Electrically conductive and optically transparent Sb-doped SnO2 STM-probe for local excitation of electroluminescence
Introduction
Nontrivial metal structures may act as optical antenna which efficiently couple an incident or outgoing optical wave to small volumes far below the diffraction limit [1]. This coupling can be understood in terms of a local enhancement of the electromagnetic field, and leads to interesting physical effects such as Surface-Enhanced Raman Scattering [2] or Surface-Enhanced Fluorescence [3]. Thus, a quantitative experimental measure of near-field effects promises both a better understanding of the underlying physics and new methods to optimize this enhancement in applications. While scanning tunnelling and atomic force microscopes enable readily atomic resolution images, a local measurement of the optical near-field remains an onerous task. In apertureless scanning near-field optical microscopy (SNOM), a resolution below 10 nm is feasible on suitable samples [4] but one has to put up with a strong global illumination of the sample. In aperture-SNOM the illumination is reduced to a small near-field spot and is therefore preferred for optically sensitive samples. Though, in order to obtain comparable resolution as in the apertureless method, ambitious probe preparation efforts have to be undertaken as e.g. in the case of Frey et al. [5] who image single fluorophores with an optical resolution of 20 nm. Both approaches share two fundamental difficulties if applied to the investigation of complex metal structures. Firstly, the tip geometry and in turn the local field distribution is not precisely known on the microscopic scale. Secondly, the optical response of the combined system tip/sample, although presenting features that are highly localised is dominated by nontrivial interactions between tip and surface, therefore it is not straightforward to extract information of the optical response of the sample without the tip.
A single fluorescent molecule as a point-like sensor for the optical near field of a structure was experimentally realised by Michaelis et al. [6]. This approach leads directly to a detected signal that can be interpreted in terms of the optical near-field of the sample. An experimentally much easier method to create a point-like light source is photon emission due to hot inelastic tunnelling of electrons in a tunnel junction between an STM tip and a surface [7]. In this case, the local coupling to outgoing plane waves is directly accessible experimentally. This approach has not yet been used to investigate near-fields because the metallic tips used in standard STM experiments optically strongly interact with the sample. This interaction can be understood in terms of the strong optical antenna formed by a metal sphere in nanometre distance from a metal plane [8] which possesses enormous field enhancement in the tunnel gap and plays an important role in the light emission process as was pointed out by Berndt and Gimzewski [9]. An extended model describing quantitatively the role of this mode for photon emission from tunnel junctions for an idealised model geometry [10] is possible but for any more complex sample geometry, the formation of this gap mode prevents access to the unperturbed optical near-field. Earlier efforts to improve the optical quality of STM tips were concentrated on optimising the photon collection efficiency [11], [12], [13] or delivery of optical power to the tip apex [14], [15]. In all of these cases, hybrid structures of transparent material with a conducting Au film on the surface proved to be favourable in terms of light throughput, still this metal film gives rise to gap modes which prevent a measurement of the unperturbed optical near-field.
The obvious solution to really access the intrinsic near-field of the sample is to use a conducting but optically transparent tip for STM experiments thus circumventing the perturbation of any metal in the vicinity of the tunnel gap. In this paper, we demonstrate that massive antimon-doped tin-oxide (Sb–SnO2)-tips are suited for such measurements. Firstly, it is demonstrated that these tips can be used in STM operation and their imaging properties are characterised. Then, light emission induced with these transparent tips is experimentally proven and characterised.
Section snippets
Experimental
The Sb–SnO2-tips were prepared from thermally degraded tin(IV)butoxide using a sol–gel process as described previously [16]. An atomically flat Au(1 1 1) surface for the investigation of the imaging properties of the tip was prepared by thermal evaporation of a gold layer with a thickness of 60 nm on a freshly cleaved mica surface at room temperature and 10−6 mbar (deposition rate 0.1 nm/s). Subsequent annealing at 650 °C for 60 s in a nitrogen atmosphere led to the formation of Au(1 1 1) terraces with
Results and discussion
In Fig. 2, STM-micrographs of the octanedithiol SAM test sample taken with the Sb–SnO2-tip and with a standard Pt/Ir tip are compared. Firstly, it is clear that STM operation with the transparent tip is possible without extra effort. Monatomic steps at the edges of gold terraces and at the rim of round etch-pitches are clearly resolved proving that the vertical resolution is significantly better than 2.36 Å, which is the distance between two Au(1 1 1) planes [19]. An upper limit for the lateral
Conclusion
To our knowledge, it has been shown for the first time that doped semiconductor tips like Sb–SnO2 prepared by a sol–gel method can be used as STM-probes. Visible light emission could be generated with these probe. This opens a promising route to use inelastically tunnelling electrons as a local light source without fundamentally changing the optical near-field as it is unavoidable when metallic tips are used. Though not experimentally shown in this work we want to stress that the same advantage
Acknowledgements
We thank G. Glasser (Max Planck-Institut für Polymerforschung, Mainz) who took the electron microscope images and Dr. A.K.A. Aliganga for the preparation of the dithiol-coated Au(1 1 1) surface. Parts of this work were financially supported by the Bundesministerium für Bildung und Forschung, (BMBF, Grant nos. 03N8702 and 03N6500), Estonian Science Foundation (Grants 5545 and 5015), and from the German-Israeli Project on Future-oriented Topics (DIP, Grant D3.1).
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