Nanoscale electrochemical kinetics & dynamics: the challenges and opportunities of single-entity measurements.

The development of nanoscale electrochemistry since the mid-1980s has been predominately coupled with steady-state voltammetric (i-E) methods. This research has been driven by the desire to understand the mechanisms of very fast electrochemical reactions, by electroanalytical measurements in small volumes and unusual media, including in vivo measurements, and by research on correlating electrocatalytic activity, e.g., O2 reduction reaction, with nanoparticle size and structure. Exploration of the behavior of nanoelectrochemical structures (nanoelectrodes, nanoparticles, nanogap cells, etc.) of a characteristic dimension λ using steady-state i-E methods generally relies on the well-known relationship, λ2 ∼ Dt, which relates diffusional lengths to time, t, through the coefficient, D. Decreasing λ, by performing measurements at a nanometric length scales, results in a decrease in the effective timescale of the measurement, and provides a direct means to probe the kinetics of steps associated with very rapid electrochemical reactions. For instance, steady-state voltammetry using a nanogap twin-electrode cell of characteristic width, λ ∼ 10 nm, allows investigations of events occurring at timescales on the order of ∼100 ns. Among many other advantages, decreasing λ also increases spatial resolution in electrochemical imaging, e.g., in scanning electrochemical microscopy, and allows probing of the electric double layer. This Introductory Lecture traces the evolution and driving forces behind the "λ2 ∼ Dt" steady-state approach to nanoscale electrochemistry, beginning in the late 1950s with the introduction of the rotating ring-disk electrode and twin-electrode thin-layer cells, and evolving to current-day investigations using nanoelectrodes, scanning nanocells for imaging, nanopores, and nanoparticles. The recent focus on so-called "single-entity" electrochemistry, in which individual and very short redox events are probed, is a significant departure from the steady-state approach, but provides new opportunities to probe reaction dynamics. The stochastic nature of very fast single-entity events challenges current electrochemical methods and modern electronics, as illustrated using recent experiments from the authors' laboratory.

Spatially Resolved Imaging on Photocarrier Generations and Band Alignments at Perovskite/PbI2 Heterointerfaces of Perovskite Solar Cells by Light-Modulated Scanning Tunneling Microscopy.

The presence of the PbI2 passivation layers at perovskite crystal grains has been found to considerably affect the charge carrier transport behaviors and device performance of perovskite solar cells. This work demonstrates the application of a novel light-modulated scanning tunneling microscopy (LM-STM) technique to reveal the interfacial electronic structures at the heterointerfaces between CH3NH3PbI3 perovskite crystals and PbI2 passivation layers of individual perovskite grains under light illumination. Most importantly, this technique enabled the first observation of spatially resolved mapping images of photoinduced interfacial band bending of valence bands and conduction bands and the photogenerated electron and hole carriers at the heterointerfaces of perovskite crystal grains. By systematically exploring the interfacial electronic structures of individual perovskite grains, enhanced charge separation and reduced back recombination were observed when an optimal design of interfacial PbI2 passivation layers consisting of a thickness less than 20 nm at perovskite crystal grains was applied.

Highly Sensitive Electrical Detection of HIV-1 Virus Based on Scanning Tunneling Microscopy.

A highly sensitive immunosensor based on scanning tunneling microscopy (STM) was developed for the first time to detect living material such as HIV-1 virus by gold (Au) nanoparticle and fragmented antibody complex. Fragmented antibodies were pre-immobilized on the Au surface, then HIV-1 virus like particles (HIV-1 VLPs) and Au-nanoparticle and fragmented antibody complexes were applied to develop sandwich assay. The developed surface morphology and the current profile of fabricated immunosensing element were characterized by Raman spectroscopy and investigated with STM. The power spectrum derived from the current profile was found to be related with concentrations of HIV-1 VLPs. Using the electrical detection method based on current mapping profile of STM, living material such as virus, HIV-1 VLPs, was able to be detected successfully. The proposed technique can be a promising method to construct the highly sensitive and efficient sensor for detecting viruses and other living materials.

Mapping image potential states on graphene quantum dots.

Free-electron-like image potential states are observed in scanning tunneling spectroscopy on graphene quantum dots on Ir(111) acting as potential wells. The spectrum strongly depends on the size of the nanostructure as well as on the spatial position on top, indicating lateral confinement. Analysis of the substructure of the first state by the spatial mapping of the constant energy local density of states reveals characteristic patterns of confined states. The most pronounced state is not the ground state, but an excited state with a favorable combination of the local density of states and parallel momentum transfer in the tunneling process. Chemical gating tunes the confining potential by changing the local work function. Our experimental determination of this work function allows us to deduce the associated shift of the Dirac point.

Measuring the elastic properties of living cells through the analysis of current-displacement curves in scanning ion conductance microscopy.

Knowledge of mechanical properties of living cells is essential to understand their physiological and pathological conditions. To measure local cellular elasticity, scanning probe techniques have been increasingly employed. In particular, non-contact scanning ion conductance microscopy (SICM) has been used for this purpose; thanks to the application of a hydrostatic pressure via the SICM pipette. However, the measurement of sample deformations induced by weak pressures at a short distance has not yet been carried out. A direct quantification of the applied pressure has not been also achieved up to now. These two issues are highly relevant, especially when one addresses the investigation of thin cell regions. In this paper, we present an approach to solve these problems based on the use of a setup integrating SICM, atomic force microscopy, and optical microscopy. In particular, we describe how we can directly image the pipette aperture in situ. Additionally, we can measure the force induced by a constant hydrostatic pressure applied via the pipette over the entire probe-sample distance range from a remote point to contact. Then, we demonstrate that the sample deformation induced by an external pressure applied to the pipette can be indirectly and reliably evaluated from the analysis of the current-displacement curves. This method allows us to measure the linear relationship between indentation and applied pressure on uniformly deformable elastomers of known Young's modulus. Finally, we apply the method to murine fibroblasts and we show that it is sensitive to local and temporally induced variations of the cell surface elasticity.

Electron-induced dynamics of heptathioether ß-cyclodextrin molecules.

Variable-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) measurements are performed on heptathioether ß-cyclodextrin (ß-CD) self-assembled monolayers (SAMs) on Au. The ß-CD molecules exhibit very rich dynamical behavior, which is not apparent in ensemble-averaged studies. The dynamics are reflected in the tunneling current-time traces, which are recorded with the STM feedback loop disabled. The dynamics are temperature independent, but increase with increasing tunneling current and sample bias, thus indicating that the conformational changes of the ß-CD molecules are induced by electrons that tunnel inelastically. Even for sample biases as low as 10 mV, well-defined levels are observed in the tunneling current-time traces. These jumps are attributed to the excitations of the molecular vibration of the macrocyclic ß-CD molecule. The results are of great importance for a proper understanding of transport measurements in SAMs.

Modulation of nanocavity plasmonic emission by local molecular states of C60 on Au(111).

We investigate the modulation of C60 monolayers on the nanocavity plasmonic (NCP) emission on Au(111) by tunneling electron excitation from a scanning tunneling microscope (STM) tip. STM induced luminescence spectra show not only suppressed emission, but also significant redshift of NCP emission bands on the C60 molecules relative to the bare metal surface. The redshift, together with the bias- and coverage-dependent emission feature, indicates that the C60 molecules act beyond a pure dielectric spacer, their electronic states are heavily involved in the inelastic tunneling process for plasmonic emission. A modified quantum cutoff relation is proposed to explain qualitatively the observed emission feature at both bias polarities. We also demonstrate molecularly resolved optical contrast on the C60 monolayer and discuss the contrast mechanism briefly.

Optical modulation of molecular conductance.

A novel scanning probe microscope stage permits break junction measurements of single molecule conductance while the molecules are illuminated with visible light. We studied a porphyrin-fullerene dyad molecule designed to form a charge separated state on illumination. A significant fraction of illuminated molecules become more conductive, returning to a lower conductance in the dark, suggesting the formation of a long-lived charge separated state on the indium-tin oxide surface. Transient absorption spectra of these molecular layers are consistent with formation of a long-lived charge separated state, a finding with implications for the design of molecular photovoltaic devices.

Electrochemical scanning tunneling microscopy.

The electrochemical scanning tunneling microscope was the first tool for the investigation of solid-liquid interfaces that allowed in situ real space imaging of electrode surfaces at the atomic level. Therefore it quickly became an important addition to the repertoire of methods for the determination of the local surface structure as well as the dynamics of reactions and processes taking place at surfaces in an electrolytic environment. In this short overview we present several examples to illustrate the powerful capabilities of the EC-STM, including the observation of clean metal surfaces as well as the adsorption of thin metal layers, specifically adsorbed anions and non-specifically adsorbed organic cations. In several cases the electrode potential has a significant influence on structure and reactivity of the surface that can be explained by the observations made with the EC-STM.

Scanning tunneling microscopy of insulators and biological specimens based on lateral conductivity of ultrathin water films.

Scanning tunneling microscopy is based on the flow of an electrical current and thus cannot be used to directly image insulating material. It has been found, however, that a very thin film of water (about one monolayer) adsorbed to a surface exhibits a surprisingly high conductivity that is sufficient to allow scanning tunneling microscope imaging at currents below 1 picoampere. Hydrophilic insulators, such as glass and mica, can thus be imaged in humid air. The same is true for biological specimens deposited on such surfaces, as demonstrated by the scanning tunneling microscope imaging of plasmid DNA on mica.

Vacancy island creation and coalescence using automated scanning tunneling microscopy.

We demonstrate that scanning tunneling microscope tip-surface crash events can be utilized as an efficient means for the creation of predefined island configurations for diffusion studies. Using this method, islands of varying size can be created and placed in close proximity, increasing the probability of initiating and observing coalescence events. Data obtained from crash initiated events on a Ag(111) surface are presented. Relaxation time exponents extracted from these data confirm that our method gives results consistent with previous, sputter-obtained island coalescence studies. We also describe an instrument-control routine developed for these measurements that utilizes commercial imaging and off-the-shelf automation software to automate the tracking of islands or other features by the microscope.

Tip-enhanced near-field Raman spectroscopy with a scanning tunneling microscope and side-illumination optics.

Conventional Raman spectroscopy (RS) suffers from low spatial resolution and low detection sensitivity due to the optical diffraction limit and small interaction cross sections. It has been reported that a highly localized and significantly enhanced electromagnetic field could be generated in the proximity of a metallic tip illuminated by a laser beam. In this study, a tip-enhanced RS system was developed to both improve the resolution and enhance the detection sensitivity using the tip-enhanced near-field effects. This instrument, by combining RS with a scanning tunneling microscope and side-illumination optics, demonstrated significant enhancement on both optical sensitivity and spatial resolution using either silver (Ag)-coated tungsten (W) tips or gold (Au) tips. The sensitivity improvement was verified by observing the enhancement effects on silicon (Si) substrates. Lateral resolution was verified to be below 100 nm by mapping Ag nanostructures. By deploying the depolarization technique, an apparent enhancement of 175% on Si substrates was achieved. Furthermore, the developed instrument features fast and reliable optical alignment, versatile sample adaptability, and effective suppression of far-field signals.

High-yield synthesis of conductive carbon nanotube tips for multiprobe scanning tunneling microscope.

We have established a fabrication process for conductive carbon nanotube (CNT) tips for multiprobe scanning tunneling microscope (STM) with high yield. This was achieved, first, by attaching a CNT at the apex of a supporting W tip by a dielectrophoresis method, second, by reinforcing the adhesion between the CNT and the W tip by electron beam deposition of hydrocarbon and subsequent heating, and finally by wholly coating it with a thin metal layer by pulsed laser deposition. More than 90% of the CNT tips survived after long-distance transportation in air, indicating the practical durability of the CNT tips. The shape of the CNT tip did not change even after making contact with another metal tip more than 100 times repeatedly, which evidenced its mechanical robustness. We exploited the CNT tips for the electronic transport measurement by a four-terminal method in a multiprobe STM, in which the PtIr-coated CNT portion of the tip exhibited diffusive transport with a low resistivity of 1.8 kOmega/microm. The contact resistance at the junction between the CNT and the supporting W tip was estimated to be less than 0.7 kOmega. We confirmed that the PtIr thin layer remained at the CNT-W junction portion after excess current passed through, although the PtIr layer was peeled off on the CNT to aggregate into particles, which was likely due to electromigration or a thermally activated diffusion process. These results indicate that the CNT tips fabricated by our recipe possess high reliability and reproducibility sufficient for multiprobe STM measurements.

Active mechanical noise cancellation scanning tunneling microscope.

We present the design and performance of an active mechanical noise cancellation scanning tunneling microscope (STM). This system features two key parts: a "twin-tip" scanner and an active mechanical noise cancellation algorithm. The twin-tip scanner functions as two independent STMs which share nearly the same mechanical transfer function, allowing both STMs to sense nearly identical background mechanical noise. Based on an adaptive digital signal processing technique, the active mechanical noise cancellation algorithm applies the noise sensed by the first STM to concurrently cancel the noise in the second STM and hence allows the second STM to acquire spectroscopy with a significantly improved signal to noise ratio. This system demonstrates long-term stability of the tip-sample tunnel junction and improved spectroscopy measurement in a mechanically noisy environment.

Development of probe-to-probe approach method for an independently controlled dual-probe scanning tunneling microscope.

We developed a method of fast probe-to-probe approach for an independently controlled dual-probe scanning tunneling microscope (STM), which is essential to measure the transport property of nanostructures, without scanning electron microscopy (SEM). In the approach method, inchworm motors are used as the coarse positioning devices, which are controlled with a personal computer. The method enables an automatic approach of the probe to the other probe within a short time (typically 30 min). After the approach, a real distance between contact points of each probe tip to a sample can be measured from the overlapped part of the STM images obtained with individual probe. The approach method without SEM is also useful to measure the charge transport in the atmosphere, which will be essential for measurement of the bio molecules.

Variable-temperature independently driven four-tip scanning tunneling microscope.

The authors have developed an ultrahigh vacuum (UHV) variable-temperature four-tip scanning tunneling microscope (STM), operating from room temperature down to 7 K, combined with a scanning electron microscope (SEM). Four STM tips are mechanically and electrically independent and capable of positioning in arbitrary configurations in nanometer precision. An integrated controller system for both of the multitip STM and SEM with a single computer has also been developed, which enables the four tips to operate either for STM imaging independently and for four-point probe (4PP) conductivity measurements cooperatively. Atomic-resolution STM images of graphite were obtained simultaneously by the four tips. Conductivity measurements by 4PP method were also performed at various temperatures with the four tips in square arrangement with direct contact to the sample surface.

Fiber optic light collection system for scanning-tunneling-microscope-induced light emission.

We report a compact light collection scheme suitable for retrofitting a scanning tunneling microscope (STM) for STM-induced light emission experiments. The approach uses a pair of optical fibers with large core diameters and high numerical apertures to maximize light collection efficiency and to moderate the mechanical precision required for alignment. Bench tests indicate that efficiency reduction is almost entirely due to reflective losses at the fiber ends, while losses due to fiber misalignment have virtually been eliminated. Photon-map imaging with nanometer features is demonstrated on a stepped Au(111) surface with signal rates exceeding 10(4) counts/s.

Simultaneous scanning tunneling microscopy and stress measurements to elucidate the origins of surface forces.

We have developed a new combined measurement system to investigate the underlying origins of forces on solid state surfaces from the viewpoint of atomic surface morphology. This system consists of two main parts: the measurements of force based on displacements and detailed atomic resolution observations of the surface morphology. The former involves a large sample cantilever and a capacitive detection method that provide sufficient resolution to detect changes of a few meV/atom or pN/atom at surfaces. For the latter, a scanning tunneling microscope was incorporated to observe structural changes occurring on the surface of the cantilever sample. Although this combined observation is not trivial, it was accomplished by carefully designing sample dimensions while suppressing the self-oscillation of the cantilever. To demonstrate the performance of this system a preliminary study of the room temperature adsorption of Br(2) on the clean Si(111)-7x7 surface is presented.

Ultrahigh vacuum, variable temperature, dual scanning tunneling microscope system operating under high magnetic field.

We present a dual scanning tunneling microscope (DSTM) system operating between 2.2 K and room temperature, in a split-coil superconducting magnetic field up to 12 T and in ultrahigh vacuum. The DSTM consists of two compact STMs, each having x, y, and z coarse positioning piezoelectric steppers with embedded capacitive positioning sensor for navigation. Each STM can be operated independently and can achieve atomic resolution. The DSTM and the sample is configured in a way that allows the magnetic field orientation to be varied continuously from normal to parallel to the sample surface. Together with the sample, the DSTM can form a nanometer scale three terminal setup for transport measurement.

Construction of a versatile ultralow temperature scanning tunneling microscope.

We constructed a dilution-refrigerator (DR)-based ultralow temperature scanning tunneling microscope (ULT-STM) which works at temperatures down to 30 mK, in magnetic fields up to 6 T and in ultrahigh vacuum (UHV). Besides these extreme operation conditions, this STM has several unique features not available in other DR-based ULT-STMs. One can load STM tips as well as samples with clean surfaces prepared in an UHV environment to a STM head keeping low temperature and UHV conditions. After then, the system can be cooled back to near the base temperature within 3 h. Due to these capabilities, it has a variety of applications not only for cleavable materials but also for almost all conducting materials. The present ULT-STM has also an exceptionally high stability in the presence of magnetic field and even during field sweep. We describe details of its design, performance, and applications for low temperature physics.