Raman spectroscopy and light emission at metal films enhanced by the optical near-field of a scanning tunneling tip

Silver and gold colloids, but also other metal nanostructures show specific optical resonances due to the excitation of localized surface plasmons (LSP). They are confined to small spatial regions and create there high electromagnetic fields. This is also the basis of the surface enhanced Raman scattering (SERS), where the rough metal surface acts simultaneously as support for the molecules and as Raman enhancer. Thus, adsorbate-substrate interaction and enhancing processes are nearly inseparably interconnected. A new concept decouples adsorption and enhancing processes. A STM tunneling tip serves as external enhancer, since localized surface plasmons can also be excited in the tip apex, providing a high near-field enhancement. For molecules in the close vicinity of the tip, but adsorbed at the metal, a tip-enhanced Raman scattering (TERS) can be observed. This approach can lead to the development of spectroscopic tools with high spatial resolution, ultimately permitting single molecule spectroscopy. TERS experiments have been conducted at two different Raman scatterers, the dye Brilliant Cresyl Blue (BCB) and the cyanide ion. If a illuminated silver tip is brought into tunneling contact with a smooth, BCB covered Au-surface, a substantial rise of the Raman intensity can be observed. The high enhancement of about 104, centered underneath the tip apex, means that only a few thousand molecules contribute to TERS. At rough Au surfaces combined SERS and TERS spectra were recorded for adsorbed cyanide ions. Their analyses prove also for this case that TERS is a local spectroscopy, averaging over a rather small ensemble of molecules. Localized surface plasmons (LSP) can also be excited by electrons tunneling through the gap between the tip apex and the metal surface. Their light emission shows a number of discrete modes extending into the near IR region. Their energies depend on the parameters of the tip and the tunnel junction including the local topography of the metal surface, information that is useful for further optimizing the TERS experiments.1 Introduction 6 2 Fundamentals 14 2.1 Raman spectroscopy 14 2.2 Surface enhanced Raman spectroscopy 16 2.3 Tip enhanced Raman spectroscopy 25 2.4 Scanning tunneling microscopy 35 2.5 Light emission from tunneling junctions 37 3 Experimental 46 3.1 The set-up 46 3.2 The optics 48 3.3 Film preparation 49 3.4 Tip preparation 51 3.5 Chemicals and reagents 55 3.6 Optics for STM-emitted light detection 56 4 Tip Enhanced Raman Spectroscopy 60 4.1 Brilliant Cresyl Blue 60 4.2 Cyanide 73 4.3 Overview on literature data concerning TERS 85 4.4 Carbon fluctations 87 5 STM light emission 96 5.1 Gold-gold junctions 96 6 Conclusions 113 References 118 Ringraziamenti 12