Surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy or surface-enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on rough metal surfaces or by nanostructures such as plasmonic-magnetic silica nanotubes. The enhancement factor can be as much as 1010 to 1011, which means the technique may detect single molecules. History SERS from pyridine adsorbed on electrochemically roughened silver was first observed by Martin Fleischmann, Patrick J. Hendra and A. James McQuillan at the Department of Chemistry at the University of Southampton, UK in 1973. This initial publication has been cited over 6000 times. The 40th Anniversary of the first observation of the SERS effect has been marked by the Royal Society of Chemistry by the award of a National Chemical Landmark plaque to the University of Southampton. In 1977, two groups independently noted that the concentration of scattering species could not account for the enhanced signal and

Porous silicon-A versatile platform for mass-production of ultrasensitive SERS-active substrates

Siarhei Zavatski

Surface-enhanced Raman scattering (SERS) spectroscopy is one of the most prospective methods combining state-of-the-art nanomaterials and optical techniques for highly sensitive express-analysis and detection of organic and bioorganic objects in liquids and gases. Special programs have been recently started all over the world to bring the SERS-spectroscopy closer to wide implementation in medical diagnostics, forensics, security, monitoring sanitary conditions, etc. Despite outstanding features of SERS-spectroscopy, its effective practical use has been particularly slowed down by moderate reproducibility, non-versatility, and restrictions imposed by commercially available SERS-active substrates to measurement and storage regimes. The present review reports SERS-active substrates constituted by noble metals & rsquo; nanoparticles (NPs) and porous silicon (PS), which potentially can be a tool to overcome the above-mentioned limitations. The PS template acts as a highly ordered host nanomaterial for the formation of a variety of metallic nanostructures, which morphological and optical properties can be easily tuned for the best performance to meet the customer requirements via managing PS synthesis regimes. An indubitable advantage of PS is the compatibility of its fabrication process with basic microelectronics operations and micro-electromechanical systems (MEMS) that make it possible to integrate SERS-active areas in a silicon chip. In contrast to the previously published reviews in the field, this one covers the most recent results on formation, characterization, and application of PS-based substrates demonstrating prominent SERS-activity that have been achieved for the last decade including modifications with graphene or Bragg structures, detection of molecules at amount down to attomolar concentration, bacteria recognition, etc.

ELSEVIER2021

Silicon Nanowire Arrays Coated with Ag and Au Dendrites for Surface-Enhanced Raman Scattering

Siarhei Zavatski

Silicon nanowires (SiNWs) were comprehensively characterized in dependence on conditions of their formation via metal (Ag) -assisted chemical etching (MACE) of monocrystalline Si. The Ag structures remained on/between SiNWs based on both n- and p-Si were found to promote surface enhancement of Raman scattering (SERS) from organic molecules adsorbed on their surface. The Ag structures on/between the SiNWs/p-Si facilitated two times higher SERS-signal from 10(-6) M rhodamine 6G than those in the SiNWs/n-Si. The activity of the SERS-substrates based on p-Si was improved by modification with small Au dendrites, which provided rich family of hot spots and prevented degradation of the SERS-activity observed for pure Ag dendrites due to formation of Ag2S during one week of storage in air. The SERS-substrates based on the Au/Ag dendrites on SiNWs/p-Si allowed to achieve nanomolar detection limit of rhodamine 6G and 5,5'-dithiobis (2-nitrobenzoic acid).

2020

Fabrication and Optoelectronic Properties of Graphene Nanoribbons

Eberhard Ulrich Stützel

Graphene, a single layer of carbon atoms, exhibits excellent charge transport properties. However, due to the absence of a band gap in this two-dimensional carbon nanostructure, graphene-based field effect transistors cannot be turned off. One strategy to increase the on/off ratio relies on patterning graphene into narrow stripes, so-called graphene nanoribbons (GNRs). The present thesis aimed at developing novel fabrication and chemical functionalization methods for GNRs. Along these lines, electrical transport studies and spectroscopic investigations were envisioned as a major tool to monitor the changes brought about by the functional groups attached to the GNRs. GNRs were fabricated with the aid of V2O5 nanofibers or CdSe nanowires as an etching mask during the plasma etching of graphene. The resulting GNRs exhibited good electrical conductivity for ribbon widths as small as 10 nm. Moreover, the transport gap in the GNRs was found to scale inversely with the ribbon width. Scanning tunneling microscopy and surface enhanced Raman spectroscopy testified a good structural quality of the GNRs. Electrical characterization of GNRs revealed a pronounced hysteresis, which was exploited for the fabrication of electrically switchable GNR memory cells. Dynamic pulse response measurements demonstrated reliable switching between two conductivity states for clock frequencies of up to 1 kHz and pulse durations as short as 500 ns for >10^7 cycles. As the most likely switching mechanism, charge trapping within a water layer in the GNR surrounding could be identified. Furthermore, the optoelectronic properties of individual GNRs were studied by scanning photocurrent microscopy. The pronounced photocurrent signal close to the nanoribbon/metal contacts was found to be directly proportional to the conductance of the devices, suggesting that a local voltage source is generated at the nanoribbon/metal interface by the photo-thermoelectric Seebeck effect. The dominance of this mechanism over charge separation via built-in electrical fields was attributed to strong local heating of the GNRs by the laser spot, combined with the reduced thermal conduction capability of the nanoribbons in comparison to extended graphene sheets. Chemical functionalization of graphene and GNRs was attempted via different gas- and liquid-phase approaches. While electrical transport and Raman measurements revealed the presence of significant doping effects, it was not possible to unequivocally prove the covalent attachment of atoms at the GNR edges, neither through changes in the charge transport characteristics, nor via scanning microscopy studies.

EPFL2013

Fabrication and Optoelectronic Properties of Graphene Nanoribbons

Eberhard Ulrich Stützel

EPFL2013