We use the nonlinear optical property of GaAs to directly visualize the path of the near infrared incident laser light
coupled into individual nanowires. We fully illuminate with near infrared pulse laser untapered and tapered GaAs
nanowires grown via the Au-assisted vapor-liquid-solid mechanism. We record second-harmonic generation (SHG)
signals in the visible spectrum. In some nanowires, an interference pattern is observed and investigated in terms of
distances between the maxima of the SHG signal taking into account the effective refractive index in such sub
wavelength structures with radius below 90 nm. We propose a model to explain the periodicity of the maxima in the
SHG interference pattern. The theoretical model includes the waveguiding and the Mie scattering theories for obtaining
the 2π periodicity fitting well the experiments. Moreover, we also measure interferences in tapererd nanowires with a
radius down to 76 nm. The possible effect of the gold in non radiative recombination and the presence of the gold
particle at the tip of some nanowires are also discussed.
Charge carrier distribution changes in solid substrates induced by the presence of biomolecules have the potential as
sensoric principle. For a high surface-to-bulk ratio as in the case of nanostructures, this effect can be used for highly
Plasmonic nanosensors represent one possible implementation: The resonance wavelength of the conductive electron
oscillation under light irradiation is changed upon molecular binding at the structure surface. This change can be detected
by spectroscopic means, even on a single nanoparticle level using microspectroscopy.
Other examples are nanowires in electrodes gaps, either by metal nanoparticles arranged in a chain-like geometry or by
rod-like semiconductor nanowires directly bridging the gap. Molecules binding at the surface will lead to changes in the
electrical conductivity which can be easily converted into an electrical readout. The various geometries will be discussed
and their sensoric potential for an electrical detection demonstrated.
Nanoscale sensors have the potential for ultrasensitive and highly parallel bioanalytical applications. Bottom up methods
like gas-phase self assembly allow for the controlled and cost-efficient preparation of numerous functional units with
nanometer dimensions. Their use in sensoric instruments, however, requires the defined integration into sensoric setups
such as electrode arrays.
We show here how to use alternating electrical fields (dielectrophoresis DEP) in order to address this micro nano
integration problem. Nanoscale units such as metal nanoparticles or semiconductor nanowires are thereby polarized and
moved into the direction of higher electrical field gradients. As result, these particles bridge an electrode gap and can so
be used for electrical sensoric using the electrical resistance through this structure as value correlated to the presence of
molecules at the sensor surface. In order to achieve high selectivity, capture molecules (such as complementary DNA or
antibodies) are used.
The connection of biomolecules like DNA to a micro scale environment such as microarrays and Lab-on-a-chip systems
is an imminent task in biochip technology. Especially in Lab-on-a-chip systems microscopic forces are used to separate
the analyte from a complex mixture for further analysis . In this contribution the sorting and manipulation of DNA
using dielectrophoresis (DEP) on micro structured chips was investigated . DEP represents an interesting approach to
manipulate and control objects at the micro- [3, 4] and nanoscale range [5-7], and especially to position them at
controlled locations in microelectrode arrangements. It could be shown that DNA can be reversible arranged but also
permanently immobilized in micro scale electrode gaps. It was also demonstrated that it is possible to stretch and align
DNA from a single molecule level to high DNA concentration in a parallel manner between microelectrodes .
Furthermore DNA was stretched between moveable electrodes.
Although functional molecular constructs promise a variety of interesting properties in combination with parallel
realization and molecular precision, the utilization requires usually integration into the macroscopic world such as
electrodes or other technical environments. Dielectrophoresis (DEP) represents an interesting approach to manipulate
and control objects at the nanoscale, and especially to position them at controlled locations in microelectrode
arrangements. Over the years this technique was established in our group and is now able to arrange either metal
nanoparticles and/or DNA into these gaps in a highly reproducible manner. Microscopic tools were optimized in order to
be able to follow single particles/molecules during the process. This ability greatly improves the potential, because now
the key parameters can be easily tuned during live imaging of the controlled objects and their behavior. It was possible to
realize bridges of nanoparticles as well as of a few stretched DNA molecules on gold microelectrode structures at chip
surfaces. Moreover, DNA positioned by DEP in electrode gaps was metallized and the resulting metal nanostructure
characterized. Work is in process to combine the various units as well as processes in order to access more complex