Though various two-dimensional (2D) materials have been identified as useful candidates for piezoresistive pressure sensors, layered vanadium pentoxide (V2O5) has been negligibly explored. Here, simple drop cast films of ultrasonicated V2O5 on copper (Cu) substrates, stacked face-to-face, are demonstrated to exhibit remarkable piezoresistive pressure sensing behaviour over a pressure range of 250-2000 Pa, as impacted by the growth of copper oxide (CuO) at V2O5/Cu interface. Significant and accelerated growth of CuO at the interface, in ambient conditions, was observed with the addition of deionized (DI) water in the solvent used for drop casting V2O5 films, while it was negligible for those deposited without DI water. The strong growth of CuO interface layer in the former, attributed to the enhanced interaction between V2O5 and Cu in the acidic medium provided by DI water, was found to induce microcracks in V2O5 films. These microcracks could be causing reduced sensitivity of the pressure sensors (similar to 0.36 kPa(-1)) with these films, compared to the sensors with films deposited without DI water (similar to 2.57 kPa(-1)). The results suggest the tunable pressure sensing performance of V2O5 films owing to their readiness to transfer electrons with other materials. In-situ impedance spectroscopic measurements under an applied pressure are also demonstrated, which revealed the major contribution of V2O5/V2O5 interface on sensor response, thereby offering insight into the sensing mechanism. Additionally, V2O5 films are demonstrated as flexible pressure/strain sensors for detecting human physiological activities like finger bending, blinking, swallowing etc.

Concomitant achievement of all three performance pillars of a supercapacitor device, namely gravimetric, areal, and volumetric capacitance is a grand challenge. Nevertheless, its fulfilment is indispensable for commercial usage. Although, high compactness is the fundamental requirement to achieve high volumetric performance, it severely affects ion transportation in thick electrodes. Such trade-off makes it extremely challenging to realize very high areal and volumetric performance simultaneously. Here, a collapsed hydrogel strategy is introduced to develop MXene/cellulose nanofiber (CNF) based densified electrodes that offer excellent ion transportation despite a massive increase in areal mass loading (>70 mg cm(-2)). Quasi-oriented MXene/CNF (MXCF) hydrogels are produced through an electric field-guided co-assembly technique. Ambient dehydration of these hydrogels incorporates numerous pores in the resultant compact electrodes due to crumpling of the MXene sheets, while CNF ensures connectivity among the locally blocked pores in different length scales. The resultant collapsed MXCF densified electrode shows a remarkably high areal capacitance of 16 F cm(-2) while simultaneously displaying a high volumetric capacitance of 849.8 F cm(-3 )at an ultrahigh mass loading of up to 73.4 mg cm(-2). The universality of strategy, including the co-assembly of hydrogel and its collapse, is further demonstrated to develop high-performance asymmetric and wearable devices.

Magnetic properties of Y0.67Sr0.33MnO3 (YSMO) and La0.67Sr0.33MnO3 (LSMO) thin films heterostructures (YSMO/LSMO) deposited on single-crystalline (001) SrTiO3 (STO), (001) LaAlO3 (LAO) and MgO substrates using pulsed laser deposition technique were investigated. Magnetization (M) vs. the applied field (H) measurements revealed shift in hysteresis loop showing the exchange bias effect of about 80 Oe for the heterostructures on LAO substrate, which was absent for those on STO substrates. In order to investigate this, single-layer LSMO layers were deposited on these substrates. X-ray diffraction patterns revealed significant compressive strain and a weaker tensile strain for LSMO films on LAO and STO substrates, respectively. Raman spectroscopy measurements further confirmed the lattice mismatch-induced strain in LSMO layer on LAO substrate. These different strain conditions in LSMO films on both the substrates are suggested to be contributing to the emergence of different magnetic ground states in these systems, which could be leading to pinning of spins at the interface and resulting in a significant exchange bias effect (EBE) on LAO substrate.

Rhenium sulfide (ReS2) is an exciting two-dimensional (2D) material employed in various optoelectronic applications due to panchromatic visible absorption properties. However, unlike other transition-metal dichalcogenides (TMDs) such as Mo(S/Se)2 and W(S/Se)2, ReS2 has not been explored for solar energy conversion applications. In this work, ReS2 nanolayers are employed as photoabsorber in both liquid and solid-state dye-sensitized type solar cells, in combination with TiO2 and SnO2 as electron transporting layers (ETLs). Various devices with compact/mesoporous TiO2 and mesoporous SnO2 ETLs were tested for photovoltaic performance with spiro-OMeTAD hole conductor as well as with different redox mediators. While no photovoltaic response was seen for mesoporous TiO2 (both liquid and solid-state), a clear photovoltaic performance was observed for mesoporous SnO2 solid-state devices. This was attributed to the energy level alignments unsuitable for charge carrier injection from ReS2 to ETL in the former and suitable in the latter cases. In the case of only compact layer TiO2 ETL, devices in both solid and liquid-state configurations exhibited photovoltaic response with latter showing higher photocurrent than the former, due to additional role played by the energy level of redox mediator. While impedance measurements revealed limitations in conductivity of ReS2 photoabsorber, presence of photovoltaic response in them indicate the significant role played by the energy level alignment of individual components. The presence of photovoltaic effect for compact layer TiO2 devices, in spite of unsuitable energy level alignment seen for TiO2, is attributed to the charge carrier injection assisted by the trap states.

Integration of pseudocapacitive nanomaterials within graphene based 3D-hydrogel network has shown suitable synergist in rate performance energy storage, but implementing the same with MXene through conventional heat involved processing is challenging due to its oxidation prone surface. Herein, we present a purely room temperature casting based approach to develop 3D-hydrogel hybrids of MXene and graphene (MGH) via metallic zinc particles induced spontaneous gelation, avoiding oxidation possibility. MGH was used as supercapacitor electrode that exhibits high mass specific capacitance of 357 F g(-1) at 10 mV s(-1) , and excellent capacity retention of 95.6% after 10000 charge-discharge cycles. MGH was used as negative electrode to develop asymmetric supercapacitor, in combination with polyaniline (PANI)-graphene hybrid hydrogel (PGH) as positive electrode, that delivers a maximum energy density of 30.3 Wh kg(-1) and a power density of 1.13 kW kg(-1) with excellent capacity retention over 10000 cycles. In comparison to compact electrodes where pseudocapacitive materials cannot display their faradic activity with full potential, the hydrated porous network of MXene-graphene hydrogels with continuous channels permit the electrolyte ions to efficiently access MXene and PANI, thereby displaying high gravimetric and rate performance. MXene-graphene hydrogels, developed via this facile and cost effective protocol, are also attractive candidate for wide application areas that requires 3D porous structure. (C) 2021 Elsevier Ltd. All rights reserved.

Nanocomposites of PEDOT:PSS with V2O5 nanoparticles are synthesized by simple physical mixing of the two with different weight percentages of the latter and their performance as supercapacitor electrode materials is verified. Best performance is obtained for an optimum weight percent of 16.8% of V2O5. The specific capacitance and specific energy of the composite with 16.8% V2O5 increases by more than two fold, with increase in specific power, as compared to that of pristine PEDOT:PSS device. This is attributed to increase in conductivity brought about by the presence of V2O5 nanoparticles, easier transportation and intimate contact of electrolyte ions with the nanolayers of V2O5 due to the intercalation of PEDOT:PSS between the layers, and additional redox reactions due to various oxidation states of vanadium element, besides redox electrolyte effects. This is further confirmed by the reduced ESR of the composite device as compared to that of pristine PEDOT:PSS device.

Reduced graphene oxide (rGO) assisted two-dimensional (2D) graphitic ZnO (gZnO) have been prepared and characterized by Raman, FTIR, Cross-sectional high resolution transmission electron microscopy (HRTEM) and X-ray absorption spectroscopy (XAS) techniques comprising of X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Raman spectroscopy and FTIR measurements suggest formation of ZnO and the same has been corroborated by XANES measurements. HRTEM and polarization dependent XANES measurements with two different sample orientations confirm the formation of gZnO/rGO sandwiched layers. Density Functional Theory simulated relaxed initial structure of bilayers of gZnO sandwiched between successive rGO layers fits the EXAFS data reasonably well, supported by formation energy calculations which indicate better stability for samples having bilayers to a few layers of gZnO between rGO layers than with monolayer. These relaxed structures also help to understand the electronic structure contributing in XANES spectrum, with the help of ligand field theory.

Charge transfer interaction at the gold (Au)/copper phthalocyanine (CuPc) interface, with Au on CuPc film, was investigated using vibrational techniques - Fourier Transform Infra Red and Raman spectroscopy, in combination with X- ray photoelectron spectroscopy. The charge transfer direction at this interface was found to be changing after annealing of the CuPc film, as compared to that before annealing. Molecule to metal charge transfer at the Au/CuPc interface observed for the as deposited film was found to be reversing to metal to molecule charge transfer for the annealed film. This reversal of charge transfer direction in the latter was attributed to the modified electronic structure/work function of the CuPc film and the varying orientation of CuPc molecules at the interface, brought about by the increased surface roughness, change in morphology, etc. The reversal in charge transfer direction was suggested to be supporting the reduction in parasitic contact resistance at the Au/CuPc interface for a top contact OFET with the annealed CuPc film, as compared to that with the as deposited film. This study demonstrates the influence of morphology of the organic semiconductor film on the charge transfer behaviour at the metal/organic semiconductor interface.

Gravimetric and volumetric capacitance are the two important matrices for any supercapacitor device, but only moderate achievement has been realized till date on the first figure-of-merit for MXene based electrodes. In this study, aided by a small quantity (5 wt%) of graphene, an interfacial layer-by-layer self-assembly of MXene has been employed to develop hydrogel based supercapacitor electrodes that display excellent gravimetric capacitance of 653.7 F g(-1) at 2 mV s(-1) in three electrode setup. Very high areal (1442 mF cm(-2) ) and volumetric capacitance (similar to 1522 F cm(-3)) were also achieved by simple tuning of respective conditions like gelation time and electrolyte protected mechanical compression. Considering the fact that MXene is prone to oxidation at elevated gelation temperatures, this purely ambient gelation protocol is established to protect its redox active titanium sites. MXene-graphene hybrid hydrogel structures consisting of network of 2D-sheets can allow full utilization of metallic surfaces and redox active sites on MXene basal plane, and thereby display outstanding performance metrics. These hybrid hydrogels also show excellent cyclic stability even with commercial scale mass loading which can pave the way to their application in practical devices. (C) 2020 Elsevier Ltd. All rights reserved.

A few layer/multilayer two-dimensional (2D) Tellurium (Te) nanoflakes were mechanically exfoliated and their structural and vibrational properties were probed. Layer thickness reduction was confirmed using optical microscopy, atomic force microscopy and kelvin probe force microscopy. Raman spectroscopy measurements revealed anomalous shift in the frequencies of their intrachain vibrational modes, where, after an initial increasing trend (blue shift) with thickness reduction from bulk, they changed to decreasing trend (red shift) on further layer thinning. Such unusual vibrational behaviour of 2D Te, exhibiting reversal from blue shift to red shift, is attributed mainly to the deformations occurring in the Te chains on layer thinning, followed by modifications in the local structures and bondlengths of the corresponding Te atoms. The deformation under relaxed conditions are also confirmed from first principle calculations. Red shift can also be correlated to lesser hole concentration for thinner flakes, as revealed by work function changes measured on the nanoflakes surfaces. Interestingly, in spite of such anomalous behavior, the difference in frequencies of both modes showed initial increase with thickness and saturation beyond a certain thickness, as generally observed in other 2D materials. Temperature dependent Raman spectroscopic measurements further confirmed anomalous behaviour for thinner flakes.

Electron–phonon interaction in bulk and nanoflakes of MoS2 is investigated using Raman spectroscopy. Resonant Raman spectroscopic studies carried out on bulk and liquid exfoliated nanoflakes of MoS2 revealed a second order Raman mode (called the “b” mode), whose frequency in the case of nanoflakes was found to be largely different from that in bulk MoS2. Temperature dependent Raman spectra show larger variation in the frequency of the “b” mode in bulk MoS2 as compared to that in nanoflakes of MoS2. This anomalous behavior of the “b” mode could be attributed to the stronger electron–phonon coupling occurring in bulk MoS2, due to higher electron concentration in the same, as compared to that in nanoflakes of MoS2. A larger sulfur vacancy in bulk MoS2 as compared to that of nanoflakes was found to be responsible for higher electron concentrations. These findings are supported by energy dispersive x-ray analysis and x-ray photoelectron spectroscopic studies carried out on bulk and nanoflakes of MoS2. The present study suggests a more sensitive probe for the estimation of electron concentrations in the low limit range by following the “b” mode in resonance Raman spectra.

Investigations on the non-covalent, charge transfer interaction between graphene oxide (GO) sheets and un-substituted copper phthalocyanine (CuPc) revealed their mutual influence on their structural, optical and magnetic properties. While charge transfer interaction is found to change the aggregation of CuPc from J to H state, and this change increasing with GO concentration, the aggregation/stacking of GO is also found to be getting modified (increasing in this study) on interaction with CuPc. The charge transfer interaction is suggested to be mostly occurring through carboxyl groups at the edges of GO sheets, which could be causing this stacking/aggregation change of GO. While GO, CuPc and the nanocomposites exhibited paramagnetic behaviour, the saturation magnetization of the nanocomposites is observed to be lower than that expected. This is correlated to the reduction in the total spin (J) of GO and its subsequent altered magnetic response, which is attributed to the charge transfer interaction with CuPc. This study shows the possibility of tuning the magnetic properties of GO through such non-covalent, charge transfer routes. The improved optical limiting behaviour of the nanocomposites is further attributed to the charge transfer interaction and the increase in stacking of GO.

Heterostructures of MoS2 and ReS2 nanocrystals were prepared in the solution phase by synthesizing the nanocomposite of these two transition metal dichalocogenides (TMDs). The heterostructure of the bulk nanocrystals of these two TMDs is shown to be forming type II heterojunction, favouring n-n junction like characteristics, exhibiting donor-acceptor like behavior. Raman spectroscopy, XPS, UV-Vis absorption measurements and EDX analysis revealed interlayer coupling, charge transfer interaction between the two TMDs and creation of sulfur vacancies in the nanocomposites. Presence of sulfur vacancies is suggested to further reduce the electron injection barrier at the ITO/MoS2 interface, favouring flow of larger current in the forward bias direction. This conclusion was supported by the electrical characteristics of the drop cast films of the nanocomposite revealing diode characteristics with the rectification ratio of about 25. The ideality factor estimated was slightly higher which could be due to defect and trap states at the interface between the two TMDs. The study shows that by simple solution phase route of the bulk like nanocrystals, diode behavior can be obtained that can be useful for bulk heterojunction solar cell applications.

Deaggregated perylenediimide (PDI) derivatives exhibit exceptionally high quantum yields, photostability and appropriate molecular features for organic electronics. This work demonstrates a metal-dye-metal framework with a large and stable negative differential resistance (NDR) at ambient conditions, built using a supramolecular strategy. The deaggregation achieved through the encapsulation of the bay-substituted phenyl groups of aggregated (l/d)-Phe-PDI dyes by the beta-CD macrocyclic host is validated through detailed spectroscopic and imaging techniques. The host-guest interaction resulted in a dramatic enhancement in the emission yield from 0.28 to 0.90. In the thin film deposits, the beta-CD/(l/d)-Phe-PDI complex displayed well-connected sheet-like morphology, whereas the uncomplexed (l/d)-Phe-PDI dye remained as scattered lumps. The large and reversible I-V characteristics displaying strong NDR behavior is attributed to the oxidation/reduction processes involving the rigid pi-rich PDI core and is stable at least for about six months at ambient conditions, a promising system for organic electronics applications.

The graphene based supercapacitors with PEDOT:PSS, a conducting polymer as the binder, were fabricated on gold coated stainless steel sheets. The performance of the devices with graphene+PEDOT:PSS composite was found to be better than that of pristine graphene, indicating that PEDOT:PSS is aiding in the conductivity of the active material, unlike other insulating polymer binders. Coin cell supercapacitors with specific capacitance of about 15 F/g were fabricated. Fast charging and discharging and high specific capacitance retention of about 90 % upto 40 cycles were observed. Turning ON of red and green LEDs could be demonstrated using two coin cells connected in series.

The simple in-plane electrical measurements of tellurium (Te) thin films, of thicknesses 8,10, 15 and 20 nm on SiO2/Si substrates, revealed strong switching characteristics at voltages less than 2V. The films exhibited clear read-write-erase (rewritable) bipolar memory behavior. The maximum ON/OFF ratio of about 10(4) to 10(5) was obtained for 10 and 15 nm thick films. Surface morphology of the thin films, investigated using scanning electron microscopy (SEM), revealed hexagonal structures with grain boundaries. These grain boundaries were suggested to be causing charge carrier trapping in Te films and hence the switching characteristics. Stable cyclic switching and good retention characteristics were demonstrated.

A hybrid solar cell consisting of nanostructured p-type porous silicon (PS) deposited with visible light absorbing dye, Copper Phthalocyanine (CuPc) has been prepared in the photoelectrochemical cell configuration. P-type PS with (100) and (111) orientations which have different porous structures were used for studying the effects of the substrate morphology on the cell efficiency. Heterostructures were prepared by depositing three different thicknesses of CuPc for optimizing the cell efficiency. Structural and surface characterizations were studied using XRD, Raman, SEM and AFM on the PS-CuPc heterostructure. XRD spectrum on both plane silicon and porous silicon indicates the pi-pi stacking of CuPc with increased disorder for CuPc film on porous silicon. Electrochemical characterizations under sun light type radiation have been carried out to evaluate the photosensitivity of the heterostructure. Between the two different substrates, (100) PS gives better photocurrent, possibly due to the higher surface area and lower series resistance of the structure. Among the (100) PS substrates, (100) PS with 15 nm CuPc film gives V-oc more than 1 V resulting in higher efficiency for the cell. The study suggests the scope for optimization of solar cell efficiency using various combinations of the substrate structure and thickness of the sensitizing layer. (c) 2017 Elsevier B.V. All rights reserved.

Surface-enhanced infrared absorption (SEIRA) studies of cytosine adsorbed on the thermally evaporated gold film on CaF2 have been carried out in transmission mode. SEIRA spectrum down to 0.1 mu M was observed owing to the plasmonic effect of the gold nano film. Cytosine molecules appear to adsorb on the film via C=O and NH groups as evidenced by the red shift observed in the stretching vibrations of the above groups. The molecules assume a perpendicular orientation with respect to the surface.

The influence of the top contact electrode on the switching characteristics of a low operating voltage organic bistable memory device, using copper phthalocyanine and gold nanoparticle thin films, was investigated using Au, Al, and Hg electrodes. While the ON/OFF ratio higher than 105 was achieved for all the devices, the nature of the memory behavior was found to be dependent on the top electrodes. Thermally evaporated Au and Al electrodes resulted in write-once read-many times (WORM) behavior, whereas Hg drop soft contact led to write read erase read (rewritable) characteristics, with the device retaining the ON state in the former and returning to the OFF state in the latter. The switching voltage was found to be influenced by the top electrode with the devices switching to the ON state at around 2 V for Hg and close to 1 V for Au and Al electrodes. Additionally, though the ON state conduction mechanism was dominated by Fowler Nordheim (FN) tunneling through AuNP trap states in all the devices, the dynamics of switching was found to be dependent on the top electrode, showing abrupt switching to the ON state for Au and Al electrodes. In contrast, a gradual increase in current at the onset of FN tunneling before switching was observed for devices with Hg electrodes. Such a significant influence of the top electrodes was mainly attributed to the difference in injection barriers between the top electrode/active layer and gold nanoparticle/active layer junctions. Devices exhibit rewritable behavior when the former is higher than the latter, while they change to WORM behavior when the two are equal. The study shows that the same device structure can be tuned to exhibit WORM or rewritable memory behavior by employing the top electrode with the work function in suitable combination with that of the nanoparticles forming trap states in the bulk of the film.

Separation of magic size clusters (MSCs) from monodisperse quantum dots (QDs) has generally been a difficult task while employing the commonly used synthesis procedure, where for preparation of PbSe QDs, Se-TOP (TOP = trioctylphosphine) is injected into Pb-oleate in 1-octadecene medium. In this study, we report for the first time a simple method to prepare MSCs, QDs and particles close to the bulk of PbSe using oleylamine (OAM) as the reducing agent, where the individual entities are efficiently separated. The chemical yield is found to be 95%. Studies on optical properties revealed the absorption and emission peaks of MSCs at fixed positions of 600 and 780 nm, respectively, while QDs exhibit significant shift to longer wavelengths for both the cases, depending on the particle size. Shift of the emission peak position for QDs is observed to be larger for initial stages of the waiting time as compared to those for longer waiting times. This can be attributed to two factors: faster growth in particle size is favoured kinetically in the initial stages, while thermodynamic stability occurs in the later stages, and reduction in surface to core contribution with increase of waiting time. QDs were found to emit at only one particular wavelength while they absorbed at two or more wavelengths. The quantum yields (QYs) of particles of sizes 4.1 and 5.1 nm are found to be 80 and 30%, respectively. The lifetime values are found to be 1.0-1.3 μs for QDs having an emission peak in the range of 1300-1500 nm. The hybrid device of PbSe (5 nm size) and MEHPPV (2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) shows increased conductivity both in the dark and in light, due to absorption in the region of NIR photons in the former and additionally in the visible region in the latter.