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Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array
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Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array Ming Tana,b,n, Yuan Denga, Yao Wanga a
Beijing Key Laboratory of Special Functional Materials and Film, School of Materials Science and Engineering, Beihang University, Beijing 100191, China b Department of Physics, College of Sciences, Tianjin University of Science & Technology, Tianjin 300222, China Received 13 June 2013; received in revised form 1 July 2013; accepted 18 July 2013
KEYWORDS
Abstract
Thermal co-evaporation technique; Bi2(Te,Se)3 film; Nanowire array; Thermoelectric properties
In this paper, novel Bi2(Te,Se)3 nanowire array structure can favorably influence the carrier and phonon transport properties. The ternary compound n-type Bi2(Te,Se)3 film, composed of ordered nanowire array, has been successfully fabricated by a simple thermal co-evaporation technique without using any templates. The composition and the microstructure of the films are studied by x-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX), and high resolution transmission electron microscopy (HRTEM). The results show that the nanowire array is composed of single-crystalline Bi2(Te,Se)3 nanowires with diameter of about 20 nm. The well-oriented nanowires are parallel to each other and uniformly distributed. The growth mechanism of such nanostructure is proposed and investigated. The inplane thermoelectric properties, i.e., electrical conductivity (s) and Seebeck coefficient (S) and thermal conductivity (κ) of the nanowire array were measured. The properties of the welloriented Bi2(Te,Se)3 nanowire array have been greatly enhanced in comparison with those of the ordinary Bi2(Te,Se)3 film. The Bi2(Te,Se)3 nanowire array film with a thermoelectric dimensionless figure-of-merit ZT=1.01 was obtained at room temperature. Introduction of such ordered nanowire array architecture into films is therefore a very promising approach. & 2013 Elsevier Ltd. All rights reserved.
Introduction n
Corresponding author at: Beijing Key Laboratory of Special Functional Materials and Film, School of Materials Science and Engineering, Beihang University, Beijing 100191, China. Tel.: +86 15222181280. E-mail addresses: [email protected], [email protected] (M. Tan).
The thermoelectric (TE) properties of a material are often summarized by the figure-of-merit ZT=(S2s/κ)T, where S, s, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. Solid state materials have typically much lower ZT's. Even the best
2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.07.009 Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
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commercial thermoelectric materials, such as Bi2Te3, have TE figure of merit ZT0.8 at room temperature [1,2]. Many different approaches have been proposed and attempted to improve the TE performance of Te compounds, namely, by tuning carrier concentration, engineering the band structure [3], suppressing the lattice thermal conductivity [4–6], and reducing the device dimensionality [7]. Theoretical calculations and experimental results have also proved that low-dimensional structures can significantly optimize the transport properties of both electrons and phonons [8–12]. Especially during recent years, the record high efficiency of 2.4 was reported for molecular beam epitaxy engineered thin films of Bi2Te3/Sb2Te3 layers, and a ZT value of 3 was also reported for an n-type PbSeTe/PbTe quantum dot superlattice, which may be difficult to use in large-scale application but convincingly demonstrated the potential for further improvements to come from nanostructured materials [6,13]. Nanowires (one-dimensional) are predicted to exhibit a better TE property than superlattice films [14,15]. But single nanowire (NW) is limited in fabrication of TE devices, while nanopillar array can realize wafer-level processing including lithography and anisotropic etching for improving performance of a wafer-scale TE device as proposed recently in Ref. [16]. Additionally, it has been found that Bismuth telluride based single crystal like bulk solid solutions, including p-type BixSb2 xTe3 and n-type Bi2Te3 ySey, still remain the best TE materials used near room temperature [17,18]. Therefore, Bi2(Te,Se)3 in the form of NW array is an excellent candidate material for TE applications. In our previous work, Bi2Te3 films, composed of ordered NW array, have been successfully synthesized by a simple thermal evaporation technique without using any templates [19]. To the best of our knowledge, the TE properties and ordered structure of Bi2(Te,Se)3 NW array film have been not reported to date. In this work, we report the novel and one-step route for the large scale formation of Bi2(Te,Se)3 NW array film by a simple thermal co-evaporation technique. Our goal here is to improve TE properties of such films. Simultaneously, we also aim to obtain further insight into the relation between NW array microstructure and film properties. This convenient physical vapor phase growth technology would be further proved to be a promising way to prepare various NW arrays in ternary compound films. Moreover, it is convenient to fabricate planar or verticaltype TE micro-devices by integrating the ordered NW array structure films using mask-assisted deposition method or lithography and anisotropic etching technology.
Experimental section In this work, n-type Bi2(Te,Se)3 NW array and ordinary films were successfully grown on SiO2 substrates at 250 1C and 200 1C deposition temperature, respectively, by a thermal co-evaporation technique. The previous researches have shown that Te deficiency was easily occurred in Bi2Te3 films [20]. The concomitant compensation for Te deficiency and doping Se element are expected to improve the transport properties in the Bi2(Te,Se)3 films. As a result, high purity (99.99%) Bi2Te3 and Se powders (The mass rate of Bi2Te3:Se is 10:1) were mounted on the evaporating dish which is connected to the alternating current (AC) power supplies.
The evaporated current were 160 A and 150 A for NW array and ordinary films, respectively. The distance between the evaporation source and the substrate was about 5 cm for these films. Before deposition, the common glass substrates were first cleaned by diluted nitric acid, and then acetone, and dried under the nitrogen airflow. After the substrates were loaded onto the substrate holder (parallel to the boat), N2 gas was introduced into the chamber and vacuumized three times to remove oxygen. The working pressure was maintained at 2 10 6 Torr in the deposition process for the films. By changing the deposition time in our experiment, the thickness of film could be controlled to be above 5 μm. The crystal structures of the Bi2(Te,Se)3 films were examined by x-ray diffraction (XRD, Rigaku D/MAX 2200) with Cu Kα radiation (λ = 0.154056 nm). The compositions were detected by energy dispersive x-ray spectroscopy (EDX). The morphology of the films was observed by field emission scanning electron microscopy (FE-SEM, Sirion 200). Further structural analyses were performed using highresolution transmission electron microscopy (HRTEM, FEI Company, Tecnai G2 F20S-Twin FEG TEM at 200 kV). The samples were put into a small ampule. Ethanol was then added until it covered the surfaces of the substrates. After ultrasonication for 1 h, the solution was drop cast onto carbon coated 200 mesh Cu grids. Surface profilometry (Ambios XP-2, USA) was used to measure the film thickness. Electrical conductivity and Seebeck coefficient of the films were examined using a ZEM-3 (Ulvac Riko, Inc.) with a selfmade test holder for film measurement in the in-plane direction. The in-plane thermal conductivity data was collected using a Laser PIT (Ulvac Riko, Inc.) at room temperature. For the 1-μm-thick film of Bi2(Te,Se)3, a specially designed specimen-holder frame was used to keep the very thin glass substrate from being damaged. The substrate and frame materials are borosilicate glass having a thickness of 30 μm and 200 μm, respectively, supplied by NIMS (Xu Group, Japan). The substrate was machined to a width of 2.5 mm and it has a length of 15 mm. It was attached to the frame with a polyimide adhesive. The Bi2(Te,Se)3 films were then deposited on a half-surface of the substrate. The specimen-holder frame was coated uniformly over the whole surface with 100-nm-thick Au by evaporation. Every measurement was carried out at room temperature in a vacuum of less than 0.01 Pa, produced using a turbo-molecular pump, to eliminate the effect of air layers. Frequencies of ac-calorimetric measurements were selected in the range of 0.1 Hz. The principle of the measurement method is described in detail in Ref. [21]. The carrier concentration and mobility were determined using a four-probe measurement based on the Hall effects (ECOPIA HMS-3000) at room temperature. The errors are 4% for electrical conductivity, 5% for Seebeck coefficient, 5% for thermal conductivity, and 10% for ZT value.
Results and discussion Ordered n-Bi2(Te,Se)3 film with NW array structure was synthesized by a simple thermal co-evaporation technique, achieving oriented growth in the evaporated current of 160 A and 250 1C deposition temperature. XRD pattern of
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array the Bi2(Te,Se)3 film is shown in Figure 1. For the NW array Bi2(Te,Se)3 film, all peaks are indexed as rhombohedral phase Bi2Te3 material (JCPDS 15-0863), implying Se atoms enter into Te vacancies or formation of other defects. The intense and sharp XRD peaks from the Bi2(Te,Se)3 film are typical signatures of a high degree of crystallinity. It reveals a single-phase product with slightly broadened reflections, which is typical for crystals with low dimensions. Compared with standard card, the intensity of (0 1 5) peak (located at 2θ = 27.761) of Bi2(Te,Se)3 is dramatically strong, indicating a highly preferential orientation of the Bi2(Te,Se)3 film along the (0 1 5) direction. Additional, the (1 0 10) peak (located at 2θ = 38.441) is obvious in the NW array. Bi2(Te,Se)3 in this growth process is not necessarily epitaxially attached to the glass substrate. The atomic interactions between the Bi2(Te, Se)3 atoms and the atoms of the amorphous SiO2 substrate can locally obstruct the formation of a perfect crystalline order. But the crystallites nucleating with the thermodynamically preferred (0 1 5) orientation can grow, thus tending to form a preferential (0 1 5) growth at special depositing conditions. Different from the NW array, the ordinary Bi2(Te, Se)3 film shows more major diffraction peaks, such as (0 1 5), (1 0 10) and (0 1 11) peaks, indicating random growth in the film with polycrystalline structure. It shows that the preferential growth of (0 1 5) direction is the essential reason for the formation of oriented NW array. Compared with the standard peaks of bulk Bi2Te3 (JCPDS 15-0863), we note that the (0 1 5) and (1 0 10) diffraction peaks of NW array and ordinary Bi2(Te,Se)3 films have slightly shifted toward higher angle. It seems reasonable to assume that the small Se atom replacing Te atom causes the lattice shrinkage. The Bi2(Te,Se)3 NW array and ordinary films are clearly shown by SEM images in Figure 2. Seen from the top view [shown in Figure 2(a)], the film is relatively dense and uniform, and some interspaces between the NWs exist in the film. From oblique view of the film [Figure 2(b)], we can observe that a large number of thin Bi2(Te,Se)3 NWs were densely grown perpendicular to the substrate, along their preferential growth direction. The observation of ordered growth is also in agreement with the XRD analysis above. The sizes of NWs are uniform in the Bi2(Te,Se)3 film. The diameter of each is estimated to be about 20 nm [Figure 2(b)], implying numerous interfaces in the film. Besides, many grains with sizes varying from nano to submicro scale distribute on the Bi2(Te,Se)3 ordinary film surface, as
Figure 1 XRD patterns of ordered nanowire array and ordinary Bi2(Te,Se)3 films.
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shown in Figure 2(c). Seen from the cross-sectional image in Figure 2(d), the film is composed of numerous disordered bulk-like particles. In order to further verify the special ordered NW array microstructure, details of the ordered NW array structure are seen in the TEM and HRTEM images shown in Figure 3(a) and (b). The NW array exhibits dense growth in Figure 3(a), which can significantly promote the carrier transport, indicating that Bi2(Te,Se)3 NW array is still stable even after a long ultrasonic treatment. Figure 3(b) is the enlarged image of selected area marked by a square in Figure 3(a), the diameter of NW is affirmed to be about 20 nm, and NWs show rough surface which can significantly suppress the phonon transport, resulting in a significantly improved ZT value. As shown in the inset, the lattice spacing of 0.336 nm corresponds to the lattice of (0 1 5) crystal plane. This confirms that the NWs grow along the preferred (0 1 5) direction, which is in accordance with the XRD result. This microstructure may play a very positive role on its high electrical conductivity. From Figure 3(c), we note that disordered particles are exhibited in the ordinary Bi2(Te,Se)3 film. Figure 3(d) is the magnified image for the selected area marked by a square in Figure 3(c), showing the ordinary film with polycrystalline structure. The lattice spacing of 0.331 nm and 0.227 nm correspond to the lattices of (0 1 5) and (1 0 10) crystal planes, as shown in Figure 3(d), which also confirms (0 1 5) and (1 0 10) major diffraction peaks in the ordinary film. Besides, a number of disordered grains boundaries exist in the ordinary film, which influences the carrier transport properties. The detailed growth mechanism of the NW array and the ordinary Bi2(Te,Se)3 film will be proposed. The growth process of the films is illustrated in Figure 4. Commonly, AV2 BVI 3 alloys have an inherently anisotropic bonding environment which causes faster growth along the top-bottom crystalline planes compared to the natural growth axis (c-axis) thus tending to form a platelet-like morphology [22]. However, in thin film growth, the growth kinetics can be influenced by temperature, supersaturation of the vapors, ion bombardment, impurity content and in case of compound deposition, by chemical composition, etc. The guiding principle in vapor phase growth is to mainly control the supersaturation of the vapors. The degree of supersaturation plays an important role in the control over a crystal growth rate leading to the morphology of the obtained NW array structure. Generally, high supersaturation leads to the growth of small powders, while medium supersaturation is favorable for the preparation of whisker and nanowires, and low supersaturation for bulk crystal growth. In our experiment, the degree of supersaturation is controlled by the evaporated currents. Bi2(Te,Se)3 in this growth process is not necessarily epitaxially attached to the glass substrate. The heterogeneous nucleation process and the fast nucleation rate are the keys to initiate the growth of these NWs. Normally, slow nucleation and fast surface diffusion lead to single crystal film growth while fast nucleation leads to island growth [23]. When the evaporated current is 160 A and the deposition temperature is 250 1C, Bi2(Te,Se)3 atoms are evaporated and deposited on the surface of the substrate, and then nucleation occurs (see Figure 4). At the early stage of film growth, thermal evaporation produces relatively high energy depositing particles due to which enhanced number of nucleation sites are produced
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
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Figure 2 SEM images of (a,b) ordered nanowire array and (c,d) ordinary Bi2(Te,Se)3 films with (a,c) surface view, (b) oblique view, and (d) cross-sectional view.
Figure 3 TEM and HRTEM images of (a,b) ordered nanowire array and (c,d) ordinary Bi2(Te,Se)3 films. (a) Enlarged image of nanowire arrays, (b) the magnified image of selected area marked by the square in (a), the inset is enlarged image of selected area marked by the square in (b); (c) enlarged image of grains in ordinary film, (d) the enlarged image for the selected area marked by the square in (c).
leading to small sized grains and island growth. Subsequently, the stabilized fast flux of incoming particles (atoms and ions) of thermal evaporation restricts the surface diffusion of the adatoms resulting in the formation of small NWs morphology
under the optimized medium supersaturation. During the deposition of the film, the vapor atoms coming from the evaporation source impinge on the substrate and get deposited on it. It should be considered that these vapor atoms do
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array not find sufficient energy to move in the lateral direction to a low energy equilibrium site, but the deposited atoms have some energy which may be beneficial for migration of deposited atoms to preferred sites for (0 1 5) texture growth at the relatively high deposition temperature (250 1C). Besides, severe kinetic restraints both in the mobility of adatoms as well as in mobility of crystallite boundaries lead to NW array film. In the case for which some mobility of adatoms is possible, competitive crystallite growth occurs. The Bi2(Te,Se)3 ordered NW array film depends on the results of affect of these factors together. For the evaporated current of 150 A and the deposition temperature of 200 1C, initially, nucleation begins and hexagonal islands form owing to the inherently anisotropic bonding environment and anisotropic diffusion. Slow nucleation and sufficient surface diffusion lead to flakes crystal formation in low supersaturation (Figure 4). Then new nucleation and growth process also happen on the already formed platelet-like structure film. The slow deposition rate implies that the nuclei have enough time to grow into flakes before a new nucleation occurred, and the deposited atoms cannot obtain enough energy for migration of deposited atoms to preferred sites, leading to random growth at the low deposition temperature of 200 1C, as a result of obtaining the bulk-like ordinary Bi2(Te,Se)3 film.
Figure 4 The growth mechanism of ordered nanowire array and ordinary Bi2(Te,Se)3 films.
Table 1
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Next, the in-plane transport properties of the ordered NW array and the ordinary Bi2(Te,Se)3 TE film were investigated. Both films are n-type with a similar effective electron concentration of 3 1019/cm3 at room temperature. However, the carrier mobility of the NW array film with preferential growth of (0 1 5) plane (97 cm2/V s) is about 2 times higher than that of the ordinary film with polycrystalline structure (49 cm2/V s), as shown in Table 1. The result of EDX confirms that both thin films have a quite similar Bi:Te:Se atomic ratio. The in-plane electrical conductivities of Bi2(Te,Se)3 NW array gradually increase as temperature increases, as shown in Figure 5(a), indicating a typical semiconductor behavior. At not too low temperatures, electric transport across the grain boundaries/interfaces can be described by thermionic emission with s(T)T 1/2exp[ EB/kT], where EB is the height of the grain boundary/interface potential barrier. Thus, for certain temperatures and grain boundary/interface potential barriers, Δs/ΔT can become positive. This behavior has recently been predicted by Nolas and coworkers for nanostructured materials where transport properties are dominated by grain boundary potential barrier scattering in combination with phonon scattering [24]. This is an important difference to the layered Bi2Te3 films [25] and the bulk Bi2Te2.7Se0.3 material [26], where the negative Δs/ΔT ratio indicates a metallic like behavior. Besides, it is slowly increasing from 4.9 104 to 6.8 104 S/m for the Bi2(Te,Se)3 NW array film in the temperature range of 30– 200 1C. We can see that the in-plane electrical conductivity of the NW array film is much lower than those obtained from state-of-the-art bulk alloys [27]. The low electrical conductivity of NW array is probably due to many interspaces between the NWs and grains boundaries/interfaces in the film. But the electrical conductivity of Bi2(Te,Se)3 NW array is higher than that of the ordinary Bi2(Te,Se)3 film [Figure 5 (a)], and comparable with the result reported by Yan et al. [26]. The short intrinsic electron mean free path suggests that in the NW array, carriers transport will not suffer much from interface or boundary scattering. Consequently, electrons may still zip through the ordered NW array. The Seebeck coefficients of the Bi2(Te,Se)3 NW array and ordinary films were measured to exhibit negative values, indicating a n-type semiconductor [shown in Figure 5(b)]. It shows that the highest Seebeck coefficient is 191 μV/K for the NW array film at the temperature of 55 1C. We find that the Seebeck coefficients of the NW array are much higher than the values of the ordinary film. The enhanced Seebeck coefficient, S, in NW array is thought to be possibly
Transport properties and compositions of the Bi2(Te,Se)3 films measured at room temperature.
Bi2(Te,Se)3 Films
Bi/Te/Se atomic ratio
Nanowire array Ordinary film
40.5/56.5/3 40.5/56.4/3.1
Carrier mobility (cm2/V s)
Electrical conductivity (104 S/m)
3.2
97
4.9
3.8
49
3
Carrier concentration (1019/cm3)
Seebeck coefficient (μV/K)
Thermal conductivity (W/m K)
ZT 300 K
189
0.52
1.01
125
0.98
0.14
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
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Figure 5 (a) Electrical conductivity, (b) Seebeck coefficient, and (c) power factor of ordered nanowire array and ordinary Bi2(Te, Se)3 films as a function of temperature.
related to the quantum confinement effect that increases the difference between the Fermi level and the average mobile carrier energy, or to be related to the energy filtering effect [28] due to the charge carrier trapped in the grain boundaries regions and the surface dangling bonds on the NWs. We believe that further optimization of our synthesis toward this ideal unique microstructure will also lead to thermopower values of up to 220 mV/K. The precondition that the thermopower of Bi2(Te,Se)3 material with high Seebeck coefficients, e.g. 4220 μV/K has been verified by other group already [27]. The in-plane power factors S2s were calculated as shown in Figure 5(c). The power factor values of the Bi2(Te,Se)3 NW array film increase at first and then decrease with increasing temperature. The highest power factor 2.18 mW/m K2 was obtained for the unique microstructure film at 120 1C. The highest power factor of the ordinary film is only 1.38 mW/ m K2 in the temperature range of 30–200 1C. It is clearly demonstrated that the Bi2(Te,Se)3 NW array show a greatly enhanced power factor compared with the ordinary film. The enhanced Seebeck coefficient, S, in the film with numerous thin NWs (20-nm-diameter) is possibly related to the quantum confinement effect and the carriers filtering effect. Furthermore, the Bi2(Te,Se)3 film with NW array structure has the high carrier mobility, i.e., 97 cm2/V s,
while the carrier mobility of ordinary film only is 49 cm2/V s due to its polycrystalline structure influence. Perhaps the NW array strong (0 1 5)-orientated plane constitutes a relatively preferential way for carriers transport and promotes the carriers mobility and electrical conductivity. It plays a positive role on the properties of the NW array film. Of course, it is the fact that NWs boundaries/interfaces act as barriers to the carriers, the NW array structured film with high grain boundary/interfaces densities and some interspaces between the NWs and dimensional confinement show a little reduced carrier mobility. Therefore, when compared to the BiTeSe and the SbBiTe alloys [26,27,29], the power factor of the Bi2(Te,Se)3 NW array film is still low and need to be further improved by increasing carrier mobility, thus enhancing in-plane power factor. The Bi2(Te,Se)3 NW array is expected to be low thermal conductivity because of its unique build-up structure. At room temperature, the in-plane thermal conductivity of the Bi2(Te,Se)3 NW array film is 0.52 W/m K (Table 1), which is much lower than that the value of the ordinary film (0.98 W/m K) and reported for the Bi2Te3-based materials [26,27,30–33]. The lattice thermal conductivity, κlatt (according the Wiedemann–Franz law κlatt =κ LsT with a Lorentz number L=1.3 10 8 W Ω K 2), is about 0.33 W/m K for the
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array Bi2(Te,Se)3 NW array, which is much less than that of the ordinary film (0.86 W/m K) and the reported values of the materials [26,27]. In the lateral direction (in plane), the thermal conductivity of the Bi2(Te,Se)3 NW array is lower than that of the materials as expected because of the novel structure. It is explained by the fact that the high densities of rough interfaces may produce thermal barriers with lower thermal conductivity in the NW array Bi2(Te,Se)3 film. The reduced lattice thermal conductivity benefits from these thin NWs and doping Se atoms due to providing effective phonon scattering centers and increasing the chance of annihilation of phonon in the NW array structured film. The in-plane ZT of the n-Bi2(Te,Se)3 NW array is estimated to be about 1.01 at room temperature, which is higher than that of the Sb2Te3 and Bi2Te3-based films and superlattices [27,31,33,34]. We consider that this enhancement in ZT is attributed to the lower thermal conductivity of the NW array. Moreover, the ZT value of the n-Bi2(Te,Se)3 NW array is superior to the reported results of Bi2(Te,Se)3 bulk materials [26,27,35] at room temperature. Such unique microstructure—the NW array structure, presenting welloriented NWs perpendicular to the substrate, including surface dangling bonds, boundaries/interfaces, interspaces and NW array—favorably influence the carrier and phonon transport properties, the unique NW array structure mainly relating to selectively scattering phonon more than carrier and the quantum confinement effect and the energy filtering effect, resulting in a significantly improved ZT value. The ZT is likely to rise even higher by controlling NWs diameter, NW array morphology, material constituents, etc. It is convenient to further fabricate planar- or vertical-type TE micro-devices by integrating the NW array using maskassisted deposition method or lithography and anisotropic etching technology.
Conclusions Unique nanowire array n-Bi2(Te,Se)3 film has been selfassembled in large scale by a simple thermal co-evaporation technique. The film consists of well-oriented NW array perpendicular to the substrate at 250 1C deposition temperature. Doping Se atoms and rough surfaces were found in the one dimension NWs. A large number of NWs were assembled into the array. The degree of supersaturation plays an important role in controlling the morphology of films. The unique array structure film exhibits attractive TE properties. The optimal TE properties were obtained in the film with Seebeck coefficient S 191 μV/K at 55 1C and power factor 2.18 mW/m K2 at 120 1C, respectively. It is clearly demonstrated that the Bi2(Te,Se)3 NW array showed a greatly enhanced power factor compared with the ordinary film. As with the NW array structure, the low thermal conductivity of Bi2(Te,Se)3 film was 0.52 W/m K at room temperature. The inplane ZT value of the NW array n-Bi2(Te,Se)3 film is estimated to be about 1.01 at room temperature, leading to a promising TE material and device for applications. With optimizing composition, NWs diameter and NW array morphology, the ZT value is likely to rise even higher. Introduction of such ordered NW array architecture into films is therefore a very promising approach.
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Acknowledgments This work was supported by the State Key Development Program for Basic Research of China (Grant no. 2012CB 933200), National Natural Science Foundation of China (Nos. 51172008 and 51002006), National natural science fund innovation group (No. 50921003), Beijing Technology Topic Program (No. Z111100066511009), Research Fund for Doctor Station Sponsored by the Ministry of Education of China (20111102110035).
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Yuan Deng received the BS and Ph.D. degrees in chemistry from Tsinghua University, China, in 1995 and 2000, respectively. During 2001—2002, he worked as postdoc in Department of Materials Science and Engineering, Tsinghua University, China.
Yao Wang received the BS degree in Materials Science and Engineering from National University of Defense and Technology, China, in 2004, and Ph.D. degree in materials science and engineering from Tsinghua University, China, in 2009. Since 2009,she has joined Prof. Yuan Deng’s group in School of Chemistry and Environment in Beihang University as a lecturer. Her recent researches focus on the growth and characterization functional films, and their applications in prototype devices.
Ming Tan received the BS degree in physics in 2004, and MS degree in materials physics and chemistry from Tianjin Normal University, China, in 2009, and Ph.D. degree in Materials Science and Engineering from Beihang University, China, in 2013. Since 2013, he has worked as a lecturer in Department of Physics,College of Sciences, Tianjin University of Science & Technology. He is working on deposition nanostructured thermoelectric films and thermoelectric power generation and cooling micro-devices.
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
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Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array Ming Tana,b,n, Yuan Denga, Yao Wanga a
Beijing Key Laboratory of Special Functional Materials and Film, School of Materials Science and Engineering, Beihang University, Beijing 100191, China b Department of Physics, College of Sciences, Tianjin University of Science & Technology, Tianjin 300222, China Received 13 June 2013; received in revised form 1 July 2013; accepted 18 July 2013
KEYWORDS
Abstract
Thermal co-evaporation technique; Bi2(Te,Se)3 film; Nanowire array; Thermoelectric properties
In this paper, novel Bi2(Te,Se)3 nanowire array structure can favorably influence the carrier and phonon transport properties. The ternary compound n-type Bi2(Te,Se)3 film, composed of ordered nanowire array, has been successfully fabricated by a simple thermal co-evaporation technique without using any templates. The composition and the microstructure of the films are studied by x-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX), and high resolution transmission electron microscopy (HRTEM). The results show that the nanowire array is composed of single-crystalline Bi2(Te,Se)3 nanowires with diameter of about 20 nm. The well-oriented nanowires are parallel to each other and uniformly distributed. The growth mechanism of such nanostructure is proposed and investigated. The inplane thermoelectric properties, i.e., electrical conductivity (s) and Seebeck coefficient (S) and thermal conductivity (κ) of the nanowire array were measured. The properties of the welloriented Bi2(Te,Se)3 nanowire array have been greatly enhanced in comparison with those of the ordinary Bi2(Te,Se)3 film. The Bi2(Te,Se)3 nanowire array film with a thermoelectric dimensionless figure-of-merit ZT=1.01 was obtained at room temperature. Introduction of such ordered nanowire array architecture into films is therefore a very promising approach. & 2013 Elsevier Ltd. All rights reserved.
Introduction n
Corresponding author at: Beijing Key Laboratory of Special Functional Materials and Film, School of Materials Science and Engineering, Beihang University, Beijing 100191, China. Tel.: +86 15222181280. E-mail addresses: [email protected], [email protected] (M. Tan).
The thermoelectric (TE) properties of a material are often summarized by the figure-of-merit ZT=(S2s/κ)T, where S, s, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. Solid state materials have typically much lower ZT's. Even the best
2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.07.009 Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
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commercial thermoelectric materials, such as Bi2Te3, have TE figure of merit ZT0.8 at room temperature [1,2]. Many different approaches have been proposed and attempted to improve the TE performance of Te compounds, namely, by tuning carrier concentration, engineering the band structure [3], suppressing the lattice thermal conductivity [4–6], and reducing the device dimensionality [7]. Theoretical calculations and experimental results have also proved that low-dimensional structures can significantly optimize the transport properties of both electrons and phonons [8–12]. Especially during recent years, the record high efficiency of 2.4 was reported for molecular beam epitaxy engineered thin films of Bi2Te3/Sb2Te3 layers, and a ZT value of 3 was also reported for an n-type PbSeTe/PbTe quantum dot superlattice, which may be difficult to use in large-scale application but convincingly demonstrated the potential for further improvements to come from nanostructured materials [6,13]. Nanowires (one-dimensional) are predicted to exhibit a better TE property than superlattice films [14,15]. But single nanowire (NW) is limited in fabrication of TE devices, while nanopillar array can realize wafer-level processing including lithography and anisotropic etching for improving performance of a wafer-scale TE device as proposed recently in Ref. [16]. Additionally, it has been found that Bismuth telluride based single crystal like bulk solid solutions, including p-type BixSb2 xTe3 and n-type Bi2Te3 ySey, still remain the best TE materials used near room temperature [17,18]. Therefore, Bi2(Te,Se)3 in the form of NW array is an excellent candidate material for TE applications. In our previous work, Bi2Te3 films, composed of ordered NW array, have been successfully synthesized by a simple thermal evaporation technique without using any templates [19]. To the best of our knowledge, the TE properties and ordered structure of Bi2(Te,Se)3 NW array film have been not reported to date. In this work, we report the novel and one-step route for the large scale formation of Bi2(Te,Se)3 NW array film by a simple thermal co-evaporation technique. Our goal here is to improve TE properties of such films. Simultaneously, we also aim to obtain further insight into the relation between NW array microstructure and film properties. This convenient physical vapor phase growth technology would be further proved to be a promising way to prepare various NW arrays in ternary compound films. Moreover, it is convenient to fabricate planar or verticaltype TE micro-devices by integrating the ordered NW array structure films using mask-assisted deposition method or lithography and anisotropic etching technology.
Experimental section In this work, n-type Bi2(Te,Se)3 NW array and ordinary films were successfully grown on SiO2 substrates at 250 1C and 200 1C deposition temperature, respectively, by a thermal co-evaporation technique. The previous researches have shown that Te deficiency was easily occurred in Bi2Te3 films [20]. The concomitant compensation for Te deficiency and doping Se element are expected to improve the transport properties in the Bi2(Te,Se)3 films. As a result, high purity (99.99%) Bi2Te3 and Se powders (The mass rate of Bi2Te3:Se is 10:1) were mounted on the evaporating dish which is connected to the alternating current (AC) power supplies.
The evaporated current were 160 A and 150 A for NW array and ordinary films, respectively. The distance between the evaporation source and the substrate was about 5 cm for these films. Before deposition, the common glass substrates were first cleaned by diluted nitric acid, and then acetone, and dried under the nitrogen airflow. After the substrates were loaded onto the substrate holder (parallel to the boat), N2 gas was introduced into the chamber and vacuumized three times to remove oxygen. The working pressure was maintained at 2 10 6 Torr in the deposition process for the films. By changing the deposition time in our experiment, the thickness of film could be controlled to be above 5 μm. The crystal structures of the Bi2(Te,Se)3 films were examined by x-ray diffraction (XRD, Rigaku D/MAX 2200) with Cu Kα radiation (λ = 0.154056 nm). The compositions were detected by energy dispersive x-ray spectroscopy (EDX). The morphology of the films was observed by field emission scanning electron microscopy (FE-SEM, Sirion 200). Further structural analyses were performed using highresolution transmission electron microscopy (HRTEM, FEI Company, Tecnai G2 F20S-Twin FEG TEM at 200 kV). The samples were put into a small ampule. Ethanol was then added until it covered the surfaces of the substrates. After ultrasonication for 1 h, the solution was drop cast onto carbon coated 200 mesh Cu grids. Surface profilometry (Ambios XP-2, USA) was used to measure the film thickness. Electrical conductivity and Seebeck coefficient of the films were examined using a ZEM-3 (Ulvac Riko, Inc.) with a selfmade test holder for film measurement in the in-plane direction. The in-plane thermal conductivity data was collected using a Laser PIT (Ulvac Riko, Inc.) at room temperature. For the 1-μm-thick film of Bi2(Te,Se)3, a specially designed specimen-holder frame was used to keep the very thin glass substrate from being damaged. The substrate and frame materials are borosilicate glass having a thickness of 30 μm and 200 μm, respectively, supplied by NIMS (Xu Group, Japan). The substrate was machined to a width of 2.5 mm and it has a length of 15 mm. It was attached to the frame with a polyimide adhesive. The Bi2(Te,Se)3 films were then deposited on a half-surface of the substrate. The specimen-holder frame was coated uniformly over the whole surface with 100-nm-thick Au by evaporation. Every measurement was carried out at room temperature in a vacuum of less than 0.01 Pa, produced using a turbo-molecular pump, to eliminate the effect of air layers. Frequencies of ac-calorimetric measurements were selected in the range of 0.1 Hz. The principle of the measurement method is described in detail in Ref. [21]. The carrier concentration and mobility were determined using a four-probe measurement based on the Hall effects (ECOPIA HMS-3000) at room temperature. The errors are 4% for electrical conductivity, 5% for Seebeck coefficient, 5% for thermal conductivity, and 10% for ZT value.
Results and discussion Ordered n-Bi2(Te,Se)3 film with NW array structure was synthesized by a simple thermal co-evaporation technique, achieving oriented growth in the evaporated current of 160 A and 250 1C deposition temperature. XRD pattern of
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array the Bi2(Te,Se)3 film is shown in Figure 1. For the NW array Bi2(Te,Se)3 film, all peaks are indexed as rhombohedral phase Bi2Te3 material (JCPDS 15-0863), implying Se atoms enter into Te vacancies or formation of other defects. The intense and sharp XRD peaks from the Bi2(Te,Se)3 film are typical signatures of a high degree of crystallinity. It reveals a single-phase product with slightly broadened reflections, which is typical for crystals with low dimensions. Compared with standard card, the intensity of (0 1 5) peak (located at 2θ = 27.761) of Bi2(Te,Se)3 is dramatically strong, indicating a highly preferential orientation of the Bi2(Te,Se)3 film along the (0 1 5) direction. Additional, the (1 0 10) peak (located at 2θ = 38.441) is obvious in the NW array. Bi2(Te,Se)3 in this growth process is not necessarily epitaxially attached to the glass substrate. The atomic interactions between the Bi2(Te, Se)3 atoms and the atoms of the amorphous SiO2 substrate can locally obstruct the formation of a perfect crystalline order. But the crystallites nucleating with the thermodynamically preferred (0 1 5) orientation can grow, thus tending to form a preferential (0 1 5) growth at special depositing conditions. Different from the NW array, the ordinary Bi2(Te, Se)3 film shows more major diffraction peaks, such as (0 1 5), (1 0 10) and (0 1 11) peaks, indicating random growth in the film with polycrystalline structure. It shows that the preferential growth of (0 1 5) direction is the essential reason for the formation of oriented NW array. Compared with the standard peaks of bulk Bi2Te3 (JCPDS 15-0863), we note that the (0 1 5) and (1 0 10) diffraction peaks of NW array and ordinary Bi2(Te,Se)3 films have slightly shifted toward higher angle. It seems reasonable to assume that the small Se atom replacing Te atom causes the lattice shrinkage. The Bi2(Te,Se)3 NW array and ordinary films are clearly shown by SEM images in Figure 2. Seen from the top view [shown in Figure 2(a)], the film is relatively dense and uniform, and some interspaces between the NWs exist in the film. From oblique view of the film [Figure 2(b)], we can observe that a large number of thin Bi2(Te,Se)3 NWs were densely grown perpendicular to the substrate, along their preferential growth direction. The observation of ordered growth is also in agreement with the XRD analysis above. The sizes of NWs are uniform in the Bi2(Te,Se)3 film. The diameter of each is estimated to be about 20 nm [Figure 2(b)], implying numerous interfaces in the film. Besides, many grains with sizes varying from nano to submicro scale distribute on the Bi2(Te,Se)3 ordinary film surface, as
Figure 1 XRD patterns of ordered nanowire array and ordinary Bi2(Te,Se)3 films.
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shown in Figure 2(c). Seen from the cross-sectional image in Figure 2(d), the film is composed of numerous disordered bulk-like particles. In order to further verify the special ordered NW array microstructure, details of the ordered NW array structure are seen in the TEM and HRTEM images shown in Figure 3(a) and (b). The NW array exhibits dense growth in Figure 3(a), which can significantly promote the carrier transport, indicating that Bi2(Te,Se)3 NW array is still stable even after a long ultrasonic treatment. Figure 3(b) is the enlarged image of selected area marked by a square in Figure 3(a), the diameter of NW is affirmed to be about 20 nm, and NWs show rough surface which can significantly suppress the phonon transport, resulting in a significantly improved ZT value. As shown in the inset, the lattice spacing of 0.336 nm corresponds to the lattice of (0 1 5) crystal plane. This confirms that the NWs grow along the preferred (0 1 5) direction, which is in accordance with the XRD result. This microstructure may play a very positive role on its high electrical conductivity. From Figure 3(c), we note that disordered particles are exhibited in the ordinary Bi2(Te,Se)3 film. Figure 3(d) is the magnified image for the selected area marked by a square in Figure 3(c), showing the ordinary film with polycrystalline structure. The lattice spacing of 0.331 nm and 0.227 nm correspond to the lattices of (0 1 5) and (1 0 10) crystal planes, as shown in Figure 3(d), which also confirms (0 1 5) and (1 0 10) major diffraction peaks in the ordinary film. Besides, a number of disordered grains boundaries exist in the ordinary film, which influences the carrier transport properties. The detailed growth mechanism of the NW array and the ordinary Bi2(Te,Se)3 film will be proposed. The growth process of the films is illustrated in Figure 4. Commonly, AV2 BVI 3 alloys have an inherently anisotropic bonding environment which causes faster growth along the top-bottom crystalline planes compared to the natural growth axis (c-axis) thus tending to form a platelet-like morphology [22]. However, in thin film growth, the growth kinetics can be influenced by temperature, supersaturation of the vapors, ion bombardment, impurity content and in case of compound deposition, by chemical composition, etc. The guiding principle in vapor phase growth is to mainly control the supersaturation of the vapors. The degree of supersaturation plays an important role in the control over a crystal growth rate leading to the morphology of the obtained NW array structure. Generally, high supersaturation leads to the growth of small powders, while medium supersaturation is favorable for the preparation of whisker and nanowires, and low supersaturation for bulk crystal growth. In our experiment, the degree of supersaturation is controlled by the evaporated currents. Bi2(Te,Se)3 in this growth process is not necessarily epitaxially attached to the glass substrate. The heterogeneous nucleation process and the fast nucleation rate are the keys to initiate the growth of these NWs. Normally, slow nucleation and fast surface diffusion lead to single crystal film growth while fast nucleation leads to island growth [23]. When the evaporated current is 160 A and the deposition temperature is 250 1C, Bi2(Te,Se)3 atoms are evaporated and deposited on the surface of the substrate, and then nucleation occurs (see Figure 4). At the early stage of film growth, thermal evaporation produces relatively high energy depositing particles due to which enhanced number of nucleation sites are produced
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
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Figure 2 SEM images of (a,b) ordered nanowire array and (c,d) ordinary Bi2(Te,Se)3 films with (a,c) surface view, (b) oblique view, and (d) cross-sectional view.
Figure 3 TEM and HRTEM images of (a,b) ordered nanowire array and (c,d) ordinary Bi2(Te,Se)3 films. (a) Enlarged image of nanowire arrays, (b) the magnified image of selected area marked by the square in (a), the inset is enlarged image of selected area marked by the square in (b); (c) enlarged image of grains in ordinary film, (d) the enlarged image for the selected area marked by the square in (c).
leading to small sized grains and island growth. Subsequently, the stabilized fast flux of incoming particles (atoms and ions) of thermal evaporation restricts the surface diffusion of the adatoms resulting in the formation of small NWs morphology
under the optimized medium supersaturation. During the deposition of the film, the vapor atoms coming from the evaporation source impinge on the substrate and get deposited on it. It should be considered that these vapor atoms do
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array not find sufficient energy to move in the lateral direction to a low energy equilibrium site, but the deposited atoms have some energy which may be beneficial for migration of deposited atoms to preferred sites for (0 1 5) texture growth at the relatively high deposition temperature (250 1C). Besides, severe kinetic restraints both in the mobility of adatoms as well as in mobility of crystallite boundaries lead to NW array film. In the case for which some mobility of adatoms is possible, competitive crystallite growth occurs. The Bi2(Te,Se)3 ordered NW array film depends on the results of affect of these factors together. For the evaporated current of 150 A and the deposition temperature of 200 1C, initially, nucleation begins and hexagonal islands form owing to the inherently anisotropic bonding environment and anisotropic diffusion. Slow nucleation and sufficient surface diffusion lead to flakes crystal formation in low supersaturation (Figure 4). Then new nucleation and growth process also happen on the already formed platelet-like structure film. The slow deposition rate implies that the nuclei have enough time to grow into flakes before a new nucleation occurred, and the deposited atoms cannot obtain enough energy for migration of deposited atoms to preferred sites, leading to random growth at the low deposition temperature of 200 1C, as a result of obtaining the bulk-like ordinary Bi2(Te,Se)3 film.
Figure 4 The growth mechanism of ordered nanowire array and ordinary Bi2(Te,Se)3 films.
Table 1
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Next, the in-plane transport properties of the ordered NW array and the ordinary Bi2(Te,Se)3 TE film were investigated. Both films are n-type with a similar effective electron concentration of 3 1019/cm3 at room temperature. However, the carrier mobility of the NW array film with preferential growth of (0 1 5) plane (97 cm2/V s) is about 2 times higher than that of the ordinary film with polycrystalline structure (49 cm2/V s), as shown in Table 1. The result of EDX confirms that both thin films have a quite similar Bi:Te:Se atomic ratio. The in-plane electrical conductivities of Bi2(Te,Se)3 NW array gradually increase as temperature increases, as shown in Figure 5(a), indicating a typical semiconductor behavior. At not too low temperatures, electric transport across the grain boundaries/interfaces can be described by thermionic emission with s(T)T 1/2exp[ EB/kT], where EB is the height of the grain boundary/interface potential barrier. Thus, for certain temperatures and grain boundary/interface potential barriers, Δs/ΔT can become positive. This behavior has recently been predicted by Nolas and coworkers for nanostructured materials where transport properties are dominated by grain boundary potential barrier scattering in combination with phonon scattering [24]. This is an important difference to the layered Bi2Te3 films [25] and the bulk Bi2Te2.7Se0.3 material [26], where the negative Δs/ΔT ratio indicates a metallic like behavior. Besides, it is slowly increasing from 4.9 104 to 6.8 104 S/m for the Bi2(Te,Se)3 NW array film in the temperature range of 30– 200 1C. We can see that the in-plane electrical conductivity of the NW array film is much lower than those obtained from state-of-the-art bulk alloys [27]. The low electrical conductivity of NW array is probably due to many interspaces between the NWs and grains boundaries/interfaces in the film. But the electrical conductivity of Bi2(Te,Se)3 NW array is higher than that of the ordinary Bi2(Te,Se)3 film [Figure 5 (a)], and comparable with the result reported by Yan et al. [26]. The short intrinsic electron mean free path suggests that in the NW array, carriers transport will not suffer much from interface or boundary scattering. Consequently, electrons may still zip through the ordered NW array. The Seebeck coefficients of the Bi2(Te,Se)3 NW array and ordinary films were measured to exhibit negative values, indicating a n-type semiconductor [shown in Figure 5(b)]. It shows that the highest Seebeck coefficient is 191 μV/K for the NW array film at the temperature of 55 1C. We find that the Seebeck coefficients of the NW array are much higher than the values of the ordinary film. The enhanced Seebeck coefficient, S, in NW array is thought to be possibly
Transport properties and compositions of the Bi2(Te,Se)3 films measured at room temperature.
Bi2(Te,Se)3 Films
Bi/Te/Se atomic ratio
Nanowire array Ordinary film
40.5/56.5/3 40.5/56.4/3.1
Carrier mobility (cm2/V s)
Electrical conductivity (104 S/m)
3.2
97
4.9
3.8
49
3
Carrier concentration (1019/cm3)
Seebeck coefficient (μV/K)
Thermal conductivity (W/m K)
ZT 300 K
189
0.52
1.01
125
0.98
0.14
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
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Figure 5 (a) Electrical conductivity, (b) Seebeck coefficient, and (c) power factor of ordered nanowire array and ordinary Bi2(Te, Se)3 films as a function of temperature.
related to the quantum confinement effect that increases the difference between the Fermi level and the average mobile carrier energy, or to be related to the energy filtering effect [28] due to the charge carrier trapped in the grain boundaries regions and the surface dangling bonds on the NWs. We believe that further optimization of our synthesis toward this ideal unique microstructure will also lead to thermopower values of up to 220 mV/K. The precondition that the thermopower of Bi2(Te,Se)3 material with high Seebeck coefficients, e.g. 4220 μV/K has been verified by other group already [27]. The in-plane power factors S2s were calculated as shown in Figure 5(c). The power factor values of the Bi2(Te,Se)3 NW array film increase at first and then decrease with increasing temperature. The highest power factor 2.18 mW/m K2 was obtained for the unique microstructure film at 120 1C. The highest power factor of the ordinary film is only 1.38 mW/ m K2 in the temperature range of 30–200 1C. It is clearly demonstrated that the Bi2(Te,Se)3 NW array show a greatly enhanced power factor compared with the ordinary film. The enhanced Seebeck coefficient, S, in the film with numerous thin NWs (20-nm-diameter) is possibly related to the quantum confinement effect and the carriers filtering effect. Furthermore, the Bi2(Te,Se)3 film with NW array structure has the high carrier mobility, i.e., 97 cm2/V s,
while the carrier mobility of ordinary film only is 49 cm2/V s due to its polycrystalline structure influence. Perhaps the NW array strong (0 1 5)-orientated plane constitutes a relatively preferential way for carriers transport and promotes the carriers mobility and electrical conductivity. It plays a positive role on the properties of the NW array film. Of course, it is the fact that NWs boundaries/interfaces act as barriers to the carriers, the NW array structured film with high grain boundary/interfaces densities and some interspaces between the NWs and dimensional confinement show a little reduced carrier mobility. Therefore, when compared to the BiTeSe and the SbBiTe alloys [26,27,29], the power factor of the Bi2(Te,Se)3 NW array film is still low and need to be further improved by increasing carrier mobility, thus enhancing in-plane power factor. The Bi2(Te,Se)3 NW array is expected to be low thermal conductivity because of its unique build-up structure. At room temperature, the in-plane thermal conductivity of the Bi2(Te,Se)3 NW array film is 0.52 W/m K (Table 1), which is much lower than that the value of the ordinary film (0.98 W/m K) and reported for the Bi2Te3-based materials [26,27,30–33]. The lattice thermal conductivity, κlatt (according the Wiedemann–Franz law κlatt =κ LsT with a Lorentz number L=1.3 10 8 W Ω K 2), is about 0.33 W/m K for the
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009
Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array Bi2(Te,Se)3 NW array, which is much less than that of the ordinary film (0.86 W/m K) and the reported values of the materials [26,27]. In the lateral direction (in plane), the thermal conductivity of the Bi2(Te,Se)3 NW array is lower than that of the materials as expected because of the novel structure. It is explained by the fact that the high densities of rough interfaces may produce thermal barriers with lower thermal conductivity in the NW array Bi2(Te,Se)3 film. The reduced lattice thermal conductivity benefits from these thin NWs and doping Se atoms due to providing effective phonon scattering centers and increasing the chance of annihilation of phonon in the NW array structured film. The in-plane ZT of the n-Bi2(Te,Se)3 NW array is estimated to be about 1.01 at room temperature, which is higher than that of the Sb2Te3 and Bi2Te3-based films and superlattices [27,31,33,34]. We consider that this enhancement in ZT is attributed to the lower thermal conductivity of the NW array. Moreover, the ZT value of the n-Bi2(Te,Se)3 NW array is superior to the reported results of Bi2(Te,Se)3 bulk materials [26,27,35] at room temperature. Such unique microstructure—the NW array structure, presenting welloriented NWs perpendicular to the substrate, including surface dangling bonds, boundaries/interfaces, interspaces and NW array—favorably influence the carrier and phonon transport properties, the unique NW array structure mainly relating to selectively scattering phonon more than carrier and the quantum confinement effect and the energy filtering effect, resulting in a significantly improved ZT value. The ZT is likely to rise even higher by controlling NWs diameter, NW array morphology, material constituents, etc. It is convenient to further fabricate planar- or vertical-type TE micro-devices by integrating the NW array using maskassisted deposition method or lithography and anisotropic etching technology.
Conclusions Unique nanowire array n-Bi2(Te,Se)3 film has been selfassembled in large scale by a simple thermal co-evaporation technique. The film consists of well-oriented NW array perpendicular to the substrate at 250 1C deposition temperature. Doping Se atoms and rough surfaces were found in the one dimension NWs. A large number of NWs were assembled into the array. The degree of supersaturation plays an important role in controlling the morphology of films. The unique array structure film exhibits attractive TE properties. The optimal TE properties were obtained in the film with Seebeck coefficient S 191 μV/K at 55 1C and power factor 2.18 mW/m K2 at 120 1C, respectively. It is clearly demonstrated that the Bi2(Te,Se)3 NW array showed a greatly enhanced power factor compared with the ordinary film. As with the NW array structure, the low thermal conductivity of Bi2(Te,Se)3 film was 0.52 W/m K at room temperature. The inplane ZT value of the NW array n-Bi2(Te,Se)3 film is estimated to be about 1.01 at room temperature, leading to a promising TE material and device for applications. With optimizing composition, NWs diameter and NW array morphology, the ZT value is likely to rise even higher. Introduction of such ordered NW array architecture into films is therefore a very promising approach.
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Acknowledgments This work was supported by the State Key Development Program for Basic Research of China (Grant no. 2012CB 933200), National Natural Science Foundation of China (Nos. 51172008 and 51002006), National natural science fund innovation group (No. 50921003), Beijing Technology Topic Program (No. Z111100066511009), Research Fund for Doctor Station Sponsored by the Ministry of Education of China (20111102110035).
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Yuan Deng received the BS and Ph.D. degrees in chemistry from Tsinghua University, China, in 1995 and 2000, respectively. During 2001—2002, he worked as postdoc in Department of Materials Science and Engineering, Tsinghua University, China.
Yao Wang received the BS degree in Materials Science and Engineering from National University of Defense and Technology, China, in 2004, and Ph.D. degree in materials science and engineering from Tsinghua University, China, in 2009. Since 2009,she has joined Prof. Yuan Deng’s group in School of Chemistry and Environment in Beihang University as a lecturer. Her recent researches focus on the growth and characterization functional films, and their applications in prototype devices.
Ming Tan received the BS degree in physics in 2004, and MS degree in materials physics and chemistry from Tianjin Normal University, China, in 2009, and Ph.D. degree in Materials Science and Engineering from Beihang University, China, in 2013. Since 2013, he has worked as a lecturer in Department of Physics,College of Sciences, Tianjin University of Science & Technology. He is working on deposition nanostructured thermoelectric films and thermoelectric power generation and cooling micro-devices.
Please cite this article as: M. Tan, et al., Ordered structure and high thermoelectric properties of Bi2(Te,Se)3 nanowire array, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.07.009