Competition of hydrogen desorption and migration on graphene surface in alternating electric field: Multiscale molecular dynamics and diffusion study

Xie Z., Yu X., Deng Y., Zhang Y., Zhou W., Song K., Liu M., Mo Z., Jia P.

The thermoelectric properties of hybrid graphane and graphene nanoribbons (GGNRs) are studied by the Landauer formula within the atomistic non-equilibrium Green's function. It is shown that hybrid GGNRs present the tunable band gap depending upon the width ratio of their components. Modulation of hybrid GGNRs can give rise to the lower thermal conductance of phonons and the better thermoelectric properties compared to pure graphene nanoribbons and graphane nanoribbons. Introducing the hydrogen defects can enhance the maximum of ZT (ZTmax) in both armchair GGNRs (AGGNRs) and zigzag GGNRs (ZGGNRs). Such the enhancement of ZTmax sensitively depends upon the defect concentrations. Among different defect concentrations, ZTmax for the 20 % defect concentration has the highest value, which can be enhanced to 0.42 and 0.24 at T = 300 K in AGGNRs and ZGGNRs, respectively. A laconic analysis of these results is given.

Wu B.

Fournier T., Cruz K., Monthioux M., Lassagne B., Petit L., Moyano S., Puech P., Piazza F.

Creating defects in graphene by hydrogenation, either to achieve hydrogen chemisorption or partial etching, is a way to open an electronic band gap in graphene. Understanding the range of stability conditions of partially etched or hydrogenated graphene is crucial for application, as processing conditions (e.g. temperature) and quality control (characterization) conditions may result in modifying the material through partial or full dehydrogenation, and subsequent alteration of its electronic properties. This work reports a study of various dehydrogenation conditions of hydrogenated or hydrogen-etched monolayer graphene (1LG), either free-standing or supported by an interferential (SiO2/Si) substrate, using incremental annealing under nitrogen atmosphere up to 400 {\textdegree}C. Materials were investigated by Raman spectroscopy. Indeed, it has been known since 2012 that the intensity ratio of two Raman bands activated by double resonance, D over D' (ID/ID') can be used to identify the type of defects in defective graphene. It is shown that hydrogenated 1LG, characterized by a large ID/ID' ratio (~9-15), is stable provided annealing remains below 300 {\textdegree}C. On the other hand, defective 1LG resulting from hydrogen etching remains stable up to 400 {\textdegree}C, whether the 1LG is hydrogenated on one side or both sides, while a modification in the type and proportions of defects is likely. Experimental conditions for the safe use of Raman spectroscopy, otherwise able to induce specimen overheating because of the laser energy and power, are also determined and discussed.

Tong J., Fu Y., Domaretskiy D., Della Pia F., Dagar P., Powell L., Bahamon D., Huang S., Xin B., Costa Filho R.N., Vega L.F., Grigorieva I.V., Peeters F.M., Michaelides A., Lozada-Hidalgo M.

AbstractThe basal plane of graphene can function as a selective barrier that is permeable to protons1,2 but impermeable to all ions3,4 and gases5,6, stimulating its use in applications such as membranes1,2,7,8, catalysis9,10 and isotope separation11,12. Protons can chemically adsorb on graphene and hydrogenate it13,14, inducing a conductor–insulator transition that has been explored intensively in graphene electronic devices13–17. However, both processes face energy barriers1,12,18 and various strategies have been proposed to accelerate proton transport, for example by introducing vacancies4,7,8, incorporating catalytic metals1,19 or chemically functionalizing the lattice18,20. But these techniques can compromise other properties, such as ion selectivity21,22 or mechanical stability23. Here we show that independent control of the electric field, E, at around 1 V nm−1, and charge-carrier density, n, at around 1 × 1014 cm−2, in double-gated graphene allows the decoupling of proton transport from lattice hydrogenation and can thereby accelerate proton transport such that it approaches the limiting electrolyte current for our devices. Proton transport and hydrogenation can be driven selectively with precision and robustness, enabling proton-based logic and memory graphene devices that have on–off ratios spanning orders of magnitude. Our results show that field effects can accelerate and decouple electrochemical processes in double-gated 2D crystals and demonstrate the possibility of mapping such processes as a function of E and n, which is a new technique for the study of 2D electrode–electrolyte interfaces.

Katin K.P., Podlivaev A.I., Kochaev A.I., Kulyamin P.A., Bauetdinov Y., Grekova A.A., Bereznitskiy I.V., Maslov M.M.

We have computationally studied eight carbon monolayer materials, including the recently synthesized biphenylene, graphyne, and DHQ-graphene, as precursors of the bilayer C2H-diamanes. The interlayer C-C bonds of about 1.6 Å confirmed the strong covalent bonding between the monolayers. Density functional theory calculations revealed that the considered diamanes have a wide range of band gaps ranging from 1.5 to 4.2 eV. Diamanes, which are based on graphene allotropes, significantly expand the range of electronic and optical properties of conventional graphene derivatives and other traditional carbon materials. Tight-binding molecular dynamics simulations showed that diamanes are less stable than monolayers due to tendency of interlayer bonds tend to break. Out of the eight considered structures, only three diamonds were identified as certain stable systems suitable for processing at elevated temperatures of about 500 K. The nudged elastic band approach provided an understanding of the rate-defined thermal decomposition steps and corresponding energy barriers, which are equal to 2.79, 4.86, and 5.41 eV for the three stable diamanes. The elastic constants of the considered diamanes are comparable to those of graphene. The absorbance spectra of diamanes are calculated using linear response time-dependent density functional theory.

Tong Y., Yang Y.

Demirci S., Gorkan T., Aktürk E., Ciraci S.

Boateng E., Thiruppathi A.R., Hung C., Chow D., Sridhar D., Chen A.

Graphene and its derivatives are attractive solid-state candidates to meet the needs of next-generation energy and hydrogen storage technologies. Although impressive storage capacities have been demonstrated, materials meeting all targets established by the United States Department of Energy have yet to be identified. In this review, we provide an overview of recent developments in functionalized graphene-based nanomaterials for energy and hydrogen storage systems. First, considerations for the most common synthetic approaches to graphene-based nanomaterials are briefly summarized. Then, the effects of traditional functionalization strategies on the structural characteristics and properties of pristine graphene are discussed. Lastly, recent advances and progress in energy applications using functionalized graphene-based nanomaterials including supercapacitors, batteries and hydrogen storage are highlighted, and new directions are discussed.

Pozzo M., Turrini T., Bignardi L., Lacovig P., Lizzit D., Tosi E., Lizzit S., Baraldi A., Alfè D., Larciprete R.

Sun P.Z., Xiong W.Q., Bera A., Timokhin I., Wu Z.F., Mishchenko A., Sellers M.C., Liu B.L., Cheng H.M., Janzen E., Edgar J.H., Grigorieva I.V., Yuan S.J., Geim A.K.

Graphite is one of the most chemically inert materials. Its elementary constituent, monolayer graphene, is generally expected to inherit most of the parent material’s properties including chemical inertness. Here, we show that, unlike graphite, defect-free monolayer graphene exhibits a strong activity with respect to splitting molecular hydrogen, which is comparable to that of metallic and other known catalysts for this reaction. We attribute the unexpected catalytic activity to surface corrugations (nanoscale ripples), a conclusion supported by theory. Nanoripples are likely to play a role in other chemical reactions involving graphene and, because nanorippling is inherent to atomically thin crystals, can be important for two-dimensional (2D) materials in general.

Chen J., Ge W.

The thermal properties of graphane, hydrogenated graphene, are surprisingly underreported, and there are commonly held opposing views on its thermal conductivity. While hydrogenation negatively affects the thermal conductivity, the exact nanoscale thermal transport properties of graphane are still poorly understood and the mechanisms involved remain obscure. The thermal transport properties of graphane nanoribbons were investigated theoretically by performing molecular dynamics simulations. The effect of hydrogenation degree was evaluated, and comparisons of thermal conductivity were carried out between graphene and graphane or between different forms of spatial isomerism. The order-of-magnitude difference in thermal conductivity between graphene and graphane ribbons was determined accordingly. The rule of mixtures was applied to provide the theoretical upper and lower bounds on the thermal conductivity. A theoretical analysis was made to assess the correlation between the thermal conductivity and the peak amplitude of out-of-plane vibration. The results indicated that graphane shows great promise for flexible tuning of thermal properties. The extent of hydrogenation admits a convenient chemical dial for tuning the thermal conductivity. The thermal conductivity of graphane isomers decreases successively in the order of the chair, boat, and stirrup configurations. The thermal conductivity of graphene and graphane differs by one order of magnitude or more, depending upon spatial isomerism and out-of-plane vibration amplitudes. For the stirrup configuration, the thermal conductivity is decreased by a factor of at least 20, and by about a factor of 10 when semi-hydrogenation occurs. The thermal conductivity of partially hydrogenated graphene can be predicted by applying the rule of mixtures. The results have significant implications for understanding of the relations between spatial isomerism and thermal properties. • Graphane shows great promise for flexible tuning of thermal properties. • The thermal conductivity differs by one order of magnitude or more. • The difference depends on spatial isomerism and out-of-plane vibration amplitudes. • The thermal conductivity of graphane isomers varies considerably. • The theoretical upper and lower bounds on the thermal conductivity are provided.

Sung Y., Vejayan H., Baddeley C.J., Richardson N.V., Grillo F., Schaub R.

On-surface synthesis with designer precursor molecules is considered an effective method for preparing graphene nanoribbons (GNRs) of well-defined widths and with tunable electronic properties. Recent reports have shown that the band gap of ribbons doped with heteroatoms (such as boron, nitrogen, and sulfur) remains unchanged in magnitude in most cases. Nevertheless, theory predicts that a tunable band gap may be engineered by hydrogenation, but experimental evidence for this is so far lacking. Herein, surface-confined hydrogenation studies of 7-armchair graphene nanoribbons (7-AGNRs) grown on Au(111) surfaces, in an ultrahigh vacuum environment, are reported. GNRs are first prepared, then hydrogenated by exposure to activated hydrogen atoms. High resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM) images reveal a self-limited hydrogenation process. By means of a combination of bond-resolved scanning tunneling microscopy (BRSTM) imaging and tip-induced site-specific dehydrogenation, the hydrogenation mechanism is studied in detail, and density-functional theory (DFT) calculation methods are used to complement the experimental findings. In all cases, the results demonstrate the successful modification of the electronic properties of the GNR/Au(111) system by edge and basal-plane hydrogenation, and a mechanism for the hydrogenation process is proposed.

Hu S., Liu X., Chen D., Lian C., Wang E., Meng S.

As one of the most striking phenomena in many-body systems, the breakdown of the adiabatic Born-Oppenheimer (ABO) approximation is essential for understanding and predicting the dynamics of quantum materials. Here, we demonstrate that strong nonadiabatic (NA) effects can emerge in electron-phonon coupling (EPC) and resultant superconductivity, yielding a surprisingly large contribution (up to 20%) to the total EPC strength \ensuremath{\lambda} of doped graphene/graphane and modulating the superconductivity transition temperature ${T}_{c}$ by \ensuremath{\sim}20%. The NA EPC, with a detectable isotopic effect, results from the renormalized EPC matrix elements (up to 40%) combined with the Fermi surface modulation, due to electron distributions deviating from the equilibrium. Our results imply that nonadiabaticity plays a wider and more important role than previously conceived, offering new understandings and strategies for dynamic engineering of quantum materials.

Dehaghani M.Z., Habibzadeh S., Farzadian O., Kostas K.V., Saeb M.R., Spitas C., Mashhadzadeh A.H.

Optimization of thermal conductivity of nanomaterials enables the fabrication of tailor-made nanodevices for thermoelectric applications. Superlattice nanostructures are correspondingly introduced to minimize the thermal conductivity of nanomaterials. Herein we computationally estimate the effect of total length and superlattice period ( $$l_{p}$$ ) on the thermal conductivity of graphene/graphane superlattice nanoribbons using molecular dynamics simulation. The intrinsic thermal conductivity ( $$\kappa_{\infty }$$ ) is demonstrated to be dependent on $$l_{p}$$ . The $$\kappa_{\infty }$$ of the superlattice, nanoribbons decreased by approximately 96% and 88% compared to that of pristine graphene and graphane, respectively. By modifying the overall length of the developed structure, we identified the ballistic-diffusive transition regime at 120 nm. Further study of the superlattice periods yielded a minimal thermal conductivity value of 144 W m−1 k−1 at $$l_{p}$$  = 3.4 nm. This superlattice characteristic is connected to the phonon coherent length, specifically, the length of the turning point at which the wave-like behavior of phonons starts to dominate the particle-like behavior. Our results highlight a roadmap for thermal conductivity value control via appropriate adjustments of the superlattice period.

Betti M.G., Placidi E., Izzo C., Blundo E., Polimeni A., Sbroscia M., Avila J., Dudin P., Hu K., Ito Y., Prezzi D., Bonacci M., Molinari E., Mariani C.

Conversion of free-standing graphene into pure graphane─where each C atom is sp3 bound to a hydrogen atom─has not been achieved so far, in spite of numerous experimental attempts. Here, we obtain an unprecedented level of hydrogenation (≈90% of sp3 bonds) by exposing fully free-standing nanoporous samples─constituted by a single to a few veils of smoothly rippled graphene─to atomic hydrogen in ultrahigh vacuum. Such a controlled hydrogenation of high-quality and high-specific-area samples converts the original conductive graphene into a wide gap semiconductor, with the valence band maximum (VBM) ∼ 3.5 eV below the Fermi level, as monitored by photoemission spectromicroscopy and confirmed by theoretical predictions. In fact, the calculated band structure unequivocally identifies the achievement of a stable, double-sided fully hydrogenated configuration, with gap opening and no trace of π states, in excellent agreement with the experimental results.