Browsing by Author "Ahmad, Muhammad"
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Item Open Access Colloidal doping of thick nanoplatelets(2022-12) Ahmad, MuhammadSemiconductor nanoplatelets (NPLs) make an interesting group of nanocrystals with unique optical properties as a result of their quasi 2-dimensional (2D) electronic structure. Such emerging fascinating optical features of NPLs include high absorption cross-section, narrow emission linewidths, and reduced Auger recombination, making them a superior choice compared to conventional semiconductor nanocrystals for optoelectronic applications. Doping of these materials with transition metals, such as silver and copper, provides great opportunities to modify and tune the electronic structure of these NPLs for various devices including light-emitting diodes and luminescent solar concentrators. Such doping with transition metals allows for manipulation of the photoluminescence from these NPLs, control of the recombination processes of the photogenerated carriers in these NPLs, and observation of the giant Zeeman effect as a result of exchange interactions between the dopants and carriers in these NPLs. Previously, CdSe NPLs have been doped with copper and silver only up to vertical thickness of 5 monolayers (ML). However, doping of thicker NPLs has not been possible to date. In this thesis work, we successfully doped thick CdSe NPLs having 7 ML in thickness with silver and copper using partial cation exchange to obtain large Stokes-shifted emission in the near-infrared (NIR) region. Here, the effect of precursor ratio and reaction temperature were systematically studied to tune the resulting emission. For both copper and silver dopants, we successfully quenched fully the band-edge emission, and purely dopantinduced emission was obtained. We also co-doped these NPLs with silver and copper, and we successfully obtained both copper- and silver-induced emissions from these NPLs. We further grew the CdZnS shell on 7 ML CdSe core by hot injection method and doped the resulting CdSe/CdZnS core/shell NPLs with silver and copper to push their emission further towards longer wavelengths in the NIR region. These thick doped-NPLs with large Stokes shift and emission in the NIR region present a promising platform for light-emitting and -harvesting applications.Item Open Access Thermodynamic silver doping of core/shell colloidal quantum wells imparted with paramagnetic properties emitting at near-infrared(American Chemical Society, 2023-05-29) Shabani, Farzan; Ahmad, Muhammad; Kumar, Satish; Delikanlı, Savaş; Işık, Furkan; Bhattacharya, A.; Petrou, A.; Demir, Hilmi VolkanTwo-dimensional (2D) core/shell nanoplatelets (NPLs) synthesized via the hot-injection method provide excellent thermal and chemical stability for high-temperature doping, where an expanded and flexible lattice is required. Here, a thermodynamic approach toward silver doping of these NPLs is proposed and demonstrated, which previously proved to be challenging due to the fast self-purification of the dopants with the introduction of the shell. Maintaining the doping procedure in the reversible regime ensured the integrity of the NPLs and allowed a high level of doping; however, the equilibrium condition is further complicated by environmental factors that affect the chemical activity of the cations and the surface composition of the NPLs. Two main deterioration mechanisms in the irreversible regime were observed: ZnS-shelled NPLs suffered preferential etching, while CdS-shelled NPLs underwent cleavage and fragmentation. Alloying of the shell minimized both mechanisms for CdZnS-shelled NPLs and preserved the metastable state of the NPLs, including their 2D shape and crystalline structure. Distribution of silver ions in the lattice of the NPLs directly affected the recombination dynamics and enabled fine-tuning of the near-infrared emission beside the exciton confinement. These silver-doped CdZnS-shelled NPLs are shown further to exhibit enhanced paramagnetic properties with Zeeman splitting and Brillouin-like bound-exciton polarization as a function of the magnetic field, critical for spintronic applicationsItem 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 VolkanWe 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.