Browsing by Author "Dang, C."

Now showing 1 - 14 of 14

Open Access
All-optical control of exciton flow in a colloidal quantum well complex
(Springer Nature, 2020) Yu, J.; Sharma, Manoj; Sharma, A.; Delikanlı, Savaş; Demir, Hilmi Volkan; Dang, C.
Excitonics, an alternative to romising for processing information since semiconductor electronics is rapidly approaching the end of Moore’s law. Currently, the development of excitonic devices, where exciton flow is controlled, is mainly focused on electric-field modulation or exciton polaritons in high-Q cavities. Here, we show an all-optical strategy to manipulate the exciton flow in a binary colloidal quantum well complex through mediation of the Förster resonance energy transfer (FRET) by stimulated emission. In the spontaneous emission regime, FRET naturally occurs between a donor and an acceptor. In contrast, upon stronger excitation, the ultrafast consumption of excitons by stimulated emission effectively engineers the excitonic flow from the donors to the acceptors. Specifically, the acceptors’ stimulated emission significantly accelerates the exciton flow, while the donors’ stimulated emission almost stops this process. On this basis, a FRET-coupled rate equation model is derived to understand the controllable exciton flow using the density of the excited donors and the unexcited acceptors. The results will provide an effective all-optical route for realizing excitonic devices under room temperature operation.
Open Access
Anisotropic stimulated emission from aligned CdSe/CdS dot-in-rods
(IEEE, 2014-10) Gao, Y.; Ta, V. D.; Zhao, X.; Wang, Y.; Chen, R.; Zhao, Y.; Dang, C.; Sun, X.; Sun, H.; Demir, Hilmi Volkan
Anisotropic optical properties of CdSe/CdS dot-in-rods loaded in a capillary tube are demonstrated, suggesting nanorods' alignment with a microfluidic approach. Polarized emissions from photoluminescence and whispering gallery mode lasing show promising applications for lighting and displays. © 2014 IEEE.
Open Access
Coreless fiber‐based whispering‐gallery‐mode assisted lasing from colloidal quantum well solids
(Wiley-VCH Verlag, 2020-01) Sak, Mustafa; Taghipour, Nima; Delikanlı, Savaş; Shendre, S.; Tanrıöver, İbrahim; Gao, Y.; Yu, J.; Yanyan, Z.; Yoo, S.; Dang, C.; Demir, Hilmi Volkan; Foroutan, Sina
Whispering gallery mode (WGM) resonators are shown to hold great promise to achieve high‐performance lasing using colloidal semiconductor nanocrystals (NCs) in solution phase. However, the low packing density of such colloidal gain media in the solution phase results in increased lasing thresholds and poor lasing stability in these WGM lasers. To address these issues, here optical gain in colloidal quantum wells (CQWs) is proposed and shown in the form of high‐density close‐packed solid films constructed around a coreless fiber incorporating the resulting whispering gallery modes to induce gain and waveguiding modes of the fiber to funnel and collect light. In this work, a practical method is presented to produce the first CQW‐WGM laser using an optical fiber as the WGM cavity platform operating at low thresholds of ≈188 µJ cm−2 and ≈1.39 mJ cm−2 under one‐ and two‐photon absorption pumped, respectively, accompanied with a record low waveguide loss coefficient of ≈7 cm−1 and a high net modal gain coefficient of ≈485 cm−1. The spectral characteristics of the proposed CQW‐WGM resonator are supported with a numerical model of full electromagnetic solution. This unique CQW‐WGM cavity architecture offers new opportunities to achieve simple high‐performance optical resonators for colloidal lasers.
Open Access
Electrically control amplified spontaneous emission in colloidal quantum dots
(American Association for the Advancement of Science, 2019) Yu, J.; Shendre, S.; Koh, W.; Liu, B.; Li, M.; Hou, S.; Hettiarachchi, C.; Delikanlı, S.; Hernandez-Martinez, P.; Birowosuto, M. D.; Wang, H.; Sum, T.; Demir, Hilmi Volkan; Dang, C.
Colloidal quantum dots (CQDs) are highly promising materials for light amplification thanks to their efficient photoluminescence, tunable emission wavelength and low-cost synthesis. Unfortunately, CQDs are suffering from band-edge state degeneracy which demands multiple excitons to achieve population inversion. As a result, non-radiative Auger recombination increases the lasing threshold and limits the gain lifetime. Here, benefiting from the negative charging, we demonstrate that the amplified spontaneous emission (ASE) threshold is controllable in a device where CQD film is exposed to an external electric field. Specifically, singly charged CQDs lower the threshold due to the preexisting electron in the conduction band, while strongly enhanced Auger recombination in doubly charged CQDs stymies the ASE. Experimental results and kinetic equation model show that ASE threshold reduces 10% even if our device only charges ~17% of the CQD population. Our results open new possibilities for controlling exciton recombination dynamics and achieving electrically pumped CQD lasers.
Open Access
Highly flexible, electrically driven, top-emitting, quantum dot light-emitting stickers
(American Chemical Society, 2014) Yang X.; Mutlugun, E.; Dang, C.; Dev, K.; Gao, Y.; Tan, S.T.; Sun X.W.; Demir, Hilmi Volkan
Flexible information displays are key elements in future optoelectronic devices. Quantum dot light-emitting diodes (QLEDs) with advantages in color quality, stability, and cost-effectiveness are emerging as a candidate for single-material, full color light sources. Despite the recent advances in QLED technology, making high-performance flexible QLEDs still remains a big challenge due to limited choices of proper materials and device architectures as well as poor mechanical stability. Here, we show highly efficient, large-area QLED tapes emitting in red, green, and blue (RGB) colors with top-emitting design and polyimide tapes as flexible substrates. The brightness and quantum efficiency are 20 000 cd/m2 and 4.03%, respectively, the highest values reported for flexible QLEDs. Besides the excellent electroluminescence performance, these QLED films are highly flexible and mechanically robust to use as electrically driven light-emitting stickers by placing on or removing from any curved surface, facilitating versatile LED applications. Our QLED tapes present a step toward practical quantum dot based platforms for high-performance flexible displays and solid-state lighting. © 2014 American Chemical Society.
Open Access
Light extraction efficiency enhancement of colloidal quantum dot light-emitting diodes using large-scale nanopillar arrays
(Wiley-VCH Verlag, 2014) Yang, X.; Dev, K.; Wang, J.; Mutlugun, E.; Dang, C.; Zhao Y.; Liu, S.; Tang, Y.; Tan S.T.; Sun, X. W.; Demir, Hilmi Volkan
A colloidal quantum dot light-emitting diode (QLED) is reported with substantially enhanced light extraction efficiency by applying a layer of large-scale, low-cost, periodic nanopillar arrays. Zinc oxide nanopillars are grown on the glass surface of the substrate using a simple, efficient method of non-wetting templates. With the layer of ZnO nanopillar array as an optical outcoupling medium, a record high current efficiency (CE) of 26.6 cd/A is achieved for QLEDs. Consequently, the corresponding external quantum efficiency (EQE) of 9.34% reaches the highest EQE value for green-emitting QLEDs. Also, the underlying physical mechanisms enabling the enhanced light-extraction are investigated, which leads to an excellent agreement of the numerical results based on the mode theory with the experimental measurements. This study is the first account for QLEDs offering detailed insight into the light extraction efficiency enhancement of QLED devices. The method demonstrated here is intended to be useful not only for opening up a ubiquitous strategy for designing high-performance QLEDs but also with respect to fundamental research on the light extraction in QLEDs.
Open Access
Light-Emitting diodes with Cu-Doped colloidal quantum wells: from ultrapure green, tunable dual-emission to white light
(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Liu, B.; Sharma, Manoj; Yu, J.; Shendre, S.; Hettiarachchi, C.; Sharma, Ashma; Yeltik, Aydan; Wang, L.; Sun, H.; Dang, C.; Demir, Hilmi Volkan
Copper‐doped colloidal quantum wells (Cu‐CQWs) are considered a new class of optoelectronic materials. To date, the electroluminescence (EL) property of Cu‐CQWs has not been revealed. Additionally, it is desirable to achieve ultrapure green, tunable dual‐emission and white light to satisfy the various requirement of display and lighting applications. Herein, light‐emitting diodes (LEDs) based on colloidal Cu‐CQWs are demonstrated. For the 0% Cu‐doped concentration, the LED exhibits Commission Internationale de L'Eclairage 1931 coordinates of (0.103, 0.797) with a narrow EL full‐wavelength at half‐maximum of 12 nm. For the 0.5% Cu‐doped concentration, a dual‐emission LED is realized. Remarkably, the dual emission can be tuned by manipulating the device engineering. Furthermore, at a high doping concentration of 2.4%, a white LED based on CQWs is developed. With the management of doping concentrations, the color tuning (green, dual‐emission to white) is shown. The findings not only show that LEDs with CQWs can exhibit polychromatic emission but also unlock a new direction to develop LEDs by exploiting 2D impurity‐doped CQWs that can be further extended to the application of other impurities (e.g., Mn, Ag).
Open Access
Low-cost, large-scale, ordered ZnO nanopillar arrays for light extraction efficiency enhancement in quantum dot light-emitting diodes
(IEEE, 2014) Yang, X.; Dev, K.; Wang, J.; Mutlugün, E.; Dang, C.; Zhao, Y.; Tan, S. T.; Sun, X. W.; Demir, Hilmi Volkan
We report a QLED with enhanced light outcoupling efficiency by applying a layer of periodic ZnO nanopillar arrays. The resulting QLED reaches the record external quantum efficiency (EQE) of 9.34% in green-emitting QLEDs with a similar device structure.
Open Access
Mutual energy transfer in a binary colloidal quantum well complex
(American Chemical Society, 2019) Yu, J.; Sharma, Manoj; Delikanlı, Savaş; Birowosuto, M. D.; Demir, Hilmi Volkan; Dang, C.
Förster resonance energy transfer (FRET) is a fundamental process that is key to optical biosensing, photosynthetic light harvesting, and down-converted light emission. However, in total, conventional FRET in a donor–acceptor pair is essentially unidirectional, which impedes practical application of FRET-based technologies. Here, we propose a mutual FRET scheme that is uniquely bidirectional in a binary colloidal quantum well (CQW) complex enabled by utilizing the d orbital electrons in a dopant–host CQW system. Steady-state emission intensity, time-resolved, and photoluminescence excitation spectroscopies have demonstrated that two distinct CQWs play the role of donor and acceptor simultaneously in this complex consisting of 3 monolayer (ML) copper-doped CQWs and 4 ML undoped CQWs. Band-edge excitons in 3 ML CQWs effectively transfer the excitation to excitons in 4 ML CQWs, whose energy is also harvested backward by the dopants in 3 ML CQWs. This binary CQW complex, which offers a unique mutual energy-transfer mechanism, may unlock revolutionary FRET-based technologies.
Open Access
Stable and low ‐ threshold optical gain in CdSe/CdS quantum dots: an all ‐ colloidal frequency up ‐ converted laser
(Wiley-VCH Verlag, 2015) Güzeltürk, B.; Keleşemur, Y.; Güngor, K.; Yeltik, A.; Akgül, M. Z.; Wang, Y.; Chen R.; Dang, C.; Sun, H.; Demir, Hilmi Volkan
An all-solution processed and all-colloidal laser is demonstrated using tailored CdSe/CdS core/shell quantum dots, which exhibit highly stable and low-threshold optical gain owing to substantially suppressed non-radiative Auger recombination.
Open Access
Strong plasmon-wannier mott exciton interaction with high aspect ratio colloidal quantum wells
(Cell Press, 2020) Yu, J.; Hou, S.; Sharma, Manoj; Tobing, L. Y. M.; Song, Z.; Delikanlı, Savaş; Hettiarachchi, C.; Zhang, D.; Fan, W.; Birowosuto, M. D.; Wang, H.; Demir, Hilmi Volkan; Dang, C.
The strong interaction between excitons and plasmons, manifested as Rabi splitting of the eigen energies, is of fundamental interest for manipulating photons in nanoscale devices. Thanks to their enhanced photostability and minimal inhomogeneous broadening compared with organic molecules, inorganic emitters are preferred for practical applications. However, a relatively small Rabi splitting with inorganic materials severely hinders the active plasmonic operation, considering its weak optical nonlinearity and slow energy interexchange. Here, we circumvent this problem in a hybrid system consisting of high aspect ratio colloidal quantum wells (HARCQWs) and an individual plasmonic silver nanocube. By taking advantages of a highly in-plane oriented exciton, enhanced exciton binding energy, and non-stacking properties in HARCQWs, we demonstrate an unprecedented giant Rabi splitting energy up to 400 meV under ambient conditions, which is observed not only in scattering but also in photoluminescent spectra. These findings are a key step toward achieving inorganic plasmonic devices.
Open Access
Ultrafast control of the optical transition in type-II colloidal quantum wells
(American Chemical Society, 2023-04-21) Yu, J.; Durmusoglu, E. G.; Wang, Y.; Sharma, M.; Demir, Hilmi Volkan; Dang, C.
Manipulating the optical transition in semiconductors at ultrashort timescales is of both fundamental interest and central importance for emerging photonic applications. Traditionally, this manipulation is realized by electrostatic gating via Stark effects or band-gap renormalizations. Here, we report an ultrafast and all-optical route to engineer an indirect transition in core–crown colloidal quantum wells (CQWs), namely, CdSe/CdTe, with a type-II band alignment. Following the intense laser pulse excitation, the indirect band transition energy exhibits a pronounced blueshift–redshift crossover on the picosecond timescale, stemming from the formation and dissipation of the transient electric field (E-field) that forms upon photoexcitation to compensate for the driving force provided by the band offsets. Both the energy shift and dynamics of the transient E-field can be modulated optically by tuning the laser pulse excitation fluence. Our finding demonstrates a strong analogy between the type-II heterojunction and a p–n junction with respect to carrier equilibrium processes, which holds promise to facilitate the integration of CQWs within optical switching networks.
Open Access
Ultrahigh-efficiency aqueous flat nanocrystals of CdSe/CdS@Cd1−xZnxS colloidal core/crown@alloyed-shell quantum wells
(Royal Society of Chemistry, 2019) Shendre, S.; Delikanlı, Savaş; Li, M.; Dede, Didem; Pan, Z.; Ha, S. T.; Fu, Y. H.; Hernández-Martínez, Pedro L.; Yu, J.; Erdem, Onur; Kuznetsov, A. I.; Dang, C.; Sum, T. C.; Demir, Hilmi Volkan
Colloidal semiconductor nanoplatelets (NPLs) are highly promising luminescent materials owing to their exceptionally narrow emission spectra. While high-efficiency NPLs in non-polar organic media can be obtained readily, NPLs in aqueous media suffer from extremely low quantum yields (QYs), which completely undermines their potential, especially in biological applications. Here, we show high-efficiency water-soluble CdSe/CdS@Cd1−xZnxS core/crown@shell NPLs formed by layer-by-layer grown and composition-tuned gradient Cd1−xZnxS shells on CdSe/CdS core/crown seeds. Such control of shell composition with monolayer precision and effective peripheral crown passivation, together with the compact capping density of short 3-mercaptopropionic acid ligands, allow for QYs reaching 90% in water, accompanied by a significantly increased photoluminescence lifetime (∼35 ns), indicating the suppression of nonradiative channels in these NPLs. We also demonstrate the controlled attachment of these NPLs without stacking at the nanoscale by taking advantage of their 2D geometry and hydrophilicity. This is a significant step in achieving controlled assemblies and overcoming the stacking process, which otherwise undermines their film formation and performance in optoelectronic applications. Moreover, we show that the parallel orientation of such NPLs achieved by the controlled attachment enables directed emission perpendicular to the surface of the NPL films, which is highly advantageous for light extraction in light-emitting platforms..
Open Access
Ultrathin highly luminescent two-monolayer colloidal CdSe nanoplatelets
(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Delikanlı, Savaş; Yu, G.; Yeltik, Aydan; Bose, S.; Erdem, Talha; Yu, J.; Erdem, Onur; Sharma, Manoj; Sharma, Vijay Kumar; Quliyeva, Ulviyya; Shendre, S.; Dang, C.; Zhang, D.; Sum, T.; Fan, W.; Demir, Hilmi Volkan
Surface effects in atomically flat colloidal CdSe nanoplatelets (NLPs) are significantly and increasingly important with their thickness being reduced to subnanometer level, generating strong surface related deep trap photoluminescence emission alongside the bandedge emission. Herein, colloidal synthesis of highly luminescent two‐monolayer (2ML) CdSe NPLs and a systematic investigation of carrier dynamics in these NPLs exhibiting broad photoluminescence emission covering the visible region with quantum yields reaching 90% in solution and 85% in a polymer matrix is shown. The astonishingly efficient Stokes‐shifted broadband photoluminescence (PL) emission with a lifetime of ≈100 ns and the extremely short PL lifetime of around 0.16 ns at the bandedge signify the participation of radiative midgap surface centers in the recombination process associated with the underpassivated Se sites. Also, a proof‐of‐concept hybrid LED employing 2ML CdSe NPLs is developed as color converters, which exhibits luminous efficacy reaching 300 lm Wopt−1. The intrinsic absorption of the 2ML CdSe NPLs (≈2.15 × 106 cm−1) reported in this study is significantly larger than that of CdSe quantum dots (≈2.8 × 105 cm−1) at their first exciton signifying the presence of giant oscillator strength and hence making them favorable candidates for next‐generation light‐emitting and light‐harvesting applications.