arXiv subjects

Amandeep Sagwal

Publications and source records attributed to Amandeep Sagwal.

7 recordsLinked to original sources

arXiv2026

Raman-detected quantum dot microscopy for nanoscale electrostatic potential imaging

Quantification of electrostatic potentials at the nanoscale is crucial for understanding the principles governing properties of materials across multiple length scales. Currently, one of the most successful approaches relies on the charging response of a molecular quantum dot, suspended on a tip of a scanning probe microscope and measured using dynamic force spectroscopy. We investigate the possibility of an optical detection, aiming to improve the speed and reduce the complexity of this measurement scheme. We show that the integrated tip-enhanced Raman scattering intensity strongly correlates with the charge state of the quantum dot, and use it to map the electrostatic potential of a single atom. A quantitative equivalence with the established force spectroscopy method is found. We address the underlying photophysical principle of this new method by measuring the Raman spectra as a function of excitation wavelength and the molecular quantum dot charge. We reveal that the observed Raman intensity variations are primarily driven by transitions between resonant and non-resonant Raman scattering regimes of the molecule.

cond-mat.mes-hall↗

arXiv2026

Time-of-flight photon spectroscopy for scanning tunneling microscopy luminescence

We build and commission a time-of-flight photon spectrometer (TOFS) for scanning tunneling microscopy luminescence (STML). We obtain the spectrum by exploiting the wavelength dependent refractive index of a long dispersive optical fiber that converts photon arrival times into wavelength information; blue photons are delayed more than red photons. The setup uses a pulsed excitation source, either laser flashes or voltage pulses, to launch the photons from the junction into a single photon detector. The TOFS calibration can be performed and transferred to the STML setup from a separate benchtop experiment using pulsed light sources with known wavelength. We verify the TOFS during STML operation by simultaneously recording luminescence from the Ag-Ag(111) plasmon using a conventional grating spectrometer. Our experiments show that the TOFS is a straightforward and cost-effective addition to existing STML setups with good performance in the near-infrared range. The TOFS is compatible with a drop-in replacement of the single photon detector such as a superconducting nanowire single photon detector or even bolometers that may expand the useful spectral range beyond the abilities of current STML setups.

cond-mat.other↗

arXiv2026

Locally-Induced Stark Shifts of Collective Excitonic Modes in Polyradical Aggregates

Active control of dark long-lived excitonic states in molecular aggregates using local electric fields is a pivotal challenge for advancing nanoscale optoelectronics and quantum device engineering. This experimental study investigates the collective excitonic states in aggregates composed of radical chromophores. With the strong optical enhancement provided by tip-enhanced photoluminescence (TEPL) spectroscopy, bright and dark excitonic modes are observed emerging due to interexciton coupling and induce changes in their spectra with the electric field locally applied within the nanocavity gap. Proportionally scaling Stark shifts are revealed as well as the emission peak sharpening of the dark states and a divergent behavior of the bright states in asymmetric measurement positions of the nanocavity above the aggregates. The observed complex behavior is discussed in terms of influence of the field, molecule arrangement, nanocavity coupling, dark mode lifetimes and electrostatic charge inhomogeneities in the clusters. This sensitivity to the external parameters demonstrates an effective means of control over radical excitonic aggregates.

physics.atm-clus↗

arXiv2026

Adsorption-Driven Symmetry Lowering in Single Molecules Revealed by Ångstrom-scale Tip-Enhanced Raman Imaging

The vibrational landscape of adsorbed molecules is central to understanding surface interactions at the atomic scale, influencing phenomena from catalysis to molecular electronics. Recent advances in atomic-scale tip-enhanced Raman spectroscopy (TERS) have enabled vibrational mapping of single molecules with sub-nanometer spatial resolution, providing unprecedented insights into molecule-surface interactions by confining light in plasmonic picocavities. Here, we exploit TERS in a cryogenic scanning tunneling microscope junction to perform Raman hyperspectral mapping of single iron phthalocyanine (FePc) molecules in three non-equivalent adsorption configurations on Ag surfaces. We explore the changes in the vibrational modes of FePc molecules adsorbed on two distinct silver crystal terminations with differing symmetry, Ag(111) and Ag(110), revealing how subtle variations in the adsorption geometry due to substrate anisotropy can strongly influence molecular vibrations, lifting the degeneracy of individual normal modes. Our findings not only demonstrate the first use of sub-nanometer TERS mapping across different symmetry configurations but also provide a deeper understanding of how site-specific vibrational properties are intimately linked to local atomic environments. This capability paves the way for precisely tailoring surface interactions and controlling chemical reactions at the atomic scale.

cond-mat.mes-hall↗

arXiv2025

Disentangling the components of a multiconfigurational excited state in isolated chromophore

Studying the excited states of doublets is challenging for their typically multiconfigurational character. We employ light-scanning-tunneling microscopy (light-STM) to investigate photon-induced currents on a single open-shell PTCDA anion molecule placed into a plasmonic nanocavity between a tip and a substrate, irradiated by laser. Submolecular mapping reveals a zero-bias bidirectional photocurrent strongly varying with the lateral position of the tip apex above the molecule. We elucidate the mechanism in terms of a theoretical model in which a multiconfigurational doublet state is excited and decays back to the anion ground state through sequential electron transfers with the tip and the substrate. The correspondence of the experimental and theoretical contrast proves the correlated character of the excited state which can be described as a superposition of two dominating electronic configurations. By applying bipolar voltage on the junction with the molecule, we switch the dominant recombination pathway from one of the configurations to the other, effectively disentangling the multiconfigurational state individual components through visualization of their Dyson orbitals, as corroborated by theoretical modelling.

physics.atm-clus↗

arXiv2023

Resonant TERS of a Single-Molecule Kondo System

Single-molecule tip-enhanced Raman spectroscopy (TERS) under ultra-high vacuum (UHV) and cryogenic conditions enables exploration of the relations between the adsorption geometry, electronic state, and vibrational fingerprints of individual molecules. TERS capability of reflecting spin states in open-shell molecular configurations is yet unexplored. Here we use the tip of a scanning probe microscope to lift a perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecule from a metal surface to bring it into an open-shell spin one-half anionic state. We reveal a correlation between the appearance of a Kondo resonance in the differential conductance spectroscopy and concurrent characteristic changes captured by the TERS measurements. Through a detailed investigation of various adsorbed and tip-contacted PTCDA scenarios, we infer that the Raman scattering on the suspended PTCDA is resonant with a higher excited state. Theoretical simulation of the vibrational spectra enables a precise assignment of the individual TERS peaks to high-symmetry Ag modes, including the fingerprints of the observed spin state. These findings highlight the potential of TERS in capturing complex interactions between charge, spin, and photophysical properties in nanoscale molecular systems, and suggest a pathway for designing spin-optical devices using organic molecules.

cond-mat.mes-hall↗

arXiv2023

Single-molecule time-resolved spectroscopy in a tunable STM nanocavity

The spontaneous fluorescence rates of single-molecule emitters are typically on the order of nanoseconds. However coupling them with plasmonic nanostructures can substantially increase their fluorescence yields. The confinement between the tip and sample of a scanning tunneling microscope creates a tunable nanocavity, an ideal platform for exploring the yields and excitation decay rates of single-molecule emitters depending on the coupling strength to the nanocavity. With this setup we estimate the excitation lifetimes from the direct time-resolved measurements of the fluorescence decays of phthalocyanine adsorbates, decoupled from the metal substrates by ultrathin NaCl layers. It is found that nanosecond-range lifetimes prevail for the emitters away from the nanocavity, whereas for the tip approached to a molecule, we find a substantial effect of the nanocavity coupling, which reduces the lifetimes to a few picoseconds. An analysis is performed to investigate the crossover between the far-field and tip-enhanced photoluminescence regimes. This approach overcomes the drawbacks associated with the estimation of lifetimes for single molecules from their respective emission linewidths.

cond-mat.mes-hall↗