Browsing by Subject "Silicon nanocrystals"

Now showing 1 - 9 of 9

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
Atomistic structure simulation of silicon nanocrystals driven with suboxide penalty energies
(American Scientific Publishers, 2008) Yılmaz, Dündar E.; Bulutay, Ceyhun; Çağın, T.
The structural control of silicon nanocrystals embedded in amorphous oxide is currently an important technological problem. In this work, an approach is presented to simulate the structural behavior of silicon nanocrystals embedded in amorphous oxide matrix based on simple valence force fields as described by Keating-type potentials. After generating an amorphous silicon-rich-oxide, its evolution towards an embedded nanocrystal is driven by the oxygen diffusion process implemented in the form of a Metropolis algorithm based on the suboxide penalty energies. However, it is observed that such an approach cannot satisfactorily reproduce the shape of annealed nanocrystals. As a remedy, the asphericity and surface-to-volume minimization constraints are imposed. With the aid of such a multilevel approach, realistic-sized silicon nanocrystals can be simulated. Prediction for the nanocrystal size at a chosen oxygen molar fraction matches reasonably well with the experimental data when the interface region is also accounted. The necessity for additional shape constraints suggests the use of more involved force fields including long-range forces as well as accommodating different chemical environments such as the double bonds.
Open Access
Electronic and Optical Properties of Silicon Nanocrystals
(Wiley-VCH, 2010) Bulutay, C.; Ossicini S.
[No abstract available]
Open Access
Electronic structure and optical properties of silicon nanocrystals along their aggregation stages
(Elsevier B.V., 2007) Bulutay, C.
The structural control of silicon nanocrystals is an important technological problem. Typically, a distribution of nanocrystal sizes and shapes emerges under the uncontrolled aggregation of smaller clusters. The aim of this computational study is to investigate the evolution of the nanocrystal electronic states and their optical properties throughout their aggregation stages. To realistically tackle such systems, an atomistic electronic structure tool is required that can accommodate about tens of thousand nanocrystal and embedding lattice atoms with very irregular shapes. For this purpose, a computationally efficient pseudopotential-based electronic structure tool is developed that can handle realistic nanostructures based on the expansion of the wavefunction of the aggregate in terms of bulk Bloch bands of the constituent semiconductors. With this tool, the evolution of the electronic states as well as the polarization-dependent absorption spectra correlated with the oscillator strengths over their aggregation stages is traced. The low-lying aggregate nanocrystal states develop binding and anti-binding counterparts of the isolated states. Such information may become instrumental with the maturity of the controlled aggregation of these nanocrystals.
Open Access
Low dimensional structures for optical and electrical applications
(2008) Akça, İmran
Low dimensional structures such as quantum dots have been particularly attractive because of their fundamental physical properties and their potential applications in various devices in integrated optics and microelectronics. This thesis presents optical and electrical applications of low dimensional structures. For this purpose we have studied silicon and germanium nanocrystals for flash memory applications and InAs quantum dots for optical modulators. As a quantum dot, nanocrystals can be used as storage media for carriers in flash memories. Performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. Charge and discharge dynamics of PECVD grown nanocrystals were studied. Electron and hole charge and discharge currents were observed to differ significantly and strongly depend on annealing conditions chosen for the formation of nanocrystals. Our experimental results revealed that, discharge currents were dominated by the interface layer acting as a quantum well for holes and route for direct tunneling for electrons. On the other hand, possibility of obtaining quantum dots with enhanced electro-optic and/or electro-absorption coefficients makes them attractive for use in light modulation. Therefore, waveguides of multilayer InAs quantum dots were studied. Electro-optic measurements were conducted at 1.5 µm and clear Fabry-Perot resonances were obtained. The voltage dependent Fabry-Perot measurements revealed that 6 V was sufficient for full on/off modulation. Electroabsorption measurements were conducted at both 1.3 and 1.5 µm. Since the structure lases at 1285 nm, high absorption values at 1309 nm were obtained. The absorption spectrum of the samples was also studied under applied electric field. Absorption spectra of all samples shift to lower photon energies with increasing electric field.
Open Access
A plasmonic enhanced photodetector based on silicon nanocrystals obtained through laser ablation
(Institute of Physics Publishing, 2012-10-18) Alkis, S.; Oruç, F. B.; Ortaç, B.; Koşger, A. C.; Okyay, Ali Kemal
We present a proof-of-concept photodetector which is sensitive in the visible spectrum. Silicon nanocrystals (Si-NCs) obtained by laser ablation are used as the active absorption region. Si-NC films are formed from a polymeric dispersion. The films are sandwiched between thin insulating films to reduce the electrical leakage current. Furthermore, Ag nanoparticles are integrated with the photodetector to enhance the visible response using plasmonic effects. The measured photocurrent is resonantly enhanced, which is explained in terms of enhanced local fields caused by localized plasmons. The UV-vis spectrum of Ag nanoparticles is also measured to verify the resonance.
Open Access
Post-Treatment od Silicon Nanocrystals Produced by Ultra-Short Pulsed Laser Ablation in Liquid: Toward Blue Luminescent Nanocrystal Generation
(American Chemical Society, 2012-01-11) Alkis, S.; Okyay, Ali Kemal; Ortac, B.
Blue luminescent colloidal silicon nanocrystals (Si-NCs) were produced in a two-stage process. In the first step, synthesis of Si-NCs was achieved by femtosecond pulsed laser ablation of a silicon wafer, which was immersed in deionized water. The size and the structural and the chemical characteristics of colloidal Si-NCs were investigated by TEM and EDAX analyses, and it is found out that the Si-NCs are in spherical shape and the particle diameters are in the range of 5-100 nm. In the second step, ultrasonic waves and filtering chemical-free post-treatment of colloidal Si-NCs solution was performed to reduce the particle size. High-resolution TEM (HRTEM) studies on post-treated colloidal solution clearly show that small (1-5.5 nm in diameter) Si-NCs were successfully produced. Raman spectroscopy results clearly confirms the generation of Si nanoparticles in the crystalline nature, and the Raman scattering study of post-treated Si-NCs confirms the reduction of the particle size. The UV-vis absorption and photoluminescence (PL) spectroscopy studies elucidate the quantum confinement effect of Si-NCs on the optical properties. The colloidal Si-NCs and post-treated Si-NCs solutions present strong absorption edge shifts toward UV region. Broadband PL emission behavior is observed for the initial colloidal Si-NCs, and the PL spectrum of post-treated Si-NCs presents a blue-shifted broadband PL emission behavior due to the particle size reduction effect.
Open Access
Silicon nanocrystal doped polymer nanowire arrays
(2013) Çelebi, Muhammet
In this thesis, we successfully produced silicon nanocrystal embedded polymer micro and nanowire arrays by using a new top—to—bottom nanofabrication approach. Silicon nanocrystal (Si-Nc) quantum dots are photoluminescent materials that give bright optical illumination under UV light excitation. Si-Ncs were used to fabricate large area luminescent thin polymer ﬁlms before production of the ﬁbers. Among many of Si—Nc fabrication methods that are available, we chose a chemical route which takes the advantage of high product yield and ease of production steps, although the resultant size distribution is not uniform as other methods such as electrochemical treatment of Si wafers. Dopant Si—Ncs in polymer sheets shows some improved properties compared to free standing silicon nanocrystals, like longer luminescent life time in normal atmospheric conditions and in high temperatures as high as 300 °C. With utilizing these properties, thermal drawing of Si—Nc doped polymer ﬁbers is possible without harming the luminescence properties. Hence, throughout the work, different types of ﬁlms were investigated and polycarbonate ﬁlms were chosen for both their thermal and optical properties such as durable luminescence at high temperatures and low absorption at visible wavelengths. Consequently, with combining these properties with our iterative thermal size reduction method, we successfully produced silicon nanocrystal doped polymer micro and nanowire arrays. In literature, there are similar works treating the same idea of producing luminescent ﬁbers, which were realized with different techniques and material sets, like dye/QD doped nanoﬁbers or ﬁbers produced with conjugated polymers. However, the methods used to produce these type of geometries lacks in some aspects such as limited length, uniformity, alignment, reproducibility, etc. On the other hand, our iterative thermal drawing method is very successful for producing indeﬁnitely long, uniform and easily aligned ﬁbers. Our production steps can be summed in ﬁve steps which are: Si—Nc synthesis, ﬁlm preparation, ﬁlmrolling, consolidation, and two consecutive ﬁber drawing. Keeping the track of characterization of the product in each step is important. Hence, for silicon nanocrystals, we took photoluminescence (PL) intensity measurements, SEM/TEM images and temperature dependent PL measurements. Also for doped ﬁlms, we performed temperature dependent PL measurements and for the resultant ﬁbers we carried out cross—section SEM and PL characterizations. Silicon nanocrystal embedded micro and nanowires can be utilized as ﬁber gain medium, single photon source, directional emitter, light emitting diodes and optical sensing elements. Also, they increases light extraction efﬁciencies with guiding advantages and this can result to ﬂuorescence enhancement for luminescent active material dopants.
Open Access
Systematic spatial and stoichiometric screening towards understanding the surface of ultrasmall oxygenated silicon nanocrystal
(Elsevier, 2016-11) Niaz, S.; Zdetsis, A. D.; Koukaras, E. N.; Gülseren, O.; Sadiq, I.
In most of the realistic ab initio and model calculations which have appeared on the emission of light from silicon nanocrystals, the role of surface oxygen has been usually ignored, underestimated or completely ruled out. We investigate theoretically, by density functional theory (DFT/B3LYP) possible modes of oxygen bonding in hydrogen terminated silicon quantum dots using as a representative case of the Si29 nanocrystal. We have considered Bridge-bonded oxygen (BBO), Doubly-bonded oxygen (DBO), hydroxyl (OH) and Mix of these oxidizing agents. Due to stoichiometry, all comparisons performed are unbiased with respect to composition whereas spatial distribution of oxygen species pointed out drastic change in electronic and cohesive characteristics of nanocrytals. From an overall perspective of this study, it is shown that bridge bonded oxygenated Si nanocrystals accompanied by Mix have higher binding energies and large electronic gap compared to nanocrystals with doubly bonded oxygen atoms. In addition, it is observed that the presence of OH along with BBO, DBO and mixed configurations further lowers electronic gaps and binding energies but trends in same fashion. It is also demonstrated that within same composition, oxidizing constituent, along with their spatial distribution substantially alters binding energy, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gap (up to 1.48 eV) and localization of frontier orbitals.