Composition Modulation of Ag2Te Nanowires for Tunable Electrical

Aug 26, 2014 - ... Nanoparticle Composites with Tunable Melting Temperature and High Thermal Conductivity for Phase-Change Thermal Storage. Minglu Liu...
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Letter pubs.acs.org/NanoLett

Composition Modulation of Ag2Te Nanowires for Tunable Electrical and Thermal Properties Haoran Yang,† Je-Hyeong Bahk,‡ Tristan Day,§ Amr M. S. Mohammed,‡,∥ Bokki Min,† G. Jeffrey Snyder,§ Ali Shakouri,‡,∥ and Yue Wu*,†,⊥ †

School of Chemical Engineering and ‡Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States § Materials Science, California Institute of Technology, Pasadena, California 91125, United States ∥ School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, United States ⊥ Department of Chemical and Biological Engineering, Iowa State University, Ames, Iowa 50011, United States S Supporting Information *

ABSTRACT: In this article, we demonstrated that composition modulation of Ag2Te nanowires can be achieved during the self-templated transformation of Te nanowires into Ag2Te nanowires during solution phase synthesis, which provides a mean to tune the carrier density of the Ag2Te nanowires. Both nearly stoichiometric and Ag-rich nanowires have been synthesized, which give rise to ptype and n-type Ag2Te nanocomposites after hot press, respectively. The electrical and thermal properties of the two kinds of samples have been measured. Theoretical modeling based on the near-equilibrium Boltzmann transport equations has been used to understand the experimental results. We found that ZT of the heavily doped n-type sample reaches 0.55 at 400 K, which is the highest ZT value reported for Ag2Te at the same temperature mainly due to the reduced thermal conductivity by the nanostructures. Theoretical analysis on the carrier transport shows that the power factor is also very well optimized in the doped Ag2Te sample considering the reduced carrier mobility by the nanostructures. KEYWORDS: Thermoelectric, nanowires, silver telluride, Ag2Te, nanocomposites

T

boundaries.7−9 Nanocomposites are usually fabricated via consolidation of nanosized building blocks obtained by either top-down (ball milling) or bottom-up (chemical synthesis) approaches, and the successful examples include ball-milled BiSbTe alloys (ZT = 1.4 at 100 °C),10 SiGe alloy (ZT = 0.95 at 900−950 °C),11 and chemically synthesized Bi0.5Sb1.5Te3 nanoplates (ZT = 1.16 at room temperature).12 In a typical top-down route, carrier density can be easily controlled through doping during the melt metallurgy step prior to ball milling. On the contrary, it is more difficult to dope nanomaterials to a desired level in chemical synthesis. In the field of solution phase synthesis-based thermoelectric nanomaterials, the reports on intentional tuning of carrier density are still limited. Silver telluride (Ag2Te) is a narrow-band gap semiconductor (Eg ∼ 0.05 eV at room temperature), which possesses many unique properties to make itself as a promising TE materials at low temperature: (1) low thermal conductivity associated with the disordered structure of the Ag atoms in the lattice of Ag2Te due to the high mobility of Ag atoms;13,14 (2) high power factor due to high electron mobility, which is also observed in other silver chalcogenide;13,15,16 (3) tunable carrier density modu-

hermoelectric (TE) power generation has been considered a viable solution to improve energy efficiency of a heat engine through converting waste heat into electricity. However, for a TE generator to become competitive with conventional energy conversion systems, materials with high performance are needed, which is evaluated by a dimensionless quantity called the thermoelectric figure of merit or ZT = σS2T/κ, where σ, κ, S, and T stand for electrical conductivity, thermal conductivity, Seebeck coefficient, and absolute temperature, respectively. T is the average temperature between the hot and the cold sides.1,2 A high ZT requires simultaneous presence of a large Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity in the material, which has been a great challenge to achieve for a long time due to the fundamental intercorrelation between these parameters.3 Recently, enhanced ZT values have been achieved in many nanostructured materials, among which nanograined materials represent the simplest design and are sometimes referred as nanocomposites for short. Utilizing nanoscale grain boundaries to introduce effective phonon scattering, the nanocomposites usually exhibit significantly reduced thermal conductivity, which is the main reason for the ZT enhancement.3−6 In some cases, nanocomposites were also reported to exhibit the so-called energy filtering effect that enhances the power factor (σS2) though the filtering of hot-energy carriers at the grain © 2014 American Chemical Society

Received: July 7, 2014 Revised: August 12, 2014 Published: August 26, 2014 5398

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Figure 1. Synthesis and characterizations of Te and Ag2Te nanowires. (a) Scheme of the two step synthesis of Ag2Te nanowires; (b) XRD, (c−e) TEM images, (f−h) HRTEM images with FFT inserted of Te (c,f), Ag2Te_2xAg (d,g), and Ag2Te_3xAg (e,h) nanowires.

temperature and is mainly due to preserved nanostructures and optimized carrier density. Results and Discussion. Structural Characterizations. The two-step synthesis approach is schematically shown in Figure 1a in which Te nanowires are first synthesized and then converted into Ag2Te through self-templated chemical transformation. A similar strategy to synthesize Ag2Te nanowires was reported by Shu-Hong Yu group, but their synthesis is based on hydrothermal reactions27−29 whereas ours is done under atmosphere pressure. The synthesis of Te nanowires is based on our previous reports,30−32 and conversion of Te into Ag2Te is based on the scaling-up the method published elsewhere.33 It is found that to convert Te nanowires into Ag2Te, twice of the stoichiometric amount of AgNO3 is needed in order to avoid the formation of Ag-deficient phases of silver tellurides such as Ag7Te4 or Ag5Te3, and near stoichiometric Ag2Te nanowires can be obtained, which are denoted as Ag2Te_2xAg throughout this paper. To produce Ag-rich Ag2Te nanowires, three times of stoichiometric amount of AgNO3 is added to the same amount of Te nanowires and the final sample is denoted as Ag2Te_3xAg. The Te nanowires and the Ag2Te nanowires synthesized in each step have been characterized with X-ray diffraction (XRD) and transmission electron microscopy (TEM). Figure 1b shows

lated by nonstoichiometric compositions of Ag2Te in which the defects including completely ionized silver atoms at interstitial sites and vacancies can create donor levels and acceptor levels, respectively;17 (4) a phase transition at low temperature from monoclinic phase to cubic phase at around 418 K.18 The TE properties of bulk Ag2Te has been investigated by several groups and the maximum ZT has reached 0.6 at 700 K.13,14,19−21 Through alloying with PbTe or Ag2Se, ZT can be even promoted to 1 or 1.2, respectively.13,22 A few attempts on Ag2Te nanocrystals or nanowires have also been reported and the ZT has reached 0.66 at 450 K.23−26 Here, we show a low-cost and scalable solution-phase synthesis of Ag2Te nanowires by self-templated transformation of Te nanowires. We also demonstrate that both nearly stoichiometric and Agrich Ag2Te nanowires can be obtained by tuning the Ag/Te ratio during the synthesis. Through composition modulation, the carrier density of the Ag2Te nanowires can be tuned from near intrinsic to heavily n-type doped. Unlike the previous reports that mainly studied the high-temperature cubic phase Ag2Te, we focus on the low-temperature monoclinic phase from 300 to 400 K below the phase transition temperature and show that enhanced ZT of 0.55 at 400 K is achieved, which is the highest ZT value reported for Ag2Te at the same 5399

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(Figure 1g) and Ag2Te_3xAg (Figure 1h) samples can be easily indexed as monoclinic Ag2Te. Ag2Te nanowires become polycrystalline and various growth directions are observed (data not shown), which corresponds to the curved features of these nanowires. The formation of polycrystalline Ag2Te nanowires indicates that heterogeneous nucleation along the Te nanowires takes place during transformation, that is, nucleation and growth of Ag2Te happens at multiple spots on each of the individual Te nanowires simultaneously. The comparison between the 3xAg sample and the 2xAg sample reveals that the grain size of the 3xAg sample is much larger than that of the 2xAg sample, which results from the ripening process and is consistent with the low-magnification TEM study. Note that Figure 1h only shows a part of a nanowire because the diameter of the nanowire is larger than 40 nm. Thermoelectric Properties of the Bulk Nanocomposites. After removing the capping surfactants, the dry powder of Ag2Te nanowires with different compositions (Ag2Te_2xAg and Ag2Te_3xAg) are hot pressed into 18 mm × 2 mm diskshape pellets, respectively. The densities and average compositions determined by energy dispersive X-ray spectroscopy of the samples are shown in Table 1. The two samples have similar relative densities around 89%. The 2xAg sample shows a composition close to the stoichiometry, while the 3xAg sample shows 1.8% excess Ag.

the XRD patterns of the Te nanowires (top), Ag2Te_2xAg (middle) ,and Ag2Te_3xAg nanowires (bottom), which are compared to the standard powder diffraction files (PDF). All the peaks from Te nanowires match well to the hexagonal phase of Te (JCPDS 36-1452). Notably, the XRD pattern shows an enhancement of the reflection of (100), (110), (200), and (210) planes, which can be attributed to substrate-induced orientation of the Te nanowires. Similar phenomena have been reported in many other nanowires.34−36 After conversion of Te to Ag2Te, both of the 2xAg and 3xAg samples obtained are characterized as monoclinic phase of Ag2Te (β-Ag2Te, JCPDS 34-0142) without any noticeable impurities peaks from Te, Ag7Te4, or Ag5Te3, which indicates the transformation from Te to Ag2Te has proceeded to completion. In comparison, if only stoichiometric amount Ag precursor is added in the synthesis, Te-deficient phases will form, as shown in Supporting Information Figure S1. Because of the finite sizes of the nanowires, both the Te nanowires and the Ag2Te_2xAg nanowires show significant peak broadening. In comparison, the 3xAg sample shows sharper peaks, which suggests that the average size of the nanowires of the 3xAg sample could be larger than that of the 2xAg sample. The shape and size of the Te and Ag2Te nanowires have been analyzed with TEM. Figure 1c−e are the TEM images of Te, Ag2Te_2xAg, and Ag2Te_3xAg nanowires, respectively. The Te nanowires are very uniform in size with an average length of 1028 ± 38 nm and an average diameter of 28 ± 2 nm. After transformation, the Ag2Te_2xAg sample (Figure 1d) showed curved nanowire shape, which is similar to several previously reported Ag2Te nanowires grown by various methods,23,37−41 and the average length and diameter becomes 1154 ± 253 and 32 ± 3 nm, respectively. The larger distribution in length could be explained by two facts. First, the Ag2Te nanowires are curved which could introduce significant measurement error. Second, very short nanowires have been observed, which are the broken pieces from the parent Te nanowires during transformation. The growth in length and thickness of the Ag2Te nanowires is attributed to the 98% volume expansion from hexagonal Te to monoclinic Ag2Te, which is well studied previously.33 The large volume expansion also introduces significant mechanical stress during transformation, which is released by the breaking and curving of the wires. The Ag2Te_3xAg sample (Figure 1e) showed significant ripening and welding of the nanowires, the diameters of the nanowires fall in the range of 30 to 160 nm, and some branched structures could also observed. These features imply that the reaction kinetics is significantly changed when overdosed AgNO3 is added, and the exact reaction mechanism is subjected to future study. High-resolution TEM (HRTEM) study has been performed to study the growth directions of the Te and Ag2Te nanowires. Figure 1f displays HRTEM image of a typical Te nanowire and the inset shows its fast Fourier transform (FFT) image. The FFT pattern can be indexed as hexagonal Te, which is consistent with XRD study. The Te nanowires are single crystalline and are grown along the [001] direction, which further explains the enhancement of certain peaks in XRD. Because the nanowires lie preferentially in parallel to the substrate, these planes in parallel with the nanowire growth direction, such as (100), (110), (200), and (210), are therefore preferentially aligned in parallel to the substrate. Consequently, the reflection of these planes is enhanced. After Te to Ag2Te conversion, both of the HRTEM images of the Ag2Te_2xAg

Table 1. Densities and Compositions of the Hot Pressed Ag2Te nanocomposites 2xAg 3xAg

density (g/cm3)

relative density

Ag atomic %

Te atomic %

7.38 7.42

88.8% 89.2%

66.7 ± 0.3 67.1 ± 0.3

33.3 ± 0.3 32.9 ± 0.3

To confirm no phase change during the hot press, the hotpressed Ag2Te pellets have been characterized with XRD again. The XRD patterns (Figure 2a) show no impurity peaks, which confirms that hot press has not introduced any noticeable phase change. Each of the hot-pressed samples shows similar degree of peak broadening in XRD patterns compared with their raw materials, which implies grain growth during the hot press is insignificant. Figure 2b,c is typical SEM images on the fresh fracture cross sections of the hot-pressed Ag2Te_2xAg and Ag2Te_3xAg pellets showing nanosized grains. For both of the samples, the SEM images show that the nanowires of the raw materials are fuzzed together yet the polycrystalline nature of the nanowires lead to the isotropic disk composed of nanorods or nanoparticles broken off from the nanowires after hot press. The grain sizes of each sample are close to the diameters of the nanowires as the raw materials, which is consistent with the XRD study. HRTEM images showing the lattice fringes further confirm the nanocrystalline structure of our samples. As shown in Figure 2d, the Ag2Te_2xAg sample consists of randomly oriented crystalline domains with sizes around tens of nanometers, and all the domains can be indexed as monoclinic Ag2Te. Figure 2e,f shows two kinds of crystalline features of the Ag2Te_3xAg sample. Single crystalline regions larger than 40 nm by 40 nm generally dominate in the Ag2Te_3xAg sample, which is consistent with the thick nanowires observed in the initial synthesis. Meanwhile, polycrystalline regions with grains around tens of nanometers are also observed, which might come from the small branches grown on the nanowires. The microstructure analysis suggests that the hot-pressed disks are 5400

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Figure 2. (a) XRD of the hot pressed Ag2Te nanocomposites; (b,c) cross-section SEM and (d−f) cross-section HRTEM of the hot pressed Ag2Te_2xAg (b,d) and Ag2Te_3xAg (c,e,f).

phase-pure nanocrystalline Ag2Te, which gives rise to the very low thermal conductivity for these samples. The electrical and thermal transport properties have been measured from 300 to 380 K for the Ag2Te_2xAg sample and from 300 to 400 K for the Ag2Te_3xAg sample. These two samples are found stable within these temperature ranges, and the results are presented along with their theoretical fitting curves in Figure 3, which exhibit completely different properties. As shown in Figure 3a, the electrical conductivity (σ) of the 2xAg sample increases from 3758 S/m to 6884 S/m as temperature rises from 300 to 380 K. After careful fitting of the experimental data, we find that 2xAg sample has higher hole density than the electron density over the entire temperature range. The hole concentration in 2xAg sample is found to be increasing with temperature from 8 × 1017 cm−3 at 300 K to 3.1 × 1018 cm−3 at 380 K, whereas the electron density is lower than the hole density but still steadily increasing with temperature due to the increased thermal excitation over the band gap from ∼1 × 1017 cm−3 at 300 K to 2 × 1017 cm−3 at 380 K. We suspect that the unintentional p-type doping is mainly due to the Ag vacancies in the matrix by accumulation of Ag atoms at the grain boundaries and the redistribution of Ag atoms with increasing temperature can be responsible for further increase of Ag vacancies, which leads to the rapid

increase of hole density at higher temperature, although this hypothesis will require future HRTEM and elemental mapping to confirm. This p-type doping in 2xAg sample is confirmed by the positive values of Seebeck coefficients for this sample as shown in Figure 3b. The Seebeck coefficient in two-type carrier transport is given by42 S=

Seσe + S hσh S nb + S hp = e σe + σh nb + p

(1)

where n and p are, respectively, electron and hole densities, the subscripts e and h denote the partial properties of electrons and holes, respectively, and b is the mobility ratio, that is, b = μe/μh. The partial Seebeck coefficients of electrons and holes have opposite signs, so that the magnitude of total Seebeck coefficient becomes smaller than those of partial values according to eq 1. Although the hole density is quite larger than the electron density at 300 K in 2xAg sample, the contribution of electrons to the total Seebeck coefficient is significant due to higher mobility of electrons than that of holes (μe = 960 cm2 V−1 s−1, and μh = 180 cm2 V−1 s−1 for sample 2xAg at 300 K. b = 5−8 over the entire temperature range). The contribution of electrons to total Seebeck coefficient is decreased as temperature increases, which results in increased 5401

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Figure 3. Experimental and theoretical fitting of thermoelectric properties of Ag2Te nanocomposites. (a) Electrical conductivity, (b) Seebeck coefficient, (c) Hall coefficient, (d) thermal conductivity, (e) power factor, and (f) ZT.

measurements. Also, the uncertainties about the valence band structure can impose errors in the modeling of bipolar transport in the material. On the other hand, the 3xAg sample shows one-type carrier transport behavior in the transport properties as the sample is highly doped with excess Ag atoms. In the case of n ≫ p, eq 2 becomes RH = −1/(en), which gives a nearly constant electron density of n = 1.65 × 1018 cm−3 from the Hall coefficients over the entire temperature range for the 3xAg sample as shown in Figure 3c. The electrical conductivity steadily decreases with increasing temperature for the 3xAg sample as shown in Figure 3a, because the predominant acoustic phonon scattering for carriers in Ag2Te becomes stronger at higher temperatures. The electrical conductivity of the 3xAg sample is higher than that of the 2xAg sample mainly because the majority carriers, electrons, have much higher mobility in the 3xAg sample than the majority carriers, holes, in the 2xAg sample. The Seebeck coefficient of the 3xAg sample only slightly increases in magnitude from −102 to −119 μV/K as temperature increases from 300 to 400 K, which can be very well fitted by the theory with a constant electron density of 1.65 × 1018 cm−3 as shown in Figure 3b. Figure 3d presents the thermal conductivities of the two samples. For the 3xAg sample, the electronic thermal

magnitude of Seebeck coefficient at higher temperature, reaching ∼190 μV/K at 360−380 K in the 2xAg sample as shown in Figure 3b. The Hall coefficient in two-type carrier transport is given by43 RH =

p − nb2 e(p + nb)2

(2)

As shown in eq 2, Hall coefficient can be negative even if the hole density is larger than the electron density, because the electron density is multiplied by b2 (typically b ≫ 1). This is the reason that the 2xAg sample exhibits negative Hall coefficients over the entire temperature range as shown in Figure 3c even though the sample is p-type doped. However, the magnitude of Hall coefficient decreases as temperature increases because the hole density increases more rapidly than the electron density with increasing temperature. The slight discrepancy between the experimental Hall coefficient and its theoretical fitting for the 2xAg sample in Figure 3c is probably because the Hall coefficient is dependent on the magnetic field intensity used in the Hall effect measurements in the case of two-type carrier transport. Note that eq 2 is only valid in the low magnetic field limit. We used 2 T in the Hall effect 5402

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conductivity is calculated using the Wiedemann−Franz relation with the Lorenz number obtained from our electron transport modeling. The Lorenz number is found to be 1.6 × 10−8 W Ω K−2 at 300 K, which decreases to 1.3 × 10−8 W Ω K−2 at 400 K for the 3xAg sample. These values are much lower than the conventional value, 2.44 × 10−8 W Ω K−2. Then, the calculated electronic thermal conductivity is subtracted from the measured total thermal conductivity to obtain the lattice thermal conductivity, and the bipolar thermal conductivity is assumed to be negligibly small because the 3xAg sample is heavily doped. As shown in Figure 3d, the lattice thermal conductivity steadily decreases with temperature and reaches 0.12 W/mK at 400 K for the 3xAg sample, which is very similar to the bulk Ag2Te.13 Although the phonon scattering at grain boundaries is expected to suppress the lattice conductivity, this effect is not very significant in this study because of the intrinsically low lattice thermal conductivity of Ag2Te. Nevertheless, the total thermal conductivity of our samples is much lower than that of the bulk Ag2Te due to the low electronic contribution, which is still beneficial for thermoelectric applications. For the case of the 2xAg sample, bipolar transport may have a significant contribution to the total thermal conductivity because the transport is highly bipolar in this sample. We find that the bipolar thermal conductivity is very difficult to quantify in the 2xAg sample as this quantity is very sensitive to the band structure of this material and the valence band of Ag2Te has not been well studied so far. A further investigation might be necessary both theoretically and experimentally for analysis of bipolar thermal conductivity in Ag2Te. The electronic thermal conductivity excluding the bipolar term for the 2xAg sample by the Wiedemann−Franz relation, however, is as low as 0.03 W/mK at 380 K due to the very low electrical conductivity as well as the smaller Lorenz number. As a much higher electrical conductivity is measured for the 3xAg sample with a heavy n-type doping, the power factor of the 3xAg sample is found to be much larger than that of the 2xAg sample as shown in Figure 3e. As a result, a higher ZT is found in the 3xAg sample although both samples possess similar thermal conductivity over the entire temperature range. A ZT ∼ 0.55 is achieved at 400 K for the 3xAg sample as shown in Figure 3f, which is about a 57% enhancement over the best value reported for Ag2Te nanocrystals (ZT = 0.35 at 400 K)24 and about 6% enhancement over the best value reported for bulk nanostructured Ag2Te (ZT = 0.52 at 400 K)13 thus far at the same temperature. A ZT ∼ 0.37 is achieved at 380 K for the 2xAg sample, and this value is predicted to keep increasing with temperature by the theory as shown in Figure 3f if the hole density steadily increases with temperature as theory anticipated. It is worth noting that p-type Ag2Te has rarely been reported, thus our p-type Ag2Te sample is of interest in terms of completing a thermoelectric device, where both p- and n-type materials are required, and also in terms of understanding the valence band of this material. A further material optimization is possible using the developed theoretical transport model at a given temperature with varying carrier density and a constant lattice thermal conductivity assumed. Figure 4 shows the material optimization results at 400 K. The scattering parameters are found by fitting the mobility of the 3xAg sample and fixed for varying electron density. The lattice thermal conductivity (0.12 W/mK) extracted from the thermal conductivity analysis for the 3xAg sample is assumed to remain constant. As shown in Figure 4, a

Figure 4. Theoretical prediction of material optimization with varying electron density for the n-type Ag2Te nanocomposites at 400 K. The symbols are experimental properties for sample 3xAg with 1.65 × 1018 cm−3 electron density.

maximum ZT ∼ 0.57 is possible according to the theory at 1.4 × 1018 cm−3 electron density, which is within the measurement uncertainty range with the measured ZT ∼ 0.55 for our 3xAg sample that has 1.65 × 1018 cm−3 electron density. This indicates that indeed we have achieved near-optimal power factor and ZT values with the 3xAg sample at this temperature. However, further improvement of ZT is still possible through optimizing the sintering of the nanowires to achieve higher electron mobility, because the estimated electron mobility of this 3xAg sample (1550 cm2 V−1 s−1 at room temperature) is still much lower than that of the bulk counterpart (∼4000 cm2 V−1 s−1).13 Summary. In summary, we have studied the solution phase synthesis of Ag2Te nanowires by self-templated transformation of Te nanowires and explored the potential application of these nanowires for thermoelectric power generation by hot-press fabrication of the nanocomposite disks. We have shown that the carrier density of the Ag2Te nanowires can be tuned from near intrinsic to heavily n-doped, which leads to a p-type to ntype transition of Seebeck coefficient and a several-fold increase in electrical conductivity, while maintaining low thermal conductivity. Through carrier density modulation, we have achieved an increase of ZT from 0.37 in the p-type sample to 0.55 in the heavily n-doped sample, which is the highest ZT value reported for Ag2Te at the same temperature. The thermoelectric transport properties have been interpreted with Boltzmann Transport Equations. Our findings suggest a potential approach to achieve superior thermoelectric nanocomposites though low-cost and large scale wet synthesis. Further improvement in the performance and high-temperature stability of the Ag2Te nanocomposites is possible through improving the hot press conditions or utilizing post heat treatments, which is subjected to future research. A final consideration is about the scarcity of tellurium; future research might include the investigation on the thermoelectric properties of Ag2Se nanostructures because bulk Ag2Se shows a very promising ZT around 144 and the earth reservation of selenium is 50 times higher than that of tellurium.45

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental processes, theoretical modeling procedures, and XRD pattern of the Ag2Te_1xAg nanowires showing 5403

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(21) Aliev, F. F.; Kerimova, E. M.; Aliev, S. A. Semiconductors 2002, 36 (8), 869−873. (22) Drymiotis, F.; Day, T. W.; Brown, D. R.; Heinz, N. A.; Jeffrey Snyder, G. Appl. Phys. Lett. 2013, 103, 143906. (23) Li, F.; Hu, C.; Xiong, Y.; Wan, B.; Yan, W.; Zhang, M. J. Phys. Chem. C 2008, 112 (41), 16130−16133. (24) Cadavid, D.; Ibanez, M.; Shavel, A.; Dura, O. J.; Lopez de la Torre, M. A.; Cabot, A. J. Mater. Chem. A 2013, 1 (15), 4864−4870. (25) Zhou, W.; Zhao, W.; Lu, Z.; Zhu, J.; Fan, S.; Ma, J.; Hng, H. H.; Yan, Q. Nanoscale 2012, 4 (13), 3926−3931. (26) Dong, G.-H.; Zhu, Y.-J. CrystEngComm 2012, 14 (5), 1805− 1811. (27) Liang, H.-W.; Liu, J.-W.; Qian, H.-S.; Yu, S.-H. Acc. Chem. Res. 2013, 46 (7), 1450−1461. (28) Liu, J.-W.; Xu, J.; Liang, H.-W.; Wang, K.; Yu, S.-H. Angew. Chem., Int. Ed. 2012, 51 (30), 7420−7425. (29) Wang, K.; Yang, Y.; Liang, H.-W.; Liu, J.-W.; Yu, S.-H. Mater. Horiz. 2014, 1 (3), 338−343. (30) Fang, H.; Feng, T.; Yang, H.; Ruan, X.; Wu, Y. Nano Lett. 2013, 13 (5), 2058−2063. (31) Zhang, G.; Fang, H.; Yang, H.; Jauregui, L. A.; Chen, Y. P.; Wu, Y. Nano Lett. 2012, 12 (7), 3627−3633. (32) Zhang, G.; Kirk, B.; Jauregui, L. A.; Yang, H.; Xu, X.; Chen, Y. P.; Wu, Y. Nano Lett. 2011, 12 (1), 56−60. (33) Moon, G. D.; Ko, S.; Xia, Y.; Jeong, U. ACS Nano 2010, 4 (4), 2307−2319. (34) Jiang, F.; Liu, J.; Li, Y.; Fan, L.; Ding, Y.; Li, Y. Adv. Funct. Mater. 2012, 22 (11), 2402−2411. (35) Kuno, M.; Ahmad, O.; Protasenko, V.; Bacinello, D.; Kosel, T. H. Chem. Mater. 2006, 18 (24), 5722−5732. (36) Grebinski, J. W.; Hull, K. L.; Zhang, J.; Kosel, T. H.; Kuno, M. Chem. Mater. 2004, 16 (25), 5260−5272. (37) Samal, A. K.; Pradeep, T. J. Phys. Chem. C 2009, 113 (31), 13539−13544. (38) Qin, A.; Fang, Y.; Tao, P.; Zhang, J.; Su, C. Inorg. Chem. 2007, 46 (18), 7403−7409. (39) Xiao, F.; Chen, G.; Wang, Q.; Wang, L.; Pei, J.; Zhou, N. J. Solid State Chem. 2010, 183 (10), 2382−2388. (40) Li, G.; Tang, X.; Zhou, S.; Li, N.; Yuan, X. Nanoscale Res. Lett. 2013, 8 (1), 356. (41) Pei, J.; Chen, G.; Jia, D. C.; Yu, Y. G.; Sun, J. X.; Xu, H. M.; Qiu, Z. Z. New J. Chem. 2014, 38 (1), 59−62. (42) Rowe, D. M. Modern thermoelectrics; Reston Publishing Co.: Reston, VA, 1983. (43) Hamaguchi, C. Basic Semiconductor Physics; Springer: New York, 2009. (44) Aliev, F. F.; Jafarov, M. B.; Eminova, V. I. Semiconductors 2009, 43 (8), 977−979. (45) Abundance in Earth’s crust: periodicity by WebElements. http://www.webelements.com/periodicity/abundance_crust/ (accessed August 11, 2014).

Ag-deficient impurity phases. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

Y.W. led the project. H.Y. was the main contributor for the Ag2Te nanowires synthesis, device fabrications, and structural characterizations. J.-H.B. and A.S. performed the theoretical modeling. T.D. and G.J.S. performed the Hall measurements. H.Y. and A.M.S.M. performed Seebeck coefficient measurements. B.M. contributed to the mass production of the nanowires. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the U.S. Air Force Office of Scientific Research (Award Number FA9550-12-1-0061). The electrical property measurement system is funded by DARPA/Army Research Office under the contract no. W911NF0810347.

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REFERENCES

(1) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7 (2), 105−114. (2) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z. F.; Fleurial, J. P.; Gogna, P. Adv. Mater. 2007, 19 (8), 1043−1053. (3) Shakouri, A. Annu. Rev. Mater. Res. 2011, 41 (1), 399−431. (4) Kanatzidis, M. G. Chem. Mater. 2010, 22 (3), 648−659. (5) Vineis, C. J.; Shakouri, A.; Majumdar, A.; Kanatzidis, M. G. Adv. Mater. 2010, 22 (36), 3970−3980. (6) Liu, W.; Yan, X.; Chen, G.; Ren, Z. Nano Energy 2012, 1 (1), 42− 56. (7) Bahk, J.-H.; Bian, Z.; Shakouri, A. Phys. Rev. B 2013, 87 (7), 075204. (8) Soni, A.; Shen, Y.; Yin, M.; Zhao, Y.; Yu, L.; Hu, X.; Dong, Z.; Khor, K. A.; Dresselhaus, M. S.; Xiong, Q. Nano Lett. 2012, 12 (8), 4305−4310. (9) Martin, J.; Wang, L.; Chen, L.; Nolas, G. S. Phys. Rev. B 2009, 79 (11), 115311. (10) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. Science 2008, 320 (5876), 634−638. (11) Joshi, G.; Lee, H.; Lan, Y. C.; Wang, X. W.; Zhu, G. H.; Wang, D. Z.; Gould, R. W.; Cuff, D. C.; Tang, M. Y.; Dresselhaus, M. S.; Chen, G.; Ren, Z. F. Nano Lett. 2008, 8 (12), 4670−4674. (12) Mehta, R. J.; Zhang, Y.; Karthik, C.; Singh, B.; Siegel, R. W.; Borca-Tasciuc, T.; Ramanath, G. Nat. Mater. 2012, 11 (3), 233−240. (13) Pei, Y.; Heinz, N. A.; Snyder, G. J. J. Mater. Chem. 2011, 21 (45), 18256−18260. (14) Capps, J.; Drymiotis, F.; Lindsey, S.; Tritt, T. M. Philo. Mag. Lett. 2010, 90 (9), 677−681. (15) Day, T.; Drymiotis, F.; Zhang, T.; Rhodes, D.; Shi, X.; Chen, L.; Snyder, G. J. J. Mater. Chem. C 2013, 1 (45), 7568−7573. (16) Ishiwata, S.; Shiomi, Y.; Lee, J. S.; Bahramy, M. S.; Suzuki, T.; Uchida, M.; Arita, R.; Taguchi, Y.; Tokura, Y. Nat. Mater. 2013, 12 (6), 512−517. (17) Jafarov, M.; Aliev, F.; Hasanova, R.; Saddinova, A. Semiconductors 2012, 46 (7), 861−865. (18) Karakaya, I.; Thompson, W. T. J. Phase Equilib. 1991, 12 (1), 56−63. (19) Fujikane, M.; Kurosaki, K.; Muta, H.; Yamanaka, S. J. Alloys Compd. 2005, 393 (1−2), 299−301. (20) Aliev, F. F. Semiconductors 2003, 37 (9), 1057−1060. 5404

dx.doi.org/10.1021/nl502551c | Nano Lett. 2014, 14, 5398−5404