Article pubs.acs.org/JPCC
Enhancement of Inelastic Electron Tunneling Conductance Caused by Electronic Decoupling in Iron Phthalocyanine Bilayer on Ag(111) Naoka Ohta,† Ryuichi Arafune,‡ Noriyuki Tsukahara,† Maki Kawai,† and Noriaki Takagi*,† †
Department of Advanced Materials Science, Graduate School of Frontier Science, The University of Tokyo, Kashiwa 5-1-5, Chiba, 277-8561 Japan ‡ International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Ibaraki, 304-0044 Japan S Supporting Information *
ABSTRACT: The effect of electronic decoupling on the inelastic electron tunneling process of iron phthalocyanine (FePc) molecules on Ag(111) was investigated using scanning tunneling microscopy (STM). A drastic difference in the inelastic electron tunneling to individual FePc molecules was found for the first and the second layer molecules grown on Ag(111). The spectrum of the first layer molecule is essentially structureless, whereas the second layer molecules provide giant conductance changes reaching several tens % due to the vibrational excitations. This is the first clear example to demonstrate, by using inelastic tunneling spectroscopy with STM, the enhancement of vibrational inelastic tunneling driven through the electronic decoupling of the molecules from the substrate.
1. INTRODUCTION The vibrational spectrum is utilized for chemical identification as a fingerprint of a molecule so that vibrational spectroscopy is one of the indispensable tools in broad areas of molecular science including biochemistry and physiological chemistry. The advent of scanning tunneling microscopy (STM) enables us to identify chemical species at conducting surfaces on an atomic scale. In particular, the inelastic electron tunneling spectroscopy (IETS) with STM provides vibrational spectra of individual molecules.1−3 When the sample voltage (V) relative to the STM tip meets the condition of |V| > ℏω/e, where ℏω is the energy of the molecular vibration and e is the elemental charge, not only the elastic tunneling but also the inelastic tunneling contributes to the total tunneling current (I), leading to the conductance change in the dI/dV spectrum at |V| = ℏω/e. The conductance change usually gives rise to a step structure in the dI/dV spectrum. Since the changes associated with the vibrational excitations are usually small (at most a few %), they are often buried in the background noise. As a consequence, it is still demanding to pick up the conduction change experimentally in spite of the current leap in the STM instrumentation. The vibrational excitation in the inelastic tunneling process is understood based on the resonant tunneling mechanism.4−9 When an electron tunnels from an STM tip to a substrate through a molecule, it is trapped in the molecular state with a certain lifetime. The formation of this transient state leads to a change of internuclear potential which induces the deformation of the molecular structure and then leaving the vibrationally excited state of the molecule in the electronic ground state after the electron escapes into the substrate. Based on the resonant © 2013 American Chemical Society
tunneling mechanism, the inelastic excitation process is governed by two factors: One is the lifetime of the transient state and the other is the accessibility to the molecular states from the Fermi level. Since one can elongate the lifetime by decoupling the molecule from the substrate,10 it should be possible to enhance the conductance change associated with the vibration excitation through tailoring the strength of the coupling at the molecule−substrate interface. Several studies have demonstrated that the progression of vibronic states is observed for individual molecules on metal substrates by electronically decoupling them from the substrate.11−13 In these studies, the molecules are electronically isolated from the substrate electronic systems by inserting an ultrathin oxide layer11 and monolayer of organic molecules12 or by a self-decoupling scheme where a subunit of the molecule itself works as an electronic decoupler.13 As a result, the lifetime of the temporal anion and/or cation state is sufficiently long that the vibrational ladder of the anion state appears in the density of state spectrum. In contrast, few works have been reported to shed light on the impact of the electronic decoupling on the inelastic excitation of vibration. A molecular bilayer on a metal substrate is a model system to verify the hypothesis about the impact of the electronic decoupling on the IETS process. The first layer works as a buffer layer to isolate the second layer electronically from the substrate electronic system to elongate the lifetime of the electronic state in the second layer molecule. The planar organic molecule such as phthalocyanine (see Figure 1a) and Received: June 26, 2013 Revised: September 24, 2013 Published: October 11, 2013 21832
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molecule in the first layer appears as a four-lobe pattern with a central bright spot. The central bright spot is Fe and the four lobes are aromatic rings. This feature indicates that the molecules adsorb with the molecular plane parallel to the substrate. One pair of the aromatic rings facing each other is aligned along the [110̅ ] direction. Molecules form a quasisquare lattice incommensurate with the substrate. These adsorption features agree with the previous works of FePc adsorbed on Ag(111).23,24 Two types of adsorption configurations are observed for FePc molecules in the second layer, denoted as α and β hereafter. The α species appears as a four-lobe pattern similar to the first layer molecule. The Fe atom of the α species is located just above the Fe atom of the first layer molecule; the molecules form the same periodic array and orientation as those in the first layer. However, the four lobes are not identical, indicating that the molecular plane is slightly tilted with respect to the surface plane. In contrast, the β species looks slightly different from the α species and the first layer molecule. While four aromatic rings of the α species are clearly observed, those for the β species are faint; the β species appears more like a round protrusion. The Fe atom of the β species is located directly above the Fe atom of the molecule of the first layer. Taking a closer look at the STM image, four lobes of the β species are resolved and they are rotated 23 ± 10° with respect to the first layer molecule just below. The α species tend to form large two-dimensional domains, and the β species often make double-line structures at the boundary of the α domains. Based on the STM observation, the structure models of the FePc molecule in the first layer and α and β species are shown in Figure 1c. The IETS spectra are found to strongly depend on the adsorption configuration. Figure 2 shows dI/dV spectra of the FePc
Figure 1. (a) Schematic model of FePc. (b) STM topographic image of molecules in the first and the second layer on Ag(111) measured at 6 K. The scan area was 11.2 × 11.2 nm2. The image was obtained with constant current mode at V = −98 mV and I = 99 pA. (c) Adsorption models for (i) the first layer molecule and (ii) α and (iii) β species in the second layer. Gray dots depict Ag atoms. Red (blue) line describes the molecular shape of the molecule in the first (second) layer.
porphyrine has two advantages to make the model bilayer system. These molecules are characterized with relatively small energy gap (≤2 eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO),14 which guarantees the availability of molecular states required for the inelastic tunneling.4−9 Furthermore, these molecules adsorbed on metal surfaces take a flat-lying configuration with the molecular plane parallel to the surface in both first and second layers.15−25 The configuration with respect to the flow of tunneling current is basically the same for the first and the second layers, which allows us to focus on the effect of interface coupling on the vibrational excitation straightforwardly. We investigated the relation of the electronic decoupling with the IETS process for iron phthalocyanine (FePc) molecules on Ag(111) using STM-IETS. We have found that the giant conductance changes due to the vibrational excitations appear only for the molecules in the second layer, whereas the conductance changes are negligible for the first layer molecules.
2. EXPERIMENTAL METHODS All of the experiments were performed in an ultrahigh vacuum chamber with base pressure of 5 × 10−11 Torr. A Ag(111) surface was cleaned by repeated cycles of Ar ion sputtering (600 eV) and annealing at 720 K. FePc molecules were deposited by heating (593 K) FePc powder onto the clean Ag(111) surface held at room temperature. An electro-chemically etched tungsten wire was used as an STM tip. All of the STM measurements were performed at 6 K. The dI/dV spectra were measured with a lockin technique by imposing a sinusoidal modulation voltage Vmod of 0.4−4 mV at 312.6 Hz to the sample bias voltage V.
Figure 2. IETS spectra of the (top) first layer FePc and (middle) α and (bottom) β species in the second layer. Black lines show the spectra taken at 6 K with the feedback loop switched off at V = −100 mV and I = 1 nA and the green broken lines are the fit with eq 2 (see text). The modulation voltage of 0.4 mV was imposed to the sample voltage for the lock-in measurement. Blue, red, and cyan lines are the symmetric, asymmetric, and background components, respectively.
molecule in the first layer and α and β species in the second layer measured by holding the STM tip above the Fe atom. The spectrum of the first layer molecule does not show noticeable features. In contrast, several conductance steps arising from the inelastic tunneling excitations appear in the spectra of the α and β species. Some of the steps are surprisingly large, so that we do not need to measure the second derivative of the tunneling current using a lock-in technique to identify the excitations. The spectral
3. RESULTS AND DISCUSSION An STM image of FePc on Ag(111) is shown in Figure 1b. The FePc molecules in the first and second layer are observed in the right darker area and the left brighter area, respectively. The 21833
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β species. Δσ/σ is 58, 29 and 32% at 51, 103, and 121 mV, respectively. Such large Δσ/σ is often accompanied by the change in the molecular conformation induced by the tunneling electrons27,28 as demonstrated for a pyrrolidine molecule on Cu(001).27 When the energy of tunneling electrons exceeds a certain threshold, the conformation change is excited and then Δσ/σ drastically changes at the threshold where low- and highconductance states are switched. In this case, the STM image of the molecule in the low conductance state is different from that in the high-conductance state. In our case however, the STM images of both α and β species are identical in the voltage range between −200 and +200 mV. Thus, we think the large Δσ/σ observed for both species is not associated with the conformational change. Now let us see the large Δσ/σ qualitatively from the viewpoint of the electronic decoupling of the second layer molecules from the substrate. Considered in a simplest condition of the resonant tunneling mechanism, which the electron conducts a single molecular state, Δσ/σ is simplified as follows:4
features of both α and β species are well reproduced and do not depend basically on the local environments where the molecules are located. For example, the spectrum of the α species neighboring the β species is essentially the same as those of the other α species. To gain better understanding of the IETS process, we compared the spectral line shape with a theory of Paulsson et al.5 The spectral shapes for both α and β species are well reproduced by the theory as shown in Figure 2. Following the theory, the total current which includes the contribution of both elastic and inelastic current due to vibration excitations is described as Itot =
e 2V τE + πℏ F +
∑ ASymλ I Sym(V , ℏωλ , T , nλ)
∑A
Asym
λ λI
Asym
(V , ℏωλ , T ) (1)
λ
where I Sym = I Asym =
ℏωλ − eV ℏωλ + eV e (2eVnλ + (ℏω − eV ) − (ℏω + eV )/ kT πℏ e λ − 1 e λ )/ kT − 1 e 2π ℏ
ΔE2 − ((Δs + Δt )/2)2 Δσ δε 2 = σ ΔE2 + ((Δs + Δt )/2)2 ΔE2 + ((Δs + Δt )/2)2
−∞
∫∞
[nF(E) − nF(E − eV )]H {nF(E′ + ℏωλ)
(3)
− nF(E′ − ℏωλ)}(E) dE
Here, δε is the electron−phonon coupling, ΔE is the energy separation between the molecular state and the Fermi level, and Δs(t) represents the coupling between the molecule and the substrate (tip). Equation 3 indicates that the conductance change is enhanced when Δs decreases, i.e., the coupling is reduced. In general, the first layer molecules are more strongly coupled to the substrate than the second layer molecules. We validate that this general trend holds for FePc on Ag(111) in the following. Figure 3 shows a wide-range STS spectra of the first layer FePc and α and β species measured right above the Fe atom. Molecules examined are the same molecules whose IETS spectra are displayed in Figure 2. For the first layer FePc, some structures are observed near the Fermi level. The α species show a large peak ranging from +0.5 to +1.5 eV together with the small features around the Fermi level similar to those observed for the first layer molecule. In addition, the α species shows the negative differential conductance around +1.5 eV. For the β species, a peak is observed at +1.1 eV with a shoulder structure at +0.7 eV as well as the dip structure around the Fermi level due to the inelastic excitation of the molecular vibrations as discussed above. The spectral evolution from the first layer to the second layer reasonably matches with the results of Gopakumar et al.23 who measured STS spectra of FePc on Ag(111) as a function of the FePc coverage. According to their STS results together with the density functional calculations, the electronic structure around the Fermi level observed for the first layer FePc is explained by the hybridization of the Fe 3d states with the Ag 5s states. The energy positions of the Fe 3d states according to ref 23 are marked by the bars in Figure 3. The existence of the interface states near the Fermi level with the Fe 3d character is also reported by Petraki et al.,25 who studied the same system using a combination of X-ray absorption spectroscopy and photoelectron spectroscopy. The rather sharp structures at +0.5 to +1.5 eV observed for the second layer FePc must also have the Fe 3d character because the spectrum is obtained at the Fe site. The appearance of the clear structure and evolution of the gap around the Fermi level in STS spectra of α and β species, thus, indicate the decoupling of the electronic state from the
The index λ specifies a phonon mode occupied by nλ phonons with energy ℏωλ, nF is the Fermi-Dirac function, and H{f(E′)}(E) = 1/πP ∫ f(E′)/(E′−E) dE′. τEF is the transmittance at the Fermi level. The coefficients ASymλ and AAsymλ are given by the Green’s function at the Fermi level and the electron−phonon coupling. Differentiating eq 1 yields the spectral function as dItot /dV =
∑ ASym λ λ
+
dI Sym(V , ℏωλ , T , nλ) dV
∑ AAsym λ λ
dI Asym(V , ℏωλ , T ) + background dV
(2)
We assumed a parabolic curve as the background and set nλ = 0 for all of the phonon modes because they are not thermally populated at the cryogenic temperature of 6 K. The fitting parameters are ℏωλ, ASymλ, AAsymλ, and kTλ, which characterize the λ mode. Note that Tλ is an effective temperature to include the broadening caused by the sample temperature, modulation voltage applied to the sample bias for the lock-in measurement, and the vibrational lifetime. Optimum values for kTλ were 1 and 2 meV for the spin and vibration excitations, respectively. We had to include nine (ten) modes to fit the IETS spectrum of the α (β) species by using eq 2. ISym mainly determines the inelastic current, but the finite value of IAsym is required to reproduce the spectral shape (Supporting Information). Table 1 shows the list of the excitation energies of the steps observed for the α and β species together with the candidates of their origins (the origins of the steps at 2 and 6 meV will be discussed later). The assignments were made by comparing the step energies with the vibrational energies calculated for a free FePc molecule.26 Note that neither overtones nor combination modes were taken into account in the fitting procedure. As summarized in Table 1, the conductance steps of the α and β species appear at different energies though they are located in the second layer. Large conductance change (Δσ/σ) is observed for both α and β species. For the α species, Δσ/σ is 23, 9 and 9% at 12, 115, and 130 mV, which is larger than the values reported previously in the literatures.1−3 Giant Δσ/σ is observed for the 21834
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Table 1. Step Energiesa in the IETS Spectra of (a) α and (b) β Species and Their Assignments Together with the Vibrational Energies Calculated by Liu et al.b (b)
(a) calculationsb
exp. (meV) 2 6 12
45
65
85
115
130
140
a
ℏω (meV(cm−1))
13.8 (111) 14.6 (118) 15.1 (122) 43.2 (348) 43.2 (348) 63.0 (508) 68.6 (553) 83 (669) 83.7 (675) 86.6 (698) 88.4 (713) 114.6 (924) 114.7 (925) 115.8 (934) 127.2 (1026) 130.9 (1056) 137.4 (1108) 138.5 (1117) 139.7 (1127) 142.8 (1152) 143.1 (1154)
sym.
calculationsb
exp. ℏω (meV) ℏω (meV(cm−1))
assignment
B2g
spin excitation spin excitation Iso. IPB
2 6 18
Eu
Iso. IPB
38
Eg
Iso. OPB
A2u
ring NM OPB
Eu
Iso. NM IPB
Eu
Iso. NM IPB
B1g
Ben. exp., NM str.
A1g
CNmC IPB, Iso. exp.
B2g
Ben. def.
Eg
CH Py. OPB
A2u
CH Py. OPB
Eg
CH OPB
A2u
CH OPB
B2g
NM CNmC IPB, Iso. def.
B2g
NM IPB, ring str., Ben. def.
103
Eu
Ben. def. CH IPB, CNm CαCβ str.
121
Eu
CH IPB, CαCβ str.
A1g
CH IPB, Iso. NM str.
B1g
CH IPB
A1g
CH IPB
Eu
CH IPB
51
65
85
96
20.1 (162) 36.6 (295) 37.1 (299) 52.2 (421) 53.8 (434) 63.0 (508) 68.6 (553) 83.0 (669) 83.7 (675) 86.6 (698) 88.4 (713) 93.7 (756) 95.8 (773) 98.1 (791) 102.5 (827) 118.5 (956) 123.7 (998) 123.7 (998) 123.9 (999)
sym.
assignment
A2u
spin excitation spin excitation Iso. NM OPB
Eu
ring boa.
A2u
NM iso. OPB
Eg
CβCγCδ OPB
A2u
CβCγCδ OPB
Eu
Iso. NM IPB
B1g
Ben. exp., NM str.
A1g
CNmC IPB, Iso. exp.
B2g
Ben. def.
Eg
CH Py. OPB
A2u
CH Py. OPB
A2u
CH OPB
B1g
NM Iso. str.
Eu
NM str., Ben. def., Iso. bre.
A1g
NM Py. str., Ben. exp., CNmC IPB
Eg
CH OPB
Eu
CH IPB, Ben. exp
B1g
CγH IPB, Ben. exp.
A1g
CγH IPB, Ben. exp.
The step energies are determined by fitting the IETS spectra with eq 2 (see text). bReference 26. The abbreviations: IPB., OPB., str., def., exp., bre., boa., Iso., Py., and Ben. represent in-plane bending, out-of-plane bending, stretching, deformation, expanding, breathing, boating, isoindole, pyrrole, and benzene, respectively.
substrate compared to the first layer FePc, which interacts strongly with the substrate. Interestingly, the step energies of the α and β species are different except for steps at 65 and 85 meV. The difference is rationalized by (i) the shifts in the vibrational energies due to the difference in the adsorption configurations (if the same modes are excited) or (ii) the excitation of the different vibrational modes. The α and β species are located in the second layer and are weakly interacting with the first layer
molecules. Although the STM images are slightly different, the vibrational signatures of these species should resemble each other. It is unlikely that the vibrational energies of the same vibrational modes are largely different. Thus, the latter explanation is more favorable; the different vibrational modes are excited. The underlying mechanism of such mode-selective excitations is not well understood in the present stage. The difference in the stacking relative to the first layer FePc may give a key to solve this puzzle. While the α species is just 21835
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configurations (α and β) exist. The spectra of both species are different in the step energy and the conductance change caused by the vibrational excitations. The giant conductance changes reaching several tens % are observed for the β species. The large conductance change in the IETS spectra of the second layer molecules is qualitatively rationalized by the decoupling from the substrate electronic system. These results provide the possibility to manipulate the IETS process by tuning the interface coupling.
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ASSOCIATED CONTENT
* Supporting Information S
Values of each parameter for the fit shown in Figure 2. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
Figure 3. STS spectra of the (top) first layer FePc and (middle) α and (bottom) β species. Each spectrum was taken at 6 K with the feedback loop switched off at V = −500 mV and I = 0.3 nA. The modulation voltage of 4 mV was imposed to the sample voltage for the lock-in measurement. The black bars indicate the energies where Fe 3d states exist according to calculations in ref 23.
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported partly by Grants-in-Aid for Scientific Research (No. 21225001) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and World Premier International Research Center Initiative (WPI), MEXT, and N.O. thanks financial support by Grants-in-Aid for Scientific Research (No. 24·5853) from Japan Society for the Promotion of Science (JSPS).
overlapped with the first layer FePc, the β species is rotated with 23 ± 10°. If the inelastic tunneling takes place mainly through the 3dzx and 3dyz orbitals, the rotation may enhance the decoupling of these orbitals with their counterparts of the first layer FePc, resulting in the excitations of the different vibrational modes. Unfortunately, the selection rules of STMIETS have not been established yet. Thus, we cannot provide the microscopic picture on the relation of the enhancement in conduction with lifetime elongation (decoupling), the modeselective excitations, and the difference in the IETS spectra between the α and β species in the present stage. Detailed theoretical analysis is desired in order to uncover these points. Finally, we discuss the origins of the steps observed at 2 and 6 meV at 6 K, which are well resolved at ±1.4 (±1.5) and ±6.0 (±6.0) meV for the α (β) species when measured at 400 mK (not shown). Applying a magnetic field perpendicularly to the sample surface, the steps of the α species move to 2.0 and 6.3 meV at 11 T. Thus, these steps are assigned to spin excitations between the spin sublevels of the triplet ground state split by the zero field splitting (ZFS).29 A similar doublestep structure has been observed for FePc adsorbed on the oxidized Cu(110) surface.30 On the Cu(110) (2 × 1)-O surface, two steps associated with the ZFS appear at ±1.9 (±4.1) and ±4.7 (±9.0) meV, which energy values depend on the adsorption configuration (in the parentheses are for a rotated species). This result also supports the assignment of the steps observed for the α species. The magnetic field evolution of the steps for the β species is not observed clearly compared to the α species. However, the step energies are almost the same as those of the α species so that they are also considered as being associated with the spin excitations of the β species.
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REFERENCES
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4. SUMMARY We investigated the IETS spectra of individual FePc molecules in the first and the second layers grown on the Ag(111) surface. It is found that the IETS spectra significantly depend on the environment where FePc molecules are placed. The spectra taken for the first layer molecules are essentially structureless, whereas multiple conductance steps appeared in the spectra for the molecules in the second layer where two types of 21836
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The Journal of Physical Chemistry C
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dx.doi.org/10.1021/jp406317t | J. Phys. Chem. C 2013, 117, 21832−21837
