APPLIED PHYSICS LETTERS 89, 063118 共2006兲
Engineering patterns of Co nanoclusters on thin film Al2O3 / NiAl„100…
using scanning tunneling microscopy manipulation techniques
Shrikrishina D. Sartale
Department of Physics, National Central University, Jungli, 32054 Taiwan
Ku-Liang Lin
Department of Physics, National Sun Yat-Sen University, Kaohsiung, 804 Taiwan
Chou-I Chiang and Meng-Fan Luoa兲
Department of Physics, National Central University, Jungli, 32054 Taiwan
Chien-Cheng Kuob兲
Department of Physics, National Sun Yat-Sen University, Kaohsiung, 804 Taiwan
共Received 8 February 2006; accepted 23 May 2006; published online 10 August 2006兲
We present precise engineering of patterns of Co nanoclusters grown on ordered Al2O3 / NiAl共100兲
surface using the scanning tunneling microscopy 共STM兲 manipulation technique. The clusters are
attracted to the STM tip by lowering the bias below a threshold value and translated and relocated
to another position by reversing the polarity. This facile manipulation technique in combination with
the self-organized patterning on this system reported earlier might play a decisive role in
nanotechnology for various applications where patterned nanoclusters are desired. © 2006
American Institute of Physics. 关DOI: 10.1063/1.2221394兴
One of the ultimate goals of nanotechnology is directed
at controlled positioning of individual atoms and molecules
for building or modifying desired nanostructures.1 Since the
pioneer work of controlled positioning of xenon atoms on
Ni共110兲 surface,2 much progress has been made in developing techniques for manipulating a variety of atoms and molecules on crystal surfaces.3–8 Among all the approaches,
scanning tunneling microscopy 共STM兲 is so far the simplest
and the most general technique. This advantage arises from
its capability to perform atomically resolved imaging and
manipulation using the same equipment. Thus it is possible
to image the surface, zoom in on a particular feature of interest, and induce atomic or molecular scale modifications,
and then image and study the properties of the structures. By
using the STM manipulation technique most research efforts
until now have been made on engineering of single atom or
molecule3–16 and recently a few large molecules7,8,17,18 supported by single crystal metals or semiconductor surfaces.
However, for various technological applications, there is an
immediate need to extend this enticing technique to engineering of the matter at macromolecular or cluster scale on
synthetic or processed surfaces, which may eventually allow
fabrication of a more diverse range of nanostructures to be
used in electronic and magnetic nanodevices and nanomechanical applications.
Recently we have reported patterning of Co nanoclusters formed from vapor deposition over ordered
Al2O3 / NiAl共100兲 surface.19–21 The uniform Co nanoclusters
are formed only on crystalline Al2O3 films and highly
aligned by protrusion structures of the crystalline Al2O3.
Through simple thermal treatments we control the geometry
of the crystalline Al2O3 film as well as the protrusion networks on it and thereby manipulate the patterns of the Co
a兲
Electronic mail: mfl28@phy.ncu.edy.tw
Electronic mail: cckuo@mail.nsysu.edu.tw
b兲
nanoclusters. However, precise control of the positions of the
Co nanoclusters is not possible.
In this letter, we show that engineering of nanocluster
patterns on ordered oxide surfaces is possible by using the
STM manipulation technique. In particular, we demonstrate
precise engineering of Co nanocluster patterns on Al2O3 thin
films grown on NiAl共100兲 surface. We placed the tip over a
specific cluster and reduced the applied bias below a threshold value to attract the cluster and subsequently remove it
from the patterns. Systematically through this approach, we
tailored the patterns of the supported Co clusters. The removed clusters can be relocated to other positions by reversing the polarity. This technique in combination with the selforganized patterning reported previously20,21 enables one to
fabricate desired cluster patterns and consequently investigate their physical properties, for instance, the magnetic
properties of patterned Co clusters, which are a critical issue
in the development of nanostorage systems.22
The experiments were performed in an ultrahigh vacuum
共UHV兲 environment with base pressure lower than
10−9 mbar. The detailed experimental procedure for the
growth of Al2O3 films on NiAl共100兲 and formation of Co
clusters over it has been reported elsewhere.19–21 In short
the NiAl共100兲 surface, cleaned by repeated cycles of
Ar ion bombardment 共2 keV, 4 ␮A sample current, 300 K,
45 min兲 and annealing at 1000 K for 45 min, was exposed to
1000 L 共1 L = 10−6 Torr s兲 O2 at 1000 K and annealed at
1000– 1100 K for 1 h. The sample was quenched to 300 K
to deposit Co atoms by evaporating a Co rod using an
e-beam evaporator 共Oxford Scientific兲. The Co atoms nucleate on crystalline Al2O3 surface with preferable uniform
cluster sizes19–21 共a diameter of 2.6 nm and a height of
0.7 nm兲 and the clusters are well aligned along the protrusion
stripes. In contrast, in the amorphous region, sizable Co clusters appear only at high Co coverages.20,21 The STM images
reported here were taken at 90 K with an RHK UHV STM300 unit in constant current mode using electrochemically
0003-6951/2006/89共6兲/063118/3/$23.00
89, 063118-1
© 2006 American Institute of Physics
Downloaded 08 Sep 2006 to 140.117.120.241. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
063118-2
Sartale et al.
Appl. Phys. Lett. 89, 063118 共2006兲
FIG. 1. 共Color online兲 STM images illustrating removal of the Co clusters
by reducing the bias during scanning. 共a兲 STM image of Co clusters grown
on ordered Al2O3 / NiAl共100兲 surface 共2.4 V , 0.8 nA兲. 共b兲 Zoom in of the
area shown by the square in 共a兲. The bias was reduced to different values as
indicated in the figure 共I = 0.8 nA兲 during scanning. 共c兲 STM image of the
same surface region as in 共a兲 after zoom out 共2.4 V , 0.8 nA兲. The inset
cartoon illustrates the procedure.
etched tungsten tips. The cluster manipulation was conducted
at the same temperature. The applied bias refers to the
sample voltage with respect to the tip.
Figures 1共a兲–1共c兲 present a sequence of STM images
illustrating removal of the Co clusters by reducing the bias
during scanning. Figure 1共a兲 shows the STM image obtained
at 2.4 V bias and 0.8 nA tunneling current, where aligned Co
nanoclusters are formed on crystalline Al2O3.20 We zoomed
in an area shown by the square and reduced the bias to different values 关Fig. 1共b兲兴 during scanning 共from left to right兲.
As the tunneling current was kept the same, this process
brought the tip close to the clusters, as illustrated schematically in the cartoon inset. We found a threshold bias value of
0.8 V to induce motion of the Co clusters. We can see in
parts of the image where the bias was above the threshold
value of 0.8 V that the Co clusters were in fixed and welldefined locations. On the contrary, the regions where the bias
was below the threshold value are seen cleared. Figure 1共c兲
is the same surface region as shown in Fig. 1共a兲, scanned just
after zoom out with 2.4 V bias. The part of the surface where
the bias was below the threshold value is seen cleared without the Co clusters. When a higher tunneling current was set,
we found a higher threshold bias. For instance, when 1.6 nA
is used, the threshold bias is 1.6 V.
Based on this result we can engineer precisely desired
Co cluster patterns. Figures 2共a兲–2共g兲 demonstrate, the potential of this approach. Figures 2共a兲 and 2共b兲 show that specific clusters were pulled out from the edge of crystalline
region to create voids or cavities in the pattern. The arrows in
Fig. 2共a兲 stand for the process that the tip was first moved
over the specific clusters 共the end sides of the arrows兲 and
brought close to them, by reducing the bias, to induce the
motion of the Co clusters, and the tip was translated to the
other location to remove them from the pattern. Figures
2共c兲–2共e兲 show that we can trim a long cluster chain and
fabricate cluster chains with various lengths. Figure 2共d兲 is
the zoom in image of Fig. 2共c兲 and Fig. 2共e兲 is the image
FIG. 2. 共Color online兲 The cluster patterns tailored by the STM manipulation technique. Creation of void or cavity in the pattern by removing the
specific Co clusters. 共a兲 before and 共b兲 after removal of the clusters. Fabrication of cluster chains by trimming a long Co clusters chain on a crystalline
Al2O3 strip. 关共c兲 and 共d兲兴 before and 共e兲 after trimming the chain. 共f兲 STM
image for the rectangular cluster chains cut by removing sections of the
chains. 共g兲 STM image after removing rows of the Co clusters from the long
cluster chains. 共0.8 V , 0.8 nA兲 and 共1.6 V , 1.6 nA兲兲 were used for the removal of the Co clusters and 共2.4 V, 0.8 nA兲 was used for the imaging.
after the trimming processes denoted by the arrows in Fig.
2共d兲. This approach is effective not only for small-scale engineering but also for large-scale pattern modification. Figure
2共f兲 shows two sections of the rectangular cluster chains
were removed. Figure 2共g兲 displays that long cluster chains
were trimmed into short ones by removing rows of Co clusters. The uncovered protrusion stripes are clearly seen and
aligned with the remaining Co cluster chains, confirming directly that the protrusion structures are preferable nucleation
centers.20 These results not only promise precise engineering
of the cluster patterns but also imply an opportunity to study
pattern-dependent physical properties of the nanostructures.
For example, one may study the length-dependent electronic
structures of the clusters-composed nanowires or the magnetism variations induced by the interactions between the magnetic nanoclusters.22–25
We believe that the tip attracts the clusters when it is
brought closer by lowering the bias below the threshold
value, and carries them with it. It might resemble the removal of Pd atoms on Al2O3 / NiAl共100兲 surface during scanning with lower bias.26 Although we observed that the tip
could also pull or push the Co clusters on the oxide surface,
in most cases the clusters are attracted and adsorbed by the
tip. These adsorbed clusters can be relocated on the surface
Downloaded 08 Sep 2006 to 140.117.120.241. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
063118-3
Appl. Phys. Lett. 89, 063118 共2006兲
Sartale et al.
In conclusion, we have shown that it is possible to engineer precisely the patterns of Co nanoclusters grown on ordered Al2O3 / NiAl共100兲 surface by using the STM manipulation technique. The STM tip can attract and carry the
clusters in order to tailor the patterns. The attracted clusters
on the tip can also be relocated on the surface. This manipulation technique in combination with the self-organized patterning on this system may open a door to fundamental researches and applications in nanodevices where patterned
nanoclusters are desired.
The authors thank K. R. Hu for technical support. This
work was supported by the National Science Council of Taiwan under Grant No. NSC 94-2112-M-008-032.
K. Tanaka, Thin Solid Films 341, 120 共1999兲.
D. M. Eigler and E. K. Schweizer, Nature 共London兲 344, 524 共1990兲.
P. Avouris, Acc. Chem. Res. 28, 95 共1995兲.
4
J. K. Gimzewski and C. Joachim, Science 283, 1683 共1999兲.
5
S.-W. Hla and K.-H. Rieder, Superlattices Microstruct. 31, 63 共2002兲.
6
A. A. Tseng, A. Notargiacomo, and T. P. Chen, J. Vac. Sci. Technol. B 23,
877 共2005兲.
7
F. Moresco, Phys. Rep. 399, 175 共2004兲.
8
S.-W. Hla, J. Vac. Sci. Technol. B 23, 1351 共2005兲.
9
G. Meyer, S. Zöphel, and K.-H. Rieder, Phys. Rev. Lett. 77, 2113 共1996兲.
10
L. Bartels, G. Meyer, and K.-H. Rieder, Phys. Rev. Lett. 79, 697 共1997兲.
11
H. J. Lee and W. Ho, Science 286, 1719 共1999兲.
12
T. W. Fishlock, A. Oral, R. G. Egdell, and J. B. Pethica, Nature 共London兲
404, 743 共2000兲.
13
N. Nilius, T. M. Wallis, and W. Ho, Science 297, 1853 共2002兲.
14
J. M. LeDue, H. J. Kreuzer, S. A. Burke, M. H. Jericho, and J. C. Neima,
Surf. Sci. 538, L465 共2003兲.
15
J. A. Stroscio and R. J. Celotta, Science 306, 242 共2004兲.
16
P. A. Sloan and R. E. Palmer, Nature 共London兲 434, 367 共2005兲.
17
P. H. Beton, A. W. Dunn, and P. Moriarty, Appl. Phys. Lett. 67, 1075
共1995兲.
18
F. Moresco, C. Joachim, and K.-H. Rieder, Surf. Interface Anal. 36, 109
共2004兲.
19
W. C. Lin, C. C. Kuo, M. F. Luo, K. J. Song, and M. S. Lin, Appl. Phys.
Lett. 86, 043105 共2005兲.
20
M. F. Luo, C. I. Chiang, H. W. Shiu, S. D. Sartale, and C. C. Kuo,
Nanotechnology 17, 360 共2006兲.
21
M. F. Luo, C. I. Chiang, H. W. Shiu, S. D. Sartale, T. Y. Wang, P. L. Chen,
and C. C. Kuo, J. Chem. Phys. 124, 164709 共2006兲.
22
J. Bansmanna, S. H. Baker, C. Binns, J. A. Blackman, J.-P. Bucher, J.
Dorantes-Dávila, V. Dupuis, L. Favre, D. Kechrakos, A. Kleibert, K.-H.
Meiwes-Broer, G. M. Pastor, A. Perez, O. Toulemonde, K. N. Trohidou, J.
Tuaillon, and Y. Xie, Surf. Sci. Rep. 56, 189 共2005兲.
23
S. S. P. Parkin, N. More, and K. P. Roche, Phys. Rev. Lett. 64, 2304
共1990兲.
24
O. Pietzsch, A. Kubetzka, and R. Wiesendanger, Phys. Rev. Lett. 92,
057202 共2004兲.
25
J. P. Pierce, M. A. Torija, Z. Gai, Shi Junren, T. C. Schulthess, G. A.
Farnan, J. F. Wendelken, E. W. Plummer, and J. Shen, Phys. Rev. Lett. 92,
237201 共2004兲.
26
N. Nilius, T. M. Wallis, and W. Ho, Phys. Rev. Lett. 90, 046808 共2003兲.
27
L. Olesen, M. Brandbyge, M. R. Sørensen, K. W. Jacobsen, E. Lægsgaard,
I. Stensgaard, and F. Besenbacher, Phys. Rev. Lett. 76, 1485 共1996兲.
28
U. Dürig, O. Zürig and D. W. Pohl, Phys. Rev. Lett. 65, 349 共1990兲.
1
2
3
FIG. 3. 共Color online兲 The clusters unloaded from the tip to the surface.
Relocation of the specific cluster from the crystalline Al2O3 to the amorphous region. 共a兲 before picking up the cluster and 共b兲 after relocating it on
the amorphous region. The cluster is indicated by the circle. The inset manifests the creation of void after removing the specific cluster. 共c兲 STM image
for a number of Co clusters released from the tip. All the images were
obtained at 2.4 V and 0.8 nA.
by reversing the bias polarity when the tip is brought closer
to the surface. Figures 3共a兲 and 3共b兲 show that we picked up
a Co cluster from the crystalline Al2O3 surface and relocated
it to the amorphous region. Adsorbing too many clusters
sometimes causes multitip effect or deteriorates the imaging.
We can refresh the tip simply by removing the adsorbed
clusters. Figure 3共c兲 shows a number of the Co clusters unloaded from the tip. It is noted that the sizes of the Co
clusters unloaded from the tip are similar to those on the
surface. In the context of the STM atom/molecule manipulation, the process reported here is more like the vertical manipulation by making atom/molecule mechanical contact.6 As
the tip-cluster distance is reduced to below 2.5 Å during patterning 共estimated by a simple tunneling model27,28兲, the tipcluster interaction involves a combination of the electric field
and chemical bonding, which must be stronger than the adhesion force between the Co cluster and the oxide surface,
about 35 eV/ Å for a single cluster with the mean size,20,21
derived from density-functional-theory calculations. The
electric field might provide a directional driving force and
unload the cluster after reversing the bias polarity.6 In line
with the argument, the STM tip may not move the clusters
while competing with much stronger cluster-oxide interactions, such as Pt clusters on ordered Al2O3 / NiAl共100兲 observed previously.20
Downloaded 08 Sep 2006 to 140.117.120.241. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp