Pyramid instead of a sphere: non-standard clustering of gold atoms

The world around us is a joint result of many phenomena and processes from a variety of sciences, the most important of which is virtually impossible to single out. Despite a certain degree of rivalry, many aspects of various sciences have similar features. Let's take geometry as an example: everything that we see has a certain shape, of which one of the most common in nature is a circle, circle, sphere, ball (tendency towards the face). The desire to be spherical manifests itself both in planets and in atomic clusters. But there is always an exception to the rule. Scientists from the University of Leuven (Belgium) found that gold atoms form not spherical, but pyramidal clusters. What is the reason for such an unusual behavior of gold atoms, what properties do precious pyramids possess, and how can this discovery be applied in practice? We learn about this from the report of scientists. Go.

Study basis

The existence of unusual clusters of gold atoms has been known for quite some time. These structures have non-standard chemical and electronic properties, which is why interest in them only increased over the years. Most studies have focused on the study of size dependencies, however, such a study needs controlled synthesis and high-precision measurements.

Naturally, there are different types of clusters, but Au has become the most popular for study.20, that is, a cluster of 20 gold atoms. Its popularity is due to the highly symmetrical tetrahedral * structure and surprisingly large HOMO-LUMO (HL) gap (gap)*.

Tetrahedron* – a polyhedron with four triangles as faces. If one of the faces is considered the basis, then the tetrahedron can be called a triangular pyramid.

HOMO-LUMO gap (gap) * – HOMO and LUMO are types of molecular orbitals (a mathematical function that describes the wave behavior of electrons in a molecule). HOMO stands for highest occupied molecular orbital, and LUMO stands for lowest unoccupied molecular orbital. The electrons of the molecule in the ground state fill all the orbitals with the lowest energies. The orbital, which among the filled has the highest energy, is called HOMO. In turn, LUMO is the least energy orbital. The difference in the energies of these two types of orbitals is called the HOMO-LUMO gap.

Au photoelectron spectroscopy20 showed that the HOMO-LUMO gap is 1.77 eV.

Based on the theory of the density functional (the method of calculating the electronic structure of systems), modeling showed that a similar energy difference can be achieved exclusively through the tetrahedral pyramid of Td symmetry (tetrahedral symmetry), which is the most stable geometry for the Au cluster20.

Scientists note that a previous study of Au20 gave extremely inaccurate results, due to the complexity of this process. Earlier, a transmission scanning electron microscope was used, the high energy of the beam of which distorted the observation results: a constant oscillation of Au was observed20 between different structural configurations. In 5% of the obtained images, the Au cluster20 was tetrahedral, and on the rest its geometry was completely disordered. Therefore, the existence of the tetrahedral structure of Au20 on a substrate of, for example, amorphous carbon, it was difficult to call one hundred percent proven.

In the study we are considering today, scientists decided to use a more gentle method for studying Au20namely, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Au clusters were observed20 on ultrathin NaCl films. STM allowed us to confirm the triangular symmetry of the pyramidal structure, and STS data made it possible to calculate the HOMO-LUMO gap, which amounted to as much as 2.0 eV.

Study preparation

The NaCl layer was grown on an Au (111) substrate using chemical vapor deposition at 800 K in an STM chamber under ultrahigh vacuum.

Cluster Au ions20 were obtained by magnetron sputtering and selected by size using a quadrupole mass filter. The spray source operated continuously and produced a large fraction of the charged clusters, which subsequently entered the quadrupole mass filter. The selected clusters were deposited on a NaCl / Au (111) substrate. For low-density deposition, the cluster flux was 30 pA (picoampere), and the deposition time was 9 minutes; for high-density deposition, 1 nA (nanoampere) and 15 minutes. The pressure in the chamber was 10-9 mbar.

Research results

Au Mass Anion Clusters20 with a very low density, the coatings deposited at room temperature on ultrathin islands of NaCl, including 2L, 3L, and 4L (atomic layers).


Image No. 1

On the 1A it can be seen that most of the grown NaCl has three layers, areas with two and four layers occupy a smaller area, and the 5L regions are practically absent.

Au Clusters20 were found in areas with three and four layers, but on 2L they were not. This is because Au20 can pass through 2L NaCl, but in the case of 3L and 4L NaCl they linger on their surface. At a low coating density in the region of 200 x 200 nm, 0 to 4 clusters were observed without any signs of Au agglomeration (accumulation)20.

Due to too high resistance 4L NaCl and instability when scanning individual Au20 on 4L NaCl, scientists focused on the study of clusters on 3L NaCl.


Image No. 2

Microscopy of clusters on 3L NaCl showed that their height is 0.88 ± 0.12 nm. This indicator agrees well with simulation results that predicted a height of 0.94 ± 0.01 nm (2A) Microscopy also showed that some clusters have a triangular shape with a protruding single atom at the apex, which in practice confirms theoretical studies regarding the pyramidal shape of the Au structure20 (2B)

Scientists note that when visualizing extremely small three-dimensional objects, such as Au clusters20, it is extremely difficult to avoid certain inaccuracies. In order to obtain the most accurate images (both from the atomic and geometric points of view), it was necessary to use a perfectly atomically sharp Cl-functionalized microscope needle. The pyramidal shape was identified in two clusters (1B and 1C), three-dimensional images of which are shown in 1D and 1E, respectively.

Despite the fact that the triangular shape and height distribution show that the deposited clusters retain their pyramidal shape, STM images (1B and 1C) do not show perfect tetrahedral structures. The largest angle in the picture 1B is about 78 °. And this is 30% more compared to 60 ° for an ideal tetrahedron with Td symmetry.

There can be two reasons for this. Firstly, these are inaccuracies in the visualization itself, caused both by the complexity of this process and the fact that the tip of the microscope needle is not rigid, and this can also distort images. The second reason is related to the internal distortion of the supported Au20. When Au Clusters20 with Td symmetry land on a square NaCl lattice, a mismatch in symmetry distorts the ideal tetrahedral structure of Au20.

In order to find out what is the reason for such deviations in the images, the scientists analyzed the data on the symmetry of three optimized Au structures20 on NaCl. As a result, it was found that the clusters are only slightly distorted from the ideal tetrahedral structure with Td symmetry with a maximum deviation in the position of atoms of 0.45. Consequently, the distortions in the images are the result of the inaccuracy of the visualization process itself, and not any deviations in the deposition of clusters on the substrate and / or the interaction between them.

Not only topographic data are clear signs of the pyramidal structure of the Au cluster20, but also a sufficiently large HL gap (of the order of 1.8 eV) in comparison with other Au20isomers * with lower energy (in theory, below 0.5 eV).

Isomers * – structures identical in atomic composition and molecular weight, which differ in their structure or arrangement of atoms.

Analysis of the electronic properties of clusters deposited on a substrate by scanning tunneling spectroscopy (1F) allowed us to obtain the differential conductivity spectrum (dI / dV) of the Au cluster20with a large forbidden zone visible (Eg), equal to 3.1 eV.

Since the cluster is electrically split by insulating NaCl films, a double-barrier tunnel junction (DBTJ) is formed, which causes the effects of tunneling of one electron. Therefore, the discontinuity in the dI / dV spectrum is the result of the joint work of the quantum HL discontinuity (EHl) and classical Coulomb energy (Ec) Measurements of the discontinuities in the spectrum showed from 2.4 to 3.1 eV for seven clusters (1F) The observed discontinuities are larger than the HL discontinuities (1.8 eV) in the Au gas phase20.

The variability of breaks in different clusters is determined by the measurement process itself (the position of the needle relative to the cluster). The largest gap measured in the dI / dV spectra was 3.1 eV. In this case, the needle was located far from the cluster, from which the electric capacitance between the needle and the cluster was less than between the cluster and the Au (111) substrate.

Next, we calculated the HL breaks of free Au clusters20 and those located on 3L NaCl.

Graph 2C shows a curve of the simulated density of states for a gas-phase tetrahedron Au20, Whose HL gap is 1.78 eV. When the cluster is located on 3L NaCl / Au (111), distortions increase and the HL gap decreases from 1.73 to 1.51 eV, which is comparable to the 2.0 eV gap obtained during experimental HL measurements.

In previous studies, it was found that Au isomers20 with a Cs-symmetric structure have an HL gap of about 0.688 eV, and structures with amorphous symmetry are 0.93 eV. Given these observations and the results of measurements, scientists came to the conclusion that a large forbidden zone is possible only in conditions of a tetrahedral pyramidal structure.

The next stage of the study was the study of cluster-cluster interactions, for which more Au was deposited on a 3L NaCl / Au (111) substrate20 (increased density).


Image No. 3

On the image 3A A topographic STM image of deposited clusters is shown. In the scanning region (100 nm × 100 nm), about 30 clusters are observed. The sizes of interacting clusters on 3L NaCl are either greater than or equal to the sizes of those studied in experiments with single clusters. This can be explained by diffusion and agglomeration (clusters) on the surface of NaCl at room temperature.

Cluster accumulation and growth can be explained by two mechanisms: Ostwald ripening (recondensation) and Smoluchowski ripening (island enlargement). In the case of Ostwald ripening, larger clusters grow due to smaller ones, when the latter atoms are separated from them and diffuse into neighboring ones. When Smoluchowski ripens, larger particles are formed as a result of migration and agglomeration of entire clusters. One type of maturation can be distinguished from another as follows: when Ostwald ripens, the cluster size distribution expands and is continuous, and when Smoluchowski ripens, the size is distributed discretely.

On the charts 3B and 3C results of analysis of more than 300 clusters are shown, i.e. size distribution. The range of observed cluster heights is quite wide, however, three groups of the most common (3C): 0.85, 1.10 and 1.33 nm.

As seen in the graph 3B, there is a correlation between the height and width of the cluster. The observed cluster structures demonstrate the features of Smolukhovskii ripening.

There is also a correlation between clusters in experiments with high and low deposition densities. Thus, a group of clusters with a height of 0.85 nm is consistent with an individual cluster with a height of 0.88 nm in experiments with low density. Therefore, clusters from the first group were assigned the value Au20and the clusters from the second (1.10 nm) and third (1.33 nm) were assigned the values ​​of Au40 and Au60, respectively.


Image No. 4

In the picture 4A we can see the visual differences of the three categories of clusters whose dI / dV spectra are shown in the graph 4B.

As Au Merges20 clusters into a larger energy gap in the dI / dV spectrum decreases. So, for each group the following indices of discontinuities were obtained: Au20 – 3.0 eV, Au40 – 2.0 eV and Au60 – 1.2 eV. Given these data, as well as topographic images of the studied groups, it can be argued that the geometry of cluster agglomerates is closer to spherical or hemispherical.

To estimate the number of atoms in spherical and hemispherical clusters, N can be useds = [(h/2)/r]3 and Nh = 1/2 (h / r)3where h and r represent the cluster height and radius of one Au atom. Given the Wigner-Seitz radius for a gold atom (r = 0.159 nm), we can calculate their number for the spherical approximation: the second group (Au40) – 41 atoms, the third group (Au60) – 68 atoms. In the hemispherical approximation, the estimated number of atoms 166 and 273 is much larger than in the Au40 and Au60 spherical approximations. Therefore, we can conclude that the geometry of Au40 and Au60 has a spherical rather than hemispherical shape.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.

Epilogue

In this study, scientists combined scanning tunneling spectroscopy and microscopy, which allowed them to obtain more accurate data regarding the geometry of clusters of gold atoms. It was found that the Au cluster20deposited on a 3L NaCl / Au (111) substrate retains its gas-phase pyramidal structure with a large HL gap. It was also found that the main mechanism for the growth and association of clusters into groups is the maturation of Smolukhovsky.

Scientists call one of the main achievements of their work not so much the results of atomic cluster studies as the very method of conducting these studies. Previously, a transmission scanning electron microscope was used, which, due to its properties, distorted the results of observations. However, the new method described in this work provides accurate data.

Among other things, the study of cluster structures allows us to understand their catalytic and optical properties, which is extremely important for their use in cluster catalysts and optical devices. At the moment, clusters are already used in fuel cells and in carbon capture. However, according to the scientists themselves, this is not the limit.

Thank you for your attention, remain curious and have a good working week, guys. 🙂

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