More sta­ble light comes from in­ten­tion­ally ‘squashed’ quan­tum dots

Tehran Times - - SCIENCE - (Source: phys.org)

Novel col­loidal quan­tum dots are formed of an emit­ting cad­mium/se­le­nium (Cd/Se) core en­closed into a com­po­si­tion­ally graded CdxZn1-xSe shell wherein the frac­tion of zinc ver­sus cad­mium in­creases to­wards the dot’s pe­riph­ery. In­ten­tion­ally “squash­ing” col­loidal quan­tum dots dur­ing chem­i­cal syn­the­sis cre­ates dots ca­pa­ble of sta­ble, “blink-free” light emis­sion that is fully com­pa­ra­ble with the light pro­duced by dots made with more com­plex pro­cesses.

The squashed dots emit spec­trally nar­row light with a highly sta­ble in­ten­sity and a non-fluc­tu­at­ing emis­sion en­ergy. New re­search at Los Alamos Na­tional Lab­o­ra­tory sug­gests that the strained col­loidal quan­tum dots rep­re­sent a vi­able al­ter­na­tive to presently em­ployed nanoscale light sources, and they de­serve ex­plo­ration as sin­gle-par­ti­cle, nanoscale light sources for op­ti­cal “quan­tum” cir­cuits, ul­tra­sen­si­tive sen­sors, and med­i­cal di­ag­nos­tics.

“In ad­di­tion to ex­hibit­ing greatly im­proved per­for­mance over traditional pro­duced quan­tum dots, these new strained dots could of­fer un­prece­dented flex­i­bil­ity in ma­nip­u­lat­ing their emis­sion color, in com­bi­na­tion with the un­usu­ally nar­row, ‘sub­ther­mal’ linewidth,” said Vic­tor Klimov, lead Los Alamos re­searcher on the project. The “squashed dots also show com­pat­i­bil­ity with vir­tu­ally any sub­strate or em­bed­ding medium as well as var­i­ous chem­i­cal and bi­o­log­i­cal en­vi­ron­ments.”

The new col­loidal pro­cess­ing techniques al­low for prepa­ra­tion of vir­tu­ally ideal quan­tum-dot emit­ters with nearly 100 per­cent emis­sion quan­tum yields shown for a wide range of vis­i­ble, in­frared and ul­tra­vi­o­let wave­lengths. These ad­vances have been ex­ploited in a va­ri­ety of light-emis­sion tech­nolo­gies, re­sult­ing in suc­cess­ful com­mer­cial­iza­tion of quan­tum-dot dis­plays and TV sets.

The next fron­tier is ex­plo­ration of col­loidal quan­tum dots as sin­gle-par­ti­cle, nanoscale light sources. Such fu­ture “sin­gle-dot” tech­nolo­gies would re­quire par­ti­cles with highly sta­ble, non­fluc­tu­at­ing spec­tral char­ac­ter­is­tics.

■ Ran­dom vari­a­tions in emis­sion

Re­cently, there has been con­sid­er­able progress in elim­i­nat­ing ran­dom vari­a­tions in emis­sion in­ten­sity by pro­tect­ing a small emit­ting core with an es­pe­cially thick outer layer. How­ever, these thick-shell struc­tures still ex­hibit strong fluc­tu­a­tions in emis­sion spec­tra.

In a new pub­li­ca­tion in the jour­nal Na­ture Ma­te­ri­als, Los Alamos re­searchers demon­strated that spec­tral fluc­tu­a­tions in sin­gle-dot emis­sion can be nearly com­pletely sup­pressed by ap­ply­ing a new method of “strain en­gi­neer­ing.” The key in this ap­proach is to com­bine in a core/shell mo­tif two semi­con­duc­tors with di­rec­tion­ally asym­met­ric lat­tice mis­match, which re­sults in an­iso­tropic com­pres­sion of the emit­ting core.

This mod­i­fies the struc­tures of elec­tronic states of a quan­tum dot and thereby its light emit­ting prop­er­ties. One im­pli­ca­tion of these changes is the re­al­iza­tion of the regime of lo­cal charge neu­tral­ity of the emit­ting “ex­ci­ton” state, which greatly re­duces its cou­pling to lat­tice vi­bra­tions and fluc­tu­at­ing elec­tro­static en­vi­ron­ment, key to sup­press­ing fluc­tu­a­tions in the emit­ted spec­trum. An ad­di­tional ben­e­fit of the mod­i­fied elec­tronic struc­tures is dra­matic nar­row­ing of the emis­sion linewidth, which be­comes smaller than the room-tem­per­a­ture ther­mal en­ergy.

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