APPLIED PHYSICS LETTERSVOLUME 79, NUMBER 530 JULY 2001

Selective adsorption and patterning of Si nanoparticles fabricated by laser
ablation on functionalized self-assembled monolayer
K. Hata,a) M. Fujita, S. Yoshida, S. Yasuda, T. Makimura, K. Murakami,and H. Shigekawab)
Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST),
University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8573, Japan

W. Mizutani and H. Tokumoto
Joint Research Center for Atom Technology, National Institute for Advanced Interdisciplinary Research,Higashi 1-1-4, Tsukuba 305-8562, Japan共Received 20 March 2001; accepted for publication 4 June 2001兲
We demonstrate an in situ selective adsorption of Si nanoparticles fabricated by laser ablation on afunctionalized self-assembled monolayer 共SAM兲. Si nanoparticles adsorbed on –CH3 terminated aSAM while Si particles did not adsorb on –NH2 , –F, –OH, and –COOH, terminated SAMs. The
end group of a SAM solely determines the selectivity against Si nanoparticle adsorption. We utilized
the screening ability of functionalized SAMs to pattern Si nanoparticles onto desired locations on aSi substrate. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1388025兴

Within the emerging paradigm of nanotechnology, artificial nanostructures such as nanoparticles, nanotubes, andself-assembled monolayers 共SAMs兲 are considered to offer
great potential as basic building blocks to realize nanodevices and molecular electronics. In order to fulfill this goal, it
would be necessary to develop technologies to assemble artificial nanostructures into highly functional complexities of
the next level. One promising way to immobilize, assemble,and pattern nanostructures is to control the interaction between the nanostructures and substrate by changing theirchemical functionality.1– 6 SAM is an ideal candidate for thispurpose because it possess several promising properties including: 共1兲 a wide range of addressable terminating end
groups that provides surfaces with various chemical identities, 共2兲 a variety of substrates possible to fabricate a SAM
and 共3兲 the ability to pattern a chemically functionalized
SAM by the microcontact printing 共␮CP兲 method.7,8 The key
point to successfully assemble and pattern nanostructures ona functionalized SAM is to achieve selective adsorption, i.e.,
to find a complementary pair of SAMs with different chemical end groups, where the target nanostructure does adsorb
on one SAM while it does not on the other. While most of
the previous research has pursued combinations of materials
that stick together, searching combinations of materials that
do not, is also a critical factor in achieving selective adsorption. One general route to achieve selective adsorption is toutilize the electrostatic interaction between oppositely
charged nanomaterials and SAMs.4 – 6 In this case, selectiveadsorption can be readily achieved by changing the polarityof a SAM. Another way is to fabricate textured surfaces withdifferent wettabilities, where nanomaterials preferably adsorb on regions with a specific wettability.3 In these methods,a兲Present address: Department of Chemistry and Chemical Biology, Harvard
University, Cambridge, MA 02138; electronic mail:khata@cmliris.harvard.edub兲Electronic mail: hidemi@ims.tsukuba.ac.jp, World Wide Web: http://dora.ims.tsukuba.ac.jp.

nanomaterials are suspended in a fluid and transferred to thesurface, thus, intrinsically, the methods are wet processes.
Utilizing these approaches, selective adsorption and subsequently patterning have been achieved on molecules,3 semiconductor nanowires,4 and metal particles3,5,6 so far.
Here we report on the in situ selective adsorption of Si
nanoparticles fabricated by laser ablation9–11 on a functionalized SAM. The Si nanoparticle is a very interesting andimportant nanomaterial12–14 possesses many promising properties, e.g., it shows visible photoluminescence more efficient than in normal Si due to the quantum confinement effect. Different from earlier reports, selective adsorption of Si
nanoparticles on a SAM we report here is achieved in a dry
ambient and interestingly the driving force for adsorption isneither electrostatic interaction nor caused by different wettabilities of the SAM surface.
SAM substrates were loaded into a laser ablation chamber filled with Ar gas. A pulsed laser was focused and irradiated to a target Si wafer that was placed on the same planeof a SAM substrate 关inset of Fig. 1共b兲兴. By laser ablation, it
is possible to fabricate pure, well characterized, and relatively uniform, Si nanoparticles.9–11 Si nanoparticles formedby the collision of the hot Si atoms in the plasma with theinert gas are supposed to land on the SAM surface. Figure 1shows typical scanning tunneling microscopy 共STM兲 imagesof two SAM surfaces with different end groups taken afterlaser ablation. Laser ablation was carried out concurrently ina configuration shown in the inset of Fig. 1共b兲. White protrusions that did not exist before ablation were observed on–CH3 terminated SAM 关Fig. 1共a兲兴. We confirmed that thewhite protrusions are Si nanoparticles through an extensive
series of STM, atomic force microscopy 共AFM兲, and photoluminescence experiments.15 The average size of Si nanoparticles was 7 nm when the Ar ambient is 5 Torr, while the size
could be readily controlled, in the region where quantum
confinement effect is important, by changing the Ar ambientpressure. Photoluminescence measurements showed an emission peak centered at 623 nm, a value that is in the range of0003-6951/2001/79(5)/692/3/$18.00
692© 2001 American Institute of Physics
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Hata et al.

Appl. Phys. Lett., Vol. 79, No. 5, 30 July 2001

FIG. 1. Selective adsorption of Si nanoparticles on a functionalized SAM.
共a兲 A STM image of a –CH3 共hexanethiol兲 terminated SAM and 共b兲 a STM
image of an –COOH 关HS共CH2兲10COOH] terminated SAM, after laser ablation, recorded with tunneling current ⫽ 0.5 nA and surface voltage ⫽ 500
mV. No eminent dependence on tunneling conditions was observed. Ablation conditions: Ar ambient pressure 5 Torr, shot number of pulsed laser 100.
SAMs were prepared on Au共111兲/mica 共thickness 50 nm兲 by adsorption ofthiols from millimolar solutions in ethanol for 12–24 h, then rinsed withethanol and dried under a flow of N2 . Inset shows configuration of the SAM
substrates 共hatched rectangles兲 and the target Si wafer 共square兲 during laserablation. This configuration ensures that the laser ablation is carried out on
the same condition for the two SAM substrates. The cross indicates theablation spot. Typical distance from the ablation spot to the SAM substrates
was approximately 5 mm. The laser ablation chamber was filled with Ar gas
of 2–10 Torr. Yttrium–aluminum–garnet pulsed laser shots 共10 ns, 25 mJ兲were focused and irradiated to the Si target at frequencies of 1 or 10 Hz.

the characteristic emission wavelengths of Si nanoparticles
fabricated by our laser ablation conditions. Si nanoparticles
were firmly attached to a SAM: they were not removed byethanol rinsing, and remained stable for at least a couple ofmonths. These features would be preferred in any practicaluse, and would enable one to implement many sequential
processes to fabricate more complex nanoarchitectures. Incontrast, no Si nanoparticles were observed on a –COOH
terminated SAM 关Fig. 1共b兲兴. The screening effect of a–COOH terminated SAM 共and others described later兲 to theadsorption of Si nanoparticles was complete, and not a single
Si nanoparticle was observed on the surface regardless of theposition and for many experimental runs carried out on various laser ablation conditions.
To elucidate the adsorption mechanism of Si nanoparticles on a SAM, we carried out laser ablation on several
SAMs that have different end groups with various polarities
and wettabilities. Each time, as a reference, laser ablation
was concurrently carried out on a –CH3 terminated SAM
substrate in a configuration shown in Fig. 1共b兲. Results arelisted in Table I. Interestingly, we found that Si nanoparticles
only adsorb on a –CH3 terminated SAM while Si particles
do not absorb on –NH2 , –F, –OH, and –COOH terminatedSAMs. Since Si nanoparticles did not adsorb on SAMs withTABLE I. Adsorption characteristics of various functionalized SAMs
against Si nanoparticles listed with the polarity and wettability properties ofthe end group of SAMs. Functionalized SAMs were fabricated from
HS共CH2兲8OH, HS共CH2兲10COOH, HS共CH2兲8NH2, and HS共CF2兲7共NH2兲2CF3.
End group

Si particle ?

Polarity

Hydro-–CF3–CH3–NH2–OH–COOH

⫻䊉
⫻
⫻
⫻

–0
⫹
––

PhobicPhobicPhilicPhilicPhilic693

FIG. 2. Selective adsorption of Si nanoparticles on a functionalized SAM/Siand Si. 共a兲 An AFM image of an oxidized Si surface and 共b兲 an AFM imageof an –NH2 terminated SAM fabricated on a Si substrate, both taken afterlaser ablation. 共c兲 Cross section of AB showing adsorption of Si nanoparticles. 共d兲 Cross section of CD, no Si nanoparticles adsorbed. The scale bars
in 共c兲 and 共d兲 correspond to 4 nm.

both negative and positive charged end groups, while didadsorb on a –CH3 terminated SAM that has a neutral polarity, we exclude electrostatic interaction as the driving force
of adsorption. Another possibility is that hydrophobic Si
nanoparticles might feel an attractive force to the hydrophobic –CH3 terminated SAM provided an ultrathin water film
exists on the surface. However, we rule out this possibility
since 共1兲 no Si nanoparticles adsorbed on the most hydrophobic –F terminated SAM, and 共2兲 no increase of Si nanoparticle adsorption was observed on a –CH3 terminated SAM
intentionally exposed to water ambient before laser ablation.
Our results indicate that neither electrostatic interaction nor
different wettabilities of the SAM surfaces are responsible
for the selective adsorption characteristics of Si nanoparticles
on a functionalized SAM. More investigations are required
to sort out the mechanism.
We confirmed that the length of the alkyl chain has nothing to do with the selective adsorption of Si nanoparticles.
The Si nanoparticles did adsorb on –CH3 terminated SAMs
that have different lengths of alkyl chains 共pentanethiol, hexanethiol, and octanethiol兲, while Si particles did not adsorb
on SAMs terminated with other end groups regardless thelengths of alkyl chains 共Table I兲. This result strongly implies
that the end group solely determines the selective adsorption
characteristic. To explore this aspect further, we turned to thetechnologically more important Si substrate, and sought
whether selective adsorption could be achieved there. As Si
nanoparticles easily and firmly adhere to the native oxide
layer on Si, here we focused on the screening property offunctionalized SAMs. We carried out laser ablation on an–NH2 terminated SAM substrate 关Fig. 2共b兲兴 and on anotheroxidized Si substrate 关Fig. 2共a兲兴, concurrently. Both surfaces
were atomically flat before ablation. After ablation, photoluminescence measurements showed a spectrum similar to that

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694

Hata et al.

Appl. Phys. Lett., Vol. 79, No. 5, 30 July 2001

FIG. 3. Patterning of Si nanoparticles. 共a兲 and 共b兲 AFM images demonstrating a successful patterning of Si nanoparticles. Patterned SAMs were fabricated using patterned polydimethylsiloxane 共PDMS兲 stamps that were madeby the method described in Ref. 8. Si wafers were cleaned in a
H2SO4 :HNO4⫽4:1 solution for 30 min. A PDMS stamp inked with
3-共aminoethylamino兲propyltrimethoxysilane 共solvent hexane兲 was brought
into contact with cleaned Si substrates for 30 min with a mass of ⬃0.5 kg on
top of the stamp-substrate system under N2 atmosphere. The patterned
SAM–Si substrate was characterized by AFM prior to laser ablation.

observed for Si nanoparticles on –CH3 terminated SAMs.
AFM images and their cross sections 关Figs. 2共c兲 and 2共d兲兴
show protrusions on the oxidized Si substrate that did notexist before laser ablation, and we attribute them to Si nanoparticles. On the other hand, almost no Si nanoparticles wereobserved on an –NH2 terminated SAM, and the surface remained atomically flat as before ablation. These results
strongly indicate that the selective adsorption characteristic
is determined solely by the terminating end group of a SAM,a point that would be of extreme importance for any future
practical use and that highlights the generality of our approach.
We utilized this screening ability of functionalized
SAMs to pattern Si nanoparticles onto desired locations. Thekey point here is to use the ␮CP method to fabricate a pattered SAM on an oxidized Si substrate, whereby Si nanoparticles selectively adhere to the bare oxidized Si regions while
they are screened at regions covered with a functionalized

SAM. Figure 3 shows AFM images 共after laser ablation兲 of a
Si substrate on which a patterned –NH2 terminated SAM
was fabricated by the ␮CP method.6,7 Si nanoparticles solelyadsorbed on the oxidized Si regions and not on regions covered with a SAM, a result that demonstrates a successful
patterning of Si nanoparticles.
In conclusion, we found that Si nanoparticles selectivelyadsorb on functionalized SAMs. The selective adsorption
characteristic is determined solely by the end group of aSAM, thus a similar selective adsorption should be obtainable on many different substrates. Significantly, this selectiveadsorption is achieved in the dry ambient during laser ablation, and we believe that the wide range of available SAMs
combined with the wide variety of nanostructures possible tofabricate by laser ablation would open up an opportunity toassemble these nanomaterials into high functional complexities of the next level.
The support of a Grant-in-Aid for Scientific Researchfrom the Ministry of Education, Science and Culture of Japan is acknowledged.
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