Journal of the American Chemical Society
Communication
approaches.28 Possible reasons are the sub-micrometer-scale
crystallite sizes (∼500 nm) and lower carrier concentration (as
described below) of our PbTe materials. However, compared to
the iodine-doped PbTe nanocrytals (<50 nm) previously
reported,30 the electrical conductivity of the PbTe materials
in this work is greater by a factor of 100. We believe that the
reasons for this difference are the relatively larger nano-
crystallite size and easier ligand removal.
NSF-funded Materials Research Facilities Network, www.mrfn.
REFERENCES
■
(1) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.;
Wang, D. Z.; Ren, Z. F.; Fleurial, J. P.; Gogna, P. Adv. Mater. 2007, 19,
1043.
(2) Minnich, A. J.; Dresselhaus, M. S.; Ren, Z. F.; Chen, G. Energy
Environ. Sci. 2009, 2, 466.
Energy-dispersive X-ray (EDX) spectroscopy on a FEI XL40
Sirion scanning electron micrograph was used to analyze the
compositions of the materials, as synthesized, after being
pressed and after high-temperature measurements, respectively.
The halogen concentrations in all the materials were generally
below the EDX detection limit (<1%). Nevertheless, the
presence of Br in the PbTe materials synthesized in [P4 4 4 4]Br
was qualitatively confirmed (Figure S15) by SEM-EDX.
The Van der Pauw method was used to characterize the
carrier concentration of the materials. The unintentional n-type
doping by the halogens, both Cl and Br, in PbTe gives a carrier
concentration of about 4 × 1017/cm3. To achieve an
appreciable figure of merit, an optimal doping level is necessary.
For the n-type PbTe and PbSe, the optimal halogen doping
level is determined to be in the range of 1019−1020/cm3.27,28,31
The doping level obtained in our materials is orders of
magnitude lower than the desired optimal value. By optimizing
the n-type doping levels in our approach, the thermoelectric
performance might be further enhanced.
In summary, we have demonstrated the microwave-assisted
ionothermal solution synthesis of metal chalcogenide micro-/
nanostructures based on simple chemistry. Various metal
chalcogenides were successfully synthesized from their
elemental precursors on a relatively large scale. The excellent
thermal and chemical stabilities of the phosphonium ionic
liquids as well as their high chalcogen solubility make possible a
rapid reaction between chalcogens and metal powders at
elevated temperatures. Furthermore, the ionic liquids can be
easily removed from the products via washing because of the
weak-binding feature of the ionic liquids, a feature that is
potentially beneficial for many electronic applications, such as
thermoelectrics and topological insulators.
(3) Vineis, C. J.; Shakouri, A.; Majumdar, A.; Kanatzidis, M. G. Adv.
Mater. 2010, 22, 3970.
(4) Pei, Y. Z.; Wang, H.; Snyder, G. J. Adv. Mater. 2012, 24, 6125.
(5) Zhang, H. J.; Liu, C. X.; Qi, X. L.; Dai, X.; Fang, Z.; Zhang, S. C.
Nat. Phys. 2009, 5, 438.
(6) Moore, J. E. Nature 2010, 464, 194.
(7) Qi, X. L.; Zhang, S. C. Rev. Mod. Phys. 2011, 83, 1057.
(8) Malik, M. A.; Afzaal, M.; O’Brien, P. Chem. Rev. 2010, 110, 4417.
(9) Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V.
Chem. Rev. 2010, 110, 389.
(10) Ma, Y.; Hao, Q.; Poudel, B.; Lan, Y. C.; Yu, B.; Wang, D. Z.;
Chen, G.; Ren, Z. F. Nano Lett. 2008, 8, 2580.
(11) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y. C.; Minnich, A.; Yu, B.;
Yan, X. A.; Wang, D. Z.; Muto, A.; Vashaee, D.; Chen, X. Y.; Liu, J. M.;
Dresselhaus, M. S.; Chen, G.; Ren, Z. F. Science 2008, 320, 634.
(12) Pei, Y. Z.; Shi, X. Y.; LaLonde, A.; Wang, H.; Chen, L. D.;
Snyder, G. J. Nature 2011, 473, 66.
(13) Biswas, K.; He, J. Q.; Blum, I. D.; Wu, C. I.; Hogan, T. P.;
Seidman, D. N.; Dravid, V. P.; Kanatzidis, M. G. Nature 2012, 489,
414.
(14) Antonietti, M.; Kuang, D. B.; Smarsly, B.; Yong, Z. Angew.
Chem., Int. Ed. 2004, 43, 4988.
(15) Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123.
(16) Ma, Z.; Yu, J. H.; Dai, S. Adv. Mater. 2010, 22, 261.
(17) Freudenmann, D.; Wolf, S.; Wolff, M.; Feldmann, C. Angew.
Chem., Int. Ed. 2011, 50, 11050.
(18) Bradaric, C. J.; Downard, A.; Kennedy, C.; Robertson, A. J.;
Zhou, Y. H. Green Chem. 2003, 5, 143.
(19) Sowmiah, S.; Srinivasadesikan, V.; Tseng, M. C.; Chu, Y. H.
Molecules 2009, 14, 3780.
(20) Adamova, G.; Gardas, R. L.; Rebelo, L. P. N.; Robertson, A. J.;
Seddon, K. R. Dalton Trans. 2011, 40, 12750.
(21) Adamova, G.; Gardas, R. L.; Nieuwenhuyzen, M.; Puga, A. V.;
Rebelo, L. P. N.; Robertson, A. J.; Seddon, K. R. Dalton Trans. 2012,
41, 8316.
(22) Meyer, B. Chem. Rev. 1976, 76, 367.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and Figures S1−S15. This material is
(23) Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A.
G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A.;
Guralnick, B. W.; Park, J.; Somogyi, A.; Theato, P.; Mackay, M. E.;
Sung, Y. E.; Char, K.; Pyun, J. Nat. Chem. 2013, 5, 518.
(24) Dong, G. H.; Zhu, Y. J.; Chen, L. D. J. Mater. Chem. 2010, 20,
1976.
(25) Mehta, R. J.; Zhang, Y. L.; Karthik, C.; Singh, B.; Siegel, R. W.;
Borca-Tasciuc, T.; Ramanath, G. Nat. Mater. 2012, 11, 233.
(26) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105.
(27) Wang, H.; Pei, Y. Z.; LaLonde, A. D.; Snyder, G. J. Proc. Natl.
Acad. Sci. U.S.A. 2012, 109, 9705.
(28) (a) LaLonde, A. D.; Pei, Y. Z.; Snyder, G. J. Energy Environ. Sci.
2011, 4, 2090. (b) Nikolic, M. V.; Paraskevopoulos, K. M.;
Hatzikraniotis, E.; Nikolic, N.; Vujatovic, S. S.; Aleksic, O. S.; Zorba,
T. T.; Kyratsi, Th.; Menicanin, A.; Nikolic, P. M. AIP Conf. Proc.
(ICT2011) 2012, 1449, 143.
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AUTHOR INFORMATION
Corresponding Author
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Center for Energy Efficient
Materials, an Energy Frontier Research Center funded by the
U.S. Department of Energy, Office of Science, Basic Energy
Sciences under Award No. DE-SC0001009, and by the
National Science Foundation (DMR 0805148). We thank
Cytec Industries Inc. for kindly providing us several types of
phosphonium ionic liquids without cost. The MRL Shared
Experimental Facilities are supported by the MRSEC Program
of the NSF under Award No. DMR 1121053(a member of the
(29) Rhyee, J. S.; Ahn, K.; Lee, K. H.; Ji, H. S.; Shim, J. H. Adv. Mater.
2011, 23, 2191.
(30) Fang, H. Y.; Luo, Z. Q.; Yang, H. R.; Wu, Y. Nano Lett. 2014,
14, 1153.
(31) Parker, D.; Singh, D. J.; Zhang, Q. Y.; Ren, Z. F. J. Appl. Phys.
2012, 111, 123701.
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