"Long before it's in the papers"
January 27, 2015


Stacking 2-D materials leads to surprises

May 17, 2013
Courtesy of MIT News Office/
David L. Chandler
and World Science staff

A nearly per­fectly flat ma­te­ri­al, graph­ene, has daz­zled sci­en­tists since its dis­cov­ery more than a dec­ade ago. It has un­equalled elec­tron­ic prop­er­ties, strength and light weight. 

But one long-sought goal has proved elu­sive: how to in­still it with a prop­er­ty called a band gap, nec­es­sary to use graph­ene for tran­sis­tors and oth­er elec­tron­ic de­vices. A band gap is an en­er­gy dif­fer­ence be­tween the elec­trons—carr­iers of elec­tric charge—that are trapped in at­oms, and those that are free to cir­cu­late, mak­ing a cur­rent.

Graphene consists of a lay­er of car­bon atoms ar­ranged in a hon­ey­comb-like pa­ttern. (Image cour­tesy Berk­eley Lab)

Now, sci­en­tists say they have tak­en a ma­jor step to­ward mak­ing graph­ene with the cov­eted fea­ture. But their re­sults al­so con­tra­dict some the­o­ret­i­cal ex­pecta­t­ions, sug­gest­ing some ac­cept­ed ideas in graphene phys­ics are wrong.

The re­search­ers put sheets of graph­ene—a carbon-based ma­te­ri­al just one at­om thick­—on top of hexa­gonal bo­ron ni­tride, anoth­er one-at­om-thick ma­te­ri­al with si­m­i­lar prop­er­ties. The re­sult­ing ma­te­ri­al shares graph­ene’s amaz­ing abil­ity to con­duct elec­trons, they ex­plain, while adding the band gap.

“We cre­at­ed a hy­brid ma­te­ri­al that has dif­fer­ent prop­er­ties than ei­ther of the two,” said Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy phys­i­cist Pa­blo Jarillo-Herrero, who with col­leagues re­ported the find­ings in the re­search jour­nal Sci­ence.

Graph­ene is an ex­tremely good con­ductor of elec­trons, while bo­ron ni­tride is a good in­su­la­tor, block­ing their pas­sage. The com­bina­t­ion is a ma­te­ri­al whose prop­er­ties are in be­tween, a semicon­ductor—the ba­sis for mod­ern elec­tron­ics. “We made a high-qual­ity semicon­ductor by put­ting them to­geth­er,” Jarillo-Herrero said.

To make the hy­brid work, the re­search­ers had to align, with near per­fection, the at­omic lat­tices of the two ma­te­ri­als, which both con­sist of a se­ries of hexa­gons. The size of the hexa­gons, known as the lat­tice con­stant, in the two ma­te­ri­als is al­most the same, but not quite: Those in bo­ron ni­tride are 1.8 per­cent larg­er. So while you can line the hexa­gons up al­most per­fectly in one place, over a larg­er ar­ea the pat­tern goes in and out of reg­is­ter.

For now, the re­search­ers say they must rely on chance to get the an­gu­lar alignment for the de­sired elec­tron­ic prop­er­ties in the re­sult­ing stack. How­ev­er, the alignment turns out to be cor­rect about one time out of 15, they say.

Oth­ers have made graph­ene in­to a semicon­ductor by etch­ing the sheets in­to nar­row rib­bons, but this de­grades graph­ene’s elec­trical prop­er­ties; the new ap­proach seems not to, said MIT phys­i­cist Ray Ashoori, anoth­er mem­ber of the re­search group.

What’s most “spec­tac­u­lar,” he adds, is that the prop­er­ties of the re­sult­ing semicon­ductor can be “tuned” by just slightly ro­tat­ing one sheet rel­a­tive to the oth­er, al­low­ing for a spec­trum of ma­te­ri­als with var­ied elec­tron­ic char­ac­ter­is­tics.

The band gap cre­at­ed so far is smaller than that needed for prac­ti­cal elec­tron­ic de­vices, and find­ing ways of in­creas­ing it will re­quire more work, the re­search­ers say. “If … a large band gap could be en­gi­neered, it could have ap­plica­t­ions in all of dig­it­al elec­tron­ics,” Jarillo-Herr­ero said. But even at its pre­s­ent lev­el, he adds, this ap­proach could be ap­plied to some optoe­lec­tron­ic ap­plica­t­ions, such as pho­tode­tec­tors.

The re­sults “sur­prised us pleas­ant­ly,” Ashoori said, and will re­quire some ex­plana­t­ion by the­o­rists. Be­cause of the dif­fer­ence in lat­tice con­stants of the two ma­te­ri­als, the re­search­ers had pre­dicted that the hy­brid’s prop­er­ties would vary from place to place. In­stead, they found a con­stant, and un­ex­pectedly large, band gap across the whole sur­face.

In ad­di­tion, Jarillo-Herrero said, the mag­ni­tude of the change in elec­trical prop­er­ties pro­duced by put­ting the two ma­te­ri­als to­geth­er “is much larg­er than the­o­ry pre­dicts.”

The team al­so saw an in­ter­est­ing new phe­nom­e­non. When ex­posed to a mag­net­ic field, the ma­te­ri­al ex­hibits frac­tal prop­er­ties—called a Hof­stadt­er but­terfly en­er­gy spec­trum—de­scribed dec­ades ago by the­o­rists, but thought im­pos­si­ble in the real world. There is in­tense re­search in this ar­e­a; two oth­er re­search groups al­so re­port on these Hof­stadt­er but­terfly ef­fects this week in the jour­nal Na­ture.

* * *

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A nearly perfectly flat material, graphene, has dazzled scientists since its discovery more than a decade ago. It has unequalled electronic properties, strength and light weight. But one long-sought goal has proved elusive: how to instill it with a property called a band gap, necessary to use graphene for transistors and other electronic devices. A band gap is an energy difference between electrons—carriers of electric charge—that are trapped in atoms, and those that are free to circulate, making a current. Now, scientists say they have taken a major step toward making graphene with the coveted feature. But their results also contradict some theoretical expectations, suggesting some accepted ideas in graphene physics are wrong. The researchers put sheets of graphene — a carbon-based material just one atom thick—on top of hexagonal boron nitride, another one-atom-thick material with similar properties. The resulting material shares graphene’s amazing ability to conduct electrons, they explain, while adding the band gap. “We created a hybrid material that has different properties than either of the two,” said MIT physicist Pablo Jarillo-Herrero, who with colleagues reported the findings in the research journal Science. Graphene is an extremely good conductor of electrons, while boron nitride is a good insulator, blocking their passage. The combination is a material whose properties are in between, a semiconductor—the basis for modern electronics. “We made a high-quality semiconductor by putting them together,” Jarillo-Herrero said. To make the hybrid work, the researchers had to align, with near perfection, the atomic lattices of the two materials, which both consist of a series of hexagons. The size of the hexagons, known as the lattice constant, in the two materials is almost the same, but not quite: Those in boron nitride are 1.8 percent larger. So while you can line the hexagons up almost perfectly in one place, over a larger area the pattern goes in and out of register. For now, the researchers say they must rely on chance to get the angular alignment for the desired electronic properties in the resulting stack. However, the alignment turns out to be correct about one time out of 15, they say. Others have made graphene into a semiconductor by etching the sheets into narrow ribbons, but this degrades graphene’s electrical properties; the new approach seems not to, said MIT physicist Ray Ashoori, another member of the resarch group. What’s most “spectacular,” he adds, is that the properties of the resulting semiconductor can be “tuned” by just slightly rotating one sheet relative to the other, allowing for a spectrum of materials with varied electronic characteristics. The band gap created so far is smaller than that needed for practical electronic devices, and finding ways of increasing it will require more work, the researchers say. “If … a large band gap could be engineered, it could have applications in all of digital electronics,” Jarillo-Herrero said. But even at its present level, he adds, this approach could be applied to some optoelectronic applications, such as photodetectors. The results “surprised us pleasantly,” Ashoori said, and will require some explanation by theorists. Because of the difference in lattice constants of the two materials, the researchers had predicted that the hybrid’s properties would vary from place to place. Instead, they found a constant, and unexpectedly large, band gap across the whole surface. In addition, Jarillo-Herrero said, the magnitude of the change in electrical properties produced by putting the two materials together “is much larger than theory predicts.” The team also saw an interesting new phenomenon. When exposed to a magnetic field, the material exhibits fractal properties — called a Hofstadter butterfly energy spectrum —described decades ago by theorists, but thought impossible in the real world. There is intense research in this area; two other research groups also report on these Hofstadter butterfly effects this week in the journal Nature.