Yexin Feng

Nuclear quantum effects of hydrogen bonds probed by tip-enhanced inelastic electron tunneling

Quantum effects in single hydrogen bonds Hydrogen bonds are a combination of electrostatics with a nuclear quantum contribution arising from the light mass of hydrogen nuclei. However, the quantum states of hydrogen nuclei are extremely sensitive to coupling with local environments, and these effects are broadened and averaged with conventional spectroscopic or diffraction techniques. Guo et al. show that quantum effects change the strength of individual hydrogen bonds in water layers adsorbed on a salt surface. These effects are revealed in inelastic tunneling spectra obtained with a chlorine-terminated scanning tunneling microscope tip. Science, this issue p. 321 Quantum effects in water hydrogen bonding are revealed with a chlorine-terminated scanning tunneling microscope tip. We report the quantitative assessment of nuclear quantum effects on the strength of a single hydrogen bond formed at a water-salt interface, using tip-enhanced inelastic electron tunneling spectroscopy based on a scanning tunneling microscope. The inelastic scattering cross section was resonantly enhanced by “gating” the frontier orbitals of water via a chlorine-terminated tip, so the hydrogen-bonding strength can be determined with high accuracy from the red shift in the oxygen-hydrogen stretching frequency of water. Isotopic substitution experiments combined with quantum simulations reveal that the anharmonic quantum fluctuations of hydrogen nuclei weaken the weak hydrogen bonds and strengthen the relatively strong ones. However, this trend can be completely reversed when a hydrogen bond is strongly coupled to the polar atomic sites of the surface.

Direct observation of ordered configurations of hydrogen adatoms on graphene.

Ordered configurations of hydrogen adatoms on graphene have long been proposed, calculated, and searched for. Here, we report direct observation of several ordered configurations of H adatoms on graphene by scanning tunneling microscopy. On the top side of the graphene plane, H atoms in the configurations appear to stick to carbon atoms in the same sublattice. Scanning tunneling spectroscopy measurements revealed a substantial gap in the local density of states in H-contained regions as well as in-gap states below the conduction band due to the incompleteness of H ordering. These findings can be well explained by density functional theory calculations based on double-sided H configurations. In addition, factors that may influence H ordering are discussed.

Inverse Temperature Dependence of Nuclear Quantum Effects in DNA Base Pairs

Despite the inherently quantum mechanical nature of hydrogen bonding, it is unclear how nuclear quantum effects (NQEs) alter the strengths of hydrogen bonds. With this in mind, we use ab initio path integral molecular dynamics to determine the absolute contribution of NQEs to the binding in DNA base pair complexes, arguably the most important hydrogen-bonded systems of all. We find that depending on the temperature, NQEs can either strengthen or weaken the binding within the hydrogen-bonded complexes. As a somewhat counterintuitive consequence, NQEs can have a smaller impact on hydrogen bond strengths at cryogenic temperatures than at room temperature. We rationalize this in terms of a competition of NQEs between low-frequency and high-frequency vibrational modes. Extending this idea, we also propose a simple model to predict the temperature dependence of NQEs on hydrogen bond strengths in general.

Nuclear quantum effects on the high pressure melting of dense lithium

Using a self-developed combination of the thermodynamic integration and the ab initio path-integral molecular dynamics methods, we quantitatively studied the influence of nuclear quantum effects (NQEs) on the melting of dense lithium at 45 GPa. We find that although the NQEs significantly change the free-energies of the competing solid and liquid phases, the melting temperature (Tm) is lowered by only ∼15 K, with values obtained using both classical and quantum nuclei in close proximity to a new experiment. Besides this, a substantial narrowing of the solid/liquid free-energy differences close to Tm was observed, in alignment with a tendency that glassy states might form upon rapid cooling. This tendency was demonstrated by the dynamics of crystallization in the two-phase simulations, which helps to reconcile an important conflict between two recent experiments. This study presents a simple picture for the phase diagram of lithium under pressure. It also indicates that claims on the influence of NQEs on phase diagrams should be carefully made and the method adopted offers a robust solution for such quantitative analyses.

The collective and quantum nature of proton transfer in the cyclic water tetramer on NaCl(001)

Proton tunneling is an elementary process in the dynamics of hydrogen-bonded systems. Collective tunneling is known to exist for a long time. Atomistic investigations of this mechanism in realistic systems, however, are scarce. Using a combination of ab initio theoretical and high-resolution experimental methods, we investigate the role played by the protons on the chirality switching of a water tetramer on NaCl(001). Our scanning tunneling spectroscopies show that partial deuteration of the H2O tetramer with only one D2O leads to a significant suppression of the chirality switching rate at a cryogenic temperature (T), indicating that the chirality switches by tunneling in a concerted manner. Theoretical simulations, in the meantime, support this picture by presenting a much smaller free-energy barrier for the translational collective proton tunneling mode than other chirality switching modes at low T. During this analysis, the virial energy provides a reasonable estimator for the description of the nuclear quantum effects when a traditional thermodynamic integration method cannot be used, which could be employed in future studies of similar problems. Given the high-dimensional nature of realistic systems and the topology of the hydrogen-bonded network, collective proton tunneling may exist more ubiquitously than expected. Systems of this kind can serve as ideal platforms for studies of this mechanism, easily accessible to high-resolution experimental measurements.

Hydrogenation Facilitates Proton Transfer through Two-Dimensional Honeycomb Crystals

Recent experiments have triggered a debate about the ability of protons to transfer easily through individual layers of graphene and hexagonal boron nitride (h-BN). However, state-of-the-art computer calculations have shown that the barriers to proton penetration can, at >3 eV, be excessively high. Despite considerable interest the origin of this apparent anomaly between experiment and simulation remains unclear. We offer a new perspective on this debate and show on the basis of first-principles calculations that the barrier for proton penetration is significantly reduced, to <1 eV, upon hydrogenation, even in the absence of pinholes in the lattice. Although hydrogenation has not been offered as an explanation before, analysis reveals that the barrier is reduced because hydrogenation destabilizes the initial state (a deep-lying chemisorption state) and expands the honeycomb lattice through which the protons penetrate. This study offers a rationalization of the fast proton transfer observed in experiments and highlights the ability of proton transport through single-layer materials in hydrogen-rich solutions.

Hydrogen induced contrasting modes of initial nucleations of graphene on transition metal surfaces

Our first-principles calculations reveal that there exist contrasting modes of initial nucleations of graphene on transition metal surfaces, in which hydrogen plays the role. On Cu(100) and Cu(111) surfaces, an sp2-type network of carbons can be automatically formed with the help of hydrogen under very low carbon coverages. Thus, by tuning the chemical potential of hydrogen, both of the nucleation process and the following growth can be finely controlled. In contrast, on the Ni(111) surface, instead of hydrogen, the carbon coverage is the critical factor for the nucleation and growth. These findings serve as new insights for further improving the poor quality of the grown graphene on transition metal substrates.

Ab initio path-integral molecular dynamics and the quantum nature of hydrogen bonds*

The hydrogen bond (HB) is an important type of intermolecular interaction, which is generally weak, ubiquitous, and essential to life on earth. The small mass of hydrogen means that many properties of HBs are quantum mechanical in nature. In recent years, because of the development of computer simulation methods and computational power, the influence of nuclear quantum effects (NQEs) on the structural and energetic properties of some hydrogen bonded systems has been intensively studied. Here, we present a review of these studies by focussing on the explanation of the principles underlying the simulation methods, i.e., the ab initio path-integral molecular dynamics. Its extension in combination with the thermodynamic integration method for the calculation of free energies will also be introduced. We use two examples to show how this influence of NQEs in realistic systems is simulated in practice.