Self-assembled monolayers of thioether substituted subphthalocyanine [BClSubpc(Sn-C12H25)6] were prepared by deposition from solution. The adsorption kinetics of the molecules on gold substrates was characterized in situ and in real time by optical second harmonic generation (SHG). The obtained data have been fitted with three standard kinetic models and best agreement has been found using the first order Langmuir kinetic model. Subsequently, ellipsometry measurements have been performed to determine the thickness of the formed molecular layers. The extracted thickness was 13.0 nm, demonstrating the formation of a monolayer.
The detection of pollutant chemicals in water, from waste water up to drinking water, is of worldwide interest. Fast
response chemical sensors based on Raman spectroscopy are well suited for a rapid identification and quantification of
such substances. Because of the weak Raman scattering intensity surface-enhanced Raman scattering (SERS) was
applied to achieve the high sensitivity necessary for trace detection. In the European Commission project SENSEnet, a
SERS sensor based on a naturally grown Ag nanoparticle ensemble was developed and adapted for in-situ detection of
polycyclic aromatic hydrocarbons (PAHs) in water.
Silver nanoparticle ensembles with surface plasmon resonance (SPR) wavelengths around 488 nm were prepared under
ultrahigh vacuum condition by Volmer-Weber growth on quartz plates. The laboratory set-up for Raman spectroscopy
contains a microsystem frequency-doubled diode-laser which generates two emission wavelengths, 487.61 nm and
487.91 nm, thus the system was configured also for shifted excitation Raman difference spectroscopy (SERDS). The
optical output power is up to 20 mW. The SERS substrate is located inside a flow-through cell which provides
continuous flow conditions of an analyte solution. The SERS spectra were recorded using a laboratory spectrograph with
a back-illuminated deep depletion CCD-detector.
We present an atomic force microscopic image of the developed SERS substrates as well as results for the SERS activity
and the limit of detection of selected PAHs, e.g. pyrene, in water with respect to the SPR wavelength. SERS/SERDS
measurement of water samples containing mixture of several PAHs (e.g. pyrene and fluoranthene) down to the detection
limit of 2 nmol/l will be discussed.
In this contribution we present surface enhanced Raman scattering (SERS) measurements of pyrene as a function of the
surface plasmon resonance position of noble metal nanoparticle ensembles, which served as SERS substrates. The noble
metal nanoparticle ensembles were prepared under ultrahigh vacuum (UHV) conditions by Volmer-Weber growth on
quartz substrates. For the SERS measurements, the substrates were mounted in a flow-through cell as part of the optical
Raman set-up. A diode laser microsystem with an emission wavelength of 488 nm was used. The system generates two
slightly different emission wavelengths (Δλ ≈ 0.3 nm) with a spectral width of ≈ 10 pm and an optical power of ≈ 10
mW. With this set-up SERS as well as shifted excitation Raman difference spectroscopy (SERDS) can be carried out.
For trace analysis of pyrene in water SERS/SERDS experiments were accomplished as a function of molecule
concentration and spectral position of the plasmon resonance. The best results with a limit of detection of 2 nmol of
pyrene were obtained with a nanoparticle ensemble with a plasmon resonance in the vicinity of the excitation wavelength
of λ = 488 nm. If the plasmon resonance frequency is slightly off-resonance the detection limit is significantly lower.
The latter results are discussed in more detail and we will demonstrate that the morphology and the optical properties of
the SERS substrates crucially influence the LOD.
Electrical as well as photoelectrical properties of the metal nanoparticle ensembles (MNEs) were investigated. The
exponential temperature dependence of the conductivity in the MNEs was obtained. Since lnIc depends linear on 1/T, an
activation energy concept of the conductivity can be applied. An activation energy of Eact = 0.50 eV for Ag nanoparticles
has been extracted. The photoconductivity of MNEs has been related to their extinction spectra. A clear evidence of the
photoinduced conductivity of the MNEs was obtained. Considerable enhancement of the photocurrent was observed for
those wavelengths that are in resonance with plasmon excitations localized in the metal nanoparticles forming the MNE.
The increased photoconductivity can be explained by the additional energy placed in the nanoparticles due to the
excitation of an LSPPR.
In this paper, we present systematic measurements of the ultrafast dephasing time T2 of surface plasmon excitations in silver nanoparticles exposed to different chemical environments. The objective of the measurements is,
whether or not different chemical environments infuence independently the damping of the plasmon resonance,
i.e., clarify if the Matthiessen law can always be applied. For this purpose, measurements of T2 in the size
range between Req = 7 nm and 18 nm were carried out for nanoparticles on different substrates and in different
chemical environments. Subsequently, the damping parameter A, which quantifes the infuence of extrinsic and
intrinsic size effects of the different damping mechanisms on T2, has been determined. While A = 0.13 nm/fs has
been determined for quasi-free nanoparticles, the A parameter increases to approximately A = 0.55 nm/fs for
nanoparticles on a quartz substrate, and further to A = 1.8 nm/fs for supported nanoparticles covered with SO2.
Most importantly, the well known Matthiessen law cannot be applied to the nanoparticle systems investigated
here, because different chemical damping channels do not contribute independently to the damping of the surface
plasmon resonance.
We have studied the fluence dependence of the laser tailoring of colloidal gold particles with radii below R = 25 nm. For this purpose gold nanoparticles in solution have been irradiated with nanosecond-pulsed laser light applying fluences between F = (25 ± 2) mJ/cm2 and F = (49 ± 2) mJ/cm2. In general, laser tailoring is based on the size and shape dependent localized surface plasmon polariton resonance (LSPPR) of metal nanoparticles.
Thus, irradiation with a given laser photon energy is absorbed only by nanoparticles whose LSPPR coincides with the photon energy of the laser light. The absorbed light is rapidly converted into heat, leading to diffusion and evaporation of surface atoms, which permits selective tailoring of nanoparticles. In this contribution, we demonstrate that irradiation of small gold nanoparticles with ns-pulsed laser light, at moderate fluences between F = (25 ± 2) mJ/cm2 and F = (41 ± 2) mJ/cm2, results in a shape change from non-spherical towards spherical
particles. At the same time a defined size reduction of the nanoparticles from (R) = 17.2 nm to (R) = 14.8 nm takes place. Higher fluences initiate nanoparticle coalescence.
Modification of metal nanoparticles with laser light has been a well-known technique for several years. In this
paper, we present our recent studies on tailoring the shape of colloidal gold particles with nanosecond-pulsed laser
light. The underlying method is based on the shape and size dependent optical properties of metal nanoparticles,
i.e. the excitation of surface plasmon polariton resonances. Thus, laser irradiation with a pre-determined photon
energy excites and heats nanoparticles of certain shapes and sizes selectively. This heating leads to diffusion
and, for sufficiently high fluences, to evaporation of surface atoms. In our experiments, colloidal gold particles
were prepared by chemical reduction of a gold salt resulting in nanoparticles with different sizes and shapes.
Subsequently, the colloidal gold particles were irradiated with nanosecond-pulsed laser light. In all cases, a
significant reduction of the line width of the surface plasmon polariton resonance has been observed. This
reflects a narrowing of the shape distribution of the particles. For example, irradiation with a photon energy of
2.16 eV and a fluence of (58 ± 2) mJ/cm2 leads to a reduction of the width of the surface plasmon resonance
from 0.30 eV to 0.22 eV (HWHM) due to a reshaping of the nanoparticles. This has also been confirmed by
TEM measurements. Also, a size reduction of the nanoparticles has been observed.
In this contribution, we present new tailor-made substrates for surface-enhanced Raman scattering (SERS). They are based on precise control of the mean shape and the average diameter of nanoparticles prepared by self-assembly of atoms on dielectric supports. Tailoring of the SERS substrates have been achieved by precise tuning of the localized surface plasmon polariton resonance of silver nanoparticles to the vicinity of the laser wavelength used for SERS excitation. The
underlying method relies on control of the growth kinetics of supported metal nanoparticles which causes a pronounced shape change as a function of particle size. Additionally, the strong dependence of the energetic position of surface plasmon excitation on the shape of the particles is exploited. With this preparation method, SERS substrates with optimized plasmon resonances and field enhancement can be easily produced for specific excitation wavelengths and detection ranges. The nanoparticles have been characterized by optical spectroscopy and atomic force microscopy (AFM). Silver nanoparticles with a plasmon resonance at about 2.4 eV were prepared with and without a protective CaF2 coating. SERS spectra of pyrene were obtained with excitation at 514.5 nm. They exhibit a good reproducibility. Furthermore, the substrates did not show degradation during the measurements and those with protective coating still yielded 70% of the SERS intensity of uncoated substrates, indicating their potential usefulness for an analytical detection of specific molecules. Further tailoring of supported metal nanoparticles for SERS applications by laser irradiation will be discussed.
Modification of metal nanoparticles with laser light has been a well-known technique for several years. Still, selective tailoring of certain sizes or shapes of nanoparticles has remained a challenge. In this paper, we present recent studies on tailoring the size and shape of supported nanoparticles with continuous-wave and femtosecond
pulsed laser light and compare them to our results obtained with ns pulsed laser light. The underlying method is based on the size and shape dependent plasmon resonance frequencies of the nanoparticles. In principle, irradiation with a given laser photon energy excites and heats nanoparticles of certain sizes or/and shapes and leads to diffusion and evaporation of surface atoms. Thus, tailoring the dimensions of the nanoparticles can be accomplished. In our experiments, gold and silver nanoparticles were prepared under ultrahigh vacuum conditions by deposition of atoms and subsequent diffusion and nucleation, i.e. Volmer-Weber growth. This gives particle ensembles with size and shape distributions of approximately 30% - 40%. The nanoparticle ensembles were irradiated with laser light either during or after growth. It turns out, that irradiation with cw or ns laser light makes possible selective modification of the nanoparticles. In contrast, application of fs laser pulses results in non-selective modification. For example, post-grown irradiation of supported gold nanoparticles with ns laser pulses (photon energy = 1.9 eV) causes a clear reduction of the width of the surface plasmon resonance from 0.52 eV to 0.20 eV (HWHM). Similar experiments were carried out with fs pulsed laser light (photon energy = 1.55 eV), which result in a slightly reduced line width but also, to an overall decrease of the extinction. A
comparison of all experiments revealed, that for size or shape tailoring of supported metal nanoparticles best results have been achieved with ns pulsed laser light.
Surface plasmon excitation in metal nanoparticles has found great interest in the past. This collective oscillation of the conduction electrons can be stimulated with light and is characterized by resonances, whose positions depend on the material, the dimensions and the dielectric surrounding of the particles. Recently, the investigation of the ultrafast dephasing time T2 and of the decay mechanisms of surface plasmon excitation have become of particular importance, an essential reason being that T2 is proportional to the enhancement factor of the electric field in the vicinity of the nanoparticle surface. This enhancement plays a key role in a great variety of applications. The present paper presents an overview of our recent experiments on the ultrafast decay of surface plasmon excitation, in particular by using a technique that allows us to measure the homogeneous line widths of surface plasmon resonances in the presence of inhomogeneous broadening and thus determine T2. The method is based on persistent spectral hole burning in the absorption profiles of supported metal nanoparticles by nanosecond laser pulses. The technique has been systematically applied to silver and, more recently, to gold nanoparticles on different substrates. Size and shape dependent dephasing times ranging from 2.6 to 15 fs have been extracted from the experimental results using a theoretical model. The values reflect the reduced dimensions of the nanoparticles and we conclude that additional damping mechanisms, in particular surface scattering and chemical interface damping, come into play.
In this contribution, we present measurements of the ultrafast dephasing time T2 of surface plasmon polariton excitation in gold nanoparticles by means of persistent spectral hole burning. T2 is an essential parameter that does not only reflect the role of different dephasing and deexcitation mechanisms but also allows one to determine the field enhancement factor that is of great importance for many applications of nanoparticles. In our experiments gold nanoparticles were first fabricated in ultrahigh vacuum on sapphire substrates by deposition of atoms, followed by diffusion and nucleation, i.e. Volmer-Weber growth. Subsequently, systematic measurements of T2 in the size range between r = 7 nm and 14 nm were carried out. The most essential among the numerous results is the observation of the influence of the reduced dimension on the dephasing time. While T2 = 14 fs has been measured for r = 12 nm which is, within the error bars, consistent with the damping
contained in the bulk dielectric function, the value of T2 shrinks to, for example, T2 = 11 fs for r = 7 nm. This reduction of T2 can be attributed to surface scattering of the electrons. Further experiments are in progress to confirm the predicted 1/r law for the variation of T2.
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