Centre d’Élaboration de Matériaux et d’Etudes Structurales (UPR 8011)


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STM-induced light emission

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When the metallic tip of a scanning tunneling microscope (STM) is biased with a few volts and scans the conductive surface of a metallic or semiconductor sample, an STM-induced luminescence can be excited and observed. This extremely confined photon source offers a unique access to the study of inelastic processes in the tunneling junction at a nanometric scale.

Contact : renaud.pechou[at]cemes.fr

Experiments are performed at CEMES on a customized hybrid near field microscope (AFM-STM-PSTM) working in ambient conditions in the clean room of the lab. Emitted photons are collected by high numerical aperture optical fibres brought in close proximity to the STM junction (< 1 mm) and connected to two detectors. The first one is a cooled photomultiplier operating in the photon counting mode in the 185-900 nm range and used to draw local maps of the emission yield over the scanned area. The second one is a spectroscope (diffraction grating coupled to a N2-cooled CCD camera operating in the 300-1050 nm range) giving access to the spectral features of the emitted light. This experimental setup allows simultaneous acquisition of STM topography, related photon mapping and spectrum of the emitted light.

Figure 1 : Examples of STM topography and photon map of the light emitted by a tunnelling junction formed by a gold tip scanning gold islands deposited on a MoS2 substrate. © CEMES-CNRS

 

Radiative electron-hole recombinations are responsible of the STM-induced luminescence in the case of metal-semiconductor tunnelling junctions. However, in the case of a metal-metal tunnelling junctions, light emission is due to the radiative decay of an interface plasmon mode excited by inelastic electrons tunnelling through the vacuum gap.

Figure 2 : Example of STM spectra of the light emitted by a tunnelling junction formed by a gold tip scanning gold islands deposited on a MoS2 substrate. © CEMES-CNRS

 

The spectral features of this light emission are thus intimately linked to the morphologic and electronic properties of the tunnelling junction and consequently of the sample nanostructure. STM-induced light emission will be used to study nanometric gold structures such as nanorods or nanoprisms. This original approach, linked with other near field techniques developed in the lab, should give a unique access to fundamental physical properties of single nanoscale objects, such as their plasmonic density of states. The modifications of their morphological parameters, the presence of localized defects (holes or hots spots due to deposited colloid) or the coupling between nanostructures should also be studied using this technique.

 

[1] A. Carladous et al., Phys. Rev. B 66, 045401 (2002).

[2] R. Péchou et al., Appl. Phys. Lett. 72, 671 (1998).

[3] C. Maurel et al., Surf. Sci. 600, 442 (2006).