Index matching at the nanoscale: light scattering by core-shell Si/SiO x nanowires.

Silicon nanowires (SiNWs) show strong resonant wavelength enhancement in terms of absorption as well as scattering of light. However, in most optoelectronic device concepts the SiNWs should be surrounded by a contact layer. Ideally, such a layer can also act as an index matching layer which could nearly halve the strong reflectance of light by silicon. Our results show that this reduction can be overcome at the nanometer scale, i.e. SiNWs embedded in a silica (SiO x ) layer can not only maintain their high scattering cross sections but also their strong polarization dependent scattering. Such effects can be useful for light harvesting or optoelectronic applications. Moreover, we show that it is possible to optically determine the diameters of the embedded nanoscale silicon (Si) cores.

High precision attachment of silver nanoparticles on AFM tips by dielectrophoresis.

AFM tips are modified with silver nanoparticles using an AC electrical field. The used technique works with sub-micron precision and also does not require chemical modification of the tip. Based on the electrical parameters applied in the process, particle density and particle position on the apex of the tip can be adjusted. The feasibility of the method is proven by subsequent tip-enhanced Raman spectroscopy (TERS) measurements using the fabricated tips as a measurement probe. Since this modification process itself does not require any lithographic processing, the technique can be easily adapted to modify AFM tips with a variety of nanostructures with pre-defined properties, while being parallelizable for a potential commercial application.

Dielectrophoretic positioning of single nanoparticles on atomic force microscope tips for tip-enhanced Raman spectroscopy.

Tip-enhanced Raman spectroscopy, a combination of Raman spectroscopy and scanning probe microscopy, is a powerful technique to detect the vibrational fingerprint of molecules at the nanometer scale. A metal nanoparticle at the apex of an atomic force microscope tip leads to a large enhancement of the electromagnetic field when illuminated with an appropriate wavelength, resulting in an increased Raman signal. A controlled positioning of individual nanoparticles at the tip would improve the reproducibility of the probes and is quite demanding due to usually serial and labor-intensive approaches. In contrast to commonly used submicron manipulation techniques, dielectrophoresis allows a parallel and scalable production, and provides a novel approach toward reproducible and at the same time affordable tip-enhanced Raman spectroscopy tips. We demonstrate the successful positioning of an individual plasmonic nanoparticle on a commercial atomic force microscope tip by dielectrophoresis followed by experimental proof of the Raman signal enhancing capabilities of such tips.

Combined dielectrophoresis-Raman setup for the classification of pathogens recovered from the urinary tract.

Rapid and effective methods of pathogen identifications are of major interest in clinical microbiological analysis to administer timely tailored antibiotic therapy. Raman spectroscopy as a label-free, culture-independent optical method is suitable to identify even single bacteria. However, the low bacteria concentration in body fluids makes it difficult to detect their characteristic molecular fingerprint directly in suspension. Therefore, in this study, Raman spectroscopy is combined with dielectrophoresis, which enables the direct translational manipulation of bacteria in suspensions with spatial nonuniform electrical fields so as to perform specific Raman spectroscopic characterization. A quadrupole electrode design is used to capture bacteria directly from fluids in well-defined microsized regions. With live/dead fluorescence viability staining, it is verified, that the bacteria survive this procedure for the relevant range of field strengths. The dielectrophoretic enrichment of bacteria allows for obtaining high quality Raman spectra in dilute suspensions with an integration time of only one second. As proof-of-principle study, the setup was tested with Escherichia coli and Enterococcus faecalis, two bacterial strains that are commonly encountered in urinary tract infections. Furthermore, to verify the potential for dealing with real world samples, pathogens from patients' urine have been analyzed. With the additional help of multivariate statistical analysis, a robust classification model could be built and allowed the classification of those two strains within a few minutes. In contrast, the standard microbiological diagnostics are based on very time-consuming cultivation tests. This setup holds the potential to reduce the crucial parameter diagnosis time by orders of magnitude.

DNA hybridization assay at individual, biofunctionalized zinc oxide nanowires.

Reliable and efficient identification of DNA is a major goal in on-site diagnostics. One dimensional nanostructures like nanowires (NW) represent potential sensor structures due to their extreme surface-to-bulk ratio, enabling enhanced biomolecule binding which results in optimal signals. While silicon NW are already well studied, NW made from other materials with promising properties like ZnO are not yet established as NW sensor material for bioanalytics. Here we demonstrate the DNA functionalization of ZnO NW even at the single NW level and their successful application in a DNA hybridization assay.

Far-field imaging for direct visualization of light interferences in GaAs nanowires.

The optical and electrical characterization of nanostructures is crucial for all applications in nanophotonics. Particularly important is the knowledge of the optical near-field distribution for the design of future photonic devices. A common method to determine optical near-fields is scanning near-field optical microscopy (SNOM) which is slow and might distort the near-field. Here, we present a technique that permits sensing indirectly the infrared near-field in GaAs nanowires via its second-harmonic generated (SHG) signal utilizing a nonscanning far-field microscope. Using an incident light of 820 nm and the very short mean free path (16 nm) of the SHG signal in GaAs, we demonstrate a fast surface sensitive imaging technique without using a SNOM. We observe periodic intensity patterns in untapered and tapered GaAs nanowires that are attributed to the fundamental mode of a guided wave modulating the Mie-scattered incident light. The periodicity of the interferences permits to accurately determine the nanowires' radii by just using optical microscopy, i.e., without requiring electron microscopy.

G-wire synthesis and modification with gold nanoparticle.

DNA molecules are well known for containing the genetic information of an individual. Furthermore, DNA is a biopolymer with the potential of building up nanoscale structures. These structures can be addressed sequence specifically and, therefore, they allow connecting and arranging with subnanometer accuracy.The extended work of the group of Nadrian Seeman (Nature 421:427-431, 2003) has shown that the self-assembly of DNA molecules offers great potential for the creation of bottom-up nanostructures for nanoelectronics, biosensors, and programmable molecular machines. Rothemund (Nature 440:297-302, 2006) has shown that it is possible to generate a wide variety of 2D nanostructures by the assembly of synthetic desoxyoligonucleotides and M13mp18 DNA via Watson-Crick base pairing. Furthermore, DNA can form three- and four-stranded structures which offer even more possibilities for molecular construction. This chapter will deal with four-stranded DNA structures (G-wires) created from 10-bp deoxynucleotide units. Our focus will be especially on the synthesis, individualization, modification with gold nanoparticles, and characterization by high-resolution scanning force microscopy (AFM).

Optical properties of individual silicon nanowires for photonic devices.

Silicon is a high refractive index material. Consequently, silicon nanowires (SiNWs) with diameters on the order of the wavelengths of visible light show strong resonant field enhancement of the incident light, so this type of nanomaterial is a good candidate for all kinds of photonic devices. Surprisingly enough, a thorough experimental and theoretical analysis of both the polarization dependence of the absorption and the scattering behavior of individual SiNWs under defined illumination has not been presented yet. Here, the present paper will contribute by showing optical properties such as scattering and absorption of individual SiNWs experimentally in an optical microscope using bright- and dark-field illumination modes as well as in analytical Mie calculations. Experimental and calculation results are in good agreement, and both reveal a strong correlation of the optical properties of individual SiNWs to their diameters. This finding supports the notion that SiNWs can be used in photonic applications such as for photovoltaics or optical sensors.

Dielectrophoretic manipulation of DNA in microelectrode gaps for single-molecule constructs.

The construction with biomolecules and their manipulation represent a key step for developing new miniaturized structures. Such micro or nanometer systems promise a variety of novel features. Dielectrophoresis (DEP) is a powerful tool for trapping and orienting individual molecules in microelectrode arrangements, and was demonstrated to be applicable to DNA. This relatively rigid biomolecule could (after defined immobilization) act as template for further modifications and functionalizations, e.g. metallization. Parameters of the DEP process were adapted to the given electrode layout and for trapping a few or even a single DNA strand. Characterization with atomic force microscopy (AFM) extends the standard method of fluorescence imaging by resolving the resulting structures with single molecule resolution.