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| 2. |
- Forsberg, Viviane, 1981-, et al.
(författare)
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Cellulose stabilizers for 2D materials inkjet inks
- 2017
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Konferensbidrag (refereegranskat)abstract
- To date, the most promising scalable method for achieving 2D materials dispersions is through liquidbasedexfoliation of nanosheets in solvents. We study the use of high throughput shear exfoliation insteadof sonication to exfoliate water dispersions of MoS2 using environmental friendly stabilizers based oncellulose. The resulted dispersion was then concentrated and inkjet printed on a flexible substrate. We usedethyl cellulose, cellulose nanofibers (CNF) and ultra-fine cellulose nanofibers (UF-CNF). The stability wasevaluated by measuring the differences in concentration over time. The particle size distribution (PSD) ofthe dispersed particles was evaluated using statistical methods applied to SEM images of the dispersions(See Fig 1 and 2). The zeta potential and the mechanisms of stabilization involved was evaluated (See Fig4). All three stabilizers appear to work very well for MoS2 nanosheets even though the mechanisms ofstabilization were different i.e. steric stabilization for MoS2-EC and electrostatic stabilization for MoS2-CNF and MoS2-UF-CNF dispersions. For the MoS2-EC dispersions we achieved a broader PSD (Fig. 1)and higher stability. Thin nanosheets was observed from the SEM image of MoS2-EC dispersions depositedonto cellulose filters by vacuum filtration (Fig. 5) which demonstrated that the exfoliation technique usedwas successful. The estimated concentration of the MoS2-EC dispersion after 8 days of sample preparationwas 0.24 mg/mL, 77% of the initial concentration (see Fig. 6) and it was relatively steady after 40 days ofsample preparation (0.22 mg/mL). To adjust the concentration and the viscosity of the MoS2-EC dispersion,we concentrated it using a rotary evaporator solvent exchange technique. For this we used terpineol andadjusted the viscosity using ethanol. This paper presents the results of an inkjet 2D material ink usingenvironmental friendly components different than previous 2D materials inks that used organic solvents orwater based dispersions containing surfactants.
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| 3. |
- Forsberg, Viviane, 1981-, et al.
(författare)
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Exfoliated MoS2 in Water without Additives
- 2016
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Ingår i: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 11:4
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Forsberg, Viviane, 1981-, et al.
(författare)
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Liquid Exfoliation of Layered Materials in Water for Inkjet Printing
- 2016
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Ingår i: Printing for Fabrication 2016. - USA : Curran Associates, Inc.. - 9780892083220 - 9780892083237 - 9780892083213 ; 60:4, s. 1-7
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 6. |
- Forsberg, Viviane, 1981-
(författare)
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Liquid Exfoliation of Molybdenum Disulfide for Inkjet Printing
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since the discovery of graphene, substantial effort has been put toward the synthesis and production of 2D materials. Developing scalable methods for the production of high-quality exfoliated nanosheets has proved a significant challenge. To date, the most promising scalable method for achieving these materials is through the liquid-based exfoliation (LBE) of nanosheetsin solvents. Thin films of nanosheets in dispersion can be modified with additives to produce 2D inks for printed electronics using inkjet printing. This is the most promising method for the deposition of such materials onto any substrate on an industrial production level. Although well-developed metallic and organic printed electronic inks exist on the market, there is still a need to improve or develop new inks based on semiconductor materials such as transition metal dichalcogenides (TMDs) that are stable, have good jetting conditions and deliver good printing quality.The inertness and mechanical properties of layered materials such as molybdenum disulfide (MoS2) make them ideally suited for printed electronics and solution processing. In addition,the high electron mobility of the layered semiconductors, make them a candidate to become a high-performance semiconductor material in printed electronics. Together, these features make MoS2 a simple and robust material with good semiconducting properties that is also suitable for solution coating and printing. It is also environmentally safe.The method described in this thesis could be easily employed to exfoliate many types of 2D materials in liquids. It consists of two exfoliation steps, one based on mechanical exfoliation of the bulk powder utilizing sand paper, and the other inthe liquid dispersion, using probe sonication to liquid-exfoliate the nanosheets. The dispersions, which were prepared in surfactant solution, were decanted, and the supernatant was collected and used for printing tests performed with a Dimatix inkjetprinter. The printing test shows that it is possible to use the MoS2 dispersion as a printed electronics inkjet ink and that optimization for specific printer and substrate combinations should be performed. There should also be advances in ink development, which would improve the drop formation and break-off at the inkjet printing nozzles, the ink jetting and, consequently, the printing quality.
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| 7. |
- Forsberg, Viviane, 1981-
(författare)
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Liquid-Phase Exfoliation of Two-Dimensional Materials : Applications, deposition methods and printed electronics on paper
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 8. |
- Forsberg, Viviane, 1981-, et al.
(författare)
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Photodetector of multilayer exfoliated MoS2 deposited on polyimide films
- 2018
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We fabricated a photodetector based on multilayer molybdenum disulfide (MoS2) by micromechanical cleavage of a molybdenite crystal using a polyimide film. We deposited 40 nm of gold by vacuum sputtering and copper tape was used for the contacts. Without any surface treatment, we achieved high responsivity at different incident optical power. The calculated responsivity was 23 mA/W of incident optical power in the range between 400 and 800 nm. For the responsivity measurement it was estimated that MoS2 have a bandgap of 1.6 eV, which lies between monolayer and multilayer films. The thickness of the MoS2 thin film was determined by Raman spectroscopy evaluating the difference between the in plane and out of plane Raman modes. The measurement of IV curves indicated Ohmic contacts in respect to the Au regardless of the incident optical power. Our device fabrication was much simpler than previous reported devices and can be used to test the light absorption and luminescence capabilities of exfoliated MoS2.
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| 9. |
- Forsberg, Viviane, 1981-, et al.
(författare)
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Printability of functional inkjet inks onto commercial inkjet substrates and a taylor made pigmented coated paper
- 2018
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Printed electronics are of increasing interest. The substrates used have primarily been plastics although the interest for cellulose-based substrates is increasing due to the environmental aspect as well as cost. The requirements of substrates for electronically active inks differs from graphical inks and therefore we have investigated a custom-made pigment based coated paper and compared it to commercial photo-papers and a coated PE film.Our goal with the study of different substrates was to select the most suitable substrate to print water based 2D materials inkjet inks for flexible electronics.The discovery of graphene, a layered material achieved from the exfoliation of graphite, has resulted in the study of other materials with similar properties to cover areas where graphene could not be used due to the absence of a bandgap in the material. For example in thin film transistors (TFT) a semiconductor layer is essential to enable turn on and off the device. This semiconductor layer can be achieved using various materials but particular interest have been dedicated to abundant and cheap 2D materials such as the transition metal dichalcogenide (TMD) molybdenum disulfide (MoS2). To date, most of the dispersions based on TMDs use organic solvents or water solutions of surfactants. Previously we focus on the study of environmental friendly inks produced by liquid phase exfoliation (LPE) of MoS2 in water using cellulose stabilizers such as ethyl cellulose (EC), cellulose nanofibrils (CNF) and nanofibrilcellulose (NFC). We have study various aspects of the ink fabrication includi ng pH range, the source of MoS2, nanosheets thickness, particle size distribution, ink stabilizers, ink concentration, viscosity and surface tension. These inks have very low concentration requiring a number of printing passes to cover the substrate. Therefore the substrate selection is crucial as a large amount of solvent is to be absorb by the substrate. Our goal was to use such an ink to print electrodes of MoS2 into a paper substrate after substrate selection.Commercial photo papers, a commercial coated PE film and a tailor made multilayer pigment coated paper substrate were used for the substrate selection analysis. We print the substrates using a DIMATIX inkjet printer with a 10 pL printing head using the distillated water waveform supplied by the printer manufacturer. The voltage used was 23V and 4 nozzles were used for the print outs. The inkjet ink used was the organic PEDOT:PSS. We printed lines ranging from 1 pixel to 20 pixels with 1, 2 and 3 printing passes. The printing quality was evaluated through measurements of the waviness of the printed lines measured after imaging the printed samples with a SEM microscope. The line width measurement was done using the software from the SEM.We also evaluated the structure of the coatings using SEM and topography measurements. The ink penetration through the substrates was evaluated using Raman Spectroscopy. For the pigmented coated sample we measured 4% of ink penetration through the substrate for the 1pxl printed line printed once onto the paper. Cross-section SEM images of the printed lines were made to visualize the ink penetration into the substrate.Regarding the electrical conductivity of the printed samples, the differences in resistivity varying the width of the printed lines and the number of printed passes were evaluated. The resistivity of the printed electrodes was evaluated using the 2-points probe method. Before the resistivity measurements, the printed substrates were heated at 50°C and 100°C for 30 minutes in an oven.We choose the PEDOT:PSS ink because it is a low price ink compared to metal nanoparticles inks for printed electronics. The print outs had low resistivity at a few printing passes with no need for sintering at high temperatures. The MoS2 ink has a very high resistance at a few printing passes due to lower coverage of the substrate therefore for this ink these measurements were not possible to be made. The main pigment composition of the paper coatings of the substrates was evaluated using FT-IR and EDX, these data plus the coating structure evaluated by SEM was related to the print quality.The best in test papers were used to print MoS2 electrodes. After the printing tests, another step for the optimization of the MoS2 ink properties shall be carried out in future studies for better print quality. We also evaluated the surface energy of the substrates through contact angle measurements to match the surface tension of the PEDOT:PSS ink and later the MoS2 ink. Although the pigmented coated printing substrate did not show better results than the commercial photo papers and PE foil in terms of line quality, it shows the lowest resistivity and sufficient results for low cost recyclable electronics, which do not require high conductivity. Nevertheless, the substrate was very thin and it could even be used in magazines as traditional lightweight coated papers (LWC) are used but with the additional of a printed electronic feature.
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