Single Domain Size Research Articles

Magnetic Tunability via Control of Crystallinity and Size in Polycrystalline Iron Oxide Nanoparticles.

Iron oxide nanoparticles (IONPs) arewidely used for biomedical applications due to their unique magnetic properties and biocompatibility. However, the controlled synthesis of IONPs with tunable particle sizes and crystallite/grain sizes to achieve desired magnetic functionalitiesacross single-domain and multi-domain size ranges remains an important challenge. Here, a facile synthetic method is used to produceiron oxide nanospheres (IONSs) with controllable size and crystallinity for magnetic tunability. First, highly crystalline Fe3O4 IONSs (crystallite sizes above 24nm) having an averagediameter of 50 to 400nm are synthesized with enhanced ferrimagnetic properties. The magnetic properties of these highly crystalline IONSs are comparable to those of their nanocube counterparts, whichtypically possess superior magnetic properties. Second, the crystallite size can be widely tuned from 37 to 10nm while maintaining the overall particle diameter, thereby allowing precise manipulation from the ferrimagnetic to the superparamagnetic state. In addition, demonstrations of reaction scale-up and the proposed growth mechanism of the IONSs are presented. This study highlights the pivotal role of crystal size in controlling the magnetic properties of IONSs and offers a viable means to produce IONSs with magnetic properties desirable for wider applications in sensors, electronics, energy, environmental remediation, and biomedicine.

Acoustic-assisted deposition of ordered self-assembled silica opal layers on hydrophilic single crystal diamond substrates

We deposited self-assembled films of a few monolayers of silica spheres from the water-based colloid on vertically moving hydrophilic single crystal diamond substrates. The use of acoustic agitation of the water suspension with the SiO2 nanospheres significantly improved the long-range order of the films with opal structure. The study examines the evolution of the structure, specifically the size of single crystal domains, with the assistance of sound frequency (0–2500 Hz) and intensity (0–125 dB). Optimal (resonance) parameters are determined based on scanning electron microscopy and optical reflection spectroscopy of the films. The production of high-quality photonic crystals, including templates for diamond-based ordered composites and inverse opal structures, is a promising application of acoustic crystallization of opals on diamond substrates.

Stabilization of various structural phases and magnetic properties of chemically synthesized Co-Ni-Ga nanoparticles

Polycrystalline Co-Ni-Ga ferromagnetic shape memory alloy nanoparticles with phase single austenite (A), martensite (M) and gamma (γ) structures have been obtained for the first time through a facile template-less chemical method. Average crystallite size of A, M and γ phase nanoparticles are 25 nm, 22 nm and 21 nm, respectively. All the synthesized nanoparticles exhibit soft ferromagnetic behavior with saturation magnetization varying from 21.22 to 77.65 emu/g and effective magnetic anisotropy of ∼105 J/m3 at 5 K. Ab initio studies have been performed to evaluate the magnetic moments of single A, M and γ phase. All the single phase nanoparticles have been ascertained to be in single-domain regime by estimating the single-domain size limit. This work showcases a simple methodology to prepare pure crystal phases in these nanoparticles. It also provides insight on the role of elemental precursors and processing conditions in designing Co-Ni-Ga nanoparticles for ferromagnetic shape memory applications.

Remagnetization of magnetite-bearing rocks in the Eastern Qiangtang Terrane, Tibetan Plateau (China): Mechanism and diagnosis

Remagnetization is a common yet notorious phenomenon that interferes with paleogeographic reconstruction. Classical paleomagnetic field tests are helpful in detecting remagnetization but their diagnostic power is limited: remagnetization may occur before folding, the tilting age may be ambiguous, or protracted remagnetization may yield dual polarities. Rock magnetic information can provide other constraints on our understanding of the origin of natural remanent magnetization (NRM). Here we focus on the rock magnetic properties of acknowledged remagnetized limestones and unremagnetized rocks of the Zaduo area in the Eastern Qiangtang Terrane, Tibetan Plateau (China). Chemical remanent magnetization is suggested as a more frequent mechanism than the thermoviscous resetting of the NRM. The secondary NRM resides in authigenic magnetite of stable single domain and superparamagnetic (SP) size which grew during post-depositional burial processes. Both high-field and low-field thermomagnetic runs reveal the alteration of existing iron sulfides to magnetite in the remagnetized limestones. NRM decay curves show that the maximum unblocking temperature of the remagnetized samples is significantly lower than that of the unremagnetized samples. Component analysis of acquisition curves of the isothermal remanent magnetization (IRM) reveals a hard component that represents SP magnetite in remagnetized limestones. This component is absent in unremagnetized rocks. End-member modelling reveals a convex curve in the coefficient of determination versus the number of end-members plot for the unremagnetized limestones, whereas the remagnetized rocks exhibit both near-linear and convex shapes. In addition, quantitative analysis of the hysteresis loop shape for different lithologies indicates its validity in detecting remagnetization. Furthermore, we show the differences in the hysteresis data distributions of the two rock types on the Day plot, the Néel diagram, the Borradaile diagram, and the Fabian diagram. Our research emphasizes that rock magnetic properties can serve as tools to diagnose remagnetization in magnetite-dominated rocks. We recommend a comprehensive rock magnetic study to discriminate remagnetization, involving the Day plot, Fabian diagram, thermal demagnetization curves, IRM component analysis and end member modelling.

Deformation behavior of SmCo compounds via amorphization and recrystallization

In recent years, nanocrystalline magnets based on the critical single domain size theory have received increasing attention. However, nanocrystalline magnets are difficult to enhance the remanent magnetization by forming textures with conventional magnetic field orientation. Texturing of nanocrystalline magnets is usually done using the hot pressing and hot deformation technique, but there are some difficulties in applying this technique to form texture in SmCo systems. To discover and solve the difficulties of obtaining texturized nanocrystalline SmCo-based magnets during the deformation process, samarium cobalt magnets with different degrees of deformation were first experimentally prepared by low strain rate hot deformation method. Then, molecular dynamics simulations were utilized to study the microstructural changes and the multi-phase formation process during hot deformation. The process of deformation together with the reason why it is difficult to form textures in SmCo systems by low-temperature hot deformation is discussed, and the strain energy densities of SmCo multiphase in multiple directions were calculated, which gives a feasible method to form low-temperature hot deformation textures in SmCo systems experimentally.

Low-temperature cluster spin glass transition in the single-domain NiCr2O4 nanoparticles

NiCr2O4 nanoparticles with average particle size ∼15 nm, a single-domain size maintains the bulk canted antiferromagnetic ground state, were synthesized by a microwave combustion method. The magnetic behavior was carefully investigated by static and dynamic magnetic susceptibility measurements. In addition to a spin-glass-like behavior below paramagnetic-ferrimagnetic transition at T C, the NiCr2O4 nanoparticles demonstrate a low-temperature cluster spin glass transition below the spin canting transition T S, which manifests itself as a magnetic anomaly peak around ∼12 K (at 100 Oe) in the zero-field cooled magnetization with a relatively stronger field dependence in a ‘de Almeida-Thouless’ line for spin glasses. The AC susceptibility analyses in different approaches demonstrate a larger relative peak temperature variation per frequency decade and a longer characteristic relaxation time in the order of 0.04 and 10−7 s, against 0.01 and 10−9 s for the high-temperature blocking, indicating the slow spin dynamics for the low-temperature cluster glassy phase. A field-temperature magnetic phase diagram is proposed for the single-domain NiCr2O4 nanoparticles.

Synthesis and physical properties of pure NiO and Ni1–2xMgxMxO (M= Cu, Ru) nanoparticles: Role of growth temperature

Herein, low-cost chloride precursors, EDTA, and NaOH were employed through a facile coprecipitation synthesis route to prepare pure and codoped NiO nanoparticles: Ni1–2xMgxMxO, where M = Cu and Ru and x = 1, 4, and 8 at%. To examine the impact of reaction temperature on the thermal, structural, morphological, optical, and magnetic properties of the prepared samples, the precursor aqueous solutions were heated at 60, 70, and 80 ºC. Data analysis revealed the strong dependence of all the studied properties on reaction temperature and codoping. Thermogravimetric analysis (TGA) was performed to evaluate the thermal stability of the prepared samples. The face-centered cubic (fcc) crystallization of NiO was maintained at all the applied temperatures and doping concentrations. X-ray diffraction (XRD) patterns showed the good incorporation of Mg2+ and Cu2+ ions with the host Ni2+ ions in Ni1–2xMgxCuxO samples, while peaks of RuO2 appeared as an impurity phase in Ni1–2xMgxRuxO samples. Tiny spherical nanoparticles with a crystallite size of 6 nm were produced at 80 ºC and 8 at% (Mg, Ru) codoping. Energy dispersive X-ray (EDX) analysis confirmed that the elemental compositions of the synthesized samples were in stoichiometric ratios. The UV-Vis spectra demonstrated a monotonic increase in band gap with reaction temperature and codoping. The estimated Herve-Vandamme refractive index (nHV), the high (ε∞) and the static (εo) dielectric constants showed a decreasing trend with increasing reaction temperature. Weak ferromagnetic behavior was observed for the pure NiO samples, while the codoped samples with x > 4% showed linear M-H curves. The coercivety (Hc) decreased with reaction temperature, implying single-domain magnetization and sizes below the critical one. The magnetocrystalline anisotropy (K) indicated the softness of the prepared samples.

Structural and magnetic properties of Sb substituted LTP Mn-Bi annealed ribbons

We optimized the composition of Mn-Bi alloy, examined the effects of third element substitution of Sn, Cu, Al, and Sb on the magnetic properties, and investigated the microstructures and magnetic properties of Sb substituted Mn-Bi annealed ribbons. The optimized composition was Mn54Bi46, and it was found that the Sb substitution had the most positive effects on the magnetic properties among the substitution elements. Thus, we accomplished a detailed investigation on Mn54Bi46-xSbx (0.0 ≤ x ≤ 5.0) annealed ribbons. The fracture planes of the annealed ribbons revealed that the grain size drastically decreased from a few tens of microns to submicrons close to the single domain size, which substantially enhanced the coercivity (Hc), remanent magnetization (Mr), and maximum energy product ((BH)max). As a result, as the Sb content changed from 0 to 1.5, the Mr, Hc, and (BH)max of randomly oriented samples increased significantly from 14.7 emu/g, 0.50 kOe, and 0.15 MGOe to 37.2 emu/g, 8.15 kOe, and 3.27 MGOe, respectively. Furthermore, the temperature dependence of Hc on the Sb substitution showed significant enhancement over a wide temperature range from 223 to 378 K, suggesting that Mn-Bi alloy can be used in electrical vehicles.

Two-dimensional confinement engineering of SiO2 nanosheets supported nano-cobalt for high-efficiency microwave absorption

Advanced electromagnetic (EM) absorbers containing magnetic and dielectric components have garnered substantial attention due to rapid expansion of wireless communication equipment. However, the EM absorbing performance of magnetic materials is greatly hindered by the substantial decrease in permeability at gigahertz frequencies, commonly referred to the Snoek limit. Confinement engineering provides effective strategy for precisely modulating particle size in a confined region to enhance the surface anisotropy and thereby surpass the Snoek limit. Herein, a novel space-confined strategy is proposed to develop two-dimensional (2D) SiO2 nanosheets that involves manipulating the topological exfoliation of CaSi2 with CoCl2 and thereafter high-temperature reduction. The resulting SiO2 supported nano-Co (Co-2DSiO2) heterostructure exhibits uniform dispersion and nearly single domain size. Contributing to the magnetic-dielectric synergistic effect, fascinating EM properties can be achieved by regulating the size of nano-Co at varied temperatures, and the Co-2DSiO2 delivers the optimal reflection loss of –51.6 dB at 2.5 mm and the effective absorption bandwidth of 4.6 GHz. Electronic structures of Co-2DSiO2 were simulated by theoretical calculation, further verifying the potential mechanism of enhanced dielectric loss and analyzing the formation of heterostructure. The proposed confinement strategy lays the groundwork for the development of advanced absorbers and provides a universal approach for other metal-based SiO2 nanosheets with tunable structures.

Effects of Ti–V pair substitution and grain boundary modification on physical and magnetic properties of Sm(Fe0.8Co0.2)12 bulk magnet

We successfully fabricated high-density Sm(Fe0.8Co0.2)12–2xTixVx (0.6 ≤ x ≤ 1.0) bulk magnets with high-purity ThMn12-type phase through the bulking of amorphous powder and subsequent annealing process. Depending on the Ti–V pair concentrations, the physical and magnetic properties including lattice constants, crystallized phases, magnetizations (M), and coercivities (Hc) of the fabricated samples were investigated. The highest maximum energy product, (BH)max, of 7.293 MGOe was obtained with x = 0.6; however, its Hc was limited to 3379 Oe due to the absence of a grain boundary phase, despite the average grain size being within 81 nm, which is close to the single domain size. Thus, an intergranular phase was developed by introducing Cu–Ga additive, i.e., Sm(Fe0.8Co0.2)10.8Ti0.6V0.6 + y wt. % Cu–Ga (1 ≤ y ≤ 3), to enhance the Hc. Uniform grain boundaries with Sm-rich phases were crystallized for y ≥ 2, and their thicknesses increased with higher y; the thickness of the grain boundary for the sample with y = 2 was 12 nm. The Hc of the samples with uniform grain boundaries were significantly enhanced from 3379 Oe to 4054 and 4317 Oe for y = 0 and y = 2 and 3, respectively. This paper presents the results of Ti–V pair substitution and grain boundary modification in an effort to improve magnetic properties by controlling crystalline phases, grain sizes, and grain boundaries.

Comprehensive magnetic analysis of the tephras in Middle-Late Pleistocene loess records of Serbia, and implications for tephra identification, correlation and loess chronology

Tephras are highly valuable for achieving precise chronologies and secure correlations for sedimentary sequences. Rock magnetic investigations provide a simple, rapid tool for tephra identification especially for weathered tephra, cryptotephra (hidden tephra), thin and dispersed distal tephra. But so far, the most diagnostic magnetic properties for tephra identification and for differentiation of specific tephras are still uncertain. Three laterally continuous and representative tephras/cryptotephras embedded in Serbian loess deposits provide a rare opportunity to explore application of magnetic approaches in tephrostratigraphy. Systematic rock magnetic and petrographic comparison of tephra and its host loess reveal that the characteristic magnetic mineral of the studied tephras is moderate-Ti titanomagnetite of magmatic origin, with a dominant magnetic grain size of non-interacting single domain (SD) and pseudo-single domain (PSD) state. We identify the most diagnostic magnetic properties for tephra identification in loess and many other sediments. The Curie temperature of titanomagnetite is the most diagnostic property of tephras. A low-temperature alternating current (AC) magnetic susceptibility peak at ∼20–100 K is a sensitive indicator of coarse-grained titanomagnetite. SIRM-χfd and χARM-χfd plots can easily differentiate tephras from loess-paleosol sequences and possibly other pedogenesis-influenced sediments. Furthermore, our results suggest Curie temperatures can be a promising tool of regional tephra correlation. Our magnetic evidence and mineral composition analysis support the same origin of Tephra-L5 as Tephra-L4 from Alban Hills Volcanic District, possibly from the ∼457 ka eruption, which would promote a robust chronostratigraphic model of Danube loess.

Tunable Subnanometer Gaps in Self-Assembled Monolayer Gold Nanoparticle Superlattices Enabling Strong Plasmonic Field Confinement.

Nanoparticle superlattices produced with controllable interparticle gap distances down to the subnanometer range are of superior significance for applications in electronic and plasmonic devices as well as in optical metasurfaces. In this work, a method to fabricate large-area (∼1 cm2) gold nanoparticle (GNP) superlattices with a typical size of single domains at several micrometers and high-density nanogaps of tunable distances (from 2.3 to 0.1 nm) as well as variable constituents (from organothiols to inorganic S2-) is demonstrated. Our approach is based on the combination of interfacial nanoparticle self-assembly, subphase exchange, and free-floating ligand exchange. Electrical transport measurements on our GNP superlattices reveal variations in the nanogap conductance of more than 6 orders of magnitude. Meanwhile, nanoscopic modifications in the surface potential landscape of active GNP devices have been observed following engineered nanogaps. In situ optical reflectance measurements during free-floating ligand exchange show a gradual enhancement of plasmonic capacitive coupling with a diminishing average interparticle gap distance down to 0.1 nm, as continuously red-shifted localized surface plasmon resonances with increasing intensity have been observed. Optical metasurfaces consisting of such GNP superlattices exhibit tunable effective refractive index over a broad wavelength range. Maximal real part of the effective refractive index, nmax, reaching 5.4 is obtained as a result of the extreme field confinement in the high-density subnanometer plasmonic gaps.

Thermal stimulated structural transformation of cubic Zn0.78Cd0.22S nanoparticles to ZnO nano-hexagons: Tailoring of optical band gap and emission spectra for optoelectronic implementations

Ternary alloys of Zn0.78Cd0.22S nanoparticles (NPs) are synthesized via the facile co-precipitation technique. The as-synthesized sample exhibits a zincblende-type cubic phase with an average crystalline size of 2 nm. Thermal annealing and UV irradiation are utilized as post-treatment processes for tailoring the optical properties of Zn0.78Cd0.22S NPs. The as-synthesized sample exhibits a stable cubic phase up to 400 °C, during which partial phase transformation to a hexagonal structure is observed at a higher temperature of 500 °C. The oxidation of Zn0.78Cd0.22S NPs to mixed oxide phases with the majority of ZnO begins at 600 °C which induces morphology transformation to a relatively large nano-hexagon with a single crystalline domain size. The increase of the annealing temperature is accompanied by a decrease of Zn0.78Cd0.22S NPs optical band gap (Eg) due to the weakness of size confinement as well as the formation of localized states below the mobility band edges. The brief UV irradiation results in the increase of Eg whereas a further increase in exposure time is accompanied by a reduction of Eg. The photoluminescence (PL) spectrum of the as-synthesized sample covers a wide spectral range from UV to visible. Thermal annealing has a slight effect on the PL emission at high excitation energy, whereas low excitation energy reveals higher sensitivity to deep-state emission. Thermal oxidation incorporates a high concentration of oxygen-related defects that induce strong enhancement in the green emission at the expense of UV emission. This indicates the thermal-induced bleaching of shallow trapping sites and the evolution of other deep trapping or recombination sites.

Large-Area Ultrathin Moiré Chiral Metamaterials by Thermal-Tape-Transfer Printing

Moiré chiral metamaterials (MCMs) consisting of stacked plasmonic nanohole arrays with twist angles exhibit strong chiroptical response, thereby opening doors to ultrathin chiroptical sensors, switches, and detectors. The existing fabrication of MCMs on the basis of colloidal lithography results in significant inhomogeneity within the structure, which impairs the responsiveness and tunability of its optical chirality. Here, we develop thermal-tape-transfer printing to enable the fabrication of large-scale and homogeneous MCMs with arbitrary twist angles and tunable optical chirality. As a demonstration, large-scale (100 × 100 μm2) gold MCMs with an ultrathin thickness (∼40 nm) were fabricated, which marked a 10-fold increase in single domain size over the colloidal lithographic method, while showing spatially uniform and strong chiroptical response. With the ultrathin thickness and high tunability, the MCMs developed by our fabrication method will advance a variety of biological, photonic, and optoelectronic applications.

A novel modified nano-alumina composite sol for potential application in forest firefighting.

Herein, modified ammonium polyphosphate wrapped nano-alumina (mAPP@Als) was first synthesized and then dispersed in traditional fire extinguishing solution (FES) to fabricate a FES-mAPP@Als composite sol. It was found that the phosphorus-silica containing units were attached onto the nano-alumina surface, and the mAPP@Als particles showed excellent dispersion level in FES with a single-domain particle size distribution range. Due to the synergistic effects of the phosphorus-nitrogen and silica-alumina flame retardant components, FES-mAPP@Als (5% concentration) coated wood exhibited improved limiting oxygen index (33.2%) and carbonization ability, and depressed heat release (41.9%) and smoke production (10.7%), as compared to the pristine wood. In addition, the FES-mAPP@Als composite sol showed enhanced fire-extinguishing and anti-reignition capacities compared to the FES. This research offers a novel composite sol fire extinguishing agent for fighting forest fires.

Intrinsic hard magnetism and thermal stability of a ThMn12-type permanent magnet

Herein, we theoretically demonstrate that simple metal (Ga and Al) substitutional atoms, rather than the conventional transition metal substitutional elements, not only stabilize the ThMn12-type SmFe12 and Sm(Fe,Co)12 phases thermodynamically but also further improve their intrinsic magnetic properties such that they are superior to those of the widely investigated SmFe11Ti and Sm(Fe,Co)11Ti magnets, and even to the state-of-the-art permanent magnet Nd2Fe14B. More specifically, the quaternary Sm(Fe,Co,Al)12 phase has the highest uniaxial magnetocrystalline anisotropy (MCA) of about 8 MJ m−3, anisotropy field of 18.2 T, and hardness parameter of 2.8 at room temperature and a Curie temperature of 764 K. Simultaneously, the Al and Ga substitutional atoms improve the single-domain size of the Sm(Fe,Co)12 grains by nearly a factor of two. Numerical results of MCA and MCA-driven hard magnetic properties can be described by the strong spin-orbit coupling and orbital angular momentum of the Sm 4f-electron orbitals.

Space-confined fabrication of hydrophobic magnetic carbon nanofibers for lightweight and enhanced microwave absorption

Advanced microwave absorbers (MAs) containing magnetic particles and carbon nanofibers (CNFs) have raised widespread attention in the rapid popularization of electronic communication equipment. However, it is quite challenging to control the growth of magnetic particles in CNFs to improve electromagnetic waves (EMWs) loss and attenuation. Herein, a novel space-confined strategy was adopted to develop Co/CoO/SiO2/CNFs, in which Co/CoO nanoparticles (NPs) with single domain size and dispersed distribution could achieve enhanced magnetic loss and dielectric loss. The introduction of nano-SiO2 in the system could enhance the dielectric confinement effect of the nanofibers (NFs), while an interfacial confinement network could also be constructed to realize the refinement of Co/CoO NPs, enhance the electromagnetic (EM) loss, and ultimately ameliorate the microwave absorption performance of as-obtained NFs. When the additive amount of nano-SiO2 in the spinning solution was 1.0 mg, the as-prepared Co/CoO/SiO2/CNFs reached the optimal reflection loss (RL) of −52.9 dB at 12.64 GHz with a thickness of 2.5 mm, and the effective bandwidth (EBW) reached 5.67 GHz (10.46–16.13 GHz). Furthermore, the excellent hydrophobic properties of the CNFs endow them with potential self-cleaning function. The confinement strategy proposed in this work lays a foundation for the design of low-filling ratio and high-performance CNFs-based microwave absorbers, and presents a general approach for the synthesis of one-dimensional composite materials with controllable structures.

Spontaneous-polarization-induced photovoltaic effect in rhombohedrally stacked MoS2

Stacking order in van der Waals materials determines the coupling between atomic layers and is therefore key to the materials' properties. By exploring different stacking orders, many novel physical phenomena have been realized in artificial vdW stacks. Recently, 2D ferroelectricity has been observed in zero-degree aligned hBN and graphene-hBN heterostructures, holding promise in a range of electronic applications. In those artificial stacks, however, the single domain size is limited by the stacking-angle misalignment to about 0.1 to 1 $\mu$m, which is incompatible with most optical or optoelectronic applications. Here we show MoS$_2$ in the rhombohedral phase can host a homogeneous spontaneous polarization throughout few-$\mu$m-sized exfoliated flakes, as it is a natural crystal requiring no stacking and is, therefore free of misalignment. Utilizing this homogeneous polarization and its induced depolarization field (DEP), we build a graphene-MoS$_2$ based photovoltaic device with high efficiency. The few-layer MoS$_2$ is thinner than most oxide-based ferroelectric films, which allows us to maximize the DEP and study its impact at the atomically thin limit, while the highly uniform polarization achievable in the commensurate crystal enables a tangible path for up-scaling. The external quantum efficiency of our device is up to 16% at room temperature, over one order larger than the highest efficiency observed in bulk photovoltaic devices, owing to the reduced screening in graphene, the exciton-enhanced light-matter interaction, and the ultrafast interlayer relaxation in MoS$_2$. In view of the wide range of bandgap energy in other TMDs, our findings make rhombohedral TMDs a promising and versatile candidate for applications such as energy-efficient photo-detection with high speed and programmable polarity.

Spontaneous Polarization induced Photovoltaic Effect In rhombohedrally stacked MoS2

Stacking order in van der Waals materials determines the coupling between atomic layers and is therefore key to the materials' properties. By exploring different stacking orders, many novel physical phenomena have been realized in artificial vdW stacks. Recently, 2D ferroelectricity has been observed in zero-degree aligned hBN and graphene-hBN heterostructures, holding promise in a range of electronic applications. In those artificial stacks, however, the single domain size is limited by the stacking-angle misalignment to about 0.1 to 1 $\mu$m, which is incompatible with most optical or optoelectronic applications. Here we show MoS$_2$ in the rhombohedral phase can host a homogeneous spontaneous polarization throughout few-$\mu$m-sized exfoliated flakes, as it is a natural crystal requiring no stacking and is, therefore free of misalignment. Utilizing this homogeneous polarization and its induced depolarization field (DEP), we build a graphene-MoS$_2$ based photovoltaic device with high efficiency. The few-layer MoS$_2$ is thinner than most oxide-based ferroelectric films, which allows us to maximize the DEP and study its impact at the atomically thin limit, while the highly uniform polarization achievable in the commensurate crystal enables a tangible path for up-scaling. The external quantum efficiency of our device is up to 16% at room temperature, over one order larger than the highest efficiency observed in bulk photovoltaic devices, owing to the reduced screening in graphene, the exciton-enhanced light-matter interaction, and the ultrafast interlayer relaxation in MoS$_2$. In view of the wide range of bandgap energy in other TMDs, our findings make rhombohedral TMDs a promising and versatile candidate for applications such as energy-efficient photo-detection with high speed and programmable polarity.

Enhanced epitaxial growth of two-dimensional monolayer WS2 film with large single domains

The emerging of graphene has stimulated the exploration of two-dimensional (2D) nanomaterials. As a kind of important semiconducting 2D materials, WS2 shows great potentials in electronics, optoelectronics, and photonics, etc.; however, the synthesis of high quality, large area, uniform and epitaxial 2D WS2 monolayer films is still a challenge. Herein, we report the epitaxial growth of WS2 on sapphire with excellent electrical and optoelectronic properties via an enhanced chemical vapor deposition method. With the assistance of Na2WO4 as well as the investigation of related growth mechanism, large single crystal triangular WS2 monolayer flakes with well-defined orientations are achieved. The corresponding single crystal domain size is larger than 1 mm together with the centimeter-scale continuous film realized. Once fabricated into transistors, the monolayers exhibit excellent device properties, such as high mobility and large on/off current ratio. Also, these devices give respectable photoresponse properties when operated in the photoconductive mode. The peak responsivity can reach 4.6 A W−1. All these results demonstrate the high quality of as-synthesized WS2 monolayers, indicating the bright future of this synthesis technology for 2D materials towards practical applications.