Time-Resolved X-Ray Diffraction Studies of CdSe:CdS Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals (NCs) are increasingly used in photonic and electronic applications due to their tunable properties, which can be adjusted by altering their size, shape, composition, or surface chemistry. As such, a thorough understanding of their nanoscale thermal properties is essential for maintaining device performance and stability during operation. Previous studies have reported that the thermal conductivity of nanocrystal films range from 0.1-0.6 W m^-1 K^-1, nearly two orders of magnitude lower than their bulk counterparts. This slow thermal transport in NC thin films can negatively affect device performance, reducing the lifespan and efficiency of NC-based optoelectronics such as lasers or LEDs. Additionally, a better understanding of nanoscale thermal transport could improve the development of NC-based thermoelectrics, which convert thermal gradients into electrical power. Real-time characterization of temperature changes as excited charges relax in device active layers is needed to help unlock these applications.<br/><br/>In this study, we use time-resolved x-ray diffraction (TR-XRD) measurements on CdSe:CdS NC thin films to directly measure thermal conductivity in samples that model a NC laser cavity. We compare experimental results with thermal transport models to determine thermal conductivity. Previous TR-XRD studies of measure thermal conductivity have been focused on bulk materials, epitaxial films, or small flakes of 2D materials, without a focus on NC thin film assemblies. Our work builds upon earlier studies of NC thin film thermal conductivity, which used conventional methods such as 3ω and time or frequency domain thermoreflectance to capture this information in a contact-less approach. Using TR-XRD to study NC thin films will enable direct monitoring of structural dynamics and thermal transport in photoexcited NC thin films and actual NC-based devices.