Au20: A Tetrahedral Cluster

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Science  07 Feb 2003:
Vol. 299, Issue 5608, pp. 864-867
DOI: 10.1126/science.1079879


Photoelectron spectroscopy revealed that a 20-atom gold cluster has an extremely large energy gap, which is even greater than that of C60, and an electron affinity comparable with that of C60. This observation suggests that the Au20 cluster should be highly stable and chemically inert. Using relativistic density functional calculations, we found that Au20 possesses a tetrahedral structure, which is a fragment of the face-centered cubic lattice of bulk gold with a small structural relaxation. Au20 is thus a unique molecule with atomic packing similar to that of bulk gold but with very different properties.

Small clusters often have different physical and chemical properties than their bulk counterparts. Materials assembled from finite-sized clusters have been intensively sought ever since the discovery of C60 (1). One of the criteria for a cluster to be used as a potential building block for cluster-assembled materials is its chemical stability relative to other reagents and to other clusters of the same material. A closed electron configuration with a large energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is a prerequisite for the chemical stability of a cluster. Besides its high symmetry, the large HOMO-LUMO gap of C60 is responsible for its chemical inertness and its ability to assemble into molecular crystals (2).

Gold is undoubtedly an important material, and small clusters of gold have also attracted great attention. Although gold colloids have been used for centuries to stain glass (3), more recently it has been shown that gold clusters have unusual catalytic properties for selective oxidation of CO (4–7), are oxidation-resistant (8), enable selective binding of DNA (9), and have potential applications in nanoelectronics (10–16). Small Au cluster cations possess planar structures up to Au7 +(17), whereas Au cluster anions are planar up to at least Au12 (18, 19). We have probed the electronic and geometrical structures of small Au clusters using anion photoelectron spectroscopy (PES) and computer simulation. The improved instrumental resolution (20) and the ability to produce cold clusters (21) enabled us to obtain considerably more detailed electronic structure information than was previously possible (22). We found that 20-atom gold clusters exhibit a HOMO-LUMO gap even greater than that of C60. Relativistic density functional calculations predict that Au20 possesses a tetrahedral geometry, similar to that of a fragment of the bulk face-centered cubic (fcc) crystal of gold.

Details of our PES apparatus have been described elsewhere (20, 23). Small Aun clusters were produced by means of laser vaporization of a pure gold target with a helium carrier gas and were mass-analyzed with time-of-flight mass spectrometry. Pure Au20 clusters were selected and decelerated before photodetachment by a pulsed laser beam. Figure 1 shows the PES spectra of Au20 at three photon energies. The 193-nm spectrum of Au20 (Fig. 1C) displays a weak peak around 2.7 eV (labeled X), followed by a large energy gap and more discrete transitions at higher binding energies (A, B, C…). This spectral pattern suggests that neutral Au20 is a closed-shell molecule with a large HOMO-LUMO gap.

Figure 1

Photoelectron spectra of Au20 . (A) At 355 nm (3.496 eV). (B) At 266 nm (4.661 eV). (C) At 193 nm (6.424 eV). The 355- and 266-nm photons were from a Nd–yttrium-aluminum-garnet laser, and the 193-nm photons were from an ArF excimer laser. Photoelectrons were analyzed with a magnetic bottle–type photoelectron spectrometer and calibrated using the known spectrum of Rh. The electron kinetic energy resolution was about 2.5%; that is, ∼25 meV for 1-eV electrons.

The extra electron that enters the LUMO of Au20 is removed upon photodetachment of the anion, yielding the neutral ground state (X in Fig. 1). The feature A corresponds to the lowest triplet excited state of the neutral. Thus, theA-X separation, measured to be 1.77 eV (Fig. 1B), represents the excitation energy of the first triplet excited state of neutral Au20 but is also an approximate experimental measure of the HOMO-LUMO gap. This energy gap in Au20 is very large, about 0.2 eV greater than that in C60 (1.57 eV) (24) (Fig. 2). However, electron signals were observed in the HOMO-LUMO gap region in the 266-nm spectrum (Fig. 1B), owing to autodetachment, as a result of a photoexcited Au20 * upon absorption of a 266-nm photon. Similar autodetachment signals were also observed previously in C60 (Fig. 2A) (24). The 355-nm spectrum of Au20 revealed a very sharp peak at the ground state transition (autodetachment signals were also observed at this detachment energy), suggesting that there is very little geometry change between the ground states of Au20 and neutral Au20. This is different from C60 , whose PES spectra exhibit vibrational features due to structural distortions of the anion ground state (25). The 355-nm spectrum yielded a vertical detachment energy of 2.751 ± 0.010 eV and an adiabatic detachment energy of 2.745 ± 0.015 eV for Au20 . The latter is the electron affinity (EA) of Au20: a measure of how tightly the cluster can bind an electron. The EA of Au20 is higher than that of C60 (2.689 eV) (24), so Au20 is even more electronegative than C60.

Figure 2

Comparison of the photoelectron spectra of Au20 with those of C60 . (A) The 266-nm spectrum of C60 . “AD” stands for autodetachment signals. (B) The 266-nm spectrum of Au20 . (C) The 193-nm spectrum of C60 . (D) The 193-nm spectrum of Au20 . C60 data are from (24).

According to the electron shell model (26), Au20 with 20 valence electrons should represent a major shell closing. What is surprising is the magnitude of the HOMO-LUMO gap. With the exception of Au2 and Au6, the HOMO-LUMO gap observed for Au20 is the largest among all known coinage-metal clusters (22). It is also larger than that observed in the recently discovered 18-electron icosahedral W@Au12 cluster (27, 28).

The large HOMO-LUMO gap suggests that Au20 should be very inert and may possess a highly symmetric geometry. To elucidate its structure and bonding, we carried out an extensive structural search for neutral and negatively charged Au20, using relativistic density functional calculations (29–33). We started from the highest symmetry possible [the Platonic dodecahedron with icosahedral (Ih ) symmetry and octahedron with octahedral (Oh ) symmetry] to their various important subgroups, as well as the ring and bowl structures known for C20 (34) (Table 1and Fig. 3). We also tested a capped decahedron (C 2v) structure (Fig. 3C) and an amorphous (C 1) structure (Fig. 3B), which were found as “global” minima in previous calculations (35, 36). The Ih and Oh Au20 structures are open-shell structures and would be subject to Jahn-Teller instability; a string-bag–like cage, a bowl, and a ring structure are closed-shell but are highly unstable, with small HOMO-LUMO gaps (Table 1). Because smaller Aun (n < 13 atoms) clusters prefer planar geometries (18,19), we also calculated a planar Au20 structure (Fig. 3D), as well as a linear Au20 chain, which has recently been formed on a NiAl surface and studied with scanning tunneling microscopy (STM) (37). Although less stable than the tetrahedral structure, the planar structure was found to be more stable than any other isomers except the amorphous cluster (Fig. 3B) and the capped decahedron (Fig. 3C). The linear chain is highly unstable, with almost no HOMO-LUMO gap, which is consistent with its metallic behavior observed by STM (37). The most stable structure we found was the ideal tetrahedral (Td ) structure, which is more stable than the previously suggested “global minima” C 1and C 2v structures by 1.4 and 1.8 eV, respectively. The Td Au20 structure is closed-shell, with a HOMO-LUMO gap of 1.8 eV, in excellent agreement with the experiment. The Au-Au distances (0.268, 0.271, 0.283, 0.297, and 0.312 nm) in the calculated Td Au20 structure are close to those in bulk gold (0.288 nm), yielding a tetrahedral edge around 1 nm. Frequency calculations for the Td Au20 structure confirmed that it is a minimum on the potential energy surface.

Figure 3

Selected optimized Au20 structures. (A) Tetrahedral structure (Td ). (B) Amorphous structure (C 1). (C) Capped decahedron (C 2v). (D) Planar structure (C 2h). (E) Octahedral structure (Oh ). (F) Dodecahedral structure (Ih ).

Table 1

Optimized molecular structures, point group symmetries, electronic configurations, HOMO-LUMO energy gaps (ΔE HL), relative scalar-relativistic energies (E SR), and EAs of Au20. The relative scalar-relativistic energies of the optimized anions are also listed [E SR(anion)]. All energies are in eV. The total energies of the various isomers of Au20 and Au20 are relative to those of the neutral tetrahedral Au20.

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To facilitate comparison with experimental results, we also optimized the geometries of the anions for all the isomers (Table 1). Consistent with the experiment, very little structural change was observed upon electron addition to Td Au20: The Jahn-Teller distortion energy (∼0.02 to 0.04 eV for distortion to the D 2d andC 3v symmetries) is much smaller than the spin-orbit coupling energy (0.16 eV), so that the geometry distortion is quenched. The total energy difference between the anion and the neutral defines the theoretical EA. The calculated EA forTd Au20 is 2.61 eV. However, when spin-orbit coupling is included, we obtain a theoretical EA of 2.741 eV, which is in excellent agreement with the experimental value of 2.745 eV, whereas the calculated EAs for all other structures deviate considerably from the experimental measurement (Table 1).

Because the X-A gap (Fig. 1) represents the excitation energy of the lowest triplet excited state, we also calculated this quantity for Td Au20. The calculated excitation energy for the lowest triplet state (3A1) is 1.777 eV, in close agreement with the experimentally determined value of 1.77 eV. The excellent agreement between the calculated EA and excitation energy and the experimental measurements can probably be attributed to the fact that very little change in geometry exists between the anion ground state and the neutral ground and excited states, and it confirms unequivocally that Au20 possesses a tetrahedral structure. Further confirmation of the Td structure is provided by the theoretical detachment spectrum (Fig. 4), which shows that major PES features are all well reproduced in the simulated spectrum for Td Au20 (38).

Figure 4

The simulated photoelectron spectrum of Au20 . The simulated spectrum was constructed by fitting the distribution of the calculated detachment transition energies with unit-area Gaussian functions of 0.05 eV at full width at half maximum.

Tetrahedral Au20 is a small piece of bulk gold with a small relaxation. Each of the four faces represents a (111) surface of fcc gold. It has a very high surface area (all the atoms are on the cluster surface) and a large fraction of corner sites with low coordination. The three different kinds of atoms in theTd structure, 4 at the apexes, 4 at the center of each face, and 12 along the edges (Fig. 3A), have different coordination environments and may provide ideal surface sites to bind different molecules for catalysis (such as CO, O2, and CO2) (39). The large HOMO-LUMO gap of Au20 suggests that it is a highly inert and stable molecule and may possess novel chemical and physical properties; its unique tetrahedral structure makes Au20 an ideal model for gold surfaces.

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