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Researchers report blackest black yet made

Jan. 23, 2008
World Science staff

In the mov­ie This is Spi­nal Tap, a less-than-cerebral rock gui­tar­ist, up­on view­ing a rec­ord-al­bum cov­er de­signed as sol­id black, de­liv­ers an im­promp­tu speech. “It’s like, how much more black could this be? And the an­swer is none,” he pro­claims. “None more black.”

The new ma­te­ri­al (right), along­side a ma­te­ri­al or­di­nar­i­ly seen as very black (left). The one on the left is meas­ured to have 1.4 per­cent re­flect­ance, darker than char­coal. The light­ing and im­age bright­ness have been set so that this ma­te­ri­al looks more gray than black. Still, the new ma­te­ri­al is not­i­ceab­ly dark­er. (Cour­te­sy Rens­se­laer P.I.)
His in­ar­tic­u­lateness is matched, sad­ly, by an ig­no­rance of phys­ics. You can get much black­er than black card­board, which re­flects a good deal of light where­as true black re­flects none. 

Find­ing an ab­so­lutely black ob­ject on Earth, though, is as likely as en­coun­ter­ing the rock group of that 1982 film. Both are only fic­tions.

On the oth­er hand, re­search­ers now say they have cre­at­ed the dark­est ma­te­ri­al ev­er made by ma­n. The ma­te­ri­al, a thin car­bon coat­ing, re­flects less than 0.1 per­cent of in­com­ing light. It ab­sorbs the rest. 

It could one day serve to boost the ef­fec­tive­ness and ef­fi­cien­cy of so­lar en­er­gy tech­no­logy, in­fra­red sen­sors, and oth­er de­vices, said the re­search­ers, who have ap­plied for a Guin­ness World Rec­ord.

“This discov­ery will al­low us to in­crease the ab­sorp­tion ef­fi­cien­cy of light as well as the over­all radia­t­ion-to-electri­city ef­fi­cien­cy of so­lar en­er­gy con­serva­t­ion,” said Shawn-Yu Lin, a phys­i­cist at Rens­se­laer Pol­y­tech­nic In­sti­tute in Troy, N.Y., who led the re­search. 

The key was to cre­ate a car­pet of car­bon nan­o­tubes, struc­tures not much thicker than an at­om, he said. It al­so re­quired build­ing in a bit of “sur­face ran­dom­ness,” he added which served to min­i­mize re­flec­tion.

All ma­te­ri­als re­flect some light. Or­di­nary black paint re­flects 5 to 10 per­cent. The dark­est ma­nmade ma­te­ri­al, pri­or to the Lin group’s discov­ery, re­flected just un­der a fifth of a per­cent, Lin said. The new ma­te­ri­al has a to­tal re­flective in­dex of less than a twen­ti­eth of a per­cent—in total, 0.045 per­cent, he said.

Its unique properties are due to “the loosely-packed for­est of car­bon nan­o­tubes, which is full of nano­scale [roughly ato­mic-sized] gaps and holes to col­lect and trap light,” Lin said. “Such a nan­otube ar­ray not only re­flects light weak­ly, but al­so ab­sorbs light strongly. These com­bined fea­tures make it an ide­al can­di­date for one day real­iz­ing a super-black ob­ject.”

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In the movie Spinal Tap, a less-than-cerebral rock guitarist, upon viewing a record album cover designed as solid black, delivers an impromptu speech. “It’s like, how much more black could this be? And the answer is none,” he proclaims, before pausing to add: “None more black.” His inarticulateness is matched, sadly, by an ignorance of physics. You can get much blacker than black cardboard, which reflects a good deal of light whereas true black reflects none. Finding an absolutely black object on Earth, though, is as likely as encountering the fictional rock group of that 1982 film. It doesn’t exist. On the other hand, researchers now say they have created the darkest material ever made by man. The material, a thin coating comprised of low-density arrays of loosely vertically-aligned carbon nanotubes, reflects just 0.1 percent of incoming light, that is, absorbs more than 99.9 percent. It could one day serve to boost the effectiveness and efficiency of solar energy conversion, infrared sensors, and other devices, said the researchers, who have applied for a Guinness World Record. “This discovery will allow us to increase the absorption efficiency of light as well as the overall radiation-to-electricity efficiency of solar energy conservation,” said Shawn-Yu Lin, a physicist at Rensselaer Polytechnic Institute in Troy, N.Y., who led the research. The key was to create a carpet of carbon nanotubes, structures not much thicker than an atom, he said. It also required building in a bit of “surface randomness,” he added which served to minimize reflection. All materials reflect some light. Ordinary black paint reflects 5 to 10 percent of light. The darkest manmade material, prior to the Lin group’s discovery, reflected just under a fifth of a percent, Lin said. The new material has a total reflective index of less than a twentieth of a percent—or 0.045 percent, he said. “The loosely-packed forest of carbon nanotubes, which is full of nanoscale gaps and holes to collect and trap light, is what gives this material its unique properties,” Lin said. “Such a nanotube array not only reflects light weakly, but also absorbs light strongly. These combined features make it an ideal candidate for one day realizing a super-black object.”