arXiv subjects

Stefan Müllegger

Publications and source records attributed to Stefan Müllegger.

10 recordsLinked to original sources

arXiv2026

Tunneling-induced translation of intact $π$-radical clusters on Au(111)

The scanning tunneling microscope (STM) is a powerful tool for investigating and manipulating molecules on surfaces. We demonstrate with a low-temperature STM operated at 6.2 K the controlled manipulation of ternary clusters of persistent molecular $π$ radicals as a whole. The ternary clusters $-$ each self-assembled from three $α,γ$-bisdiphenylene-$β$-phenylallyl (BDPA) molecules on Au(111) $-$ maintain their natural cluster structure throughout tip-induced translation and rotation relative to the surface. Sustained and repeated dragging of radical clusters is shown to facilitate the construction of artificial assemblies of several clusters. Our results provide new opportunities for the creation and investigation of radical-based spin assemblies on surfaces.

cond-mat.mes-hall↗

arXiv2024

Microwaves reveal the nanoscale ion intercalation and edge activity of 2D catalyst

The accelerated demand for electrochemical energy storage urges the need for new, sustainable, stable and lightweight materials able to store high energy densities rapidly and efficiently. Development of these functional materials requires specialized techniques that can provide a close insight into the electrochemical properties at the nanoscale. For this reason, we have introduced the electrochemical scanning microwave microscopy (EC-SMM) enabling local measurement of electrochemical properties with nanometer spatial resolution and sensitivity down to atto-Ampere electrochemical currents. The exceptional power of EC-SMM operated at radio frequency is exemplified here by the successful detection of the electrochemical activity and dynamics of molecularly thin NiCo(OH)2 flakes with a spatial resolution of 16 +/- 1 nm, uncovering the location of the active sites and providing atomistic details on the catalytic process that controls the electrocatalytic performance. Our results pinpoint the factors required to tune the thermodynamics of ion intercalation and to optimize the surface adsorption.

physics.chem-ph↗

arXiv2023

Calibrated Microwave Reflectance in Low-Temperature Scanning Tunneling Microscopy

We outline calibrated measurements of the microwave reflection coefficient from the tunnel junction of an ultra-high vacuum low temperature scanning tunneling microscope. The microwave circuit design is described in detail, including an interferometer for enhanced signal-to-noise and a demodulation scheme for lock-in detection. A quantitative, in-situ procedure for impedance calibration based on the numerical 3-error-term model is presented. Our procedure exploits the response of the microwave reflection signal due to the change of the tunneling conductance caused by sub-nm variation of the tunneling distance. Experimental calibration is achieved by a least-squares numerical fit of simultaneously measured conductance and microwave reflection retraction curves at finite conductance. Our method paves the way for nanoscale microscopy and spectroscopy of dielectric surface properties at GHz frequencies and cryogenic temperatures. This opens a promising pathway even for dielectric fingerprinting at the single molecule limit.

physics.ins-det↗

arXiv2021

Frequency-independent voltage amplitude across a tunnel junction

Radio-frequency (rf) scanning tunneling microscopy has recently been advanced to methods such as single-atom spin resonance. Such methods benefit from a frequency-independent rf voltage amplitude across the tunnel junction, which is challenging to achieve due to the strong frequency dependence of the rf attenuation in a transmission line. Two calibration methods for the rf amplitude have been reported to date. In this Note, we present an alternative method to achieve a frequency-independent rf voltage amplitude across the tunnel junction and show the results of this calibration. The presented procedure is applicable to devices that can deliver rf voltage to a tunnel junction.

physics.ins-det↗

arXiv2021

Stable $π$-radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) adsorbed at the elbows of $22\times\sqrt{3}$ reconstructed Au(111)

Stable organic radicals serve as model systems for investigating metal-free magnetic phenomena at (sub)nanometer length scales. We have investigated at the single-molecule level the stable $π$-radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) adsorbed at the elbow sites of the $22\times\sqrt{3}$ reconstructed Au(111) surface. Low-temperature scanning tunneling microscopy (STM) and -spectroscopy under ultrahigh vacuum conditions at 8~K reveal structural as well as frontier-orbital related electronic details of DPPH/Au(111). A Kondo-like spectroscopic signature indicates preservation of the unpaired electron spin state.

physics.ins-det↗

arXiv2020

Attoampere Nanoelectrochemistry

Local electrochemical measurements and imaging at the nanoscale are crucial for the future development of molecular devices, sensors, materials engineering, electrophysiology and various energy applications from artificial photosynthesis to batteries. The ultimate step towards single-molecule sensitivity requires the measurement of aA currents, which is three orders of magnitude below that of current state-of-the-art measurement abilities. Here, we show electrochemical measurements at the sub aA level and <80 nm spatial resolution, that we reach by exploiting the ultra-high sensitivity of our GHz microscope for local faradaic interface capacitances. We demonstrate this for a well-known surface-bound ferrocene alkyl monolayer, a system that cannot be studied at the nanoscale unless large nanoarrays are used11. We report the simultaneous measurement of local cyclic voltammograms (CV) which provide atomistic information on the respective electron transfer reaction and reveal two molecular configurations with a similar redox energy potential -- insights inaccessible by electrochemical ensemble measurements.

physics.chem-ph↗

arXiv2016

Atomic-scale sensor for charge- and atomic-lattice-dynamics on surfaces

We present here a powerful method providing simultaneous atomic spatial and nanosecond temporal resolution for investigating dynamics and structure on the atomic scale, in general. We reveal the dynamic reorganization of surface (ad)atoms induced by radio frequency alternating charging and decharging of a metal. Our method utilizes taylor-made nano-fabricated two-dimensional islands of physisorbed argon atoms, acting as motion sensors, probed by a radio frequency low-temperature scanning tunneling microscope.

cond-mat.mes-hall↗

arXiv2015

Mechanism for nuclear and electron spin excitation by radio frequency current

Recent radio frequency scanning tunneling spectroscopy (rf-STS) experiments have demonstrated nuclear and electron spin excitations up to $\pm12\hbar$ in a single molecular spin quantum dot (qudot). Despite the profound experimental evidence, the observed independence of the well-established dipole selection rules is not described by existing theory of magnetic resonance -- pointing to a new excitation mechanism. Here we solve the puzzle of the underlying mechanism by presenting all relevant mechanistic steps. At the heart of the mechanism, periodic transient charging and electric polarization due to the rf-modulated tunneling process cause a periodic asymmetric deformation of the qudot, enabling spin transitions via spin-phonon-like coupling. The mechanism has general relevance for a broad variety of different spin qudots exhibiting internal mechanical degrees of freedom (organic molecules, doped semiconductor qudots, nanocrystals, etc.).

cond-mat.mes-hall↗

arXiv2014

Nuclear and electronic resonance spectroscopy of single molecules by radio-frequency scanning tunnelling microscopy

The ongoing miniaturization in nanoscience and -technology challenges the sensitivity and selectivity of experimental analysis methods to the ultimate level of single atoms and molecules. A promising new approach, addressed here, focuses on the combination of two well-established complementary techniques that have proven to be very successful in their own fields: (i) low-temperature scanning tunneling microscopy (STM), offering high spatial resolution for imaging and spectroscopy together with the capability of manipulating single atoms and molecules in a well-controlled manner; (ii) radio-frequency (rf) magnetic resonance techniques, providing paramount analytical power based on a high energy resolution combined with the versatility of being sensitive to a great variety of different properties of matter. Here, we demonstrate the successful resonant excitation and detection of nuclear and electronic magnetic transitions of a single quantum spin in a single molecule by rf tunneling of electrons applied through the tip of a modified STM instrument operated at 5 K. The presented rf-STM approach allows the unrivalled spectroscopic investigation of electronic hyperfine levels in single molecules with simultaneous sub-molecular spatial resolution. The achieved single-spin sensitivity represents a ten orders of magnitude improvement compared to existing methods of magnetic resonance - offering, atom-by-atom, unprecedented analytical power and spin control with impact to physics, chemistry, biology, medicine, nanoscience and -technology.

cond-mat.mes-hall↗

arXiv2012

'Kondo state' and Kondo resonance in a two-dimensional electron gas

The delicate balance of spin-screening and spin-aligning interactions determines many of the peculiar properties of dilute magnetic systems. We study a surface-supported all-organic multi-impurity Kondo spin system at the atomic scale by low-temperature scanning tunnelling microscopy and -spectroscopy. The model system consists of spin-1/2 radicals that are aligned in one-dimensional chains and interact via a ferromagnetic RKKY interaction mediated by the 2DEG of the supporting substrate. Due to the RKKY-induced enhanced depopulation of one spin-subband in the 2DEG, we finally succeeded to detect the so far unobserved 'Kondo state' as opposed to the well-established Kondo resonance. Its cloud of screening electrons, that are virtually bound to the radicals below the Kondo temperature, represents the extended exchange hole of the ferromagnetically polarized spin chain imaged here in real space.

cond-mat.str-el↗