Optical properties of femtosecond laser-synthesized Silicon nanoparticles in deionized water

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Published: February 08, 2011
r2011 American Chemical Society
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pubs.acs.org/JPCC
Optical Properties of Femtosecond Laser-Synthesized Silicon
Nanoparticles in Deionized Water
R. Intartaglia,*,†K. Bagga,†F. Brandi,†G. Das,†A. Genovese,†E. Di Fabrizio,†,‡and A. Diaspro†
†Italian Institute of Technology (IIT), via Morego 30, 16152 Genoa, Italy
‡BIONEM lab, University Magna Graecia, Viale Europa Catanzaro, Italy
ABSTRACT: Silicon nanoparticles were prepared by ultrafast
laser ablation of a silicon target in deionized water. The
nanoparticles were characterized by using optical absorption,
Raman spectroscopy, and transmission electron microscopy.
Themeansizeisfoundtovaryfrom60to2.5nmintheabsence
of any reducing chemical reagents, decreasing the pulse energy
value. High-resolution transmission electron microscopy to-
gether with Raman spectroscopy confirms the crystalline
structure of the generated silicon nanoparticles. The energy
confinement of carriers which is evaluated from optical experi-
mentsvariesfrom90to550meVwhenthemeannanoparticles
size decreases from 60 to 2.5 nm. In particular, the evaluated nanoparticle sizes from optical analysis and the LCAO theoretical
modelarefoundinagreementwithtransmissionelectronmicroscopyandRamanmeasurementsforthesiliconnanoparticleswitha
size less than 6 nm. Finally, we present stability studies which show that the smallest nanoparticles aggregate over time.
1. INTRODUCTION
Silicon nanoparticles (Si-NPs) present a growing interest due
to their particular size-dependent optical properties leading to
important applications such as light-emitting devices,1energy
source,2andinbiomedicine.3-7Theintegrationofultrathinfilms
of Si-NPs on silicon solar cells is found to enhance the power
performance of polycrystalline cells by 60% in the UV range.
Recently, Si-NPs have been shown to be able to generate singlet
oxygen under irradiation, making them promising candidates for
photodynamic therapy. A variety of chemical8-11and physical12-17
methods havebeenemployedtoprepareSi-NPs. Among them, wet
chemistry routes are attractive because particle size and surface
properties can be controlled simultaneously. Nevertheless, the
obtained nanoparticles are contaminated with the residual byprod-
ucts such as ions and reducing agents, which is not suitable for
biological application of nanoparticles. Laser ablation synthesis in
liquid environment provides the advantage to reduce the risk of
contamination.Biocompatibilityimprovementofthelaser-produced
nanoparticles is predicted due to their restricted surface contamina-
tionsincethesynthesiscanbecarriedoutinwaterorinsolutionofa
biocompatible ligand, which is a key to the subsequent successful
functionalization of the nanoparticle surface.18
There are many irradiation parameters which should be taken
into account for controlling the size and shape of nanoparticles.
Someoftheseparametersincludelaserwavelength,pulseenergy,
pulse duration, repetition rate, and liquid environments.19-22In
particular, the laser pulse duration is found to affect directly the
ablation, nucleation, growth, and aggregation mechanisms. Long
laser pulses (nanosecond) release energy slowly on a time scale
comparable to the thermal relaxation processes of the target,
while femtosecond laser pulses release energy to electrons in the
target on a time scale much faster than electron-phonon
thermalization processes. Local heating on the target in this
waycanbereducedinthecaseoffemtosecondpulses.Moreover,
temporal overlap between laser pulse duration and the time of
material evaporation induces thermodynamic instability of the
plasma during this expansion.23Consequently, some differences
are observed in the generated nanoparticles produced by means
of laser with different pulse durations.22Only few works have
been reported on generation of Si-NPs in liquid environment.
Nanosecond laser ablation of silicon shows generation of nano-
particles which stabilize into clusters due to the agglomeration
effect;i.e.,Si-NPsareheldtogetherbyanirregularnetwork.24-26
Strong agglomeration of the produced nanoparticles is a major
barrier to most of the applications requiring nonagglomerated
substrate-free nanoparticles. Recently, isolated Si-NPs have also
been produced using a UV femtosecond pulse laser source.27
The present paper focuses on the infrared femtosecond laser
ablation of silicon in deionized water aiming to clarify the
possibility to get isolated Si-NPs with controllable sizes. We
report the production of the Si-NPs with a mean size ranging
from 60 to 2.5 nm in the absence of any reducing chemical
reagents, decreasing the pulse energy value. High-resolution
transmission electron microscopy (HR-TEM) together with
Raman spectroscopy confirm the crystalline structure of the
generated Si-NPs. The energy confinement of carriers which is
Special Issue: Laser Ablation and Nanoparticle Generation in Liquids
Received:
Revised:
September 29, 2010
December 14, 2010

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evaluated from optical experiments varies from 90 to 550 meV
when the mean NP size decreases from 60 to 2.5 nm. In
particular, the evaluated NP sizes from optical analysis and
LCAO theoretical model are found in agreement with TEM
and Raman measurements for the Si-NPs with a size less than 6
nm. Moreover, stability studies show that the smallest nanopar-
ticles aggregate over time.
2. EXPERIMENTAL SECTION
Laser ablation experiments were carried out using a Ti:
sapphire laser system providing pulses centered at 800 nm, with
a maximum energy of 1 mJ/pulse at a repetition rate of 1 kHz.
Thelaser pulsesweregeneratedbyaMicraTi:sapphireoscillator
(500 mW, 80 MHz) followed by a regenerative amplifier and a
compressor (Coherent’s Legend Elite). The amplified laser
pulses have a pulse duration of 100 fs. The target material
(99.999 % Si from Alpha Aesar) in the form of a cylinder with
diameter of 6 mm and thickness of 10 mm was placed on the
bottomofaquartzcuvette(dimension10?10?100mm).The
cuvette was filled with deionized water (1 mL) which corre-
sponds to 1 cm of liquid above the surface of the target. The
target was mechanically polished, and then it was washed by
deionizedwaterseveraltimestoremoveimpuritiesfromthesurface.
The laser beam was focused below the target surface using a short
focal length lens (10 cm) and kept fixed on one point of the target
during all ablation. The beam diameter of 8 mm and the distance
betweentargetsurfaceandfocalplane,5mm,giveusaestimationof
thespotsize,500μm.Pulseenergiesfrom0.05to0.5mJwereused,
controlled by a set of fixed and variable attenuators. Ablation was
carriedoutfor60min,andtheformationofNPscouldbeestimated
by the slight change of the color of water during ablation (only at
high pulse energy). All characterization measurements were per-
formed 1 day after preparation of the colloidal solution.
Transmission electron microscopy (TEM) images were ac-
quired on a JEOL Jem 1011 microscope working at an accelera-
tion voltage of 100KeV. High resolution and scanning transmis-
sion electron microscopy (HRTEM and STEM, respectively)
analyses were performed on a JEOL Jem 2200FS microscope
equipped with a field emission electron gun working at 200 KV,
and with a JED-2300 Energy Dispersive X-ray Spectrometer
(EDS) and spherical aberration corrector system (Cs-corrector)
for objective lens. The Cs-corrector improves the spatial resolu-
tion of the optical system that reaches the detectable spatial
frequency of 0.9 Å; instead, the Z-contrast STEM measurements
were acquired recording the images with a high angle annular
dark field detector (HAADF) with a camera length of 50 cm.
TEM/STEM samples were prepared by dropping colloidal
solution directly onto a carbon-coated 300 mesh copper grids
and allowing the solution to evaporate.
Microprobe Raman measurements (Jobin Yvon, model:
LabRam) were excited by a 632 nm He-Ne laser (power = 3
mW and accumulation time = 20 s) in backscattering configura-
tion with spectral resolution of about 1.1 cm-1. The experi-
mental setup consists of a grating with 600 lines/mm. The
sample was deposited by a drop coating deposition (DCD)
technique,inwhichthesubstancewasdroppedoverthesubstrate
and waited for evaporation of excess liquid. This leads to the
formation of a coffee ring. To be noted, the material property of
the sample in liquid and after the evaporation of excess liquid
remains the same.28Thereafter, various measurements were
performed at different positions of the coffee ring.
Optical absorption spectra were recorded in quartz cuvette
(10 mm, Helma), using a Cary 6000 UV-VIS-NIR double beam
spectrophotometer. The scan range was 200-1000 nm with a
600 nm/min rate. Absorption spectra are corrected for water
absorption, by subtracting the contribution of water from the
recorded spectrum.
3. RESULTS
3.1.OpticalMeasurements. Si-NPsolutions were produced
by laser ablation in deionized water of a solid target, using
femtosecond laser pulses emitting at 800 nm, by varying pulse
energy from 0.09 to 0.40 mJ. Figure 1 shows UV-visible
absorption measurements of obtained solution. All absorption
spectraappeartohaveabroadcontinuousbandbetween200and
800nmandadistinctiveshoulderwithaminimumabsorbanceat
around 400 nm, except for the solution obtained at lower pulse
energy. For the clarity of the figure, the absorption spectra were
normalized to 1 at the shoulder maximum peak value. When the
pulse energy value decreases, the spectra are characterized by a
blue-shift of the position of the shoulder peak together with an
increase ofthe relative absorption intensity intheUVrange. The
position of the shoulder peak shifts from 480 to 440 nm. The
shifting of the absorption edge is ascribed to the change in
nanoparticle size which is an effect of quantum confinement
(estimated in the next section).29The bandgap energy of Si-NPs
increases (i.e., decrease of the wavelength) with reduction of
their size. The UV absorption band is attributedto Si-NPs witha
size varying from 1 to 3 nm, similar to the published research
work on Si-NPs in the past.8,30
3.2. Transmission Electron Microscopy. Detailed informa-
tion about the size and size distribution of the produced
Si-NPsinthesolutionwasobtainedbyTEMandSTEManalyses.
Figure 2a, Figure 2b and Figure 2c show, respectively, one TEM
and two STEM images of the particles in the colloidal solution
obtainedbyfemtosecondlaserablationatdifferentpulseenergies.
WecanobserveisolatedSi-NPs(i.e.,nonagglomeratedsubstrate-
freeNPs)withapseudosphericalmorphologyandsmoothsurface.
To be noted, SiOxamorphous material essentially observed at
higher pulse energy probably is formed during the laser ablation
process (Figure 2a). Since temperature is locally high (around
Figure 1. Absorption spectra of the Si-NPs produced via femtosecond
laser ablation of a silicon target in deionized water.

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5000K)duringthelaser ablation process,itisnotsurprisingthat
silicon oxidation occurs when silicon and water come into
contact, as observed by Sverck et al.24Nevertheless, contrary
to nanosecond laser ablation of the silicon target,24the SiOx
amorphous structure is found well separated from the Si-NPs
(inset of Figure 2a) and may be responsible for NPs stability, as
discussed in the next section.
The mean and distribution sizes of particles obtained by
counting more than 300 particles in the electron microscopy
images are shown below each corresponding microphotograph.
The hypothesis on the energy dependence of the NP size is
confirmed andrevealedadrastic particle sizereduction whenthe
pulse energy decreases. The mean particle size drops from 60 to
2.5 nm as pulse energy decreases from 0.40 to 0.16 mJ.
3.3. HR-TEM and Raman Spectroscopy. The crystallinity of
the synthesized Si-NPs via laser ablation was further investigated
by high-resolution transmission electron microscopy (HR-TEM
image) and Raman spectroscopy. Figure 3 shows the HR-TEM
of one isolated Si-NP distributed onto the carbon film. We can
Figure 2. TEM and STEM analyses of the obtained solution by femtosecond laser ablation using different pulse energies: (a) 0.40 mJ TEM image and
(b) 0.27 mJ and (c) 0.16 mJ STEM images.
Figure 3. HRTEM image of a single synthesized Si-NP showing the
(111) lattice sets and its corresponding numerical electron diffraction
pattern (inset).
Figure 4. Raman scattering spectra of the Si-NPs synthesized via laser
ablation in deionized water at a pulse energy of 0.27 mJ.

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observe the crystalline structure of NPs that displays the (111)
latticesetswithaninterplanaryspacingof3.12Å,characteristicof
Si bulk. Raman spectroscopy was performed to get an analyzed
volume of the NP solution considerably higher than the one
analyzed by TEM (Figure 4). The Raman spectrum shows a
sharp band centered at around 520 cm-1with the asymmetricity
toward the lower frequency side. Asymmetricity in the Si Raman
band in Figure 4 confirms the presence of Si-NPs within the
sample.31,32This Raman shift is attributed to the quantum
confinement of electronic wave function in Si-NPs and can
provide the characteristic dimensions of nanocrystalline struc-
tures. The Si-NP size and the dispersion in size distribution were
estimated by deconvoluting the Raman spectrum using three
distribution functions: Lorentzian distribution, Phonon confine-
ment model, proposed by Ritcher et al.33and modified by De
Santos et al.,34and the Gaussian distribution function. These
three independent peaks are: (i) Si sharp peak in the vicinity of
520 cm-1(bulk Si having crystalline size >9 nm); (ii) an inter-
mediate peak between 510 and 520 cm-1originating from
relatively small Si crystallites (i.e., Si-NPs), estimated after fitting
the peak using phonon confinement model; and (iii) an amor-
phous Si peak at approximately 480 cm-1.35Si-NP size is found
to be around 41 A with the size dispersion of 1 A, in agreement
with the STEM image (Figure 2b). The Raman peak in the
vicinity of 520 cm-1indicates the presence of crystalline Si with
the grain size higher than 9 nm. Nevertheless, the STEM images
and the histograms originating from their analysis shown in
Figure 2 demonstrate that the vastmajorityof the producedNPs
have very small average sizes (∼ 4 nm) but that a small
percentage of larger NPs are always present due to the particu-
larity of the laser synthesis.
3.4. Optical Analysis. Si-NPs generated so far by laser
ablation (using long pulse) or with other methods were con-
firmed to be indirect band gap semiconductors.25,36The data in
Figure5havebeenplottedasR(ω)1/2asafunctionofthephoton
energy pω. In such plots, linear behavior is expected for
semiconductorswithanindirectband gap, forwhichoneexpects
anenergydependenceofthefundamentalabsorptionasgivenby
R(pω) ? ω-1(pω - Eg)2. As we can see from Figure 5, the
absorption spectra in such plots show linear behavior, except for
Figure5. AbsorptiondataforSi-NPssynthesizedviafemtosecondlaser
ablation of the silicon target in deionized water at different pulse
energies, plotted as R(ω)1/2vs (pω), corresponding to an indirect
semiconductor. Dashed lines, extrapolation for determination of the
electronic band gap.
Table 1. Si-NP Parameters Obtained from TEM and Absorption Spectroscopya
pulse energy (mJ)Eg(NPs) (eV)
ΔE (meV)D(NPs)TEM(nm) (σ)D(NPs)abs(nm)
0.16
0.27
0.40
1.67(0.01
1.56(0.01
1.21(0.01
550(20
440(20
90(20
2.5(1.2)
3.5(1.7)
60(0.6)
3.96( 0.1
4.65( 0.2
14.5( 3
aEg(NPs)corresponds totheexperimentalvalueofNPbandgapobtainedbyabsorptionanalysis.ΔEcorrespondstothequantumenergyconfinement
and experimental error (including error linear fit), 20 meV. D(NPs)TEMcorresponds to the NP mean size obtained by TEM analysis. σ corresponds to
thestandarddeviation.D(NPs)AbscorrespondstothecalculatedNPsizefromabsorptionmeasurements(ΔE)andempiricallaw,ΔE=πβ/(Dγ).Model
uncertainty is reported.
Figure 6. (a) Absorption spectra of Si-NPs in deionized water as
synthesized (black line), after 15 days (red line), plotted as R(ω)1/2vs
(pω).(b) TEMimageoftheSi-NPsolutionafter15days.Theagglomera-
tioneffectisclearlyobservedbytheformationofaclusterwithasizearound
50 nm, having bulk properties.

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the Si-NPs of bigger size. The observed shift can be due to the
effect of particle size distribution as observed in the TEM image
and/or SiOxamorphous absorption structure.37The band gap
energy of the obtained NP solution has been determined using
linear approximation of the previous relation, as shown in
Figure 5, and consequently the NP quantum energy confine-
ment, ΔE. To validate the accuracy of our method, we have used
the known electronic band gap of bulk silicon, Eg= (1.12 ( 0.01
eV). Therefore, we assume that the method has an experimental
inaccuracyΔEof20meV,includingthelinearfiterror.Thevalue
of the band gap energy and the quantum energy confinement
(ΔE) of the obtained NPs solution are reported in Table 1. It is
possible to estimate the Si-NP size from ΔE, using the empirical
lawEg(d)=Eg0þπβ/(Dγ).βandγdependonthenatureofthe
surface as well as size and symmetry of NPs, and D corresponds
to NP size. For the NP size estimation, we used the values (β =
3.73, γ = 1.39) which have been calculated by Delarue et al. for
relatively small spherical Si-NPs (<10 nm) using a linear
combination of atomic orbitals (LCAO).30It can be seen in
Table 1 that the NP diameter determined from the optical
experiments is in agreement with the NP diameter obtained by
TEM analysis for smaller NPs, while the diameter is under-
estimated for larger NPs. One has to take into account that the
analyzed volume of the NP solution from absorption spectros-
copyisconsiderablyhigherthantheoneofTEM.ForNPsbelow
5nm,thedifferenceinsizeisobviouslycausedbytheparticlesize
distribution, inwhich particlessubstantially larger than the mean
particle size observed in the STEM image (Figure 2b) always
contribute to the absorption, thus increasing the calculated
NP diameter. For a NP population of bigger size, several factors
can influence the accuracy of the calculated NP size determinate
from optical experiments and the LCAO theoretical model: (i)
From a theoretical model point of view (β = 3.73, γ = 1.39),
parameters have been calculated by Delerue et al., for a popula-
tion of NPs less than 10 nm and could be not available for
bigger NPs. (ii) The estimated mean size from a STEM image
analysis histogram (Figure 2b) can be misleading since it is not
possible to analyze all the TEM grid surface and consequently
all the NPs solutions; i.e., the density of NPs with a size around
20 nm could be higher than those observed in the acquired
electron microscopy photographs. (iii) Finally, it is known that
the surroundings of the NPs (Figure 2a) and, consequently, the
chemical nature of the NP surface can affect the optical proper-
ties of NP solutions, i.e., quantum confinement.38-40
4. STABILITY OF NANOPARTICLE SOLUTION: AGING
EFFECT
The aging of NP solution leading to aggregation is known to
bedirectlylinkedtothechemicalnatureoftheNPsurface.Inthis
section, we report the agglomeration effect of the Si-NPs
obtained at different pulse energy values (i.e., different NP size)
in deionized water. The Si-NP solution of bigger size (i.e.,
obtained at higher pulse energy) did not show any sign of
agglomeration within several months after the preparation. The
stability could be attributed to the presence of a surface oxide
layer41,42as observed in TEM images (Figure 2a). In contrary,
theSi-NPsproducedatlowerpulseenergy(sizelessthan10nm)
are found to be less stable, indicating a different chemical nature
of the NP surface. In Figure 6a is reported the absorption
spectra of the NP solution with size less than 4 nm, performed
just after the synthesis and after 2 weeks. The absorption edge is
found to shift to lower energy, exhibiting the features associated
to Si bulk materials. This observation is characteristic of an
agglomeration effect of the Si-NPs and can be understood in
terms of an evolution from isolated (localized) NPs to collective
electronic states delocalized within a finite number of NPs.43
TEM analyses of the aged Si-NPs solution are also reported in
Figure 6b. We can observe aggregates with a size around 50 nm
composed of nanospheres, in agreement with the optical mea-
surements.
5. CONCLUSIONS
Ultrafast laser ablation of a Si target in deionized warter was
employed to prepare size-controlled silicon nanoparticles in
solution. Contrary to nanosecond laser ablation of the silicon
target, the SiOxamorphous structure is found well separated
from the silicon nanoparticles. We obtain isolated Si-NPs (i.e.,
nonagglomerated substrate-free nanoparticles) with pseudo-
spherical morphology and smooth surface. The mean size is
found to vary from 60 to 2.5 nm in the absence of any reducing
chemical reagents, decreasing the pulse energy value. The
crystalline structure of the generated silicon nanoparticles has
been confirmed by high-resolution transmission electron micro-
scopy and raman spectroscopy. The energy confinement of
carriers evaluated from optical experiments varies from 90 to
550meVwhenthemeanNPsizedecreasesfrom60to2.5nm.In
particular,theevaluatedNPsizefromopticalanalysisandLCAO
theoretical model are found in agreement with TEM and Raman
measurements for the Si-NPs with a size less than 6 nm. Finally,
stability studies show that the smallest nanoparticles aggregate
over time.
’AUTHOR INFORMATION
Corresponding Author
*E-mail: romuald.intartaglia@iit.it.
’ACKNOWLEDGMENT
This work was supported by the Italian Institute of Technol-
ogy. The authors gratefully acknowledge the technical help of
Marco Scotto d' Abbusco.
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