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Intrinsic Lattice Relationship of Catalyst/Nanowire Interfaces by Heating High-Resolution Transmission Electron Microscopy Tingting Wang,† Junli Wang,*,† YaJie Qiao,† Junhao Zhang,‡ Hua Tang,† Xiaofei Yang,† Kangmin Chen,† Guiwu Liu,† and Guanjun Qiao*,† †

School of Materials Science & Engineering, Jiangsu University, Zhenjiang 212013, P. R. China School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, P. R. China

‡

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S Supporting Information *

ABSTRACT: The clarification of intrinsic, atom-level catalyst/nanowire interfacial microstructures is of great help in understanding the underlying catalytic mechanisms and realizing controllable nanowire growth. Herein, we investigate the interfacial microstructures and lattice relationship between body-centered cubic (bcc) superionic-phase α-Ag2Se and facecentered cubic (fcc) zinc blende (ZB) ZnSe for the Ag2Se/ ZnSe catalyst/nanowire growth system by high-resolution imaging in heating high-resolution transmission electron microscopy (heating HRTEM). It is found that the bcc αAg2Se (110) and fcc ZB-ZnSe (111) pair planes make up the most preferred adjoining interface, which theoretically have a considerable lattice misfit but probably exhibit a nearly complete, two-dimensional (2D) planar coherent relationship through the lattice straining of the ZB-ZnSe (111) plane along a specific crystallographic orientation (i.e., [110]). A reasonable atomic model, consistent with the heating HRTEM results, is presented to demonstrate the strained, 2D coherent α-Ag2Se/ZB-ZnSe catalyst/nanowire interface. Meanwhile, we rationally explain the formation of this specific α-Ag2Se (110)/ZB-ZnSe (111) interface from the views of the atomic closest packing and nanowire growth habits, the interfacial energy and activity, the small size effect on lattice strain and deformation, as well as the effect of the solubility of the nanowire in the catalyst on interdiffusion.

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nanowires,14−18 which are composed of a catalyst head and wire stem and thus an ideal platform for investigating interfacial structure information between a nonmetal catalyst and nanowire. However, the superionic-phase α-Ag2Se is a high-temperature (HT) phase.12,13,15,19 As the temperature cooled to the room temperature (RT), the α-phase of Ag2Se catalyst heads was found to convert to a low-temperature (LT) metastable phase, tetragonal phase (t-Ag2Se),14−17 whereas metal selenide nanowire stems, for example, ZnSe, mainly crystallizing in the face-centered cubic (fcc) zinc blende (ZB) form,14−16 still keep their crystal phase unchanged. Obviously, owing to the temperature-dependent α-Ag2Se → t-Ag2Se phase transition, the interfacial microstructures and lattice relation of α-Ag2Se/ ZB-ZnSe cannot be detected by using the conventional RT transmission electron microscopy (TEM). The heating TEM, which can be operated at different temperatures to record the phase transition and crystal structure of solids,15,20−22 will be an effective tool to reveal the microstructural details and lattice relationship of the α-Ag2Se/ZB-ZnSe catalyst/nanowire interface at high temperatures at which α-Ag2Se is stable.

INTRODUCTION Semiconductor nanowires are one of the central building blocks in the revolutionary nanoscience and nanotechnology for electronic, photonic, photoelectronic, and bioelectronic applications.1−3 Catalytic growth mechanisms have been considered as the most efficient methods to prepare various kinds of crystalline semiconductor nanowires, where catalyst nanoparticles guide the growth characteristics of nanowires at the highly reactive catalyst/nanowire interface.4−11 The atomic-scale microstructure details and the structural correlation of catalyst/nanowire interface play critical roles in the activation and reactivity of growth interface4−7 and even dominate the nanowire growth dynamics, growth direction, as well as the formation of planar defects in nanowires.5−11 Recently, it was found that the nonmetal solid-state superionicphase silver selenide (α-Ag2Se), crystallizing in the bodycentered cubic (bcc) crystal structure,12,13 is able to catalyze nanowire growth of metal selenides,14−18 such as ZnSe, CdSe, and MnSe, in solution at a moderate temperature (100−220 °C). Like the Au-catalyzed vapor−liquid−solid (VLS)4 and Bicatalyzed solution−liquid−solid (SLS)5 mechanisms, a solution−solid−solid (SSS) growth model was proposed to describe the growth of these metal selenide nanowires catalyzed by a solid state catalyst in a solution phase.15 As a characteristic feature, the catalyst particles of Ag2Se are retained atop the resultant nanowires to form hetero© XXXX American Chemical Society

Received: December 27, 2017 Revised: July 7, 2018 Published: July 16, 2018 A

DOI: 10.1021/acs.cgd.7b01798 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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of catalyst head and nanowire stem, which is consistent with the nanowire growth characteristic of catalytic mechanisms.4−11,14−18 The EDS elemental analyses confirmed that the darker head is Ag2Se (Ag/Se = 1.96:1) and the stem is ZnSe (Zn/Se = 0.97:1) (Figure 1b), in agreement with their stoichiometric ratios. It is known that Ag2Se has three types of crystal structures, that is, cubic (α), orthorhombic (β), and tetragonal (t). Thermodynamically, the α phase is a HT phase, whereas both β and t phases are two LT phases.12−19,23,24 For the Ag2SeZnSe catalyst/nanowire growth system, it has been proven that ZnSe nanowires mainly exhibit the fcc ZB crystal structure (Scheme 1a),14−16 a typical phase for the II−VI semiconductor

In this paper, we present our results of the heating highresolution transmission electron microscopy (HRTEM) studies on the detailed microstructures of α-Ag2Se/ZB-ZnSe interfaces. On the basis of three representative examples of heating HRTEM imaging, a prevailing interface that contains the bcc α-Ag2Se (110) and fcc ZB-ZnSe (111) habit planes has been determined, and a strained, coherent crystallographic relationship is rationally proposed to reveal this specific catalyst/nanowire interface; that is, a large elastic lattice deformation may take place in the ZB-ZnSe {111} plane and enables a close lattice match (including atom packing patterns and spacings) with the α-Ag2Se (110) plane. As such, our theoretical simulation and calculation of atomic configurations in the α-Ag2Se (110) and strained ZB-ZnSe (111) planes well agree with the heating HRTEM results. The reasons for the formation of the favored α-Ag2Se (110)/ZB-ZnSe (111) interface are also explicated.

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Scheme 1. Crystal Structures: (a) fcc ZB ZnSe (a = 5.67 Å); (b) bcc α-Ag2Se (a = 4.98 Å). Silver: Ag, yellow: Se, green: Zn

EXPERIMENTAL SECTION

Ag2Se-catalyzed ZnSe nanowires were prepared according to our previously reported method.15 The Ag/Zn molar ratio was set at 10%, and the temperature of synthesis was 160 °C, producing a high ratio of Ag2Se-ZnSe hetero-nanowires in the products (see Figure 1). The

compounds,10 and that Ag2Se is stable in the superionic-state bcc α phase at the preparation temperatures (100−220 °C).15 For α-Ag2Se (Scheme 1b), Se atoms are orderly arranged in a high-symmetry bcc lattice with Ag atoms statistically, predominantly distributed over tetrahedral interstitial sites.12,13 Being a HT phase, α-Ag2Se is unstable at RT and undergoes a structure transition to the LT β or t phase.13−16,23,24 For the nanosized α-Ag2Se catalytic heads atop the nanowires, it was found that they finally convert to the LT t phase14−17 when the temperature declined from the preparation temperature to RT. The α → t phase transition is found to be reversible, and it is reported that the t → α transition process takes place around the temperature of 101− 109 °C.15,17,23,24 For Ag2Se/ZnSe hetero-nanowires, the microstructures of the t-Ag2Se/ZB-ZnSe interface can be easily accessed through the conventional RT TEM.14−16 Because of the α → t phase transition, however, if we want to observe the intrinsic microstructural details of α-Ag2Se/ZB-ZnSe interfaces, the operation temperature of TEM has to be higher than the t → α phase transition temperature, that is, 101−109 °C,15,23,24 above which Ag2Se is stabilized in its HT bcc-structured α form. Fortunately, the heating TEM, which can be performed at different temperatures through a heating specimen-holder, enables detailed interfacial microstructures and HT crystal phases to be characterized at elevated temperatures.15,20−22 By using heating TEM, we successfully obtained the HRTEM images of α-Ag2Se/ZB-ZnSe interfaces above the t → α phase transition temperature, typically at 115 and 135 °C. Figure 2a shows a typical HRTEM image of Ag2Se/ZnSe interface recorded at 115 °C by heating TEM (the raw HRTEM images without any marks for Figures 2a, 3a, and 4a are also shown in the Supporting Information as Figure S1a−c for reference). The results of this HRTEM imaging have been briefly described in our previous report,15 and here we make a

Figure 1. (a) TEM image and (b) EDS spectra of Ag2Se-ZnSe heteronanowires prepared using a 10% Ag/Zn precursor ratio. products were dispersed in ethanol and then dropped onto a carboncoated Cu grid for TEM characterization. The TEM, energydispersive X-ray spectroscopy (EDS), and heating HRTEM experiments were carried out on a JEOL 2010F field-emission TEM at 200 kV (not aberration corrected), which was equipped with a Gatan model 901 heating specimen holder. The heating rate was 5 °C/min, and when reaching the target temperature the time was further kept for 3 min for the heating HRTEM characterization. Because the heating temperature we performed is rather low (115−135 °C), the sample drift in TEM is not apparent. The crystal structure and atomic configurations were constructed by using Materials Studio 7.0. All FFT diffractograms were extracted from the corresponding HRTEM images in the Digital Micrograph software by directly clicking “reduced FFT”. The measurements of the interplanar spacings were carried out on the real-space HRTEM micrographs by averaging the total distance of 11 linear periodic atoms (atom columns) in a given crystallographic direction through the Digital Micrograph software. The error for the measurement and instrument within 5% may exist in as-obtained data. Despite this, we measured and calculated interplanar spacings, lattice mismatch, and lattice strain by directly using the measured raw data without a correction process of errors.

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RESULTS AND DISCUSSION Ag2Se-ZnSe hetero-nanowires (or hetero-nanorods) were synthesized through recently developed Ag2Se-catalyzed mechanism.14−18 A high proportion of hetero-nanowires can be obtained at a high Ag/Zn precursor molar ratio, for example, 10% (Figure 1a). These hetero-nanowires are formed B

DOI: 10.1021/acs.cgd.7b01798 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 2. (a) HRTEM image of Ag2Se/ZnSe catalyst/nanowire recorded at 115 °C. (b−d) Different-region FFT patterns of (b) ZnSe nanowire, (c) Ag2Se/ZnSe interface, and (d) Ag2Se catalyst head marked with red, green, and yellow rectangles, respectively. (e) An enlarged part of interfacial HRTEM image in panel (a). (f) Atomic model of α-Ag2Se/ZB-ZnSe interface in panel (e), observed from the α-Ag2Se [11̅1]//ZB-ZnSe [11̅0] zone axis.

Figure 3. (a) HRTEM image of Ag2Se/ZnSe catalyst/nanowire recorded at 135 °C. (b, d) FFT patterns of Ag2Se and ZnSe HRTEM parts within yellow and pink framed regions. (c) An enlarged HRTEM interfacial region marked with a yellow arrow in panel (a).

Figure 4. (a) HRTEM image of another Ag2Se/ZnSe catalyst/ nanowire obtained at 135 °C. (b, d) FFT patterns for Ag2Se and ZnSe HRTEM parts within the yellow and pink rectangles. (c) An enlarged interfacial part of HRTEM image marked with a white frame in panel (a).

more detailed analysis on the microstructures of both α-Ag2Se and ZB-ZnSe domains, as well as on their interfacial lattice correlation. It can be observed that the Ag2Se catalyst head and ZnSe nanowire are both highly crystallized, and a clear, planar

heterogeneous interface is formed between them. The HRTEM image of ZnSe nanowire part (see the red rectangle), along with its fast Fourier transform (FFT) pattern (Figure C

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in the ZnSe nanowire, consistent with the previous reports.14,16 By combining FFT patterns (Figure 3b,d), it is determined that the HRTEM image for α-Ag2Se part is taken from the [001] zone axis, with interplanar distances of 2.50 Å for the (020) plane and 3.50 Å for (110) plane; the HRTEM image for ZB-ZnSe segment is recorded from the [01̅1] zone axis, with interplanar distances of 3.25 Å for the (111) plane and 2.81 Å for the (2̅00) plane (not labeled in the figure). It is found that the FFT spot of α-Ag2Se (110) plane is nearly parallel to that of ZnSe (111) plane, indicating that two such kinds of crystal planes are the connect planes of Ag2Se/ZnSe catalyst/nanowire, although some lattice distortions and dislocations exist at the interface (marked with a yellow arrow in Figure 3a). Meanwhile, the enlarged HRTEM image (Figure 3c) clearly shows that the neighboring atom distances for two materials at the interface are very close and have a good fit (misfit degree δ = (3.50−3.45)/3.45 = 1.45%) and almost no lattice strain, as observed from the α-Ag2Se [001]// ZB-ZnSe [01̅1] zone direction. Consequently, the orientation relationship of α-Ag2Se and ZB-ZnSe at the interface in Figure 3a can be concluded: α-Ag2Se (110)//ZB-ZnSe (111) and αAg2Se [001]//ZB-ZnSe [01̅1]. The HRTEM image collected at 135 °C for another different Ag2Se-ZnSe heteronanowire is depicted in Figure 4a. Our careful assignment of this HRTEM image and the FFT patterns for Ag2Se and ZnSe parts (Figure 4b,d) reveals that the HRTEM image for Ag2Se part is taken from the α-Ag2Se [11̅3] zone axis, while that for ZnSe part is from the ZB-ZnSe [12̅1] zone axis. d110 = 3.50 Å (α-Ag2Se) and d111 = 3.27 Å (ZB-ZnSe) are established. The FFT spots of α-Ag2Se (110) and ZB-ZnSe (111) planes appear in the same direction, suggesting that such a pair of planes constitute the Ag2Se/ZnSe catalyst/nanowire interface, which accords with the results shown in Figures 2 and 3 (For ZB-ZnSe, the (111) plane is equivalent to the (111̅) plane, both of which belong to the {111} plane group). The results in Figure 4 reveal that α-Ag2Se and ZB-ZnSe have a crystallographic orientation relationship at their adjoining interface: α-Ag2Se (110)//ZB-ZnSe (111) and α-Ag2Se [11̅3]//ZB-ZnSe [12̅1]. An enlarged part of the resultant HRTEM image is displayed in Figure 4c, where the atom spacings at the interface are measured to be 2.05 Å for αAg2Se (corresponding to 2d3−3−2) and 2.00 Å for ZB-ZnSe (corresponding to d20−2). Such data disclose that the interfacial atoms projected from the α-Ag2Se [11̅3]//ZB-ZnSe [12̅1] zone axis have a good match (misfit degree δ = 2.5%) and thus lead to a very low or negligible lattice strain at the α-Ag2Se (110)/ZB-ZnSe (111) interface. The above heating HRTEM results and analyses reveal that (i) the α-Ag2Se/ZB-ZnSe catalyst/nanowire interface is composed of the α-Ag2Se (110) and ZB-ZnSe (111) pair planes, (ii) such an interface is a preferred adjoining interface of catalyst and nanowire, (iii) this specific interface is coherent through a strained or nonstrained lattice match. However, the HRTEM imaging results only record the one-dimensional (1D) coherency of the interface from different specific zone axis directions. In order to have a comprehensive and clear understanding of the 2D interfacial coherency,31,32 it is required to clarify the detailed atomic configurations in both α-Ag2Se (110) and ZB-ZnSe (111) crystal planes and their 2D lattice matching relationship. It is known that the low-index planes in crystals have densely packed atoms and high symmetry. Therefore, we first illustrate the atomic packings and lattice symmetry and parameters in

2b), is consistent with the atomic arrangement projected from the [11̅0] zone axis of ZB-ZnSe. The lattice fringe distance marked with red lines is measured to be nearly 3.27 Å, which corresponds to the interplanar spacing of the fcc ZB-ZnSe (111̅) plane (d11−1). The growth direction for this nanowire can be accordingly determined to be the [111̅] orientation of ZB-ZnSe. For the Ag2Se catalyst head, clear two-dimensional (2D) lattice fringes are recorded by HRTEM imaging, and the neighboring fringe distance is approximately 3.52 Å, in good agreement with d110 of bcc-structured α-Ag2Se. A selected HRTEM image area of α-Ag2Se head is highlighted by the yellow rectangle, and its corresponding FFT pattern is displayed in Figure 2d. The diffraction spots display 6-fold symmetry with an angle of 60° between two adjacent closest spots, and the distances for the six closest diffraction spots to the center are equal, confirming that this HRTEM imaging is projected from the [11̅1] zone axis of bcc α-Ag2Se. Figure 2c shows the FFT pattern for the interfacial HRTEM image marked with the green frame, in which a superposition of α-Ag2Se and ZB-ZnSe FFT spots can be distinguished (FFT pattern with scale bar in the style of black dots on white background is displayed in Figure S2). It is found that the diffraction spots for the α-Ag2Se (110) and ZB-ZnSe (111̅) planes appear in the same direction. This means that such two sets of crystal planes are parallel to each other, and they make up the α-Ag2Se/ZB-ZnSe catalyst/nanowire conjugate interface. From the crystal growth point of view, this also indicates that ZB-ZnSe nanowire grows on the α-Ag2Se (110) surface through the atomic packing of the (111̅) planes along the [111̅] direction. The crystallographic orientation relationship of α-Ag2Se and ZB-ZnSe at the interface can be determined according to the results of Figure 2a−d: α-Ag2Se (110)//ZBZnSe (111̅ ) and α-Ag2Se [11̅ 1]//ZB-ZnSe [11̅ 0]. The formation of (110)bcc/(111)fcc interface has been often observed between bcc and fcc metals, which shows a so-called Nishiyama−Wassermann (N−W) or Kurdjumov−Sachs (K− S) crystallographic interfacial relationship.25−31 Shown in Figure 2e is an enlarged HRTEM image of the αAg2Se/ZB-ZnSe interface. On the both sides of interface, the distances for two neighboring atoms parallel to the interface are measured to be 4.05 Å for α-Ag2Se and 4.02 Å for ZBZnSe, respectively. The measured interatomic spacing of 4.05 Å corresponds to two times the interplanar distance of the (1̅12) planes for α-Ag2Se (2d−112), whereas that of 4.02 Å in ZB-ZnSe should be assigned to three times the interplanar distance of the (224) plane (3d224), which actually has a theoretically normal value of 3.47 Å for ZB-ZnSe (a = 5.67 Å), as simulated in Figure 2f. Obviously, there is a large lattice strain (ε = (4.02−3.47)/3.47 = 15.85%) in the ZB-ZnSe [112] direction in order to meet the requirement for the formation of a coherent match at the α-Ag2Se/ZB-ZnSe interface. The absence of misfit dislocations on both sides of the interface in the HRTEM images (Figure 2a,e) can prove the formation of coherent interface when observed from the α-Ag2Se [11̅1]// ZB-ZnSe [11̅0] zone axis direction. To obtain more microstructural and crystallographic orientation information, another two heating HRTEM imaging results recorded on different Ag2Se/ZnSe catalyst/nanowire adjoining interfaces from different zone axes are further demonstrated. Figure 3a displays a representative HRTEM image for Ag2Se/ZnSe hetero-nanowires collected at 135 °C. Highly crystalline Ag2Se and ZnSe are observed, and they compose a clear interface. Twins and stacking faults are present D

DOI: 10.1021/acs.cgd.7b01798 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Scheme 2. Atomic Arrangements of (100), (110), and (111) Low-Index Planes: (a1−a3) α-Ag2Se; (b1−b3) ZB-ZnSe. Silver: Ag, Yellow: Se, Green: Zn

the (100), (110), and (111) planes of both α-Ag2Se and ZBZnSe by taking Se sublattices as models (Scheme 2). The packing patterns of Se atoms in the (100) plane for either αAg2Se or ZB-ZnSe have 4-fold symmetry, but the interatomic distances for these two square lattices have a misfit of 12.14% (Scheme 2a1 and b1). In Scheme 2a2 and b2, the (110) planes for α-Ag2Se and ZB-ZnSe both displays 2-fold symmetry, and the rectangular Se sublattices with different interatomic distances can be constructed. The mismatches in two orthogonal orientations for the rectangular lattices marked in the solid black lines are 24.25% [2d200 or d100 (Ag2Se, 4.980 Å)/2d220 (ZnSe, 4.008 Å)] and 24.26% [2d110 (Ag2Se, 7.043 Å)/d100 (ZnSe, 5.668 Å)], respectively. However, the rectangular lattices highlighted in the dotted red lines have small misfits, which are 1.47% [2d110 (Ag2Se, 7.043 Å)/6d422 (ZnSe, 6.941 Å)] and 1.46% [2d100 or 4d200 (Ag2Se, 9.960 Å)/ 3d111 (ZnSe, 9.817 Å)]. Furthermore, as illustrated in Scheme 2a3 and b3, Se sublattices in the (111) planes for α-Ag2Se and ZB-ZnSe both show 6-fold symmetry atomic packing but have a large difference in the interatomic distances of hexagonal lattices, whereas the rectangular lattices with small misfits can be built as highlighted in the dotted red line, that is, 1.47% in one direction [2d110 (Ag2Se, 7.043 Å)/6d422 (ZnSe, 6.941 Å)] and 1.45% in the orthogonal direction [6d211 (Ag2Se, 12.198 Å)/6d220 (ZnSe, 12.024 Å)]. Although the lattices with the same symmetry and nearly equal spacings can be constructed respectively in the (110) and (111) planes of α-Ag2Se and ZBZnSe, the atom configurations and the number of atoms in these lattices are greatly different, which is not favorable for the formation of 2D, fully coherent interface.31,32 From the above theoretical calculations and analyses, we think that there is a very small possibility or no possibility to form a complete 2D coherent interface between α-Ag2Se and ZB-ZnSe through the crystal planes of the same index, due to the large differences in the lattice symmetry and parameters or the atom arrangement patterns. However, a good 2D lattice match, including the atom patterns and spacings, can be reasonably built between the αAg2Se (110) and ZB-ZnSe (111) planes when lattice strain occurs in the ZB-ZnSe (111) plane. Considering the simplicity and lattice periodicity, we use the Se single-layer sublattice to represent the atomic configurations in α-Ag2Se (110) and ZB-

ZnSe (111) planes (Scheme 3a,b). It is reported that the (110)bcc and (111)fcc planes are usually able to compose the adjoining interface between bcc and fcc crystals, especially metals, and they exhibit N−W or K−S crystallographic orientation relationship (Scheme 3c,d),25−31 which only differ by an in-plane rotation angle of 5.26° (Scheme 3e).25,27,29,31 The N−W and K−S relationships often occur at the grain boundaries or the interfaces for the film epitaxial growth and the structural transformation between two materials (or two phases) that have bcc and fcc crystal structures.25−31 It is thought that for the bcc α-Ag2Se (110)/fcc ZB-ZnSe (111) interface, though there are a large number of small regions that have a close lattice fit (Scheme 3c,d, marked with arrows), the formation of a 2D perfectly coherent (110)/(111) interface between bcc α-Ag2Se and fcc ZB-ZnSe is very difficult or even not possible if no lattice deformation occurs or the lattice is rigid, for more atoms are not well matched through the N−W or K−S orientated relationship. It has been shown in many reports that a considerable lattice deformation by elastic straining can appear at the micro- and nanoscale interface or boundaries,28−31,33−36 and in some extreme situations the elastic strain will surpass the theoretical limit.37 We here propose that the atom spacing of ZB-ZnSe (111) plane in the [01̅1] direction expands from 4.008 to 4.98 Å to accommodate the match of atom configurations with the α-Ag2Se (110) plane, which results in a 2D nearly fully coherent bcc α-Ag2Se (110)/fcc ZB-ZnSe (111) interface (Scheme 3f). This proposed 2D coherency formed through the strained, elastic lattice deformation is a new possible interfacial lattice relationship for nanoscale bcc and fcc crystal systems and can serve as a complement for the traditional N−W or K−S crystallographic relationships. Comparing the strained ZB-ZnSe (111) plane with the unstrained one as depicted in Scheme 4, theoretically the tensile lattice strain in the [01̅1] direction is the maximum and up to 24.25%, and that in the orthogonal [21̅1̅] direction is the lowest (0%). It can be speculated from Scheme 4b that if the HRTEM image is recorded parallel to, i.e., projected from the [11̅2]bcc//[12̅1]fcc,s orientation, the corresponding strained lattice deformation value is 6.59% in the nearly orthogonal direction ([11̅1̅]bcc//[101̅]fcc,s, labeled as Case 1). Case 1 can be compared with the HRTEM result shown in Figure 4. As E

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Scheme 3. (a, b) Atomic Configurations of Single-Layer Se Sublattice in bcc α-Ag2Se (110) and fcc ZB-ZnSe (111) Planes, Respectively. (c, d) Nishiyama−Wassermann (N−W) and Kurdjumov−Sachs (K−S) Orientation Relationships at the α-Ag2Se (110)/ZB-ZnSe (111) Interface. (e) Illustration of the Difference in N−W and K−S Orientations by an in-Plane Rotation Angle (θ) of 5.26°. (f) Simulated Coherent Epitaxy between α-Ag2Se (110) and ZB-ZnSe (111) Planes through the Tensile, Elastic Lattice Strain of ZnSe (111) Plane along the [01̅1] Direction

shown in Figure 4, it is detected that the HRTEM images of αAg2Se and ZB-ZnSe parts are taken from the parallel [11̅3]bcc//[12̅1]fcc zone axes. In comparison with Scheme 4b, ZB-ZnSe in Figure 4 and Scheme 4b shows the same zone axis, [12̅1], but α-Ag2Se [11̅3] in Figure 4 is different from the theoretically zone axis orientation α-Ag2Se [11̅2] in Scheme 4b. Such a difference does not support the lattice-strained model in Scheme 4b. The reason for this difference is a small parallel rotation of the α-Ag2Se (110) plane above the ZBZnSe (111) plane. It is calculated that the rotation angle from α-Ag2Se [11̅2] to α-Ag2Se [11̅3] is 10.02°, through which these two orientations can be mutually transformed. The plane-parallel rotation demonstrates the variation of contact fashions between α-Ag2Se (110)/ZB-ZnSe (111) planes and can optimize the interfacial energy and reduce the lattice strain.34 The lattice strain of ZB-ZnSe in Figure 4 (the

measured d20−2 is approximately 2.00 Å, very close to the theoretical value of d220, 2.0046 Å) is much smaller than the theoretical value (6.59%) obtained from the atomic model in Scheme 4b. However, the strained atomic model is in good agreement with the HRTEM results shown in Figures 2 and 3. As seen from Scheme 4b, if the HRTEM image of ZB-ZnSe is recorded from the directions, for example, [11̅0] or [01̅1], theoretically the corresponding lattice strain should be 18.67% in the nearly orthogonal [112̅] direction (Case 2) or 0% in the orthogonal [21̅1̅] direction (Case 3). Such two cases are consistent with the HRTEM imaging results in Figure 2 and Figure 3, respectively. Briefly, as displayed in Figure 2a,e,f, the obtained HRTEM for ZB-ZnSe image is projected from the [11̅0] zone axis direction, the same as that in Scheme 4b, and the measured lattice strain in the ZB-ZnSe [112] direction is F

DOI: 10.1021/acs.cgd.7b01798 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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(111) interface, which offers a highly reactive platform for growing nanowire. (ii) For many cubic ZB-structured crystals including ZnSe, the (111) plane has a lower surface free energy than the (100) and (110) planes.38 The growth in the [111] direction through the atomic stacking of the (111) plane can minimize the total energy of nanowire and therefore is a most favorable fashion in wire growth for the ZB-structured crystals.9,39,40 ZnSe nanowires have the tendency to grow along the [111] direction by the atomic stacking of the (111) planes, which will lead to the fact that the α-Ag2Se/ZB-ZnSe catalyst/ nanowire interface objectively contains the ZnSe (111) plane. It is reported that due to the low stacking fault energies in the [111] direction,40,41 a high density of planar defects, such as twin boundaries and stacking faults, often takes place in the [111]-grown nanowires10,39,40,42 This phenomenon is also observed in the as-obtained ZnSe nanowires (Figure 3).14,16 (iii) In the catalytic regimes, the nanowire growth requires as well as starts at the highly reactive catalyst/nanowire interface. It is known that the coherent interface has the lowest interfacial energy.31,32 However, the lattice strain of ZB-ZnSe (111) plane increases the coherently interfacial energy to some degree and meets with the requirement of the high reactivity of catalyst/nanowire interface. The existence of lattice strain may not only contribute to the reactivity but also to the instability of Ag2Se/ZnSe catalyst/nanowire interface. During our HRTEM studies, when irradiated under a long duration of electron beam or under the electron diffraction collection (∼10s of seconds to several minutes), Ag2Se is sometimes decomposed from the interface (Figure S3 in the Supporting Information). (iv) It is found that when the dimensions of materials are reduced to the micrometer or even nanometer regime, the elastic lattice strain can reach a large value and may be beyond the theoretical limit, which often appears at the interface or boundary of two different materials or phases.33−37 ZnSe nanowires catalyzed by Ag2Se have a relative small crosssection area (i.e., thin diameters within several to 10s of nanometers14−16). For example, the nanowire in Figure 2a has a diameter of about 28 nm at the catalyst/nanowire interface, and thus its circular cross-section area can be calculated to be approximately 615 nm2. Consequently, it is possible and reasonable that a maximum 24.25% elastic strain appears in the ZB-ZnSe (111) plane along a certain crystallographic orientation (e.g., [01̅1] in Scheme 4b) to realize the interfacial lattice match and form the 2D coherent interface with the αAg2Se (110) plane. The lattice strain is eliminated from the interface to the ZnSe nanowire body domain along the [111] direction, and the transition width is about 5−7 atomic layers (∼2 nm, Figure 2f). Such a strained width is thin, and it will not induce the HRTEM imaging to obviously deviate from the perfect zone axis. Therefore, our theoretical calculation and analysis on HRTEM results and the atomic model of strained lattice in Scheme 4b are reasonable. For nanoscale metals, both the theoretical prediction and experiment have shown that the lattice deformation strain can surpass 30% before the formation of dislocations.37,43 Within semiconductor colloidal nanocrystals, it has been reported that the highly strained (CdTe)ZnSe core@shell heterostructures (14.4% lattice mismatch) are formed through heteroepitaxy without the yield of crystalline defects at the interface.44 For the first epilayer of ZB-ZnSe on the CdTe core (3.8 nm), it means that the tensile lattice strain may be up to 14.4%. Such a large

Scheme 4. Schematic Illustrations of Atom Patterns and Several Specific Crystallographic Orientations in the SingleLayer Se Sublattice: (a) the Unstrained ZB-ZnSe (111) Plane, and (b) the Nearly Perfect 2D Coherency between the Superposed Planes of α-Ag2Se (110) and Strained ZBZnSe (111)

about 15.85% (Figure 2f), very close to the theoretically strained value 18.67%, in the equivalent [112̅] direction in Scheme 4b (Case 2). Meanwhile, the zone axis direction for αAg2Se in Figure 2 accords with that in Scheme 4b, namely, [11̅1]. As for Figure 3c, the HRTEM image for ZB-ZnSe is collected from the [01̅1] zone axis direction, and the measured 3d4−2−2 in the [21̅1̅] direction is 3.45 Å, very close to the theoretical value of 3.47 Å for 3d4−2−2 (= 3d422). This means that nearly no strain takes place in the ZB-ZnSe [21̅1̅] direction, which is consistent with Scheme 4b (Case 3, 0% lattice strain in the [21̅1̅] direction). At the same time, the zone axis direction for α-Ag2Se in Figure 3 agrees with that in Scheme 4b, namely, [001]. Overall, the crystallographic relationship simulation and our theoretical calculation and analysis presented in Scheme 4 can well interpret the heating HRTEM results and help understand the formation of 2D, nearly fully coherent α-Ag2Se (110)/ZB-ZnSe (111) interface by the [01̅1]-oriented lattice strain of ZB-ZnSe (111) plane ([01̅1] is equivalent to [110] for ZB-ZnSe). Finally, several reasons are proposed to explain the formation of such a favored, strained coherent α-Ag2Se (110)/ZB-ZnSe (111) interface: (i) As seen in Scheme 2, the (110) and (111) planes are the closest-packed planes respectively for the bcc and fcc crystals.31 From the thermodynamics and crystallographic points of view, the atomic packing prefer to proceed in such two kinds of planes. Consequently, the transport, diffusion, binding, and incorporation of different kinds of atoms or ions (e.g., Ag+, Zn2+, and Se2−) tend to occur at the α-Ag2Se (110)/ZB-ZnSe G

DOI: 10.1021/acs.cgd.7b01798 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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lattice strain maybe comes from the considerably high lattice compressibility (and its reverse) of ZB-ZnSe.44 Meanwhile, another mechanism causing high elastic strain is the coherent heteroepitaxial growth,35,36,44 which ensures the yield of lattice strain and its stabilization and at the same time is beneficial to the measurement and determination of straining degree. In this regard, it is reasonable that a tensile elastic strain as large as 24.25% occurs in the ZB-ZnSe (111) plane along the [110] direction when epitaxially grown on the α-Ag2Se (110) plane at the nanoscale level. (v) In addition, ZnSe has a very low solubility at the growing temperature (100−220 °C) in Ag2Se according to the ZnSe− Ag2Se phase diagram.15,45 This limits the interdiffusion between ZnSe and Ag2Se and therefore is favorable for the formation planar, abrupt interface. A similar case was measured in the axial Si−Ge nanowire heterojunctions.33,46 The planar, abrupt interface is the basis to form strained interfacial coherency between two different materials or phases. It should be pointed out that some lattice imperfects (e.g., lattice dislocations, screw step, or in-plane twist34) are highly possible to appear at α-Ag2Se/ZB-ZnSe interfaces, which will reduce and relax the lattice strain at/near the heterointerfaces to some degree. Consequently, the actual lattice strain in the ZB-ZnSe (111) plane may be not as large as theoretically proposed in Scheme 4.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01798. The raw TEM images of Figures 2a, 3a and 4a, FFT pattern, and the TEM results of Ag2Se-ZnSe heteronanowires before and after a relatively long time of electron diffraction collection (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.W.) E-mail: [email protected]. *(G.Q.) E-mail: [email protected]. ORCID

Junli Wang: 0000-0002-3775-539X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21571086, 21201086, 51572112, 51572111), the China Postdoctoral Science Foundation (2014M550267, 2015T80501), the Natural Science Foundation of Jiangsu Province (BK20141297), the Cultivating Project of Young Academic Leader and the Research Foundation of Jiangsu University (11JDG071).

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CONCLUSIONS In conclusion, we investigated the microstructures and crystallographic orientation relationship of α-Ag2Se/ZB-ZnSe catalyst/nanowire interfaces by means of the heating HRTEM technique. The results revealed that the closest-packed bcc αAg2Se (110) and fcc ZB-ZnSe (111) planes compose the catalyst/nanowire adjoining interface, which is considered a preferred, habit interface of nanowire growth. Besides the known N−W and K−S relationships, we proposed a new possible lattice relationship for the bcc α-Ag2Se (110)/fcc ZBZnSe (111) interface, in which the 2D, nearly perfect interfacial coherency can occur given that a maximum 24.25% elastic lattice strain appears in the ZB-ZnSe (111) plane along the [110] direction. An atomic model was illustrated to interpret such a proposal. Our proposal is supported by the heating HRTEM imaging results, although the lattice dislocations, twist, and screw steps caused by large lattice mismatch and strain cannot be excluded at the catalyst/ nanowire interfaces. To the best of our knowledge, the (110)bcc/(111)fcc preferred interface was rarely reported for catalyst/nanowire crystal growth systems composed of two inorganic compounds. However, such a kind of habit interface is observed in the α-Ag2Se/ZB-ZnSe catalyst/nanowire by means of heating HRTEM imaging technique. Moreover, we explained the reasons for the formation of favored, strained αAg2Se (110)/ZB-ZnSe (111) coherent interface from the several aspects, including the crystallographic habits of crystal growth and atom packing, the interfacial energy and activity, the size effect on the lattice strain, and the solubility of nanowire in catalyst. We expect that our findings and understanding about the catalyst/nanowire interfacial structures will help unravel nanowire catalytic mechanisms from the interfacial microstructure point of view, and that this work will inspire insightful thoughts and strategies for realizing the controllable nanowire growth.

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DOI: 10.1021/acs.cgd.7b01798 Cryst. Growth Des. XXXX, XXX, XXX−XXX