Shueh-Lin Yau

Conformations of polyaniline molecules adsorbed on Au(111) probed by in situ STM and ex situ XPS and NEXAFS.

In situ scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) have been used to examine the conformation of a monolayer of polyaniline (PAN) molecules produced on a Au(111) single-crystal electrode by anodization at 1.0 V [vs reversible hydrogen electrode (RHE)] in 0.10 M H(2)SO(4) containing 0.030 M aniline. The as-produced PAN molecules took on a well-defined linear conformation stretching for 500 A or more, as shown by in situ and ex situ STM. The XPS and NEXAFS results indicated that the linear PAN seen at 1.0 V assumed the form of an emeraldine salt made of PAN chains and (bi)sulfate anions. Shifting the potential from 1.0 to 0.7 V altered the shape of the PAN molecules from straight to crooked, which was ascribed to restructuring of the Au(111) electrified interface on the basis of voltammetric and XPS results. In situ STM showed that further decreasing the potential to 0.5 V transformed the crooked PAN threads into a mostly linear form again, with preferential alignment and formation of some locally ordered structures. PAN molecules could be reduced from emeraldine to leucoemeraldine as the potential was decreased to 0.2 V or less. In situ STM showed that the fully reduced PAN molecules were straight but mysteriously shortened to approximately 50 A in length. The conformation of PAN did not recuperate when the potential was shifted positively to 1.0 V.

In-situ scanning tunneling microscopy of bromine adlayers on Pt(111)

Abstract In-situ scanning tunneling microscopy (STM) and cyclic voltammetry have been used to examine chemisorbed Br adlayers on a well-defined Pt(111) surface in a 10 mM KClO4 solution (pH 4) in the presence of 3 mM KBr. The adlayer structures of the chemisorbed Br were found to be potential dependent. Asymmetric and hexagonal (3 × 3)-Br(νBr = 0.444) structures were observed at potentials in the double-layer charging region. These two structures seemed to appear randomly on the Pt(111) surface. No clear transformation between them was observed. Atomic STM images observed at potentials slightly negative with respect to the first characteristic sharp peak were not consistent with the previously proposed model for a (4 × 4) structure. All Br atoms were aligned parallel to the Pt(111) lattice and appeared with an equal corrugation height. Incommensurate-like Br adlayers were believed to form at potentials more negative than the first characteristic sharp peak. The Pt(111)-(1 × 1) structure was clearly discerned in high resolution STM images at potentials near the hydrogen evolution potential. An identical structure was observed in HClO4 and H2SO4 solutions.

Electrochemical etching of Si(001) in NH4F solutions: Initial stage and {111} microfacet formation

In situ scanning tunneling microscopy (STM) has been used to examine the etching of an n‐Si(001) electrode in 0.1 M NH4F. Cathodic polarization facilitated chemical etching of Si(001) to give {111} microfacets as a result of the tendency of Si to form a monohydride terminated surface. Time‐dependent in situ STM atomic images were obtained to demonstrate the preferential etching at the kinks and steps. From the results of the time‐dependent imaging, local etching rates were evaluated for the specific crystallographic directions. A Si(001):H‐(1×1) square structure was also obtained, demonstrating the presence of dihydride configuration in the beginning of the etching.

Atomic Scale Etching Processes of n-Si(111) in NH4F Solutions: In Situ Scanning Tunneling Microscopy

In situ scanning tunneling microscopy (STM) was employed to examine the electrochemical etching process of an n‐Si(111) electrode in dilute NH4F solutions under potential control. Time‐dependent STM images have revealed prominent effects of microscopic structures of Si on the rate of its dissolution. Multiple hydrogen‐terminated Si atoms at the kink and step sites were eroded more rapidly than the monohydride Si step. This presumably resulted from the difference in reactivity of these hydrogen‐terminated Si species. It is demonstrated that the density of kinks plays a main role in controlling the etching rate of Si. In the absence of kinks, not only the monohydride but also the dihydride steps were found to be stable. The etching rate of the monohydride step is substantially increased from a negligible value to 15 nm/min by the introduction of kink sites. The average etching rate for a dihydride step was 32 nm/min. Overall, the difference in the reactivity guides the dissolution of Si in a layer‐by‐layer ...

In-situ scanning tunneling microscopy of well-ordered Rh(111) electrodes

A simple procedure is described for the preparation of an atomically flat surface of Rh(111) without the use of ultrahigh vacuum techniques or iodine protective adlayers, as described in previous literature. The well-defined Rh(111) electrode was characterized by cyclic voltammetry and in-situ scanning tunneling microscopy (STM). A highly reversible feature, the so-called “butterfly” peak, appeared at 0.64 V vs. RHE in the cyclic voltammogram in 0.1 M HClO4. An atomically flat terrace-step structure was consistently observed by STM on the well-ordered Rh(111) surface. Atomic images of a Rh(111)-(1 × 1) structure were found at potentials positive end of the butterfly peak and in the hydrogen adsorption region. Dipping the Rh(111) electrode into a 1 mM iodine solution resulted in the formation of a Rh(111)(√3 × √3)R30°-I overlayer. The electrochemical activation of the Rh(111) surface by potential cycling is also described.

Sensitivity Enhancement for Quantitative Electrochemical Determination of a Trace Amount of Accelerator in Copper Plating Solutions

An accelerator is an indispensable organic additive for the bottom-up filling of copper electroplating in nano- or micro-scale features. However, its effective concentration is too low to be easily determined and controlled. Herein, a new electrochemical analysis method based on self-assembly monolayers of thiol molecules on a gold electrode was developed to accurately determine a trace amount of accelerator. The accelerator employed in copper plating solutions is bis-(3-sulfopropyl) disulfide (SPS), which is the most common accelerator for the filling of vias and trenches of interconnects. The SPS concentration in copper plating solutions ranged from 0.3 to 9.0 ppm. Following selective chemisorption of SPS onto the gold electrode, the SPS-modified gold electrode was transferred into a specific electrolyte composed of CuSO 4 , H 2 SO 4 , polyethylene glycol and chloride ions to run cyclic voltammetry (CV) for copper deposition and stripping. A specific peak current of copper reduction formed in the CV, and its peak area depended on the SPS concentration in the copper plating solution. A good linear calibration line was obtained by using this electrochemical analysis method, which can determine a trace amount of SPS in a concentration range of 0.3-1.0 ppm, which is a significant challenge for traditional instruments.

3-Mercapto-1-propanesulfonic acid and Bis(3-sulfopropyl) disulfide adsorbed on Au(111): in situ scanning tunneling microscopy and electrochemical studies.

3-Mercapto-1-propanesulfonic acid (MPS) and bis(3-sulfopropyl) disulfide (SPS) adsorbed on a Au(111) electrode were studied by using in situ scanning tunneling microscopy (STM). Although the adsorptions of MPS and SPS are known to be oxidative and reductive, respectively, on an Au(111) electrode, these two admolecules behave similarly in terms of phase evolution, surface coverage, potential for stripping, and characteristics of cyclic voltammetry. However, different adsorption mechanisms of these molecules result in different structures. Raising electrode potential causes more MPS and SPS molecules to adsorb, yielding ordered adlattices between 0.67 and 0.8 V (vs reversible hydrogen electrode). The ordered adlattices of MPS and SPS appear as striped and netlike structures with molecules adsorbed parallel to the Au(111) surface. Switching potential to 0.9 V or more positive still does not result in upright molecular orientation, possibly inhibited by electrostatic interaction between the end group of -SO(3)(-) and the Au(111) electrode. Lowering the potential to 0.4 V disrupted the ordered adlayer. Stripping voltammetric experiments show that MPS and SPS admolecules are desorbed from Au(111) at the same potential, suggesting that these molecules are both adsorbed via their sulfur headgroups. The S-S bond in SPS is likely broken upon its adsorption on Au(111).

In situ scanning tunneling microscopy of GaAs(001), (111)A, and (111)B surfaces in sulfuric acid solution

In situ electrochemical scanning tunneling microscopy (STM) was used to examine n‐type GaAs(001), (111)A, and (111)B surfaces in 0.05 M sulfuric acid. Cathodic polarization of the GaAs electrodes effectively inhibited the oxidation of the surface, making it possible to acquire STM images with atomic resolution. Atomically‐flat terrace‐step structures were consistently observed on all surfaces prepared by the chemical etching method. Steps observed on these surfaces are composed of double layers with step heights of 0.28 and 0.33 nm for the (001) and (111) surfaces, respectively. In situ STM atomic images revealed that those surfaces have (1×1) structures with the square and hexagonal lattices, respectively.

In Situ STM of 3-Mercaptopropanesulfonate Adsorbed on Pt(111) Electrode and Its Effect on the Electrodeposition of Copper

The adsorption of 3-mercaptopropanesulfonate (MPS) molecule on a Pt(111) single-crystal electrode and its effect on the deposition of Cu have been examined using in situ scanning tunneling microscopy (STM). MPS admolecules were irreversibly adsorbed in a largely disordered adlayer on bare Pt(lll) in 0.1 M HClO 4 , irrespective of the presence of chloride, the concentration of MPS, and the applied potential. In comparison, the MPS admolecules readily formed a highly ordered molecular structure identified as (4 × 2 √ 3)rect on Pt(111) precoated with a monolayer of Cu adatoms. The MPS admolecules were adsorbed upright on Pt(111). The cyclic voltammetric results show that the MPS adlayer on Pt(111) would inhibit Cu deposition because the addition of 10 μM MPS to the electrolyte of 0.1 M HClO 4 + 1 mM KCl + 1 mM Cu(ClO 4 ) 2 reduced the amount of the Cu deposit by half, even in the presence of chloride. The texture of the Cu deposit also varied with the surface state of the Pt(111) electrode as the Cu film grew in three-dimensional islands and smooth flakelike morphology on the MPS-modified and Cu-coated Pt(111) electrodes, respectively. In situ STM results indicated that the MPS admolecules stayed afloat rather than buried by the Cu deposit.

In situ STM imaging of the structures of pentacene molecules adsorbed on Au(111).

In situ scanning tunneling microscope (STM) was used to examine the spatial structures of pentacene molecules adsorbed onto a Au(111) single-crystal electrode from a benzene dosing solution containing 16-400 microM pentacene. Molecular-resolution STM imaging conducted in 0.1 M HClO(4) revealed highly ordered pentacene structures of ( radical31 x radical31)R8.9 degrees , (3 x 10), ( radical31 x 10), and ( radical7 x 2 radical7)R19.1 degrees adsorbed on the reconstructed Au(111) electrode dosed with different pentacene solutions. These pentacene structures and the reconstructed Au(111) substrate were stable between 0.2 and 0.8 V [vs reversible hydrogen electrode, RHE]. Increasing the potential to E > 0.8 V lifted the reconstructed Au(111) surface and disrupted the ordered pentacene adlattices simultaneously. Ordered pentacene structures could be restored by applying potentials negative enough to reinforce the reconstructed Au(111). At potentials negative of 0.2 V, the adsorption of protons became increasingly important to displace adsorbed pentacene admolecules. Although the reconstructed Au(111) structure was not essential to produce ordered pentacene adlayers, it seemed to help the adsorption of pentacene molecules in a long-range ordered pattern. At room temperature (25 degrees C), approximately 100 pentacene molecules seen in STM images could rotate and align themselves to a neighboring domain in 10 s, suggesting that pentacene admolecules could be mobile on Au(111) under the STM imaging conditions of -150 mV in bias voltage and 1 nA in feedback current.