C3N van der Waals p–n

Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 858−862

pubs.acs.org/JPCL

Predicting Two-Dimensional C3B/C3N van der Waals p−n Heterojunction with Strong Interlayer Electron Coupling and Enhanced Photocurrent Chunmei Zhang,† Yalong Jiao,† Tianwei He,† Steven Bottle,† Thomas Frauenheim,‡ and Aijun Du*,† †

School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Gardens Point Campus, Brisbane, Queensland 4001, Australia ‡ Bremen Center for Computational Materials Science, University of Bremen, Am Falturm 1, 28359 Bremen, Germany S Supporting Information *

ABSTRACT: The interlayer coupling in 2D van der Waals (vdW) heterostructures (HTS) plays the main role in generating new physics. However, the interlayer coupling is often weak, and little information on the strength of interlayer interaction in HTS is available. On the basis of density functional theory, we demonstrate that an effective electron coupling can be created in 2D C3B/C3N vdW HTS. The experimentally synthesized monolayers C3B and C3N are p- and n-type doped large gap semiconductors, respectively. However, the formed vdW HTS exhibits novel Dirac fermion. The strong interlayer electron coupling results in a large interlayer built-in electric field and improved optical properties of the 2D C3B/C3N vdW HTS. Moreover, a simple tight-binding model of C3B/C3N HTS with the nonzero interlayer hopping parameters captures the physical picture of interlayer coupling. Our results demonstrate the importance of interlayer electron coupling in the modulation of materials properties of 2D vdW HTS.

T

semiconductor (n-type), while C3B monolayer is hole abundant (p-type). Both monolayer C3N and C3B possess large indirect band gaps (1.09 and 1.83 eV), which makes them less attractive for optoelectronics. The arrangement of single layer n-type C3N and p-type C3B monolayer into a vdW HTS, on the contrary, suggests large interlayer coupling and interesting physics. We examined the prospects of such novel 2D vdW p−n junction materials through systematic studies of 2D C3B/C3N HTS based on density functional theory (DFT) and GW approximation in conjunction with the Bethe−Salpeter equation (BSE) approaches. We find that the expected electron interlayer coupling in the C3B/C3N HTS is very strong and thus results in the development of a large interlayer built-in electric field. Furthermore, 2D C3B/C3N HTS possess Dirac cone-like electronic structures with a small gap opening, which is similar to the gated bilayer graphene.22 Nevertheless, large electric fields are demanded in the bilayer graphene to create an imbalance in the charge distribution, which would cause adverse effect on the stability in this configuration. In C3B/C3N HTS, the generated large built-in electric field by strong electron coupling can induce an intrinsic band inversion. In addition, our theoretical calculations are complemented with a simple tight-binding (TB) model by introducing nonzero interlayer hopping parameters to capture the strong electron

wo dimensional (2D) van der Waals (vdW) heterostructures (HTS), which are typically formed by vertically stacking different types of 2D materials together, can introduce distinct optical and electronic properties.1−8 In 2D vdW HTS, strong covalent bonds provide in-plane stability for crystals, while the weak vdW force (∼10 meVÅ−2)9 keeps layers stacked together. Because the interlayer electron coupling in 2D vdW HTS is generally weak, the overall electronic properties remain largely unchanged10,11 compared with isolated monolayers. Hence, tuning the interlayer electron coupling is a critical task to endow 2D HTS with attractive electronic properties. In general, strong interlayer coupling can be established by a large charge transfer in a system.12,13 Thus an effective way to realize strong electron coupling is to design vdW HTS with p− n junctions composed of the hole- and electron-doped 2D materials. The concept of p−n junction materials is of great interest in low-dimensional condensed-matter physics.14 However, the large-scale synthesis of p- and n-type 2D materials has not yet been demonstrated, limiting the development of this field. Chemical doping15 has been reported to be valuable in tailoring surface potentials of 2D vdW p−n junction materials, for example, vertically stacked B-doped graphene and N-doped graphene.16,17 However, the weak coupling between layers remains and the low doping concentrations generate a negligible impact on the electronic properties of such 2D vdW HTS.18 Single-layer C3B19,20 and C3N21 are structurally similar to graphene and have been successfully synthesized in recent experiments. The C3N monolayer is an electron-abundant © XXXX American Chemical Society

Received: December 31, 2017 Accepted: February 6, 2018 Published: February 6, 2018 858

DOI: 10.1021/acs.jpclett.7b03449 J. Phys. Chem. Lett. 2018, 9, 858−862

Letter

The Journal of Physical Chemistry Letters

Figure 1. Top views of monolayer (a) C3B and (b) C3N. The brown, green, and gray spheres represent C, B, and N atoms, respectively. (c,d) Band structure of C3B and C3N monolayers calculated by HSE06 method.

Figure 2. (a−d) Top view and side view of four patterns (AA-, AB-, AC-, and AD-stacking) for C3B/C3N HTS, respectively. The brown, green, and gray spheres represent C, B, and N atoms, respectively. (e−g) Band structure of C3B/C3N HTS calculated by HSE06 method.

which are consistent with previous theoretical studies.19,21 From this analysis it is clear that these new members of the graphene family are n-type (C3N) and p-type (C3B) semiconductors, which can easily form a p−n junction HTS and might offer novel applications for electronics and photonics. It is also important to consider prospective structural arrangements for C3B/C3N HTS. Potential stacking geometries for the HTS are shown in Figure 2a−d. There are four typical configurations, namely, AA-, AB-, AC-, and AD-stacking order, and the AD-stacking is the most energetic favorable arrangement. These stacking arrangements yield three different electronic band structure, as plotted in Figure 2e−g. We observe Dirac cone-like bands located around M points with a small direct band gap, and this is in sharp contrast with individual materials displaying a large indirect gap. The relative energy, interlayer distance, and band gap of C3B/C3N HTS with different stacking orders are listed in Table s1. To understand atomic orbital contributions to CBM and VBM of 2D C3B/C3N HTS, we calculated the orbital resolved

coupling features. The valence-band maximum (VBM) and conduction-band minimum (CBM) in 2D C3B/C3N HTS are located in different layers, indicating well-separated charges for the photoexcited electrons and holes4,23−25 and longer electron−hole recombination lifetimes.26 The calculated optical transition of the C3B/C3N HTS using the GW+BSE approach clearly demonstrates the enhanced photocurrent production ranging from the near-infrared to the ultraviolet. Our results propose a practical strategy to realize a strong electron coupling toward the applications in nanoelectronics and photonics First, we discuss the electronic properties of isolated C3B and C 3 N monolayers. Monolayer C 3 N 27 and C 3 B 19,20 are structurally related to graphene (see Figure 1a,b) and have been fabricated in recent experiments. The calculated band structure using HSE methods are presented in Figure 1c,d, with relatively large indirect band gaps of 1.83 and 1.09 eV for monolayers C3B and C3N, respectively. The PBE method predicted band gaps of 0.67 and 0.39 eV (see Figure S1) and lattice constants of a = 5.17 and c = 4.86 Å for C3B and C3N, 859

DOI: 10.1021/acs.jpclett.7b03449 J. Phys. Chem. Lett. 2018, 9, 858−862

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The Journal of Physical Chemistry Letters

Figure 3. (a,b) Orbital-resolved band structure of C3B/C3N HTS and (c) 3D charge density difference for the C3B/C3N HTS with respect to C3B and C3N monolayer. Brown, green, and gray atoms represent C, B, and N atoms, respectively. Yellow and cyan isosurfaces represent electron accumulation and depletion in the 3D space with an isovalue of 0.001 e/Å3.

band structures, as shown in Figure 3. It was found that the dominant contribution to the highest occupied orbital (VBM) in the first BZ is from the B_pz and C_pz orbitals of C3B, while the lowest unoccupied orbital (CBM) mainly consists of N_pz and C_pz orbitals of C3N (Figure 3a,b). The CBM and VBM of the four HTS are contributed by the C3B and C3N layers, respectively, suggesting a good charge separation. More interestingly, we notice that the VB and CB edges are inverted around the M point. Figure 3 c presents a 3D charge density difference for the C3B/C3N HTS with respect to C3B and C3N monolayer. Clearly, the electron-rich and hole-rich regions are distributed within C3B and C3N layers, respectively, indicating the significant flow of electrons from the N-doped layer to the B-doped one. To understand the origin of the band inversion presented in Figure 3a,b, we further scrutinized the band structure of 2D C3B/C3N HTS. We found that the two parabolic Dirac cone bands around the Fermi level resemble gated bilayer graphene22 (Figure s2c). If an external electric field is applied perpendicular to the gapless bilayer graphene (Figure s2b), then a band gap opens, as demonstrated in recent experiments.28,29 The band gap strongly depends on the strength of the perpendicular electric field and can reach values of 0.2 to 0.25 eV at a high field strength.30,31 In C3B/C3N HTS, stacking of C3N on C3B gives rise to an electrostatic potential difference between C3B and C3N layers, leading to an intrinsic electric field pointing from C3B layer to the C3N layer. The effective electrical potential difference in 2D vdW C3B/C3N HTS was further calculated by subtracting the total potential of single-layer C3B and monolayer C3N from that of 2D C3B/C3N HTS and then taking an average in the plane along the Z axis, as shown in Figure 4. The electric potential difference of the HTS exhibited a repeated sawtooth-like profile along the Z axis. When stacking C3B and C3N together, the induced largest band gap was as high as 0.23 eV, which is nearly the same order as that for gated bilayer graphene, suggesting that a strong built-in electrical field played a critical role in opening the large band gap. This built-in electric field is calculated to be ∼0.4 V/Å based on eq 1,30 where V2−1 is the planar average of the DFT-calculated electrostatic potential energy difference between C3B and C3N layers (Figure 4). It should be noted that a previous study reported that the application of an external electric field on fewlayer phosphorene can also induce a specific transition from the normal insulator to a band-inverted topological insulator.32

Figure 4. Average of 3D electrostatic potential difference between 2D C3B/C3N HTS and separate C3B, C3N monolayers along the Z axis. The red and green dots denote the positions of C3N and C3B layers, respectively.

Here the built-in electric field (∼0.4 V/Å) leads to the band inversion in 2D C3B/C3N HTS. To the best of our knowledge, this is the first demonstration of band inversion in HTS without an external electrical field, which would avoid the adverse effect on the stability of gated bilayer structures.

E=

1 dV2 − 1 e dz

(1)

The large charge transfer from C3N layer to C3B layer also suggests a strong interlayer coupling.12,13 To shed new physical insight into the interlayer coupling of the C3B/C3N HTS, a simple TB model was constructed33 by incorporating two nearest-neighbors in-plane (intraplane) hopping parameters γ0 and γ5 and four out-of-plane (interlayer) hopping parameters γ1, γ2, γ3, and γ4. The Hamilton can be written as ⎡ ϵ γ0*f1* γ1 γ2*f1 ⎤ ⎢ 1 ⎥ ⎢ γ *f ⎥ γ f γ f ϵ * * 2 0 1 3 1 4 2⎥ H = ⎢⎢ γ γ3*f1* ϵ3 γ5*f1 ⎥ ⎢ 1 ⎥ ⎢ ⎥ * * * ⎣ γ2*f1 γ4*f 2 γ5*f1 ϵ2 ⎦

(2)

Here we only discuss the interlayer coupling parts that refer to the upper-right and lower-left square 2 × 2 blocks of H. We fit our HSE bands in the first BZ with our TB model, as shown in Figure s4. The calculated parameter γ1 = −2.5 eV demonstrated that the vertical coupling between N_pz and B_pz is strong. The other interlayer couplings γ2, γ3, and γ4 are nonzero parameters, which also indicated that the interlayer coupling is 860

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HTS with strong interlayer electron coupling that promises exciting applications in novel electronics and photoelectronics. The calculations were performed using DFT within generalized gradient approximation of the Perdew−Burke− Ernzerhof (PBE) functional, as implemented in the Vienna ab initio simulation package (VASP).34−36 The hybrid functional method based on the Heyd−Scuseria−Ernzerhof (HSE)37 was adopted to accurately calculate the band structure. A dispersion correction of total energy (DFT-D3 method)38 was used to incorporate the long-range vdW interaction. To study 2D systems under the periodic boundary condition, a vacuum layer with a thickness 18 Å and a plane-wave basis set with an energy cutoff of 500 eV was set to minimize artificial interactions between neighboring layers. The structures studied here were fully relaxed until energy and force converged to 10−6 eV and 0.001 eV/Å, respectively. The band structures of C3B, C3N and C3B/C3N HTS were calculated with Monkhorst k-point meshes of 0.0015 Å−1. Electron−hole interaction is accounted by solving the BSE within the Tamm−Dancoff approximation39,40 on top of single-shot G0W0 procedure.

significant (further details are supplied in the Supporting Information). When 2D vdW C3N/C3B HTS are exploited for optoelectronic devices, both C3B and C3N would be expected to suffer from poor light emission due to the large indirect band gaps. Encouragingly, the HTS experiences an intriguing indirect-todirect band gap transition when stacking C3B and C3N layer together, which is expected to significantly enhance the production of photocurrent. The optical absorption spectrum was then calculated by solving the G0W0+BSE for 2D C3B, C3N, and C3N/C3B HTS, as shown in Figure 5. Clearly, four

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b03449. Band structure of monolayer C3N and C3B calculated by the PBE exchange correlation functional; lattice parameters, relative energy, interlayer distance, and band gap of 2D vdW C3N/C3B HTS in different stacking order; band structures for monolayer/bilayer graphene; and more details of our TB model fitting with HSE results. (PDF)

Figure 5. (a−c) Calculated optical adsorption spectra for 2D C3B, C3N, and C3B/C3N HTS, respectively, by the BSE approach. The black and red lines indicate the intralayer and interlayer, respectively.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

additional adsorption peaks (named the A, B, C, and D) in the low-energy regions emerged for 2D vdW C3N/C3B HTS, and these were mainly attributed to interlayer optical transition between VBM and CBM around M point. The peaks from 2 to 5 eV are in good agreement with the absorption peaks of singlelayer C3B and C3N, respectively, indicating that the optical characteristics of the monolayers are expected to be retained in the HTS. Thus the HTS is capable of extending absorbing light ranging from the near-infrared through the ultraviolet. It should be noted that the mixing of optical transitions is enhanced over a simple summation of the individual monolayer spectra and furthermore that the electron and hole are localized to different layers, which suggests a significant electron−hole recombination lifetime from these exciting new hybrid materials.26 In conclusion, on the basis of first-principles studies, we have systematically investigated the structural, electronic, and optical properties of 2D vdW C3B/C3N HTS. The strong interlayer electron coupling between 2D C3B and C3N induces a large built-in electric field, which gives rise to an intrinsic band inversion in the HTS. The stacking of semiconducting C3B and C3N monolayers also exhibits the potential for enhanced photocurrent production and significantly reduces charge recombination. Most importantly, C3B and C3N monolayers have already been synthesized in experiments, and thus assembling C3B/C3N van der Waals HTS is highly feasible.3 Our findings portray the 2D vdW C3N/C3B HTS as a prototypical example of atomically thin 2D semiconducting

ORCID

Steven Bottle: 0000-0003-0436-2044 Aijun Du: 0000-0002-3369-3283 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.D acknowledges the financial support by Australian Research Council under Discovery Project (DP170103598) and computer resources provided by high-performance computer time from computing facility at the Queensland University of Technology, NCI National Facility, and The Pawsey Supercomputing Centre through the National Computational Merit Allocation Scheme supported by the Australian Government and the Government of Western Australia.

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