Defect engineering of thermoelectric (TE) materials enables the alteration of their crystal lattice by creating an atomic-scale disorder, which can facilitate a synergistic modulation of the electrical and phonon transport, leading to the enhancement of their TE properties. This work employs a compositional nonstoichiometry strategy for manipulation of Nivacancies and Ni-interstitials through Ni-deficient and Ni-excess compositions of (Zr, Hf)Ni 1±x Sn-based half-Heusler (HH) alloys to realize a stateof-the-art TE figure-of-merit (ZT) of ∼1.4 at 873 K in 4 atomic % Ni-excess HH composition, which corresponds to a remarkable TE conversion efficiency of ∼12%, estimated using the cumulative temperature dependence model. These alloys are synthesized employing arc-melting followed by spark plasma sintering and are characterized for their phase, morphology, structure, and composition along with electrical and thermal transport properties to examine the implication of Niexcess and Ni-deficiency on the TE properties of the synthesized Zr 0.6 Hf 0.4 NiSn HH alloy. A significant enhancement (∼30%) of ZT is observed in the low doping limit of Ni-excess HH compositions over their stoichiometric counterpart due to Ni-interstitials and in situ full-Heusler precipitation, which enable a strong phonon scattering for a drastic reduction in lattice thermal conductivity and lead to an enhancement of ZT. However, Ni-deficient HH compositions exhibit a deterioration in the TE properties owing to the vacancy-induced bipolarity. The defect-mediated optimization of electrical and thermal transport, thus, opens up promising avenues for boosting the TE performance of HH alloys.
The bulk carrier excitation influences the phonon dynamics, which can alter and modulate the surface charge density of topological insulators such as bismuth selenide (Bi2Se3). This work investigates the charge carrier and phonon dynamics in Bi2Se3 grown on various substrates. The orientation of the substrate, the size of the crystallites, and the misfit of the lattice affect the carrier and phonon dynamics. Bi2Se3 thin films are grown under the same growth conditions on SiO2, Si(111), and SiN. Bi2Se3 overlayers exhibit substrate‐dependent charge carrier relaxation channels and phonon dynamics. It is evident from Raman spectroscopy and ultrafast transient absorption spectroscopy that the heterointerface interactions of all three samples affect the vibration modes of Bi2Se3 and coherent acoustic phonon oscillations in the NIR range. At 13.6, 41.2, and 34.4 GHz, the vibration modes of SiO2, Si(111), and SiN are equivalent. The propagation depth of phonon waves is shown by measuring the speed of sound in Bi2Se3 overlayers on SiO2, Si(111), and SiN. This study demonstrates that the surface and bulk‐bound charge carriers of a topological insulator determine the frequency and velocity of the generated sound.
We present an integral study on the photonic response of AuGe nanoparticles (NPs) to establish a correlation between different parameters such as NP size, volume, and distribution over different substrates (n + GaAs, semiconducting GaAs) having different film thicknesses (∼5, ∼10 nm) at different annealing temperatures (573 and 673 K) subjected to repeated and stepped annealing cycles. The structural characterization of overlayer growth and formation of AuGe nanoclusters/ NPs is correlated with unusual plasmonic response measured during photonic characterization. The morphological changes induced by stepped annealing of the AuGe/GaAs system resulted in enhanced light emission and favorable tuning of plasmon frequency. A larger photoluminescence enhancement is measured with the fifth anneal, which is found to be related to the enhanced NP reflectance and is in good theoretical agreement with the Drude−Lorentz model. The numerical negative real permittivity "ε 1 " confirms the plasmonic impact, while X-ray photoelectron spectroscopy studies reveal the unique intermixing phenomena of AuGe and GaAs at the interface due to annealing. Additionally, ultrafast studies depict the periodic absorbance in the below-band-gap region of GaAs resembling an EL2-state-like behavior. The AuGe NPs embedded/adhered on a light-emitting surface thus offer both the ease and flexibility for tuning the plasmon resonance frequency, to attain tweaked and enhanced light-matter coupling, which can be concluded as plasmon-induced interfacial charge-transfer transition phenomena. The enhanced light coupling enhances both the photovoltaic conversion efficiency and quantum efficiency, thereby enabling increased solar cell fill factor and responsivity enhancement in photovoltaic devices such as photodetectors and thermal imagers.
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