🕑 10:20 - Invited

Optical interaction between nanocavities and single emitters beyond the dipolar approximation

Ruben ESTEBAN
Centro Mixto de Física de Materiales CSIC-UPV/EHU

Plasmonic resonances in metallic nanostructures confine light strongly and interact very efficiently with excitons and vibrations in quantum dots, molecules or other single emitters. We first introduce how plasmonic nanocavities characterized by (sub)nanometer gaps are able to strongly enhance the optical fields in extremely small regions, which can even reach the atomic-scale when exploiting the lighting-rod effect of atomic protrusions. We describe the quantum effects that appear in these systems, as well as the Quantum Corrected Model and the Feibelman approach that have been developed to treat typical experimental conditions. We then focus on the optical response of single emitters placed at such (sub)nanometer plasmonic nanocavities. We consider situations where the emitters cannot be described by the standard dipolar approximation, for example due to the extreme field confinement or to electronic coupling.

🕑 11:10 - Invited

Hyper-resolved fluorescence microscopy of molecular and extend systems

Guillaume SCHULL
Université de Strasbourg, CNRS, IPCMS

The electric current traversing the junction of a scanning tunneling microscope (STM) can be used to generate sub-molecularly resolved fluorescence maps of individual molecules decoupled from a metallic substrate by a thin insulating NaCl layer. Combined with spectral selection and time-correlated measurements, this hyper-resolved fluorescence microscopy approach [1] allowed us to scrutinize the vibronic structure of individual molecules [2] in a very similar way than in the recent TERS reports, without requiring an optical excitation. We used this approach to characterize the photonics properties of charged species [3] to track the motion of hydrogen atoms within free-based phthalocyanine molecules [4], and to address exciton diffusion between complex chromophore architectures [5], graphene nanoribons [6], or 2D semiconductors [7].

[1] A. Roslawska et al., Phys. Rev. X 12, 011012 (2022)
[2] B. Doppagne et al. Nature Nanotechnol,15, 207 (2020).
[3] B. Doppagne et al., Phys. Rev. Lett. 118, 127401 (2017)
[4] B. Doppagne et al. Science, 361, 251 (2018)
[5] S. Cao et al. Nature Chem. 12, 766 (2021)
[6] S. Jiang et al., arXiv:2209.01471
[7] L. Parra Lopez et al., arXiv:2204.14022.

🕑 13:40

Overcoming the fluorescence quenching of single molecules adsorbed directly on a metal substrate

Vibhuti RAI
Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology (KIT)

Self-decoupled single molecule light emitting sources have been challenging to achieve in-spite of the advancement made in the synthesis of tailored molecules. This clearly points toward the complexity of such molecules. The basic idea to achieve such self-decoupled molecules involves anchoring a chromophore with a spacer group which can bind to the metallic electrodes while providing enough electronic decoupling to the chromophore. However, in previous studies, the van der Waals forces between the chromophores and the substrate overcome the binding energy of the anchoring groups forcing the molecules to lie flat on the substrate and hence quenching the fluorescence. Here, we discuss a novel design of naphthalene diimide derivatives (NDI) with an anchoring group that overcomes the van der Waals forces between the chromophores and the substrate. This allows us to perform systematic experiments in the STM so far only possible for the molecules decoupled via insulating layers.

🕑 14:00

A complete picture of excitation-relaxation pathways in STML

Daniel WEGNER
Radboud University, Nijmegen

Despite many STM-induced luminescence (STML) studies of zinc phthalocyanine (ZnPc) on NaCl ultrathin films, there are still controversies regarding the excitation and relaxation pathways that lead to light emission in the tunnel junction. We recently discovered that ZnPc on NaCl/Ag(111) emits light when in its neutral and both charged states (i.e. cationic and anionic), depending on the tip-sample bias polarity. By carefully mapping the molecular frontier orbitals over a wide energy range, we were able to correlate them with the various onset energies observed in STML. Supported by TD-DFT calculations, we propose an alternative charging and intramolecular relaxation mechanism that consistently explains all observed STML features of ZnPc in a unified many-body energy diagram. The results have relevance, beyond STM experiments, for any electroluminescent device.

🕑 14:20

Single-molecule phosphorescence and intersystem crossing in a coupled exciton-plasmon system

Abhishek GREWAL
Max-Planck-Institut für Festkörperforschung

The coupling between phosphors and plasmons is a promising route for their decay rate enhancement, improving efficiency and stability in OLED. The exciton-plasmon interactions (X-P) behind these observations occur at sub-molecular length scales. Therefore, we combine optical spectroscopy with STM, which enables electronic characterization and X-P control at an atomic resolution.
We observe both fluorescence (Fl) and phosphorescence (Ph) in STM-luminescence. We demonstrate that resonant photo-Fl can also lead to Ph. Since direct excitation of the spin-triplet (T1) state is forbidden for incident light, we report intersystem crossing from the spin-singlet (S1) to the T1 state in a tip-controlled single-molecule experiment. The X-P leads to a significant variation in the Ph intensity. We propose that the tip-induced Stark and Purcell effects tune the relative Ph intensities.
The experimental findings of this study are relevant to techniques employing control of S1 and T1 exciton dynamics.

🕑 14:40

Illuminating an individual non-fluorescent molecule

Tzu-Chao HUNG
Department of Physics, University of Regensburg

Combining scanning tunneling microscopy, spectroscopy and STM-induced luminescence (STML) allows us to study the optoelectronic properties down to the atomic scale. However, current STML studies are limited to fluorescent molecules. Generalizing the atomic control and imaging capabilities of STML to phosphorescent or even non-fluorescent molecules can provide a new route to fundamentally understand the photophysical properties of an individual molecule. Nickel phthalocyanine (NiPc) is a non-fluorescent molecule. Light emission from the ligand-centered excited state known as Q-band is quenched due to fast relaxation into the nonradiative metal-centered excited state. Hence, the transition energy of the Q-band can only be determined by absorption spectroscopy. Here, we propose an alternative approach to activate radiative decay of the Q-band of NiPc by utilizing STML in combination with control of the local environment and discuss the involved excitation and relaxation pathways.

🕑 15:20

Modifying plasmon resonances at nanocavities by molecular adsorption: A Scanning Tunnelling Microscopy Study (STM)

Óscar JOVER ARRATE
Universidad Autónoma de Madrid

STM is a promising tool to study the properties of plasmonic nano and pico-cavities, as it allows atomic-scale control of the cavity size. STM electroluminescence spectra holds information about the optical properties of the cavity and the electronic structure of the system. Our group has developed a normalization technique to separate these two factors by combining luminescence and I(V) measurements. In this work, we use this technique to investigate the optical modifications in nanocavities between a Au tip and noble metal surfaces, due to the adsorption of two different organic molecules (BPEN, BPEA). Our results show that most of the changes in the raw spectra can be attributed to electronic structure factors, but still true optical changes in the plasmonic gap modes due to the presence of the molecules can be observed in the normalized spectra. As possible origins, Pauli repulsion from the molecule electron density or coupling of plasmonic and excitonic modes will be discussed.

🕑 15:40

Plasmon-driven Reactivity and Selectivity on Metal Nanoparticles Assessed by Quantum Chemical Simulations

Sadaf EHTESABI
Institute of Physical Chemistry, Friedrich Schiller University, Jena

The control of chemical reactions by plasmon-driven catalytic processes is a highly active field in the chemical community. In this regard, the Kurouski group performed in-depth TERS studies of 2-nitro-5-thiolobenzoic acid (2-N-5TBA) surface-immobilized on either gold or palladium-coated gold nanoparticles. Depending on the experimental setup, branching pathways yielding different products (and yields), e.g. various azo-species obtained upon dimerization. Therefore, a comprehensive understanding of the underlying mechanism is necessary to control the selectivity on the plasmon-driven reaction.
We aim at modeling the chemical interaction of a plasmonic Au and Pd surface with educt. Thus, we study how plasmonic-hybrid systems behave and how plasmons cause different reactions that result in product variations. The driving forces for various pathways and the directionality of light-driven charge transfer allowed us to elucidate the production braching at a quantum mechanical level.

🕑 09:30 - Invited (online)

Atomic-scale investigation of photocurrent generation in a single molecule

Miyabi IMAI-IMADA
Surface and Interface Science Laboratory, RIKEN

Given its central role in light energy conversion, photo-induced electron transfer (PET) from an excited molecule has been widely studied. To deepen understanding of PET, microscopic photocurrent measurement methods have been developed. However, the spatial resolution has been insufficient to resolve individual molecules, so the fundamentals remain elusive. In this study1, we report an atomic-scale investigation of photocurrent generation in a single phthalocyanine molecule using a scanning tunnelling microscope. The localized-plasmon field driven by a tuneable laser efficiently excites the molecule2,3, and PET from its excited state was distinctly detected via photoinduced tunnelling current through the tip. We find that the direction and the spatial distribution of the photocurrent depend on the applied voltage, and detect counterflowing photocurrent channels. In this presentation, the detailed mechanism of photocurrent generation will be discussed.

[1] M. Imai-Imada, et al, Nature 603, 829 (2022).
[2] H. Imada, M. I-Imada, et al, Science 373, 95 (2021).
[3] R. B. Jaculbia, et al, Nat. Nanotechnol. 15, 105 (2020).

🕑 10:40

Energy funneling at the molecular scale

Anna ROSŁAWSKA
IPCMS CNRS

We study STM-induced luminescence from phthalocyanine molecules adsorbed on a few-monolayer NaCl film epitaxially grown on Ag(111) substrate. We show how the atomically-confined electromagnetic field at the STM tip apex acts as a “picocavity” for localized plasmons and both enables optical studies with atomic-scale precision and interacts with the emitter. Profiting from that resolution, we investigate a critical mechanism in the photosynthesis process – resonant energy transfer in multichromophoric architectures. We use individual phthalocyanines as ancillary, passive or blocking elements to promote and direct resonant energy transfer between distant donor and acceptor units. Such an approach constitutes a powerful model to study the role of the relative dipole orientation, distance and chemical nature of the chromophores on the efficiency of the energy transfer.

🕑 11:00

Evidence of trion-libron coupling in chirally adsorbed single molecules

Jiří DOLEŽAL
Institute of Physics, Czech Academy of Sciences

Interplay between motion of nuclei and excited electrons in molecules plays a key role both in biological and artificial nanomachines. Here we provide a detailed analysis of coupling between quantized librational modes (librons) and charged excited states (trions) on single phthalocyanine dyes adsorbed on a surface. By means of tunnelling electron-induced electroluminescence, we identify libronic progressions on a {\mu}eV energy range in spectra of chirally adsorbed phthalocyanines, which are otherwise absent from spectra of symmetrically adsorbed species. Experimentally measured libronic spectra match very well the theoretically calculated libron eigenenergies and peak intensities (Franck-Condon factors) and reveal an unexpected depopulation channel for the zero libron of the excited state that can be effectively controlled by tuning the size of the nanocavity. Our results showcase the possibility of characterizing the dynamics of molecules by their low-energy molecular modes using {\mu}eV-resolved tip-enhanced spectroscopy.

🕑 11:20

Mapping Lamb, Stark and Purcell effects at a chromophore-picocavity junction with atomically resolved fluorescence microscopy

Tomas NEUMAN
Institut des Sciences Moléculaires d'Orsay

We present a correlated theoretical and experimental study of the optical properties of an individual free-base phthalocyanine molecule excited electrically by the plasmonic tip of a STM [1,2,3]. We theoretically and experimentally show that the variation of the spectral position (energy) and width of the electroluminescence emission line of the molecular exciton (recorded as a function of the tip position) can be linked with the tip-position-dependent DC Stark effect induced by the static voltage applied across the tip-substrate gap and the plasmonic Purcell effect induced by the dynamical response of the plasmonic picocavity [4] formed by the tip, respectively. Our theory allows us to interpret the tip-position-dependent spectral line shift map as an image of the difference between the excited- and ground-state electron densities of the molecule, and the tip-position-dependent map of the excitonic line-width variation as an image of the transition electron density.

[1] Chen, C., et al., 2010. Phys. Rev. Lett., 105(21), 217402.
[2] Doppagne, B., et al., 2020, Nat. Nanotechnol., 15, 207-211.
[3] Roslawska, A., et al., 2022, Phys. Rev. X, 12(1), 011012.
[4] Benz, F., et al., 2016, Science, 354(6313), 726-729.

🕑 11:40

Nano-spectroscopic investigation of interlayer interaction in van der Waals heterostructures

Otakar FRANK
J. Heyrovsky Institute of Physical Chemistry, Czech Academy of Sciences

Two-dimensional van der Waals materials and heterostructures formed by deliberate stacking of their individual layers on top of each other belong to the forefront of current materials research. However, their spectroscopic investigation relies mostly either on diffraction-limited microRaman or photoluminescence (PL), which do not properly capture local structural variations caused by nanometre-scale heterogeneities originating from contamination trapped either between the bottom-most layer and its substrate or in the interlayer galleries.
To this end, tip-enhanced spectroscopy methods enable the access to information on the local lattice deformation and also the interaction between the individual layers composing the heterostructure. In this work, we will show specific spectroscopic Raman and PL signatures that allow such nanoscale characterization of monolayer transition metal dichalcogenides and their heterobilayers.

🕑 13:40

Real space visualization of entangled excitonic states in charged molecular assemblies

Martin ŠVEC
Institute of Physics, Czech Academy of Sciences

Entanglement of excitons holds great promise for the future of quantum computing, which would use individual molecular dyes as building blocks of their circuitry. Studying entangled excitonic eigenstates emerging in coupled molecular assemblies in the near-field with submolecular resolution has the potential to bring insight into the photophysics of these fascinating quantum phenomena. In contrast to far-field spectroscopies, near-field spectroscopic mapping permits direct identification of the individual eigenmodes, type of exciton coupling, including excited states otherwise inaccessible in the far field (dark states). Here we combine tip-enhanced spectromicroscopy with atomic force microscopy to inspect delocalized single-exciton states of charged molecular assemblies engineered from individual perylenetetracarboxylic dianhydride (PTCDA) molecules. Hyperspectral mapping of the eigenstates and comparison with calculated many-body optical transitions reveals a second low-lying excited state of the anion monomers and its role in the exciton entanglement within the assemblies. We demonstrate control over the exciton coupling by switching the assembly charge states. Our results reveal the possibility of tailoring excitonic properties of organic dye aggregates for advanced functionalities and establish the methodology to address them individually at the nanoscale.

🕑 14:00

Ultra-broadband Highly Sensitive Semiconductor Nanomaterials-Graphene Photodetectors

Golam HAIDER
J. Heyrovsky Institute of Physical Chemistry, Czech Academy of Sciences

Recently designed quantum dot (QD)-graphene-based hybrid photodetector has shown a great promise with an ultra-high photoresponsivity (106 times higher than commercial Si-based photodiode). In such design, the QDs harvest photons producing electron-hole pairs. Due to the built-in electric field at the graphene-QD interface, one kind of photogenerated charge gets transferred to the graphene layer contributing to the photocurrent. The remanent charge at the QD produces a capacitance effect that helps in amplifying the photocurrent in graphene. We observed that the performance of such devices can be enormously tuned to design broadband ultra-violet to infra-red photodetectors utilizing suitable light-harvesting materials. Additionally, photons having energy much higher than the bandgap of the light-harvesting materials can produce space charges in the device, which dramatically influence the current in graphene channel.

🕑 14:20

Visualization of surface-stabilized nitrene after tip- and light-induced transformation of aromatic azides on Au(111)

Federico FREZZA
Institute of Physics, Czech Academy of Sciences

Organic azides attracted interest as versatile precursors for the synthesis of several nitrogen-containing compounds. The high reactivity of azides makes them challenging to study even on surfaces. In this work, we deposit 2,6-diazidoanthracene-9,10-dione molecules on Au(111) under ultra-high vacuum conditions, observing the ordered self-assembly of intact molecules. Scanning probe microscopies at low temperature allow to control the tip-molecule interaction at the nanometric scale. Here, we induce the removal of molecular nitrogen (N2) from the azide groups of single molecules either with STM tip or by illuminating with UV light (266 nm) at 5 K. This leads to to the formation of nitrenes stabilized by Au atoms through a mechanism that involves the injection of electrons into the LUMO. We characterized the system by STM, STS and nc-AFM supported by DFT calculations.

🕑 14:40

Construction of a THz Scanning Tunneling Microscope

Christian LOTZE
Freie Universität Berlin

We have set up a THz-based ultrafast pump-probe experiment, that will allow us to investigate ultrafast processes on the atomic scale [1]. Therefore, we couple ultrashort VIS/IR and THz pulses into the junction of a low temperature (<10K) scanning tunneling microscope.
We measure THz gated photocurrents as proposed in [2, 3] and find THz pulses inside the STM allow for a time resolution of about 550 fs. Within the tunneling regime the effect of THz induced transient bias voltages on features with non-linear I-V characteristics, we can estimate the THz electric field amplitude in the STM junction to reach some 100 mV. Also here, we find from sending two time delayed THz pulses, again, a confirmation of above’s possible time resolution.

[1] T. L. Cocker, D. Peller, P. Yu, J. Repp & R. Huber: Nature 539, 263 (2016)
[2] S. Yoshida et al.: ACS Photonics 6, 1356 (2019)
[3] M. Muller, N. M. Sabanés, T. Kampfrath, and M. Wolf: ACS Photonics 7, 2046 (2020)

🕑 15:30

Coherent phonon spectroscopy on the nanoscale

Melanie MÜLLER
Fritz Haber Institute of the Max Planck Society

Coherent phonon (CP) spectroscopy is a powerful tool to monitor ultrafast lattice dynamics under nonequilibrium conditions, providing insight into microscopic interactions that dictate macroscopic material properties. In imperfect crystals, the excitation and relaxation of CPs will be susceptible to the nanoscale environment, calling for real-space observation of ultrafast lattice dynamics. We demonstrate nanoscale coherent phonon spectroscopy by means of ultrafast laser-induced scanning tunneling microscopy (STM) in a plasmonic junction. Comparison of the CP spectra with tip-enhanced Raman spectroscopy allows us to identify the involved phonon modes. In contrast to the Raman spectra, the relative CP intensities exhibit strong nanoscale spatial variations, which correlate with changes in the local density of states recorded via scanning tunneling spectroscopy. Our work introduces a new approach to study the ultrafast structural response at solid surfaces using optical STM.

🕑 15:50

Tracking self-trapped holes at the SrTiO3 (001) surface

Martin SETVÍN
Charles University, Prague

Perovskites are versatile materials that attract attention in many research fields. One interesting property is that many perovskites possess excellent capabilities for electron-hole separation, which makes them hot materials for photovoltaics or photocatalysis. Here, we study the behaviour of charge carriers on bulk-terminated SrTiO3 (001) surfaces prepared by cleaving. When the as-cleaved surface is illuminated by UV light, the local contact potential shifts by several tenths of eV. This shift persists over many days at cryogenic temperatures and we show it originates from holes self-trapped at the surface. We discuss the possible approaches for localizing these self-trapped charge carriers by noncontact AFM.

🕑 16:10

Emergence of sulfur quantum dots, its room temperature phosphorescence, and efficiency to show fluorescence depended circular dichroism.

Karthika S SUNIL
Leibniz Institute of Photonic Technology, Jena

Sulfur dots (SQDs) are recently developed luminescent non-metallic nanomaterial and, we have synthesized highly luminescent, blue emitting dots through a single step, facile method using a kitchen blender. Since we are using this method of mechanical grinding, the synthesis is not consuming additional energy in terms of heat, time, pressure, and harsh chemical treatment. Hence it is very advantageous over all the known previous methods of synthesis of SQDs. The morphological and photophysical features of SQDs were characterized through high-resolution transmission electron microscopy(HR-TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), photoluminescence spectroscopy, and UV-Visible spectroscopy, etc. For the first time, we have found an efficient room-temperature phosphorescence and long-term afterglow (5 sec) while embedded in the B2O3 and biuret matrix. Also for the first time in SQDs, we studied the reversible transformation of room temperature.

Ultrafast terahertz scanning tunneling microscope for the investigation of quantum defects in 2D semiconductors

Jonas ALLERBECK
nanotechATsurfaces Laboratory, EMPA

Ultrafast lightwave-driven tunneling microscopy is an emerging technology combining atomic spatial reso-lution of a scanning tunneling microscope with sub-picosecond time resolution enabling the investigation of nanoscale dynamics at the frontier of spatiotemporal resolution. We currently design and implement a next-generation THz scanning tunneling microscope with single-cycle THz pulses at variable repetition rates up to 41 MHz, allowing measurable currents even in the single electron per pulse regime. Advanced pulse shaping methods allow continuous control of the carrier-envelope phase of THz pulses for energy selective field-driven tunneling. Our state-of-the-art low-temperature STM enables simultaneous investiga-tion of ultrafast light-wave driven currents, THz pump-probe experiments, and single emitter lumines-cence. These new capabilities will enable to explore exciting physics of atomic quantum systems in 2D materials and beyond, aiming at yet unexplored dynamic properties.

Modeling of molecular aggregates excitonic states probed by near-field spectroscopy

Sofia CANOLA
Institute of Physics, Czech Academy of Sciences

Aggregates of interacting molecules display coherently delocalized electronic excitations (excitons). Tailoring the structure of aggregates, probing their optical response and modeling the underlying electronic structure at a microscopic level are crucial aspects to analyze and engineer aggregates with desired properties. Scanning tunneling microscopy-induced luminescence (STML) can detect light emission with submolecular resolution providing information not available in the standard far-field measurements. A computational framework combining molecular quantum-chemical calculations with a theoretical description of the near-field optical response is a valuable instrument for the interpretation of STML data. Here we theoretically address the experimentally observed delocalized excitonic states of aggregates formed by conjugated molecules, revealing detailed information on the excitonic states and how they evolve from the excited states of the interacting chromophores.

Detecting spin state transition by tip-enhanced Raman spectroscopy

Rodrigo FERREIRA
Institute of Physics, Czech Academy of Sciences

Tip-enhanced Raman spectroscopy (TERS) is a valuable technique that has been proving to unveil fundamental features of single molecules at the nanoscale level via near-field spectroscopy. The localized plasmonic field in the STM tip apex atomically confines light in the nanocavity and can be used as a probe to obtain an essential understanding of molecular structural and electronic properties. Here, the STM-controlled TERS technique in a UHV environment at low temperature was performed and verified for the perylene tetracarboxylic dianhydride (PTCDA) and naphthalene tetracarboxylic dianhydride (NTCDA), in which was measured the Raman fingerprint for a single molecule while following the controlled detachment from its metal carrier. The precise control of geometry and spin state of the suspended molecule together with the Raman fingerprint revealed an abrupt change in the spectra from singlet to decoupled free spin-1/2 ground state. We correlate them to the presence of Kondo signature and discuss the implication for other systems.

Integrated simulation package for scanning probe microscopy

Prokop HAPALA
Institute of Physics, Czech Academy of Sciences

Scanning probe microscopy (SPM) opens a door to single-molecular science and engineering of molecular machines with unparalleled levels of atomic control. But simulations required to provide insight into these experiments are currently unnecessarily costly and laborious due to lack of optimized software. Our new integrated package (https://github.com/ProkopHapala/FireCore) aims to streamline tasks, such as search for absorption geometries of molecules on substrate and simulation of respective SPM images including AFM, STM and light-STM microscopy, to enable high throughput screening of chemical reactions and molecular architectures on surface and to simplify the simulations to such a degree which allows it to be used by non-experts and experimentalists. The necessary speed-up is achieved by combination of classical/quantum approach (QM/MM) and by extensive use of GPU acceleration.

Electron-stimulated photon emission on TMD defects

Lysander HUBERICH
EMPA

Due to their exceptionally long electron spin coherence and spin-selective optical readout nitrogen-vacancy centers in diamond are considered a key building block in quantum sensing and quantum-cryptography applications. However, defects in bulk materials suffer from limited tunability and placement control, poor photon extraction efficiency, and coherence degradation close to the surface. 2D materials such as monolayer transitional metal dichalcogenides (TMDs) are expected to overcome these challenges while offering new synthetic strategies for the bottom-up design of solid-state defects. Here we present an NV- center analog in 2D; the dopant vacancy complex (ReMo + VacS)- in MoS2. We investigate its electronic states using scanning tunneling spectroscopy and present atomically-resolved photon emission maps of single TMD defects by means of STM luminescence.

Self-decoupled tetrapodal Zinc-porphyrin molecules for luminescence studies of isolated emitters on Au(111)

Talha IJAZ
Shaanxi Normal University

Light-emitting molecules directly adsorbed on metal surfaces do not emit photons due to fluorescence quenching. To avoid such a quenching effect chemical decoupling is one way by functionalizing emitter molecules with anchor groups. It also provides an additional advantage to designing the molecular dipole along the axial direction to facilitate the strong coupling between nanocavity plasmons and emitters. In this work, self-decoupled tetrapodal(ZnP4TPM) molecules were designed, synthesized, and deposited on Au(111) substrate by electrospray ionization technique. Preliminary STML indicates the molecule-specific emissions from isolated single ZnP4TPM molecules. The significant emission at the molecular center suggests that there is a considerable vertical component of the transition dipole of ZnP4TPM along the tip axial direction. Our results may open up a route for the realization of nanolight sources and plasmonic devices based on organic molecules.

STM-induced luminescence from single graphene nanoribbons

Song JIANG
Université de Strasbourg, CNRS, IPCMS

Graphene nanoribbons (GNRs),[1] quasi-one-dimensional narrow strips of graphene, have emerged as promising candidates for high-performance nanoelectronic devices due to their tunable energy band gaps resulting from lateral quantum confinement and edge effects. The recent development of on-surface synthesis (OSS) has achieved various types of atomically precise GNRs, revealing fascinating electronic, magnetic, and mechanical properties. Their optical properties, on the other hand, remain largely unexplored, with only few reports on the averaging fluorescence/absorbance measurements over GNR film/solution. The intrinsic luminescence properties of atomically precise GNR remains remain to be addressed at single molecule level. Here, excitonic emission from atomically precise GNRs synthesized on a metal surface is probed using a scanning tunneling microscopy (STM) approach.[2] A STM-based strategy to transfer the GNRs to a partially insulating surface is used to prevent light emission quenching of the ribbons by the metal substrate. Sub-nanometer resolved STM-induced fluorescence spectra reveal emission from localized dark excitons build upon the topological end states of the GNRs accompanying with a series of vibronic peaks. The vibronic peaks at high energy (>1000 cm-1) are assigned to the specific vibrations of sp2 carbon materials and insensitive with the ribbon length. On the other hand, the low-frequency vibronic emission comb changes with the GNR length and is attributed to longitudinal acoustic modes confined to a finite box. Overall, our study provides a novel path to investigate the interplay between excitons, vibrons and topology in atomically precise graphene nanostructures.

[1] J. Cai, P. Ruffieux; R. Jaafar, et al., Atomically precise bottom-up fabrication of graphene Nanoribbons. Nature 466, 470-473 (2010).
[2] S. Jiang, T. Neuman, A, Boeglin, et al. Topologically localized excitons in single graphene nanoribbons. Submitted.

The impact of field- and chemical-enhancement on TERS for CuNc

Amirhassan KHODADADI
Friedrich Schiller University of Jena

Tip- and Surface- Enhanced Raman Spectroscopy are powerful plasmon-based techniques employed due to their high sensitivity and specificity. The increase of the Raman signal is usually explained by two major mechanisms known as chemical enhancement mechanism (CM) arising from charge-transfer within the molecule or between the molecule and surface/tip, and electromagnetic field enhancement mechanism (EM) originating from localized plasmon resonance. The advantage of TERS compared to SERS is the spatial resolution provided by the tip[1, 2].
In this study, we aim to assess both the CM and the EM contribution to TERS signals for copper naphthalocyanine (CuNc) in a holistic approach. A system including one CuNc molecule and a single gold atom mimicking the plasmonic tip was considered. Excited-state properties were obtained by time-dependent DFT. Furthermore, the EM contribution was approximated by a spatially inhomogeneous static electric field centered around of the gold tip.
Raman maps were simulated using non-resonant as well as resonant conditions (upon 720 nm excitation) and compared to experimental data [3].

[1] Cao Y, et al., Reviews in Physics 100067 (2022)
[2] Fiederling K, et al., Nanoscale 12, 6346-6359 (2020)
[3] Jaculbia RB, et al., Nature nanotechnology 15, 105-110 (2020)

Scanning tunneling microscope induced luminescence of an atomically thin-semiconductor

Luis Enrique PARRA LOPEZ
Fritz Haber Institut of the Max Planck Society

Atomically thin semiconductors made from transition metal dichalcogenides are appealing systems for investigating efficient light matter interaction. Their optical properties are dominated by tightly bound excitons with a typical spatial extension of ~ 1 nm. This makes excitons sensitive to the presence of atomic-scale inhomogeneities whose individual effects cannot be addressed with traditional diffraction-limited techniques. Here, we present an approach using a low-temperature STM to induce the luminescence of a MoSe2/FLG heterostructure. We correlate the atomic-scale landscape with the locally induced optical response of the heterostructure and demonstrate STM-induced luminescence from neutral, charged, and localized excitons. We report sizeable variations in the emission of the heterostructure between different nm-sized areas suggesting an interface-dependent emission. Our study paves the way for novel investigations of van der Waals heterostructure with sub-nm resolution.

Photoactivity of anydrides on a semiconductor surface

Ana SÁNCHEZ-GRANDE
Insitute of Physics, Czech Academy of Sciences

During the last decade the on-surface synthesis field has emerged giving rise to the formation of unprecedented carbon-based nanomaterials on noble metal surfaces. Recently, on-surface synthesis has been expanded to the study of chemical reactions on inert surfaces in view of the requirements needed for technological applications, where the absence of a metal surface that catalyzes the chemical reaction makes it challenging. The adsorption energy on inert surfaces is lower than on metal surfaces, which limits the viability of thermally activated reactions. Typically, molecular desorption upon thermal activation on inert surfaces takes place before carbon-carbon coupling occurs, thus, another alternative as photochemistry has to be considered. Here, we present the selective photoactivation of tetraphenyl phthalic anhydride (TPPA) on an inert SnSe surface grown on Au(111). Upon irradiation of the TPPA on SnSe with UV-light at room temperature we observe the photo-induced decarboxylation and decarbonylation of the molecules. The structural and electronic characterization of the tetraphenyl ortho-benzyne was performed by scanning tunneling microscopy (STM) together with density functional theory (DFT) calculations.

Ferroelectric control of molecular states in 2D limit

Kezilebieke SHAWULIENU
University of Jyväskylä

The use of electric fields is a powerful approach to manipulating molecular electronic/magnetic states, and consequently, optical properties, adsorption structures, vibrational frequencies, oxidation states, and chemical reactivity. However, it is normally very challenging to achieve this at a single molecular scale in single-phase materials.
We overcome these limitations by coupling single molecules with two-dimensional ferroelectric (2D-FE) materials. By controlling the charge polarization of the FE using STM, one can tune the electric field experienced by the molecule and consequently, control the electronic states of molecules.
Our study not only provides a well-defined, controllable platform for manipulating molecular electronic states with an electric field but also has great potential for practical applications in molecular electronics and spintronics devices.

STM imaging of a decoupled porphyrin molecule in the HOMO-LUMO gap

Liqing ZHENG
Max Planck Institute for solid-state research, Stuttgart

The electronic structure of molecules can be well-resolved by scanning tunneling microscopy (STM). Here, single porphyrin (PtOEP) molecules adsorbed on ultrathin insulating NaCl films have been examined by low-temperature STM. Different features of PtOEP molecules in the HOMO-LUMO gap were captured by STM imaging when molecules were adsorbed at different sites of NaCl layers. When the center of PtOEP is located on a Na atom, it appears as chiral lobes and a dark center, together with a dark rim. When the molecule is located on a Cl atom, it shows a protrusion in the middle with less chirality in lobes. The calculations suggest that the contributions from molecular orbitals at energies well below the HOMO-LUMO gap are significant to the in-gap features, which allows for understanding the molecular in-gap structure. Understanding the in-gap structure of molecules would provide insights into the mechanism of up conversion electroluminescence that involves tunneling through HOMO-LUMO gap.

Aji ALEXANDER
Charles University, Prague

Jan BERGER
Institute of Physics, Czech Academy of Sciences

Rahul BHUYAN
University of Gothenburg, Sweden

Latifeh EIRI
Institute of Physical Chemistry, Friedrich Schiller University, Jena

Karl-Heinz ERNST
EMPA

Pavel JELÍNEK
Institute of Physics, Czech Academy of Sciences

Petr KAHAN
Institute of Physics, Czech Academy of Sciences

Katharina KAISER
IPCMS - CNRS, Department of Surfaces and Interfaces

Pablo MERINO
ICMM-CSIC / ICN2

Aitor MUGARZA
Institut Català de Nanociència i Nanotecnologia

Ruben PEREZ
Universidad Autónoma de Madrid

Klaus RENZIEHAUSEN
Friedrich-Schiller-Universität Jena

Amandeep SAGWAL
Institute of Physics, Czech Academy of Sciences

Fabian SCHULZ
CIC nanoGUNE San Sebastian - Donostia

Hosein SHADRAM
Friedrich Schiller University, Jena

Irena G STARÁ
Institute of Chemistry and Biochemistry, Czech Academy of Sciences