Papers by Keyword: Quantum Dot

Abstract: Semiconductor nanostructures are referred to semiconductor heterostructures confined in one, two, or all three dimensions, which are known as quantum wells, quantum wires, and quantum dots (QDs), respectively. QDs are semiconductor nanocrystals with significant potential for high-performance photonic and electronic devices based on III–V semiconductor alloys. To fabricate these structures, several methods have been developed, including chemical synthesis of colloidal QDs, Stranski–Krastanov (S–K) growth technology, and droplet epitaxy (DE). DE is an epitaxial technique primarily employed for fabrication of nanostructures based on III–V semiconductors for quantum information technology applications. This work presents a DE technology of growth of InGaP nanocrystals on GaP surface. The technology includes the electrochemical deposition of group III metals on the III–V semiconductor surface, followed by annealing in inert gas atmosphere. The photoelectric and photonic properties of the resulting nanomaterials are analyzed. Based on the experimental results and literature data, the growth mechanism of InGaP nanocrystals on the GaP surface is described, and a phenomenological model for the formation of InGaP/GaP nanostructures is proposed.

Abstract: In this work, an intermediate band solar cell (IBSC) model consisting of MAPbI₃ quantum dots (QD) and MAPbCl₃ barrier material is explored analytically with MATLAB. Titanium di-oxide (TiO₂) is used as transport layer for electron and Spiro-OMeTAD (2,2',7,7'-tet-rakis (N,N'-di-p-methoxyphenylamine)–9,9' spirobifluorene) is used as transport layer for hole. Fluorine-doped tin oxide (FTO) and Silver (Ag) is used as top and bottom contact. The impact of QD size and dot spacing on the key parameters of MAPbI₃ QD-IBSC is illustrated throughout this paper. In order to identify the number of IB in a single regime, Schrödinger equation is solved as a function of host energy gap using Kronig–Penney model. The detailed balance limit assumptions with unity fill factor are applied to extract highest efficiency from the system. For any case, face centered cubic (FCC) crystal structure is assumed. The (100) crystal orientation is considered as charge carriers from n–region to p–region transport in this orientation. Major performance indicators of the device such as photocurrent intensity J_sc, open circuit voltage V_oc and power conversion efficiency η have been delineated. Highest efficiency of 63% is attained for dot size of 4 nm and dot spacing of 1.5 nm.

Abstract: A simple aqueous based synthesis technique at room temperature was performed for preparation manganese-doped zinc sulfide quantum dots. Under 4 eV excitation quantum dots show photoluminescence bands at 2.11 and 3.1 eV corresponded to Mn²⁺ and intrinsic ZnS emission respectively. ZnS quantum dots were used as the luminescent sensing element for methane detection in aqueous media. The luminescent sensor response occurs due to photoinduced electron transfer from QDs to methane molecule resulting in QD luminescence quenching.

Abstract: Fluorescent zinc blende structured pure and Mn²⁺ doped ZnS quantum dots were prepared by simple aqueous based technique at room temperature. Under UV-excitation the quantum dots show photoluminescence bands at 2.1 and 3.0 eV corresponded to Mn²⁺ and ZnS intrinsic defect emission, respectively. The photocatalytic activity was tested for the photodegradation of methylene blue in aqueous solution. The influence of the Mn²⁺ concentration on the dye decolorization efficiency was studied. The highest photocatalytic degradation rate of methylene blue was obtained for ZnS quantum dots in glutathione shell doped with 0.5 at.% of Mn²⁺. The mechanisms of photoluminescence and photocatalytic activity were discussed.

Abstract: New approaches to enhance properties of silicon based quantum dot heterostructures for optical device application were developed. That is strain driven heteroepitaxy, small-sized quantum dots, elemental compositions of the heterointerface, virtual substrate, plasmonic effects, and the quantum dot charging occupation with holes in epitaxially grown Ge quantum dots (QDs) on Si (100). Experiments have shown extraordinary optical properties of Ge/Si QDs heterostructures and mid-infrared quantum dot photodetectors performance.

Abstract: This research reports a theoretical investigation on the role of filtered optical feedback (FOF) in the quantum dot light emitting diode (QD-LED). The underlying dynamics is affected by a sidle node, which returns to an elliptical shape when the wetting layer (WL) is neglected. Both filter width and time delay change the appearance of different dynamics (chaotic and mixed mode oscillations ,MMOs). The results agrees with the experimental observations. Here, the fixed point analysis for QDs was done for the first time. For QD-LED with FOF, the system transits from the coherence collapse (CC) case in conventional optical feedback (COF) to a coherent case with a filtered mode in FOF. It was found that the WL washes out the modes which is an unexpected result. This may attributed to the longer capture time of WL compared with that between QD states. Thus, WL reduces the chaotic behavior.

Abstract: Quantum dots of CdSe, CdS and ZnS QDs were prepared by chemical reaction and used to fabricate organic quantum dot hybrid junction device. QD-LEDs were fabricated using ITO/TPD: PMMA/CdSe/Al, ITO/TPD: PMMA/CdS/Al and ITO/TPD: PMMA/ZnS/Al QDs devices which synthesized by phase segregation method. The hybrid white light emitting devices consists, of two-layers deposited successively on the ITO glass substrate; the first layer was of N, N’-bis (3-methylphenyl)-N, N’-bis (phenyl) benzidine (TPD) polymer mixed with polymethyl methacrylate (PMMA) polymers in ratio 1:1, while the second layer was 0.5wt% from each type of the (CdSe, CdS and ZnS) QDs for each device.The optical properties of QDs were characterized by UV-Vis. and photoluminescence (PL) spectrometer. The results show that the prepared QDs were nanocrystalline with defects formation. The E_g calculated from PL were 2.38, 2.69 and 3.64 eV for CdSe, CdS and ZnS respectively. The generated white light properties with acceptable efficiency using confinement effect that makes the energy gap larger, thus the direction of the light sites are toward the centre of white light color.The hybrid junction devices (EL devices) were characterized by room temperature PL and electroluminescence (EL). Current-voltage (I–V) characteristics indicate that the output current is good compared to the few voltages ( 8-11.5 V) used which gives acceptable results to get a generation of white light. The EL spectrum reveals a broad emission band covering the range from 350 - 700 nm. The emissions causing this white luminescence were identified depending on the chromaticity coordinates (CIE 1931). The correlated color temperature (CCT) was found to be about 5500, 4885 and 3400K respectively. Fabrication of EL-devices from semiconductors material (CdSe, CdS and ZnS QDs) with hole injection organic polymer (TPD) was effective in white light generation. The recombination processes and I-V characteristics gives rises to the output current is good compared to the few voltages used which gives acceptable results to get a generation of white light.

Abstract: Cadmium based quantum dots (Cd QDs) is well established and extensively used for various applications, but found limiting usage in bioimaging application, due to its high toxicity and insolubility in aqueous solution. Passivation and/or encapsulation of the Cd QDs with a non-toxic and biocompatible polymer are common practice to overcome the drawback. In our work, CdSe is synthesized as a core and encapsulated with ZnS to produce a hydrophobic colloidal core/shell CdSe/ZnS QD. Then, a biocompatible PEGylated amphiphilic polymer as matrices is used to encapsulate CdSe/ZnS QD for converting it into water dispersible property which required for in-vitro imaging applications. The cell viability and cellular uptake of the biocomposite were studied against cancerous and non-cancerous cells. Also, peak of emission spectrum was recorded to determine the photostability of this biocomposites under continuous UV light illumination up to 100 minutes.

Abstract: The nonlinear absorption and nonlinear refraction properties of water soluble ZnS semiconductor quantum dots are studied by using Z-scan technique with the action of 400 nm and 532 nm femtosecond laser pulses. The experimental results show that the three order nonlinear effect of ZnS semiconductor quantum dot is stronger in 400 nm laser pulse, and it is weaker in 532 nm laser pulse. The three order nonlinear polarization coefficient of ZnS semiconductor quantum dot is 4.58×10^-13 esu with the action of 400 nm laser pulse, it is 9.22×10^-14 esu with the action of 532nm laser pulse. The ZnS semiconductor quantum dot has a relatively good three order optical nonlinear effect with the action of 400nm wavelength laser pulse.

Abstract: α-Cobratoxin is the main neurotoxin in the cobra Naja kaouthia venom; it binds efficiently and selectively with neuronal α7 and muscle type nicotinic acetylcholine receptor and can be used for specific labeling and visualization of these receptors in organs and tissues. For these applications we have prepared conjugates of α-cobratoxin with CdSe quantum dots which have many benefits as compared to organic fluorescent labels. To prepare the conjugate, CdSe quantum dots with ZnS shell were functionalized using a tripeptide glutathione and coupled to toxin using water soluble carbodiimide. The conjugate was purified by gel-filtration chromatography and tested for biological activity. It was found that conjugate preserved the capacity to interact with both neuronal α7 and muscle type nicotinic acetylcholine receptor. Its cytotoxicity to mammalian cells was not higher than that of functionalized quantum dots.