Using Quantum Dots for Liquid-Phase Thermometry at Near-Infrared Wavelengths in Silicon Devices

The need for new thermal management technologies to cool electronic components with their ever-increasing density and power requirements has renewed interest in techniques for measuring liquid-phase coolant temperatures, especially nonintrusive techniques with micron-scale spatial resolution. A variety of optical liquid-phase thermometry techniques exploit the changes in the emission characteristics of fluorescent, phosphorescent or luminescent tracers suspended in a liquid-phase coolant. Such techniques are nonintrusive and have micron-scale spatial resolution, but they also require optical access to both excite and image the emissions. Silicon (Si), the leading material for electronic devices, is opaque at visible wavelengths, but is partially transparent in the near-infrared (IR). To date, the only tracers that emit at near-IR wavelengths with reasonable quantum yield are IR quantum dots (IRQD), colloidal nanocrystals of semiconductor materials such as lead sulfide (PbS). Previous work has shown that the intensity of emissions at 1.55 μm from PbS IRQD suspended in toluene are temperature-sensitive, decreasing by as much as 15% as the temperature increased from 20 °C to 60 °C. The accuracy of temperature measurements using PbS IRQD was estimated to be about 5 °C, based on 95% confidence intervals, where the major limit on the accuracy of the technique was the poor photostability of this material [1]. Recently, a new method for creating a cadmium sulfide (CdS) overcoat layer on PbS “cores” has been developed [2]. The experimental results presented here on the temperature sensitivity of these PbS/CdS core-shell infrared quantum dots with an emission peak around 1.35 μm and a diameter of 5.7 nm (with a core diameter of 4 nm) suggest that these new core-shell structures are more temperature-sensitive than the PbS cores. These core-shell quantum dots, when suspended in toluene, were found to have a 0.5% decrease in emission power per °C increase in temperature at suspension temperatures ranging from 20 °C to 60 °C. The uncertainty in the liquid-phase temperatures derived from these emissions was estimated to be less than 0.3 °C based on the standard deviation. Furthermore, the PbS/CdS quantum dots were highly photostable, with a consistent response more than 100 days after suspension. These results imply that that these new IRQD can be used to measure liquid-phase coolant temperatures without disturbing the flow of coolant at an accuracy comparable to commercially available thermocouples in monolithic Si devices.