Liquid phase exfoliation

First demonstrated in 2008,[1] Liquid-phase exfoliation (LPE) is a solution-processing method which is used to convert layered crystals into 2-dimensional nanosheets in large quantities.[2] It is currently one of the pillar methods for producing 2D nanosheets.[3] According to IDTechEx, the family of exfoliation techniques which are directly or indirectly descended from LPE now make up over 60% of global graphene production capacity.[4]

This method involves adding powdered layered crystals, for example of graphite, to appropriate solvents and inserting energy, often by ultrasonication, although high-shear mixing[5] is often commonly used. The addition of energy causes a combination of fragmentation and exfoliation resulting in the removal of small nanosheets from the layered crystals.[6] In this way graphite can be converted into large quantities of graphene nanosheets.[7] In general, these nanosheets tend to be a few monolayers thick and of lateral sizes ranging from tens of nanometers to many microns.[8] These dispersed nanosheets form quasi stable suspensions so long as solvents used have surface energies similar to that of the nanosheets. Dispersed concentrations of order 1 gram per litre can be achieved. In addition to solvents, it is also possible to use molecular stabilizers, for example surfactants or polymers to coat the nanosheets and stabilise them against regaggregation.[9] This has the advantage that it allows nanosheets to be suspended in water.

Although this method was first applied to exfoliate graphite to yield graphene nanosheets, it has since been used to produce a wide range of 2D materials including molybdenum disulfide, tungsten diselenide, boron nitride, nickel(II) hydroxide, germanium monosulfide, SnP3, and black phosphorus. The liquid suspensions produced by liquid phase exfoliation can be used to create a range of functional structures. For example, they can be printed into thin films and networks using standard techniques such as inkjet printing.[10]

Printed structures have been used in a range of applications in areas included printed electronics, sensors and nanocomposites. Related methods include exfoliation by wet ball milling, homogenization, microfluidization and wet jet milling.[11] Liquid phase exfoliation is different from other liquid exfoliation methods, for example the production of graphene oxide, because it is much less destructive, leaving minimal defects in the basal planes of the nanosheets. It has recently emerged that LPE can also be used to convert non-layered crystals into quasi-2D nanoplatelets.[12]

Origins
One of the earliest transmission electron microscope images of a graphene nanosheet produced by liquid phase exfoliation (exfoliated by the Dublin group in 2007).[13]

Liquid phase exfoliation was first described in detail in a paper by a research team in Ireland in 2008,[14] although a very short description of a similar process was published by the Manchester group around the same time.[15] While other papers had previously described methods to exfoliate layered crystals in liquids,[16] these papers were the first to describe exfoliation in liquids without any previous ion intercalation or chemical treatment.

Exfoliation methods

LPE involves inserting layered crystals into appropriate stabilizing liquids and then adding energy to remove nanosheets from the layered crystals. A number of different methods have been used to supply energy to the liquid. The earliest and most common is ultrasonication.[17] In order to scaleup the process, high shear mixing was introduced in 2014.[18] This method proved extremely useful and inspired a number of other methods of generating shear in the suspension, including wet ball milling, homogenization, microfluidization and wet jet milling.[19]

Stabilisers

The simplest stabilizing liquids are solvents with surface energy close to the layered crystal being exfoliated. In practice, liquids with surface tensions close to 70 mJ/m2 are used.[20] In addition aqueous surfactant solutions are often used.[21] Less common, but useful for certain applications, is using molecular or polymeric additives to stabilise the exfoliated nanosheets.[22][23][24]

LPE of 2D materials beyond graphene

A very wide range of 2D materials have been produced by LPE. The first material to be exfoliated was graphene in 2008. This was followed in 2011 by the exfoliation of BN, MoS2 and WS2.[25] Since, the a wide range of 2D materials have been exfoliated including molybdenum diselenide, tungsten diselenide, gallium sulphide, molybdemum trioxide, nickel(II) hydroxide, germanium monosulfide, SnP3, black phosphorus etc.[26]

LPE of non-layered materials

Recent work has shown that liquid phase exfoliation can be used to produce 2D-nanoplatelets from non-layered 3D-strongly bonded bulk materials.[27] This is intuitively unexpected as these 3D-solid bulk crystals consists of strong bonds in all the three-directions. Nevertheless, many non-layered materials such as boron, silicon, germanium, iron disulfide, iron oxide, iron trifluoride, manganese telluride, have been converted to 2D nanoplatelets when sonicated in appropriate solvents.[28] This raises many open questions on the mechanism of liquid-phase exfoliation process.[29] For layered materials, the energy required to break inter-plane (perdominately van der Waals) bonds forces is small compared to that required to break in-plane ionic or covalent bonds. Then, the exfoliation procedure results in the formation of 2D-nanosheets.[30] However, for non-layered 3D-strongly bonded materials, with minimal difference in bonding between different atomic planes, there is no "easily exfoliated" direction and sonication should yield quasi spherical particles.[31] Nevertheless, near isotropic materials such as silicon have been exfoliated to give high-aspect ratio platelets.[32] Therefore, developing an understanding of the mechanisms by which non-layered materials are exfoliated will be important, in particular because the application scope of such nonlayered 2D-nanoplatelets is broad, ranging from biomedical applications to energy storage to opto-electronics.[33]

References
  1. Hernandez, Yenny; Nicolosi, Valeria; Lotya, Mustafa; Blighe, Fiona M.; Sun, Zhenyu; De, Sukanta; McGovern, I. T.; Holland, Brendan; Byrne, Michele; Gun'Ko, Yurii K.; Boland, John J.; Niraj, Peter; Duesberg, Georg; Krishnamurthy, Satheesh; Goodhue, Robbie; Hutchison, John; Scardaci, Vittorio; Ferrari, Andrea C.; Coleman, Jonathan N. (September 2008). "High-yield production of graphene by liquid-phase exfoliation of graphite". Nature Nanotechnology. 3 (9): 563–568. arXiv:0805.2850. Bibcode:2008NatNa...3..563H. doi:10.1038/nnano.2008.215. PMID 18772919. S2CID 205443620.
  2. Coleman, Jonathan N.; Lotya, Mustafa; O’Neill, Arlene; Bergin, Shane D.; King, Paul J.; Khan, Umar; Young, Karen; Gaucher, Alexandre; De, Sukanta; Smith, Ronan J.; Shvets, Igor V.; Arora, Sunil K.; Stanton, George; Kim, Hye-Young; Lee, Kangho; Kim, Gyu Tae; Duesberg, Georg S.; Hallam, Toby; Boland, John J.; Wang, Jing Jing; Donegan, John F.; Grunlan, Jaime C.; Moriarty, Gregory; Shmeliov, Aleksey; Nicholls, Rebecca J.; Perkins, James M.; Grieveson, Eleanor M.; Theuwissen, Koenraad; McComb, David W.; Nellist, Peter D.; Nicolosi, Valeria (4 February 2011). "Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials". Science. 331 (6017): 568–571. Bibcode:2011Sci...331..568C. doi:10.1126/science.1194975. hdl:2262/66458. PMID 21292974. S2CID 23576676.
  3. Ferrari, Andrea C.; Bonaccorso, Francesco; Fal'ko, Vladimir; Novoselov, Konstantin S.; Roche, Stephan; Bøggild, Peter; Borini, Stefano; Koppens, Frank H. L.; Palermo, Vincenzo; Pugno, Nicola; Garrido, José A.; Sordan, Roman; Bianco, Alberto; Ballerini, Laura; Prato, Maurizio; Lidorikis, Elefterios; Kivioja, Jani; Marinelli, Claudio; Ryhänen, Tapani; Morpurgo, Alberto; Coleman, Jonathan N.; Nicolosi, Valeria; Colombo, Luigi; Fert, Albert; Garcia-Hernandez, Mar; Bachtold, Adrian; Schneider, Grégory F.; Guinea, Francisco; Dekker, Cees; Barbone, Matteo; Sun, Zhipei; Galiotis, Costas; Grigorenko, Alexander N.; Konstantatos, Gerasimos; Kis, Andras; Katsnelson, Mikhail; Vandersypen, Lieven; Loiseau, Annick; Morandi, Vittorio; Neumaier, Daniel; Treossi, Emanuele; Pellegrini, Vittorio; Polini, Marco; Tredicucci, Alessandro; Williams, Gareth M.; Hee Hong, Byung; Ahn, Jong-Hyun; Min Kim, Jong; Zirath, Herbert; van Wees, Bart J.; van der Zant, Herre; Occhipinti, Luigi; Di Matteo, Andrea; Kinloch, Ian A.; Seyller, Thomas; Quesnel, Etienne; Feng, Xinliang; Teo, Ken; Rupesinghe, Nalin; Hakonen, Pertti; Neil, Simon R. T.; Tannock, Quentin; Löfwander, Tomas; Kinaret, Jari (2015). "Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems". Nanoscale. 7 (11): 4598–4810. Bibcode:2015Nanos...7.4598F. doi:10.1039/C4NR01600A. hdl:2117/27112. PMID 25707682.
  4. "China to dominate graphene commercialization". IDTechEx. 18 January 2018.
  5. Paton, Keith R.; Varrla, Eswaraiah; Backes, Claudia; Smith, Ronan J.; Khan, Umar; O’Neill, Arlene; Boland, Conor; Lotya, Mustafa; Istrate, Oana M.; King, Paul; Higgins, Tom; Barwich, Sebastian; May, Peter; Puczkarski, Pawel; Ahmed, Iftikhar; Moebius, Matthias; Pettersson, Henrik; Long, Edmund; Coelho, João; O’Brien, Sean E.; McGuire, Eva K.; Sanchez, Beatriz Mendoza; Duesberg, Georg S.; McEvoy, Niall; Pennycook, Timothy J.; Downing, Clive; Crossley, Alison; Nicolosi, Valeria; Coleman, Jonathan N. (June 2014). "Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids" (PDF). Nature Materials. 13 (6): 624–630. Bibcode:2014NatMa..13..624P. doi:10.1038/NMAT3944. PMID 24747780. S2CID 43256835.
  6. Li, Zheling; Young, Robert J.; Backes, Claudia; Zhao, Wen; Zhang, Xun; Zhukov, Alexander A.; Tillotson, Evan; Conlan, Aidan P.; Ding, Feng; Haigh, Sarah J.; Novoselov, Kostya S.; Coleman, Jonathan N. (22 September 2020). "Mechanisms of Liquid-Phase Exfoliation for the Production of Graphene". ACS Nano. 14 (9): 10976–10985. doi:10.1021/acsnano.0c03916. hdl:2262/93628. PMID 32598132. S2CID 220269811.
  7. Galindo-Uribe, Carlos Daniel; Calaminici, Patrizia; Solorza-Feria, Omar (1 June 2022). "Revisión sobre la síntesis de grafeno por exfoliación en fase líquida: Mecanismos, factores y técnicas". Uniciencia. 36 (1): 1–14. doi:10.15359/ru.36-1.35. S2CID 249696830.
  8. Backes, Claudia; Campi, Davide; Szydlowska, Beata M.; Synnatschke, Kevin; Ojala, Ezgi; Rashvand, Farnia; Harvey, Andrew; Griffin, Aideen; Sofer, Zdenek; Marzari, Nicola; Coleman, Jonathan N.; O’Regan, David D. (25 June 2019). "Equipartition of Energy Defines the Size–Thickness Relationship in Liquid-Exfoliated Nanosheets". ACS Nano. 13 (6): 7050–7061. arXiv:2006.14909. doi:10.1021/acsnano.9b02234. PMID 31199123. S2CID 189813507.
  9. Hu, Chen-Xia; Shin, Yuyoung; Read, Oliver; Casiraghi, Cinzia (2021). "Dispersant-assisted liquid-phase exfoliation of 2D materials beyond graphene". Nanoscale. 13 (2): 460–484. doi:10.1039/d0nr05514j. PMID 33404043. S2CID 230784148.
  10. Torrisi, Felice; Hasan, Tawfique; Wu, Weiping; Sun, Zhipei; Lombardo, Antonio; Kulmala, Tero S.; Hsieh, Gen-Wen; Jung, Sungjune; Bonaccorso, Francesco; Paul, Philip J.; Chu, Daping; Ferrari, Andrea C. (24 April 2012). "Inkjet-Printed Graphene Electronics" (PDF). ACS Nano. 6 (4): 2992–3006. doi:10.1021/nn2044609. PMID 22449258. S2CID 8624837.
  11. Del Rio Castillo, A. E.; Pellegrini, V.; Ansaldo, A.; Ricciardella, F.; Sun, H.; Marasco, L.; Buha, J.; Dang, Z.; Gagliani, L.; Lago, E.; Curreli, N.; Gentiluomo, S.; Palazon, F.; Prato, M.; Oropesa-Nuñez, R.; Toth, P. S.; Mantero, E.; Crugliano, M.; Gamucci, A.; Tomadin, A.; Polini, M.; Bonaccorso, F. (2018). "High-yield production of 2D crystals by wet-jet milling". Materials Horizons. 5 (5): 890–904. arXiv:1804.10688. doi:10.1039/c8mh00487k. S2CID 96459022.
  12. Kaur, Harneet; Coleman, Jonathan N. (September 29, 2022). "Liquid‐Phase Exfoliation of Nonlayered Non‐Van‐Der‐Waals Crystals into Nanoplatelets". Advanced Materials. 34 (35): 2202164. doi:10.1002/adma.202202164. hdl:2262/101345. PMID 35470487. S2CID 248390135.
  13. Hernandez, Yenny; Nicolosi, Valeria; Lotya, Mustafa; Blighe, Fiona M.; Sun, Zhenyu; De, Sukanta; McGovern, I. T.; Holland, Brendan; Byrne, Michele; Gun'Ko, Yurii K.; Boland, John J.; Niraj, Peter; Duesberg, Georg; Krishnamurthy, Satheesh; Goodhue, Robbie; Hutchison, John; Scardaci, Vittorio; Ferrari, Andrea C.; Coleman, Jonathan N. (September 2008). "High-yield production of graphene by liquid-phase exfoliation of graphite". Nature Nanotechnology. 3 (9): 563–568. arXiv:0805.2850. doi:10.1038/nnano.2008.215. PMID 18772919. S2CID 205443620.
  14. Hernandez, Yenny; Nicolosi, Valeria; Lotya, Mustafa; Blighe, Fiona M.; Sun, Zhenyu; De, Sukanta; McGovern, I. T.; Holland, Brendan; Byrne, Michele; Gun'Ko, Yurii K.; Boland, John J.; Niraj, Peter; Duesberg, Georg; Krishnamurthy, Satheesh; Goodhue, Robbie; Hutchison, John; Scardaci, Vittorio; Ferrari, Andrea C.; Coleman, Jonathan N. (September 2008). "High-yield production of graphene by liquid-phase exfoliation of graphite". Nature Nanotechnology. 3 (9): 563–568. arXiv:0805.2850. Bibcode:2008NatNa...3..563H. doi:10.1038/nnano.2008.215. PMID 18772919. S2CID 205443620.
  15. Blake, Peter; Brimicombe, Paul D.; Nair, Rahul R.; Booth, Tim J.; Jiang, Da; Schedin, Fred; Ponomarenko, Leonid A.; Morozov, Sergey V.; Gleeson, Helen F.; Hill, Ernie W.; Geim, Andre K.; Novoselov, Kostya S. (1 June 2008). "Graphene-Based Liquid Crystal Device". Nano Letters. 8 (6): 1704–1708. arXiv:0803.3031. Bibcode:2008NanoL...8.1704B. doi:10.1021/nl080649i. PMID 18444691. S2CID 14620203.
  16. Nicolosi, Valeria; Chhowalla, Manish; Kanatzidis, Mercouri G.; Strano, Michael S.; Coleman, Jonathan N. (21 June 2013). "Liquid Exfoliation of Layered Materials". Science. 340 (6139): 1226419. doi:10.1126/science.1226419. hdl:2262/69769. S2CID 177513486.
  17. Hernandez, Yenny; Nicolosi, Valeria; Lotya, Mustafa; Blighe, Fiona M.; Sun, Zhenyu; De, Sukanta; McGovern, I. T.; Holland, Brendan; Byrne, Michele; Gun'Ko, Yurii K.; Boland, John J.; Niraj, Peter; Duesberg, Georg; Krishnamurthy, Satheesh; Goodhue, Robbie; Hutchison, John; Scardaci, Vittorio; Ferrari, Andrea C.; Coleman, Jonathan N. (September 2008). "High-yield production of graphene by liquid-phase exfoliation of graphite". Nature Nanotechnology. 3 (9): 563–568. arXiv:0805.2850. Bibcode:2008NatNa...3..563H. doi:10.1038/nnano.2008.215. PMID 18772919. S2CID 205443620.
  18. Paton, Keith R.; Varrla, Eswaraiah; Backes, Claudia; Smith, Ronan J.; Khan, Umar; O’Neill, Arlene; Boland, Conor; Lotya, Mustafa; Istrate, Oana M.; King, Paul; Higgins, Tom; Barwich, Sebastian; May, Peter; Puczkarski, Pawel; Ahmed, Iftikhar; Moebius, Matthias; Pettersson, Henrik; Long, Edmund; Coelho, João; O’Brien, Sean E.; McGuire, Eva K.; Sanchez, Beatriz Mendoza; Duesberg, Georg S.; McEvoy, Niall; Pennycook, Timothy J.; Downing, Clive; Crossley, Alison; Nicolosi, Valeria; Coleman, Jonathan N. (June 2014). "Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids" (PDF). Nature Materials. 13 (6): 624–630. Bibcode:2014NatMa..13..624P. doi:10.1038/nmat3944. PMID 24747780. S2CID 43256835.
  19. Del Rio Castillo, A. E.; Pellegrini, V.; Ansaldo, A.; Ricciardella, F.; Sun, H.; Marasco, L.; Buha, J.; Dang, Z.; Gagliani, L.; Lago, E.; Curreli, N.; Gentiluomo, S.; Palazon, F.; Prato, M.; Oropesa-Nuñez, R.; Toth, P. S.; Mantero, E.; Crugliano, M.; Gamucci, A.; Tomadin, A.; Polini, M.; Bonaccorso, F. (2018). "High-yield production of 2D crystals by wet-jet milling". Materials Horizons. 5 (5): 890–904. arXiv:1804.10688. doi:10.1039/c8mh00487k. S2CID 96459022.
  20. Hernandez, Yenny; Lotya, Mustafa; Rickard, David; Bergin, Shane D.; Coleman, Jonathan N. (2 March 2010). "Measurement of Multicomponent Solubility Parameters for Graphene Facilitates Solvent Discovery". Langmuir. 26 (5): 3208–3213. doi:10.1021/la903188a. PMID 19883090.
  21. Lotya, Mustafa; Hernandez, Yenny; King, Paul J.; Smith, Ronan J.; Nicolosi, Valeria; Karlsson, Lisa S.; Blighe, Fiona M.; De, Sukanta; Wang, Zhiming; McGovern, I. T.; Duesberg, Georg S.; Coleman, Jonathan N. (18 March 2009). "Liquid Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions". Journal of the American Chemical Society. 131 (10): 3611–3620. arXiv:0809.2690. doi:10.1021/ja807449u. PMID 19227978. S2CID 16624132.
  22. Ciesielski, Artur; Samorì, Paolo (August 2016). "Supramolecular Approaches to Graphene: From Self-Assembly to Molecule-Assisted Liquid-Phase Exfoliation" (PDF). Advanced Materials. 28 (29): 6030–6051. doi:10.1002/adma.201505371. PMID 26928750. S2CID 205266229.
  23. Ciesielski, Artur; Samorì, Paolo (2014). "Grapheneviasonication assisted liquid-phase exfoliation". Chem. Soc. Rev. 43 (1): 381–398. doi:10.1039/c3cs60217f. PMID 24002478.
  24. May, Peter; Khan, Umar; Hughes, J. Marguerite; Coleman, Jonathan N. (24 May 2012). "Role of Solubility Parameters in Understanding the Steric Stabilization of Exfoliated Two-Dimensional Nanosheets by Adsorbed Polymers". The Journal of Physical Chemistry C. 116 (20): 11393–11400. doi:10.1021/jp302365w.
  25. Coleman, Jonathan N.; Lotya, Mustafa; O’Neill, Arlene; Bergin, Shane D.; King, Paul J.; Khan, Umar; Young, Karen; Gaucher, Alexandre; De, Sukanta; Smith, Ronan J.; Shvets, Igor V.; Arora, Sunil K.; Stanton, George; Kim, Hye-Young; Lee, Kangho; Kim, Gyu Tae; Duesberg, Georg S.; Hallam, Toby; Boland, John J.; Wang, Jing Jing; Donegan, John F.; Grunlan, Jaime C.; Moriarty, Gregory; Shmeliov, Aleksey; Nicholls, Rebecca J.; Perkins, James M.; Grieveson, Eleanor M.; Theuwissen, Koenraad; McComb, David W.; Nellist, Peter D.; Nicolosi, Valeria (4 February 2011). "Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials". Science. 331 (6017): 568–571. Bibcode:2011Sci...331..568C. doi:10.1126/science.1194975. hdl:2262/66458. PMID 21292974. S2CID 23576676.
  26. Hu, Chen-Xia; Shin, Yuyoung; Read, Oliver; Casiraghi, Cinzia (2021). "Dispersant-assisted liquid-phase exfoliation of 2D materials beyond graphene". Nanoscale. 13 (2): 460–484. doi:10.1039/d0nr05514j. PMID 33404043. S2CID 230784148.
  27. Kaur, Harneet; Coleman, Jonathan N. (September 2022). "Liquid‐Phase Exfoliation of Nonlayered Non‐Van‐Der‐Waals Crystals into Nanoplatelets". Advanced Materials. 34 (35): 2202164. doi:10.1002/adma.202202164. hdl:2262/101345. PMID 35470487. S2CID 248390135.
  28. Kaur, Harneet; Coleman, Jonathan N. (September 2022). "Liquid‐Phase Exfoliation of Nonlayered Non‐Van‐Der‐Waals Crystals into Nanoplatelets". Advanced Materials. 34 (35): 2202164. doi:10.1002/adma.202202164. hdl:2262/101345. PMID 35470487. S2CID 248390135.
  29. Li, Zheling; Young, Robert J.; Backes, Claudia; Zhao, Wen; Zhang, Xun; Zhukov, Alexander A.; Tillotson, Evan; Conlan, Aidan P.; Ding, Feng; Haigh, Sarah J.; Novoselov, Kostya S.; Coleman, Jonathan N. (22 September 2020). "Mechanisms of Liquid-Phase Exfoliation for the Production of Graphene". ACS Nano. 14 (9): 10976–10985. doi:10.1021/acsnano.0c03916. hdl:2262/93628. PMID 32598132. S2CID 220269811.
  30. Backes, Claudia; Campi, Davide; Szydlowska, Beata M.; Synnatschke, Kevin; Ojala, Ezgi; Rashvand, Farnia; Harvey, Andrew; Griffin, Aideen; Sofer, Zdenek; Marzari, Nicola; Coleman, Jonathan N.; O’Regan, David D. (25 June 2019). "Equipartition of Energy Defines the Size–Thickness Relationship in Liquid-Exfoliated Nanosheets". ACS Nano. 13 (6): 7050–7061. arXiv:2006.14909. doi:10.1021/acsnano.9b02234. PMID 31199123. S2CID 189813507.
  31. Kaur, Harneet; Coleman, Jonathan N. (September 2022). "Liquid‐Phase Exfoliation of Nonlayered Non‐Van‐Der‐Waals Crystals into Nanoplatelets". Advanced Materials. 34 (35): 2202164. doi:10.1002/adma.202202164. hdl:2262/101345. PMID 35470487. S2CID 248390135.
  32. Wang, Mengxia; Zhang, Fang; Wang, Zhengping; Xu, Xinguang (9 January 2019). "Liquid-Phase Exfoliated Silicon Nanosheets: Saturable Absorber for Solid-State Lasers". Materials. 12 (2): 201. doi:10.3390/ma12020201. PMC 6356386. PMID 30634424.
  33. Kaur, Harneet; Coleman, Jonathan N. (September 2022). "Liquid‐Phase Exfoliation of Nonlayered Non‐Van‐Der‐Waals Crystals into Nanoplatelets". Advanced Materials. 34 (35): 2202164. doi:10.1002/adma.202202164. hdl:2262/101345. PMID 35470487. S2CID 248390135.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.