Just three years ago, we were reporting on the first tentative steps in producing a two-dimensional (2D) form of boron that came to be known as borophene. Since then most of the work with borophene has been aimed at synthesis as well as characterization of the material’s properties.
Now researchers at Northwestern University—led by Mark Hersam, a Northwestern professor at the forefront of investigating the potential of a variety of 2D materials—have taken a significant step beyond merely characterizing borophene and have started to move towards making nanoelectronic devices from it.
In research described in the journal Science Advances, Hersam’s team has for the first time combined borophene with another material to create a heterostructure, which is a fundamental building block for electronic devices. Since this work represents the first demonstration of a borophene-based heterostructure, the researchers believe that it will guide future and ongoing research into using borophene for nanoelectronic applications.
Heterostructures are the combination of multiple heterojunctions, which is where layers of different 2D materials meet. By stacking layers of materials with different properties on top of each other—such as a conductor with an insulator—you can tailor the electronic properties of the heterostructures to create functional devices.
Of course, there is a growing list of 2D materials from which to form heterostructures, but borophene offers a fairly rare quality in the “flatlands” of 2D materials: it’s a 2D metal.
“As a 2D metal, borophene helps fill a void in the family of 2D nanoelectronic materials,” said Hersam in an e-mail interview with IEEE Spectrum. “Fundamentally, borophene is also interesting since there is no 3D layered version of boron (i.e., there is no boron version of graphite). Therefore, borophene is relatively unique among 2D materials in that it only exists in the atomically thin limit.”
While this is an interesting property of borophene, it also makes it a challenge to synthesize because you can only make it in pristine, ultrahigh vacuum environments. The relatively high chemical reactivity of borophene also presents challenges for handling it in ambient conditions.[shortcode ieee-pullquote quote="Borophene offers a fairly rare quality in the "flatlands" of 2D materials: it's a 2D metal." float="left" expand=1]
Hersam believes that one of the key outcomes from this most recent research was not just combining a borophene with a semiconductor, but also better understanding the chemistry of borophene so that it will become easier to manipulate.
“We are at an early stage of the development cycle of borophene,” said Hersam. “The material was first synthesized fairly recently, and we are now just learning about its chemistry and how to integrate it with other materials. More work is required before the full potential of borophene is realized in electronic applications.”
While the Northwestern researchers have developed a completely ultrahigh vacuum (UHV)-based process for forming borophene-based heterostructures, they can only reliably handle the material in UHV processes, which creates experimental challenges.
Hersam recognizes that they will need to develop reliable encapsulation and/or passivation schemes that allow the borophene to be removed from a UHV environment so that a practical device could actually be fabricated and tested.
Another big challenge: how to transfer borophene from the present silver growth substrate to an electrically insulating substrate.
Hersam added: “When borophene is on silver (both of which are metallic), the silver substrate electrically shorts out the borophene, which creates serious problems for any device applications.”