Natural Superlattice Structure Research Articles

Excellent thermoelectric performance predicted in Sb2Te with natural superlattice structure* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11974300, 11974299, and 11704319), the Natural Science Foundation of Hunan Province, China (Grant No. 2021JJ30645), the Scientific Research Fund of Hunan Provincial Education Department, China (Grant Nos. 20K127, 20A503, and 20B582), the Program for Changjiang Scholars and Innovative

Using first-principles calculations combined with the Boltzmann transport theory, we explore the thermoelectric properties of natural superlattice (SL) structure Sb2Te. The results show that n-type Sb2Te possesses larger Seebeck coefficient of 249.59 (318.87) μV/K than p-type Sb2Te of 219.85 (210.38) μV/K and low lattice thermal conductivity of 1.25 (0.21) W/mK along the in-plane (out-of-plane) direction at 300 K. The excellent electron transport performance is mainly attributed to steeper density of state around the bottom of conduction band. The ultralow lattice thermal conductivity of Sb2Te is mainly caused by low phonon group velocity and strong anharmonicity. Further analysis shows that the decrease of group velocity comes from flatter dispersion curves which are contributed by the Brillouin-zone folding. The strong anharmonicity is mainly due to the presence of lone-pair electrons in Sb2Te. Combining such a high Seebeck coefficient with the low lattice thermal conductivity, maximum n-type thermoelectric figure of merit (ZT) of 1.46 and 1.38 could be achieved along the in-plane and out-of-plane directions at room temperature, which is higher than the reported values of Sb2Te3. The findings presented here provide insight into the transport property of Sb2Te and highlight potential applications of thermoelectric materials at room temperature.

Anomalously Low Heat Conduction in Single‐Crystal Superlattice Ceramics Lower Than Randomly Oriented Polycrystals

AbstractHeat conduction in ceramics is attributed to phonon propagation, which can be strongly suppressed at boundaries. Usually, polycrystals show lower thermal conductivity (κ) than single crystals, as polycrystals contain many grain boundaries. For functional applications in thermal management technologies, ceramics with low thermal conductivity are required. While grain boundary engineering is effective for reducing κ, its utilization is limited by the fact that other functional properties are often damaged. Here it is shown that single‐crystalline oxide of InGaO3(ZnO)m with a natural superlattice structure exhibits anomalously lower κ than polycrystals. Single‐crystalline films of InGaO3(ZnO)m (m = integer), which has a superlattice structure of InO2−/GaO(ZnO)m+ stacking along the c‐axis with a controllable layer thickness of m, are fabricated. It is found that the κ perpendicular to the superlattices decreases with decreasing m‐value, and the minimum κ is 1.1 W m−1 K−1 (m = 4, 5), which is lower than randomly oriented polycrystalline InGaO3(ZnO)m. On the other hand, the κ parallel to the superlattices have similar values with those of polycrystalline. The present finding suggests that layer boundaries between different components inside single crystal can also function as thermal resistance, which will be useful for the material design of thermal management technologies.

Colossal seebeck coefficient in Aurivillius phase-perovskite oxide composite

We propose an inexpensive, scalable approach for achieving extremely high values of the Seebeck coefficient (α) by exploiting the natural superlattice structure in Aurivillius phase oxides. In particular, we report an α ≈ 319 mV/K at 300 K in a composite of Aurivillius phase compound SrBi4Ti4O15 (as a matrix) and a perovskite phase material (e.g., La0.7Sr0.3MnO3 or, La0.7Sr0.3CoO3 as filler). Such a colossal value of α may be attributed to contributions from the enhanced density of states due to the effective low dimensional character of the Bi2O2 layer. The corresponding thermal conductivity (κ) and the electrical conductivity (σ) lies in the range 0.7–1.25 W/m-K and 10–100 μS/m, respectively at 300 K. Attributed to the high α values, such oxide composites can be used as suitable materials for thermopile sensors and highly sensitive bolometric applications. We anticipate that the demonstration of colossal α in oxide composites using a simple synthesis strategy also sets the stage for future material innovations for high temperature thermoelectric applications.

Effect of multisite alloying and chloride doping for realizing a high thermoelectric performance in misfit-layered chalcogenide

Abstract Misfit-layered chalcogenide has captured large attention due to its unique opportunity for optimizing figure of merit zT originating from the natural superlattice structure. Existing strategies mainly focus on single element doping at cation site, while multisite alloying, especially for the doping at anion site, has not yet properly understood. In this work, chloride doping at anion site as a novel method is utilized to create excess electrons for manipulating the carrier concentration, thus achieving an advancement on electronic performance (up to 1.0 mW K−2 m−1) by ∼35% in oriented textured (SnS)1.2(TiS2)2 alloys. Such a chloride anion doping also enables a reduction on the lattice thermal conductivity to 0.82 and 0.35 W K−1 m−1 in the in-plane and out-of-plane directions, respectively. In addition, the multisite alloying strengthen the phonon scattering for a further reduction on lattice thermal conductivity, thus leading to an enhancement of thermoelectric performance. Consequently, a record zT∼0.45 is realized in the in-plane direction, which demonstrates (SnS)1.2(TiS2)2 based material as a promising thermoelectric candidate and the multisite alloying as an effective strategy for zT enhancement.

InFeO3 photoelectrode with two-dimensional superlattice for visible- and ultraviolet-light-driven water splitting

Thin film of indium iron oxide, InFeO3, with a two-dimensional natural superlattice structure was proposed as one of the candidate photoelectrodes for visible- and ultraviolet-light-driven water splitting. A first principle calculation shows that InFeO3 is a charge transfer-type insulator with a bandgap energy (Eg) of 2.5 eV. Flat band potentials are calculated from Mulliken electronegativities, and they show an appropriately placed conduction band to enable H2 evolution. Despite its wide Eg, the photocurrent of InFeO3 in the visible region is comparable to that of α-Fe2O3 (Eg = 2.1 eV), a conventional oxide photoelectrode, possibly owing to its intrinsic structural anisotropy, reducing recombination losses of photogenerated electron–hole pairs. Furthermore, InFeO3 shows enhanced light absorption and photocurrent in the ultraviolet region compared with α-Fe2O3. The onset potential of photocurrent of InFeO3 is significantly lower than that of α-Fe2O3. These properties suggest that InFeO3 is a promising candidate for water splitting by visible and ultraviolet light.

Atomic structure of cobalt doped copper ferrite thin film

Abstract Copper ferrite (CuFe2O4) has been well known because of its interesting physical properties, and it has been used as a candidate for many applications such as Li-ion storage, gas sensors, catalysts, and magnetic devices. Copper ferrite (CuFe2O4) structure is of significant importance in the context of engineered materials and their structure development due to the natural superlattice structure. In this work, copper ferrite (CuFe2O4) nanostructures with cobalt doped were prepared in the form on glass substrates by a spin-coating technique. The different doped amounts of cobalt (Co) doped copper ferrite thin films were used to prepare thin film at room temperature and then post-annealed at 500◦C in the air up to 5 hours. The nanostructures of fabricated thin films are characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS). The chemical composition of nanostructure in thin film form can be extracted XPS due to their surface sensitivities. The property modification by cobalt doping in copper ferrite (CuFe2O4) thin film was systematically investigated for various mixing ratios and post-anneal temperatures. It was found that the XPS spectra of copper 2p peak exhibited the significant change for the modified atomic arrangement of nanostructure thin film after etching.

Enhancement of the Thermoelectric Properties of BiCuSeO via In Doping and Powder Size Controlling

BiCuSeO is regarded as one of the most promising oxygenated thermoelectric materials because of its special natural superlattice structure and corresponding ultra-low thermal conductivity. In order to improve the thermoelectric performance, we should pay attention to both composition adjustment and structure adjustment. In this paper, we report a study which combines the two ways to improve the electrical transporting performance and suppress the thermal transporting performance, and we have effectively improved the thermoelectric performance. Firstly, by adjusting the composition with In doping at the Bi site, the band gap widens, the energy offset between heavy band and light band increases, and the mobility and electrical transporting performance improves correspondingly. The maximum ZT value is increased to 0.61. Based on this, the thermoelectric properties are further improved in the whole temperature range by increasing the mechanical alloying strength to refine the powders and adjusting the proportion of fine powder and coarse powder. Grain refinement increases the Cu vacancies and corresponding electrical properties. In addition, Grain refinement contributes to enhancing phonon scattering and effectively reduces the lattice thermal conductivity. The thermal conductivity decreases significantly in the whole temperature range because grain refinement can significantly enhance phonon scattering and reduce the lattice thermal conductivity. The appropriate proportion of coarse powder and fine powder can make full use of the fine grains to enhance phonon scattering while enlarging the frequency range of scattered phonons, thus further reducing the thermal conductivity. Finally, the ZT value increased to 1.08.

Improved spectral and temporal response of MSM photodetectors fabricated on MOCVD grown spontaneous AlGaAs superlattice

A co-planar metal-semiconductor-metal nonsymmetrical back to back Schottky diode photodetector using natural superlattice AlGaAs grown by metalorganic vapor phase epitaxy on GaAs (100) has been reported. The detection efficiency and photoresponse of the superlattice based device are found significantly superior compared to the one based on high temperature annealed homogeneous AlGaAs. Under a forward bias of 1 V, the peak values of responsivity, detectivity and sensitivity were 10.133 mA/W, 7.6 × 1011 cmHz1/2W−1, 81.06 cm2/W and 1.14 mA/W, 7.05 × 1010 cmHz1/2W−1, 2.82 cm2/W for the device with as-grown natural superlattice and homogeneous composition of AlGaAs, respectively. Besides, the device with natural superlattice structure showed much faster response to the pulsed light with rise and decay time of 560 μs and 1 ms as compared to 2 and 7 ms, respectively for the device with disordered bulk AlGaAs. The superior spectral and temporal characteristics of the device are explained by a model based on a third diode representing the net effect due to the superlattice modulations along with two Schottky diodes at the metal-semiconductor junctions. The third barrier, which is basically due to the periodic modulation in aluminium composition, plays an important role in enhancement of the photocurrent owing to the activation of the superlattice channels under light while keeping the dark current small. The fast sweeping of the photogenerated carries by the intrinsic electric field at the heterointerfaces in the active semiconducting layer makes the characteristic times of the device with the superlattice structures much smaller than one with homogeneous AlGaAs. Degradation in photoresponse and speed is attributed to the interdiffusion as an effect of thermal annealing.

Anisotropic magnetic property and exchange bias effect in a homogeneous Sillen-Aurivillius layered oxide

A new Sillen-Aurivillius oxide Bi7Fe2Ti2O17Cl (BFTOCl), in which four perovskite layers are sandwiched by Sillen slabs, is designed and prepared by the solid-state reaction. Compared with pure Aurivilius structure, the Sillen slab requires one less positive charge for charge compensation due to the intercalation of Cl anions between two (Bi2O2)2+, which increases the concentration of magnetic Fe3+ ions in perovskite slab, and may depress the interaction of Fe3+ ions existing in neighboring perovskite slabs. This unique natural superlattice structure and the highly orientated texture of the ceramic result in a complex spin structure and an interesting magnetic anisotropy. The magnetism in in-plane direction (0.00487 emu/g at 100 K) is about 2.5 times larger than that in out-of-plane direction. Remarkably, exchange bias anisotropy has been observed in the BFTOCl ceramic, of which the exchange bias fields in in-plane and out-of-plane directions are 345 and 174 Oe at 100 K, respectively.

Enhanced thermoelectric performances in BiCuSeO oxyselenides via Er and 3D modulation doping

BiCuSeO is a kind of promising oxide-based thermoelectric material with special natural superlattice structure. Herein, the effects of Er doping and modulation doping on the thermoelectric properties of BiCuSeO have been investigated. The results indicate that Er doping can tune the Fermi level, widen the band gap, reduce the energy offset between the heavy band and the light band, and then greatly improve the electrical conductivity, power factor and ZT. Modulation doping based on Er doping can increase carrier concentration and power factor significantly while maintaining high carrier mobility. Meanwhile, due to the special heterostructure of the modulation doping sample, the phonon scattering is greatly enhanced, and the corresponding lattice thermal conductivity is significantly reduced. Finally, the maximum ZT value reaches 0.99 for the modulation doped Bi0.92Er0.08CuSeO bulk at 873 K, which was ~1.8 times as that of the pristine BiCuSeO sample.

Superconductivity in SnSb with a natural superlattice structure

We report the results of electrical resistivity, magnetic, and thermodynamic measurements on polycrystalline SnSb, whose structure consists of stacks of Sb bilayers and Sn4Sb3 septuple layers along the c-axis. The material is found to be a weakly coupled, fully gapped, type-II superconductor with a bulk Tc of 1.50 K, while showing a zero resistivity transition at a significantly higher temperature of 2.48 K. The Sommerfeld coefficient and upper critical field, obtained from specific heat measurements, are 2.29 mJ mol−1 K−2 and 520 Oe, respectively. Compositional inhomogeneity and strain effect at the grain boundaries are proposed as possible origins for the difference in resistive and bulk superconducting transitions. In addition, a comparison with the rock-salt structure SnAs superconductor is presented. Our results provide the first clear evidence of bulk superconductivity in a natural superlattice derived from a topological semimetal.

Characterization of the ScAlMgO4 cleaving layer by X-ray crystal truncation rod scattering

ScAlMgO4—easily cleaved in c-plane—forms a natural superlattice structure of a ScO2 layer and two Al0.5Mg0.5O layers stacking along c-axis. ScAlMgO4 is one of the RAMO4-type layered multicomponent oxides and a promising lattice-matching substrate material for InGaN and ZnO. Identification of the topmost layer and the surface atomic structure of the cleaved ScAlMgO4 (0001) are investigated by the X-ray crystal truncation rod scattering method. It is confirmed that ScAlMgO4 is cleaved between the two Al0.5Mg0.5O layers. The two parts separated at this interlayer are inversion symmetric to each other and without surface charge. This prevents parallel-plate-capacitor-like electrostatic force during the cleavage. Two different mechanisms are proposed for the two types of cleavage caused by the impact of a wedge and by the in-plane stress due to an overgrown thick GaN film. It is also revealed that about 10%–20% of the topmost O atoms are desorbed during a surface cleaning at 600 °C in ultra-high vacuum. Surface observations using reflection high-energy electron diffraction are possible only after the high-temperature cleaning because the electrical conduction caused by the oxygen deficiency prevents the charge-up of the insulating sample.

4p-YG-9 自然超格子GaInPの無秩序化過程 : TEMその場観察

4p-YG-9 自然超格子GaInPの無秩序化過程 : TEMその場観察

Mixed phase delafossite structured p type CuFeO2/CuO thin film on FTO coated glass and its Schottky diode characteristics

Delafossite structures are of significant importance in the recent context of development of structured and engineered materials because of their natural super lattice structure. The delafossite structured CuFeO2 mixed with crystalline CuO phase has been prepared by the simple chemical method of sol-gel technique on fluorine doped tin oxide (FTO) coated glass substrate. The mixed phase of delafossite structured CuFeO2 with crystalline CuO was appeared on annealing the film at 723K. The prepared film was characterized with X-ray diffraction measurements, atomic force microscopy, UV–Vis-NIR spectrophotometry and electrical I–V measurements. The optical energy band gap of 2.63eV was obtained from the UV–Vis-NIR spectrophotometric measurements. On electrical characterization of the film, the (I–V) measurements show a Schottky diode like characteristics.

Synthesis of homologous compounds Fe2O3(ZnO)m (m=6, 8, 34) by various selective microwave heating conditions

Homologous compounds, Fe2O3(ZnO)m, with a modulated natural superlattice structure, were synthesized within 10min at 1200°C in a simple solid-state reaction under 2.45 and 28GHz microwave irradiation. Transmission electron microscopy and energy dispersive X-ray spectroscopy results revealed that m decreased with increasing the synthesis temperature and microwave frequency. In the Zn–Fe–O system, γ-Fe2O3 was heated in H and E-field and ZnO was heated in E-field under 2.45GHz microwave irradiation. However, α, γ-Fe2O3 and calcined ZnO were heated under 28GHz microwave irradiation. This selective microwave heating resulted in the different diffusion conditions than conventional heating; therefore, homologous compounds with low m values were rapidly synthesized at low temperature.

First-principles Study of Intergrowth Bismuth Layer-structured Ferroelectric Bi7Ti4NbO21

Intergrowth bismuth layer-structured ferroelectric Bi7Ti4NbO21 material (iBTN) is a natural super lattice structure with excellent properties. Ferroelectric iBTN and parent compounds Bi3TiNbO9 (BTN) and Bi4Ti3O12 (BiT) were studied with first-principles method based on density functional theory (DFT). The band structures and the density of states of different compounds were calculated based on their optimized structures. The enthalpy of formation and band gaps of iBTN, BTN and BiT are -56.21, -30.72, -43.32 eV and 0.796, 2.535, 2.436 eV, respectively. The analysis shows that the intergrowth structure iBTN stays at a thermodynamic metastable state in relative to BTN and BiT. The electronic conductivity of iBTN is mainly depended on the perovskite layers, and the narrowed band gap is attributed to the down shift of the bottom of the conduction band. Detailed partial density of states of iBTN shows that different perovskite layers have different effects on the conduction band which suggests that the electrons choose their transportation channels in this material. This study can improve the understanding of the intergrowth mechanism and the influence between the structure and the electronic properties of iBTN.

Thermoelectric performance enhancement of (BiS)1.2(TiS2)2 misfit layer sulfide by chromium doping

A misfit layer sulfide (BiS)1.2(TiS2)2 with a natural superlattice structure has been shown to be a promising thermoelectric material, but its high carrier concentration should be reduced so as to further optimize the thermoelectric performance. However, ordinary acceptor doping has not succeeded because of the non-parabolic band structure. In this paper, we have successfully doped chromium ions into the Ti sites, which can maintain or even enhance the high effective mass of electrons so as to effectively improve ZT value. X-ray diffraction analysis, coupled with X-ray photoelectron spectroscopy, shows that chromium has been substituted into titanium sites in TiS2 layers and confirms its ionic state. The chromium doping has successfully reduced the carrier concentration with the subsequent reduction of electrical conductivity. Unlike other acceptor dopants (alkaline earth metals), chromium also enhances Seebeck coefficient and the effective mass, which can possibly be attributed to the formation of additional resonant states near Fermi level. Though the power factor does not improve, the significant reduction in the electronic part of the thermal conductivity leads to a measurable improvement in ZT.

Room temperature multiferroicity in Bi4.2K0.8Fe2O9+δ

Magnetoelectric multiferroics are materials that have coupled magnetic and electric dipole orders, which can bring novel physical phenomena and offer possibilities for new device functions. In this report, single-crystalline Bi4.2K0.8Fe2O9+δ nanobelts which are isostructural with the high-temperature superconductor Bi2Sr2CaCu2O8+δ are successfully grown by a hydrothermal method. The regular stacking of the rock salt slabs and the BiFeO3-like perovskite blocks along the c axis of the crystal makes the Bi4.2K0.8Fe2O9+δ nanobelts have a natural magnetoelectric–dielectric superlattice structure. The most striking result is that the bulk material made of the Bi4.2K0.8Fe2O9+δ nanobelts is of multiferroicity near room temperature accompanied with a structure anomaly. When an external magnetic field is applied, the electric polarization is greatly suppressed, and correspondingly, a large negative magnetocapacitance coefficient is observed around 270 K possibly due to the magnetoelectric coupling effect. Our result provides contributions to the development of single phase multiferroics.

Intercalation: Building a Natural Superlattice for Better Thermoelectric Performance in Layered Chalcogenides

A natural superlattice with composition (SnS)1.2(TiS2)2, built by intercalating a SnS layer into the van der Waals gap of layered TiS2, has been directly observed by high-resolution transmission electron microscopy (HRTEM). The thermoelectric performance is improved in the direction parallel to the layers because the electron mobility is maintained while simultaneously suppressing phonon transport, which is attributed to softening of the transverse sound velocities due to weakened interlayer bonding. In the direction perpendicular to the layers, the lattice thermal conductivity of (SnS)1.2(TiS2)2 is even lower than the predicted minimum thermal conductivity, which may be caused by phonon localization due to the translational disorder of the SnS layers parallel to the layers. Moreover, we propose a large family of misfit-layer compounds (MX)1+x(TX2)n (M = Pb, Bi, Sn, Sb, rare-earth elements; T = Ti, V, Cr, Nb, Ta; X = S, Se; n = 1, 2, 3) with a natural superlattice structure as possible candidate high-performance thermoelectric materials.

Structural and ferroelectric properties of epitaxial Bi5Ti3FeO15 and natural-superlattice-structured Bi4Ti3O12–Bi5Ti3FeO15 thin films

Bismuth-layer-structured multiferroic Bi5Ti3FeO15 (BTFO15) (number of pseudoperovskite blocks, m=4) and natural-superlattice-structured Bi4Ti3O12–Bi5Ti3FeO15 (BIT-BTFO15) (m=3 or 4) thin films were prepared on (001)-oriented and (110)-oriented La-doped (3.73 wt %) SrTiO3 (La-STO) single-crystal substrates using pulsed laser deposition. X-ray diffraction patterns of these thin films on (001) La-STO single crystals indicated that the obtained thin films were single-phase (00l)-oriented layered perovskite, and that the BIT-BTFO15 (m=3 or 4) natural-superlattice structure was also obtained. On (110) La-STO single crystal, layered perovskite (11l)-oriented thin films were also obtained. Moreover, cross-sectional transmission electron microscope images of these thin films revealed four pseudoperovskite blocks (m=4) in the BTFO15 thin films, and an intergrowth structure of three and four pseudoperovskite blocks (m=3 or 4) in BIT-BTFO15 sandwiched by two (Bi2O2)2+ layers. In ferroelectric D-E hysteresis loop measurements, BTFO15 (m=4) and BIT-BTFO15 (m=3 or 4) thin films on (110) La-doped STO single crystals produced clear ferroelectric hysteresis loops with double remanent polarizations (2Pr) of 49 μC/cm2 and 44 μC/cm2, respectively. However, BTFO15 and BIT-BTFO15 thin films on (001) La-doped STO single crystals had paraelectric characteristics. These results demonstrate that the spontaneous polarization vectors of the BTFO15 and BIT-BTFO15 thin films were parallel to the a axis of their unit cells.