Novel behavior of monolayer quantum gases on graphene, graphane and fluorographene

Effect of zero-point motion on properties of quantum particles adsorbed on a substrate

We qualitatively investigate the effect of zero-point motion (ZPM) on the structure and properties of a film composed of quantum particles adsorbed on a graphite substrate. The amplitude of ZPM is controlled by a change of the particle mass while keeping the interactions fixed. In that sense it is assumed that the interactions can be controlled by future doping methods. The worm-algorithm path integral Monte Carlo (WAPIMC) method is applied to simulate this system in the grand-canonical ensemble, where particles can be exchanged with the external particle reservoir. Another method, namely the multiconfigurational time-dependent Hartree method for bosons is additionally applied to verify some of the WAPIMC results and to provide further information on the entropy and the condensate fraction. Several important findings are reported. It is found that ZPM plays an important role in defining order and disorder in the crystalline structure of the adsorbed film. The total energy of the film drops with a reduction in the amplitude of ZPM, that is, it becomes more negative which is an indication to stronger adsorption. For a few particle numbers, a significant condensate fraction is detected that however drops sharply at critical values of the ZPM amplitude. Most importantly, a connection is established between chaos, in coordinate as well as momentum space, and the Heisenberg uncertainty principle. The importance of the present study lies in the fact that adsorbed two-dimensional films serve as an excellent experimental testbed for demonstrating low-dimensional quantum phenomena in the ground state. The present examination contributes also to a further understanding of the properties of heavy quantum particles adsorbed on substrates.

Adsorption of Helium on Small Cationic PAHs: Influence of Hydrocarbon Structure on the Microsolvation Pattern

The adsorption of up to ∼100 helium atoms on cations of the planar polycyclic aromatic hydrocarbons (PAHs) anthracene, phenanthrene, fluoranthene, and pyrene was studied by combining helium nanodroplet mass spectrometry with classical and quantum computational methods. Recorded time-of-flight mass spectra reveal a unique set of structural features in the ion abundance as a function of the number of attached helium atoms for each of the investigated PAHs. Path-integral molecular dynamics simulations were used with a polarizable potential to determine the underlying adsorption patterns of helium around the studied PAH cations and in good general agreement with the experimental data. The calculated structures of the helium-PAH complexes indicate that the arrangement of adsorbed helium atoms is highly sensitive toward the structure of the solvated PAH cation. Closures of the first solvation shell around the studied PAH cations are suggested to lie between 29 and 37 adsorbed helium atoms depending on the specific PAH cation. Helium atoms are found to preferentially adsorb on these PAHs following the 3 × 3 commensurate pattern common for graphitic surfaces, in contrast to larger carbonaceous molecules like corannulene, coronene, and fullerenes that exhibit a 1 × 1 commensurate phase.

Adsorption of Organic Molecules to van der Waals Materials: Comparison of Fluorographene and Fluorographite with Graphene and Graphite

Understanding strength and nature of noncovalent binding to surfaces imposes significant challenge both for computations and experiments. We explored the adsorption of five small nonpolar organic molecules (acetone, acetonitrile, dichloromethane, ethanol, ethyl acetate) to fluorographene and fluorographite using inverse gas chromatography and theoretical calculations, providing new insights into the strength and nature of adsorption of small organic molecules on these surfaces. The measured adsorption enthalpies on fluorographite range from -7 to -13 kcal/mol and are by 1-2 kcal/mol lower than those measured on graphene/graphite, which indicates higher affinity of organic adsorbates to fluorographene than to graphene. The dispersion-corrected functionals performed well, and the nonlocal vdW DFT functionals (particularly optB86b-vdW) achieved the best agreement with the experimental data. Computations show that the adsorption enthalpies are controlled by the interaction energy, which is dominated by London dispersion forces (∼70%). The calculations also show that bonding to structural features, like edges and steps, as well as defects does not significantly increase the adsorption enthalpies, which explains a low sensitivity of measured adsorption enthalpies to coverage. The adopted Langmuir model for fitting experimental data enabled determination of adsorption entropies. The adsorption on the fluorographene/fluorographite surface resulted in an entropy loss equal to approximately 40% of the gas phase entropy.