Browsing by Author "Humayun, Muhammad Hamza"

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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
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
Well-controlled modification of emission kinetics of colloidal semiconductor quantum wells
(2021-09) Humayun, Muhammad Hamza
Colloidal quantum wells (CQWs) belong to an important quasi-2-dimensional sub-family of semiconductor nanocrystals. Thanks to their uniquely tight quantum conﬁnement of only few monolayers extending across their vertical thickness, CQWs possess giant oscillator strength, substantially increasing their absorption cross-section along with their large lateral size expanded over tens to hundreds of nm’s on one side. These together make CQWs excellent candidates for light-harvesting applications. In this thesis, to utilize CQWs’ superior light-harvesting capability, we investigated the alteration and control of photoluminescence de-cay lifetimes of the CQWs in a variety of hybrid absorbing systems. In particular, we proposed and demonstrated the nonradiative energy transfer from strongly-absorbing CQWs to indirect-bandgap bulk semiconductors as weak ab-sorbers, e.g., bulk silicon. To this end, we systematically studied and showed the well-controlled modiﬁcation of the emission kinetics of these CQWs that are self-assembled into a single-layer all-face-down oriented ensemble in the vicinity of silicon with a ﬁne-tuned dielectric separator (of thickness d). We found the Förster resonance energy transfer (FRET) to be the chief underlying mechanism for the observed modiﬁcations in the emission kinetics of the CQWs, which we further modeled and explained using full electromagnetic solutions. We showed that the rate of the resultant energy transfer from these CQWs to the bulk silicon scales slowly with d−1 in space. Finite element method (FEM) based computation revealed that this inverse relationship is caused by the delocalization of the electric ﬁeld in the CQW layer and the substrate due to strong in-plane dipoles present in the CQWs. To address the shortcomings of silicon-based photo-detecting platforms, in a proof-of-concept hybrid device we fabricated, we experimentally demonstrated that the photosensitization using such a single-layer CQW-ﬁlm enhances the photocurrent collected in the silicon by up to 3-folds. The ﬁndings in this thesis are expected to help further exploit the amazing light-harvesting potential of CQWs in optoelectronic applications.