Sungsoo Yoon

On the individuality of the iris biometric

Biometric authentication has been considered a model for quantitatively establishing the discriminative power of biometric data. The dichotomy model classifies two biometric samples as coming either from the same person or from two different people. This paper reviews features, distance measures, and classifiers used in iris authentication. For feature extraction we compare simple binary and multi-level 2D wavelet features. For distance measures we examine scalar distances such as Hamming and Euclidean, feature vector and histogram distances. Finally, for the classifiers we compare Bayes decision rule, nearest neighbor, artificial neural network, and support vector machines. Of the eleven different combinations tested, the best one uses multi-level 2D wavelet features, the histogram distance, and a support vector machine classifier.

Room-Temperature Chemical Welding and Sintering of Metallic Nanostructures by Capillary Condensation

Room-temperature welding and sintering of metal nanostructures, nanoparticles and nanowires, by capillary condensation of chemical vapors have successfully been demonstrated. Nanoscale gaps or capillaries that are abundant in layers of metal nanostructures have been found to be the preferred sites for the condensation of chemically oxidizing vapor, H2O2 in this work. The partial dissolution and resolidification at such nanogaps completes the welding/sintering of metal nanostructures within ∼10 min at room-temperature, while other parts of nanostructures remain almost intact due to negligible amount of condensation on there. The welded networks of Ag nanowires have shown much improved performances, such as high electrical conductivity, mechanical flexibility, optical transparency, and chemical stability. Chemically sintered layers of metal nanoparticles, such as Ag, Cu, Fe, Ni, and Co, have also shown orders of magnitude increase in electrical conductivity and improved environmental stability, compared to nontreated ones. Pertinent mechanisms involved in the chemical welding/sintering process have been discussed. Room-temperature welding and sintering of metal nanostructures demonstrated here may find widespread application in diverse fields, such as displays, deformable electronics, wearable heaters, and so forth.

On binary similarity measures for handwritten character recognition

Similarity and dissimilarity measures play an important role in pattern classification and clustering. For a century, researchers have searched for a good measure. Here, we review, categorize, and evaluate various binary vector similarity/dissimilarity measures for character recognition. One of the most contentious disputes in the similarity measure selection problem is whether the measure includes or excludes negative matches. While inner-product based similarity measures consider only positive matches, other conventional measures credit both positive and negative matches equally. Hence, we propose an enhanced similarity measure that gives variable credits and show that it is superior to conventional measures in an offline handwritten character recognition application. Finally, the proposed similarity measure can be further boosted by applying weights and we demonstrate that it outperforms the weighted Hamming distance.

Switchable wettability of vertical Si nanowire array surface by simple contact-printing of siloxane oligomers and chemical washing

The wettability of a vertical Si nanowire (SiNW) array surface, prepared by a metal-assisted chemical etching technique, has been systematically investigated. The simple contact printing of siloxane oligomers has converted a SiNW array surface from superhydrophilic to superhydrophobic. In addition to the contact-printing time of siloxane oligomers, the nanowire length and its aggregation or bundling state have been found to have a profound effect on the wettability. The superhydrophobic state of the SiNWs with printed siloxane oligomers has shown long-term stability, even after prolonged dipping in water. Furthermore, the wettability could be switched reversibly many times, i.e., from superhydrophilic to superhydrophobic or vice versa, by the simple contact printing of siloxane oligomers and chemical washing with sulfuric acid repeatedly. The demonstrated reversible switching of the SiNWs’ surface wettability may be very helpful in many applications.

Facile and Clean Release of Vertical Si Nanowires by Wet Chemical Etching Based on Alkali Hydroxides

A simple method to release Si nanowires (SiNWs) from a substrate, with their original length almost intact, is demonstrated. By exploiting the unique chemistry involved for the fabrication of vertical arrays of SiNWs in metal-assisted chemical etching (MaCE) based either on HF/AgNO3 or HF/H2O2 chemistries, wet etching with alkali hydroxides such as NaOH or KOH preferentially attacks the bottom part of the vertical SiNWs. A protective layer of Si oxide is found to exist on the outer wall of the SiNWs and to play the key role of etch mask during the release-etching by alkali hydroxides. The clean release of SiNWs also enables the repeated use of the Si substrate for the fabrication of vertical SiNW arrays by MaCE. The released SiNWs are further used for the fabrication of field-effect transistors on a flexible plastic substrate. The method developed here, when combined with a suitable assembling technique, can be very useful in implementing flexible electronics, or in the fabrication of SiNW composites with other functional materials.

Evaluation of biometric identification in open systems

This paper concerns the generalizability of biometric identification results from small-sized closed systems to larger open systems. Many researchers have claimed high identification accuracies on closed system consisting of a few hundred or thousand members. Here, we consider what happens to these closed identification systems as they are opened to non-members. We claim that these systems do not generalize well as the non-member population increases. To support this claim, we present experimental results on writer and iris biometric databases using Support Vector Machine (SVM) and Nearest Neighbor (NN) classifiers. We find that system security (1-FAR) decreases rapidly for closed systems when they are tested in open-system mode as the number of non members tested increases. We also find that, although systems can be trained for greater closed-system security using SVM rather than NN classifiers, the NN classifiers are better for generalizing to open systems due to their superior capability of rejecting non-members.

Writer profiling using handwriting copybook styles

Handwriting originates from a particular copybook style such as Palmer or Zaner-Bloser that one learns in childhood. Since questioned document examination plays an important investigative and forensic role in many types of crime, it is important to develop a system that helps objectively identify a questioned documents handwriting style. We proposed a handwriting analysis system that can assist a document examiner in the identification of the writers handwriting style and therefore of his/her origin or nationality. We collected 33 English alphabet copybook styles from 18 countries. Here, we extend the analysis using several data mining techniques to discover important information that can be gleaned from a handwriting copybook style image database, e.g., the most information bearing alphabet characters for the purpose of copybook style identification and the relationship between geographical regions and similarity based clusters of copybook styles.

Facile Patterning of Ag Nanowires Network by Micro-Contact Printing of Siloxane

A simple, low-cost, scalable patterning method has been demonstrated for chemically welded Ag nanowires (AgNWs) network. The chemically welded network of AgNWs on substrates has been patterned by modified microcontact printing (μCP). As an ink for the μCP, uncured high-viscosity siloxane polymer has been applied. Using elastomeric polydimethylsiloxane (PDMS) stamp that has been replicated from micromachined Si master mold by metal-assisted chemical etching, the printed siloxane ink materials have been cured by simple UV/ozone exposure for 3 min, which acts as an etch barrier in ensuing wet-removal of exposed AgNWs network. The proposed patterning technique has no limitation in the choice of substrates and pattern shape, in addition to high resolution. The patterned AgNWs network electrodes have shown excellent optical, electrical, and mechanical performances, such as high flexibility (up to ∼10%) and stretchability (up to 40%). Finally, the patterned AgNWs network electrodes have been applied as a transparent heater, which can be used for rapid raindrop removal or deicing of car windows and outside mirrors. This can be a valuable help for driving safety under harsh weather conditions.

Direct Visualization of Etching Trajectories in Metal-Assisted Chemical Etching of Si by the Chemical Oxidation of Porous Sidewalls

We demonstrate a simple method for the visualization of trajectories traced by noble metal nanoparticles during metal-assisted chemical etching (MaCE) of Si. The nanoporous Si layer formed around drilled pores is converted into SiO2 by simple chemical oxidation. Etch removal of the remaining Si using alkali hydroxide leaves SiO2 nanostructures that are the exact replica of those drilled pores or etching trajectories. The differences in etching characteristics between Ag and Au have been investigated using the proposed visualization method. The shape and chemical stability of metal nanoparticles used for MaCE have been found to be critical in determining etching paths. The proposed method would be very helpful in studying the fundamental mechanism of MaCE as well as in micro/nanostructuring of the Si surface for various applications. This approach can also be used for the generation of straight or helical SiO2 nanotubes.

The importance of interfacial adhesion in the buckling-based mechanical characterization of materials

The effect of interfacial adhesion on the buckling-based mechanical characterization of materials has been elucidated and practical guides for the correct measurement of the modulus by the buckling method have been given accordingly. When a stiff film on a compliant substrate is compressed, the film buckles into well-ordered sinusoidal surface undulations, over a large area, for the case of good interfacial adhesion and the resulting modulus value agrees well with that observed via the microtensile test. On the other hand, the modulus value obtained from the buckling measurement deviates significantly from that of the microtensile method for the case of poor interfacial adhesion. The nanoscale interface debonds in the poor adhesion case and led to the reduction of effective sample dimension, and this finite width effect has been found to be the reason for the observed discrepancy. Furthermore, simple ways to avoid or reduce the adhesion-related artefacts in the buckling method have been demonstrated. Although the well-known PS/PDMS system has been used in the present work, due to its simplicity to control the interfacial adhesion simply by changing the mixing ratio of PDMS substrate, we believe that the present results should bear the same importance in any other buckling system. Therefore, great care should be taken when one uses or reports the mechanical properties of a material obtained from the buckling method, as well as in studies of physics and mechanics for a given buckling system.