Article pubs.acs.org/crystal
Multiple Twinning and Stacking Faults in Silver Dendrites Vuk V. Radmilović,*,† Josh Kacher,‡ Evica R. Ivanović,§ Andrew M. Minor,∥,¶ and Velimir R. Radmilović⊥,# †
Innovation Center, Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia Department of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § Faculty of Agriculture, University of Belgrade, Nemanjina 6, Zemun, 11000 Belgrade, Serbia ∥ Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, United States ¶ National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, P.O.B. 3503, 11120 Belgrade, Serbia # Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000 Belgrade, Serbia ‡
ABSTRACT: Detailed defect structure of dendrite formation was studied in order to connect the mesoscopic with the atomistic structure. It was demonstrated that twinning and stacking fault formation play a central role in the growth of electrodeposited Ag dendrites. The broad faces of Ag dendrites and the main trunk growth direction were found to be (11̅ 1) and [1̅12̅], respectively. Dendrite branches also formed and grew from the main trunk parallel to the [121̅] and [2̅1̅1̅] crystallographic directions. Twins and stacking faults were found to reside on the {111} crystallographic planes, as expected for a face centered cubic (FCC) Ag crystal. Using electron back scattered diffraction (EBSD) we found two variants of in-plane 60° rotational twin domains in the (1̅11) broad dendrite surface plane. The intersections of twins and stacking faults with dendrite arm surfaces are perpendicular to the ⟨112⟩ arm growth directions. However, occasionally twins on the {111} planes parallel to the ⟨112⟩ arm growth directions were also observed. Although defect assisted dendrite growth is facilitated by twinning and stacking fault formation on {111} planes, the growth directions of the trunk and branches are not of the ⟨111⟩ type, but rather close to ⟨112⟩. The ⟨112⟩ growth directions are maintained by breaking dendrite facets into thermodynamically stable 111 and 200 steps and structural ledges of different length.
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INTRODUCTION A common property of low stacking fault metals is their susceptibility to the formation of stacking faults and twins, regardless of the driving forces. This can include, for example, temperature gradients during solidification or solid-state phase transformation, severe plastic deformation, overpotential in electrochemical deposition etc. As Ag has a low stacking fault energy, ∼16 mJ/m2,1 it is subject to profuse twinning and stacking fault formation. For example, two sets of twins, (1̅1̅1) [112] and (111)[112̅] have been observed in Ag nanowires with [111] growth direction, while in the Ag nanowires with a [112̅] growth orientation, twins and stacking faults parallel to the nanowire axis have been reported.2 Electrochemical synthesis of various shapes of silver nanostructures has an advantage over other synthesis methods as the electrochemical driving force, i.e., the overpotential, can be easily controlled. It has been reported that increasing the driving force for crystallization results in crystal shapes varying from strongly faceted nanoparticles, to various hierarchical structures like dendrites.3 Dendritic structures can be fabricated with high purity and yield,4 and in reproducible ways in © 2015 American Chemical Society
potentiostatic mode, i.e., keeping the voltage constant during deposition. Therefore, electrodeposition represents a powerful processing route that provides versatility in tailoring the architecture of metals on the micro/nanoscale. Dendritic growth during electrochemical deposition has been modeled by Mullins and Sekerka’s linear stability analysis,5 Barton and Bockris,6 and using a twin plane reentrant edge (TPRE) mechanism or Wagner, Hamilton, and Seidensticker (WHS) model.7,8 These models are used to explain how two parallel planes can create very high aspect ratios in silver nanoparticles.9 According to the TPRE mechanism, crystal growth takes place through the formation of reentrant grooves at the intersections of twin planes. Once a reentrant groove is formed, it becomes a favorable site for further nucleation and growth by the lateral motion of {111} planes. Consequently, the dendrite can be considered as a twinned single crystal with at least one twin boundary, which extends throughout the Received: October 12, 2015 Published: November 16, 2015 467
DOI: 10.1021/acs.cgd.5b01459 Cryst. Growth Des. 2016, 16, 467−474
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Figure 1. Schematic representation of the Ag dendrite sample mounted onto a 45° pretilted specimen holder, relative to the ion beam, electron beam, and EBSD detector, for material surface preparation: (a) Sample stage was tilted 7° for surface oxide removal, during which the ion beam was parallel to the Ag dendrite broad surface. (b) Sample stage was tilted 45° for SEM imaging, during which the electron beam was perpendicular to the Ag dendrite broad surface. (c) Sample stage was rotated 180° and tilted 25° for EBSD analysis, during which the Ag dendrite broad surface was at 70° with respect to incident electron beam.
The aim of this paper is to develop a reliable method for accurate identification and indexing of twin domain orientations on broad surfaces of Ag dendrites. In addition, our focus is to elucidate the presence of defects, such as twins and stacking faults, and their type, size, shape, and distribution in electrodeposited Ag dendrites. The main objectives of the present work are to confirm the crystallographic growth directions and to analyze distribution of twins and stacking faults in Ag dendrites and correlate them with anomalous diffraction effects, showing the presence of the 1/3 ⟨422⟩ reflection, forbidden in perfect face centered cubic metal crystal lattices, already reported for various silver nanoparticle shapes such as triangular platelets,37 nanodiscs,38 plate-belt nanoheterostructures,39 nanoprisms,40,41 and dendrites.35,42
dendrite while the twinning plane is parallel to the broad dendrite surface. Nano- and mesoscale metallic structures are of great interest to various fields of research due to their unique combination of properties and the ability to tune these properties by varying the structure geometries.10−12 For example, the optical properties of Ag nanostructures depend strongly on their size, shape, aggregation state, and local environment,13,14 and can be tunable through visible light and near-infrared regions of the spectrum.15 Silver nanostructures can be synthesized in a variety of shapes, such as platelets,16,17 nanoprisms,18 cubes,19 wires,20 dendrites,21,22 and many others.23−26 Because of their unique optical and electrical properties in all anisotropic nanostructural forms the investigation of Ag dendrites has attracted many different research groups around the world. It includes surface-enhanced Raman spectroscopy (SERS),27,28 electronics,29 catalysis,28,30 photonics,31 chemical and biological sensing,32 fabrication of superhydrophobic surfaces,33 as well as transparent electrodes for organic solar cell applications.34 Unfortunately, reproducible control of silver nanostructure morphology is not always an easy task and remains an active area of research. In addition, due to their size constraints it is not straightforward to characterize in an accurate and reliable way their defect structures, such as twin and stacking fault distribution, and crystallography in Ag dendrites. For example, some authors reported that the growth directions of electrodeposited silver dendrite main trunks and arms coincide with ⟨211⟩ directions.35 However, other authors reported, based on high resolution imaging close to ⟨110⟩ zone axis, that Ag dendrites, synthesized using a similar processing route, have the growth direction of the trunk along ⟨111⟩.22 We believe that the use of high resolution imaging in only one zone axis, in tandem with fast Fourier transforms (FFTs), to identify Ag dendrite preferential growth direction, does not necessarily provide accurate findings. This does not mean that the trunk growth direction in FCC metals and alloys cannot be of the ⟨111⟩ type, because depending on composition and processing route, the crystallography of dendrites in FCC metals can be different and can be governed by the interface instability and the anisotropy in interfacial properties and growth kinetics.36 It requires the use of several imaging and crystallography tools, such as conventional and high resolution electron microscopy, in both phase and Z contrast modes, orientation imaging, focused ion beam, channeling enhanced contrast scanning electron microscopy, and so forth.
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EXPERIMENTAL SECTION
The electrodeposition procedure used for Ag dendrite synthesis in this study has been reported elsewhere.43 After rinsing in distilled water and ethanol, the as-deposited Ag dendrites were kept in ethanol for further characterization. The morphology, topography, microstructure, and crystallography of Ag dendrites were evaluated using scanning electron microscopy (SEM). Orientation imaging microscopy (OIM), based on electronbackscattered diffraction (EBSD), was used to characterize the crystallography of the Ag dendrites. The use of OIM for analyzing the crystallography of Ag dendrites, grown by electrodeposition, is uncommon and, to our knowledge, has never been performed on freestanding dendrites of submicron size. To prepare the Ag dendrites for OIM analysis, the surface oxide layer was removed using focused ion beam (FIB) machining in a DB235 dual beam system. The dual beam system is equipped with an EBSD detector, facilitating OIM analysis without breaking the vacuum and exposing the sample to air.44,45 For FIB-machining and OIM analysis, the deposited Ag dendrites were mounted onto a 45° pretilted specimen holder. The tilting procedure for cleaning and analysis is shown in Figure 1. The 45° holder facilitates orientation of the Ag dendrite parallel to the ionbeam for oxide-layer removal (Figure 1a). The sample was tilted 45° for electron beam imaging (Figure 1b), and then to 70° with respect to the electron beam for EBSD analysis (Figure 1c). OIM results were obtained using a TSL-EDAX OIM system at 20 kV, with a spot size of 5, step size of 100 nm, and working distance of ∼10 mm. No data cleaning (interpolation) was performed on the data sets, but lowconfidence points were filtered out from the data. As the dendrites did not lie flat on the surface, the orientation data, once collected, was rotated to align ⟨111⟩ orientation normal to broad dendrite surface. Pole figures of the orientation data were constructed using a harmonic series expansion with a resolution of 5°. For transmission electron microscopy (TEM) characterization, the Ag dendrites were rinsed in ethanol and drop-cast on copper lacey 468
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carbon grids. Phase contrast high resolution TEM (HRTEM) and high angle annular dark field (HAADF) imaging were performed in the Titan-X and the aberration corrected TEAM 0.5 transmission electron microscopes operated at 200 kV and 300 kV, respectively.
Figure 3d are associated with two variants of 60° rotational twins, indicating the presence of multiple twin domains in each Ag dendrite single crystal. Figure 4a is a HAADF transmission electron micrograph of an Ag dendrite. All microstructural features, including the trunk and primary and secondary branches, exhibit similar contrast, indicating they all have similar thicknesses. The exceptions are bright white dots along the trunk, which indicate that some of the primary branches grew vertical with respect to the trunk and the broad dendrite surface. Since the broad dendrite surface has a {111} orientation (see Figure 5b), it seems that protrusions from this surface must grow along the ⟨111⟩, unless the growth direction is altered by the presence of twins and stacking faults. An enlarged segment shown in Figure 4b demonstrates the presence of a high density of lines indicated by white arrows, at 30° with respect to the trunk axis. These lines represent defects, either {111} stacking faults or {111} ⟨ 112⟩ growth twins lying on inclined {111} planes with respect to {111} broad dendrite surfaces. A conventional bright-field (BF) transmission electron micrograph and SAED (selected area electron diffraction) pattern of an Ag dendrite are shown in Figures 5a and b, respectively. The SAED pattern shows that the [1̅11] direction is normal to broad dendrite surface, i.e., the broad face of the Ag dendrite is (1̅11). Diffraction patterns taken from the same dendrite in several different orientations show that this is a single crystal with a FCC structure. Based on literature data available for Ag nanostructures,47 the SAED pattern shown in Figure 5b suggests that the Ag dendrite contains planar defects such as twins and stacking faults. Two sets of reflections are present in the SAED image in Figure 5b: very bright spots of the 220 type, which are typical for a FCC crystal, and faint spots closer to the transmitted beam. These faint spots are formally forbidden reflections in perfect FCC crystals, of the type 1/3 ⟨422⟩ (inner hexagon connected by white dotted lines), previously observed in trigonal lamellar particles (nanoprisms) of Au and Ag.9,37 The origin of these faint spots are stacking faults and nano- and microtwins on {111} planes parallel to the broad surface of the Ag dendrite and extending through the entire dendrite, similar to the diffraction effects observed in similarly synthesized Ag dendrites.35 The same SAED patterns have been obtained at every position along the main trunk of the dendrite in Figure 5a. By tilting one of the Ag dendrite arms onto a ⟨110⟩ zone axis, we can confirm the presence of a high density of defects, stacking faults, and twins, as shown by the high resolution phase contrast image in Figure 6. This is in agreement with previous observations in electrodeposited Ag dendrites.22,48−51 The angles between the trunk and the primary branches, as well as between primary and secondary branches, are all 60°. This angular relationship between branches indicates that the main axis of the dendrite is parallel to the [1̅12̅] crystallographic direction, and two sets of primary branches are parallel to [121̅] and [21̅ 1̅ ]̅ crystallographic directions, as shown in the SAED pattern in Figure 5b. Geometric Phase Analysis (GPA) was used to analyze local lattice rotation/distortion in the silver FCC lattice created by the presence of a high density of defects, twins, and stacking faults using high-resolution electron microscope (HREM) imaging. GPA is based on the calculation of local lattice distortion with respect to the undistorted lattice (referred to as
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RESULTS AND DISCUSSION Typical Ag dendrite topography and morphology, characterized by tilting individual Ag dendrites from 0° (perpendicular to the broad dendrite surface) to 90° (parallel to the broad dendrite surface) in the SEM, are shown in Figure 2. Highly anisotropically branched Ag dendrites, such as the one shown in Figure 2, consist of a trunk, primary, secondary, and, occasionally, ternary arms (branches).
Figure 2. Scanning electron micrographs of electrodeposited Ag dendrites taken at different tilt angles: 0° (perpendicular to the broad dendrite surface), 75°, and 90° (parallel to the broad dendrite surface).
By comparing SEM images taken at 0° and 90° tilt, it is clear that Ag dendrites are two-dimensional structures with large aspect ratios of width and length of approximately 10:1, with respect to the protrusion height, clearly visible at 90° tilt. Due to their large aspect ratio, they are often considered twodimensional (2D) crystals.46 By inspecting many different Ag dendrites, we have found that every dendrite contains multiple thin micro- and nanotwins and stacking fault lamellae parallel to the broad dendrite surface. These microtwins and stacking faults are clearly visible in the cross-sectional image montage shown in Figure 3d, obtained by FIB machining and electron beam channeling enhanced imaging, after protective carbon layer deposition. Figure 3d shows twins of different thicknesses due to random twinning events. The dark and light gray regions in 469
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Figure 3. (a,b) SEM images of an Ag dendrite taken at approximately 0° and 52° tilt, respectively; (c) SEM cross-sectional image recorded at 52° after protective carbon layer deposition and FIB cutting along the main trunk; (d) enlarged area indicated by white rectangle in (c) showing high density of twins; white arrows indicate 60° rotational twin boundaries.
Figure 4. HAADF transmission electron micrographs of an Ag dendrite showing: (a) white spherical islands along the trunk; (b) series of parallel lines indicated by white arrows, perpendicular to the dendrite arm axis, at 30° with respect to the trunk axis.
reference lattice), which allows mapping of displacement fields and strain fields.52 A high resolution electron micrograph of an Ag dendrite arm taken close to 110 zone axis, an appropriate moiré pattern, and a lattice rotation angle map, obtained by GPA are shown in Figures 6 a,b,c, respectively. The dotted white line in Figure 6a indicates traces of {111} planes. From the moiré pattern in Figure 6b and lattice rotation angle map in Figure 6c, it is very easy to identify the positions of stacking faults and twin planes running parallel to the silver dendrite arm axis. In addition to clear identification of twins and stacking faults, both images, Figure 6b and c, show lattice distortion within the same slab of twin segments. The results of the orientation imaging analysis are shown in Figure 7. The SEM micrograph shown in Figure 7a was recorded after the surface oxide layer was removed via FIB machining for EBSD analysis. From the lookup stereographic triangle in Figure 7b it is obvious that the broad dendrite
surface orientation is of the 111 type. White lines in Figures 7b and c indicate the positions and distribution of twin boundaries, with two twin variants shown in the orientation imaging colorcoded map in Figure 7c. Pole figures constructed from the orientation data also confirmed the presence of 60° rotational twins (Figures 7d and e). This can be clearly seen from the threefold symmetry of the white squares and triangles in Figures 7d and e, which represent 100 and 111 poles in the 111 standard stereographic projections, respectively. In order to generate either 100 or 111 pole figure of the twinned dendrite in Figure 7a, it is necessary to rotate 100 and 111 poles 60° around the 111 zone axis, i.e., 60° around the center of the 111 standard stereographic projection, as shown by the black squares and triangles, in Figures 7d and e, respectively. By comparing Figures 7a with b and c, it is clear that some details, such as the small, closely spaced dendritic branches, are lost in the orientation map due to the limited resolution of 470
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Figure 5. (a) BF TEM image taken close to [1̅11] and (b) SAED pattern of an Ag dendrite, showing (1̅11) broad surface plane, with ⟨112⟩ growth directions of the trunk and the branches indicated; the strong 220 reflections connected by solid white line hexagon and weak 1/3 422 reflections, forbidden in perfect single-crystal FCC metal structures, connected by dotted white line hexagon.
Figure 6. (a) HREM image of Ag dendrite arm taken close to 110 zone axis (the dotted white line indicates traces of {111} planes, (b) moiré pattern, and (c) lattice rigid rotation angle map; lookup table is given in radians.
The observed defect population suggests that Ag dendrite growth is facilitated by the formation of planar defects, including twinning and stacking fault formation on {111} planes. This is made possible by the low stacking fault energy of Ag (∼16 mJ/m2).1 The growth directions of a trunk and branches are not close to ⟨111⟩ as would be expected from the habitat planes of the twins and stacking faults, but rather close to ⟨112⟩, as was shown in the SAED pattern presented in Figure 5b. The HRTEM micrograph taken with the (110) plane perpendicular to the electron beam near the root of a dendrite branch (Figure 8) shows narrow twin bands and illustrates how the TPRE mechanism takes place and how twins are related to dendrite trunk and branch growth direction. Generally, growth kinetics of Ag crystals are limited by the slow growth of {111} facets. However, due to the low stacking fault energy of Ag, random twinning events may occur on the (111)-oriented
OIM analysis. However, the maps in Figures 7b and c clearly demonstrate a random distribution of two Ag twin variants rotated 60° around the 111 direction, perpendicular to the dendrite {111} broad surface plane, as shown in Figures 7d and e. These results confirm that the growth direction of the dendrite trunk is close to [1̅12̅] and the directions of the branches on both sides of the trunk are close to [121]̅ and [2̅1̅1]̅ . These results also show that, besides the commonly accepted presence of a twin plane parallel to the broad dendrite surface plane,17,18 the Ag dendrite structure is also composed of two twin variants related to each other by a 60° rotation around the [1̅11] crystallographic direction (sometimes called 180° rotational twins), as shown in the OIM color coded map (Figure 7c). The twins appear to be randomly distributed throughout the sample. 471
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Figure 7. (a) Scanning electron image, recorded after the FIB polishing of a dendrite for which the orientation map is seen in (b) lookup stereographic triangle shows crystallographic orientation of the dendrite; white lines indicate twin boundaries; (c) the orientation imaging color coded map showing two twin variants; (d,e) the 100 and 111 pole figures constructed from the orientation data. The color legend is a log-scale in units of random times; squares and triangles represent 100 and 111 poles in the 111 standard stereographic projections, respectively.
Figure 8. HRTEM micrograph of twins in the zone between the dendrite trunk and branch; inset is a FFT taken from the area indicated by the white square; on the right: schematic illustration of the role of twinning in the dendrite branch nucleation and growth.
direction are also observed. No visible deviation in orientation between the twinning bands has been observed. The presence of a two-twin configuration is shown schematically on the right in Figure 8, as was originally proposed for germanium dendrite formation.8 It appears that the ⟨112⟩ growth directions are maintained by breaking dendrite facets into thermodynamically stable 111 and 200 steps and structural ledges of appropriate length.
trunk facets, which create a reentrant corner and protruding twin junctions (ridge morphology) with angles of 141° and 219°, respectively, characterized by rapid nucleation and growth. Once multiple twinning events initiate, the dendrite branch begins to form, leaving behind the concave surface between the trunk and arm, as can be seen in the high resolution electron micrograph shown in Figure 8. It was stated a long time ago that the growth of metal crystals by reentrant edge mechanism requires the presence of two or more twin planes.7,8,53 Based on this fact, it can be concluded that the formation of dendrite branches must be accompanied by multiple twinning events. Most of the HRTEM images show the presence of a high density of nanotwins on the {111} planes. These twins are predominantly perpendicular to the ⟨112⟩ dendrite branch growth direction, as shown in Figure 8. However, twins on {111} plane parallel to the ⟨112⟩ dendrite branch growth
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CONCLUSIONS In order to elucidate the presence of defects, their type, size, shape, and distribution, electrodeposited Ag dendrites have been characterized by SEM, SAED, HRTEM, HAADF, and OIM electron microscopy techniques. Our findings demonstrate the importance of defect assisted growth in Ag dendrites. We demonstrate that Ag dendrites, besides twins and stacking faults running parallel to the {111} broad dendrite 472
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surface, also exhibit the presence of well-defined randomly distributed twin domains of irregular size and shape related to each other by in-plane 60° rotations about the ⟨111⟩ direction perpendicular to the broad dendrite surface. Stacking faults and nanotwins serve as heterogeneous nucleation sites stimulating dendrite growth by a ledge growth assisted mechanism. This leads to the creation of the broad surface of Ag dendrites and their large width- and length-to-thickness aspect ratios. The formation of Ag dendrite branches is not possible without the twinning of the {111} dendrite trunk facets. Although defect assisted dendrite growth is facilitated by twinning and stacking fault formation on {111} planes, the growth directions of the trunk and branches are not close to ⟨111⟩ as one would expect, but rather close to ⟨112⟩. The ⟨112⟩ growth directions are maintained by breaking dendrite facets into thermodynamically stable 111 and 200 steps and structural ledges of different length.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS V.V.R. and V.R.R. acknowledge support by the Ministry of Education, Science and Technological Development of the Republic of Serbia, under contract Nos. III45019 and 172054, respectively. V.R.R. acknowledges support by Serbian Academy of Sciences and Arts under contract #F-141. Electron microscopy was performed at the Molecular Foundry, which is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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DOI: 10.1021/acs.cgd.5b01459 Cryst. Growth Des. 2016, 16, 467−474
