Enhancing the thermopower and tuning the resistivity in Bi2Se3 with Fe-doping

Chemical Physics Letters 638 (2015) 94–98

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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Enhancing the thermopower and tuning the resistivity in Bi2 Se3 with Fe-doping Rejaul Sk a , Mandar M. Shirolkar b , Barun Dhara a , Sulabha Kulkarni a , Aparna Deshpande a,∗ a b

Indian Institute of Science Education and Research, Pune, Dr. Homi Bhabha Road, Pashan, Pune 411008, India Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, PR China

a r t i c l e

i n f o

Article history: Received 4 May 2015 In final form 11 August 2015 Available online 20 August 2015

a b s t r a c t We report the synthesis of undoped bismuth selenide (Bi2 Se3 ) and iron (Fe) doped (upto 30%) bismuth selenide (Bi2 Se3 ) powder samples by polyol method. The samples consist of submicron size platelets with thickness ranging between 5 and 10 nm. The temperature dependent thermopower measurements reveal all these materials to be n-type in nature and the room temperature thermopower increases with Fe concentration. The electrical resistivity of undoped Bi2 Se3 shows metallic behavior in 5–300 K range, but the Fe-doped samples show semiconducting behavior. Thus we find Fe-doping of Bi2 Se3 to be a promising route for tuning the properties of Bi2 Se3 for applications. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Thermolectric materials have been around since more than a hundred years with applications related to cooling and power generation. The most commonly used thermoelectric materials were typically semiconductors like tellurides and selenides of bismuth, lead, antimony and their alloys. A recent revival of interest in the study of these materials has been propelled by the global needs for alternative sources of energy and by the advances in field of novel material synthesis and characterization [1]. In particular, bismuth selenide (Bi2 Se3 ) has been characterized as an efficient n-type thermoelectric materials [2]. Another intriguing property of Bi2 Se3 is that it behaves like a topological insulator with a conducting surface and an insulating bulk. This has also led to extensive studies on electronic properties of Bi2 Se3 [3,4]. The thermoelectric effect or Seebeck effect is the direct conversion of heat into electric energy when two junctions made up of two dissimilar conductors are held at two different temperatures, T1 and T2 , with T1 > T2 lead to a voltage being developed between the two junctions. The ratio of the developed voltage and the applied temperature gradient across a material is defined as Seebeck coefficient (S). The efficiency of heat conversion into electrical energy is denoted by a dimensionless quantity called the thermoelectric figure of merit, defined as ZT = (S2 /)T, where

∗ Corresponding author. E-mail address: [email protected] (A. Deshpande). http://dx.doi.org/10.1016/j.cplett.2015.08.029 0009-2614/© 2015 Elsevier B.V. All rights reserved.

 is electric conductivity,  is thermal conductivity and T is the average operating temperature [2]. A high figure of merit (ZT) is desirable to get efficient conversion of heat into electrical energy as this material can be used to generate power from waste heat. To improve ZT the three intrinsic parameters S,  and  have to be tuned. These three quantities are interrelated in such a way that one cannot control these parameters independently. The increase in S will result in a decrease in  and according to Wiedemann–Franz law, a decrease in  will decrease  [2]. However the change in the electronic density of states for low dimensional materials on quantum confinement can provide a way to control these parameters quasi-independently. Another way to improve ZT is to introduce interfaces. The large number of interfaces inside the material will scatter the phonons more efficiently than the electrons, which lowers  without decreasing . From the recent theoretical prediction and experiment [5] it is understood that quantum confinement and introduction of an interface in a microcomposite material can give us a material with high power factor (PF) defined by S2  and low thermal conductivity [6–9]. Also defects, vacancies and doping can increase S in any material by changing the carrier concentration. Switching the material from n-type to p-type and vice versa can be achieved only by doping [10]. Doping of magnetic ions with local magnetic moment also modifies S as the charge carriers interact with these moments via different exchange interaction and effectively act as scattering centers [1]. Here we report the synthesis of microcomposites of pure Bi2 Se3 and Fe-doped Bi2 Se3 with Fe in varying concentration. Previous Fe-doping studies have shown the opening of a band gap

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in Bi2 Se3 as a consequence of the breaking of the surface state band dispersion of the Dirac point. From angle-resolved photoemission spectroscopy(ARPES) measurements the gap was found to be 50 meV for 12% Fe-doping and 44 meV for 16% Fe-doping[11]. Other studies involving Fe-doping have mostly focused on magnetic properties with weak Fe-doping [12]. Theory calculations have shown Fe-doped Bi2 Se3 to be weakly antiferromagnetic [13]. Not much has been reported on the resistivity and thermoelectric properties of heavy Fe-doped Bi2 Se3 . Our work aims to fill up this void and broaden our understanding of this material for applications. 2. Experiment Bismuth selenide (Bi2 Se3 ) samples were synthesized by polyol method as reported by Zhang et al. [14]. All the reagents used in the synthesis were purchased from Sigma–Aldrich and used without any further purification. Fe-doped Bi2−x Fex Se3 (x = 0.1, 0.2 and 0.3 or 10%, 20% and 30% respectively) were synthesized by the same method with the addition of stoichiometric amount of Fe(NO3 )3 ·9H2 O (refer to the supplementary information for preparation details). Hereafter throughout the discussion 0% (pure), 10%, 20% and 30% Fe-doped Bi2 Se3 samples are termed as BS1 , BS2 , BS3 and BS4 respectively. The samples were characterized by Xray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), and Energy Dispersive X-Ray Analysis (EDXA). For measurements of transport properties the samples were made into solid pellets by applying 1.5 GPa pressure in a hydraulic press. The density of our sample is 5.26 g/cm3 where the density of bulk Bi2 Se3 is about 6.8 g/cm3 . The room temperature hall effect measurements

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were performed to detect the type of charge carriers and the carrier concentration. The resistivity data was taken by using the standard four probe technique. The thermopower measurements were performed by the differential technique where a small temperature gradient was created across the sample and the voltage developed between the hot end and cold end was measured [15]. 3. Results and discussion The XRD pattern of BS1 and BS2 samples (Fig. 1a and b) matches quite well those of standard Bi2 Se3 phase (JCPDF 00-033-0214). They do not show any impurity peaks thus confirming that these samples are of good quality and Fe-doping in the Bi2 Se3 lattice was successful in the BS2 sample (see supplementary material for details). However, this scenario is different for the BS3 and BS4 samples, the XRD data show extra peaks other than those of the Bi2 Se3 phase (Fig. 1a and b). These extra peaks have been identified to be due to the presence of orthorhombic FeSe2 marcasite phase (JCPDF 00-079-1892) [16] that in turn has been estimated to have a concentration of 2.3 ± 0.03% and 1.6 ± 0.08%, respectively. These scenarios indicate clearly that the solubility limit of Fe in Bi2 Se3 is up to 10% only which is being reached and thereby leading to the appearance of impurity phase in BS3 and BS4 samples. Other structural parameters were extracted from Rietveld refinement of the XRD patterns using the Fullprof suite (Version September 2014). All the details of lattice parameters of undoped and doped samples are listed in Table 1. These parameters revealed ¯ that all the samples were crystallized in the rhombohedral R3m space group with 15 atoms per unit cell. The Bi occupancies, bond

Figure 1. (a) XRD patterns of BS1 , BS2 , BS3 , and BS4 samples. (b) Zoomed view of the XRD patterns in (a) showing the existence of FeSe2 phase along with Bi2 Se3 phase for BS3 and BS4 samples. (c) SEM image of the pure Bi2 Se3 sample. (d) SEM image of BS4 sample. Scale bars in both (c) and (d) are 1 ␮m.

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Table 1 Room temperature lattice parameters of synthesized BS1 , BS2 , BS3 , and BS4 samples. Sample

a (Å)

c (Å)

V (Å3 )

BS1 BS2 BS3 BS4

4.136 4.136 4.129 4.136

28.569 28.558 28.566 28.567

423.24 423.08 421.77 423.01

Table 2 Elemental composition data by EDX analysis for BS1 , BS2 , BS3 , and BS4 samples. Sample

Element

Weight percent

Atomic percent

BS1

Bi Se Fe

67.39 32.61 –

43.85 56.15 –

BS2

Bi Se Fe

64.14 33.93 1.92

39.80 55.73 4.47

BS3

Bi Se Fe

59.94 36.74 3.32

35.34 57.33 7.33

BS4

Bi Se Fe

58.61 36.93 4.45

33.87 56.49 9.63

angles and Bi Se bond lengths undergo systematic decrement (see Table S1 in supplementary material) suggesting the Bi2 Se3 lattice distortion with an increase in Fe concentration. More lattice distortions and randomness of Fe occur in BS4 sample compared to BS3 sample. The morphology of the samples was examined using Field Emission Scanning Electron Microscope (FESEM, Zeiss ultra plus). SEM images in Figure 1c and d show the formation of sub-micrometersized particles of platelets of different shapes and sizes. From the ambient pressure AFM images it is confirmed that the thickness of the as grown particles ranges between 5 and 10 nm (see Fig. S1 in the supplementary information for details). The elemental composition and sample purity were determined and estimated from an average of twenty different points of the pellets made by applying 1.5 GPa pressure (see Fig. S2 in supplementary information for EDXA spectra). The elemental composition data for undoped and doped samples from EDXA are listed in Table 2. EDXA composition analysis shows that the as-prepared BS1 (pure) sample is Bi rich and it has Se vacancies. The selenium vacancies are seen to be common in Bi2 Se3 samples grown by various techniques [17]. For the doped samples it is observed that all the doped samples show successful Fe-doping. For BS2 and BS3 samples the Fe-doping percentage is in good agreement with the precursor ratio. BS4 shows the Fe-doping amount a little bit less than the precursor amount. But there is a good increasing trend of Fe-doping percentage for the doped samples BS2 , BS3 , and BS4 . For BS3 and BS4 , there is FeSe2 impurity phase with very low amount (≤2.3%). So a small amount of Fe contributes to form FeSe2 but most of it gets doped in bulk Bi2 Se3 . We derived the electron density(ED) of unit cell based on the XRD analysis. This is very important since there is a correlation between electron density change and properties of materials [18,19]. Figure 2 shows the two-dimensional reciprocal space electron density maps of pure and doped samples. It can be seen that in BS1 , Bi atoms are coordinated with six Se atoms forming a Se octahedra around Bi atom, and it contains Se vacancies. However, such coordination was observed to be disturbed in case of Fe-doped samples. The ED study in the doped samples reveals the following facts: (i) Se vacancies are reduced with Fe-doping. In the host lattice, the sites which have Fe coordination or octahedra with Se are observed to have a larger octahedral volume than that of the host

octahedra. The reduction in Se vacancies with doping has been also observed in resistivity measurements (discussed later). (ii) Some of the Bi-sites in BS2 sample were observed to be replaced with Fe, which gives a partial coordination (octahedra) between Fe and Se atoms along with Bi Se octahedra. (iii) The ED also changes with doping concentration. BS3 sample contains Fe Se octahedra along with Bi Se octahedra and partial Fe Se octahedral coordination. (iv) The ED was decreased in BS4 . However, it maintains the coordination similar to sample BS3 . Thus, the electron density study confirms the Fe-doping at Bi site in Bi2 Se3 lattice in agreement with that of the XRD result. The room temperature Hall effect measurements indicate that all samples are n-type in nature. The room temperature carrier concentration (n) of BS1 , BS2 , BS3 and BS4 samples were respectively 1.21 × 1020 cm−3 , 2.46 × 1020 cm−3 , 5.75 × 1019 cm−3 and 2.29 × 1020 cm−3 . The electrical resistivity of all the samples was measured in the temperature range of ∼5–300 K and is shown in Figure 3a. The resistivity of BS1 sample attains a minimum at 25 K (37.28 m cm) but increases upto 300 K (45.66 m cm). The increase in resistivity with temperature is typical of metallic behavior that can be correlated with the presence of selenium vacancies that are doubly charged and act as electron donors [10,20]. The origin of a slight upturn in the resistivity on decreasing the temperature below 25 K is still not clear to us. Such an upturn has been previously observed in the single crystal sample [10] and the thin film sample [21] in the same temperature range 20–30 K. This kind of behavior has also been observed in the single crystal sample with varying carrier concentration [22]. In our case it might be the carrier concentration at low temperature is playing a crucial role for such an increase in the resistance or it might be due to some undetectable impurity. The electrical behavior of Fe-doped samples changes systematically with an increase in doping (Fig. 3a). As the Fe concentration increases from BS2 to BS4 samples, the resistivity behavior evolves from less semiconducting nature to increasingly semiconducting behavior, as if doping in combination with the impurities present in the sample is enhancing the semiconducting characteristics for the whole temperature range of study. This semiconducting behavior was not observed in previous studies of Fe-doped Bi2 Se3 single crystal [23,24]. The only observed semiconducting behavior so far is in Cr-doped Bi2 Se3 thin film [25]. While the resistivity in BS2 sample is nearly same as that in pure BS1 sample at 300 K, it is higher in BS3 and BS4 samples. We could imagine two scenarios: (i) in pure Bi2 Se3 samples Se vacancies lead to enhancement in the resistivity with a decrease in the temperature and (ii) Fe-doped Bi2 Se3 samples have more Se vacancies in the host matrix due to the formation of FeSe2 phase along with intrinsic Se vacancies. Hence, in BS3 and BS4 samples, comparatively a large change in resistivity with a decrease in the temperature has been observed. The change is more prominent in BS4 sample. Metallic conduction has been reported for Bi2 Se3 single crystals as well as various nanostructures like nanoribbons, nanowires, nanoparticles [10,24,26,27]. The effective resistivity reflecting semiconducting behavior in the our samples is attributed to the chemical synthesis route followed, over-doping, impurity, grain-boundary effect [28] and morphology of platelets formed. To better understand the electrical transport behavior of these samples, Seebeck coefficients S were studied. They are shown in Figure 3b. It is clear that sign of the Seebeck coefficients in these samples is negative, indicating that these materials are n-type. The absolute values of S, |S|, at 300 K are rather high ranging from ∼67.7 to 83.8 ␮V/K, in good agreement with those reported earlier for Bi2 Se3 nanostructure [8] and single crystal [24]. The |S| in BS1 sample decreases nearly linearly from 300 K down to about 170 K. The slope of S changes below 170 K rather slowly with less negative slope without any dip expected in elemental metals due to phonon

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Figure 2. Reciprocal space two-dimensional electron density plot of the Bi2 Se3 unit cell in pure and doped samples. The black, brown and violet circular dotted regions indicate Bi Se octahedra, Fe Se coordination and Fe Se octahedra respectively.

Figure 3. Measurements for BS1 , BS2 , BS3 , and BS4 samples. (a) Electrical resistivity, (b) Seebeck coefficient (S), and (c) power factor (PF) as a function of temperature, (d) plots of S and PF as a function of Fe concentration at 300 K.

drag. However, below about 40 K, the higher slope returns but it is less than that above 170 K. As a result, the possible phonon drag feature is nearly absent. Similar trends are seen in BS2 sample also, showing that the nature of electrical transport in these samples is nearly identical. Such trends are slightly deviated in the case of BS3 and BS4 samples especially the slopes, but with moderate enhancement in the values of S with the largest being in the BS4 sample at 300 K. The room temperature data for Seebeck coefficient (S), resistivity (), and power factor (PF) are obtained from respective graphs (Fig. 3) and listed in Table 3. From a previous report a decreasing order of thermopower is observed with Fe-doping concentration in Bi2 Se3 single crystal [24]. Thus a systematic evolution of S with Fe-doping is evident. Also, for these samples it is expected that since it is a microcomposite material the thermal conductivity will be lower than their bulk values. The changing scenarios of electron transport with doping, impurity, grain size, shape, grain boundaries, and morphology contribute to this. The evolution of resistivity and Seebeck coefficients with doping may have a strong bearing on the PF of these materials. This

Table 3 Room temperature data of carrier concentration (n), resistivity (), Seebeck coefficient (S), and power factor (S2 ) for BS1 , BS2 , BS3 , and BS4 samples. Sample

n (cm−3 )

 (m cm)

S (␮V/K)

S2  (␮W m−1 K−2 )

BS1 BS2 BS3 BS4

1.21 × 1020 2.46 × 1020 5.75 × 1019 2.29 × 1020

45.56 46.12 48.95 49.67

−67.7 −69.14 −79.38 −83.84

100.65 103.02 128.85 141.42

assessment is performed using Figure 3c. There is a clear increase of PF from ∼100 to 141 ␮W/m K2 . Since PF has S2 factor and S is reasonably large, the trend of PF nearly follows that of S. These are clear from the plots of S and PF versus × (Fe-doping concentration) in Figure 3d. Accordingly, since S is maximal in BS4 , PF attains its maximum of ∼141 ␮W/m K2 at 300 K. Overall, PF also shows an evolution similar to S. They reveal that there is strong influence

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Fe-doping, structural disorder, grain size, shape, morphology and impurity present on the thermoelectric properties of Bi2 Se3 . 4. Conclusions In this study, we have synthesized pure and Fe-doped Bi2 Se3 submicron sized platelets. Fe dissolves fully upto 10% in Bi2 Se3 lattice. However, above it, FeSe2 phase in very low percentage appears. The resistivity measurement of pure Bi2 Se3 samples shows metallic behavior down to 25 K. However, Fe-doping results in semiconducting nature of increasing order as the dopant concentration increases. The Seebeck coefficient and power factor show concomitant temperature and dopant dependence that exhibits very attractive trends with doping. Thus, Fe-doping in Bi2 Se3 is promising with a desirable trend in electronic and thermoelectric properties. Acknowledgments The authors thank DST, India Nano-Mission Initiative Project SR/NM/NS-42/2009. We acknowledge gratefully Dr. Gunadhor S. Okram (UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452001, Madhya Pradesh, India) for resistivity and thermopower measurements at UGC-DAE Indore and for valuable inputs. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2015.08.029. References [1] D.M. Rowe, Thermoelectrics Handbook: Macro to Nano, CRC Press, 2005. [2] G.S. Nolas, J. Sharp, H.J. Goldsmid, Thermoelectrics: Basic Principles and New Materials Developments, Springer, 2001.

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