Photoninduced charge redistribution of graphene determined by edge structures in the infrared region

Journal Pre-proof Photoninduced charge redistribution of graphene determined by edge structures in the infrared region

Jian Chai, Xijiao Mu, Jing Li, Liangxin Zhu, Kunpeng Zhai, Mengtao Sun, Yuee Li PII:

S1386-1425(19)31248-X

DOI:

https://doi.org/10.1016/j.saa.2019.117858

Reference:

SAA 117858

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date:

15 March 2019

Revised date:

21 November 2019

Accepted date:

23 November 2019

Please cite this article as: J. Chai, X. Mu, J. Li, et al., Photoninduced charge redistribution of graphene determined by edge structures in the infrared region, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy(2019), https://doi.org/10.1016/ j.saa.2019.117858

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© 2019 Published by Elsevier.

Journal Pre-proof

Photoninduced Charge Redistribution of Graphene determined by Edge Structures in the Infrared Region

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Jian Chai,1,+ Xijiao Mu,2,+ Jing Li,3 Liangxin Zhu1, Kunpeng Zhai,2 Mengtao Sun2,* , Yuee Li1,* School of information science and engineering, Lanzhou University, China

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School of Mathematics and Physics, Beijing Key Laboratory for Magneto-Photoelectrical

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Composite and Interface Science, University of Science and Technology Beijing, Beijing 100083,

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute

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China

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of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China Contributed Equally.

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Corresponding Authors. Email: [email protected] (Y. E. Li) and [email protected] (M.

ABSTRACT

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T. Sun).

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By using the ab initio density-functional theory method, we investigated the charge redistribution of monolayer graphene with ZigZag and/or ArmChair edges upon infrared excitation. The photoinduced charge redistribution is strongly dependent on edge types. The priority of electrons transfer has been revealed by charge density difference. To further investigate the influence of edge types on optical properties, the dielectric constants and absorption coefficient of graphene with various edge types have been calculated. The edge types have a non-negligible influence on

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Journal Pre-proof optical properties of graphene, and the Zigzag edge graphene owns stronger optical absorption in infrared region. Our results are potentially beneficial for designing graphene nanodevices in the infrared region. 1.

INTRODUCTION

Due to its atomically thin nature and excellent optoelectronic properties, such as ultra-high carrier mobility [1,2], high optical transparency [3] (with an absorptivity of 2.3% in visible and

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Near-Infrared regions), ultrawideband tenability [4] and strong nonlinearity [5-7], the graphene

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has witnessed many exciting applications for design of optoelectronic devices ranging from

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visible to THz frequencies. Notably, the growing application of graphene in infrared photonics

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has been shown in numerous results, including photodetectors [8-12], graphene plasmonics [13-16], optical modulator [17-20], optical frequency converters [21,22] and ultrafast pulse laser

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[23,24].

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In the course of designing nano-scale devices, it is found that the graphene nanostructures behave distinctly due to minor deviations of edges [25,26]. For example, the observation of

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surface plasmons in graphene nanoribbons revealed that the graphene with different edges

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presented different plasmonic resonance (greater plasmon broadening for the ZigZag edge than the ArmChair edge) [27-32]. The edge of graphene, as the surface of a 3D crystal, plays a crucial role in the determination of its physical, electronic and chemical properties [26]. With the development of nano technology, the graphene with well-defined edges was prepared and the edge effects have been widely investigated experimentally [33-36]. That is, the edges of finite size graphene play an important role in giving unconventional electronic structure, which demonstrates different physical properties [37-39]. It is primarily important to investigate the physical mechanism of edge effects in specific working wavelength for further understanding the

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Journal Pre-proof finite-size effect and flexibly designing graphene-based nanodevices. To my best knowledge, the physical mechanism of photoinduced charge redistribution on electronic transitions has not been well revealed for single layer graphene with ZigZag or ArmChair edge in the infrared (IR) region. In this paper, we try to explore how the charge redistribution is dependent on the edge structure of graphene by analyzing the charge density difference. We have shown that how the

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edge type determinates the occurrence of electrons transfer. The priority of electrons transfer

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orientation upon optical excitation with ZigZag and/or ArmChair edges is revealed for graphene

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in IR region. And our calculation indicates that Zigzag edge graphene owns excellent optical

COMPUTATIONAL DETAILS

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absorption properties in infrared region.

Gaussian 09 software is used for calculations of the charge density difference between the

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excited and ground states for graphene with different edges in IR region [40]. The geometry of

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graphene is optimized with density functional theory (DFT) [41], B3LYP [42] functional and 6-31G(d) [43] basis set. The electronic transitions of graphene were calculated with time

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dependent DFT (TD-DFT) [44], CAM-B3LYPfunctional and 6-31G(d) basis set. The long-range-corrected functional (CAM-B3LYP) was employed for analyzing the long-range part of exchange functional. The electrons transfer on the electronic transition is visualized with charge density difference [45]. Calculations for optical parameters of graphene in IR region were performed using the Vienna Ab initio Simulation Package (VASP) [46] with the projector augment wave (PAW) method [47]. The exchange and correlation potential were described with the Perdew Burke Ernzerhof (PBE) [48] the generalized gradient approximation (GGA) [49]. A Gamma centered grid

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Journal Pre-proof of 3×3×1 mesh for K-points is sampled. The cutoff energy for electronic wave functions was set to

450 eV. The convergence criteria of energy and force were set to 10-5eV/atom and 0.01eV/Å during self-consistent calculations. The Oscillator strength (f ) [50] is related to the molar absorption coefficient ε(ν) [51] 2𝑚𝑒

f0k =

3ħ2

(𝐸𝑘 − 𝐸0 )|𝒓0𝑘 |2

4𝑚𝑒 𝑐 0

=

3𝑁𝐴 𝑒 2

∫ 𝜀(𝜈) 𝑑𝑣

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Where, Ek - E0 represents the energy difference between the kth excited and ground states, me is the electron mass, and 𝒓0𝑘 = 0 |𝒓|𝑘  is the transition dipole matrix element between the ground state |0  and the kth excited state |𝑘 . In order to investigate the edge effects, four possible single layer graphene models were selected for consideration. The first model is a right triangle (the acute angles are 60ºand 30º)

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with one ArmChair edge and two ZigZag edges, named a1,b1,c1 respectively (Fig.1(a)). The

C2

respectively (Fig.1(b)). The other two models are the

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ArmChair edges, named a2, b2,

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second model is a right triangle (the acute angles are 60ºand 30º) with one ZigZag edge and two

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equilateral triangles that own three ArmChair edges (Fig. 1(c)) or three ZigZag edges (Fig. 1(d)).

Fig. 1. The models of single layer graphene with different edge types: (a) the right triangle graphene with one ArmChair edge and two ZigZag edges; (b) the right triangle graphene with one ZigZag edge and two

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3.

RESULTS AND DISCUSSION

3.1 Optical properties for the right triangle graphene in IR region For the graphene model shown in Fig. 1(a), we calculated the electronic transitions in IR region

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and the optical absorption spectrum is shown in Fig. 2(a). There are four optical absorption peaks, among which two strong ones are observed at 2595.7 nm and 2171.4 nm, and corresponding

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oscillator strength (f) are 0.059, 0.025 respectively. The charge density difference (CDD) [45]

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between excited state and ground state is used to study the alteration in the electronic structures

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of a system upon excitation. The 3D distribution of holes and electrons for these four electronic

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transitions is visualized with CDD, as shown in Fig. 2(b). CDD at 2595.7 nm in Fig. 2(b) reveals the electrons (red color) transfer from c1 edge (ZigZag) to the a1 edge (ArmChair) and the inner

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part; while CDD at 2171.4 nm reveals that the electrons transfer from b1 and c1 edges (ZigZag)

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to the inner part and the a1 edge (ArmChair). Two absorption peaks at 1338.2 nm and 1270.8 nm are very close because the energy difference between these electronic transitions is about 0.05 eV.

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CDDs show that the electrons transfer for these two transitions is in the opposite direction. Further, in IR region from 2000 nm to 3000 nm, all electrons transfer from the ZigZag edge to the inner part or the ArmChair edge. The graphene plasmon produced by excitation coupling between photon and electrons on electronic transitions can result in the strong electron-hole separation. However, in IR region from 1000 nm to 2000 nm, two degenerated electronic transitions own the opposite electrons transfer orientation, which will break the electronic collective oscillations. Consequently, the graphene plasmon produced by excitation coupling between photon and electron is destroyed by the opposite orientations of electrons transfer.

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Fig. 2. (a) Calculated absorption spectra of the first model in IR region by using the

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TD-DFT/CAM-B3LYP/6-31G(d) calculations of excited transitions (the half width of peak is set as 50 cm-1); (b) Hole (green) and electron (red) distributions corresponding electronic transitions. ( λ is the wavelength and f is the Oscillator strength.)

In addition, we calculated electronic transitions for the second model with one ZigZag (a2) edge and two ArmChair(b2,c2) edges in Fig. 1(b). Its optical absorption spectrum and electronic transitions are shown in Fig.3 (a) and Fig.3 (b). It is found that 4059 nm is associated with S1 state (f=0.0888). CDD reveals that the holes are also localized on the ZigZag edge (a2), while electrons transfer to the ArmChair (b2, c2) edges and the inner part, as well as the farthest rings of

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Journal Pre-proof the ZigZag edge. Therefore, for both two cases in Fig. 2(b) and Fig. 3(b)), the photoinduced

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electrons show a tendency transferring from ZigZag edges to ArmChair edges and the center.

Fig. 3. (a) Calculated absorption spectra of the second model in IR region by using the

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TD-DFT/CAM-B3LYP/6-31G(d) calculations of excited transitions (the half width of peak is set as 50 cm-1); (b) Hole (green) and electron (red) distributions corresponding electronic transitions. (λ is the wavelength and f is the Oscillator strength)

Nature of optical properties of graphene with different edges To further explore electronic transitions between ZigZag edges and ArmChair edges of the single layer graphene, the optical absorptions of the other two models in Fig. 1(c) and (d) are calculated. It is found that no optical absorption occurs for the graphene with ArmChair edges in IR region. The first four electronic transitions have been calculated, which are in the visible

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Journal Pre-proof region. CDD revealed that the ArmChair edge is not excited, and the electrons and holes are localized in the center, see Fig. 4. Due to the high symmetry of the structure, the distributions of

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electron-hole are almost the same, which belong to the local excitations.

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Fig. 4. Calculated absorption spectra of equilateral triangles that own three ArmChair edges in visible region

as 50 cm-1)

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by using the TD-DFT/CAM-B3LYP/6-31G(d) calculations of excited transitions (the half width of peak is set

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However, the equilateral triangle single layer graphene with three ZigZag edges is different.

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We calculated optical absorption spectrum and electronic transitions in IR region, as shown in Fig. 5(a), (b). Four strong electronic transitions clearly show up in IR region. CDDs Fig. 5(b) informs that the photoinduced charge redistribution occurs for these electronic transitions. In addition, it can be seen that all the holes are localized on the ZigZag edge of graphene, and the electrons are delocalized on both center and edge of graphene.

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Fig. 5. Calculated absorption spectra of equilateral triangles that own three ZigZag edges the in IR region by using the TD-DFT/CAM-B3LYP/6-31G(d) calculations of excited transitions (the half width of peak is set as 50 cm-1); (b) Hole (green) and electron (red) distributions corresponding electronic transitions. (λ is the wavelength and f is the Oscillator strength)

Therefore, for graphene nanostructure, the edge type of ZigZag and/or ArmChair determines whether the graphene can be optical electronic state excited in IR region. At the same time, the edge type also governs where the excitation occurs, on the edge or the inner part. Our calculation gives a deep insight into previous experimental results in theoretical way [52-57].

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Journal Pre-proof More importantly, the electronic transition revealed the degree and orientation of the electrons transfer for graphene with ZigZag and/or ArmChair edge structures. 3.3 Optical parameters To further investigate optical properties of single layer zigzag-armchair graphene above, the complex dielectric constant () = 1()+ i2()of graphene with edge structures in Fig. 1(c) (d) are calculated, as shown in Fig. 6. This curve shows similar trend in the mid-IR region with our

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previous publication46. Notably, the graphene with ZigZag edges presents relatively large ε2(Fig.

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6(e)), which means that much larger losses must be considered for the graphene with ZigZag

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edges in IR region, which is very important for designing optical devices based on graphene.

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Fig. 6. Real part (1) of the dielectric function of Equilateral triangular graphene with: (a) zigzag, (c) ArmChair

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edges; Imaginary part (2) of the dielectric function of Equilateral triangular graphene with: (b) zigzag, (d)

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ArmChair edges. (e) Imaginary part (2) of equilateral triangle graphene with zigzag edges is compared

with Imaginary part (2) of ArmChair edges

1/2 4𝐸 (21 +22 )1/2 −1   ℎ𝑐 2

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α(E)=

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Furthermore, the absorption coefficient [α(E)] can be evaluated by using the expression [58]

Where, () = 1()+ i2() is the frequency-dependent complex dielectric function, ε1 and ε2

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represent the real and imaginary parts, respectively. Because of the structural anisotropy, the optical absorption of single layer graphene was expected with α(E)xx = α(E)yy ≠ α(E)zz.

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Correspondingly, the absorption coefficient was divided into two parts: parallel (α) and

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perpendicular (α) to the plane of single layer graphene. The optical absorptions for the graphene with two different edges have been calculated, as shown in Fig. 7. The results indicate that, in the IR region, the graphene with ZigZag edges presents strong absorption while the ArmChair edge graphene shows weak absorption. Moreover, comparing two absorption spectra, the absorption peak of ArmChair edge graphene locates at higher energy in visible region. This confirms the results of charge difference density analysis above (Fig.4).

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Fig. 7. Comparison of absorption spectrum of equilateral triangle graphene with ZigZag and ArmChair edges

CONCLUSIONS

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In this paper, by analyzing the charge density difference based on DFT analysis, we demonstrate the

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photoinduced charge redistribution of the graphene with various edge types in IR region. The nature of

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edge structures determinates the orientation of photoinduced electrons transfer in this region. Moreover,

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the complex dielectric constant and the absorption spectrum confirm and extend our conclusions deduced by charge density difference analysis. Our results can not only promote deeper understanding of the

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light-graphene interaction for the region with specific edges, but also can inspire the graphene’s potential applications for designing nanophotonic devices in IR region.

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ACKNOWLEDGMENT

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This work was supported by the National Science Foundation of China (Grant No. 61405083 91436102, 11374353) and the Fundamental Research Funds for the Central Universities (lzujbky-2018-129). REFERENCE: [1] K. I. Bolotin, K. J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kim and H. J. S. S. C. Stormer, 2008, 146, 351-355. [2] J. Wang, X. Mu, X. Wang, N. Wang, F. Ma, W. Liang and M. J. M. T. Sun, 2018, 5, 29-57. [3] K. F. Mak, L. Ju, F. Wang and T. F. Heinz, Solid State Communications, 2012, 152, 1341-1349. [4] F. Bonaccorso, Z. Sun, T. Hasan and A. C. Ferrari, Nature Photonics, 2010, 4, 611-622. [5] S. A. Mikhailov and K. Ziegler, Phys Rev Lett, 2007, 99, 016803.

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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Graphical Abstract

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The charge redistribution upon infrared excitation on edge types (ZigZag and/or ArmChair) of monolayer graphene was investigated using the ab initio density-functional theory method. The electrons transfer priority upon optical excitation has been revealed. The Zigzag edge graphene shows stronger optical absorption in infrared region.

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Highlights

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The photoinduced charge redistributions of the graphene. The edge structure determinates photoinduced electrons transfer. The Zigzag edge structure owns stronger optical absorption.

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