This research was initiated yet in my pre-Binghamton career with focus the role of controlled defect interactions on the thin-film growth modes taking place at electrode / electrolyte interface. The ultimate goal of the research is to learn how to manipulate growth parameters such that layer-by-layer or self-organized islanding growth becomes the kinetically preferred growth mode. Inspired by earlier work in ultrahigh vacuum the effort on this project resulted in the development of two approaches for epitaxial thin film growth. The first one termed electrochemical defect-mediated growth relies on co-deposition of the metal of interest with a reversibly deposited mediator metal. This work was published in the journal Science (1999). In another approach called surfactant-mediated growth, we exploited the ability of a pre-deposited monolayer fraction of a "surfactant" metal to float on the surface of the depositing metal thereby facilitating the interlayer transport in the course of growth.
With my joining the ranks of BU-SUNY faculty the long-term research
interest in epitaxial thin film growth motivated a new and different research
activity emphasizing the development of a new method for epitaxial growth by
Surface Limited Redox replacement (SLRR). This method has been developed in
parallel with the founder of Electrochemical Atomic Layer deposition, John
Stickney (University of Georgia) and is based upon multiple application of a "building block"
deposition event consisting of decoupled potential-controlled deposition
of "sacrificial" monolayer of metal U (U = Tl, Cd, Pb and Bi) and
electroless redox replacement of this layer by more-noble metal ions Gz+
(G = Ag+, Au3+, Pt2+, Pd2+
and Cu2+). While similar approaches have been first introduced
by R. Adzic et al. (Brookhaven National Laboratory) for sub-monolayer to a
monolayer surface modification, the new outcome that warrants the novelty of
our study is associated with the application of this strategy for metal thin
film deposition. Proof-of-concept experiments employing the replacement of
Pb sacrificial layers for the growth of Ag and Cu on Au (111) validated SLRR as
a viable pathway toward the deposition of stoichiometrically adequate number of
epitaxial monolayers with perfectly uniform surface morphology and thickness
controlled in the sub-nanometer range. Results of this stage were published in Electrochemical
and Solid State Letters (2005), Journal of
the Electrochemical Society (2006), and Journal of Physical Chemistry C (2007).
Currently, in collaboration
with a group at University of Bristol, SLRR protocols are being studied
and successfully employed for controlled deposition of thin alloy films and
metal multilayers aimed at final applications in hydrogen catalysis and
electronic industry. Work in this direction has been done on the application of
the SLRR method to the homo-and hetero-epitaxial growth of Pt and Au ultra thin
films. These activities clearly validate the applicability of SLRR to systems
of practical interest and served as a road map to further interdisciplinary
projects associated with application in the design of new catalysts and their
testing in fuel cell and battery applications. Recent results relevant to this
topic were published in Langmuir (2011),
Electrochimica Acta (2012 and 2013), and Electrocatalysis (2013). The fundamental importance
of the proposed program is manifested primarily by the unique nature of the
new SLRR method that for the first time combines sequential
potential-controlled and electroless steps to encompass a single deposition
event. Challenges that have surfaced
with the SLRR development reveal a diverse phenomenology and invoke more complex approaches for
understanding the balance between thermodynamic and kinetic factors governing
the "building block" reaction. Ongoing fundamental research addressing some of
these challenges include experiments aimed at understanding the immediate
mechanism of SLRR, deposition effort in systems with more noble metal growing
on less noble substrates, deposition of functional alloys, and application of
SLRR for coating of high aspect ratio porous structures. This method is now
being applied for the growth of thin metal films and/or multilayers of Ag, Au, Cu,
Pt and Pd by many research groups in the USA and worldwide.
(2006, Materials World Network, funded in by NSF-DMR)
De-alloying or selective alloy dissolution is a solid-state separation process in which the most electrochemically active constituent is selective removed from the system. This process results in the formation of a nanoporous structure composed almost entirely of the more-noble alloy constituents. Earlier results summarizing the progress of the analytical, simulation and experimental work on this subject were reported in Nature (2001) magazine.
Electrochemical selective alloy dissolution and potential controlled cementation have been employed since the very beginning of my appointment at BU-SUNY to design a variety of porous structures at nanometer length scales. In this research understanding the role of different factors controlling the porous structure is strongly emphasized. Key points of interest are associated with both, the transition from nucleated clusters/ligaments to 3D porosity structure with a defined length scale and the limitations in the growth evolution of the porous layer in vertical direction. While a striking similarity is observed between morphologies generated by de-alloying and potential controlled cementation the differences in the length scale in identical systems processed by both approaches is not yet understood. Work to understand the mechanism and limitations of these phenomena was done by the development of potential controlled displacement method based on Cu-Ag and Cu-Au systems. It was clearly seen that the developing length scale in accordingly processed nanoporous Ag and Au was coarser in immiscible metal systems (Cu-Ag) and finer in systems where both components demonstrate ideal solubility (Cu-Au). Results and discussion on controlling factors, resulting morphology retention of the less noble component and extent of length scale tunability of the accordingly processed structures have been presented in Journal of the Electrochemical Society (2007 and 2010), Langmuir (2008), Journal of Physical Chemistry C (2009).
Ongoing collaborations of my research
group with the University of South Australia (UniSA) and with Sofia University
(SU), Bulgaria have been essential to the assessment of the applied aspect of
this effort. The joint work with UniSA includes
summer exchange of graduate students and is aimed at hydrophobzing porous
substrates processed by the above means relying on the periodical surface
roughness to render them suprhydrophobic ("lotus leaf" effect). The SU
collaboration is dedicated to studying and understanding of the de-alloying in
amorphous metal mixtures (alloys) and to the development of alloys for hydrogen
storage. Papers published through the collaborative activities include, Langmuir
(2009), Journal of Alloys and Compounds (2009), Journal of Power
Sources (2008) and International Journal of Hydrogen Energy (2008).
(2013, funded in by
NSF-Chemistry, Catalysis)
Motivated by the ever-increasing demand for development of cost-effective, highly active and durable catalysts with application in fuel cells, batteries and environmental remediation, a research founded on my expertise with nanoporous metals, introduces all-electrochemical synthetic approach for catalysts that could be an alternative to the most established nanoparticle based ones. The new synthetic route enables production of ultra thin and continuous nanoporous catalysts with tunable thickness and length scale of interconnected porosity along with reliable composition control and high processing efficiency. The protocols under development include electrodeposition of ultra thin and continuous film of single-phase alloy followed by a selective electrochemical dissolution (de-alloying) intended for the less-noble component removal. The de-alloying generates a nanoporous metal (NPM) with thickness of less than 20 nm. Depending upon its nature, the NPM could then be either used as catalyst or electrochemically functionalized with a catalytically active layer ensuring complete coating coverage over the NPM matrix. Accordingly synthesized catalysts will be assessed for activity and durability in established tests for oxygen reduction reaction, organic fuel oxidation, and nitrate electroreduction. The work on each catalyst of interest is separated in four sequential modules with development (first two), functionalization and application emphasis respectively. Initial work of my group relevant to this matter has been published in ACS Advanced Materials and Interfaces (2011), Journal of Physical Chemistry C (2012), Langmuir (2011), and Electrochimica Acta (2013).
Challenges that have emerged with preliminary results of this method development like (i) identifying the chemistry and plating conditions for deposition of single-phase alloys of interest, (ii) electrodepositing thin and continuous alloy layer on carbon based surfaces, (iii) ensuring reliable adhesion of these layer to the carbon carrier, (iv) generating a continuous coating of more noble functional layers on less noble NPM frameworks, and (v) optimizing the functional layer composition for best catalytic performance, reveal a variety of thermodynamic and kinetic limitations and invoke complex basic research approaches for understanding the balance between factors impacting the respective processes. Testing and assessment of the catalytic performance of accordingly synthesized catalysts has been based on known protocols like formic acid oxidation, oxygen reduction and nitrate reduction. My group's work in catalysis is summarized in papers published in Journal of the Electrochemical Society (2003), Analytical Chemistry (2008), Angewandte Chemie (2010), Electrocatalysis (2013), and ACS Catalysis (2013).
The development of electroplating technologies for Cu has been research focus of the activity of many groups in the last two decades. It has been shown that a viable electroplating bath would mandatory contain Cl- ions and organic additives that render the plated Cu particularly suitable for fabrication of high reliability interconnect structures. Solder joints coupling these interconnect structures have been considered to be highly reliable for decades, but the advent of Pb-free solder and higher processing temperatures has revealed that Cu/solder joints are susceptible to premature failure under shock loading. This failure is associated with void formation at the Cu/solder intermetallic compound interface. Proprietary work pointed to void nucleation associated exclusively with electroplated Cu. A reason for the voiding was sought in impurity incorporation. Our initial work involved careful Cu electroplating experiments that ascertained a clear correlation between the voiding propensity and controlled plating parameters.
In a recent study, Cu samples were electroplated galvanostatically from a generic solution, containing Cl- ions, as well as a suppressor (Polyethylene Glycol, PEG), and a brightener (bis(3-sulfopropyl) disulfide, SPS) as additives. Overpotential transients were measured during electroplating with a wide range of current densities in baths with various compositions. Effects of the bath chemistry on the Cu surface morphology, as well as on the propensity for voiding after soldering, were also investigated. Elemental analysis of selected samples was performed by SIMS. A key achievement was the control gained over Cu growth of "void-proof" and "void-prone" deposits. The trends in voiding were attributed to different rates of consumption of PEG and SPS and respective changes in the contaminants being incorporated in the deposits. It was also found that differences in the voiding behavior could be predicted by monitoring characteristic overpotential transient signatures. Thus it was ascertained that we can control electroplating baths at the laboratory scale so as to vary the degree of voiding, i.e. to control the degree of badness of plated Cu samples. The latest research assessed effects of temperature, grain orientation, bath chemistry and age as parameters that one way or another govern the outcome of the growth process. A comprehensive analysis combining the results of this study establishes a clear correlation between plating rate, temperature, overpotential, and voiding propensity. Based on this correlation, ranges of overpotentials where either "void prone" or "void-free" Cu could be deposited are clearly identified in two separate acidic Cu plating solutions containing different additive combinations. The proposed analysis enables further prediction and control of the voiding propensity of solder joints with electrodeposited Cu layers. Recent double-layer and impedance measurements are carried out to reveal details on the correlation between the potential of zero charge of different copper faces and possible preferential incorporation of impurities during the Cu deposition. Results of this activity were published in Journal of Applied Electrochemistry (2008 and 2011), IEEE Transactions (2009), Journal of the Electrochemical Society (2010), and Journal of Electronic Materials (2012).
In earlier developments the UPD in the systems Cu2+/AgxAu(1-x) (111), Ag+/CuxAu(1-x) (111) and Pb2+/Cu-Al poly was investigated as a function of the alloy composition. A linear dependence of the UPD coverage on the composition was found in the case of ideal separation of the alloying constituents. A power law function was found to describe the UPD as a function of the alloy composition in the case of a randomly mixed alloy. These findings were successfully applied as an analytical tool for determining the alloy composition of the investigated substrates.
During my pre-tenured time at BU-SUNY, an analytical method was developed exploiting simultaneous NO3-ion electroreduction and metal UPD on Cu substrates for the analysis and monitoring of heavy metal content in natural waters. A quantitative study and modeling work shed light on such scenario taking place on Cu (111) electrode at open circuit potential. The method is based on observations that a complete Pb UPD layer inhibits the electroreduction of NO3- ions on bare Cu (111) electrode observed earlier in a purely kinetic study. It has been found that an inexpensive polycrystalline copper electrode is sensitive enough for analytical detection of lead traces in electrolytes down to 1x10-8 M. Similar approach is now being considered for analysis of trace amounts of Tl and Sn. Also, more recent research has been focused on the quantitative development of a method determination of surface area on porous metal substrates. Similarly to the gas-phase BET method, this approach takes advantage of the surface limited nature of the UPD process leading to the formation of exactly one monolayer of foreign metal. The new method results are thus obtained by comparison of UPD coverage on high-surface area flat metal surfaces. Recent developments have allowed for exploring the quantitative aspects of the new method including comprehensive validation and comparison with results from modeling work on the same subject. The nature of substrate, the transport limitations through the porous structure and the decoupling between double-layer charging/discharging and UPD effects were found to play role as limiting factors in that method development. Finally, a method for quantifying the Pt dissolution during testing of catalysts has been also developed. Key results from these activities were published in Journal of Electroanalytical Chemistry (2005), Analytical Chemistry (2008), Journal of Physical Chemistry C (2009), and ACS Catalysis (2013).