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Nano Energy (2014) 8, 223–230

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Enhanced thermoelectric properties of n-typeBi2Te2.7Se0.3 thin films through theintroduction of Pt nanoinclusions by pulsedlaser deposition

Ting Suna, Majid Kabiri Samanib, Narjes Khosravianb,Kok Ming Anga, Qingyu Yana, Beng Kang Tayb, Huey Hoon Hnga,n

aSchool of Materials Science and Engineering, Nanyang Technological University, Singapore 639798,SingaporebNovitas, Nanoelectronics Centre of Excellence, School of Electrical & Electronic Engineering, NanyangTechnological University, Singapore 639798, Singapore

Received 5 February 2014; received in revised form 28 April 2014; accepted 10 June 2014Available online 21 June 2014

KEYWORDSThermoelectric;Bi2Te2.7Se0.3;Thin film;Nanoinclusion;Pulsed laserdeposition

0.1016/j.nanoen.2lsevier Ltd. All rig

thor. Tel.: +65 67shhhng@ntu.edu.

AbstractThis work demonstrates the first preparation of textured n-type Bi2Te2.7Se0.3 thin films withcontent-adjustable Pt nanoinclusions by pulsed laser deposition. Characterization results revealthat metallic Pt nanoinclusions are embedded in the semiconductor matrix at grain boundaries.Addition of Pt nanoinclusions results in a higher in-plane power factor based on thesimultaneous increase in electrical conductivity and absolute Seebeck coefficient. Power factorof the optimized nanocomposite thin film reaches 3.51� 10–3 W/mK2 at room temperature,which is a more than 20% enhancement as compared to the single phase Bi2Te2.7Se0.3 thin film.An even greater improvement in the in-plane ZT can be expected from a reduced thermalconductivity, as indicated by cross-plane thermal property measurement. This work highlightsthe feasibility of combining nanocomposite engineering with textured thin films to furtherimprove thermoelectric performance.& 2014 Elsevier Ltd. All rights reserved.

014.06.011hts reserved.

904140; fax: +65 67909081.sg (H.H. Hng).

Introduction

The ever increasing energy demand and the global environmentconcerns have created a significant interest in exploiting novelenergy materials against waste, pollution and global warming.

T. Sun et al.224

With the capability to transform a temperature difference intoelectricity, and vice-versa, thermoelectric (TE) materials arepotential candidates for waste heat recovery [1] and solid statecooling [2] in a silent and reliable manner. The performance ofa TE material is governed by the dimensionless figure of merit,ZT ¼ ðS2σ=κÞ T, where S, σ and T are the Seebeck coefficient,electrical conductivity and absolute temperature, respectively,and κ is the thermal conductivity, which is contributed fromboth electronic carriers (κe) and lattice vibration (κL). Thestrong interdependence of the electronic transport properties(S, σ and κe) has limited ZT to about unity for commercialdevices for a long time.

Currently, the best commercial-available TE materials forroom temperature applications are still bismuth telluride basedintermetallic semiconductor materials, typically p-typeBi0.5Sb1.5Te3 and n-type Bi2Te2.7Se0.3, that are established asTE candidates for more than 50 years ago [3]. Over the past twodecades, nanostructures, such as superlattice thin films [4,5],nanowires [6], nanograins [7–10], have been introduced toenhance the TE property. At this stage, their major contributionwas reckoned to be a significant reduction in κL through thephonon scattering at the grain boundaries. Very recently, nano-engineering has been further extended to heterostructurednanocomposite materials. Typically, theoretical calculations[11,12] have revealed that the metallic nanoinclusions, assecondary phase embedded in the semiconductor matrix, canintroduce energy-dependent carrier scattering effect, wherethe band bending at the metal/semiconductor interface canscatter low-energy (cold) carriers to enhance the Seebeckcoefficient without deteriorating the electrical conductivitysignificantly. Such energy filtering effect can realize thesimultaneous improvement in power factor (PF=Sσ2) andsuppression of the thermal conductivity to achieve higher ZT.This has been successfully demonstrated in Bi/Bi2Te3 [13] andBi2Te3/Bi2S3 [14] nanocomposites, Pt/Sb2Te3 [15] and AgxTey/Sb2Te3 [16] dropcast films, PbTe/Bi2Te3 barbell nanowires [17],etc., based on various wet-chemical synthesis methods. How-ever, in order to achieve good TE performance, wet-chemicalpreparations face the challenges of oxidization, surfactantremoval and high pressure densification. In comparison, physi-cal vapor deposition techniques, such as pulsed laser deposition(PLD) [18,19], sputtering [20,21], and thermal evaporation [22],cater for dense and textured thin films with coherent grainboundaries, ensuring excellent electrical conductivity compar-able to bulk single crystals [19]. In addition, they facilitate theMicro-Electro-Mechanical-system (MEMS) processing for on-chipdevices [23]. Unfortunately, the power factor improvement isstill a great challenge in Bi2Te3-based n-type polycrystallinematerials due to their strong anisotropic dependence on texture[24,25]. In this study, for the first time, we report thepreparation of n-type Bi2Te2.7Se0.3 textured thin films with Ptnanoinclusions via a one-step PLD method. Our success in thephysical deposition of nanocomposite thin films and the greatTE enhancement provide an alternative strategy in the nano-engineering of novel TE materials towards their promisingapplications.

Figure 1 XRD patterns of the as-prepared thin films ascompared with the powder XRD pattern. Inset is the schematicillustration of the nanocomposite thin film preparation by thePLD method.

Experimental

Bi2Te2.7Se0.3-based thin films with high c-axis texture wereprepared on fused silica substrates at 350 1C in 20 Pa

(150 mTorr) argon pressure by using a KrF excimer laser(Lambda Physik Compex, λ=248 nm) under 3 Hz repetitionwith �1.2 J/cm2 laser energy density for each pulse.In order to prepare nanocomposite thin film with Ptnanoinclusions, a commercially available Bi2Te2.7Se0.3 tablettarget was partially covered by a 0.1 mm thin platinumstripe (99.99%, Goodfellow) along the diameter. As illu-strated in the inset of Figure 1, the rotation of the targetsetup during deposition ensured alternate deposition ofboth materials. Instead of using a pre-mixed compositetarget, our method can mostly avoid the simultaneousdeposition and inter-reaction of Pt and Bi–Te–Se materialsin a mixed plume. In addition, variable amount of Ptnanoinclusions could be achieved by changing the width ofthe platinum stripe (1–3 mm). The thickness of each thinfilm was measured using a surface profiler (Alpha-Step IQ),while the Pt amount was detected by electron probe micro-analyzer (EPMA) equipped on a JEOL JXA-8530F Fieldemission microprobe. The thickness, Pt content and thenominal composition of each thin film are listed in Table 1.

The crystal structure of the thin films were characterizedby X-ray diffraction (XRD) in a Bruker D8 Advance diffract-ometer with a Cu Kα radiation (λ=1.54178 Å). Surface analysisof the thin films was performed by an X-ray photoelectronspectroscopy (XPS) using Theta Probe XPS (Thermo FisherScientific) equipped with an aluminum anode (15 kV, 100 W,hv=1486.6 eV). Cross-sectional transmission electron micro-scopy (TEM) images, selected-area election diffraction (SAED)patterns and compositional analysis were taken using a JEOL2100F at 200 kV equipped with Oxford X-ray energy dispersivespectroscopy (EDX) detector.

The in-plane electrical conductivity and Seebeck coefficientof the thin films were measured by an Ulvac ZEM-3 using thestandard steady state method. Room temperature carrierconcentration and mobility of each thin film was determinedby the Hall measurement (Bio Rad HL5500) with four-point-probe technique using van der Pauw geometry. The thermalconductivity of the thin films was measured by well-developed

Table 1 Thickness, Pt content and nominal composi-tion of the thin films.

Sample Thickness(nm)

Pt content(wt%)

Composition

N0 253 0 Bi2Te2.7Se0.3N1 247 0.62 Pt0.025/

Bi2Te2.7Se0.3N2 254 1.13 Pt0.046/

Bi2Te2.7Se0.3N3 251 1.46 Pt0.060/

Bi2Te2.7Se0.3

225Enhanced thermoelectric properties of n-type Bi2Te2.7Se0.3 thin films

3ωmethod [26] with details given in Supplmentary information.In brief, a SiO2 film with 600 nm thickness was prepared byelectron beam evaporation on top of a thin film. Then a goldwire with 1 mm length, 20 μm width and 150 nm thickness,used as the heater, was fabricated by photo-lithographytechnique. Identical fabrication was also performed on a baresubstrate as a reference test.

Results and discussion

Figure 1(a) shows the XRD patterns of the as-prepared thinfilms. All of which can be clearly indexed to the wellcrystallized rhombohedral structure (R-3m) of Bi2Te2.7Se0.3as previously reported [13]. In contrast with the powder XRDpattern, remarkably enhanced (00l) peaks, as well as thevery weak peak of (015) at 2θ�281, can be observed in thethin film patterns, which indicates their strong c-axistexture perpendicular to the substrate. For the thin filmswith Pt nanoinclusions, no diffraction peaks related toplatinum metal phase can be detected in their XRD pat-terns, which is mostly because of the small amount of Ptnanoinclusions in the matrix phase (o1.5 wt%, as detectedby EPMA and listed in Table 1).

Pt species in the nanocomposite thin films can be detectedby XPS surface analysis. Figure 2(a) shows the XPS fine scan ofPt 4f in thin film N2. The binding energy of Pt doublets arelocated at 74.74 and 71.25 eV for Pt 4f (5/2) and (7/2),respectively, with the spin energy separation about 3.49 eV.These values are characteristic for metallic platinum [27],showing that the Pt species in the Bi2Te2.7Se0.3 matrix are freefrom oxidization and formation of telluride compounds. InFigure 2(b), the low-magnification cross-sectional TEM imageshows a uniform thin film with column structures perpendicularto the substrate surface. The high-magnification image inFigure 2(c) reveals that the column structure corresponds to awell crystallized Bi2Te2.7Se0.3 matrix phase with their layeredlattice parallel to the substrate. The column edges are mostlycomposed of grain boundaries of the matrix phase with thepresence of a great amount of stacking faults and structuraldefects [19]. The SAED pattern in Figure 2(e), taken from thesquare area in Figure 2(c), clearly indicates a Bi2Te2.7Se0.3 singlecrystal pattern with c-axis perpendicular to thesubstrate. This is consistent with the c-axis texture in the XRDpattern, and the observation is similar to reportedBi2Te3-based thin films [19,20]. Besides the Bi2Te2.7Se0.3 matrixphase, some dark contrast regions (about 5–10 nm in width and

10–20 nm in length) at the matrix phase boundaries can also beobserved in Figure 2(c). These regions can be clearly identifiedas Pt nanoinclusions based on the EDX line scan analysis. Inaddition, some of the Pt nanoinclusions are crystallized, asindicated by the well-ordered lattice in the circled area inFigure 2(c), as well as its corresponding fast Fourier transform(FFT) pattern in Figure 2(d).

TEM-EDX analysis indicates that the Pt nanoinclusions arelocated only at the Bi2Te2.7Se0.3 grain boundaries but notinside the matrix grains. This may originate from the thinfilm growth mechanism of the Bi2Te3-based thin films as wellas our method of introducing Pt nanoinclusions. Based onphysical vapor deposition techniques [20], Bi2Te3-basedmaterials has a strong tendency to stack along the c-axisduring the deposition process, and leaves the high energydangling bonds of ab-directions at the lateral grain bound-aries. This is usually followed by an extensive growth alongthe ab-directions on a hot substrate surface [19], which canbe regarded as a ripening process. In our PLD fabrication ofnanocomposite thin films, the laser ablation takes placealternately on Bi2Te2.7Se0.3 target and on the platinumstripe. Thus, the mixing of and the reaction between Ptand Bi–Te–Se are largely suppressed. When the successivelaser-ablated Pt atoms reach the growing thin film surface,the high energy sites at Bi2Te2.7Se0.3 matrix phase grainboundaries are highly favorable in order to reduce thesystem energy. Since the Pt content in the sample is quitelow, and most of the c-plane surfaces for matrix grains arenot contaminated, the texture growth for the laterBi2Te2.7Se0.3 layers is unaffected. In the thin film ripeningprocess, however, the grain growth of the matrix phase(along ab-directions) may be slightly interrupted at the siteof Pt nanoinclusions, and thus leaving the site as part of thematrix grain boundaries.

Figure 3(a)–(c) summarizes the electrical transport propertiesof the thin film samples from room temperature to 550 K.Owing to the high c-axis texture and well crystallization ofthese thin films, their electrical conductivity (σ) and absoluteSeebeck coefficient (|S|), 4–8� 104 S/m and 150–240 μV/K,respectively, are found to be comparable to the typical valuesof Bi2Te2.7Se0.3 textured bulk materials [13,24], and higher thanthe reported data for nanoparticles-sprayed films [10]. Figure 3(a) clearly shows that σ increases monotonically with increasingPt content. As compared to the pure phase thin film N0, N3exhibits an enhancement in σ of about 14% at room tempera-ture and an even higher enhancement of about 68% at 550 K.In contrast, the effects of Pt nanoinclusions on Seebeckcoefficient show a more complicated trend: |S| is slightlyenhanced by the addition of a small amount of Pt addition(r1.13 wt%), but decreases dramatically by a higher Ptcontent (1.46 wt%) for the thin film N3. Based on the remark-able increase in the electrical conduction, the power factor ofthin films can be greatly improved, as shown in Figure 3(c).Typically, N2 exhibits the highest power factor of3.51� 10�3 W/mK2 at room temperature (303 K), which corre-sponds to a more than 20% enhancement as compared to N0,owing to the simultaneous increase in σ and |S|. Our furtherinterpretation on the transport behavior of the nanocompositethin films can be realized based on the study of roomtemperature Hall mobility (μ) and carrier concentration (n)via Hall effect measurements. From Figure 3(d), it can be seenthat the Hall mobility of the thin films decreases up to 30% by

Figure 2 (a) XPS Pt 4f scan, (b) low- and (c) high-magnification TEM images of the thin film N2, and (d) FFTand (e) SAED patterns ofthe circled and squared area in (c), respectively.

T. Sun et al.226

the addition of Pt nanoinclusions, while the carrier concentra-tion increases by up to 1.6 times. The decrease in Hall mobilitystrongly indicates the scattering effect on the charge carriers atthe metal/semiconductor interface, where the band bendingaround Pt nanoinclusions can act as energy filter to block thelow energy carriers and allow the high energy electrons through[11]. Meanwhile, the electrons accumulated in Pt near theenergy barrier can provide n-type carrier flows to the conduc-tion band of Bi2Te2.7Se0.3, and therefore leads to an increasedcarrier concentration. In fact, both theoretical calculation [12]and experimental study [13] have suggested that the metallicnanoinclusions are quite efficient in adding electrons to theconduction band of semiconductor matrix to provide a higherelectron concentration for better electrical conduction.

The enhanced Seebeck coefficient with a small amount ofPt nanoinclusions can be regarded as another evidenceof the energy filtering effect in these nanocomposite thinfilms [13]. The room temperature |S| plotted as a functionof carrier concentration is shown in Figure 4. The dashedlines in Figure 4 are calculated Seebeck coefficient as afunction of carrier concentration based on the Boltzmannexpressions [24],

S¼7kBe

ðrþ5=2ÞFrþ3=2ðξÞðrþ3=2ÞFrþ1=2ðξÞ

�ξ

� �ð1Þ

n¼ 4π2mnkBT

h2

� �3=2

F1=2ðξÞ ð2Þ

where kB is the Boltzmann constant, e is the electroniccharge, r is scattering constant (also known as the relaxa-tion time exponent), h is Planck constant, ξ is reducedFermi level ξ=EF / kBT and Fi (ξ) is the Fermi integral, whichcan be written as,

Fi ðξÞ ¼Z 1

0

xi

1þex� ξdx ð3Þ

For the purpose of illustration, the calculation is based onthe hypothesis of a single parabolic band structure and a simplepower-law dependence of the relaxation time. The scatteringparameter r takes the values r=�1/2 for acoustic-phononscattering and r=0 for neutral impurity scattering, both ofwhich are widely applied to explain the situation for Bi2Te3-based n-type semiconductors [25]. For a degenerate semicon-ductor, a higher carrier density normally results in a lower |S|,as indicated by the dashed line. Instead, the |S|�n points ofour nanocomposite thin films show higher positions in Figure 5,indicating a varied scattering mechanism due to the introduc-tion of Pt nanoinclusions. As mentioned above, the potentialbarrier due to band bending at the Pt/Bi2Te2.7Se0.3 interfacesfacilitates the “selective” blocking of low-energy electrons,

Figure 3 Temperature dependence of in-plane (a) electrical conductivity σ, (b) absolute Seebeck coefficient |S|, (c) power factorPF and (d) the room temperature Hall mobility μ and carrier concentration n plotted as a function of Pt content.

Figure 4 The room temperature |S| plotted as a function ofcarrier concentration. The dashed lines are calculations frombasic transport theory assuming power law relaxition time withrelaxition factor r=–1/2 and r=0 corresponding to acousticphonon and neutral impurity scattering, respectively. Hollowedsymbols denote the recent reported values for S-doped Bi2Te3[7], Bi2Te3 with Bi nanoinclusions [13], Cu-doped Bi2Te2.7Se0.3[24] and Zn-added Bi2Te2.7Se0.3 [25] for comparison.

227Enhanced thermoelectric properties of n-type Bi2Te2.7Se0.3 thin films

which results in an increase in the average carrier energy andthus the increase in |S|. Basically, the energy barrier at themetal/semiconductor interfaces is a Schottky barrier and ismostly dependent on the difference between the work functionof metal and the electron affinity of the semiconductor. Inaddition to its chemical inertness, another advantage forselecting platinum in this work is its high work function(�5.65 eV) [15], which ensures the Schottky barrier formationwhen combined with the Bi2Te3-based n-type semiconductor(electron affinity of around 4.5 eV [13]). Interestingly, theenergy filtering effect has also been observed in Pt/Sb2Te3nanocomposites [15], where the Ohmic contact seems to beinevitable by considering a metal work function higher than thep-type electron affinity. It should be emphasized that theestablishment and exact height of the Schottky barrier in areal case is rather complex. Relevant factors include thesurface state of the semiconductors and the possible formationof a transition layer at the interface due to diffusion or reactionbetween both phases. Therefore, although a potential barrieraround 0.1 eV is predicted to be optimal in the power factorenhancement according to the previous study [11], verificationand pursuit based on materials selection and synthesis are stillgreat challenges. And investigations on those beyond the workfunction considerations are highly aspired in the nano-engineering of TE materials. In addition, it is also worth tonote that the energy filtering effect on the electrical transport

Figure 5 Thermal conductivity κ and estimated ZT plotted asfunctions of Pt content for the pure and nanocomposite thinfilms. (Cross-plane thermal conductivity κ?, of the thin filmswas measured by the 3ω method, while the in-plane thermalconductivity κO, and ZT were approximated based on κ?,Wiedemann–Franz law and the anisotropic factors.)

T. Sun et al.228

properties is also dependent on the volume percentage of thenanoinclusions [12]. The Pt content in N2 of 1.13 wt% corre-sponds to 0.7 vol% (calculated based on the atomic radii of theelements), is speculated to be a favorable level in theoptimization of power factor. With a higher Pt content in N3,the enhancement of |S| via the energy filtering effect is largelycompensated by the negative influence of the additionalincrease in carrier concentration, and hence resulted inmoderate |S| values.

In addition to the significant increase in power factor,a reduction in thermal conductivity is also expected for thesenanocomposite thin films, which can further increase ZT. Thecross-plane thermal conductivity, κ?, of the thin films, mea-sured by 3ω technique at room temperature, are compared inFigure 5. By introducing Pt nanoinclusions with size anddistribution in a few to hundreds nanometer scale, the mid-to long-wavelength phonons can be effectively scattered. Inorder to estimate the in-plane ZT values of these textured thinfilms, the in-plane thermal conductivity, κO, values were derivedby the approximation based on the measured κ?, the Wiede-mann–Franz law (with the Lorentz number [9] L=2.0� 10�8

V2K�2) and the assumption that the anisotropic factors of thesetextured thin films are the same as single crystals(κO,e/κ?,e=4.3and κO,L/κ?,L=1.6 [28]). The approximation calculation detailscan be found elsewhere [29]. As plotted in Figure 5, theestimated in-plane thermal conductivity of the thin filmsdecreases from 1.16 W/(mK) for the pure phase thin film toas low as 0.86 W/(mK) by the addition of Pt nanoinclusions. As aresult, the estimated in-plane ZT of our textured nanocompo-site thin films reaches 1.17 at room temperature, whichcorresponds to a more than 50% enhancement. In fact, thein-plane thermal conductivity of our nanocomposite thin filmscould be even lower than those plotted in Figure 5. Since the Ptnanoinclusions are mostly located at the grain boundariesbetween ab-planes, scattering of the in-plane phonons (trans-ferring along ab-directions) would be more efficient than thatof the cross-plane phonons. Therefore, an even higher gain forthe in-plane ZT value can be expected. This is largely due tothe great increase in power factor as well as the simultaneousreduction in thermal conductivity, based on the introduction ofnanocomposite structure into the textured thin films.

Conclusion

In conclusion, textured n-type Bi2Te2.7Se0.3 thin films with Ptnanoinclusions have been successfully prepared via pulsedlaser deposition. The Pt nanoinclusions are found embeddedat the grain boundaries of the semiconductor matrix. Byintroducing Pt nanoinclusions, power factor of the nano-composite thin film can be greatly improved due to theenergy filtering effect, while the thermal conductivity isreduced by scattering the long wave-length phonons.Through the Pt content optimization, the power factorreaches 3.51� 10–3 W/mK2 at room temperature with anestimated ZT of �1.17. This preparation method facilitatesthe nano-engineering in texture semiconductors and ispromising in the development of anisotropic materials forthermoelectric and other clean-energy applications.

Acknowledgments

The authors would like to acknowledge the financial supportfrom the Future Systems and Technology Directorate,Singapore under Project no. 9010100257. The authors alsothank Dr. Qing Liu and Prof. Chee Lip Gan from TemasekLaboratory, Nanyang Technological University (TL@NTU),Singapore, for their help in TEM sample preparation byusing Focused Ion Beam (FIB). The electron microscopy,EPMA and XRD work were performed at the Facility forAnalysis, Characterization, Testing, and Simulation (FACTS)in Nanyang Technological University, Singapore.

Appendix A. Supplementary information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2014.06.011.

References

[1] L.E. Bell, Science 321 (2008) 1457–1461.[2] B.C. Sales, Science 295 (2002) 1248–1249.[3] H.J. Goldsmid, Thermoelectric Refrigeration, Plenum Press,

New York, 1964.[4] T.C. Harman, P.J. Taylor, M.P. Walsh, B.E. LaForge, Science 297

(2002) 2229–2232.[5] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O'Quinn,

Nature 413 (2001) 597–602.[6] M. Martin-Gonzalez, G.J. Snyder, A.L. Prieto, R. Gronsky,

T. Sands, A.M. Stacy, Nano Lett. 3 (2003) 973–977.[7] R.J. Mehta, Y.L. Zhang, C. Karthik, B. Singh, R.W. Siegel,

T. Borca-Tasciuc, G. Ramanath, Nat. Mater. 11 (2012) 233–240.[8] B. Poudel, Q. Hao, Y. Ma, Y.C. Lan, A. Minnich, B. Yu, X.A. Yan,

D.Z. Wang, A. Muto, D. Vashaee, X.Y. Chen, J.M. Liu, M.S. Dresselhaus, G. Chen, Z.F. Ren, Science 320 (2008) 634–638.

[9] S.F. Fan, J.N. Zhao, J. Guo, Q.Y. Yan, J. Ma, H.H. Hng, Appl.Phys. Lett. 96 (2010) 182104.

[10] Z. Lu, L.P. Tan, X. Zhao, M. Layani, T. Sun, S. Fan, Q. Yan,S. Magdassi, H.H. Hng, J. Mater. Chem. C 1 (2013) 6271–6277.

[11] S.V. Faleev, F. Léonard, Phys. Rev. B 77 (2008) 214304.[12] M. Zebarjadi, K. Esfarjani, A. Shakouri, J.-H. Bahk, Z. Bian,

G. Zeng, J. Bowers, H. Lu, J. Zide, A. Gossard, Appl. Phys.Lett. 94 (2009) 202105.

229Enhanced thermoelectric properties of n-type Bi2Te2.7Se0.3 thin films

[13] S. Sumithra, N.J. Takas, D.K. Misra, W.M. Nolting, P.F.P. Poudeu, K.L. Stokes, Adv. Energy Mater. 1 (2011) 1141–1147.

[14] M.K. Han, S. Kim, H.Y. Kim, S.J. Kim, RSC Adv. 3 (2013) 4673–4679.[15] D.-K. Ko, Y. Kang, C.B. Murray, Nano Lett. 11 (2011) 2841–2844.[16] Y. Zhang, M.L. Snedaker, C.S. Birkel, S. Mubeen, X. Ji, Y. Shi,

D. Liu, X. Liu, M. Moskovits, G.D. Stucky, Nano Lett. 12 (2012)1075–1080.

[17] H.Y. Fang, T.L. Feng, H.R. Yang, X.L. Ruan, Y. Wu, Nano Lett.13 (2013) 2058–2063.

[18] A. Li Bassi, A. Bailini, C.S. Casari, F. Donati, A. Mantegazza,M. Passoni, V. Russo, C.E. Bottani, J. Appl. Phys. 105 (2009)124307.

[19] H.-C. Chang, C.-H. Chen, Y.-K. Kuo, Nanoscale 5 (2013)7017–7025.

[20] Y. Deng, H.M. Liang, Y. Wang, Z.W. Zhang, M. Tan, J.L. Cui,J. Alloys Compd. 509 (2011) 5683–5687.

[21] D. Bourgault, C.G. Garampon, N. Caillault, L. Carbone,J.A. Aymami, Thin Solid Films 516 (2008) 8579–8583.

[22] L.M. Goncalves, C. Couto, P. Alpuim, A.G. Rolo, F. Volklein,J.H. Correia, Thin Solid Films 518 (2010) 2816–2821.

[23] G.J. Snyder, J.R. Lim, C.-K. Huang, J.-P. Fleurial, Nat. Mater. 2(2003) 528–531.

[24] W.S. Liu, Q.Y. Zhang, Y.C. Lan, S. Chen, X. Yan, Q. Zhang,H. Wang, D.Z. Wang, G. Chen, Z.F. Ren, Adv. Energy Mater. 1(2011) 577–587.

[25] S.Y. Wang, H. Li, R.M. Lu, G. Zheng, X.F. Tang, Nanotechnology24 (2013) 285702.

[26] S.M. Lee, David G. Cahill, J. Appl. Phys. 81 (1997) 2590–2595.[27] F. Sen, G. Gokagac, J. Phys. Chem. C 111 (2007) 5715–5720.[28] H. Scherrer, S. Scherrer, Bismuth telluride, antimony telluride,

and their solid solutions, in: D.M. Rowe (Ed.), CRC Handbook ofThermoelectrics, CRC Press, Boca Raton, 1995, p. 235.

[29] M. Takashiri, S. Tanaka, K. Miyazaki, Thin Solid Films 519(2010) 619–624.

Ting Sun received his B.Eng. and M. Eng. fromZhejiang University, China, in 2003 and 2006respectively, and obtained his Ph.D. from theSchool of Materials Science and Engineering,Nanyang Technological University (NTU), Sin-gapore, in 2011. His research covers ceramicsprocessing, thin film deposition, nano-compo-sites, materials characterization, device fab-rication and thermoelectric application. He iscurrently a research fellow at NTU and works

on thermoelectric materials and devices based on intermetallicnanostructured bulk materials and thin films.

Majid Kabiri Samani received his B.S.(1999) and M.S. (2002) degrees in solid statephysics from Isfahan University and TarbiatMoalem University in Iran. He joinedNanyang Technological University (NTU) in2009 as a research assistant. Currently he isa PhD. Student at Novitas, NanoelectronicsCentre of Excellence in School of Electricaland Electronic Engineering at NTU. Hiscurrent research interest focuses on funda-

mental studies of heat transfer at the nanoscale materials, usingoptical and electrical experimental methods.

Narjes Khosravian obtained her B.S. (2000)and M.S. (2003) degrees in solid statephysics from Kashan University and TarbiatMoalem University of Tehran in Iran, respec-tively. She received her Ph.D. (2008) incomputational Physics from Institute forResearch in Fundamental Sciences (IPM) inIran. She joined Nanyang Technological Uni-versity (NTU) in 2009 as a research fellow.Her research interest focuses on atomistic

simulation of heat transfer in nano-dimensional materials.

Kok Ming Ang received his B.Eng. (Hons.) inMaterials Engineering from School of Mate-rials Science and Engineering, NanyangTechnological University, Singapore, in2013, and is currently doing his Ph.D. underAssociate Professor Huey Hoon Hng there.His research interest lies in the synthesisand characterization of nanostructures. Heis currently working on thermoelectricmaterials and devices.

Qingyu Yan is an assistant professor atSchool of Materials Science and Engineer-ing, Nanyang Technological University, Sin-gapore. He received his B.Sc. in MaterialsScience and Engineering from Nanjing Uni-versity, China in 1999 and Ph.D. from Mate-rials Science and Engineering Department ofState University of New York at Stony Brook,in 2004. He then joined the MaterialsScience and Engineering Department of

Rensselaer Polytechnic Institute as a postdoctoral research associ-ate before joining Nanyang Technological University in 2007. Hisresearch interests focused on nanostructured materials and theirintegration/assembly for electrode materials for energy storagedevices, thermoelectric module, magnetic devices and photovoltaicmodule applications.

Beng Kang Tay obtained his B.E. (1985) andM.S. (1989) in Electrical engineering fromNational University of Singapore. Hereceived Ph.D. from Nanyang TechnologicalUniversity (NTU) in 1999. Currently he is aprofessor at Novitas, Nanoelectronics Cen-tre of Excellence in School of Electrical andElectronic Engineering at NTU. His is cur-rently the Deputy Director of the CNRSInternational NTU Thales Research Alliance

(CINTRA), and concurrently holding the appointment of AssociateDean (Research), College of Engineering, NTU. His research interestfocuses on the heat transfer experimentally and theoretically innanoscale materials and development of nanostructure for nanoe-lectronics application.

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Huey Hoon Hng received her B.Sc. (Hons)degree and M.Sc. in Materials Science fromNational University of Singapore, and laterher Ph.D. in Materials Science and Metallurgyat University of Cambridge, UK. She has beenwith the School of Materials Science andEngineering in NTU since 1999. She is cur-rently the Associate Chair (Academic) for theSchool, and also holds the post of Director,Facility for Analysis, Characterization, Testing

and Simulation (FACTS). Her research interest is in understanding theprocessing-microstructure-property relationships of materials. Theresearch covers a wide range of experimental analytical techniquessuch as electron microscopy and X-ray diffraction analysis. Suchtechniques enable the characterization of nanometre scale phasesand provide an in-depth understanding of the materials' properties.Her current research interest is focused on the synthesis of inorganicmaterials using chemical and mechanical processing techniques. Thematerials of interest are thermoelectric materials and materials forenergy storage.