Colloidal inorganic nano- and microparticles for passive daytime radiative cooling.

Compared to traditional cooling systems, radiative cooling (RC) is a promising cooling strategy in terms of reducing energy consumption enormously and avoiding severe environmental issues. Radiative cooling materials (RCMs) reduce the temperature of objects without using an external energy supply by dissipating thermal energy via infrared (IR) radiation into the cold outer space through the atmospheric window. Therefore, RC has a great potential for various applications, such as energy-saving buildings, vehicles, water harvesting, solar cells, and personal thermal management. Herein, we review the recent progress in the applications of inorganic nanoparticles (NPs) and microparticles (MPs) as RCMs and provide insights for further development of RC technology. Particle-based RCMs have tremendous potential owing to the ease of engineering their optical and physical properties, as well as processibility for facile, inexpensive, and large area deposition. The optical and physical properties of inorganic NPs and MPs can be tuned easily by changing their size, shape, composition, and crystals structures. This feature allows particle-based RCMs to fulfill requirements pertaining to passive daytime radiative cooling (PDRC), which requires high reflectivity in the solar spectrum and high emissivity within the atmospheric window. By adjusting the structures and compositions of colloidal inorganic particles, they can be utilized to design a thermal radiator with a selective emission spectrum at wavelengths of 8-13 µm, which is preferable for PDRC. In addition, colloidal particles can exhibit high reflectivity in the solar spectrum through Mie-scattering, which can be further engineered by modifying the compositions and structures of colloidal particles. Recent advances in PDRC that utilize inorganic NPs and MPs are summarized and discussed together with various materials, structural designs, and optical properties. Subsequently, we discuss the integration of functional NPs to achieve functional RCMs. We describe various approaches to the design of colored RCMs including structural colors, plasmonics, and luminescent wavelength conversion. In addition, we further describe experimental approaches to realize self-adaptive RC by incorporating phase-change materials and to fabricate multifunctional RC devices by using a combination of functional NPs and MPs.

Dynamically tunable multicolor emissions from zero-dimensional Cs3LnCl6 (Ln: europium and terbium) nanocrystals with wide color gamut.

This study demonstrates dynamically tunable multicolor emissions from a single component, zero-dimensional (0-D) cesium europium chloride (Cs3EuCl6) and cesium terbium chloride (Cs3TbCl6) nanocrystals (NCs). Highly uniform colloidal Cs3EuCl6 and Cs3TbCl6 NCs are synthesized via the heating-up method. Excitation-wavelength-dependent multicolor emissions from Cs3EuCl6 and Cs3TbCl6 NCs are observed. Under excitation of 330-400 nm, both NCs exhibit blue photoluminescence (PL). Under wavelengths shorter than 330 nm, characteristic red and green emissions are observed from Cs3EuCl6 and Cs3TbCl6, respectively, owing to the atomic emissions from the f-orbitals in trivalent europium (Eu3+) and terbium (Tb3+) ions. Cs3EuCl6 and Cs3TbCl6 NCs exhibit broadband excitation spectra and enhanced absorption properties. Particularly, Cs3EuCl6 NCs exhibit a very narrow full-width at half-maximum in both blue and red PL and no overlap between the two spectra. The photophysical properties of these NCs are further investigated to understand the multicolor PL origins by time-resolved and temperature-dependent PL measurements. Finally, the potential applications of Cs3EuCl6 and Cs3TbCl6 NCs as anti-counterfeiting inks for high-level security are demonstrated. Given their broadband excitation with enhanced absorption properties and dynamically tunable colors with a wide color gamut, Cs3EuCl6 and Cs3TbCl6 NCs have great potential as novel multicolor NC emitters for many emerging applications.

N,N-Dimethylformamide-Assisted Shape Evolution of Highly Uniform and Shape-Pure Colloidal Copper Nanocrystals.

In this paper, the N,N-dimethylformamide (DMF)-assisted shape evolution of highly uniform and shape-pure copper nanocrystals (Cu NCs) is presented for the first time. Colloidal Cu NCs are synthesized via the disproportionation reaction of copper (I) bromide in the presence of a non-polar solvent mixture. It is observed that the shape of Cu NCs is systematically controlled by the addition of different amounts of DMF to the reaction mixture in high-temperature reaction conditions while maintaining a high size uniformity and shape purity. With increasing amount of DMF in the reaction mixture, the morphology of the Cu NCs change from a cube enclosed by six {100} facets, to a sphere with mixed surface facets, and finally, to an octahedron enclosed by eight {111} facets. The origin of this shape evolution is understood via first-principles density functional theory calculations, which allows the study of the change in the relative surface stability according to surface-coordinating adsorbates. Further, the shape-dependent plasmonic properties are systematically investigated with highly uniform and ligand-exchanged colloidal Cu NCs dispersed in acetonitrile. Finally, the facet-dependent electrocatalytic activities of the shape-controlled Cu NCs are investigated to reveal the activities of the highly uniform and shape-pure Cu NCs in the methanol oxidation reaction.

Sub-100-nm Nearly Monodisperse n-Paraffin/PMMA Phase Change Nanobeads.

In this study, we demonstrate the colloidal synthesis of nearly monodisperse, sub-100-nm phase change material (PCM) nanobeads with an organic n-paraffin core and poly(methylmethacrylate) (PMMA) shell. PCM nanobeads are synthesized via emulsion polymerization using ammonium persulfate as an initiator and sodium dodecylbenzenesulfonate as a surfactant. The highly uniform n-paraffin/PMMA PCM nanobeads are sub-100 nm in size and exhibit superior colloidal stability. Furthermore, the n-paraffin/PMMA PCM nanobeads exhibit reversible phase transition behaviors during the n-paraffin melting and solidification processes. During the solidification process, multiple peaks with relatively reduced phase change temperatures are observed, which are related to the phase transition of n-paraffin in the confined structure of the PMMA nanobeads. The phase change temperatures are further tailored by changing the carbon length of n-paraffin while maintaining the size uniformity of the PCM nanobeads. Sub-100-nm-sized and nearly monodisperse PCM nanobeads can be potentially utilized in thermal energy storage and drug delivery because of their high colloidal stability and solution processability.

Post-synthetic oriented attachment of CsPbBr3 perovskite nanocrystal building blocks: from first principle calculation to experimental demonstration of size and dimensionality (0D/1D/2D).

Post-synthesis engineering methods that employ oriented attachment to precisely control the size and dimensionality (0D/1D/2D) of as-synthesized CsPbBr3 nanocrystals (NCs) are demonstrated. We investigated the chemical effects of the properties of polar solvents, including their immiscibility, polarity, and boiling point, on the surfaces of NCs, as well as their effect on the structural and optical properties of NCs. Appropriate exploitation of the solvent properties made it possible to use a polar solvent to mildly affect the NCs indirectly such that they discarded their ligands and became attached to proximal NCs without being destroyed. Based on our observations, we developed a method whereby a solution of the NCs in a non-polar solvent is mixed with a polar solvent to form an immiscible phase to induce epitaxial growth of CsPbBr3 NCs. The method enables the size of NCs to be easily regulated from 5 to 50 nm by controlling the engineering time. Taking advantage of the minimal effect of a mild solvent, we also developed a self-assembly method that operates at the liquid-air interface to systematically control the dimensionality. At this interface, the NCs self-assemble in the horizontal direction and grow into micron-sized, single-crystalline, defect-free nanowires (1D) and nanoplates (2D) via oriented attachment. Finally, we discuss the origin of the non-destructive oriented attachment phenomenon and the surface chemistry of a perovskite NC using density functional theory (DFT) simulations and a proposed model system.

Heating-up synthesis of cesium bismuth bromide perovskite nanocrystals with tailored composition, morphology, and optical properties.

This study represents the heating-up synthesis of lead-free cesium bismuth bromide perovskite nanocrystals (NCs). CsBr and BiBr3 precursors are used to synthesize uniform and phase-pure cesium bismuth bromide NCs, and the reaction is performed via an injection-free, heating-up method in the presence of a solvent mixture with a high boiling point. The size and composition of cesium bismuth bromide NCs are readily controlled by changing the reaction time, temperature, and amount of surfactant added to the reaction mixture. Upon heating, sequential phase evolution occurs, resulting in the formation of kinetically stable BiOBr in the early reaction stages, which transformed into the thermodynamically stable Cs3BiBr6 and Cs3Bi2Br9 with an increase in either the reaction time or the reaction temperature. Furthermore, the absorption and photoluminescence properties of Cs3BiBr6 and Cs3Bi2Br9 NCs are characterized to investigate their composition-dependent optical properties. This work provides the potential to synthesize various types of lead-free perovskite NCs by tailoring the size and compositions.

Nanobiosensing Platforms for Real-Time and Non-Invasive Monitoring of Stem Cell Pluripotency and Differentiation.

Breakthroughs in the biomedical and regenerative therapy fields have led to the influential ability of stem cells to differentiate into specific types of cells that enable the replacement of injured tissues/organs in the human body. Non-destructive identification of stem cell differentiation is highly necessary to avoid losses of differentiated cells, because most of the techniques generally used as confirmation tools for the successful differentiation of stem cells can result in valuable cells becoming irrecoverable. Regarding this issue, recent studies reported that both Raman spectroscopy and electrochemical sensing possess excellent characteristics for monitoring the behavior of stem cells, including differentiation. In this review, we focus on numerous studies that have investigated the detection of stem cell pluripotency and differentiation in non-invasive and non-destructive manner, mainly by using the Raman and electrochemical methods. Through this review, we present information that could provide scientific or technical motivation to employ or further develop these two techniques for stem cell research and its application.