Scanning Tunneling Microscope Junction Research Articles

Origin of photoinduced DC current and two-level population dynamics in a single molecule.

Studying the photoinduced changes of materials with atomic-scale spatial resolution can provide a fundamental understanding of light-matter interaction. A long-standing impediment has been the detrimental thermal effects on the stability of the tunneling gap from intensity-modulated laser irradiation of the scanning tunneling microscope junction. Photoinduced DC current transduces photons to an electric current and is widely applied in optoelectronics as switches and signal transmission. Our results revealed the origin of the light-induced DC current and related it to the two-level population dynamics and related nonlinearity in the conductance of a single molecule. Here, we compensated for the near-visible laser-induced thermal effects to demonstrate photoinduced DC current spectroscopy and microscopy and to observe the persistent photoconductivity of a two-level pyrrolidine molecule. The methodology can be generally applied to the coupling of light to scan probes to investigate light-matter interactions at the atomic scale.

Inelastic Light Scattering in the Vicinity of a Single-Atom Quantum Point Contact in a Plasmonic Picocavity.

Electromagnetic fields can be confined in the presence of metal nanostructures. Recently, subnanometer scale confinement has been demonstrated to occur at atomic protrusions on plasmonic nanostructures. Such an extreme field may dominate atomic-scale light-matter interactions in "picocavities". However, it remains to be elucidated how atomic-level structures and electron transport affect plasmonic properties of a picocavity. Here, using low-temperature optical scanning tunneling microscopy (STM), we investigate inelastic light scattering (ILS) in the vicinity of a single-atom quantum point contact (QPC). A vibration mode localized at the single Ag adatom on the Ag(111) surface is resolved in the ILS spectrum, resulting from tip-enhanced Raman scattering (TERS) by the atomically confined plasmonic field in the STM junction. Furthermore, we trace how TERS from the single adatom evolves as a function of the gap distance. The exceptional stability of the low-temperature STM allows to examine distinctly different electron transport regimes of the picocavity, namely, in the tunneling and QPC regimes. This measurement shows that the vibration mode localized at the adatom and its TERS intensity exhibits a sharp change upon the QPC formation, indicating that the atomic-level structure has a crucial impact on the plasmonic properties. To gain microscopic insights into picocavity optomechanics, we scrutinize the structure and plasmonic field in the STM junction using time-dependent density functional theory. The simulations reveal that atomic-scale structural relaxation at the single-atom QPC results in a discrete change of the plasmonic field strength, volume, and distribution as well as the vibration mode localized at the single atom. These findings give a qualitative explanation for the experimental observations. Furthermore, we demonstrate that strong ILS is a characteristic feature of QPC by continuously forming, breaking, and reforming the atomic contact and how the plasmonic resonance evolves throughout the nontunneling, tunneling, and QPC regimes.

Directing and Understanding the Translation of a Single Molecule Dipole.

Understanding the directed motion of a single molecule on surfaces is not only important in the well-established field of heterogeneous catalysis but also for the design of artificial nanoarchitectures and molecular machines. Here, we report how the tip of a scanning tunneling microscope (STM) can be used to control the translation direction of a single polar molecule. Through the interaction of the molecular dipole with the electric field of the STM junction, it was found that both translations and rotations of the molecule occur. By considering the location of the tip with respect to the axis of the dipole moment, we can deduce the order in which rotation and translation take place. While the molecule-tip interaction dominates, computational results suggest that the translation is influenced by the surface direction along which the motion takes place.

Oxide-Metal Interaction Probed by Scanning Tunneling Microscope Manipulation of Cr2O7 Clusters on Au(111).

Interfacial interaction plays a crucial rule in catalysis over supported catalysts, and the catalyst-support interaction needs to be explored at microscopic scale. Here, we use the scanning tunneling microscope (STM) tip to manipulate Cr2O7 dinuclear clusters on Au(111) and find that the Cr2O7-Au interaction can be weakened by an electric field in the STM junction, facilitating rotation and translation of the individual clusters at the imaging temperature (78 K). Surface alloying with Cu makes the manipulation of the Cr2O7 clusters hard due to the enhanced Cr2O7-substrate interaction. Density functional theory calculations reveal that barrier for translation of a Cr2O7 cluster on the surface can be increased by surface alloying, influencing the tip manipulation. Our study demonstrates that the oxide-metal interfacial interaction can be probed by STM tip manipulation of supported oxide clusters, which provides a new method to investigate the interfacial interaction.

(Invited) Development of THz-Field-Driven Scanning Tunneling Luminescence Spectroscopy for Future Investigation of Exciton Dynamics

Exciton dynamics is responsible for the photophysical and photochemical functions, such as energy harvesting, light emission, and photosynthesis. In particular, energy conversions, transfers, and dissipations at interfaces and nanostructures govern the dynamics of excitons. Nevertheless, the spatial resolutions of conventional optical methods are limited to several hundred nanometers due to the diffraction limit of light. Luminescence measurements combined with scanning tunneling microscopy (STM) enable to investigate optical properties with sub-nanometer spatial resolution in various systems such as metals[1], semiconductors[2], 2D materials[3], and molecules[4-5]. In this method, the scanning tunneling luminescence (STL) spectroscopy, photons induced by the tunneling current of STM are detected. Since the tunneling current can be controlled with atomic spatial resolution, it is possible to investigate optical phenomena in localized regions. While STL spectroscopy is a powerful tool for investigating exciton dynamics with atomic-scale spatial resolution, the time resolution of STL spectroscopy is limited because it uses “steady-state” tunneling current for the excitation. Therefore, this method cannot be applicable to track the ultrafast exciton dynamics in real-time.For overcoming the limitation of time resolution of STM, a single-cycle terahertz (THz) electric field has recently been installed to drive tunneling electrons[6-9]. By coupling the THz pulse to an atomically sharp metal tip of STM, the electric field is confined to the tip, and it enables to manipulate the tunneling electrons with extremely high spatiotemporal resolution. This method, THz-STM, has been utilized to track the ultrafast carrier dynamics[6-8] and motion of a molecule[9] with sub-picosecond time resolution. In this work, we combined a photon detection system with THz-STM to establish THz-STL spectroscopy (Fig. 1a). We measured visible photon emission from the radiative decay of a localized plasmon as the first demonstration of THz-STL spectroscopy[10]. We anticipate that the THz-STL spectroscopy will open new avenues for “real-time” and “real-space” investigations of exciton dynamics in near future.In this work, we generated single-cycle THz pulses via optical rectification of laser pulses from a Yb fiber laser in a LiNbO3 prism using a tilted-pulse-front configuration (Fig. 1b). The generated THz pulses were guided into a low-temperature STM and focused to the STM junction (Fig. 1a). Figure 1c shows a current trace on Ag(111) surface, where the current induced by the THz pulses was measured as 2.65 pA. Based on the simulation of electron tunneling probability, the maximum magnitude of the voltage applied by the THz pulse was estimated to be ~6.5 V. Figure 1d shows STL spectra when the THz pulse was either introduced to the STM (red) or blocked (grey). Whereas no peak is seen in the grey spectrum, a broad peak ranging from 1.3 eV to 2.3 eV appears in the red spectrum, which is originated from the radiative decay of a plasmon localized in the gap.In this presentation, we would like to discuss the detailed mechanism of plasmon excitation by THz-field-driven electrons as well as the challenges of applying THz-STL to the real-time investigation of exciton dynamics in a molecule.[1] R. Berndt et al., Phys. Rev. Lett. 67, 3796 (1991). [2] J. K. Gimzewski et al., Z. Phys. B Condensed Matter 72, 497 (1988). [3] Schuler et al., Sci. Adv. 6, eabb5988 (2020). [4] H. Imada et al., Nature 538, 364 (2016). [5] K. Kimura et al., Nature 570, 210 (2019). [6] T. L. Cocker et al., Nat. Photon. 7, 620 (2013). [7] K. Yoshioka et al., Nat. Photon. 10, 762 (2016). [8] V. Jelic et al., Nature Physics 13, 591 (2017). [9] D. Peller et al., Nature 585, 58 (2020). [10] K. Kimura et al., ACS photon. 8, 982 (2021). Figure 1

Interplay of boundary states of graphene nanoribbons with a Kondo impurity

We investigate the interplay of two highly localized, nearly degenerate electronic states, namely, a zero-energy edge mode in a graphene nanoribbon on the one hand and an Abrikosov-Suhl resonance located at a Kondo impurity on the other. On-surface synthesis of the ribbon structures in combination with intercalation of single-atom Kondo impurities by atomic manipulation in a scanning tunneling microscope junction offer full control of the atomic geometry of the system. Density functional theory provides the microscopic description to scrutinize the electronic features observed in experiment. We find the interaction of the two localized states and the resulting signatures of Kondo physics to be very sensitive to the placing of the atom, suggesting its use as a laboratory to study the interplay of the Kondo effect with other zero-bias anomalies as well as to tailor these states by controlling the atomic-scale coupling.

Confinement-Induced Catalytic Dissociation of Hydrogen Molecules in a Scanning Tunneling Microscope.

The catalytic scission of single chemical bonds has been induced by the nanoscale confinement in a scanning tunneling microscope (STM) junction. Individual hydrogen molecules sandwiched between the STM tip and a copper substrate can be dissociated solely by the reciprocating movement of the tip. The reaction rate depends sensitively on the local molecular environment, as exemplified by the effects of a nearby carbon monoxide molecule or a gold adatom. Detailed mechanisms and the nature of the transition states are revealed by density functional theory (DFT) calculations. This work provides insights into chemical reactions at the atomic scale induced by localized confinement applied by the STM tip. Furthermore, a single diatomic molecule can act as a molecular catalyst to enhance the reaction rate on a surface.

Controlling Localized Plasmons via an Atomistic Approach: Attainment of Site-Selective Activation inside a Single Molecule.

Chemical reactions such as bond dissociation and formation assisted by localized surface plasmons (LSPs) of noble metal nanostructures hold promise in solar-to-chemical energy conversion. However, the precise control of localized plasmons to activate a specific moiety of a molecule, in the presence of multiple chemically equivalent parts within a single molecule, is scarce due to the relatively large lateral distribution of the plasmonic field. Herein, we report the plasmon-assisted dissociation of a specific molecular site (C-Si bond) within a polyfunctional molecule adsorbed on a Cu(100) surface in the scanning tunneling microscope (STM) junction. The molecular site to be activated can be selected by carefully positioning the tip and bringing the tip extremely close to the molecule (atomistic approach), thereby achieving plasmonic nanoconfinement at the tip apex. Furthermore, multiple reactive sites are activated in a sequential manner at the sub-molecular scale, and different sets of products are created and visualized by STM topography and density functional theory (DFT) modeling. The illustration of site-selective activation achieved by localized surface plasmons implies the realization of molecular-scale resolution for bond-selected plasmon-induced chemistry.

Ultrafast Photon-Induced Tunneling Microscopy.

Unification of the techniques of ultrafast science and scanning tunneling microscopy (STM) has the potential of tracking electronic motion in molecules simultaneously in real space and real time. Laser pulses can couple to an STM junction either in the weak-field or in the strong-field interaction regime. The strong-field regime entails significant modification (dressing) of the tunneling barrier of the STM junction, whereas the weak-field or the photon-driven regime entails perturbative interaction. Here, we describe how photons carried in an ultrashort pulse interact with an STM junction, defining the basic fundamental framework of ultrafast photon-induced tunneling microscopy. Selective dipole coupling of electronic states by photons is shown to be controllable by adjusting the DC bias at the STM junction. An ultrafast tunneling microscopy involving photons is established. Consolidation of the technique calls for innovative approaches to detect photon-induced tunneling currents at the STM junction. We introduce and characterize here three techniques involving dispersion, polarization, and frequency modulation of the laser pulses to lock-in detect the laser-induced tunneling current. We show that photon-induced tunneling currents can simultaneously achieve angstrom scale spatial resolution and sub-femtosecond temporal resolution. Ultrafast photon-induced tunneling microscopy will be able to directly probe electron dynamics in complex molecular systems, without the need of reconstruction techniques.

Anti-Stokes Light Scattering Mediated by Electron Transfer Across a Biased Plasmonic Nanojunction

Light scattering from plasmonic nanojunctions is routinely used to assess their optical properties. However, the microscopic mechanism remains imperfectly understood, and an accurate description requires the experiment in a well-defined environment with a highly precise control of the nanojunction. Here we report on inelastic light scattering (ILS) from plasmonic scanning tunneling microscope (STM) junctions under ultrahigh vacuum and cryogenic conditions. We particularly focus on anti-Stokes continuum generation in the ILS spectra with a narrowband continuous-wave laser excitation, which appears when an electrical bias is applied between the tip and the surface. This anti-Stokes continuum is commonly observed for various STM junctions at ∼10 K, corroborating that it is a universal phenomenon in electrically biased plasmonic nanojunctions. We propose that the microscopic mechanism underlying the anti-Stokes continuum generation is explained by ILS accompanied by electron transfer across the STM junction, whereby the excess energy is provided by the applied bias voltage. This process occurs through either photoluminescence (PL) or electronic Raman scattering (ERS). By recording the ILS spectra in parallel with STM-induced luminescence, we show that ERS becomes dominant when the excitation wavelength matches the plasmonic resonance of the STM junction, whereas PL mainly contributes to the off-resonance excitation. Our results provide an in-depth understanding of ILS by plasmonic nanojunctions and demonstrate that the anti-Stokes continuum can arise from a nonthermal mechanism.

Tracking Electron Dynamics of Single Molecules in Scanning Tunneling Microscopy Junctions with Laser Pulses.

On the basis of real-time simulations, we devise a method to extend the capability of scanning tunneling microscopy (STM) to track the electronic dynamics of molecules on a material's surface with the ultrafast temporal resolution of laser pulses. The intrinsic mechanism of visualization of electronic dynamics by measuring tunneling charge is attributed to the interference between the electronic oscillations stimulated by pump and probe pulses. The charge-transfer rate from molecule to the surrounding environment can be estimated with the decay time of electronic dynamics, which can also be detected by measuring the tunneling charge across the STM junction. Moreover, it is found that the tunneling charge can be varied precisely by changing the carrier-envelope phase (CEP) of the pulses, and this phase-dependence of tunnelling diminishes as the duration of incident laser pulses increases. The proposed scheme provides an alternative means for visualization of electron dynamics of single molecules by measuring tunnelling charges.

Laser stimulation ofMoSe2/Au(111)in an STM junction: Photoinduced versus thermally induced current response

A laser beam is focused onto the junction of a scanning tunneling microscope (STM) to probe the local photocurrent response of highly ordered $\mathrm{Mo}{\mathrm{Se}}_{2}$ islands. The islands, grown by physical vapor deposition onto Au(111), are characterized by uniform triangular shapes, Mo-terminated edges, and the typical 1.8 V band gap of monolayer $\mathrm{Mo}{\mathrm{Se}}_{2}$. The laser-induced signals are detected with nanometer spatial resolution by a lock-in amplifier, both in the form of point spectra and surface maps. To distinguish real photocurrents from current contributions, arising from periodic tip expansions due to the dissipated laser power, blind experiments are performed in which the tip height is modulated without laser irradiation. In addition, the expected current responses are simulated as a function of sample bias for the two scenarios. Comparison between experimental and simulated data indicates that periodic tip expansions dominate the detected current modulations and photoinduced signals are of minor importance. Our work provides various indicators to safely distinguish photo- and thermally induced current components in a laser-driven STM experiment and discusses means to enhance the desired photoresponse with respect to parasitic thermal effects.

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.

Phase-Resolved Detection of Ultrabroadband THz Pulses inside a Scanning Tunneling Microscope Junction.

Coupling phase-stable single-cycle terahertz (THz) pulses to scanning tunneling microscope (STM) junctions enables spatiotemporal imaging with femtosecond temporal and Ångstrom spatial resolution. The time resolution achieved in such THz-gated STM is ultimately limited by the subcycle temporal variation of the tip-enhanced THz field acting as an ultrafast voltage pulse, and hence by the ability to feed high-frequency, broadband THz pulses into the junction. Here, we report on the coupling of ultrabroadband (1–30 THz) single-cycle THz pulses from a spintronic THz emitter (STE) into a metallic STM junction. We demonstrate broadband phase-resolved detection of the THz voltage transient directly in the STM junction via THz-field-induced modulation of ultrafast photocurrents. Comparison to the unperturbed far-field THz waveform reveals the antenna response of the STM tip. Despite tip-induced low-pass filtering, frequencies up to 15 THz can be detected in the tip-enhanced near-field, resulting in THz transients with a half-cycle period of 115 fs. We further demonstrate simple polarity control of the THz bias via the STE magnetization and show that up to 2 V THz bias at 1 MHz repetition rate can be achieved in the current setup. Finally, we find a nearly constant THz voltage and waveform over a wide range of tip–sample distances, which by comparison to numerical simulations confirms the quasi-static nature of the THz pulses. Our results demonstrate the suitability of spintronic THz emitters for ultrafast THz-STM with unprecedented bandwidth of the THz bias and provide insight into the femtosecond response of defined nanoscale junctions.

Influence of Co bilayers and trilayers on the plasmon-driven light emission from Cu(111) in a scanning tunneling microscope

Light emission from the gap cavity formed by the tip of a scanning tunneling microscope (STM) and a flat metallic sample allows us to probe the dielectric response of metals at the atomic scale and presents a way to distinguish between different materials. The excitation mechanism of the charge carrier oscillations, which ultimately decay into light, is linked to inelastic electron tunneling as opposed to the mostly semiclassical picture of the electromagnetic resonance of the gap cavity. Thus, the observed light emission does not only reflect the electromagnetic resonance of the cavity but also involves the electronic density of states. In this paper, we compare light emission from Cu(111) and Co nanoislands on Cu(111). We find a strong intensity contrast but almost no alteration of the resonance wavelength except close to step edges. Our results show that the light emission from the STM junction is highly sensitive to a few atomic layers of alien material mostly due to the dielectric properties of the layer.

Real-space observation of far- and near-field-induced photolysis of molecular oxygen on an Ag(110) surface by visible light.

Dissociation of molecular oxygen is an important elementary process in heterogeneous catalysis. Here, we report on a real-space observation of oxygen photolysis on the Ag(110) surface at 78 K by far- and near-field excitation in the ultraviolet-near-infrared range using a low-temperature scanning tunneling microscope (STM) combined with wavelength-tunable laser excitation. The photolysis of isolated oxygen molecules on the surface occurs even by visible light with the cross section of ∼10-19 cm2. Time-dependent density functional theory calculations reveal optical absorption of the hybridized O2-Ag(110) complex in the visible and the near-infrared range which is associated with the oxygen photolysis. We suggest that the photolysis mechanism involves a direct charge transfer process. We also demonstrate that the photolysis can be largely enhanced in plasmonic STM junctions, and the cross section is estimated to be ∼10-17 cm-2 in the visible and the near-infrared range, which appears to be an interesting feature of plasmon-induced reactions from the perspective of photochemical conversion with the aid of solar energy.

Near-Field Manipulation in a Scanning Tunneling Microscope Junction with Plasmonic Fabry-Pérot Tips.

Near-field manipulation in plasmonic nanocavities can provide various applications in nanoscale science and technology. In particular, a gap plasmon in a scanning tunneling microscope (STM) junction is of key interest to nanoscale imaging and spectroscopy. Here we show that spectral features of a plasmonic STM junction can be manipulated by nanofabrication of Au tips using focused ion beam. An exemplary Fabry–Pérot type resonator of surface plasmons is demonstrated by producing the tip with a single groove on its shaft. Scanning tunneling luminescence spectra of the Fabry–Pérot tips exhibit spectral modulation resulting from interference between localized and propagating surface plasmon modes. In addition, the quality factor of the plasmonic Fabry–Pérot interference can be improved by optimizing the overall tip shape. Our approach paves the way for near-field imaging and spectroscopy with a high degree of accuracy.

Theoretical investigation of real-time charge dynamics in open systems coupled to bulk materials.

Environmental effects play an important role on the electron dynamics of open systems, which provide channels for dissipation of electrons and energy in the systems. However, accurate description of the environment of quantum systems is still challenging. The environment is usually assumed to be a quasi-one-dimensional reservoir in previous theoretical studies. In this work, we focus on systems that are adsorbed on bulk surfaces. Two different approaches to describe the spectral details of the environment are adopted and compared: the Lorentzian decomposition approach and the complex absorbing potential (CAP) approach. To achieve similar accuracy for the spectral density of the environment, it is shown that the Lorentzian decomposition approach is computationally more efficient than the CAP approach, especially for bulk systems. The electron dynamics is then followed using the nonequilibrium Green's function method for two systems: a modeling bulk surface system and a scanning tunneling microscope junction. Dissipation paths of excited charge carriers can be analyzed, which provide insights into the understanding of excitation dynamics in bulk materials.

Near-Field Spectral Response of Optically Excited Scanning Tunneling Microscope Junctions Probed by Single-Molecule Action Spectroscopy.

The near-field spectral response of metallic nanocavities is a key characteristic in plasmon-assisted photophysical and photochemical processes. Here, we show that the near-field spectral response of an optically excited plasmonic scanning tunneling microscope (STM) junction can be probed by single-molecule reactions that serve as a nanoscale sensor detecting the local field intensity. Near-field action spectroscopy for the cis ↔ cis tautomerization of porphycene on a Cu(110) surface reveals that the field enhancement in the STM junction largely depends on microscopic structures not only on the tip apex, but also on its shaft. Using nanofabrication of Au tips with focused ion beam, we show that the spectral response is strongly modulated through the interference between the localized surface plasmon in the junction and propagating surface plasmon polariton generated on the shaft. Furthermore, it is demonstrated that the near-field spectral response can be manipulated by precisely shaping the tip shaft.

Plasmon-Assisted Resonant Electron Tunneling in a Scanning Tunneling Microscope Junction.

We report plasmon-assisted resonant electron tunneling from a Ag or Au tip to field emission resonances (FERs) of a Ag(111) surface induced by cw laser excitation of a scanning tunneling microscope (STM) junction at visible wavelengths. As a hallmark of the plasmon-assisted resonant tunneling, we observe a downshift of the first peak in the FER spectra by a fixed amount equal to the incident photon energy. STM-induced luminescence measurement for the Ag and Au tip reveals the clear correlation between the laser-induced change in the FER spectra and the plasmonic properties of the junction. Our results clarify a novel resonant electron transfer mechanism in a plasmonic nanocavity.