There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This thesis explores transport phenomena in nanochannels on a chip. Fundamental nanofluidic ionic studies form the basis for novel separation and preconcentration applications for proteomic purposes. The measurements were performed with 50-nm-high 1D nanochannels, which are easily accessible from both sides by two microchannels. Nanometer characteristic apertures were manufactured in the bonded structure of Pyrex-amorphous silicon – Pyrex, in which the thickness of the amorphous silicon layer serves as a spacer to define the height of the nanochannels. The geometry of the nanometer-sized apertures is well defined, which simplifies the modeling of the transport across them. Compared to biological pores, the present nanochannels in Pyrex offer increased stability. Fundamental characteristics of nanometer-sized apertures were obtained by impedance spectroscopy measurements of the nanochannel at different ionic strengths and pH values. A conductance plateau (on a log-log scale) was modeled and measured, establishing due to the dominance of the surface charge density in the nanochannels, which induces an excess of mobile counterions to maintain electroneutrality. The nanochannel conductance can be regulated at low ionic strengths by pH adjustment, and by an external voltage applied on the chip to change the zeta potential. This field-effect allows the regulation of ionic flow which can be exploited for the fabrication of nanofluidic devices. Fluorescence measurements confirm that 50-nm-high nanochannels show an exclusion of co-ions and an enrichment of counterions at low ionic strengths. This permselectivity is related to the increasing thickness of the electrical double layer (EDL) with decreasing salt concentrations, which results in an EDL overlap in an aperture if the height of the nanochannel and the thickness of the EDL are comparable in size. The diffusive transport of charged species and therefore the exclusion-enrichment effect was described with a simple model based on the Poisson-Boltzmann equation. The negatively charged Pyrex surface of the nanometer characteristic apertures can be inversed with chemical surface pretreatments, resulting in an exclusion of cations and an enrichment of anions. When a pressure gradient is applied across the nanochannels, charged molecules are electrostatically rejected at the entrance of the nanometer-sized apertures, which can be used for separation processes. Proteomic applications are presented such as the separation and preconcentration of proteins. The diffusion of Lectin proteins with different isoelectric points and very similar compositions were controlled by regulating the pH value of the buffer. When the proteins are neutral at their pI value, the diffusion coefficient is maximal because the biomolecules does not interact electrostatically with the charged surfaces of the nanochannel. This led to a fast separation of three Lectin proteins across the nanochannel. The pI values measured in this experiment are slightly shifted compared to the values obtained with isoelectric focusing because of reversible adsorption of proteins on the walls which affects the pH value in the nanochannel. An important application in the proteomic field is the preconcentration of biomolecules. By applying an electric field across the nanochannel, anionic and cationic analytes were preconcentrated on the cathodic side of the nanometer-sized aperture whereas on the anodic side depletion of ions was observed. This is due to concentration polarization, a complex of effects related to the formation of ionic concentration gradients in the electrolyte solution adjacent to an ion-selective interface. It was measured that the preconcentration factor increased with the net charge of the molecule, leading to a preconcentration factor of > 600 for rGFP proteins in 9 minutes. Such preconcentrations are important in micro total analysis systems to achieve increased detection signals of analytes contained in dilute solutions. Compared to cylindrical pores, our fabrication process allows the realization of nanochannels on a chip in which the exclusion-enrichment effect and a big flux across the nanometer-sized aperture can be achieved, showing the interest for possible micro total analysis system applications. The described exclusion-enrichment effect as well as concentration polarization play an important role in transport phenomena in nanofluidics. The appendix includes preliminary investigations in DNA molecule separation and fluorescence correlation spectroscopy measurements, which allows investigating the behavior of molecules in the nanochannel itself.

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Transport phenomena in nanofluidics

Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing components for various optical and opto-electrical equipments. Extensive research in optical and biological reflectance minimization as well as the emergence of nanotechnology over the years has contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The different types of ARCs are also described in greater detail and the state-of-the-art fabrication techniques have been fully illustrated. The inspiration that ARCs derive from nature (‘biomimetics’) has been an area of major research and is discussed at length. The various materials that have been reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of application of ARCs on solar cells and modules, contemporary research and associated challenges are presented in the end to facilitate a universal understanding of the ARCs and encourage future research.

Anti-reflective coatings: A critical, in-depth review

Nature routinely produces nanostructured surfaces with useful properties, such as the self-cleaning lotus leaf, the colour of the butterfly wing, the photoreceptor in brittlestar and the anti-reflection observed in the moth eye. Scientists and engineers have been able to mimic some of these natural structures in the laboratory and in real-world applications. Here, we report a simple aperiodic array of silicon nanotips on a 6-inch wafer with a sub-wavelength structure that can suppress the reflection of light at a range of wavelengths from the ultraviolet, through the visible part of the spectrum, to the terahertz region. Reflection is suppressed for a wide range of angles of incidence and for both s- and p-polarized light. The antireflection properties of the silicon result from changes in the refractive index caused by variations in the height of the silicon nanotips, and can be simulated with models that have been used to explain the low reflection from moth eyes. The improved anti-reflection properties of the surfaces could have applications in renewable energy and electro-optical devices for the military.

/pdf/improved-broadband-and-quasi-omnidirectional-anti-reflection-28262vbtzi.pdf

Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures

This review gives a brief overview of microvalves, and focuses on the actuation mechanisms and their applications. One of the stumbling blocks for successful miniaturization and commercialization of fully integrated microfluidic systems was the development of reliable microvalves. Applications of the microvalves include flow regulation, on/off switching and sealing of liquids, gases or vacuums. Microvalves have been developed in the form of active or passive microvalves employing mechanical, non-mechanical and external systems. Even though great progress has been made during the last 20 years, there is plenty of room for further improving the performance of existing microvalves.

http://iopscience.iop.org/article/10.1088/0960-1317/16/5/R01/pdf

A review of microvalves

We report on the electrochemical growth of micro/nanowire devices using e-beam-patterned electrolyte channels, potentially enabling the controlled fabrication of individually addressable arrays. The concept of growing single wires and small arrays using this technique is demonstrated by single and double wires of Pd and polypyrrole with 500-nm and 1-μm widths up to 7-μm lengths and 200-nm thicknesses. The use of Pd wires as hydrogen sensors and polypyrrole wires as pH sensors is demonstrated.

Electrochemically Grown Wires for Individually Addressable Sensor Arrays

Found in many terahertz (THz) and submillimeter-wave systems, GaAs Schottky diodes continue to be one of the most useful THz devices. As a low-parasitic device that operates well into the THz range, Schottky diodes provide useful detection and power generation for a number of practical applications. Mixers and multipliers, working as high as ~3 THz, have already been demonstrated. This paper reviews the current status of diode technology, detailing some of the different ways for fabricating THz chips. An overview regarding the current state of technology and performance for THz frequency multipliers and mixers is presented, along with applications enabled by these diodes.

THz Diode Technology: Status, Prospects, and Applications

We have developed a new technique to fabricate an antireflection surface using silicon nanotips for use on a micro Sun sensor for Mars rovers. We have achieved randomly distributed nanotips of radii spanning from 20 to 100 nm and aspect ratio of ∼200 using a two-step dry etching process. The 30° specular reflectance at the target wavelength of 1 μm is only about 0.09%, nearly 3 orders of magnitude lower than that of bare silicon, and the hemispherical reflectance is ∼8%. When the density and aspect ratio of these nanotips are changed, a change in reflectance is demonstrated. When surfaces are covered with these nano-tips, the critical problem of ghost images that are caused by multiple internal reflections in a micro Sun sensor was solved.

A Novel Silicon Nanotips Antireflection Surface for the Micro Sun Sensor

Interest in nanowire technology has grown significantly over the past several years, due to their potentially broad applications in optics, electronics, and sensors (e.g., resonators, molecular detection, nanoconnectors, biosensors, and gas sensors). Due to their small size, sensitivity, real-time detection, and ultra-low power demands, nanowire sensors are being investigated for detection of a wide range of chemical and biochemical species. The techniques used to fabricate these sensors include laser ablation cluster formation, focused ion-beam etching of electrodes to create a small gap ( 100 nm), use of ion-track membranes, use of alumina templates, and electrochemical step-edge decoration. These techniques have drawbacks of limited controllability and manufacturability, therefore reliable and controllable nanowire fabrication remains a significant challenge. We have previously reported a method of electrodepositing Pd wires with micrometer diameters and conducting polymer nanowires of polyaniline and polypyrrole with diameters of 100 nm. In this work, single Pd nanowires are fabricated by electrodeposition in electrolyte channels patterned with electron-beam lithography, and hydrogen sensing is demonstrated. This fabrication technique can produce nanowires with controlled dimensions, positions, alignments, and chemical compositions, and potentially enables the use of a wide range of materials for creating arrays of singlenanowire sensors. Additionally, the growth of nanowires using this technique would allow for easy integration with existing silicon technology. Figure 1 shows SEM images of typical electrodeposited nanowires between electrodes. The magnified image in Figure 1b shows a single Pd nanowire with a diameter ranging from 70 to 85 nm. An EDS (energy dispersive spectroscopy) elemental analysis detected only Pd metal in the nanowire.

Investigation of a Single Pd Nanowire for Use as a Hydrogen Sensor

An antenna element suitable for integrated arrays at terahertz frequencies is disclosed. The antenna element comprises an extended spherical (e.g. hemispherical) semiconductor lens, e.g. silicon, antenna fed by a leaky wave waveguide feed. The extended spherical lens comprises a substantially spherical lens adjacent a substantially planar lens extension. A couple of TE/TM leaky wave modes are excited in a resonant cavity formed between a ground plane and the substantially planar lens extension by a waveguide block coupled to the ground plane. Due to these modes, the primary feed radiates inside the lens with a directive pattern that illuminates a small sector of the lens. The antenna structure is compatible with known semiconductor fabrication technology and enables production of large format imaging arrays.

/pdf/dielectric-covered-planar-antennas-45a3sdkwib.pdf

Choonsup Lee

Papers

Electrochemically Grown Wires for Individually Addressable Sensor Arrays

THz Diode Technology: Status, Prospects, and Applications

A Novel Silicon Nanotips Antireflection Surface for the Micro Sun Sensor

Investigation of a Single Pd Nanowire for Use as a Hydrogen Sensor

Dielectric covered planar antennas