Periodic Reporting for period 5 - SARF (Single-Atom Radio Frequency Fingerprinting)
Reporting period: 2024-04-01 to 2024-09-30
Precise investigation tools for analyzing and manipulating matter down to the scale of single atoms are the eyes, ears and fingers of nanoscience and -engineering. It is widely accepted that these technologies have the potential to change our society even more significantly in the years to come than they have already done so in the past. We take these nano-analytical „senses“ beyond the present state of the art. Our project is breaking new grounds by enabling spectral fingerprinting at the scale of single atoms and molecules with sub-nanometer spatial resolution and operating in vacuum- as well as liquid-phase environments. This combination of analytical capabilities simultaneously in a single tool is highly desirable already today: ever more, in diverse fields of present nanoscience & -technology, decisive functions like switching of electrical current, storing of information or catalyzing a chemical reaction are taken over by no more than a single individual atom or molecule - rather than ensembles of many atoms or molecules. For instance, think of spintronics, sensorics, catalysis, or medicinal drug delivery. Our project follows two visions: First, we aim at realizing the challenging goal of radio-frequency scanning tunneling microscopy (RF-STM) based resonance spectroscopy for fingerprinting (identifying) single atoms and molecules. Secondly, we try to achieve this goal by utilizing such technical means that require relatively mild monetary resources, including conventional electrical conductance measurements. We believe this to be crucial in order to make the new technology accessible to a broad community. For specific fingerprinting, our project realizes resonance spectroscopy at Giga-Hertz frequency combined with scanning tunneling microscopy. Characteristic resonance signals are locally detectable by the probe tip as small changes of electrical conductance that enable elemental and chemical identification. We develop this fingerprinting on different atomic and molecular systems to cover a broad spectrum of applications, including magnetic and nonmagnetic materials & functional organic molecules. If successful, our project will provide a controlled, versatile, fast and readily applicable “atom-by-atom” matter analysis, where single atoms or molecules are selected and identified one by one in real time and space. And all that at relatively mild equipment costs compared to other methods in order to make this new methodology accessible to a broad community.
We have achieved substantial progress in all areas foreseen & following important scientific results:
(.) We developed a novel method of nanoscale electrochemical fingerprinting at GHz frequency under ambient conditions in liquid phase, Small, 17, 2101253 (2021). SARF fingerprinting of the oxidation state of redox molecules at a solid-liquid electrolyte interface achieves local cyclic voltammetry with a sensitivity of ca. 120 molecules. SARF fingerprinting of the oxidation & reduction processes in a 2D catalyst due to ion intercalation from the electrolyte, arxiv.org/abs/2410.12719 detects the electrochemical activity & dynamics at spatial resolution of 16 nm.
(.) We got convincing experimental evidence for GHz mechanical resonance in a single-molecule junction, opening unprecedented possibilities for increasing the sensitivity of dielectric spectroscopy ultimately towards the scale of no more than a single molecule, Sci. Rep. 12, 2865 (2022).
(.) We report in Sci. Rep. 12, 6183 (2022) a 1st alternative route for single-molecule dielectric resonance, more directly and less time-consuming via local tunnel conduction. We clarify the mutual inter-dependencies of the DC conductance versus voltage characteristics of the molecule under investigation and the high-frequency dependent DC conductance response obtained via lock-in detection of the tunnel current signal.
(.) We solved technical issues of different types of artefact signals and developed improved measurement protocols for excluding artefacts. Our protocols are less complicated compared to previous methods. We substantially improved the transmission characteristics of our setup, which is crucial for achieving ultimate sensitivity.
(.) We successfully set-up and adapted our three different SARF instruments by internal and external RF circuitry, calibrating the respective signals, and achieving a typically 20 dB better transmission characteristics in a 32 GHz bandwidth. We report our improved calibration method in Rev. Sci Instr. 92, 043710 (2021).
(.) We report in Rev. Sci. Instr. 94, 103702 (2023) on a 2nd alternative route to experimentally detect the single-molecule dielectric response utilizing quantitative microwave reflection detection via a home-built interferometric circuit. Our promising results are reported in a most recent master’s thesis of one of our team members.
(.) We identified, prepared and characterized at the single-molecule level several molecules well-suited for comparative SARF experiments, including two different organic pi-radicals adsorbed on coinage metals, namely DPPH/Au(111), Surf. Sci. 700, 121676 (2020), and BDPA/Cu(100), Teeter et al. ChemPhysChem DOI: 10.1002/cphc.202400852 (2024).
(.) We developed a novel method of nanoscale electrochemical fingerprinting at GHz frequency under ambient conditions in liquid phase, Small, 17, 2101253 (2021). SARF fingerprinting of the oxidation state of redox molecules at a solid-liquid electrolyte interface achieves local cyclic voltammetry with a sensitivity of ca. 120 molecules. SARF fingerprinting of the oxidation & reduction processes in a 2D catalyst due to ion intercalation from the electrolyte, arxiv.org/abs/2410.12719 detects the electrochemical activity & dynamics at spatial resolution of 16 nm.
(.) We got convincing experimental evidence for GHz mechanical resonance in a single-molecule junction, opening unprecedented possibilities for increasing the sensitivity of dielectric spectroscopy ultimately towards the scale of no more than a single molecule, Sci. Rep. 12, 2865 (2022).
(.) We report in Sci. Rep. 12, 6183 (2022) a 1st alternative route for single-molecule dielectric resonance, more directly and less time-consuming via local tunnel conduction. We clarify the mutual inter-dependencies of the DC conductance versus voltage characteristics of the molecule under investigation and the high-frequency dependent DC conductance response obtained via lock-in detection of the tunnel current signal.
(.) We solved technical issues of different types of artefact signals and developed improved measurement protocols for excluding artefacts. Our protocols are less complicated compared to previous methods. We substantially improved the transmission characteristics of our setup, which is crucial for achieving ultimate sensitivity.
(.) We successfully set-up and adapted our three different SARF instruments by internal and external RF circuitry, calibrating the respective signals, and achieving a typically 20 dB better transmission characteristics in a 32 GHz bandwidth. We report our improved calibration method in Rev. Sci Instr. 92, 043710 (2021).
(.) We report in Rev. Sci. Instr. 94, 103702 (2023) on a 2nd alternative route to experimentally detect the single-molecule dielectric response utilizing quantitative microwave reflection detection via a home-built interferometric circuit. Our promising results are reported in a most recent master’s thesis of one of our team members.
(.) We identified, prepared and characterized at the single-molecule level several molecules well-suited for comparative SARF experiments, including two different organic pi-radicals adsorbed on coinage metals, namely DPPH/Au(111), Surf. Sci. 700, 121676 (2020), and BDPA/Cu(100), Teeter et al. ChemPhysChem DOI: 10.1002/cphc.202400852 (2024).
The vision of SARF is to enable spectral fingerprinting of single atoms for elemental identification and intra-molecular chemical analytics with sub-nanometer spatial resolution and operating at various surfaces in vacuum as well as at the solid-liquid interface at ambient conditions. On the way to achieve this goal we have developed novel and unique methodology that has already led to unprecedented radio frequency-based nanoscale electrochemical fingerprinting under ambient conditions in at the solid-liquid interface. Our inter-disciplinary collaboration between physics, chemistry and microwave engineering has enabled the successful radio-frequency fingerprinting of the oxidation state of metal-organic redox molecules, where we achieved a sensitivity of no more than about 120 molecules; compared to conventional electrochemical detection, where the redox electrical current is typically a few micro-Ampere, we have managed to detect atto-Ampere electrochemical currents. For comparison, at vacuum conditions, we have reported two unprecedented experimental routes for increasing the sensitivity of dielectric spectroscopy at GHz frequency ultimately towards the scale of no more than a single molecule. Firstly, we have succeeded to detect the dielectric relaxation of a single molecule junction indirectly via its effect of power dissipation. Secondly, we have detected it more directly via microwave reflection, as reported in a most recent master’s thesis of a SARF team member: Tiny changes of the reflection coefficient, detected by our home-built interferometer circuit, are found to be fingerprints of the electrical conductance and capacitance across the tunnel junction of the microscope, providing a quantitative fingerprint of the molecule’s relaxation in terms of the real and imaginary parts of the dielectric function. To achieve this, we have for the first time successfully implemented microwave reflectance spectroscopy in a low-temperature scanning tunneling microscope operating at GHz frequency under ultra-high vacuum and at cryogenic temperature of ca. 8 K.