CRYSTAL GROWTH & DESIGN
Aligned Growth of ZnCdSe Nano-arrowheads
2009 VOL. 9, NO. 2 803–806
Zhuang Liu,† Xitian Zhang, and Sui Kong Hark* Department of Physics, The Chinese UniVersity of Hong Kong Shatin, N. T., Hong Kong, China ReceiVed April 8, 2008; ReVised Manuscript ReceiVed May 29, 2008
ABSTRACT: Aligned ZnCdSe alloy nanostructures have been grown on GaAs (100) substrates using metalorganic chemical vapor deposition. Scanning electron microscopy observations find that they resemble nano-arrowheads in having three-blades, and their long axis is aligned along either the [11j 1] or the [111j ] direction of the substrate. Transmission electron microscopy shows that the blades of the nano-arrowhead have a (01j 13) twin relationship with each other and that their flat surface is close to (2j 110). Cathodoluminescence studies reveal that non-radiative recombination centers are localized at the twin boundaries. A four-step growth process, combining epitaxy, stacking fault generation, nanotetrapod nucleation, and vapor-liquid-solid growth, is suggested to understand the three-bladed morphology and alignment directions of the nano-arrowheads. Introduction In nanoscale fabrications, assembly and architectural control of complex shaped nanoachitectures have attracted extraordinary research interest because they are crucial steps toward the realization of functional nanosystems. The complex shaped nanoachitectures evolved from nanorods or nanoribbons offer opportunities to show both novel properties coming from their low dimensionality and potentially new phenomena arising from their 3D organization. Many complex shaped structures, including comb-like,1,2 dendritic,3,4 multiarmed,5 triple-crystal,6,7 and tetrapod nanoarchitectures,8 have been reported. Aligned growth of these nanostructures is very important if they are to be combined into nanoblocks for building devices. Template and epitaxial growth methods are widely used to realize the aligned growth of nanowires and nanorods.9-12 However, very few reports are concerned with aligned growth of complex nanoarchitectures.13,14 The difficulty in obtaining aligned growth seems to come from the variety and complexity of the growth mechanism of these nanoarchitectures. In this article, we present the fabrication and structural study of novel aligned ternary alloyed ZnCdSe nanoarrowheads (NAs). Because of their wide band gap, the Zn-Cd-Se system has been extensively studied as material for laser diodes15 and photodetectors in the blue-green region of the visible spectrum.16 Many, including us, have reported successful fabrications of 1D nanostructures in this system, including nanowires,17 nanoribbons,14 nanopyramids,18 nanorings,19 and other nanocrystals.20-22 However, only a very few are about complex nanoarchitectures.23,24 Experiments The NAs were synthesized by metalorganic chemical vapor deposition (MOCVD). The growth process is similar to that for the nanowires.17 Briefly, a thin layer (∼20 nm) of Au film was sputtered on GaAs (100) substrates before they were loaded into the horizontal growth chamber of the MOCVD system. Diethylzinc (DEZn), dimethylcadmium (DMCd), and diisopropylselenide (DIPSe) were used as precursors and 7N hydrogen as the carrier gas. The growth temperature was kept at 620 °C, and the pressure in the growth * To whom correspondence should be addressed. E-mail: skhark@ phy.cuhk.edu.hk. Phone: (852) 26096321. Fax: (852) 26035204. † Permanent address: Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences.
Figure 1. (a) SEM top view of the as-synthesized products. (b, c) Enlarged images of two NAs. (d) A pole stereograph of the zincblende crystal from [100] direction. (h) XRD pattern of the products. “i” indicates the peaks come from instruments. chamber at 500 Torr. The flow rates used were as follows: H2, 0.975 slm; DEZn, 1.08 sccm; DMCd, 4 sccm; and DIPSe, 7.2 sccm. The morphology, structure, and composition of the as-synthesized products were characterized by scanning electron microscopy (SEM, Leo 1450), X-ray diffraction (XRD, using the 1.54Å Cu KR radiation), and transmission electron microscopy (TEM, Philips CM120 and Technai 200, both attached with an energy dispersive X-ray (EDX) detector).
Results and Discussion Figure 1a shows an SEM top view of the products. Nanostructures are seen covering the substrate, about 90% of which are NAs and the remaining are nanowires. Most of the NAs are about 5 µm in length and, when projected onto the substrate, are aligned along either the [01j 1] or [011j ] direction, as indicated by the double-headed white arrow. Figures 1b and 1c are two magnified images of the NAs, revealing the three blades that intersect at a common axis along their entire length. The width of the blades is about 200 nm on the bottom and tapers toward the tip. After checking many NAs, we found that the plane of one of the blades is always perpendicular to the substrate. And according to this blade, the NAs can be divided into two kinds, satisfying a mirror symmetry with each other. Viewing top down from the substrate normal, one kind contains the
10.1021/cg800360m CCC: $40.75 2009 American Chemical Society Published on Web 12/15/2008
804 Crystal Growth & Design, Vol. 9, No. 2, 2009
perpendicular blade above the common axis, just as the one shown in Figure 1b; the other contains the perpendicular blade below the common axis, as the one shown in Figure 1c. To analyze the geometrical relationship between the NAs and the substrate, the angle θ between their long axis and the substrate normal was determined by SEM observations from different perspectives, through rotating the substrate about its surface normal and tilting it about its horizontal axis [01j 1].10 The angle θ determined is about 54.7°, which is close to that between the 〈111〉 and [100] directions (55.8°). To better show the geometrical relationship, a pole stereograph is drawn in Figure 1d. The arrow indicates that the projections of [11j 1] and [111j ] are along [01j 1] and [011j ]. Comparing Figures 1a and 1d, and considering the value of the angleθ, it is concluded that the NAs are either aligned along the [11j 1] or the [111j ] directions. The XRD pattern of the NAs, displayed in Figure 1e, shows that they are wurtzite in structure, with lattice constants of a ) 4.08 Å˙ and c ) 6.67 Å˙. On the basis of these values, their chemical composition is determined to be Zn0.68Cd0.32Se. It is noted that unlike the XRD patterns of random W-structured powders, only (101j 1) and (112j 0) peaks appear in Figure 1e. The XRD pattern is thus consistent with the ordering of the NAs.10 The peak at 31.7° is from the GaAs substrate and the one at 40.6° from the Au7Ga2 film formed on the surface of the substrate. This Au-Ga alloy resulted from the reaction between the GaAs substrate and sputtered Au on their surface.25 Figure 2a shows a typical TEM bright field image of a single NA. Because it blocks off most of the electron beam, the middle blade appears darker than the other two blades. Figure 2b is a high resolution TEM (HRTEM) image of the region near the tip of the NA in Figure 2a, clearly showing a twin boundary. Its inset is a fast Fourier transformed image of the twin boundary region. The pattern can be indexed as having the zone axis [1j54j1] of the W structure and consists of two sets of diffraction spots, which share a common (01j13) spot. Several NAs have been examined, and the crystallographic relationship of the three blades in all of them is the same. Only a coherent (01j13) twin boundary was observed, and no foreign particle was found existing between the blades. Figure 2c is an image of a NA rotated such that one of its blades is perpendicular to the observational direction. The corresponding HRTEM image shown in Figure 2d indicates that the blades’ flat surfaces are approximately the (2j110) planes. EDX spectra taken from the body and the particle attached at the end of a typical NA are shown in Figures 2e and 2f, respectively. They indicate that the body contains Zn/Cd/Se at 0.65:0.35:1.00 and that the particle contains Au/Ga at 0.77:0.23, which are consistent with the XRD results. Figure 3 shows the cathodoluminescence (CL) spectrum and images of the NAs sample. To eliminate the influence of the substrate, the CL study is performed on the NAs that were moved to a TEM grid. The CL spectrum is measured from a typical NA at room temperature, showing a broad peak centered at 720 nm and a shoulder peak at about 900 nm. For the measurement, the beam voltage is 20 keV, and the probe current is 1 nA. It is interesting that the CL panchromatic images of the NAs (the inset in Figure 3) reveal a dark contrast on the twin boundary region, which indicates the localization of nonradiative recombination centers. Because no foreign particle and dislocations were found by our TEM, this localization is suggested to result from the inherent structure of the (01j13) twins. Bere and Serra26 have simulated the atomic structure of the interface of (01j13) twins. The dangling bonds at the interface
Liu et al.
Figure 2. (a) Typical TEM image of a NA. (b) HRTEM image of the tip region in (a). The (01j 13) twin interface is indicated by the white line. The inset is a FFT of a region near the twin boundary. (c) NA having a blade perpendicular to the observational direction. (d) HRTEM image of a region near the edge of the lower-left blade of the NA in (c) along the [1000] zone axis. The inset is FFT of (d). (e, f) EDX spectra taken from the body and the tip particle of the NA in c. The strong Cu signal comes from TEM sample grid.
Figure 3. CL spectrum taken from a typical NA. The inset shows panchromatic images of the NAs.
are reconstructed to form six-coordinated channels to minimize the distortion of the tetracoordination. On the other hand, the reconstruction of dangling bonds had been suggested to explain the CL contrast observed in studying dislocations in semiconductors.27 Fan et al.6 suggested a nucleation mechanism to understand the occurrence of the three-bladed nanoarchitectures. A
Aligned Growth of ZnCdSe Nano-arrowheads
Crystal Growth & Design, Vol. 9, No. 2, 2009 805
can also explain the geometrical relationship between the blades and the substrate. In Figure 4a,b,c, we note that the (2j 110) planes of the A blade and D′ arm are the same as the (011) plane of the substrate. That is to say, one of the blades must be perpendicular to the (100) substrate. This phenomenon has been confirmed by SEM observations. It is noted that because of the 6-fold structural symmetry of the D′ arm, the perpendicular blade can appear either above or below the long axis when viewed top down from the substrate normal. Conclusions
Figure 4. (a) Growth process of the aligned NAs. (b) Schematic showing the geometrical relationship between the NA and the nanotetrapod nucleus. (c) Schematic illustrating the 〈111〉 aligned directions.
nanotetrapod nucleus first forms in a liquid droplet. When its size becomes comparable to that of the droplet, surface tension will elicit the vapor-liquid-solid (VLS) mode of growth, inducing a tetrapod to grow into three-bladed structures. However, this model can not explain the alignment of our NAs on the substrate. We suggest and illustrate the processes involved in the growth of the aligned NAs in Figure 4a. First, nanosized zincblende (Z)-structured ZnCdSe grains grow epitaxially on the substrate, nucleating in the molten Au-Ga catalysts. Second, W-structured extensions attach to the Z-structured grains through stacking faults, which leads to the formation of tetrapod nuclei. This assumption is based on the small energy difference between the Z-structured and W-structured ZnCdSe,28 and the nuclei are often in the form of tetrapod crystals during solidification of many hexagonal materials.29,30 Third, just as in Fan’s model, when the tetrapod grows to a size about that of the liquid droplet, drawing by the surface tension, the liquid droplet would be lifted up and captured by the three upper arms.6 Once the droplet is lifted up, the growth of the lower arm stops. Fourth, VLS leads to the growth of the three arms along the [0001] direction of the lower arm, which evolve into the three blades. Figure 4a are the perspective views from the [011]/[1000] (the former refers to the substrate and the latter to the lower arm) direction. The geometry relationship between the NA and the tetrapod nucleus is more clearly illustrated in Figure 4b, in which they are enlarged and slightly tilted from the [011]/ [1000] direction. The three blades of the NA are labeled as A, B, and C; the three corresponding upper arms of the tetrapod are labeled as A′, B′, and C′, and its lower arm is labeled as D′. It is noted that the long axis of the NA and the D′ arm are along the same direction. In Figure 4c it is shown that the direction [0001] of the W structure is parallel to the direction [11j 1] of the Z structure, when these two structures are connected by a stacking fault. Thus, the growth processes described above explain why the NAs align along the 〈111〉 directions. The reason for their tendency to preferentially align along only 2 of the 4 possible 〈111〉 directions may relate to the anisotropic nature of the (100) surface of the substrate.10 The suggested growth processes
In conclusion, we have achieved directionally ordered growths of three-bladed ZnCdSe NAs on GaAs (100) substrates by MOCVD. They aligned along two equivalent 〈111〉 directions of the substrate. TEM studies reveal the wurtzite crystal structure of the NA and a (01j13) twin relationship between its blades. CL images show the existence of a dark contrast along the twin boundary, indicating the localization of the non-radiative recombination centers as a result of dangling bond reconstructions. A model, combining epitaxy, nanotetrapod nucleation, stacking fault generation, and VLS growth, is proposed to explain the appearance of the morphology and the aligned growths of the NAs. 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 No. 411807) and CUHK direct grant (Project code: 2060305). Supporting Information Available: Panchromatic and monochromatic CL images of a single NA. This material is available free of charge via the Internet at http://pubs.acs.org.
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