Geeta Bhatt

Increasing the urea content of microwave-synthesized citric acid–urea carbon quantum dots raises their nitrogen content and surface-to-bulk nitrogen ratio, and introduces nitrogen- and oxygen-related sub-gap states that their emission follows.

This study theoretically investigates an optical fiber structure coated with a bilayer film of zinc oxide (ZnO) and lithium fluoride (LiF) for ethanol detection. The ZnO thin film exhibits lossy mode resonance (LMR) in the visible and near-infrared spectrum (400 – 1500nm), which serves as the basis for designing a refractive index (RI) sensor by appropriately selecting the ZnO thickness. To further enhance the resonance characteristics, a dielectric layer of LiF is introduced between the unclad fiber core and the ZnO coating. It Improves phase matching and generates sharper LMR dips with a narrower full width at half maximum. The proposed structure is also analyzed using two different core materials: pure silica and 4% Ge-doped silica. A comparative study of their sensing performance is also presented. Its sensing performance is examined by varying the thicknesses of the ZnO and LiF layers to identify the optimal structural parameters. The designed fiber-optic sensor is then evaluated for detecting ethanol concentrations in ethanol–water mixtures (RI =1.33 for 0% ethanol and RI =1.36 for pure ethanol) using both s- and p -polarized incident light. Calibration curves demonstrating the relationship between resonant wavelength shifts and the RI of the sample are presented. This sensor achieves excellent sensitivities of 1163 nm RIU ⁻¹ for s -polarization and 1475 nm RIU ⁻¹ for p -polarization, making it promising for the development of High-Figure of Merit biochemical sensors for potential applications.

Nitrogen- and sodium-doped carbon quantum dots (CQDs) were synthesized from citric acid monohydrate and urea, using varying concentrations of Ethylenediaminetetraacetic acid disodium salt dehydrate (EDTA) in a bottom-up approach with microwave irradiation. First, a comparative study of synthesis duration and EDTA concentration was conducted. Upon optimization, our results showed that E1-CQD@300sec exhibited the highest absorbance among all EDTA-CQD samples, with absorbance values following this trend: EDTA-CQD@300sec > EDTA-CQD@225sec > EDTA-CQD@165sec for most EDTA concentrations. Additionally, photoluminescence (PL) was found to follow the order: E2-CQD@165sec > E1-CQD@225sec ≈ E1-CQD@300sec. E1-CQD@300sec had a spherical morphology with an approximate size of 13 nm, as observed in high-resolution transmission electron microscopy (HR-TEM). This E1-CQD@300sec sample has a bandgap of 1.95 eV, with a highest occupied molecular orbital (HOMO) of -5.15 eV and a lowest unoccupied molecular orbital (LUMO) of -3.2 eV. X-ray diffraction (XRD) revealed a broad peak at ~ 26.45°, indicating an amorphous structure, which was confirmed by Raman spectroscopy, showing an I _D /I _G ratio of 1.11. FT-IR analysis indicated the presence of nitrogen-based functional groups, such as C-N and N-H, on the surface. With increase in EDTA concentration, the bandgap of EDTA-CQD@300sec increased, reaching a maximum of 2.29 eV for E5-CQD@300sec, with a HOMO of -5.55 eV and a LUMO of -3.26 eV, and other functional groups, such as C ≡ C and C ≡ N, were also detected in the FT-IR spectra. In addition, agglomeration was further increased in the presence of EDTA, possibly due to uncontrolled nucleation, EDTA-CQD complex formation, high EDTA concentration, and insufficient surface coverage by EDTA, as observed through dynamic light scattering (DLS). The thermal stability of ETDA-CQD@300sec also found to decreased from ~ 208°C to 40 °C. Further modifications and optimization is necessary for potential use of synthesized CQDs in the opto-electronic applications.

Attenuation characteristics of germanium (Ge) film clad planar optical waveguide are theoretically studied at the spectral wavelength of 0.6328 μm. Thin films of Ge are found to possess greater attenuation values than the other recognized materials. The existence of these greater attenuations is caused by lossy modes that are sustained by these films. It has been shown that the attenuation of the TM mode can be greatly increased by adding a high-index buffer layer between the Ge layer and the dielectric guide. It occurred because of the resonant coupling between the lossy modes supported by the Ge layer and the waveguide modes. Moreover, it is claimed that by adding a dielectric superstrate layer above the Ge thin film, the strength of the TM mode in the proposed structure can be further attenuated to an ultra-high value. In this study, a TE pass polarizer is proposed having a high extinction ratio of 30582.15 dB. Attenuation results are presented, after optimization of several parameters like thicknesses of Ge, buffer, and superstrate layers along with environmental stability of the suggested structure. The generated field distribution profiles are also in good agreement with the obtained attenuation results.

Carbon quantum dots (CQDs) are fluorescent carbon nanomaterials with unique optical and structural properties. The detailed stability analysis of CQDs is very much needed for their commercial applications.

Carbon quantum dots (CQDs) are emerging as promising materials for applications like flexible or transparent solar cell, white light emitting diodes (WLEDs), etc. due to their low cost, eco-friendliness, substantial absorption coefficient, wide absorption spectrum, tuneable optical properties, good charge transfer/separation ability, good quantum yield and large two-photon absorption cross-section. They have been employed in solar cells as active absorbing layers, electron acceptors/donors, electron sinks, electron transporting layers (ETL), hole transporting layers (HTLs), dopants, and interlayer spacing. Consequently, such solar cells have exhibited enhanced performance. In contrast to commercial rare-earth phosphors and traditional semiconductor quantum dots (SQDs) (usually toxic), CQDs exhibit wide range of emission characteristics [full width half maxima (FWHM) > 80 nm]. Interestingly, the emission characteristics of these nanomaterials are tuneable which makes them suitable for WLEDs applications. Red-CQDs are gaining importance as they are required to realize the warm WLEDs. Though a lot of work has been done to modulate the properties of CQDs in order to enhance the performance of solar cells and WLEDs, there are immense possibilities to further exploit the potential of CQDs in these applications.

Correction for ‘A review on advancements in carbon quantum dots and their application in photovoltaics’ by Pawan Kumar et al., RSC Adv., 2022, 12, 4714–4759, DOI: 10.1039/D1RA08452F

This review puts forth the in-depth understanding of the fundamentals of carbon quantum dots(CQDs), recent advancements in the field including a thorough discussion on different roles of CQDs to enhance the performance of solar cells and white-LEDs.