Semiconductors can behave like metals and even like superconductors

March 17, 2020

The crystal structure at the surface of semiconductor materials can make them behave like metals and even like superconductors, a joint Swansea/Rostock research team has shown. The discovery potentially opens the door to advances like more energy-efficient electronic devices.

Semiconductors are the active parts of transistors, integrated circuits, sensors, and LEDs. These materials, mostly based on silicon, are at the heart of today's electronics industry.

We use their products almost continuously, in modern TV sets, in computers, as illumination elements, and of course as mobile phones.

Metals, on the other hand, wire the active electronic components and are the framework for the devices.

The research team, led by Professor Christian Klinke of Swansea University's chemistry department and the University of Rostock in Germany, analysed the crystals at the surface of semiconductor materials.

Applying a method called colloidal synthesis to lead sulphide nanowires, the team showed that the lead and sulphur atoms making up the crystals could be arranged in different ways. Crucially, they saw that this affected the material's properties.

In most configurations the two types of atoms are mixed and the whole structure shows semiconducting behaviour as expected.

However, the team found that one particular "cut" through the crystal, with the so called {111} facets on the surface, which contains only lead atoms, shows metallic character.

This means that the nanowires carry much higher currents, their transistor behaviour is suppressed, they do not respond to illumination, as semiconductors would, and they show inverse temperature dependency, typical for metals.

Dr. Mehdi Ramin, one of the researchers from the Swansea/Rostock team, said:

"After we discovered that we can synthesize lead sulphide nanowires with different facets, which makes them look like straight or zigzag wires, we thought that this must have interesting consequences for their electronic properties.

But these two behaviours were quite a surprise to us. Thus, we started to investigate the consequences of the shape in more detail."

The team then made a second discovery: at low temperatures the skin of the nanostructures even behaves like a superconductor. This means that the electrons are transported through the structures with significantly lower resistance.

Professor Christian Klinke of Swansea University and Rostock University, who led the research, said:

"This behaviour is astonishing and certainly needs to be further studied in much more detail.

But it already gives new exciting insights into how the same material can possess different fundamental physical properties depending on its structure and what might be possible in the future.

One potential application is lossless energy transport, which means that no energy is wasted.

Through further optimization and transfer of the principle to other materials, significant advances can be made, which might lead to new efficient electronic devices.

The results presented in the article are merely a first step in what will surely be a long and fruitful journey towards new thrilling chemistry and physics of materials."
The research was published in "Advanced Functional Materials".


Figure: Left - Shape of nanostructures made of lead sulphide, computer reconstructed based on series of transmission electron microscopy images. The left straight stripe behaves like a semiconductor and the right zigzag nanowire behaves like a metal. Right - Electrical device consisting of two gold electrodes contacting a nanowire (in red) on a silicon chip (in blue). Figures: Hungria/Universidad de Cádiz, Ramin/DESY, Klinke/University of Rostock and Swansea University.

Notes to Editors

Read the article in Advanced Functional Materials:

"Function Follows Form: From Semiconducting to Metallic Towards Superconducting PbS Nanowires by Faceting the Crystal": Mohammad Mehdi Ramin Moayed, Sascha Kull, Angelique Rieckmann, Philip Beck, Michael Wagstaffe, Heshmat Noei, Andreas Kornowski, Ana B. Hungria, Rostyslav Lesyuk, Andreas Stierle, Christian Klinke.

Swansea University is a world-class, research-led, dual campus university offering a first class student experience and has one of the best employability rates of graduates in the UK. The University has the highest possible rating for teaching - the Gold rating in the Teaching Excellence Framework (TEF) in 2018 and was commended for its high proportions of students achieving consistently outstanding outcomes.

Swansea climbed 14 places to 31st in the Guardian University Guide 2019, making us Wales' top ranked university, with one of the best success rates of graduates gaining employment in the UK and the same overall satisfaction level as the Number 1 ranked university.

The 2014 Research Excellence Framework (REF) 2014 results saw Swansea make the 'biggest leap among research-intensive institutions' in the UK (Times Higher Education, December 2014) and achieved its ambition to be a top 30 research University, soaring up the league table to 26th in the UK.

The University is in the top 300 best universities in the world, ranked in the 251-300 group in The Times Higher Education World University rankings 2018. Swansea University now has 23 main partners, awarding joint degrees and post-graduate qualifications.

The University was established in 1920 and was the first campus university in the UK. It currently offers around 350 undergraduate courses and 350 postgraduate courses to circa 20,000 undergraduate and postgraduate students.

The University has ambitious expansion plans as it moves towards its centenary in 2020 and aims to continue to extend its global reach and realise its domestic and international potential.

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