Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

Some 3dn (4 ≤ n ≤ 7) transition metal compounds exhibit a cooperative transition between a low-spin (LS) and a high-spin (HS) state. This transition is abrupt and occurs with a thermal hysteresis, which confers a memory effect on the system. The intersite interactions and thus the cooperativity are magnified in polymeric compounds such as [Fe(Rtrz)3]A2·nH2O in which the Fe2+ ions are triply bridged by 4-R-substituted-1,2,4-triazole molecules. Moreover, in these compounds, the spin transition is accompanied by a well-pronounced change of color between violet in the LS state and white in the HS state. The transition temperatures of these materials can be fine tuned, using an approach based on the concept of a molecular alloy. In particular, it is possible to design a compound for which room temperature falls in the middle of the thermal hysteresis loop. These materials have many potential applications, for example, as temperature sensors, as active elements of various types of displays, and in information storage and retrieval.

https://science.sciencemag.org/content/sci/279/5347/44.full.pdf

Spin-Transition Polymers: From Molecular Materials Toward Memory Devices

Transparent conductors as solar energy materials: A panoramic review

Electrochromic (EC) materials are able to change their optical properties, reversibly and persistently, by the application of an electrical voltage. These materials can be integrated in multilayer devices capable of modulating the optical transmittance between widely separated extrema. We first review the recent literature on inorganic EC materials and point out that today's research is focused on tungsten oxide (colouring under charge insertion) and nickel oxide (colouring under charge extraction). The properties of thin films of these materials are then discussed in detail with foci on recent results from two comprehensive investigations in the authors' laboratory. A logical exposition is obtained by covering, in sequence, structural features, thin film deposition (by sputtering), electronic band structure, and ion diffusion. A novel conceptual model is given for structural characteristics of amorphous W oxide films, based on notions of defects in the ideal amorphous state. It is also shown that the conduction band density of states is obtainable from simple electrochemical chronopotentiometry. Ion intercalation causes the charge-compensating electrons to enter localized states, implying that the optical absorption underlying the electrochromism can be described as ensuing from transitions between occupied and empty localized conduction band states. A fully quantitative theory of such transitions is not available, but the optical absorption can be modeled more phenomenologically as due to a superposition of transitions between different charge states of the W ions (6+, 5+, and 4+). The Ni oxide films were found to have a porous structure comprised of small grains. The data are consistent with EC coloration being a surface phenomenon, most likely confined to the outer parts of the grains. Initial electrochemical cycling was found to transform hydrated Ni oxide into hydroxide and oxy-hydroxide phases on the grain surfaces. Electrochromism in thus stabilized films is consistent with reversible changes between Ni hydroxide and oxy-hydroxide, in accordance with the Bode reaction scheme. An extension of this model is put forward to account for changes of NiO to Ni2O3. It was demonstrated that electrochromism is associated solely with proton transfer. Data on chemical diffusion coefficients are interpreted for polycrystalline W oxide and Ni oxide in terms of the lattice gas model with interaction. The later part of this review is of a more technological and applications oriented character and is based on the fact that EC devices with large optical modulation can be accomplished essentially by connecting W-oxide-based and Ni-oxide-based films through a layer serving as a pure ion conductor. Specifically, we treat methods to enhance the bleached-state transmittance by mixing the Ni oxide with other oxides characterized by wide band gaps, and we also discuss pre-assembly charge insertion and extraction by facile gas treatments of the films, as well as practical device manufacturing and device testing. Here the emphasis is on novel flexible polyester-foil-based devices. The final part deals with applications with emphasis on architectural “smart” windows capable of achieving improved indoor comfort jointly with significant energy savings due to lowered demands for space cooling. Eyewear applications are touched upon as well.

/pdf/electrochromics-for-smart-windows-thin-films-of-tungsten-aw0rai8h60.pdf

Electrochromics for smart windows: thin films of tungsten oxide and nickel oxide, and devices based on these

The magnetic properties of many magnetic materials can be controlled by external stimuli. The principal focus here is on the thermal, photochemical, electrochemical, and chemical control of phase transitions that involve changes in magnetization. The molecular compounds described herein range from metal complexes, through pure organic compounds to composite materials. Most of the Review is devoted to the properties of valence-tautomeric compounds, molecular magnets, and spin-crossover complexes, which could find future application in memory devices or optical switches.

Control of magnetic properties through external stimuli.

Electrochromic degradation in nickel oxide thin film: A self-discharge and dissolution phenomenon

[Cu(hyetrz)3](ClO4)2·3H2O (hyetrz = 4-(2-hydroxyethyl)-1,2,4-triazole) represents the first structurally characterized metal(II) chain compound containing triple N1,N2-1,2,4-triazole bridges. The structure has been solved at 298 K by single-crystal X-ray analysis. catena-Poly[μ-tris(4-(2-hydroxyethyl)-1,2,4-triazole-N1,N2)copper(II)] diperchlorate trihydrate (C12H27N9Cl2O14Cu) crystallizes in the monoclinic space group P21/n, a = 13.877(3) A, b = 23.023(5) A, c = 15.351(2) A, β = 91.10(2)°, Z = 8 (Cu(II) units). The Cu(II) ions are linked by triple N1,N2-1,2,4-triazole bridges yielding a slightly alternating chain with Cu1−Cu2 = 3.853(2) A and Cu2−Cu3 = 3.829(2) A. The EXAFS results are consistent with the crystal structure. At 30 K, the EXAFS signature of the multiple scattering path Cu1−Cu2−Cu3 is clearly observed, confirming that such exceptional long distance EXAFS signals can be used to detect metal alignments in inorganic long chains when crystal structures are not available. The thermal behavior of...

Synthesis, crystal structure, EXAFS and magnetic properties of Catena [mu-tris(4-(2'-hydroxy-ethyl)-1,2,4-triazole-N1,N2) copper(II)] bis (perchlorate) trihydrate. Relevance with the structure of the Fe(II) 1,2,4-triazole spin transition molecular materials

EXAFS and X-ray powder diffraction studies of the spin transition molecular materials [Fe(Htrz)2(trz)](BF4) and [Fe(Htrz)3](BF4)2.H2O (Htrz = 1,2,4-4H-triazole; trz = 1,2,4-triazolato)

EXAFS structure and magnetic properties of a CuIINiII .mu.-oxalato mixed linear chain

http://www.cmdmc.com.br/redecmdmc/lab/arquivos_publicacoes/2628_Intense%20blue%20and%20green%20photoluminescence%20emissions%20at%20room%20temperature.pdf

Alain Michalowicz

Papers

Electrochromic degradation in nickel oxide thin film: A self-discharge and dissolution phenomenon

Synthesis, crystal structure, EXAFS and magnetic properties of Catena [mu-tris(4-(2'-hydroxy-ethyl)-1,2,4-triazole-N1,N2) copper(II)] bis (perchlorate) trihydrate. Relevance with the structure of the Fe(II) 1,2,4-triazole spin transition molecular materials

EXAFS and X-ray powder diffraction studies of the spin transition molecular materials [Fe(Htrz)2(trz)](BF4) and [Fe(Htrz)3](BF4)2.H2O (Htrz = 1,2,4-4H-triazole; trz = 1,2,4-triazolato)

EXAFS structure and magnetic properties of a CuIINiII .mu.-oxalato mixed linear chain

Intense blue and green photoluminescence emissions at room temperature in barium zirconate powders