Crystal Structure of Colloidally Prepared Metastable Ag2Se Nanocrystals

A Recipe for a Great Meal: A Benchtop Route from Elemental Se to Superior Thermoelectric β-Ag2Se

The low-temperature modification of β-Ag2Se has proven to be useful as a near-room-temperature thermoelectric material. Over the past years, research has been devoted to interstitial, vacancy, and substitutional doping into the parent β-Ag2Se structure, aiming at tuning the material's charge and heat transport properties to enhance thermoelectric performance. The transformation of β-Ag2Se into α-Ag2Se at ∼134 °C and the low solubility of dopants are the main obstacles for the doping approach. Herein, we report a facile, safe, scalable, and cost-effective benchtop approach to successfully produce metal-doped β-Ag2Se. The doped materials display a remarkable enhancement of thermoelectric performance with a record-high peak zT of 1.30 at 120 °C and an average zT of ∼1.15 in the 25-120 °C range for 0.2 at. % Zn-doped Ag2Se. The enhancement in zT is attributed to point defects created by Zn doping into Ag vacancies/interstitials, which enhances the scattering of phonons and tunes the charge carrier properties, leading to the significant suppression of thermal conductivity. The simplicity of the synthetic method developed herein and the high performance of the final products provide an avenue to produce high-quality Ag2Se-based thermoelectric materials.

Colloidal Ag2Se intraband quantum dots

With the emergence of the Internet of Things, wearable electronics, and machine vision, the exponentially growing demands for miniaturization, energy efficiency, and cost-effectiveness have imposed critical requirements on the size, weight, power consumption and cost (SWaP-C) of infrared detectors. To meet this demand, new sensor technologies that can reduce the fabrication cost associated with semiconductor epitaxy and remove the stringent requirement for cryogenic cooling are under active investigation. In the technologically important spectral region of mid-wavelength infrared, intraband colloidal quantum dots are currently at the forefront of this endeavor, with wafer-scale monolithic integration and Auger suppression being the key material capabilities to minimize the sensor's SWaP-C. In this Feature Article, we provide a focused review on the development of sensors based on Ag2Se intraband colloidal quantum dots, a heavy metal-free colloidal nanomaterial that has merits for wide-scale adoption in consumer and industrial sectors.

Strain and Surface Engineering of Multicomponent Metallic Nanomaterials with Unconventional Phases

Multicomponent metallic nanomaterials with unconventional phases show great prospects in electrochemical energy storage and conversion, owing to unique crystal structures and abundant structural effects. In this review, we emphasize the progress in the strain and surface engineering of these novel nanomaterials. We start with a brief introduction of the structural configurations of these materials, based on the interaction types between the components. Next, the fundamentals of strain, strain effect in relevant metallic nanomaterials with unconventional phases, and their formation mechanisms are discussed. Then the progress in surface engineering of these multicomponent metallic nanomaterials is demonstrated from the aspects of morphology control, crystallinity control, surface modification, and surface reconstruction. Moreover, the applications of the strain- and surface-engineered unconventional nanomaterials mainly in electrocatalysis are also introduced, where in addition to the catalytic performance, the structure-performance correlations are highlighted. Finally, the challenges and opportunities in this promising field are prospected.

Halide-driven polymorph selectivity in the synthesis of MnX (X = S, Se) nanoparticles

Devising synthetic strategies to control crystal structure is of great importance as materials properties are governed by structure. MnS is a great model system as it has three known stable polymorphs. Herein, we show the selective synthesis of colloidal wurtzite- and rock-salt-type MnS under identical reactions conditions changing only the manganese halide precursor. Mixtures of Mn halides or halide surrogate (e.g., NH₄Cl) also enabled polymorph control. Powder X-ray diffraction aliquot studies of the reactions revealed the crystal structure at the onset of nucleation and that of the final product is the same, unlike the Ostwald ripening transformation observed in other systems. The halide-driven selectivity was also observed in the synthesis of manganese selenide nanoparticles. In this system, variation of the Mn halide precursor allowed access to the wurtzite- and rock salt-type polymorphs of MnSe, as well as the pyrite-MnSe₂ phase. Based on this work, the mixing of metal salts might be a simple and effective strategy towards polymorph control and access materials with new crystal structures.

Thermoelectric Silver-Based Chalcogenides

Heat is abundantly available from various sources including solar irradiation, geothermal energy, industrial processes, automobile exhausts, and from the human body and other living beings. However, these heat sources are often overlooked despite their abundance, and their potential applications remain underdeveloped. In recent years, important progress has been made in the development of high-performance thermoelectric materials, which have been extensively studied at medium and high temperatures, but less so at near room temperature. Silver-based chalcogenides have gained much attention as near room temperature thermoelectric materials, and they are anticipated to catalyze tremendous growth in energy harvesting for advancing internet of things appliances, self-powered wearable medical systems, and self-powered wearable intelligent devices. This review encompasses the recent advancements of thermoelectric silver-based chalcogenides including binary and multinary compounds, as well as their hybrids and composites. Emphasis is placed on strategic approaches which improve the value of the figure of merit for better thermoelectric performance at near room temperature via engineering material size, shape, composition, bandgap, etc. This review also describes the potential of thermoelectric materials for applications including self-powering wearable devices created by different approaches. Lastly, the underlying challenges and perspectives on the future development of thermoelectric materials are discussed.

Tailoring the phase transition of silver selenide at the atomistic scale

Thermoelectric materials provide promising solutions for energy harvesting from the environment. Silver selenide (Ag₂Se) material attracts much attention due to its excellent thermoelectric properties under superionic phase transition. However, the optimal thermoelectric figure of merit occurs during the phase transition at high temperatures, making low-temperature devices unable to benefit from their best thermoelectric performance. Here, we tailored the phase transition process of Ag₂Se materials with various sizes, and probed the phase transition temperature by in situ transmission electron microscopy. By tuning the motion of the atoms near the surface using size-dependent surface energy, the phase transition-induced process is tailored towards low temperatures. This work paves the way for future phase transition engineering to enhance thermoelectric performance.

Synthesis, Properties and Bioimaging Applications of Silver-Based Quantum Dots

Ag-based quantum dots (QDs) are semiconductor nanomaterials with exclusive electrooptical properties ideally adaptable for various biotechnological, chemical, and medical applications. Silver-based semiconductor nanocrystals have developed rapidly over the past decades. They have become a promising luminescent functional material for in vivo and in vitro fluorescent studies due to their ability to emit at the near-infrared (NIR) wavelength. In this review, we discuss the basic features of Ag-based QDs, the current status of classic (chemical) and novel methods ("green" synthesis) used to produce these QDs. Additionally, the advantages of using such organisms as bacteria, actinomycetes, fungi, algae, and plants for silver-based QDs biosynthesis have been discussed. The application of silver-based QDs as fluorophores for bioimaging application due to their fluorescence intensity, high quantum yield, fluorescent stability, and resistance to photobleaching has also been reviewed.