Christian Leiterer

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.

A precise optical determination of nanoscale diameters of semiconductor nanowires

Electrical and optical properties of semiconducting nanowires (NWs) strongly depend on their diameters. Therefore, a precise knowledge of their diameters is essential for any kind of device integration. Here, we present an optical method based on dark field optical microscopy to easily determine the diameters of individual NWs with an accuracy of a few nanometers and thus a relative error of less than 10%. The underlying physical principle of this method is that strong Mie resonances dominate the optical scattering spectra of most semiconducting NWs and can thus be exploited. The feasibility of this method is demonstrated using GaAs NWs but it should be applicable to most types of semiconducting NWs as well. Dark field optical microscopy shows that even slight tapering of the NWs, i.e. diameter variations of a few nanometers, can be detected by a visible color change. Abrupt diameter changes of a few nanometers, as they occur for example when growth conditions vary, can be determined as well. In addition a profound analysis of the elastic scattering properties of individual GaAs NWs is presented theoretically using Mie calculations as well as experimentally by dark field microscopy. This method has the advantage that no vacuum technique is needed, a fast and reliable analysis is possible based on cheap standard hardware.

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.

High precision attachment of silver nanoparticles on AFM tips by dielectrophoresis

AbstractAFM 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. Graphical abstractSilver nanoparticles attached to AFM tips using dielectrophoresis. Comparing nanoparticles attached using 1 kHz (left) to 1 MHz (right), SEM and optical (inset) images

Applying contact to individual silicon nanowires using a dielectrophoresis (DEP)-based technique

One major challenge for the technological use of nanostructures is the control of their electrical and optoelectronic properties. For that purpose, extensive research into the electrical characterization and therefore a fast and reliable way of contacting these structures are needed. Here, we report on a new, dielectrophoresis (DEP)-based technique, which enables to apply sufficient and reliable contact to individual nanostructures, like semiconducting nanowires (NW), easily and without the need for lithography. The DEP contacting technique presented in this article can be done without high-tech equipment and monitored in situ with an optical microscope. In the presented experiments, individual SiNWs are trapped and subsequently welded between two photolithographically pre-patterned electrodes by applying varying AC voltages to the electrodes. To proof the quality of these contacts, I–V curves, photoresponse and photoconductivity of a single SiNW were measured. Furthermore, the measured photoconductivity in dependence on the wavelength of illuminated light and was compared with calculations predicting the absorption spectra of an individual SiNW.

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.

Dielectrophoresis based integration of nanostructures and their sensorial application

Here we present a technique to integrate bottom-up nanostructures for optoelectronic and chemoresistive sensing using an AC electrical field. The work focuses mainly on two types of nanostructured materials: gold nanoparticle and silicon nanowire. In terms of electrical microintegration of these structures, it is especially important to apply a reliable electrical contact with low contact-resistance, in order to be able to use them as optoelectronic or chemo resistive sensors. To achieve this, a micro integration process was developed to achieve this goal. The contacted nanostructures were characterized electrically to optimize the integration procedure and acquire best possible sensing capabilities. Silicon nanowires were demonstrated to work as wavelength sensitive optical sensors and gold nanoparticle as marker free chemo resistive sensor.

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).

Index matching at the nanoscale: light scattering by core-shell Si/SiOx 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.

DEP-Based Integration of G-quadruplex Structures

The unique properties of G4-based nucleic acids provide the base for a variety of applications.A whole set of biomedical as well as bioanalytical applications is based on the ability of G-quartets to stabilize defined three-dimensional nucleic acid structures that exhibit a high affinity to a target molecule. Such aptamers can therefore show similar binding properties as antibodies with comparable applications in diagnostics and therapy, but exhibit striking advantages like ex-vivo synthesis and increased physicochemical stability. Moreover, they can even show catalytic behavior that can be utilized for bioanalytical purposes. The chapter contains examples for applications in these fields.The structural properties described in previous chapters are the base for applications in molecular nanotechnology and –electronics. Nucleic acids represent the most promising materials in these fields, and here G4 structures show even outstanding mechanical stability and length control from the nano- into the micrometer range. An important step on the way to respective applications is the integration of G4-based nanostructures into technical environments such as microelectrodes. Here electrical field-based approaches – e.g. dielectrophoresis DEP – represent the most promising technique, which has been demonstrated for the integration and subsequent characterization of even single G4 structures. These techniques have also been used to enable the characterization of electrical properties of G4 assemblies as described in this chapter.In conclusion, by presenting various biomedical as well as nanobiotechnological demonstrations this chapter demonstrates the great application potential of G4-based nanostructures.