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Photoluminescence upconversion by defects in hexagonal boron nitride Qixing Wang, Qi Zhang, Xiaoxu Zhao, Xin Luo, Calvin Pei Yu Wong, Junyong Wang, Dongyang Wan, Thirumalai Venkatesan, Stephen J. Pennycook, Kian Ping Loh, Goki Eda, and Andrew T. S. Wee Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02804 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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Photoluminescence upconversion by defects in hexagonal boron nitride Qixing Wang†,¶, Qi Zhang†,¶, Xiaoxu Zhao‡,§, ∥,¶, Xin Luo⊥, #, Calvin Pei Yu Wong†,∥,∇, Junyong Wang†, Dongyang Wan○, T. Venkatesan†, ∥,○,◆, ⬢ , Stephen J. Pennycook†, ∥,○,◆, Kian Ping Loh‡,§, Goki Eda†,‡,§, and Andrew T. S. Wee*†,§ †

Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore ‡

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore

§

Centre for Advanced 2D Materials, National University of Singapore, Block S14, 6 Science Drive 2, Singapore 117546, Singapore ∥

NUS Graduate School for Integrative Sciences and Engineering, National University of

Singapore, 13 Centre for Life Sciences, #05-01, 28 Medical Drive, Singapore 117456, Singapore ⊥

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun Yat-sen University, Guangzhou 510275, Guangdong, People’s Republic of China

#

Department of Applied Physics, the Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, People's Republic of China

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∇

Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology, and Research (A*STAR), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Singapore ○

NUSNNI-NanoCore, National University of Singapore, 117411, Singapore

◆

Department of Materials Science & Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore

⬢

Department of Electrical and Computer Engineering, National University of Singapore, 9 Engineering Drive 1, 117575, Singapore

¶

These authors contributed equally to this work

Corresponding author: Andrew T. S. Wee (Email: [email protected], Tel: +65 66013757)

ABSTRACT: Hexagonal boron nitride (h-BN) was recently reported to display single photon emission from ultraviolet to near-infrared range due to the existence of defects. Single photon emission has potential applications in quantum information processing and optoelectronics. These findings trigger increasing research interests in h-BN defects, such as revealing the nature of the defects. Here we report another intriguing defects property in h-BN, namely photoluminescence (PL) upconversion (anti-Stokes process). The energy gain by the PL upconversion is about 162 meV. The anomalous PL upconversion is attributed to the optical phonon absorption in the one-photon excitation process, based on excitation power, excitation wavelength, and temperature dependence investigations. Possible constitutions of the defects are discussed from the results of scanning transmission electron microscopy (STEM) studies and theoretical calculations. These findings show that defects in h-BN exhibit strong defects phonon 2 ACS Paragon Plus Environment

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coupling. The results from STEM and theoretical calculations are beneficial for understanding the constitution of the h-BN defects. KEYWORDS: hexagonal boron nitride (h-BN), defects, PL upconversion, optical phonon absorption

The emitted photon energy in a semiconductor is normally less than the excitation photon energy (Stokes process or downconversion). In some cases, however, photoluminescence (PL) occurs at energies higher than that of excitation photons (anti-Stokes process), and this is referred to as PL upconversion. This phenomenon has been widely observed in lead-halide perovskites1, 2, transition metal dichalcogenides (TMDCs)3-5, quantum dots6, dye molecules7 , and carbon nanotubes8. The underlying mechanisms of the PL upconversion are proposed to be the Auger excitation9, multiple-photon absorption10 or the thermal effect (phonon absorption)8,

11

,

sometimes in conjunction with intermediate states with energies resonant with or lower than those of the excitation photons8. Potential applications of PL upconversion process include semiconductor cooling2, 11, high efficient solar cells12, 13, bio-imaging14, 15, and so on. h-BN is an insulator with an electronic bandgap of around 6 eV16, 17. Owing to its atomically flat surface, lack of dangling bonds and charge traps, h-BN is frequently used as a substrate to improve the mobility of two-dimensional field effect transistors18, 19 and to enhance the optical properties, e.g. PL quantum yield, of TMDCs20, 21. The existence of defects in h-BN may create mid-gap states in the bandgap, some of which exhibit single photon emission22-24. To date, however, most of the optical investigations of h-BN defects focused on single photon emissions.


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Here we report the observation of PL upconversion with an energy gain of 162 meV from the defects in h-BN. Excitation power, excitation wavelength, and temperature dependence investigations reveal that the anomalous PL upconversion can be attributed to the optical phonon absorption in one-photon excitation process. Scanning transmission electron microscopy (STEM) studies and theoretical calculations are also performed to discuss possible constitutions of the hBN defects. The samples we studied were prepared by mechanical exfoliation of h-BN bulk crystals (HQ Graphene) onto Si/SiO2 (300 nm) substrates. The quality of the h-BN crystal was examined by X-ray Diffraction (XRD) (see Supporting Information, Figure S1). Optical microscope (OM) image of one of the h-BN samples used is presented in Figure 1a. Thickness of the h-BN is about 4.7 nm measured by atomic force microscope (AFM) (see Supporting Information, Figure S2). The Raman peak shift of the h-BN flake is 1366.4 cm-1, with a full width at half maximum (FWHM) of 8.5 cm-1 (Figure 1b), which is consistent with the results of earlier studies22, 25. Figure 1c shows room temperature PL spectrum of the defects in h-BN. The peak at 565 nm (2.195 eV) corresponds to the zero phonon line (ZPL), while the one at 610 nm (2.033 eV) corresponds to its phonon side band (PSB)24, 26, 27. The 162 meV energy difference between ZPL and PSB corresponds to the in-plane optical (LO/TO) phonon energy, which is in agreement with the experimentally26-28 and theoretically29 reported 165±10 meV values. We also observed defects that emitted at other wavelengths, such as 630 nm. The defects that emitted at 565 nm were most frequently observed, and their PL intensities were stronger than the PL intensity of the defects that emit at other wavelengths. Hence, we chose the defects that emitted at 565 nm, which were easier for identification and measurements.


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From the PL spectrum in Figure 1c, Debye-Waller (DW) factor can also be calculated. DW factor is a parameter to estimate the extent of defect-phonon coupling, which is defined as

I ZPL , I tot

where I ZPL is the integrated PL intensity of ZPL and I tot is the total integrated PL intensity30. The larger the DW factor, the weaker the defect-phonon coupling. Lorentz fittings were used to calculate the ZPL and PSB integrated intensities (see Supporting Information, Figure S3). Based on the PL spectrum in Figure 1c, the calculated DW factor is 0.53 (Huang-Rhys factor 0.63), which indicates a very strong defect-phonon coupling22. Figure 1d,e are Raman mapping image at 1366.4 cm-1 and PL mapping image of the ZPL in the white dashed region in Figure 1a, respectively. The ZPL PL mapping image suggests a high density of defects. Because of the high density, it is impossible to spatially resolve two neighboring defects using optical methods. The resolution limited by diffration is about 500 nm. Hence, all the measurements were conducted on an ensemble of defects. The room temperature PL upconversion spectrum of the defects in h-BN is shown in Figure 1f. The defects in h-BN were excited by 610 nm (2.033 eV) laser resulting in 565 nm (2.195 eV) emissions. The energy gain in this upconversion process is 162 meV, which is much larger than room temperature thermal energy kBT (26 meV). After normalization to the excitation laser power, the intensity ratio of the upconverted PL to the normal ZPL is about 0.031. In terms of quantum yield, we could only estimate a relative external quantum yield (EQE) between the upconversion and normal PL of h-BN defects since we don’t know the exact absorption value of the defect state. The estimated relative EQE between the upconversion and normal PL of h-BN defect is about 0.0056.


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To clarify the underlying origin of the observed PL upconversion, excitation power dependence of the upconversion was studied. As the excitation power increases, the intensity of the PL upconversion is intensified and there is no obvious change of the peak shapes in the spectra (Figure 2a). The upconversion peak intensity versus excitation power is plotted in Figure 2b. The red line is a linear fitting of the data. The linear power dependence of the PL upconversion intensity at low excitation power indicates that the upconversion is a one photon absorption process, rather than a multiple-photon absorption or Auger excitation process, where the upconversion intensity excitation power dependence are superlinear (square or even more)8, 12

. In addition to the excitation power dependence, PL upconversion is also strongly associated

with the excitation wavelength. Figure 2c compares the PL upconversion spectra at different excitation wavelengths with the same excitation power. As the excitation wavelength is increased from 610 nm to 622 nm, the upconversion spectrum intensity declines. PL upconversion is absent for excitation wavelengths longer than 622 nm. The energy gain of the upconversion is in a range from 162 meV to 201 meV, which is close to the optical phonon energy (165±10 meV) 26-29. Absorption spectroscopy is a useful tool to investigate the electronic structures of materials, but it is difficult to perform for the defects in h-BN because of their low absorption cross section. Photoluminescence excitation (PLE) spectroscopy is a substitute for the absorption spectroscopy due to its superior signal-to-noise ratio31. PLE is a specific type of PL and is applied to spectroscopic measurements where the excitation wavelength is varied, and the PL is monitored at the typical emission wavelength of the sample. Peaks in the PLE spectra usually represent absorption lines of the examined material. The intensities of the normal (downconversion) and 6 ACS Paragon Plus Environment

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the upconversion PL at different excitation wavelengths are presented in the PLE spectrum (Figure 2d). From Figure 2d, we can see that there is a peak at around 523 nm (2.371 eV), with an energy spacing of 176 meV from the ZPL peak center, which almost matches the optical phonon energy (162 meV) in this report. According to Franck–Condon principle32, the resonant peaks of the phonon assisted absorption and the phonon assisted emission are mirror reflections of one another with respect to the ZPL center for linear phonon mode. In our case, the PSB at 610 nm (2.033 eV) of the PL spectrum (blue line in Figure 2d) is associated with phonon assisted emission, which is about 162 meV below the ZPL peak center at 565 nm (2.195 eV). In comparison, the peak at around 523 nm (2.371 eV) of the PLE spectrum is about 176 meV above the ZPL peak center. The PSB of the PL spectrum and the peak of the PLE spectrum at around 523 nm are almost symmetric relative to the ZPL peak center with energy differences close to the optical phonon energy (165±10 meV)

26-29

. Hence, the peak at 523 nm of the PLE spectrum is

attributed to the phonon assisted resonant absorption line, which will be further explained in the remainder of this paper. Moreover, in the long wavelength side (from 604 to 620 nm) of the PLE spectrum, there is a resonant absorption at around 607 nm (2.043 eV) (Figure 2d inset). Since the energy gain of the PL upconversion excited at 607 nm is about 152 meV, which is close to the optical phonon energy (162 meV), we conjecture that the PL upconversion is a resonance process involving one photon and one optical phonon absorption. To further validate the assumption that the PL upconversion is a resonance process involving one photon and one optical phonon absorption, temperature dependence of the PL is evaluated. Figure 3a compares the PL spectra of the defects in h-BN at different temperatures. The PL intensity decreases when the temperature is increased from 80 to 280 K. The higher PL intensity 7 ACS Paragon Plus Environment

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at lower temperature comes from the reduction of the non-radiative transitions33, 34. In contrast, the PL upconversion intensity increases as the temperature increases (Figure 3b). The stronger PL upconversion intensity at higher temperature further corroborates that the PL upconversion is a thermally assisted effect (phonon-mediated process). It means that at higher temperatures a higher population of phonons for vibronic transitions are available for the phonon-mediated upconverison34. The intensity of the PL upconversion is dependent on the average phonon population, which obeys Bose-Einstein statistics ( I (T ) = I 0 /（ exp( Eact / k BT ) − 1）). Since the optical phonon energy (165 meV) in h-BN is much larger than room temperature thermal energy (26 meV), the upconversion PL intensities at different temperatures are fitted by the simplified Bose-Einstein distribution formula ( I PL ∝ exp(− Eact / kBT ) ) (Figure 3c). The activation energy Eact is calculated from the slope of the red line. The calculated activation energy Eact ( 166.8 ± 9.7 meV) closely matches the energy gain (162 meV) in the PL upconversion process at 610 nm excitation and the optical phonon energy (162 meV), further supporting our conjectures. Configuration coordinate diagrams are often utilized to depict transitions between electronic energy levels which are coupled to lattice vibrations. To visually summarize and explain the PL and PL upconversion process of the defects in h-BN, an energy diagram of the phonon-assisted electronic transitions along the configurational coordinate Q is plotted (Figure 3d). In the configuration coordinate diagram, the horizontal axis Q corresponds to optical (LO/TO) phonons vibrations (or lattice vibrations) in h-BN24, 35. E is the electronic ground state and E ∗ is the excited state. Q0 and Q0∗ represent equilibrium lattice configurations in the ground and first excited electronic states. Figure 3d describes electronic state energy variation with respect to the 8 ACS Paragon Plus Environment

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vibrational coordinate displacement (or phonon displacement) away from the equilibrium position. The inset is an illustration of the optical (LO/TO) phonon vibrations in h-BN at a frequency of 1366.4 cm-1 (169.4 meV) showing how B and N atoms move. The blue and copper balls represent N and B atoms respectively. The upward arrows represent optical absorption of the defect without phonons ( n = 0 ) (normal PL) and with one phonon ( n = 1) (phonon assisted PL upconversion) from the ground state ( E , n ) to the excited state ( E ∗ , n∗ ) . The excited state defect will encounter fast thermalization to the vibronic ground state ( E ∗ , 0 ) , which will be followed by radiative transition to the ground state ( E , n ) (downward arrows). The orange downward arrow represents transition of ZPL to the ground state ( E , 0 ) , while the red one represents transition of PSB to the ground state ( E ,1) . The gray upward arrow represents one phonon absorption assisted upconversion from ( E ,1) to ( E ∗ , 0 ) . In the rest of this paper, we will discuss possible constitutions of the defects that emits at 565 nm based on scanning transmission electron microscopy–annular dark field (STEM-ADF) studies and theoretical calculations. The h-BN samples used for STEM-ADF imaging studies were exfoliated monolayer h-BN with high defect densities (see Supporting Information, Figure S8). Figure 4a is a large scale STEM-ADF image of a monolayer h-BN with some boron (B) vacancy ( VB ). The bright and dim atoms represent nitrogen (N) and B atoms respectively36. The employed acceleration voltage for STEM-ADF imaging was set as 60 kV, which was significantly lower than the knock-on threshold (78 kV) of B and N in h-BN37. The corresponding fast Fourier transform (FFT) pattern (Figure 4a inset) reveals a single set of hexagonally arranged spots, which indicates the exfoliated monolayer h-BN is single crystal. VB


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is frequently observed in monolayer h-BN, which is highlighted by the green dashed circles in Figure 4a. An atomic resolution STEM-ADF image (Figure 4b) further confirms the presence of

VB (dim site), and the simulated image (Figure 4c) derived from the DFT optimized model greatly resembles the experimental image at all atomic sites. In all the STEM-ADF images scanned (total investigated area is ~ 12800 nm2), there are in total four types of atomic defects, i.e. VB , N vacancy ( VN ), vacancy with neighboring B and N ( VBN ), vacancy of one N and its surrounding three B ( VB 3 N ) (see Supporting Information, Figure S10-S13). Figure 4d presents the statistical densities of the VB ( 1.1×105 µ m−2 ), VN ( 4.7 ×103 µ m−2 ), VBN ( 3.1×103 µ m−2 ), and VB 3 N ( 2.2 ×104 µ m−2 ), respectively. There are a large number of theoretical models of possible defects in h-BN that have been proposed22, 35, 38-41. But here, we just focus on the defects found in our STEM-ADF investigations. To distinguish whether VB , VN , VBN , or VB 3 N was the emitter, DFT calculations were performed. The calculated electronic band structures and absorption spectra of VB , VN , VBN , and VB 3 N are shown in Figure S10-S13 (Supporting Information), which are in consistence with previous results27, 40, 42. The calculated absorption peaks of VB , VN , VBN , and VB 3 N are 1.88 eV, 1.937eV (and 1.162 eV), 4.9eV, and 3.21 eV, respectively. The resonant absorptions at 1.88 eV ( VB ) and 1.937 eV ( VN ) indicate that VB and VN are possible candidates that emit at 2.195 eV in this paper. Since the GGA type calculations used in our work usually underestimate bandgaps of insulators, the predicted optical properties of h-BN defects may not be reliable40, 42. It has been reported that the charge neutral VB and VN are not likely to emit at around 2 eV42, 43. A more


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accurate hybrid DFT functional calculation results from Feng et al42 showed that charged VB could be responsible for the emitters that emit at around 2 eV. Additional calculations are still needed to further verify our assumptions, such as using a computationally demanding GW/BSE method that takes excitonic effects into consideration, which are out of the scope of the current paper. The STEM-ADF and DFT calculation results here cannot be applied to other experimentally observed h-BN defects emissions because of varied emission wavelengths26, 39, 44, 45

. To determine whether the electronic transitions between the defect and band edge states are

symmetry allowed, group theory analysis of the defects was performed. If the ground and excited states symmetry products of the possible transitions contain the irreducible representation of the dipole, electronic transitions are allowed46. According to a previous report40, VB has C2v symmetry, while VB− and VN have D3h symmetry. For both C2v and D3h point groups, irreducible representations exist. Therefore, the electronic transitions of VB , VB− , and VN are symmetry allowed. The PL upconversion process strongly depends on the overlap of vibrational modes in the ground and excited electronic sates, which is also known as the Frank Condon factor. The Frank Condon factor of the h-BN defect was calculated based on the model of Jungwirth et al24. For linear optical phonon modes ω = ω * , the calculated Frank Condon factor along the Frank Condon shift is shown in Figure 4e. As can be seen from Figure 4e, there is a large overlap of optical phonon vibrational modes in the ground and excited electronic sates when optical phonons are involved. Therefore, optical phonon enabled PL upconversion is possible for h-BN defects. 11 ACS Paragon Plus Environment

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We have demonstrated PL upconversion by defects in h-BN with an energy gain as high as 162 meV. Excitation power, excitation wavelength (including PLE spectrum), and temperature dependence investigations were performed to reveal the origin of the PL upconversion. The linear excitation power dependent PL upconversion indicates that the upconversion is one photon excitation process. Excitation wavelength dependence and PLE spectrum studies suggest that optical phonon absorption is involved in the one photon excitation process. Temperature dependence study further confirms that the high energy gain PL upconversion comes from one photon and one optical phonon absorption. STEM-ADF image and DFT calculation investigation results are used to discuss possible constitutions of the defects. We believe our findings provide insight into novel optical and structural properties of defects in h-BN and will trigger further relative studies.


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Figure 1. Photoluminescence (PL) and PL upconversion of the defects in h-BN. (a) Optical microscope (OM) image of the exfoliated h-BN flakes on SiO2. The white dashed region is where PL and Raman mappings are conducted. (b) Room temperature Raman spectrum of the exfoliated h-BN excited with 532 nm laser. (c) Room temperature PL spectrum of the defects in h-BN. The peak at 565 nm corresponds to the zero phonon line (ZPL), while the one at 610 nm corresponds to the phonon side band (PSB). The excited wavelength is 532 nm. (d,e) Linear scale Raman mapping at the 1366.4 cm-1 and PL mapping of the ZPL in the white dashed region in (a). The units of the color bars are CCD counts/s. The scale bar is 5 µm. (f) PL upconversion spectrum of the defects in h-BN. The 610 nm laser excites defects in h-BN resulting in 565 nm emission. A 600 nm short pass filter is used in the collection path to filter the 610 nm excitation laser.


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Figure 2. Excitation power and excitation wavelength dependences of the PL upconversion. (a) PL spectra of the upconversion with excitation powers of 15 µW, 29 µW, 47 µW, 64 µW respectively. Each spectrum is vertically shifted for comparison. The excitation wavelength is 610 nm. (b) Power dependence of the PL upconversion intensity. The red line is a linear fitting. (c) PL spectra of the upconversion at different excitation wavelengths. Each spectrum is vertically shifted for comparison. The excitation powers are 70 µW for all the excitation wavelengths. (d) Photoluminescence excitation (PLE) spectrum of the integrated ZPL (565 nm) intensity with the PL spectrum (blue) excited with 532 nm laser for comparison. The green dots are excited with the wavelength scanning from 483 nm to 543 nm (downconversion or normal 14 ACS Paragon Plus Environment

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PL). The red squares are excited from 604 nm to 620 nm (upconversion). In the PLE spectrum measurements, the 550 nm long pass and 600 nm short pass filters are used in the collection path to collect the downconversion and upconversion signals, respectively. These two filters overlap in the gray region. Inset: An enlarged PLE spectrum in the region from 604 to 620 nm. The error bars are standard deviations of twice measurements.

Figure 3. Temperature dependences of the PL and the PL upconversion. (a) PL spectra of the defects at temperatures from 80 to 280 K. Each spectrum is vertically shifted for comparison. The excitation wavelength is 532 nm. (b) PL spectra of the upconversion from 240 to 350 K. Each spectrum is vertically shifted for comparison. The excitation 15 ACS Paragon Plus Environment

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wavelength is 610 nm. (c) Logarithmic upconversion PL intensity as a function of 1/T. The red line is a linear fitting. The excitation wavelength is 610 nm. The error bars are standard deviations of twice measurements. (d) Energy diagram of the phonon-assisted electronic transitions along the configurational coordinate Q. E is the electronic ground state and E ∗ is the excited state. The upward arrows represent optical absorption of the defect without phonons ( n = 0 ) (normal PL) and with one phonon ( n = 1) (phononassisted PL upconversion) from the ground state ( E , n ) to the excited state ( E ∗ , n∗ ) . The excited state defect will encounter fast thermalization to the vibronic ground state ( E ∗ , 0 ) , which will be followed by radiative transition to the ground state ( E , n ) (downward arrows). The orange downward arrow represents transition of ZPL to the ground state

( E , 0 ) , while the red one represents transition of PSB to the ground state ( E ,1) . The gray upward arrow represents one phonon absorption assisted upconversion from ( E ,1) to

( E , 0 ) . Inset is an illustration of the optical (LO/TO) phonon vibrations in h-BN with a ∗

frequency of 1366.4 cm-1 (169.4 meV). The blue and copper balls represent N and B atoms respectively.


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Figure 4. Scanning transmission electron microscopy–annular dark field (STEM-ADF) image and Frank Condon factors of the defects in h-BN. (a) A large scale STEM-ADF image of monolayer h-BN with B vacancies ( VB ). Inset: fast Fourier transform (FFT) of h-BN. (b) Enlarged STEM-ADF image of a single VB in monolayer h-BN. (c) Simulated STEM-ADF image of a single VB in monolayer h-BN derived from the DFT optimized model. (d) Statistical densities of the VB , N vacancy ( VN ), vacancy with neighboring B and N ( VBN ), and vacancy of one N and its surrounding three B ( VB 3 N ). (e) Calculated Frank Condon factors for linear optical phonon modes of the defects in h-BN.


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Methods. Sample preparation and Characterization. The samples used in this report are exfoliated few-layer h-BN from bulk crystals (HQ Graphene). After exfoliation, the samples are used for optical studies directly without any treatment, such as annealing. The crystallinity of the bulk h-BN was characterized with a Bruker D8 Advance TXS X-rays diffractometer (XRD) equipped with Cu Kα radiation (λ=1.54 Å) source (Supplementary information). First the bulk hBN is attached to a piece of blue sticky tape (Nitto-BT-150E-KL), and then another piece of tape is placed on it. The two tapes were separated and put together several times to obtain thinner layers. Finally, one piece of tape was attached on the 300 nm Silicon wafer. The samples were first characterized by an optical microscope (Nikon Eclipse LV100D), and their thicknesses were roughly estimated from the color contrast25. Their precise thicknesses were determined by AFM (BRUKER Dimension FastScan) in the tapping mode. Raman spectrum. PL and Raman mapping: These measurements were conducted using a

commercial WITec Alpha 300 R Raman system. The 532 nm continuous wave (CW) laser spot diameter is around 1 µm using 100 × objective lens. In the Raman spectrum measurement, laser power of 0.65 mW, integration time of 1 s and accumulation times of 2 were used. The spatial distance between every data point is 500 nm for both PL and Raman mapping measurements which is close to the spatial resolution of the equipment. The laser power for PL and Raman mapping are 0.17 mW and 0.65 mW respectively. The integration time for PL and Raman mapping is 0.5 s. Room temperature and low temperature PL spectroscopy. Micro-PL spectra were obtained

using a laser confocal microscope (NT-MDT, NTEGR Spectra) in back scattering geometry with


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532 nm (CW) excitation laser using 50 × objective lens. The sample was put in a vacuum chamber coupled with a liquid nitrogen (78K) container (Janis) for low-temperature PL measurement. PLE spectroscopy. Photoluminescence excitation (PLE) spectra were obtained by a super-

continuum light as the excitation source which is coupled to a monochromator in back scattering geometry with confocal microscope (NT-MDT, NTEGR Spectra). The spot diameter is around 4 µm using 50 × object lens. The PLE experiment was conducted in vacuum (~10-5 mbar) at room temperature. The excitation intensity was kept below 1 µW in order to avoid any non-linear effect. In the upconversion PL measurement, 610 nm (CW) pump laser was obtained from a super-continuum light after passing through the monochromator. A 600 nm short pass filter is used in the collection path to filter the 610 nm excitation laser. In the PLE spectrum measurements, the 550 nm long pass and 600 nm short pass filters are used in the collection path to collect the downconversion and upconversion signals, respectively. STEM-ADF imaging. Monolayer h-BN was exfoliated onto 90 nm SiO2 and then confirmed

from their optical contrasts using the published method25. Then the exfoliated monolayer h-BN was spin coated with PMMA and etched with KOH. After etching, the monolayer h-BN with PMMA was transferred onto the Mo grid for STEM-ADF analysis. STEM-ADF imaging and EELS analysis were conducted on an aberration-corrected JEOL ARM-200F equipped with a cold field emission gun, operating at 60 kV. DFT calculations. The density functional theory calculations were performed using the

generalized gradient approximation (GGA) with the exchange correlation functional in the flavor of PBE as implemented in the Vienna Ab Initio Simulation Package (VASP). The projector-


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augmented wave (PAW) pseudopotentials were used in the calculations with the spinpolarization switch on. A plane wave cutoff of 450 eV was used for the all calculations, which were found to be sufficient to achieve the well converged physical properties. The tolerance of 10-5 eV was set for the electronic self-consistent calculations and the lattice coordinates are fully relaxed until the maximum component of Hellmann-Feynman force acting on each ions is less than 0.01 eV/ Å. A variety of defect structures are simulated with the 6x6x1 supercells. The kpoint meshes of 33x33x1 and 3x3x1 were used for the pristine perfect h-BN unit cell and the supercell, respectively. A large vacuum thickness of 16 Å was set to prevent the spurious interactions between the periodic images.

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Supporting Information. Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *Email: [email protected].

Author Contributions ¶

These authors contributed equally to this work. Q.W. conceived and designed the experiment.

Q.Z. gave some good suggestions. Q.W. prepared the sample, and performed PL, Raman, and AFM characterizations. Q.W. and Q.Z. performed power dependence, PLE, and temperature dependence studies together. Q.W. and C.P.Y.W. carried out XRD study together. X.Z. assisted 20 ACS Paragon Plus Environment

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with STEM-ADF imaging study. X.L. conducted the DFT calculation. Q.W. analyzed the data and wrote the manuscript. All authors discussed the results and commented on the manuscript.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We acknowledge support by NUS research scholarship (WQX), A-STAR 2D Pharos grant (SERC 1527000012). X.L. acknowledges the Hong Kong Polytechnic University grant (No: GUABC) REFERENCES (1) Yang, J.; Wen, X.; Xia, H.; Sheng, R.; Ma, Q.; Kim, J.; Tapping, P.; Harada, T.; Kee, T. W.; Huang, F. Nat. Commun. 2017, 8, 14120. (2) Ha, S.-T.; Shen, C.; Zhang, J.; Xiong, Q. Nat. Photonics 2016, 10, 115-121. (3) Manca, M.; Glazov, M.; Robert, C.; Cadiz, F.; Taniguchi, T.; Watanabe, K.; Courtade, E.; Amand, T.; Renucci, P.; Marie, X. Nat. Commun. 2017, 8, 14927. (4) Bai, G.; Yuan, S.; Zhao, Y.; Yang, Z.; Choi, S. Y.; Chai, Y.; Yu, S. F.; Lau, S. P.; Hao, J. Adv. Mater. 2016, 28, 7472-7477. (5) Jones, A. M.; Yu, H.; Schaibley, J. R.; Yan, J.; Mandrus, D. G.; Taniguchi, T.; Watanabe, K.; Dery, H.; Yao, W.; Xu, X. Nat. Phys. 2016, 12, 323-327. (6) Deutsch, Z.; Neeman, L.; Oron, D. Nat. Nanotechnol. 2013, 8, 649-653. (7) Zou, W.; Visser, C.; Maduro, J. A.; Pshenichnikov, M. S.; Hummelen, J. C. Nat. Photonics 2012, 6, 560-564. (8) Akizuki, N.; Aota, S.; Mouri, S.; Matsuda, K.; Miyauchi, Y. Nat. Commun. 2014, 6, 8920-8920. (9) Seidel, W.; Titkov, A.; André, J.; Voisin, P.; Voos, M. Phys. Rev. Lett. 1994, 73, 2356. (10) Monteiro, T.; Neves, A.; Soares, M.; Carmo, M.; Peres, M.; Alves, E.; Rita, E. Appl. Phys. Lett. 2005, 87, 192108. (11) Zhang, J.; Li, D.; Chen, R.; Xiong, Q. Nature 2013, 493, 504-508. (12) Gan, Z.; Wu, X.; Zhou, G.; Shen, J.; Chu, P. K. Adv. Opt. Mater. 2013, 1, 554-558. (13) De Wild, J.; Meijerink, A.; Rath, J.; Van Sark, W.; Schropp, R. Energy & Environmental Science 2011, 4, 4835-4848. (14) Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Nat. Mater. 2011, 10, 968-973. (15) Zhou, J.; Liu, Z.; Li, F. Chemical Society Reviews 2012, 41, 1323-1349. (16) Song, L.; Ci, L.; Lu, H.; Sorokin, P. B.; Jin, C.; Ni, J.; Kvashnin, A. G.; Kvashnin, D. G.; Lou, J.; Yakobson, B. I. Nano Lett. 2010, 10, 3209-3215. (17) Shi, Y.; Hamsen, C.; Jia, X.; Kim, K. K.; Reina, A.; Hofmann, M.; Hsu, A. L.; Zhang, K.; Li, H.; Juang, Z.-Y. Nano Lett. 2010, 10, 4134-4139.


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Figure 1. Photoluminescence (PL) and PL upconversion of the defects in h-BN. (a) Optical microscope (OM) image of the exfoliated h-BN flakes on SiO2. The white dashed region is where PL and Raman mappings are conducted. (b) Room temperature Raman spectrum of the exfoliated h-BN excited with 532 nm laser. (c) Room temperature PL spectrum of the defects in h-BN. The peak at 565 nm corresponds to the zero phonon line (ZPL), while the one at 610 nm corresponds to the phonon side band (PSB). The excited wavelength is 532 nm. (d,e) Linear scale Raman mapping at the 1366.4 cm-1 and PL mapping of the ZPL in the white dashed region in (a). The units of the color bars are CCD counts/s. The scale bar is 5 µm. (f) PL upconversion spectrum of the defects in h-BN. The 610 nm laser excites defects in h-BN resulting in 565 nm emission. A 600 nm short pass filter is used in the collection path to filter the 610 nm excitation laser. 189x104mm (300 x 300 DPI)

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Figure 2. Excitation power and excitation wavelength dependences of the PL upconversion. (a) PL spectra of the upconversion with excitation powers of 15 µW, 29 µW, 47 µW, 64 µW respectively. Each spectrum is vertically shifted for comparison. The excitation wavelength is 610 nm. (b) Power dependence of the PL upconversion intensity. The red line is a linear fitting. (c) PL spectra of the upconversion at different excitation wavelengths. Each spectrum is vertically shifted for comparison. The excitation powers are 70 µW for all the excitation wavelengths. (d) Photoluminescence excitation (PLE) spectrum of the integrated ZPL (565 nm) intensity with the PL spectrum (blue) excited with 532 nm laser for comparison. The green dots are excited with the wavelength scanning from 483 nm to 543 nm (downconversion or normal PL). The red squares are excited from 604 nm to 620 nm (upconversion). In the PLE spectrum measurements, the 550 nm long pass and 600 nm short pass filters are used in the collection path to collect the downconversion and upconversion signals, respectively. These two filters overlap in the gray region. Inset: An enlarged PLE spectrum in the region from 604 to 620 nm. The error bars are standard deviations of twice measurements. 191x163mm (300 x 300 DPI)


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Figure 3. Temperature dependences of the PL and the PL upconversion. (a) PL spectra of the defects at temperatures from 80 to 280 K. Each spectrum is vertically shifted for comparison. The excitation wavelength is 532 nm. (b) PL spectra of the upconversion from 240 to 350 K. Each spectrum is vertically shifted for comparison. The excitation wavelength is 610 nm. (c) Logarithmic upconversion PL intensity as a function of 1/T. The red line is a linear fitting. The excitation wavelength is 610 nm. The error bars are standard deviations of twice measurements. (d) Energy diagram of the phonon-assisted electronic transitions along the configurational coordinate Q. E is the electronic ground state and E* is the excited state. The upward arrows represent optical absorption of the defect without phonons (n=0) (normal PL) and with one phonon (n=1) (phonon-assisted PL upconversion) from the ground state (E, n) to the excited state (E*, n*). The excited state defect will encounter fast thermalization to the vibronic ground state (E*, 0), which will be followed by radiative transition to the ground state (E, n) (downward arrows). The orange downward arrow represents transition of ZPL to the ground state (E, 0), while the red one represents transition of PSB to the ground state (E, 1). The gray upward arrow represents one phonon absorption assisted upconversion from (E, 1) to (E*, 0). Inset is an illustration of the optical (LO/TO) phonon vibrations in h-BN with a frequency of 1366.4 cm-1 (169.4 meV). The blue and copper balls represent N and B atoms respectively. 163x161mm (300 x 300 DPI)


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Figure 4. Scanning transmission electron microscopy–annular dark field (STEM-ADF) image and Frank Condon factors of the defects in h-BN. (a) A large scale STEM-ADF image of monolayer h-BN with B vacancies (VB). Inset: fast Fourier transform (FFT) of h-BN. (b) Enlarged STEM-ADF image of a single VB in monolayer h-BN. (c) Simulated STEM-ADF image of a single VB in monolayer h-BN derived from the DFT optimized model. (d) Statistical densities of the VB, N vacancy (VN), vacancy with neighboring B and N (VBN), and vacancy of one N and its surrounding three B (VB3N). (e) Calculated Frank Condon factors for linear optical phonon modes of the defects in h-BN. 173x181mm (300 x 300 DPI)


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