Browsing by Author "Gheshlaghi, Negar"

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Open Access
Deep-red-emitting colloidal quantum well light-emitting diodes enabled through a complex design of core/crown/double shell heterostructure
(Wiley, 2022-02-24) Shabani, Farzan; Dehghanpour Baruj, Hamed; Yurdakul, Iklim; Delikanlı, Savaş; Gheshlaghi, Negar; Işık, Furkan; Liu, B.; Altıntaş, Yemliha; Canımkurbey, Betül; Demir, Hilmi Volkan
Extending the emission peak wavelength of quasi-2D colloidal quantum wells has been an important quest to fully exploit the potential of these materials, which has not been possible due to the complications arising from the partial dissolution and recrystallization during growth to date. Here, the synthetic pathway of (CdSe/CdS)@(1-4 CdS/CdZnS) (core/crown)@(colloidal atomic layer deposition shell/hot injection shell) hetero-nanoplatelets (NPLs) using multiple techniques, which together enable highly efficient emission beyond 700 nm in the deep-red region, is proposed and demonstrated. Given the challenges of using conventional hot injection procedure, a method that allows to obtain sufficiently thick and passivated NPLs as the seeds is developed. Consequently, through the final hot injection shell coating, thick NPLs with superior optical properties including a high photoluminescence quantum yield of 88% are achieved. These NPLs emitting at 701 nm exhibit a full-width-at-half-maximum of 26 nm, enabled by the successfully maintained quasi-2D shape and minimum defects of the resulting heterostructure. The deep-red light-emitting diode (LED) device fabricated with these NPLs has shown to yield a high external quantum efficiency of 6.8% at 701 nm, which is on par with other types of LEDs in this spectral range. © 2021 Wiley-VCH GmbH
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
Optical gain in ultrathin self‐assembled bi‐layers of colloidal quantum wells enabled by the mode confinement in their high‐index dielectric waveguides
(Wiley-VCH Verlag, 2020) Foroutan-Barenji, Sina; Erdem, Onur; Gheshlaghi, Negar; Altıntaş, Yemliha; Demir, Hilmi Volkan
This study demonstrates an ultra‐thin colloidal gain medium consisting of bi‐layers of colloidal quantum wells (CQWs) with a total film thickness of 14 nm integrated with high‐index dielectrics. To achieve optical gain from such an ultra‐thin nanocrystal film, hybrid waveguide structures partly composed of self‐assembled layers of CQWs and partly high‐index dielectric material are developed and shown: in asymmetric waveguide architecture employing one thin film of dielectric underneath CQWs and in the case of quasi‐symmetric waveguide with a pair of dielectric films sandwiching CQWs. Numerical modeling indicates that the modal confinement factor of ultra‐thin CQW films is enhanced in the presence of the adjacent dielectric layers significantly. The active slabs of these CQW monolayers in the proposed waveguide structure are constructed with great care to obtain near‐unity surface coverage, which increases the density of active particles, and to reduce the surface roughness to sub‐nm scale, which decreases the scattering losses. The excitation and propagation of amplified spontaneous emission (ASE) along these active waveguides are experimentally demonstrated and numerically analyzed. The findings of this work offer possibilities for the realization of ultra‐thin electrically driven colloidal laser devices, providing critical advantages including single‐mode lasing and high electrical conduction.
Open Access
Optical microfluidic waveguides and solution lasers of colloidal semiconductor quantum wells
(Wiley-VCH Verlag GmbH & Co. KGaA, 2021-01-25) Maskoun, Joudi; Gheshlaghi, Negar; Işık, Furkan; Savaş, Delikanlı; Onur, Erdem; Emine Yegan, Erdem; Hilmi Volkan, Demir
The realization of high-quality lasers in microfluidic devices is crucial for numerous applications, including biological and chemical sensors and flow cytometry, and the development of advanced lab-on-chip (LOC) devices. Herein, an ultralow-threshold microfluidic single-mode laser is proposed and demonstrated using an on-chip cavity. CdSe/CdS@CdxZn1−xS core/crown@gradient-alloyed shell colloidal semiconductor quantum wells (CQWs) dispersed in toluene are employed in the cavity created inside a poly(dimethylsiloxane) (PDMS) microfluidic device using SiO2-protected Ag mirrors to achieve in-solution lasing. Lasing from such a microfluidic device having CQWs solution as a microfluidic gain medium is shown for the first time with a record-low optical gain threshold of 17.1 µJ cm−² and lasing threshold of 68.4 µJ cm−² among all solution-based lasing demonstrations. In addition, air-stable SiO2 protected Ag films are used and designed to form highly tunable and reflective mirrors required to attain a high-quality Fabry–Pérot cavity. These realized record-low thresholds emanate from the high-quality on-chip cavity together with the core/crown@gradient-alloyed shell CQWs having giant gain cross-section and slow Auger rates. This microfabricated CQW laser provides a compact and inexpensive coherent light source for microfluidics and integrated optics covering the visible spectral region.
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.
Open Access
Single-mode lasing from a single 7 nm thick monolayer of colloidal quantum wells in a monolithic microcavity
(Wiley-VCH Verlag, 2021-03-03) Foroutan-Barenji, Sina; Erdem, Onur; Delikanlı, Savaş; Yağcı, Hüseyin Bilge; Gheshlaghi, Negar; Altıntaş, Yemliha; Demir, Hilmi Volkan
In this work, monolithically-fabricated vertical cavity surface emitting lasers (VCSELs) of densely-packed, orientation-controlled, atomically flat colloidal quantum wells (CQWs) using a self-assembly method is reported and single-mode lasing from a record thin colloidal gain medium with a film thickness of 7 nm under femtosecond optical excitation is demonstrated. Specially engineered CQWs are used to demonstrate these hybrid CQW-VCSELs consisting of only a few layers to a single monolayer of CQWs and are achieved the lasing from these thin gain media by thoroughly modeling and implementing a vertical cavity consisting of distributed Bragg reflectors with an additional dielectric layer for mode tuning. Accurate spectral and spatial alignment of the cavity mode with the CQW films is secured with the help of full electromagnetic computations. While overcoming the long-pending problem of limited electrical conductivity in thicker colloidal films, such ultrathin colloidal gain media can be helpful to enable fully electrically-driven colloidal lasers.
Open Access
Spectrally wide-range-tunable, efficient, and bright colloidal light-emitting diodes of quasi-2D nanoplatelets enabled by engineered alloyed heterostructures
(American Chemical Society, 2020) Altıntaş, Yemliha; Liu, B.; Hernández-Martínez, P. L.; Gheshlaghi, Negar; Shabani, Farzan; Sharma, Manoj; Wang, L.; Sun, H.; Mutlugün, Evren; Demir, Hilmi Volkan
Recently, there has been tremendous interest in the synthesis and optoelectronic applications of quasi-two-dimensional colloidal nanoplatelets (NPLs). Thanks to the ultranarrow emission linewidth, high-extinction coefficient, and high photostability, NPLs offer an exciting opportunity for high-performance optoelectronics. However, until now, the applications of these NPLs are limited to available discrete emission ranges, limiting the full potential of these exotic materials as efficient light emitters. Here, we introduce a detailed systematic study on the synthesis of NPLs based on the alloying mechanisms in core/shell, core/alloyed shell, alloyed core/shell, and alloyed core/alloyed shell heterostructures. Through the engineering of the band gap supported by the theoretical calculations, we carefully designed and successfully synthesized the NPL emitters with continuously tunable emission. Unlike conventional NPLs showing discrete emission, here, we present highly efficient core/shell NPLs with fine spectral tunability from green to deep-red spectra. As an important demonstration of these efficient emitters, the first-time implementation of yellow NPL light-emitting diodes (LEDs) has been reported with record device performance, including the current efficiency surpassing 18.2 cd A–1, power efficiency reaching 14.8 lm W–1, and record luminance exceeding 46 900 cd m–2. This fine and wide-range color tunability in the visible range from stable and efficient core/shell NPLs is expected to be extremely important for the optoelectronic applications of the family of colloidal NPL emitters.
Open Access
Thickness-tunable self-assembled colloidal nanoplatelet films enable ultrathin optical gain media
(American Chemical Society, 2020) Erdem, Onur; Foroutan, Sina; Gheshlaghi, Negar; Güzeltürk, B.; Altıntaş, Yemliha; Demir, Hilmi Volkan
We propose and demonstrate construction of highly uniform, multilayered superstructures of CdSe/CdZnS core/shell colloidal nanoplatelets (NPLs) using liquid interface self-assembly. These NPLs are sequentially deposited onto a solid substrate into slabs having monolayer-precise thickness across tens of cm2 areas. Because of near-unity surface coverage and excellent uniformity, amplified spontaneous emission (ASE) is observed from an uncharacteristically thin film having 6 NPL layers, corresponding to a mere 42 nm thickness. Furthermore, systematic studies on optical gain of these NPL superstructures having thicknesses ranging from 6 to 15 layers revealed the gradual reduction in gain threshold with increasing number of layers, along with a continuous spectral shift of the ASE peak (∼18 nm). These observations can be explained by the change in the optical mode confinement factor with the NPL waveguide thickness and propagation wavelength. This bottom-up construction technique for thickness-tunable, three-dimensional NPL superstructures can be used for large-area device fabrication.