Viscous liquid-liquid wetting and dewetting of textured surfaces.

We experimentally investigate the spreading and receding behavior of small water droplets immersed in viscous oils on grid-patterned surfaces using synchronized bottom and profile views. In particular, the evolution of apparent advancing and receding contact angles of droplets fed at constant flow rate is studied as a function of grid surface coverage and height for a wide range of external phase viscosity. Detailed examination of droplet aspect ratio during inflation process provides an averaging method for characterization of quasi-static advancing angles on heterogeneous surfaces. Droplets spreading in partial Cassie state on planar microfluidic grids are also shown to capture oil patches that further evolve into trapped oil droplets depending on grid aspect ratio. The natural retraction velocity of thin water films is examined based on external phase velocity and regime maps of trapped droplets are delineated based on control parameters.

Forced Wetting and Dewetting of Water and Oil Droplets on Planar Microfluidic Grids.

We experimentally study the wetting behavior of small water and oil droplets spreading and receding from textured surfaces made using a backside laser processing technique. A dual image acquisition system enables the three-dimensional characterization of both wetted area and dynamic contact angles. In particular, we compare droplet growth on smooth surfaces and planar microfluidic grids of various surface coverages and heights and discuss contact angle characterization. The surface texture is shown to trap liquid in microwells during the stick-and-slip motion of advancing contact lines. Receding wetting dynamics of liquid infused substrates shows similarity with forced spreading on smooth surfaces. Contact angle hysteresis is investigated as a function of surface parameters to better delineate specific wetting behaviors of water and oil on laser-processed surfaces.

Optoelectronics of Multijunction Heterostructures of Transition Metal Dichalcogenides.

Among p-n junction devices with multilayered heterostructures with WSe2 and MoSe2, a device with the MoSe2-WSe2-MoSe2 (NPN) structure showed a remarkably high photoresponse, which was 1000 times higher than the MoSe2-WSe2 (NP) structure. The ideality factor of the NPN structure was estimated to be ∼1, lower than that of the NP structure. It is claimed that the NPN structure formed a thinner depletion region than that of the NP structure because of the difference of carrier concentrations of MoSe2 and WSe2. Hence, the built-in electric field was weaker, and the motion of the photocarriers was facilitated. These behaviors were confirmed experimentally from a photocurrent mapping analysis and Kelvin probe force microscopy. The work function depended on the wavelength of the illuminator, and quasi-Fermi level was estimated. The surface photovoltage on the MoSe2 region was higher than that on WSe2 because the lower bandgap of MoSe2 induces more electron-hole pair generation.

Role of a 193 nm ArF Excimer Laser in Laser-Assisted Plasma-Enhanced Chemical Vapor Deposition of SiNx for Low Temperature Thin Film Encapsulation.

In this study, silicon nitride thin films are deposited on organic polyethylene-naphthalate (PEN) substrates by laser assisted plasma enhanced chemical vapor deposition (LAPECVD) at a low temperature (150 °C) for the purpose of evaluating the encapsulation performance. A plasma generator is placed above the sample stage as conventional plasma enhanced chemical vapor deposition (PECVD) configuration, and the excimer laser beam of 193 nm wavelength illuminated in parallel to the sample surface is coupled to the reaction zone between the sample and plasma source. Major roles of the laser illumination in LAPECVD process are to compete with or complement the plasma decomposition of reactant gases. While a laser mainly decomposes ammonia molecules in the plasma, it also contributes to the photolysis of silane in the plasma state, possibly through the resulting hydrogen radicals and the excitation of intermediate disilane products. It will also be shown that the LAPECVD with coupled laser illumination of 193 nm wavelength improves the deposition rate of silicon nitride thin film, and the encapsulation performance evaluated via the measurement of water vapor transmission rate (WVTR).

Fabrication and Characterization of Multiscale PLA Structures Using Integrated Rapid Prototyping and Gas Foaming Technologies.

Multiscale structured polymers have been considered as a promising category of functional materials with unique properties. We combined rapid prototyping and gas foaming technologies to fabricate multiscale functional materials of superior mechanical and thermal insulation properties. Through scanning electron microscope based morphological characterization, formation of multiscale porous structure with nanoscale cellular pores was confirmed. Improvement in mechanical strength is attributed to rearrangement of crystals within CO2 saturated grid sample. It is also shown that a post-foaming temperature higher than the glass transition temperature deteriorates mechanical strength, providing process guidelines. Thermal decomposition of filament material sets the upper limit of temperature for 3D printed features, characterized by simultaneous differential scanning calorimetry and thermogravimetric analysis. Porosity of the fabricated 3D structured polylactic acid (PLA) foam is controllable by suitable tuning of foaming conditions. The fabricated multiscale 3D structures have potential for thermal insulation applications with lightweight and reasonable mechanical strength.

Effect of Laser Beam Dimension on Laser-Assisted Chemical Vapor Deposition of Silicon Nitride Thin Films.

Laser assisted chemical vapor deposition based on the ultraviolet laser illuminated in parallel to sample surface has been examined as a useful means to deposit thin films without sample heating or damage. In this study, we investigate the effect of enlarged laser beam towards silicon nitride film deposition on larger sample area, relying on Argon Fluoride excimer laser beam of 193 nm wavelength to induce photolysis in SiH4/NH3 mixture. Advantages and mechanisms of enlarged beam are presented, and further parametric trends including the influences of reactant gas and laser parameters, and substrate temperature are reported.

On demand shape-selective integration of individual vertical germanium nanowires on a Si(111) substrate via laser-localized heating.

Semiconductor nanowire (NW) synthesis methods by blanket furnace heating produce structures of uniform size and shape. This study overcomes this constraint by applying laser-localized synthesis on catalytic nanodots defined by electron beam lithography in order to accomplish site- and shape-selective direct integration of vertically oriented germanium nanowires (GeNWs) on a single Si(111) substrate. Since the laser-induced local temperature field drives the growth process, each NW could be synthesized with distinctly different geometric features. The NW shape was dialed on demand, ranging from cylindrical to hexagonal/irregular hexagonal pyramid. Finite difference time domain analysis supported the tunability of the light absorption and scattering spectra via controlling the GeNW shape.

In situ TEM near-field optical probing of nanoscale silicon crystallization.

Laser-based processing enables a wide variety of device configurations comprising thin films and nanostructures on sensitive, flexible substrates that are not possible with more traditional thermal annealing schemes. In near-field optical probing, only small regions of a sample are illuminated by the laser beam at any given time. Here we report a new technique that couples the optical near-field of the laser illumination into a transmission electron microscope (TEM) for real-time observations of the laser-materials interactions. We apply this technique to observe the transformation of an amorphous confined Si volume to a single crystal of Si using laser melting. By confinement of the material volume to nanometric dimensions, the entire amorphous precursor is within the laser spot size and transformed into a single crystal. This observation provides a path for laser processing of single-crystal seeds from amorphous precursors, a potentially transformative technique for the fabrication of solar cells and other nanoelectronic devices.

Multi-parametric growth of silicon nanowires in a single platform by laser-induced localized heat sources.

Nanoscale-synthesized materials hold great promise for the realization of future generation devices. In order to fulfil this exceptional promise, new techniques must be developed that will enable the precise layout and assembly of the heterogeneous components into functional 'superblocks'. Direct synthesis of nanostructures via a laser-assisted chemical vapor deposition process is one promising route. In this paper, laser-assisted silicon nanowire growth based on a vapor-liquid-solid (VLS) mechanism is studied. Spatial confinement of the nanowire growth region via focused laser beam illumination provides a convenient way to examine multiple growth parameters (temperature, time, illumination direction, gas species composition, and pressure), thereby elucidating fundamental mechanisms of laser-assisted growth in a single sample configuration. Furthermore, the work demonstrates an advanced method for direct synthesis of nanostructures for the purpose of practical rapid patterning.

Chemical patterning of ultrathin polymer films by direct-write multiphoton lithography.

We applied 2-photon laser ablation to write subdiffraction nanoscale chemical patterns into ultrathin polymer films under ambient conditions. Poly(ethylene glycol) methacrylate brush layers were prepared on quartz substrates via surface-initiated atom-transfer radical polymerization and ablated to expose the underlying substrate using the nonlinear 2-photon absorbance of a frequency-doubled Ti:sapphire femtosecond laser. Single-shot ablation thresholds of polymer films were ~1.5 times smaller than that of a quartz substrate, which allowed patterning of nanoscale features without damage to the underlying substrate. At a 1/e(2) laser spot diameter of 0.86 µm, the features of exposed substrate approached ~80 nm, well below the diffraction limit for 400 nm light. Ablated features were chemically distinct and amenable to chemical modification.

High-throughput near-field optical nanoprocessing of solution-deposited nanoparticles.

The application of nanoscale electrical and biological devices will benefit from the development of nanomanufacturing technologies that are high-throughput, low-cost, and flexible. Utilizing nanomaterials as building blocks and organizing them in a rational way constitutes an attractive approach towards this goal and has been pursued for the past few years. The optical near-field nanoprocessing of nanoparticles for high-throughput nanomanufacturing is reported. The method utilizes fluidically assembled microspheres as a near-field optical confinement structure array for laser-assisted nanosintering and nanoablation of nanoparticles. By taking advantage of the low processing temperature and reduced thermal diffusion in the nanoparticle film, a minimum feature size down to approximately 100 nm is realized. In addition, smaller features (50 nm) are obtained by furnace annealing of laser-sintered nanodots at 400 degrees C. The electrical conductivity of sintered nanolines is also studied. Using nanoline electrodes separated by a submicrometer gap, organic field-effect transistors are subsequently fabricated with oxygen-stable semiconducting polymer.

The effect of micronscale anisotropic cross patterns on fibroblast migration.

Cell movement on adhesive surfaces is a complicated process based on myriad cell-surface interactions. Although both micron and nanoscale surface topography have been known to be important in understanding cell-materials interactions, typically only simple patterns (e.g., parallel lines or aligned posts) have been used in studying cell morphology, migration, and behavior. This restriction has limited the understanding of the multidirectional aspects of cell-surface response. The present study was performed to investigate cell morphology and motility on micronscale anisotropic cross patterns and parallel line patterns having different aspect ratios (1:2, 1:4, and 1:infinity), grid size (12-, 16-, and 24-mum distance neighboring longer side ridges), and height of ridges (3- and 10-mum). The movement characteristics were analyzed quantitatively with respect to cell migration speed, migration angle, persistence time (P) and motility coefficient (mu). A significant effect of the 1:4 grid aspect ratio cross patterns and parallel line patterns on cell alignment and directionality of migration was observed. Cell motility was also dependent on the patterned surface topography: the migration speed was significantly enhanced by the 1:2 and 1:4 cross patterns when the grid size was smaller than the size of individual cells (i.e., approximately 16 microm). In addition, the migration speed of cells on lower patterns was greater than on higher ridges. Overall, cell morphology and motility was influenced by the aspect ratio of the cross pattern, the grid size, and the height of ridges.

Fabrication of arbitrary polymer patterns for cell study by two-photon polymerization process.

Topographically patterned surfaces are known to be powerful tools for influencing cellular functions. Here we demonstrate a method for fabricating high aspect ratio ( approximately 10) patterns of varying height by using two-photon polymerization process to study contact guidance of cells. Ridge patterns of various heights and widths were fabricated through single laser scanning steps by low numerical aperture optics, hence at much higher processing throughput. Fibroblast cells were seeded on parallel line patterns of different height ( approximately 1.5-microm, approximately 0.8-microm, and approximately 0.5-microm) and orthogonal mesh patterns ( approximately 8-microm and approximately 4-microm height, approximately 5-microm and approximately 5.5-microm height, approximately 5-microm and approximately 6-microm height). Cells experienced different strength of contact guidance depending on the ridge height. Our results demonstrate that a height threshold of nearly 1 microm influences cell alignment on both parallel line and orthogonal mesh patterns. This fabrication technique may find wide application in the design of single cell traps for controlling cell behavior in microdevices and investigating signal transduction as influenced by surface topology.

Self-standing aligned fiber scaffold fabrication by two photon photopolymerization.

Development of materials and fabrication techniques lead the growth of three-dimensional cell culture matrices in biomedical engineering. In this work, we present a method for fabricating self-standing fiber scaffolds by two-photon polymerization induced by a femtosecond laser. The aligned fibers are 330 microm long with a diameter of 6-9 microm. Depending on the pitch of the aligned fibers, various cell morphologies are distinguished via three-dimensional images. Furthermore, the morphologies of fibroblast cells (NIH-3T3) and epithelial cells (MDCK) on the fiber scaffolds are studied to show the effect of high curvature (3-4.5 microm radii) on cell morphology. NIH-3T3 cells that contain straight pattern of actin microfilament bundles are extended and partly wrap single fibers or tend to reside between fibers. On the other hand, MDCK cells that contain circular pattern of actin microfilament bundles cover the fiber peripheral surface exhibiting high aspect ratio elongation. These results indicate that cell morphology on fiber scaffolds is influenced by the pattern of actin microfilament bundles.

Single cell detection using a glass-based optofluidic device fabricated by femtosecond laser pulses.

We demonstrate the fabrication of integrated three-dimensional microchannel and optical waveguide structures inside fused silica for the interrogation and processing of single cells. The microchannels are fabricated by scanning femtosecond laser pulses (523 nm) and subsequent selective wet etching process. Optical waveguides are additionally integrated with the fabricated microchannels by scanning the laser pulse train inside the glass specimen. Single red blood cells (RBC) in diluted human blood inside of the manufactured microchannel were detected by two optical schemes. The first involved sensing the intensity change of waveguide-delivered He-Ne laser light (632.8 nm) induced by the refractive index difference of a cell flowing in the channel. The other approach was via detection of fluorescence emission from dyed RBC excited by Ar laser light (488 nm) delivered by the optical waveguide. The proposed device was tested to detect 23 fluorescent particles per second by increasing the flow rate up to 0.5 microl min(-1). The optical cell detection experiments support potential implementation of a new generation of glass-based optofluidic biochip devices in various single cell treatment processes including laser based cell processing and sensing.

Imaging dielectric properties of Si nanowire oxide with conductive atomic force microscopy complemented with femtosecond laser illumination.

In most Si nanowire (NW) applications, Si oxide provides insulation or a medium of controlled electron tunneling. This work revealed both similarities and differences in the dielectric properties of NW oxide compared with that grown on wafers. The interface barrier to electron transit from the semiconductor to the dielectric and the threshold electric field for current flow are quite similar to those in the planar geometry. This is not true for the lowest currents measured which are not uniformly distributed, indicating variations of trap density in the gap of NW oxide.

Nanoscale rapid melting and crystallization of semiconductor thin films.

The current study details nanosecond laser-based rapid melting and crystallization of thin amorphous silicon (a-Si) films at the nanoscale using two different optical near-field processing schemes. Both apertureless and tapered fiber near-field scanning optical microscope probes were utilized to deliver highly confined irradiation on the target surface. The various modification regimes produced as a result of the rapid a-Si melting and crystallization transformations were shown to critically depend on the applied laser fluence. Consequently, the crystallized pattern morphology and feature size could be finely controlled. High energy density was observed to impart ablation surrounded by a narrow melt ring. At much lower incident laser energy density, single nanostructures with a lateral dimension of approximately 90 nm were defined.