Negative-charge-functionalized carbon nanodot: a low-cost smart cold emitter

Cold emission properties of carbon nanodots (CNDs) evaluated using ANSYS Maxwell software are predicted to be size-dependent and then verified experimentally. In order to correlate the electron emission properties with the size of CNDs, the work function values were determined using ultraviolet photoelectron spectroscopy. This is the first report on theoretical calculations based on density functional theory and experimental results that confirm the work function dependency on the charge state of the functional group attached on the particle surface. The smallest CND (2.5 nm) has the highest percentage of negatively charged groups as well as the lowest work function (5.18 eV). The smallest dimension with the lowest work function assures that this sample is the best suited for field emission. It shows excellent field emission properties with a high current density of ∼1.45 mA cm^-2 at 2 V μm^-1 electric field, turn-on field as low as 0.04 V μm^-1, very high field enhancement factor of 2.7 × 10⁵ and high stability. Overall, the zero-dimensional CNDs showed superior field emission activity as compared to the higher dimensional carbon nanomaterials.

Graphene-Anchored p-Type CuBO2 Nanocrystals for a Transparent Cold Cathode

CuBO₂ nanostructures were synthesized by employing a low-cost hydrothermal technique to combine into the CuBO₂-RGO nanocomposite for the first time using chemically prepared graphene sheets. The nanohybrid samples were characterized for structural information using X-ray diffraction (XRD) that revealed the proper crystalline phase formation of CuBO₂ unaltered by composite formation with graphene. Raman spectroscopic studies were employed to confirm the presence of graphene. A morphological study with field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) suggested the proper wrapping of RGO sheets over CuBO₂ nanocubes. Moreover, the close proximity of lattice planes of CuBO₂ and RGO to each other was observed in high-resolution TEM studies that were correlated with the Raman spectroscopic studies. Finally, the samples were characterized to study the field emission (FE) properties of the same using a laboratory-made high-vacuum field-emission setup. Finite-element-based theoretical simulation studies were carried out to explain and compare the field emission properties with the experimental results. The FE properties of the composite samples were found to be tuned by the nature of wrapping the RGO sheets over the CuBO₂ nanocubes, which was typically dependent upon the spiky morphology of the nanocubes.

Novel Quaternary Chalcogenide/Reduced Graphene Oxide-Based Asymmetric Supercapacitor with High Energy Density

In this work we have synthesized quaternary chalcogenide Cu₂NiSnS₄ (QC) nanoparticles grown in situ on 2D reduced graphene oxide (rGO) for application as anode material of solid-state asymmetric supercapacitors (ASCs). Thorough characterization of the synthesized composite validates the proper phase, stoichiometry, and morphology. Detailed electrochemical study of the electrode materials and ASCs has been performed. The as-fabricated device delivers an exceptionally high areal capacitance (655.1 mF cm^-2), which is much superior to that of commercial micro-supercapacitors. Furthermore, a remarkable volumetric capacitance of 16.38 F cm^-3 is obtained at a current density of 5 mA cm^-2 combined with a very high energy density of 5.68 mW h cm^-3, which is comparable to that of commercially available lithium thin film batteries. The device retains 89.2% of the initial capacitance after running for 2000 cycles, suggesting its long-term capability. Consequently, the enhanced areal and volumetric capacitances combined with decent cycle stability and impressive energy density endow the uniquely decorated QC/rGO composite material as a promising candidate in the arena of energy storage devices. Moreover, Cu₂NiSnS₄ being a narrow band gap photovoltaic material, this work offers a novel protocol for the development of self-charging supercapacitors in the days to come.