Thermal Transport in Solid State: From First-Principles Simulation to Machine Learning

Solid state materials including semiconductors and metals deeply shaped our modern world, from smartphones and light-emitting diodes to electric cars. Besides the electrical transport properties, thermal transport properties also play significant roles in various applications, such as thermoelectrics and electronics thermal management, where thermal transport properties can affect both performance and reliability of the devices. In most semiconductors and insulators, it is the quasiparticle phonon that dominates the thermal transport, which is the quanta of vibrational modes in crystals. A fundamental study of phonon in semiconducting materials is thus of great importance in order to understand how heat is transported and how to better design new nanoscale devices and materials. Electron, on the other hand, contributes significantly to the thermal transport of metals in most cases, while it usually contributes little to thermal transport in semiconductors. It can, however, play a central role in thermoelectrics, where its transport is limited by the electron-phonon scattering. In this way, a comprehensive understanding of coupled electron and phonon properties are needed to fully understand thermal transport in metals and semiconductors.Nowadays, data-driven techniques have emerged as powerful tools for materials design when analytical principles are not established between material compositions and properties. It is thus of great interest and potential to apply the data-driven machine learning techniques to the area of thermal transport in solid state materials. With more data available on thermal transport properties, using machine learning techniques can help us understand and utilize the relations between thermal transport properties and materials structures without complex analytical expressions.In the first part of the thesis, considering the phonon-phonon scattering, lattice thermal conductivities of various two-dimensional (2D) semiconductors are studied by solving the phonon Boltzmann transport equation iteratively with the help of the parameter-free first-principles calculations. We found that a bond saturation rule can work as a general strategy for the design of high lattice thermal conductivity materials. Several ultrahigh thermal conductivity materials, such as the 2D penta-CN2, three-dimensional T12-carbon and AA T12-carbon are predicted.The electron-phonon scattering is then focused in the second part, where thermoelectric properties in semiconductors and the thermal and electrical transport properties of metals are studied in a first-principles calculation scheme. The 2D blue phosphorene is found to be with a low thermoelectric figure of merit, ZT, due to its high lattice thermal conductivity. However, with nanostructures which reduce lattice thermal conductivity, the ZT can be improved dramatically to around 1. In metallic hexagonal NbN, a promising and important superconductor, the first-principles calculation predicted thermal conductivity agrees well with experimental measurements.Finally, machine learning techniques, especially the transfer learning (TL), are applied to study the phonon properties of semiconductors. Considering the fact that electron properties are much easier to obtain. TL is used to leverage electron properties to help predict phonon properties. We found that TL can help improve the prediction accuracy significantly compared with direct training, indicating a deep connection between electron and phonon. Results also indicate that TL can leverage not-so-accurate proxy properties, as long as they encode composition-property relation, to improve models for target properties – a significant feature to materials informatics in general.In general, with the help of both the first-principles calculations and data-driven machine learning techniques, the thermal transport properties of semiconductors and metals are studied. These studies provide us a clearer understanding about how those quasi-particles can interact in solid state materials and they can potentially enable us to design materials and devices with desired thermal transport properties.</p

Optical Properties of Novel Conjugated Nanohoops: Revealing the Effects of Topology and Size

The unique properties of chemical materials are usually related to the topology and size of their constituent unit. Inspired by a previous report on the synthesis of a separable catenane composed of two Möbius-conjugated nanohoops, we theoretically predicted the optical properties of the Möbius monomer, its Hückel analogue, and some structures derived from them. The underlying reasons for the effects of topology and size on molecular (hyper)­polarizability and the absorption spectrum were explored from the electronic structure level by means of several analytical methods, including the two-level model, (hyper)­polarizability density analysis, and the (hyper)­polarizability contribution decomposition. This work is conducive to providing rational design ideas for those who are committed to discovering and synthesizing nanoscale cyclic molecules with special optical properties

Effects of External Electric Field and Self-Aggregations on Conformational Transition and Optical Properties of Azobenzene-Based D-π-A Type Chromophore in THF Solution

The influence of environments (THF solvents and electric field) and molecular self-aggregations on the structure and optical properties of 4-(4-hydroxyphenylazo)nitrobenzene has been investigated by molecular dynamics (MD) simulations and quantum chemical calculations. Long-range electrostatic effects and the hydrogen bond interactions between the solute and the THF solvent molecules lead to the augments of nonlinear optical (NLO) response by about two times from the gas phase to THF solution, accompanied by considerable red-shift of more than 40 nm in the maximum absorption wavelengths of the ground (S0) and low-lying excited states (S1, S2, and S3). The solvated chromophore reorients quickly (within 300 ps) under external electric field of 1.0 V/nm, even when the direction of the applied electric field is antiparallel to the dipole moment of the solute. Nonequilibrium MD simulations demonstrate that the light-induced cis–trans isomerization in THF solution and external electric field need longer relaxation time (about 1.0 ps) than that in gas phase (about 500 fs). The dipole–dipole interactions and intermolecular hydrogen bonds facilitate the self-aggregations of solute molecules in solution. The V-shaped dimer exhibits higher hyperpolarizability value by about 1.2 times of the monomer, whereas the antiparallel alignment leads to a cancellation of dipole moment and hence dramatic decrease in hyperpolarizability (one-third of the monomer). However, the Boltzmann-weighted contribution to hyperpolarizability from these two aggregations (with 82% V-shaped and 18% antiparallel) is close to that of the monomer. Orientations of D-π-A dipoles in various environments and molecular aggregations are important to modulate the optical properties of materials

Extended First Hyperpolarizability of Quasi-Octupolar Molecules by Halogenated Methylation: Whether the Iodine Atom is the Best Choice

Inspired by a previous report, in which the quasi-octupolar molecules DPATSB, DPATSP, and (DPATSP–Me)+I– were synthesized and the iodized salt was proposed as a suitable optical limiting chromophore, we performed linear and nonlinear optical analyses on the synthesis molecules and several extension halogenated dyes ((DPATSP–Me)+X– (X = F, Cl, and Br)) by using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The energy gap of frontier molecular orbitals (FMOs) was diminished by introducing the pyridinium ion into the system, and a charge-separated state was formed in halogenated salts. The N-methylation of the dye leads to a sharp increase in response characteristics, accompanied by a red-shift of the absorption spectrum from the ultraviolet region to the visible light region. Importantly, our studies show that there exists a significant halogen atomic-species dependence of the nonlinearity of the salts. Fluorination and bromination of the molecule seem to be more potential in improving the nonlinearity in zero-frequency and frequency-dependent incident light, respectively, without causing the undesirable red-shift of the absorption band relative to their halogenated analogues. The nonlinear optical (NLO) responses calculated by different DFTs are identical in trend

Linear and Nonlinear Optical Properties of Triphenylamine–Indandione Chromophores: Theoretical Study of the Structure–Function Relationship under the Combined Action of Substituent and Symmetry Change

Linear and nonlinear optical properties of experimentally synthesized triphenylamine–indandione chromophores were investigated by time-dependent density functional theory calculations. The absorption and emission spectra, as well as the static and dynamic first hyperpolarizabilities related to the combined effect of substituent introduction and symmetry breaking, were discussed in detail. Theoretical analysis indicated the uniting of indandione acceptor group(s) with a precursor (triphenylamine, TriPhA), with the molecular symmetry destroyed simultaneously, leads to an obvious change in both the peak position and intensity of the linear spectra. The same process can also substantially magnify the molecular first hyperpolarizabilities. The triphenylamine–indandione molecules exhibit efficiencies in static first hyperpolarizability relative to that of the electron-donating TriPhA component and the electron-accepting indandione moiety. The optical nonlinearity would be further expanded under the influence of a resonance effect induced by appropriate excitation. Incident light with a wavelength nearly two times the one-photon absorption is likely to cause a greater frequency dispersion response. In particular, the first hyperpolarizabilities of the title compounds can be enlarged by about 3.2 times on average by resonance enhancement at a fundamental wavelength of 1064 nm

Table1_Evolution of the Continental Crust in the Northern Tibetan Plateau: Constraints From Geochronology and Hf Isotopes of Detrital Zircons.XLSX

To investigate the evolution of the continental crust in the northern Tibetan Plateau, detrital zircon U–Pb geochronology and Hf isotopes analysis were performed on two fluvial sand samples from North Qaidam (the Yuka and Shaliu rivers). A total of 443 detrital zircon U–Pb ages and 244 Hf isotopic results were obtained and reveal that the South Qilian, North Qaidam, and East Kunlun terranes show affinity to the western Yangtze Block. Age distributions of detrital zircons from the Yuka River cluster mainly in two age intervals of 1,000–700 and 480–400 Ma. The corresponding εHf(t) values are mostly negative, with depleted two-stage Hf model ages (TDM2) of 2.1–1.6 Ga. In contrast, age data for the Shaliu River fall in the ranges of 1,000–700, 460–380, and 260–200 Ma, with TDM2 ages of 2.0–1.6 and 1.6–1.2 Ga. In addition, zircons with Neoproterozoic ages from both river samples possess common Paleoproterozoic TDM2 ages (2.0–1.6 Ga, with a peak of 1.8–1.7 Ga), indicating that the South Qilian, North Qaidam and East Kunlun terranes were probably part of the same Neoproterozoic continent. The East Kunlun and North Qaidam terranes are inferred to include Mesoproterozoic continental crust (1.6–1.0 Ga), suggesting differences in crustal evolution between the East Kunlun–North Qaidam and Qilian terranes. Phanerozoic magmatism in these three terranes was sourced mainly from the recycling of ancient continental crust with minor contributions from the juvenile crust.</p

Open-Cage Fullerene as a Macrocyclic Ligand for Na, Pt, and Rh Metal Complexes

An open-cage [60]fullerene derivative was prepared through Malaprade oxidation of a vicinal triol moiety as the key step. Above the 17-membered orifice, there is one carboxyl group. Three ketone carbonyl groups and one lactone carbonyl group are located on the rim of the orifice. The carboxylic and carbonyl oxygen atoms around the orifice act as strong polydentate ligands for a sodium ion. These oxygen atoms also react with [Rh(CO)2Cl]2 to form various isomeric rhodium complexes with comparable stability. The fullerene CC bond on the rim of the orifice forms a stable platinum complex when treated with Pt(PPh3)4. Single crystal X-ray diffraction data reveal that one of the carboxylic oxygen atoms above the orifice forms a H-bond with the water molecule trapped in the cage