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

Effects of Heavy Ion Irradiation on the Thermoelectric Properties of In2(Te1-xSex)3 Thin Films

Ion irradiation is an exceptionally effective approach to induce controlled surface modification/defects in semiconducting thin films. In this investigation, ion-irradiated Se-Te-based compounds exhibit electrical transport properties that greatly favor the transformation of waste heat into electricity. Enhancements of both the Seebeck coefficient (S) and the power factor (PF) of In₂(Te_0.98Se_0.02)₃ films under 120 MeV Ni⁹⁺ ion irradiation were examined. The maximum S value of the pristine film was about ~221 µVK^-1. A significantly higher S value of about ~427 µVK^-1 was obtained following irradiation at 1 × 10¹³ ions/cm². The observed S values suggest the n-type conductivity of these films, in agreement with Hall measurements. Additionally, Ni ion irradiation increased the PF from ~1.23 to 4.91 µW/K²m, demonstrating that the irradiated films outperformed the pristine samples. This enhancement in the TE performance of the In₂(Te_0.98Se_0.02)₃ system is elucidated by irradiation-induced effects that are revealed by structural and morphological studies.

A setup for Seebeck coefficient measurement through controlled heat pulses

A setup is designed for measuring the Seebeck coefficient (S) of materials in the form of thin films, bars, and wires. The main feature of this setup is its control in heating and cooling cycles. In this setup, a heat pulse is used to generate the temperature gradient. To demonstrate the capabilities of this setup, S versus T of standard wire samples such as Au-Fe (0.07%), chromel, Pt, and thin films of Pt and F doped SnO₂ are presented. The standard uncertainty of the repeatability in the S measurement is found to be ∼±0.056 μV/K while the temperature stability is ∼±10 mK (at 320 K), estimated for a chromel wire sample. We have tested the setup in the temperature range 100 K-320 K, while it does not have any intrinsic limitations in going down to liquid He temperatures. For temperatures above 320 K, the limitation is due to gluing materials such as varnish.

Tuning the Electrical and Thermoelectric Properties of N Ion Implanted SrTiO3 Thin Films and Their Conduction Mechanisms

The SrTiO₃ thin films were fabricated by pulsed laser deposition. Subsequently ion implantation with 60 keV N ions at two different fluences 1 × 10¹⁶ and 5 × 10¹⁶ ions/cm² and followed by annealing was carried out. Thin films were then characterized for electronic structure, morphology and transport properties. X-ray absorption spectroscopy reveals the local distortion of TiO₆ octahedra and introduction of oxygen vacancies due to N implantation. The electrical and thermoelectric properties of these films were measured as a function of temperature to understand the conduction and scattering mechanisms. It is observed that the electrical conductivity and Seebeck coefficient (S) of these films are significantly enhanced for higher N ion fluence. The temperature dependent electrical resistivity has been analysed in the temperature range of 80–400 K, using various conduction mechanisms and fitted with band conduction, near neighbour hopping (NNH) and variable range hopping (VRH) models. It is revealed that the band conduction mechanism dominates at high temperature regime and in low temperature regime, there is a crossover between NNH and VRH. The S has been analysed using the relaxation time approximation model and dispersive transport mechanism in the temperature range of 300–400 K. Due to improvement in electrical conductivity and thermopower, the power factor is enhanced to 15 µWm⁻¹ K⁻² at 400 K at the higher ion fluence which is in the order of ten times higher as compared to the pristine films. This study suggests that ion beam can be used as an effective technique to selectively alter the electrical transport properties of oxide thermoelectric materials.

A load-based thermopower measurement setup in the temperature range of 5-330 K

This is a report on the development of an automated precision load-based measurement setup for thermoelectric power (S) of different types of samples in the temperature range of 5-330 K. The problems in the old spring-based setup have been solved in this load-based setup. This setup takes nearly 4 h for each run, and the typical error is within 5%. High quality calibration has been demonstrated using high purity platinum wires and cylinders.

Effect of Fe ion implantation on the thermoelectric properties and electronic structures of CoSb3 thin films

In the present study, thin films of single-phase CoSb₃ were deposited onto Si(100) substrates via pulsed laser deposition (PLD) method using a polycrystalline target of CoSb₃. These films were implanted by 120 keV Fe-ions with three different fluences: 1 × 10¹⁵, 2.5 × 10¹⁵ and 5 × 10¹⁵ ions per cm². All films were characterised by X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), Rutherford backscattering (RBS) spectrometry and X-ray absorption spectroscopy (XAS). XRD data revealed that the ion implantation decreased the crystalline nature of these films, which are recovered after the rapid thermal annealing process. The Seebeck coefficient S vary with the fluences in the temperature range of 300 K to 420 K, and is found to be highest (i.e., 254 μV K⁻¹) at 420 K for the film implanted with 1 × 10¹⁵ ions per cm². The high S and low resistivity lead to the highest power factor for the film implanted with 1 × 10¹⁵ ions per cm² (i.e., 700 μW m⁻¹ K⁻²) at 420 K. The changing of the sign of S from negative for the pristine film to positive for the Fe-implanted samples confirm that the Fe ions are electrically active and act as electron acceptors by replacing the Co atoms. XAS measurements confirm that the Fe ions occupied the Co site in the cubic frame of the skutterudite and exist in the 3+ oxidation state in this structure.

The power factor for the Fe ion-implanted samples is greater than that of the pristine sample with a value of 700 mW m⁻¹ K⁻² at 420 K for the I_1E15A sample.

Tuning of the Thermoelectric Properties of Bi2Te3 Nanorods Using Helium Ion Irradiation

The present study reports an enhancement of the power factor of Bi2Te3 nanorods NRs) by helium (He+) ion irradiation. High-resolution transmission electron microscopy studies revealed the formation of amorphous layers on the surface of the NRs at the high ion fluence. This amorphous nature is due to the accumulation of migrating point defect clusters at the surface of the NRs. Raman scattering experiments provide further insight to the observed structural modifications. At higher ion fluence, impurity-dominated scattering processes significantly enhance the value of the Seebeck coefficient of Bi2Te3 NRs. The He+ ion irradiation up to the ion fluence of 1 × 1016 ions/cm2 improves the thermoelectric transport properties with the highest power factor, 8.2 μW/m K2, at 390 K. Further investigations may result in the possibility of fabricating the Bi2Te3 NRs as thermoelectric generators with a high power factor for space applications.

Contributed Review: Instruments for measuring Seebeck coefficient of thin film thermoelectric materials: A mini-review

Thin film thermoelectric materials (TF TEMs) based on organic semiconductors or organic/inorganic composites exhibit unique properties such as low-temperature processability, mechanical flexibility, great freedom of material design, etc. Thus they have attracted a growing research interest. Similar to inorganic bulk thermoelectric materials (IB TEMs), the Seebeck coefficient combined with electrical conductivity and thermal conductivity is a fundamental property to influence the performance of TF TEMs. However, due to the differences in material and sample geometries, the well-established characterization devices for IB TEMs are no longer applicable to TF TEMs. And until now, a universal standard of measuring the Seebeck coefficient of TF TEMs is still lacking. This mini-review presents the development of instruments designed for measuring the Seebeck coefficient of TF TEMs in the last decade. Primary measurement methods and typical apparatus designs will be reviewed, followed by an error analysis induced by instrumentation. Hopefully this mini-review will facilitate better designs for a more accurate characterization of the Seebeck coefficient of thin film thermoelectric materials.

Atomic layer deposition of nickel-cobalt spinel thin films

We report the atomic layer deposition (ALD) of high-quality crystalline thin films of the spinel-oxide system (Co_1-xNi_x)₃O₄. These spinel oxides are ferrimagnetic p-type semiconductors, and promising material candidates for several applications ranging from photovoltaics and spintronics to thermoelectrics. The spinel phase is obtained for Ni contents exceeding the x = 0.33 limit for bulk samples. It is observed that the electrical resistivity decreases continuously with x while the magnetic moment increases up to x = 0.5. This is in contrast to bulk samples where a decrease of resistivity is not observed for x > 0.33 due to the formation of a rock-salt phase. From UV-VIS-NIR absorption measurements, a change from distinct absorption edges for the parent oxide Co₃O₄ to a continuous absorption band ranging deep into the near infrared for 0 < x ≤ 0.5 was observed. The conformal deposition of dense films on high-aspect-ratio patterns is demonstrated.

Morphological investigations on the growth of defect-rich Bi2Te3 nanorods and their thermoelectric properties

Bi₂Te₃ nanorods (NRs) have been successfully synthesized at different reaction temperatures via a surfactant-assisted hydrothermal method.

Enhancement of thermoelectrical performance in Au-ion implanted V2O5 thin films

The present study reports an enhancement of thermoelectric performance in Au ion implanted V₂O₅ thin films.

Evolution of nanostructured single-phase CoSb3 thin films by low-energy ion beam induced mixing and their thermoelectric performance

We report that a nanostructured CoSb₃ thin film in a single phase can be synthesized by ion beam processing of Co/Sb bilayer thin films with better thermoelectric properties.

Anomalous thickness-dependent optical energy gap of ALD-grown ultra-thin CuO films

Usually an inverse square relation between the optical energy gap and the size of crystallites is observed for semiconducting materials due to the strong quantum localization effect. Coulomb attraction that may lead to a proportional dependence is often ignored or considered less important to the optical energy gap when the crystallite size or the thickness of a thin film changes. Here we report a proportional dependence between the optical energy gap and the thickness of ALD-grown CuO thin films due to a strong Coulomb attraction. The ultrathin films deposited in the thickness range of 9-81 nm show a p-type semiconducting behavior when analyzed by Seebeck coefficient and electrical resistivity measurements. The indirect optical energy gap nature of the films is verified from UV-vis spectrophotometric measurements. A progressive increase in the indirect optical energy gap from 1.06 to 1.24 eV is observed with the increase in the thickness of the films. The data are analyzed in the presence of Coulomb attractions using the Brus model. The optical energy gap when plotted against the cubic root of the thickness of the films shows a linear dependence.

A Simple and Portable Setup for Thermopower Measurements

Thermoelectric energy conversion technology has received significant attention because of its promising applications and environmentally clean nature. The innovative design of efficient thermoelectric materials is assisted by simple and reliable techniques for fast and accurate testing. Here, a simple approach for rapid measurement of the Seebeck coefficient is described using commonly available materials based on hot and cold probes; both probes are heated by built-in microheaters. A spring-loaded sample mounting arrangement provides easy sample loading/unloading. The setup is suitable for measurements on a wide range of materials, such as pellets, films, and even soft surfaces without damage. Several known thermoelectric materials, such as alumel, bismuth, and silicon, yielded values close to reported ones. The setup is very compact, simple, fast, low cost, and reliable to develop as a laboratory characterization tool.

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

Thermoelectric power enhancement of ion beam synthesized Co–Sb alloy thin films.

Enhancement of thermoelectric power of PbTe:Ag nanocomposite thin films

Formation of nanostructures results in the enhancement in thermoelectric power of PbTe:Ag thin films.