Self-aligned Co nanoparticle chains supported by single-crystalline Al
2O
3/ NiAl „ 100 … template
Wen-Chin Lin
Department of Physics, National Taiwan University, 106 Taipei, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
Chien-Cheng Kuo
Department of Physics, National Taiwan University, 106 Taipei, Taiwan and Department of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan
Meng-Fan Luo
Department of Physics and Nano-Cataylst Research Center, National Central Universtiy, Jhongli City, Taoyuan, Taiwan
Ker-Jar Song
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan Minn-Tsong Lina兲
Department of Physics, National Taiwan University, 106 Taipei, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
共Received 13 September 2004; accepted 30 November 2004; published online 19 January 2005兲 We present Co nanoparticle chains grown by vapor deposition over a single-crystalline Al2O3layers on NiAl共100兲 with such features as self-limiting size distribution with the average size of ⬃2.7 nm, well-ordered alignment, and high thermal stability. We attribute these features to peculiar one-dimensional long stripes with ⬃4 nm interdistance on the surface of the ultrathin Al2O3 template. This nanostructure may open the door to numerous applications, such as catalysis and nanostorage, where large area well-ordered nanodots are desired. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1855410兴
Nanoparticles or nanodots attract much interest due to their potential applications in electronic and magnetic nan- odevices as well as for catalysts in growing further various nanomaterials such as nanotubes or nanowires. However, a realistic application often requires an extremely high achievement in size controlling, size uniformity, and even perfect ordering. In recent decades, a self-organized ap- proach has been realized to be a promising way for growing nanodots with uniform size. Nevertheless, in the area of semiconductor quantum dots, the Stranski–Krastanow growth mode is usually used to describe the presence of the wetting layer at the initial stage as well as the following clustering.1,2 This reflects the complication of the dot/
substrate surface interfacial interaction, increasing the diffi- culty for controlling nanodot characteristics, such as size uni- formity and undesired interdot contact through wetting layers. Very recently, nanodots or nanoclusters without wet- ting layers have been prepared for the metallic nanoparticles by deposition on insulators or semiconductors.3–16 For ex- ample, Gai et al. reported a self-assembly of nanometer-scale Fe dots with narrow size distributions on an insulating sub- strate 共NaCl兲3 due to strain-mediated dot–dot interaction.
The particle size in this system, however, is proportional to the deposition amount. On the other hand, another study on Co/ Si3N4/ Si共111兲4 shows the self-limiting size distribution which is attributed to a quantum size effect, manifested by local energy minima in the electronic shell structure of Co dot.
Besides the size uniformity, another key issue is to pre- pare orderly arranged nanoparticles. Ordered alignment de- termines the spatial symmetry and thus may induce aniso- tropy in physical and chemical properties. With respect to technological application, well-ordered nanoparticles are also extremely important. For instance, magnetic nanopar- ticle arrays can be a good candidate for data storage medium.
However, the patterning of the nanoparticles is still a chal- lenging work, due to the high cost and resolution limit of conventional lithography as well as the rarity of self- assembled systems. For example, although the 7⫻7 struc- ture of Si共111兲 has served as a good template for arranging magic clusters of Ga,10In,11and Al,12due to the formation of silicide, it does not work with magnetic materials like Fe, Co, and Ni.
In this letter, we present that Co nanoparticles grown on a single crystalline Al2O3layers on NiAl共100兲 can satisfy the size uniformity and ordered alignment. Moreover, the Co nanoparticles are thermally stable up to an elevated tempera- ture of 700 K. Such high thermal stability renders this sys- tem a good candidate for catalysis application, which many quantum-sized systems induced by electronic structures fail to serve.4
The experiment was performed in an ultrahigh vacuum chamber with base pressure⬍2⫻10−10mbar. After cycles of 1.5 keV Ar+sputtering and subsequent annealing at 1000 K, the substrate NiAl共100兲 was exposed to ⬃1000 L O2 and then annealed at the same temperature for 1 h. After the oxi- dation and annealing procedures, the sample was quenched to room temperature for the deposition of Co. The crystalline structure and chemical composition of the sample was char-
a兲Author to whom correspondence should be addressed; electronic-mail:
APPLIED PHYSICS LETTERS 86, 043105共2005兲
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acterized and checked by low energy electron diffraction 共LEED兲 and Auger electron spectroscopy, respectively. The morphology of the sample was monitored by scanning tun- neling microscopy 共STM兲 using bias voltage of 1.6–2.0 V and tunneling current of 0.8– 1 nA.
Figures 1共a兲–1共c兲 present the STM images for 0.13, 0.73, and 2.2 ML Co/ Al2O3/ NiAl共100兲 in different scales, respec- tively.关1 ML is the surface atom density of hcp Co共0001兲 or fcc Co共111兲 and it means 1.83⫻1015 atoms/ cm2.兴 Figure 1共a兲 shows that Co deposition produces uniform-sized nano- particles which are highly aligned like chains extending more than 200 nm. With more deposition, as shown in Figs.
1共b兲 and 1共c兲, both alignment and uniformity are sustained but particle density is highly enhanced. These STM results reveal amazing ordering and size uniformity of the grown particles. Statistical analysis demonstrates the uniformity even further. Figures 2共a兲 and 2共b兲 show the height and di- ameter distribution for 0.43 ML Co/ Al2O3/ NiAl共100兲, re- spectively. The average height is 0.9± 0.2 nm, and the aver- age diameter is 2.7± 0.3 nm. In Fig. 2共b兲, there seems to be an a-upper boundary in the diameter distribution. Smaller particles are observed共the tail of distribution at 1.5–2.7 nm兲 and nearly no larger particles can be seen共the clear cut after 3.2 nm兲. Similar distributions are also found for different amounts of Co deposition, as depicted by Figs. 2共c兲 and 2共d兲.
Both the average height and diameter of the particles are shown to be almost the same, indicating a self-limiting size distribution. More Co deposition increases the particle den-
sity and the average height to⬃0.9 nm. However, more Co deposition does not create larger particles⬎3.5 nm, but only enlarges the existing smaller ones or produce more particles, leading to a good size uniformity as shown in Fig. 1共c兲.
To investigate the magic alignment of Co nanoparticles, we look into detailed structures of the Al2O3 thin film and the early stages of Co nucleation. Figure 3共a兲 shows the LEED image of Al2O3/ NiAl共100兲. The 2⫻1 superstructure indicates the typical surface structure of single crystal Al2O3, and the streaks of the 1⫻1 spots are corresponding to the formation of domains of Al2O3 along the 共010兲 and 共001兲 directions.17 The splitting of the 2⫻1 spots, depicted in the inset of Fig. 3共a兲, is about 1/13 of the interspace between 1⫻1 spots. Since the lattices constant of NiAl共100兲 is 2.89 Å,17 this splitting gives us the average interdistance of
FIG. 1.共Color兲 STM images of 共a兲 0.13 ML, 共b兲 0.73 ML, and 共c兲 2.2 ML Co deposited on Al2O3/ NiAl共100兲. These pictures reveal that cobalt prefers to nucleate nano-sized particles which are aligned by the Al2O3/ NiAl共100兲 template with high density and uniform size. Note that the Co particles are well-ordered as one-dimensional chain for all coverages.
FIG. 2. 共a兲, 共b兲 Height and diameter distribution of 0.43 ML Co/ Al2O3/ NiAl共100兲. The average height is 0.9±0.2 nm, and the average diameter is 2.7± 0.3 nm, which are obtained from Gaussion fitting of the distribution.共c兲, 共d兲 The average heights and diameters of Co nanoparticles with different coverage.
FIG. 3.共Color兲 共a兲 LEED pattern of Al2O3/ NiAl共100兲. The inset depicts the line profile across the 2⫻1 superstructure. 共b兲 STM image of 0.05 ML Co/ Al2O3/ NiAl共100兲.
043105-2 Linet al. Appl. Phys. Lett. 86, 043105共2005兲
Downloaded 21 Jan 2005 to 140.112.101.92. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
⬃3.8 nm for one-dimensional stripes 共the domain bound- aries兲, as indicated by the STM image in Fig. 3共b兲 for 0.05 ML Co/ Al2O3/ NiAl共100兲. The background of Fig. 3共b兲 was composed of long Al2O3 domain. Such domains are sepa- rated by the one-dimensional stripes 共island boundaries or fine structure兲 and have average width ⬃4.0 nm, consistent with the result extracted from the LEED pattern. The gray and dark backgrounds correspond to two different terraces, respectively. The step height in Fig. 3共b兲 is ⬃3.0 nm.17 These observed features of Al2O3/ NiAl共100兲 are consistent with the results of previous studies.17,18
The bright dots in Fig. 3共b兲 are Co nanoparticles. We note that the nanoparticles always straddle the one- dimensional strips between long Al2O3domains. The results imply that the stripes are more favorable nucleation sites and consequently nanoparticles grow along the stripes and form a nearly perfect one-dimensional chains, as shown in Figs.
1共a兲–1共c兲. Moreover, due to the short distance in between the boundaries, nucleation occurs almost exclusively along these narrow lines and thus the spectacular alignment of the nano- particles formed. That is, the alignment is predetermined by the structure of the oxide, and does not have much to do with the mutual interactions among the nanoparticles. This also provides a natural explanation why several different metals 共Fe, Cu, Ag兲 we tried all show the same kind of spectacular alignment. We are inspired that this template has great ad- vantages for self-organized one-dimensional nanopatterning, particularly for nanoparticles.
It is interesting to compare our results to those from Al2O3/ NiAl共110兲,19,20where one did not observe the stripes.
In the study of Pd and Rh on Al2O3/ NiAl共110兲 by Bäumer et al.,21although steps and film domain boundaries are favored nucleation centers, it is difficult to create high density of one-dimensionally aligned nanoparticles because the domain boundaries is irregular and the interdistance is large. As Co/ Al2O3/ NiAl共110兲21 is concerned, the nucleation is pri- marily at point defects randomly distributed on Al2O3/ NiAl共110兲. As a result, its ordering is even worse. In contrast, deposited Co atoms on Al2O3/ NiAl共100兲 nucleate along the one-dimensional stripes as well as steps, as shown in Fig. 2共b兲, while we note the steps have no priority. Addi- tionally, the interdistance of the one-dimensional stripes is only around 4 nm. Therefore, the grown Co nanoparticles are so well aligned by the stripes even at very high density. Very similar features are also found for other materials, such as Fe, Cu, and Ag on Al2O3/ NiAl共100兲, indicating its superior- ity as a template for growing one-dimensionally well-ordered nanoparticles.
We also examined the thermal stability of Co nanopar- ticles supported by Al2O3/ NiAl共100兲. The sample was an- nealed to a selection of temperatures and as shown in Fig. 4, the main feature of Co/ Al2O3/ NiAl共100兲 can be sustained at much higher annealing temperature up to 700 K, as com- pared to that of Co/ Si3N4/ Si共111兲 共400 K兲.4 Annealing to 800 K results in sintering of particles and the appearance of larger particles due to thermal diffusion. Similar behavior of the thermal stability was also found in metal particles on Al2O3/ NiAl共110兲.22Thermal stability of the system typically depends on the deposit-template interfacial interaction and nucleation energy of the particles. To achieve detailed under- standing of the thermal stability requires further investiga- tion.
In summary, high temperature oxidation of NiAl共100兲 provided a single crystalline Al2O3 layer as a template for self-organized nanopatterning. Co nanoparticles show a self- limiting size distribution, and are directed by domain bound- aries or linear stripes on the single crystalline Al2O3surface, forming regular one-dimensional particle chains. The well- ordered Co nanoparticles were stable up to an elevated tem- perature of ⬃700 K, providing a promising application in catalysis for various purposes, such as growing patterned na- nomaterials.
The authors acknowledge P. S. Lin for the technical sup- port in this experiment. This work was supported by the National Science Council of Taiwan under Grant Nos. NSC 92-2112-M-002-028, NSC 92-2112-M-002-019, NSC 92- 2112-M-001-039, and NSC 92-2120-M-007-002, and by the Ministry of Economic Affairs of Taiwan under Grant No.
92-EC-17-A-09-S1-022.
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FIG. 4.共Color兲 共a兲–共c兲 STM images of 2.2 ML Co/Al2O3/ NiAl共100兲 after annealing at 700, 750, and 800 K, respectively, for 10 min.
043105-3 Linet al. Appl. Phys. Lett. 86, 043105共2005兲
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