STM Junction Research Articles

Nanoscale terahertz STM imaging of a metal surface

Terahertz scanning tunneling microscopy (THz-STM) has enabled studies of ultrafast dynamics in materials down to the atomic scale. However, despite recent advances, more work is needed to better understand and quantify the subpicosecond THz pulse-induced tunnel currents and corresponding THz-STM images of nanoscale features on surfaces. Here, we perform THz-STM on a metal surface and fully characterize the observed THz pulse-induced tunnel current and nanoscale imaging at atomic steps and defects using a Bardeen tunneling model in a three-dimensional (3D) tip geometry. We show that the measured steady-state STM current-voltage curves can be used in our model to accurately map the observed ultrafast THz-induced tunnel currents and calibrate the magnitude of the near-field peak transient THz voltage bias in the tunnel junction. Peak THz voltage bias transients greater than 10 V across the STM junction are achieved leading to field emission of subpicosecond tunnel currents with current densities exceeding ${10}^{9}\phantom{\rule{0.28em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}^{2}$ in THz-STM imaging of a Cu(111) surface. Our results establish an important benchmark for future studies in THz-STM by quantifying the ultrafast THz-induced currents and bias voltages in the tunnel junction and providing a 3D tunneling model for understanding and accurately simulating THz-STM images of nanoscale features on metal surfaces.

Nanoscale Mapping of Photo‐Induced Charge Carriers Generated at Interfaces of a Donor/Acceptor 2D‐Assembly by Light‐Assisted‐Scanning Tunneling Microscopy

AbstractCharge transfers between donor (D) and acceptor (A) species at their excited state in a light‐assisted STM setup (LA‐STM) are investigated. Through an all‐solution process, supramolecular architectures deposited on the Au(111) surface and made of 2D islands of PC71BM (electron acceptor) on top of a single layer of the polymer PTB7 (electron donor) are elaborated. The STM junction under modulated laser irradiation exhibits a strong background of photothermal signal attributed both to vertical and lateral expansion of the tip. However, from the analysis of differential images obtained at opposite voltages, additional photocurrent peaks located at the PTB7/PC71BM interfaces are detected, providing evidence for active charge transfer between D/A species at their excited state. This phenomenon is discussed in the framework of a charge transport model at interfaces in organic electronics systems.

Upgrade of a low-temperature scanning tunneling microscope for electron-spin resonance.

Electron spin resonance with a scanning tunneling microscope (ESR-STM) combines the high energy resolution of spin resonance spectroscopy with the atomic scale control and spatial resolution of STM. Here we describe the upgrade of a helium-3 STM with a 2D vector-field magnet (Bz = 8.0 T, Bx = 0.8 T) to an ESR-STM. The system is capable of delivering radio frequency (RF) power to the tunnel junction at frequencies up to 30 GHz. We demonstrate magnetic field-sweep ESR for the model system TiH/MgO/Ag(100) and find a magnetic moment of (1.004 ± 0.001) μB. Our upgrade enables to toggle between a DC mode, where the STM is operated with the regular control electronics, and an ultrafast-pulsed mode that uses an arbitrary waveform generator for pump-probe spectroscopy or reading of spin-states. Both modes allow for simultaneous radiofrequency excitation, which we add via a resistive pick-off tee to the bias voltage path. The RF cabling from room temperature to the 350 mK stage has an average attenuation of 18 dB between 5 and 25 GHz. The cable segment between the 350 mK stage and the STM tip presently attenuates an additional 34-3 +5 dB from 10 to 26 GHz and 38-2 +3 dB between 20 and 30 GHz. We discuss our transmission losses and indicate ways to reduce this attenuation. We finally demonstrate how to synchronize the arrival times of RF and DC pulses coming from different paths to the STM junction, a prerequisite for future pulsed ESR experiments.

Hovering and Twirling of Tethered Molecules by Confinement between Surfaces.

Through STM images, we show that azobenzene-terminated alkanethiols hover and twirl when confined between the Ag tip and Au(111) substrate of an STM junction. In contrast with mechanisms of activation used to drive molecular rotors, twirling is induced by the effective elimination of lateral corrugation in the energy landscape when molecules hover by their van der Waals attraction to the approaching tip. While in the stationary state the benzenes of the head group lie flat with an inter-ring separation of 7.5 Å, they stand on-edge as the molecule twirls and their separation contracts to 5.2 Å, close to the value of the free molecule. The captured images of motion allow the characterization of physisorption potentials.

STM-induced light emission from vacuum-evaporated gold film

A vacuum evaporation system has been used to evaporate gold film on glass substrate in order to probe the scanning tunneling microscope-light emission (STM-LE) from the evaporated film. The surface morphology of the evaporated Au film has been checked by atomic force microscope (AFM). In order to estimate the appropriate thickness of the Au film, which is essential for the enhancement of STM-LE in the prism-coupled geometry, a theoretical calculation has been performed. Our theoretical simulation revealed that the light emission from the prism-coupled STM junction is strongly enhanced when the Au film has a thickness of 40 nm. AFM observation also showed that the morphology of the gold films strongly depends on the cleanliness of glass substrates and the deposition temperature. Relatively smooth surface was observed when a 40-nm-thick Au film was evaporated at room temperature on the preannealed glass substrate. Finally, the evaporated films were deposited on the flat bottom of a hemispherical glass prism, and STM-LE from the tip–sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were measured. It was found from the experimental results that the prism-side emission is much stronger than the tip-side emission by virtue of the enhancement of the prism-coupled geometry.

FIM tips in SPM: Apex orientation and temperature considerations on atom transfer and diffusion

Atoms transferred to W(1 1 1) and W(1 1 0) tip apices from the Au(1 1 1) surface during tunneling and approach to mechanical contact experiments in STM are characterized in FIM at room temperature and at 158 K. The different activation energies for diffusion on the (1 1 1) and (1 1 0) tip planes and the experiment temperature are shown to be important factors controlling the extent of changes to the atomic structure of the tip. W(1 1 1) tips are much better suited to scanning probe studies which require the characterization of an atomically defined tip and subsequent verification of its integrity in FIM. The statistics of the observed spikes in the tunneling current when the tips are approached to Au(1 1 1) are interpreted using a simple model of adatoms diffusing through the STM junction.

The role of the tip symmetry on the STM topography of ‐conjugated molecules

We present an STM theory based on the reduced density matrix (RDM) formalism which is able to describe transport properties of an STM junction for ‐conjugated molecules on thin insulating films. It combines a very popular derivation of STM tunneling matrix elements (), based on Bardeen's tunneling formalism (), with a generalized master equation approach for interacting molecular systems. We show that this method allows the efficient implementation of different tip symmetries in STM simulations. With the example of hydrogen phthalocyanine (H2 Pc), we study the influence of s‐ and p‐wave tip symmetries on the constant‐height current maps of ‐conjugated molecules. Constant‐height STM images evaluated at the cationic resonance of H2 Pc. Left image computed by using an s‐wave tip and right image for a linear combination of px and py states.

Observation and manipulation of tris(dibenzoylmethanato)ruthenium molecules by low temperature scanning tunnelling microscopy

The behavior of tris(dibenzoylmethanato)ruthenium molecules has been studied using low temperature scanning tunneling microscopy after adsorption on Ag(111) and Cu(111). On both surfaces, the molecules present two adsorption geometries: a bi-lobed and a chiral threefold structure. This latter form can stand up to adopt a bi-lobed conformation which is oriented along the dense crystallographic directions. The molecular geometries after adsorption can be modified using the STM tip. In one of these modifications, a switch between two different conformations has been observed. The data recorded during the experiments show that this phenomenon is induced by the electric field in the STM junction.

Sensitizers in inelastic electron tunneling spectroscopy: a first-principles study of functional aromatics on Cu(111)

Low sensitivity is a key problem in inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope. Using first-principles simulations, we predict different means to tune the IETS sensitivity of symmetrical functional aromatics on a Cu(111) surface. We show how the IET-spectra of phenyl-NO2 compounds can be greatly enhanced as compared to pristine phenyl. More precisely, the NO2 substituent qualifies as a sensitizer of low-frequency wagging modes, but also as a quencher of high-frequency stretching modes. At variance, the CO2 substituent is found to suppress the whole IET-activity. The head-up (non-anchoring) and head-down (anchoring) configurations of the functional group lead to minor changes in the signals, nevertheless allowing access to discriminate configurational features. It is shown how to disentangle the electronic and steric effects of the substituent in the STM junction.

Chemically Controlled Conductivity: Torsion‐Angle Dependence in a Single‐Molecule Biphenyldithiol Junction

Stepwise regulation of the molecular conductance was observed in a series of eight biphenyldithiols with fixed torsion angles between the phenyl rings. These compounds were synthesized and their single-molecule conductance was investigated in an STM junction. A cos2 dependence was found between the interplane torsion angle and the single-molecule conductivity (see plot).

Photon mapping of MgO thin films with an STM

The light emission from an STM junction consisting of an MgO thin film on Mo(0 0 1) and an Au tip is analyzed with respect to its spatial distribution for various excitation conditions. The spectral characteristic of the light is compatible with an emission mechanism mediated by tip-induced plasmons that are excited by inelastic electron tunneling involving field-emission resonances in the tip–sample gap. The dependence of field-emission resonances on the MgO work function allows the controlled stimulation of differently thick oxide islands in the photon maps by changing the sample bias.

Prototypical single-molecule chemical-field-effect transistor with nanometer-sized gates.

A prototypical single-molecule chemical-field-effect transistor is presented, in which the current through a hybrid-molecular diode is modified by nanometer-sized charge transfer complexes covalently linked to a molecule in an STM junction. The effect is attributed to an interface dipole which shifts the substrate work function by approximately 120 meV. It is induced by the complexes from electron acceptors covalently bound to the molecule in the gap and electron donors coming from the ambient fluid. This proof of principle is regarded as a major step towards monomolecular electronic devices.

Electron Tunneling and Electrochemical Currents through Interfacial Water Inside an STM Junction

The apparent barrier height for charge transfer through an interfacial water layer between a Pt/Ir tip and a gold surface has been measured using STM technique. The average thickness of the interfacial water layer inside an STM junction was controlled by the amount of moisture. A thin water layer on the surface was formed when relative humidity was in the range of 10 to 80%. In such a case, electron tunneling through the thin water layer became the majority of charge transfers. The value of the barrier height for the electron tunneling was determined to be 0.95 eV from the current vs. distance curve, which was independent of the tip-sample distance. On the other hand, the apparent barrier height for charge transfer showed a dependence on tip-sample distance in the bias range of 0.1-0.5 V at a relative humidity of approximately 96%. The non-exponentiality for current decay under these conditions has been explained in terms of electron tunneling and electrochemical processes. In addition, the plateau current was observed at a large tip-sample distance, which was caused by electrochemical processes and was dependent on the applied voltage.

Bistability in scanning tunneling spectroscopy of Ga-terminated Si(111)

Bistable electron transport, a phenomenon usually associated with double-barrier structures, has been observed with a conventional STM junction formed between a metal tip and a Ga-terminated Si(111) surface at 77 K. Large hysteresis loops appear in the current-voltage characteristics when electrons are injected from the tip to the surface. The turn-on bias varies from -3.1 to -4.0 V and shows an inverse dependence on the tip-sample distance, indicating a strong field effect. The turn-off bias, however, is essentially pinned at a conductance threshold of -2.7 V.

Investigation On Laser-Induced Effects In Nanostructure Fabrication With Laser-Irradiated Scanning Tunneling Microscope Tips in Air Ambient

AbstractIn this paper, we report our investigation on the kinetics of nanostructure fabrication on gold films and on H-passivated Ge surfaces. The relationship between the current and the tip-sample distance of the STM junction was measured for both gold films and H-passivated Ge surfaces. The tip-sample distance for gold films under a electrochemically etched W tip is approximately 2 nm, while that for H-passivated Ge sufaces is more than 27 nm. The thermal expansion length of the tip under laser irradiation was calculated. From the comparison of the thermal expansion length and the tip-sample distance, we can reach the conclusion that for gold films, thermal mechanical indention is the primary reason of nanostructure formation, while for H-passivated Ge surfaces, optical enhancement is the only reason.

Cluster-model density functional study of a W–Cu(1 0 0) STM junction

In this article, we investigate the electronic properties of different clusters modelling a tungsten tip, the Cu(1 0 0) surface and interacting W–Cu(1 0 0) systems in STM configuration. Electronic structure calculations are carried out within the LDA approximation of the Density Functional Theory (DFT). Both integrated (densities of states) and local properties (electronic density and electrostatic potential) are considered. The study is performed for top and hollow surface sites and two different tip–sample separations.

Theoretical study of the resistance of short (Xe) n wires within an STM junction: the (Xe) 2 case

, 1921 (1996)]. The resistance versus the separation between the STM tip and the surface has been calculated, the relaxation of the Xe atoms being calculated using molecular dynamics. Two major deviations from the well-known exponential decrease of the junction resistance have been found. The first is due to an electronic interference effect. With a tight binding model, we showed that this effect comes from the destructive interference between two different tunneling paths. The second deviation is due to the sideways motion of one Xe atom under the tip compression.

Investigations of the fine structure of I(V) characteristics for highly oriented pyrolitic graphite surface by means of STM/STS at room temperature

A comprehensive room-temperature study of the electron tunneling in the STM junction between a tungsten tip and a highly oriented pyrolitic graphite surface is presented. The conductance versus voltage dependencies d I= dV.V/ ob- tained by numerical filtration and differentiation of the direct measured current - voltage characteristics I.V/ have a num- ber of narrow resistive peaks at the bias voltages located near the corresponding tungsten and HOPG phonon frequen- cies. The plotted histograms of the frequency distribution of the global minimum in d I= dV.V/ curves enable us to draw a conclusion on the phonon origin of this fine structure. A model that could qualitatively explain the observed data is discussed.

Electron tunneling across an interfacial water layer inside an STM junction: tunneling distance, barrier height and water polarization effect

surfaces in air. The electron tunneling distance ranges from 7–20 A when the junction resistance is of the order of 108 Ω. We suggest that the unusually low value of the barrier height (<1.5 eV) originates from the three-dimensional nature of the electron tunneling through the interfacial water layer present inside the junction. An asymmetric variation in barrier height is observed with respect to the applied bias voltage at a junction distance of water monolayer thickness, which reflects the polarization of the water layer on the surface.

Polarized light emission from the metal-metal STM junction

Polarized light emission from the metal-metal STM junction