Resolving single Cu nanoparticle oxidation and Kirkendall void formation with in situ plasmonic nanospectroscopy and electrodynamic simulations

Copper nanostructures are ubiquitous in microelectronics and heterogeneous catalysis and their oxidation is a topic of high current interest and broad relevance. It relates to important questions, such as catalyst active phase, activity and selectivity, as well as fatal failure of microelectronic devices. Despite the obvious importance of understanding the mechanism of Cu nanostructure oxidation, numerous open questions remain, including under what conditions homogeneous oxide layer growth occurs and when the nanoscale Kirkendall void forms. Experimentally, this is not trivial to investigate because when a large number of nanoparticles are simultaneously probed, ensemble averaging makes rigorous conclusions difficult. On the other hand, when (in situ) electron-microscopy approaches with single nanoparticle resolution are applied, concerns about beam effects that may both reduce the oxide or prevent oxidation via the deposition and cross-linking of carbonaceous species cannot be neglected. In response we present how single particle plasmonic nanospectroscopy can be used for the in situ real time characterization of multiple individual Cu nanoparticles during oxidation. Our analysis of their optical response combined with post mortem electron microscopy imaging and detailed Finite-Difference Time-Domain electrodynamics simulations enables in situ identification of the oxidation mechanism both in the initial oxide shell growth phase and during Kirkendall void formation, as well as the transition between them. In a wider perspective, this work presents the foundation for the application of single particle plasmonic nanospectroscopy in investigations of the impact of parameters like particle size, shape and grain structure with respect to defects and grain boundaries on the oxidation of metal nanoparticles

Shedding light on CO oxidation surface chemistry on single Pt catalyst nanoparticles inside a nanofluidic model pore

Investigating a catalyst under relevant application conditions is experimentally challenging and parameters like reaction conditions in terms of temperature, pressure, and reactant mixing ratios, as well as catalyst design, may significantly impact the obtained experimental results. For Pt catalysts widely used for the oxidation of carbon monoxide, there is keen debate on the oxidation state of the surface at high temperatures and at/above atmospheric pressure, as well as on the most active surface state under these conditions. Here, we employ a nanoreactor in combination with single-particle plasmonic nanospectroscopy to investigate individual Pt catalyst nanoparticles localized inside a nanofluidic model pore during carbon monoxide oxidation at 2 bar in the 450-550 K temperature range. As a main finding, we demonstrate that our single-particle measurements effectively resolve a kinetic phase transition during the reaction and that each individual particle has a unique response. Based on spatially resolved measurements, we furthermore observe how reactant concentration gradients formed due to conversion inside the model pore give rise to position-dependent kinetic phase transitions of the individual particles. Finally, employing extensive electrodynamics simulations, we unravel the surface chemistry of the individual Pt nanoparticles as a function of reactant composition and find strongly temperature-dependent Pt-oxide formation and oxygen spillover to the SiO2 support as the main processes. These results therefore support the existence of a Pt surface oxide in the regime of high catalyst activity and demonstrate the possibility to use plasmonic nanospectroscopy in combination with nanofluidics as a tool for in situ studies of individual catalyst particles

Operando detection of single nanoparticle activity dynamics inside a model pore catalyst material

Nanoconfinement in porous catalysts may induce reactant concentration gradients inside the pores due to local conversion. This leads to inefficient active material use since parts of the catalyst may be trapped in an inactive state. Experimentally, these effects remain unstudied due to material complexity and required high spatial resolution. Here, we have nanofabricated quasi-two-dimensional mimics of porous catalysts, which combine the traits of nanofluidics with single particle plasmonics and online mass spectrometry readout. Enabled by single particle resolution at operando conditions during CO oxidation over a Cu model catalyst, we directly visualize reactant concentration gradient formation due to conversion on single Cu nanoparticles inside the “model pore” and how it dynamically controls oxidation state-and, thus, activity-of particles downstream. Our results provide a general framework for single particle catalysis in the gas phase and highlight the importance of single particle approaches for the understanding of complex catalyst materials

Tunable magnetic properties of arrays of Fe(110) nanowires grown on kinetically-grooved W(110) self-organized templates

We report a detailed magnetic study of a new type of self-organized nanowires disclosed briefly previously [B. Borca et al., Appl. Phys. Lett. 90, 142507 (2007)]. The templates, prepared on sapphire wafers in a kinetically-limited regime, consist of uniaxially-grooved W(110) surfaces, with a lateral period here tuned to 15nm. Fe deposition leads to the formation of (110) 7 nm-wide wires located at the bottom of the grooves. The effect of capping layers (Mo, Pd, Au, Al) and underlayers (Mo, W) on the magnetic anisotropy of the wires was studied. Significant discrepancies with figures known for thin flat films are evidenced and discussed in terms of step anisotropy and strain-dependent surface anisotropy. Demagnetizing coeffcients of cylinders with a triangular isosceles cross-section have also been calculated, to estimate the contribution of dipolar anisotropy. Finally, the dependence of magnetic anisotropy with the interface element was used to tune the blocking temperature of the wires, here from 50K to 200 K

An African-American Resources and Needs Assessment of the West Bloomington Area, Phase 1

Over the course of the Fall 2009 semester, students in the Community Project Design and Management class worked with members of West Bloomington to recognize the growing needs within the community. Along with the African-American Focus Workgroup (AAFW) and the West Bloomington Revitalization Partnership (WBRP), the students completed Phase 1 of an African-American resources and needs assessment resulting in this report. This document reflects needs, wants, and hopes of the West Bloomington community and the AAFW. Comprehensively, this document is meant to serve as a bridge between community members and AAFW members. As Bloomington as a whole continues to grow and expand, some members within the community feel the difficulty of being left out of development. Therefore, the project‟s purpose was to help ascertain the assets and deficiencies within the West Bloomington neighborhood with the participation of community members, and to help the AAFW integrate their goals with those of the WBRP. This report can aid future efforts and provide useful information in the development of the West Bloomington area. The report includes a brief review of the literature considering race, education, income, age, public space, housing, civil society, and community-based participatory research. The report also includes sections outlining the research methods used, the research findings along the same themes used in the literature review, a discussion of limitations and barriers to research, and conclusions and recommendations. Appendices include Institutional Review Board (IRB) protocol for ethical treatment and protection of research participants, definition of the study area for drawing out Census data, and a Survey Design and Administration Protocol Handbook to aid in administering surveys drafted for AAFW member organizations to use with their staff and volunteers, patrons, and the general public. Student researchers analyzed public data and local reports to understand the background and demographics of the West Bloomington community. Reports consulted include these topics: housing conditions and issues; food justice; Census data on race, income, and age; community and economic development; and education statistics for District 87 and West Bloomington schools. Through various focus groups, four common themes appeared: the need for greater communication between and among local organizations and community members; the need for additional activities, especially for young people; a lack of accessibility to various services offered by (or in) the City of Bloomington and surrounding areas; and distressed amenities. Communication can be strengthened between the WBRP and the AAFW, between West Bloomington community members and the City of Bloomington, and among all of these groups and members. Focus group findings also show that the West Bloomington community and organizations serving it do not offer activities to people of all age ranges. In addition, all focus groups shared comments about transportation issues within West Bloomington. Regarding distressed amenities, focus group participants brought up issues concerning poor housing conditions, inadequate amenities at local parks, and old infrastructure in need of repair or replacement. Findings from key informant interviews also acknowledged several areas for enhancement within the West Bloomington community. Within the community, apathy and a lack of respect among residents were identified as negative attributes coupled with a perceived lack of police presence in the area. Residents linked these attributes to a fear of crime, and specifically, drug dealing, prostitution, and gang activity and recruitment. Resource collaboration and greater awareness of available resources also are viewed as necessary enhancements within the community and those serving it. West Bloomington residents serving as key informant interviewees identified issues of homelessness, overcrowded housing facilities, and a lack of affordable housing. Some interviewees expressed a desire for better notification of neighborhood events. Several interviewees mentioned a desire for better communication between the City of Bloomington and West Bloomington community and a better working relationship with the police department. Additionally, multiple interviewees discussed the importance of education, including the need for more job training and literacy programs. This report includes conclusions and recommendations based on findings from key informant interviews, focus groups, local and public data review, and past and future surveys. These recommendations are broken down into three categories focusing on the City of Bloomington, the West Bloomington neighborhood, and youth. City-related recommendations include the following: The Bloomington Police Department can increase its visible presence in the West Bloomington neighborhood by providing foot patrol. The City of Bloomington should develop a plan for addressing infrastructure needs in the West Bloomington area. The Bloomington-Normal Public Transit System can evaluate the current mass transit bus routes for West Bloomington. Improved civic awareness and education can increase residents‟ skills and abilities to address grievances and development issues within their community. Neighborhood recommendations advise the following: Primarily, communication among all stakeholders needs to be improved. Stakeholders can increase awareness of available services, programs, and resources that can benefit community members. A community fair can be held within the West Bloomington neighborhood. A feasibility study should be performed for a multi-purpose community center to be located in the West Bloomington neighborhood. The youth recommendations suggest the following: Encourage The Bloomington Boys and Girls Club to determine if additional services could be added to benefit the teen youth of West Bloomington. Increase the availability of summer programs to benefit both working parents and children. Explore effective strategies for identifying and dealing with barriers to development, especially those that apply to youth and crime. Stakeholders can collaborate on a campaign to bring awareness and encourage action around the need for more volunteer mentors, especially male mentors who are needed for West Bloomington male youth. In conclusion, this report sheds some light on the needs, wants, assets, and resources of the West Bloomington community. It is our hope that this report will serve as an impetus for enhanced relationships among the AAFW, the WBRP, West Bloomington residents, and the City of Bloomington, as well as provide information and materials to aid in future development

Probing the role of grain boundaries in single Cu nanoparticle oxidation by in situ plasmonic scattering

Grain boundaries determine physical properties of bulk materials including ductility, diffusivity, and electrical conductivity. However, the role of grain boundaries in nanostructures and nanoparticles is much less understood, despite the wide application of nanoparticles in nanophotonics, nanoelectronics, and heterogeneous catalysis. Here, we investigate the role of high-angle grain boundaries in the oxidation of Cu nanoparticles, using a combination of in situ single particle plasmonic nanoimaging and postmortem transmission electron microscopy image analysis, together with ab initio and classical electromagnetic calculations. We find an initial growth of a 5-nm-thick Cu2O shell on all nanoparticles, irrespective of different grain morphologies. This insensitivity of the Cu2O shell on the grain morphology is rationalized by extraction of Cu atoms from the metal lattice being the rate limiting step, as proposed by density functional theory calculations. Furthermore, we find that the change in optical scattering intensity measured from the individual particles can be deconvoluted into one contribution from the oxide layer growth and one contribution that is directly proportional to the grain boundary density. The latter contribution signals accumulation of Cu vacancies at the grain boundaries, which, as corroborated by calculations of the optical scattering, leads to increased absorption losses and thus a decrease of the scattering, thereby manifesting the role of grain boundaries as vacancy sinks and nuclei for Kirkendall void formation at a later stage of the oxidation process

Label-free nanofluidic scattering microscopy of size and mass of single diffusing molecules and nanoparticles

Nanofluidic scattering microscopy enables label-free, quantitative measurements of the molecular weight and hydrodynamic radius of biological molecules and nanoparticles freely diffusing inside a nanofluidic channel. Label-free characterization of single biomolecules aims to complement fluorescence microscopy in situations where labeling compromises data interpretation, is technically challenging or even impossible. However, existing methods require the investigated species to bind to a surface to be visible, thereby leaving a large fraction of analytes undetected. Here, we present nanofluidic scattering microscopy (NSM), which overcomes these limitations by enabling label-free, real-time imaging of single biomolecules diffusing inside a nanofluidic channel. NSM facilitates accurate determination of molecular weight from the measured optical contrast and of the hydrodynamic radius from the measured diffusivity, from which information about the conformational state can be inferred. Furthermore, we demonstrate its applicability to the analysis of a complex biofluid, using conditioned cell culture medium containing extracellular vesicles as an example. We foresee the application of NSM to monitor conformational changes, aggregation and interactions of single biomolecules, and to analyze single-cell secretomes