Imaging the dynamics of an individual hydrogen atom intercalated between two graphene sheets

The interlayer gallery between two adjacent sheets of van der Waals materials is expected to modify properties of atoms and molecules confined at the atomic interfaces. Here, we directly image individual hydrogen atom intercalated between two graphene sheets and investigate its dynamics by scanning tunnelling microscopy (STM). The intercalated hydrogen atom is found to be remarkably different from atomic hydrogen chemisorbed on the external surface of graphene. Our STM measurements, complemented by first-principles calculations, show that the hydrogen atom intercalated between two graphene sheets has dramatically reduced potential barriers for elementary migration steps. Especially, the confined atomic hydrogen dissociation energy from one of the graphene sheet is reduced to 0.34 eV, which is only about a third of a hydrogen atom chemisorbed on the external surface of graphene. This offers a unique platform for direct imaging of the atomic dynamics of confined atoms. Our results suggest that the atomic interfaces of van der Waals materials provide a confined environment to tune the dynamics process of confined atoms or molecules.

Figure 1

The structure of H confined between graphene layers. [(a) and (d)] The side view for ball-and-stick model of bilayer graphene showing the position of the H atom adsorbing. The red ball denotes the hydrogen atom. [(b) and (e)] Typical atomic STM images of hydrogen atom chemisorbed on epitaxial graphene on SiC($000\overline{1}$), corresponding to schematic diagrams shown in (a) and (d), respectively. Sample bias and set points are (0.4 V, 300 pA) and (0.07 V, 300 pA), respectively. The honeycomb structures of graphene are overlaid onto the STM images. [(c) and (f)] The simulated STM images at comparable imaging conditions show similar features to that in (b) and (e), respectively. Image size is 1.48 nm × 1.48 nm.

Figure 2

The height of H intercalated between graphene bilayer. (a)–(c) Atomic STM images of hydrogen atom (highlighted by a yellow circle) chemisorbed on the second layer of graphene. The images are recorded at sample bias of 200, 400, and 600 mV, respectively. The larger and stable bright protrusion may arise from defects of the SiC substrate. (d)–(f) The corresponding simulated images at comparable imaging conditions to that of (a)–(c). They are adjusted into the same contrast for better comparison. (g) shows the profile lines across the hydrogen atom of panels (a)–(c). The scale bar for (a)–(c) is 1 nm. The image size for (d)–(f) is 1.48 nm × 1.48 nm.

Figure 3

(a) The typical STM image of the H atom adsorbed on the outer surface of the topmost graphene layer. Sample bias and setpoint are (50 mV, 200 pA). Scale bar is 1 nm. (b) The simulated STM image at comparable imaging conditions to (a). (c) The shifted profile lines across the H corresponding to the H atom adsorbed on a different layer: outer surface of the layer1 (red line); inner surface of the layer1 (pink line); and layer 2 (purple line). It is clear that the H atom adsorbed on the outer surface of layer 1 graphene is much higher and wider in the STM image.

Figure 4

The diffusion of the confined hydrogen atom. (a)–(d) Several representative STM images showing the diffusion process of the confined hydrogen atom (highlighted by yellow circle) at 80 K. Sample bias and set points for (a), (b), and (d) are (0.6 V, 300 pA); for C, they are (1 V, 300 pA).

Figure 5

The desorption and chemisorption processes of the confined H atom. (a) The hydrogen atom chemisorbed on the inner surface of the first layer. (b) The in-between state of the hydrogen atom, which changes from that chemisorbed on the inner surface of the first layer to that chemisorbed on the second layer. (c) The hydrogen atom chemisorbed on the second layer. [(d) and (e)] The hydrogen atom chemisorbed on the inner surface of the first layer and chemisorbed on the second layer, respectively. The sample bias and set point are (0.4 V, 300 pA) for (a)–(d), and (0.6 V, 300 pA) for (e). The scale bar is 2 nm. We use a triangle and a circle to denote a hydrogen atom chemisorbed on the inner surface of the first layer and one chemisorbed on the second layer, respectively. (f) Line profiles across the H atom taken from (a)–(c) to illustrate the topographic changes.

Figure 6

Energy barriers of diffusion and desorption for the confined H atom. (a) Lattice structure of pristine bilayer graphene in Bernal stacking (side view). The up layer named layer 1, the down layer named layer 2. Two hexagonal sublattices are labeled as A and B. The red ball denotes H atom. (b) The computed barrier for migration of hydrogen atom from A site to B site on the top layer. (c) Hydrogen atom migration barrier from layer 1(A) sublattice to layer 2(B) site. During the calculation, one atom in each of two layers for in-plane direction is fixed to avoid the translation between the two layers. However, the atoms are still free in vertical direction, which allows the changing of graphene-graphene separation during the structural relaxation. (d) Hydrogen atom migration barrier from layer 1(B) site to layer 2(A) site.

Figure 7

The influence of the electric field on the transferred process of the H atom between bilayers of graphene. (a) Schematic diagram to illustrate the direction of the electric field. (b)–(d) The effect of the different electric field on the processes of layer1(A)-layer1(B), layer1(A)-layer2(B), and layer1(B)-layer2(A), respectively.