Anomalous Phonon Modes in Black Phosphorus Revealed by

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Spectroscopy and Photochemistry; General Theory

Anomalous Phonon Modes in Black Phosphorus Revealed by Resonant Raman Scattering Xingzhi Wang, Nannan Mao, Weijun Luo, Hikari Kitadai, and Xi Ling J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01098 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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The Journal of Physical Chemistry Letters

Anomalous Phonon Modes in Black Phosphorus Revealed by Resonant Raman Scattering Xingzhi Wang,a, ‡ Nannan Mao,a, b, ‡ Weijun Luo,a Hikari Kitadai,a Xi Linga ,c, d∗ a

Department of Chemistry, Boston University, Boston, MA 02215, United States

b

Department of Electrical Engineering and Computer Science, Massachusetts Institute of

Technology, Cambridge, MA 02139, United States c

Division of Materials Science and Engineering, Boston University, Boston, MA 02215,

United States d

The Photonics Center, Boston University, Boston, MA 02215, United States

∗

To whom the correspondence should be addressed.

‡

These author contributed equally to this paper.

Email address: [email protected] Phone: +1-617-358-8584

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Abstract Black phosphorus (BP), a layered material with puckered crystalline structure in each layer, drew intense interest due to its unique optical and electronic properties. In particular, the intricate Raman scattering effect in BP is intriguing and provides a platform for researchers to probe the physical properties of BP in depth. Here we report the first observation of anomalous modes with the frequency in the range of 100-900 cm-1 in BP due to the resonant Raman effect. The origin and assignment of the anomalous modes are discussed based on the excitation energy and angle-dependent Raman measurements. Density function theory (DFT) calculated electronic band structure is used to support our understanding. The newly observed phonon modes could serve as a unique probe for the fine electronic structures and the exciton-phonon couplings, which promotes a better understanding of BP for potential nanoelectronic and nanophotonic applications in the future. Table of Contents (TOC) Graphic

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Since the discovery of graphene, two-dimensional (2D) atomic layered materials emerged due to their unique electronic and optical properties in their ultrathin layers and bulk counterparts.1-8 Among the members in the 2D materials family, orthorhombic black phosphorus (BP) has a layered structure similar to graphene and MoS2-like transition metal dichalcogenides (TMDs), but with a puckered single-layer geometry.9-10 A variety of studies show that BP has unique advantages, including the thickness-dependent direct band gap from visible to infrared region,9, 11-12 relatively high carrier mobility (~1000 cm2/ V-1·S-1)13 and anisotropic properties,14-17 which render BP promising potential in nanoelectronics and nanophotonics.12-13,

18

Despite the intriguing properties, intricate

phenomenon in BP keeps being observed in recent years.16, 19-20 For example, Wang et al. reported highly anisotropic and strongly bound excitons in monolayer BP,16 and the integer quantum Hall effect was observed in BP 2D electron systems.20 Those phenomena are attributed to the unique electron and phonon behaviors in BP crystal. Therefore, probing the fine electronic structure and phonon structure is crucial to understand the novel properties of BP and lead to future applications. Raman spectroscopy is a powerful characterization tool to probe the behaviors of phonons in materials.21 It has been successfully used to reveal the structure and properties of graphene, TMDs and other 2D materials.22-24 In recent years, Raman spectroscopy has exhibited a unique feature in characterizing the structure and revealing the physics in BP. The three intralayer vibrational modes Ag1 (362 cm-1), B2g (440 cm-1) and Ag2 (467 cm-1) were widely reported and their strong laser polarization dependence was intensively studied.10,

25-30

In particular, the anisotropic Raman scattering was found strongly

dependent on the excitation wavelength, phonon modes and thickness of BP, which was intricate and the origin was discussed by several groups..10,

15, 26

Moreover, Davydov

splitting of the Ag modes were observed in few-layer BP due to the strong interlayer coupling.25 Recently, Ribeiro et al. observed the B1g and B3g at the edge of the BP flakes, which was attributed to the structural relaxation of the BP crystal.31 Favron et al. reported the defect induced second-order Raman modes near Ag1 and Ag2 modes.32 Besides, lowfrequency interlayer vibrational modes were observed and their strong thickness dependence was demonstrated.29, 33 Of particular note, Raman spectrum can provide the information about the fine electronic structure of materials through resonant Raman 3 ACS Paragon Plus Environment


scattering.22-23, 34-35 Especially, forbidden modes may be observed due to the breakdown of selection rules.34-36 For example, with suitable excitation energy closed to the exciton absorption of the material, A2u and LA mode were observed in bulk MoS2,37 and several forbidden interlayer modes were resolved in few-layer WS2.38 In this work, benefitting from multiple laser lines equipped in the Raman system, we are able to measure the Raman spectra under diverse excitation wavelengths. It is found that when using particular excitation wavelengths for the measurement, several anomalous modes in BP flakes can be resolved in the Raman spectra. Systematic studies suggest that resonant Raman effect plays an important role in this observation based on the systematic study of the azimuthal-angle-dependent and excitation-dependent Raman spectroscopy of BP, as well as the first-principle calculations. Moreover, the resonant excitation profile reveals that different phonons couple with different electronic states in BP, which offers an advanced probe for the fine electronic structure of BP. The BP flakes were mechanically exfoliated on a fused quartz substrate using Scotch Tape (Figure S1a). To prevent degradation of BP flakes, a thin layer of PMMA was spincoated on top of the prepared samples immediately after the atomic force microscopy (AFM) measurement of the thickness. Figure S1b shows an investigated BP flake with a thickness of around 65 nm (Figure S1b). All the Raman spectroscopy measurements in this work were conducted in a backscattering geometry and parallel configuration where the incident laser polarization (PL) was parallel to the polarization of the collected scattered light (Figure 1a). Because bulk orthorhombic BP belongs to the D2h point group (Figure 1b and 1c), it has 12 irreducible zone-center phonon modes denoted by Γ = 2Ag + B1g + B2g + 2B3g + Au + 2B1u + 2B2u + B3u, among which, Ag, B1g, B2g and B3g are Raman-active modes.36 When using a backscattering geometry with the incident laser propagating along b axis of the crystal for Raman measurement (which is a common configuration in most measurements), only three phonon modes, Ag1 (362 cm-1), B2g (440 cm-1) and Ag2 (467 cm-1), can be resolved in the typical Raman spectrum of BP. These three modes have been widely studied in the previous literature.10,

26-27

In our experiment, however, when the excitation laser

wavelength was 496.5 nm, 17 additional anomalous Raman modes were clearly resolved 4 ACS Paragon Plus Environment

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in the Raman spectra (Figure 1d) besides the three ordinary modes (Ag1, B2g and Ag2). The intensities of these modes were 1 to 2 orders weaker than that of Ag2 mode. Figure 1d also shows the spectra with the laser polarization along armchair and zigzag direction of BP. Obvious differences were found when comparing the two spectra, indicating the anisotropic properties of these anomalous modes. Based on the calculated phonon dispersion relation in literature,17 the frequencies and assignments of each mode were listed in Table S1. Systematic studies on the angle-dependent and excitation wavelengthdependent Raman spectra of the anomalous modes were conducted to confirm the assignments in later parts. It was widely reported that the intensities of Ag modes and B2g mode varied when the laser irradiated the sample with different polarization due to the anisotropic crystalline structure of BP.26-27, 29 This phenomenon was also observed in this work (Figure S3). Since the maximum intensity of the Ag modes can switch between zigzag and armchair in different sample thicknesses or excitation energies,15 careful identification should be carried out to obtain the crystal orientation. In this work, the crystal orientation of investigated sample was determined by the azimuthal-angle-dependent intensity of Ag mode, compared with the result in the literature using the same excitation and similar sample thickness.10 In this work, the azimuthal angle θ was defined as the angle between the armchair direction of BP and the laser polarization direction. Here, we focused on investigating the anisotropy of the newly observed anomalous phonon modes. As shown in Figure 2a, most of the anomalous peaks showed a strong dependence on the azimuthal angle. The anomalous peaks were labeled by letters from “A” to “H”. It was observed that some peaks (e.g. peak B, C and D) could be clearly observed at a certain angle, while they vanish when the sample was rotated by 90°. The angle-dependent Raman intensities of 8 anomalous peaks were plotted in Figure 2b-2i. All of them exhibited 2-fold symmetry with the maximum intensity along either zigzag or armchair direction of BP. Besides, the peak located at 867.2 cm had two secondary peaks, which behaved similarly to the Ag2 peak (Figure 2i and S3d). Anomalous phonon modes were also observed in many other materials, and various origins were discussed, including the impurities and defects in the material,39-40 5 ACS Paragon Plus Environment


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deformation of crystal structure,31 and resonant Raman effect23, 34, 38, 41. In this study, the bulk BP crystal for preparing BP flakes was newly purchased (

Anomalous Phonon Modes in Black Phosphorus Revealed by

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