Gundam Sandeep Kumar

Demonstration of Ultrarapid Interfacial Formation of 1D Fullerene Nanorods with Photovoltaic Properties

We demonstrate ultrarapid interfacial formation of one-dimensional (1D) single-crystalline fullerene C60 nanorods at room temperature in 5 s. The nanorods of ∼ 11 μm in length and ∼ 215 nm in diameter are developed in a hexagonal close-pack crystal structure, contrary to the cubic crystal structure of pristine C60. Vibrational and electronic spectroscopy provide strong evidence that the nanorods are a van der Waals solid, as evidenced from the preservation of the electronic structure of the C60 molecules within the rods. Steady state optical spectroscopy reveals a dominance of charge transfer excitonic transitions in the nanorods. A significant enhancement of photogenerated charge carriers is observed in the nanorods in comparison to pristine C60, revealing the effect of shape on the photovoltaic properties. Due to their ultrarapid, large-scale, room-temperature synthesis with single-crystalline structure and excellent optoelectronic properties, the nanorods are expected to be promising for photosensitive devices applications.

Easy extraction of water-soluble graphene quantum dots for light emitting diodes

Graphene quantum dots (GQDs) have attracted a large amount of attention due to their unique optoelectronic properties, which arise from the quantum confinement effect or edge effect or surface functionalization. Popular routes for designing GQDs are based on the reactions in aqueous phases, which are detrimental for integration of GQDs into optoelectronic devices. Hence, a critical challenge remains in utilizing the water-soluble GQDs for fabricating optoelectronic devices. Here we demonstrate for the first time a single step facile route to extract water-soluble GQDs into solid powder under reduced pressure for the fabrication of light emitting diodes (LEDs). The process avoids the need for post synthesis ex situ functionalization or the use of additional polymers to make GQDs hydrophobic retaining the intrinsic luminescent behavior of GQDs. The measured current–voltage characteristics of GQD-LED devices showed a significantly low turn-on voltage of ∼2.5 V. Bias dependent electroluminescence leading to color tunability from blue to deep cyan is demonstrated.

Induced Aggregation of AIE‐Active Mono‐Cyclometalated Ir(III) Complex into Supramolecular Branched Wires for Light‐Emitting Diodes

Aggregation-induced emission (AIE) is commonly observed in irregular bulk form. Herein, unique aggregation properties of an AIE-active complex into branched supramolecular wires are reported for the first time. Mono-cyclometalated Ir(III) complex shows in-plane J-aggregation at the air-water interface owing to the restriction of intramolecular vibration of bidentate phenylpyridinato and intramolecular rotations of monodentate triphenylphosphine ligands at air-water interface. As a consequence, a large enhancement of luminescence comparable to the solid state is obtained from the monolayers of supramolecular wires. This unique feature is utilized for the fabrication of light-emitting diodes with low threshold voltage using supramolecular wires as active layer. This study opens up the need of ordered assembly of AIE complexes to achieve optimal luminescence characteristics.

Shape-controlled cobalt phosphide nanoparticles as volatile organic solvent sensor

Here we demonstrate shape-controlled cobalt phosphide nanostructures through a one-pot synthetic approach in a single step with precise control over the size, shape and composition. We found that the ratio of the cobalt precursor, phosphorus sources, capping ligands, and annealing time play a key role in determining the morphology of the cobalt phosphide nanostructures. For instance, a zero-to-one-dimensional nanostructure transformation was observed with an increase in annealing time. The aspect ratio of one-dimensional nanorods could flexibly be tuned by a subtle balance of cobalt precursor, phosphorous source and capping ligands. Furthermore, a rod-to-hollow sphere transformation could be observed upon a decrease in the phosphorous source. Of these nanostructures, branched nanowires showed an excellent volatile organic solvent vapor sensing performance selective to benzene compared to the linear chain hydrocarbon hexane. A novel yet simple synthesis strategy with wide varieties of a controlled morphology of cobalt phosphide is expected to open up a new avenue to design other metal phosphide nanostructures, which would offer superior applications in the field of energy harvesting and sensing.

Shape Dependent Synthesis and Field Emission Induced Rectification in Single ZnS Nanocrystals

We report on the synthesis of shape controlled ZnS nanocrystals designed into nanodots, nanorods, and nanowires retaining the same diameter and crystallographic phase. We used UHV scanning tunneling microscopy and spectroscopy to study rectification behavior from single nanocrystals. The nanorod and nanowire show large tunneling current at the negative bias in comparison to the positive bias demonstrating current rectification, while the nanodot shows symmetric current-voltage behavior. We proposed a tunneling mechanism where direct tunneling is followed by resonant tunneling mechanism through ZnS nanocrystal at lower applied bias voltages. Stimulation of field emission in Fowler-Nordheim tunneling regime at higher negative bias voltages enables the rectification behavior from the ZnS nanorod or nanowire. Absence of rectification from the ZnS nanodot is associated with spherical shape where the field emission becomes less significant. Realizing functional electronic component from such shape dependent single ZnS nanocrystal may provide a means in realizing nanocrystal based miniaturized devices.

Triboelectric generator composed of bulk poly(vinylidene fluoride) and polyethylene polymers for mechanical energy conversion

We designed a stable triboelectric generator (TEG) consisting of a new combination of poly(vinylidene fluoride) (PVDF) and polyethylene (PE) films which shows prospect for easy and low cost device fabrication. Both PE and PVDF are selected from the negative side of the triboelectric series considering the difference in charge affinity. An open circuit peak-to-peak output voltage of ∼20 V and current density ∼0.34 A m−2 were obtained just by striking the TEG manually. The effect of directional forces on the TEG was studied for future anisotropic pressure sensor fabrications. The charging capacity of the TEG was checked by using a commercially available capacitor revealing the prospect of battery charging. Furthermore, the TEG powered at least 12 commercial white light emitting diodes which indicates its potential use as a carbon emission free power source suitable for portable electronic devices.

Transparent, Flexible Silicon Nanostructured Wire Networks with Seamless Junctions for High-Performance Photodetector Applications

Optically transparent photodetectors are crucial in next-generation optoelectronic applications including smart windows and transparent image sensors. Designing photodetectors with high transparency, photoresponsivity, and robust mechanical flexibility remains a significant challenge, as is managing the inevitable trade-off between high transparency and strong photoresponse. Here we report a scalable method to produce flexible crystalline Si nanostructured wire (NW) networks fabricated from silicon-on-insulator (SOI) with seamless junctions and highly responsive porous Si segments that combine to deliver exceptional performance. These networks show high transparency (∼92% at 550 nm), broadband photodetection (350 to 950 nm) with excellent responsivity (25 A/W), optical response time (0.58 ms), and mechanical flexibility (1000 cycles). Temperature-dependent photocurrent measurements indicate the presence of localized electronic states in the porous Si segments, which play a crucial role in light harvesting and photocarrier generation. The scalable low-cost approach based on SOI has the potential to deliver new classes of flexible optoelectronic devices, including next-generation photodetectors and solar cells.

Resonant energy transfer in a van der Waals stacked MoS2 – functionalized graphene quantum dot composite with ab initio validation

Graphene-based van der Waals (vdW) heterostructures can facilitate exciting charge transfer dynamics in between structural layers with the emission of excitonic quasi-particles. However, the chemical formation of such heterostructures has been elusive thus far. In this work, a simple chemical approach is described to form such van der Waals (vdW) heterostructures using few layer MoS2 sheet embedded quantum dots (QDs) and amine-functionalized graphene quantum dots (GQDs) to probe the energy transfer mechanism for tunable photoluminescence (PL). Our findings reveal an interesting non-radiative Förster-type energy transfer with the quenching of functional GQD PL intensity after GQD/MoS2 composite formation, which validates the existing charge transfer dynamics analogous to 0D and 2D systems. The non-radiative type of energy transfer characteristic from GQD into the MoS2 layer through vdW interactions has been confirmed by photoluminescence, time decay analyses and ab initio calculations with the shifting of the Fermi level in the density of states towards the conduction band in the stacked configuration. These results are encouraging for the fundamental exploration of optical properties in other chemically prepared QD/2D based heterostructures to understand the charge transfer mechanism and fingerprint luminescence quenching for future optoelectronic device and optical sensing applications.

Correction: Easy extraction of water-soluble graphene quantum dots for light emitting diodes

Correction for ‘Easy extraction of water-soluble graphene quantum dots for light emitting diodes’ by Gundam Sandeep Kumar et al., RSC Adv., 2015, 5, 27711–27716.