Electrodeposition of P-Type Sb2te3 Films and Micro-Pillar Arrays in a Multi-Channel Glass Template

materials

Article Electrodeposition of p-Type Sb2Te3 Films and Micro-Pillar Arrays in a Multi-Channel Glass Template

Ning Su 1, Shuai Guo 1, Fu Li 2,*, Dawei Liu 3 and Bo Li 1,*

1 Graduate School of Shenzhen, Tsinghua University, Shenzhen 518065, China; [email protected] (N.S.); [email protected] (S.G.) 2 Shenzhen Key Laboratory of Advanced Thin Films and Applications, College of Physics and Energy, Shenzhen University, Shenzhen 518060, China 3 Huaneng Clean Energy Research Institute, Beijing 102209, China; [email protected] * Correspondence: [email protected] (F.L.); [email protected] (B.L.); Tel.: +86-0755-2603-6360 (B.L.)

Received: 5 June 2018; Accepted: 9 July 2018; Published: 12 July 2018

Abstract: Antimony telluride (Sb2Te3)-based two-dimensional films and micro-pillar arrays are + + fabricated by electrochemical deposition from electrolytes containing SbO and HTeO2 on Si wafer-based Pt electrode and multi-channel glass templates, respectively. The results indicate that the addition of tartaric acid increases the solubility of SbO+ in acidic solution. The compositions of deposits depend on the electrolyte concentration, and the micro morphologies rely on the reduction + + potential. Regarding the electrolyte containing 8 mM of SbO and 12 mM of HTeO2 , the grain size increases and the density of films decreases as the deposition potential shifts from −100 mV to −400 mV. Sb2Te3 film with nominal composition and dense morphology can be obtained by using a deposition potential of −300 mV. However, this condition is not suitable for the deposition of Sb2Te3 micro-pillar arrays on the multi-channel glass templates because of its drastic concentration polarization. Nevertheless, it is found that the pulsed voltage deposition is an effective way to solve this problem. A deposition potential of −280 mV and a dissolve potential of 500 mV were selected, and the deposition of micro-pillars in a large aspect ratio and at high density can be realized. The deposition technology can be further applied in the fabrication of micro-TEGs with large output voltage and power.

Keywords: thermoelectric device; electrodeposition; Sb2Te3 micro-pillar arrays; multi-channel glass template

1. Introduction Thermoelectric (TE) devices, which can directly and reversibly convert heat into electricity, have attracted worldwide interest because of their promising applications in electronics, which arise from their distinct characteristics, such as no mechanical moving parts, no maintenance, and high reliability [1–17]. In particular, one of their unique advantages compared with traditional energy conversion stems is their very high speciﬁc power, which can scale well to low power levels and be useful in applications in which weight and size matter. Thus, miniaturized TE devices will display favorable and efﬁcient performance. Technology to miniaturize and integrate TE devices has become an important research direction with which to promote their materialization. Many advances in the fabrication of microscopic TE devices combined with microelectromechanical systems processing have been achieved in recent years. However, there are still some important problems that have limited the performance and application of TE micro-devices; for instance, the complex mic-fabrication process,

Materials 2018, 11, 1194; doi:10.3390/ma11071194 www.mdpi.com/journal/materials Materials 2018, 11, 1194 2 of 13 the quality of TE materials including thin films and microarrays, the aspect ratio of microarrays, and contact resistance [3,9,10,12,14,18–26]. Typically, TE modules consist of a series of electrically connected p- and n-type elements. In micro-devices, the basic components are the corresponding p- and n-type TE thin films or microarrays. Thus, the deposition of both films and microarrays is an important technology with which to obtain high-performance devices. Bismuth telluride (Bi2Te3)-based alloys are the best n-type TE materials near room temperature and the corresponding films and microarrays have been studied extensively in TE micro-devices. The electrodeposition of Bi2Te3 films has been well reported because electrochemical deposition has many advantages including comparably facile control over process parameters, low cost, low temperature, and relatively simple equipment requirements [4,9,14,17,22,25,27–30]. Additionally, electrodeposited materials can be integrated into micro-devices. Considering the compatibility factor of TE materials that influences the output power of the device, p-type antimony telluride (Sb2Te3) is a suitable TE material to match with n-type Bi2Te3 [5,30–35]. However, few Sb2Te3 films prepared by electrochemical deposition have been reported, mainly because SbO+ is difficult to dissolve in acidic solution, which leads to poor crystallization of the pure Sb2Te3 and weak TE properties of the final films. Meanwhile, few studies on p-type Sb2Te3 microarrays are available [9,29,36–38]. Especially, Sb2Te3 microarrays with high aspect ratio are difficult to obtain. Therefore, it is hard to obtain a large temperature difference between two ends of the legs, resulting in a lower output power of the final micro-device. In this work, electrodeposition of Sb2Te3 thin films on silicon (Si) wafer surface and microarrays on a multi-channel glass template are investigated. Electrodeposition on Si wafer surface-based Pt electrode is firstly carried out to clarify the possibility and process of Sb2Te3 deposition, which also acts as a reference to subsequently fabricate microarrays by electrodeposition, which are used in TE devices. Actually, in the present study, the microarrays for TE micro-devices are mainly obtained by deposition in Si micro-pore, anodic aluminum oxide (AAO), and organic photoresist templates [20,39–43]. However, the height of the micro-pores in the AAO and organic photoresist templates is limited by the processing technique, which induces a lower aspect ratio of the micro-pillars. Thus, it is difficult to increase the temperature difference between two ends of the micro-device, leading to lower conversion output power and efficiency. Moreover, the thermal conductivity of the AAO and Si templates is higher than that of TE materials. Thus, they must be removed after deposition to guarantee the energy conversion efficiency of the TE device. However, removing the micro-pore template degrades the mechanical properties of the microarrays, which tends to break the microarrays easily without the support. Therefore, here we use multi-channel glass as the template for the Sb2Te3 electrodeposition after optimization of the deposition parameters by using the films. The glass template has low thermal conductivity and is electrically insulating. It can be retained after being filled with TE materials, which simplifies the device fabrication process and also helps to enhance the rigidity of the TE modules. Notably, the multi-channel glass template used in the present work is fabricated by laser boring. Although the template aspect ratio is about 4:1 in this work, it can easily be further increased to obtain a high aspect ratio by increasing the thickness of the initial glass substrate.

2. Materials and Methods Sb-Te ﬁlms were deposited from the nitric acid (3%) (Guanghua Technology Co. Ltd., Shenzhen, China)- and tartaric acid (20 g/L) (Macklin Biochemical Co. Ltd., Shanghai, China)-supporting + + electrolyte containing Sb and HTeO2 in a three-electrode system with a capacity of 0.2 L at room temperature. The electrodeposition reaction of Sb2Te3 can be expressed as shown in Equation (1) [36].

+ + + − 2SbO + 3HTeO2 + 13H + 18e →Sb2Te3 + 8H2O (1)

The anode was polished graphite, which remained chemically stable during the deposition in acidic solution. The cathode was a Si wafer that was coated with 20 nm of Ti and 150 nm of Pt. Materials 2018, 11, 1194 3 of 13

The wafer was then cleaved into pieces with dimensions of 10 × 10 mm. To remove any organic materials from the Si wafer, it was pre-cleaned with a 2:1 mixture of concentrated sulfuric acid and hydrogen peroxide. The reference electrode was potassium chloride saturated calomel electrode (SCE), which had a constant potential of 0.2412 V, and the potential difference between the cathode and SCE could be observed by an electrochemical workstation (CHI-660E, CH Instruments Co. Ltd., Shanghai, China). All stated potentials are relative to SCE. The deposition process was controlled by a personal computer and the electrochemical workstation. Deposition on Si wafer-based Pt electrode was carried out firstly to determine the optimal electrolyte concentration and deposition potential. + + The electrolyte contained 8 mM of Sb and 8 to 15 mM of HTeO2 . Electrolyte pH was controlled at −0.1~0.1 with nitric acid according to the potential–pH graph. Deposition and dissolution peak potentials were observed by cyclic voltammetry at a scan rate of 5 mV/s. Then, Sb-Te films were fabricated by constant voltage processes and their microstructure, composition, crystal structure density, and electrical properties were investigated. The electrical resistivity and Seebeck coefficient of the films as function of temperature from 310 K to 417 K in helium atmosphere were measured by using the equipment (ZEM-3, ULVAC-RIKO, Yokohama, Japan). The optimized electrolyte concentration and potential of the Sb2Te3 films were tried to fabricate the microarrays based on the multi-channel glass. The multi-channel glass template was fabricated from alkali-free glass. Arrays of vertical holes with a diameter of 0.06 mm and depth of 0.2 mm were formed by a laser. The distance between neighboring holes was 0.2 mm. A complete template contained about 2500 holes. The template was then attached onto a Si wafer by using polyethylene terephthalate (PET) tape for the next deposition. Morphologies and chemical composition of all the Sb-Te deposited films and arrays were investigated by scanning electron microscopy (SEM; Zeiss SUPRA® 55, Berlin, Germany) and energy-dispersive spectroscopy (EDS; Oxford X-Max 20, Oxford, UK). The crystalline structure and composition of deposits were analyzed by X-ray diffraction (XRD; Rigaku, Tokyo, Japan), which is conducted using a Rint-2000V/PC diffractometer in 2θ mode with Cu-Kα radiation. Then, a microprobe test platform (RTS-9, four probes technology Co. LED, Guangzhou, China) was used to measure the potential–current curve of a single pillar to calculate its resistance.

3. Results

3.1. Electrodeposition on Si Wafer-Based Pt Electrode

+ + Sb-Te films can be deposited on the cathode in an acidic electrolyte of SbO and HTeO2 [29,44,45]. Some reports indicated that Sb was difficult to dissolve in acidic solution in the absence of complexing agents. Tartaric acid can promote the dissolution process of Sb and stabilize the solution system, because it chelates strongly to Sb. When Sb was added to nitric acid solution, a white precipitate formed immediately. As tartaric acid was added to the mixture, the white precipitate gradually disappeared. The dissolution rate was relative to the amount of tartaric acid added. Cyclic voltammetry + + scans were conducted in electrolytes containing 8 mM of SbO and 8~15 mM of HTeO2 at a scan rate of 5 mV/s at room temperature to investigate the electrochemical behavior of the Si wafer. As shown in Figure1, every cyclic voltammogram contains only a reduction peak and an oxidation peak, which represent the deposition and dissolution processes of Sb-Te, respectively. This behavior indicates that the crystallization and nucleation processes of Sb2Te3 on the Si wafer-based Pt electrode occurred at the same time, rather than Sb or Te nucleating first and then inducing the growth of Sb2Te3. The oxidation peak potential was approximately 0.6 V, and the reduction peak potential ranged from −0.25 to −0.35 V as the Te concentration of the electrolyte changed from 8 to 15 mM. The charge integral and current density of the deposition and dissolution processes both increased with Te concentration, which indicated that the reaction rate was related to the Te concentration. The electrodeposition reaction occurred more easily at lower over-potential, which corresponded to a more positive reduction peak potential. Materials 2018, 11, 1194 4 of 13 Materials 2018, 11, x FOR PEER REVIEW 4 of 13

Figure 1. Cyclic voltammetry scans of the Sb-Te films at room temperature at a scan rate of 0.05 V/s Figure 1. Cyclic voltammetry scans of the Sb-Te ﬁlms at room temperature at a scan rate of 0.05 V/s + + with electrolytes containing 8 mM of SbO+ and (a) 8, (b) 10, (c) 12, and (d) 15 mM of HTeO2+. with electrolytes containing 8 mM of SbO and (a) 8, (b) 10, (c) 12, and (d) 15 mM of HTeO2 .

The EDS and XRD results of the films deposited from electrolytes containing 8 mM of SbO+ and The EDS and XRD results of the ﬁlms deposited from electrolytes containing 8 mM of SbO+ and 8–15 mM of HTeO2+ were investigated. As illustrated in Figure 2a, the composition of the deposits 8–15 mM of HTeO + were investigated. As illustrated in Figure2a, the composition of the deposits was strongly related2 to the HTeO2+ concentration of the electrolyte, but had almost no relationship was strongly related to the HTeO + concentration of the electrolyte, but had almost no relationship with deposition potential. This is2 probably because HTeO2+ adsorbed strongly on the Pt electrode. + withTherefore, deposition the composition potential. Thisof the is deposits probably near because the electrode HTeO2 dependedadsorbed on strongly the adsorption on the Pt capacity electrode. of Therefore, the composition of the deposits near the electrode depended on the adsorption capacity of HTeO2+, rather than the over-potential during deposition. In addition, the composition of the film can + HTeObe calculated2 , rather by than Equation the over-potential (2), during deposition. In addition, the composition of the ﬁlm can be calculated by Equation (2), XTe = (iTe/zTe)/{(iSb/zSb) + (iTe/zTe)} (2)

XTe = (iTe/zTe)/{(iSb/zSb) + (iTe/zTe)} (2) In which XTe is the content of Te in deposits, iSb and iTe are the limited current values of the Sb(III) and Te(IV), respectively, and zSb = 3 and zTe = 4 are the valences of Sb and Te, respectively. iSb, iTe, zSb, and In which XTe is the content of Te in deposits, iSb and iTe are the limited current values of the Sb(III) zTe are related to the concentration of electrolyte, but no relationship with deposition potential. This and Te(IV), respectively, and z = 3 and z = 4 are the valences of Sb and Te, respectively. i , conclusion is consistent with someSb of the Tereports [46–51]. When deposited from the electrolyteSb iTe, zSb, and zTe are related to the concentration of electrolyte, but no relationship with deposition containing 8 mM of SbO+ and 12 mM of HTeO2+, the iSb = −4.59 × 10−4 Acm−2 and iTe = −9.2 × 10−4 Acm−2, potential. This conclusion is consistent with some of the reports [46–51]. When deposited from and the XTe = 60 mol % can be calculated. The electrodeposition process could be divided into two the electrolyte containing 8 mM of SbO+ and 12 mM of HTeO +, the i = −4.59 × 10−4 Acm−2 + 2 Sb steps. The first was− 4the adsorption−2 of HTeO2 on the Pt substrate, which could be expressed as and iTe = −9.2 × 10 Acm , and the XTe = 60 mol % can be calculated. The electrodeposition HTeO2+→ (HTeO2+)ads. The adsorbed HTeO2+ ion on the Si wafer based Pt electrode then reacted with process could be divided into two steps. The first was the adsorption of HTeO + on the Pt substrate, + 2 SbO in solution, producing the final compound+ of+ Sb2Te3. The percentage of Te+ in the films increased which could be expressed as HTeO2 → (HTeO2 )ads. The adsorbed HTeO2 ion on the Si wafer with the HTeO2+ concentration of the electrolyte. Nearly stoichiometric Sb2Te3 was obtained by based Pt electrode then reacted with SbO+ in solution, producing the final compound of Sb Te . + + 2 3 electrodeposition using an electrolyte consisting of 8 mM of SbO+ and 12 mM of HTeO2 scanning at The percentage of Te in the films increased with the HTeO2 concentration of the electrolyte. a potential range from −0.1 to −0.4 V for 600 s. The XRD pattern of the film deposited in 8 mM SbO+ Nearly stoichiometric Sb Te was obtained by electrodeposition using an electrolyte consisting of + 2 3 and 12 mM HTeO+ 2 at a potential of +−0.3 V is shown in Figure 2b. This pattern confirms that the 8 mM of SbO and 12 mM of HTeO2 scanning at a potential range from −0.1 to −0.4 V for 600 s. deposit is Sb2Te3 (JCPDS # 15-0874). Considering the results+ of electrochemical+ measurements and the The XRD pattern of the film deposited in 8 mM SbO and 12 mM HTeO2 at a potential of −0.3 V is EDS and XRD analyses, we used an electrolyte with 8 mM of SbO+ and 12 mM of HTeO2+ in shown in Figure2b. This pattern confirms that the deposit is Sb Te (JCPDS # 15-0874). Considering the subsequent experiments. 2 3 results of electrochemical measurements and the EDS and XRD analyses, we used an electrolyte with + + 8 mM of SbO and 12 mM of HTeO2 in subsequent experiments.

Materials 2018, 11, 1194 5 of 13 Materials 2018, 11, x FOR PEER REVIEW 5 of 13 Materials 2018, 11, x FOR PEER REVIEW 5 of 13

FigureFigure 2. 2.(a )(a EDS) EDS results results for for films films deposited deposited withwith electrolyte consisting consisting of of 8 8mM mM of of SbO SbO+ and+ and 8–15 8–15 mM mM + + ++ ++ + of HTeOFigureof HTeO2 2.;(2 (a;b ())b EDS XRD) XRD results pattern pattern for of offilms the the film depositedfilm depositeddeposited with ininelectrolyte 8 mM of of consistingSbO SbO andand 12 of 12 mM8 mMmM of of HTeO HTeOSbO2 and at2 aat 8–15potential a potential mM of −of0.3 HTeO−0.3 V. V.2+ ; (b) XRD pattern of the film deposited in 8 mM of SbO+ and 12 mM of HTeO2+ at a potential of −0.3 V. TheThe deposition deposition process process onon a Si Si wafer-based wafer-based Pt Ptelectrode electrode was was then then observed. observed. After Afterabout about60 s, the 60 s, thesurface surfaceThe of deposition of the the Pt Pt electrode electrode process becameon became a Si wafer-based gray gray and and the Pt the wholeel wholeectrode surface surface was thenexhibited exhibited observed. a metallic a After metallic aboutsheen. sheen. 60 As s, Asthe the depositionsurfacedeposition of time thetime Pt increased, increased, electrode the thebecame filmsfilms gray gotgot thickerthickerand the an and wholed denser denser surface [41]. [41 ]. exhibitedThe The thin thin filmsa filmsmetallic deposited deposited sheen. from As from the the deposition time increased, the films+ +got thicker and denser2 +[41]. The thin films deposited from the electrolyteelectrolyte containing containing 8 8 mM mM of of SbO SbO and and 1212 mMmM ofof HTeOHTeO2 atat potentials potentials of of −−100100 to to−400−400 mV mV were were + + investigatedelectrolyteinvestigated containing by by SEM. SEM. Figure 8Figure mM3 of 3shows showsSbO and the the top12top mM viewsview ofs HTeO ofof all2 the at potentialsfilms films and and ofcross-section cross-section −100 to −400 for formV the thewere film film investigatedprepared at potentialsby SEM. Figureof −300 3 mV. shows The the top top view view revealss of all that the Sb films2Te3 consistedand cross-section of equiaxed for globularthe film prepared at potentials of −300 mV. The top view reveals that Sb2Te3 consisted of equiaxed globular prepared at potentials of −300 mV. The top view reveals that Sb2Te3 consisted of equiaxed globular grains,grains, with with small small grains grains uniting uniting toto formform larger spheres [46], [46], which which is isdifferent different from from that that of laminar of laminar grains,Bi2Te3. withAt a smallpotential grains of uniting−100 mV, to formthe crystal larger grainsspheres were [46], smalwhichler, is and different the film from was that much of laminar more Bi2Te3. At a potential of −100 mV, the crystal grains were smaller, and the film was much more Bicompact2Te3. At than a potential that deposited of −100 atmV, a morethe crystal negative grains potential. were smal As ler,the anddeposition the film potential was much becomes more compact than that deposited at a more negative potential. As the deposition potential becomes compactnegative, thanthe grains that deposited size increases, at a andmore the negative films be potential.come less denseAs the owing deposition to the potentiallarger growth becomes rate negative, the grains size increases, and the films become less dense owing to the larger growth rate negative,resulting thefrom grains larger size over-potential. increases, and The the fi filmslms deposited become less at −dense100 and owing −400 to mV the were larger much growth thinner rate − − resultingresultingthan those from from deposited larger larger over-potential. atover-potential. −200 and −300 TheThe mV films fiinlms the depositeddeposited same time at atof − 20100100 min, and and because −400400 mV the mV were deposition were much much thinnercurrent thinner thanthanat thosethe those deposition deposited deposited potential at at− −200200 of andand −100 −− 300300and mV mV−400 in in mVthe the sameis same smaller time time thanof of 20 20 thatmin, min, at because − because200 and the − the deposition300 deposition mV, which current current we at theatcould the deposition depositionobtain from potential potential the cyclic of of− −100 100voltammetry andand− −400 mV mVcurves is is smaller smaller. The thanlongitudinal than that that at at − 200−section200 and and −views300−300 mV, for mV, which the which film we we couldcoulddeposited obtain obtain fromat −from300 the mV cyclicthe at cyclic 20 voltammetry min voltammetry reveal curves.that thecurves Theglobular. longitudinalThe crystalslongitudinal sectionof Sb 2sectionTe views3 stack views for along the for filmthe the normal deposited film at −depositeddirection300 mV of atat the20−300 minPt mVelectrode, reveal at 20 that min and the revealthe globular crystals that crystalsthe clos globulare to of the Sb crystalsPt2Te electrode3 stack of alongSb are2Te 3much thestack normal smaller along direction the than normal those of the Pt electrode,directionclose to the of and surface.the thePt electrode, crystals The surface close and of tothe the the crystals film Pt electrodeis smooth, close toare andthe much thePt electrode film smaller thickness are than much is those about smaller close 2.5 µm. tothan the those surface. Theclose surface to the of surface. the film The is smooth,surface of and the the film film is smooth, thickness and is the about film 2.5 thicknessµm. is about 2.5 µm.

Figure 3. Top-view SEM images of Sb-Te films deposited on Si wafer-based Pt electrode under a FigureFigureconstant 3. 3.Top-view applied Top-view voltage SEM SEM of imagesimages (a) −100, ofof ( Sb-Teb) −200, filmsfilms (c) − deposited300, deposited and (d on) on− 400Si Si wafer-basedmV; wafer-based (e) cross-sectional Pt Ptelectrode electrode view under of under the a a constantconstantfilm deposited applied applied at voltage voltage −300 mV. of of ( a(a))− −100,100, ( (bb)) −−200,200, (c) ( c−)300,−300, and and (d) − (400d) − mV;400 ( mV;e) cross-sectional (e) cross-sectional view of view the of thefi filmlm deposited deposited at at−300−300 mV. mV.

Materials 2018, 11, 1194 6 of 13 Materials 2018, 11, x FOR PEER REVIEW 6 of 13

TheseThese resultsresults suggest suggest that that the the micromorphology micromorphology and andcomposition composition of deposits of deposits are related are to related both to + boththe theconcentration concentration of HTeO of HTeO2+ in2 electrolytein electrolyte and and reduction reduction potential. potential. Sb2Te Sb32 Tefilm3 film with with nominal nominal compositioncomposition and and dense dense morphologymorphology inin thethe presentpresent work can be obtained using using reduction reduction potential potential of + + −of300 −300 mV mV with with 8 mM 8 mM of of SbO SbOand+ and 12 12 mM mMof of HTeOHTeO22+ inin subsequent. subsequent. TheThe electricalelectrical resistivityresistivity and Seebeck coefficient coefficient of of the the film film deposited deposited at at−300−300 mV mV from from the the + + electrolyteelectrolyte of of 8 8 mM mM SbOSbO+ and 12 mM HTeO HTeO22+ werewere measured measured using using ZEM-3 ZEM-3 equipment, equipment, because because the the filmsfilms were were in in direct direct contact contact with with Pt Pt electrode, electrode, which which has has higher higher electrical electrical and and thermal thermal conductivity conductivityand significantand significant influence influence on the on electrical the electrical testing testing results. results. The films The mustfilms bemust removed be removed to another to another substrate, whichsubstrate, is electric which insulation is electric andinsulation has lower and thermalhas lower conductivity. thermal conductivity. As shown As in shown Figure in4a, Figure epoxy 4a, resin wasepoxy applied resin onwas the applied surface on ofthe films, surface and of itfilms, solidified and it aftersolidified a few after hours, a few which hours, allowed which allowed it to pick it off theto films.pick off Then, the films. the sampleThen, the was sample placed was between placed between the hot andthe hot cold and sides cold of sides ZEM, of andZEM, the and electrical the connectionelectrical connection was realized was by realized silver pasteby silver and paste copper and foil, copper which foil, is shownwhich is in shown Figure in4b. Figure Figure 4b.4c,d showsFigure the 4c,d electrical shows the conductivity, electrical conductivity, Seebeck coefficient, Seebeck and coefficient, power factor and power values factor of the values aforementioned of the aforementioned films (deposited at −300 mV from the electrolyte of+ 8 mM SbO+ and 12 mM+ HTeO2+) films (deposited at −300 mV from the electrolyte of 8 mM SbO and 12 mM HTeO2 ) as function as function of temperature, respectively, because the inferior heat resistance of the epoxy resin, which of temperature, respectively, because the inferior heat resistance of the epoxy resin, which was only was only slightly below 417 K, was tested. The Seebeck coefficient is positive, which proves that the slightly below 417 K, was tested. The Seebeck coefficient is positive, which proves that the deposit deposit is p-type semiconductor. Seebeck coefficient decreases, and the electrical conductivity is p-type semiconductor. Seebeck coefficient decreases, and the electrical conductivity increases increases with the increasing temperature. Power factor value reaches its maximum of 595.93 with the increasing temperature. Power factor value reaches its maximum of 595.93 µW/mK2 at µW/mK2 at 310 K. The Seebeck coefficient and electrical resistivity have strong relationship with the 310quality K. The of Seebeckdeposits. coefficient The Seebeck and coefficient electrical of resistivity our TE films have is strong 56.2~50.2 relationship µV/K, and with the the electrical quality of deposits.resistivity The is 5.3 Seebeck × 10−6~7.9 coefficient × 10−6 Ωm of at our a temperature TE films is of 56.2~50.2 310~417 K,µV/K, which and is similar the electrical to the TE resistivity films in is −6 −6 5.3previous× 10 ~7.9study× [36,47,48].10 Ωm However,at a temperature the Seebeck of 310~417 coefficient K, which of the is similarTE films to is the far TE below films the in previousbulk studymaterials, [36,47 on,48 account]. However, of the fact the that Seebeck the TE coefficient films are much of the looser TE films than is bulk far materials. below the bulk materials, on account of the fact that the TE films are much looser than bulk materials.

FigureFigure 4. 4. ((aa,,bb)) TheThe schematic diagram diagram of of testing testing method method for for electrical electrical properties properties for forTe thin Te thin films films + + depositeddeposited at at− −300 mV from the the electrolyte electrolyte of of 8 8 mM mM SbO SbO+ andand 12 12 mM mM HTeO HTeO2+. 2Electrical. Electrical resistivity resistivity and and SeebeckSeebeck coefficient coefficient (c(c)) and and powerpower factorfactor ((d)) valuesvalues of TE film film as as function function of of temperature temperature from from 310 310 K K to 417to 417 K in K helium in helium atmosphere. atmosphere.

3.2.3.2. Electrodeposition Electrodeposition ontoonto Multi-Channel Glass Glass Templates Templates FigureFigure5 5displays displays the the schematicschematic andand scanning electron electron microgra micrographsphs of of the the multi-channel multi-channel glass glass template.template. During the the deposition deposition process, process, the template the template was attached was attached to an Si to wafer an Si by wafer PET gum, byPET which gum,

Materials 2018, 11, 1194 7 of 13 Materials 2018, 11, x FOR PEER REVIEW 7 of 13 Materials 2018, 11, x FOR PEER REVIEW 7 of 13 haswhich high has chemical high chemical stability stabilityand viscosity and viscosityin acidic solution. in acidic Cyclic solution. voltammetry Cyclic voltammetry scans were scansconducted were tohasconducted observe high chemical the toobserve electrodeposition stability the electrodepositionand viscosity behavior in acidicon behavior both solution. an onSi bothwafer Cyclic an and voltammetry Si waferglass template; and scans glass werethese template; conducted scans these are comparedtoscans observe are comparedin the Figure electrodeposition 6. in The Figure curves6. behavior Thewere curves obta onined wereboth in an obtained electrolyteSi wafer in and an with electrolyte glass 8 mM template; of with SbO + 8these and mM 12scans of mM SbO are of+ + compared+ in Figure 6.+ The curves were obtained in an electrolyte with 8 mM of SbO and 12 mM of HTeOand 122 mMat a scan of HTeO rate 2of at5 mV/s a scan at rate room of 5temperature mV/s at room from temperature 0 to −0.55 V. from The 0linear to − 0.55voltammograms V. The linear + revealedHTeOvoltammograms2 at that a scan the reduction revealedrate of 5 thatmV/s peak the ofatreduction theroom glass temperature template peak of the was from glass not 0 to templateas −obvi0.55ous V. was Theas notthat linear asfor obvious thevoltammograms Si wafer. as that The for currentrevealedthe Si wafer. density that Thethe of reduction currentthe deposition density peak procof of the theess glass deposition on the template glass process template was not on theasis lowerobvi glassous than template as thatthat on isfor lower the the Si Si thanwafer. wafer. that This The on indicatescurrentthe Si wafer. density that Thisthe of deposition indicatesthe deposition that process the proc deposition isess more on the diffic processglassult andtemplate is morerequires is difﬁcult lower a larger than and driving that requires on force the a largerSi for wafer. the driving glass This templateindicatesforce for thethanthat glass thefor deposition templatethe Si wafer. than process forTherefore, theis more Si wafer. much diffic Therefore, ulthigher and requiresover-potential much highera larger is over-potential driving needed force to promote for is needed the glass the to reactiontemplatepromote on thethan the reaction glassfor the template on Si the wafer. glass comp Therefore, templateared with compared much that onhigher the with Si over-potential that wafer. on the Si wafer. is needed to promote the reaction on the glass template compared with that on the Si wafer.

Figure 5. (a) Schematic and (b) SEM image of the multi-channel glass template. Figure 5. ((aa)) Schematic Schematic and and ( (b)) SEM SEM image image of of the the multi-channel multi-channel glass template.

Figure 6. Cyclic voltammograms of Sb-Te electrodeposition on the Si wafer-based Pt electrode and glassFigure template 6. Cyclic at voltammograms room temperature of of atSb-Te Sb-Te a scan electrodeposit electrodeposition rate of 5 mV/sion withon the an Si electrolyte wafer-based containing Pt electrode 8 mM and of + + SbOglass andtemplate 12 mM at ofroom HTeO temperature2 . at a scan rate of 55 mV/smV/s with an electrolyte containing 8 mM of + ++ SbO andand 12 12 mM mM of of HTeO HTeO22. . By adjusting the potential, a high-quality film can be obtained on the Si wafer-based Pt electrode. By adjusting the potential, a high-quality ﬁlm can be obtained on the Si wafer-based Pt electrode. In contrast,By adjusting it was thedifficult potential, to obtain a high-quality compact pillars film can on be the obtained glass template. on the Si We wafer-based found that Pt the electrode. deposit In contrast, it was difﬁcult to obtain compact pillars on the glass template. We found that the deposit densityIn contrast, was it too was low difficult to satisfy to obtain the requirements compact pillars of TE on modules.the glass template.This is because We found the micro-poresthat the deposit on density was too low to satisfy the requirements of TE modules. This is because the micro-pores on the thedensity glass was template too low were to satisfy rather the deep requirements and their of largeTE modules. aspect ratioThis isled because to drastic the micro-pores concentration on glass template were rather deep and their large aspect ratio led to drastic concentration polarization. polarization.the glass template In addition, were the rather ions indeep solution and difftheiruse large poorly, aspect because ratio the led deposition to drastic reaction concentration can only In addition, the ions in solution diffuse poorly, because the deposition reaction can only occur on the Pt occurpolarization. on the Pt In electrode, addition, whichthe ions is inlocated solution at the diff bottomuse poorly, of the because micro-pores. the deposition Pulsed voltage reaction deposition can only electrode, which is located at the bottom of the micro-pores. Pulsed voltage deposition was therefore wasoccur therefore on the Pt investigatedelectrode, which to solve is located the problem at the bottom of concentration of the micro-pores. polarization Pulsed [3,36,52].voltage deposition Figure 7 investigated to solve the problem of concentration polarization [3,36,52]. Figure7 shows the three-step showswas therefore the three-step investigated pulsed voltage-timeto solve the andproblem curren oft-time concentration curves for polarization this deposition [3,36,52]. experiment. Figure At 7 pulsed voltage-time and current-time curves for this deposition experiment. At the beginning of theshows beginning the three-step of every pulsed deposition voltage-time stage, the and current curren densityt-time curves rapidly for reached this deposition a very high experiment. value, then At every deposition stage, the current density rapidly reached a very high value, then dropped abruptly droppedthe beginning abruptly of every and depositiongradually stage,tended the to current smooth, density eventually rapidly reached reached aa verysteady high state. value, At thenthe and gradually tended to smooth, eventually reached a steady state. At the beginning, the current beginning,dropped abruptly the current and density gradually rapidly tended increases, to smooth, corresponding eventually to a quicklyreached nucleated a steady process state. Aton thethe surfacebeginning, of thethe currentsubstrate. density As the rapidly number increases, of crys cotalrresponding nucleus increases, to a quickly the nucleated diffusion process cross-section on the surface of the substrate. As the number of crystal nucleus increases, the diffusion cross-section

Materials 2018, 11, 1194 8 of 13

Materialsdensity 2018 rapidly, 11, x increases,FOR PEER REVIEW corresponding to a quickly nucleated process on the surface of the substrate.8 of 13 As the number of crystal nucleus increases, the diffusion cross-section projection of the solute near the projection of the solute near the substrate also increases, corresponding to the acceleration of crystal substrate also increases, corresponding to the acceleration of crystal growth, that is, the increase of the growth, that is, the increase of the current; subsequently, the current value slowly declines and finally current; subsequently, the current value slowly declines and ﬁnally reaches stability, corresponding to reaches stability, corresponding to the formation of concentration polarization. Eventually, the the formation of concentration polarization. Eventually, the concentration gradient of electrolyte concentration gradient of electrolyte nearing the electrode gradually becomes stable. nearing the electrode gradually becomes stable.

Figure 7. ((a)) Three-step pulsed pulsed voltage-time voltage-time curve, curve, ( (bb)) current-time current-time curve, curve, and and ( (cc)) integral integral charge charge of of deposits under pulsed voltage conditions.

Figure 88 depicts SEM images of of the the micro-pillars micro-pillars deposited deposited in in an an electrolyte electrolyte containing containing 8 mM 8 mM of + 2+ SbOof SbO+ andand 12 mM 12 mM of HTeO of HTeO2+ throughthrough pulsed pulsed voltage voltage deposition deposition at the at reduction the reduction potential potential range rangefrom −from280 to− 280−480 to mV.−480 To mV. observe To observe the inside the insideof micro-pores, of micro-pores, the filled the template filled template was packaged was packaged in epoxy in resinepoxy and resin then and polished then polished in a metallographic in a metallographic grinding/polishing grinding/polishing machine machine to expose to expose the theinside inside of micro-pores.of micro-pores. Figure Figure 8a–i8a–i were were th thee top-and top-and side-view side-view SEM SEM images images of of Sb-Te Sb-Te pillars deposited at thethe deposition potentials of −280280 mV, mV, −−380380 mV, mV, and and −−480480 mV mV with with 15 15 h, h, respectively. respectively. It It revealed revealed that that as as thethe cathode potential moved from −280280 mV mV to to −−480480 mV, mV, the the density density of of depo depositssits decreased, decreased, but but the speedspeed of of nucleation nucleation and and growth growth rose rose and the the filling filling rate rate increased. increased. This This is is because because the the current current density density and nucleation raterate andand growth growth rose rose due due to to the the higher higher over-potential over-potential and and numerous numerous crystal crystal nuclei nuclei stuck stuckto the to Si the wafer-based Si wafer-based Pt electrode Pt electrode at the bottomat the bo ofttom the template, of the template, allowing allowing more micro-pores more micro-pores to be filled. to beHowever, filled. However, the filling the quality filling was quality decreased was decreased because because of the high of the growth high rate,growth which rate, decreased which decreased density densityand led and to more led fillingto more defects. filling Thedefects. concentration The concentration gradient becamegradient more became obvious more with obvious increasing with increasingover-potential, over-potential, which caused which dendrite caused growthdendrite in gr crystalowth in grains crystal and grains the decreaseand the decrease of pillar of density. pillar density.Considering Considering these findings, these fi thendings, cathode the reductioncathode reduction potential ofpotential−280 mV of was−280 selectedmV was to selected obtain theto obtainmicro-pillars the micro-pillars with higher with density. higher Using density. the three-stepUsing the pulsedthree-step voltage pulsed regime voltage illustrated regime inillustrated Figure6, inrelatively Figure 6, compact relatively pillar compact deposits pillar were deposits obtained. were In obtained. 20 pulsed In cycles, 20 pulsed the integral cycles, the charge integral is − 0.018charge C isfrom −0.018 the current-timeC from the current-time curve shown curve in Figure shown7c. Itin takes Figure about 7c. It 15 takes h in theoryabout 15 to completeh in theory the to deposition complete theof an deposition entire micro-pillar. of an entire However, micro-pillar. actually However, the growth actually of each the pillar growth is effected of each by pillar surrounding is effected pillars; by surroundingbesides, the growth pillars; speedbesides, of individualthe growth micro-pillarsspeed of individual is much micro-pillars higher than othersis much on higher account than of others a local oncurrent account density of a concentration local current whichdensity was concentration raised by nonuniformity which was raised in micro-pores by nonuniformity forming. To in increase micro- poresthe filling forming. ratio, To we increase lengthened the filling the deposition ratio, we time lengthened to ensure the that deposition most micro-pores time to ensure were filledthat most with micro-poresthe TE material. were Whenfilled with the deposition the TE material. time was When extended the deposition to 30 h, thetime filling was extended ratio of the to 30 template h, the fillingdeposited ratio at of− the280 template mV can deposited reached 96%. at −280 The mV SEM can images reached of micro-pillars96%. The SEM deposited images of at micro-pillars−280 mV at deposited30 h were shownat −280 in mV Figure at 830j–k. h Thewere filling shown ratio in ofFigure micro-pillars 8j–k. The in thefilling condition ratio of of micro-pillars different deposition in the conditionpotentials of and different times deposition is shown in potentials Figure8l. and After times deposition, is shown abrasivein Figure paper8l. After and deposition, a metallographic abrasive papergrinding/polishing and a metallographic machine grinding/polishing were used to remove machine the excess were depositedused to remove material, the ensuringexcess deposited that the material,micro-pillars ensuring were that as long the micro-pillars as the micro-pores were as and long the as upper the micro-pores surface was and smooth. the upper The surface EDS results was smooth.showed The that EDS the micro-pillarresults showed composition that the micro-pill was similarar composition to that of the was films similar deposited to that atof the samefilms − depositedpotential. at Figure the same9 was potential. the elementary Figure mappings9 was the elementary of the Sb 2Te mappings3 pillars depositedof the Sb2Te at3 pillars280 mV deposited at 30 h; atthe − result280 mV indicated at 30 h; that the the result composition indicated of thethat whole the composition micro-pillars of was the homogeneous whole micro-pillars mainly, but was at + homogeneousthe bottom of themainly, pillars, but theat the composition bottom of ofthe Sb/ pillars, Te was the slightly composition lower of than Sb/ 2:3,Te was because slightly SbO lowerhas + thaninferior 2:3, diffusibility because SbO compared+ has inferior to HTeO diffusibility2 . compared to HTeO2+.

Materials 2018, 11, 1194 9 of 13 Materials 2018, 11, x FOR PEER REVIEW 9 of 13 Materials 2018, 11, x FOR PEER REVIEW 9 of 13

Figure 8. Top-and side-view SEM images of Sb-Te pillars deposited in a glass template at the Figure 8. Top-and side-view SEM images of Sb-Te pillars deposited in a glass template at the deposition Figuredeposition 8. Top-and parameters side-view of (a–c ) SEM−280 mV,images 15 h;of ( dSb-Te–f) −380 pill mV,ars 15deposited h; (g–i) −in480 a mV, glass 15 templateh; and (j, kat) −the280 parameters of (a–c) −280 mV, 15 h; (d–f) −380 mV, 15 h; (g–i) −480 mV, 15 h; and (j,k) −280 mV, 30 h. depositionmV, 30 h. ( lparameters) The filling of ratio (a– cin) −micro-pores280 mV, 15 h;at (thed– fcondit) −380 ionmV, of 15 different h; (g–i) deposition−480 mV, 15 potential h; and (andj,k) time.−280 (l) The ﬁlling ratio in micro-pores at the condition of different deposition potential and time. mV, 30 h. (l) The filling ratio in micro-pores at the condition of different deposition potential and time.

Figure 9. The elementary mappings of the Sb2Te3 pillars.

Figure 9. The elementary mappings of the Sb2Te3 pillars. A microprobe test platformFigure 9. wasThe used elementary to measure mappings the ofpotential–current the Sb2Te3 pillars. curve of a single pillar to calculateA microprobe its resistance, test platformas shown was in Figure used to10. measure The resistance the potential–current of the pillar with curve a length of a single of 180 pillar µm and to calculatediameter its of resistance,60 µm was as about shown 20 inΩ .Figure Considering 10. The the resistance Sb2Te3 films, of the whichpillar withhave a an length electrical of 180 resistivity µm and −6 diameterof about of10 60 Ω µmm, wasthe same about size 20 Ω with. Considering the above the pillar Sb 2willTe3 films, have whicha resistance have anof electricalapproximately resistivity 6 Ω. of about 10−6 Ωm, the same size with the above pillar will have a resistance of approximately 6 Ω.

Materials 2018, 11, 1194 10 of 13

A microprobe test platform was used to measure the potential–current curve of a single pillar to calculate its resistance, as shown in Figure 10. The resistance of the pillar with a length of 180 µm and

Materialsdiameter 2018 of, 11 60, xµ FORm was PEER about REVIEW 20 Ω . Considering the Sb2Te3 films, which have an electrical resistivity10 of 13 of about 10−6 Ωm, the same size with the above pillar will have a resistance of approximately 6 Ω. Thus, it seems that a difference exists between the the resistances resistances of of the the micro-pillars micro-pillars and and bulk bulk material. material. The resistanceresistance ofof the the micro-pillar micro-pillar was was measured measured by by the the two-point two-point method method using using a microprobe, a microprobe, so large so largecontact contact resistance resistance between between the the microprobe microprobe and and micro-pillar micro-pillar is is possible. possible. ConsideringConsidering this factor, factor, the measured measured resistance resistance of of the the micro-pillar micro-pillar is isreason reasonable.able. In addition, In addition, the crystallinity, the crystallinity, grain grain size, size,and testingand testing method method also influence also influence the resistance the resistance of micro-pillars, of micro-pillars, which which remains remains unsolved unsolved and needs and needsmore attentionmore attention in the infuture. the future. Regardless, Regardless, these theseresults results indicate indicate that micro-pillars that micro-pillars of dense of dense Sb2Te Sb3 with2Te3 highwith fillinghigh filling ratio and ratio large and largeaspect aspect ratio can ratio be can obtained be obtained by electrodeposition by electrodeposition based based on multi-channel on multi-channel glass template,glass template, which which can be can further be further applied applied in the in thefabrication fabrication of ofmicro-TEGs micro-TEGs with with large large output output voltage voltage and power.

Figure 10. (a) The test platform of a single micro-pillar and (b) the potential–current curve of a single Figure 10. (a) The test platform of a single micro-pillar and (b) the potential–current curve of a micro-pillar. single micro-pillar.

It seems that the aspect ratio of our deposits is incomparable with the nanowire that has been fabricatedIt seems by thatprevio theus aspectwork [40,53], ratio of but our it deposits also takes is incomparablenumerous of withnanowire the nanowire arrays to thatform has a thermoelectricbeen fabricated leg by when previous it is assembled work [40, 53into], buta device it also. Therefore, takes numerous the aspect of ratio nanowire of a single arrays nanowire to form isa thermoelectricmeaningless when leg when using it TE is assembleddevice. Besides, into a the device. lengths Therefore, of most thenanowires aspect ratio are too of ashort single to nanowireestablish ais large meaningless temperature when difference. using TE device. Therefore, Besides, the main the lengths advantage of most of our nanowires thermoelectric are too shortleg is to that establish it can achievea large temperaturehigh integration difference. level in Therefore, the precondition, the main such advantage that the of length our thermoelectric of the TE legs legis large is that enough it can toachieve establish high a integrationlarge temperature level in thedifference. precondition, such that the length of the TE legs is large enough to establish a large temperature difference. 4. Conclusions 4. Conclusions Sb2Te3 TE films and micro-pillars were deposited on Si wafer-based Pt electrode and a Sb Te TE films and micro-pillars were deposited on Si wafer-based Pt electrode and a microporous2 3 glass template, respectively. The results indicate that the composition of deposits microporous glass template, respectively. The results indicate that the composition of deposits depends depends on the electrolyte concentration, and the micromorphology depends on the reduction on the electrolyte concentration, and the micromorphology depends on the reduction potential. potential. With increasing Te concentration, the charge integral and current density of the deposition With increasing Te concentration, the charge integral and current density of the deposition and and dissolution processes both rose. As the deposition potential shifted to negative value, the dissolution processes both rose. As the deposition potential shifted to negative value, the composition composition of the deposit remained unchanged, but the grain size increased, and the film density of the deposit remained unchanged, but the grain size increased, and the film density decreased. As a decreased. As a result, Sb2Te3 film with nominal composition and dense morphology on Si wafer result, Sb2Te3 film with nominal composition and dense morphology on Si wafer based Pt electrode+ based Pt electrode can be obtained using the reduction potential of −300 mV+ with 8 mM of SbO and+ can be obtained using the reduction potential of −300 mV with 8 mM of SbO and 12 mM of HTeO2 . 12 mM of HTeO2+. The Seebeck coefficient of aforesaid TE film is positive, which proves that the The Seebeck coefficient of aforesaid TE film is positive, which proves that the deposit is a p-type deposit is a p-type semiconductor. However, it is difficult to obtain compact pillars on the glass semiconductor. However, it is difficult to obtain compact pillars on the glass template using the template using the same condition on the Si wafer -based Pt electrode because of the concentration same condition on the Si wafer -based Pt electrode because of the concentration polarization when polarization when fabricating the micro-pillars. Nevertheless, the pulsed voltage deposition is found to be an effective way to fabricate Sb2Te3 micro-pillars. A deposition potential of −280 mV and a dissolve potential of 500 mV were selected, which can realize the deposition of micro-pillars with a large aspect ratio and high density. The deposition technology can be further applied in the fabrication of micro-TEGs with large output voltage and power.

Materials 2018, 11, 1194 11 of 13 fabricating the micro-pillars. Nevertheless, the pulsed voltage deposition is found to be an effective way to fabricate Sb2Te3 micro-pillars. A deposition potential of −280 mV and a dissolve potential of 500 mV were selected, which can realize the deposition of micro-pillars with a large aspect ratio and high density. The deposition technology can be further applied in the fabrication of micro-TEGs with large output voltage and power.

Author Contributions: Data curation: N.S. and S.G.; formal analysis: N.S.; investigation: N.S.; resources: F.L.; software: D.L.; supervision: F.L. and B.L.; writing—original draft: N.S.; writing—review & editing: N.S. Funding: This research was funded by the National Key Research and Development Program of China (Grant No. 2017YFB0406300), the Shenzhen Science and Technology Plan Project (No. JCYJ20150827165038323), and the Natural Science Foundation of SZU (No. 2016016). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Nour, A.; Hassan, N.; Refaat, H.M.; Soliman, H.M.A.; El-Dissouky, A. Effect of reducing agent strength on the growth and thermoelectric performance of nanocrystalline bismuth telluride. Mater. Res. Express 2018, 5, 035033. [CrossRef]

2. Adam, A.M.; Petkov, P. Structural and optical properties of nano-powder-based (Sb1−xBix)2Te3 thin ﬁlms. Mater. Res. Express 2017, 4, 085029. [CrossRef] 3. Shen, S.; Zhu, W.; Deng, Y.; Zhao, H.; Peng, Y.; Wang, C. Enhancing thermoelectric properties of

Sb2Te3 ﬂexible thin ﬁlm through microstructure control and crystal preferential orientation engineering. Appl. Surf. Sci. 2017, 414, 197–204. [CrossRef] 4. Martin-Gonzalez, M.S.; Prieto, A.L.; Gronsky, R.; Sands, T.; Stacy, A.M. Insights into the electrodeposition of

Bi2Te3. J. Electrochem. Soc. 2002, 149, C546–C554. [CrossRef] 5. Rowe, D.M. CRC Handbook of Thermoelectrics; CRC: Washington, DC, USA, 1995. 6. Li, J.; Pan, Y.; Wu, C.; Sun, F.; Wei, T. Processing of advanced thermoelectric materials. Sci. China Technol. Sci. 2017, 60, 1347–1364. [CrossRef] 7. Sajid, M.; Hassan, I.; Rahman, A. An overview of cooling of thermoelectric devices. Renew. Sustain. Energy. Rev. 2017, 78, 15–22. [CrossRef] 8. Zhao, D.; Tan, G. A review of thermoelectric cooling: Materials, modeling and applications. Appl. Therm. Eng. 2014, 66, 15–24. [CrossRef] 9. Liu, D.W.; Li, J.F. Electrocrystallization process during deposition of Bi-Te ﬁlms. J. Electrochem. Soc. 2008, 155, D493–D498. [CrossRef] 10. Wu, Y.; Lin, Z.; Tian, Z.; Han, C.; Liu, J.; Zhang, H.; Zhang, Z.; Wang, Z.; Dai, L.; Cao, Y.; et al. Fabrication

of Microstructured thermoelectric Bi2Te3 thin ﬁlms by seed layer assisted electrodeposition. Mater. Sci. Semicond. Process. 2016, 46, 17–22. [CrossRef] 11. Wu, M.; Binnemans, K.; Fransaer, J. Electrodeposition of antimony from chloride-free ethylene glycol

solutions and fabrication of thermoelectric Bi2Te3/(Bi1−xSbx)2Te3 multilayers using pulsed potential electrodeposition. Electrochim. Acta 2014, 147, 451–459. [CrossRef]

12. Wang, Y.; Zhang, J.; Shen, Z.; Yang, M.; Liu, X.; Wang, W. Preparation of Bi2Te3/Nano-SiC Composite Thermoelectric Films by Electrodeposition. J. Electron. Mater. 2015, 44, 2166–2171. [CrossRef]

13. Tu, F.; Xie, J.; Cao, G.; Zhao, X. Self-Assembly of Bi2Te3-Nanoplate/Graphene-Nanosheet Hybrid by One-Pot Route and Its Improved Li-Storage Properties. Materials 2012, 5, 1275–1284. [CrossRef] 14. Goldsmid, H.J. Bismuth Telluride and Its Alloys as Materials for Thermoelectric Generation. Materials 2014, 7, 2577–2592. [CrossRef][PubMed] 15. Jood, P.; Ohta, M. Hierarchical Architecturing for Layered Thermoelectric Sulﬁdes and Chalcogenides. Materials 2015, 8, 1124–1149. [CrossRef][PubMed] 16. Merrill, D.R.; Moore, D.B.; Bauers, S.R.; Falmbigl, M.; Johnson, D.C. Misﬁt Layer Compounds and Ferecrystals: Model Systems for Thermoelectric Nanocomposites. Materials 2015, 8, 2000–2029. [CrossRef] [PubMed] 17. Hao, F.; Qiu, P.F.; Song, Q.F.; Chen, H.Y.; Lu, P.; Ren, D.D.; Shi, X.; Chen, L.D. Roles of Cu in the Enhanced

Thermoelectric Properties in Bi0.5Sb1.5Te3. Materials 2017, 10, 251. [CrossRef][PubMed] Materials 2018, 11, 1194 12 of 13

18. Li, W.; Sipe, D.M.; Yong, X.; Qiao, L. A MEMS Thermal Biosensor for Metabolic Monitoring Applications. J. Microelectromech. Syst. 2008, 17, 318–327. 19. Bar-Cohen, A.; Wang, P. On-chip Hot Spot Remediation with Miniaturized Thermoelectric Coolers. Microgravity Sci. Technol. 2009, 21, 351–359. [CrossRef] 20. Snyder, G.J.; Lim, J.R.; Huang, C.K.; Fleurial, J.P. Thermoelectric microdevice fabricated by a MEMS-like electrochemical process. Nat. Mater. 2003, 2, 528–531. [CrossRef][PubMed] 21. Huang, I.Y.; Lin, J.C.; She, K.D.; Li, M.C.; Chen, J.H.; Kuo, J.S. Development of low-cost micro-thermoelectric coolers utilizing MEMS technology. Sens. Actuators A Phys. 2008, 148, 176–185. [CrossRef] 22. Cao, Y.; Zeng, Z.G.; Liu, Y.L.; Zhang, X.P.; Shen, C.; Wang, X.H.; Gan, Z.X.; Wu, H.; Hu, Z.Y. Electrodeposition

and Thermoelectric Characterization of (00L)-Oriented Bi2Te3 Thin Films on Silicon with Seed Layer. J. Electrochem. Soc. 2013, 160, D565–D569. [CrossRef]

23. Nanda, M.M. Joining Techniques of Bi2te3 Elements in Thermoelectric Cooling Devices. J. Electrochem. Soc. 1963, 110, C54. 24. Patil, P.B.; Mali, S.S.; Kondalkar, V.V.; Mane, R.M.; Patil, P.S.; Hong, C.K.; Bhosale, P.N. Morphologically

controlled electrodeposition of fern shaped Bi2Te3 thin ﬁlms for photoelectrochemical performance. J. Electroanal. Chem. 2015, 758, 178–190. [CrossRef] 25. Datta, A.; Sangle, A.; Hardingham, N.; Cooper, C.; Kraan, M.; Ritchie, D.; Narayan, V.; Kar-Narayan, S.

Structure and Thermoelectric Properties of Bi2-xSbxTe3 Nanowires Grown in Flexible Nanoporous Polycarbonate Templates. Materials 2017, 10, 553. [CrossRef][PubMed] 26. Gierczak, M.; Prazmowska-Czajka, J.; Dziedzic, A. Thermoelectric Mixed Thick-/Thin Film Microgenerators Based on Constantan/Silver. Materials 2018, 11, 115. [CrossRef][PubMed] 27. Chen, C.L.; Chen, Y.Y.; Lin, S.J.; Ho, J.C.; Lee, P.C.; Chen, C.D.; Harutyunyan, S.R. Fabrication and Characterization of Electrodeposited Bismuth Telluride Films and Nanowires. J. Phys. Chem. C 2010, 114, 3385–3389. [CrossRef] 28. Heo, P.; Hagiwara, K.; Ichino, R.; Okido, M. Electrodeposition and thermoelectric characterization of Bi2Te3. J. Electrochem. Soc. 2006, 153, C213–C217. [CrossRef] 29. Leimkuhler, G.; Kerkamm, I.; Reineke-Koch, R. Electrodeposition of antimony telluride. J. Electrochem. Soc. 2002, 149, C474–C478. [CrossRef] 30. Trung, N.H.; Sakamoto, K.; Toan, N.V.; Ono, T. Synthesis and Evaluation of Thick Films of Electrochemically

Deposited Bi2Te3 and Sb2Te3 Thermoelectric Materials. Materials 2017, 10, 154. [CrossRef][PubMed] 31. Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; et al. High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science 2008, 320, 634–638. [CrossRef][PubMed] 32. Kuleshova, J.; Koukharenko, E.; Li, X.H.; Frety, N.; Nandhakumar, I.S.; Tudor, J.; Beeby, S.P.; White, N.M. Optimization of the Electrodeposition Process of High-Performance Bismuth Antimony Telluride Compounds for Thermoelectric Applications. Langmuir 2010, 26, 16980–16985. [CrossRef][PubMed] 33. Snyder, G.J. Application of the compatibility factor to the design of segmented and cascaded thermoelectric generators. Appl. Phys. Lett. 2004, 84, 2436–2438. [CrossRef]

34. Miyazaki, Y.; Kajitani, T. Preparation of Bi2Te3 ﬁlms by electrodeposition. J. Cryst. Growth 2001, 229, 542–546. [CrossRef] 35. Mun, H.; Lee, K.H.; Kim, S.J.; Kim, J.Y.; Lee, J.H.; Lim, J.H.; Park, H.J.; Roh, J.W.; Kim, S.W. Fe-Doping Effect

on Thermoelectric Properties of p-Type Bi0.48Sb1.52Te3. Materials 2015, 8, 959–965. [CrossRef][PubMed] 36. Schumacher, C.; Reinsberg, K.G.; Akinsinde, L.; Zastrow, S.; Heiderich, S.; Toellner, W.; Rampelberg, G.;

Detavernier, C.; Broekaert, J.A.C.; Nielsch, K.; et al. Optimization of Electrodeposited p-Doped Sb2Te3 Thermoelectric Films by Millisecond Potentiostatic Pulses. Adv. Energy Mater. 2012, 2, 345–352. [CrossRef] 37. Li, X.H.; Koukharenko, E.; Nandhakumar, I.S.; Tudor, J.; Beeby, S.P.; White, N.M. High density p-type

Bi0.5Sb1.5Te3 nanowires by electrochemical templating through ion-track lithography. Phys. Chem. Chem. Phys. 2009, 11, 3584–3590. [CrossRef][PubMed]

38. Li, F.H.; Wang, W. Electrodeposition of BixSb2-xTey thermoelectric thin ﬁlms from nitric acid and hydrochloric acid systems. Appl. Surf. Sci. 2009, 255, 4225–4231. [CrossRef] 39. Li, J.-F.; Tanaka, S.; Umeki, T.; Sugimoto, S.; Esashi, M.; Watanabe, R. Microfabrication of thermoelectric materials by silicon molding process. Sens. Actuators A Phys. 2003, 108, 97–102. [CrossRef] Materials 2018, 11, 1194 13 of 13

40. Lim, J.R.; Whitacre, J.F.; Fleurial, J.P.; Huang, C.K.; Ryan, M.A.; Myung, N.V. Fabrication method for thermoelectric nanodevices. Adv. Mater. 2005, 17, 1488–1492. [CrossRef]

41. Liu, D.-W.; Xu, Y.; Li, J.-F. Electrodeposition of Bi2Te3 ﬁlms and micro-pillar arrays on p-Si(100) wafers. Phys. Status Solidi A 2010, 207, 354–359. [CrossRef] 42. Martin, J.; Manzano, C.V.; Caballero-Calero, O.; Martin-Gonzalez, M. High-aspect-ratio and highly ordered 15-nm porous alumina templates. ACS Appl. Mater. Interfaces 2013, 5, 72–79. [CrossRef][PubMed] 43. Chu, S.Z.; Wada, K.; Inoue, S.; Segawa, H. Direct Growth of Highly Ordered Crystalline Zirconia Nanowire Arrays with High Aspect Ratios on Glass by a Tailored Anodization. J. Electrochem. Soc. 2011, 158, C148–C157. [CrossRef] 44. Kim, M.Y.; Oh, T.S. Thermoelectric Power Generation Characteristics of a Thin-Film Device Consisting of

Electrodeposited n-Bi2Te3 and p-Sb2Te3 Thin-Film Legs. J. Electron. Mater. 2013, 42, 2752–2757. [CrossRef] 45. Del Frari, D.; Diliberto, S.; Stein, N.; Boulanger, C.; Lecuire, J.-M. Comparative study of the electrochemical

preparation of Bi2Te3, Sb2Te3, and (BixSb1−x)2Te3 ﬁlms. Thin Solid Films 2005, 483, 44–49. [CrossRef] 46. Zhu, Y.-B.; Wang, W. Microstructure and thermoelectric properties of p-type Bi–Sb–Te–Se thin ﬁlms prepared by electrodeposition method. Thin Solid Films 2012, 520, 2474–2478. [CrossRef]

47. Kim, M.-Y.; Oh, T.-S. Electrodeposition and Thermoelectric Characteristics of Bi2Te3 and Sb2Te3 Films for Thermopile Sensor Applications. J. Electron. Mater. 2009, 38, 1176–1181. [CrossRef] 48. Kim, M.-Y.; Oh, T.-S. Processing and Thermoelectric Performance Characterization of Thin-Film Devices Consisting of Electrodeposited Bismuth Telluride and Antimony Telluride Thin-Film Legs. J. Electron. Mater. 2011, 40, 759–764. [CrossRef] 49. Danaher, W.J.; Lyons, L.E. The Kinetics of the Electrodeposition of Cadmium Telluride. Aust. J. Chem. 1984, 37, 689–701. [CrossRef] 50. Michel, S.; Stein, N.; Schneider, M.; Boulanger, C.; Lecuire, J.-M. Optimization of chemical and electrochemical

parameters for the preparation of n-type Bi2Te2.7Se0.3 thin ﬁlms by electrodeposition. J. Appl. Electrochem. 2003, 33, 23–27. [CrossRef] 51. Kroger, F.A. Cathodic Deposition and Characterization of Metallic or Semiconducting Binary Alloys or Compounds. J. Electrochem. Soc. 1978, 125, 2028–2034. [CrossRef]

52. Zou, Z.G.; Cai, K.F.; Chen, S.; Qin, Z. Pulsed electrodeposition and characterization of Bi2Te3−ySey ﬁlms. Mater. Res. Bull. 2012, 47, 3292–3295. [CrossRef] 53. Pinisetty, D.; Gupta, M.; Karki, A.B.; Young, D.P.; Devireddy, R.V. Fabrication and characterization of electrodeposited antimony telluride crystalline nanowires and nanotubes. J. Mater. Chem. 2011, 21, 4098–4107. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).