The next break­through in highly effi­cient bat­tery tech­nolo­gies and solar cells may very well be nanoscopic crys­tals of sil­icon assem­bled like sky­scrapers on wafer-​​scale sub­strates. An impor­tant route for growth of these nanoscale “whiskers”—or nanowires—involves alloyed metal droplets.

Mon­eesh Upmanyu, an asso­ciate pro­fessor in the Depart­ment of Mechan­ical and Indus­trial Engi­neering, has been using com­pu­ta­tional tools to under­stand the atomic-​​scale inter­ac­tions between these droplets and the growth of nanowires.

The droplet is able to mul­ti­task on sev­eral levels, and that is the beauty of this growth tech­nique,” said Upmanyu. “It cat­alyzes and then absorbs the growing species from the sur­rounding vapor, gets sat­u­rated, and even­tu­ally guides the nucle­ation of the growing nanowire, not unlike a jet that leaves a crys­talline nanowire in its wake.”

The tech­nique was devel­oped decades ago for growing sil­icon “whiskers” that used a droplet of liq­ue­fied metal to trick vapor­ized sil­icon par­ti­cles into solid­i­fying as whiskers. The syn­thetic route is now widely used for growing nanowires for a variety of tech­no­log­i­cally impor­tant materials.

The droplet ulti­mately gives absolute con­trol on the growth form, yet no one knew exactly how it sculpts the nanowires into spe­cific shapes and sizes,” said Hai­long Wang, a former post-​​doctoral stu­dent within Upmanyu’s group and the first author on a recently pub­lished paper on this research in the journal Nature Com­mu­ni­ca­tions. The study was per­formed in col­lab­o­ra­tion with researchers at Lawrence Liv­er­more National Lab­o­ra­tory and Col­orado School of Mines.

There was no under­standing at the atomic scale, mostly assump­tions,” added Upmanyu.“Unmasking them is crit­ical as it allows us to con­trol the growth form and, as is the case at these small scales, form invari­ably dic­tates function.

A droplet of liquefied metal collects vaporized silicon particles from the surrounding environment, spurring the synthesis of a silicon nano whisker that has a hexagonal cross section. Image courtesy of Moneesh Upmanyu.

A droplet of liq­ue­fied metal col­lects vapor­ized sil­icon par­ti­cles from the sur­rounding envi­ron­ment, spurring the syn­thesis of a sil­icon nano whisker that has a hexag­onal cross sec­tion. Image cour­tesy of Mon­eesh Upmanyu.

The researchers dis­cov­ered that the droplet does not uni­formly wrap around the nanowire. Rather, it coaxes the growing end of the nanowire to facet into unevenly beveled edges. “This col­lec­tion of trun­cated edges serves the same pur­pose as the Archimedean spi­rals that facil­i­tate the growth of macroscale crys­tals, and that is a key part of the puzzle for large-​​scale growth of these crys­tals with pre­scribed form,” Upmanyu said. As the droplet col­lects the vapor­ized par­ti­cles in its liquid state, they begin to sat­u­rate the system and pre­cip­i­tate out to form the solid wire. The pre­cip­i­ta­tion is much faster on the beveled edges, which ulti­mately lead to layer-​​by-​​layer growth of the nanowire.

With this new under­standing, researchers can begin to develop very spe­cific crys­talline structures—ranging from effi­cient solar panels to LED lighting—at rel­a­tively inex­pen­sive price points. Upmanyu has already begun col­lab­o­rating with other researchers at North­eastern, from physi­cists to biol­o­gists, to “sculpt” nanowires with par­tic­ular properties.

A fun­da­mental under­standing of nanocrystal growth remains a chal­lenge, as the key processes require an inter­dis­ci­pli­nary effort,” Upmanyu said. “Besides cutting-​​edge com­pu­ta­tional tools and algo­rithms, it involves ele­ments of growth chem­istry, alloy met­al­lurgy, and sur­face science.”