Browsing by Author "Baruj, Hamed Dehghanpour"

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Open Access
Blue-emitting CdSe nanoplatelets enabled by sulfur-alloyed heterostructures for light-emitting diodes with low turn-on voltage
(American Chemical Society, 2021-12-28) İzmir, M.; Sharma, A.; Shendre, S.; Durmuşoğlu, E. G.; Sharma, V. K.; Shabani, Farzan; Baruj, Hamed Dehghanpour; Delikanlı, Savaş; Sharma, M.; Demir, Hilmi Volkan
Colloidal nanoplatelets (NPLs) have emerged as the last class of semiconductor nanocrystals for their potential optoelectronic applications. The heterostructures of these nanocrystals can achieve high photoluminescence quantum yield and enhanced photostability, along with color purity. Such advantages make them a promising candidate for solution-processable light-emitting diodes (LEDs). However, to date, blue-emitting CdSe nanoplatelets (NPLs) exhibit poor photoluminescence quantum yield and also typically suffer from a rolled-up morphology. To mitigate these problems in this work, we propose and demonstrate efficient alloyed 4 ML CdSe1–xSx nanoplatelets having a CdS crown with enhanced photoluminescence quantum yields (up to 60%) in the blue region (462–487 nm). We successfully used these NPLs as an electrically driven active emitter in the blue-emitting NPL-LEDs with a low turn-on voltage of ∼4 V. The Commission Internationale de L’Eclairage (CIE) coordinates of (0.23, 0.14) were obtained for these blue-emitting NPL-LEDs. These emitters could potentially open up the opportunity for full-color displays using these NPL-based blue LEDs in conjunction with the red and green ones.
Open Access
Gradient Type-II CdSe/CdSeTe/CdTe Core/Crown/Crown heteronanoplatelets with asymmetric shape and disproportional excitonic properties
(Wiley-VCH Verlag GmbH & Co. KGaA, 2023-01-17) Shabani, Farzan; Hernandez Martinez, P. L.; Shermet, Nina; Korkut, Hilal; Sarpkaya, İbrahim; Baruj, Hamed Dehghanpour; Delikanlı, Savaş; Işık, Furkan; Durmuşoğlu, E. G.; Demir, Hilmi Volkan
Characterized by their strong 1D confinement and long-lifetime red-shifted emission spectra, colloidal nanoplatelets (NPLs) with type-II electronic structure provide an exciting ground to design complex heterostructures with remarkable properties. This work demonstrates the synthesis and optical characterization of CdSe/CdSeTe/CdTe core/crown/crown NPLs having a step-wise gradient electronic structure and disproportional wavefunction distribution, in which the excitonic properties of the electron and hole can be finely tuned through adjusting the geometry of the intermediate crown. The first crown with staggered configuration gives rise to a series of direct and indirect transition channels that activation/deactivation of each channel is possible through wavefunction engineering. Moreover, these NPLs allow for switching between active channels with temperature, where lattice contraction directly affects the electron–hole (e–h) overlap. Dominated by the indirect transition channels over direct transitions, the lifetime of the NPLs starts to increase at 9 K, indicative of low dark-bright exciton splitting energy. The charge transfer states from the two type-II interfaces promote a large number of indirect transitions, which effectively increase the absorption of low-energy photons critical for nonlinear properties. As a result, these NPLs demonstrate exceptionally high two-photon absorption cross-sections with the highest value of 12.9 × 10$^{6}$ GM and superlinear behavior.
Open Access
Highly directional highly efficient solution processed light emitting diodes of all face down
(Wiley-VCH Verlag GmbH & Co. KGaA, 2023-04-06) Baruj, Hamed Dehghanpour; Bozkaya, İklim; Canımkurbey, Betül; Işık, Ahmet Tarık; Shabani, Farzan; Delikanlı, Savaş; Shendre, S.; Erdem, Onur; Işık, Furkan; Demir, Hilmi Volkan
Semiconductor colloidal quantum wells (CQWs) provide anisotropic emission behavior originating from their anisotropic optical transition dipole moments (TDMs). Here, solution-processed colloidal quantum well light-emitting diodes (CQW-LEDs) of a single all-face-down oriented self-assembled monolayer (SAM) film of CQWs that collectively enable a supreme level of IP TDMs at 92% in the ensemble emission are shown. This significantly enhances the outcoupling efficiency from 22% (of standard randomly-oriented emitters) to 34% (of face-down oriented emitters) in the LED. As a result, 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 the hybrid organic-inorganic evaporation-based CQW-LEDs and all other best solution-processed LEDs. This SAM-CQW-LED architecture allows for a high maximum brightness of 19,800 cd m$^{−2}$ with a long operational lifetime of 247 h at 100 cd m$^{−2}$ as well as a stable saturated deep-red emission (651 nm) with a low turn-on voltage of 1.7 eV at a current density of 1 mA cm$^{−2}$ and a high J$_{90}$ of 99.58 mA cm$^{−2}$. These findings indicate the effectiveness of oriented self-assembly of CQWs as an electrically-driven emissive layer in improving outcoupling and external quantum efficiencies in the CQW-LEDs.
Open Access
Modulating emission properties in a host–guest colloidal quantum well superlattice
(Wiley-VCH Verlag GmbH & Co. KGaA, 2021-12-19) Yu, J.; Sharma, Manoj; Wang, Y.; Delikanlı, S.; Baruj, Hamed Dehghanpour; Sharma, A.; Demir, Hilmi Volkan
Self-assembly of colloidal nanocrystals into ordered superlattices is a powerful approach to enable novel collective properties which are not available in individual colloids. However, to date, it remains a major challenge to develop a practical route to modulate such collective properties for potential photonic applications. Herein, it is shown that the collective emission properties in colloidal quantum well (CQW) superlattices, including emission color and anisotropy, can be effectively modulated in a binary host–guest architecture. The experimental and theoretical results reveal that excitons of the host (i.e., the undoped CQWs) generated by photoexcitation can be controllably harvested by the guest (i.e., the Cu-doped CQWs) for light emission, owing to an exciton hopping assisted exciton trapping process. Such a nano-building block with tunable collective optical properties may enlighten novel colloidal material-based photonic applications, including optical anti-counterfeiting, next-generation liquid crystal displays, and multifunctional biological markers.
Open Access
Ultralow threshold optical gain enabled by quantum rings of inverted type-I CdS/CdSe core/crown nanoplatelets in the blue
(Wiley-VCH Verlag GmbH & Co. KGaA, 2021-04-20) Delikanlı, Savaş; Işık, Furkan; Shabani, Farzan; Baruj, Hamed Dehghanpour; Taghipour, Nima; Demir, Hilmi Volkan
Achieving low-threshold optical gain for solution-processed materials is crucial for their real-life applications and deployment as gain media. However, the realization of low gain threshold in the blue region has shown to be technically an extremely challenging task using colloidal nanocrystals as a result of fast nonradiative Auger rates in smaller nanocrystals. Here, ultralow-threshold blue amplified spontaneous emission (ASE) (≈2.7 µJ cm−2) accompanied with a large net modal gain coefficient of 360 cm−1 in the blue enabled by blue-emitting (≈455–465 nm) colloidal quantum rings (QRs) of inverted type-I CdS/CdSe core/crown nanoplatelets (NPLs) is proposed and demonstrated. The synthesized QRs with controlled crown size outperform the best reported ASE thresholds and net modal gain coefficients from the solution-processed materials by ≈2.5- and ≈4-fold, respectively, in the similar blue spectral window. Utilizing this QR architecture, it is also shown that the ASE peak can be spectrally tuned by controlling the lateral size of the crown and hence quantum confinement in the lateral direction. These outstanding results support the prospects of these solution-processed QRs made of 2D hetero-NPLs in the challenging blue region as colloidal gain.