Ultimate-resolution thermal spectroscopy in time domain thermoreflectance (TDTR)

Abstract

Time domain thermoreflectance is a very well-suited technique for thermophysical property measurements. The heterodyne or asynchronous optical sampling versions of the technique allow transient temperature measurements from hundreds of femtoseconds up to tens of nanoseconds. The corresponding spectral material response expands from 100MHz up to 10THz. However, the ultimate bandwidth of the technique is not limited by optical sampling but by the presence of the metal transducer deposited on top of the material of interest. In this work, we investigated the thermal transparency of several metal transducer thin films. We implemented and solved numerically the two-temperature model for a set of Al, Au, Ag, Cu, Cr, and Pt thin (50 and 150 nm) layers. This numerical study showed that Al thin films are better suited for spectroscopic lattice temperature measurements up to 10GHz, while noble metals like Au allow the measurement of hot carrier dynamics up to 5THz.

Molecular Dynamics Simulations of Wettability, Thermal Transport, and Interfacial Liquid Structuring at the Nanoscale in Polar Solid–Liquid Interfaces

“…The authors reported atypical water spreading that hindered their MD calculations of contact angles and attributed the abnormal droplet shape to the reduced number of water molecules. However, the atypical nanodroplet formation reported in ref could have been induced by overestimated electrostatic interactions. A proper partial charge modeling is critical in ionic compounds, such as alumina, as it relates to the solid–liquid affinity and the formation of structures, such as hydrogen bond (H-bond) networks.…”

“…Recent developments in nanotechnology have demonstrated the potential of aluminum oxide or alumina (Al 2 O 3 ) for various applications due to its good chemical stability, wide band gap, and biological compatibility. − Special attention is drawn to alumina in aqueous environments, as the behavioral understanding of this solid–liquid interface is crucial for dielectric nanocomposites, water filtration, drug delivery, biosensing devices, combustion, among others. − In particular, the new developments in anodic alumina membranes, enabled by the progress in nanofabrication techniques, could be beneficial for filtration and separation of molecules, cells, or proteins, and evaporation in thermal management applications, see Figure (a). − Characterizing and tuning the inner nanopore geometry and electrostatic properties in alumina membranes facilitate molecular filtration, , while the interplay between the nanopore surface and the absorbed liquid thin film is critical for the evaporation performance . It has been suggested that most of the interfacial transport (specifically momentum and thermal) is strongly influenced by a thin layer, which usually encompasses no more than a few nanometers from the interfacial region. , Due to the reduced scale, the thickness of this region hinders accurate probing using the current experimental capabilities; therefore, atomistic-level simulations can provide useful insight into the interface and transport properties. Although different methods exist to model and characterize solid–liquid interfaces, there is no consensus on the proper approach to model realistic interfacial systems, especially when functional groups and electrostatic interactions are considered, i.e., surface chemistry and topography must be as close as possible to real interfaces in atomistic scale simulations.…”

How to Measure Hot Electron and Phonon Temperatures with Time Domain Thermoreflectance Spectroscopy?

“…At long time scale, typically after 10 ps, for which the thermal equilibrium between electrons and phonons is reached, i.e., Δ T ph = Δ T e = Δ T , the measured signal can classically be written as Δ R R false( t , λ , F false) true| t > 10 ps = ( κ e false( λ , F false) + κ ph false( λ , F false) ) normalΔ T = κ normale + ph ( λ , F ) normalΔ T Consequently, a one-temperature model (1TM) is then sufficient to describe the system, and, at long time scale, the thermoreflectance measurements can be converted into temperature measurements provided that the thermoreflectance coefficient κ e+ph (λ, F ) is known. , …”

“…We have numerically solved 17 the 2TM to predict the phonon and electron contributions in the thermoreflectance signal for different kinds of transducers. In particular, it has been shown that the choice of the transducer material is crucial with Al better suited for spectroscopic phonon temperature measurements, while Au is more adapted when studying hot carrier transport.…”

“…Our previous work 17 focused on temperature simulation; here, we propose to experimentally investigate temperature measurements of both electrons and phonons, investigating a possible effect of the probe. Consequently, in this paper, we first consider the long-time-scale regime when electron and phonon are thermalized.…”