Browsing by Subject "nanocrystals"
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Item Open Access Environmental friendly InP/ZnS nanocrystals(Bilkent University, 2012) Coşkun, YaseminSemiconductor nanocrystals are nanometer scale fluorescent crystallites with tunable optical properties, which can be controlled by the material composition and particle size. They can be prepared using various synthesis techniques and find applications in many different areas ranging from life sciences to electronics. In this thesis, indium phosphide based nanocrystals are studied for LED applications. The thesis research work focuses on the colloidal synthesis method and material characterization of these nanocrystals. Using one pot synthesis method, the indium phosphide/zinc sulfide (InP/ZnS) core/shell nanocrystal structures are synthesized. This synthesis technique allows for a reproducible and tunable preparation method. The material characterization techniques used in this thesis include UV-Vis spectroscopy, photoluminescence spectroscopy, transmission electron microscopy (TEM), X-ray diffraction, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-mass spectrometry (ICP-MS), and Zeta Sizer (or DLS). These properties make InP/ZnS nanocrystals comparable to their cadmium (Cd) containing counterparts with respect to their optical properties, and InP/ZnS nanocrystals offer the potential to replace them because of environmental concerns in LED applications.Item Open Access Localized plasmon-coupled semiconductor nanocrystal emitters for innovative device applications(Bilkent University, 2007) Soğancı, İbrahim MuratQuantum confinement allows for the development of novel luminescent materials such as colloidal semiconductor quantum dots for a variety of photonic applications spanning from biomedical labeling to white light generation. However, such device applications require efficient photoluminescence. To this end, in this thesis we investigate the spontaneous emission characteristics of semiconductor nanocrystal emitters under different conditions and their enhancement and controlled modification via plasmonic resonance coupling, placing metallic nanoparticles in their proximity, for innovative device applications. We first present our theoretical and experimental work on the optical characterization of nanocyrstals (e.g., CdSe, CdS, and CdSe/ZnS) including absorption/photoluminescence, time-resolved luminescence, and excitation spectra measurements. Here we demonstrate very strong electromodulation (up to 90%) of photoluminescence and absorption of such nanocrystals (nanodots and nanorods) for optical modulator applications. Second, we present our electromagnetic modeling on the optical response of metal nanoparticles using finite-difference-time-domain method. For the first time, using localized plasmons of metal nanoisland films (nano-silver) carefully spectrally and spatially tuned for optimal coupling conditions, we report very significant controlled modifications of nanocrystal emission including the peak emission wavelength shift (by 14nm), emission linewidth reduction (by 10nm with 22% FWHM reduction), photoluminescence intensity enhancement (15.1- and 21.6-fold compared to the control groups of the same nanocrystals with no plasmonic coupling and those with identical nano-silver but no dielectric spacer in the case of non-radiative energy transfer, respectively), and selectable peaking of surface-state emission at desired wavelengths. Such localized plasmonic engineering of nanocrystal emitters opens new possibilities for our lightemitting and photovoltaic devices.Item Open Access A near-infrared range photodetector based on indium nitride nanocrystals obtained through laser ablation(IEEE, 2014) Tekcan, B.; Alkis, S.; Alevli, M.; Dietz, N.; Ortac, B.; Bıyıklı, Necmi; Okyay, Ali KemalWe present a proof-of-concept photodetector that is sensitive in the near-infrared (NIR) range based on InN nanocrystals. Indium nitride nanocrystals (InN-NCs) are obtained through laser ablation of a high pressure chemical vapor deposition grown indium nitride thin film and are used as optically active absorption region. InN-NCs are sandwiched between thin insulating films to reduce the electrical leakage current. Under-1 V applied bias, the recorded photoresponsivity values within 600-1100-nm wavelength range are as high as (3.05 × 10-2) mA/W. An ultrathin layer of nanocrystalline InN thin film is, therefore, a promising candidate for NIR detection in large area schemes. © 2014 IEEE.Item Open Access Novel light-sensitive nanocrystal skins(Bilkent University, 2013) Akhavan, ShahabLight sensing devices traditionally made from crystalline or amorphous silicon, operating at the visible and near-infrared wavelengths, have led to a multibillion-dollar annual market. However, silicon faces various limitations including weak detection at long wavelengths (insufficient beyond 1.1 µm) with a cut-off at short wavelengths (in the ultraviolet) and small-area applications. On the other hand, solution-processed semiconductor nanocrystals (NCs), also known as colloidal quantum dots, offer large-area light sensing platforms with strong absorption cross-section. In this thesis we propose and demonstrate a new class of large-area, semi-transparent, light-sensitive nanocrystal skin (LS-NS) devices intended for large-surface applications including smart transparent windows and light-sensitive glass facades of smart buildings. These LS-NS platforms, which are fabricated over areas up to many tens of cm2 using spraycoating and several cm-squares using dip-coating, are operated on the basis of photogenerated potential buildup, as opposed to conventional charge collection. The close interaction of the monolayer NCs of the LS-NS with the top interfacing metal contact results in highly sensitive photodetection in the absence of external bias, while the bottom side is isolated using a high dielectric spacing layer. In operation, electron-hole pairs created in the NCs of the LS-NS are disassociated and separated at the NC monolayer - metal interface due to the difference in the workfunctions. As a result, the proposed LS-NS platforms perform as highly sensitive photosensors, despite using a single NC monolayer, which makes the device semi-transparent and reduces the noise generation Furthermore, because of the band gap tunability, it is possible to construct cascaded NC layers with a designed band gap gradient where the NC diameters monotonically change. Here we present the first account of exciton funneling in an active device, which leads to significant performance improvement in the device. We show highly photosensitive NC skins employing the exciton funneling across the multiple layers of NC film. To further enhance the device photosensitivity performance, we demonstrate embedding plasmonic nanoparticles into the light-sensitive skins of the NCs. In addition, we exhibit the LS-NS device sensitivity enhancement utilizing the device architecture of semi-transparent tandem skins, the addition of TiO2 layer for increased charge carrier dissociation, and the phenomenon of multiexciton generation in infrared NCs. With fully sealed NC monolayers, LS-NS is found to be highly stable under ambient conditions, promising for low-cost large-area UV/visible sensing in windows and facades of smart buildings. We believe the findings presented in this thesis have significant implications for the future design of photosensing platforms and for moving toward next generation large-surface light-sensing platforms.Item Open Access Novel nanocrystal-integrated LEDs utilizing radiative and nonradiative energy transfer for high-quality efficient light generation(Bilkent University, 2011) Nizamoğlu, SedatTo combat environmental issues escalating with the increasing carbon footprint, combined with the energy problem of limited resources, innovating fundamentally new ways of raising energy efficiency and level of energy utilization is essential to our energy future. Today, to this end, achieving lighting efficiency is an important key because artificial lighting consumes about 19% of total energy generation around the globe. There is a large room for improving lighting efficacy for potential carbon emission cut. However, the scientific challenge is to reach simultaneously high-quality photometric performance. To address these problems, we proposed, developed and demonstrated a new class of color-conversion light emitting diodes (LEDs) integrated with nanophosphors of colloidal quantum dots. The favorable properties of these semiconductor nanocrystal quantum dots, including size-tuneable and narrow-band emission with high photostability, have provided us with the ability of achieving highquality, efficient lighting. Via using custom-design combinations of such nanocrystal emitters, we have shown that targeted white luminescence spectra can be generated with desired high photometric performance, which is important for obtaining application-specific white LEDs, e.g., for indoors lighting, street lighting, and LED-TV backlighting. Furthermore, dipole-dipole coupling capability of these semiconductor nanocrystals has allowed us to realize novel device designs based on Förster-type nonradiative energy transfer. By mastering exciton-exciton interactions in color-conversion LEDs, we have demonstrated enhanced color conversion via recycling of trapped excitons and white light generation based on nonradiative pumping of nanocrystal quantum dots for color conversion. This research work has led to successful demonstrators of semiconductor nanocrystal quantum dots that photometrically outperform conventional rareearth phosphor powders in terms of color rendering, luminous efficacy of optical radiation, color temperature and scotopic/photopic ratio for the first time.Item Open Access Targeted self-assembly of nanocrystal quantum dot emitters using smart peptide linkers on light emitting diodes(Bilkent University, 2008) Zengin, GülisSemiconductor nanocrystal quantum dots find several applications in nanotechnology. Particularly in device applications, such quantum dots are typically required to be assembled with specific distribution in space for enhanced functionality and placed at desired spatial locations on the device which commonly has several diverse material components. In conventional approaches, self-assembly of nanocrystals typically takes place nonspecifically without surface recognition of materials and cannot meet these requirements. To remedy these issues, we proposed and demonstrated uniform, controlled, and targeted self-assembly of quantum dot emitters on multi-material devices by using cross-specificity of genetically engineered peptides as smart linkers and achieved directed immobilization of these quantum dot emitters decorated with peptides only on the targeted specific regions of our color-conversion LEDs. Our peptide decorated quantum dots exhibited 270 times stronger photoluminescence intensity compared to their negative control groups.