Characterizing the excited states of individual atoms through the combination of tunneling microscopy and pulsed laser
The characterization of the dynamics of the excited states of a single atom localized on a surface remains to this day an experimental challenge. By combining a tunable pulsed laser at the junction of a low-temperature tunneling microscope, researchers have highlighted rapid photocurrent signals that can be attributed to the dynamics of excited states of an individual erbium atom deposited on the silicon surface.
In a vast majority of physico-chemical processes, the excited states of atoms are obtained either by absorbing light or by interacting with electrons. The most common method for measuring the characteristics of the quantum state of an excited atom is to analyze the light emitted during its relaxation to its ground state. In a gas, it is possible to observe many atoms relaxing simultaneously and thus collect enough light signal. When it comes to performing this type of analysis on a single atom localized on a surface, the emitted light signal is often too weak to be detected. It is therefore essential to be able to couple the optical measurement of the excited state with another type of measurement, via interaction with electrons. However, the electronic readout of the excited state of a nano-object is often hindered by its rapid dynamics, and the readout disturbs its state, posing several fundamental and technical challenges that have not been addressed until now.
By locally coupling a tunable pulsed laser to excite erbium atoms deposited on a silicon surface at the junction of a tunneling microscope, researchers from the Institute of Molecular Sciences of Orsay (ISMO, CNRS / Université Paris-Saclay) have demonstrated that the tunneling current measured above each erbium atom simultaneously contains information regarding the excited states of both the surface and the atom itself. Thanks to the high precision and stability of this type of microscope, physicists were able to distinguish between the origins of the two sources of photocurrent (surface versus atom). Furthermore, by conducting precise measurements of photocurrent spectra on two types of erbium adsorption conformations on silicon, it is also possible to establish a precise relationship between the photocurrent peaks and the electronic structure (quantum state) of each erbium conformation. Through collaboration with the Franche-Comté Institute of Electronics, Mechanics, Thermics and Optics - Sciences and Technologies (FEMTO-ST, CNRS / Université de Bourgogne Franche-Comté), numerical calculations using density functional theory were performed, taking into account the relativistic aspect of erbium's electronic interactions as well as spin-orbit couplings. These calculations confirmed that the experimental measurements acquired by scanning the laser wavelength indeed contain information about the most probable electronic transitions of the excited erbium in relation to the photocurrent peaks observed experimentally.
All these results suggest that the reading of erbium's excited states via the tunnel current of the STM occurs during their de-excitation (optical relaxation), locally leading to the dissociation of excitons (electron-hole pairs) created by the laser and strongly localized at the erbium atoms on the surface of silicon. This sudden dissociation induces a burst of photoelectrons measured in the tunnel current, which occurs at the optical resonance of the probed excited state of erbium as the tunable laser sweeps wavelengths from 800 nm to 1200 nm. The researchers were able to define that the spectral resolution of this new spectroscopic method is less than 5 cm-1, primarily limited by the resolution of the laser used.
These findings, published in ACS Nano, may pave the way for new tools to explore the dynamics of future nano-objects such as atomic or molecular assemblies. This technique thus offers new perspectives for experimental and theoretical explorations aimed at controlling model devices by preparing specific quantum states. Reference Optoelectronic Readout of Single Er Adatom’s Electronic States Adsorbed on the Si(100) Surface at Low Temperature (9 K)
Eric Duverger and Damien Riedel
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