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Helium nanodroplets for material science research

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Helium nanodroplets are fascinating objects that can be used as microscopic laboratories to form new types of nanomaterials.

 

Researchers in COSY COST Action’s Working Group 4 , which recently celebrated its second anniversary, combine quantum, semiclassical and classical methods to investigate the physico-chemical properties of these droplets under extremely well-controlled conditions

 

Visit their site: https://cost-cosy.eu/

 

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Hello and welcome to Research Pod! Thank you for listening and joining us today.

 

In this episode we’re looking at Helium Nanodroplets and how they can be used in Science and Engineering to form new nanomaterials.

 

Helium nanodroplets are fascinating objects that exist at extremely low temperatures, just a fraction of a degree above absolute zero (that is  minus 273.15 degree Celsius , or minus 459.67 degrees fahrenheit). These droplets can be made up of anywhere from thousands to millions of helium atoms. Moreover, they exhibit unique properties due to their superfluid nature. Superfluidity is an unusual state of matter observed only in liquid helium and characterised by its apparently frictionless flow. This means that helium nanodroplets have vanishing viscosity, extreme chemical inertness and they are transparent to infrared up to ultraviolet light.

 

The COSY COST Action is an interdisciplinary research network of researchers and innovators working on “Confined molecular systems: from a new generation of materials to the stars”, or COSY for short, supported by COST – the European Cooperation in Science and Technology. This network, which recently celebrated its second anniversary,   aims to provide a sound foundation for understanding and controlling confined molecular systems.

 

Researchers in COSY COST Action’s Working Group 4 are exploring how these helium nanodroplets can be used as microscopic laboratories to form new types of nanomaterials. They are also employing theoretical tools that combine quantum, semiclassical and classical methods to investigate the physico-chemical properties of these droplets under extremely well-controlled conditions. Here are some examples of the innovative work currently being undertaken by four international research teams as part of this initiative:

 

In the first study, a group of researchers from Austrian and Croatia have been focusing on the self-assembly of diamondoid ether molecules within helium nanodroplets. Diamondoid molecules are structured in the same way as diamonds. These researchers have synthesized three different diamondoid ethers and introduced them into the droplets. They then went on to analyse the mass spectra of the resulting clusters. One of their most notable observations was the formation of "magic number" clusters that produced particularly stable arrangements of the ether molecules. These specific numbers, or magic numbers of molecules in the cluster, offer insights into the intermolecular interactions that govern the self-assembly process.

 

The researcher team also found out that the presence of trace water molecules had a significant impact on both the stability and the magic number distribution of the ether clusters. They discovered that the water molecules were acting as hydrogen bond donors and competing with the ether molecules for hydrogen bonding interactions. This led to the disruption of the initial organisation of the ether diamondoid clusters. This finding highlights how these systems are exceptionally sensitive to even small amounts of impurities and emphasises the importance of a controlled environment. A research paper describing the findings of this investigation has been published in the research journal Physical Chemistry Chemical Physics.

 

In another investigation, the results of which have also been published in the Physical Chemistry Chemical Physics journal, a researcher team from Spain has explored a technique called "soft-landing deposition". This involved the use of helium droplets to gently deposit atoms or molecules onto surfaces so they can later be used for example as catalysers or ultrasensitive sensors.

 

The researchers investigated the deposition of silver atoms (both neutral and positively charged) onto a graphene surface. (The carbon allotrope graphene is made up of a single layer of carbon atoms that are bound tightly in a hexagonal lattice similar to a honeycomb.)

A combination of computational methods, including high-level ab initio calculations to establish the dispersion-dominated silver–graphene interaction potential, and quantum dynamical simulations were employed. These enabled the researchers to develop an understanding of the superfluid helium nanodroplet-assisted collision dynamics of the positively charged silver atoms with the target surface. The research team was then able to simulate the dynamics of the silver atoms within the helium droplets and their subsequent interaction with the graphene surface.

 

The results of this study show how the superfluid nature of the helium droplets plays a crucial role in achieving the soft-landing of the silver atoms. Here, the droplets act as a buffer and absorb most of the kinetic energy of the impacting atoms. This prevents them from damaging or penetrating the delicate graphene surface. In addition, the simulations revealed that larger helium droplets are more effective at dissipating energy, which leads to a higher probability of successful soft-deposition.

 

Another team of researchers from Spain, Austria and Italy have recently published an account of their research in the International Journal of Hydrogen Energy. These researchers have been exploring the possibility of using organic molecules as materials for hydrogen storage, and the potential use of coronene in particular. Coronene is considered to be the smallest prototype of graphene and is often referred to as a “magic material” due to its extraordinary and unique properties.

 

The researchers used a combination of experimental and computational methods to investigate the adsorption of hydrogen molecules onto both bare and sodium-decorated protonated coronene. Their experiments revealed that the presence of one single sodium atom on the coronene was enough to significantly enhance its hydrogen storage capacity. The team’s analysis of the experimental data that was obtained through mass spectrometry disclosed that at higher hydrogen pressures, the sodium-decorated coronene retained a significantly larger number of hydrogen molecules compared to the bare coronene.

 

The researchers went on to carry out further theoretical calculations to confirm their discoveries. They were able to establish that the sodium atom alters the electronic structure of coronene and creates a more favourable environment for hydrogen adsorption. These breakthroughs highlight the potential of using alkali metal decoration to enhance the hydrogen storage capabilities of organic materials.

 

In the fourth study, a team of researchers from Denmark and Germany have examined a fascinating phenomenon known as interatomic Coulombic decay, or ICD for short. ICD occurs in weakly bound matter, such as helium nanodroplets.

 

When a helium droplet absorbs a photon that possesses sufficient energy, an electron can become excited to a higher energy level. This excited state can then decay through ICD, and the excess energy is transferred to a neighbouring atom or molecule, which leads to its ionization.

 

The team used a combination of high-resolution electron spectroscopy and coincidence imaging techniques to investigate the ICD process in helium nanodroplets that had been exposed to synchrotron radiation. Synchrotron radiation is electromagnetic radiation, emitted by a special type of electron accelerator, can be tuned to very short wavelengths in the ultraviolet and even x-ray regions of the spectrum where no other light source provides radiation.”

 

The researchers observed that ICD was occurring both below and above the ionisation threshold of helium. This suggested that different mechanisms were at play. When they explored what was happening below the ionisation threshold, they found that resonant excitation of multiple helium atoms within the droplet led to efficient ICD.

 

They observed, however, that ICD was occurring through a more complex process above the threshold. This involving electron-ion recombination into excited states which subsequently decayed via ICD. These findings, published in the Physical Review Research journal, highlight the importance of secondary processes, such as electron scattering and recombination, in driving ICD in extended systems.

 

Helium nanodroplets are fascinating quantum fluid objects whose extraordinary properties offer great potential for technological applications.

 

The research being carried out by the COSY COST Action’s Working Group 4 demonstrates how Helium nanodroplets can be ideal test tubes for forming new nanomaterials and investigating their properties. From facilitating the self-assembly of molecules at ultra-low temperatures to acting as “air cushions” for soft-deposition and enhancing hydrogen storage, these studies demonstrate the versatility and potential of helium droplets as microscopic laboratories for scientific exploration.

 

That’s all for this episode – thanks for listening. Links to the publications mentioned in this episode can be found in the show notes for this episode. And, as always, stay subscribed to ResearchPod for more of the latest science.

 

See you again soon.