Electrochemical Scanning Tunneling Spectroscopy Research Articles

Reversible Redox‐Driven Crystallization in a Paracyclophane Monolayer at a Solid–Liquid Interface

AbstractThe development and integration of cyclophanes into future functional materials require a detailed understanding of the physicochemical principles that underlie their properties, phase behavior, and in particular the relationship between structure and function. Here, electrochemically switchable crystallization of a ferrocene‐bearing 3D Janus tecton (M‐Fc) at the interface between highly oriented pyrolytic graphite (HOPG) and an electrolyte solution is demonstrated. The M‐Fc adlayer is successfully visualized under both ambient and electrochemical conditions using scanning tunneling microscopy. Voltammetric measurements show a surface‐confined redox process for the M‐Fc modified surface that drives the phase transition between a visible 2D ordered linear phase (M‐Fc0, with ferrocene in the neutral state) and an invisible gas‐like adsorption layer with high mobility when ferrocene is oxidized, M‐Fc+, and a “square scheme” mechanism explains the data. Analogous experiments in a ferrocene‐free tecton adlayer show no phase transition and confirm that the dynamics in M‐Fc are redox‐driven. On‐surface 3D nanoarchitectures are also demonstrated by forming inclusion complexes between M‐Fc and β‐cyclodextrin and device behavior through electrochemical scanning tunneling spectroscopy (STS). These results showcase the functional potential of this class of cyclophanes, which can find use in actuators, optical crystals, and other smart materials.

The Protein Matrix of Plastocyanin Supports Long-Distance Charge Transport with Photosystem I and the Copper Ion Regulates Its Spatial Span and Conductance.

Charge exchange is the fundamental process that sustains cellular respiration and photosynthesis by shuttling electrons in a cascade of electron transfer (ET) steps between redox cofactors. While intraprotein charge exchange is well characterized in protein complexes bearing multiple redox sites, interprotein processes are less understood due to the lack of suitable experimental approaches and the dynamic nature of the interactions. Proteins constrained between electrodes are known to support electron transport (ETp) through the protein matrix even without redox cofactors, as the charges housed by the redox sites in ET are furnished by the electrodes. However, it is unknown whether protein ETp mechanisms apply to the interprotein medium present under physiological conditions. We study interprotein charge exchange between plant photosystem I (PSI) and its soluble redox partner plastocyanin (Pc) and address the role of the Pc copper center. Using electrochemical scanning tunneling spectroscopy (ECSTS) current-distance and blinking measurements, we quantify the spatial span of charge exchange between individual Pc/PSI pairs and ETp through transient Pc/PSI complexes. Pc devoid of the redox center (Pcapo) can exchange charge with PSI at longer distances than with the copper ion (Pcholo). Conductance bursts associated with Pcapo/PSI complex formation are higher than in Pcholo/PSI. Thus, copper ions are not required for long-distance Pc/PSI ETp but regulate its spatial span and conductance. Our results suggest that the redox center that carries the charge in Pc is not necessary to exchange it in interprotein ET through the aqueous solution and question the canonical view of tight complex binding between redox protein partners.

Mapping the electronic structure of polypyrrole with image‐based electrochemical scanning tunneling spectroscopy

AbstractConducting polymers are semiconductors whose applications cover a wide range of devices. Their versatility is due, in addition to other factors, to properties that can be easily modulated according to the intended application. It is therefore important to study and map the electronic structure of these materials for a better correlation between structure and properties. Electrochemical scanning tunneling spectroscopy (EC‐STS) can be a powerful tool to characterize the electronic structure of the semiconductor interface. In this work, we have used image‐based EC‐STS (IB‐EC‐STS) to describe quantitatively the band structure of an electrochemically deposited polypyrrole film. IB‐EC‐STS located the band edge of the polymer's valence band (VB) at 0.95 V vs. RHE (‐5.33 eV in the absolute potential scale) and the intragap polaron states formed when the polymer is doped, at 0.46 V vs. RHE (‐4.84 eV). The IB‐EC‐STS data were cross‐checked with electrochemical impedance spectroscopy (EIS) and Mott‐Schottky analysis of the interfacial capacitance. The DOS spectrum obtained from EIS data is consistent with the STS‐deduced location of the VB and the polarons.

Controlled assembly and single electron charging of monolayer protected Au144 clusters: an electrochemistry and scanning tunneling spectroscopy study.

Single gold particles may serve as room temperature single electron memory units because of their size dependent electronic level spacing. Here, we present a proof-of-concept study by electrochemically controlled scanning probe experiments performed on tailor-made Au particles of narrow dispersity. In particular, the charge transport characteristics through chemically synthesized hexane-1-thiol and 4-pyridylbenzene-1-thiol mixed monolayer protected Au(144) clusters (MPCs) by differential pulse voltammetry (DPV) and electrochemical scanning tunneling spectroscopy (EC-STS) are reported. The pyridyl groups exposed by the Au-MPCs enable their immobilization on Pt(111) substrates. By varying the humidity during their deposition, samples coated by stacks of compact monolayers of Au-MPCs or decorated with individual, laterally separated Au-MPCs are obtained. DPV experiments with stacked monolayers of Au(144)-MPCs and EC-STS experiments with laterally separated individual Au(144)-MPCs are performed both in aqueous and ionic liquid electrolytes. Lower capacitance values were observed for individual clusters compared to ensemble clusters. This trend remains the same irrespective of the composition of the electrolyte surrounding the Au(144)-MPC. However, the resolution of the energy level spacing of the single clusters is strongly affected by the proximity of neighboring particles.

In situ ESTM and ESTS studies of the nanostructure and energetic properties of Fe-20% Cr-40% Ni and Fe-20% Cr-70% Ni alloy surfaces

Fe-20% Cr-40% Ni and Fe-20% Cr-70% Ni alloys are studied in air and in 0.01 N HCl and 0.1 N H2SO4 solutions at the controlled potentials of the specimen and needle using in situ techniques of electrochemical scanning tunneling microscopy (ESTM) and electrochemical scanning tunneling spectroscopy (ESTS). The changes in the potential of Fe-20% Cr-40% Ni alloy specimen fall in ranges of −0.3 to 1.1 V and 0–0.3 V (N.H.E.) in 0.01 N HCl and 0.1 N H2SO4, respectively, and the changes in the potential of Fe-20% Cr-70% Ni specimen, in ranges of 0.09–0.94 V and −0.1 to 0.5 V in 0.01 N HCl and 0.1 N H2SO4, respectively. Local spectral dependences of the tunnel current on the tunnel voltage are obtained and processed and the coefficients that characterize the local electrophysical properties of the surfaces are calculated on their basis. It is found that, in the studied acid solutions at the controlled potential, the surfaces are more energetically homogeneous than in air, where a substantially larger dispersion of the properties is observed. It is concluded that the energetic characteristics of separate surface sites of these alloys are determined on the atomic scale by the mutual effects of the neighboring atoms involved in its composition, i.e., by the matrix effect, which was discovered previously in the case of Kh18N10T stainless steel [1].

ESTM study of nanostructure and surface energetic properties of individual chromium and nickel

Individual chromium and nickel in air and at interfaces to 0.01 N HCl and 0.1 N H2SO4 solutions are studied with the use of electrochemical scanning tunneling microscopy (ESTM) and electrochemical scanning tunneling spectroscopy (ESTS) at the controlled potentials of the specimen and tip. The potential range of chromium specimens was 0.1 to 0.9 V in 0.01 N HCl and 0.3 to 1.1 V in 0.1 N H2SO4; the potential range of nickel was −0.01 to 0.64 V in 0.01 N HCl and 0.184 to 0.834 V (N.H.E.) in 0.1 N H2SO4. At the controlled potential, the surfaces in the studied acid solutions are smoother than in air. The potential of the specimen affects the electrophysical properties, which determine the electron tunneling transfer, more noticeably than the nanorelief of the metals. The obtained changes in the It,dZ spectra dependence on the distance in air on a field of 1 nm2 at a step of 0.04 nm enabled us to estimate the effective barrier of tunneling Kef between neighbor atoms, which is in good agreement with the known dependence of Kef on the work function.

Nanostructure and energy properties of the stainless steel X18N10T surface studied by scanning tunneling microscopy and scanning tunneling spectroscopy. 2. Measurements at different potentials in HCl and H2SO4 solutions

Stainless steel Kh18N10T is studied by electrochemical scanning tunneling microscopy (ESTM) and electrochemical scanning tunneling spectroscopy (ESTS) at the interface with solutions of 0.01 N HCl and 0.1 N H2SO4 at the controlled sample and tip potentials. The sample potential varied from −0.46 to +1.14 V in 0.01 N HCl and from −0.06 to +0.74 V (NHE) in 0.1 N H2SO4. It is shown that although in the studied acid solutions at the controlled potential the surface is smother than in air, this does not allow one to determine the extremal values of parameters α, β, and γ corresponding to individual properties of Fe, Cr, and Ni or their oxides. It is shown that compared to the steel nanorelief, the sample potential has a stronger effect on the electrophysical properties that determine the tunneling transfer of electrons.

Scanning tunneling microscopic and scanning tunneling spectroscopic studies of nanostructure and surface energy properties of stainless steel Kh18N10T. 1. Measurements at interfaces with air and 0.01 N HCl solution

Stainless steel #Kh18N10T is studied by electrochemical scanning tunneling microscopy (ESTM) and electrochemical scanning tunneling spectroscopy (ESCS) methods at the interface with air and for the fixed sample and probe potentials in 0.1 N HCl. Local spectral dependences of tunneling current on the tunneling voltage ( I t , U t ) are studied in either air or solution in the vicinity of potentials corresponding to stable properties. The local tunneling conductivity ( G O ), the coefficient of the probability of tunneling transfer of an electron to the tip ( α ), and the logarithmic-dependence slope ( β = U t / ln ( I t ) are calculated. As for Fe-Cr alloys, it is shown that at the atomic level, the local energy properties of a steel #Kh18N10T are determined by the mutual effect of neighboring atoms (the matrix effect).