AFM Manipulation of Redox states, Measurement of Electron Tunneling Rates and Voltage/Redox-State Controlled Atomic Conductance Switches via Adsorbates on Rutile TiO2(110) Surface

Ivan Štich (Institute of Physics, SAS)

Place: Pavilón QUTE, Auditórium – 2. poschodie, FÚ SAV

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Transition metal oxide surfaces (TMOS) are among the most important catalytic systems with titanium dioxide (TiO2) being the most important and studied TMOS. We have studied number of adsorbates (H, O, Au) on the rutile TiO2 surface. We will focus mainly on oxygen adsorbates, time permitting, we will briefly mention also Au clusters adsorbed on the TiO2 surfaces at 78K, the latter being an important catalytic system. Atomic force microscopy (AFM) suggests that oxygen adsorbs on TiO2 in form of molecular reactive oxygen species (ROS), such as peroxide (O22-), superoxide (O2-), or hydroxyl species (OH) or in form of single-atom quantum dots (SaQDs) [1]. The ground-state redox state of ROSs and SaQDs is 2- but the redox state can be reversibly manipulated by a combination of AFM/KPFM (Kelvin Probe Force Microscopy) techniques between 2- and 1-. The properties of the two oxygen redox states are drastically different. For example, using AFM tip we are able to switch locally and repeatedly the SaQDs between two conductance regimes: opened (redox state 1-, binary 1 state) and locked (redox state 2-, 0 state) and create with AFM+KPFM a desired binary arrangement of 0’s and 1’s, a nanosymbol encoded by means of SaQDs on the surface, which can be read out/decoded either by AFM or STM tip. Both the on-surface electrochemistry of ROS as well as the encoding of binary information by SaQDs require tip-tuning of their quantum states, an ability quantified by the tunnelling rates and, hence, measuring the tunnelling rates becomes of utmost importance.

For that purpose, we have developed a novel technique, a Fast-Cycling KPFS (FC-KPFS) [1], a method an order of magnitude faster than the alternative methods which, given the stochastic nature of the tunnelling processes, allows us to collect enough statistics even at 78K, compared to the ultra-low 5K temperatures ubiquitous in all alternative measurements. We found that the rate measurements are very sensitive indicators of the electronic structure of the SaQDs, being able to uncover differences for dots appearing equal in their AFM images. Finally, we will show that oxygen adatoms can be operated as voltage/redox-state controlled ultra-fast reversible conductance switches [2], setting the ultimate length and time scales. We note that while on-surface molecular switches have been frequently constructed and studied, the atomic conductance switches have been extremely rarely realized or studied, in spite of the fact that their switching times would be orders of magnitude faster (electron tunneling times in atomic vs. atomic vibration times during a conformal change in molecular systems).

[1] Y. Adachi, J. Brndiar, H. F. Wen, Q.Z. Zhang, M. Miyazaki, Y. Sugawara, H.Q. Sang, Y.J. Li, I. Stich, L. Kantorovich, Electron dynamics of tip-tunable oxygen species on TiO2 surface, Communications Materials (Nature), accepted (2021).

[2] Q. Zhang, J. Brndiar, Y. Adachi, M. Konôpka, H. F. Wen, M. Miyazaki, Y. Sugawara, R. Xu, Z. H. Cheng, H. Sang, Y. J. Li, L. Kantorovich, and I. Stich, Voltage and Redox State Triggered Oxygen Adatom Conductance Switch, submitted (2021).

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