Letter pubs.acs.org/NanoLett
Topological Surface State Enhanced Photothermoelectric Effect in Bi2Se3 Nanoribbons Yuan Yan,† Zhi-Min Liao,*,†,‡ Xiaoxing Ke,§ Gustaaf Van Tendeloo,§ Qinsheng Wang,∥ Dong Sun,∥,‡ Wei Yao,⊥ Shuyun Zhou,‡,⊥ Liang Zhang,† Han-Chun Wu,#,△ and Da-Peng Yu*,†,‡ †
State Key Laboratory for Mesoscopic Physics, Department of Physics and ∥International Center for Quantum Materials, Peking University, Beijing 100871, P. R. China ‡ Collaborative Innovation Center of Quantum Matter, Beijing, P. R. China § EMAT (Electron Microscopy for Materials Science), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium ⊥ Department of Physics, Tsinghua University, Beijing 100084, P. R. China # School of Physics, Beijing Institute of Technology, Beijing, 100081, P. R. China △ King Saud University, Riyadh, 11451, Saudi Arabia S Supporting Information *
ABSTRACT: The photothermoelectric effect in topological insulator Bi2Se3 nanoribbons is studied. The topological surface states are excited to be spin-polarized by circularly polarized light. Because the direction of the electron spin is locked to its momentum for the spin-helical surface states, the photothermoelectric effect is significantly enhanced as the oriented motions of the polarized spins are accelerated by the temperature gradient. The results are explained based on the microscopic mechanisms of a photon induced spin transition from the surface Dirac cone to the bulk conduction band. The as-reported enhanced photothermoelectric effect is expected to have potential applications in a spin-polarized power source. KEYWORDS: Topological insulator, Bi2Se3, nanoribbons, photothermoelectric effect, helical surface states
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thermoelectric material,20 to which light−matter interactions can induce a significant photothermoelectric effect. It has been pointed out that the acoustic phonon-mediated cooling of twodimensional Dirac Fermions in Bi2Se3 implies that it could be possible to create long-lived hot photocarriers to be used for high efficiency photothermoelectric applications.21 However, the transport properties of the optically excited spin-helical surface states under thermoelectric potential are still unknown, and the photothermoelectric effect of individual Bi 2 Se 3 nanoribbons has not been studied to date. Here we report the influence of the surface states on the photothermoelectric effect of individual Bi2Se3 nanoribbons. Combining the properties of the surface states on the topological insulator and the thermoelectric properties through CPL excitation, the photothermoelectric effect is significantly enhanced. The results suggest that the devices may have potential applications in a spin-polarized carrier source. The Bi2Se3 nanostructures including nanoribbons and nanoplates used in this work were synthesized by a chemical vapor deposition (CVD) method, and the Dirac Fermions on
opological insulators have attracted widespread research interest in recent years.1−10 Bi2Se3 is a model threedimensional strong topological insulator, which has a bulk bandgap of ∼0.3 eV and conducting surface states.7,8 The surface states are helically chiral, presenting a locking between the electron spin direction and its momentum direction.9,10 The surface states are protected by time-reversal symmetry to be free of backscattering by nonmagnetic impurities and therefore have great applications in spintronics.1−10 Angleresolved photoemission spectroscopies (ARPES) have revealed that the linear energy-momentum dispersion relation of the surface states is in the form of the so-called Dirac cone.11,12 Transport measurements demonstrate the nature of surface Dirac Fermions through the Aharonov−Bohm effect and Shubnikov−de Haas oscillations.13−16 Moreover, the coupling of photons to the spin-helical surface states opens a new route to study and utilize the spin in Bi2Se3 for spintronics. The transfer of a photon angular momentum to the surface spin can induce a surface spin polarization, by means of circularly polarized light (CPL) with a well-defined angular momentum. The spin-polarized surface states in Bi2Se3 can emit photoelectrons with high spin polarization.17 Owing to the helicity property, CPL excited surface spin polarization can introduce a translational motion of the surface carriers and thus result in a spin-polarized photocurrent.18,19 Bi2Se3 is also an important © 2014 American Chemical Society
Received: April 7, 2014 Revised: July 15, 2014 Published: July 21, 2014 4389
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Figure 1. Characterization of the as-synthesized Bi2Se3 nanostructures. (a) Cross-sectional HAADF-STEM image of a Bi2Se3 nanoribbon. (b) Diffraction pattern acquired from the cross section along the [100] zone axis. (c) High-resolution HAADF-STEM image of the nanoribbon surface, where the QL layer is indicated. (Bi represented by red dots; Se represented by green dots.) (d) ARPES spectrum measured at room temperature.
Figure 2. Photovoltage generation from a single Bi2Se3 nanoribbon device with vertically incident linearly polarized light. (a) Typical optical image of a Bi2Se3 nanoribbon device. The red symbols “±” mark the positive/ground electrodes. The numbered circles mark the irradiated positions. (b) I−V curve of a typical Bi2Se3 nanoribbon device. (c) Photovoltage response with switching on/off the laser illumination. The curves with different marks correspond to the illuminated positions marked in a.
our sample surface were also revealed through low-temperature quantum transport.16 To characterize the crystalline quality and detect the eventual presence of Se vacancies of the Bi2Se3 nanoribbons, individual Bi2Se3 nanoribbons were transferred onto a SrTiO3 substrate. To study the layered microstructure of the Bi2Se3 nanoribbons, we then prepared a cross-sectional sample (perpendicular to the long axis, i.e., the [100] direction of the nanoribbon) using a focused ion beam (FIB). The microstructure of the cross-sectional sample was characterized using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) on a FEI Titan microscope fitted with an aberration corrector for the probeforming lens, operated at 120 kV. The cross section of a Bi2Se3
nanoribbon is shown in Figure 1a. The diffraction pattern in Figure 1b, acquired from the cross-section along the [100] zone axis, indicates that the as-synthesized nanoribbon is single crystalline. A high resolution HAADF-STEM image of the nanoribbon surface is revealed in Figure 1c at the atomic scale. The basic unit of the quintuple layer (QL) in sequence with −Se−Bi-Se−Bi−Se− can be clearly observed along the [100] zone axis. Energy dispersive X-ray spectroscopy (EDS) was simultaneously acquired in STEM mode using a Super-X detector, giving the atomic ratio of Bi to Se ∼ 41.2:58.8 with a ∼2−3% error, which could be due to a small amount of Se vacancies in the sample. ARPES data taken through the Γ point (Figure 1d) reveal the conical-like dispersion from the surface 4390
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Figure 3. Photovoltage generation from a single Bi2Se3 nanoribbon device with vertically incident LP, RCP, and LCP light. (a) Optical image of a typical Bi2Se3 nanoribbon device. (b) Sketch of the experimental set up. (c) Photovoltage response with switching laser illumination on Bi2Se3 near the positively connected electrode. (d) Photovoltage response with switching laser illumination on Bi2Se3 near the ground electrode.
states typically observed in Bi2Se3. Although the bands are broadened due to the fact that the ARPES results sum up collective signals from all individual Bi2Se3 nanostructures in the probed area (the ARPES beam size is of a few hundred μm), the measured dispersions are overall in good agreement with those from Bi2Se3 single crystals.8 The Fermi level is ∼0.35 eV higher than the Dirac point, indicating an n-type doping which is usually attributed to Se vacancies. Fortunately, in this work the photocurrent generated from the surface states is highly sensitive to the polarized state of the incident light, while the bulk states only contribute a constant background. Therefore, the effect of the surface states can be identified even in the coexistence of the inevitable bulk states. To fabricate a device for further photothermoelectric measurements, the Bi2Se3 nanostructures were transferred onto a Si substrate with a 300 nm SiO2 layer. Cr/Au (50/ 100 nm) electrodes were attached to the individual Bi2Se3 nanoribbons using electron beam lithography techniques. Figure 2a shows an optical image of a typical Bi2Se3 nanoribbon device. All of the electrical measurements in this work were performed at room temperature in air conditions. The linear current−voltage (I−V) curve in Figure 2b, measured using an Agilent B2912A source-meter unit without laser illumination, indicates an Ohmic contact between the electrodes and the Bi2Se3 nanoribbon. The conductance of this sample is 5.7 e2/h. It has been reported that the minimum conductivity of Bi2Se3 surface states is about 2 to 5 e2/h.22 Thus, the conductance from the surface states in our sample (its dimensions obtained by atomic force microscope (AFM) measurement) is ∼0.96− 2.4 e2/h, which is smaller than the total conductance. Without applying any external power source, the photothermoelectric effect was studied by measuring the voltage response with an incident linearly polarized 514 nm laser with a spot size of ∼1 μm and a power of ∼0.15 mW. The red symbols “±” in Figure 2a mark the positive/ground electrodes for the generated voltage measurements. The colored and numbered spots demonstrate the positions where the laser illumination was directed. For the on/off switching cycles of the laser illumination, the generated voltages were recorded, as shown in Figure 2c. The voltage vs time curves of different marks
correspond to the excited spot regions with the same marks in Figure 2a, respectively. The largest voltage (∼400 μV) was generated when illuminating near the electrode/nanoribbon interface. When illuminating at the central area of the nanoribbon between two electrodes, the measured voltage was approximately zero since the thermal flow was symmetrically transported toward the two electrodes. The generated voltage changed its sign as the illumination position was drawn across the middle point between the two electrodes. Upon illumination away from the measurement electrodes, a photothermoelectric effect was still observed (Supporting Information, Figure S1), which rules out a possible photovoltaic effect due to the electron−hole separation under a build-in electric field at the Bi2Se3/metal interface. The photothermoelectric effect generated voltage increases with increasing the incident laser intensity (Supporting Information, Figure S2), and the voltage has a linear dependence with the laser illumination intensity, suggesting that the energy conversion efficiency is constant within the measurement range. Similar results about laser illumination generated voltage due to the photothermoelectric effect are general in our Bi2Se3 nanoribbons and have been observed in more than 10 devices. To study whether there is any surface states related photothermoelectric effect in Bi2Se3 nanoribbons, the voltage response of circularly polarized laser illumination was measured. Figure 3a is a typical optical image of a Bi2Se3 nanoribbon device. Marks fabricated beside the nanoribbon were used to locate the illumination position. We chose the wider and thicker ribbons for measurement to reduce the uncertainty of the illumination position in the transverse direction and to reduce the carrier excitation from the bottom surface. The width and thickness of the sample are about 8 μm and 120 nm, respectively (Supporting Information, Figure S3). A schematic illustration of the experimental setup is shown in Figure 3b. The laser is incident along the y−z plane. The rightcircular polarization (RCP), linear polarization (LP), and leftcircular polarization (LCP) of the incident laser can be obtained via tuning the λ/4 waveplate, and the illumination power was precisely controlled to keep a constant using a tunable attenuator. The angle Θ between the incident light and 4391
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Figure 4. Photovoltage generation from a single Bi2Se3 nanoribbon device with switching obliquely incident LP, RCP, and LCP light in the y−z plane with a 30° angle between the direction of light and the z-axis. (a) The laser illumination on Bi2Se3 near the positively connected electrode. The schematic diagrams show the mechanism of the enhancement of the photovoltage by RCP light excitation. (b) The laser illumination on Bi2Se3 near the ground electrode. The schematic diagrams show the mechanism of the enhancement of the photovoltage by LCP light excitation.
z axis in the y−z plane was chosen to be 0° and 30°. In Figure 3c,d when the angle Θ = 0°, a little difference in the generated voltage can be observed when illuminated by the laser with RCP, LP, and LCP, respectively. The vertically incident RCP and LCP laser cannot excite the in-plane spin polarization of the surface states. Therefore, no additional photovoltage related to the spin-polarized surface states can be measured. The influence of spatial variations of the light position on the output photovoltage was also measured. The transverse line scan across the nanoribbon shows that the light position with 500 nm variation in the middle of the sample only results in ∼0.5% difference in the photovoltage (Supporting Information, Figure S4). In Figure 4a, when Θ = 30°, the laser is focused on the Bi2Se3 nanoribbon near the electrode connected to the positive terminal of the voltmeter. It is observed that the RCP light leads to an enhanced photo thermoelectric voltage with respect to the LP light, while the LCP and LP lights induce similar photothermoelectric voltages. As Θ = 30° and the laser is focused on the Bi2Se3 nanoribbon near the electrode connected to the ground terminal of the voltmeter, the RCP, LCP, and LP lights induced photothermoelectric voltages are shown in Figure 4b. It is found that the LCP light leads to an enhanced photothermoelectric voltage, while the RCP and LP lights induce similar photo thermoelectric voltages. We discuss the microscopic mechanism of the light helicitydependent photothermoelectric effect below. As shown in Figure 5, in the Dirac cone, the spin orientation along the −ky axis was defined as spin-up. Therefore, the electrons in the
Figure 5. Physical mechanism for the light polarization-selective transition. Here, the spin orientation in the Dirac cone along the −ky axis is defined as spin-up. The local orbital angular momentum in the Dirac cone is marked by the magnetic quantum number (ml) of +1 or −1. In the transition process, the total angular momentum of the surface states and the circularly polarized phonons is conserved.
Dirac cone with positive slope correspond to the spin-up states, and the electrons in the branch with negative slope correspond to the spin-down states. Considering the component of LCP or RCP light in the sample surface along the −y axis, the angular momentum with magnetic quantum number (ml) of +1 or −1 can be transferred to the surface states during the spinconserving dipole transitions. Recently, it is proposed that the 4392
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accelerated by the temperature gradient induced by the illumination heating, which may significantly increase the spin polarized current and provide a new means to produce a spinpolarized source. In conclusion, the photothermoelectric effect of individual Bi2Se3 nanoribbons has been studied. The photothermoelectric effect was further enhanced via CPL illumination. The CPL selectively excites the surface sates with special local angular momentum to meet the conservation of the total angular momentum, resulting in additional spin-polarized surface states that can be driven by the temperature gradient. The photothermoelectric effect induced motion of the spinpolarized surface carriers paves a way to opto-spin-caloritronics.
orbital angular momentum of the surface states is related to the p-like wave functions and shows a chiral texture.23−26 The quantum numbers of the orbital angular momentum for the two branches of the surface Dirac band are +1 and −1, respectively.23−26 An electric transition between surface states and s-orbital (l = 0) bulk states excited by a photon can occur. The σ- photons (RCP light) selectively excite electrons from the surface band with ml = 1 to the bulk conduction bands, while the transitions from ml = −1 surface band are forbidden, resulting in a nonequilibrium spin-up polarization in the positive-slope branch in the Dirac cone. Although the conduction bands of Bi2Se3 are complex, the photoexcited electrons from the surface states to the bulk bands do not contribute to the helicity-dependent portion of the photovoltages. For the highly n-doped Bi2Se3, although the hexagonal warping of the Dirac cone induces a complicated momentumspin relation of the surface electrons, a large portion of spin can hold on with respect to momentum to produce observable effects in our samples. Therefore, the RCP laser excitation will result in the in-plane spin polarization of the surface state along the −y axis, as shown by the sketch in the middle panel of Figure 4a. Due to the spin-momentum locking of the surface states in the topological insulator, the electrons have a momentum directed along the +x axis.9 The additional surface electrons excited by the RCP have the same motion direction with the electrons driven by the temperature gradient and therefore significantly enhances the generated voltage. When the Bi2Se3 nanoribbon is irradiated by the LCP laser, the electronic states in the Dirac cone with the ml = −1 branch can absorb photons with a σ+ angular momentum and hop to the bulk conduction band, resulting in a nonequilibrium spin-down polarization in the Dirac cone. The spin-down electrons have an in-plane spin orientation along the +y axis and a momentum direction along the -x axis, as shown by the lower panel in Figure 4a. The surface electrons excited by the LCP laser initially have the opposite motion direction with the electrons driven by the temperature gradient. The electrons with a deceleration can either reach to the left electrode or slow down to zero and then have a reverse acceleration toward the right electrode, which makes the situation complex. A similar analysis can be made for the laser illumination position near the ground electrode in Figure 4b. Here, the illumination heating induced temperature gradient is along the −x axis. The RCP laser excitation induced additional surface electrons driven by the opposite forces contribute little to the generated voltage, as shown by the sketch in the middle panel of Figure 4b. When illuminated by the LCP laser, the additional surface electrons with a momentum direction along the −x axis, accelerated by the temperature gradient, result in a significant enhancement of the photothermoelectric effect, as shown by the lower panel in Figure 4b. In order to compare with the CPL dependent photothermoelectric effect using nonuniform illumination, the CPL dependent photocurrent was studied using homogeneous illumination. A helicity-dependent photocurrent was observed (Supporting Information, Figure S5), which is very similar to that presented in ref 18. Obliquely incident CPL produces inplane spin polarization. In-plane spin polarization corresponds to an oriented motion of the surface carriers. Usually, the spinpolarized current is small due to the lifetime limitation of the photon generated carriers, the presence of scattering, and the small carrier momentum. By means of the photothermoelectric effect, one favorable type of the spin polarized carriers can be
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ASSOCIATED CONTENT
S Supporting Information *
Detailed photovoltage generation from different positions of the sample, the photovoltage dependence with illumination intensity, the AFM image, the line scan of the photovoltage, and the photocurrent with uniform illumination. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected] (Z.M.L.). *E-mail: [email protected] (D.P.Y.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by MOST (Nos. 2013CB934600, 2013CB932602), NSFC (Nos. 11274014, 11234001), and the Program for New Century Excellent Talents in University of China (No. NCET-12-0002). X.K. and G.V.T. are grateful to funding from the European Research Council under the Seventh Framework Program (FP7), ERC Advanced Grant No. 246791-COUNTATOMS.
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REFERENCES
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Topological Surface State Enhanced Photothermoelectric Effect in Bi2Se3 Nanoribbons Yuan Yan,† Zhi-Min Liao,*,†,‡ Xiaoxing Ke,§ Gustaaf Van Tendeloo,§ Qinsheng Wang,∥ Dong Sun,∥,‡ Wei Yao,⊥ Shuyun Zhou,‡,⊥ Liang Zhang,† Han-Chun Wu,#,△ and Da-Peng Yu*,†,‡ †
State Key Laboratory for Mesoscopic Physics, Department of Physics and ∥International Center for Quantum Materials, Peking University, Beijing 100871, P. R. China ‡ Collaborative Innovation Center of Quantum Matter, Beijing, P. R. China § EMAT (Electron Microscopy for Materials Science), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium ⊥ Department of Physics, Tsinghua University, Beijing 100084, P. R. China # School of Physics, Beijing Institute of Technology, Beijing, 100081, P. R. China △ King Saud University, Riyadh, 11451, Saudi Arabia S Supporting Information *
ABSTRACT: The photothermoelectric effect in topological insulator Bi2Se3 nanoribbons is studied. The topological surface states are excited to be spin-polarized by circularly polarized light. Because the direction of the electron spin is locked to its momentum for the spin-helical surface states, the photothermoelectric effect is significantly enhanced as the oriented motions of the polarized spins are accelerated by the temperature gradient. The results are explained based on the microscopic mechanisms of a photon induced spin transition from the surface Dirac cone to the bulk conduction band. The as-reported enhanced photothermoelectric effect is expected to have potential applications in a spin-polarized power source. KEYWORDS: Topological insulator, Bi2Se3, nanoribbons, photothermoelectric effect, helical surface states
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thermoelectric material,20 to which light−matter interactions can induce a significant photothermoelectric effect. It has been pointed out that the acoustic phonon-mediated cooling of twodimensional Dirac Fermions in Bi2Se3 implies that it could be possible to create long-lived hot photocarriers to be used for high efficiency photothermoelectric applications.21 However, the transport properties of the optically excited spin-helical surface states under thermoelectric potential are still unknown, and the photothermoelectric effect of individual Bi 2 Se 3 nanoribbons has not been studied to date. Here we report the influence of the surface states on the photothermoelectric effect of individual Bi2Se3 nanoribbons. Combining the properties of the surface states on the topological insulator and the thermoelectric properties through CPL excitation, the photothermoelectric effect is significantly enhanced. The results suggest that the devices may have potential applications in a spin-polarized carrier source. The Bi2Se3 nanostructures including nanoribbons and nanoplates used in this work were synthesized by a chemical vapor deposition (CVD) method, and the Dirac Fermions on
opological insulators have attracted widespread research interest in recent years.1−10 Bi2Se3 is a model threedimensional strong topological insulator, which has a bulk bandgap of ∼0.3 eV and conducting surface states.7,8 The surface states are helically chiral, presenting a locking between the electron spin direction and its momentum direction.9,10 The surface states are protected by time-reversal symmetry to be free of backscattering by nonmagnetic impurities and therefore have great applications in spintronics.1−10 Angleresolved photoemission spectroscopies (ARPES) have revealed that the linear energy-momentum dispersion relation of the surface states is in the form of the so-called Dirac cone.11,12 Transport measurements demonstrate the nature of surface Dirac Fermions through the Aharonov−Bohm effect and Shubnikov−de Haas oscillations.13−16 Moreover, the coupling of photons to the spin-helical surface states opens a new route to study and utilize the spin in Bi2Se3 for spintronics. The transfer of a photon angular momentum to the surface spin can induce a surface spin polarization, by means of circularly polarized light (CPL) with a well-defined angular momentum. The spin-polarized surface states in Bi2Se3 can emit photoelectrons with high spin polarization.17 Owing to the helicity property, CPL excited surface spin polarization can introduce a translational motion of the surface carriers and thus result in a spin-polarized photocurrent.18,19 Bi2Se3 is also an important © 2014 American Chemical Society
Received: April 7, 2014 Revised: July 15, 2014 Published: July 21, 2014 4389
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Figure 1. Characterization of the as-synthesized Bi2Se3 nanostructures. (a) Cross-sectional HAADF-STEM image of a Bi2Se3 nanoribbon. (b) Diffraction pattern acquired from the cross section along the [100] zone axis. (c) High-resolution HAADF-STEM image of the nanoribbon surface, where the QL layer is indicated. (Bi represented by red dots; Se represented by green dots.) (d) ARPES spectrum measured at room temperature.
Figure 2. Photovoltage generation from a single Bi2Se3 nanoribbon device with vertically incident linearly polarized light. (a) Typical optical image of a Bi2Se3 nanoribbon device. The red symbols “±” mark the positive/ground electrodes. The numbered circles mark the irradiated positions. (b) I−V curve of a typical Bi2Se3 nanoribbon device. (c) Photovoltage response with switching on/off the laser illumination. The curves with different marks correspond to the illuminated positions marked in a.
our sample surface were also revealed through low-temperature quantum transport.16 To characterize the crystalline quality and detect the eventual presence of Se vacancies of the Bi2Se3 nanoribbons, individual Bi2Se3 nanoribbons were transferred onto a SrTiO3 substrate. To study the layered microstructure of the Bi2Se3 nanoribbons, we then prepared a cross-sectional sample (perpendicular to the long axis, i.e., the [100] direction of the nanoribbon) using a focused ion beam (FIB). The microstructure of the cross-sectional sample was characterized using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) on a FEI Titan microscope fitted with an aberration corrector for the probeforming lens, operated at 120 kV. The cross section of a Bi2Se3
nanoribbon is shown in Figure 1a. The diffraction pattern in Figure 1b, acquired from the cross-section along the [100] zone axis, indicates that the as-synthesized nanoribbon is single crystalline. A high resolution HAADF-STEM image of the nanoribbon surface is revealed in Figure 1c at the atomic scale. The basic unit of the quintuple layer (QL) in sequence with −Se−Bi-Se−Bi−Se− can be clearly observed along the [100] zone axis. Energy dispersive X-ray spectroscopy (EDS) was simultaneously acquired in STEM mode using a Super-X detector, giving the atomic ratio of Bi to Se ∼ 41.2:58.8 with a ∼2−3% error, which could be due to a small amount of Se vacancies in the sample. ARPES data taken through the Γ point (Figure 1d) reveal the conical-like dispersion from the surface 4390
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Figure 3. Photovoltage generation from a single Bi2Se3 nanoribbon device with vertically incident LP, RCP, and LCP light. (a) Optical image of a typical Bi2Se3 nanoribbon device. (b) Sketch of the experimental set up. (c) Photovoltage response with switching laser illumination on Bi2Se3 near the positively connected electrode. (d) Photovoltage response with switching laser illumination on Bi2Se3 near the ground electrode.
states typically observed in Bi2Se3. Although the bands are broadened due to the fact that the ARPES results sum up collective signals from all individual Bi2Se3 nanostructures in the probed area (the ARPES beam size is of a few hundred μm), the measured dispersions are overall in good agreement with those from Bi2Se3 single crystals.8 The Fermi level is ∼0.35 eV higher than the Dirac point, indicating an n-type doping which is usually attributed to Se vacancies. Fortunately, in this work the photocurrent generated from the surface states is highly sensitive to the polarized state of the incident light, while the bulk states only contribute a constant background. Therefore, the effect of the surface states can be identified even in the coexistence of the inevitable bulk states. To fabricate a device for further photothermoelectric measurements, the Bi2Se3 nanostructures were transferred onto a Si substrate with a 300 nm SiO2 layer. Cr/Au (50/ 100 nm) electrodes were attached to the individual Bi2Se3 nanoribbons using electron beam lithography techniques. Figure 2a shows an optical image of a typical Bi2Se3 nanoribbon device. All of the electrical measurements in this work were performed at room temperature in air conditions. The linear current−voltage (I−V) curve in Figure 2b, measured using an Agilent B2912A source-meter unit without laser illumination, indicates an Ohmic contact between the electrodes and the Bi2Se3 nanoribbon. The conductance of this sample is 5.7 e2/h. It has been reported that the minimum conductivity of Bi2Se3 surface states is about 2 to 5 e2/h.22 Thus, the conductance from the surface states in our sample (its dimensions obtained by atomic force microscope (AFM) measurement) is ∼0.96− 2.4 e2/h, which is smaller than the total conductance. Without applying any external power source, the photothermoelectric effect was studied by measuring the voltage response with an incident linearly polarized 514 nm laser with a spot size of ∼1 μm and a power of ∼0.15 mW. The red symbols “±” in Figure 2a mark the positive/ground electrodes for the generated voltage measurements. The colored and numbered spots demonstrate the positions where the laser illumination was directed. For the on/off switching cycles of the laser illumination, the generated voltages were recorded, as shown in Figure 2c. The voltage vs time curves of different marks
correspond to the excited spot regions with the same marks in Figure 2a, respectively. The largest voltage (∼400 μV) was generated when illuminating near the electrode/nanoribbon interface. When illuminating at the central area of the nanoribbon between two electrodes, the measured voltage was approximately zero since the thermal flow was symmetrically transported toward the two electrodes. The generated voltage changed its sign as the illumination position was drawn across the middle point between the two electrodes. Upon illumination away from the measurement electrodes, a photothermoelectric effect was still observed (Supporting Information, Figure S1), which rules out a possible photovoltaic effect due to the electron−hole separation under a build-in electric field at the Bi2Se3/metal interface. The photothermoelectric effect generated voltage increases with increasing the incident laser intensity (Supporting Information, Figure S2), and the voltage has a linear dependence with the laser illumination intensity, suggesting that the energy conversion efficiency is constant within the measurement range. Similar results about laser illumination generated voltage due to the photothermoelectric effect are general in our Bi2Se3 nanoribbons and have been observed in more than 10 devices. To study whether there is any surface states related photothermoelectric effect in Bi2Se3 nanoribbons, the voltage response of circularly polarized laser illumination was measured. Figure 3a is a typical optical image of a Bi2Se3 nanoribbon device. Marks fabricated beside the nanoribbon were used to locate the illumination position. We chose the wider and thicker ribbons for measurement to reduce the uncertainty of the illumination position in the transverse direction and to reduce the carrier excitation from the bottom surface. The width and thickness of the sample are about 8 μm and 120 nm, respectively (Supporting Information, Figure S3). A schematic illustration of the experimental setup is shown in Figure 3b. The laser is incident along the y−z plane. The rightcircular polarization (RCP), linear polarization (LP), and leftcircular polarization (LCP) of the incident laser can be obtained via tuning the λ/4 waveplate, and the illumination power was precisely controlled to keep a constant using a tunable attenuator. The angle Θ between the incident light and 4391
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Figure 4. Photovoltage generation from a single Bi2Se3 nanoribbon device with switching obliquely incident LP, RCP, and LCP light in the y−z plane with a 30° angle between the direction of light and the z-axis. (a) The laser illumination on Bi2Se3 near the positively connected electrode. The schematic diagrams show the mechanism of the enhancement of the photovoltage by RCP light excitation. (b) The laser illumination on Bi2Se3 near the ground electrode. The schematic diagrams show the mechanism of the enhancement of the photovoltage by LCP light excitation.
z axis in the y−z plane was chosen to be 0° and 30°. In Figure 3c,d when the angle Θ = 0°, a little difference in the generated voltage can be observed when illuminated by the laser with RCP, LP, and LCP, respectively. The vertically incident RCP and LCP laser cannot excite the in-plane spin polarization of the surface states. Therefore, no additional photovoltage related to the spin-polarized surface states can be measured. The influence of spatial variations of the light position on the output photovoltage was also measured. The transverse line scan across the nanoribbon shows that the light position with 500 nm variation in the middle of the sample only results in ∼0.5% difference in the photovoltage (Supporting Information, Figure S4). In Figure 4a, when Θ = 30°, the laser is focused on the Bi2Se3 nanoribbon near the electrode connected to the positive terminal of the voltmeter. It is observed that the RCP light leads to an enhanced photo thermoelectric voltage with respect to the LP light, while the LCP and LP lights induce similar photothermoelectric voltages. As Θ = 30° and the laser is focused on the Bi2Se3 nanoribbon near the electrode connected to the ground terminal of the voltmeter, the RCP, LCP, and LP lights induced photothermoelectric voltages are shown in Figure 4b. It is found that the LCP light leads to an enhanced photothermoelectric voltage, while the RCP and LP lights induce similar photo thermoelectric voltages. We discuss the microscopic mechanism of the light helicitydependent photothermoelectric effect below. As shown in Figure 5, in the Dirac cone, the spin orientation along the −ky axis was defined as spin-up. Therefore, the electrons in the
Figure 5. Physical mechanism for the light polarization-selective transition. Here, the spin orientation in the Dirac cone along the −ky axis is defined as spin-up. The local orbital angular momentum in the Dirac cone is marked by the magnetic quantum number (ml) of +1 or −1. In the transition process, the total angular momentum of the surface states and the circularly polarized phonons is conserved.
Dirac cone with positive slope correspond to the spin-up states, and the electrons in the branch with negative slope correspond to the spin-down states. Considering the component of LCP or RCP light in the sample surface along the −y axis, the angular momentum with magnetic quantum number (ml) of +1 or −1 can be transferred to the surface states during the spinconserving dipole transitions. Recently, it is proposed that the 4392
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accelerated by the temperature gradient induced by the illumination heating, which may significantly increase the spin polarized current and provide a new means to produce a spinpolarized source. In conclusion, the photothermoelectric effect of individual Bi2Se3 nanoribbons has been studied. The photothermoelectric effect was further enhanced via CPL illumination. The CPL selectively excites the surface sates with special local angular momentum to meet the conservation of the total angular momentum, resulting in additional spin-polarized surface states that can be driven by the temperature gradient. The photothermoelectric effect induced motion of the spinpolarized surface carriers paves a way to opto-spin-caloritronics.
orbital angular momentum of the surface states is related to the p-like wave functions and shows a chiral texture.23−26 The quantum numbers of the orbital angular momentum for the two branches of the surface Dirac band are +1 and −1, respectively.23−26 An electric transition between surface states and s-orbital (l = 0) bulk states excited by a photon can occur. The σ- photons (RCP light) selectively excite electrons from the surface band with ml = 1 to the bulk conduction bands, while the transitions from ml = −1 surface band are forbidden, resulting in a nonequilibrium spin-up polarization in the positive-slope branch in the Dirac cone. Although the conduction bands of Bi2Se3 are complex, the photoexcited electrons from the surface states to the bulk bands do not contribute to the helicity-dependent portion of the photovoltages. For the highly n-doped Bi2Se3, although the hexagonal warping of the Dirac cone induces a complicated momentumspin relation of the surface electrons, a large portion of spin can hold on with respect to momentum to produce observable effects in our samples. Therefore, the RCP laser excitation will result in the in-plane spin polarization of the surface state along the −y axis, as shown by the sketch in the middle panel of Figure 4a. Due to the spin-momentum locking of the surface states in the topological insulator, the electrons have a momentum directed along the +x axis.9 The additional surface electrons excited by the RCP have the same motion direction with the electrons driven by the temperature gradient and therefore significantly enhances the generated voltage. When the Bi2Se3 nanoribbon is irradiated by the LCP laser, the electronic states in the Dirac cone with the ml = −1 branch can absorb photons with a σ+ angular momentum and hop to the bulk conduction band, resulting in a nonequilibrium spin-down polarization in the Dirac cone. The spin-down electrons have an in-plane spin orientation along the +y axis and a momentum direction along the -x axis, as shown by the lower panel in Figure 4a. The surface electrons excited by the LCP laser initially have the opposite motion direction with the electrons driven by the temperature gradient. The electrons with a deceleration can either reach to the left electrode or slow down to zero and then have a reverse acceleration toward the right electrode, which makes the situation complex. A similar analysis can be made for the laser illumination position near the ground electrode in Figure 4b. Here, the illumination heating induced temperature gradient is along the −x axis. The RCP laser excitation induced additional surface electrons driven by the opposite forces contribute little to the generated voltage, as shown by the sketch in the middle panel of Figure 4b. When illuminated by the LCP laser, the additional surface electrons with a momentum direction along the −x axis, accelerated by the temperature gradient, result in a significant enhancement of the photothermoelectric effect, as shown by the lower panel in Figure 4b. In order to compare with the CPL dependent photothermoelectric effect using nonuniform illumination, the CPL dependent photocurrent was studied using homogeneous illumination. A helicity-dependent photocurrent was observed (Supporting Information, Figure S5), which is very similar to that presented in ref 18. Obliquely incident CPL produces inplane spin polarization. In-plane spin polarization corresponds to an oriented motion of the surface carriers. Usually, the spinpolarized current is small due to the lifetime limitation of the photon generated carriers, the presence of scattering, and the small carrier momentum. By means of the photothermoelectric effect, one favorable type of the spin polarized carriers can be
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ASSOCIATED CONTENT
S Supporting Information *
Detailed photovoltage generation from different positions of the sample, the photovoltage dependence with illumination intensity, the AFM image, the line scan of the photovoltage, and the photocurrent with uniform illumination. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected] (Z.M.L.). *E-mail: [email protected] (D.P.Y.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by MOST (Nos. 2013CB934600, 2013CB932602), NSFC (Nos. 11274014, 11234001), and the Program for New Century Excellent Talents in University of China (No. NCET-12-0002). X.K. and G.V.T. are grateful to funding from the European Research Council under the Seventh Framework Program (FP7), ERC Advanced Grant No. 246791-COUNTATOMS.
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REFERENCES
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