Maximum surface area, minimum thickness: Research teams from materials science and physics are growing ultra-thin, inorganic materials consisting of only a single layer of atoms. In combination with nanoscale optical antennas, they are tailoring 2D materials to form photonic nanomaterials.
By Ute Schönfelder
Despite the fact that the crystals are less than one nanometre thick, they are incredibly robust. As they hardly have any volume, they are usually just referred to as ›2D materials‹. Similar to graphene, the first 2D material to be artificially produced, the ultrathin molybdenum or tungsten disulphide sheets have a number of extraordinary properties. »These ultimately thin semiconductor materials are suitable for various applications, such as new types of electronic and optoelectronic components, chemical sensor components and catalysts,« explains Andrey Turchanin.
Turchanin synthesizes the 2D materials in his laboratory by heating and evaporating the precursor materials at over 700°C. These materials are transported through a glass tube in a stream of argon and hydrogen gas at a defined pressure and temperature, which causes a reaction. The material crystallizes on a substrate within the tube, similar to the way in which water forms frostwork on a cold window pane. The ›ice crystals‹ made of tungsten disulphide are shaped like equilateral triangles (see picture below), while other 2D materials crystallize in the shape of stars or ›snowflakes‹.
When laser light interacts with a nonlinear material (e.g. a crystal with a certain non-centrosymmetric lattice structure), radiation is generated with twice the frequency of the incident light. When the frequency is doubled, the light’s wavelength is halved. In this way, for example, green light with a wavelength of 532 nm can be produced from the infrared radiation of an Nd:YAG laser with a wavelength of 1,064 nm. This phenomenon is used in laser pointers. The effect is caused by the oscillations of charge carriers in the crystal, which are produced through interactions with intense laser light.
As nonlinear frequency doubling is also referred to as ›second-harmonic generation‹, it is often abbreviated as SHG.