There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

At present, the actual mechanism of the photoluminescence (PL) of fluorescent carbon dots (CDs) is still an open debate among researchers. Because of the variety of CDs, it is highly important to summarize the PL mechanism for these kinds of carbon materials; doing so can guide the development of effective synthesis routes and novel applications. This review will focus on the PL mechanism of CDs. Three types of fluorescent CDs were involved: graphene quantum dots (GQDs), carbon nanodots (CNDs), and polymer dots (PDs). Four reasonable PL mechanisms have been confirmed: the quantum confinement effect or conjugated π-domains, which are determined by the carbon core; the surface state, which is determined by hybridization of the carbon backbone and the connected chemical groups; the molecule state, which is determined solely by the fluorescent molecules connected on the surface or interior of the CDs; and the crosslink-enhanced emission (CEE) effect. To give a thorough summary, the category and synthesis routes, as well as the chemical/physical properties for the CDs, are briefly introduced in advance.

The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): current state and future perspective

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Carbon Nanomaterials for Biological Imaging and Nanomedicinal Therapy

To address aggravating energy and environment issues, inexpensive, highly active, and durable electrocatalysts as noble metal substitutes both at the anode and cathode are being actively pursued. Among them, heteroatom-doped graphene-based materials show extraordinary electrocatalytic performance, some even close to or outperforming the state-of-the-art noble metals, such as Pt- and IrO2-based materials. This review provides a concise appraisal on graphene doping methods, possible doping configurations and their unique electrochemical properties, including single and double doping with N, B, S, and P. In addition, heteroatom-doped graphene-based materials are reviewed as electrocatalysts for oxygen reduction, hydrogen evolution, and oxygen evolution reactions in terms of their electrocatalytic mechanisms and performance. Significantly, three-dimensional heteroatom-doped graphene structures have been discussed, and those especially can be directly utilized as catalyst electrodes without extra binders and s...

Heteroatom-Doped Graphene-Based Materials for Energy-Relevant Electrocatalytic Processes

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.

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

Small Pd cluster adsorbed double vacancy defect graphene sheet for hydrogen storage: A first- principles study

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.

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

Dispersive force corrected density functional theory is used to map the oxygen reduction reaction (ORR) kinetics of six kinds of graphyne (Gy) and graphdiyne (Gdy) systems (namely αGy, βGy, γGy, δGy, 6,6,12Gy, RGy and Gdy) with substitutional boron (B) atom doping. To this end, the most favorable sites for B doping of each structures are determined by comparing their formation energies and then the best configuration for di-oxygen (O2) adsorption is computed by analyzing the corresponding adsorption energies. Even though oxygen adsorption is found to be energetically favorable on all of these and all Gys and Gdy are found to distinctly favor the four electron pathways for ORR, a reaction scheme with monotonically exothermic ΔG is observed only for B doped RGy. Further computations performed by varying electrode potential indicated this monotonically exothermic nature of the ΔG of B doped RGy to persist in the range 0–0.22 V and also indicated the first (H+ + e) transfer step to be the rate limiting step.

Implications of boron doping on electrocatalytic activities of graphyne and graphdiyne families: a first principles study

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.

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

Dipayan Sen

Papers

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

Small Pd cluster adsorbed double vacancy defect graphene sheet for hydrogen storage: A first- principles study

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

Implications of boron doping on electrocatalytic activities of graphyne and graphdiyne families: a first principles study

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