Ag2 Te Research Articles

High performance magnesium-based plastic semiconductors for flexible thermoelectrics

Low-cost thermoelectric materials with simultaneous high performance and superior plasticity at room temperature are urgently demanded due to the lack of ever-lasting power supply for flexible electronics. However, the inherent brittleness in conventional thermoelectric semiconductors and the inferior thermoelectric performance in plastic organics/inorganics severely limit such applications. Here, we report low-cost inorganic polycrystalline Mg3Sb0.5Bi1.498Te0.002, which demonstrates a remarkable combination of large strain (~ 43%) and high figure of merit zT (~ 0.72) at room temperature, surpassing both brittle Bi2(Te,Se)3 (strain ≤ 5%) and plastic Ag2(Te,Se,S) and organics (zT ≤ 0.4). By revealing the inherent high plasticity in Mg3Sb2 and Mg3Bi2, capable of sustaining over 30% compressive strain in polycrystalline form, and the remarkable deformability of single-crystalline Mg3Bi2 under bending, cutting, and twisting, we optimize the Bi contents in Mg3Sb2-xBix (x = 0 to 1) to simultaneously boost its room-temperature thermoelectric performance and plasticity. The exceptional plasticity of Mg3Sb2-xBix is further revealed to be brought by the presence of a dense dislocation network and the persistent Mg-Sb/Bi bonds during slipping. Leveraging its high plasticity and strength, polycrystalline Mg3Sb2-xBix can be easily processed into micro-scale dimensions. As a result, we successfully fabricate both in-plane and out-of-plane flexible Mg3Sb2-xBix thermoelectric modules, demonstrating promising power density. The inherent remarkable plasticity and high thermoelectric performance of Mg3Sb2-xBix hold the potential for significant advancements in flexible electronics and also inspire further exploration of plastic inorganic semiconductors.

Mechanical and thermoelectric properties in Te-rich Ag2(Te,S) meta-phases

Ductile Ag2(Te,S) pseudobinary compounds have attracted great attention in thermoelectric community since they can be fabricated into high-performance flexible and hetero-shaped thermoelectric devices. However, in spite of the numerous studies, the ‘brittle–ductile’ transition boundary in Ag2(Te,S) is still unclear. In this work, a series of Te-rich Ag2(Te,S) pseudobinary compounds have been prepared. The structure characterizations confirm they belong to the new-concept of meta-phase. The systematically investigation on the mechanical properties demonstrate that the ‘brittle–ductile’ transition boundary appears around x = 0.1. Unexpected good ductility is observed in the Te-rich Ag2Te1–xSx crystalizing in the Ag2Te room-temperature monoclinic structure and high-temperature cubic structure, which are thought to be brittle before. Likewise, Ag content is found to be a very critical parameter determining the ductility of Te-rich Ag2Te1–xSx. Very slight Ag-deficiency can greatly deteriorate the ductility. The thermoelectric properties of these ductile Te-rich Ag2Te1–xSx pseudobinary compounds are investigated. A maximum thermoelectric figure-of-merit of 0.6 is obtained for Ag2Te0.9S0.1 at 600 K. This work sheds light on the future investigation of Ag2(Te,S) pseudobinary compounds.

Synergetic Enhancement of Strength-Ductility and Thermoelectric Properties of Ag2 Te by Domain Boundaries.

Simultaneously improving the mechanical and thermoelectric (TE) properties is significant for the engineering applications of inorganic TE materials. In this work, a novel nanodomain strategy is developed for Ag2 Te compounds to yield 40% and 200% improved compressive strength (160MPa) and fracture strain (16%) when compared to domain-free samples (115MPa and 5.5%, respectively). The domained samples also achieve a 45% improvement in average ZT value. The domain boundaries (DBs) provide extra sites for dislocation nucleation while pinning the dislocation movement, resulting in superior strength and ductility. In addition, phonon scattering induced by DBs suppresses the lattice thermal conductivity of Ag2 Te and also reduces the weighted mobility. These findings provide new insights into grain and DB engineering for high-performance inorganic semiconductors with robust mechanical properties.

Defect-Engineering-Stabilized AgSbTe2 with High Thermoelectric Performance.

Thermoelectric (TE) generators enable the direct and reversible conversion between heat and electricity, providing applications in both refrigeration and power generation. In the last decade, several TE materials with relatively high figures of merit (zT) have been reported in the low- and high-temperature regimes. However, there is an urgent demand for high-performance TE materials working in the mid-temperature range (400-700 K). Herein, p-type AgSbTe2 materials stabilized with S and Se co-doping are demonstrated to exhibit an outstanding maximum figure of merit (zTmax ) of 2.3 at 673 K and an average figure of merit (zTave ) of 1.59 over the wide temperature range of 300-673 K. This exceptional performance arises from an enhanced carrier density resulting from a higher concentration of silver vacancies, a vastly improved Seebeck coefficient enabled by the flattening of the valence band maximumand the inhibited formation of n-type Ag2 Te, and ahighly improved stability beyond 673 K. The optimized material is used to fabricate a single-leg device with efficiencies up to 13.3% and a unicouple TE device reaching energy conversion efficiencies up to 12.3% at a temperature difference of 370 K. These results highlight an effective strategy to engineer high-performance TE material in the mid-temperature range.

Synthesis of Ultrathin Topological Insulator β-Ag2 Te and Ag2 Te/WSe2 -Based High-Performance Photodetector.

β-Ag2 Te has attracted considerable attention in the application of electronics and optoelectronics due to its narrow bandgap, high mobility, and topological insulator properties. However, it remains a significant challenge to synthesize 2D Ag2 Te because of the non-layered structure of Ag2 Te. Herein, the synthesis of large-size, ultrathin single crystal topological insulator 2D Ag2 Te via the van der Waals epitaxial method for the first time is reported. The 2D Ag2 Te crystal exhibits p-type conduction behavior with high carrier mobility of 3336 cm2 V-1 s-1 at room temperature. Taking advantage of the high mobility and perfect electron structure of Ag2 Te, the Ag2 Te/WSe2 heterojunctions are fabricated via mechanical stacking and show an ultrahigh rectification ratio of 2×105 . Ag2 Te/WSe2 photodetector also exhibits self-driven properties with a fast response speed (40 µs/60 µs) in the near-infrared region. High responsivity (219mA W-1 ) and light ON/OFF ratio of 6×105 are obtained under the photovoltaic mode. The overall performance of the Ag2 Te/WSe2 photodetector is significantly competitive among all reported 2D photodetectors. These results indicate that 2D Ag2 Te is a promising candidate for future electronic and optoelectronic applications.

Au-Doped Ag2 Te Quantum Dots with Bright NIR-IIb Fluorescence for In Situ Monitoring of Angiogenesis and Arteriogenesis in a Hindlimb Ischemic Model.

Fluorescence located in 1500-1700nm (denoted as the near-infrared IIb window, NIR-IIb) has drawn great interest for bioimaging, owing to its ultrahigh tissue penetration depth and spatiotemporal resolution. Therefore, NIR-IIb fluorescent probes with high photoluminescence quantum yield (PLQY) and stability along with high biocompatibility are urgently pursued. Herein, a novel NIR-IIb fluorescent probe of Au-doped Ag2 Te (Au:Ag2 Te) quantum dots (QDs) is developed via a facile cation exchange method. The Au dopant concentration in the Ag2 Te QDs is tunable from 0% to 10% by controlling the ratio of supplied Au precursor to Ag2 Te QDs, resulting in a wide range of PL emission in the NIR-IIb window and a much-enhanced PL intensity. After surface modification, the Au:Ag2 Te QDs possess bright NIR-IIb emission, high colloidal stability and photostability, and decent biocompatibility. Further, in vivo monitoring of the process of angiogenesis and arteriogenesis in an ischemic hindlimb is successfully performed.

Low Temperature Activation of Tellurium and Resource-Efficient Synthesis of AuTe2 and Ag2 Te in Ionic Liquids.

The low temperature syntheses of AuTe2 and Ag2Te starting from the elements were investigated in the ionic liquids (ILs) [BMIm]X and [P66614]Z ([BMIm]+=1‐butyl‐3‐methylimidazolium; X = Cl, [HSO4]−, [P66614]+ = trihexyltetradecylphosphonium; Z = Cl−, Br−, dicyanamide [DCA]−, bis(trifluoromethylsulfonyl)imide [NTf2]−, decanoate [dec]−, acetate [OAc]−, bis(2,4,4‐trimethylpentyl)phosphinate [BTMP]−). Powder X‐ray diffraction, scanning electron microscopy, and energy‐dispersive X‐ray spectroscopy revealed that [P66614]Cl is the most promising candidate for the single phase synthesis of AuTe2 at 200 °C. Ag2Te was obtained using the same ILs by reducing the temperature in the flask to 60 °C. Even at room temperature, quantitative yield was achieved by using either 2 mol % of [P66614]Cl in dichloromethane or a planetary ball mill. Diffusion experiments, 31P and 125Te‐NMR, and mass spectroscopy revealed one of the reaction mechanisms at 60 °C. Catalytic amounts of alkylphosphanes in commercial [P66614]Cl activate tellurium and form soluble phosphane tellurides, which react on the metal surface to solid telluride and the initial phosphane. In addition, a convenient method for the purification of [P66614]Cl was developed.

Controlled Synthesis of Ag2 Te@Ag2 S Core-Shell Quantum Dots with Enhanced and Tunable Fluorescence in the Second Near-Infrared Window.

Fluorescence in the second near-infrared window (NIR-II, 900-1700 nm) has drawn great interest for bioimaging, owing to its high tissue penetration depth and high spatiotemporal resolution. NIR-II fluorophores with high photoluminescence quantum yield (PLQY) and stability along with high biocompatibility are urgently pursued. In this work, a Ag-rich Ag2 Te quantum dots (QDs) surface with sulfur source is successfully engineered to prepare a larger bandgap of Ag2 S shell to passivate the Ag2 Te core via a facile colloidal route, which greatly enhances the PLQY of Ag2 Te QDs and significantly improves the stability of Ag2 Te QDs. This strategy works well with different sized core Ag2 Te QDs so that the NIR-II PL can be tuned in a wide range. In vivo imaging using the as-prepared Ag2 Te@Ag2 S QDs presents much higher spatial resolution images of organs and vascular structures as compared with the same dose of Ag2 Te nanoprobes administrated, suggesting the success of the core-shell synthetic strategy and the potential biomedical applications of core-shell NIR-II nanoprobes.

Are Cu2 Te-Based Compounds Excellent Thermoelectric Materials?

Most of the state-of-the-art thermoelectric (TE) materials exhibit high crystal symmetry, multiple valleys near the Fermi level, heavy constituent elements with small electronegativity differences, or complex crystal structure. Typically, such general features have been well observed in those well-known TE materials such as Bi2 X3 -, SnX-, and PbX-based compounds (X = S, Se, and Te). The performance is usually high in the materials with heavy constituent elements such as Te and Se, but it is low for light constituent elements such as S. However, there is a great abnormality in Cu2 X-based compounds in which Cu2 Te has much lower TE figure of merit (zT) than Cu2 S and Cu2 Se. It is demonstrated that the Cu2 Te-based compounds are also excellent TE materials if Cu deficiency is sufficiently suppressed. By introducing Ag2 Te into Cu2 Te, the carrier concentration is substantially reduced to significantly improve the zT with a record-high value of 1.8, 323% improvement over Cu2 Te and outperforms any other Cu2 Te-based materials. The single parabolic band model is used to further prove that all Cu2 X-based compounds are excellent TE materials. Such finding makes Cu2 X-based compounds the only type of material composed of three sequent main group elements that all possess very high zT s above 1.5.

Cell Membrane-Camouflaged NIR II Fluorescent Ag2 Te Quantum Dots-Based Nanobioprobes for Enhanced In Vivo Homotypic Tumor Imaging.

The advantages of fluorescence bioimaging in the second near-infrared (NIR II, 1000-1700 nm) window are well known; however, current NIR II fluorescent probes for in vivo tumor imaging still have many shortcomings, such as low fluorescence efficiency, unstable performance under in vivo environments, and inefficient enrichment at tumor sites. In this study, Ag2 Te quantum dots (QDs) that emit light at a wavelength of 1300 nm are assembled with poly(lactic-co-glycolic acid) and further encapsulated within cancer cell membranes to overcome the shortcomings mentioned above. The as-prepared ≈100 nm biomimetic nanobioprobes exhibit ultrabright (≈60 times greater than that of free Ag2 Te QDs) and highly stable (≈97% maintenance after laser radiation for 1 h) fluorescence in the NIR II window. By combining the active homotypic tumor targeting capability derived from the source cell membrane with the passive enhanced permeation and retention effect, improved accumulation at tumor sites ((31 ± 2)% injection dose per gram of tumor) and a high tumor-to-normal tissue ratio (13.3 ± 0.7) are achieved. In summary, a new biomimetic NIR II fluorescent nanobioprobe with ultrabright and stable fluorescence, homotypic targeting and good biocompatibility for enhanced in vivo tumor imaging is developed in this study.

Understanding the Chemical Reactivity of Phosphonium-Based Ionic Liquids with Tellurium.

The chemical reactivity of phosphonium based ionic liquids (ILs) towards tellurium at temperatures above 220 °C was systematically investigated by a series of dissolution experiments, tracking the solute tellurium species by nuclear magnetic resonance, and characterizing the reaction products by X-ray diffraction and scanning electron microscopy. The initial step is the thermal elimination of an alkyl group of the phosphonium cation of the ILs, most probably via an SN 2 mechanism. The addition of tellurium follows to form trialkylphosphane tellurides as evidenced by 31 P and 125 Te NMR spectroscopic experiments. The trialkylphosphane tellurides can serve as a tellurium reservoir for the formation of metal tellurides, like Bi2 Te3 and Ag2 Te. It was observed that trihexyltetradecylphosphonium chloride ([P6 6 6 14 ]Cl) shows a very weak reactivity that is reflected by a low solubility of tellurium, while trihexyltetradecylphosphonium dicyanamide/decanoate ([P6 6 6 14 ][N(CN)2 ]/[P6 6 6 14 ][decanoate]) and tetrabutylphosphonium decanoate ([P4 4 4 4 ][decanoate]) dissolve tellurium to a much higher extent. We attribute these observations to the different Lewis basicity of the anions of the ILs as main influencing factor.

Stable Te isotope fractionation in tellurium-bearing minerals from precious metal hydrothermal ore deposits

The tellurium isotope compositions of naturally-occurring tellurides, native tellurium, and tellurites were measured by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) and compared to theoretical values for equilibrium mass-dependent isotopic fractionation of representative Te-bearing species estimated with first-principles thermodynamic calculations. Calculated fractionation models suggest that 130/125Te fractionations as large as 4‰ occur at 100°C between coexisting tellurates (Te VI) and tellurides (Te −II) or or native tellurium Te(0), and smaller, typically <1‰, fractionations occur between coexisting Te(−I) or Te(−II) (Au,Ag)Te2 minerals (i.e., calaverite, krennerite) and (Au,Ag)2Te minerals (i.e., petzite, hessite). In general, heavyTe/lightTe is predicted to be higher for more oxidized species, and lower for reduced species.Tellurides in the system Au–Ag–Te and native tellurium analyzed in this study have values of δ130/125Te=−1.54‰ to 0.44‰ and δ130/125Te=−0.74‰ to 0.16‰, respectively, whereas those for tellurites (tellurite, paratellurite, emmonsite and poughite) range from δ130/125Te=−1.58‰ to 0.59‰. Thus, the isotopic composition for both oxidized and reduced species are broadly coincident. Calculations of per mil isotopic variation per amu for each sample suggest that mass-dependent processes are responsible for fractionation. In one sample of coexisting primary native tellurium and secondary emmonsite, δ130/125Te compositions were identical. The coincidence of δ130/125Te between all oxidized and reduced species in this study and the apparent lack of isotopic fractionation between native tellurium and emmonsite in one sample suggest that oxidation processes cause little to no fractionation.Because Te is predominantly transported as an oxidized aqueous phase or as a reduced vapor phase under hydrothermal conditions, either a reduction of oxidized Te in hydrothermal liquids or deposition of Te from a reduced vapor to a solid is necessary to form the common tellurides and native tellurium in ore-forming systems. Our data suggest that these sorts of reactions during mineralization may account for a ∼3‰ range of δ130/125Te values. Based on the data ranges for Te minerals from various ore deposits, the underpinning geologic processes responsible for mineralization seem to have primary control on the magnitude of fractionation, with tellurides in epithermal gold deposits showing a narrower range of isotope values than those in orogenic gold and volcanogenic massive sulfide deposits.

Solubility and microstructure in the pseudo-binary PbTe–Ag 2Te system

The solvus lines of the PbTe and Ag 2Te phases in the pseudo-binary PbTe–Ag 2Te system have been determined using diffusion couples and unidirectional solidification by the Bridgman method. The solubilities of both Ag 2Te in PbTe and PbTe in Ag 2Te decrease with decrease in temperature. For the former, this change is from 14.9 at% Ag (694 °C) to 0.5 at% Ag (375 °C), while for the latter it is from 12.4 at% Pb (650 °C) to 3.1 at% Pb (375 °C). The decrease in solubilities leads to the formation of precipitates of Ag 2Te in PbTe and PbTe in Ag 2Te. In particular, fast atomic diffusion in Ag 2Te results in the precipitation of PbTe even in quenched samples. From the temperature dependence of these solubilities, heats of solution have been determined. In the diffusion couple, the phase boundary moves toward PbTe. In the region between the phase boundary and the initial interface, PbTe transforms to β-Ag 2Te (cubic) retaining the cube-on-cube orientation relationship.

The effect of Ag concentration on the structural, electrical and thermal transport behavior of Pb:Te:Ag:Se mixtures and improvement of thermoelectric performance via Cu doping

Pb, Te, Ag and Se, when reacted in a 1:1: x:1 ( x = 1.9, 2.0, 2.01) molar ratio, form a two phase composite which consists of a phase which crystallizes in the fcc cubic PbSe structure and a phase that crystallizes in the Ag 2Te structure. In this article, we demonstrate that by varying the Ag concentration, we can manipulate which variant of the Ag 2Te structure stabilizes at room temperature (monoclinic α-Ag 2Te or cubic β-Ag 1.9Te) and can consequently manipulate the electrical and thermal transport behavior of the composite and hence the thermoelectric performance. Additionally, we show that Cu-doping results in an overall improvement in thermoelectric performance. Our results suggest that formation of composites is a viable path for achieving a phonon-glass-electron-crystal (PGEC) alloy.

Microstructures and liquidus projection of the ternary Ag–Sb–Te system

Ag–Sb–Te alloys are important for thermoelectric applications. Fifty-one Ag–Sb–Te ternary alloys were prepared, and their primary solidification phases were analyzed. The liquidus troughs of the liquidus projection of the ternary Ag–Sb–Te system are determined based on the experimental results and the phase diagrams of the three binary constituent systems. There are 13 primary solidification phase regions. In addition to the three terminal solid solution phases and nine binary compounds, there is one ternary compound, AgSbTe 2. A unique microstructure with bright spherical phases uniformly dispersed in a matrix caused by a miscibility gap in the liquid phase is found in the γ-Ag 2Te primary solidification phase regime. A very fine microstructure with nanometer size Ag 2Te is also observed, resulting from the class I reaction, liquid = δ + Ag 2Te + AgSbTe 2, at 496.5 °C, and the liquid composition of Ag–40.0 at%Sb–36.0 at%Te.

Microstructure and magnetoresistance of heterogeneous gold-rich Ag 3Au 1.1Te 2

Dispersions of very small non-magnetic metal particles or inclusions in a non-magnetic semiconductor matrix are well known to produce unusually large and linear magnetoresistance effects. So far these materials were limited to the binary silver-rich chalcogenides Ag 2Se and Ag 2Te. In this contribution Ag 3AuTe 2 was selected as a first candidate for a ternary matrix material, thus offering enhanced capabilities for the generation of heterogeneous microstructure and spatially varying composition on the nanoscale. In gold-rich Ag 3Au 1.1Te 2 two kinds of inhomogeneities are present, namely Au deposits with a size on the micron scale and an inhomogeneous distribution of Au and Ag within the matrix. The matrix consists of micron-sized grains with the structure type of Ag 3AuTe 2 as studied by electron microscopy. Like the binary silver chalcogenide phases, the material also shows a large and linear magnetoresistance effect. The transversal magnetoresistance effect was measured between 20 K and 270 K in magnetic fields up to B = 5 T. The results are discussed on the basis of existing models for a large and linear positive MR effect.

Simple synthesis of ultra-long Ag 2Te nanowires through solvothermal co-reduction method

Ultra-long single crystal β-Ag 2Te nanowires with the diameter of about 300 nm were fabricated through a solvothermal route in ethylene glycol (EG) system without any template. The long single crystal wires were curves, with high purity, well-crystallized, and dislocation-free and characterized by using X-ray powder diffraction (XRD), Differential scanning calorimetry (DSC) analysis, X-ray photoelectron spectroscope (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high-resolution transmission microscopy (HRTEM). The detailed topotactic transformation process from particles into single crystal wires was studied. Furthermore, the electrical conductivity and Seebeck coefficient have been systematically studied between 300 and 600 K.

Morphological evolution of Ag 2Te precipitates in thermoelectric PbTe

The precipitation of Ag 2Te in a PbTe matrix is investigated using electron microscopy and atom probe tomography. We observe the formation of oriented nanoscale Ag 2Te precipitates in PbTe. These precipitates initially form as coherent spherical nanoparticles and evolve into flattened semi-coherent disks during coarsening. This change in morphology is consistent with equilibrium shape theory for coherently strained precipitates. Upon annealing at elevated temperatures these precipitates eventually revert to an equiaxed morphology. We suggest this shape change occurs once the precipitates grow beyond a critical size, making it favorable to relieve the elastic coherency strains by forming interfacial misfit dislocations. These investigations of the shape and coherency of Ag 2Te precipitates in PbTe should prove useful in the design of nanostructured thermoelectric materials.

The Ag effect on magnetic properties and microstructure of FePt/Ag 2Te particulate films

A [FePt (1 nm)/Ag 2Te( t)] 10 (thickness t = 0.1–0.3 nm) multilayer was deposited alternately on glass substrate and subsequently annealed by a rapid thermal process (RTP). After the RTP, the interface between FePt and Ag 2Te was intermixed, forming particulate films. The L1 0 FePt grain size decreases from 23 to 14 nm as t of the Ag 2Te intermediate layer increases from 0.1 to 0.3 nm. The (FePt/Ag 2Te) 10 particulate film shows perpendicular magnetization. Compared to (FePt/Ag 2Te) 10, the Ag/(FePt/Ag 2Te) 10/Ag multilayer also shows perpendicular magnetization with less c-axis dispersion. The Ag capping and seed layers reduce the ordering temperature of FePt but facilitate its grain growth during RTP. As a result, the FePt grains are refined and well-separated by the Ag 2Te phase, but change to a continuous film after inserting Ag capping and seed layers.

Improved thermoelectric properties of AgSbTe 2 based compounds with nanoscale Ag 2Te in situ precipitates

Ternary Ag x Sb 2− x Te 3− x thermoelectric materials have been prepared with x value varying from 0.78 to 0.93. By adjusting the Ag 2Te ratio, double-phased in situ nanocomposites were obtained with the nanostructured Ag 2Te embedded in the AgSbTe 2 matrix when x > 0.81. The high-resolution transmission electron microscopy observation showed that the Ag 2Te precipitates were in situ formed as nanodots and nanoscale lamellar structures. Compared with the single-phased samples of x = 0.78 and 0.81, the Seebeck coefficient of the nanocomposite samples exhibited significant improvement over the entire measured temperature range and the electrical conductivity also slightly increased, resulting in the increase of the power factor. At the same time, the thermal conductivity of the nanocomposite samples slightly decreased. The optimization of both the power factor and the thermal conductivity contributed to the notable improvement in figure of merit ZT by a factor of ∼40% at 670 K.