Credits: the figure combines electron microscopy data provided by Rongting Wu and an image of the calculated structure of borophene on copper provided by Stephen Eltinge.
Material properties – for example whether they conduct electricity and heat, are rigid or flexible, transparent or opaque – are determined by the type of constituent atoms, their spatial arrangement, and how they share their electrons. We know that copper conducts electricity much better than rubber, stainless steel is hard, and iron is magnetic. The importance of the crystalline arrangement in a material can be even better appreciated if we look at different structural forms of the same material, i.e. at the so-called allotropes. Take carbon for example: diamond is transparent and hard, while graphite, which is a stack of weakly bound atomically thin sheets forming a honeycomb lattice, is opaque in bulk and cleaves so easily that it is used for writing or as a lubricant.
How about having a mono-elemental material whose structure and electronic properties can be tuned at will and used in a device? Well, it turns out that borophene, a two-dimensional (2D) boron allotrope, might just be that ideal material. Do we already have working devices and gadgets using this material? Not yet, but we have just made important progress on the way to reaching this dream goal, by demonstrating that the structure of borophene can be tuned by the choice of the substrate, and by synthesizing large, device-size single crystals of borophene.
The structure of borophene can be easily envisaged starting from a 2D honeycomb layer of B atoms: draw an arbitrary unit cell, fill some of the empty hexagon centers with B atoms and now translate this super-cell so that you fill the entire 2D plane. You have just built a potential borophene layer. How about its stability? A key point is that out of the myriad options to do this there is still a large number of stable structures satisfying these building rules. But for many years, these layers existed only in theory. The prospect of producing them was very appealing though because several of the predicted electronic or elastic properties rivalled those of well-known graphene. Since the most stable allotrope form of boron is the 3D solid, it was not even clear whether borophene can be ever synthesized.
Using Ag substrates two pioneering works demonstrated successful synthesis of atomically thin borophene organized in islands tens of nanometers in size. From the device fabrication perspective larger domains are needed. In our group we used a different substrate material which we hoped would strike the balance between promoting larger crystals but not be so reactive and form boride compounds. Copper satisfied these requirements. Using advanced synthesis tools along with real-space imaging and diffraction capabilities of low-energy electron microscopy we were able to produce single-crystal domains of borophene with a novel structure and up to 100 mm2 in size. A series of tour-de-force tunneling experiments, where the only way we could get enough detail at the atomic scale was by hanging a carbon-oxide molecule on an atomically sharp metallic tip and raster it across the sample, allowed to identify the structure of our layers. We still had to sift through the candidate structures that were in principle compatible with our data. Theory and extensive numerical work helped us do just that.
It was a tortuous and painstaking work but the results are exciting. We are not at the end of the road and, obviously, there are a few more milestones to conquer. The first step is to transfer these borophene layers from the metallic surfaces they have been grown on so far onto insulating, device-compatible substrates. Then one would like to understand in detail how to tune from one structure to another and obtain on-demand electric or mechanical properties. Then we can indeed carry in our pockets fast devices based on all-borophene transistors or flexible screens that we can unfold and watch our favorite movie.