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Observation and manipulation of visible edge plasmons in Bi2Te3 nanoplates Xiaowei Lu, Qunqing Hao, Mengjia Cen, Guanhua Zhang, Ju-Long Sun, Libang Mao, Tun Cao, Chuanyao Zhou, Peng Jiang, Xueming Yang, and Xinhe Bao Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00023 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018
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Observation and manipulation of visible edge plasmons in Bi2Te3 nanoplates
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Xiaowei Lu∆,#,†, Qunqing Hao§,†, Mengjia Cen¶, Guanhua Zhang§, Julong Sun§, Libang Mao¶,
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Tun Cao¶,*, Chuanyao Zhou§, Peng Jiang∆,*, Xueming Yang§, Xinhe Bao∆,*
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∆
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Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023,
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China
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§
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Physics, Chinese Academy of Science, Dalian, Liaoning 116023, China
State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience, Dalian
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical
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¶
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116024, China
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#
Department of Biomedical Engineering, Dalian University of Technology, Dalian, Liaoning
University of Chinese Academy of Sciences, Beijing 100049, China
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ABSTRACT
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Noble metals, like Ag and Au, are the most intensively studied plasmonic materials in
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the visible range. Plasmons in semiconductors, however, are usually believed to be in the
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infrared wavelength region due to the intrinsic low carrier concentrations. Herein, we observe
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the edge plasmon modes of Bi2Te3, a narrow-band gap semiconductor, in the visible spectral
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range using photoemission electron microscopy (PEEM). The Bi2Te3 nanoplates excited by
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400 nm femtosecond laser pulses exhibit strong photoemission intensities along the edges,
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which follow a cos4 dependence on the polarization state of incident beam. Due to the phase
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retardation effect, plasmonic response along different edges can be selectively exited. The
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thickness-dependent photoemission intensities exclude the spin-orbit induced surface states
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as the origin of these plasmonic modes. Instead, we propose that the interband
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transition-induced non-equilibrium carriers might play a key role. Our results not only
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experimentally demonstrate the possibility of visible plasmons in semiconducting materials,
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but also open up a new avenue for exploring the optical properties of topological insulator
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materials using PEEM.
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KEYWORDS
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Bi2Te3, edge plasmon, selective excitation, interband transition, PEEM
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Plasmonics play a significant role in nanophotonics,1,2 high-sensitivity detector3 and
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high-resolution imaging.4 The most widely studied plasmonic materials are noble metals,
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such as Ag and Au, which support strong electric field enhancements in the visible region.5 2
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However, because of the high resistive loss and lack of tunability of metals, it is imperative to
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search for novel semiconductors as alternative plasmonic materials.6,7 Due to the low carrier
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concentrations, the plasmonic resonance of semiconductors usually lies in the near-infrared
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and mid-infrared range.8-10 Notably, beyond the traditional doping scheme,8 the metal-like
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response can be achieved by non-equilibrium carriers induced by interband transition.11-14
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Specifically when the oscillator strength of the interband transition is strong enough, the
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negative real part of the permittivity will emerge on the higher frequency side of the
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maximum interband absorption, as illustrated in some organic plasmonic materials11,12 and
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p-block metals.13,14
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Three-dimensional (3D) topological insulators (TIs) are insulating materials with
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metallic surface states protected by time-reversal symmetry.15 In the past few years, many
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fascinating physical properties have been discovered in TIs, such as the quantum spin Hall
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effect,16 quantum anomalous Hall effect17 and Majorana fermions.18,19 Recently, the plasmons
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of TIs are also attracting great attention,9,20-23 considering the potential applications in the
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fields of nanoelectronics and nanophotonics. Several characterization methods, including
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electron energy loss spectroscopy (EELS),21,22 cathodoluminescence (CL) spectroscopy,21 and
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scattering-type scanning near-field optical microscope (s-SNOM),23 have been utilized to
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study the plasmon mode of TIs nanostructures from the ultraviolet to visible regions, which
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was proposed to originate from the combined contribution of the metallic surface states and
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non-equilibrium carriers induced the interband transition.9 These characterization techniques,
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however, have a difficulty in investigating the polarization-dependent behavior of plasmon,
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which is vital to confirm and understand the plasmonic properties. 3
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Femtosecond laser pulse excited photoemission electron microscopy (fs-PEEM) presents
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the promising tool for mapping the near-field distribution of plasmon mode at the nanometer
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scale.24-26 Multiphoton photoemission (MPPE) can be initiated due to the high peak power
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density of the femtosecond laser (fs-laser). For the non-resonance planar sample, the yield of
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MPPE is relatively low. On the contrary, the plasmon resonance can dramatically enhance the
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electric field, which, in turn, remarkably promote the photoelectron emission, because in the
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MPPE, the photoelectron yield is proportional to 2nth power of electric field intensity (E2n),
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where n is the number of the photons absorbed. In addition, for PEEM, it is facile to tune the
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incident laser to study the polarization-dependent behavior of plasmons. Furthermore, in
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comparison with s-SNOM, the absences of AFM tip and line-by-line scanning allows the
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fs-PEEM to operate in real time. These advantages render the fs-PEEM a powerful tool to
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observe and manipulate the plasmonic behaviors beyond the diffraction limit.
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In this work, we applied the 400 nm fs-laser excited PEEM to study the plasmonic
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modes on the vapor-solid-grown Bi2Te3 nanoplates with lateral size on the micron (µm) scale.
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Oscillatory plasmonic hotspots along the edges of nanoplates were clearly observed in both
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the experiment and theory. We also illustrated that such hotspots are strongly associated with
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the polarization of the incident photon, which can selectively excite the plasmonic hotspots
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along different edges. Systematic studies on the thickness dependence of the photoemission
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intensity indicates the plasmonic response of Bi2Te3 at λ = 400 nm is mainly associated with
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the interband transition-induced non-equilibrium carriers rather than the surface states.
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Bi2Te3 nanoplates with the different lateral sizes and thicknesses were synthesized by the
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well-studied vapor-solid (VS) mechanism (see Methods for details). Boron-doped Si(100) 4
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wafer with a resistivity less than 0.005 ohm·cm was chosen as the substrate to avoid charging
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effect during PEEM measurement. A scanning electron microscopy (SEM) operated in
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backscattered electron mode was employed to characterize the morphological and
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compositional information of the Bi2Te3 nanoplates (Figure 1a). The lateral size of nanoplates
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with the uniform composition is in the range of 1-15 µm. AFM topography indicates that the
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thickness of these nanoplates varies from several nanometers to hundreds of nanometers. The
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crystal quality was characterized by the micro-Raman spectroscopy. In Figure S1a, two
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vibrational modes of E 2g (101 cm-1) and A 21g (131.9 cm-1) can be clearly recognized.
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Furthermore, the Raman mapping of the peak intensity of E 2g
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demonstrates a uniform distribution, which suggests a high crystallinity.
mode (Figure S1b)
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Prior to the PEEM characterization, we experimentally investigated the optical
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properties of interconnected Bi2Te3 nanoplates by a variable angle spectroscopic ellipsometer.
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In order to minimize the perturbation of inhomogeneous thickness, the light spot was focused
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down to ~80 µm. Figure 1b displays the retrieved complex dielectric function across the
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spectral range from 240 to 1300 nm, from which three crucial features can be derived. Firstly,
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the connection between the real part (εr) and imaginary part (εi) of the permittivity follows the
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Kramers–Kronig relations.9,11-14 Secondly, the εi exhibits a peak with extremely high value (~
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46.5) at λ = 1120 nm, which results in the strong absorption of electromagnetic radiation
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originating from the interband transition at the Z-point in the Brillouin zone.27 Finally, the εr
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is negative in a spectral range from 240 to 798 nm, caused by the strong oscillator strength of
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the interband transition. The negative εr implies the possibility of plasmonic resonance in this
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region. Because of the low thermodynamic formation energies of intrinsic defects, Bi2Te3 5
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nanoplates grown by VS mechanism is unintentionally n-type doped with the bulk carrier
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density of the order of 1019 cm-3.28 According to the study on the surface plasmon of doped
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semiconductor,29 free carrier density with 1019 cm-3 only leads to the negative εr in the
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far-infrared region, and its effect on the optical response in the ultraviolet-visible (UV-Vis)
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region can be neglected.13
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To confirm the existence of visible range plasmonics, we investigated the near-field
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distributions of optical modes on the individual Bi2Te3 nanoplates using PEEM equipped with
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400 nm (ℎ = 3.1 eV) femtosecond laser pulses. At λ = 400 nm, Bi2Te3 possesses a negative
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εr, while maintaining a relatively small εi that indicates the small loss. The work function of
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Bi2Te3 is approximately 5.3 eV, larger than the photon energy of the incident light.30
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Therefore, at least two-photon absorption is required to excite a photoelectron. The schematic
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diagram of the PEEM setup is illustrated in Figure 1c. The fs-laser illuminates the sample
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surface at a grazing angle of 16o, and the polarization state of the laser is tuned by a
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half-wave plate.
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Figure 2a shows a fs-PEEM image on a truncated triangular nanoplates with lateral size
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of about 2.8 µm under a p-polarized laser illumination. Photoemission hotspots along the two
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short edges are observed, whereas they are absent in one photon photoemission process with
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Hg lamp as light source (Figure 2c). The Hg arc lamp is an incoherent light source, and the
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contrast in Hg-PEEM is mainly related to the work function of materials. In Figure 2b,
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oscillatory intensity distribution, i.e., secondary spots, with interval space of 182 nm can be
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resolved, due to the high spatial resolution capability of PEEM. Afterwards, the topography
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of the same nanoplate was studied by AFM to exclude the laser radiation damage. As 6
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indicated in Figure 2d, the thickness of this nanoplate is uniform and its surface remains
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atomically flat, indicating that the sample was not damaged by the laser irradiation and
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exhibited a high crystal quality. Importantly, the two short edges exhibiting the photoemission
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hotspots are smooth, thus, the oscillatory feature doesn’t result from any artifact.
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In 2PPE PEEM, the photoemission intensity is directly proportional to fourth power of
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electric field intensity. Accordingly, the observed photoemission hotspots indicate that the
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local electric field is enhanced along the edges. In terms of PEEM, the electric field
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enhancement mechanism can be ascribed to either lightning rod effect or surface plasmon
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resonance.31 Lightning rod effect is brought about by a geometric extremity (i.e., tips of
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polygon) where electric charge is accumulated. Nevertheless, the highest photoemission
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intensity appears at the edge of Bi2Te3 prism, not the sharp corner (Figure 2a). Furthermore,
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on a diabolo-like Bi2Te3 nanoplate, the long edge and short edge can be concurrently excited
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(Figure S2). These experimental results corroborate that the electric field enhancement
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observed here is not from the lightning rod effect, but from the plasmon resonance. Note that
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the size of the studied nanoplates is much larger than the wavelength of excitation, thus, our
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case is different from the PEEM study on Au nanoblock with smaller size compared to the
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incident wavelength, in which localized dipole mode and quadrupole mode are observed.26
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Given that the electric field confinement only appears along the edges, we refer the observed
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plasmonic mode as edge plasmon, which has been observed on Bi2Se3 nanoplate,22 Au
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nanoplate32 and Ag thin film by EELS.33 For edge plasmon, the propagation of electron
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density wave is bound to the boundaries due to the existence of geometric edge, which breaks
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the translation symmetry and leads to the difference between the dispersion relation of edge 7
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plasmon and surface plasmon.33,34 We only focus on the planar Bi2Te3 nanoplates with flat
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surface.35 Further investigations on vertical nanoplates can be performed to study the impact
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of crystal orientation on edge plasmon dispersion.36,37
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To further understand the physical properties of the edge plasmon, we calculated the
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wavelength of the propagating surface plasmon polaritons (SPPs) using the dielectric
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function of Bi2Te3 at λ0=400 nm taken from Figure 1b ( = −7.55 + 8.14). According
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to the formula:38
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=
(1)
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where λ0 is the incident light’s wavelength, εr is the real part of dielectric function and εvac is
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the dielectric constant of vacuum, the calculated λSPP is 373 nm. This value is twice of the
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interval space between the adjacent photoemission maxima (182 nm) in Figure 2b, indicating
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for the short edge (~550 nm), the intensity modulation mainly comes from the interference
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between forward-propagating plasmon and the plasmon wave reflected by the corner.22,32,39
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However, the situation is different for the long edge. As evident in Figure S3, along the long
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edge (~3.2 µm) of another nanoplate, the interval between the adjacent photoemission
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intensity maxima is about 860 nm. In this case, beating pattern, i.e., the interference of
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incident light and plasmon wave, should be taken into account to explain this
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phenomenon.40,41 We treat our case as a one-dimensional (1D) model in which both the wave
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vector of the beating pattern (kB) and that of SPP (kSPP) are along the edge of the nanoplate.
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The beating period is calculated using:41
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!
=
"##
−
‖ cos ()),
./
+,-+ =0 !
1
8
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where k‖ is the horizontal projection of the incident wave vector, φ is the angle between the k‖
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and kSPP (φ ≈ 52o in Figure S3). We obtain the λcalc ≈ 833 nm, consistent with the observed B
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beating period. This demonstrates the 1D propagating edge plasmon with plasmon
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wavelength of 373 nm can elucidate the experimental phenomena.
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Next, we explored the effect of polarization of incident laser on the photoemission
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patterns. Figure 3a shows the fs-PEEM image of another nanoplate with the horizontal
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projection of the incident wave vector aligned with the triangle’s height. Coinciding with the
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photoemission behavior presented in Figure 2a, the highest photoemission intensities are
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observed at the two edges, which are far away from the light source with p-polarization. This
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phenomenon is related to the phase retardation, an effect that should be considered when the
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object size is much larger than the incident wavelength.26,42 Herein, the polarization field
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experienced by the object is no longer homogeneous. In Figure 3b, the dependence of the
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intensity of each plasmonic hotspot on the polarization angle (θ) is presented, which can be
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fitted using:43
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2 = 3 × 567 (8 + )) + 9
(4)
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where A, φ and B are fitting parameters. A good agreement with the correlation coefficient
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above 0.99 can be obtained for both spots. The cos4 dependence manifests that a coherent
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two-photon photoemission process is involved.43,44 Interestingly, there is a phase shift of
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about 41o between the polarization-dependent photoemission behaviors at the two edges,
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which can be utilized to selectively excite individual plasmonic hotspot. Similar phenomenon
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has been observed in the noble metals.24,25,42 In Figure 3c, when the polarization angle is
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rotated counterclockwise by 40o relative to the p-polarization (θ = –50o), only the upper right 9
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edge can be excited, and the photoemission at the lower right edge is switched off. This
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process can be reversible by changing the polarization angle to 40o (Figure 3d). For a given
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linear polarization state, it can be decomposed along two orthogonal directions (s- and
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p-polarized directions). Accordingly, the induced electric field distribution (:;
