RESEARCH NEWS Borophene: a new paradigm! Sanjoy Mukherjee and Pakkirisamy Thilagar The chemistry of boron-containing com- tremendously complex structure, with The synergistic feedback between pounds is quite unique and incomparable greater than 300 atoms per hexagonal theory and experiment has greatly enriched to that of any other element in the peri- unit cell that consists of a combination of boron chemistry. For instance, Prasad odic table. The strong tendency of boron icosahedra and fused icosahedra (Figure and Jemmis10 had proposed theoretical to form 3D clusters is quite noticeable 1). However, a phase transition from models for stabilizing fullerene-like bo- from the library of boranes and metallo- -boron to other phases by cooling or ron clusters in 2008 and in 2014, Wang boranes1. Owing to their stability, annealing at ambient pressure has never et al.11 published the first report on the 3D-aromaticity, etc., these boron-rich been reported. However, the unresolved experimental observation of an all boron- compounds find important applications issues of partial occupancies (as fullerene. The intriguing 3D-structural in medicine (e.g. BNCT or boron- observed by X-ray crystallography) at features of boron and its clusters make it neutron-capture-therapy, etc.) as well as two different sites even of this structure quite unintuitive to even stumble upon in material research (e.g. high tempera- has spurred a large number of theoretical the idea of 2D boron sheets. However, in ture stable materials, luminescent com- investigations9. Even though our under- 2014, Wang et al.12 proposed theoretical ponents, LASERs, etc.)2. On the other standing of the structure of elemental models based on experimental observa- hand, a significant recent development in boron is incomplete, the progress of tions which predicted planar hexagonal the intriguing organometallic chemistry boron chemistry has not ceased and con- B36 as a potential basis for extended of boron is the emergence of Frustrated tinued to expand. In understanding single-atom layer of boron sheets (Figure Lewis acids, containing electron- boron, theoretical knowledge and com- 1). The vacancies and the bending of the deficient boron centres and their applica- putational approaches have contributed overall structure are expected to be of tions in catalysis3. Further, these Lewis to a great extent1b. Electronic predictions fundamental importance for the enhanced 12,13 acid-base adduct systems and simple and validations of experimental struc- stability of the B36 cluster . Based on donor–acceptor (D–A) architectures of tures have often compelled experimental the results, the authors predicted that the boron-containing molecules are also of chemists to get back to their work electronic structure of the 2D boron sheets interest from as materials in optoelec- benches. can be either metallic or semiconducting. tronic devices4. The vast chemistry of boronic-acids as reagents and drugs is also expanding at a rapid pace5. Inor- ganic boron-compounds (e.g. boron- nitride (BN)) are finding importance in nano-sciences6. Last but not the least, boron-containing organic materials, e.g. triarylboranes (TABs), boron-dipyrro- methenes (BODIPY) have been success- fully utilized in the developments of families of bright luminescent, photo- stable and fine-tunable dyes, which find applications in solid-state lighting, secu- rity, forensic analysis, sensing, stimuli- responsive and light-harvesting systems7. Verily, boron-chemistry has grown by leaps and bounds and is full of surprises which continue to astound researchers. In a previous article, we had described different bonding tendencies of boron in the formation of various stable organic/ 8 organometallic compounds . Even after two centuries of the discovery of boron, there is no consensus among chemists in Figure 1. The crystal structure of -rhombohedral boron in terms of B84 and B10 units. describing the phase diagrams of elemen- a, B84 consists of a central B12 icosahedron (blue) surrounded by 12 half icosahedra 9 (pink). b, B84 units in adjacent rhombohedral unit cells connected via B10 cluster units tal boron . Although boron displays (gold). This view is from the c-axis (perpendicular to the page). These layered structures a large number of allotropes, only a few stick to each other along this axis, and interstitial boron atoms (not drawn) lie between of them (e.g. -rhombohedral boron, B10 clusters connecting the layers. c, A schematic view of part of an extended one-atom- -rhombohedral boron, -boron and thick boron sheet, named borophene, as constructed from the planar hexagonal B36 unit. T-192) have been confirmed to be ther- The circles represent the apex atoms in the B36 unit that are shared by three units. Re- moval of these atoms would lead to the a-sheet. Adapted with permission from ref. 9 modynamically stable. The thermo- (Copyright 2013, American Chemical Society) and ref. 12 (Copyright 2014, Nature Pub- dynamically favourable -boron has lishing Group). 1302 CURRENT SCIENCE, VOL. 111, NO. 8, 25 OCTOBER 2016 RESEARCH NEWS This idea of achieving a 2D sheet of or- dered boron atoms is quite intriguing. Current research on 2D materials such as graphene, boronitrene layers, phos- phorene, silicene, or transition-metal dichalcogenides is focused on the tuning of their electronic and photonic proper- ties combined with high-quality film syn- theses (Figure 2)14. Major goals are the fabrication of semiconducting films for nanoscale electronic devices and applica- tions to improve the performance of energy conversion and storage sys- tems14c. The mechanical properties of graphene and MoS2 are also being con- sidered for their nano- and meso-scale tribological aspects. The prospect of exploiting the fascinating electrodyna- mics of the quantum realm using two- dimensional (2D) materials has initiated a paradigm shift towards the exploration of sophisticated 2D systems14b. The outstanding achievement in 2D materials has been the development of graphene, the hexagonal planar allotrope Figure 2. Chemical structure of single layers of (a) graphene, (b) borene, (c) bo- of carbon which has driven the hunt for ronitrene. 3D views showing conformations of (d) silicene, germene, stanene and (e) phosphorene. Adapted with permission from ref. 16a. Copyright 2010; Elsevier. high speed nano-electronics. Its isolated 2D nature gives rise to some of its strik- ing and favourable properties that are absent in its bulk counterpart (graph- ite)14–16. The superior electronic proper- ties of graphene arise because of the quantum confinement of the carriers along the plane and carriers acting as relativistic massless Dirac fermions, where they can cover remarkable dis- tances without scattering15. Even at a free carrier concentration of zero, the presence of such Dirac fermions ensures a minimum (quantum) electrical conduc- tivity. These remarkable aspects of gra- phene are projected to be the basis for the future generation of nano-electronics. However, the semi-metallic and chemi- cally inactive nature of graphene remains an impediment in utilizing its full poten- tial. In recent years, elemental sheets of silicon and germanium (silicene and germanene respectively) have been developed as strong contenders in the realm of 2D materials16. Although semi- metallic, silicene and germanene, unlike graphene, do not form van der Waals layered structures in their bulk phase and this has posed a major hurdle in realizing Figure 3. a, Schematics of distorted B7 cluster and growth setup with atomic structure their applications. Hence, they do not ex- model and STM topography rendering. b, Top and side views of the low-energy ist as freestanding sheets and till now can monolayer structure (unit cell indicated by green box). c, Simulated empty states STM be only synthesized as ad-layer struc- image (Vsample = 1.0 V), with overlaid atomic structure and unit cell of 0.500 nm by tures on ordered substrates. Recently, 0.289 nm and d, Experimental STM images (Vsample = 0.1 V, It = 1.0 nA), with overlaid unit cell of 0.51 nm by 0.29 nm. e–g, Series of large-scale STM topography (left) images phosphorene (the fundamental 2D struc- of borophene sheets. Adapted with permission from ref. 18b. Copyright 2015, The ture that constitutes black phosphorus American Association for the Advancement of Science. CURRENT SCIENCE, VOL. 111, NO. 8, 25 OCTOBER 2016 1303 RESEARCH NEWS crystals) which exists as a freestanding conductors at standard conditions, only Berner, S. et al., Angew. Chem. Int. Ed., 2D nano-sheet and displays exceptional becoming metallic at enormously high 2007, 46, 5115. semiconductor with graphene like prop- pressures. It is expected that the reactiv- 7. (a) Ulrich, G., Ziessel, R. and Harriman, erties, has been added to the family of ity of the boron atoms can be beneficial A., Angew. Chem., Int. Ed., 2008, 47, elemental material17. in the further modifications of borophene 1184; (b) Jakle, F., Chem. Rev., 2010, 110, 3985; (b) Loudet, A. and Burgess, Very recently in 2015, Guisinger et by introducing other chemical groups or 18b K., Chem. Rev., 2007, 107, 4891; (c) al. first demonstrated experimental by sandwiching it between other materi- Mukherjee, S. and Thilagar, P., J. Mater. characterization of boron monolayers by als to fine-tune its properties. Boron con- Chem. C, 2016; 4, 2647. scanning tunnelling microscopy and low- taining compounds are well known for 8. (a) Mukherjee, S. and Thilagar, P., Curr. energy electron diffraction (Figure 3). their extreme hardness and thus they may Sci., 2013, 104, 1601; (b) Thilagar, P., The authors used physical vapour deposi- be easier to handle than the more fragile Curr. Sci., 2015, 109, 1541. tion (PVD) method to create atomically 2D materials such as silicene and germa- 9. Ogitsu, T., Schwegler, E. and Galli, G., thin, borophene sheets under ultrahigh- nene. Chem. Rev., 2013, 113, 3425. vacuum (UHV) conditions, using a solid These explorations have remarkably 10. Prasad, D. L. V. K. and Jemmis, E. D., boron atomic source (99.9999% purity) brought boron to the forefront of the Phys.