Atom-Resolved Evidence of Anisotropic Growth in ZnS Nanotetrapods

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Atom-Resolved Evidence of Anisotropic Growth in ZnS Nanotetrapods Wei Liu,|| Ning Wang,|| and Rongming Wang* Key Laboratory of Micro-nano Measurement-Manipulation and Physics (Ministry of Education), Department of Physics, School of Chemistry and Environment, Beijing University of Aeronautics and Astronautics, Beijing 100191, People’s Republic of China

Shishir Kumar and Georg S. Duesberg School of Chemistry and Center for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland

Hongzhou Zhang* School of Physics, Center for Research on Adaptive Nanostructures and Nanodevices (CRANN), and CRANN Advanced Microscopy Laboratory (CRANN AML), Trinity College Dublin, Dublin 2, Ireland

Kai Sun Department of Material Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States ABSTRACT: ZnS nanotetrapods were investigated by atom-resolved microscopy characterization and quantitative simulation. The octahedron core enclosed with Znand S-terminated surfaces was verified. Four hexaprism-shaped arms were selectively grown from Zn-terminated surfaces of the core by alternately stacking zinc blende and wurtzite structures. The stacking order change at the core/arm interface is significant to activate the arm growth. The anisotropic growth mechanism was proposed and further proved by the synthesis of ZnS nanoparticles and nanobelts. KEYWORDS: ZnS, nanotetrapod, quantitative TEM, anisotropic growth

A

s a class of anisotropically shaped nanocrystals in threedimensional, tetrapods (TP) offer the possibility of creating materials with integrated functionalities under the “bottom-up” architecture.1,2 They are of great interest in recent years because of their novel behaviors exhibited in charge migration,3,4 field electron emission,5,6 multiterminal sensing,7 and DNA transfection.8 In addition, asymmetric morphology makes TPs facile to be orderly aligned with one arm perpendicular to the substrate, which ensures them as more versatile building blocks to realize improved performance in photovoltaic and nanoelectronic devices compared to their symmetric counterparts.9
11 To form a tetrapodal architecture, an enhanced anisotropic crystal growth should be activated despite using any approach.12,13 Such a requirement is well satisfied by group II
VI materials, in which each atom is tetrahedrally coordinated by ones of the opposite species. This intrinsic character makes TP a favorite choice for these materials, including ZnO,11,14 CdS,15
17 CdSe,17,18 and CdTe.19,20 These nanocrystals can exist in either hexagonal wurtzite (WZ), or in combination of cubic zinc blende (ZB) and WZ. As known as so far, two models have been developed for the interpretation of the tetrapodal structure, the octa-twin model for WZ TPs21
23 and the model of the ZB core epitaxial growth of WZ arms.12,15 For both cases, the intrinsic crystal growth behaviors of ZB and WZ phases, such as the twinning r 2011 American Chemical Society

within the same phase, the variation of stacking sequences of close-packed planes (between ZB and WZ), and the asymmetric growth on polar surfaces, result in various of stacking faults and twin boundaries, thereby significantly promote the complexity of such a three-dimensional organization. These defects can form interfaces of either core/arm or within each part. These interfaces can introduce heterogeneous strain distribution among the TPs and therefore have critical impact on the band alignment and electron delocalization.18 Recently, a strain-induced nonradiative recombination of cathodoluminescence at the interface of defects of single ZnO TP was observed.11 On the other hand, when exposed in a nonhydrostatic stress field, the bonds between atoms can be compressed/stretched nonisotropically owning to the inherent asymmetry of TPs, which leads to the overlap/split of the wave function of bonded ions, resulting in an optically detectable blue/ red shift of energy band. Nonisotropic strain sensitive photoluminescence emission in CdSe/CdS TPs was also reported.24 These novel phenomena are prominently dependent upon the crystal structure and leave us great desire to figure out the detailed atoms arrangement in a TP to establish a precise model for interpretation as well as further tailoring their optomechanical properties. However, most of the Received: May 11, 2011 Published: June 10, 2011 2983

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Figure 1. TEM images of ZnS TPs. (a) An overview TEM image revealing the tetrapodal morphology of the material; the inset is the diffraction pattern of the selected area. (b) An enlarged image of a single ZnS TP. (c,d) HRTEM images of the vertical arm and part of the horizontal arm, respectively.

Figure 2. (a) HRTEM image of a single TP and its corresponding FFT pattern (inset). (b) Phase image reconstructed from marked area of image (a), right half of the core is marked out with a white dashed triangle; the middle inset is the enlarged phase image of the ZB core from the marked area of image (b).

materials synthesized up to now (generally more than tens of nanometers) failed to satisfy weak phase object approximation (sample thickness should be less than ∼10 nm) required in TEM to reach the atomic resolution, which blocks direct investigations of the atoms arrangement of individual TP. In this report, ZnS TPs with arms less than 5 nm in diameters were synthesized via a solvothermal method. The atomic structures were quantitatively investigated by high-resolution transmission electron microscopy (HRTEM) combined with focal series reconstruction and image simulation. Direct evidence revealed the polarized growth of the arms from an octahedron core. On the basis of the microscopy analysis, a surfactant-controlled anisotropic growth mechanism was proposed and discussed. The ZnS nanostructures were synthesized via a solvothermal method. Briefly, ethylene glycol (EG) solution of 10 mL with Zn(Ac)2 3 2H2O (0.219 g) was added into a mixtures of 55 mL EG with Na2S 3 9H2O (0.4804 g) and various amount of ethylenediamine (EDA). The system was kept in autoclave at 200
C for 10 h. The yielding ZnS nanostructures were all more than 0.08 g per autoclave. The products were nanoparticles, nanotetrapods, and nanobelts, depending on the concentration of EDA in the reaction. The annealing treatment of ZnS nanobelts was conducted on a tubular furnace (Carbolite) under nitrogen. The products were dispersed onto the grids and characterized by TEM (FEI Titan 80
300 at 300 kV). Focal series reconstruction was performed on the platform of True Image software (FEI Ver. 1.1.0). Image simulation was conducted by using software package of MacTempas (Version 2.2.8). As shown by Figure 1a, ZnS tetrapodal nanostructure was produced on a large scale and with high uniformity. The four arms are of 40
50 nm in length and ∼5 nm in diameter (Figure 1a,b). Selected area diffraction (SAD) pattern (inset of Figure 1a) demonstrates the coexistence of both ZB and WZ phases. The atoms arrangement was also studied from a vertical arm (Figure 1c), showing a hexaprism morphology with six equivalent {1010} lateral facets. HRTEM (Figure 1d) of the horizontal arm reveals that the single arm is grown by alternately stacking the ZB and WZ phases along Æ111æZB/Æ0001æWZ direction, where the WZ phase takes a higher volume percentage. A detailed study of the core that connects with four arms is essential for the understanding of crystal growth mechanism of the tetrapodal structure. Figure 2a shows a HRTEM image of single TP with incident beam perpendicular to the plane set by two arms. A ZB core in size of ∼5 nm was directly observed, from which the arms were epitaxially grown by stacking ZB and WZ

phase alternately. The orientation relationships between the ZB core and the two arms (considered as WZ phase for less confusion) were determined as Æ112æZB//Æ1010æWZ and (111)ZB//(0001)WZ (for the arm above); Æ112æZB//Æ0110æWZ and (111)ZB//(0001)WZ (for the arm below). Polarized growth has been discovered in the ZnO nanostructures,25,26 while direct observation of the polarized growth at atomic scale on ZnS nanostructures has rarely been reported. The satisfaction of weak phase object approximation (WPOA) owed to the finite size of the as-synthesized TPs justifies focal series reconstruction (FSR) as an efficient technique for obtaining the atom-resolved structure information. To further investigate the atom arrangements at the core/arm interfaces, the exit wave function of the TP in Figure 2a was reconstructed with FSR method based on the experience of our former work.27 In a typical procedure, 20 focal series of HRTEM images with defocus step of 3 nm were taken to reconstruct the exit wave function of the ZnS nanotetrapod. By compensating the aberration and astigmatism introduced by the microscope, the atom-resolved phase image of the core and part of the arms is obtained (Figure 2b and the middle inset). It is distinctly demonstrated that the TP has a single crystal ZB core with two equivalent {111} Zn-terminated surfaces connecting the two in-plane arms by changing stacking order at the interfaces. The stacking sequences can be interpreted as the following: for arm above, (ABC)ZB(CAC)WZ(CAB)ZB; for arm below, (ABC)ZB(CA)WZ(AB)ZB, (BA)WZ, where the layer label connecting two neighboring brackets corresponds to the same layer shared by the two phases. Such stacking behavior is rather interesting and might be related to the arm growth from the core. It will be discussed in the growth mechanism section. At the surface of the TP, low-indexed facets (Figure 2b and the middle inset) were formed during the alternate growth of ZB and WZ phases, such as (111), (1010), and (0110), which minimized the surface energy. Meanwhile, some small high-indexed atomic steps were formed due to the frequent switch of stacking between ZB and WZ structures, such as {001} of ZB phase and {1012} of WZ phase. The stacking switch is more popular at the core/arm joint areas, which can induce lattice stress and electron delocalization due to the breaking of crystal symmetry. It is thereby expected to affect the optoelectronic properties of the TPs. Considering the symmetry feature of the tetrapodal morphology as well as the structure of the visible part of the ZB core, the crystal model of the core was drawn as shown in Figure 3a,b. 2984

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Figure 3. Schematic illustration of the crystallographic features and HRTEM simulation of individual ZnS TP. (a) The crystal demonstration of the core with four equivalent {111} Zn-terminated surfaces marked as gray; (b) atoms stacking model of an entire TP with four wurtzite arms (regardless of ZB phase for simplicity) growing on a octahedral core (stroked with white dashed lines), the cross-section evolved from triangle to hexagon; (c) simulated HRTEM image of a TP with 41 993 atoms indexed by [110]ZB of the octahedral core under a defocus of
50 nm and a limited resolution of 1.2 Å; (d) simulated HRTEM image of the vertical arm indexed by [111]ZB under a defocus of
300 nm and a limited resolution of 1.6 Å.

The core of the ZnS TP forms an octahedral morphology enclosed by four Zn-terminated surfaces separated by another four S-terminated surfaces (Figure 3a). The four arms grow on the four Zn-terminated surfaces following orientation relationship of Æ111æZB/Æ0001æWZ by stacking ZB and WZ phases alternately (Figure 3b). To get further verification of the octahedral model of the ZB core, an atoms stacking model in real size was built and indexed by the same crystal orientation (Æ110æZB) with the TP in Figure 2. The corresponding simulated HRTEM image calculated with all of the 41 993 atoms in this model is shown in Figure 3c and it demonstrates the same features with the measurements of Figure 2. The left half of the core cannot be resolved due to the overlap of top arm (much “thicker” in that orientation), which makes the planes spacing in this area (∼1.1 Å) much smaller than the resolution limit of microscope (g1.6 Å) and it is even worse in the experimental result due to the slight deviation of the top arm from the exact orientation. Moreover, the gradient of the contrast implies it is thinner for the area nearer to the right tip of the core, serving as another evidence of the octahedral shape. Besides, it was expected to observe the triprism shaped arms induced by the triangular facets of the octahedron core, however, the three edges of the arm degenerate during the crystal growth and the large defocus value of the microscope makes only the hexagonal tip of the vertical arm clearly identified, which is supported by the simulated HRTEM image of the TP indexed by ZB (Figure 2d). The vertical view of a single

Figure 4. (a,b) SEM image and SAD pattern of as-synthesized ZnS nanoparticles. (c) SEM image of the WZ ZnS nanobelts. (d) HRTEM image of a ZnS nanobelts after annealing at 200
C in nitrogen, which is enclosed with low index facets on the sides; the inset is the corresponding SAD pattern indexed to wurtzite single crystal structure.

arm appears a hexagonal section, which is more stable for the hexagonal WZ structure. 2985

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Figure 5. Schematic illustration to demonstrate three paths (0, 10, and 30 mL EDA was used) of structure evolution for ZnS nanoparticles, nanotetrapods, and nanobelts, respectively. Stage (I) shows the original formation of ZnS monomers. Stage (II) shows the three types of clusters (ZB nanoparticles, ZB octahedrons, and WZ nanoparticles) formed under different concentration of EDA. Stage (III) shows final ZnS nanostructures formed from kinetic crystal growth and assembly.

It is well-known that the kinetic control of the monomer concentration and the crystal growth rate is the key factor to manipulate the particle size as well as material shapes initiated from the anisotropic growth.28 EDA acts as not only a capping agent29 and but also a complex ligand30,31 in the chemical synthesis of II
VI hierarchical structures. Similarly, the complex of precursor and EDA in our solution system can be described as follows32,33 (m is the positive integer) ZnðAcÞ2 þ mEDA f ½ZnðEDAÞm 2þ þ 2Ac


ð1Þ

½ZnðEDAÞm 2þ f Zn2þ þ mEDA

ð2Þ

Zn2þ þ Na2 S f ZnS þ 2Naþ

ð3Þ

In a basic environment with EDA, ionized Zn atoms are complexed with EDA molecules to form the soluble coordinated ions [Zn(EDA)m]2þ, which further decompose into Zn2þ ions at an elevated temperature (>90
C) according to reactions 1 and 2. Thus the [Zn(EDA)m]2þ ions serves as a buffer to control the release rate of Zn2þ ions and hence the formation rate of ZnS nuclei is controlled. Additional experiments were performed to explore the influence of EDA on the anisotropic growth of ZnS nanotetrapods. In one case, by removing the usage of EDA, ZnS nanoparticles were synthesized (Figure 4a). These nanoparticles are uniform with an average size of ∼8 nm. The SAD pattern has three rings indexed to {111}, {220}, and {311} planes of ZB structure (Figure 4b). No diffraction rings concerning on WZ phase was observed. In another case, when a large volume of EDA (30 mL) was introduced, ZnS nanobelts with ∼1 μm length 100
120 nm width were obtained (Figure 4c). TEM analysis reveals that the as-prepared nanobelts are assembled by small WZ ZnS nanoparticles (6
8 nm) in an ordered orientation relationship along Æ0001æ direction. Great promotion of crystallinity was realized

when these nanobelts were annealed at 200
C in Nitrogen for 5 h (Figure 4d). An individual nanobelt split into narrower branches dominantly enclosed with equivalent {1011} planes of which the surface energy locates between those of {1210} and {1010} planes in the original nanobelts and thus serves a transition to stabilize the one-dimensional structure. On the basis of the results mentioned above, a kinetically controlled anisotropic growth mechanism assisted with EDA comes to be clear as illustrated in Figure 5. In the case when no EDA was introduced, a high concentration environment of ZnS monomer was formed from the rapid reaction of Zn2þ and S2-. Then their average size was kinetically “focused” into a narrow distribution at the supersaturation stage of ZnS monomers in solution34 (Figure 5 stage II
III). The final ZnS nanoparticles favors ZB phase due to the relatively lower surface energy for small nanoparticles.35,36 In the other case that produces ZnS TP when less EDA (∼10 mL) was used, the monomer concentration was still comparable with the non-EDA case at the initial stage of reaction, which yielded first ZB nanoparticles through a similar “focusing” procedure; however, the monomer concentration dropped rapidly due to the complex of Zn 2þ with EDA (reaction 1), which initiated the crystal growth process.28 At this stage, the reduced release rate of Zn2þ ions from decomposition of the coordinated ions [Zn(EDA)m]2þ (reactions 2 and 3) drove the as-formed ZB nanoparticles to grow into an octahedral shape enclosed with {111} polar facets (close-stacking planes) to achieve minimized surface energy (Figure 5 stage II). Then, the arms were grown anisotropically on the four Zn-terminated surfaces owned to their higher chemical activity compared to S-terminated ones (Figure 5 stage III). The tetrapodal morphology was finally formed. Hereby some points are notable. First, the growth of arms on the Zn-terminated surfaces of the core is generally initiated by changing the stacking sequences from ABCABC (ZB) to one/two periods of ABAB (WZ). This interesting manner indicates that the crystal growth of the ZB core is no longer maintained when its size reached 2986

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Nano Letters ∼5 nm, which should be a threshold in the reaction system. WZ phase was preferred for the growth of larger nanocrystals. Then the change of stacking order occurred on the Zn-terminated surfaces of the core due to their higher activity and finally the TP structure forms. Therefore, the stacking order change at the core/arm interface serves as a transition to activate the onedimensional growth of the arms. Second, the alternate stacking of ZB and WZ phases in the arms can be attributed to the kinetic variation of the parameters in solution around the phase transition point, such as the monomer concentration and the system pressure.37,38 In addition, as an intrinsic anisotropic structure, the arms favored anisotropic growth along the unique c-axis (WZ) and meanwhile minimized surface energy through forming the {1010} closed hexaprism shape. In the third case that a large amount of EDA (∼30 mL) was added, strong complex effects result in a pretty slow release rate of Zn2þ ions and hence further reduced the concentration of monomers; WZ nanoparticles were favorable originally for this anisotropic growth environment. As particles concentration increases, ZnS nanobelts were assembled from these nanoparticles along the c-axis. This behavior coincided with the intrinsic anisotropic nature of WZ materials. Consequently, the amount of EDA serves as a critical factor to kinetically manipulate the anisotropic growth of ZnS materials in the solvothermal system. In conclusion, ZnS TPs of ∼5 nm in diameter were synthesized via a solvothermal approach. The atom-resolved structure was studied by applying exit wave reconstruction on TEM. The ZB octahedral core as the center of TP was attested by microscopy analysis combined with quantitative simulation. The four arms are selectively grown on the four Zn-terminated surfaces of the core by changing stacking order. This interesting behavior is significant to initiate the arm growth by ending the ZB growth of the core. The alternate stacking of ZB and WZ phases in the arms is also expected to introduce novel mechanical and electronic properties. Besides, the anisotropic growth mechanism kinetically controlled with EDA was proposed to interpret the formation of ZnS TPs. Following this scheme, uniform ZB ZnS nanoparticles as well as pure WZ ZnS nanobelts were all synthesized in the same solution system. This elaborate investigation of ZnS TPs could be extended for the understanding of other tetrapodal materials with both cubic and hexagonal phases.

’ AUTHOR INFORMATION Corresponding Authors

*E-mail: (R.W.) [email protected]; (H.Z.) [email protected] )

Author Contributions

These authors contributed equally.

’ ACKNOWLEDGMENT The work is supported by National Natural Science Foundation of China (No. 50971011 & 50902007), Beijing Natural Science Foundation (No. 1102025) and Research Fund for the Doctoral Program of Higher Education of China (No. 20091102110038), the Innovation Foundation of BUAA for PhD Graduates and the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), School of Physics (TCD) Science Foundation Ireland under Grant 07/SK/I1220a, the

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INSPIRE program of National Development Plan 2007
2013 and the SFI under Contract No. 08/CE/I1432. SK supported by SFI IRCSET fellowship. We would like to acknowledge Dr. Markus Boese at the CRANN Advanced Microscopy Laboratory at Trinity College for his assistance and useful discussions.

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