Lukas M. Eng

Monodisperse Platinum Nanospheres with Adjustable Diameters from 10 to 100 nm: Synthesis and Distinct Optical Properties

We present a facile and reproducible method for synthesizing monodisperse platinum (Pt) spheres with sizes ranging from 10 to 100 nm in diameter and exceptionally small standard deviations of 3% for large spheres. The reaction takes place in aqueous solution using a multistep seed-mediated approach. The Pt nanospheres consist of several small crystallites resulting in a surface roughness of 5-10 nm. Extinction spectra are measured from particles dispersed in water and calculated for single particles which are found to be in excellent agreement. We obtain a linear correlation between the plasmon extinction maximum (from UV to the visible regions) and the particle diameter which might be of value for experimentalists in the field.

The Route to Functional Graphene Oxide

We report on an easy-to-use, successful, and reproducible route to synthesize functionalized graphite oxide (GO) and its conversion to graphene-like materials through chemical or thermal reduction of GO. Graphite oxide containing hydroxyl, epoxy, carbonyl, and carboxyl groups loses mainly hydroxyl and epoxy groups during reduction, whereas carboxyl species remain untouched. The interaction of functionalized graphene with fluorescent methylene blue (MB) is investigated and compared to graphite, fully oxidized GO, as well as thermally and chemically reduced GO. Optical absorption and emission spectra of the composites indicate a clear preference for MB interaction with the GO derivatives containing a large number of functional groups (GO and chemically reduced GO), whereas graphite and thermally reduced GO only incorporate a few MB molecules. These findings are consistent with thermogravimetric, X-ray photoelectron spectroscopic, and Raman data recorded at every stage of preparation. The optical data also indicate concentration-dependent aggregation of MB on the GO surface leading to stable MB dimers and trimers. The MB dimers are responsible for fluorescence quenching, which can be controlled by varying the pH value.

Nanoscale reconstruction of surface crystallography from three-dimensional polarization distribution in ferroelectric barium–titanate ceramics

The three-dimensional orientation of polarization vectors at the surface of ferroelectric barium–titanate (BaTiO3) ceramics is evaluated using voltage-modulated scanning force microscopy (SFM). By applying an ac voltage to the conductive SFM tip, we measure the relative amount of the three orthogonal components Px, Py, and Pz, of the polarization vector at any surface point. The measured polarization orientation together with the actual domain-wall orientation allows a precise reconstruction of the crystallographic orientation of the investigated grains down to a 40 nm resolution. Excellent agreement is obtained when comparing this orientation with the crystallographic reconstruction revealed by etch patterns from the sample surface topography. We show that the surface topography manifests a domain structure, which was present in the past, while the actual ferroelectric domain configuration is revealed by the modulation technique.

Differentiating 180° and 90° switching of ferroelectric domains with three-dimensional piezoresponse force microscopy

Three-dimensional (3D) piezoresponse force microscopy is applied in order to differentiate 90° and 180° domain switching in PbTiO3 (PTO) thin films. The 3D domain configuration is recorded both statically, revealing the surface crystallographic orientation of PTO films on the nanometer scale, and dynamically by simultaneously mapping the in-plane and out-of-plane hysteresis loops. We show that exclusively 180° switching occurs, also switching only half of the grain volume.

Imaging three-dimensional polarization in epitaxial polydomain ferroelectric thin films

Voltage-modulated scanning force microscopy (Piezoresponse microscopy) is applied to investigate the domain structure in epitaxial PbZr0.2Ti0.8O3 ferroelectric thin films grown on (001) SrTiO3. By monitoring the vertical and lateral differential signals from the photodetector of the atomic force microscope it is possible to separate out and observe the out-of-plane and in-plane polarization vectors in the thin film individually. The relative orientation of the polarization vectors across a 90° domain wall is observed. Nucleation of new reversed 180° domains at the 90° domain wall is studied and its impact on the rotation of polarization within the a domain is analyzed as a function of reversal time.

Néel-type skyrmion lattice with confined orientation in the polar magnetic semiconductor GaV4S8.

Following the early prediction of the skyrmion lattice (SkL)--a periodic array of spin vortices--it has been observed recently in various magnetic crystals mostly with chiral structure. Although non-chiral but polar crystals with Cnv symmetry were identified as ideal SkL hosts in pioneering theoretical studies, this archetype of SkL has remained experimentally unexplored. Here, we report the discovery of a SkL in the polar magnetic semiconductor GaV4S8 with rhombohedral (C3v) symmetry and easy axis anisotropy. The SkL exists over an unusually broad temperature range compared with other bulk crystals and the orientation of the vortices is not controlled by the external magnetic field, but instead confined to the magnetic easy axis. Supporting theory attributes these unique features to a new Néel-type of SkL describable as a superposition of spin cycloids in contrast to the Bloch-type SkL in chiral magnets described in terms of spin helices.

Depolarizing-field-mediated 180° switching in ferroelectric thin films with 90° domains

Switching of the out-of-plane and in-plane polarizations in polydomain epitaxial PbZr0.2Ti0.8O3 thin films is studied using three-dimensional piezoresponse force microscopy (PFM). It is found that, under an electric field induced between the PFM tip and the bottom electrode, the 180° switching occurs in both c and a domains. After the removal of this field, the spontaneous reversal of the out-of-plane and in-plane polarizations back to the initial orientations takes place, evolving via heterogeneous development of antiparallel 180° domains. The switching of in-plane polarization inside a domains and the preferential formation of reversed 180° domains at 90° domain walls are explained by the effects of the depolarizing fields caused by transient polarization charges appearing on these domain walls.

Near-field coupling of a single fluorescent molecule and a spherical gold nanoparticle

Near-field coupling of a single gold nanoparticle (GNP) to a single fluorescent molecule is investigated here for varying separation d between the two. While the emission quantum efficiency of the coupled system generally decreases for d!0, a pronounced near-field enhancement is observed under certain conditions, partly outweighing the efficiency loss at small distances. We report on optimizing these conditions by varying the excitation field direction and the three-dimensional relative configuration between the GNP and the fluorophore. Furthermore, we examine how the sphere diameter, the surrounding medium, as well as the absorption and emission wavelengths of the molecular dipole influence the fluorescence yield. Our results are of high practical relevance for all GNP-mediated application fields such as fluorescence microscopy, scattering near-field optical microscopy, bioanalytics, and medical applications.

Distance Dependent Spectral Tuning of Two Coupled Metal Nanoparticles

The spectral properties of two spherical metallic nanoparticles of 80 nm in diameter are examined with regard to the interparticle distance and relative polarization of the excitation light. One Au nanoparticle is attached to a scanning fiber probe and the second to a scanning substrate. This configuration allows three-dimensional and arbitrary manipulation of both distance and relative orientation with respect to the incident light polarization. As supported by numerical simulations, a periodic modulation of the coupled plasmon resonance is observed for separations smaller than 1.5 microm. This interparticle coupling affects the scattering cross section in terms of spectral position and spectral width as well as the integral intensity of the Mie-scattered light.

Ferroelectric lithography: bottom-up assembly and electrical performance of a single metallic nanowire.

We report on both the assembly of noble-metal nanowires by means of the nanotechnological and large-scale integrable approach of ferroelectric lithography and their performance testing upon electrical transport. Our results on LiNbO(3) single crystal templates show that the deposition of different elemental metals from ionic solutions by photochemical reduction is confined to the ferroelectric 180 degrees domain walls. Current-voltage-characteristics recorded from such nanowires of typically 30-300 microm in length revealed an Ohmic behavior that even improved with time. Additionally, we also examined the local topographic and potentiostatic properties of such wires using dynamic scanning force microscopy in combination with Kelvin probe force microscopy.