Orthogonal Lithography for Halide Perovskite Optoelectronic

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Orthogonal Lithography for Halide Perovskite Optoelectronic Nanodevices Chun-Ho Lin, Bin Cheng, TingYou Li, Jose Ramon Duran Retamal, Tzu-Chiao Wei, Hui-Chun Fu, Xiaosheng Fang, and Jr-Hau He ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05859 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on December 27, 2018

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Orthogonal Lithography for Halide Perovskite Optoelectronic Nanodevices Chun-Ho Lin†§, Bin Cheng†§, Ting-You Li†, José Ramón Durán Retamal†, Tzu-Chiao Wei†, Hui-Chun Fu†, Xiaosheng Fang‡*, and Jr-Hau He†*

†Computer, Electrical, and Mathematical Sciences and Engineering (CEMSE) Division, King Abdullah University of Science & Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. ‡Department of Materials Science, Fudan University, Shanghai 200433, P. R. China.

§These authors contributed equally to this work. *E-mail: [email protected]., *E-mail: [email protected].


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ABSTRACT: Three-dimensional (3D) organic-inorganic hybrid halide perovskites have attracted great interest due to their impressive optoelectronic properties. Recently, the emergence of two-dimensional (2D) layered hybrid perovskites, with their excellent and tunable optoelectronic behavior, has encouraged researchers to develop the next generation of optoelectronics based on these 2D materials. However, device fabrication methods of scalable patterning on both types of hybrid perovskites are still lacking as these materials are readily damaged by the organic solvents in standard lithographic processes. In this report, we conceived the orthogonal processing and patterning method: Chlorobenzene and hexane, which are orthogonal to hybrid perovskites, are utilized in modified electron beam lithography (EBL) processes to fabricate perovskite-based devices without compromising their electronic or optical characteristics. As a proof-of-concept, we used the orthogonal EBL technique to fabricate a 2D layered single-crystal (C6H5C2H4NH3)2PbI4 photodetector featuring nanoscale patterned electrodes and superior photodetection ability with responsivity of 5.4 mA/W and detectivity of 1.07 × 1013 cm Hz1/2/W. Such orthogonal processing and patterning method are believed to fully enable the high-resolution, high-throughput fabrication of complex perovskite-based electronics in the near future.

Keywords: perovskite, lithography, orthogonal, patterning, photodetector. 3 ACS Paragon Plus Environment

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Organic-inorganic hybrid halide perovskites (MAPbX3, MA = CH3NH3+; X = Cl-, Br- or I-) have become one of the most studied and fastest-growing fields of optoelectronic materials to date.1-7 In the past few years, researchers have increased the power efficiency of perovskite-based solar cells to over 20%.8-10 Their superior properties, such as long carrier lifetimes, high carrier mobility, low trap-state density, and outstanding light-absorption,11-14 make hybrid perovskites extremely promising for various device applications, including transistors,15 photovoltaics,16 photosensing,17 light emission,18 and lasing.19 Recently, 2D layered organic-inorganic hybrid perovskites ((ANH3)2(CH3NH3PbX3)n-1PbX4, in which ANH3 is a functional organic group, and n is the number of perovskite layers between two ANH3 groups in the 2D quantum well (n = 1, 2, 3, 4, ∞)) have garnered increased attention due to their improved stability with moisture compared to 3D hybrid perovskites, as well as their highly performed device properties, which can be tuned by changing the number of perovskite layers in the 2D quantum well (n).20-22 For these reasons, hybrid perovskites have great potential to become key compounds for next-generation optoelectronics.

Hybrid perovskites are known for their capability to be synthesized by the all-solution process, which is one of the major advantages of perovskite electronics compared to those based on inorganic materials.23,24 However, the solubility of perovskites is also somewhat of a double-edged sword. Contact with most polar solvents will damage the perovskite, or even etch it completely, leading to significant


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restrictions on subsequent solution processing steps. This means that perovskites cannot be exposed to conventional lithographic solvents, including water, acetone, and even isopropyl alcohol for scaling down and scalability.25 Accordingly, most perovskite-based devices have been fabricated by using layer-by-layer processes including spin-coating, e-beam evaporation, sputtering, atomic layer deposition, shadow mask deposition, or printing to avoid the solvents involved in lithographic processing, significantly hampering the use of perovskite in integrated system due to the lack of the device scaling down capability, flexibility, wafer scalability and controllability.23,25-28

It is worth noting that owing to the superior optoelectronic properties, perovskite nanowires, nanoplates, and other nanostructures have also been utilized in lasing, LED, photodetectors, etc.29-32 However, due to the lack of applicable lithography technique, researchers only used shadow mask deposition or dropping the perovskite nanostructures over pre-patterned electrodes to fabricate perovskite nanodevices, which are not flexible.33,34 As a result, developing a perovskite compatible lithography process is the most pressing demand to enable flexible and numerous nanostructured perovskite devices for advanced optoelectronics applications. Recently, the conventional lithography to fabricate the perovskite microdevices using isopropyl alcohol and methyl isobutyl ketone has been carried out with limited success as the perovskite is degraded in highly polar solvents and the deterioration of materials’ performance occurs although it is a highly reproducible process.35 Moreover,


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researchers employed a modified lithography method to fabricate perovskite devices using protective interlayers and plasma etching to avoid direct contact of the perovskite with the solvents.36 However, the transfer of protective interlayers involves additional steps, which are complicated, expensive, and time-consuming. Therefore, the use of aggressive organic solvents deteriorating perovskite’ performance during the photoresist deposition, development and removal stages should be avoided for achieving advanced perovskite devices.

Chemical orthogonality is the vital concept in the organic semiconductor fabrication process: solvents are selected so that the resist layer can be deposited or removed without damaging the underlying perovskite.37-39 The challenge in patterning organic materials originates from the limited number of options regarding orthogonal solvents. In this report, in the search for universal, perovskite-friendly device fabrication process, we have identified benign two orthogonal solvents combined with specifically tailored patterning materials as a possible solution to this complex problem: chlorobenzene as aggressive solvent for dissolving the poly(methyl methacrylate) (PMMA) resist and hexane as non-effective solvent for cleaning the perovskites, so that acetone, isopropyl alcohol, and other materials commonly employed in EBL can be substituted for fabricating the perovskite-based devices. With an appropriate combination of those two orthogonal solvents in EBL process, we successfully fabricated 9.7-nm-thick 2D perovskite photodetectors featuring excellent photoresponse


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with 380-nm in channel length, various patterned structures, and atomically flat surface without etching, demonstrating feasibility, reliability, and reproducibility of the orthogonal lithography and the robustness of the perovskite during the orthogonal EBL process. The results demonstrated here may open the door to the era of the perovskite nanoelectronics.

Results and Discussion

First, we tested the orthogonality of 12 common solvents (Table 1) on a 3D hybrid CH3NH3PbBr3 perovskite (3D perovskite), as well as a 2D layered (C6H5C2H4NH3)2PbI4 perovskite (2D perovskite), whose crystal structures and powder X-ray diffraction (XRD) spectra are shown in Fig. 1 (single-crystal XRD results are given in Tables S1–S4). Among those solvents, methyl isobutyl ketone, chlorobenzene, ethyl ether, dichloromethane, N,N-dimethylformamide, and dimethyl sulfoxide can be used to attack PMMA, while deionized water, acetone, isopropyl alcohol, ethanol, methanol, and hexane are commonly used to clean the samples. To study how various solvents impacted the perovskites, we performed photoluminescence (PL) spectroscopy before and after immersing the crystals in different solvents. Our results demonstrated that there were no detectable changes in the PL spectra or the morphology of the perovskite crystals that had been immersed in hexane and chlorobenzene for 2 min (Fig. 2a,b), indicating these two solvents do not cause any damage or swelling to either perovskite. In 7 ACS Paragon Plus Environment

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contrast, after immersing these materials into the other ten solvents studied, the perovskite PL signals disappeared or were significantly weakened, and the crystals dissolved or eroded in a short time, as demonstrated in Fig. 2a, 2b, S1 and S2. Especially, we noted that the 2D perovskite can be damaged by these solvents in an extremely short time (3 sec) due to its layered structure and nanoscale thickness.

To further examine the perovskites’ chemical orthogonality toward hexane and chlorobenzene, we deposited Au electrodes on both the 3D and 2D bulk single crystals to measure their electrical properties. After immersion in hexane and chlorobenzene for 2 min, which is longer than the 100 s required for typical development and lift-off steps during lithographic patterning, the perovskites’ photocurrents and dark currents featured no significant change (Fig. S3), again confirming that these materials are not damaged by hexane or chlorobenzene. Thus, we can conclude that hexane and chlorobenzene feature excellent chemical orthogonality with the studied perovskites, as demonstrated by their unaffected electrical and optical characteristics after exposure to these solvents, as well as the retention of their crystal morphologies. This finding is particularly important for the fabrication of perovskite-based electronic and optoelectronic devices, since patterned materials can be added and processed using these orthogonal solvents without damaging the active material.

This chemical orthogonality can be explained by the relations between ionic property of perovskites and the polarity of solvents. The ionic property of perovskites comes from two parts: the 8 ACS Paragon Plus Environment

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chemical bonds and mobile ions in perovskites. Due to the large electronegativity difference between the organic groups and inorganic PbX frameworks, ionic bonds are the primarily interaction force in the perovskite crystal, leading to the ionic property of perovskite.40 Additionally, a considerable number of mobile ionic carriers have been observed in perovskites, such as ions (MA+, Pb2+, X-) and vacancies (VMA, VPb, VX), which cause the famous hysteresis effect and enhance the ionic property of materials.41-44 According to the principles of solubility, ionic materials tend to be dissolved in polar solvents. Therefore, most polar solvents can dissolve both normal and layered hybrid perovskites (an example illustration is shown in Fig. 2c), as described in equations (1) and (2), respectively: Polar solvents CH 3 NH 3PbX 3   PbX 2 (s)  + CH 3 NH 3 + X 

(1)

Polar solvents (ANH 3 ) 2 (CH 3 NH 3PbX 3 ) n-1PbX 4   nPbX 2 (s)  + (n-1)CH 3 NH 3+ + 2ANH 3+ + (n+1)X  (2)

Therefore, we must consider the polarities of the 12 solvents tested in this study in order to explain their different effects on the perovskites (Table 1).45-47 Hexane has almost no polarity due to its symmetric molecular structure and nonpolar bonds,48 making it an ideal solvent for processing and cleaning perovskite samples after different lithographic steps. On the other hand, chlorobenzene contains a very electronegative chlorine atom, but the molecule achieves a resonance stabilized state by delocalizing the chlorine electrons.49 As a result, chlorobenzene also possesses a very small polarity, which makes it


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unable to dissolve polar hybrid perovskites, and together with its ability to attack PMMA resists, makes it highly suitable for use in EBL processes for these compounds.

In this study, we chose the 2D perovskite nanosheet thinner than 20 nm as the active material to test the orthogonal EBL process, since the demand for a steady strategy of high-resolution fabrication on these materials due to their small size and ultrathin thickness. Before the orthogonal EBL testing, the absorption spectrum of 2D (C6H5C2H4NH3)2PbI4 perovskite was characterized by UV-visible measurement (Fig. 3a), which shows a sharp band edge cut-oﬀ corresponds to the PL obtained bandgap of 2.36 eV. Time-resolved photoluminescence (TRPL) measurement was performed to evaluate the carrier lifetime of 2D perovskite, in which the PL decay can be fitted by equation (3), and the obtained short lifetime (τ1) and long lifetime (τ2) are 1.9 ns and 24.4 ns, respectively (Fig. 3b).

 t   t  I  t  = C1exp    C2 exp    τ1   τ2 

(3)

Carrier mobility is another important factor for the optoelectronics materials. However, because the surface functional organic group of (C6H5C2H4NH3)2PbI4 perovskite is hydrophobic and not very conductive, the conductivity of (C6H5C2H4NH3)2PbI4 is significantly lower than 3D perovskites, making it difficult to determine the carrier mobility through conventional electrical measurements, such as time-of-flight, hall effect, and transistor measurement. Up to now, only optical method have been


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reported to measure the mobility of (C6H5C2H4NH3)2PbI4 perovskite, which is estimated around 10 cm2V-1s-1 from the optical pump−THz probe measurement.50 Nevertheless, the hydrophobic surface organic group also brings some benefits to the 2D perovskite. For examples, the perovskite’s stability in the air is enhanced, and the high dielectric confinement from organic group results in the strong exciton binding energy, which is favorable for lighting applications.51

In an ideal lithographic approach, the resist, developer, and stripping solvents should all be orthogonal to the perovskite in order to minimize any possible damage to the single crystal material. Otherwise, the perovskite could decompose to PbX2, resulting in considerably decreased device performance. To demonstrate the practicality of hexane- and chlorobenzene-based lithography for hybrid perovskite, we utilized these solvents to fabricate a photodetector using a modified EBL technique, which typically includes five steps: resist spin-coating, exposure, development, metal deposition, and lift-off, as shown in Fig. 4a. First, we prepared the 2D perovskite nanosheets on a SiO2/Si substrate using mechanical exfoliation and a dry transfer method (see Methods). We then employed a PMMA bilayer on the 2D perovskite nanosheet because of its good sensitivity under exposure to the electron beam and its ability to be easily lifted off. To make a metal-semiconductor-metal photodetector, we then patterned the sample using an electron beam system, followed by the development process with a 1:3 ratio of chlorobenzene and hexane solution. Because


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PMMA is highly soluble in chlorobenzene, the developer solution was diluted and weakened with hexane to ensure that only the exposed regions of the PMMA were removed. In the next step, we used an e-beam evaporator to deposit a 10 nm Ti adhesion layer, followed by 80 nm of Au electrodes for ohmic contact with the perovskite. After metal deposition, the entire sample was placed in chlorobenzene and heated to 60 °C to assist in the lift-off process until the unwanted regions of metal were fully removed with the dissolving PMMA layer. Finally, the samples were cleaned by hexane (the optical microscope images and PL characterizations of 2D perovskite before and after orthogonal EBL fabrication are shown in Fig S4).

A scanning electron microscopy (SEM) image of the orthogonal EBL-fabricated 2D perovskite photodetector is shown in Fig. 4b. More perovskite devices with various patterned structures are shown in Fig. S5 to demonstrate the reliability and reproducibility of this orthogonal EBL method. Briefly speaking, under the e-beam exposure conditions employed, the 380 nm channel length could be achieved without extensive optimization of the EBL parameters, which demonstrates this technique may have practical value for realizing nanoscale features for perovskite devices. To achieve even higher resolution using this orthogonal patterning method, more efforts are needed to optimize the orthogonal EBL parameters. Furthermore, we surveyed the topography of the device using atomic force microscopy (AFM) as shown in Fig. 4c, in which we found the surface of the sample remained atomically flat (3.2


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nm of the root mean square roughness of 2D perovskite nanosheet), demonstrating the robustness of the perovskite during the orthogonal EBL process. Because all organic-inorganic hybrid halide perovskites possess similar crystal structure and strong ionic bonding between the organic group and the inorganic PbX framework, they are expected to have similar chemically orthogonality as the 3D and 2D perovskites tested in this report, and the proposed orthogonal EBL method is suitable for various perovskites (a 2D (HOC2H4NH3)2PbI4 perovskite device is shown in Fig. S6 as an example).

The I-V curves of the completed perovskite photodetector under dark and solar AM 1.5G illuminated conditions are shown in Fig. 4d. Our results demonstrate that the photocurrent becomes saturated at 3.5 V. Due to the small size and thickness of the device, there are fewer photo-generated carriers in the 2D perovskite nanosheet compared to the bulk 3D perovskite, leading to a saturation of photocurrent at a much smaller bias voltage. Moreover, because the 2D perovskite featured hydrophobic phenylethyl ammonium as its surface functional organic group, the dark current was less than 1 picoampere, which minimizes power consumption. The photo-to-dark current ratio of the device is 10.8 at a bias of 5 V, confirming its excellent photosensing ability even after orthogonal processing and patterning. The perovskite photodetector also demonstrates reversible photoswitching behavior, indicating its stability during operation (Fig. 4e). The responsivity (R) of the photodetector is 5.4 mA/W at 5 V, based on equation R = Ip/(Pin × A), where Ip is the photogenerated current, Pin is the light


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intensity, and A is the active area.52 Generally, the responsivity of 2D perovskite photodetector increases with the number of perovskite layers (n).53,54 Compared with other shadow mask fabricated 2D perovskite nano-photodetectors with n = 1 (responsivities from 2 mA/W to 8 mA/W),53,54 our lithography fabricated device shows the similar responsivity. Detectivity (D*) is another important figure of merit for characterizing the sensitivity of photodetectors. Because the background dark current is greatly reduced by the functional organic group, the 2D perovskite photodetector in this study can reach high detectivity of 1.07 × 1013 cm Hz1/2/W, calculated using D* = R/(2eJd)1/2, where e is the electron charge and Jd is the dark current density.55 This detectivity value is much higher than 2D perovskite nano-photodetectors using other fabrication methods (detectivities: 4 × 1011 ~ 8 × 1011 cm Hz1/2/W),53 and even comparable with 3D perovskite based photodetectors (detectivities: 1012 ~ 1.37 × 1013 cm Hz1/2/W).56-59 To sum up, the high performance of our 2D perovskite photodetector in terms of responsivity and detectivity demonstrates the practicality, reliability, and reproducibility of the orthogonal lithography and the robustness of the perovskite during the orthogonal EBL process, enabling large scale, high-density, high-throughput fabrication of perovskite-based electronics.

In addition to patterning flexibility, EBL is regarded as the key technique for scaling down the device size and optimizing the performance of hybrid perovskite based electronics. Although the mean free path of the carriers in bulk crystal of 3D perovskite is long,60 the strong quantum confinement in


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nanometer thick nanosheet of both 2D and 3D perovskite will largely decrease the mean free path.50 As a result, benefiting from shorter carrier transport channel in perovskite nanodevices fabricated by the orthogoanl EBL technique, faster operation and high sensitivity are expected to achieve after the EBL process optimization. On the other hand, compared to the polycrystalline thin film device, our single crystal nanosheet device ruled out the domain boundaries, reducing the scattering effect on the moving carriers. Scalable, efficient and well prescribed orthogonal lithography processes demonstrated here shall shed light on the highly performing lead halide perovskite-based optoelectronic devices.

Conclusion In summary, we have shown that hexane and chlorobenzene are ideal orthogonal solvents even for single crystalline perovskites without deteriorating their optical and electrical properties. Hexane with almost no polarity due to its symmetric molecular structure and nonpolar bonds is an ideal solvent for processing and cleaning perovskite during lithographic steps while chlorobenzene possessing a very small polarity and ability to attack PMMA resists makes it unable to dissolve polar hybrid perovskites, and highly suitable for use in EBL processes for patterning perovskites. A reversible photoswitching device based on the 9.7-nm-thick single crystal 2D perovskite nanosheet with channel length as small as 380 nm has successfully fabricated without surface etching, thus realizing a nanoscale perovskite electronic device. The orthogonal EBL method is capable of patterning various structures on the


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perovskite, demonstrating its reliability and reproducibility. After the orthogonal lithography, the surface of the perovskite remained atomically flat, demonstrating the robustness of the perovskite during the EBL process. The orthogonal EBL process would open the door to high-resolution perovskite-based optoelectronics for mass production.

Methods Materials. The following reagents and materials were purchased from Sigma-Aldrich: PMMA, methyl isobutyl ketone (98.5%), chlorobenzene (anhydrous, 99.8%), diethyl ether (anhydrous, ≥ 99.0%), dichloromethane

(anhydrous,

≥

99.8%),

N,N-dimethylformamide

dimethylsulfoxide (≤ 0.02% water), isopropyl alcohol (≥

99.7%), ethanol (≥

(anhydrous,

99.8%),

99.8%), hydrobromic

acid (48 wt% in H2O, ≥ 99.99%), hydriodic acid (57 wt% in H2O, 99.99%), methylamine (40 wt% in H2O), phenethylamine (99%), lead (II) bromide (PbBr2; ≥ 98%), and lead (II) iodide (99%). We also purchased acetone (≥ 99.5%), methanol (≥ 99.9%), and hexane (HPLC) from Fisher Chemical.

Synthesis of 3D single crystal CH3NH3PbBr3 perovskite. First, methylammonium bromide was crystallized by adding ethanol to equimolar amounts of hydrobromic acid and methylamine. Then, we took these methylammonium bromide crystals and dissolved them with an equimolar amount of PbBr2 powder in N,N-dimethylformamide at 90 °C on a hotplate for 24 h. The resulting CH3NH3PbBr3 was then recrystallized by cooling the solution to room temperature. 16 ACS Paragon Plus Environment

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Synthesis of millimeter-sized 2D single crystal (C6H5C2H4NH3)2PbI4 perovskite. 0.5 g of PbI2 powder was added to hydriodic acid. The solution was heated at 90 °C on a hotplate until the PbI2 powder had completely dissolved. Then 0.1 ml phenethylamine was added to the solution very slowly. We continued to heat the reaction at 90 °C for 24 h, after which we cooled it down to room temperature. After 1 h, (C6H5C2H4NH3)2PbI4 single crystals as large as 10 mm2 appeared, which were filtered from the solution and cleaned by hexane to remove any hydriodic acid residue. All procedures were carried out in a 24 °C room that featured a humidity of 50%.

Preparation of few-layered nanosheets of 2D (C6H5C2H 4NH3)2PbI4 perovskite on SiO2/Si substrate. Thin nanosheets of the 2D perovskite were prepared by mechanical exfoliation using Scotch tape and then transferred to a polydimethylsiloxane (PDMS) film (Gel Film from Gel-Pak Company), which was fixed on a glass slide. Using optical microscopy, we selected nanosheets that were > 100 um2 in size and featured low thickness (according to their color contrast). A selected piece of the 2D perovskite nanosheet was then transferred to 50 nm thick SiO2, which was thermally grown on a Si substrate. The PDMS-assisted dry transfer method was used to ensure the sample was free of tape residue.61 The exact thickness of the nanosheet was determined by AFM. Only nanosheets thinner than 20 nm were selected for the EBL process.


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Preparation of 3D CH3NH3PbBr3 perovskite on SiO2/Si substrate for orthogonality tests. An equimolar amount of PbBr2 powder and the methylammonium bromide crystals were dissolved in N,N-dimethylformamide. Then a small drop of solution was added to the SiO2/Si substrate and heated to 60 °C on a hotplate to evaporate the N,N-dimethylformamide solvent. The microscale CH3NH3PbBr3 perovskite single crystals appear on the substrate after the solvent has fully evaporated.

Solvent

orthogonality

tests.

Before

the

orthogonality

tests,

twelve

CH3NH3PbBr3

and

(C6H5C2H4NH3)2PbI4 perovskite samples on SiO2/Si substrates were characterized with optical microscopy and PL spectroscopy. Then the CH3NH3PbBr3 perovskite samples were immersed in hexane, chlorobenzene, acetone, isopropyl alcohol, ethanol, methanol, deionized water, methyl isobutyl ketone, N,N-dimethylformamide, dimethyl sulfoxide, dichloromethane, and ethyl ether for 2 mins, respectively, while the (C6H5C2H4NH3)2PbI4 perovskite samples were immersed in hexane, chlorobenzene for 2 mins and the other ten solvents for 3 secs (due to almost immediate dissolution). After immersion, the samples were heated to 60 °C on a hotplate to remove the solvents. Then, we characterized the samples with optical microscopy and PL spectroscopy again to examine the status of the perovskites.

Orthogonal EBL processes for fabricating the 2D single crystal (C6H5C2H4NH3)2PbI4 photodetector. First, 495 PMMA (A4) was spin-coated on the perovskite sample at 4500 rpm for 30 s, followed by baking at 60 °C for 2 h. Then, another 950K PMMA (A4) layer was spin-coated using the 18 ACS Paragon Plus Environment

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same conditions. The sample was then put on a hotplate at 60 °C for 12 h. This baking temperature of 60 °C was used instead of the common 180 °C condition because (C6H5C2H4NH3)2PbI4 and CH3NH3PbBr3 will be severely degraded at temperatures above 100 °C and 140 °C, respectively.62 Note that the PMMA baking time must be long enough (> 2 h) to avoid bubble formation in the resist, which may result in cracking of the PMMA layer during later stages of the lithographic process, particularly e-beam evaporation of the electrodes. We then used an EBL system (Crestec, CABL-9000C series) to pattern the PMMA on the perovskite using a beam current of 1000 pA and a dose value of 50 µC/cm2. After e-beam exposure, the sample was put in the developer solution (chlorobenzene:hexane = 1:3) for 100 s to remove the exposed PMMA. In the next step, we used an e-beam evaporator to deposit metal Ti/Au (10 nm/80 nm) on the sample. After metal deposition, the whole sample was put into chlorobenzene and heated to 60 °C to perform the lift-off process. After unwanted parts of the metal were fully removed, the sample was then cleaned with hexane. Note that the 495 PMMA can be changed to other PMMA with smaller molecular weight, which can increase the success rate of the fabrication.

Single crystal and powder XRD. The structural details of the single crystal CH3NH3PbBr3 and (C6H5C2H4NH3)2PbI4 perovskites were surveyed using a Bruker KAPPA APEX DUO Diffractometer featuring IμS Cu radiation at 296 K (λ = 0.71073 Å), an APEX II 4K CCD detector, and a microfocus


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X-ray source. The powder XRD spectra were measured using a Bruker D8 Advance diffractometer (Bragg-Brentano geometry) equipped with a Cu Ka X-ray tube.

PL spectroscopy. A fluorescence microscope system (NTEGRA Spectra, NT-MDT) was used to investigate the PL spectra of single crystal CH3NH3PbBr3 and (C6H5C2H4NH3)2PbI4 with a 473 nm wavelength diode-pumped solid-state laser and a spot size of ~0.5 μm in diameter.

AFM characterization. The surface morphology of the 2D (C6H5C2H4NH3)2PbI4 samples was examined with a commercial multifunction AFM (Agilent 5400 AFM/SPM) using Bruker (RFESPA-75) Al-coated cantilevers. The tip curvature radius was ∼8 nm, and the resonance frequency was ∼75 kHz.

SEM characterization. The SEM images of the perovskite device were taken using a Quanta 200 SEM (FEI) at a voltage of 5 kV.

I-V characterization. A Keithley 4200-SCS semiconductor characterization system in combination with an EverBeing Cryogenic Probe Station CG-196-200 was used to measure the I-V curves of the perovskite device in the dark and under solar light AM 1.5G illumination in vacuum.


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Table 1. The 12 solvents used in the perovskite orthogonality tests and their polarity. The relative polarity characterizes the polarity of solvents relative to water, where the water is given the polarity index of 1. Purpose

Cleaning samples

Dissolving PMMA

Solvent

Relative polarity Reference

Hexane

0.009

[45]

Isopropyl alcohol

0.382

[45]

Acetone

0.5

[45]

Methanol

0.5

[45]

Ethanol

0.578

[46]

Water

1

Chlorobenzene

0.188

[47]

Ethyl ether

0.275

[45]

Dichloromethane

0.304

[45]

Methyl isobutyl ketone

0.412

[45]

N,N-dimethylformamide

0.627

[45]

Dimethyl sulfoxide

0.706

[45]


–

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Figure 1. The crystal structure and powder XRD spectra of the 3D CH3NH3PbBr3 and 2D (C6H5C2H4NH3)2PbI4

perovskites.

The

molecular

models

of

(a)

CH3NH3PbBr3

and

(b)

(C6H5C2H4NH3)2PbI4 perovskites, featuring cubic and layered triclinic crystal structures, respectively. The sharp peaks in the XRD patterns show the high purity of (c) CH3NH3PbBr3 and (d) (C6H5C2H4NH3)2PbI4 perovskites.


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Figure 2. Solvent orthogonality tests of the 3D and 2D perovskites. The PL spectra and optical images of (a) 3D and (b) 2D perovskites before and after immersion in hexane, chlorobenzene (CB), acetone, and isopropyl alcohol (IPA). The length of the scale bars is 10 µm. The orthogonality tests of 8 other


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solvents can be found in Fig. S1 and S2 in the Supporting Information. (c) Schematic diagram of the perovskite being dissolved by a polar solvent.

Figure 3. Optical characterizations of 2D (C6H5C2H4NH3)2PbI4 perovskite. (a) UV-visible absorption spectrum of 2D perovskite. (b) TRPL spectra at 526 nm. The fast and slow carrier lifetimes are 1.9 ns and 24.4 ns, respectively.


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Figure 4. EBL processes on the 2D perovskite and the resulting photodetector. (a) Illustration of the EBL processes on the perovskite using chlorobenzene (CB) and hexane. (b) SEM image of the 2D perovskite photodetector with EBL patterned Ti/Au (10 nm/80 nm) electrodes. (c) AFM topographical map of the perovskite device. The inset shows the topographical height along the line profile, which confirms the thickness of the perovskite nanosheet is around 9.7 nm. (d) I-V curves of the 2D perovskite photodetector in the dark and under AM 1.5G solar light. (e) The time-resolved photoresponse of the device at 5 V bias.


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ASSOCIATED CONTENT Supporting Information Additional experimental data is shown in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *J.H.H.: E-mail, [email protected]. *X.F.: E-mail, [email protected]. Author Contributions C.H.L., T.Y.L., X.F. and J.H.H. conceived the experiments. T.Y.L. and B.C. synthesized the perovskite crystals. B.C. executed the dry transfer of the perovskites. C.H.L., B.C., and T.Y.L. performed the perovskite orthogonality tests. C.H.L. and J.R.D.R. carried out the orthogonal EBL fabrication process. C.H.L. and T.Y.L. measured the I-V characteristics of the perovskite photodetector. C.H.L., B.C., T.C.W., H.C.F., and J.H.H. performed the characterization of materials and the data analysis. C.H.L.,


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B.C., and J.H.H. wrote the manuscript. All authors discussed the results and commented on the manuscript.

The authors declare no competing interests.

ACKNOWLEDGMENT

This work was financially supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) (OSR-2016-CRG5-3005), KAUST solar center (FCC/1/3079-08-01), and KAUST baseline funding.

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