Stable Red Emission from Nanosheets of Molecularly Doped

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C: Physical Processes in Nanomaterials and Nanostructures

Stable Red Emission from Nanosheets of Molecularly Doped Hexagonal Boron Nitride Vikash Kumar, Niharika Joshi, Barun Dhara, Plawan Kumar Jha, Shammi Rana, Prasenjit Ghosh, and Nirmalya Ballav J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07660 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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The Journal of Physical Chemistry

Stable Red Emission from Nanosheets of Molecularly Doped Hexagonal Boron Nitride Vikash Kumar,§ Niharika Joshi,# Barun Dhara,§ Plawan Kumar Jha,§ Shammi Rana,§ Prasenjit Ghosh,# and Nirmalya Ballav§,* §

Department of Chemistry, Indian Institute of Science Education and Research (IISER), Dr.

Homi Bhabha Road, Pune – 411 008, India #

Department of Physics, Indian Institute of Science Education and Research (IISER), Dr. Homi

Bhabha Road, Pune – 411 008, India

Corresponding Author *E-mail: [email protected] Tel. no.: +91 20 2590 8215

ACS Paragon Plus Environment

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Achieving visible photo-luminescence (PL) from hexagonal boron nitride nanosheets (hBN-ns) is synthetically very much challenging due to its intrinsically high electronic band gap. We have explored the concept of molecular-doping in turning-on PL from hBN-ns

in

the

red-region

of

visible

spectrum.

Solution

of

organic

electrophile,

tetracyanoquinodimethane (TCNQ), was mixed with the dispersion of exfoliated hBN-ns and molecularly doped hBN nanosheets (md-hBN-ns) were isolated. Detailed mechanistic investigations evidenced the presence of dicyanotoluoylcyanide (DCTC) anion – an oxidative decay product of TCNQ – as the dopant species. Density functional theory (DFT) calculations revealed a selective and strong bonding scenario between the negatively charged DCTC moiety with the positively charged boron vacancy (VB) in hBN, giving rise to optically active electronic states within the hBN forbidden gap, which were assigned to be the primary source of such an unusual emission from chemically derived md-hBN-ns.

1. INTRODUCTION Upon exfoliation of three dimensional (3D) layered materials, two dimensional (2D) counterparts are obtained. Changing the dimension from 3D to 2D, chemical composition in the material remains same but a new set of physical properties emerge which are absent in bulk.1, 2 This observation is primarily attributed to the change in electronic band structure.3 The dimension-property relationship is well known since decades. However, in the domain of 2D materials it was only realized in 2004 when graphene, a single layer of graphite was discovered.4 Since then an enormous interest is being triggered in the field. This is because 2D materials owing to their superlative properties enables them a promising candidates for photonics, electronics, magnetic, photovoltaics, catalysis, and sensing applications.1, 2, 5-7 Some well-known


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2D materials that emerged after graphene are various transition metal dichalcogenides (TMDs), phosphorene, and hexagonal boron nitride (hBN) covering a wide range of the band gap from ~0 eV to ~6 eV.8 hBN is isoelectronic and isostructural with graphene.9 The presence of ionicity in hBN, due to electronegativity difference of B and N, localizes the electronic states resulting in a large band gap (~6 eV) with supplements of high thermal and chemical stability.6 hBN based materials are mainly used as lubricants, nanofillers and as dielectric gate layers in field effect transistors (FETs).2, 6 hBN also provides ideal platform to explore exotic hyperbolic phonon polaritons.8, 10, 11

Recently, a great deal of attention is paid to bring optically active defect states within the

forbidden gap giving rise to an interesting photoluminescence (PL) including the quantum emission phenomenon.7, 12-15 In most of the studies, origin of PL from hBN was assigned to nitrogen/boron vacancy (VN/VB) and/or anti-site nitrogen vacancy (NBVN) defects (color centers)7, 12 which were generated and activated in some ways, for example, by employing harsh physical conditions like UV irradiation, laser ablation, ion implantation and treatment at hightemperatures.7, 12, 16, 17 Elegant alternatives in this direction could be molecular-doping and/or functionalization with hetero-atoms which has been successfully used in the past to tune band gaps as well as physicochemical properties of various 2D materials.18-22 Typical examples include (i) tuning PL properties of molybdenum disulfide (MoS2)22 and tungsten disulfide (WS2)18 by moleculardoping with small organic molecule–tetracyanoquinodimethane (TCNQ); and (ii) modifying the electronic and magnetic properties of hBN upon functionalization with hetero-atoms (H, C, O and F).19, 21 Herein, for the first time, we have employed the concept of molecular-doping in nanosheets of hBN to turn-on PL in the red-region of visible spectrum.


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2. METHODS Chemicals. Hexagonal boron nitride (hBN) (1 μm, 98%), 7,7,8,8-Tetracyanoquinodimethane (TCNQ) were purchased from Sigma-Aldrich. Sodium nitrite (NaNO2) was purchased from TCI. IPA (iso-propyl alcohol), DMF (N,N-Dimethylformamide), Acetone, Water were used as solvent. Exfoliation of hBN. 450 mg of pristine hBN powder added in 150 ml of co-solvent IPA:Water (v:v = 1:1). The solution was sonicated (BRANSON 5510-MTH) with the frequency of 40 kHz for 4.5 hr. The resulting dispersion was centrifuged at 1000 rpm for 10 min, and then supernatant was collected followed by another centrifugation at 4000 rpm for 10 min to further remove nonexfoliated hBN. The obtained supernatant solution was labeled as hBN-ns and used for further experiments. Synthesis of md-hBN-ns (in situ). 10 ml of hBN-ns and TCNQ (1 mM) in 10 ml DMF were mixed and kept in a vial for 3 weeks and the pink color precipitate was collected. Synthesis of Na-DCTC. 5.2 mmol of Sodium nitrite (NaNO2) was dissolved in 16 ml water and added into warm solution (70⁰ C) of 200 ml Acetone containing 2.96 mmol of TCNQ. The resulting dark red solution was evaporated and Na-DCTC powder was obtained. Synthesis of md-hBN-ns (ex situ). 20 ml of hBN-ns and Na-DCTC solution (1 ml) were mixed and kept in a vial for 3 weeks and the fade orange precipitate was collected. Characterization. Raman spectra (λex = 488 nm) were recorded at Raman microscope (LabRAM HR, Horiba JobinYvon) with a 20X objective lens. XPS spectra were recorded by using ESCA0 (Omicron) equipped with mono chromator source (Al Kα= 1486.6 eV). The


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morphology were analyzed by employing Zeiss Ultra Plus FESEM. Solid state UV-vis absorption spectra were recorded on Shimadzu UV- 3600 UV-VIS-NIR spectrophotometer (mdhBN-ns were directly drop casted on quartz glass and spectra were collected). Liquid state UVvis absorption spectra were recorded on Chemito Spectroscan UV-vis 2600 spectrophotometer. The photoluminescence study was performed on a Fluorolog-3 spectrofluorometer (HORIBA Scientific). md-hBN-ns were directly drop casted on quartz glass and PL spectra were collected. Cyclic voltammetry was performed in a conventional three-electrode setup, glassy carbon, Pt, and Graphite rod were working, reference, and counter electrode respectively by using PARSTAT MC Potentiostat, PMC 2000. 0.1 M NaClO4 was used as supporting electrolyte for recording the CV spectra. Fourier transform infrared (FTIR) spectroscopy was carried out in a NICOLET 6700 FTIR spectrophotometer. KBr pellet was prepared and then md-hBN-ns was drop casted on the pellet and then spectra was collected. Confocal imaging done by Zeiss LSM 710 microscope having spectral detector (QUASAR). The laser power was 3.14 mW after objective and the power density was 2.86 mW/cm2. During measurement Laser power was used 20%.The filter was MBS 488/561 and the objective was Plan-Apochromat 10X/0.45.The Nanosurf AFM was used for height profile measurement. The imaging was done using continuous wave green laser (λex = 561±5 nm) and images were collected at the difference of wavelength (λ) ~10 nm, then these images were used for plotting the graph by selecting the region and calculating the corresponding mean intensity value. Fiji software was used for evaluating all Confocal PL images. Computational Details. We have performed ab initio spin polarized DFT calculations using plane-wave based Quantum ESPRESSO software package23 for understanding the optical transitions in the molecularly doped hBN nanosheets (md h-BN ns). For treating the electron ion


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interaction we have used ultrasoft pseudopotentials.24 The kinetic energy cutoffs for wave function and charge density used in our calculations are 35 Ry and 280 Ry, respectively. The electron-electron exchange-correlation potential is described by Perdew, Burke and Ernzerh of (PBE) parametrization which uses the generalized gradient approximation (GGA).25 The Brillouin zone integrations are performed with a 3×3×1 Monkhorst Pack shifted k-point grid for 8×8 supercell of h-BN monolayer.26 For speeding up the calculations we have used MarzariVanderbilt smearing of width 0.005 Ry.27 A vacuum separation of around 10 Å is used to minimize the spurious interaction between the periodic images of md-hBN-ns. In order to include van der Waal interactions, which otherwise is absent in the simple PBE-GGA formalism, between the molecule and h-BN monolayer we have used Grimme’s dispersion correction in all our calculations.28, 29 For the treatment of charged molecule (DCTC anion) on the h-BN monolayer we have used effective screening medium method with the following boundary condition for the electrostatic potential along the perpendicular direction to the surface (V(z))30: V(gǁ, z)|z=z1 = 0 ,

(1)

∂zV(gǁ,z)|z=-∞ = 0

(2)

Where, gǁis the absolute value of the wave vector in the Brillouin zone parallel to the surface and z1 is the boundary denoting interface between vacuum and the medium. For the above boundary condition the relative permittivity (ε(z)) is defined as, ε(z) = 1 if z≤z1 and ε(z) = ∞, if z≥z1.


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3. RESULTS AND DISCUSSION hBN layers were exfoliated upon sonication in a mixed solvent of isopropyl alcohol and water to produce nanosheets (hBN-ns) (Figure S1, Supporting Information).31 Subsequent addition of TCNQ solution in dimethylformamide (DMF) significantly changed the color (Figure 1) and gradually resulted in the formation of a pink colored precipitate. Finally, after filtration the solid was isolated and hereafter, the material is referred as molecularly doped hBN nanosheets (md-

Mixing

md-hBN-ns

+

TCNQ

hBN-ns).

hBN-ns

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic illustrating md-hBN-ns synthesis with chemical diagrams of TCNQ and hBN (black, blue and pink spheres represent C, N and B atoms).

The structure was investigated by powder X-ray diffraction (PXRD). As shown in Figure 2a, the main characteristic diffraction peak of hBN at 26.4º arising from (002) plane showed a significantly reduced intensity in hBN-ns indicating the presence of few layers of hBN with less


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- stacking in the c direction.32 Notably, structural integrity was observed to be retained in the md-hBN-ns system. Raman spectra were collected using 488 nm laser excitation and hBN showed a peak at 1364.9 cm-1 which is characteristic of B-N E2g vibrational mode (Figure 2b).33 A blue shift from 1364.9 to 1365.4 cm-1 for hBN-ns was noted and found to be consistent with earlier literature.34 Interestingly, upon reaction of hBN-ns with TCNQ, the Raman peak again red shifted to 1364.9 cm-1 which could be due to presence of more number of layers in the md-hBNns system compared to hBN-ns. Indeed, height profile measurements employing atomic force microscopy (AFM) revealed thickness of ~10 nm for the hBN-ns (Figure S2) while that of the md-hBN-ns system was ~20 nm (Figure S3).The increase in height is probably due to interaction between polar functional groups of molecular dopant and hBN. The chemical bonding environment of B and N in md-hBN-ns was studied by X-ray photoelectron spectroscopy (XPS). Appearance of B 1s and N 1s photoemission signals at ~190.5 eV and ~398.2 eV, respectively confirmed the presence of B-N bond characteristic of hBN (Figure 2c, d).35 Also, absence of photoemission signals at higher as well as lower binding energy values indicated that B-O, N-O, N-N, B-C and N-C bonds were almost absent in the md-hBN-ns material. The Field Emission Scanning Electron Microscope (FESEM) images of hBN-ns and md-hBN-ns showed very similar disc-like morphological patterns suggesting retention of the layered-structure after molecular doping (Figure 2e, f).The histogram of FESEM images of hBN-ns (Figure S4a) and md-hBN-ns (Figure S4b) constructed using their respective FESEM images is given in Figures S5 and S6 and the distribution showed heterogeneity in both the samples of hBN-ns and mdhBN-ns.


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c

e XPS B1s

Intensity (a.u.)

Intensity (a.u.)

a md-hBN-ns hBN-ns

Raw Fit

hBN

21

24

27

30

400 nm

33

194

2(degree)

192

190

188

186

B.E. (eV)

d

f

Intensity (a.u.)

b Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


md-hBN-ns

hBN-ns

XPS N1s Raw Fit

hBN

1350

1360

1370

1380

402

400

-1

398

396

394

400 nm

B.E. (eV)

Raman shift (cm )

Figure 2. (a) PXRD patterns of hBN, hBN-ns and md-hBN-ns. (b) Raman spectra of hBN, hBNns and md-hBN-ns. (c) B 1s XPS data recorded in md-hBN-ns (yellow line: fit). (d) N 1s XPS data recorded in md-hBN-ns (yellow line: fit). XPS data were fitted using the Gaussian function in Origin. The Quality of fit (R2) values are 0.997 for B 1s and 0.998 for N 1s. (e) FESEM image of hBN-ns. (f) FESEM image of md-hBN-ns.

A drastic change in color of hBN-ns (white) to that of md-hBN-ns (pink) is a prime indication of the presence of charge transfer (CT) type interaction in the material. The solid-state UV-vis spectrum (Figure 3a) of md-hBN-ns showed two new high-intensity bands at ~545 nm and ~660 nm and a broad band at ~850 nm which were neither present in TCNQ nor in hBN-ns. While the first two absorption peaks (relatively sharp) could be of molecular origin, the broad peak at higher wavelength (~850 nm) is a characteristic of a CT band.36 In fact, a wide range of


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absorption characteristics starting from UV-vis to Near-Infrared Region (NIR) were realized in the md-hBN-ns and motivated us to record emission spectra. hBN and TCNQ did not show any emission in the visible region, however, md-hBN-ns did show prominent emissions at ~616 nm (~2 eV) with a shoulder at ~672 nm (~1.85 eV) (Figure 3b). Optically, red emission from mdhBN-ns was also visualized under UV-lamp (Figure 3b).

Absorbance (a.u.)

a

1.0 hBN TCNQ md-hBN-ns

0.8 0.6 0.4 0.2 0.0 200

6

b Intensity (a.u.) x 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0

400

600 800 1000 Wavelength (nm)

ex = 545 nm

1200

md-hBN-ns Fit

1.5

 ex = 354 nm

1.0 0.5 0.0 600

hBN

TCNQ

650

700

750

800

Wavelength (nm)

Figure 3. (a) Solid-state UV-vis spectra of hBN, TCNQ and md-hBN-ns. (b) Emission spectra of hBN, TCNQ and md-hBN-ns with excitation wavelength of 545 nm (red dotted lines: fits).Graph of md-hBN-ns was fitted using Gaussian function in Origin and the R2 value was 0.997. Inset: optical image of md-hBN-ns under UV lamp (354 nm).


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We further employed confocal microscopy to record live imaging of PL. Green laser (λex= 561±5 nm) was used for the excitation purpose and the images were taken for all three systems hBN, TCNQ and md-hBN-ns (Figure 4a). The emission spectra (Figure 4b) was extracted using the PL images (Figure S7), collected at an interval of λ ~10 nm and the spectra was plotted using mean intensity value of selected area (10 different areas). The md-hBN-ns did show emission in the red region whereas no PL could be observed for the parent compounds. To further compliment the PL data on powder md-hBN-ns sample, drop casted thin film of md-hBN-ns was fabricated and PL imaging was performed under identical conditions (Figure S8a, b). The extracted spectrum presented in Figure S8c showed similar features likewise the spectrum in Figure 4b. Notably, upon extensive exposure of excitation laser (100 cycles of dose) to mdhBN-ns (Figure S9a), more than 80% retention (Figure S9b) of the fluorescence intensity collected at 632 nm was observed which is remarkable and as good as the standard organic fluorophore, Rhodamine B. The extracted emission profiles (Figure S9c) across various wavelengths also complimented the cycling stability plot. Recently, hBN has emerged as an excellent bio-compatible nanomaterial and such a stability of our md-hBN-ns system against photo-bleaching appears promising for various biological applications.37 At this point two important questions arise: What is the chemical species acting as molecular dopant? How does molecular-doping bring visible emission? To address the first question various complementary experiments were performed and the results are discussed below. Earlier, upon exposure to H2O/O2 an oxidative decay product of TCNQ namely α, α-dicyano-ptoluoylcyanide (DCTC) was observed.38-40 Recently, TCNQ on graphene surface was also gradually converted to DCTC upon aerial oxidation.41 Keeping this notion in mind, we


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separately prepared Na-DCTC salt (Figure S10) and investigated the supernatant solution (mother liquor giving md-hBN-ns) which was left over along with the precipitate.

a

hBN

TCNQ

md-hBN-ns (in situ)

md-hBN-ns (ex situ)

c

hBN TCNQ md-hBN-ns Fit

120 100 80 60 40 20 0 570

600

630

660

690

50

Mean Intensity (a.u.)

b Mean Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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720

hBN-DCTC (ex-situ) Fit

40 30 20 10 0 570

Wavelength (nm)

600

630

660

690

720

Wavelength (nm)

Figure 4. (a) Confocal laser scanning microscopy images of hBN, TCNQ, md-hBN-ns (in situ) and md-hBN-ns (ex situ), along with bright field images (black and white) and optical photographs of samples. Each scale bar represents a length of 223 μm. (b) Emission spectra of hBN, TCNQ and md-hBN-ns (in situ) plotted using mean intensity of each PL images corresponding to different wavelength. Graphs were fitted (red line) using the Gaussian function in Origin, with R2 value of 0.986. Error bars were indicated in the graph. (c) Emission spectrum of md-hBN-ns (ex situ) plotted using mean intensity of each PL images corresponding to different wavelength. Graphs were fitted (red line) using the Gaussian function in Origin, with R2 value of 0.842. Error bars were indicated in the graph.


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Since supernatant of the reaction was primarily comprised of ternary solvent (DMF+IPA+H2O) we dissolved TCNQ in same ternary solvent and analyzed it by cyclic voltammetry (CV). The redox peaks of TCNQ were clearly visible in CV spectra (Figure S11). Note that TCNQ can exist in three different redox active forms: neutral TCNQ, TCNQ monoanion and TCNQ dianion.42 However, after two days, the redox features of TCNQ disappeared and a flat plateau in the CV plot was observed, thereby depicting transformation of a redox species to non-redox species in the mentioned potential range. Liquid state UV-vis absorption spectrum of the supernatant of md-hBN-ns matched well with that of the solution of Na-DCTC – an intense absorption at 480 nm was consistently observed (Figure S12), thereby indicated the predominant existence of DCTC in the supernatant.38 Raman spectra of the supernatant and Na-DCTC also looked very similar, specifically, characteristic C=O of DCTC at 1642 cm-1 was consistently present (Figure S13).43 In our system, H2O played a crucial role in generating the md-hBN-ns material. TCNQ was dissolved in three different solvents: DMF, IPA and DMF+IPA – without addition of H2O (Figure S14). Interestingly, when hBN-ns dispersion in DMF was mixed with TCNQ in DMF (also TCNQ in IPA and TCNQ in DMF+IPA), the color of the respective supernatant (green) as well precipitate (white) did not change – unlike the isolation of md-hBN-ns in presence of H2O. In the Fourier transform infrared (FTIR) spectrum of md-hBN-ns, characteristic peaks of TCNQ were absent and new peaks at 1235 cm-1, 1279 cm-1 and 1104 cm-1 appeared corroborating the FTIR spectrum of Na-DCTC powder (Figure S15).43 Finally, we mixed NaDCTC solution with dispersion of hBN-ns and an orange colored precipitate was isolated, here named as md-hBN-ns (ex situ) (Figure S16). Confocal PL measurements on md-hBN-ns (ex situ)


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(Figure 4a, c) were observed to be very similar to that of md-hBN-ns (in situ) (Figure 4a, b), even, both the materials – md-hBN-ns (ex situ) and md-hBN-ns (in situ) – exhibited red color emission under UV-lamp (Figure S16 and Figure 3, respectively). The emission spectra (Figure 4c) was extracted using the PL images (Figure S17), collected at an interval of λ ~10 nm and the spectra was plotted using mean intensity value of selected area (10 different areas).Thus, based on the above mentioned results we have assigned the molecular dopant species as DCTC. To address the second question, on the origin of such unusual emission from md-hBN-ns, we have performed density functional theory (DFT) calculations. We have considered three plausible configurations: (1) DCTC anion with defect free hBN layer, (2) DCTC anion with hBN layer having nitrogen vacancy (VN), and (3) DCTC anion with hBN layer having boron vacancy (VB). The defect free hBN and DCTC anion were found to be ~3.24 Å (Figure S18a) apart, thereby exhibiting primarily van der Waals type interaction. The density of state (DOS) plot (Figure S18b) showed that the contributions in highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) mainly originate from DCTC; and no defect and/or new states were observed within the gap (Figure S18c). The interaction of DCTC with hBN (VN) was also observed to be van der Waals type and the distance between DCTC and hBN (VN) was found to be ~2.93 Å (Figure S19a) – an indication of relatively stronger interaction in comparison to the former configuration. Due to intrinsic nitrogen vacancy, various defect states were observed on the DOS plot of the hBN (VN)-DCTC configuration (Figure S19b). The probable spin-preserving transition (1.95 eV) (Figure S19c) closely matched with the experimental value of 2 eV corresponding to emission peak at 616 nm, however, the other state related to the emission at 672 nm was absent.


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c

Conduction band

1.8 eV

a

Valence band

b 40

20

hBN hBN-VB

10

DOS

DCTC Total

DOS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.96 eV

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0

0 -10 -20

-40 -4

-2

0

2 vac (eV)

4

-2

-1

vac (eV)

0

Figure 5. (a) Schematic of the energy optimized geometry (top and side views) revealing strong interaction of DCTC with hBN-VB. (b) Total DOS plot along with the contributions of the hBN, hBN-VB and DCTC components. A zoomed-in portion of the DOS plot is present on the right side. (c) Schematic of valence band and conduction band with possible transition states within the gap derived from the DOS plot.

Finally, we have analyzed the scenario of interaction of DCTC anion with the boron vacancy (VB) in hBN layer which turned out to be very interesting. DCTC anion strongly interacted with


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the boron vacancy (VB). DCTC was appeared to be covalently bonded to VB forming three C-N bonds and as a result the planarity of hBN was locally lifted by ~1.3 Å from the plane (Figure 5a). Such chemical activation of the VB generated new electronic states (both in spin up and spin down channels) in between the valence and conduction band edges of hBN (Figure 5b). Amongst these new states, though, the highest occupied state (HOMO) and lowest unoccupied one (LUMO) are localized primarily on DCTC, they are in different spin channels, thereby making optical transitions between them forbidden. However, there are two states in the spin down channel at 1.96 eV and 1.8 eV that results from the hybridization of the atomic states of N of hBN-VB and the molecular orbitals of DCTC. Upon excitation, electrons from these two states are probably transferred to the LUMO of the molecule giving rise to transitions at ~1.96 eV and ~1.8 eV within the gap (Figure 5c) which can be assigned to the observed emission peaks at ~616 nm ( 2 eV) and ~672 nm ( 1.85 eV), respectively (Figure 3b). 4. CONCLUSIONS In conclusion, we have successfully demonstrated room-temperature stable and red-emission from molecularly doped hBN-ns. Dopant species was realized to be a chemically converted TCNQ moiety namely DCTC. DFT calculations revealed strong electronic interaction between boron vacancy (VB) and DCTC anion in the md-hBN-ns system giving rise to new states at 1.96 eV, 1.8 eV within the gap corroborating the emission peaks at ~616 nm ( 2 eV) and ~672 nm ( 1.85 eV), respectively. Our approach of molecular-doping by solution phase reactions at ambient conditions appears robust and can be used to tune optoelectronic properties of other 2D materials for various practical applications.


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ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional data on experimental, AFM, FESEM, FTIR, CV, Raman, UV-vis, PL and DFT calculations (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel. no.: +91 20 2590 8215 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Financial support from SERB (India, Project No. EMR/2016/ 001404), MHRD-FAST (India, Project − CORESUM), DST Nano-Mission (India, Project No. SR/NM/NS- 1285/2014 and SR/NM/NS-15/2011) and IISER Pune is thankfully acknowledged. The authors thank CDAC Pune, India and Center for Computational Materials Science, Tohoku University, Japan for providing computational resource. V.K. thanks DST and Infosys foundation for Fellowships. N.J., P.K.J., and S.R. thank IISER Pune for providing Senior Research Fellowships. We also thank the Micro-Imaging Center at IISER Pune for the Confocal Microscopy facility.


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Stable Red Emission from Nanosheets of Molecularly Doped

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