Peter Broekmann

Atomic structures and dynamics of a Cu(100) electrode in dilute hydrobromic acid: An in situ STM study

Abstract The surface electrochemistry of Cu(100) in 10 mM hydrobromic acid electrolyte has been studied by means of cyclic voltammetry and in situ STM. In the potential range between the onset of the anodic copper dissolution at positive and the hydrogen evolution at negative electrode potentials, the CV of Cu(100) in 10 mM HBr is characterized only by the double-layer charge. Within this potential regime a highly ordered (√2×√2)R45°-superstructure is seen in the STM experiments assigned to specifically adsorbed bromide anions. No desorption of the bromide adlayer has been found in these STM experiments even at extremely negative potentials at the onset of hydrogen evolution. Therefore the bromide desorption potential is concluded to lie within the potential regime of massive hydrogen evolution at even more negative potentials. Adsorbed bromide induces a drastic restructuring and faceting of the surface topography depending on the applied potential. The driving force of this process is the formation of thermodynamically favored copper steps aligned parallel to close packed 〈100〉 directions of the bromide adsorbate. Dynamic processes like copper dissolution and deposition are also strongly influenced by the geometry of the (√2×√2)R45° bromide adlayer. Corrosion as well as deposition of copper material follows the close packed 〈100〉 directions of the bromide adsorbate. For moderate reaction rates an additional anisotropy between the [001]- and [010]-direction is observed due to the nonequivalence of two different kinds of bromide stabilized copper steps. The origin of these two kinds of steps is the phase relation of close packed adsorbate rows of adjacent terraces. The deposition of copper material does not only start at the lower but unusually, also at the upper sites of step edges leading to the formation of microfacets. Not only the growth of monoatomically high islands is observed but also a double-layer and multilayer growth of copper.

Electrochemical control of single-molecule conductance by Fermi-level tuning and conjugation switching.

Controlling charge transport through a single molecule connected to metallic electrodes remains one of the most fundamental challenges of nanoelectronics. Here we use electrochemical gating to reversibly tune the conductance of two different organic molecules, both containing anthraquinone (AQ) centers, over >1 order of magnitude. For electrode potentials outside the redox-active region, the effect of the gate is simply to shift the molecular energy levels relative to the metal Fermi level. At the redox potential, the conductance changes abruptly as the AQ unit is oxidized/reduced with an accompanying change in the conjugation pattern between linear and cross conjugation. The most significant change in conductance is observed when the electron pathway connecting the two electrodes is via the AQ unit. This is consistent with the expected occurrence of destructive quantum interference in that case. The experimental results are supported by an excellent agreement with ab initio transport calculations.

STM investigation of specific anion adsorption on Cu(111) in sulfuric acid electrolyte

The reversible specific adsorption of sulfate on Cu(111) was investigated using atomic resolution scanning tunneling microscopy (STM). Continuous change of the sample potential during a scan allowed imaging surface phase transitions. The appearance and disappearance of a sulfate adlayer forming a Moire pattern were observed.

Surface redox chemistry of adsorbed viologens on Cu(100)

The surface redox-chemistry of adsorbed viologens is studied by means of cyclic voltammetry (CV) in combination with in situ scanning tunneling microscopy (STM). 1,1′-Dibenzyl-4,4′-bipyridinium molecules (DBV2+) adsorb on a chloride modified Cu(100) electrode surface under formation of a laterally well ordered 2D array of supramolecular cavitand ensembles. Each cavitand consists at least of 4 individual DBV2+ sub-units which are arranged in a certain circular manner making this supramolecular cavitand chiral. Both possible enantiomeric forms are found in two mirror domains at the surface. Reducing the di-cationic DBV2+(ads) species to the corresponding radical mono-cation DBV•+(ads) causes a phase transition from the pre-existing DBV2+(ads) cavitand phase to a stripe pattern following a nucleation and growth mechanism. DBV•+(ads) species are adsorbed with their main molecular axis parallel to the surface in a side-on adsorption geometry. Enhanced intermolecular π–π-interactions are identified as the main driving force for the formation of 1D oligomer and polymer chains as the characteristic structural motif of the DBV•+(ads) phase. These structural motifs are generally independent of the electronic and structural substrate properties. Chloride desorption through the viologen film is discussed as the reason for an order–disorder transition within the viologen film at even more negative potentials.

Electrochemical CO2 Reduction - A Critical View on Fundamentals, Materials and Applications.

The electrochemical reduction of CO(2) has been extensively studied over the past decades. Nevertheless, this topic has been tackled so far only by using a very fundamental approach and mostly by trying to improve kinetics and selectivities toward specific products in half-cell configurations and liquid-based electrolytes. The main drawback of this approach is that, due to the low solubility of CO(2) in water, the maximum CO(2) reduction current which could be drawn falls in the range of 0.01-0.02 A cm(-2). This is at least an order of magnitude lower current density than the requirement to make CO(2)-electrolysis a technically and economically feasible option for transformation of CO(2) into chemical feedstock or fuel thereby closing the CO(2) cycle. This work attempts to give a short overview on the status of electrochemical CO(2) reduction with respect to challenges at the electrolysis cell as well as at the catalyst level. We will critically discuss possible pathways to increase both operating current density and conversion efficiency in order to close the gap with established energy conversion technologies.

Charge Transport in C60-Based Dumbbell-type Molecules: Mechanically Induced Switching between Two Distinct Conductance States

Single molecule charge transport characteristics of buckminsterfullerene-capped symmetric fluorene-based dumbbell-type compound 1 were investigated by scanning tunneling microscopy break junction (STM-BJ), current sensing atomic force microscopy break junction (CS-AFM-BJ), and mechanically controlled break junction (MCBJ) techniques, under ambient conditions. We also show that compound 1 is able to form highly organized defect-free surface adlayers, allowing the molecules on the surface to be addressed specifically. Two distinct single molecule conductance states (called high G(H)(1) and low G(L)(1)) were observed, depending on the pressure exerted by the probe on the junction, thus allowing molecule 1 to function as a mechanically driven molecular switch. These two distinct conductance states were attributed to the electron tunneling through the buckminsterfullerene anchoring group and fully extended molecule 1, respectively. The assignment of conductance features to these configurations was further confirmed by control experiments with asymmetrically designed buckminsterfullerene derivative 2 as well as pristine buckminsterfullerene 3, both lacking the G(L) feature.

Suppressor Effects during Copper Superfilling of Sub- 100 nm Lines

The effects of suppressor and accelerator on the superfilling of copper are studied for two commercial chemistries. The potential transients during galvanostatic plating were obtained for injections of the industrially recommended dose of accelerator and various doses of suppressor. The potential increase immediately after the injection was found to be strongly dependent on the type of suppressor as well as the amount of suppressor injected. While the transient of the full dose of suppressor represents what happens at the mouth of the feature and the field outside the feature, the bottom of the feature can be simulated by the case in which a fraction of suppressor is injected. The filling results in sub-100 nm lines are well correlated with the differences observed in the potential transients.

ATOMIC STRUCTURES OF A Cu(111) SURFACE UNDER ELECTROCHEMICAL CONDITIONS: AN IN-SITU STM STUDY

The atomic structures of a Cu(111) electrode in dilute sulfuric electrolyte have been studied using in-situ STM. At anodic potentials near the copper dissolution the adsorbed sulfate anions form a characteristic anisotropic Moire pattern. The appearance of the long range Moire modulation is explained by a sulfate-induced reconstruction (expansion) of the topmost copper layer and the resulting misfit between the first and the second copper layer. For the first time it was possible to image not only the sulfate adsorbate but also the underlying reconstructed copper substrate at the same anodic working potential by a systematical variation of the tunneling parameters. On an atomic scale the observed sulfate structure on Cu(111) is very similar to those found for other fcc(111) surfaces (Au, Pt and Rh). On these electrodes sulfate anions form a regular superstructure. For the Cu(111) surface a -like unit cell is found which is slightly distorted. Also for the first time it was possible to image the atomic structure of an electrode during a massive hydrogen evolution current at cathodic potentials. Even under such extreme electrochemical conditions far from the thermodynamic equilibrium an ordered superstructure of hydronium cations is found. At the anodic end of the cyclic voltammogram a pit-etching mechanism is induced by a fast copper corrosion.

Electrochemical reactions at a porphyrin–copper interface

The structure and reactivity of a Cu(100) single crystal electrode surface covered with free base meso-tetra (N-methyl-4-pyridinium) porphyrin (abbreviated as H(2)TMPyP) as a function of electrode potential have been investigated with cyclic voltammetry (CV), electrochemical scanning tunneling microscopy (ECSTM), and UV-Vis and Raman spectroscopy. The well-ordered self-assembled layer of the porphyrin is consistent with the adsorption of the reduced porphyrin species after the first two-electron reduction step. The copper dissolution reaction in the presence of the stable self-assembled porphyrin layer starts at step edges on both upper and lower terraces and coincides with the preferential oxidation of reduced porphyrin species at step sites. The dissolved copper cations are incorporated into the free base porphyrin molecules leading to the formation of CuTMPyP. As a consequence this new species accumulates in the solution with time and a copper redeposition in the cathodic potential scan is lacking.

High-Resolution Chemical Depth Profiling of Solid Material Using a Miniature Laser Ablation/Ionization Mass Spectrometer

High-resolution chemical depth profiling measurements of copper films are presented. The 10 μm thick copper test samples were electrodeposited on a Si-supported Cu seed under galvanostatic conditions in the presence of particular plating additives (SPS, Imep, PEI, and PAG) used in the semiconductor industry for the on-chip metallization of interconnects. To probe the trend of these plating additives toward inclusion into the deposit upon growth, quantitative elemental mass spectrometric measurements at trace level concentration were conducted by using a sensitive miniature laser ablation ionization mass spectrometer (LIMS), originally designed and developed for in situ space exploration. An ultrashort pulsed laser system (τ ∼ 190 fs, λ = 775 nm) was used for ablation and ionization of sample material. We show that with our LIMS system, quantitative chemical mass spectrometric analysis with an ablation rate at the subnanometer level per single laser shot can be conducted. The measurement capabilities of our instrument, including the high vertical depth resolution coupled with high detection sensitivity of ∼10 ppb, high dynamic range ≥10(8), measurement accuracy and precision, is of considerable interest in various fields of application, where investigations with high lateral and vertical resolution of the chemical composition of solid materials are required, these include, e.g., wafers from semiconductor industry or studies on space weathered samples in space research.