Materials synthesis in the liquid phase, or wet-chemical synthesis, utilizes a solution medium in which the target materials are generated from a series of chemical and physical transformations. Although this route is central in organic chemistry, for materials synthesis the low operational temperature range of the solvent (usually below 200 °C, in extreme 350 °C) is a serious restriction. Here, salt melt synthesis (SMS) which employs a molten inorganic salt as the medium emerges as an important complementary route to conventional liquid phase synthesis. Depending on the nature of the salt, the operational temperature ranges from near 100 °C to over 1000 °C, thus allowing the access to a broad range of inorganic crystalline materials and carbons. The recent progress in SMS of inorganic materials, including oxide ceramic powders, semiconductors and carbon nanostructures, is reviewed here. We will introduce in general the range of accessible materials by SMS from oxides to non-oxides, and discuss in detail based on selected examples the mechanisms of structural evolution and the influence of synthetic conditions for certain materials. In the later sections we also present the recent developments in SMS for the synthesis of organic solids: covalent frameworks and polymeric semiconductors. Throughout this review, special emphasis is placed on materials with nanostructures generated by SMS, and the possible modulation of materials structures at the nanoscale in the salt melt. The review is finalized with the summary of the current achievements and problems, and suggestions for potential future directions in SMS.

Low-dimensional (LD) materials demonstrate intriguing optical properties, which lead to applications in diverse fields, such as photonics, biomedicine and energy. Due to modulation of electronic structure by the reduced structural dimensionality, LD versions of metal, semiconductor and topological insulators (TIs) at the same time bear distinct nonlinear optical (NLO) properties as compared with their bulk counterparts. Their interaction with short pulse laser excitation exhibits a strong nonlinear character manifested by NLO absorption, giving rise to optical limiting or saturated absorption associated with excited state absorption and Pauli blocking in different materials. In particular, the saturable absorption of these emerging LD materials including two-dimensional semiconductors as well as colloidal TI nanoparticles has recently been utilized for Q-switching and mode-locking ultra-short pulse generation across the visible, near infrared and middle infrared wavelength regions. Beside the large operation bandwidth, these ultrafast photonics applications are especially benefit from the high recovery rate as well as the facile processibility of these LD materials. The prominent NLO response of these LD materials have also provided new avenues for the development of novel NLO and photonics devices for all-optical control as well as optical circuits beyond ultrafast lasers.

Efficient synthetic routes are continuously pursued for graphene in order to implement its applications in different areas. However, direct conversion of simple monomers to graphene through polymerization in a scalable manner remains a major challenge for chemists. Herein, a molten-salt (MS) route for the synthesis of carbon nanostructures and graphene by controlled carbonization of glucose in molten metal chloride is reported. In this process, carbohydrate undergoes polymerization in the presence of strongly interacting ionic species, which leads to nanoporous carbon with amorphous nature and adjustable pore size. At a low precursor concentration, the process converts the sugar molecules (glucose) to rather pure few-layer graphenes. The MS-derived graphenes are strongly hydrophobic and exhibit remarkable selectivity and capacity for absorption of organics. The methodology described may open up a new avenue towards the synthesis and manipulation of carbon materials in liquid media.

Inspired by black powder explosion, a mixture of sugar and nitrate is diluted with inert molten salt, such that their rapid reaction is moderated, allowing capturing of the intermediacy product: highly porous N-doped graphene nanoplates. The methodology is extended to S-doped porous carbon sheets by replacing the nitrate with sulfate. These MS-derived carbons show promising performances towards electrocatalysis and capacitive energy storage.

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Transfer of energy occurs endlessly in our universe by means of radiation. Compared to energy transfer (ET) in free space, in solid state materials the transfer of energy occurs in a rather confined manner, which is usually mediated by real or virtual particles, including not only photons, but also electrons, phonons, and excitons. In the present review, we discuss the recent advances in optical ET by resonance mediated with photons in solid materials as well as their nanoscale counterparts, with focus on the photoluminescence behavior pertaining to ET between optically active centers, such as rare earth (RE) ions. This review begins with a brief discussion on the classification of optical ET together with an overview of the theoretical formulations and experimental method for the examination of ET. We will then present a comprehensive discussion on the ET in practical systems in which normal photoluminescence, upconversion and quantum cutting resulted from ET involving metal ions, QDs, organic species, 2D materials and plasmonic nanostructures. Diverse ET systems are therefore simply categorized into cases of ion-ion interactions and non-ion interactions. Special attention has been paid to the progress in the manipulation of spatially confined ET in nanostructured systems including core-shell structures, as well as the ET in multiple exciton generation found in QDs and organic molecules, which behave quite similarly to resonance ET between metal ion centers. Afterwards, we will discuss the broad spectrum of applications of ET in the aforementioned systems, including solid state lighting, solar energy utilization, bio-imaging and diagnosis, and sensing. In the closing part, along with a short summary, we discuss further research focus regarding the problems and possible future directions of optical ET in solids.

We report that non-contact self-referencing temperature sensors can be realized with the use of core-shell nanostructures. These lanthanide-based nanothermometers (NaGdF4:Yb(3+)/Tm(3+)@Tb(3+)/Eu(3+)) exhibit higher sensitivity in a wide range from 125 to 300 K based on two emissions of Tb(3+) at 545 nm and Eu(3+) at 615 nm under near-infrared laser excitation.

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