Ions Adsorbed at Amorphous Solid/Solution Interfaces Form Wigner Crystal-like Structures.

When a surface is immersed in a solution, it usually acquires a charge, which attracts counterions and repels co-ions to form an electrical double layer. The ions directly adsorbed to the surface are referred to as the Stern layer. The structure of the Stern layer normal to the interface was described decades ago, but the lateral organization within the Stern layer has received scant attention. This is because instrumental limitations have prevented visualization of the ion arrangements except for atypical, model, crystalline surfaces. Here, we use high-resolution amplitude modulated atomic force microscopy (AFM) to visualize the lateral structure of Stern layer ions adsorbed to polycrystalline gold, and amorphous silica and gallium nitride (GaN). For all three substrates, when the density of ions in the layer exceeds a system-dependent threshold, correlation effects induce the formation of close packed structures akin to Wigner crystals. Depending on the surface and the ions, the Wigner crystal-like structure can be hexagonally close packed, cubic, or worm-like. The influence of the electrolyte concentration, species, and valence, as well as the surface type and charge, on the Stern layer structures is described. When the system parameters are changed to reduce the Stern layer ion surface excess below the threshold value, Wigner crystal-like structures do not form and the Stern layer is unstructured. For gold surfaces, molecular dynamics (MD) simulations reveal that when sufficient potential is applied to the surface, ion clusters form with dimensions similar to the Wigner crystal-like structures in the AFM images. The lateral Stern layer structures presented, and in particular the Wigner crystal-like structures, will influence diverse applications in chemistry, energy storage, environmental science, nanotechnology, biology, and medicine

Released micromachined beams utilizing laterally uniform porosity porous silicon

© 2014, Sun et al.; licensee Springer. Abstract: Suspended micromachined porous silicon beams with laterally uniform porosity are reported, which have been fabricated using standard photolithography processes designed for compatibility with complementary metal-oxide-semiconductor (CMOS) processes. Anodization, annealing, reactive ion etching, repeated photolithography, lift off and electropolishing processes were used to release patterned porous silicon microbeams on a Si substrate. This is the first time that micromachined, suspended PS microbeams have been demonstrated with laterally uniform porosity, well-defined anchors and flat surfaces. PACS: 81.16.-c; 81.16.Nd; 81.16.R

Micromachined microbeams made from porous silicon for dynamic and static mode sensing

Through a controlled variation of the applied current during porous silicon formation, newly developedprocesses enable previously unattainable structural integrity of all-mesoporous silicon microelectrome-chanical systems (MEMS) structures. Such structures are desirable for applications such as sensing wherethe large surface area and low Young’s modulus of the high porosity layer enable ultra-high sensitivitydetection of adsorbed species. In this work, micromachined all-mesoporous silicon microbeams werereleased, allowing both the dynamic and static sensing modes to be studied using such porous struc-tures. Resonant frequencies (50–250 kHz) of released doubly clamped porous silicon microbeams weremeasured, allowing mechanical properties to be extracted. Static mode sensing of vapour at the 1100 ppmlevel was also performed, with the released porous silicon cantilevers showing a significant 6.5 m (3.7%of a 175 m beam length) and repeatable deflection after exposure

Fabrication of uniform porosity, all-porous-silicon microstructures and stress/stress gradient control

All-mesoporous silicon microstructures were released with standard micromachining processes. The extremely high porosity of the films allows control of the mechanical properties as well as providing a platform material for devices with extremely large surface area. To pattern and release devices from these highly porous structural layers, pore filling, photoresist mask adhesion and electropolishing techniques were developed. The internal stress of porous silicon was characterized under repeated thermal annealing and HF immersion treatments, allowing a stable, slightly tensile stress of 2.0  ±  0.4 MPa to be achieved. A method to independently control the stress gradient induced curvature in the porous MEMS devices was developed, which achieved released PS structures that were flat to within 78 nm over a range of 100 µm. This is the first time that fully released, stress gradient adjusted all-mesoporous-silicon structures have been reported

Stress control of porous silicon films for microelectromechanical systems

© 2015 Elsevier Inc. All rights reserved. Control of stress in porous silicon (PS) through porosity changes was studied using X-ray diffraction rocking curve measurements. The effect of thermal annealing on the stress was also investigated with both X-ray diffraction and radius of curvature measurements. Annealed films could achieve compressive or tensile stress. The effect of annealing was reversed by a short HF dip, except in the case of nitridised samples (annealed in N2 at temperatures above 500 °C). The effect of hydrogen desorption, oxidation and nitridation, modified via annealing temperature and ambient, was studied to understand the evolution of physical properties and the mechanism of the stress modification. The effect of stress on PS microbeams was studied to determine the influence when PS films are used as the structural layer in a micromachined device. When modelling the effect of stress changes on the order of those observed during thermal annealing, the results indicated that for PS-based microbeams, stress is a significant factor in determining resonant frequency, far more than found in nonporous materials, illustrating the need for accurate control of stress

Pt/GaN Schottky diode for propene (C3H6) gas sensing

Pt/GaN Schottky diode based gas sensors were fabricated and characterized for their sensitivity towards propene (C3H6) at high operating temperatures. The GaN epitaxial layer was deposited onto sapphire substrates by metal-organic chemical vapor deposition (MOCVD). Current-voltage (I-V) characteristics and the effective change in the barrier height of the sensors were investigated. The effective change of the barrier height was found to be 60.52 meV for 1% propene at 530°C. Dynamic response of the sensors at different propene concentrations in synthetic air was measured and a voltage shift of 231 mV and response time of 18s for 1% propene was obtained at temperatures as high as 530°C

Mercury(II) selective sensors based on AlGaN/GaN transistors

This work presents the first polymer approach to detect metal ions using AlGaN/GaN transistor-based sensor. The sensor utilised an AlGaN/GaN high electron mobility transistor-type structure by functionalising the gate area with a polyvinyl chloride (PVC) based ion selective membrane. Sensors based on this technology are portable, robust and typically highly sensitive to the target analyte; in this case Hg²⁺. This sensor showed a rapid and stable response when it was introduced to solutions of varying Hg²⁺ concentrations. At pH 2.8 in a 10⁻²M KNO₃ ion buffer, a detection limit below 10⁻⁸M and a linear response range between 10⁻⁸ M-10⁻⁴ M were achieved. This detection limit is an order of magnitude lower than the reported detection limit of 10⁻⁷M for thioglycolic acid monolayer functionalised AlGaN/GaN HEMT devices. Detection limits of approximately 10⁻⁷M and 10⁻⁶M in 10⁻²M Cd(NO₃)₂ and 10⁻²M Pb(NO₃)₂ ion buffers were also achieved, respectively. Furthermore, we show that the apparent gate response was near-Nernstian under various conditions. X-ray photoelectron spectroscopy (XPS) experiments confirmed that the sensing membrane is reversible after being exposed to Hg²⁺ solution and rinsed with deionised water. The success of this study precedes the development of this technology in selectively sensing multiple ions in water with use of the appropriate polymer based membranes on arrays of devices.7 page(s