Electrochemical Quartz Crystal Nanobalance Research Articles

Exosomal Biomarkers-Based Biosensors for the Noninvasive Early Cancer Detection

Exosomes are extracellular vesicles released from cells, which contain in their membranes many ligands/receptors and membrane proteins, such as tetraspanins or major histocompatibility complex (MHC), that characterize the cell of origin. They can also carry various biomarkers depending on the type and state of the originating cell (proteins, mRNA, miRNA).Recently, it has been found, that the serum-derived exosomes from patients with malignant gliomas contain survivin protein (Sur) on their surface [1]. Survivin is the smallest member of the inhibitor of apoptosis proteins (IAP) family. It is strongly expressed in embryos and malignant tumors but very weakly in the normal cells. Due to its active role in tumor growth and metastasis, Sur became a prime cancer biomarker [2].It was also found by Li at al. [3] that the exosomal programmed cell death ligand 1 (Exo-PD-L1) level was correlated with non-small cell lung cancer (NSCLC) disease progression. The programmed cell-death ligand 1 (PD-L1) is an immuno-suppressive molecule commonly upregulated on the surface of tumor cells.In this work, we have investigated the interactions of proteins expressed on the surface of exosomes with the anti-CD274 and anti-survivin antibodies immobilized on a gold piezoelectrode using the electrochemical quartz crystal nanobalance (EQCN) technique.The biosensors developed in this work have enabled gaining further insides into the peculiar nature of the exosomes and their great potential for enhancing clinical diagnostics and therapy by identifying cancer biomarkers for early cancer detection without any invasive biopsies.We would like to thank for financial support from the National Science Centre (NCN), Poland: Program OPUS Grant No. 2017/25/B/ST4/01362.

Selective conversion of CO into ethanol on Cu(511) surface reconstructed from Cu(pc): Operando studies by electrochemical scanning tunneling microscopy, mass spectrometry, quartz crystal nanobalance, and infrared spectroscopy

A polycrystalline copper, surface-terminated by a well-defined (511)-oriented facet, was electrochemically generated by a series of step-wise surface reconstruction and iterations of mild oxidative-reductive processes in 0.1 M KOH. The electrochemical reduction of CO on the resultant stepped surface was investigated by four surface-sensitive operando methodologies: electrochemical scanning tunneling microscopy (STM), electrochemical quartz crystal nanobalance (EQCN), differential electrochemical mass spectrometry (DEMS), and polarization-modulation infrared spectroscopy (PMIRS). The stepped surface catalyzed the facile conversion of CO into ethanol, the exclusive alcohol product at a low overpotential of −1.06 V (SHE) or − 0.3 V (RHE). The chemisorption of CO was found to be a necessary prelude to ethanol production; i.e. the surface coverages, rather than solution concentrations, of CO and its surface-bound intermediates primarily dictate the reaction rates (current densities). Contrary to the expected predominance of undercoordinated step-site reactivity over the coordination chemistry of vicinal surfaces, vibrational spectroscopic evidence reveals the involvement of terrace-bound CO adsorbates during the multi-atomic transformations associated with the production of ethanol.

Role of Additive Structure on Controlling Ceria/Substrate Interactions Relevant to Shallow Trench Isolation (STI) Chemical Mechanical Planarization (CMP)

Chemical Mechanical Polishing (CMP) has emerged as a critical process step for achieving global planarization in advanced integrated circuit manufacturing and has lead to the extension of Moore’s Law. Specifically, Shallow Trench Isolation (STI) CMP is used in the electrical isolation of the active components in an integrated circuit by actively removing TEOS and stopping on Si3N4 to achieve angstrom level uniformity and limit defectivity. Planarization of the overburden material is achieved through a balance of an applied mechanical force and the delivery of a colloidal dispersion (slurry) containing ceria (CeO2) nanoparticles, complexing additives, and rheology modifiers to the substrate surface. Currently, STI slurries contain chemical additives that selectively boost material removal rate (MRR) as well as enhance selectivity via non-covalent passivation films formed at the polishing substrate surface. A crucial factor in maintaining a high MRR is the presence of CeO2 surface oxygen vacancies, commonly associated with Ce3+surface oxidation state, which can be achieved either from pre-dispersion particle processing or though additive/nanoparticle surface reactions present in the slurry formulation. This study probed the impact of the slurry additive functionality, such as carboxylic acids, amines, and phenols, on the removal rate and post-CMP defectivity. Results suggest that the Ce3+ oxidation state is critical to performance but the Ce4+ oxidation state is also a viable reaction site as long as it remains uncomplexed and free of any site-blocking additives. The additive adsorption to the CeO2 surface will result in altering the dynamic interplay of the abrasive particle/chemistry at the substrate surface shifting CMP performance. Employing and correlating a suite of modified analytical methods, such as atomic force microscopy (AFM), electrochemical quartz crystal nanobalance (EQCN), dynamic contact angle measurements, fluorescence microscopy, pre/post characterization of particle properties (size and zeta potential), and dynamic contact angle, a strong correlation between surface energy and CMP performance metrics has been uncovered.

(Invited) Unraveling Synergistic Interactions driving the Chemical Mechanical Planarization Process

Due to the emergence of sub-10 nm technologies, next generation CMP slurry formulations have continued to increase in additive (nanoparticle and chemistry) complexity to meet stringent device specifications. Therefore, it is essential to probe the molecular level interactions at the nanoparticle/slurry chemistry/substrate interface and in turn correlate them to key performance metrics such as removal rate, post CMP defects, planarization efficiency. More specifically, this work has developed a suite of techniques that explore the adsorption/complexation dynamics that evolve during the CMP process. Employing a suite of techniques such as modified atomic force microscopy (AFM), electrochemical quartz crystal nanobalance (EQCN), dynamic contact angle measurements, fluorescence microscopy, pre/post polish characterization of particle properties (size and zeta potential), and in-situ rotating disk electrode open circuit potential measurement the critical static and dynamic mechanisms can be exposed. Correlation of this information gives way to modulation (desired or undesired) of the CMP performance potentially providing tunabilty (process and formulation) at advanced technology nodes to meet stringent electrical, topography, and defect requirements. Furthermore, these methods have shown great promise as a vehicle to probe the molecular level interactions at the nanoparticle/chemistry/ substrate interface. Results have shown excellent correlation to key process performance metrics and have provided insight into relevant surface chemistry changes that impact critical interactions during the CMP process. This presentation will address key interactions through a series of case studies focusing on probing the impact of inhibitor structure on the substrate surface energy relevant to Cu CMP, the role of slurry chemistry on controlling defectivity during STI post-CMP cleaning, and effect of organic additives on the modulation of nanoparticle surface properties to enhance STI CMP performance.

Probing the Effect of Point of Use Filtration on Cu CMP Slurry Health

Chemical mechanical planarization (CMP) is a process that balances chemical surface modification and mechanical removal utilizing a nanoparticle dispersion (slurry) to limit defects and irregularities on the wafer surface. Implementing point of use (POU) filtration prior to the CMP process is crucial in capturing rogue particle aggregates that are known to induce defects such as scratching and pitting. As technology continues to advance CMP slurries are increasing in complexity, making it necessary to unravel the dynamic synergy between the formulated slurry and filtration media. More specifically, this work set out to investigate the non-covalent interactions such as hydrogen bonding, pi interactions, or dispersion forces that occur at the interface of a fundamental Cu CMP slurry and a polyamide and polypropylene based media. The slurry utilized in this study contained SiO2 abrasive particles due to its selectivity for Cu, glycine as a complexing agent, benzotriazole (BTA) as a passivating agent, H2O2 as an oxidizing agent, and pH adjusters. It has been known that Cu-glycine complexes increase the hydroxyl radical concentration, which in turn enhances the MRR. Initial results revealed that there was a subtle difference in the MRR post-filtration independent of the media used, meaning that both filters were capable of interacting with the chemistry in the slurry during the filtration process. In order to determine what chemistry was being removed through adsorption the following techniques were employed as a function of the filtration process, dynamic light scattering (DLS), zeta potential, electrochemical quartz crystal nanobalance (EQCN), UV-Vis spectroscopy, potentiodynamic scan analysis, and measuring Cu removal rate. Ultimately, results show that both polymeric media, despite the difference in their characteristics (backbone structure, hydrophilicity, and pore size) were not only effective in removing large particle aggregates but were able to alter the slurry integrity by removing key complexing chemistry, which significantly impacts the Cu film formation and removal processes.

Unraveling Slurry Chemistry/Nanoparticle/Polymeric Membrane Adsorption Relevant to Cu Chemical Mechanical Planarization (CMP) Filtration Applications

Due to the emergence of sub-10 nm technologies, next generation slurries have continued to increase in complexity to meet stringent device performance demands. Prior to the chemical mechanical planarization (CMP) process, point-of-use filtration (POU) is implemented in order to limit particle aggregates and ultimately decrease surface defects. This study probes the non-covalent interactions at the interface of a fundamental Cu slurry and a polyamide and polypropylene-based membranes. Results indicate that independent of the membrane used, material removal rate (MRR) showed a subtle decrease as a result of filtration (time and ΔP), demonstrating that the synergistic balance between the nanoparticle and slurry additives is disrupted during the filtration process. Corrosion current measurements (Icorr) decreased by at least 85% post-filtration, indicating a rapid adsorption of glycine to the filter membrane. Regardless of the filter membrane, glycine adsorption was further validated using a modified electrochemical quartz crystal nanobalance (EQCN) technique. Since Cu-glycine complexes are integral in controlling MRR, a widely reported method of tracking *OH production was employed. Results show a decrease in the concentration of *OH, which in turn can be correlated to a decrease in the Cu-glycine complexes, altering the overall Cu MRR.

Probing Interactions at the Polymeric Filtration Media/ CMP Slurry Interface Using Dynamic Electrochemical Quartz Crystal Nanobalance (EQCN) and Atomic Force Microscopy (AFM)

As devices continue to shrink, the demands on chemical mechanical polishing (CMP) become more stringent, requiring slurries to be modulated in order to enhance surface quality. Achieving angstrom level uniformity and minimizing defects, results in the need to develop polymeric filtration media that also increases polishing efficiency. Probing the interactions that can occur at the particle/polymer/slurry interface is essential in order to understand the impact of altering the chemistry and the filtration media on the filterability, rate of diffusion, adhesion force, surface roughness, and activation energy (EA). This work focuses on investigating the interactions between common abrasive particles, such as ceria and silica (in various chemistries), as well as different polymeric filtration media through the use of an electrochemical quartz crystal nanobalance (EQCN) and atomic force microscopy (AFM). The AFM tip is effectively functionalized in solution using epoxy glue to mimic a nanoparticle, which is used to measure the adhesion force between the particle and membrane via force distance curves. These nanoparticles can be reduced, oxidized, or functionalized and can be used to measure the adhesion force at different filtration time intervals, monitoring the change in the particle-polymer interaction. Preliminary results suggest that altering the complexing agent in the slurry can heavily influence the EA of the interaction between the slurry and filter membrane. Specifically, the activation energy for a copper CMP slurry and an amine-based filter was found to be negative, indicating a barrier-less reaction, due to non-covalent interactions present at the slurry-polymer interface. Changing the complexing agent from glycine to L-alanine and β-alanine revealed the EA to be positive and negative, respectively, which also changed with the structure of the membrane. Ultimately, both the backbone of the filter and the complexing agent alter the interactions on the surface of the polymer membrane and have shown to have a direct impact on CMP performance.

Potential-Dependent Adsorption of CO and Its Low-Overpotential Reduction to CH3CH2OH on Cu(511) Surface Reconstructed from Cu(pc): Operando Studies by Seriatim STM-EQCN-DEMS

Operando scanning tunneling microscopy first revealed that application of a CO2-reduction potential to a Cu(pc) electrode in 0.1 M KOH resulted in the reconstruction of the selvedge to an x-layer stack of well-ordered Cu(100) terraces, Cu(pc)-x[Cu(100)]. Subsequent Cu↔Cu2O oxidation-reduction cycles between −0.90 V and 0.10 V SHE converted the reconstructed region to a stepped Cu(S)-[3(100) × (111)], or Cu(511), surface. Differential electrochemical mass spectrometry showed that reduction of CO produced only CH3CH2OH at the lowest overpotential. Later application of STM and surface infrared spectroscopy uncovered a potential, above which no CO adsorption occurs. In this study, electrochemical quartz crystal nanobalance was combined with STM and DEMS as a prelude to the acquisition of CO coverages as continuous functions of concentration and potential; in heterogeneous catalysis, surface coverage are important since the reaction rate are functions of those quantities. Also equally critical is the knowledge of the packing arrangement at the onset of the reaction because, if “CO dimers” were indeed the precursors to C2+ products, reduction can only be initiated when the adlayer consists of closely packed CO; otherwise, dimerization will not transpire if the molecules were far apart. The results indicate that the catalysis lags the adsorption, and starts only when CO adsorption is saturated.

Towards Practical Applications of EQCN Experiments to Study Pt Anchor Sites on Carbon Surfaces

This work investigates the viability and outlines the current challenges in electrochemical quartz crystal nanobalance (EQCN) experiments on supported Pt catalysts. EQCN experiments involving Pt supported on 2-D “surface-treated graphite sputtered onto quartz crystal” (Pt/MFG-H) catalysts were compared to standard polycrystalline Pt (Ptpoly), which showed similarities in frequency versus potential trends; however, the Pt/MFG-H catalysts obtained higher frequencies due to the support capacitance. The physical characterizations (XRD and XPS) and electrochemical responses, mainly cyclic voltammetry in acidic media and the ferri/ferrocyanide couple, of the 2-D Pt/MFG-H were compared to the representative 2-D Pt supported on treated highly orientated pyrolytic graphite (Pt/HOPG-H), in order to make assertions on the similarities between the two catalysts. The XRD diffraction patterns and the XPS valence band structure for the treated and untreated MFG (-H and -P, respectively) and HOPG (-H and -P, respectively) demonstrated similarities. Nevertheless, the cyclic voltammograms and peak positions of the ferri/ferrocyanide couple between the treated and untreated MFG and HOPG catalysts were dissimilar. However, EQCN may be used qualitatively between the two different 2-D catalysts since the same trends in electrochemical responses before and after treatment of the MFG and HOPG catalysts were seen. Hence, the EQCN technique can be used in future studies as an alternative method to study degradation mechanisms of Pt and carbon for PEFCs.Graphical ᅟ

Influence of the Surface Roughness of Platinum Electrodes on the Calibration of the Electrochemical Quartz-Crystal Nanobalance.

The electrochemical quartz-crystal nanobalance (EQCN) is an in situ technique that measures mass changes (Δm) associated with interfacial phenomena. Analysis of Δm sheds light on the mass balance (in addition to the charge and energy balances) and provides new insight into the nature of electrochemical processes. The EQCN measures changes in frequency (Δf) of a quartz-crystal resonator, which are converted into Δm using the Sauerbrey equation containing the characteristic constant (Cf). The value of Cf is determined by physical parameters of the crystal and refers to an atomically smooth surface. However, real resonators are not smooth and electrodes have their intrinsic roughness. Thus, the conversion of Δf to Δm should be done using an experimentally determined characteristic constant (Cf,exp) for a given value of the surface roughness factor (R). Here, we calibrate the system using Ag electrodeposition on Pt electrodes of gradually increasing R; the latter is adjusted through Pt electrodeposition. The surface morphology of the Pt substrates prior to and after Ag electrodeposition is examined using atomic force microscopy. The values of Cf,exp are determined by analyzing the slopes of charge density versus Δf plots for the Ag electrodeposition. They are different than Cf and increase logarithmically with R. The Cf and Cf,exp values are used in a comparative analysis of the mass changes (δΔm) for complete cyclic voltammetry profiles covering the 0.05-1.40 V range. This reveals that the employment of Cf instead of Cf,exp provides inaccurate values of δΔm, and the magnitude of the discrepancy increases with R.

Study of the surface mass changes during the redox transformations of copper(II) phthalocyanine-tetrasulfonic acid on gold in acidic media

An electrochemical quartz crystal nanobalance (EQCN) has been used to study the deposition (adsorption) and dissolution (desorption) during the redox transformations of the water-soluble copper phthalocyanine-tetrasulfonic acid (CuPcTS) on a gold electrode in contact with aqueous acid solutions. The anomalous cyclic voltammetric responses that had been observed invoked the application of the EQCN technique. Indeed, it became evident that during the electroreduction in certain potential ranges, deposition/dissolution processes take place on the gold surface. Surprisingly, complex changes have been observed even when the cyclic voltammetric curves seem to be regular, i.e., characteristic to the responses of soluble redox pairs. Beside the cyclic voltammetry, EQCN was combined with galvanostatic measurements detecting the chronopotentiometric curves with the simultaneous mass changes. A reaction sequence has been suggested for the elucidation of the events occurring during the reduction and reoxidation of CuPcTS.

Electrochemical Quartz Crystal Nanobalance (EQCN) Based Biosensor for Sensitive Detection of Antibiotic Residues in Milk.

An electrochemical quartz crystal nanobalance (EQCN), which provides real-time analysis of dynamic surface events, is a valuable tool for analyzing biomolecular interactions. EQCN biosensors are based on mass-sensitive measurements that can detect small mass changes caused by chemical binding to small piezoelectric crystals. Among the various biosensors, the piezoelectric biosensor is considered one of the most sensitive analytical techniques, capable of detecting antigens at picogram levels. EQCN is an effective monitoring technique for regulation of the antibiotics below the maximum residual limit (MRL). The analysis of antibiotic residues requires high sensitivity, rapidity, reliability and cost effectiveness. For analytical purposes the general approach is to take advantage of the piezoelectric effect by immobilizing a biosensing layer on top of the piezoelectric crystal. The sensing layer usually comprises a biological material such as an antibody, enzymes, or aptamers having high specificity and selectivity for the target molecule to be detected. The biosensing layer is usually functionalized using surface chemistry modifications. When these bio-functionalized quartz crystals are exposed to a particular substance of interest (e.g., a substrate, inhibitor, antigen or protein), binding interaction occurs. This causes a frequency or mass change that can be used to determine the amount of material interacted or bound. EQCN biosensors can easily be automated by using a flow injection analysis (FIA) setup coupled through automated pumps and injection valves. Such FIA-EQCN biosensors have great potential for the detection of different analytes such as antibiotic residues in various matrices such as water, waste water, and milk.

Interfacial structure of atomically flat polycrystalline Pt electrodes and modified Sauerbrey equation.

The electrochemical quartz-crystal nanobalance (EQCN) measures in situ mass changes associated with interfacial electrode processes. Real electrodes are not atomically flat, thus their surface roughness affects the conversion of frequency variations (Δf) to mass changes (Δm) associated with electrochemical processes. Here, we analyze Δm associated with the electrochemical H adsorption/desorption and surface oxide formation/reduction on Pt electrodes of gradually increasing surface roughness using the EQCN and cyclic-voltammetry in an aqueous H2SO4 solution. These two interfacial processes are ideal to probe changes in the electrochemically active surface area. The surface roughness of Pt-coated resonators is fine-tuned through Pt electrodeposition and examined using atomic force microscopy. The results acquired using Pt electrodes of increasing roughness factor (1.61 ≤ R ≤ 13.0) reveal a linear relationship between Δm and R. Extrapolation of this relationship to R = 1.00 leads to the determination of Δm associated with H adsorption/desorption and oxide formation/reduction on an atomically flat polycrystalline Pt electrode. The values of Δm associated with these processes are analyzed in terms of the number of H, O, water, and ionic species interacting with each Pt atom of the electrode surface. We find that the charge densities associated with these electrochemical processes and mass variations do not scale up by the same factor. This leads to a modified version of the Sauerbrey equation for Pt electrodes, which takes into account the intrinsic surface roughness.

Limits of Detection and Quantification of Electrochemical Quartz-Crystal Nanobalance in Platinum Electrochemistry and Electrocatalysis Research.

The electrochemical quartz-crystal nanobalance has been used in electrochemistry research for over three decades. It provides an atomic/molecular level insight into the nature of interfacial electrochemical phenomena by measuring in situ mass changes on the nanogram scale. The sensitivity of this technique remains unknown because there have been no attempts to determine its limits of detection (LOD) or quantification (LOQ). We propose an experimental approach for determining the values of LOD and LOQ for Pt electrodes in aqueous H2SO4 solutions that employs cyclic voltammetry and frequency variation measurements. However, this methodology is also appropriate to other electrode materials and electrolytes. The LOD and LOQ values depend on the electrolyte concentration and decrease (i.e., the sensitivity increases) as the concentration decreases. Knowledge of the LOD and LOQ values determines the applicability of this technique in research on the oxidation and degradation of Pt catalysts employed in fuel cells.

Preliminary Advancement for an Electrochemical Biosensor for Early Diagnosis of Alpha-1-Antitrypsin Deficiency

Electrochemical biosensors present a potential advance in the field of disease diagnosis by providing immediate and accurate results in the detection of pre-specified targets. In most cases, the successful diagnosis and treatment of diseases is reliant upon out-of-date testing instruments that are no longer adequate. For this reason, it is important to explore the novel combination of established testing methods with modern technology. For the case of alpha-1-antitrypsin (A1AT) deficiency (A1AD), a complete diagnosis currently requires a minimum of three individual tests performed using blood serum. The use of a specialized biosensor to detect a deficiency and the form of A1AT could potentially have a dramatic increase in diagnosing carriers of genetic mutations in the A1AT gene sequence that predispose them to early-onset emphysema (1). The main purpose of A1AT is to counter human neutrophil elastase (HNE). HNE is responsible for the removal of harmful bacteria in the lungs as well as other undesirable particles that may be present. When there is a lack of A1AT resulting in A1AD, the HNE will become in relative excess, resulting in the degradation of elastin, the elastic material of the lungs which may lead to emphysema development (2). Due to the challenges of diagnosing A1AD, it is often discovered after one exhibits symptoms of emphysema. The three separate tests that are currently used to diagnose A1AD are the determination of A1AT in the blood, followed by protein phenotyping, and then A1AT genotyping (3). All three tests are time intensive and require a patient’s serum for sampling. The use of an electrochemical biosensor could replace the need for all three of these tests within one instrument. The biosensor would incorporate an electrochemical quartz crystal nanobalance (EQCN) with an electrode surface that has been functionalized with a biological agent that would bind to an analyte that confirms A1AD. A visual representation of the electrode surface is provided in Figure 1 for the binding of an analyte on the functionalized gold surface. Lomant’s Reagent, dithiobis(succinimidyl propionate) (DSP), has been used in several other biosensors to functionalize the gold surface of the electrode (4). DSP is a preferred reagent due to the functionality provided by the carboxyl group as well as the length of the carbon backbone. An electrochemical biosensor allows for the measurement of a biological sample through redox responses by measuring both conductivity or resistance through the electrode as well as any changes in mass on the surface of the electrode by the EQCN. The dual functionality of the EQCN in measuring the electric and mass parameters allows for accurate detection of biological analytes and greatly reduces the concern for false negatives or positives. It is the goal of this work to develop and test an electrochemical biosensor with an EQCN that will be readily available in the medical field to allow accurate and timely results for the detection of A1AD. This technology has the potential to increase early detection and diagnosis of many different diseases which allows for early treatment and could potentially improve outcomes for those afflicted. Initial work has been performed to build a firm foundation of knowledge of the responses of the EQCN in appropriate media. Calibration testing has been established for the quartz crystals through numerous experiments. Thus far, the study is progressing into attaching DSP to the quartz crystal surface and measuring the electrochemical response of the system in preparation for biological analyte detection. Figure 1

Probing the Role of Slurry Chemistry on Nanoparticle-Media Adsorption Relevant to Cu CMP Filtration Applications

As the complexity of nanoparticle dispersions used in next-generation CMP slurries increases to meet stringent performance demands, it is necessary to develop polymeric filtration media that capture rouge particles in order to reduce overall surface defectivity without sacrificing planarization efficiency. Therefore, it is essential to probe the molecular level interactions at the nanoparticle/slurry chemistry/polymer filtration media interface and in turn correlate this to the key performance metrics such as substrate removal rate, post CMP defects, planarization efficiency. More specifically, simulated Cu CMP slurries with a wide range of film formation chemistries for performance control were evaluated with respect to the filterability through various polymeric filtration media. It was determined that a synergy exists between the structure of the additive (small organic molecule vs. surfactant-like additive) and the filter medias ability to regulate the dynamic adsorption processes of the slurry nanoparticles. Utilizing atomic force microscopy (AFM), electrochemical quartz crystal nanobalance (EQCN), modified ATR-IR spectroscopy, and in-situ rotating disk electrode corrosion measurements the static and dynamic nanoparticle binding mechanisms can be determined and correlated to the filterability studies. Furthermore, monitoring the pre/post filtration zeta potential and particle size distributions, bulk copper removal rate, and corrosion current as a function of filtration pressure, the role of filtration media slurry chemistry health can be surveyed on CMP performance based scale.

Nanoparticle-Polymer Surface Adsorption Relevant to CMP Applications

Chemical Mechanical Polishing (CMP) has emerged as a critical technology for the planarization of precision optical components, data storage media, and integrated circuit semiconductor devices. This process is a delicate balance between chemical surface modification and mechanical removal utilizing a nanoparticle dispersion (slurry) to remove defects/irregularities with the goal to attain angstrom level uniformity. As technology continues to advance, next generation nanoparticle dispersions and CMP slurries will increase in complexity to meet stringent performance demands, making it necessary to develop polymeric media that reduce overall surface defectivity and increase planarization efficiency. To uncover dynamics amongst the slurry additives, abrasives, polymeric interfaces, and the CMP substrate the molecular level interactions at the nanoparticle-polymer interface need to be explored. This work set out to explore these interaction mechanisms by using a modified electrochemical quartz crystal nanobalance (EQCN) technique. This modified technique utilizes thin film deposition to place a polymer on the electrode and then by applying the Sauerbrey equation it is possible to gain insight about the surface adsorption at the nanoparticle-polymer interface. The synergistic interaction of the slurry chemistry and the polymeric substrate is further explored to determine the role of surface reactions that alter the polymeric medium’s surface energy and subsequently alter the nanoparticle binding mechanisms.

Identification and Analysis of Electrochemical Instrumentation Limitations through the Study of Platinum Surface Oxide Formation and Reduction.

Anodic polarization of Pt electrodes in aqueous H2SO4 leads to the formation of a surface oxide (PtO). Herein, the surface oxide growth is accomplished using three different approaches: (i) chronoamperometry (CA); (ii) chronocoulometry (CC); and (iii) a combination of cyclic voltammetry (CV) and CA. The PtO reduction is accomplished potentiodynamically using voltammetry. The oxide growth takes place at defined polarization potentials (E(p)), polarization times (t(p)), and temperatures (T). The oxide charge density (q(ox)) is determined for both the formation (q(ox,form)) and reduction (q(ox,red)) processes. The oxide reduction CV profiles are integrated to determine the charge density values for oxide reduction (q(ox,red,CV)) which are compared with the q(ox,form,CA) and q(ox,form,CC) values. The values of q(ox,form,CC) are greater than those of q(ox,form,CA), but both potentiotatic methods (CA and CC) produce q(ox,form) values that are consistently lower than those of q(ox,red,CV). In the case of oxide formation with combined CV and CA, the values of q(ox,form,CV+CA) are found to be lower than the values of q(ox,red,CV), although the difference is small. Electrochemical quartz crystal nanobalance (EQCN) is used to monitor the mass variation at the electrode surface during the oxide formation and reduction process at E(p) = 1.20 V with various t(p) values. Equal mass changes during oxide formation and reduction are detected by the EQCN. The nature of the differences in q(ox,form) and q(ox,red) encountered with the different experimental methods are discussed in terms of instrumental limitations.

Tailoring polypyrrole supercapacitive properties by intercalation of graphene oxide within the layer

In this work polypyrrole was prepared using different solutions of graphene oxide as the supporting electrolyte. Two different samples of graphene oxide (GO1 and GO2) were prepared from different graphite precursors. The obtained samples were characterised by means of scanning electron microscope (SEM), atomic force microscope (AFM) and X-ray photoemission spectroscopy (XPS), which showed a significant difference between the two structures considering their morphology characteristics, sheet size and type of oxygen functionalities. However, all the graphene oxide samples used in this work contain appropriate amounts of COO⿿ groups essential for compensation of positive charge generated at the polymer backbone.The polymer layers were prepared by cyclic voltammetry and chronoamperometry methods from GO1 (PPy/GO1), GO2 (PPy/GO2) and dialysed GO2 (PPy/D-GO2) supporting electrolyte solutions. The examinations were carried out in appropriate graphene oxide or Na2SO4 solutions. The results obtained reveal that the layers are electrochemically active in graphene oxide solution. However, the polymer response is significantly improved in Na2SO4 solution. Electrochemical quartz crystal nanobalance (EQCN) measurements suggest that graphene oxide is involved in the polypyrrole redox reaction while the morphology characteristics suggest that graphene oxide is incorporated within the polymer layer. The properties of the layer depend on the type of graphene oxide solution used during the synthesis. The specific capacitance value of the PPy/D-GO2 layer prepared by the chronoamperometry method was found to be dependent on the deposited material mass and for 1.20 to 0.38μg of polypyrrole it ranged between 149.51 and 221.76Fg⿿1, respectively.

Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt

The effect of elastic strain on catalytic activity of platinum (Pt) toward oxygen reduction reaction (ORR) is investigated through dealloyed Pt–Cu thin films; stress evolution in the dealloyed layer and the mass of the Cu removed are measured in real-time during electrochemical dealloying of (111)-textured thin-film PtCu (1:1, atomic ratio) electrodes. In situ stress measurements are made using the cantilever-deflection method, and nanogravimetric measurements are made using an electrochemical quartz crystal nanobalance. Upon dealloying via successive voltammetric sweeps between −0.05 and 1.15 V vs standard hydrogen electrode, compressive stress develops in the dealloyed Pt layer at the surface of thin-film PtCu electrodes. The dealloyed films also exhibit enhanced catalytic activity toward ORR compared with polycrystalline Pt. In situ nanogravimetric measurements reveal that the mass of dealloyed Cu is approximately 210 ± 46 ng/cm2, which corresponds to a dealloyed layer thickness of 1.2 ± 0.3 monolayers or 0.16 ± 0.04 nm. The average biaxial stress in the dealloyed layer is estimated to be 4.95 ± 1.3 GPa, which corresponds to an elastic strain of 1.47% ± 0.4%. In addition, density functional theory calculations have been carried out on biaxially strained Pt(111) surface to characterize the effect of strain on its ORR activity; the predicted shift in the limiting potentials due to elastic strain is found to be in good agreement with the experimental shift in the cyclic voltammograms for the dealloyed PtCu thin film electrodes.