Heterostructure System Research Articles

Magnetoelectric Coupling Induced Orbital Reconstruction and Ferromagnetic Insulating State in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 Heterostructures.

Novel phenomena at the ferromagnetic/ferroelectric interface have generated much interest. Here, a ferromagnetic insulating state with the Curie temperature about 268-286 K in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 heterostructures is induced and modulated by varying the PbZr0.52Ti0.48O3 thickness. An abnormally enlarged c/a ratio in La0.67Sr0.33MnO3 by strain-based coupling effect leads to d3z2-r2 orbital preferable occupancy. This orbital reconstruction modulates effective electron transfer and finally leads to a ferromagnetic insulating state. Valence change induced by charge-based coupling effect could be partially responsible for change in the Curie temperature in the strongly correlated electron system of La1-xSrxMnO3. This work provides a deeper understanding of strain effects near the ferromagnetic/ferroelectric interface, especially in a PbZr1-yTiyO3/La1-xSrxMnO3 heterostructure system.

Large Photoresponsivity in the Amorphous‐TiO2/SrRuO3 Heterostructure

Thin‐film heterostructures are effective in enhancing the performance or triggering novel physical phenomena of optoelectronic applications. For example, the epitaxial heterostructures of ultraviolet‐light‐sensitive TiO2 with metallic SrRuO3 can acquire visible‐light functionalities using the hot carrier injection mechanism. Therefore, the TiO2/SrRuO3 heterostructure system has recently attracted increasing interest. Herein, the amorphous‐TiO2/SrRuO3 heterostructure is fabricated and compared to the epitaxial TiO2/SrRuO3. As opposed to the occurrence of the visible‐light photovoltaic effect in the epitaxial TiO2/SrRuO3, the amorphous‐TiO2/SrRuO3 heterostructure herein exhibits different optoelectronic functionality, specifically the photoresistor behavior. The amorphous‐TiO2/SrRuO3 heterostructure achieves a photoresponsivity of 6.56 A W−1 at 1 V. Such a performance is hardly obtained in typical oxide‐based photoresistors. The analyses of the crystalline and electronic structures show that it is due to the defect‐induced high electron doping in the amorphous TiO2 with hot carrier injection mechanism. This study discusses the correlation between the hot carrier injection and band diagram, which provides more degrees of freedom in designing potential optoelectronic devices.

Origin of the enhanced photocatalytic activity of graphitic carbon nitride nanocomposites and the effects of water constituents

A metal-free heterostructure system composed of graphitic carbon nitride (g-C3N4), reduced graphene oxide (rGO), and cyclooctasulfur (α-S) is developed as a facile route for establishing efficient photocatalysts. We aim to identify the governing factors that contributed to the photochemical performance of g-C3N4/rGO/α-S nanocomposites, which are important to advance potential applications but remain unexplored. The results indicate that the constituent ratio of each component in g-C3N4/rGO/α-S composites leads to varying surface microstructure, band alignment, and photochemical properties. The enhanced visible-light photocatalytic activity originates from an upward shift of the conduction bands toward higher energies, higher content of sp2-hybridized pyridine nitrogen in triazine rings (CN–C), and a lower amount of hydrogen bonds. The established structural integration informs a guiding framework for the design of emerging g–C3N4–based nanocomposites. Additionally, the influence of humic acid (HA) on photocatalytic decontamination was studied and shows an overall detrimental effect on the photocatalytic activity of g-C3N4 nanocomposites. Although HA advanced the photoexcited electrons, and therefore the reactive oxygen species, as well as enhanced the adsorption of the pollutants onto g-C3N4, especially at lower pH, attenuation of oxygen transfer as a result of active site competition between oxygen and target pollutants was found.

Comparison of NiFe-LDH based heterostructure material towards photocatalytic rhodamine B and phenol degradation with water splitting reactions

This work deals with the synthesis of photocatalytically superactive NiFe-LDH based heterostructure system by adopting co-precipitation, weight impregnation, electrostatic self-assembly, in-situ photoreduction and in-situ hydrothermal route, which signifies their capability towards photodegradation of Rhodamine B (RhB), and Phenol with H2 and O2 evolution. The synthesized heterostructures includes g-C3N4/NiFe-LDH, Ag@Ag3PO4/g-C3N4/NiFe-LDH, g-C3N4/N-rGO/NiFe-LDH, and MoS2/NiFe-LDH, respectively. Consequently, this comparative work describes NiFe-LDH based heterostructure reflected its superior efficiency in the field of environment abatement, and energy conversion.

High electron density β-(Al0.17Ga0.83)2O3/Ga2O3 modulation doping using an ultra-thin (1 nm) spacer layer

This report discusses the design and demonstration of β-(Al0.17Ga0.83)2O3/Ga2O3 modulation doped heterostructures to achieve high sheet charge density. The use of a thin spacer layer between the Si delta-doping and the heterojunction interface was investigated in a β-(AlGa)2O3/Ga2O3 modulation doped structure. It is shown that this strategy enables a higher two-dimensional electron gas (2DEG) sheet charge density up to 4.7 × 1012 cm−2 with an effective mobility of 150 cm2/V s. The presence of a degenerate 2DEG channel was confirmed by the measurement of a low temperature effective mobility of 375 cm2/V s and the lack of carrier freeze out from low temperature capacitance voltage measurements. The electron density of 4.7 × 1012 cm−2 is the highest reported 2DEG density obtained without parallel conducting channels in a β-(AlxGa(1−x))2O3/Ga2O3 heterostructure system.

Construction of Z-Scheme CdS/WO3 Heterostructure on Photonic Crystals with High-Efficiency Photocatalysis Performance

Tungsten oxide (WO3) has been widely used as a photocatalyst for its suitable bandgap and relative stability. However, the high recombination rate of photo-generated carriers impedes raising the photocatalytic efficiency of tungsten oxide (WO3) beyond its current value. In this work, a cadmium sulfide (CdS) and tungsten oxide (WO3) Z-scheme heterostructure system was constructed which shows improved photocatalytic performance due to better hole-electron separation. In addition, photonic crystal fluorine-doped SnO2 (FTO) was applied to the photoanode as a supporting framework for CdS and WO3, which can also play a role in assisting photocatalytic materials to assimilate light at the same time. As a result, the overall photocatalytic performance of a Z-scheme system on a photonic crystal framework has been significantly increased beyond that of conventional tungsten oxide.

Dynamic Epitaxial Crystallization of SnSe2 on the Oxidized SnSe Surface and Its Atomistic Mechanisms.

Surface oxidation of SnSe sharply reduces its thermoelectric properties though the bulk single-crystalline materials of SnSe claim the record high zT values. Investigation on the oxidation behaviors of SnSe together with the subsequent phase transition and element migration is fundamentally important to maintaining the ultrahigh zT values, with a potential for further improvement. In this work, we disclose the dynamic epitaxial crystallization of SnSe2 on the amorphous surface of partially oxidized SnSe crystals and the corresponding atomistic mechanisms via transmission electron microscopy (TEM). It is revealed that the thermally annealed amorphous surface crystallized to SnO2 and SnSe2 in the outermost and secondary layers, respectively, forming distinctive SnSe/SnSe2/SnO2 multilayer heterostructures with specific orientation relationships between the two selenides. By means of in situ scanning TEM (STEM), the dynamic epitaxial crystallization process of SnSe2 was revealed when the oxidized SnSe surface was subjected to electron beam irradiation. Through the atomic-scale characterization and modeling analysis, we find that the exposed dangling Se diatoms on the SnSe surface serve as nucleation sites for lateral epitaxial crystallization of SnSe2. The same valence and similar coordination configuration of Se atoms in these two phases are supposed to facilitate the sharing of Se atoms, with lattice distortions in the SnSe2/SnSe interface. These findings are valuable for understanding the surface oxidation behavior of SnSe and revealing the interface structures of SnSe2/SnSe heterojunctions and also offering new routes for SnSe-related multilayer or heterostructure system design.

Effects of doping and biaxial strain on the electronic properties of GaN/graphene/WS2 trilayer vdW heterostructure

Based on the calculation using first-principles, we discussed adjustment for electronic properties of the GaN/graphene/WS2 trilayer vdW heterostructure by doping and biaxial strain. Mg or Se doping can regulate the band gap of the GaN/graphene/WS2 trilayer vdW heterostructure and achieve p-type or n-type dopant in graphene and the trilayer heterostructure system. Band gap decreases with the increase in positive strain, and a p-type Schottky barrier is always maintained. As the negative strain increases, the band gap reaches its maximum at e = − 3% and then gradually decreases. And after |e| ≥ | − 5|%, it changes to an indirect band gap. When |e| ≥ | − 7|%, the Schottky contact type changes from p-type to n-type. Electrons are transferred from GaN layer to graphene and WS2 layer, and transfer increases with the increase in strain from negative to positive. More electrons are transferred to WS2 with positive strain, and more electrons are transferred to graphene with negative strain. The results will provide valuable information for the design of trilayer Schottky devices.

Novel g-C3N4/Fe-ZnO/RGO nanocomposites with boosting visible light photocatalytic activity for MB, Cr (VI), and outstanding catalytic activity toward para-nitrophenol reduction

A novel g-C3N4/Fe-ZnO/RGO nanocomposite has been synthesized using the facile solvothermal method to boost the catalytic efficiency of ZnO. The structure and morphology of nanocomposites were examined by a wide range of characterization methods. The obtained g-C3N4/Fe-ZnO/RGO nanocomposite (Z-scheme heterostructure) exhibits improved photocatalytic activity toward the photodegradation of MB, Cr (VI) under visible-light irradiation and 4-nitrophenol reduction. The enhancement in activity of nanocomposite is ascribed to a unique heterostructure system, which facilitates excellent transport and separation of the photogenerated charge carrier, resulting in prolonged lifetime leading to continuous generation of reactive species. Moreover, the synergistic effect on the interface of ZnO and g-C3N4 and the introduction of reduced graphene oxide (RGO) serve as a booster for charge separation at the Z-scheme, which ultimately speeds up the degradation of pollutants. The present study provides a novel and facile approach for establishing an efficient nanocomposite for environmental remediation.

Spin-layer locking of interlayer excitons trapped in moiré potentials.

Van der Waals heterostructures offer attractive opportunities to design quantum materials. For instance, transition metal dichalcogenides (TMDs) possess three quantum degrees of freedom: spin, valley index and layer index. Furthermore, twisted TMD heterobilayers can form moiré patterns that modulate the electronic band structure according to the atomic registry, leading to spatial confinement of interlayer excitons (IXs). Here we report the observation of spin-layer locking of IXs trapped in moiré potentials formed in a heterostructure of bilayer 2H-MoSe2 and monolayer WSe2. The phenomenon of locked electron spin and layer index leads to two quantum-confined IX species with distinct spin-layer-valley configurations. Furthermore, we observe that the atomic registries of the moiré trapping sites in the three layers are intrinsically locked together due to the 2H-type stacking characteristic of bilayer TMDs. These results identify the layer index as a useful degree of freedom to engineer tunable few-level quantum systems in two-dimensional heterostructures.

High performance, 3D-hierarchical CoS2/CoSe@C nanohybrid as an efficient electrocatalyst for hydrogen evolution reaction

Abstract Electrolysis, driven by renewables, is ideally a direct and clean route to generate the hydrogen. However, for the efficient H2 generation designing active, stable and low cost electrocatalyst system to replace expensive Pt is of paramount importance. Here, a hetero-structured system composed of CoS2 and CoSe nanostructures on carbon matrix (CoS2/CoSe@C) is proposed as an active electrocatalyst for hydrogen evolution reaction (HER) in acid medium. The composition of sulfur and selenium in the derived CoS2/CoSe@C electrocatalyst is analytically controlled for the enhanced HER performance. The prepared electrocatalyst offers the low overpotential of 164 mV at the current density of 10 mA cm−2 with a small Tafel slope of 42 mV dec−1 as a result of proper synergy between the active components CoS2 and CoSe. Moreover, the interconnected network in the carbon matrix of CoS2/CoSe@C provides better conductivity ensured by adequate contact area between the electrocatalyst and electrolyte. Besides the convincing electrochemical activity, catalyst demonstrates the good stability for at least 10 h. This work highlights importance of composite materials for HER via synchronizing catalytically active materials (Ni, Co, Mo) and highly conductive supports (CNT, C, rGO).

Structural and electronic properties of two-dimensional freestanding BaTiO3/SrTiO3 heterostructures

The successful preparation of the freestanding perovskite materials down to the monolayer limit [Ji et al., Nature (London) 570, 87 (2019)] provided the opportunity to make the two-dimensional (2D) oxide and heterostructure, which could be significantly distinctive from the conventional oxide superlattices and other 2D van der Waals heterostructures. By stacking one unit-cell $\mathrm{BaTi}{\mathrm{O}}_{3}$ (BTO) and one unit-cell $\mathrm{SrTi}{\mathrm{O}}_{3}$ (STO) on top of each other, we constructed two isolated bilayers of the 2D heterostructure systems. From our density functional theory simulation, their ground states exhibit an in-plane ferroelectricity in both BTO layer and STO layer, while the antiferrodistortive mode of the STO layer is totally suppressed. These two systems show band gaps in the range of 2--2.5 eV (by using HSE06), which are smaller than their monolayer and bulk phases. The layer arrangement strongly influences their electronic properties. We reveal that they adopt the type-II electronic band alignment. The tensile biaxial strain can strongly promote the ferroelectricity and increase the band gaps of these systems. Our results will contribute to the further understanding of layered materials based on the transition metal oxide perovskites and developing relevant experimental devices.

Hybridization of g-C3N4 quantum dots with 1D branched TiO2 fiber for efficient visible light-driven photocatalytic hydrogen generation

The hybrid 1D branched TiO2 loaded with g-C3N4 QDs was successfully fabricated that plays a significant role in photocatalysis. The 1D branched TiO2 prepared by electrospinning followed by alkali-hydrothermal process, and g-C3N4 QDs were grafted over it by a chemical vapor deposition method. The composite display enhancement in photocatalytic hydrogen evolution is about 10.57 mmol. g−1.h−1 in comparison to the g-C3N4 sample that only produces 0.32 mmol. g−1.h−1 while the HBTiO2 sample evolved a negligible amount of hydrogen under visible light. The composite sample shows quantum efficiency for HER at 420 nm light is 18.6% that is much higher than the other two samples. The specific surface area of the composite sample is 92.39 m2g-1 that is about 13 times more than bulk g-C3N4. The bandgap of HBTiO2/g-C3N4 QDs, g-C3N4, and HBTiO2 samples calculated as 2.71 eV, 2.67eV, and 3.24eV, respectively. The TRPL spectra imply that the duration of the lifetime of composite becomes longer which effectually overwhelm the electron-hole recombination. The 1D branched TiO2 fiber reduces the charge recombination by fast transfer of electron while g-C3N4 QDs facilitate the visible light absorption by improving the optical properties. The formation of the type II heterostructure system remarkably promotes the separation and transfer of electron holes and facilitates the photo-reduction reaction.

In situ decoration of g-C3N4 quantum dots on 1D branched TiO2 loaded with plasmonic Au nanoparticles and improved the photocatalytic hydrogen evolution activity

The hybrid 1D branched TiO2 loaded with surface plasmonic Au and decorated with g-C3N4 QDs were successfully fabricated which played a significant role in photocatalysis. The 1D branched TiO2 prepared by electrospinning followed by an alkali-hydrothermal process. The Au particles decorated by photo deposition and after that g-C3N4 QDs were grafted over it by a chemical vapor deposition method. The composite shows great improvement in photocatalytic hydrogen evolution and evolved about 2.22 mmol g−1 h−1 hydrogen. The composite sample has a quantum efficiency of 19.5% for hydrogen production at 420 nm light that depicts the performance behavior of the composite. The 1D branched TiO2 fiber facilitates the fast transfer of electrons and Au reduces the charge recombination due to its surface plasmonic effect while g-C3N4 QDs facilitate the visible light absorption by improving the optical properties and reduces the bandgap. The formation of the type II heterostructure system remarkably promotes the separation and transfer of electron holes and facilitates the photo-reduction reaction.

Classical phase diagram of the Rashba-Hubbard model in the strongly correlated limit on square lattice

In this work, we investigate the interacting electrons on a square lattice in the presence of Rashba spin-orbit coupling. We first obtain the effective spin model from the Rashba-Hubbard model in the strongly correlated limit using the second perturbation theory. The effective spin model includes isotropic Heisenberg terms, nearest- and next-nearest-neighbor interactions, as well as the anisotropic ones as Kane-Mele and Dzyaloshinski-Moriya interactions. We proceed to study the influence of Rashba spin-orbit coupling on the stability of the magnetic phases of isotropic Heisenberg using Luttinger-Tisza and variational minimization classical methods. Our classical calculations show that the anisotropic terms leads to the instability of the Neel, classical degenerate and collinear phases of the isotropic Heisenberg model on the square lattice into an incommensurate planar phase. The spiral magnetic order in the two-dimensional frustrated magnets can be disordered by considering the quantum fluctuations. In a heterostructure including a noncollinear magnet and a singlet superconductor, singlet Cooper pairs can be converted to triplet pairings due to the broken spin rotational symmetry. Therefore, we can engineer a topological superconductor using noncollinear magnet in a heterostructure system.

The Improved Photoelectrochemical Performance of WO3/BiVO4 Heterojunction Thin‐Film Photoanodes via Thermal Treatment

Improvement in the photoelectrochemical (PEC) performance of WO3/BiVO4 heterojunction photoanodes is achieved by thermal treatment. The heterostructure systems consist of WO3 and BiVO4 layers spin coated onto fluorine‐doped tin oxide (FTO)‐coated glass substrates. The morphology and optical properties are modified by varying annealing conditions and calcining time. The crystal structure, morphology, and optical properties are studied using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and UV–vis spectroscopy, respectively. The results show that the WO3/BiVO4 photoanodes obtain an interconnected structure which helps promote the PEC activity by increasing the active area for reaction and shortening the distance for carrier diffusion. Also, the preannealing treatment can improve film qualities, namely crystallinity, uniformity, and optical absorbance, leading to an enhancement of the PEC performance. The optimal thermal treatment is preannealing at 60 °C for 15 min, followed by calcining at 400 °C for 5 h. The photoanode with such an optimal treatment exhibits the highest photocurrent density under light‐emitting diode (LED) illumination, and the efficiency is increased by 3–4 times compared with the others. Herein, a simple method is offered to improve the PEC performance of a heterojunction photoelectrode.

ZnO-CuxO heterostructure photocatalyst for efficient dye degradation

A simple aqueous solution process was used for the synthesis of Cu-loaded ZnO nanorod structures in which highly crystalline and hexagonal wurtzite ZnO, cubic CuO and Cu2O phases coexist. Absorption/emission characteristics identify band gap narrowing, which facilitates visible light absorption, as a consequence of hybridization of O 2p bands with Cu 3 d bands of the ZnO-CuxO heterostructure system. With use of the specific band energy position of the individual components (i.e., ZnO, Cu2O, and CuO), photogenerated electrons and holes could be made to migrate separately and thereby the carrier recombination could be delayed, making them amply available for photocatalytic reactions. The available electrons plausibly produce superoxide radical anion (O2·−), while holes generate hydroxyl radicals (·OH), which are reactive agents for dye degradation. Furthermore, the formation of a depletion region via Cu-induced charge separation within the ZnO-CuxO heterostructure and thermal-energy-mediated abstraction of the electric field barriers and subsequent migration of the positively charged defects from the subsurface region toward the interface lead to efficient photocatalytic performance.

Influence of morphology and interfacial interaction of TiO2-Graphene nanocomposites on the visible light photocatalytic performance

In most cases, different morphologies of the same composites correspond to different properties. In this paper, the TiO2/rGO heterostructure system with three morphologies was obtained by one-step hydrothermal method through regulating hydrothermal time. Compared with TiO2 nanoparticle/reduced graphene oxide (TNP/rGO) and TiO2 nanobelt/reduced graphene oxide (TNB/rGO), TiO2 nanotube/reduced graphene oxide (TNT/rGO) showed higher photocatalytic activity (78.5%, 2 ​h) toward degrading Rhodamine B and 2, 4-Dichlorophenol under visible light. The mechanism of enhancing photocatalytic activities of the TiO2/rGO system was confirmed by free radicals trapping experiment, ESR, PL and photocurrent measurement. It is revealed that the hole (h+) is the main reactive specie in TNT/rGO heterojunction and rapid transfer of photogenerated electrons is responsible for enhancing photocatalytic activity. This work not only emphasizes the important role of morphology in improving photocatalytic activity, but also provides a simple strategy to alter the morphology of TiO2.

Visible light responsive CuS/ protonated g-C3N4 heterostructure for rapid sterilization

As the environment deterioration is becoming more serious, bacterial pollution is threatening the health of human beings. Hence, it is vital to develop rapid and safe sterilization strategy. Herein, CuS/protonated g-C3N4(CuS/PCN) composites were synthesized by simple hydrothermal method and electrostatic adsorption. This heterostructured system exhibited enhanced photocatalytic properties under visible light compared with CuS or g-C3N4 alone, ascribing to the rapid separation of photogenerated electron-hole pairs. Meanwhile, the obvious photothermal effects of CuS/PCN were achieved and the temperature increased with the increased amount of CuS in the composites due to the more light absorption. However, when the CuS content is more than 10 %, photocurrent density is decreased with increasing the amount of CuS, indicating the increased recombination of photogenerated electron-hole pairs. When the CuS content is 20 %, the composite can perform the optimized synergistic effects of both photothermal action and photocatalysis under light irradiation for 20 min. The corresponding bacteria-killing efficiency against Staphylococcus aureus and Escherichia coli is 98.23 % and 99.16 %, respectively. The underlying mechanism is that the bacterial membrane can be weakened by reactive oxygen species and bacterial activities are inhibited by hyperthermia. This CuS/PCN heterojunction is promising for environmental disinfection including water and public facilities sterilization.

Study on Ca Segregation toward an Epitaxial Interface between Bismuth Ferrite and Strontium Titanate.

Segregation is a crucial phenomenon, which has to be considered in functional material design. Segregation processes in perovskite oxides have been the subject of ongoing scientific interest, since they can lead to a modification of properties and a loss of functionality. Many studies in oxide thin films have focused on segregation toward the surface using a variety of surface-sensitive analysis techniques. In contrast, here we report a Ca segregation toward an in-plane compressively strained heterostructure interface in a Ca- and Mn-codoped bismuth ferrite film. We are using advanced transmission electron microscopy techniques, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. Ca segregation is found to trigger atomic and electronic structure changes at the interface. This includes the reduction of the interface strain according to the Ca concentration gradient, interplanar spacing variations, and oxygen vacancies at the interface. The experimental results are supported by DFT calculations, which explore two segregation scenarios, i.e., one without oxygen vacancies and Fe oxidation from 3+ to 4+ and one with vacancies for charge compensation. Comparison with electron energy loss spectroscopy (EELS) measurements confirms the second segregation scenario with vacancy formation. The findings contribute to the understanding of segregation and indicate promising effects of a Ca-rich buffer layer in this heterostructure system.