Walker, Amy V.

Permanent URI for this collectionhttps://hdl.handle.net/10735.1/3996

Amy Walker is an Associate Professor in the Department of Materials Science and Engineering. Her research interests include: "Physical chemistry, analytical chemistry, surface science, materials science, nanoscience, catalysis, metal-organic thin films, new methods for the deposition of metals and other materials on organic surfaces, patterning of organic surfaces, molecular electronics, time-of-flight secondary ion mass spectrometry (TOF SIMS), mass spectrometric imaging." More information about Dr. Walker can be found here.


Recent Submissions

Now showing 1 - 7 of 7
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    Polytype Control of MoS₂ Using Chemical Bath Deposition
    (American Institute of Physics Inc., 2019-05-01) Hedlund, Jenny K.; Walker, Amy V.; 0000-0003-2114-3644 (Walker, AV); Hedlund, Jenny K.; Walker, Amy V.
    Molybdenum disulfide (MoS₂) has a wide range of applications from electronics to catalysis. While the properties of single-layer and multilayer MoS₂ films are well understood, controlling the deposited MoS₂ polytype remains a significant challenge. In this work, we employ chemical bath deposition, an aqueous deposition technique, to deposit large area MoS₂ thin films at room temperature. Using Raman spectroscopy and x-ray photoelectron spectroscopy, we show that the deposited MoS₂ polytype can be changed from semiconducting 2H MoS₂ on hydrophobic -CH₃ and -CO₂C₆F₅ terminated self-assembled monolayers (SAMs) to semimetallic 1T MoS₂ on hydrophilic -OH and -COOH terminated SAMs. The data suggest that the deposition of MoS₂ polytypes is controlled by the substrate surface energy. High surface energy substrates stabilize 1T MoS₂ films, while 2H MoS₂ is deposited on lower surface energy substrates. This effect appears to be general enabling the deposition of different MoS₂ polytypes on a wide range of substrates. ©2019 Author(s).
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    Law and Disorder: Special Stacking Units—Building the Intergrowth Ce₆ Co₅ Ge₁₆
    (American Chemical Society, 2019-04-22) Felder, Justin B.; Weiland, Ashley; Hodovanets, H.; McCandless, George T.; Estrada, Tania G.; Martin, Thomas J.; Walker, Amy V.; Paglione, J.; Chan, Julia Y.; 0000-0003-4434-2160 (Chan, JY); 0000-0002-2528-1967 (Felder, JB); 0000-0001-7198-3559 (Weiland, A); 0000-0003-2114-3644 (Walker, AV); Felder, Justin B.; Weiland, Ashley; McCandless, George T.; Estrada, Tania G.; Martin, Thomas J.; Walker, Amy V.; Chan, Julia Y.
    A new structure type of composition Ce₆ Co₅ Ge₁₆ was grown out of a molten Sn flux. Ce₆ Co₅ Ge₁₆ crystallizes in the orthorhombic space group Cmcm, with highly anisotropic lattice parameters of α = 4.3293(5) Å, b = 55.438(8) Å, and c = 4.3104(4) Å. The resulting single crystals were characterized by X-ray diffraction, and the magnetic and transport properties are presented. The Sn-stabilized structure of Ce₆ Co₅ Ge₁₆ is based on the stacking of disordered Ce cuboctahedra and is an intergrowth of existing structure types including AlB₂ , BaNiSn₃, and AuCu₃. The stacking of structural subunits has previously been shown to be significant in the fields of superconductivity, quantum materials, and optical materials. Herein, we present the synthesis, characterization, and complex magnetic behavior of Ce₆ Co₅ Ge₁₆ at low temperature, including three distinct magnetic transitions. © 2019 American Chemical Society.
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    Toward Understanding Weak Matrix Effects in TOF SIMS
    (American Vacuum Society) Gelb, Lev D.; Walker, Amy V.; 0000 0001 3758 9240 (Walker, AV); 0000-0003-0291-5098 (Gelb, LD); Gelb, Lev D.; Walker, Amy V.
    Chemical imaging methods, including imaging mass spectrometry, are increasingly used for the analysis of samples ranging from biological tissues to electronic devices. A barrier to wider adoption of imaging mass spectrometry is the presence of matrix effects which complicate quantitative analysis. Interactions between an analyte molecule and its surroundings (the "matrix") can substantially alter both the yield and type of ions observed. Furthermore, such "intrinsic" effects can be confused with nonlinear response due to detector saturation and other instrument-related complications. As a result, quantitative analyses of time-of-flight secondary ion mass spectrometry (TOF SIMS) data that attempt to account for matrix effects are rare. The authors discuss analysis of such data using maximum a posteriori reconstruction based on physically motivated models, and present progress toward the quantitative extraction of chemical concentration profiles and component spectra in the presence of matrix effects, using mixed self- assembled alkanethiolate monolayers as a test system. The authors demonstrate that the incorporation of matrix effects to lowest order using a series-expansion approach is an effective strategy and that doing so provides improved quantitative performance in measuring surface compositions and can also yield information about interactions between species during the SIMS process. Published by the AVS.
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    Application of Visible-Light Photosensitization to Form Alkyl-Radical-Derived Thin Films on Gold
    Quarels, Rashanique D.; Zhai, Xianglin; Kuruppu, Neepa; Hedlund, Jenny K.; Ellsworth, Ashley A.; Walker, Amy V.; Garno, Jayne C.; Ragains, Justin R.; Hedlund, Jenny K.; Ellsworth, Ashley A.; Walker, Amy V.
    Visible-light irradiation of phthalimide esters in the presence of the photosensitizer [Ru(bpy)₃]²⁺ and the stoichiometric reducing agent benzyl nicotinamide results in the formation of alkyl radicals under mild conditions. This approach to radical generation has proven useful for the synthesis of small organic molecules. Herein, we demonstrate for the first time the visible-light photosensitized deposition of robust alkyl thin films on Au surfaces using phthalimide esters as the alkyl radical precursors. In particular, we combine visible-light photosensitization with particle lithography to produce nanostructured thin films, the thickness of which can be measured easily using AFM cursor profiles. Analysis with AFM demonstrated that the films are robust and resistant to mechanical force while contact angle goniometry suggests a multilayered and disordered film structure. Analysis with IRRAS, XPS, and TOF SIMS provides further insights.
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    Photochemical CVD of Ru on Functionalized Self-Assembled Monolayers from Organometallic Precursors
    (American Institute of Physics Inc, 2018-08-20) Johnson, K. R.; Rodriguez, Paul Arevalo; Brewer, C. R.; Brannaka, J. A.; Shi, Zhiwei; Yang, Jing; Salazar, Brian; McElwee-White, L.; Walker, Amy V.; 0000 0001 3758 9240 (Walker, AV); 0000-0003-3079-7367 (Salazar, B); Rodriguez, Paul Arevalo; Shi, Zhiwei; Yang, Jing; Salazar, Brian; Walker, Amy V.
    Chemical vapor deposition (CVD) is an attractive technique for the metallization of organic thin films because it is selective and the thickness of the deposited film can easily be controlled. However, thermal CVD processes often require high temperatures which are generally incompatible with organic films. In this paper, we perform proof-of-concept studies of photochemical CVD to metallize organic thin films. In this method, a precursor undergoes photolytic decomposition to generate thermally labile intermediates prior to adsorption on the sample. Three readily available Ru precursors, CpRu(CO)₂Me, (η³-allyl)Ru(CO)₃Br, and (COT)Ru(CO)₃, were employed to investigate the role of precursor quantum yield, ligand chemistry, and the Ru oxidation state on the deposition. To investigate the role of the substrate chemistry on deposition, carboxylic acid-, hydroxyl-, and methyl-terminated self-assembled monolayers were used. The data indicate that moderate quantum yields for ligand loss (φ ≥ 0.4) are required for ruthenium deposition, and the deposition is wavelength dependent. Second, anionic polyhapto ligands such as cyclopentadienyl and allyl are more difficult to remove than carbonyls, halides, and alkyls. Third, in contrast to the atomic layer deposition, acid-base reactions between the precursor and the substrate are more effective for deposition than nucleophilic reactions. Finally, the data suggest that selective deposition can be achieved on organic thin films by judicious choice of precursor and functional groups present on the substrate. These studies thus provide guidelines for the rational design of new precursors specifically for selective photochemical CVD on organic substrates.
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    Room Temperature Atomic Layerlike Deposition of ZnS on Organic Thin Films: Role of Substrate Functional Groups and Precursors
    (AVS Science and Technology Society) Shi, Z.; Walker, Amy V.; 0000 0001 3758 9240 (Walker, AV)
    The room temperature atomic layerlike deposition (ALLD) of ZnS on functionalized self-assembled monolayers (SAMs) was investigated, using diethyl zinc (DEZ) and in situ generated H₂S as reactants. Depositions on SAMs with three different terminal groups, -CH₃, -OH, and -COOH, were studied. It was found that the reaction of DEZ with the SAM terminal group is critical in determining the film growth rate. Little or no deposition is observed on -CH₃ terminated SAMs because DEZ does not react with the methyl terminal group. ZnS does deposit on both -OH and -COOH terminated SAMs, but the grow rate on -COOH terminated SAMs is ∼10% lower per cycle than on -OH terminated SAMs. DEZ reacts with the hydroxyl group on -OH terminated SAMs, while on -COOH terminated SAMs it reacts with both the hydroxyl and carbonyl bonds of the terminal groups. The carbonyl reaction is found to lead to the formation of ketones rather than deposition of ZnS, lowering the growth rate on -COOH terminated SAMs. SIMS spectra show that both -OH and -COOH terminated SAMs are covered by the deposited ZnS layer after five ALLD cycles. In contrast to ZnO ALLD where the composition of the film differs for the first few layers on -COOH and -OH terminated SAMs, the deposited film composition is the same for both -COOH and -OH terminated SAMs. The deposited film is found to be Zn-rich, suggesting that the reaction of H₂S with the Zn-surface adduct may be incomplete.
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    Partially Fluorinated Oxo-Alkoxide Tungsten(VI) Complexes as Precursors for Deposition of WOx Nanomaterials
    (Royal Society of Chemistry) Bonsu, R. O.; Kim, H.; O'Donohue, C.; Korotkov, R. Y.; McClain, K. R.; Abboud, K. A.; Ellsworth, Ashley A.; Walker, Amy V.; Anderson, T. J.; McElwee-White, L.; 0000 0001 3758 9240 (Walker, AV)
    The partially fluorinated oxo-alkoxide tungsten(VI) complexes WO(OR) 4 [4; R = C(CH3)2CF3, 5; R = C(CH3)(CF3)2] have been synthesized as precursors for chemical vapour deposition (CVD) of WOx nanocrystalline material. Complexes 4 and 5 were prepared by salt metathesis between sodium salts of the fluoroalkoxides and WOCl4. Crystallographic structure analysis allows comparison of the bonding in 4 and 5 as the fluorine content of the fluoroalkoxide ligands is varied. Screening of 5 as a CVD precursor by mass spectrometry and thermogravimetric analysis was followed by deposition of WOx nanorods.

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