Amorphous Te Research Articles

Atomistic oxidation mechanism of Bi0.5Sb1.5Te3 (0001) surface

Surface oxidation can potentially influence transport properties of thermoelectric materials and service performance of thermoelectric devices. Here, the oxidation mechanism of the (0001) surface of a widely used p-type TE material, Bi0.5Sb1.5Te3 (BST), was investigated using spherical aberration corrected scanning transmission electron microscopy (STEM). Combining atomic-resolution STEM and density function theory (DFT), it was revealed that oxidation occurs at room temperature in the dry air through O diffusion into the surface quintuple layer, facilitating formation of both cation and anion vacancies, weakening the Bi-Te and Sb-Te bonds, leading to oxidation product Sb2O3, amorphous Te, and Bi-rich BST. Formation of a thin O-rich cation-deficient quintuple layer on the surface and the Bi-rich BST region can significantly reduce the interfacial reaction between BST and Ni electrode, as the reaction layer thickness is reduced by 75% after the surface oxidation. This work provides essential structural information on surface oxidation mechanism and its influence on properties and performance of TE materials and devices.

Amorphous Engineering and In Situ Atomic‐Scale Deciphering of Lithium‐Ion Storage Mechanism in Tellurium

AbstractLithium‐ion batteries (LIBs) using tellurium (Te) as electrode material are appealing because of their high capacities, conductivities, and lithium‐ion diffusivity relative to those of silicon. However, crystalline Te electrode suffers from mechanical instability and poor cyclability during Li+ insertion and extraction. Moreover, the reaction mechanisms governing Te electrode during the electrochemical charge and discharge are poorly understood. Here, an amorphous Te phase is deliberately conducted and the results of comparative operando experiments on the crystalline and amorphous Te phases are reported. The lithiation of the crystalline Te phase results in grains with concomitant pulverization. On the lithiation‐induced volumetric expansion and aggregation of the intrinsic stress, the Te crystalline phase undergoes bending, fracture, and finally collapse. In addition to the Li‐rich phase (Li2Te), a new Li‐deficient phase (LiTe3) that may be associated with incomplete lithiation owing to the poor ion conductivity of pulverized lithiation product is also detected. However, the amorphous Te specimens show promising lithiation/delithiation properties, particularly no pulverization behavior or structural damage, suggesting better capacity and reversibility. The different performances of crystalline and amorphous Te can be ascribed to the ordered and disordered structures. The findings will serve as a reference for the design of Te‐containing LIBs.

The changeable coordination of structural and bonding characteristics in amorphous Cu2Te from ab initio molecular dynamics simulations

Crystalline Cu2Te has recently attracted a great deal of attention owing to its good performance in thermoelectric materials. Yet, knowledge of the amorphous phase is still insufficient, which may restrict its practical application. Here, we have studied the structural and bonding characteristics of amorphous Cu2Te by ab initio molecular dynamics simulations. We show that, compared with its crystal phase, the Cu atoms bond more Cu than Te atoms in amorphous Cu2Te and Te atoms predominantly bond with Cu atoms. In detail, the amorphous Cu2Te is made up of Cu–Te network structures and Cu–Cu high-coordinated configurations, presenting the hexagonal and icosahedral structures, respectively. This result is probably ascribed to both the stronger bonding ability of Cu–Cu bonds and the multivalence of Te atoms. Our findings enrich the knowledge of the local structure and the bonding nature of amorphous Cu2Te, which can guide the design of good performance Cu2Te based thermoelectric devices further.

Amorphous Tellurium‐Embedded Hierarchical Porous Carbon Nanofibers as High‐Rate and Long‐Life Electrodes for Potassium‐Ion Batteries

Tellurium (Te) is a promising electrode active material for potassium-ion batteries due to its intrinsic electrical conductivity and ultra-high theoretical volumetric capacity. Nevertheless, Te-based electrodes usually exhibit low capacity at high rates and poor cycling stability caused by the large volume expansion and severe polytellurides dissolution. Herein, hierarchical porous carbon nanofibers (HPCNFs) film is utilized as a multifunctional Te substrate. The free-standing Te@HPCNFs electrode renders an outstanding K-ion storage performance with a high-rate capacity of 1294.4 mAh cm-3 (207.1 mAh g-1 Te ) at 14C and ultra-long lifespan for 4500 cycles at 7C, and K-ion full batteries coupled with KSn alloy anode also exhibit good cyclability. Such a superior performance benefits from the space confinement of HPCNFs to load amorphous Te in the micropores for accommodating the volume change, where the interconnected conductive frameworks and residual hierarchical pores enable fast ion/electron diffusion kinetics. In situ UV-vis absorption spectra confirm that the detachment of polytellurides and K2 Te from the electrode is effectively suppressed, and ex situ X-ray photoelectron spectroscopy analysis reveals the conversion of Te into K5 Te3 and K2 Te. This work presents the significance of porous structure design of carbon matrix to construct high performance Te electrodes, which will be instructive for chalcogens-based energy-storage materials.

Sterically Hindered Tellurium(IV) Catecholate as a Lewis Acid.

Sterically hindered tellurium catecholate Te(Cat36)2 (Cat36 = 3,6-di-tert-butyl-catecholate) was synthesized with the reaction of amorphous Te with 3,6-di-tert-butyl-o-benzoquinone. Adducts of Te(Cat36)2 with various O- and N-donors were isolated and characterized by means of single-crystal X-ray diffraction along with IR, UV-vis, and NMR (1H, 13C, and 125Te) spectroscopies. In the crystal structure of the adduct with 2,2'-bipyridine (bipy), the unprecedented μ-κ2N,N':κ2N,N'-bridging coordination mode of bipy was observed. Various intermolecular interactions Te...O, Te...N, and Te...C in adducts were analyzed using density functional theory calculations and quantum theory of atoms in molecules analysis. The estimated strength for appropriate short contacts varies from 0.9 to 5.3 kcal/mol, and they are attractive and purely non-covalent.

Conductometric NO2 gas sensor based on nanolayered amorphous tellurium for room temperature operation

A fast operating NO2 sensor based on amorphous Te layer of a nanometric thickness, enclosed between Pt electrodes has been developed and investigated. The gas sensor operates at room temperature and by sensitivity ~ 60% / ppm it exhibits a response time around 5 s. The long – term stability of the sensor tested by operating it 12 weeks showed no appreciable changes in the characteristics. To elucidate the mechanism of so fast gas detection, the sensor structure and active material have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, energy-dispersive X-ray spectroscopy (EDX) and atomic force microscopy (AFM), being followed with its characterization via studying the current - voltage characteristics, dynamic response, long – term stability, effects of temperature, humidity and other gases. It is shown that combination of used materials as well as the developed sensor design allow the simultaneous involvement of contact, and surface phenomena in sensor mechanism of operation, making possible the gas induced modulation of charge carriers simultaneously on Te film surface, accumulation regions at contacts, which include the portions of the degenerate (metallic) p-Te, as well as in contact gaps (transition regions), originated from microscopically roughness, formed since during electrode deposition. The last referred process leads to increasing the portion of the semiconducting Te nanolayer turned into metal of p-type Te, which at closed – circuit conditions results in a sharp increasing of the current flow through the device. Such explanation of obtained results reflects the fundamentals of metal – semiconductor junctions, the properties of used materials and meets the modern models proposed for interpretation of adsorption processes in similar devices.

Structure, bonding nature and transition dynamics of amorphous Te

Te has drawn many interest due to the excellent electrical and optical performances in both bulk and 2D scale. Although amorphous phase usually generates some particular characteristics, relevant studies are still inadequate. By using ab initio molecular dynamics (AIMD) simulations, we have explored the dynamics and structure of Te during amorphization process. The large fraction of 2- and 3-coordinated Te results in an average coordination number of 2.43 in amorphous Te (a-Te), deviating from the “8-N rule”. The 2-coordinated chain structures and the 3- and 4-coordinated defective octahedral clusters, together with 25.3% voids, make a-Te a flexible and fragile glass. As a result, a mid-gap state is observed in the electronic density of states. Our study deepens the insight on a-Te, which is useful for the design of novel electronic and optical devices.

A wide range of CO : H2 syngas ratios enabled by a tellurization-induced amorphous telluride–palladium surface

An amorphous Te–Pd surface was realized via surface tellurization on Pd NPs, on which the adsorption of CO2 is promoted while the binding strengths of CO* and H* are weakened, thus producing a wide range of CO : H2 syngas during CO2 electroreduction.

RETRACTED: Restorations of fresh surfaces for topological materials by de-capping Te

Abstract The topological categorization originates from a depiction of the sequencing of bands around the band gap of the material in comparison to the normal vacuum type band ordering. The structure can either be normal or inverted, and thus topologically trivial or non-trivial. An odd number of Dirac fermions on the surface occurs in this type of material. Time-reversal symmetry is responsible to preserve the surface bands. On the other hand, topological crystalline insulator is protected by mirror symmetry; this system has an even number of Dirac surface state per BZ. Capping layer of Te or Se is necessary due to the high sensitivity of films surfaces to atmospheric pressure. The capping layer works as a protection from oxidation. In Bi2Te3 samples, the bonds between Bi and Te are covalent bonds while the bonds between Te-Te at a certain position are van der Waals bonds. Thereby, the bonds between amorphous Te cap and sample surface is weaker than van der Waals bonds within the sample structure. However, the top boost cleaving method was successful. But, for Pb1-xSnxTe samples the top boost cleaving method unsuccessfully results. The bonds between the metallic Pb1-xSnxTe sample surface and amorphous Te cap are stronger than van der Waals bonds in BaF2 substrate, i.e. F-F van der Waals bonds. In this paper, we present different techniques to recover fresh surfaces, from classical methods (such as sputtering and annealing) working for all types of films and top boost methods, which could be less used due to the strong bonds in some films. Moreover, we were able to recover the immaculate surfaces for non-capped samples. We present here in a systematic study by low energy electron diffraction (LEED), x-ray photoemission spectroscopy (XPS), and angle-resolved photoemission spectroscopy (ARPES).

Gas-sensing features of nanostructured tellurium thin films.

Nanocrystalline and amorphous nanostructured tellurium (Te) thin films were grown and their gas-sensing properties were investigated at different operating temperatures with respect to scanning electron microscopy and X-ray diffraction analyses. It was shown that both types of films interacted with nitrogen dioxide, which resulted in a decrease of electrical conductivity. The gas sensitivity, as well as the response and recovery times, differed between these two nanostructured films. It is worth mentioning that these properties also depend on the operating temperature and the applied gas concentration on the films. An increase in the operating temperature decreased not only the response and recovery times but also the gas sensitivity of the nanocrystalline films. This shortcoming could be solved by using the amorphous nanostructured Te films which, even at 22 °C, exhibited higher gas sensitivity and shorter response and recovery times by more than one order of magnitude in comparison to the nanocrystalline Te films. These results were interpreted in terms of an increase in disorder (amorphization), leading to an increase in the surface chemical activity of chalcogenides, as well as an increase in the active surface area due to substrate porosity.

The environmental stability characterization of exfoliated few-layer CrXTe3 (X = Si, Ge) nanosheets

Two dimensional van der Waals layered CrXTe3 (X = Si, Ge) crystals have attracted great attention recently due to the observation of fascinating long range magnetic order. However, the detailed environmental stability of surface chemical reactivity of CrXTe3 has not been explored yet. Herein, we have studied the ambient air oxidation of exfoliated few-layer CrXTe3 (X = Si, Ge) nanosheets using optical microscopy, atomic force microscopy and Raman spectra. The optical contrast decreases, and surface roughness and thickness increase with exposure time for few-layer CrXTe3. The stability of CrGeTe3 flakes is superior to that of CrSiTe3. CrSiTe3 nanosheets degrade almost immediately after exposure to air, while CrGeTe3 above 11 nm can be air stable for 24 h. Under high power laser excitation or exposure to the air for sufficient time, it is found that amorphous Te and TeOx are formed initially and continually converted to TeO2, resulting in the degradation of CrXTe3. Our results shed light on environmental effects on the surface evolution of CrXTe3 and highlight the need for developing suitable protecting strategies in future CrXTe3-based devices.

Surface structures of tellurium on Si(111)–(7×7) studied by low-energy electron diffraction and scanning tunneling microscopy

The Te-covered Si(111) surface has received recent interest as a template for the epitaxy of van der Waals (vdW) materials, e.g. Bi2Te3. Here, we report the formation of a Te buffer layer on Si(111)–(7×7) by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While deposition of several monolayer (ML) of Te on the Si(111)–(7×7) surface at room temperature results in an amorphous Te layer, increasing the substrate temperature to 770 K results in a weak (7×7) electron diffraction pattern. Scanning tunneling microscopy of this surface shows remaining corner holes from the Si(111)–(7×7) surface reconstruction and clusters in the faulted and unfaulted halves of the (7×7) unit cells. Increasing the substrate temperature further to 920 K leads to a Te/Si(111)–(23×23)R30° surface reconstruction. We find that this surface configuration has an atomically flat structure with threefold symmetry.

Permittivity of Ge, Te and Se thin films in the 200–1500 nm spectral range. Predicting the segregation effects in silver

Optical properties of well-known bulk materials can be significantly modified by decreasing dimensions to nm-size. Using Molecular Beam Epitaxy (MBE) and e-beam Physical Vapour Deposition (PVD) we have fabricated 20–30 nm-thick amorphous Ge, Te and Se films. The permittivities of investigated layers have been extracted from measurements of the Ψ and Δ ellipsometric azimuths. We found that for all of the investigated films, the intensity of all bands in the permittivity spectrum is smaller than for bulk materials or thick (> 100 nm) films. Using the acquired optical constants along with the permittivity of a 20 nm-thick silver film, we have applied the Maxwell-Garnett equation to predict the permittivities of a silver film with Ge, Se or Te segregated in its structure. Implementing the parameters of 20 nm-thick Ge results in an 81 nm redshift of the segregation-induced band with respect to the experimental value, while implementing the parameters of 2 nm-thick Ge film results in a 95 nm blueshift of this band.

Analysis of the threshold switching mechanism of a Te–SbO selector device for crosspoint nonvolatile memory applications

The threshold switching mechanism of Te–SbO thin films with a unique microstructure in which a Te nanocluster is present in the SbO matrix is analyzed. During the electro-forming process, amorphous Te filaments are formed in the Te nanocluster. However, unlike conventional Ovonic threshold switching (TS) selector devices, it has been demonstrated that the off-current flows along the filament. Numerical calculations show that the off-current is due to the trap present in the filament. We also observed changes in TS parameters through controls in the strength or volume of the filaments.

CdTe/Zn(Mg)(Se)Te quantum dots for single photon emitters grown by MBE

We report on pseudomorphic MBE growth of CdTe/Zn(Mg)(Se)Te quantum dot (QD) structures on InAs(100) substrates and studies of their structural and optical properties. The QDs were fabricated by using a thermal activation technique comprising deposition of a strained CdTe 2D layer, covering it with amorphous Te, followed by fast thermal desorption of the Te layer, which results in a 2D-3D RHEED pattern transition. The QDs exhibit the surface density as low as ~1010cm−2. The influence of MBE growth parameters and the structure design on photoluminescence properties of the QDs are discussed. Single QD photoluminescence was observed at T=8K from the 200-nm-wide mesa-structures made of the CdTe QD structures, and the antibunching effect with g(2)(0)=0.16±0.04 was demonstrated. The peculiarities of MBE growth of ZnTe/MgTe/MgSe short-period superlattices nearly lattice-matched to InAs, which could serve as wide gap barriers for efficient electron and hole confinement in the CdTe/Zn(Mg)(Se)Te QDs, are also described.

Heterostructures with CdTe/ZnTe quantum dots for single photon emitters grown by molecular beam epitaxy

We report on the molecular beam epitaxy (MBE) of heterostructures with CdTe/ZnTe quantum dots (QDs) with relatively low surface density, which could be used as single-photon emitters. The QDs were formed on the surface of a 3.1- to 4.5-monolayer-thick two-dimensional strained CdTe layer by depositing amorphous Te layer and its fast thermal desorption. Subsequent thermal annealing of the surface with QDs in the absence of external Te flux led to strong broadening and short-wavelength shift of the QD photoluminescence (PL) peak. Measurement of the micro-PL spectra of individual CdTe/ZnTe quantum dots in fabricated mesastructures with a diameter of 200—1000 nm allowed estimation of the QD surface density as ~1010 cm–2.

Evolution of thermal conductivity of In3Sbβ Teγ thin films up to 550 °C

The temperature dependent thermal conductivity of In–Sb–Te thin films has been measured by modulated photothermal radiometry in the 20–550 °C range for samples with different Te content. Significant changes with temperature are observed and ascribed to a sequence of structural transformations on the basis of in‐situ Raman spectra. The data suggest that the as‐deposited material consisting of a mixture of polycrystalline InSb0.8Te0.2and amorphous Te first undergoes a progressive crystallization of the amorphous part, mostly above 300 °C. Further increase in temperature above 460 °C leads, for higher Te content in the alloy, to the formation of crystalline In3SbTe2, intertwined with a less conductive compound, possibly InTe and/or InSb. Upon cooling to room temperature, the initial polycrystalline InSb0.8Te0.2phase is mostly recovered along with other compounds, with a slightly higher thermal conductivity than that of the as deposited material. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Photo-induced oxidation and amorphization of trigonal tellurium: A means to engineer hybrid nanostructures and explore glass structure under spatial confinement

Controlled photo-induced oxidation and amorphization of elemental trigonal tellurium are achieved by laser irradiation at optical wavelengths. These processes are monitored in situ by time-resolved Raman scattering and ex situ by electron microscopies. Ultrathin TeO2 films form on Te surfaces, as a result of irradiation, with an interface layer of amorphous Te intervening between them. It is shown that irradiation, apart from enabling the controllable transformation of bulk Te to one-dimensional nanostructures, such as Te nanotubes and hybrid core-Te/sheath-TeO2 nanowires, causes also a series of light-driven (athermal) phase transitions involving the crystallization of the amorphous TeO2 layers and its transformation to a multiplicity of crystalline phases including the γ-, β-, and α-TeO2 crystalline phases. The kinetics of the above photo-induced processes is investigated by Raman scattering at various laser fluences revealing exponential and non-exponential kinetics at low and high fluence, respectively. In addition, the formation of ultrathin (less than 10 nm) layers of amorphous TeO2 offers the possibility to explore structural transitions in 2D glasses by observing changes in the short- and medium-range structural order induced by spatial confinement.

Peculiarities of Ultrathin Amorphous and Nanostructured Te Thin Films by Gas Sensing

Peculiarities of Ultrathin Amorphous and Nanostructured Te Thin Films by Gas Sensing

Photoemission study of amorphous and crystalline GeTe and (Ge,Mn)Te semiconductors

Resonant photoemission spectroscopy was applied to compare the valence band structure of Ge0.9Mn0.1Te and GeTe semiconductor layers deposited on BaF2 substrate in monocrystalline and amorphous forms. In (Ge,Mn)Te the contribution of Mn 3d5 electronic orbitals to density of states was found in three binding energy regions: below the top of the valence band (Eb<4.2eV), at the binding energy range 4.2–4.4eV, and in many-body satellite at 9–13eV. The comparative analysis of the photoemission spectra based on configuration interaction model showed that p–d hybridization effects, important for magnetic and optical properties of (Ge,Mn)Te, are stronger in monocrystalline than in amorphous (Ge,Mn)Te layers.