Ritu Srivastava

Luminscent Graphene Quantum Dots for Organic Photovoltaic Devices

Recent research in organic photovoltaic (OPV) is largely focused on developing low cost OPV materials such as graphene. However, graphene sheets (GSs) blended conjugated polymers are known to show inferior OPV characteristics as compared to fullerene adduct blended with conjugated polymer. Here, we demonstrate that graphene quantum dots blended with regioregular poly(3-hexylthiophene-2,5-diyl) or poly(2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene) polymer results in a significant improvement in the OPV characteristics as compared to GSs blended conjugated polymers. This work has implications for inexpensive and efficient solar cells as well as organic light emitting diodes.

Synthesis and characterization of 9,10-bis(2-phenyl-1,3,4-oxadiazole) derivatives of anthracene: Efficient n-type emitter for organic light-emitting diodes

With a general aim to make anthracene derivatives multifunctional (n-type emitter) and also study their suitability as electron transport layers for organic light emitting diodes (OLED), and with a more specific interest to understand the charge transport and packing pattern in the solid state due to the rotating side rings, we report the synthesis and characterization of six novel molecules (5–10) in which the 9 and 10 positions of anthracene have been directly substituted by phenyloxadiazole groups. We have carried out detailed studies of these molecules including photophysical, electrochemical, electroluminescent studies and solid state structure determination through crystallographic techniques. The electron affinity is very high, around 3.1–3.2 eV, and the ionization potential is around 5.9–6.0 eV, comparable to the more commonly used electron transport electroluminescent layer Alq3. The studies reveal that the new molecules being reported by us, in addition to the high thermal stability, are quite efficient in a two layer unoptimized device with the device structure ITO/α-NPD/5–10/LiF/Al and have an emission in pure green. They also show very high efficiency as electron transport layer in device structure ITO(120nm)/α-NPD(30nm)/Ir(ppy)3 doped CBP(35nm)/BCP(6nm)/5(28nm)/Al(150nm). From these studies we conclude that the anthracene derivatives also have considerable potential as multifunctional layers and as electron transport layers in OLED.

EPR STUDY OF NANOSTRUCTURED ZINC FERRITE

Bulk zinc ferrite has been studied in the past. In the bulk form zinc ferrite crystallizes in the normal spinel structure with all the zinc ions occupying the tetrahedral and all the iron occupying the octahedral sites, respectively. But, nanosize zinc ferrite is believed to crystallize in a different way. It is now well established that in a nanosize zinc ferrite there is partial inversion in the cation occupancy, leading to some kind of magnetic ordering in this system much above its Neel temperature of 10 K. Hence, there is ample scope of maneuvering the electrical and magnetic properties of this ferrite. In the present study nanosize zinc ferrite was prepared by a chemical reaction involving the nitrates of Zn and Fe and using the citric acid as the host. The sintering of the precursor was done for 1 h at various temperatures ranging from 300°C to 1000°C. These samples were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), and electron paramagnetic resonance (EPR). The average particle sizes in these sintered samples measured by TEM are found to vary from ≈ 10 to 62 nm. The XRD measurements show the formation of single-phase spinel structure in all the samples. The X-band EPR spectrum of the precursor specimen at room temperature consists of two EPR signals: an intense signal at geff = 2.02 having peak-to-peak line width △HPP = 425 Gauss and other a weak signal at geff = 4.51 having an asymmetric line shape. As the samples are heat-treated at different temperatures, the EPR signal corresponding to the intense peak gets stabilized at geff ≈ 2.03, whereas the weak signal gets suppressed progressively. The spin–spin relaxation time in these samples have values ≈ 3 × 10-10s. The effect of sintering temperature on the properties of the nanostructured specimen of zinc ferrite and the underlying mechanism would be discussed.

Enhancement of light extraction efficiency of organic light emitting diodes using nanostructured indium tin oxide

Improved outcoupling efficiency of organic light emitting diodes (OLEDs) is demonstrated by incorporating a nanostructured indium tin oxide (NSITO) film between a conducting anode and a glass substrate. NSITO film was fabricated using rf-sputtering at oblique angle (85°). Significant reduction in refractive index and improved transmission of NSITO film was observed. OLEDs were then fabricated onto NSITO film to extract the ITO-glass waveguided modes. Extraction efficiency was enhanced by 80% without introducing any detrimental effects to operating voltage, current density, and angular invariance of emission spectra of OLEDs.

Engineering fused coumarin dyes: a molecular level understanding of aggregation quenching and tuning electroluminescence via alkyl chain substitution

Simple molecular structures capable of emitting over the entire visible range are still a challenge. Planar molecular structures have the drawback of fluorescence quenching in the solid state thus limiting their application fields. Combining long range excimer/exciplex emissions with a compound emission have been used to get white light. In this work, a series of new coumarin derivatives having a planar structure have been synthesized and characterized. The effects of systematic variation in alkyl chain functionalization providing morphological variations that permit interesting solid state emitting properties have been discussed simultaneously with electrochemical behavior and OLED (organic light emitting diode) device applications. Carbon chains containing 0–16 carbon atoms have been studied in order to conclude the results that systematic changes in alkyl group substitution can be utilized as a tool to tune the emitting color of these planar coumarins. Alkyl chains were introduced by O-acylation and O-benzoylation reaction on the hydroxyl group of parent coumarin 5. Thus the present strategy is also helpful in establishing a template to control the unproductive interchromophore electronic couplings. Solid state fluorescence properties support the crystal studies. Theoretical studies are also in agreement with experimental data. Electroluminescence of Device 2 with a turn on voltage (Von) around 5–6 V having s-CBP doped with 1% of 8 having alkyl substitution of 2-carbons is found to exhibit white emission with CIE co-ordinates of (0.29, 0.34) which is close to white emission while the alkyl substitution of 14-carbons (compound 17) in Device 7 (Von = 7 V) exhibited green emission. Thus a strategy helpful to tune the electroluminescence has been discussed.

Energy transfer process between exciton and surface plasmon: Complete transition from Forster to surface energy transfer

The energy transfer process between surface plasmons and excitons was studied by varying the filling fraction of gold (Au) nano-clusters (NCs) and by placing a spacer of different thickness between Au NC and organic semiconductor layer. The intensity enhancement has occurred for 10%-50% filling fractions and 4-14 nm spacer thicknesses. Energy transfer mechanism was found to switch from Forster type to surface type by increase in filling fraction. Transverse electric field for Au NCs was simulated and we observed that for filling fraction 60%, they behave like 2-dimensional dipoles.

Large Area Fabrication of Semiconducting Phosphorene by Langmuir-Blodgett Assembly

Phosphorene is a recently new member of the family of two dimensional (2D) inorganic materials. Besides its synthesis it is of utmost importance to deposit this material as thin film in a way that represents a general applicability for 2D materials. Although a considerable number of solvent based methodologies have been developed for exfoliating black phosphorus, so far there are no reports on controlled organization of these exfoliated nanosheets on substrates. Here, for the first time to the best of our knowledge, a mixture of N-methyl-2-pyrrolidone and deoxygenated water is employed as a subphase in Langmuir-Blodgett trough for assembling the nanosheets followed by their deposition on substrates and studied its field-effect transistor characteristics. Electron microscopy reveals the presence of densely aligned, crystalline, ultra-thin sheets of pristine phosphorene having lateral dimensions larger than hundred of microns. Furthermore, these assembled nanosheets retain their electronic properties and show a high current modulation of 104 at room temperature in field-effect transistor devices. The proposed technique provides semiconducting phosphorene thin films that are amenable for large area applications.

High yield synthesis of electrolyte heating assisted electrochemically exfoliated graphene for electromagnetic interference shielding applications

Herein, we demonstrate a facile one pot synthesis of graphene nanosheets by electrochemical exfoliation of graphite. In the present study, we report a significant increase in the yield of graphene by electrolyte heating assisted electrochemical exfoliation method. The obtained results of heating assisted electrochemically exfoliated graphene (utilizing H2SO4 + KOH + DW) synthesis clearly exhibit that the yield increases ∼4.5 times i.e. from ∼17% (room temperature) to ∼77% (at 80 °C). A plausible mechanism for the enhanced yield based on lattice expansion and vibration of intercalated ions has been put forward and discussed in details. The quality of graphene was examined by Raman, XPS, FTIR, AFM, SEM, TEM/HRTEM and TGA techniques. The Raman as well as morphogenesis results confirm the quality of the graphene nanosheets. We have used this graphene as electromagnetic interference shielding material where a comparatively large quantity of graphene is required. This graphene exhibits enhanced shielding effectiveness (46 dB at 1 mm thickness of stacked graphene sheets in frequency region 12.4 to 18 GHz) as compared to conventional electromagnetic interference shielding materials, which is greater than the recommended limit (∼30 dB) for techno-commercial applications. Thus the present work is suggestive for future studies on enhancement of yield of high quality graphene by proposed method and the use of synthesized graphene in electromagnetic interference shielding and other possible applications.

Effect of doping of 8-hydroxyquinolinatolithium on electron transport in tris(8-hydroxyquinolinato)aluminum

Effect of doping of 8-hydroxyquinolinatolithium (Liq) on the electron transport properties of tris(8-hydroxyquinolinato)aluminum (Alq3) has been investigated as a function of temperature and doping concentration by fabricating electron only devices. It has been observed that current density in the devices increases with the doping of Liq up to a doping concentration of 33 wt. % and then decreases. Current density-voltage (J-V) characteristics of 0, 15, and 33 wt. % Liq doped Alq3 devices were found to be bulk limited and analyzed on the basis of trap charge limited conduction model. The J-V characteristics of 50 and 100 wt. % Liq doped Alq3 devices were found to be injection limited and were analyzed using the Fowler-Nordheim model. The increase in current density with doping up to 33 wt. % was found to be due to an increase in electron mobility upon doping, whereas the decrease in current density above 33 wt. % was due to the switching of transport mechanism from bulk limited to injection limited type du...

Spatial coherence properties of electroluminescence from Alq3-based organic light emitting diodes

We report the measurement of spatial coherence properties of light emitted by organic electroluminescent devices based on tris-(8-hydroxyquinoline) aluminum. Coherence measurements were performed using Young’s double slit experiment. Fourier-transform technique was used for the measurement of visibility of the interference fringes from which the modulus of the degree of spatial coherence was determined. Experimental results were compared with the theoretical values and it is shown that the coherence properties of the light emitted by a simple organic light emitting device match with those of practical Lambertian sources.