Junghoon Yeom - Academia.edu (original) (raw)
Papers by Junghoon Yeom
We present a versatile technique to create arrays of micrometer-size patterns of photosensitive p... more We present a versatile technique to create arrays of micrometer-size patterns of photosensitive polymers like photoresists (PRs) on the surface of soft stamps and to transfer these patterns to planar, curved, and porous substrates. A PR film was patterned through detachment lithography, utilizing the difference in adhesion to induce the fracture in the film. Various 3D structures were fabricated on glass cylinders as well as flat Si substrates. The detached patterns were printed on a nanoporous membrane, facilitating the guided assembly of nanomaterials.
Physical Chemistry of Semiconductor Materials and Interfaces XX
In most of ionic field-effect bio-sensors currently available in the literature, the major role o... more In most of ionic field-effect bio-sensors currently available in the literature, the major role of flow rectification or switching is played by electrostatic gatings through a single electrode. In this paper, however, we propose a paradigm for ionic field-effect sensing devices, where both the hydrodynamic and the electrostatic behaviors are leveraged by employing a double-gate configuration. Ultrahigh sensitivity of the double-gate device with respect to the single-gate bio-sensing devices is demonstrated for field effects through numerical simulations.
This work reports the synthesis and materials characterization of novel heterogeneous nanostructu... more This work reports the synthesis and materials characterization of novel heterogeneous nanostructures consisting of zinc oxide (ZnO) nanowires and silicon (Si) nanocrystals. Hydrothermal and non-thermal plasma synthesis techniques were utilized to create ZnO nanowires and Si nanocrystals, respectively. The heterostructures of ZnO nanowires and Si nanocrystals were formed and characterized using scanning electron microscopy, transmission electron microscopy, the Raman spectroscopy, and photoluminescence. The heterostructures have been evaluated for the photodegradation performance, demonstrating that the addition of Si nanocrystals to ZnO nanostructures improves the degradability under visible light.
Scientific Reports
Non-linear effects of the Navier–Stokes equations disappear under the Stokes regime of Newtonian ... more Non-linear effects of the Navier–Stokes equations disappear under the Stokes regime of Newtonian fluid flows disallowing a flow rectification behavior. Here we show that passive flow rectification of Newtonian fluids is obtainable under the Stokes regime of both compressible and incompressible flows by introducing nonlinearity into the otherwise linear Stokes equations. Asymmetric flow resistances arise in shallow nozzle/diffuser microchannels with deformable ceiling, in which the fluid flow is governed by a non-linear coupled fluid–solid mechanics equation. The proposed model captures the unequal deflection profile of the deformable ceiling depending on the flow direction under the identical applied pressure, permitting a larger flow rate in the nozzle configuration. Ultra-low aspect ratio microchannels sealed by a flexible membrane have been fabricated to demonstrate passive flow rectification for low-Reynolds-number flows (0.001
Microsystems & Nanoengineering
Exploiting nonlinear characteristics in micro/nanosystems has been a subject of increasing intere... more Exploiting nonlinear characteristics in micro/nanosystems has been a subject of increasing interest in the last decade. Among others, vigorous intermodal coupling through internal resonance (IR) has drawn much attention because it can suggest new strategies to steer energy within a micro/nanomechanical resonator. However, a challenge in utilizing IR in practical applications is imposing the required frequency commensurability between vibrational modes of a nonlinear micro/nanoresonator. Here, we experimentally and analytically investigate the 1:2 and 2:1 IR in a clamped–clamped beam resonator to provide insights into the detailed mechanism of IR. It is demonstrated that the intermodal coupling between the second and third flexural modes in an asymmetric structure (e.g., nonprismatic beam) provides an optimal condition to easily implement a strong IR with high energy transfer to the internally resonated mode. In this case, the quadratic coupling between these flexural modes, originat...
Nanomaterials
Numerous semiconductor-based hybrid nanostructures have been studied for improved photodegradatio... more Numerous semiconductor-based hybrid nanostructures have been studied for improved photodegradation performance resulting from their broadband optical response and enhanced charge separation/transport characteristics. However, these hybrid structures often involve elements that are rare or toxic. Here, we present the synthesis and material characterization of hybrid nanostructures consisting of zinc oxide (ZnO) nanowires (NWs) and silicon nanocrystals (Si-NCs), both abundant and environmentally benign, and evaluate them for photodegradation performance under various illumination conditions. When incorporating Si-NCs into the vertically-aligned ZnO NWs immobilized on substrates, the resulting photocatalysts exhibited a narrowed band gap, i.e., more responsive to visible light, and enhanced charge separation at the interface, i.e., more reactive species produced for degradation. Consequently, the hybrid Si-NCs/ZnO-NWs displayed a superior photodegradability for methylene blue under UV ...
Journal of Micromechanics and Microengineering
Sensors
A conventional approach to making miniature or microscale gas chromatography (GC) components reli... more A conventional approach to making miniature or microscale gas chromatography (GC) components relies on silicon as a base material and MEMS fabrication as manufacturing processes. However, these devices often fail in medium-to-high temperature applications due to a lack of robust fluidic interconnects and a high-yield bonding process. This paper explores the feasibility of using metal additive manufacturing (AM), which is also known as metal 3D printing, as an alternative platform to produce small-scale microfluidic devices that can operate at a temperature higher than that which polymers can withstand. Binder jet printing (BJP), one of the metal AM processes, was utilized to make stainless steel (SS) preconcentrators (PCs) with submillimeter internal features. PCs can increase the concentration of gaseous analytes or serve as an inline injector for GC or gas sensor applications. Normally, parts printed by BJP are highly porous and thus often infiltrated with low melting point metal....
Microfluidics and Nanofluidics
The emerging field of deformable microfluidics widely employed in the Lab-on-a-Chip and MEMS comm... more The emerging field of deformable microfluidics widely employed in the Lab-on-a-Chip and MEMS communities offers an opportunity to study a relatively under-examined physics. The main objective of this work is to provide a deeper insight into the underlying coupled fluid-solid interactions of a low-Reynolds-number, i.e. Re ∼ O(10 −2-10 +1) , fluid flow through a shallow deformable microchannel with ultra-low height-to-width-ratios, i.e. O(10 −3). The fabricated deformable microchannels of several microns in height and few millimeters in width, whose aspect ratio is about two orders of magnitude smaller than that of the previous reports, allow us to investigate the fluid flow characteristics spanning a variety of distinct regimes from small wall deflections, where the deformable microchannel resembles its corresponding rigid one, to wall deflections much larger than the original height, where the height-independent characteristic behavior emerges. The effects of the microchannel geometry, membrane properties, and pressure difference across the channel are represented by a lumped variable called flexibility parameter. Under the same pressure drop across different channels, any difference in their geometries is reflected into the flexibility parameter of the channels, which can potentially cause the devices to operate under distinct regimes of the fluid-solid characteristics. For a fabricated microchannel with given membrane properties and channel geometry, on the other hand, a sufficiently large change in the applied pressure difference can alter the flow-structure behavior from one characteristic regime to another. By appropriately introducing the flexibility parameter and the dimensionless volumetric flow rate, a master curve is found for the fluid flow through any long and shallow deformable microchannel. A criterion is also suggested for determining whether the coupled or decoupled fluid-solid mechanics should be considered.
Nano-Micro Letters
• ZnO nanowires were securely immobilized onto a floatable photocatalytic platform, which had a u... more • ZnO nanowires were securely immobilized onto a floatable photocatalytic platform, which had a uniform diameter (55 ± 5 nm) and length (1.5 ± 0.3 μm). • An additional 20% of the probe pollutant (methylene blue) was degraded by piezocatalysis-assisted photocatalytic degradation. • The crude oil pollutant was decomposed up to 20% within 6 h.
Langmuir : the ACS journal of surfaces and colloids, Oct 31, 2017
Self-assembly of colloidal nanospheres combined with various nanofabrication techniques produces ... more Self-assembly of colloidal nanospheres combined with various nanofabrication techniques produces an ever-increasing range of two-dimensional (2D) ordered nanostructures, although the pattern periodicity is typically bound to the original interparticle spacing. Deformable soft lithography using controlled deformation of elastomeric substrates and subsequent contact printing transfer offer a versatile method to systematically control the lattice spacing and arrangements of the 2D nanosphere array. However, the anisotropic nature of uniaxial and biaxial stretching as well as the strain limit of solvent swelling makes it difficult to create well-separated, ordered 2D nanosphere arrays with large-area hexagonal arrangements. In this paper, we report a simple, facile approach to fabricate such arrays of polystyrene nanospheres using a custom-made radial stretching apparatus. The maximum stretchability and spatial uniformity of the poly(dimethylsiloxane) (PDMS) elastomeric substrate is sys...
Procedia Manufacturing
Abstract Miniaturization of chemical system has attracted tremendous interests in chemistry and b... more Abstract Miniaturization of chemical system has attracted tremendous interests in chemistry and biology due to many advantages such as enhancement in heat/mass transfer rates, reduction in expensive reagents and hazardous wastes, and facilitation of massive parallelization in reaction/catalyst screening and optimization Fabrication of microchemical systems (μCSs) has matured around materials such as silicon, glass and polymers, which are not particularly suitable for harsh environment such as high pressure, high temperature, and corrosive reactants. The main objective of this paper is to introduce a new ceramic-based fabrication framework for such harsh environment. This paper demonstrates the feasibility of the new technique by constructing a micro-burner, which can be used as on-chip heater or sensor. The proposed technique overcomes one major roadblock in fabrication of ceramic-based μCSs. The unreliable joining and assembling of simple ceramic components are eliminated by utilizing a fugitive phase machined into an intricate shape of an internal combustion chamber and channels. The main fabrication strategy is to use the advantage of powder, offering the fluidity to flow into a complex cavity or around the solid phase as well as the rigidity to remove the undesirable sections. The paper demonstrates the unique method of fabricating a micro-burner that can sustain an oxy-hydrogen flame for an extensive period and under cyclic operations.
Nanotechnology
Nanosphere lithography (NSL) has been widely used as an inexpensive method to create periodic arr... more Nanosphere lithography (NSL) has been widely used as an inexpensive method to create periodic arrays of metallic nanoparticles or nanodiscs on substrates. However, most nanodisc arrays derived from a NSL template are restricted to hexagonally-ordered triangular arrays because the metal layer is deposited onto the interstices between the nanospheres. Metallic nanodisc arrays with the same arrangement as the original nanosphere array have been rarely reported. Here, we demonstrate a facile, low-cost method to fabricate large-area hexagonal arrays of metallic nanodiscs using an NSL template combined with a two-step lift-off process. We employ a bi-layer of two dissimilar metals to create a re-entrant sidewall profile to undercut the sacrificial layer and facilitate the final lift-off of the metallic nanodiscs. The quality of the nanodisc pattern and the array periodicity is determined using statistical image analysis and compared to the original nanosphere array in terms of size distribution, surface smoothness, and array pitch. This nanodisc array is used as an etch mask to create a vertically-aligned Si nanowire array. This combined approach is a scalable and inexpensive fabrication method for creating relatively large-area, ordered arrays of various nanostructures.
Scientific Reports
Ceramic-based microchemical systems (μCSs) are more suitable for operation under harsh environmen... more Ceramic-based microchemical systems (μCSs) are more suitable for operation under harsh environments such as high temperature and corrosive reactants compared to the more conventional μCS materials such as silicon and polymers. With the recent renewed interests in chemical manufacturing and process intensification, simple, inexpensive, and reliable ceramic manufacturing technologies are needed. The main objective of this paper is to introduce a new powder-based fabrication framework, which is a one-pot, cost-effective, and versatile process for ceramic μCS components. The proposed approach employs the compaction of metal-oxide sub-micron powders with a graphite fugitive phase that is burned out to create internal cavities and microchannels before full sintering. Pure alumina powder has been used without any binder phase, enabling more precise dimensional control and less structure shrinkage upon sintering. The key process steps such as powder compaction, graphite burnout during partial sintering, machining in a conventional machine tool, and final densification have been studied to characterize the process. This near-full density ceramic structure with the combustion chamber and various internal channels was fabricated to be used as a micro-burner for gas sensing applications. Miniaturization of chemical system has garnered significant attentions in chemistry and biology due to many advantages such as enhancement in heat/mass transfer rates at small scale, reduction in expensive reagents and hazardous wastes, and facilitation of massive parallelization in reaction/catalyst screening and optimization 1-3. The significant technological advancements for micro-chemical systems (μ CSs) have been focused on chemical reactions, separation, and sensing in a low-to-medium temperature range (20 °C to 600 °C). One notable example is a lab-on-a-chip or micro total analytical system, in which the total sequence of laboratory processes is integrated to perform chemical synthesis, transport, and analysis, and it has profound influence in chemistry and biomedical areas 4,5. In some cases, the microreactors and heat exchangers in μ CS need to be operated at high temperatures (> 600 °C) and/or under highly corrosive environments like solid-oxide fuel cells 6-9 , fuel reformers 10-12 , combustion burners 13,14 , and gasifiers 15,16. However, high-temperature μ CSs with sophisticated design and similar level of integration found in low-temperature counterpart have rarely been realized mainly because the conventional μ CSs such as silicon, glass, polymers, metals and conventional metal alloys are not stable at these high operating temperatures. Ceramic materials offer excellent high-temperature compatibility and corrosion resistances, but pose significant manufacturing challenges due to their hardness and brittleness. Several groups have demonstrated the promise of ceramic-based microreactors for medium-to-high temperature reactions such as hydrogen production from continuous reforming of propane 10,17,18 , oxidative coupling of methane 19,20 , catalytic combustion 21 , and nanoparticle synthesis 22. One of the paramount challenges in fabricating ceramic μ CSs is that the microfabrication techniques borrowed from well-established microelectronics and microelectromechanical system (MEMS), which are very effective for silicon-or polymer-based μ CSs, are not compatible with ceramic materials. Instead, the conventional and non-conventional ceramic processing techniques have been utilized to create ceramic microreactors and other components of μ CSs. These techniques include rapid prototyping using low-pressure injection molding 19,23 , micromachining 20,24 , sol-gel/nanoparticle casting 25,26 , and tape casting 27,28. In the rapid prototyping process, a negative silicone mold is first created from the
Physical Review B
Implementation of geometric nonlinearity in micro-electro-mechanical system (MEMS) resonators off... more Implementation of geometric nonlinearity in micro-electro-mechanical system (MEMS) resonators offers a flexible and efficient design to overcome the limitations of linear MEMS by utilizing beneficial nonlinear characteristics not attainable in a linear setting. Integration of nonlinear coupling elements into an otherwise purely linear microcantilever is one promising way to intentionally realize geometric nonlinearity. Here, we demonstrate that a nonlinear, heterogeneous micro-resonator system, consisting of a silicon micro-cantilever with a polymer attachment exhibits strong nonlinear hardening behavior not only in the first flexural mode but also in the higher modes (i.e., second and third flexural modes). In this design, we deliberately implement a drastic and reversed change in the axial vs. bending stiffness between the Si and polymer components by varying the geometric and material properties. By doing so, the resonant oscillations induce the large axial stretching within the polymer component, which effectively introduces the geometric stiffness and damping nonlinearity. The efficacy of the design and the mechanism of geometric nonlinearity are corroborated through a comprehensive experimental, analytical, and numerical (Finite Element) analysis on the nonlinear dynamics of the proposed system.
ACS applied materials & interfaces, Jan 22, 2017
As a new family member of two-dimensional layered materials, black phosphorus (BP) has attracted ... more 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 Al2O3 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 th...
Advanced Functional Materials, Jan 22, 2010
We present a versatile technique to create arrays of micrometer-size patterns of photosensitive p... more We present a versatile technique to create arrays of micrometer-size patterns of photosensitive polymers like photoresists (PRs) on the surface of soft stamps and to transfer these patterns to planar, curved, and porous substrates. A PR film was patterned through detachment lithography, utilizing the difference in adhesion to induce the fracture in the film. Various 3D structures were fabricated on glass cylinders as well as flat Si substrates. The detached patterns were printed on a nanoporous membrane, facilitating the guided assembly of nanomaterials.
Physical Chemistry of Semiconductor Materials and Interfaces XX
In most of ionic field-effect bio-sensors currently available in the literature, the major role o... more In most of ionic field-effect bio-sensors currently available in the literature, the major role of flow rectification or switching is played by electrostatic gatings through a single electrode. In this paper, however, we propose a paradigm for ionic field-effect sensing devices, where both the hydrodynamic and the electrostatic behaviors are leveraged by employing a double-gate configuration. Ultrahigh sensitivity of the double-gate device with respect to the single-gate bio-sensing devices is demonstrated for field effects through numerical simulations.
This work reports the synthesis and materials characterization of novel heterogeneous nanostructu... more This work reports the synthesis and materials characterization of novel heterogeneous nanostructures consisting of zinc oxide (ZnO) nanowires and silicon (Si) nanocrystals. Hydrothermal and non-thermal plasma synthesis techniques were utilized to create ZnO nanowires and Si nanocrystals, respectively. The heterostructures of ZnO nanowires and Si nanocrystals were formed and characterized using scanning electron microscopy, transmission electron microscopy, the Raman spectroscopy, and photoluminescence. The heterostructures have been evaluated for the photodegradation performance, demonstrating that the addition of Si nanocrystals to ZnO nanostructures improves the degradability under visible light.
Scientific Reports
Non-linear effects of the Navier–Stokes equations disappear under the Stokes regime of Newtonian ... more Non-linear effects of the Navier–Stokes equations disappear under the Stokes regime of Newtonian fluid flows disallowing a flow rectification behavior. Here we show that passive flow rectification of Newtonian fluids is obtainable under the Stokes regime of both compressible and incompressible flows by introducing nonlinearity into the otherwise linear Stokes equations. Asymmetric flow resistances arise in shallow nozzle/diffuser microchannels with deformable ceiling, in which the fluid flow is governed by a non-linear coupled fluid–solid mechanics equation. The proposed model captures the unequal deflection profile of the deformable ceiling depending on the flow direction under the identical applied pressure, permitting a larger flow rate in the nozzle configuration. Ultra-low aspect ratio microchannels sealed by a flexible membrane have been fabricated to demonstrate passive flow rectification for low-Reynolds-number flows (0.001
Microsystems & Nanoengineering
Exploiting nonlinear characteristics in micro/nanosystems has been a subject of increasing intere... more Exploiting nonlinear characteristics in micro/nanosystems has been a subject of increasing interest in the last decade. Among others, vigorous intermodal coupling through internal resonance (IR) has drawn much attention because it can suggest new strategies to steer energy within a micro/nanomechanical resonator. However, a challenge in utilizing IR in practical applications is imposing the required frequency commensurability between vibrational modes of a nonlinear micro/nanoresonator. Here, we experimentally and analytically investigate the 1:2 and 2:1 IR in a clamped–clamped beam resonator to provide insights into the detailed mechanism of IR. It is demonstrated that the intermodal coupling between the second and third flexural modes in an asymmetric structure (e.g., nonprismatic beam) provides an optimal condition to easily implement a strong IR with high energy transfer to the internally resonated mode. In this case, the quadratic coupling between these flexural modes, originat...
Nanomaterials
Numerous semiconductor-based hybrid nanostructures have been studied for improved photodegradatio... more Numerous semiconductor-based hybrid nanostructures have been studied for improved photodegradation performance resulting from their broadband optical response and enhanced charge separation/transport characteristics. However, these hybrid structures often involve elements that are rare or toxic. Here, we present the synthesis and material characterization of hybrid nanostructures consisting of zinc oxide (ZnO) nanowires (NWs) and silicon nanocrystals (Si-NCs), both abundant and environmentally benign, and evaluate them for photodegradation performance under various illumination conditions. When incorporating Si-NCs into the vertically-aligned ZnO NWs immobilized on substrates, the resulting photocatalysts exhibited a narrowed band gap, i.e., more responsive to visible light, and enhanced charge separation at the interface, i.e., more reactive species produced for degradation. Consequently, the hybrid Si-NCs/ZnO-NWs displayed a superior photodegradability for methylene blue under UV ...
Journal of Micromechanics and Microengineering
Sensors
A conventional approach to making miniature or microscale gas chromatography (GC) components reli... more A conventional approach to making miniature or microscale gas chromatography (GC) components relies on silicon as a base material and MEMS fabrication as manufacturing processes. However, these devices often fail in medium-to-high temperature applications due to a lack of robust fluidic interconnects and a high-yield bonding process. This paper explores the feasibility of using metal additive manufacturing (AM), which is also known as metal 3D printing, as an alternative platform to produce small-scale microfluidic devices that can operate at a temperature higher than that which polymers can withstand. Binder jet printing (BJP), one of the metal AM processes, was utilized to make stainless steel (SS) preconcentrators (PCs) with submillimeter internal features. PCs can increase the concentration of gaseous analytes or serve as an inline injector for GC or gas sensor applications. Normally, parts printed by BJP are highly porous and thus often infiltrated with low melting point metal....
Microfluidics and Nanofluidics
The emerging field of deformable microfluidics widely employed in the Lab-on-a-Chip and MEMS comm... more The emerging field of deformable microfluidics widely employed in the Lab-on-a-Chip and MEMS communities offers an opportunity to study a relatively under-examined physics. The main objective of this work is to provide a deeper insight into the underlying coupled fluid-solid interactions of a low-Reynolds-number, i.e. Re ∼ O(10 −2-10 +1) , fluid flow through a shallow deformable microchannel with ultra-low height-to-width-ratios, i.e. O(10 −3). The fabricated deformable microchannels of several microns in height and few millimeters in width, whose aspect ratio is about two orders of magnitude smaller than that of the previous reports, allow us to investigate the fluid flow characteristics spanning a variety of distinct regimes from small wall deflections, where the deformable microchannel resembles its corresponding rigid one, to wall deflections much larger than the original height, where the height-independent characteristic behavior emerges. The effects of the microchannel geometry, membrane properties, and pressure difference across the channel are represented by a lumped variable called flexibility parameter. Under the same pressure drop across different channels, any difference in their geometries is reflected into the flexibility parameter of the channels, which can potentially cause the devices to operate under distinct regimes of the fluid-solid characteristics. For a fabricated microchannel with given membrane properties and channel geometry, on the other hand, a sufficiently large change in the applied pressure difference can alter the flow-structure behavior from one characteristic regime to another. By appropriately introducing the flexibility parameter and the dimensionless volumetric flow rate, a master curve is found for the fluid flow through any long and shallow deformable microchannel. A criterion is also suggested for determining whether the coupled or decoupled fluid-solid mechanics should be considered.
Nano-Micro Letters
• ZnO nanowires were securely immobilized onto a floatable photocatalytic platform, which had a u... more • ZnO nanowires were securely immobilized onto a floatable photocatalytic platform, which had a uniform diameter (55 ± 5 nm) and length (1.5 ± 0.3 μm). • An additional 20% of the probe pollutant (methylene blue) was degraded by piezocatalysis-assisted photocatalytic degradation. • The crude oil pollutant was decomposed up to 20% within 6 h.
Langmuir : the ACS journal of surfaces and colloids, Oct 31, 2017
Self-assembly of colloidal nanospheres combined with various nanofabrication techniques produces ... more Self-assembly of colloidal nanospheres combined with various nanofabrication techniques produces an ever-increasing range of two-dimensional (2D) ordered nanostructures, although the pattern periodicity is typically bound to the original interparticle spacing. Deformable soft lithography using controlled deformation of elastomeric substrates and subsequent contact printing transfer offer a versatile method to systematically control the lattice spacing and arrangements of the 2D nanosphere array. However, the anisotropic nature of uniaxial and biaxial stretching as well as the strain limit of solvent swelling makes it difficult to create well-separated, ordered 2D nanosphere arrays with large-area hexagonal arrangements. In this paper, we report a simple, facile approach to fabricate such arrays of polystyrene nanospheres using a custom-made radial stretching apparatus. The maximum stretchability and spatial uniformity of the poly(dimethylsiloxane) (PDMS) elastomeric substrate is sys...
Procedia Manufacturing
Abstract Miniaturization of chemical system has attracted tremendous interests in chemistry and b... more Abstract Miniaturization of chemical system has attracted tremendous interests in chemistry and biology due to many advantages such as enhancement in heat/mass transfer rates, reduction in expensive reagents and hazardous wastes, and facilitation of massive parallelization in reaction/catalyst screening and optimization Fabrication of microchemical systems (μCSs) has matured around materials such as silicon, glass and polymers, which are not particularly suitable for harsh environment such as high pressure, high temperature, and corrosive reactants. The main objective of this paper is to introduce a new ceramic-based fabrication framework for such harsh environment. This paper demonstrates the feasibility of the new technique by constructing a micro-burner, which can be used as on-chip heater or sensor. The proposed technique overcomes one major roadblock in fabrication of ceramic-based μCSs. The unreliable joining and assembling of simple ceramic components are eliminated by utilizing a fugitive phase machined into an intricate shape of an internal combustion chamber and channels. The main fabrication strategy is to use the advantage of powder, offering the fluidity to flow into a complex cavity or around the solid phase as well as the rigidity to remove the undesirable sections. The paper demonstrates the unique method of fabricating a micro-burner that can sustain an oxy-hydrogen flame for an extensive period and under cyclic operations.
Nanotechnology
Nanosphere lithography (NSL) has been widely used as an inexpensive method to create periodic arr... more Nanosphere lithography (NSL) has been widely used as an inexpensive method to create periodic arrays of metallic nanoparticles or nanodiscs on substrates. However, most nanodisc arrays derived from a NSL template are restricted to hexagonally-ordered triangular arrays because the metal layer is deposited onto the interstices between the nanospheres. Metallic nanodisc arrays with the same arrangement as the original nanosphere array have been rarely reported. Here, we demonstrate a facile, low-cost method to fabricate large-area hexagonal arrays of metallic nanodiscs using an NSL template combined with a two-step lift-off process. We employ a bi-layer of two dissimilar metals to create a re-entrant sidewall profile to undercut the sacrificial layer and facilitate the final lift-off of the metallic nanodiscs. The quality of the nanodisc pattern and the array periodicity is determined using statistical image analysis and compared to the original nanosphere array in terms of size distribution, surface smoothness, and array pitch. This nanodisc array is used as an etch mask to create a vertically-aligned Si nanowire array. This combined approach is a scalable and inexpensive fabrication method for creating relatively large-area, ordered arrays of various nanostructures.
Scientific Reports
Ceramic-based microchemical systems (μCSs) are more suitable for operation under harsh environmen... more Ceramic-based microchemical systems (μCSs) are more suitable for operation under harsh environments such as high temperature and corrosive reactants compared to the more conventional μCS materials such as silicon and polymers. With the recent renewed interests in chemical manufacturing and process intensification, simple, inexpensive, and reliable ceramic manufacturing technologies are needed. The main objective of this paper is to introduce a new powder-based fabrication framework, which is a one-pot, cost-effective, and versatile process for ceramic μCS components. The proposed approach employs the compaction of metal-oxide sub-micron powders with a graphite fugitive phase that is burned out to create internal cavities and microchannels before full sintering. Pure alumina powder has been used without any binder phase, enabling more precise dimensional control and less structure shrinkage upon sintering. The key process steps such as powder compaction, graphite burnout during partial sintering, machining in a conventional machine tool, and final densification have been studied to characterize the process. This near-full density ceramic structure with the combustion chamber and various internal channels was fabricated to be used as a micro-burner for gas sensing applications. Miniaturization of chemical system has garnered significant attentions in chemistry and biology due to many advantages such as enhancement in heat/mass transfer rates at small scale, reduction in expensive reagents and hazardous wastes, and facilitation of massive parallelization in reaction/catalyst screening and optimization 1-3. The significant technological advancements for micro-chemical systems (μ CSs) have been focused on chemical reactions, separation, and sensing in a low-to-medium temperature range (20 °C to 600 °C). One notable example is a lab-on-a-chip or micro total analytical system, in which the total sequence of laboratory processes is integrated to perform chemical synthesis, transport, and analysis, and it has profound influence in chemistry and biomedical areas 4,5. In some cases, the microreactors and heat exchangers in μ CS need to be operated at high temperatures (> 600 °C) and/or under highly corrosive environments like solid-oxide fuel cells 6-9 , fuel reformers 10-12 , combustion burners 13,14 , and gasifiers 15,16. However, high-temperature μ CSs with sophisticated design and similar level of integration found in low-temperature counterpart have rarely been realized mainly because the conventional μ CSs such as silicon, glass, polymers, metals and conventional metal alloys are not stable at these high operating temperatures. Ceramic materials offer excellent high-temperature compatibility and corrosion resistances, but pose significant manufacturing challenges due to their hardness and brittleness. Several groups have demonstrated the promise of ceramic-based microreactors for medium-to-high temperature reactions such as hydrogen production from continuous reforming of propane 10,17,18 , oxidative coupling of methane 19,20 , catalytic combustion 21 , and nanoparticle synthesis 22. One of the paramount challenges in fabricating ceramic μ CSs is that the microfabrication techniques borrowed from well-established microelectronics and microelectromechanical system (MEMS), which are very effective for silicon-or polymer-based μ CSs, are not compatible with ceramic materials. Instead, the conventional and non-conventional ceramic processing techniques have been utilized to create ceramic microreactors and other components of μ CSs. These techniques include rapid prototyping using low-pressure injection molding 19,23 , micromachining 20,24 , sol-gel/nanoparticle casting 25,26 , and tape casting 27,28. In the rapid prototyping process, a negative silicone mold is first created from the
Physical Review B
Implementation of geometric nonlinearity in micro-electro-mechanical system (MEMS) resonators off... more Implementation of geometric nonlinearity in micro-electro-mechanical system (MEMS) resonators offers a flexible and efficient design to overcome the limitations of linear MEMS by utilizing beneficial nonlinear characteristics not attainable in a linear setting. Integration of nonlinear coupling elements into an otherwise purely linear microcantilever is one promising way to intentionally realize geometric nonlinearity. Here, we demonstrate that a nonlinear, heterogeneous micro-resonator system, consisting of a silicon micro-cantilever with a polymer attachment exhibits strong nonlinear hardening behavior not only in the first flexural mode but also in the higher modes (i.e., second and third flexural modes). In this design, we deliberately implement a drastic and reversed change in the axial vs. bending stiffness between the Si and polymer components by varying the geometric and material properties. By doing so, the resonant oscillations induce the large axial stretching within the polymer component, which effectively introduces the geometric stiffness and damping nonlinearity. The efficacy of the design and the mechanism of geometric nonlinearity are corroborated through a comprehensive experimental, analytical, and numerical (Finite Element) analysis on the nonlinear dynamics of the proposed system.
ACS applied materials & interfaces, Jan 22, 2017
As a new family member of two-dimensional layered materials, black phosphorus (BP) has attracted ... more 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 Al2O3 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 th...
Advanced Functional Materials, Jan 22, 2010