The fabrication of well‐separated, narrow, and relatively smooth silicon nanowires with good periodicity is demonstrated, using non‐close‐packed arrays of nanospheres with precisely controlled diameters, pitch, and roughness. Controlled reactive ion etching in an inductively coupled plasma reduces the self‐assembled nanospheres to approximately a tenth of their original diameter, while retaining their surface smoothness and periodic placement. A titanium adhesion layer between the silicon substrate and gold film allows much thinner catalyst layers to be continuous, facilitating the film liftoff and formation of the perforated pattern without influencing catalyzed etching of silicon. Using these methods, a periodic array of silicon nanowires with a large pitch and small diameter (e.g., a 490 nm pitch and 55 nm diameter) is created, a combination not typically found in the open literature. This approach extends the types and quality of silicon nanostructures that can be fabricated using the combined nanosphere lithography and metal‐assisted chemical etching techniques.

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When etching high-aspect-ratio silicon features using deep reactive ion etching ͑DRIE͒, researchers find that there is a maximum achievable aspect ratio, which we define as the critical aspect ratio, of an etched silicon trench using a DRIE process. At this critical aspect ratio, the apparent etch rate ͑defined as the total depth etched divided by the total elapsed time͒ no longer monotonically decreases as the aspect ratio increases, but abruptly drops to zero. In this paper, we propose a theoretical model to predict the critical aspect ratio and reveal its causal mechanism. The model considers aspect ratio dependent transport mechanisms specific to each of the reactant species in the three subprocesses of a time-multiplexed etch cycle: deposition of a fluorocarbon passivation layer, etching of the fluorocarbon polymer at the bottom of the trench, and the subsequent etching of the underlying silicon. The model predicts that the critical aspect ratio is defined by the aspect ratio at which the polymer etch rate equals the product of the deposition rate and the set time ratio between the deposition and etching phases for the time-multiplexed process. Several DRIE experiments were performed to qualitatively validate the model. Both model simulations and experimental results demonstrate that the magnitude of the critical aspect ratio primarily depends on ͑i͒ the relative flux of neutral species at the trench opening, i.e., the microloading effect, and ͑ii͒ aspect ratio dependent transport of ions during the polymer etching subprocess of a DRIE cycle.

This paper describes a strategy to impart brittle conductive patterns composed of silver nanoparticles with high stretchability and structure‐dependent electrical characteristics. Silver nanoinks are printed on an elastomeric polyurethane acrylate substrate in the form of planar serpentine structures that can effectively mitigate strain concentration. The relative changes in resistance (∆R/R 0) and stretchability are found to strongly depend on the serpentine radius (r) that determines the strain relieving efficiency. Features with small radius of curvature show colossal ∆R/R 0 and hold great promise as ultrasensitive stretchable strain gauges. A record high gauge factor of 107 is achieved at 12% strain with r = 200 µm. Devices with larger radius of curvature exhibit higher stretchability and much more stable conductance, thus can be used as stretchable conductors. The results demonstrate the versatile functionalities that can be acquired from conventional materials by judicious structural designs.

As a new family member of two-dimensional layered materials, black phosphorus (BP) has attracted significant attention for chemical sensing applications due to its exceptional electrical, mechanical, and surface properties. However, producing air-stable BP sensors is extremely challenging because BP atomic layers degrade rapidly in ambient conditions. In this study, we explored the humidity sensing properties of BP field-effect transistors fully encapsulated by a 6 nm-thick AlO encapsulation layer deposited by atomic layer deposition. The encapsulated BP sensors exhibited superior ambient stability with no noticeable degradation in sensing response after being stored in air for more than a week. Compared with the bare BP devices, the encapsulated ones offered long-term stability with a trade-off in slightly reduced sensitivity. Capacitance-voltage measurement results further reveal that instead of direct charge transfer, the electrostatic gating effect on BP flakes arising from the dipole moment of adsorbed water molecules is the basic mechanism governing the humidity sensing behavior of both bare and encapsulated BP sensors. This work demonstrates a viable approach for achieving air-stable BP-based humidity or chemical sensors for practical applications.

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The design, fabrication and characterization of a silicon microheater for an integrated MEMS gas preconcentrator Junghoon Yeom, Christopher R Field, Byunghoon Bae et al. Numerical modeling of three-dimensional compressible gas flow in microchannnels V Jain and C X Lin Modeling of liquid-gas meniscus for textured surfaces: effects of curvature and local slip length Anvesh Gaddam, Mayank Garg, Amit Agrawal et al. Experimental observations and lattice Boltzmann method study of the electroviscous effect for liquid flow G H Tang, Zhuo Li, Y L He et al. Stokes flow through a rectangular array of circular cylinders C Y Wang Low Reynolds number flow across an array of cylindrical microposts in a microchannel and figure-of-merit analysis of micropost-filled microreactors AbstractMicropost-filled reactors are commonly found in many micro-total analysis system applications because of their large surface area for the surrounding volume. Design rules for micropost-filled reactors are presented here to optimize the performance of a micro-preconcentrator, which is a component of a micro-gas chromatography system. A key figure of merit for the performance of the micropost-filled preconcentrator is to minimize the pressure drop while maximizing the surface-area-to-volume ratio for a given overall channel geometry. Several independent models from the literature are used to predict the flow resistance across the micropost-filled channels for low Reynolds number flows. The pressure drop can be expressed solely as a function of a couple of design parameters: β = a/s, the ratio of the radius of each post to the half-spacing between two adjacent posts, and N, the number of microposts in a row. Pressure drop measurements are performed to experimentally corroborate the flow resistance models and the optimization scheme using the figure of merit. As the number of microposts for a given β increases in a given channel size, a greater surface-area-to-volume ratio will result for a fixed pressure drop. Therefore, increasing the arrays of posts with smaller diameters and spacing will optimize the microreactor for larger surface area for a given flow resistance, at least until Knudsen flow begins to dominate.

A technique to create arrays of micrometer‐sized patterns of photosensitive polymers on the surface of elastomeric stamps and to transfer these patterns to planar and nonplanar substrates is presented. The photosensitive polymers are initially patterned through detachment lithography (DL), which utilizes the difference in adhesion forces to induce the mechanical failure in the film along the edges of the protruded parts of the mold. A polydimethylsiloxane (PDMS) stamp with a kinetically and thermally adjustable adhesion and conformal contact can transfer the detached patterns to etched or curved substrates, as well as planar ones. These printed patterns remain photochemically active for further modification via photolithography, and/or can serve as resists for subsequent etching or deposition, such that photolithography can be used on highly nonconformal and nonplanar surfaces. Various 3D structures fabricated using the process have potential applications in MEMS (micro‐electromechanical systems) sensors/actuators, optical devices, and microfluidics.