Optical UV Research Articles

Microplasma nanoengineering of emission-tuneable colloidal nitrogen-doped graphene quantum dots as smart environmental-responsive nanosensors and nanothermometers

Nitrogen-doped graphene quantum dots (NGQDs) with controlled emission properties are useful materials for fundamental research and applications. However, the scalable and sustainable production of NGQDs with defined nanostructures from biomass precursors remains elusive. Here a bottom-up microplasma synthesis of emission-controlled colloidal NGQDs used as the multifunctional nanosensors for temperature and selective metal ion sensing is demonstrated. The emission-tuneable NGQDs are synthesized by engineering the N-doping configurations and surface functional groups on the NGQDs surfaces by carefully controlling the plasma conditions and electrolyte chemistry. In situ optical emission and UV–Vis absorbance spectroscopy measurement reveals that the plasma-induced hydroxyl radicals and solvated electrons define the N-dopant configuration and surface functionalization of NGQDs, leading to emission-tuneable NGQDs. The emission-tuneable NGQDs are applied as multifunctional nanosensors for selective Fe3+, Cu2+, and Hg2+ detection revealing broad linear detection range (0.5–300 μM) and low limit of detection (LOD) of 47.9 nM for Hg2+ and sensitive temperature detection from 10 to 80 °C. The developed PL sensing method shows high throughput of 5000 detections per hour. Our work demonstrates the possibility of the plasma-enabled nanoengineering of NGQDs with controlled optical properties and a step towards sustainable and scalable synthesis of graphene nanomaterials from renewable bioresources.

Methodology for estimating the energy reserve of a mobile ad-hoc communication network with UV-C channel

Communication systems of the solar-blind ultraviolet range UV-C from 200 to 280 nm provide the possibility of communication in the absence of line of sight due to the strong scattering of UV radiation in the atmosphere. For the effective use of UV-C communication systems, an adequate assessment of the energy budget of the UV channel is necessary. Analytical expressions are obtained that characterize the energy reserve of the UV-C system in relation to the specified characteristics of the optical receiver, transmitter, and UV channel. Based on the analytical and numerical model of losses in the UV channel, the energy reserve of the UV-C communication system is modeled on the distance between the transmitter and receiver, the elevation angles and the azimuthal deviation of the transmitter and receiver. The results obtained can be used to calculate and optimize ad-hoc communication networks with UV-C channel, characterized by changes in the spatial location of network nodes during operation.

Gamma ray interaction of optical, chemical, physical behavior of bismuth silicate glasses and their radiation shielding proficiency using Phy-X / PSD program

Three compositions of lithium bismuth silicate glasses containing either 2% PbO, BaO or SrO (all in wt. %) were studied to investigate some of their optical, chemical, and physical properties before and after gamma irradiation. Optical UV transmission, FTIR and ESR spectra have been analyzed and discussed. Also, chemical durability before and after gamma irradiation in acidic, alkaline and neutral solutions has been studied. The results showed relative stability of the glasses on UV-visible, FTIR and ESR spectra. Additionally, good chemical durability in the three leaching solutions was observed especially in the alkaline medium. Many shielding parameters have been investigated such as; mass attenuation coefficient that has been measured experimentally and calculated theoretically using NaI (Tl) detector and Phy-X/PSD software, respectively and the results showed a good agreement of the experimental and theoretical values. Also, some other important shielding parameters have been calculated using the Phy-X/PSD program.

Tunable band gap energy of single-walled zigzag ZnO nanotubes as a potential application in photodetection

ZnO nanotubes play a substantial role in various optoelectronic applications such as light-emitting diodes, solar cells and photodetectors. In this study, the structural, electronic, and optical effects of (n,0) single-walled ZnO nanotubes with different radius are discussed. An ab-initio approach of density functional theory with the generalized gradient approximation has been done by Quantum ESPRESSO software on nanotubes with n = 3 to 6. All simulated ZnO nanotubes illustrate semiconducting behavior, which due to the quantum confinement effect, the band gap value decreases by the increase in diameter. Moreover, optical characteristics including dielectric function ε(ω), refractive index n(ω), optical absorption coefficient α(ω), conductivity σ(ω), energy loss spectrum L(ω) and reflectivity R(ω) have been analyzed and compared. The predicted optical features show that the mentioned nanotubes transmit light in perpendicular polarization better than parallel one and present superior reflect characters. Additionally, ZnO NTs act as a good absorbent in the visible wavelength which they can absorb various colors of the visible spectrum or even UV emission by altering the number of n. This ability turns ZnO NTs into remarkable photodetectors. Furthermore, according to optical characteristics like energy loss spectrum and dielectric function, it is obvious that results are approximately similar for different nanotubes and despite the miner red-shift of the position of the main peak, their appearance is close to each other. Therefore, ZnO NTs can be utilized in various applications such as optical filters, polarizers, and UV shields.

Optical absorption modeling of bilayer photoanode based on Cu@TiO2 plasmonic dye sensitized solar cells towards photovoltaic applications

A periodic array of core–shell Cu@TiO2 nanoparticle for plasmonic dye sensitized solar cells (DSSCs) in the wavelength range between 350 and 750 nm was studied. The size of copper nanospheres was 70 nm while the length and diameter of the copper nanorods were 100 and 10 nm, respectively. The UV–Visible absorption spectrum showed that the photo-anode based copper added TiO2 has 29.3% absorption capability compared with copper-free TiO2. TiO2 with shell thickness of 5 nm coated copper exhibited the absorption efficiency of 71.9%, while short circuit current density of 17.52 mA cm−2 for Ci@TiO2 photo-anode. This was attributed to a strong localized electric field around ultra-thin TiO2-coated copper nanospheres. The UV–Visible results of different geometries indicated that the spherical-shaped Cu@TiO2 nanoparticles induced the high absorption capability of 3.4% compared to rod-shaped Cu@TiO2 nanoparticles. The hybrid nanorods/nanospheres bilayer photo-anode showed the high optical UV–Visible absorption of 11.42% as compared with nanospheres/nanorods, ascribed to the large surface area for dye-loading excellent light scattering.

Morphological and structural study of crystals in black-to-brown glazes of Yaozhou ware (Song dynasty) using imaging and spectroscopic techniques

The Yaozhou kiln complex (Shaanxi, China) is a famous black-to-brown ceramic production center due to its significant volume of production, long manufacturing history (from Tang Dynasty to Yuan Dynasty) and variety of decorations. In this work, four representative types of black-to-brown ware from the Yaozhou kilns were selected to study the morphology, structure and distribution of the precipitated crystals in the glazes using a series of imaging and analytical techniques (optical microscopy, XRF, SEM-EDS, TEM-EDS, μRS and UV–vis-IR). The results show that the rare metastable ε-Fe2O3 phase, already observed in typical sauce glaze, was also detected in other types of brown glaze decorations, such as light brown stripes on a darker background or irregular brown spots on a black background. Cross-section analyses also showed a three well-separated layered crystalline distribution in the glazes of lighter brown colors as it has been noted in glazes from Qilizhen kilns. Raman analyses revealed that the crystals contained in third layer are of varying nature: ε-Fe2O3 in the sauce glaze and magnetite in the light brown stripes. The peak shifts and line broadenings observed in the Raman spectra of ε-Fe2O3 crystals were also investigated. They are the result of Al, Ti and Mg substitutions, which were identified using TEM-EDS. Such ionic substitutions would stabilize the metastable ε-Fe2O3 crystals by increasing the cationic disorder. In addition, Mg-, Ca- and Fe-rich spinel crystals were observed for the first time in a black glaze of Yaozhou wares.

Multigram‐scale HPLC enantioseparation as a rescue pathway for circumventing racemization problem during enantioselective synthesis of ethyl 3,4‐dihydro‐2H‐1,4‐benzoxazine‐2‐carboxylate

Racemic ethyl 3,4-dihydro-2H-1,4-benzoxazine-2-carboxylate is a key synthon for the design of promising therapeutic drugs. It is mainly synthesized from racemic ethyl 2,3-dibromopropionate and 2-aminophenol in presence of K2 CO3 in refluxed acetone. Unfortunately, synthesis of (R)- and (S)-enantiomers using the enantioselective version of this reaction, which should normally be performed with a double SN 2 mechanism, is unsuitable due to a racemization process involving the dehydrobromination of enantiopure ethyl 2,3-dibromopropionate into ethyl 2-bromoacrylate. For the first time, the enantioselective version is studied (ee ≈ 55-66%), and the percentage of racemization process has estimated to around 34-46% after determination of the optimal experimental conditions for analytical HPLC enantioseparation of racemate. The influence of the experimental and purification conditions on the racemization rate is also studied. The results indicate that racemization occurs faster at the beginning of the reaction but the initiation of the double SN 2 process takes place more faster to limit the racemization rate. The study of the influence of experimental conditions (reaction times, temperature, solvent or type of base, etc.) on the degree of racemization of the (R)- enantiomer is performed and shows that despite changes in the experimental conditions, the synthesis of the (R)- enantiomer is always accompanied by a racemization rate which is difficult in reducing. In parallel, (R)- and (S)-enantiomers are obtained in high enantiopurity (ee > 99.5%) by preparative HPLC enantioseparation of racemate on multigram scale and characterized by optical rotation measurements, ECD and UV spectra.

A new and facile route to prepare gold nanoparticle clusters on anodic aluminium oxide as a SERS substrate

In the present study, a new SERS-active gold nanoparticle clusters having a flower-shape have been prepared easily on nano porous anodic aluminium oxide (AAO) by immersing it in auric chloride solution without any need for complex production steps. In this process, presented for the first time, the metallic aluminum which were released under the influence of chloride ions due to pitting corrosion act as a reducing agent, while gold ions were reduced onto the AAO layer based on the difference in standard reduction potentials between aluminum and gold. Gold nanoparticle clusters on AAO layer formed “hot spots” providing enhanced Raman signal. Optical microscope, SEM, EDX, AFM, and UV–vis spectrophotometer have been used to characterize the substrate. In order to demonstrate applicability of the method, label free SERS measurements of nitrate ion was performed on the proposed sensing platform. A high sensitivity with 1.03 ppm of limit of detection level and the enhancement factor of 2.9 × 105 were obtained for nitrate ion. In addition, remarkable recoveries ranging from 98.4% to 106.8% were obtained for nitrate spiked into drinking water samples. The inter-day and intra-day precisions of the method as relative standard deviation (RSD) were determined as 3.3% and 5.2%, respectively. The sensor platform, developed using a facile method and a low-cost base material (aluminum), can be a good alternative for SERS based sensing applications.

Synthesis, characterization, Electrostatic Molecular Potential, H-bond Energy, spectroscopic properties and phase transitions of NH3(CH2)2NH3SnCl6

The β-NH3(CH2)2NH3SnCl6 compound is a new crystal belonging to the large hybrid organic-inorganic compounds family, which has been synthesized by the solvent evaporation method and characterized by single crystal. It crystallizes in the zero-dimensional (3-D) triclinic P1¯ space group. The cell dimensions are: a = 7.038(1) Å, b = 7.112(6) Å, c = 26.412(3) Å, α=87.70(2) °, β=85.96(1) °, γ=60.28(3) °. The crystal structure was determined. It is described by a deformed lattice and the hydrogen bonds N–H···Cl cause a weak layer type solid. The cohesion and stability of the atomic arrangement result from the establishment of hydrogen bonds between (C2H10N2)2+ cations and isolated (SnCl6)2– anions. The NMR spectra show the presence of two organic groups C2H10N2, and three types of tin atom in the structure which is in agreement with the XRD structure. Furthermore, the room temperature Raman spectrum of the title compound was analyzed on the basis of data found in the literature. Approximately linear relations have been found between X-H···Y distances and the X–H stretching frequencies in the systems N–H···Cl. The optical proprieties of the crystal were studied using optical absorption UV–visible, which were investigated at room temperature exhibited one band at 321 nm. The results show that this compound exhibits three phases transitions at 123 K, 164 K and 264 K which was characterized by, differential scanning calorimetry (DSC), X-rays powder diffraction (DRX) and Raman spectroscopy. The Raman spectra of this compound were recorded from ambient temperature through the phase's transitions at 123 K, 165 K and 264 K. The observed phase transitions may be of the order–disorder type and probably of first-order character.

An Investigation of the Thermal and Dielectric Properties of Nickel Sulfate Hexahydrate Single Crystal for Sensors, Bandpass Filters and Optical Applications

In the present work, a good-quality nickel sulfate hexahydrate (NSH) single crystal measuring 30 × 20 × 15 mm3 was grown via a slow evaporation solution technique (SEST). NSH single crystal is an inorganic material and is widely used in optical applications, UV sensors, and missile detection systems, among others. The grown crystal was characterized using powder x-ray diffraction (PXRD), UV-visible spectroscopy, microhardness test and etching analysis. Photoacoustic spectroscopy (PAS) was used to identify the thermal diffusivity of the grown crystal. The thermal diffusivity of the NSH crystal was found to be superior to that of various common nonlinear optical (NLO) materials, and is thus suitable for use in optical and sensor applications. Differential scanning calorimetry (DSC) was utilized to determine the material decomposition at a certain temperature. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed to identify the melting point of the grown crystal. The dielectric analysis showed that the crystal has low dielectric loss, fewer defects, and better optical properties, and is suitable for NLO applications. Thermal analysis showed that the grown crystal can be utilized for a wide array of applications including optical communications, UV bandpass filters, UV sensors, harmonic generation, and missile detection systems.

Structural and optical modifications of CdS properties in CdS-Au thin films prepared by CBD

In this work highlights the modifications of the crystalline structure and properties of the nanostructured cadmium sulphide (CdS) thin films, including gold quantum dots grown to form a hybrid metal-semiconductor system. By means of chemical bath deposition (CBD) good quality thin films are obtained, with the advantage of doping in-synthesis with no required additional steps. Film thicknesses around 100 nm were determined by ellipsometry. The binding energies of elements in the CdS and CdS-Au samples were measured by X-ray photoelectronic spectroscopy (XPS). The materials crystalline structure was studied by X-ray diffraction (XRD). A change from disperse like-single-crystalline (highly oriented) CdS NP’s films to monodisperse CdS NP’s–Au QD’s like-single-crystalline samples was observed. This behaviour was confirmed by TEM micrographs. The samples properties pass from an intermediate quantum confinement regime (IQCR) in CdS to a strong one (SQCR) in CdS-Au. The Raman vibrational spectra allowed the analysis of the phonon emissions in CdS, where the Raman shift gives structural information and confirms the effects of IQCR in CdS and the SQCR in CdS-Au. The optical UV–Vis characterization describes the effect of the structural modifications, with an optical band gap shift at higher energy in the CdS-Au sample related to the SQCR. Photoluminescence (PL) measured at room-temperature shows a decrease of PL intensity in CdS-Au film due to via defects-recombination, with respect to that in CdS. This fact is interpreted as an effect of a decreasing of surface defects due to passivation by the Au presence.

The influence of Cu2ZnSnS4 thin films with characteristics of treatment conditions on spray pyrolysis technique for solar cells applications

Generally, thin-film solar cells are used in CdTe or CIGS based on light-absorbing materials that it is non-toxic and cost. It is best on the materials for CZTS cost-effective, the high absorption coefficient of over 104 cm−1. CZTS based on electrical (light) conversion efficiency of 0.6% of the solar cell is an important growth in this research area was reached in the last five years. Cu2ZnSnS4 thin films have been prepared by spray pyrolysis technique for different substrate temperatures of 300 °C, 350 °C, 375 °C, 400 °C, and 450 °C with varying precursor concentrations. The prepared Cu2ZnSnS4 thin films for various temperatures were characterized for various parameters like its structure using X-ray diffraction (XRD), phase constituents with energy dispersive analysis (EDS), morphology by scanning electron microscope (SEM), and optical properties UV–vis spectrophotometry technique. The XRD and EDS results are embarked as a peak at a 2θ position of 28.5° corresponding to (112) planes. The excellent polycrystalline kieserite structure is obtained from XRD for the Cu2ZnSnS4 films. The organized harvests also exposed the morphology with size ranging from the formation of smooth, compact, and even surface of CZTS. The composition of the Cu2ZnSnS4 thin film fundamentally agrees with the theoretical value. The absorption coefficient in the order of 104 cm−1 is obtained. The experimental bandgap of 1.5 eV corresponds well with the substance Cu2ZnSnS4 materials analysis of SEM, and EDAX is making them excellent aspirants as absorbent materials for the thermal solar cell process.

О возможности применения фторидных неодим-активированных кристаллов в качестве светоуправляемого затворв для излучения ArF-лазера (193 nm)

Сообщено об оптических свойствах кристаллов фторидов KY3F10:Nd3+, Na3Y6F22:Nd3+, SrF2:Nd3+ и CaF2:Nd3+ в УФ и ВУФ областях спектра. Спектры поглощения из основного (4I9/2) и из возбужденного (4F3/2) состояний зарегистрированы в диапазоне длин волн от 190 до 240 nm. Спектры поглощения из возбужденного состояния показывают существенное приращение коэффициентов поглощения исследованных образцов в области длин волн 193 nm при накачке состояния 4F3/2 ионов Nd3+ ИК излучением. Полученные результаты демонстрируют возможность применения этих кристаллов в качестве оптических затворов для УФ и ВУФ диапазона, управляемых внешним лазерным ИК излучением, например для излучения эксимерного ArF-лазера с длиной волны 193 nm, которое начало применяться в фотолитографии. Ключевые слова: фторидный кристалл, ионы неодима, ВУФ, УФ, спектр поглощения, поглощение из возбужденного состояния, оптический затвор, 193 nm, эксимерный ArF-лазер.

Coordination Polymers Integrating Metalloimmunology with Immune Modulation to Elicit Robust Cancer Chemoimmunotherapy

Nanoscale coordination polymers (NCPs) constructed by metal ions and organic ligands via metalligand bonds have attracted great attention for their biomedical application. Herein, a new type of NCP...

Hierarchical amphiphilic high‐efficiency oil–water separation membranes from fermentation derived cellulose and recycled polystyrene

AbstractA high‐efficiency separation of oil and water can be achieved by using specially designed amphiphilic porous membrane. However, the preparation of such membranes often involves complex multistep chemical processes. Herein, we report an amphiphilic composite membrane (polystyrene [PS]/bacterial cellulose [BC] membrane) consisting of hydrophobic recycled PS and hydrophilic BC, fabricated by a facile in situ fermentation process. Not only these membranes exhibit a combination of contrasting wettability but also comprise of a hierarchical network of microfibers and nanofibers, which makes them ideal for oil–water separation. The structural and morphological properties of as‐produced BC, recycled PS membrane, and PS/BC composite membrane were studied by Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. The ability of the membranes to separate oil and water was tested by using an emulsion of hexane‐in‐water as the feed and the collected filtrates were characterized by optical microscopy and UV–Vis spectroscopy. PS membranes were unable to separate oil and water, while the PS/BC membrane efficiently separated water from the emulsion. PS/BC composite membranes showed a high water recovery of more than 90%, against only 57% recovery shown by BC. Mechanisms of oil–water separation for each membrane are discussed. The reusability of the PS/BC composite membrane was also demonstrated.

Lifting the curtain: The Seyfert galaxy Mrk 335 emerges from deep low-state in a sequence of rapid flare events

Context. The narrow-line Seyfert 1 galaxy Mrk 335 was one of the X-ray brightest active galactic nuclei, but it has systematically faded since 2007. Aims. We report the discovery with Swift of a sequence of bright and rapid X-ray flare events that reveal the emergence of Mrk 335 from its ultra-deep multiyear low state. Methods. Results are based on our dedicated multiyear monitoring of Mrk 335 with Swift. Results. Unlike other bright active galactic nuclei, the optical–UV is generally not correlated with the X-rays in Mrk 335 on a timescale of days to months. This fact either implies the absence of a direct link between the two emission components; or else implies that the observed X-rays are significantly affected by (dust-free) absorption along our line of sight. The UV and optical, however, are closely correlated at the 99.99% confidence level. The UV is leading the optical by Δt = 1.5 ± 1.5 d. The Swift X-ray spectrum shows strong deviations from a single power law in all brightness states of the outbursts, indicating that significant absorption or reprocessing is taking place. Mrk 335 displays a softer-when-brighter variability pattern at intermediate X-ray count rates, which has been seen in our Swift data since 2007 (based on a total of 590 observations). This pattern breaks down at the highest and lowest count rates. Conclusions. We interpret the 2020 brightening of Mrk 335 as a decrease in column density and covering factor of a partial-covering absorber along our line of sight in the form of a clumpy accretion-disk wind that reveals an increasing portion of the intrinsic emission of Mrk 335 from the disk and/or corona region, while the optical emission-line regions receive a less variable spectral energy distribution. This then also explains why Mrk 335 was never seen to change its optical Seyfert type (not “changing look”) despite its factor ∼50 X-ray variability with Swift.

Growth and Characterization of Single Crystals of l-Histidine Hydrochloride Monohydrate for Nonlinear Optical Applications

In the present study, we have focused on the growth of semi-organic single crystals, as they play a vital role in the generation of a terahertz pulse and its potential applications. The single crystals of l-histidine hydrochloride monohydrate (LMHCL) were grown by slow evaporation solution growth by using deionized water as a solvent in a controlled atmosphere. Good quality crystals of the required size were obtained within 2 weeks. To estimate the lattice dimensions and get the structural information, powder x-ray diffraction (PXRD) study was performed in which we have found that the crystal belongs to the orthorhombic crystal system with space group P212121. The functional groups and the corresponding vibrational mode were confirmed using Fourier transform infrared (FTIR) and Raman spectroscopy, respectively. To study the optical properties UV–Vis transmission spectrum and photoluminescence (PL) were recorded. It was observed that the single crystal has a high value of transmission over a long range of wavelength which signifies that the crystal is a good candidate for nonlinear optical (NLO) applications. The UV cut-off wavelength is found to be 236 nm. The grown single crystals were studied by time-domain terahertz spectroscopy (THz-TDS) for photonic applications and the refractive indices were calculated and it is found that the refractive index is nearly equal to 3.4.

Tracking the year-to-year variation in the spectral energy distribution of the narrow-line Seyfert 1 galaxy Mrk 335

ABSTRACT Multiwavelength monitoring of Mrk 335 with Swift between 2007 and 2019 are used to construct annual spectral energy distributions (SEDs) and track year-to-year changes. Non-contemporaneous archival data prior to 2007 are used to build a bright state SED. In this work, the changes are examined and quantified to build the foundation for future SED modelling. The yearly SEDs trace a downward trend on the average, with the X-ray portion varying significantly and acquiring further lower values in the past two years when compared to the optical/UV portion of SED. The bolometric Eddington ratios derived using optical/UV to X-ray SEDs and the calculated X-ray luminosities show a gradual decrease over the monitoring period. Changes in the parameters over time are examined. Principal component analysis suggests that the primary variability is in the X-ray properties of Mrk 335. When looking at the broader picture of Mrk 335 and its behaviour, the X-rays, accounting most of the variability in the 13-yr data, are possibly driven by physical processes related to the corona or absorption whereas the modest optical–UV variations suggest their origin within the accretion disc. These results are consistent with the previous interpretation of Mrk 335 using the timing analyses on the monitoring data and spectral modelling of deep observations.

Spiropyran‐Functionalized Polymer–Carbon Nanotube Hybrids for Dynamic Optical Memory Devices and UV Sensors

AbstractNanohybrids of purified semiconducting single‐walled carbon nanotubes and conjugated polymers with attached spiropyran moieties are created as photoresponsive materials for optical memory devices and ultraviolet (UV) light sensors. The hybrids respond to UV light exposure in air with photoisomerization of the spiropyran groups to merocyanine, resulting in significant and persistent p‐doping of the nanotubes, as confirmed by photoluminescence quenching and trion emission. The increased carrier concentration after illumination and the inherently high mobility of carbon nanotubes enable an up to two orders of magnitude increased conductivity of the nanotube/polymer hybrid networks in simple two‐terminal devices operated at very low bias. The photodoping decays very slowly in the dark and increases further with every illumination dose. However, it can be completely reversed by heating for a few seconds. Thin film devices based on these photoresponsive polymer/nanotube hybrids show great potential as simple and resettable UV dosimeters and optical memory elements.

Correlation between optical and γ-ray flux variations in bright flat spectrum radio quasars

ABSTRACT Blazars are known to show flux variations over a range of energies from low-energy radio to high-energy γ-rays. Cross-correlation analysis of the optical and γ-ray light curves in blazars shows that flux variations are generally correlated in both bands, however, there are exceptions. We explored this optical–GeV connection in four flat spectrum radio quasars by a systematic investigation of their long-term optical and γ-ray light curves. On analysis of the four sources, namely 3C 273, 3C 279, PKS 1510−089, and CTA 102, we noticed different behaviours between the optical and GeV flux variations. We found instances when (i) the optical and GeV flux variations are closely correlated, (ii) there are optical flares without γ-ray counterparts, and (iii) γ-ray flares without optical counterparts. To understand these diverse behaviours, we carried out broad-band spectral energy distribution (SED) modelling of the sources at different epochs using a one-zone leptonic emission model. The optical–UV emission is found to be dominated by emission from the accretion disc in the sources PKS 1510−089, CTA 102, and 3C 273, while in 3C 279, the synchrotron radiation from the jet dominates the optical–UV emission. Our SED analysis indicates that (i) correlated optical and γ-ray flux variations are caused by changes in the bulk Lorentz factor (Γ), (ii) γ-ray flares without optical counterparts are due to increase in Γ and/or the electron energy density, and (iii) an optical flare without γ-ray counterpart is due to increase in the magnetic field strength.