High thermoelectric properties of (Sb, Bi)2Te3 nanowire arrays by tilt-structure engineering

Applied Surface Science 443 (2018) 11–17

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High thermoelectric properties of (Sb, Bi)2Te3 nanowire arrays by tilt-structure engineering Ming Tan a,b,⇑, Yanming Hao a, Yuan Deng b, Jingyi Chen c a

Department of Physics, College of Sciences, Tianjin University of Science & Technology, Tianjin 300457, China Beijing Key Laboratory of Special Functional Materials and Films, School of Materials Science and Engineering, Beihang University, Beijing 100191, China c College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China b

a r t i c l e

i n f o

Article history: Received 28 November 2017 Revised 26 January 2018 Accepted 19 February 2018 Available online 21 February 2018 Keywords: Thermal evaporation technique Tilt-structure Nanowire array Thermoelectric properties

a b s t r a c t In this paper, we present an innovative tilt-structure design concept for (Sb, Bi)2Te3 nanowire array assembled by high-quality nanowires with well oriented growth, utilizing a simple vacuum thermal evaporation technique. The unusual tilt-structure (Sb, Bi)2Te3 nanowire array with a tilted angle of 45° exhibits a high thermoelectric dimensionless figure-of-merit ZT = 1.72 at room temperature. The relatively high ZT value in contrast to that of previously reported (Sb, Bi)2Te3 materials and the vertical (Sb, Bi)2Te3 nanowire arrays evidently reveals the crucial role of the unique tilt-structure in favorably influencing carrier and phonon transport properties, resulting in a significantly improved ZT value. The transport mechanism of such tilt-structure is proposed and investigated. This method opens a new approach to optimize nano-structure in thin films for next-generation thermoelectric materials and devices. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction Owing to the increasing concern on the global energy crisis, the thermoelectric (TE) technology has been paid much attention in recent years as an alternative energy source to reduce the conventional oil and fossil-fuel consumption. The ability of a TE material to convert heat into electricity is determined by the figure-ofmerit ZT = (S2r/j)T, where S, r, j and T are Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively [1–5]. There exists a strong coupling of TE parameters j, r and S. Many research efforts to overcome the conventional r-S and j-r trade-off have been made in attempts to obtain a high ZT value during recent years [6–9]. Theoretical and experimental analyses have shown that the low-dimensional structure can significantly optimize the transport properties of electrons and phonons, which are to break through the limitation of the electron-phonon coupling and provide an effective pathway past a low-dimensional structure material, such as, the record high efficiency of 2.4 was reported for the Bi2Te3/Sb2Te3 superlattice, and a ZT value of 3 was also reported for the PbSeTe/PbTe quantum dot superlattice. However, One-dimensional nanowires (NWs) are

⇑ Corresponding author at: Department of Physics, College of Sciences, Tianjin University of Science & Technology, Tianjin 300457, China. E-mail address: [email protected] (M. Tan). https://doi.org/10.1016/j.apsusc.2018.02.193 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.

predicted to exhibit a better TE property than superlattices [10– 12]. Bi-Sb-Te materials are the best TE materials near room temperature, which are anisotropic with a layered structure. Their thermal and electrical conductivities along the a-axis (in the c-plane) are approximately two and four times higher, respectively, than those along the c-axis of Bi2Te3-based materials. But Seebeck coefficients are less dependent on the crystallography [13–16]. Therefore, an improved ZT value can be expected when utilizing anisotropic thermal and electrical transport properties. Our previous results show that the Sb2Te3 pillar array structure can selectively scatter phonon more than carrier, resulting in an improved in-plane ZT value [17]. In addition, we also find that unique NW array structuring can induce a change of the Fermi level of the Bi2(Te, Se)3 and favorably influence the carrier and phonon transport properties, thus dramatically enhancing an in-plane ZT result [18]. The previous studies have witnessed the feasibility of controlling novel microstructures to modify TE properties of Sb2Te3 and Bi2Te3based alloys. However, it is noted that the Sb2Te3 pillar array and the Bi2(Te, Se)3 NW array are grown perpendicular to the substrates and possess relatively high densities of interspaces, which degrade the in-plane thermopower to some extent. Sun and Stranz reported that a wafer-scale vertical nanopillar arrays or NW arrays can be realized by lithography and anisotropic etching for improving the performance of TE cross-plane devices as proposed recently [19,20]. But it is hardly to overcome a problem of carrier and

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phonon transport along the in-plane direction and measure crossplane TE properties for vertical nanopillar arrays or NW arrays films. These adverse factors need to be further improved in films, enabling films to show better in-plane properties. Some vertically aligned nanowire arrays or nanopillar arrays have been synthesized by the electrochemical deposition with templates or the anisotropic etching and lithography method, however, this kind of NW array with tilt-structure has never been reported, let alone the TE (Sb, Bi)2Te3 material. This motivates us to further explore the effect of tilt-structure on the (Sb, Bi)2Te3 NW array. Hence, in this work, we aim to control the tilt-angle of (Sb, Bi)2Te3 NW array based on the construction of one-dimensional NWs. A simple thermal evaporation technique was carried out, to our best knowledge, for the first time on the tilt-growth of (Sb, Bi)2Te3 NW arrays. The unusual structure (Sb, Bi)2Te3 NW array with a tilted angle of 45° exhibits a greatly high in-plane ZT = 1.72 at room temperature. It is believed that the interrelationship between the tilt-structure and the properties of films uncovered by this work may help to better understand unique tilt structuring of this kind of material. Furthermore, it provides a new avenue to control the structural configuration of materials with possible relevance to improvement of their properties. It is also convenient to further fabricate cross-plane or in-plane devices by integrating the (Sb, Bi)2Te3 NW array with tilted structure using the anisotropic etching or lithography method or mask-assisted deposition technology.

2. Experimental section In this work, in order to successfully grow the tilted and the vertical (Sb, Bi)2Te3 NW arrays on SiO2 glass substrates by the thermal evaporation technique, the corresponding angles between the substrate holder plane and the horizontal plane are supposed to be about 45°, 30° and 0°, respectively. The compensation for Te deficiency and doping Bi element are expected to improve the transport properties in the (Sb, Bi)2Te3 films. The high purity (99.99%) Sb2Te3, Te and Bi powders (The mass rate of Sb2Te3:Te:Bi is 10:1:1) were mounted on the evaporating dish which is connected to the alternating current power supplies, and the evaporated current was 165A for all NW arrays. Before deposition, the common glass substrate was first cleaned by diluted nitric acid, and then acetone, and dried under the nitrogen gas flow. After the substrate was loaded onto the substrate holder, N2 gas was introduced into the chamber and vacuumized three times to remove oxygen. The deposition temperature was set at 250 °C, the working pressure was maintained at 2
106 Torr in the deposition process for the films. All TE films were grown to thickness of 1.5 lm by adjusting the deposition rate and deposition time in our experiments. The crystal structure characterizations for the NW arrays were measured using X-ray diffraction (XRD, Rigaku D/MAX 2200) with Cu Ka radiation (k = 0.154056 nm). The morphology and composition of the samples were investigated using a field emission scanning electron microscope (FE-SEM, Sirion 200) equipped with an energy dispersive X-ray spectroscope (EDX). Further structural analyses were performed using high-resolution transmission electron microscopy (HRTEM, FEI Company, Tecnai G2 F20S-Twin FEG TEM at 200 kV). 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 self-made test holder for film measurement in the inplane direction. The in-plane thermal conductivity data was collected using a Laser PIT (Ulvac Riko, Inc.) at room temperature. 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. All tests for transport

properties were repeated at least 5 times. The errors are 4% for electrical conductivities, 5% for Seebeck coefficients, 5% for thermal conductivities, and 10% for ZT values.

3. Results and discussion The morphologies of the (Sb, Bi)2Te3 NW arrays were studied by SEM, respectively. The SEM images (Fig. 1a and b) reveal that the (Sb, Bi)2Te3 NW array with a tilted angle of 45° has been perfectly prepared by a simple thermal evaporation technique. Seen from the cross-sectional image of the tilted film (Fig. 1a), a large number of (Sb, Bi)2Te3 NWs are densely grown tilted to the substrate, along their oriented growth direction. It clearly shows that the angle between the radial direction of NW and the substrate plane is about 45°. The diameters of tilt-growth NWs are estimated to be <20 nm, implying a large number of unique interfaces in the NW array. Some interspaces between NWs are found, but the adjacent NWs have been tilted and interconnected closely to give a very good contact each other, guaranteeing carriers transport in the in-plane direction of the film. Seen from the top view (Fig. 1b), the sizes of NWs are uniform in the tilt-structure film, which is composed of NW array based on the construction of onedimensional NWs. By controlling growth parameter, NW arrays microstructures have obviously changed as shown in Fig. 1. With the angle between the substrate plane and the horizontal plane reduces to about 30°, the (Sb, Bi)2Te3 NW with a tilted angle of 60° has been successfully fabricated (Fig. 1c and d). From Fig. 1c, we note that numerous NWs are tilted growth on the substrate and the angle between the radial direction of NW and the substrate plane is approximately 60°. Seen from the surface SEM image (Fig. 1d), some nano-scaled open gaps between NWs can be found in the NW array. When the angle between the substrate plane and the horizontal plane is about 0°, that is, the substrate plane approximatively parallels to the horizontal plane. The NW array is uniformly grown perpendicular to the substrate, which exhibits that the angle between the radial direction of vertical NWs and the substrate plane is about 90° (Fig. 1e and f). This phenomenon is the same to the reported result for the vertically aligned Bi2(Te, Se)3 NW array [18]. In order to gain insight into the crystal structure, NW arrays were examined by XRD. Fig. 2 presents XRD patterns of all (Sb, Bi)2Te3 NW arrays. As shown in Fig. 2, a single Sb2Te3 phase, consistent with the standard card (JCPDS 71-0393) of the Sb2Te3, was obtained in NW arrays samples, implying Bi atoms enter into Sb vacancies or formation of other defects. A preferential orientation (0 1 5) peak (located at 2h = 28.13°) was mainly observed in these NW arrays. Compared with the standard card, the major (0 1 5) diffraction peaks of (Sb, Bi)2Te3 films have slightly shifted toward lower angle. It seems reasonable to assume that the large Bi atom replacing Sb atom causes the lattice to expand. The intensity of (0 0 1 5) texture of the NW array with a tilted angle of 45° is dramatically strong. With increasing the tilted angle, the (0 0 1 5) peak becomes weak, while the intensity of (1 0 1 0) peak becomes strong in the NW array with a tilted angle of 60°. When the angle becomes large to 90°, the intensity of (1 0 1 0) peak of the NW array becomes greatly strong and the (0 0 1 5) peak is disappeared. This seems to indicate that the tilt-growth is associated with the (0 0 1 5) and (1 0 1 0) peaks of NW arrays. The atom lateral mobility increase on the surface due to a decrease in the angle between the deposition direction and the substrate plane may be responsible for the structure change. The growing grains can be sufficiently mobile to migrate to the preferred sites for crystallization growth. The microstructure details of the special NW array with a tilted angle of 45° are observed in TEM and HRTEM images, as depicted in Fig. 3. A microstructure of (Sb, Bi)2Te3 NWs varies continuously

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Fig. 1. SEM images of the cross section (a, c, e) and surface (b, d, f) of (Sb, Bi)2Te3 NW arrays with tilted angles of (a, b) 45°, (c, d) 60° and (e, f) 90°.

Fig. 2. XRD patterns of (Sb, Bi)2Te3 NW arrays with tilted angles of 45°, 60° and 90°.

from the NW surface to the unique interface shown in Fig. 3a, which can significantly influence the carrier transport. It clearly shows that NWs tightly connect side-by-side to assemble into NW array, which would be necessary to improve the transport

property by preserving the high quality channel region from the ordered NW region through the alignment of tilted interfaces, as shown in Fig. 3b. The lattices of (0 1 5) and (0 0 1 5) crystal planes are shown in Fig. 3c which originates from the magnified image of the selected area in Fig. 3b. This confirms that the NWs grow along the preferred direction, which is fully consistent with the above XRD result. The diameter of NWs is confirmed to be <20 nm, and NWs show rough surfaces which can greatly suppress phonon transport. Furthermore, the interface between NWs remains substantially coherent, as shown in Fig. 3c. This implies that rough interfaces periodically exist in the NW array. Thus, while phonons are strongly scattered, the carrier transport is only little impeded in the tilted NW array. This unique microstructure would play an extremely positive role on its TE properties. It is of considerable interest to be able to tune electronic transport properties by modifying the morphology and crystal structure of films. The in-plane electrical conductivities of the tilted and the vertical (Sb, Bi)2Te3 NW arrays were investigated, respectively. As shown in Fig. 4a, the (Sb, Bi)2Te3 NW array with a tilted angle of 45° has maximum electrical conductivity of 9.1
104 S/m in the temperature range of 30–200 °C, which is higher than that of the vertical NW array and those obtained from materials [14,22–24]. It can be obviously noted that the in-plane electrical conductivities of NW arrays increase as the tilted angle of NW arrays decreases.

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Fig. 3. TEM and HRTEM images of the (Sb, Bi)2Te3 NW array with a tilted angle of 45°. (a) The enlarged image of the NW array; (b) and (c) images of selected area respectively marked by a square in (a) and (b), respectively. (The interface between NWs marked by the dot line, as shown in (c)).

Fig. 4. (a) Electrical conductivity, (b) Seebeck coefficient, and (c) power factor of (Sb, Bi)2Te3 NW arrays as a function of temperature.

The enhanced electrical conductivities in the NW array are thought to be mainly related to unique tilt-structure of NW arrays. Besides, the (Sb, Bi)2Te3 material is a well-known narrow band gap semiconductor, there are surface states at an energetic position above

the conduction band edge, which leads to a charge transfer from the surface state into the bulk [25]. Thus, an electron layer accumulates at the surface region of (Sb, Bi)2Te3 NWs with a high surface-to-volume ratio, leading to significantly increased electri-

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2–24,27–31,33–36], and comparable with the value (1.86) of the bulk Bi1.5Sb0.5Te3 material as reported recently [37]. Tiltstructuring opens great opportunities for an effective modification of the interrelated TE transport properties at NW arrays with tilted growth, where the size effect [38], energy filtering effect [39], and preferential route effect can have a profound influence on the TE transport properties. The different behavior of the tilted NW array in terms of thermal and electrical conduction strongly suggests that the tilted (Sb, Bi)2Te3 NW array promises to be a most efficient structure for use in TE devices. The results of this study provide insights for the structural design and synthesis of TE materials, which will be very important for future development of functional materials. With optimization of the NWs diameter, tilted angles, periodic rough interfaces, etc., ZT is likely to rise even higher. In order to further verify novel tilt-structure NW array playing a greatly important role to optimize TE transport properties, the mobility and concentration of carriers were examined by a fourprobe measurement based on the Hall effects at room temperature. The composition analysis by EDX was firstly considered due to the composition of films affecting the carrier concentration. The result of EDX confirms that all thin films have a quite similar Sb:Bi:Te atomic ratio. It can be seen that the mobility and carrier concentration exhibit obviously different among these NW arrays, as shown in Table 1. The concentrations of carriers are 3.1
1019 cm3 and 4.2
1019 cm3 and 4.8
1019 cm3 for the (Sb, Bi)2Te3 NW arrays with tilted angles of 45° and 60° and 90°, respectively. It clearly exhibits that the concentrations of carriers decrease owing to the formation of a relatively perfect crystalline structure at a proper deposition condition. Here, the vertical (Sb, Bi)2Te3 NW array obtains the relatively high value of the carrier concentration possibly due to formation of defects in quantity, which negatively effects on Seebeck coefficients. On the other hand, the tilted NW arrays with tilted angles of 45° and 60° have high mobility of 175 cm2/V s and 115 cm2/V s, respectively, which is prominently higher than that of the vertical NW array (70 cm2/V s), as shown in Table 1. It clearly shows that carrier mobility is closely related to the tilt-angle and the unique tilt-structure is more beneficial for the carrier mobility in comparison with the vertical structure, then enhancing power factors. A detailed discussion will continue at the end of this section. The in-plane transport properties of all NW arrays are measured along the x direction, the transport mechanism of the tilt-structure NWs is proposed as schematic diagram shown in Fig. 5. The NWs model, in which the unique tilt-structure played an important role in TE property enhancement, mainly explains our experimental results. At a given diameter of NW, the size of the surface area clearly depends on the length of the NW. The length of the tilted NW is 1/sina times longer than that of the vertical NW in the films with the same thickness. Therefore, the ratio of the surface area between the tilted (S) and the vertical (S0 ) NWs is 1/sina. The length and the surface area for the tilted and the vertical NW 0 respectively follow l ¼ h= sin a ¼ h = sin a and S ¼ S0 = sin a. When tilt-angles become small, the length and the surface area for the NW increase. It is noted that the long NWs can provide the large contact area for the adjacent NWs, leading to the increase in the chance of carriers transport. Besides, the length of the effective

cal conductivities. Moreover, it may be speculated that dramatically oriented (0 1 5) and (0 0 1 5) lattices and tilt-structure of NW array can allow the top-priority route for carriers transport, enhancing the carrier mobility and the electrical conductivity in the tilted NW arrays. In Fig. 4b, the temperature dependent Seebeck coefficients of the (Sb, Bi)2Te3 NW arrays are presented, respectively, which are positive values for all samples, indicating p-type semiconductor. It shows that the highest Seebeck coefficient reaches to 255 lV/K for the (Sb, Bi)2Te3 NW array with a tilted angle of 45° at the temperature of 200 °C. An outstanding increase in the Seebeck coefficient is observed with a tilt-structure growth in NW arrays. The Seebeck coefficient value in the present work is much higher in comparison to the reported results of Sb2Te3- and Bi2Te3-based materials [14,22–24,26–30]. The reason for this greatly difference could be the change in the microstructure observed in this work compared to the reported structure. This tilt-structure can possibly promote the carrier mobility in the in-plane direction, leading to large Seebeck coefficients. The power factor S2r versus temperature for the (Sb, Bi)2Te3 NW arrays are plotted in Fig. 4c. It exhibits that a high average power factor of 5.08 mW/m K2 was obtained for the unique structure film with a tilted angle of 45° between 30 and 200 °C, along with the maximum power factor value of 5.33 mW/m K2 at 30 °C, which also implies that the S2r nearly remains constant with temperature. In contrast to Sb-Bi-Te materials [14,22–24,26–30], whose S2r values increase or decrease as the temperature increases, the temperature stability of S2r of the (Sb, Bi)2Te3 NW arrays is useful in practical use. It is clearly demonstrated that the tilted (Sb, Bi)2Te3 NW array show a largely enhanced power factor compared with the vertical NW array and previous results [28–32]. There is no doubt that the tilt-structure architecture significantly aids in achieving high power factors. However, these samples were still considered to have better TE properties due to greatly reduce thermal conductivities originating from numerous rough interfaces, defects, etc. The unique film is composed of 1D NWs, which is expected to be low thermal conductivity, especially for the in-plane thermal conductivity. The in-plane thermal conductivity of the tilted (Sb, Bi)2Te3 NW arrays is 0.93 and 0.89 W/m K at room temperature (see Table 1), respectively, which is slightly higher than values of the vertical NW array (0.71 W/m K) due to the influence of the tilt-structure of NW array. However, the presented thermal conductivities are lower than the reported results for Sb-Bi-Te materials [24,31,33]. It is explained by the fact that thin NWs, coherent grain boundaries, periodic rough interfaces, surface dangling bonds, antisite defects as well as other defects are responsible for scattering phonon with a variety of wavelengths, leading to lower thermal conductivity in these unique NW arrays, as SEM and TEM microstructure above. According to the measured electrical conductivity, Seebeck coefficient, and thermal conductivity, the ZT value was calculated at room temperature, as shown in Table 1. The in-plane ZT of the (Sb, Bi)2Te3 NW array with a tilted angle of 45° was about 1.72 at room temperature, which is superior to that of the NW array with a tilted angle of 60° (1.22) and the vertical NW array (0.9) and the reported results of Sb-Bi-Te film and bulk materials [14,2

Table 1 Transport properties and compositions of (Sb, Bi)2Te3 NW arrays measured at room temperature. Tilt angle

Sb/Bi/Te atomic ratio

Carrier concentration (1019/cm3)

Carrier mobility (cm2/V s)

Electrical conductivity (104 S/m)

Seebeck coefficient (lV/K)

Thermal conductivity (W/m K)

ZT 300 K

45° 60° 90°

36.4/3.3/60.3 36.3/3.2/60.5 36.5/3.3/60.2

3.1 4.2 4.8

175 115 70

8.8 7.4 5.1

246 221 204

0.93 0.89 0.71

1.72 1.22 0.9

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[3]

[4]

[5]

[6] [7] Fig. 5. The in-plane transport mechanism of the tilt-structure (Sb, Bi)2Te3 NW array in a schematic diagram.

pathway is d/sina along x direction (in-plane direction) in each tilted NW, leading to lengthening the easy transport journey of carriers. Therefore, it is easily understood that the in-plane carrier mobility of NW arrays increases as the tilted angle of NW arrays decreases. At the same time, the preferential (0 1 5) and (0 0 1 5) planes have also provided a preferential way for carriers transport in the in-plane direction. The low thermal conductivity can be explained by the fact that the high densities of rough interfaces may produce thermal barriers in the (Sb, Bi)2Te3 NW arrays. The high carrier mobility and the excellent performance depend on the results of affect of these factors together. Taking advantage of optimization to structure and a new avenue to adjust the carrier and phonon transport, the enhanced TE performance has been achieved. It is believed that this direction of research will inspire a flurry of interest in exploring effective approaches to fabricate such novel tilt-structure TE materials using simple, scalable, and controllable synthesis processes.

[8]

[9]

[10] [11] [12]

[13] [14]

[15]

[16] [17]

[18]

4. Conclusions The uniquely tilted (Sb, Bi)2Te3 NW array has been fabricated by a simple thermal evaporation technique. The unusual (Sb, Bi)2Te3 NW array with a tilted angle of 45° exhibits the highest Seebeck coefficient of 255 lV K1 at 200 °C, the maximum power factor value of 5.33 mW/m K2 at 30 °C, the average power factor of 5.08 mW/m K2 between 30 and 200 °C, along with the high TE dimensionless figure-of-merit ZT = 1.72 at room temperature. The oriented (0 1 5) and (0 0 1 5) lattices and tilt-structure of NW array can provide the top-priority route for carriers transport, enhancing TE properties in the NW array with a tilted angle of 45°. With optimizing the NWs diameter, tilted angles, periodic rough interfaces, the ZT value is likely to rise even higher. It provides a new control over the structural configuration of materials with relevance to improvement of their properties.

[19] [20]

[21]

[22]

[23]

[24]

[25]

[26]

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 61474082), State Key Program of National Natural Science Foundation of China (No. 61534001), Science and Technology Planning Project of Zhejiang Province, China (No. 2016C37062), Science and Technology Achievement Award Project for the Universities of Tianjin, China.

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