Orthogonal Lithography for Halide PerovskiteOptoelectronic NanodevicesChun-Ho Lin,†,§ Bin Cheng,†,§ Ting-You Li,† Jose Ramon Duran 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

*S Supporting Information

ABSTRACT: 3D organic−inorganic hybrid halide perovskites have attracted great interestdue to their impressive optoelectronic properties. Recently, the emergence of 2D layeredhybrid perovskites, with their excellent and tunable optoelectronic behavior, has encouragedresearchers to develop the next generation of optoelectronics based on these 2D materials.However, device fabrication methods of scalable patterning on both types of hybridperovskites are still lacking as these materials are readily damaged by the organic solvents instandard lithographic processes. We conceived the orthogonal processing and patterningmethod: Chlorobenzene and hexane, which are orthogonal to hybrid perovskites, areutilized 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 superiorphotodetection ability with responsivity of 5.4 mA/W and detectivity of 1.07 × 1013 cm Hz1/2/W. Such orthogonalprocessing and patterning methods are believed to fully enable the high-resolution, high-throughput fabrication ofcomplex perovskite-based electronics in the near future.KEYWORDS: perovskite, lithography, orthogonal, patterning, photodetector

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 thepast few years, researchers have increased the power efficiencyof perovskite-based solar cells to >20%.8−10 Their superiorproperties, such as long carrier lifetimes, high carrier mobility,low trap-state density, and outstanding light absorption,11−14

make hybrid perovskites extremely promising for variousdevice applications, including transistors,15 photovoltaics,16

photosensing,17 light emission,18 and lasing.19 Recently, 2Dl a y e r ed o r g an i c− i n o r g an i c hyb r i d pe ro v s k i t e s((ANH3)2(CH3NH3PbX3)n−1PbX4, in which ANH3 is afunctional organic group and n is the number of perovskitelayers between two ANH3 groups in the 2D quantum well (n =1, 2, 3, 4, ∞)) have garnered increased attention due to theirimproved stability with moisture compared with 3D hybridperovskites as well as their high-performing device properties,which can be tuned by changing the number of perovskitelayers in the 2D quantum well (n).20−22 For these reasons,hybrid perovskites have great potential to become keycompounds 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 withthose based on inorganic materials.23,24 However, the solubilityof perovskites is also somewhat of a double-edged sword.Contact with most polar solvents will damage the perovskite oreven etch it completely, leading to significant restrictions onsubsequent solution-processing steps. This means that perov-skites cannot be exposed to conventional lithographic solvents,including water, acetone, and even isopropyl alcohol for scalingdown and scalability.25 Accordingly, most perovskite-baseddevices have been fabricated by using layer-by-layer processesincluding spin-coating, e-beam evaporation, sputtering, atomiclayer deposition, shadow mask deposition, or printing to avoidthe solvents involved in lithographic processing, significantlyhampering the use of perovskite in the integrated system dueto 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 optoelectronicproperties, perovskite nanowires, nanoplates, and othernanostructures have also been utilized in lasing, light-emittingdiodes, photodetectors, and so on.29−32 However, because of

Received: August 1, 2018Accepted: December 27, 2018Published: December 27, 2018

Artic

lewww.acsnano.orgCite This: ACS Nano 2019, 13, 1168−1176

© 2018 American Chemical Society 1168 DOI: 10.1021/acsnano.8b05859ACS Nano 2019, 13, 1168−1176

Dow

nloa

ded

via

KIN

G A

BD

UL

LA

H U

NIV

SC

I T

EC

HL

GY

on

Febr

uary

27,

201

9 at

12:

37:5

5 (U

TC

).

See

http

s://p

ubs.

acs.

org/

shar

ingg

uide

lines

for

opt

ions

on

how

to le

gitim

atel

y sh

are

publ

ishe

d ar

ticle

s.

the lack of applicable lithography technique, researchers onlyused shadow mask deposition or dropping the perovskitenanostructures over prepatterned electrodes to fabricateperovskite nanodevices, which are not flexible.33,34 As a result,developing a perovskite-compatible lithography process is themost pressing demand to enable flexible and numerousnanostructured perovskite devices for advanced optoelec-tronics applications. Recently, the conventional lithographyto fabricate the perovskite microdevices using isopropylalcohol and methyl isobutyl ketone has been carried out withlimited success because the perovskite is degraded in highlypolar solvents and the deterioration of materials’ performanceoccurs, although it is a highly reproducible process.35

Moreover, researchers employed a modified lithographymethod to fabricate perovskite devices using protectiveinterlayers and plasma etching to avoid direct contact of theperovskite with the solvents.36 However, the transfer ofprotective interlayers involves additional steps, which arecomplicated, expensive, and time-consuming. Therefore, theuse of aggressive organic solvents deteriorating perovskites’performance during the photoresist deposition, development,and removal stages should be avoided to achieve advancedperovskite devices.Chemical orthogonality is the vital concept in the organic

semiconductor fabrication process: Solvents are selected sothat the resist layer can be deposited or removed withoutdamaging the underlying perovskite.37−39 The challenge inpatterning organic materials originates from the limitednumber of options regarding orthogonal solvents. In thisArticle, in the search for a universal, perovskite-friendly devicefabrication process, we have identified two benign orthogonalsolvents combined with specifically tailored patterningmaterials as a possible solution to this complex problem,chlorobenzene as an aggressive solvent for dissolving thepoly(methyl methacrylate) (PMMA) resist and hexane as anoneffective solvent for cleaning the perovskites, so thatacetone, isopropyl alcohol, and other materials commonlyemployed in EBL can be substituted for fabricating theperovskite-based devices. With an appropriate combination ofthose two orthogonal solvents in the EBL process, wesuccessfully fabricated 9.7 nm thick 2D perovskite photo-detectors featuring excellent photoresponse with 380 nm inchannel length, various patterned structures, and atomically flatsurface without etching, demonstrating the feasibility, reli-ability, and reproducibility of the orthogonal lithography andthe robustness of the perovskite during the orthogonal EBLprocess. The results demonstrated here may open the door toan era of perovskite nanoelectronics.

RESULTS AND DISCUSSIONFirst, we tested the orthogonality of 12 common solvents(Table 1) on a 3D hybrid CH3NH3PbBr3 perovskite (3Dperovskite) as well as a 2D layered (C6H5C2H4NH3)2PbI4perovskite (2D perovskite), whose crystal structures andpowder X-ray diffraction (XRD) spectra are shown in Figure1 (single-crystal XRD results are given in Tables S1−S4).Among those solvents, methyl isobutyl ketone, chlorobenzene,ethyl ether, dichloromethane, N,N-dimethylformamide, anddimethyl sulfoxide can be used to attack PMMA, whereasdeionized water, acetone, isopropyl alcohol, ethanol, methanol,and hexane are commonly used to clean the samples. To studyhow various solvents impacted the perovskites, we performedphotoluminescence (PL) spectroscopy before and after

immersing the crystals in different solvents. Our resultsdemonstrated that there were no detectable changes in thePL spectra or the morphology of the perovskite crystals thathad been immersed in hexane and chlorobenzene for 2 min(Figure 2a,b), indicating that these two solvents do not causeany damage or swelling to either perovskite. In contrast, afterimmersing these materials into the other 10 solvents studied,the perovskite PL signals disappeared or were significantlyweakened, and the crystals dissolved or eroded in a short time,as demonstrated in Figure 2a,b and Figures S1 and S2. Inparticular, we noted that the 2D perovskite can be damaged bythese solvents in an extremely short time (3 s) due to itslayered structure and nanoscale thickness.To further examine the perovskites’ chemical orthogonality

toward hexane and chlorobenzene, we deposited Au electrodeson both the 3D and 2D bulk single crystals to measure their

Table 1. 12 Solvents Used in the Perovskite OrthogonalityTests and Their Polaritya

purpose solventrelativepolarity reference

cleaning samples hexane 0.009 45isopropyl alcohol 0.382 45acetone 0.5 45methanol 0.5 45ethanol 0.578 46water 1

dissolving PMMA chlorobenzene 0.188 47ethyl ether 0.275 45dichloromethane 0.304 45methyl isobutyl ketone 0.412 45N,N-dimethylformamide 0.627 45dimethyl sulfoxide 0.706 45

aRelative polarity characterizes the polarity of solvents relative towater, where the water is given the polarity index of 1.

Figure 1. Crystal structure and powder XRD spectra of the 3DCH3NH3PbBr3 and 2D (C6H5C2H4NH3)2PbI4 perovskites. Themolecu l a r mode l s o f (a) CH3NH3PbBr 3 and (b)(C6H5C2H4NH3)2PbI4 perovskites, featuring cubic and layeredtriclinic crystal structures, respectively. The sharp peaks in theXRD patterns show the high purity of (c) CH3NH3PbBr3 and (d)(C6H5C2H4NH3)2PbI4 perovskites.

ACS Nano Article

DOI: 10.1021/acsnano.8b05859ACS Nano 2019, 13, 1168−1176

1169

electrical properties. After immersion in hexane and chlor-obenzene for 2 min, which is longer than the 100 s required fortypical development and lift-off steps during lithographicpatterning, the perovskites’ photocurrents and dark currentsfeatured no significant change (Figure S3), again confirmingthat these materials are not damaged by hexane orchlorobenzene. Thus we can conclude that hexane andchlorobenzene feature excellent chemical orthogonality withthe studied perovskites, as demonstrated by their unaffectedelectrical and optical characteristics after exposure to thesesolvents as well as the retention of their crystal morphologies.This finding is particularly important for the fabrication ofperovskite-based electronic and optoelectronic devices becausepatterned materials can be added and processed using theseorthogonal solvents without damaging the active material.This chemical orthogonality can be explained by the

relations between the ionic property of perovskites and thepolarity of solvents. The ionic property of perovskites comesfrom two parts: the chemical bonds and mobile ions inperovskites. Because of the large electronegativity differencebetween the organic groups and inorganic PbX frameworks,ionic bonds are the primary interaction force in the perovskitecrystal, leading to the ionic property of perovskite.40

Additionally, a considerable number of mobile ionic carriershave 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 materi-als.41−44 According to the principles of solubility, ionicmaterials tend to be dissolved in polar solvents. Therefore,most polar solvents can dissolve both normal and layeredhybrid perovskites (an example illustration is shown in Figure2c), as described in eqs 1 and 2, respectively

CH NH PbX PbX CH NH X3 3 3polar solvents

2(s) 3 3⎯ →⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ ↓ + ++ −

(1)

n n n

(ANH ) (CH NH PbX ) PbX

PbX ( 1)CH NH 2ANH ( 1)X

n3 2 3 3 3 1 4

polar solvents2(s) 3 3 3⎯ →⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ ↓ + − + + +

−

+ + −

(2)

Therefore, we must consider the polarities of the 12 solventstested in this study to explain their different effects on theperovskites (Table 1).45−47 Hexane has almost no polarity dueto its symmetric molecular structure and nonpolar bonds,48

making it an ideal solvent for processing and cleaningperovskite samples after different lithographic steps. On thecontrary, chlorobenzene contains a very electronegativechlorine atom, but the molecule achieves a resonance stabilizedstate by delocalizing the chlorine electrons.49 As a result,chlorobenzene also possesses a very small polarity, whichmakes it unable to dissolve polar hybrid perovskites and,

Figure 2. Solvent orthogonality tests of the 3D and 2D perovskites. The PL spectra and optical images of (a) 3D and (b) 2D perovskitesbefore and after immersion in hexane, chlorobenzene (CB), acetone, and isopropyl alcohol (IPA). The length of the scale bars is 10 μm. Theorthogonality tests of eight other solvents can be found in Figures S1 and S2. (c) Schematic diagram of the perovskite being dissolved by apolar solvent.

ACS Nano Article

DOI: 10.1021/acsnano.8b05859ACS Nano 2019, 13, 1168−1176

1170

together with its ability to attack PMMA resists, makes ithighly 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 EBLprocess because of the demand for a steady strategy of high-resolution fabrication on these materials due to their small sizeand ultrathin thickness. Before the orthogonal EBL testing, theabsorption spectrum of 2D (C6H5C2H4NH3)2PbI4 perovskitewas characterized by UV−visible measurement (Figure 3a),

which shows that a sharp band-edge cutoff corresponds to thePL obtained band gap of 2.36 eV. Time-resolved photo-luminescence (TRPL) measurement was performed toevaluate the carrier lifetime of 2D perovskite, in which thePL decay can be fitted by eq 3, and the obtained short lifetime(τ1) and long lifetime (τ2) are 1.9 and 24.4 ns, respectively(Figure 3b).

ikjjjjj

y{zzzzz

ikjjjjj

y{zzzzzI t C

tC

t( ) exp exp1

12

2τ τ= − + −

(3)

Carrier mobility is another important factor for theoptoelectronics materials. However, because the surfacefunctional organic group of (C6H5C2H4NH3)2PbI4 perovskiteis hydrophobic and not very conductive, the conductivity of(C6H5C2H4NH3)2PbI4 is significantly lower than that of 3Dperovskites, making it difficult to determine the carrier mobilitythrough conventional electrical measurements, such as time-of-flight, Hall effect, and transistor measurement. Up to now, onlythe optical method has been reported to measure the mobilityof (C6H5C2H4NH3)2PbI4 perovskite, which is estimated to be∼10 cm2 V−1 s−1 from the optical pump−THz probemeasurement.50 Nevertheless, the hydrophobic surface organicgroup also brings some benefits to the 2D perovskite. Forexample, the perovskite’s stability in the air is enhanced, andthe high dielectric confinement from the organic group resultsin the strong exciton binding energy, which is favorable forlighting applications.51

In an ideal lithographic approach, the resist, developer, andstripping solvents should all be orthogonal to the perovskite tominimize any possible damage to the single-crystal material.Otherwise, the perovskite could decompose to PbX2, resultingin considerably decreased device performance. To demonstratethe practicality of hexane- and chlorobenzene-based lithog-raphy for hybrid perovskite, we utilized these solvents tofabricate a photodetector using a modified EBL technique,which typically includes five steps: resist spin-coating,exposure, development, metal deposition, and lift-off, asshown in Figure 4a. First, we prepared the 2D perovskite

nanosheets on a SiO2/Si substrate using mechanical exfoliationand a dry-transfer method (see Methods). We then employeda PMMA bilayer on the 2D perovskite nanosheet because of itsgood sensitivity under exposure to the electron beam and itsability to be easily lifted off. To make a metal−semi-conductor−metal photodetector, we then patterned the sampleusing an electron beam system, followed by the developmentprocess with a 1:3 ratio of chlorobenzene and hexane solution.Because PMMA is highly soluble in chlorobenzene, thedeveloper solution was diluted and weakened with hexane toensure that only the exposed regions of the PMMA wereremoved. In the next step, we used an e-beam evaporator todeposit a 10 nm Ti adhesion layer, followed by 80 nm of Auelectrodes for ohmic contact with the perovskite. After metaldeposition, the entire sample was placed in chlorobenzene andheated to 60 °C to assist in the lift-off process until theunwanted regions of metal were fully removed with thedissolving PMMA layer. Finally, the samples were cleaned byhexane. (The optical microscope images and PL character-izations of 2D perovskite before and after orthogonal EBLfabrication are shown in Figure S4.)A scanning electron microscopy (SEM) image of the

orthogonal EBL-fabricated 2D perovskite photodetector isshown in Figure 4b. More perovskite devices with variouspatterned structures are shown in Figure S5 to demonstrate thereliability and reproducibility of this orthogonal EBL method.Briefly speaking, under the e-beam exposure conditionsemployed, the 380 nm channel length could be achievedwithout extensive optimization of the EBL parameters, whichdemonstrates that this technique may have practical value forrealizing nanoscale features for perovskite devices. To achieveeven higher resolution using this orthogonal patterningmethod, more efforts are needed to optimize the orthogonalEBL parameters. Furthermore, we surveyed the topography ofthe device using atomic force microscopy (AFM), as shown inFigure 4c, in which we found that the surface of the sampleremained atomically flat (3.2 nm of the root-mean-squareroughness of 2D perovskite nanosheet), demonstrating therobustness of the perovskite during the orthogonal EBLprocess. Because all organic−inorganic hybrid halide perov-skites possess a similar crystal structure and strong ionicbonding between the organic group and the inorganic PbXframework, they are expected to have similar chemicallyorthogonality as the 3D and 2D perovskites tested in thisreport, and the proposed orthogonal EBL method is suitablefor various perovskites. (A 2D (HOC2H4NH3)2PbI4 perovskitedevice is shown in Figure S6 as an example.)The I−V curves of the completed perovskite photodetector

under dark and solar AM 1.5G illuminated conditions areshown in Figure 4d. Our results demonstrate that thephotocurrent becomes saturated at 3.5 V. Because to thesmall size and thickness of the device, there are fewerphotogenerated carriers in the 2D perovskite nanosheetcompared with the bulk 3D perovskite, leading to a saturationof photocurrent at a much smaller bias voltage. Moreover,because the 2D perovskite featured hydrophobic phenyl-ethylammonium as its surface functional organic group, thedark current was <1 pA, which minimizes power consumption.The photo-to-dark current ratio of the device is 10.8 at a biasof 5 V, confirming its excellent photosensing ability even afterorthogonal processing and patterning. The perovskite photo-detector also demonstrates reversible photoswitching behavior,indicating its stability during operation (Figure 4e). The

Figure 3. Optical characterizations of 2D (C6H5C2H4NH3)2PbI4perovskite. (a) UV−visible absorption spectrum of 2D perovskite.(b) TRPL spectra at 526 nm. The fast and slow carrier lifetimes are1.9 and 24.4 ns, respectively.

ACS Nano Article

DOI: 10.1021/acsnano.8b05859ACS Nano 2019, 13, 1168−1176

1171

responsivity (R) of the photodetector is 5.4 mA/W at 5 Vbased on the equation R = Ip/(Pin × A), where Ip is thephotogenerated current, Pin is the light intensity, and A is theactive area.52 In general, the responsivity of 2D the perovskitephotodetector increases with the number of perovskite layers(n).53,54 Compared with other shadow mask fabricated 2Dperovskite nanophotodetectors with n = 1 (responsivities from2 to 8 mA/W),53,54 our lithography fabricated device showsthe similar responsivity. Detectivity (D*) is another importantfigure of merit for characterizing the sensitivity of photo-detectors. Because the background dark current is greatlyreduced by the functional organic group, the 2D perovskitephotodetector in this study can reach a high detectivity of 1.07× 1013 cm Hz1/2/W, calculated using D* = R/(2eJd)

1/2, where eis the electron charge and Jd is the dark current density.

55 Thisdetectivity value is much higher than that of 2D perovskitenanophotodetectors using other fabrication methods (detec-tivities: (4 to 8) × 1011 cm Hz1/2/W)53 and even comparableto 3D perovskite-based photodetectors (detectivities: 1012 to1.37 × 1013 cm Hz1/2/W).56−59 In summary, the highperformance of our 2D perovskite photodetector in terms of

responsivity and detectivity demonstrates the practicality,reliability, and reproducibility of the orthogonal lithographyand the robustness of the perovskite during the orthogonalEBL process, enabling large-scale, high-density, and 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 optimizingthe performance of hybrid-perovskite-based electronics.Although the mean free path of the carriers in bulk crystal of3D perovskite is long,60 the strong quantum confinement inthe nanometer-thick nanosheets of both 2D and 3D perov-skites will largely decrease the mean free path.50 As a result,benefiting from the shorter carrier transport channel inperovskite nanodevices fabricated by the orthogonal EBLtechnique, faster operation and high sensitivity are expected tobe achieved after the EBL process optimization. On thecontrary, compared with the polycrystalline thin-film device,our single-crystal nanosheet device ruled out the domainboundaries, reducing the scattering effect on the movingcarriers. Scalable, efficient, and well-prescribed orthogonallithography processes demonstrated here shall shed light on

Figure 4. EBL processes on the 2D perovskite and the resulting photodetector. (a) Illustration of the EBL processes on the perovskite usingchlorobenzene (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 thatthe thickness of the perovskite nanosheet is ∼9.7 nm. (d) I−V curves of the 2D perovskite photodetector in the dark and under AM 1.5Gsolar light. (e) Time-resolved photoresponse of the device at 5 V bias.

ACS Nano Article

DOI: 10.1021/acsnano.8b05859ACS Nano 2019, 13, 1168−1176

1172

the high-performing lead-halide perovskite-based optoelec-tronic devices.

CONCLUSIONSIn summary, we have shown that hexane and chlorobenzeneare ideal orthogonal solvents even for single-crystallineperovskites without deteriorating their optical and electricalproperties. Hexane with almost no polarity due to itssymmetric molecular structure and nonpolar bonds is anideal solvent for processing and cleaning perovskite duringlithographic steps, whereas chlorobenzene possessing a verysmall polarity and ability to attack PMMA resists makes itunable to dissolve polar hybrid perovskites and highly suitablefor use in EBL processes for patterning perovskites. Areversible photoswitching device based on the 9.7 nm thicksingle-crystal 2D perovskite nanosheet with channel length assmall as 380 nm has been successfully fabricated withoutsurface etching, thus realizing a nanoscale perovskite electronicdevice. The orthogonal EBL method is capable of patterningvarious structures on the perovskite, demonstrating itsreliability and reproducibility. After the orthogonal lithography,the surface of the perovskite remained atomically flat,demonstrating the robustness of the perovskite during theEBL process. The orthogonal EBL process would open thedoor to high-resolution perovskite-based optoelectronics formass production.

METHODSMaterials. 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-dimethylfor-mamide (anhydrous, 99.8%), dimethyl sulfoxide (≤0.02% water),isopropyl alcohol (≥99.7%), ethanol (≥99.8%), hydrobromic acid (48wt % 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 alsopurchased 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 ethanolto equimolar amounts of hydrobromic acid and methylamine. Then,we took these methylammonium bromide crystals and dissolved themwith an equimolar amount of PbBr2 powder in N,N-dimethylforma-mide at 90 °C on a hot plate for 24 h. The resulting CH3NH3PbBr3was then recrystallized by cooling the solution to room temperature.Synthesis of Millimeter-Sized 2D Single-Crystal

(C6H5C2H4NH3)2PbI4 Perovskite. 0.5 g of PbI2 powder was addedto hydriodic acid. The solution was heated to 90 °C on a hot plateuntil the PbI2 powder had completely dissolved. Then, 0.1 mL ofphenethylamine was added to the solution very slowly. We continuedto heat the reaction to 90 °C for 24 h, after which we cooled it toroom temperature. After 1 h, (C6H5C2H4NH3)2PbI4 single crystals aslarge as 10 mm2 appeared, which were filtered from the solution andcleaned by hexane to remove any hydriodic acid residue. Allprocedures were carried out in a 24 °C room that featured ahumidity of 50%.Preparation of Few-Layered Nanosheets of 2D

(C6H5C2H4NH3)2PbI4 Perovskite on SiO2/Si Substrate. Thinnanosheets of the 2D perovskite were prepared by mechanicalexfoliation using Scotch tape and then transferred to a polydime-thylsiloxane (PDMS) film (Gel Film from Gel-Pak Company), whichwas fixed on a glass slide. Using optical microscopy, we selectednanosheets that were >100 um2 in size and featured low thickness(according to their color contrast). A selected piece of the 2Dperovskite nanosheet was then transferred to 50 nm thick SiO2, whichwas thermally grown on a Si substrate. The PDMS-assisted dry

-transfer method was used to ensure that the sample was free of taperesidue.61 The exact thickness of the nanosheet was determined byAFM. Only nanosheets thinner than 20 nm were selected for the EBLprocess.

Preparation of 3D CH3NH3PbBr3 Perovskite on SiO2/SiSubstrate for Orthogonality Tests. An equimolar amount ofPbBr2 powder and the methylammonium bromide crystals wasdissolved in N,N-dimethylformamide. Then, a small drop of solutionwas added to the SiO2/Si substrate and heated to 60 °C on a hot plateto evaporate the N,N-dimethylformamide solvent. The microscaleCH3NH3PbBr3 perovskite single crystals appeared on the substrateafter the solvent had fully evaporated.

Solvent Orthogonality Tests. Before the orthogonality tests, 12CH3NH3PbBr3 and (C6H5C2H4NH3)2PbI4 perovskite samples onSiO2/Si substrates were characterized with optical microscopy and PLspectroscopy. Then, the CH3NH3PbBr3 perovskite samples wereimmersed in hexane, chlorobenzene, acetone, isopropyl alcohol,ethanol, methanol, deionized water, methyl isobutyl ketone, N,N-dimethylformamide, dimethyl sulfoxide, dichloromethane, and ethylether for 2 min, respectively, whereas the (C6H5C2H4NH3)2PbI4perovskite samples were immersed in hexane, chlorobenzene for 2min, and the other 10 solvents for 3 s (due to almost immediatedissolution). After immersion, the samples were heated to 60 °C on ahot plate to remove the solvents. Then, we characterized the sampleswith optical microscopy and PL spectroscopy again to examine thestatus of the perovskites.

Orthogonal Electron Beam Lithography Processes forFabricating the 2D Single-Crystal (C6H5C2H4NH3)2PbI4 Photo-detector. First, 495 PMMA (A4) was spin-coated on the perovskitesample at 4500 rpm for 30 s, followed by baking at 60 °C for 2 h.Then, another 950 K PMMA (A4) layer was spin-coated using thesame conditions. The sample was then put on a hot plate at 60 °C for12 h. This baking temperature of 60 °C was used instead of thecommon 180 °C condition because (C6H5C2H4NH3)2PbI4 andCH3NH3PbBr3 will be severely degraded at temperatures above 100and 140 °C, respectively.62 Note that the PMMA baking time must belong enough (>2 h) to avoid bubble formation in the resist, whichmay result in cracking of the PMMA layer during later stages of thelithographic process, in particular, e-beam evaporation of theelectrodes. We then used an EBL system (Crestec, CABL-9000Cseries) to pattern the PMMA on the perovskite using a beam currentof 1000 pA and a dose value of 50 μC/cm2. After e-beam exposure,the sample was put in the developer solution (chlorobenzene/hexane1:3) for 100 s to remove the exposed PMMA. In the next step, weused an e-beam evaporator to deposit metal Ti/Au (10 nm/80 nm)on the sample. After metal deposition, the whole sample was put intochlorobenzene and heated to 60 °C to perform the lift-off process.After unwanted parts of the metal were fully removed, the sample wasthen cleaned with hexane. Note that the 495 PMMA can be changedto other PMMA with smaller molecular weight, which can increasethe success rate of the fabrication.

Single-Crystal and Powder X-ray Diffraction. The structuraldetails of the single-crystal CH3NH3PbBr3 and (C6H5C2H4NH3)2PbI4perovskites were surveyed using a Bruker KAPPA APEX DUOdiffractometer featuring IμS Cu radiation at 296 K (λ = 0.71073 Å),an APEX II 4K CCD detector, and a microfocus X-ray source. Thepowder XRD spectra were measured using a Bruker D8 Advancediffractometer (Bragg−Brentano geometry) equipped with a Cu KαX-ray tube.

Photoluminescence Spectroscopy. A fluorescence microscopesystem (NTEGRA Spectra, NT-MDT) was used to investigate the PLspectra of single-crystal CH3NH3PbBr3 and (C6H5C2H4NH3)2PbI4with a 473 nm wavelength diode-pumped solid-state laser and a spotsize of ∼0.5 μm in diameter.

Atomic Force Microscopy Characterization. The surfacemorphology of the 2D (C6H5C2H4NH3)2PbI4 samples was examinedwith a commercial multifunction AFM (Agilent 5400 AFM/SPM)using Bruker (RFESPA-75) Al-coated cantilevers. The tip curvatureradius was ∼8 nm, and the resonance frequency was ∼75 kHz.

ACS Nano Article

DOI: 10.1021/acsnano.8b05859ACS Nano 2019, 13, 1168−1176

1173

Scanning Electron Microscopy Characterization. The SEMimages 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 cryogenicprobe station CG-196-200 was used to measure the I−V curves of theperovskite device in the dark and under solar light AM 1.5Gillumination in vacuum.

ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.8b05859.

Additional experimental data (PDF)

AUTHOR INFORMATIONCorresponding Authors*J.-H.H.: E-mail: jrhau.he@kaust.edu.sa.*X.F.: E-mail: xshfang@fudan.edu.cn.ORCIDXiaosheng Fang: 0000-0003-3387-4532Jr-Hau He: 0000-0003-1886-9241Author Contributions§C.-H.L. and B.C. contributed equally to this work. 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 thedry transfer of the perovskites. C.-H.L., B.C., and T.-Y.L.performed the perovskite orthogonality tests. C.-H.L. andJ.R.D.R. carried out the orthogonal EBL fabrication process.C.-H.L. and T.-Y.L. measured the I−V characteristics of theperovskite photodetector. C.-H.L., B.C., T.-C.W., H.-C.F., andJ.-H.H. performed the characterization of materials and thedata analysis. C.-H.L., B.C., and J.-H.H. wrote the manuscript.All authors discussed the results and commented on themanuscript.NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSThis work was financially supported by the King AbdullahUniversity of Science and Technology (KAUST) Office ofSponsored Research (OSR) (OSR-2016-CRG5-3005),KAUST solar center (FCC/1/3079-08-01), and KAUSTbaseline funding. X.F. acknowledges the support from Scienceand Technology Commission of Shanghai Municipality(18520744600, 18520710800, and 17520742400).

REFERENCES(1) Stranks, S. D.; Snaith, H. J. Metal-halide Perovskites forPhotovoltaic and Light-emitting Devices. Nat. Nanotechnol. 2015, 10,391−402.(2) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.;Seok, S. I. Compositional Engineering of Perovskite Materials forHigh-performance Solar Cells. Nature 2015, 517, 476−480.(3) Zhang, W.; Eperon, G. E.; Snaith, H. J. Metal Halide Perovskitesfor Energy Applications. Nat. Energy 2016, 1, 16048.(4) Meng, X.; Cui, X.; Rager, M.; Zhang, S.; Wang, Z.; Yu, J.; Harn,Y. W.; Kang, Z.; Wagner, B. K.; Liu, Y.; Yu, C.; Qiu, J.; Lin, Z.Cascade Charge Transfer Enabled by Incorporating Edge-EnrichedGraphene Nanoribbons for Mesostructured Perovskite Solar Cellswith Enhanced Performance. Nano Energy 2018, 52, 123−133.(5) He, M.; Li, B.; Cui, X.; Jiang, B.; He, Y.; Chen, Y.; O’Neil, D.;Szymanski, P.; EI-Sayed, M. A.; Huang, J.; Lin, Z. Meniscus-Assisted

Solution Printing of Large-Grained Perovskite Films for High-Efficiency Solar Cells. Nat. Commun. 2017, 8, 16045.(6) Stoumpos, C. C.; Kanatzidis, M. G. Halide Perovskites: PoorMan’s High-performance Semiconductors. Adv. Mater. 2016, 28,5778−5793.(7) He, M.; Pang, X.; Liu, X.; Jiang, B.; He, Y.; Snaith, H. J.; Lin, Z.Monodisperse Dual-Functional Upconversion Nanoparticles EnabledNear-Infrared Organolead Halide Perovskite Solar Cells. Angew.Chem., Int. Ed. 2016, 55, 4280−4284.(8) National Renewable Energy Laboratory. Best Research-CellEfficiencies Chart https://upload.wikimedia.org/wikipedia/commons/3/35/Best_Research-Cell_Efficiencies.png.(9) Park, N. G. Organometal Perovskite Light Absorbers Toward a20% Efficiency Low-cost Solid-state Mesoscopic Solar Cell. J. Phys.Chem. Lett. 2013, 4, 2423−2429.(10) Bi, D.; Yi, C.; Luo, J.; Decoppet, J. D.; Zhang, F.; Zakeeruddin,S. M.; Li, X.; Hagfeldt, A.; Gratzel, M. Polymer-templated Nucleationand Crystal Growth of Perovskite Films for Solar Cells with EfficiencyGreater than 21%. Nat. Energy 2016, 1, 16142.(11) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin,A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.;Zhang, X.; Dowben, P. A.; Mohammed, O. F.; Sargent, E. H.; Bakr, O.M. Low Trap-state Density and Long Carrier Diffusion in OrganoleadTrihalide Perovskite Single Crystals. Science 2015, 347, 519−522.(12) Lin, C. H.; Li, T. Y.; Cheng, B.; Liu, C.; Yang, C. W.; Ke, J. J.;Wei, T. C.; Li, L. J.; Fratalocchi, A.; He, J. H. Metal Contact andCarrier Transport in Single Crystalline CH3NH3PbBr3 Perovskite.Nano Energy 2018, 53, 817−827.(13) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.;Alcocer, M. J.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J.Electron-hole Diffusion Lengths Exceeding 1 Micrometer in anOrganometal Trihalide Perovskite Absorber. Science 2013, 342, 341−344.(14) Wei, T. C.; Mokkapati, S.; Li, T. Y.; Lin, C. H.; Lin, G. R.;Jagadish, C.; He, J. H. Nonlinear Absorption Applications ofCH3NH3PbBr3 Perovskite Crystals. Adv. Funct. Mater. 2018, 28,1707175.(15) Li, F.; Ma, C.; Wang, H.; Hu, W.; Yu, W.; Sheikh, A. D.; Wu, T.Ambipolar Solution-processed Hybrid Perovskite Phototransistors.Nat. Commun. 2015, 6, 8238.(16) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.;Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-super-structured Organometal Halide Perovskites. Science 2012, 338, 643−647.(17) Ho, K. T.; Leung, S. F.; Li, T. Y.; Maity, P.; Cheng, B.; Fu, H.C.; Mohammed, O. F.; He, J. H. Surface Effect on the 2-DimensionalHybrid Perovskite Crystals: Perovskites Using Ethanolamine OrganicLayer as an Example. Adv. Mater. 2018, 30, 1870351.(18) Tan, Z. K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler,R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington,D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright Light-emitting Diodes Based on Organometal Halide Perovskite. Nat.Nanotechnol. 2014, 9, 687−692.(19) Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba,D.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Low-temperature Solution-processed Wavelength-tunable Perovskites for Lasing. Nat. Mater.2014, 13, 476−480.(20) Dou, L.; Wong, A. B.; Yu, Y.; Lai, M.; Kornienko, N.; Eaton, S.W.; Fu, A.; Bischak, C. G.; Ma, J.; Ding, T.; Ginsberg, N. S.; Wang, L.W.; Alivisatos, A. P.; Yang, P. Atomically Thin Two-dimensionalOrganic-inorganic Hybrid Perovskites. Science 2015, 349, 1518−1521.(21) Blancon, J. C.; Tsai, H.; Nie, W.; Stoumpos, C. C.; Pedesseau,L.; Katan, C.; Kepenekian, M.; Soe, C. M. M.; Appavoo, K.; Sfeir, M.Y.; Tretiak, S.; Ajayan, P. M.; Kanatzidis, M. G.; Even, J.; Crochet, J.J.; Mohite, A. D. Extremely Efficient Internal Exciton Dissociationthrough Edge States in Layered 2D Perovskites. Science 2017, 355,1288.(22) Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.;Karunadasa, H. I. A Layered Hybrid Perovskite Solar-cell Absorber

ACS Nano Article

DOI: 10.1021/acsnano.8b05859ACS Nano 2019, 13, 1168−1176

1174

with Enhanced Moisture Stability. Angew. Chem. 2014, 126, 11414−11417.(23) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S.I. Solvent Engineering for High-performance Inorganic-organicHybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897−903.(24) Tsai, M. L.; Li, M. Y.; Retamal, J. R. D.; Lam, K. T.; Lin, Y. C.;Suenaga, K.; Chen, L. J.; Liang, G.; Li, L. J.; He, J. H. SingleAtomically Sharp Lateral Monolayer P-N Heterojunction Solar Cellswith Extraordinarily High Power Conversion Efficiency. Adv. Mater.2017, 29, 1701168.(25) Cheng, H. C.; Wang, G.; Li, D.; He, Q.; Yin, A.; Liu, Y.; Wu,H.; Ding, M.; Huang, Y.; Duan, X. Van Der Waals HeterojunctionDevices Based on Organohalide Perovskites and Two-dimensionalMaterials. Nano Lett. 2016, 16, 367−373.(26) Chueh, C. C.; Li, C. Z.; Jen, A. K. Y. Recent Progress andPerspective in Solution-processed Interfacial Materials for Efficientand Stable Polymer and Organometal Perovskite Solar Cells. EnergyEnviron. Sci. 2015, 8, 1160−1189.(27) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient PlanarHeterojunction Perovskite Solar Cells by Vapour Deposition. Nature2013, 501, 395−398.(28) Hwang, K.; Jung, Y. S.; Heo, Y. J.; Scholes, F. H.; Watkins, S.E.; Subbiah, J.; Jones, D. J.; Kim, D. Y.; Vak, D. Toward Large ScaleRoll-to-roll Production of Fully Printed Perovskite Solar Cells. Adv.Mater. 2015, 27, 1241−1247.(29) Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.;Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X. Y. Lead HalidePerovskite Nanowire Lasers with Low Lasing Thresholds and HighQuality Factors. Nat. Mater. 2015, 14, 636−642.(30) Xing, J.; Liu, X. F.; Zhang, Q.; Ha, S. T.; Yuan, Y. W.; Shen, C.;Sum, T. C.; Xiong, Q. Vapor Phase Synthesis of Organometal HalidePerovskite Nanowires for Tunable Room-Temperature Nanolasers.Nano Lett. 2015, 15, 4571−4577.(31) Fu, Y.; Zhu, H.; Schrader, A. W.; Liang, D.; Ding, Q.; Joshi, P.;Hwang, L.; Zhu, X. Y.; Jin, S. Nanowire Lasers of FormamidiniumLead Halide Perovskites and Their Stabilized Alloys with ImprovedStability. Nano Lett. 2016, 16, 1000−1008.(32) Ma, D.; Fu, Y.; Dang, L.; Zhai, J.; Guzei, I. A.; Jin, S. Single-Crystal Microplates of Two-Dimensional Organic−Inorganic LeadHalide Layered Perovskites for Optoelectronics. Nano Res. 2017, 10,2117−2129.(33) Gao, L.; Zeng, K.; Guo, J.; Ge, C.; Du, J.; Zhao, Y.; Chen, C.;Deng, H.; He, Y.; Song, H.; Niu, G.; Tang, J. Passivated Single-Crystalline CH3NH3PbI3 Nanowire Photodetector with HighDetectivity and Polarization Sensitivity. Nano Lett. 2016, 16, 7446−7454.(34) Xiao, R.; Hou, Y.; Fu, Y.; Peng, X.; Wang, Q.; Gonzalez, E.; Jin,S.; Yu, D. Photocurrent Mapping in Single-Crystal MethylammoniumLead Iodide Perovskite Nanostructures. Nano Lett. 2016, 16, 7710−7717.(35) Zhang, N.; Sun, W.; Rodrigues, S. P.; Wang, K.; Gu, Z.; Wang,S.; Cai, W.; Xiao, S.; Song, Q. Highly Reproducible OrganometallicHalide Perovskite Microdevices Based on Top-down Lithography.Adv. Mater. 2017, 29, 1606205.(36) Tan, Z.; Wu, Y.; Hong, H.; Yin, J.; Zhang, J.; Lin, L.; Wang, M.;Sun, X.; Sun, L.; Huang, Y.; Liu, K.; Liu, Z.; Peng, H. Two-dimensional (C4H9NH3)2PbBr4 Perovskite Crystals for High-perform-ance Photodetector. J. Am. Chem. Soc. 2016, 138, 16612−16615.(37) Zakhidov, A. A.; Lee, J. K.; DeFranco, J. A.; Fong, H. H.;Taylor, P. G.; Chatzichristidi, M.; Ober, C. K.; Malliaras, G. G.Orthogonal Processing: A New Strategy for Organic Electronics.Chem. Sci. 2011, 2, 1178−1182.(38) Lee, C. P.; Lai, K. Y.; Lin, C. A.; Li, C. T.; Ho, K. C.; Wu, C. I.;Lau, S. P.; He, J. H. A Paper-Based Electrode Using a Graphene Dot/PEDOT:PSS Composite for Flexible Solar Cells. Nano Energy 2017,36, 260−267.(39) Wei, W. R.; Tsai, M. L.; Ho, S. T.; Tai, S. H.; Ho, C. R.; Tsai, S.H.; Liu, C. W.; Chung, R. J.; He, J. H. Above-11%-EfficiencyOrganic−Inorganic Hybrid Solar Cells with Omnidirectional Harvest-

ing Characteristics by Employing Hierarchical Photon-TrappingStructures. Nano Lett. 2013, 13, 3658−3663.(40) Noel, N. K.; Abate, A.; Stranks, S. D.; Parrott, E. S.; Burlakov,V. M.; Goriely, A.; Snaith, H. J. Enhanced Photoluminescence andSolar Cell Performance via Lewis Base Passivation of Organic−inorganic Lead Halide Perovskites. ACS Nano 2014, 8, 9815−9821.(41) Eames, C.; Frost, J. M.; Barnes, P. R. F.; O’Regan, B. C.; Walsh,A.; Islam, M. S. Ionic Transport in Hybrid Lead Iodide PerovskiteSolar Cells. Nat. Commun. 2015, 6, 7497.(42) Zhao, Y.; Liang, C.; Zhang, H.; Li, D.; Tian, D.; Li, G.; Jing, X.;Zhang, W.; Xiao, W.; Liu, Q.; Zhang, F.; He, Z. Anomalously LargeInterface Charge in Polarity-switchable Photovoltaic Devices: AnIndication of Mobile Ions in Organic−inorganic Halide Perovskites.Energy Environ. Sci. 2015, 8, 1256−1260.(43) Unger, E. L.; Hoke, E. T.; Bailie, C. D.; Nguyen, W. H.;Bowring, A. R.; Heumuller, T.; Christoforo, M. G.; McGehee, M. D.Hysteresis and Transient Behavior in Current−voltage Measurementsof Hybrid-perovskite Absorber Solar Cells. Energy Environ. Sci. 2014,7, 3690−3698.(44) Azpiroz, J. M.; Mosconi, E.; Bisquert, J.; De Angelis, F. DefectMigration in Methylammonium Lead Iodide and Its Role inPerovskite Solar Cell operation. Energy Environ. Sci. 2015, 8, 2118−2127.(45) Polarity Index. http://macro.lsu.edu/howto/solvents/polarity%20index.htm (accessed August 1, 2018).(46) HPLC Solvent Properties, Polarity Index (Snyder). http://www.sanderkok.com/techniques/hplc/eluotropic_series_extended.html(accessed August 1, 2018).(47) Miller’s Home, Solvent Polarity Table 1, https://sites.google.com/site/miller00828/in/solvent-polarity-table (accessed August 1,2018).(48) Marcus, Y. The Properties of Organic Liquids That AreRelevant to Their Use as Solvating Solvents. Chem. Soc. Rev. 1993, 22,409−416.(49) Berliner, E. Electrophilic Aromatic Substitution Reactions. InProgress in Physical Organic Chemistry 2; Interscience: New York,1964; pp 253−321.(50) Milot, R. L.; Sutton, R. J.; Eperon, G. E.; Haghighirad, A. A.;Martinez Hardigree, J.; Miranda, L.; Snaith, H. J.; Johnston, M. B.;Herz, L. M. Charge-carrier Dynamics in 2D Hybrid Metal−HalidePerovskites. Nano Lett. 2016, 16, 7001−7007.(51) Cheng, B.; Li, T. Y.; Maity, P.; Wei, P. C.; Nordlund, D.; Ho, K.T.; Lien, D. H.; Lin, C. H.; Liang, R. Z.; Miao, X.; Ajia, I.; Yin, J.;Sokaras, D.; Javey, A.; Roqan, I.; Mohammed, O.; He, J. H. ExtremelyReduced Dielectric Confinement in Two-Dimensional HybridPerovskites with Large Polar Organics. Commun. Phys. 2018, 1, 80.(52) Lin, C. H.; Fu, H. C.; Lien, D. H.; Hsu, C. Y.; He, J. H. Self-Powered Nanodevices for Fast UV Detection and Energy HarvestingUsing Core-Shell Nanowire Geometry. Nano Energy 2018, 51, 294−299.(53) Wang, K.; Wu, C.; Yang, D.; Jiang, Y.; Priya, S. Quasi-Two-Dimensional Halide Perovskite Single Crystal Photodetector. ACSNano 2018, 12, 4919−4929.(54) Zhou, J.; Chu, Y.; Huang, J. Photodetectors Based on Two-Dimensional Layer-Structured Hybrid Lead Iodide Perovskite Semi-conductors. ACS Appl. Mater. Interfaces 2016, 8, 25660−25666.(55) Lin, C. H.; Fu, H. C.; Cheng, B.; Tsai, M. L.; Luo, W.; Zhou,L.; Jang, S. H.; Hu, L.; He, J. H. A Flexible Solar-Blind 2D BoronNitride Nanopaper-Based Photodetector with High ThermalResistance. npj 2D Mater. Appl. 2018, 2, 23.(56) Leung, S. F.; Ho, K. T.; Kung, P. K.; Hsiao, V. K. S.; Alshareef,H. N.; Wang, Z. L.; He, J. H. A Self-Powered and FlexibleOrganometallic Halide Perovskite Photodetector with Very HighDetectivity. Adv. Mater. 2018, 30, 1704611.(57) Sutherland, B. R.; Johnston, A. K.; Ip, A. H.; Xu, J.; Adinolfi, V.;Kanjanaboos, P.; Sargent, E. H. Sensitive, Fast, and Stable PerovskitePhotodetectors Exploiting Interface Engineering. ACS Photonics 2015,2, 1117−1123.

ACS Nano Article

DOI: 10.1021/acsnano.8b05859ACS Nano 2019, 13, 1168−1176

1175

(58) Ji, C. H.; Kim, K. T.; Oh, S. Y. High-Detectivity Perovskite-Based Photodetector Using a Zr-Doped TiOx Cathode Interlayer.RSC Adv. 2018, 8, 8302−8309.(59) Zhang, M.; Zhang, F.; Wang, Y.; Zhu, L.; Hu, Y.; Lou, Z.; Hou,Y.; Teng, F. High-Performance Photodiode-Type PhotodetectorsBased on Polycrystalline Formamidinium Lead Iodide PerovskiteThin Films. Sci. Rep. 2018, 8, 11157.(60) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.;Huang, J. Electron-hole Diffusion Lengths> 175 μm in SolutionGrown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970.(61) Madaria, A. R.; Kumar, A.; Ishikawa, F. N.; Zhou, C. Uniform,Highly Conductive, and Patterned Transparent Films of a PercolatingSilver Nanowire Network on Rigid and Flexible Substrates Using aDry Transfer Technique. Nano Res. 2010, 3, 564−573.(62) Berhe, T. A.; Su, W. N.; Chen, C. H.; Pan, C. J.; Cheng, J. H.;Chen, H. M.; Tsai, M. C.; Chen, L. Y.; Dubale, A. A.; Hwang, B. J.Organometal Halide Perovskite Solar Cells: Degradation andStability. Energy Environ. Sci. 2016, 9, 323−356.

ACS Nano Article

DOI: 10.1021/acsnano.8b05859ACS Nano 2019, 13, 1168−1176

1176