Trigonal Tellurium Research Articles

Mechanochemical synthesis of binary phosphorus telluride: Short range structure and thermal properties

A binary phosphorus telluride sample (20 at% Te) was synthesized by mechanochemical solid state reaction from elemental powders. The X-ray diffraction pattern and neutron total structure factor show no detectable Bragg peaks, indicating that the sample is amorphous with respect to X-ray and neutron diffraction. The neutron and X-ray total pair distribution functions show correlation peak arising from PTe heteropolar bonds in addition to those arising from PP and TeTe bonds. The TeTe bond length is slightly shorter than that in the crystal structure of trigonal tellurium, suggesting a different environment from a polymeric chain structure. The thermal analysis results show that the binary phosphorus telluride is thermally unstable, that is, it decomposes above 200 °C into trigonal tellurium crystals and gaseous phosphorus, where phosphors begins to gasify at a lower temperature than the sublimation temperature of red phosphors.

Pancharatnam-Berry phase and kinetic magnetoelectric effect in trigonal tellurium

We study the kinetic magnetoelectric effect (current-induced magnetization including both the orbital and spin contributions) in three-dimensional conductors, specializing to the case of p-doped trigonal tellurium. We include both intrinsic and extrinsic contributions to the effect, which stem from the band structure of the crystal, and from disorder scattering, respectively. Specifically, we determine the dependence of the kinetic magnetoelectric response on the hole doping in tellurium, and show that the intrinsic and extrinsic effects dominate for low and high levels of doping, respectively. The results of this work imply that three-dimensional helical metals are promising for spintronics applications, in particular, they can provide robust control over current-induced magnetic torques.

Gyrotropic effects in trigonal tellurium studied from first principles

We present a combined ab initio study of several gyrotropic effects in p-doped trigonal tellurium (effects that reverse direction with the handedness of the spiral chains in the atomic structure). The key ingredients in our study are the k-space Berry curvature and intrinsic orbital magnetic moment imparted on the Bloch states by the chirality of the crystal structure. We show that the observed sign reversal with temperature of the circular photogalvanic effect can be explained by the presence of Weyl points near the bottom of the conduction band acting as sources and sinks of Berry curvature. The passage of a current along the trigonal axis induces a rather small parallel magnetization, which can nevertheless be detected by optical means (Faraday rotation of transmitted light) due to the high transparency of the sample. In agreement with experiment, we find that when infrared light propagates antiparallel to the current at low doping the current-induced optical rotation enhances the natural optical rotation. According to our calculations the plane of polarization rotates in the opposite sense to the bonded atoms in the spiral chains, in agreement with a recent experiment that contradicts earlier reports.

Observation of current-induced bulk magnetization in elemental tellurium

The magnetoelectric effect in bulk matter is of growing interest both fundamentally and technologically. Since the beginning of the century, the magnetoelectric effect has been studied intensively in multiferroic materials. However, magnetoelectric phenomena in materials without any (anti-)ferroic order remain almost unexplored. Here we show the observation of a new class of bulk magnetoelectric effect, by revisiting elemental trigonal tellurium. We demonstrate that elemental tellurium, which is a nonmagnetic semiconductor, exhibits current-induced magnetization. This effect is attributed to spin splitting of the bulk band owing to the lack of inversion symmetry in trigonal tellurium. This finding highlights magnetoelectricity in bulk matter driven by moving electrons without any (anti-)ferroic order. Notably, current-induced magnetization generates a magnetic field that is not circular around but is parallel to the applied current; thus, this phenomenon opens a new area of magnetic field generation beyond Ampere’s law that may lead to industrial applications.

Robust zero-energy bound states in a helical lattice

Atomic-scale helices exist as motifs for several material lattices. We examine a tight-binding model for a single one-dimensional monatomic chain with a $p$-orbital basis coiled into a helix. A topologically nontrivial phase emerging from this model supports a chiral symmetry-protected zero-energy mode localized to a boundary, always embedded within a continuum band, regardless of termination site. We identify a topological invariant for this phase that is related to the number of zero energy end modes by means of the bulk-boundary correspondence, and give strict conditions for the existence of the bound state. An additional class of gapped edge modes in the model spectrum has practical consequences for surface states in, e.g., trigonal tellurium and selenium and other van der Waals-bonded one-dimensional semiconductors.

Composite Weyl nodes stabilized by screw symmetry with and without time-reversal invariance

We classify the band degeneracies in 3D crystals with screw symmetry $n_m$ and broken $\mathcal P*\mathcal T$ symmetry, where $\mathcal P$ stands for spatial inversion and $\mathcal T$ for time reversal. The generic degeneracies along symmetry lines are Weyl nodes: Chiral contact points between pairs of bands. They can be single nodes with a chiral charge of magnitude $|\chi|=1$ or composite nodes with $|\chi|=2$ or $3$, and the possible $\chi$ values only depend on the order $n$ of the axis, not on the pitch $m/n$ of the screw. Double Weyl nodes require $n=4$ or 6, and triple nodes require $n=6$. In all cases the bands split linearly along the axis, and for composite nodes the splitting is quadratic on the orthogonal plane. This is true for triple as well as double nodes, due to the presence in the effective two-band Hamiltonian of a nonchiral quadratic term that masks the chiral cubic dispersion. If $\mathcal T$ symmetry is present and $\mathcal P$ is broken there may exist on some symmetry lines Weyl nodes pinned to $\mathcal T$-invariant momenta, which in some cases are unavoidable. In the absence of other symmetries their classification depends on $n$, $m$, and the type of $\mathcal T$ symmetry. With spinless $\mathcal T$ such $\mathcal T$-invariant Weyl nodes are always double nodes, while with spinful $\mathcal T$ they can be single or triple nodes. $\mathcal T$-invariant triples nodes can occur not only on 6-fold axes but also on 3-fold ones, and their in-plane band splitting is cubic, not quadratic as in the case of generic triple nodes. These rules are illustrated by means of first-principles calculations for hcp cobalt, a $\mathcal T$-broken, $\mathcal P$-invariant crystal with $6_3$ symmetry, and for trigonal tellurium and hexagonal NbSi$_2$, which are $\mathcal T$-invariant, $\mathcal P$-broken crystals with 3-fold and 6-fold screw symmetry respectively.

Solution-Processed Tellurium Nanowires for Stretchable and Wearable Piezoelectric Device

Nanowires made of materials with non-centrosymmetric crystal structure are under investigation for their piezoelectric properties and suitability for next-generation self-powered nanodevices. In this work, we report a systematic study on synthesis of trigonal tellurium nanowires with well-controlled dimensions. After assembling the tellurium nanowires on the stretchable substrates, a stretchable, soft piezoelectric nanogenerator has been developed with the capability to efficiently convert mechanical stimuli into electric power. We revealed the correlation between the piezoelectric outputs from the integrated devices and the diameters of as-produced tellurium nanowires. We further studied the feasibility of using the as-fabricated piezoelectric device for biomedical applications, such as gesture recognition. This work establishes the process-structure-property-performance relationship in the piezoelectric nanogenerator that may pave way for the nanomanufacturing and practical application of piezoelectric nanomaterials.

Band splitting and Weyl nodes in trigonal tellurium studied by angle-resolved photoemission spectroscopy and density functional theory

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on trigonal tellurium consisting of helical chains in the crystal. Through the band-structure mapping in the three-dimensional Brillouin zone, we found a definitive evidence for the band splitting originating from the chiral nature of crystal. A direct comparison of the band dispersion between the ARPES results and the first-principles band-structure calculations suggests the presence of Weyl nodes and tiny spin-polarized hole pockets around the H point. The present result opens a pathway toward studying the interplay among crystal symmetry, band structure, and exotic physical properties in chiral crystals.

Weyl Node and Spin Texture in Trigonal Tellurium and Selenium.

We study Weyl nodes in materials with broken inversion symmetry. We find based on first-principles calculations that trigonal Te and Se have multiple Weyl nodes near the Fermi level. The conduction bands have a spin splitting similar to the Rashba splitting around the H points, but unlike the Rashba splitting the spin directions are radial, forming a hedgehog spin texture around the H points, with a nonzero Pontryagin index for each spin-split conduction band. The Weyl semimetal phase, which has never been observed in real materials without inversion symmetry, is realized under pressure. The evolution of the spin texture by varying the pressure can be explained by the evolution of the Weyl nodes in k space.

Red bayberry-like ZnTe microstructures: Controlled synthesis, growth mechanism and enhanced photocatalytic performance

Uniform Red bayberry-like ZnTe microstructures have been synthesized by a facile solvothermal method. The effects of synthetic conditions, such as the amount of NaOH, the zinc source and reaction time were investigated. The results showed that the morphology of the ZnTe microstructures can be controlled by altering the concentration of NaOH. The growth mechanism of Red bayberry-like ZnTe microstructures was as follows: At the initial stage, the decomposition of Na2TeO3 generates Te nucleus. Trigonal tellurium has a highly anisotropic crystal structure consisting of helical chains of covalently bound atoms, thus resulting in Te nanowires are firstly formed in the reaction. Subsequently, the Te nanowires were reduced by CH3CHO generated from ethylene glycol, meanwhile the generated Te2− ions combined with Zn2+ ions generates ZnTe nucleus, finally forming Red bayberry-like ZnTe. When used as a catalytic of organic pollutants, the as-obtained Red bayberry-like ZnTe showed strong photocatalytic capability reached up to about 90%. The high photocatalytic activities of the Red bayberry-like ZnTe would have a potential photocatalytic application in environmental purification.

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.

High‐Power Density Piezoelectric Energy Harvesting Using Radially Strained Ultrathin Trigonal Tellurium Nanowire Assembly

A high-yield solution-processed ultrathin (<10 nm) trigonal tellurium (t-Te) nanowire (NW) is introduced as a new class of piezoelectric nanomaterial with a six-fold higher piezoelectric constant compared to conventional ZnO NWs for a high-volume power-density nanogenerator (NG). While determining the energy-harvesting principle in a NG consisting of t-Te NW, it is theoretically and experimentally found that t-Te NW is piezoelectrically activated only by creating strain in its radial direction, along which it has an asymmetric crystal structure. Based upon this mechanism, a NG with a monolayer consisting of well-aligned t-Te NWs and a power density of 9 mW/cm(3) is fabricated.

Novel Family of Chiral-Based Topological Insulators: Elemental Tellurium under Strain

Employing ab initio electronic structure calculations, we predict that trigonal tellurium consisting of weakly interacting helical chains undergoes a trivial insulator to strong topological insulator (metal) transition under shear (hydrostatic or uniaxial) strain. The transition is demonstrated by examining the strain evolution of the band structure, the topological Z2 invariant and the concomitant band inversion. The underlying mechanism is the depopulation of the lone-pair orbitals associated with the valence band via proper strain engineering. Thus, Te becomes the prototype of a novel family of chiral-based three-dimensional topological insulators with important implications in spintronics, magneto-optics, and thermoelectrics.

Controlled synthesis of tellurium nanowires and nanotubes via a facile, efficient, and relatively green solution phase method

Single-crystal trigonal tellurium nanowires and nanotubes were synthesized using a facile, efficient, and relatively green solution phase method with ethylene glycol as solvent and an alternative reducing agent in the presence of NaOH. Large-scale production of slender nanowires and hollow nanotubes with average diameters of 72 and 240 nm, respectively, was achieved by increasing the temperature from 170 °C to 200 °C. Tellurium morphology from nanowires to nanotubes was also controlled by adjusting the NaOH dosage. Scanning electron micrographs show that low temperature or NaOH insufficiency promotes the formation of slender nanowires but counteracts the synthesis of tellurium nanotubes. Detailed reaction equations based on the NaOH dosage were obtained. Preferential growth orientation of [001] was observed in the nanowire and nanotube through high-resolution transmission electron microscopy. Further formation mechanisms dependent on time were systematically studied.

Growth of tellurium nanowire bundles from an ionic liquid precursor

Trigonal tellurium nanowire bundles have been successfully synthesized from an ionic liquid precursor via a directed-growth route. The as-obtained samples were well characterized by X-ray diffraction, scanning electron microscope and high-resolution transmission electron microscope. Moreover, the influences of experimental parameters (i.e., ionic liquid, precursor concentration, and reaction temperature) on the morphology of final samples are discussed. Furthermore, the formation mechanism of trigonal tellurium nanowire bundles are well explained from the viewpoints of Ostwald ripening and kinetics of ionic liquid ([TEAH]+[BF4]−). Our methodology provides a simple and convenient route to inorganic nanomaterials with high hierarchical nanostructures. In addition, our obtained trigonal tellurium nanowire bundles may potentially act as templates to produce metal telluride-based thermoelectric materials (i.e., Bi2Te3 and PbTe) with excellent thermoelectric performance.

Pressure dependence of Raman modes in double wall carbon nanotubes filled with 1D Tellurium

The preparation of highly anisotropic one-dimensional (1D) structures confined into carbon nanotubes (CNTs) in general is a key objective in nanoscience. In this work, capillary effect was used to fill double wall carbon nanotubes (DWCNTs) with trigonal Tellurium. The samples are characterized by high resolution transmission electronic microscopy and Raman spectroscopy. In order to investigate their structural stability and unravel the differences induced by intershell interactions, unpolarized Raman spectra of radial and tangential modes of DWCNTs filled with 1D nanocrystalline Te excited with 514nm were studied at room temperature and high pressure. Up to 11GPa we found a pressure coefficient of 3.7cm−1GPa−1 for the internal tube and 7cm−1GPa−1 for the external tube. In addition, the tangential band of the external and internal tubes broaden and decrease in amplitude. All findings lead to the conclusion that the outer tube acts as a protection shield for the inner tube (at least up 11GPa). No pressure-induced structural phase transition was observed in the studied range.

Ageing effect on mechanically alloyed ZnTe nanocrystals

Abstract After three years and three months keeping mechanically alloyed Zn 50 Te 50 sample and its annealed portion under ambient conditions, these samples were re-investigated using X-ray diffraction, differential scanning calorimetry and Raman spectroscopy measurements to verify changes on structural, thermal and optical properties with ageing. The X-ray diffraction patterns of the aged-as-milled samples showed that the zinc blende ZnTe phase fraction decreased and the minority trigonal tellurium and hexagonal ZnO phase fractions increased with ageing. Similar (but less intense) behavior was observed for the aged-annealed sample, except by the nucleation of the β -TeO 2 phase with ageing. Structural parameters, phase fractions, average crystallite sizes and microstrains of all crystalline phases were obtained from Rietveld analyses of the X-ray patterns for both aged samples. The DSC results corroborate the trigonal Te phase fraction variation with ageing found by XRD analyses. Raman measurements of aged ( as-milled and annealed ) samples showed an huge intensity decreasing and even disappearing of the zinc blende ZnTe phase signals with ageing, which cannot be explained by the zinc blende phase fraction variations observed by X-ray diffraction. Then, as trigonal Te features are predominant in the Raman spectra of aged samples (there are no evidences of Raman signals of the minority phases), one can suggests a kind of Te capping effect on the ZnTe, ZnO and TeO 2 nanocrystals.

X-ray diffraction, Raman, and photoacoustic studies of ZnTe nanocrystals

Nanocrystalline ZnTe was prepared by mechanical alloying. X-ray diffraction (XRD), energy dispersive spectroscopy, Raman spectroscopy, and photoacoustic absorption spectroscopy techniques were used to study the structural, chemical, optical, and thermal properties of the as-milled powder. An annealing of the mechanical alloyed sample at 590 °C for 6 h was done to investigate the optical properties in a defect-free sample (close to bulk form). The main crystalline phase formed was the zinc-blende ZnTe, but residual trigonal tellurium and hexagonal ZnO phases were also observed for both as-milled and annealed samples. The structural parameters, phase fractions, average crystallite sizes, and microstrains of all crystalline phases were obtained from Rietveld analyses of the X-ray patterns. Raman results corroborate the XRD results, showing the longitudinal optical phonons of ZnTe (even at third order) and those modes of trigonal Te. Nonradiative surface recombination and thermal bending heat transfer mechanisms were proposed from photoacoustic analysis. An increase in effective thermal diffusivity coefficient was observed after annealing and the carrier diffusion coefficient, the surface recombination velocity, and the recombination time parameters remained the same.

Superlong High-Quality Tellurium Nanotubes: Synthesis, Characterization, and Optical Property

Single-crystalline trigonal tellurium (t-Te) nanotubes with sloping cross-section and hexagonal cross-section can be selectively synthesized on a large scale by a simple solvothermal reduction route, using tellurium dioxide (TeO 2) as tellurium source and ethylene glycol (EG) as both a reducing agent and a solvent in the presence of cetyltrimethyl ammonium bromide (CTAB) and cellulose acetate (CA), respectively. The individual Te nanotubes with cylindrical morphology and open ends have outer diameters of 100–500 nm, wall thicknesses of 50–100 nm, and lengths of 150–200 µm. Both kinds of Te nanotubes grow along the [001] direction and have excellent crystallinity. The optical properties and the stability in ethanol of the t-Te nanotubes with sloping cross section have been investigated.

Preparation of Fluorescent Tellurium Nanowires at Room Temperature

We have demonstrated a simple approach to the synthesis of fluorescent trigonal tellurium (t-Te) nanowires in aqueous solution at room temperature. The t-Te nanowires were prepared from the reduction of tellurium dioxide (TeO2) with concentrated hydrazine solution through deposition of Te atoms that were reduced from telluride ions (Te2−) and dissolved from amorphous tellurium (a-Te) nanoparticles onto t-Te nanocrystallines. By carefully controlling the growth time from 40 to 120 min, we prepared different sizes of t-Te nanowires; the length changed from 251 to 879 nm, while the diameter only grew from 8 to 19 nm. The absorption wavelength against the diameter of t-Te nanowires displays the diameter dependence of band I (<300 nm). On the other hand, the absorption wavelength against the length of t-Te nanowires displays the length dependence of band II (>600 nm). By increasing the size of t-Te nanowires, their absorption is under a red shift. For example, the maximum wavelengths of the absorption bands for the t-Te nanowires with the lengths of 251 nm are 271 and 602 nm, while those for 879 nm t-Te nanowires are at 280 and 687 nm, respectively. Deconvolution of the photoluminescence profile for t-Te nanowires obtained at 120 min yields four Gaussian peaks centered at 334, 397, 460, and 507 nm.