Colloidal Synthesis of Te-Doped Bi Nanoparticles ... - ACS Publications

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Colloidal Synthesis of Te-Doped Bi Nanoparticles: Low-Temperature Charge Transport and Thermoelectric Properties Da Hwi Gu,† Seungki Jo,† Hyewon Jeong,† Hyeong Woo Ban,† Sung Hoon Park,† Seung Hwae Heo,† Fredrick Kim,† Jeong In Jang,‡ Ji Eun Lee,*,‡ and Jae Sung Son*,† †

School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ‡ Thermoelectric Conversion Research Center, Korea Electrotechnology Research Institute (KERI), Changwon 51543, Republic of Korea S Supporting Information *

ABSTRACT: Electronically doped nanoparticles formed by incorporation of impurities have been of great interest because of their controllable electrical properties. However, the development of a strategy for n-type or p-type doping on sub-10 nm-sized nanoparticles under the quantum confinement regime is very challenging using conventional processes, owing to the difficulty in synthesis. Herein, we report the colloidal chemical synthesis of sub-10 nm-sized tellurium (Te)-doped Bismuth (Bi) nanoparticles with precisely controlled Te content from 0 to 5% and systematically investigate their low-temperature charge transport and thermoelectric properties. Microstructural characterization of nanoparticles demonstrates that Te ions are successfully incorporated into Bi nanoparticles rather than remaining on the nanoparticle surfaces. Lowtemperature Hall measurement results of the hot-pressed Te-doped Bi-nanostructured materials, with grain sizes ranging from 30 to 60 nm, show that the charge transport properties are governed by the doping content and the related impurity and nanoscale grain boundary scatterings. Furthermore, the low-temperature thermoelectric properties reveal that the electrical conductivity and Seebeck coefficient expectedly change with the Te content, whereas the thermal conductivity is significantly reduced by Te doping because of phonon scattering at the sites arising from impurities and nanoscale grain boundaries. Accordingly, the 1% Tedoped Bi sample exhibits a higher figure-of-merit ZT by ∼10% than that of the undoped sample. The synthetic strategy demonstrated in this study offers the possibility of electronic doping of various quantum-confined nanoparticles for diverse applications. KEYWORDS: doped nanoparticles, bismuth, colloidal synthesis, charge carrier transport, thermoelectric properties

1. INTRODUCTION

Furthermore, its relatively low thermal conductivity, arising from the heavy mass of a Bi atom that efficiently scatters phonons, makes it a great candidate for low-temperature n-type thermoelectric materials.12 The thermoelectric efficiency of materials can be evaluated by the figure-of-merit, ZT = σS2T/κ, where σ is the electrical conductivity, S is the Seebeck coefficient, κ is the thermal conductivity, and T is the absolute temperature.13 Recently, it has been reported that the semimetal-to-semiconductor transition can occur for lowdimensional Bi nanostructures such as quantum wells or quantum wires arising from the quantum confinement effect.14 Because hole carriers in semimetals such as Bi contribute to compensate for the Seebeck coefficient of electrons, a great enhancement in ZT has been predicted in Bi quantum nanostructures under the quantum confinement regime because of this transition. For example, Dresselhaus et al.

Doping, the intentional introduction of impurities into a material, lies at the heart of materials science because it provides unique tools to control the electronic, optical, and magnetic properties of materials. For example, an impurity with one more or one less valence electron can donate one more electron or hole carrier to a host material, which then has available charge carriers to carry current. With the recent advances in nanomaterials,1−3 doping of low-dimensional nanomaterials has also attracted tremendous attention. Because the potential applicability of nanoscale electronic materials, such as semiconductors and semimetals under the quantum confinement regime, ultimately depends on tailoring the electronic properties by doping and tuning the sizes, the development of a nanoscale doping methodology is crucial for advanced electronic and energy applications.4−8 Bismuth (Bi) has been recognized as an important electronic material because of the extremely small effective mass of electron (m*e , m*e = 0.001m0), long electron mean-free-path at the L-point, and high electron mobility (35 000 cm2/V·s).9−11 © 2017 American Chemical Society

Received: March 28, 2017 Accepted: May 16, 2017 Published: May 16, 2017 19143

DOI: 10.1021/acsami.7b04404 ACS Appl. Mater. Interfaces 2017, 9, 19143−19151

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∼10%. This promising strategy for electronic doping in nanostructured electronic materials will provide an additional degree of control over the charge transport and thermoelectric properties in addition to dimensionality.

predicted that the ZT values of tellurium (Te)-doped Bi quantum wells or quantum wires can increase up to more than 13 by the increase in the density of states near the Fermi level.11,14−16 In addition to dimensionality, another important parameter for highly efficient n-type Bi thermoelectric materials is the electron concentration that can be controlled by doping because the electrical conductivity and the Seebeck coefficient are inversely proportional to the carrier concentration; therefore, the optimum doping content needs to be determined for the fabrication of highly efficient thermoelectric materials.17−19 However, despite the importance of doping on Bi nanostructures, studies on the properties of sub-10 nm-sized doped Bi nanostructures under the quantum confinement regime have rarely been reported because of difficulties in synthesis. Moreover, most studies have focused on the thermoelectric properties of Bi nanostructures with grain sizes larger than hundreds of nanometers20,21 or Bi nanowires with diameter >40 nm,10,11,22 which restrict the realization of theoretically predicted highly efficient low-temperature thermoelectric materials. Colloidal chemistry routes are widely utilized for the synthesis of various nanoparticles such as metals, semiconductors, and ceramics because of the ease in controlling their sizes and shapes, scale-up, and the ability for surface functionalization.1,23−25 Although there have been recent advances in the synthesis of doped nanoparticles, most reports have focused on the introduction of magnetic ions such as manganese (Mn) and cobalt (Co) into semiconductor quantum dots, whereas n-type or p-type electronic doping by the introduction of impurities has rarely been studied.4−8,26 In fact, nanoparticles with n-type or p-type dopants undergo selfpurification and remove the dopants out of atomic lattices or exhibit electrochemical instability to trap the charge carriers on the surfaces.4,27 The recently suggested concept of remote doping, where extra electrons are injected into undoped nanoparticles by molecular attachment on nanoparticles or by an electrochemical route, can provide an alternative to the electronic doping of nanoparticles.6,7,28,29 However, the doping content in remotely doped nanoparticles is not maintained sustainably and is sensitive to environmental changes.6,7 As an alternative route, Geyer et al. proposed the chemical deposition of a Cd layer on the surface of InAs nanocrystal to achieve doping.30 Here, we report the development of a process for the synthesis of sub-10 nm-sized Te-doped Bi nanoparticles, where the Te content was precisely tuned from 0 to 5%. Te ion is a well-known n-type dopant that can donate one electron into a Bi crystal.17,18 We demonstrated that Te ions were successfully incorporated into Bi nanoparticles, rather than remaining on the surfaces. These Te-doped Bi nanoparticles can provide a great model system for studying the low-temperature charge transport and thermoelectric properties of low-dimensional nanostructures with controlled doping content. Low-temperature Hall measurements on the hot-pressed Te-doped Binanostructured materials revealed that the charge transport properties were strongly dependent on the doping content and the related impurity and grain boundary scatterings, and the electrical conductivity and Seebeck coefficient predictably changed with the doping content. Furthermore, the thermal conductivity of the doped samples decreased significantly compared with that of the undoped samples because of the impurity and grain boundary scatterings of phonons, which led to an increase in ZT of the Te-doped Bi nanoparticles by

2. EXPERIMENTAL PROCEDURES 2.1. Materials. Bismuth neodecanoate (technical grade), 1dodecanethiol (DDT, ≥98%), 1-octadecene (90%), tri-n-octylphosphine (TOP, 90%), and ethanethiol (97%) were purchased from Aldrich Chemical Co. Te powder (99.999%) was purchased from 5N Plus. All materials were used without further purification. 2.2. Synthesis of Te-Doped Bi Nanoparticles. All synthetic processes for Te-doped Bi nanoparticles were based on conventional air-free techniques using a Schlenk line and a nitrogen-filled glovebox. Typically, 10 mmol of bismuth neodecanoate was dissolved in 50 mL of 1-octadecene and was heated to 393 K under vacuum for 2 h. After that, the reaction mixture was cooled to 353 K. At this temperature, 2.4 mL of DDT was added, and the reaction was continued for 5 min. After the addition of DDT, the colorless solution turned yellow, indicating the formation of a bismuth dodecanethiolate complex.31 For the synthesis of the Te precursor solution, the desired amount of Te powder, depending on the doping concentration, was dissolved into 10 mL of TOP. After vigorous stirring for 12 h, the color of this solution turned yellow. This Te solution in TOP was injected into the reaction mixture at 353 K, and it was immediately cooled down to 338 K. The yellow-colored reaction mixture turned black immediately after the injection of the Te solution in TOP, which indicated the formation of Bi nanoparticles. After aging the reaction mixture for 40 min at 338 K, the mixture was centrifuged with excess acetone. The precipitate was redispersed in 10 mL of hexane, followed by centrifugation with excess acetone. Typically, we obtained 1.5 g of powderlike Te-doped Bi nanoparticles in a single batch reaction (Figure S1). 2.3. Preparation of Pellets. For measuring the electrical properties of Te-doped Bi nanoparticles, we fabricated disk-shaped pellets by hot-pressing. Typically, a dodecanethiol-capped Te-doped Bi nanoparticle solution in hexane was mixed with ethanethiol (20% volume fraction of nanoparticle solution), followed by vigorous stirring for 1 h. After the ligand exchange, the nanoparticles were precipitated at the bottom of the flask because of the agglomeration due to the shorter interparticle distance. Approximately 0.8 g of the dried nanoparticle powder was loaded into a graphite mold (12.7 mm diameter) and was pressed at 393 K under a pressure of 600 psi in an N2-filled glovebox with a holding time of 10 min, producing a 1 mmthick disk-shaped pellet. 2.4. Materials Characterization. The microstructural characterization of the Te-doped Bi nanoparticles was conducted using a transmission electron microscope (JEM-2100, JEOL) operated at 200 kV. Elemental mapping images of Te-doped Bi nanoparticles were obtained using energy dispersive X-ray spectroscopy (EDS) by a JEM2100F, JEOL instrument. Hot-pressed pellets were characterized using a field emission scanning electron microscope (Nano-FESEM, FEI Nova-NanoSEM 230, FEI) operated at 10 kV. Elemental mapping images of hot-pressed pellets were obtained using EDS (analysis by using a Nova-NanoSEM 230, FEI). X-ray diffraction (XRD) patterns for Te-doped Bi nanoparticles were obtained using a high power XRD instrument (HPXRD, D/MAX2500V/PC, Rigaku) with a Cu Kα Xray source, which has a distinctive wavelength of 1.5418 Å, operating at 40 kV and 200 mA. To estimate the grain sizes of the hot-pressed Tedoped Bi-nanostructured materials, the Scherrer equation was used with the equipment full width at half maximum of 0.087° in 2θ, obtained using a Si bulk crystal reference. The molar ratio of Te in Tedoped Bi nanoparticles was characterized using inductively coupled plasma optical emission spectrometry (ICP-OES, 700-ES, Varian). The Fourier transform infrared (FT-IR) spectrum was obtained using an FT-IR spectrometer (Varian 670). 2.5. Low-Temperature Hall Effect and Thermoelectric Property Measurements. Carrier transport properties in the temperature range of 50−300 K were measured by Hall measurement using a physical property measurement system (PPMS, PPMS19144


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ACS Applied Materials & Interfaces EverCool-II, Quantum Design) in magnetic fields of ±1, 3, and 5 T, and the averaged values are displayed. Thermoelectric properties in the temperature range of 50−300 K were measured using a PPMS with a thermal transport option.

elemental mapping analysis of two separate regions for the 11 nm-sized nanoparticles and larger nanoparticles in 3% Tedoped Bi nanoparticles using EDS (Figure S2a,b). The elemental mapping image of 11 nm-sized nanoparticles indicates the presence of Bi with its content approaching 99%, whereas a significant Te content was observed in the larger nanostructures. The elemental mapping sum spectra of these two nanostructures exhibited 0.4 mol % of Te in the smaller one and 4.9 mol % in the larger one (Table S1). These results indicate that the larger nanostructures are heavily doped, which were mixed with the less doped Bi nanoparticles. The XRD patterns (Figure 1e) of all nanoparticles, regardless of the Te content, correspond to that of bulk rhombohedral Bi crystals (JCPDS 85-1331), whereas the peaks become slightly sharper with increasing Te content because of an increase in the concentration of larger nanoparticles. In addition, the peaks related to the secondary phases such as Bi-rich BiTe or Te were not observed. This indicates that Te ions are atomically doped in the Bi nanoparticles, rather than forming secondary structures. More importantly, the diffraction peaks in the XRD patterns (Figure 1f,g) progressively shifted to higher 2θ angles with increasing Te content, demonstrating the integration of Te ions inside of an atomic lattice in the Bi nanoparticles. To study the charge transport and thermoelectric behaviors of Te-doped Bi nanoparticles, we prepared disk-shaped pellets by pressing nanoparticle powders at 393 K under an inert atmosphere (Figure S3). The hot-pressing even under mild conditions (T ≈ 393 K) was found to be advantageous to significantly improve the electrical properties of Te-doped Binanostructured materials. As shown in Figure S4, the electrical conductivity of the hot-pressed samples is an order of magnitude higher than that obtained from the cold-pressed samples, whereas their Seebeck coefficients give similar values. To further improve the electrical properties, long-hydrocarbon-chain dodecanethiol ligand-capped Te-doped Bi nanoparticles were exchanged with short-chain ethanethiol before the hot-pressing process. These short-chain alkylthiol ligands allow for efficient electronic communication and effective contact among the Bi nanograins, significantly improving the electrical properties. Son et al. reported that the electrical conductivity of the pressed Bi nanoparticles increased upon decreasing the chain length of the alkylthiol capping ligands.12 Furthermore, the organic ethanethiol to the detriment of electrical properties can be further evaporated during the drying process under vacuum because of its low boiling temperature (308 K) that is close to room temperature. The FT-IR absorption spectra of both dried and heat-treated Te-doped Bi nanoparticles at 393 K (Figure S5) show no peaks related to the bands corresponding to the S−H and C−H stretch modes of ethanethiol. The structural characteristics of the prepared samples were characterized using scanning electron microscopy (SEM) and XRD. The SEM images of the hot-pressed Te-doped Binanostructured materials (Figure 2a−d) show that the size of the grains ranges from several tens to several hundreds of nanometers, which is indicative of the grain growth of Tedoped Bi nanoparticles during the pressing process. Interestingly, the grain sizes apparently decrease with increasing Te content. Because the impurity ions can sometimes either promote or disturb the grain growth by adjusting the atomic diffusion rate, this phenomenon can be attributed to the effect of Te ions on the diffusion of Bi atoms.33 The XRD patterns of

3. RESULTS AND DISCUSSION The synthesis of Te-doped Bi nanoparticles was carried out by modifying a method used for the synthesis of Bi nanoparticles reported by Son et al.12 In this reaction, a Bi−thiolate complex was reduced to form Bi nanoparticles with the mild reducing agent TOP. In addition, TOP can act as an efficient solvent for dissolving Te and can form a TOP−Te complex, which is a widely used Te precursor for the synthesis of metal telluride nanoparticles.18,32 Accordingly, we utilized the TOP−Te solution as the Te precursor and the reducing agent for the synthesis of Te-doped Bi nanoparticles. The Te content in the TOP−Te solution was controllably varied from 0 to 5% in molar ratio to the Bi precursor. The Te-doping content in the synthesized Bi nanoparticles, confirmed using the ICP-OES analysis (Table 1), was slightly higher than the initial content of Table 1. Comparison of Te Precursor Concentration for the Synthesis of Te-Doped Bi Nanoparticles and the Measured Te-Doping Concentration by ICP-OES Analysis Te molar ratio Te precursor concentration (mol %)

Te concentration obtained by ICP-OES analysis (mol %)

1% 3% 5%

1.77% 4.25% 6.05%

the Te precursor. This can be attributed to the difference in the reactivity of the Bi and Te precursors. To confirm the Te doping inside of the lattice of the Bi nanoparticles, we performed the surface treatment of the 5% Te-doped Bi nanoparticles with excess TOP, which effectively binds to the Te ions placed on the surface of Bi nanoparticles. The treated and purified Te-doped Bi nanoparticles exhibited the same Te content as the untreated nanoparticles, which demonstrated the doping of Te ions into the atomic lattices in the Bi nanoparticles (Table 2). Table 2. Te Molar Ratio to Bi Nanoparticles before and after TOP Treatment Te molar ratio before TOP treatment

after TOP treatment

6.05%

5.92%

The structural characteristics of Te-doped Bi nanoparticles were investigated using transmission electron microscopy (TEM) and XRD analysis. The TEM image of undoped Bi nanoparticles (Figure 1a) shows uniformly sized spherical nanostructures with an average size of 11 nm, which agrees with the result reported in a previous study by Son et al.12 Interestingly, we found larger nanostructures with a size of 10− 30 nm mixed with the spherical 11 nm-sized structures, as shown in the TEM images of the Te-doped Bi nanoparticles (Figure 1b−d). The concentration of the former structures increased with increasing Te content, which indicates that there is a relationship between the large nanostructures and the Tedoping content. To identify these structures, we conducted an 19145


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Figure 1. TEM images of Te-doped Bi nanoparticles with various Te concentrations of (a) 0%, (b) 1%, (c) 3%, and (d) 5%. (e) Full-scale XRD patterns and (f) enlarged patterns of Te-doped Bi nanoparticles with various Te contents. (g) Plot for the (012) peak shift of Te-doped Bi nanoparticles in the XRD patterns.

On the other hand, Te-doped Bi samples exhibited almost 1 order of magnitude higher electron concentrations of 1.4 × 1020 cm−3 for 1%, 1.7 × 1020 cm−3 for 3%, and 1.9 × 1020 cm−3 for 5% Te-doped samples. In addition, the temperature dependence of the electron concentrations of Te-doped samples was much weaker than that of the undoped sample. This phenomenon can be attributed to the occupancy of thermally activated charge carriers in the total carrier concentration. The doped samples should have much more extra electrons generated from the shallow doping states near the conduction band edge, which should not be strongly temperature dependent. On the other hand, the number of electrons in the undoped sample can be governed by the thermal activation of electrons across the entire bandgap. These results show the great possibility of precise control of the carrier concentration of nanostructured materials by the chemical incorporation of dopants inside of the nanoparticles. The undoped Bi-nanostructured materials exhibited a mobility as high as 610 cm2/V·s at room temperature (Figure 3b). Furthermore, the mobility dramatically increased with decreasing temperature and reached 1400 cm2/V·s at 50 K. On the other hand, the electron mobilities of Te-doped Binanostructured materials were much lower than those of the undoped samples, that is, between 120 and 240 cm2/V·s. As Te-doping content increased, the mobility decreased over the entire temperature range and showed a much weaker temperature dependence. These results suggest that the charge transport properties of Te-doped Bi samples are primarily dominated by the Te-doping content. The effects of Te doping on the temperature-dependent electrical behaviors can be summarized by two factors in the microstructures: (1) the increase in the impurity concentration arising from Te doping in Bi-nanostructured materials and (2) as aforementioned, the

all samples (Figure 2e) were indexed to Bi rhombohedral structures, whereas the diffraction peaks broaden with increasing Te content. The grain sizes of the Te-doped Bi pellets, evaluated from the (012) peaks in the XRD patterns of the samples using the Scherrer equation (Figure 2f), ranged from 30 to 60 nm and decreased with increasing Te content, which was in agreement with the results obtained using the SEM analysis. In addition, the peaks related to the impurity or the secondary phase were not observed. This suggests that Te ions were integrated well into the Bi lattices, rather than forming a secondary phase. To further investigate the homogeneous doping of Te, the hot-pressed Te-doped Binanostructured materials were characterized using SEM−EDS. The EDS mapping results (Figure S6a−d and Table S2) show that Te was detected on the entire surface of the specimen, but the strong signals coming from the Te phase were not observed. The weak signals of Te on the undoped sample and the slightly higher Te content in the doped samples are attributed to artifacts, considering the composition of the materials. These results demonstrate homogeneous doping of Te ions in Bi-nanostructured materials. To study the charge transport properties of the hot-pressed Te-doped Bi-nanostructured materials at low temperatures, we measured the carrier concentration (n) and mobility (μ) by Hall measurement because doping of impurities to provide extra charge carriers in semiconductors or semimetals critically adjusts the parameters that depend on temperature. The Hall coefficient (RH) (Figure S7) exhibited negative values over the entire temperature range from 50 to 300 K, indicating the ntype electrical behavior of the hot-pressed Te-doped Binanostructured materials. The electron concentration of an undoped Bi sample was 3.3 × 1019 cm−3 at room temperature and swiftly decreased with decreasing temperature (Figure 3a). 19146


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Figure 3. Temperature dependences of (a) electron concentration and (b) mobility of Te-doped Bi pellets with various Te contents of 0% (black square), 1% (red circle), 3% (blue upward triangle), and 5% (green downward triangle).

factor. In this rule, the total scattering is the sum of each contribution from different electron-scattering processes. Typically, the electron−phonon (lattice) scattering, which is generally observed in intrinsic semiconductors, strongly depends on the temperatures. However, the impurity- and defect-scattering processes are almost temperature independent.34−37 In the current study, the mobility of the undoped sample shows a strong dependence on temperature, which indicates that the electron−phonon scattering is the dominantscattering process in its charge transport, being further supported by the low electron concentration. On the other hand, the Te-doped samples show an almost constant mobility regardless of the temperature. Because the Te-doped samples have much higher impurity concentrations and smaller nanoscale grain sizes, their temperature-dependent charge transport can be understood by the impurity and grain boundary scatterings of the charge carriers. The much lower mobility of the Te-doped samples than the undoped sample should be attributed to these scattering processes. In the current study, the Te-doping content in Bi nanoparticles was precisely controlled by a chemical approach, and it also provides an additional degree of control over the grain sizes on the nanoscale. This promising chemical approach may provide an effective route for realizing ideal Bi-nanostructured thermoelectric materials with optimized doping content and grain sizes. In this regard, we measured the thermoelectric properties of the hot-pressed Te-doped Binanostructured materials with 0, 1, 3, and 5% Te-doping contents at temperatures ranging from 50 to 300 K using the PPMS. The electrical conductivity of the undoped sample was 3.2 × 105 S/m at room temperature and decreased with decreasing

Figure 2. SEM images of Te-doped Bi pellets prepared from Te-doped Bi nanoparticles with various Te contents of (a) 0%, (b) 1%, (c) 3%, and (d) 5%. (e) XRD patterns of Te-doped Bi pellets prepared from Te-doped Bi nanoparticles with various Te contents of 0, 1, 3, and 5%. (f) Mean grain sizes of the pellets estimated from the (012) peak in the XRD patterns using the Scherrer equation.

decrease in the grain sizes of Te-doped samples with increasing Te-doping content. These complex microstructural characteristics affecting the charge transport properties can be explained by the carrier-scattering mechanism. According to Matthiessen’s rule, the contribution of carrier-scattering factors to the total mobility can be qualitatively estimated by the following equation.34,35 μT−1 = μI−1 + μL−1 + μD−1 + ...

where μT is the total carrier mobility, μI is the impurityscattering factor, μL is the lattice-scattering factor, and μD is the defect including the grain boundary or the alloy-scattering 19147


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the relationship that the Seebeck coefficient is inversely proportional to the carrier concentration, the samples show a decrease, from −56.2 μV/K for the undoped sample to −29.2 μV/K for the 5% Te-doped sample, with increasing Te-doping content. In addition, considering that the scattering probability of charge carriers at the sites of ionized impurity is reciprocally proportional to the energy of the carriers, the low energy charge carriers can be more scattered than the high energy ones.38 Because the low energy charge carriers are known to contribute to the Seebeck coefficient negatively, the ionized impurity scattering can enhance the Seebeck coefficient by the energy filtering effect. In the current samples, although the Seebeck coefficients decreased with increasing content of Te doping, the intensified ionized impurity scattering might somewhat compensate the reduction in the Seebeck coefficient. The power factors of all samples increased with increasing temperatures because of the temperature dependence of the Seebeck coefficient (Figure 4c). The 1% Te-doped sample exhibits the highest value of 13.2 μW/cm·K2 at room temperature because of the highest electrical conductivity and a moderately high Seebeck coefficient. The thermal conductivity (κ) of the undoped sample was 7.32 W/m·K at room temperature and increases with decreasing temperatures (Figure 5a). On the other hand, the 1% Te-doped sample exhibits the highest thermal conductivity of 8.88 W/m·K at room temperature, and it decreases with decreasing temperatures. The 3 and 5% Te-doped samples show thermal conductivities of 7.37 and 5.23 W/m·K, respectively, and show positive temperature dependences. The lattice thermal conductivity (κL) of the samples was evaluated based on the equation κ = κE + κL, where κE is the electronic contribution to total thermal conductivity, calculated using the Wiedemann−Franz law κE/σ = LT (Figure 5b). We used the Lorenz number (L) 2.4 × 10−8 V2/K2 for degenerate semiconductors.13,39 The lattice thermal conductivity of the undoped sample is 5.03 W/m·K at room temperature, and it increases with decreasing temperatures; whereas the thermal conductivities of the Te-doped samples are lower than that of the undoped sample over the entire temperature range and exhibit a weaker temperature dependence. The lattice thermal conductivity decreases with increasing Te-doping content. In general, the phonon scattering for determining the thermal conductivity can be understood using several mechanisms such as the Umklapp phonon−phonon scattering, phonon−impurity scattering, and phonon−boundary scattering.34−36 The decreased thermal conductivity with increasing Te-doping content suggests that the phonon transport properties of these materials are dominated by Te impurity scattering and grain boundary scattering, which is independent of temperature. On the other hand, the Umklapp scattering is strongly dependent on temperature, which is responsible for the strong temperature dependence of the thermal conductivity of the undoped sample. Furthermore, the decrease in the thermal conductivity of the Te-doped samples can be attributed to the higher concentration of impurities and smaller grains available for phonon scattering. These results demonstrate that the Te doping on Bi nanoparticles can be advantageous for reducing the thermal conductivity by phonon scattering at impurities and grain boundaries. The temperature-dependent ZT values of all samples were calculated from the measured electrical conductivities, Seebeck coefficients, and thermal conductivities (Figure 5c). All samples exhibit the highest ZT values at room temperature, and the

temperature (Figure 4a). This positive temperature dependence can be observed in the electrical properties of an intrinsic

Figure 4. Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient, and (c) power factor of Te-doped Bi pellets with various Te contents of 0% (black square), 1% (red circle), 3% (blue upward triangle), and 5% (green downward triangle).

semiconductor. This result was in agreement with the temperature dependence of electron concentration because the electrical conductivity (σ) of the materials can be determined by the equation σ = neμ for unipolar transport, where e is the electron charge. On the other hand, 1 and 3% Te-doped samples exhibited the highest electrical conductivity of 5.1 × 105 S/m at room temperature, which was around 2 times higher than that of the undoped sample and comparable to the value of bulk Bi (6.7 × 105 S/m).35 In addition, all Tedoped samples showed almost constant or negative temperature dependences. This phenomenon can be understood by considering that metals or degenerate semiconductors exhibit scattering dominant electrical properties rather than the number of charge carriers generated by thermal energy. The Seebeck coefficients for all samples have negative values, indicating n-type characteristics (Figure 4b). As predicted by 19148


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characterization of the synthesized Te-doped Bi nanoparticles clearly demonstrated the doping of Te ions inside of the Bi atomic lattices rather than their attachment on their surfaces. The homogeneously doped Te ions in the hot-pressed samples were confirmed using an elemental analysis. In addition, we found that the Te ion impurity significantly disturbed the grain growth of Bi, so that the grain sizes were controllable on the nanoscale by the Te-doping content. The low-temperature charge transport properties of the hot-pressed Te-doped Binanostructured materials were somewhat different from those of the undoped sample, which exhibited typical semiconducting properties, and the developed doped materials were arguably heavily doped degenerate semiconductors. We found that the properties of the doped samples were governed by the impurity and grain boundary scatterings of the charge carriers, as demonstrated by the weak temperature dependences of the carrier mobility and concentration. These interesting properties of Te-doped Bi-nanostructured materials were directly reflected in the thermoelectric properties, where the electrical conductivity increased and the Seebeck coefficient decreased with increasing Te content. The thermal conductivity decreased with increasing Te content because of phonon scattering at the sites of impurities and grain boundaries. The 1% Te-doped sample exhibited a higher ZT value than the undoped sample over a wide temperature range of 170−300 K. These low-temperature charge transport and thermoelectric properties of Te-doped Bi nanostructures will extend our understanding of the fundamental charge transport and thermoelectric properties in nanostructured electronic materials. Furthermore, the currently developed synthetic route for Te-doped Bi nanoparticles shows a new method for the precise control of extra charge carriers in nanostructures without performing electrochemical reactions on the surfaces. We believe that this methodology will be widely applicable for the production of doped semiconductor nanoparticles and nanostructured thermoelectric materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04404. Photograph of 1.57 g of Te-doped Bi nanoparticles synthesized in a single batch, TEM and EDS mapping images, Te molar ratio to 11 nm-sized smaller and larger Bi nanoparticles estimated from EDS mapping sum spectrum, photograph of Te-doped Bi pellets prepared from ∼0.8 g of nanoparticle powder, electrical conductivity and Seebeck coefficient of the cold-pressed and hot-pressed Te-doped Bi samples with various Te-doping contents, FT-IR spectra of dried (room temperature) and heat-treated (393 K) Bi nanoparticles, SEM images of Te-doped Bi pellets and their EDS map of Bi, Te molar ratio in hot-pressed Te-doped Bi-nanostructured materials estimated from EDS, and temperature dependence of Hall coefficients of Te-doped Bi pellets with various Te-dopant concentrations (PDF)

Figure 5. Temperature dependence of (a) thermal conductivity, (b) lattice thermal conductivity, and (c) ZT of Te-doped Bi pellets with various Te contents of 0% (black square), 1% (red circle), 3% (blue upward triangle), and 5% (green downward triangle).

values decrease with decreasing temperatures. The highest value of 0.045 was achieved by the 1% Te-doped sample, which was ∼10% higher than that of the undoped sample. In addition, the 1% doped Te sample exhibited higher ZT values than the undoped sample over a wide range of temperatures from 170 to 300 K. Although the absolute value of ZT in the current study is not so high as to compete with those of traditional bulk Bi (ZT = 0.16, 300 K) and recently developed nanostructured materials,15,40−43 the currently developed promising chemical route will offer an additional degree of control over the doping content in nanostructured thermoelectric materials with a more optimized and enhanced efficiency.

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AUTHOR INFORMATION

Corresponding Authors

4. CONCLUSIONS In summary, we successfully developed a method for the synthesis of Te-doped Bi nanoparticles and systematically investigated the charge transport and thermoelectric properties of the hot-pressed samples at low temperatures. The structural

*E-mail: [email protected] (J.E.L.). *E-mail: [email protected] (J.S.S.). ORCID

Jae Sung Son: 0000-0003-3498-9761 19149


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Center for Advanced Meta-Materials (CAMM) funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (NRF2014M3A6B3063704) in Republic of Korea, and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education [2015R1C1A1A01053599 (J.S.S.), 2014R1A1A3053206 (J.E.L.)].

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