Physica B 407 (2012) 4154–4159

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Physica B

0921-45

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n Corr

E-m

journal homepage: www.elsevier.com/locate/physb

The effect of Te doping on the electronic structure and thermoelectricproperties of SnSe

Song Chen a, Kefeng Cai a,n, Wenyu Zhao b

a Functional Materials Research Laboratory, Tongji University, 1239 Siping Road, Shanghai 200092, Chinab State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China

a r t i c l e i n f o

Article history:

Received 7 May 2012

Received in revised form

20 June 2012

Accepted 29 June 2012Available online 20 July 2012

Keywords:

SnSe

Doping

Thermoelectric

First-principle theory

26/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.physb.2012.06.041

esponding author.

ail address: kfcai@tongji.edu.cn (K. Cai).

a b s t r a c t

SnSe1�xTex (x¼0, 0.0625) bulk materials were fabricated by melting Sn, Se and Te powders and then

hot pressing them at various temperatures. The phase compositions of the materials were determined

by X-ray diffraction (XRD) and the crystal lattice parameters were refined by the Rietveld method

performed with DBWS. XRD analysis revealed that the grains in the materials preferentially grew along

the (l 0 0) directions. The structural behavior of SnSe1�xTex (x¼0, 0.0625) was calculated using CASTEP

package provided by Materials Studio. We found that the band gap of SnSe reduced from 0.643 to

0.608 eV after Te doping. The calculated results were in good agreement with experimental results. The

electrical conductivity and the Seebeck coefficient of the as-prepared materials were measured from

room temperature to 673 K. The maximum power factor of SnSe is �0.7 mW cm�1K�2 at 673 K.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

With the increasing global energy demands and declining fossilfuel reserves, development of renewable energy alternatives isextremely urgent. Thermoelectric (TE) devices are solid-state energyconverters that can transform waste heat directly into electricity.However, until now, TE devices only have niche applications mainlydue to low efficiency and high cost. The efficiency of such devicesmainly depends on the figure of merit, ZT (¼a2sT/k, where a, s, T,and k are the Seebeck coefficient, the electrical conductivity, theabsolute temperature, and the thermal conductivity, respectively) ofthe materials used. Bi2Te3-based materials are the best TE materialsaround room temperature [1,2]. However, Bi and Te elements arerare in the earth and their prices are increasing with the develop-ment of LED industry. Therefore, it is necessary to find newTE materials. Sn is an earth-abundant and environment friendlyelement, and Se is more abundant than Te in the earth. Like Bi2Te3,SnSe also has a layered structure, hence, it may be of interest tostudy the TE properties of SnSe.

SnSe has orthorhombic crystal structure with space groupPmna and lattice parameters: a¼1.150 nm, b¼0.415 nm andc¼0.444 nm. A unit cell of SnSe contains eight atoms placed inpositions by the scaled co-ordinates (u, 1/4, v) and (1/2, 1/4, 1/2v).The Sn and Se atoms form double layers made up of two planes ofzigzag Sn–Se chains perpendicular to the a-axis. Within either

ll rights reserved.

double layer, each atom has three nearest neighbors and two nextnearest neighbors. The layers pile up with a weak van der Waals-like coupling along the a-axis direction. Bulk SnSe has an indirectband gap of 0.90 eV and a direct band gap of 1.30 eV [3].

Recently, SnSe has received much attention because it could beused in many application fields, such as solar cells [4], phase-changealloys for electronic memory [5], and as a cathodic material inlithium intercalation batteries, due to the anisotropic character [6].Several methods have been developed to prepare SnSe, such asfluxing [7], chemical vapor deposition [8], electrodeposition [9],electron beam irradiation [10], direct vapor transport [11], and one-pot chemical synthesis [12]. Theoretically, the ab-initio calculationswere used to study the band structure and density of states (DOS) ofSnSe. Makinistian et al. [13] reported that spin–orbit does not affectthe band gap in the Brillouin zone of SnSe. The structural behavior ofSnSe under the hydrostatic pressure using a constant pressure abinitio technique was also studied, and a structural second orderphase transition at 7 GPa was found [14].

In fact, the TE properties of single crystalline SnSe grown by adirect vapor transport technique have already been studied inRefs. [15,16]. It is found that the SnSe has very high Seebeckcoefficient that increases with the increasing temperature (from�200 mV/K at 308 K to 1300 mV/K at 573 K) and low electricalconductivity (�0.00055 S/cm at 308 K) [15]. As the pressureincreases, the Seebeck coefficient of the single crystalline SnSedecreases from �47 mV/K at 0.5 GPa to 24 mV/K at 8 GPa andS-doping decreases the Seebeck coefficient of the SnSe. And thevalues of the band gap decrease with the increasing pressure andthe materials become more conducting [16]. However, the TE

S. Chen et al. / Physica B 407 (2012) 4154–4159 4155

properties of polycrystalline SnSe and Te doped SnSe have not beenreported, and the relationship between electronic structure and TEproperties of SnSe has also not been studied. In this work, the effectof Te doping on the electronic structure and TE properties of SnSewere investigated by both experimentally and theoretically.

Fig. 1. XRD patterns of samples SnSe before (a) and after (b) hot pressing at

575 1C, and (c) SnSe0.9375Te0.0625 hot pressed at 575 1C.

2. Experimental

Te powder (Alfa Aesar, 99.99%), Se powder (Alfa Aesar, 99.95%),and Sn powder (Sinopharm Chemical Reagent, 99.5%) were usedas received. All manipulations were performed in an argon-filledglove box. The elements, Te:Se:Sn, were placed in a 5-mL BNpowder-lined graphite crucible in the molar ratios of x:1�x:1(x¼0, 0.0625), respectively. The reactants were sealed in quartzampoules under 1/5 atm argon atmosphere and placed in a high-temperature programmable furnace. The furnace was heated to1023 K with a rate of 10 K/min, then held at 1023 K for 1 h,followed by slowly cooling (�2 K/min) to room temperature. Theingots obtained had obviously metallic luster and layer-structure,which is in agreement with that reported in Ref. [10]. The ingotswere broken and ground into powders under an Ar atmosphere inglove box, and finally hot pressed into pellet (10 mm in diameterand �3 mm in thickness) in vacuum (about 10�2 Pa) for 2 h atdifferent temperatures under 80 MPa.

The phase composition of the samples before and after hotpressing was determined by XRD (Rigaku DMAX2500VPC) with CuKa radiation (40 kV, 200 mA). The DBWS suite of programs was usedfor Rietveld fitting of the powder XRD data. A pseudo-voigt functionwith axial divergence was used to describe the peak shape. Thefracture surface of the crystals was observed by field emissionscanning electron microscopy (FESEM, Quanta 200FEG), equippedwith electron energy dispersive X-ray spectroscopy (EDS, Oxford7582). Hall effect measurement was carried out at room tempera-ture using a Hall effect measurement system (HMS 3000, Ecopia)with a magnetic field of 0.55 T. The differential scanning calorimeter(DSC) and thermogravimetric analysis (TG) of the samples wereperformed at a heating rate of 10 K/min in a flowing nitrogen.The pellets were cut into rectangles (10�2�2 mm3) for s and ameasurements. The measurements were carried out using a home-made computer control test system from room temperature to673 K under argon atmosphere. The s measurement was performedby a steady-state four-probe technique with a square wave current(�10 mA in amplitude). The a value was determined by the slope ofthe linear relationship between the thermal electromotive force andtemperature difference (�10 K) between the two ends of eachsample. The density of the samples was determined by measuringthe Archimedes method.

Fig. 2. Final Rietveld refinement of powder XRD data for the SnSe sample hot

pressed at 575 1C.

3. Computation method

The calculations were performed using CASTEP package pro-vided by Materials Studio. Our calculations were based on thedensity functional theory (DFT) in generalized gradient approx-imations (GGA) with Perdew–Burke–Ernzerhof (PBE) exchange-correlation potential. The electronic structure was calculated byoptimizing all the atoms of the crystal using ultrasoft pseudopo-tentials for the core electrons. The cut-off energy in plane waveexpansion was 300 eV. The total energy, maximum stress, max-imum force and maximum displacement were converged to lessthan 1�10�5 eV/atom, 0.05 GPa, 0.3 eV/nm and 0.0001 nm,respectively. The tolerance in the self-consistent field (SCF)calculation was set to 10�6 eV/atom. We also used 2�3�3k-point Monkhorst–Pack mesh for the bulk. The calculations werebased on the experimental crystal structure data after refinement.

We began our calculations with 1�2�2 supercell consisting of16 Sn atoms and 16 Se atoms, and for the Te-doped SnSe,we replaced one of the Se atoms in the supercell with a Te atom.

4. Results and discussion

Fig. 1(a) and (b) shows the XRD patterns of the SnSe sample beforeand after hot pressing at 575 1C, and Fig. 1(c) shows the XRD patternof the Te doped sample hot pressed at 575 1C (the XRD pattern of theother SnSe samples is almost the same as that shown in Fig. 1(b)).There is not much difference in the patterns of the SnSe samplesbefore and after hot pressing. All the XRD peaks can be indexed to thestandard data of SnSe (JCPDS card, 48-1224). This indicates that pureSnSe was prepared. Compared with the standard data, the intensity ofthe (l 0 0) plane peaks of the samples is much increased, whichindicates that the grains in the samples preferentially grew along the(l 0 0) planes. By comparing Fig. 1(b) with (c), it is known that Tedoping does not introduce any impurity phase into SnSe.

In order to obtain the crystal lattice parameters, Rietveldrefinement was performed with DBWS, and the final calculated,observed, and residual patterns are shown in Fig. 2. The details of

Table 1Refined atomic coordinates and thermal parameters at room temperature for the sample SnSe hot pressed at 575 1C.

Space group Pnma 62

Atom Position Atomic coordinates Occu. Temperature parameter Biso(A2)

x y z

Se 4 0.8559(1) 0.2500 0.4836(1) 1 0.022294

Sn 4 0.1180(2) 0.2500 0.1043(5) 0.98 0.025842

Cell dimensions and angles

a¼1.1500(7) nm b¼0.4154(2) nm c¼0.4446 (2) nm a¼b¼g¼90o

U.C. density d¼6.203 (1) g/cm3

R-P 9.87%

R-Wp 14.80%

S 3.56

Fig. 3. (a) FESEM image of the fracture surface of the sample SnSe hot pressed at

575 1C and (b) EDS spectrum recorded on the marked area in (a).

S. Chen et al. / Physica B 407 (2012) 4154–41594156

the refinement are summarized in Table 1. The fit reliabilitywas converged to give R-wp¼14.80%, R-p¼9.87% and S¼3.56.The refinement result shows that the sample containing a littleamount of impurity: 1.2(4) wt% of Sn (JCPDS card 65-7657), whichis confirmed by DSC analysis result (Supporting material Fig. S1).

FESEM image (Fig. 3(a)) also shows that the samples havelayered structure. This is ascribed to the dominant van der Waalscharacter of the bonds between adjacent layers [10,11]. EDSanalysis (Fig. 3(b)) reveals that the samples consist of Sn and Se,and quantitative EDS analysis indicates that the atomic ratio ofSn/Seo1, which agrees with the refinement result. All the dataabove indicate that the samples prepared mainly contain Sn1�ySe(0oyo1) mixed with a little amount of Sn.

The structure parameters of the SnSe1�xTex (x¼0, 0.0625) sam-ples hot pressed at 575 1C were optimized and listed in Table 2.Compared with the refined experimental result, the calculated latticeparameters a, b and c are slightly larger with relative errors of 1.3%,1.1% and 0.7%, respectively. And the calculated lattice parameters ofthe Te-doped sample were also listed in Table 2, with maximumrelative error of 1.6%. Thus, our calculated results are reasonable. Boththe calculated and experimental results indicate that the unit cell sizeof SnSe was enlarged after doping Te, because the Te atom was largerthan Se atom.

To better understand the electronic nature of this material, wehave calculated the band structure of SnSe along high symmetrylines corresponding to a 1�2�2 super cell shown in Fig. 4. Inthis figure, the G, F, Q, Z and B are (0 0 0), (0 1/2 0), (0 1/2 1/2),(0 0 1/2) and (1/2 0 0) high symmetry points, respectively. It canbe seen from Fig. 4 that the valence band maximum (VBM)locates between Z- and G-point, and the conduction band mini-mum (CBM) between G- and F-point, indicating an indirect bandgap material. It is obvious that the Fermi energy level is on theVBM and the band gap is 0.643 eV, which is lower than theexperimental result (about 0.9 eV) [3].The low value is caused bya well-known drawback using standard DFT calculation [17]. Theband structure of SnSe0.9375Te0.0625 has also been calculated (notshown here), and it reveals that it is also an indirect band gapmaterial, with band gap of 0.608 eV, which is somewhat smallerthan that of SnSe.

In addition, we also plotted the DOS and the decomposedpartial density of states (PDOS) of each atom for SnSe0.9375Te0.0625

in Fig. 5. We chose the DOS of the SnSe0.9375Te0.0625 as positivevalue and that of the SnSe as negative one to clearly showthe doping effect of Te. From the total DOS for the SnSe, thevalence band can generally be divided into lower valence bandfrom �14.5 to �11.5 eV, middle one from �8.5 to �5.0 eV andupper one from �5.0 to 0.0 eV (Fig. 5(a)). Moreover, the lowervalence band is mainly contributed to the Se(4s) states, the upperone is mainly from Se(4p) states, and the middle one is dominated

by Sn(5s) (Fig. 5(b)), which agree with the results reported inRef. [14]. In addition, the conduction band from 0.5 to 2.5 eVis primarily ascribed to the Sn(5p) orbit (Fig. 5(c)). Comparedwith the SnSe, the SnSe0.9375Te0.0625 raises the valence band from�11.5 to �10.5 eV on the DOS curve, which is mainly occupiedby Te(5s) with admixture of Sn(5s). As it is far away fromthe Fermi energy level, Te(5s) and Sn(5s) interact and form astrong covalent bond in this region. As the DOS curve of theSnSe0.9375Te0.0625 is similar to that of the SnSe and in order toclearly show the peak slightly shifts after Te doping, we added

Fig. 4. Calculated GGA-PBE band structure of SnSe.

Fig. 5. (a) DOS of SnSe (negative) and SnSe0.9375Te0.0625 (positive), (b) partial DOS

of Se(4s), (4p), Te(5s) (5p), and (c) partial DOS of Sn(5s), (5p) in SnSe0.9375Te0.0625

and SnSe from the GGA calculation. (For interpretation of the references to color in

this figure, the reader is referred to the web version of this article.)

Table 2The calculated and the experimental lattice parameters of the SnSe1�xTex (x¼0,

0.0625) samples hot pressed at 575 1C.

Sample a (nm) b (nm) c (nm) a (deg.) b (deg.) g (deg.)

SnSe

Exp 1.1500 0.8308 0.8892 90 90 90

Cal 1.1650 0.8403 0.8956 90.0 90.0 90.3

Error 1.3% 1.1% 0.7% 0.3%

SnSe0.9375Te0.0625

Exp 1.1559 0.8311 0.8990 90 90 90

Cal 1.1700 0.8446 0.8972 90.0 90.0 90.3

Error 1.2% 1.6% �0.2% 0.3%

S. Chen et al. / Physica B 407 (2012) 4154–4159 4157

green and blue lines in Fig. 5(a). It should be noticed that theconduction band peak of the SnSe0.9375Te0.0625 shifts to leftcompared with that of the SnSe, but the upper valence bands donot. So we presumed that the reduction of the band gap is due tothe splitting of the conduction band [18], so-called band gaprenormalization, which results in a gap narrowing [19]. Thesefindings are helpful to understand the optical properties of SnSe.If one of the Se atoms in SnSe is substituted by a Te atom, sincethe electronegativity of Te is weaker than that of Se, the energylevel will be broadened to form impurity-induced bands. This isthe result from the interaction between Sn(5s) and Te(5s) orbitscombined with CBM. Furthermore, CBM moves down to the Fermilevel and then the band gap reduces.

To explore the bonding character in SnSe, the charge densitydistribution was studied. The counter plot of the valence chargedensity on the (1 0 0) plane is shown in Fig. 6. It can be seen fromFig. 6 that the charge density around Se atom exhibits a directionaldistribution toward Sn atom. In addition, the charge density aroundSe atom is much higher than that around Sn atom, which indicatesthat the Se–Sn bond has a strong polarization covalent character dueto the hybridization effect between the Sn(5s) and Se(4s) states,which is also reflected in the DOS of SnSe0.9375Te0.0625. The chargedensity around Te atom is much lower than that around Se atom,which means that the bonding of Te–Sn atoms has covalent characterdue to the hybridization effect between the Te(5s) and Sn(5s) states.

The effective mass of the carrier can be expressed as follows:

mn ¼h

2p

� �2 d2E

dk2

!�1

ð1Þ

where mn is the effective mass of electron, h is the Planck’s constant,E is the energy level and k is the wave vector. We calculated the

effective mass of carrier on the CBM band at G-point along b–c plane(average of G–F and Z–G) and along the a-axis direction (G–B), allthe results are listed in Table 3.

For electron-conduction, the carrier concentration (n) can beexpressed as follows:

n¼1

V

Z 1Ec

f ðEÞgcðEÞdE ð2Þ

where f Eð Þ ¼ 1=1þexp½ðE�EF Þ=kT�, and it is Fermi–Dirac distribu-tion function; V is the supercell volume; gc(E) is the DOS function.Note that we chose the point where E40, DOS40.3 (states/eV) andis nearest to Fermi-Point as Ec during the calculation. We calculatedthe Seebeck coefficient using the formula as follows [20]:

a¼ 8p2k2B

3eh2mnT

p3n

� �2=3ð3Þ

where kB is the Boltzmann constant and e is the electronic charge.The calculated results are given in Table 3.

Table 3Some physical properties of the samples prepared.

mn/m0 a (mV/K) n0 s0 m0

y–z x y–z x (1017/cm3) (S/cm) (cm2/Vs)

SnSe

Cal. 4.026 1.06 �161.6 �44.5 �3.5

Exp. �165.2 �96.2 �2.9 0.23 4.88

SnSe0.9375Te0.0625

Cal. 4.045 �168.9 �3.3

Exp. �168.3 �2.1 0.14 4.21

Fig. 7. Temperature dependence of (a) electrical conductivity and (b) the Seebeck

coefficient of the samples hot pressed at different temperatures.

Fig. 6. Charge density maps for SnSe1�xTex (x¼0, 0.0625) on (1 0 0) plane.

S. Chen et al. / Physica B 407 (2012) 4154–41594158

It is seen from Table 3 that all the calculated values agree withthe experimental results. Hall measurement indicates that theSnSe has higher mobility than the SnSe0.9375Te0.0625 (Table 3). Asthe SnSe sample is rich in Se, its carrier concentration is higherthan that of the SnSe single crystals reported in Refs. [15,16], andalso the conduction changes from p-type to n-type.

Fig. 7 shows the temperature dependence of electrical conduc-tivity and the Seebeck coefficient of the samples prepared. All thecurves have the same change trend: as temperature increases, theelectrical conductivity rapidly increases first, then decreases a little,and finally increases again. The electrical conductivity of the SnSesample at room temperature is higher than that of the single crystalSnSe reported in Ref. [15], which should be because the presentsample is nonstoichiometric and mixed with some Sn impurity. Therelative density of all the samples is �95%; therefore, we neglectedthe effect of porosity on the TE properties of the samples.

It can be seen from Fig. 7 that when To550 K, at a giventemperature, the absolute Seebeck coefficient of the SnSe samplesincreases with the increasing hot pressing temperature, whereasthe electrical conductivities of the samples are close near roomtemperature after which these increase first then decrease with theincreasing hot pressing temperature at intermediate temperatures(400oTo550 K). Based on the results, we deduce that the hotpressing process introduced deep level defects in the samples andthe concentration of the defects increased with increasing hotpressing temperature. The defects increase the scattering factorwhich results in the increasing Seebeck coefficient. On the otherhand, the deep level defects can be excited when the temperature ishigh enough (�T4400 K), leading to increased carrier concentra-tion. When the carrier concentration is high enough, it will increasethe scattering and hence decrease the mobility. Therefore, at a giventemperature (550 KoT) the changing trend of the electrical con-ductivity for the samples is not the same as that of the Seebeckcoefficient.

As the measurement temperature increases, the changingtrend of the absolute Seebeck coefficient is consistent with thatof the electrical conductivity of the samples, which is differentfrom the behavior of the common TE materials. This should beascribed to the phonon-drag effect as carrier concentration of thesamples is less than 1017 cm�3[21]. The SnSe sample hot pressedat 575 1C shows higher power factor at a given temperature. Themaximum value of power factor is 0.7mW cm�1 K�2 at 673 K andit is close to that of FeSi2-based TE material [22].

Combined with the DSC result, it is known that the s–T anda–T curves of the samples both changing around 500 K are relatedto the melting of Sn impurity in the samples. At about 550–600 K,

S. Chen et al. / Physica B 407 (2012) 4154–4159 4159

both the electrical conductivity and the absolute Seebeck coeffi-cient rapidly increase. If it is because of intrinsic excitation, theabsolute Seebeck coefficient should decrease. In addition, SnSehas a large band gap (�0.9 eV), hence we excluded the possibilityof intrinsic excitation at this temperature range.

To know the effect of the impurity, Sn, on thermal stability ofthe samples, recycle measurements were carried out (Fig. S2). Theresults indicate that the TE properties of the samples are stable. Inaddition, the Seebeck coefficient of the sample hot pressed at575 1C was measured along the directions perpendicular (?) andparallel (J) to the hot pressing direction, and the results (Fig. S3)show that the a? is higher than the aJ, indicating anisotropictransport behavior of the SnSe due to the anisotropic structure.

By combining the DSC analysis (Fig. S1) and recycle measure-ment (Fig. S2) results, we think that the strange behavior of a ands as T�550–600 K may not have resulted from Sn and/or Sevolatilization. Nariya et al. [15] also found that the absoluteSeebeck coefficient of the SnSe single crystals increased signifi-cantly above �530 K. Hence, we deduce that the electronicstructure of SnSe may change at high temperatures, which needsto be further studied.

It is seen from Fig. 7 that at a given temperature although theTe-doped sample has lower electrical conductivity than theundoped ones (due to the lower carrier concentration andmobility, see Table 3), its temperature dependence of electricalconductivity is similar to that of undoped samples; however, itsabsolute Seebeck coefficient starts to decrease at first and thendecreases more significantly. In general, like S-doping in SnSe[16], Te-doping also does not improve the electrical transportproperties of SnSe. It may be a good choice to dope at the Sn-sitewith proper elements or even better to study other selenides. Forexample, more recently, Liu et al. [23] reported that Cu2Se hasexcellent TE properties with ZT¼1.5 at 1000 K.

5. Conclusions

In summary, polycrystalline SnSe1�xTex (x¼0, 0.0625) sampleswere fabricated by melting followed by hot-pressing at varioustemperatures and their TE properties were measured from roomtemperature to 673 K. The electronic structures of SnSe1�xTex

(x¼0, 0.0625) were calculated by the first-principle theory. Theband gap and the band structure near the band gap of SnSe can beadjusted by doping element Te. The maximum power factor for

SnSe was 0.7 mW cm�1 K�2 at 673 K. Doping Te on Se site doesnot improve the power factor of SnSe.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (50872095), Doctoral Fund of Ministry ofEducation of China, the foundation of the State Key Lab of AdvancedTechnology for Materials Synthesis and Processing, Wuhan Uni-versity of Technology.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.physb.2012.06.041.

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