@article{marusak_johnston-peck_wu_anderson_tracy_2017, title={Size and Composition Control of CoNi Nanoparticles and Their Conversion into Phosphides}, volume={29}, ISSN={["1520-5002"]}, url={https://doi.org/10.1021/acs.chemmater.6b04335}, DOI={10.1021/acs.chemmater.6b04335}, abstractNote={The synthesis of binary rather than unary metal nanoparticles (NPs) introduces challenges in controlling the chemistry and opportunities to tune the properties of the products. Ligand-stabilized CoNi NPs were synthesized by heating mixtures of Ni(acac)2 and Co(acac)2 (acac = acetylacetonate), oleylamine, trioctylphosphine, and trioctylphosphine oxide to 240 °C. Varying the amounts of the Co and Ni precursors allows for control over the NP size, giving diameters of 6–18 nm and compositions (XCo) of ≤0.7. The products are enriched with Ni, in comparison with the Co:Ni ratio of the precursors. Co and Ni are both dispersed throughout the NPs, while the shells are enriched with Co. The magnetic properties of CoNi NPs are between those of magnetically soft Ni and magnetically harder Co, with additional effects caused by oxidation under ambient atmosphere, which gives rise to exchange bias. When the reaction mixture for synthesizing CoNi NPs is heated to 300 °C, trioctylphosphine decomposes, and conversion into ...}, number={7}, journal={CHEMISTRY OF MATERIALS}, publisher={American Chemical Society (ACS)}, author={Marusak, Katherine E. and Johnston-Peck, Aaron C. and Wu, Wei-Chen and Anderson, Bryan D. and Tracy, Joseph B.}, year={2017}, month={Apr}, pages={2739–2747} }
 @article{medford_johnston-peck_tracy_2013, title={Nanostructural transformations during the reduction of hollow and porous nickel oxide nanoparticles}, volume={5}, ISSN={["2040-3372"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000313347200017&KeyUID=WOS:000313347200017}, DOI={10.1039/c2nr33005a}, abstractNote={Size-dependent nanostructural transformations occurring during the H(2)-mediated reduction of hollow and porous NiO nanoparticles were investigated for controlled nanoparticle sizes of ~10 to 100 nm. Transmission electron microscopy reveals that the location and number of reduction sites strongly depend on the nanoparticle size and structure.}, number={1}, journal={NANOSCALE}, author={Medford, John A. and Johnston-Peck, Aaron C. and Tracy, Joseph B.}, year={2013}, pages={155–159} }
 @article{jie_niskala_johnston-peck_krommenhoek_tracy_fan_you_2012, title={Laterally patterned magnetic nanoparticles}, volume={22}, ISSN={["1364-5501"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000298970700034&KeyUID=WOS:000298970700034}, DOI={10.1039/c1jm14612b}, abstractNote={Laterally patterning magnetic nanoparticles (MNPs) through self-assembly and simple solution processing constitutes an important step toward inexpensive nanoparticle-based devices. In this work, MNPs were laterally patterned on metal thin films using laterally patterned self-assembled monolayers (SAMs) as a template. SAMs of inactive molecules were first patterned on an Au thin film using the soft-lithographic technique, microcontact printing. The active, bifunctional molecules, 1,10-decanedithiol or 4-(11-mercaptoundecyl)benzene-1,2-diol, were then patterned through backfilling. The MNPs selectively bind to the terminal thiols or modified catechols when the substrates are submerged into a solution of MNPs. By adjusting the deposition conditions, both monolayers and partial multilayers were controllably formed. Co, Ni, Fe3O4, and FePt MNPs, as well as Au non-magnetic nanoparticles were successfully patterned by this process. This generalized approach is anticipated to be adaptable to many other kinds of nanoparticlesvia judicious selection of the substrates, surfactant ligands (on the nanoparticle), and/or surface-bound monolayers.}, number={5}, journal={JOURNAL OF MATERIALS CHEMISTRY}, author={Jie, Yanni and Niskala, Jeremy R. and Johnston-Peck, Aaron C. and Krommenhoek, Peter J. and Tracy, Joseph B. and Fan, Huiqing and You, Wei}, year={2012}, pages={1962–1968} }
 @article{johnston-peck_tracy_2012, title={Phase transformation of alumina-coated FePt nanoparticles}, volume={111}, ISSN={["1089-7550"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000303282400219&KeyUID=WOS:000303282400219}, DOI={10.1063/1.3676419}, abstractNote={Monolayers of FePt nanoparticles (NPs) were coated with 10 nm-thick layers of Al2O3 by pulsed laser deposition (PLD). The Al2O3 coating prevents diffusion and stabilizes the NP monolayer against sintering. Annealing converts the FePt NPs from an alloy phase (A1) into intermetallic phases. Atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) shows that Pt-rich A1 Fe29Pt71 NPs convert into the FePt3 L12 intermetallic phase upon annealing and develop faceted morphologies. HAADF-STEM also reveals the presence of structural defects and Pt-rich shells. These results, in conjunction with prior studies of Al2O3 deposition by atomic layer deposition (ALD), demonstrate that coating arrays of FePt NPs with thin Al2O3 films by PLD or ALD imparts thermal stability and provides comparable results.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Johnston-Peck, Aaron C. and Tracy, Joseph B.}, year={2012}, month={Apr} }
 @article{sarac_wilson_johnston-peck_wang_pearce_klein_melechko_tracy_2011, title={Effects of Ligand Monolayers on Catalytic Nickel Nanoparticles for Synthesizing Vertically Aligned Carbon Nanofibers}, volume={3}, ISSN={["1944-8244"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000289762400003&KeyUID=WOS:000289762400003}, DOI={10.1021/am101290v}, abstractNote={Vertically aligned carbon nanofibers (VACNFs) were synthesized using ligand-stabilized Ni nanoparticle (NP) catalysts and plasma-enhanced chemical vapor deposition. Using chemically synthesized Ni NPs enables facile preparation of VACNF arrays with monodisperse diameters below the size limit of thin film lithography. During pregrowth heating, the ligands catalytically convert into graphitic shells that prevent the catalyst NPs from agglomerating and coalescing, resulting in a monodisperse VACNF size distribution. In comparison, significant agglomeration occurs when the ligands are removed before VACNF growth, giving a broad distribution of VACNF sizes. The ligand shells are also promising for patterning the NPs and synthesizing complex VACNF arrays.}, number={4}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Sarac, Mehmet F. and Wilson, Robert M. and Johnston-Peck, Aaron C. and Wang, Junwei and Pearce, Ryan and Klein, Kate L. and Melechko, Anatoli V. and Tracy, Joseph B.}, year={2011}, month={Apr}, pages={936–940} }
 @article{johnston-peck_wang_tracy_2011, title={Formation and Grain Analysis of Spin-Cast Magnetic Nanoparticle Monolayers}, volume={27}, ISSN={["0743-7463"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000289321000097&KeyUID=WOS:000289321000097}, DOI={10.1021/la200005q}, abstractNote={Ligand-stabilized magnetic nanoparticles (NPs) with diameters of 4-7 nm were spin-cast into monolayers on electron-transparent silicon nitride (SiN) substrates. SiN membranes facilitate detailed high-resolution characterization of the spin-cast monolayers by transmission electron microscopy (TEM) and approximate spin-casting onto wafers. Suspending the NPs in hexanes and pretreating the substrate with ultraviolet light and ozone (UVO) gives the best results. Computer-aided analysis of the arrays elucidates their grain structures, including identification of the grain boundaries and defects and measurements of the grain orientations and translational correlation lengths. Narrow NP size distributions result in close-packed arrays with minimal defects and large grains containing thousands of NPs. Edge dislocations, interstitials, vacancies, and overlapping NPs were observed. Deviations from close packing occur as the normalized standard deviation of the sample's size distribution increases above approximately 11%. Polydisperse size distributions and deviations from spherical NP shapes frustrate assembly and prevent ordered packing.}, number={8}, journal={LANGMUIR}, author={Johnston-Peck, Aaron C. and Wang, Junwei and Tracy, Joseph B.}, year={2011}, month={Apr}, pages={5040–5046} }
 @article{chhetri_kozek_johnston-peck_tracy_oldenburg_2011, title={Imaging three-dimensional rotational diffusion of plasmon resonant gold nanorods using polarization-sensitive optical coherence tomography}, volume={83}, ISSN={["1550-2376"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000289521900001&KeyUID=WOS:000289521900001}, DOI={10.1103/physreve.83.040903}, abstractNote={We demonstrate depth-resolved viscosity measurements within a single object using polarized optical scattering from ensembles of freely tumbling plasmon resonant gold nanorods (GNRs) monitored with polarization-sensitive optical coherence tomography. The rotational diffusion coefficient of the GNRs is shown to correlate with viscosity in molecular fluids according to the Stokes-Einstein relation. The plasmon resonant and highly anisotropic properties of GNRs are favorable for microrheological studies of nanoscale properties.}, number={4}, journal={PHYSICAL REVIEW E}, author={Chhetri, Raghav K. and Kozek, Krystian A. and Johnston-Peck, Aaron C. and Tracy, Joseph B. and Oldenburg, Amy L.}, year={2011}, month={Apr} }
 @article{johnston-peck_scarel_wang_parsons_tracy_2011, title={Sinter-free phase conversion and scanning transmission electron microscopy of FePt nanoparticle monolayers}, volume={3}, ISSN={["2040-3372"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000295618200024&KeyUID=WOS:000295618200024}, DOI={10.1039/c1nr10567a}, abstractNote={Thermally robust monolayers of 4-6 nm diameter FePt nanoparticles (NPs) were fabricated by combining chemical synthesis and atomic layer deposition. Spin-cast monolayers of FePt NPs were coated with thin, 11 nm-thick layers of amorphous Al(2)O(3), followed by annealing to convert the FePt NPs from an alloy (A1) into intermetallic FePt (L1(0)) and FePt(3) (L1(2)) phases. The Al(2)O(3) layer serves as a barrier that prevents sintering between NPs during annealing at temperatures up to 730 °C. Electron and X-ray diffraction in conjunction with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) show that as-synthesized A1 FePt NPs convert into L1(0) and L1(2) phase NPs through annealing. HAADF-STEM measurements of individual NPs reveal imperfect ordering and show that the NP composition determines which intermetallic phase is obtained. Mixed-phase NPs with L1(0) cores and FePt(3) L1(2) shells were also observed, as well as a smaller number of unconverted A1 NPs. These results highlight the need for improved control over the compositional uniformity of FePt NPs for their use in bit-patterned magnetic recording.}, number={10}, journal={NANOSCALE}, author={Johnston-Peck, Aaron C. and Scarel, Giovanna and Wang, Junwei and Parsons, Gregory N. and Tracy, Joseph B.}, year={2011}, pages={4142–4149} }
 @article{shore_wang_johnston-peck_oldenburg_tracy_2011, title={Synthesis of Au(Core)/Ag(Shell) Nanoparticles and their Conversion to AuAg Alloy Nanoparticles}, volume={7}, ISSN={["1613-6829"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000286997600011&KeyUID=WOS:000286997600011}, DOI={10.1002/smll.201001138}, abstractNote={Metal nanoparticles (NPs) are of great interest due to their special optical, [ 1–3 ] electronic, [ 4–8 ] and catalytic [ 9,10 ] properties. [ 11 ] Among metal NPs, Au NPs have been investigated most extensively because of their facile preparation, resistance to oxidation, and surface plasmon resonance (SPR) band that can absorb and scatter visible light. [ 3 ] Core/ shell and alloy bimetallic NPs are especially interesting because they provide opportunities to tune the NPs’ optical and catalytic properties [ 12–15 ] and are potentially useful as taggants for security applications. [ 2 ] The AuAg system is of particular interest because the SPR band is tunable between ∼ 520 nm for Au [ 11 ] and ∼ 410 nm for Ag. [ 16 ] Several syntheses for AuAg alloy, [ 15 , 17–37 ] Au(core)/Ag(shell), [ 22 , 25 , 31 , 33 , 35–45 ] and Ag(core)/Au(shell) NPs [ 28 , 31 , 33–35 , 42 , 46–48 ] have already been reported. Here, we report a facile, stoichiometrically controlled synthesis of Au(core)/Ag(shell) and AuAg alloy NPs through digestive ripening, [ 49–51 ] which is a potentially general method for synthesizing alloy NPs. [ 37 , 52,53 ] Au(core)/Ag(shell) NPs were synthesized and annealed to form AuAg alloy NPs, followed by elemental analysis and structural and optical characterization. Methods utilizing Au rather than Ag NPs as the seed particles are advantageous: obtaining monodisperse Ag NPs is signifi cantly more challenging [ 16 ] because it is harder to control the nucleation of Ag NPs and to avoid oxidation. Huang and co-workers recently demonstrated the conversion of Au(core)/Ag(shell) to AuAg alloy NPs as a part of a larger study showing the generality of digestive ripening for synthesizing alloy NPs, but very limited data without quantitative elemental analysis or optical characterization of the AuAg alloy NPs was provided. [ 37 ] In this study, we compare a twostep synthesis of Au(core)/Ag(shell) NPs and their conversion to AuAg alloy NPs through annealing with a one-step, direct conversion of Au NPs to AuAg alloy NPs. The products of}, number={2}, journal={SMALL}, author={Shore, Matthew S. and Wang, Junwei and Johnston-Peck, Aaron C. and Oldenburg, Amy L. and Tracy, Joseph B.}, year={2011}, month={Jan}, pages={230–234} }
 @article{railsback_johnston-peck_wang_tracy_2010, title={Size-Dependent Nanoscale Kirkendall Effect During the Oxidation of Nickel Nanoparticles}, volume={4}, ISSN={["1936-086X"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000276956800020&KeyUID=WOS:000276956800020}, DOI={10.1021/nn901736y}, abstractNote={The transformation of Ni nanoparticles (NPs) of different sizes (average diameters of 9, 26, and 96 nm) during oxidation to hollow (single void) or porous (multiple voids) NiO through the nanoscale Kirkendall effect was observed by transmission electron microscopy. Samples treated for 1-4 h at 200-500 degrees C show that the structures of the completely oxidized NPs do not depend on the temperature, but oxidation proceeds more quickly at elevated temperatures. For the Ni/NiO system, after formation of an initial NiO shell (of thickness approximately 3 nm), single or multiple voids nucleate on the inner surface of the NiO shell, and the voids grow until conversion to NiO is complete. Differences in the void formation and growth processes cause size-dependent nanostructural evolution: For 9 and 26 nm NPs, a single void forms beneath the NiO shell, and the void grows by moving across the NP while conversion to NiO occurs opposite the site where the void initially formed. Because of the differences in the Ni/NiO volume ratios for the 9 and 26 nm NPs when the void first forms, they have distinct nanostructures: The 9 nm NPs form NiO shells that are nearly radially symmetric, while there is a pronounced asymmetry in the NiO shells for 26 nm NPs. By choosing an intermediate oxidation temperature and varying the reaction time, partially oxidized Ni(core)/NiO(shell) NPs can be synthesized with good control. For 96 nm NPs, multiple voids form and grow, which results in porous NiO NPs.}, number={4}, journal={ACS NANO}, author={Railsback, Justin G. and Johnston-Peck, Aaron C. and Wang, Junwei and Tracy, Joseph B.}, year={2010}, month={Apr}, pages={1913–1920} }
 @article{wang_johnston-peck_tracy_2009, title={Nickel Phosphide Nanoparticles with Hollow, Solid, and Amorphous Structures}, volume={21}, ISSN={["1520-5002"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000270461700018&KeyUID=WOS:000270461700018}, DOI={10.1021/cm901073k}, abstractNote={Conversion of unary metal nanoparticles (NPs) upon exposure to oxygen, sulfur, selenium, and phophorus precursors usually produces hollow metal oxide, sulfide, selenide, or phosphide NPs through the Kirkendall effect. Here, nanostructural control of mixed-phase Ni2P/Ni12P5 (represented as NixPy) NPs prepared through the thermolysis of nickel acetylacetonate using trioctylphosphine (TOP) as a ligand and phosphorus precursor is reported. The P:Ni molar ratio controls the NP size and is the key factor in determining the nanostructure. For P:Ni molar ratios of 1−3, nickel NPs form below 240 °C and subsequently convert to crystalline-hollow NixPy NPs at 300 °C. For higher P:Ni ratios, a Ni-TOP complex forms that requires higher temperatures for NP growth, thus favoring direct formation of NixPy rather than nickel. Consequently, for P:Ni molar ratios of >9, amorphous-solid NixPy NPs form at 240 °C and become crystalline-solid NixPy NPs at 300 °C. For intermediate P:Ni molar ratios of ∼6, both growth mechanisms ...}, number={19}, journal={CHEMISTRY OF MATERIALS}, author={Wang, Junwei and Johnston-Peck, Aaron C. and Tracy, Joseph B.}, year={2009}, month={Oct}, pages={4462–4467} }
 @article{johnston-peck_wang_tracy_2009, title={Synthesis and Structural and Magnetic Characterization of Ni(Core)/NiO(Shell) Nanoparticles}, volume={3}, ISSN={["1936-086X"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000266323600007&KeyUID=WOS:000266323600007}, DOI={10.1021/nn900019x}, abstractNote={A size series of ligand-stabilized Ni nanoparticles (NPs) with diameters between 8-24 nm was prepared by solution chemistry, followed by solution-phase oxidation with atmospheric oxygen at 200 degrees C to form Ni(core)/NiO(shell) NPs with shell thicknesses of 2-3 nm. In comparison with the oxidation of Fe and Co NPs, Ni NPs require higher temperatures for significant conversion to NiO. Transmission electron microscopy and electron diffraction show polycrystalline cores with predominantly amorphous shells. SQUID magnetometry measurements were performed to assess the effects of coupling between the ferromagnetic Ni cores and antiferromagnetic NiO shells. After intentional oxidation, the Ni(core)/NiO(shell) NPs have decreased superparamagnetic blocking temperatures (T(B)) and no exchange shift (H(EB)), but a small enhancement in the coercivity (H(C)) signifies weak exchange bias. These effects originate from the amorphous structure of the NiO shells and their thin layer thickness that renders the NiO moments incapable of pinning the core moment in moderate applied fields. The magnetocrystalline anisotropy constants before and after oxidation approach the value for bulk Ni and depend on the Ni core size and NiO shell thickness.}, number={5}, journal={ACS NANO}, author={Johnston-Peck, Aaron C. and Wang, Junwei and Tracy, Joseph B.}, year={2009}, month={May}, pages={1077–1084} }