Anomalous Pressure Characteristics of Defects in Hexagonal Boron

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Anomalous Pressure Characteristics of Defects in Hexagonal Boron Nitride Flakes Yongzhou Xue, Hui Wang, Qinghai Tan, Jun Zhang, Tongjun Yu, Kun Ding, Desheng Jiang, Xiuming Dou, Jun-jie Shi, and Bao-quan Sun ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02970 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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Anomalous Pressure Characteristics of Defects in Hexagonal Boron Nitride Flakes

Yongzhou Xue,*,⁑,‡ Hui Wang,⁂,‡ Qinghai Tan,*,⁑ Jun Zhang,* Tongjun Yu,⁂ Kun Ding,* Desheng Jiang,* Xiuming Dou,*,† Jun-jie Shi,⁂,† and Bao-quan Sun*,†

*

State Key Laboratory for Superlattices and Microstructures, Institute of

Semiconductors, Chinese Academy of Sciences, Beijing 100083, China ⁑

College of Materials Science and Opto-Electronic Technology, University of Chinese

Academy of Sciences, Beijing 100049, China ⁂

State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, School of

Physics, Peking University, Beijing 100871, China

†

Address

correspondence

to

[email protected],

[email protected],

[email protected] ‡Yongzhou

Xue and Hui Wang contributed equally to the work.

ABSTRACT: Research on hexagonal boron nitride (hBN) has been intensified recently due to the application of hBN as a promising system of single-photon emitters. To date, the single photon origin remains under debate even though many experiments and theoretical calculations have been performed. We have measured the pressuredependent photoluminescence (PL) spectra of hBN flakes at low temperatures by using a diamond anvil cell (DAC) device. The absolute values of the pressure coefficients of discrete PL emission lines are all below 15 meV/GPa, which is much lower than the pressure-induced 36 meV/GPa redshift rate of the hBN bandgap. These A


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PL emission lines originate from atom-like localized defect levels confined within the bandgap of the hBN flakes. Interestingly, the experimental results of the pressuredependent PL emission lines present three different types of pressure responses corresponding to a redshift (negative pressure coefficient), a blueshift (positive pressure coefficient) or even a sign change from negative to positive. Density functional theory calculations indicate the existence of competition between the intralayer and interlayer interaction contributions, which leads to the different pressure-dependent behaviors of the PL peak shift.

KEYWORDS: hexagonal boron nitride flakes, single photon emissions, deep-level defects, photoluminescence, hydrostatic pressure

B


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Solid-state single-photon emissions (SPEs) based on quantum dot (QD)-like emitters are one of the most promising SPEs because they have narrow transition lines, long coherence times, and extreme photostability, among other advantages, which are important characteristics for miniaturization and integration.1 One of the most prominent SPE systems is atomic point defects. So far, reported atomic point defect SPEs have been color centers in wide-band-gap semiconductors (such as diamond,2,3 ZnO4-6 and SiC7,8) and defects in thin layers of semiconducting transition metal dichalcogenides (TMDs) (such as WSe29,10 and WS211). Recently, SPEs in hexagonal boron nitride (hBN) have attracted strong research interest due to their ultrabrightness, photostability and possibility of operating at room temperature.12-17 On the one hand, hBN could exist as two-dimensional monolayer and few-layered flakes (like TMDs), thus making it possible to integrate SPEs with van der Waals heterostructured devices; on the other hand, hBN is characterized by a large optical bandgap (Eg ≈ 5.5 eV, similar to that of SiC and diamond), which can host optically active defects with ground and excited states within the bandgap.18,19 To comprehensively understand the hBN defects, many experiments have been performed to study their optical and physical behaviors, such as the temperature dependence of emissions,20 magnetooptical characterization,17 polarization measurements of optical absorption and emission,21 strain tuning,22 exciton-phonon interaction23 and direct optical imaging.24 Additionally, many theoretical calculation studies have been performed, such as the density functional theory (DFT) calculations from the first-principles investigation.12,25 However, the origin of SPEs in hBN remains a subject of debate, and further C


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experimental studies are needed for a better understanding. Here, we present experimental results based on the application of a hydrostatic pressure technique in the low-temperature photoluminescence (PL) measurement of hBN flakes. The absolute values of the obtained pressure coefficients are all found to be below 15 meV/GPa, which is far less than the approximate 36 meV/GPa bandgap redshift rate of hBN.26 Notably, the use of hydrostatic pressure is particularly powerful since it allows one to vary the ratio of different interactions in the material and, thus, helps to identify the nature of the interactions involved in the various physical properties.27 Interestingly, the experimental results herein present three different types of pressure responses: pressure coefficients with negative or positive signs, and pressure coefficients for some emissions lines that change sign from negative to positive under high pressure. The origin of the pressure characteristics of the emission lines is attributed to excitons bound to a complex defect, NBVN (a nitrogen atom occupied the boron site near a nitrogen vacancy), which is confirmed by our theoretical calculations, and competition between the intralayer and interlayer interaction contributions induces the blueshift and redshift of the PL peak.

RESULTS AND DISCUSSION We started by probing hBN multilayer samples dispersed on a SiO2/Si substrate using conventional Raman spectroscopy and optical microscopy with a λ=532 nm laser at room temperature, as shown in Figure 1. The Raman spectra shown in Figure 1a exhibits a characteristic E2g phonon mode at 1366.4 cm-1, with a full-width at halfD


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maximum (FWHM) of 9.8 cm-1, which is consistent with the results of an earlier study indicating that the sample is a high-quality crystal.28,29 Figure 1b presents typical confocal PL mapping of the hBN flake sample, showing only one bright emission center. The photon energy of the excitation laser is lower than the hBN band gap energy, which indicates that the origin of the emission center is point defects in hBN, which provide electronic states within the band gap.13,15 The inset of Figure 1b shows a corresponding optical microscope image of the hBN flake, where the red square indicates the location of defects. Next, we studied low-temperature (5 K) micro-PL of the hBN flakes excited by a 532 nm laser. Similar to other reports, many bright discrete emission lines are densely distributed in a broad wavelength range from approximately 570 to 730 nm.14,20 We chose 11 representative discrete emission lines from the hBN flakes to roughly cover the whole wavelength range, as shown in Figure 2b and c. The single-photon character of these emission lines was confirmed by measuring the second-order correlation function, g2(τ), with a Hanbury-Brown and Twiss (HBT) setup. Figure 2b displays the distribution of g2(0) values for these emission lines, which are all below 0.2 (see Supporting Information, more g2(τ) functions with long delay times are presented in Figure S1), which indicates that the discrete emission lines are derived from separated atom-like point defects and have good single-photon properties. The experimental g2(τ) data are fitted using a two-level model: 𝑔2 (𝜏) = 1 − 𝑎 ∗ 𝑒

−|𝜏| 𝑡

, where parameter a is

the fitting parameter, and t is the fitting lifetime of the excited state of defects.30 The obtained t values of the emission lines are distributed in a range from 0.64 to 4.46 ns, E


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as shown in Figure 2c. We chose three typical lines, named A, B, and C, and displayed their PL spectra, as shown in Figure 2a. Their peak wavelengths are at 572.60, 659.92 and 721.69 nm, and the FWHM values are 0.17, 0.75 and 0.53 meV, respectively, and the insets show the g2(τ) of these three PL lines with g2(0) values of 0.05, 0.06 and 0.19, respectively. Evidently, Figure 2 suggests that the quantum emitters in hBN are related to different types of point defects, with different lifetime values and a broad energy distribution, displaying discrete emission lines. Identification of the type of point defects is helpful to understand the electronic structure and excitation process related to single-photon emissions. In general, the defects in semiconductor materials have two different kinds of electronic levels within the band gap, either shallow defects or deep defects. The wave function of the former is mainly composed of an extended wave function, which is related to the band edge, whereas the latter is composed of more localized wave functions, which do not follow any particular band edge. Hydrostatic pressure is a powerful technique to distinguish different kinds of defects based by their different pressure coefficients.10,31-35 Hydrostatic pressure variations are in situ applied by the home-built piezoelectric actuator (PZT)-driven diamond anvil cell (DAC) device at a temperature of 20 K to track the single defect emission lines under pressure. Notably, the obtained pressure coefficients present an anomalous pressure response of these emission lines: both a redshift and a blueshift of the PL emission lines are observed in the high-pressure experiment, as shown in Figure 3a and b, where the PL emission line is blueshifted (Figure 3a) or redshifted (Figure 3b), and the pressure coefficients are 14.1±0.2 and F


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5.9±0.1 meV/GPa, respectively. Here, we normalized the PL intensity for clearer presentation, and the arrows are used to indicate the direction of the shift in the PL line. Figure 3c shows peak energies as a function of pressure for 9 typical PL emission lines, in a wavelength range approximately from 570 to 730 nm. The absolute values of the obtained positive and negative pressure coefficients are all less than 15 meV/GPa, and most pressure coefficients are lower than 6 meV/GPa. The pressure dependence of the bandgap of hBN is approximately -36 meV/GPa,26 and the absolute value is much larger than those of the PL emission lines derived from point defects. Therefore, similar to the color centers in diamond,2 SPEs in the hBN flakes are attributed to atom-like defects whose electronic levels are deep within the bandgap and, thus, are called deep center states. Identification of deep center states is consistent with the pressure characteristic of the PL peak energy shift of the deep defects.32 In addition, we found that for PL lines with negative pressure coefficients of less than approximately 2 meV/GPa (here, the applied pressure is below 4 GPa), a pressureinduced sign change of the pressure coefficient from negative to positive was observed, as shown in Figure 4. In Figure 4a and c, the two PL lines redshift initially, and then blueshift with increasing pressure (see Supporting Information, the PL lines with a longer wavelength scale are presented in Figure S2). Their PL peak energies as a function of pressure are summarized in Figure 4b and d, respectively, where the corresponding pressure coefficients are approximately -1.31±0.07 (peak A) and 1.13±0.03 meV/GPa (peak C) for the redshift, and 0.72±0.04 (peak B) and 0.67±0.09 G


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meV/GPa (peak D) for the blueshift. We investigate several important point defects, such as VN, VB, CN, NBVN and CBVN by using the powerful first-principles calculations based on DFT to clarify the origin of the anomalous strain-induced single-photon emission peak shift in hBN nanosheets. According to the optical responses in Figure 5a, the calculated peak at approximately 1.93 eV without strain, which is attributed to the NBVN in monolayer hBN, is close to the observed value (1.75-2.15 eV, Figure 3) and is in good agreement with previous results.12,22 We thus consider only the NBVN defect in our following calculations. With the difference between intralayer interactions with covalent or ionic bonding and interlayer interactions with van der Waals (vdW) bonding, we further investigate bilayer NBVN defects as a representation of few-layer hBN. The calculated imaginary dielectric tensor of bilayer hBN with NBVN defects is shown in Figure 5b, where an obvious emission peak at approximately 2.01 eV is found without strain. Generally, the strain tensor can be expressed as 𝑎 − 𝑎0 , 𝑎0 𝑐 − 𝑐0 = . 𝑐0

𝜀11 = 𝜀22 = 𝜀33

We optimize the lattice parameter of an hBN monolayer based on minimization of the total energy and obtain 𝑎0 = 2.512 Å , which is in good agreement with the experimental value of 𝑎0 = 2.51 Å.36 In the calculation of a bilayer defect system, the optimized interlayer spacing is 𝑐0 = 3.368 Å . According to previous experimental values,37,38 hydrostatic pressure can induce isotropic in-plane strain components (ℰ11 and ℰ22 ) and an out-of-plane strain component (ℰ33 ), as shown in Table 1. Because of H


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the weak vdW interaction between the two hBN monolayers, ℰ33 is tens of times larger than ℰ11 or ℰ22 under applied pressure. Hence, strain is more easily induced along the c-axis. Moreover, we find that the ratio of ℰ33 to ℰ11 decreases as the pressure increases. The contribution of the interlayer interaction is dominant at first, and then, the intralayer interaction gradually increases with pressure and to finally become the most prominent factor. Let us now investigate the pressure effect on energy shifts in both monolayer and bilayer hBN with NBVN defects. We adopted isotropic pressure to simulate strained 2D hBN nanosheets in our calculations. For monolayer hBN with both B and N atoms in a single-atom layer, such as graphene, the pressure vertical to the hexagonal plane (along the a3-axis, see Figure 5c) retains all atoms in the same plane without vertical strain, and the pressure induces only in-plane strain (along the a1- and a2-axes). Thus, the intralayer strain determines the optical response in monolayer hBN. For bilayer hBN, the strain elements along three directions ( ℰ11 , ℰ22 and ℰ33 ) are applied according to Table 1, and both the intralayer and interlayer components determine the energy shift characteristics. Figure 5d shows the calculated peak energies as a function of pressure, and there are two opposite pressure responses. We note that the peak energy blueshifts with a pressure coefficient of 1.3 meV/GPa and redshifts with a pressure coefficient of -11.9 meV/GPa for monolayer and bilayer defect systems, respectively. The corresponding calculated NBVN defect levels of the monolayer and bilayer hBN at zero and 4 GPa are given in Supplementary information (see Figure S3), showing that applied pressure causes an increase or decrease in the energy difference I


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of the defect levels of monolayer or bilayer hBN, respectively. Thus, for monolayer hBN with NBVN defects, the strain-induced intralayer interaction is dominant and causes a blueshift, while for bilayer defects, the interlayer interaction component is dominant and leads to a redshift in the peak energy, which explains both the blueshift and redshift of the PL emission lines in the pressure experiments, as shown in Figure 3. In addition, there is competition between the intralayer and interlayer interaction contributions. As the pressure increases, the dominant interlayer interaction induces a redshift due to the large vdW gap in the beginning, and the intralayer interaction becomes the dominant factor, inducing a blueshift of the peak energy, which clarifies the physical origin of the anomalous phenomenon in which an initial redshift in the PL peak energy occurs and then a blueshift with pressure occurs, as observed in our experiments (see Figure 4).

CONCLUSION We studied the PL spectra of multilayer hBN flakes, combined with their pressure dependence measured by using DAC hydrostatic pressure technology at low temperature. The single-photon emission lines distributed in a broad spectral range roughly between 570 and 730 nm are identified. The measured pressure-dependent results reveal that there are three types of pressure responses for PL emission lines coming from point defects in hBN flakes. The pressure coefficient may be negative (redshift), positive (blueshift), or change sign from negative to positive (redshift to blueshift). Our DFT calculations indicate that competition between the intralayer and J


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interlayer interaction contributions is responsible for the observed PL peak shift.

METHODS All hBN samples were prepared from an ethanol/water solution with nanosized pristine flakes purchased from Graphene Supermarket. Their lateral size distribution ranges from 50 to 200 nm, and the thickness ranges from 1 to 5 monolayers. The solution purity in the dry phase is > 99%, and all the physical and chemical properties are stable under ambient conditions. PL measurements under high pressure and at 20 K were performed by using an improved DAC device in combination with a Montana Instruments cryostat.10,39,40 The DAC device was developed by combining a classical DAC with a PZT, and the PZT-driven device can continuously generate pressure between approximately 0.4 and 4 GPa for the samples studied at a low temperature depending on the bias voltage of the PZT. The core of the DAC consists of a pair of diamonds with a culet diameter of approximately 500 µm and a 500-µm-thick stainless steel gasket. A hole in the gasket with diameter of 250 µm forms the pressure chamber into which the sample is placed together with a small chip of ruby for the in situ pressure measurement. The applied pressure is derived from the shift of the R1 line of 7.665

the ruby fluorescence using the equation P = 248.4 × [(𝜆⁄𝜆 ) 0

− 1], where P is

the pressure in GPa, λ0 and λ represent the wavelengths of the ruby R1 line at zero pressure and at a certain pressure P, respectively.41 The gasket hole is filled with condensed argon as the pressure-transmitting medium at low temperature. The first step in our experiment was to prepare a bare SiO2/Si substrate mechanically K


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thinned to a total thickness of approximately 20 μm. Then, we dropped a small amount of the hBN flake solution on the SiO2/Si wafer and let it dry in air. Then, we cut the SiO2/Si wafer containing hBN multilayer flakes into pieces approximately 100 × 100 µm2 in size to fit the DAC chamber. Micro-PL measurements were taken at low temperature using a home-built optical confocal microscopy setup, and a 532 nm laser with a power of a few mWs was focused on the sample in the DAC device using an objective (NA=0.35). The emitted PL was collected by the same objective and analyzed using a 0.5 m monochromator equipped with a silicon charge-coupled device (CCD). Additionally, an HBT setup equipped with two silicon avalanche photo diodes (APDs) was used to perform auto-correlation measurements, which were employed to verify the single-photon property.42 The theoretical calculations are based on DFT performed by using the VASP code. 43 The generalized gradient approximation (GGA) with the Perdew, Burke and Ernzerhof (PBE) functional is employed.44,45 All calculations are spin-polarized, and the electron– ion interaction is described using the projector augmented wave (PAW) method. During our DFT calculations, supercells consisting of 7 × 7 and 5 × 5 primitive cells are used for monolayer and bilayer hBN, respectively, and the vacuum along the surface normal direction is held at approximately 20 Å. Sufficient k-point sampling (6 × 6 × 1) is used, and the energy cutoff is set to 450 eV. The structural optimization is carried out until the maximum energy difference and residual forces converge to 10−5 eV and 0.01 eV/Å. To correctly describe the effect of vdW interactions, the Becke88 optimization (optB88) exchange functional is adopted.46 L


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ASSOCIATED CONTENT Supporting Information Four g2(τ) functions for defect emission lines with a long delay time in Figure S1, two PL plots with longer wavelength scale in Figure S2, spin-polarized band structures of monolayer and bilayer hBN with complex defect NBVN in Figure S3.

ACKNOWLEDGMENTS We acknowledge support from the National Key R&D Program of China under Grant Nos. 2016YFA0301202 and 2017YFA0206303, the National Natural Science Foundation of China (Grant Nos. 11474275, 11474012, 61674135, 61774008 and 91536101), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDPB0603).

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Defects. Nanoscale 2017, 9, 13575-13582. (26) Akamaru, H.; Onodera, A.; Endo, T.; Mishima, O. Pressure Dependence of The Optical-Absorption Edge of AlN and Graphite-Type BN. J. Phys. Chem. Solids 2002, 63, 887-894. (27) Gauthier, M.; Polian, A.; Besson, J. M.; Chevy, A. Optical Properties of Gallium Selenide under High Pressure. Phys. Rev. B 1989, 40, 3837-3854. (28) Kubota, Y.; Watanabe, K.; Tsuda, O.; Taniguchi, T. Deep Ultraviolet Light– emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure. Science 2007, 317, 932-934. (29) Gorbachev, R. V.; Riaz, I.; Nair, R. R.; Jalil, R.; Britnell, L.; Belle, B. D.; Hill, E. W.; Novoselov, K. S.; Watanabe, K.; Taniguchi, T.; Geim, A. K.; Blake, P. Hunting for Monolayer Boron Nitride: Optical and Raman Signatures. Small 2011, 7, 465-468. (30) Michler, P.; Imamoglu, A.; Mason, M. D.; Carson, P. J.; Strouse, G. F.; Buratto, S. K. Quantum Correlation Among Photons from a Single Quantum Dot at Room Temperature. Nature 2000, 406, 968-970. (31) Kadri, A.; Zitouni, K.; Konczewicz, L.; Aulombard, R. L. Shallow Donorlike Impurity States in n-Type InP in Magnetic Field and under Hydrostatic Pressure. Phys. Rev. B 1987, 35, 6260-6269. (32) Hong, R. D.; Jenkins, D. W.; Ren, S. Y.; Dow, J. D. Hydrostatic-Pressure Dependencies of Deep Impurity Levels in Zinc-Blende Semiconductors. Phys. Rev. B 1988, 38, 12549-12555. (33) Gorczyca, I.; Svane, A.; Christensen, N. E. Calculated Defect Levels in GaN and Q


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S


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Figure 1. Optical characterization performed with a 532-nm excitation laser at room temperature. (a) The Raman spectra exhibit a characteristic E2g phonon mode at 1366.4 cm-1 with a FWHM of 9.8 cm-1. (b) Typical confocal PL intensity mapping of hBN flakes. Inset: Optical microscope image of the hBN flakes, where the red square indicates the location of a defect.

T


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Figure 2. The results of the optical characterization performed with a 532-nm excitation laser at 5 K. (a) PL spectra of three typical defect emission lines, named A, B, and C, at wavelengths of 572.60, 659.92 and 721.69 nm, respectively. The corresponding FWHW is 0.17, 0.75, and 0.53 meV. Insets: Second-order correlation functions g2(τ) of the corresponding emission lines with g2(0) values of 0.05, 0.06, and 0.19, respectively. (b) and (c) A statistical diagram of the obtained g2(0) (red circles) and fitted lifetime (blue circles) values of 11 defect-related emission lines distributed at different wavelengths from 570 to 730 nm.

U


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Figure 3. Pressure-dependent energy blueshift (a) and redshift (b) for two different defect emission lines, which have positive and negative pressure coefficients of 14.1(2) and -5.9(1) meV/GPa, respectively. The arrows give the direction of the PL line shift, where the blue (red) arrow means the blue (red)-shift. (c) PL peak energy as a function of pressure for the emission lines from 9 typical defects between 0.5 to 3.5 GPa. The purple (golden) solid circle data points present the blue (red)-shift of the peak energies, respectively, which are fitted by linear functions y=Ax+B, and parameter A presents a pressure coefficient. The red (blue) fitting lines correspond to red (blue) shift. The numbers in the right column represent the pressure coefficients, correspondingly.

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Figure 4. (a) and (c): Defect emission lines as a function of pressure, showing a redshift at a rate of 1.31(7) (peak A) and 1.33(3) meV/GPa (peak C) initially as well as a subsequent blueshift at a rate of 0.72(4) (peak B) and 0.67(9) meV/GPa (peak D), respectively. (b) and (d): Fitting data of the PL peak energies as a function of pressure. Here, red and blue arrows, named A, C and B, D, present the directions of the PL line shift under pressure, and the purple solid circles present the blueshift and the golden solid circles present the redshift.

W


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𝑦𝑦

Figure 5. Calculated imaginary part of the dielectric function 𝜀2 (𝜀2𝑥𝑥 , 𝜀2 and 𝜀2𝑧𝑧 ) for monolayer (a) and bilayer (b) hBN with anti-site nitrogen vacancy complex defects (NBVN), and the corresponding lattice structures (see insets). (c) Schematic diagram for the strain applied in monolayer (upper part) and bilayer (lower part) geometries. The isotropic hydrostatic pressure is exerted in both monolayer and bilayer hBN defect systems. For monolayer hBN, the pressure along the a3-axis keeps all atoms in the same plane without vertical strain because of the single-atom-layer structure, similar to graphene. Hence, only in-plane (along a1- and a2-axis) strains exist, as indicated by solid arrows. (d) The calculated optical response (peak energy shift of 𝜀2 ) as a function of pressure for monolayer (blue) and bilayer (red) hBN.

X


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Table 1. Strain tensor 11, 22, 33, and 33/11 as a function of pressure P in the units of GPa, according to Ref. 38

P (GPa)

1

2

3

4

5

6

ℰ11 (ℰ22 )

0.040%

0.080%

0.160%

0.200%

0.240%

0.280%

ℰ33

2.417%

4.459%

6.245%

7.807%

9.203%

10.45%

60.43

55.74

39.03

39.04

38.35

37.32

ℰ33 ⁄ℰ 11

Y


Anomalous Pressure Characteristics of Defects in Hexagonal Boron

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