Some metal alloys exhibit sharp, inducible transitions in their optical and electronic states. However, such alloys are difficult to handle on the nanoscale. Zengguang Cheng, Fudan University, Shanghai, China, and University of Oxford, UK, Harish Bhaskaran, University of Oxford, and colleagues have discovered that pure antimony has excellent optoelectronic properties, too—especially when made into a nanofilm. Such nano-optical materials are of interest for potential future applications, such as photonic computing or high-speed holographic displays.

Optical Phase-Changers

Phase-changing materials switch between two phases when the temperature is raised, or films of them are laser-bombarded with photons. Some semiconducting alloys can switch between their obscure amorphous and bright crystalline phase, which gives a sharp optical contrast. These alloys are useful for electrical and optical storage technologies, such as CDs or Blu-rays, as well as photodetectors. However, they tend to lose their integrity at reduced dimensions, which makes miniaturization down to the nanoscale problematic.

The team looked for phase-changers made of single elements. They found what they were looking for in antimony: This gray and brittle semimetal forms a black, amorphous solid when deposited from the gas phase. Then, it slowly turns into its gray, crystalline form with a layered hexagonal lattice.

New Territory on the Nanoscale

This transition is textbook knowledge. So, it may come as a surprise that antimony’s electrical and optical properties as a thin film are relatively unknown. “An optical property change of pure antimony could be expected during the phase transition, yet this has never been studied,” the researchers said.

They found that sputtered thin films of antimony were amorphous and obscure. However, the films changed their phase upon annealing to become more transparent and crystalline. This optical contrast was found to be sharper and more pronounced than for established phase-changing alloys such as germanium telluride. However, it was also dependent on thickness: The thinner the film of antimony (below 15 nanometers), the sharper the contrast.

As a possible explanation for the contrast, the team proposes that local clusters of antimony make the bonding in the amorphous phase more isotropic, which affects the electrical properties of the film. However, they also note that a deeper knowledge of the interfaces and the thin-film behavior of antimony would be useful.

Electro-Optical Applications

To explore possible applications, the researchers tested the films in a reflective display setup: They sandwiched antimony films between two indium tin oxide (ITO) layers deposited in sequence on a platinum mirror. Based on the thicknesses of both the amorphous film and the ITO layers, the display appeared in different colors. The researchers then switched the color by annealing. In the amorphous state, the display shone dark blue, but after annealing, it was pale brown.

Then, the team integrated the reflective display setup in an atomic force microscope and made the conductive AFM tip raster-scan a micron-sized area. The applied voltage induced changes in the optical contrast at high resolution. The researchers used this technique to reproduce miniature grayscale fine-art pictures with a resolution below 200 nm per pixel.

Future applications, such as photonic computing, demand not only that the lateral resolution be high, but also that the response time be ultra-short. Accordingly, the researchers applied femtosecond laser pulses to crystallized antimony thin films. A single pulse re-amorphized the antimony, producing dark dots of the amorphous phase on a bright crystalline background. However, this photon-induced contrast did not last: After keeping the samples for several days at room temperature, the team observed a slow decay of the amorphized film. It recrystallized, and the dark areas became bright again.

The researchers see great potential for these antimony nanofilms, especially for applications where metals showing optical contrast on a miniature size are important. However, they also note that the molecular basis underlying the electro-optical behaviors still requires further exploration.

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• Antimony thin films demonstrate programmable optical nonlinearity,
  Zengguang Cheng, Tara Milne, Patrick Salter, Judy S. Kim, Samuel Humphrey, Martin Booth, Harish Bhaskaran,
  Sci. Adv. 2021, 7, eabd7097.
  https://doi.org/10.1126/sciadv.abd7097