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for isotopically purified silicon – from a former Soviet-era isotope manufacturing plant . ( Haller founded Berkeley Lab ’ s DOE-funded Electronic Materials Program in 1984 and was a senior faculty scientist in Berkeley Lab ’ s Materials Sciences Division and a professor of materials science and mineral engineering at UC Berkeley . He died in 2018 .)
This led to a series of pioneering experiments , including a 2006 study published in Nature , whereby Ager and Haller fashioned silicon-28 into single crystals , which they used to demonstrate quantum memory storing information as quantum bits or qubits , units of data stored simultaneously as a one and a zero in an electron ’ s spin .
Subsequently , semiconducting thin films and single crystals made with Ager ’ s and Haller ’ s silicon isotope material were shown to have a 10 % higher thermal conductivity than natural silicon – an improvement , but from the computer industry ’ s point of view , probably not enough to justify spending a thousand times more money to build a computer from isotopically pure silicon , according to Ager .
However , Ager knew that the silicon isotope materials were of scientific importance beyond quantum computing . So , he kept what remained in a safe place at Berkeley Lab , just in case other scientists might need it , because few people have the resources to make or even purchase isotopically pure silicon , he reasoned .
A PATH TOWARD COOLER TECH WITH SILICON-28 About three years ago , Wu and his graduate student Penghong Ci were trying to come up with new ways to improve the heat transfer rate in silicon chips .
One strategy to make more efficient transistors involves using a type of nanowire called a Gate-All-Around Field Effect Transistor . In these devices , silicon nanowires are stacked to conduct electricity , and heat is generated simultaneously , Wu explained . “ And if the heat generated is not extracted out quickly , the device would stop working , akin to a fire alarm blaring in a tall building without an evacuation map ,” he says .
But heat transport is even worse in silicon nanowires , because their rough surfaces – scars from chemical processing – scatter or “ confuse ” the phonons even more , he explains .
“ And then one day we wondered , ‘ What would happen if we made a nanowire from isotopically pure silicon-28 ?’” Wu says .
Silicon isotopes are not something one can easily buy on the open market , and word had it that Ager still had some silicon isotope crystals in storage at Berkeley Lab – not a lot , but still enough to share “ if someone has a great idea about how to use it ,” Ager says . “ And Junqiao ’ s new study was such a case .”
Thor Swift / Berkeley Lab ; photo of Joel Ager courtesy of UC Berkeley )
Matthew R . Jones and Muhua Sun / Rice University )
From left : Junqiao Wu and Joel Ager .
Transmission electron microscopy image showing a silicon-28 nanowire coated with silicon dioxide ( SiO2 ).
A SURPRISING BIG REVEAL WITH NANO TESTS “ We ’ re really fortunate that Joel happened to have the isotopically enriched silicon material ready to use for the study ,” Wu says .
Using Ager ’ s silicon isotope materials , the Wu team tested the thermal conductivity in bulk 1-millimeter-size silicon-28 crystals versus natural silicon – and again , their experiment
confirmed what Ager and his collaborators discovered years ago – that bulk silicon-28 conducts heat only 10 % better than natural silicon .
Now for the nano test . Using a technique called electroless etching , Ci made natural silicon and silicon-28 nanowires just 90 nanometers ( billionths of a meter ) in diameter – about a thousand times thinner than a single strand of human hair .
To measure the thermal conductivity , Ci suspended each nanowire between two microheater pads outfitted with platinum electrodes and thermometers , and then applied an electrical current to the electrode to generate heat on one pad that flows to the other pad via the nanowire .
“ We expected to see only an incremental benefit – something like 20 % – of using isotopically pure material for nanowire heat conduction ,” Wu says . But Ci ’ s measurements astonished them all . The Si-28 nanowires conducted heat not 10 % or even 20 %, but 150 % better than natural silicon nanowires with the same diameter and surface roughness .
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