Non-shivering thermogenesis through mitochondrial proton uncoupling is one of the dominant thermoregulatory mechanisms crucial for normal cellular functions. The metabolic pathway for intracellular temperature rise has widely been considered as steady-state substrate oxidation. Here, we show that a transient proton motive force (pmf) dissipation is more dominant than steady-state substrate oxidation in stimulated thermogenesis. Using transient intracellular thermometry during stimulated proton uncoupling in neurons of Aplysia californica , we observe temperature spikes of ~7.5 K that decay over two time scales: a rapid decay of ~4.8 K over ~1 s followed by a slower decay over ~17 s. The rapid decay correlates well in time with transient electrical heating from proton transport across the mitochondrial inner membrane. Beyond ~33 s, we do not observe any heating from intracellular sources, including substrate oxidation and pmf dissipation. Our measurements demonstrate the utility of transient thermometry in better understanding the thermochemistry of mitochondrial metabolism.

Measuring temperatures within a biological cell requires a sensor with small thermal mass and microscale or smaller size that is electrically and chemically inert to the cell's environment, and is thermally isolated from the surroundings. We investigate how such requirements can be satisfied in a microscale thermocouple probe that is fabricated using the techniques of silicon-based microelectromechanical systems. Previous reports of invasive probes lacked either the required spatial resolution (< 5 µm) or response time (< 4 ms). Here, we report 1 µm thick silicon nitride supported probes with a 5 µm tip that has a response time of 32 µs. These figures enable future transient thermometry of cell organelles. To reduce calibration errors, we devise an on-chip calibration in a vacuum cryostat. We find that the accuracy of our measurements is ±54 mK for 300 ± 10 K. This work paves the way toward future thermometry at a subcellular level.

The specularity of phonons at crystal surfaces is of direct importance to thermal transport in nanostructures and to dissipation in nanomechanical resonators. Wave scattering theory provides a framework for estimating wavelength dependent specularity, but experimental validation remains elusive. Widely available thermal conductivity data presents poor validation since the involvement of infinitude of phonon wavelengths in thermal transport presents an underconstrained test for specularity theory. Here, we report phonon specularity by measuring the lifetimes of individual coherent longitudinal acoustic phonon modes excited in ultrathin (36-205 nm) suspended silicon membranes at room temperature over the frequency range ∼ 20-118 GHz. Phonon surface scattering dominates intrinsic Akhiezer damping at frequencies 60 GHz, enabling measurements of phonon boundary scattering time over wavelengths ∼72-140 nm. We obtain detailed statistics of the surface roughness at the top and bottom surfaces of membranes using HRTEM imaging. We find that the specularity of the excited modes are in good agreement with solutions of wave scattering only when the TEM statistics are corrected for projection errors. The often cited Ziman formula for phonon specularity also appears in good agreement with the data, contradicting previous results. This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.

The thermoelectric properties of nanostructured silicon are not fully understood despite their initial promise. While the anomalously low thermal conductivity has attracted much work, the impact of nanostructuring on the power factor has mostly escaped attention. While initial reports did not find any significant changes to the power factor compared to the bulk, subsequent detailed measurements on p-type silicon nanowires showed a stark reduction in the Seebeck coefficient when compared to similarly doped bulk. The reduction is consistent with the disappearance of the phonon drag contribution, due to phonon boundary scattering. Here, we report measurements on a different nanostructure, holey silicon films, to test if similar loss of phonon drag can be observed. By devising experiments where all properties are measured on the same sample, we show that though these films possess electrical conductivity close to that in the bulk at comparable doping, they exhibit considerably smaller thermopower. The data are consistent with loss of phonon drag. At neck distances between 120 -230 nm, the power factor at optimal doping is ∼ 50 percent that of the bulk. These insights are useful in the practical design of future thermoelectric devices based on nanostructured silicon.

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