8912
J. Phys. Chem. C 2008, 112, 8912–8916
Quadra-Twin Model for Growth of Nanotetrapods and Related Nanostructures Zhuang Liu, Xitian Zhang, and Sui Kong Hark* Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, N. T., Hong Kong, China ReceiVed: February 11, 2008; ReVised Manuscript ReceiVed: March 26, 2008
We fabricated ZnCdSe alloy three-bladed nanoarrowheads (TBNAs) on GaAs (001) substrates by metalorganic chemical vapor deposition. On the basis of crystallographic analyses of their structure, we found that they are closely related to nanotetrapods, despite the apparent difference in morphology. A new quadra-twin core model is proposed to understand the growth of TBNAs. This model also agrees with reported results of a subclass of nanotetrapods that cannot be explained by conventional zinc blende and octa-twin core models. Furthermore, a simple method to distinguish the core structures by TEM is suggested. Introduction Complex semiconductor nanocrystals grown or evolved from a central core, such as nanotetrapods,1 multiarmed structures,2 and tricrystal structures,3 are potential functional elements and interconnections for future integrated nanodevices and are expected to play important roles in the “bottom-up” approach to nanotechnology. Many compounds, including CdTe,1 CdS,2 ZnO,4 and ZnS,3,5 have been found to form complex nanocrystals. It has been demonstrated that CdSe-based nanotetrapods are more effective than nanorods in enhancing the performance of composite polymer/nanocrystal solar cells.6 It is predicted that if good control of their size, morphology, and alignment could be achieved, these unique nanocrystals could be widely used in optoelectronics and in field emission, gas sensing, and piezoelectric devices. It is commonly suggested that the growth of these nanocrystals proceeds via a core. The simplest case is that of the nanotetrapod:1 a symmetric structure with four arms extending from an octahedral core. Other nanocrystals with a more complex morphology, such as multiarmed2 and tricrystal structures,3 can be considered to be evolved from it. The core may contain a single element or several elements. Two main models, different in the assumed shape, packing arrangement, and structure of the core elements, usually are used to explain the occurrence of nanotetrapods in the literature. Shiojiri and Kaito proposed a zinc blende core model in which the octahedral core contains a single zinc blende structured nanocrystal.7 The octahedron exposes eight {111} surfaces, four of which are zinc-terminated and chemically more reactive than the other four oxygen-terminated ones.8 As a result, wurtzite structured arms readily grow on the zinc-terminated surfaces through the introduction of stacking faults. Gong et al. reported TEM evidence of the existence of this zinc blende core.5 However, a zinc blende phase is not always observed in many nanotetrapods reported.10,11 An octa-twin core model in which the octahedral core is made up of eight tetrahedral wurtzite elements, instead of a single zinc blende one, has been suggested. Each element is bound by three {112j2} pyramidal facets and one (0001) basal facet. These elements are joined together, making the pyramidal facets the twin boundaries and the basal facets the outer surfaces of * Corresponding author. Tel.: (852) 26096321; fax: (852) 26035204; e-mail:
[email protected].
an octahedron. However, the eight elements cannot perfectly fit into an octahedron because the angle between two {112j2} twin planes is not exactly 90°. Thus, a deformation of the elements must be assumed to accommodate the misfits, which results in extra strain energy. The other point that should be noted is that, similar to the zinc blende core model, four of the surfaces must be zinc-terminated and the remaining four oxygenterminated, to produce the tetrapod morphology in this model. The elements on the two sides of the twin planes must obey a head-tail twin relationship.9 Dai et al. reported the first direct TEM observations of an octa-twin core in ZnO nanotetrapods, supporting this model.10 The two conventional models are quite successful in explaining the growth of some nanotetrapods. However, observations of other nanotetrapods that cannot be explained by these models were reported recently. Ronning et al. found that the boundaries in the core part of their ZnO nanotetrapods appear as the letter H when they were observed by TEM along C2, the two-fold symmetry axis,11 a feature incompatible with the two conventional models. Instead of {112j2} twins, Hu et al. observed {01j13} twins in their ZnO nanotetrapods.12 Fan et al. also reported observing a (01j13) twin relationship between the branch ribbons in their ZnS tricrystal nanodarts and suggested that their growth might share a nucleation mechanism related to those of the nanotetrapods.3 To explain the observed geometrical relationship between the arms of their ZnO nanotetrapod, Zhang et al. suggested the existence of {033j8} twins, which are close to the {01j13} twins.13 These results conflict with the zinc blende and octa-twin models, indicating that an additional model, consisting of a multiply {01j13} twinned core, should exist in some nanotetrapods and related nanostructures. In this paper, we present a quadra-twin core model to understand the growth of ZnCdSe alloy three-bladed nanoarrowheads (TBNAs) and the results of other groups regarding the {01j13} twins. The TBNAs have three blades, any pair of which shares a (01j13) twin boundary. The crystallographic relationship among the blades is the same as that among the arms of a nanotetrapod. The quadra-twin model can be extended to cover those nanotetrapods and observations that could not be explained by the two conventional models. Moreover, we found a simple rule to judge as to which of the three cores, ours and the two conventional ones, is involved in the formation of the nanotretrapods in question.
10.1021/jp801235k CCC: $40.75 2008 American Chemical Society Published on Web 05/21/2008
Quadra-Twin Model for Nanotetrapods
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8913
Figure 2. TEM observations from a direction perpendicular to the right blade of TBNA. (a) Low-magnification image of TBNA. Inset shows the corresponding diffraction pattern taken from the right blade. The arrows indicate the directions of the long axis of TBNA and [101j0]. (b) HRTEM image taken from the area indicated by the open rectangle. Arrows indicate directions of the axis and [0001].
Figure 1. (a) SEM image of TBNA sample. The inset is a top view of a single TBNA. (b) Two TEM images observed from different directions. (c) EDX spectrum taken from a single TBNA. The Cu signal comes from the TEM grid.
Experimental Procedures TBNAs were synthesized in a horizontal reactor metalorganic chemical vapor deposition system. Briefly, a thin layer (∼20 nm) of Au film was sputtered on the GaAs (001) substrates, serving as the catalyst. When the reactor was heated to a growth temperature of 620 °C, precursor vapors of diethylzinc, dimethylcadmium, and diisopropylselenide were introduced into the reactor by a carrier gas (7 N hydrogen). The growth temperature was kept for 3600 s before the reactor was cooled to room temperature. The total pressure was kept at 500 Torr during the whole fabrication process. After growth, the products were characterized by SEM (Leo 1450) and TEM (Philips CM120 and Technai 200). Results and Discussion Figure 1 shows the morphology of the ZnCdSe alloy TBNAs. Figure 1a is an SEM image of the products grown, ∼90% of which are TBNAs and the remaining nanowires. The inset in Figure 1a shows a top view of a TBNA, showing its three blades that make an angle of ∼120° with each other and combine at a common axis. Figure 1b shows two representative TEM images. The upper image in Figure 1b shows TBNA having a darker contrasted band running along its left edge. The lower image in Figure 1b shows the same TBNA after it was rotated about its axis by ∼28°. As a result, the darker band moved to the middle and became narrower, confirming the three-bladed shape from another perspective. It was determined that the dimensions of the blades are ∼200 nm in width, 20 nm in thickness, and several micrometers in length. Figure 1c shows a typical EDX spectrum taken from a single TBNA, revealing its chemical compositions as Zn0.68Cd0.32Se. To examine the three-bladed structure in more detail, we looked at its crystal structure and growth direction by TEM from
Figure 3. TEM observations from a direction parallel to the middle blade. (a) Low-magnification image of TBNA. (b) Corresponding diffraction pattern taken from the whole TBNA. (c) Dark field image of TBNA in panel a formed by the (101j0)r spot and (d) dark field image of the same TBNA formed from the twin spot (101j0)l. (e) HRTEM image of another TBNA from the same direction as in panel a. The arrow in the inset indicates the region from where the HRTEM image was obtained. White lines indicate atomic planes (01j13) and (101j0).
two characteristic directions, one roughly perpendicular to and the other parallel to a blade. These directions are the same as those for obtaining images shown in Figure 1b. The images and diffraction patterns are shown in Figures 2 and 3. Figure 2a is an image taken from a direction roughly perpendicular to the right blade. The inset in Figure 2a shows the corresponding SAED pattern obtained along the [1j21j0] zone axis. Analyses of the pattern and morphology of TBNA indicate that its long
8914 J. Phys. Chem. C, Vol. 112, No. 24, 2008
Liu et al.
Figure 4. Quadra-twin model to understand the structure of TBNA. (a) Top view and (b) side view drawings of TBNA. The rectangles in dashed lines are representatives of three {01j13} twin planes. Panels a and b also show the angular relationship between the twin planes and the three blades. Panel c shows as to how a nanotetrapod evolves into TBNA.
axis makes an angle of ∼18° with the [101j0] direction. The HRTEM image in Figure 2b was taken from the region labeled by the open rectangle in Figure 2a, showing the different crystallographic orientations of the blades. For most complex nanostructures with blade-like elements, catalytic particles are usually found in the interconnected region of the blades.14,15 However, in our case, the three blades connect coherently without any particles in between. The white arrow in Figure 2b indicates the [0001] direction, which makes an angle of ∼72° to the long axis of TBNA. Figure 3 shows the images of TBNA observed from a direction parallel to its middle blade. Figure 3a is a low-magnification image of the whole TBNA, and Figure 3b shows the corresponding SAED pattern. The latter consists of two sets of spots having a common [1j21j1] zone axis and a mirror symmetry about the (01j13) twin plane. The spots joined by the right parallelogram (yellow in Figure 3b) are from the right blade, and those by the left parallelogram (green in Figure 3b) are from the left. The subscripts r and l of the indices represent the blade from which the spots originated. The (01j13) plane was predicted theoretically as a twin boundary with a low interfacial energy in the wurtzite structure16 and was first observed experimentally by Wang et al.17 This twin plays an importantroleinconnectingelementsofcomplexnanostructures.9,17 Figure 3c is a dark field image formed from using the (101j0)r spot and Figure 3d from using the (101j0)l spot. The inverted contrast of the two images again reveals the twin relationship of the left and right blades. The uneven contrast within each blade indicates the existence of strain and/or structural defects. The diffraction pattern and dark field image of the middle blade that is parallel to the electron beam cannot be shown because it appears to be too thick from this perspective. The lattice resolved image of the interfacial area between the two blades clearly shows the twin boundary (Figure 3e). The (01j13) and (101j0) planes are indicated by white lines in Figure 3e. The arrow in the inset of Figure 3e indicates the region from where this image was taken. The darker band in the middle disappears because a section of the middle blade is missing here. On the basis of our TEM observations and its three-fold symmetry, TBNA can be understood as being built up from a triply twin structure. Figure 4 illustrates this structure: panel a is a top view of a schematic of TBNA, and three {01j13} twin planes are indicated by dashed lines. Panel b is a drawing showing the geometrical relationship between the blades and the twin planes, with the arrow showing the deduced [303j1] axial direction referenced to the blades. All experimentally observed angular relationships mentioned previously can now be understood in this model. First, the observed angle between the {01j13} twin planes is 126°, close to the 120° deduced from the three-fold symmetry. Second, the angle between the long
axis and the [101j0] crystal direction is calculated as 17.4°, consistent with the observed 18°, shown in Figure 2a. We found that it is interesting that the [0001] directions of the three blades make an angle of ∼110° to each other and also to the long axis of TBNA (Figure 2b), which is close to 109°28′, the angle between two arms in an ideal tetrapod. Thus, TBNA may be considered to be related to the nanotetrapod. TBNA will lead to a nanotetropod if the blades grow preferentially along the [0001] direction. The blades, A-C, of TBNA will become three of the four arms, A′-C′, of the nanotetrapod, as shown in Figure 4b,c. Fan et al. reported a (01j13) twin relationship between the ribbons in a nanodart,3 a nanostructure that closely resembles ours; however, the direction of the long axis was determined to be [22j01] ([22j1] in their paper), different from [303j1] deduced by us. We note that while [303j1] is contained within {01j13} twin planes, [22j01] is not. Encouraged by the crystallographic analysis of the three blades of TBNA, we proposed a quadra-twin model of the core structure to understand the growth of nanotetrapods. In this model, every two arms are joined at a (01j13) twin plane. There are a total of six twin planes among the four arms. Figure 5a shows a schematic of this quadra-twin core, in which four identical core elements form an octahedron. For clarity, one of the elements that constitutes the core is highlighted by broad (red in Figure 5a) lines. In Figure 5, A denotes the apex, F the face center, and O the body center of the octahedron. The core elements in our model are heptahedra, each bound by three shared faces: A1F1OF3, A2F1OF2, and A3F2OF3; one complete octahedral face, A1A2A3; and three partially octahedral faces, A1F1A2, A2F2A3, and A3F3A1, whose area is 1/3 that of the original octahedral face. Figure 5b shows as to how the four elements combine to form an octahedron. For comparison, the zinc blende and octa-twin cores are shown in Figure 5c. In the zinc blende and octa-twin core models, the four selectively faster growing arms of the nanotetrpod are attributed to the polarity of the {111} planes of the zinc blende core and the (0001) plane of the wurtzite structure. However, in our model, no polarity is needed. The facets of our octahedron core are naturally divided into two groups, of which Figure 5d shows the planes as examples from each group. A1A2A3 exposes just the (0001) plane, and A1A2A5 exposes three twin boundaries. Growth of the four (0001) planes is preferred over the four planes containing exposed twin boundaries. This way, a nanotetrapod is formed, consisting of four arms extending along the directions. This model suggests that it is possible even for nonpolar materials to form nanotetrapods. To compare our model with the zinc blende and octa-twin models in more detail, we list in Table 1, n, the number of elements in the octahedral core; r, the ratio between the areas
Quadra-Twin Model for Nanotetrapods
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8915
Figure 6. (a and b) Schematics of grain boundaries in the cores of octa- and quadra-twin models, respectively, as observed along the C2 symmetry axis. Dashed lines indicate the cores and boundaries. Solid lines indicate the arms of the nanotetrapods. (c) Schematic of a nanotetrapod with arms of a hexagonal cross-section. The crystallographic properties of surface AB are dependent on the models.
Figure 5. (a) Quadra-twin core, (b) connection of its four core elements, (c) zinc blende and octa-twin cores, and (d) two kinds of surfaces in the quadra-twin core.
TABLE 1: Comparisons of Twin Planes, n, R, e, and f of Three Core Models core structure
twin planes
n
R
e (mJ m-2)
f
zinc blende octa-twin quadra-twin
none {112j2} {01j13}
1 8 4
0.5 0.87 0.41
8.0 96 530
0 -0.14 0.08
of the grain boundaries to the surface of the core (in the zinc blende model, the area of the grain boundaries is taken as the area of the stacking fault planes); and e, the interfacial energy of the grain boundary per unit surface area. The interfacial energy is estimated as the product of the area of the grain boundaries and the energy of stacking faults or twins. Instead of calculating for the ZnCdSe alloy, where many energy values are not well-known, we use GaN as an example because its twin energy is most studied. The results obtained from the latter also should be applicable to the former. For GaN, we found the following energies: stacking fault energy of 16 mJ m-2, (112j2) twin of 110 mJ m-2, and (01j13) twin of 1300 mJ m-2.16,18,19 We note that the surface energy of the interfaces between the elements in the quadra-twin model is larger than the corresponding energies in the other two models. However, strain energy resulting from accommodating the misfits also should be taken into account. Presumably, the relatively smaller (by half) interfacial area in the quadra-twin model would result in reduced strain, as compared to the case in the octa-twin model. To show this, a filling factor f ) ∆V/V, where V is the volume of the octahedron and ∆V the difference between the volumes of the octahedron and the total volume of the core elements, is tabulated. We found that the magnitude of f for the quadra-twin model is 40% smaller than that for the octatwin model. This smaller filling factor indicates that it is possible for the quadra-twin core to have a lower total energy, even though the interfacial energy of its {01j13} twin planes is higher. In studying the core structures of the nanotetrapods, TEM observations were usually performed along their C2 symmetry
axis. From this direction, different configurations of the boundaries can be seen. In the octa-twin model (Figure 6a), the four {112j2} boundaries appear as a cross.5 In the zinc blende model, no boundary exists in the core. However, Ding et al. reported the appearance of a cross that was attributed to the oppositely connected arms.9 In the quadra-twin model (Figure 6b), the configuration of the{01j13} boundaries appears as H shaped, which was first observed by Ronning et al.11 The connection between this H configuration and the multiple {01j13} twins was, however, not discussed until now. More interestingly, we found that the crystallographic relationships among the arms and the core are different in different models, which enables us to deduce as to which model is appropriate for the core structure of a nanotetrapod by TEM. Figure 6c is a top view of the nanotetrapod along its C3 symmetry axis. AB represents the (101j0) plane in our quadratwin model and the zinc blende model, but it represents the (112j0) plane in the octa-twin model. The arms usually have either a hexagonal or a triangular cross-section, depending on the details of the growth process.4,5,12 However, this shape difference does not affect the crystallographic relationships. A zinc blende core can be ascertained directly by its electron diffraction patterns. The octa-twin core and quadra-twin core can be differentiated by taking SAED patterns of the arms along the C3 symmetry axis of the nanotetrapod. The advantage of this method is that it can be performed for all nanotetrapods, including those in which the core cannot be observed directly.20,21 Conclusion We fabricated ZnCdSe alloy TBNA on GaAs (001) substrates. On the basis of crystallographic analyses of their structure, we found that they are closely related to the nanotetrapods, despite their apparently different morphology. A new quadra-twin core model is proposed to understand the growth of TBNAs. This model also explains the results of a subclass of nanotetrapods reported in the literature that cannot be explained by conventional zinc blende and octa-twin core models. Furthermore, we also suggested a simple way to distinguish the core structures by TEM.
8916 J. Phys. Chem. C, Vol. 112, No. 24, 2008 Acknowledgment. The work described in this paper was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project 411807) and a CUHK direct grant (Project 2060305). References and Notes (1) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Paulalivisatos, A. Nat. Mater. 2003, 2, 382. (2) Gao, F.; Lu, Q.; Xie, S.; Zhao, D. AdV. Mater. 2002, 14, 1537. (3) Fan, X.; Meng, X. M.; Zhang, X. H.; Shi, W. S.; Zhang, W. J.; Zapien, J. A.; Lee, C. S.; Lee, S. T. Angew. Chem., Int. Ed. 2006, 45, 2568. (4) Newton, M. C.; Warburton, P. A. Mater. Today 2007, 10, 50. (5) Gong, J.; Yang, S.; Huang, H.; Duan, J.; Liu, H.; Zhao, X.; Zhang, R.; Du, Y. Small 2006, 2, 732. (6) Sun, B.; Marx, E.; Greenham, N. C. Nano Lett. 2003, 3, 961. (7) Shiojiri, M.; Kaito, C. J. Cryst. Growth 1981, 52, 173. (8) Wang, Z. L.; Kong, X. Y.; Ding, Y.; Gao, P.; Hughes, W. L.; Yang, R.; Zhang, Y. AdV. Funct. Mater. 2004, 14, 943. (9) Ding, Y.; Wang, Z. L.; Sun, T.; Qiu, J. Appl. Phys. Lett. 2007, 90, 153510. (10) Dai, Y.; Zhang, Y.; Wong, Z. L. Solid State Commun. 2003, 126, 629.
Liu et al. (11) Ronning, C.; Shang, N. G.; Gerhards, I.; Hofsass, H.; Seibt, M. J. Appl. Phys. 2005, 98, 34307. (12) Hu, J.; Bando, Y.; Golberg, D. Small 2005, 1, 95. (13) Zhang, Z.; Yuan, H.; Gao, Y.; Wang, J.; Liu, D.; Shen, J.; Liu, L.; Zhou, W.; Xie, S.; Wang, X.; Zhu, X.; Zhao, Y.; Sun, L. Appl. Phys. Lett. 2007, 90, 153116. (14) Zou, K.; Qi, X. Y.; Duan, X. F.; Zhou, S. M.; Zhang, X. H. Appl. Phys. Lett. 2005, 86, 13103. (15) Meng, X. M.; Jiang, Y.; Liu, J.; Lee, C. S.; Bello, I.; Lee, S. T. Appl. Phys. Lett. 2003, 83, 2244. (16) Bere, A.; Serra, A. Phys. ReV. B: Condens. Matter Mater. Phys. 2003, 68, 33305. (17) Moore, A.; Ding, Y.; Wang, Z. L. Angew. Chem., Int. Ed. 2006, 45, 5150. (18) Wright, A. F. J. Appl. Phys. 1997, 82, 5259. (19) Yan, Y.; AlJassim, M. M.; Chisholm, M. F.; Boatner, L. A.; Pennycook, S. J.; Oxley, M. Phys. ReV. B: Condens. Matter Mater. Phys. 2005, 71, 41309. (20) Shen, G. Z.; Bando, Y.; Hu, J. Q.; Golberg, D. Appl. Phys. Lett. 2007, 90, 123101. (21) Fischer, A. M.; Srinivasan, S.; Garcia, R.; Ponce, F. A.; Guano, S. E.; Di Lello, B. C.; Moura, F. J.; Solorzano, I. G. Appl. Phys. Lett. 2007, 91, 121905.
JP801235K
