Phononic topological insulators based on six-petal holey silicon structures.

Since the discovery of the Quantum Spin Hall Effect, electronic and photonic topological insulators have made substantial progress, but phononic topological insulators in solids have received relatively little attention due to challenges in realizing topological states without spin-like degrees of freedom and with transverse phonon polarizations. Here we present a holey silicon-based topological insulator design, in which simple geometric control enables topologically protected in-plane elastic wave propagation up to GHz ranges with a submicron periodicity. By integrating a hexagonal lattice of six small holes with one central large hole and by creating a hexagonal lattice by themselves, our design induces zone folding to form a double Dirac cone. Based on the hole dimensions, breaking the discrete translational symmetry allows the six-petal holey silicon to achieve the topological phase transition, yielding two topologically distinct phononic crystals. Our numerical simulations confirm inverted band structures and demonstrate backscattering-immune elastic wave transmissions through defects including a cavity, a disorder, and sharp bends. Our design also offers robustness against geometric errors and potential fabrication issues, which shows up to 90% transmission of elastic waves even with 6% under-sized or 11% over-sized holes. These findings provide a detailed understanding of the relationship between geometry and topological properties and pave the way for developing future phononic circuits.

TSV-integrated thermoelectric cooling by holey silicon for hot spot thermal management.

The trends toward higher power, higher frequency, and smaller scale electronics are making heat dissipation ever more challenging. Passive thermal management based on high thermal conductivity materials or through-silicon vias (TSVs) may not provide sufficient cooling for hot spots reaching 1 kW cm-2, and active thermal management by thermoelectric cooling (TEC) may require large power consumption or suffer from a large off-state thermal resistance of thermoelectric materials. Here we address these issues by integrating a holey silicon-based TEC with a TSV that directly draws heat from a hot spot to combine active and passive cooling approaches. Our simulations of the TSV-integrated TEC demonstrate exceptional cooling performance, which reduces the hot spot temperature from 154 °C to 68 °C while dissipating a heat flux of 1 k W cm-2 and consuming 0.5 W for TEC operation. The off-state hot spot temperature, 154 °C, is 24 °C lower than that of the same TEC with no TSV, and the on-state hot spot temperature, 68 °C, is 67 °C lower than that of the same TEC with no TSV. We also investigate the cooling prospects of metal-filled holey silicon by modeling the electron-phonon coupling and size dependent transport phenomena, which can further increase the thermal conductivity anisotropy and improve the TEC performance depending on the metal-to-silicon interfacial resistance. These results show the combined passive and active cooling in TSV-integrated TEC offers effective hot spot thermal management solutions for advanced electronics.

Thermal conductivity anisotropy in holey silicon nanostructures and its impact on thermoelectric cooling.

Artificial nanostructures have improved prospects of thermoelectric systems by enabling selective scattering of phonons and demonstrating significant thermal conductivity reductions. While the low thermal conductivity provides necessary temperature gradients for thermoelectric conversion, the heat generation is detrimental to electronic systems where high thermal conductivity are preferred. The contrasting needs of thermal conductivity are evident in thermoelectric cooling systems, which call for a fundamental breakthrough. Here we show a silicon nanostructure with vertically etched holes, or holey silicon, uniquely combines the low thermal conductivity in the in-plane direction and the high thermal conductivity in the cross-plane direction, and that the anisotropy is ideal for lateral thermoelectric cooling. The low in-plane thermal conductivity due to substantial phonon boundary scattering in small necks sustains large temperature gradients for lateral Peltier junctions. The high cross-plane thermal conductivity due to persistent long-wavelength phonons effectively dissipates heat from a hot spot to the on-chip cooling system. Our scaling analysis based on spectral phonon properties captures the anisotropic size effects in holey silicon and predicts the thermal conductivity anisotropy ratio up to 20. Our numerical simulations demonstrate the thermoelectric cooling effectiveness of holey silicon is at least 30% greater than that of high-thermal-conductivity bulk silicon and 400% greater than that of low-thermal-conductivity chalcogenides; these results contrast with the conventional perception preferring either high or low thermal conductivity materials. The thermal conductivity anisotropy is even more favorable in laterally confined systems and will provide effective thermal management solutions for advanced electronics.

Hydrogen bonded and ionically crosslinked high strength hydrogels exhibiting Ca2+-triggered shape memory properties and volume shrinkage for cell detachment.

A hydrogen bonded and calcium ion crosslinked hydrogel, termed PVDT-PAA, was synthesized by one-step photo-polymerization of 2-vinyl-4,6-diamino-1,3,5-triazine (VDT), acrylic acid (AA), and polyethylene glycol diacrylate (PEGDA, Mn = 4000). Combined physical crosslinking from inter-diaminotriazine and coordination of Ca2+ with carboxyls contributed to a significant enhancement in the mechanical properties of the PVDT-PAA hydrogels. Furthermore, reversible Ca2+ crosslinking imparted shape memory properties to the hydrogel allowing it to firmly memorize multiform shapes and return to its initial state in response to Ca2+. Interestingly, PVDT-PAA hydrogels with weaker H-bonding interactions demonstrated a sharp volume change phenomenon induced by Ca2+. This volume change could be utilized to trigger unharmful cell detachment from the hydrogel surface, which was thought to be due to Ca2+-induced marked variation in mechanotransduction between the cells and the substrate interface. This H-bonding and ionic crosslinking strategy opens up a new opportunity for designing and constructing multifunctional high strength hydrogels for biomedical applications.