Atomically Thin Films Grow In Number

Films of materials just a few atoms thick or less are expected to usher in a new generation of smaller, faster electronics and more powerful energy-storage devices. This year, researchers took important steps in that direction by developing methods for preparing a number of these atomically thin layers, called two-dimensional materials (see page 11). Deji Akinwande of the University of Texas, Austin, and Alessandro Molle of the National Research Council of Italy led a team that made field-effect transistors built with a single layer of silicon atoms serving as the channel, which is the circuit component through which charge flows from the source to the drain electrodes (Nat. Nanotechnol. 2015, DOI: 10.1038/nnano.2014.325). The team grew the 2-D film, dubbed silicene because of its similarity to graphene, on a silver support and capped it with a protective layer of alumina. Meanwhile, at Northwestern University, Mark C. Hersam and colleagues demonstrated a method for producing large quantities of ultrathin flakes of black phosphorus, a sought-after semiconductor. The team showed that the slow manual method typically used to isolate the thin flakes can be replaced with a faster method that relies on ultrasonicating chunks of black phosphorus in anhydrous solvents (ACS Nano 2015, DOI: 10.1021/acsnano.5b01143). Also this year, researchers led by Drexel University’s Yury Gogotsi and Michel W. Barsoum reported that the family of strong, flexible, conducting 2-D transition-metal carbides and nitrides known as MXenes (pronounced “maxenes”) could be much larger than previously thought. At the start of 2015, scientists knew about roughly 70 MXenes containing one transition metal. The Drexel team predicted that 26 or more members of a new family of sandwichlike MXenes containing two different transition metals should be stable, including Mo₂TiC₂T_x and two others, which they synthesized; T represents OH, O, and F surface terminations (ACS Nano 2015, DOI: 10.1021/acsnano.5b03591).