Artificial Heterostructures Research Articles

Weak localization in graphene sandwiched by aligned h-BN flakes

Charge carriers in graphene exhibit distinct characteristics from those in other two-dimensional materials because of their chiral nature. Additionally, multiple Dirac cones that emerge in graphene superlattices have been regarded as an interesting point in condensed-matter physics in recent years. Here, we report an investigation of the magneto-conductance in graphene encapsulated on the top and bottom by aligned h-BN. The bottom h-BN is precisely aligned with graphene, while the top h-BN is rotated a very small angle relative to it. Such a heterostructure could spoil the commensurate state existing in precisely aligned graphene while the giant moiré superlattice remains. A clear signature of weak localization and weak anti-localization is observed at multiple Dirac cones. Both the weak (anti)localization and the universal conductance fluctuations exhibit strong dependencies on the carrier density, temperature and channel length. This artificial heterostructure allows one to explore quantum interference in graphene with a wide spectrum of electronic properties.

Bi-dimensional materials for THz frequency nanodevices

Although artificial semiconductor heterostructures have long been the core material system for the generation, detection and manipulation of carriers, at TeraHertz (THz) frequencies, the discovery of graphene and the related intriguing abilities have triggered an unprecedented interest in inorganic two-dimensional (2D) materials, as black phosphorus and boron nitride, amongst many others. They offer a unique platform for developing efficient devices, without the need of lattice matching, and with a variety of physical properties, that can be engineered from scratch, exploiting the material structures, the layer thickness or their inherent anisotropy.

Artificial oxide heterostructures with non-trivial topology

In the quest for topological insulators with large band gaps, heterostructures with Rashba spin–orbit interactions come into play. Transition metal oxides with heavy ions are especially interesting in this respect. We discuss the design principles for stacking oxide Rashba layers. Assuming a single layer with a two-dimensional electron gas (2DEG) on both interfaces as a building block, a two-dimensional topological insulating phase is present when negative coupling between the 2DEGs exists. When stacking multiple building blocks, a two-dimensional or three-dimensional topological insulator is artificially created, depending on the intra- and interlayer coupling strengths and the number of building blocks. We show that the three-dimensional topological insulator is protected by reflection symmetry, and can therefore be classified as a topological crystalline insulator. In order to isolate the topological states from bulk states, the intralayer coupling term needs to be quadratic in momentum. It is described how such a quadratic coupling could potentially be realized by taking buckling within the layers into account. The buckling, thereby, brings the idea of stacked Rashba system very close to the alternative approach of realizing the buckled honeycomb lattice in [111]-oriented perovskite oxides.

Heterogeneous integration of single-crystalline complex-oxide membranes.

Complex-oxide materials exhibit a vast range of functional properties desirable for next-generation electronic, spintronic, magnetoelectric, neuromorphic, and energy conversion storage devices1-4. Their physical functionalities can be coupled by stacking layers of such materials to create heterostructures and can be further boosted by applying strain5-7. The predominant method for heterogeneous integration and application of strain has been through heteroepitaxy, which drastically limits the possible material combinations and the ability to integrate complex oxides with mature semiconductor technologies. Moreover, key physical properties of complex-oxide thin films, such as piezoelectricity and magnetostriction, are severely reduced by the substrate clamping effect. Here we demonstrate a universal mechanical exfoliation method of producing freestanding single-crystalline membranes made from a wide range of complex-oxide materials including perovskite, spinel and garnet crystal structures with varying crystallographic orientations. In addition, we create artificial heterostructures and hybridize their physical properties by directly stacking such freestanding membranes with different crystal structures and orientations, which is not possible using conventional methods. Our results establish a platform for stacking and coupling three-dimensional structures, akin to two-dimensional material-based heterostructures, for enhancing device functionalities8,9.

Large Polarization and Susceptibilities in Artificial Morphotropic Phase Boundary PbZr1−xTixO3 Superlattices

AbstractThe ability to produce atomically precise, artificial oxide heterostructures allows for the possibility of producing exotic phases and enhanced susceptibilities not found in parent materials. Typical ferroelectric materials either exhibit large saturation polarization away from a phase boundary or large dielectric susceptibility near a phase boundary. Both large ferroelectric polarization and dielectric permittivity are attained wherein fully epitaxial (PbZr0.8Ti0.2O3)n/(PbZr0.4Ti0.6O3)2n (n = 2, 4, 6, 8, 16 unit cells) superlattices are produced such that the overall film chemistry is at the morphotropic phase boundary, but constitutive layers are not. Long‐ (n ≥ 6) and short‐period (n = 2) superlattices reveal large ferroelectric saturation polarization (Ps = 64 µC cm−2) and small dielectric permittivity (εr ≈ 400 at 10 kHz). Intermediate‐period (n = 4) superlattices, however, exhibit both large ferroelectric saturation polarization (Ps = 64 µC cm−2) and dielectric permittivity (εr = 776 at 10 kHz). First‐order reversal curve analysis reveals the presence of switching distributions for each parent layer and a third, interfacial layer wherein superlattice periodicity modulates the volume fraction of each switching distribution and thus the overall material response. This reveals that deterministic creation of artificial superlattices is an effective pathway for designing materials with enhanced responses to applied bias.

Nanodevices at terahertz frequency based on 2D materials

Artificial semiconductor heterostructures played a pivotal role in modern electronic and photonic technologies, providing a highly effective mean for the manipulation and control of carriers, from the visible to the terahertz frequency range. Despite their exceptional versatility, they commonly require challenging epitaxial growth procedures, due to the need of clean and abrupt interfaces, lattice matching or limited and controlled lattice mismatch, which proved to be major obstacles for the development of room-temperature devices, like sources, detectors or modulators, especially in the far-infrared. The discovery of graphene and the related fascinating capabilities have triggered an unprecedented interest in inorganic two-dimensional materials. Layered materials such as graphene, hexagonal boron nitride, transition metal dichalcogenides, and the more recently re-discovered black phosphorus display an exceptional technological potential for engineering nano-electronic and nano-photonic devices and components ‘by design’, offering a unique platform for developing devices with a variety of properties. Here, I review our latest achievements in the design and developments of graphene based nanodetectors, saturable absorbers and near field probes operating across the far-infrared.

Dynamic electromagnonic crystal based on artificial multiferroic heterostructure

One of the main challenges for the modern magnonics, which, as opposed to the conventional electronics, operates with quanta of spin waves in magnetically ordered materials—magnons—is energy efficient control of magnon transport on small time and space scales. The magnon propagation in a time-dependent periodic spatial potentials—dynamic magnonic crystals—paves a way to this aim. To date, dynamic manipulation of the magnonic crystals has been realized with electric current and optic control influence. However, both approaches show limited potential for reduction in energy consumption and miniaturization of magnonic circuits. Voltage (or electric field) control of magnon currents promises to be fast and low energy consuming. It can be achieved in ferrite-ferroelectric (multiferroic) heterostructures, where strong coupling of magnons and microwave photons constitutes new quasiparticles called electromagnons. Here, we present an experimental realization of a voltage-controlled dynamic electromagnonic crystal operating with electromagnons at microwave frequencies.

Electronic Structure of a Graphene-like Artificial Crystal of NdNiO3.

Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes with typical information depths of a few angstroms. Here, we use a combination of bulk-sensitive soft X-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard X-ray photoelectron spectroscopy (HAXPES), and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO3 confined between two 4-unit cell-thick layers of insulating LaAlO3. The momentum-resolved dispersion of the buried Ni d states near the Fermi level obtained via SX-ARPES is in excellent agreement with the first-principles calculations and establishes the realization of an antiferro-orbital order in this artificial lattice. The HAXPES measurements reveal the presence of a valence-band bandgap of 265 meV. Our findings open a promising avenue for designing and investigating quantum states of matter with exotic order and topology in a few buried layers.

Direct Epitaxial Synthesis of Selective Two-Dimensional Lateral Heterostructures.

Two-dimensional (2D) heterostructured or alloyed monolayers composed of transition metal dichalcogenides (TMDCs) have recently emerged as promising materials with great potential for atomically thin electronic applications. However, fabrication of such artificial TMDC heterostructures with a sharp interface and a large crystal size still remains a challenge because of the difficulty in controlling various growth parameters simultaneously during the growth process. Here, a facile synthetic protocol designed for the production of the lateral TMDC heterostructured and alloyed monolayers is presented. A chemical vapor deposition approach combined with solution-processed precursor deposition makes it possible to accurately control the sequential introduction time and the supersaturation levels of the vaporized precursors and thus reliably and exclusively produces selective and heterogeneous epitaxial growth of TMDC monolayer crystals. In addition, TMDC core/shell heterostructured (MoS2/alloy, alloy/WS2) or alloyed (Mo1-xWxS2) monolayers are also easily obtained with precisely controlled growth parameters, such as sulfur introduction timing and growth temperature. These results represent a significant step toward the development of various 2D materials with interesting properties.

Magnetoelectric Coupling in Ni–Mn–In/PLZT Artificial Multiferroic Heterostructure and Its Application in Mid-IR Photothermal Modulation by External Magnetic Field

Here we show fabrication of an artificial multiferroic heterostructure by a sputter deposited ferromagnetic Ni0.5Mn0.35In0.15 (Ni–Mn–In) layer followed by a ferroelectric Pb0.96La0.04(Zr0.52Ti0.48)...

Photonic Heterostructures for Spin-Flipped Beam Splitting

Enhanced chiral light-matter interaction in metastructures could benefit applications in polarimetry and biosensing, but requires a high-performance chiral beam splitter that separates an incident beam into two circularly polarized ones of opposite chirality. This work unveils a photonic heterostructure that creates such chirality by stacking metasurfaces that are individually achiral, and hence provides an alternative approach. Analyses of symmetry, reciprocity, and microscopic dipolar interactions reveal the mechanism of interlayer coupling, extending the realm of artificial heterostructures into chiral photonics.

Build-in electric field induced step-scheme TiO2/W18O49 heterojunction for enhanced photocatalytic activity under visible-light irradiation

Artificial Z-scheme heterostructure photocatalysts, inspired by the natural photosynthesis, have been explored for water contamination owing to the efficiency of charge carriers separation and enhanced redox capacity. Herein, a novel step-scheme TiO2/W18O49 heterojunction integrating TiO2 nanosheets and spindle-like W18O49 was prepared via a two-step solvothermal method. The TiO2/W18O49 heterojunction with the optimum ratio exhibited excellent photodegradation of RhB higher by a factor of 2.26 and TC degradation higher by a factor of 3.32 than the pristine TiO2 when exposed to visible light. The reactive free radical trapping experiment indicated •O2− was determined to the dominant reactive species. Furthermore, XPS analysis and •O2− quantification measurements confirmed a build-in electric field induced step-scheme structure in TiO2/W18O49 composites, which effectively enhanced the separation of carriers, thus improved photocatalytic activity. The present study will offer a new perception related to the formation of step-scheme heterostructure for photocatalytic degradation.

Disentangling Highly Asymmetric Magnetoelectric Effects in Engineered Multiferroic Heterostructures

One of the main strategies to control magnetism by voltage is the use of magnetostrictive-piezoelectric hybrid materials, such as ferromagnetic-ferroelectric heterostructures. When such heterostructures are subjected to an electric field, piezostrain-mediated effects, electronic charging, and voltage-driven oxygen migration (magnetoionics) may simultaneously occur, making the interpretation of the magnetoelectric effects not straightforward and often leading to misconceptions. Typically, the strain-mediated magnetoelectric response is symmetric with respect to the sign of the applied voltage because the induced strain (and variations in the magnetization) depends on the square of the ferroelectric polarization. Conversely, asymmetric responses can be obtained from electronic charging and voltage-driven oxygen migration. By engineering a ferromagnetic-ferroelectric hybrid consisting of a magnetically soft 50-nm thick Fe75Al25 (at. %) thin film on top of a (110)-oriented Pb(Mg1/3Nb2/3)O3-32PbTiO3 ferroelectric crystal, a highly asymmetric magnetoelectric response is obtained and the aforementioned magnetoelectric effects can be disentangled. Specifically, the large thickness of the Fe75Al25 layer allows dismissing any possible charge accumulation effect, whereas no evidence of magnetoionics is observed experimentally, as expected from the high resistance to oxidation of Fe75Al25, leaving strain as the only mechanism to modulate the asymmetric magnetoelectric response. The origin of this asymmetric strain-induced magnetoelectric effect arises from the asymmetry of the polarization reversal in the particular crystallographic orientation of the ferroelectric substrate. These results are important to optimize the performance of artificial multiferroic heterostructures.

High-throughput Production of ZnO-MoS2-Graphene Heterostructures for Highly Efficient Photocatalytic Hydrogen Evolution.

High-throughput production of highly efficient photocatalysts for hydrogen evolution remains a considerable challenge for materials scientists. Here, we produced extremely uniform high-quality graphene and molybdenum disulfide (MoS2) nanoplatelets through the electrochemical-assisted liquid-phase exfoliation, out of which we subsequently fabricated MoS2/graphene van der Waals heterostructures. Ultimately, zinc oxide (ZnO) nanoparticles were deposited into these two-dimensional heterostructures to produce an artificial ZnO/MoS2/graphene nanocomposite. This new composite experimentally exhibited an excellent photocatalytic efficiency in hydrogen evolution under the sunlight illumination (), owing to the extremely high electron mobilities in graphene nanoplatelets and the significant visible-light absorptions of MoS2. Moreover, due to the synergistic effects in MoS2 and graphene, the lifetime of excited carriers increased dramatically, which considerably improved the photocatalytic efficiency of the ZnO/MoS2/graphene heterostructure. We conclude that the novel artificial heterostructure presented here shows great potential for the high-efficient photocatalytic hydrogen generation and the high throughput production of visible-light photocatalysts for industrial applications.

Sensitivity enhancement of surface plasmon resonance sensors with 2D franckeite nanosheets

Abstract As a latest novel two-dimensional (2D) material with naturally occurring van der Waals heterostructure, franckeite has the unique advantages compared with the artificial stacking heterostructures. Herein, we propose a surface plasmon resonance (SPR) sensor with 2D franckeite nanosheets to improve the sensitivity. In this structure, franckeite is covered on the surface of 50 nm silver film, so that the SPR sensitivity is related to the thickness of the franckeite overlayer. The sensitivity as high as 188 °/RIU in theory has been obtained, which is a 62% increasing compared with the conventional SPR biosensor. Moreover, an additional film of graphene is considered for the virtue of bio-compatible, and the sensitivity of the franckeite-graphene heterostructures is furthermore improved. As an air-stable 2D material, we believe that 2D franckeite nanosheets could have potential applications in the field of chemical and biological sensors in the future.

Van der Waals integration before and beyond two-dimensional materials.

Material integration strategies, such as epitaxial growth, usually involve strong chemical bonds and are typically limited to materials with strict structure matching and processing compatibility. Van der Waals integration, in which pre-fabricated building blocks are physically assembled together through weak van der Waals interactions, offers an alternative bond-free integration strategy without lattice and processing limitations, as exemplified by two-dimensional van der Waals heterostructures. Here we review the development, challenges and opportunities of this emerging approach, generalizing it for flexible integration of diverse material systems beyond two dimensions, and discuss its potential for creating artificial heterostructures or superlattices beyond the reach of existing materials.

Large orbital polarization in nickelate-cuprate heterostructures by dimensional control of oxygen coordination

Artificial heterostructures composed of dissimilar transition metal oxides provide unprecedented opportunities to create remarkable physical phenomena. Here, we report a means to deliberately control the orbital polarization in LaNiO3 (LNO) through interfacing with SrCuO2 (SCO), which has an infinite-layer structure for CuO2. Dimensional control of SCO results in a planar-type (P–SCO) to chain-type (C–SCO) structure transition depending on the SCO thickness. This transition is exploited to induce either a NiO5 pyramidal or a NiO6 octahedral structure at the SCO/LNO interface. Consequently, a large change in the Ni d orbital occupation up to ~30% is achieved in P–SCO/LNO superlattices, whereas the Ni eg orbital splitting is negligible in C–SCO/LNO superlattices. The engineered oxygen coordination triggers a metal-to-insulator transition in SCO/LNO superlattices. Our results demonstrate that interfacial oxygen coordination engineering provides an effective means to manipulate the orbital configuration and associated physical properties, paving a pathway towards the advancement of oxide electronics.

III-VI van der Waals heterostructures for sustainable energy related applications.

van der Waals (vdW) heterostructures, achieved by binding various two-dimensional (2D) materials together via vdW interaction, expand the family of 2D materials and show fascinating possibilities. In this work, we have systematically investigated the geometrical structures, electronic structures, and optical properties of III-VI (MX, M = Ga, In and X = S, Se, Te) vdW heterostructures and their corresponding applications in sustainable energy related areas based on first principles calculations. It is highlighted that different heterostructure types can be achieved in spite of the similar electronic structures of MX monolayers. Meanwhile, the potential applications of the heterostructures for sustainable energy related areas have been further unraveled. For instance, type-II InS/GaSe and GaS/GaSe vdW heterostructures can separately produce hydrogen and oxygen at the opposite parts. On the other hand, a type-II GaSe/GaTe heterostructure with a direct band gap compatible with silicon has been proposed to be a potential solar cell material with a power conversion efficiency over 18%. Furthermore, a gapless type-IV semi-metallic InTe/GaS heterostructure has been predicted to be a Li-ion battery anode material based on three-step lithiated analysis. The present results will shed light on the sustainable energy applications of such remarkable artificial MX vdW heterostructures in the future.

Distinguishing charge and strain coupling in ultrathin (001)-La0.7Sr0.3MnO3/PMN-PT heterostructures

Interfacial charge and strain distributions inside artificial perovskite ABO3 heterostructures often affect intriguing physical properties that are important to device performance. Normally, both charge and strain coexist across the interfaces, and their exact roles in determining the properties remain elusive. In the present work, La0.7Sr0.3MnO3 (LSMO) ultrathin films were grown on (001)-0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMNPT) single-crystal substrates to discriminate between the effect of charge and strain on the transport and magnetoelectric properties. In heterostructures with thicker LSMO films, the strain effect dominates the resistance and the magnetic moment depends on the external electric field. With the decreasing LSMO thickness, the butterfly-like resistance–electric-field (R-E) and magnetization–electric-field (M-E) curves become loop-like, indicating that charge effects dominate strain effects in determining the electric field that controls the transport and magnetic properties. Furthermore, soft-x-ray absorption spectra of 32 and 4 nm LSMO/PMNPT samples at the Mn L edge under an applied electric field of ±6 kV/cm indicate that orbital reconstruction also plays an important role in interfacial magnetoelectric coupling.

Influence of chemical composition and crystallographic orientation on the interfacial magnetism in BiFeO3 / La1−xSrxMnO3 superlattices

The emergence of magnetism unique to the interface between the multiferroic $\mathrm{BiFe}{\mathrm{O}}_{3}$ (BFO) and ferromagnetic $\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}\mathrm{x}}\mathrm{S}{\mathrm{r}}_{\mathrm{x}}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) offers an opportunity to control magnetism in nanoscale heterostructures with electric fields. In this paper, we investigate the influence of chemical composition and crystallographic orientation on the interfacial magnetism of BFO/LSMO superlattices. Our results reveal that the induced net magnetic moment in the BFO layers increases monotonically with increasing saturation magnetization of the LSMO layers. For the (100)-BFO/LSMO ($x=0.2$) superlattice, the induced moment reaches a record high value of $\ensuremath{\sim}2.8\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe}$. No interfacial magnetization is observed at the (100)-BFO/LSMO interface when LSMO is an antiferromagnet. In contrast to (100)-oriented superlattices, no induced moment is observed in (111)-BFO layers. Our results suggest the interfacial structural reconstruction may not be a sufficient condition for the enhanced net moment in BFO layer. Instead, spin canting induced by interfacial exchange coupling is proposed in the (100)- but not in the (111)-BFO, leading to the large net magnetization at the (100)-oriented interface. This work further demonstrates the importance of exchange coupling across heterointerfaces for spin canting in nominally antiferromagnets, providing a pathway to control the magnetic properties of artificial oxide heterostructures.