Thin Film Heterostructures Research Articles

High-performance (Na0.5Bi0.5)(Ti0.97Fe0.03)O3-based heterostructure thin films for energy storage capacitors

This work presents a simple process to fabricate Na0.5Bi0.5(Ti0.97Fe0.03)O3/Ba(1–x)SrxTiO3–based heterostructure thin films with a compositional graded sequence, and their energy storage properties are investigated systematically. A simulation technique is used to predict the experimental results. Interestingly, improved energy storage properties are obtained in the “up–graded” film, which is associated with the stress/strain. Furthermore, the “up–graded” film after aging with treating exhibits a higher breakdown strength (EBD = 3176.4 kV/cm), recoverable energy storage density (Wrec = 67.18 J/cm3) and efficiency (η = 75.65%). This can be attributed to the ordered defect dipoles induced by aging with treating driving domain switching, which is favorable for high (Pm–Pr) and large Wrec. These results provide an approach and guidance to design thin film-based devices with optimal energy storage performance.

Local Atomic Configuration Control of Superconductivity in the Undoped Pnictide Parent Compound BaFe2As2

Emergent superconductivity is strongly correlated with the symmetry of local atomic configuration in the parent compounds of iron-based superconductors. While chemical doping or hydrostatic pressure can change the local geometry, these conventional approaches do not provide a clear pathway in tuning the detailed atomic arrangement predictably, due to the parent compounds complicated structural deformation in the presence of the tetragonal-to-orthorhombic phase transition. Here, we demonstrate a systematic approach to manipulate the local structural configurations in BaFe2As2 epitaxial thin films by controlling two independent structural factors orthorhombicity (in-plane anisotropy) and tetragonality (out-of-plane/in-plane balance) from lattice parameters. We tune superconductivity without chemical doping utilizing both structural factors separately, controlling local tetrahedral coordination in designed thin film heterostructures with substrate clamping and bi-axial strain. We further show that this allows quantitative control of both the structural phase transition, associated magnetism, and superconductivity in the parent material BaFe2As2. This approach will advance the development of tunable thin film superconductors in reduced dimension.

Exploring a high-carrier-mobility black phosphorus/MoSe2 heterostructure for high-efficiency thin film solar cells

Transition metal dichalcogenides (TMDCs) with self-passivated surfaces, suitable bandgaps and high optical-absorption coefficients are very promising for thin film solar cells, but seriously hindered by low carrier mobility. Herein, we propose interface-engineered two-dimensional van der Waals heterostructure composed of high-carrier-mobility black phosphorus (BP) layer and MoSe2 layer, and demonstrate a new-type BP/MoSe2 heterostructure thin film solar cell with high efficiency. The electronic structure and optical properties of both BP/MoSe2 and BP/MoS2 heterostructures are systematically investigated. Compared with BP/MoS2 heterostructure, BP/MoSe2 heterostructure shows enhanced photoelectric characteristics. Additionally, BP/MoSe2 heterostructure has greater light absorption intensity as well as a wider absorption range, achieving a much higher power conversion efficiency of up to 23.04%. It is found that the built-in electric field at the interface of BP/MoSe2 heterostructure accelerates the separation of electron-hole pairs. This work provides a feasible strategy for using the BP/MoSe2 heterostructure for next-generation high-specific-power thin film solar cells.

Highly orientated one-dimensional Cu2O/TiO2 heterostructure thin film for photoelectrochemical photoanode and photocatalytic degradation applications

A highly orientated one-dimensional TiO2 nanorod (NR) and TiO2 NR coupled with Cu2O growth on a fluorine doped tin oxide (FTO) substrate were fabricated by the hydrothermal method. A p-n Cu2O/TiO2 heterostructure photoanode was applied for photoelectrochemical water splitting and photocatalytic organic dye degradation application. The purpose of using Cu2O is to extend the visible light absorption capacity and improve the charge transport and charge carrier lifetime of the TiO2 based photoanode. The materials were characterized by various methods. Scanning electron microscopy image revealed that the Cu2O/TiO2 was well aligned perpendicular to the FTO substrate. Energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed that Cu2O was coated on the TiO2 surface. The absorption spectra showed the energy bandgap of pristine TiO2 decreased after the Cu2O enhancement. Electrochemical impedance spectroscopy revealed that the charge carrier and charge transport of the Cu2O/TiO2 material were improved. Overall photocurrent density generated by the Cu2O/TiO2 photoanodes were higher than the pristine TiO2 photoanode when they were applied as the photoanode in the photoelectrochemical cell. For the thin film Rhodamine B degradation, the Cu2O/TiO2 photocatalyst showed a better performance than the pure TiO2 NR thin film by ∼60%.

Capacitive Photodetector Thin-Film Cells of Cu-As2S3-Cu as Revealed by Dielectric Spectroscopy.

The As2S3-Cu interface was studied by dielectric spectroscopy measurements on Cu-As2S3-Cu thin film heterostructure samples to assess the charge carriers’ contribution to the electrical properties of such an interface. Three-dimensional printed masks ensured good reproducibility during the PLD deposition of heterostructure samples. The samples were tested for electrical conductivity and AC photoconductivity by dielectric spectroscopy measurements. DC bias voltages and light were applied to the samples. The electrical capacity of the thin film heterostructure can be modified electrically and optically. We observed long-term photoconductivity with a time dependency that was not exponential, and a quick change of the electrical capacity, indicating the potential of the heterostructure cells as photodetector candidates.

Structural, optical and electrical characterization of MoS2/TiO2 heterostructured thin films by chemical bath deposition technique

MoS2/TiO2 heterostructures thin films are successfully deposited by chemical bath deposition (CBD) technique. The structural, optical and electrical properties of prepared films are characterized by X-ray diffraction (XRD), Raman spectroscopy, UV-vis spectrophotometry, photoluminescence spectroscopy (PL), and Four point probe technique, respectively. Raman Spectra and XRD confirmed the formation of hexagonal MoS2 and anatase TiO2. UV-Vis spectrophotometry confirmed the band gap energy (Eg) of MoS2 and TiO2 thin films are 1.14 eV and 3.44 eV, respectively. The Eg of films is changed according to the material deposited onto them i.e. it increased by depositing TiO2 onto the MoS2 and decreased the other way round. MT (Titania on Molybdenum disulfide) and TM (Vice Versa) have band gaps of 2.81 eV and 1.5 eV, respectively. The photoluminescence spectra showed that photoluminescence emission increased for TiO2 in the MoS2/TiO2 and TiO2/MoS2 heterostructures films. The exchange of trion to neutral excitons by charges transfer from MoS2 to TiO2 in heterostructures leads to increase the PL intensity. The average sheet resistivity of TiO2, MoS2, glass/MoS2/TiO2 and glass/TiO2/MoS2 films are 2.41 × 107 (Ω-m), 6.44 × 104 (Ω-m), 1.93 × 106 (Ω-m) and 2.35 × 104 (Ω-m), respectively. CBD is low cost, simple, and large area deposition technique and by this research the heterostructures films can easily be deposited for industrial purpose.

Magnetic skyrmion crystal at a topological insulator surface

We consider a magnetic skyrmion crystal formed at the surface of a topological insulator. Incorporating the exchange interaction between the helical Dirac surface states and the periodic N\'eel or Bloch skyrmion texture, we obtain the resulting electronic band structure and discuss the constraints that symmetries impose on the energies and Berry curvature. We find substantive qualitative differences between the N\'eel and Bloch cases, with the latter generically permitting a multiband low energy tight-binding representation whose parameters are tightly constrained by symmetries. We explicitly compute the associated Wannier orbitals, which resemble the ringlike chiral bound states of helical Dirac fermions coupled to a single skyrmion in a ferromagnetic background. We construct a two-band tight-binding model with real nearest-neighbor hoppings which captures the salient topological features of the low-energy bands. Our results are relevant to magnetic topological insulators (TIs), as well as to TI-magnetic thin film heterostructures, in which skyrmion crystals may be stabilized.

Enhanced Valley Polarization in WS2/LaMnO3 Heterostructure

AbstractMonolayer transition metal dichalcogenides have attracted great attention for potential applications in valleytronics. However, the valley polarization degree is usually not high because of the intervalley scattering. Here, a largely enhanced valley polarization up to 80% in monolayer WS2 under nonresonant excitation at 4.2 K is demonstrated using WS2/LaMnO3 thin film heterostructure, which is much higher than that for monolayer WS2 on SiO2/Si substrate with a valley polarization of 15%. Furthermore, the greatly enhanced valley polarization can be maintained to a high temperature of about 160 K with a valley polarization of 53%. The temperature dependence of valley polarization is strongly correlated with the thermomagnetic curve of LaMnO3, indicating an exciton–magnon coupling between WS2 and LaMnO3. A simple model is introduced to illustrate the underlying mechanisms. The coupling of WS2 and LaMnO3 is further confirmed with an observation of two interlayer excitons with opposite valley polarizations in the heterostructure, resulting from the spin‐orbit coupling induced splitting of the conduction bands in monolayer transition metal dichalcogenides. The results provide a pathway to control the valleytronic properties of transition metal dichalcogenides by means of ferromagnetic van der Waals engineering, paving a way to practical valleytronic applications.

Skyrmion Stabilization at the Domain Morphology Transition in Ferromagnet/Heavy Metal Heterostructures with Low Exchange Stiffness

AbstractHerein, the experimental observation of micrometer‐scale magnetic skyrmions at room temperature in several Pt/Co‐based thin film heterostructures designed to possess low exchange stiffness, perpendicular magnetic anisotropy, and a modest interfacial Dzyaloshinskii–Moriya interaction (iDMI) is reported. It is found both experimentally and by micromagnetic and analytic modeling that a low exchange stiffness and modest iDMI eliminates the energetic penalty associated with forming domain walls in thin films. When the domain wall energy density approaches negative values, the remanent morphology transitions from a uniform state to labyrinthine stripes. A low exchange stiffness, indicated by a sub‐400 K Curie temperature, is achieved in Pt/Co, Pt/Co/Ni, and Pt/Co/Ni/Re structures by reducing the Co thickness to the ultrathin limit (<0.3 nm). Similar effects occur in thicker Pt/Co/NixCu1−x structures when the Ni layer is alloyed with Cu. At this transition in domain morphology, skyrmion phases are stabilized by small (<1 mT), perpendicular magnetic fields, and skyrmion motion in response to spin–orbit torque is observed. While the temperature and thickness‐induced morphological phase transitions observed are similar to the well‐studied spin reorientation transition that occurs in the ultrathin limit, the underlying energy balances are substantially modified by the presence of an iDMI.

Photon spin angular momentum driven magnetization dynamics in ferromagnet/heavy metal bilayers

Thin-film magnetization controlled by optical helicity has been recently reported. Although circularly polarized light has spin angular momentum, helicity-dependent all-optical magnetization switching is mediated by the stochastic thermal process, such as magnetic circular dichroism, and the effect of photon spin angular momentum is considered to be a secondary role. Conversely, the inverse Faraday effect in ferromagnetic thin films and photon spin angular momentum injection into heavy metal thin films have been observed, which can induce torque on metallic thin-film magnets. In this study, we show photon spin angular momentum driven magnetization dynamics in bilayers of Co/(Pt, Au) thin films with various thicknesses. The heavy metal Pt, Au, and ferromagnetic Co layer thickness dependencies of photon spin angular momentum driven torques are discussed in terms of field-like torque owing to the inverse Faraday effect and spin-transfer torque caused by photon spin angular momentum injection into the heavy metal layer with details of optical and magnetic properties. This study provides a better understanding of photon spin angular momentum induced magnetization dynamics in metallic thin-film heterostructures for efficient photon-driven magnetization manipulation.

MnBi2Se4-Based Magnetic Modulated Heterostructures

Thin films of magnetic topological insulators (TIs) are expected to exhibit a quantized anomalous Hall effect when the magnetizations on the top and bottom surfaces are parallel and a quantized topological magnetoelectric effect when the magnetizations have opposite orientations. Progress in the observation of these quantum effects was achieved earlier in the films with modulated magnetic doping. On the other hand, the molecular-beam-epitaxy technique allowing the growth of stoichiometric magnetic van der Waals blocks in combination with blocks of topological insulator was developed. This approach should allow the construction of modulated heterostructures with the desired architecture. In the present paper, based on the first-principles calculations, we study the electronic structure of symmetric thin film heterostructures composed of magnetic MnBi2Se4 blocks (septuple layers, SLs) and blocks of Bi2Se3 TI (quintuple layers, QLs) in dependence on the depth of the magnetic SLs relative to the film surface and the TI spacer between them. Among considered heterostructures we have revealed those characterized by nontrivial band topology.

Magnetic Properties of Ultrathin As-deposited and Annealed Ta/CoFeB/TaOx Heterostructures

In order to meet the ever-increasing demand of magnetic recording industry, it is important to developferromagnetic thin film heterostructure compatible for magnetic memory device. Here, we have developed ultrathin ferromagnetic film of transition metal borides(CoFeB) which has huge potential to be integrated in magnetic memory devices. In particular, we have studied the surface roughness and magnetic properties of sputter deposited Substrate/1 nm Ta/1.5 nm CoFeB/1nm TaOx heterostructures. Magnetic properties investigation of as-deposited and 300 ⁰C annealed Ta/CoFeB/TaOx heterostructure using vibrating sample magnetometer indicates the presence of in-plane anisotropy in both the film stacks and a reasonable increase in the saturation magnetization of annealed film stack. Importantly, possible boron diffusion as well as partial crystallization of CoFeB layer due to annealing play crucial roles in governing the magnetic properties in these film stacks. These results provide in-depth insight into the factors affecting saturation magnetization of such ultrathin film heterostructures.

Toward controlling the Al2O3/ZnO interface properties by in situ ALD preparation.

An Al2O3/ZnO heterojunction was grown on a Si single crystal substrate by subsequent thermal and plasma-assisted atomic layer deposition (ALD) in situ. The band offsets of the heterointerface were then studied by consecutive removal of the layers by argon sputtering, followed by in situ X-ray photoelectron spectroscopy. The valence band maximum and conduction band minimum of Al2O3 are found to be 1.1 eV below and 2.3 eV above those of ZnO, resulting in a type-I staggered heterojunction. An apparent reduction of ZnO to elemental Zn in the interface region was detected in the Zn 2p core level and Zn L3MM Auger spectra. This suggests an interface formation different from previous models. The reduction of ZnO to Zn in the interface region accompanied by the creation of oxygen vacancies in ZnO results in an upward band bending at the interface. Therefore, this study suggests that interfacial properties such as the band bending as well as the valence and conduction band offsets should be in situ controllable to a certain extent by careful selection of the process parameters.

High thermoelectric performance of ZrTe2/SrTiO3 heterostructure

Achieving high thermoelectric performance in thin film heterostructures is essential for integrated and miniatured thermoelectric device applications. In this work, we demonstrate a mechanism and device performance of enhanced thermoelectric performance induced by interfacial effect in a transition metal dichalcogenides-SrTiO3 (STO) heterostructure. Owing to the formed conductive interface and elevated conductivity, the ZrTe2/STO heterostructure presents large thermoelectric power factor of 3.7 × 105 μWcm−1K−2 at 10 K. Formation of quasi-two-dimensional conductance at the interface is attributed for the large Seebeck coefficient and high electrical conductivity, leading to high thermoelectric performance which is demonstrated by a prototype device attaining 3 K cooling with 100 mA current input to this heterostructure. This superior thermoelectric property makes this heterostructure a promising candidate for future thermoelectric device.

Magnetoelectric effect in Ni[formula omitted]ZnxFe2O4/PZT thin film heterostructures

Magnetoelectric coupling effect of Ni1-xZnxFe2O4/PZT (NZFO/PZT) thin film heterostructures is investigated by tuning the Zn content in NZFO film. The highest magnetoelectric coupling coefficient of NZFO/PZT heterostructures is achieved at x=0.15 with the value of 328 mV/cm Oe. This various magnetoelectric effect in NZFO/PZT heterostructures with different Zn content is induced by distinct magnetostriction of NZFO film, which is related to piezomagnetic coefficient q and the initial permeability μi of NZFO layer, as well as the exchange interaction of ions at tetrahedral and octahedral sites. This study is of helpful for understanding the magnetoelectric effects of epitaxial magnetoelectric heterostructures that involves NZFO ferrite films.

Interfacial adhesion strength of III-N heterostructures

Wide bandgap semiconductors such as group III-nitrides and SiC are considered as key materials for the fabrication of smaller, more reliable and efficient power electronics. Fabrication of robust and durable power devices requires an optimized design based on the understanding of the interfacial adhesion properties of the constituent thin-film heterostructures. In this study, the adhesion properties of GaN/AlN layers grown on Si substrates were investigated. Particularly, the influence of the AlN buffer layers, necessary for GaN growth on Si, on the delamination response was determined. The interfacial adhesion strength was obtained using cross-sectional nanoindentation (CSN) and four-point bending (4PB) tests. Analytical models based on beam- and elastic plate theory which were applied respectively to calculate the interfacial fracture energy (Gic) for both methods are found to be in good agreement provided the loading conditions are similar. Detailed transmission and scanning electron microscopy investigations prior and subsequent to delamination reveal the microstructural details of the relevant interfaces and provide insights into the encountered mechanisms of interfacial failure. Finally, the probability of delamination along the weakest interface is discussed based on a fracture mechanics model.

Spinning disc photoreactor based visible-light-driven Ag/Ag2O/TiO2 heterojunction photocatalyst film toward the degradation of amoxicillin

The presence of antibiotics in waste and drinking water is causing increasing concern around the world, thereby an advanced sustainable technology needs to be developed to eliminate the antibiotics from water resources. Hence, an efficient spinning disc photoreactor (SDPR) equipped with visible light-activated Ag/Ag2O/TiO2 heterostructure thin film photocatalyst was assessed for the degradation of amoxicillin (AMX) as a typical antibiotic. The surface morphology, optoelectronic and structural features of Ag/Ag2O/TiO2 heterojunction were characterized by TEM, BET, mott Schottky, FESEM, EDS, AFM, XRD, UV–Vis-DRS, and contact angle measurements. Results confirm that Ag and Ag2O have a significant effect on the photocharge carrier separation and transfer of the as-developed photocatalyst system. The operative variables including illumination time, rotational speed, solution flow rate, aeration rate, pH, and initial AMX concentration were optimized by CCD. The results displayed the maximum AMX photodegradation (97.91%) could be achieved at optimal conditions involving illumination time of 80 min, a rotational speed of 225 rpm, the solution flow rate of 0.6 L/min, aeration rate of 20 L/min, pH = 6, and initial AMX concentration of 20 mg/L. Interestingly, more than 79% COD and 64% TOC were removed under optimum conditions during 80 min illumination time, respectively. Active species tests confirmed the dominant role of ·OH and ·O2− in AMX degradation. finally, the XRD pattern confirmed that the reusability assessments of the heterojunction film could successfully retain its stability for six consecutive photocatalytic degradation runs. This work demonstrates the feasibility of utilizing visible-light-driven thin-film photocatalysts in spinning disc photoreactors in treating the tenacious antibiotic pollutants.

Recent Progress in Devices Based on Magnetoelectric Composite Thin Films.

The strain-driven interfacial coupling between the ferromagnetic and ferroelectric constituents of magnetoelectric (ME) composites makes them potential candidates for novel multifunctional devices. ME composites in the form of thin-film heterostructures show promising applications in miniaturized ME devices. This article reports the recent advancement in ME thin-film devices, such as highly sensitive magnetic field sensors, ME antennas, integrated tunable ME inductors, and ME band-pass filters, is discussed. (Pb1−xZrx)TiO3 (PZT), Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), Aluminium nitride (AlN), and Al1−xScxN are the most commonly used piezoelectric constituents, whereas FeGa, FeGaB, FeCo, FeCoB, and Metglas (FeCoSiB alloy) are the most commonly used magnetostrictive constituents in the thin film ME devices. The ME field sensors offer a limit of detection in the fT/Hz1/2 range at the mechanical resonance frequency. However, below resonance, different frequency conversion techniques with AC magnetic or electric fields or the delta-E effect are used. Noise floors of 1–100 pT/Hz1/2 at 1 Hz were obtained. Acoustically actuated nanomechanical ME antennas operating at a very-high frequency as well as ultra-high frequency (0.1–3 GHz) range, were introduced. The ME antennas were successfully miniaturized by a few orders smaller in size compared to the state-of-the-art conventional antennas. The designed antennas exhibit potential application in biomedical devices and wearable antennas. Integrated tunable inductors and band-pass filters tuned by electric and magnetic field with a wide operating frequency range are also discussed along with miniaturized ME energy harvesters.

Valley polarization of trions in monolayer MoSe2 interfaced with bismuth iron garnet

Interfacing atomically thin van der Waals semiconductors with magnetic substrates enables additional control on their intrinsic valley degree of freedom and provides a promising platform for the development of novel valleytronic devices for information processing and storage. Here we study circularly polarized photoluminescence in heterostructures of monolayer MoSe2 and thin films of ferrimagnetic bismuth iron garnet (BIG). We observe strong emission from charged excitons with circular polarization opposite to that of the pump and demonstrate contrasting response to left and right circularly polarized excitation, associated with finite out-of-plane magnetization in the substrate. We propose a theoretical model accounting for magnetization-induced imbalance of charge carriers in the two valleys of MoSe2, as well as for valley-switching scattering from B to A excitons and fast formation of trions with extended valley relaxation times, which shows excellent agreement with the experimental data. Our results establish monolayer MoSe2 interfaced with BIG as a promising system for valley control of charged excitons.

Dynamic transition of current-driven single-skyrmion motion in a room-temperature chiral-lattice magnet

Driving and controlling single-skyrmion motion promises skyrmion-based spintronic applications. Recently progress has been made in moving skyrmionic bubbles in thin-film heterostructures and low-temperature chiral skyrmions in the FeGe helimagnet by electric current. Here, we report the motion tracking and control of a single skyrmion at room temperature in the chiral-lattice magnet Co9Zn9Mn2 using nanosecond current pulses. We have directly observed that the skyrmion Hall motion reverses its direction upon the reversal of skyrmion topological number using Lorentz transmission electron microscopy. Systematic measurements of the single-skyrmion trace as a function of electric current reveal a dynamic transition from the static pinned state to the linear flow motion via a creep event, in agreement with the theoretical prediction. We have clarified the role of skyrmion pinning and evaluated the intrinsic skyrmion Hall angle and the skyrmion velocity in the course of the dynamic transition. Our results pave a way to skyrmion applications in spintronic devices.