Signatures in vibrational and UV-visible absorption spectra for identifying cyclic hydrocarbons by graphene fragments

The nature of small molecules adsorbed on defective carbon nanotubes

In this work, we perform a comprehensive theoretical study on adsorption of representative 10-electron molecules H₂O, CH₄ and NH₃ onto defective single-walled carbon nanotubes. Results of adsorption energy and charge transfer reveal the existence of both chemical adsorption (CA) and physical adsorption (PA). While PA processes are common for all molecules, CA could be further achieved by the polar molecule NH₃, whose lone-pair electrons makes it easier to be bonded with the defective nanotube. Our systematic work could contribute to the understanding on intermolecular interactions and the design of future molecular detectors.

Mechanism of Charge Separation and Frontier Orbital Structure in Graphitic Carbon Nitride and Graphene Quantum Dots

Graphene quantum dots (GQDs) and carbon nitride quantum dots (CNQDs), the latest addition to the carbon material family, are promising materials for numerous novel applications in optical sensing, photocatalysis, biosensing, and photovoltaics. However, understanding the photocatalytic capability of CNQDs compared to GQDs requires investigations of the charge behavior on the excited state energy surface. In this work, through time-dependent density functional tight binding (TD-DFTB) calculations, we show that CNQDs exhibit superior ground state frontier orbitals (FOs) localization. Strong localization of the FOs and excited state charge separation observed in the first excited state are caused by the relaxation of the structure. Excited energy surface investigations reveal spatial confinement of FOs to the stretched C-N bonds due to excited state structural relaxation. On the other hand, no such localized FOs structure was found for GQDs, presumably caused by its strong π-conjugated configuration not allowing large structural changes upon excitation. The optical absorption and emission of CNQDs is sensitive to size and does not show large variations with the shape of the QD. Our approach provides an explanation for the origin of the enhanced photocatalytic performance of CNQDs over GQDs and their characteristic FOs localization.

Computational study of the NO, SO2, and NH3 adsorptions on fragments of 3N-graphene and Al/3N graphene

The adsorption properties of common gas molecules (NO, NH₃, and SO₂) on the surface of 3N-graphene and Al/3N graphene fragments are investigated using density functional theory. The adsorption energies have been calculated for the most stable configurations of the molecules on the surface of 3N-graphene and Al/3N graphene fragments. The adsorption energies of Al/3N graphene-gas systems are -220.5 kJ mol^-1 for Al/3NG-NO, -111.9 kJ mol^-1 for Al/3NG-NH₃, and -347.7 kJ mol^-1 for Al/3NG-SO₂, respectively. Compared with the 3N-graphene fragment, the Al/3N graphene fragment has significant adsorption energy. Furthermore, the molecular orbital, density of states, and electron densities distribution were used to explore the interaction between these molecules and the surface. We found that orbital hybridization exists between these molecules and the Al/3N graphene surface, which indicates that doping Al significantly increases the interaction between the gas molecules and Al/3N graphene. In addition, compared with Li, Al can more powerfully enhance adsorption of the 3N-graphene fragment. The results indicate that Al/3N graphene can be viewed as a new nanomaterial adsorbent for NO, NH₃, and SO₂.

Theoretical study of the CO, NO, and N2 adsorptions on Li-decorated graphene and boron-doped graphene

The adsorption properties of common gas molecules (CO, NO, and N2) on the surface of Li-decorated pristine graphene and Li-decorated boron doped graphene are investigated using density functional theory. The adsorption energy, charge transfer, and density of states of gas molecules on three surfaces have been calculated and discussed, respectively. The results show that Li-decorated pristine graphene has strong interaction with CO and N2. Compared with Li-decorated pristine graphene, Li-decorated boron doped graphene exhibit a comparable adsorption ability of CO and N2. Moreover, Li-decorated boron doped graphene have a more significant adsorption energy to NO than that of Li-decorated pristine graphene because of the chemical interaction of the NO gas molecule. The strong interaction between the NO molecule and substrate (Li-decorated boron doped graphene) induces dramatic changes to the electrical conductivity of Li-decorated boron doped graphene. The results indicate that Li-decorated boron doped graphene would be an excellent candidate for sensing NO gas.

Excited state dynamics study of the self-trapped exciton formation in silicon nanosheets

After excitation to S₁ (1), the exciton takes ∼450–850 femtoseconds to relax into the self-trapped (ST) state (2) with the occurrence of strong localization and a large Stokes shift, due to the significant stretching of the Si–Si bonds.

Restoring the Electrical Properties of CVD Graphene via Physisorption of Molecular Adsorbates

Chemical vapor deposition (CVD) is a powerful technique to produce graphene for large-scale applications. Polymer-assisted wet transfer is commonly used to move the graphene onto silicon substrates, but the resulting devices tend to exhibit p-doping, which decreases the device quality and reproducibility. In an effort to better understand the origin of this effect, we coated graphene with n-methyl-2-pyrrolidone (NMP) and hexamethyldisilazane (HMDS) molecules that exhibit negligible charge transfer to graphene but bind more strongly to graphene than ambient adsorbents. Using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electrical transport measurements, and quantum mechanical computer simulations, we show that the molecules help in the removal of p-doping, and our data indicate that the molecules do this by replacing ambient adsorbents (typically O₂ and water) on the graphene surface. This very simple method of improving the electronic properties of CVD graphene by passivating its surface with common solvent molecules will accelerate the development of CVD graphene-based devices.

Approximate quantum chemical methods for modelling carbohydrate conformation and aromatic interactions: β-cyclodextrin and its adsorption on a single-layer graphene sheet

Adsorption of carbohydrates on graphene has the potential to improve graphene dispersibility in water. Here we assess the ability of DFTB-based and NDDO-based quantum chemical methods to model β-cyclodextrin conformations and interactions with graphene.

Water film inside graphene nanosheets: electron transfer reversal between water and graphene via tight nano-confinement

Electron transfer reversal between water and grapheneviatight nano-confinement.