CoSb3 Thin Films Research Articles

Electronic and magnetic properties of CoSb3, Cr-doped CoSb3, and related compound thin films

The electronic, lattice, and magnetic properties of CoSb3, Cr-doped CoSb3, and related compound thin films were calculated by using the total energy pseudopotential method. A vacuum region and slab as a thin film were periodically repeated in the supercell. We considered three slab structures. One is symmetric, another is asymmetric, and the third is unusual. They are free-standing in the supercell. The magnetic moment of doped Cr atom is large in all Cr-doped cases. Magnetic moments of several Co atoms are enhanced in the asymmetric and unusual cases. The calculation results in this study indicate that electronic states of most thin films are ferromagnetic and they are energetically more favorable than nonmagnetic cases.

Enhanced Thermoelectric Performance of CoSb3 Thin Films by Ag and Ti Co-Doping.

The Skutterudites CoSb3 material has been the focus of research for the conversion applications of waste heat to electricity due to its ability to accommodate a large variety of ions in the cages that have been proven effective in improving the thermoelectric performance. Although the co-doped CoSb3 bulk materials have attracted increasing attention and have been widely studied, co-doped CoSb3 thin films have been rarely reported. In this work, Ag and Ti were co-doped into CoSb3 thin films via a facile in situ growth method, and the influence of doping content in the thermoelectric properties was investigated. The results show that all the Ag and Ti co-doped CoSb3 thin films contain a pure well-crystallized CoSb3 phase. Compared to the un-doped thin film, the co-doped samples show simultaneous increase in the Seebeck coefficient and the electrical conductivity, leading to a distinctly enhanced power factor. The high power factor value can reach ~0.31 mWm-1K-2 at 623 K after appropriate co-doping, which is two times the value of the un-doped thin film we have been obtained. All the results show that the co-doping is efficient in optimizing the performance of the CoSb3 thin films; the key point is to control the doping element content so as to obtain high thermoelectric properties.

Structural and electrical transport properties of Ge implanted CoSb3 thin films and their conduction mechanisms

Structural and electrical transport properties of Ge implanted CoSb3 thin films and their conduction mechanisms

Enhancing thermoelectric properties of p-type CoSb3 skutterudite by Fe doping

In this report, we have successfully synthesized thin films of Co1-xFexSb3 (0 ≤ x ≤ 0.1) using electrochemical deposition technique. The electrical and thermoelectric properties of CoSb3 thin films have been investigated by varying the concentration of Fe. X-ray diffraction analysis confirms formation of single phase of Co1-xFexSb3 (0 ≤ x ≤ 0.05) with increase in lattice parameter indicating incorporation of Fe at the Co site. The X-ray photoelectron spectroscopy study reveals that Fe2+ substitutes Co3+ thus generating excess hole carriers, further confirmed by the positive signs of Seebeck and Hall coefficients. It is noted that the thermoelectric properties are mainly controlled by Fe substitution in terms of increasing carrier effective mass as well as phonon scattering. Fe substitution does not significantly alter the carrier mobility and thus confining the transport properties to be mainly leveraged by carrier concentration. Scanning thermal microscopy is used to study the effect of Fe incorporation on the thermal conductivity of nanostructured Co1-xFexSb3 thin films. Enhanced value of power factor and relatively lower value of average thermal conductivity for Co1-xFexSb3 sample is achieved at an optimized value of 5% Fe, as a result of enhanced electron transport and phonon scattering. Furthermore, the improved thermoelectric performance makes this natural-abundant, non-toxic and cheap CoSb3:Fe material a very promising candidate for low cost thermoelectric energy generation.

Rational band engineering and structural manipulations inducing high thermoelectric performance in n-type CoSb3 thin films

Owing to the earth-abundancy, eco-friendliness and high thermoelectric performance, CoSb3 skutterudites have been employed in thermoelectric devices with a high energy conversion efficiency. However, the thermoelectric performance of CoSb3-based thin films is still relatively low within the medium temperature range. In this work, we report a record high ZT of ~0.65 at 623 K in the n-type Ag/In co-doped CoSb3 thin films, fabricated by a facile magnetron sputtering technique. Extensive characterizations and computational results indicate both Ag and In as fillers prefer to occupy the interstitial sites in the CoSb3 lattice. A 0.2% Ag doping induces impurity states in the band structure of CoSb3, boosts the density-of-states near the Fermi level and enhances the absolute Seebeck coefficient up to ~198 μV K−1. Simultaneously, a 4.2% In doping further tunes the bandgap, increases the electrical conductivity up to ~75 S cm−1, and contributes to an optimized power factor of ~2.94 μW cm−1 K−2 at 623 K. In addition, these interstitial dopants accompanying with dense grain boundaries contribute an ultra-low thermal conductivity of ~0.28 W m−1 K−1 at 623 K, leading to a high ZT in the film system. This work demonstrates that rational band engineering and structural manipulations can achieve high performance in n-type CoSb3-based thin films, which possess full potential for applying to miniature thermoelectric devices.

Drastic power factor improvement by Te doping of rare earth-free CoSb3-skutterudite thin films.

In the present study, we have focused on the elaboration of control of Te-doped CoSb3 thin films by RF magnetron sputtering which is an attractive technique for industrial development of thermoelectric (TE) thin films. We have successfully synthesized sputtering targets with a reliable approach in order to obtain high-quality films with controlled stoichiometry. TE properties were then probed and revealed a reliable n-type behavior characterized by poor electrical transport properties. Tellurium substitution was realized by co-sputtering deposition and allowed obtaining a significant enhancement of the power factor with promising values of PF ≈ 0.21 mW m−1 K−2 near room temperature. It is related to the Te doping effect which leads to an increase of the Seebeck coefficient and the electrical conductivity simultaneously. However, despite this large improvement, the properties remained far from the bulk material and further developments are necessary to improve the carrier mobility reduced by the thin film formatting.

Nano structure Ti-doped skutterudite CoSb3 thin films through layer inter-diffusion for enhanced thermoelectric properties

Ti-doped CoSb3 thin films were prepared through layer inter-diffusion method and the prepared thin films have unique architecture, containing nano-size grains. According to the Raman analysis, the films have weaker atomic vibration after Ti doping. This nano-structure and the weak atomic vibration can scatter a wide-range spectrum of phonons, resulting the significantly decrease of thermal conductivity. In addition, this material design also leads to a very important improvement of the absolute Seeback coefficient value, which increases from about 50 μVK−1 to over 100 μVK-1. As the result, the maximum ZT value is 0.86 at 523 K for a Ti doped sample, which is six times higher than the un-doped sample and is comparable to the best value for the CoSb3 based thin films. A flexible CoSb3 based thin film thermoelectric generator was also fabricated and shows that the response time of all the touch events is below 1 s.

Effects of Ag-doped content on the microstructure and thermoelectric properties of CoSb3 thin films

In this work, Ag doped CoSb3 thin films were grown directly on the heated substrate by the simple and effective magnetron sputtering process. The micro-structure and thermoelectric properties of the doped thin films are found to strongly depend on the Ag doping content. High-resolution transmission electron microscope image and corresponding selected-area electron diffraction pattern confirm that the doped thin film has single CoSb3 crystal structure with well-crystallized. The high crystal quality of the doped thin film will lead to good thermoelectric properties. As expected, both the electrical conductivity and Seebeck coefficient are greatly enhanced after Ag doping, resulting in the enhancement of power factor, suggesting that the proposed strategy can be applied to prepare high-performance thermoelectric thin films.

Effect of heating cycle on cobalt-antimonide-based thin films for high-temperature thermoelectric energy conversion applications

In this study, a comparative analysis of the structural and thermoelectric properties of all possible Co-Sb-based phases in thin-film form for high-temperature applications is presented and discussed. A series of thin-film samples were prepared via radiofrequency co-sputtering on an oxidized silicon substrate. For improved crystallinity, the thin-film samples were subjected to heat treatment in the form of a high-temperature heating cycle where the temperature was slowly ramped up to about 760 K. The films were characterized using X-ray diffraction for phase identification and crystalline quality. Energy-dispersive spectroscopy was performed to determine the stoichiometric composition and homogeneity, whereas the surface morphology and microstructure were examined using scanning electron microscopy and atomic force microscopy. To investigate film performance at high temperatures, the electrical resistivity and Seebeck coefficient were measured at regular temperature intervals, and results were correlated with identified phases, composition, and crystalline quality. The CoSb phase was observed to have metallic features, the CoSb2 phase had doped semiconductor characteristics, whereas the CoSb3-phase thin films showed typical semiconductor behavior. The CoSb2 and CoSb3 samples shows a transition of the Seebeck coefficient from positive to negative at a temperature range of 500–550 K and 550–600 K respectively. The thermoelectric power factor was evaluated from the measured parameters and maximum values of 3.2, 7.8, and 3.8 mWm−1·K−2 for the CoSb, CoSb2, and CoSb3 thin films, respectively, are reported. We observed a wide range of structural and thermoelectric properties in the series of samples, demonstrating a route to tailoring and improving properties for maximal performance.

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.

Structure and Transport Properties of Nickel-Implanted CoSb3 Skutterudite Thin Films Synthesized via Pulsed Laser Deposition

The present study reports the effect of Ni ion implantation on the structural, compositional, electrical, and thermoelectric properties and electronic structures of CoSb3 skutterudite thin films deposited on Si substrate by pulsed laser deposition. Ni ions were implanted at 200 keV in CoSb3 thin films at three different fluences: 3 × 1015, 6 × 1015, and 1.5 × 1016 ions/cm2. X-ray diffraction of Ni-implanted films shows an additional phase of Co0.75Ni0.25Sb3. The electrical measurement of pristine films exhibits typical semiconductor behavior, while the Ni-implanted films show an abrupt increase in resistivity, which may be attributed to the formation of Co0.75Ni0.25Sb3 and material modification by the energetic ion beam. The Seebeck coefficient measurements imply that all the films are n-type with maximum Seebeck coefficient of ∼75 μV/K at ∼410 K for 2% Ni-implanted film which is about 7 times higher than the pristine CoSb3 film. The X-ray absorption study of Ni, Co, and Sb spectrum confirms that Ni ions ...

An enhanced power factor via multilayer growth of Ag-doped skutterudite CoSb3 thin films

The Seebeck coefficient and electrical conductivity of the CoSb3 thin films are enhanced after Ag doping, leading to substantial enhancement of the power factor.

Improvement of Power Factor of CoSb3 Thermoelectric Thin Films via Microstructure Optimization

Skutterudite CoSb3 has emerged as one of the most studied candidate materials for thermoelectric applications. In this work, CoSb3 thin films were prepared by radio frequency sputtering, and their microstructure was investigated with emphasis on the effect of target composition and deposition temperature. The goal was to enhance the thermoelectric properties of CoSb3 thin films via microstructure optimization. Results showed that the Sb content of films gradually decreased with increasing deposition temperature. Although the thin films prepared by the target with a Co and Sb element ratio of 1:3.5 approached the ideal stoichiometric ratio, they showed poor thermoelectric properties due to the formation of an additional Sb phase. By contrast, the thin films obtained with insufficient Sb showed a single CoSb3 phase and good thermoelectric properties. The mechanism behind this difference was studied accordingly. The power factor of the thin films was enhanced due to their dense structure and good crystallization.

Thermoelectric properties and micro-structure characteristics of nano-sized CoSb3 thin films prefabricating by co-sputtering

Skutterudite CoSb3 material exhibits excellent thermoelectric properties for thermoelectric application. In this work, CoSb3 thin films were fabricated by direct current magnetron co-sputtering. The composition of thin films was controlled by regulating the deposition power of both Co and Sb. Then the as-deposited Sb-excess sample was chosen to process post-annealing. High-resolution transmission electron microscopy and X-ray diffraction, together with Raman analysis, demonstrate that the annealed samples have well-crystallized and nano-sized skutterudite CoSb3 structure. Hall measurement shows that specimens are P-type semiconductor. Both of the increasing of Seebeck coefficient and electric conductivity can be obtained when the samples annealed at appropriate temperature, indicating that thermally can improve its thermoelectric properties due to well-crystallized and nano-sized structure.

Thermoelectric properties of co-sputtered CoSb3 thin films as a function of stoichiometry

The skutterudite compound CoSb3 exhibits excellent thermoelectric properties for thermoelectric application. The composition plays an important role in affecting the microstructural and thermoelectric behavior of CoSb3, especially in the form of thin film. In this study, the Co-excess and Sb-excess CoSb3 thin films were prepared by radio frequency and direct magnetron co-sputtering technique. It was found that the thin film with excess Co exhibits a significant n-type conduction behavior and transforms to p-type conduction behavior with Sb rich. The X-ray diffraction patterns show that either the Co-excess or Sb-excess samples contain skutterudite CoSb3 and a secondary phase CoSb2. Additionally, Sb can be found in the Sb-excess samples, yet no Co can be observed in the Co-excess samples.

Effect of Sb content on the thermoelectric properties of annealed CoSb3 thin films deposited via RF co-sputtering

A series of CoSb3 thin films with Sb contents in the range 70–79at.% were deposited at room temperature via RF co-sputtering. The thin films were amorphous in the as-deposited state and annealed at 300°C for 3h to obtain crystalline samples. The annealed thin films were characterized using scanning electron microscopy and X-ray diffraction (XRD), and these data indicate that the films exhibited good crystallinity. The XRD patterns indicate single-phase CoSb3 thin films in the Sb-rich samples. For the Sb-deficient samples, however, mixed-phase thin films consisting of CoSb2 and CoSb3 components were obtained. The electrical and thermoelectric properties were measured at temperatures up to 760K and found to be highly sensitive to the phases that were present. We observed a change in the thermoelectric properties of the films from p-type at low temperatures to n-type at high temperatures, which indicates potential applications as n-type thermoelectric thin films. A large Seebeck coefficient and power factor was obtained for the single-phase CoSb3 thin films. The CoSb2 phase thin films were also found to possess a significant Seebeck coefficient, which coupled with the much smaller electrical resistivity, provided a larger power factor than the single-phase CoSb3 thin films. We report maximum power factor of 7.92mW/mK2 for the CoSb2-containing mixed phase thin film and 1.26mW/mK2 for the stoichiometric CoSb3 thin film.

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

Skutterudites are emerging as potential candidates that show high efficiency and thus provide an ideal platform for research. The properties of nanostructured films of skutterudites are different from those of the corresponding bulk. The present study reports the evolution of nanostructured single-phase CoSb3 fabricated by using low-energy ion irradiation of Co/Sb bilayer films and subsequent annealing at an optimized temperature and their Seebeck coefficients (S). The effects of ion beam parameters with annealing on the phase evolution and nanostructure modifications were studied. An increase in Xe+ ion fluence resulted in complete mixing of Co/Sb on postannealing forming flower-like nanostructures of single phase CoSb3. The temperature-dependent electrical resistivity (ρ) increases with the ion fluence because of defect creation which further increases on postannealing due to surface nanostructuring. The S of these films of CoSb3 is found to be higher and this is attributed to the formation of a uniform layer of nanostructured CoSb3 alloy thin film. The S and Hall coefficients of all these films are negative implying that they are n-type semiconductors.

Thermoelectric properties of skutterudite CoSb3 thin films

The thermoelectric properties of Co-Sb thin films with different Sb content and with a thickness of 30 nm were investigated with respect to the composition and the corresponding structural properties of these films. The films were prepared by molecular beam deposition either by codeposition on heated substrates or room temperature deposition followed by a post-annealing step. It was found that the prepared films exhibit bipolar conduction, indicated by a positive Hall constant and a negative Seebeck coefficient. The obtained results can be well explained by using the bipolar model, assuming heavy electrons and light holes, which was finally confirmed experimentally by the preparation of p- and n-type doped CoSb3 thin films. Furthermore, variable range hopping was identified by temperature dependent transport measurements as dominant conduction mechanism at low temperatures.

Thermoelectric Characterization of Ti and In Double-Doped Cobalt Antimony Thin Films

CoSb3 thermoelectric thin films were prepared on polyimide flexible substrate by radio frequency (RF) magnetron sputtering technology using a cobalt antimony alloy target. Ti and In were added into CoSb3 thin films by co-sputtering. The influence of Ti and In on the thermoelectric properties of CoSb3 thin films was investigated. X-ray diffraction result shows that the major diffraction peaks of all the thin films match the standard peaks related to the CoSb3 phase. The sample has best thermoelectric properties when the Ti sputtering time was 1min and In sputtering time was 30 seconds.

From thermoelectric bulk to nanomaterials: Current progress for Bi2Te3 and CoSb3

Bi2Te3 and CoSb3 based nanomaterials were synthesized and their thermoelectric, structural, and vibrational properties analyzed to assess and reduce ZT‐limiting mechanisms. The same preparation and/or characterization methods were applied in the different materials systems. Single‐crystalline, ternary p‐type Bi15Sb29Te56, and n‐type Bi38Te55Se7 nanowires with power factors comparable to nanostructured bulk materials were prepared by potential‐pulsed electrochemical deposition in a nanostructured Al2O3 matrix. p‐type Sb2Te3, n‐type Bi2Te3, and n‐type CoSb3 thin films were grown at room temperature using molecular beam epitaxy and were subsequently annealed at elevated temperatures. This yielded polycrystalline, single phase thin films with optimized charge carrier densities. In CoSb3 thin films the speed of sound could be reduced by filling the cage structure with Yb and alloying with Fe yielded p‐type material. Bi2(Te0.91Se0.09)3/SiC and (Bi0.26Sb0.74)2Te3/SiC nanocomposites with low thermal conductivities and ZT values larger than 1 were prepared by spark plasma sintering. Nanostructure, texture, chemical composition, as well as electronic and phononic excitations were investigated by X‐ray diffraction, nuclear resonance scattering, inelastic neutron scattering, Mössbauer spectroscopy, and transmission electron microscopy. For Bi2Te3 materials, ab‐initio calculations together with equilibrium and non‐equilibrium molecular dynamics simulations for point defects yielded their formation energies and their effect on lattice thermal conductivity, respectively.Current advances in thermoelectric Bi2Te3 and CoSb3 based nanomaterials are summarized. Advanced synthesis and characterization methods and theoretical modeling were combined to assess and reduce ZT‐limiting mechanisms in these materials.