An instrument to investigate femtochemistry on metal surfaces in real space

Locally varying formation of nanoclusters across a low-intensity ultra-short laser spot

Ultra-short laser illumination is an intriguing tool for engineering material by light. It is usually employed at or above the ablation threshold. Practical applications profit from tailoring surface properties, for instance, by structural changes to the surface layer of an irradiated target. A target-orientated restructuring of surfaces on the nanoscale is much less explored. In particular, an intrinsic intensity variation across a laser spot has not yet been considered or employed. We image the unexpected nanoscale clusters formed on the Cu(111) surface upon illumination of a Cu sample far below its ablation threshold by femtosecond laser light, employing a specifically-developed multi-scale approach. We unravel that these clusters vary significantly in size and shape across the micrometer-scale 400 nm 50 fs laser spot (repetition rate: 250 kHz). There are three qualitatively different regions separated by sharp changes. The observations highlight the importance of local fluence for specific types of nanoclusters. Ultra-short laser illumination represents a non-trivial interplay between photo-thermal and electron-induced mechanisms, transport of heat and electrons, and material properties, which we discuss for identifying the underlying principles. Our study demonstrates that a multitude of as yet unconsidered processes are involved in the tailoring of nanoscale materials by ultra-short laser light.

The Influence of a Changing Local Environment during Photoinduced CO2 Dissociation

Though largely influencing the efficiency of a reaction, the molecular-scale details of the local environment of the reactants are experimentally inaccessible hindering an in-depth understanding of a catalyst's reactivity, a prerequisite to maximizing its efficiency. We introduce a method to follow individual molecules and their largely changing environment during a photochemical reaction. The method is illustrated for a rate-limiting step in a photolytic reaction, the dissociation of CO₂ on two catalytically relevant surfaces, Ag(100) and Cu(111). We reveal with a single-molecule resolution how the reactant's surroundings evolve with progressing laser illumination and with it their propensity for dissociation. Counteracting processes lead to a volcano-like reactivity. Our unprecedented local view during a photoinduced reaction opens the avenue for understanding the influence of the products on reaction yields on the nanoscale.

Real-space observation of surface structuring induced by ultra-fast-laser illumination far below the melting threshold

Intense short laser pulses are an intriguing tool for tailoring surface properties via ultra-fast melting of the surface layer of an irradiated target. Despite extensive studies on the interaction of femto-second laser interaction with matter, the initial steps of the morphological changes are not yet fully understood. Here, we reveal that substantial surface structure changes occur at energy densities far below the melting threshold. By using low-temperature scanning tunneling microscopy we resolve atomic-scale changes, i.e. the creation of nanosized adatom and vacancy clusters. The two cluster types have distinct non-linear fluence-dependencies. A theoretical analysis reveals their creation and motion to be non-thermal in nature. The formation of these atomistic changes, individually resolved here for the first time, recast our understanding of how surfaces respond to low-intensity ultra-short laser illumination. A visualization and control of the initial morphological changes upon laser illumination are not only of fundamental interest, but pave the way for the designing material properties through surface structuring.

Common structures of CO2 on structurally different coin metal surfaces

We investigate superstructures formed by CO2 on Ag(100) and Cu(111) from small clusters forming at 21 K up to multilayers grown at 43 K by low temperature scanning tunneling microscopy. On both surfaces, CO2 nucleates only at defects, here at co-adsorbed CO. At the lower adsorption temperature, superstructures of different symmetry coexist on both surfaces at submonolayer coverage, while the superstructures formed at the higher adsorption temperature differ largely for the two surfaces. On Ag(100), the CO2 monolayer exhibits a long-range order interrupted by antiphase domain boundaries. On Cu(111), a random distribution of domain structures of different symmetry leads to a monolayer without long-range order. Surprisingly, the degree of ordering is inverted for the 2nd layer of CO2. On Ag(100), the coexistence of different superstructures in the 2nd layer leads to reduced long-range order. On Cu(111), a hexagonal 2nd layer exhibits long-range order. A layer of a similar superstructure, hexagonal with long-range order, exists as the 3rd layer of Ag(100). Despite the different substrates, a multitude of common structural features of CO2 exist. Hexagonal layers grow with a long-range order on less ordered layers on both surfaces. Our results suggest that the preferred structure of a CO2 layer is hexagonal.

Anomalously Low Barrier for Water Dimer Diffusion on Cu(111)

A molecular-scale description of water and ice is important in fields as diverse as atmospheric chemistry, astrophysics, and biology. Despite a detailed understanding of water and ice structures on a multitude of surfaces, relatively little is known about the kinetics of water motion on surfaces. Here, we report a detailed study on the diffusion of water monomers and the formation and diffusion of water dimers through a combination of time-lapse low-temperature scanning tunnelling microscopy experiments and first-principles electronic structure calculations on the atomically flat Cu(111) surface. On the basis of an unprecedented long-time study of individual water monomers and dimers over days, we establish rates and mechanisms of water monomer and dimer diffusion. Interestingly, we find that the monomer and the dimer diffusion barriers are similar, despite the significantly larger adsorption energy of the dimer. This is thus a violation of the rule of thumb that relates diffusion barriers to adsorption energies, an effect that arises because of the directional and flexible hydrogen bond within the dimer. This flexibility during diffusion should also be relevant for larger water clusters and other hydrogen-bonded adsorbates. Our study stresses that a molecular-scale understanding of the initial stages of ice nanocluster formation is not possible on the basis of static structure investigations alone.

Understanding the Enhancement of Surface Diffusivity by Dimerization

Beyond dilute coverage, the collective diffusion of molecules might enhance material transport. We reveal an enhanced mobility of molecular dimers by separating two motions, diffusion and rotation, of CO dimers on elemental Ag(100) as well as on a dilute Cu alloy of Ag(100). From time-lapsed scanning tunneling microscopy movies recorded between 15 and 25 K, we determine the activation energy of dimer diffusion on elemental Ag(100) to be, at (40±2)  meV, considerably smaller than the one for monomer diffusion, at (72±1)  meV. The alloyed Cu atoms reduce the dimer mobility facilitating to determine their rotational barrier separately to be (39±3)  meV. Disentangling different degrees of freedom suggests that a rotational motion is at the origin of enhanced dimer diffusivity.

Near-Field Enhanced Photochemistry of Single Molecules in a Scanning Tunneling Microscope Junction

Optical near-field excitation of metallic nanostructures can be used to enhance photochemical reactions. The enhancement under visible light illumination is of particular interest because it can facilitate the use of sunlight to promote photocatalytic chemical and energy conversion. However, few studies have yet addressed optical near-field induced chemistry, in particular at the single-molecule level. In this Letter, we report the near-field enhanced tautomerization of porphycene on a Cu(111) surface in a scanning tunneling microscope (STM) junction. The light-induced tautomerization is mediated by photogenerated carriers in the Cu substrate. It is revealed that the reaction cross section is significantly enhanced in the presence of a Au tip compared to the far-field induced process. The strong enhancement occurs in the red and near-infrared spectral range for Au tips, whereas a W tip shows a much weaker enhancement, suggesting that excitation of the localized plasmon resonance contributes to the process. Additionally, using the precise tip-surface distance control of the STM, the near-field enhanced tautomerization is examined in and out of the tunneling regime. Our results suggest that the enhancement is attributed to the increased carrier generation rate via decay of the excited near-field in the STM junction. Additionally, optically excited tunneling electrons also contribute to the process in the tunneling regime.

Imaging the Bonds of Dehalogenated Benzene Radicals on Cu(111) and Au(111)

Dissociative adsorption of doubly substituted benzene molecules leads to formation of benzyne radicals. In this study, co-adsorbed hydrogen molecules are used in scanning tunneling hydrogen microscopy to enhance the contrast of the meta- and the para-isomers of these radicals on Cu(111) and Au(111). Up to three hydrogen molecules are attached to one radical. One hydrogen molecule reveals the orientation of the carbon ring and its adsorption site, allowing discrimination between the two radicals. Two hydrogen molecules reflect the bond picture of the carbon skeleton and reveals that adsorption on Cu(111) distorts the meta- isomer differently from its gas-phase distortion. Three hydrogen molecules allow us to determine the bond picture of a minor species.

Experimental Evidence for a Three-Body Interaction between Diffusing CO Molecules

The diffusion of carbon monoxide molecules on Cu(111) is investigated in time-lapsed scanning tunneling microscopy in a temperature range from 30 to 38 K. An asymptotic theory of adsorbate diffusion predicted a trio interaction that changes the diffusion barrier of three particles diffusing in close proximity beyond the change induced by the long-range interaction between three pairs of molecules. Distance-dependent variations in the diffusion energy confirm this theoretical prediction. In future, the theory can better assist experiments for a broader exploration, not only for diffusion, but also for nucleation and reaction.

Temperature calibration for diffusion experiments to sub-Kelvin precision

Arrhenius plots are often used to determine energy barriers and prefactors of thermally activated processes. The precision of thus determined values depends crucially on the precision of the temperature measurement at the sample surface. We line out a procedure to determine the absolute temperature of a metal sample in a cryogenic scanning tunneling microscope between 5 K and 50 K with sub-Kelvin precision. We demonstrate the dependence of prefactor and diffusion energy on this calibration for diffusion of CO on Cu(111) and on Ag(100) measured in the temperature range from 30 K to 38 K and 19 K to 23 K, respectively.

Physisorption versus chemisorption of oxygen molecules on Ag(100)

We compare the adsorption of oxygen molecules on Ag(100) at 60 K and at 100 K. At both temperatures, the molecules form islands. Differences between the species adsorbed at the two temperatures in both low-temperature scanning tunneling microscopy and inelastic electron tunneling spectroscopy are attributed to two different adsorption states, a chemisorbed state after 100 K adsorption and a physisorbed state after 60 K adsorption.

Visualizing Optoelectronic Processes at the Nanoscale

In this issue of ACS Nano, Nienhaus et al. report the optoelectronic properties of carbon nanotube chiral junctions with nanometer resolution in the presence of strong electric fields (∼1 V/nm). Here, we provide an overview of recent studies that combine scanning tunneling microscope (STM) and laser or microwave illumination. These techniques reveal nanoscale laser- or microwave-induced phenomena utilizing the intrinsic atomic resolution of the tunneling current, and do not require substantial modification of the STM itself. The merits of atomic-scale spatial resolution and chemical sensitivity of the laser or microwave spectroscopes make these techniques useful for nanoscale characterization.

Complex Surface Diffusion Mechanisms of Cobalt Phthalocyanine Molecules on Ag(100)

We used time-lapsed scanning tunneling microscopy between 43 and 50 K and density functional theory (DFT) to explore the basic surface diffusion steps of cobalt phthalocyanine (CoPc) molecules on the Ag(100) surface. We show that the CoPc molecules translate and rotate on the surface in the same temperature range. Both processes are associated with similar activation energies; however, the translation is more frequently observed. Our DFT calculations provide the activation energies for the translation of the CoPc molecule between the nearest hollow sites and the rotation at both the hollow and the bridge sites. The activation energies are only consistent with the experimental findings, if the surface diffusion mechanism involves a combined translational and rotational molecular motion. Additionally, two channels of motion are identified: the first provides only a channel for translation, while the second provides a channel for both the translation and the rotation. The existence of the two channels explains a higher rate for the translation determined in experiment.

Consecutive Mechanism in the Diffusion of D2O on a NaCl(100) Bilayer

The motion of D2O monomers is investigated on a NaCl(100) bilayer on Ag(111) between 42.3 and 52.3 K by scanning tunneling microscopy. The diffusion distance histogram reveals a squared diffusion lattice that agrees with the primitive unit cell of the (100) surface. From the Arrhenius dependence, we derive the diffusion energy, the pre-exponential factor, and the attempt frequency. The mechanism of the motion is identified by comparison of the experimental results to theoretical calculations. Via low temperature adsorption site determination in connection with density functional theory, we reveal an influence of the metallic support onto the intermediate state of the diffusive motion.

Differences between thermal and laser-induced diffusion

A combination of femtosecond laser excitation with a low-temperature scanning tunneling microscope is used to study long-range interaction during diffusion of CO on Cu(111). Both thermal and laser-driven diffusion show an oscillatory energy dependence on the distance to neighboring molecules. Surprisingly, the phase is inverted; i.e., at distances at which thermal diffusion is most difficult, it is easiest for laser-driven diffusion and vice versa. We explain this unexpected behavior by a transient stabilization of the negative ion during diffusion as corroborated by ab initio calculations.

Size dependence of the dispersion relation for the interface state between NaCl(100) and Ag(111)

This study investigates the interface state electron dispersion relation between NaCl(100) islands and Ag(111) dependent upon NaCl island size. Both onset energy and effective mass are size dependent. However, these dependencies are relevant at different island sizes. We trace back this effective mass dependency to a misfit-induced strain based on atomically resolved images. Our results open up new avenues for the development of nanodevices by tuning the effective electron mass via strain of the insulating component.

A switch based on self-assembled thymine

The DNA base thymine is deposited at 100 K on Cu(111) and investigated and manipulated by low-temperature scanning tunneling microscopy at 5 K. At submonolayer coverage paired rows are observed. At monolayer coverage a hexagonal commensurate self-assembled layer with the methyl group pointing away from the surface forms. A reversible local manipulation of molecules within the self-assembled layer is demonstrated. This manipulation is interpreted as an out-of-plane relaxation of molecules within the layer induced by the change of the adsorption geometry of individual molecules between two meta-stable orientations. A positive field of 2-4 V leads to this local change in the molecular arrangement, while a field larger than 4 V restores the original geometry.

Coulomb attraction during the carpet growth mode of NaCl

The submonolayer growth of NaCl bilayer high-rectangular shaped islands on Ag(111) is investigated at around room temperature by using low temperature scanning tunneling microscopy. The growth at the step edges is preferred. Two kinds of islands are observed. They either grow with their non-polar edge at the step edge of Ag(111) or the islands overgrow in a carpet-like mode with the polar direction parallel to the edge. In the latter case, the Ag step is rearranged and considerable, while the NaCl layer is bent. This study clarifies the nature of the interaction of an alkali halide nanostructure with a metal step edge.

On the interpretation of IETS spectra of a small organic molecule

We have investigated vibrational spectra of nitrobenzene molecules adsorbed on Cu(111) by low temperature inelastic electron tunneling spectroscopy. This molecule, which should support 39 internal modes, only gives rise to seven peaks in the spectra. We outline a comparison with ensemble IR data and interpret the small number of vibrational peaks by the superposition of a multitude of almost isoenergetic vibrational modes. The non-detectability of further modes cannot be understood in terms of symmetry considerations. Additional modes in the spectra are attributed to external molecular-metal vibrations.

Anomalous scaling in heteroepitaxial island dynamics on Ag(100)

Diffusion and decay of alloyed Cu/Ag islands are investigated in the size range from 1 to 40  nm2 on Ag(100) at room temperature with fast-scanning tunneling microscopy and density functional theory. While islands at sizes above 7  nm2 show the diffusion and decay behavior expected for dynamics based on single atom hopping, islands smaller than 4  nm2 diffuse faster and decay slower than predicted by standard theory. This anomalous behavior at unexpected large island sizes is related to a size dependent dealloying of the Cu/Ag islands.

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.

Dissociation of oxygen on Ag(100) induced by inelastic electron tunneling

Scanning tunneling microscopy (STM) is used to study the dissociation of molecular oxygen on Ag(100) induced by inelastic electron tunneling (IET) at 5 K. This dissociation is possible above 3.3 V with a yield of (3.63 ± 0.47) × 10(-9) per electron. Dissociation leads to three different types of hot atom motion: lateral motion, a cannon ball mechanism, and abstractive dissociation. Analysis of the I-t characteristics during dissociation suggests that the dissociation is proceeded by an adsorption site change.

Diffusion and dimer formation of CO molecules induced by femtosecond laser pulses

We investigate two fundamental steps of a nonadiabatic surface process, the photo-induced movement and approach of CO molecules on the Cu(111) surface, at a hitherto unachieved single-molecule level through scanning tunneling microscope imaging. For the close approach of two CO molecules, we not only determine the nonadiabatic diffusion barrier (87 meV), but also discover a femto-second-laser-induced transient attraction (30 meV) of the usually repelling CO molecules.

Local investigation of femtosecond laser induced dynamics of water nanoclusters on Cu(111)

We explore the dynamics of low temperature interfacial water nanoclusters on Cu(111) by femtosecond-laser excitation, scanning tunneling microscopy and density functional theory. Laser illumination is used to induce single molecules to diffuse within water clusters and across the surface, breaking and reforming hydrogen bonds. A linear diffusion probability with laser fluence is observed up to 0.6 J/m2 and we suggest that diffusion is initiated by hot electron attachment and detachment processes. The density functional calculations shed light on the detailed molecular mechanism for water diffusion that is determined by the local structure of the water clusters.

Isomerization reactions on single adsorbed molecules

Molecular switches occur throughout nature. In one prominent example, light induces the isomerization of retinal from the compact 11-cis form to the elongated all-trans form, a conversion that triggers the transformation of light into a neural impulse in the eye. Applying these natural principles to synthetic systems offers a promising way to construct smaller and faster nanoelectronic devices. In such systems, electronic switches are essential components for storage and logical operations. The development of molecular switches on the single-molecule level would represent a major step toward incorporating molecules as building units into nanoelectronic circuits. Molecular switches must be both reversible and bistable. To meet these requirements, a molecule must have at least two different thermally stable forms and a way to repeatedly interconvert between those forms based on changes in light, heat, pressure, magnetic or electric fields, pH, mechanical forces, or electric currents. The conversion should be connected to a measurable change in electronic, optical, magnetic, or mechanical properties. Because isomers can differ significantly in physical and chemical properties, isomerization could serve as a molecular switching mechanism. Integration of molecular switches into larger circuits will probably require arranging them on surfaces, which will require a better understanding of isomerization reactions in these environments. In this Account, we describe our scanning tunneling microscopy studies of the isomerization of individual molecules adsorbed on metal surfaces. Investigating chlorobenzene and azobenzene derivatives on the fcc(111) faces of Ag, Cu, and Au, we explored the influence of substituents and the substrate on the excitation mechanism of the isomerization reaction induced by inelastically tunneling electrons. We achieved an irreversible configurational (cis-trans) isomerization of individual 4-dimethyl-amino-azobenzene-4-sulfonic acid molecules on Au(111), a reversible configurational (cis-trans) isomerization of amino-nitro-azobenzene on Au(111), a constitutional (meta-ortho) isomerization of chloronitrobenzene molecules adsorbed on Cu(111) and Au(111), and a constitutional (meta-para) isomerization of dichlorobenzene molecules adsorbed on Cu(111) and Ag(111). These studies demonstrate that we can induce a variety of isomerization reactions by electron excitation on a metal surface. Our model isomerization studies provide a way to manipulate properties of single molecules, changing both their geometric structure and their physicochemical properties. The control of isomerization of single molecules will advance the development of single-molecule electronics and other nanoscale processes.

Electron damage to supported ice investigated by scanning tunneling microscopy and spectroscopy

We study the interaction of low-energy electrons with crystalline ice (D2O) on Cu(111) by low-temperature scanning tunneling microscopy and spectroscopy. Electrons induce dissociation of the molecules with a threshold of approximately 3 eV. The large dissociation yield of the order of 10(-8)/electron and the extended area of dissociation are attributed to a shift in conduction band during the dissociation. Voltage dependent differences in imaging of ice and dissociated ice are reflected in the spectroscopic signature.

Manipulation and control of hydrogen bond dynamics in absorbed ice nanoclusters

Inelastic electron tunneling is used to explore the dynamics of ice nanoclusters adsorbed on Ag(111). The diffusion of entire nanoclusters or internal hydrogen bond rearrangement can be selectively controlled by injecting electrons either directly into the clusters themselves or indirectly ("indirect inelastic electron tunneling") into the substrate at distances of up to 20 nm from them; a reaction probability that oscillates with the tip-cluster lateral distance presents evidence that surface state electrons mediate the excitation. Density functional theory calculations reveal a strong sensitivity of the computed activation energies of the individual processes to the applied electrical field.

Faceting during the transformation of amorphous to crystalline ice

We study the thermally activated transition from amorphous to crystalline ice (D2O) on Cu(111) with high-resolution scanning tunneling microscopy. Annealing of amorphous solid water up to the desorption temperature of 149 K results subsequently in monomer decorated double bilayers with different superstructure, a faceted surface, pyramidal islands, and nanocrystallites of distinct height at different coverages. Though all structures are truncations from crystalline water ice, for none of them is the ice bilayer found to be the terminating surface.

Amino-nitro-azobenzene dimers as a prototype for a molecular-level machine

Low-temperature scanning tunnelling microscope measurements of an X-shaped dimer, consisting of two amino-nitro-azobenzene molecules, on a Au(111)-surface are presented. Electron-induced manipulation switches either the upper- or the lower-lying molecule. These reversible switching processes are based on the cis-trans isomerization of the azobenzene molecules. The switching of the upper molecule moves the lower-lying molecule in a controlled way. Thus mechanical work is performed. This proof-of-principle shows the ability of azobenzene molecules to act as a molecular-level machine.

Ice nanoclusters at hydrophobic metal surfaces

Studies of the structure of supported water clusters provide a means for obtaining a rigorous molecular-scale description of the initial stages of heterogeneous ice nucleation: a process of importance to fields as diverse as atmospheric chemistry, astrophysics and biology. Here, we report the observation and characterization of metal-supported water hexamers and a family of hydrated nanoclusters--heptamers, octamers and nonamers--through a combination of low-temperature scanning tunnelling microscopy experiments and first-principles electronic-structure calculations. Aside from achieving unprecedented resolution of the cyclic water hexamer--the so-called smallest piece of ice--we identify and explain a hitherto unknown competition between the ability of water molecules to simultaneously bond to a substrate and to accept hydrogen bonds. This competition also rationalizes previous structure predictions for water clusters on other substrates.

Contribution of the surface state to the observation of the surface Kondo resonance

Tunneling spectra obtained on and near Co atoms adsorbed on Ag(111) show at 5 to 6 K a Kondo resonance that appears as a characteristic dip around the Fermi energy. The feature is present up to 1.5 nm around Co atoms adsorbed on terraces with the surface state onset in the occupied region of the density of states. On a narrow terrace, where the surface state onset lies in the unoccupied region of the density of states, it is only present up to 0.5 nm. This difference demonstrates directly the importance of the surface state electrons in the observation of the surface Kondo resonance.

Impact of ice structure on ultrafast electron dynamics in D2O clusters on Cu(111)

The structure of D2O clusters on a Cu(111) surface and the femtosecond dynamics of photoexcited excess electrons are investigated by low-temperature scanning tunneling microscopy and two-photon photoemission spectroscopy. Two types of amorphous ice clusters, porous and compact, which exhibit characteristic differences in electron dynamics, are identified. By titration with Xe we show that in both structures solvated electrons preferentially bind on the cluster surface.