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Nanoscale Surface Photovoltage Mapping of 2D Materials and Heterostructures by Illuminated Kelvin Probe Force Microscopy

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Nanoscale Surface Photovoltage Mapping of 2D Materials and Heterostructures by Illuminated Kelvin Probe Force Microscopy Nanoscale Surface Photovoltage Mapping of 2D Materials and Heterostructures by Illuminated Kelvin Probe Force Microscopy Item Type Article Authors Shearer, Melinda J.; Li, Ming-yang; Li, Lain-Jong; Jin, Song; Hamers, Robert J Citation Shearer MJ, Li M-Y, Li L-J, Jin S, Hamers RJ (2018) Nanoscale Surface Photovoltage Mapping of 2D Materials and Heterostructures by Illuminated Kelvin Probe Force Microscopy. The Journal of Physical Chemistry C. Available: http://dx.doi.org/10.1021/acs.jpcc.7b12579. Eprint version Post-print DOI 10.1021/acs.jpcc.7b12579 Publisher American Chemical Society (ACS) Journal The Journal of Physical Chemistry C Rights This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/ acs.jpcc.7b12579. Download date 28/09/2021 13:53:18 Link to Item http://hdl.handle.net/10754/627064 Subscriber access provided by King Abdullah University of Science and Technology Library Article Nanoscale Surface Photovoltage Mapping of 2D Materials and Heterostructures by Illuminated Kelvin Probe Force Microscopy Melinda J. Shearer, Ming-Yang Li, Lain-Jong Li, Song Jin, and Robert J Hamers J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12579 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 6, 2018 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 23 The Journal of Physical Chemistry 1 2 3 4 Nanoscale Surface Photovoltage Mapping of 2D Materials and 5 6 7 Heterostructures by Illuminated Kelvin Probe Force Microscopy 8 9 10 Melinda J. Shearer,1 Ming-Yang Li,2,3 Lain-Jong Li,2 Song Jin1* and Robert J. Hamers1* 11 12 13 14 1 15 Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, 16 17 United States 18 19 2 Physical Sciences and Engineering Division, King Abdullah University of Science and 20 21 22 Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia 23 24 3Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan 25 26 27 ABSTRACT 28 29 30 Nanomaterials are interesting for a variety of applications, such as optoelectronics and 31 32 photovoltaics. However, they often have spatial heterogeneity, i.e. composition change or physical 33 34 change in the topography or structure, which can lead to varying properties that would influence 35 36 37 their applications. New techniques must be developed to understand and correlate spatial 38 39 heterogeneity with changes in electronic properties. Here we highlight the technique of surface 40 41 photovoltage-Kelvin probe force microscopy (SPV-KFM), which is a modified version of non- 42 43 44 contact atomic force microscopy capable of imaging not only the topography and surface potential, 45 46 but also the surface photovoltage on the nanoscale. We demonstrate its utility in probing 47 48 monolayer WSe2-MoS2 lateral heterostructures, which form an ultrathin p-n junction promising 49 50 for photovoltaic and optoelectronic applications. We show surface photovoltage maps 51 52 53 highlighting the different photoresponse of the two material regions as a result of the effective 54 55 charge separation across this junction. Additionally, we study the variations between different 56 57 58 59 1 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 2 of 23 1 2 3 heterostructure flakes and emphasize the importance of controlling the synthesis and transfer of 4 5 6 these materials to obtain consistent properties and measurements. 7 8 9 10 11 12 13 INTRODUCTION 14 1-3 4- 15 Nanomaterials are interesting for solar energy conversion and other optoelectronic applications 16 8 9-11 17 for a variety of reasons: tunability of electronic properties based on size, high surface area that 18 19 allows for manipulation of properties via surface chemistry,12-14 and the ability to form unique 20 21 15 22 heterojunctions due to minimization of lattice strain. However, many nanomaterials have spatial 23 24 heterogeneity at the nanoscale that can lead to drastic variations in material properties. Spatial 25 26 heterogeneity can involve both macroscopic properties, such as varying chemical composition and 27 28 29 lack of uniformity in the size and shape as well as localized features, such as step edges, adsorbed 30 31 atoms, and defects. The formation of heterostructures or doping of nanomaterials can also create 32 33 spatially-varying properties based on changes in the chemical composition. All of these types of 34 35 36 spatial heterogeneity can contribute to varying charge transfer properties, and it is critical to study 37 38 how these changes influence the overall photoresponse of individual nanomaterials. Indeed, as 39 40 new nanomaterials and heterostructures are synthesized, understanding charge transfer and 41 42 16 dynamics within these materials is critically important, and high-resolution, high-throughput 43 44 45 techniques with this capability are required. 46 47 Surface photovoltage (SPV) spectroscopy is a powerful technique for understanding the 48 49 influence of surface electronic states on the photoresponse of materials,17-18 and has been used 50 51 19-22 52 extensively to understand charge dynamics in a variety of semiconductor materials. This 53 54 technique involves illuminating a semiconductor material and measuring the resulting surface 55 56 57 58 59 2 60 ACS Paragon Plus Environment Page 3 of 23 The Journal of Physical Chemistry 1 2 3 photovoltage, typically using a metal-insulator-semiconductor configuration. A limitation of this 4 5 6 technique, however, is its inability to measure spatial variation, which makes it impractical for 7 8 studying individual nanostructures. One method in particular can help fill this void — surface 9 10 photovoltage-Kelvin probe force microscopy (SPV-KFM), also referred to as illuminated KFM or 11 12 13 microscopic SPV. With this method, standard Kelvin probe force microscopy is used to measure 14 15 the surface potential of the material both in the dark and under illumination, the net difference of 16 17 which creates surface photovoltage maps. It has so far been demonstrated on a small number of 18 19 23-24 25-28 29-30 materials such as silicon wafers, organic solar cells and inorganic thin films, but viable 20 21 31-33 22 demonstrations on individual nanostructures have been limited. Additionally, in a typical 23 24 SPV-KFM setup, the sample is illuminated from below, thus requiring the use of a transparent 25 26 conductive substrate, which is not always feasible or practical for device applications. 27 28 29 In this work, we have developed the SPV-KFM technique with illumination of the sample 30 31 from above, and have demonstrated its efficacy on a monolayer lateral heterostructure of WSe2- 32 33 MoS2. WSe2 and MoS2 are layered transition metal dichalcogenides (MX2), which have received 34 35 34-35 36 much research interest recently due to the ability to tune their properties based on the number 37 36-37 38 38 and rotation of layers as well as their strong absorption at only a monolayer thickness. 39 40 Therefore, they have been demonstrated for a variety of electronic,39-40 optoelectronic,41-42 solar 41 42 energy conversion2 and catalysis applications.43-45 Additionally, these 2D materials can be 43 44 45 synthesized as lateral heterostructures, allowing for the formation of covalently-bonded nanoscale 46 47 p-n junctions that are promising for ultra-thin electronics.46-50 Extensive structural and electronic 48 49 characterization of individual or heterojunction devices has been collected,46-48 but only rarely has 50 51 52 there been information collected that combines charge transfer properties of these materials with 53 51-54 54 spatial resolution. In order to see the effects of both heterostructure formation as well as 55 56 57 58 59 3 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 4 of 23 1 2 3 variation among nanostructures, we have mapped the surface photovoltage of lateral WSe2-MoS2 4 5 6 heterostructures and revealed the heterogeneity in photoresponse between the two materials as a 7 8 result of the charge transfer across the p-n junction that they form.
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