Based on the concept of band bending at metal/semiconductor interfaces as an energy filter for electrons, we present a theory for the enhancement of the thermoelectric properties of semiconductor materials with metallic nanoinclusions. We show that the Seebeck coefficient can be significantly increased due to a strongly energy-dependent electronic scattering time. By including phonon scattering, we find that the enhancement of $ZT$ due to electron scattering is important for high doping, while at low doping it is primarily due to a decrease in the phonon thermal conductivity.

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Theory of enhancement of thermoelectric properties of materials with nanoinclusions.

We have probed the local thermoelectric power of semiconductor nanostructures with the use of ultrahigh-vacuum scanning thermoelectric microscopy. When applied to a p-n junction, this method reveals that the thermoelectric power changes its sign abruptly within 2 nanometers across the junction. Because thermoelectric power correlates with electronic structure, we can profile with nanometer spatial resolution the thermoelectric power, band structures, and carrier concentrations of semiconductor junctions that constitute the building blocks of thermoelectric, electronic, and optoelectronic devices.

Profiling the Thermoelectric Power of Semiconductor Junctions with Nanometer Resolution

Metallic alloy nanoparticles are synthesized by combining two or more different metals. Bimetallic or trimetallic nanoparticles are considered more effective than monometallic nanoparticles because of their synergistic characteristics. In this review, we outline the structure, synthesis method, properties, and biological applications of metallic alloy nanoparticles based on their plasmonic, catalytic, and magnetic characteristics.

Synthesis, Properties, and Biological Applications of Metallic Alloy Nanoparticles.

Emerging Theory, Materials, and Screening Methods: New Opportunities for Promoting Thermoelectric Performance

Wearable and flexible thin film thermoelectric module for multi-scale energy harvesting

We report on an experimental setup for the simultaneous measurement of the thermoelectric power (TEP) of two samples in the temperature range from 77 K to 500 K using optimum electronic instruments. The setup consists of two rectangular copper bars in a bridge arrangement for sample mounting, two surface mount (SM) chip resistors for creating alternate temperature gradient, and a type E thermocouple in differential geometry for gradient temperature (ΔT) measurement across the samples. In addition, a diode arrangement has been made for the alternate heating of SM resistors using only one DC current source. The measurement accuracy of ΔT increases with the differential thermocouple arrangement. For the calibration of the setup, measurements of TEP on a high purity (99.99%) platinum wire and type K thermocouple wires Chromel and Alumel have been performed from 77 K to 500 K with respect to copper lead wires. Additionally, this setup can be utilized to calibrate an unknown sample against a sample of known absolute TEP.

An experimental setup for the simultaneous measurement of thermoelectric power of two samples from 77 K to 500 K.

The present study focuses on the enhancement of thermoelectric power of PbTe:Ag nanocomposite thin films, synthesized by the thermal evaporation technique. Thermoelectric measurements were carried out from room temperature to 400 K. It is observed that Ag addition improves the thermoelectric power and crystalline nature of the PbTe thin films. Synchrotron based X-ray diffraction was performed to confirm the phases of the Pb–Ag–Te alloy. This was further reconfirmed by X-ray photoelectron spectroscopy (XPS) and showed the precipitation of Pb on the surface of the PbTe:Ag films. The enhancement of thermoelectric power is thus attributed to the formation of Ag2−xTe alloy and the precipitation of Pb nanostructures on the surface. The origin of such enhancement is understood based on the phenomenon of energy dependent filtering of charge carriers.

Enhancement of thermoelectric power of PbTe:Ag nanocomposite thin films

An investigation was carried out to understand the phase evolution and study the structural, morphological, optical and electrical properties of Co–Sb alloys fabricated by two different approaches: (a) thermal annealing and (b) ion-beam mixing followed by post annealing. The as-deposited and 100 MeV Ag ion beam irradiated Co/Sb bilayer thin films were subjected to thermal annealing from 200 to 400 °C for 1 hour. The Rutherford backscattering spectrometry (RBS) results showed partial mixing for the thermally annealed films and complete mixing for the irradiated and post annealed films at 400 °C. The XRD and RAMAN measurements indicated the formation of Co–Sb alloy, with ∼70% concentration of CoSb3 phase in the irradiated post annealed sample at 400 °C. The band gaps of the annealed and post irradiated annealed Co–Sb alloys were determined using UV-visible spectroscopy. Electrical and thermoelectric power measurements were performed in the temperature range of 300–420 K. It was observed that the alloys formed by ion-beam induced mixing exhibited higher electrical conductivity and thermoelectric power than the as-deposited and thermally annealed Co/Sb bilayer thin films.

Phase evolution and electrical properties of Co–Sb alloys fabricated from Co/Sb bilayers by thermal annealing and ion beam mixing

In the present study, we investigated the formation of AuFe alloy nanoparticles embedded in a silica matrix by the cosputtering of silica, Au and Fe with two different metal fractions using an atom beam source. The increase in the metal fraction in the thin film results in the formation of AuFe alloy nanoparticles. The absence of surface plasmon resonance peak for the Au nanoparticles in the optical spectra, structural studies and transmission electron microscopy results confirmed the existence of AuFe alloy nanoparticles. The nanocomposite is ferromagnetic at 2 K with a symmetric hysteresis loop. The formation of AuFe alloy nanoparticles in the thin film is explained on the basis of interatomic distance and diffusion during deposition.

Synthesis and characterization of Au–Fe alloy nanoparticles embedded in a silica matrix by atom beam sputtering

Swift heavy ion (SHI) irradiation of damaged germanium (d-Ge) layer results in porous structure with voids aligned along ion trajectory due to local melting and resolidification during high electronic energy deposition. The present study focuses on the irradiation temperature- and incident angle-dependent growth dynamics and shape evolution of these voids due to 100 MeV Ag ions irradiation. The d-Ge layers were prepared by multiple low-energy Ar ion implantations in single crystalline Ge with damage formation of ~7 displacements per atom. Further, these d-Ge layers were irradiated using 100 MeV Ag ions at two different temperatures (77 and 300 K) and three different angles (7°, 30° and 45°). After SHI irradiation, substantial volume expansion of d-Ge layer is detected which is due to formation of nanoscale voids. The volume expansion is observed to be more in the samples irradiated at 77 K as compared to 300 K at a given irradiation fluence. It is observed that the voids are of spherical shape at low ion irradiation fluence. The voids grow in size and change their shape from spherical to prolate spheroid with increasing ion fluence. The major axis of spheroid is observed to be aligned approximately along the ion beam direction which has been confirmed by irradiation at three different angles. The change in shape is a consequence of combination of compressive strain and plastic flow developed due to thermal spike generated along ion track. Post-SHI irradiation annealing shows increase in size of voids and reversal of shape from prolate spheroid towards spherical through strain relaxation. The stability of voids was studied with the effect of post-growth annealing.

Manju Bala

Papers

An experimental setup for the simultaneous measurement of thermoelectric power of two samples from 77 K to 500 K.

Enhancement of thermoelectric power of PbTe:Ag nanocomposite thin films

Phase evolution and electrical properties of Co–Sb alloys fabricated from Co/Sb bilayers by thermal annealing and ion beam mixing

Synthesis and characterization of Au–Fe alloy nanoparticles embedded in a silica matrix by atom beam sputtering

Effect of ion beam parameters on engineering of nanoscale voids and their stability under post-growth annealing