Chiral 2D Perovskites with High Degree of Circularly Polarized

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Chiral 2D Perovskites with High Degree of Circularly Polarized Photoluminescence Jiaqi Ma, Chen Fang, Chao Chen, Long Jin, Jiaqi Wang, Shuai Wang, Jiang Tang, and Dehui Li ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00302 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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Chiral 2D Perovskites with High Degree of Circularly Polarized Photoluminescence Jiaqi Ma1, Chen Fang1, Chao Chen2, Long Jin1, Jiaqi Wang1, Shuai Wang1, Jiang Tang2 and Dehui Li1, 2* 1School

of Optical and Electronic Information, Huazhong University of Science and

Technology, Wuhan, 430074, China; 2Wuhan

National Laboratory for Optoelectronics, Huazhong University of Science

and Technology, Wuhan, 430074, China *Correspondence to: Email: [email protected].

1

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Abstract: Chiral materials are of particular interest and have a wide range of potential applications in life science, material science, spintronic and optoelectronic devices. Two-dimensional (2D) hybrid organic-inorganic lead halide perovskites have attracted increasing attention. Incorporating the chiral organic ligands into the layered leadiodide frameworks would introduce strong chirality in pure 2D perovskites for potential applications in circularly polarized light (CPL) emission and detection; nonetheless, studies on those aspects are still in their infancy. Here we report on the strong CPL emission and sensitive CPL detection in the visible wavelength range in pure chiral (R/S-MBA)2PbI4 (MBA=C6H5C2H4NH3) 2D perovskites, which are successfully synthesized with a needle shape and millimeter size by incorporating the chiral molecules. The chiral 2D perovskites (R-MBA)2PbI4 and (S-MBA)2PbI4 exhibit average degree of circularly polarized photoluminescence (PL) of 9.6% and 10.1% at 77 K, respectively, and a maximum degree of the circularly polarized PL of 17.6% is achieved in (S-MBA)2PbI4. The degree of circularly polarized PL dramatically decreases with increasing temperature, implying that the lattice distortion induced by the incorporated chiral molecules and/or temperature dependent spin flipping might be the origin for the observed chirality. Finally, CPL detection has been achieved with decent performance in our chiral 2D perovskite microplate/MoS2 heterostructural devices. The high degree of the circularly polarized PL and excellent CPL detection together with the layered nature of pure chiral 2D perovskites enables them to be a class of very promising materials for developing and exploring spin associated electronic devices based on the chiral 2D perovskites.

Keywords: chiral two-dimensional perovskites, degree of circularly polarized photoluminescence, chirality transfer, circularly polarized light emission, circularly polarized light detection

Chirality is a common property of the nature and refers to the phenomenon that the real substance cannot overlap with its mirror image. Such property not only have 2


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important applications in pharmaceutical and biological science,1-4 but also closely relates to spintronics which is of great importance for the quantum technology.5-10 Various drugs as well as many of proteins, and RNA are also chiral compounds and thus chirality plays an indispensable role in the fields of pharmacology, biology and medicine.11 In particular, chiral materials exhibit optical response to the circularly polarized light (CPL) described by the circular dichroism (CD) which is defined as the differential absorption of left-handed and right-handed CPL.9, 12, 13 Nevertheless, the variety of chiral inorganic matters that nature can provide to us is very limited. To this end, it is vital to develop chiral materials for both fundamental investigations and practical applications. Two-dimensional (2D) Ruddlesden–Popper type lead halide perovskites have recently been in the spotlight mainly due to their better long-term stability compared with their 3D counterparts.14-17 Additionally, 2D perovskites also exhibit optoelectronic properties including highly tunable band gap, strong quantum confinement effect as well as high optical absorption coe cients,16,

18-22

which render 2D perovskites

promising candidates for developing high-performance optoelectronic devices including light-emitting diodes,23-27 photonic lasers,28-30 photodetectors31-36 and solar cells.19, 37 In particular, benefiting from the diversity of available organic molecules and the ionic composition as well as the flexible crystal structure, 2D perovskites provide a versatile platform to form various semiconducting crystals with different optical and electronic properties. It is anticipated that chiral 2D perovskites possess the merits of both chiral materials and lead halide perovskite frameworks and thus would be a class of chiral semiconductors for the next-generation optical and spintronic devices.38-41 Chiral 2D perovskite single crystals and thin films have been synthesized with oppositely-signed CD by incorporating the chiral organic molecules.12,

39, 40

Recently, the circularly

polarized second harmonic generation has been achieved in chiral 2D perovskite nanowires39 while CD and CPL have been demonstrated in reduced-dimensional chiral perovskites both with and without applying the external magnetic field.40 Nevertheless, the degree of the circularly polarized photoluminescence (PL) is achieved to be as small 3


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as 3% at 2 K in the reduced-dimensional chiral perovskites and no CPL detection has been achieved in pure chiral 2D perovskites up to date.40 To this end, it is essential to study the circularly polarized PL and CPL detection in pure chiral 2D perovskites which are believed to have a stronger chirality due to the large mole fraction of the chiral ligands. Here we report on the synthesis of chiral 2D perovskite (S- and R-MBA)2PbI4 crystals (MBA=C6H5C2H4NH3) with high degree of circularly polarized PL and sensitive CPL detection, where S- and R-MBA represent the molecules with opposite chirality (Figure 1a). The as-synthesized (S- and R-MBA)2PbI4 crystals both have needle-shape and exhibit strong oppositely-signed CD signals. The average degree of circularly polarized PL is around 10 % with a maximum value of 17.6% at 77 K, nearly six times larger than that in the reduced-dimensional chiral perovskites at 2 K, which is 3% at zero magnetic field.40 The degree of the circularly polarized PL reduces with temperature, implying the reduction of the chirality transfer or the increase of the spin flip as the temperature increases. Finally, we have also demonstrated CPL detections in our chiral 2D perovskites with a responsivity of 0.45 A/W and a detectivity of 2.2×1011 Jones. Results and Discussion The schematic illustrations of pure chiral 2D perovskite crystal structures are shown in Figure 1a, where each corner-sharing [PbI6]4- octahedral layer is sandwiched by two layers of chiral organic chains and these basic units are held together by the weak Van der Waals force. In such way, the multi-quantum well structure will be naturally formed with the chiral organic chains embedded in. The chiral organic chains can be either lefthanded (S-MBA) or right-handed (R-MBA) enantiomers, endowing the resultant crystals with the oppositely-signed CD due to the chirality transfer from chiral organic chains to [PbI6]4- octahedra layers.12 In pure chiral 2D perovskites, the chiral organic chains have a larger mole fraction of chiral molecules of 67 % (Figure S1) and thus would be expected to exhibit stronger chirality compared with the reduced-dimensional perovskites.40 The chiral 2D perovskites were synthesized via a solution method16 by using S-MBA 4


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synthesized (S-, R-, and rac-MBA)2PbI4 crystals. The band edges of the chiral perovskites locate around 533 nm for both (S-MBA)2PbI4 and (R-MBA)2PbI4 crystals, consistent with those in the reported extinction spectra of the 2D perovskite films on soda lime glass.12 Nevertheless, the absorption edge shifts to 555 nm for the racemic (rac-MBA)2PbI4 crystals probably due to the rather different morphology of (racMBA)2PbI4 crystals compared with that of (S-MBA)2PbI4 and (R-MBA)2PbI4 crystals (Figure 2a). Figure 2b exhibits the steady-state PL spectra of the perovskite microplates excited by a linearly polarized 473-nm solid-state laser. The emission peaks of (SMBA)2PbI4, (R-MBA)2PbI4 and (rac-MBA)2PbI4 microplates are all around 510 nm with slight shifts, agreeing well with the absorption spectra (Figure 2a). According to previous reports, the emission for all three types of 2D perovskites originates from the free exciton emission.42, 43 The long tail of the PL spectra at the long wavelength side might be attributed to the self-trapped states, which have been commonly observed in 2D perovskites.44 b

(S-MBA)2PbI4 (rac-MBA)2PbI4

500

600

700

Wavelength (nm)

(R-MBA)2PbI4

Normalized PL (a.u.)

(R-MBA)2PbI4

c 200

800

(R-MBA)2PbI4


CD (mdeg)

a

Absorbance (a.u.)

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100


0

-100 500

600

700

800

Wavelength (nm)

500

600

700

800

Wavelength (nm)

Figure 2. Optical properties of our chiral 2D perovskites. (a, b) Normalized absorption (a) and steady-state PL spectra (b) of (R-MBA)2PbI4, (S-MBA)2PbI4 and (rac-MBA)2PbI4 microplates obtained by mechanical exfoliation. (c) CD spectra of (R-, S- and rac-MBA)2PbI4 films. We have further applied CD spectroscopy to confirm the chirality of our chiral perovskites. The CD measurement was carried out in the chiral and achiral thin films since it is impossible to directly measure the CD signals from the very thick assynthesized perovskite crystals. We prepared the continuous (S-, R-and rac-MBA)2PbI4 thin films via dissolving the as-synthesized crystals into DMF solution and then spincoating on the quartz substrates (see Methods). Strong CD signals have been observed at nearly same position but with oppositely-signed values at the peak positions for (S6


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MBA)2PbI4 and (R-MBA)2PbI4 thin films while (rac-MBA)2PbI4 shows a featureless flat signal (Figure 2c). The CD spectra of both (S-MBA)2PbI4 and (R-MBA)2PbI4 change the sign close to the exciton absorption edge, which can be interpreted as the Cotton effect.45 The presence of the oppositely-signed CD in (S-MBA)2PbI4 and (RMBA)2PbI4 verifies that the incorporation of chiral molecules can indeed introduce the chirality into 2D perovskites.12, 13, 39, 40 Furthermore, the observed CD could be ascribed to the transition of electrons in the electronic bands supported by the observed Cotton effect.13, 46 Thus, we anticipate that circularly polarized PL would be observed in our chiral 2D perovskites. a

b

40k

(R-MBA)2PbI4

+

20k

10k

+

5k

0

0 500 Wavelength (nm) 3k

500 Wavelength (nm)

550

550

d -

(rac-MBA)2PbI4

+

2k 1k 0 500 Wavelength (nm)

550

Sample Quantity

c

(S-MBA)2PbI4

-

Intensity (a.u.)

Intensity (a.u.)

-

Intensity (a.u.)

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(R-MBA)2PbI4 (S-MBA)2PbI4

10

5

8 6

5

3

3

1 0

4~7

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3 1

7~9 9~12 12~1515~18

|P| (%)

Figure 3. Polarization-sensitive PL spectra of our chiral 2D perovskites. (a-c) Circularly polarized PL spectra of (R-MBA)2PbI4 (a), (S-MBA)2PbI4 (b), and (racMBA)2PbI4 (c) excited by a 473-nm laser at 77 K. (d) Statistical histogram of the degree of circularly polarized PL |P| for (R- and S-MBA)2PbI4 excited by a 473-nm laser at 77 K. In order to explore the circularly polarized PL characteristics of the chiral 2D perovskites, we carried out the polarization-sensitive PL studies in the as-exfoliated (S-, R-, and rac-MBA)2PbI4 at 77 K. We have carefully calibrated our system in advance to rule out any possible artifacts by using a depolarized halogen tungsten light. The PL peaks for (S-, R-, and rac-MBA)2PbI4 all locate around 505 nm as shown in Figure 3a-c. 7


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The obvious intensity difference is observed between the left-handed ( -) and righthanded ( +) circularly polarized PL for the chiral (S-MBA)2PbI4 and (R-MBA)2PbI4 microplates (Figure 3a, b). In contrast, the racemic (rac-MBA)2PbI4 microplates exhibit the same PL intensity between the left-handed and right-handed circularly polarized PL (Figure 3c). The circularly polarized PL spectra are consistent with the CD measurement, further confirming that the chirality was introduced in (S-MBA)2PbI4 and (R-MBA)2PbI4 by incorporating the chiral molecules.40 To quantify the degree of the circularly polarized PL, we introduce a parameter P defined as P=(Ileft-Iright)/(Ileft+Iright), wherein Ileft and Iright represent the intensity of the left-handed and right-handed circularly polarized PL, respectively.5,

10, 40

Figure 3d

displays the statistical histogram of the estimated |P| of (S-MBA)2PbI4 and (RMBA)2PbI4 microplates at 77 K. We have measured more than 16 microplates for each type of the chiral perovskites and the average |P| of (R-MBA)2PbI4 and (S-MBA)2PbI4 are 9.6% and 10.1%, respectively (Figure 3d and Figure S5). The relatively large variation of |P| among microplates might be due to the different thicknesses of the microplates since the absorbance difference between right-handed and left-handed CPL relies on the thickness of the samples. Strikingly, the maximum |P| value of (SMBA)2PbI4 can achieve 17.61%, which is more than 5 times larger than that in the reduced-dimensional perovskites at 2 K, which is 3% under zero magnetic field.40 This is expected since the pure chiral 2D single crystal perovskites have a larger mole fraction of chiral molecules (67%) leading to a stronger chirality compared with that in the reduced-dimensional polycrystal perovskite films. Furthermore, their polycrystal films contain hybrid perovskite phases leading to the energy funneling effect. During this process, the spin would undergo flip, which would further reduce the chirality of their samples.47 In addition, the grain boundary in their film samples would also enhance the spin flip. To study how the degree of the circularly polarized PL P evolves with temperature which is essential for light emitting applications, the temperature dependent polarization-sensitive PL measurement was performed from 77 K to 290 K in a 20 K step. For both (R-MBA)2PbI4 and (S-MBA)2PbI4 microplates, P value monotonously 8


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decreases as the temperature increases and nearly vanishes at room temperature (Figure 4 and Figure S6), which suggests that the chirality transfer from chiral molecules to perovskites is reduced with the increasing of the temperature. The exact underlying mechanism for such reduction of the chirality transfer with temperature is unclear yet. One possible reason is that the chirality is a result of the lattice distortion induced by the incorporated chiral molecules.48 Since the energy of excitation source (473-nm laser) we used to measure PL spectra is too small to directly excite the chiral molecules, it is impossible for the polarized electrons or holes to directly transfer from chiral molecules. Rather, the polarized electrons and holes should be directly generated inside the inorganic layer of the 2D perovskites, which is supported by the presence of the Cotton effect near the free exciton emission wavelength. The temperature dependent P value also suggests that the lattice distortion induced by the chiral molecules is likely to be the dominant factor for the chirality. As the temperature increases, the enhanced electron-phonon interact and thermal expansion interaction would reduce the lattice distortion and thus the reduced chirality (Figure S7).49 As a result, the measured P value gradually decreases with temperature. Besides, spin flip would be a possible origin for the decrease of the PL polarization with the increasing temperature.47 Nonetheless, further investigations are demanded to clarify the exact underlying mechanism. 20 (S-MBA)2PbI4

10

P (%)

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0 -10 |

(R-MBA)2PbI4

50 100 150 200 250 300 Temperature (K)

Figure 4. Degree of circularly polarized PL P as a function of temperature of two microplates for each type chiral 2D perovskites. Finally, we have successfully demonstrated that our as-synthesized chiral pure 2D perovskites can efficiently and selectively detect CPL at room temperature even though the degree of circularly polarized PL almost vanishes at room temperature. To efficiently separate the photogenerated carries, hBN/chiral perovskite/MoS2 9


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under a 518-nm + CPL irradiation with a power density of 1.25 mW/cm2. (e) The spectral response of the hBN/(R-MBA)2PbI4/MoS2 and hBN/(S-MBA)2PbI4/MoS2 device under different CPL illumination. (f-h) Transient photocurrent response (f), frequency-dependent photoresponse (g) and detectivity (D*) spectrum (h) of the hBN/(R-MBA)2PbI4/MoS2. All measurements are performed at a bias of 3 V and the excitation source for transient photocurrent response and frequency-dependent photoresponse is a 518-nm + CPL with a power density of 1.25 mW/cm2. The optical switch characteristic indicates the excellent repeatability and stability of our hBN/(R-MBA)2PbI4/MoS2 device with an Ion/Ioff ratio of ~14 under the 518-nm right-handed CPL illumination with a power density of 1.25 mW/cm2 at a bias of 3 V (Figure 5d). The response speed, characterized by the rise and fall time,50 is estimated to be around 100 ms for both rise time and fall time of the as-fabricated hBN/(RMBA)2PbI4/MoS2 device under a 518-nm right-handed CPL illumination (Figure 5e). Impressively, the responsivities (defined as R=Iph/P, where Iph is photocurrent and P is the incident light power)34 of the hBN/(R-MBA)2PbI4/MoS2 and hBN/(SMBA)2PbI4/MoS2 device exhibit an opposite trend when sensing the right-handed and left-handed CPL with the peak value around 519 nm (Figure 5f), which excellently matches that of CD spectra shown in Figure 2c. The peak value of the responsivity can be as large as 0.45 A/W, which is almost two order of magnitude larger than that in metamaterial based CPL detectors.51 Figure 5g displays the normalized photocurrent versus the input frequency under a 518-nm right-handed CPL illumination at a bias of 3 V, from which the 3-dB frequency is estimated to be 11 Hz.52 To calculate the specific detectivity, the noise equivalent power (NEP) spectrum of our photodetector have been measured according to the reported method (Figure S9).34, 53 It is obvious that the noise current of our hBN/(RMBA)2PbI4/MoS2 device is estimated to be 1.25×10-13 A at 11 Hz. Specific detectivity (D*), a figure of merit used to characterize performance for photodetectors,35, 50, 53 is calculated and shown in Figure 5h (See Methods). The peak value of D* in our CPL detector is evaluated to be 2.2×1011 Jones around 519 nm, which is on par with that in 3D perovskite/MoS2 photodetectors and one order of magnitude smaller than that of 3D perovskite/WS2 photodetectors.54, 55 Similar results of the hBN/(S-MBA)2PbI4/MoS2 device are achieved (Figure S8). To demonstrate that the CPL detection is not from 11


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multi-layer MoS2, we performed identical measurements on multilayer MoS2 twoprobe devices and found that the device shows a same responsivity for both left-handed and right-handed CPL illumination, suggesting that the CPL photodetections were due to the chiral 2D perovskites as the photoactive ingredients rather than MoS2 (Figure S10). Conclusions In summary, we have successfully demonstrated circularly polarized PL in pure 2D chiral perovskites with a maximum P value of 17% by the incorporation of chiral organic molecules. The as-synthesized crystals exhibit excellent crystalline quality and phase purity confirmed by the XRD and low-temperature PL studies. The degree of circularly polarized PL continuously decreases with the increase of the temperature, suggesting that lattice distortion induced by the chiral molecules might be response to the observed chirality in our samples. Finally, we achieved the CPL detection with decent performance by using our pure chiral 2D perovskites. Our studies not only develop the chirality associated optoelectronic applications and spintronic devices based on pure chiral 2D perovskites, but also shed light on exploring heterostructures by integrating the chiral 2D perovskites with other layered materials to achieve desired functionalities as demanding. Methods Reagents and Materials Used. 16301L30P0 P0

*

*

(S-MBA, 98%), and 1Q30P0

(R-MBA, 98%), (S)-(-)*

(rac-MBA, 99%)

were purchased from DiBai Chemistry Company (Shanghai, China). Lead oxide (PbO, 99%) was purchased from greagent Ltd. (Shanghai, China). Hypophoaphoeous acid (H3PO2) was purchased from Aladdin Ltd. (Shanghai, China). N,N-anhydrous dimethylformamide (DMF, 99.9%) and 57% aqueous hydriodic acid (HI) solution were purchased from Sigma-Aldrich (St. Louis, MO, USA). Materials Preparation. The 2 mmol/ml precursor R-MBAI/S-MBAI/rac-MBAI solution was synthesized by neutralizing R-MBA/S-MBA/rac-MBA with superfluous HI (57 wt % in H2O) aqueous solution in the ice bath under strong magnetic stirring for 2 h. To prepare (R-MBA)2PbI4, (S-MBA)2PbI4 and (rac-MBA)2PbI4, the as-synthesized 12


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R-MBAI/S-MBAI/rac-MBAI solution was mixed with PbO powder at 2:1 stoichiometric ratio in an aqueous HI/H3PO2 (4:1) mixture solvent under constant magnetic stirring at 150°C. Subsequently, the stirring was stopped and the final yellowcolored homogeneous solutions was naturally cooled down to room temperature. The resultant solution was left overnight to complete the growth before being isolated by suction filtration and thoroughly dried in the oven. Thin Film Preparation. First, the clean quartz substrates (2×2 cm2) were washed using ethanol in a sonicator for 20 min and then treated in a plasma-cleaner with oxygen plasma for 3 min. Second, the as-synthesized perovskite crystals were dissolved in DMF with a certain concentration (70 wt%) as the precursor solution and the continuous thin films were prepared on substrates by a spin-coating method at 2000 rpm for 30 s. Finally, the films were annealed at 65 °C for 3 min on a hot-plate to induce crystallization. Device Fabrication. The chiral 2D perovskite devices were fabricated on a 300-nm SiO2/Si substrate pre-treated by an oxygen plasma cleaner for 3 min. 5-nm Cr/50-nm Au two-probe electrodes were first defined by photolithography with a channel length of 10 m and followed by thermal evaporation and lift-off process. To fabricate MoS2/perovskite heterostructures, a few-layer MoS2 flake was first aligned and transferred onto one stripe of the predefined Au electrodes under the aid of optical microscope and manipulator. Subsequently, a perovskite flake with a thickness of ~100 nm was exfoliated from bulk crystals by using Scotch tape reported previously and then was transferred onto the MoS2 and the other stripe of Au electrodes to sever as the main light absorbent layer. Finally, an exfoliated hBN flake was stacked on the top of the asprepared heterostructure to protect it from being degraded in air. Material Characterizations. The structure was characterized by X-ray diffraction (XRD, Panalytical X’pert PRO MRD, Holland). Optical microscopy (OM) images were acquired by an Olympus BX53M system microscope. The scanning electron microscopy (SEM) images were captured in a TESCAN scanning electron microscope. Ultraviolet–vis absorbance spectra were performed on a CRAIC 20 micro-spectra photometer with an aperture size of 10 µm. The PL spectra were measured in a 13


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backscattering geometry on a Horiba HR550 Raman spectrometer equipped with a 600 gr/mm granting excited by a linearly polarized 473-nm laser with a power of 0.13 W. The circularly polarized PL was analyzed by sending the emission through a quarterwave plate followed by a polarizer. The transmission CD data was collected using a CD spectrometer (J-810, JASCO). Device Characterizations. Optoelectronic characterizations of the photodetectors were carried out on a home-built photoconductivity measurement system in ambient condition. A 518-nm laser with a power density of 1.25 mW/cm2 was used as the light source for the current-voltage and optical switch characteristics measurements. The incident light power was tuned by a neutral filter and monitored by a pyroelectric detector (Gentec, model APM (D)). For the spectral response measurement, a Horiba JY HR320 monochromator coupled with a quartz tungsten halogen lamp (250 W) was used to provide the monochromatic light beam. The output monochromatic light beam was perpendicularly irradiated onto our devices. The photocurrent was amplified by a low-noise amplifier (Stanford SR570) and subsequently read by a lock-in amplifier (Stanford SR830) coupled with a mechanical chopper (Stanford SR540). The response speed and bandwidth were measured by a digital oscilloscope (Tektronix MDO3032) coupled with a computer-controlled analogue-to-digital converter (National Instruments model 6030E). The noise equivalent power (NEP) calculated by NEP = (Sn) 1/2/(RB1/2),

where Sn is the noise spectral density, R is the responsivity and B is the

device bandwidth. The noise spectral density and device bandwidth were recorded by a lock-in amplifier (Stanford SR830). Specific detectivity (D*) of the as-fabricated device can be calculated by D*=(AB)1/2/NEP, where A is the area of the photosensitive region of the detectors. Acknowledgments This work was supported by the NSFC (61674060) and the Fundamental Research Funds for the Central Universities, HUST (2017KFYXJJ030, 2017KFXKJC003, 2017KFXKJC002). We thank Hong Cheng engineer in the Analytical and Testing Center of Huazhong University of Science and Technology for the support in CD 14


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measurement and thank the Center of Micro-Fabrication of WNLO for the support in device fabrication. Associated Content Supporting Information Supporting Information Available: The mole fraction of the chiral ligands; OM image; SEM-EDX images; XRD patterns; scatter diagram of circularly polarized PL; variable temperature degree of circular polarization P; variable temperature PL spectral; optoelectronic properties and

noise power

density

spectrum of

the

(S-

MBA)2PbI4/MoS2/hBN device; optoelectronic properties of the MoS2 device. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author * E-mail: [email protected] Conflict of Interest The authors declare no conflict of interest.

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Chiral 2D Perovskites with High Degree of Circularly Polarized

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