Systematic measurement of pentacene assembled on Cu(111) by bimodal dynamic force microscopy at room temperature

A wide range of depositions of pentacene on a Cu(111) surface was studied by frequency modulation bimodal dynamic force microscopy at room temperature. The morphologies and stabilities of the pentacene films were systematically studied via detections of the vertical and lateral tip-sample interactions at sub-molecular-scale resolution. At low coverage, pentacene adsorbs flat on the surface where two phases of molecular assemblies were observed. At the boundary between the different phase domains, a large magnitude of the dissipation energy, arising from the high mobility of pentacene, was measured while no significant increase of the dissipation energy was observed at the domain boundary of different orientations in the same phase. By increasing the amount of the deposited molecules, the second layer with the flatly aligned molecule tilted with a longitudinal molecular axis was observed over a small area, but most of the molecules moved to a certain site and formed crystallites with bulk structure on top of the first flat layer. We found that the work function of the bulk surface was strongly affected by the first flat layer.

Figure 1

(a) DFM topographic image of the pentacene film growth on Cu(111) in an area of $300\times 290$ nm${}^2$. The arrows show the self-assembled pentacene islands. (b) Line profile along A-B. (c) Schematic drawing of the first flat layer of pentacene. Measurement parameters: $\Delta f_{\text{2nd}}=-8.0\mathrm{Hz}$, $A_{\text{2nd}}=1\mathrm{n}\mathrm{m}$, and $V_{\text{bias}}=0.079\mathrm{V}$. The resonance frequency $f_{\text{2nd}}=930315\mathrm{Hz}$, the mechanical quality factor $Q_{\text{2nd}}=8467$, and the spring constant $k_{\text{2nd}}=1400\mathrm{N}/\mathrm{m}$.

Figure 2

(a) DFM topographic image in an area of $50\times 50$ nm${}^2$, indicated by an open box in Fig. 1a. Simultaneously recorded dissipation energy $E_{\text{ts}}$ map for (b) the vertical interaction and (c) frequency shift of the torsional mode $\Delta f_{\text{TR}}$ and (d) the dissipation energy map for the lateral interaction. Schematic drawings of (e) phase I and (f) phase II. Measurement parameters: $50\times 50$ nm${}^2$, $\Delta f_{\text{2nd}}=-110\mathrm{Hz}$, $A_{\text{2nd}}=700\mathrm{p}\mathrm{m}$, $f_{\text{TR}}=1.485786\mathrm{M}\mathrm{Hz}$, $A_{\text{TR}}=50\mathrm{p}\mathrm{m}$, $Q_{\text{TR}}=146379$, and $V_{\text{bias}}=-0.115\mathrm{V}$. The other parameters are the same as those used in Fig. 1.

Figure 3

(a) DFM topographic image and (b) simultaneously recorded $E_{\text{ts}}$ measured via the second flexural mode and (c) $\Delta f_{\text{TR}}$ map and (d) $E_{\text{ts}}$ measured via the torsional mode, near the boundary of the same phase with different orientations, related to the symmetry of the Cu(111) substrate. Measurement parameters: $19\times 19$ nm${}^2$, $\Delta f_{\text{2nd}}=-40\mathrm{Hz}$, $A_{\text{2nd}}=3\mathrm{n}\mathrm{m}$, and $V_{\text{bias}}=-0.383\mathrm{V}$. The other parameters are the same as those used in Fig. 1.

Figure 4

(a) DFM topographic image and (b) simultaneously recorded $E_{\text{ts}}$ map of the 2-ML-coverage pentacene-molecule self-assembly on Cu(111) over an area of $200\times 200$ nm${}^2$. Measurement parameters: $\Delta f_{\text{1st}}=-10\mathrm{Hz}$, $A_{\text{1st}}=10\mathrm{n}\mathrm{m}$, $f_{\text{1st}}=149190\mathrm{Hz}$, $Q_{\text{1st}}=33895$, and $V_{\text{bias}}=-0.306\mathrm{V}$.

Figure 5

(a) DFM topographic image, (b) simultaneously recorded $E_{\text{ts}}$ map of the vertical tip-sample interaction, (c) $\Delta f_{\text{TR}}$ map, and (d) $E_{\text{ts}}$ map of the lateral tip-sample interaction, magnified around the center of Fig. 4. (e) Line profile along A-B in (c). (f) Schematic drawing of the second pentacene layer. The image size is $15\times 15$ nm${}^2$. Arrows show the direction of the self-assembly. Measurement parameters: $\Delta f_{\text{1st}}=-20\mathrm{Hz}$, $f_{\text{TR}}=1.485752\mathrm{M}\mathrm{Hz}$, $A_{\text{TR}}=50\mathrm{p}\mathrm{m}$, $Q_{\text{TR}}=146379$, and $V_{\text{bias}}=-0.306\mathrm{V}$. The other parameters are the same as those used in Fig. 4.

Figure 6

(Color online) (a) DFM topographic image of the sample, where more than 1 ML of pentacene was deposited, in an area of $700\times 700$ nm${}^2$. (b) Corresponding line profile along A-B. (c) Schematic drawing of the crystallized pentacene structure. Measurement parameters: $\Delta f_{\text{2nd}}=-17\mathrm{Hz}$, $A_{\text{2nd}}=1\mathrm{n}\mathrm{m}$, $f_{\text{2nd}}=930324\mathrm{Hz}$, $Q_{\text{2nd}}=8281$, and $V_{\text{bias}}=0.079\mathrm{V}$.

Figure 7

(Color online) (a) DFM topographic image and (b) simultaneously recorded $\Delta f_{\text{TR}}$ map of the crystallized pentacene in an area of $6\times 6$ nm${}^2$. Measurement parameters: $\Delta f_{\text{2nd}}=-260\mathrm{Hz}$, $A_{\text{2nd}}=600\mathrm{p}\mathrm{m}$, $f_{\text{TR}}=1.485789\mathrm{M}\mathrm{Hz}$, $A_{\text{TR}}=50\mathrm{p}\mathrm{m}$, $Q_{\text{TR}}=147021$, and $V_{\text{bias}}=-0.2\mathrm{V}$. The other parameters are the same as those used in Fig. 6.

Figure 8

(Color online) (a) DFM topographic image, (b) simultaneously recorded contact potential difference map, (c) schematic drawing of the first flat and the second standing layers, and (d) corresponding histogram of the CPD map. Measurement parameters: $\Delta f_{\text{1st}}=-100\mathrm{Hz}$, $A_{\text{1st}}=3\mathrm{n}\mathrm{m}$, $f_{\text{1st}}=149750\mathrm{Hz}$, $Q_{\text{1st}}=34122$, and $f_{\text{ac}}=140\mathrm{Hz}$,, and $V_{\text{ac}}=0.5\mathrm{V}$.