Intramolecular bonds resolved on a semiconductor surface

Noncontact atomic force microscopy (NC-AFM) is now routinely capable of obtaining submolecular resolution, readily resolving the carbon backbone structure of planar organic molecules adsorbed on metal substrates. Here we show that the same resolution may also be obtained for molecules adsorbed on a reactive semiconducting substrate. Surprisingly, this resolution is routinely obtained without the need for deliberate tip functionalization. Intriguingly, we observe two chemically distinct apex types capable of submolecular imaging. We characterize our tip apices by “inverse imaging” of the silicon adatoms of the $\mathrm{Si}\left(111\right)-7\times 7$ surface and support our findings with detailed density functional theory (DFT) calculations. We also show that intramolecular resolution on individual molecules may be readily obtained at 78 K, rather than solely at 5 K as previously demonstrated. Our results suggest a wide range of tips may be capable of producing intramolecular contrast for molecules adsorbed on semiconductor surfaces, leading to a much broader applicability for submolecular imaging protocols.

Figure 1

(a) STM image taken of an NTCDI molecule adsorbed on the $\text{Si}\left(111\right)-7\times 7$ surface at 78 K, image parameters ${\text{V}}_{\text{gap}}=+1.5$ V, $I_t=40$ pA. (b) Constant height NC-AFM $\Delta f$ image, taken with the same tip. Oscillation amplitude $A_0=0.28$ nm. Image size 3.8 nm $\times$ 3.8 nm. Green arrows highlight nearby passivated silicon adatoms. (c) Ball and stick model of NTCDI adsorbed on the $\text{Si}\left(111\right)-7\times 7$ unit cell.

Figure 2

(a)–(d) Series of constant height images taken at decreasing tip-sample separation with a tip demonstrating attractive adatom contrast. Relative tip heights 0, $-0.075,-0.14$, and $-0.20$ nm, respectively. $A_0=0.11$ nm. Image size $2.2\text{nm}\times 2.2$ nm. (e)–(h) Series of constant height images with a tip demonstrating repulsive adatom contrast. Relative tip heights 0, $-0.08,-0.19$, and $-0.28$ nm, respectively. $A_0=0.28$ nm. Image size $3.5\text{nm}\times 3.5$ nm. Datasets acquired at 5 K.

Figure 3

Additional images of NTCDI acquired with tip apices which yield repulsive adatom contrast at (a) 78 K and (b) and (c) 5 K. The left-hand color bars give the $\Delta f$ scales for the upper parts of each images (molecules), the right-hand bars for the lower part (adatoms). Note the tip in image (b) shows a slight double feature over the molecule suggesting either multiple, or tilted, molecular adsorption on the tip.

Figure 4

(a) Experimental tip-sample forces $F\left(z\right)$ on an NTCDI molecule and a $\text{Si}\left(111\right)-7\times 7$ adatom for attractive and repulsive adatom contrasts (data taken from the sequences shown in Fig. 2). Note that the attractive adatom data has been shifted +80 pm on the $z$ axis to align the minima of the molecular interaction curves of the two datasets. Inset: Experimental data from (a) showing detail of the molecular force turnaround. (b) Calculated $F\left(z\right)$ curves modeled using DFT for the four tip models shown in (c)–(e), over a silicon adatom. Fixed tip atoms are grayed out while surface atoms which are free to relax are shaded in green.

Figure 5

(a) Simulated $F\left(z\right)$ for three additional tip terminations: two additional NTCDI tips (side down and planar) and an additional H-terminated H3-type silicon cluster (tip models shown inset). (b) Simulated $F\left(z\right)$ for a bare silicon dimer tip over the oxygen atom of a surface-adsorbed NTCDI molecule (location marked with an “x” in the inset diagram).

Figure 6

Simulated constant height force slices for an NTCDI terminated tip on a $\text{Si}\left(111\right)-7\times 7$ adatom. (a) Calculated constant height force slice with the O-down tip shows the adatom as a bright repulsive feature. The red dashed box marks the calculated points which are then bred onto the rest of the image. The green dotted box shows the uninterpolated pixel size, the $F\left(z\right)$ at this point is shown in (c) with an inset electron density different plot illustrating the electron repulsion between the tip and sample at an isovalue of 0.02 $e/{\mathrm{bohr}}^3$. (e) Progression of constant height slices at decreasing tip-sample separations at heights marked by i–iv in (c). (b), (d), and (f) Equivalent data for an H-down tip orientation.

Figure 7

Image acquired of two NTCDI molecules with a “double lobe” attractive tip. Arrows indicate positions where the tip height was altered relative to the starting tip height (slow scan direction bottom to top). Right-hand color bar corresponds to contrast over the molecules, left-hand bar to contrast over the adatoms. $A_0=0.275$ nm. Image acquired at 78 K.