Browsing by Author "Malko, Anton V."
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Item Carrier Recombination in Perovskites 3D Through 0D(2022-05-01T05:00:00.000Z) Zheng, Yangzi; Campbell, Zachary; Malko, Anton V.; Gartstein, Yuri; Lv, Bing; Shi, Xiaoyan; Zheng, JieLead-halide perovskites have long been demonstrated as materials with exceptional structural and optoelectronic properties. As an important crystalline material, lead-halide perovskites have potential applications in lasers, light-emitting diodes (LEDs), photovoltaic solar cells, photon detectors and biosensors, etc. By tailoring the morphological dimensionality, low-dimensional possessed distinct properties from their bulk (3D) counterparts. Due to the strong quantum confinement and octahedral site isolation, these low dimensional metal halide hybrids at the molecular level exhibit remarkable and unique properties that are significantly different from those of ABX3 perovskites. Considering the rapid development of low dimensional metal halide perovskites, we will discuss the synthesis, characterization, application, computational studies and compare various metal halide perovskites ranging from 3D through 0D. Finally, we show that a modified atomic layer deposition technique may be successfully used to protect 0D perovskite against external environment.Item Chemical Bonding and Defect States of LPCVD Grown Silicon-Rich Si₃N₄ for Quantum Dot Applications(A V S: Science & Technology of Materials, Interfaces, and Processing, 2014-03) Mohammed, Shakil; Nimmo, Michael T.; Malko, Anton V.; Hinkle, Christopher L.; 0000 0001 2678 9765 (Malko, AV); Mohammed, Shakil; Nimmo, Michael T.; Malko, Anton V.; Hinkle, Christopher L.Si-rich Si₃N₄ (SRN) thin films were investigated to understand the various defect states present within the SRN that can lead to reduced performance in quantum dot based devices made of these materials. The SRN films, deposited by low pressure chemical vapor deposition followed by furnace anneals over a range of temperatures, were determined to be comprised of two distinct phase separated SRN regions with different compositions (precipitates within a host matrix). Photoluminescence (PL) spectra showed multiple peaks convoluted together within the visible and near-visible range. Depending on deposition and annealing conditions, the films displayed changes in PL peak intensities which were correlated with chemical bonding utilizing x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, spectroscopic ellipsometry, and capacitance-voltage measurements. It is found that the PL originates from defect-state to defect-state and band edge to defect-state electronic transitions.Item Energy Transfer from Colloidal Nanocrystals into Si Substrates Studied via Photoluminescence Photon Counts and Decay Kinetics(2013-08-16) Nguyen, H. M.; Seitz, Oliver; Gartstein, Yuri N.; Chabal, Yves J.; Malko, Anton V.; 0000 0001 2678 9765 (Malko, AV); Nguyen, H. M.; Seitz, Oliver; Gartstein, Yuri N.; Chabal, Yves J.; Malko, Anton V.We use time-resolved photoluminescence (PL) kinetics and PL intensity measurements to study the decay of photoexcitations in colloidal CdSe/ZnS nanocrystals grafted on SiO₂ - Si substrates with a wide range of the SiO₂ spacer layer thicknesses. The salient features of experimental observations are found to be in good agreement with theoretical expectations within the framework of modification of spontaneous decay of electric-dipole excitons by their environment. Analysis of the experimental data reveals that energy transfer (ET) from nanocrystals into Si is a major enabler of substantial variations in decay rates, where we quantitatively distinguish contributions from nonradiative and radiative ET channels. We demonstrate that time-resolved PL kinetics provides a more direct assessment of ET, while PL intensity measurements are also affected by the specifics of the generation and emission processes.Item Energy Transfer from Colloidal Nanocrystals to Strongly Absorbing Perovskites(Royal Society of Chemistry, 2018-06-01) Cabrera, Yasiel; Rupich, Sara M.; Shaw, Ryan; Anand, Benoy; Villa, Manuel de Anda; Rahman, Rezwanur; Dangerfield, Aaron; Gartstein, Yuri N.; Malko, Anton V.; Chabal, Yves J.; 0000-0002-6435-0347 (Chabal, YJ); Cabrera, Yasiel; Rupich, Sara M.; Shaw, Ryan; Anand, Benoy; Villa, Manuel de Anda; Rahman, Rezwanur; Dangerfield, Aaron; Gartstein, Yuri N.; Malko, Anton V.; Chabal, Yves J.Integration of colloidal nanocrystal quantum dots (NQDs) with strongly absorbing semiconductors offers the possibility of developing optoelectronic and photonic devices with new functionalities. We examine the process of energy transfer (ET) from photoactive CdSe/ZnS core/shell NQDs into lead-halide perovskite polycrystalline films as a function of distance from the perovskite surface using time-resolved photoluminescence (TRPL) spectroscopy. We demonstrate near-field electromagnetic coupling between vastly dissimilar excitation in two materials that can reach an efficiency of 99% at room temperature. Our experimental results, combined with electrodynamics modeling, reveal the leading role of non-radiative ET at close distances, augmented by the waveguide emission coupling and light reabsorption at separations >10 nm. These results open the way to combining materials with different dimensionalities to achieve novel nanoscale architectures with improved photovoltaic and light emitting functionalities.Item Existence and Spatio-temporal Patterns of Periodic Solutions to Non-autonomous Second Order Equivariant Delayed Systems(2022-05-01T05:00:00.000Z) Ye, Xiaoli; Malko, Anton V.; Krawcewicz, Wieslaw; Lou, Yifei; Ohsawa, Tomoki; Balanov, Zalman I.In this dissertation, we study the existence and spatio-temporal symmetric patterns of peri- odic solutions to second order reversible equivariant non-autonomous periodic systems with multiple delays under the Hartman-Nagumo growth conditions. Our method is based on the usage of the Brouwer D1 × Z2 × Γ-equivariant degree theory, where D1 is related to the reversing symmetry, Z2 is related to the oddness of the right-hand-side and Γ reflects the symmetric character of the coupling in the corresponding network. Abstract results are supported by a concrete example with Γ = Dn – the dihedral group of order 2n.Item HIF-1α-PDK1 Axis-Induced Active Glycolysis Plays an Essential Role in Macrophage Migratory Capacity(American Physical Society) Anand, Benoy; Sampat, Siddharth; Danilov, E. O.; Peng, Weina; Rupich, Sara M.; Chabal, Yves J.; Gartstein, Yuri N.; Malko, Anton V.; 0000 0001 1969 6683 (Gartstein, YN); 0000 0001 2678 9765 (Malko, AV); 170647442 (Gartstein, YN); Anand, Benoy; Sampat, Siddharth; Peng, Weina; Rupich, Sara M.; Chabal, Yves J.; Gartstein, Yuri N.; Malko, Anton V.Ultrafast transient pump-probe measurements of thin CH₃NH₃PbI₃ perovskite films over a wide spectral range from 350 to 800 nm reveal a family of photoinduced bleach (PB) and absorption (PA) features unequivocally pointing to the fundamentally multiband character of the underlying electronic structure. Excitation pump-energy dependent kinetics of three long-lived PB peaks at 1.65, 2.55, and 3.15 eV along with a broad PA band shows the involvement of band-edge thermalized carriers in all transitions and at least four, possibly more, electronic bands. The evolution of the transient signatures is described in terms of the redistribution of the conserved oscillator strength of the whole system. The multiband perspective opens up different directions for understanding and controlling photoexcitations in hybrid perovskites.Item Influence of Growth Temperature on Bulk and Surface Defects in Hybrid Lead Halide Perovskite Films(Royal Society of Chemistry, 2015-12-14) Peng, Weina; Anand, Benoy; Liu, Lihong; Sampat, Siddharth; Bearden, Brandon E.; Malko, Anton V.; Chabal, Yves J.The rapid development of perovskite solar cells has focused its attention on defects in perovskites, which are gradually realized to strongly control the device performance. A fundamental understanding is therefore needed for further improvement in this field. Recent efforts have mainly focused on minimizing the surface defects and grain boundaries in thin films. Using time-resolved photoluminescence spectroscopy, we show that bulk defects in perovskite samples prepared using vapor assisted solution process (VASP) play a key role in addition to surface and grain boundary defects. The defect state density of samples prepared at 150 °C (~10¹⁷ cm⁻³) increases by 5 fold at 175 °C even though the average grains size increases slightly, ruling out grain boundary defects as the main mechanism for the observed differences in PL properties upon annealing. Upon surface passivation using water molecules, the PL intensity and lifetime of samples prepared at 200 °C are only partially improved, remaining significantly lower than those prepared at 150 °C. Thus, the present study indicates that the majority of these defect states observed at elevated growth temperatures originates from bulk defects and underscores the importance to control the formation of bulk defects together with grain boundary and surface defects to further improve the optoelectronic properties of perovskites.Item Influence of the Core Size on Biexciton Quantum Yield of Giant CdSe/CdS Nanocrystals(The Royal Society of Chemistry, 2014-02-06) Mangum, B. D.; Sampat, Siddharth; Ghosh, Y.; Hollingsworth, J. A.; Htoon, H.; Malko, Anton V.; 0000 0001 2678 9765 (Malko, AV); Sampat, Siddharth; Malko, Anton V.We present a systematic study of photoluminescence (PL) emission intensity and biexciton (BX) quantum yields (QY_{BX}) in individual "giant" CdSe/CdS nanocrystals (g-NCs) as a function of g-NC core size and shell thickness. We show that g-NC core size significantly affects QY_{BX} and can be utilized as an effective tuning parameter towards higher QY_{BX} while keeping the total volume of the g-NC constant. Specifically, we observe that small-core (2.2 nm diameter) CdSe/CdS NCs with a volume of ~200 nm³ (shell comprises 4 CdS monolayers) show very low average and maximum QY_{BX}'s of ~3 and 7%, respectively. In contrast, same-volume medium-core (3 nm diameter) NCs afford higher average values of ~10%, while QY_{BX}'s of ~30% are achieved for same-volume large-core (5.5 nm diameter) CdSe/CdS NCs, with some approaching ~80%. These observations underline the influence of the g-NC core size on the evolution of PL emissive states in multi-shell NCs. Moreover, our study also reveals that the use of long anneal times in the growth of CdS shells plays a critical role in achieving high QY_{BX}.Item Propagation and Absorption of Light in Planar Dielectric Waveguides with Two-Dimensional Semiconductors(Optical Soc Amer, 2017-09-12) Gartstein, Yuri N.; Malko, Anton V.; 0000 0001 2678 9765 (Malko, AV); Gartstein, Yuri N.; Malko, Anton V.Strong optical responses of atomically thin two-dimensional (2D) semiconductors make them attractive candidates for integration into various photonic and optoelectronic structures. We discuss some fundamental effects of such integration into planar dielectric waveguides by demonstrating that a substantial modification of the spectrum of waveguide modes can occur due to high in-plane polarizability of 2D layers. Our calculations illustrate both the conceptual possibilities associated with sharp excitonic resonances as well as the results obtained with the experimentally assessed polarizability of monolayer MoS2 over a broad spectral range. We point out that waveguide structures could also enable the tunable absorption by 2D semiconductors of the light that propagates along them, a modality quite different from the traditional light harvesting geometry.Item Ultrafast Charge Recombination Mechanism in Single 0D All-inorganic Perovskite Nanocrystals(August 2023) Zhou, Xiaohe 1995-; Malko, Anton V.; Nicholas, Dillon; Gartstein, Yuri; Du, Shengwang; Zhang, Fan; Slinker, Jason D.Nanoscale semiconductors possess many attractive properties as compared to bulk counterparts. Main advantages are tunable energy levels, size-controlled carrier-carrier interactions and, often times, facile synthesis methods offering broad possibilities in optoelectronic applications. A rigorous understanding of the elemental physical properties in these nanos- tructures would provide deep insights for designing and improving the quality of materials. Among many spectroscopic detection methods, single particle spectroscopy is a powerful and sensitive tool for understanding interactions of multiple charge-carrier species in a wide range of materials, from bulk crystals down to individual nanoparticles . In this work, we study various types of so-called “0D” cesium-based perovskite nanocrystals, whose photo and environmental stability has been reported to exceed the conventional 3D perovskites nanocrystals. Due to specific, nearly complete isolation of halide octahedra in 0D structures, their optical properties bear strong resemblance to molecular-type defects, necessitating use of single particle detection methods. The following work concerns with perovskites’ ultrafast charge-carrier dynamics, stability and change of photoluminescence quantum yield (PLQY) inferred from single particle blinking signatures and modified by application of nanoscale alumina layers via the atomic layer deposition (ALD) encapsulation.Item Using Shape to Turn off Blinking for Two-Colour Multiexciton Emission in CdSe/CdS Tetrapods(2017-05-12) Mishra, Nimai; Orfield, Noah J.; Wang, Feng; Hu, Zhongjian; Krishnamurthy, Sachidananda; Malko, Anton V.; Casson, Joanna L.; Htoon, Han; Sykora, Milan; Hollingsworth, Jennifer A.; Krishnamurthy, Sachidananda; Malko, Anton V.Semiconductor nanostructures capable of emitting from two excited states and thereby of producing two photoluminescence colours are of fundamental and potential technological significance. In this limited class of nanocrystals, CdSe/CdS core/arm tetrapods exhibit the unusual trait of two-colour (red and green) multiexcitonic emission, with green emission from the CdS arms emerging only at high excitation fluences. Here we show that by synthetic shape-tuning, both this multi-colour emission process, and blinking and photobleaching behaviours of single tetrapods can be controlled. Specifically, we find that the properties of dual emission and single-nanostructure photostability depend on different structural parameters-arm length and arm diameter, respectively-but that both properties can be realized in the same nanostructure. Furthermore, based on results of correlated photoluminescence and transient absorption measurements, we conclude that hole-trap filling in the arms and partial state-filling in the core are necessary preconditions for the observation of multiexciton multi-colour emission.