⇦Chinese Physics Letters, 2020, Vol. 37, No. 7, Article code 076203 Synthesis of Highly Stable One-Dimensional Black Phosphorus/h-BN Heterostructures: A Novel Flexible Electronic Platform Jingyan Song (宋京岩)†, Shuai Duan (段帅)†, Xin Chen (陈欣)*, Xiangjun Li (李相君), Bingchao Yang (杨兵超), and Xiaobing Liu (刘晓兵)* Affiliations Laboratory of High Pressure Physics and Material Science (HPPMS), School of Physics and Physical Engineering, Qufu Normal University, Qufu 273100, China Received 18 May 2020; accepted 30 May 2020; published online 21 June 2020 Supported by the National Natural Science Foundation of China (Grant Nos. 11804184, 11974208, and 21905159) and the Shandong Provincial Science Foundation (Grant Nos. ZR2019MA054, 2019KJJ020, and ZR2019BA010).
†The authors contributed equally to this work.
*Corresponding authors. Email: xiaobing.phy@qfnu.edu.cn; chenxin@qfnu.edu.cn Citation Text: Song J Y, Duan S, Chen X, Li X J and Yang B C et al. 2020 Chin. Phys. Lett. 37 076203    Abstract Layered black phosphorus (BP) has recently emerged as a promising semiconductor because of its tunable band gap, high carrier mobility and strongly in-plane anisotropic properties. One-dimensional (1D) BP materials are attractive for applications in electronic and thermal devices, owing to their tailored charge and phonon transports along certain orientations. However, the fabrication of 1D BP materials still remains elusive thus far. We herein report the successful synthesis and characterization of nanotube-like BP for the first time by a selective composite with hexagonal boron nitride (h-BN) nanotubes under high pressure and high temperature conditions. The produced 1D BP/h-BN composites possess flexible diameter, length and thickness by adjusting the experimental synthesis parameters. Interestingly, it is important to notice that the stability of our BP sample has been significantly improved under the formation of heterostructures, which can actively promote their commercial applications. Our experimental work, together with first-principles calculations, presents a new scalable strategy of designing 1D tube-like BP/h-BN heterostructures that are promising candidates for flexible and high efficiency electronic platform. DOI:10.1088/0256-307X/37/7/076203 PACS:62.50.-p, 61.50.Ks, 71.20.-b © 2020 Chinese Physics Society Article Text Since the successful exfoliation of few layers,[1] black phosphorus (BP) has recently attracted considerable interest as a promising two-dimensional (2D) semiconductor for electrical and optical applications.[2,3] In contrast to the gapless character of graphene,[4] BP presents a tunable direct band gap ($E_{\rm g}$) from $\sim $0.3 eV for its bulk form to $\sim $2.0 eV for monolayer phase,[5–7] as well as the high carrier mobility ($\sim $1000 cm$^{2}$$\cdot$V$^{-1}$s$^{-1}$) and large ON/OFF current ratio ($\sim$$10^{5}$),[8] satisfying different technological requirements. Monolayer or few-layer BP based materials have been demonstrated to have promising applications in many fields such as next-generation electronics,[9] supercapacitors,[10] thermoelectric devices,[11,12] sensors,[13] optoelectronic devices.[14,15] Nevertheless, alongside these remarkable properties, the existing drawback in BP materials is their poor stability in air under ambient atmospheric conditions, which has severely hampered the further commercial applications.[16–19] It is therefore highly critical to develop an effective passivation technique to avoid deterioration of BP flakes for high performance new BP-based devices. Moreover, due to its unique structure, BP crystal displays strongly in-plane anisotropic electronic, optical and thermal properties[20] that have ensured novel uses of BP in electronic and thermoelectric applications. It is of great interest to investigate and fabricate one-dimensional (1D) BP materials and comprehensively explore unprecedented properties, owing to the tailored charge and phonon transports along certain orientations. For example, their relatively independent control of electrical conductivity ($\sigma$) and thermal conductivity ($\kappa$) provides a promising candidate[11,21] for flexible and high performance thermoelectrics by designing 1D materials curled along certain directions of phosphorene (e.g., zigzag direction). However, it is rather challenging to synthesize 1D BP materials via a conventional route, which cannot overcome the high energy barrier. Thus far, it is still a lack of effective way to directly fabricate 1D BP materials. Previous theoretical and experimental studies[22–26] have found that designing heterostructures enables the tailoring of physical properties in BP-based structures. Hexagonal boron nitride (h-BN),[27] with the structure similar to graphene, possesses a wide bandgap of up to 5.9 eV, high thermal stability and chemical inertness. The hybridized BP/h-BN structures were theoretically predicted to be an ideal way for an excellent protection for air-stable BP-based devices.[28,29] Motivated by this fact, we expect to design and fabricate 1D heterostructures through epitaxial growth for BP layers on some commercial h-BN nanotubes to solve aforementioned issues of BP. Here we show that we have successfully synthesized uniform and high-quality 1D BP/h-BN heterostructures by epitaxial growth method with a wide range of tube size and chirality. Importantly, these tubes are very stable, which may enable other applications, in addition to thermoelectrics. In our experiment, red phosphorus (99.999% purity, Alfa Aesar) was used as the starting source for the synthesis of BP. We took h-BN nanotubes, $\sim $100–2500 nm in length and 50–300 nm in diameter, used as tube substrates. The produced samples were studied by an optical microscope (Leica 205 C), scanning electron microscopy (SEM, Zeiss Sigma 500), Raman spectroscopy (Horiba, LabRAM HR revolution), powder x-ray diffraction (X'Pert3 diffractometer with a Cu-$K\alpha$ target), TEM (JEM-2100Plus) and x-ray photoelectron spectroscopy (Thermo Scientific ESCALAB 250Xi). The structural relaxation and the electronic structure calculations were performed based on density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) package.[30,31] The projector-augmented wave (PAW) method[32] and the generalized gradient approximation (GGA)[33] of Perdew–Burke–Ernzerhof (PBE)[34] was employed to describe the electron-electron interaction. Hybrid HSE06 (Heyd–Scuseria–Ernzerhof) functionals[35,36] are also used for more accurate band gap calculations. The cutoff energy of plane-wave was set to be 550 eV and Monkhorst–Pack uniform $k$-point spacing of 0.10 Å$^{-1}$ was chosen throughout to ensure the total energy convergence of 10$^{-5}$–$10^{-6}$ eV. The geometries and atomic coordinates were fully optimized until the Hellmann–Feynman forces on each atom are less than 0.01 eV/Å. Based on the electronic structure, the transport properties were derived by using the BoltzTraP code[37] which is based on the semiclassical Boltzmann theory[38] in the framework of relaxation time approximation. 3. Result and discussion
cpl-37-7-076203-fig1.png
Fig. 1. Illustration of the fabrication of hybrid BP/h-BN nanotubes; (a)–(d) Mixture of RP powders were gasified in a sealing tube and coated on the employed h-BN nanotubes. (e)–(f) The reaction samples were pressed into dense cylindrical sample and treated in a cubic large volume press for BP growth. (g) Typical SEM image of the HPHT productions.
To fabricate 1D BP materials, we carried out synthesis experiments by epitaxial growth on h-BN nanotubes under high pressure and high temperature (HPHT) conditions, as illustrated in Fig. 1. In our growth, we used h-BN nanotubes, $\sim $100–2500 nm in length and 50–300 nm in diameter, as tube substrates for BP growth. High-purity red phosphorus (RP) powders (99.999 wt.%) were gasified in a sealing tube at 847 K for 24 hours for phosphorus coating on h-BN nanotubes (Figs. 1(a)–1d). This enables the produced composites to be easily tuned by adjusting the experimental parameters. For example, we can control the thickness for coating phosphorus by mixing ratio, reaction temperature and reacting time, while the lengths and diameters are mainly depended on the h-BN substrate. Then the reacted samples were treated at 473 K for two hours in an Ar and N$_{2}$ environment to remove the remaining white phosphorus (WP). The reaction samples were pressed into dense cylindrical sample and treated at 2 GPa and 1373 K for 15 min for BP growth in a cubic large volume press (Fig. 1(f)).[9] After growth, the HPHT productions were ground to powders in a glove box filled with Ar gas, and then were carefully exfoliated using an ultrasonic crusher noise isolating machine and extracted to remove the remaining BP nanoflakes. Figure 1(g) shows an SEM image of the fabricated tube-like composites, indicating the possible synthesis of 1D BP-based nanomaterials. Further transmission electron microscopy (TEM) observations exhibit that the length and diameter of the produced tube-like composites are in the ranges of 200–2500 nm and 50–400 nm (Figs. 2(a)–2(d)), respectively, which can be tunable by changing the size of h-BN substrates. Figure 2(e) is a typical TEM image of tube-like composite in the HPHT productions. High-resolution TEM (HRTEM) and corresponding fast Fourier transform (FFT) images of the interface structures indicate that our produced composites mainly consist of BP layers grown on 1D h-BN substrates, as shown in Figs. 2(f)–2(g). We find that multiple layers are typically arranged orderly with no obvious deformation or dislocations. Most tube-like BP layers with $\sim $2–8 nm in thickness were grown along the [012] orientation at the interfaces between BP layers and the h-BN nanotubes (Figs. S1 in the Supplementary Materials). Therefore, we can suggest that our productions have been formed as 1D tube-like BP/h-BN heterostructures.
cpl-37-7-076203-fig2.png
Fig. 2. Tube-like BP through epitaxial growth on h-BN nanotubes. (a)–(e) Typical TEM images showing the BP/h-BN heterostructures with different sizes fabricated at 2 GPa and 1373 K. (f)–(g) HRTEM image of the interface between the hybrid BP/h-BN structure, corresponding to the position to marked with red box in (a). Inset: corresponding FFT images of BP (right top) and h-BN (left bottom). (c) Microstructure in the top of BP/h-BN heterostructures, showing a high curved BP layer on the h-BN tube.
cpl-37-7-076203-fig3.png
Fig. 3. (a) XRD spectra of the HPHT produced BP/h-BN composite and BP crystals. The BP and h-BN are marked as blue stars and red balls, respectively. (b) The Raman spectrum shows the characteristic $A_{\rm 1g}$, $B_{\rm 2g}$ and $A_{\rm 2g}$ of the produced BP/h-BN composite (black curve), BP (red curve) and in-plane mode of h-BN (blue curve). (c) EDS spectrum of the hybrid BP/h-BN heterostructures. Inset: schematic diagram of the hybrid BP/h-BN heterostructures. (d) XPS spectra of P 2$p$ core-level region for BP crystal. (e)–(g) XPS spectra from the as-grown BP/h-BN composite of P 2$p$, B 1$s$ and N 1$s$ core levels, respectively.
The x-ray diffraction (XRD) spectra in Fig. 3(a) demonstrate that the HPHT productions are pure BP/h-BN composites. Figure 3(b) shows a typical Raman spectrum of the BP/h-BN composite (top curve). Compared with that of the pure BP (red curve) and h-BN (blue curve), the BP/h-BN composite shows a lower wavenumber in the Raman spectrum (Figure 1(c)) by $\sim $4, 4, 4.5 cm$^{-1}$ for the $A_{\rm 1g}$, $B_{\rm 2g}$, $A_{\rm 2g}$ modes in BP, respectively, and 0.5 cm$^{-1}$ for the in-plane mode in h-BN. The intensity of $A_{\rm 2g}$ is about twice the value of pure BP, while the in-plane mode for h-BN has been significantly suppressed and broadened. In combination of the energy-dispersive spectroscopy (EDS) results in Fig. 3(c), our Raman data support the model of composite structures in the HPHT productions. We further performed an x-ray photoelectron spectroscopy (XPS) measurement to investigate the chemical bonds and structure in the produced tube-like composites. Figures 3(d)–3(f) show the XPS spectrum of phosphorus, boron and nitrogen elements, respectively, from a pure BP crystal and as-grown BP/h-BN composite. The shapes of P 2$s$ and B 1$s$ are very different from the pure BP and h-BN,[27,39] implying that the BP/h-BN heterostructures were formed in the produced tube-like composites. The P 2$p$ XPS core-level spectrum in the as-grown BP/h-BN composite shows that a shoulder at 128.5 eV appears on the left side of the P 2$p_{3/2}$, implying a contribution from the bonding configurations of boron and nitrogen atoms with lower electronegativity. The main peak of the N $1s$ is at 398.03 eV (Fig. 3(f)), close to the pure h-BN,[27] where three nitrogen atoms surround one boron atom. We can clearly observe three new small peaks of B 1$s$ in Fig. 3(g) at 182.3, 184.7 and 186.9 eV in the produced BP/h-BN composite, indicating that the main bonding configuration at the interface between BP layers and employed h-BN nanotubes is B–P bonds. These characterizations provide strong evidence that we have successfully prepared hybrid BP/h-BN nanotubes with B–P covalent bonds, which is consistent with the results of Raman spectra and EDS.
cpl-37-7-076203-fig4.png
Fig. 4. Stability of the produced BP/h-BN composite. (a) Obtained Raman spectrum of BP/h-BN after exposure in air for 4–10 days. (b) Thermal stability measurement of BP/h-BN composite. (c) A resistance-versus-temperature curves from 1.8 to 100 K for produced BP/h-BN composites with weight ratios $1\!:\!1$, $2\!:\!1$, $3\!:\!1$ and $4\!:\!1$ measuring $5 \times 5$ mm and 1 mm thick. The top inset is the enlarged images of for the BP(4)/h-BN(1) heterostructures.
Since BP can be easily oxidized and hydrolyzed in air, the practical application of the BP-based devices has severely been hampered by the poor stability for the long run. It is thus important to investigate the stability of the hybrid BP/h-BN composite. Strikingly, we notice that the stability of our BP sample has been significantly improved under the formation of heterostructures under ambient conditions. Figure 4(a) shows the Raman spectra of the produced BP/h-BN composite as a function of ambient time in air to elucidate the chemical degradation,[18] which usually leads to the weakening of Raman intensity. After 10-day exposure, the intensities for all the three characteristic peaks of the $A_{\rm 1g}$, $B_{\rm 2g}$, $A_{\rm 2g}$ in BP remain stable and almost constant with only a slight downshift to the lower wavenumbers, indicating high stability of BP/h-BN heterostructures. Moreover, it is worth noting that the synthesized composite structures exhibit high stability up to 637 K (Fig. 4(b)), indicating a wider scope of applications in electrics. It was expected[9,17,18,40] to improve the stability of BP by effective passivating the P atoms located on the puckered surface. Here we suggest that the significant improvement on the stability of the BP/h-BN composite can be attributed to the B–P bonding, as confirmed by the above Raman spectrum and XPS results. In combination of their tunable electrical transport properties by controllable ratios of BP and h-BN (Fig. 4(c)), the experimental realization of semiconducting BP/h-BN heterostructures provides an exciting new electronic platform that may be of interest in many areas including especially thermoelectrics. Since these BP/h-BN heterostructures can be experimental synthesized, we turn to analyze their electronic and transport properties by first-principle calculations for potential applications in electronic devices. We construct [100] and [010] oriented BP/h-BN shell-core heterostructured nanotubes, composed of a single-wall (1L) and double-wall (2L) BP tube, in order to investigate the effect of crystallographic orientation and thickness on electronic and transport properties. Here, [100] and [010] orientations represent the roll-up vector along zigzag and armchair directions in monolayer black phosphorene or h-BN, respectively, to curl into the tube. The optimized structures are depicted in Figs. 5(a) and 5(b) and Fig. S2 in the supplementary material. The diameters of all the 1L nanotubes are around 2 nm, and that for the 2L tube is about 3.1 nm. More detailed structural information of constructed nanotubes can be found in Table S1. Calculated electronic band structure and density of states (DOS) in Figs. 5(c), 5(d) and Fig. S3 demonstrate that all the heterostructured nanotubes are narrow band-gap semiconductors, and the band gaps can be easily modulated by the tube orientation and wall thickness, beneficial for application in electronics. Moreover, we notice that electronic states near the Fermi level are most primarily from P $p$ orbitals in outer BP shells. Therefore, the physical properties that is related with band structure, like electrical transport etc., are mainly determined by BP shell in heterostructures. This is very favorable for real applications since BP/h-BN nanotubes have been proved to be more stable by our experiments, only if they can maintain the high mobility as that in layered BP.
cpl-37-7-076203-fig5.png
Fig. 5. Theoretical calculations on BP/h-BN heterostructured nanotubes. Here (a) and (b) represent crystal structures of BP/h-BN tubes along [100] and [010] directions, respectively. Left and right images are top- and side-views. The calculated band structures and DOS for (c) [100] and (d) [010] BP/h-BN nanotube. (e) Calculated Seebeck coefficients at room temperature as a function of carrier concentration. The calculated values for monolayer black phosphorene along armchair and zigzag direction are included for comparison.
Based on the above discussion, we calculate the Seebeck coefficient of BP/h-BN nanotubes, which is a kernel for transport properties and thermoelectric applications. It isworth noting from Fig. 5(e) that BP/h-BN nanotubes have distinctly improved Seebeck coefficient compared with monolayer black phosphorene, especially those along [010] orientation, since the interface confinement effect, arising from lattice mismatch of monolayer BP and h-BN, lead to the flatter band structures and steeper DOS near the Fermi level in [010] BP/h-BN tubes (Figs. 5(d) and S3(c)). To be specific, for a heavy doping of 0.2$e$/u.c., Seebeck coefficient for 2L [010] BP/h-BN within p- and n-type can reach up to 223 and 247 µV/K, six and three times larger than that in monolayer BP, respectively. More strikingly, [010] BP/h-BN tubes show multiple band extrema close enough in energy to all be relevant for transport, very favorable for electrical conductivity. The results imply that [010]-oriented BP/h-BN has great potential as a high-performance thermoelectrics. Although at this stage the huge BP/h-BN atomic system hinders quantitatively evaluating thermoelectric figure of merit ZT values, it is highly reasonable to believe that the BP/h-BN nanotubes could be promising candidates for potential thermoelectric applications because their thermal transport could be effectively reduced by hybridized BP with the h-BN substrate, which is generally employed as thermal insulation materials in the HPHT experiments.[9,41,42] In conclusion, we have experimentally synthesized stable BP/h-BN heterostructures and investigated its thermoelectric potential by first-principles calculations and Boltzmann transport theory. This is ascribed to effect of dimensional reduction, which leads to multi-valley band structures near the Fermi level and enhanced Seebeck coefficient. Our results present a new scalable strategy of designing and fabricating curled BP/h-BN heterostructures that are promising electronic platform for flexible, eco-friendly and high-performance thermoelectric devices. The calculations were performed at the High Performance Computing Center of Qufu Normal University. References Isolation and characterization of few-layer black phosphorusApplications of Phosphorene and Black Phosphorus in Energy Conversion and Storage DevicesLattice Opening upon Bulk Reductive Covalent Functionalization of Black PhosphorusThe rise of grapheneElectronic Bandgap and Edge Reconstruction in Phosphorene MaterialsHighly anisotropic and robust excitons in monolayer black phosphorusHigh-mobility transport anisotropy and linear dichroism in few-layer black phosphorusBlack phosphorus field-effect transistorsTe-Doped Black Phosphorus Field-Effect TransistorsFlexible All-Solid-State Supercapacitors based on Liquid-Exfoliated Black-Phosphorus NanoflakesEnhanced Thermoelectric Performance in Black Phosphorus Nanotubes by Band Modulation through Tailoring Nanotube ChiralityLayered Black Phosphorus as a Selective Vapor SensorBlack Phosphorus–Monolayer MoS 2 van der Waals Heterojunction p–n DiodePhotoluminescence Quenching in Self-Assembled CsPbBr 3 Quantum Dots on Few-Layer Black Phosphorus SheetsControl of Surface and Edge Oxidation on PhosphoreneStabilizing black phosphorus nanosheets via edge-selective bonding of sacrificial C60 moleculesCovalent functionalization and passivation of exfoliated black phosphorus via aryl diazonium chemistryDegradation Chemistry and Stabilization of Exfoliated Few-Layer Black Phosphorus in WaterAnisotropic in-plane thermal conductivity observed in few-layer black phosphorusHigh thermoelectric performance can be achieved in black phosphorusvan der Waals Bonded Co/h-BN Contacts to Ultrathin Black Phosphorus DevicesMultipurpose Black-Phosphorus/hBN HeterostructuresTransport properties of ultrathin black phosphorus on hexagonal boron nitrideNonvolatile Floating-Gate Memories Based on Stacked Black Phosphorus-Boron Nitride-MoS 2 HeterostructuresHeterostructured hBN-BP-hBN Nanodetectors at Terahertz FrequenciesAtomic layers of hybridized boron nitride and graphene domainsAir-Stable Room-Temperature Mid-Infrared Photodetectors Based on hBN/Black Arsenic Phosphorus/hBN HeterostructuresSimple fabrication of air-stable black phosphorus heterostructures with large-area hBN sheets grown by chemical vapor deposition methodEfficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis setEfficient iterative schemes for ab initio total-energy calculations using a plane-wave basis setProjector augmented-wave methodIterative minimization techniques for ab initio total-energy calculations: molecular dynamics and conjugate gradientsGeneralized Gradient Approximation Made SimpleHybrid functionals based on a screened Coulomb potentialInfluence of the exchange screening parameter on the performance of screened hybrid functionalsBoltzTraP. A code for calculating band-structure dependent quantitiesIn Situ Doping of Black Phosphorus by High-Pressure SynthesisAzide Passivation of Black Phosphorus Nanosheets: Covalent Functionalization Affords Ambient Stability EnhancementBoron–oxygen complex yields n-type surface layer in semiconducting diamondBand structure manipulated by high pressure-assisted Te doping realizing improvement in thermoelectric performance of BiCuSeO system
[1] Castellanos-Gomez A, Vicarelli L, Prada E, Island J Q, Narasimha-Acharya K L and Blanter S I et al. 2014 2D Mater. 1 025001
[2] Pang J, Bachmatiuk A, Yin Y, Trzebicka B, Zhao L, Fu L et al. 2018 Adv. Energy Mater. 8 1702093
[3] Wild S, Fickert M, Mitrovic A, Lloret V, Neiss C, VidaloMoya J A et al. 2019 Angew. Chem. Int. Edit. 58 5763
[4] Geim A K and Novoselov K S 2007 Nat. Mater. 6 183
[5] Liang L, Wang J, Lin W, Sumpter B G, Meunier V and Pan M 2014 Nano Lett. 14 6400
[6] Wang X, Jones A M, Seyler K L, Tran V, Jia Y, Zhao H et al. 2015 Nat. Nanotechnol. 10 517
[7] Qiao J, Kong X, Hu Z X, Yang F and Ji W 2014 Nat. Commun. 5 4475
[8] Li L, Kim J, Jin C, Ye G J, Qiu D Y, Jornada F H et al. 2014 Nat. Nanotechnol. 9 372
[9] Yang B, Wan B, Zhou Q, Wang Y, Hu W, Lv W et al. 2016 Adv. Mater. 28 9408
[10] Hao C, Yang B, Wen F, Xiang J, Li L, Wang W et al. 2016 Adv. Mater. 28 3194
[11]Duan S, Cui Y, Chen X, Yi W and Liu X 2019 Adv. Funct. Mater. 29 1904346
[12] Chen X, Duan S, Yi W, Singh D J, Guo J and Liu X 2020 Small (in press)
[13] Mayorga-Martinez C C, Sofer Z and Pumera M 2015 Angew. Chem. Int. Edit. 54 14317
[14] Deng Y, Luo Z, Conrad N J, Liu H and Ye P D 2014 ACS Nano 8 8292
[15] Subas M, Padmini P, Gayathri D, Rohit B, Thripuranthaka M, Kothari D C et al. 2018 Angew. Chem. Int. Edit. 130 7808
[16] Kuntz K L, Wells R A, Hu J, Yang T, Dong B, Guo H et al. 2017 ACS Appl. Mater. & Interfaces 9 9126
[17] Zhu X, Zhang T, Jiang D, Duan H, Sun Z, Zhang M et al. 2018 Nat. Commun. 9 4177
[18] Ryder C R, Wood J D, Wells S A, Jariwala D, Marks T J et al. 2016 Nat. Chem. 8 597
[19] Zhang T, Wan Y, Xie H, Mu Y, Du P, Wang D et al. 2018 J. Am. Chem. Soc. 140 7561
[20] Luo Z, Maassen J, Deng Y, Du Y, Garrelts R P, Lundstrom M S et al. 2015 Nat. Commun. 6 8572
[21] Zhang J, Liu H J, Cheng L, Wei J, Liang J L, Fan D D et al. 2016 J. Mater. Chem. C 4 991
[22] Avsar A, Tan J Y, Luo X, Khoo K H, Yeo Y, Watanabe K et al. 2017 Nano Lett. 17 5361
[23] Constantinescu G C and Hine N D 2016 Nano Lett. 16 2586
[24] Doganov R A, Koenig S P, Yeo Y, Watanabe K, Taniguchi T and Zyilmaz B 2015 Appl. Phys. Lett. 106 083505
[25] Li D, Wang X, Zhang Q, Zou L, Xu X, Zhang Z 2016 Adv. Funct. Mater. 25 7360
[26] Viti L, Hu J, Coquillat D, Politano A, Consejo C, Knap W et al. 2016 Adv. Mater. 28 7390
[27] Ci L, Song L, Jin C, Jariwala D, Wu D, Li Y et al. 2010 Nat. Mater. 9 430
[28] Yuan S, Shen C, Deng B, Chen X, Guo Q, Ma Y et al. 2018 Nano Lett. 18 3172
[29] Sinh S, Takabayashi Y, Shinohara H and Kitaura R 2016 2D Mater. 3 035010
[30] Kresse G and Furthmüller J 1996 J. Comput. Mater. Sci. 6 15
[31] Kresse G and Hafner J 1996 Phys. Rev. B 54 11169
[32] Blöchl P E B 1994 Phys. Rev. B 50 17953
[33] Payne M C, Arias T A and Joannopoulos J D 1992 Rev. Mod. Phys. 64 1045
[34] Perdew J P, Burke K and Ernzerhof M 1996 Phys. Rev. Lett. 77 3865
[35] Heyd J, Scuseria G E and Ernzerhof M 2003 J. Chem. Phys. 118 8207
[36] Krukau A V, Vydrov O A, Izmaylov A F and Scuseria G E 2006 J. Chem. Phys. 125 224106
[37] Madsen G K H and Singh D J 2006 Comput. Phys. Commun. 175 67
[38]Chelikowsky J R and Louie S G 1996 Quantum Theory of Real Materials (Berlin: Springer Science & Business Media) vol 348 p 219
[39] Antonatos N, Bouša D, Shcheka S, Beladi-Mousavi S M, Pumera M and Sofer Z 2019 Inorg. Chem. 58 10227
[40] Liu Y, Gao P, Zhan T, Zhu X, Zhang M, Chen M et al. 2019 Angew. Chem. Int. Edit. 58 1479
[41] Liu X, Chen X, David S, Richard S, Wu J, Petitgirard S et al. 2019 Proc. Natl. Acad. Sci. USA 116 7703
[42] Liu Z, Guo X, Li R, Li H, Chen X et al. 2019 J. Materiomics 5 649