materials Review From 2-D to 0-D Boron Nitride Materials, The Next Challenge Luigi Stagi , Junkai Ren and Plinio Innocenzi * Laboratorio di Scienza dei Materiali e Nanotecnologie, CR-INSTM, Dipartimento di Chimica e Farmacia, Università di Sassari, Via Vienna 2, 07100 Sassari, Italy; [email protected] (L.S.); [email protected] (J.R.) * Correspondence: [email protected] Received: 8 October 2019; Accepted: 22 November 2019; Published: 26 November 2019 Abstract: The discovery of graphene has paved the way for intense research into 2D materials which is expected to have a tremendous impact on our knowledge of material properties in small dimensions. Among other materials, boron nitride (BN) nanomaterials have shown remarkable features with the possibility of being used in a large variety of devices. Photonics, aerospace, and medicine are just some of the possible fields where BN has been successfully employed. Poor scalability represents, however, a primary limit of boron nitride. Techniques to limit the number of defects, obtaining large area sheets and the production of significant amounts of homogenous 2D materials are still at an early stage. In most cases, the synthesis process governs defect formation. It is of utmost importance, therefore, to achieve a deep understanding of the mechanism behind the creation of these defects. We reviewed some of the most recent studies on 2D and 0D boron nitride materials. Starting with the theoretical works which describe the correlations between structure and defects, we critically described the main BN synthesis routes and the properties of the final materials. The main results are summarized to present a general outlook on the current state of the art in this field. Keywords: boron nitride; 2D materials; quantum dots; fluorescence; nanocomposites 1. Introduction Boron nitride (BN) is a chemically stable material exhibiting four different polymorphs: hexagonal (h-BN), cubic (c-BN), rhombohedral (r-BN), and wurtzite (w-BN) [1–5]. The different BN allotropes have attracted considerable interest as a possible alternative to diamond for their great hardness and high thermal stability [3–6]. Despite a thermal stability higher than diamond (1473 K) and a low reactivity with steel, c-BN still presents a more moderate hardness in comparison with diamond (HV = 40–60 GPa) [3]. Nevertheless, C2-BN nanocomposites (c-BN: diamond, 1:2) can reach a very high hardness, HV = 85 GPa. More recently, w-BN displayed interesting hardness properties, comparable with the other allotropes [5]. Pure w-BN, treated at a pressure of 10–20 GPa and high temperature (400–1900 ◦C) reached a hardness as high as HV = 46 GPa. A two-stage shear mechanism is responsible for the unusual hardness of w-BN [6]. h-BN presents an sp2 hybridization of B–N bonds, and it crystallizes in a layered structure like graphene (Figure1). While B and N atoms are held together with strong covalent bonds densely covering the plane, the different BN layers interact via Van der Waals forces. Contrary to graphene, B–N is strongly polarized with electronegative N and almost vacant B. BN nanoplates are known to preserve their thermal stability up to 1000 ◦C, undergoing oxidation in the range between 1000–1200 ◦C, with the formation of B2O3. For thin BN nanosheets the oxidation process starts at 850 ◦C[7–10]. 1 h-BN monolayers present a thermal conductivity of 751 W mK− at 25 ◦C conjugating high thermal conductivity with electrical insulation as a good candidate for heat dissipation in future electronic devices [11]. Materials 2019, 12, 3905; doi:10.3390/ma12233905 www.mdpi.com/journal/materials Materials 2019, 12, 3905 2 of 22 Materials 2018, 11, x FOR PEER REVIEW 2 of 22 Figure 1. ((AA)) hexagonal hexagonal boron boron nitride nitride structures. structures. Layer Layer distance distance and and crystal crystal parameter. parameter. ( (B,CB,C)) Different Different edge terminations. ( D,ED,E) different different boron nitridenitride shapes:shapes: BN nanotubes and BN fullerene.fullerene. Copyright 2012 and 2014, with permission of refs, [[7]7,8 and]. [8]. Optical spectroscopyspectroscopy measurements measurements on on h-BN h-BN crystals crystals have have demonstrated demonstrated evidence evidence for an indirectfor an bandindirect gap band of 5.955 gap eVof 5.955 [12]. eV More [12]. recently, More recently, first principle first principle calculations calculations described described the evolution the evolution of band gapof band as a gap function as a offunction the numbers of the ofnumbers layers, predictingof layers, predicting the crossover the tocrossover direct band to direct gap at band a limit gap of at one a layerlimit of [13 one], later layer verified [13], later experimentally verified experimentally by Elias et al. by [14 Elias]. et al. [14]. Boron and nitrogen show the ability to create ordered superstructures under ultra-high vacuum conditions. Borazine, widely used in film film depositi depositions,ons, has the tendency to decompose on Ru and Rh substrates, forming a self-assembled regular mesh [[15–17].15–17]. The derived structure presents hexagonal pores at distances of 3.2 nm and 2 nm and is produced upon BN adhesion on the metal surface. Correspondingly, 13 13 B orB Nor atomsN atoms are locatedare located on 12 Rhon atoms,12 Rh whichatoms, in turnwhich determines in turn determines the corrugation the ofcorrugation the BN system. of the BN The system. periodic The repetition periodic of repetition peaks and of valleyspeaks and originates valleys inoriginates a uniform in BNa uniform single layerBN single with layer potential with applications potential applications in spintronics, in spintronics, quantum computing, quantum computing, and photochemistry. and photochemistry. It has been demonstratedIt has been demonstrated that the nanomeshes that the nanomeshes could work ascould templates work foras templates molecules for and molecules metal nanoparticles. and metal Thenanoparticles. molecules tendThe tomolecules be trapped tend into to the be pores trapped preserving into the their pores structures preserving and properties;their structures this opens and upproperties; new opportunities this opens up for new molecular opportunities electronics for [molecular18]. electronics [18]. h-BN represents a candidatecandidate for p-typep-type layerslayers inin nitridenitride deep-ultravioletdeep-ultraviolet (DUV)(DUV) emitters.emitters. Even though the lattice mismatch of h-BN with respect to the c-plane of AlN is more than 19%, fivefive lattice constants ofof BNBN alignalign with with four four lattice lattice constants constants of of AlN, AlN, reducing reducing the the mismatching mismatching down down to 0.58% to 0.58% and makingand making feasible feasible a h-BN a /h-BN/w-AlNw-AlN heterojunction. heterojunction. Moreover, Moreover, h-BN h-BN is a good is a e-blockinggood e-blocking and p-contact and p- layercontact with layer w-AlN, with w-AlN, showing showing a type II a alignment type II alignment [19]. [19]. Because of their negative electron affinity, affinity, BNBN nanotubesnanotubes displaydisplay interestinginteresting electron emission properties. DespiteDespite thethe higher higher turn-on turn-on field field compared compared to carbonto carbon nanotubes nanotubes (CNT), (CNT), BN nanotubesBN nanotubes have betterhave better thermal thermal stability stability and can and work can work at high at temperaturehigh temperature in an oxygenin an oxygen environment environment [20]. [20]. The great versatilityversatility ofof BNBN materials materials enables enables their their application application as as neutron neutron detectors detectors [21 [21]] because because of 10 theof the large large cross cross section section of Bof for 10 thermalB for thermal neutrons. neutrons. Maity etMaity al. developed et al. developed a h-BN detector, a h-BN composed detector, ofcomposed two bilayers of two Ni /bilayersAu as ohmic Ni/Au contacts. as ohmic They contac exhibitedts. They an exhibited efficiency an of efficiency 58% when of exposed 58% when to a calibratedexposed to neutron a calibrated source neutron [22]. source [22]. A porous amorphous BN has been recently proposed as photocatalyst for the reduction of CO . A porous amorphous BN has been recently proposed as photocatalyst for the reduction of CO2. It works in in the the UV UV region region without without employing employing any any coca cocatalysttalyst [23]. [23 Continuous]. Continuous endeavors endeavors to extend to extend the BN the BNapplication application to the to thevisible visible range range have have been been made made to totrigger trigger the the reactions reactions under under solar solar exposition exposition [24]. [24]. Several semiconductors in hybrid systems with BN were proposed such as TiO , γ-C N , ZnO, In S Several semiconductors in hybrid systems with BN were proposed such as TiO2 2, -C33N44, ZnO, In22S33 and manymany othersothers [24[24].]. InIn the the composites, composites, BN BN plays plays a cruciala crucial role, role, promoting promoting the the reduction reduction of band of band gap, throughgap, through structural structural strain strain and inhibitingand inhibiting electron electron hole recombinationhole recombination [25]. [25]. Since the sixties, BNBN hashas attractedattracted considerableconsiderable attentionattention inin aerospaceaerospace applicationsapplications [[26].26]. Most recently, BNBN materials materials have have been been the the focus focus of a renewedof a renewed interest interest in the formin the of form BN nanotubes