Fully Inkjet-Printed, Mechanically Flexible MoS2 Nanosheet

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Fully Inkjet-Printed, Mechanically Flexible MoS2 Nanosheet Photodetectors Jung-Woo T Seo, Jian Zhu, Vinod K. Sangwan, Ethan B Secor, Shay G Wallace, and Mark C Hersam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19817 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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ACS Applied Materials & Interfaces

Fully Inkjet-Printed, Mechanically Flexible MoS2 Nanosheet Photodetectors Jung-Woo Ted Seo1, Jian Zhu1,†, Vinod K. Sangwan1, Ethan B. Secor1,‡, Shay G. Wallace1, and Mark C. Hersam1,2,3,*

1

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois

60208, United States 2

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States

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Department of Electrical Engineering and Computer Science, Northwestern University,

Evanston, Illinois 60208, United States *E-mail: [email protected]

Keywords: solution processing, two-dimensional materials, transition metal dichalcogenide, printed electronics, optoelectronics, photonic annealing

Abstract Solution-processed two-dimensional materials offer a scalable route towards nextgeneration printed devices. In this report, we demonstrate fully inkjet-printed photodetectors using molybdenum disulfide (MoS2) nanosheets as the active material and graphene as the electrodes. Percolating films of semiconducting MoS2 with high electrical conductivity are achieved with an ethyl cellulose-based ink formulation. Two classes of photodetectors are fabricated, including 1

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thermally annealed devices on glass with fast photoresponse of 150 µs and photonically annealed devices on flexible polyimide with high photoresponsivity exceeding 50 mA/W. The photonically annealed photodetector also reduces the curing time to milliseconds and maintains functionality over 500 bending cycles.

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The superlative properties of two-dimensional (2D) materials suggest their utility in a range of electronic devices. Specifically, van der Waals heterojunction devices based on 2D materials, such as semiconducting transition metal dichalcogenides (TMDCs), conductive graphene, and insulating hexagonal boron nitride, provide diverse electronic functionality.1, 2 For example, highquality TMDCs and graphene produced by micromechanical exfoliation have enabled prototype photovoltaic devices,3 high-performance photodetectors,4, 5 and light-emitting diodes.6 These cases highlight the compatibility between TMDCs and graphene as the active semiconductor and electrode components, respectively, which results in efficient charge carrier extraction from TMDC layers into graphene electrodes3. While micromechanically exfoliated devices provide the highest quality graphene/TMDC interfaces, this fabrication approach lacks the large-area scalability required for most technologies. Liquid-phase exfoliation from bulk layered materials provides an alternative route for the mass production of 2D nanosheets.7, 8 This process involves the application of shear forces in solution, often with polymeric or ionic stabilizers that facilitate exfoliation and minimize restacking of nanosheets.8 Following exfoliation, additional post-processing steps such as centrifugal separation can be utilized to refine the physical attributes (e.g., thickness and size) of the resulting 2D nanosheets. By further tuning the rheological properties of the 2D nanosheet dispersions, 2D material inks have recently been reported with promising results for printed electronics.9, 10 For example, vertical heterostructure photodetectors have been demonstrated by sequentially inkjet printing water-based graphene and tungsten disulfide inks, resulting in photoresponsivities on the order of 1 mA/W.11 Lateral heterostructures have also been explored based on printed MoS2 nanosheets as the photoactive channel material with graphene12, 13 or silver nanoparticles14 as electrodes with photoresponsivities ranging from 36 µA/W12 to 300 mA/W,13

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and rise times of 60 ms.14 The MoS2 and graphene inks in these studies were prepared by sonicating the raw powders in organic solvents, followed by centrifugation of the dispersion to isolate thin nanosheets. Subsequently, multiple steps involving stabilizer addition and solvent exchange were employed to obtain suitable viscosity and surface tension for inkjet printing.14 While these early studies have established the feasibility of printable 2D material inks for optoelectronic applications, the devices either fall short of fundamental materials limits or present processing challenges such as long sonication times, suggesting the need for further innovation in ink and processing design. In addition, the post-printing annealing conditions in previous work have often been incompatible with oxidation-sensitive materials and flexible substrates, thus necessitating the development of alternative curing conditions in order to facilitate roll-to-roll manufacturing. Herein, we present alternative 2D material ink formulations and post-printing curing conditions that enable high-performance, fully inkjet-printed MoS2-graphene photodetectors on both rigid and flexible substrates. The MoS2 ink utilizes the polymer stabilizer ethyl cellulose (EC) that has previously been shown to provide several advantages for inkjet-printed graphene.15 In particular, EC allows efficient exfoliation of pristine MoS2 powders in ethanol, leading to scalable production of nanosheets via shear mixing, centrifugation for size-selection (thickness < 6 nm, lateral size < 100 nm), and flocculation by mixing with aqueous NaCl solution. While the MoS2:EC ratio is tunable by varying the initial loading of EC for shear mixing, relatively high content of MoS2 in the flocculated powder was utilized here to facilitate subsequent processing steps, with the MoS2 fraction (~44 wt%) being confirmed with thermal gravimetric analysis (Supporting Information, Figure S1). To optimize the ink viscosity for inkjet printing, the dried MoS2/EC powder was dispersed in a dual solvent system of 85:15 v/v cyclohexanone/terpineol at different loadings (Figure 1). The

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cyclohexanone/terpineol dual solvent system was chosen because the cyclohexanone offers high solubility of nanomaterial/EC complexes while terpineol improves droplet formation, minimizes coffee-ring effects, and enhances printed feature resolution.9, 13 The MoS2/EC ink possesses a super-linear dependence of viscosity on mass loading (Supporting Information, Figure S2) and reduced viscosities at elevated temperatures, which follows established rheological models for polymer dispersions in organic solvents.16 Considering the ideal viscosity range of 8-15 mPa∙s for inkjet printing of EC-based nanomaterial formulations,9 a total MoS2/EC loading of 35 mg/mL was used for inkjet printing experiments. The characteristics of representative inkjet-printed MoS2/EC lines are shown in Figure 1b-d. (Supporting Information, Figure S3) The thickness of inkjet-printed MoS2/EC layers scaled linearly with the number of printing passes with a thickness of ~100 nm for each layer. Following printing, annealing is required to decompose the EC and enhance electrical contact between the nanosheets in the percolating film. For EC-based graphene films, both thermal and photonic annealing have proven to be effective at yielding high electrical conductivity.15 The inkjet-printed MoS2 films were similarly subjected to the following conditions: (1) thermal annealing (TA) at 400°C in Ar/H2 environment for 3 hours, and (2) photonic annealing (PA) at 2.8 kV for 1.36 ms in air. The TA conditions were carefully selected to minimize oxidation of MoS2, which becomes highly evident above 250°C in air.17 As an alternative to the traditional TA approach, photothermal PA treatments have recently emerged as a highly efficient method for desorbing residual solvent and decomposing polymeric binders from graphene-based composite films.18, 19 This method employs a high-intensity pulsed light source to rapidly heat the active, light-absorbing material with minimal thermal load on the underlying substrate. While most commonly applied to thermally sensitive substrates (e.g., plastic substrates), photonic annealing is

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also effective on rigid substrates (e.g., silicon or glass substrates).18,

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One morphological

characteristic of photonically annealed films is their increased porosity, which is attributed to the rapid gas-phase evolution of adsorbed solvent residues and binder decomposition products. This is similarly observed in MoS2/EC films as the height profile and SEM image of photonically annealed films indicate higher macroscopic roughness compared to thermally annealed films (Supporting Information, Figure S4). Figure 2 shows electrical and chemical characterization of inkjet-printed MoS2/EC films following different TA and PA treatments. Careful tailoring of the annealing conditions is necessary to minimize oxidation of MoS2 since oxidation leads to disordered structures, increases undesired metallic characteristics, and ultimately results in decreased photoexcitation in photodetectors. The oxidation of MoS2 nanosheets under different annealing conditions was initially probed using charge transport measurements (Figure 2a). For TA in Ar/H2 environments and mild PA conditions in air, the film conductance is enhanced by approximately two orders of magnitude and is attributed to the effective removal of EC with minimal MoS2 oxidation. In contrast, for more aggressive PA conditions in air, the conductance dramatically increases by more than six orders of magnitude, which suggests significant oxidation of MoS2. Previously, the products of MoS2 oxidation, namely MoO3 and MoO2, have been shown to possess comparably higher conductivity than MoS2 due to their degenerately doped semiconducting and metallic properties, respectively.20-22 To chemically probe the oxidation of MoS2 following different annealing conditions, Xray photoelectron spectroscopy (XPS) was performed. Figure 2b,c shows the Mo 3d and S 2p spectra for MoS2/EC films as a function of annealing conditions. For TA in Ar/H2 environments and mild PA conditions in air, no measurable changes are observed for the Mo4+ and S2- XPS peaks,

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thereby confirming the preservation of the integrity of the semiconducting MoS2 phase (Mo 3d5/2 = 229 eV, Mo 3d3/2 = 232 eV; S 2p3/2 = 161.9 eV , S 2p1/2 = 163 eV). The total amount of dissipated energy in PA can be controlled via the input voltage and duration of the light pulse. The optimized PA condition that minimizes MoS2 oxidation and achieves a similar electrical conductivity as the TA treatment involves a single light pulse with input voltage of 2.8 kV for 1.36 ms (equivalent to an energy of 4.06 J/cm2). One noteworthy observation is that the degree of oxidation depends on the duration over which the entire amount of energy is applied, which is also likely applicable to other 2D materials prone to oxidation. For example, when a printed MoS2/EC film was photonically annealed with the same nominal energy of 4.06 J/cm2 but with higher input voltage and shorter pulse duration (i.e., 3 kV for 1.16 ms), the measured conductance showed an increase of over 6 orders of magnitude, and the XPS revealed the emergence of peaks associated with MoOx. This effect is more evident at a higher energy of 4.62 J/cm2 (i.e., 3 kV for 1.32 ms), which leads to a highly disordered mixture of phases with dominant composition of the oxidized species (a quantitative summary of the MoS2 XPS analysis as a function of annealing is provided in the Supporting Information, Table S1).23 Ultimately, these results indicate that PA is as effective as TA in removing the EC binders from inkjet-printed MoS2/EC films, albeit with more intricate process optimization than graphene-based systems due to the propensity for MoS2 oxidation. To demonstrate the functionality of inkjet-printed MoS2/EC, we fabricated two types of fully printed photodetectors utilizing graphene (Gr) as the electrode material on rigid glass substrates with thermal annealing (MoS2-Gr TA) and flexible polyimide substrates with photonic annealing (MoS2-Gr PA). Graphene is utilized as the electrode material since it can be printed and cured under the same conditions as MoS2. In particular, Figure 3a shows the configuration of the MoS2-Gr TA device on glass, in which MoS2/EC layers were first inkjet printed and thermally

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annealed, followed by inkjet printing of graphene/EC layers on top and finally thermally annealing the entire device at the aforementioned optimized TA condition. The charge transport results in Figure 3b reveal a high dark bulk conductivity of 2.73 × 10-3 S/m and photocurrent over 100 nA at an irradiation intensity of 0.6 W/cm2 (channel length and width of ~100 and ~165 µm, respectively). This dark bulk conductivity value is 2-3 orders of magnitude greater than previously reported for inkjet-printed MoS2 films12, 14 and 4 orders of magnitude greater than that of solutionprocessed MoS2 films.24 This superior conductivity can be attributed to the densely percolating MoS2 network, suitable contact interface between graphene and MoS2 layers (SEM images provided in the Supporting Information, Figure S6), and favorable carbonaceous EC residues for flake-flake contacts, which is consistent with the observations from graphene/EC films (XPS analysis provided in the Supporting Information, Figure S7). Representative photoresponse data for the MoS2-Gr TA device are shown in Figure 3b-d, which was measured in vacuum with a power-tunable laser at λ = 515.6 nm. The responsivity (R = Ipc/Plaser) of ~1 mA/W for the MoS2-Gr TA photodetector is found to have a relatively weak dependence on the laser intensity (Figure 3c). Previously, the dependence of responsivity on irradiation intensity in MoS2 photodetectors has been explained by various photoconductivity models depending on the materials properties and device architecture.25 The MoS2-Gr TA device possesses a moderate power-law coefficient of ~1.13 from the intensity dependence of photocurrent and a response time of less than 150 µs (Supporting Information, Figures S8-S9). The net effect is a weak power dependence of responsivity (R ~ Pγ, γ ~ 0), which contrasts the sublinear power dependence in monolayer CVD-grown MoS2 phototransistors (γ < 0) that has been attributed to bimolecular recombination, and the super-linear power dependence in previous solution-processed MoS2 composite films (γ > 0) that has been explained by a two-center

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Shockley-Read-Hall model.26 Super-linear power dependence can also arise from photothermal effects, although photothermal phenomena tend to show a slow time response (which will become even more evident later for the PA annealed photodetectors on polyimide substrates).27 Therefore, the combination of a relatively weak power dependence and fast photoresponse suggests a balance between bimolecular and trap-mediated recombination in MoS2-Gr TA photodetectors.25 In addition, the detectivity (D) has been calculated to be ~4.31 × 107 Jones (D = (A1/2R)/(2qIdark)1/2), where A is the effective detector area, q is the electron charge, and Idark is the dark current. It should be noted that semiconductor photodetectors with spatially homogeneous doping are known to possess relatively low detectivity. However, in applications where higher detectivity is required, this issue can be addressed with spatially varying doping profiles (e.g., photodiode device architectures). The versatility of inkjet-printed MoS2/EC is further illustrated through the fabrication of fully printed photodetectors on flexible polyimide substrates using photonic annealing (MoS2-Gr PA). While the device architecture is the same as MoS2-Gr TA (Figure 4a), the MoS2/EC film in this case is first photonically annealed at 2.8 kV for 1.36 ms (4.06 J/cm2), followed by inkjet printing of graphene electrodes and a milder second PA treatment at 2 kV for 0.75 ms (1.03 J/cm2). Since graphene more efficiently absorbs light during photonic annealing than MoS2, milder PA conditions were sufficient to remove EC from the printed graphene layers while avoiding oxidation of MoS2 during the second PA treatment (Supporting Information, Figure S11). The MoS2-Gr PA photodetector characteristics were measured in vacuum (channel length and width of ~100 µm and ~180 µm, respectively), showing dark bulk conductivity of 1.07 × 10-2 S/m, photocurrent close to 5.5 µA, and responsivity exceeding 50 mA/W at an irradiation intensity of 0.6 W/cm2. The increased dark bulk conductivity of MoS2-Gr PA can likely be attributed to the improved contact

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interface between the intermixing surfaces of the photonically annealed MoS2 and graphene layers. Previously, similar observations were noted in graphite-polymer composite films with carbonbased electrodes where increased surface intermixing led to lower contact resistance.28 Increased photoresponsivity also leads to a higher detectivity (D ~ 3.18 × 109 Jones) for the polyimidesupported devices compared to glass-supported devices. As shown in Figure 4b-d, the photoresponse of MoS2-Gr PA exhibits distinct behavior from the thermally annealed devices, particularly a super-linear intensity dependence for the photocurrent and a slower photoresponse time. Similar behavior has been previously reported for solution-processed composite MoS2 films and CdS/CdSe heterostructures,12,

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and has been

attributed to a two-center Shockley-Read-Hall model.12 However, the power-law fitting of photocurrent as function of laser power intensity for MoS2-Gr PA leads to a coefficient of 4.35 (Supporting Information, Figure S8), which cannot be fully explained with the two-center Shockley-Read-Hall model.26,

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Therefore, it appears that there is also a contribution from

photothermally generated charge carriers, which are expected to be more prominent for the MoS2Gr PA devices due to their lower thermal conductivity (~0.12 W/m•K for polyimide compared to ~1 W/m•K for glass) and larger heat capacity (~1.09 J/gram•K for polyimide compared to ~0.84 J/gram•K for glass). The increased porosity of the photonic-annealed MoS2 film on polyimide further reduces its thermal conductivity. Photothermal effects are also consistent with the more than ten-fold greater photocurrent rise time for MoS2-Gr PA (~5 ms) compared to MoS2-Gr TA (~0.15 ms) (Figure 4d). Since the MoS2-Gr PA photodetectors are fabricated on mechanically flexible polyimide substrates, their performance under bending is assessed in Figure 4e, revealing invariant sensitivity (Ipc/Idark) over 500 bending cycles at a radius of curvature of 8.1 mm. In Figure 4f, the distinctive

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photodetector metrics for both MoS2-Gr TA and MoS2-Gr PA are compared to previously reported inkjet-printed MoS2 photodetectors.11 This comparative plot illustrates that both MoS2-Gr TA and MoS2-Gr PA have clear advantages compared to literature precedent with MoS2-Gr TA providing more than 2 orders of magnitude faster time response and MoS2-Gr PA providing ~1,000-fold higher responsivity. Finally, it is noted that the scope of this work is limited to the development of a phototransistor device architecture, which traditionally has relatively high dark current values. For photosensing applications that require lower dark currents, n-type semiconducting MoS2 can be integrated with other solution-processed p-type semiconducting materials (e.g., single-walled carbon nanotubes) in a photodiode architecture. In summary, an inkjet printable MoS2 ink has been developed utilizing EC as a binder that imparts tunability in viscosity and enhanced bulk conductivity following optimized thermal or photonic annealing. The printed MoS2/EC films are compatible with inkjet-printed graphene/EC electrodes, which enabled fully inkjet-printed photodetectors on rigid glass substrates following thermal annealing or flexible polyimide substrates following photonic annealing. Thermally annealing offers ultrafast photoresponse times under 150 µs, whereas photonic annealing provides a mechanically flexible device with high responsivity over 50 mA/W. Significantly, both figures of merit are orders of magnitude higher than previously reported inkjet-printed MoS2 photodetectors.14 These results thus set a new standard for fully printed MoS2 photodetectors that complement other emerging nanomaterial-based systems for next-generation optoelectronic applications.30-33

ASSOCIATED CONTENT Supporting Information: 11



Experimental methods and supporting characterization data accompany this paper and are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *Correspondence should be addressed to: [email protected] Present Addresses: †

Jian Zhu

School of Materials Science and Engineering, Nankai University, 38 Tongyan Road, Tianjin, 300350, China ‡

Ethan B. Secor

Sandia National Laboratories Albuquerque, NM 87185, USA

AUTHOR CONTRIBUTIONS J.-W.T.S., J.Z., and M.C.H. conceived the idea. J.-W.T.S. and J.Z. developed, prepared, and printed the MoS2/EC powders and inks. E.B.S. prepared the graphene/EC inks. J.-W.T.S. and S.G.W. printed the graphene/EC inks. J.-W.T.S. developed and performed the thermal and photonic annealing treatments. J.-W.T.S. performed the materials characterization and analyzed the results. J.-W.T.S. and V.K.S. performed and analyzed the electrical and photocurrent measurements. M.C.H. oversaw the development and execution of the research. All coauthors discussed the results and contributed to the writing and editing of the manuscript.

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NOTES Competing financial interests: The inkjet printable MoS2/EC ink is available commercially from MilliporeSigma as Product Number 901187.

ACKNOWLEDGEMENTS This work was primarily supported by MilliporeSigma. In addition, the graphene/EC ink electrodes were developed under the support of the Air Force Research Laboratory under agreement number FA8650-15-2-5518. This research made use of the EPIC and Keck-II facility of the Northwestern University NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois. Rheometry and thermal gravimetric analysis were performed in facilities supported by the NSF MRSEC Program (DMR1720139) of the Materials Research Center at Northwestern University. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the sponsors.

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15. Secor, E. B.; Ahn, B. Y.; Gao, T. Z.; Lewis, J. A.; Hersam, M. C. Rapid and Versatile Photonic Annealing of Graphene Inks for Flexible Printed Electronics. Adv. Mater. 2015, 27, 6683-6688. 16. Sadeghi, R. Segment-Based Eyring-Wilson Viscosity Model for Polymer Solutions J. Chem. Thermodyn. 2005, 37, 445-448. 17. Sim, D. M.; Kim, M.; Yim, S.; Choi, M.-J.; Choi, J.; Yoo, S.; Jung, Y. S. Controlled Doping of Vacancy-Containing Few-Layer MoS2 via Highly Stable Thiol-Based Molecular Chemisorption ACS Nano 2015, 9, 12115-12123. 18. Arapov, K.; Bex, G.; Hendriks, R.; Rubingh, E.; Abbel, R.; de With, G.; Friedrich, H. Conductivity Enhancement of Binder‐Based Graphene Inks by Photonic Annealing and Subsequent Compression Rolling. Adv. Eng. Mater. 2016, 18, 1234-1239. 19. Secor, E. B.; Gao, T. Z.; Dos Santos, M. H.; Wallace, S. G.; Putz, K. W.; Hersam, M. C. Combustion-Assisted Photonic Annealing of Printable Graphene Inks via Exothermic Binders. ACS Appl. Mater. Interfaces 2017, 9, 29418-29423. 20. Inzani, K.; Nematollahi, M.; Vullum-Bruer, F.; Grande, T.; Reenaas, T. W.; Selbach, S. M. Electronic Properties of Reduced Molybdenum Oxides. Phys. Chem. Chem. Phys. 2017, 19, 92329245. 21. Guo, Y.; Robertson, J. Origin of the High Work Function and High Conductivity of MoO3. Appl. Phys. Lett. 2014, 105, 222110. 22. López-Carreño, L. D.; Pardo, A.; Zuluaga, M.; Cortés-Bracho, O. L.; Torres, J.; Alfonso, J. E. Electrical Transport Properties of MoO3 Thin Films Prepared by Laser Assisted Evaporation. Phys. Status Solidi C 2007, 4, 4064-4069. 23. Spevack, P. A.; McIntyre, N. S. A Raman and XPS Investigation of Supported Molybdenum Oxide Thin Films. 2. Reactions with Hydrogen Sulfide. J. Phys. Chem. 1993, 97, 11031-11036. 24. Cunningham, G.; Lotya, M.; McEvoy, N.; Duesberg, G. S.; van der Schoot, P.; Coleman, J. N. Percolation Scaling in Composites of Exfoliated MoS2 Filled with Nanotubes and Graphene. Nanoscale 2012, 4, 6260-6264. 25. Kufer, D.; Konstantatos, G. Highly Sensitive, Encapsulated MoS2 Photodetector with Gate Controllable Gain and Speed. Nano Lett. 2015, 15, 7307-7313. 26. Shockley, W.; Read, W. T. Statistics of the Recombinations of Holes and Electrons. Phys. Rev. 1952, 87, 835-842. 27. Bube, R. H. Infrared Quenching and a Unified Description of Photoconductivity Phenomena in Cadmium Sulfide and Selenide. Phys. Rev. 1955, 99, 1105-1116. 28. Avasarala, B.; Haldar, P. Effect of Surface Roughness of Composite Bipolar Plates on the Contact Resistance of a Proton Exchange Membrane Fuel Cell. J. Power Sources 2009, 188, 225229. 29. Grimmeiss, H. G. Photoelectronic Properties of Semiconductors. By Richard H. Bube, Cambridge University Press, Cambridge 1992, 318 pp., ISBN 0-521-40681-1. Adv. Mater. 1993, 5, 65-66. 30. Song, J.; Zeng, H. Transparent Electrodes Printed with Nanocrystal Inks for Flexible Smart Devices. Angew. Chem. Int. Ed. Engl. 2015, 54, 9760-9774. 31. Song, J.; Xu, L.; Li, J.; Xue, J.; Dong, Y.; Li, X.; Zeng, H. Monolayer and Few‐Layer All‐ Inorganic Perovskites as a New Family of Two‐Dimensional Semiconductors for Printable Optoelectronic Devices. Adv. Mater. 2016, 28, 4861-4869.

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32. Song, J.; Li, J.; Xu, L.; Li, J.; Zhang, F.; Han, B.; Shan, Q.; Zeng, H. Room‐Temperature Triple‐Ligand Surface Engineering Synergistically Boosts Ink Stability, Recombination Dynamics, and Charge Injection toward EQE‐11.6% Perovskite QLEDs. Adv. Mater. 2018, 30, 1800764. 33. Li, J.; Xu, L.; Wang, T.; Song, J.; Chen, J.; Xue, J., Dong, Y., Cai, B., Shan, Q., Han B., Zeng, H. 50‐Fold EQE Improvement up to 6.27% of Solution‐Processed All‐Inorganic Perovskite CsPbBr3 QLEDs via Surface Ligand Density Control. Adv. Mater. 2017, 29, 1603885.

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FIGURES

Figure 1. Inkjet printing characteristics of MoS2/EC inks. (a) Photograph of the MoS2/EC ink (inset) and its viscosity dependence on solid loading and temperature. (b) Inkjet-printed MoS2/EC lines on glass (top) and polyimide (bottom). The width of the printed lines can be tuned with the number of rows of droplets per line, as indicated by the red circles. The scale bar is 100 µm. (c) Average height measured by profilometry from the as-printed (initial), thermally annealed (TA), and photonically annealed (PA) MoS2/EC lines. Inset: Height profiles from the initial (black), TA (red, 177.65 ± 14.52 nm), and PA (blue, 124.68 ± 49.63 nm) lines after 7 printing passes, illustrating larger standard deviation in thickness and morphological roughness after PA. (d) SEM images of inkjet-printed MoS2/EC films following TA and PA. The scale bars are 1 µm. 17



Figure 2. Electrical and chemical characterization of inkjet-printed MoS2/EC films following different annealing conditions. (a) Electrical conductance measured from inkjet-printed MoS2/EC lines following different annealing conditions. The as-printed lines were ~165 µm wide and ~700 nm thick, and the electrical conductance was measured with two probes spaced by 300 µm. A: no annealing (initial); B: TA at 400°C for 3 hr under Ar/H2; C: PA at 2.8 kV for 1.36 ms (4.06 J/cm2 with long pulse duration); D: PA at 3 kV for 1.16 ms (4.06 J/cm2 with short pulse duration); E: PA at 3 kV for 1.32 ms (4.62 J/cm2). (b,c) XPS analysis of the Mo 3d and S 2p spectral regions, respectively, for the inkjet-printed MoS2/EC films under the same annealing conditions as (a). The red curves indicate the pristine MoS2 peaks, whereas the other colors represent oxidized states of MoOx.

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Figure 3. Fully printed MoS2-Gr photodetectors prepared with thermal annealing. (a) Optical microscopy image of the MoS2/EC channel (vertical line) and graphene/EC electrodes (horizontal lines) after annealing. The scale bar is 100 µm. (b) Current-voltage curves of a MoS2-Gr TA photodetector showing the dark and illuminated (λ = 515.6 nm, laser intensity = 0.6 W/cm2) currents, and the corresponding photocurrent. (c) Photocurrent and responsivity as a function of intensity at a bias voltage of 40 V. (d) Temporal response showing current modulation as the laser is switched on and off at a bias voltage of 10 V.

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Figure 4. Fully printed MoS2-Gr photodetectors prepared with photonic annealing. (a) Optical microscopy image of the inkjet-printed photodetector on polyimide (PI) after photonic annealing, with a vertical MoS2/EC channel and horizontal graphene/EC electrodes. The scale bar is 100 µm. (b,c) Photocurrent and responsivity as a function of laser intensity at a bias voltage of 40 V. (d) Temporal response showing current modulation as the laser is switched on and off at a bias voltage of 10 V. (e) Bending test over 500 cycles showing invariant sensitivity. Inset: photograph of the flexible MoS2-Gr device. The scale bar is 3 mm. (f) Comparison of photodetector characteristics from MoS2-Gr TA on glass (red), MoS2-Gr PA on PI (blue), and literature precedent for MoS2-Ag thermally annealed on SiO2/Si.14

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TOC IMAGE

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Fully Inkjet-Printed, Mechanically Flexible MoS2 Nanosheet

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