Inter-atomic photon emission during contact-electrification

Contact electrification can arise when physical contact occurs between two materials. In a new report now published on Science Advances, Ding Li, and a team of scientists in nanoscience, nanoenergy and materials science in China and the U.S., detailed atomic-featured photon emission spectra between two solid materials. Electron transfer can take place at the interface from an atom in one material to another atom in another material, alongside photon emission, during contact electrification.

This process can assist contact electrification induced interface photon emission spectroscopy (CEIIPES) to detect spectroscopy corresponding to contact electrification at an interface, and impact the awareness of interactions between solids, liquids and gases. The physics of this research can be expanded to X-ray emission, Auger electron excitation and electron emission during contact electrification, which remains to be explored. The work leads to a general field known as contact electrification induced interface spectroscopy (CEIIS).

Triboelectrification

Contact electrification is a scientific term used for the well-known phenomenon of triboelectrification and defines the charges produced by physical contact. The concept is universal in both daily life and in nature, occurring between shoes and the ground, when clouds move in the air and when the Earth shakes. While the process was first recorded more than 2600 years ago, scientists still debate the mechanism behind the process. Research in the field has evolved with modern technologies to describe the true complexity of the phenomenon, although some observations are inexplicable or contradictory. In this work, Li et al observed atomic featured photon emission spectra during contact electrification at a solid-solid interface by contacting fluorinated ethylene propylene (FEP) with acrylic, or FEP with quartz. Compared to triboluminescence, the characteristic photon emission induced by contact electrification can carry abundant information about the energy structure at interfaces. Li et al suggested three possible physical processes to understand photon emission arising from electron charge transferred during charge electrification. The process is known as contact electrification induced interface photon spectroscopy (CEIIPES) and can allow researchers to study electronic transitions at solid-solid interfaces.

The working principle of contact electrification (CE)

Li et al formed the core parts of a hollow cylinder sandwiched between a metal cover and a metal base, within which they drove four metal fans using a motor. The team attached the materials for contact electrification (CE) to the metal fans or to the cylinder, and induced CE at the interface during fan rotation. They measured the pressure using a pressure motor and controlled the differential flow of the inlet and outlet of the vacuum chamber through flow meters. If a photon signal originated from the core, they could record it using a spectrometer with a sensitive charge-coupled device detector. Li et al noted photon emission associated with the physical processes of CE. For instance, photon emissions with atomic spectra features were associated with electron transitions during contact electrification and the scientists defined this phenomenon as contact electrification induce interface photon emission spectroscopy (CEIIPES).

Physical processes of electron transfer

The team next illustrated the physical processes underlying photon emission lines relative to the energy levels and electron transitions in the setup. For example, when the FEP material contacted quartz, electron transitions occurred for these materials including transitions between atoms such as hydrogen and oxygen at the surface of quartz. The team summarized the possible physical routes for electron transitions among different atoms during contact electrification and noted two possible methods for electron transition to excited states, including (1) electron transition from molecular orbit to the excited state of an atom, or (2) the excitation of an atom from a lower energy level to higher energy level inside an atom. Furthermore, an excited state electron can transit to a lower energy level by emitting a photon. Contact electrification induced interface photon emission spectroscopy (CEIIPES) is different from fluorescent spectra for molecules, where CEIIPES is associated with photon emission relative to electron transfer between two atoms. Comparatively, fluorescence spectra are associated with electron transition between molecular levels with many vibrational levels. The team then highlighted the role of the hydrogen atom during contact electrification, where H atoms possessed unique roles during the experiments. The present studies only demonstrated photon emission relative to CEIIPES at solid-solid interfaces, the team intend to use the method and reveal more interesting phenomena at solid-liquid, solid-gas, gas-gas and gas-liquid, as well as liquid-liquid interfaces.

Outlook

In this way, Ding Li and colleagues observed atomic featured photon emission spectra during contact electrification between two solids. During the work, electrons transferred from one atom of a specific material to another atom in another material at the interface during contact electrification in a process known as contact electrification induced interface photon emission spectroscopy (CEIIPES). The process occurred through energy resonance transfer when atoms from different materials were brought close to each other. The team analyzed the processes underlying contact electrification to better understand how two materials were charged after contact electrification to assess interactions between liquids, solids, and gases. The work is specific to solid-solid interfaces and is applicable for more general cases such as X-ray emission and Auger electron excitation.