Examples of my scientific work

Monodisperse PMMA-beads as a model for DWBA effects

Two samples with the same PMMA-beads were compared by means of GISAXS: A) with a loose package of beads and B) with compact arrangment. Both are monolayers and have the same bead diameter of 340 nm. The following figure shows the two different situations, how the x-ray beam (red) passes the particles and which scattering terms (yellow) will appear. In the upper case all four terms are possible. While term 1 has its orign around the direct beam, the other two strong terms 2 and 3 will scatter around the reflected beam. This will lead to a strong oszillating scattering around the specular beamstop, with interferences between term 2 and 3, as they have a slightly phase shift.

DWBA scattering events for a loose (top) and a dense arrangement (bottom) of relatively large PMMA-beads on a highly reflecting silicon substrate

I used these models for simulating GISAXS scattering patterns and compared them with a GIUSAXS experiment. The simulation and experiment are in good agreement. Most of the maxima in the patterns arise from the form factor terms, and not from the 2D-lattice.

Experiment and simulation for a loose (top) and a dense arrangement (bottom) of PMMA-bead monolayers)

(click images for original publication)

Filled block copolymer micelles and plasma treatment

The next experiment show patterns and simulations for SiO2 loaded cores in a micellar monolayer before and after plasma treatment. Assuming the depicted model of core/shell spheres or half spheres on a substrate the simulation leads to almost identical patterns. This allows determining the lateral distances between the particles. Additionally the height, shape, and density of the particles are revealed as well as the all over thickness of the coating. Before plasma treatment, the still existing polymer scatters at a lower critical angle than the substrate, which leads to an additional scattering maximum at lower qz. Hence the polymer shell has the same lateral periodicity as the filled cores, the reflections along qy do not change their position. Before O2 plasma treatment, the 2-dimensional hexagonal array has a period of 38.5 nm, the SiO2 particle radius is 9 nm and the height of the coating 22 nm. After removing the organic shell via oxygen plasma the height of the array decreases to 13 nm, while the lateral values are constant. This means that not only the organic shell on top of the SiO2 particles is completely removed but also the particles are etched from the top by 5 nm.

Experiment and simulation before and after O2-plasma treatment of SiO2 filled diblock copolymer micelles on a silicon wafer

The microscopic methods SEM and AFM are not suitable to investigate the surface morphology on the scale of several mm. Micrographs show only a small part of the sample and for better statistics additional methods are needed. Here GISAXS is the first choice, as it reveals information from a large part (several mm2) of the surface. The scattering curves can be compared with the FFTs of the microscopic images, to demonstrate that the same nanostructure is uniformly spread over a large surface area.

SEM FFT spectra and GISAXS out of plane scattering curves (at Yoneda maximum) from silica filled micelles before and after O2-plasma etching

(click images for original publication)

Sputtering masks for magnetic dot production

Magnetic nanostructure arrays are created using diblock copolymer micelles with silica loaded cores. The number of preparation steps is kept as low as possible to simplify the formation of the nanostructure array. The SiO2 filled micelles are deposited on a Co/Pt-multilayer film. After removal of the organic shell by oxygen etching, the structure of the cores is transferred to the film via ion milling under normal incidence. The generated dots were made of (Co/Pt)2-multilayers. They show different magnetic behaviour, depending on their size, interparticle distance and milling time

MOKE investigations of a nanopatterned (Co/Pt)n-multilayer film at different ion milling times and scheme of milling process

The etching mask was transformed into dots of Co/Pt. The magnetic behaviour changes during the milling process. Further investigations are in progress to understand the mechanism. The GISAXS patterns change drastically after ion milling. The strong oscillations along qz disappeared, indicating that the complete Co/Pt multilayer was eroded between the metal oxide particles while along qy the interference increased due to the formation of the Co/Pt dot array.

GISAXS pattern of SiO2 particles on Co/Pt multilayer before sputtering (left) and of the fabricated magnetic dot array by Ar+-ion milling (right).

(click images for original publication)

Thermal ligand degradation at CoPt3-nanoparticles and their arrangement in monolayers

After deposition of the CoPt3-nanoparticles on the substrates, the particles self assemble into highly ordered 2-dimensional hexagonal arrays. The organic shell prevents the interparticle contact. To remove the organic ligands, the samples were heated to different temperatures for 2 hours. The stability of the particles and of the 2-dimensional arrangement is important for further applications, i.e. magnetic quantum dots. The characterisation of the obtained 2-dimensional nanoparticle-arrays surfaces has be done by scanning electron microscopy (SEM) and GISAXS.

Thermal degradation of a 2D nanoparticle array at different temperatures.

left: GISAXS patterns; mid: intensity profile cuts along qy and Scatter simulation with parameters; right: SEM images (200 nm x 200 nm; 20 keV)

(click image for original publication)

At 300 C the interparticle distance decreases, while the quality of the array is constant. At higher temperatures, the particles are starting to melt and form partial aggregates, which lead to a higher deviation of the particle form factor. Additionally the diffuse scattering increases and the order parameters show a stronger displacement of the particles from their ideal lattice points.

Highly ordered 2D-arrays of FeOx-nanoparticle/polymer composites

The distance between oxide nanoparticles can be tuned by different organic ligands attached on their surface. After synthesys oleic acid form the organic shell of iron oxide particles. This short ligand leads to small lateral distances, as one can observe in the left TEM micrograph. In reciprocal space, the GISAXS pattern of such a monolayer show reflections at higher qy. After ligand exchange against a long polystyrene block, the distance gets larger by preservation of the the long distance order, as indicated by the higher order reflections in the right GISAXS pattern.

Distance control inside 2D-arrays of iron oxide nanoparticles by ligand exchange

(click image for original publication)

Contents provided by Dr. A. Meyer - Institute of Physical Chemistry - University of Hamburg