Far-Field Optical Nanothermometry via Individual Luminescent Nanoparticles
As device length scales trend downward, poor thermal dissipation more frequently leads to nanoscale hotspots that limit overall performance. To address this challenge, developing nanoscale thermometry tools is key. One promising option utilizes the temperature-dependent optical response of individual lanthanide-doped upconverting nanoparticles. When an isolated nanoparticle is excited using a diffraction-limited laser beam, the temperature-dependent emission comes only from the nanoparticle, thereby circumventing the optical diffraction limit. This approach allows for single-point temperature measurements with spatial resolution governed by the nanoparticle size. My colleges and I demonstrated the first such measurements using individual sub-50 nm particles.
Apparent Self-Heating of Individual Upconverting Nanoparticle Thermometers
Individual upconverting nanoparticles require high excitation intensities, but the possibility of single-particle self-heating has received limited attention because even conservative thermal estimates predict negligible self-heating. Unexpectedly, we observed that the commonly used “ratiometric” thermometry signal of individual 50 x 50 x 50 nm3 NaYF4:Yb3+,Er3+ nanoparticles increases with excitation intensity, implying a temperature rise greater than 50 K if interpreted as thermal. Luminescence lifetime thermometry, which we demonstrated for the first time using individual NaYF4:Yb3+,Er3+ nanoparticles, indicates a similar temperature rise. To resolve this apparent contradiction between model and experiment, we systematically varied the nanoparticle’s thermal environment by changing the substrate thermal conductivity, the nanoparticle-substrate contact resistance via the application of several coatings, and the nanoparticle size. The apparent self-heating remains unchanged in all cases, and we demonstrate that this effect is an artifact rather than a true temperature rise. By performing rate equation modeling, we show that this artifact instead results from increased radiative and non-radiative relaxation from higher-lying Er3+ energy levels. This work has important implications for the calibrations required to achieve accurate single-nanoparticle thermometry.
Cathodoluminescence Thermometry in a Scanning Electron Microscope
An alternative method for overcoming the optical diffraction limit in the context of nanothermometry is to employ the temperature-dependent cathodoluminescence (electron beam-induced luminescence) of nanoparticles, rather than photoluminescence. In this scenario, the electron scattering volume in the material of interest limits the spatial resolution of the temperature measurement. In collaboration with Prof. Naomi Ginsberg’s group in the Department of Chemistry at UC Berkeley, we demonstrated cathodoluminescence thermometry via thin layers of upconverting nanoparticles for the first time in a commercial scanning electron microscope (SEM). We explored two different thermometry schemes, one based on temperature-dependent spectral changes in the cathodoluminescence, and a second based on changes in the cathodoluminescence luminescence lifetime with temperature.
Flexible Reduced Graphene Oxide Films for Lighting and Thermoelectric Applications up to 3,000 K
The U.S. Energy Information Administration estimates that industrial and residential lighting accounts for approximately 7% of total US energy consumption. Furthermore, a large fraction of energy used across all sectors is wasted as heat. New materials that more efficiently emit light or scavenge waste heat can help reduce energy use. My collaborators in Prof. Liangbing Hu's group at the University of Maryland developed a new flexible reduced-graphene oxide (RGO) material that can withstand temperatures of 3,000 K, enabling both incandescent lighting and high-temperature thermoelectric energy conversion.
High operating temperatures allow for white light emission and increase lighting efficiency; low thermal conductivity materials also are more efficient emitters. These same features also increase thermoelectric efficiency. Prior to our work, no thermoelectric material could operate higher than ~1500 K.
By doubling this limit, we benefit from inherent efficiency advantages of high temperature operation and enable applications like thermoelectric topping of combustion power cycles and electricity generation from concentrated solar power. The RGO’s high operation temperature is thus advantageous, but also poses challenges for conventional thermometry and thermal conductivity metrology. The RGO temperature can instead be determined by fitting emission spectra to the Planck distribution. I also developed a variation on traditional electrothermal self-heating methods by combining experimental intensity profiles with numerical analysis to extract the RGO thermal conductivity – in essence, a generalization of IR thermography into the visible wavelength range.
Onsager Reciprocity Relation for Ballistic Phonon Heat Transport in Thin Films with Arbitrary Anisotropy
Ballistic phonon transport leads to the breakdown of Fourier's law of heat conduction in thin films, which in turn reduces the thin film thermal conductivity relative to the corresponding bulk value. Boltzmann transport equation (BTE) models are used to characterize this thermal conductivity suppression due to boundary scattering in thin films with symmetries such that the heat flux is parallel to the temperature gradient. However, analogous solutions are also needed for lower-symmetry, anisotropic thin films of arbitrary orientation for which the heat flux and temperature gradient are no longer parallel, giving rise to off-diagonal terms in the thermal conductivity tensors.
By solving the BTE using the relaxation time approximation for in-plane and cross-plane phonon transport in thin films with arbitrary anisotropy, we show that the thermal conductivity tensor remains symmetric from the diffusive to the ballistic regime. We also calculate thermal conductivity suppression functions in order to demonstrate this reciprocity. I developed a matrix-inversion approach that can be used to numerically solve the deviational form of the BTE.
Localized Reflow of Lead-Free Solder/Magnetic Nanoparticle Composites
With the introduction of the Restriction of Hazardous Substances (RoHS) and Waste in Electrical and Electronic Equipment (WEEE) directives in the European Union, there has been a pressing need to find suitable alternatives to traditional lead-tin solder. Sn-Ag-Cu (SAC) alloys are a promising option, but their higher reflow temperature and longer reflow period leads to reliability concerns. Coupling RF fields to magnetic nanoparticles (MNPs) that have been added to lead-free solder enables localized magnetic reflow, which can alleviate these issues. I developed finite element models to assess the thermal and thermomechanical stress effects of this localized reflow process. We demonstrate that solder-MNP composites with relatively low MNP loading can achieve reflow when exposed to AC magnetic fields without damaging other components of an area-array package.
Other related publications:
S. Xu, A.H. Habib, A.D. Pickel, and M.E. McHenry, “Magnetic Nanoparticle-based Solder Composites for Electronic Packaging Applications,” Progress in Materials Science 67, 95-160 (2015).
S. Xu, A. Prasitthipayong, A.D. Pickel, et al., “Mechanical Properties of FeCo Magnetic Particles-based Sn-Ag-Cu Solder Composites,” Applied Physics Letters 102, 251909 (2013).