[tt] Complex Patterning by Vertical Interchange Atom Manipulation Using AFM

[Eugen Leitl]

http://www.sciencemag.org/cgi/content/full/322/5900/413

Science 17 October 2008:

Vol. 322. no. 5900, pp. 413 - 417

DOI: 10.1126/science.1160601
	
Complex Patterning by Vertical Interchange Atom Manipulation Using Atomic
Force Microscopy

Yoshiaki Sugimoto,1 Pablo Pou,2 Oscar Custance,3* Pavel Jelinek,4 Masayuki
Abe,1,5 Ruben Perez,2 Seizo Morita1

The ability to incorporate individual atoms in a surface following
predetermined arrangements may bring future atom-based technological
enterprises closer to reality. Here, we report the assembling of complex
atomic patterns at room temperature by the vertical interchange of atoms
between the tip apex of an atomic force microscope and a semiconductor
surface. At variance with previous methods, these manipulations were produced
by exploring the repulsive part of the short-range chemical interaction
between the closest tip-surface atoms. By using first-principles
calculations, we clarified the basic mechanisms behind the vertical
interchange of atoms, characterizing the key atomistic processes involved and
estimating the magnitude of the energy barriers between the relevant atomic
configurations that leads to these manipulations.

1 Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, 565-0871
Suita, Osaka, Japan.

2 Departamento de Física Teórica de la Materia Condensada, Universidad
Autónoma de Madrid, 28049 Madrid, Spain.

3 National Institute for Materials Science, 1-2-1 Sengen, 305-0047 Tsukuba,
Ibaraki, Japan.

4 Institute of Physics, Academy of Sciences of the Czech Republic,
Cukrovarnicka 10, 1862 53 Prague, Czech Republic.

5 Precursory Research for Embryonic Science and Technology, Japan Science and
Technology Agency (JST), 332-0012 Saitama, Japan.

* To whom correspondence should be addressed. E-mail:
custance.oscar{at}nims.go.jp

Scanning tunneling microscopy (STM) has proven to be the method of excellence
for creating nanostructures on surfaces, manipulating atoms and molecules one
at a time (1–3). A new panorama has recently been opened by the capability of
atomic force microscopy (AFM) to create similar nanostructures atom by atom
(4) and to quantify the forces involved in these lateral manipulations (5,
6).

When exploring a surface with these scanning probe methods, the apex of the
probe can be contaminated with atomic species present at the surface (7) by
picking up atoms in accidental or intended mechanical contacts with the
surface. Advantage could be taken of this situation, and an atomic version of
the dip-pen nanolithography (8) may be implemented: Atoms wetting the tip
apex could be individually deposited to write patterns at heterogeneous
surfaces. We provide evidence that such an atomic pen can be implemented by
using AFM.

We performed the AFM experiments (9) in dynamic mode under the frequency
modulation detection scheme (10), keeping the cantilever oscillation
amplitude constant. Commercial silicon cantilevers, which have very sharp
tips at their free ends, were used to image the Sn/Si(111) – Formula surface
(11) by detecting the short-range chemical interaction force between the
closest tip and surface atoms (9).

The inset of Fig. 1A shows topographic images of a single atomic layer of tin
(Sn) atoms, which appear as bright protrusions, grown over a silicon (111)
single-crystal substrate. Among the atomic defects this surface exhibits
(11), the most representative ones are substitutional silicon (Si) atoms (12)
at the perfect Sn atomic layer, and these appear as protrusions with
diminished contrast. We have observed that these Si defects can be vertically
manipulated during force spectroscopy (13, 14) experiments. After imaging the
surface and positioning the AFM tip with a lateral precision better than ±
0.1 Å (15) over the topmost part of the marked Si atom, we moved the sample
toward the oscillating AFM probe. At a given tip-surface distance, an
instability in the frequency shift occurs, as highlighted by the arrow in the
graph. In an image taken after the sample was retracted, the Si atom was no
longer visible, and a Sn atom was found to occupy the corresponding lattice
position instead (Fig. 1A, bottom right inset). One hypothesis to explain
this event is that the Si atom at the surface has been replaced by a Sn atom
originally located at the tip apex, as sketched out by the illustration in
Fig. 1A. The same procedure can be consecutively applied to the freshly
deposited Sn atom (marked with a circle in Fig. 1B, left inset), resulting in
the replacement of this surface atom by a Si atom coming from the tip and in
a partial loss of atomic contrast (Fig. 1B, bottom right inset). Because all
the images shown in Fig. 1 were acquired under the same experimental
parameters, this contrast change should correspond to a modification of the
tip apex (fig. S4) (9). This tip modification is, however, not irreversible.
By scanning aside a region that neighbors the imaged area at vertical
distances slightly closer to the surface than that used for imaging, we
recovered good atomic contrast again for most of the test performed (fig. S1)
and were able to repeat these manipulation processes multiple times (fig.
S2). Dissipation signals (16) of up to 1.2 eV per cycle accompanied these
atomic interchanges between the tip and surface (fig. S1). Although the
vertical manipulations described here were performed at room temperature,
they have also been accomplished at low temperature (80 K).


Figure 1 	Fig. 1. Alternate vertical interchange atom manipulations.
(A) Frequency shift ({Delta}f) signal upon approach (black) and retraction
(red) of the tip over the Si atom marked with a white circle in the left
inset image. In a consecutive topographic image to the curve acquisition
(bottom right inset), a Sn atom appears at the same surface location instead.
The Si atom was replaced by a Sn atom coming from the tip (Sn deposition).
(B) Frequency shift signal upon approach (black) and retraction (red) of the
tip above the Sn atom deposited in (A), pointed out by a black circle (left
inset). After the curve acquisition, the replacement of this Sn atom by a Si
atom coming from the tip (Si deposition) and a partial loss of atomic
contrast are obtained (bottom right inset). For comparison with other curves,
the normalized frequency shift ({gamma}) (19) is displayed in the vertical
axis on the right. The black arrows in the plots indicate instabilities
representative of the corresponding concerted vertical interchange of atoms
between the tip and surface. The origin of the horizontal axes denotes the
point of maximum proximity between the tip and sample; this criterion was
adopted for all the experimental curves shown in this work. The illustrations
are representations of the corresponding vertical atomic interchange, with
yellow and gray spheres symbolizing Sn and Si atoms, respectively; they do
not bear any realistic information about the tip-apex structure or
composition. For the acquisition parameters and analysis of the short-range
forces associated with these manipulations, see fig. S1. [View Larger Version
of this Image (30K GIF file)]
 

This vertical interchange of strongly bound atoms between the tip and the
surface differs from methods previously reported using STM, in which an atom
weakly bonded on a metallic surface can be reversibly transferred between the
tip and the surface by applying an appropriate bias voltage (17). It also
diverges from other methods of controlled atom manipulations recently
achieved with AFM (4–6) that make use of the attractive part of the
tip-surface interaction to laterally manipulate atoms without any active
participation of the tip, which is only used to tune the interaction of the
manipulated atom with the surface. In contrast, the mechanism of the
manipulations reported here is based on a process in which the vertical
interchange of atoms is controlled by the mechanical properties of a hybrid
tip-surface structure formed in the repulsive regime of the tip-surface
interaction force.

Although imaging and manipulation in the attractive regime involve mainly the
bonding interaction between the closest tip-surface atoms, in the repulsive
regime a larger contact involving several atoms is expected, leading to a
very complex energy landscape. The structure of the contact at the closest
approach determines the most likely outcome among the different competing
processes: atom interchange, atom transfer to the tip, or deposition of tip
atoms when the tip is pulled out from the surface. However, the
reproducibility of the vertical atomic interchange and the resemblance of
recorded frequency shift and force curves in experiments performed with
different tips (figs. S1 and S2) point toward a common basic microscopic
mechanism.

To characterize the atomistic processes, we performed simulations of the
tip-surface approach and retraction events. These simulations (9) are based
on density functional theory first-principles calculations implemented with a
local orbital basis using the FIREBALL code (18). To model the experimental
tip apex, we considered a rigid tip (Fig. 2B) on which only the two atoms in
the dimer defining the apex were allowed to relax upon interaction with the
surface. We imposed this constraint on the tip model in order to simplify the
complex configuration space associated with the tip mechanical response.


Figure 2 	Fig. 2. First-principles simulations of vertical interchange
atom manipulation proceses. (A) Evolution of the total energy upon two
consecutive approach-and-retraction cycles using a model rigid tip (only the
two atoms in the dimer defining the apex are allowed to relax) over the same
location of a Sn/Si(111) – Formula surface, which results in the alternate
deposition of a Sn atom [first cycle (continuous lines)] and a Si atom
[second cycle (dashed lines)], respectively. Upon increasing the load of the
tip apex over the surface and consecutive retraction, atoms at both the tip
and surface undergo a series of structural relaxations that manifest in jumps
between different solutions that correspond to local energy minima. (B and C)
Atomic configurations associated with the transitions between energy branches
labeled in (A) showing the most relevant atomistic details involved in the
concerted vertical interchange of atoms between the tip and surface. A detail
of the bonding configuratitill keep their original bonding topology. Upon
retraction from any tip-surface distance larger than the one corresponding to
A3, the system follows the same energy curve back to the original structure
A1. Approaching the tip beyond A3, the system undergoes a discontinuous jump
to a new energy branch B (triangles) with a substantially different bonding
topology (Fig. 2B, image B1). During further approach and consecutive
retraction, the system follows this energy branch up to B2, where a jump
takes it to another energy solution [branch C (circles)], leading to the
bonding topology (structure C2) and the atomic interchange. The Si deposition
case presents the same basic features. During approach, the system follows
the C branch until reaching C3, where it jumps to branch D (pentagons).
Retraction from any distance along the D branch after this jump leads to a
new jump from D2 to A2 and to the atom interchange. Comparing the two
deposition cases, although the atomistic details are slightly different,
overall the atom interchange mechanism seems to be the same. The key step in
these processes is to reach the dimer-like structure shown in B1 and D1. In
these atomic configurations, the outermost atom of the tip and the atom at
the surface now have an equal number of bonds with the surrounding atoms,
losing their association as being part of the tip or the surface. Our
simulations confirm that the dimer structure that minimizes the stored
elastic energy under compression is the lowest energy configuration for other
tip-surface orientations and even for different tip structures.

The energy landscape of these atomic interactions sheds light on some of the
features observed in the associated experimental curves. The frequency shift
signals (Fig. 1) display a shoulder at closer tip-surface distances that
develops into a double well structure in the corresponding short-range
chemical interaction forces (figs. S1 and S2), in contrast to canonical force
spectroscopy curves that for this system can be found in figures 3 and 5 of
(12). The presence of seve gained just by approaching our initial tip model
with a more realistic situation. To this end, we have allowed the four
outermost atoms of the tip apex to relax. This option considerably enlarges
the configuration space with processes including tip modifications and
extraction of atoms from the surface (fig. S6) (9) that lead the system to a
final state of energy higher than that for the vertical atomic interchange.
Although at room temperature thermodynamics would favor the lowest energy
final configurations, the feasibility of the process is controlled by the
energy barriers among the different local minima. Figure 4A depicts the
energy for an approach (squares) and retraction (triangles) cycle over a Si
atom resulting in a tip change, in which the Sn atom at the apex is deposited
over the surface atoms. Upon retraction, the system crosses an energy branch
(circles) that corresponds to a tip retraction in which the surface Si atom
is located at the dimer structure forming the tip apex and the tip Sn atom is
now at the surface in a vertical atomic interchange similar to the one shown
in Fig. 1A. Using the nudge elastic band method (25), we studied the
transition between two atomic configurations very close in energy (points
{alpha} and β) belonging to these two different energy solutions: The
starting atomic arrangement ({alpha}) is a dimer-like configuration in which
both atoms have lost their association as being part of the tip or the
surface; the final state (β) is the deposition of the Sn atom. We found that
there is an energy barrier of 0.4 eV for the transition between these two
configurations (Fig. 4B), which corresponds to the breaking of the remaining
bond of the Si atom with the surface and the formation of a second bond of
the Sn atom with the surface (Fig. 4C, images 4 to 6). This relatively small
energy barrier does not prevent the process from taking place at room
temperature, accounting as well for the presence of a considerable
dissipation signal (fig. S1) at the closest tip-surface distances (16, 23,
24). Therefore, in th the high loads predicted in Fig. 2 for the atomic
interchange taking place; just by exploring tip-surface distances close to
the location of the repulsive zero short-range force (near the crossing point
of the different energy branches available for the system), we are very
likely to obtain a thermally activated vertical interchange atom
manipulation.


Figure 4 	Fig. 4. Typical energy barriers involved in the concerted
vertical interchange of atoms between the tip and surface. (A) Total energy
solutions (squares and triangles) for a tip-surface approach-and-retraction
cycle over a Si atom producing a tip modification in which the Sn atom at the
apex is lost and left on the surface. The energy solution upon retraction
(triangles) crosses an energy branch (circles) that would result in the
concerted vertical interchange of these atoms. (B) Transition energy between
two atomic configurations close in total energy labeled as {alpha} and β in
(A). Energy barriers of a magnitude similar to the one shown in (B) can be
easily overcome by atomic thermal fluctuations at room temperature. The
atomic configurations corresponding to the points marked in (A) and (B) are
displayed on the right. (C) Details of the bonding configurations, reaction
coordinate (9) in (B), through the minimum energy path for the transition
from state {alpha} to state β. The area displayed matches the dashed-line
rectangles shown on the images on the right. [View Larger Version of this
Image (44K GIF file)]
 

The results reported here provide evidence that AFM can be used for the
controlled deposition of individual atoms in semiconductor surfaces with the
possibility of patterning complex atomic structures. Although our results
focus on the Sn/Si system, we have found these vertical interchange atomic
manipulations in other semiconductor surfaces (fig. S8). This manipulation
technique may pave the way toward selective semiconductor doping (26),
practical implementation of quantum computing (27), or atomic-based
spintronics (28). The possibility of combining sophisticated vertical and
lateral atom manipulations (4, 6) with the capabili chemical identification
(14) may bring closer the advent of future atomic-level applications, even at
room temperature.


References and Notes

    * 1. D. M. Eigler, E. K. Schweizer, Nature 344, 524 (1990). [CrossRef]
[ISI]

    * 2. M. F. Crommie, C. P. Lutz, D. M. Eigler, Science 262, 218
(1993).[Abstract/Free Full Text]

    * 3. A. J. Heinrich, C. P. Lutz, J. A. Gupta, D. M. Eigler, Science 298,
1381 (2002).[Abstract/Free Full Text]

    * 4. Y. Sugimoto et al., Nat. Mater. 4, 156 (2005). [CrossRef] [Medline]

    * 5. Y. Sugimoto et al., Phys. Rev. Lett. 98, 106104 (2007). [CrossRef]
[Medline]

    * 6. M. Ternes, C. Lutz, C. Hirjibehedin, F. Giessibl, A. Heinrich,
Science 319, 1066 (2008).[Abstract/Free Full Text]

    * 7. O. Paz, I. Brihuega, J. M. Gómez-Rodríguez, J. M. Soler, Phys. Rev.
Lett. 94, 056103 (2005). [CrossRef] [Medline]

    * 8. R. D. Piner, J. Zhu, F. Xu, S. Hong, C. A. Mirkin, Science 283, 661
(1999).[Abstract/Free Full Text]

    * 9. Materials and methods are available as supporting online material on
Science Online. In AFM experiments we used the easyPLL Plus (Nanosurf,
Liestal, Switzerland) for the detection and regulation of the cantilever
dynamics, and data acquisition and representation was performed using a
Dulcinea SPM controller (Nanotec Electrónica, Madrid, Spain) (29).

    * 10. T. R. Albrecht, P. Grütter, D. Horne, D. Rugar, J. Appl. Phys. 69,
668 (1991). [CrossRef] [ISI]

    * 11. S. T. Jemander, N. Lin, H. M. Zhang, R. I. G. Uhrberg, G. V.
Hansson, Surf. Sci. 475, 181 (2001). [CrossRef]

    * 12. Y. Sugimoto et al., Phys. Rev. B 73, 205329 (2006). [CrossRef]

    * 13. M. A. Lantz et al., Science 291, 2580 (2001).[Abstract/Free Full
Text]

    * 14. Y. Sugimoto et al., Nature 446, 64 (2007). [CrossRef] [ISI]
[Medline]

    * 15. M. Abe, Y. Sugimoto, O. Custance, S. Morita, Appl. Phys. Lett. 87,
173503 (2005). [CrossRef]

    * 16. N. Oyabu et al., Phys. Rev. Lett. 96, 106101 (2006). [CrossRef]
[Medline]

    * 17. D. M. Eigler, C. P. Lutz, W. E. Rudger, Nature 352, 600 (1991).
[CrossRef] [ISI]

    * 18. P. Je 71, 235101 (2005). [CrossRef]

    * 19. F. J. Giessibl, Phys. Rev. B 56, 16010 (1997). [CrossRef]

    * 20. J. E. Sader, S. P. Jarvis, Appl. Phys. Lett. 84, 1801 (2004).
[CrossRef]

    * 21. H. Hölscher et al., Phys. Rev. B 64, 075402 (2001). [CrossRef]

    * 22. U. Dürig, Appl. Phys. Lett. 75, 433 (1999). [CrossRef]

    * 23. N. Sasaki, M. Tsukada, Jpn. J. Appl. Phys. 39, L1334 (2000).
[CrossRef] [ISI]

    * 24. L. N. Kantorovich, T. Trevethan, Phys. Rev. Lett. 93, 236102
(2004). [CrossRef] [Medline]

    * 25. G. Mills, H. Jónsson, Phys. Rev. Lett. 72, 1124 (1994). [CrossRef]
[ISI] [Medline]

    * 26. T. Shinada, S. Okamoto, T. Kobayashi, I. Ohdomari, Nature 437, 1128
(2005). [CrossRef] [ISI] [Medline]

    * 27. B. E. Kane, Nature 393, 133 (1998). [CrossRef] [ISI]

    * 28. D. Kitchen, A. Richardella, J.-M. Tang, M. E. Flatté, A. Yazdani,
Nature 442, 436 (2006). [CrossRef] [ISI] [Medline]

    * 29. I. Horcas et al., Rev. Sci. Instrum. 78, 013705 (2007). [CrossRef]
[Medline]

    * 30. S.Y., O.C., M.A., and S.M. acknowledge support from grants in aid
for scientific research from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), JST, Handai Frontier Research Center, the
Project Atomic Technology funded by MEXT, and Global Center of Excellence
program "Center for Electronic Devices Innovation." O.C. also acknowledges
support from the Air Force Office of Scientific Research–Asian Office of
Aerospace Research and Development. The work of Y.S. is supported by the
Frontier Research Base for Global Young Researchers funded by MEXT. P.P. and
R.P. acknowledge the support of Ministerio de Educación y Ciencia (Spain) and
computer time provided by Red Española de Supercomputación at the Barcelona
Supercomputing Center. P.J. acknowledges the support of Grantová Agentura
Akademie Ved (Czech Republic)

Supporting Online Material

www.sciencemag.org/cgi/content/full/322/5900/413/DC1

Materials and Methods

Figs. S1 to S8

References

Received for publication 16 May 2008. Accepted for publication 9 September
2008.