Nanoscale Hotspots Research Articles

Ultrafast Chemistry under Nonequilibrium Conditions and the Shock to Deflagration Transition at the Nanoscale

We use molecular dynamics simulations to describe the chemical reactions following shock-induced collapse of cylindrical pores in the high-energy density material RDX. For shocks with particle velocities of 2 km/s we find that the collapse of a 40 nm diameter pore leads to a deflagration wave. Molecular collisions during the collapse lead to ultrafast, multistep chemical reactions that occur under nonequilibrium conditions. Exothermic products formed during these first few picoseconds prevent the nanoscale hotspot from quenching. Within 30 ps, a local deflagration wave develops; it propagates at 0.25 km/s and consists of an ultrathin reaction zone of only ∼5 nm, thus involving large temperature and composition gradients. Contrary to the assumptions in current models, a static thermal hotspot matching the dynamical one in size and thermodynamic conditions fails to produce a deflagration wave indicating the importance of nonequilibrium loading in the criticality of nanoscale hot spots. These results provide...

Disparate quasiballistic heat conduction regimes from periodic heat sources on a substrate

We report disparate quasiballistic heat conduction trends for periodic nanoscale line heaters deposited on a substrate, depending upon whether measurements are based on the peak temperature of the heaters or the temperature difference between the peak and the valley of two neighboring heaters. The degree of quasiballistic transport is characterized by the effective thermal conductivities of the substrate which are obtained by matching the diffusion solutions to the phonon Boltzmann transport equation results. We find that while the ballistic heat conduction effect based on the peak temperature diminishes as the two heaters become closer, it becomes stronger based on the peak-valley temperature difference. Our results also show that the collective behavior of closely spaced heaters can counteract the nonlocal effects caused by an isolated nanoscale hot spot. These results are relevant to thermal conductivity spectroscopy techniques under development and also have important implications for understanding nonlocal heat conduction in integrated circuits and carbon nanotube array thermal interface materials.

Formation of super-resolution spot through nonlinear Fabry–Perot cavity structures: theory and simulation

This study explores how interference manipulation breaks through the diffraction limit and induces super-resolution nano-optical hot spots through the nonlinear Fabry-Perot cavity structure. The theoretical analytical model is established, and the numerical simulation results show that when the thickness of the nonlinear thin film inside the nonlinear Fabry-Perot cavity structure is adjusted to centain value, the constructive interference effect can be formed in the central point of the spot, which causes the nanoscale optical hot spot in the central region to be produced. The simulation results also tell us that the hot spot size is sensitive to nonlinear thin film thickness, and the accuracy is required to be up to nanometer or even subnanometer scale, which is very large challenging for thin film deposition technique, however, slightly changing the incident laser power can compensate for drawbacks of low thickness accuracy of nonlinear thin films. Taking As(2)S(3) as the nonlinear thin film, the central hot spot with a size of 40nm is obtained at suitable nonlinear thin film thickness and incident laser power. The central hot spot size is only about λ/16, which is very useful in super-high density optical recording, nanolithography, and high-resolving optical surface imaging.

Temperature Enhancement Through Interaction of Thermal Waves for Phonon Transport in Silicon Thin Films

Recently, Siemens et al. found that the Fourier law may overestimate the energy transported away from one hot spot in the thin film. In this work, the lattice Boltzmann method is employed to investigate phonon transport in silicon thin films. It is found that in the transitional or ballistic regime when the thermal waves initiated from temperature disturbances on two surfaces of the thin film meet in the inner region, the temperature rises significantly, which is different from the case when the heat conduction is induced by the temperature disturbance on one surface of the thin film. Therefore, the interaction of thermal waves induced by the thermal transport from two nanoscale hot spots separated by a nanoscale distance inside the thin film may temporarily cause a higher temperature in some regions between the two hot spots than that predicted by the Fourier law.

Detection of a Nanoscale Hot Spot by Hot Carriers in a Poly-Si TFT Using Polydiacetylene-Based Thermoresponsive Fluorometry

The thermal distribution of polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) with sizes close to the nanoregime is analyzed by means of a fluorescent nanothermographic imaging technique based on the heat-induced fluorescence feature of polydiacetylene (PDA) supramolecules. The direct detection of nanoscale hot spots generated by hot carriers in poly-Si TFTs is demonstrated with sufficient spatial resolution. The thermal information of poly-Si TFTs under operation is recorded in a PDA-embedded polyvinyl alcohol film in the fluorescence state. The proposed thermal analysis method for poly-Si TFTs overcomes the fundamental spatial resolution limitation of conventional infrared detection systems and guarantees nanoscale spatial resolution. This approach, which offers cost effectiveness, nontoxicity, and simplicity of calibration steps, can be useful for further analysis of the degradation mechanism and reliability issues of submicrometer poly-Si TFTs.

Incidence of in situ annealing on the nanoscale topographical/electrical properties of the tunnel barrier in sputtered epitaxial Fe/MgO/Fe multilayers

Technological improvements in the magnetotransport performance of Fe/MgO/Fe stacks require nanoscale control over the topographical and electrical properties of the ultrathin MgO barrier. We have statistically investigated the incidence of in situ annealing of the lower Fe layer on the nanoscale topographical/electrical properties of Fe/MgO bilayers and the structural and magnetic properties of Fe/MgO/Fe/Co multilayers prepared by sputtering. This annealing step improves the crystal quality of both the lower Fe and the upper Fe/Co layers, leading to an enhanced saturated magnetic moment. Finally, this annealing step substantially mitigates the presence of nanohills on the lower Fe layer and improves the uniformity of the height and/or the thickness of the MgO tunnel barrier. Our results pave the way for studies of nanoscale transport on micrometre-sized devices through a better understanding of, and control over, nanoscale hotspots in the tunnel barrier.

Quasi-ballistic thermal transport from nanoscale interfaces observed using ultrafast coherent soft X-ray beams

Fourier theory of thermal transport considers heat transport as a diffusive process where energy flow is driven by a temperature gradient. However, this is not valid at length scales smaller than the mean free path for the energy carriers in a material, which can be hundreds of nanometres in crystalline materials at room temperature. In this case, heat flow will become 'ballistic'--driven by direct point-to-point transport of energy quanta. Past experiments have demonstrated size-dependent ballistic thermal transport through nanostructures such as thin films, superlattices, nanowires and carbon nanotubes. The Fourier law should also break down in the case of heat dissipation from a nanoscale heat source into the bulk. However, despite considerable theoretical discussion and direct application to thermal management in nanoelectronics, nano-enabled energy systems and nanomedicine, this non-Fourier heat dissipation has not been experimentally observed so far. Here, we report the first observation and quantitative measurements of this transition from diffusive to ballistic thermal transport from a nanoscale hotspot, finding a significant (as much as three times) decrease in energy transport away from the nanoscale heat source compared with Fourier-law predictions.

Experimental Investigation of Scaling Effect on Thermal Transport in Nanoscale Hot Spots

This article reports experimental investigation of the scaling effect on thermal transport in platinum nanoheaters deposited on a single crystalline silicon substrate. The size dependency of the thermal resistances of nanoheaters with dimensions from 5 μm down to 100 nm was experimentally obtained. Additional joule heating and electrical resistance thermometry in heater/sensor pairs of various dimensions and spacing (100 to 400 nm) were carried out in order to study the temperature distribution in the vicinity of these nanoheaters. The results of these two sets of measurements confirm that the thermal resistances of smaller nanoheaters are dominated by the interface resistance between the platinum and silicon substrate. Furthermore, analytical solutions of the BTE around a hot spot were used to evaluate and examine the significance of the localized heating or subcontinuum transport around a hot spot.

Lattice-Boltzmann finite-difference model with optical phonons for nanoscale thermal conduction

A recently developed multiscale model is used to study thermal conduction in silicon. In this work, the role of optical phonons is included in the nanoscale by introducing phonons with zero velocity in the lattice-Boltzmann domain. In the model, only the optical phonons are heated, and the energy transfer rate from optical to acoustic phonons is described with a relaxation time. As a test case, a nanoscale hot spot is introduced into the system, and thermal conduction to ambient medium is calculated. The results show a temperature step at the spot boundary, while elsewhere the results are identical to thermal diffusion. Optical phonons are seen to increase the spot boundary thermal resistance, which is also heavily dependent on the relaxation time of the optical-acoustic phonon scattering.

Chemically Patterned Microspheres for Controlled Nanoparticle Assembly in the Construction of SERS Hot Spots

An important challenge in developing ultra-sensitive surface enhanced Raman spectroscopy (SERS) platforms lies in the creation of nanoscale hot spots, locations where the electromagnetic field is greatly concentrated. Even when this is successful, finding these hot spots is a difficult task. We describe a novel chemical microsphere patterning technique utilizing contact masking during silanization for bonding silver nanoparticles preferentially to geometrically restricted sites. Small bifunctional linkers (i.e., <2 nm) are used to bind silver particles on different microspheres drawing the two together, thereby forming small nanoparticle aggregates containing one or more hot spots with the dimensions of the linker at the junction. The microspheres limit the extent of nanoparticle aggregation and are large enough to be visible by optical microscopy leading in most cases directly to the location of the hot spots and hence the most intense SERS signals.

Transient thermal modeling of a nanoscale hot spot in multilayered film

A subcontinuum based lattice Boltzmann method is used to accurately model the transient thermal response of a nanoscale hot spot in solids. We developed the numerical scheme for the hot spot in a thin uniform material and extended the approach to study the multilayered materials. We observed that subcontinuum effects of high temperature rise become more prominent as the size of the film reduces to the scale of carrier mean free path. The thermal transport through a double layer is also considered, both for constant temperature difference across the double layer and hot-spot generation in one of the layers, using the diffusive mismatch scattering model at the interface. A finite temperature jump is observed at the interface whose magnitude depends upon the dimensions and properties of the material on the either side of the interface. The insight into the nanoscale thermal modeling, acquired in this work via a relatively simple model, will be critical for the design and operation of complex data storage and electronic systems, dealing with subcontinuum systems.