Density functional calculation of the molecular properties of the [Au20-C60-Au20]n (n = 0, 1, 2, 3) model complexes

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Highlights

  • Electrostatic interactions determine the stability and properties of the complexes.

  • All the complexes exist as a pair of different energy degenerate structures.

  • [Au20-C60-Au20] singlet complexes have delocalized orbitals near the C60 Fermi level.

  • The [Au20-C60-Au20] singlet complexes are the most suitable for electron-conduction.

Abstract

We have calculated molecular geometries, HOMO-LUMO gaps, Fermi levels and vibrational frequencies for the σ-linear, σ-bent and π-coordinated [Au20-C60-Au20]n (n = 0–3) complexes using the density functional B3LYP method and LANL2DZ basis sets. Our calculations showed that the σ-bent η1(b)-[Au20-C60-Au20]3− complex is the global minimum structure with Au-C distances of 2.27 and 2.97 Å, respectively. The C60 calculated bouncing frequencies for the [Au20-C60-Au20]n (n = 0–3) complexes appear at 18–60 cm−1 and compare well with the experimental value of 40 cm−1. The calculations of Mulliken charges indicate that the electrostatic interaction of the C60n− ions with the Au20 clusters plays a major role in the stability and electron-conducting properties of the [Au20-C60-Au20]n− (n = 0–3) complexes. The orbital analysis and calculated Fermi levels of the complexes indicate that the neutrally-charged singlet [Au20-C60-Au20] complexes have the HOMO-1 and LUMO+1 delocalized orbitals with Fermi levels at −5.18 eV that line up with the free C60 Fermi level.

Introduction

The study of electronic conduction at the molecular-scale limit has grown substantially in the last two decades and promises to be a fertile ground for amazing futuristic technologic applications [1], [2]. Recent electron transport measurements have provided evidence for the coupling of the C60 molecule vibrational bouncing mode and the single electron transfer processes through gold surfaces [3]. This novel electron transfer phenomena modulated through nanomechanical oscillations of a single C60 molecule, with a frequency of about 40 cm−1 [3] has not previously been observed in quantum dots [3], [4], [5].

The proposed mechanism for the electronic nuclear coupling and the transfer of electrons to the gold surface suggest that initially an e jumps onto a C60n ion-molecule [3], [5]. This produces an attraction between the C60n ion-molecule and the positive image charge inside the gold surface. This interaction causes the C60n molecule to approach closer to the gold surface resulting in nanomechanical oscillations of the C60n against the gold surface. The new electronic nuclear coupling and the electron transfer process, observed in the C60n nanomechanical oscillations against the gold surface, appears to be different from that observed in standard molecules. Many authors have suggested that these effects in the nanoscale regime give a unique electron transfer property to these nanoparticles and imply the possibility of new applications in nanodevices [3], [5].

Giving the relevance in many technological applications of the future [1], [2], [3], the understanding of the electron conduction mechanism of a bouncing fullerene molecule against a gold electrode surface, is a very important problem. The current tendency, in the microelectronics and the semiconductor industry, is to shrink the size of electronic devices, while keeping their functionality to produce more powerful computers [1], [2], [3], [4]. The current research in the field shows that for the year 2030, the cell size of the random-access memory devices will be less than the size of an atom [4]. This suggests that the understanding of electron conduction mechanisms where single molecules interact with metallic surfaces has enormous technological implications in the development of new technologies, not only in microelectronics, semiconductor electronics, and computer industry, but also in new fields such as molecular electronics, and nanotechnology [2], [3], [5], [6].

Electron conduction in fullerenes is highly dependent on the bonding geometries of fullerenes with the metal electrodes [7]. High spread and low conductivity have been observed to depend on the orientation of the molecule on the metal surface, in the alignment and coupling of the frontier molecular orbitals with the Fermi level of the metal [8], [9], [10]. Recent theoretical developments in molecular electronics [11] have demonstrated that the analysis of the phase and amplitude of frontier orbitals of the molecule at the Fermi level of the metal-molecule-metal system is a powerful tool, which allows evaluating the electron transport properties in molecular devices [11], [12], [13], [14]. It has been demonstrated that for a weak interaction regime of the C60 molecule and metal electrodes, the Fermi level of the gold-C60-gold system is in the energy range of HOMO and LUMO orbital energies [15]. In this situation, delocalized molecular orbitals of the system with the right symmetries and energies very closed to the Fermi level, are the best channels for electron conduction in the π-conjugated systems [12], [13], [14].

For systems where organic molecules interact with gold and silver surfaces, the experimental and theoretical literature [16], [17], [18], [19], [20], [21], [22], suggest that simplified models of the system, using small transition metal clusters to represent the metal surface, can describe the fundamental physics, of the local interaction of the molecule with the metal surface. This suggests that ab initio calculations in the [Au20-C60-Au20]n (n = 0–3) model complexes can be a very convenient tool to study the interaction of the C60n with the gold surfaces. These studies could help in bringing more understanding of the electron conduction mechanism [3], [4], [5] of the bouncing C60n molecule on the gold surfaces and in the interpretation of the electrochemical and photoelectron experiments reported for these phenomena [6].

The objective of this work is to study the interaction of the C60 molecule with a gold surface using the [Au20-C60-Au20]n (n = 0–3) model complexes. We will be presenting theoretical calculations of the Au-C60 bonding interaction types, geometries, the C60 bouncing frequencies, the HOMO-LUMO gaps, the Fermi levels, and the Mulliken charges. We will also present the analysis of the phase and amplitude of the orbitals near the Fermi level of the complexes. The analysis of the properties outlined above will provide the basis for the understanding of the electron transfer and conduction properties of the gold-C60 systems.

Section snippets

Methodology

The interaction of the C60 molecule with the gold surface will be described using the symmetric model of the [Au20-C60-Au20]n (n = 0–3) complexes. This model mimics the interaction of a single C60 molecule with two gold clusters (Au20) of pyramidal shape. The different types of the [Au20-C60-Au20]n (n = 0–3) complexes studied, shown in Fig. 1, have the following multiplicities: the neutral singlet structure for n = 0, the singly-charged doublet structure for n = 1, the doubly-charged singlet

Results

We have carried out calculations of geometries, frequencies, the C60 bouncing frequency, and the HOMO-LUMO energy differences of the [Au20-C60-Au20]n (n = 0–3) model complexes. In Table 1, we present a summary of our calculations carried out using the B3LYP/LANL2DZ method. We have calculated the electronic energies of all complexes relative to the σ-bent coordinated η1(b)-[Au20-C60-Au20]3− complex, which is the global minimum structure with relative energy set to 0.0 kcal/mol. The analysis of

Conclusions

We have carried out the B3LYP/LANL2DZ calculations in the [Au20-C60-Au20]n (n = 0–3) complexes to explore the different binding modes of the gold-C60 interaction. The results of our calculations indicate that global minimum is the linear σ-bent coordinated η1(b)-[Au20-C60-Au20]3− complex with 1.0 kcal/mol lower than the linearly σ-coordinated η1-[Au20-C60-Au20]3− complex. Except for the triply-charged quartet complexes, all the linearly σ-coordinated η1-[Au20-C60-Au20]n (n = 0–3) complexes

CRediT authorship contribution statement

Jairo Castillo-Chará: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Visualization, Investigation, Supervision, Validation, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation grant number OCI-1053575 and the Pittsburgh Supercomputing Center. The authors also want to acknowledge the partial support of Fayetteville State University through the ITS.

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