Palladium-based micellar nanohybrids: Preparation and nonlinear optical response

Article abstract of DOI:10.1039/c3ra46132g

In the present work the synthesis and physicochemical characterisation of a new class of Pd-containing micellar nanohybrid systems are presented. The new systems consist of metallic Pd nanoparticles and well-defined diblock copolymers, possessing carbazole and β-ketoester side-chain functionalities, namely poly[2-(N-carbazolyl) ethyl methacrylate]-block-poly[2- (acetoacetoxy) ethyl methacrylate] (CbzEMA_x-b-AEMA_y). Block copolymer synthesis has been carried out by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerisation. The presence of the β-ketoester groups within these diblock copolymers enabled the complexation and stabilisation of the Pd nanoparticles in tetrahydrofuran. Visualisation of the Pd-containing micellar nanohybrids was realised by means of transmission electron microscopy (TEM), providing information on the metal nanoparticle sizes. The nonlinear optical response of these polymer-metal nanohybrid systems has been systematically studied both in solution and in thin films, using the Z-scan technique, employing 35 ps, visible (532 nm) and infrared (1064 nm) laser excitation. Discussion in regards to the spectral position of the surface plasmon resonance (SPR) of the Pd nanoparticles and its effect on the nonlinear optical response of the micellar systems has been carried out in terms of a two-photon process. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3ra46132g

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Page 1 of 27
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DOI: 10.1039/C3RA46132G
Palladiumꢀbased Micellar Nanohybrids: Preparation and Nonlinear
Optical Response
Irene Papagiannoulia,b, Maria Demetriouc, Theodora KrasiaꢀChristoforouc* and
Stelios Courisa,b∗
aDepartment of Physics, University of Patras, Patras 26504, GREECE
bInstitute of Chemical Engineering Sciences (ICEHT), Foundation for Research and
TechnologyꢀHellas (FORTH), P.O. Box 1414, Patras 26504, GREECE
cDepartment of Mechanical and Manufacturing Engineering, University of Cyprus,
P.O. Box 20537, 1678 Nicosia, CYPRUS
submitted for publication in RSC Advances
Revised January 2014
∗Corresponding authors eꢀmails: krasia@ucy.ac.cy, couris@iceht.forth.gr
1

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Abstract
In the present work the synthesis and physicochemical characterisation of a new class
of Pdꢀcontaining micellar nanohybrid systems are presented. The new systems consist
of metallic Pd nanoparticles and wellꢀdefined diblock copolymers, possessing
carbazole and βꢀketoester sideꢀchain functionalities namely poly[2ꢀ(Nꢀcarbazolyl)
ethyl methacrylate]ꢀblockꢀpoly[2ꢀ(acetoacetoxy) ethyl methacrylate] (CbzEMAxꢀbꢀ
AEMAy). Block copolymer synthesis has been carried out by Reversible Additionꢀ
Fragmentation chain Transfer (RAFT) polymerisation. The presence of the βꢀ
ketoester groups within these diblock copolymers enabled the complexation and
stabilisation of the Pd nanoparticles in tetrahydrofuran. Visualisation of the Pdꢀ
containing micellar nanohybrids was realised by means of transmission electron
microscopy (TEM), providing information on the metal nanoparticle sizes. The
nonlinear optical response of these polymerꢀmetal nanohybrid systems has been
systematically studied both in solution and in thin films, using the Zꢀscan technique,
employing 35 ps, visible (532 nm) and infrared (1064 nm) laser excitation. Discussion
in regards to the spectral position of the surface plasmon resonance (SPR) of the Pd
nanoparticles and its effect on the nonlinear optical response of the micellar systems
has been carried out in terms of two photon process.
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1. Introduction
Polymerꢀbased materials exhibiting interesting optical and electronic properties have
attracted significant attention due to their potential use in various applications
including organicꢀbased photovoltaics1,2, polymer light emitting diodes3, thin film
transistors and sensors4, optical switching5, optical data storage and information
processing6,7.
Carbazoleꢀcontaining polymers in which the carbazole unit is located in either the
main or in the side chain, belong to one of the most investigated systems in the area of
polymerꢀbased optoelectronic materials8. Such materials present good electroꢀ and
photoactive properties, high charge carrier mobilities, hole transporting capability and
electroluminescent properties. Moreover, these materials are characterised by high
thermal, chemical and environmental stability. As a result, there are many reports in
the literature concerning the photorefractive and nonlinear optical (NLO) properties of
carbazoleꢀcontaining polymers. As an example, Fuks et al. have investigated the
thirdꢀorder NLO properties of poly(Nꢀvinylcarbazole) by means of degenerate fourꢀ
wave mixing (DFWM) employing visible laser excitation9. Zhang and coꢀworkers
have investigated the optical properties of main chain polymers containing carbazole
NLOꢀphores10. Moreover, Zhan et al. have studied the offꢀresonant third order NLO
response of conjugated copolymers containing carbazole and fluorine groups via
DFWM measurements under infrared excitation11.
During the last years, metal nanoparticles (NPs) have also attracted considerable
attention because of their sizeꢀdependent properties, their high surfaceꢀtoꢀvolume
ratios and their plasmonic effects, all these characteristics contributing to very
interesting chemical and electrochemical properties. Consequently, NPs are among
the most promising candidates for several applications in photonics, optoelectronics,
sensing, catalysis etc. In particular, concerning applications related to the optical
properties of NPs, the most investigated systems have been noble metals12,13 and
semiconducting14ꢀ16 NPs.
A promising strategy for the development of new materials exhibiting attractive
optoelectronic properties is the combination of electroꢀoptical polymers with NPs. In
that view, the combination of polymers with transition NPs17,18 has attracted the
scientific interest very recently. Palladium (Pd) characterised by nanoscale
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dimensions is considered to be one of the most attractive metals since besides its NLO
properties that are relatively unexplored and rather poorly understood19ꢀ22, it also
exhibits very interesting catalytic, hydrogen storage23 and sensing properties24,25. The
use of wellꢀdefined nanostructured domains such as amphiphilic block copolymer
micelles as stabilisers for Pd nanoparticles in solution provides an ideal nanoꢀ
environment for the controlled nanoparticle formation and NP localisation in the
nanoscale26.
Recently we have been focusing on the synthesis and characterisation of micellar
organicꢀinorganic nanohybrids based on a relatively new family of coordination
copolymers bearing βꢀketoester side chain functionalities27,28. Our initial studies on
the investigation of the NLO response were performed on nanohybrid systems based
on diblock copolymer micelles consisting of a highly hydrophobic monomer namely
lauryl methacrylate (LauMA) and the metal chelating monomer 2ꢀ(acetoacetoxy)
ethyl methacrylate (AEMA) employed for the complexation and stabilisation of Pd
NPs in nꢀhexane. These systems have been found to exhibit rather low thirdꢀorder
susceptibility χ(3) values when excited with 35 ps laser light in the visible or in the
infrared, while their NLO response was found to remain unaffected by changing the
size of the encapsulated Pd NPs 29.
In an effort to develop new polymerꢀbased hybrid nanomaterials with enhanced
optical nonlinearity, a new block copolymer family has been synthesised in which the
AEMA metal binding groups are combined with 2ꢀ(Nꢀcarbazolyl) ethyl methacrylate
(CbzEMA) units. A simple and costꢀeffective synthetic approach involving Reversible
Addition Fragmentation chain Transfer (RAFT) polymerisation has been employed
towards this purpose30.
For the first time AEMA is combined with CbzEMA to yield wellꢀdefined functional
diblock copolymers presenting electroactive and metal chelating properties. Hence,
the present work describes the unique combination of a relatively rare example of a
metal chelating monomer polymerised by RAFT, with an electroꢀ and photoactive
polymethacrylate, the controlled polymerisation of which has been rarely reported31ꢀ
34. The RAFT process was readily employed to prepare wellꢀdefined CbzEMAx
homopolymers and CbzEMAxꢀbꢀAEMAy diblock copolymers, that were further
characterised in regards to their molecular and compositional features. The
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CbzEMAxꢀbꢀAEMAy diblock copolymers were further used as macromolecular steric
stabilisers for Pd NPs in organic media resulting in the generation of Pdꢀcontaining
block copolymer micelles. To the best of our knowledge, this is the first systematic
investigation of the NLO properties of these nanohybrids in the infrared (1064 nm)
and in the visible (532 nm) regions under 35 ps laser excitation, both in solution and
in thin films.
2. Experimental
Materials and Methods
Benzene (Fluka, ≥ 99.5%) and tetrahydrofuran (THF) (Scharlau, 99.9%) were stored
over CaH2 (Merck, 99.9%) and distilled under reduced pressure immediately prior to
the polymerisation reactions. Methanol (LabScan, 99.9%), nꢀhexane (LabScan, 99%),
toluene (Scharlau, 99.9%), dimethylformamide (DMF) (Aldrich, 99.8%), cyclohexane
(SigmaꢀAldrich, ≥ 99.9%), dichloromethane (SigmaꢀAldrich, ≥ 99.5%), HCl (Merck,
37% solution), diethyl ether (LabScan, 99.5%), benzoic acid (Merck, 99%), aqueous
solution of tetrabutylammonium hydroxide (SigmaꢀAldrich, 40% w.t.), triethylamine
(Merck, ≥ 99%), hydrazine monohydrate (SigmaꢀAldrich, 98%), benzyl chloride
(SigmaꢀAldrich, 99%), carbon tetrachloride (Riedel de Haën, ≥ 99.8 %), αꢀ
methylstyrene (SigmaꢀAldrich, 99%), sodium methoxide (Aldrich, 30% solution in
methanol), carbazole (Sigma, ≥ 95%), ethylene carbonate (Aldrich, ≥ 98%),
methacryloyl chloride (Fluka, ≥ 97%) and deuterated chloroform (Merck) were used
as received. The following inorganic compounds were used without further
purification: Sulfur (Aldrich, powder ~ 100 mesh), silica gel (Aldrich, 60 Å, 70ꢀ230
mesh), NaOH pellets (Scharlau, 99%), KOH pellets (HiMedia, 85%), Na2SO4
(HiMedia, 99%), NaHCO3 (SigmaꢀAldrich, 99.5%), NaCl (HiMedia, ≥ 99.0%),
Pd(CH3COO)2 (SigmaꢀAldrich, 99.98%), anhydrous MgSO4 (Scharlau, 98%). 2ꢀ
(acetoacetoxy) ethyl methacrylate (AEMA) (SigmaꢀAldrich, 95%) was passed
through a basic alumina column prior to the polymerisations and used without further
purification. 2,2ꢀAzobis(isobutylnitrile) (AIBN) (SigmaꢀAldrich, 95%) was
recrystallised twice from ethanol and dried under vacuum at room temperature for
three days. Cumyl dithiobenzoate (CDTB) was synthesised following a procedure
reported by Le et al.35.
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Synthesis
2ꢀ(Nꢀcarbazolyl) ethyl methacrylate (CbzEMA). The synthesis of the monomer
CbzEMA was performed in two steps by following a modified methodology reported
in the literature31,36,37. A DMF solution of carbazole (19 g, 0.114 mol) and ethylene
carbonate (11 g, 0.125 mol) was placed under reflux for 8h in the presence of KOH
(1.9 g). After been cooled down to room temperature, the reaction mixture was
filtered and then poured into a large amount of water (4 L). The precipitated white
crystalline 2ꢀ(Nꢀcarbazolyl) ethanol (yield: 38%) was collected and dried in a vacuum
o
oven for 12h at 40 C. Subsequently, it was recrystallised twice in a 1:1 v/v
benzene/cyclohexane mixture.
The purified 2ꢀ(Nꢀcarbazolyl) methanol (5 g, 0.024 mol) was then dissolved in
dichloromethane (80 mL). In the reaction flask, triethylamine (2.8 mL, 0.028 mol)
and methacryloyl chloride (4 mL, 0.028 mol) were added. The addition of
triethylamine and methacryloyl chloride was carried out in a 20% excess, for a more
quantitative reaction. The methacryloyl chloride was added dropwise during stirring
o
at 0 C. During the addition, an exotherm was observed and sediment was formed,
namely hydrochloric triethylamine (Et3NHCl), which is a byꢀproduct of the reaction.
The reaction mixture was stirred for 24h. The hydrochloric triethylamine was filtered
off by using a glass frit filter. After filtration, water (10 mL) was added in the mixture
for the hydrolysis of the excess methacryloyl chloride into methacrylic acid and the
segregation of the organic phase from the aqueous. The product remained in the
organic phase. Subsequently, dichloromethane (40 mL) was added in the solution and
the mixture was extracted two times with water (5 and 10 mL), then three times with
NaHCO3 solution (5% in water, 3 x 5 mL) and again three times with water (3 x 5
mL). The obtained monomer was recrystallised twice in methanol (yield: 42%).
1H NMR (300MHz, CDCl3) δ (ppm): 8.1 (d, ꢀ carbazole aromatic protons), 7.46 (m,
ꢀ carbazole aromatic protons), 7.24 (m, ꢀ carbazole aromatic protons), 5.92 (s, =CH2),
5.48 (s, =CH2), 4.64 (t, N–CH2), 4.53 (t, O–CH2), 1.94 (s, CH3).
CbzEMA homopolymers. CbzEMA (733 mg, 2.63 mmol) was placed in a roundꢀ
bottom flask and dissolved in freshly distilled benzene (0.7 mL) under a dry nitrogen
atmosphere. CDTB (7.2 mg, 0.026 mmol) and AIBN (2.36 mg, 0.014 mmol)
dissolved in benzene were then transferred into the flask via a syringe. The reaction
mixture was degassed by three freeze–evacuate–thaw cycles and placed under a dry
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nitrogen atmosphere at 65 oC for 20h. The polymerisation was terminated by cooling
the reaction down to room temperature. The produced homopolymer (555 mg, 76%
polymerisation yield) was retrieved by precipitation in methanol and was left to dry
under vacuum at room temperature for 24h.
1H NMR (300MHz, CDCl3) δ (ppm): 7.1ꢀ7.9 (s, ꢀPh), 4.15 (s, NꢀCH2), 3.98 (s, Oꢀ
CH2), 1.6 (s, ꢀCH2), 0.86 (m, ꢀCH3).
CbzEMAxꢀbꢀAEMAy diblock copolymers. In a roundꢀbottom flask, the macroꢀCTA,
CbzEMA82, (MnSEC=23075 g.molꢀ1, 493 mg, 0.020 mmol) was placed and dissolved
in freshly distilled THF (0.9 mL) under a dry nitrogen atmosphere. AIBN (1.6 mg,
0.0096 mmol) dissolved in THF and AEMA (0.2 ml, 0.99 mmol) were then
transferred into the flask via a syringe. The reaction mixture was degassed by three
freeze–evacuate–thaw cycles and placed under a dry nitrogen atmosphere at 65 oC for
20 hours. The polymerisation was terminated by cooling the reaction down to room
temperature. The produced diblock copolymer (603 mg, 36% polymerisation yield)
was retrieved by multiple precipitations in methanol and was left to dry under vacuum
at room temperature for 24 hours.
1H NMR (300MHz, CDCl3) δ (ppm): 7.9 (br, ꢀPh), 7ꢀ7.2 (br, ꢀPh), 4.35 (br, ꢀCH2),
4.18 (br, NꢀCH2), 3.97 (s, OꢀCH2), 3.75 (br, ꢀCH2), 3.56 (br, ꢀCH2), 2.29 (br, ꢀCH3),
0.86ꢀ2.00 (m, ꢀCH2, ꢀCH3).
CbzEMAxꢀbꢀAEMAy/Pd(0) micellar nanohybrids. CbzEMA82ꢀbꢀAEMA47 (20 mg,
0.0284 mmol of AEMA units) was dissolved in THF (5 mL). After complete
dissolution of the polymer, triethylamine (40ꢁL, 0.2870 mmol) was added to the
solution. Subsequently, the resulting solution was transferred in a glass vial
containing Pd(CH3COO)2 (3.2 mg, 0.0142 mmol) and left to stir at room temperature
until complete solubilisation of the salt. Finally, hydrazine monohydrate was added to
the solution upon stirring. The reduction of Pd(II) ions to noble Pd was accompanied
by a color change of the solution from yellow to dark brown.
Film fabrication. The Pdꢀcontaining micellar nanohybrids were spinꢀcoated on glass
substrates using a SchaeferꢀTech AG/SCIꢀ50 spinꢀcoater. The spin velocity was 4500
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o
rpm and the spinning time 40 sec. Selected films have been also annealed at 170 C
for 1 hour.
Characterisation Methods
NMR spectra were recorded in CDCl3 using an Avance Brucker 300 MHz
spectrometer equipped with an Ultrashield magnet. Tetramethylsilane (TMS) was
used as an internal reference in CDCl3. The molecular weights and molecular weight
distributions (MWD) of the polymers were determined by size exclusion
chromatography (SEC) using equipment supplied by Polymer Standards Service
(PSS). All measurements were carried out at room temperature using Styragel HR 3
and Styragel HR 4 columns. The mobile phase was THF, delivered at a flow rate of 1
mL min−1 using a Waters 515 isocratic pump. The SEC data were obtained using a
refractive index detector (Waters 2414) supplied by PSS. The instrumentation was
calibrated using poly(methyl methacrylate) (PMMA) standards with narrow
polydispersity indices (MWs of 102, 450, 670, 1580, 4200, 14400, 31000, 65000,
126000, 270000, 446000, 739000 g molꢀ1) supplied by PSS. Dynamic Light
Scattering (DLS) measurements were carried out using a DLS 90Plus Brookhaven
scattering spectrometer operating at 633 nm (power: 30 mW). DLS experiments were
performed at a 90o scattering angle. Solution concentrations maintained at 1g/L. All
polymer solutions were filtered through PTFE microfilters (pore size: 0.45 ꢁm) prior
to measurements. Transmission electron microscopy (TEM) studies were performed
on a 1010 JEOL microscope (200 kV). The micellar solutions were left to dry on a
carbon coated copper grid to allow the TEM investigation.
The nonlinear optical properties of the Pdꢀcontaining micellar systems have been
studied be means of the Zꢀscan technique38, which can provide simultaneously the
sign and the magnitude of the nonlinear absorption and refraction of a sample.
Briefly, during the Zꢀscan measurements, the sample moves through the focal plane,
along the propagation direction of a focused laser beam, thus experiencing different
laser intensity at each position. The transmitted laser beam is then divided in two
parts, e.g. by a beam splitter, allowing two different experimental configurations, the
soꢀcalled “open” and “closedꢀaperture” Zꢀscans respectively. In the former
configuration, the transmitted laser light is totally collected (e.g. using a large
diameter lens), while in the latter, only a part of the transmitted laser beam is
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collected, after it has passed through a small aperture placed in the far field. From the
division of the “closedꢀaperture” Zꢀscan recording by the corresponding “openꢀ
aperture” one, the soꢀcalled “divided” Zꢀscan is obtained, from which the total
variation of the transmission, i.e. the ꢂΤpv, is determined. This last quantity is related
to the onꢀaxis nonlinear phase shift ꢂΦ0 through the following relation:
ꢀTpv
0.406(1− S)0.27
ꢀΦ0 =
(1)
where S is the fraction of beam transmitted through the aperture. Then, the nonlinear
refractive index parameter γ′ can be deduced through the relation:
ꢀΦ0
kI0Leff
′
γ
=
(2)
where I0 is the laser peak intensity at the focus, Leff =1−exp(−a0L) a0 is the effective
path length of the sample which is related to the linear absorption coefficient α0 of the
sample and the sample thickness L and k=2π/λ is the wave vector.
From the “openꢀaperture” Zꢀscan the nonlinear absorption coefficient β can be
obtained by fitting with the following relation38:
m
∞
−q (0,0)
[
]
0
(3)
T =
∑
(m +1)3/2
m=0
where q 0,0 =
β I0 Leff . Finally, with the parameters β and γ′ known, the real
(
)
0
(Reχ(3)) and the imaginary (Imχ(3)) parts of the thirdꢀorder nonlinear susceptibility can
be easily calculated using the following relations:
c(m s)n02
γ
'(m2 W)
Re
Im
χ
χ
(3) (esu) =
(3) (esu) =
480π 2
(4)
(5)
c2 (m s)2 n02
960π 2ω(s−1)
β(m W)
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where n0 is the linear refractive index (which is the refractive index of the solvent in
the case of solutions), c is the speed of light, ε0 is the permittivity of free space and ω
is the angular frequency of the laser radiation.
For the measurements a 35 ps modeꢀlocked Nd:YAG laser (Quantel YG900)
operating at 1064 and 532 nm, at a repetition rate of 10 Hz was employed. A more
detailed description of the analysis of the experimental data can be found
elsewhere39,40. The solutions of the investigated Pdꢀcontaining micellar samples were
placed into 1 mm thick quartz cells while their UVꢀVisꢀNIR optical absorption spectra
were regularly measured by a spectrophotometer (Hitachi U3000), in order to ensure
that no photoꢀdegradation has occurred during the measurements. The laser beam was
focused in the sample by means of a 20 cm focal length quartz lens.
3. Results and discussion
Figure 1 illustrates the chemical structures and names of the two monomers and the
chain transfer agent (CTA) used in polymer synthesis.
Fig. 1 Chemical structures of the two monomers (AEMA and CbzEMA) and the CTA
(CDTB) used in polymer synthesis.
Molecular weights and composition of the CbzEMAx homopolymers and the
CbzEMAxꢀbꢀAEMAy diblock copolymers. CbzEMAx homopolymers and
CbzEMAxꢀbꢀAEMAy diblock copolymers were successfully synthesised following
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typical RAFT methodologies already described in the Experimental Section. The
molecular characterisation of these materials was carried out by means of size
exclusion chromatography (SEC) and proton nuclear magnetic resonance
spectroscopy (1H NMR). Table 1 summarises the chemical structures of all
homopolymers and diblock copolymers prepared in this study along with their
molecular weight (MW) and composition characteristics. Narrow to moderate
molecular weight distributions (MWD) – defined as the ratio of the weight average
(Mw) to the number average (Mn) molecular weight – were observed for the
homopolymers and diblock copolymers, varying between ∼1.2 – 1.5.
Table 1 Characteristics of the homopolymers and block copolymers based on CbzEMA and
AEMA obtained by RAFT (conversion percentages, molecular weights and molecular weight
distributions (MWD)).
Experimental
Structurea
Conversion
(%)
Theor. MWb
gmolꢀ1
SECc
Mn (gmolꢀ1) MWD
CbzEMA82
76
36
21388
33374
41830
n.d.
26590
31800
29158
22950
10883
12561
1.23
1.31
1.50
1.33
1.22
1.34
CbzEMA82ꢀbꢀAEMA 47
CbzEMA104
75
CbzEMA104ꢀbꢀAEMA 44
CbzEMA38
n.d.
81
11534
12483
CbzEMA38ꢀbꢀAEMA 50
75
1
b
adetermined by SEC and H NMR; [(g. monomer)/(mol RAFT agent)]×(polymerisation
yield)
+
MW of CTA (for homopolymers) and [(g. monomer)/(mol CTA
agent)]x(polymerisation yield) + Mn of macroꢀCTA (for diblock copolymers); cSEC
calibrated with PMMA standards; Mn = number average molecular weight; MWD =
molecular weight distribution; CbzEMA = 2ꢀ(Nꢀcarbazolyl) ethyl methacrylate; AEMA = 2ꢀ
(acetoacetoxy)ethyl methacrylate; n.d.: not determined.
The expected chemical structure of the CbzEMAxꢀbꢀAEMAy diblock copolymers was
confirmed by 1H NMR spectroscopy. Fig. 2 exemplarily shows the 1H NMR spectrum
of the CbzEMA82ꢀbꢀAEMA47 diblock copolymer. The peak assignments are shown in
the spectrum. The AEMA comonomer compositions were determined from the ratio
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of the areas under the characteristic signals of the AEMA and the CbzEMA,
appearing at 2.27 (f) and 7.9 (h) respectively.
1
Fig. 2 H NMR spectrum of the CbzEMA82ꢀbꢀAEMA47 diblock copolymer recorded in
CDCl3.
CbzEMAxꢀbꢀAEMAy/Pd(0) micellar nanohybrids. The CbzEMAxꢀbꢀAEMAy/Pd(0)
hybrid micelles were prepared in tetrahydrofuran (THF). The latter is a nonꢀselective
solvent (i.e. a good solvent) for both, the CbzEMA and AEMA block segments.
Therefore, the CbzEMAxꢀbꢀAEMAy block copolymers exist only as unimers in this
solvent in the absence of the Pd NPs. Micellisation is induced upon the addition of the
Pd(CH3COO)2 in the solution followed by reduction in the presence of hydrazine
monohydrate, as schematically depicted in Fig. 3.
Fig. 3 Formation of micellar nanohybrids consisting of CbzEMAxꢀbꢀAEMAy diblock
copolymers and Pd NPs in an organic solvent (THF).
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Similar observations i.e. micelle formation upon complexation and/or reduction have
been reported by various groups. For example, L. M. Bronstein et al. have reported on
the formation of micellar structures in aqueous media upon interaction of certain
metal compounds such as Pd, Au and Pt with poly(ethylene oxide)ꢀbꢀpoly(ethylene
imine) doubleꢀhybrophilic block copolymers41. Moreover, H. Zhao and coꢀworkers
have reported on the induction of micellisation in THF upon coordination of Cd2+
metal ions with the 2VP units of poly(styreneꢀbꢀ2ꢀvinylpyridine) (PSꢀbꢀP2VP) block
copolymer. In the absence of the ions both block segments are soluble in THF, i.e. no
micelles are present, whereas, upon the addition of Cd2+ to the PSꢀbꢀP2VP/THF
solution, complexed 2VP segments become insoluble in THF and collapse resulting to
micelle formation14.
Dynamic Light Scattering (DLS) was used for determining the hydrodynamic
diameters (DH) of the CbzEMA82ꢀbꢀAEMA47 and CbzEMA104ꢀbꢀAEMA44 hybrid
micelles that were further investigated in respect to their NLO response in solution.
The obtained results are summarised in Table 2.
Table 2 Hydrodynamic diameters of the CbzEMAxꢀbꢀAEMAy micellar nanohybrids
generated in THF after the addition of Pd(CH3COO)2 followed by reduction.
Experimental Structure
DH (theoretical)*
DH (no Pd) (nm)
DH (nm)
CbzEMA82 ꢀbꢀAEMA47
CbzEMA104ꢀbꢀAEMA44
66
75
∼ 3
∼ 3
72
91
91
[AEMA:Pd(0)]=2:1
DH (nm)
[AEMA:Pd(0)]=1:1
114
*assuming the generation of micelles with spherical morphology and fullyꢀextended
chains
As seen in Table 2, upon increasing the inorganic content within the micelles, the
hydrodynamic diameter increases. As previously reported41, in cases where micelles
are formed owing to complexation with a metal compound, a change in metal loading
may significantly affect the micellar size. The obtained DH values exceed the
maximum theoretical micelle diameters calculated assuming spherical morphology
and fullyꢀextended chains, suggesting the presence of micellar aggregates in solution.
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The latter is also confirmed by TEM analysis. Exemplarily, TEM images of
CbzEMAxꢀbꢀAEMAy micellar aggregates having Pd NPs incorporated within their
cores are presented in Fig. 4. Using the Digimizer Image Analysis Software, the
average particle sizes were calculated to be 63.9 ± 10 nm for the CbzEMA82ꢀAEMA47
/Pd(0) (1:1) system and 78.5 ± 9 nm for the CbzEMA104ꢀAEMA44 /Pd(0) (2:1) system.
Fig. 4 TEM images of CbzEMAxꢀbꢀAEMAy/Pd(0) micellar nanohybrids generated in THF.
Nonlinear Optical (NLO) Properties. For the determination of the NLO properties
of the Pdꢀcontaining micellar nanohybrid systems, various solutions with different
CbzEMA82ꢀbꢀAEMA47/Pd(0) and CbzEMA104ꢀbꢀAEMA44/Pd(0) concentrations in
THF have been prepared. In addition, Pdꢀcontaining micellar systems with different
polymerꢀtoꢀmetal molar ratios (i.e. AEMA to Pd) have been synthesised and
investigated, called hereafter 1:1 and 2:1 systems, with the 2:1 micellar systems
containing half of the Pd load compared to the 1:1 ones. The NLO properties of the
prepared systems have been investigated under both visible (532 nm) and infrared
(1064 nm), 35 ps laser excitation. Some representative UVꢀVisꢀNIR absorption
spectra of the prepared micellar solutions are presented in Fig. 5, together with the
absorption spectrum of the pristine copolymer CbzEMA82ꢀbꢀAEMA47 for comparison
purposes. As seen from the spectra, the copolymer was found to exhibit negligible
absorption in the visible and near infrared, while the Pdꢀcontaining micelles exhibited
sizeable absorption, this difference attributed to the Pd presence in the micellar
nanohybrid systems. As a matter of fact, the absorption spectra of the samples shown
in Fig. 5, having the same Pd concentration (i.e. 1 mM), exhibited the same
absorbance in the 400 to 800 nm spectral range, independently of the copolymer type
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employed. For these reasons, in the next, the reported samples’ concentrations will be
referred to the Pd content of each solution instead of the concentration of the Pdꢀ
containing micellar entities in solution.
Fig. 5 UVꢀVisꢀNIR absorption spectra of the copolymer CbzEMA82ꢀbꢀAEMA47, and the
CbzEMA82ꢀbꢀAEMA47/Pd(0) and CbzEMA104ꢀbꢀAEMA44/Pd(0) micellar solutions in THF.
In order to check if the observed NLO response had contributions arising from the
solvent and/or the copolymer, Zꢀscan measurements of the neat solvent (i.e. THF) and
of various concentration’ solutions of the CbzEMAxꢀbꢀAEMAy copolymers in THF
were performed, at both excitation wavelengths, under identical experimental
conditions to those used for the investigation of the Pdꢀcontaining micellar systems.
In all cases, the CbzEMAxꢀbꢀAEMAy copolymer solutions exhibited negligible NLO
response for the range of incident laser energies used, whereas THF exhibited only
nonlinear refraction, which was measured separately in order to be taken into account
for the determination of the nonlinear response of the Pdꢀcontaining micellar
solutions. The magnitude of the thirdꢀorder susceptibility χ(3) of THF was determined
to be (2.8±0.6)×10ꢀ22 and (2.5±0.2)×10ꢀ22 m2/V2 under 35 ps, 532 and 1064 nm laser
excitation respectively in very good agreement with other published reports42,43
.
The present study has revealed that the investigated micellar nanohybrid systems had
substantial NLO response, comparable to that reported recently in another study of
similar systems, where a different block copolymer, namely the poly(lauryl
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methacrylate)ꢀblockꢀpoly(2ꢀ(acetoacetoxy)
ethyl
methacrylate)
(LauMAxꢀbꢀ
AEMAy)29 has been employed. In particular, the LauMAxꢀbꢀAEMAy/Pd micellar
systems were found to exhibit both nonlinear absorption and refraction when excited
with 532 and 1064 nm laser light.
Fig. 6 Representative “divided” and “openꢀaperture” Zꢀscans of THF Pdꢀcontaining micellar
nanohybrid solutions, under 35 ps, 532 nm (a, b) and 1064 nm (c, b) laser excitation.
In Figure 6b, the “openꢀaperture” Zꢀscan recording of a 0.91 mM solution of the 2:1
CbzEMA104ꢀbꢀAEMA44/Pd(0) micellar system, measured under 532 nm excitation is
shown. As shown, this micellar system was found to exhibit important transmission
minimum suggesting the presence of strong nonlinear absorption. Similar behaviour
was observed for the other micellar systems as well. Similarly, important nonlinear
absorption was observed under infrared excitation as shown from the “openꢀaperture”
Zꢀscans in Fig. 6d, corresponding to a 1.7 mM 2:1 CbzEMA104ꢀbꢀAEMA44/Pd(0)
micellar system solution. It should be mentioned at this point that in all infrared
excitation measurements, significantly larger laser energies and concentrations were
used compared to those used under visible excitation. Based on these experimental
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evidences, it can be reasonably assumed that under visible excitation, a resonance
enhancement should be operational. In addition, all Pdꢀcontaining micellar
nanohybrid systems have been found to exhibit positive sign nonlinear absorption,
corresponding to reverse saturable like behaviour (RSA) both in the visible and in the
infrared.
Concerning the nonlinear refraction of the micellar nanohybrids, from the analysis of
the Zꢀscan data, it was concluded that the Pdꢀcontaining micellar systems and the
solvent exhibited opposite sign nonlinear refraction under visible excitation, while
they exhibited same sign nonlinear refraction under infrared excitation. As a matter of
fact, since THF exhibited valleyꢀpeak transmission configuration, indicating positive
sign nonlinear refraction (i.e. selfꢀfocusing) at both excitation wavelengths as shown
by the shape of the corresponding Zꢀscan recordings presented in Fig. 6a and 6c, it
becomes evident that the micellar systems should exhibit negative nonlinear
refraction, (i.e., selfꢀdefocusing) under 532 nm excitation and positive nonlinear
refraction (i.e., selfꢀfocusing) under 1064 nm excitation.
In conclusion, the micellar nanohybrids were found to exhibit both nonlinear
refraction and absorption under both visible and infrared excitation conditions, while
their nonlinear refraction under infrared excitation was significantly weaker to that
observed under visible excitation from similar concentration solutions. In general,
both the nonlinear refraction and absorption were found to be more important in the
visible than in the infrared, indicating most probably the presence of resonance
enhancement. In all cases, the analysis of the Zꢀscan measurements revealed χ(3)/ a0
values ranging between (7ꢀ18.2)×10ꢀ25 m3/V2 in the visible and (1.4ꢀ2. 1)×10ꢀ25 m3/V2
in the infrared, while a good linear dependence of the χ(3) versus Pd content was
obtained, with no significant dependence with the size and the AEMAy to Pd(0) ratio.
These experimental findings could be attributed to the fact that the Pd NPs generated
within the cores of the different micellar systems, did not exhibit any significant size
differences (based on TEM data), thus resulting to similar χ(3) values regardless of the
hydrodynamic micellar size. This is in agreement with some recent results suggesting
similar weak dependence of the NLO response on the size of Pd core encapsulated
within the block copolymer micelles, under both visible and infrared laser
excitation44.
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In a recent study19, the NLO properties of commercially available Pd NPs in colloidal
suspensions in water and ethanol, with sizes of about 3ꢀ5 nm have been reported. The
excitation was performed using 792 nm, 120 fs and 210 ps laser light. In the case of
excitation using the 210 ps laser pulses, which are more relevant to the present
experimental conditions, a positive sign nonlinear refractive index of 9×10−15 m2W−1
and negligible nonlinear absorption have been observed. Similarly, in another study,
investigating the NLO properties of similar Pd suspensions, using 1064 nm, 50 ps
laser excitation, negative nonlinear refraction and positive nonlinear absorption have
been reported45. In particular, the nonlinear refractive parameter γ′ and the nonlinear
absorption coefficient β were determined to be −1.2×10−19 m2W−1 and 5.4×10−13
mW−1 respectively for a 20 mM Pd suspension. From the above, it becomes evident
that despite the disagreement about the sign of the nonlinear refraction, a quite
satisfactory agreement exists concerning the magnitude of both the refractive and
absorptive parts of the thirdꢀorder optical nonlinearity.
In order to further investigate the NLO properties of the Pdꢀcontaining nanohybrids
and shed more light on the issue of the controversial literature results concerning the
sign of the nonlinear refraction, thin films of these micellar systems have been
prepared and studied.
The films have been prepared using the spinꢀcoating technique and have been
deposited on glass substrates, using a spin velocity of 4500 rpm for 40 sec. Some of
the spinꢀcoated films have been also annealed at 170 oC for 1 hour, while in all cases,
the film thickness was kept small (e.g. of the order of few nm) in order to keep linear
aborption low. In Fig. 7, some representative UVꢀVisꢀNIR absorption spectra of the
prepared films are presented.
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Fig. 7 UVꢀVisꢀNIR absorption spectra of annealed and nonꢀannealed thin films of Pdꢀ
polymer nanohybrids deposited on glass substrates via spinꢀcoating.
The “divided” Zꢀscans of two representative films, namely of 1:1 CbzEMA104ꢀbꢀ
AEMA44 and 2:1 CbzEMA82ꢀbꢀAEMA47 nonꢀannealed films obtained under 35 ps,
532 nm laser excitation are presented in Fig. 8a and 8c. The incident laser energy used
was 6.8 ꢁJ. As shown, both films displayed a “peakꢀvalley” transmission
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configuration, indicative of negative sign nonlinear refraction, similar to what has
been found in the Zꢀscan studies of the micellar solutions. The corresponding “openꢀ
aperture” Zꢀscans, also shown in Fig. 8b and 8d, displayed negligible nonlinear
absorption. In fact, the nonlinear absorption of the micellar nanohybridꢀderived thin
films has become observable at incident laser energies higher than 12 ꢁJ, where
however, nonlinear refraction started to saturate, thus preventing the accurate
determination of the NLO parameters. In addition, analogous Zꢀscan measurements
performed using 1064 nm laser excitation did not reveal any measurable NLO
response for incident laser energy up to 50 ꢁJ, where ablation started to become
observable.
Fig. 8 “Divided” (a), (c) and “openꢀaperture” (b), (d) Zꢀscans of thin films generated via spinꢀ
coating of two different Pdꢀcontaining micellar systems, obtained using 6.8 ꢁJ, 35 ps, 532 nm
laser excitation.
From the absorption spectra of the Pdꢀcontaining micellar systems (in solution and/or
in thin films) shown in Figures 5 and 7, it becomes clear that their featureless
character in the 350 to 800 nm spectral region, does not provide any evidence of the
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presence of surface plasmon resonance (SPR). The very weak shoulder at 380 nm (in
the films’ spectra) is most probably correlated with the presence of Pd(II) complexes,
that have not been entirely reduced to Pd(0). Similar featureless absorption spectra
have been also reported by two other groups46,47 exploring size control strategies of
polyhedral Pd NPs and selfꢀorganisation issues. The same situation has been reported
recently in the case of Pdꢀcontaining LauMAxꢀbꢀAEMAy micellar systems29 where (in
the UVꢀVis absorption spectra of the solutions) two broad absorption features
apperring at 350 and 400 nm were attributed to Pd(II) complexes, not been entirely
reduced to Pd(0).
In contrast to the above, some other studies have reported the observation of Pd
surface plasmon resonance feature, although often the findings are rather conflicting.
In the case of Pd nanoparticles with diameters of a few nm found in colloidal
suspensions19,45, the SPR has been reported at 245 nm and 305 nm, while in the case
of similar size Pd nanoparticles incorporated in the pores of silica glasses48, the SPR
peak has been reported at 320 nm. Furthermore, in another study, the SPR peak of 8
nm sized Pd nanocubes coated with polyvinylpyrrolidone (PVP) has been reported
lying between 200 and 240 nm, while for larger size nanocubes, c.a. 25 and 50 nm,
the SPR has been reported to be at 330 and 390 nm respectively49. Finally, in another
study investigating the shapeꢀcontrolled synthesis of Pd icosahedra NPs50 the SPR
peak has been observed to lie at about 286 nm.
Based on the above, it becomes probable that the large absorption below 350 nm of
the Pdꢀcontaining micellar systems studied in this work, due to the block copolymers,
might prevent the clear and unambiguous observation of the SPR, in case it is located
in this spectral area. Nevertheless, the exact position of the SPR of Pd NPs is still not
certain, in the sense that the various synthetic protocols result in NPs having different
shapes and dimensions, affecting strongly the position and the spectral width of
SPR49. Moreover, the use of surfactants and/or other chemicals (e.g., protective
layers, solvent, etc) can further complicate the situation as their absorption may
prevent the observation of the SPR peak as well.
Taking into account our experimental findings and considering the absence of the
SPR in the visible and that no strong transitions occur in the visible part of the
absorption spectrum, the observed enhancement could be explained in terms of a twoꢀ
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photon process. In fact, such a situation has been previously considered by several
groups investigating multiphoton processes in metallic nanoparticles51,52. Among
those, in a recent work, studying the NLO absorption and refraction of Pd NPs, the
authors attributed their experimental findings to such processes53. In fact, they have
suggested that the twoꢀphoton process originated from the interband transitions
between the d band and sꢀp conduction band of the Pd. Therefore, in the present work,
in order for two 532 nm photons to be absorbed, the SPR should lie in the UV region.
In addition, since similar enhancement, although weaker, can also be achieved under
near resonant conditions, the SPR can be placed in a relatively larger spectral region
extended c.a. from about 220 to 290 nm, according to the results reported by several
literature reports as discussed above. In total, based on the present findings, the SPR
of the Pdꢀcontaining micellar systems cannot be located with better accuracy.
However, further work is in progress, using variable wavelength laser source, in order
to detect the position of the SPR of these Pdꢀcontaining micellar systems.
4. Conclusions
In the present work a versatile synthetic route was employed for the preparation of
Pdꢀcontaining micellar nanohybrid systems making use of new, wellꢀdefined,
methacrylateꢀbased amphiphilic block copolymers comprised of carbazole and βꢀ
ketoester sideꢀchain functionalities. These micellar nanohybrids could be of great
interest for optoelectronics and other photonic applications, since the promotion of the
confinement of metallic NPs within wellꢀdefined nanostructured domains, becomes a
valuable tool for controlling their size and shape, therefore allowing for the tuning of
their nonlinear optical response.
In that view, the NLO properties of the block copolymerꢀPd micellar nanohybrids,
both in solution and in thin films, have been thoroughly investigated using
picosecond, visible and infrared laser excitation. Their NLO response has been found
to be larger under visible than under infrared excitation. In particular, negative sign
NLO refraction (i.e. selfꢀdefocusing) has been observed under visible excitation,
while positive sign NLO refraction (i.e. selfꢀfocusing) has been observed in the case
of infrared excitation, while under both excitation wavelengths, positive sign NLO
absorption has been found. In all cases, no sign of SPR peak has been observed in the
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spectral range from 350 to 1100 nm. This fact, together with the larger NLO optical
response of the Pdꢀcontaining micellar systems under 532 nm excitation, suggest that
the SPR of the Pd nanoparticles should be located below 350 nm and most probably
in the spectral region 220 to 290 nm, allowing twoꢀphoton resonant or near resonant
conditions to occur, resulting in the enhancement of the NLO response under visible
excitation.
Acknowledgements
This work has received partial financial support from the Research Promotion
Foundation (RPF) of Cyprus (Program Technology/THEPIS/0308(ΒΕ)/06). We thank
Dr. Rodica, P. Turcu (National Institute for Isotopic and Molecular Technologies,
ClujꢀNapoca, Romania) for the TEM analyses and the A. G. Leventis Foundation for
a generous donation that enabled the purchase of the NMR spectrometer of the
University of Cyprus. This research has been also coꢀfinanced by the European Union
(European Social Fund – ESF) and Greek national funds through the Operational
Program "Education and Lifelong Learning" of the National Strategic Reference
Framework (NSRF) ꢀ Research Funding Programs: THALIS and Heracleitus II.
Investing in knowledge society through the European Social Fund.
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TOC Figure
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