Dipayan Sen

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.

Rules of Boron–Nitrogen Doping in Defect Graphene Sheets: A First‐Principles Investigation of Band‐Gap Tuning and Oxygen Reduction Reaction Catalysis Capabilities

Introduction of defects and nitrogen doping are two of the most pursued methods to tailor the properties of graphene for better suitability to applications such as catalysis and energy conversion. Doping nitrogen atoms at defect sites of graphene and codoping them along with boron atoms can further increase the efficiency of such systems due to better stability of nitrogen at defect sites and stabilization provided by B-N bonding. Systematic exploration of the possible doping/codoping configurations reflecting defect regions of graphene presents a prevalent doping site for nitrogen-rich BN clusters and they are also highly suitable for modulating (0.2-0.9 eV) the band gap of defect graphene. Such codoped systems perform significantly better than the platinum surface, undoped defect graphene, and the single nitrogen or boron atom doped defect graphene system for dioxygen adsorption. Significant stretching of the O-O bond indicates a lowering of the bond breakage barrier, which is advantageous for applications in the oxygen reduction reaction.

Cu2O/g-C3N4 nanocomposites: an insight into the band structure tuning and catalytic efficiencies

We demonstrate an easy and scalable room-temperature synthesis of Cu2O nanoparticle incorporated graphitic carbon nitride composites without the aid of any inert atmosphere. First principles calculations based upon density functional theory, in addition to the experimental validations, have been employed to investigate the electronic and optical properties of the nanocomposites. An insight into the band structure tunability, phase stabilisation and the dependancy of the catalytic properties of the nanocomposites upon the amount of Cu loading, in the form of Cu oxides, have been provided in this work.

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.

Unique quasi-vertical alignment of RGO sheets under an applied non-uniform DC electric field for enhanced field emission

In this paper, we report unique quasi-vertical alignment of reduced graphene oxide (RGO) sheets under an applied non-uniform high DC electric field deposited on a carbon cloth (CC) substrate, which eventually resulted in enhanced field emission characteristics. Significant improvement in enhancement factor was achieved with major increment in current due to the alignment of thin RGO edges under an applied non-uniform electric field. The non-uniform field was generated by the conical shape of the top electrode (anode). Moreover, the experimental findings have been validated through simulated modeling of the electric field over RGO sheets using the finite element method for both planar and conical top electrode geometrical configurations. This observed phenomenon could have a potential fundamental impact on field emission applications with vertically aligned graphene and RGO based materials in the coming years.

Chalcogen doping at anionic site: A scheme towards more dispersive valence band in CuAlO2

Using first-principles calculations, we propose to enhance the dispersion of the top of valence band at high-symmetry points by selective introduction of chalcogen (Ch) impurities at oxygen site. As ab-plane hole mobility of CuAlO2 is large enough to support a band-conduction model over a polaronic one at room temperature [M. S. Lee et al. Appl. Phys. Lett. 79, 2029, (2001); J. Tate et al. Phys. Rev. B 80, 165206, (2009)], we examine its electronic and optical properties normal to c-axis. Intrinsic indirectness of energy-gap at Γ-point can be effectively removed along with substantial increase in density of states near Fermi level (EF) upon Ch addition. This can be attributed to S 2p-Cu 3d interaction just at or below EF, which should result in significantly improved carrier mobility and conductivity profile for this important p-type TCO.

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.