Browsing by Author "Dehghanpour Baruj, Hamed"

Now showing 1 - 4 of 4

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
Solution-processed light-emitting diodes of monolayers of colloidal quantum wells
(2022-06) Dehghanpour Baruj, Hamed
Semiconductor colloidal quantum wells (CQWs) make an exciting quasi-2D class of nanocrystals thanks to their unique properties emerging from their atomically-flat verticallythin geometry including highly anisotropic optical transition dipole moment (TDM), giant oscillator strength, and extraordinarily high absorption cross-section. This regular shape of the CQWs enables them to assemble on the surface of a liquid with a high degree of packing, which creates an almost fully uniform monolayer film with the desired orientation. Nearly all of the emission in a single CQW, again because of its geometry, comes from in-plane (IP) transition dipole moments (TDMs). Thus, an assembled film of such CQWs with an all-facedown orientation may show a highly anisotropic and directional emission. Using such a film in an emissive layer (EML) of an electroluminescent device, in this thesis work our intention is therefore to substantially boost photon outcoupling efficiency in a light-emitting device of oriented monolayer of CQWs. Thus far, research efforts have been conducted to investigate the properties of the deposited film of CQWs with self-assembly. However, in all of the previous studies, the property of CQW monolayer has been investigated only in a passive film; therefore, investigation of the self-assembled film in an active device is lacking. In this thesis, we developed and demonstrated an all-solution-processed colloidal quantum well light-emitting diodes (CQW-LEDs) using a single all-face-down oriented self-assembled monolayer (SAM) film of CQWs that enables a high level of IP TDMs of 92%. This film significantly enhances the outcoupling efficiency of LEDs from 22% (of standard randomlyoriented emitters) to 34% (of face-down oriented emitters). As a result of the outcoupling efficiency increased by 1.55 times, along with the enhanced charge injection and reduced reabsorption in the case of using a single SAM of CQWs, the external quantum efficiency reaches a record high level of 18.1% for the solution-processed type of CQW-LEDs, putting their efficiency performance on par with those of the hybrid organic-inorganic evaporationbased CQW-LEDs and all other best solution-processed LEDs. This EQE value is 65% higher than that in the devices fabricated by spin-coating with the same procedure and structure. Also, the SAM-CQW-LED architecture enables a high maximum brightness of 19,800 cd/m2 with a long operational lifetime of 247 h at 100 cd/m2, as well as a stable saturated deep-red emission (651 nm) with a low turn-on voltage of 1.7 eV and a high J90 of 99.58 mA/cm2. These findings indicate the effectiveness of oriented self-assembly of CQWs as electrically-driven emissive layers in improving outcoupling and external quantum efficiencies in the CQW-LEDs.
Open Access
Ultrahigh green and red optical gain cross sections from solutions of colloidal quantum well heterostructures
(American Chemical Society, 2021-03-11) Delikanli, Savaş; Erdem, Onur; Işık, Furkan; Dehghanpour Baruj, Hamed; Shabani, Farzan; Yağcı, Hüseyin Bilge; Durmuşoğlu, E. G.; Demir, Hilmi Volkan
We demonstrate amplified spontaneous emission (ASE) in solution with ultralow thresholds of 30 μJ/cm2 in red and of 44 μJ/cm2 in green from engineered colloidal quantum well (CQW) heterostructures. For this purpose, CdSe/CdS core/crown CQWs, designed to hit the green region, and CdSe/CdS@CdxZn1–xS core/crown@gradient-alloyed shell CQWs, further tuned to reach the red region by shell alloying, were employed to achieve high-performance ASE in the visible range. The net modal gain of these CQWs reaches 530 cm–1 for the green and 201 cm–1 for the red, 2–3 orders of magnitude larger than those of colloidal quantum dots (QDs) in solution. To explain the root cause for ultrahigh gain coefficient in solution, we show for the first time that the gain cross sections of these CQWs is ≥3.3 × 10–14 cm2 in the green and ≥1.3 × 10–14 cm2 in the red, which are two orders of magnitude larger compared to those of CQDs.
Open Access
Vertically oriented self-assembly of colloidal CdSe/CdZnS quantum wells controlled via hydrophilicity/lipophilicity balance: optical gain of quantum well stacks for amplified spontaneous emission and random lasing
(Royal Society of Chemistry, 2023-05-28) Dikmen, Zeynep; Işık, Ahmet Tarık; Bozkaya, İklim; Dehghanpour Baruj, Hamed; Canımkurbey, Betül; Shabani, Farzan; Ahmad, Muhammad; Demir, Hilmi Volkan
We propose and demonstrate vertically oriented self-assembly of colloidal quantum wells (CQWs) that allows for stacking CdSe/CdZnS core/shell CQWs in films for the purposes of amplified spontaneous emission (ASE) and random lasing. Here, a monolayer of such CQW stacks is obtained via liquid–air interface self-assembly (LAISA) in a binary subphase by controlling the hydrophilicity/lipophilicity balance (HLB), a critical factor for maintaining the orientation of CQWs during their self-assembly. Ethylene glycol, as a hydrophilic subphase, orients the coalition of these CQWs into self-assembled multi-layers in the vertical direction. Stacking CQWs into large micron-sized areas as a monolayer is facilitated by adjusting HLB with diethylene glycol addition as a more lyophilic subphase during LAISA. ASE was observed from the resulting multi-layered CQW stacks prepared via sequential deposition onto the substrate by applying the Langmuir–Schaefer transfer method. Random lasing was achieved from a single self-assembled monolayer of the vertically oriented CQWs. Here, highly rough surfaces resulting from the non-close packing nature of the CQW stack films cause strongly thickness-dependent behavior. We observed that in general a higher roughness-to-thickness ratio of the CQW stack films (e.g., thinner films that are intrinsically rough enough) leads to random lasing, while it is possible to observe ASE only in thick enough films even if their roughness is relatively higher. These findings indicate that the proposed bottom-up technique can be used to construct thickness-tunable, three-dimensional CQW superstructures for fast, low-cost, and large-area fabrication.