pubs.acs.org/NanoLett
High Electrical Conductivity Antimony Selenide Nanocrystals and Assemblies Rutvik J. Mehta,† C. Karthik,† Wei Jiang,† B. Singh,† Yunfeng Shi,† Richard W. Siegel,† Theo Borca-Tasciuc,‡ and Ganpati Ramanath*,† †
Department of Materials Science and Engineering and ‡ Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, United States ABSTRACT Antimony selenide is a promising thermoelectric material with a high Seebeck coefficient, but its figure of merit is limited by its low electrical conductivity. Here, we report a rapid and scalable (gram-a-minute) microwave synthesis of one-dimensional nanocrystals of sulfurized antimony selenide that exhibit ∼104-1010 times higher electrical conductivity than non-nanostructured bulk or thin film forms of this material. As the nanocrystal diameter increases, the nanowires transform into nanotubes through void formation and coalescence driven by axial rejection of sulfur incorporated into the nanowires from the surfactant used in our synthesis. Individual nanowires and nanotubes exhibit a charge carrier transport activation-energy of ∼150 nm exhibit cylindrical voids about the axis at the nanowire tips (Figure 1e), having wedgelike appearance. For diameters in the 150-350 nm range, the void fraction increases with increasing diameter (Figure 1f). For diameters >350 nm, the voids extend along the entire nanowire length, transforming the nanowires into nanotubes (Figure 1g). The nanowire-to-nanotube conversion mechanism supported by Monte Carlo simulations is discussed below. TEM (Figure 1j) and electron diffraction analyses (Figure 1k) reveal that all the one-dimensional nanostructures we analyzed were single crystals with an orthorhombic structure with the axis oriented along the high charge carrier mobility [010] direction.9 Additionally, the nanocrystals are coated with a thin amorphous layer attributable to TGA capping, © 2010 American Chemical Society
corroborated by spectroscopic analyses, also described below. X-ray diffractograms showing only reflections corresponding to the orthorhombic Pnma structure of Sb2Se320 (see Figure 2a) are consistent with the electron diffraction results. Altering the TGA/TOP-Se2- ratio cTGA/cTOP-Se shifts the Bragg peaks, without the emergence of any additional peaks (see Figure 2a,b). The lattice parameter values range from that of bulk antimony selenide with 350 nm diameter nanotubes, suggests that nanotubes form by void nucleation and coalescence24 resulting from axial sulfur redistribution during synthesis. This inference is supported by Monte Carlo simulations (see Experimental Section in Supporting Information for details) showing that an axial
FIGURE 4. (a) Sulfur-selenium ratio along the axis of a nanowire (red) and a nanotube (blue) determined by EDX in a transmission electron microscope. The curves are only to guide the eye. Insets show bright field micrographs from a nanowire (top) and nanotube (bottom). (b) Monte Carlo simulation snapshots capturing void nucleation and axial growth leading to nanotube formation, shown alongside representative TEM images (scale bars ) 100 nm) that exhibit these features. Red, blue, and green denote antimony selenide solid, solution, and sulfur, respectively. © 2010 American Chemical Society
4420
DOI: 10.1021/nl1020848 | Nano Lett. 2010, 10, 4417-–4422
R4p ∼ 10 kΩ, which translates to σ ∼ 10 Ω-1 m-1 for a 10 µm thick film. Although this σ is comparable to the lower end of values obtained for individual nanowires due to film porosity, it corresponds to ∼102-106 higher σ than that of the bulk material. The highest σ we obtain for the individual nanocrystals is comparable to that of high ZT Bi2Te3-based bulk thermoelectrics.10-12 However, we also retain low thermal conductivity κ because of the low electronic component of thermal conductivity κe, and nanostructuring-induced lowering of the lattice-scattering component of thermal conductivity κL. From the Weidemann-Franz law, we estimate κe ∼ 0.135 W/mK for σ ∼ 3 × 104 Ω-1 m-1 using a conservative Lorenz number L ) 1.5 × 10-8 WΩ/K2 for a degenerately doped semiconductor. Since κL∼ 2.65 W/mK for bulk Sb2Se3, κe/κL < ∼0.05, which is significantly lower than κe/κL ∼ 0.3 for high ZT bulk Bi2Te3 alloys.10-12 Furthermore, since nanostructuring can be used to decrease κL several-fold from the bulk value,14 our results show that a σ increase is possible together with a κ decrease. Indeed our preliminary measurements on individual antimony selenide nanocrystals reveal κ < 1 W/mK, which is more than 2.5-fold lower than bulk κ, while retaining high σ and high R ∼ -750 µV/K. These features of our sulfurized antimony selenide nanocrystals are thus quite attractive for thermoelectric applications. The resistance-temperature characteristics (Figure 5c) of the one-dimensional nanostructures indicate semiconducting behavior described by σ ) σ0 exp(-Ea/kT) where Ea ∼ 15-60 meV for 200 K < T e 300 K, and Ea ∼ 4-10 meV at lower temperatures. Such low activation energies, when compared with intrinsic Ea ∼ 700 meV for bulk single crystals2 and Ea ∼ 100-400 meV for selenium-rich Sb2Se32,9 at 155 < T e 300 K, point to shallow donor levels near the conduction band edge.29 Nanocrystals with low (e.g., 10 MΩ), contrary to the high σ expected for low sulfur contents from suggestions of lattice-sulfur-substitution-induced energy band gap increase,1,15 but consistent with high resistivity Sb2S3 nanowires,30 which again confirms that S is substituting Se in Sb2Se3. Two-probe measurements and modeling31 of Au-nanowire-Au Schottky junctions (Supporting Information Figure S2) of individual nanowires reveal a majority carrier concentration of ∼1018 cm-3 in air, confirming degenerate doping consistent with low Ea. This carrier concentration for σ ∼ 1500-2500 Ω-1 m-1 implies a mobility µ ∼ 100-150 cm2/(V s), which is about 10-fold higher than that in the bulk.12 Electrical measurements in a 10-7 Torr vacuum show a ∼15-20 times lower nanowire resistance than that in air, and this conductivity change is reversible upon repeated cycling between air and vacuum ambients. In contrast, nanotube resistance is insensitive to the ambient. Since the absence of axial sulfur gradients is the key difference between nanotubes and nanowires, the observed retention of high σ and sensitivity to the ambient points to the role of
FIGURE 5. (a) Current-voltage characteristics from an individual nanowire (red) and a nanotube (blue) of sulfurized Sb2Se3 in air (dashed lines) and in 10-7 Torr vacuum (solid lines). Inset is a SEM micrograph of a test device with a 1 µm scale bar. (b) Current-voltage plot from a thin film assembly of nanowires and nanotubes synthesized in 90 s shown with an optical image (inset) of the thin film device used for our measurements. (c) Temperature-dependent resistance measurements of a nanowire (red) and a nanotube (blue).
tance R4p in the 10 kΩ < R4p < 100 kΩ range (Figure 5a). These values correspond to σ ∼ 10-104 Ω-1 m-1, which are 4-10 orders of magnitude higher than that reported for bulk and thin film forms of this material.2,9,13,16 Thin film assemblies28 of the sulfurized Sb2Se3 nanowires and nanotubes also show semiconducting behavior (Figure 5b) with © 2010 American Chemical Society
4421
DOI: 10.1021/nl1020848 | Nano Lett. 2010, 10, 4417-–4422
sulfur-induced surface states, for example, arising from structural changes induced by sulfur incorporation, similar to that seen32 in orthorhombic chalcogenide nanowires. These results indicate that surface-sulfur induced donor states close to the conduction band edge offset the low conductivity expected due to band gap broadening arising from sulfur incorporation in the nanocrystal lattice. In summary, we have developed a rapid and scalable surfactant-directed microwave-activated synthesis method for fabricating sulfurized orthorhombic antimony selenide nanowires and nanotubes with enhanced electrical conductivity over non-nanostructured forms. The nanowires with high diameter transform into nanotubes by void nucleation and axial growth arising from axial redistribution of excess sulfur incorporated from the antimony-ligated thioglycolic acid surfactant at the nanowire tip growth front. Surfactant capping of the nanocrystal surfaces not only inhibits oxidation, but also creates shallow donor states leading to 104-1010 higher electrical conductivity and about 10-fold higher charge carrier mobility in the one-dimensional nanocrystals than the respective bulk values. The sulfurized Sb2Se3 nanocrystals could serve as attractive building blocks to realize novel nanomaterials with enhanced electrical conductivity and decreased thermal conductivity, thereby circumventing extant limitations of the material in its bulk form and paving the way for applications in thermoelectric devices.
(4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)
Acknowledgment. This work was supported through a gift from IBM through the Rensselaer Nanotechnology Center, a NRI-NIST award through the Index Center at the University at Albany, a grant from the S3TEC EFRC supported by DOE office of Basic Energy Sciences, NSF Awards DMR 0519081 and CBET 0348613, and the New York State Foundation for Science, Technology, and Innovation.
(22) (23) (24) (25) (26)
Supporting Information Available. We have provided detailed experimental procedures in addition to data on Vegard’s fit of lattice parameters, Fourier transform infrared spectra, and two-probe electrical measurements on the antimony selenide nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.
(27) (28) (29) (30)
REFERENCES AND NOTES (1) (2) (3)
Bin Yang, R.; Bachmann, J.; Pippel, E.; Berger, A.; Woltersdorf, J.; Go¨sele, U.; Nielsch, K. Adv. Mater. 2009, 21, 3170–3174. Gribnyak, L. G.; Ivanova, T. B. Inorg. Mater. 1987, 23, 478–482. Platakis, N. S.; Gatos, H. C. Phys. Status Solidi 1972, 13, K1–K4.
© 2010 American Chemical Society
(31) (32)
4422
Messina, S.; Nair, M. T. S.; Nair, P. K. J. Electrochem. Soc. 2009, 156, H327–H332. Fernandez, A. M.; Merino, M. G. Thin Solid Films 2000, 366, 202– 206. Minnich, A. J.; Dresselhaus, M. S.; Ren, Z. F.; Chen, G. Energy Environ. Sci. 2009, 2, 466–479. 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, 1043–1053. Arun, P.; Vedeshwar, A. G.; Mehra, N. C. J. Phys. D 1999, 32, 183– 190. Chakraborty, B. R.; Ray, B.; Bhattacharya, R.; Dutta, A. K. J. Phys. Chem. Solids 1980, 41, 913–917. Rowe, D. M. Thermoelectrics handbook: Macro to nano; CRC Press: Boca Raton, Fl, 2005. Nolas, G. S.; Sharp, J.; Goldsmid, H. J. Thermoelectrics: Basic principles and new materials developments; Springer: New York, 2001. Rowe, D. M. CRC handbook of thermoelectrics; CRC Press: Boca Raton, FL, 1995. Zheng, X. W.; Xie, Y.; Zhu, L. Y.; Jiang, X. C.; Jia, Y. B.; Song, W. H.; Sun, Y. P. Inorg. Chem. 2002, 41, 455–461. Boukai, A. I.; Bunimovich, Y.; Tahir-Kheli, J.; Yu, J. K.; Goddard, W. A.; Heath, J. R. Nature 2008, 451, 168–171. Deng, Z. T.; Mansuripur, M.; Muscat, A. J. Nano Lett. 2009, 9, 2015–2020. Suarez-Sandoval, D. Y.; Nair, M. T. S.; Nair, P. K. J. Electrochem. Soc. 2006, 153, C91–C96. Purkayastha, A.; Yan, Q. Y.; Raghuveer, M. S.; Gandhi, D. D.; Li, H. F.; Liu, Z. W.; Ramanujan, R. V.; Borca-Tasciuc, T.; Ramanath, G. Adv. Mater. 2008, 20, 2679–2683. Purkayastha, A.; Kim, S.; Gandhi, D. D.; Ganesan, P. G.; BorcaTasciuc, T.; Ramanath, G. Adv. Mater. 2006, 18, 2958–2963. We note that if the microwave power is insufficient to heat the solution to temperatures above 180 °C, precipitation is suppressed. Antimony selenide: JCDPS 15-861; antimony sulfide: JCPDS 06474. Izquierdo, R.; Sacher, E.; Yelon, A. Appl. Surf. Sci. 1989, 40, 175– 177. Iwakuro, H.; Tatsuyama, C.; Ichimura, S. Jpn. J. Appl. Phys. 1982, 21, 94–99. We compared the ratio of the antimony sulfide to the antimony selenide sub-band intensities to estimate the thiolation of surface antimony atoms. Fan, H. J.; Go¨sele, U.; Zacharias, M. Small 2007, 3, 1660–1671. Bar-Sadan, M.; Kaplan-Ashiri, I.; Tenne, R. Eur. Phys. J. 2007, 149, 71–101. Zhang, B.; Jung, Y.; Chung, H. S.; Van Vugt, L.; Agarwal, R. Nano Lett. 2010, 10, 149–155. Wark, S. E.; Hsia, C. H.; Son, D. H. J. Am. Chem. Soc. 2008, 130, 9550–9555. Nanocrystal suspensions were chemically cleaned with a solution of 10% hydrazine in acetonitrile to remove the capping TGA layers prior to thin film deposition. The sulfurized antimony selenide nanocrystals and assemblies are n-type as verified by Seebeck coefficient measurements. Bao, H. F.; Cui, X. Q.; Li, C. M.; Song, Q. L.; Lu, Z. S.; Guo, J. J. Phys. Chem. C 2007, 111, 17131–17135. Zhang, Z. Y.; Jin, C. H.; Liang, X. L.; Chen, Q.; Peng, L. M. Appl. Phys. Lett. 2006, 88, No. 073102. Peng, H. L.; Xie, C.; Schoen, D. T.; Cui, Y. Nano Lett. 2008, 8, 1511–1516.
DOI: 10.1021/nl1020848 | Nano Lett. 2010, 10, 4417-–4422
