Thermoelectric Properties of Solution Synthesized

0 downloads 0 Views 766KB Size Report
Apr 24, 2015 - Adam Heller and Elton J. Cairns ... Joanna B. Dahl, Jung-Ming G. Lin, Susan J. Muller, and Sanjay Kumar pppppppppppp 293. Biocatalysis: A ...
CH06CH12-Wu

ARI

2 July 2015

ANNUAL REVIEWS

6:57

Further

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

Click here for quick links to Annual Reviews content online, including: • Other articles in this volume • Top cited articles • Top downloaded articles • Our comprehensive search

Thermoelectric Properties of Solution Synthesized Nanostructured Materials Scott W. Finefrock,1 Haoran Yang,1 Haiyu Fang,1 and Yue Wu2 1 School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907; email: sfi[email protected], [email protected], [email protected] 2 Department of Chemical and Biological Engineering, Iowa State University, Ames, Iowa 50010; email: [email protected]

Annu. Rev. Chem. Biomol. Eng. 2015. 6:247–66

Keywords

First published online as a Review in Advance on April 24, 2015

ZT, nanotechnology, bottom-up, nanocomposite, chalcogenides, wet-chemistry

The Annual Review of Chemical and Biomolecular Engineering is online at chembioeng.annualreviews.org This article’s doi: 10.1146/annurev-chembioeng-061114-123348 c 2015 by Annual Reviews. Copyright  All rights reserved

Abstract Thermoelectric nanocomposites made by solution synthesis and compression of nanostructured chalcogenides could potentially be low-cost, scalable alternatives to traditional solid-state synthesized materials. We review the progress in this field by comparing the power factor and/or the thermoelectric figure of merit, ZT, of four classes of materials: (Bi,Sb)2 (Te,Se)3 , PbTe, ternary and quaternary copper chalcogenides, and silver chalcogenides. We also discuss the thermal conductivity reduction associated with multiphased nanocomposites. The ZT of the best solution synthesized materials are, in several cases, shown to be equal to or greater than the corresponding bulk materials despite the generally reduced mobility associated with solution synthesized nanocomposites. For the solution synthesized materials with the highest performance, the synthesis and processing conditions are summarized to provide guidance for future work.

247

CH06CH12-Wu

ARI

2 July 2015

6:57

INTRODUCTION

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

Considerable effort toward the generation of electricity from sustainable sources has been made recently. Thermoelectric generators could play niche roles by using the Seebeck effect to convert waste heat into electricity (1, 2). Meanwhile, thermoelectric coolers using the Peltier effect could potentially replace compressor-based refrigerators, thereby reducing the use of hydrochlorofluorocarbon refrigerants (1, 2). The efficiency of these devices is governed largely by the properties of the materials used. Analysis and optimization of a thermoelectric device led to the dimensionless number, ZT, as a convenient figure of merit by which to evaluate a material’s performance. The figure of merit is given by ZT = S2 σ T/κ, where S is the Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is absolute temperature (3). The power factor (S2 σ ) is then a useful quantity for comparing electrical properties of thermoelectric materials. Amid much optimism about thermoelectrics in the 1950s and 1960s, several general criteria for good thermoelectric materials were established: (a) They should be semiconductors with band gaps of ∼4 kB T, where kB is the Boltzmann constant and T is the intended operating temperature; (b) they should be comprised of heavy elements; (c) they should be solid solutions with disorder on the anion site and cation site for n-type and p-type materials, respectively; and (d ) they should have low effective mass if acoustic phonon scattering dominates conduction and high effective mass if ionized impurity scattering dominates conduction (4). Given these criteria, several materials, including (Bi,Sb)2 (Te,Se)3 , PbTe, AgSbTe2 , alloys of these materials, and others, were found to have ZT values of approximately one at various temperatures (4). One strategy to increase the ZT of these materials is to employ boundary scattering to reduce thermal conductivity. Although this was suggested in 1968, it was not employed extensively until after 1993, when Hicks & Dresselhaus predicted that one- and two-dimensional materials could possess significantly increased ZT values owing to boundary scattering as well as increased power factor owing to quantum confinement (5–7). Whereas the 1993 predictions were based on truly confined one- and two-dimensional materials, research has expanded to the area of bulk materials with nanoscale grains, which are often referred to as nanocomposites. In principal, grain boundaries in these materials could scatter heat-carrying phonons more than charge carriers, thus leading to improved ZT (8). Nanocomposites could potentially benefit from the phenomenon of energy filtering, in which the Seebeck coefficient is enhanced owing to the preferential scattering of low energy charge carriers (9). Whereas the effectiveness of energy filtering in nanocomposites is under debate (10), the use of boundary scattering of phonons to increase ZT has become widely accepted (11). To incorporate a high density of grain boundaries, several methods are employed. Crystal growth from the melt, which was the only thermoelectric materials synthesis method to receive widespread use from the 1950s through the 1980s (12), has been employed recently to form nanocomposites with embedded nanoprecipitates of a secondary phase (11). Ball milling of ingots followed by high-temperature compression has recently been used to create high-performance nanocomposites of many materials (13). In the past fifteen years, solution synthesis has emerged as another route to make nanocomposite thermoelectric materials. Benefits of solution synthesis methods include low synthesis temperatures, short synthesis times, and compatibility with largescale chemical synthesis practices in industry. Despite these potential benefits, solution synthesis methods receive little attention in reviews of thermoelectric materials (11, 13, 14). Therefore, this review focuses on thermoelectric materials synthesized in solution and shows that their thermoelectric properties are sometimes comparable or even superior to solid-state synthesized materials. The preparation of solution synthesized materials generally entails three steps: (a) solution synthesis, (b) purification of the product, and (c) consolidation or film coating. A review of the

248

Finefrock et al.

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

CH06CH12-Wu

ARI

2 July 2015

6:57

methods for synthesizing thermoelectric materials in solution has been published elsewhere (15). A few methods include synthesis in boiling or sub-boiling solutions of water and/or ethylene glycol at atmospheric pressure (16, 17), hydro- and solvothermal synthesis (18, 19), microwave synthesis (20), and ligand-based synthesis in a long-chain hydrocarbon-based solvent (21). After the synthesis step, the material is washed, generally by adding an antisolvent and centrifuging to cause the solid product to precipitate, redispersing the solid in a solvent, and repeating the process as necessary to obtain pure solid product. Often, the material is washed with hydrazine, which is known to remove electrically insulating ligands (22). Finally, the purified material is formed into a measurable geometry by hot pressing, spark plasma sintering, or cold pressing and annealing. These methods form millimeter-scale discs that could, in principal, be diced into pillars suitable for the traditional thermoelectric device architecture (14). In some cases, solution processable materials are cast into films that would be more suitable for thin film thermoelectrics (23). The thermoelectric properties of the materials can then be measured using a variety of methods discussed elsewhere (24, 25). In the remaining sections of this review, five classes of solution synthesized materials are discussed. The electronic properties of each class of materials are evaluated based on plots of the Seebeck coefficient versus electrical conductivity near room temperature. Plots of ZT versus temperature are employed to compare the overall performance of several of the best materials. The review emphasizes comparison with high-performance bulk single-crystal and large-grained polycrystalline materials. Within each section, the methods involved in creating the highest performance solution synthesized materials are discussed to better understand the key factors in obtaining high-ZT materials. The review concludes with a summary of the field and recommendations for future work.

(Bi,Sb)2 (Te,Se)3 Bismuth telluride has been widely studied since the 1950s and has excellent thermoelectric properties in the temperature range of 200–450 K (26, 27). Bismuth telluride has a low thermal conductivity of ∼1.4 W/mK at 300 K and a sufficiently high mobility such that bulk ingots obtained by traditional solid state routes possess ZT values in the range of 0.9–1.1 (14, 26). Binary Bi2 Te3 can be p-type owing to Bi vacancies or BiTe antisite defects. Alternatively, it can be n-type owing to Te vacancies or TeBi antisite defects (28, 29). Alloying with Sb is an effective way to ensure p-type behavior, whereas alloying with Se tends to lead to n-type behavior (30). The carrier concentration can be adjusted using the fraction of Sb and Se (31, 32). The solution synthesis of (Bi,Sb)2 (Te,Se)3 nanostructures began at least as early as 2003 (33). Research on the thermoelectric properties of bulk nanocomposites made by solution synthesized nanoscale powder has progressed quickly since then. In Figure 1a, the near–room temperature Seebeck coefficient versus the near–room temperature electrical conductivity of p-type solution synthesized (Bi,Sb)2 (Te,Se)3 and bulk single crystalline (Bi,Sb)2 (Te,Se)3 is shown along lines of equal power factor (18, 20, 21, 34–52). Immediately apparent is the wide range of Seebeck coefficients observed in the various materials, which is indicative of a wide range of carrier concentrations (31). For (Bi,Sb)2 (Te,Se)3 to exhibit high ZT, the Seebeck coefficient at 300 K should be in the range of 120–290 μV/K, corresponding to carrier concentrations of 4 × 1018 –7 × 1019 /cm3 (26, 31). Therefore, more than half of the samples represented in Figure 1a appear to have carrier concentrations that are within the optimal range. The figure also shows that only one of the solution synthesized samples has a power factor equal to that of the single crystals. This is likely related to the reduced mobility associated with small grains and porosity in the solution synthesized samples. Still, several www.annualreviews.org • Nanostructured Materials

249

CH06CH12-Wu

ARI

2 July 2015

6:57

a

b 2.0

300

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

S (µV K–1)

200

150

42 37

43

100

Ref. 14 Ref. 3 Ref. 20 Ref. 20 Ref. 18 Ref. 21 Ref. 21 Ref. 48 Ref. 41

1.5 18

50 21 49 43 36 35

45 44

47 48 34 41 38 20 40

44 51

39 21

46 46 46

ZT

250

3 mW/mK 2 1 mW/mK 2 0.1 mW/mK 2 0.01 mW/mK 2 Bulk (Ref. 52) Solution synthesized

1.0

20 46 0.5

50

0

1,000

10,000

σ (S m–1)

100,000

0.0 100

200

300

400

500

600

T (K)

Figure 1 (a) Seebeck coefficient versus electrical conductivity near room temperature for p-type (Bi,Sb)2 (Te,Se)3 . (b) ZT versus temperature for solution synthesized (symbols) and bulk (lines) p-type (Bi,Sb)2 (Te,Se)3 .

solution synthesized materials possess power factors of nearly 3 mW/mK2 , which, combined with low thermal conductivity owing to nanoscale grains, leads to high ZT values. In Figure 1b, the ZT versus temperature for seven of the best solution synthesized ptype (Bi,Sb)2 (Te,Se)3 materials is plotted along with that of two solid-state synthesized p-type (Bi,Sb)2 (Te,Se)3 samples without intentional nanostructuring (3, 14, 18, 20, 21, 41, 48). Clearly, solution synthesized (Bi,Sb)2 (Te,Se)3 can exhibit excellent performance, even exceeding that of bulk (Bi,Sb)2 (Te,Se)3 . The seven high-ZT solution synthesized materials share a few common themes. First, all incorporate Sb. Second, all preclude the presence of electrically insulating large molecule surfactants in the final nanocomposites by either a surfactant removal process involving hydrazine or the use of small molecule structure–directing agents instead of large molecules. Third, nearly all involve a postsynthesis heat treatment at 300◦ C or higher. Fourth, nearly all final materials have relative densities above 90%. The seven solution synthesized materials differ greatly with regard to their synthesis and compression methods. Of the seven materials, four are made by hydrothermal synthesis (18, 41, 48), two are made by microwave synthesis (20), and one is made by moderate temperature synthesis involving long-chain hydrocarbon-based solvents and ligands (21). Regarding compression methods, four are cold pressed and annealed (20, 48), two are hot pressed (18, 21), and one is spark plasma sintered (41). Figure 2a shows the near–room temperature electronic properties of n-type solution synthesized (Bi,Sb)2 (Te,Se)3 and large-grained (Bi,Sb)2 (Te,Se)3 grown by a traditional solid-state method (16, 17, 19, 20, 37, 38, 40, 43, 50, 53–79). For n-type (Bi,Sb)2 (Te,Se)3 , the highest ZT values are achieved by materials with carrier concentrations in the range of 3 × 1018 –9 × 1019 /cm3 , which are associated with Seebeck coefficients in the range of 100–290 μV/K at 300 K (26, 32). Most of the 53 samples represented in Figure 2a have Seebeck coefficients in the appropriate range. As with p-type (Bi,Sb)2 (Te,Se)3 , only one solution synthesized n-type sample has a power factor 250

Finefrock et al.

CH06CH12-Wu

ARI

2 July 2015

6:57

a

b 1.2

–300 mW/mK 2

–250

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

S (µV K–1)

–200

1.0 62 58

55

–150 75 –100

–50

1,000

60

72

64 76 20 74 19 64 54 50 61 76 65 16 50 37 68 54 53 69 70 79 67 73 78 59 50 78 68 58 58 50 79 63 20 58 38 56 17 71 43 40

57

0

66

72 53

10,000

σ (S m–1)

100,000

0.8

ZT

3 1 mW/mK 2 0.1 mW/mK 2 0.01 mW/mK 2 Bulk (Ref. 77) Solution synthesized

0.4

Ref. 3 Ref. 14 Ref. 20 Ref. 76 Ref. 76 Ref. 74 Ref. 69 Ref. 68 Ref. 68 Ref. 16 Ref. 61 Ref. 67 Ref. 50

0.2

0.0 100

200

300

400

500

600

T (K)

Figure 2 (a) Seebeck coefficient versus electrical conductivity near room temperature for n-type (Bi,Sb)2 (Te,Se)3 . (b) ZT versus temperature for solution synthesized (symbols) and bulk (lines) n-type (Bi,Sb)2 (Te,Se)3 .

equal to that of bulk (Bi,Sb)2 (Te,Se)3 . All others have power factors less than 3 mW/mK2 , likely a result of reduced mobility. However, some power factors are very nearly equal to 3 mW/mK2 , leading to good ZT values. Figure 2b shows the temperature dependence of ZT for 11 of the best solution synthesized n-type (Bi,Sb)2 (Te,Se)3 samples and 2 bulk reference samples (3, 14, 16, 20, 50, 61, 67–69, 74, 76). As with p-type (Bi,Sb)2 (Te,Se)3 , the performance of n-type solution synthesized (Bi,Sb)2 (Te,Se)3 can be as good as and even better than that of bulk (Bi,Sb)2 (Te,Se)3 synthesized by solid-state methods. The 11 high-ZT solution synthesized materials shown here share two common themes. First, all either are treated with a hydrazine-based surfactant removal step or are synthesized using small-molecule structure directing agents instead of large molecule surfactants. Second, all 11 samples are treated at 250◦ C or higher at some point after the solution synthesis step. In other ways, the materials are quite different. Some but not all of the materials incorporate Se. With regard to the solution synthesis, seven samples are synthesized in boiling or near-boiling water or ethylene glycol at atmospheric pressure (16, 50, 61, 67, 68, 76), two are synthesized by coprecipitation of Bi and Te oxides in water at room temperature followed by calcination and high-temperature reduction (69), one is synthesized at moderate temperature with a long-chain hydrocarbon solvent and ligands (74), and one is made by microwave synthesis (20). Regarding compression methods, seven are spark plasma sintered (16, 61, 68, 69, 74, 76), three are hot pressed (50, 67), and one is cold pressed and annealed (20).

Pbte PbTe is one of the best thermoelectric materials with peak performance at 600–950 K (80). It was investigated thoroughly in the 1950s and 1960s, but its maximum ZT was understated for www.annualreviews.org • Nanostructured Materials

251

CH06CH12-Wu

ARI

2 July 2015

6:57

a

b 350

300

87 86 89 86 87 87

1.4

90 250

Ref. 82 Ref. 86 Ref. 87 Ref. 88 Ref. 89 Ref. 90

1.2 90

90

1.0

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

200

88

ZT

S (µV K–1)

88

150

100

50

0.6

3 mW/mK 2 1 mW/mK 2 0.1 mW/mK 2 0.01 mW/mK 2 Bulk (Ref. 82, 91, 92) Solution synthesized

0 1,000

10,000

0.4 0.2

100,000

σ (S m–1)

0.8

1,000,000

0.0

300

400

500

600

700

800

T (K)

Figure 3 (a) Seebeck coefficient versus electrical conductivity near room temperature for p-type PbTe. (b) ZT versus temperature for solution synthesized (symbols) and bulk (lines) p-type PbTe.

many years owing to inaccurate estimation of κ at high temperature (81). Recently, the ZT was reevaluated and found to be ∼1.4 at ∼750 K for both p-type and n-type bulk polycrystalline PbTe (82, 83). Being composed of heavy elements, PbTe benefits from a low lattice thermal conductivity of 1.5–2.3 W/mK at 300 K (82, 83). Highly doped p-type PbTe benefits from the presence of two valence bands, which converge at high temperature to result in a high Seebeck coefficient even for materials with carrier concentrations of ∼1 × 1020 /cm3 (82). Meanwhile, n-type PbTe benefits from high electron mobility (83). PbTe is stable in a very small range of compositions (84). Therefore, whereas excess Te leads to p-type behavior, dopants such as Na, K, or Tl are required to achieve optimized carrier concentrations of ∼1 × 1020 /cm3 . Similarly, whereas excess Pb leads to n-type behavior, dopants such as I, La, or Bi are required to achieve optimized carrier concentrations of ∼2–5 × 1019 /cm3 (81). Solution synthesis of PbTe nanostructures dates back to at least 2000 (85). Over the following years, many methods for the solution synthesis of PbTe nanostructures were published. To our best knowledge, the first thorough investigation of the thermoelectric properties of solution synthesized PbTe was completed by Nolas and published in 2007 (86). Several reports of the thermoelectric properties of solution synthesized PbTe have followed, which we summarize here. Figure 3a presents the electronic properties of p-type solution synthesized PbTe at or near 300 K (86–90). Also shown are the properties of several Na-doped single-crystalline and large-grained polycrystalline bulk PbTe samples and lines of equal power factor (82, 91, 92). This graph reveals several important trends. First, the vast majority of materials made by solution synthesis have electrical conductivities less than 10,000 siemens/m, whereas highly doped bulk samples possess electrical conductivities in excess of 200,000 S/m. Second, nearly all materials made by solution synthesis have Seebeck coefficients greater than 200 μV/K, and doping during solid-state synthesis allows the Seebeck coefficient to be tuned from ∼250 down to ∼50 μV/K. 252

Finefrock et al.

ARI

2 July 2015

6:57

Third, all solution synthesized materials have power factors that are lower than those of Na-doped large-grained PbTe. The optimal range of carrier concentration for bulk p-type PbTe is associated with room temperature Seebeck coefficients in the range of 50–65 μV/K (80, 82). Therefore, the Seebeck coefficient data for solution synthesized p-type PbTe suggest that their carrier concentrations are far below the optimal range. Clearly, improved doping strategies are necessary. Figure 3b shows the thermoelectric figure of merit versus temperature for the highestperformance solution synthesized p-type PbTe along with Na-doped bulk polycrystalline PbTe optimized for maximum ZT at temperatures above 600 K (82, 86–90). At 300 K, several solution synthesized materials possess a higher ZT than bulk PbTe at the same temperature. The improvement is due in part to a reduced thermal conductivity; however, much of this reduction is associated with the electronic component. Still, in a few cases, there is also a reduction in the lattice thermal conductivity by 20–50% compared with bulk values owing to fine-grain structures and porosity (90). Unfortunately, the ZT of most of the solution synthesized materials has not been measured at high temperature. Our group performed measurements up to 400 K on spark plasma sintered PbTe nanowires and observed a maximum ZT of 0.33 at 350 K. Although this is higher than ZT of highly doped PbTe at the same temperature, the doping level in our material achieved by excess Te is insufficient to suppress minority carrier contributions and achieve high ZT above 600 K. This further highlights the need to develop effective p-type doping methods for solution synthesized PbTe. Figure 4a shows the electronic properties of n-type solution synthesized PbTe at or near 300 K (51, 93–95). Properties of bulk n-type PbTe synthesized by solid-state methods are shown along with lines of equal power factor for comparison (83). The trends illustrated are similar to those of p-type PbTe. Solution synthesized n-type PbTe tends to have significantly lower electrical conductivity, a higher Seebeck coefficient, and a lower power factor compared with solid-state

b

a –350

1.4

95

Ref. 83 Ref. 93 Ref. 93 Ref. 93 Ref. 93 Ref. 51 Ref. 94

–300 1.2 –250 1.0 –200

–150

–100

–50

0 10

94 94

3 mW/mK 2 1 mW/mK 2 0.1 mW/mK 2 0.01 mW/mK 2 Bulk (Ref. 83) Solution synthesized 100

93 93 94 93 93 94

ZT

S (µV K–1)

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

CH06CH12-Wu

0.8 0.6 0.4

51 0.2

1,000

10,000

σ (S m–1)

100,000

1,000,000

0.0

300

400

500

600

700

800

T (K)

Figure 4 (a) Seebeck coefficient versus electrical conductivity near room temperature for n-type PbTe. (b) ZT versus temperature for solution synthesized (symbols) and bulk (lines) n-type PbTe. www.annualreviews.org • Nanostructured Materials

253

ARI

2 July 2015

6:57

synthesized PbTe. The optimal carrier concentration is associated with Seebeck coefficients in the range of 40–75 μV/K at 300 K (83). Thus, n-type solution synthesized PbTe appears to exhibit insufficient doping even though several of the materials represented in Figure 4a are doped with Bi. Figure 4b shows the thermoelectric figure of merit versus temperature for the solution synthesized n-type PbTe with the highest performance along with I-doped polycrystalline PbTe optimized for maximum ZT at temperatures above 600 K (51, 83, 93, 94). The performance of Cao’s solution synthesized undoped PbTe nearly parallels that of bulk PbTe up to 575 K (93). Although these results are impressive, a close inspection of the thermoelectric properties of the material reveals some peculiar phenomena. For example, the electrical conductivity of the best material increases slightly with temperature from 300–575 K, which is quite unlike the drastic decrease observed in bulk PbTe (83). Also, the Seebeck coefficients at 300 K suggest carrier concentrations of 2 × 1018 –5 × 1018 /cm3 , but at the temperatures employed in their synthesis and consolidation steps, the PbTe phase diagram predicts carrier concentrations of less than 2 × 1017 /cm3 (84). Such deviations from expectation require significant additional experimentation to both verify and better explain the results. To summarize, some progress has been made in the area of solution synthesized p- and n-type PbTe thermoelectrics, but the lack of effective doping strategies has prevented the realization of high ZT at temperatures above 600 K where PbTe is best suited.

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

CH06CH12-Wu

TERNARY AND QUATERNARY COPPER CHALCOGENIDES During the past few years, ternary and quaternary copper chalcogenides have received attention as promising thermoelectric materials that are composed of earth-abundant elements. Materials of interest, such as Cu2 SnSe3 (CTSe), Cu2 ZnSnSe4 , Cu2 ZnGeSe4 (CZGSe), Cu2 CdSnSe4 (CCTSe), and Cu2 HgSnSe4 , have much larger band gaps than typical thermoelectric materials, but their complex crystal structures lead to low lattice thermal conductivities and moderate ZT values in the temperature range of 600–850 K (96, 97). Stoichiometric compounds tend to have unsuitably low electrical conductivities, but doping with In, Ga, or excess Cu leads to an increase in electrical conductivity by as much as two orders of magnitude and often a simultaneous decrease in total thermal conductivity owing to increased disorder within the crystal lattice (96, 97). Therefore, highly doped materials tend to have the largest ZT values. In some cases, loading too much excess Cu leads to phase segregation of CuSe and Cu2−x Se (98). Figure 5a shows the near–room temperature electronic properties of several ternary and quaternary solution synthesized copper chalcogenides and three of the best corresponding bulk materials (96–111). All materials have positive Seebeck coefficients owing to copper excess and/or chalcogen deficiency. The near–room temperature electrical conductivities and power factors of the materials presented are significantly lower than those of low–band gap materials such as (Bi,Sb)2 (Te,Se)3 and doped PbTe, a phenomenon that is mostly due to low mobility. This agrees with the general trend of mobility having an inverse relationship with band gap (3). Within the group of CTSe materials, the In doping used in the bulk material clearly leads to higher electrical conductivities and lower Seebeck coefficients than are possible in the solution synthesized materials. However, in one case, the near–room temperature power factor observed in solution synthesized CTSe is actually larger than that of bulk CTSe (103). Regarding CCTSe and CZGSe, carrier concentration adjustment by excess Cu can be easily achieved in bulk materials by simply adjusting the ratio of elemental starting materials, as shown by the broad range of electrical conductivities and Seebeck coefficients observed. Interestingly, carrier concentration adjustment using excess Cu can also be achieved in solution synthesized nanomaterials. The Cabot group determined that during the synthesis, Cu-Se-type nanocrystals form first, followed by the addition of Cd, Zn, Sn, or Ge. Therefore, the amount of excess Cu 254

Finefrock et al.

CH06CH12-Wu

ARI

2 July 2015

6:57

a

b 250

1.2 111

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

150

100

50

0 10

1.0

1 mW/mK 0.1 mW/mK 2 0.01 mW/mK 2 102 103 Bulk CTSe (Ref. 97) Solution 102 synthesized CTSe 108 Bulk CCTSe 104 99 (Ref. 96) 101 106 102 Solution 105 synthesized CCTSe Bulk CZGSe 109 110 100 (Ref. 98) Solution synthesized CZGSe 99 107 Solution 102 synthesized other 100

1,000

σ (S m–1)

10,000

100,000

0.8

ZT

S (µV K–1)

200

Ref. 97 CTSe Ref. 105 CTSe Ref. 103 CTSe Ref. 96 CCTSe Ref. 101 CCTSe Ref. 102 CCTSe Ref. 98 CZGSe Ref. 99 CZGSe

0.6

0.4

0.2

0.0

300

400

500

600

700

800

900

T (K)

Figure 5 (a) Seebeck coefficient versus electrical conductivity near room temperature for ternary and quaternary copper chalcogenides. (b) ZT versus temperature for solution synthesized (symbols) and bulk (lines) ternary and quaternary copper chalcogenides.

can be conveniently controlled by adjusting the reaction time and temperature (99, 102). The success of this method is evidenced by the wide range of electrical conductivities and Seebeck coefficients observed in these solution synthesized materials. In the case of CCTSe, the solution synthesized materials possess power factors far below those of bulk CCTSe. Conversely, solution synthesized CZGSe samples have similar and even larger power factors than bulk CZGSe near room temperature. Figure 5b shows the ZT versus temperature for several of the best solution synthesized materials along with bulk materials with similar compositions (96–99, 101–103, 105). Interestingly, all bulk materials possess similar ZT values from room temperature to 700 K, with CCTSe possessing the best properties in that range. ZT values as high as 1.14 at 850 K have been observed in CTSe, but stable measurements at these temperatures require that samples are coated with glass to prevent Se sublimation (97). Such a coating would act as a thermal shunt in real devices. Despite the reduced power factor observed in some cases in Figure 5a, solution synthesized materials possess ZT values nearly equal to or greater than bulk counterparts owing to thermal conductivity reduction. A close inspection of the five papers on solution synthesized materials shown reveals several similarities and differences. All five materials are synthesized by reactions of dissolved Se with solutions of the appropriate cation precursors in the temperature range of 180–300◦ C in oleylamine, 1-octadecene, or a combination of the two. All five synthesis methods produce nanocrystals with sizes in the range of 5–25 nm, which experience some extent of grain growth during the subsequent hot pressing or annealing steps at 350–550◦ C. With regard to the issue of insulating ligands, three of the materials experience a thorough hydrazine washing in which nanocrystals that are initially dissolved in hexane are transferred to an 85% hydrazine phase with repeated hexane rinses. Meanwhile, two of the five materials are treated with repeated dissolution in chloroform and precipitation using isopropanol until the nanocrystals are no longer www.annualreviews.org • Nanostructured Materials

255

CH06CH12-Wu

ARI

2 July 2015

6:57

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

soluble in organic solvent; they are then annealed in Ar at 500◦ C. It is interesting that both the hydrazine-based and non-hydrazine-based methods yield high ZT values. A comparison of the two solution synthesized CCTSe materials with similar Seebeck coefficients reveals that the hydrazine wash followed by hot pressing leads to a room-temperature electrical conductivity that is ∼2.4 times greater than that of material that is not hydrazine washed but rather is annealed and cold pressed (101, 102). But, as the room-temperature thermal conductivity of the hydrazine-washed material is ∼3.2 times larger, the ZT values are quite similar. The necessity of using highly toxic hydrazine is therefore debatable for these materials; this topic deserves more investigation. In summary, the area of solution synthesized ternary and quaternary copper chalcogenides has experienced rapid progress in the past few years, and ZT values similar to bulk values have already been achieved.

Ag2 Te AND Ag2 Se Ag2 Te and Ag2 Se were both recognized as good n-type thermoelectric materials around 1960 owing to their high electron mobility and low lattice thermal conductivity (112, 113). Belonging to the silver chalcogenide family, the two compounds show similarity in many aspects. First, Ag2 Te and Ag2 Se undergo a phase change at 418 K and 406 K, respectively, from low-symmetric phases to cubic phases (114, 115). Second, both Ag2 Te and Ag2 Se are narrow–band gap semiconductors (Eg = 0.05 eV for Ag2 Te and Eg = 0.075 eV for Ag2 Se) with electron mobilities as high as 105 cm2 /Vs in the low-temperature phases (112, 116). Third, silver is highly mobile in the two compounds, which results in the low lattice thermal conductivity (approaching as low as 0.1 W/mK for both Ag2 Te and Ag2 Se near room temperature) (114, 115). Fourth, the cubic phases of Ag2 Te and Ag2 Se show superionic conduction (114, 117). Fifth, the carrier density in Ag2 Te and Ag2 Se is commonly controlled by tuning the composition; excess Ag acts as an electron donor, whereas excess Te or Se acts as an electron acceptor (112, 113). The highest ZT values reported are 0.64 in the range of 575–650 K for Ag2 Te (114, 118) and 0.99 at room temperature for Ag2 Se (119). The high-temperature performance for these compounds deteriorates owing to thermally excited minority carriers, especially for Ag2 Se, which exhibits metallic transport above the phase transition (113). Solution synthesis of A2 Te and Ag2 Se nanostructures dates back to at least 2001 and 1996, respectively (120, 121). Since then, various nanostructures, such as nanocrystals, nanowires, and dendrites, from various methods have been reported. However, only a handful of studies on their thermoelectric properties have been published. The near–room temperature electrical properties of some solution synthesized n-type Ag2 Te and Ag2 Se nanostructures are plotted along with the best bulk data in Figure 6a (112, 116–119, 122–127, 129). Bulk Ag2 Te and Ag2 Se clearly exhibit the highest room temperature power factor and generally have electrical conductivities above 100,000 S/m. Meanwhile, the electrical conductivity of the solution synthesized nanostructures is often at least one order lower. In only a few cases have Ag2 Te nanocrystal-based nanocomposites shown electrical conductivities above 100,000 S/m and power factors that approach the best bulk values (122, 123). The Seebeck coefficient versus carrier concentration curves for Ag2 Te and Ag2 Se are not well established. Also, in most cases the carrier density and mobility data are not reported. So it is not possible to generally conclude whether the low electrical conductivity of the Ag2 Te and Ag2 Se nanostructures is associated with low carrier density or low carrier mobility. Our group created nanocomposites of hot-pressed Ag2 Te nanowires, which exhibit a mobility of 1,550 cm2 /Vs (124). Meanwhile, a bulk sample with a similar carrier concentration possessed a mobility as high as 4,000 cm2 /Vs (114). Zhou (125) reported an even lower electron mobility of 13–199 cm2 /Vs for 256

Finefrock et al.

CH06CH12-Wu

ARI

2 July 2015

6:57

a

b 1.2 –150

127 117

Ref. 114 Ag2Te Ref. 124 Ag2Te Ref. 125 Ag2Te Ref. 122 Ag2Te Ref. 119 Ag2Se Ref. 129 Ag2Se Ref. 117 Ag2Se

127 1.0

–125

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

125 125 –75

–50

–25

0 10,000

0.8

124 122 125 3 mW/mK 1 mW/mK 2 0.1 mW/mK 2 Bulk Ag2Te (Ref. 112, 118) Solution synthesized Ag2Te Bulk Ag2Se (Ref. 116, 119, 129) Solution synthesized Ag2Se

ZT

S (µV K–1)

–100

126

0.6

123 0.4 126

100,000

σ (S m–1)

0.2

0.0

100

200

300

400

500

600

T (K)

Figure 6 (a) Seebeck coefficient versus electrical conductivity near room temperature for Ag2 Te and Ag2 Se. (b) ZT versus temperature for solution synthesized (symbols) and bulk (lines) Ag2 Te and Ag2 Se.

sulfur-doped Ag2 Te nanocrystals, albeit at higher carrier concentration. The deteriorated electron mobility in these cases might be related to surface properties of the nanostructures or possibly the sintering techniques. Another issue for nanostructured Ag2 Te and Ag2 Se is the lattice thermal conductivity. In bulk Ag2 Te and Ag2 Se, the lattice thermal conductivities are already as low as 0.1–0.4 W/mK and 0.2–0.6 W/mK, respectively, in the temperature range of 300–400 K (114, 115, 128, 129). Thus, there is little room for the improvement that is generally associated with phonon scattering in nanostructured materials. Although some of the nanostructured Ag2 Te and Ag2 Se samples exhibit lower total thermal conductivity than the bulk counterparts, the reduction is primarily due to the reduced electrical contribution. Figure 6b shows the temperature dependence of ZT for several solution synthesized Ag2 Te and Ag2 Se samples along with bulk values (114, 117, 119, 122, 124, 125, 129). Although none of the solution synthesized nanostructures have ZT values that are consistently higher than those of bulk, the solution synthesized Ag2 Te samples have higher values at certain temperatures. At the temperatures of optimal ZT, the nanostructured samples of Ag2 Te exhibit electrical conductivities of 24,000–33,000 S/m (122, 125), which are close to the corresponding bulk value of 42,000 S/m (114), indicating that grain boundary scattering of electrons has a relatively weak effect as temperature increases. The slightly lower electrical conductivity is approximately compensated by the slightly lower total thermal conductivity in the nanostructured samples, yielding comparable ZT values for the nanostructured and bulk samples of Ag2 Te. Meanwhile, there is room for improvement in the area of solution synthesized Ag2 Se. To our best knowledge, only one paper reported the full characterization of temperature-dependent thermoelectric properties, and the maximum ZT is 0.23 at 408 K, which is far from the best bulk value of ∼1 (117, 129).

www.annualreviews.org • Nanostructured Materials

257

700

CH06CH12-Wu

ARI

2 July 2015

6:57

MULTIPHASED NANOCOMPOSITES

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

Despite their complexity, multiphased nanocomposites have been pursued enthusiastically by researchers owing in part to the record-breaking ZT values of 2.2 reported for PbTe/SrTe and 2.4 reported for Bi2 Te3 /Sb2 Te3 superlattices (80, 130). The significant enhancement of ZT is mostly the result of the substantial thermal conductivity reduction caused by the lattice mismatch between the two phases and scattering of mid– to long–mean free path phonons by nanoinclusions in the matrix. Of equal importance to the enhancement of ZT is the retention of high charge carrier mobility owing to proper energy band alignment between the two phases (131). The common methods of making multiphased nanocomposites through solution synthesis can be divided into two major categories: physically blending two separately grown nanoparticles and directly solution synthesizing binary-phased heterostructures. In either case, the hybrid nanopowder can be consolidated into multiphase nanocomposites that usually possess much smaller thermal conductivity than the corresponding majority phase. Many solution synthesized multiphased nanocomposites are comprised of two traditional thermoelectric materials, such as Ag2 Te/PbTe (132), but less traditional materials have also been included, as in PbTe/PtTe2 (133). Table 1 summarizes the room-temperature lattice thermal conductivity comparison between multiphased and single-phase materials (90, 124, 134–139). Overall, a 20–60% reduction is observed in multiphased nanocomposites, which rivals the ∼30% reduction observed in PbTe/SrTe (80). The relative densities of the multiphased and single-phase materials are also shown in Table 1 because porosity significantly reduces lattice thermal conductivity. In Table 1, most pairs of similar materials have nearly equal relative density. In those cases, the reduction in lattice thermal conductivity of the material with an additional phase is likely associated with the additional phase and not with a difference in porosity. Still, the porosity brings another issue: a reduction in electrical conductivity (136, 137, 140, 141). Because porosity can be caused by retained surfactants (140, 141), several groups have developed surfactant-free syntheses, which lead to relative densities of 85–93% (72, 142). Another group addressed the issue of porosity by postannealing solution synthesized nanopowder to obtain a relative density of 97% (138). Table 1 Lattice thermal conductivity comparison between multiphased nanocomposite and corresponding matrices fabricated with the same method Lattice thermal conductivity (W/m-K)

Relative density (%)

Bi2 Te3

0.40

97

Bi2 Te3 /CNT

0.28

96

Sb2 Te3 -Te

0.85



Ag/oxide/Sb2 Te3 -Te

0.65



Material

258

PbS

1.20

80

(PbS)0.72 /(PbTe)0.28

0.69

80

Reference 138 134 135

PbTe

0.94

85

90

(PbTe)0.96 /(Bi2 Te3 )0.04

0.55

76

136

PbTe

1.90



PbTe/3 wt.% graphene

0.81



139

Ag2 Te

0.3

89

124

(Ag2 Te)0.95 /(Bi2 Te3 )0.05

0.24

87

137

Finefrock et al.

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

CH06CH12-Wu

ARI

2 July 2015

6:57

One important aspect still lacking in solution synthesized binary-phase nanocomposites is the optimization of carrier concentration. To the best of our knowledge, a systematic study on the optimization of carrier concentration in solution synthesized multiphased nanocomposites cannot be found in the literature. When carrier concentrations are reported, they are often more than one order of magnitude different from the appropriate range of the matrix material (44, 141, 143). The situation is complicated in some cases by the doping of one phase by the other. For example, PtTe2 acts as an n-type dopant in PbTe and Ag acts as a p-type dopant in Bi2 Te3 (44, 133). Despite this challenge, future research on multiphased composites should address carrier concentration optimization; this will ultimately lead to higher figures of merit. Although there are many challenges and uncertainties in the area of solution synthesized multiphased nanocomposites, these materials represent some of the more convincing cases of the energy filtering effect. Ko and coworkers (143) fabricated Pt/Sb2 Te3 nanocrystal films that show a simultaneous increase in Seebeck coefficient and carrier concentration compared with Sb2 Te3 nanocrystal films, which could be attributed to the energy filtering effect. Zhang and coworkers (134) developed a Ag/Sb2 Te3 -Te composite in which a thin oxide layer minimized Ag diffusion into the Sb2 Te3 -Te matrix. As a result, the Seebeck coefficient of Ag/oxide/Sb2 Te3 -Te was improved compared with the same matrix at a lower carrier concentration. This result was interpreted using theoretical curves of the Seebeck coefficient versus carrier concentration calculated using different energy filtering potentials. Overall, it appears that the field of solution synthesized multiphased nanocomposites is still in its initial stage; there is much room for improvement. Future efforts should focus on optimization of surfactant removal, consolidation, and carrier concentration, as well as on the development of new methods to take advantage of the potential benefits of the energy filtering effect.

CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK Remarkable milestones have been achieved in the solution synthesis of thermoelectric materials in the past 10 to 15 years. In particular, solution synthesized p-type and n-type (Bi,Sb)2 (Te,Se)3 , ternary and quaternary copper chalcogenides, and Ag2 Te have reached and in some cases even surpassed the ZT of the corresponding large-grained bulk materials. For these materials that have achieved high performance, future research should begin to consider other important variables, such as mechanical properties, thermal cycling stability, and overall synthesis cost, to more thoroughly prove that thermoelectric devices made using these materials are commercially viable. In the case of PbTe, significant progress has been made, but effective doping methods and high ZT at high temperature have yet to be demonstrated. In light of the accomplishments in this research area, it is important to note that the electronic properties of the high-ZT materials were measured using a variety of instruments, including ULVAC-Riko ZEM2 and ZEM3, Harman systems, several different home-built systems, and others. Thermal properties were often measured using laser flash systems and differential scanning calorimetry, but transient plane source and several home-built systems were used in some cases. This is a concern because recent round-robin studies on identical samples showed significantly different results obtained for the same material using different instruments from different research groups (24, 25). Furthermore, many studies of disc samples involved in-plane electrical properties measurements and cross-plane thermal diffusivity measurements, and the results were used to calculate ZT without first proving that the discs were isotropic. This could result in significant overestimation, such as erroneous ZT values as high as 1.9 (144). The concerns of the verification of measurement instrument accuracy and sample isotropy should be addressed in future work on www.annualreviews.org • Nanostructured Materials

259

CH06CH12-Wu

ARI

2 July 2015

6:57

solution synthesized thermoelectric materials. The recent publication by the Ramanath group (20) provides an excellent example of such research practices. SUMMARY POINTS 1. In a few cases, solution synthesized p-type and n-type (Bi,Sb)2 (Te,Se)3 materials possess ZT values equal to or greater than those of bulk (Bi,Sb)2 (Te,Se)3 synthesized by solidstate methods. 2. Solution synthesized PbTe is relatively underexplored and suffers from insufficient carrier concentration, which leads to low ZT at high temperatures. Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

3. Research on solution synthesized ternary and quaternary copper chalcogenides has expanded rapidly and has achieved ZT values essentially equal to those of corresponding bulk materials. 4. Solution synthesized Ag2 Te quickly achieved similar performance to bulk pure-phase Ag2 Te, but the already low lattice thermal conductivity of the bulk material could perhaps limit further improvement. 5. Multiphased nanocomposites are complex; thus, although there appear to be several examples of lattice thermal conductivity reduction in these materials, improvement in ZT will likely require optimization of relative density, carrier concentration, and other variables. 6. High-temperature treatment is an essential postsynthesis step to create high ZT materials.

DISCLOSURE STATEMENT The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

ACKNOWLEDGMENTS This work is supported by the US Air Force Office of Scientific Research (award no. FA9550-121-0061). LITERATURE CITED 1. 2. 3. 4. 5. 6.

Joffe AF. 1958. The revival of thermoelectricity. Sci. Am. 199(5):31–37 Snyder GJ. 2008. Small thermoelectric generators. Interface 17(3):54–56 Goldsmid HJ. 2010. Introduction to Thermoelectricity. Heidelberg, Ger.: Springer Wood C. 1988. Materials for thermoelectric energy conversion. Rep. Prog. Phys. 51:459–539 Goldsmid HJ, Penn A. 1968. Boundary scattering of phonons in solid solutions. Phys. Lett. A 2(8):523–24 Hicks LD, Dresselhaus MS. 1993. Thermoelectric figure of merit of a one-dimensional conductor. Phys. Rev. B 47(24):16631 7. Hicks LD, Dresselhaus MS. 1993. Effect of quantum-well structures on the thermoelectric figure of merit. Phys. Rev. B 47(19):12727 8. Medlin DL, Snyder GJ. 2009. Interfaces in bulk thermoelectric materials. Curr. Opin. Colloid Interface Sci. 14(4):226–35 9. Zebarjadi M, Esfarjani K, Dresselhaus MS, Ren Z, Chen G. 2012. Perspectives on thermoelectrics: from fundamentals to device applications. Energy Environ. Sci. 5(1):5147–62

260

Finefrock et al.

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

CH06CH12-Wu

ARI

2 July 2015

6:57

10. Bachmann M, Czerner M, Heiliger C. 2012. Ineffectiveness of energy filtering at grain boundaries for thermoelectric materials. Phys. Rev. B 86(11):1–6 11. Sootsman JR, Chung DY, Kanatzidis MG. 2009. New and old concepts in thermoelectric materials. Angew. Chem. Int. Ed. 48(46):8616–39 12. Goldsmid HJ. 1986. Electronic Refrigeration. London: Pion 13. Lan Y, Minnich A, Chen G, Ren Z. 2010. Enhancement of thermoelectric figure-of-merit by a bulk nanostructuring approach. Adv. Funct. Mater. 20(3):357–76 14. Snyder GJ, Toberer ES. 2008. Complex thermoelectric materials. Nat. Mater. 7(2):105–14 15. Zhao Y, Dyck JS, Burda C. 2011. Toward high-performance nanostructured thermoelectric materials: the progress of bottom-up solution chemistry approaches. J. Mater. Chem. 21(43):17049 16. Li D, Qin XY, Liu YF, Wang NN, Song CJ, Sun RR. 2013. Improved thermoelectric properties for solution grown Bi2 Te3−x Sex nanoplatelet composites. RSC Adv. 3(8):2632 17. Finefrock SW, Fang H, Yang H, Darsono H, Wu Y. 2014. Large-scale solution-phase production of Bi2 Te3 and PbTe nanowires using Te nanowire templates. Nanoscale 6:7872–76 18. Chen Z, Lin MY, Xu GD, Chen S, Zhang JH, Wang MM. 2014. Hydrothermal synthesized nanostructure Bi–Sb–Te thermoelectric materials. J. Alloys Compd. 588:384–87 19. Data in Figure 2a adapted with perimission from Zhang Y, Hu L, Zhu T. 2013. High yield Bi2 Te3 single crystal nanosheets with uniform morphology via a solvothermal synthesis. Cryst. Growth Des. 13(2):645–51 20. Mehta RJ, Zhang Y, Karthik C, Singh B, Siegel RW, et al. 2012. A new class of doped nanobulk highfigure-of-merit thermoelectrics by scalable bottom-up assembly. Nat. Mater. 11(3):233–40 21. Gupta RP, Sharp J, Peng A, Perera S, Ballinger C, et al. 2012. Inorganic colloidal solution-based approach to nanocrystal synthesis of (Bi,Sb)2 Te3 . J. Electron. Mater. 41(6):1573–78 22. Talapin DV, Murray CB. 2005. PbSe nanocrystal solids for n- and p-channel thin film field-effect transistors. Science 310(5745):86–89 23. Chowdhury I, Prasher R, Lofgreen K, Chrysler G, Narasimhan S, et al. 2009. On-chip cooling by superlattice-based thin-film thermoelectrics. Nat. Nanotechnol. 4(April):235–38 24. Wang H, Porter WD, Bottner H, Konig J, Chen L, et al. 2013. Transport properties of bulk ¨ ¨ thermoelectrics—an international round-robin study, part i: Seebeck coefficient and electrical resistivity. J. Electron. Mater. 42(4):654–64 25. Wang H, Porter WD, Bottner H, Konig J, Chen L, et al. 2013. Transport properties of bulk ther¨ ¨ moelectrics: an international round-robin study, part ii: thermal diffusivity, specific heat, and thermal conductivity. J. Electron. Mater. 42(6):1073–84 26. Kutasov VA, Lukyanova LN, Vedernikov MV. 2006. Shifting the maximum figure-of-merit of (Bi, Sb)2(Te, Se)3 thermoelectrics to lower temperatures. In Thermoelectrics Handbook: Macro to Nano, ed. DM Rowe, pp. 37-1–37-18. Boca Raton, FL: Taylor & Francis 27. Heremans JP, Dresselhaus MS, Bell LE, Morelli DT. 2013. When thermoelectrics reached the nanoscale. Nat. Nanotechnol. 8(7):471–73 28. Oh MW, Son JH, Kim BS, Park SD, Min BK, Lee HW. 2014. Antisite defects in n-type Bi2 (Te,Se)3 : experimental and theoretical studies. J. Appl. Phys. 115(13):133706 29. Scanlon DO, King PDC, Singh RP, de la Torre A, Walker SM, et al. 2012. Controlling bulk conductivity in topological insulators: key role of anti-site defects. Adv. Mater. 24(16):2154–58 30. Scherrer H, Scherrer S. 2006. Thermoelectric properties of bismuth antimony telluride solid solutions. In Thermoelectrics Handbook: Macro to Nano, ed. DM Rowe, pp. 27-1–27-18. Boca Raton, FL: Taylor & Francis 31. Xie W, Wang S, Zhu S, He J, Tang X, et al. 2012. High performance Bi2 Te3 nanocomposites prepared by single-element-melt-spinning spark-plasma sintering. J. Mater. Sci. 48(7):2745–60 32. Wang S, Xie W, Li H, Tang X. 2011. Enhanced performances of melt spun Bi2 (Te,Se)3 for n-type thermoelectric legs. Intermetallics 19(7):1024–31 33. Ge J-P, Li Y-D. 2003. Ultrasonic synthesis of nanocrystals of metal selenides and tellurides. J. Mater. Chem. 13(4):911–15 www.annualreviews.org • Nanostructured Materials

Data in Figure 2a adapted with permission from Reference 19. Copyright 2013 American Chemical Society.

261

CH06CH12-Wu

ARI

2 July 2015

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

Data in Figure 1a adapted with permission from Reference 36. Copyright 2010 American Chemical Society.

Data in Figures 1a and 2a adapted with permission from Reference 37. Copyright 2010 American Chemical Society.

Data in Figures 1a and 2a adapted with permission from Reference 38. Copyright 2010 American Chemical Society. Data in Figures 1a and 2a adapted with permission from Reference 40. Copyright 2010 American Chemical Society.

Data in Figures 1a and 2a adapted with permission from Reference 43. Copyright 2012 American Chemical Society.

Data in Figure 1a adapted with permission Reference 44. Copyright 2012 American Chemical Society.

Data in Figure 1a adapted with permission from Reference 45. Copyright 2012 American Chemical Society.

262

6:57

34. Liu C-J, Liu G-J, Tsao C-W, Huang Y-J. 2009. Improvement of thermoelectric power factor of hydrothermally prepared Bi0.5 Sb1.5 Te3 compared with its solvothermally prepared counterpart. J. Electron. Mater. 38(7):1499–503 35. Dong G-H, Zhu Y-J, Chen L-D. 2010. Microwave-assisted rapid synthesis of Sb2 Te3 nanosheets and thermoelectric properties of bulk samples prepared by spark plasma sintering. J. Mater. Chem. 20(10):1976 36. Scheele M, Oeschler N, Veremchuk I, Reinsberg K, Kreuziger A, et al. 2010. ZT enhancement in solution-grown Sb(2-x)BixTe3 nanoplatelets. ACS Nano 4(7):4283–91 37. Zhao Y, Dyck JS, Hernandez BM, Burda C. 2010. Enhancing thermoelectric performance of ternary nanocrystals through adjusting carrier concentration. J. Am. Chem. Soc. 132(14):4982– 83 38. Zhao Y, Dyck JS, Hernandez BM, Burda C. 2010. Improving thermoelectric properties of chemically synthesized Bi2 Te3 -based nanocrystals by annealing. J. Phys. Chem. C 114(26):11607–13 39. Ren W, Cheng C, Ren Z, Zhong Y. 2010. The effect of the precursor nanopowder size on the thermoelectric properties of nanostructured Bi–Sb–Te bulk materials. Phys. B Condens. Matter. 405(24):4931– 36 40. Kovalenko MV, Spokoyny B, Lee J-S, Scheele M, Weber A, et al. 2010. Semiconductor nanocrystals functionalized with antimony telluride zintl ions for nanostructured thermoelectrics. J. Am. Chem. Soc. 132(19):6686–95 41. Zhang Y, Xu G, Mi J, Han F, Wang Z, Ge C. 2011. Hydrothermal synthesis and thermoelectric properties of nanostructured Bi0.5 Sb1.5 Te3 compounds. Mater. Res. Bull. 46(5):760–64 42. Liu C-J, Liu G-J, Liu Y-L, Chen L-R, Kaiser AB. 2011. Enhanced thermoelectric performance of compacted Bi0.5 Sb1.5 Te3 nanoplatelets with low thermal conductivity. J. Mater. Res. 26(15):1755–61 43. Ganguly S, Zhou C, Morelli D, Sakamoto J, Brock SL. 2012. Synthesis and characterization of telluride aerogels: effect of gelation on thermoelectric performance of Bi2 Te3 and Bi2−x Sbx Te3 nanostructures. J. Phys. Chem. C 116(33):17431–39 44. Zhang Y, Snedaker ML, Birkel CS, Mubeen S, Ji X, et al. 2012. Silver-based intermetallic heterostructures in Sb2 Te3 thick films with enhanced thermoelectric power factors. Nano Lett. 12(2):1075–80 45. Schulz S, Heimann S, Friedrich J, Engenhorst M, Schierning G, Assenmacher W. 2012. Synthesis of hexagonal Sb2 Te3 nanoplates by thermal decomposition of the single-source precursor (Et2 Sb)2 Te. Chem. Mater. 24(11):2228–34 46. Pelz U, Kaspar K, Schmidt S, Dold M, J¨agle M, et al. 2012. An aqueous-chemistry approach to nanobismuth telluride and nano-antimony telluride as thermoelectric materials. J. Electron. Mater. 41(6):1851– 57 47. Dyck JS, Mao B, Wang J, Dorroh S, Burda C. 2012. Effect of sintering on the thermoelectric transport properties of bulk nanostructured Bi0.5 Sb1.5 Te3 pellets prepared by chemical synthesis. J. Electron. Mater. 41(6):1408–13 48. Liu C-J, Lai H-C, Liu Y-L, Chen L-R. 2012. High thermoelectric figure-of-merit in p-type nanostructured (Bi,Sb)2 Te3 fabricated via hydrothermal synthesis and evacuated-and-encapsulated sintering. J. Mater. Chem. 22(11):4825 49. Sun S, Peng J, Jin R, Song S, Zhu P, Xing Y. 2013. Template-free solvothermal synthesis and enhanced thermoelectric performance of Sb2 Te3 nanosheets. J. Alloys Compd. 558:6–10 50. Lu Z, Tan LP, Zhao X, Layani M, Sun T, et al. 2013. Aqueous solution synthesis of (Sb, Bi)2 (Te, Se)3 nanocrystals with controllable composition and morphology. J. Mater. Chem. C 1(39):6271 51. Chai Z, Wang H, Suo Q, Wu N, Wang X, Wang C. 2014. Thermoelectric metal tellurides with nanotubular structures synthesized by the Kirkendall effect and their reduced thermal conductivities. Cryst. Eng. Comm. 16(17):3507 52. Testardi LR, Bierly JN, Donahoe FJ. 1962. Transport properties of p-type Bi2 Te3 -Sb2 Te3 alloys in the temperature range 80–370 k. J. Phys. Chem. Solids 23:1209–17 53. Ni HL, Zhu TJ, Zhao XB. 2005. Hydrothermally synthesized and hot-pressed Bi2 (Te,Se)3 thermoelectric alloys. Phys. B Condens. Matter 364(1–4):50–54 Finefrock et al.

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

CH06CH12-Wu

ARI

2 July 2015

6:57

54. Ni HL, Zhu TJ, Zhao XB. 2005. Thermoelectric properties of hydrothermally synthesized and hot pressed n-type Bi2 Te3 alloys with different contents of Te. Mater. Sci. Eng. B 117(2):119–22 55. Dirmyer MR, Martin J, Nolas GS, Sen A, Badding JV. 2009. Thermal and electrical conductivity of size-tuned bismuth telluride nanoparticles. Small 5(8):933–37 56. Scheele M, Oeschler N, Meier K, Kornowski A, Klinke C, Weller H. 2009. Synthesis and thermoelectric characterization of Bi2 Te3 nanoparticles. Adv. Funct. Mater. 19(21):3476–83 57. Mi J-L, Lock N, Sun T, Christensen M, Søndergaard M, et al. 2010. Biomolecule-assisted hydrothermal synthesis and self-assembly of Bi2 Te3 nanostring-cluster hierarchical structure. ACS Nano 4(5):2523–30 58. Wang RY, Feser JP, Gu X, Yu KM, Segalman RA, et al. 2010. Universal and solution-processable precursor to bismuth chalcogenide thermoelectrics. Chem. Mater. 22(6):1943–45 59. Sun Z, Liufu S, Chen L. 2010. Synthesis and characterization of nanostructured bismuth selenide thin films. Dalton Trans. 39(45):10883–87 60. Xu H, Chen G, Jin R, Chen D, Pei J, Wang Y. 2013. Electrical transport properties of microwavesynthesized Bi2 Se3−x Tex nanosheet. Cryst. Eng. Comm. 15(28):5626 61. Kim C, Kim DH, Han YS, Chung JS, Park S, et al. 2011. Development of bismuth tellurium selenide nanoparticles for thermoelectric applications via a chemical synthetic process. Mater. Res. Bull. 46(3):407– 12 62. Zhang G, Kirk B, Jauregui LA, Yang H, Xu X, et al. 2011. Rational synthesis of ultrathin n-type Bi2 Te3 nanowires with enhanced thermoelectric properties. Nano Lett. 12(1):56–60 63. Kim C, Kim DH, Han YS, Chung JS, Park S, Kim H. 2011. Fabrication of bismuth telluride nanoparticles using a chemical synthetic process and their thermoelectric evaluations. Powder Technol. 214(3):463–68 64. Kim C, Kim DH, Kim JS, Han YS, Chung JS, Kim H. 2011. A study of the synthesis of bismuth tellurium selenide nanocompounds and procedures for improving their thermoelectric performance. J. Alloys Compd. 509(39):9472–78 65. Fu J, Song S, Zhang X, Cao F, Zhou L, et al. 2012. Bi2 Te3 nanoplates and nanoflowers: synthesized by hydrothermal process and their enhanced thermoelectric properties. Cryst. Eng. Comm. 14(6):2159 66. Soni A, Yanyuan Z, Ligen Y, Khor M, Aik K, Dresselhaus MS. 2012. Enhanced thermoelectric properties of solution grown Bi2 Te3−x Sex nanoplatelet composites. Nano Lett. 12(3):1203–9 67. Zhang Y, Day T, Snedaker ML, Wang H, Kr¨amer S, et al. 2012. A mesoporous anisotropic n-type Bi2 Te3 monolith with low thermal conductivity as an efficient thermoelectric material. Adv. Mater. 24(37):5065–70 68. Kim C, Kim D, Kim H, Chung J. 2012. Significant enhancement in the thermoelectric performance of a bismuth telluride nanocompound through brief fabrication procedures. ACS Appl. Mater. Interfaces 4(6):2949–54 69. Saleemi M, Toprak MS, Li S, Johnsson M, Muhammed M. 2012. Synthesis, processing, and thermoelectric properties of bulk nanostructured bismuth telluride (Bi2 Te3 ). J. Mater. Chem. 22(2):725 70. Li D, Qin XY, Dou YC, Li XY, Sun RR, et al. 2012. Thermoelectric properties of hydrothermally synthesized Bi2 Te3−x Sex nanocrystals. Scr. Mater. 67(2):161–64 71. Bai T, Li C, Liang D, Li F, Jin D, et al. 2013. Synthesis of various metal selenide nanostructures using the novel selenium precursor 1,5-bis(3-methylimidazole-2-selone)pentane. Cryst. Eng. Comm. 15(33):6483 72. Min Y, Roh J, Yang H, Park M. 2013. Surfactant-free scalable synthesis of Bi2 Te3 and Bi2 Se3 nanoflakes and enhanced thermoelectric properties of their nanocomposites. Adv. Mater. 25(10):1425–29 73. Song S, Fu J, Li X, Gao W, Zhang H. 2013. Facile synthesis and thermoelectric properties of selfassembled Bi2 Te3 one-dimensional nanorod bundles. Chem. A Eur. J. 19(8):2889–94 74. Stavila V, Robinson DB, Hekmaty MA, Nishimoto R, Medlin DL, et al. 2013. Wet-chemical synthesis and consolidation of stoichiometric bismuth telluride nanoparticles for improving the thermoelectric figure-of-merit. ACS Appl. Mater. Interfaces 5(14):6678–86 75. Xu H, Chen G, Jin R, Chen D, Wang Y, et al. 2014. Enhancement of the Seebeck coefficient in stacked Bi2 Se3 nanoplates by energy filtering. Eur. J. Inorg. Chem. 2014(16):2625–30 76. Kim C, Kim DH, Lee YK, Kim JT, Han YS, Kim H. 2014. Study of reaction mechanisms and synthetic manipulations of bismuth tellurium selenide nanomaterials for enhanced thermoelectric performance. J. Alloys Compd. 584:108–13 www.annualreviews.org • Nanostructured Materials

Data in Figure 2a adapted with permission from Reference 57. Copyright 2010 American Chemical Society.

Data in Figure 2a adapted with permission from Reference 58. Copyright 2010 American Chemical Society.

Data in Figure 2a adapted with permission from Reference 62. Copyright 2011 American Chemical Society.

Data in Figure 2a adapted with permission from Reference 66. Copyright 2012 American Chemical Society.

Data in Figure 2 adapted with perimission from Reference 68. Copyright 2012 American Chemical Society.

Data in Figure 2 adapted with permission from Reference 74. Copyright 2013 American Chemical Society.

263

CH06CH12-Wu

ARI

2 July 2015

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

Data in Figure 2a adapted with permission from Reference 79. Copyright 2010 American Chemical Society.

Data in Figure 3 adapted with permission from Reference 90. Copyright 2014 American Chemical Society.

Data in Figure 4a adapted with permission from Reference 95. Copyright 2014 American Chemical Society.

Data in Figure 5 adapted with permission from Reference 97. Copyright 2010 American Chemical Society.

Data in Figure 5 adapted with permission from Reference 98. Copyright 2012 American Chemical Society.

Data in Figure 5 adapted with permission from Reference 99. Copyright 2012 American Chemical Society.

Data in Figure 5 adapted with permission from Reference 101. Copyright 2011 American Chemical Society.

264

6:57

77. Yim W, Fitzke E, Rosi F. 1966. Thermoelectric properties of Bi2 Te3 -Sb2 Te3 -Sb2 Se3 pseudo-ternary alloys in the temperature range 77 to 300 k. J. Mater. Sci. 1(1):52–65 78. Ji XH, Zhao XB, Zhang YH, Lu BH, Ni HL. 2005. Synthesis and properties of rare earth containing Bi2 Te3 based thermoelectric alloys. J. Alloys Compd. 387(1–2):282–86 79. Datta A, Paul J, Kar A, Patra A, Sun Z, et al. 2010. Facile chemical synthesis of nanocrystalline thermoelectric alloys based on Bi−Sb−Te−Se. Cryst. Growth Des. 10(9):3983–89 80. Zhao L-D, Dravid VP, Kanatzidis MG. 2014. The panoscopic approach to high performance thermoelectrics. Energy Environ. Sci. 7(1):251–68 81. Ravich YI, Efimova BA, Smirnov IA. 1970. Semiconducting Lead Chalcogenides. New York: Plenum 82. Pei Y, LaLonde A, Iwanaga S, Snyder GJ. 2011. High thermoelectric figure of merit in heavy hole dominated PbTe. Energy Environ. Sci. 4(6):2085–89 83. LaLonde AD, Pei Y, Snyder GJ. 2011. Reevaluation of PbTe1−x Ix as high performance n-type thermoelectric material. Energy Environ. Sci. 4(6):2090 84. Muhlberg M, Hesse D. 1983. Tem precipitation studies in te-rich as-grown PbTe single crystals. Phys. ¨ Status Solidi 76:513–24 85. Zhang W, Zhang L, Cheng Y, Hui Z, Zhang X. 2000. Synthesis of nanocrystalline lead chalcogenides PbE (E = S, Se, or Te) from alkaline aqueous solutions. Mater. Res. Bull. 35:2009–15 86. Martin J, Nolas GS, Zhang W, Chen L. 2007. PbTe nanocomposites synthesized from PbTe nanocrystals. Appl. Phys. Lett. 90(22):222112 87. Paul B, Banerji P. 2009. Grain structure induced thermoelectric properties in PbTe nanocomposites. Nanosci. Nanotechnol. Lett. 1:208–12 88. Martin J, Wang L, Chen L, Nolas GS. 2009. Enhanced Seebeck coefficient through energy-barrier scattering in PbTe nanocomposites. Phys. Rev. B 79(11):115311 89. Paul B, V AK, Banerji P. 2010. Embedded Ag-rich nanodots in PbTe: enhancement of thermoelectric properties through energy filtering of the carriers. J. Appl. Phys. 108(6):064322 90. Finefrock SW, Zhang G, Bahk J-H, Fang H, Yang H, et al. 2014. Structure and thermoelectric properties of spark plasma sintered ultrathin PbTe nanowires. Nano Lett. 14(6):3466–73 91. Crocker A, Rogers L. 1967. Interpretation of the Hall coefficient, electrical resistivity and Seebeck coefficient of p-type lead telluride. Br. J. Appl. Phys. 18:563 92. LaLonde AD, Ikeda T, Snyder GJ. 2011. Rapid consolidation of powdered materials by induction hot pressing. Rev. Sci. Instrum. 82(2):025104 93. Cao YQ, Zhu TJ, Zhao XB. 2009. Low thermal conductivity and improved figure of merit in fine-grained binary PbTe thermoelectric alloys. J. Phys. D. Appl. Phys. 42(1):015406 94. Popescu A, Datta A, Nolas GS, Woods LM. 2011. Thermoelectric properties of Bi-doped PbTe composites. J. Appl. Phys. 109(10):103709 95. Fang H, Luo Z, Yang H, Wu Y. 2014. The effects of the size and the doping concentration on the power factor of n-type lead telluride nanocrystals for thermoelectric energy conversion. Nano Lett. 14(3):1153–57 96. Liu M-L, Chen I-W, Huang F-Q, Chen L-D. 2009. Improved thermoelectric properties of Cu-doped quaternary chalcogenides of Cu2 CdSnSe4 . Adv. Mater. 21(37):3808–12 97. Shi X, Xi L, Fan J, Zhang W, Chen L. 2010. Cu−Se bond network and thermoelectric compounds with complex diamondlike structure. Chem. Mater. 22(22):6029–31 98. Zeier WG, LaLonde A, Gibbs ZM, Heinrich CP, Panthofer M, et al. 2012. Influence of a nano ¨ phase segregation on the thermoelectric properties of the p-type doped stannite compound Cu(2+x) Zn(1−x) GeSe4 . J. Am. Chem. Soc. 134(16):7147–54 99. Iba´ nez M, Zamani R, LaLonde A, Cadavid D, Li W, et al. 2012. Cu2 ZnGeSe4 nanocrystals: ˜ synthesis and thermoelectric properties. J. Am. Chem. Soc. 134(9):4060–63 100. Xue D-J, Jiao F, Yan H-J, Xu W, Zhu D, et al. 2013. Synthesis of wurtzite Cu2 ZnGeSe4 nanocrystals and their thermoelectric properties. Chem. Asian J. 8(10):2383–87 101. Fan F-J, Yu B, Wang Y-X, Zhu Y-L, Liu X-J, et al. 2011. Colloidal synthesis of Cu2 CdSnSe4 nanocrystals and hot-pressing to enhance the thermoelectric figure-of-merit. J. Am. Chem. Soc. 133(40):15910–13 Finefrock et al.

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

CH06CH12-Wu

ARI

2 July 2015

6:57

102. Iba M, Cadavid D, Zamani R, Garc´ıa-Castello N, Izquierdo-Roca V, et al. 2012. Composition control and thermoelectric properties of quaternary chalcogenide nanocrystals: the case of stannite Cu2 CdSnSe4 . Chem. Mater. 24(3):562–70 103. Song J-M, Liu Y, Niu H-L, Mao C-J, Cheng L-J, et al. 2013. Hot-injection synthesis and characterization of monodispersed ternary Cu2 SnSe3 nanocrystals for thermoelectric applications. J. Alloys Compd. 581:646–52 104. Ib´anez ˜ M, Cadavid D, Anselmi-Tamburini U, Zamani R, Gorsse S, et al. 2013. Colloidal synthesis and thermoelectric properties of Cu2 SnSe3 nanocrystals. J. Mater. Chem. A 1(4):1421 105. Fan F-J, Wang Y-X, Liu X-J, Wu L, Yu S-H. 2012. Large-scale colloidal synthesis of non-stoichiometric Cu2 ZnSnSe4 nanocrystals for thermoelectric applications. Adv. Mater. 24(46):6158–63 106. Ibaez M, Zamani R, Li W. 2012. Crystallographic control at the nanoscale to enhance functionality: polytypic Cu2 GeSe3 nanoparticles as thermoelectric materials. Chem. Mater. 24(23):4615–22 107. Li W, Ib´anez ˜ M, Cadavid D, Zamani RR, Rubio-Garcia J, et al. 2014. Colloidal synthesis and functional properties of quaternary Cu-based semiconductors: Cu2 HgGeSe4 . J. Nanoparticle Res. 16(3):2297 108. Li W, Ib´anez ˜ M, Zamani RR, Garc´ıa-Castello´ N, Gorsse S, et al. 2013. Cu2 HgSnSe4 nanoparticles: synthesis and thermoelectric properties. Cryst. Eng. Comm. 15(44):8966 109. Shavel A, Cadavid D, Iba´ nez ˜ M, Carrete A, Cabot A. 2012. Continuous production of Cu2 ZnSnS4 nanocrystals in a flow reactor. J. Am. Chem. Soc. 134(3):1438–41 110. Yang H, Jauregui LA, Zhang G, Chen YP, Wu Y. 2012. Nontoxic and abundant copper zinc tin sulfide nanocrystals for potential high-temperature thermoelectric energy harvesting. Nano Lett. 12(2):540–45 111. Liang D, Ma R, Jiao S, Pang G, Feng S. 2012. A facile synthetic approach for copper iron sulfide nanocrystals with enhanced thermoelectric performance. Nanoscale 4(20):6265–68 112. Taylor PF, Wood C. 1961. Thermoelectric properties of Ag2 Te. J. Appl. Phys. 32(1):1 113. Conn JB, Taylor RC. 1960. Thermoelectric and crystallographic properties of Ag2 Se. J. Electrochem. Soc. 107(12):977 114. Pei Y, Heinz NA, Snyder GJ. 2011. Alloying to increase the band gap for improving thermoelectric properties of Ag2 Te. J. Mater. Chem. 21(45):18256 115. Day T, Drymiotis F, Zhang T, Rhodes D, Shi X, et al. 2013. Evaluating the potential for high thermoelectric efficiency of silver selenide. J. Mater. Chem. C 1(45):7568 116. Ferhat M, Nagao J. 2000. Thermoelectric and transport properties of β-Ag2 Se compounds. J. Appl. Phys. 88(2):813 117. Xiao C, Xu J, Li K, Feng J, Yang J, Xie Y. 2012. Superionic phase transition in silver chalcogenide nanocrystals realizing optimized thermoelectric performance. J. Am. Chem. Soc. 134(9):4287–93 118. Capps J, Drymiotis F, Lindsey S, Tritt TM. 2010. Significant enhancement of the dimensionless thermoelectric figure of merit of the binary Ag2 Te. Philos. Mag. Lett. 90(9):677–81 119. Aliev FF, Jafarov MB, Eminova VI. 2009. Thermoelectric figure of merit of Ag2 Se with Ag and Se excess. Semiconductors 43(8):977–79 120. Jiang Y, Wu Y, Yang Z, Xie Y, Qian Y. 2001. Room temperature growth of rod-like nanocrystalline Ag2 Te in mixed solvent. J. Cryst. Growth 224:1–4 121. Monnoyer P, Nagy JB, Buschmann V, Fonseca A, Jeunieau L, et al. 1996. Preparation of colloidal nanocrystals of AgX and Ag2 Se from microemulsions. In Nanoparticles in Solids and Solutions, ed. JH Fendler, I Dekany, pp. 505–17. Dordrecht: Kluwer Acad. 122. Cadavid D, Ib´anez de la Torre MA, Cabot A. 2013. Organic ligand ˜ M, Shavel A, Dur´a OJ, Lopez ´ displacement by metal salts to enhance nanoparticle functionality: thermoelectric properties of Ag2 Te. J. Mater. Chem. A 1(15):4864–70 123. Pei J, Chen G, Jia D, Yu Y, Sun J, et al. 2014. Crooked Ag2 Te nanowires with rough surfaces: facile microwave-assisted solution synthesis, growth mechanism, and electrical performances. New J. Chem. 38(1):59 124. Yang H, Bahk J-H, Day T, Mohammed AMS, Min B, et al. 2014. Composition modulation of Ag2 Te nanowires for tunable electrical and thermal properties. Nano Lett. 14(9):5398–404 125. Zhou W, Zhao W, Lu Z, Zhu J, Fan S, et al. 2012. Preparation and thermoelectric properties of sulfur doped Ag2 Te nanoparticles via solvothermal methods. Nanoscale 4(13):3926–31 www.annualreviews.org • Nanostructured Materials

Data in Figure 5a adapted with permission from Reference 106. Copyright 2012 American Chemical Society.

Data in Figure 5a adapted with permission from Reference 109. Copyright 2012 American Chemical Society.

Data in Figure 5a adapted with permission from Reference 110. Copyright 2012 American Chemical Society.

Data in Figure 6 adapted with permission from Reference 117. Copyright 2012 American Chemical Society.

Data in Figure 6 adapted with permission from Reference 124. Copyright 2014 American Chemical Society.

265

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

CH06CH12-Wu

ARI

2 July 2015

Data in Table 1 adapted with permission from Reference 135. Copyright 2013 American Chemical Society.

Data in Table 1 adapted with perimission from Reference 136. Copyright 2013 American Chemical Society.

Data in Table 1 adapted with permission from Reference 137. Copyright 2014 American Chemical Society.

266

6:57

126. Dong G-H, Zhu Y-J. 2012. Room-temperature solution synthesis of Ag2 Te hollow microspheres and dendritic nanostructures, and morphology dependent thermoelectric properties. Cryst. Eng. Comm. 14(5):1805 127. Pei J, Chen G, Jia D, Jin R, Xu H, Chen D. 2013. Rapid synthesis of Ag2 Se dendrites with enhanced electrical performance by microwave-assisted solution method. New J. Chem. 37(2):323 128. Jung D, Kurosaki K, Ohishi Y. 2012. Effect of phase transition on the thermoelectric properties of Ag2 Te. Mater. Trans. 53(7):1216–19 129. Mi W, Qiu P, Zhang T, Lv Y, Shi X, Chen L. 2014. Thermoelectric transport of Se-rich Ag2 Se in normal phases and phase transitions. Appl. Phys. Lett. 104(13):133903 130. Venkatasubramanian R, Siivola E, Colpitts T. 2001. Thin-film thermoelectric devices with high roomtemperature figures of merit. Nature 413:597–602 131. Zhao L-D, He J, Hao S, Wu C-I, Hogan TP, et al. 2012. Raising the thermoelectric performance of p-type PbS with endotaxial nanostructuring and valence-band offset engineering using CdS and ZnS. J. Am. Chem. Soc. 134(39):16327–36 132. Cadavid D, Ib´anez AM, Cirera A, et al. 2012. Bottom-up processing of thermoelectric ˜ M, Gorsse S, Lopez ´ nanocomposites from colloidal nanocrystal building blocks: the case of Ag2 Te-PbTe. J. Nanoparticle Res. 14(12):1328 133. Zhou W, Zhu J, Li D, Hng HH, Boey FYC, et al. 2009. Binary-phased nanoparticles for enhanced thermoelectric properties. Adv. Mater. 21(31):3196–200 134. Zhang Y, Bahk J-H, Lee J, Birkel CS, Snedaker ML, et al. 2014. Hot carrier filtering in solution processed heterostructures: a paradigm for improving thermoelectric efficiency. Adv. Mater. 26(17):2755–61 135. Iba´ nez ˜ M, Zamani R, Gorsse S, Fan J. 2013. Core-shell nanoparticles as building blocks for the bottom-up production of functional nanocomposites: PbTe-PbS thermoelectric properties. ACS Nano 7(3):2573–86 136. Fang H, Feng T, Yang H, Ruan X, Wu Y. 2013. Synthesis and thermoelectric properties of compositional-modulated lead telluride-bismuth telluride nanowire heterostructures. Nano Lett. 13(5):2058–63 137. Fang H, Yang H, Wu Y. 2014. Thermoelectric properties of silver telluride-bismuth telluride nanowire heterostructure synthesized by site-selective conversion. Chem. Mater. 26(10):3322–27 138. Kim KT, Choi SY, Shin EH, Moon KS, Koo HY, et al. 2013. The influence of CNTs on the thermoelectric properties of a CNT/Bi2 Te3 composite. Carbon 52:541–49 139. Dong J, Liu W, Li H, Su X, Tang X, Uher C. 2013. In-situ synthesis and thermoelectric properties of PbTe/graphene nanocomposites by utilizing a facile and novel wet chemical method. J. Mater. Chem. A 1:12503–11 140. Jin R, Chen G, Pei J, Sun J, Wang Q. 2012. Controllable synthesis and thermoelectric transport properties of binary-phased PbTe/PbSe nanocrystals. Cryst. Eng. Comm. 14(13):4461 141. Scheele M, Oeschler N, Veremchuk I, Peters S-O, Littig A, et al. 2011. Thermoelectric properties of lead chalcogenide core-shell nanostructures. ACS Nano 5(11):8541–51 142. Shi Y, Zhang F, Snedaker M, Ding K. 2011. Surfactant-free synthesis of heterostructure with enhanced thermoelectric figure of merit. ACS Nano 5(4):3158–65 143. Ko D, Kang Y, Murray CB. 2011. Enhanced thermopower via carrier energy filtering in solutionprocessable Pt-Sb2 Te3 nanocomposites. Nano Lett. 11(7):2841–44 144. Hu L-P, Zhu T-J, Wang Y-G, Xie H-H, Xu Z-J, Zhao X-B. 2014. Shifting up the optimum figure of merit of p-type bismuth telluride-based thermoelectric materials for power generation by suppressing intrinsic conduction. NPG Asia Mater. 6(2):e88

Finefrock et al.

CH06-Frontmatter

ARI

14 July 2015

20:14

Annual Review of Chemical and Biomolecular Engineering

Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

Contents

Volume 6, 2015

A Conversation with Adam Heller Adam Heller and Elton J. Cairns p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 1 An Integrated Device View on Photo-Electrochemical Solar-Hydrogen Generation Miguel A. Modestino and Sophia Haussener p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p13 Synthetic Biology for Specialty Chemicals Kelly A. Markham and Hal S. Alper p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p35 Chemical Looping Technology: Oxygen Carrier Characteristics Siwei Luo, Liang Zeng, and Liang-Shih Fan p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p53 Gasification of Woody Biomass Jianjun Dai, Jean Saayman, John R. Grace, and Naoko Ellis p p p p p p p p p p p p p p p p p p p p p p p p p p p p p77 Design Criteria for Future Fuels and Related Power Systems Addressing the Impacts of Non-CO2 Pollutants on Human Health and Climate Change James Jay Schauer p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 101 Graphene Mechanics: Current Status and Perspectives Costas Galiotis, Otakar Frank, Emmanuel N. Koukaras, and Dimitris Sfyris p p p p p p p p p p 121 Smart Manufacturing Jim Davis, Thomas Edgar, Robert Graybill, Prakashan Korambath, Brian Schott, Denise Swink, Jianwu Wang, and Jim Wetzel p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 141 Current Trends and Challenges in Biointerfaces Science and Engineering A.M. Ross and J. Lahann p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 161 Defects in the Self-Assembly of Block Copolymers and Their Relevance for Directed Self-Assembly Weihua Li and Marcus Muller ¨ p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 187 Clean Water for Developing Countries Aniruddha B. Pandit and Jyoti Kishen Kumar p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 217

vii

CH06-Frontmatter

ARI

14 July 2015

20:14

Thermoelectric Properties of Solution Synthesized Nanostructured Materials Scott W. Finefrock, Haoran Yang, Haiyu Fang, and Yue Wu p p p p p p p p p p p p p p p p p p p p p p p p p p p 247 Group Contribution Methods for Phase Equilibrium Calculations Jurgen ¨ Gmehling, Dana Constantinescu, and Bastian Schmid p p p p p p p p p p p p p p p p p p p p p p p p p p p 267 Microfluidic Strategies for Understanding the Mechanics of Cells and Cell-Mimetic Systems Joanna B. Dahl, Jung-Ming G. Lin, Susan J. Muller, and Sanjay Kumar p p p p p p p p p p p p 293 Annu. Rev. Chem. Biomol. Eng. 2015.6:247-266. Downloaded from www.annualreviews.org Access provided by University of California - Santa Barbara on 08/15/15. For personal use only.

Biocatalysis: A Status Report Andreas S. Bommarius p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 319 Computational Modeling of Multiphase Reactors J.B. Joshi and K. Nandakumar p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 347 Particle Formation and Product Formulation Using Supercritical Fluids ˇ ˇ Zeljko Knez, Maˇsa Knez Hrnˇciˇc, and Mojca Skerget p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 379 Indexes Cumulative Index of Contributing Authors, Volumes 2–6 p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 409 Cumulative Index of Article Titles, Volumes 2–6 p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 412 Errata An online log of corrections to Annual Review of Chemical and Biomolecular Engineering articles may be found at http://www.annualreviews.org/errata/chembioeng

viii

Contents