Browsing by Author "Keleştemur, Y."

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Open Access
Continuously tunable emission in inverted type ‐ I CdS/CdSe core/crown semiconductor nanoplatelets
(Wiley, 2015-07-15) Delikanlı, S.; Güzeltürk, B.; Hernandez - Martinez, P. L.; Erdem, T.; Keleştemur, Y.; Olutas M.; Akgül, M. Z.; Demir, Hilmi Volkan
The synthesis and unique tunable optical properties of core/crown nanoplatelets having an inverted Type-I heterostructure are presented. Here, colloidal 2D CdS/CdSe heteronanoplatelets are grown with thickness of four monolayers using seed-mediated method. In this work, it is shown that the emission peak of the resulting CdS/CdSe heteronanoplatelets can be continuously spectrally tuned between the peak emission wavelengths of the core only CdS nanoplatelets (421 nm) and CdSe nanoplatelets (515 nm) having the same vertical thickness. In these inverted Type-I nanoplatelets, the unique continuous tunable emission is enabled by adjusting the lateral width of the CdSe crown, having a narrower bandgap, around the core CdS nanoplatelet, having a wider bandgap, as a result of the controlled lateral quantum confinement in the crown region additional to the pure vertical confinement. As a proof-of-concept demonstration, a white light generation is shown by using color conversion with these CdS/CdSe heteronanoplatelets having finely tuned thin crowns, resulting in a color rendering index of 80. The robust control of the electronic structure in such inverted Type-I heteronanoplatelets achieved by tailoring the lateral extent of the crown coating around the core template presents a new enabling pathway for bandgap engineering in solution-processed quantum wells.
Open Access
Experimental determination of the asbsorption cross-section and molar extinction coefficient of colloidal CdSe nanoplatelets
(American Chemical Society, 2015) Yeltik A.; Delikanlı, S.; Olutas M.; Keleştemur, Y.; Güzeltürk, B.; Demir, Hilmi Volkan
There has been a strong interest in solution-processed two-dimensional nanomaterials because of their great potential in optoelectronics. Here, the absorption cross-section and molar extinction coefficient of four and five monolayer thick colloidal CdSe nanoplatelets (NPLs) having various lateral sizes are reported. The absorption cross-section of these NPLs and their corresponding molar extinction coefficients are found to strongly depend on the lateral area. An excellent agreement is observed between the experimental results and the calculated values based on the small-particle light absorption model. With these optical properties, NPLs hold great promise for optoelectronic applications. © 2015 American Chemical Society.
Open Access
Lateral size-dependent spontaneous and stimulated emission properties in colloidal CdSe nanoplatelets
(American Chemical Society, 2015) Olutaş, M.; Güzeltürk, B.; Keleştemur, Y.; Yeltik A.; Delikanlı, S.; Demir, Hilmi Volkan
Here, we systematically investigated the spontaneous and stimulated emission performances of solution-processed atomically flat quasi-2D nanoplatelets (NPLs) as a function of their lateral size using colloidal CdSe core NPLs. We found that the photoluminescence quantum efficiency of these NPLs decreases with increasing lateral size while their photoluminescence decay rate accelerates. This strongly suggests that nonradiative channels prevail in the NPL ensembles having extended lateral size, which is well-explained by the increasing number of the defected NPL subpopulation. In the case of stimulated emission the role of lateral size in NPLs influentially emerges both in the single- and two-photon absorption (1PA and 2PA) pumping. In the amplified spontaneous emission measurements, we uncovered that the stimulated emission thresholds of 1PA and 2PA exhibit completely opposite behavior with increasing lateral size. The NPLs with larger lateral sizes exhibited higher stimulated emission thresholds under 1PA pumping due to the dominating defected subpopulation in larger NPLs. On the other hand, surprisingly, larger NPLs remarkably revealed lower 2PA-pumped amplified spontaneous emission thresholds. This is attributed to the observation of a "giant" 2PA cross-section overwhelmingly growing with increasing lateral size and reaching record levels higher than 10(6) GM, at least an order of magnitude stronger than colloidal quantum dots and rods. These findings suggest that the lateral size control in the NPLs, which is commonly neglected, is essential to high-performance colloidal NPL optoelectronic devices in addition to the vertical monolayer control.
Open Access
Nonradiative energy transfer in colloidal CdSe nanoplatelet films
(Royal Society of Chemistry, 2015) Güzeltürk, B.; Olutas M.; Delikanlı, S.; Keleştemur, Y.; Erdem, O.; Demir, Hilmi Volkan
Nonradiative energy transfer (NRET) has been extensively studied in colloidal nanocrystal (quantum dots) and nanorod (quantum wires) assemblies. In this work, we present the first account of spectroscopic evidence of NRET in solid thin films of CdSe based colloidal nanoplatelets (NPLs), also known as colloidal quantum wells. The NRET was investigated as a function of the concentration of two NPL populations with different vertical thicknesses via steady state and time resolved spectroscopy. NRET takes place from the NPLs with smaller vertical thickness (i.e., larger band gap) to the ones with a larger vertical thickness (i.e., smaller band gap) with efficiency up to ∼60%. Here, we reveal that the NRET efficiency is limited in these NPL solid film assemblies due to the self-stacking of NPLs within their own population causing an increased distance between the donor-acceptor pairs, which is significantly different to previously studied colloidal quantum dot based architectures for nonradiative energy transfer. © The Royal Society of Chemistry 2015.
Open Access
Stable and efficient colour enrichment powders of nonpolar nanocrystals in LiCl
(Royal Society of Chemistry, 2015) Erdem, T.; Soran-Erdem Z.; Sharma, V. K.; Keleştemur, Y.; Adam, M.; Gaponik N.; Demir, Hilmi Volkan
In this work, we propose and develop the inorganic salt encapsulation of semiconductor nanocrystal (NC) dispersion in a nonpolar phase to make a highly stable and highly efficient colour converting powder for colour enrichment in light-emitting diode backlighting. Here the wrapping of the as-synthesized green-emitting CdSe/CdZnSeS/ZnS nanocrystals into a salt matrix without ligand exchange is uniquely enabled by using a LiCl ionic host dissolved in tetrahydrofuran (THF), which simultaneously disperses these nonpolar nanocrystals. We studied the emission stability of the solid films prepared using NCs with and without LiCl encapsulation on blue LEDs driven at high current levels. The encapsulated NC powder in epoxy preserved 95.5% of the initial emission intensity and stabilized at this level while the emission intensity of NCs without salt encapsulation continuously decreased to 34.7% of its initial value after 96 h of operation. In addition, we investigated the effect of ionic salt encapsulation on the quantum efficiency of nonpolar NCs and found the quantum efficiency of the NCs-in-LiCl to be 75.1% while that of the NCs in dispersion was 73.0% and that in a film without LiCl encapsulation was 67.9%. We believe that such ionic salt encapsulated powders of nonpolar NCs presented here will find ubiquitous use for colour enrichment in display backlighting. © The Royal Society of Chemistry 2015.
Open Access
Type-II colloidal quantum wells: CdSe/CdTe core/crown heteronanoplatelets
(American Chemical Society, 2015) Keleştemur, Y.; Olutas M.; Delikanlı, S.; Güzeltürk, B.; Akgül, M. Z.; Demir, Hilmi Volkan
Solution-processed quantum wells, also known as colloidal nanoplatelets (NPLs), are emerging as promising materials for colloidal optoelectronics. In this work, we report the synthesis and characterization of CdSe/CdTe core/crown NPLs exhibiting a Type-II electronic structure and Type-II speci fic optical properties. Here, based on a core-seeded approach, the CdSe/CdTe core/crown NPLs were synthesized with well-controlled CdTe crown coatings. Uniform and epitaxial growth of CdTe crown region was verified by using structural characterization techniques including transmission electron microscopy (TEM) with quantitative EDX analysis and X-ray diffraction (XRD). Also the optical properties were systematically studied in these Type-II NPLs that reveal strongly red-shifted photoluminescence (up to ∼150 nm) along with 2 orders of magnitude longer fluorescence lifetimes (up to 190 ns) compared to the Type-I NPLs owing to spatially indirect excitons at the Type-II interface between the CdSe core and the CdTe crown regions. Photoluminescence excitation spectroscopy confirms that this strongly red-shifted emission actually arises from the CdSe/CdTe NPLs. In addition, temperature-dependent time-resolved fluorescence spectroscopy was performed to reveal the temperature-dependent fluorescence decay kinetics of the Type-II NPLs exhibiting interesting behavior. Also, water-soluble Type-II NPLs were achieved via ligand exchange of the CdSe/CdTe core/crown NPLs by using 3-mercaptopropionic acid (MPA), which allows for enhanced charge extraction efficiency owing to their shorter chain length and enables high quality film formation by layer-by-layer (LBL) assembly. With all of these appealing properties, the CdSe/CdTe core/crown heterostructures having Type-II electronic structure presented here are highly promising for light-harvesting applications. (Graph Presented). © 2015 American Chemical Society.