Browsing by Subject "Silica"

Now showing 1 - 20 of 47

Open Access
Analysis of strain fields in silicon nanocrystals
(American Institute of Physics, 2009) Yilmaz, D. E.; Bulutay, C.; Çaǧın, T.
Strain has a crucial effect on the optical and electronic properties of nanostructures. We calculate the atomistic strain distribution in silicon nanocrystals up to a diameter of 3.2 nm embedded in an amorphous silicon dioxide matrix. A seemingly conflicting picture arises when the strain field is expressed in terms of bond lengths versus volumetric strain. The strain profile in either case shows uniform behavior in the core, however, it becomes nonuniform within 2-3 Å distance to the nanocrystal surface: tensile for bond lengths whereas compressive for volumetric strain. We reconcile their coexistence by an atomistic strain analysis.
Open Access
Chalcogenide integrated hollow-core optical fibers for infrared light guidance
(2022-12) Khan, Asfandyar
The low-loss light transmission and broad bandwidth of hollow-core negative curvature fibers (NCFs) have a variety of applications in infrared (IR) light guidance, such as chemical detection, biomedical surgery, and laser delivery. Although silica is a material of choice for light guidance in the visible and near-IR spectra, transmission losses increase drastically in the mid-IR region; thus, other mid-IR transparent materials, such as chalcogenide glasses, are potentially preferred to guide the light. In this thesis, various cladding designs of arsenic trisulfide (As2S3) and arsenic triselenide (As2Se3) chalcogenide NCFs are numerically explored for low-loss transmission in the mid-IR region. A detailed numerical investigation in the optimization of As2S3 NCFs with tubular and elliptical cladding elements was performed, and a low-loss ellipse-nested tubular NCF design is proposed for mid-IR guidance. The effect on the transmission loss due to cladding elements of the proposed low-loss As2Se3 ellipse-nested tubular fiber design was investigated. Confinement and total loss of all fiber designs were numerically studied, and the single-mode light guidance performance of the proposed low-loss fiber design was explored. The bending loss performance of the fiber was analyzed in the targeted spectrum, and a dispersion control study was carried out to investigate the effect of the primary design parameters on the dispersion performance. A fabrication tolerance study was performed to investigate the effects of common fabrication issues on the proposed design’s guidance properties. In the second part of the thesis, NCFs with silica, chalcogenide, and chalcogenide-coated silica cladding elements were numerically investigated for low-loss near and mid-IR transmission. As2S3 coated silica NCF was compared to simple silica and simple As2S3 fiber to understand the effect of the As2S3 coating on the transmission loss of silica NCF. Fabrication of silica NCF through the stack-and-draw technique followed by micro-coating with As2S3 solution was performed to improve the transmission performance of the As2S3 coated silica glass-based NCF. Further modifications in the fabrication of the NCFs were realized for a thorough comparison with the numerical investigations.
Open Access
Chalcogenide microresonators tailored to distinct morphologies by the shaping of glasses on silica tapers
(OSA - The Optical Society, 2017) Aktaş, O.
Production of chalcogenide (As2Se3) microresonators in sphere, loop, and bottle morphologies by the shaping of glasses at appropriate temperatures between cleaved silica tapers is reported. The quality factors exceed QS = 6.2×105, QB = 6.7 × 105, and QL = 1.6 × 104 for the sphere, bottle, and loop microresonators, respectively. All-optical thermally assisted tuning with a rate of 0.61 nm/mW is demonstrated for a bottle microcavity pumped via a silica taper at a wavelength of 670 nm. This technique enables practical and robust in situ production of chalcogenide microresonators thermally spliced to silica fibers in several morphologies with a wide tuning range of size.
Open Access
Charging/discharging of Au (core)/silica (shell) nanoparticles as revealed by XPS
(American Chemical Society, 2005) Tunc, I.; Demirok, U. K.; Süzer, Şefik; Correa-Duatre, M. A.; Liz-Marzan, L. M.
By recording XPS spectra while applying external voltage stress to the sample rod, we can control the extent of charging developed on core-shell-type gold nanoparticles deposited on a copper substrate, in both steady-state and time-resolved fashions. The charging manifests itself as a shift in the measured binding energy of the corresponding XPS peak. Whereas the bare gold nanoparticles exhibit no measurable binding energy shift in the Au 4f peaks, both the Au 4f and the Si 2p peaks exhibit significant and highly correlated (in time and magnitude) shifts in the case of gold (core)/silica (shell) nanoparticles. Using the shift in the Au 4f peaks, the capacitance of the 15-nm gold (core)/6-nm silica (shell) nanoparticle/nanocapacitor is estimated as 60 aF. It is further estimated that, in the fully charged situation, only 1 in 1000 silicon dioxide units in the shell carries a positive charge during our XPS analysis. Our simple method of controlling the charging, by application of an external voltage stress during XPS analysis, enables us to detect, locate, and quantify the charges developed on surface structures in a completely noncontact fashion. © 2005 American Chemical Society.
Open Access
Computational modeling of quantum-confined impact ionization in Si nanocrystals embedded in SiO2
(2007) Sevik, C.; Bulutay, C.
Injected carriers from the contacts to delocalized bulk states of the oxide matrix via Fowler-Nordheim tunneling can give rise to quantum-confined impact ionization (QCII) of the nanocrystal (NC) valence electrons. This process is responsible for the creation of confined excitons in NCs, which is a key luminescence mechanism. For a realistic modeling of QCII in Si NCs, a number of tools are combined: ensemble Monte Carlo (EMC) charge transport, ab initio modeling for oxide matrix, pseudopotential NC electronic states together with the closed-form analytical expression for the Coulomb matrix element of the QCII. To characterize the transport properties of the embedding amorphous SiO2, ab initio band structure and density of states of the α-quartz phase of SiO2 are employed. The confined states of the Si NC are obtained by solving the atomistic pseudopotential Hamiltonian. With these ingredients, realistic modeling of the QCII process involving a SiO2 bulk state hot carrier and the NC valence electrons is provided.
Open Access
Cyclodextrin-functionalized mesostructured silica nanoparticles for removal of polycyclic aromatic hydrocarbons
(Academic Press Inc., 2017) Topuz, F.; Uyar, T.
Polycyclic aromatic hydrocarbons (PAHs) are the byproducts of the incomplete combustion of carbon-based fuels, and have high affinity towards DNA strands, ultimately exerting their carcinogenic effects. They are ubiquitous environmental contaminants, and can accumulate on tissues due to their lipophilic nature. In this article, we describe a novel concept for PAH removal from aqueous solutions using cyclodextrin-functionalized mesostructured silica nanoparticles (CDMSNs) and pristine mesostructured silica nanoparticles (MSNs). The adsorption applications of MSNs are greatly restricted due to the absence of surface functional groups on such particles. In this regard, cyclodextrins can serve as ideal functional molecules with their toroidal, cone-type structure, capable of inclusion-complex formation with many hydrophobic molecules, including genotoxic PAHs. The CDMSNs were synthesized by the surfactant-templated, NaOH-catalyzed condensation reactions of tetraethyl orthosilicate (TEOS) in the presence of two different types of cyclodextrin (i.e. hydroxypropyl-β-cyclodextrin (HP-β-CD) and native β-cyclodextrin (β-CD)). The physical incorporation of CD moieties was supported by XPS, FT-IR, NMR, TGA and solid-state 13C NMR. The CDMSNs were treated with aqueous solutions of five different PAHs (e.g. pyrene, anthracene, phenanthrene, fluorene and fluoranthene). The functionalization of MSNs with cyclodextrin moieties significantly boosted the sorption capacity (q) of the MSNs up to ∼2-fold, and the q ranged between 0.3 and 1.65 mg per gram CDMSNs, of which the performance was comparable to that of the activated carbon.
Open Access
Cytotoxicity of multifunctional surfactant containing capped mesoporous silica nanoparticles
(Royal Society of Chemistry, 2016) Yildirim, A.; Turkaydin, M.; Garipcan, B.; Bayındır, Mehmet
This paper reports the synthesis of silica capped surfactant (cetyltrimethylammonium bromide; CTAB) and dye (Rose Bengal; RB) containing mesoporous silica nanoparticles (MSNs). Capping the pores of the surfactant containing MSNs with a thin silica layer decreased the immediate surfactant originated cytotoxicity of these particles without affecting their long term (3 days) cytotoxicity. Also, the silica capping process almost completely prevented the hemolytic activity of the surfactant containing MSNs. In addition, improved uptake of silica capped MSNs compared to the uncapped particles by cancer cells was demonstrated. The delayed cytotoxicity, low hemolytic activity, and better cellular uptake of the silica capped MSNs make them promising for the development of safe (i.e. with fewer side effects) yet efficient theranostic agents. These nanocarriers may release the loaded cytotoxic molecules (CTAB) mostly after being accumulated in the tumor site and cause so minimal damage to the normal tissues and blood components. In addition, the nanoscale confinement of RB molecules inside the pores of MSNs makes the particles brightly fluorescent. Furthermore, it was demonstrated that due to the singlet oxygen generation capability of the RB dye the silica capped MSNs can be also used for photodynamic therapy of cancer. © 2016 The Royal Society of Chemistry.
Open Access
Differential charging in SiO2/Si systems as determined by XPS
(American Chemical Society, 2004) Karadas, F.; Ertas, G.; Süzer, Şefik
The Si2p binding and the SiKLL kinetic energy difference between the SiO2 layer and Si substrate is shown to be influence by application of external voltage bias to the sample holder due to the differential charging as was already reported earlier (Ulgut, B.; Suzer, S. J. Phys. Chem. B 2003, 107, 2939). The cause of this bias induced (physical)-shift is now proven to be mostly due to partial neutralization by the stray electrons within the vacuum system by (i) introducing additional stray electrons via a filament and following their influence on the measured binding energy as a function of the applied voltage, (ii) measuring and Auger parameter. It is also shown that citrate-capped gold nanoclusters deposited on the SiO2/Si system experience differential charging similar to that of the oxide layer rather than the silicon substrate.
Open Access
Differential charging in x-ray photoelectron spectroscopy: a nuisance or a useful tool?
(American Chemical Society, 2003) Süzer, Şefik
We apply a negative bias to the sample while recording an XPS spectrum to enhance differential (positive) charging. The enhanced differential charging is due to the repulsion of stray electrons from the sample, which normally cause partial neutralization of the poorly conducting samples or regions accumulating positive charging, as a consequence of the photoelectron emission. This enhanced differential charging (obtained by negative biasing) is shown to have the ability to separate otherwise overlapping peaks of PDMS layer from that of the SiO2/Si substrate. Each layer experiences different charging that can be used to derive information related to dielectric properties of the layers, proximity of the atoms within composite multilayers, or both. Hence, differential charging in XPS, which is usually considered as a nuisance, is turned into a useful tool for extracting additional information from nanometer-size surface structures.
Open Access
Femtosecond laser fabrication of fiber based optofluidic platform for flow cytometry applications
(SPIE, 2017) Serhatlioglu, Murat; Elbuken, Çağlar; Ortac, Bülend; Solmaz, Mehmet E.
Miniaturized optofluidic platforms play an important role in bio-analysis, detection and diagnostic applications. The advantages of such miniaturized devices are extremely low sample requirement, low cost development and rapid analysis capabilities. Fused silica is advantageous for optofluidic systems due to properties such as being chemically inert, mechanically stable, and optically transparent to a wide spectrum of light. As a three dimensional manufacturing method, femtosecond laser scanning followed by chemical etching shows great potential to fabricate glass based optofluidic chips. In this study, we demonstrate fabrication of all-fiber based, optofluidic flow cytometer in fused silica glass by femtosecond laser machining. 3D particle focusing was achieved through a straightforward planar chip design with two separately fabricated fused silica glass slides thermally bonded together. Bioparticles in a fluid stream encounter with optical interrogation region specifically designed to allocate 405nm single mode fiber laser source and two multi-mode collection fibers for forward scattering (FSC) and side scattering (SSC) signals detection. Detected signal data collected with oscilloscope and post processed with MATLAB script file. We were able to count number of events over 4000events/sec, and achieve size distribution for 5.95μm monodisperse polystyrene beads using FSC and SSC signals. Our platform shows promise for optical and fluidic miniaturization of flow cytometry systems. © 2017 SPIE.
Open Access
Fluorescent aerogel films for TNT sensing
(2009) Yıldırım, Adem
Silica aerogels are unique materials with extraordinary properties such as, high porosity, large surface area and low refractive indices. Due to these properties, they can be applied to a wide range of areas including, insulation, catalyst support, sensors and dielectric materials. However, until know because of their poor mechanical properties and costly production a few applications of aerogels were realized. Ambient pressure drying method is a promising way to produce low cost aerogels and thus expanding the realized application areas of aerogels. This method is based on lowering the surface tension on the gel network, in order to minimize the collapse of the gel during drying. For this purpose gel surface can be modified to make it hydrophobic. In the first part of this work, ambient pressure production of fluorescent aerogel thin films are described. The produced fluorescent films were characterized to identify their morphological, optical and surface properties. The gel was produced by using methlyltrimethoxysilane (MTMS) to produce hydrophobic gel. A porphyrin derivative (TCPPH2) was simply mixed with the sol before gelation for the fluorescence property. After gelation and aging the produced gels are homogenized and spin coated on glass substrates. The produced films were found to be highly porous (60.3-77.1%), fluorescent and transparent in visible region (82-89%). In the second part, sensing performances of the films were examined by using the common explosive trinitrotoluene (TNT). All films show fluorescence quenching based sensing against TNT exposure. The quenching efficiency of the films is highly thickness dependent. For the thinnest film (120 nm) the quenching efficiency was found to be 8.6% in 10 seconds and for the thickest film (1100 nm) film 2.1% in 10 seconds.
Open Access
Genetically programmed engineered cells for biomaterials synthesis
(2021-01) Kırpat, Büşra Merve
Several organisms can process nanomaterials and producing in various sizes and morphologies in mild conditions by utilizing specific proteins. In sea sponges, silicatein proteins play a key role in synthesizing silica nanoparticles the precursor silicic acid. Silaffin proteins in diatoms can also biomineralize silica. One subunit of silaffin called R5 peptide has a key role for nucleation and initiation of the nanoparticle formation and it has been shown that bacteria synthesized R5 peptide has ability to precipitate silica structures. These silica nanostructures can be utilized in many areas. Silica-based cements take attentions to make them useful in restorative dentistry and endodontics. In this work, a synthetic cell system has reprogrammed autotransporter (Ag43) system to display R5 peptide fused with fluorescent proteins. After displaying the fused proteins on the surface of bacteria or secreting them into environment, whole cell or the proteins are used to precipitate silica in the presence of precursor such as tetramethyl orthosilicate (TMOS). These silica structures are used to evaluate their in vitro effects on the proliferation of dental pulp stem cells (DPSCs) and their osteogenesis.
Open Access
Growth of Ge nanoparticles on SiO2 / Si interfaces during annealing of plasma enhanced chemical vapor deposited thin films
(Elsevier B.V., 2007) Foss, S.; Finstad, T. G.; Dana, A.; Aydınlı, Atilla
Multilayer germanosilicate (Ge:SiO2) films have been grown by plasma enhanced chemical vapor deposition. Each Ge:SiO2 layer is separated by a pure SiO2 layer. The samples were heat treated at 900 °C for 15 and 45 min. Transmission electron microscopy investigations show precipitation of particles in the layers of highest Ge concentration. Furthermore there is evidence of diffusion between the layers. This paper focuses mainly on observed growth of Ge particles close to the interface, caused by Ge diffusion from the Ge:SiO2 layer closest to the interface through a pure SiO2 layer and to the interface. The particles grow as spheres in a direction away from the interface. Particles observed after 15 min anneal time are 4 nm in size and are amorphous, while after 45 min anneal time they are 7 nm in size and have a crystalline diamond type Ge structure.
Open Access
High-Q silicon-on-insulator optical rib waveguide racetrack resonators
(Optical Society of American (OSA), 2005) Kiyat I.; Aydınlı, Atilla; Dagli, N.
In this work, detailed design and realization of high quality factor (Q) racetrack resonators based on silicon-on-insulator rib waveguides are presented. Aiming to achieve critical coupling, suitable waveguide geometry is determined after extensive numerical studies of bending loss. The final design is obtained after coupling factor calculations and estimation of propagation loss. Resonators with quality factors (Q) as high as 119000 has been achieved, the highest Q value for resonators based on silicon-on-insulator rib waveguides to date with extinction ratios as large as 12 dB. © 2005 Optical Society of America.
Open Access
Influence of gold-silica nanoparticles on the performance of small-molecule bulk heterojunction solar cells
(Elsevier BV * North-Holland, 2015) Xu, X.; Kyaw, A. K. K.; Peng, B.; Xiong, Q.; Demir, Hilmi Volkan; Wang Y.; Wong, T. K. S.; Sun, X. W.
Light trapping by gold (Au)-silica nanospheres and nanorods embedded in the active layer of small-molecule (SM) organic solar cell has been systematically compared. Nanorod significantly outperforms nanosphere because of more light scattering and higher quality factor for localized surface plasmon resonance (LSPR) triggered by nanorods. The optimum concentration of nanorod was characterized by charge carrier transport and morphology of the active layers. At optimum nanorod concentration, almost no change in the morphology of the active layer reveals that LSPR and scattering effects rather than the morphology are mainly responsible for the enhanced power conversion efficiency. In addition, the preliminary lifetime studies of the SM solar cells with and without Au-silica nanorods were conducted by measuring the current density-voltage characteristics over 20 days. The results show that plasmonic device with nanorods has no adverse impact on the device stability
Open Access
Matrix density effect on morphology of germanium nanocrystals embedded in silicon dioxide thin films
(Materials Research Society, 2011) Alagoz, A. S.; Genisel, M. F.; Foss, Steinar; Finstad, T. G.; Turan, R.
Flash type electronic memories are the preferred format in code storage at complex programs running on fast processors and larger media files in portable electronics due to fast write/read operations, long rewrite life, high density and low cost of fabrication. Scaling limitations of top-down fabrication approaches can be overcome in next generation flash memories by replacing continuous floating gate with array of nanocrystals. Germanium (Ge) is a good candidate for nanocrystal based flash memories due its small band gap. In this work, we present effect of silicon dioxide (SiO 2) host matrix density on Ge nanocrystals morphology. Low density Ge+SiO 2 layers are deposited between high density SiO 2 layers by using off-angle magnetron sputter deposition. After high temperature post-annealing, faceted and elongated Ge nanocrystals formation is observed in low density layers. Effects of Ge concentration and annealing temperature on nanocrystal morphology and mean size were investigated by using transmission electron microscopy. Positive correlation between stress development and nanocrystal size is observed at Raman spectroscopy measurements. We concluded that non-uniform stress distribution on nanocrystals during growth is responsible from faceted and elongated nanocrystal morphology.
Open Access
Mesoscopic model of nucleation and Ostwald ripening/stepping: Application to the silica polymorph system
(American Inst of Physics, Woodbury, NY, United States, 2000) Ozkan, G.; Ortoleva, P.
Precipitation is modeled using a particle size distribution ~PSD! approach for the single or multiple polymorph system. A chemical kinetic-type model for the construction of the molecular clusters of each polymorph is formulated that accounts for adsorption at a heterogeneous site, nucleation, growth, and Ostwald ripening. When multiple polymorphs are accounted for, Ostwald stepping is also predicted. The challenge of simulating the 23 order of magnitude in cluster size ~monomer, dimer, . . . , 1023-mer! is met by a new formalism that accounts for the macroscopic behavior of large clusters as well as the structure of small ones. The theory is set forth for the surface kinetic controlled growth systems and it involves corrections to the Lifshitz–Slyozov, Wagner ~LSW! equation and preserves the monomer addition kinetics for small clusters. A time independent, scaled PSD behavior is achieved both analytically and numerically, and the average radius grows with Rave}t1/2 law for smooth particles. Applications are presented for the silica system that involves five polymorphs. Effects of the adsorption energetics and the smooth or fractal nature of clusters on the nucleation, ripening, and stepping behavior are analyzed. The Ostwald stepping scenario is found to be highly sensitive to adsorption energetics. Long time scaling behavior of the PSD reveals time exponents greater than those for the classical theory when particles are fractal. Exact scaling solutions for the PSD are compared with numerical results to assess the accuracy and convergence of our numerical technique. © 2000 American Institute of Physics. @S0021-9606~00!70123-1#
Open Access
A method of two-scale chemo-thermal-mechanical coupling for concrete
(CIMNE, 2011) Wu, T.; Temizer, İlker; Wriggers, P.
The Alkali Silica Reaction(ASR) is one of the most important reasons to cause damage in cementitious constructions, which can be attributed to the expansion of hydrophilic gel produced in the reaction. In this contribution, the chemical extent is described depending on the temperature and it has influences on damage parameters. Expansions of the gel are assumed to only happen in the micropores of Hardened Cement Paste. Afterwards, the homogenization of damage in the microscale is initialized and the effective damage can be applied in the mesoscale directly. Moreover, parameter identification is implemented to extract the effective inelastic consititutive equation. In all, 3D multiscale chemo-thermo-mechanical coupled model is set up to describe the damage in the concrete due to ASR.
Open Access
A multiscale method to analyze the deterioration due to alkali silica reaction considering the effects of temperature and relative humidity
(International Center for Numerical Methods in Engineering, 2013) Wu, T.; Temizer I.; Wriggers P.
This work presents a three-dimensional multiscale framework to investigate the deterioration resulting from alkali silica reaction (ASR) in the concrete. In this contribution, 3D micro-CT scan of hardened cement paste (HCP) and aggregates with a random distribution embedded in a homogenized cement paste matrix represent the microscale and mesoscale of the concrete respectively. A 3D hydro-chemo-thermo-mechanical model based on staggered method is developed at the mesoscale of the concrete, yet taking into account the deterioration at the microscale due to ASR.
Open Access
Nano-structured organically modified silica thin films for functional surfaces
(2011) Bayındır, Mehmet; Yıldırım, Adem; Budunoglu, Hülya; Yaman, Mecit; Deniz, Hakan; Güler, Mustafa O.
We report a template-free sol-gel method for preparation of nanoporous ormosil thin films at ambient conditions. The thin films are coated to the surfaces by using colloidal suspensions of ormosil gels. Gels are synthesized by using a trifunctional organosilane monomer, methyltrimethoxysilane (MTMS), with a two-step acid base reaction. We prepared several ormosil thin films on glass, metal, plastic and paper surfaces with different functionalities like superhydrophobic, antireflective, antifogging and ice retarding properties, from gels prepared in different conditions. Also films on flexible substrates exhibits durable surface properties after several bending cycles. In addition, we also demonstrate that these thin films can be used for fluorescent sensing of explosives by doping them with fluorescent dyes.