Synergetic Enhancement of Thermoelectric Performance by Selective

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Synergetic Enhancement of Thermoelectric Performance by Selective Charge Anderson Localization-Delocalization Transition in n-type Bi-doped PbTe/Ag2Te Nanocomposite Min Ho Lee, Jae Hyun Yun, Gareoung Kim, Ji Eun Lee, Su-Dong Park, Heiko Reith, Gabi Schierning, Kornelius Nielsch, Wonhee Ko, An-Ping Li, and Jong-Soo Rhyee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08579 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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ACS Nano

Synergetic

Enhancement

of

Thermoelectric

Performance by Selective Charge Anderson Localization-Delocalization Transition in n-type Bi-doped PbTe/Ag2Te Nanocomposite

Min Ho Lee†,‡, Jae Hyun Yun†, Gareoung Kim†, Ji Eun Lee§, Su-Dong Park§, Heiko Reith‡, Gabi Schierning‡, Konelius Nielsch‡, Wonhee Ko⊥, An-Ping Li⊥, Jong-Soo Rhyee*,†.

†Department

of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yong-

In 17104, Korea. ‡Leibniz

Institute for Solid State and Materials Research, Helmholtzstr. 20, Dresden 01069,

Germany. §Thermoelectric

Conversion Research Centre, Korea Electrotechnology Research Institute,

Changwon 51543, Korea. ⊥Centre

for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge,

KEYWORDS: thermoelectric, bulk composite, Anderson localization, thermal conductivity, figure-of-merit.

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ABSTRACT: Considerable efforts have been devoted to enhancing thermoelectric (TE) performance, by employing phonon scattering from nano-structural architecture, and material design using phonon-glass and electron-crystal concepts. The nanostructural approach helps to lower thermal conductivity but has limited effect on the power factor. Here, we demonstrate selective charge Anderson localization as a route to maximize the Seebeck coefficient while simultaneously preserving high electrical conductivity and lowering the lattice thermal conductivity. We confirm the viability of interface potential modification in an n-type Bi-doped PbTe/Ag2Te nanocomposite, and the resulting enhancement in thermoelectric figure-of-merit ZT. The introduction of random potentials via Ag2Te nanoparticle distribution using extrinsic phase mixing was determined using scanning tunneling spectroscopy measurements. When the Ag2Te undergoes a structural phase transition (T > 420 K) from monoclinic β-Ag2Te to cubic α-Ag2Te, the band gap in the α-Ag2Te increases due to the p-d hybridization. This results in a decrease in the potential barrier height, which gives rise to partial delocalization of the electrons, while wave packets of the holes are still in a localized state. Using this strategic approach, we achieved an exceptionally high thermoelectric figure-of-merit in n-type PbTe materials, a ZT greater than 2.0, suitable for waste heat power generation.


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Rising global energy demands and resulting environmental problems have led to the intensive study of thermoelectric devices as alternative energy saving systems, and solid-state cooling technologies that can operate without environmentally hazardous cooling agents. Thermoelectric energy conversion efficiency is described by the dimensional figure-of-merit ZT, defined as ZT = S2σT/(κph + κel), where S, σ, T, and κph (κel) are the Seebeck coefficient, electrical conductivity, absolute temperature, and lattice (electronic) thermal conductivity, respectively. Recent investigations of thermoelectric devices have mainly focused on decreasing the lattice thermal conductivity κph by various means, including adopting nanoscale phonon scattering centres,1 using the anharmonicity of phonons,2 nanoscale grain boundaries,3-5 nanoprecipitation, and point defects.6-10 Other efforts to increase the power factor S2σ have focused on the influence of band structure engineering, such as resonant states, band convergence,11-13 and energy filtering in nanocomposites.14-17 Peierls distortion both lowers thermal conductivity while optimizing the power factor.18-20 The reason it is difficult to achieve higher ZT values has to do with the trade-off relationship between the Seebeck coefficient S, electrical conductivity σ, and (electronic) thermal conductivity κel.21 The trade-off relationship is based on the linear-response relation between the carrier and phonons from the Onsager matrix within a single transport channel.22,23 A recent theoretical study has proposed that the Seebeck coefficient and ZT value can be significantly enhanced on the insulating side with a small, non-zero, electrical conductivity due to Anderson localization, when the chemical potential resides below the localization threshold within a single mobility edge.24 Considering two mobility edges, if the two mobility edge thresholds approach each other with increasing disorder, the Seebeck coefficient and ZT value are increased. This approach is very promising as a strategy to enhance thermoelectric


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performance, but it has not yet been experimentally realized. The approach also does not exclude a compromising effect between Seebeck coefficient and electrical conductivity, because it can be applied to the insulating side at low temperature. Here, we propose a concept to improve thermoelectric performance by synergistically increasing the Seebeck coefficient while preserving electrical conductivity, and lowering the thermal conductivity by selective carrier Anderson localization. Many good thermoelectric materials, such as (Bi,Sb)2(Te,Se)3 and PbTe-based compounds, are narrow band gap semiconductors, which inherently have bipolar diffusion effects at high temperatures due to the thermal excitation of electrons and holes. The minority carrier diffusion of electrons (holes) in p-type (n-type) matrices degrades the thermoelectric performance by compensating the Seebeck coefficient, and increasing the bipolar thermal conductivity κbi. This makes it necessary to suppress the bipolar transport contribution as much as possible.25,26 We theoretically suggest that minority carrier Anderson localization can help increase the ZT value because it has three distinct effects on the thermoelectricity. First, the minority carrier Anderson localization selectively localizes the minority carriers while preserving the dispersive majority carriers, which results in the suppression of the bipolar diffusion effect. Second, because the majority carriers are in an extended state, the suppression of the electrical conductivity is minimized compared with the enhancement of the Seebeck coefficient. Third, the random distribution of potential centres affects the phonon scattering due to nanoprecipitation, resulting in a decrease in the lattice thermal conductivity.


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Figure 1. Schematic representation of minority carrier Anderson localization (a, b) and Boltzmann transport calculation results (c ~ e); (a) the Fermi-level resides near the bottom of the conduction band minimum (pink concave parabola) with the mobility edge (flattened sky blue convex parabola) in the minority hole localization. (b) The randomly distributed peaks are disordered positive potentials compared to the matrix. When the chemical potential of electrons is high enough in the n-type conduction carriers, the electronic wave function becomes an extended state (upper side) while the minority carrier holes become localized (lower localized wave packets) if the minority carrier holes hybridize with the potential well. (c) Normalized electric conductivity σ(Ec)/σ(Ec=0) (black line) and normalized electronic thermal conductivity κ(Ec)/κ(0) (red line), (d) normalized Seebeck coefficient S(Ec)/S(0) (black line) and normalized power factor PF(Ec)/PF(0), and (e) normalized ZT value ZT(Ec)/ZT(0) with respect to cut-off energy in the valence band (hole localization).

RESULTS/DISCUSSION


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Anderson localization is a form of charge localization due to random potential in disordered lattices.27,28 It is observed in many disordered semiconductors,29-32 polycrystals,33,34 and even single crystals.35 Two carrier-selective Anderson localizations can be realized when the chemical potential of the majority carriers resides far from the band edge, while the chemical potential of the minority carriers is located near the band edge. For example, when the chemical potential (or Fermi energy EF) of the majority carrier electrons is near the conduction band minimum, as shown in Figure 1a, which is far from the mobility edge with a cut-off energy Ec, and the energy scale of the minority carrier holes is sufficient to hybridize with the random potential well, the electronic wave function is in an extended state. This is because the Fermi level is far from the band edge of the hole. The hole wave packets become localized due to the random potential well in Figure 1b. Likewise, in the case of majority carrier holes, if the Fermi level resides near the top of the valence band maximum, the minority carrier electrons localize due to hybridization with the random potential well on the electron side (Figure S1 in supplementary information; SI). This is likely for narrow energy gap Eg semiconductors or semimetals within the parabolic band assumption. Many thermoelectric materials follow the parabolic band assumption in the Pisarenko plot well.36 The cut-off energy Ec of the minority carriers depends on the potential hybridization with minority carriers and disorder from Anderson localization. The thermoelectric properties of the electrical conductivity σ, Seebeck coefficient S, and electronic thermal conductivity κ are calculated from the Boltzmann transport theory as follows:37 ∞

[

∂𝑓

]

𝜎(𝑇) = ∫ ―∞𝑑𝐸𝜎0(𝐸) ― ∂𝐸

(1)


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[

∞

∂𝑓

]

𝑆(𝑇) = ∫ ―∞𝑑𝐸(𝐸 ― 𝐸𝐹)𝜎0(𝐸) ― ∂𝐸 /𝑒𝜎(𝑇)𝑇

{

[

∞

∂𝑓

]

2 𝑘2𝐵 ∫ ―∞𝑑𝐸(𝐸 ― 𝐸𝐹) 𝜎0(𝐸) ― ∂𝐸

𝜅(𝑇) = 𝜎(𝑇)𝑇 𝑒2

𝜎(𝑇)𝑇2

―

[

(2)

[

∞

∂𝑓

]2

∫ ―∞𝑑𝐸(𝐸 ― 𝐸𝐹)𝜎0(𝐸) ― ∂𝐸 𝜎(𝑇)𝑇

]}

(3)

where f is the Fermi-Dirac distribution function, kB is the Boltzmann constant, e is the electronic charge, and σ0 is the electronic conductivity at zero temperature. We adopted the mobility edge of holes, as shown in Figure 1a, with 𝜎0 = 𝐴(𝐸 ― 𝐸𝑚)𝑥, where A is a constant pre-factor, Em is the mobility edge in the Anderson localized state or the band edge in the normal state, and x is the scaling exponent of the characteristic length scale (x = 1.5 in the parabolic band case). We set the Fermi-level EF ≅ 1019 e/cm3, which is known to be the optimal carrier density for thermoelectricity at T = 300 K. The energy band gap Eg = 30 meV is similar to the real band gap scales of many thermoelectric materials, such as Bi2Te3 and PbTe-based compounds. All the transport coefficients are given by the contribution of electron and hole bands as: σ = 𝜎𝑒 + 𝜎ℎ, 𝜅 = 𝜅𝑒 + 𝜅ℎ, and 𝑆 = (𝜎𝑒𝑆𝑒 + 𝜎ℎ𝑆ℎ)/(𝜎𝑒 + 𝜎ℎ), where e and h are the electron and hole bands, respectively. The calculated thermoelectric properties of the scaled electrical conductivity from the zero cutoff energy σ(Ec)/σ(0) (black line) and electronic thermal conductivity κe(Ec)/κe(0) (red line) decreased with the increase in cut-off energy induced by the minority hole band Anderson localization, as presented in Figure 1c. The scaled electronic conductivity decreased more rapidly than the electronic thermal conductivity. On the other hand, the scaled Seebeck coefficient S(Ec)/S(0) drastically increased with the cut-off energy Ec (Figure 1d). This was caused by the suppression of the bipolar diffusion effect between the electrons and holes, and


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from the increase in the effective band gap Eeff, which is influenced by the real band gap and cutoff energy Eeff = Eg + Ec. As a result, the power factor S2σ increased significantly, such that the power factor at Ec = 0.1 eV was as much as 10 times higher than that of the pristine parabolic symmetric band. The scaled value of the electronic ZT without considering the lattice thermal conductivity followed the enhancement of the power factor (Figure 1e). Even for small cut-off energies from localization (Ec = 0.04 eV, comparable to a temperature scale of 500 K), which is similar to the localization energy scale of 3-dimensional LixFe7Se8 single crystals,35 the electronic ZT value increased as much as 6 times over the case without selective hole Anderson localization. Anderson localization was observed in the disordered lattice, and the phonon scattering from the disordered lattice thus decreased the lattice thermal conductivity further, resulting in the enhancement of the ZT value. As a candidate model system, we adopted Ag2Te nanoparticles distributed in a Bi-doped PbTe matrix. Small (0.2%) Bi-doping in PbTe is intended to decrease the offset of the conduction band minimum compared with that of Ag2Te.38 The Ag2Te is an appropriate candidate to use to check the effect of the Anderson localization-delocalization transition, because it undergoes a structural phase transition from β-Ag2Te (monoclinic) to the α-Ag2Te (cubic) phase at 417 K, implying a change in potential height with the structural phase transition.39 The theoretical electronic band structure of Ag2Te predicts that the antifluorite β-Ag2Te is a topological insulator with a gapless Dirac surface state, and the α-Ag2Te is anticipated to have a sizable band gap due to band inversion driven by p-d hybridization.40 The theoretical calculation expects that the β-Ag2Te has a high carrier mobility with a small effective mass (10-2 me)41 and


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exhibits gapless anisotropic Dirac surface states.40,42 In addition, the lattice parameters of the Bidoped PbTe, β-Ag2Te, and α-Ag2Te are 6.575 Å, 6.558 Å, and 6.741 Å, respectively. Since the lattice mismatch between the Bi-doped PbTe and Ag2Te is relatively small (β-Ag2Te: -0.3% and α-Ag2Te: 2.5%), there may be a coherent interface between them.

Figure 2. HR-TEM images and strain field mapping of PbTe, Bi-doped PbTe, and Ag2Te dispersed Bi-doped PbTe. The high resolution TEM (HR-TEM) images of PbTe (a) and Bi-doped PbTe (d). (b) and (e) are the inverse Fouriertransformed images of (a) and (d), respectively. (c) and (f) are the lattice strain field mapping results of (a) and (d), respectively. (g) and (h) are the scanning transmission electron microscopy (S-TEM) images of 15 % Ag2Te nanoparticle dispersed PbBi0.002Te composites with different length scales. The striped phase in (g) and (h) is not a phase separation but an artifact of ion milling by focused ion beam. (i) is the expanded elemental mapping image from the energy dispersive X-ray spectroscopy (EDS) of (h). We confirmed the clear phase separation of Ag2Te (red dashed line in (g) and inset of (h)) in a matrix without Ag diffusion.


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Figure 2 presents the high-resolution transmission electron microscopy (HR-TEM) images of PbTe (Figure 2a) and Bi-doped PbTe (PBT) (Figure 2d), and their inverse Fourier transform images (Figure 2b and 2e) and strain field maps (Figure 2c and 2f) which confirm a more disordered lattice in the PBT than that of the parent compound PbTe. The disordered PBT matrix reduces the electron-phonon interaction and increases the electron-impurity scattering. Transmission electron microscope (TEM) images of the 15% distributed Ag2Te composite show random distributions of nanoparticles in a matrix and a clear phase separation, as shown in Figure 2g~2i. The typical sizes of the Ag2Te nanoprecipitates are approximately 20~50 nm, and the mean distance among the nanoprecipitates is approximately 100 nm. The striped phase is not a phase separation but surface damage that occurred with focused ion beam (FIB) polishing of the surface. The powder x-ray diffraction (XRD) patterns shown in Figure S2 (SI) indicate that the lattice parameters have not significantly changed following Bi-doping and Ag2Te dispersions in the Bidoped PbTe (PBT) matrix, as presented in Table S1 (SI). This is consistent with theh clear phase separation of Ag2Te, rather than Ag-doping in the matrix. The room temperature Hall carrier concentration nH and Hall mobility μH are drastically decreased in the Ag2Te/PBT composite, by one order of magnitude (Table S1). With respect to electron-electron scattering, the Hall carrier concentration is inversely proportional to the Hall mobility. The simultaneous declines in Hall carrier concentration and Hall mobility indicate carrier disorder scattering by grain boundaries and defects.


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Figure 3. Scanning Tunnelling Microscopy (STM) and Scanning Tunnelling Spectroscopy (STS) of Bi-doped PbTe (PBT) and PBT/AT 24% composites. Topographic images of the composite from STM measurements (a), height profile near the PBT/AT interface (b), and the dI/dV map near the PBT/AT interface from the STS measurement (c).

The interface potential difference between Ag2Te and the Bi-doped PbTe matrix can be identified from the scanning tunnelling microscopy (STM) measurements, as presented in Figure 3a for the PBT/AT 24% composite near the Ag2Te agglomerated region. The height profile along the line, as indicated by the arrow, shows a distinctive region, as presented in Figure 3b. The dI/dV map from the scanning tunnelling spectroscopy (STS) measurements of the region corresponds to the energy band gap in Figure 3c with a fine tuning of the conduction band offset between the matrix and Ag2Te phase. Since the Fermi level of the composite resides near the conduction band minimum with a small offset of the conduction bands between the PBT and AT compounds, the composites exhibit n-type behaviour. The estimated energy band gaps of the PBT matrix and Ag2Te region are 1.3 V and 1.0 V, respectively, from the dI/dV STS measurement. Since the Fermi level resides near the bottom of the conduction band, the


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electronic band is the majority carrier with an extended state. The random nanoscale distribution of Ag2Te can localize the minority hole carriers when the energy scale of the holes hybridizes with the potential well.27

Figure 4. Thermoelectric properties of the Bi-doped PbTe (PBT), Ag2Te (AT), and PBT/AT composites. Temperature-dependent electric conductivity σ(T) (a), Seebeck coefficient S(T) (b), power factor S2σ (c), thermal conductivity κ(T) (d), and lattice thermal conductivity κL(T) (E), and thermoelectric figure-of-merit ZT (F). The inset in (E) represents the temperature-dependent Lorenz number L(T).

Figure 4 presents the thermoelectric properties of the composites. The temperature-dependent electrical conductivity σ(T) reveals that the pristine PBT and Ag2Te (AT) compounds exhibit metallic behaviour (Figure 4a). It is noteworthy that with a small addition of semi-metallic Ag2Te the composites of the metallic PBT compound (PBT/AT composite) show insulating or gap-like behaviour in σ(T) (Figure 4a). In particular, the electrical conductivity drastically decreased near 450 K, showing insulating properties at room temperature.


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The temperature dependent Seebeck coefficient S(T) of the PBT/AT composites also exhibits a step-like decrease of S(T) with increasing temperature for T ≥ 450 K, while pristine PBT and AT compounds show a monotonic behaviour with temperature, as shown in Figure 4b. The significant decrease in the electrical conductivity in the PBT/AT composite compared with the pristine PBT and AT compounds is an exceptionally exotic phenomenon in several respects. First, the significant decrease in σ(T) with a small addition of distributed AT nanoparticles cannot be solely attributed to carrier scattering near the grain boundary. Second, the broad decrease in σ(T) and the abrupt change in S(T) in Figure 4b near 450 K are related to the structural phase transition of Ag2Te, which causes a change in potential. The exceptional behavior of the S(T) of the PBT/AT 25 % distributed compound in Figure 4(b) may be due to Ag2Te agglomeration. When we increased the Ag2Te concentration, the microscale agglomeration became significant, as presented in Figure S7 in the supplementary information. Therefore, a higher concentration of distributed Ag2Te nanoparticles is not likely to account for the intrinsic thermoelectric properties of the nanocomposites. Even though the Seebeck coefficients of the dispersed Ag2Te compound showed the suppression of Seebeck coefficient for T ≥ 450 K, the step-like increase in the power factor in Figure 4c indicates that the enhancement in power factor was mainly determined by the increase in electrical conductivity in the temperature range. The insulating behaviour of σ(T) for small additions (5 mol.%) of Ag2Te nanoparticles to the PBT matrix can be attributed to carrier localization due to the random distribution of Ag2Te nanoparticles. Theoretical predictions of selective charge localization have also been reported in Titania composites with band alignment.43 The interface potential between the anatase and rutile structures of TiO2 can also give rise to electron and hole localization. The band gap difference


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between the Bi-doped PbTe (PBT) and Ag2Te (AT) is similar to the case of Titania composites with band alignment in that the interfacial potential difference between the PBT and AT can localize the carriers. Therefore, the random distribution of interfacial potential over the PBT matrix can cause Anderson localization, which is not limited by a low-temperature limit (𝜇 ― 𝐸𝑐 ≫ 𝑘𝐵𝑇).44 The temperature-dependent, macroscopic electrical conductivity and the Seebeck coefficient can be calculated from the Thouless picture within the tunnel-junction model; thus, the interface potential within a matrix can be considered to be tunnel junction.37

Figure 5. Low temperature thermoelectric properties: temperature-dependent electrical conductivity σ(T) (a) and its expanded vertical scale (b), Seebeck coefficient S(T) (c), and thermal conductivity κ(T) (d) of the samples as indicated.

Within the Thouless theory of the tunnel-junction model in terms of the Sommerfeld expansion, the low- and high-temperature limit of the Seebeck coefficient is given by:37


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𝜋2𝑥𝑇

𝜋4

𝑇3

𝑆𝑙𝑜𝑤(𝑇) ≈ 3𝑒(𝜇 ― 𝐸𝑚) + 45𝑥(𝑥 ― 1)(𝑥 ― 7)𝑒(𝜇 ― 𝐸

𝑚)

1

𝑆ℎ𝑖𝑔ℎ ≈ 𝑒[2log (2) + 𝑥]

3

(4)

(5)

where x is the critical exponent for localization. The S(T) cannot be fit with the above equations exactly, because it is not an ideal random distribution of the interface potential. However, the non-linear behaviour with temperature in S(T) and saturation in the high-temperature region qualitatively corresponds to Anderson localization in the Thouless theory of the tunnel-junction model for Ag2Te composites, as shown in Figure 5c. Figure 5a also shows that the low temperature electrical conductivity σ(T) gives rise to strong charge localization with a small addition of Ag2Te (5 mol.%). Interestingly, the low temperature σ(T) increases with increasing Ag2Te concentration, as shown in Figure 5b. The dilute concentration of Ag2Te prefers a more random distribution of particles in a matrix than the non-diluted case, which is consistent with Anderson localization. The strong carrier-impurity scattering by localization in the PBT/Ag2Te (5 mol.%) composite suppresses the Umklapp scattering of phonons, as presented in the suppression of freeze-out of Umklapp phonon scattering behaviour in κ(T) near 25 K (Figure 5d). On the other hand, there are phase transitions near 420 K in the σ(T) and S(T) of the PBT/AT composites (Figure 4a & 4b), related to the structural phase transition of Ag2Te from the lowtemperature monoclinic β-Ag2Te to the high-temperature cubic α-Ag2Te (BCC) phase.45 From the first principle calculation of the α-Ag2Te phase, it was revealed to be a zero-gap semiconductor. However, if we consider sufficiently strong p-d hybridization, the Te-5p states are pushed up with band inversion, resulting in a sizable energy band gap opening.40 This finding


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is consistent with the experimental results showing a decrease in electrical conductivity at high temperatures (T ≥ 420 K) induced by the increase in band gap due to band inversion from p-d hybridization.

Figure 6. Selective Anderson localization before and after the Ag2Te structural phase transition: Charge localization of electron and hole in the β-Ag2Te (T ≤ 420 K) phase (a) and its energy band gap diagram with sizable potential height in the valence band (c). Extended electronic wave with hole localization in α-Ag2Te (T ≥ 420 K) (b) and its energy band gap diagram with decreasing barrier height on the hole side due to the increasing energy band gap of αAg2Te (d).

Within the context of Anderson localization, the PBT composite with β-Ag2Te (T ≤ 420 K) has a sizable random potential height resulting in charge localization of the electron and hole, as shown in Figure 6a and 6c. This is because the randomly distributed potential localizes carriers. For the high temperature structural phase transition in α-Ag2Te (T ≥ 420 K), the increase in the energy band gap in the α-Ag2Te phase decreases the potential barrier height in the PBT/AT composites (Figure 6b and 6d). If we assume that there is no charge localization, the electrical conductivity σ(T) should decrease more at the temperature of α-Ag2Te (T ≥ 420 K) than at β-


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Ag2Te (T ≤ 420 K) because the electrical conductivity of the α-Ag2Te is smaller than that of the β-Ag2Te. But the experimental results of σ(T) are contrary to our assumption; therefore, it is likely that there is a localization effect in the PBT/AT composites. The increase in the energy band gap in α-Ag2Te decreases the energy barrier height between the PBT and AT interfaces, inducing charge delocalization (Figure 6b). This results in an increase in the electrical conductivity at high temperature (Figure 4a). When the potential height is decreased, the localization effect becomes weak, so that electrons easily become itinerant. The chemical potential of the electron, which is nearly same as Fermi level when that electron is the major carrier, resides in the bottom of the conduction band, while the chemical potential of the hole will still be affected by the potential well (on the hole side, it is not a potential barrier but a potential well). Therefore, holes are still in a localized state with an itinerant n-type carrier in the α-Ag2Te phase region. The decrease in κ(T) with increasing temperature indicates the contribution of acoustic phonons to thermal conductivity (Figure 4d). From the thermal diffusivity TD and specific heat Cp measurements at high temperature, as presented in Figure S11 in the supplementary information, the reduction in thermal conductivity with increasing Ag2Te concentration mainly results from the decrease in thermal diffusivity, because the specific heat increased slightly with increasing Ag2Te concentration. By subtracting the electronic thermal conductivity κ = κL + κel from the Wiedemann-Franz law κel = LσT, where L is the Lorenz number, we obtain the lattice thermal conductivity κL (Figure 4e). The inset in Figure 4e shows the temperature-dependent Lorenz number calculated from the Fermi integral formalism under the assumption of a single parabolic band model.36 The


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nanoparticle inclusion in the matrix (PBT/AT) decreases the lattice thermal conductivity compared with pristine PBT compounds. The κL systematically decreased, from 1.92 W m-1 K-1 to 1.38 W m-1 K-1, with increasing Ag2Te nanoparticle concentration, owing to phonon scattering by the nano-precipitates. Because the Ag2Te nanoparticle distribution reduces the lattice thermal conductivity with high power factor, the Ag2Te 15% distributed composite exhibited an exceptionally high ZT of 2.05 at 800 K (Figure 4f). This high thermoelectric performance was thermally stable, as confirmed by repetitive heat cycling measurements using differential scanning calorimetry up to 800 K (Figure S4 in SI). We measured the electrical conductivity and Seebeck coefficient of the 10% Ag2Te nanoparticle dispersed PbBi0.002Te/Ag2Te composite several times, and found that the thermoelectric properties did not significantly change, and the ZT value was reproducible over repeated measurements, as shown in Figure S5 of SI. This value of ZT 2.05 at 800 K is exceptionally high for power generation in n-type materials because the ZT values of n-type PbTe-based compounds typically remain below 1.4~1.8 in the mid-temperature range.46-48 Even though there was a report of an exceptionally high ZT of 2.2 at 800 K in AgPbmSbTe2+m (m = 18),49 the work has not been reproduced due to difficulty controlling the nano-phase separation of the AgSbTe2 in the PbTe matrix. The Anderson localization scenario is not likely in natural phase separation during heat treatment because the intrinsic doping does not create a sharp potential interface. For this reason, the charge localization behaviour in electrical transport is not observed in the PbTe/Ag2Te and PbTe/AgSbTe2 composites with natural phase separation.46-49


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A previous report of an Ag 4%-doped PbTe/Ag2Te composite showed different temperature dependent thermoelectric behaviours with a relatively lower ZT value of 1.3 at 750 K,46 with no indication of selective carrier Anderson localization. The thermodynamic phase separation of PbTe and Ag2Te during the heating treatment in the previous study cannot guarantee random potential within the matrix, due to atomic diffusion at the interfaces. In contrast, we carefully controlled a clear phase separation without atomic diffusion using low-temperature sintering (below the phase separation temperature) and extrinsic phase mixing with separate Bi-doped PbTe and Ag2Te powders (Figures S6-S8). In addition, the size effect of the Ag2Te particle distribution is critical for high thermoelectric performance. When we investigated the thermoelectric properties of the PBT/AT composites with micrometer sized Ag2Te particles, as presented in Figures S9 and S10, the thermoelectric performance was lower than the nano-particle distributed composite. This also supports the conclusion of Anderson localization, because micrometer sized particle distribution does not permit random potential, because the electron coherence length in Anderson localization is on the order of nanometers. The Anderson localization itself at T ≤ 420 K is not suitable for high thermoelectric performance (Figure 4f). However, the charge delocalization resulting from the reduction in the potential barrier in the α-Ag2Te phase region (T ≥ 420 K) can give rise to selective charge delocalization. Because we tuned the chemical potential to reside near the conduction band minimum, the electronic charges are much more delocalized than the holes. Therefore, the electronic state becomes extended with a small perturbation of localized holes. This is a manifestation of selective charge localization (Figure 1b). Even though we increased the chemical potential of the PbTe with Bi-doping, a bipolar diffusion effect still exists for the S(T)


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and κ(T) of PBT in the high-temperature region (T ≥ 620 K), as shown in Figure 4b & 4d. The bipolar diffusion effect disappeared with even a small addition of Ag2Te nanoparticles, which is an indirect indication of selective charge carrier localization.

CONCLUSIONS In summary, we propose that selective charge Anderson localization can significantly enhance thermoelectric performance in the high-temperature region (T ≥ 420 K) by enhancing the power factor. When we examined the thermoelectric properties of the prepared PBT/AT composites, we observed a strong charge localization in the β-Ag2Te phase at temperatures below 420 K. Scanning tunneling microscopy and scanning tunneling spectroscopy measurements revealed a sharp potential difference between the PBT matrix and the Ag2Te phases. At high temperature, the structural phase transition to α-Ag2Te increases the energy band gap, resulting in a decrease in the potential barrier. Since the chemical potential resides near the bottom of the conduction band, the decrease in the potential barrier significantly delocalizes the electronic states. Therefore, an extended electronic state that preserves the localized hole carriers can increase the power factor. In addition, the distribution of Ag2Te nanoparticles suppresses the lattice thermal conductivity. By adopting this concept, we achieved an exceptionally high thermoelectric figure-of-merit, a ZT ≥ 2.0 at 800 K. We believe that the selective charge Anderson localization can provide a useful strategy for maximizing thermoelectric performance. This material design concept can be applied to many material platforms for thermoelectric energy harvesting technologies.


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EXPERIMENTAL METHODS 1. Material synthesis 1.1.

Ag2Te nano particle synthesis

The Ag2Te nanoparticles were synthesized according to a previously reported method.50 10 mmol of silver nitrate (AgNO3 1.7 g, 10 mmol) and H2O (100 mL) were mixed with dodecanethiol (2.4 mL) and toluene (100 mL). The mixture was stirred for 1 h at room temperature, and then the aqueous phase was removed. After the addition of oleylamine (100 mL), the solution was heated up to 469 K, the temperature was maintained for 10 min for toluene evaporation, and 5 mL of trioctylphosphine (TOP)-Te (1 M) was quickly injected. When the colour of the solution changed to black following the injection, the reaction was quenched within 1 min by placing the flask in cold water. The as-synthesized nanoparticles were washed with ethanol several times by centrifuging. The Ag2Te nanoparticles and PbTe powder were mixed at different ratios. The ligand of the nanoparticles was exchanged by mercaptopropionic acid.

1.2. Synthesis of bulk composites In order to synthesize the bulk composite, we first synthesized a Bi-doped PbTe (PBT) ingot. The PBT compound was synthesized by melting high purity Pb (99.999 %), Bi (99.999 %), Ag (99.999 %) and Te (99.999 %) granules. For the synthesis of the PBT, the stoichiometric elements were inserted in evacuated quartz ampoules and melted at 1273 K for 12 h, followed by furnace cooling. The Ag2Te ingot, for comparison, was melted at 1273 K for 12 h, cooled to 823 K, and kept at this temperature for 24 h followed by room temperature water quenching.


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Secondly, the extrinsic phase mixing for the n-type PbBi0.002Te/Ag2Te (PBT/AT) composites was performed by hand grinding the pulverized PBT and AT nanoparticles for 6 h in an agate mortar. The powders were mixed in a tube cell with n-hexane solution at molar ratios of Ag2Te = 5 %, 10 %, 12 %, 15 %, and 25 %. The mixed solutions were uniformly stirred with a vibrator and naturally dried at room temperature for 1 day. The dried powder mixtures were inserted in a graphite die and sintered by spark plasma sintering (SPS) under a vacuum environment (below 10-5 torr) and a uniaxial pressure of 50 MPa at 873 K for 5 min.

2. Measurement and characterization Phase identification and structural characterization were obtained using powder X-ray diffraction (XRD) with Cu Kα radiation. The samples were prepared for transmission electron microscopy (TEM), which was carried out with a Nova 600 Nanolab Focused Ion Beam (FEI) system with a 5–30 kV ion beam. TEM images (high resolution images/STEM/ED pattern) and energy dispersive X-ray spectroscopy (EDS/EDX) measurements were obtained using a JEOL2100F at 200 kV. The Seebeck coefficient and electrical resistivity were measured by the four-probe point contact method using a thermoelectric measurement system (ZEM-3 ULVAC, Japan). The thermal conductivity was determined using the relation of κ = ρsλCp, where ρs, λ, and Cp are the sample density, thermal diffusivity measured by a laser flash method, and specific heat measured by a physical property measurement system (PPMS, Quantum Design, USA), respectively. The Hall resistivity ρxy was measured by the four-probe contact method using the physical property measurement system.


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3. Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STMSTS) measurements For STM measurement, cryogenic four-probe scanning tunneling microscope (4P-STM) was used, and all measurements were performed at ultra-high vacuum (5×10-10 torr) and a temperature of 82 K.51,52 Among the four STM tips in 4P-STM, two of them were used as voltage supply while one contacted the sample to supply bias voltage, and the other was connected to the preamplifier and controlled by feedback loop to stay in tunneling mode for scanning and spectroscopy. After loading the sample in a UHV chamber, it was sputtered with Ne ions by ion sputtering gun with an ion beam energy of 1 keV, Ne gas pressure of 5×10-6 torr, and 3 minute sputtering time, before transferring to the STM stage to remove any oxide layers and adsorbed gases. The conventional lock-in technique was used to measure the dI/dV spectra with a modulation voltage of 10 mV and a modulation frequency of 731 Hz.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publication website at DOI: Schematic diagram of Anderson localization of electron in p-type compound. Characterization of the composites such as X-ray diffraction, transmission electron microscope images, scanning electron microscope images, differential scanning calorimetry, and scanning transmission electron microscopy images. Thermoelectric properties of the nano-composites such as the repetitive measurements for reproducibility, thermal diffusivity, and heat capacity. Thermoelectric properties of the micro-composites.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT J.S. Rhyee was supported by the Materials and Components Technology Development Program of MOTIE/KEIT (10063286) Republic of Korea and a grant from Kyung Hee University in 2017 (20171203). A portion of the work (STM/STS) was conducted at the Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.

REFERENCES (1) Lan, Y.; Minnich, A. J.; Chen, G.; Ren, Z. Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach. Adv. Mater. 2010, 20, 357-376. (2) Zhao, L. D.; Lo, S. H.; Zhang, Y.; Sun, H.; Tan, G.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. Z. Ultralow Thermal Conductivity and High Thermoelectric Figure of Merit in SnSe Crystals. Nature. 2014, 508, 373-378. (3) Wu, H. J.; Zhao, L.-D.; Zheng, F. S.; Wu, D.; Pei, Y. L.; Tong, X.; Kanatzidis, M. G.; He, J. Q. Broad Temperature Plateau for Thermoelectric Figure of Merit ZT>2 in Phase-Separated PbTe0.7S0.3. Nature Commun. 2014, 5, 4515. (4) Tan, G.; Shi, F.; Hao, S.; Zhao, L.-D.; Chi, H.; Zhang, X.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Non-equilibrium Processing Leads to Record High Thermoelectric Figure of Merit in PbTe–SrTe. Nature Commun. 2016, 7, 12167.


Page 24 of 31

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(5) Wang, H.; Bahk, J. H.; Kang, C.; Hwang, J.; Kim, K.; Kim, J.; Burke, P.; Bowers, J. E.; Gossard, A. C.; Shakouri, A.; Kim, W. Right Sizes of Nano- and Microstructures for HighPerformance and Rigid Bulk Thermoelectrics. Proc. Natl. Acad. Sci. 2014, 111, 10949-10954. (6) Orabi, R. A.; Hwang, J.; Lin, C.-C.; Gautier, R.; Fontaine, B.; Kim, W.; Rhyee, J.-S.; Wee, D.; Fornari. Ultralow Lattice Thermal Conductivity and Enhanced Thermoelectric Performance in SnTe:Ga Materials. Chem. Mater. 2017, 29, 612-620. (7) Lin, C.-C.; Lydia, R.; Yun, J. H.; Lee, H. S.; Rhyee, J.-S. Extremely Low Lattice Thermal Conductivity and Point Defect Scattering of Phonons in Ag-doped (SnSe)1–x(SnS)x Compounds. Chem. Mater. 2017, 29, 5344-5352. (8) Ginting, D.; Lin, C.-C.; Lydia, R.; So, H. S.; Lee, H.; Hwang, J.; Kim, W.; Orabi, R. A. R. A.; Rhyee, J.-S. High Thermoelectric Performance in Pseudo Quaternary Compounds of (PbTe)0.95−x(PbSe)x(PbS)0.05 by Simultaneous Band Convergence and Nano precipitation. Acta Mater. 2017, 131, 98-109. (9) Lee, M. H.; Byeon, D.-G.; Rhyee, J.-S.; Ryu, B. Defect Chemistry and Enhancement of Thermoelectric Performance in Ag-doped Sn1+δ−xAgxTe. J. Mater. Chem. A. 2017, 5, 2235-2242. (10) Ginting, D.; Lin, C.-C., Lydia, R.; Yun, J. H.; Yu, B. K.; Kim, S.-J.; Rhyee, J.-S. High Thermoelectric Performance due to Nano-Inclusions and Randomly Distributed Interface Potentials in N-type (PbTe0.93−xSe0.07Clx)0.93(PbS)0.07 Composites. J. Mater. Chem. A. 2017, 5, 13535-13543. (11) Pei, Y.; Shi, X.; LaLonde, A.; Wang, H.; Chen, L.; Snyder, G. J. Convergence of Electronic Bands for High Performance Bulk Thermoelectrics. Nature. 2011, 473, 66-69.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(12) Heremans, J. P.; Jovovic, V.; Toberer, E. S.; Saramat, A.; Kurosaki, K.; Charoenphakdee, A.; Yamanaka, S.; Snyder, G. J. Enhancement of Thermoelectric Efficiency in PbTe by Distortion of the Electronic Density of States. Science. 2008, 321, 554-557. (13) Heremans, J. P.; Wiendlocha, B.; Chamoire, A. M. Resonant Levels in Bulk Thermoelectric Semiconductors. Energy Environ. Sci. 2012, 5, 5510-5530. (14) Bahk, J.-H.; Bian, Z.; Zebarjadi, M.; Santhanam, P.; Ram, R.; Shakouri, A. Thermoelectric Power Factor Enhancement by Ionized Nanoparticle Scattering. Appl. Phys. Lett. 2011, 99, 072118. (15) Bahk, J.-H.; Santhanam, P.; Bian, Z.; Ram, R.; Shakouri, A. Resonant Carrier Scattering by Core-Shell Nanoparticles for Thermoelectric Power Factor Enhancement. Appl. Phys. Lett. 2012, 100, 012102. (16) Liu, M.; Qin, X. Y. Enhanced Thermoelectric Performance Through Energy-Filtering Effects in Nanocomposites Dispersed with Metallic Particles. Appl. Phys. Lett. 2012, 101, 132103. (17) Nishio, Y.; Hirano, T. Improvement of the Efficiency of Thermoelectric Energy Conversion by Utilizing Potential Barriers. Jpn. J. Appl. Phys. 1997, 36, 170-174. (18) Rhyee, J.-S.; Lee, K. H.; Lee, S. M.; Cho, E.; Kim, S. I.; Lee, E. S.; Kwon, Y. S.; Shim, J. H.; Kotliar, G. Force-Induced Activation of Covalent Bonds in Mechanoresponsive Polymeric Materials. Nature. 2009, 459, 965-968. (19) Rhyee, J. S.; Ahn, K.; Lee, K. H.; Ji, H. S.; Shim, J. H. Enhancement of the Thermoelectric Figure-of-Merit in a Wide Temperature Range in In4Se3–xCl0.03 Bulk Crystals. Adv. Mater. 2011, 23, 2191-2194.


Page 26 of 31

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(20) Rhyee, J. S.; Kim, J. H. Chemical Potential Tuning and Enhancement of Thermoelectric Properties in Indium Selenides. Materials. 2015, 8, 1283-1324. (21) Mahan, J. D.; Sofo, J. O. The Best Thermoelectric, Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 7436-7439. (22) Onsager, L. Reciprocal Relations in Irreversible Processes. I. Phys. Rev. 1931, 37, 405. (23) Callen, H. B. The Application of Onsager's Reciprocal Relations to Thermoelectric, Thermomagnetic and Galvanomagnetic Effects. Phys. Rev. 1948, 73, 1349. (24) Yamamoto, K.; Aharony, YaA.; Wohlman, O. E.; Hatano, N. Thermoelectricity Near Anderson Localization Transitions. Phys. Rev. B. 2017, 96, 155201. (25) Gong, J. J.; Hong, A. J.; Shuai, J.; Li, L.; Yan, Z. B.; Ren, Z. F.; Liu, J.-M. Investigation of the Bipolar Effect in The Thermoelectric Material CaMg2Bi2 using a First-principles. Phys. Chem. Chem. Phys. 2016, 18, 16566-16574. (26) Zhao, L. D.; Wu, H. J.; Hao, S. Q.; Wu, C. I.; Zhou, X. Y.; Biswas, K.; He, J. Q.; Hogan, T. P.; Uher, C.; Wolverton, C.; Dravid, V. P.; and Kanatzidis, M. G. All-Scale Hierarchical Thermoelectrics: MgTe in PbTe Facilitates Valence Band Convergence and Suppresses Bipolar Thermal Transport for High Performance. Energy Environ. Sci. 2013, 6, 3346-3355. (27) Anderson, P. W. Absence of Diffusion in Certain Random Lattices. Phys. Rev. 1958, 109, 1492–1505. (28) Edwards, P. P.; Sienko, M. J. Universality Aspects of the Metal-Nonmetal Transition in Condensed media. Phys. Rev. B. 1978, 17, 2575–2581. (29) Raj, S.; Hashimoto, D.; Matsui, H.; Souma, S.; Sato, T.; Takahashi, T. D.; Sarma, D.; Mahadevan,

P.;

Oishi,S.

Angle-Resolved

Photoemission


Spectroscopy

of

the

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Insulating NaxWO3: Anderson Localization, Polaron Formation and Remnant Fermi Surface. Phys. Rev. Lett. 2006, 96, 147603. (30) Siegrist, T.; Jost, P.; Volker, H.; Woda, M.; Merkelbach, P.; Schlockermann, C.; Wuttig, M. Effects of Stoichiometry on the Transport Properties of Crystalline Phase-Change Materials. Nat. Mater. 2011, 10, 202–208. (31) Zhang, W.; Thiess, A.; Zalden, P.; Zeller, R.; Dederichs, P. H.; Raty, J.-Y.; Wuttig, M.; Blügel, S.; Mazzarello, R. Role of Vacancies in Metal–Insulator Transitions of Crystalline Phase-Change Materials. Nat. Mater. 2012, 11, 952–956. (32) Jost, P.; Volker, H.; Poitz, A.; Poltorak, C.; Zalden, P.; Schäfer, T.; Lange, F. R. L.; Schmidt, R. M.; Holländer, B.; Wirtssohn, M. R. Disorder‐Induced Localization in Crystalline Pseudo‐Binary GeTe–Sb2Te3 Alloys between Ge3Sb2Te6 and GeTe. Adv. Funct. Mater. 2015, 25, 6399–6406. (33) Williams, A. J.; McQueen, T. M.; Ksenofontov, V.; Felser, C.; Cava, R. J. The Metal– Insulator Transition in Fe1.01−xCuxSe. J. Phys. Condens. Matter. 2009, 21, 305701. (34) Chadov, S.; Schärf, D.; Fecher, G. H.; Felser, C.; Zhang, L.; Singh, D. Electronic Structure, Localization, and Spin-State Transition in Cu-Substituted FeSe:Fe1−xCuxSe. Phys. Rev. B. 2010, 81, 104523. (35) Ying, T.; Gu, Y.; Chen, X.; Wang, X.; Jin, S.; Zhao, L.; Zhang, W.; Chen, X. Anderson Localization of Electrons in Single Crystals: LixFe7Se8. Sci. Adv. 2016, 2, e1501283. (36) Lin, C.-C.; Ginting, D.; Lydia, R.; Lee, M. H.; Rhyee, J.-S. Thermoelectric Properties and Extremely Low Lattice Thermal Conductivity in P-type Bismuth Tellurides by Pb-Doping and PbTe Precipitation. J. Alloys & Compd. 2016, 671, 538-544.


Page 28 of 31

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(37) Imry, Y.; Amir, A. in 50 Years of Anderson Localization, ed. E. Abrahams, World Scientific, Singapore. 2011, Ch. 9, 191-212. (38) Popescu, A.; Datta, A.; Nolas, G. S.; Woods, L. M. Thermoelectric Properties of Bi-doped PbTe Composites. J. Appl. Phys. 2011, 109, 103709. (39) Capps, J.; Drymiotis, F.; Lindsey, S.; Tritt, T. M. Significant Enhancement of the Dimensionless Thermoelectric Figure of Merit of the Binary Ag2Te. Philos. Mag. Lett. 2010, 90, 677-681. (40) Zhang, W.; Yu, R.; Feng, W.; Yao, Y.; Weng, H.; Dai, X.; Fang, Z. Topological Aspect and Quantum Magnetoresistance of β−Ag2Te. Phys. Rev. Lett. 2011, 106, 156808. (41) Dalven, R. Fundamental Optical Absorption in β-Silver Telluride. Phys. Rev. Lett. 1966, 16, 311. (42) Lee, S.; In, J.; Yoo, Y.; Jo, Y.; Park, Y. C.; Kim, H.-J.; Koo, H. C.; Kim, J.; Kim, B.; Wang, K. L. Single Crystalline β-Ag2Te Nanowire as a New Topological Insulator. Nano Lett. 2012, 12, 4194-4199. (43) Nolan, M.; Deskins, N. A.; Schwartzenberg, K. C.; Gray, K. A. Local Interfacial Structure Influences Charge Localization in Titania Composites: Beyond the Band Alignment Paradigm. J. Phys. Chem. C. 2016, 120, 1808-1815. (44) Yamamoto, K.; Aharony, A.; Wohlman, O. E.; Hatano, N. Thermoelectricity Near Anderson Localization Transitions. Phys. Rev. B. 2017, 96, 155201. (45) Jung, D.-Y.; Kurosaki, K.; Ohishi, Y.; Muta, H.; Yamanaka, S. Effect of Phase Transition on the Thermoelectric Properties of Ag2Te. Mater. Trans. 2012, 53, 1216-1219.


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(46) Pei, Y.; Lensch-Falk, J.; Toberer, E. S.; Medlin, D. L.; Snyder, G. J. High Thermoelectric Performance in PbTe Due to Large Nanoscale Ag2Te Precipitates and La Doping. Adv. Funct. Mater, 2011, 21, 241-249. (47) Pei, Y.; May, A. F.; Snyder, G. J. Self‐Tuning the Carrier Concentration of PbTe/Ag2Te Composites with Excess Ag for High Thermoelectric Performance. Adv. Energy Mater. 2011, 1, 291-296. (48) Kim, M.-S.; Lee, W.-J.; Cho, K.-H.; Ahn, J.-P.; Sung, Y.-M. Spinodally Decomposed PbSePbTe Nanoparticles for High-Performance Thermoelectrics: Enhanced Phonon Scattering and Unusual Transport Behavior. ACS Nano. 2016, 10, 7197-7207. (49) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidks, M. G. Cubic AgPbmSbTe2+m: Bulk Thermoelectric Materials with High Figure of Merit. Science 2004, 303, 818-821. (50) Zhou, W.; Zhao, W.; Lu, Z.; Zhu, J.; Fan, S.; Ma, J.; Hng, H. H.; Yan, Q. Preparation and Thermoelectric Properties of Sulfur doped Ag2Te Nanoparticles via Solvothermal Methods. Nanoscale. 2012, 4, 3926–3931. (51) Kim, T.-H.; Wang, Z.; Wendelken, J. F.; Weitering, H. H.; Li, W.; Li, A.-P. A Cryogenic Quadraprobe Scanning Tunneling Microscope System with Fabrication Capability for Nanotransport Research. Rev. Sci. Instrum. 2007, 78, 123701. (52) Li, A.-P.; Clark, K. W.; Zhang, X. G.; Baddorf, A. P. Electron Transport at the Nanometer Scale Spatially Revealed by Four-Probe Scanning Tunneling Microscopy. Adv. Funct. Mater. 2013, 23, 2509.


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Synergetic

Enhancement

of

Thermoelectric

Performance by Selective Charge Anderson Localization-Delocalization Transition in n-type Bi-doped PbTe/Ag2Te Nanocomposite

Min Ho Lee†,‡, Jae Hyun Yun†, Gareoung Kim†, Ji Eun Lee§, Su-Dong Park§, Heiko Reith‡, Gabi Schierning‡, Konelius Nielsch‡, Wonhee Ko⊥, An-Ping Li⊥, Jong-Soo Rhyee*,†.

TOC Graphic


Synergetic Enhancement of Thermoelectric Performance by Selective

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