Raman Anisotropy and Polarization-Sensitive Photodetection in 2D Bi2O2Se-WSe2 Heterostructure.

Two-dimensional (2D) bismuth oxyselenide (Bi2O2Se) has attracted increasing attention due to its high mobility, tunable band gap, and air stability. The surface reconstruction of cleaved Bi2O2Se due to the electrostatic interlayer interactions can lead to the in-plane anisotropic structure and physics. In this work, we first discovered the strong anisotropy in phonon modes through the angle-resolved polarized Raman (ARPR) spectra. Benefiting from the anisotropic feature, a high-performance polarization-sensitive photodetector has been achieved by constructing a heterostructure composed of the multilayer Bi2O2Se as polarized-light sensitizers and 2D WSe2 as a photocarrier transport channel. The detectors exhibit broadband response spectra from 405 to 1064 nm along with high responsivity, fast speed, and high sensitivity owing to the photogating effect in this device architecture. More importantly, the photocurrent shows strong light polarization dependence with the maximum dichroism ratio of 4.9, and a reversal is observed for the angle-dependent photocurrent excited by polarized 405 and 635 nm light. This work provides new insight in terms of optical and photocurrent anisotropy of exfoliated Bi2O2Se and expands its applications in angle-resolved electronics and optoelectronics.

Introduction

For the two-dimensional (2D) materials with an in-plane anisotropic crystalline structure, the electronic and band structures show high dependence on orientations, leading to the strong anisotropy in effective mass, optical absorption, and electrical transport, giving rise to the unique feature of linear dichroism in these classes of materials.[1−7] The linear dichroism endows them as promising platforms for angle-resolved optoelectronic applications such as polarization spectroscopy imaging, polarized photodetectors, and optical radar.[8−10] In the past, the optical media with meta-surface structures has been widely utilized to manipulate the light phase and polarization,[11] while the complex and costly manufacturing process is the limiting factor for practical applications. Due to the geometric anisotropy, the one-dimensional (1D) nanowires or nanobelts such as ZnO, InP, and Sb2S3 can also exhibit the linear dichroism effect and capability of polarization photodetection; however, the small aspect ratios hamper the dichroism ratio and limit the diversity of device fabrication.[12−14]

Recently, the low-symmetric 2D materials have emerged as centers for polarization-dependent optical and electrical applications due to their intrinsic in-plane anisotropic properties.[1] For example, the black phosphorus (b-P) with a puckered honeycomb structure has shown the distinct anisotropy in optical absorption, photoluminescence (PL), carrier mobility, and thermoelectric transport,[6,15,16] realizing the polarization-sensitive photodetectors, field-effect transistors, etc. Inspired by the anisotropic feature of b-P, the black arsenic (b-As) from the same group V was also reported to exhibit in-plane optical and electrical anisotropy with a dichroism ratio of 2.68 in electron mobility.[17] In addition, the germanium selenide (GeSe),[18] germanium arsenic (GeAs),[19] and low-symmetry transition-metal dichalcogenides (TMDs) such as Td-WTe2,[20] 1T-MoTe2,[21] ReS2,[22] and ReSe2[23] have also been established as anisotropic material systems possessing strong linear dichroism. On the other hand, highly symmetric 2D materials such as graphene and 2H-TMDs with a hexagonal crystal structure have been regarded as in-plane isotropy lacking the potential in polarization-dependent applications.

As new emerging 2D ternary materials, layered bismuth oxyselenide (Bi2O2Se) has been rapidly developed with great success in electronic and optoelectronic applications including field-effect transistors, near-infrared photodetectors, thermal electronics, and topological quantum devices, benefitting from their ultrahigh mobility, outstanding stability, and tunable band gaps.[24] The electron mobility can reach as high as 20 000 cm2/(V s) at low temperature and 450 cm2/(V s) at room temperature, enabling the detection of Shubnikov–de Haas quantum oscillations.[25,26] The corresponding photodetectors exhibited broadband, highly sensitive, and ultrafast photodetection performance from near-infrared to terahertz.[27] Although a fascinating development, the linear dichroism and polarized electronics based on Bi2O2Se have been rarely studied and remain elusive.

As we know, Bi2O2Se belongs to a body-centered tetragonal phase (I4/mmm) group with strong anisotropy, consisting of positively charged [Bi2O2]2 layers that are alternately sandwiched with the negatively charged [Se]2, as shown in Figure a. Different from other van der Waals (vdW) layer materials, the interaction between [Bi2O2]2 and [Se]2 is the electrostatic force instead of vdW force,[28] making it hard to be mechanically exfoliated. Currently, the chemical vapor deposition (CVD) method is the most common approach to obtain the thin and large-area Bi2O2Se with the lateral size of up to hundreds of micrometers.[25,29,30] We note that the CVD-grown samples have atomically smooth surfaces; in contrast, the Bi2O2Se samples obtained through the cleavage method show a complicated surface reconstruction due to the existence of interlayer electrostatic interactions, which led to the difference in surface morphology and band structure.[31] When the crystal is mechanically cleaved, 50% of the Se atoms are attached to each Bi plane because of the electroneutrality requirement.[32] Through the scanning tunneling spectroscopy, the interlinked weave pattern of Se vacancies and morphology of steps have been observed on the Bi2O2Se surface,[32] which can offer an opportunity for exploring unique physical properties and device functionality. For example, the strong surface reconstruction due to the interlayer electrostatic force can result in a remarkable spontaneous in-plane anisotropy and electric polarization.[33]

Figure 1

(a) Atomic structure of Bi2O2Se. (b) Optical microscopy (OM) image of the device based on the Bi2O2Se/WSe2 heterostructure. (c) Raman spectra of the heterostructure and the components. (d) PL spectral of the heterostructures. (e, f) Atomic force microscopy (AFM) image and thickness profile of Bi2O2Se and WSe2 layers.

In this work, we prepared the multilayer Bi2O2Se using the mechanically exfoliated method. Through the angle-resolved polarized Raman spectroscopy (ARPR), we first discovered the significant anisotropic Raman modes of the exfoliated Bi2O2Se. The intensities of Raman modes show a marked periodic feature (180°) with a two-lobed shape. Furthermore, Bi2O2Se as a photosensitive layer has been integrated with WSe2 layers acting as a photocarrier channel, the constructed heterostructures exhibit high polarization-sensitive photodetection performance. The dichroism ratio of photocurrent for 405 and 635 nm light can reach 2.2 and 4.9, respectively. It is interesting that the orientations at which the photocurrent under 405 and 635 nm polarized light reach the maximum are 0 and 90°, respectively, implying the optical reversal of linear dichroism when switching light from 405 to 635 nm. This is consistent with the predicted polarization-resolved absorption spectra where the linear dichroism reversal also occurs.[33] Due to the photogating effect at the interface and a suitable band gap of Bi2O2Se, the devices also exhibit broadband photoresponse ranging from 405 to 1064 nm with a high responsivity of 44 A/W, fast speed of less than 20 ms, and high detectivity of up to 3 × 1013 Jones. The first observation of Raman anisotropy and high-performance polarized-light photodetection can make layered Bi2O2Se promising candidates for angle-dependent optoelectronic applications.

Results and Discussion

The multilayer Bi2O2Se was exfoliated on the Si/SiO2 substrate and then transferred on top of the WSe2 layers (Figure b). The clear diffraction selected area electron diffraction (SAED) pattern is observed (Figure S1), demonstrating the high crystalline quality of the Bi2O2Se nanosheets. The detailed process can be seen in the Experimental Section. The purpose of constructing the heterostructure is to form the out-of-plane built-in field at the interface, inducing the photogating effect for highly efficient photodetection, which will be discussed later. Figure c shows the Raman spectra of Bi2O2Se, WSe2, and their heterostructures. The Raman peaks located at 158.8 and 252 cm–1 are ascribed to the A1g mode of Bi2O2Se and WSe2, respectively, which is consistent with previous reports.[34,35] The PL spectrum was obtained under the excitation of a 532 nm laser, showing a strong PL peak at 920 nm (Figure d), corresponding to an indirect optical band gap of multilayer WSe2, while the PL is absent in Bi2O2Se layers. In the heterostructures, the thickness of the Bi2O2Se and WSe2 layers is 36.6 and 68.2 nm from AFM images as shown in Figure e,f, respectively.

Angle-resolved polarized Raman spectroscopy (ARPR) has been regarded as a facile technique to identify the anisotropic crystal structure. To further investigate the crystal structure symmetry and the electron–phonon interactions on the surface of Bi2O2Se, we performed the ARPR measurement with both parallel and cross-polarization configurations. To realize this, we inserted a linear polarizer in front of the Raman detection system and detected the scattered light along the parallel or perpendicular to the incident light polarization. The ARPR was measured by rotating the sample with 30° steps. The rotation angle θ is defined as the angle between the a-axis and laser polarization direction, and Figure S2 shows the optical microscopy images of the Bi2O2Se sample under each rotation angle from 0 to 330°. Figure a,b shows the Raman spectra for Bi2O2Se at different rotation angles under the parallel or cross-polarized configuration, respectively. Figure c,d shows the polarized Raman mapping at different rotation angles from 0 to 330° under parallel and cross-polarization configuration, respectively. Under the parallel configuration, one pronounced Raman peak at 159.7 cm–1 is observed, while two Raman peaks at 159.73 and 158.07 cm–1 appear under cross configuration. In both cases, we found that the intensities of these A1g modes present a strong correlation with rotation angles, indicating the in-plane anisotropic phonon vibration and lattice structure. The Raman intensities were then extracted to be plotted in polar coordinates as shown in Figure e,f. The data were well fitted using the formulawhere θ is the angle of the linear polarization laser. bmin and bmax are the minimum and maximum fitting parameters, respectively. The Raman intensities of A1g mode show a period of 180° with a two-lobed shape under both configurations. The dichroism ratios defined as the ratio of maximum intensity at 0° divided by the minimum value at 90° are 3.6 and 3.2 for parallel or cross-polarized configuration, respectively.

Figure 2

(a, b) Raman spectra of Bi2O2Se at different rotation angles under the parallel and cross-polarized configuration, respectively. (c, d) Polarized Raman mapping and (e, f) plots in polar coordinate at the varying rotation angles from 0 to 330° under parallel and cross-polarization configuration, respectively.

To further investigate the potential application of the novel linear dichroism feature, we then assembled the Bi2O2Se nanoflakes with multilayer WSe2 and fabricated the heterostructure-based photodetectors as shown in Figure b. The architecture of this device is shown in Figure a, the underlying WSe2 can act as a photocarrier transport layer and top Bi2O2Se acts as polarization-sensitive layers. We note that the photoconductor based on individual exfoliated Bi2O2Se has poor photoresponse due to the extremely high dark current, which can hamper the study of the polarized light detection. Thus, the purpose of constructing heterostructure structure is to introduce the photogating effect for highly efficient photodetection. Now we explain the corresponding photodetection scheme. Through the transfer curves of the field-effect transistors based on the individual materials (Figure S3), Bi2O2Se and WSe2 exhibit n-type and p-type behavior, respectively. Thus, out-of-plane p–n junctions can form at the interface. Figure b shows the surface potential difference (SPD) at the interface obtained by Kelvin probe force microscopy (KPFM) measurement. The SPD between AFM tip and Bi2O2Se (WSe2) is defined aswhere W is the work function. The Fermi level difference (ΔEF) between Bi2O2Se and WSe2 can be calculated by the formulaThus, ΔEF can be calculated to be 20.7 meV. Based on the energy band and electron affinities of Bi2O2Se and WSe2,[28,36] we plot the band diagram of the heterostructure as shown in Figure c, depicting the type II band alignment, facilitating the charge separation. Under light illumination, the photons are absorbed by Bi2O2Se and the excited electron–hole pairs are separated under the built-in electric field (Figure d). The holes are transferred into the WSe2 channel layers and the electrons reside in top Bi2O2Se, resulting in the photogating effect. Within the lifetime, the holes can recirculate multiple times in the channel before recombining with electrons, leading to a large photo gain. A similar scheme is also reported in 2D/0D hybrids and 2D MoS2 with an out-of-plane homojunction.[37,38]

Figure 3

(a) Schematic diagram of Bi2O2Se/WSe2 heterostructure devices. (b) KPFM of the heterostructure at the interface. (c, d) Band diagram of the heterostructure before and after contact, respectively.

We then performed the photodetection measurement using the lasers with different wavelength and power densities at room temperature and ambient environment. Figure a–c shows the dynamic photoresponse of the device under laser irradiation with wavelengths of 405, 635, and 808 nm, demonstrating a significant photocurrent and a fast response speed. The temporal response is less than 20 ms for both rise and decay processes, which can satisfy the requirement of video imaging applications. The photoswitching ratio defined as Ilight/Idark can reach 103 and 104 for 405 and 635 nm light, respectively, as a result of low dark current, implying a high sensitivity. The device can also respond to the near-infrared laser with wavelength of 980 and 1064 nm as shown in Figure S4 (Supporting Information) due to the small band gap of Bi2O2Se layers, indicating a broad spectral coverage from 405 to 1064 nm. In addition, the photocurrent mapping of the photodetector is also measured (Figure S5), showing the strong photocurrent at the area between drain electrodes and the Bi2O2Se layer.

Figure 4

(a–c) Dynamic photoresponse of the device under laser irradiation with wavelengths of 405, 635, and 808 nm, respectively. (d, e) Time-dependent photoresponse as a function of light power density under 405 and 635 nm light, respectively. (f) Photocurrent as a function of light power density. The solid curves are the power-law fitting.

The measured time-dependent photoresponse as a function of light power density under 405 and 635 nm light is shown in Figure d,e, respectively, demonstrating gradually decreased photocurrent with decreasing light power. The photocurrent is then extracted and plotted in Figure f. The curves are fitted by the power-law equation of Iph ∼ P∂, where P is the laser power and ∂ is the fitting exponent. We can see that the exponents ∂ are fitted to be ∼0.66 and ∼0.75 for 405 and 635 nm light, respectively, indicating a highly efficient conversion efficiency from incident photons to collected electron–hole pairs.

As key figures of merit for photodetectors, the responsivity (R), external quantum efficiency (EQE), and detectivity (D*) are also evaluated as followswhere Iph is the photocurrent, S is the effective area of the detector, P is the optical power density, hc is the Planck constant, e is the unit charge, λ is the incident wavelength, C is the speed of light, and Idark is the dark current.

Figure a shows R as a function of light power densities for 450 and 635 nm light. In both cases, R decreases with increased light power owing to the gradual saturation of the photosensing states in the heterostructure.[38] The R can reach up to 43 and 44 A/W for 405 and 635 nm, respectively, at weak light illumination. The EQE as a function of light power density is shown in Figure b, the maximum EQE up to 1.3 × 104 and 8.6 × 103% can be achieved for 405 and 635 nm, respectively. The high responsivity results from the huge photogain due to the existence of the photogating effect in the heterostructure as discussed above. The detectivity D* is a common figure of merit to characterize the sensitivity of a detector with the unit of cm Hz1/2/W or Jones. The D* of the device at 405 and 635 nm light is shown in Figure c, which can reach up to 6.2 × 1012 Jones at 405 nm light and 3.1 × 1013 Jones at 635 nm light, exceeding the sensitivity of most photodetectors based on 2D materials. The R, EQE, and D* for the light wavelength of 808, 980, and 1064 nm as a function of light power have also been performed as shown in Figure S6, demonstrating moderate performance.

Figure 5

(a) Responsivity, (b) EQE, and (c) detectivity as functions of light power density for 405 and 635 nm light.

From the angle-resolved polarized Raman, Bi2O2Se has an anisotropic in-plane crystal structure, the polarization absorption spectrum has also been measured, indicating that exfoliated Bi2O2Se flakes have the linear dichroism in the visible light (Figure S7), which can enable a polarization-sensitive photodetection. To investigate the polarization sensitivity of the heterostructure device under the polarized light illumination, we performed the measurement using the 405 and 635 nm polarized light achieved through a polarizer. The polarization angle was varied by rotating the polarizer with 30° steps. Figure a,b shows photocurrent as a function of polarization angles at VDS of 1 V for the 405 and 635 nm light, exhibiting the periodic variations with varying polarization angles. However, the angle-varied photocurrent exhibits different phases with a difference of about 90° for the 405 and 635 nm light. Based on the data, the angle-dependent photocurrent is plotted in polar coordinates (Figure c,d), showing a two-lobed shape with a period of 180°, which is consistent with the periodicity of Raman modes. It is interesting that the angle at which the photocurrent reaches the maximum is 0° for 635 nm and 90° for 405 nm, which may be attributed to the reversal of linear dichroism of Bi2O2Se.[33] To further confirm the reversal of polarized photocurrent, we prepared other heterostructures with thicker Bi2O2Se. The angle-dependent photocurrents for 405 and 635 nm light are shown in Figure S8 (Supporting Information), also demonstrating polarization-sensitive photocurrent and reverse phenomenon. A similar phenomenon of reversal in optical absorption and anisotropic photocurrent has also been observed in GeAs-based polarized photodetectors.[19] The dichroism ratio of photocurrent, defined as the value of maximum photocurrent divided by the minimum photocurrent, is calculated to be 2.2 and 4.9 for 405 and 635 nm light, respectively, which implies a high polarization-sensitive performance and is at par with that of other 2D low-symmetric materials.[39−41]Figure e,f shows the mapping plot of I–V curves at 405 and 635 nm light with varying light polarization angles, also depicting strong anisotropic photocurrent.

Figure 6

(a, b) Photocurrent as a function of polarization angles for the 405 and 635 nm light. (c, d) Same as (a, b) but in polar coordinates. (e, f) Mapping plot of I–V curves at 405 and 635 nm light with varying light polarization angles.

As control experiments, we also measured the angle-dependent photocurrent of individual WSe2 devices. The photocurrent as a. function of angles is shown in polar coordinates, presenting no polarization angle dependence (Figure S9). This indicates that the WSe2 component is in-plane isotropic materials, thus the anisotropic- and polarization-sensitive photocurrent of the heterostructures can be contributed from the Bi2O2Se component.

Conclusions

In summary, we prepared multilayer Bi2O2Se using the mechanically exfoliated method. The intensities of phonon vibration modes show significant angle dependence with a period of 180° through the angle-resolved polarized Raman spectra, this indicates that the surface reconstruction led to the anisotropic crystal structure. We then assembled Bi2O2Se, which acted as a polarized light sensitizer with 2D WSe2 layers as a photocarrier transport channel. Due to the p-type and n-type features of WSe2 and Bi2O2Se, respectively, an out-of-plane p–n heterojunction can be formed, resulting in an out-of-plane built-in field that can facilitate the separation of photoexcited electron–hole pairs. As a result, superior photodetection performance in terms of high responsivity, fast speed, and high detectivity has been achieved. The detectors exhibit a broadband spectral coverage from 405 to 1064 nm. More importantly, the photoresponse of the device is also polarization-sensitive due to the linear dichroism character of the Bi2O2Se layers. The dichroism ratio for photocurrent can reach as high as 2.2 and 4.9 for 405 and 635 nm, respectively. In addition, the photodetection performance is much superior compared to that of other reported polarization-sensitive photodetectors (Tables S1 and S2). We discovered the linear dichroism characters of exfoliated Bi2O2Se through the ARPR and polarization absorption measurement, which is successfully utilized for polarization-sensitive photodetector applications. This work expands the family of low-symmetry 2D materials and offers a new opportunity for the development of angle-resolved electronics and optoelectronics.

Experimental Section

Device Fabrication of Bi2O2Se/WSe2 Heterostructure

Multilayer Bi2O2Se and WSe2 were achieved by the mechanical exfoliation method from the bulk crystal (purchased from Shanghai OnWay Technology Co., Ltd.) on the 300 nm SiO2/Si substrate. The details of the exfoliation process can be found in the Supporting Information (Figure S10). Then, the Bi2O2Se layer was stacked on top of the WSe2 layer using the poly(vinyl alcohol) (PVA)/poly(dimethylsiloxane) (PDMS) assisted dry-transfer method. Briefly, the PVA solvent was injected on top of the PDMS placed on the glass, then the solid PVA/PDMS film was obtained by heating at 50 °C for 10 minutes. The film was stacked on Bi2O2Se. After heating at 90 °C for 5 min, the PDMS attached with Bi2O2Se was separated from the PVA by tweezers. Then the PDMS/PVA/Bi2O2Se film was stacked on the target WSe2. Finally, the PVA can be removed by soaking in water. The contact electrodes of Ti/Au (10/60 nm) were fabricated by an ultraviolet maskless photolithography machine (TuoTuo Technology Co., Ltd.).

Characterization

Raman spectra were obtained using a confocal microscope spectrometer (Nost Technology Co., Ltd., a laser excitation of 532 nm). The thickness and surface potential differences were measured by AFM and KPFM (Dimension FastScan from Bruker Co., Ltd.). The electrical and optical characteristics were measured using a probe stage equipped with a semiconductor device analyzer (Keithley 2636B) and a four-channel laser. All of the measurements were performed in ambient conditions.