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Liquid-Phase Exfoliation of Two-Dimensional Materials: Applications, deposition methods and printed electronics on paper
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

After the unprecedented success of graphene research, other materials that can also be exfoliated into thin layers, like Transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2), have also become the subjects of extensive studies. As one of the most promising methods for large scale production of such materials, liquid-phase exfoliation (LPE) has also been the subject of extensive research and is maturing as a field to the point that devices using additive manufacturing and printed nanosheets are often reported. The stability of the nanosheets in environmentally friendly solvents, particularly in water, with or without stabilizers, is still a focus of great interest for sustainable and commercial production. In this thesis, different methods of LPE in water with and without stabilizers are investigated and discussed. Stabilizers such as surfactant sodium dodecyl sulfate (SDS) and modified cellulose2-hydroxyethyl cellulose (HEC), were employed. Because waterdoes not have surface energy parameters that match those of2D materials, the dispersions in water do not usually have a high yield. Therefore, to circumvent the use of organic solvents that are known to be able to successfully exfoliate and stabilize nanosheets of two-dimensional materials, this thesis focuses on water as the solution-process medium for exfoliation and the assisting stabilizers used to keep the exfoliated nanomaterials in dispersion with a long half-time. Surfactant-assisted dispersions are discussed together with test-printing resultsusing inkjet to deposit the material. Process parameters for the LPE method using HEC as a stabilizer are presented together with thin nanosheets characterized by Raman spectroscopy. Dispersions using HEC presented the longest half-time among the studied methods, higher than previously reported values for methods using mixed low-boiling-point solvents. Devices using exfoliated nanosheets have been fabricated and presented in the present study. The photoconductivity of MoS2 using a device fabricated with LPE MoS2 nanosheets and the cathodoluminescence of LPE MoS2 are discussed. Although fabricated with mechanically exfoliated nanosheets and not LPE ones, another photodetector fabricated with one of the MoS2 grades used in this thesis is presented to highlight the excellent photoresponse of this material. A method of producing thin nanosheets with-out stabilizers by pre-processing the MoS2 grades withs and papers is introduced. With this method, nanosheets with a lateral size of around 200nm and a concentration around 0.14 g L−1 - that is half the concentration at the same processing conditions in solvent n-methyl pyrrolidone (NMP) - are discussed. Inkjet printing as a deposition method is discussed together with the requirements for the 2D inks. Printed organic electronics using the conductive polymer PEDOT:PSS are compared to those using commercially-available graphene ink, with a focus on printing on paper substrates. In order to bring the thesis into perspective from materials to device fabrication, I study the suitability of inkjet paper substrates for printed electronics, by extensively characterizing the chemical and physical properties of their ink-receiving layers (IRLs) and their impact on the electronic properties of the conductive printed lines.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University , 2019. , p. 125
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 305
Keywords [en]
molybdenum disulfide (MoS2), transition metal dichalcogenide (TMD), thin films, inkjet printing, 2D inks, liquid-phase exfoliation, shear exfoliation, printed electronics, organic electronics, PEDOT:PSS, photoresponse, photocurrent, cathodoluminescence
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:miun:diva-37287ISBN: 978-91-88947-19-2 (print)OAI: oai:DiVA.org:miun-37287DiVA, id: diva2:1353377
Public defence
2019-10-18, O102, Sundsvall, 10:15 (English)
Opponent
Supervisors
Note

Vid tidpunkten för disputationen var följande delarbete opublicerat: delarbete 3 (inskickat).

At the time of the doctoral defence the following paper was unpublished: paper 3 (submitted).

Available from: 2019-09-23 Created: 2019-09-23 Last updated: 2019-09-23Bibliographically approved
List of papers
1. Exfoliated MoS2 in Water without Additives
Open this publication in new window or tab >>Exfoliated MoS2 in Water without Additives
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2016 (Swedish)In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 11, no 4, article id 0154522Article in journal (Refereed) Published
Abstract [en]

Many solution processing methods of exfoliation of layered materials have been studied during the last few years; most of them are based on organic solvents or rely on surfactants andother funtionalization agents. Pure water should be an ideal solvent, however, it is generallybelieved, based on solubility theories that stable dispersions of water could not be achievedand systematic studies are lacking. Here we describe the use of water as a solvent and thestabilization process involved therein. We introduce an exfoliation method of molybdenumdisulfide (MoS2) in pure water at high concentration (i.e., 0.14±0.01 g L−1). This was achieved by thinning the bulk MoS2by mechanical exfoliation between sand papers and dis-persing it by liquid exfoliation through probe sonication in water. We observed thin MoS2nanosheets in water characterized by TEM, AFM and SEM images. The dimensions of thenanosheets were around 200 nm, the same range obtained in organic solvents. Electropho-retic mobility measurements indicated that electrical charges may be responsible for the sta-bilization of the dispersions. A probability decay equation was proposed to compare thestability of these dispersions with the ones reported in the literature. Water can be used as asolvent to disperse nanosheets and although the stability of the dispersions may not be ashigh as in organic solvents, the present method could be employed for a number of applications where the dispersions can be produced on site and organic solvents are not desirable.

Keywords
liquid exfoliation; MoS2; solar cells; water exfoliation
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-27560 (URN)10.1371/journal.pone.0154522 (DOI)000374976200086 ()2-s2.0-85002584946 (Scopus ID)
Projects
Paper Solar Cells
Available from: 2016-04-28 Created: 2016-04-28 Last updated: 2019-09-23Bibliographically approved
2. Liquid Exfoliation of Layered Materials in Water for Inkjet Printing
Open this publication in new window or tab >>Liquid Exfoliation of Layered Materials in Water for Inkjet Printing
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2016 (English)In: Journal of Imaging Science and Technology, ISSN 1062-3701, E-ISSN 1943-3522, Vol. 60, no 4, p. 1-7, article id 040405Article in journal (Refereed) Published
Abstract [en]

MoS2 is a layered material which is abundant and non-toxic and has been increasingly studied during the last few years as a semiconducting alternative to graphene. While most studies have been performed on single MoS2 nanosheets, for example to demonstrate high-performance electronic transistors, more work is needed to explore the use of MoS2 in printed electronics. The importance of using MoS2 as a printed electronic material could be understood by considering the several orders higher electron mobility in MoS2, even in several nanometer thick layers, compared to the organic and other materials used today. In the few studies performed so far on printing MoS2, the developed dispersions used mainly organic solvents that might be detrimental for the environment. Here, we show an environmentally friendly liquid-based exfoliation method in water where the solution was stabilized by sodium dodecyl sulfate (SDS) surfactant. The dispersions consisted of very thin MoS2 nanosheets with average lateral size of about 150 nm, surface tension of 28 mN m(-1), and a shelf life of a year. Although both the concentration and viscosity was less than optimal, we were able to inkjet print the MoS2 solution on paper and on PET films, using multiple printing passes. By tuning the concentration/viscosity, this approach might lead to an environmentally friendly MoS2 ink suitable for printed electronics.

National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-28750 (URN)10.2352/J.ImagingSci.Technol.2016.60.4.040405 (DOI)000381636200006 ()2-s2.0-85016315340 (Scopus ID)STC (Local ID)STC (Archive number)STC (OAI)
Conference
Printing for Fabrication 2016 (JIST First Paper)
Available from: 2016-09-12 Created: 2016-09-12 Last updated: 2019-09-23Bibliographically approved
3. Process parameters of liquid-phase exfoliation of MoS2 in waterstabilized with 2-hydroxyethyl cellulose
Open this publication in new window or tab >>Process parameters of liquid-phase exfoliation of MoS2 in waterstabilized with 2-hydroxyethyl cellulose
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:miun:diva-37286 (URN)
Available from: 2019-09-23 Created: 2019-09-23 Last updated: 2019-09-23Bibliographically approved
4. Photoconductivity of acid exfoliated and flash-light-processed MoS2 films
Open this publication in new window or tab >>Photoconductivity of acid exfoliated and flash-light-processed MoS2 films
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2018 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 3296Article in journal (Refereed) Published
Abstract [en]

MoS2 has been studied intensively during recent years as a semiconducting material in several fields, including optoelectronics, for applications such as solar cells and phototransistors. The photoresponse mechanisms of MoS2 have been discussed but are not fully understood, especially the phenomenon in which the photocurrent slowly increases. Here, we report on a study of the photoresponse flash-light-processed MoS2 films of different thicknesses and areas. The photoresponse of such films under different light intensities and bias voltages was measured, showing significant current changes with a quick response followed by a slow one upon exposure to pulsed light. Our in-depth study suggested that the slow response was due to the photothermal effect that heats the MoS2; this hypothesis was supported by the resistivity change at different temperatures. The results obtained from MoS2 films with various thicknesses indicated that the minority-carrier diffusion length was 1.36 mu m. This study explained the mechanism of the slow response of the MoS2 film and determined the effective thickness of MoS2 for a photoresponse to occur. The method used here for fabricating MoS2 films could be used for fabricating optoelectronic devices due to its simplicity.

National Category
Physical Sciences
Identifiers
urn:nbn:se:miun:diva-33302 (URN)10.1038/s41598-018-21688-0 (DOI)000425380900079 ()29459668 (PubMedID)2-s2.0-85061713034 (Scopus ID)
Available from: 2018-03-19 Created: 2018-03-19 Last updated: 2019-09-23Bibliographically approved
5. Electronic performance of printed PEDOT:PSS lines correlated to the physical and chemical properties of coated inkjet papers
Open this publication in new window or tab >>Electronic performance of printed PEDOT:PSS lines correlated to the physical and chemical properties of coated inkjet papers
2019 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 9, no 41, p. 23925-23938Article in journal (Refereed) Published
Abstract [en]

PEDOT:PSS organic printed electronics chemical interactions with the ink-receiving layer (IRL) of monopolar inkjet paper substrates and coating color composition were evaluated through Raman spectroscopy mapping in Z (depth) and (XY) direction, Fourier transform infrared spectroscopy (FTIR) and energy dispersive X-ray spectroscopy (EDS). Other evaluated properties of the IRLs were pore size distribution (PSD), surface roughness, ink de-wetting, surface energy and the impact of such characteristics on the electronics performance of the printed layers. Resin-coated inkjet papers were compared to a multilayer coated paper substrate that also contained an IRL but did not contain the plastic polyethylene (PE) resin layer. This substrate showed better electronic performance (i.e., lower sheet resistance), which we attributed to the inert coating composition, higher surface roughness and higher polarity of the surface which influenced the de-wetting of the ink. The novelty is that this substrate was rougher and with somewhat lower printing quality but with better electronic performance and the advantage of not having PE in their composite structure, which favors recycling. © 2019 The Royal Society of Chemistry.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-36895 (URN)10.1039/c9ra03801a (DOI)000478947000054 ()2-s2.0-85070373982 (Scopus ID)
Available from: 2019-08-20 Created: 2019-08-20 Last updated: 2019-09-23Bibliographically approved

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