Diamonds are known for many things: hardness, luster, and their reputation for being a “girl’s best friend.” But the gems have important scientific uses, too. New research suggests that a certain type of artificial diamond can be used as a nanoscale temperature probe with unmatched precision over time and space.
“I think this work is a real advance,” says materials scientist Daniel Jaque of the Autonomous University of Madrid, who was not involved in the study. “It’s a good paper on a hot topic.”
The tiny diamond probes can measure temperatures ranging from 120 K to 900 K (–153°C to 627°C)—as cold as the poles of Mars and almost 200° hotter than the surface of Venus. They can also detect temperature changes across distances as small as 5 μm (roughly the size of a sperm cell’s head) and on timescales as short as 800 picoseconds (0.0000000008 seconds). Scientists discovered the properties of the probes—reported in the current issue of Applied Physics Letters—when they set out to investigate a unique defect in diamonds grown using nickel precursors. The technique incorporates some nickel atoms into the diamond’s crystal structure, forming what is called an “S3 defect center.” Like many other diamond defects, the S3 center emits a glow when struck by a pulse of laser light. Scientists can then use the lifetime of the resulting luminescence to calculate the temperature of the probe: As the temperature drops, the diamond glows for longer periods of time.
Luminescent temperature probes aren’t a totally new idea, but what makes the S3 defect so appealing is that it combines speed and precision across a wide range of temperatures, says materials scientist Estelle Homeyer of the University of Lyon in France and lead author of the paper. Her co-author, spectroscopist Christophe Dujardin of the University of Lyon, adds: “There are many kinds of impurities in diamond, and this particular defect was the most interesting. It’s more universal. You combine all the purposes in one probe.”
The superior versatility of the S3 defect comes from its electronic structure, which can be excited at two different energy levels. This produces luminescence at two separate wavelengths that have lifetimes ranging from 277 millionths of a second to about 100 billionths of a second. This difference makes the nickel-doped diamond luminescence extremely sensitive to fluctuations in temperature.
Researchers say the diamond probes could be used for a wide range of applications, but Jaque suspects they’ll be most useful for observing the nanoscopic world, in particular the minute temperature fluctuations in living cells. But this might be limited to thin layers of cells in laboratory settings, since the visible light emitted by the diamond probes—a faint green glow—does not penetrate whole human tissue very well. “Only infrared light can penetrate into your body. You cannot do that by using visible light,” Jaque says. Still, a micron-scale look at the thermodynamics of human cells with picosecond time resolution would be a tremendous tool for scientists.
The probes could have applications for material sciences, too, says co-author Gilles Ledoux of the University of Lyon, especially in measuring the friction between two materials at very small scales—an area of study currently not very well understood. But the team points out that the probes are still in their infancy. For starters, scientists don’t know precisely how to make the S3 defect centers. Current techniques rely on growing diamonds with a nickel precursor and hoping the defects show up. “We do not know how to prepare it. We just collect it from many diamonds, [and] some of them have this effect. It’s a long path,” Dujardin says. Now, the technique gives a temperature reading accurate to 2, but a more refined approach might allow researchers to standardize the size of diamond particles and the number of defects to increase precision even further.