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A Van Der Waals P-N Heterojunction Based on Polymer-2D Layered MoS for Solution Processable Electronics 2

Abdus Salam Sarkar, and Suman Kalyan Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07132 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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A van der Waals p-n Heterojunction Based on Polymer-2D Layered MoS2 for Solution Processable Electronics Abdus Salam Sarkar and Suman Kalyan Pal* School of Basic Sciences, Indian Institute of Technology Mandi, Kamand, Mandi-175005, Himachal Pradesh, India. Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi175005, Himachal Pradesh, India.

ABSTRACT Organic-inorganic heterostructures are emerging materials for developing high performance, solution processable organic electronic and optoelectronic devices. In particular, the heterostructures of semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) are interesting due to quantum confinement effect, extended solar light absorption and tunable optoelectronic properties. Here, we report a facile and fully solution processable method called semiconductive Polymer assisted chemical exfoliation (SPACE) of synthesizing polymerMoS2 nanoheterostructures. Synthesized polymer grafted MoS2 (PG-MoS2) nanoheterostructures consist of few layers of MoS2 and are chemically quite stable. The phase integrity of MoS2 in PG-MoS2 is confirmed by various microscopic and spectroscopic techniques. Efficient 1 ACS Paragon Plus Environment

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dissociation of excitons in PG-MoS2 heterostructures is reported and explained based on of electron and hole transfers. Strong photovoltaic effects are observed in the polymer-MoS2 heterojunction devices due to photocarrier generation at the interface. In addition, we demonstrate a bipolar resistive switching effect with very high ON/OFF ratio (~104) in a PGMoS2 sandwiched device. A thorough carrier transport study has been undertaken to understand the switching effect. Our results reveal that the heterostructures of polymer and layered TMDs could be the basis for up-and-coming solution processable solar cells and memory devices.

INTRODUCTION After the successful isolation of graphene in 2004, two-dimensional (2D) materials have received significant attention due to their intriguing electrical, optical and mechanical properties.1-3 However, semi-metallic nature (with zero band gap) of graphene have restricted its applications in electronic and optoelectronic devices. Recently, 2D layered transition metal dichalcogenides (TMDs) with the chemical formula MX2, where the transitional metal atom (M) is sandwiched between two chalcogen atoms (X), offer an exciting platform for exploring new semiconductor technologies.4-7 Unique optoelectronic properties of TMDs including a non-zero direct or indirect band gap (that can be tuned by varying the number of layers) and strong light–matter interactions make them more interesting.4, 7 Among all TMDs, molybdenum disulfide (MoS2) is the one in which the semiconducting 2H-phase exhibits layer dependent band gap energy between 1.3 and 1.8 eV leading to interesting absorption and emission features, which could be exploited in optoelectronic device applications.8-10 Monolayer MoS2 possesses outstanding carrier mobility

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(~200 cm2 V-1 s-1), which increases to 500 cm2 V-1 s-1 for few-layer.10, 11 Moreover, ultrathin (1.7 V) the slope is ~3.72. Slope of more than 2 suggests that the carrier transport is governed by the exponentially distributed trap controlled SCLC in the voltage range 1.7 to 2 V. On contrary, the linear fitting of the I-V curve (within 2 to 0 V) for the LRS (ON state) with a slope of m~1 implies Ohomic carrier conduction over entire positive voltage sweep. Plenty of nanoheterojunctions are formed in the device due to the vertical stacking of PG1-MoS2 nanosheets while forming the PG1-MoS2 layer on FTO. Considering this fact, we

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propose a possible charge carrier transport mechanism and a general model of carrier tunneling across the polarization potential barriers.46 The carrier pathway through the device can be divided into various voltage regimes (Supporting Information Figure S6). The presence of barrier potentials at the FTO interface and multiple nanoheterojunctions sets the device initially in the HRS by limiting the carrier transport (Supporting Information Scheme a, Figure S6). At a relatively low applied electric field, charge carriers are injected from the electrode (FTO) following the TE and accumulated in polymer wells (Supporting Information Scheme b, Figure S6). A polarization domain is formed due to the difficulty of carrier tunneling in HRS. With increasing the applied electric field accumulation of charges (polarizations) is increased. This intern reduces the barrier height of the potential well leading to the carrier transport via SCLC (Supporting Information Scheme c, Figure S6). At a particular positive electric field, charge carriers fill all the potential wells by making the carrier path conductive and setting the device in LRS (Supporting Information Scheme d, Figure S6). Because of the existence of the accumulated charges and the internal polarization field the conductive pathway is well-set until a reverse applied electric field is high enough to rupture the polarized wells. During the reverse voltage sweep, when the number of accumulated charges is reduced to a certain level, the junction barriers start hindering the charge transport through the device (Supporting Information Scheme d, Figure S6). The demolition of the conductive pathway during negative voltage sweep reset the device from LRS to HRS. Therefore, PG-MoS2 nanoheterostructures could be used in memory devices as the trapping and detrapping of charge carriers in alternate p-n heterojunctions take place because of the presence of bound charges and internal polarization fields. It is worth noting that none of the previous reports have shown bipolar characteristics of heterostructures of organic and 2D materials. Recently, Hersam and co-workers47 reported

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‘antiambipolar’ behavior in bilayer heterojunctions of single-walled carbon nanotube (SWCNT) and single-layer MoS2. They referred the unusual gate voltage dependency of the transfer characteristics of the heterojunction made field effect transistor (FET) as an ‘antiambipolar’ behavior. It was proposed that the observed ‘antiambipolar’ behavior originates at the FET channel consisting of two p and n semiconductors in series. Variable gate bias voltage changes the resistance of each layer which affects the net series resistance leading to the resulting ‘antiambipolar’ characteristics of the junction. CONCLUSIONS In summary, we have successfully synthesized 2D nanoscale-heterojunctions, PG-MoS2 via semiconductive polymer assisted chemical exfoliation. The microscopic and X-ray diffraction results confirm the presence of hexagonal 2H (semiconducting) MoS2 phase in PG1-MoS2 nanoheterostructures. PL of both polymer and MoS2 nanosheets are quenched due to electron and hole transfer, respectively at p-n junctions in PG-MoS2. Efficient exciton generation and dissociation leads to photovoltaic activity in PG1-MoS2 devices. Moreover, a PG1-MoS2 diode shows sustainable bipolar switching behavior with high ON/OFF current ratio (~104) due to carrier transport through polarization induced tunneling in vertically stacked multiple p-n junctions. We believe that because of the facile synthesis in non-polar solvent and high chemical stability, polymer-2D TMD nanoheterostructures could find applications in solution processable electronic devices, especially in solar cells and memory devices.

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Supporting Information The supporting Information is available free of charge on the ACS Publication website. Additional information including supplemental spectroscopic, microscopic data and schematic of the carrier transport mechanism. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +91 1905 267040; Fax: +91 1905 237924 ■ Acknowledgements This work is supported by the Science and Engineering Research Board (SERB), Government of India under Grant No. SB/S1/OC-48/2014. A S Sarkar is thankful to IIT Mandi for his fellowship and Advanced Materials Research Centre for the experimental facilities. ■ REFERENCES (1)

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