Infrared quantum dots for liquid-phase thermometry in silicon

The need for new technologies such as forced-liquid convection and evaporative cooling to cool microelectronics has renewed interest in nonintrusive liquid-phase thermometry techniques with micron-scale resolution. A number of such techniques exploit changes in the emission characteristics of photoluminescent species, but require optical access to both excite and image the emissions. Silicon, the leading material for microelectronics, is opaque at visible wavelengths. It is, however, partially transparent in the near-infrared (IR). Quantum dots (QD), or colloidal semiconductor nanocrystals, of lead sulfide (PbS) emit in the near-IR. Although the emissions from PbS IRQD vary with temperature, the poor photostability of these IRQD in a colloidal suspension due to surface oxidation made them impractical for liquid-phase thermometry. Recently, new methods have made it possible to overcoat a layer of cadmium sulfide (CdS) on PbS 'cores'. Experiments to evaluate the temperature sensitivity of CdS-coated PbS IRQD emitting around 1.35 mu m suggest that the emissions from these core-shell structures, when suspended in toluene and illuminated by 488 nm, change 0.5% per degrees C at temperatures of 20-60 degrees C. The uncertainty in the liquid-phase temperatures using these tracers was estimated to be less than 0.3 degrees C. Moreover, the IRQD had a consistent temperature response for more than 100 days after suspension.