Te Atomic Layer Research Articles

Magnetic Anisotropy of Cr2Te3: Competition between Surface and Middle Layers

Recently, the orbital coupling in 2D materials has been demonstrated to have a significant impact on the magnetic anisotropy (MA) of CrTe2. The Te atomic layers on the surface layers determine the magnetocrystalline anisotropy (MCA) of the system due to orbital coupling. Herein, is proposed to investigate the surface and middle layers of Te atoms on MA. The MCA of consists of in‐plane and out‐of‐plane components, which are contributed by the surface layer and middle layer, respectively. Due to the lack of Cr–Te–Cr chemical bonds in the z‐axis, the surface layer produces coupling and results in the in‐plane MA. In addition, a tensile strain can enhance the coupling on the middle layer and lead to the out‐of‐plane MCA. At the same time, the out‐of‐plane MCA breaks the vertical mirror symmetry and an anomalous Hall effect has been detected on monolayer (1L) . Based on this result, an anomalous Hall device is designed to record and read information. The opposing contribution of the surface and middle layers provides a completely new way to understand 2D materials.

Boosting thermoelectric performance of PEDOT: PSS/Bi2Te3 hybrid films via structural and interfacial engineering

In this work, we synthesized poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) and PEDOT: PSS/Bi2Te3 hybrid composite film using a spin coating method. The maximum Seebeck coefficient (22 μVK−1) and power factor (57.18 μWm-1 K−2 around 300 K) were achieved at 0.4 wt% Bi2Te3. The electrical conductivity (σ) reached a maximum of 1467 Scm−1 at 300 K for 0.6 wt% Bi2Te3, which is more than three times higher than that of pure PEDOT: PSS. Two critical components contribute to the improved electrical transport performance, as identified by XRD, Raman spectroscopy, XPS, AFM, and SEM. First, the conductive polymer undergoes a structural transformation from a benzenoid to a quinoid configuration, enhancing conductivity. This transformation is due to the interaction between the π bonds of PEDOT: PSS and the Van der Waals forces between the tellurium (Te) atom layers of Bi2Te3. Second, the interfacial barrier between PEDOT: PSS and Bi2Te3 creates an energy-filtering effect that increases the Seebeck coefficient.

Processes to enable hysteresis-free operation of ultrathin ALD Te p-channel field-effect transistors.

Recently, tellurium (Te) has been proposed as a promising p-type material; however, even the state-of-the-art results couldn't overcome the critical roadblocks for its practical applications, such as large I-V hysteresis and high off-state leakage current. We developed a novel Te atomic layer deposition (ALD) process combined with a TeOx seed layer and Al2O3 passivation to detour the limitations of p-type Te semiconducting materials. Also, we have identified the origins of high hysteresis and off current using the 77 K operation study and passivation process optimization. As a result, a p-type Te field-effect transistor exhibits less than 23 mV hysteresis and a high field-effect mobility of 33 cm2 V-1 s-1 after proper channel thickness modulation and passivation. Also, an ultralow off-current of approximately 1 × 10-14 A, high on/off ratios in the order of 108, and a steep slope subthreshold swing of 79 mV dec-1 could be achieved at 77 K. These enhancements strongly indicate that the previously reported high off-state current was originated from interfacial defects formed at the metal-Te contact interface. Although further studies concerning this interface are still necessary, the findings herein demonstrate that the major obstacles hindering the use of Te for ultrathin p-channel device applications can be eliminated by proper process optimization.

Structural and topological phase transitions in Se doping-controlled intrinsic magnetic topological material FeBi2Te4

Defects profoundly affect the magnetic, electronic structure and topological behaviour of intrinsic magnetic topological insulators. Here, we investigate the magnetic, structural stability and topological properties of different Te atomic sites in the intrinsic magnetic topological insulator FeBi2Te4 with Se substitution of the FM-z magnetic order. When Se replaces the outermost site of the septuple layer (the Te atomic layer connected by van der Waals bonds), the Se–Bi bond length formed with its nearest neighbour Bi is drastically shortened and the lattice constant and volume contract accordingly. We speculate that this defect-induced chemical bond enhancement is related to the strong electronegativity of Se over Te. In contrast, the system has lower formation energy, a more stable structure, and enhanced magnetic moment when Se replaces the Te atomic layer inside the septuple layer. Also, we reveal a variety of topological phase transitions due to substitution defects. FeBi2Te4 is considered to belong to the topological classification of higher-order topological insulators with z 4 = 2. The system after Se substitution can be transformed into Weyl semimetals, strong 3D topological insulators, and unknown topological materials without symmetry-base indicators, respectively.

Atomically constructing a van der Waals heterostructure of CrTe2/Bi2Te3 by molecular beam epitaxy

A 2D heterostructure with proximity coupling of magnetism and topology can provide enthralling prospects for hosting new quantum states and exotic properties that are relevant to next-generation spintronic devices. Here, we synthesize a delicate van der Waals (vdW) heterostructure of CrTe2/Bi2Te3 at the atomic scale via molecular beam epitaxy. Low-temperature scanning tunneling microscopy/spectroscopy measurements are utilized to characterize the geometric and electronic properties of the CrTe2/Bi2Te3 heterostructure with a compressed vdW gap. Detailed structural analysis reveals complex interfacial structures with diversiform step heights and intriguing moiré patterns. The formation of the interface is ascribed to the embedded characteristics of CrTe2 and Bi2Te3 by sharing Te atomic layer upon interfacing, showing intercoupled features of electronic structure for CrTe2 and Bi2Te3. Our study demonstrates a possible approach to construct artificial heterostructures with different types of ordered states, which may be of use for achieving tunable interfacial Dzyaloshinsky–Moriya interactions and tailoring the functional building blocks in low dimensions.

Non-volatile control of topological phase transition in an asymmetric ferroelectric In2Te2S monolayer.

The coupling of topological electronic states and ferroelectricity is highly desired due to their abundant physical phenomenon and potential applications in multifunctional devices. However, it is difficult to achieve such a phenomenon in a single ferroelectric (FE) monolayer because the two polarized states are topologically equivalent. Here, we demonstrate that the symmetry of polarized states can be broken by constructing a Janus structure in a FE monolayer. We illustrate such a general idea by replacing a layer of Te atoms in the In2Te3 monolayer with S atoms. Using first-principles calculations, we show that the In2Te2S monolayer has two asymmetric polarized states, which are characterized by a metal and semiconductor, respectively. Importantly, as the spin-orbit coupling is included, a band gap (50.4 meV) is created in the metallic state, resulting in a non-trivial topological phase. Thus, it proves to be a feasible method to engineer non-volatile FE control of topological order in a single-phase system. We also demonstrate the underlying physical mechanism of topological phase transition, which is unveiled to be related to the weakened intrinsic electric field resulting from charge transfer. These interesting results provide a general way to design asymmetric FE materials and shed light on their potential application in non-volatile multifunctional devices.

Spectroscopic signature of surface states and bunching of bulk subbands in topological insulator ( Bi0.4Sb0.6)2Te3 thin films

High quality thin films of the topological insulator (Bi$_{0.4}$Sb$_{0.6}$)$_2$Te$_3$ have been deposited on SrTiO$_3$ (111) by molecular beam epitaxy. Their electronic structure was investigated by in situ angle-resolved photoemission spectroscopy and in situ scanning tunneling spectroscopy. The experimental results reveal striking similarities with relativistic ab-initio tight binding calculations. We find that ultrathin slabs of the three-dimensional topological insulator (Bi$_{0.4}$Sb$_{0.6}$)$_2$Te$_3$ display topological surface states, surface states with large weight on the outermost Te atomic layer, and dispersive bulk energy levels that are quantized. We observe that the bandwidth of the bulk levels is strongly reduced. These bunched bulk states as well as the surface states give rise to strong peaks in the local density of states.

Pressure-induced topological quantum phase transition in the magnetic topological insulator MnBi2Te4

In this paper, topological quantum phase transition was reported in the magnetic topological insulator MnBi2Te4 under pressure strain. Electronic and topological properties of the bulk anti-ferromagnetic MnBi2Te4 were investigated by first-principles calculations. We found that the band structure of MnBi2Te4 changes with the strain, resulting in a phase transition between metal and insulator. From the variation of charge-density distribution with strain, it was found that hydrostatic tensile strain is beneficial for increasing the interlayer spacing, thereby reducing the anti-ferromagnetic interaction between layers. On the contrary, the compressive strain promotes the strengthening of the bonding between the Te and Bi atomic layers. It was worth noting that the phase transition occurs at 2.12% strain when the band crossing is observed at Γ point, suggesting that the band gap has just closed. In addition, through the calculation of surface states, it is observed that, after the action of 2.12% strain, the bulk band gap of the system closes with the surface band gap reopens, achieving an intrinsic mechanism of strain modulation of the MnBi2Te4 antiferromagnetic bulk structure to undergoes a topological quantum phase transition. Our results provide feasible guidance not only for pressure-strain engineering of MnBi2Te4 experimentally but also for developing a meaningful strain-control mechanism for the electronic structures of other potential intrinsic magnetic insulators.

Electrostatic Potential Anomaly in 2D Janus Transition Metal Dichalcogenides

AbstractSymmetry breaking induced exotic physical properties is an eternal topic in scientific community. Due to lack of mirror symmetry, 2D Janus transition metal dichalcogenides (TMDs) exhibit many bizarre features; however, the physical mechanisms of most of these intrinsic properties are still unclear. Herein, a generalized and effective approach is developed to disclose the physical mechanism of electrostatic potential anomaly in 2D Janus TMDs, based on fast Fourier transform and Moore–Penrose generalized inverse matrix for separating Hartree potential and ionic potential from electrostatic potential, and conversely, calculating charge density distribution through Hartree potential. Through extensive numerical simulations and theoretical analyses, the electrostatic potential anomaly is expounded successfully, which is a pending issue in 2D Janus TMDs: the electrostatic potential energy at Se atomic layer is larger than that at S and Te atomic layers, which breaks the periodic law. Such an anomaly could be attributed to the competition between Hartree potential energy and ionic potential energy that emerges as a result of asymmetric charge transfer, atomic layer distance, and atomic species. This approach possesses universality, and is proved to be a robust method in dealing with the issues related to electrostatic potential.

Photoinduced Vacancy Ordering and Phase Transition in MoTe2.

We show that non-equilibrium dynamics plays a central role in the photoinduced 2H-to-1T' phase transition of MoTe2. The phase transition is initiated by a local ordering of Te vacancies, followed by a 1T' structural change in the original 2H lattice. The local 1T' region serves as a seed to gather more vacancies into ordering and subsequently induces a further growth of the 1T' phase. Remarkably, this process is controlled by photogenerated excited carriers as they enhance vacancy diffusion, increase the speed of vacancy ordering, and are hence vital to the 1T' phase transition. This mechanism can be contrasted to the current model requiring a collective sliding of a whole Te atomic layer, which is thermodynamically highly unlikely. By uncovering the key roles of photoexcitations, our results may have important implications for finely controlling phase transitions in transition metal dichalcogenides.

Reduced thermal conductivity of Bi-In-Te thermoelectric alloys in a eutectic lamellar structure

Directional solidification of Bi2Te3-In2Te3 eutectic alloys allows for exploiting both crystal anisotropy and internal interface density for the enhancement of thermoelectric properties. A lamellar structure with a spacing of 2 μm was produced for the eutectic alloy Bi-16.5In-60Te (at%). Coherent Bi2Te3-In2Te3 interfaces sharing the same Te atomic layer were found. The Bi2Te3-In2Te3 composites are n-type thermoelectric materials with lower thermal conductivity than each of the constituent phases. The eutectic alloy with a higher interface density exhibits a lower phonon thermal conductivity than off-eutectic alloys. The disparity between experimentally determined values and calculations using composite models concerning the thermoelectric properties demonstrates the enhanced potential of internal interfaces for improving thermoelectric properties.

Pressure-driven superconductivity in the transition-metal pentatellurideHfTe5

Layered transition-metal tellurides have attracted considerable attention because of their rich physics; for example, tungsten ditelluride WTe2 exhibits extremely large magnetoresistance; the tritelluride ZrTe3 shows a charge density wave transition at low temperature; and the pentatelluride ZrTe5 displays an enigmatic resistivity anomaly and large thermoelectric power. Recently some transition-metal tellurides are predicted to be quantum spin Hall insulators (e.g. ZrTe5 and HfTe5) or Weyl semimetals (e.g. WTe2 and MoTe2) and were subjected to intensive investigations. Here, we report on the discovery of superconductivity in hafnium pentatelluride HfTe5 under high pressure. We observe two structural phase transitions and metallization with superconductivity developing at around 5 GPa. A maximal critical temperature of 4.8 K is attained at a pressure of 20 GPa, and superconductivity persists up to the maximum pressure in our study (42 GPa). Theoretical calculations indicate that the superconductivity develops mainly in the Te atom layers at medium pressure and in the Te atom chains at high pressure.

Information on real-structure phenomena in metastable GeTe-rich germanium antimony tellurides (GeTe)nSb2Te3 (n ≥ 3) by semi-quantitative analysis of diffuse X-ray scattering

Abstract Quenching cubic high-temperature polymorphs of (GeTe)nSb2Te3 (n ≥ 3) yields metastable phases whose average structures can be approximated by the rocksalt type with 1/(n + 3) cation vacancies per anion. Corresponding diffraction patterns are a superposition of intensities from individual twin domains with trigonal average structure but pseudo-cubic metrics. Their four orientations are mirrored in structured diffuse streaks that interconnect Bragg reflections along the [001] directions of individual disordered trigonal domains. These streaks exhibit a “comet-like” shape with a maximum located at the low-angle side of Bragg positions (“comet head”) accompanied by a diffuse “comet tail”. 2D extended cation defect ordering leads to parallel but not equidistantly spaced planar faults. Based on a stacking fault approach, the diffuse scattering was simulated with parameters that describe the overall metrics, the concentration and distribution of cation defect layers, atom displacements in their vicinity and the stacking sequence of Te atom layers around the planar defects. These parameters were varied in order to derive simple rules for the interpretation of the diffuse scattering. The distance between Bragg positions and “comet heads” increases with the frequency of planar faults. A sharp distance distribution of the planar faults leads to an intensity modulation along the “comet tail” which for low values of n approximates superstructure reflections. The displacement of atom layers towards the planar defects yields “comets” on the low-angle side of Bragg positions. A rocksalt-type average structure is only present if the planar defects correspond to missing cation layers in the “cubic” ABC stacking sequence of the Te atom layers. An increasing amount of hexagonal ABA transitions around the defect layers leads to increasing broadening and splitting of the Bragg reflections which then overlap with the diffuse scattering. Based on these rules, the diffuse scattering of (GeTe)nSb2Te3 (n = 2, 4, 5, 12) crystals was analyzed by comparing simulated and experimental reciprocal space sections as well as selected streaks extracted from synchrotron data. With decreasing n, both the average distance between faults and thus the slab thickness decrease, whereas the probability of hexagonal ABA transitions increases. The quenched metastable phases can be understood as intermediates between the stable high-temperature phases, which exhibit a rocksalt-type structure with randomly disordered cations and vacancies on the cation position, and the trigonal layered structures, which are stable at room temperature and consist of distorted rocksalt-type slabs separated by equidistant defect layers.

Novel superstructure of the rocksalt type and element distribution in germanium tin antimony tellurides

A superstructure of the rocksalt-type observed in quenched CVT-grown single crystals of Ge3.25(7)Sn1.10(3)Sb1.10(3)Te6 was elucidated by X-ray diffraction using fourfold twinned crystals (space group P3¯m1, a=4.280(1)Å, c=20.966(3)Å). The structure is built up of distorted rocksalt-type building blocks typical for long-range ordered GST materials and substitution variants thereof. In contrast to those phases, an exclusive ABC-type cubic stacking sequence of the Te-atom layers is present. High-resolution electron microscopy reveals spheroidal domains with this structure (average diameter 25nm) whose stacking direction is perpendicular to the 〈111〉 directions of the basic rocksalt-type structure. Additional slab-like domains with a lateral extension up to 1µm occasionally result in a hierarchical structure motif. Due to the similar electron counts of the elements involved, resonant diffraction was used in order to elucidate the element distribution in rocksalt-type building blocks of the stable layered compound 39R-Ge3SnSb2Te7 (R3¯m, a=4.24990(4)Å, c=73.4677(9)Å). Sb tends to occupy the atom site close to the van der Waals gaps while Ge concentrates in the center of the building blocks.

DFT Studies of Pristine Hexagonal Ge1Sb2Te4(0001), Ge2Sb2Te5(0001), and Ge1Sb4Te7(0001) Surfaces

Ternary alloys of germanium, antimony, and tellurium (GexSbyTez) are not only prominent phase-change data-storage materials with intriguing properties; they also form precisely ordered crystal structures if one allows them to do so. Here, we study atomically pristine surfaces of representative GexSbyTez alloys using density functional theory (DFT) and suitable plane-wave-based slab models. In particular, we look at the hexagonal (0001) surfaces of Ge1Sb2Te4, Ge2Sb2Te5, and Ge1Sb4Te7 but with the intent to draw some general conclusions beyond those explicit compositions. The conditions for thermodynamic stability of the ternary surfaces (i.e., the space of permissible chemical potentials) are delineated, and a simple general expression to estimate the latter is given. Because of weak van der Waals-type interactions between Te atomic layers, we assess how surface energy computations are affected by the choice of exchange–correlation functional and also by the use of dispersion corrections (here, Grimme’s DFT+D2 scheme). LDA and PBE+D2 predicted surface energies are similar to a degree that is rather unexpected; generally, Te terminations are lowest in surface energy regardless of the compound and methodology. Implications for further surface studies of “less ideal’’ GexSbyTez surfaces are discussed.

Temperature dependent resonant X-ray diffraction of single-crystalline Ge2Sb2Te5

The element distribution in the crystal structure of the stable phase of the well-known phase-change material Ge2Sb2Te5 was determined at temperatures up to 471 °C using single crystals synthesized by chemical transport reactions. Because of the similar electron count of Sb and Te, the scattering contrast was enhanced by resonant diffraction using synchrotron radiation (beamline ID11, ESRF). A simultaneous refinement on data measured at the K-absorption edges of Sb and Te as well as at additional wavelengths off the absorption edges yielded reliable occupancy factors of each element on each position (a = 4.2257(2) A, c = 17.2809(18) A, Pm1, R1 (overall) = 0.037). The dispersion correction terms Δf′ were refined and match experimental ones obtained from fluorescence spectra by the Kramers–Kronig transform. The structure contains distorted rocksalt-type blocks of nine alternating cation and anion layers, respectively, which are separated by van der Waals gaps between Te atom layers. Ge atoms prefer the cation positions near the center of the rocksalt-type block (occupancy factors Ge0.60(4)Sb0.36(2)), Sb atoms the one near the van der Waals gap (Ge0.33(7)Sb0.66(4)). Anti-site disorder is not significant. During heating up to 471 °C and subsequent cooling, a reversible structural distortion was observed. The refinements show that with increasing temperature the first pair of anion and cation layers next to the van der Waals gap becomes slightly detached from the block and increasingly resembles a GeTe-type layer. Thus, the difference between interatomic distances in the 3 + 3 cation coordination sphere of the mixed Ge–Sb position next to the gap becomes more pronounced. The element distribution, in contrast, neither changes during the heating experiment nor upon long-time annealing. Thus, the behavior of 9P-Ge2Sb2Te5 single crystals is predominantly under thermodynamic control.

In depth analysis of complex interfacial processes: in situ electrochemical characterization of deposition of atomic layers of Cu, Pb and Te on Pd electrodes

A combination of cyclic voltammetry, electrogravimetry, and electrochemical impedance spectroscopy has been used to characterize, in situ, the underpotential deposition (UPD) of atomic layers of Cu, Pb and Te on Pd electrode surfaces. This approach provides co-adsorption and competitive adsorption of anions to be measured and quantified during the UPD processes, highlighting the complex competitive processes that can e.g. hinder the design of new catalysts. The formed Cu, Pb and Te atomic layers on the Pd electrode showed no evidence of anion co-adsorption or surface alloying effects, which indicates that these systems, when formed in a perchlorate medium, could act as building blocks for catalysts. The mode of deposition was found to vary greatly for each overlayer. Cu was found to form a compact monolayer on the Pd surface, while Te formed a bilayer structure on the Pd surface, of which ∼1/4 of a monolayer was found to be irreversibly adsorbed. The formation of Pb overlayers was complicated by background UPD of hydrogen and its absorption to the underlying Pd substrate. While perchloric acid media are suitable for the formation of the overlayer, catalytic application of the formed Pb-layers would require a higher pH to negate such processes.

Cu(111) Surface Passivation with Atomic Layers of Te, Se or I, Studied Using Auger Spectrscopy

Efforts are described to identify an atomic layer (AL) passivating agent for Cu, allowing it to be transferred from solution, between solutions, or from solution into vacuum without oxidation. Atomic layers investigated included tellurium, selenium and iodine. Potentials and pH used to form the AL were investigated. Elemental ratios from auger electron spectroscopy (AES), such as O/Cu, and low energy electron diffraction (LEED) patterns were used to characterize the Te, Se and I AL before and after exposure to oxygen. The AL for all three elements resulted in 1/3 ML coverage (√3x√3)R300 structures. The ability of an AL to passivate the Cu(111) surface was investigated by exposing it to solution vapor in 1 atm of UHP Ar for 5 minutes within the EC ante-chamber. The resulting surface was then characterized in the analysis chamber. The intent was to mimic transfer between tools in a FAB. In addition, AL coated Cu(111) substrates were exposed to atmospheric oxygen for over 10 minutes, followed by characterization. Exposure to solution vapor and 1 atm UHP Ar resulted in no significant oxygen uptake, though small oxygen signals were detected for the Se AL and the I AL. However, only the Te AL provided passivation to atmospheric oxygen for 10 minutes.

Enhancement of the emission from TeO2 nanorods by encapsulation with ZnO

AbstractTeO2‐core/ZnO‐shell nanorods were synthesized by a two–step process comprising thermal evaporation of Te powders and atomic layer deposition of ZnO. Scanning electron microscopy images exhibit that the core‐shell nanorods are 50 ‐ 150 nm in diameter and up to a few tens of micrometers in length, respectively. Transmission electron microscopy and X‐ray diffraction analysis revealed that the cores and shells of the core‐shell nanorods were polycrystalline simple tetragonal TeO2 and amorphous ZnO with ZnO nanocrystallites locally, respectively. Photoluminescence measurement revealed that the TeO2 nanorods had a weak broad violet band at approximately 430 nm. The emission band was shifted to a yellowish green region (∼540 nm) by encapsulation of the nanorods with a ZnO thin film and the yellowish green emission from the TeO2‐core/ZnO‐shell nanorods was enhanced significantly in intensity by increasing the shell layer thickness. The highest emission was obtained for 125 ALD cycles (ZnO coating layer thickness: ∼15 nm) and its intensity was much higher than that of the emission from the uncapsulated TeO2 nanorods. The origin of the enhancement of the emission by the encapsulation is discussed in detail. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of Ge Nanofilms Using Electrochemical Atomic Layer Deposition, with a “Bait and Switch” Surface-Limited Reaction

Ge nanofilms were deposited from aqueous solutions using the electrochemical analog of atomic layer deposition (ALD). Direct electrodeposition of Ge from an aqueous solution is self-limited to a few monolayers, depending on the pH. This report describes an E-ALD process for the growth of Ge films from aqueous solutions. The E-ALD cycle involved inducing a Ge atomic layer to deposit on a Te atomic layer formed on Ge, via underpotential deposition (UPD). The Te atomic layer was then reductively stripped from the deposit, leaving the Ge and completing the cycle. The Te atomic layer was bait for Ge deposition, after which the Te was switched out, reduced to a soluble telluride, leaving the Ge (one "bait and switch" cycle). Deposit thickness was a linear function of the number of cycles. Raman spectra indicated formation of an amorphous Ge film, consistent with the absence of a XRD pattern. Films were more stable and homogeneous when formed on Cu substrates, than on Au, due to a larger hydrogen overpotential, and the corresponding lower tendency to form bubbles.