Over past 50 years, silicon has been a core material in the semiconductor industry. So far, the number of microchips that can be integrated in a unit area has continued to increase, but it is now facing a fundamental limit due to the natural properties of silicon. It would thus be timely and important to look for another material with better properties for achieving higher performance and even smaller devices. In this respect, “two-dimensional (2D) materials” is attracting rapidly growing interests in the study of condensed matter physics and materials science. 2D materials are in the thinnest limit of materials, only few atoms thick, which can be made, for example, by exfoliating a couple of surface layers from layered materials. The material of only a few atomic layers exhibits a unique property, different from their 3D counterparts. Some of these unique properties are useful for device applications, which may impact our future electronic and optoelectronics technology.
Graphene is a prototypical 2D material, consisting of carbon atoms, in the planar honeycomb lattice. It is stronger than steel yet many times lighter, more conductive than copper and more flexible than rubber. All these properties combined make it a tremendous conductor of heat and electricity. A defect-free layer is also impermeable to all atoms and molecules. This amalgamation makes it a terrifically attractive material to apply to scientific developments in a wide variety of fields, such as electronics, aerospace and sports. For all its dazzling promise there is however a disadvantage; graphene has no band gap.
The band gap, which is the forbidden energy region of electrons, is a fundamental property of insulators. It can be regarded as a barrier in the flow of electrons from one end to the other. If the band gap is large enough it’s difficult to induce an efficient current, whereas it is much easier with the band gap close to zero. The material with a moderate band gap (e.g., silicon has the band gap of 1.1 eV) is a semiconductor, and one can effectively control its current. For the case of graphene, the band gap is zero, making the control of current highly inefficient.
Ever since graphene was first isolated from graphite in 2004, inducing a band gap has been a major research goal. However, it has been difficult to obtain a sizable band gap in graphene without degrading the electronic quality. Therefore, a key issue in the study of 2D materials is controlling their electronic states to overcome the limit of natural properties. Furthermore, the band gap of a material has been widely believed to be almost “impossible” to change over a wide range.
An alternative approach to this problem is to find a different 2D material that exhibits a band gap in its natural state, that is, 2D semiconductors. Just over a year ago such a material was found: phosphorene. Phosphorene is a 2D material of phosphorus (P) atoms, where the honeycomb lattice, similar to graphene, is strongly puckered. This unique puckered structure renders its electronic state susceptible to external perturbations, such as applied strain and electric field. That is, phosphorene looks much less obstinate, as compared to graphene. Our research group in POSTECH and collaborators thought that it might be pretty interesting, if one can start with phosphorene and somehow manipulate its electronic state to mimic that of graphene. This is an alternative approach to generating a band gap in graphene.
Soon after this gedanken experiment could be realized in our experiments. We doped the surface of black phosphorus, which consists of layers of phosphorene, by depositing alkali-metal atoms in the ultrahigh vacuum. The adsorbed dopants induce a strong electric field, thereby changing the charge configuration near the surface phosphorene layers. Variations in the band gap were then monitored by angle-resolved photoemission spectroscopy with the synchrotron radiation. As a result, the band gap of phosphorene could be controlled over the wide range of 0 ~ 0.6 eV by selectively adjusting the concentration of dopants. As the band gap approaches zero, the well-known electronic state of graphene, which is responsible for its unique properties, could also be created in phosphorene. It is very exciting to see that what has been impossible with graphene now becomes possible with phosphorene[Science 349, 723 (2015)] .
Still there are many remaining problems in the various fields of research. It is time to challenge on your gedanken experiments to make another impossible possible!