Publications with Fulltext
Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6
Browse
17 results
Search Results
Publication Open Access Emergence of near-infrared photoluminescence via ZnS shell growth on the AgBiS2 nanocrystals(American Chemical Society, 2024) Department of Chemistry; Department of Electrical and Electronics Engineering; Önal, Asım; Kaya, Tarık Safa; Metin, Önder; Nizamoğlu, Sedat; Department of Chemistry; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of EngineeringAgBiS2 nanocrystals (NCs), composed of nontoxic, earth-abundant materials and exhibiting an exceptionally high absorption coefficient from visible to near-infrared (>105 cm(-1)), hold promise for photovoltaics but have lack of photoluminescence (PL) due to intrinsic nonradiative recombination and challenging shell growth. In this study, we reported a facile wet-chemical approach for the epitaxial growth of ZnS shell on AgBiS2 NCs, which triggered the observation of PL emission in the near-infrared (764 nm). Since high quality of the core is critical for epitaxial shell growth, we first obtained rock-salt structured AgBiS2 NCs with high crystallinity, nearly spherical shape and monodisperse size distribution (<6%) via a dual-ligand approach reacting Ag-Bi oleate with elemental sulfur in oleylamine. Next, a zincblende ZnS shell with a low-lattice mismatch of 4.9% was grown on as-prepared AgBiS2 NCs via a highly reactive zinc (Zn(acac)(2)) precursor that led to a higher photoluminescence quantum yield (PLQY) of 15.3%, in comparison with a relatively low reactivity precursor (Zn(ac)(2)) resulting in reduced PLQY. The emission from AgBiS2 NCs with ultrastrong absorption, facilitated by shell growth, can open up new possibilities in lighting, display, and bioimaging.Publication Open Access Enhanced sinterability, thermal conductivity and dielectric constant of glass-ceramics with PVA and BN additions(Multidisciplinary Digital Publishing Institute (MDPI), 2022) Akkasoğlu, Ufuk; Çiçek, Buğra; N/A; Department of Chemistry; Arıbuğa, Dilara; Balcı, Özge; Researcher; Department of Chemistry; Koç University AKKİM Boron-Based Materials _ High-technology Chemicals Research _ Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; College of Sciences; N/A; 295531With the rapid development of the microelectronics industry, many efforts have been made to improve glass-ceramics' sinterability, thermal conductivity, and dielectric properties, which are essential components of electronic materials. In this study, low-alkali borosilicate glass-ceramics with PVA addition and glass-BN composites were prepared and successfully sintered at 770 degrees C. The phase composition, density, microstructure, thermal conductivity, and dielectric constant were investigated. It was shown that PVA addition contributes to the densification process of glass-ceramics (~88% relative density, with closed/open pores in the microstructure) and improves the thermal conductivity of glass material from 1.489 to 2.453 W/K.m. On the other hand, increasing BN addition improves microstructures by decreasing porosities and thus increasing relative densities. A glass-12 wt. % BN composite sample exhibited almost full densification after sintering and presented apparent and open pores of 2.6 and 0.08%, respectively. A high thermal conductivity value of 3.955 W/K.m and a low dielectric constant of 3.00 (at 5 MHz) were observed in this material. Overall, the resulting glass-ceramic samples showed dielectric constants in the range of 2.40-4.43, providing a potential candidate for various electronic applications.Publication Open Access Improved superconducting properties in the Mg11B2 low activation superconductor prepared by low-temperature sintering(Nature Publishing Group (NPG), 2016) F. Cheng, Y.; Liu, Z. Ma; Hossain, M. S.; Department of Chemistry; Somer, Mehmet Suat; Faculty Member; Department of Chemistry; College of Sciences; 178882Mg11B2 has a great application prospect in the superconducting coils for fusion reactor as the “low activation superconductors”. The un-doped Mg11B2 and Cu-doped Mg11B2 bulks using 11B as a boron precursor were fabricated by low-temperature sintering in present work. It was found that the prepared Mg11B2 low activation superconductors exhibit better Jc performance than all of other Mg11B2 samples reported in previous studies. As for Cu doped Mg11B2, minor Cu addition can obviously improve the Mg11B2 grain crystallization and reduce the amount of MgO impurity. Hence, improved grain connectivity and higher Jc at low fields is obtained in Cu doped Mg11B2 samples. For un-doped samples, refined grains and more MgO impurity with proper size brought about more flux pinning centers, resulting in better Jc performance at high fields.Publication Open Access Development of highly luminescent and cytocompatible near-IR-emitting aqueous Ag2S quantum dots(Royal Society of Chemistry (RSC), 2012) Çizmeciyan, M. Natali; Erdem, Rengin; Özen, Can; Kurt, Adnan; N/A; Department of Physics; Department of Chemistry; Hocaoğlu, İbrahim; Sennaroğlu, Alphan; Acar, Havva Funda Yağcı; PhD Student; Faculty Member; Department of Physics; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; N/A; 23851; 178902Colloidally stable and highly luminescent near-IR emitting Ag2S quantum dots (NIRQDs) were prepared by a very simple aqueous method using 2-mercaptopropionic acid (2MPA) as a coating. Emission of Ag2S-2MPA NIRQDs can be tuned between 780 and 950 nm. These NIRQDs have photoluminescence quantum yields (PLQY) around 7–39% and exhibit excellent cytocompatibility even at 600 mg mL 1 in NIH/3T3 cells. With such improved properties, Ag2S-2MPA NIRQDs have a great potential in practical bio-applications.Publication Open Access Meso-2,3-dimercaptosuccinic acid: from heavy metal chelation to CdS quantum dots(Royal Society of Chemistry (RSC), 2012) Özen, Can; N/A; Department of Chemistry; Sevinç, Esra; Ertan, Fatoş Sibel; Ulusoy, Gülen; Acar, Havva Funda Yağcı; Faculty Member; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; 178902DMSA (meso-2,3-dimercaptosuccinic acid) a prescription drug and a heavy-metal chelating agent, is shown to act both as a sulfur source and a capping agent in the aqueous synthesis of CdS quantum dots under mild conditions. Release of sulfur from DMSA depends on the solution pH and the reaction temperature. Combination of 70 C and pH 7.5 was determined as the best reaction conditions for a well-controlled reaction. Changing the SH/Cd ratio from 2.5 to 7 provides QDs emitting from blue to orange with 6–9% quantum yield with respect to Rhodamine 2B. Viability tests performed with HeLa and MCF-7 cell lines indicate a very low cytotoxicity. Mild reaction conditions and biocompatibility makes these particles valuable candidates for bio applications.Publication Open Access Determination of the surface electronic structure of Fe3O4(111) by soft X-ray spectroscopy(Elsevier, 2015) Ogasawara, Hirohito; Nilsson, Anders; Department of Chemistry; Kaya, Sarp; Faculty Member; Department of Chemistry; College of Sciences; 116541The determination of surface terminations in transition metal oxides is not trivial because many structural configurations could be possible. They exhibit various terminations depending on the oxidation states of metal cations exposed to the surface. Fe3O4 is one example in which octahedrally and tetrahedrally coordinated Fe2+ and Fe3+ cations coexists with oxygen anions. For the identification of the surface termination of Fe3O4(1 1 1) grown on Pt(1 1 1) we have employed surface sensitive synchrotron based X-ray photoelectron and absorption spectroscopy. It has been shown that the topmost surface is octahedrally coordinated Fe3+ rich. (C) 2014 Elsevier B.V. All rights reserved.Publication Open Access Bosonic helium droplets with cationic impurities: onset of electrostriction and snowball effects from quantum calculations(American Institute of Physics (AIP) Publishing, 2007) Coccia, E.; Bodo, E.; Marinetti, F.; Gianturco, F. A.; Yıldırım, E.; Yurtsever, M.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Variational Monte Carlo and diffusion Monte Carlo calculations have been carried out for cations such as Li(+), Na(+), and K(+) as dopants of small helium clusters over a range of cluster sizes up to about 12 solvent atoms. The interaction has been modeled through a sum-of-potential picture that disregards higher order effects beyond atom-atom and atom-ion contributions. The latter were obtained from highly correlated ab initio calculations over a broad range of interatomic distances. This study focuses on two of the most striking features of the microsolvation in a quantum solvent of a cationic dopant: electrostriction and snowball effects. They are discussed here in detail and in relation with the nanoscopic properties of the interaction forces at play within a fully quantum picture of the cluster features.Publication Open Access Solvation of carbonaceous molecules by para-H2 and ortho-D2clusters. II. Fullerenes(American Institute of Physics (AIP) Publishing, 2016) Calvo, F.; Department of Chemistry; Yurtsever, İsmail Ersin; Doctor; Department of Chemistry; College of Sciences; 7129The coating of various fullerenes by para-hydrogen and ortho-deuterium molecules has been computationally studied as a function of the solvent amount. Rotationally averaged interaction potentials for structureless hydrogen molecules are employed to model their interaction with neutral or charged carbonaceous dopants containing between 20 and 240 atoms, occasionally comparing different fullerenes having the same size but different shapes. The solvation energy and the size of the first solvation shell obtained from path-integral molecular dynamics simulations at 2 K show only minor influence on the dopant charge and on the possible deuteration of the solvent, although the shell size is largest for ortho-D-2 coating cationic fullerenes. Nontrivial finite size effects have been found with the shell size varying non-monotonically close to its completion limit. For fullerenes embedded in large hydrogen clusters, the shell size and solvation energy both follow linear scaling with the fullerene size. The shell sizes obtained for C-60(+) and C-70(+) are close to 49 and 51, respectively, and agree with mass spectrometry experiments.Publication Open Access Solvation of carbonaceous molecules by para-H-2 and ortho-D-2 clusters. I. Polycyclic aromatic hydrocarbons(American Institute of Physics (AIP) Publishing, 2016) Calvo, F.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129This work theoretically examines the progressive coating of planar polycyclic aromatic hydrocarbon (PAH) molecules ranging from benzene to circumcoronene (C54H18) by para-hydrogen and ortho-deuterium. The coarse-grained Silvera-Goldman potential has been extended to model the interactions between hydrogen molecules and individual atoms of the PAH and parametrized against quantum chemical calculations for benzene-H-2. Path-integral molecular dynamics simulations at 2 K were performed for increasingly large amounts of hydrogen coating the PAH up to the first solvation shell and beyond. From the simulations, various properties were determined such as the size of the first shell and its thickness as well as the solvation energy. The degree of delocalization was notably quantified from an energy landscape perspective, by monitoring the fluctuations among inherent structures sampled by the trajectories. Our results generally demonstrate a high degree of localization owing to relatively strong interactions between hydrogen and the PAH, and qualitatively minor isotopic effects. In the limit of large hydrogen amounts, the shell size and solvation energy both follow approximate linear relations with the numbers of carbon and hydrogen in the PAH.Publication Open Access Vacuum space charge effects in sub-picosecond soft X-ray photoemission on a molecular adsorbate layer(American Institute of Physics (AIP) Publishing, 2015) Dell'Angela, M.; Anniyev, T.; Beye, M.; Coffee, R.; Fohlisch, A.; Gladh, J.; Kaya, S.; Katayama, T.; Krupin, O.; Nilsson, A.; Nordlund, D.; Schlotter, W. F.; Sellberg, J. A.; Sorgenfrei, F.; Turner, J. .J.; Ostrom, H.; Ogasawara, H.; Wolf, M.; Wurth, W.; Department of Chemistry; Kaya, Sarp; Faculty Member; Department of Chemistry; College of Sciences; 116541Vacuum space charge induced kinetic energy shifts of O 1s and Ru 3d core levels in femtosecond soft X-ray photoemission spectra (PES) have been studied at a free electron laser (FEL) for an oxygen layer on Ru(0001). We fully reproduced the measurements by simulating the in-vacuum expansion of the photoelectrons and demonstrate the space charge contribution of the high-order harmonics in the FEL beam. Employing the same analysis for 400 nm pump-X-ray probe PES, we can disentangle the delay dependent Ru 3d energy shifts into effects induced by space charge and by lattice heating from the femtosecond pump pulse.