Article pubs.acs.org/JPCC
Atomic Bonding between Metal and Graphene Hongtao Wang,†,‡ Qiong Feng,‡ Yingchun Cheng,§ Yingbang Yao,† Qingxiao Wang,† Kun Li,† Udo Schwingenschlögl,§ Xi Xiang Zhang,*,† and Wei Yang‡ †
Advanced Nanofabrication, Imaging and Characterization Core Lab and §Materials Science and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 239955, Kingdom of Saudi Arabia ‡ Institute of Applied Mechanics, Zhejiang University, Hangzhou 310027, P. R. China S Supporting Information *
ABSTRACT: To understand structural and chemical properties of metal−graphene composites, it is crucial to unveil the chemical bonding along the interface. We provide direct experimental evidence of atomic bonding between typical metal nano structures and graphene, agreeing well with density functional theory studies. Single Cr atoms are located in the valleys of a zigzag edge, and few-atom ensembles preferentially form atomic chains by self-assembly. Low migration barriers lead to rich dynamics of metal atoms and clusters under electron irradiation. We demonstrate no electron-instigated interaction between Cr clusters and pristine graphene, though Cr has been reported to be highly reactive to graphene. The metal-mediated etching is a dynamic effect between metal clusters and pre-existing defects. The resolved atomic configurations of typical nano metal structures on graphene offer insight into modeling and simulations on properties of metal-decorated graphene for both catalysis and future carbon-based electronics. observations8,13−20 and theoretical studies.7,21−27 So far, our understanding of the atomic bonding mostly relies on calculations based on density functional theory (DFT). Although the simulations offer quantitative information on the interactions, the results of such models depends heavily on the initial setup. Atomic resolution transmission electron microscopy (TEM) is one of most direct methods to unveil the structural information of the metal-graphene interface. However, traditional TEM requires high operating voltages (>100−200 kV) for resolving the graphene lattice, which results in substantial damage to the graphene. Fast structural change, due to the high energy transferred from energetic electrons, leads to great difficulties in capturing snapshots of atomic configurations. By using aberration-corrected and monochromated TEM, we give direct experimental evidence of atomic bonding between Cr and graphene at an operation voltage of 60 kV. Metastable configurations of a single atom, a few-atom ensemble, and subnanometer clusters on graphene edges have been carefully resolved which unveil detailed C-metal bonding, consistent with our DFT simulations. Structural evolution uncovers the dynamic behavior of clusters on defective graphene, which must be taken into account in understanding the mechanisms of metal-mediated etching of graphene.28−31
1. INTRODUCTION Graphene, a monolayer of C atoms bound in a planar lattice, has attracted great interest for its unique structural and physical properties.1,2 Particularly, a high theoretical specific surface area of ∼2600 m2/g makes it attractive to be utilized as a twodimensional catalyst support.3,4 The chemical reactivity can be tuned by decorating graphene or its edge with metal atoms and clusters, which has profound consequences on various practical applications. It has been demonstrated that reduced graphene oxide as a conducting support can anchor semiconductor and metal nanoparticles and realize redox reactions, which opens new opportunities for designing novel catalysts.5 Recently, graphene nano ribbons (GNRs) decorated with metal atoms has been investigated.6−8 Large spin polarizations and magnetoresistance have been found in metal-terminated GNRs. Also GNR-based sensors with edges decorated by metal clusters have shown enhanced sensitivity and selectivity.8 Downsizing the nanoparticles to clusters or single atoms is highly desirable for catalytic reactions because the ultimate low coordination often enhances the efficiency.9,10 For this purpose, our recent research demonstrates that both vacancies and edges in graphenes are effective trapping centers for dispersing single atoms.11,12 However, up to date, there is limited experimental insight into the detailed bonding between metals and graphene, although the interfacial chemical bonding is known to be a key factor in determining the structural and physical properties of metal−graphene composites, as predicted by experimental © 2013 American Chemical Society
Received: November 27, 2012 Revised: February 1, 2013 Published: February 11, 2013 4632
dx.doi.org/10.1021/jp311658m | J. Phys. Chem. C 2013, 117, 4632−4638
The Journal of Physical Chemistry C
Article
Figure S1, ∼0.01 eV) is less than the thermal activation at room temperature, which makes it unobservable to TEM observation. Vacancies and free edges in graphene were efficiently fabricated by adopting the two-step method proposed by Wang et al.12 Figure 1a shows a snapshot of a single Cr atom located in the
2. EXPERIMENTAL AND SIMULATION METHODS TEM studies have shown that surface-adsorbed hydrocarbons, vacancies, and free edges are generally the nucleation sites for metal clusters due to their low affinity to the pristine graphene. 12,14,16,32 In this work, we choose Cr as a representative metal because of its importance as a “wetting layer” in the fabrication of electrical contacts on graphene and its reported strong interaction with graphene.14,16,28 Also, it has been demonstrated to stay metallic on graphene, instead of forming a metal oxide, even after a short exposure to air at room temperature.14 To explore the interaction of metal clusters with the perfect graphene lattice, Cr was deposited for a nominal thickness of about 1−2 Å using a sputtering tool. For comparison, the metal on defective graphene was prepared by adopting the two-step method, proposed by Wang et al., in doping graphene with atomic substitutions.12 Vacancies and free edges were first introduced by bombarding the freestanding graphene with energetic particles and metal deposition was followed by lowering the particle energy. The process can be realized in a common pulsed laser deposition chamber.12 TEM imaging is carried out using an aberration-corrected and monochromated FEI Titan 80-300 microscope with a typical electron beam current density of 7 × 106 e·s−1 nm−2 (i.e., 100 A/cm2). To minimize the knock-on damage to graphene,33 the TEM was operated at 60 kV, corresponding to a maximum transferred energy (11.3 eV) far below the threshold energy (16−17 eV).34,35 However, it is noted that defects may nucleate at contamination sites under electron irradiation with lower energies, which can be caused by beam-induced chemical reactions to adsorbents, such as water or oxygen residues.35,36 The third-order spherical aberration is carefully tuned to about 1 μm. Because the point resolution and the information limit are more sensitive to the chromatic aberration at lower operation voltage, the gun monochromatorwas excited to 1.8 to reduce the electron energy spread to
