Henrik Schneidewind

Probing Innovative Microfabricated Substrates for their Reproducible SERS Activity

New types of microfabricated surface-enhanced Raman spectroscopy (SERS) active substrates produced by electron beam lithography and ion beam etching are introduced. In order to achieve large enhancement factors by using the lightning rod effect, we prepare arrays consisting of sharp-edged nanostructures instead of the commonly used dots. Two experimental methods are used for fabrication: a one-stage process, leading to gold nanostar arrays and a two-stage process, leading to gold nanodiamond arrays. Our preparation process guarantees high reproducibility. The substrates contain a number of arrays for practical applications, each 200x200 microm2 in size. To test the SERS activity of these nanostar and nanodiamond arrays, a monolayer of the dye crystal violet is used. Enhancement factors are estimated to be at least 130 for the nanodiamond and 310 for the nanostar arrays.

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.

Doubly resonant optical nanoantenna arrays for polarization resolved

We report that rhomb-shaped metal nanoantenna arrays support multiple plasmonic resonances, making them favorable bio-sensing substrates. Besides the two localized plasmonic dipole modes associated with the two principle axes of the rhombi, the sample supports an additional grating-induced surface plasmon polariton resonance. The plasmonic properties of all modes are carefully studied by far-field measurements together with numerical and analytical calculations. The sample is then applied to surface-enhanced Raman scattering measurements. It is shown to be highly efficient since two plasmonic resonances of the structure were simultaneously tuned to coincide with the excitation and the emission wavelength in the SERS experiment. The analysis is completed by measuring the impact of the polarization angle on the SERS signal.

Investigation on the second part of the electromagnetic SERS enhancement and resulting fabrication strategies of anisotropic plasmonic arrays.

In general, the electromagnetic mechanism is understood as the strongest contribution to the overall surface-enhanced Raman spectroscopy (SERS) enhancement. Due to the excitation of surface plasmons, a strong electromagnetic field is induced at the interfaces of a metallic nanoparticle leading to a drastic enhancement of the Raman scattering cross-section. Furthermore, the Raman scattered light expierences an emission enhancement due to the plasmon resonances of the nanoantennas. Herein, this second part of the electromagnetic enhancement phenomenon is investigated for different Raman bands of crystal violet by utilizing the anisotropic plasmonic character of gold nanorhomb SERS arrays. We aim at evaluating the effects of localized and propagating surface plasmon polariton modes as well as their combination on the scattered SERS intensity. From that point of view, design and fabrication strategies towards the fabrication of SERS arrays for excitation wavelengths in the visible and near-infrared (NIR) spectral region can be given, also using a double-resonant electromagnetic enhancement.

Ultrafast plasmon dynamics and evanescent field distribution of reproducible surface-enhanced Raman-scattering substrates

AbstractSurface-enhanced Raman scattering (SERS) is a potent tool in bioanalytical science because the technique combines high sensitivity with molecular specificity. However, the widespread and routine use of SERS in quantitative biomedical diagnostics is limited by tight requirements on the reproducibility of the noble metal substrates used. To solve this problem, we recently introduced a novel approach to reproducible SERS substrates. In this contribution, we apply ultrafast time-resolved spectroscopy to investigate the photo-induced collective charge-carrier dynamics in such substrates, which represents the fundamental origin of the SERS mechanism. The ultrafast experiments are accompanied by scanning-near field optical microscopy and SERS experiments to correlate the appearance of plasmon dynamics with the resultant evanescent field distribution and the analytically relevant SERS enhancement. FigureUltrafast time-resolved differential absorption spectroscopy combined with scanning near-field optical microscopy (left) and atomic force microscopy (right) yields insight into the photoinduced charge-carrier dynamics in innovative reproducible SERS-substrates

Towards multiple readout application of plasmonic arrays

Summary In order to combine the advantages of fluorescence and surface-enhanced Raman spectroscopy (SERS) on the same chip platform, a nanostructured gold surface with a unique design, allowing both the sensitive detection of fluorescence light together with the specific Raman fingerprint of the fluorescent molecules, was established. This task requires the fabrication of plasmonic arrays that permit the binding of molecules of interest at different distances from the metallic surface. The most efficient SERS enhancement is achieved for molecules directly adsorbed on the metallic surface due to the strong field enhancement, but where, however, the fluorescence is quenched most efficiently. Furthermore, the fluorescence can be enhanced efficiently by careful adjustment of the optical behavior of the plasmonic arrays. In this article, the simultaneous application of SERS and fluorescence, through the use of various gold nanostructured arrays, is demonstrated by the realization of a DNA detection scheme. The results shown open the way to more flexible use of plasmonic arrays in bioanalytics.

Improvement of sensor performance of high-T/sub C/ thin film planar SQUID gradiometers by ion beam etching

The sensor performance of galvanically coupled Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) dc SQUID gradiometers on 24/spl deg/ bicrystal substrates has been improved by thickness reduction in the region of the grain boundary Josephson junctions using ion beam etching. The prepared etching mask allows the reduction of the critical current by more than one order of magnitude while the SQUID inductance is slightly increased. This treatment shifts the SQUID parameter /spl beta//sub L/ from values above 10 to the proposed optimum around 1. The authors observed with decreasing critical current and increasing normal resistance a reduced I/sub C/R/sub N/ product with values between 300 and 400 /spl mu/V at 150-nm film thickness changing to values near 150 /spl mu/V at 50-nm film thickness. Despite this fact, the white flux noise level as well as the low-frequency noise is reduced. With their galvanically coupled 4/spl times/8 mm/sup 2/ dc SQUID gradiometer the authors obtained a white noise level of 4.2 /spl mu//spl Phi//sub 0///spl radic/Hz corresponding to a field gradient sensitivity of 430 fT/cm/spl radic/Hz at 77 K after the trimming process.

2212-Tl-Ba-Ca-Cu-O films on 3" sapphire wafers for high frequency filters

Due to their higher critical temperature of above 100 K compared to Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ thin films, high temperature superconducting Tl/sub 2/Ba/sub 2/Ca/sub 1/Cu/sub 2/O/sub 8/ films are well suited for applications with regard to cryocooler operation. In the case of high frequency devices there exists not only the demand for high quality films but also to prepare them on suitable substrate materials such as sapphire. We report the preparation of high quality Tl/sub 2/Ba/sub 2/Ca/sub 1/Cu/sub 2/O/sub 8/ films on both sides of cerium oxide buffered 2 inch and for the first time on single side buffered 3 inch sapphire wafers. The films were prepared by means of the well known two-step process. This technology allows us to prepare crack-free films with a thickness of up to 400 nm. The best 2 inch films reach a surface resistance of about 50 /spl mu//spl Omega/ at 5.6 GHz and 77 K as well as a critical current density as high as 4 MA/cm/sup 2/ at 77 K and critical temperatures distinctly above 100 K, whereas the 3 inch films show values of 340 /spl mu//spl Omega/ at 3.9 GHz and 77 K, 1 MA/cm/sup 2/, and 105 K. We prepared 4-pole band pass filters from the 2 inch double-sided films which were able to operate up to 88 K within their designed performance.

Superconducting Hg-Ba-Ca-Cu-O thin films on lanthanum aluminate and sapphire substrates

Epitaxial c-axis-oriented HgBa/sub 2/CaCu/sub 2/O/sub 6+/spl delta// (Hg-1212) thin films have been prepared on lanthanum aluminate as well as on CeO/sub 2/ buffered r-plane sapphire substrates using a Tl-Hg cation-exchange process. Films on LaAlO/sub 3/ exhibited superconducting transition temperatures (T/sub c/) up to 122 K and critical current densities in the range of 2.2-3.1 MA/cm/sup 2/ at 77 K and under self-field. For the microwave surface resistance (R/sub s/) values of /spl sim/4 m/spl Omega/ at 17.86 GHz and 77 K have been obtained. In comparison, films on sapphire substrates with T/sub c/ values up to 118 K showed critical current densities in the range of 0.7-1.0 MA/cm/sup 2/ and R/sub s/ values of /spl sim/7 m/spl Omega/. With further optimization of processing an improvement of superconducting properties of Hg-1212 films on sapphire substrates is expected.

Optimization of surface morphology and electrical parameters of Tl-Ba-Ca-Cu-O thin films for high frequency devices

We prepared double-sided Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ (Tl-2212) thin films on large area sapphire substrates (up to 3 inch) for high frequency filters, which are intended to be used in future communication systems. For high frequency operation one demands film thicknesses three or four times the penetration depth /spl lambda/, but the thin film preparation process has several limits for the film thickness. Beside the crack formation on sapphire substrates due to different thermal expansion coefficients the surface morphology with parameters as roughness or the formation of precipitates depend on the film thickness. We show results of an optimization process to achieve the best suitability of the films for high frequency devices. The investigations were done in terms of SEM and AFM surface inspections as well as the measurement of electrical film parameters as critical temperature T/sub c/, critical current density J/sub c/, or surface resistance R/sub s/.