Laser-induced preferential dehydrogenation of graphane

Abstract

We have used first-principles simulations based on time-dependent density functional theory to show that short laser pulses can trigger preferential hydrogen desorption from the upper or lower side of suspended graphane (H-terminated graphene). This control is achieved by using intense ultrashort p-polarized laser pulses (∼2 fs) with an asymmetric time envelope. The dynamical Stark effect induced by the pulse creates an asymmetric charge distribution and force field on the H ions, even at low laser fluence. At finite temperatures the carbon-hydrogen stretching softens, favoring H desorption from one side. This transient geometry can be modified by halogen functionalization, which results in a two-dimensional dipolar structure. Single-layer graphene is fabricated by mechanically peeling off a layer of carbon from pyrolytic graphite.1 Graphene's unique electronic, chemical, and mechanical properties and its potential applications have been extensively studied. Chemically modified graphene has been investigated, 2 and hydrogen (H)-terminated graphene, known as graphane, has also been found to be theoretically stable.3 Further theoretical studies have examined its structural, magnetic, and optical properties, 4-9 and its experimental synthesis has also been investigated. 10,11 Graphene that is H terminated on only one side has also been proposed for use in spin-polarized devices with the structure known as graphone, 12 or as a gapped semiconductor material with full H coverage. 13In this Rapid Communication we propose a way of H desorption, which is mainly from one side of a suspended graphane sheet, with the use of an asymmetric pulse of the femtosecond laser by performing first-principles simulations. Further chemical modification of this side with other reactive species can reach the heterogeneously terminated graphene, which was recently studied.14 We emphasize that this asymmetrically biased desorption is thermodynamically difficult and thus has not been considered in former theoretical studies for stability, 15,16 because the carbon-hydrogen (C-H) bond is the same on both sides. The femtosecond laser pulse has an asymmetric electric field (E-field) time envelope in order to induce dehydrogenation on one side of the graphane in our first-principles molecular dynamics calculations, based on the Ehrenfest time-dependent density functional theory (E-TDDFT) framework. 17,18 In this work, the z axis is set normal to the graphane sheet with the sheet at z = 0, and z > 0 and z < 0 are taken as the upper and lower regions, respectively (Fig. 1). An E-field polarized normal to the graphane sheet with the envelope shown in Fig. 2 results in a positive (upward) force on the ions and a negative force on the electrons. The H atoms in the upper region are desorbed from the sheet by this pulse, whereas the H atoms in the lower region remain bound. Thus, the upper graphene region remains chemically active, making this graphone sheet transient, which can be used for further functionalization. 19 We provide a complete microscopi...

Flexible Organic Photovoltaic Cells with In Situ Nonthermal Photoreduction of Spin‐Coated Graphene Oxide Electrodes

“…Upon excitation with ns pulses e−h recombination occurs during the pulse and heat reduction of GO dominates. It should be noted that the threshold fluence required to reduce the sheet resistance in our experiments, 3.5 mJ cm −2 (corresponding to 1 mW output power), is close to the recently predicted theoretical value for non‐thermal removal of oxygen atoms from the GO lattice 48…”

“…With the progression of laser irradiation, thermal effects become more and more pronounced46 giving rise to photohermal breakage of carbon bonds, which increases the number of defects in the lattice and in turn leads to the formation of smaller and smaller crystalline graphene domains 47. It is concluded that removal of oxygen atoms from graphene sheets takes place without causing any damage to the lattice by precise tuning of the laser power and pulse number, which is consistent with recent theoretical calculations 48. The presence of non‐thermal and thermal components during the reduction process is further supported by two additional experimental findings: a) contrary to the fs case, the D/G peak intensity is rapidly and monotonically increasing upon irradiation with nanosecond (ns) laser pulses of the same incident fluence (Supporting Information Figure S5); and, b) the ablation threshold was shifted to lower fluences when the pulse repetition rate is increased from kHz to MHz (Supporting Information Figure S6) indicating cumulative effects due to successive pulses.…”

Photoreduction of Graphene Oxides: Methods, Properties, and Applications

“…Later, the femtosecond laser irradiation‐induced effective reduction of GO was confirmed by performing a first‐principles simulation of electron–ion dynamics based on the time‐dependent density functional theory . In the simulations, a laser shot was mimicked by an alternating electric field (E field), and electron–ion dynamics were performed for a GO sheet.…”

Novel graphene-based nanostructures: physicochemical properties and applications

“…Dehydrogenation of graphane is also possible under the action of short (*2 fs) laser pulses. 121 The systems thus fabricated can be represented by a GNR confined between high potential barriers originated from graphane. 51 Therefore, one can expect that the electronic properties of`nanoroads' will be similar to those of GNRs.…”