Graphene Heterojunction for Efficient

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Heavy Mediator at Quantum Dot/Graphene Heterojunction for Efficient Charge Carrier Transfer: Alternative Approach for HighPerformance Optoelectronic Devices

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Rapti Ghosh,†,§,∥,¶ Kanchan Yadav,†,⊥,□ Monika Kataria,†,§,∥,¶ Hung-I Lin,#,¶ Christy Roshini Paul Inbaraj,⊥,¶,△ Yu-Ming Liao,⊥,¶ Yen Nguyen,#,¶ Cheng-Hsin Lu,▲ Mario Hofmann,¶ Raman Sankar,‡,■ Wei-Heng Shih,▲ Ya-Ping Hsieh,*,†,∥ and Yang-Fang Chen*,¶,○ †

Institute of Atomic and Molecular Sciences and ∥Molecular Science and Technology Program, Taiwan International Graduate Program, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 115, Taiwan ‡ Institute of Physics, Academia Sinica, Taipei 11529, Taiwan § Department of Physics, National Central University, Chung-Li 320, Taiwan ⊥ Nano Science and Technology Program, Taiwan International Graduate Program, Institute of Physics, Academia Sinica, Taipei 106, Taiwan # Graduate Institute of Applied Physics, ¶Department of Physics, and □Department of Chemistry, National Taiwan University, Taipei 106, Taiwan ■ Centre for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan △ Department of Engineering and System Sciences, National Tsing Hua University, Hsinchu 30013, Taiwan ▲ Department of Material Sciences and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States ○ Advanced Research Centre for Green Materials Science and Technology, National Taiwan University, Taipei, Taiwan S Supporting Information *

ABSTRACT: Two-dimensional (2D) material nanocomposites have emerged as a material system for discovering new physical phenomena and developing novel devices. However, because of the low density of states of most twodimensional materials such as graphene, the heterostructure of nanocomposites suffers from an enhanced depletion region, which can greatly reduce the efficiency of the charge carrier transfer and deteriorate the device performance. To circumvent this difficulty, here we propose an alternative approach by inserting a second 2D mediator with a heavy effective mass having a large density of states in-between the heterojunction of 2D nanocomposites. The mediator can effectively reduce the depletion region and form a type-II band alignment, which can speed up the dissociation of electron−hole pairs and enhance charge carrier transfer. To illustrate the principle, we demonstrate a novel stretchable photodetector based on the combination of graphene/ReS2/ perovskite quantum dots. Two-dimensional ReS2 acts as a mediator in-between highly absorbing perovskite quantum dots and a high-mobility graphene channel and a thiol-based linker between the ReS2 and the perovskite. It is found that the optical sensitivity can be enhanced by 22 times. This enhancement was ascribed to the improvement of the charge transfer efficiency as evidenced by optical spectroscopy measurements. The produced photosensors are capable of reaching the highest reported value of photoresponsivity (>107 A W−1) and detectivity compared to previously studied stretchable devices. Mechanical robustness with tolerable strain up to 100% and excellent stability make our device ideal for future wearable electronics. KEYWORDS: heavy mediator, 2D−0D interface, ReS2, stretchable device, optoelectronic device The dissociation of photocarriers at heterointerfaces forms the basic building block of optoelectronic devices for light harvesting and light detecting applications.1 Due to this relevance to important applications, ongoing effort is devoted to improve the dissociation efficiency.2,3 It is well established that the device performance and speed are limited by the © XXXX American Chemical Society

Received: May 13, 2019 Accepted: July 8, 2019 Published: July 8, 2019 A

DOI: 10.1021/acsami.9b08294 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic diagram of the ReS2 bonded PQD solution/graphene hybrid rippled structure photodetector. (b) HR-TEM image of ReS2 flake bonded with PQD. (c) (i) Cross-sectional SEM image of the rippled structure. (ii) Lateral SEM image of the rippled structure. (d) Schematic diagram of change in depletion region before (i) and after (ii) the insertion of ReS2 flake. (e) Theoretically simulated zoomed-in image of the depletion region (i) without and (ii) with ReS2 flake.

introduce a novel approach for overcoming the limited charge transfer efficiency in 2D materials by adding a mediator in the heterointerface. In traditional pn-junctions graded dopant distributions are employed to improve the junction electrostatics and enhance the carrier transfer process.14 We adapt this idea and insert a second 2D material at the interface inbetween 2D material (graphene) and photosensitizer (perovskite quantum dots). The mediator’s role is to decrease the extent of the depletion region that forms in the graphene due to its low density of states and thus enhance the built-in fields. In addition to that, with the selection of an appropriate 2D material, the formation of type-II band alignment at the heterostructure interface can assist the separation of photogenerated electrons and holes, which is also beneficial for the dissociation of photocarriers.15 Out of the large range of available 2D materials, rhenium disulfide (ReS2) is well suited for this purpose. Due to the extended nature of the rhenium’s d-orbital, states at the

transit mechanism through the heterointerface which is a focus of current research.4,5 Two-dimensional (2D) materials and quantum dots heterostructures have shown immense promise in enhancing the photodissociation compared to traditional bulk materials due to their increased absorption,6,7 high carrier mobility,8 and atomically clean interfaces.9 Unfortunately, the unique electrostatics of 2D materials heterojunctions results in significantly enhanced depletion region, which reduces the achievable efficiency of carrier transfer across the heterointerface. Several interesting approaches, including conjunction of metal ion with black phosphorus,9 implementation of TiO2 layer between HgTe colloidal quantum dots−MoS2 interface,10 doping of MoO3 in 2D perovskite layer through covalent bonding,11 and proper electrolytic concentration with 2D InSe nanosheets12 and BP nanosheets,13 have been employed to enhance charge carrier mobility. We here B


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ACS Applied Materials & Interfaces Fermi level exhibit large effective masses (around 0.5 me; me = electronic mass)16 which results in a large density of states. Rhenium is a member of group-VII transition metal family with its electronic configuration quite dissimilar to other transition metals like molybdenum (Mo) and tungsten (W). Because an extra valence electron prevails in rhenium, both Re−S and Re−Re bonds are formed in ReS2. The existence of this metal−metal chain gives rise to Peierls distortion forming a distorted octahedral structure17 with this reduced crystal symmetry, and it makes the atomic layers of ReS2 vibrationally decoupled to each other. Due to the presence of this property, the bandgap of ReS2 remains direct, which is independent of the layer thickness. The associated high carrier concentrations18 and the large dielectric constant of the material lead to a decreased depletion width. Moreover, the thickness-independent band gap of 1.5 eV makes it ideal as a staggered gap material between the sensitizer and graphene. The choice of sensitizer also plays an important role, and here perovskite quantum dots (PQD) are employed to unravel the mechanism at the heterostructure interface. The organolead halide perovskite quantum dots have some interesting features because of having dual functionality of both organic and inorganic complexes where the organic part provides good mechanical strength and the inorganic part is known for providing good electronic mobility.19 Exploiting the intriguing features of heavy mediator ReS2 and sensitizer PQD, we demonstrate the potential of our approach by producing a highly sensitive and stretchable photodetector with unprecedented responsivities above 107 A W−1 which is nearly 103 times higher than that of pure graphene/QD interface as reported earlier by Tetsuka et al.20 and Zheng et al.21 In addition, there is 22 times improvement by incorporating a thiol-based linker between ReS2 and the PQDs, which was found to originate from improved charge transfer efficiency at the ReS2/PQD interface. The resulting optimized device exhibits an impregnable photoconductive gain of 7.5 × 107 A W−1 and detectivity of 1014 Jones, which are superior to previously reported stretchable photodetectors22−26 and open up novel routes to high-performance wearable electronics. Figure 1a represents the concept of our novel approach. A ReS2 layer is added in-between graphene and PQDs. The indepth description of fabricating a wavy pattern graphenebased ReS2 conjugated with PQD (CH3NH3PbBr3) hybrid stretchable photodetector is illustrated in the Supporting Information (Figure S1) depicting the procedure of transferring CVD-grown single-layer graphene27 on a prestretched PDMS substrate along with the formation of the wavy pattern after releasing the stretch. Figure 1a represents the schematic diagram of the wavy patterned ReS2 and PQD (bonded)/ graphene stretchable photodetector. The scanning electron microscopy (SEM) image (Figure 1c) of a 100% prestretched PDMS substrate depicts the formation of the wavy pattern of an average height of 150 nm whereas the inset shows the top view of the wavy formation which confirms the uniformity of the wavy pattern throughout the substrate. The zoom in schematic view of one ReS2 flake bonded with PQD is further illustrated in the high-resolution transmission electron microscope (HRTEM) image (Figure 1b), showing the top view of a ReS2 flake bonded with PQD. After optimization (Figure S2(i)), the thickness of the ReS2 flake and PQD layer is ∼180 nm as estimated by the SEM image (Figure S2(ii)).

The HRTEM image (Figure S2(iii)) and EDAX image (Figure S2(iv)) reveal the elemental distribution of the optimized ReS2 flake and PQD layer. The ReS2 flake is further characterized by HRTEM (Figure S3(i)) representing a few-layers ReS2 flake. The fast Fourier transform (FFT) pattern (inset) of ReS2 flake confirms the regular crystallographic arrangement of atoms.28 The Raman spectrum of ReS2 flake (Figure S3(ii)) matches well with the previously reported values.17 Due to the band alignment,29 excitons will dissociate and electrons will transfer from the sensitizer to the ReS2. This process is expected to be fast due to the small extent of the depletion region in the ReS2 around the quantum dot (Figure 1d) which is further proved by theoretical simulation using COMSOL software (Figure 1e). To realize this structure, the organic−inorganic hybrid PQD has been bonded with ReS2 flake through thiol (disulfide) bonding which eventually helps in efficient charge transfer. Figure S4 illustrates the detailed steps of bonding PQD with ReS2 flake, where the zeta potential study in Figure S5 provides experimental evidence of the bonding of PQD with ReS2 flake. The PQDs are further characterized to obtain their morphology. The HRTEM image of PQD (Figure S6) and the corresponding FFT pattern (inset) of a single nanoparticle clearly reveal the crystallographic arrangement of atoms without any trace of the amorphous layer. The photoluminescence (PL) spectrum of PQD (Figure S6(ii)) exhibits a narrow emission peak at around 530 nm which is consistent with other reported values.30 On the other hand, the PL spectrum of few-layer ReS2 flake (Figure S3(iii)) reveals a band edge emission at around 1.45 eV which is also similar to previous studies.31 Under illumination of 325 nm laser, the graphene channel (45 μm × 205 μm) with high carrier mobility (∼103 cm2 V−1 S−1)32 shows a significant increase in conductivity (Figure 2a) for ReS2−PQD bonded active material in the presence of bifunctional linker thiol from MPTMS (mercaptopropyltrimethoxysilane) with a short response time (Figure 2b) of the order of 200 ms. The photocurrent enhancement is more than 22 times compared to the ReS2/graphene interface, and a significant 12 times improvement occurs compared to the unbounded ReS2−PQD/graphene interface whereas it is 15 times for PQD/graphene interface (Figure 2a). According to the proposed mechanism, the device performance is limited by the charge transfer, and we subsequently attempt to improve the understanding of the underlying mechanism. The interface between ReS2 and PQDs is known to exhibit the Van der Waals gap that acts as an insulating layer for charge transfer.33 The presence of this layer hinders the flow of electrons from the QDs to the ReS2 nanoflake (Figure 3c(i) and (ii)). In order to remove this difficulty, the QDs are tightly bound with ReS2 nanoflake through disulfide bonding. Thiol provides a short distance between the QDs and ReS2 resulting in high tunneling probability. Now, the QDs are generally attached with ligands having a long polymeric chain. Since the tunneling current between the QDs decreases exponentially with the length of the cross-linker, the ligand used during the synthesis of QDs acts as an insulating layer for the charge transferring process between the QDs. To eradicate this difficulty, the long polymeric ligand is exchanged by a short-length-ligand thiol. The QDs come in close proximity because of the short-ligand thiol causing tunneling current flow between the QDs.34 C


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charge transferring process more easily.37,38 Therefore, upon illumination, electrons can easily flow toward ReS2, and holes remain in PQD for the sample with the ligand. The improvement in charge transfer efficiency can be confirmed from photoluminescence (PL) spectra (Figure 3d) of the QDs: When QDs are deposited in the vicinity of a ReS2 flake, little charge transfer occurs, and the PL intensity is high. For bonded QDs, however, exciton dissociation at the interface represents a more efficient nonradiative decay pathway which removes photoexcited carriers before they can recombine and results in a lower PL intensity.39 Compared with previous reports, the weak PL intensity and broad spectra shown in Figure 3d can be attributed to charge carrier transfer due to the type-II band alignment between PQD and ReS2 flakes and the size distribution of PQDs. Next, to increase the mechanical robustness, the structure was transferred onto prestrained PDMS (polydimethylsiloxane). Upon release, the device exhibited periodic ripples (Figure 1c). Figure 4a represents the typical output characteristic curve of the device where the difference between the on-current (ION) and off-current (IOFF) is defined as the photocurrent (IPh), i.e., IPh = |ION − IOFF | with an applied bias voltage of (VDS) 1 V under the illumination of 325 nm laser of different power having a spot size of 0.06 cm2 in an active device area of 45 μm × 205 μm. Figure 4b illustrates the dynamic photoresponse (I vs T) curve at different power of 325 nm laser illumination under bias voltage (VDS) 0.1 V. The responsivity (RP) of a photodetector represents its sensitivity that is the efficiency of the detector to convert the incident photon into photocurrent, which can be calculated according to the formula reported earlier.40 The thus-produced device exhibited a maximum photoresponsivity of 1.96 × 107 A W−1 and a corresponding photoconductive gain of 7.5 × 107 (Figure 4c), resulting in a detectivity of 2.5 × 1014 Jones (Figure 4d) under a bias voltage of 1 V at 325 nm laser illumination calculated using previously reported methods (detailed calculation in Figure S7).41 These values represent the highest reported values for any stretchable photosensors (Table S1). Another important factor for this extraordinary value of photoresponsivity is the wavy structure of the device which multiply reflects the incoming photons generating photon confinement effect.26,40 Because of this virtue, the detector is capable of sensing photons as low as 1 nW. The decrease of photoresponsivity with increasing illumination power can be attributed to the screening effect of the built-in electric field as reported previously.40 The high value of photocurrent gain is achieved at low-power illumination because the built-in electric field is larger, which can draw more electrons to the graphene channel. These electrons further shift the Fermi level of graphene as evidenced by the PL spectra of ReS2−PQD/ graphene (Figure S8) where the quenching effect occurs due to the charge transfer from ReS2−PQD active layer to the graphene channel. Notably, the photoresponse of the device also varies with the wavelengths (Figure S9) depending upon the absorbance of the graphene and the ReS2−PQD active layer, since graphene is capable of absorbing in the whole spectral regime whereas ReS2 and PQD have pronounced absorption in the NIR and visible range, respectively. Finally, we demonstrate the suitability of our device for wearable electronics. The wearable devices set a new criterion for the upcoming optoelectronic generation because of their impressive stretchability for being adhesively attached to

Figure 2. (a) Output I−V curve and (b) dynamic photoresponse curve of ReS2/graphene, PQD/graphene, ReS2 and PQD (not bonded)/graphene, and ReS2 and PQD (bonded)/graphene hybrid devices.

Thus, the conductivity of the QD layer is enhanced, and the passivation effect of thiol (disulfide) bonding35 also helps to decrease trap centers. Moreover, the employed S−2 ions in the thiol group are a weak ligand resulting in a positive dipole moment from the perovskite in the direction of the ReS2 as depicted in the schematic diagram (Figure 3b(i)). Because of the positive dipole moment, thiol molecules pull electrons out from the QDs and transfer them to the ReS2 surface.36 In addition, the presence of thiol group causes a large enhancement of photocurrent because the ligand field can induce the interaction of ligand orbitals and the atomic orbitals of the transition metal complexes. Depending upon the splitting energy of the unoccupied d orbital of the TMD complex, the bonding and antibonding of the molecular orbitals occur. The two major axes dx2−y2 and dz2 of the ReS2 octahedral complex have a strong interaction with the orbitals of the ligand causing an enhancement of the energy of the d orbital. This added electric field reduces the barrier height for charge injection further enhancing the charge transfer. In addition, the type-II band diagram formed at the interface (Figure 3a and b) of ReS2 and PQD helps to assist the D


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Figure 3. (a) Band diagram of ReS2 and PQD hybrid system before illumination. (b) (i) After illumination, band bending of ReS2 and PQD hybrid system with ligand, (ii) schematic diagram of ReS2 and PQD hybrid system with ligand. (c) (i) After illumination, band bending of ReS2 and PQD hybrid system without ligand, (ii) schematic diagram of ReS2 and PQD hybrid system without ligand. (d) Photoluminescence (PL) spectra of ReS2 and PQD (not bonded and bonded) devices.

different body parts.42 For studying the strain dependency, a device of 100 μm × 100 μm channel length is examined under 80 nW power of 325 nm laser illumination with bias voltage of 0.1 V. The dynamical photoresponse of the device, where change in current vs time (Figure 5a) and change in

current vs voltage (Figure 5b) are recorded under 0%, 25%, 50%, 75%, and 100% longitudinal strain, clearly reveals the fact of decrement of photocurrent with the increase of strain. The basic physics behind this behavior is photon trapping which comes into play because of the wavy pattern. Multiple E


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Figure 4. Photoresponse characteristics of the hybrid photodetector: (a) photocurrent generation at different illumination power density of 325 nm laser. (b) Temporal photoresponse at different illumination power density of 325 nm laser. (c) Variation of responsivity and photocurrent gain with power. (d) Dependence of detectivity at different illumination power density of 325 nm.

internal reflections of photons at the side wall cause the enhancement of current because of high radiation density illustrated by the schematic diagram in Figure S10. The quantitative analysis of strain dependency on the conductivity ratio is illustrated in Figure S11. This can be further proved by Figure S12 showing the transmittance (%) of a regular and wavy patterned device where it is clearly visible that the optical transmittance is more for a planner device compared to a wavy pattern device because light trapping occurs in the crest and trough of the wave. When the device gets stretched from 0% to 100% for up to 120 times, the photocurrent did not change significantly as depicted in Figure 5c which shows the durability of the device. The less intense D peak of the Raman spectrum of graphene over the wavy patterned PDMS (Figure S13) also depicts the defect-free structure of monolayer graphene even after extreme stretching. The important reason behind this durability is the presence of PMMA layer beneath the graphene. This layer is capable of withstanding large tensile and flexural strength because of the long polymeric chain. The mechanical bendability is the key feature of any wearable device. To examine the bending strain, the device composed of PDMS substrate is placed on different cylindrical apparatus (Figure S14) having a different radius of curvatures such as r = 0.35, 0.5, 0.6, and 1 cm and infinity (planner). The device having 100 μm × 100 μm channel length is subjected to 325 nm laser with a power of 80 nW, and a bias voltage of 0.1 V is applied on it. Figure 5d and 5e represent the dynamic photoresponse of the device with the

variation of time and voltage respectively under different bending strain. The bending strain (φ) is calculated using the formula φ = −(y/r), where r signifies the radius of curvature and y is the distance from the neutral axis. After probing into the result of photoresponsive behavior under various bending strain, it can be concluded that the photocurrent is decreased little with the increase of strain. The reason behind this decrement is the reduction of the incident radiation density after the application of strain. When the wavy patterned device is bent circularly, the waves become broadened and shortened simultaneously. As a result, light trapping is reduced as the light starts falling tangentially after bending and cannot be trapped efficiently because of the distortion of wave shape. But the change in photocurrent is very little in the case of bending strain which makes this device a potential candidate for future flexible optoelectronics applications. Upon repeating the applied bending strain of 14% for up to 120 times, there is no significant change in the photocurrent as shown in Figure 5f which reveals the stability of the device. This factor is attributed to the excellent mechanical property of the underlying PMMA layer which has high impact grade. In addition to the mechanical stability, the device was also found to be chemically stable over extended periods of time, and the photocurrent remains virtually unchanged even after 90 days as shown in Figure S15. In conclusion, we have introduced a new approach in improving the photoexcited charge transfer at the graphene/ perovskite quantum dot interface by introducing a 2D ReS2 F


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Figure 5. (a) Dynamic photoresponse and (b) photocurrent of the hybrid device under different longitudinal strain with schematic diagram of ripple formation (inset). (c) Photocurrent generation vs strain and bending cycle. (d) Dynamic photoresponse and (e) photocurrent of the hybrid device under different curvature strain. (f) Photocurrent generation vs strain and bending cycle.

approach shown here, two major points have to be fulfilled, including type-II band alignment between quantum dots and 2D materials and the heavy effective mass of 2D materials to provide a high density of states and reduce the depletion region.

mediator, which possesses a heavy effective mass with a large density of states. Because of the enhanced charge transfer efficiency, it results in the highest reported photoresponsivity of 1.96 × 107 A W−1 among all stretchable photodetectors. In addition, the resulting photodetector exhibits high detectivity, fast response time, mechanical robustness under large strain, and excellent stability under ambient conditions. Our study, therefore, demonstrates the potential of engineering complex 2D materials interfaces for enhanced sensing performance and opens up new possibilities for modern, wearable optoelectronic devices. Our methodology can be extended to many other composite systems consisting of quantum dots and 2D materials. However, before implementing the

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EXPERIMENTAL SECTION

Required Chemicals. The chemicals octylammonium bromide and methylammonium bromide were purchased from Shanghai MaterWin New Materials. Re powder (99.99%), S (99.999%), I2 (99.99%), TeBr4 (99.99%), DMF (dimethylformamide), tetraethyl orthosilicate (TEOS), mercaptopropyltrimethoxysilane (MPTMS), G


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ACS Applied Materials & Interfaces ammonium hydroxide, cyclohexane, acetone, ethanol, and 99.98% pure copper foil were purchased from Sigma-Aldrich. Material Preparation. Synthesis of Perovskite Quantum Dots (PQDs). The synthesis procedure of CH3NH3PbBr3 PQD was adapted from the reported procedure in the literature.30 First 96 μL of oleic acid was added into 2 mL of octadecene. After the solution was stirred and heated at 80 °C, 0.06 mmoL of octylammonium bromide was added following the addition of N,2-dimethylformamide of 200 μL volume. After this addition, 0.04 mmol volume of methylammonium bromide and lead(II) bromide of 0.1 mmol were added in the solution subsequently in such a way that the later precursor solution has to be added only after the complete dissolution of the former precursor solution. A yellow dispersion was obtained after 5 min reaction time which thereby was purified further by adding acetone for the precipitation of nanoparticles following 10 min centrifugation at 3000 rpm. Finally discarding the supernatant, the precipitates were redispersed in toluene for future usage. Synthesis of ReS2 Bulk Crystal Growth. In the present study, ReS2 crystal growth was carried out by using the chemical vapor transport method using I2 and TeBr4 as transport agents. We have used Re powder (99.99%), S (99.999%), I2 (99.99%), and TeBr4 (99.99%) as precursors. The reactants were taken in a fixed ratio and sealed in evacuated quartz tubes under the pressure of 10−5 mbar. These quartz ampules were kept in the furnace and melted for 24 h in order to form the pure ReS2 powder. Further, these pure phase powders were taken into 40 cm quartz tubes along with I2 and TeBr4 transport agents (added in a 1:0.05 g ratio) and sealed. These sealed tubes were placed in the 2 Zone furnace to maintain 1000 and 950 °C of the hot and cold side of the furnace for 200 h. The large crystals were formed in the case of TeBr4, whereas I2 is able to form very thin layered crystals of ReS2. Synthesis of ReS2 Nanoflakes (ReS2 NFs). ReS2 nanoflakes were obtained from bulk ReS2 by employing liquid exfoliation technique by several steps of optimization. In the procedure, 100 mg of bulk ReS2 was mixed with 200 mL of DMF in serum bottle, and the solution was kept for continuous liquid exfoliation via sonication for 24 h to exfoliate bulk ReS2 by sonicator machine with an output power of 250 W. Later, the solution was kept for few hours at room temperature, followed by decanting the top 2/3 of the solution into another round-bottom flask. Finally, the sample was centrifuged several times at 2000 rpm for 5 min in order to separate the supernatant from the pellet. The supernatant obtained was ReS2 NFs which were collected for further characterization. Conjugation Steps of ReS2 Nanoflake with Perovskite Quantum Dots. The schematic diagram in Figure S4 depicts the step by step detailed description of conjugation. Silica Coating of Perovskite QDs (To Form Silica-PQDs). The perovskite quantum dots (PQDs) were subsequently coated with TEOS to obtain a thin and uniform silica layer on the PQDs (referred to as silica PQDs). This coating provided the silanol groups (Si−OH) on the PQDs surface for the next modification of the silane-based functionalization. In the optimized protocol, about 30 mg mL−1 of PQDs was dissolved in 7 mL of cyclohexane and sonicated for 30 min. At the same time, 500 μL of Igepal dissolved in 3 mL of cyclohexane was stirred for 30 min and then added into the solution containing PQDs. Next, 80 μL of ammonium hydroxide was added to the mixture and sonicated for another 30 min, followed by adding 30 μL of TEOS and stirring the mixture for 20 h. Finally, the solution was precipitated by acetone and isolated via centrifugation. The pellets were washed twice with an ethanol:water (1:1) mixture to remove impurities. Surface Modification for MPTMS-Conjugated PQDs (To Form SH-PQDs). Silica PQDs were functionalized with MPTMS to introduce thiol groups (−SH) on the PQDs (referred to as SH− PQDs). In the optimized protocol, silica−PQDs (30 mg) was added into 15 mL of water:ethanol (5:95%) solution containing 25 μL of MPTMS. The solution was transferred to a three-necked roundbottom flask (50 mL), followed by stirring for 24 h. Finally, the solution was precipitated by acetone (20 mL) and isolated via

centrifugation. The pellets were washed twice with an ethanol:water (1:1) mixture to remove impurities. Conjugation of ReS2 NFs and PQDs To Form ReS2−PQDs. SH− PQDs (10 mg mL−1) were mixed with 1 mg mL−1 of ReS2 nanoflakes for 12 h at room temperature, where the thiol group bonded with the sulfur vacancies, which exist intrinsically in ReS2 NFs to form ReS2−PQDs (via disulfide bonding). Synthesis of Graphene. A pristine quality single-layer graphene was synthesized using traditional chemical vapor deposition (CVD) technique on a pre-etched copper substrate. The surface morphology and purity level of copper play a key role in the growth of graphene on it. Pure copper foil (99.98%) was polished by electrolysis of 85% H3PO4 at 1.5 V for 15 min in order to reduce surface roughness. The same quality Cu foil has been used as both cathode and anode to avoid the doping of the host atom in the Cu. After completion of etching, the polished Cu foil is placed inside the CVD furnace for graphene growth. During the growth process, at first, the Cu foil is exposed to H2 flow at 60 sccm rate for 60 min at 1000 °C followed by the CH4 flow at 3.4 sccm rate for 30 min keeping all the parameters unchanged. The furnace was allowed to cool up to room temperature. Finally, because of the chemical reaction of H2 and CH4, a pristine quality SLG was evolved on the Cu foil. Substrate Preparation. PDMS precursor’s polymer from SigmaAldrich was mixed by continuous stirring with the curing agent at a ratio of 10:1 by weight and was poured into a Petri dish uniformly. The Petri dish was placed inside the desiccator for degassing for 1 h. After the bubbles were removed, PDMS filled Petri dish was kept inside the thermal at 70 °C for 2 h. Finally, a highly stretchable, uniform, and pristine quality PDMS can be removed from the Petri dish. Device Fabrication. The method of making stretchable wavy pattern photodetector is illustrated in Figure S1. A PDMS substrate was stretched (about twice its length) and was clamped by clips on a glass slide. After the growth of monolayer graphene on Cu foil, a thin layer of PMMA was spin-coated over it. This PMMA/ graphene/Cu was made to float on FeCl3 solution for Cu etching, and after a few minutes PMMA/graphene was transferred on distilled water with the help of glass slide. From there PMMA/ graphene was flipped transferred over prestretched PDMS substrate. After the completion of this step, the Ag electrode of desired channel length (45 and 100 μm) was deposited on the top of graphene with the help of the shadow masking technique. For getting good adherence, the thermal evaporator chamber was kept under ultrahigh vacuum of 10−7 Torr. After the fabrication of the basic structure of the device, the active material ReS2 with bonded PQD solution was spin-coated on top of it obeying the optimized parametric condition. Finally, the clip was released, and the wavy patterned PQD/ReS2/graphene/PMMA/PDMS hybrid device was fabricated. Optoelectronic Measurements. The electronic measurements of the device were done using a Keithley 2400 electrometer, and for photoluminescence measurements 20 ps pulsed 374 nm laser of frequency 40 MHz and a Horiba Jobin Yvon TRIAX 320 spectrometer were used. Instrumentation Detail for Zeta Potential Measurements. The zeta potentials of the ReS2 and the various steps of ReS2−PQD formed conjugates were measured by using a Zetasizer Nano-ZS (Malvern Instrument Inc., U.K.). Instrumentation Detail for TEM Measurements. TEM images were recorded on a JEOL JEM-1400 (120 kV) TEM.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b08294. Schematic diagram of device fabrication; optimization parameters for the ReS2−PQD solution; characterization of ReS2 flake and PQD; schematic for ReS2− H


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PQD flake synthesis; zeta potential measurement of ReS2−PQD flake; detail calculation of responsivity, photoconductive gain and detectivity; PL spectra of ReS2−PQD flake with and without graphene; photoresponsivity at different wavelengths; photoconfinement mechanism for wavy pattern device; quantitative analysis of photoconfinement effect in wavy pattern device; comparison of transmission spectra of planner and wavy pattern PDMS; Raman spectra of graphene on wavy pattern PDMS; demonstration of stretchability of the device;measurement of stability of the device at different periods of time; table of comparison of different stretchable photodetectors (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Rapti Ghosh: 0000-0002-0999-6217 Monika Kataria: 0000-0003-4912-340X Hung-I Lin: 0000-0001-9849-3138 Christy Roshini Paul Inbaraj: 0000-0002-8776-9917 Yu-Ming Liao: 0000-0002-8696-7313 Mario Hofmann: 0000-0003-1946-2478 Raman Sankar: 0000-0003-4702-2517 Ya-Ping Hsieh: 0000-0002-6065-751X Author Contributions

R.G. and Y.F.C. conceived the idea of ReS2−PQD flake. K.Y. synthesized the ReS2 nanoflake and ReS2−PQD bonded nanoflake, performed the material characterization (zeta potential measurement), and provided the experimental and characterization writing details. R.S. synthesized bulk ReS2 crystal. C.H.L and W.H.S. synthesized PQD. Y.N and M.H did simulation for ReS2−PQD/graphene composite. R.G, H.I.L., and C.R.P.I took PL spectra. R.G and Y.M.L took transmission spectra. R.G. and M.K. performed electrical measurement of stretchability together. R.G, Y.P.H, W.H.S, M.F., and Y.F.C. discussed the mechanism of ReS2-mediated PQD/graphene stretchable device. Y.F.C. supervised the project. R.G, Y.P.H., and Y.F.C. contributed in writing the manuscript through the input of all the authors. All the authors were involved in analyzing the data. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the “Advanced Research Centre for Green Materials Science and Technology” from the Featured Area Research Centre Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 107-3017-F-002001).

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Graphene Heterojunction for Efficient

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