Exploiting a single intramolecular conformational switching Ni-TPP molecule to probe charge transfer dynamics at the nanoscale on bare Si(100)-2 × 1.

Acquiring quantitative information on charge transfer (CT) dynamics at the nanoscale remains an important scientific challenge. In particular, CT processes in single molecules at surfaces need to be investigated to be properly controlled in various devices. To address this issue, the dynamics of switching molecules can be exploited. Here, nickel-tetraphenylporphyrin adsorbed on the Si(100) surface is used to study the CT process ruling the reversible activation of two chiral molecular conformations. Via the electrons of a scanning tunneling microscope (STM), a statistical study of molecular switching reveals two specific locations of the molecule for which their efficiency is optimized. The CT mechanism is shown to propagate from two lateral aryl groups towards the porphyrin macrocycle inducing an intramolecular movement of two symmetric pyrroles. The measured switching efficiencies can thus be related to a Markus-Jordner model to estimate relevant parameters that describe the dynamics of the CT process. Numerical simulations provide a precise description of the molecular conformations and unveil the molecular energy levels that are involved in the CT process. This quantitative method opens a completely original approach to study CT at the nanoscale.

Optoelectronic Readout of Single Er Adatom's Electronic States Adsorbed on the Si(100) Surface at Low Temperature (9 K).

Integrating nanoscale optoelectronic functions is vital for applications such as optical emitters, detectors, and quantum information. Lanthanide atoms show great potential in this endeavor due to their intrinsic transitions. Here, we investigate Er adatoms on Si(100)-2×1 at 9 K using a scanning tunneling microscope (STM) coupled to a tunable laser. Er adatoms display two main adsorption configurations that are optically excited between 800 and 1200 nm while the STM reads the resulting photocurrents. Our spectroscopic method reveals that various photocurrent signals stem from the bare silicon surface or Er adatoms. Additional photocurrent peaks appear as the signature of the Er adatom relaxation, triggering efficient dissociation of nearby trapped excitons. Calculations using density functional theory with spin-orbit coupling correction highlight the origin of the observed photocurrent peaks as specific 4f→4f or 4f→5d transitions. This spectroscopic technique can facilitate optoelectronic analysis of atomic and molecular assemblies by offering insight into their intrinsic quantum properties.

A scanning tunneling microscope as a tunable nanoantenna for atomic scale control of optical-field enhancement.

The high stability of a low temperature (9 K) scanning tunneling microscope junction is used to precisely adjust the enhancement of an external pulsed vacuum ultraviolet (VUV) laser. The ensuing VUV optical-field strength is mapped on an hydrogenated Si(100) surface by imprinting locally one-photon atomic scale hydrogen desorption. Subsequent to irradiation, topography of the Si(100):H surface at the reacted area revealed a desorption spot with unprecedented atomic precision. Our results show that the shapes, positions, and sizes of the desorption spots are correlated to the calculated optical-field structure, offering real control of the optical-field distribution at molecular scale.

Two-Dimensional Functionalized Ultrathin Semi-Insulating CaF2 Layer on the Si(100) Surface at a Low Temperature for Molecular Electronic Decoupling.

The ability to precisely control the electronic coupling/decoupling of adsorbates from surfaces is an essential goal. It is important for fundamental studies not only in surface science but also in several applied domains including, for example, miniaturized molecular electronic or for the development of various devices such as nanoscale biosensors or photovoltaic cells. Here, we provide atomic-scale experimental and theoretical investigations of a semi-insulating layer grown on a silicon surface via its epitaxy with CaF2. We show that, following the formation of a wetting layer, the ensuing organized unit cells are coupled to additional physisorbed CaF2 molecules, periodically located in their surroundings. This configuration shapes the formation of ribbons of stripes that functionalize the semiconductor surface. The obtained assembly, having a monolayer thickness, reveals a surface gap energy of ∼3.2 eV. The adsorption of iron tetraphenylporphyrin molecules on the ribbons of stripes is used to estimate the electronic insulating properties of this structure via differential conductance measurements. Density functional theory (DFT) including several levels of complexity (annealing, DFT + U, and nonlocal van der Waals functionals) is employed to reproduce our experimental observations. Our findings offer a unique and robust template that brings an alternative solution to electronic semi-insulating layers on metal surfaces such as NaCl. Hence, CaF2/Si(100) ribbon of stripe structures, whose lengths can reach more than 100 nm, can be used as a versatile surface platform for various atomic-scale studies of molecular devices.

Surface-isomerization dynamics of trans-stilbene molecules adsorbed on Si(100)-2 x 1.

Photoinduced trans-cis isomerization studies of stilbene molecules in the gas phase have led to a precise understanding of the corresponding molecular dynamics. Yet, when such molecules are adsorbed on surfaces, these reactions are expected to be strongly modified as compared to what is know in the gas phase. In this work, a low temperature (5 K) scanning tunneling microscope (STM) is used to image the trans-stilbene molecules deposited on a Si(100)-2 x 1 surface at 12 K. trans-Stilbene undergoes conformational changes during the adsorption process such that four different stilbene conformers are observed: trans-stilbene (TS), cis-stilbene (CS), and two new conformers I(1) and I(2). Furthermore, electronic excitation of individual stilbene molecules, by means of tunnel electrons, is shown to activate specific reversible molecular surface isomerization (TS <--> I(1) and CS <--> I(2)) combined with diffusion across the surface. Calculated STM topographies, using the tight binding method, indicate that the CS and TS molecules are physisorbed. The molecular conformations of the surface isomers I(1) and I(2) are suggested to be analogous to transient states conformations of the stilbene molecule when stabilized by the silicon surface. The measurements of the molecular surface isomerization and diffusion reaction yields are used to build a qualitative potential energy surface of the various stilbene reactions. The molecular surface-isomerization dynamics is shown to be influenced by the type of dopant (n or p). This is related to surface charging, which reveals modifications in the stilbene ionization potential.

Selective scanning tunnelling microscope electron-induced reactions of single biphenyl molecules on a Si(100) surface.

Selective electron-induced reactions of individual biphenyl molecules adsorbed in their weakly chemisorbed configuration on a Si(100) surface are investigated by using the tip of a low-temperature (5 K) scanning tunnelling microscope (STM) as an atomic size source of electrons. Selected types of molecular reactions are produced, depending on the polarity of the surface voltage during STM excitation. At negative surface voltages, the biphenyl molecule diffuses across the surface in its weakly chemisorbed configuration. At positive surface voltages, different types of molecular reactions are activated, which involve the change of adsorption configuration from the weakly chemisorbed to the strongly chemisorbed bistable and quadristable configurations. Calculated reaction pathways of the molecular reactions on the silicon surface, using the nudge elastic band method, provide evidence that the observed selectivity as a function of the surface voltage polarity cannot be ascribed to different activation energies. These results, together with the measured threshold surface voltages and the calculated molecular electronic structures via density functional theory, suggest that the electron-induced molecular reactions are driven by selective electron detachment (oxidation) or attachment (reduction) processes.

Probing charge transfer dynamics in a single iron tetraphenylporphyrin dyad adsorbed on an insulating surface.

Although the dynamics of charge transfer (CT) processes can be probed with ultimate lifetime resolution, the inability to control CT at the nanoscale is one of the most important roadblocks to revealing some of its deep fundamental aspects. In this work, we present an investigation of CT dynamics in a single iron tetraphenylporphyrin (Fe-TPP) donor/acceptor dyad adsorbed on a CaF2/Si(100) insulating surface. The tip of a scanning tunneling microscope (STM) is used to create local ionic states in one fragment of the dyad. The CT process is monitored by imaging subsequent changes in the neighbor acceptor molecule and its efficiency is mapped, revealing the influence of the initial excited state in the donor molecule. In the validation of the experiments, simulations based on density functional theory show that holes have a higher donor-acceptor CT rate compared to electrons and highlight a noticeable initial state dependence on the CT process. We leverage the unprecedented spatial resolution achieved in our experiments to show that the CT process in the dyad is governed via molecule-molecule coherent tunneling with negligible surface-mediated character.

A two-dimensional ON/OFF switching device based on anisotropic interactions of atomic quantum dots on Si(100):H.

Controlling the properties of quantum dots at the atomic scale, such as dangling bonds, is a general motivation as they allow studying various nanoscale processes including atomic switches, charge storage, or low binding energy state interactions. Adjusting the coupling of individual silicon dangling bonds to form a 2D device having a defined function remains a challenge. Here, we exploit the anisotropic interactions between silicon dangling bonds on n-type doped Si(100):H surface to tune their hybridization. This process arises from interactions between the subsurface silicon network and dangling bonds inducing a combination of Jahn-Teller distortions and local charge ordering. A three-pointed star-shaped device prototype is designed. By changing the charge state of this device, its electronic properties are shown to switch reversibly from an ON to an OFF state via local change of its central gap. Our results provide a playground for the study of quantum information at the nanoscale.

Atomic scale control of hexaphenyl molecules manipulation along functionalized ultra-thin insulating layer on the Si(1 0 0) surface at low temperature (9 K).

Ultra-thin CaF2 layers are grown on the Si(1 0 0) surface by using a Knudsen cell evaporator. These epitaxial structures are studied with a low temperature (9 K) scanning tunneling microscope and used to electronically decouple hexaphenyl molecules from the Si surface. We show that the ultra-thin CaF2 layers exhibit stripe structures oriented perpendicularly to the silicon dimer rows and have a surface gap of 3.8 eV. The ultra-thin semi-insulating layers are also shown to be functionalized, since 80% of the hexaphenyl molecules adsorbed on these structures self-orients along the stripes. Numerical simulations using time-dependent density functional theory allow comparison of computed orbitals of the hexaphenyl molecule with experimental data. Finally, we show that the hexaphenyl molecules can be manipulated along or across the stripes, enabling the molecules to be arranged precisely on the insulating surface.

Atomic scale study of corrugating and anticorrugating states on the bare Si(1 0 0) surface.

In this article, we study the origin of the corrugating and anticorrugating states through the electronic properties of the Si(1 0 0) surface via a low-temperature (9 K) scanning tunneling microscope (STM). Our study is based on the analysis of the STM topographies corrugation variations when related to the shift of the local density of states (LDOS) maximum in the [Formula: see text] direction. Our experimental results are correlated with numerical simulations using the density-functional theory with hybrid Heyd-Scuseria-Ernzerhof (HSE06) functional to simulate the STM topographies, the projected density of states variations at different depths in the silicon surface as well as the three dimensional partial charge density distributions in real-space. This work reveals that the Si(1 0 0) surface exhibits two anticorrugating states at +0.8 and +2.8 V that are associated with a phase shift of the LDOS maximum in the unoccupied states STM topographies. By comparing the calculated data with our experimental results, we have been able to identify the link between the variations of the STM topographies corrugation and the shift of the LDOS maximum observed experimentally. Each surface voltage at which the STM topographies corrugation drops is defined as anticorrugating states. In addition, we have evidenced a sharp jump in the tunnel current when the second LDOS maximum shift is probed, whose origin is discussed and associated with the presence of Van Hove singularities.

Single molecule manipulation at low temperature and laser scanning tunnelling photo-induced processes analysis through time-resolved studies.

This paper describes, firstly, the statistical analysis used to determine the processes that occur during the manipulation of a single molecule through electronically induced excitations with a low temperature (5 K) scanning tunnelling microscope (STM). Various molecular operation examples are described and the ability to probe the ensuing molecular manipulation dynamics is discussed within the excitation context. It is, in particular, shown that such studies can reveal reversible manipulation for tuning dynamics through variation of the excitation energy. Secondly, the photo-induced process arising from the irradiation of the STM junction is also studied through feedback loop dynamics analysis, allowing us to distinguish between photo-thermally and photo-electronically induced signals.

Imaging molecular orbitals by scanning tunneling microscopy on a passivated semiconductor.

Decoupling the electronic properties of a molecule from a substrate is of crucial importance for the development of single-molecule electronics. This is achieved here by adsorbing pentacene molecules at low temperature on a hydrogenated Si(100) surface (12 K). The low temperature (5 K) scanning tunneling microscope (STM) topography of the single pentacene molecule at the energy of the highest occupied molecular orbital (HOMO) tunnel resonance clearly resembles the native HOMO of the free molecule. The negligible electronic coupling between the molecule and the substrate is confirmed by theoretical STM topography and diffusion barrier energy calculations.