Roland Mattheis

The morphology of silver nanoparticles prepared by enzyme-induced reduction

Summary Silver nanoparticles were synthesized by an enzyme-induced growth process on solid substrates. In order to customize the enzymatically grown nanoparticles (EGNP) for analytical applications in biomolecular research, a detailed study was carried out concerning the time evolution of the formation of the silver nanoparticles, their morphology, and their chemical composition. Therefore, silver-nanoparticle films of different densities were investigated by using scanning as well as transmission electron microscopy to examine their structure. Cross sections of silver nanoparticles, prepared for analysis by transmission electron microscopy were additionally studied by energy-dispersive X-ray spectroscopy in order to probe their chemical composition. The surface coverage of substrates with silver nanoparticles and the maximum particle height were determined by Rutherford backscattering spectroscopy. Variations in the silver-nanoparticle films depending on the conditions during synthesis were observed. After an initial growth state the silver nanoparticles exhibit the so-called desert-rose or nanoflower-like structure. This complex nanoparticle structure is in clear contrast to the auto-catalytically grown spherical particles, which maintain their overall geometrical appearance while increasing their diameter. It is shown, that the desert-rose-like silver nanoparticles consist of single-crystalline plates of pure silver. The surface-enhanced Raman spectroscopic (SERS) activity of the EGNP structures is promising due to the exceptionally rough surface structure of the silver nanoparticles. SERS measurements of the vitamin riboflavin incubated on the silver nanoparticles are shown as an exemplary application for quantitative analysis.

Advanced giant magnetoresistance technology for measurement applications

Giant magnetoresistance (GMR) sensors are considered one of the first real applications of nanotechnology. They consist of nm-thick layered structures where ferromagnetic metals are sandwiched by nonmagnetic metals. Such multilayered films produce a large change in resistance (typically 10 to 20%) when subjected to a magnetic field, compared with a maximum change of a few per cent for other types of magnetic sensors. This technology has been intensively used in read heads for hard disk drives and now increasingly finds applications due to the high sensitivity and signal-to-noise ratio. Additionally these sensors are compatible with miniaturization and thus offer a high spatial resolution combined with a frequency range up to the 100 MHz regime and simple electronic conditioning. In this review, we first discuss the basics of the underlying magnetoresistance effects in layered structures and then present three prominent examples for future applications: in the field of current sensing the new GMR sensors offer high bandwidth and good accuracy in a space-saving open loop measurement configuration. In rotating systems they can be used for multiturn angle measurements, and in biotechnology the detection of magnetic particles enables the quantitative measurement of biomolecule concentrations.

A New Four Bit Magnetic Domain Wall Based Multiturn Counter

A multiturn counter is proposed which is based on domain wall movement. This sensor has a true-power-on functionality and works gear-less. Due to the new design of the multiturn, the drawbacks of the former design, namely the limitation to 12 turns in maximum and a large die size, are overcome, allowing counting of up to 64 turns at reasonable die size. Parameters as the maximum frequency of the rotating field to be detected and the magnetic working window fit the demands in industrial application.

360/spl deg/ domain wall investigation for sensor applications

The stability of 360/spl deg/ domain walls is suitable for future sensor applications. We present a new idea for such a sensor based on special spin valve geometries, where 360/spl deg/ domain walls can be generated with an appropriate magnetic field and stored. Micro magnetic simulations and the first experimental structures show the feasibility of the idea. The magnetoresistive effect in our spin-valve sandwich is up to 0.5% for one 360/spl deg/ domain wall.

Multiturn Counter Using the Movement and Storage of 180

We describe a novel giant magnetoresistive (GMR) stack geometry which enables the counting of the number of turns of a rotating magnetic field. A race-track-like spiral of N turns acts as a 180deg domain-wall memory. At one end of the spiral, an enlarged film area works as a domain-wall generator. Due to the small width of the spirals themselves in the order of 100-200 nm, there is only domain-wall movement within the spiral. By rotating the magnetic field with/against the sense of the spiral, every half turn increases/decreases the number of domains by one. The number of domain walls have a one-to-one correspondence with the resistance of the spiral due to the GMR effect

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We investigated silver-covered polymer based nanogratings as substrates for surface-enhanced Raman spectroscopy (SERS), in particular with respect to the thickness of the plasmonically active silver film. In order to obtain accurate geometrical input data for the simulation process, we inspected cross sections of the gratings prepared by breaking at cryogenic temperature. We noticed a strong dependence of the simulation results on geometrical variations of the structures. Measurements revealed that an increasing silver film thickness on top of the nanogratings leads to a blue shift of the plasmonic resonance, as predicted by numerical simulations, as well as to an increased field enhancement for an excitation at 488 nm. We found a clear deviation of the experimental data compared to the simulated results for very thin silver films due to an island-like growth at a silver thickness below 20 nm. In order to investigate the SERS activity. we carried out measurements with crystal violet as a model analyte at an excitation wavelength of 488 nm. The SERS enhancement increases up to a silver thickness of about 30 nm, whereas it remains nearly constant for thicker silver films.

Magnetic Domain Walls

Summary form only given. Exchange bias is widely used for fixing a magnetic soft layer within a GMR or TMR spin valve system. However there is a big lack of knowledge concerning the microscopic description and the determination of local properties like the distribution of exchange bias field strength H/sub eb/ and direction responsible for the reversal mechanism, and asymmetry of the reversal. In this paper the interaction at the ferromagnet/ antiferromagnet interface was studied in detail by MOKE and AMR investigations.

The effect of silver thickness on the enhancement of polymer based SERS substrates

Growth temperature effects on the microstructure of Nb-doped BaTiO3 thin films of the composition BaTi0.98Nb0.02O3 are studied using X-ray diffraction and transmission electron microscopy (TEM). Reciprocal space maps and electron diffraction patterns show that the a-axis lattice parameter increases and the c-axis parameter decreases with increasing growth temperature, indicating a decrease of tetragonality. Bright-field TEM images show low and high densities of threading defects in films grown at low and high temperatures, respectively. The observations are discussed in terms of a hindering of the cubic-to-tetragonal phase transition by a high defect density and a high unit cell volume.

MOKE and AMR investigations on exchange bias field strength and direction in ferromagnet/antiferromagnet systems

We report on a galvanic isolated current sensor that operates on the principle of the giant magnetoresistive effect. Due to an optimized combination of uniaxial and unidirectional magnetic anisotropy of the sense layer system the sensor response is temperature independent and linear at low currents. Higher currents are measured with a reduced sensitivity. By optimizing the sensor stack a good accuracy with an error less than 0.3% is obtained in the industrial temperature range. The sensor system is capable of measuring dc, ac, and pulse currents up to 20 MHz.

Growth temperature dependence of crystal symmetry in Nb-doped BaTiO3 thin films

In order to increase the inherent weak but molecular specific Raman signature the exceptional optical properties of plasmonic nanostructures are used. This technique is termed as surface enhanced Raman spectroscopy (SERS) and combines the fingerprint specificity with potential single molecule sensitivity. However, the non-application of SERS in routine analytics is often linked with the low reproducibility of metallic nanostructures or metallic surfaces. Within this contribution, the fabrication of innovative plasmonic arrays with homogenous signal enhancement is introduced, which is an important contribution towards powerful SERS diagnostics. Here, the application of plasmonic arrays for DNA sensing is demonstrated.