disorder was verified by 31P solid state NMR spectroscopy. Characterization of the transport properties indicated that the Ga-substituted samples exhibit higher Seebeck coefficients and electrical resistivities compared to its pristine counterpart, in line with the expected reduction of the hole concentration due to Ga/Cu substitution. Moderate improvements in the thermoelectric power factor and overall figure-of-merit were observed for samples with 6.9% and 3.8% Ga/ Mtotal as compared to those for the pristine Ba8Cu16P30 clathrate. Band structure calculations shed light on how Ga substitution affects the electronic structure and thermoelectric properties of studied clathrates.

(Invited) Ferroelectric Effect in SrTiO3 Promotes Photoelectrochemical Water Oxidation

Abstract: Ferroelectric materials, such as BaTiO3 or BiFeO3, retain an electric polarization after exposure to an external electric field. In semiconductor photoelectrodes, such a ferroelectric effect (FE) could be potentially used to enhance the carrier separation and photoelectrochemical activity under photoexcitation. However, to date, examples of FE-enhanced photoelectrochemistry are rare. Here, we report the discovery of a FE effect in oxygen vacancy rich SrTiO3 nanoparticles and single crystals, and their application for improved photoelectrochemical water oxidation. Single crystals terminated by [111] facets were subjected to hydrogen annealing at 1000 C to induce oxygen vacancies, as confirmed with optical absorption and X-ray photoelectron spectroscopy. In 0.5 M aqueous Na2SO4 solution 1.0 cm2 large crystals produce a 2.1 mA photocurrent at 0.6 V RHE and under 55 mW cm-2 UV illumination. Prior exposure to 10 kV cm-1 electric field increases the photocurrent to 4.5 mA or decreases it to 0.05 mA, depending on the orientation of the polarization. The effect of the electric polarization on the carrier separation direction is confirmed with surface photovoltage spectra. At room temperature, the FE in SrTiO3 is relatively short lived; samples stored in argon retain 85% of the FE effect after 24h, but samples stored in air loose 100% of the FE, likely due to oxygen uptake and disappearance of the vacancies. To obtain a microscopic interpretation of the observed FE effect we investigated ferroelectricity in oxygen-deficient SrTiO3 by density functional theory with effective many-body corrections. These calculations applied to SrTiO3 bulk and surfaces provide a rationale to interpret the experimental results. The finding of an FE in oxygen vacancy rich SrTiO3 is significant because it may be applicable to other metal oxides in the cubic perovskite structure type. Figure 1

Editorial for Issue 2/3 by Pamela M. Norris, Editor-in-Chief

Abstract: What exciting times we are living in! Our ability to measure, model, predict, and influence the thermophysical properties of materials at the microand nanoscale is now enabling exciting new advances with potential positive impacts in many important areas such energy, environment, information, medicine, and transportation. This journal, which was begun in 1997 by my post-doctoral advisor, Chang-Lin Tien, under the title “Microscale Thermophysical Engineering” was established at the very beginning of the nanotechnology revolution and it has always been a forum for thought leadership.