PI: Dongxia Shi, Shixuan Du, Li Gao, and Hongjun GaoNanoscale Physics & Devices Laboratory, Institute of Physics, Chinese Academy of Sciences, P.O. BOX 603, Beijing 100080, China

NSFC Grant No.: 90406022, 60621061, 60620120443

Functional organic molecules have attracted tremendous scientific and technological interests due to their promising potential applications in nano-electronics and nano-optoelectronics. One of the most important basic issues is to control the molecular nanostructures and then modulate the related properties. Understanding the molecular structure specifics and controllably fabricating the functional unit cell on the nanometer scale have great promotion for nanodevices and quantum modulation, which are full of research interests as well as challenges.

The adsorptions of functional organic molecular systems on single crystal metal surfaces are systematically investigated recently. The competition of intermolecular interaction and the interaction between molecules and substrates are finely examined in the molecular self-assembly from single molecule, sub-monolayer to monolayer. Two valuable ways are put forward for the modulation of molecular structures and properties. The first way is to change the length of molecular non-functional lateral chains and adjust the molecular structure. The second way is to change the physical and chemical properties of the substrates, which adjust the molecular selectively adsorbed sites on substrate.

The adsorption of Quinacridone derivatives (QA) with 4 and 16 alkyl chains on an Ag(110) substrate was investigated by in situ MBE-LEED, STM, first-principles calculations, and molecular mechanics simulations. The interaction of QAnC and the Ag substrate is primarily due to the chemical bonds between oxygen and silver. The spatial match or mismatch of the O atoms of the molecules with the underlying Ag substrate may determine the orientation and adsorption sites of the molecular backbones. It is therefore in essence determined by geometrical constraints. The lateral alkyl chains are perpendicular to the substrate plane when the number of carbon atoms is less than six. In this case, the ordered structure of the self-assembled molecular monolayer is unaffected by the alkyl chains. However, longer lateral chains tend to be parallel to the molecular plane due to the interaction between QAnC molecules and the substrate. This feature then determines the actual distance and the orientation of the molecules. Figure 1 shows the different adsorbed molecular structures of QA molecules with 4 and 16 carbon alkyl chains on Ag(110) substrates.  Fig. 1(a) and (b) are the predicted geometry of QA4C and QA16C monolayers, respectively, by density-functional-theory and molecular mechanics theoretical calculations. Fig. 1(c) and (d) are the STM images of QA4C and QA16C monolayers, respectively. For QA4C monolayer, the lateral alkyl chains are perpendicular to the substrate plane, which have no influence on the monolayer structure. The chemical bonds between O atoms and the underlying silver atoms determine QA4C monolayer structure. For QA16C monolayer, the longer lateral chains are parallel to the molecular plane, which determines the actual molecular distance and monolayer structure. It is obvious that QAnC molecules can be arranged in a wide variety of distances and arrangements, depending only on the length of the alkyl chains. Taking into account the role of lateral alkyl chains found in the work, the distance between the large aromatic molecules on metal surfaces could be controlled in a discrete manner, which is helpful for designing molecular nanostructures and their applications in molecular devices.

Figure 1 Comparison of simulated and experimental STM images of QA4C and QA16C monolayers on Ag (110). (a) and (b) Predicted geometry of QA4C and QA16C monolayers on Ag (110). (c) STM image of QA4C and scale bar: 5 nm. (Insert: high resolution experimental scans and scale bar: 1 nm) (d) STM image of QA16C and scale bar is 1 nm. The STM images fit theoretical simulations very well.

Non-template selective adsorption of different molecules on different facets was demonstrated. By suitable control, ordered PTCDA structures exclusively on flat (111) facets and DMe-DCNQI structures exclusively on stepped (221) facets were achieved on Ag(775) substrate. Figure 2 shows STM images of PTCDA and DMe-DCNQI on a Ag(775) substrate. (a) 50 nm × 50 nm image of DMe-DCNQI and PTCDA, evaporating DMe-DCNQI first. The (221) facets are covered with DMe-DCNQI molecules while the (111) facets are covered with PTCDA molecules. (b) 30 nm × 30 nm image of DMe-DCNQI/Ag(221) and PTCDA/Ag(111). The parameter-free first-principles density-functional calculations were also used to probe the selective adsorption mechanisms of PTCDA, DMe-DCNQI, and other similar molecules on facets with varying terrace widths and step heights. It is found that the binding energy of PTCDA is larger on the flat (111) facet whereas the binding energy of DMe-DCNQI is larger on the stepped (221) facet, which is in agreement with experimental observations. And more significantly, bonding occurs through only the end-atoms and that the mid-region benzene rings arch away from the substrate. It means that the key factor for adsorption selectivity is bonding via the end-atoms of molecules and substrate. These results offer guidance for the design of non-template facet-selective adsorption of molecules on nanocrystals and bring out a new method for the epitaxial growth of A/B ordered nanostructures. It is demonstrated that the non-template selective adsorption has specific for the adjustment of molecular nanostructures and related properties.

Figure 2. STM images of PTCDA and DMe-DCNQI on a Ag(775) substrate. (a) 50 nm × 50 nm image of DMe-DCNQI and PTCDA, evaporating DMe-DCNQI first. DMe-DCNQI adsorbed on (221) facet, while PTCDA adsorbed on (111) facet. Uncovered (111) areas are visible (triangular frame). (b) 30 nm × 30 nm image of DMe-DCNQI/Ag(221) and PTCDA/Ag(111). High-resolution images and related schematic structures are also displayed.
 
Currently, Kondo resonances with Kondo temperature above room temperature are found for iron phthalocyanine (FePc) molecules on Au(111) substrates in scanning tunneling spectroscopy measurements. It is also shown that the signal of the Kondo resonance depends strongly on the adsorption site of the molecule. Experimental data are verified by extensive numerical simulations, which establish that the coupling between iron states and states of the substrate depends strongly on the adsorption configuration. Figure 3 shows the STM image of FePc molecules adsorbed on Au (111) substrate with two configurations (I and II) and the comparison of their dI/dV spectra measured at the molecular center. The STM image of a single FePc molecule is a “cross” with a bright spot at the center, indicating a flat-lying adsorption configuration. The enhanced brightness at the molecular center is ascribed to the d-orbital character of the Fe(II) d6 system near the Fermi level. It was found that there are two molecular adsorption configurations on the Au(111) substrate. For one configuration, called Configuration I, the “cross” is directed in the   and   directions of the gold substrate; for the other configuration, called Configuration II, the “cross” rotates with respect to the molecular center by ~15° compared to Configuration I. Meanwhile, dI/dV spectra show different features for the two adsorption configurations, a narrow peak for Configuration I but a narrow dip for Configuration II near the Fermi level. There are two other broad resonances, Fe-d (d orbitals of Fe atom) and SS (the surface state of the gold substrate) below the Fermi level. The most likely explanation for the peak and dip features near the Fermi level is that they are signatures of a Kondo resonance. The fitted Fano resonance width Г yields the Kondo temperature TK due to kBTK = Г. It was found that the Kondo temperature TK is 357 ± 21 K for Configuration I and 598 ± 19 K for Configuration II. The results show that both the Kondo temperature and the line shape of dI/dV spectra are greatly influenced by the molecular adsorption configuration on the Au(111) substrate. This implies that it is feasible to control the local spin coupling and the competition between different tunneling channels in molecular Kondo effect by changing the molecular adsorption configuration. The result is significant not only from a fundamental physical perspective, but also for applications in spintronic devices.

Figure 3. STM image of isolated FePc molecules on Au(111) surface and dI/dV spectra for configurations I and II measured at the molecular center. The direction of Au(111) substrate is determined by surface reconstruction. The overlayed grid represents the gold substrate lattice, showing a shift of 1/2 unit cell between the adsorption sites of two types (I and II) of molecules. Fe-d is the Fe-3d state, SS is the surface state and FR is the Fano resonance.

References

1) S.X. Du, H.-J. Gao et al., Phys. Rev. Lett. 2006, 97, 156105

2) D.X. Shi, H.-J. Gao et al., Phys. Rev. Lett. 2006, 96, 226101

3) L. Gao, H.-J. Gao et al., Phys. Rev. Lett. 2007, 99, 106402