10842

J. Phys. Chem. B 2001, 105, 10842-10846

Self-Assembled Monolayer as a Template to Deposit Silicon Nanoparticles Fabricated by
Laser Ablation
K. Hata,*,†,‡ S. Yoshida, M. Fujita, S. Yasuda, T. Makimura, K. Murakami, and H. Shigekawa§
Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST),
UniVersity of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8573, Japan
ReceiVed: February 28, 2001; In Final Form: June 17, 2001

We demonstrate a use of self-assembled monolayer (SAM) as a template to deposit in-situ Si nanoparticles
fabricated by laser ablation. Scanning tunneling microscopy, atomic force microscopy observations, andphotoluminescence measurements show that the Si nanoparticles deposited in situ on SAMs are round shaped,
firmly attached to the surface, and remain stable for at least a couple of months. Control over the average
size of the Si nanoparticles could be achieved, in the region where quantum confinement effect is important,by changing the Ar ambient pressure. Our results show that SAM endures the fierce heat, ions, and plasma
generated during the laser ablation process, and the use of SAM could be extended as a substrate to deposit
in-situ materials fabricated by laser ablation. In-situ deposition is important because it would facilitate fabricationof high functional nanoarchitectures based on this easily oxidized material. We believe that the wide rangeof available SAMs, different in chemical and electronic functionality, combined with the wide variety ofnanostructures possible to fabricate by laser ablation, would open up a new opportunity to assemble thesenanomaterials into high functional complexities of the next level.

1. Introduction
The self-assembled monolayer (SAM) stands as a unique andpromising building block to fabricate rational nanodevices. One
route to fabricate highly functional nanoarchitectures by SAM
is to use SAM as a template to deposit and immobilize functional
nanostructures or molecules rather than exploring the use of
SAM itself. Even though this direction has not been explored
so intensely so far, the ease of control of the chemicalfunctionality of the surface of SAM and the ability to pattern
SAM by the microcontact printing method1 might prove
important in the future. Some examples of nanostructures
immobilized and templated on SAM include: Au nanoparticles,2,3 carbon nanotubes,4 polymers,5 and organic molecules.6
Here we introduce a new entry to this list, Si nanoparticles.
Nanoparticles readily exhibit quantum phenomena and are
hopefully the ideal candidate as building blocks for the next
generation devices.7 Silicon nanoparticle is a promising candidate because (1) it shows visible photoluminescence8 that is
related to a quantum confinement effect, (2) the wavelength ofphotoluminescence is tunable,9 and (3) the reason an indirect
band gap material as Si shows photoluminescence with highquantum efficiency when formed into a nanostructure is notclearly understood, a fact that poses a challenge to basic physics
and chemistry.10 The most simple and prevailed method tofabricate Si nanoparticles is to etch Si wafers in a solution ofhydrofluoric acid, a procedure that produces porous Si.8
Basically, colloidal Si nanoparticles can be produced from thisporous Si and transferred to another template,11 though the sizeand surface states of these particles are highly heterogeneous.
* Author to whom correspondence should be addressed.
† E-mail: khata@cmliris.harvard.edu.
‡ Present address: Department of Chemistry and Chemical Biology,
Harvard University, Cambridge, MA 02138.
§ E-mail: hidemi@ims.tsukuba.ac.jp, http://dora.ims.tsukuba.ac.jp.

This aspect is critically undesirable because these two factors
mainly determine the optical and electronic properties of aparticle.10
Laser ablation is an alternative method that has the ability tofabricate pure, well characterized, and relatively uniform Si
nanoparticles.12-15 The laser ablation method possesses its own
unique strengths, including (1) control over the size of thenanoparticle,13,14 (2) wide variety of nanostructures possible tofabricate, including, fluerene,16 carbon nanotubes,17 nanowires,18
and Si nanoparticles,12-15 and (3) the ability to fabricate
nanostructures with impurities embedded.19,20 The last point
might become important in future aspects of nanoparticle
engineering and is difficult to achieve by colloidal synthesis.
On the other hand, heat, plasma, and ions generated during
the laser ablation process are harmful and might prevent theuse of soft organic materials as a deposition substrate. Actually,in the past, most of the previous studies have used robustmaterials such as oxidized Si layers for a deposition substrate.
Here we demonstrate that it is possible to deposit in situ Si
nanoparticles fabricated by laser ablation on SAM substrates.
Our results demonstrate that the use of SAM as a template could
be extended even into the very fierce environment of laser
ablation. Achievement of in situ deposition would be very
important in future applications since sequential processes could
be carried out in a vacuum after deposition without exposing
these easily oxidized material to air. We believe that the wide
range of available SAMs, different in chemical and electronic
functionality, combined with the wide variety of nanostructures
possible to fabricate by laser ablation would open up a newopportunity to assemble these nanomaterials into new nanoarchitectures.
2. Experimental Section
2.1. Materials. Epitaxial Au(111) surfaces (50 nm thick) wereprepared by resistive evaporation at a rate of 0.06 nm/s onto

10.1021/jp010760b CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/12/2001

Deposition of Si Nanoparticles Fabricated by Laser Ablation

J. Phys. Chem. B, Vol. 105, No. 44, 2001 10843

Figure 1. A schematic of the laser ablation chamber.

Figure 2. A typical STM image of the SAM (hexanethiol) substrate
exposed to ablation. This image was taken approximately 1 cm fromthe ablation spot. White protrusions observed are Si nanoparticles
deposited on SAM. Ablation conditions: Ar ambient pressure 5 Torr,
100 shots. Tunneling conditions: surface bias ) 500 mV, tunnelingcurrent ) 500 pA. No eminent dependence on tunneling conditions
was observed.

freshly cleaved mica heated to 500 °C. The vacuum pressureduring the evaporation was 6 × 10-6 Torr. Before use, Au(111)
surfaces were cleaned by hydrogen flame annealing. A terraceand stepped surface with grain sizes of 100-200 nm wasobserved by scanning tunneling microscope (STM) and theexistence of the herringbone structure was confirmed in somehigh-resolution images. SAM was prepared on these Au(111)substrates by adsorption of hexanethiol (Wako, used as received)from millimolar solutions in ethanol at room temperature for
12 to 24 h. All monolayes were rinsed with ethanol after
adsorption and dried under a flow of N2. All the resulting
samples were characterized using a scanning probe microscope(Nanoscope III system, Digital Instruments), and a tapping mode
noncontact atomic force microscope (AFM) (SPA 300HV
system (Seiko)).
2.2. Laser Ablation. A schematic of the ablation chamberis shown in Figure 1. A part of a Si wafer was placed close to
the focal point of the laser beam. The SAM substrate was placedon the same plane of the target wafer and the typical distancefrom the ablation spot was approximately 5 mm. The chamber
was evacuated to 5 × 10-7 Torr, and then filled with Ar gas(purity 99.9999%), with a pressure ranging from 2 to 10 Torr.
A predetermined number of YAG pulsed laser shots (10 ns, 5
J/cm2) were focused and irradiated to the target Si wafer at
frequencies of 1 or 10 Hz. Si nanoparticles formed by collisionof the hot Si atoms in the plasma with the inert gas are supposedto land softly on the SAM.
3. Results and Discussion
Figure 2 shows a typical STM image of the surface of SAM
exposed to laser ablation. Before ablation, the surface is covered
with numerous etch-pits. Their density and size are similar to

Figure 3. A set of STM images of the same SAM (hexanethiol)
substrate exposed to ablation taken at different distances from theablation shot. Ablation conditions: Ar ambient pressure 5 Torr, 100
shots. (e) Schematic showing the distance of each image from theablation spot. Histogram schematically shows the density of the Si
nanoparticles versus distance from the ablation spot.

previous reports,21 which we consider as an evidence of a
successful formation of SAM. After ablation, many brightprotrusions are observed that have a size of 5 to 10 nm whenthe Ar ambient is 5 Torr. On the other hand, the heights of the
protrusions in the STM images were around 0.4 nm, a value
substantially smaller than their lateral size,22 a point that would
be discussed later. The density of the protrusions depends
strongly on the distance from the ablation spot. This aspect isdemonstrated by a set of STM images in Figure 3 taken on thesame sample at different distances from the ablation spot. Veryclose to the ablation spot (Figure 3a) only a very small numberof protrusions were observed, and also the density of the etchpits had significantly decreased (close region). At a distance of
6 mm from the ablation spot (Figure 3b), a small number ofprotrusions were observed which density increases with distance.
The density of the protrusions maximize at a distance approximately 1 cm from the ablation spot (Figure 3c), and thendecreases with distance (middle region). At 2 cm from theablation spot (Figure 3d) no protrusions were observed, and asurface was clean SAM (far region). When the fact that almost
no SAM and protrusions were observed very close to theablation spot and the fierce circumstance during the ablation
process (heat, ions, and plasma) are considered, it is possiblethat SAM, particularly those close to the ablation spot, isdemolished or rearranged and observed as protrusions in the

10844 J. Phys. Chem. B, Vol. 105, No. 44, 2001

Hata et al.

Figure 4. (a) Case 1: white protrusions are Si nanoparticles depositedon SAM. (b) Case 2: protrusions are SAM islands on SAM. (c) Case
3: Protrusions are SAM islands on Au. (d) Case 4: Protrusions are Si
nanoparticles on Au.

Figure 5. (a) An STM image of a SAM (hexanethiol) substrate exposed
to ablation before ethanol rinse. (b) An STM image of the same sample
after ethanol rinse.

STM images. Figure 4 shows schematics of the possible casesthat must be considered. Case 1: SAM endures the ablation
process and the protrusion observed are indeed Si nanoparticles
deposited on SAM (Si nanoparticles on SAM). Case 2:
Protrusions observed are fragments of SAM blown away fromsome part of the SAM (probably closer to the ablation spot)
during the ablation process (SAM islands on SAM - noparticles). Case 3: SAM is partly blown way (SAM island on
Au - no particles). Case 4: SAM is completely blown away
and the protrusions are Si nanoparticles deposited on Au(particles on Au). Several tests based on known characteristicsof SAM were carried out to eliminate case 2 to 4, and to verify
the successful deposition of Si nanoparticles on SAM.
3.1. Case 2: SAM Islands on SAM. It is possible that SAM
close to the ablation spot is blown away during the laser ablation
process and redeposit onto undamaged SAM farther from theablation spot. From this standpoint, the observed dependenceof the density of the protrusions in the STM images with
distance from the ablation spot is readily explained. No SAM
was observed very close to the ablation spot because they were
blown away. They redeposit at locations farther and rearrange
and are observed as protrusions in the STM images. Regions
far away remains untouched, hence a clean SAM is observed
there. We eliminate this possibility by considering the physicalproperties of bilayer SAM. SAM only weakly physisorbs on
SAM and is easily washed away by rinsing in ethanol. Actually,
ethanol rinsing is used at the final stage of SAM preparation toremove any possible SAM on SAM. Regarding this point, werinsed a SAM substrate after laser ablation in ethanol andobserved the middle region by STM. STM images of the same
sample before and after rinse are displayed in Figure 5.
Immediately, it is it clear that the protrusions survive ethanol
rinse. No obvious decrease in size of the protrusions was noticed,
though their density might have slightly decreased. From thisexperimental result, we exclude case 2, thus the protrusions are
not SAM islands on SAM. Also the result shows that the
protrusions are firmly adsorbed to the SAM substrates, a featurethat would be preferred in any practical use.

Figure 6. (a) An STM image of SAM islands fabricated by the vapor
method. (b) Enlarged STM image. (c) STM image taken 1 day after.
(d) STM image of a SAM (hexanethiol) substrate taken two months
after exposure to ablation

3.2. Case 3: SAM Islands. Another possibility is that SAM
in the middle region is partly damaged and the protrusions are
SAM islands on Au. In this scenario, (1) SAM close to theablation spot is totally blown away, (2) SAM in the middle
region is partly blown away, and (3) the remaining SAM islands
are observed as protrusions. SAM in the far regions is notdamaged. No Si nanoparticles exist on the surface. We exclude
this possibility by fabricating real SAM islands and by comparing the characteristics of SAM islands with the protrusions. SAM
islands can be easily made by exposing an Au substrate to vaporof hexanethiol. Typical STM images of SAM islands fabricatedby the vapor method are shown in Figure 6a and Figure 6b.
Even though the size of the SAM islands is on the same orderof that of the protrusions, their shapes differ significantly; theprotrusions are round shaped while the shape of the SAM islands
resembles triangles. Another, and critical difference between
the SAM island and protrusion is their stabilities. SAM islands
are very unstable and desorb easily leaving a clean Au surface,
while the protrusions are very stable. This aspect is demonstrated
in Figure 6c, an STM image of a substrate where the SAM
islands were observed one day before, and Figure 6d, an STM
image of the protrusions taken two months after laser ablation.
While the SAM islands dissipate in a day, the protrusions remain
even for two months. No obvious decrease in size of the
protrusions with time was noticed, though their density gradually
decreased. From this experimental result, we exclude case 3,
thus the protrusions are not SAM islands on Au. Also the resultshows that the protrusions adsorbed to the SAM substrates arestable, a point that would be important in practical use.
3.3. Case 4: Si Nanoparticles on Au. The last possibility
is that the SAM is completely blow away and the protrusions
are Si nanoparticles deposited on Au. We exclude this possibility
by listing up some evidence that shows or indicates existenceof SAM on the surface. First, and most directly, in some mosthigh-resolution STM images, the atomic structure of SAM wasobserved. Second, the density, shape, and size of the etch-pits
on the substrate exposed to ablation in the middle region (atwhere the protrusions are observed) were the same with a clean
SAM. In situations where the SAM is supposed not to exist,
the density of etch-pits has significantly decreased (Figure 3a,Deposition of Si Nanoparticles Fabricated by Laser Ablation
an image taken close to the ablation spot) or no etch-pits wereobserved (Figure 6c, an image of a substrate where SAM islands
were observed one day before). We thus consider the existenceof the etch-pit as an indirect evidence of the presence of SAM.
From these experimental result, we exclude case 4, thus the
protrusions are not Si nanoparticles on Au.
3.4. Atomic Force Microscopy Observations. The experimental results presented so far are easily explained if we
consider the protrusion as Si nanoparticles, though are difficult
to explain in other cases. The problem of the STM observations
is that the height of the protrusion is measured around 0.4 nm,
a value that is substantially smaller than the lateral size. It should
be noted that the height of the protrusions did not change on
SAMs that have different lengths of alkyl chain (SAM madeof pentanethiol, hexanethiol, and octanethiol), a result that gives
another evidence that the protrusions are not composed from
SAM. As STM measures the electronic structure, the height ofa feature in STM does not necessarily correspond to their real
topographic height. This is particularly true for organic materials
or semiconductor nanoparticles, and the generally tendency isthat the height measured by STM is smaller than the real height.
To address this problem, we turned to atomic force microscopy(AFM). The lateral feature resolution obtained by AFM isdetermined in large part by the size and shape of the imaging
probe tip, though the measured height of a feature reflects theactual height fairly well. A typical noncontact AFM image(Figure 7a) of the protrusions and their cross section (Figure7b) shows that the height of the protrusions are around 7 nm,
a value substantially larger than the height measured by STM.
Taking into considerations these points, we observed the same
SAM substrate exposed to laser ablation by STM and AFM,
from which the lateral size histogram (Figure 7c) of the
protrusions was constructed from the STM images, and theheight histogram (Figure 7d) was made from the AFM data.
Immediately, it is clear that the mean lateral size and height ofthe protrusions are very similar, implying that the shape of the
protrusions is close to a sphere.22 The height of the protrusions
measured by AFM gives another strong evidence that the
protrusions are not composed from SAM. In addition, aphotoluminescence measurement of these particles presented in
Figure 8 shows an emission peak centered at 623 nm, a valuethat is in the range of the characteristic emission wavelengthsof Si nanoparticles. This result implies that Si nanoparticles exist
on the surface. Summarizing all of the experimental results anddiscussions, we conclude that the protrusions are Si nanoparticles
on SAM.
3.5. Control of Size of Si Nanoparticles. We demonstratethat it is possible to control the average size of nanoparticles
deposited on SAM. Size of the nanoparticle is the mostimportant factor that determines the degree of the quantum
confinement effect, and consequently, the electronic and opticalproperties of the nanoparticles. Control over the size of Si
nanoparticles deposited on Si substrates has been achieved bya simple method of changing the ambient pressure.13,14 To
demonstrate this aspect of nanoparticle engineering, we fabricated Si nanoparticles in different Ar gas ambient pressures anddeposited them on SAM. A substantial increase of the size ofthe nanoparticles fabricated at 10 Torr (Figure 9b) was observed
compared to those fabricated at 2 Torr (Figure 9a), showingthat it is possible to control the size of the Si nanoparticles
deposited on SAM, a point that is of extreme importance tofabricate layers of Si nanoparticles with artificially controlledcharacteristics.

J. Phys. Chem. B, Vol. 105, No. 44, 2001 10845

Figure 7. (a) A NC-AFM image of a SAM (hexanethiol) substrate
exposed to ablation. (b) Cross section of Si nanoparticles. (c) Histogramof widths of Si nanoparticles measured from STM images. (d)Histogram of heights of Si nanoparticles measured from AFM images.
Both histograms were constructed from observations carried out on
the same sample.

Figure 8. Photoluminescence of the Si nanoparticle deposited on SAM
excited at 325 nm.

4. Conclusion
We report that SAM endures fierce conditions during the laser
ablation process, and that it is possible to deposit in-situ Si
nanoparticles fabricated by laser ablation on SAM. Si nanoparticles deposited on SAM show several promising characteristics that would be appreciated in any future nanodevice
applications. They were round shaped, firmly attached to thesurface, and remained stable for at least a couple of months.

10846 J. Phys. Chem. B, Vol. 105, No. 44, 2001

Hata et al.
adsorption of Si nanoparticles through which deposition of
nanoparticles on desired locations could be achieved.23
Acknowledgment. The support of a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science
and Culture of Japan is also acknowledged.
References and Notes

Figure 9. Control over the average size of the Si nanoparticles. STM
images of SAM (hexanethiol) substrates exposed to ablation withdifferent ambient pressures. (a) 2 Torr. (b) 10 Torr.

Control over the size of the Si nanoparticles, in the region where
quantum confinement effect is important, could be easily
achieved by changing the Ar ambient pressure. This encouraging
result suggests the possibility of fabricating thin layers of Si
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In-situ deposition is important since subsequent processes
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exposing these easily oxidized materials to air. When oxidized,
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devices. The oxidized layer might also determine the physicalproperties of the nanoparticle, hindering the ability to artificiallycontrol the properties of nanoparticle by changing their size.
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