Clustering of Chemisorbed H(D) Atoms on the Graphite (0001) Surface due to Preferential Sticking

We present scanning tunneling microscopy experiments and density functional theory calculations which reveal a unique mechanism for the formation of hydrogen adsorbate clusters on graphite surfaces. Our results show that diffusion of hydrogen atoms is largely inactive and that clustering is a consequence of preferential sticking into specific adsorbate structures. These surprising findings are caused by reduced or even vanishing adsorption barriers for hydrogen in the vicinity of already adsorbed H atoms on the surface and point to a possible novel route to interstellar ${\mathrm{H}}_2$ formation.

Figure 1

STM images of the graphite surface after D atom deposition. During deposition and in the STM the sample temperature is kept below 210 K. (a) STM image after deposition at low integrated flux ($F\sim {10}^{12}\mathrm{atoms}/{\mathrm{cm}}^2$). (b) STM image after heating the sample in (a) to room temperature (RT) for 10 min followed by recooling in the STM. (c) STM image after D atom deposition at high integrated flux ($F\sim {10}^{14}\mathrm{atoms}/{\mathrm{cm}}^2$). (d) STM image after heating the sample in (c) to RT for 10 min followed by recooling in the STM. Imaging parameters: (a) $I_t=-0.58\mathrm{nA}$, $V_t=-312\mathrm{mV}$, (b) $I_t=-0.45\mathrm{nA}$, $V_t=-1250\mathrm{mV}$, (c) $I_t=-0.43\mathrm{nA}$, $V_t=-1250\mathrm{mV}$, (d) $I_t=-0.26\mathrm{nA}$, $V_t=-1486\mathrm{mV}$.

Figure 2

STM images of the graphite surface after increasing doses of D atoms showing the formation of increasingly more complex adsorption clusters. (a) Zoom in on one of the bright protrusions in Fig. 1a. (b) STM image showing the formation of hydrogen dimer structures after a 1 min D atom dose at a flux of $F={10}^{15}\mathrm{atoms}/{\mathrm{cm}}^2\mathrm{s}$. (c) STM image showing the formation of larger and more complex adsorption clusters after a 6 min D atom dose at a flux of $F={10}^{15}\mathrm{atoms}/{\mathrm{cm}}^2\mathrm{s}$. (d) STM image showing the graphite surface at saturation coverage after a 24 min D atom dose at a flux of $F={10}^{15}\mathrm{atoms}/{\mathrm{cm}}^2\mathrm{s}$. The underlying carbon lattice is no longer visible and the image is dominated by the electronic reconstruction induced by the hydrogen adsorbates. Imaging parameters: (a) $I_t=-0.71\mathrm{nA}$, $V_t=-156\mathrm{mV}$, (b) $I_t=-0.16\mathrm{nA}$, $V_t=-884\mathrm{mV}$, (c) $I_t=-0.150\mathrm{nA}$, $V_t=-743\mathrm{mV}$, (d) $I_t=-0.53\mathrm{nA}$, $V_t=-1051\mathrm{mV}$.

Figure 3

(a) Potential energy curves for adsorption of a single H atom monomer (solid line), shown in (b), and for the three dimer configurations shown in (c), the orthodimer (dash-dotted line), the next neighbor site (dashed line), and the paradimer (dash-double dotted line).

Figure 4

Potential energy curves for adsorption of a third H atom in the vicinity of a paradimer at sites s1–s5 shown in the inset. The reaction coordinate is a sum over all position changes of all atoms in the calculation.