15603
2008, 112, 15603–15606 Published on Web 09/12/2008
Detecting a Molecule-Surface Hybrid State by an Fe-Coated Tip with a Non-s-Like Orbital Zhenpeng Hu, Lan Chen, Aidi Zhao, Zhenyu Li, Bing Wang, Jinlong Yang,* and J. G. Hou* Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science & Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed: July 24, 2008
A tungsten tip and an iron-coated tungsten tip were used to investigate cobalt phthalocyanine molecules absorbed on a Au(111) surface. Similar STM images but different STS curves were obtained above the central part of the molecules. Theoretical analysis points out that the delocalized orbital of ligands hybridized with the Au surface at 0.4 eV below Fermi level can be detected and enhanced with an iron-coated tungsten tip due to the extended spatial distribution of the frontier orbital in the tip. As the spatial distribution of the frontier orbital in the tungsten tip is localized, the hybrid state cannot be detected above the central part of the molecule. These results indicate that the appropriateness of selection and preparation of the STM tip can probe richer chemistry and physics on surfaces. In the past two decades, the scanning tunneling microscopy (STM) has become a powerful tool in surface science.1 With STM, it is now a routine task to see a molecule adsorbed on a surface. At high resolution, it is even possible to identify the conformation state and the local density of states (LDOS) of single molecules on surfaces by the topography image and scanning tunneling spectroscopy (STS).2 In principle, the structure of the STS spectrum is determined by the interaction of the sample state and the tip state.3 Practically due to the poor matching between the wave functions of the sample and the tip used, some sample states are very difficult to observe by STS.4,5a Some attempts have been reported to design specific STM tips to probe the target surface states based on molecule adsorption on tips.5 In this Letter, we present a different simple approach to modify a tungsten (W) tip by coating iron (Fe) layers to change the electronic states of the tip. The wavefront of the coated tip is changed from a localized s-like orbital (s, pz, and dz2) to a more extended mix state of dyz(xz) and dxy(x2-y2). The coated tip shows advantages when used to get the STS of cobalt phthalocyanine (CoPc) molecules adsorbed on Au; the intramolecularSTSdataareposition-independent,andanewmolecule-surface hybrid state can be enhanced and detected. In our experiments, the CoPc molecules were thermally evaporated to the Au(111) substrate. Two tips were adopted; one was a tungsten tip which was chemically etched and cleaned by argon ion sputtering, and the other one was a tungsten tip coated by hundreds of nanometers of iron atoms through evaporation (Fe/W tip). Figure 1a shows that CoPc adsorbates disperse on the Au(111) surface with a coverage of about 0.1 monolayer. Figure 1b and c shows topographic images obtained at -1.3 V and 0.4 nA with the two different tips, where color spots point out the positions to detect the differential conductance (dI/dV) spectra. These similar images with a bright spot at the center of the molecule in the negative bias are consistent with previous works1i,6 and show that both tips can provide a good spatial resolution. Although the STM images measured * To whom correspondence should be addressed. E-mail: jlyang@ ustc.edu.cn (J.Y.);
[email protected] (J.G.H.).
10.1021/jp8065508 CCC: $40.75
with these two tips are almost identical, the STS obtained above the center of the molecule (Figure 1d) shows a remarkable difference in the negative bias region. The dI/dV curve measured with the Fe/W tip has four main peaks centered at about -0.1, -0.4, -0.75, and -1.35 V, which are marked with IFe, IIFe, IIIFe, and IVFe, respectively. The peak at -0.4 V has never been reported before and disappears when probing by a W tip. There are only three main peaks in the STS curve acquired with the W tip, which are marked with IW, IIW, and IIIW, corresponding to IFe, IIIFe, and IVFe, and they are similar to those from a previous study at a different temperature.6a The clean Au surface STS is also plotted in Figure 1d, and there are not large differences for the two tips, which indicates that there is no resonant state introduced by the STM tip as mentioned in ref 3b. Away from the center of CoPc, a series of dI/dV spectra measured at different positions is present in Figure 1e. Though measured at different positions, data for the Fe/W tip show nearly the same character, four main peaks with the highest peak at -0.4 V, whereas the peak at -0.1 V become weaker when the tip is farther from the center of the molecule. These data indicate that the peak at -0.1 V may be relative to the electronic state of the central Co ion, and the other three peaks may be relative to the combined electronic states of both the ligand and the Co ion. However, data for the W tip are quite different at different tip positions. There is no clear peak at -0.4 V in all cases, and a slight shoulder appears at -0.4 V only in the B case of Figure 1e. These data show that the W tip is sensitive to the probing positions and the -0.4 V related state may be poorly matching with the tip states. To get insight into the experimental observations, we have performed first-principles calculations with the local density approximation (LDA) using the DMol3 package.7 The structure model and computational method are the same as those in our previous work.1i The calculated density of states (DOS) and energy level diagram are shown in Figure 2a, where the four positions corresponding to the peaks in the STS with the Fe/W tip are marked with I, II, III, and IV, respectively. Since the main differences between the two dI/dV spectra in Figure 1d are in the negative bias region, we focus on the occupied states. As shown in the 2008 American Chemical Society
15604 J. Phys. Chem. C, Vol. 112, No. 40, 2008
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Figure 1. a) STM topographic image obtained with a W tip at a 1.6 V sample bias voltage and with a 0.4 nA set current. b) STM topographic image obtained at -1.3 V and 0.4 nA with a W tip. The red dots are the positions for STS measurements. c) STM topographic image obtained at -1.3 V and 0.4 nA with the Fe/W tip. The blue dots are the positions for STS measurements. d) and e) STS measured above CoPc molecule. Blue curves for the Fe/W tip, and the red curves are for the W tip. The positions where the tip was put are marked above the STS curves. The two STS curves at the bottom of d) were measured for a clean Au surface. The STS curves are shifted vertically for a clearer presentation.
Figure 2. a) Density of states and the energy level diagram of the molecule-substrate system. The lengths of the magenta lines represent orbital populations of CoPc. b) Density of states of the adsorbed CoPc molecule. c) Isosurfaces of the electronic charge density with an isovalue of 1.0 × 10-4 e/Å3 for the orbital at -0.13 (I), -0.42 (II), -0.72 (III), and -1.38 eV (IV), respectively. At -0.72 eV, there are two degenerate orbitals; for easy understanding, we only show one here; the other one can be obtained by rotating the figure by 90°.
energy level diagram, there are molecule-substrate hybrid states at position II, for which the molecular components are small. More directly, we have projected the DOS on the CoPc molecule (Figure 2b) to find the origin of the peaks observed in the experiments. At positions I, III, and IV, there are peaks with large Co contributions, corresponding to the three peaks detected by both the W and the Fe/W tips. There is only a shoulder in the ligand-projected DOS at position II, which should correspond to peak IIFe. Charge densities of the corresponding molecular orbital to the four peaks in DOS are shown in Figure 2c. There are clearly Co contributions in the molecular orbitals at energy position I, III, and IV. However, the electronic density mainly distributes on the ligand at energy position II.
To compare with experiment directly, we performed dI/dV spectra simulation with the tip electronic structure taken into account. Cluster models were used for the two STM tips. Frontier orbitals of the tips were obtained from the results of DFT-LDA calculations. Following the modified Bardeen approach,3a the STM tunneling current can be expanded as a summation of tunneling matrix elements given by the Fermi golden rule
I)
2πe p
∑ [f(εt) - f(εs)]|Mst|2δ(εt - εs + eV)
(a)
ts
where f(ε) is the Fermi-Dirac distribution function, εt and εs are energies of tip and sample relative to the Fermi levels, and V is the bias voltage applied on the STM sample. Mst is the
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J. Phys. Chem. C, Vol. 112, No. 40, 2008 15605
Figure 3. a) Simulated dI/dV spectrum using the W cluster tip model. b) Simulated dI/dV spectrum using the Fe cluster tip model. c) Left: isosurfaces of the electronic charge density with an isovalue of 1.0 × 10-4 e/Å3 for the LUMO of the W tip. Right: cluster model of the W tip. d) Left: isosurfaces of the electronic charge density with an isovalue of 1.0 × 10-4 e/Å3 for the LUMO+1 of the Fe tip. Right: cluster model of the Fe tip.
Bardeen tunneling matrix element between the state of the tip (ψt) and the state of the sample (φs)
Mst ) -
p2 2m
∫Σ [ψt/ ∇ φs - φ/s ∇ ψt]dSb
(b)
where Σ is a separate surface located in the vacuum region between the tip and sample. A simulated dI/dV spectrum with the W tip is presented in Figure 3a, where there are three peaks agreeing well with the experimental data. Figure 3b shows the simulated dI/dV spectrum with the Fe tip, which has four main peaks at almost the same positions as those from the experimental data shown in Figure 1d. The good agreement between theory and experiment indicates that our tip models are reasonable. To know how the spatial distribution of the STM tip orbital influences the STS data, we calculated the frontier orbitals of the two tips. The charge density of the lowest unoccupied molecular orbital (LUMO) in the W tip is shown in Figure 3c, and it is a typical s-like orbital (thus, the Tersoff-Hamann approach8 should be valid). Since the LUMO+1 orbital is the one contributing most to the peak at -0.4 V, the charge density of LUMO+1 in the Fe tip is presented in Figure 3d. Unlike the LUMO of the W tip, it is a more delocalized dyz-like orbital. Due to this nonlocal character, it matches the orbital at about -0.4 eV well and leads to the corresponding peak in the dI/dV spectrum. However, for the W tip, the s-like character dominates the spatial distribution of the active electron around the apex atom. Such a distribution leads to a poor match between the tip and the nonlocal orbital of CoPc at about -0.4 eV and the disappearance of the -0.4 V peak in the dI/dV spectra when measured above the central part of the CoPc molecule with a W tip. As shown in eq a, two factors influence the total tunneling current; one is the DOS, and the other is the tunneling matrix. In our case, the latter plays an important role. As analyzed above, there is a lack of Co contribution at -0.4 eV below the Fermi level; however, a higher peak in the Fe/W tip STS has been obtained in both the experiment and theoretical simulation.
It arises from the good matching between the tip orbital and the sample orbital, which contributes a large tunneling matrix element to enhance the peak in the dI/dV spectrum. Therefore, even if the Fe/W tip is not put above the central part of the molecule, the peak at -0.4 V is still observable due to the nonlocal distributions of the orbitals in both the tip and the molecule. Contrastingly, for the W tip with the localized frontier orbital, only the local electronic information can be detected by dI/dV spectrum. In conclusion, we have presented the dI/dV spectra measured above the CoPc molecule absorbed on a Au(111) surface using two different STM tips and the simulated results with DFT calculations. With large tunneling matrix elements, the Fe/W tip can detect the molecule-surface hybrid state at about 0.4 eV below the Fermi level. As the frontier orbital in the W tip is more localized, it is not easy to probe this state using the W tip. Our results show that some molecule-surface hybrid states can only be probed by a specifically designed STM tip. We hereby demonstrate that there is plenty of room in STM tip preparation to probe richer chemistry and physics on the surface. Experimental Section 1: In our experiments, a piece of Au(111) film on mica of 180 nm thickness was cleaned by argon ion sputtering at 1000 V for 10 min and annealed at 300 °C for 10 min to form a Au(111)-22 × 3 reconstruction. The CoPc molecules, bought from Aldrich Chem. Co., were thermally evaporated to the substrate in an UHV chamber. Then, the sample was transferred to an OMICRON low-temperature STM chamber with a base pressure below 3.0 × 10-11 Torr. The STM measurements were performed at 4.5 K. The W wire of 0.25 mm was purchased from Goodfellow, and the Fe particles were purchased from MaTecK GmbH, Germany. W tips were etched chemically and cleaned by circular argon ion sputtering at 800 V for about 10 min. Fe was coated on the W tip by thermal evaporation from Fe particles in a heated W basket winding.
15606 J. Phys. Chem. C, Vol. 112, No. 40, 2008 2: Our calculations were performed using density functional theory with the local density approximation (LDA) implemented with the DMol3 package. The Vosko-Wilk-Nusair local correlation functional (VWN) was used. The basis sets were double numerical atomic orbitals augmented by polarization functions (DNP), and density functional semicore pseudopotentials (DSPP) were used to take the relativistic effect into account. The self-consistent field procedure was carried out with a convergence criterion of 1.0 × 10-5 au on the energy and electron density. Geometry optimizations were performed with convergence criterions of 5 × 10-3 au/Å on the gradient, 5 × 10-3 Å on the displacement, and 5 × 10-5 au on the energy. Medium grid mesh points were employed for the matrix integrations, and a single Å point was used to calculate the total energy and charge density due to the numerical limitations. Acknowledgment. This work is partially supported by the National Natural Science Foundation of China (50721091, 20533030, 50532040), by National Key Basic Research Program under Grant No. 2006CB922000, by the Shanghai Supercomputer Center, by the USTC-HP HPC project, and by the SCCAS. Supporting Information Available: Experimental and computational details. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Fernandez-Torrente, I.; Monturet, S.; Franke, K. J.; Fraxedas, J.; Lorente, N.; Pascual, J. I. Phys. ReV. Lett. 2007, 99, 176103. (b) Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J. W.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Fre´chet, J. M. J.; Trauner, D.; Louie, S. G.; Crommie, M. F. Phys. ReV. Lett. 2007, 99, 038301. (c) Grobis, M.; Khoo, K. H.; Yamachika, R.; Lu, X.; Nagaoka, K.; Louie, S. G.; Crommie, M. F.; Kato, H.; Shinohara, H. Phys. ReV. Lett. 2005, 94, 136802. (d) Lu, X. H.;
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