J. W. Dailey

Arizona State University, Phoenix, Arizona, United States

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Publications (4)7.27 Total impact

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    ABSTRACT: A rich variety of nanostructures can be synthesized by varying pressure and temperature during vapor–liquid–solid (VLS) epitaxy on Si. The chemical vapor deposition (CVD) growth by VLS of epitaxial Ge nanowires and nanopillars seeded by metallic nanodots on (1 1 1) and (1 0 0) oriented Si is reported with an emphasis on analyses by ion backscattering and channeling in combination with scanning electron microscopy. The nanostructures are grown using digermane in an Ultra High Vacuum (UHV) system at pressures from 10−6 to 10−2 Torr and temperatures between 400 and 600 °C. Au nanodots with diameters of 10–50 nm are formed by vapor deposition on H-terminated Si surfaces. The Ge growth kinetics and morphology are observed to depend strongly on pressure. At lower pressures the Au seeds the growth of layered heteroepitaxial islands (referred to here as nanopillars) which grow both vertically and laterally. At higher pressures a transition to rapid 〈1 1 1〉 axial nanowire epitaxial growth occurs with a growth rate that scales linearly with pressure. We contrast quantitative measurements of the kinetics for VLS nanowire growth with that for uniform CVD layer growth.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2006; 242(s 1–2):205–208. · 1.27 Impact Factor
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    ABSTRACT: The pressure and temperature dependencies for vapor-liquid-solid (VLS) growth of Ge nanostructures on Si using chemical vapor deposition are reported. Gold nanodots self-assembled by evaporation on clean hydrogen-terminated and heated Si substrates are used to seed the liquid eutectic VLS growth. Digermane pressures are varied from 4×10−5 to 1×10−2 Torr and substrate temperatures from 400 to 600°C for heteroepitaxial growth on Si(111). Two types of nanostructures are identified, nanowires and nanopillars, with a transition from nanopillar growth to nanowire growth occurring with increasing pressure. Nanowires are characterized by rapid vertical growth, long-aspect-ratio structures, and linear dependence of the growth rate on pressure. At lower pressures a transition to nanopillars is observed; these exhibit both vertical and lateral growth with typical aspect ratios of 1:2. For Si(111) substrates nanowires grow epitaxially with their growth axis along the 〈111〉 direction. High-resolution transmission electron microscopy shows that the Ge nanowires are relaxed to their equilibrium lattice spacings a short distance from the Si substrate interface.
    Journal of Applied Physics 12/2004; 96(12):7556-7567. · 2.21 Impact Factor
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    ABSTRACT: A rough surface morphology is shown to significantly amplify the light-induced change in water contact angle of a photoresponsive surface. Smooth Si surfaces and fractally rough Si nanowire surfaces grown on a Si substrate were studied, both coated with a hydrophobic monolayer containing photochromic spiropyran molecules. Under visible irradiation the spiropyran is in a closed, hydrophobic form, whereas UV irradiation converts the spiropyran to a polar, hydrophilic form, reducing the contact angle. The superhydrophobic nanowire surface both amplifies the light-induced contact angle change by a factor of 2 relative to a smooth surface and reduces the contact angle hysteresis. As a result the UV-induced advancing contact angle is lower than the receding contact angle under visible irradiation, allowing water drops to be moved solely under the influence of a UV−visible light gradient. The amplification of the reversible light-induced wetting angle change was predicted using the Wenzel model for fractally rough surfaces. The model and amplification effects are expected to apply to other types of stimuli-induced contact angle changes such as that by heat or electrical potentials.
    Journal of Physical Chemistry B - J PHYS CHEM B. 07/2004; 108(34).
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    ABSTRACT: We report epitaxial growth of Ge nanopillars ( NPs ) on Si (100) by vapor-liquid-solid ( VLS ) growth from digermane. This growth morphology is characterized by short, low-aspect-ratio pillars and is markedly different from the long, narrow nanowires ( NWs ) previously reported for VLS growth. The NP growth mode occurs at low digermane pressures. It is attributed to surface-diffusion-induced lateral growth in combination with an insufficient Ge concentration gradient in the AuGe eutectic to catalyze NW growth. High resolution electron microscopy confirms that the NPs are epitaxial with the Si (100) substrate and are fully relaxed and strain free.
    Applied Physics Letters 07/2004; · 3.79 Impact Factor