Controlling the Kondo Effect of an Adsorbed Magnetic Ion Through Its Chemical Bonding

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Science  02 Sep 2005:
Vol. 309, Issue 5740, pp. 1542-1544
DOI: 10.1126/science.1113449


We report that the Kondo effect exerted by a magnetic ion depends on its chemical environment. A cobalt phthalocyanine molecule adsorbed on an Au(111) surface exhibited no Kondo effect. Cutting away eight hydrogen atoms from the molecule with voltage pulses from a scanning tunneling microscope tip allowed the four orbitals of this molecule to chemically bond to the gold substrate. The localized spin was recovered in this artificial molecular structure, and a clear Kondo resonance was observed near the Fermi surface. We attribute the high Kondo temperature (more than 200 kelvin) to the small on-site Coulomb repulsion and the large half-width of the hybridized d-level.

The Kondo effect arises from the coupling between localized spins and conduction electrons, and at sufficiently low temperatures, it can lead to change in the transport properties through scattering or resonance effects (1). The Kondo effect is often studied in systems where spins are permanently introduced into the sample through magnetic ions, and recently the Kondo effect has been controlled in quantum dot systems by changing their charging and hence the spin state of the dots (2-13).

We show here that the Kondo effect arising from magnetic ions on the surface of a nonmagnetic conductor can be controlled by changing their chemical environment. In particular, we show that Co ions, when adsorbed on a gold surface as cobalt phthalocyanine (CoPc), do not interact strongly with conduction electrons and exhibit no Kondo effect. However, after dehydrogenation of the ligand by voltage pulses from a scanning tunneling microscope (STM) tip, the Kondo effect is recovered.

Single CoPc molecules adsorbed on the terraces of an Au(111) surface exhibit a protruding four-lobed structure that is consistent with the molecular symmetry (Fig. 1, A and D) (14). Dehydrogenation of a CoPc molecule was realized with a local high-voltage pulse from the STM tip in a manner similar to the case of benzene on copper surfaces (15, 16). We initially used a constant current mode with relatively low bias voltage and tunneling current (typically voltage |V| < 2 V and current I < 0.5 nA) to image isolated CoPc molecules. We then placed the STM tip directly over the edge of a lobe, temporarily suspended the feedback loop, and applied a positive high-voltage pulse (Fig. 1B). A typical current trace simultaneously measured during the application of a 3.6-V pulse on one of the four lobes of a CoPc molecule (Fig. 1C) shows two sudden drops in the current signal, indicating the sequential dissociation of the two H atoms from the benzene ring. We found the dehydrogenation threshold voltage to be in the range of 3.3 to 3.5 V, depending on the structure of the tip apex.

Fig. 1.

STM tip-induced dehydrogenation of a single CoPc molecule. (A) Structural formula of the CoPc. Hydrogen atoms 2 and 3 of one lobe were dissociated in our experiments. (B) Diagram of the dehydrogenation induced by the STM current. (C) Current versus time during two different voltage pulses on the brink of one lobe. Black and red lines correspond to 3.3 V and 3.6 V, respectively. (D to H) STM images of a single CoPc molecule during each step of the dehydrogenation process, from (D) an intact CoPc to (H) d-CoPc. Image area, 25 Å by 25 Å. The color scale represents apparent heights, ranging from 0 Å (low) to 2.7 Å (high).

Topographic images of the dehydrogenation product show that the bright lobes disappear sequentially (Fig. 1, E to H). The apparent height of the molecular center (the Co ion) initially increases slightly (by ∼0.15 Å), while the intact CoPc (Fig. 1D) is converted to a three-lobes-dehydrogenated CoPc (Fig. 1G). After the last step, when all four lobes were cut to obtain the final dehydrogenated CoPc (d-CoPc) molecule (Fig. 1H), a marked increase of ∼0.8 Å in apparent height at the center indicated either a strong conformational change of the molecular structure or a redistribution of the local density of states of the molecule. Moreover, the d-CoPc molecule on the Au(111) surface was difficult to move with the STM tip, indicating a strong interaction between the molecule and substrate (figs. S1 and S2).

Typical differential conductance dI/dV spectra near the Fermi level (EF) (Fig. 2A) were measured precisely at the center of an intact CoPc and a d-CoPc molecule with the same tip. The dI/dV spectra were obtained by sinusoidally modulating the bias voltage (4 mV in amplitude) with the first-harmonic current signal detected through a lock-in amplifier. For the intact CoPc molecule at 5 K, there is a broad resonance centered around 150 meV below EF with a full-width at half-maximum of ∼260 meV, which has been well characterized as the Co Embedded Image orbital-mediated tunneling (OMT) peak (17-21). This peak disappears completely in the dI/dV spectrum of d-CoPc. Instead, an intense resonance peak arises immediately below EF (-6 ± 3 meV), with an asymmetric shape and a narrow width of ∼50 meV. The amplitude of this peak decreased continuously as the dI/dV spectrum was measured at an increasing distance from the Co center. The peak eventually vanished at the edge of d-CoPc. This resonance was observed with nearly identical height and width in more than 50 d-CoPc molecules. After we elevated the temperature from 5 to 150 K, the resonance peak height for d-CoPc decreased by about a factor of 4 (Fig. 2A), but the height of the Embedded Image OMT resonance peak for the intact CoPc varied by only ∼15%.

Fig. 2.

Kondo resonance of d-CoPc at different temperatures. (A) Typical dI/dV spectra measured at the centers of a CoPc molecule at 5 K (black line), showing a Embedded Image OTM resonance, and a d-CoPc molecule at 5, 90, and 150 K (colored lines), showing strong resonance near EF. Spectra from bare Au(111) (gray line) is shown for comparison. (B) Topographic three-dimensional view of CuPc and d-CuPc, together with the corresponding dI/dV spectra measured at their centers. All spectra in (A) and (B) were taken with the same set point of V = 600 mV and I = 0.4 nA. (C) A fit (red line) to the resonance at 5 K in (A) according to the Fano model, with parameters of width ∼ 44 meV, q ∼ -6, and α ∼ -5 meV. Black symbols indicate experimental results. (D) The resonance width against measured temperature. Error bars represent standard deviations. (E) The temperature-dependent height of the Kondo resonance peak, which decreases approximately logarithmically from 20 to 150 K and becomes nearly saturated at lower temperatures.

The peak position, the line shape, and the temperature-dependent peak intensity all suggest that the resonance near EF for d-CoPc molecules likely arises through the Kondo effect. The good fit of the peak at different temperatures in the Fano model (22), which has been successfully applied to surface Kondo systems to describe the quantum interference between a localized magnetic impurity and a continuum (23, 24), further supports the notion of the Kondo effect (Fig. 2, C to E). The Fano model here can be described by the relation [Embedded Image, where R is the transition rate, Embedded Image represents the energy parameter as a function of the resonance energy and width, q is the interference parameter that controls the resonance shape, e is the elementary charge, α is the energy shift of the resonance center with respect to EF, and Γ = 2kBTK is the width of the resonance, where kB is the Boltzmann constant and TK the Kondo temperature.

At 5 K, fitting all the dI/dV spectra for different tips and d-CoPc molecules to the Fano model gives the average values α = -4 ± 3 meV, Γ = 49 ± 5 meV, and q = -9 ± 4 (Fig. 2C). The temperature-dependent resonance width also shows a good fit to an approximate formula [Embedded Image, where T is the measurement temperature] developed from Fermi liquid theory (25) and gives a TK of ∼208 K (Fig. 2D). The TK value obtained here is much higher than any previously reported temperature for magnetic atoms (23, 24, 26-29) or clusters (30) on surfaces. For comparison, we also studied CuPc molecules, which have a nonmagnetic ion center, in contrast with CoPc. The CuPc molecules adsorbed on a Au(111) surface can also be dehydrogenated by the same method. The central part of an STM image of the CuPc molecule is a hole (Fig. 2B), but it is a protrusion in a fully dehydrogenated CuPc (d-CuPc), and there is no noticeable resonance appearing near EF in the dI/dV spectra.

In order to understand qualitatively our experimental observations, we carried out first-principles studies on the structural and electronic properties of CoPc and d-CoPc molecules adsorbed on Au(111) (31). We used a slab model for the adsorption system, consisting of three atomic layers with 56 Au atoms each for the Au substrate and a vacuum seven atomic layers thick (Fig. 3, A and B). The distance between the molecule and the gold substrate is ∼3.0 Å. The interaction between the molecule and substrate clearly changes the electronic structure and magnetic property of the CoPc molecule. In a free CoPc molecule, the Co atom has unpaired d electrons and the magnetic moment of the Co atom is 1.09 Bohr magnetons (μB). In the CoPc adsorption system, the magnet moment is completely quenched by the molecule-substrate interaction. The spin-polarized partial density of states (PDOS) of the Co atom in the CoPc adsorption system (Fig. 3C), and in a free CoPc molecule (Fig. 3D), revealed that the spin-down states were filled more than the spin-up states for the free CoPc molecule. However, the filling difference disappeared for the CoPc adsorbed on Au(111). The theoretical STM image of a CoPc molecule on Au(111) simulated with the Tersoff-Hamann formula (32) (Fig. 3E) reproduces the main feature of the experimental image (Fig. 1D).

Fig. 3.

The geometric and electronic structures of CoPc on Au(111). (A and B) Top and side views, respectively, of the optimized computational model for the CoPc/Au(111) adsorption system. The dashed line represents the unit cell, which contains 56 Au atoms per layer. (C) The PDOS of the Co atom in a CoPc molecule on a Au(111) surface. The black line is the total PDOS; the red, green, and blue lines represent its m = 0, |m| = 1, and |m| = 2 components, respectively. E, electron energy. (D) The PDOS of the Co atom in a free CoPc molecule is shown. (E) The simulated STM image of CoPc/Au(111). arb., arbitrary.

Dehydrogenation induces a marked change of the molecular structure (Fig. 4, A and B), so that the d-CoPc molecule on Au(111) is no longer planar. The smallest separation between the end C atoms of the benzene ring and the gold substrate is ∼1.9 Å, leading to a much stronger binding to the gold substrate. The central Co atom in the d-CoPc molecule shifts upward remarkably (the dCo-Au distance is ∼3.8 Å for d-CoPc but 3.0 Å for CoPc).

Fig. 4.

The geometric and electronic structures of d-CoPc on Au(111). (A and B) Top and side views, respectively, of the optimized structure model for the d-CoPc/Au(111) adsorption system. The dashed line stands for the unit cell. (C) The PDOS of the Co atom in a d-CoPc molecule on a Au(111) surface. The black line is the total PDOS; the red, green, and blue lines represent its m = 0, |m| = 1, and |m| = 2 components, respectively. (D) The simulated STM image of d-CoPc/Au(111). (E) Comparison of the total PDOS of an isolated Co atom on a hollow site of a Au(111) surface with that of a d-CoPc molecule on Au(111). Arrows indicate the energy positions of the spin-polarized PDOS centroids of the Co atom.

More importantly, the magnetic moment is recovered for the d-CoPc adsorption system. The spin-polarized PDOS of the Co atom in the d-CoPc adsorption system (Fig. 4C) near EF has an empty minority spin peak that comes from the magnetic quantum number m = 0 (Embedded Image) states. This peak is consistent with our experimental spectra measured at different temperatures, in which an observable peak appears near 135 meV (fig. S3). The magnetic moment of the d-CoPc molecule is now 1.03 μB, very close to the value of a free CoPc molecule. The simulated STM image with a large bright spot for the d-CoPc adsorption system (Fig. 4D) agrees quite well with the observed image (Fig. 1H).

To understand the high Kondo temperature in the d-CoPc/Au(111) system, we compared its PDOS with that of a single Co adatom on an Au(111) surface (33) (Fig. 4E). The average spin splitting of the d-CoPc/Au(111) system is smaller than that of Co/Au(111). The on-site Coulomb repulsion U is proportional to this splitting, so the U of the d-CoPc/Au(111) system is smaller than that of the Co/Au(111) system. Moreover, the crystal field splitting of the Co d-level of the d-CoPc/Au(111) system is greater than that of the Co/Au(111) system (Fig. 4E), so the half-width Δ of the hybridized d-level of the d-CoPc/Au(111) system is greater than that of the Co/Au(111) system. According to theoretical models for the Kondo temperature TK (33, 34), TK increases monotonically as U decreases or as Δ increases [Embedded Image, where D is a prefactor and M is the degeneracy number]. Previous experiments (24) reported that the TK for Co/Au(111) is ∼75 K; thus, our experimental finding of a higher TK for the d-CoPc on Au(111) is in qualitative agreement with theory.

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