Journal of C Carbon Research Review A Review on van der Waals Boron Nitride Quantum Dots Amit Acharya 1,2, Sambhawana Sharma 1,2, Xiuling Liu 1,3, Dongyan Zhang 1,2,* and Yoke Khin Yap 1,2 1 Department of Physics, Michigan Technological University, Houghton, MI 49931, USA; [email protected] (A.A.); [email protected] (S.S.); [email protected] (X.L.); [email protected] (Y.K.Y.) 2 Elizabeth and Richard Henes Center for Quantum Phenomena, Michigan Technological University, Houghton, MI 49931, USA 3 StabiLux Biosciences, Inc., Ann Arbor, MI 48103, USA * Correspondence: [email protected]; Tel.: +1-906-487-2086 Abstract: Boron nitride quantum dots (BNQDs) have gained increasing attention for their versatile fluorescent, optoelectronic, chemical, and biochemical properties. During the past few years, sig- nificant progress has been demonstrated, started from theoretical modeling to actual application. Many interesting properties and applications have been reported, such as excitation-dependent emission (and, in some cases, non-excitation dependent), chemical functionalization, bioimaging, phototherapy, photocatalysis, chemical, and biological sensing. An overview of this early-stage research development of BNQDs is presented in this article. We have prepared un-bias assessments on various synthesis methods, property analysis, and applications of BNQDs here, and provided our perspective on the development of these emerging nanomaterials for years to come. Keywords: quantum dots; boron nitride; van der Waals; fluorescent; sensing; photocatalysis Citation: Acharya, A.; Sharma, S.; 1. Introduction Liu, X.; Zhang, D.; Yap, Y.K. A Review Low dimensional boron nitride (BN) materials such as BN nanosheets and BN nan- on van der Waals Boron Nitride otubes (BNNTs) have gained substantial attention [1–9]. BN materials are isoelectric to car- Quantum Dots. C 2021, 7, 35. bon solids. For example, the three-dimensional (3D) cubic phase (c-BN), two-dimensional https://doi.org/10.3390/c7020035 (2D) hexagonal phase (h-BN), and one-dimensional (1D) BN nanotubes (BNNTs) are struc- turally like diamonds, graphene, and carbon nanotubes (CNTs), respectively. Despite Academic Editor: these structural similarities, BN materials offer different physical and chemical properties. Jean-François Morin For instance, all BN materials are electrically insulating with about 6eV bandgaps, while most carbon nanomaterials are metallic, except semiconducting single-walled CNTs [10]. Received: 26 February 2021 Like other 2D materials such as graphene [11], and transitional metal dichalcogenides Accepted: 23 March 2021 Published: 27 March 2021 (TMDC), 2D BN nanosheets have gained significant research attention [12–14]. Boron ni- tride nanoribbons, the 1D version of BN nanosheets, are also attractive for their spintronics, Publisher’s Note: MDPI stays neutral magnetic, and electronic properties [15–18]. More recently, zero-dimensional (0D) BN with regard to jurisdictional claims in quantum dots (BNQDs) or BN dots (BNDs) have emerged as the new BN nanomaterials, published maps and institutional affil- with extremely high surface/volume ratios. Many optical and electronic properties of iations. BNQDs are yet to be clarified and therefore are expected to stimulate a lot more theoretical and experimental investigation. BNQDs are a 0D version of h-BN nanosheets. Due to their small dimension, the ratio of dangling bonds at the edges of BNQDs to the saturated sp2 h-BN bonds on the bulk of the 0D structure is high. It is expected that these edges will alter the physical and chemical Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. properties of the h-BN nanosheets. Their high chemical stability and the non-toxic nature This article is an open access article of BN materials are expected to allow for many biomedical applications in BNQDs. The distributed under the terms and theoretical study of electronic structures and spin polarization in BNQDs was first con- conditions of the Creative Commons ducted in 2011 [19] based on first-principle calculations and an extended Hubbard model. Attribution (CC BY) license (https:// Later in 2013, similar studies [20,21] on the structural stability, electronic, optical, and mag- creativecommons.org/licenses/by/ netic properties of BNQDs were performed. Experimentally, BNQDs synthesis [22,23] was 4.0/). reported thereafter. As shown in Figure1, the number of peer-reviewed publications has C 2021, 7, 35. https://doi.org/10.3390/c7020035 https://www.mdpi.com/journal/carbon C 2021, 7, x FOR PEER REVIEW 2 of 26 C 2021, 7, 35 2 of 26 was reported thereafter. As shown in Figure 1, the number of peer-reviewed publications has significantly increased during the past two years. This publication statistic has con- sideredsignificantly papers increased consisting during of the the keyword past two “boron years. nitride This publication quantum dots”, statistic and has did considered not in- cludepapers papers consisting with other of the 0D keyword boron nitride “boron nanoparticles. nitride quantum Two dots”, recent and review did not articles include briefly papers discussedwith other 0D 0D BN boron mate nitriderials along nanoparticles. with other Two BN recentnanoparticles review articles[24,25]. briefly In this discussed article, we 0D willBN focus materials on reviewing along with the otherrecent BN development nanoparticles of BNQDs, [24,25]. Infrom this the article, synthesis, we will character- focus on ization,reviewing and various the recent applications. development of BNQDs, from the synthesis, characterization, and various applications. Figure 1. The number of papers published with title or topic containing boron nitride quantum dots Figure(BNQDs) 1. The based number on Web of papers of Science publishedTM analysis with title with or the topic keyword containing “boron boron nitride nitride quantum quantum dots”. dots (BNQDs) based on Web of Science TM analysis with the keyword “boron nitride quantum dots”.2. Crystal Structures BN is the lightest III-IV compound and is isoelectric to carbon structures by sharing 2.the Crystal same Structure total numbers of electrons between the neighboring atoms [26]. As shown in FigureBN2 isa, the BN lightest has different III-IV polymorphs,compound and including is isoelectric hexagonal to carbon BN (h-BN), structures cubic by BN sharing (c-BN), therhombohedral same total number BN (r-BN), of electrons and wurtzite between BN the (w-BN) neighboring [27–31]. atoms The h-BN [26]. andAs shown r-BN have in sp2 Figurehybridized 2a, BN B-N has bondsdifferent while polymorphs, the c-BN andincluding w-BN hexagonal are sp3 hybridized. BN (h-BN), The cubic h-BN BN phase (c- BN),is the rhombohedral most stable BN phase (r-BN), under and standard wurtzite conditions,BN (w-BN) consists[27–31]. The of strong h-BN and boron–nitrogen r-BN have spcovalent2 hybridized intralayer B-N bonds bonds while making the c hexagonal-BN and w planar-BN are geometry sp3 hybridized. and weak The van h-BN der phase Waals is interactionthe most stable between phase layers under in thestandard bulk state conditions, as shown consists in Figure of2 strongb. There boron is also–nitrogen electrostatic co- valentinteraction intralayer due bonds to the electronegativitymaking hexagonal difference planar geometry between and the boronweak andvan nitrogender Waals atoms in- teractionbetween between adjacent layers layers in [ 32the] inbulk the state interlayer as shown stacking. in Figure Unlike 2b. electronThere is distributionalso electrostatic in the interactiongraphene due C-C to bond, the electronegativity the electrons in thedifference B-N bond between move the towards boronthe and electronegative nitrogen atoms N betweenatoms vacating adjacent the layers boron [32] atoms in the and interlayer provide s atacking. partial Unlike ionic character electron through distribution polarization in the grapheneof the B-N C-C bond. bond, Thethe electrons basal planes in the in B h-BN-N bond are stackedmove towards in an alternatingthe electronegative AA’AA’ N... atomssequence vacating which the is boron different atoms when and compared provide a withpartial graphite ionic character in which through hexagonal polarization nanosheets ofare the stacked B-N bond. in the The ABAB basal sequence. planes in Theh-BN w-BN are stack has aed diamond-like in an alternating tetrahedral AA’AA structure’… se- quencewith similar which AA’AA’is different staking when to compared that of h-BN. with The graph r-BNitee has in which a three-fold hexagonal stacking nanosheets sequence areABCABC stacked ...in theasshown ABAB insequence Figure2.a. The Similarly, w-BN has the stackinga diamond sequence-like tetrahedral for c-BN isstructure like that withof r-BNsimilar but AA’AA’ has a tetrahedral staking to that structure. of h-BN. In The this r- review,BN has wea three will-fold discuss stacking only sequence the stable ABCABC…h-BN materials. as shown in Figure 2a. Similarly, the stacking sequence for c-BN is like that of r-BN but has a tetrahedral structure. In this review, we will discuss only the stable h-BN materials. CC2021 2021, ,7 7,, 35 x FOR PEER REVIEW 33 of of 26 26 FigureFigure 2.2. ((aa)) Crystal Crystal structure structuress of of h- h-BN,BN, w w-BN,-BN, r-BN, r-BN, and and c-BN. c-BN. Boron Boron atoms atoms are areshown shown in red in, red, whilewhile nitrogennitrogen atoms atoms are are shown shown in in blue blue [28 [28]]. Reproduced. Reproduced with with permission. permission. 2007 2007 IOP IOP Publishing Publishing Ltd. (Ltd.b) Structural (b) Structural basis basis for two-dimensional for two-dimensional h-BN h nanostructures,-BN nanostructures, where where B-N bondsB-N bonds have have the length the of 0.145length nm, of the0.145 distance nm, the between distance two between adjacent two borazine adjacent rings borazine is 0.250 rings nm, is and 0.250 the nm, interlayer and the distance inter- is 0.333layer nm.distance The edge is 0.333 of the nm. monolayer The edgeBN of the nanosheet monolayer can beBN either nanosheet zigzag can or armchair be either [13zigzag].