Browsing by Subject "Quantum dot"

Now showing 1 - 13 of 13

Open Access
Bipolaronic phase in polar semiconductor quantum dots: An all-coupling approach
(Elsevier B.V., 2007) Krishna, P. M.; Mukhopadhyay, S.; Chatterjee, A.
An all-coupling variational calculation has been performed to explore the formation and stability of a bipolaron in a polar semiconductor quantum dot. It has been shown that quantum confinement in general leads to a broadening of the bipolaron stability region. It has been furthermore shown for the first time that stable bipolarons can exist in realistic parabolic quantum dots of polar semiconductors like GaAs, CdS, CdTe and CdSe if they are fabricated in certain range of sizes.
Open Access
Color generation and enhancement using large-scale compatible metamaterial design architectures
(2022-01) Köşger, Ali Cahit
Metamaterials are a type of artificial matt that can impose exotic functionalities beyond natural materials. These specifically designed sub-wavelength structures acquire these functionalities from their collective geometric arrangement rather than their individual single-unit properties. As a result, metamaterials have shown promising applications, including negative refraction, artificial magnetism, asymmetric transmission, lasing, and cloak of invisibility. Among all these applications, the concept of color generation and enhancement using metamaterial designs have attracted much attention in recent years. We can achieve color generation from two primary sources: i) filtering white light, and ii) generating light from emitting materials such as quantum dots. In color generation using white light, a metamaterial design reflects or transmits a narrow portion of the incident spectrum. Thus, the design acts as a color filter. However, the source is already a narrowband color light in the second category. Thus metamaterials merely amplify the color intensity rather than manipulate its spectral response. In this thesis, metamaterial structures are designed, fabricated, and characterized in both categories mentioned above; The content of this thesis consists of two parts; i) In the first part, we generated additive red-green-blue (RGB) colors in reflectance mode with near-unity amplitude. For this purpose, we designed a multilayer structure made of metal-insulator-metal-semiconductor-insulator (MIMSI) stacks to achieve >0.9 reflection peaks with full-width-at-half-maximum (FWHM) values <0.3λpeak. The proposed design also shows near-zero reflection in off-resonance spectral ranges, which, in turn, leads to high color purity. Finally, we fabricated the optimized designs and verified the simulation and theoretical results with characterization findings. This work demonstrates the potential of multilayer tandem cavity designs in realizing lithography-free large-scale compatible functional optical coatings. ii) In the second part, we utilized a large-scale compatible plasmonic nanocavity design platform to achieve almost an order of magnitude photoluminescence enhancement from light-emitting quantum dots. The proposed design is multi-sized/multi-spacing gold (Au) nano units that are uniformly wrapped with thin aluminum oxide (Al2O3) layer as a foreign host to form a metal-insulator-semiconductor (MIS) cavity, as we coated them with semiconductor quantum dots (QDs). Our numerical and experimental data demonstrate that, in an optimal insulator layer thickness, the simultaneous formation of broadband Fabry-Perot (FP) resonances and plasmonic hot spots leads to enhanced light absorption within the QD unit. This improvement in absorption response leads to the PL enhancement of QDs. This work demonstrates the potential and effectiveness of a host comprised of random plasmonic nanocavities in the realization of lithography-free efficient emitters. Overall, this thesis presents an alternative perspective on applying large-scale compatible metamaterials in color generation. Furthermore, the proposed designs and routes can be extended toward other functional photoelectronic designs, where high performances can be acquired in scaleable architectures.
Open Access
Disordered plasmonic nanocavity enhanced quantum dot emission
(Institute of Physics, 2023-08-31) Kosger, Ali Cahit; Ghobadi, Amir; Omam, Zahra Rahimian; Soydan, Mahmut Can; Ulusoy Ghobadi, Türkan Gamze; Özbay, Ekmel
In this paper, a large-scale compatible plasmonic nanocavity design platform is utilized to achieve a nearly order of magnitude photoluminescence (PL) enhancement. The proposed design is made of multi-sized/multi-spacing gold (Au) nanounits that are uniformly wrapped with a thin aluminum oxide (Al2O3) layer, as a foreign host to form a metal-insulator-semiconductor cavity, as they are coated with semiconductor quantum dots (QDs). Our numerical and experimental data demonstrate that, in an optimal insulator layer thickness, the simultaneous formation of broadband Fabry-Perot resonances and plasmonic hot spots leads to enhanced light absorption within the QD unit. This improvement in absorption response leads to the PL enhancement of QDs. This work demonstrates the potential and effectiveness of a random plasmonic nanocavities host in the realization of lithography-free efficient emitters. © 2023 IOP Publishing Ltd
Open Access
Formation of quantum structures on a single nanotube by modulating hydrogen adsorption
(American Physical Society, 2003) Gülseren, O.; Yildirim, T.; Çıracı, Salim
Using first-principles density functional calculations we showed that quantum structures can be generated on a single carbon nanotube by modulating the adsorption of hydrogen atoms. The band gap of the hydrogen-free zone of the tube widens in the adjacent hydrogen covered zone. The sudden variation of the band gap leads to band offsets at the conduction- and valence-band edges. At the end, the band gap of the whole system is modulated along the axis of the tube, which generates quantum wells or quantum dots. Specific electronic states are confined in these quantum wells. The type and radius of the nanotube and the extent and sequence of hydrogen-free and hydrogen-covered zones can provide several options to design a desired optoelectronic nanodevice.
Open Access
Förster resonance energy transfer enhanced color-conversion using colloidal semiconductor quantum dots for solid state lighting
(American Institute of Physics, 2009-10-15) Nizamoglu, S.; Demir, Hilmi Volkan
In this paper, we present Förster resonance energy transfer (FRET)-enhanced color-conversion using colloidal semiconductor quantum dot nanocrystals (NCs) to make reddish-orange light-emitting diodes for use in ultraefficient solid state lighting. To achieve FRET enhancement at 614 nm, we use an energy gradient hybrid structure made of cyan- and orange-emitting CdSe/ZnS NCs (λPL =492 and 588 nm in solution, respectively). This enables recycling of trapped excitons using FRET and achieves a relative quantum efficiency enhancement of 15.1% in reddish-orange full color-conversion for the integrated hybrid cyan-orange NC layer with respect to the case of full color-conversion using only orange NCs without FRET.
Open Access
MaterialVis: material visualization tool using direct volume and surface rendering techniques
(Elsevier Inc., 2014) Okuyan, E.; Güdükbay, Uğur; Bulutay, C.; Heinig, Karl-Heinz
Visualization of the materials is an indispensable part of their structural analysis. We developed a visualization tool for amorphous as well as crystalline structures, called MaterialVis. Unlike the existing tools, MaterialVis represents material structures as a volume and a surface manifold, in addition to plain atomic coordinates. Both amorphous and crystalline structures exhibit topological features as well as various defects. MaterialVis provides a wide range of functionality to visualize such topological structures and crystal defects interactively. Direct volume rendering techniques are used to visualize the volumetric features of materials, such as crystal defects, which are responsible for the distinct fingerprints of a specific sample. In addition, the tool provides surface visualization to extract hidden topological features within the material. Together with the rich set of parameters and options to control the visualization, MaterialVis allows users to visualize various aspects of materials very efficiently as generated by modern analytical techniques such as the Atom Probe Tomography.
Open Access
Nanoengineering InP quantum dot-based photoactive biointerfaces for optical control of neurons
(Frontiers Media S.A., 2021-06-23) Karatum, O.; Aria, M. M.; Eren, G. Ö.; Yıldız, E.; Melikov, R.; Srivastava, S. B.; Sürme, S.; Bakış Doğru, I.; Jalali, H. B.; Ulgut, Burak; Şahin, A.; Kavaklı, İ. H.; Nizamoğlu, S.
Light-activated biointerfaces provide a non-genetic route for effective control of neural activity. InP quantum dots (QDs) have a high potential for such biomedical applications due to their uniquely tunable electronic properties, photostability, toxic-heavy-metal-free content, heterostructuring, and solution-processing ability. However, the effect of QD nanostructure and biointerface architecture on the photoelectrical cellular interfacing remained unexplored. Here, we unravel the control of the photoelectrical response of InP QD-based biointerfaces via nanoengineering from QD to device-level. At QD level, thin ZnS shell growth (∼0.65 nm) enhances the current level of biointerfaces over an order of magnitude with respect to only InP core QDs. At device-level, band alignment engineering allows for the bidirectional photoelectrochemical current generation, which enables light-induced temporally precise and rapidly reversible action potential generation and hyperpolarization on primary hippocampal neurons. Our findings show that nanoengineering QD-based biointerfaces hold great promise for next-generation neurostimulation devices.
Open Access
Phonon-mediated electron-electron interaction in confined media: low-dimensional bipolarons
(2000-09) Senger, R. Tuğrul
We study the criterion for the formation of confined large bipolarons and their stability. In order to deal with this specific subject of polaron theory, it is required to adopt some particular approximation methods, because the polaronic systems do not admit exact analytic solutions in general. Those approximation techniques, which are applied to the low-dimensional one-polaron problems, are presented to some extent to form a working basis for our main theme, bipolarons. As the model of confined bipolaron, the electrons are treated as bounded within an external potential while being coupled to one another via the Fröhlich interaction Hamiltonian. Within the framework of the bulk-phonon approximation, the model that we use consists of a pair of electrons immersed in a reservoir of bulk LO phonons and confined within an anisotropic parabolic potential box, the barrier slopes of which can be tuned arbitrarily from zero to infinity. Thus, encompassing the bulk and all low-dimensional geometric configurations of general interest, we obtain an explicit tracking of the critical values of material parameters for the bipolarons to exist in confined media. First, in the limit of strong electron-phonon coupling, we present a unified insight into the stability criterion by applying the Landau-Pekar strong coupling approximation. This crude approximation provides us the condition on the ratio of dielectric constants (η = epsilon substcript infinity/epsilon substrcript 0) for large values of electron-phonon coupling constant α. For more reliable results, we consider the path-integral formulation of the problem adopting the Feynman-polaron model to derive variational results over a wide range of the Coulomb interaction and phonon coupling strengths. It is shown that the critical values of α and η exhibit some non-trivial features as the effective dimensionality is varied, and the path integral results conform to those of strong coupling approximation in the limit of large α.
Open Access
Polaronic effects in a gaussian quantum dot
(Elsevier, 2008) Yanar, S.; Sevim, A.; Boyacioglu, B.; Saglam, M.; Mukhopadhyaya, S.; Chatterjee, A.
The problem of an electron interacting with longitudinal-optical (LO) phonons is investigated in an N-dimensional quantum dot with symmetric Gaussian confinement in all directions using the Rayleigh-Schrödinger perturbation theory, a variant of the canonical transformation method of Lee-Low-Pines, and the sophisticated apparatus of the Feynman-Haken path-integral technique for the entire range of the coupling parameters and the results for N = 2 and N = 3 are obtained as special cases. It is shown that the polaronic effects are quite significant for small dots with deep confining potential well and the parabolic potential is only a poor approximation of the Gaussian confinement. The Feynman-Haken path-integral technique in general gives a good upper bound to the ground state energy for all values of the system parameters and therefore is used as a benchmark for comparison between different methods. It is shown that the perturbation theory yields for the ground state polaron self-energy a simple closed-form analytic expression containing only Gamma functions and in the weak-coupling regime it provides the lowest energy because of an efficient partitioning of the Gaussian potential and the subsequent use of a mean-field kind of treatment. The polarization potential, the polaron radius and the number of virtual phonons in the polaron cloud are obtained using the Lee-Low-Pines-Huybrechts method and their variations with respect to different parameters of the system are discussed.
Open Access
Resonant and coherent transport through Aharonov-Bohm interferometers with coupled quantum dots
(The American Physical Society, 2005) Moldoveanu, V.; Ţolea, M.; Aldea, A.; Tanatar, Bilal
A detailed description of the tunneling processes within Aharonov-Bohm (AB) rings containing two-dimensional quantum dots is presented. We show that the electronic propagation through the interferometer is controlled by the spectral properties of the embedded dots and by their coupling with the ring. The transmittance of the interferometer is computed by the Landauer-Büttiker formula. Numerical results are presented for an AB interferometer containing two coupled dots. The charging diagrams for a double-dot interferometer and the Aharonov-Bohm oscillations are obtained, in agreement with the recent experimental results of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)] We identify conditions in which the system shows Fano line shapes. The direction of the asymetric tail depends on the capacitive coupling and on the magnetic field. We discuss our results in connection with the experiments of Kobayashi et al. [Phys. Rev. Lett. 88, 256806 (2002)] in the case of a single dot. ©2005 The American Physical Society.
Open Access
Robust whispering-gallery-mode microbubble lasers from colloidal quantum dots
(American Chemical Society, 2017) Wang Y.; Ta, V. D.; Leck K.S.; Tan, B. H. I.; Wang, Z.; He T.; Ohl, C.-D.; Demir, Hilmi Volkan; Sun, H.
Microlasers hold great promise for the development of photonics and optoelectronics. Among the discovered optical gain materials, colloidal quantum dots (CQDs) have been recognized as the most appealing candidate due to the facile emission tunability and solution processability. However, to date, it is still challenging to develop CQD-based microlasers with low cost yet high performance. Moreover, the poor long-term stability of CQDs remains to be the most critical issue, which may block their laser aspirations. Herein, we developed a unique but generic approach to forming a novel type of a whispering-gallery-mode (WGM) microbubble laser from the hybrid CQD/poly(methyl methacrylate) (PMMA) nanocomposites. The formation mechanism of the microbubbles was unraveled by recording the drying process of the nanocomposite droplets. Interestingly, these microbubbles naturally serve as the high-quality WGM laser resonators. By simply changing the CQDs, the lasing emission can be tuned across the whole visible spectral range. Importantly, these microbubble lasers exhibit unprecedented long-term stability (over one year), sufficient for practical applications. As a proof-of-concept, the potential of water vapor sensing was demonstrated. Our results represent a significant advance in microlasers based on the advantageous CQDs and may offer new possibilities for photonics and optoelectronics.
Open Access
Synthesis of blue-shifted luminescent colloidal GaN nanocrystals through femtosecond pulsed laser ablation in organic solution
(Springer Netherlands, 2016-05) Demirel, A.; Öztaş T.; Kurşungöz, C.; Yılmaz, İ.; Ortaç, B.
We demonstrate the synthesis of GaN nanocrystals (NCs) with the sizes of less than the doubled exciton Bohr radius leading quantum confinement effects via a single-step technique. The generation of colloidal GaN nanoparticles (NPs) in organic solution through nanosecond (ns) and femtosecond (fs) pulsed laser ablation (PLA) of GaN powder was carried out. Ns PLA in ethanol and polymer matrix resulted in amorphous GaN-NPs with the size distribution of 12.4 ± 7.0 and 6.4 ± 2.3 nm, respectively, whereas fs PLA in ethanol produced colloidal GaN-NCs with spherical shape within 4.2 ± 1.9 nm particle size distribution. XRD and selected area electron diffraction analysis of the product via fs PLA revealed that GaN-NCs are in wurtzite structure. Moreover, X-ray photoelectron spectroscopy measurements also confirm the presence of GaN nanomaterials. The colloidal GaN-NCs solution exhibits strong blue shift in the absorption spectrum compared to that of the GaN-NPs via ns PLA in ethanol. Furthermore, the photoluminescence emission behavior of fs PLA-generated GaN-NCs in the 295–400 nm wavelength range is observed with a peak position located at 305 nm showing a strong blue shift with respect to the bulk GaN.
Open Access
Ten million-atom InGaAs embedded quantum dot electron g factor calculations using semi-empirical pseudopotentials
(2022-10) Kahraman, Mustafa
Quantum technologies rely on key capabilities such as electron spin control over the full-Bloch sphere, generation of indistinguishable single photons, or entangled photon pairs. For all these purposes, arguably the most established semiconductor structure currently is the self-assembled InGaAs quantum dots (QDs). In this thesis, electron ground state g tensors of embedded InGaAs QDs are calculated employing an atomistic empirical pseudopotential method. Computed QDs have varied size, shape, indium molar fraction but uniform strain. The components of the g tensor do not show appreciable deviation even though the shape is anisotropic for some of the studied QDs. Universality is observed when family of g factor curves is plotted with respect to energy gap which generalizes the findings of a recent study under more restricted conditions. Our work expands its applicability to alloy QDs with different shapes, and finite confinement putting it on a more realistic foundation by allowing penetration to the matrix material. Our regression model shows that the effect of magnetic field on the electron in an InGaAs QD will be the minimal when the so-called, s-shell optical transition energy is around 1.13 eV. Furthermore, low indium molar fraction is unfavorable in terms of g factor tunability. Our findings could be beneficial in the fabrication of g-near-zero QDs or other desired g values aimed for spintronic or electron spin resonance applications.