Gold-Nanoparticle-Decorated Boron Nitride Nanosheets: Structure and Optical Properties

Synergetic cooperation of individual components of the nanocomposites (NCs) is responsible for their novel properties that lead to various technological applications. A simple chemical process depicting the deposition of functionalized gold nanoparticles on the surface of boron nitride nanosheets (BNNSs) in solution is reported. The structure, chemical composition, and optical properties of nanosheets are systematically studied. The deposition of Au nanoparticles on BNNS (BNNSAu) results in plasmonic band modulation, thus altering the optoelectronic properties of BNNSs. The intense surface plasmon absorption band of BNNSAu is narrowed and red‐shifted relative to the absorption band of as synthesized monometallic BNNSs. The observations reflect the strong interfacial interaction between BNNS and Au nanoparticles. This approach constitutes a basis for a simple process leading to the preparation of functionalized BNNSs and their utilization as nanoscale templates for assembly and integration with other nanoscale materials for futuristic optoelectronic devices.

A facile process, depicting the assembly of functionalized gold (BNNSAu) nanoparticles at the surface of boron nitride nanosheets (BNNSs) by a simple chemical route resulting in plasmonic band modulation is reported. This approach constitutes a basis for the preparation of functionalized BNNSs and their utilization as nanoscale templates for assembly and integration with other nanoscale materials for optoelectronic devices.

nanocomposites; plasmonic bandgaps; boron nitride; noble metal nanoparticles

The underlying physics, chemistry, and applications of 2D materials such as graphite, h‐BN, and dichalcogenides are quite rich.[1] , [2] , [3] , [4] Various methods have been implemented for engineering the physical properties of these planar nanostructures.[5] , [6] , [7] , [8] A key feature that has enabled many of the developments concerning most explored 2D material, e.g., carbon nanotubes (CNTs), graphene, and boron nitride nanosheets (BNNS), is the ability for their surface to be chemically modified or functionalized.[9] The surface modification with chemical moieties of interest generates multifucntionality and capability to form various nanocomposite and assemblies.[10] , [11] This also provides avenues for manipulation of physico‐chemical properties for desired applications.[8] Their counterpart the boron nitride nanotubes (BNNTs) are not only structurally similar to carbon nanotubes (CNTs),[12] but their mechanical properties are also theoretically and experimentally found to be comparable to those of the CNTs.[13] , [14] However compared to carbon nanostructures, the BN nanosystems are electrically insulating (a bandgap of ≈5–6 eV)[15] , [16] , [17] and have profound chemical and thermal stabilities.[18] , [19]

The lack of studies on these systems may be primarily attributed to the fact that the synthesis and subsequent functionalization of BN‐based nanostructures is a much more challenging task compared to well‐established synthetic routes of carbon nanotubes and graphene formation. High level resistance to oxidation of BNNTs in comparison to CNTs also justifies their candidacy for the promising FE applications,[8] which unfortunately suffers with low field‐emission current density.[18] It has been theoretically predicted that the electronic properties in BNNTs can be tuned by means of mechanical deformation or chemical alteration.[6] , [7] It has been shown that hydrogenation of h‐BN sheets leads to reduction in the bandgap.[20] Bhattacharya et al. have theoretically proven that surface functionalization of single layer h‐BN sheet by various groups, such as H, F, OH, CH3, CHO, CN, and NH2, dictates the electronic bandgap.[21] Furthermore the surface modification of BNNTs doesl not only modify their electronic properties but also alter their dispersion in suitable solvent.[19] , [22] , [23] , [24] , [25] , [26] , [27]

Compared to the more common nanotubes form (BNNT), the thin unrolled film version (BNNS) can achieve higher sensitivity and efficiency because of the increased surface area which is beneficial to light absorption/emission or electron transfer and the generation of high densities of functional groups at their surface. This makes them potential candidates as nanoscale templates for integration with other nanoscale materials to form assemblies/composites for chemical/biochemical applications, electronic device and composite materials.

We present a facile chemical route based strategy to assemble and organize Au nanoparticles on BNNS using specific functional groups. The hybridized heterostructure composite of Au and BNNS exhibited tailored optical properties. The chemical modification of BNNS with Au nanoparticles follow the Lewis base interaction with hydrophilic chains of amine functionalized Au NPs forming complexes with the electron‐deficient boron atoms on h‐BN (hexagonal boron nitride). The complexation of a Lewis base with h‐BN also facilitates the further exfoliation of the layered structure of the bulk material, resulting in thinner planar nanosheets that are readily dispersible in organic solvents.

To the best of our knowledge this is the first report on binding Au functionalized nanoparticles with BNNS by chemical route with a detailed study of the structural, functional, and optical properties. The bandgap modulation has been observed and possible reasons are discussed based upon the energy band structure and plasmonic interactions. The schematic presented in Figure[NaN] depicts the synthesis route.

X‐ray diffraction (XRD) was used to investigate the crystal structure and phase purity of BNNS and Au‐BNNS. The hBN powder was ball milled for about 20 h using a high energy ball mill and ZrO2 balls at 500 rpm (details are provided in the Experimental Section) to obtain exfoliated BNNS. Figure[NaN] a shows the XRD pattern of hBN (curve 1) and as synthesized BNNS after ball milling (curve 2). The unmilled hBN powder shows a characteristic diffraction peaks corresponding to (100), (101), (102) planes for typical hBN structure.[28] After ball milling for 20 h all the diffraction peaks of hBN structure diminish, except the (002) plane (Figure [NaN] a, curve 2). The result indicates that the (002) basal plane of BN remains intact retaining the short rangec‐axis orientation of BN nanostructure.[29] The structural nature of BNNS was examined by Fourier transformation infrared spectroscopy (FTIR). The spectrum of as‐produced BNNSs (Figure [NaN] b, curve 1) exhibits two characteristic vibrational modes of BNNSs; namely, the in‐plane axial B‐N‐B vibration of the nanosheets at ≈1400 cm−1 and the out‐of‐plane B‐N vibration at around ≈800 cm−1. Ball‐milling is known to break down crystalline h‐BN creating disorders and create defect structures thus is responsible for slight shifts in the characteristic peaks[30] , [31] (Figure [NaN] b, curve 1).

The fact that the all nanosheets show a selective orientation as compared to the mixed orientation of h‐BN powder is also evident by the scanning electron microscopy (SEM) images shown in Figure[NaN] . The SEM images presented in Figure [NaN] a,b and c,d elucidate the morphology of pristine h‐BN and BNNSs at low and high magnification, respectively. It is evident from the images that the ball milling and ultrasonication is able to peel off BNNSs from the BN particles. The thickness of BN particles has been reduced along with the lateral sizes of the BNNSs compared to pure h‐BN. This reduction in size and separation of BNNS is attributed to mild shear forces generated by ball milling and sonication. Figure[NaN] shows the transmission electron microscopy (TEM) and atomic force microscopy (AFM) images of BNNS, which further highlight the thin morphology and orientation of the nanosheets. The BNNSs images under the transmission beam shows low contrast due to the thin structure, with few of the BNNSs being nearly transparent to the electron beam, as shown in Figure [NaN] a. It can be observed that despite the presence of the overlapping of sheets the mass thickness contrast is not very noticeable. The lateral sizes of most BNNSs were found to be less than few micrometers; however, some smaller sheets were also occasionally seen. Similar to graphene, BNNSs also exhibits folded edges, which allow us to directly measure the number of layers using high‐resolution TEM (HRTEM). The lattice image shown in Figure [NaN] b elucidates the crystalline nature of the films wherein the area consisting of a single sheet has been marked as (1) and the corresponding selected area electron diffraction (SAED) is depicted in inset on the left hand side. The crystallinity of the films could be evaluated from the SAED pattern clearly exhibiting the distinctive hexagonal structure. The pattern shows the well‐known reflections of the BNNS (0002) plane corresponding to 0.34 nm lattice distance. We can also mention that the BNNS films exhibits the arrangement with (0002) plane oriented perpendicular to the surface. The diffraction pattern shown as right side inset has been captured from area on the micrograph marked (2). Presence of double spots indicates the presence of more than one layer of BNNS. The middle inset of Figure [NaN] b shows a typical atomic HRTEM image of BNNS in which the resolved features (drawn using inverse fast Fourier transform (IFFT)) of the region marked (1) corresponds to a lattice distance of 0.22 nm corresponding to interplanar distance of (1100) plane. The structural buckling/entanglements of the microstructures also suggests the abundant presence of topological defects perhaps even including the less energetically favorable Stone–Wales defects.[30] Most of the measured sample consisted of fewer than 5 layers (≈2 nm) and large particles were rarely seen in the samples; however, some of the films were composed of only one to two layers. The thinnest BNNSs observed consisting of two layers is illustrated in Figure [NaN] c. The average thickness of a BNNS exhibit centrifugation speed dependent behavior. At 5000 rpm, most of the sheets have less than 20 layers, i.e., the thickness is less than 17 nm, whereas at 8000 rpm the thickness is reduced to 5 nm (less than 6 layers). The microscopy images shown in Figure [NaN] a–c also indicate that the film is quite uniform and continuous except for some wrinkles, which may be introduced due to the high speed spinning of the thin films. As compared to graphene, which shows excessive wrinkling on formation of films by spin coating, the absence of π bonds in BN and the higher percentage of covalency make it less flexible thus preventing the formation of wrinkles. The thickness of BNNS and the related height profile estimated from the AFM image is shown in Figure [NaN] d. On the basis of AFM measurements on large numbers of BNNSs, it was found that nearly all the BNNS are thin sheets with thicknesses less than 2 nm. Inset of Figure [NaN] d shows the line‐scan profile of a single sheet, indicating the as‐grown film with thickness of about 1.1 nm.

Both HRTEM and AFM images are a clear evidence of the presence of monolayered BN nanosheets. In the former, the lightest‐contrasted structure extruding from a larger (thicker) structure seems to be of a single layer. We would like to add that due to the poor contrast, the identification and imaging of free‐standing monolayered species was quite challenging. The speed of centrifugation during the extraction process seemed to affect the thicknesses, i.e., the sheet dispersion, and the lateral sizes of the fractionated h‐BN nanosheets.[32]

The binding of BNNS with gold nanoparticles was examined by Fourier transform infrared (FTIR) spectroscopy. The spectrum of the BNNSAu (Figure [NaN] b, curve 2) exhibits a spectrum similar to that of the as‐produced BNNSs, shown in curve 1; however, the heterogeneous environment of the surface‐bound functional groups coupled with the large number of sheets dominate the IR spectra, making identification of all the present surface‐bound groups nontrivial. The shift from axial (1400 cm−1) and tangential (800 cm−1) B‐N vibrations is believed to be a consequence of the increased atomic disorder following the interaction of the BNNSs. The boron atoms at defect sites are likely to be more reactive toward amino groups on the surface of Au NPs than those on an unperturbed h‐BN plane as the defect boron atoms (especially those at vacancy defects or edge) are more polar (or more positively charged) and thus are more vulnerable toward Lewis base complexation than the facial boron atoms. After the interaction of the milled sheets with functionalized gold (Figure [NaN] b, curve 2) there is a prominent appearance of symmetrical and asymmetrical stretching of (methylene) C‐H group (due to the functionalization of Au nanoparticles) at around 2850 cm−1 indicating that the hydrocarbon chains are in a close‐packed, crystalline state.[33] Appearance of absorption at 3420 cm−1 is due to the N‐H stretching of the amine group of the functionalized molecule.[34] , [35] , [36]

The stability and the confirmation of formation of bond between the BNNSs and the functionalized nanoparticles synthesized was further emphasized using thermogravimetric analysis (TGA) by heating a known weight of BN nanosheets powder in air (at 10 °C/min) up to a temperature of 1000 °C where the weight loss and the onset temperatures were determined. The flow rate of air was kept constant at 100 mL/min. Because of the excellent thermal stability and oxidation resistivity of h‐BN, the organic groups could be selectively removed thermally by heating the solid samples to above their evaporation/decomposition temperatures in either an inert or oxidative atmosphere. Figure[NaN] a shows the TGA of both pure and functionalized BN nanosheets synthesized. From these graphs it is seen that the onset temperature for the oxidation of pure BN nanosheets is ≈625 °C. The weight loss of pure BN nanosheets up to 600 °C is only about 3.5%. This weight loss is mainly due to moisture present in the nanosheets. However, in the case of BNNS NCs the weight loss up to 500 °C was about 3.2%, and the onset temperature for oxidation of nanosheets was <600 °C. The weight loss was mainly due to the elimination of some functional groups present on the surface of BN nanosheets. The BN nanosheets synthesized were found to be stable around till 600 °C with and thereafter we notice the evidence of oxidation on the surface.[37]

Figure [NaN] b shows the typical Raman spectrum at room temperature of BNNS with a dominant peak at 1365.5 cm−1,which is attributed to the E2g mode, the so‐called counter phase of BN vibrational mode within the BN basal plane. This peak is similar to that of bulk hBN structure in the range 1366–1370 cm−1[38] –[40] but shift towards the lower wave number, perhaps due to the curvature‐induced E2g mode softening.[39] The full width at half maximum of the peak is 18.1 cm−1, which is broader than that observed in single crystalline bulk h‐BN (8 cm−1)[41] and that observed in pure BN nanotubes (13 cm−1).[42]

Figure[NaN] a–c shows the TEM images of Au decorated BNNSs with different concentrations of Au nanoparticles. The BNNS: Au ratio by weight was varied from 1:1 to 1:20. In case of the lower concentration of Au, the nanoparticles (up to BNNS:Au = 1:2) are separated from each other as shown in Figure [NaN] a but as the concentration of Au goes on increasing the adjacent Au nanoparticles on BNNSs start to connect and form islands. Figure [NaN] d–f shows HRTEM images of the Au decorated BNNS. It can be clearly noticed in Figure [NaN] d that the BNNSAu shows the presence of BN lattice along with the atomic images of Au nanoparticles. Few portions depicting the lattice are marked in the figure and the corresponding IFFT images are also shown as insets (i) and (ii). We can also see the image contrast in the Au nanoparticle marked as “Au” with half of the particle under one of the sheet. It is evident from Figure [NaN] e that almost all of the Au nanoparticles have a twin structure with interplanar spacing of 0.24 nm corresponding to (111) plane. The multilayer structure of BNNS (showing with four layers) is marked with white arrow. Figure [NaN] f shows a single Au nanoparticle which appears to be beneath a thin BNNS as the BN lattice is clearly visible throughout the cross section. The thin single BNNS is also allowing the lattice of Au nanoparticle to be clearly seen at the edges. The distortion at the periphery of the Au nanoparticle might be due to the functionalization of the Au nanoparticle.

With increase in concentration, the dimensions of Au islands expand and many of them get interlinked. As per the conventional percolation theory, the conductance of noble metal NP‐decorated BNNSs is expected to deviate from that of pure BNNSs. Figure S1 (Supporting Information) shows the conductance of Au‐decorated BNNTs with respect to the concentration of Au added. The relationship between the conductance and the concentration of nanoparticles can be fitted perfectly with an exponential function for lower concentration of nanoparticles. However for concentration of more than (1:5) for BNNS: Au, the conductance linearly increases with the concentration of nanogold. The linear relationship suggests ohmic contact between the nanosheet and electrodes. This perfectly relates to the TEM analyses, demonstrating that with higher concentration of gold there is higher coverage of NP on the surface of BNNSs.

The luminescence properties of both BN bulk crystals and BN polycrystalline films have been investigated with various methods, such as cathodoluminescence (CL), photoluminescence (PL), thermoluminescence, electroluminescence, and ionoluminescence.[43] We have studied the optical properties of BNNS and BNNSAu via Raman spectroscopy, UV‐vis absorption, and photoluminescence. The result reveals strong emission in the ultraviolet range, indicating that BNNS and BNNSAu are highly promising for applications in optoelectronic devices.

The UV‐vis absorption properties reflect the electronic state of the materials and are widely used to analyze the optical properties. The analysis of the available literature shows that the measured values of the BN bandgap energy varies in the range from 3.6 to 7.1 eV for BN samples with different structures. The variation in values can also be related to synthesis condition, purity of sample, and the variation of their microstructures. It is believed that structural changes in morphology and size may result in changes in the electronic state due to the changes in the vibration modes, and resuling in changes in the bandgap energy.

Figure[NaN] a depicts the UV‐vis optical absorption spectra of BNNSs and the BNNSAu. Curve 1, Figure [NaN] a, represents the absorption spectrun of BNNS (An enlarged spectrum is shown as smaller inset) exhibiting an absorption peak around 230 nm and indicate that these sheets are highly transparent in the wavelength range of 300–700 nm. The optical bandgap value can be estimated to be 5.6 eV compared to the theoretical value of 6 eV of single layer BN‐sheets.[42] The reduction in the band gap can be attributed to the multilayer structure of BNNS. UV–vis absorption spectrum of the as synthesized Au NPs shows a sharp characteristic surface plasmonic absorbance peak centered at 520 nm (Figure [NaN] a, curve 2) indicating the formation of spherical gold nanoparticles. Figure [NaN] a (curve 3) exhibits a prominent red shift in the absorption peak of the BNNSs to the wavelength of 310 nm on chemical modification with the Au NPs. On the other hand, a blue shift in the surface plasmon absorption of Au NPs was also observed in BNNSAu sample. The shifting of excitonic peak of BNNS at 230 nm to 310 nm for BNNSAu is attributed to the reduction in bandgap as a result of Au NP attachment. The reduction in the bandgap of BNNS could be explained in the framework of surface plasmon induced local fields at the interface of Au NPs and BNNS. The plasmonic nanostructures provide the development of resonant surface plasmons in response to a photon flux and localizing electromagnetic energy close to their interfacing surfaces. Theoretically, it is shown that application of transverse electric field to BN‐nanotubes results in gradual reduction in bandgap and is attributed to the iconicity of BN bonds.[44] Penetrating surface plasmonic fields into interfacing medium with spatial decay kinetics modifies the dielectric/optical properties of the medium.[45] The plasmonic field existing at the interface as the transverse electric field component is attributed to decrease in band gap of BNNSAu, which is in agreement with theoretical predictions.[46] Electronic rearrangement at metal semiconductor interface results in equalizing the Fermi levels and lowering the energy bandgap considering the work functions at interface (Figure [NaN] b). The magnitude of modified energy bandgap depends on the radially penetrating depth and propagation of transient SPP waves at interface. The absorption spectroscopy clearly indicates that optoelectronics properties of BNNS could be tuned by using plasmonics.[47]

Figure[NaN] a shows the photoluminescence excitation (PLE) spectrum recorded in the range 260 to 380 nm at an emission wavelength of 410 nm. It is observed that the PLE spectrum is broad and has the peak at ≈333 nm. The incorporation of Au nanoparticles to BNNS results in the increase in intensity of absorption peak at ≈333 nm by five orders of magnitude. However, a slight red shift of ≈7 nm is observed for the pristine BNNSs and BNNSAu. The deconvolution of the PLE spectrum of the BNNSAu has also been shown in the figure and it has been divided into six UV peaks, grouped in three sets, A, B, and C. The set A is composed of the excitation wavelength 267 nm, which is attributed to second excited states of BO− ion. The set B includes the excitation wavelengths 289, 301, and 313 nm, which are attributed to BO− ion's deep level transitions from the ground state to first excited state as reported previously.[44] in the case of h‐BN nanoplates. The set C is composed of the excitation wavelengths 345, 362, and 380 nm, which are attributed to B‐ or N‐vacancy‐type defect trapped states as reported previously.[46] , [47]

Figure [NaN] b shows the photoluminescence (PL) spectra of pristine BNNS and BNNSAu recorded in the range 350–700 nm and registered at the excitation wavelength of 333 nm. The PL spectrum shows a relatively narrow profile peaking at ≈410 nm with a full width at half‐maximum of ≈100 nm for BNNSAu with a significant increase in the emission intensity by five orders of magnitude as compared to BNNS. Figure [NaN] b also shows the deconvoluted PL spectrum, which comprises of three resolved peaks at 406, 430 and 459 nm. As the spacing among the three observed peaks is found to be regular, it has been ascribed that the PL peaks observed at 430 nm and 459 nm represent the phonon replicas of the band at 406 nm, with respective 1 and 2 emitted phonons. The ratio of area under the curve to the number of phonons is estimated to be ≈0.18 eV (1370 cm−1), which is in good agreement with the B‐NE2g vibrational mode phonon frequency of 1369 cm−1 of the BNNS measured by Raman spectroscopy as shown in Figure [NaN] b.

The transmittance of a tetrahydrofuran (THF) dispersion of spermine‐BNNS concentration of ≈0.1 mg/mL (data not shown) was over 85% at 400 nm and above. This confirmed the expectation that the h‐BN nanosheets are excellent low‐colored/transparent materials in the visible and near‐IR. These values are consistent with the literature data on bulk h‐BN with no functionalities,[48] , [49] suggesting that the functioanalization and the exfoliation had little effect on the electronic structure of h‐BN.

In summary, a facile method is reported for the modification of optoelectronic properties of BNNS by simple chemical route. It is worth mentioning that this route, which consists of Lewis acid‐base interactions, leads to the integration of the Au nanoparticles with BNNS for the formation of NC. A precise control of density of nanoparticles has been demonstrated, which in turn tunes the optoelectronic properties of BNNS. The reported approach strongly implicates the rich chemistry of these novel 2D nanomaterials, paving the way for future exploitation in their wet processing for a variety of electronics and nanocomposite‐related applications.

Chemicals: Hexagonal boron nitride (h‐BN) powder (1 μm particle size), di‐methylformamaide (DMF), chloroauric acid (HAuCl4),, sodium borohydride (NaBH4),, and octadecyamine (ODA, CH3(CH2)17NH2) were obtained from Sigma Aldrich and used as received.

Characterization: X‐ray diffraction (XRD) of the BNNS and BNNSAu was performed using a Rigaku Miniflex II desktop X‐ray diffractometer. IR studies were performed using a Nicolet 5700 FTIR spectrometer with pure KBr as the background. The nanoparticle powder was filtered, repeated washed and dried before mixing with KBr. The mixture was dried and compressed into a transparent tablet for measurement. HRTEM and SAED were performed using Tecnai G2 F30 S‐Twin field emission HRTEM. The preparation of samples for TEM analysis involved depositing a drop (20 L) of the relevant dispersion or suspension onto carbon‐coated 400 mesh copper grids. SEM was performed using Zeiss EVO MA10. The sample was drop cast on the silicon wafer and selective areas with good contrast were sampled to record the images. Oxford INCA energy 250 system attached to the Zeiss SEM was used to perform EDX. The micro‐Raman studies were carried out using an Ar ion laser of wavelength 514.5 nm, Model Innova 70, Coherent make, from USA. The signal was acquired using a single stage Jobin Yvon‐Spex monochromator through a flexible optical fiber coupling. The monochromator is coupled to a water cooled Hamamatsu, (R943 PMT) phototube for detecting the Raman signal. AFM imaging of the BNNS and BNNS‐Au nancomposite was conducted with Nanoscope IV multimode V instrument. PL excitation and emission spectra of unfunctionalized, amine functionalized, and noble metal nanoparticle integrated BNNS were recorded using an Edinburgh luminescence Spectrometer (Model F900) equipped with a xenon lamp. The excitation and emission spectra were recorded in the fluorescence mode over the range of 200–700 nm. Electrical measurements were performed using a Keithley 4200 (Semiconductor characterization system). The measurements were performed in linear sweep mode from –10 V to 10 V. Optical absorption measurement (UV‐vis spectroscopy) was performed using Hitachi U‐3900H. TGA studies were conducted using TGA DSC 1/1600/LF, Mettler Toledo, Switzerland.

Synthesis of BNNSs: BNNSs were synthesized using a number of methods, i.e., micromechanical cleavage of hexagonal boron nitride,[25] , [26] , [27] chemical exfoliation, and chemical vapor deposition of thin h‐BN films on a suitable substrate.[25] ,[40] Here, BNNSs were prepared using the modified chemical exfoliation of hBN powder with dimethylformamide (DMF) as the solvent. Prior to exfoliation the hBN powder was ball milled for about 20 h using a high energy ball mill and ZrO2 balls at 500 rpm. The balls to powder weight ratio was kept constant at 10:1 during high energy ball milling. It must be mentioned that it is rather easy to introduce defect sites in sp2 carbon allotropes via chemical oxidation,[33] [48] but h‐BN is inert towards such treatments. Therefore, in order to create defects on h‐BN, high energy ball milling was used to mechanically break down the hexagonal crystalline structures.

The ball milled h‐BN powder (0.5 g) was ultransonicated for 4 h in DMF and then centrifuged at 8000 rpm for 10 min. The supernatant solution containing thin BNNSs was collected. This extraction cycle was repeated 5–8 times on the residue from the centrifugation. The exfoliation and dispersion of BNNS was due to the combination of solvent polarity effect and the sonication assisted hydrolysis of h‐BN. The supernatants thus collected were used for the integration of Au and Ag nanoparticles (Figure [NaN] b, step 1,2).

Synthesis of Gold Nanoparticles: In a typical experiment, 100 mL of 10−4 M aqueous solution of chloroauric acid was reduced using 0.01 g of NaBH4 at room temperature, resulting in a ruby‐red solution indicating the formation of Au‐NPs.[50] The as‐synthesized NPS were immediately transferred to the organic phase using octadecylamine (ODA). In a typical experiment 10−3 M ODA in chloroform was taken and with solution of Au NPs and was subjected to vigorous stirring which resulted in transfer of nanoparticles from aqueous to organic phase as shown schematically in Figure [NaN] c. The purified AuNPs were washed with ethanol several times to remove uncoordinated ODA and were redispersed in organic phase for further experiments.

Attachment of Au with BNNSs: The as‐synthesized BNNS were interacted with functionalized Au and Ag nanoparticles using a simple solution route by ultrasonication of both the solutions for 10–12 h and keeping the solution overnight. The resultant solution was used for the formation of spin coated films of BNNS attached with nanoparticles (labelled BNNSAu). The driving force for the interaction is believed to be the Lewis acid base reaction between the BNNS and the anime group capping the nanoparticles. These interactions are effective during the binding of ODA to ball milled BNNS which contain the active defect sites (i.e., vacancy defects, topological defects, or exposed as also shown by Lin et al.[51] (Figure [NaN] b, step 3–5). These films were highly transparent with a shade of red in case of Au nanoparticles. The thickness of the BN nanosheets could be tuned by adjusting the synthesis and reaction parameters and these can be deposited on a large area. Schematic in Figure [NaN] shows the various steps involved in the reaction.

V.K. thanks the University Grant Commission, Govt. of India for the research fellowship. The authors greatly acknowledge Prof. R. C. Budhani, Director NPL for the kind support and valuable suggestions. They would also like to acknowledge Dr. Haranath's support for the luminescence studies. Sincere thanks are due to Mr. R. K. Seth for TGA analysis of BNNSs synthesized in the present work.

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Graph: Schematic not to scale: a) depiction of the incorporation of defects in BNNS, b) the synthesis route, and c) the steps involved in the synthesis of Au nanoparticles.

Graph: Spectroscopy studies: a) XRD pattern of hBN (curve 1) and as synthesized BNNS after milling (curve 2). b) FTIR spectra of ball‐milled BNNS (curve 1) and of BNNSAu (curve 2).

Graph: SEM images: a,b) pure h‐BN powder and c,d) BNNSs at low magnification and at high magnification.

Graph: TEM and AFM images: a–c) The representative images of BNNS. a) TEM image showing the overlapping thin BNNSs. b) HRTEM image depicting the lattice structure; the inset shows the SAED pattern captured from single and multilayer portion and the IFFT image. c) TEM image showing the presence of layers of BNNS. d) AFM image showing the surface morphology of one BNNS; the inset shows the line‐scan profile.

Graph: a) TGA of as synthesized and Au NP attached BN nanosheets. b) Raman spectrum of BN nanosheets.

Graph: Morphological analysis. TEM images of BNNSAu at different concentrations of BNNS and Au: a) 1:2, b) 1:6, and c) 1:10. d) HRTEM images showing the BN lattice (with outline); the inset shows the corresponding IFFT image and the Au (marked Au) nanoparticles on and under the sheet. e) Image showing the multilayer sheets and the Au nanoparticles with twin defects. f) HRTEM image elucidating the Au nanoparticle beneath the lattice resolved single BNNS.

Graph: a) UV‐vis optical absorption spectrum of BNNSs (curve 1), Au nanoparticles (curve 2), and the BNNSAu (curve 3). The inset shows the enlarged spectra for lower wavelength region for BNNS. b) Energy band diagram of BNNSAu.

Graph: a) PLE spectrum and b) PL spectra of BNNS and BNNSAu. Curve 1 shows the spectrum of pristine BNNS.

Graph: Supplementary