Damiano Nardi

A New Regime of Nanoscale Thermal Transport: Collective Diffusion Increases Dissipation Efficiency

Significance A complete description of nanoscale thermal transport is a fundamental problem that has defied understanding for many decades. Here, we uncover a surprising new regime of nanoscale thermal transport where, counterintuitively, nanoscale heat sources cool more quickly when placed close together than when they are widely separated. This increased cooling efficiency is possible when the separation between nanoscale heat sources is comparable to the average mean free paths of the dominant heat-carrying phonons. This finding suggests new approaches for addressing the significant challenge of thermal management in nanosystems, with design implications for integrated circuits, thermoelectric devices, nanoparticle-mediated thermal therapies, and nanoenhanced photovoltaics for improving clean-energy technologies. Understanding thermal transport from nanoscale heat sources is important for a fundamental description of energy flow in materials, as well as for many technological applications including thermal management in nanoelectronics and optoelectronics, thermoelectric devices, nanoenhanced photovoltaics, and nanoparticle-mediated thermal therapies. Thermal transport at the nanoscale is fundamentally different from that at the macroscale and is determined by the distribution of carrier mean free paths and energy dispersion in a material, the length scales of the heat sources, and the distance over which heat is transported. Past work has shown that Fourier’s law for heat conduction dramatically overpredicts the rate of heat dissipation from heat sources with dimensions smaller than the mean free path of the dominant heat-carrying phonons. In this work, we uncover a new regime of nanoscale thermal transport that dominates when the separation between nanoscale heat sources is small compared with the dominant phonon mean free paths. Surprisingly, the interaction of phonons originating from neighboring heat sources enables more efficient diffusive-like heat dissipation, even from nanoscale heat sources much smaller than the dominant phonon mean free paths. This finding suggests that thermal management in nanoscale systems including integrated circuits might not be as challenging as previously projected. Finally, we demonstrate a unique capability to extract differential conductivity as a function of phonon mean free path in materials, allowing the first (to our knowledge) experimental validation of predictions from the recently developed first-principles calculations.

Probing Thermomechanics at the Nanoscale: Impulsively Excited Pseudosurface Acoustic Waves in Hypersonic Phononic Crystals

High-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces. Here we study this phenomenon in the hypersonic frequency limit. By modeling the thermomechanics from first-principles, we calculate the system’s initial heat-driven impulsive response and follow its time evolution. A scheme is introduced to quantitatively access frequencies and lifetimes of the composite system’s excited eigenmodes. A spectral decomposition of the calculated response on the eigemodes of the system reveals asymmetric resonances that result from the coupling between surface and bulk acoustic modes. This finding allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes and expands our understanding of the scattering of surface waves in mesoscale metamaterials. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on one-dimensional and two-dimensional surface phononic crystals, probed using light at extreme ultraviolet and near-infrared wavelengths.

Pseudosurface acoustic waves in hypersonic surface phononic crystals

We present a theoretical framework allowing to properly address the nature of surfacelike eigenmodes in a hypersonic surface phononic crystal, a composite structure made of periodic metal stripes of nanometer size and periodicity of 1 mu m, deposited over a semi-infinite silicon substrate. In surface-based phononic crystals there is no distinction between the eigenmodes of the periodically nanostructured overlayer and the surface acoustic modes of the semi-infinite substrate, the solution of the elastic equation being a pseudosurface acoustic wave partially localized on the nanostructures and radiating energy into the bulk. This problem is particularly severe in the hypersonic frequency range, where semi-infinite substrates surface acoustic modes strongly couple to the periodic overlayer, thus preventing any perturbative approach. We solve the problem introducing a surface-likeness coefficient as a tool allowing to find pseudosurface acoustic waves and to calculate their line shapes. Having accessed the pseudosurface modes of the composite structure, the same theoretical frame allows reporting on the gap opening in the now well-defined pseudo-SAW frequency spectrum. We show how the filling fraction, mass loading, and geometric factors affect both the frequency gap, and how the mechanical energy is scattered out of the surface waveguiding modes.

Design of a surface acoustic wave mass sensor in the 100 GHz range

A design for photoacoustic mass sensors operating above 100 GHz is proposed. The design is based on impulsive optical excitation of a pseudosurface acoustic wave in a surface phononic crystal with nanometric periodic grating and on time-resolved extreme ultraviolet detection of the pseudosurface acoustic wave frequency shift upon mass loading the device. The present design opens the path to sensors operating in a frequency range currently unaccessible to electro-acoustical transducers, providing enhanced sensitivity, miniaturization, and incorporating time-resolving capability while forgoing the piezoelectric substrate requirement.

Interface nano-confined acoustic waves in polymeric surface phononic crystals

The impulsive acoustic dynamics of soft polymeric surface phononic crystals is investigated here in the hypersonic frequency range by near-IR time-resolved optical diffraction. The acoustic response is analysed by means of wavelet spectral methods and finite element modeling. An unprecedented class of acoustic modes propagating within the polymer surface phononic crystal and confined within 100 nm of the nano-patterned interface is revealed. The present finding opens the path to an alternative paradigm for characterizing the mechanical properties of soft polymers at interfaces and for sensing schemes exploiting polymers as embedding materials.

Ab initio thermodynamics calculation of all-optical time-resolved calorimetry of nanosize systems: Evidence of nanosecond decoupling of electron and phonon temperatures

The thermal dynamics induced by ultrashort laser pulses in nanoscale systems, i.e., all-optical time-resolved nanocalorimetry is theoretically investigated from 300 to 1.5 K. We report ab initio calculations describing the temperature dependence of the electron-phonon interactions for Cu nanodisks supported on Si. The electrons and phonons temperatures are found to decouple on the ns time scale at similar to 10 K, which is two orders of magnitude in excess with respect to that found for standard low-temperature transport experiments. By accounting for the physics behind our results we suggest an alternative route for overhauling the present knowledge of the electron-phonon decoupling mechanism in nanoscale systems by replacing the mK temperature requirements of conventional experiments with experiments in the time domain.

Probing limits of acoustic nanometrology using coherent extreme ultraviolet light

Photoacoustic nanometrology using coherent extreme ultraviolet (EUV) light detection is a unique and powerful tool for probing ultrathin films with a wide range of mechanical properties and thicknesses well under 100 nm. In this technique, short wavelength acoustic waves are generated through laser excitation of a nano-patterned metallic grating, and then probed by diffracting coherent EUV beams from the dynamic surface deformation. Both longitudinal and surface acoustic waves within thin films and metallic nanostructures can be observed using EUV light as a phase-sensitive probe. The use of nanostructured metal transducers enables the generation of particularly short wavelength surface acoustic waves, which truly confine the measurement within the ultrathin film layer of interest, to thicknesses < 50 nm for the first time. Simultaneous measurement of longitudinal and transverse surface wave velocities yields both the Young’s modulus and Poisson’s ratio of the film. In the future, this approach will make possible precise mechanical characterization of nanostructured systems at sub-10 nm length scales.

Temperature dependence of the thermal boundary resistivity of glass-embedded metal nanoparticles

The temperature dependence of the thermal boundary resistivity is investigated in glass-embedded Ag particles of radius 4.5 nm, in the temperature range from 300 to 70 K, using all-optical time-resolved nanocalorimetry. The present results provide a benchmark for theories aiming at explaining the thermal boundary resistivity at the interface between metal nanoparticles and their environment, a topic of great relevance when tailoring thermal energy delivery from nanoparticles as for applications in nanomedicine and thermal management at the nanoscale.

Nondestructive Measurement of the Evolution of Layer-Specific Mechanical Properties in Sub-10 nm Bilayer Films

We use short wavelength extreme ultraviolet light to independently measure the mechanical properties of disparate layers within a bilayer film for the first time, with single-monolayer sensitivity. We show that in Ni/Ta nanostructured systems, while their density ratio is not meaningfully changed from that expected in bulk, their elastic properties are significantly modified, where nickel softens while tantalum stiffens, relative to their bulk counterparts. In particular, the presence or absence of the Ta capping layer influences the mechanical properties of the Ni film. This nondestructive nanomechanical measurement technique represents the first approach to date able to distinguish the properties of composite materials well below 100 nm in thickness. This capability is critical for understanding and optimizing the strength, flexibility and reliability of materials in a host of nanostructured electronic, photovoltaic, and thermoelectric devices.

Disentangling thermal and nonthermal excited states in a charge-transfer insulator by time- and frequency-resolved pump-probe spectroscopy

Time-and-frequency resolved pump-probe optical spectroscopy is used to investigate the effect of the impulsive injection of delocalized excitations through a charge-transfer process in insulating CuGeO3. A large broadening of the charge-transfer edge is observed on the sub-ps timescale. The modification of this spectral feature can not be attributed to the local increase of the effective temperature, as a consequence of the energy absorbed by the pump pulse. The measured modifications of the optical properties of the system are consistent with the creation of a non-thermal state, metastable on the ps timescale, after the pump-induced impulsive modification of the electron interactions.