van Hove Singularity Enhanced Photochemical Reactivity of Twisted

Letter pubs.acs.org/NanoLett

van Hove Singularity Enhanced Photochemical Reactivity of Twisted Bilayer Graphene Lei Liao,† Huan Wang,† Han Peng,‡ Jianbo Yin,† Ai Leen Koh,§ Yulin Chen,‡ Qin Xie,† Hailin Peng,*,† and Zhongfan Liu*,† †

Center for Nanochemistry, Beijing Science and Engineering Centre for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡ Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, U.K. § Stanford Nano Shared Facilities, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: Twisted bilayer graphene (tBLG) exhibits van Hove singularities (VHSs) in the density of states that can be tuned by changing the twist angle (θ), sparking various novel physical phenomena. Much effort has been devoted to investigate the θ-dependent physical properties of tBLG. Yet, the chemical properties of tBLG with VHSs, especially the chemical reactivity, remain unexplored. Here we report the first systematic study on the chemistry of tBLG through the photochemical reaction between graphene and benzoyl peroxide. Twisted bilayer graphene exhibits θ-dependent reactivity, and remarkably enhanced reactivity is obtained when the energy of incident laser matches with the energy interval of the VHSs of tBLG. This work provides an insight on the chemistry of tBLG, and the successful enhancement of chemical reactivity derived from VHS is highly beneficial for the controllable chemical modification of tBLG as well as the development of tBLG based devices. KEYWORDS: van Hove singularity, twisted bilayer graphene, chemical modification, enhanced photochemical reactivity We here, for the first time, present a systematic study on the chemistry of tBLG with VHS through the photochemical reaction between graphene and benzoyl peroxide (BPO). The chemical reactivity of tBLG was found to be highly dependent on the twist angle (θ). When the energy of the incident laser matched the energy interval between two VHSs, enhanced reactivity was observed, which was attributed to the higher probability for both the excitation of electron in tBLG and the tunneling of electron from tBLG to BPO. The chemical reaction between graphene and benzoyl peroxide was utilized to investigate the reactivity of tBLG with different twist angles (Figure 1a). The 0.1% benzoyl peroxide solution in alcohol was spin-coated on the surface of tBLG. Then the laser of Raman spectrometer was utilized to induce the reaction. The power of 514, 633, and 785 nm lasers were 0.68, 1.02, 1.13 mW, respectively. The diameter of laser spot was ∼1 μm. Under the irradiation of laser, the phenyl free radicals dissociated from the BPO molecules covalently linked onto the surface of bilayer graphene, followed by the structural transformation of C atoms from sp2 to sp3 configuration.9,19,20

T

wisted bilayer graphene (tBLG), formed by stacking two graphene monolayers with a certain twist angle (θ), has great potential for the development of graphene devices because of its tunable interlayer coupling and band structure.1,2 In tBLG, the Dirac cones in two layers slightly shift relative to each other and then overlap, resulting in the reconstruction of band structure (Figure 1a).3,4 The bending of energy bands of the two graphene layers and the possible opening of bandgap at the point between two cones may give rise to a logarithmic divergence in the density of states (DOS), known as the van Hove singularity (VHS), one of the most attractive characteristics of tBLG.3,5 The VHS leads to intriguing properties including the enhanced optical absorption and G band resonance in tBLG.3,4,6 Despite the knowledge of electronic and optical characteristics of tBLG with VHS, the influence of VHS on the chemical property of tBLG is still lacking. Understanding the chemical property of tBLG with VHS, especially the chemical reactivity, is highly helpful to the controllable chemical modification of tBLG, which may provide a promising approach to tune the band structure and electronic properties of tBLG by breaking the symmetry through converting the hybridization of carbon atoms from sp2 to sp3.7−18 © XXXX American Chemical Society

Received: June 7, 2015 Revised: June 26, 2015

A

DOI: 10.1021/acs.nanolett.5b02240 Nano Lett. XXXX, XXX, XXX−XXX

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the edges and 12.3° from the SEAD result, showing a good consistency. Therefore, due to the convenience and uncompromised accuracy, twist angles of tBLG in this work were measured from the edges of the two hexagon layers (Figure 1b) unless otherwise indicated. The electronic band structure of tBLG could be approximated as a superposition of two single layer graphene bands with a relative rotation.3,4 The micro-ARPES data (Figure 1d) revealed clear linear dispersion of two Dirac cones from the two stacked monolayer graphene and the unambiguous emergence of a VHS when two Dirac cones intersected at higher electron binding energy. Due to the attenuation of photoelectrons caused by the screening of the top layer of graphene, the dispersions of the bottom layer graphene (the weaker dispersions of the Dirac cone on the right of Figure 1d) exhibited slightly lower intensity. Two Dirac cones were observed for tBLG with 18.6° and the energy interval between the VHSs (Δ) was 3.17 eV, agreeing well with the earlier calculations.5 Raman spectroscopy was performed to evaluate the quality of our CVD grown tBLG samples with different twist angles. Under the irradiation of 514 nm laser, the Raman characteristics of tBLG samples were found to be highly dependent on the twist angle (Figure 1e). Remarkably, 13° tBLG showed a very strong G band intensity. The systematic investigation of G band intensities of tBLG with different twist angles was summarized in Figure 1f. A peak (θpeak) appeared at ∼13° when using 514 nm laser as the light source. Such enhanced G band intensity was attributed to the G band resonance, which occurred when the energy of laser (Elaser) matched with the critical energy interval of tBLG’s VHSs. Altering the wavelength of incident laser resulted in the shift of resonated energy. Since VHS originated from the coupling effect of tBLG, which was influenced by the twist angle (θ), the energy interval of VHSs was also θ-dependent. Therefore, θpeak changed with the wavelength of incident laser. When the wavelength of incident lasers were changed to 633 and 785 nm, θpeak shifted to 10° and 8°, respectively. As shown in Figure 2a, with the photochemical reaction induced by 514 nm laser, D bands arose at 1340 cm−1, indicating the successful modification of tBLG.22 Specially, 13° tBLG showed the highest D band intensity, implying the most effective modification. Time evolution of Raman spectra was thoroughly carried out for tBLG samples with different twist angles and was summarized in Figure 2b. The D band intensity ascended as the reaction went on with the appearance of a distinctive red band around 13°, indicating the prominently higher phenyl group coverage for tBLG with twist angles around 13°. As clearly illustrated in Figure 2c, the photochemical reaction rate for 13° tBLG was also much faster, for which the reaction was completed within only ∼10 s. Such increased chemical reactivity was assumed to be resulted from the enhanced absorption of incident energy arising from the VHS of tBLG, which led to a high probability for the excitation of electron in tBLG and thus the higher reactivity. Since the enhancement of optical absorption was expected when the energy of the incident laser matched with the energy interval between two VHSs, the high reactivity band could be tuned by alerting the wavelength of the irradiating laser. As shown in Figure 2d,e, when using 633 and 785 nm lasers to trigger the photochemical reaction, the red bands shifted to 10° and 8°, respectively. Therefore, the reactivity for the twist angle of interest could be selectively enhanced simply by choosing

Figure 1. (a) Scheme of the reaction between tBLG and BPO. The right top image shows the structure of tBLG with a twist angle of θ. The right bottom image demonstrates the band structure of tBLG. The Dirac cones from each layer are separated in k space and interact with each other, leading to the bending of band structure and the possible opening of bandgap at VHSs. (b) The twist angle measured from the edge of the domain and the selected area electron diffraction (SAED) patterns. The left two columns are the optical images of CVD-grown tBLG transferred on a 90 nm-thick SiO2/Si substrate; the scale bars are 20 μm. The right top image is the transmission electrons microscopy (TEM) image of tBLG. The right bottom image is the SAED result of the sample in top right. (c) Typical high-resolution transmission electron microscopy image of tBLG. The inset is the fast Fourier transform (FFT) pattern, which reveals that the twist angle is ∼9°. The dashed line shows the Moiré pattern of twist angle, whose periodicity is ∼1.57 nm. (d) Microspot angle-resolved photoemission spectroscopy (micro-ARPES) image of 18.6° tBLG. The plot on the right is the integrated density of state (DOS) of the 2D band dispersions on the left. (e) The Raman spectra of tBLG with different twist angles before reaction under the irradiation of 514 nm laser. (f) The G band intensity of tBLG with different twist angles normalized with that of AB-stacked bilayer graphene.

Twisted bilayer graphene used in this work was grown by a chemical vapor deposition (CVD) method.21 The regular hexagon shape with smooth edges (Figure 1b) and the clear moiré pattern in the high-resolution transmission electrons microscopy (TEM) image (Figure 1c) confirmed the high quality of our tBLG samples. The twist angle of tBLG could be measured from either the angle between the edges of the two hexagon graphene layers or the selected area electron diffraction (SAED) result of TEM images. As shown in Figure 1b (the right column), the angles were measured as 12° from B

DOI: 10.1021/acs.nanolett.5b02240 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 2. (a) Raman spectra of tBLG with different twist angles after 20 s reaction between tBLG and BPO under the irradiation of 514 nm laser. (b,d,e) D band intensity evaluation of tBLG with different twist angles and reaction time under the irradiation of 514 nm (b), 633 nm (d), and 785 nm (e) laser, respectively; the step of twist angle is 1°, and the steps of time are 6 s (b), 10 s (d), and 1 min (e), respectively. (c) The D band intensity of tBLG with different twist angles after different reaction time under the irradiation of 514 nm laser. (f) The relationship between the D band intensity (normalized with that of AB-stacked bilayer graphene) and the twist angle (514 nm, 15 s; 633 nm, 30 s; 785 nm, 6 min).

Figure 3. (a,c,e) Raman spectra for modified 10° and 13° tBLG samples collected using a 785 nm laser. (b,d,f) Corresponding statistical counts of the ID/IG ratio collected from 50 samples. The photochemical reaction is induced by 514 nm (a,b), 633 nm (c,d), and 785 nm (e,f) lasers, respectively.

collecting laser (785 nm) was not able to resonate with either of the two tBLG samples, the larger ID/IG ratio of 13° tBLG could thus be attributed to the higher coverage of phenyl groups, which was derived from the higher reactivity under the irradiation of 514 nm laser. When using 633 nm laser to trigger the photochemical reaction, similar D bands were observed after 20 s of irradiation (Figure 3c). The 10° tBLG sample presented a higher ID/IG ratio than that of 13° tBLG, indicating a higher reactivity for 10° tBLG with the irradiation of 633 nm laser. The reaction rate here was 2 times slower than that under the irradiation of 514 nm laser because the lower incident energy was unfavorable for the electron transfer between tBLG and BPO.9 Therefore, when using 785 nm laser as the irradiation laser, which exhibited a further lower incident energy, it took a much longer time (2 min) to complete the reaction. D bands with similar intensities arose after irradiation of 785 nm laser (Figure 3e), and the average ID/IG ratios were of no big difference for 10° and 13° tBLG samples (Figure 3f), indicating the unbiased chemical reactivity when irradiated by a laser that could not resonate with the VHSs of either tBLG sample. In brief conclusion, by using 785 nm laser, which could not resonate with the VHSs of 10° or 13° tBLG sample, to collect Raman spectra, the sp3 defects introduced by chemical modification were responsible for the D band intensity. In this case, the ID/IG ratios were determined by the coverage of phenyl groups. Therefore, the higher coverage and the enhanced reactivity for 13° tBLG under irradiation of 514

the appropriate lasers (Figure 2f), highly facilitating the controllable modification and property modulation of tBLG. Unlike that of single layer graphene, the D band intensity of tBLG was not only determined by the coverage of sp3 defects, but also affected by the resonance between the incident laser and the VHSs of tBLG.23−25 Besides, the D-like band, which presented at a similar frequency to D band but was induced by the unavoidable presence of long-range defects (including charge impurities absorbed at the tBLG’s surface and strain induced during the superlattice formation), could also contribute to a high D band intensity.26 Fortunately, the Dlike band could only be observed in tBLG samples with large twist angles (>20°) (Figure S1).26 Therefore, it was reasonable that the influence of D-like band was negligible. To further rule out the influence of resonance, the 785 nm laser, which could not resonate with VHS of either 10° or 13° tBLG sample (Figure S2), was utilized to collect the Raman spectra. As shown in Figure 3a, 10° and 13° tBLG samples were first irradiated by 514 nm laser for 10 s to induce the photochemical reaction, and then the Raman spectra were collected using 785 nm laser with the acquisition time of 2 min. D bands at 1315 cm−1 arose after modification for both tBLG samples, and 13° tBLG exhibited a higher intensity, which was also confirmed by the statistical results (Figure 3b). Because the C

DOI: 10.1021/acs.nanolett.5b02240 Nano Lett. XXXX, XXX, XXX−XXX

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variable that determines the tunneling of hot electron is the distribution of hot electron, i.e., f(E, ℏω), which is dependent on the DOS of tBLG. When incident energy (Elaser) approaches the Δ of tBLG, a larger distribution of hot electron is obtained due to the larger DOS, resulting in the higher tunneling probability of hot electrons. Thus, a higher reactivity of tBLG could be expected. As in our study, with the irradiation of 514 nm laser, Elaser is approximately equal to the Δ of 13° tBLG, exhibiting the enhanced optical absorption (Figure S3) as well as the G band resonance (Figure 1f). The matching between Elaser and Δ consequently reaches a larger DOS and a greater contribution of parallel band transition, leading to the higher probability of electron excitation in tBLG. Meanwhile, the resonance between the incident laser and the VHSs of tBLG is also favorable for the tunneling of hot electrons into the unoccupied orbital of BPO. Therefore, the probability for both electron excitation and tunneling are increased; hence the photochemical reactivity of tBLG is enhanced. The alteration of the laser wavelength results in the change of energy matching point. Therefore, the twist angle exhibiting the highest reactivity shifts to 10° when using 633 nm laser and 8° when using 785 nm laser, respectively. In summary, for the first time, the influence of VHS on the chemical reactivity of tBLG has been comprehensively explored through the reaction between CVD grown tBLG and BPO, which can be explained by the hot electron transfer mechanism. When the incident energy matches with the energy interval of VHSs, the chemical reactivity of tBLG could be greatly enhanced because of the higher probability for both the excitation of electron in tBLG and the tunneling of electron from tBLG to BPO. This study on the angle-dependent reactivity of tBLG sheds light on the chemistry of tBLG, and we believe such selective enhancement of chemical reactivity provides an effective approach to controllable modification of tBLG with tunable band structure and electrical properties.

nm laser, as was the case for 10° tBLG under irradiation of 633 nm laser, was thus confirmed. The enhanced chemical reactivity derived from VHS of tBLG could be explained by the hot electron transfer mechanism.9 The UV−vis spectrum of BPO indicated that BPO was transparent in the range of visible light;27 hence, tBLG was the only light absorbing material in the reaction. According to the hot electron transfer reaction,9,28−31 graphene absorbs the light and excites the electron to the excitation state. Then the excited hot electrons transfer onto the lowest unoccupied molecular orbital (LUMO) orbital of BPO, resulting in the dissociation of BPO molecules. Thus, the dissociation of BPO is determined by two separate steps (Figure 4): (i) excitation of the hot electron of graphene; (ii) tunneling of hot electrons into the unoccupied orbital of the BPO molecule.32−34

Figure 4. Hot electron transfer mechanism of the dissociation of BPO.

For the first step of hot electron transfer reaction (i), the probability of electron excitation in tBLG is highly related to the DOS and the parallel band transition. The DOS of tBLG reaches a maximum when the incident energy (E) matches with the energy interval of the VHSs of tBLG (Δ), i.e., E = Δ. A large DOS at VHSs is favorable for a higher possibility of electron excitation. Meanwhile, when the incident energy matches with the energy of parallel band transition (a value similar to Δ), the parallel band transition could benefit for the excitation of electrons.3 Therefore, with an incident energy approximate to Δ, both DOS and the parallel band transition could effectively contribute to the higher possibility of electron excitation, resulting in the higher reactivity. In other words, with the presence of VHS, the absorption of incident energy could be greatly enhanced at a certain point where E = Δ, leading to the distinctive enhancement of the chemical reactivity of tBLG. For the second step of hot electron transfer reaction (ii), tunneling of hot electrons into the unoccupied orbital of BPO is described by eq 1:32 R(ω) ∝

∫E

Ef +ℏω f

2

f (E , ℏω)T (E) e−(Eaff − E)

/2s 2

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ASSOCIATED CONTENT

S Supporting Information *

Methods and contrast spectra of tBLG. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b02240.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: zfl[email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

dE

Notes

(1)

The authors declare no competing financial interest.

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where the f(E, ℏω) is the distribution of hot electron caused by the optical absorption and the possible multiple exciton generation process, which is affected by the DOS and thus varies with the change of twist angle; T(E) is the tunneling probability at energy E, determined by the potential barrier between tBLG and BPO, which is roughly constant for different 2 2 twist angles; the third term (e−(Eaff − E) /2s ) is related to the energy broadening of the LUMO orbital of BPO (Eaff) and could also be treated as a constant for BPO. Therefore, the only

ACKNOWLEDGMENTS We are grateful to Dr. Jinying Wang in our lab for her helpful discussions about the reaction mechanism and to Dr. Hongduan Huang for her help on manuscript preparation and discussion. We acknowledge financial support from the National Natural Science Foundation of China (Grants 51432002, 21222303, and 21473001) and the Ministry of Science and Technology of China (Grants 2014CB932500, D

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DOI: 10.1021/acs.nanolett.5b02240 Nano Lett. XXXX, XXX, XXX−XXX