Browsing by Subject "Semiconductor nanocrystals"

Now showing 1 - 20 of 26

Open Access
Anisotropic stimulated emission from aligned CdSe/CdS dot-in-rods
(IEEE, 2014-10) Gao, Y.; Ta, V. D.; Zhao, X.; Wang, Y.; Chen, R.; Zhao, Y.; Dang, C.; Sun, X.; Sun, H.; Demir, Hilmi Volkan
Anisotropic optical properties of CdSe/CdS dot-in-rods loaded in a capillary tube are demonstrated, suggesting nanorods' alignment with a microfluidic approach. Polarized emissions from photoluminescence and whispering gallery mode lasing show promising applications for lighting and displays. © 2014 IEEE.
Open Access
Brightly luminescent Cu-Zn-In-S/ZnS Core/shell quantum dots in salt matrices
(De Gruyter, 2019) Lox, J. F. L.; Eichler, F.; Erdem, Talha; Adam, M.; Gaponik, N.; Demir, Hilmi Volkan; Lesnyak, V.; Eychmüller, A.
In the past decades cadmium-free quantum dots (QDs), among which are quaternary colloidal Cu-Zn-In-S/ZnS (CZIS/ZnS) core/shell nanocrystals (NCs), have attracted great scientific interest. Particularly, their low toxicity and the possibility to tune their photoluminescence (PL) properties by varying the composition in the multicomponent system make them highly attractive for applications in light-emitting diodes (LEDs). Thus, the demands for high quality CZIS/ZnS QDs and methods to process them into bulk materials stimulate investigations of these nanomaterials. Herein, we demonstrate the synthesis of CZIS/ZnS core/shell NCs via a surfactant induced nucleation process, which emit in various colors covering the range from 520 nm to 620 nm possessing high photoluminescence quantum yields (PLQYs) up to 47%. Furthermore, the as synthesized NCs were successfully integrated into two different salt matrices [Na2B4O7 (Borax) and LiCl] using two different approaches. The commonly used incorporation of the NCs into Borax salt led to salt crystals emitting from 540 nm to 600 nm with PLQYs up to 24%. By encapsulating the QDs into LiCl, brightly emitting NCs-in-LiCl powders with the PL covering a range from 520 nm to 650 nm with PLQYs of up to 14% were obtained. As a proof of concept, the fabrication of a color conversion LED using NCs encapsulated into LiCl demonstrated the applicability of the encapsulated NCs.
Open Access
Colloidal nanoplatelet/conducting polymer hybrids: excitonic and material properties
(American Chemical Society, 2016) Guzelturk, B.; Menk, F.; Philipps, K.; Kelestemur Y.; Olutas M.; Zentel, R.; Demir, Hilmi Volkan
Here we present the first account of conductive polymer/colloidal nanoplatelet hybrids. For this, we developed DEH-PPV-based polymers with two different anchor groups (sulfide and amine) acting as surfactants for CdSe nanoplatelets, which are atomically flat semiconductor nanocrystals. Hybridization of the polymers with the nanoplatelets in the solution phase was observed to cause strong photoluminescence quenching in both materials. Through steady-state photoluminescence and excitation spectrum measurements, photoluminescence quenching was shown to result from dominant exciton dissociation through charge transfer at the polymer/nanoplatelet interfaces that possess a staggered (i.e., type II) band alignment. Importantly, we found out that sulfide-based anchors enable a stronger emission quenching than amine-based ones, suggesting that the sulfide anchors exhibit more efficient binding to the nanoplatelet surfaces. Also, shorter surfactants were found to be more effective for exciton dissociation as compared to the longer ones. In addition, we show that nanoplatelets are homogeneously distributed in the hybrid films owing to the functional polymers. These nanocomposites can be used as building blocks for hybrid optoelectronic devices, such as solar cells.
Open Access
Colloidal synthesis and doping of semiconductor nanocrystals
(2015-07) Akgül, Mehmet Zafer
Colloidal semiconductor nanocrystals have drawn great interest for application areas in photonics and optoelectronics thanks to their superior optical properties including strong bandgap emission and tunability. Also, their suitability for solution-based processing has made them highly attractive for low-cost production of light-emitting diodes and lasers. Our objective in this thesis is to show the potential and versatility of semiconductor nanocrystals via colloidal synthesis and post-processing methods. The thesis work includes the synthesis of colloidal quantum dot and well structures and their post-doping and investigates their exciton decay dynamics. In this thesis a novel colloidal approach for the doping of zinc blende colloidal quantum wells was proposed and demonstrated for the first time. This new doping method uniquely relies on atomic layer deposition (ALD) process. Here we achieved the worlds first manganese-doped CdSe@CdS core@shell nanoplatelets using our technique of ALD-assisted doping. Also, we studied silver-doped CdTe quantum dots under different conditions. Our experimental work proved that the quantum yield enhancement of silver-doped CdTe quantum dots is a strong function of the nanocrystal size and doping concentration. Tuning the nanocrystal size and doping level, our aqueous core-only CdTe nanocrystals reached a record high photoluminescence quantum efficiency of 68%. For these quantum dots, various decay kinetics were proposed and the enhancement in the quantum yield was attributed to the trap state annihilation. The methods and results provided in this thesis contribute to the fundamental understanding of semiconductornanocrystals and pave the way for high-performance colloidal platforms and devices.
Open Access
Colloidal synthesis of Ag(I)-doped CdSe nanoplatelets with partial cation exchange method
(2019-01) Bozdoğan, İrfan Selim
Colloidal nanoplatelets (NPLs) exhibit strong one-dimensional quantum confinement in the vertical direction. This makes them a highly attractive host system for studying variable doping techniques and effects without variation in the quantum confinement effect. Earlier, core-only CdSe NPLs were converted into Cu2Se and HgSe NPLs, and also CdSe/CdS core/shell NPLs were transformed into Cu2Se/Cu2S, ZnSe/ZnS, and PbSe/PbS NPLs by using full cation exchange (CE) methods. Recently, core-only CdSe NPLs have been doped with Cu(I) ions using high-temperature nucleation doping and post-synthesis partial CE approaches. On the other hand, unlike Cu(I), such monovalent doping with Ag(I) ions has previously not been possible in NPLs as a host system, although silver doping had been widely studied in other host systems. Therefore, there has been no previous report on the doping of Ag(I) into CdSe NPLs to date. To address this gap, in this thesis, Ag(I) doping in CdSe NPLs by using a postsynthesis partial CE technique was developed. A systematic study was carried out to investigate the effects of dopant precursor reactivities, reaction timing, and temperature on the evolution of dopant-related emission as compared to the excitonic emission. In controlled experiments, the excitonic emission peak was eliminated and only dopant-related emission peak was successfully obtained. Finally, temperaturedependent emission kinetics of the as-synthesized Ag(I)-doped CdSe NPLs at varied temperatures ranging from 25 to 298 K were investigated. It was observed that both excitonic and dopant-related emission peaks were blue-shifted and their intensities were considerably increased with the decreasing temperature. As a new dopant-host system, these Ag(I)-doped CdSe NPLs hold a great promise for further systematic spectroscopic studies and possibly various optoelectronic applications.
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
Emerging fields of colloidal nanophotonics for quality lighting to versatile lasing
(Springer Verlag, 2018) Demir, Hilmi Volkan
Solution-processed semiconductor nanocrystals have attracted increasingly greater interest in optoelectronics including color conversion and enrichment in quality lighting and display backlighting. Optical properties of these colloidal nanocrystals can be conveniently controlled by tailoring their shape, composition, and size in an effort to realize high-performance light generation and lasing. We now witness the expanding deployment of semiconductor nanocrystals in consumer products being adapted by giant electronics companies. Based on the rational design and control of excitonic processes in these nanocrystals, it is possible to achieve highly efficient light-emitting diodes and optically pumped lasers. In this chapter, we introduce an emerging field of nanocrystal optoelectronics with applications from quality lighting to versatile lasing. We look into the performance limits of color conversion using colloidal nanocrystals. Here we introduce a new concept of all-colloidal lasers developed by incorporating nanocrystal emitters as the optical gain media intimately into fully colloidal cavities. As an extreme case of solution-processed tightly-confined quasi-2D colloids, we also show that the atomically flat nanoplatelets uniquely offer record high optical gain coefficients and ultralow threshold stimulated emission. Given the recent accelerating progress in colloidal nanophotonics, solution-processed quantum materials now hold great promise to challenge their conventional epitaxial counterparts in the near future.
Open Access
Exciton dynamics in colloidal quantum-dot LEDs under active device operations
(American Chemical Society, 2018) Shendre, S.; Sharma, V. K.; Dang C.; Demir, Hilmi Volkan
Colloidal quantum-dot light-emitting diodes (QLEDs) are lucrative options for color-pure lighting sources. To achieve high-performance QLEDs, besides developing high-efficiency quantum dots (QDs), it is essential to understand their device physics. However, little understanding of the QD emission behavior in active QLEDs is one of the main factors hindering the improvement of device efficiency. In this work, we systematically studied the exciton dynamics of gradient composition CdSe@ZnS QDs during electroluminescence in a working QLED. With time-resolved photoluminescence analyses using fluorescence lifetime imaging microscopy we analyzed a large population of QDs spatially spreading over an extended area inside and outside the device. This allows us to reveal the statistically significant changes in the behavior of QD emission in the device at different levels of applied voltages and injection currents. We find that the QD emission efficiency first drops in device fabrication with Al electrode deposition and that the QD exciton lifetime is then statistically reduced further under the QLED's working conditions. This implies the nonradiative Auger recombination process is active in charged QDs as a result of imbalanced charge injection in a working QLED. Our results help to understand the exciton behavior during the operation of a QLED and demonstrate a new approach to explore the exciton dynamics statistically with a large QD population.
Open Access
Exciton harvesting systems of nanocrystals
(2011) Mutlugün, Evren
Semiconductor nanocrystals, also known as colloidal quantum dots, have gained substantial scientific interest for innovative light harvesting applications including those in biolabeling. Organic dyes and fluorescent proteins are widely used in biotargeting and live cell imaging, but their intrinsic optical properties, such as narrow excitation windows, limit their potential for advanced applications, e.g., spectral multiplexing. Compared to these organic fluorophores, favorable properties of the quantum dots including high photoluminescence quantum yields together with tunable emission peaks and narrow spectral emission widths, high extinction coefficients, and broad absorption bands enable us to discover and innovate light harvesting composites. In such systems, however, the scientific challenge is to achieve high levels of energy transfer from one species to the other, with additional features of versatility and tunability. To address these problems, as a conceptual advancement, this thesis proposes and demonstrates a new class of versatile light harvesting systems of semiconductor nanocrystals mediated by excitonic interactions based on Förstertype nonradiative energy transfer. In this thesis, we synthesized near-unity efficiency colloidal quantum dots with as-synthesized photoluminescence quantum yields of >95%. As proof-of-concept demonstrations, we studied and achieved highly efficient exciton harvesting systems of quantum dots bound to fluorescent proteins, where the excitons are zipped from the dots to the proteins in the composite. This led to many folds of light harvesting (tunable up to 15 times) in the case of the green fluorescent protein. Using organic dye molecules electrostatically interacting with quantum dots, we showed high levels of exciton migration from the dots to the molecules (up to 94%). Furthermore, we demonstrated stand-alone, flexible membranes of nanocrystals in unprecedentedly large areas (> 50 cm × 50 cm), which paves the way for highend, large-scale applications. In the thesis, we also developed exciton-exciton coupling models to support the experimental results. This thesis opens up new possibilities for exciton-harvesting in biolabeling and optoelectronics.
Open Access
High-efficiency optical gain in type-II semiconductor nanocrystals of alloyed colloidal quantum wells
(American Chemical Society, 2017) Guzelturk, B.; Kelestemur Y.; Olutas M.; Li, Q.; Lian, T.; Demir, Hilmi Volkan
Colloidal nanocrystals having controlled size, tailored shape, and tuned composition have been explored for optical gain and lasing. Among these, nanocrystals having Type-II electronic structure have been introduced toward low-threshold gain. However, to date, their performance has remained severely limited due to diminishing oscillator strength and modest absorption cross-section. Overcoming these problems, here we realize highly efficient optical gain in Type-II nanocrystals by using alloyed colloidal quantum wells. With composition-tuned core/alloyed-crown CdSe/CdSexTe1-x quantum wells, we achieved amplified spontaneous emission thresholds as low as 26 μJ/cm2, long optical gain lifetimes (τgain ≈ 400 ps), and high modal gain coefficients (gmodal ≈ 930 cm-1). We uncover that the optical gain in these Type-II quantum wells arises from the excitations localized to the alloyed-crown region that are electronically coupled to the charge-transfer state. These alloyed heteronanostructures exhibiting remarkable optical gain performance are expected to be highly appealing for future display and lighting technologies.
Open Access
Highly stable, near-unity efficiency atomically flat semiconductor nanocrystals of CdSe/ZnS hetero-nanoplatelets enabled by ZnS-Shell hot-injection growth
(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Yemliha, Yemliha; Quliyeva, Ulviyya; Güngör, Kıvanç; Erdem, Onur; Kelestemur, Yusuf; Mutlugün, Evren; Kovalenko, M.; Demir, Hilmi Volkan
Colloidal semiconductor nanoplatelets (NPLs) offer important benefits in nanocrystal optoelectronics with their unique excitonic properties. For NPLs, colloidal atomic layer deposition (c‐ALD) provides the ability to produce their core/shell heterostructures. However, as c‐ALD takes place at room temperature, this technique allows for only limited stability and low quantum yield. Here, highly stable, near‐unity efficiency CdSe/ZnS NPLs are shown using hot‐injection (HI) shell growth performed at 573 K, enabling routinely reproducible quantum yields up to 98%. These CdSe/ZnS HI‐shell hetero‐NPLs fully recover their initial photoluminescence (PL) intensity in solution after a heating cycle from 300 to 525 K under inert gas atmosphere, and their solid films exhibit 100% recovery of their initial PL intensity after a heating cycle up to 400 K under ambient atmosphere, by far outperforming the control group of c‐ALD shell‐coated CdSe/ZnS NPLs, which can sustain only 20% of their PL. In optical gain measurements, these core/HI‐shell NPLs exhibit ultralow gain thresholds reaching ≈7 µJ cm−2. Despite being annealed at 500 K, these ZnS‐HI‐shell NPLs possess low gain thresholds as small as 25 µJ cm−2. These findings indicate that the proposed 573 K HI‐shell‐grown CdSe/ZnS NPLs hold great promise for extraordinarily high performance in nanocrystal optoelectronics.
Open Access
Layer-by-layer self-assembled semiconductor nanocrystal composites with nonradiative resonance energy transfer for innovative architectural precise color tuning and control
(2009-08) Çiçek, Neslihan
In recent years semiconductor quantum dot nanocrystals (NC) have attracted significant interest and have found numerous important optoelectronic device applications mainly because of their highly tunable optical properties. For example, precisely tuning shades of color chromaticity is critically important in solid state lighting to achieve ultra-efficient, application-specific, spectrallyengineered illumination. To date such color tuning and control of NC emitters have been investigated and demonstrated only based on their composition, shape, and size (using the quantum confinement effect). All of these parameters are, however, limited to be controlled and set during the synthesis process. As a post-synthesis alternative, we proposed and demonstrated the precise and broad control and tuning of color chromaticity by strongly modifying photoluminescence decay kinetics of NC emitters solely based on nonradiative Förster resonance energy transfer (FRET) in layer-by-layer self-assembled NC composite structures. Locating NC emitters in such a layered architecture with a targeted gradient of bandgap in the close proximity (<10 nm) of each other and spatially interspacing them at the nanoscale (with a precision of <1 nm) enabled us to fine-tune and master FRET at a desired efficiency level of nonradiative energy transfer from electronically excited donor NCs to luminescent acceptor iv NCs. These proof-of-concept experimental demonstrations, combined with our numerical modeling and simulation results, proved a highly sensitive tuning capability based on FRET to span a broad color area in Commission Internationale De L’Eclairage (CIE) chromaticity diagram in principle beyond the limits of each of the commonly used LED epitaxial material systems. This innovative architectural tuning opens up a new direction for the photometric engineering of color-conversion LEDs.
Open Access
Liquid-liquid diffusion ‐ assisted crystallization: a fast and versatile approach toward high quality mixed quantum dot ‐ salt crystals
(Wiley-VCH Verlag, 2015) Adam, M.; Wang, Z.; Dubavik, A.; Stachowski, G. M.; Meerbach, C.; Soran-Erdem, Z.; Rengers, C.; Demir, Hilmi Volkan; Gaponik N.; Eychmuller, A.
Here, a new, fast, and versatile method for the incorporation of colloidal quantum dots (QDs) into ionic matrices enabled by liquid-liquid diffusion is demonstrated. QDs bear a huge potential for numerous applications thanks to their unique chemical and physical properties. However, stability and processability are essential for their successful use in these applications. Incorporating QDs into a tight and chemically robust ionic matrix is one possible approach to increase both their stability and processability. With the proposed liquid-liquid diffusion-assisted crystallization (LLDC), substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process allows to incorporate even the less stable colloids including initially oil-based ligand-exchanged QDs into salt matrices. Furthermore, in a modified two-step approach, the seed-mediated LLDC provides the ability to incorporate oil-based QDs directly into ionic matrices without a prior phase transfer. Finally, making use of their processability, a proof-of-concept white light emitting diode with LLDC-based mixed QD-salt films as an excellent color-conversion layer is demonstrated. These findings suggest that the LLDC offers a robust, adaptable, and rapid technique for obtaining high quality QD-salts.
Open Access
Low-threshold optical gain and lasing of colloidal nanoplatelets
(IEEE, 2014-10) Keleştemur, Yusuf; Güzeltürk, Burak; Olutaş, Murat; Delikanlı, Savaş; Demir, Hilmi Volkan
Semiconductor nanocrystals, which are also known as colloidal quantum dots (CQDs), are highly attractive materials for high performance optoelectronic device applications such as lasers. With their size, shape and composition tunable electronic structure and optical properties, CQDs are highly desired for achieving full-color, temperature-insensitive, low-threshold and solution-processed lasers [1, 2]. However, due to their small size, they suffer from the nonradiative multiexciton Auger Recombination (AR), where energy of a bound electron-hole pair is transferred to a third particle of either an electron or a hole instead of radiative recombination. Therefore, CQDs having suppressed AR are strongly required for achieving high quality CQD-based lasers. To address this issue, CQDs having different size, shape and electronic structure have been synthesized and studied extensively [3-5]. Generally, suppression of AR and lower optical gain thresholds are achieved via reducing the wavefunction overlap of the electron and hole in a CQD. However, the separation of the electron and hole wavefunctions will dramatically decrease the oscillator strength and optical gain coefficient, which is highly critical for achieving high performance lasers. Therefore, colloidal materials with suppressed AR and high gain coefficients are highly welcomed. Here, we study optical gain performance of colloidal quantum wells [6] of CdSe-core and CdSe/CdS core/crown nanoplatelets (NPLs) that demonstrate remarkable optical properties with ultra-low threshold one- and two-photon optical pumping. As a result of their giant oscillator strength, superior optical gain and lasing performance are achieved from these colloidal NPLs with greatly enhanced gain coefficient [7]. © 2014 IEEE.
Open Access
Near-field energy transfer into silicon inversely proportional to distance using quasi-2D colloidal quantum well donors
(Wiley-VCH Verlag GmbH & Co. KGaA, 2021-09-12) Humayun, Muhammad Hamza; Hernandez-Martinez, Pedro Ludwig; Gheshlaghi, Negar; Erdem, Onur; Altıntaş, Yemliha; Shabani, Farzan; Demir, Hilmi Volkan
Silicon is the most prevalent material system for light-harvesting applications; however, its inherent indirect bandgap and consequent weak absorption limits its potential in optoelectronics. This paper proposes to address this limitation by combining the sensitization of silicon with extraordinarily large absorption cross sections of quasi-2D colloidal quantum well nanoplatelets (NPLs) and to demonstrate excitation transfer from these NPLs to bulk silicon. Here, the distance dependency, d, of the resulting Förster resonant energy transfer from the NPL monolayer into a silicon substrate is systematically studied by tuning the thickness of a spacer layer (of Al2O3) in between them (varied from 1 to 50 nm in thickness). A slowly varying distance dependence of d−1 with 25% efficiency at a donor–acceptor distance of 20 nm is observed. These results are corroborated with full electromagnetic solutions, which show that the inverse distance relationship emanates from the delocalized electric field intensity across both the NPL layer and the silicon because of the excitation of strong in-plane dipoles in the NPL monolayer. These findings pave the way for using colloidal NPLs as strong light-harvesting donors in combination with crystalline silicon as an acceptor medium for application in photovoltaic devices and other optoelectronic platforms.
Open Access
Near-unity efficiency energy transfer from colloidal semiconductor quantum wells of CdSe/cdS nanoplatelets to a monolayer of MoS2
(American Chemical Society, 2018) Taghipour, N.; Martinez, P. L. H.; Ozden, A.; Olutas M.; Dede, D.; Gungor K.; Erdem, O.; Perkgoz, N. K.; Demir, Hilmi Volkan
A hybrid structure of the quasi-2D colloidal semiconductor quantum wells assembled with a single layer of 2D transition metal dichalcogenides offers the possibility of highly strong dipole-to-dipole coupling, which may enable extraordinary levels of efficiency in Förster resonance energy transfer (FRET). Here, we show ultrahigh-efficiency FRET from the ensemble thin films of CdSe/CdS nanoplatelets (NPLs) to a MoS2 monolayer. From time-resolved fluorescence spectroscopy, we observed the suppression of the photoluminescence of the NPLs corresponding to the total rate of energy transfer from ∼0.4 to 268 ns-1. Using an Al2O3 separating layer between CdSe/CdS and MoS2 with thickness tuned from 5 to 1 nm, we found that FRET takes place 7- to 88-fold faster than the Auger recombination in CdSe-based NPLs. Our measurements reveal that the FRET rate scales down with d-2 for the donor of CdSe/CdS NPLs and the acceptor of the MoS2 monolayer, d being the center-to-center distance between this FRET pair. A full electromagnetic model explains the behavior of this d-2 system. This scaling arises from the delocalization of the dipole fields in the ensemble thin film of the NPLs and full distribution of the electric field across the layer of MoS2. This d-2 dependency results in an extraordinarily long Förster radius of ∼33 nm.
Open Access
Optical gain and lasing of colloidal semiconductor quantum wells intimately integrated into optical cavities
(2019-07) Sak, Mustafa
Colloidal semiconductor quantum wells, also known as nanoplatelets (NPLs), attract an increasingly greater deal of interest as a promising material platform for light-generating applications. The superior optical properties of NPLs including their ultra-large absorption cross-sections, purely homogeneous broadening, and suppressed Auger recombination make them highly attractive for solution-processable color convertors, LEDs and lasers. In this thesis, we studied the optical gain properties and performance levels of tailored heterostructures of such NPLs intimately integrated into various optical cavities. To do so, we systematically measured their amplified spontaneous emission under one- and twophoton absorption excitations. Also, with these hetero-NPLs as the gain media, we have proposed and demonstrated a new class of practical whispering gallery mode (WGM) NPL-fiber architecture with high stability and low lasing thresholds enabled by record low waveguide loss coefficients in its class. Moreover, we have developed a single-mode vertical-cavity surface-emitting laser (VCSEL) of these hetero-NPLs closely integrated into the wedge cavity of a pair of distributed Bragg reflectors, leading to a record low lasing threshold in its class. The findings obtained in these WGM NPL-laser and NPLVCSEL structures indicate that these NPLs are excellent for high-performance colloidal lasing.
Open Access
Orientation-controlled nonradiative energy transfer to colloidal nanoplatelets: engineering dipole orientation factor
(American Chemical Society, 2019) Erdem, Onur; Güngör, Kıvanç; Güzeltürk, Burak; Tanrıöver, İbrahim; Sak, Mustafa; Olutaş, Murat; Dede, Didem; Kelestemur, Yusuf; Demir, Hilmi Volkan
We proposed and showed strongly orientation-controlled Förster resonance energy transfer (FRET) to highly anisotropic CdSe nanoplatelets (NPLs). For this purpose, we developed a liquid–air interface self-assembly technique specific to depositing a complete monolayer of NPLs only in a single desired orientation, either fully stacked (edge-up) or fully nonstacked (face-down), with near-unity surface coverage and across large areas over 20 cm2. These NPL monolayers were employed as acceptors in an energy transfer working model system to pair with CdZnS/ZnS core/shell quantum dots (QDs) as donors. We found the resulting energy transfer from the QDs to be significantly accelerated (by up to 50%) to the edge-up NPL monolayer compared to the face-down one. We revealed that this acceleration of FRET is accounted for by the enhancement of the dipole–dipole interaction factor between a QD-NPL pair (increased from 1/3 to 5/6) as well as the closer packing of NPLs with stacking. Also systematically studying the distance-dependence of FRET between QDs and NPL monolayers via varying their separation (d) with a dielectric spacer, we found out that the FRET rate scales with d–4 regardless of the specific NPL orientation. Our FRET model, which is based on the original Förster theory, computes the FRET efficiencies in excellent agreement with our experimental results and explains well the enhancement of FRET to NPLs with stacking. These findings indicate that the geometrical orientation of NPLs and thereby their dipole interaction strength can be exploited as an additional degree of freedom to control and tune the energy transfer rate.
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
Self-resonant microlasers of colloidal quantum wells constructed by direct deep patterning
(American Chemical Society, 2021-06-09) Gheshlaghi, Negar; Foroutan-Barenji, Sina; Erdem, Onur; Altintas, Yemliha; Shabani, Farzan; Humayun, Muhammad Hamza; Demir, Hilmi Volkan
Here, the first account of self-resonant fully colloidal μ-lasers made from colloidal quantum well (CQW) solution is reported. A deep patterning technique is developed to fabricate well-defined high aspect-ratio on-chip CQW resonators made of grating waveguides and in-plane reflectors. The fabricated waveguide-coupled laser, enabling tight optical confinement, assures in-plane lasing. CQWs of the patterned layers are closed-packed with sharp edges and residual-free lifted-off surfaces. Additionally, the method is successfully applied to various nanoparticles including colloidal quantum dots and metal nanoparticles. It is observed that the patterning process does not affect the nanocrystals (NCs) immobilized in the attained patterns and the different physical and chemical properties of the NCs remain pristine. Thanks to the deep patterning capability of the proposed method, patterns of NCs with subwavelength lateral feature sizes and micron-scale heights can possibly be fabricated in high aspect ratios.