Mind the band gap! – researchers create new nanoscale forms of elementary semiconductor with tunable electronic properties

Researchers have demonstrated that by using a semiconductor with flexible bonds, the material can be moulded into various structures using nano containers, without altering its composition, the discovery could lead to the design of a variety of customised electronic devices using only a single element. 

Semiconductors are vital to our daily lives, as they are found in nearly every electronic device. One of the key characteristics of semiconductors is their band gap, which determines how they conduct electric current. The band gap is typically engineered for specific applications by breaking chemical bonds or introducing additional elements into the material. However, these processes can be complex and energy-intensive. 

Researchers from the University of Nottingham, the EPSRC SuperSTEM facility, Ulm University in Germany, and BNNT LLC in the USA imaged new forms of selenium using transmission electron microscopy, employing nanotubes as tiny test tubes. The study has been published today in Advanced Materials.

Dr Will Cull, research fellow in School of Chemistry, Univesity of Nottingham, who carried out the experimental work, said, ‘Selenium is an old semiconductor with a rich history, having been used in the first solar cells. In our research, we have revitalised selenium by discovering new forms that can emerge when confined to the nanoscale.’

Selenium can exist as nanowires, with its structure and bonding varying by diameter. Below a certain size, the bonding between selenium atoms changes, increasing bond angles. This causes straightening of the initially helical structure, ultimately constricting it into atomically thin wires. 

Dr Will Cull said, ‘We successfully imaged new forms of selenium using transmission electron microscopy, employing nanotubes as tiny test tubes. This approach allowed us to create a new phase diagram that connects the atomic structure of selenium to the diameter of the nanowires.’ 

The Nottingham group previously reported using nano test tubes to image chemical reactions of individual molecules and to observe phase transitions in semiconductors. This approach enables real-time filming of chemistry at the atomic level.

Dr Will Cull said, ‘To our astonishment, we observed that the nano test tube became thinner as we imaged it! Before our very eyes, we witnessed the selenium nanowire inside the nanotube being squeezed like toothpaste, stretching and thinning. This serendipitous discovery allowed us to establish mechanisms for the transformation of one type of nanowire to another, which have implications for their electronic properties, with near-atomic precision.’  

The band gap is a crucial property of semiconductors that significantly impacts their use in various devices, including solar cells, transistors, and photocatalysts. Professor Quentin Ramasse, director of EPSRC SuperSTEM, said, ‘By utilising atomically resolved scanning transmission electron microscopy coupled with electron energy loss spectroscopy, we were able to measure the band gaps of individual chains of selenium. These measurements enabled us to establish a relationship between the diameter of these nanowires and their corresponding band gaps.’ 

Professor Quentin Ramasse said, ‘Traditionally, carbon nanotubes have been used as nano test tubes; however, their outstanding energy absorption properties can obscure the electronic transitions of the material inside. In contrast, a newer type of nano test tube, boron nitride nanotubes, is transparent, allowing us to observe the band gap transitions in selenium nanowires contained within them.’

The famous Moore's Law states that the number of transistors on an integrated circuit doubles approximately every two years. As a result, electronic components must become smaller. Professor Andrei Khlobystov, School of Chemistry, University of Nottingham, said, ‘We have investigated the ultimate limit for nanowire size while preserving useful electronic properties. This is possible for selenium because the phenomenon of quantum confinement can be effectively balanced by distortions in the atomic structure, thus allowing the band gap to remain within a useful range.’

The researchers hope that these new materials will be incorporated into electronic devices in the future. Accurately tuning the band gap of selenium by changing the diameter of the nanowire could lead to the design of a variety of customised electronic devices using only a single element.

This work is co-funded by the EPSRC Programme Grant ‘Metal atoms on surfaces and interfaces (MASI) for sustainable future’ www.masi.ac.uk and the Leverhulme Trust project ‘Taming the Radicals’.

The University of Nottingham has a strong track record in championing nanoscience and nanotechnologies. The Nanoscale & Microscale Research Centre (nmRC) provides a unique set of instrumentation and allied expertise, all under one roof, to support a number of cross-disciplinary research projects from functional materials to quantum technologies to healthcare.

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