Oriented Growth of Ultrathin Single Crystals of 2D Ruddlesden

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Functional Nanostructured Materials (including low-D carbon)

Oriented-Growth of Ultrathin Single Crystals of 2D Ruddlesden-Popper Hybrid Lead Iodide Perovskite for High-Performance Photodetectors Xianxiong He, Yaguang Wang, Kun Li, Xi Wang, Peng Liu, Yijun Yang, Qing Liao, Tianyou Zhai, Jiannian Yao, and Hongbing Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01825 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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ACS Applied Materials & Interfaces

Oriented-Growth of Ultrathin Single Crystals of 2D Ruddlesden−Popper Hybrid Lead Iodide Perovskite for High-Performance Photodetectors Xianxiong He †, Yaguang Wang‡, Kun Li§, Xi Wang, Peng Liu†, Yijun Yang, Qing Liao§, Tianyou Zhai‡, Jiannian Yao§,†, Hongbing Fu*,†,§ † Institute of Molecular Plus, Department of Chemistry, Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China. ‡ State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering Huazhong University of Science and Technology, Wuhan 430074, P. R. China. § Beijing Key Laboratory for Optical Materials and Photonic Devices, Department of Chemistry, Capital Normal University, Beijing 100048, P. R. China.  Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing Jiaotong University, Beijing, P. R. China. KEYWORDS: Orientated growth, 2D perovskite, single crystal film, mechanical exfoliation, photodetector

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ABSTRACT: As compared with their 3D counterparts, layered 2D Ruddlesden-Popper perovskites (2D RPPs) in the formula of (A′)2An-1PbnX3n+1 exhibit improved photo- and moisture-stability. To develop flexible single-crystalline electronics and wearable devices, costeffective growth of large-area single crystals of 2D RPPs with large lateral size, ultrathin thickness, parallel orientation, and well-defined and controlled n values are highly desirable, but remain still a challenge. Here, we modified the space-confined aqueous solution growth method to fabricate single-crystalline films of (BA)2(MA)n−1PbnX3n+1 and n = 1, 2, 3, respectively, with the lateral size reaching millimeter and the thickness about 400-1000 nm. Moreover, the quantum well layers are found to be parallel to the substrate, well-suitable for lateral transport requirement. We successfully integrated ultrathin single-crystals of (BA)2(MA)n−1PbnX3n+1 (n=13) into a metal-semiconductor-metal two-terminal photodetector. The photoresponsive wavelength range was extended from 525 nm for n = 1, 590 nm for n = 2 to 630 nm for n = 3, respectively, as a result of the reduced exciton bandgap. The device of n = 2 single crystals shows the highest responsivity of 2.96 A/W and a detectivity of 1013 Jones, while the device of n = 3 shows the lowest dark current 10-14 A and therefore an on/off ratio reaching 2862 at the bias voltage of 1 V.

1. Introduction Three-dimensional (3D) lead-halide perovskites of APbX3 [A = methylammonium (MA) CH3NH3+ ,Cs+ or formamidinium (FA) HC(NH2)2+ and X = I, Br, or Cl] are solution-processable semiconductors with attractive properties, such as low trap state densities, high photoluminescence (PL) efficiencies, large absorption cross-sections and long carrier diffusion lengths, enabling great progresses in high-efficiency solar cells1-4, light emitting diodes5-7, solid-


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state lasers8-11 and photodetectors12-18. Nonetheless, the device instability owing to halide ion migration and atmospheric moisture represents a significant challenge in the commercialization of 3D-perovskite technologies. Recently, 2D Ruddlesden-Popper perovskites (RPPs) in the formula of (A′)2An-1PbnX3n+1 have been demonstrated to show improved photo- and chemicalstability than their 3D counterparts, because the perovskite framework [An-1MnX3n+1]2- layers are encapsulated and therefore protected between two hydrophobic organic A′ bulky cation layers1922.

The power conversion efficiency of 2D-RPPs solar cells had been reported as high as 15.3%

with no hysteresis and obviously improved moisture and heat stability23-26. Moreover, lightemitting diodes made by 2D-RPPs had also demonstrated electroluminescence quantum efficiency > 10%27-31. Note that layered 2D-RPPs naturally form quantum wells (QWs) owing to effective confinement of electron-hole pairs (excitons) within the corner-sharing lead-halide octahedra [An-1PbnX3n+1]2- layer, where the integer n value determines the QW thickness and consequently the exciton features, such as the band-gap of exciton absorption and emission, the exciton binding-energy, as well as the charge carrier dynamics20,23. The n = 1 case of (A′)2PbX4 has been the mostly studied layered perovskites in several optoelectronic devices32,33, whereas the 3D APbX3 is actually an extreme case of 2D-RPPs with n = . In most cases, solution-casting 2DRPP thin films for a specific stoichiometry ( > n > 1) are usually composed of a mixture of multiple QWs, which exhibit a wide distribution of n values and consequently a spread of exciton bandgaps27-29. In order to investigate the intrinsic optoelectronic properties, it is imperative to prepare single crystals of 2D-RPPs with pure n values. Although various methods have been developed to fabricate millimeter-size single crystals of 3D perovskites, previous solution-phase growth of 2D-RPP single crystals was limited to the n =


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1 cases34-38. Recently, Kanatzidis and co-workers synthesized (BA)2(MA)n−1PbnI3n+1 bulk crystals with n = 1, 2, 3, and 4, respectively, by adding C4H9NH3I and CH3NH3Cl to a hot aqueous solution containing HI, H3PO2, and PbO followed by slow-cooling process20. Unfortunately these bulk single crystals do not allow solution processing for large-area, thin-film optoelectronics. By modifying the hot aqueous solution cooling process, Leng et.al fabricated the controlled centimetre-sized (BA)2(MA)n−1PbnI3n+1 crystals with n =1-4, and even exfoliated the corresponding single quantum well layers39. Feng et.al demonstrated a capillary-bridge asymmetric-wettability topographical template method to fabricate single-crystalline nanowire arrays of (BA)2(MA)n−1PbnI3n+1 (n = 25)40. In these cases, the quantum-well [MAn-1PbnX3n+1]2layers are perpendicular to the substrate and separated from each other by organic BA layers, leading to high resistance in the interior of the nanowires and high conductivity at the edges of the nanowires. To develop flexible single-crystalline electronics and wearable devices with the lateral transport requirement, cost-effective growth of large-area single crystals of 2D-RPPs with large lateral size, ultrathin thickness, parallel orientation, and well-defined and controlled n values are highly desirable, but remain still a challenge. Here, we modified the space-confined aqueous solution growth method to fabricate single-crystalline films of (BA)2(MA)n−1PbnX3n+1 (n = 1, 2, 3) with the lateral size reaching millimeter, the thickness about 400-1000 nm and the quantum well layers being parallel to the substrate41. Furthermore, the thickness can be reduced to < 250 nm by using the classical mechanically exfoliating protocol. We successfully integrated exfoliated ultrathin single-crystals of (BA)2(MA)n−1PbnX3n+1 (n=1, 2, 3) into a metalsemiconductor-metal two-terminal photodetector and systematically investigate the photodetective properties at ambient condition. The photoresponsive wavelength range was extended from 525 nm for n = 1, 590 nm for n = 2 to 630 nm for n = 3, respectively, as a result of the


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reduced exciton bandgap. The device of n = 2 single crystals shows the highest responsivity of 2.96 A/W and a detectivity of 1013 Jones, while the device of n = 3 shows the lowest dark current 10-14 A and therefore an on/off ratio reaching 2862 at the bias voltage of 1 V.

2. Experimental Section 2.1 Chemicals and reagents. n-Butylamine [CH3(CH2)3NH2, 99+%] is purchased from acros. Lead(Ⅱ) oxide, 99.9% is purchased from alfa. Hydriodic acid, 57wt.% solution in H2O, 55-57% contains ≤1.5% hypophosphorous acid as stabilizer is purchased from Innochem. CH3NH3Cl is purchased from Shanghai MaterWinNew Materials Corporation. All salts and solvent were used as received without any further purification. 2.2 (BA)2(MA)n−1PbnI3n+1 (n=1, 2, 3) hydriodic acid solutions preparation. 10 mmol PbO powder was dissolved in 10 mL Hydriodic acid, 57wt.% solution in H2O, 55-57% contains ≤1.5% hypophosphorous acid as stabilizer by heating to boiling under constant magnetic stirring for about 5min, which formed a bright yellow solution A. In a separate beaker, 10mmol nCH3(CH2)3NH2 was neutralized with 10 mL Hydriodic acid, 57wt.% solution in H2O, 55-57% contains ≤1.5% hypophosphorous acid as stabilizer in an ice bath resulting in a clear pale yellow solution B. The hydriodic acid solutions of (BA)2PbI4 was synthesized by adding 500uL solution B into 1mL solution A heating to boiling under constant magnetic stirring. And addition of a mixture of 57% w/w aqueous HI solution 1mL in the boiling solution preserves there are not precipitation of perovskites when the solution cool to room temperature. The hydriodic acid solutions of (BA)2(MA)Pb2I7 was synthesized by adding 16.43mg CH3NH3Cl powder and 350uL solution B into 1mL solution A heating to boiling under constant magnetic


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stirring. And addition of a mixture of 57% w/w aqueous HI solution 1mL in the boiling solution preserves there are not precipitation of perovskites when the solution cool to room temperature. The hydriodic acid solutions of (BA)2(MA)2Pb3I10 was synthesized by adding 39.12mg CH3NH3Cl powder and 350uL solution B into 1mL solution A heating to boiling under constant magnetic stirring. And addition of a mixture of 57% w/w aqueous HI solution 1mL in the boiling solution preserves there are not precipitation of perovskites when the solution cool to room temperature. 2.3 Mechanical exfoliated 2D Ruddlesden-Popper hybird lead iodide perovskite single crystals. (BA)2(MA)n−1PbnX3n+1 (n=1, 2, 3) single crystal films with two-dimensional inorganic quantum well grow parallel to the substrate were attached to a regular scotch tape and pressed firmly to the tape for several seconds. The tape was gently unfolded leaving the single crystal films on the tape. Then press the adhesive section of the same piece of tape with single crystal films on the shiny-side of the Si/SiO2 substrate and pressed firmly to the tape for several seconds. It was then gently removed. 2.4 Solvent exfoliated 2D Ruddlesden-Popper hybird lead iodide perovskite single crystals translated on TEM grids. A piece of (BA)2(MA)n−1PbnX3n+1 (n=1, 2, 3) single crystal films on the glass substrate was added to 2 mL of anhydrous toluene and the mixture was ultrasonicated for 30 min to fully exfoliate and disperse the material. 10 μL of the suspension was then dropped on the TEM grids and dried at 60 °C for 10 min. 2.5 Characterization. The SEM and TEM images, X-ray diffraction (XRD) patterns, diffused absorption and emission spectra of (BA)2(MA)n−1PbnX3n+1 (n=1, 2, 3) single crystal films were examined using the same equipment and methods in our previous work46. The thickness of the


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exfoliated perovskite single crystals was characterized by atomic force microscope (SPM9700, Shimadzu). 2.6 Metal-semiconductor-metal photodetector fabrication and characterization. The mechanical exfoliated (BA)2(MA)n−1PbnX3n+1 (n=1, 2, 3) single crystals on the Si/SiO2 substrate were covered with TEM grids. 100nm of Au was deposited on the substrate by Nexdep, Angstrom Engineering. Then remove the TEM grids. The I-V characteristic of the devices were measured by semiconductor parameter analyzer (4200-SCS, Keithley). The devices was placed in probe station (CRX-6.5K, Lake Shore) and kept in a dark environment. For the light characteristic, a broadband laser-driven light source (LDLS, EQ-1500, Energetiq) calibrated by a UV-enhanced silicon photodiode as an incident light source. The response speed of devices was conducted using a probe station (TTPX, Lakeshore) together with a semiconductor analyzer (B1500A, Agilent).

3. Results and Discussion We modify the space-confined aqueous solution growth method to fabricate large orientation controlled (BA)2(MA)n−1PbnX3n+1 (n=1, 2, 3) single crystal films, shown in Figure 1a. First, 20μL (BA)2(MA)n−1PbnX3n+1 (n=1, 2, 3) hydriodic acid solutions drop on a 2×2μm clean glass substrate covered with another. Then place the apparatus in the drying oven at 80℃ until all the H2O and HI volatilized. As the solutions confined between two glass slips, the solvents only could volatilize at the edges of glass substrates, which controlled the evaporation rate of solvents slowly, in favour of the nucleation and growth of single crystal films. Figure 1(b, c, d) presents the

bright-field

microscopy

images

and

fluorescence

pictures

(inset)

of

(BA)2(MA)n−1PbnI3n+1(n=1,2,3) single crystal films, respectively. According to the bright-field


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microscopy images, we can see the clean uniform surfaces of fabricated single crystal films with few long cracks, and length of sides reaching millimeter range. The fluorescence pictures show the color of single crystal films turn from green to yellow and red, as the value increase from 1 to 2 and 3. Figure 1e presents the AFM image of a single crystal film. And the thickness vs width graph of three points in Figure 1e were shown in Figure 1f. It presents the uniform thickness about 400nm with sharp edges. Figure 1g shows the low-magnification and highmagnification (inset) SEM images of the single crystal film. We can also see the uniform clean surface and sharp edge with thickness about 500nm. Figure S1 presents the high-magnification SEM images of several different thickness (BA)2(MA)n−1PbnI3n+1 single crystal films. We can see the thickness of the films are about 400-1000nm. To investigate the crystal structure of these single crystal films, we used the solution exfoliated method to prepare small thin single crystals transferred on TEM grids. Figure 2(a, b, c) present high-magnification TEM images and corresponding selected-area electron diffraction patterns (inset) of solution exfoliated (BA)2(MA)n−1PbnI3n+1 (n=1-3) single crystals, respectively. We can see the stratified structure at the fractured edges from the high-magnification TEM images. And the selected-area electron diffraction patterns (inset) show the clear strong diffraction points presenting the high-crystalline of these single crystals. The calculated average in-plane lattice constants are a = 8.92, b=8.69Å for (BA)2PbI4 and a= 9.12, 9.05Å, c=8.96, 8.94Å for (BA)2(MA)n−1PbnI3n+1 (n=2, 3), respectively. The calculated lattice constants are little bigger than the works reported by Kanatzidis et.al20. The reasons might be the lattice thermal expansion when the electron beams striking on the samples. Figure 2e presents the XRD profiles of (BA)2(MA)n−1PbnI3n+1 (n=1-3) single crystal films grown on the glass substrates. All the XRD peaks can be perfectly indexed to the orthorhombic phase of (BA)2(MA)n−1PbnI3n+1 (n=1-3)


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perovskites. Due to the crystal growth method are different with the hot aqueous solution cooling process20, in the XRD profile of (BA)2(MA)2Pb3I10, there are some bare characteristic peaks of (BA)2(MA)Pb2I7 marked by red star. Furthermore, the XRD patterns of these perovskites are dominated by the (001) series of peaks for (BA)2PbI4 and (010) series of peaks for (BA)2(MA)n−1PbnI3n+1 (n=2, 3), which demonstrates the (001) and (010) planes parallel to the substrate, with out-of-plane d spacing are 1.33, 1.94, 2.61nm, respectively, well corresponding to the reported data of single crystals23. Figure 2d reveals the steady-state absorption and photoluminescence (PL) spectra of (BA)2(MA)n−1PbnI3n+1 (n=1-3) perovskite single crystal films on glass substrates. The excitation wavelength for recording the PL spectra is 400nm. The exciton absorption peaks at 512, 571, and 610nm and the PL peaks at 520, 581, and 620nm for (BA)2(MA)n−1PbnI3n+1 (n=1-3) perovskite single crystal films, respectively, well corresponding to data of the exfoliated crystals23. As the above-mentioned results demonstrate the large high-crystalline (BA)2(MA)n−1PbnI3n+1 (n=1-3) perovskite single crystal films with the layered halide perovskites are parallel to the substrate and separated by long BA+ ligands, in which the adjacent layers are stacked together by weak Vander Waals forces. Inspiring by the classical mechanical exfoliated method to prepare graphene41, we used the similar method to exfoliate the large (BA)2(MA)n−1PbnI3n+1 (n=1-3) perovskite single crystal films for fabricating the corresponding large fresh thin single crystals. Figure 3a presents the bright-field microscopy image of mechanical exfoliated large (BA)2(MA)n−1PbnI3n+1 (n=1-3) single crystals evaporated with 100nm Au electrode on silicon wafer. Figure S3 demonstrates the fluorescence and bright-field images of mechanical exfoliated (BA)2(MA)n−1PbnI3n+1 (n=1-3 ) single crystals. It shows the lateral length of exfoliated single crystals reaching 150μm. As the channel width between two Au electrodes about 30μm, we


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measure the thickness of dozens of exfoliated single crystals with the lateral length above 40μm by AFM, and the thickness vs counts distribution histogram shown in Figure 3b. It can be seen that 95% of the exfoliated single crystals thickness below 300nm, the final thin even below 50nm, and the most within the interval of 50-250nm. Figure(3c, S4) present the AFM image of several single crystals. And the thickness vs width graph of three points in Figure 3c show in Figure 1d. It presents the uniform thickness about 180nm with sharp edges. Figure 3e shows the scheme of carrier dynamics in the photodetector of single-crystalline (101)-oriented (BA)2(MA)Pb2I7 perovskite. And another devices show the similar structure. This device design realizes the carriers transport in the two-dimensional inorganic quantum well. We investigated the photodetector performance of devices above mentioned. Figure 4a demonstrates the I-V curves of mechanical exfoliated (BA)2(MA)n−1PbnI3n+1 (n=1-3) single crystals at dark state and the bias voltage from -1 to 1V, respectively. We can see the dark current Id of these single crystal devices are 0.572, 0.225, 0.0145 pA, respectively, at bias voltage of 1V. Figure 4b presents the I-V curves of corresponding single crystals at 500, 570, 590nm light sources in illumination intensity of 700μW/cm2 and the bias voltage from -1 to 1V, respectively. It shows the light currents Iph are 14.1, 89.77, 41.5pA, respectively, at bias voltage of 1V. According to Id and Iph of mechanical exfoliated (BA)2(MA)n−1PbnI3n+1 (n=1-3) single crystals at bias voltage of 1V, we calculate the nearly 24, 399, 2862 on/off current ratio, respectively. The lowest Id and highest on/off current ratio are 1.45×10-14A and 2862, when value n=3. Figure 4c presents the lg-lg relationships of illumination power density P vs light current Iph of mechanical exfoliated (BA)2(MA)n−1PbnI3n+1 (n=1-3) single crystals, respectively, operated at a bias voltage of 1 V. It shows the liner relationships of these three perovskite single crystals, and


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the slopes k of these liner relationships are 0.179, 0.396, and 0.585 for n=1, 2, 3, respectively. It means as value n increase from 1 to 3, the more photocurrent made as one unit illumination power density increase. At low illumination power density, we can see the Iph of (BA)2(MA)n−1PbnI3n+1 (n=1, 2) at same order of magnitudes are as dozens of times high as (BA)2(MA)2Pb3I10, which are well corresponding to the Id. From the bright-field microscopy images of devices shown in Figure S5, we measure the active area of 300, 600, 420μm2 for mechanical exfoliated (BA)2(MA)n−1PbnI3n+1 (n=1-3) single crystals, respectively. Then according to the definition of responsivity (R) R=(Iph-Id)/PA and simplified detectivity (D*) D*=R/(2qAId)-1/2, where P is illumination power density, A the active area of device, q the elementary charge, we plot the lg-lg relationships of illumination power density P vs responsivity (R) and detectivity (D*) as shown in Figure 4d,e. It also shows the liner relationships, and the values of R and D* decrease with illumination power density P increase. For responsivity (R) and detectivity (D*), (BA)2(MA)Pb2I7 shows dozens of times higher than (BA)2PbI4, but (BA)2(MA)2Pb3I10 shows weaker properties than (BA)2(MA)Pb2I7. The highest R and D* are 2.96A/W and 2.7×1013Jones when value n=2, which are the best of 2D RuddlesdenPopper hybird lead photodetectors that don’t have the protection and assistance of other material such as graphene and hBN have been reported and comparable with the typical 3D hybird perovskite single crystal photodetectors42,43,44. Figure 4f plots the normalized responsivity vs wavelength curves of mechanical exfoliated (BA)2(MA)n−1PbnI3n+1

(n=1-3)

single

crystal

devices.

The

highest

R

values

of

(BA)2(MA)n−1PbnI3n+1 (n=1-3) single crystal devices are at 500, 560, and 595nm. Comparing with the corresponding single crystals exciton absorption peaks at 510, 570, and 610nm, there are about 10nm blue shift. Different with the spectral responses of the typical 3D hybird


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perovskite single crystal photodetectors are consistent with the absorption spectrum, the 10nm blue shift maybe attribute to the exciton state of carriers in (BA)2(MA)n−1PbnI3n+1 (n=1-3) single crystals. There is more energy to dissociate the exciton into free electrons and holes as photocurrent. Figure 4g shows the I-T curves of mechanical exfoliated (BA)2(MA)Pb2I7 single crystals at a bias voltage of 1V with different illumination power density. It can be seen that the light current are stable at lower illumination power density and shows sharp peaks as the illumination switching on at high illumination power density. This maybe attribute to the pyroelectric effect of the materials as Wang et al have been reported45. Figure (4h, S6) demonstrate the I-T curves of mechanical exfoliated (BA)2(MA)n-1PbnI3n+1 (n=1-3) single crystals during on-off illumination switching, operated at a bias voltage of 1V and illumination power density P=676μW/cm2. We can see the devices of (BA)2(MA)n-1PbnI3n+1 (n=1, 2) single crystals show the pyroelectric effect, different with (BA)2(MA)2Pb3I10. The relationships of value n with the pyroelectric effect need more detail investigation. Figure 4i presents the enlarged view of the photocurrent response of (BA)2(MA)Pb2I7 during on-off illumination switching. As the limit of our experimental facilities, we only can test the response speed τr/d

Oriented Growth of Ultrathin Single Crystals of 2D Ruddlesden

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