{"id":2240,"date":"2022-04-28T21:14:49","date_gmt":"2022-04-28T21:14:49","guid":{"rendered":"http:\/\/chemweb.unl.edu\/sinitskii\/?p=2240"},"modified":"2023-04-27T21:28:59","modified_gmt":"2023-04-27T21:28:59","slug":"unl-news-team-demonstrates-rare-form-of-ferroelectricity-in-ultra-thin-material","status":"publish","type":"post","link":"http:\/\/chemweb.unl.edu\/sinitskii\/unl-news-team-demonstrates-rare-form-of-ferroelectricity-in-ultra-thin-material\/","title":{"rendered":"[UNL News] Team demonstrates rare form of ferroelectricity in ultra-thin material"},"content":{"rendered":"

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[Team demonstrates rare form of ferroelectricity in ultra-thin material<\/em> | University Communications | 04\/27\/2022]
\nThe nanoscopic equivalent of stacking a deck of cards \u2014 layering materials a mere few atoms thick atop one another \u2014 has emerged as a favorite pastime of material scientists and electrical engineers worldwide.<\/p>\n

Just as cards can differ by suit and value, the properties of those atomically thin 2D materials can vary, too: electronically, magnetically, optically or in any number of other ways. And much like combining the right cards can yield valuable hands, the right combinations of 2D materials can yield technologically valuable outcomes.
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\nAlexey Lipatov \/ npj 2D Materials and Applications\/ Springer Nature<\/em>\n<\/div>\n

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\nThe University of Nebraska\u2013Lincoln\u2019s Alexei Gruverman, Alex Sinitskii and colleagues have now demonstrated that one particular 2D material, already considered a face card, actually ranks as an ace in the hole.<\/p>\n

That material is molybdenum disulfide, or MoS2<\/sub>. Alongside partners from Luxembourg, China and France, the Husker researchers have shown that MoS2<\/sub> possesses a long-theorized property that could help computers, phones and other microelectronics save both power and their exact electrical states, even after being turned off.<\/p>\n

MoS2<\/sub>\u2019s power-saving, state-saving promise comes courtesy of a prized but uncommon property known as ferroelectricity. The vertical separation and arrangement of negative vs. positive charges in ferroelectric materials can be instantaneously flipped just by applying some voltage. Those oppositely aligned, or polarized, states can be read or stored as the 1s and 0s of binary data, with the states remaining even when a power source has been cut.<\/p>\n

That set-it-and-forget-it advantage is compounded by the fact that voltage can flip polarization, and encode a respective 1 or 0, while drawing far less energy than the magnetic fields often used to encode digital data. Collectively, those benefits have positioned ferroelectric materials as a prominent player in a future even more dependent on microelectronics.<\/p>\n

Theory-backed simulations had suggested that MoS2<\/sub> was just such a material. As with other 2D materials, though, proving it had proven fiendishly difficult. But by prodding flakes of molybdenum disulfide with a nanoscopic needle that simultaneously excited the material with an electric field, the Husker-led team has managed to confirm that MoS2<\/sub> is, in fact, ferroelectric. The material\u2019s polarized states held for up to weeks at a time, the researchers said, and were observed with the MoS2<\/sub> flakes sitting atop any one of several other materials.<\/p>\n

\u201cFerroelectricity in two-dimensional materials is, in general, a new phenomenon,\u201d said Sinitskii, professor of chemistry at Nebraska. \u201cIt was discovered fairly recently, and the examples of two-dimensional systems that exhibit ferroelectric polarization are still very limited.\u201d<\/p>\n

Ferroelectricity alone, then, would be enough to vault molybdenum disulfide up the rankings of 2D materials. Yet MoS2<\/sub> features other properties that appeal to the engineers tasked with building better devices. It\u2019s relatively easy to grow, first in bulk, then by peeling off atomically thin layers with the aid of Scotch tape. Unlike many of its 2D counterparts, it holds up when exposed to air and plays well with the oxygen-rich materials found in many electronic components.<\/p>\n

Beyond all that, it\u2019s a semiconducting material in the vein of silicon \u2014 the longstanding choice for integrated circuits, or microchips \u2014 meaning that its flow of electric current can be triggered and halted with minimal effort. That sets MoS2<\/sub> apart from most ferroelectrics, Gruverman said.<\/p>\n

In the wake of the team\u2019s study, which appeared in the journal npj 2D Materials and Applications<\/a>, MoS2<\/sub> now joins just a handful of materials that boast high-yet-controllable conductivity and easily switchable polarization, the researchers said.<\/p>\n

\u201cThere was always this striving to combine semiconducting and ferroelectric properties in one material, because that would make it a very powerful material \u2014 a holy grail, if you will \u2014 for the semiconductor industry,\u201d said Gruverman, Charles Mach University Professor of physics and astronomy.<\/p>\n

\u2018The structure that we observed was clearly unprecedented\u2019<\/strong>
\nThe atoms of a material can take on different configurations that generate different properties. The most famous example of the phenomenon might be carbon, which can range from a soft black lump of coal to a nigh-indestructible, transparent diamond.<\/p>\n

Molybdenum disulfide, which consists of one molybdenum atom for every two sulfur, is no exception. In its most stable state, known as 2H, the material acts as a semiconductor yet actually lacks ferroelectricity. But prodding the MoS2<\/sub> with a minuscule point shifted some of the sulfur atoms upward, the team found, altering the distances between those atoms and the molybdenum. That, in turn, altered the distribution of the atoms\u2019 electron clouds, ultimately transforming the semiconducting 2H into a more conductive, ferroelectric phase known as 1T\u201d.<\/p>\n

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\nAlexey Lipatov \/ npj 2D Materials and Applications
\nA top-down rendering of molybdenum disulfide\u2019s atomic structure in its 1T\u201d configuration, with triangular groupings of purple molybdenum atoms shifting closer to the green, upward-displaced sulfur atoms. (The blue molybdenum atoms and yellow sulfur atoms are relatively undisturbed.)<\/em><\/p>\n

To switch the polarization of MoS2<\/sub>, the researchers exploited the so-called flexo-electric effect: a change in the electrical behavior of a material when it begins straining under the force of a mechanical stress. For more than a half-century, physicists have known that the more variable the strain \u2014 that is, the greater the disparities in how various areas of a material will deform under stress \u2014 the more pronounced the electric polarization will be. Thicker materials tend to experience fairly uniform strains, Gruverman said, resulting in limited polarization and usefulness for encoding binary data.<\/p>\n

A 2D material such as MoS2<\/sub> \u2014 especially one pricked with the finest of fine points \u2014 is a much different prospect, yielding a huge disparity in strains and, consequently, a massive flexo-electric effect.<\/p>\n

\u201cIn materials as thin as MoS2<\/sub>, this flexo-electric effect is very profound,\u201d Gruverman said. \u201cWhat\u2019s important is that this approach could be used as a very effective tool to control polarization states in ferroelectrics.<\/p>\n

\u201cNow we\u2019ve demonstrated that, in addition to the electric field, we can use mechanical stress as a way of controlling or tuning the electronic properties of these heterostructures.\u201d<\/p>\n

The team also discovered a surprise that could work in MoS2<\/sub>\u2019s favor. Though the flakes that Sinitskii and his colleagues fabricated were virtually pristine, the team occasionally encountered polarization signals that were substantially weaker than they expected. Curious, Sinitskii had the idea to flip the flakes over and measure the signals again, hoping to glean insights on the ultra-thin third dimension of the essentially 2D material.<\/p>\n

When they did, the researchers determined that the flakes contained randomly alternating layers of polarization \u2014 some with positive charges at the top and negative charges at the bottom, others vice versa.<\/p>\n

\u201cThe structure that we observed was clearly unprecedented, because none of the two-dimensional ferroelectric structures that people observed before exhibited this kind of arrangement of ferroelectric domains,\u201d Sinitskii said.<\/p>\n

The existence of those randomly alternating layers implied another surprise. In some cases, like-signed charges are butting up against one another \u2014 positive to positive or negative to negative \u2014 without repelling each other, as they would normally be expected to. How? The team suspects that the especially high conductivity of 1T\u201d MoS2<\/sub> promotes the flow of enough charges between those layers to prevent the repulsion. It\u2019s possible, Gruverman said, that the intra-layer currents could be controlled by flipping the polarization of the MoS2<\/sub> flakes, offering another, hyper-localized way to encode data.<\/p>\n

\u201cIt\u2019s quite unusual to have these layers of a material where polarization in one layer doesn\u2019t care about the polarization state in the adjacent layer,\u201d Gruverman said. \u201cUsually, this kind of head-to-head and tail-to-tail configuration would be very unfavorable. Yet it seems that, here, these layers are absolutely non-sensitive to the polarization state in the neighboring layers.\u201d<\/p>\n

But the full promise of molybdenum disulfide may only reveal itself, Sinitskii said, when material scientists \u2014 now knowing the true value of MoS2<\/sub> \u2014 manage to play it in just the right hands.<\/p>\n

\u201cThis is a very hot topic right now,\u201d Sinitskii said. \u201cThere are many people who are really shuffling these different layers and stacking them on top of each other. Now they have another kind of two-dimensional material that could be added to those stacks and make them more diverse, more programmable and, eventually, more useful.\u201d<\/p>\n

Gruverman and Sinitskii authored the npj 2D Materials and Applications<\/a> study with Nebraska\u2019s Evgeny Tsymbal, Ding-Fu Shao, Alexey Lipatov, Pradeep Chaudhary, Haidong Lu and Gang Li; the University of Luxembourg\u2019s Jorge \u00cd\u00f1iguez; Zhao Guan of East China Normal University; the University of Strasbourg\u2019s Olivier Cr\u00e9gut, Kokou Dodzi Dorkenoo and Salia Cherifi-Hertel; and Roger Proksch of Asylum Research.<\/p>\n","protected":false},"excerpt":{"rendered":"

Team demonstrates rare form of ferroelectricity in ultra-thin material<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"aside","meta":{"jetpack_post_was_ever_published":false,"jetpack_publicize_message":"","jetpack_is_tweetstorm":false,"jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","enabled":false}}},"categories":[4,1],"tags":[],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p5qrjz-A8","_links":{"self":[{"href":"http:\/\/chemweb.unl.edu\/sinitskii\/wp-json\/wp\/v2\/posts\/2240"}],"collection":[{"href":"http:\/\/chemweb.unl.edu\/sinitskii\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chemweb.unl.edu\/sinitskii\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chemweb.unl.edu\/sinitskii\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/chemweb.unl.edu\/sinitskii\/wp-json\/wp\/v2\/comments?post=2240"}],"version-history":[{"count":13,"href":"http:\/\/chemweb.unl.edu\/sinitskii\/wp-json\/wp\/v2\/posts\/2240\/revisions"}],"predecessor-version":[{"id":2255,"href":"http:\/\/chemweb.unl.edu\/sinitskii\/wp-json\/wp\/v2\/posts\/2240\/revisions\/2255"}],"wp:attachment":[{"href":"http:\/\/chemweb.unl.edu\/sinitskii\/wp-json\/wp\/v2\/media?parent=2240"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chemweb.unl.edu\/sinitskii\/wp-json\/wp\/v2\/categories?post=2240"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chemweb.unl.edu\/sinitskii\/wp-json\/wp\/v2\/tags?post=2240"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}