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“Smoothest” mirror could lead to new microscopes

Sept. 22, 2008
Courtesy Plataforma SINC
and World Science staff

Phys­i­cists have cre­at­ed the smoothest sur­face ev­er made, called a “quan­tum sta­bi­lised at­om mir­ror,” ac­cord­ing to this week’s edi­tion of the re­search jour­nal Ad­vanced Ma­te­ri­als.

The scientists from the Au­ton­o­mous Uni­ver­s­ity of Ma­drid and the Ma­drid In­sti­tute of Ad­vanced Stud­ies in Na­no­sci­ence say the in­nova­t­ion is be­ing used to de­sign the world’s first at­omic mi­cro­scope.

Despite small holes and “is­lands,” the mir­ror has a mostly smooth sur­face that re­flects an im­ag­in­ary mole­cu­lar beam (each mo­le­cule with four atoms). (Pho­to: Bar­re­do et al.)
The mirror is designed to reflect beams of atoms. One of the stu­dy’s au­thors, Rodolfo Mi­ran­da of the Au­ton­o­mous Uni­ver­s­ity, said the mir­ror re­flects “ex­tra­or­di­narily well” most of these at­oms, through the use of ma­te­ri­als of near-zero thick­ness whose prop­er­ties are dom­i­nat­ed by quan­tum, or at­omic-scale ef­fects.

The mir­ror re­sem­bles a curved wa­fer. It is made up of a thin sil­i­con crys­tal with a thick­ness of one-twentieth of a mil­li­me­ter, and cov­ered with a lay­er of lead one or two mil­lionths of a mil­li­me­ter thick. 

To study the re­flec­tion on this met­al, the sci­en­tists used he­li­um at­oms. Un­til now mir­rors made solely from sil­i­con re­flected one per­cent of he­li­um at­oms, but by adding the lay­er of lead they have man­aged to achieve a re­flec­tion of up to 67 per­cent, the sci­en­tists re­ported.

The lead is de­posited on the sil­i­con at a tem­per­a­ture of be­tween -173º and -133º C which, to­geth­er with the mi­nute thick­ness of the lead, al­lows its quan­tum prop­er­ties to “come to the sur­face,” Mi­ran­da said. Then, in an “as­ton­ish­ing and spon­ta­neous” way, bumps on the sur­face even out and a su­per-flat lay­er emerges, he added.

“The ex­tra­or­di­nary thing about this pro­cess is that when the ma­te­ri­al is heat­ed to room tem­per­a­ture, it does not dis­tort or break, but in­stead be­co­mes even flat­ter, en­hanc­ing its re­flec­tion prop­er­ties.”

These types of mir­rors are vi­tal for ma­n­u­fac­tur­ing fu­ture at­omic mi­cro­scopes, he added. Un­til now elec­tron­ic mi­cro­scopes have achieved the high­est res­o­lu­tions when it comes to view­ing ob­jects, but with the dis­ad­vant­age that the ac­cel­er­ated elec­trons they use de­stroy the most del­i­cate bi­o­log­i­cal sam­ples. Electron microscopes work in general by shooting beams of electrons at a sample, then “reading” the reflection of the beam or the response of the material.

“With at­omic mi­cro­scopes we hope to achieve the same res­o­lu­tion but with­out dam­ag­ing sam­ples,” said Mi­randa.

The Span­ish re­search­ers, to­geth­er with a sci­en­tists at the Uni­ver­s­ity of Cam­bridge and the Uni­ver­s­ity of Graz in Aus­tria, are work­ing on pro­to­types of at­omic mi­cro­scopes that use quan­tum sta­bi­lised mir­rors, and say the first im­ages that re­sult should be ready next year.

* * *

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Physicists have created have the the smoothest surface ever, called a “quantum stabilised atom mirror,” according to this week’s edition of the research journal Advanced Materials. The researchers from the Autonomous University of Madrid and the Madrid Institute of Advanced Studies in Nanoscience say the innovation is being used to design the world’s first atomic microscope. One of the study’s authors, Rodolfo Miranda of the Autonomous University, said the mirror reflects “extraordinarily well” most of the atoms that affect it, through the use of materials of near-zero thickness whose properties are dominated by quantum, or atomic-scale effects. The mirror resembles a curved wafer. It is made up of a thin silicon crystal with a thickness of one-twentieth of a millimeter, and covered with a layer of lead one or two millionths of a millimeter thick. To study the reflection on this metal, the scientists used helium atoms. Until now mirrors made solely from silicon reflected 1% of helium atoms, but by adding the layer of lead they have managed to achieve a reflection of up to 67%, the scientists reported. The lead is deposited on the silicon at a temperature of between -173º and -133º C which, together with the nanometric thickness of the lead, allows its quantum properties to “come to the surface,” Miranda said. Then, in an “astonishing and spontaneous” way, bumps on the surface become evened out and a super flat layer is created, he added. “The extraordinary thing about this process is that when the material is heated to room temperature, it does not distort or break, but instead becomes even flatter, enhancing its reflection properties.” These types of mirrors are vital for manufacturing future atomic microscopes, he added. Until now electronic microscopes have achieved the highest resolutions when it comes to viewing objects, but with the disadvantage that the accelerated electrons they use destroy the most delicate biological samples. “With atomic microscopes we hope to achieve the same resolution but without damaging samples,” said the physicist. The Spanish researchers, together with a scientists at the University of Cambridge and the University of Graz in Austria, are working on prototypes of atomic microscopes that use quantum stabilised mirrors, and say the first images that result should be ready next year.