Abstract

The very recent development of highly selective techniques making possible the synthesis and experimental characterization of subnanometric (subnanometer-sized) metal clusters (even single atoms) is pushing our understanding far beyond the present knowledge in materials science, driving these clusters as a new generation of quantum materials at the lower bounds of nanotechnology. When the size of the metal cluster is reduced to a small number of atoms, the d-band of the metal splits into a subnanometric d-type molecular orbitals network in which all metal atoms are inter-connected, with the inter-connections having the length of a chemical bond (1–2 Å). These molecular characteristics are at the very core of the high stability and novel properties of the smallest metal clusters, with their integration into colloidal materials interacting with the environment having the potential to further boost their performance in applications such as luminescence, sensing, bioimaging, theranostics, energy conversion, catalysis, and photocatalysis. Through the presentation of very recent case studies, this Feature Article is aimed to illustrate how first-principles modelling, including methods beyond the state-of-the-art and an interplay with cutting-edge experiments, is helping to understand the special properties of these clusters at the most fundamental level. Moreover, it will be discussed how superfluid helium droplets can act both as nano-reactors and carriers to achieve the synthesis and surface deposition of metal clusters. This concept will be illustrated with the quantum simulation of the helium droplet-assisted soft-landing of a single Au atom onto a titanium dioxide (TiO2) surface. Next, it will be shown how the application of first-principles methods have disclosed the fundamental reasons why subnanometric Cu5 clusters are resistant to irreversible oxidation, and capable of increasing and extending into the visible region the solar absorption of TiO2, of augmenting its efficiency for photo-catalysis beyond a factor of four, also considering the decomposition and photo-activation of CO2 as a prototypical (photo-) catalytic reaction. Finally, I will discuss how the modification of the same material with subnanometric Ag5 clusters has converted it into a “reporter” of a surface polaron property as well as a novel two-dimensional polaronic material.

Highlights

  • Recent joint theoretical–experimental research has shown that subnanometric metal clusters [1], and more generally atomically precise clusters [2] or even single atoms [3], possess special chemical and physical properties due to singular quantum effects which are not present in nanoparticles and bulk materials. The study of these properties can offer novel solutions to many reactions in very important fields such as sensing [5], bio-imaging,[6] theranostics [7,8], energy conversion [9], catalysis [10,11,12,13,14,15,16] and photocatalysis [15,17]. When these clusters are supported on titanium dioxide [17,18], photon energy is temporarily stored in the form of charge pairs in the direct vicinity of the surface which is a perfect prerequisite for follow-up chemistry [15,19]

  • In Ref. [49], we presented experiments and molecular dynamics (MD) simulations of the deposition and aggregation of silver clusters synthesized within helium droplets onto an amorphous carbon surface at room temperature

  • In order to calculate photo-induced properties of subnanometric metal clusters adsorbed on semiconductor surfaces, one practical choice is the employment of the Reduced Density Matrix (RDM) approach in the Redfield approximation [112], based on orbitals taken from the periodic calculations

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Summary

Introduction

Recent joint theoretical–experimental research has shown that subnanometric metal clusters [1], and more generally atomically precise clusters [2] or even single atoms [3], possess special chemical and physical properties due to singular (e.g., collective) quantum effects which are not present in nanoparticles and bulk materials (see, e.g., [4] for a recent review). Hauser’s and Ernst’s groups have reported on the successful helium droplet-mediated synthesis and thermal-induced restructuring of bimetallic core–shell nanoparticles of sub-10 nm size [44,45,46] while Orlov, Lindsay and collaborators have demonstrated that catalytic active gold clusters, down to the subnanometer scale, can be produced and soft-landed using superfluid helium droplets of different sizes [43] Due to all these progresses, the helium-droplet mediated technique is current experience a rapid expansion in an effort to provide essential ingredients to better control the synthesis and deposition of metal clusters.

The helium droplet-mediated synthesis and surface deposition technique
HexetÀsurfaceðzÞ
HeÀAu and HeÀsurface ext denote the
A brief overview of selected computational methods
Selected computational approaches combining periodic and cluster models
Photo-induced properties using a reduced density matrix approach
À CÀj 1
Equilibrium thermodynamics
Results and discussion
Final remarks
Full Text
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