Tunable Single-Photon Emission by Defective Boron-Nitride

Simultaneous Near-unity Efficiency and Indistinguishability In Semiconductor Single- photon Sources. Nat. Photonics 2017, 11, 521–526. (13) Bhattach...
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C: Physical Processes in Nanomaterials and Nanostructures

Tunable Single-Photon Emission by Defective BoronNitride Nanotubes for High-Precision Force Detection Wei Hu, Xinrui Cao, Yujin Zhang, Tian-Duo Li, Jun Jiang, and Yi Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01651 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 15, 2019

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Tunable Single-Photon Emission by Defective Boron-Nitride Nanotubes for High-precision Force Detection Wei Hu,†,‡,⊥ Xinrui Cao,¶,⊥ Yujin Zhang,§,⊥ Tianduo Li,† Jun Jiang,∗,‡ and Yi Luo‡,k †Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Jinan, Shandong 250353, P. R. China ‡Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. ¶Institute of Theoretical Physics and Astrophysics, Department of Physics, Xiamen University, Xiamen 361005, China §School of Electronic and Information Engineering (Department of Physics), Qilu University of Technology, Jinan, Shandong 250353, P. R. China kDepartment of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, S-106 91 Stockholm, Sweden ⊥Contributed equally to this work E-mail: [email protected]

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Abstract Boron-Nitride nanotubes (BNNTs) hold great potential for electronic, optical and mechanical applications. By introducing a NB VN defect of removing one nitrogen atom while replacing one boron by nitrogen atom, we examined the use of defective NB VN @BNNTs as a novel type of single photon emission (SPE) material. Using firstprinciples calculations to reveal the electronic structures of NB VN @BNNTs, we found that SPE with 1.45∼2.29 eV in energy can be generated in NB VN @BNNTs with size ranging from (5,0) to (10,0). It is also intriguing to find that their SPE responses are sensitive to the external forces, as indicted by the computed potential energy surfaces and dielectric tensors. Specifically, the (7,0) NB VN @BNNT can serve as an ideal force detector due to its sensitivity and linear response to external force. While the (5,0) and (6,0) NB VN @BNNTs exhibit insensitive SPE with respect to force applying, and the detection ability of the (8,0), (9,0) and (10,0) NB VN @BNNTs are limited due to the emergence of new photon emissions when tensions become larger than 10 nN. These findings would open a new door for utilizing defective BNNTs for SPE and mechanical detecting applications.

Introduction Single-photon emission (SPE) is an important light resource for many leading technologies, such as quantum photonics, quantum information, and high-precision measuring. 1–3 Fundamentally, confining electron in a small nano-scale with high energy barriers can lead to atomic-like discrete energy levels, which is essential for SPE. 4–6 Based on this physical principle, a number of SPE materials have been developed, including single quantum dots (QDs), 7–10 impurity centers in semiconductors, 11–13 single nitrogen-vacancy (NV) centers in diamond, 14–17 and so on. QDs attract widespread attention due to their advantages in optical stability, 18 sharp luminescence line 19 and controllability of wavelength, 20 which are unfortunately limited by the difficulties in manufacturing high-quality QD materials. 21 In

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contrast, NV centers could be addressed in relatively simple experimental configurations, 15 providing appropriate alternatives for SPE source. Hexagonal boron nitride (h-BN), known as ”white graphite”, is an insulator material with an bandgap of nearly 6 eV. 22 It can host optically active defects that have ground and excited states within its intrinsic big bandgap, serving as single photon emitters at room temperature. 23–27 Recently, the quantum emission from defects in single-crystalline h-BN is observed in the range of 4∼ 6 eV. 28,29 Intriguingly, its two-dimensional (2D) counterparts, i.e., h-BN multilayer and monolayer, 26,30 have shown advantages in narrow line widths, short excited-state lifetime and high brightness in a recent research reported by Tran et al.. Moreover, another interest fact was revealed by Tran et al. 26 with their experimental data in combined with first-principles calculations, that the NB VN defect of a boron site occupied by nitrogen and a missing nitrogen atom is responsible for the observed bright photonemission at 623 nm (1.9 eV). Meanwhile, the nitrogen vacancy (VN ) and the boron vacancy (VB ) defects are found to emit high energy photons that can not match the experimental observation. So far, extensive studies have been performed on three- and two- dimensional h-BN systems. It is of abroad interests to examine the photophysical and SPE properties of the one-dimensional (1D) counterpart.

Computational Details The perfect single-walled hexagonal boron nitride nanotubes (BNNTs) were firstly constructed by repeating 8 primitive unit cells along the axial direction. An orthorhombic unit cell with a≈20 ˚ A, b≈20 ˚ A and c≈35 ˚ A is used to ensure the interaction between the adjacent BNNTs can be negligible. Then one nitrogen atom was removed and a boron atom was substituted with a nitrogen to produce the NB VN defect, as shown in the insert in Figure 1. In this way, the density of the defect is set to be almost 3/nm. To investigate the effect of the defect concentration on the performance of the SPE, two same defects are made in the same

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BNNTs, resulting a defect density of 6/nm. The geometries of defected BNNTs are optimized using the PBC model implemented in the Vienna ab initio simulation package (VASP). 31 The projector augmented wave pseudopotentials were employed to represent the interaction between the core ions and the valence electrons. 32 Meanwhile, the exchange-correlation effects were mainly described by the Perdew-Burke-Ernzerhof generalized-gradient approximation (GGA-PBE) 33 with a plane-wave basis cutoff of 400 eV. A 2 × 2 × 1 Monkhurst–Pack reciprocal space grid is employed. To calculate the external forces applied on the BNNTs, we continuously change the axial length of cell. All atoms are relaxed and the PES was obtained. External forces were then calculated as the derivative of the PES with respect to the displacement.

Results and Discussion Single Photon Emission

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Figure 1: (a, b, c, d, e, f) The calculated imaginary dielectric tensors of the defected (5,0), (6,0), (7,0), (8,0), (9,0) and (10,0) NB VN @BNNT, respectively. The red line represent the z component, while the black line represent the x plus y components. The insert is the corresponding optimized structures of NB VN @BNNT. 4

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BNNTs with diameters from (3,0) to (12,0) have been examined, among which we found that the (3,0) and (4,0) tubes could not support SPE, while the (11,0) and (12,0) BNNTs offers nearly SPE identical properties as (10,0). Here in this work, we focus on BNNTs from (5,0) to (10,0) to examine the structure-properties relationship. Density functional theory (DFT) calculations find their band gap as 1.97, 2.57, 3.14, 3.33, 3.59, 3.92 eV (details in Supporting Figure S1), suggesting possible photon emission in the visible range. Calculations of the dielectric tensors are employed to reveal the optical properties. As illustrated in Figure 1, clear anisotropic optical properties ware found, suggesting good SPE abilities for these NB VN @BNNT. As for the (5,0) NB VN @BNNT, its first single photon emission peak is located at 1.44 eV along the z component, while the lowest excitation for the x+y component is located at 1.74 eV. Increasing the nanotube diameter from (6,0) to (10,0), the calculated SPE peak shows a blue shift of energies from 1.88 to 2.36 eV. Additionally, the intensities of their dominant SPE peaks also increases as the diameter increases from (5,0), (6,0) to (7,0)

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Figure 2: (a, b, c, d, e, f) Density of states of the (5,0), (6,0), (7,0), (8,0), (9,0) and (10,0) NB VN @BNNT. Meanwhile, the introduction of NB VN defect in BNNTs induces strong magnetic moments of about 2µB for all of these defected BNNTs, showing a typical spin-polarized character. 5

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The computed density of states (DOS) in Figure 2 show that the photon excitations from the spin down channels always hold a lower excitation energy. The energy differences between spin up and down channels for all these NB VN @BNNTs are in the range of 0.17∼0.30 eV. Here, only the spin-preserving transitions are allowed. 26,30 It should be noted that the lowest excitation energy increases as the diameters increases. Therefore, changing the diameters of the defected BNNTs can lead to tunable emitted single photons. 0.4

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Figure 3: The influence of the defect density on the SPE performance for (7,0) NB VN @BNNT. (a, b) represent a density of 3/nm and 6/nm along the z direction, respectively. It is known that the defect concentration can effectively affect electronic properties, as well as their possible photon emission properties. Here the (7,0) NB VN @BNNT was taken as an example to evaluate this effect. Figure 3 presents the comparison of the optical properties of the (7,0) NB VN @BNNT with the defect density of 3/nm and 6/nm. The predicted lowest excitation is the same, while the intensity of single photon emission is doubled in 6/nm than in 3/nm, implying a good scaling effect.

Mechanical Response Aiming at designing high-precision force detectors, we have investigated the responses of such SPE to the external forces on tubes. The potential energy surfaces (PES) of the whole tube along the z direction were simulated. As shown in Figure 4a, a perfect harmonic PES is obtained for the (7,0) NB VN @BNNT. Similar PES features were found for (5,0), (6,0), (8,0), (9,0) and (10,0) NB VN @BNNT (details in Supporting Figure S2). We only 6

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Figure 4: (a) The potential energy surface (PES) along the z direction for (7,0) NB VN @BNNT (black line) with different external forces (red line, calculated as the first derivative of PES). (b) The relationship between the energies of the emitted single photon and the external forces imposed on the (7,0) NB VN @BNNT. considered the axial pressure/tension forces because BNNTs are elastic along z direction. On the other hand, BNNTs are inelastic along x or y direction and will be rearranged if a tiny pressure/tension force is applied. Furthermore, to avoid the bending of the nanotubes, the applied external forces are in the magnitude order of nN. We then calculated the derivative of the PES with respect to the displacement induced by external forces on the nanotubes. As shown in Figure 4a, a good linear relationship is found between the forces and the displacement. The predicted elastic coefficients for defected (5,0), (6,0), (7,0), (8,0), (9,0) and (10,0) NB VN @BNNT are 8.5, 10.7, 12.8, 14.8, 16.8 and 18.8 nN/˚ A, respectively, in proportional to their tube diameters. It is noted that if a very long BNNTs (10-100 nm) is used, there is a big possibility that the nanotubes bend and the forces are not homogeneous. As a result, the BNNTs must be short enough to avoid bending deformation. To examine the performance of NB VN @BNNTs as force detector, the optical properties of every single point along the PES are studied. The (7,0) NB VN @BNNT possesses the highest SPE intensity and with a relatively low excitation energy. Figure 5 shows the calculated imaginary dielectric tensors of the (7,0) NB VN @BNNT with selective external forces. It is good to see that the anisotropic optical property is preserved no matter how much pressure or tension are applied. As shown in Figure 5, the SPE intensity increases with an increasing

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Figure 5: Calculated imaginary dielectric tensors of the defected (7,0) NB VN @BNNT with selective external forces. (a, b, c) represent pressure forces applied, while (d, e, f) are for tensions. pressure, while applying stronger tensions would decrease its corresponding intensity. Furthermore, the peak position of the lowest excitation varies if external forces were applied. High pressure tends to lower down excitation energy, while a blue shift is found by applying tensions. If a pressure of 17 nN is applied on the (7,0) NB VN @BNNT, the emitted single photon is located at 1.87 eV. While a tension of 16 nN leads to a SPE of 2.41 eV, as shown in Figure 5. Obviously, the SPE is very sensitive to the external force. A linear relationship between the SPE energy and external force is obtained in Figure 4b. To be specific, the energy variation in response to external force is found to be 0.016 eV/nN for the defected (7,0) NB VN @BNNT, demonstrating the potential of being used as high-precision force detector. For NB VN @BNNTs with diameters smaller than (7,0), the energy of single photon excitation is insensitive to the applied forces. For instance, the emitted single photon of the (5,0) NB VN @BNNT is always at around 1.45 eV no matter how much pressure or tension is applied (details in Supporting Figure S3), making it unavailable for force detecting. On the other hand, NB VN @BNNTs with larger diameters exhibit similar mechanical response to

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external forces as (7,0) NB VN @BNNT. It should be noted that a new emission band emerges in the low energy range when large tensions are applied to (9,0) NB VN @BNNT (details in Supporting Figure S4), which might affect the detection precision.

Conclusion In summary, by applying first-principles simulations for the potential energy surface and dielectric tensors, we systematically studied six defective single-walled hexagonal boron nitride nanotubes (BNNTs). Our calculations show that the single photon emission can be realized by introducing the NB VN defect, where a nitrogen occupies the boron site and another nitrogen atom is missing. It is found that the single photon emission can be regulated with changing nanotube diameters. The mechanical response of the NB VN @BNNTs is found to be linear. It is thus demonstrated that the defected (7,0) NB VN @BNNTs could be used as high-precision force detector with the measurement accuracy of 0.016 eV/nN.

Acknowledgement This work is supported by the Ministry of Science and Technology of China (2017YFA0303500, 2018YFA0208603), National Natural Science Foundation of China (21703223, 11704209, 21633007), Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province. The Swedish National Infrastructure for Computing (SNIC) and University of Science and Technology (USTC) is acknowledged for computer time.

Supporting Information Available The related results (including the DOS of pure BNNTs, PES of NB VN @BNNTs, response of SPE to external forces) are provided in the Supporting Information.

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(27) Hong, J.; Jin, C.; Yuan, J.; Zhang, Z. Atomic Defects in Two-Dimensional Materials: From Single-Atom Spectroscopy to Functionalities in Opto-/Electronics, Nanomagnetism, and Catalysis. Adv. Mater. 29, 1606434. (28) Cassabois, G.; Valvin, P.; Gil, B. Hexagonal Boron Nitride is an Indirect Bandgap Semiconductor. Nat. Photonics 2016, 10, 262–266. (29) Watanabe, K.; Taniguchi, T.; Kanda, H. Direct-bandgap Properties and Evidence for Ultraviolet Lasing of Hexagonal Boron Nitride Single Crystal. Nat. Mater. 2004, 3, 404–409. (30) Tran, T. T.; Elbadawi, C.; Totonjian, D.; Lobo, C. J.; Grosso, G.; Moon, H.; Englund, D. R.; Ford, M. J.; Aharonovich, I.; Toth, M. Robust Multicolor Single Photon Emission from Point Defects in Hexagonal Boron Nitride. ACS Nano 2016, 10, 7331– 7338. (31) Kresse, G.; Furthm¨ uller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186. (32) Bl¨ochl, P. E. Projector Augmented-wave Method. Phys. Rev. B 1994, 50, 17953–17979. (33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868.

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