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Publication Open Access 3D printing of elastomeric bioinspired complex adhesive microstructures(Wiley, 2021) Dayan, Cem Balda; Chun, Sungwoo; Krishna Subbaiah, Nagaraj; Drotlef, Dirk Michael; Akolpoğlu, Mükrime Birgül; Department of Mechanical Engineering; Sitti, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; School of Medicine; 297104Bioinspired elastomeric structural adhesives can provide reversible and controllable adhesion on dry/wet and synthetic/biological surfaces for a broad range of commercial applications. Shape complexity and performance of the existing structural adhesives are limited by the used specific fabrication technique, such as molding. To overcome these limitations by proposing complex 3D microstructured adhesive designs, a 3D elastomeric microstructure fabrication approach is implemented using two-photon-polymerization-based 3D printing. A custom aliphatic urethane-acrylate-based elastomer is used as the 3D printing material. Two designs are demonstrated with two combined biological inspirations to show the advanced capabilities enabled by the proposed fabrication approach and custom elastomer. The first design focuses on springtail- and gecko-inspired hybrid microfiber adhesive, which has the multifunctionalities of side-surface liquid super-repellency, top-surface liquid super-repellency, and strong reversible adhesion features in a single fiber array. The second design primarily centers on octopus- and gecko-inspired hybrid adhesive, which exhibits the benefits of both octopus- and gecko-inspired microstructured adhesives for strong reversible adhesion on both wet and dry surfaces, such as skin. This fabrication approach could be used to produce many other 3D complex elastomeric structural adhesives for future real-world applications.
Publication Metadata only A quantum mechanical study of the electrochemical polymerization of pyrrole(Elsevier Science Sa, 2001) Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Mechanism for the electrochemical polymerization of pyrrole is studied using accurate density functional theory; (DFT) calculations. The primary emphasis is on the structures and stability of intermediates generated during various mechanisms. Structures of the radical cations, which play role in reactions, an optimized to elucidate radical-radical and radical-neutral pathways. The competing probabilities of reactions between various size oligomers are discussed in terms of their thermodynamical stability.Publication Metadata only AuPt alloy nanoparticles supported on graphitic carbon nitride: in situ synthesis and superb catalytic performance in the light-assisted hydrolytic dehydrogenation of ammonia borane(Elsevier, 2022) N/A; Department of Chemistry; Department of Chemistry; Aksoy, Merve; Korkut, Sibel Eken; Metin, Önder; PhD Student; Researcher; Faculty Member; Department of Chemistry; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); N/A; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM) / Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; N/A; 46962Addressed herein is the enhancement of catalytic activity of Pt-based nanocatalysts in the hydrolysis of ammonia borane (AB) via in-situ synthesis of bimetallic AuPt alloy nanoparticles (NPs) supported on graphitic carbon nitride (gCN). The presented in-situ synthesis protocol yielded gCN/AuxPt100-x (x = 0, 8, 15, 33) nanocatalysts with highly dispersed AuxPt100-x NPs having the average particle sizes varied in the range of 1.6-2.6 nm over the gCN nanosheets. The generated gCN/Pt92Au8 (600.3 mol H-2 mol Pt-(1) min(-1)) and gCN/Pt85Au15 (587.1 mol H2 mol Pt-1 min(-1)) nanocatalysts showed higher catalytic activity compared to gCN/Pt-100 (525.7 mol H-2 mol Pt-1 min(-1)) under white-light irradiation, attributed to the synergistic effects aroused in the AuPt alloy NPs and heterojunctions formed between gCN and AuPt alloy NPs. The detailed characterization of photophysical properties of gCN/AuxPt100-x nanocatalysts revealed that their boosted catalytic activity is attributed to the improved charge kinetics, higher light absorption, and effective electron transfer channels from gCN to the bimetallic AuPt alloy NPs. The role of photogenerated carriers in the photocatalytic AB dehydrogenation was also elucidated via scavenger studies. This study shows that gCN/AuxPt100-x nanocatalysts can be prepared in situ during the hydrolysis of AB at room temperature and the yielded nanocatalysts have a significant role in boosting the hydrogen production from the light-assisted hydrolysis of AB.Publication Metadata only Development of highly stable and luminescent aqueous CdS quantum dots with the poly(acrylic acid)/mercaptoacetic acid binary coating system(Amer Scientific Publishers, 2009) Lieberwirth, I.; Department of Chemistry; N/A; Department of Chemical and Biological Engineering; Acar, Havva Funda Yağcı; Çelebi, Serdar; Serttunalı, Nazlı İpek; Faculty Member; Master Student; Undergraduate Student; Department of Chemistry; Department of Chemical and Biological Engineering; College of Sciences; Graduate School of Sciences and Engineering; College of Engineering; 178902; N/A; N/AHighly stable and luminescent CdS quantum dots (QD) were prepared in aqueous solutions via in situ capping of the crystals with the poly(acrylic acid) (PAA) and mercaptoacetic acid (MAA) binary mixtures. The effect of reaction temperature and coating composition on the particle size, colloidal stability and luminescence were investigated and discussed in detail. CdS QDs coated with either PAA or MAA were also prepared and compared in terms of properties. CdS-MAA QDs were highly luminescent but increasing reaction temperature caused an increase in the crystal size and a significant decrease in the quantum yield (QY). Although less luminescent and bigger than CdS-MAA, CdS-PAA QDs maintained the room temperature size and QY at higher reaction temperatures. CdS-MAA QDs lacked long term colloidal stability whereas CdS-PAA QDs showed excellent stability over a year. Use of PAA/MAA mixture as a coating for CdS nanoparticles during the synthesis provided excellent stability, high QY and ability to tune the size and the color of the emission. Combination of all of these properties can be achieved only with the mixed coating. CdS coated with PAA/MAA at 40/60 ratio displayed the highest QY (50% of Rhodamine B) among the other compositions.Publication Metadata only Dx2-y2 superconductivity and the Hubbard model(Taylor & Francis, 2002) Department of Physics; Bulut, Nejat; Faculty Member; Department of Physics; College of Sciences; 4963The numerical studies of d(x2-y2)-wave pairing in the two-dimensional (2D) and the 2-leg Hubbard models are reviewed. For this purpose, the results obtained from the determinantal Quantum Monte Carlo and the Density-Matrix Renormalization-Group calculations are presented. These are calculations which were motivated by the discovery of the high-T-c cuprates. In this review, the emphasis is placed on the microscopic many-body processes which are responsible for the d(x2-y2)-wave pairing correlations observed in the 2D and the 2-leg Hubbard models. In order to gain insight into these processes, the results on the effective pairing interaction as well as the magnetic, density and the single-particle excitations will be reviewed. In addition, comparisons will be made with the other numerical approaches to the Hubbard model and the numerical results on the t-J model. The results reviewed here indicate that an effective pairing interaction which is repulsive at (pi,pi) momentum transfer, and enhanced single-particle spectral weight near the (pi,0) and (0,pi) points of the Brillouin zone, create optimum conditions for d(x2)-(y2)-wave pairing. These are two effects which act to enhance the d(x2-y2)-wave pairing correlations in the Hubbard model. Finding additional ways is an active research problem.Publication Metadata only Effect of Zn(NO3)(2) concentration in hydrothermal-electrochemical deposition on morphology and photoelectrochemical properties of ZnO nanorods(Elsevier, 2016) N/A; N/A; Department of Chemistry; Akkaya, Ceren Yılmaz; Ünal, Uğur; PhD Student; Faculty Member; Department of Chemistry; N/A; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; 42079Zn(NO3)(2) concentration had been reported to be significantly influential on electrodeposition of ZnO structures. In this work, this issue is revisited using hydrothermal-electrochemical deposition (HED). Seedless, cathodic electrochemical deposition of ZnO films is carried out on ITO electrode at 130 degrees C in a closed glass reactor with varying Zn(NO3)(2) concentration. Regardless of the concentration of Zn2+ precursor (0.001-0.1 M) in the deposition solution, vertically aligned 1-D ZnO nanorods are obtained as opposed to electrodepositions at lower temperatures (70-80 degrees C). We also report the effects of high bath temperature and pressure on the photoelectrochemical properties of the ZnO films. Manipulation of precursor concentration in the deposition solution allows adjustment of the aspect ratio of the nanorods and the degree of texturation along the c-axis; hence photoinduced current density. HED is shown to provide a single step synthesis route to prepare ZnO rods with desired aspect ratio specific for the desired application just by controlling the precursor concentration.Publication Open Access Liquid crystal structure of supercooled liquid gallium and eutectic gallium-indium(Wiley, 2021) Yunusa, Muhammad; Adaka, Alex; Aghakhani, Amirreza; Shahsavan, Hamed; Guo, Yubing; Alapan, Yunus; Jakli, Antal; Department of Mechanical Engineering; Sitti, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; School of Medicine; 297104Understanding the origin of structural ordering in supercooled liquid gallium (Ga) has been a great scientific quest in the past decades. Here, reflective polarized optical microscopy on Ga sandwiched between glasses treated with rubbed polymers reveals the onset of an anisotropic reflection at 120 degrees C that increases on cooling and persists down to room temperature or below. The polymer rubbing usually aligns the director of thermotropic liquid crystals (LCs) parallel to the rubbing direction. On the other hand, when Ga is sandwiched between substrates that align conventional LC molecules normal to the surface, the reflection is isotropic, but mechanical shear force induces anisotropic reflection that relaxes in seconds. Such alignment effects and shear-induced realignment are typical to conventional thermotropic LCs and indicate a LC structure of liquid Ga. Specifically, Ga textures obtained by atomic force and scanning electron microscopy reveal the existence of a lamellar structure corresponding to a smectic LC phase, while the nanometer-thin lamellar structure is transparent under transmission polarized optical microscopy. Such spatial molecular arrangements may be attributed to dimer molecular entities in the supercooled liquid Ga. The LC structure observation of electrically conductive liquid Ga can provide new opportunities in materials science and LC applications.Publication Open Access Machine learning-based and experimentally validated optimal adhesive fibril designs(Wiley, 2021) Son, Donghoon; Liimatainen, Ville; Department of Mechanical Engineering; Sitti, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; School of Medicine; 297104Setae, fibrils located on a gecko's feet, have been an inspiration of synthetic dry microfibrillar adhesives in the last two decades for a wide range of applications due to unique properties: residue-free, repeatable, tunable, controllable and silent adhesion; self-cleaning; and breathability. However, designing dry fibrillar adhesives is limited by a template-based-design-approach using a pre-determined bioinspired T- or wedge-shaped mushroom tip. Here, a machine learning-based computational approach to optimize designs of adhesive fibrils is shown, exploring a much broader design space. A combination of Bayesian optimization and finite element methods creates novel optimal designs of adhesive fibrils, which are fabricated by two-photon-polymerization-based 3D microprinting and double-molding-based replication out of polydimethylsiloxane. Such optimal elastomeric fibril designs outperform previously proposed designs by maximum 77% in the experiments of dry adhesion performance on smooth surfaces. Furthermore, finite-element-analyses reveal that the adhesion of the fibrils is sensitive to the 3D fibril stem shape, tensile deformation, and fibril microfabrication limits, which contrast with the previous assumptions that mostly neglect the deformation of the fibril tip and stem, and focus only on the fibril tip geometry. The proposed computational fibril design could help design future optimal fibrils with less help from human intuition.Publication Metadata only Multiple bipolaron formation in oligothiophenes(Elsevier Science Sa, 1999) Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Semi-empirical calculations on large oligomers of thiophene containing up to 24 monomer units are carried out. The geometry of neutral as well as highly oxidized structures with charges of +2, +4, and +6 are optimized in order to understand the charge localization in the absence of counter ions. The bond length alternation in the polymer backbone is shown to provide a useful tool for studying bipolaron formation. The effects of defects such as branches, alpha-beta bondings or deviations from planarity on the structures of oxidized forms are investigated. The bipolarons are found to spread over six monomer units. Defects disturb the bipolaron formation and possibly impede the mobility of them.Publication Open Access Optimization of room-temperature continuous-wave cr (4+)-Doped solid-state lasers: experiment and modeling(Institute of Electrical and Electronics Engineers (IEEE), 2001) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851A novel study to determine the optimum crystal and resonator parameters for Cr4+ doped lasers subjected to lifetime thermal loading was permormed. The comparison of the results of lasing threshold, pump absorption saturation, and power efficiency measurements was done with the predictions of a theoretical model to determine the laser cross sections. Using the best-fit cross-section values, numerical optimization studies were carried out to determine the optimum crystal absorption, resonator reflectivity, and crystal length which maximized the output power.