Rattling-induced suppression of thermal transport in cubic In2O3with Sn-Ga diatomic defect.

We present the first principles study of cubic In2O3with a diatomic defect composed of a Sn atom substituting the In atom at theb-site and a Ga atom embedded in the nearestc-site (structural vacancy) with lattice positions according to the Wyckoff notations. Structural, electronic, phononic and thermal properties were investigated within density functional theory formalism. The lattice anharmonicity effects were taken into account for all possible three-phonon scattering processes. The phonon transport was considered within the Peierls-Boltzmann transport equation with relaxation time approximation. In the relaxed lattice, a strong rearrangement of the initial positions of the atoms in the defect vicinity was revealed, which primarily manifests itself in the displacement of the Sn atom toward another interstitial site. Thus, a cage is formed around the defect by 12 O and 12 In atoms. The calculations of elastic constants and mean square displacements of cage region atoms showed the rattling-like behavior of the Sn atom. Bader charge analysis and electron localization function allowed a deeper understanding and explanation of such behavior. Phonon energy spectra as compared to In2O3and In2O3:(Sn) demonstrated flattening of phonon branches with spatial localization of phonon modes. They also revealed a decrease in average group velocities of phonons, including those of acoustic type, the presence of avoided-crossing features in the low energy range, and an increase of available phase space for three-phonon scattering. Accounting for all these vibrational features due to defect atoms resulted in a thermal conductivity drop at room temperature by more than seven times compared to In2O3.

Thermal Transport Evolution Due to Nanostructural Transformations in Ga-Doped Indium-Tin-Oxide Thin Films.

We report on a comprehensive theoretical and experimental investigation of thermal conductivity in indium-tin-oxide (ITO) thin films with various Ga concentrations (0-30 at. %) deposited by spray pyrolysis technique. X-ray diffraction (XRD) and scanning electron microscopy have shown a structural transformation in the range 15-20 at. % Ga from the nanocrystalline to the amorphous phase. Room temperature femtosecond time domain thermoreflectance measurements showed nonlinear decrease of thermal conductivity in the range 2.0-0.5 Wm-1 K-1 depending on Ga doping level. It was found from a comparison between density functional theory calculations and XRD data that Ga atoms substitute In atoms in the ITO nanocrystals retaining Ia-3 space group symmetry. The calculated phonon dispersion relations revealed that Ga doping leads to the appearance of hybridized metal atom vibrations with avoided-crossing behavior. These hybridized vibrations possess shortened mean free paths and are the main reason behind the thermal conductivity drop in nanocrystalline phase. An evolution from propagative to diffusive phonon thermal transport in ITO:Ga with 15-20 at. % of Ga was established. The suppressed thermal conductivity of ITO:Ga thin films deposited by spray pyrolysis may be crucial for their thermoelectric applications.

Energetic, structural and electronic features of Sn-, Ga-, O-based defect complexes in cubic In2O3.

Defect energy formation, lattice distortions and electronic structure of cubic In2O3 with Sn, Ga and O impurities were theoretically investigated using density functional theory. Different types of point defects, consisting of 1-4 atoms of Sn, Ga and O in both substitutional and interstitial (structural vacancy) positions, were examined. It was demonstrated, that formation of substitutional Ga and Sn defects are spontaneous, while formation of interstitial defects requires an activation energy. The donor-like behavior of interstitial Ga defects with splitting of conduction band into two subbands with light and heavy electrons, respectively, was revealed. Contrarily, interstitial O defects demonstrate acceptor-like behavior with the formation of acceptor levels or subbands inside the band gap. The obtained results are important for an accurate description of transport phenomena in In2O3 with substitutional and interstitial defects.

In-Plane Thermal Conductivity of Radial and Planar Si/SiOx Hybrid Nanomembrane Superlattices.

Silicon, although widely used in modern electronic devices, has not yet been implemented in thermoelectric applications mainly due to its high thermal conductivity, κ, which leads to an extremely low thermoelectric energy conversion efficiency (figure of merit). Here, we present an approach to manage κ of Si thin-film-based nanoarchitectures through the formation of radial and planar Si/SiOx hybrid nanomembrane superlattices (HNMSLs). For the radial Si/SiOx HNMSLs with various numbers of windings (1, 2, and 5 windings), we observe a continuous reduction in κ with increasing number of windings. Meanwhile, the planar Si/SiOx HNMSL, which is fabricated by mechanically compressing a five-windings rolled-up microtube, shows the smallest in-plane thermal conductivity among all the reported values for Si-based superlattices. A theoretical model proposed within the framework of the Born-von Karman lattice dynamics to quantitatively interpret the experimental data indicates that the thermal conductivity of Si/SiOx HNMSLs is to a great extent determined by the phonon processes in the SiOx layers.

Engineering of the thermodynamic properties of bilayer graphene by atomic plane rotations: the role of the out-of-plane phonons.

We investigated theoretically the specific heat of graphene, bilayer graphene and twisted bilayer graphene taking into account the exact phonon dispersion and density of states for each polarization branch. It is shown that contrary to a conventional belief the dispersion of the out-of-plane acoustic phonons - referred to as ZA phonons - deviates strongly from a parabolic law starting from the frequencies as low as ∼100 cm(-1). This leads to the frequency-dependent ZA phonon density of states and the breakdown of the linear dependence of the specific heat on temperature T. We established that ZA phonons determine the specific heat for T ≤ 200 K while contributions from both in-plane and out-of-plane acoustic phonons are dominant for 200 K ≤ T ≤ 500 K. In the high-temperature limit, T > 1000 K, the optical and acoustic phonons contribute approximately equally to the specific heat. The Debye temperature for graphene and twisted bilayer graphene was calculated to be around ∼1861-1864 K. Our results suggest that the thermodynamic properties of materials such as bilayer graphene can be controlled at the atomic scale by rotation of the sp(2)-carbon planes.

Thermal conductivity of twisted bilayer graphene.

We have investigated experimentally the thermal conductivity of suspended twisted bilayer graphene. The measurements were performed using an optothermal Raman technique. It was found that the thermal conductivity of twisted bilayer graphene is lower than that of monolayer graphene and the reference, Bernal stacked bilayer graphene in the entire temperature range examined (∼300-700 K). This finding indicates that the heat carriers - phonons - in twisted bilayer graphene do not behave in the same manner as that observed in individual graphene layers. The decrease in the thermal conductivity found in twisted bilayer graphene was explained by the modification of the Brillouin zone due to plane rotation and the emergence of numerous folded phonon branches that enhance the phonon Umklapp and normal scattering. The results obtained are important for understanding thermal transport in two-dimensional systems.