We have developed a scheme to manipulate metallic aerosol particles on silicon dioxide substrates using an atomic force microscope. The method utilizes the noncontact mode both for locating and moving nanoparticles of size 10–100 nm. The main advantage of our technique is the possibility of “seeing” the moving particle in real time. Our method avoids well sticking problems that typically hamper the manipulation in the contact mode.Peer reviewe

Hakonen, Pertti J.

Kauppinen, Esko I.

Martin, M.

Paalanen, M.

Parts, U.

Roschier, L.

Schleicher, B.

M. Martin

L. Roschier

P. Hakonen

Ü. Parts

M. Paalanen

B. Schleicher

E. I. Kauppinen

Crossref

Manipulation of Ag nanoparticles utilizing noncontact atomic force microscopy

Aaltodoc Publication Archive

This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Martin, M. & Roschier, L. & Hakonen, Pertti J. & Parts, U. &Paalanen, M. & Schleicher, B. & Kauppinen, E. I.Title: Manipulation of Ag nanoparticles utilizing noncontact atomic forcemicroscopyYear: 1998Version: Final published versionPlease cite the original version:Martin, M. & Roschier, L. & Hakonen, Pertti J. & Parts, U. & Paalanen, M. & Schleicher,B. & Kauppinen, E. I.. 1998. Manipulation of Ag nanoparticles utilizing noncontact atomicforce microscopy. Applied Physics Letters. Volume 73, Issue 11. 1505-1507. ISSN0003-6951 (printed). DOI: 10.1063/1.122187Rights: © 1998 AIP Publishing. This article may be downloaded for personal use only. Any other use requires priorpermission of the authors and the American Institute of Physics. The following article appeared in AppliedPhysics Letters, Volume 73, Issue 11 and may be found athttp://scitation.aip.org/content/aip/journal/apl/73/11/10.1063/1.122187.All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Manipulation of Ag nanoparticles utilizing noncontact atomic force microscopyM. Martin, L. Roschier, P. Hakonen, Ü. Parts, M. Paalanen, B. Schleicher, and E. I. Kauppinen  Citation: Applied Physics Letters 73, 1505 (1998); doi: 10.1063/1.122187 View online: http://dx.doi.org/10.1063/1.122187 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/73/11?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Anomalous polarized absorbance spectra of aligned Ag nanorod arrays Appl. Phys. Lett. 89, 053117 (2006); 10.1063/1.2267161  Mapping substrate/film adhesion with contact-resonance-frequency atomic force microscopy Appl. Phys. Lett. 89, 021911 (2006); 10.1063/1.2221404  Tribonanolithography of silicon in aqueous solution based on atomic force microscopy Appl. Phys. Lett. 85, 1766 (2004); 10.1063/1.1773620  Atomic force microscope anodic oxidation studied by spectroscopic microscopy Appl. Phys. Lett. 81, 2842 (2002); 10.1063/1.1509121  Self-assembled-monolayer film islands as a self-patterned-mask for SiO 2 thickness measurement with atomicforce microscopy Appl. Phys. Lett. 70, 3398 (1997); 10.1063/1.119183    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.22 On: Mon, 28 Sep 2015 07:50:56Manipulation of Ag nanoparticles utilizing noncontact atomic forcemicroscopyM. Martin,a) L. Roschier, P. Hakonen, U¨ . Parts, and M. PaalanenHelsinki University of Technology, Low Temperature Laboratory, FIN-02015 HUT, FinlandB. Schleicher and E. I. KauppinenVTT Chemical Technology, VTT Aerosol Technology Group, P.O. Box 1401, FIN-02044 VTT, Finland~Received 8 June 1998; accepted for publication 10 July 1998!We have developed a scheme to manipulate metallic aerosol particles on silicon dioxide substratesusing an atomic force microscope. The method utilizes the noncontact mode both for locating andmoving nanoparticles of size 10–100 nm. The main advantage of our technique is the possibility of‘‘seeing’’ the moving particle in real time. Our method avoids well sticking problems that typicallyhamper the manipulation in the contact mode. © 1998 American Institute of Physics.@S0003-6951~98!00337-4#It is rather well established by now that that scanningprobe microscopy ~SPM! can be utilized to manufacturenanostructures. Several approaches have been, and continueto be, studied. These methods provide one of the most prom-ising ways to produce room-temperature mesoscopic quan-tum devices in an efficient fashion. Reliable manufacturingprocesses are a must before there is any hope for prolifera-tion of such nanoscale devices, e.g., single electron transis-tors ~SET!, in the future.In our laboratory, we concentrate on nanostructuresmade of nanometer-sized aerosol particles. It was demon-strated recently that an atomic force microscope ~AFM! canbe used to arrange and manipulate Au clusters1 and aerosolparticles2 as well as evaporated nanoscale platelets.3 In theseundertakings, the AFM tip was used as a tool to push par-ticles along smooth substrates. Tips as sharp as possible wereemployed in order to minimize the adhesion force betweenthe particles and the tip. Nevertheless, severe problems withparticle sticking were observed. In spite of these shortcom-ings, good control for positioning of aerosol particles wasachieved, and even basic SET operation could be demon-strated.In this letter, we report a method to position aerosolparticles of 10–100 nm in diameter. Our method employs thenoncontact ~NC! mode both to locate and to move particleson smooth silicon dioxide substrates. Using the NC mode weencounter fewer problems with apex wear and tip degrada-tion due to particle sticking as compared with the work byJunno et al.2 The main advantage of our method, however, isthe possibility of knowing the particle position, even when itis moving.We produce nanometer-sized particles suspended in acarrier gas ~aerosol! by a technique called spray pyrolysis.4A solution of about 4.5 g of AgNO3 in 1 l of ultrapure wateris sprayed in the form of small droplets into a N2 carrier gasusing a constant output collision atomizer. The mean size ofa droplet is about 800 nm. The nitrogen picks up the particlesand transports them in a ceramic tube ~inner diameter: 8 cm!through a hot flow reactor ~three zone furnace, Lindberg55666, heated length ;90 cm! heated to about 700° C at aflow rate of about 3 l/min at room temperature and atmo-spheric pressure. In the reactor water evaporates, AgNO3 de-composes and an Ag particle is formed from each precursordroplet. The particles leaving the furnace are not uniform insize, but have a lognormal size distribution. These particlesare charged in a diffusion charger and subsequently a differ-ential mobility analyzer ~DMA! extracts a desired particlesize by their electric mobility. The 95% monodisperse,charged aerosol particles leaving the DMA are removed fromthe gas and precipitated in an electric field on a SiO2 sub-strate ~100 nm of thermally grown oxide on top of bulk ^100&Si! that was swept clean with an AFM in contact mode be-fore the deposition.Because of nearly spherical shape of the aerosol par-ticles, their adhesion force to the sample surface is muchsmaller than in the case of evaporated dots of the same di-mensions. Imaging of these particles with contact modeAFM is not really possible unless stabilization by sintering isdone.5 The adhesion force fails to keep the particle stationaryduring contact mode scanning and, as a result, the particle iseither moved unintentionally or lifted away from the surfaceby the tip.2 Hence, only the noncontact mode imaging can beused to locate particles on the substrate. In the NC mode, thetip is vibrating at a constant frequency, slightly above itsresonance frequency. The vibration amplitude diminishes asthe distance decreases, so that one can image the topographyin a safe way, as the van der Waals force between the tip andthe sample is about a few of piconewtons.We employed our AFM ~PSI Autoprobe CP! in NCmode all through this work. This differs from the earlierwork2 where a combination of contact and noncontact modeswas used. The AFM was operated in ambient air ~relativehumidity 50%! using cantilevers ~PSI Ultralevers! with reso-nance frequency of f res5104 kHz. The spring constant ofthese triangular silicon cantilevers is k52 N/m and the Qvalue is about 100. The cantilever deflection was measuredcontinuously by a laser beam which reflected from the can-tilever to a double channel (A/B) photodetector. The tip ra-a!Permanent address: CNRS-CRTBT, BP 166X, F-38042 Grenoble Cedex 9,France. Electronic-mail: michmart@boojum.hut.fiAPPLIED PHYSICS LETTERS VOLUME 73, NUMBER 11 14 SEPTEMBER 199815050003-6951/98/73(11)/1505/3/$15.00 © 1998 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.22 On: Mon, 28 Sep 2015 07:50:56dius Rt , nominally 20 nm, was verified afterwards using ascanning electron microscope ~SEM!; in the worst case itwas found to be 50 nm.The algorithm we developed to move particles is de-scribed as follows ~see Fig. 1!: first, an image of sampletopography is taken to check the particle position. The feed-back loop is still on and a line, scanned at a rate of 2 Hz, isselected in the desired direction for the chosen particle. Thelength of the line scan is about two or three times the particlediameter. An offset is made to place the end of the scanabove the particle; this guarantees that the movement beginsin the desired direction. The feedback loop is then switchedoff to have direct control of the z axis. The sample height isincreased gradually until there is a clear change in the vibra-tion amplitude of the tip. The point at which the tip touchesthe particle is easy to find as the vibration amplitude be-comes zero. Depending on how the particle reacts, one canchoose either to lift or lower the sample in order to achieve asuitably strong tip-particle interaction. As the tip touches theparticle, it tries to push it towards the end of the scan. Thedisplacement of the particle may take place in one singlepush or it may take a few scans. As we monitor the vibrationamplitude in real time, it is easy to follow the reaction con-tinuously and, thereby, the position of the particle. After theparticle has been moved, a new offset is given to the scanline to continue the movement until the final position isreached. At this point, it is possible to select another particlefor movement without taking a topography image as the po-sition of the displaced particle is well known.The tip-particle interaction during the movement, as seenby the difference A-B of the photodetector, is illustrated inFig. 2. Three different regimes can be resolved in the trace ofthe oscillation amplitude. First, on the left the vibration am-plitude is determined almost solely by the drive ( f dr5 f res10.5 kHz) and the response of the cantilever, although thepresence of the substrate slightly decreases the swing; forfree vibration the amplitude was set to 100 nmpeak-peak . In thesecond regime, where the tip starts to feel the van der Waalsforce more strongly, the resonance frequency shifts and theoscillation amplitude is reduced substantially owing to thereduced coupling of the fixed-frequency drive. As the tip-particle separation decreases further, the tip is eventuallytrapped by the surface ~‘‘negative deflection’’! and the vibra-tion amplitude becomes zero. The tip then starts to track theparticle, but the pushing begins only after the cantilever de-flection has become positive. The hysteresis observed in thecontact/off-contact points was rather small. This we interpretso that there is not much water adsorbed on the particles atour operating conditions.6The scan illustrated in Fig. 2 was made 5 nm below thetop of the sphere. During this scan the particle did not move,although the parameters were rather close to those at whichmovement starts. The horizontal force Fx can be estimated ina straightforward way without friction. Using the parameter-ization given in Fig. 1, the force may be written asFx5k~sin u2sin u0!Rptan u.According to this formula, the lateral force needed to inducemovement is about 2 nN for 55 nm silver particles. Themeasured threshold force depended on the particle size andvaried strongly among different positions of the particles,i.e., it depended on the variation in the roughness of thesubstrate (;0.8 nmrms). Compared with typical atomic fric-tion forces,7 our values are larger by an order of magnitude.The accuracy of our displacement method depends onthe particle radius. Provided that the particle and tip radii areon the same order, one can rather simply move particles topredefined locations within about 10% of their diameter.However, it is quite difficult to get below this limit. It is notclear at all if the particles can be moved Å by Å as thesurface potential may contain a complicated distribution oflocal minima. The operating range can be easily varied: par-ticles can be pushed over several micrometers or only a fewnanometers. Figure 3 illustrates the resolution achieved whenwriting the letters LTL using 45 nm diameter silver particles.Under optimum operation conditions, the structure shown inFig. 3 can be constructed in a few hours. Since the particlescan be followed well during the movement process, we don’tsee any real obstacle in speeding up our manipulationmethod by automation.FIG. 1. Schematic view of our scan method to move particles utilizing thenoncontact mode. The trace in the inset illustrates an overall scan, made firstto locate particles on the substrate; the line scan, selected to move a particlewith radius Rp , is denoted by the dashed vertical lines. The oscillationamplitude on the left depicts the vertical movement of the tip over the scanline when the feedback is switched off. The tip is brought closer to thesubstrate till the particle starts to move at the right end of the scan. Anglesu0 and u parameterize the ‘‘zero force’’ and the scan reversal points on thesphere, respectively.FIG. 2. Trace of the differential photodetector signal A-B as the tip (Rt;50 nm) approaches a 55 nm particle and moves on top of it. The cantile-ver is excited at a constant drive frequency f dr5 f res10.5 kHz. The horizon-tal line indicates the equilibrium height of the cantilever, i.e., the positionwhere the vertical force is zero.1506 Appl. Phys. Lett., Vol. 73, No. 11, 14 September 1998 Martin et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.22 On: Mon, 28 Sep 2015 07:50:56We have also tried the contact mode by Junno et al.2Using this method we encountered severe sticking problemsas was the case in their own work. Clearly, the contact moderequires extremely sharp tips for which the adhesion forcetowards the apex is smaller than towards the substrate. In ourmethod, on the contrary, we have been able to move 8 nmparticles using a tip having Rt537 nm almost without anysticking problems. The lateral force with 8 nm particles wasclearly less than with 45 nm particles.It is not clear to us why our method avoids so well theproblem of sticking. Inertial forces due to tip vibration mightfavor sticking to the stationary substrate. Our numericalsimulations, however, do not support this view. Any materialdependence is excluded by the fact that we have stickingwhen using the contact mode scheme. This leaves capillarycondensation as the only possible source for the difference inthe sticking behavior. Hence, we conjecture that sticking iscaused mostly by condensed water and, in our case, the con-tact time of 50-100 ms is not long enough to produce abinding between the adsorbed water layers. This conjecturewill be tested in the future in a dry nitrogen atmosphere.In summary, we have developed a method to manipulatenanometer size aerosol particles using an AFM microscope.Our method is based on continuous scanning of a line andsimultaneous acquisition of the vibration amplitude of thecantilever. The main advantage of this technique is the pos-sibility of ‘‘seeing’’ the moving particle in real time, whichallows the control of position down to 10% of the particlediameter. Surprisingly, minimal sticking of silver particleson silicon tips was found under our experimental conditions,even when moving particles ten times smaller than the apexof the tip.The authors want to thank P. Davidson, T. Junno, L.Montelius, U. Tapper, and R. Vuorinen for useful discus-sions and assistance. This work was supported by the Acad-emy of Finland, by TEKES of Finland via the Nanotechnol-ogy program, and by the Human Capital and MobilityProgram ULTI of the European Community.1 D. M. Schaefer, R. Reifenberger, A. Patil, and R. P. Andres, Appl. Phys.Lett. 66, 1012 ~1995!.2 T. Junno, K. Deppert, L. Montelius, and L. Samuelsson, Appl. Phys. Lett.66, 3627 ~1995!.3 L. Montelius, T. Junno, S.-B. Carlsson, and L. Samuelsson, Microelec-tron. Eng. 35, 281 ~1997!.4 A. Gurav, T. T. Kodas, T. Pluym, and Y. Xiong, J. Aerosol Sci. 19, 411~1993!.5 T. Junno, S. Anand, K. Deppert, L. Montelius, and L. Samuelsson, Appl.Phys. Lett. 66, 3295 ~1995!.6 With a considerable amount of condensed water, even dragging of par-ticles with a sharp tip is possible.7 T. Gyalog and H. Thomas, Z. Phys. B 104, 669 ~1997!.FIG. 3. Low temperature laboratory ~LTL! written in block letters with 45nm silver particles on top of silicon dioxide substrate.1507Appl. Phys. Lett., Vol. 73, No. 11, 14 September 1998 Martin et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.22 On: Mon, 28 Sep 2015 07:50:56

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Martin, M. & Roschier, L. & Hakonen, Pertti J. & Parts, U. &
Paalanen, M. & Schleicher, B. & Kauppinen, E. I.
Title: Manipulation of Ag nanoparticles utilizing noncontact atomic force
microscopy
Year: 1998
Version: Final published version
Please cite the original version:
Martin, M. & Roschier, L. & Hakonen, Pertti J. & Parts, U. & Paalanen, M. & Schleicher,
B. & Kauppinen, E. I.. 1998. Manipulation of Ag nanoparticles utilizing noncontact atomic
force microscopy. Applied Physics Letters. Volume 73, Issue 11. 1505-1507. ISSN
0003-6951 (printed). DOI: 10.1063/1.122187
Rights: © 1998 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior
permission of the authors and the American Institute of Physics. The following article appeared in Applied
Physics Letters, Volume 73, Issue 11 and may be found at
http://scitation.aip.org/content/aip/journal/apl/73/11/10.1063/1.122187.
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
be duplicated by you for your research use or educational purposes in electronic or print form. You must
obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or
otherwise to anyone who is not an authorised user.
Powered by TCPDF (www.tcpdf.org)
Manipulation of Ag nanoparticles utilizing noncontact atomic force microscopy
M. Martin, L. Roschier, P. Hakonen, Ü. Parts, M. Paalanen, B. Schleicher, and E. I. Kauppinen 
 
Citation: Applied Physics Letters 73, 1505 (1998); doi: 10.1063/1.122187 
View online: http://dx.doi.org/10.1063/1.122187 
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/73/11?ver=pdfcov 
Published by the AIP Publishing 
 
Articles you may be interested in 
Anomalous polarized absorbance spectra of aligned Ag nanorod arrays 
Appl. Phys. Lett. 89, 053117 (2006); 10.1063/1.2267161 
 
Mapping substrate/film adhesion with contact-resonance-frequency atomic force microscopy 
Appl. Phys. Lett. 89, 021911 (2006); 10.1063/1.2221404 
 
Tribonanolithography of silicon in aqueous solution based on atomic force microscopy 
Appl. Phys. Lett. 85, 1766 (2004); 10.1063/1.1773620 
 
Atomic force microscope anodic oxidation studied by spectroscopic microscopy 
Appl. Phys. Lett. 81, 2842 (2002); 10.1063/1.1509121 
 
Self-assembled-monolayer film islands as a self-patterned-mask for SiO 2 thickness measurement with atomic
force microscopy 
Appl. Phys. Lett. 70, 3398 (1997); 10.1063/1.119183 
 
 
 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.233.216.22 On: Mon, 28 Sep 2015 07:50:56
Manipulation of Ag nanoparticles utilizing noncontact atomic force
microscopy
M. Martin,a) L. Roschier, P. Hakonen, U¨ . Parts, and M. Paalanen
Helsinki University of Technology, Low Temperature Laboratory, FIN-02015 HUT, Finland
B. Schleicher and E. I. Kauppinen
VTT Chemical Technology, VTT Aerosol Technology Group, P.O. Box 1401, FIN-02044 VTT, Finland
~Received 8 June 1998; accepted for publication 10 July 1998!
We have developed a scheme to manipulate metallic aerosol particles on silicon dioxide substrates
using an atomic force microscope. The method utilizes the noncontact mode both for locating and
moving nanoparticles of size 10–100 nm. The main advantage of our technique is the possibility of
‘‘seeing’’ the moving particle in real time. Our method avoids well sticking problems that typically
hamper the manipulation in the contact mode. © 1998 American Institute of Physics.
@S0003-6951~98!00337-4#
It is rather well established by now that that scanning
probe microscopy ~SPM! can be utilized to manufacture
nanostructures. Several approaches have been, and continue
to be, studied. These methods provide one of the most prom-
ising ways to produce room-temperature mesoscopic quan-
tum devices in an efficient fashion. Reliable manufacturing
processes are a must before there is any hope for prolifera-
tion of such nanoscale devices, e.g., single electron transis-
tors ~SET!, in the future.
In our laboratory, we concentrate on nanostructures
made of nanometer-sized aerosol particles. It was demon-
strated recently that an atomic force microscope ~AFM! can
be used to arrange and manipulate Au clusters1 and aerosol
particles2 as well as evaporated nanoscale platelets.3 In these
undertakings, the AFM tip was used as a tool to push par-
ticles along smooth substrates. Tips as sharp as possible were
employed in order to minimize the adhesion force between
the particles and the tip. Nevertheless, severe problems with
particle sticking were observed. In spite of these shortcom-
ings, good control for positioning of aerosol particles was
achieved, and even basic SET operation could be demon-
strated.
In this letter, we report a method to position aerosol
particles of 10–100 nm in diameter. Our method employs the
noncontact ~NC! mode both to locate and to move particles
on smooth silicon dioxide substrates. Using the NC mode we
encounter fewer problems with apex wear and tip degrada-
tion due to particle sticking as compared with the work by
Junno et al.2 The main advantage of our method, however, is
the possibility of knowing the particle position, even when it
is moving.
We produce nanometer-sized particles suspended in a
carrier gas ~aerosol! by a technique called spray pyrolysis.4
A solution of about 4.5 g of AgNO3 in 1 l of ultrapure water
is sprayed in the form of small droplets into a N2 carrier gas
using a constant output collision atomizer. The mean size of
a droplet is about 800 nm. The nitrogen picks up the particles
and transports them in a ceramic tube ~inner diameter: 8 cm!
through a hot flow reactor ~three zone furnace, Lindberg
55666, heated length ;90 cm! heated to about 700° C at a
flow rate of about 3 l/min at room temperature and atmo-
spheric pressure. In the reactor water evaporates, AgNO3 de-
composes and an Ag particle is formed from each precursor
droplet. The particles leaving the furnace are not uniform in
size, but have a lognormal size distribution. These particles
are charged in a diffusion charger and subsequently a differ-
ential mobility analyzer ~DMA! extracts a desired particle
size by their electric mobility. The 95% monodisperse,
charged aerosol particles leaving the DMA are removed from
the gas and precipitated in an electric field on a SiO2 sub-
strate ~100 nm of thermally grown oxide on top of bulk ^100&
Si! that was swept clean with an AFM in contact mode be-
fore the deposition.
Because of nearly spherical shape of the aerosol par-
ticles, their adhesion force to the sample surface is much
smaller than in the case of evaporated dots of the same di-
mensions. Imaging of these particles with contact mode
AFM is not really possible unless stabilization by sintering is
done.5 The adhesion force fails to keep the particle stationary
during contact mode scanning and, as a result, the particle is
either moved unintentionally or lifted away from the surface
by the tip.2 Hence, only the noncontact mode imaging can be
used to locate particles on the substrate. In the NC mode, the
tip is vibrating at a constant frequency, slightly above its
resonance frequency. The vibration amplitude diminishes as
the distance decreases, so that one can image the topography
in a safe way, as the van der Waals force between the tip and
the sample is about a few of piconewtons.
We employed our AFM ~PSI Autoprobe CP! in NC
mode all through this work. This differs from the earlier
work2 where a combination of contact and noncontact modes
was used. The AFM was operated in ambient air ~relative
humidity 50%! using cantilevers ~PSI Ultralevers! with reso-
nance frequency of f res5104 kHz. The spring constant of
these triangular silicon cantilevers is k52 N/m and the Q
value is about 100. The cantilever deflection was measured
continuously by a laser beam which reflected from the can-
tilever to a double channel (A/B) photodetector. The tip ra-
a!Permanent address: CNRS-CRTBT, BP 166X, F-38042 Grenoble Cedex 9,
France. Electronic-mail: michmart@boojum.hut.fi
APPLIED PHYSICS LETTERS VOLUME 73, NUMBER 11 14 SEPTEMBER 1998
15050003-6951/98/73(11)/1505/3/$15.00 © 1998 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.233.216.22 On: Mon, 28 Sep 2015 07:50:56
dius Rt , nominally 20 nm, was verified afterwards using a
scanning electron microscope ~SEM!; in the worst case it
was found to be 50 nm.
The algorithm we developed to move particles is de-
scribed as follows ~see Fig. 1!: first, an image of sample
topography is taken to check the particle position. The feed-
back loop is still on and a line, scanned at a rate of 2 Hz, is
selected in the desired direction for the chosen particle. The
length of the line scan is about two or three times the particle
diameter. An offset is made to place the end of the scan
above the particle; this guarantees that the movement begins
in the desired direction. The feedback loop is then switched
off to have direct control of the z axis. The sample height is
increased gradually until there is a clear change in the vibra-
tion amplitude of the tip. The point at which the tip touches
the particle is easy to find as the vibration amplitude be-
comes zero. Depending on how the particle reacts, one can
choose either to lift or lower the sample in order to achieve a
suitably strong tip-particle interaction. As the tip touches the
particle, it tries to push it towards the end of the scan. The
displacement of the particle may take place in one single
push or it may take a few scans. As we monitor the vibration
amplitude in real time, it is easy to follow the reaction con-
tinuously and, thereby, the position of the particle. After the
particle has been moved, a new offset is given to the scan
line to continue the movement until the final position is
reached. At this point, it is possible to select another particle
for movement without taking a topography image as the po-
sition of the displaced particle is well known.
The tip-particle interaction during the movement, as seen
by the difference A-B of the photodetector, is illustrated in
Fig. 2. Three different regimes can be resolved in the trace of
the oscillation amplitude. First, on the left the vibration am-
plitude is determined almost solely by the drive ( f dr5 f res
10.5 kHz) and the response of the cantilever, although the
presence of the substrate slightly decreases the swing; for
free vibration the amplitude was set to 100 nmpeak-peak . In the
second regime, where the tip starts to feel the van der Waals
force more strongly, the resonance frequency shifts and the
oscillation amplitude is reduced substantially owing to the
reduced coupling of the fixed-frequency drive. As the tip-
particle separation decreases further, the tip is eventually
trapped by the surface ~‘‘negative deflection’’! and the vibra-
tion amplitude becomes zero. The tip then starts to track the
particle, but the pushing begins only after the cantilever de-
flection has become positive. The hysteresis observed in the
contact/off-contact points was rather small. This we interpret
so that there is not much water adsorbed on the particles at
our operating conditions.6
The scan illustrated in Fig. 2 was made 5 nm below the
top of the sphere. During this scan the particle did not move,
although the parameters were rather close to those at which
movement starts. The horizontal force Fx can be estimated in
a straightforward way without friction. Using the parameter-
ization given in Fig. 1, the force may be written as
Fx5
k~sin u2sin u0!Rp
tan u
.
According to this formula, the lateral force needed to induce
movement is about 2 nN for 55 nm silver particles. The
measured threshold force depended on the particle size and
varied strongly among different positions of the particles,
i.e., it depended on the variation in the roughness of the
substrate (;0.8 nmrms). Compared with typical atomic fric-
tion forces,7 our values are larger by an order of magnitude.
The accuracy of our displacement method depends on
the particle radius. Provided that the particle and tip radii are
on the same order, one can rather simply move particles to
predefined locations within about 10% of their diameter.
However, it is quite difficult to get below this limit. It is not
clear at all if the particles can be moved Å by Å as the
surface potential may contain a complicated distribution of
local minima. The operating range can be easily varied: par-
ticles can be pushed over several micrometers or only a few
nanometers. Figure 3 illustrates the resolution achieved when
writing the letters LTL using 45 nm diameter silver particles.
Under optimum operation conditions, the structure shown in
Fig. 3 can be constructed in a few hours. Since the particles
can be followed well during the movement process, we don’t
see any real obstacle in speeding up our manipulation
method by automation.
FIG. 1. Schematic view of our scan method to move particles utilizing the
noncontact mode. The trace in the inset illustrates an overall scan, made first
to locate particles on the substrate; the line scan, selected to move a particle
with radius Rp , is denoted by the dashed vertical lines. The oscillation
amplitude on the left depicts the vertical movement of the tip over the scan
line when the feedback is switched off. The tip is brought closer to the
substrate till the particle starts to move at the right end of the scan. Angles
u0 and u parameterize the ‘‘zero force’’ and the scan reversal points on the
sphere, respectively.
FIG. 2. Trace of the differential photodetector signal A-B as the tip (Rt
;50 nm) approaches a 55 nm particle and moves on top of it. The cantile-
ver is excited at a constant drive frequency f dr5 f res10.5 kHz. The horizon-
tal line indicates the equilibrium height of the cantilever, i.e., the position
where the vertical force is zero.
1506 Appl. Phys. Lett., Vol. 73, No. 11, 14 September 1998 Martin et al.
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We have also tried the contact mode by Junno et al.2
Using this method we encountered severe sticking problems
as was the case in their own work. Clearly, the contact mode
requires extremely sharp tips for which the adhesion force
towards the apex is smaller than towards the substrate. In our
method, on the contrary, we have been able to move 8 nm
particles using a tip having Rt537 nm almost without any
sticking problems. The lateral force with 8 nm particles was
clearly less than with 45 nm particles.
It is not clear to us why our method avoids so well the
problem of sticking. Inertial forces due to tip vibration might
favor sticking to the stationary substrate. Our numerical
simulations, however, do not support this view. Any material
dependence is excluded by the fact that we have sticking
when using the contact mode scheme. This leaves capillary
condensation as the only possible source for the difference in
the sticking behavior. Hence, we conjecture that sticking is
caused mostly by condensed water and, in our case, the con-
tact time of 50-100 ms is not long enough to produce a
binding between the adsorbed water layers. This conjecture
will be tested in the future in a dry nitrogen atmosphere.
In summary, we have developed a method to manipulate
nanometer size aerosol particles using an AFM microscope.
Our method is based on continuous scanning of a line and
simultaneous acquisition of the vibration amplitude of the
cantilever. The main advantage of this technique is the pos-
sibility of ‘‘seeing’’ the moving particle in real time, which
allows the control of position down to 10% of the particle
diameter. Surprisingly, minimal sticking of silver particles
on silicon tips was found under our experimental conditions,
even when moving particles ten times smaller than the apex
of the tip.
The authors want to thank P. Davidson, T. Junno, L.
Montelius, U. Tapper, and R. Vuorinen for useful discus-
sions and assistance. This work was supported by the Acad-
emy of Finland, by TEKES of Finland via the Nanotechnol-
ogy program, and by the Human Capital and Mobility
Program ULTI of the European Community.
1 D. M. Schaefer, R. Reifenberger, A. Patil, and R. P. Andres, Appl. Phys.
Lett. 66, 1012 ~1995!.
2 T. Junno, K. Deppert, L. Montelius, and L. Samuelsson, Appl. Phys. Lett.
66, 3627 ~1995!.
3 L. Montelius, T. Junno, S.-B. Carlsson, and L. Samuelsson, Microelec-
tron. Eng. 35, 281 ~1997!.
4 A. Gurav, T. T. Kodas, T. Pluym, and Y. Xiong, J. Aerosol Sci. 19, 411
~1993!.
5 T. Junno, S. Anand, K. Deppert, L. Montelius, and L. Samuelsson, Appl.
Phys. Lett. 66, 3295 ~1995!.
6 With a considerable amount of condensed water, even dragging of par-
ticles with a sharp tip is possible.
7 T. Gyalog and H. Thomas, Z. Phys. B 104, 669 ~1997!.
FIG. 3. Low temperature laboratory ~LTL! written in block letters with 45
nm silver particles on top of silicon dioxide substrate.
1507Appl. Phys. Lett., Vol. 73, No. 11, 14 September 1998 Martin et al.
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130.233.216.22 On: Mon, 28 Sep 2015 07:50:56


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Manipulation of Ag nanoparticles utilizing noncontact atomic force microscopy

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