Browsing by Subject "Nucleation"

Now showing 1 - 8 of 8

Open Access
Ab initio temperature dependent studies of the homoepitaxial growth on Si(0 0 1) surface
(2001) Dağ, S.; Çıracı, Salim; Kılıç, Ç.; Fong, C. Y.
We performed ab initio zero temperature and finite temperature molecular dynamics calculations to investigate the homoepitaxial growth on the Si(0 0 1) surface. How do the deposited atoms (adatoms) form addimers and how do the addimers reach their favorable positions at the nucleation site of the growth process are presented. Once two epitaxial addimers, one over the dimer row and oriented perpendicular to the surface dimer bonds and the other over the adjacent trough, are aligned at high temperature, the nucleation site of the growth process is formed. The concerted bond exchange between these addimers and the reconstructed surface dimers is found to be the atomistic mechanism that leads to the homoepitaxial growth. © 2001 Elsevier Science B.V.
Open Access
Area-selective atomic layer deposition using an inductively coupled plasma polymerized fluorocarbon layer: A case study for metal oxides
(American Chemical Society, 2016) Haider, A.; Deminskyi, P.; Khan, T. M.; Eren, H.; Bıyıklı, Necmi
Area-selective atomic layer deposition (AS-ALD) has attracted immense attention in recent years for self-aligned accurate pattern placement with subnanometer thickness control. Here, we demonstrate a methodology to achieve AS-ALD by using inductively couple plasma (ICP) grown fluorocarbon polymer film as hydrophobic blocking layer for selective deposition. Our approach has been tested for metal-oxide materials including ZnO, Al2O3, and HfO2. Contact angle, X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometer, and scanning electron microscopy (SEM) measurements were performed to investigate the blocking ability of plasma polymerized fluorocarbon layers against ALD-grown metal-oxide films. A considerable growth inhibition for ZnO has been observed on fluorocarbon coated Si(100) surfaces, while the same polymerized surface caused a relatively slow nucleation for HfO2 films. No growth selectivity was obtained for Al2O3 films, displaying almost the same nucleation behavior on Si and fluorocarbon surfaces. Thin film patterning has been demonstrated using this strategy by growing ZnO on lithographically patterned fluorocarbon/Si samples. High resolution SEM images and XPS line scan confirmed the successful patterning of ZnO up to a film thickness of ∼15 nm. © 2016 American Chemical Society.
Open Access
Femtosecond laser assisted synthesis of silicate-1 zeolite
(2022-01) Hagverdiyev, Mehdi
Zeolites are microporous (pore sizes < 2 nm) inorganic aluminosilicate materials with well-defined molecular pores and high surface areas used widely for various chemical processes, primarily as catalysts, sorbents, and ion exchangers. Aside from 40 types of natural zeolite, 253 different synthetic zeolitic framework types are synthesized and recognized by the International Zeolite Association (IZA). Zeolite synthesis requires moderate temperatures between 50°C - 270°C and high pressures (up to 120 bar). A fundamental challenge in zeolite synthesis is to elucidate and control the nucleation and growth of the zeolite crystals. The main reason for this is the fast kinetics of zeolite synthesis and rapid conformational transitions between quasi-equilibrium phases. Zeolite synthesis is a complex process because more than 40 different types of silica polymerization and depolymerization reactions occur simultaneously in a reaction mixture (i.e., precursor suspension). Reaction time scales of the silica polymerization are within the range of picoseconds and femtoseconds. Using the traditional hydrothermal synthesis method, which is occurring near thermal equilibrium, it is impossible to control the system at the time scale of these simultaneous polymerization reactions due to the slow energy deposition, which can be from 24 hours to several days. Other types of zeolite synthesis methods such as microwave synthesis are capable of depositing high energies in short time scales. However, the synthesis method is lacking the control of the excess heating of the full volume of the precursor suspension. During microwave heating, several hot spots form inside the precursor suspension, causing the boiling of the liquid. Growth by inhomogeneous heating leads to the formation of fused (i.e., interconnect) crystals instead of the discrete ones that dominate the end product in microwave-assisted synthesis method of zeolites. Here, we introduce a novel femtosecond laser-assisted synthesis method for the synthesis of Silicalite-1 zeolites. Femtosecond laser pulses ensure the delivery of a precise amount of energy per area within a given time interval, and therefore the spatiotemporal control over the energy delivered to the precursor suspension could be done on the time scale of the polymerization reactions of the zeolite synthesis. Thanks to the femtosecond laser pulses, the appropriate environment for zeolite synthesis, such as local high temperature and local high pressure (shock waves), has been created. In the laser-assisted synthesis method, the time required for zeolite synthesis decreased drastically compared to the hydrothermal method, overall control and product quality increased compared to the microwave synthesis method of zeolites. Unlike other rapid synthesis methods such as microwave synthesis, the uncontrollable temperature rise over the full volume of precursor suspension was not observed, resulting in 'discrete' crystals in the final product. Energy intake of the transparent precursor suspension was achieved through multiphoton absorption of the femtosecond laser pulses inducing steep spatiotemporal thermal gradients. Since surface tension of fluid is a function of temperature, surface tension gradients form as well, causing Marangoni flow. The ‘stirring effect’ of these flows leads to the distribution of the formed clusters evenly to the system, which is not attained by static hydrothermal synthesis of zeolites. It is proposed that vigorous flow induced in the laser-assisted synthesis assembles nuclei/polymerized clusters much faster than the other synthesis methods, which may be the reason for the drastically reduced reaction times compared to hydrothermal synthesis (i.e., 30 - 48h for hydrothermal vs. 3h - 5h for laser-assisted syntheses). Growth kinetics of the Silicalite-1 zeolite was examined in detail. In addition, templatefree nanosized microporous Zeolite Y, and mesoporogen-free hierarchical ZSM-5 zeolites with micro and meso-porosity were synthesized with reduced reaction times through laser-assisted synthesis method (i.e., 24 - 45h for hydrothermal vs. 1 - 5h for laser-assisted syntheses), which is important in terms of green synthesis approaches drawing attention in recent years.
Open Access
Growth of high crystalline quality semi-insulating GaN layers for high electron mobility transistor applications
(2006) Yu H.; Caliskan, D.; Özbay, Ekmel
Semi-insulating character (sheet resistivity of 3.26 × 10 11 Ω/sq) of thick GaN layers was developed for AlGaN/GaN high electron mobility transistor (HEMT) applications on an AlN buffer layer. Electrical and structural properties were characterized by a dark current-voltage transmission line model, x-ray diffraction, and atomic force microscope measurements. The experimental results showed that compared to semi-insulating GaN grown on low temperature GaN nucleation, the crystal quality as well as surface morphology were remarkably improved. It was ascribed to the utilization of a high quality insulating AlN buffer layer and the GaN initial coalescence growth mode. Moreover, the significant increase of electron mobility in a HEMT structure suggests that this is a very promising method to obtain high performance AlGaN/GaN HEMT structures on sapphire substrates. © 2006 American Institute of Physics.
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
Preparation of Au and Au-Pt nanoparticles within PMMA matrix using UV and X-ray irradiation
(2009) Ozkaraoglu, E.; Tunc, I.; Süzer, Şefik
Au and Au-Pt alloy nanoparticles are prepared and patterned at room temperature within the PMMA polymer matrix by the action of 254 nm UV light or X-rays. The polymer matrix enables us to entangle the kinetics of the photochemical reduction from the nucleation and growth processes, when monitored by UV-vis spectroscopy. Accordingly, increase of the temperature to 50 °C of the reaction medium increases the nucleation and growth rates of the nanoparticle formation by more than one order of magnitude, due to enhanced diffusion and nucleation at the higher temperature, but has no effect on the photochemical reduction process. Presence of Pt ions also increases the same rate, but by a factor two only. Similar photochemical reduction and particle growth take also place within the PMMA matrix, when these metal ions are subjected to prolonged exposure to X-rays, as evidenced by XPS analysis. Both angle-resolved and charge-contrast measurements using XPS reveal that the resultant Au and Pt species are in close proximity to each other, indicating the Au-Pt alloy formation to be the most likely case.
Open Access
Theories of nanoparticle and nanostructure formation in liquid phase
(Elsevier, 2018) Karatutlu, A.; Barhoum, A.; Sapelkin, A.
Nanoparticles (NPs) and nanostructured materials exhibit shape- and size-dependent properties that are desired for a wide variety of applications, such as catalysis, sensing, drug delivery, energy production, and storage. In view of this, it is essential to produce well-defined NPs and nanostructures with desired characteristics, to understand their formation and growth mechanisms, and to define the critical size below which they act differently from bulk materials in order to develop synthetic strategies. For example, quantum dots (below 20nm) are mainly single nanocrystals characterized by a single-domain crystalline lattice without grain boundaries. These tiny individual crystals differ drastically from bulk polycrystalline materials. In fact, existing investigations indicated that ordered polycrystalline particles are preferably formed at high supersaturations, where rapid nucleation generates many NPs, which subsequently tend to aggregate randomly at high NP concentrations. Single crystals, such as quantum dots, form at low supersaturations. The reduction of the supersaturation to a level at which primary NPs are still formed in solution yields mesocrystals. This chapter discusses the advanced nucleation and growth theories that are used to explain the growth of the obtained nanoparticles and nanostructures to the desired structures.
Open Access
X-ray photoelectron spectroscopic investigation of gold particles deposited on (formula) system
(2003) Karadaş, Ferdi
Gold particles on SiO2/Si system were investigated by X-ray Photoelectron Spectroscopy (XPS) technique. A suitable reference point was first established in order to investigate the physical/chemical factors affecting chemical shift of gold particles. Gold particles were: i) deposited directly from aqueous solution, ii) capped with citrate agent and then deposited, iii) reduced chemically by NaBH4 and deposited on SiO2/Si system. In addition, gold particles were deposited onto different substrates (quartz, glass). Similar chemical shift of Si4+ 2p and Au 0 4f peak upon the application of external bias gave a strong evidence to the assumption that SiO2 could be chosen as reference. In addition, the derived Auger Parameters have shown that chemical shifts observed during the application of external bias are solely due to charging. It was shown that reduction and nucleation processes occur at the same time during X-ray exposure when gold particles are deposited from aqueous solution. Differential charging of gold particles was investigated by measuring the changes in: i) binding energy, ii) FWHM and iii) intensity values of Au 0 and Si4+ peaks. Our findings obtained from Angle Resolved XPS method supported the assumption that gold particles deposited from aqueous solution prefer to grow three-dimensionally. Assuming the Si 2p binding energy of Si4+ peak as a reference, the binding energy of gold particles is: i) 84.30 ± 0.05 eV when gold is deposited from aqueous solution, ii) pdfMachine trial version V 84.00 ± 0.05 when citrate capped gold particles are used, iii) 84.10 ± 0.05 when gold is chemically reduced by NaBH4. Vis-absorption and electrophoresis methods have shown that capped gold particles have negative charges and they aggregate reversibly (i.e. without coagulation) when they are deposited on SiO2/Si system from their aqueous solution (and transferred back).