A passive turnoff Passive radiative cooling technology uses the infrared atmospheric window to allow outer space to be a cold sink for heat. However, this effect is one that is only helpful for energy savings in the warmer months. Wang et al . and Tang et al . used the metal-insulator transition in tungsten-doped vanadium dioxide to create window glass and a rooftop coating that circumvents this problem by turning off the radiative cooling at lower temperatures. Because the transition is simply temperature dependent, this effect also happens passively. Model simulations suggest that these materials would lead to energy savings year-round across most of the climate zones in the United States. —BG

Room-temperature optoelectronic devices that operate at shortwave and midwave infrared wavelengths (1-8 μm) can be used for numerous applications 1-5 . To achieve the operating wavelength range needed for a given application, a combination of materials with different bandgaps (e.g. superlattice/heterostructure) 6,7 or the variation of semiconductor alloy composition during growth 8,9 is used; however, these approaches involve fabrication complexity and the operating range is fixed post-fabrication. Although wide-range, active, and reversible tunability of the operating wavelengths in optoelectronic devices after fabrication is a highly desirable feature, no such platform has been yet developed. Here, we demonstrate high-performance room-temperature infrared optoelectronics with actively variable spectra by presenting black phosphorus (bP) as an ideal candidate. Enabled by the * � � 𝐸𝐸 𝑔𝑔 𝑘𝑘 𝐵𝐵 𝑇𝑇 �� (2)where 𝑚𝑚 𝑒𝑒 * and 𝑚𝑚 ℎ * are the effective masses of electrons and holes, respectively, 𝑘𝑘 𝐵𝐵 is Boltzmann's constant, and 𝑇𝑇 is temperature 36,37 . Since 𝑚𝑚 𝑒𝑒 * and 𝑚𝑚 ℎ * in bP have similar values, the effective mass ratio (𝑚𝑚 𝑒𝑒 * /𝑚𝑚 ℎ * ) is much higher than that of other small bandgap semiconductors.According to equation (2), this results in suppressed Auger recombination (longer Auger lifetime), which leads to bP's theoretical QY limit being much higher than that of other small bandgap semiconductors in the high injection regime.

The critical size limit of voltage-switchable electric dipoles has extensive implications for energy-efficient electronics, underlying the importance of ferroelectric order stabilized at reduced dimensionality. We report on the thickness-dependent antiferroelectric-to-ferroelectric phase transition in zirconium dioxide (ZrO 2 ) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO 2 , conventionally a paraelectric material, notably persists down to a film thickness of 5 angstroms, the fluorite-structure unit-cell size. This approach to exploit three-dimensional centrosymmetric materials deposited down to the two-dimensional thickness limit, particularly within this model fluorite-structure system that possesses unconventional ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale nonvolatile ferroelectric memory on silicon. Additionally, it is also indicative of hidden electronic phenomena that are achievable across a wide class of simple binary materials.

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In this study, a hierarchical TiO2 nanostructure with densely-packed and omnidirectional branches grown by a hydrothermal method is introduced. This morphology is achieved via high-concentration TiCl4 treatment of upright backbone nanowires (NWs) followed by hydrothermal growth. Secondary nanobranches grow in all directions from densely distributed, needle-like seeds on the jagged round surface of the backbone NWs. In addition, hierarchical, flower-like branches grow on the top surface of each NW, greatly increasing the surface area. For dye-sensitized solar cell (DSSC) applications, the TiO2 nanostructure demonstrated a photoconversion efficiency of up to 6.2%. A parametric study of the DSSC efficiency showed that branched TiO2 DSSCs can achieve nearly four times the efficiency of non-branched TiO2 nanowire DSSCs, and up to 170% the efficiency of previously-reported sparsely-branched TiO2 NW DSSCs.

We develop a digital direct writing method for ZnO NW micro-patterned growth on a large scale by selective laser decomposition of zinc acetate. For ZnO NW growth, by replacing the bulk heating with the scanning focused laser as a fully digital local heat source, zinc acetate crystallites can be selectively activated as a ZnO seed pattern to grow ZnO nanowires locally on a larger area. Together with the selective laser sintering process of metal nanoparticles, more than 10,000 UV sensors have been demonstrated on a 4 cm × 4 cm glass substrate to develop all-solution processible, all-laser mask-less digital fabrication of electronic devices including active layer and metal electrodes without any conventional vacuum deposition, photolithographic process, premade mask, high temperature and vacuum environment.

The new physics of magic-angle twisted bilayer graphene (TBG) motivated extensive studies of flat bands hosted by moiré superlattices in van der Waals structures, inspiring the investigations into their photonic counterparts with potential applications including Bose-Einstein condensation. However, correlation between photonic flat bands and bilayer photonic moire ´ systems remains unexplored, impeding further development of moire ´ photonics. In this work, we formulate a coupled-mode theory for low-angle twisted bilayer honeycomb photonic crystals as a close analogy of TBG, discovering magic-angle photonic flat bands with a non-Anderson-type localization. Moreover, the interlayer separation constitutes a convenient degree of freedom in tuning photonic moire ´ bands without high pressure. A phase diagram is constructed to correlate the twist angle and separation dependencies to the photonic magic angles. Our findings reveal a salient correspondence between fermionic and bosonic moiré systems and pave the avenue toward novel applications through advanced photonic band or state engineering.

In this study, we perform drop-on-demand (DOD) inkjet printing and laser reductive sintering of precrystallized NiO nanoparticle (NP) ink under ambient conditions to obtain NiO/Ni hybrid electrode patterns on a highly localized area. By formulating an inkjet-printable and laser-reducible NiO NP ink, and by exploring the optimum conditions of inkjet printing parameters, we generate stable droplets, enabling arbitrary shapes of NiO NP dot arrays or line patterns to be deposited. Subsequent short-time low-temperature sintering produces highly crystalline NiO electrodes. Furthermore, laser reductive sintering applied on deposited NiO NP patterns can successfully realize a selective transformation of NiO into Ni electrodes under ambient conditions. Therefore, we can define either NiO or Ni electrodes, or a combination of the two on specific areas with precise amounts of ink. In addition, we identify the characteristics of the synthesized NPs, NP ink, NiO and Ni electrodes using various analytical methods.