This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 59.185.236.52

This content was downloaded on 23/09/2014 at 02:40

Please note that terms and conditions apply.

Thermoelectric performance of layered SrxTiSe2 above 300 K

View the table of contents for this issue, or go to the journal homepage for more

2014 J. Phys.: Condens. Matter 26 445002

(http://iopscience.iop.org/0953-8984/26/44/445002)

Home Search Collections Journals About Contact us My IOPscience

1 © 2014 IOP Publishing Ltd Printed in the UK

1. Introduction

The enhancement of the dimensionless figure  of merit, ZT (= α2σ/κ) in existing thermoelectric materials requires advanced approaches to either increase the power factor; α2σ where α is Seebeck coefficient and σ is electrical conductivity, or reduce the thermal conductivity (κ) in the same material. All these parameters are determined by the details of the electronic structure (band gap, band shape and band degeneracy near the Fermi level) and the scattering of the charge carriers (electrons and holes), hence they cannot be determined independently. To achieve a maximum α2σ, development of a new class of thermoelectric material or optimization of the thermoelectric parameters in the existing material using various dopants is required. The transportation of electrons and phonons in the lat-tice determines the thermal conductivity of the material, thus it

is strongly affected by the presence of interfaces, surfaces and grain boundaries in nanostructures or low dimensional mate-rials. The energy filtering effect of these interfaces preferably scatters the phonon and blocks the low energy electrons which effectively suppress the κ value [1–3]. Thus, to attain minimum thermal conductivity, attempts may include the modification of microstructure in terms of nanostructuring, lamellar growth or superlattices in the existing thermoelectric material using var-ious material preparation methods such as ball milling, spark plasma sintering, chemical synthesis, etc [4–6].

Recent work on thermoelectricity shows that researchers have paid a lot of attention to materials with properties of a phonon- glass-electron-crystal such as rattling semiconduc-tors (skutterdites, clathrates, low-dimensional materials), nanocomposites, etc [6–8]. Among the various types of materials, layered two-dimensional 2D metal chalcogenides have attracted considerable interest due to several inter-esting physical properties, such as charge density waves

Journal of Physics: Condensed Matter

Thermoelectric performance of layered SrxTiSe2 above 300 K

Ranu Bhatt1,6, Miral Patel1,2, Shovit Bhattacharya1, Ranita Basu1, Sajid Ahmad1,3, Pramod Bhatt4, A K Chauhan 1, M Navneethan5, Y Hayakawa5, Ajay Singh1, D K Aswal1 and S K Gupta1

1 Technical Physics Division, Bhabha Atomic Research Center, Mumbai-400 085, India2 Department of Physics, Maulana Azad National Institute of Technology, Bhopal – 46205, India3 Astrophysical Sciences Division, Bhabha Atomic Research Centre, Mumbai-400 085, India4 Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India5 National University Corporation, Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu-432 8011, Japan

E-mail: rbhatt@barc.gov.in

Received 2 July 2014, revised 5 August 2014Accepted for publication 12 August 2014Published 22 September 2014

AbstractIn this paper we report the thermoelectric performance of Sr intercalated TiSe2 above 300 K. Refined x-ray diffraction, high resolution transmission electron microscopy and scanning electron microscopy images show well oriented polycrystalline grains along a (0 0 l) direction and layered growth of the sample. Intercalation of Sr in TiSe2 shows an improved Seebeck coefficient (α) value without altering the polarity of the majority charge carrier. A drastic reduction in the thermal conductivity (κ) from 3.8 W m K−1 to 1.2 W m K−1 (at 650 K) was observed which is ascribed to the: (i) scattering of the phonon by natural layer interfaces, grain boundaries and lattice defects and (ii) rattling of intercalated Sr atoms among weakly bound TiSe2 layers. This led to the maximum ZT of ~0.08 at 650 K for SrxTiSe2 (x > 0.1) which is almost twice as high as the parent TiSe2.

Keywords: thermoelectric materials, layered structure, thermal conductivity, figure-of- merit

(Some figures may appear in colour only in the online journal)

R Bhatt et al

Printed in the UK

445002

cm

© 2014 IOP Publishing Ltd

2014

26

J. Phys.: condens. matter

cm

0953-8984

10.1088/0953-8984/26/44/445002

Papers

44

Journal of Physics: condensed matter

mm

6 Author to whom all correspondence should be addressed.

0953-8984/14/445002+8$33.00

doi:10.1088/0953-8984/26/44/445002J. Phys.: Condens. Matter 26 (2014) 445002 (8pp)

R Bhatt et al

2

[9], superconductors [10], topological insulators [11] and advanced applications like thermoelectric materials [12], elec-tronics and optoelectronics [13], battery materials etc [14]. To achieve the optimum thermoelectric properties, complex sys-tems based on group V and VI transition metal dichalcoge-nides (TMDs), transition metal oxides (TMOs), and other 2D compounds such as Bi2Te3, Bi2Se3 have also been deliberately investigated [15, 16].

The TMDs exhibit a layered lattice structure where layers of metal atoms (M) are sandwiched between two layers of chalcogen atoms (X) resulting in MX2 stoichiometry. In such a material (X-M-X) there exists covalent bonding within the chalcogen trilayer sheets, while the adjacent sheets stack together through weak van der Waals forces. Depending on the coordination and oxidation state of the metal atoms, TMDs can be metallic (TiTe2), semi metallic (TiSe2, WSe2) or semiconducting (TiS2, MoS2). Due to this interesting layer feature of TMDs the interlayer bonding of these materials per-mits the process of intercalation of guest species which effec-tively alters the structural and electronic properties of these materials. Titanium diselenide (TiSe2) is one of the known members of this family which is electronically semi-metallic in nature and possesses a small band gap value of 0.2 eV [17]. Intercalation of 3d transition metal species such as (Co, Cu, Fe, Ni etc) among the layers in a TiX2 material shows signifi-cant changes in their physical and thermal transport properties [18–20]. There are very few reports available on the effect of alkali earth metal ion intercalation in the TMD compounds [21]. In this work, we investigated the effect of strontium alkali earth metal intercalation on the electrical and thermal transport properties of titanium diselenide materials.

2. Experimental Section

Polycrystalline compounds of SrxTiSe2 (x = 0.04, 0.08, 0.1, 0.14, 0.2) were synthesized using a solid-state reaction method. In the first step, to prepare TiSe2, the stoichiometric amounts of Ti (purity (99.99%) and Se (purity 99.999%) powder were mixed and sealed in a graphite bottle inside a quartz ampoule under a vacuum of the order of 2   ×  10−5 Torr. The sealed material was heat treated in a tubular furnace at 920 K for a period of one week and cooled to room temperature under a low controlled cooling rate. In the second step, the stochio-metric amounts of Sr (99.99%) and the prepared TiSe2 com-pound were mixed and ground for a few hours using a mortar presser in an Ar atmosphere in a glove box. The mixed powder material was further sealed in an evacuated quartz ampoule and heated at 920 K for 50 h. To obtain compact dense pel-lets of SrxTiSe2, the powder compound was sintered using a hot press method at a sintering temperature of 1020 K for a duration of 60 min, with an applied load of 2.5 Kg. Structural characterizations of the samples were carried out by x-ray diffraction (XRD) using Cu-Kα radiation on a Rigaku instru-ment and analyzed using a Reitveld refinement programme [22]. Microstructure and elemental analysis of the samples was performed using high resolution electron microscopy (HRTEM), a scanning electron microscope (SEM) (TESCAN

VEGA) and Energy Dispersive x-ray Analysis (EDXA). The electrical resistivity and Seebeck coefficient of the samples was measured using a Leinsies Seebeck Thermopower (LSR) instrument in a temperature range of 300–650 K. The thermal conductivity (κ) of the samples was calculated using the rela-tion κ = C D d. .p where Cp is specific heat, D is diffusivity and d is material density [23]. The thermal diffusivity and specific heat were measured in a LFA-1000 instrument in a temperature range of 300–650 K. For measuring specific heat a comparative method was employed where a high purity graphite disc was used as a standard. The specific heat of the sample was measured using the equation:

=CC

(sample). slope.mass (reference)

slope.mass (sample)p

p(1)

To minimize error in the measurement, the mass of the standard disc and the sample were approximately equal, the remaining parameters (laser energy, amplitude etc) were constant and the measuring conditions were identical. The material density of the sample was measured using the Archimedes method. The tem-perature dependent Raman spectra of the samples were recorded between 300 −493 K using Ar laser wavelength at 514.5 nm and a power of 400 mW. The work function of these compounds was studied using the Kelvin probe method at 300 K.

3. Results and discussion

3.1. Phase and microstructure analysis

Figures 1(a) and 1(b) shows the schematic side and top view of the crystal structure and the fitted XRD pattern of the pure TiSe2 material. The figure  shows that TiSe2 has a natural superlattice structure with trigonal symmetry, in which the Ti metal atoms are surrounded by two hexagonal sheets of Se chalcogenide atoms to give an octahedral co-ordination around the Ti atoms. Individual TiSe2 layers are stacked together in such a way that the Ti-Se atom has strong covalent bonding whereas the layers are inter-bonded by a weak van der Waals attraction among the Se layers. The inset in figure1 (b) shows the layer microstructure present in the parent TiSe2.

The added guest atom has a maximum probability to occupy the octahedral position between these TiSe2 layers. Figure  2(a) shows a Reitveld refined XRD pattern for the SrxTiSe2 (x = 0.04, 0.08, 0.1, 0.2) samples. The XRD data for all compounds were fitted using a FullProf

Program [24] and the analysis shows that it can be refined to a space group P-3 m1 (164). The observed intense peaks strongly demonstrate that all the compounds are preferably oriented in a (0 0 l) direction. Moreover, all the fitted Bragg peaks (x = 0, 0.04 0.1, 0.2) match well with the pure TiSe2 phase [JCPDS no. 830980] suggesting a similar (CdI2-type) crystal structure for all the samples. The Reitveld refined parameter shown in table 1 depicts a systematic increase in the lattice parameter c with increasing Sr content. The increase in lattice parameter c clearly shows that the intercalated Sr atom goes to the vacant octahedral position existing in the van der Waals gaps between the layers.

J. Phys.: Condens. Matter 26 (2014) 445002

R Bhatt et al

3

The morphologies of the Sr intercalated TiSe2 (x = 0.04, 0.08, 0.1, 0.2) fractured samples were examined using SEM, as shown in figure 2(b). The cross sectional view of the images clearly illustrates the existence of layer morphology with a large area uniformity in the range of microns. It is worth noticing that the growth of the layer structure preferably lies in the direction of the applied load of the sintering process. This is in agreement with the XRD data showing grain growth with a preferred orientation of (0 0 l) along the c-axis.

Figures 3 (a)–(d) shows the HRTEM images for the sam-ples TiSe2 and Sr0.2TiSe2. The sample TiSe2 shows polycrys-talline sharp lattice planes exhibiting dislocations and point defects. However the sample Sr0.2TiSe2 shows randomly ori-ented crystalline grains exhibiting grain boundaries, disloca-tions and long range and short range order defects.

The elemental distribution of the sample measured using EDXA, for the representative Sr0.08TiSe2 sample, is shown in figure 4. The figure shows the uniform distribution of all three elements over the whole fractured cross section, depicting the homogeneity of the prepared material.

It was reported earlier that the nanostructuring in the mate-rial scatters the phonons with a short and medium free-path (30–100 nm), however to scatter phonons with a higher mean free-path, micron sized grain boundaries or layer structures are desirable [25]. Interface and grain boundaries play an important role in altering the electrical and thermal transport

properties by introducing barriers for filtering charge carriers of various levels of energy.

3.2. Electrical properties

Figure 5(a) shows the temperature dependent resistivity plot for the SrxTiSe2 (x = 0, 0.04, 0.08, 0.1, 0.14, 0.2). The TiSe2 sample shows a small resistivity value of the order of micro ohm-m and follows a common metallic trend. Such metallic behaviour may exist due to two reasons, (a) partial overlapping of the Ti d-band with the Se p-band or, (b) off-stoichiometric composition. Upon Sr intercalation, the resis-tivity increases due to lattice deformation as indicated by the increasing lattice parameter c, however parametric variation of resistivity as a function of Sr content shows a systematic reduction. The electronic band structure in TiSe2 originates from a strong interlayer hybridisation between the metal dz

2 and dxy, dx

2 − y2 bands, leading to the band gap value of nearly 0.2 eV. The intercalated Sr sitting in the van der Waals gap between the layers transfers one or two electrons to the upper narrow d-band. This increases the density of states near the Fermi level resulting in decreased electrical resistivity in addi-tion to expansion in lattice parameter c.

Figure 5(b) shows the temperature dependence of the ther-mopower (α) for the SrxTiSe2 samples with x = 0, 0.04, 0.08, 0.1, 0.14 and 0.2. Interestingly, Sr intercalation does not alter

Table 1. Structural parameters of Reitveld refined XRD patterns of SrxTiSe2 compounds. (x, y and z denote the fractional coordinates).

SrxTiSe2 Atom X y z Occupancy a = b (Å) c/a

X = 0 Ti 0.0 0.0 0.0 0.56 3.53(6) 1.65Se 0.32(4) 0.64(5) 0.24(5) 1.21

x = 0.04 Ti 0.0 0.0 0.0 0.74 3.54(7) 1.66Se 0.33(4) 0.67(5) 0.257(5) 1.82

x = 0.08 Ti 0.0 0.0 0.0 0.63 3.54(7) 1.67 Se 0.33(3) 0.67(6) 0.260(4) 1.34

x = 0.1 Ti 0.0 0.0 0.0 0.405 3.53(4) 1.67Se 0.39(8) 0.68(7) 0.259(4) 0.867

x = 0.2 Ti 0.0 0.0 0.0 0.55 3.54(8) 1.72Se 0.32(5) 0.65(2) 0.24(9) 1.85

a Reitveld refined parameter.

Figure 1. Schematic (a) side view and top view of the TiSe2 crystal structure generated by Diamond software and (b) fitted XRD pattern of pure TiSe2 sample. The inset in figure 1(b) is a SEM image of TiSe2.

J. Phys.: Condens. Matter 26 (2014) 445002

R Bhatt et al

4

Figure 3. (a) HRTEM for the sample TiSe2 (b) enlarged image of the dotted square in the sample TiSe2, (c) HRTEM for the sample Sr0.2TiSe2 where the dotted line shows the grain boundaries and (d) enlarged image of the sample Sr0.2TiSe2 showing defects in the crystalline phase.

Figure 2. (a) Representative fitted XRD patterns and (b) SEM image of a SrxTiSe2 (x = 0.04, 0.08, 0.1, 0.2) fractured sample.

J. Phys.: Condens. Matter 26 (2014) 445002

R Bhatt et al

5

the polarity of the majority charge carrier. For all compounds, α is positive in the entire temperature range revealing that the holes are the majority charge carriers. A rough estimation of the number of charge carriers calculated using the Hall coef-ficient measurement lies in the range of1018 − 1019 cm−3.

The Seebeck coefficient curve shows a systematic increase with increasing temperature which is a characteristic of heavily doped semiconductors. In SrxTiSe2, the thermopower values increase with x up to ~90 µV K−1, this is slightly higher than TiSe2. The temperature dependence of the power factor (α2σ) of various Sr content is also shown in figure  5(c). A maximum power factor of almost 0.3 mW m−1 K2 is obtained for SrxTiSe2 (x = 0, 0.14) at 650 K. However, a lining up of thermal transport properties with improved α values on Sr intercalation in TiSe2 may improve the thermoelectric perfor-mance of this material.

3.3. Thermal transport properties

Thermal conductivity is an integral property where heat is transferred by a phonon with a broad distribution of mean free path. The presence of a large number of layer inter-faces in addition to randomly oriented polycrystalline grains exhibiting grain boundaries and dislocations may signifi-cantly affect the thermal conductivity of these materials. Figure  6(a) shows the temperature dependent total thermal conductivity (κ) for SrxTiSe2 samples. The TiSe2 material shows a thermal conductivity value of nearly 3.4 W m−1 K at room temperature which is comparable to the thermal con-ductivity value reported for other diselenide polycrystalline materials. Interestingly, a drastic reduction in thermal conduc-tivity is observed with increasing Sr content in the SrxTiSe2 compound. In general, the thermal conductivity of heavily doped semiconductors to metal has two major contributions occurring due to the heat transfer through (a) phonons (lattice

thermal conductivity; κl) and (b) electrons (electronic thermal conductivity; κe), where κe itself consists of the polar thermal conductivity (κp) and the bipolar component (κbip) written as; κ κ κ κ κ κ= + = + +( ) .e l p lbip The κp of the samples can be esti-mated using the Weidmann–Franz law (κp = L0σT = neµLT; where L0 is the Lorentz number, (2.44  ×  10−8 J−2 C−2 K−2, for free electrons), σ is the electrical conductivity, n is the carrier concentration and µ is mobility). The temperature dependent variation of phonon thermal conductivity, kl i.e. (k − ke) with Sr intercalation, is shown in figure 6(b). The thermal conductivity of TiSe2 exhibits a major contribution occurring due to the electronic contribution. However, the thermal conductivity in Sr intercalated samples shows a negligible contribution occur-ring due to electrons. This suggests that although Sr intercala-tions among the TiSe2 layers do not contribute to any major charge transfer it effectively contributes towards increasing

Figure 4. Elemental mapping of Sr, Ti and Se in the Sr0.08TiSe2 sample.

Figure 5. Temperature dependence of (a) resistivity, (b) Seebeck coefficient, (c) power factor for the SrxTiSe2 samples (x = 0, 0.04, 0.08, 0.1, 0.14, and 0.2).

J. Phys.: Condens. Matter 26 (2014) 445002

R Bhatt et al

6

the scattering points in the structure which increases the phonon scattering. As reported by Wan et al the thermal con-ductivity of misfit layer compounds such as (MS)1+x(TiX2)2 (M- transition metal, organic molecule or alkali earth metals and X –S,Se,Te) are expected to be comparatively lower which may be associated with the weak interlayer bonding between the layers and a disruption of periodicity along the perpen-dicular direction, due to intercalated foreign atoms [26].

Since SrxTiSe2 grows with a layered structural property it sustains a large number of scattering interfaces for phonons having a wavelength in microns. Additionally, the presence of a large number of defects, dislocations, grain boundaries and rattling behaviour of loosely bound Sr in the TiSe2 layers acts as a potential source for the scattering of lower wave-length phonons. Altogether they effectively increase the scat-tering canters which lowers the mean free path of the phonons

Figure 6. (a) Temperature dependent total thermal conductivity (κe + κl) data for SrxTiSe2 samples (x = 0, 0.04, 0.08, 0.1, 0.14 and 0.2) and (b) shows the calculated phonon thermal conductivity (κl) contribution. The inset in figure 6(b) shows the ∆κl plotted with transition temperature as a function of Sr content.

Figure 7. (a) Temperature dependent Raman spectra for the sample Sr0.2TiSe2, (b) Raman active modes for a one phonon process in unit cell, (c) temperature dependent peak frequency of A1g + Eg and SrSe modes and (d) area ratio (SrSe/A1g + Eg) as a function of temperature.

J. Phys.: Condens. Matter 26 (2014) 445002

R Bhatt et al

7

transferring heat. The charge carriers and phonons transfer-ring through these interfaces encounter coherent interfaces leading to minimal heat carrier mobility. A reduction of almost 30–50% in the magnitude of phonon thermal conductivity is observed in the SrxTiSe2 samples as compared to TiSe2. The reduction in κ is comparable to the reported thermal conduc-tivity data for other complex compounds such as skutterd-ites and layered structure materials such as (PbS)1.2(TiS2), Nd0.025TiS2 etc [26–28]. One may also observe a sharp transi-tion in κ at a temperature of ~ 430 K upon Sr intercalation. The inset in figure 6(b) shows the change in thermal conductivity (∆kl) plotted for different Sr concentrations at a transition tem-perature of 430 K. It is clearly observed that (∆kl) increases systematically with increasing Sr content. This suggests the intercalated Sr is responsible for the observed transition peak in the κ value which has been further investigated using a tem-perature dependent Raman in the transition temperature range. After the transition temperature a further moderate increase in thermal conductivity is attributed to the bipolar diffusion originating at a higher temperature due to intrinsic excitation which enhances the carrier concentration giving rise to bipolar thermal conductivity.

In order to understand the reasons behind the observed thermal conductivity peak, temperature dependent Raman spectra have been recorded in the κ transition temperature range. Figure 6(a) shows the temperature dependent Raman spectra of the samples Sr0.2TiSe2 recorded at different tem-peratures around that peak. The crystal structure of the layered TiSe2 belongs to a D3d3 space group and the Raman active zone centre phonon modes can be represented by the fol-lowing irreducible representations [29],

Γ = + +A E E2g g g1 2 1 (2)

Figure 7(b) shows the atomic displacement of a possible Raman active mode in TiSe2. The initial Raman spectra show peaks at 153 cm−1, 196 cm−1 and 233 cm−1 which is in agree-ment with the previously reported values for the (A1g + Eg), A1g and Eg modes respectively [29]. An additional Raman peak observed at 257 cm−1 corresponds to the Raman mode

of SrSe [30]. With increasing temperature we observed a sup-pression of the (A1g + Eg) mode whereas the Sr-Se mode starts dominating in the temperature range of enhanced thermal con-ductivity. Figure 7(c) shows the peak position of the A1g + Eg and SrSe modes plotted as a function of temperature. With increasing temperature up to 413 K, the red shift in the peak position of (A1g + Eg) is observed which suggests that the inter-action of the (A1g + Eg) frequency of phonons with incident photons has decreased. At a higher temperature above 413 K (transition temperature for κ) a red shift in the peak position of Sr-Se and a blue shift in the peak position of A1g + Eg is observed suggesting a type of structural transition or defor-mation at this temperature. The area ratio of these two peaks (SrSe/A1g + Eg) plotted in a transition temperature range of κ, shown in figure 7(d), shows a maxima at 430 K which indi-cates the SrSe mode to be the dominant factor responsible for the observed thermal conductivity peak. The weakly bound Sr-Se may reduce the decoupling among the TiSe2 layers and thus can contribute to reducing the scattering of phonons by the rattling of Sr atoms. At a higher temperature (above 430 K) κ again reduces due to a suppressed SrSe phonon mode; in addition the larger thermal activation energy may induce a large fluctuation in the bond energy of Sr-Se causing Sr to act again like a free rattler.

The 3D surface plots for the work function ɸ measured using Kelvin probe microscopy on some representative fractured SrxTiSe2 (x- 0, 0.08, 0.2) samples are shown in figure 8(a). All the samples show uniform work function dis-tribution. In the p-type material the Fermi level remains close to the valence band; the reduction of ɸ with increasing Sr con-tent reveals a shifting of the Fermi level towards the conduc-tion band resulting in increased electrical conductivity. The temperature dependent variation in a dimensionless figure of merit, ZT, for SrxTiSe2 is shown in figure 8(b). All the sam-ples show a consistent increasing trend with increasing tem-perature. Intercalation of Sr in TiSe2 shows a significant improvement in the ZT value of the material as compared to pure TiSe2. The maximum ZT value of ~0.08 is obtained for x > 0.1 Sr content. The improvement in ZT of the material can

Figure 8. (a) Work function plot for SrxTiSe2 (x = 0.08, 0.2) and (b) dimensionless figure of merit, ZT for SrxTiSe2 (x = 0, 0.04, 0.08, 0.1, 0.14 and 0.2).

J. Phys.: Condens. Matter 26 (2014) 445002

R Bhatt et al

8

be ascribed mainly to the substantial decrease in the thermal conductivity values.

4. Conclusion

We have investigated the effect of Sr intercalation on the ther-moelectric properties of a layered TiSe2 material exhibiting a p-type conducting nature. The intercalation of Sr in TiSe2 shows a significant improvement in the ZT value due to: (i) an improve-ment in the thermo power values and (ii) a strong reduction (of 30–50%) in thermal conductivity. The intrinsically layered structure of the TiSe2 material, the presence of grain bound-aries, defects and the subsequent Sr atoms’ (which act like rattlers) intercalation gives a strong scattering to the phonons of various mean free paths. A maximum ZT of ~ 0.08 was achieved for the Sr content (x > 0.1) which is twice as high as the parent TiSe2.

References

[1] Medlin DL and Synder GJ 2009 Curr. Opin. Colloid Interface Sci. 14 226–35

[2] Zebarjadi M, Shakouri A and Esfarjani K 2006 Phys. Rev. B 74 195331

Ko D-K, Kang Y and Murray C B 2011 Nano Lett. 11 2841 [3] Moyzhes B and Nemchinsky V 1998 Appl. Phys. Lett. 73 1895 [4] Soni A, Shen Y, Yin M, Zhao Y, Yu L, Hu X, Dong Z, Khor K A,

Dresselhaus M S and Xiong Q 2012 Nano Lett. 12 4305 [5] Putri Y K, Wan C, Zhang R, Mori T and Koumoto K J 2013

Adv. Ceram. 2 42–8 [6] Hsu K F, Loo S, Guo F, Chen W, Dyck J S, Uher C, Hogan T,

Polychroniadis E K and Kanatzidis M G 2004 Science 303 818

[7] Nolas G S, Morelli DT and Tritt TM 1999 Annu. Rev. Mater. Sci. 29 89–116

[8] Mahan G Sales B and Sharp J 1997 Phys. Today 50 42–7 [9] Wilson J A and Yoffe A D 1969 Adv. Phys. 28 193–335 Wilson J A, di Salvo F J and Mahajan S 1975 Adv. Phys. 24

117–201 Morris R C 1975 Phys. Rev. Lett. 34 1164 Rossnagel K 2011 J. Phys.: Condens. Matter 23 213001[10] Morosan E, Zandbergen H W, Dennis B S, Bos J W G,

Onose Y, Klimczuk T, Ramirez A P, Ong N P and Cava R J 2006 Nat. Phys. 2 544

[11] Tretiakov O A, Abanov A r, Murakami S and Sinova J 2010 Appl. Phys. Lett. 97 073108

[12] Hor Y S, Richardella A, Roushan P, Xia Y, Checkelsky J G, Yazdani A, Hasan M Z, Ong N P and Cava R J 2009 Phys. Rev. B 79 195208

[13] Radisavljevic B, Radenovic A, Brivio J, Giacometti V and Kis A 2011 Nat. Nanotechnol. 6 147

Wang Q H, Kalantar-Zadeh K, Kis A, Coleman J N and Strano M S 2012 Nat. Nanotechnol. 7 699–712

[14] Whittingham M S 2004 Chem. Rev. 104 4271 Whittingham M S 1976 Science 192 1126

[15] Chung D-Y, Hogan T, Brazis P, Rocci-Lane M, Kannewurf C, Bastea M, Uher C and Kanatzidis M G 2000 Science 287 1024

[16] Taskin A A, Lavrov A N and Ando Y 2003 IEEE 22nd International conf. on Thermoelectrics (La Grande Motte, France, 21 August 2003)

Koumoto K, Terasaki I and Funahashi R, 2006 MRS Bull. 31 206

[17] Friend RH, Jérome D and Yoffe AD 1982 J. Phys. C15 2183

[18] Bhatt R et al 2013 Appl. Phys. A 111 465 Guilmeau E, Bréard Y and Maignan A 2011 Appl. Phys. Lett.

99 052107[19] Holgate T C, Zhu S, Zhou M, Bangarigadu-Sanasy S,

Kleinke H, He J, Tritt T M 2013 J. Solid State Chem. 197 273–8

Titov A N, Suvorova O N, Ketkov S Yu, Titova S G and Merentsov A I 2006 Phys. Solid State 48 1466

[20] Kuranov A V, Pleshchev V G, Titov A N, Baranov N V and Krasavin L S 2000 Phys. Solid State 42 2089

[21] Julien C M 2003 Mater. Sci. Eng. R 40 47–102 Yoffe A D 1984 Phys. Chem. Electron. Ions Condens. Matter NATO ASI Series 130 437–58

[22] Rietveld H M 1969 J. Appl. Cryst. 2 65–71[23] Patel M, Bhatt R, Bhattacharya S, Basu R, Haque F Z,

Singh A, Aswal D K and Gupta S K 2013 AIP Conf. Proc. 1536 373

[24] Rodriguez-Carvajal J 2007 FULLPROF, November www.ill.eu/sites/fullprof/

[25] Bathula S, Jayasimhadri M, Singh N, Srivastava A K, Pulikkotil J, Dhar A and Budhani R C 2012 Appl. Phys. Lett. 101 213902

Biswas K, He J, Blum I D, Wu C I, Hogan T P, Seidman D N, Dravid V P and Kanatzidis M G 2012 Nature 489 414–8

Szczech J R, Higgins J M and Jin S 2011 J. Mater. Chem. 21 4037

Bux S K, Fleurial J-P and Kaner R B 2010 Chem. Commun. 46 8311

Vaqueiro P and Powell A V 2010 J. Mater. Chem. 20 9577

[26] Wan C, Wang Y, Wang N, Norimatsu W, Kusunoki M and Koumoto K 2011 J. Electron. Mater. 40 1271

[27] Wan C, Wang Y, Wang N and Koumoto K 2010 Materials 3 2606–17

[28] Li D, Qin XY, Zhang J and Li H J 2006 Phys. Lett. A 348 379–5

[29] Jaswal S S 1979 Phys. Rev. B 20 5297[30] Chen J and Shen W Z 2006 J. Appl. Phys. 99 013513

J. Phys.: Condens. Matter 26 (2014) 445002