Thermoelectric characteristics of X$$_2$$YH$$_2$$ monolayers (X=Si, Ge; Y=P, As, Sb, Bi): a first-principles study

Ever since global warming emerged as a serious issue, the development of promising thermoelectric materials has been one of the main hot topics of material science. In this work, we provide an in-depth understanding of the thermoelectric properties of X$$_2$$ 2 YH$$_2$$ 2 monolayers (X=Si, Ge; Y=P, As, Sb, Bi) using the density functional theory combined with the Boltzmann transport equation. The results indicate that the monolayers have very low lattice thermal conductivities in the range of 0.09−0.27 Wm$$^{-1}$$ - 1 K$$^{-1}$$ - 1 at room temperature, which are correlated with the atomic masses of primitive cells. Ge$$_2$$ 2 PH$$_2$$ 2 and Si$$_2$$ 2 SbH$$_2$$ 2 possess the highest mobilities for hole (1894 cm$$^2$$ 2 V$$^{-1}$$ - 1 s$$^{-1}$$ - 1 ) and electron (1629 cm$$^2$$ 2 V$$^{-1}$$ - 1 s$$^{-1}$$ - 1 ), respectively. Si$$_2$$ 2 BiH$$_2$$ 2 shows the largest room-temperature figure of merit, $$ZT=2.85$$ Z T = 2.85 in the n-type doping ( $$\sim 3\times 10^{12}$$ ∼ 3 × 10 12 cm$$^{-2}$$ - 2 ), which is predicted to reach 3.49 at 800 K. Additionally, Si$$_2$$ 2 SbH$$_2$$ 2 and Si$$_2$$ 2 AsH$$_2$$ 2 are found to have considerable ZT values above 2 at room temperature. Our findings suggest that the mentioned monolayers are more efficient than the traditional thermoelectric materials such as Bi$$_2$$ 2 Te$$_3$$ 3 and stimulate experimental efforts for novel syntheses and applications.

Ultrafast carrier’s dynamics with scattering rate saturation in Ge thinfilms

<p>An innovative theoretical approach for deeper understanding of the ultrafast spectroscopy experiments through solution of the Boltzmann transport equation coupled with various nonlinear scattering mechanisms, overcoming the limitations offered by DFT, RT-TDDFT and molecular based methods, is reported. A clear advantage of the real-time approach is that it does not make a priori assumptions about specific scattering, relaxation mechanisms and has capabilities to capture the full real-time carrier’s dynamics, including the superposition of all electron–electron, electron-lattice and electron–phonon scatterings etc. No such method with advances in theoretical treatments to explain ultrafast spectroscopy has been reported previously as per the author’s knowledge.</p>

Ultrafast carrier’s dynamics with scattering rate saturation in Ge thinfilms

<p>An innovative theoretical approach for deeper understanding of the ultrafast spectroscopy experiments through solution of the Boltzmann transport equation coupled with various nonlinear scattering mechanisms, overcoming the limitations offered by DFT, RT-TDDFT and molecular based methods, is reported. A clear advantage of the real-time approach is that it does not make a priori assumptions about specific scattering, relaxation mechanisms and has capabilities to capture the full real-time carrier’s dynamics, including the superposition of all electron–electron, electron-lattice and electron–phonon scatterings etc. No such method with advances in theoretical treatments to explain ultrafast spectroscopy has been reported previously as per the author’s knowledge.</p>

Heat Transport Driven by the Coupling of Polaritons and Phonons in a Polar Nanowire

Heat transport guided by the combined dynamics of surface phonon-polaritons (SPhPs) and phonons propagating in a polar nanowire is theoretically modeled and analyzed. This is achieved by solving numerically and analytically the Boltzmann transport equation for SPhPs and the Fourier’s heat diffusion equation for phonons. An explicit expression for the SPhP thermal conductance is derived and its predictions are found to be in excellent agreement with its numerical counterparts obtained for a SiN nanowire at different lengths and temperatures. It is shown that the SPhP heat transport is characterized by two fingerprints: (i) The characteristic quantum of SPhP thermal conductance independent of the material properties. This quantization appears in SiN nanowires shorter than 1 μm supporting the ballistic propagation of SPhPs. (ii) The deviation of the temperature profile from its typical linear behavior predicted by the Fourier’s law in absence of heat sources. For a 150 μm-long SiN nanowire maintaining a quasi-ballistic SPhP propagation, this deviation can be as large as 1 K, which is measurable by the current state-of-the-art infrared thermometers.

Study of Phonon Transport Across Si/Ge Interfaces Using Full-Band Phonon Monte Carlo Simulation

A Full Band Monte Carlo simulatorhas been developed to considerphonon transmission across interfaces disposedperpendicularlyto the heat flux. This solver of the Boltzmann transport equation does not require any assumption on the shape the phonon distribution and can naturally consider all phonon transport regimes from the diffusive to the fully ballistic regime. This simulatoris used to study single and double Si/Ge heterostructures from the micrometer scale downto the nanometer scale,i.e. in all phonon transport regime from fully diffusive toballistic.A methodology to determine the thermal conductivity atthermal interfaces is presented.