Size Selection in Self-Assembled and Oriented Al Nanocrystals Grown

an Al−Co−Ni decagonal substrate. The resulting mismatch causes the formation of well-oriented nanometric single crystals of Al, satisfying the epi...
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NANO LETTERS

Nanoepitaxy: Size Selection in Self-Assembled and Oriented Al Nanocrystals Grown on a Quasicrystal Surface

2003 Vol. 3, No. 12 1717-1721

Thomas Flu1 ckiger,* Yves Weisskopf, Mehmet Erbudak, Rouven Lu1 scher, and A. Refik Kortan† Laboratorium fu¨r Festko¨rperphysik, ETHZ, CH-8093 Zurich, Switzerland Received September 8, 2003; Revised Manuscript Received October 28, 2003

ABSTRACT The unusual interfacial lattice-mismatch properties of a periodic film and a quasiperiodic substrate are investigated in the Al films grown on an Al−Co−Ni decagonal substrate. The resulting mismatch causes the formation of well-oriented nanometric single crystals of Al, satisfying the epitaxial conditions on a local scale. Total-energy calculations based on a rigid-lattice atomic model for the interface between a cubic and the decagonal surface reproduce the low-energy electron diffraction results concerning the size of the clusters, their distribution, and the details of the orientational alignment of aluminum islands. Such systems can be used in self-size-selecting growth of nanocrystals.

The hetero-epitaxial growth of dissimilar materials is normally expected to reveal a wealth of interesting behavior. The equilibrium structure of the film in such systems is enforced by the relative strengths of various competing interactions, which occasionally results in a new and unusual epitaxy-stabilized structure. An extreme example of such a system would be the growth of a periodic, crystalline material on an aperiodic, quasicrystalline substrate.1-4 These systems are not only very rich in fundamental issues but may hold promise in a number of important applications that enhance the magnetic, optical, and other properties. The 10-fold-symmetry surface of Al70Co15Ni15 (Al-CoNi) decagonal (d-) quasicrystal (QC) is easy to prepare, atomically flat, defect free, and offers an ideal substrate surface for the film-growth experiments. The film growth in the submonolayer regime on this surface has been previously reported. A 0.8-monolayer (ML) film of Au and Pt forms oriented AuAl2 or PtAl2 crystallites, after annealing at 400 K1, whereas Sb and Bi grow as a QC up to 1 ML.3 Theoretical predictions for crystalline adsorbate orientations on crystalline substrate structures have been very successful.5,6 For QCs, however, no analogous attempts were known until the energy calculations of a QC-crystal interface were made for an Al-Cu-Fe-Cr thin film grown on sapphire (0001).7 A reliable calculation of such an interface would primarily require the atomic coordinates of the QC structure as an input. For Al-Co-Ni, several atomic* Corresponding author: Tel.: +41 1 633 21 51; Fax.: +41 1 633 10 96; E-mail: [email protected] † Guest scientist. 10.1021/nl034751l CCC: $25.00 Published on Web 11/14/2003

© 2003 American Chemical Society

structure models have been proposed,8-13 which show some subtle differences in the atomic positions. To our knowledge, no calculations have been attempted that can account for the experimental observations for the growth of adsorbates on Al-Co-Ni surfaces. In this paper, we report the formation of an unusual structure in the Al layers deposited on a d-Al-Co-Ni surface. We present low-energy electron diffraction (LEED) and secondary electron imaging (SEI) results, showing that this formation starts at few atomic layers and is stable up to a thickness of 120 Å. This nanostructure consists of uniformly nanosized Al crystallites, with their fcc (111) planes aligned parallel to the d-Al-Co-Ni surface. Within the interface plane, the nanocrystallites are oriented along one of the 10-fold directions with a slight rotational angle. We also present results of energy calculations for the crystal-QC interface formed between an Al(111) layer and Al-Co-Ni. The calculations reproduce the growth mode of Al on Al-Co-Ni and, additionally, show that for a domain size of ∼30 Å the angular locations of energy minima lead to the development of two groups of Al crystallites on the substrate, which are separated by 2.8°. We find that the uniform nanosize is a clear consequence of the unusual lattice mismatch of this system. A d-Al-Co-Ni QC with dimensions 5 × 3 × 1 mm3 was oriented by means of the X-ray Laue method with an accuracy of (0.5° along the d-symmetry axis and cut and polished mechanically to an optical finish.14 The surface of Al-Co-Ni was cleaned in ultrahigh vacuum in the lower 10-9 Torr region by cycles of heat treatment (700 K for 30

min) and sputtering with Ar+ ions (1.5 keV, 4.5 × 10-7 A/mm2 for 30 min).15 The geometric structure and the structural quality of the investigated surface were determined by LEED. The SEI technique16 was performed to examine the average local atomic order in the near-surface region. In both techniques, the same back-view LEED display system with a total acceptance angle of 100° was used. The patterns were recorded with a SBIG ST-9E charged-coupled device camera. To survey the chemical composition in the near-surface region, Auger electron spectroscopy (AES) was applied with a primary electron energy EP ) 2.4 keV. Al was evaporated from a power-regulated atomic-beam source. The evaporation rate was 0.8 ( 0.1 Å/min, calibrated using the L2.3VV AES signal of Al and taking a mean-free path of 5 Å for the 68 eV electrons after Al deposition on a Cu sample.17 The calibration of the flux from the atom source has been done by comparing the changes in the AES signals with the changes in the mass of a quartz microbalance for a given condition of operation of the source.18 This calibration is periodically confirmed and is consistent with the mean free path used here. During the Al deposition the substrate temperature was held at 300 K. As an input to the calculations, we have used the coordinates of atoms given by Saitoh et al.12 For the single Al(111) layer we have used the bulk fcc coordinates with a lattice parameter of 4.05 Å.19 Energy calculations were performed using a Lennard-Jones (L-J) potential for the interaction between Al adsorbate atoms and the QC substrate, consisting of a surface and a subsurface layer. Thus, our calculations depend on three parameters: the interstructural distance h between the Al-Co-Ni surface and the Al overlayer, the depth , and the hard-core radius σ of a standard L-J potential V(r) ) 4[(σ/r)12 - (σ/r)6], where r is the distance between a substrate and an adsorbate atom. For the results presented below, we have set h ) 2.34 Å, which is the bulk interplanar distance of Al(111). Our calculations show that the exact value of h does not influence the results qualitatively as long as it remains within reasonable limits. The term σ was deduced from calculations of the pair potential for Al-Al.20 It was shown that the absolute minimum for Al-Al is located at r ) 2.86 Å, which results in σ ) r/21/6 ) 2.55 Å for the Al growth on Al.  is a scaling factor. Since we are interested in the relative positions of energy minima, we have set  ) 0.25 eV for reasons of simplicity. The substrate diameter was chosen as large as 50 Å in order to minimize boundary effects, while the adsorbate cluster diameter was increased incrementally up to 36 Å. Figure 1a shows a LEED pattern from the d-Al-Co-Ni surface at EP ) 55 eV in near-normal incidence. The pattern displays 10-fold symmetry, while the analysis of diffractionspot profiles indicates a terrace size of ∼100 Å, in accordance with STM investigations21 (cf. solid line in Figure 1f). Figure 1b depicts a LEED pattern measured at EP ) 55 eV after 6 Å Al deposition. We observe that Al deposition leads to the formation of an unusual diffraction pattern, while the contribution from Al-Co-Ni is still discernible near the 1718

Figure 1. LEED patterns obtained from (a) the clean decagonal Al-Co-Ni surface at EP ) 55 eV; (b) an Al layer with an average thickness of 6 Å deposited onto the decagonal Al-Co-Ni surface at EP ) 55 eV; (c) 120 Å Al on Al-Co-Ni at EP ) 142 eV. (d) SEI pattern obtained at near-normal incidence after depositing 120 Å of Al. Line scans along (e) the tangential direction; (f) the radial direction (omitting 2π) obtained from the d-Al-Co-Ni substrate (solid lines) and after deposition of Al (dashed lines). The radial intensity profiles were analyzed according to refs. 23, 24.

rim of the screen. The fact that the pattern from the adsorbate azimuthally locks to that from the substrate proves that the underlying Al-Co-Ni surface serves as a structural template for the Al film growth. The persistence of diffraction spots from the underlying substrate up to an Al thickness of 12 Å suggests a cluster growth but not a layer-by-layer growth. Figure 1c shows a LEED pattern at EP ) 142 eV after depositing 120 Å Al on the d-surface of Al-Co-Ni. In this pattern we observe a set of 30 broad diffraction spots lying on an inner and another set lying on an outer circle. The bright patches in each set are rotated by 6° relative to each other. The relative position and the mutual distances between parallel rows of scatterers reveal that the deposited Al forms the native fcc structure exposing the (111) face. Spot-profile analysis of the broad diffraction patches reveal that they consist of two diffraction spots separated azimuthally by ∼2.5° (cf. dashed line in Figure 1e) and that the domain size is about 32 Å (cf. dashed line in Figure 1f). Figures 1b and 1c reveal that Al forms two sets of five (or its multiples) fcc (111) domains. Nano Lett., Vol. 3, No. 12, 2003

Figure 1d presents an SEI pattern obtained from the same sample as in Figure 1c. Secondary electrons leaving the crystal into vacuum with polar angles of 20° and 35° are concentrated in two diffuse bright circles, while the rim of the screen represents 50°. In the outer regions of the two circles 30 discrete patches are discernible. A single-domain fcc (111) surface structure would lead to three spots at a polar angle of 35.3°, signaling the 〈110〉 directions.16 Since 30 bright spots exist on the outer ring, two sets of not five, but ten Al(111) domains must be present on the surface. Thus, the combination of LEED and SEI shows that Al crystallizes in two sets of ten domains, each about 32 Å in size, in the native fcc structure with the (111) face aligned parallel to the d-surface of the substrate. Within each set, the domains are rotated by 36° increments as a consequence of either 5-fold-symmetric terraces each inverted by 180° or a local 10-fold symmetry of the substrate. The two sets are displaced by ∼2.5° relative to each other to lead to the azimuthal elongation of diffraction spots. Different atomic species within Al-Co-Ni were not taken into consideration in the calculations, because deposition of 0.5 ML Al already shows a metallic behavior, signaling that Al adsorbate atoms interact more strongly with each other rather than with the substrate. The transition to metallic character results in a shift by almost 2 eV to higher kinetic energy of the L2,3VV Auger transitions in Al. By measuring this shift in the AES signal during Al deposition, we find that already 0.5 ML thick Al is metallic. Both these electronic and structural observations support our conclusion that the Al growth takes place as discrete island growth type. We therefore assume that the interaction between an adsorbate and an individual substrate atom is averaged out, and differences in the chemical identity of the substrate atoms do not affect the results appreciably. Hence, the quasicrystalline substrate is thought to act only as a structural template for the growth process of Al, as it is also observed for the growth of Ag on the pentagonal surface of Al-Pd-Mn and the d-surface of Al-Co-Ni.22 Similarly, on these surfaces a rough growth mode has also been observed by scanning tunneling microscopy for low Ag coverages. In our modeling we first determine the positions at which Al nucleation takes place on the substrate. This is done by scanning an Al atom in steps of 0.1 Å over the substrate, while calculating the energy at each point. Figure 2a shows the resulting constant total energy contours for an area of 20 × 20 Å2, which is within the limits of the “quasi-unit cell”, a building block put forward for the Al-Co-Ni structure.10 The concentric circles point to locations of the top layer surface substrate atoms for which r < σ results in a positive total energy. There are large plateaus of constant energy with a negative value. Some relative minima can be found in between; however, only at a few selected locations do equivalent absolute minima exist, e.g., at (x, y) ) (-7.8 Å, 1.8 Å) or (5.2 Å, 6.1 Å). We note that such locations occur at 2-fold symmetry sites, which are marked with an arrow in Figure 2a. Similar to a step-flow growth process, during which adsorbates are attracted to the step edges, Al atoms can find these minimum energy sites on the QC Nano Lett., Vol. 3, No. 12, 2003

Figure 2. (a) Constant-energy contour plots in the x-y plane. Arrows point to equivalent locations of absolute energy minima. (b) The energy as a function of the adsorbate rotation angle θ in a range of 20 to 140°. The adsorbate structure has a diameter of 24 Å, and the center of rotation is set at one of the absolute energy minima plotted in (a). θ ) 0° is an orientation of the Al(111) plane with respect to the substrate in agreement with experimental results.

surface. We assume that these locations act as a seed for the growth of Al clusters. In the next step, we have calculated the rotational alignment of the fcc (111) layer with respect to the QC surface. For a fixed Al-Co-Ni substrate, we have rotated the Al(111) layer through an angle θ in the range of 20 to 140° around one of the points of total energy minimum marked with an arrow in Figure 2a. For each adsorbate position, the energy is calculated. Figure 2b illustrates the results as a function of θ for a cluster of diameter 24 Å. We note several features in this figure. There exists a 60° periodicity in the alignment of the adsorbate layer due to the symmetry of its 〈111〉 surface. We also note that each 60° period is 5-fold modulated by local minima, signaling an n × 5 symmetry of the Al-Co-Ni substrate. Further, the energy curve shows a mirror symmetry around 78 and 1719

Figure 3. Results of the energy calculation in a θ range of -4° to 40°. Different plots belong to different adsorbate cluster sizes. An Al cluster with a diameter of 28 and 33 Å generates the experimentally observed separation of ∼2.5° (38.8°-36° ) 2.8°). For Al clusters larger than 33 Å in diameter, two different locations of energy minimum are favored.

138°, thus doubling the local rotational symmetry of the substrate surface from 5-fold to 10-fold. Hence, our calculations based on this atomic model12 reproduce both the 6-fold adsorbate and the local 10-fold substrate symmetry, i.e., the atomic model contains the necessary structural properties to reproduce the experimental observations in the calculations presented here. It is impossible that an extended crystal grows in registry with an aperiodic substrate. To find the critical adsorbate size at which a commensurate growth breaks down, we have repeated our calculations for increasing adsorbate diameter. Figure 3 presents a series of computational results in the θ range -4° to 40° as a function of the cluster diameter. The cluster represented in the top left curve is the same as that analyzed in Figure 2b. We observe that the position of the two absolute minima changes between 36° and 40.5°, with a cluster diameter up to 33 Å. The angle θ ) 36° implies a 10-fold symmetry in the distribution of Al domains due to the distribution of the columnar clusters.10 A deviation from this value is an indication of the Al growth into two different sets, which takes place already for a cluster diameter of 26 Å. For a diameter of 28 and 33 Å, a separation of ∼2.5° (38.8° - 36° ) 2.8°) is calculated that reproduces the experimentally observed value. We note that the domain size and the separation are interrelated. A further increase in the adsorbate cluster size leads to a crack, i.e., the two absolute total-energy minima are located at different θ positions, as can be observed in the lowest right-hand side curve in Figure 3. The agreement of computational results with those determined in the experiment for both of these values approves the reliability of our procedures. The remarkable success of this simple phenomenological L-J potential can be understood by a relatively weak interaction of aluminum adsorbate atoms with the substrate, enabled by the chemical inertness of the atomically flat d-surface. Complementary energy calculations have also been performed with different lateral positions for the Al(111) film, but these results exhibited only one absolute energy minimum within each 60° period (cf. Figure 2b). This suggests that the specific anchoring of the Al(111) is responsible for the 1720

occurrence of two sets of orientational alignment. The presented experimental results undoubtedly show the necessity of two equivalent energy locations. These findings further confirmed that the film nucleates at the 2-fold site. In summary, we have characterized the growth of aluminum films on a strongly lattice mismatched quasicrystalline d-Al-Co-Ni substrate. Aluminum adatoms on this surface start clustering into islands above 1/2-ML coverage as evidenced by their metallic character and LEED results. With increasing thickness, the Al film gradually develops the native fcc structure in a remarkable nanosized, 32-Å large domain structure, with the most densely packed, i.e., (111), surfaces of the domains oriented parallel to the substrate surface. Within the plane, these domains are oriented along two sets of 10 distinct orientations, defined by the decagonal substrate. The particular size selection of the nanodomains and their well-defined distinct orientations are all elegantly explained within the context of the rigid-lattice atomic model.6 Remarkably, the comparison between the experiment and theory, proved to be extremely sensitive to the structural details, furnishing a critical test for the quality of structural models. The success of this modeling also suggests that the underlying physics of a QC-crystal interface is similar to that of a crystal-on-crystal epitaxy. We have shown that energy considerations of this unusual crystal/QC interface give rise to some favored positions on the d-QC surface for the nucleation and growth. Since these minimum-energy sites are part of the d-surface structure, they are also in an aperiodic and self-similar order. Therefore, we suggest that these 32-Å diameter Al nanocrystals themselves are assembled on the surface in a quasiperiodic order. Quite obviously, our conjecture must await experimental verification using, e.g., secondary-electron microscopy. These results are presented in the hope of offering some encouragement for such experiments. Acknowledgment. We mention a special thank you to Dr. K. Saitoh for providing us with the atomic coordinates for his Al-Co-Ni model. The authors thank ETH Zu¨rich and Schweizerischer Nationalfonds for financial support. References (1) Shimoda, M.; Sato, T. J.; Tsai, A. P.; Guo, J. Q. Phys. ReV. B 2000, 62, 11288. Surf. Sci. 2002, 507-510, 276. (2) Bolliger, B.; Dmitrienko, V. E.; Erbudak, M.; Lu¨scher, R.; Nissen, H.-U.; Kortan, A. R. Phys. ReV. B 2001, 63, 052203. (3) Franke, K. J.; Sharma, H. R.; Theis, W.; Gille, P.; Ebert, P.; Rieder, K. H. Phys. ReV. Lett. 2002, 89, 156104. (4) Fourne´e, V.; Cai, T. C.; Ross, A. R.; Lograsso, T. A.; Evans, J. W.; Thiel, P. A. Phys. ReV. B 2003, 67, 033406. (5) van der Merve, J. H. Philos. Mag. A 1982, 45, 145. (6) Ramirez, R.; Rahman, A.; Schuller, I. K. Phys. ReV. B 1984, 30, 6208. (7) Widjaja, E. J.; Marks, L. D. Philos. Mag. Lett. 2003, 83, 47. (8) Steurer, W.; Haibach, T.; Zhang, B.; Kek, S.; Lu¨ck, R. Acta Crystallogr. Sect. B 1993, 49, 661. (9) Yamamoto, A.; Weber, S. Phys. ReV. Lett. 1997, 78, 4430. (10) Steinhardt, P. J.; Jeong, H.-C.; Saitoh, K.; Tanaka, M.; Abe, E.; Tsai, A. P. Nature 1998, 396, 55. (11) Yan, Y.; Pennycook, S. J.; Tsai, A. P. Phys. ReV. Lett. 1998, 81, 5145. (12) Saitoh, K.; Tsuda, K.; Tanaka, M. J. Phys. Soc. Jpn. 1998, 67, 2578. (13) Abe, E.; Saitoh, K.; Takakura, H.; Tsai, A. P.; Steinhardt, P. J.; Jeong, H.-C. Phys. ReV. Lett. 2000, 84, 4609. Nano Lett., Vol. 3, No. 12, 2003

(14) Kortan, A. R.; Thiel, F. A.; Chen, H. S.; Tsai, A. P.; Inoue, A.; Masumoto, T. Phys. ReV. B 1989, 40, 9397. (15) Bolliger, B.; Erbudak, M.; Hochstrasser, M.; Kortan, A. R.; Zurkirch, M. Phys. ReV. B 1996, 54, R15598. (16) Erbudak, M.; Hochstrasser, M.; Wetli, E.; Zurkirch, M. Surf. ReV. Lett. 1997, 4, 179. (17) Zangwill, A. Physics at Surfaces; Cambridge University Press: Cambridge, 1988. (18) Erbudak, M.; Hochstrasser, M.; Wetli, E. J. Electron Spectrosc. Relat. Phenom. 1995, 76, 529. (19) Donohue, J. The Structures of the Elements; John Wiley & Sons: New York, 1974.

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(20) Mihalkovic, M.; Al Lehyani, I.; Cockayne, E.; Henley, C. L.; Moghadam, N.; Moriarty, J. A.; Wang, Y.; Widom, M. Phys. ReV. B 2002, 65, 104205. (21) Cox, E. J.; Ledieu, L.; McGrath, R.; Diehl, R. D.; Jenks, C. J.; Fisher, I. Mater. Res. Soc. Symp. Proc. 2001, 643, K11.3.1. (22) Fourne´e, V.; Ross, A. R.; Lograsso, T. A.; Evans, J. W.; Thiel, P. A. Surf. Sci. 2003, 537, 5. (23) Gierer, M.; Mikkelsen, M.; Gra¨ber, M.; Gille, P.; Moritz, W. Surf. Sci. 2000, 463, L654. (24) Horn von Hoegen, M. Z. Kristallogr. 1999, 214, 591.

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