Scott Walck | U.S. Army Research Laboratory (original) (raw)

Papers by Scott Walck

Thin Solid Films, Feb 1, 2000

ABSTRACT Zinc oxide (ZnO) is well known to the electronic industry as a piezoelectric material. R... more ABSTRACT Zinc oxide (ZnO) is well known to the electronic industry as a piezoelectric material. Recent research from this laboratory also indicates the potential of ZnO as a tribological material. The current work describes the evolution of microstructure with deposition parameters in pulsed laser deposited ZnO thin films, specifically targeted for friction and wear applications. Films were characterized by high resolution scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Friction and wear measurements were made using a ball-on-disk tribometer. Films were grown in vacuum (V) as well as in 5 mTorr of oxygen (O2), while the substrates were kept at room temperature (RT). The RT/V ZnO films have (002) columnar texture with an average column width of 20 nm. The RT/O2 films also are nanoclumnar with (002) texture, but each column is a mosaic of low-angle boundaries. Deformation mechanisms associated with nanocrystalline grain structure were analyzed with particular reference to sliding contact. Mechanisms to provide the observed low friction of RT/O2 films (μ=0.15–0.20) have been activated by its mosaic structure.

MRS Proceedings, 1984

Accurate depth profiling of implanted hydrogen and its isotopes in metals is extremely important.... more Accurate depth profiling of implanted hydrogen and its isotopes in metals is extremely important. Field ion microscopy and atom-probe techniques provide the most accurate depth profiling analytical method of any available. In addition, they are extremely sensitive to hydrogen. This paper reports our early work on hydrogen trapping at defects in metals using the Field Ion Microscope/Imaging Atom Probe (FIM/IAP). Our results deal primarily with the control experiments required to overcome instrumental difficulties associated with in situ implantation and the influence of a high electric field. Transmission Electron Microscopy (TEM) has been used extensively to independently examine the influence of high electric fields on emitters.

Journal of Applied Physics, Feb 8, 2021

Aluminum nanoparticles (nAl) have the potential as energetic additives in explosive/propellant fo... more Aluminum nanoparticles (nAl) have the potential as energetic additives in explosive/propellant formulations. Scalable methodologies must be pursued to mitigate the inactive amorphous alumina shell surrounding the active aluminum (Al) core with modified surface morphology and chemistry for increased combustion effects. This paper explores the feasibility of making reactive core/shell nAl with thinned oxide shells and modified surface coatings via a two-step atmospheric plasma surface treatment process in a custom dielectric barrier discharge plasma reactor. The commercial nAl of nominal average size ∼40–60 nm was first treated with helium (He) followed by He/carbon monoxide (CO) plasmas for different durations. The resultant samples were characterized via high-resolution transmission electron microscopy (HRTEM) and Fourier transform IR (FTIR) spectra. HRTEM images revealed sporadic patchy γ-alumina deposits on particle surfaces and in gaps among particles for all samples, suggesting the non-uniform plasma effects of the He/CO glow. Nanoscale chemical analyses via scanning transmission electron microscopy elemental mapping and x-ray energy dispersive spectroscopy were further performed. Although no carbon-associated structure appeared in electron energy loss spectroscopy (EELS) spectra, the presence of carbonaceous materials was confirmed as a thin dispersive layer evenly distributed on the nAl surface suggesting either its amorphous nature or is present at a level insufficient to generate satisfactory EELS spectra. The trend of intensity profiles for key elements acquired by drawing lines across a single particle on the elemental maps confirmed that carbonaceous materials only existed on the surface and they were most likely carboxylates that increased with increased He/CO treatment duration, as evident by FTIR results. This work demonstrated the success of atmospheric plasma-treated reactive nAl with comprehensively characterized surface features via advanced microscopy and spectroscopy.

Journal of Electronic Materials, Apr 1, 1997

The growth of low temperature (LT) GaAs by molecular beam epitaxy has been studied using ellipsom... more The growth of low temperature (LT) GaAs by molecular beam epitaxy has been studied using ellipsometryo Different regimes of growth were observed in the data, depending on film thickness. Epitaxial growth ofpseudomorphic LT-GaAs occurred immediately above the substrate, followed by a layer with changing dielectric properties. This upper layer can be modeled as a two-phase region consisting ofepitaxial LT-GaAs and small grained, polycrystalline GaAs, which increases in volume fraction with increasing layer thickness. For sufficiently thick LT layers, cross-sectional transmission electron microscopy analysis showed pyramidal defects that were composed primarily of highly twinned regions. The ellipsometry data showed a deviation from the homogeneous growth model at a thickness less than the thickness at which the pyramidal defects nucleated in all samples.

Journal of Materials Research, 1994

Films of PbO/MoS2, grown by pulsed laser deposition, exhibit a significant improvement in tribolo... more Films of PbO/MoS2, grown by pulsed laser deposition, exhibit a significant improvement in tribological performance compared to MoS2 films grown by the same process. The microstructure and crystallography of PbO/MoS2 composite films were investigated using transmission electron microscopy (TEM) to identify the features responsible for this tribological improvement. Self-supporting samples were prepared from pulsed laser deposited, PbO/MoS2 thin films grown on single crystal sodium chloride substrates. Films deposited at room temperature exhibited a two-phase microstructure with one of the phases being amorphous. X-ray microanalysis results showed that the crystalline phase had significantly higher concentration ratios of Mo/Pb, Mo/S, and Pb/S than did the amorphous phase. Films grown at 300 °C were polycrystalline, with a grain size of about 20 nm, and had a NaCl type structure which was isomorphous to PbS. The grains had rectangular shape, and exhibited preferred orientation with the sodium chloride substrate. The concentration of S for these films was approximately 80% of the S concentration for films grown at room temperature. Both the high temperature and room temperature films had S concentrations which were higher than expected from the MoS2 in the target; this was attributed to gettering of the S in the vacuum chamber by Pb. The electron diffraction results, together with previously published results, suggest that the crystal structure of the phases in these films is not responsible for the improvement in tribological properties. However, the microstructural components formed during film growth do determine the wear-induced chemical reaction pathways.

Bulletin of the American Physical Society, 2018

Journal of Applied Physics, 2021

Microscopy and Microanalysis, 2001

With the advent of field emission gun equipped SEMs and TEMs with clean vacuum systems, it has be... more With the advent of field emission gun equipped SEMs and TEMs with clean vacuum systems, it has been shown repeatedly that the surface cleanliness of samples exposed to the small, high current density electron beam available in these instruments is critical with respect to controlling hydrocarbon contamination under the electron beam. in previous studies of dedicated plasma cleaners for TEM samples by one of the authors (SDW), it was shown that surface analytical techniques are the best methods for determining the effectiveness of such cleaners in removing hydrocarbons from the samples’ surfaces. in the current study, some of the experiments that were used in these previous studies were repeated using the XEI Evactron® system in order to compare the relative effectiveness of this system for surface cleaning of EM samples.Figure 1 shows a picture of the sample introduction chamber of a VG Scientific ESCALAB Mkll system (XPS) with the Evactron® in place. The Evactron® system consists o...

Journal of Materials Chemistry A, 2018

The high cost and scarcity of platinum (Pt) materials have considerably hindered their use as cat... more The high cost and scarcity of platinum (Pt) materials have considerably hindered their use as catalysts for the oxygen reduction reaction (ORR) and thus wide-scale implementation in fuel cells for practical applications.

MRS Advances, 2019

ABSTRACTThis work describes exploration of mitigating the parasitic amorphous alumina (Al2O3) she... more ABSTRACTThis work describes exploration of mitigating the parasitic amorphous alumina (Al2O3) shell of aluminum nanoparticles (n-Al) and modifying the surface using different plasmas, leading to n-Al with thinner shell and different coatings including carbons and oxidizing salt called aluminum iodate hexahydrate (AIH), respectively. The approach exploits a prototype atmospheric non-thermal plasma reactor with dielectric barrier discharge (DBD) configuration for nanoparticle surface modifications using n-Al of 80 nm average diameter as an example. Preliminary results indicate that the amorphous Al2O3 shell surrounding the active aluminum core can be mitigated with inert plasmas by as much as 40% using either helium (He) or argon (Ar). The particle surface becomes carbon-rich with carbon monoxide (CO) / He plasmas. By immersing the plasma-treated n-Al in an iodic acid (HIO3) solution, AIH crystals can be formed on the n-Al surface. Transmission electron microscopy (TEM) is used as a m...

Thin Solid Films, 1993

ABSTRACT The deposition rate in a pulsed laser deposition system is known to depend on a variety ... more ABSTRACT The deposition rate in a pulsed laser deposition system is known to depend on a variety of system parameters including wavelength, energy density, target material, target exposure, geometry and repetition rate. In an attempt to dynamically monitor the progress during deposition of tribological thin films, an off-axis quartz crystal monitor was used to determine the thickness of the growing film. A Hitachi S-900 high resolution, field emission scanning electron microscope was used to determine the thickness of films for various laser parameters. Consecutive depositions of MoS2 using the same crystal over several days showed that the calibration factor was constant. The calibration factor did not change appreciably when films of NbSe2 totaling approximately 3.8 μm were deposited between two groups of MoS2 depositions. It increased significantly when approximately 1.1 μm of WS2 films were deposited. Decreasing the energy density by changing the focus of the laser was also shown to decrease the calibration factor. Suggestions for the calibration of the off-axis quartz crystal monitor when the same crystal is to be used for different targets are presented.

Journal of the American Chemical Society, Jan 24, 2014

With the aim to achieve rapid and efficient topochemical polymerizations in the solid state, via ... more With the aim to achieve rapid and efficient topochemical polymerizations in the solid state, via solution-based processing of thin films, we report the integration of a diphenyldiacetylene monomer and a poly(styrene-b-acrylic acid) block copolymer template for the generation of supramolecular architectural photopolymerizable materials. This strategy takes advantage of non-covalent interactions to template a topochemical photopolymerization that yields a polydiphenyldiacetylene (PDPDA) derivative. In thin films, it was found that hierarchical self-assembly of the diacetylene monomers by microphase segregation of the block copolymer template enhances the topochemical photopolymerization, which is complete within a 20 s exposure to UV light. Moreover, UV-active cross-linkable groups were incorporated within the block copolymer template to create micropatterns of PDPDA by photolithography, in the same step as the polymerization reaction. The materials design and processing may find pote...

Macromolecules, 2014

ABSTRACT This work incorporates ML complexes as unbound entities that interact with ML complexes ... more ABSTRACT This work incorporates ML complexes as unbound entities that interact with ML complexes bound to the backbone of the polymer. The π?π interactions and Coulombic forces between bound and unbound ML complexes hold the ML-rich phase together and result in improved mechanical properties over polymers containing only the bound ML complexes. The ML-rich phase formed ordered, cylindrical domains. The storage modulus, surface elastic modulus, and high temperature stability of these metallopolymers increased with increasing concentration of ML complex in the polymer while an optimal concentration and morphology are necessary to improve the strength and creep resistance of the polymer. Ultimately, the successful addition and patterning of unbound ML complexes as a hard phase in a polymer matrix provides an important template for the design of a new type of supramolecular nanocomposite.

Thin Solid Films, 2000

Surface and Coatings Technology, 1995

Graphite fluoride (CFx) is investigated as an additive for WS2 thin films to reduce its sensitivi... more Graphite fluoride (CFx) is investigated as an additive for WS2 thin films to reduce its sensitivity to moisture. The films are grown onto hardened 440C stainless steel disks by pulsed laser deposition using the 248 nm line from an excimer laser. Substrate temperature and additive concentration are varied to control film chemistry and crystal structure. The effect of relative humidity (i.e., < 1 to 85% RH) on friction is evaluated. Coatings grown at RT from targets with a low concentration of CF.~ exhibit ultralow friction (ULF) behavior in dry air (i.e., # ~< 0.01), but friction increases with RH. Mechanisms for the ULF behavior are proposed which suggest that further reductions in friction are possible. Films grown at 300 °C or with higher concentrations of CFx are relatively insensitive to humidity, but have more typical friction coefficients (~L ~< 0.04) in dry air.

Journal of Materials Science Letters, 1996

Applied Physics Letters, 1994

The objective of this letter is to report on the successful deposition of SiC by pulsed laser dep... more The objective of this letter is to report on the successful deposition of SiC by pulsed laser deposition at room temperature. Deposition of SiC films is accomplished by ablating a 6H-SiC target, using the 248 nm radiation from a KrF* excimer laser. X-ray photoelectron spectroscopy data conclusively show that the films are silicon carbide. The Si 2p peaks are observed from a film at 100.3 eV, from a 6H-SiC standard at 100.3 eV, and from a Si standard at 99.7 eV. Similar scans of the C 1s peak reveal a shift in binding energy from 284.7 eV for a graphite standard, to 283.3 eV for a deposited film, and 283.4 eV for the SiC standard. Further, the integrated areas and shapes of the peaks from the film and the SiC standard are equivalent. Transmission electron microscopy reveals a film microstructure which is largely amorphous, but which contains a significant volume fraction of SiC crystallites. Analysis of the electron diffraction patterns indicates that the crystallites are β-SiC. The ...