Improvement of Scanning Tunneling Microscopy Resolution with H-Sensitized Tips

Recent scanning tunneling hydrogen microscopy (STHM) experiments on PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride)/Au(111) have shown unprecedented intramolecular and intermolecular spatial resolution. The origin of this resolution is studied using an accurate STHM theoretical simulation technique that includes a detailed description of the electronic structure of both the tip and sample. Our results show that ${\mathrm{H}}_2$ molecules are dissociated on the Au tip; the adsorbed H atoms change the density of states at the Fermi level ($E_F$) of the tip, increasing its $p$-orbital character and reducing the $s$-orbital contribution. Also, due to the interaction with the H-decorated tip, $E_F$ is shifted to the middle of the PTCDA lowest unoccupied molecular orbital peak, increasing dramatically the density of states of the sample at $E_F$. These effects give rise to the enhanced STHM resolution.

Figure 1

Comparison of experimental [9] and theoretical STM images for PTCDA on Au(111): (A) Theoretical (t) and experimental (e) STM image for the clean Au tip. (B) $dI/dV$ images at $V_s=0$ for a Au tip with a ${\mathrm{H}}_2$ molecule adsorbed between tip and sample: (t.1) total; (t.2) ${\mathrm{H}}_2$ contribution. (C.e) Experimental STHM image. Top: schematic representation of the last few atoms for the tips used in the theoretical images. Scanning height: (A.t): 3.8 Å; (B.t1 and B.t2): 4.5 Å.

Figure 2

$dI/dV$ images at $V_s=0$ for the herringbone monolayer of PTCDA molecules on Au(111). Scanning height: (C.t1, C.t3): 3.8 Å; (C.t2): 3.5 Å. (C.e) Experimental STHM image [8]. Bottom panel shows the normalized $dI/dV$ along different high-symmetry scanning lines (see bottom left) for the C.t1 image (solid line), and the Au apex (see Fig. 3) contribution (dashed line).

Figure 3

(A) DOS on the PTCDA monolayer. The interaction with the Au tip shifts $E_F$ upwards with respect to the molecular DOS; for the H-decorated Au tip (shown in the inset) $E_F$ is in the middle of the narrow LUMO peak. Middle: different contributions to the STM image of Fig. 1B.t1 coming from the Au apex (also separated into $s$, $p$ and $d$ contributions) and the apex H atom. (B) DOS on the Au apex atom around $E_F$ for the clean ($E_{F_{\mathrm{no}\mathrm{tip}}}$) and H-decorated ($E_{F_{(\mathrm{Au}/\mathrm{H})\mathrm{\text{−}}\mathrm{tip}}}$) Au tips: (i) total; (ii) $d$-orbitals; (iii) $s$-orbitals; (iv) $p$-orbitals.

Figure 4

Potential energy surface for the lateral displacement of the H atom at the tip apex.