Communication pubs.acs.org/cm
Synthesis and Characterization of Ternary SnxGe1−xSe Nanocrystals Jannise J. Buckley, Federico A. Rabuffetti, Hannah L. Hinton, and Richard L. Brutchey* Department of Chemistry and the Center for Energy Nanoscience and Technology, University of Southern California, Los Angeles, California 90089-0744, United States S Supporting Information *
KEYWORDS: tin selenide, germanium selenide, semiconductor, nanocrystal, alloy
T
While the SnxGe1−xSe alloy has been synthesized in the bulk form with complete solid solubility (0 ≤ x ≤ 1),12 it has yet to be synthesized in the nanocrystal form. Herein, we communicate the first synthesis of ternary SnxGe1−xSe nanocrystals, and demonstrate tunable compositions throughout the entire alloy range (0 ≤ x ≤ 1). Compositional tuning of the lattice parameters, band gaps, and morphologies was demonstrated and the alloy formation mechanism was investigated. Both the ternary SnxGe1−xSe nanocrystals and the SnSe/ GeSe end-members were synthesized using a di-tert-butyl diselenide precursor, as previously reported by our group for the synthesis of a variety of metal selenide nanocrystals.13 In short, all ternary SnxGe1−xSe nanocrystals were prepared by injecting di-tert-butyl diselenide (0.10 mmol) and hexamethyldisilazane (HMDS, 10 mmol) into a dodecylamine solution (13 mmol) containing varying ratios of GeI4 and SnI4 (0.40 mmol total) at 95 °C. The reaction was then heated to 225 °C and held at that temperature for 4.75 h. It is interesting to note that without the addition of HMDS, the reaction produces phase pure SnSe nanocrystals with no evidence of germanium incorporation (see the Supporting Information, Figure S1). Therefore, the addition of HMDS was found to be essential for alloy formation as it may aid in the reduction of Ge(IV) to Ge(II), which is consistent with the more negative reduction potential of Ge(IV) relative to Sn(IV). The SnSe and GeSe end-members were synthesized using identical procedures to that described above with the exclusion of either GeI4 or SnI4, respectively. Additionally, obtaining crystalline GeSe in the orthorhombic Pnma phase required the omission of HMDS and higher injection temperatures (180 °C). When GeSe was synthesized in the presence of HMDS, an unknown phase of germanium selenide was obtained with a 50:50 Ge/Se stoichiometry (see the Supporting Information, Figure S2). The elemental composition of the SnxGe1−xSe nanocrystals was determined using energy dispersive X-ray spectroscopy (EDS). The composition of the ternary SnxGe1−xSe nanocrystals was modulated by varying the stoichiometry of the SnI4 and GeI4 precursors used during synthesis (see the Supporting Information, Table S1, Figure S3). A slight deviation between the Sn/Ge ratio introduced during synthesis and the Sn/Ge
he ability to tune IV−VI semiconductor nanocrystals through alloying can have a direct effect on their optoelectronic properties, which can in turn affect device performance.1 For example, Alivisatos et al. described the use of ternary PbSxSe1−x nanocrystals in Schottky junction photovoltaic (PV) devices that demonstrated their compositionally dependent device performance.1a Schottky devices made with these PbSxSe1−x nanocrystals gave higher power conversion efficiencies (3.3%) than either of the PbS (∼1.8%) or PbSe (∼1.4%) end-member lead chalcogenides. Although the potential utility of alloyed ternary semiconductor nanocrystals is clear, their synthesis is often difficult.2 Inherent differences in precursor reaction kinetics often lead to inhomogeneous internal structures (i.e., gradient or core/shell structures).3 A number of strategies have been developed to balance the reactivity of the precursors and obtain homogeneous alloys in the following IV−VI systems: PbSe x Te 1−x , PbS x Se 1−x , PbxSn1−xTe, PbSxTe1−x, PbxSn1−xS, and SnSxSe1−x.4 The layered IV−VI semiconductor nanocrystals (i.e., SnS, SnSe, GeS, GeSe) have recently gained popularity as potential alternatives to the lead chalcogenides, due in part to their relatively higher stability and environmental sustainability.5 In the bulk, SnSe is a native p-type semiconductor with a high absorption coefficient (∼1 × 104 cm−1) and narrow indirect and direct band gaps (Eg = 0.9 and 1.3 eV, respectively) that are close to the optimal band gap values for single junction solar cells.6,7 Similar to the lead chalcogenides, SnSe has a relatively large Bohr exciton radius (>23 nm) and exhibits strong sizedependent optical properties enforced by quantum confinement effects.8 However, with the exception of the work by Hanrath and co-workers, it has thus far been difficult to tune the band gap of resulting SnSe nanocrystals through nanocrystal size control within the quantum confined regime.9 Another potential way of tuning the optoelectronic properties of the SnSe nanocrystals is to alloy SnSe and GeSe to make ternary SnxGe1−xSe nanocrystals. Alloyed semiconductor nanocrystals can display compositionally tunable properties, distinct from both their bulk alloys and binary nanocrystalline endmembers. Bulk GeSe exhibits similar properties to SnSe. Both materials possess a thermodynamically preferred orthorhombic crystal structure described as a highly distorted rock salt structure consisting of strongly bound double layers.10 In addition to being a native p-type semiconductor with a high absorption coefficient, GeSe has a slightly larger band gap (Eg = 1.1 − 1.2 eV) than SnSe with reduced lattice parameters.11 © 2012 American Chemical Society
Received: July 26, 2012 Revised: August 20, 2012 Published: August 27, 2012 3514
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Communication
ratio incorporated into the isolated SnxGe1−xSe nanocrystals was observed. In compositions where x < 0.9, it was observed that greater than nominal germanium concentrations were incorporated into the SnxGe1−xSe nanocrystals at the expense of tin incorporation. Furthermore, slight stoichiometric deviations from the expected 50:50 metal(loid)/chalcogen ratio were also observed in all samples. All of the nanocrystals were selenium-poor (with deviations between 2.8 and 10%) given the selenium-limited (2:1 metal(loid)/selenium mole ratio) reaction conditions required for alloy formation. Powder X-ray diffraction (XRD) revealed that all of the SnxGe1−xSe nanocrystals were prepared phase pure (Figure 1a).
Diffuse reflectance UV−vis−NIR spectroscopy was used to investigate the optical properties of the SnxGe1−xSe nanocrystals. It was observed that as the composition of the SnxGe1‑xSe nanocrystals becomes more germanium rich, the onset of absorption systematically blue-shifted from ca. 1400 nm (Figure 2). Applying Kubelka−Munk functions to the reflectance
Figure 2. Diffuse reflectance spectra for SnxGe1−xSe nanocrystals. The spectra have been offset for clarity.
spectra allowed indirect band gap transitions to be estimated ([F(R)hν]0.5), which were found to range from Eg = 0.87−1.13 eV as the composition of the SnxGe1−xSe nanocrystals was tuned from x = 1.0 − 0.2, respectively (see the Supporting Information, Figures S6 and S7). These values generally fall within the range of bulk indirect band gaps previously reported for SnSe and GeSe nanocrystals.9b,15 Homogeneous nanocrystal alloys can result when the nucleation and growth rates of the two constituent materials are similar. Here, it is apparent that SnI4 is kinetically more reactive than GeI4 toward the active selenium source, as evidenced by a shorter nucleation time for the SnSe nanocrystal synthesis (i.e., 1 h for the GeSe nanocrystal synthesis). This obvious reactivity difference prompted an investigation into the mechanism of alloy formation. Timed-aliquot XRD studies were performed to monitor the change in nanocrystal composition during growth. For example, when SnI4 and GeI4 were added to the reaction in a 4:1 mol ratio, the initial XRD pattern of the aliquot taken ca. 20 min after nucleation appeared close to that of SnSe (a = 11.44 Å, b = 4.16 Å, c = 4.35 Å). Subsequent XRD patterns of aliquots taken at longer time intervals display a gradual shift to higher 2θ values as the reaction proceeds to completion. After 4.75 h, the nanocrystals exhibit lattice constants of a = 11.28 Å, b = 4.04 Å, and c = 4.43 Å, as expected from nanocrystals with an approximate Sn0.6Ge0.4Se composition (vide supra; see the Supporting Information, Figures S8 and S9). This implies that nucleation of the crystalline material begins as SnSe (or a tinrich selenide) and gradually incorporates germanium over the growth period, suggesting the potential involvement of a cation exchange mechanism. This also points to the difference in the nucleation and growth kinetics, where the nucleation kinetics under the current conditions favor the formation of SnSe, whereas germanium incorporation or exchange occurs during growth until the final composition is achieved. To investigate the possibility of a cation exchange mechanism, we first synthesized SnSe nanocrystals and then
Figure 1. Structural characterization of SnxGe1−xSe nanocrystals. (a) XRD patterns for select SnxGe1−xSe compositions showing the orthorhombic Pmna crystal structure. (b) Dependence of the lattice constants and unit-cell volume on germanium content.
Nanocrystals of both SnSe and GeSe were indexed to the orthorhombic phase of the GeS structure, with a Pnma space group (PDF#01−075−6133 and PDF#01−075−1802 for SnSe and GeSe, respectively). The calculated lattice constants extracted from Rietveld analysis for SnSe (a = 11.5040(9) Å, b = 4.1563(4) Å, c = 4.4301(5) Å) and GeSe (a = 10.841(2) Å, b = 3.8335(8) Å, c = 4.3877(12) Å) closely match literature values. The SnxGe1‑xSe nanocrystals display the same overall diffraction patterns as the SnSe and GeSe end-members, indicating that they are isostructural with the orthorhombic Pnma crystal structure. As expected with the smaller cationic radius of Ge2+ (cationic radii of Ge2+ and Sn2+ are 0.73 Å and 0.93 Å, respectively14), a gradual shift to higher 2θ values was observed as the germanium content in the SnxGe1−xSe nanocrystals increased. The lattice constants and unit cell volumes of the SnxGe1−xSe nanocrystals were determined from the XRD patterns using Rietveld analysis (see the Supporting Information, Figures S4 and S5, Table S2) and compared against the experimentally determined elemental compositions (Figure 1b). The a and b lattice constants clearly demonstrate a linear dependence on the composition as they decrease monotonically with increasing germanium content. This linear relationship is consistent with Vegard’s law and establishes the compositional homogeneity of the nanocrystals in these crystallographic directions. Noteworthy, however, is the fact that the c lattice constant varies slightly from linearity and tends to bow with compositional changes even though the overall unit cell volume appears to vary linearly with composition. The bowing phenomenon of the c lattice constant is in accord with previously published literature for bulk material.12b 3515
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ACKNOWLEDGMENTS This work is supported by the National Science Foundation under DMR-1205712. J.J.B. acknowledges National Science Foundation for a Graduate Research Fellowship and R.L.B. acknowledges the Research Corporation for Science Advancement for a Cottrell Scholar Award.
allowed them to react with the germanium source via injection of GeI4 in dodecylamine after 1 h. Powder XRD and EDS were used to monitor the structural and compositional changes before and after addition of the GeI4 to the SnSe nanocrystals. The XRD pattern taken before introduction of GeI4 matches that of SnSe, while the XRD pattern taken 3.75 h after introduction of GeI4 suggests germanium incorporation with lattice constants of a = 11.14 Å, b = 3.96 Å, and c = 4.43 Å, in addition to some unreacted SnSe (see the Supporting Information, Figure S10). EDS confirms the compositions before and after introduction of GeI4 as nearly 50:50 Sn/Se and 25:25:50 Sn/Ge/Se, respectively. These results suggest that germanium may indeed alloy into SnSe through a possible cation exchange mechanism. This is similar to a recent report by Schaak and co-workers that described the transformation of SnSe to SnTe through an anion exchange mechanism.16 Interesting morphological changes also resulted as the SnxGe1−xSe nanocrystal composition was varied. Transmission electron microscopy (TEM) was used to monitor the morphological differences between the sheetlike SnSe nanocrystals and the rodlike morphologies exhibited by the germanium-rich Sn0.1Ge0.9Se nanocrystals (Supporting Information, Figure S11). The SnSe nanosheets adopted rectangular morphologies with lateral dimensions varying between 0.7−1.0 μm × 0.5−1.0 μm in size. The Sn0.1Ge0.9Se nanorods vary in size with a range of dimensions from 70−100 nm in width to 0.5−1.5 μm in length. All intermediate compositions inherited morphological features from both compositional extremes; forming intermediate sheetlike aggregates comprised of bundled nanorods. This phenomenon has been previously reported in PbxSn1−xS, where the PbS and SnS end-members exhibit cube and spherical morphologies, respectively, and the intermediate Pb0.85Sn0.15S nanocrystals inherit aspects of both end-members with truncated cube morphologies.4c In summary, a facile synthetic method for compositional control in SnxGe1−xSe nanocrystals has been described for the first time. Homogeneous internal structures were obtained, despite the inherent relative differences in the cation precursor reaction rates. Investigation into the formation mechanism revealed the initial formation of SnSe or tin-rich selenide followed by a partial cation exchange with Ge2+. The band gaps, lattice parameters, and morphologies of the alloys were tuned via nanocrystal composition. This alloying approach provides an additional means of band gap tuning that is otherwise difficult to achieve in the layered IV−VI semiconductors; making these materials potential candidates as photovoltaic materials.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details; XRD pattern of alloy synthesized without HMDS and GeSe synthesized with HMDS; EDS composition data; Rietveld analysis of XRD patterns; calculated indirect band gaps and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.
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Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 3516
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