Frank Hubenthal

Noble Metal Nanoparticles: Synthesis and Optical Properties

After a general introduction of noble metal nanoparticles, the fundamental and unique optical properties of noble metal nanoparticles are explained, based on the Mie application. The well-known quasistatic approximation is derived and subsequently extended to more general cases. In addition, the field enhancement of noble metal nanoparticles and the factors that influence the dephasing of a surface plasmon resonance are explained. The most common top-down and bottom-up preparation techniques are explained. The chapter concludes with three recently developed applications of noble metal nanoparticles.

Nanoparticles and their tailoring with laser light

Monodisperse noble metal nanoparticles are of tremendous interest for numerous applications, such as surface-enhanced Raman spectroscopy, catalysis or biosensing. However, preparation of monodisperse metal nanoparticles is still a challenging task, because typical preparation methods yield nanoparticle ensembles with broad shape and/or size distributions. To overcome this drawback, tailoring of metal nanoparticles with laser light has been developed, which is based on the pronounced shape- and size-dependent optical properties of metal nanoparticles. I will demonstrate that nanoparticle tailoring with ns-pulsed laser light is a suitable method to prepare nanoparticle ensembles with a narrow shape and/or size distribution. While irradiation with ns-pulsed laser light during nanoparticle growth permits a precise shape tailoring, post-grown irradiation allows a size tailoring. For example, the initial broad Gaussian size distribution of silver nanoparticles on quartz substrates with a standard deviation of σ= 30% is significantly reduced to as little as σ= 10% after tailoring. This paper addresses teachers of undergraduate and advanced school level as well as students. It assumes some fundamental knowledge in solid-state physics, thermodynamics and resonance vibration.

PREPARATION OF GOLD NANOPARTICLES WITH NARROW SIZE DISTRIBUTIONS AND WELL DEFINED SHAPES

In this contribution, we present the results of recent experiments with the objective of tailoring the size and shape of gold nanoparticles with nanosecond laser pulses. The technique is based on the size and shape dependent surface plasmon resonance frequencies of metal nanoparticles. In our recent experiments gold nanoparticles were prepared by deposition of atoms on dielectric substrates followed by diffusion and nucleation. This usually results in ensembles of oblate nanoparticles with a broad size and shape distribution. Irradiating the gold particles during growth with nanosecond laser pulses makes it possible to produce nanoparticles with a predetermined axial ratio independent of size. For example, irradiating gold nanoparticles with a photon energy of 1.65 eV during growth stabilizes an axial ratio of a/b = 0.14, a being the short axis and b the long axis of the ellipsoidal nanoparticles. Furthermore, post-growth irradiation permits tailoring the average size of the nanoparticles by laser induced surface diffusion and evaporation of atoms. In principle, it is possible to eliminate all particles of undesired sizes by choosing the appropriate photon energies. We demonstrate that narrowing of the width of the surface plasmon resonance from initially 0.52 eV (half width at half maximum) to 0.2 eV is possible by using a single laser frequency. Combining both methods, i.e. laser irradiation during and after growth, finally results in a narrow size and shape distribution of the particles.

Parallel generation of nanochannels in fused silica with a single femtosecond laser pulse: Exploiting the optical near fields of triangular nanoparticles

We present experiments to prepare highly ordered nanochannels with subdiffraction dimensions on fused silica surfaces with femtosecond laser light. For this purpose, we exploit the strongly enhanced near field of highly ordered triangular gold nanoparticles. We demonstrate that after a single laser shot, 6 μm long nanochannels with a mean depth of 4 nm and an average width of 96 nm, i.e., well below the diffraction limit, are generated. These nanochannels are prepared by ablation, caused by the localization of the near field. The crucial parameters, besides the applied fluence, are the polarization direction of the incoming laser light with respect to the triangular nanoparticles and the size of the nanoparticles.

Irradiation of supported gold and silver nanoparticles with continuous-wave, nanosecond, and femtosecond laser light: a comparative study

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.

Shaping nanoparticles with laser light: a multi-step approach to produce nanoparticle ensembles with narrow shape and size distributions

In this paper we present a multi-step laser tailoring process in order to narrow the size distribution of self-assembled supported metal nanoparticles. The method exploits the shape and size dependent optical absorption coefficients of metal particles. Silver nanoparticles were prepared by deposition of atoms on dielectric substrates followed by diffusion and nucleation. This results in ensembles of oblate nanoparticles with broad size and shape distributions. Post-growth irradiation allows tailoring the average size of the nanoparticles by laser-induced surface diffusion and evaporation of surface atoms of the nanoparticles. This makes it possible to shrink large particles to the desired size and remove all nanoparticles that are too small. We demonstrate here that the size distribution of silver nanoparticles can be narrowed significantly by subsequently applying different laser wavelengths.

The influence of the reduced dimension on the dephasing time of surface plasmon excitation in gold nanoparticles

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.

Photoconductivity of Silver Nanoparticle Ensembles on Quartz Glass (SiO2) Supports Assisted by Localized Surface Plasmon Excitations

We have studied the conductivity and photoconductivity in silver nanoparticle ensembles on quartz glass substrates. We observed a significant increase of the photoconductivity if the localized surface plasmon resonance in the metal nanoparticles was excited. A detailed analysis of the temperature dependence of the conductivity as well as dependences of the conductivity and photoconductivity on the amount of deposited metal led to the mechanism of the charge transfer in these structures. We found that the primary role in this mechanism is due to defects in the quartz glass structure which act as traps for electrons.

Determination of Fundamental Morphological Parameters of Supported Nanoparticle Ensembles

A new model to extract important morphological parameters of noble metal nanoparticle ensembles with a broad size and shape distribution is presented. The technique is based on a rigorous simulation of the inhomogeneously broadened extinction profiles of nanoparticle ensembles. As input data, only experimentally accessible parameters, such as the amount of deposited material, the nanoparticle number density, and the relative size distribution of the nanoparticles, are used. The model can be applied to oblate nanoparticles, which exhibit a strong correlation between their shape and size, e.g., to supported nanoparticles generated, for example, by deposition of atoms and subsequent nucleation or by gas phase deposition. Both methods are standard preparation techniques to generate well-defined nanoparticle ensembles under ultra high vacuum conditions. We apply our model to gold and silver nanoparticles on sapphire and TiO2 supports and obtain a perfect agreement between the calculated and experimental data. More importantly, we could extract the functional dependence between the axial ratio and the radius of the nanoparticles within the ensemble and, therewith, the most probable axial ratio in the ensemble. In addition, the extinction spectrum of a nanoparticle ensemble irradiated with nanosecond pulsed laser light during growth has been successfully modeled. This demonstrates, that the model is able to describe shape changes of resonantly heated nanoparticles within the ensemble. By using the coverage as a free parameter, we could calculate from the extinction spectrum the average particle radius as well as the amount of desorbed atoms after irradiation with laser light. In summary, the model allows a fast, easy, but extensive morphological characterization of nanoparticle ensembles that exhibit a broad size and shape distribution.

Chemical damping of the localized surface plasmon polariton resonance: infuence of different chemical environments

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.