Letter pubs.acs.org/NanoLett
Catalyst Incorporation at Defects during Nanowire Growth Eric R. Hemesath,† Daniel K. Schreiber,† Emine B. Gulsoy,† Christian F. Kisielowski,‡ Amanda K. Petford-Long,†,§ Peter W. Voorhees,† and Lincoln J. Lauhon*,† †
Department of Materials Science and Engineering, Northwestern University, 1881 Sheridan Road, Evanston, Illinois 60208, United States ‡ National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States § Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States S Supporting Information *
ABSTRACT: Scanning and transmission electron microscopy was used to correlate the structure of planar defects with the prevalence of Au catalyst atom incorporation in Si nanowires. Site-specific high-resolution imaging along orthogonal zone axes, enabled by advances in focused ion beam cross sectioning, reveals substantial incorporation of catalyst atoms at grain boundaries in ⟨110⟩ oriented nanowires. In contrast, (111) stacking faults that generate new polytypes in ⟨112⟩ oriented nanowires do not show preferential catalyst incorporation. Tomographic reconstruction of the catalyst−nanowire interface is used to suggest criteria for the stability of planar defects that trap impurity atoms in catalyst-mediated nanowires. KEYWORDS: Nanowire, TEM, grain boundary, impurity, tomography, crystal growth
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mpurities in crystals tend to segregate to grain boundaries1 with dramatic consequences for their physical behaviors. The relationship between impurity and defect distributions is thus a foundation of materials engineering for structural,2−4 energy,5,6 and electronic7−9 applications. Recently, catalyst-mediated nanowire growth processes10,11 have been used to realize new polytypes composed of ordered planar defects, specifically twinning superlattices.12−15 Polytypes are compositionally identical materials with different crystal structures and therefore different electronic properties,16 offering the potential to engineer band structure and new types of heterojunctions through defect engineering. Given the consequences of unintentional17 and/or nonuniform18 impurity incorporation for physical properties, it is important to characterize the prevalence of planar defects in nanowires and their tendency to trap impurity atoms from the catalyst19 or the growth environment. For example, twinning was controlled by the presence or absence of dopant impurities in the work of Algra et al,12 but neither the precise role nor the impurity distribution was established. In addition, we provided definitive evidence of new polytypes arising from (111) stacking faults in Aucatalyzed Si nanowires,14 but our earlier observation of single Au atoms decorating Σ3 (111) twin boundaries19 raises the question of whether one should expect catalyst atom incorporation in nanowires with planar defects, and if so, at which types of defects? Here we report the substantial incorporation of metal catalyst atoms at grain boundaries in Si nanowires grown by the vapor− liquid−solid (VLS) process. Correlated aberration-corrected © 2011 American Chemical Society
scanning and transmission electron microscopy (STEM and TEM) and tomography allow us to discriminate between planar defects that do and do not trap catalyst atoms. Site-specific high-resolution imaging along orthogonal zone axes, enabled by advances in focused ion beam cross sectioning, reveals substantial incorporation of catalyst atoms at grain boundaries in ⟨110⟩ oriented nanowires. In contrast, (111) stacking faults that generate new polytypes in ⟨112⟩ oriented nanowires do not show preferential catalyst incorporation, indicating that these materials may be suitable for electronic devices. Tomographic reconstruction of the catalyst−nanowire interface is used to suggest criteria for the stability of planar defects that trap impurity atoms in catalyst-mediated nanowires. Si nanowires were grown from Au nanoparticle catalysts and silane (SiH4) at 2 Torr partial pressure in a hot-walled 1 in. tube furnace at 450 °C. N2 was used as a dilutant gas to maintain a total pressure of 50 Torr at a total flow rate of 50 sccm. Immediately following nanowire growth, while still at the growth temperature, the chamber was evacuated and filled to 20 Torr with O2 to promote SiOx formation, as silicon oxide has been observed to suppress Au surface migration and therefore preserve the catalyst for ex situ imaging. Nanowires were suspended in solution by sonicating the growth substrate in isopropyl alcohol and were pipetted directly onto lacey carbon grids for (S)TEM analysis. Microscopy was conducted Received: September 18, 2011 Revised: November 2, 2011 Published: November 23, 2011 167
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(111) stacking fault arrays, or polytypes,14 in ⟨112⟩ oriented nanowires. Single-atom resolved imaging by aberration-corrected Zcontrast scanning transmission electron microscopy (STEM) confirmed that Au atoms do not decorate (111) stacking faults, but rather the termination of these faults at internal grain boundaries. A focal series of Z-contrast STEM images in another ⟨110⟩ oriented nanowire locates Au atoms (Figure 2a,
on the JEOL 2100F at the NUANCE facility at Northwestern University and also at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory on the TEAM 0.5, a modified FEI Titan microscope operated at 300 kV, equipped with a high-brightness Schottky field emission electron source, a gun monochromator, a high-resolution GIF Tridiem energy filter, and two CEOS hexapole-type spherical aberration correctors. A focused ion beam was used to perform site-specific cross-sectioning following identification of regions of interest in traditional plan-view imaging. A detailed description of the process will be reported elsewhere.20 Parallel lines of strong high-angle scattering were consistently observed in Z-contrast plan-view STEM images of ⟨110⟩ oriented nanowires whose diffraction patterns indicated the presence of planar defects parallel to the growth direction (Figure 1a,b). Cross-sectional images of the nanowire segment
Figure 1. ⟨110⟩ oriented Si nanowire with Au-decorated grain boundary: Plan view HRTEM (a) and Z-contrast STEM (b) images. Diffraction pattern inset in (a) is consistent with a bicrystalline nanowire. Cross-sectional HRTEM (c) and Z-contrast STEM (d) images. Dashed lines indicate ⟨11-2⟩ directions. The misorientation is 13°. Scale bars are 10 nm.
shown in Figure 1a,b reveal that columns of Au impurity atoms decorate a low-angle (13°) grain boundary (Figure 1c,d). The periodicity is consistent with impurity incorporation at periodic edge dislocations that can accommodate Au atoms at their termini. Given the nonsymmetric nature of the grain boundary, the exact structure is not known but is expected to be composed of a series of edge dislocations with mutually perpendicular Burger’s vectors. We note that not all Au lines are visible in Figure 1d due to strong channeling effects that enhance scattering from the Si matrix at this orientation. In addition to the grain boundary defect, there are many (111) stacking faults within each crystal, but no high-angle scattering is observed in excess of the Si background signal, which suggests that these defects do not trap impurities. Consistent with this claim, Z-contrast imaging of tens of nanowires with planar defects found that Au atoms were commonly incorporated in ⟨110⟩ oriented nanowires but not ⟨112⟩ oriented nanowires, despite the frequent observation of ordered
Figure 2. Distribution of Au atoms in Si nanowire with planar defects: (a) Plan view aberration-corrected Z-contrast image resolving individual Au atoms. Scale bar is 5 nm. (b) Corresponding phase component of an exit wave reconstruction with yellow lines indicating regions of distinct spatial periodicity. (c) Cross-sectional HRTEM image with superposed Au atom distribution extracted from Z-contrast focal series.
movie M1, Supporting Information) concentrated at boundaries between regions of distinct spatial periodicity in highresolution TEM (HRTEM) images (Figure 2b). While the phase image in Figure 2b was derived from an exit wave 168
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iterative reconstruction technique (SIRT). The data set was segmented into four regions, namely the background, the Si wire body, the Au droplet, and noise using expectation maximization/maximization of posterior marginals (EM/ MPM) method.25 The Au droplet region was then further processed by histogram thresholding. Figure 3a,b shows two images from the tilt series used in reconstructing the Au distribution. The entire series of images may be viewed as movie M2, Supporting Information. The growth interface appears faceted, with the Au lines terminating at a central trough (Figure 3a and Figure S1, Supporting Information). Interestingly, an additional third line of finite length can also be seen, indicating that the grain boundary may rearrange locally but is globally stable. Cross-sectional HRTEM imaging and diffraction (Figure 3c) establishes that the trough is located at a bisecting grain boundary between two crystals with common [11̅0] growth directions and a relative misorientation about the growth axis of ∼30°. An image generated by a filtered inverse fast Fourier transform (iFFT, Figure 3d) identifies the two crystals. The upper crystal (red shading) is nearly defect free, whereas the twinned lower bicrystal (blue and green shading) contains several faults on (111) planes approximately normal to the grain boundary. It is intriguing to note that despite the distinct orientations of the two halves of the nanowire and the twinning in the bottom crystal, the faceting of the Au−Si interface on either side of the grain boundary is remarkably similar (Figure 3a). We hypothesize that Au-trapping grain boundaries are common in defective ⟨110⟩ oriented nanowires because low-index, low-energy liquid−solid interface planes can be found whose intersections are parallel to the grain boundary and perpendicular to the growth direction. The grain boundary can act as a source of steps, kinetically stabilizing growth parallel to the grain boundary. To explore this hypothesis, we compare the faceting of the growth interface in our ⟨110⟩ nanowires with that of ⟨111⟩ and ⟨112⟩ nanowires, all of which exhibit {111} growth interfaces. To investigate the faceting more quantitatively, we generated stereographic projections of the distribution of surface normals (Figure 4c) associated with the growth interface (Figures 4a,b) from each half of the nanowire, with the caveat that we assume the interface structure examined in postgrowth imaging is similar to what would be observed during growth as examined by in situ microscopy.26 The growth interface identified in the segmentation analysis was transformed into a triangular mesh using the built-in functions of interactive data language (IDL), and the probability of finding a normal of a given orientation of each triangle on the solid−liquid interface was plotted on a stereographic projection. We find that the right-hand liquid− solid interface (red, Figure 4b) is composed of two {111} interfaces whose intersection is parallel to the (001̅) termination at the grain boundary. These two {111} planes are the lowest index planes closest to the [11̅0] growth direction and therefore reduce the liquid−solid interfacial energy. The left-hand crystal (blue, Figure 4b) exhibits a qualitatively similar shape: There are two major peaks in the surface normal distribution, and these two peaks have the same angular separation of those in the right-hand crystal projection. The outermost interface is identified as a {112} type plane that is common to both crystals (blue and green regions of Figure 3e). Remarkably, the innermost interface is comparably “flat”, that is the probability density of surface normals is highly concentrated, though we cannot assign this feature to a single low-index plane. Furthermore, the angle between the two
reconstruction that could be used, in principle, to solve the structure, we took a more direct approach by performing focused ion beam cross-sectioning of the nanowire region of interest.20 HRTEM imaging of the cross-sectioned nanowire revealed that the varying spatial frequencies in the phase image (Figure 2b) arise from a combination of stacking faults and finite crystal size along the beam direction along the plan-view imaging direction.21 Significantly, the horizontal locations of the Au atoms in Figure 2a coincide with the positions of the grain boundaries visible in Figure 2c. A simple analysis of the scattering intensity variation of single Au atoms with focal depth in the Z-contrast imaging19,22 confirms that Au atoms decorate the grain boundaries (parallel to (112̅)) between the two crystals but not the (111) stacking faults within the left-hand crystal (Figure 2c). (The density of Au atoms observed away from the central defect is approximately equal in the faulted and unfaulted crystal.) While the atomic configuration of the grain boundary is not known, this observation is consistent with the differences in atomic structure of the symmetric Σ3 (111) and Σ3 (112̅) grain boundaries in Si; whereas bond lengths and angles are preserved in the Σ3 (111) structure, a relative translation of the two crystals at the Σ3 (112̅) boundary generates a dislocation that can accommodate solute atoms.23 In bulk specimens, one typically analyzes the partitioning of impurities between the bulk and interfacial phases by comparing their relative solubilities,1 but the solid solubility of Au in Si is negligible at the growth temperature,19,24 so there should be very little ‘bulk’ Au to segregate to the grain boundary. Instead, the Au is incorporated directly from the liquid catalyst during growth, implying an intimate connection between growth direction, defect structure, and the prevalence of catalyst incorporation. To prove this assertion, we located several nanowires in which grain boundaries persisted to the intact catalyst tip. Lines of incorporated Au atoms terminating at the growth interface are visible for the example shown in Figure 3, providing direct
Figure 3. Impurity incorporation from the growth interface: (a) Zcontrast image of nanowire with lines of Au impurity leading to catalyst. (b) View of nanowire from (a), rotated about the growth axis. Scale bar is 25 nm. (c) Cross-sectional HRTEM image of the same nanowire. Scale bar is 10 nm. (d) Filtered inverse FFT image identifying upper crystal (red) and lower twinned crystal (blue, green). Reciprocal space filter shown in upper left inset. Equivalent ⟨111⟩ directions shown as blue and green arrows in (c).
evidence of incorporation from the catalyst during growth. A tilt series of high-angle annular dark field (HAADF) STEM images was collected with a 2 Å uncorrected electron probe at multiple angles of rotation about the growth axis (±68° in 2° increments) to enable tomographic reconstruction and subsequent segmentation analysis of the Au distribution. The aligned images were reconstructed using the simultaneous 169
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be the largest,30 contrary to observations. This points to the predominance of the kinetics of step nucleation at the grain boundary and VLS trijunction in establishing the interface shape in defective ⟨112⟩ and ⟨110⟩ oriented nanowires, in agreement with recent in situ measurements.26 Our findings of grain boundary segregation in free-standing nanoscale crystals are consistent with expectations of interfacial segregation in bulk polycrystalline materials,1 but the strong correlation between growth direction, defect structure, and impurity segregation is a distinct aspect of these onedimensional materials. These results suggest that growth conditions should be selected to promote ⟨111⟩ over ⟨110⟩ orientations to avoid persistent grain boundaries. We emphasize, however, that Au is not found at (111) stacking faults, whose ordering in ⟨112⟩ oriented nanowires produces new polytypes12,13 with altered bandstructures that could be used to make new types of electrical junctions for light emission and energy harvesting applications. In this context, the need for studies of defect-correlated dopant distribution is underscored, given that the segregation of impurities smaller than the Au atoms studied here may not be limited to relatively open grain boundaries.31,32 Our three-dimensional analysis of impurity distributions by correlated plan-view and cross-sectional electron microscopy and tomography establishes the feasibility and importance of such investigations.
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ASSOCIATED CONTENT S Supporting Information * Movies and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 4. Faceting of growth interface. (a) Schematics of growth interfaces for different growth directions. White lines indicate planar defects. At right, reconstruction of nanowire from Figure 3. (b) Reconstruction of central portion of Au−Si interface. (c) Stereographic projections of the probability density of the normals from the interfaces in (b).
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AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected].
dominant interfaces is precisely the same as that between the two {111} interfaces in the right-hand crystal. Apparently, the thermodynamic and kinetic criteria that establish the interface shape lead to a similar solution for each crystal, with a ‘pseudofacet’ forming for the twinned crystal. The interface shape and growth direction are determined by the combined influences of facet growth kinetics and energetics. For a crystal growing from another phase, where the mobility of the interface is rate limiting, the growth shape at long times is dominated by the slowest moving facet and must be convex.27 By contrast, the solid−liquid interfaces in a nanowire intersect other interfaces that can act as nucleation sites for steps and thereby alter this result. For ⟨111⟩ oriented nanowires (Figure 4a) the solid−liquid interface is predominantly the slow growth rate low interfacial energy {111} facet of Si.28 In this case, a small corner facet forms at the VLS trijunction that promotes nucleation of new crystal planes.26,29 For ⟨112⟩ oriented wires, the low interfacial energy {111} facets are present, but the interface is concave with a twin boundary at the bottom of the trough (Figure 4a, Figure S2, Supporting Information). It is well-known that Si dendrites grow in the ⟨112⟩ direction with a concave interface, but more than one twin is required for steady-state growth. In the ⟨112⟩ oriented nanowire, the required second source of steps is likely the VLS trijunction. The ⟨110⟩ oriented nanowires with Au-trapping grain boundaries appear to be stabilized by the same mechanisms, with the grain boundary acting as a source of steps on {111} planes. The larger surface energy of the VS surface compared to the twin or grain boundary suggests that the outer facet should
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ACKNOWLEDGMENTS We acknowledge support of the National Science Foundation through DMI-0507053 and DMR-1006069. Microscopy in this work was performed, in part, at the EPIC facility of NUANCE Center at Northwestern University. The NUANCE Center is supported by NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University. Additional microscopy was performed at NCEM, which is supported by the Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under contract no. DE-AC0205CH11231.
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