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MOCVD of Bi2Te3, Sb2Te3 and Their Superlattice Structures for Thin-Film Thermoelectric Applications
Abstract
The characteristics of metalorganic chemical vapor deposition (MOCVD) of Bi2Te3, Sb2Te3 and their superlattice structures are discussed in this paper. We have grown c-oriented films on both hexagonal sapphire and fcc GaAs substrates, with specular morphology and occasional stacking faults. Single crystallinity was confirmed by X-ray diffraction and low-energy electron diffraction (LEED). The stoichiometry (Bi:Te = 2:3, Sb:Te = 2:3) of the films were confirmed by X-ray photo-emission spectroscopy (XPS) and Rutherford back-scattering. We have also attempted to grow short-period (∼ 10 to 80 Å) superlattice structures in the Bi2Te3Sb2Te3 materials system. X-ray diffraction data indicating the quality of these layered structures is presented. The advantages offered by the nature of chemical bonding in these materials, along the growth direction, for obtaining abrupt interfaces is discussed. The electrical transport properties of the MOCVD-grown p-type Bi2Te3Sb2Te3 structures and other thermoelectric properties including thermal conductivity and Seebeck coefficient are discussed. The initial results on the performance parameter known as figure-of-merit of the superlattice structures, measured parallel to the plane of the superlattice interfaces, are significantly higher than in conventional bulk materials. These initial results suggest a significant potential for MOCVD-based materials technology for high-performance, thin-film, thermoelectric refrigeration.
... For metals, figure of merit ZT $ 10 À3 and for semiconductors, a maximum ZT of 2.4 is achieved in the Bi 2 Te 3 =Sb 2 Te 3 superlattice. 30 The device used to convert thermal energy into electrical energy is known as thermoelectric generator (TEG). A thermoelectric generator made up of conductors has very low conversion efficiency. ...
... As a result, quantum confinement and enhanced phonon scattering results in high ZT. Bi 2 Te 3 =Sb 2 Te 3 superlattice fabricated by Metal Organic CVD gives ZT nearly 2:4 at room temperature with a period of nearly 6 nm: 30 Reduction of the total in-plane thermal conductivity in n-type Bi 2 Te 3 /Bi 2 ðSe 0:12 Te 0:88 Þ 3 superlattice (SL period 10 nm) has been observed as compared to homogeneous Bi 2 Te 3 : 254 High ZT can be obtained with a less superlattice period and high nano-dot areal density. 255 In the case of PbSeTe/PbTe QDSLs, (i) quantum confinement of electrons leads to large increment of power factor (ii) thermal conductivity is lowered due to phonon scattering because of Te nano-precipitates and superlattice interfaces. ...
Various techniques to enhance the performance of thermoelectric materials have been reviewed in an unified way. The influence of synthesis techniques, post-synthesis treatment, microstructure, nanostructure, doping, and interface on thermoelectric materials' transport properties has been discussed. The research ideas given by researchers are presented in tabular forms so that young researchers and engineers can find the potential research gaps and best practices in this field. Conclusions drawn from this review would give research directions to the new researchers working in thermoelectric materials.
... Two-dimensional superlattices can also have low thermal conductivities due to the increased interface scattering. Venkatasubramanian et al. 49 reported a ZT of B2.4 at room temperature with Bi 2 Te 3 /Sb 2 Te 3 superlattices. They obtained a cross-plane lattice thermal conductivity of 0.22 W/m-K, which is only 50% of that of a Bi 2-x Sb x Te 3 bulk alloy. ...
... Thus, most thin-film thermoelectric devices are manufactured with high-figure-of-merit inorganic semiconducting materials. 16,31,49,74 Superlattices that have superior thermoelectric properties in the cross-plane direction have also been used for thin-film thermoelectric devices. 75 On the basis of a new approach to increase the figure of merit by lowering the dimensionality of thermoelectric materials, 3 many studies have investigated 1D thermoelectric nanowires of silicon 17,19 or bismuth. ...
Surface area has a directly relationship with the efficiency of energy devices. Hierarchical nanostructuring has the potential to greatly increase surface area, and their electrical properties are favourable, not only to energy generation and storage, but also energy-consuming electronic circuits.
This book provides systematic coverage of how nanostructured materials can be applied to energy devices, with an emphasis on the process of generation to storage and consumption. The fundamentals (including properties, characterisation and synthesis) are clearly presented across the first chapters of the book, providing readers new to the field with a clear overview of this expanding topic. The detailed discussion of applications will be an inspiration to those already well-versed in the field.
The editors have more than a decade of experience in working on all aspects of energy generation and storage - in academia, national laboratories and industry. The book presents a balanced view from all sectors and is presented in a format accessible by postgraduate students and professional researchers alike.
... Though not as conformal as ALD, the MOCVD process typically lends itself to manufacturing applications due to the relatively high throughput achieved as the growth rate can be significantly faster than the ALD process. Moreover, MOCVD enables the ability to tailor thin-film compositions through alloying and complex superlattice structures [19,20] to mitigate strain that occurs during the volume changes associated with electrochemical alloying that limits cycle life [21]. ...
... Gas flow ratios of DipTe/TMBi and DipTe/TDSb were 4.4 and 2.2 for the Bi 2 Te 3 and Sb 2 Te 3 films, respectively. This pressure and gas ratios have been used for growing epitaxially registered Bi 2 Te 3 and Sb 2 Te 3 films on GaAs substrates used for thermoelectric applications described elsewhere [19,20]. An initial study varying the growth temperature at 250, 325 and 400 C for 1 h was performed to determine a suitable temperature for the Bi 2 Te 3 and Sb 2 Te 3 films on the Cr/Au coated Si (001) substrates. ...
Vapor deposition techniques are particularly attractive for lithium-ion battery materials, for both powder coatings as well as thin-film electrodes. To increase capacity, alloying anodes represent a new class of materials for energy-dense films. Herein, we use metal organic chemical vapor deposition to prepare bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) thin-film electrodes, materials which have not been studied extensively for lithium-ion batteries, and assess their lithium storage performance. A deposition process was developed to yield continuous thin films with thicknesses ranging from 0.5 to 1.4 μm on electrically conductive substrates. By varying the growth time of the deposition process, capacities up to ∼0.3 mA h cm⁻² were obtained with a growth time of 60 min. The Sb2Te3 thin-film electrode outperforms Bi2Te3 for similar deposition conditions, which is primarily attributed to the high performance of native Sb that exhibits high capacity and stable cycling. Although the final phase of these thin-film electrodes separates into a biphasic domain, it is evident that the starting compound Sb2Te3 is stable up to 50 cycles. Yet, since cracks are still observed from post-cycling scanning electron microscopy, it is evident that a proper balance between film thickness and cycling must be remediated.
... The thermoelectric properties of Bi x Sb 2 − x Te 3 can be improved by controlling the grain size, doping concentration, type of defects, preferred growth orientation, etc. 9,11,12,14 Adjusting the deposition parameters offers possibilities to tune both S and ρ in the films. 15 Numerous methods have been used for the preparation of nanostructured TE thin films such as flash evaporation, 16 molecular beam technique, 17 magnetron Co-sputtering, 18,19 metal organic chemical vapor deposition (MOCVD), 20,21 and pulsed laser deposition. 22,23 Enhanced TE performance in the p-type Bi 2 Te 3 -based nanocrystalline bulk material has been demonstrated by Poudel et al., with ZT ∼ 1.4 at 100°C. ...
... 22,23 Enhanced TE performance in the p-type Bi 2 Te 3 -based nanocrystalline bulk material has been demonstrated by Poudel et al., with ZT ∼ 1.4 at 100°C. 24 So far, the highest ZT ∼ 2.0 is reported for bilayered Bi 2 Te 3 /Sb 2 Te 3 thin films, 20 which suggests that Sb addition has a positive effect on the thermoelectric properties of Bi 2 Te 3 -based thin films. To date, various approaches have been widely proposed to synthesize binary Bi 2 Te 3 and Sb 2 Te 3 alloy thin films, but research on the synthesis of nanostructured Bi x Sb 2 − x Te 3 thin films with tunable microstructure and composition is still limited. ...
The present study focuses on the enhancement of the Seebeck coefficient (S) of BiSbTe alloy thin films on post-deposition annealing. It is demonstrated that thermal treatment leads to about twofold enhancement in the S of BiSbTe alloy thin films deposited using DC magnetron sputtering. Investigation of the enhanced thermoelectric properties has been done by studying their phase, compositional, and structural properties. The x-ray diffraction patterns show the presence of a mixed BixSb2 − xTe3 phase, which crystallizes in the Sb-rich phase on annealing. The surface morphology of the as-deposited samples exhibit spherical features that grow in the form of hexagonal rods on increasing the annealing temperature to 300 °C. However, on further increasing the annealing time to 3 h at 300 °C, distorted cubical microstructures were observed. The microstructures had a higher Sb/Bi ratio, implying that these structures were Sb rich. The thermoelectric properties of the nanostructured BixSb2 − xTe3 films were studied as a function of annealing temperature and time. An enhancement of about two orders of magnitude is observed both in the S and power factor for the samples annealed at 300 °C for 3 h. This enhancement is attributed to the energy filtering of charge carriers at the junction of the BixSb2 − xTe3 matrix and Sb-rich inclusions. These results indicate that annealing is an efficient way of tuning the growth of microstructures and the S of BixSb2 − xTe3 thin films.
... The great pullulation of Large Scale Integration was accompanied by development of nano-fabrication technologies, and it rebounded on a couple of subjects of physics researches on nano-constitutions beyond integrated circuits. Epitaxy had been appreciated as effective TE and thermal-insulating material in very early years [10][11][12][13][14][15][16] when dense boundaries in it are found to effectively scatter phonons. For example the well-known Pb-SeTe/PbTe quantum dots superlattice stood out as a PbTe-family material with = 0.62 W m −1 K −1 and ZT = 1.3 at room temperature as early as 2000. ...
The development of nano–manufacture technology in the twenty‐first century has paved the way for artificial nanostructure constructions like man–made superlattices, providing historical breakthroughs in thermal physics and thermoelectrics by the modulation of phonons. Still, high–performance thermal insulators haven't come into operation due to the arduousness, costing and unscalability of artificiality. Herein, intentional engineering on a so–called ‘natural superlattice’ with alternating PbSe– and Bi2Se3–layer crystal structure is brought forth to recreate the mechanism of artificial superlattices and boost phonon localization. The thermal conductivity notably shows a direction–specific reduction, leading to minimum approaching and enhanced anisotropy. The modification of the natural framework and its effects have been supported by various transport and structure studies. This work sets a generalizable example for natural layered material engineering that bridges between the inflexible, changeless but self–assembled natural layered compounds, and the highly efficient, delicately tailored but unscalable artificial superlattice complexes. The methodology promises new horizons for practicable thermal management.
... Nevertheless, metal organic chemical vapour deposition (MOCVD) is the most frequent approach for producing Bi 2 Te 3 /Sb 2 Te 3 superlattices, and it shows tremendous promise in the field of thin film creation because of the abundance of Te organometallic precursors and low breaking temperatures [529]. MOCVD preparation of Bi 2 Te 3 /Sb 2 Te 3 superlattices was first experimented by Venkatasubramanian et al supported by pyrolytic reaction of trimethylbismuth and diisopropyltelluride for the tpyrolytic reaction of trisdimethylaminoantimony and diisopropyltelluride for the growth of Sb 2 Te 3 [530]. Following that, by using diisopropyl telluride, trimethyl bismuth and triethyl antimony as precusors, Bi 2 Te 3 epitaxial films were deposited by CVD on GaAs substrate and their thermoelectric properties were studied [531]. ...
The continuous depletion of fossil fuels and the increasing demand for eco-friendly and sustainable energy sources have prompted researchers to look for alternative energy sources. The loss of thermal energy in heat engines (100-350 ºC), coal-based thermal plants (150-700 ºC), heated water pumping in the geothermal process (150-700 ºC), and burning of petrol in the automobiles (150-250 ºC) in form of untapped waste-heat can be directly and/or reversibly converted into usable electricity by means of charge carriers (electrons or holes) as moving fluids using thermoelectric (TE) technology, which works based on typical Seebeck effect. The enhancement in TE conversion efficiency has been a key challenge because of the coupled relation between thermal and electrical transport of charge carriers in a given material. In this review, we have deliberated the physical concepts governing the materials to device performance as well as key challenges for enhancing the TE performance. Moreover, the role of crystal structure in the form of chemical bonding, crystal symmetry, order-disorder and phase transition on charge carrier transport in the material has been explored. Further, this review has also emphasized some insights on various approaches employed recently to improve the TE performance, such as, (i). carrier engineering via band engineering, low dimensional effects, and energy filtering effects and (ii). Phonon engineering via doping/alloying, nano-structuring, embedding secondary phases in the matrix and microstructural engineering. Wehave also briefed the importance of magnetic elements on thermoelectric properties of the selected materials and spin Seebeck effect. Furthermore, the design and fabrication of TE modules and their major challenges are also discussed. As, thermoelectric figure of merit, zT does not have any theoretical limitation, an ideal high performance thermoelectric device should consist of low-cost, eco-friendly, efficient, n- or p-type materials that operate at wide- temperature range and similar coefficients of thermal expansion, suitable contact materials, less electrical/ thermal losses and constant source of thermal energy. Overall, this review provides the recent physical concepts adopted and fabrication procedures of TE materials and device so as to improve the fundamental understanding and to develop a promising TE device.
... The value of 'ZT' around (2 to 3) is good for practical applications. Highest value of 'ZT' has been achieved to 2.4 [3]. Figure of merit which is expressed as ZT = S 2 σT/(k L +k e ), where S is Seebeck coefficient, σ is electrical conductivity, T is absolute temperature and k L , k e are the thermal conductivity due to lattice and electronic contributions respectively. ...
There is an increasing demand for the development of new thermoelectric materials with high figure of merit. This is due to the fact that convectional energy generating methods causes many environmental problems. Thermoelectric materials have capacity to harvest waste energy as they convert directly any type of waste heat into electricity. Telluride based materials are known as best thermoelectric materials as they have high figure of merit. Present work is based on multilayered structure of bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) thin films. In this work thin film of single layered Bi2Te3, Sb2Te3, their bilayers (Bi2Te3-Sb2Te3), 5 layers and 10 layers have been deposited on clean glass substrate by e-beam evaporation method. X- Ray diffraction (XRD) has been done for the characterization of these thin films. In this paper, studies of electrical properties are also reported which have been carried out by Hall and I-V measurements while optical properties have been studied by employing by UV – Vis spectrometer. Thermoelectric characteristics have been measured by laboratory thermoelectric measurement setup.
... 14,15 Due to the high cost and low yield, the use of the MBE process in the industry is limited. Several other techniques are employed for synthesis, such as evaporation, [16][17][18] pulsed laser deposition (PLD), 19,20 chemical vapor deposition (CVD), 21,22 and sputtering. 23,24 Synthesis of thin films with topological surface states through cost-effective techniques is crucial for commercial ARTICLE scitation.org/journal/adv ...
Polycrystalline thin films of Bi 2 Te 3 , a well-known topological insulator (TI), grown by RF sputtering shows metallic-like transport for a wide range of temperatures, T = 50 K to T = 225 K. For T > 225 K, the sample shows activated transport.. The metallic-like behavior at low temperatures can be understood within a model of overlapping surface states of the TI nanocrystallites in the film, suggesting that TI thin films of polycrystalline nature may also stabilize topologically protected states.
... The QAHE has been observed in Cr-and V-doped (Bi,Sb) 2 Te 3 [24][25][26][27][28], completing the quantum Hall effect trio [24]. Thin films of undoped and doped Sb 2 Te 3 have been prepared on a number of substrates, such as Si(111) [29], c-plane sapphire [30,31], and GaAs(100) [32], in part in the context of thermoelectrics research [33,34]. The topological surface state in Sb 2 Te 3 bulk crystals as well as molecular beam epitaxy (MBE) grown thin films [29] has been predicted [7] and experimentally confirmed by ARPES [35]. ...
Chromium-doped Sb2Te3 is a magnetic topological insulator (MTI), which belongs to the (Sb,Bi)2(Se,Te)3 family. When doped with the transition metals V, Cr, and Mn this family displays long-range ferromagnetic order above liquid nitrogen temperature and is currently intensely explored for quantum device applications. Despite the large magnetic ordering temperature, the experimental observation of dissipationless electrical transport channels, i.e., the quantum anomalous Hall effect, is limited in these materials to temperatures below ≈2 K. Inhomogeneities in the MTI have been identified as a major concern, affecting the coupling between the Dirac states and the magnetic dopants. Nevertheless, details on the local magnetic order in these materials are not well understood. Here, we report the study of the magnetic correlations in thin films using a combination of muon spin relaxation (μSR), and magnetic soft x-ray spectroscopy and imaging. μSR provides two key quantities for understanding the microscopic magnetic behavior: The magnetic volume fraction, i.e., the percentage of the material that is ferromagnetically ordered, and the relaxation rate, which is sensitive to the magnetic static (≈μs) and dynamic disorder. By choosing different implantation depths for the muons, one can further discriminate between near-surface and bulk properties. No evidence for a surface enhancement of the magnetic ordering is observed, but, instead, we find evidence of small magnetically ordered clusters in a paramagnetic background, which are coupled. The significant magnetic field shift that is present in all samples indicates a percolation transition that proceeds through the formation and growth of magnetically ordered spin clusters. We further find that fluctuations are present even at low temperatures, and that there appears to be a transition between superparamagnetism and superferromagnetism.
... [20,50] The pristine Sb 2 Te 3 thin film sample investigated in the present study has properties comparable to those reported for Sb 2 Te 3 thin films. [8,18] As mentioned earlier, different approaches such as nanocomposite, doping, core-shell, and multilayer structure have been reported for improving the response of Bi 2 Te 3 , Sb 2 Te 3, and similar compounds. [18,57,90,91] However, the present methodology of using a multilayer structure seems to be more effective as it enhances the power factor along with a simultaneous reduction in thermal conductivity values. ...
Efficient thermoelectric (TE) conversion of waste heat to usable energy is a holy grail promising to address major societal issues related to energy crisis and global heat management. For these to be economical, synthesis of a solid‐state material with a high figure‐of‐merit (ZT) values is the key, with characterization methods quantifying TE and heat transport properties being indispensable for guiding the development of such materials. In the present study, a large enhancement of the TE power factor in Sb2Te3/MoS2 multilayer structures is reported. A new approach is used to simultaneously experimentally determine the values of in‐plane (kxy) and out‐of‐pane (kz) thermal conductivities for multilayer samples with characteristic layer thickness of few nanometres, essential for the quantification of the ZT, the key parameter for the TE material. Combining simultaneous enhancement in the value of in‐plane power factor (to (4.9 ± 0.4) × mWm⁻¹ K⁻²) and reduction of the in‐plane value of the thermal conductivity (to 0.7 ± 0.1 Wm⁻¹ K⁻¹) for Sb2Te3/MoS2 multilayer sample led to high values of ZT of 2.08 ± 0.37 at room temperature. The present study, therefore, sets the foundation for a new methodology of exploiting the properties of 2D/3D interfaces for the development of novel fully viable thermoelectric materials.
... They measured ZT of the superlattice structures, parallel to the plane of the superlattice interfaces and reported significantly higher ZT than conventional bulk materials. [18] Annealing process with a high aspect ratio for BiÀ Te nanostructures used to enhance the Seebeck coefficients. [19] A maximum ZT of 0.8 is optimized at 225 K for appropriately doped CsBi 4 Te 6 , which makes it an outstanding candidate for low-temperature TE applications. ...
The effect of nanoparticle size on thermoelectric properties of Bi2Te3 nanoparticles is theoretically analysed using a phonon scattering mechanism. The size‐dependent thermoelectric properties in Bi2Te3 nanoparticles give an opportunity to tune the nanoparticles′ size, which helps to optimize the figure of merit (ZT=S²σT/κ). The thermoelectric effect in Bi2Te3 nanoparticles is investigated using the phonon scattering effect. Reduction in nanoparticle size increases interface volume ratio, which increases the scattering of phonons with grain boundaries. An increase in phonon‐grain boundary scatterings decreases the thermal conductivity (κ) due to the shortening of the phonon mean free path. The Seebeck coefficient (S) and simultaneously ZT increase significantly because of a decrease in lattice thermal conductivity. The highest value of ZT=0.45 is obtained at temperature T=400 K for 150 nm Bi2Te3 nanoparticles, which has been enhanced up to ZT=0.92 by reducing the nanoparticles size up to 30 nm at the same temperatures. The results obtained from the present model are in good agreement with the experimental data and reflect that the thermoelectric properties are dominated by the phonon scattering mechanism and depend on the density of interfaces in the material. Numerical analysis of thermoelectric properties from the present investigation will help in designing efficient thermoelectric materials.
... Thermoelectric materials hold promise as a basis of solid-state devices for capturing waste heat from industrial processes and automotive engines. [1][2][3][4] Strategies for increasing thermoelectric efficiency are being actively researched, using experiments and computation, and they usually aim to maximize the power factor PF = σS 2 (where σ is the electrical conductivity and S the Seebeck coefficient) and/or to reduce the thermal conductivity κ. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Increasing PF has been a long-standing and challenging goal since σ and S have typically opposite dependence on the carrier concentration. [4] Several approaches to increase the PF have been proposed based on the idea of engineering the shape of electronic bands. ...
The Seebeck coefficient and electrical conductivity are two central quantities to be optimized simultaneously in designing thermoelectric materials, and they are determined by the dynamics of carrier scattering. Here a new regime is uncovered where the presence of multiple electron bands with different effective masses, crossing near the Fermi level, leads to strong energy‐dependent carrier lifetimes due to intrinsic electron–phonon scattering. In this anomalous regime, electrical conductivity decreases with carrier concentration, Seebeck coefficient reverses sign even at high doping, and power factor exhibits an unusual second peak. The origin and magnitude of this effect is explained using a general simplified model as well as first‐principles Boltzmann transport calculations in recently discovered half‐Heusler alloys. General design rules for using this paradigm to engineer enhanced performance in thermoelectric materials are identified. First principles calculations reveal that electron–phonon inter‐band scattering can cause anomalous reversal of the Seebeck effect and sharp increase in thermoelectric power factor of semiconductors, even at high doping concentrations. This opens a new design dimension for thermoelectric materials.
... The Te-Te distance (0.275 nm) between two quintuple layers in Sb 2 Te 3 is shorter than twice the theoretical vdW radius of Te (0.206 nm), suggesting a stronger interaction between adjacent Te layers compared to purely vdW bonded structures [40,41]. The material is a p-type semiconductor with a narrow direct gap of 180 meV [9] and it offers great potential for applications in thermoelectrics [42], optoelectronics [27,43] and phase change memory devices [44,45]. In addition, Sb 2 Te 3 is a topological insulator [46], which offers application in spintronics, including quantum computing [47]. ...
Two-dimensional layered materials have attracted a lot of attention as building block in memristive devices owing to their high downscaling potential, easy stacking due to van der Waals forces and mechanical flexibility. In this study, memristive switching is explored in vertical device structures based on layered Sb2Te3. For this, epitaxial 2D-like Sb2Te3 thin films with thicknesses of ∼20 nm were directly grown on conductive p-type Si (111) substrates by pulsed laser deposition. Analog programmability mimicking neuromorphic operation, stable multilevel retention and endurance performance with a memory window larger than one order of magnitude are achieved by utilizing Ag as electrode metal. However, Cu top electrodes lead to a memristive switching with generally smaller memory window and volatility of programmed states. Devices with both electrode metals offer forming-free operation and self-compliance. Structural and chemical characterization reveal a diffusion of Ag and Cu into the Sb2Te3. It is suggested that charge trapping is involved in the memristive switching mechanism. Overall, this work shows the high potential of thin layered Sb2Te3 for neuromorphic computing and offers a scalable method for integration into the existing Si platform.
... Therefore strategies have evolved to try to find a "phonon-glass electron-crystal", i.e. a material that conducts electricity like a single crystal but conducts heat like an amorphous solid. 4 In the past decade, research breakthroughs have seen lab-based thermoelectric materials punch through the ZT = 2 ceiling, however, these systems are normally based on compounds such as PbTe [5][6][7][8][9] or Bi 2 Te 3 10 which contain rare or toxic elements. Earth abundant alternatives have been emerging, including SnSe which has yielded the highest ZT measured thus far of 2.6, 11 however widespread adoption of SnSe in industrial modules is still a long way off. ...
... As the MFP of phonons is considerably larger than that of electrons, this has the potential to improve thermal resistance without affecting the electrical properties of the material. Methods of doing this include creating porous materials [30] or through the use of superlattices [31], composed of alternating layers of different thermoelectric materials. Isoelectronic substitution, the replacements of elements with other elements with similar electron configuration, can reduce phonon transport without affecting electron movement. ...
Micro thermoelectric generators (μTEGs) are solid-state devices that directly convert thermal power into electrical power through the Seebeck effect, a solid-state transduction mechanism. Through this effect, μTEGs can harvest power from temperature gradients available in their operating environment. They are capable of providing a robust and long-term power solution for remote sensing and internet of things (IoT) applications where there exists high servicing costs, harsh environments, or the need for long-term device operation. In particular, thin-film based μTEGs are desirable due to ease of process integration, high throughput, material quality, and reproducibility. However, thickness constraints inherent to thin-film processes limit their potential usage. In conventional TEGs, the thickness of the thermoelectric film itself determines the separation distance between the hot and cold terminals. A very thin thermoelectric film thus creates a thermal short. This reduces the temperature difference across the device, limiting power output. The focus of this dissertation is to remove this thermal limitation in thin-film generators. This is accomplished through a new μTEG design that decouples the height of the thermoelectric elements from the film thickness. Central to the implementation of the proposed design is the creation of thermoelectric (TE) films deposited over the sidewalls of high-aspect vertical columns. In this way, the height of the columns, and not the thickness of the TE film, sets the separation distance between the hot and cold ends of the thermocouples. In this thesis, performance of this new μTEG design is analyzed. Bi2Te3 and Sb2Te3 thermoelectric films compatible with the proposed design are developed and integrated into functional μTEGs. The impact of column material, thermocouple height, and fill factor on μTEG performance are also presented. The thermoelectric films used in this design are industry standard Bi2Te3 and Sb2Te3. The crystal structure of these films grown on vertical surfaces was found to differ significantly from that grown on standard planar substrates. Potential causes for this difference and impact on μTEG performance are investigated. Additionally, factors that impact Bi2Te3 and Sb2Te3 film growth are studied. These factors include surface topology, substrate material, deposition temperature, and the presence of a seed layer. Key components required for the successful fabrication of μTEGs utilizing high-aspect column designs were developed. These include the creation of thermally insulating high-aspect columns, contact formation between N & P thermoelectric elements, and die attachment. The fabricated μTEGs have thermocouple heights of 20 μm using 2 μm thick films and a fill factor of 17.5%. The measured power output of the fabricated generators is 4-5 μW/K2/cm2. These μTEGs use thermoelectric films grown over sidewall surfaces. The power factors of the sidewall films were 0.85 and 1.36 mW/K2m for the N and P type films, respectively. Sidewall film performance was poorer in comparison to N and P type thermoelectric films grown under similar conditions on planar surfaces. These planar films had power factors of 3.63 and 1.30 mW/K2m for the N and P type materials.
... The results show that, Bi 2 Te 3 @ZnO composite is highly stable photoanode during PEC water splitting activity in the KOH electrolyte. Bi 2 Te 3 thin films have been prepared by different techniques such as thermal evaporation [26], metal organic chemical vapor deposition (MOCVD) [27], pulsed laser deposition (PLD) [28], sputtering [29], electrochemical deposition [30], chemical bath deposition (CBD) [31], etc. Among these various techniques available for the preparation of Bi 2 Te 3 thin films, the electrodeposition is one of the significant techniques for synthesis of uniform nanocrystalline Bi 2 Te 3 thin film [1]. ...
In this research work, we have synthesized nanocrystalline Bi2Te3 thin films for photoelectrochemical (PEC) solar cell application by potentiostatic electrodeposition method on stainless steel substrates. Subsequently films were irradiated with electron beam irradiation (EBI) of energy 4.5 MeV at various doses of irradiation ranging from 10 to 50 kGy. The experimental results showed that synthesized Bi2Te3 thin films having polycrystalline nature and rhombohedral crystal structure with momentous morphological digression through various doses of irradiation. The electrochemical active surface area (ECSA) and photoelectrochemical (PEC) performance of Bi2Te3 thin films were improved well at irradiation dose of 40 kGy. The calculated values of photoconversion efficiency and fill factor for Bi2Te3 thin film were found to be 0.1837% and 0.5383, respectively. The values of photoconversion efficiency and fill factor of same electrode before irradiation have values 0.0987% and 0.3979, respectively. The enhancement in efficiency and fill factor due to EBI were found 53% and 74%, respectively. The enhancement in PEC properties is due to increased active surface area of the Bi2Te3 films after irradiation dose. The results showed that, EBI are capable to strengthen the quality of electrode for PEC solar cell application.
... Several superlattice structures have been synthesized, and their TE performances were reported. Metal organic chemical vapor deposition (MOCVD)-grown Bi 2 Te 3 /Sb 2 Te 3 superlattices were studied by Venkatasubramanian [57], which shows a high zT value of $2 at 300 K. They found that the superlattices have very low thermal conductivity, while the hole mobility is higher compared with the individual TE materials. ...
Over recent years, two-dimensional (2D) materials have gained enormous interest as high-performance thermoelectric (TE) materials for efficient and technoeconomic conversion of energy in small-packed power generators and cooling devices. TE devices generate electrical voltage by converting thermal energy across a TE material via the Seeback effect. The 2D materials, including graphene, graphene oxide (GO), transition metal dichalcogenides (TMDCs), phosphorene, MXene, boron nitride, and other layered materials, are sought to be potential candidates for promising TE devices. The quantum effect and scattering phenomenon in 2D materials are advantageous to design better TE materials with tailorable performance. The 2D materials have shown high TE figure of merit (zT value; a dimensionless parameter that defines the performance of the TE materials). The study of the quantum effect and thickness-dependent properties of low-dimensional materials led to significant progress in TE performance. Most of the 2D materials are still in the early stages of development for use as TE materials. Moreover, the requirement of facile synthesis methods of 2D materials and simplistic device integration process are the key challenges that require more rigorous investigations to move toward commercialization.
... Various methods have been used to fabricate antimony telluride thin films: thermal evaporation [6], electrochemical atomic layer epitaxy [7], sputtering [8], electrochemical method [9], flash evaporation [10], metalorganic chemical vapor deposition [11], and molecular-beam epitaxy [12]. ...
Sb2Te3 is an endpoint of the GeTe-Sb2Te3 quasi-binary tie-line that represents phase-change alloys widely used in optical and non-volatile phase-change memory devices. In the crystalline form it is also a prototypical topological insulator with a layered structure where covalently bonded quintuple layers are held together by weak van der Waals forces. One of the ways to fabricate a crystalline phase is solid-state crystallization of an amorphous film, whereby the three-dimensional (3D) structure relaxes to the two-dimensional (2D) crystalline phase. The mechanism of the 3D-2D transformation remains unclear. In this work, we performed a study of relaxation processes in thin Sb2Te3 films in both amorphous and crystalline phases. We found that both phases possess two kinds of relaxators (type I and type II), where the type I fragments are identical in the two phases, while the relaxation of type II fragments are shifted to lower temperature in the amorphous phases. The activation energies of the associated relaxation processes and the values of the Havriliak–Negami parameters were determined. The differences between the relaxation processes in the two phases are discussed. The obtained result will contribute to better understanding of the 3D-2D transformation during the crystallization of van der Waals solids.
... 1 Nanostructure engineering progress has further improved their performance, which has drawn much attention to thermoelectric devices. 2,3 Indeed, low dimensional materials are interesting due to efficiency enhancement by thermal conductivity reduction and their integration into microdevices. 4,5 Although nand p-type bismuth telluride nanowires (NWs) have been the subject of numerous studies, only few approaches to build TE devices based on NW have been reported in the literature. ...
This paper presents a study of the contact resistance between a metal M (M = Ni, Pt, and Au) and an array of n-type Bi2Te3−xSex thermoelectric nanowires deposited through the electrodeposition process in the alumina membrane. Contact resistances between different metals and thermoelectric nanowires have been tested and characterized after optimization of the mechanical thinning and polishing process of the top part of the membrane. A low areal contact resistance of 87 µΩ cm² obtained with Au as the contact electrode is very encouraging for the development of thermoelectric modules based on nanowires in their membranes.
... The efficiency of thermoelectrics is characterized by the figure of merit ZT = σS 2 T /κ, where σ is the electrical conductivity, S -the Seebeck coefficient, T -the absolute temperature, and κ -the thermal conductivity. Strategies for increasing the figure of merit are being actively researched, using experiments and computation, and they typically aim to maximize the power factor PF= σS 2 and/or to reduce the lattice thermal conductivity [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] . Increasing PF is challenging since σ and S have typically opposite dependence on the carrier concentration 4 . ...
The Seebeck coefficient and electrical conductivity are two critical quantities to optimize simultaneously in designing thermoelectric materials, and they are determined by the dynamics of carrier scattering. We uncover a new regime where the co-existence at the Fermi level of multiple bands with different effective masses leads to strongly energy-dependent carrier lifetimes due to intrinsic electron-phonon scattering. In this anomalous regime, electrical conductivity decreases with carrier concentration, Seebeck coefficient reverses sign even at high doping, and power factor exhibits an unusual second peak. We discuss the origin and magnitude of this effect using first-principles Boltzmann transport calculations and simplified models. We also identify general design rules for using this paradigm to engineer enhanced performance in thermoelectric materials.
... Synthesis and optimization of such bismuth chalcogenide thermoelectric materials in low dimensional form is essential due to their applications to the fabrication of integrated microscale thermoelectric generators and active Peltier cooling devices 15,16 . Several technologies for the preparation of Bi 2 Te 3 films, such as thermal evaporation [17][18][19][20][21] , sputtering 16,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] , pulse laser deposition (PLD) [38][39][40] , electrochemical deposition 41,42 , metal organic chemical vapor deposition (MOCVD) 43 and molecular beam epitaxy (MBE) 44 , have been investigated extensively. For practical device applications and easier integration with microsystems, high quality epitaxial films with sophisticated microstructure and composition control can be obtained using the expensive MOCVD and MBE deposition techniques. ...
N-type bismuth telluride (Bi2Te3) thin films were prepared on an aluminum nitride (AlN)-coated stainless steel foil substrate to obtain optimal thermoelectric performance. The thermal co-evaporation method was adopted so that we could vary the thin film composition, enabling us to investigate the relationship between the film composition, microstructure, crystal preferred orientation and thermoelectric properties. The influence of the substrate temperature was also investigated by synthesizing two sets of thin film samples; in one set the substrate was kept at room temperature (RT) while in the other set the substrate was maintained at a high temperature, of 300 °C, during deposition. The samples deposited at RT were amorphous in the as-deposited state and therefore were annealed at 280 °C to promote crystallization and phase development. The electrical resistivity and Seebeck coefficient were measured and the results were interpreted. Both the transport properties and crystal structure were observed to be strongly affected by non-stoichiometry and the choice of substrate temperature. We observed columnar microstructures with hexagonal grains and a multi-oriented crystal structure for the thin films deposited at high substrate temperatures, whereas highly (00 l) textured thin films with columns consisting of in-plane layers were fabricated from the stoichiometric annealed thin film samples originally synthesized at RT. Special emphasis was placed on examining the nature of tellurium (Te) atom based structural defects and their influence on thin film properties. We report maximum power factor (PF) of 1.35 mW/m K² for near-stoichiometric film deposited at high substrate temperature, which was the highest among all studied cases.
... Sb 2 Te 3 belongs to the class of layered van der Waals solids, where covalently bonded quintuple layers with the Te-Sb-Te-Sb-Te stacking sequence are held together by weak van der Waals interactions. Various methods have been used to fabricate antimony telluride thin films: thermal evaporation [6], electrochemical atomic layer epitaxy [7], sputtering [8], electrochemical method [9], flash evaporation [10], metalorganic chemical vapor deposition [11] and molecular-beam epitaxy [12]. ...
Sb 2 Te 3 is an end-point of the GeTe-Sb 2 Te 3 quasibinary tie-line that represents phase-change alloys widely used in optical and non-volatile phase-change memory devices. In the crystalline form it is also a prototypical topological insulator with a layered structure where covalently bonded quintuple layers are held together by weak van der Waals forces. One of the ways to fabricate a crystalline phase is solid-state crystallisation of an amorphous film, whereby the three-dimensional (3D) structure relaxes to the two-dimensional (2D) crystalline phase. The mechanism of the 3D-2D transformation remains unclear. In this work, we performed a study of relaxation processes in thin Sb 2 Te 3 films in both amorphous and crystalline phases. We found that both phases possess two kinds of relaxators (type I and type II), where the type I fragments are identical in the two phases, while the relaxation of type II fragments are shifted to lower temperature in the amorphous phases. The activation energies of the associated relaxation processes and the values of the Havriliak-Negami parameters were determined. The differences between the relaxation processes in the two phases are discussed. The obtained result will contribute to better understanding of the 3D-2D transformation during the crystallisation of van der Waals solids.
... The possibilities of increasing the thermoelectric figure of merit by obtaining low-dimensional structures [8,9] are being actively studied in recent years. The use of low-dimensional structures such as thin films [10,11], superlattices [10,12], whiskers [13], nanoscale structures [14,15], quantum wells [16,17], quantum wires [18,19,20], etc. leads to a decrease in the thermal conductivity of the lattice or to an increase in the density of electronic states [21]. However, there are significant technological difficulties in obtaining such materials. ...
The effect of hot pressing modes (pressing pressure and holding time under pressure) on the thermoelectric properties of n-type bismuth telluride Bi 2 Te 2,4 Se 0,6 doped with Hg 2 Cl 2 was investigated. Samples were obtained by powder metallurgy technology - the synthesis of a chemical compound followed by hot pressing. A change in the hot pressing modes does not significantly influence the value of the thermo-EMF and conductivity of the samples. A change in the hot pressing mode significantly influences on the value of thermal conductivity. Both the increase of pressing pressure and the increase of the holding time under pressure leads to a decrease in the thermal conductivity of the material. Thus, the thermoelectric figure of merit of bismuth telluride can be increased by increasing the pressing pressure, holding time under pressure, or both parameters simultaneously. The increase of the thermoelectric figure of merit was 15% in the investigated samples. As a result of the tests of thermoelectric generator batteries, it was found that the output power of the battery made from a material with a high figure of merit was 27 W at temperatures 70 °C on the cold side, 300 °C on the hot side. The output power of the battery which was made from the material with a lower figure of merit was 25 W at a similar temperature regime.
... VdW heterostructure versatility can be further expanded by intercalating atoms or molecules between the layers, or functionalizing the layer surfaces [35,36]. In particular, changing growth stoichiometry of QL materials yields natural superlattices with for instance intercalation of Bi 2 vdW layers in Bi 2 Se 3 stacks, or alternation of Bi 2 Te 3 and Sb 2 Te 3 layers with controllable periodicity [37][38][39]. Overall, this brings interesting potentials for scalable manufacturing and integration with the current CMOS technology [40]. In sum, state of the art nanofabrication now yields heterostructures, i.e., composite vdW materials with atomic precision on the out-of-plane dimension, which can be processed as scalable and in situ tunable devices. ...
Coherent phonons can be launched in materials upon localized pulsed optical excitation, and be subsequently followed in time-domain, with a sub-picosecond resolution, using a time-delayed pulsed probe. This technique yields characterization of mechanical, optical, and electronic properties at the nanoscale, and is taken advantage of for investigations in material science, physics, chemistry, and biology. Here we review the use of this experimental method applied to the emerging field of homo- and heterostructures of van der Waals materials. Their unique structure corresponding to non-covalently stacked atomically thin layers allows for the study of original structural configurations, down to one-atom-thin films free of interface defect. The generation and relaxation of coherent optical phonons, as well as propagative and resonant breathing acoustic phonons, are comprehensively discussed. This approach opens new avenues for the in situ characterization of these novel materials, the observation and modulation of exotic phenomena, and advances in the field of acoustics microscopy.
... 因此, 打开石墨烯带隙或者寻找其 他二维层状半导体材料一度成为研究的热点 [3] . 前 者可以依靠二维异质结的构建实现 [4] , 后者则引导 大家发现诸多具有高载流子迁移率的二维材料候 选, 如h-BN [5] , 过渡金属二硫化物 [6] , 黑磷 [7] 等. 其 中得益于高周期的外壳层p轨道影响, 二维碲化物 常拥有较小的带隙值, 广泛应用于热电(如Bi 2 Te 3 [8] ) ...
Two-dimensional (2D) niobium silicon telluride (Nb2SiTe4) was recently proposed as a promising candidate in infrared detector, photoelectric conversion, polarized optical sensor and ferroelastic switching application due to its narrow bandgap, long-term air stability, high carrier mobility, etc. However, the in-plane strains and interfacial defects induced by the lattice misfits between functional layers are harmful to 2D heterojunction nanodevice performance, making the crystal-lattice regulation and strain engineering necessary to achieve lattice matching and strain-controllable interface. Here, using first-principles calculations and elemental substitutions, i.e., replacing cations (anions) with elements in the same group of periodic table, we identify three new and stable single-layer A2BX4 analogues (Nb2SiSe4, Nb2SnTe4 and Ta2GeTe4) as appealing candidates in manipulating the lattice parameters of Nb2SiTe4. The controllable lattice parameters are 6.04 Å ≤ a ≤ 6.81 Å and 7.74 Å ≤ b ≤ 8.15 Å. Among them, Ta2GeTe4 exhibits similar lattice parameters to Nb2SiTe4 but smaller bandgap, yielding better response in far-infrared region. Strain engineering shows that the external biaxial tensile stress narrows the bandgaps of A2BX4 due to the downshifting in energy of conduction band minimum (CBM). External biaxial compressive stress induces valance band maximum (VBM) orbital inversion for Nb2SiTe4, Nb2GeTe4 and Ta2GeTe4, which pushes up VBM and discontinues the trend of corresponding bandgap increase. In this case, the bandgap change depends on the competition between energy upshifts of both CBM and VBM. In the Nb2SiSe4 and Nb2SnTe4 cases, the d-p antibonding coupling in valance band is so strong that no valance band inversion appears while the bandgap increases by ~0.3 eV under −5% compressive strain. Regarding Nb2SiTe4, Nb2GeTe4 and Ta2GeTe4, their bandgaps can hardly change under −5% compressive strain, indicating that the energy upshift in VBM equals that in CBM. Such a valance band inversion is attributed to Te outmost p orbital overlapping, which introduces more dispersive VBM and smaller effective mass of hole. Our findings suggest that Nb2SiTe4 can be alloyed with Nb2SiSe4, Nb2SnTe4 and Ta2GeTe4 to achieve controllable device lattice matching while maintaining its superior properties at the same time. The use of external biaxial compressive stress can promote the hole diffusion and improve the device performance.
... This ideal situation can only be achieved through the optimisation of the conflicting properties embedded in the dimensionless figure-of-merit, zT = S 2 σT/(κ e + κ ph ), where S is the Seebeck coefficient, σ is the electrical conductivity, κ e is the electronic thermal conductivity, κ ph is the phonon thermal conductivity and T is the temperature 2 . The best material for room temperature applications is bismuth telluride (Bi 2 Te 3 ), which has been synthetized employing a plethora of methods, such as co-evaporation 3,4 , molecular beam epitaxy (MBE) 5 , Metal Organic Chemical Vapour Deposition (MOCVD) 6 , pulsed layer deposition (PLD) 7 and hydrothermal synthesis 8 . Among them, the electrodeposition technique offers high deposition rates and scalability, as well as the ability to tune multiple parameters for the desired output 9 . ...
The best known thermoelectric material for near room temperature heat-to-electricity conversion is bismuth telluride. Amongst the possible fabrication techniques, electrodeposition has attracted attention due to its simplicity and low cost. However, the measurement of the thermoelectric properties of electrodeposited films is challenging because of the conducting seed layer underneath the film. Here, we develop a method to directly measure the thermoelectric properties of electrodeposited bismuth telluride thin films, grown on indium tin oxide. Using this technique, the temperature dependent thermoelectric properties (Seebeck coefficient and electrical conductivity) of electrodeposited thin films have been measured down to 100 K. A parallel resistor model is employed to discern the signal of the film from the signal of the seed layer and the data are carefully analysed and contextualized with literature. Our analysis demonstrates that the thermoelectric properties of electrodeposited films can be accurately evaluated without inflicting any damage to the films.
Bi2Te3 based alloys have attracted considerable attention in low-temperature thermoelectric materials due to their superior
electrical transport properties and low thermal conductivity. However, the conversion efficiency of thermoelectric power generation or
refrigeration devices made of Bi2Te3-based alloys is still low. It is thus crucial to further improve the thermoelectric figure of merit zT
of Bi2Te3 based materials. There are some correlations between lattice thermal conductivity and electrical performance parameter.
Reducing the lattice thermal conductivity by phonon engineering becomes an important method to increase zT without deteriorating
the electrical performance. This review summarized the main research progress in recent years to optimize the thermoelectric
properties of Bi2Te3 based materials by phonon engineering, such as nano-modification, superlattice structure, nanocomposing,
doping and adding dislocation arrays. The effect of the optimization method on the specific heat capacity, phonon group velocity and
phonon mean free path was discussed. The specific heat capacity, phonon group velocity or phonon mean free path of Bi2Te3 based
materials can be significantly reduced by the optimization methods. As a result, the lattice thermal conductivity of Bi2Te3 based
materials is significantly reduced, leading to a significant increase in the thermoelectric properties.
Nano-modification: The interface density is increased via low dimensionalization and grain refinement, resulting in an enhanced
scattering of low-frequency phonons, reduced phonon mean free path and phonon group velocity, significantly reducing the lattice thermal conductivity. Nowadays, the grain size and morphology of Bi2Te3-based alloys can be precisely controlled by mechanical
milling, MBE method, hydrothermal method, chemical vapour deposition and wet chemical method. Bi2Te3 nanostructures with a
ultra-low grain size can be prepared. However, heat conduction cannot be completely prevented even at rather small sizes due to the
incomplete suppression of low-frequency phonons. Therefore, it is difficult to improve zT by a single low-dimensionalisation and
grain refinement method. The specific surface area can be increased either by combining the phases of different nanostructures to
form a heterogeneous interface or by introducing special nanostructures such as twins and nanopores, which can strongly scatter the
low-frequency phonons caused by lattice mismatch and lattice vibration, and further reduce the lattice thermal conductivity.
Bi2Te3-based superlattice structure: Furthermore, in the microscopic and mesoscopic scales, the superlattice structure generated
by artificially controlling the ordered arrangement of atomic layers of Bi2Te3-based materials is considered as the optimum candidate
material to achieve the ‘phonon glass-electron crystal’ thermoelectric material standard. Especifically, the quantum well structure in
the thin film superlattice produces an intense boundary scattering effect and a quantum confinement effect on phonons, resulting in a
decrease in the mean free path and group velocity of phonons, greatly reducing the thermal conductivity of Bi2Te3-based
thermoelectric materials. The bulk superlattice structure of the Bi2Te3-based alloy relies on its complex crystal structure and
acousto-optic coupling effect, causing a low cut-off frequency of phonon mode and an intense phonon resonance scattering, and
resulting in a low intrinsic thermal conductivity. In addition, the low phonon group velocity caused by chemical bond softening and
lattice anharmonicity is also a reason for the low intrinsic thermal conductivity of the bulk superlattice structure of Bi2Te3-based alloy.
Nanocomposites, doping modification and dislocation arrays: Nanocomposite and doping modification are the main approaches
to improve the thermoelectric properties of Bi2Te3-based materials. In the nanocomposite process, the nanoparticles dispersed in the
thermoelectric material do not enter the matrix lattice, but attach onto the surface of the matrix grain, forming a heterogeneous
interface with the matrix. Also, introducing nanoparticles increases the grain boundary density, resulting in a further scattering of low
frequency phonons. Doping modification is achieved by doping elements entering the Bi2Te3 lattice, replacing Bi site (or Te site) or
entering the van der Waals gap for interstitial doping. This process can introduce multiple scattering centers such as grain boundaries,
point defects, in-situ precipitates and dislocations into Bi2Te3-based alloy, enhancing a multi-scale scattering of phonons and
effectively reducing the mean free path of phonons. Combined with the preparation process such as sintering, the dislocation array is
increased, which leads to a higher scattering intensity of intermediate frequency phonons, further reducing the lattice thermal
conductivity.
Summary and prospects The phonon specific heat capacity, phonon group velocity and mean free path of the Bi2Te3-based alloy
could be effectively controlled by phonon engineering including nano-modification, superlattice structure, nanocomposite, doping and
introduction of the dislocation array to achieve a sufficiently low lattice thermal conductivity. The electrical transport properties were
optimized by the carrier engineering and energy band engineering techniques, and the synergistic regulation of the electrical and
thermal transport properties was realized. As a result, the thermoelectric properties of Bi2Te3-based alloys were significantly improved.
This provided a scientific and technical support for the application of thermoelectric devices. However, a lot of work remained in
improving the thermoelectric properties of Bi2Te3 materials by phonon, carrier and energy band engineering. Nevertheless, the
large-scale commercialization of Bi2Te3-based alloys as thermoelectric materials still attracts much attention. Also, interdisciplinary
research in thermoelectricity and other disciplines becomes popular. The future research and large-scale application of Bi2Te3
thermoelectric materials are expected.
Topological insulators (TIs) are widely recognized as a new state of quantum matter with semiconductor‐like or insulator‐like bulk states (BSs) and Dirac‐like surface states (SSs). Herein, the terahertz (THz) properties of the SSs and BSs in Bi 2 Te 3 ‐based TIs, as well as the effects of temperature on them, are examined. By using THz time–domain spectroscopy, the optical conductance of Bi 2 Te 3 is obtained for TI thickness of about 10 and 20 nm. It is shown that the THz responses of the SSs and BSs can be distinguished by purely optical means in a wide temperature range (80–280 K). A Drude formula, which includes two terms corresponding to the SSs and BSs, is applied to describe the behavior of the optical conductance. Moreover, the evidence of bulk‐to‐surface scattering and additional electron injection from the BSs to the SSs is also observed without the help of electric measurement. These results are helpful to gain an in‐depth understanding of the THz properties of TIs and to explore new applications in topologically protected plasmonic and optoelectronic devices.
Solid-state thermoelectric devices offer the possibility of exploiting waste heat from engines and power plants and converting it into electrical energy. One of the greatest challenges in the development of thermoelectric material systems is to find new thermoelectric materials with high thermoelectric figures of merit ZT. In this work, the structural, electronic and thermoelectric properties of PbSe[Formula: see text]S[Formula: see text] ([Formula: see text], 0.25, 0.50, 0.75 and 1) semiconductors are investigated by applying density functional theory in a full potential linearized augmented plane wave method (FP-LAPW). Calculations of structural properties were completed using the generalized gradient approximation GGA of Perdew Burke and Ernzerhof PBE to get reliable lattice constant results with experimental values. The obtained electronic results show that the PbSe[Formula: see text]S[Formula: see text] material is a narrow band gap semiconductor. In addition, the thermoelectric properties are studied on the basis of the fully iterative solution of the Boltzmann transport equation. PbSe[Formula: see text]S[Formula: see text] had a high figure of merit indicating that our materials are promising candidates in thermoelectric applications.
III-Nitrides, especially InGaN, are promising for high-efficiency thermoelectric (TE) components operating at high temperatures (HTs), playing a critical role in the recovery of waste heat for sustainable energy development. However, the performance of InGaN TE materials is limited by the high thermal conductivity (k) and the conflict coupling between the power factor (PF) and the k. Here, the previously unstudied two-dimensional InGaN/GaN SL structured TE device with a high In composition of 31% was developed and demonstrated to improve the TE figure of merit (ZT, Z=PF/k) by reducing the k value without deteriorating the PF. The Seebeck coefficient (S) exhibited a value of -365 μV/K due to the increased density of electron states near the Fermi level by the low dimensional construction. Simultaneously, a relatively low k was obtained as 7.7 W/m·K, benefitting from the alloying and SL interface scattering effect of high energy phonons. Moreover, enhancement of the Umklapp process by the space confinement effect further lowers the k. Accordingly, a record ZT value of 0.089 at 300 K was achieved, which was better than previously reported values for GaN-based TE film materials. This work provides a new material system for improving the performance of nitride TE materials at HTs and extends the fields of application in electricity harvesting from waste heat.
The emergence of two-dimensional (2D) materials enables enormous progress in the development of high-performance chemical sensors facilitating exotic structural and material properties. In this review, we focus on the rational design and synthesis strategies of various 2D metal oxide-based for chemiresistive gas sensors. We first discuss various synthesis strategies for 2D metal oxides such as thin-film manufacturing, exfoliation of layered metal oxides, templating route using sacrificial layer, and template-free synthesis route to elucidate the basic design principles of metal oxide nanosheets both from the top-down and bottom-up perspectives and their efficacy toward gas sensing applications. Then, we discuss assembly strategies of 2D metal oxide nanosheets for hierarchical and hybrid nanostructures with increased design complexity in terms of morphology and/or composition, which boosted their sensing performances. Finally, we conclude by providing an outlook of development in 2D metal oxides for realizing practical gas sensing devices. Through this article, not only did we elucidate the representative synthesis strategies for 2D metal oxides for applications in gas sensors, but we also provided a rich insight into their fundamental design principles to help propel the future development of high-performance gas sensors.Graphic abstract
It is found that many two-dimensional chalcogenides are exhibiting a superlative thermoelectric behavior because of their versatile characters in terms of stability, conductivity, bandgap tunability, etc. In this paper, the thermoelectric properties of monolayer Ge2Se2 have been calculated using first-principles calculations with all involved electrical and thermal transport properties in the parameter-free framework. It has been found that the Ge2Se2 offers a bandgap of 1.13 eV with a sufficiently high thermoelectric coefficient. The thermoelectric transport parameters have been calculated by solving the linearized Boltzmann Transport Equation and found optimum to qualify for good thermoelectric materials. The investigated material exhibit outstanding thermoelectric performance in terms of an ultra low lattice thermal conductivity of the order of 0.5 Wm−1K−1 and TE Figure of merit around two (2) at 1000 K. For better accuracy in results, we employed hybrid exchange–correlation (HSE06) throughout the calculations and, instead of approximating the relaxation time, computed the accurate relaxation time using the deformation potential approach. The research findings prove its excellent behavior and worthy guideline for the experimentalists looking for materials with high FoM to design thermoelectric devices.
Flexing computer memory
Phase change materials leverage changes in structure into differences in electrical resistance that are attractive for computer memory and processing applications. Khan et al . developed a flexible phase change memory device with layers of antimony telluride and germanium telluride deposited directly on a flexible polyimide substrate. The device shows multilevel operation with a low switching current density. The combination of phase change and mechanical properties is attractive for the large number of emerging applications for flexible electronics. —BG
Scalable synthetic strategies for high-quality and reproducible thermoelectric (TE) materials is an essential step for advancing the TE technology. We present here very rapid and effective methods for the synthesis of nanostructured bismuth telluride materials with promising TE performance. The methodology is based on an effective volume heating using microwaves, leading to highly crystalline nanostructured powders, in a reaction duration of two minutes. As the solvents, we demonstrate that water with a high dielectric constant is as good a solvent as ethylene glycol (EG) for the synthetic process, providing a greener reaction media. Crystal structure, crystallinity, morphology, microstructure and surface chemistry of these materials were evaluated using XRD, SEM/TEM, XPS and zeta potential characterization techniques. Nanostructured particles with hexagonal platelet morphology were observed in both systems. Surfaces show various degrees of oxidation, and signatures of the precursors used. Thermoelectric transport properties were evaluated using electrical conductivity, Seebeck coefficient and thermal conductivity measurements to estimate the TE figure-of-merit, ZT. Low thermal conductivity values were obtained, mainly due to the increased density of boundaries via materials nanostructuring. The estimated ZT values of 0.8–0.9 was reached in the 300–375 K temperature range for the hydrothermally synthesized sample, while 0.9–1 was reached in the 425–525 K temperature range for the polyol (EG) sample. Considering the energy and time efficiency of the synthetic processes developed in this work, these are rather promising ZT values paving the way for a wider impact of these strategic materials with a minimum environmental impact.
In this chapter, we discuss traditional thermoelectric materials such as chalcogenide, Zintl phase, Heusler compounds, Si-Ge alloys, Bi2Te3, oxide-based thermoelectric materials, etc. Dependence of thermoelectric parameters such as Seebeck coefficient, conductivity, and power factor of bulk and nanostructured materials on carrier concentration is also explored. The doping effect on the power factor and the figure of merit of different thermoelectric materials, the relation of Seebeck coefficient, and electrical conductivity with effective mass are also entertained in this chapter. In nanostructured materials, the quantum confinement effect directly affects the figure of merit and, therefore, the power factor changes with dimensionality.
Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.
This work demonstrates the production of high-performing p-type and n-type hybrid AgxTe/poly(3,4-ethylenedioxythiopene):polystyrene sulfonic acid (PEDOT:PSS) thermoelectric materials from the same Te/PEDOT:PSS parent structure during aqueous-based synthesis. All samples were solution-processed and analyzed in thin-film architectures. We were able to demonstrate a power factor of 44 μW m–1 K–2 for our highest-performing n-type material. In addition, we were also able to realize a 68% improvement in the power factor of our p-type compositions relative to the parent structure through manipulation of the inorganic nanostructure composition. We demonstrate control over the thermoelectric properties by varying the stoichiometry of AgxTe nanoparticles in AgxTe/PEDOT:PSS hybrid materials via a topotactic chemical transformation process at room temperature. This process offers a simple, low-temperature, and cost-effective route toward the production of both efficient n-type and p-type hybrid thermoelectric materials.
Antimony telluride (Sb2Te3) nanoparticles of different sizes were fabricated by mechanical alloying (MA) of elemental Sb and Te powers for different durations. The powder nanostructures were pelletized, annealed in Ar ambient, and characterized by XRD, FESEM, TEM to study the effect of milling time and thermal treatment on particle size, grain growth, and crystallinity. The annealed and unannealed pelletized nanostructures were analyzed in a PPMS to study the effect of grain growth on their electrical and thermoelectric properties. Room temperature electrical conductivity of the p-type semiconductor nanostructures improved significantly (from ∼10³ to ∼ 10⁵ mho/m) due to thermal annealing and results in the considerable improvement in thermoelectric figure of merit (ZT). Thermal annealing-induced grain growth also transforms the semiconducting nature of the sample to metallic. The reduced thermal conductivity of the nanostructures with reduced grain size improves the ZT. The temperature-dependent Lorenz number (Leffective) is used to find the electronic contribution of total thermal conductivity, and it is explained by the non-parabolic Kane model.
Coordination polymers (CPs) are potential thermoelectric (TE) materials to replace the sometimes costly, brittle and toxic heavy metal inorganic TEs for near-ambient-temperature applications. Air-stable and highly conductive p-type thermoelectric CPs are relatively well known, but the their n-type counterparts are only now emerging and both are needed for most practical applications. This perspective reviews recent advances in the development of n-type thermoelectric CPs, particularly the 1D and 2D metal bisdithiolenes, and introduces a relatively new class of guest@metal-organic framework(MOF)-based composites. Low dimensional CPs with reasonable n-type thermelectric performance are emerging with good charge mobility and air-stability but still relatively low electrical conductivity.
This paper reports the microstructures and thermoelectric properties of In-doped (HgTe)0.55(PbTe)0.45 composites. X-ray diffraction and optical microscopy confirmed that all composites consisted of the HgTe and PbTe phases and formed a dendritic structure. Transmission electron microscopy revealed that In mostly displaced Hg in the HgTe. For x = 0.01 and 0.03, the electrical resistivity of (Hg1-xInxTe)0.55(PbTe)0.45 composites decreased significantly, while the sample with x = 0.05 showed increased value, compared with the parent composite. The measured Seebeck coefficient demonstrated that electrons dominated the thermoelectric transport in these composites. A considerable reduction in thermal conductivity was found after In doping, while heat conduction was dominated by lattice contribution in all composites. The maximum figure of merit and power factor values of 0.25 and 20 μW/cmK², respectively, were obtained at 300 K for the x = 0.01 composite, which were almost an order of magnitude higher than those of the parent composite.
Van der Waals layered GeTe/Sb2Te3 chalcogenide superlattices have demonstrated outstanding performance for use in dynamic resistive memories in what is known as interfacial phase change memory devices due to their low power requirement and fast switching. These devices are made from the periodic stacking of nanometer thick crystalline layers of chalcogenide phase change materials. The mechanism for this transition is still debated, though it varies from that of traditional phase change melt-quench transition observed in singular layers of GeTe and Sb2Te3. In order to better understand the mechanism and behavior of this transition, a thorough study on each constituent layer and the parameters for growth via molecular beam epitaxy was performed. In this work, the authors show the effect of tellurium overpressure and substrate temperature on the growth of thin film GeTe and Sb2Te3 on (100) GaAs. The authors demonstrate the significant role during growth that tellurium overpressure plays in the transport properties of both GeTe and Sb2Te3, as well as the negligible impact this has on both the structural and optical properties. The highest mobility recorded was 466 cm²/V s with a p-type bulk carrier concentration of 1.5 × 10¹⁹ cm⁻³ in Sb2Te3. For GeTe, the highest achieved was 55 cm²/V s at a p-type bulk carrier concentration of 8.6 × 10²⁰ cm⁻³. The authors discuss transport properties, orientation, and crystal structure and the parameters needed to achieve high mobility chalcogenide thin films.
Crystal growth conditions of Bi2Te3 narrow bandgap semiconductors have been studied using molecular beam epitaxy method. It was applied to the growth of Bi2Te3 on Bridgman single-crystal substrate Sb2Te3. Substrate ingots were taken from the natural cleavage along the (0001) plane. The deposited conditions have been studied as a function of substrate temperature (Ts) and flux ratio (. The quality of deposited layers was controlled by X-ray diffraction, scanning electron microscope (SEM), secondary ion mass spectroscopy (SIMS) depth profiling and energy-dispersive X-ray (EDX) microanalyser. The sticking coefficients Ks(Te) and Ks(Bi) of the elements that compose Bi2Te3 were determined. It was found that the stoichiometry of deposited layers depended on substrate temperature and flux ratio. It was observed that all deposited layers were single-crystal in the orientation of their substrates with a small shift due to the stress in layer.
Epitaxial GaSe films have been prepared on WSe 2 (0001) substrates with 14% lattice mismatch and characterized by photoelectron spectroscopy, electron diffraction, and ex situ by tunneling microscopy. The films grow in the Frank–van der Merve growth mode. The best films with perfect azimuthal orientation are formed after an annealing step at 720 K. The basic mechanisms of this van der Waals epitaxy are qualitatively discussed in terms of thermodynamic and kinetic parameters.
The thermal conductivity of sputtered a-Si:H thin films for a hydrogen
content of 1-20 % and a film thickness of 0.2-1.5 μm is determined in
the temperature range 80-400 K using an extension of the 3ω
measurement technique. The reliability of the method is demonstrated on
1-μm-thick a-SiO2 thermally grown on Si. Scattering of
phonons at the interface between the a-Si:H film and the substrate
places a simple upper limit on the heat transport by long-wavelength
phonons and facilitates the comparison of the experimental data to
recent numerical solutions of a Kubo formula using harmonic vibrations.