Browsing by Subject "Nanocrystals"

Now showing 1 - 20 of 95

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
Bio-nanohybrids of quantum dots and photoproteins facilitating strong nonradiative energy transfer
(Royal Society of Chemistry, 2013-05-21) Seker U.O.S.; Mutlugun, E.; Hernandez-Martinez, R. L.; Sharma, V. K.; Lesnyak, V.; Gaponik N.; Eychmuller, A.; Demir, Hilmi Volkan
Utilization of light is crucial for the life cycle of many organisms. Also, many organisms can create light by utilizing chemical energy emerged from biochemical reactions. Being the most important structural units of the organisms, proteins play a vital role in the formation of light in the form of bioluminescence. Such photoproteins have been isolated and identified for a long time; the exact mechanism of their bioluminescence is well established. Here we show a biomimetic approach to build a photoprotein based excitonic nanoassembly model system using colloidal quantum dots (QDs) for a new bioluminescent couple to be utilized in biotechnological and photonic applications. We concentrated on the formation mechanism of nanohybrids using a kinetic and thermodynamic approach. Finally we propose a biosensing scheme with an ON/OFF switch using the QD-GFP hybrid. The QD-GFP hybrid system promises strong exciton-exciton coupling between the protein and the quantum dot at a high efficiency level, possessing enhanced capabilities of light harvesting, which may bring new technological opportunities to mimic biophotonic events.
Open Access
CdSe/ZnS core-shell nanocrystal based scintillators for enhanced detection in UV
(IEEE, 2006) Demir, Hilmi Volkan; Soğancı, İbrahim Murat; Mutlugün, Evren
In this work, we present a novel scintillator that incorporates CdSe/ZnS core-shell nanocrystals to utilize both their high quantum fluorescence efficiency in the visible and high optical absorption in the UV for the purpose of facilitating UV imaging on a Si platform. Here we demonstrate enhancement in UV detection up to 800% with respect to the host polymer in the UV.
Open Access
Charge retention in quantized energy levels of nanocrystals
(Elsevier B.V., 2007) Dâna, A.; Akça, I.; Ergun, O.; Aydınlı, Atilla; Turan, R.; Finstad, T. G.
Understanding charging mechanisms and charge retention dynamics of nanocrystal (NC) memory devices is important in optimization of device design. Capacitance spectroscopy on PECVD grown germanium NCs embedded in a silicon oxide matrix was performed. Dynamic measurements of discharge dynamics are carried out. Charge decay is modelled by assuming storage of carriers in the ground states of NCs and that the decay is dominated by direct tunnelling. Discharge rates are calculated using the theoretical model for different NC sizes and densities and are compared with experimental data. Experimental results agree well with the proposed model and suggest that charge is indeed stored in the quantized energy levels of the NCs.
Open Access
Colloidal nanocrystals embedded in macrocrystals: methods and applications
(American Chemical Society, 2016) Adam, M.; Gaponik N.; Eychmüller A.; Erdem, T.; Soran-Erdem, Z.; Demir, Hilmi Volkan
Colloidal semiconductor nanocrystals have gained substantial interest as spectrally tunable and bright fluorophores for color conversion and enrichment solids. However, they suffer from limitations in processing their solutions as well as efficiency degradation in solid films. As a remedy, embedding them into crystalline host matrixes has stepped forward for superior photostability, thermal stability, and chemical durability while simultaneously sustaining high quantum yields. Here, we review three basic methods for loading the macrocrystals with nanocrystals, namely relatively slow direct embedding, as well as accelerated methods of vacuum-assisted and liquid-liquid diffusion-assisted crystallization. We discuss photophysical properties of the resulting composites and present their application in light-emitting diodes as well as their utilization for plasmonics and excitonics. Finally, we present a future outlook for the science and technology of these materials.
Open Access
Colloidal Nanocrystals Embedded in Macrocrystals: Robustness, Photostability, and Color Purity
(American Chemical Society, 2012-09-14) Otto, T.; Mueller, M.; Mundra, P.; Lesnyak, V.; Demir, Hilmi Volkan; Gaponik N.; Eychmuller, A.
The incorporation of colloidal quantum dots (QDs) into ionic crystals of various salts (NaCl, KCl, KBr, etc.) is demonstrated. The resulting mixed crystals of various shapes and beautiful colors preserve the strong luminescence of the incorporated QDs. Moreover, the ionic salts appear to be very tight matrices, ensuring the protection of the QDs from the environment and as a result providing them with extraordinary high photo- and chemical stability. A prototype of a white light-emitting diode (WLED) with a color conversion layer consisting of this kind of mixed crystals is demonstrated. These materials may also find applications in nonlinear optics and as luminescence standards.
Open Access
Colloidal optoelectronics of self-assembled quantum well superstructures
(2020-06) Erdem, Onur
Advances in the colloidal nanocrystal synthesis enabled creation of quasi twodimensional colloidal quantum wells (CQWs) in the last decade. These CQWs possess similar properties to those of epitaxially grown quantum wells while at the same time oﬀering the beneﬁts of low-cost synthesis and solubility in various solvents. Their atomically precise thickness and one-dimensional quantum conﬁnement grant them favorable properties such as narrow emission linewidth, reduced inhomogeneous broadening and giant oscillator strength. In addition, due to their quasi-two dimensional shape, they display intrinsic anisotropy. Because of this anisotropy, the particle interactions in closely packed ﬁlms depend greatly on the orientation of these CQWs. To fully utilize the interaction of CQWs with each other or with other particles in their proximity, we develop a selfassembly technique, which is used to deposit highly uniform thin CQW ﬁlms onto various solid substrates. This self-assembly technique allows us to deposit CQWs as a continuous monolayer while at the same time controlling their orientation throughout the substrate, thereby modifying their packing factor as well as nearﬁeld dipole-dipole interactions. This self-assembly technique is also employed to create large-area CQW ﬁlms of any desired thickness, simply by applying the same deposition technique on the same substrate as many times as desired. We use these self-assembled CQW ﬁlms to study the two main aspects of nanocrystal optoelectronics, namely, Förster resonance energy transfer (FRET) and optical gain, with CQWs. By using the orientation-controlled CQW monolayers, we show that the rate of FRET from colloidal quantum dots (QDs) to a monolayer of CQWs can be tuned via dipole-dipole interactions between QDs and CQWs. We use Förster’s theory of nonradiative energy transfer while taking into account the anisotropy of the excitonic CQW excitonic state as well as its delocalization throughout the CQW to account for our results. Next, we show that our multilayered CQW ﬁlms display optical gain in uncharacteriscally low thicknesses (as small as 40 nm) due to the tight packing and extremely uniform deposition of the CQWs. We furthermore study systematically the observed threshold of ampliﬁed spontaneous emission (ASE) in these CQW multilayers as a function of the ﬁlm thickness (i.e., the number of monolayers), and demonstrate that the gain threshold drops with increasing thickness, accompanied by the red-shift of the ASE peak. These trends can be explained by the varying degree of optical mode conﬁnement, which is a function of both the ﬁlm thickness as well as the wavelength of propagating mode. Our self-assembly technique allows to study and make use of the favorable properties of the CQWs including anisotropy and enhanced optical gain. Since this technique enables us to produce large-area ﬁlms displaying excellent homogeneity, it can be a benchmark building block for creating device-scale 2- or 3-dimensional superstructures from CQWs as well as from other types of colloidal nanocrystals to be utilized in both in- and out-of-plane optical applications.
Open Access
Colloidal semiconductor quantum well supraparticles as low-threshold and photostable microlasers
(Wiley-VCH Verlag GmbH & Co. KGaA, 2025-01-08) Alves, P. U.; Quinn, G.; Strain, M. J.; Durmuşoğlu, E. G.; Sharma, M.; Demir, Hilmi Volkan; Edwards, P. R.; Martin, R. W.; Dawson, M. D.; Laurand, N.
This study introduces and compares the lasing performance of micron-sized and sphere-shaped supraparticle (SP) lasers fabricated through bottom-upassembly of II-VI semiconductor colloidal quantum wells (CQWs) with their counterparts made of quantum dots (CQDs). CQWs consist of a 4-monolayers thick CdSe core and an 8-monolayers thick $Cd_xZn_{1-x}S$ shell with a nominal size of 14 × 15 × 4.2 nm, and CQDs of $CdS_xSe_{1-x}/ZnS$with 6 nm diameter. SPs areoptically characterized with a 0.76 ns pulse laser (spot size: 2.88 × 10$^{−7}$ cm$^2$ )at 532 nm, and emit in the 620–670 nm spectral range. Results show thatCQW SPs have lasing thresholds twice as low (0.1–0.3 nJ) as CQD SPs(0.3–0.6 nJ), and stress tests using a constant 0.6 nJ optical pump energy demonstrate that CQW SPs withstand lasing emission for longer than CQDSPs. Lasing emission in CQW and CQD SPs under continuous operation yields half-lives of $𝝉_{CQW SP}$ ≈150 min and $𝝉_{CQD SP}$ ≈22 min, respectively. The half-life of CQW SPs is further extended to $𝝉_{QW}$ ≈385 min when optically pumped at 0.5 nJ. Such results compare favorably to those in the literature and highlight the performance of CdSe-based CQW SPs for laser applications.
Open Access
Comparison of electron and hole charge-discharge dynamics in germanium nanocrystal flash memories
(AIP Publishing, 2008-02) Akça, İmran B.; Dâna, Aykutlu; Aydınlı, Atilla; Turan, R.
Electron and hole charge and discharge dynamics are studied on plasma enhanced chemical vapor deposition grown metal-oxide-silicon germanium nanocrystal flash memory devices. Electron and hole charge and discharge currents are observed to differ significantly and depend on annealing conditions chosen for the formation of nanocrystals. At low annealing temperatures, holes are seen to charge slower but to escape faster than electrons. They discharge slower than electrons when annealing temperatures are raised. The results suggest that discharge currents are dominated by the interface layer acting as a quantum well for holes and by direct tunneling for elec-trons.
Open Access
Computational modeling of quantum-confined impact ionization in Si nanocrystals embedded in SiO2
(2007) Sevik, C.; Bulutay, C.
Injected carriers from the contacts to delocalized bulk states of the oxide matrix via Fowler-Nordheim tunneling can give rise to quantum-confined impact ionization (QCII) of the nanocrystal (NC) valence electrons. This process is responsible for the creation of confined excitons in NCs, which is a key luminescence mechanism. For a realistic modeling of QCII in Si NCs, a number of tools are combined: ensemble Monte Carlo (EMC) charge transport, ab initio modeling for oxide matrix, pseudopotential NC electronic states together with the closed-form analytical expression for the Coulomb matrix element of the QCII. To characterize the transport properties of the embedding amorphous SiO2, ab initio band structure and density of states of the α-quartz phase of SiO2 are employed. The confined states of the Si NC are obtained by solving the atomistic pseudopotential Hamiltonian. With these ingredients, realistic modeling of the QCII process involving a SiO2 bulk state hot carrier and the NC valence electrons is provided.
Open Access
Computational study of excitons and biexcitons in semiconductor core/shell nanocrystals of type I and type II
(2013) Yerli, Ozan
In this thesis, we studied electronic structure and optical properties of Type-I, Type-II, and quasi Type-II semiconductor nanocrystals (also known as colloidal quantum dots). For a parametric study, we developed quantum mechanical models and solved them using both analytical and numerical techniques. The simulation results were compared to the experimental findings. We showed that charge carrier localization at di↵erent spatial locations could be tuned by controlling size parameters of the core and shell. While tuning charge localization, we also predicted photoluminescence peaks of these core/shell nanocrystals using our theoretical and numerical calculations. We demonstrated that Type-II nanocrystals exhibit di↵erent tuning trends compared to the Type-I ones. We also investigated biexcitonic properties of nanocrystals using quantum mechanical simulations, which are important especially in lasing applications. We showed that two-photon absorption mechanism can be tuned by changing the core and shell size in quantum dots. We calculated at which core and shell sizes biexcitons in quantum dots show attractive or repulsive interaction. The computational studies presented in this thesis played an important role in the experimental demonstrations and understanding of controlling excitonic features of core/shell nanocrystals.
Open Access
Computational study of power conversion and luminous efficiency performance for semiconductor quantum dot nanophosphors on light-emitting diodes
(Optical Society of America, 2012-01-30) Erdem, T.; Nizamoglu, S.; Demir, Hilmi Volkan
We present power conversion efficiency (PCE) and luminous efficiency (LE) performance levels of high photometric quality white LEDs integrated with quantum dots (QDs) achieving an averaged color rendering index of >= 90 (with R9 at least 70), a luminous efficacy of optical radiation of >= 380 lm/W-opt a correlated color temperature of <= 4000 K, and a chromaticity difference dC <0.0054. We computationally find that the device LE levels of 100, 150, and 200 lm/W-elect can be achieved with QD quantum efficiency of 43%, 61%, and 80% in film, respectively, using state-of-the-art blue LED chips (81.3% PCE). Furthermore, our computational analyses suggest that QD-LEDs can be both photometrically and electrically more efficient than phosphor based LEDs when state-of-the-art QDs are used. (C) 2012 Optical Society of America
Open Access
Critical role of CdSe nanoplatelets in color-converting CdSe/ZnS nanocrystals for InGaN/GaN light-emitting diodes
(OSA - The Optical Society, 2016) Hasanov N.; Sharma, V. K.; Martinez, P. L. H.; Tan S.T.; Demir, Hilmi Volkan
Here we report CdSe nanoplatelets that are incorporated into color-converting CdSe/ZnS nanocrystals for InGaN/GaN light-emitting diodes. The critical role of CdSe nanoplatelets as an exciton donor for the color conversion was experimentally investigated. The power conversion efficiency of the hybrid light-emitting diode was found to increase by 23% with the incorporation of the CdSe nanoplatelets. The performance enhancement is ascribed to efficient exciton transfer from the donor CdSe nanoplatelet quantum wells to the acceptor CdSe/ZnS nanocrystal quantum dots through Fï¿½rster-type nonradiative resonance energy transfer.
Open Access
Dc-switchable and single-nanocrystal-addressable coherent population transfer
(2010) Gunceler, D.; Bulutay, C.
Achieving coherent population transfer in the solid-state is challenging compared to atomic systems due to closely spaced electronic states and fast decoherence. Here, within an atomistic pseudopotential theory, we offer recipes for the stimulated Raman adiabatic passage in embedded silicon and germanium nanocrystals. The transfer efficiency spectra display characteristic Fano resonances. By exploiting the Stark effect, we predict that transfer can be switched off with a dc voltage. As the population transfer is highly sensitive to structural variations, with a choice of a sufficiently small two-photon detuning bandwidth, it can be harnessed for addressing individual nanocrystals within an ensemble. © 2010 American Institute of Physics.
Open Access
Effects of rapid thermal annealing on the structural and local atomic properties of ZnO: Ge nanocomposite thin films
(A I P Publishing LLC, 2015) Ceylan, A.; Rumaiz, A. K.; Caliskan, D.; Ozcan, S.; Özbay, Ekmel; Woicik, J. C.
We have investigated the structural and local atomic properties of Ge nanocrystals (Ge-ncs) embedded ZnO (ZnO: Ge) thin films. The films were deposited by sequential sputtering of ZnO and Ge thin film layers on z-cut quartz substrates followed by an ex-situ rapid thermal annealing (RTA) at 600 °C for 30, 60, and 90 s under forming gas atmosphere. Effects of RTA time on the evolution of Ge-ncs were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), hard x-ray photoelectron spectroscopy (HAXPES), and extended x-ray absorption fine structure (EXAFS). XRD patterns have clearly shown that fcc diamond phase Ge-ncs of sizes ranging between 18 and 27 nm are formed upon RTA and no Ge-oxide peak has been detected. However, cross-section SEM images have clearly revealed that after RTA process, Ge layers form varying size nanoclusters composed of Ge-ncs regions. EXAFS performed at the Ge K-edge to probe the local atomic structure of the Ge-ncs has revealed that as prepared ZnO:Ge possesses Ge-oxide but subsequent RTA leads to crystalline Ge structure without the oxide layer. In order to study the occupied electronic structure, HAXPES has been utilized. The peak separation between the Zn 2p and Ge 3d shows no significant change due to RTA. This implies little change in the valence band offset due to RTA. © 2015 AIP Publishing LLC.
Open Access
Electroluminescence efficiency enhancement in quantum dot light-emitting diodes by embedding a silver nanoisland layer
(Wiley-VCH Verlag, 2015) Yang, X.; Hernandez-Martinez, P. L.; Dang C.; Mutlugün, E.; Zhang, K.; Demir, Hilmi Volkan; Sun X. W.
A colloidal quantum dot light-emitting diode (QLED) is reported with substantially enhanced electroluminescence by embedding a thin layer of Ag nanoislands into hole transport layer. The maximum external quantum efficiency (EQE) of 7.1% achieved in the present work is the highest efficiency value reported for green-emitting QLEDs with a similar structure, which corresponds to 46% enhancement compared with the reference device. The relevant mechanisms enabling the EQE enhancement are associated with the near-field enhancement via an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Ag nano-islands, which are found to lead to good agreement between the simulation results and the experimental data, providing us with a useful insight important for plasmonic QLEDs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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
Elements of nanocrystal high-field carrier transport modeling
(Wiley, 2007) Sevik, Cem; Bulutay, Ceyhun
Embedded semiconductor nanocrystals (NCs) within wide bandgap oxide materials are being considered for light emission and solar cell applications. One of the fundamental issues is the high-field transport in NCs. This requires the combination of a number of tools: ensemble Monte Carlo carrier transport simulation, ab initio band structure of the bulk oxide, Fermi's golden rule modeling of impact ionization and Auger processes and the pseudopotential-based atomistic description of the confined NC states. These elements are outlined in this brief report.
Open Access
Enhanced memory effect via quantum confinement in 16 nm InN nanoparticles embedded in ZnO charge trapping layer
(AIP Publishing, 2014) El-Atab, N.; Cimen, F.; Alkis, S.; Ortac, B.; Alevli, M.; Dietz, N.; Okyay, Ali Kemal; Nayfeh, A.
In this work, the fabrication of charge trapping memory cells with laser-synthesized indium-nitride nanoparticles (InN-NPs) embedded in ZnO charge trapping layer is demonstrated. Atomic layer deposited Al2O3 layers are used as tunnel and blocking oxides. The gate contacts are sputtered using a shadow mask which eliminates the need for any lithography steps. High frequency C-Vgate measurements show that a memory effect is observed, due to the charging of the InN-NPs. With a low operating voltage of 4 V, the memory shows a noticeable threshold voltage (Vt) shift of 2 V, which indicates that InN-NPs act as charge trapping centers. Without InN-NPs, the observed memory hysteresis is negligible. At higher programming voltages of 10 V, a memory window of 5 V is achieved and the Vt shift direction indicates that electrons tunnel from channel to charge storage layer. © 2014 AIP Publishing LLC
Open Access
Enhanced spontaneous emission in semiconductor nanocrystal solids using resonant energy transfer for integrated devices
(IEEE, 2008-11) Nizamoğlu, Sedat; Demir, Hilmi Volkan
Size-tuneable optical properties of semiconductor nanocrystal (NC) quantum dots make them attractive for a wide range of device applications. However, in these device applications, nanocrystals typically suffer from relatively low quantum efficiency (QE) when they are cast into solid form. To reduce the effect of this problem, we propose and demonstrate the enhancement of spontaneous emission in nanocrystal solids by recycling their trapped excitons through resonant nonradiative Forster energy transfer (ET) for hybrid integrated devices. For this purpose, we designed closely packed CdSe/ZnS core/shell nanocrystal emitters with an energy gradient of approximately 160 meV integrated on LEDs.