Uttam Kumar Ghorai

Amino-functionalized graphene quantum dots: origin of tunable heterogeneous photoluminescence

Graphene quantum dots are known to exhibit tunable photoluminescence (PL) through manipulation of edge functionality under various synthesis conditions. Here, we report observation of excitation dependent anomalous m-n type fingerprint PL transition in synthesized amino functionalized graphene quantum dots (5-7 nm). The effect of band-to-band π*-π and interstate to band n-π induced transitions led to effective multicolor emission under changeable excitation wavelength in the functionalized system. A reasonable assertion that equi-coupling of π*-π and n-π transitions activated the heterogeneous dual mode cyan emission was made upon observation of the PL spectra. Furthermore, investigation of incremented dimensional scaling through facile synthesis of amino functionalized quantum graphene flakes (20-30 nm) revealed it had negligible effect on the modulated PL pattern. Moreover, an effort was made to trace the origin of excitation dependent tunable heterogeneous photoluminescence through the framework of energy band diagram hypothesis and first principles analysis. Ab initio results suggested formation of an interband state as a manifestation of p orbital hybridization between C-N atoms at the edge sites. Therefore comprehensive theoretical and experimental analysis revealed that newly created energy levels can exist as an interband within the energy gap in functionalized graphene quantum structures yielding excitation dependent tunable PL for optoelectronic applications.

Surface modification of amorphous carbon nanotubes with copper phthalocyanine leading to enhanced field emission

Amorphous carbon nanotubes (ACNTs) were synthesized by following a simple chemical route with ammonium chloride and ferrocene taken as starting materials and heating at 250 °C under an open atmosphere. The as-prepared samples were oxidized and chemically activated by simple acid treatment and further functionalized by copper phthalocyanine (CuPc). The attachment of CuPc at the walls of the nanotubes was investigated by Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy, transmission and scanning electron microscopy. The field emission properties of ACNTs was found to be greatly enhanced after coating them with CuPc. The effects of inter-electrode distance on the field emission properties of the functionalized samples were also studied.

Highly oriented cupric oxide nanoknife arrays on flexible carbon fabric as high performing cold cathode emitter

Sharp knife edged copper oxide (CuO) nano architectures were directly grown on the surface of flexible carbon fabric by a facile chemical route with the assistance of nonionic surfactant PEG-6000. The carbon fabric substrate preserves its high flexibility even after the growth of the entire nanostructure. Moreover, it can be rolled-up and twisted to a large degree without affecting the electrical characteristics. The phase purity and degree of crystallinity of the developed nanostructures were systemically supported by X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy and high-resolution transmission electron microscopy. A proposed growth mechanism has been offered by thoroughly analyzing the field emission scanning electron microscopic images of the nanostructures grown at different concentrations of PEG. Among the as grown nanostructures, copper oxide nanoknives have exhibited remarkable field emission properties and high stability with a lower turn-on field of 0.9 V μm−1 (10 μA cm−2) at room temperature, which is sufficient for electron emission based devices like field emission displays and vacuum nano-electronic devices.

Bane to boon: tailored defect induced bright red luminescence from cuprous iodide nanophosphors for on-demand rare-earth-free energy-saving lighting applications

The long standing controversy concerning the defect band in cuprous iodide (CuI) has been addressed in this paper from a technological point of view of its solid state lighting application. Recently, solid state lighting technology using nanophosphors has been proposed as the prime candidate in the energy saving lighting paradigm. Herein, we demonstrate a novel rare-earth free and non-toxic CuI nanophosphor, which has been synthesized via a facile solvothermal route. These nanophosphors are able to show ultra-bright and stable red emission under near UV excitation. The spectral features of this easily derived nanophosphor are not less than any rare-earth or cadmium based conventional phosphor. Furthermore, it has been conclusively verified that the deep red emission is strongly related to the excess iodine induced optimized defect level engineering in the band structure. The concepts and results presented in this paper clearly establish that the CuI nanophosphor is a promising ‘green’ material for the state-of-the-art rare-earth free lighting and display applications.

Experimental and theoretical investigation of enhanced cold cathode emission by plasma-etched 3d array of nanotips derived from CuPc nanotube

In the present work, we report fabrication and field emission responses of 3D copper phthalocyanine (CuPc) nanotip arrays synthesized over nanotube walls by facile plasma treatment. Significant field emission enhancement is confirmed for a nanotip–nanotube hybrid system (turn-on field 4.2 V μm−1@10 μA cm−2) as compared to pristine CuPc nanotubes (turn-on field 6.8 V μm−1@10 μA cm−2). Root of the observed enhanced cold cathode emission performances is further probed by a finite element method based simulation protocol that computed local electric field distribution for a single tube without and with plasma etching in a manner parallel to the experimental setup. Our obtained results strongly suggest that CuPc nanotip–nanotube hybrid nanostructures are a major potential candidate as field emitters for vacuum nanoelectronics and cold cathode based emission display applications.

Local Field Enhancement-Induced Enriched Cathodoluminescence Behavior from CuI-RGO Nanophosphor Composite for Field-Emission Display Applications.

Field-emission displays (FEDs) constitute one of the major foci of the cutting edge materials research because of the increasingly escalating demand for high-resolution display panels. However, poor efficiencies of the concurrent low voltage cathodoluminescence (CL) phosphors have created a serious bottleneck in the commercialization of such devices. Herein we report a novel CuI-RGO composite nanophosphor that exhibits bright red emission under low voltage electron beam excitation. Quantitative assessment of CL spectra reveals that CuI-RGO nanocomposite phosphor leads to the 4-fold enhancement in the CL intensity as compared to the pristine CuI counterpart. Addition of RGO in the CuI matrix facilitates efficient triggering of luminescence centers that are activated by local electric field enhancement at the CuI-RGO contact points. In addition, conducting RGO also reduces the negative loading problem on the surface of the nanophosphor composite. The concept presented here opens up a novel generic route for enhancing CL intensity of the existing (nano)phosphors as well as validates the bright prospects of the CuI-RGO composite nanophosphor in this rapidly growing field.

Tailored defect-induced sharp excitonic emission from microcrystalline CuI and its ab initio validation

Defect rich structures of metal halide-based ionic semiconductors are a major problem owing to their prospective applications in a broad range of light emitting devices, such as ordinary light emitting diodes to more exotic laser diodes. As a consequence of the inherently low formation energies of the native defects, especially in cuprous iodide (CuI), it is difficult to achieve band edge excitonic emission as it is often quenched by a defect center-mediated radiative recombination. Herein, we report an in situ room temperature technique for fabricating highly crystalline CuI films on transparent and flexible polyethylene terephthalate (PET) substrates. The as-prepared samples were found to exhibit signatures of sharp excitonic emission. Optimization of the reaction parameters revealed the pH of the solution to be a pivotal parameter for controlling such excitonic emissions. Although all as-grown films were observed to be highly crystalline in nature, varying concentrations of iodine were found to manifest its effect by evolving a crystal morphology from microrods to polyhedrons. Further theoretical investigations using density functional theory were also carried out to investigate how the breakage of the Cu–I bond contributes to the development of such defects. The less defective films with a sharp excitonic band are speculated to be a potential candidate for solid state light emitting devices.

Facile synthesis of ZnPc nanoflakes for cold cathode emission

The challenge of developing two dimensional metal phthalocyanine nanostructures by controlling the reaction protocols is successfully addressed in the present work by synthesizing zinc phthalocyanine (ZnPc) novel nanoflakes using a simple low temperature hydrothermal route. The as synthesized samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultra-violet visible spectrometer (UV-Vis), X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscope (FESEM). The field emission or cold cathode emission characteristics of these phthalocyanine nanostructures have been reported for the first time here and it is shown that as prepared nanoflakes can act as electron field emitter having a turn-on field 4.7 V μm−1 at a current density of 1 μA cm−2 for an inter electrode distance of 130 μm. The local electric field distributions around nanoflakes were also further studied theoretically using a finite element method. The obtained results indicate that ZnPc nanoflakes are the potential candidate for electron emission based applications such as vacuum nanoelectronic devices and field emission display devices.

Observation of bright green luminescence in an Eu2+ complexed graphene oxide composite through reduction of Eu3+

Red light harvesting with Eu3+ graphene complexes are well known in the literature. But embedding Eu2+ in a graphene oxide matrix is a difficult proposition and could be of paramount interest because of emission tunability. Herein, we report the observation of highly visible green luminescence for Eu2+ which occurs due to the transition between the 4f7 (8S7/2) ground state and the 4f6 5d excited state with nanosecond time decay through reduction of Eu3+ using a simple chemical approach. This newly formed luminescence complex may be used as a major potential candidate for application in optoelectronics and nanobiotechnology.

A scheme of simultaneous cationic–anionic substitution in CuCrO2 for transparent and superior p-type transport

Considering CuCrO2 to be a promising p-type transparent conducting oxide, unprecedented simultaneous cationic–anionic doping is carried out to achieve superior hole transport while maintaining its transparency. Magnesium and sulphur are doped at Cr and O-sites respectively by solid-state approach (CuCr1−x Mg x O1−y S y , x, y ranging 0–5 atomic %) with significant doping confirmed by Rietveld refinement. UV–Vis spectroscopy is observed to imply promising optical properties of engineered materials. DC conductivity of co-doped CuCr0.95Mg0.05O1.9S0.1 is observed to be twice as large compared to CuCr0.95Mg0.05O2 at 300 K, which is consistent with the lower frequency shift of the negative differential susceptance () and the admittance peak, indicating higher metallicity upon co-doping. Hole mobility of 16.26 cm2 V−1 s−1 at 300 K is observed for the co-doped CuCrO2. This strategy combines an established doping scheme at the cationic site with our newly developed anionic chalcogen doping, aiming to overcome a long-standing transport bottleneck in the field of semiconductor oxides.