The new germanium nanofilm not only shimmers like an opal but is hard as a crystal, exceptionally thin and highly porous. Photo: Andreas Battenberg/TUM.
The new germanium nanofilm not only shimmers like an opal but is hard as a crystal, exceptionally thin and highly porous. Photo: Andreas Battenberg/TUM.

Using a new procedure, researchers at the Technical University of Munich (TUM) and the Ludwig Maximillians University of Munich (LMU) can now produce extremely thin and robust yet highly porous semiconductor nanofilms. These nanofilms could help produce small, light-weight, flexible solar cells or electrodes for improving the performance of rechargeable batteries.

Not only does the semiconductor nanofilm shimmer like an opal, but it also has amazing properties: hard as a crystal, exceptionally thin and – since it is highly porous – light as a feather. By integrating suitable organic polymers into the pores of the nanofilm, the researchers can tailor the electrical properties of the resultant hybrid material. This design both saves space and creates large interface surfaces that improve overall effectiveness.

"You can imagine our raw material as a porous scaffold with a structure akin to a honeycomb," explains Thomas Fässler, chair of inorganic chemistry with a focus on novel materials at TUM. "The walls comprise inorganic, semiconducting germanium, which can produce and store electric charges. Since the honeycomb walls are extremely thin, charges can flow along short paths." Fässler is senior author of a paper on this work in Angewandte Chemie.

To transform brittle, hard germanium into a flexible and porous nanofilm required the researchers to apply a few tricks. Traditionally, etching processes are used to structure the surface of germanium. However, this top-down approach is difficult to control at an atomic level. The new procedure solves this problem.

Together with his team, Fässler established a synthesis methodology to fabricate the desired structures very precisely and reproducibly. The raw material is germanium with atoms arranged in clusters of nine. Since these clusters are electrically charged, they repel each other as long as they are dissolved, only coming together when the solvent evaporates.

This can be done by heating the solvent to 500°C or it can be chemically induced, by adding germanium chloride, for example. By using other chlorides like phosphorous chloride, the germanium structures can also easily be doped. This allows the researchers to adjust the properties of the resulting nanomaterials in a very targeted manner.

To give the germanium clusters the desired porous structure, LMU researcher Dina Fattakhova-Rohlfing developed a novel methodology for nanostructuring. This involves producing three-dimensional templates with tiny polymer beads in an initial step. Next, the germanium-cluster solution fills the gaps between the beads. As soon as stable germanium networks have formed on the surface of the tiny beads, the templates are removed by applying heat, leaving the highly porous nanofilm.

The deployed polymer beads have a diameter of 50–200nm and form an opal structure. The germanium scaffold that emerges on the surface acts as a negative mold, forming an inverse opal structure that causes the nanofilm to shimmer like an opal.

"The porous germanium alone has unique optical and electrical properties that many energy relevant applications can profit from," says Fattakhova-Rohlfing, who developed the material with Fässler. "Beyond that, we can fill the pores with a wide variety of functional materials, thereby creating a broad range of novel hybrid materials."

"When combined with polymers, porous germanium structures are suitable for the development of a new generation of stable, extremely lightweight and flexible solar cells that can charge mobile phones, cameras and laptops while on the road," explains team member Peter Müller-Buschbaum, professor of functional materials at TU Munich.

Manufacturers around the world are on the lookout for lightweight and robust materials to use in portable solar cells. To date, they have primarily used organic compounds, which are delicate and have relatively short lifetimes: heat and light decompose the compounds and degrade their performance. These thin but robust germanium hybrid nanofilms thus offer a real alternative.

Next, the researchers want to use the new technology to manufacture highly porous silicon nanofilms, which are currently being tested as anodes for rechargeable batteries. By replacing the graphite layers currently used as anodes, they could improve the energy capacity of the batteries.

This story is adapted from material from the Technical University of Munich, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.