Spin-Triplet-Mediated Up-Conversion and Crossover Behavior in Single-Molecule Electroluminescence

Spin-state switching of indium-phthalocyanine on Pb(100)

Indium(iii) phthalocyanine chloride deposited on Pb(100) is studied by scanning tunnelling spectroscopy at cryogenic temperatures. The Cl ions are dissociated and the remaining indium phthalocyanine (InPc) is observed in two states with the metal ion pointing to (↓) or away (↑) from the substrate. Isolated molecules and islands with a superstructure and a unit cell of four inequivalent molecules, namely one InPc↑ and three InPc↓ in different sites, are observed. Using atomic resolution images of the substrate the adsorption sites and azimuthal orientation of InPc are determined and a structure model is proposed. Conductance spectra of the lowest unoccupied molecular orbital reveal differences that depend on the adsorption sites and azimuthal orientations of the complexes. Only InPc↑ molecules exhibit Shiba states, indicating the presence of a localized spin. By electron extraction isolated complexes as well as molecules in islands are converted from InPc↑ to InPc↓. At the same time, their spin state changes, as indicated by the disappearance of the Shiba states.

Selective excitation of vibrations in a single molecule

The capability to excite, probe, and manipulate vibrational modes is essential for understanding and controlling chemical reactions at the molecular level. Recent advancements in tip-enhanced Raman spectroscopies have enabled the probing of vibrational fingerprints in a single molecule with Ångström-scale spatial resolution. However, achieving controllable excitation of specific vibrational modes in individual molecules remains challenging. Here, we demonstrate the selective excitation and probing of vibrational modes in single deprotonated phthalocyanine molecules utilizing resonance Raman spectroscopy in a scanning tunneling microscope. Selective excitation is achieved by finely tuning the excitation wavelength of the laser to be resonant with the vibronic transitions between the molecular ground electronic state and the vibrational levels in the excited electronic state, resulting in the state-selective enhancement of the resonance Raman signal. Our approach contributes to setting the stage for steering chemical transformations in molecules on surfaces by selective excitation of molecular vibrations.

Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions

Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron-hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a ~2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10-6 S) and low power density regime (<102 W cm-2), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface.

Scanning Tunneling Microscopy for Molecules: Effects of Electron Propagation into Vacuum

Using scanning tunneling microscopy (STM), we experimentally and theoretically investigate isolated platinum phthalocyanine (PtPc) molecules adsorbed on an atomically thin NaCl(100) film vapor deposited on Au(111). We obtain good agreement between theory and constant-height STM topography. We theoretically examine why strong distortions of STM images occur as a function of distance between the molecule and the STM tip. The images of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) exhibit for increasing distance, significant radial expansion due to electron propagation in the vacuum. Additionally, the imaged angular dependence is substantially distorted. The LUMO image has substantial intensity along the molecular diagonals where PtPc has no atoms. In the electronic transport gap, the image differs drastically from HOMO and LUMO even at energies very close to these orbitals. As the tunneling becomes increasingly off-resonant, the eight angular lobes of the HOMO or of the degenerate LUMOs diminish and reveal four lobes with maxima along the molecular axes, where both, HOMO and LUMO have little or no weight. These images are strongly influenced by low-lying PtPc orbitals that have simple angular structures.

Activating the Fluorescence of a Ni(II) Complex by Energy Transfer

Luminescence of open-shell 3d metal complexes is often quenched due to ultrafast intersystem crossing (ISC) and cooling into a dark metal-centered excited state. We demonstrate successful activation of fluorescence from individual nickel phthalocyanine (NiPc) molecules in the junction of a scanning tunneling microscope (STM) by resonant energy transfer from other metal phthalocyanines at low temperature. By combining STM, scanning tunneling spectroscopy, STM-induced luminescence, and photoluminescence experiments as well as time-dependent density functional theory, we provide evidence that there is an activation barrier for the ISC, which, in most experimental conditions, is overcome. We show that this is also the case in an electroluminescent tunnel junction where individual NiPc molecules adsorbed on an ultrathin NaCl decoupling film on a Ag(111) substrate are probed. However, when an MPc (M = Zn, Pd, Pt) molecule is placed close to NiPc by means of STM atomic manipulation, resonant energy transfer can excite NiPc without overcoming the ISC activation barrier, leading to Q-band fluorescence. This work demonstrates that the thermally activated population of dark metal-centered states can be avoided by a designed local environment at low temperatures paired with directed molecular excitation into vibrationally cold electronic states. Thus, we can envisage the use of luminophores based on more abundant transition metal complexes that do not rely on Pt or Ir by restricting vibration-induced ISC.

Anomalously bright single-molecule upconversion electroluminescence

Efficient upconversion electroluminescence is highly desirable for a broad range of optoelectronic applications, yet to date, it has been reported only for ensemble systems, while the upconversion electroluminescence efficiency remains very low for single-molecule emitters. Here we report on the observation of anomalously bright single-molecule upconversion electroluminescence, with emission efficiencies improved by more than one order of magnitude over previous studies, and even stronger than normal-bias electroluminescence. Intuitively, the improvement is achieved via engineering the energy-level alignments at the molecule-substrate interface so as to activate an efficient spin-triplet mediated upconversion electroluminescence mechanism that only involves pure carrier injection steps. We further validate the intuitive picture with the construction of delicate electroluminescence diagrams for the excitation of single-molecule electroluminescence, allowing to readily identify the prerequisite conditions for producing efficient upconversion electroluminescence. These findings provide deep insights into the microscopic mechanism of single-molecule upconversion electroluminescence and organic electroluminescence in general.

Light-Emitting Electrochemical Cells Based on Nanogap Electrodes

In a light-emitting electrochemical cell (LEC), electrochemical doping caused by mobile ions facilitates bipolar charge injection and recombination emissions for a high electroluminescence (EL) intensity at low driving voltages. We present the development of a nanogap LEC (i.e., nano-LEC) comprising a light-emitting polymer (F8BT) and an ionic liquid deposited on a gold nanogap electrode. The device demonstrated a high EL intensity at a wavelength of 540 nm corresponding to the emission peak of F8BT and a threshold voltage of ∼2 V at 300 K. Upon application of a constant voltage, the device demonstrated a gradual increase in current intensity followed by light emission. Notably, the delayed components of the current and EL were strongly suppressed at low temperatures (<285 K). The results clearly indicate that the device functions as an LEC and that the nano-LEC is a promising approach to realizing molecular-scale current-induced light sources.

Charge-state lifetimes of single molecules on few monolayers of NaCl

In molecular tunnel junctions, where the molecule is decoupled from the electrodes by few-monolayers-thin insulating layers, resonant charge transport takes place by sequential charge transfer to and from the molecule which implies transient charging of the molecule. The corresponding charge state transitions, which involve tunneling through the insulating decoupling layers, are crucial for understanding electrically driven processes such as electroluminescence or photocurrent generation in such a geometry. Here, we use scanning tunneling microscopy to investigate the decharging of single ZnPc and H2Pc molecules through NaCl films of 3 to 5 monolayers thickness on Cu(111) and Au(111). To this end, we approach the tip to the molecule at resonant tunnel conditions up to a regime where charge transport is limited by tunneling through the NaCl film. The resulting saturation of the tunnel current is a direct measure of the lifetimes of the anionic and cationic states, i.e., the molecule's charge-state lifetime, and thus provides a means to study charge dynamics and, thereby, exciton dynamics. Comparison of anion and cation lifetimes on different substrates reveals the critical role of the level alignment with the insulator's conduction and valence band, and the metal-insulator interface state.

Character of Electronic States in the Transport Gap of Molecules on Surfaces

We report on scanning tunneling microscopy (STM) topographs of individual metal phthalocyanines (MPc) on a thin salt (NaCl) film adsorbed on a gold substrate, at tunneling energies within the molecule's electronic transport gap. Theoretical models of increasing complexity are discussed. The calculations for MPcs adsorbed on a thin NaCl layer on Au(111) demonstrate that the STM pattern rotates with the molecule's orientations─in excellent agreement with the experimental data. Thus, even the STM topography obtained for energies in the transport gap represent the structure of a one atom thick molecule. It is shown that the electronic states inside the transport gap can be rather accurately approximated by linear combinations of bound molecular orbitals (MOs). The gap states include not only the frontier orbitals but also surprisingly large contributions from energetically much lower MOs. These results will be essential for understanding processes, such as exciton creation, which can be induced by electrons tunneling through the transport gap of a molecule.

Many-Body Description of STM-Induced Fluorescence of Charged Molecules

A scanning tunneling microscope is used to study the fluorescence of a model charged molecule (quinacridone) adsorbed on a sodium chloride (NaCl)-covered metallic sample. Fluorescence from the neutral and positively charged species is reported and imaged using hyperresolved fluorescence microscopy. A many-body model is established based on a detailed analysis of voltage, current, and spatial dependences of the fluorescence and electron transport features. This model reveals that quinacridone adopts a palette of charge states, transient or not, depending on the voltage used and the nature of the underlying substrate. This model has a universal character and clarifies the transport and fluorescence mechanisms of molecules adsorbed on thin insulators.

Activating Electroluminescence of Charged Naphthalene Diimide Complexes Directly Adsorbed on a Metal Substrate

Electroluminescence from single molecules adsorbed on a conducting surface imposes conflicting demands for the molecule-electrode coupling. To conduct electrons, the molecular orbitals need to be hybridized with the electrodes. To emit light, they need to be decoupled from the electrodes to prevent fluorescence quenching. Here, we show that fully quenched 2,6-core-substituted naphthalene diimide derivative in a self-assembled monolayer directly deposited on a Au(111) surface can be activated with the tip of a scanning tunneling microscope to decouple the relevant frontier orbitals from the metallic substrate. In this way, individual molecules can be driven from a strongly hybridized state with quenched luminescence to a light-emitting state. The emission performance compares in terms of quantum efficiency, stability, and reproducibility to that of single molecules deposited on thin insulating layers. Quantum chemical calculations suggest that the emitted light originates from the singly charged cationic pair of the molecules.

Plasmon Squeezing in Single-Molecule Junctions

Scanning tunneling microscope (STM)-induced luminescence provides an ideal platform for electrical generation and the atomic-scale manipulation of nonclassical states of light. However, despite its extreme importance in quantum technologies, squeezed light emission with reduced quantum fluctuations has hitherto not been demonstrated in such a platform. Here, we theoretically predict that the emitted light from the plasmon mode can be squeezed in an STM single molecular junction subject to an external laser drive. Going beyond the traditional paradigm that generates squeezing with the quadratic interaction of photons, our prediction explores the molecular coherence involved in an anharmonic energy spectrum of a coupled plasmon-molecule-exciton system. Furthermore, we show that, by selectively exciting the energy ladder, the squeezed plasmon can show either sub- or super-Poissonian statistical properties. We also demonstrate that, following the same principle, the molecular excitonic mode can be squeezed simultaneously.

Selectively Addressing Plasmonic Modes and Excitonic States in a Nanocavity Hosting a Quantum Emitter

Controlling the interaction between the excitonic states of a quantum emitter and the plasmonic modes of a nanocavity is key for the development of quantum information processing devices. In this Letter we demonstrate that the tunnel electroluminescence of electrically insulated C₆₀ nanocrystals enclosed in the plasmonic nanocavity at the junction of a scanning tunneling microscope can be switched from a broad emission spectrum, revealing the plasmonic modes of the cavity, to a narrow band emission, displaying only the excitonic states of the C₆₀ molecules by changing the bias voltage applied to the junction. Interestingly, excitonic emission dominates the spectra in the high-voltage region in which the simultaneously acquired inelastic rate is low, demonstrating that the excitons cannot be created by an inelastic tunnel process. These results point toward new possible mechanisms for tunnel electroluminescence of quantum emitters and offer new avenues to develop electrically tunable nanoscale light sources.

Light Emission from M-Type Enantiomer of 2,13-bis(hydroxymethyl)[7]-thiaheterohelicene Molecules Adsorbed on Au(111) and C60/Au(111) Surfaces Investigated by STM-LE

Light emission from the M-type enantiomer of a helicene derivative (2,13-bis(hydroxymethyl)[7]-thiaheterohelicene) adsorbed on the clean Au(111) and the C₆₀-covered Au(111) surfaces were investigated by tunneling-current-induced light-emission technique. Plasmon-originated light emission was observed on the helicence/Au(111) surface and it was strongly suppressed on the area where the helicene molecules were adsorbed at the edges of the Au(111) terraces. To avoid luminescence quenching of excited helicene molecules and to suppress strong plasmon light emission from the Au(111) surface, C₆₀ layers were used as decoupling buffer layers between helicene molecules and the Au(111) surface. Helicene molecules were adsorbed preferentially on the Au(111) surface rather than on the C₆₀ buffer layers due to the small interaction of the molecules and C₆₀ islands. This fact motivated us to deposit a multilayer of helicene molecules onto the C₆₀ layers grown on the Au(111) surface, leading to the fact that the helicene/C₆₀ multilayer showed strong luminescence with the molecules character. We consider that such strong light emission from the multilayer of helicene molecules has a plasmon origin strongly modulated by the molecular electronic states of (M)-[7]TH-diol molecules.

Single-molecule nano-optoelectronics: insights from physics

Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.

Anionic character of the conduction band of sodium chloride

The alkali halides are ionic compounds. Each alkali atom donates an electron to a halogen atom, leading to ions with full shells. The valence band is mainly located on halogen atoms, while, in a traditional picture, the conduction band is mainly located on alkali atoms. Scanning tunnelling microscopy of NaCl at 4 K actually shows that the conduction band is located on Cl- because the strong Madelung potential reverses the order of the Na+ 3s and Cl- 4s levels. We verify this reversal is true for both atomically thin and bulk NaCl, and discuss implications for II-VI and I-VII compounds.

Vibrationally Resolved Absorption and Luminescence Spectra of Mass-Selected Free-Base and Zinc Phthalocyanine Radical Cations Isolated in Solid Ne

We report the first vibrationally well-resolved absorption and laser-induced fluorescence spectra of the radical cations of free-base phthalocyanine (H₂Pc⁺) and zinc phthalocyanine (ZnPc⁺) isolated in 5 K neon matrices and compare them to the spectral properties of the corresponding neutrals. The samples were generated by low-energy deposition of the mass-selected ions. The spectra are also discussed in terms of time-dependent density functional theory calculations and compared with recently reported scanning tunneling microscopy-induced single-molecule luminescence of the same species adsorbed on NaCl-covered Au(111) or Ag(111) single crystal supports.

Internal Stark effect of single-molecule fluorescence

The optical properties of chromophores can be efficiently tuned by electrostatic fields generated in their close environment, a phenomenon that plays a central role for the optimization of complex functions within living organisms where it is known as internal Stark effect (ISE). Here, we realised an ISE experiment at the lowest possible scale, by monitoring the Stark shift generated by charges confined within a single chromophore on its emission energy. To this end, a scanning tunneling microscope (STM) functioning at cryogenic temperatures is used to sequentially remove the two central protons of a free-base phthalocyanine chromophore deposited on a NaCl-covered Ag(111) surface. STM-induced fluorescence measurements reveal spectral shifts that are associated to the electrostatic field generated by the internal charges remaining in the chromophores upon deprotonation.

The internal Stark effect, a shift of the spectral lines of a chromophore induced by electrostatic fields in its close environment, plays an important role in nature. Here the authors observe a Stark shift in the fluorescence spectrum of a phthalocyanine molecule upon charge modifications within the molecule itself, achieved by sequential removal of the central protons with a STM tip.

Structural Reconstruction of Optically Invisible State in a Single Molecule via Scanning Tunneling Microscope

Molecular dark states, participating in various energy- and electron-transfer processes, are typically beyond direct optical-spectroscopic measurements because of the forbidden transition dictated by the selection rule. In this work, we demonstrate a direct profile of the dark-state transition density of a single molecule on the subnanometer scale by using a scanning tunneling microscope. Our method allows one to resolve the four-lobe configuration in a 1 nm region for the example molecule. The current proposal will bring about a new methodology to study the single-molecule properties in electro-optical devices and light-assisted biological processes.

Synthesis and Surface Behaviour of NDI Chromophores Mounted on a Tripodal Scaffold: Towards Self-Decoupled Chromophores for Single-Molecule Electroluminescence

This paper reports the efficient synthesis, absorption and emission spectra, and the electrochemical properties of a series of 2,6-disubstituted naphthalene-1,4,5,8-tetracarboxdiimide (NDI) tripodal molecules with thioacetate anchors for their surface investigations. Our studies showed that, in particular, the pyrrolidinyl group with its strong electron-donating properties enhanced the fluorescence of such core-substituted NDI chromophores and caused a significant bathochromic shift in the absorption spectrum with a correspondingly narrowed bandgap of 1.94 eV. Cyclic voltammetry showed the redox properties of NDIs to be influenced by core substituents. The strong electron-donating character of pyrrolidine substituents results in rather high HOMO and LUMO levels of -5.31 and -3.37 eV when compared with the parental unsubstituted NDI. UHV-STM measurements of a sub-monolayer of the rigid tripodal NDI chromophores spray deposited on Au(111) show that these molecules mainly tend to adsorb flat in a pairwise fashion on the surface and form unordered films. However, the STML experiments also revealed a few molecular clusters, which might consist of upright oriented molecules protruding from the molecular island and show electroluminescence photon spectra with high electroluminescence yields of up to 6×10-3 . These results demonstrate the promising potential of the NDI tripodal chromophores for the fabrication of molecular devices profiting from optical features of the molecular layer.

Single-molecule laser nanospectroscopy with micro-electron volt energy resolution

Ways to characterize and control excited states at the single-molecule and atomic levels are needed to exploit excitation-triggered energy-conversion processes. Here, we present a single-molecule spectroscopic method with micro-electron volt energy and submolecular-spatial resolution using laser driving of nanocavity plasmons to induce molecular luminescence in scanning tunneling microscopy. This tunable and monochromatic nanoprobe allows state-selective characterization of the energy levels and linewidths of individual electronic and vibrational quantum states of a single molecule. Moreover, we demonstrate that the energy levels of the states can be finely tuned by using the Stark effect and plasmon-exciton coupling in the tunneling junction. Our technique and findings open a route to the creation of designed energy-converting functions by using tuned energy levels of molecular systems.

Plasmon-Driven Motion of an Individual Molecule

We demonstrate that nanocavity plasmons generated a few nanometers away from a molecule can induce molecular motion. For this, we study the well-known rapid shuttling motion of zinc phthalocyanine molecules adsorbed on ultrathin NaCl films by combining scanning tunneling microscopy (STM) and spectroscopy (STS) with STM-induced light emission. Comparing spatially resolved single-molecule luminescence spectra from molecules anchored to a step edge with isolated molecules adsorbed on the free surface, we found that the azimuthal modulation of the Lamb shift is diminished in case of the latter. This is evidence that the rapid shuttling motion is remotely induced by plasmon-molecule coupling. Plasmon-induced molecular motion may open an interesting playground to bridge the nanoscopic and mesoscopic worlds by combining molecular machines with nanoplasmonics to control directed motion of single molecules without the need for local probes.

Influence of an atomistic protrusion at the tip apex on enhancing molecular emission in tunnel junctions: A theoretical study

Light emission from the gap of a scanning tunneling microscope can be used to investigate many optoelectronic processes at the single-molecule level and to gain insight into the fundamental photophysical mechanisms involved. One important issue is how to improve the quantum efficiency of quantum emitters in the nanometer-sized metallic gap so that molecule-specific emission can be clearly observed. Here, using electromagnetic simulations, we systematically investigate the influence of an atomic-scale protrusion at the tip apex on the emission properties of a point dipole in the plasmonic nanocavity. We found that such an atomistic protrusion can induce strong and spatially highly confined electric fields, thus increasing the quantum efficiency of molecular fluorescence over two orders of magnitude even when its dipole is oriented parallel to the metal surface, a situation occurring in most realistic single-molecule electroluminescence experiments. In addition, our theoretical simulations indicate that due to the lightning rod effect induced by the protrusion in a plasmonic nanocavity, the quantum efficiency increases monotonically as the tip approaches the dipole to the point of contact, instead of being quenched, thus explaining previous experimental observations with ever-enhancing fluorescence. Furthermore, we also examine in detail how the protrusion radius, height, and material affect the protrusion-induced emission enhancement. These results are believed to be instructive for further studies on the optoelectronic properties of single molecules in tip-based plasmonic nanocavities.

Probing Molecular Excited States by Atomic Force Microscopy

By employing single charge injections with an atomic force microscope, we investigated redox reactions of a molecule on a multilayer insulating film. First, we charged the molecule positively by attaching a single hole. Then we neutralized it by attaching an electron and observed three channels for the neutralization. We rationalize that the three channels correspond to transitions to the neutral ground state, to the lowest energy triplet excited states and to the lowest energy singlet excited states. By single-electron tunneling spectroscopy we measured the energy differences between the transitions obtaining triplet and singlet excited state energies. The experimental values are compared with density functional theory calculations of the excited state energies. Our results show that molecules in excited states can be prepared and that energies of optical gaps can be quantified by controlled single-charge injections. Our work demonstrates the access to, and provides insight into, ubiquitous electron-attachment processes related to excited-state transitions important in electron transfer and molecular optoelectronics phenomena on surfaces.

Exciton-Trion Conversion Dynamics in a Single Molecule

Charged optical excitations (trions) generated by charge carrier injection are crucial for emerging optoelectronic technologies as they can be produced and manipulated by electric fields. Trions and neutral excitons can be efficiently induced in single molecules by means of tip-enhanced spectromicroscopic techniques. However, little is known of the exciton-trion dynamics at single molecule level as this requires methods permitting simultaneous subnanometer and subnanosecond characterization. Here, we investigate exciton-trion dynamics by phase fluorometry, combining radio frequency modulated scanning tunnelling luminescence with time-resolved single photon detection. We generate excitons and trions in single Zinc Phthalocyanine (ZnPc) molecules on NaCl/Ag(111), and trace the evolution of the system in the picosecond range. We explore the dependence of effective lifetimes on bias voltage and describe the conversion mechanism from neutral excitons to trions, via charge capture, as the primary pathway to trion formation. We corroborate the dynamics of the system by a causally deterministic four-state model.

Probing intramolecular vibronic coupling through vibronic-state imaging

Vibronic coupling is a central issue in molecular spectroscopy. Here we investigate vibronic coupling within a single pentacene molecule in real space by imaging the spatial distribution of single-molecule electroluminescence via highly localized excitation of tunneling electrons in a controlled plasmonic junction. The observed two-spot orientation for certain vibronic-state imaging is found to be evidently different from the purely electronic 0–0 transition, rotated by 90°, which reflects the change in the transition dipole orientation from along the molecular short axis to the long axis. Such a change reveals the occurrence of strong vibronic coupling associated with a large Herzberg–Teller contribution, going beyond the conventional Franck–Condon picture. The emergence of large vibration-induced transition charges oscillating along the long axis is found to originate from the strong dynamic perturbation of the anti-symmetric vibration on those carbon atoms with large transition density populations during electronic transitions.

Vibronic coupling is a key feature of molecular electronic transitions, but its visualization in real space is an experimental challenge. Here the authors, using scanning tunneling microscopy induced luminescence, resolve the effect of vibronic coupling with different modes on the electron distributions in real space in a single pentacene molecule.

What can single-molecule Fano resonance tell?

In this work, we showcase applications of single-molecule Fano resonance (SMFR) measurements beyond the determination of molecular excitonic energy and associated dipole orientation. We use the SMFR measurement to probe the local influence of a man-made single chlorine vacancy on the molecular transition of a single zinc phthalocyanine, which clearly reveals the lifting-up of the double degeneracy of the excited states due to defect-induced configurational changes. Furthermore, time-trace SMFR measurements at different excitation voltages are used to track the tautomerization process in a free-base phthalocyanine. Different behaviors in switching between two inner-hydrogen configurations are observed with decreasing voltages, which helps to reveal the underlying tautomerization mechanism involving both the molecular electronic excited states and vibrational excited states in the ground state.

Fluorescence and Electroluminescence of J-Aggregated Polythiophene Monolayers on Hexagonal Boron Nitride

The photophysics of a semiconducting polymer is manipulated through molecular self-assembly on an insulating surface. Adsorption of polythiophene (PT) monolayers on hexagonal boron nitride (hBN) leads to a structurally induced planarization and a rebalancing of inter- and intrachain excitonic coupling. This conformational control results in a dominant 0-0 photoluminescence peak and a reduced Huang-Rhys factor, characteristic of J-type aggregates, and optical properties which are significantly different to both PT thin films and single polymer strands. Adsorption on hBN also provides a route to explore electroluminescence from PT monolayers though incorporation into hybrid van der Waals heterostructures whereby the polymer monolayer is embedded within a hBN tunnel diode. In these structures we observe up-converted singlet electroluminescence from the PT monolayer, with an excitation mechanism based upon inelastic electron scattering. We argue that surface adsorption provides a methodology for the study of fundamental optoelectronic properties of technologically relevant polymers.

Effective Control of Photon Statistics from Electroluminescence by Fano-like Interference Effect

The photon blockade induced by optical nonlinearity has been widely used to generate single-photon emission under optical driving in quantum optics. However, the same approach is difficult to achieve in electrically driven molecular junctions. Here we propose a scheme for tuning photon statistics via Fano-like interference effect in a system consisting of two molecules within one optical cavity. Under electrical pumping, a transition from photon bunching to antibunching takes place as a manifestation of the Fano-like interference. This effect persists even in the presence of the dipole-dipole interaction between molecules based on the parameters extracted from the experiments. Our proposal can be realized in current-carrying scanning tunneling microscope junctions.

Boosting Light Emission from Single Hydrogen Phthalocyanine Molecules by Charging

Interest in electroluminescence of single molecules is stimulated by the prospect of possible applications in novel light emitting devices. Recent studies provide valuable insights into the mechanisms leading to single molecule electroluminescence. Concrete information on how to boost the intensity of the emitted light, however, is rare. By combining scanning tunnelling microscopy (STM) and quantum chemical calculations, we show that the light emission efficiencies of an individual hydrogen-phthalocyanine molecule can be increased by a factor of ≈19 upon charging. This boost in intensity can be explained by the development of a vertical dipole moment normal to the substrate facilitating out-coupling of the local excitation to the far field. As this effect is not related to the specific nature of hydrogen-phthalocyanine, it opens up a general way to increase light emission from molecular junctions.

Mechano-Optical Switching of a Single Molecule with Doublet Emission

The ability to control the emission from single-molecule quantum emitters is an important step toward their implementation in optoelectronic technology. Phthalocyanine and derived metal complexes on thin insulating layers studied by scanning tunneling microscope-induced luminescence (STML) offer an excellent playground for tuning their excitonic and electronic states by Coulomb interaction and to showcase their high environmental sensitivity. Copper phthalocyanine (CuPc) has an open-shell electronic structure, and its lowest-energy exciton is a doublet, which brings interesting prospects in its application for optospintronic devices. Here, we demonstrate that the excitonic state of a single CuPc molecule can be reproducibly switched by atomic-scale manipulations permitting precise positioning of the molecule on the NaCl ionic crystal lattice. Using a combination of STML, AFM, and ab initio calculations, we show the modulation of electronic and optical bandgaps and the exciton binding energy in CuPc by tens of meV. We explain this effect by spatially dependent Coulomb interaction occurring at the molecule-insulator interface, which tunes the local dielectric environment of the emitter.

Atomic-Scale Dynamics Probed by Photon Correlations

Light absorption and emission have their origins in fast atomic-scale phenomena. To characterize these basic steps (e.g., in photosynthesis, luminescence, and quantum optics), it is necessary to access picosecond temporal and picometer spatial scales simultaneously. In this Perspective, we describe how state-of-the-art picosecond photon correlation spectroscopy combined with luminescence induced at the atomic scale with a scanning tunneling microscope (STM) enables such studies. We outline recent STM-induced luminescence work on single-photon emitters and the dynamics of excitons, charges, molecules, and atoms as well as several prospective experiments concerning light-matter interactions at the nanoscale. We also describe future strategies for measuring and rationalizing ultrafast phenomena at the nanoscale.

Excited-State Imaging of Single Particles on the Subnanometer Scale

At the intersection of spectroscopy and microscopy lie techniques that are capable of providing subnanometer imaging of excited states of individual molecules or nanoparticles. Such approaches are particularly important for imaging macromolecules or nanoparticles large enough to have a high probability of containing a defect. These inevitable defects often control properties and function despite an otherwise ideal structure. We discuss real-space imaging techniques such as using scanning tunneling microscopy tips to enhance optical measurements and electron energy-loss spectroscopy in a scanning transmission electron microscope, which is based on focused electron beams to obtain high-resolution spatial information on excited states. The outlook for these methods is bright, as they will provide critical information for the characterization and improvement of energy-switching, electron-switching, and energy-harvesting materials.

Energy, Particle, and Photon Fluxes in Molecular Junctions

Electroluminescence from a current-carrying molecular junction at steady state is simulated. (Charge) particle conservation and energy conservation are satisfied by a perturbative expansion in the radiation/matter coupling. Our approach makes it possible to adopt standard tools of traditional (equilibrium) spectroscopy to study signals from open systems such as molecular junctions. The nonperturbative analysis of spontaneous light emission signals coincides with the perturbative approach for weak molecule-field coupling.

Triplet Excitation and Electroluminescence from a Supramolecular Monolayer Embedded in a Boron Nitride Tunnel Barrier

We show that ordered monolayers of organic molecules stabilized by hydrogen bonding on the surface of exfoliated few-layer hexagonal boron nitride (hBN) flakes may be incorporated into van der Waals heterostructures with integral few-layer graphene contacts forming a molecular/two-dimensional hybrid tunneling diode. Electrons can tunnel through the hBN/molecular barrier under an applied voltage V_SD, and we observe molecular electroluminescence from an excited singlet state with an emitted photon energy hν > eV_SD, indicating upconversion by energies up to ∼1 eV. We show that tunneling electrons excite embedded molecules into singlet states in a two-step process via an intermediate triplet state through inelastic scattering and also observe direct emission from the triplet state. These heterostructures provide a solid-state device in which spin-triplet states, which cannot be generated by optical transitions, can be controllably excited and provide a new route to investigate the physics, chemistry, and quantum spin-based applications of triplet generation, emission, and molecular photon upconversion.

Spontaneous Light Emission from Molecular Junctions: Theoretical Analysis of Upconversion Signal

Spontaneous light emission from a current-carrying molecular junction is analyzed. There are two leading processes, fluorescence and electroluminescence, as defined using Liouville space diagrams within the perturbative method, that contribute to the light emission from junctions. This allows us to identify a general mechanism that explains the origin of the so-called upconversion electroluminescence (UCEL) signal, which has been observed in a variety of molecular junctions [Umera et al. Chem. Phys. Lett. 2007, 448, 232; Dong et al. Nat. Photonics 2010, 4, 50]. Here, we show that a double-peak signal, one at energy less than the applied bias and the other at higher energy (UCEL), is generated due to overlap between two processes: one is electron transfer to create the required excited state, and the other is radiative relaxation of the excited state. The lifetimes induced by the lead interactions play a crucial role in determining the required overlap between these processes. Our analysis shows that, unlike the higher-energy signal, the lower-energy peak is sensitive to the applied bias and does not correspond to any optical resonance in the junction. The signal at higher energy is enhanced as the temperature is increased. We demonstrate our findings using nonperturbative analytic results for a model system.

Charge Carrier Injection Electroluminescence with CO-Functionalized Tips on Single Molecular Emitters

We investigate electroluminescence of single molecular emitters on NaCl on Ag(111) and Au(111) with submolecular resolution in a low-temperature scanning probe microscope with tunneling current, atomic force, and light detection capabilities. The role of the tip state is studied in the photon maps of a prototypical emitter, zinc phthalocyanine (ZnPc), using metal and CO-metal tips. CO-functionalization is found to have an impact on the resolution and contrast of the photon maps due to the localized overlap of the p-orbitals on the tip with the molecular orbitals of the emitter. The possibility of using the same CO-functionalized tip for tip-enhanced photon detection and high resolution atomic force is demonstrated. We study the electroluminescence of ZnPc, induced by charge carrier injection at sufficiently high bias voltages. We propose that the distinct level alignment of the ZnPc frontier orbitals with the Au(111) and Ag(111) Fermi levels governs the primary excitation mechanisms as the injection of electrons and holes from the tip into the molecule, respectively. These findings put forward the importance of the tip status in the photon maps and contribute to a better understanding of the photophysics of organic molecules on surfaces.

Scanning Tunneling Microscope-Induced Excitonic Luminescence of a Two-Dimensional Semiconductor

The long sought-after goal of locally and spectroscopically probing the excitons of two-dimensional (2D) semiconductors is attained using a scanning tunneling microscope (STM). Excitonic luminescence from monolayer molybdenum diselenide (MoSe_{2}) on a transparent conducting substrate is electrically excited in the tunnel junction of an STM under ambient conditions. By comparing the results with photoluminescence measurements, the emission mechanism is identified as the radiative recombination of bright A excitons. STM-induced luminescence is observed at bias voltages as low as those that correspond to the energy of the optical band gap of MoSe_{2}. The proposed excitation mechanism is resonance energy transfer from the tunneling current to the excitons in the semiconductor, i.e., through virtual photon coupling. Additional mechanisms (e.g., charge injection) may come into play at bias voltages that are higher than the electronic band gap. Photon emission quantum efficiencies of up to 10^{-7} photons per electron are obtained, despite the lack of any participating plasmons. Our results demonstrate a new technique for investigating the excitonic and optoelectronic properties of 2D semiconductors and their heterostructures at the nanometer scale.