Browsing by Author "Kumar, A."
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Item Open Access Cd-free Cu-doped ZnInS/ZnS Core/Shell nanocrystals: Controlled synthesis and photophysical properties(SpringerOpen, 2018) Kaur, Manpreet; Sharma, Ashma; Olutaş, Murat; Erdem, Onur; Kumar, A.; Sharma, Manoj; Demir, Hilmi VolkanHere, we report efficient composition-tunable Cu-doped ZnInS/ZnS (core and core/shell) colloidal nanocrystals (CNCs) synthesized by using a colloidal non-injection method. The initial precursors for the synthesis were used in oleate form rather than in powder form, resulting in a nearly defect-free photoluminescence (PL) emission. The change in Zn/In ratio tunes the percentage incorporation of Cu in CNCs. These highly monodisperse Cu-doped ZnInS CNCs having variable Zn/In ratios possess peak emission wavelength tunable from 550 to 650 nm in the visible spectrum. The quantum yield (QY) of these synthesized Cd-free CNCs increases from 6.0 to 65.0% after coating with a ZnS shell. The CNCs possessing emission from a mixed contribution of deep trap and dopant states to only dominant dopant-related Stokes-shifted emission are realized by a careful control of stoichiometric ratio of different reactant precursors during synthesis. The origin of this shift in emission was understood by using steady state and time-resolved fluorescence (TRF) spectroscopy studies. As a proof-of-concept demonstration, these blue excitable Cu-doped ZnInS/ZnS CNCs have been integrated with commercial blue LEDs to generate white-light emission (WLE). The suitable combination of these highly efficient doped CNCs results led to a Commission Internationale de l’Enclairage (CIE) color coordinates of (0.33, 0.31) at a color coordinate temperature (CCT) of 3694 K, with a luminous efficacy of optical radiation (LER) of 170 lm/Wopt and a color rendering index (CRI) of 88.Item Open Access Development of CuAg/Cu2O nanoparticles on carbon nitride surface for methanol oxidation and selective conversion of carbon dioxide into formate(Elsevier, 2020) Nazir, Roshan; Kumar, A.; Saad, M. A. S.; Ali, S.Herein we report a catalyst consisting of CuAg/Cu2O nanoparticles (NPs), synthesized on the two-dimensional carbon nitride (Csingle bondN) surface via galvanic exchange route for electrocatalytic methanol oxidation and carbon dioxide reduction. The lower reduction potential of copper ([Cu+(aq) + e− → Cu(s)], + 0.52 eV) compared to Ag ([Ag+(aq) + e− → Ag(s)], +0.80 eV) makes Cu(0) easily exchangeable by Ag+ ions via galvanic exchange without applying any external bias. In a two-step process, the Cu NPs are synthesized first on Csingle bondN surface by adsorbing Cu2+ precursors and reducing them by NaBH4. Due to unstable nature of Cu2+ in aqueous medium some Cu2O NPs (a mixed phase of Cu/CuO) were also formed. Thereafter in the second step, Ag+ precursors are brought in contact with the already synthesized Cu and Cu2O nanoparticles (NPs). The Cu and Cu2O NPs present on the surface of Csingle bondN are partially exchanged by Ag atoms to generate bimetallic CuAg/Cu2O NPs. Two atoms of Ag are expected to be deposited for every Cu atom replaced. As galvanic replacement occurs on the solid surface of carbon nitride, there is only a partial replacement of Cu and Cu2O atoms. The catalysts Csingle bondN/Cu/Cu2O and Csingle bondN/CuAg/Cu2O were evaluated for their performance towards methanol oxidation and carbon dioxide reduction. Csingle bondN/CuAg/Cu2O showed twice the current density for methanol oxidation than Csingle bondN/Cu/Cu2O in a 0.5 M methanol solution. Probably the reason for high activity of Ag than Cu is related to the weak bond of oxygen on silver substrate for oxidation reactions and strong binding affinity on copper substrate. In case of carbon dioxide reduction (CO2 reduction) the product was identified to be formate by oxidizing the product (formate) on a Pt ring electrode. The results revealed Csingle bondN/CuAg/Cu2O shows a better selectivity towards formic acid formation than Csingle bondN/Cu/Cu2O using the rotating ring disc electrode (RRDE). A probable reason may be the strain induced by alloy formation which could favor a reduced coverage of adsorbed hydrogen and a decrease in oxophilicity of the compressively strained copper.Item Open Access Nanosheet synthesis of mixed Co3O4/CuO via combustion method for methanol oxidation and carbon dioxide reduction(American Chemical Society, 2020) Nazir, Roshan; Khalfani, A.; Abdelfattah, O.; Kumar, A.; Saad, M. A. S.; Ali, S.This paper represents a study of mixed Co3O4/CuO nanosheet (NS) synthesis via solution combustion synthesis for oxidation of methanol and carbon dioxide (CO2) conversion. The mixed oxide NS of Co3O4/CuO is a hybrid structure of Co3O4 and CuO NSs. We applied this mixed oxide NS of Co3O4/CuO for methanol oxidation and carbon dioxide (CO2) conversion, and the results revealed that the activity of the mixed oxide NS surpassed the activity of the corresponding individual Co3O4 and CuO metal oxide NSs, both in methanol oxidation and in CO2 conversion. The mass activity of the mixed Co3O4/CuO NS produced at 0.627 V versus Ag/AgCl during methanol oxidation (0.5 M) was 12 mA g–1, which is 2.4 times better than that of Co3O4, whose mass activity is 5 mA g–1, and 4 times better than that of the CuO NS, whose mass activity is 3 mA g–1. The methanol oxidation peak at 0.62 V versus Ag/AgCl was also more intense than individual oxides. The trend in performance of methanol oxidation follows the order: Co3O4/CuO > Co3O4 > CuO. In the case of CO2 reduction, we experienced that our product was formate, and this was proved by formate oxidation (formate is formed as a product during the reduction of CO2) on the surface of the Pt ring of a rotating ring-disc electrode. Similar to methanol oxidation, Co3O4/CuO also showed superior activity in carbon dioxide reduction. It was experienced that at −1.5 V, the current density rises to −24 mA/cm2 for the Co3O4/CuO NS, that is, 0.6 times that of the CuO NS, which is −15 mA/cm2, and 3 times more than that of the Co3O4 NS, which is 8 mA/cm2. The trend in performance of CO2 reduction follows the order: Co3O4/CuO > CuO > Co3O4.Item Open Access Shouji: a fast and efficient pre-alignment filter for sequence alignment(Oxford University Press, 2019) Alser, Mohammed; Hassan, H.; Kumar, A.; Mutlu, Onur; Alkan, CanThe ability to generate massive amounts of sequencing data continues to overwhelm the processing capability of existing algorithms and compute infrastructures. In this work, we explore the use of hardware/software co-design and hardware acceleration to significantly reduce the execution time of short sequence alignment, a crucial step in analyzing sequenced genomes. We introduce Shouji, a highly parallel and accurate pre-alignment filter that remarkably reduces the need for computationally-costly dynamic programming algorithms. The first key idea of our proposed pre-alignment filter is to provide high filtering accuracy by correctly detecting all common subsequences shared between two given sequences. The second key idea is to design a hardware accelerator that adopts modern field-programmable gate array (FPGA) architectures to further boost the performance of our algorithm. Shouji significantly improves the accuracy of pre-alignment filtering by up to two orders of magnitude compared to the state-of-the-art pre-alignment filters, GateKeeper and SHD. Our FPGA-based accelerator is up to three orders of magnitude faster than the equivalent CPU implementation of Shouji. Using a single FPGA chip, we benchmark the benefits of integrating Shouji with five state-of-the-art sequence aligners, designed for different computing platforms. The addition of Shouji as a pre-alignment step reduces the execution time of the five state-of-the-art sequence aligners by up to 18.8×. Shouji can be adapted for any bioinformatics pipeline that performs sequence alignment for verification. Unlike most existing methods that aim to accelerate sequence alignment, Shouji does not sacrifice any of the aligner capabilities, as it does not modify or replace the alignment step.Item Open Access Synthesis of hydroxide nanoparticles of Co/Cu on carbon nitride surface via galvanic exchange method for electrocatalytic CO2 reduction into formate(Elsevier, 2020-04-06) Nazir, Roshan; Kumar, A.; Saad, M. A. S.; Ashok, A.; Nazir, RoshanHerein we report Co and Cu based metal hydroxides (Co(OH)2/Cu(OH)2) on carbon nitride (C3N4) surface via replacement of Co nanoparticles (NPs) through galvanic exchange method for electrocatalytic carbon dioxide reduction. The lower value of reduction potential in case of cobalt ([Co+(aq) + 2e− → Co(s)], −0.28 eV) compared to copper ([Cu2+(aq) +2 e− → Cu(s)], +0.34 eV) makes Co(0) easily susceptible to galvanic exchange process. On the basis of this significant difference in the reduction potential of Cu(0) and Co(0), 0.62 V, Cu2+ can replace Co(0) via galvanic exchange without using any external bias. The synthesis of (Co(OH)2/Cu(OH)2) involves two steps, where in the first step on surface of C3N4, Co NPs were synthesized via reducing of Co2+ ions with a strong reducing agent NaBH4. In presence of aqueous medium, formation of cobalt hydroxide also takes place. In the second step these cobalt nanoparticles on C3N4 were subjected to the process of galvanic exchange in which the sacrificial Co NPs were exchanged by Cu atoms and forming Cu(OH)2 in presence of an aqueous medium. Overall, the whole synthesis process results in deposition of hydroxides of cobalt and copper (C3N4/(Co(OH)2/Cu(OH)2) on C3N4 surface. The synthesized materials were characterized using PXRD, EDS, XPS, TEM and SEM. The two electrocatalysts C3N4/(Co/Co(OH)2 C3N4/(Co(OH)2/Cu(OH)2 were evaluated for their performance towards carbon dioxide reduction. C3N4/(Co(OH)2/Cu(OH)2 showed superior performance with electrocatalytic activity more than three times of C3N4/(Co/Co(OH)2. The product of CO2 electro-reduction was identified, using a rotating ring disc electrode (RRDE) system, to be primarily formate.Item Open Access Understanding the suitable alloying conditions for highly efficient Cu- and Mn-doped Zn1-xCdxS/ZnS core-shell quantum dots(Elsevier B.V., 2023-10-20) Kaur, Manpreet; Sharma, Ashma; Erdem, Onur; Kumar, A.; Demir, Hilmi Volkan; Sharma, M.Doping of alloyed colloidal quantum dots (QDs) has garnered significant attention for providing tunable and Stokes-shifted emission. By alloying the host semiconductor nanocrystals (NCs), their band gap can be tuned. With the specific addition of dopant ions, these NCs can emit tunable emissions within the visible spectrum. However, while doped and alloyed quantum dots (QDs) have shown promise for tunable emissions, their emission qualities have not been consistent across the spectrum. Here, we report the synthesis of high-quality Cu- and Mn-doped ZnxCd1-xS (x = 0–1) alloyed QDs by a colloidal non-injection method. In this study, we examined the effect of different dopant ions on the optical properties of similar alloyed nanocrystals. The deposition of a ZnS shell on these doped QDs significantly improves their quantum yield (QY), increasing it from 7.0 % to 50.0 % for Cu-doped QDs and from 30.0 % to 80.0 % for Mn-doped QDs. The Cu-doped QDs exhibit tunable emission from green to red across the visible spectrum by varying the Zn/Cd ratio, whereas the Mn-doped QDs show a fixed orange emission. Interestingly, the Cu-doped alloyed QDs show a contrasting trend in quantum yield (QY) compared to those of Mn-doped QDs when the amount of Cd in ZnCdS alloyed QDs is systematically changed. As the amount of Cd increases in the ZnCdS alloyed QDs, the Cu-doped QDs show both an increase in average lifetime and an increase in QY. In contrast, for the Mn-doped QDs, the decay lifetime values remain fairly constant for different amounts of Cd in the ZnCdS alloyed QDs, but the QY decreases as the amount of Cd increases. The results of this study may facilitate the design of optimal alloying combinations for Cu/Mn-doped QDs in optoelectronic applications. © 2023 The Authors