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Tomohiro Shiraki, Rioe Saito, Hayato Saeki, Naoki Tanaka, [Koji Harano](https://orcid.org/0000-0001-6800-8023), Tsuyohiko Fujigaya

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[Defect Photoluminescence from Alkylated Boron Nitride Nanotubes](https://mdr.nims.go.jp/datasets/00503a82-8f9d-46f2-af80-1d38ced09955)

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Defect Photoluminescence from Alkylated Boron Nitride NanotubesTomohiro Shiraki,*1,2 Rioe Saito,1 Hayato Saeki,1 Naoki Tanaka,1Koji Harano,3 and Tsuyohiko Fujigaya*1,2,41Department of Applied Chemistry, Graduate School of Engineering, Kyushu University,744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan2International Institute for Carbon-Neutral Energy Research (WPI-I2CNER),Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan3Research Center for Advanced Measurement and Characterization,National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan4Center for Molecular Systems (CMS), Kyushu University,744 Motooka, Nishi-ku, Fukuoka 819-0395, JapanE-mail: shiraki.tomohiro.992@m.kyushu-u.ac.jp (T. Shiraki), fujigaya.tsuyohiko.948@m.kyushu-u.ac.jp (T. Fujigaya)Boron nitride nanotubes (BNNTs) are chemically function-alized by a reductive alkylation reaction for defect doping tocreate luminescent defects. The hexyl group attachment on theBNNTwall results in sp3 boron atom defect formation in the BNnetwork, by which defect photoluminescence from the function-alized BNNTs is newly observed in a UV­vis region. Thischemistry-based defect doping technique offers an attractive toolfor bandgap engineering of BNNTs.Keywords: Boron nitride nanotubes | Defect |PhotoluminescenceBoron nitride nanotubes (BNNTs) are nanocylinders com-posed of rolled-up hexagonal boron nitride (h-BN) sheets, whichwere theoretically predicted in 19941,2 and experimentallysynthesized in 1995.3 The BN framework consists of alternatelyconnected boron (B) and nitrogen (N) atoms with sp2 hybrid-ization, by which a strong covalent bond with an ionic characteris formed because of their electronegativity difference. Accord-ingly, BNNTs exhibit unique properties such as high mechanicalstrength, thermal stability, and a wide bandgap semiconductingfeature.4 Thus, BNNTs have been utilized for various applica-tions including reinforced materials and thermally conductivecomposites, and also recently used as a key component for one-dimensional (1-D) van der Waals heterostructure fabrication.5,6Because of the large bandgap (µ6 eV) independent of theBNNT diameter and chirality, BNNTs show optical absorptionand emission based on the band edge transition in deep UVregions (around 200 nm) for pristine tubes.7 The observedoptical responses are reported to be strongly influenced byexcitonic effects.8,9 In typical BNNT samples, atomic vacanciesare formed in the BN framework structure and are reported toyield longer wavelength photoluminescence (PL) in UV­visregions.7 This phenomenon indicates a prospect for PL modula-tion of BNNTs based on defect engineering approaches. Forcarbon nanotubes (CNTs), which are structural analogues ofBNNTs, chemical functionalization for defect engineering hasbeen developed to produce near-infrared defect PL with longerwavelengths and enhanced quantum yields from the function-alized CNTs (locally functionalized single-walled CNTs: lf-SWCNTs).10­12 To synthesize the lf-SWCNTs, a small amountof chemical functionalization is applied to dope local sp3 carbondefects in the sp2 carbon network of CNTs for the formation ofluminescent defects. Chemical functionalization for BNNTs13has been conducted to modify their surface properties as analternative to a physical functionalization method.14 Thecovalently attached molecules on the BNNT surfaces allowedsolubilization of the mostly insoluble BNNTs in various solventsand enhancement of miscibility with matrices in compositefabrication.13 In this study (Figure 1), chemical functionalizationof BNNTs is newly utilized for defect doping to modulate theiroptical properties; a reductive alkylation reaction producesalkylated BNNTs that emit defect PL based on sp3 boron atomdefect doping for the luminescent defect formation.BNNTs were purified by heating at 800 °C in air for 2 h,followed by washing with hot water, by which impurities suchas amorphous boron are removed.15 Scanning electron micro-scope (SEM) images of purified and unpurified BNNTs areshown in Figures 2a and S1, respectively. For the unpurifiedBNNTs, one-dimensional (1-D) tube-like structures were ob-served together with spherical particles (a few hundred nm indiameter). In contrast, for the purified BNNTs, only the 1-Dstructures were confirmed, indicating removal of the impurityparticles. Transmission electron microscopy (TEM) showed thatthe 1-D structures were multiwalled tubes (the common layernumber is double-wall) with a diameter of 2­6 nm, as shown inFigures 2b and S2. The purified BNNTs were used for chemicalfunctionalization experiments.Reductive alkylation of BNNTs was conducted based on areported procedure.16 Specifically, BNNTs were reduced by theFigure 1. Synthetic scheme for the h-BNNT synthesis. N andB atoms are depicted in blue and green colors, respectively.T. Shiraki R. SaitoK. Harano T. FujigayaCL-220467 Received: October 27, 2022 | Accepted: November 18, 2022 | Web Released: November 25, 202244 | Chem. Lett. 2023, 52, 44–47 | doi:10.1246/cl.220467 © 2023 The Chemical Society of Japanhttps://doi.org/10.1246/cl.220467addition of naphthalene and sodium metal in dry tetrahydro-furan, then mixed with 1-bromohexane. After quenching thereaction, the product was collected by filtration and washed withsolvents, providing hexylated BNNTs (h-BNNTs). In the Fouriertransform infrared (FT-IR) spectra of h-BNNTs and BNNTs(Figure S3), sharp peaks were observed at µ790 and µ1362cm¹1, attributed to the out-of-plane B­N­B bending and the in-plane B­N stretching modes, respectively, in the BN frame-work.16,17 The latter signal of h-BNNTs was slightly shiftedfrom that of BNNTs (1371 cm¹1), indicating partial distortionof the B­N framework by sp3 boron atom formation.16 Theh-BNNTs showed a distinct peak around 1090 cm¹1, attributedto the vibration modes based on B­C bond formation, togetherwith peaks at 2852, 2924, and 2959 cm¹1, attributed to the C­Hstretching modes of CH2 and CH3 groups.16,17 Because thosesignals are not observed for BNNTs, the results indicate thealkylation of BNNTs by this reaction.The X-ray photoelectron spectroscopy (XPS) spectra forB 1s, N 1s, and C 1s orbitals of h-BNNTs and BNNTs areshown in Figure 3, in which peak deconvolution was conductedusing GL (mixture of Gauss and Lorentz) functions. In the XPSsurvey spectra (Figure S4) and the atomic concentration analysis(Table S1), a small carbon signal for BNNTs was detected,indicating the existence of some carbon impurities. In contrast,h-BNNTs showed a distinct C 1s peak in the spectrum and alarge increase in the C content, indicating hexyl group modifica-tion. Both h-BNNTs and BNNTs showed obvious peaks at 190and 398 eV in the B 1s and N 1s, respectively, assigned to B­Nbonding in the BN framework.17 Small signals relating to someoxidized species were detected. For h-BNNTs, new peaksappeared at 189 and 284 eV in the B 1s and C 1s, respectively,assigned to B­C bonding. This result shows covalent bondformation between the boron atom in the BNNT and the hexylgroup through the reaction based on electrons (from the sodiumreductant) localized on the empty p orbitals of boron atoms.17Therefore, the reductive alkylation of BNNTs was confirmedthrough the B­C bond formation, which induced partial hybrid-ization conversion of boron atoms from sp2 to sp3 as a role ofdefect doping to the crystalline BN framework. A TEM imageof multiple h-BNNTs shows that the BNNT surfaces arecovered with amorphous organic residues (Figure 4a), suggest-ing that the BNNT surfaces are modified by the hexyl groups.In the TEM observation, we found a sparsely modified regionwhere a single hexyl group covalently bonded to the outersurface of the double-walled BNNTs was observed (Figure 4b),supported by comparison with the molecular model (Figure 4c)and the simulated image (Figure 4d). The TEM image alsodemonstrates that the alkylation reaction could mostly maintainthe tubular BN network structures in the sp3 boron atomformation process.Figure 2. (a) SEM and (b) TEM images of purified BNNTs.Scale bars: 1¯m and 5 nm, respectively.Figure 3. XPS spectra for (a) B 1s, (b) N 1s, and (c) C 1s orbitals of h-BNNTs (upper) and BNNTs (lower). Black and colored linesshow measured and deconvoluted spectra, respectively. Peak deconvolution was conducted using GL functions.Chem. Lett. 2023, 52, 44–47 | doi:10.1246/cl.220467 © 2023 The Chemical Society of Japan | 45https://doi.org/10.1246/cl.220467Thermogravimetric (TG) curves (Figure S5) showed that aweight loss occurred for h-BNNTs around 200­400 °C, althoughBNNTs exhibited negligible weight changes due to their greatthermal stability.4 Based on the observed weight loss forh-BNNTs, the amount of modified hexyl groups was estimatedto be 13.5wt%, corresponding to one hexyl chain per 22 B­Nunits in the BN framework.The spectroscopic analyses were conducted using BNNTand h-BNNT solutions, which were prepared using an aqueoussolution of cetyltrimethylammonium bromide (CTAB). In theUV­vis absorption measurements (Figure S6), BNNTs andh-BNNTs showed a clear peak at µ204 nm, attributed to abandgap transition of BNNTs.19 For h-BNNTs, the absorbanceincreased in the region from 208 to 450 nm, exhibiting partialelectronic structure variation by the alkylation reaction. In the PLspectra (Figure 5a), h-BNNTs and BNNTs exhibited PL peaks at228 nm, assignable to the near bandgap excitonic states and/orquasi donor­acceptor pair emission and free bound transitions,and 282 nm, assignable to the atomic vacancies or impurities(carbon or oxygen).7,20 Characteristically, h-BNNTs emitted newPL at 334 and 413 nm. In an excitation spectrum for 413 nmPL of h-BNNTs (Figure 5b), not only the bandgap transition(207 nm) but also lower energy absorption bands (233 and275 nm) contributed to the 413 nm PL, which were similarlyobserved in the 334 nm PL (Figure S7). These results suggest thatthe alkylation creates some energy levels in the intrinsically widebandgap of BNNTs (Figure S8), and photogenerated excitonswould migrate in the 1-D tubes and be partially trapped at thedefects with the lower energy levels for the defect PL emission,resembling phenomena observed in lf-SWCNTs.10­12When the amounts of chemical reagents were increased byfive times for the BNNT alkylation, the amount of the attachedalkyl group increased based on a TG result showing 19.5wt%loss by the alkyl group degradation. However, the defect PLintensity at 413 nm decreased (Figure S9), possibly indicatingthat the chemical reaction condition could be an importantcontrol factor to create luminescent defects in the BN framework.To date, chemical reactions to change the electronicstructures of BNNTs have been rarely reported; Zhi et al. usedan amidation reaction using acyl chlorides for amino groupanchors possibly introduced in BNNT sidewalls and open endsthrough a synthetic process of chemical vapor deposition usingNH3 gas, and observed optical property changes explained by aproposed charge transfer-driven doping effect.19,21 In this study,we use the reductive alkylation for chemical modification ofthe inherent BN framework and, directly and partly convertthe orbital hybridization of the boron atoms from sp2 to sp3 as ameans of defect doping, which offers a new way to createluminescent defects in BNNTs.In conclusion, the reductive alkylation of BNNTs newlyproduced defect PL at 334 and 413 nm in h-BNNTs. Spectro-scopic measurements, thermal analysis, and TEM observationsconfirmed the alkyl group functionalization, in which the B­Cbond selectively formed to produce sp3 boron atom defects inthe BN framework for the luminescent defect formation.Consequently, the defect doping functionalization generatednew absorption and emission bands. This is the first report of aluminescent defect formation in BNNTs based on defect dopingby atomic orbital hybridization conversion using a chemicalfunctionalization technique. Chemical functionalization wouldenable the versatile design of molecular structures of the defect-doped sites in the functionalized BNNTs, which would offernovel band engineering techniques of BNNTs and produce awide variety of molecularly functionalized BNNTs for advancedoptical applications such as single-photon emitters in quantumtechnologies.We thank Dr. Koji Kimoto (National Institute for MaterialsScience) for assistance with the TEM observations. This studywas supported by Grants-in-Aid, the Groundbreaking ResearchProject from the Faculty of Engineering of Kyushu University,the JSPS KAKENHI (Grant Numbers JP22H01910 andJP21H01758), and the Nanotechnology Platform Project, fromthe Ministry of Education, Culture, Sports, Science andTechnology, Japan.Supporting Information is available on https://doi.org/10.1246/cl.220467.References1 A. Rubio, J. L. Corkill, M. L. Cohen, Phys. Rev. B 1994, 49,5081.Figure 4. TEM characterization of h-BNNTs. (a) TEM imageof h-BNNTs. Scale bar: 20 nm. (b) Atomic-resolution TEMimage of a single hexyl chain (indicated by an arrow) attached toa double-walled BNNT. Scale bar: 1 nm. (c) A model of a hexylgroup on a double-walled BNNT presented as an atomic-number-correlated molecular model.18 (d) A corresponding TEMsimulation image.Figure 5. 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