Silver Chalcogenides Research Articles

Bendable thermoelectric generators composed of p- and n-type silver chalcogenide nanoparticle thin films

In this study, we design and construct lateral thermoelectric generators on bendable substrates with pairs of p-Ag2Te and n-Ag2Se chalcogenide nanoparticle (NP) thin films, and investigate their thermoelectric performances in ambient atmosphere. The individual Seebeck coefficient and lateral thermal conductivity are 1110 μV/K and 0.67 W/m K, respectively, for a single p-Ag2Te thin film, and − 290 μV/K and 0.55 W/m K, respectively, for a single n-Ag2Se NP thin film. A ring-shaped thermoelectric generator made with 50 pn modules of these NP thin films at a temperature difference of 15 K generates a Seebeck voltage of 1.16 V and an output power of 106 mW/cm2. Moreover, the mechanical durability of our thermoelectric generator is verified by repeated bending tests of up to 5000 cycles.

A study on the resistance switching of Ag2Se and Ta2O5 heterojunctions using structural engineering

The resistive random access memory (RRAM) devices with heterostuctures have been investigated due to cycling stability, nonlinear switching, complementary resistive switching and self-compliance. The heterostructured devices can modulate the resistive switching (RS) behavior appropriately by bilayer structure with a variety of materials. In this study, the bipolar resistive switching characteristics of the bilayer structures composed of Ta2O5 and Ag2Se, which are transition-metal oxide (TMO) and silver chalcogenide, were investigated. The bilayer devices of Ta2O5 deposited on Ag2Se (Ta2O5/Ag2Se) and Ag2Se deposited on Ta2O5 (Ag2Se/Ta2O5) were fabricated for investigation of the RS characteristics by stacking sequence of Ta2O5 and Ag2Se. All operating voltages were applied to the Ag top electrode with the Pt bottom electrode grounded. The Ta2O5/Ag2Se device showed that a negative voltage sweep switched the device from high resistance state (HRS) to low resistance state (LRS) and a positive voltage sweep switched the device from LRS to HRS. On the contrary, for the Ag2Se/Ta2O5 device a positive voltage sweep switched the device from HRS to LRS, and a negative voltage sweep switched it from LRS to HRS. The polarity dependence of RS was attributed to the stacking sequence of Ta2O5 and Ag2Se. In addition, the combined heterostructured device of both bilayer stacks, Ta2O5/Ag2Se and Ag2Se/Ta2O5, exhibited the complementary switching characteristics. By using threshold switching devices, sneak path leakage can be reduced without additional selectors. The bilayer heterostructures of Ta2O5 and Ag2Se have various advantages such as self-compliance, reproducibility and forming-free stable RS. It confirms the possible applications of TMO and silver chalcogenide heterostructures in RRAM.

Equivalence of effective medium and random resistor network models for disorder-induced unsaturating linear magnetoresistance

A linear unsaturating magnetoresistance at high perpendicular magnetic fields, together with a quadratic positive magnetoresistance at low fields, has been seen in many different experimental materials, ranging from silver chalcogenides and thin films of InSb to topological materials like graphene and Dirac semimetals. In the literature, two very different theoretical approaches have been used to explain this classical magnetoresistance as a consequence of sample disorder. The phenomenological random resistor network model constructs a grid of four terminal resistors, each with a varying random resistance. The effective medium theory model imagines a smoothly varying disorder potential that causes a continuous variation of the local conductivity. Here, we demonstrate numerically that both models belong to the same universality class and that a restricted class of the random resistor network is actually equivalent to the effective medium theory. Both models are also in good agreement with experiments on a diverse range of materials. Moreover, we show that in both cases, a single parameter, i.e., the ratio of the fluctuations in the carrier density to the average carrier density, completely determines the magnetoresistance profile.

Ultraquantum magnetoresistance in the Kramers-Weyl semimetal candidateβ−Ag2Se

In the history of condensed matter physics, reinvestigation of a well-studied material with enhanced quality sometimes led to important scientific discoveries. A well-known example is the discovery of fractional quantum Hall effect in high quality GaAs/AlGaAs heterojunctions. Here we report the first single crystal growth and magnetoresistance (MR) measurements of the silver chalcogenide $\beta $-Ag$_2$Se (Naumannite), a compound has been known for the unusual, linear-field-dependent MR in its polycrystalline form for over a decade. With the quantum limit (QL) as low as 3 Tesla, a moderate field produced by a superconductor magnet available in many laboratories can easily drive the electrons in Ag$_2$Se to an unprecedented state. We observed significant negative longitudinal MR beyond the QL, which was understood as a `charge-pumping' effect between the novel fermions with opposite chiralities. Characterization of the single-crystalline Ag$_2$Se and the fabrication of electric devices working above the QL, will represent a new direction for the study of these exotic electrons.

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures.

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

Thermoelectric properties and chemical potential tuning by Cu-doping in n-type ionic conductors CuxAg2−xSe0.5Te0.5

Copper and silver chalcogenides with superionic conduction behavior have shown impressively high ZT values, but there has been no intensive effort to optimize their carrier density to further improve their ZT values. Here, we prepared polycrystalline CuxAg2−xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples using high temperature melting followed by hot-press sintering, and characterized their thermoelectric properties. We demonstrated that Cu substitution for Ag was achieved with <10% Cu content for CuxAg2−xSe0.5Te0.5 and the Cu doping was quite effective and significantly enhanced the compound's n-type carrier density, which was one order of magnitude higher than the pristine Ag2Se0.5Te0.5 (4.10 × 1018 cm−3). Impressively, the enhancement in electrical conductivity with increasing Cu content was greater than the decrease in absolute value of the Seebeck coefficient in the superionic conduction state. This led to relatively high power factors for Cu0.1Ag1.99Se0.5Te0.5, ranging between 1.10 and 1.30 mW m−1 K−2 over the broad temperature range of 400–560 K, and resulted in the highest ZT of 0.85 at 560 K. Furthermore, ZT values approached >0.7 over a wide temperature range of 460–560 K for x > 0.05. We suggest that the unusual Cu doping effect in Ag2Se0.5Te0.5 can be attributed to the creation of Cu ion conduction in addition to Ag ion conduction, and the optimization of the compound's n-type carrier density.

Fusion of Image Storage and Operation Based on Ag-Chalcogenide Memristor with Synaptic Plasticity

With the discovery of the memristor devices, most researchers have focused on the simulation and application of memristive characteristics, especially the nonvolatile storage potential of the memristor. However, the fusion of image storage and operation is seldom discussed. Therefore, this paper focuses on the electrical characterization of silver chalcogenide based on memristor devices, which can be used as a synapse to mimic the STDP and SRDP learning rules in neuromorphic systems. Then on this basis, a improved memristive cross array structure is designed, whose storage and operation are set in a single unit. Furthermore, this improved memristive cross array and image processing are combined with adjusting the memristor resistance by synaptic plasticity to achieve the integration of storage and processing. Finally, the effectiveness of the design is verified by the LTSPICE experiments of the nonlinear grayscale transformation and logic operation of the image. The design breaks the gap between the processing and storage in the Von Neumann architecture, which will be helpful to the development of the parallel computing in the future.

Thermoelectric Properties of a Single Crystalline Ag2Te Nanowire

Silver chalcogenides have received much attention in potential thermoelectric materials research because of high carrier mobility and low effective mass. Among them, in Ag2Te, it was reported that the phase transition from monoclinic to cubic phase occurs at relatively low temperatures, so that extensive research for effective application using this material has been aroused. In this work, we investigated how 1-dimensional nanostructure affects the thermoelectric properties through as-synthesized single crystalline Ag2Te nanowires. Adopting well-defined thermoelectric MEMS device structure and transferring an individual Ag2Te nanowire, we measure electrical resistance and Seebeck coefficient as a function of temperature. When the phase changes from monoclinic to cubic, the resistance increases, while absolute Seebeck coefficient value decreases. These results are compared with previous reports for Ag2Te bulk and film, suggesting the increased density of states of the carriers due to nanowire structure.

Facile room temperature solventless synthesis of high thermoelectric performance Ag2Se via a dissociative adsorption reaction

The facile room temperature yet solventless synthesis process, characterized by its simplicity, speed, low energy demand, low cost, opens a new avenue for the preparation of high quality silver chalcogenide samples.

Pressure tuning the lattice and optical response of silver sulfide

Binary transition metal chalcogenides have attracted increasing attention for their unique structural and electronic properties. High pressure is a powerful tool for tuning the lattice and electronic structure of transition metal chalcogenides away from their pristine states. In this work, we systematically studied the in situ structural and optical behavior of silver sulfide (Ag2S) under pressure by synchrotron X-ray diffraction and infrared spectroscopy measurements in a diamond anvil cell. Upon compression, Ag2S undergoes structural symmetrization accompanied by a series of structural transitions while the crystallographic inequivalence of the two Ag sites is maintained. Electronically, pressure effectively tunes the ambient semiconducting Ag2S into a metal at ∼22 GPa. Drude model analysis shows that the optical conductivity evolves significantly, reaching the highest value of 100 Ω−1 cm−1 at ∼40 GPa. Our results highlight the structural and electronic tunability of silver chalcogenides as a function of pressure and suggest the potential of Ag2S as a platform for developing optical and opto-electronic applications.

Highly luminescent monolayer protected Ag56Se13S15clusters

Highly luminescent monolayer protected mixed silver chalcogenide clusters showing high sensitivity towards mercury ions.

Effect of pressures on the electrical properties of multicomponent copper and silver chalcogenides

The effect of high pressures (up to 45 GPa) and frequencies of the alternating-current (ac) electric field (10 Hz–10 MHz) on the electrical properties of multicomponent copper and silver chalcogenides has been studied. The behavior of the direct-current (dc) electrical resistivity of the samples has been analyzed under uniaxial pressure along the measuring electric field.

Synthesis and characterization of nanosized silver chalcogenides under ultrasonic irradiation

Synthesis and characterization of nanosized silver chalcogenides under ultrasonic irradiation

Single-step synthesis of silver sulfide nanocrystals in arsenic trisulfide

Silver sulfide nanocrystals and chalcogenide glasses (ChGs) are two distinct classes of semiconductor materials that have been exploited for new infrared technologies. Each one exhibits particular optoelectronic phenomena, which could be encompassed in a hybrid material. However, the integration of uniformly distributed crystalline phases within an amorphous matrix is not always an easy task. In this paper, we report a single step method to produce Ag2S nanocrystals (NCs) in arsenic trisulfide (As2S3) solution. The preparation is carried out at room temperature, using As2S3, AgCl and propylamine resulting in highly crystalline Ag2S nanoparticles in solution. These solutions are spin-coated on glass and silicon substrates to produce As2S3/Ag2S metamaterials for optoelectronics.

Dependence of the spectra of charge carriers on the concentration of defects in silver telluride

A change in the band gap as a function of the concentration of electrically active defects in silver telluride has been considered. It has been found that the electronegative and electropositive defects lead to an increase and a decrease in the band gap \(\varepsilon _{g_0 } \), respectively. It has been revealed that, in Ag2Te with Te concentrations of higher than or equal to 0.75 at %, the band gap \(\varepsilon _{g_0 } \) has an unusually low value (∼0.008 eV). In the case where the band gap is characterized by the temperature dependence ɛg = (0.008−7 × 10−5T) eV, at T > 100 K, there is a gapless state. Owing to the changes in the concentration of electrically active defects, silver telluride can exhibit n- and p-type conductivities, unlike other silver chalcogenides (Ag2S and Ag2Se).

Modification of polyamide-CdS-CdSe composite material films with Ag using a cation–cation exchange reaction

Thin mixed CdSe-CdS-Ag2Se-Ag2S films were deposited on a polyamide 6 (PA) surface by successfully using a cation-exchange reaction between Cd2+ and Ag+ to convert CdSe-CdS into Ag2Se-Ag2S. These were deposited using a K2SeS2O6 precursor solution at 60°C followed by cadmium acetate (Cd(CH3COO)2). An aqueous AgNO3 solution was used as the Ag source. XRD patterns showed a complex PA-Cd-S-Se-Ag film crystalline composition with CdS, CdSe, Ag2S and Ag2Se peaks. Calculated dislocation density ranged within 5–15×1013 lines·m−2 indicating high quality atomic layers. Atomic Absorption Spectroscopy (AAS) showed five- to ten-fold excess of chalcogens to metals in the thin films formed. No chalcogenides were observed on the sample surface during XPS analysis after Ag exchange due to the desorption of CdS and CdSe layers, not diffused into the bulk of the polymer suggesting that silver chalcogenides were located subsurface, as opposed to the outermost layer, likely comprised of Ag2O.

Polymorphic transformations in Cu2Se, Ag2Se, AgCuSe and the role of partial cation-cation and anion-anion replacement in stabilizing their modifications

A review of studies of the crystalline structure of copper and silver chalcogenides is given. These materials have diverse physical properties making them promising in practical applications. Physical properties of crystals are determined by chemical composition, crystalline structure, and the influence of external conditions. In this work we carried out the analysis of published results on the crystalline structures of copper and silver chalcogenides at high and low temperatures. The inconsistency of published data on the crystal lattice parameters is noted. We also present our own results on the temperature dependence and the features of phase transitions in copper and silver chalcogenides with different composition.

Thermoelectric Properties of Solution Synthesized Nanostructured Materials.

Thermoelectric nanocomposites made by solution synthesis and compression of nanostructured chalcogenides could potentially be low-cost, scalable alternatives to traditional solid-state synthesized materials. We review the progress in this field by comparing the power factor and/or the thermoelectric figure of merit, ZT, of four classes of materials: (Bi,Sb)2(Te,Se)3, PbTe, ternary and quaternary copper chalcogenides, and silver chalcogenides. We also discuss the thermal conductivity reduction associated with multiphased nanocomposites. The ZT of the best solution synthesized materials are, in several cases, shown to be equal to or greater than the corresponding bulk materials despite the generally reduced mobility associated with solution synthesized nanocomposites. For the solution synthesized materials with the highest performance, the synthesis and processing conditions are summarized to provide guidance for future work.

Photoreduction of Ag+ in Ag/Ag2S/Au memristor

Silver halides and chalcogenides are excellent memristor materials that have been extensively used in the past as photosensitive layers in photography. Here we examine the effect of illumination on the operating voltages and switching speed of Ag/Ag2S/Au memristors using a green laser (473–523nm). Our results indicate that illumination decreases the average switching time from high to low resistance states by ∼19% and decreases the turn-off voltages dramatically from −0.8V to −0.25V that we attribute to the change in sulfur valency and a photo-induced change in its oxidation/reduction potential. Photo-induced reduction of silver in Ag2S may be used in three dimensional optical memories that can be electronically read and reset.