Jiuxiang Dai

The genesis of ferroelectricity in thick (GeTe) _m (Sb₂Te₃)₁ lamellae is uncovered. The resilience of the quasi 2D character of GeTe-rich (GeTe) _m (Sb₂Te₃) _n films is demonstrated. It paves the way for the design of novel van der Waals heterostructures and multifunctional devices.

Abstract

The possibility to engineer (GeTe) _m (Sb₂Te₃) _n phase-change materials to co-host ferroelectricity is extremely attractive. The combination of these functionalities holds great technological impact, potentially enabling the design of novel multifunctional devices. Here an experimental and theoretical study of epitaxial (GeTe) _m (Sb₂Te₃) _n with GeTe-rich composition is presented. These layered films feature a tunable distribution of (GeTe) _m (Sb₂Te₃)₁ blocks of different sizes. Breakthrough evidence of ferroelectric displacement in thick (GeTe) _m (Sb₂Te₃)₁ lamellae is provided. The density functional theory calculations suggest the formation of a tilted (GeTe) _m slab sandwiched in GeTe-rich blocks. That is, the net ferroelectric polarization is confined almost in-plane, representing an unprecedented case between 2D and bulk ferroelectric materials. The ferroelectric behavior is confirmed by piezoresponse force microscopy and electroresistive measurements. The resilience of the quasi van der Waals character of the films, regardless of their composition, is also demonstrated. Hence, the material developed hereby gathers in a unique 2D platform the phase-change and ferroelectric switching properties, paving the way for the conception of innovative device architectures.

Light: Science & Applications, Published online: 22 November 2023; doi:10.1038/s41377-023-01327-8

Photonic micropatterns are created through single-step growth of colloidal crystals on lithographically prepatterned substrates with planar surfaces and nano-needle arrays. When the colloidal assembly is mediated by depletion attraction, the planar surfaces effectively cause heterogeneous nucleation and crystal growth, whereas the nano-needle arrays inhibit it. This results in regioselective crystallization on the planar areas, yielding high-resolution structural-color graphics.

Abstract

Colloidal crystals display photonic stopbands that generate reflective structural colors. While micropatterning offers significant value for various applications, the resolution is somewhat limited for conventional top-down approaches. In this work, a simple, single-step bottom-up approach is introduced to produce photonic micropatterns through depletion-mediated regioselective growth of colloidal crystals. Lithographically-featured micropatterns with planar surfaces and nano-needle arrays as substrates are employed. Heterogeneous nucleation is drastically suppressed on nano-needle arrays due to minimal particle-to-needles overlap of excluded volumes, while it is promoted on planar surfaces with large particle-to-plane volume overlap, enabling regioselective growth of colloidal crystals. This strategy allows high-resolution micropatterning of colloidal photonic crystals, with a minimum feature size as small as 10 µm. Stopband positions, or structural colors, are controllable through concentration and depletant and salt, as well as particle size. Notably, secondary colors can be created through structural color mixing by simultaneously crystallizing two different particle sizes into their own crystal grains, resulting in two distinct reflectance peaks at controlled wavelengths. The simple and highly reproducible method for regioselective colloidal crystallization provides a general route for designing elaborate photonic micropatterns suitable for various applications.

Through optimizing band structure and thus increasing the population of electrons at states near the Fermi level, entropy engineering can significantly improve the SERS performance of 2D metal phosphorus trichalcogenides. The optimal MnFeCuAgInPS₃ nanosheets exhibit remarkable SERS signals with a detection limit down to 10⁻⁹ m and excellent stability for trace analysis.

Abstract

Surface-enhanced Raman scattering (SERS) spectroscopy is an ultrasensitive detection technique for molecular identification in both biology and chemistry. 2D materials have displayed increasing potentials as SERS substrates based on chemical enhancement, where optimization of the band structure to promote the charge transfer is exceptionally important. Here, the regulation of band structure of 2D metal phosphorus trichalcogenides (MPCh₃) via entropy engineering is demonstrated. The optimized high-entropy MnFeCuAgInPS₃ nanosheets (NSs) with narrowed bandgap (E _g) show significant SERS performance with a low detection limit with 10⁻⁹ m for both rhodamine 6G and crystal violet. Combined spectral characterizations and density functional theory (DFT) calculations reveal that the combination of multiple hetero-element provides continuous d orbitals and endows high-entropy MPCh₃ NSs with high population of electrons at the energies near Fermi level (E _F), which allows highly efficient photo-induced charge transfer (PICT) between the SERS substrates and target molecules. This work affords a new strategy for high-performance 2D SERS materials and also reveals the origin of the band structure regulation by entropy engineering.

This paper proposes a modified molten salt assisted exfoliation (MMSAE) strategy to prepare high-quality 2D materials. The MMSAE approach involves using high-frequency disturbance to create a shear force that results in the successful and efficient exfoliation of 2D material. Taking h-BN as an example, an average lateral size of 6.30 µm could be achieved.

Abstract

2D materials hold great promise for numerous applications, and the physical properties of individual 2D materials depend strongly on their thickness and lateral size. Currently, there is a lack of a straightforward method to produce 2D materials with both high-quality and ultra-high aspect ratios. Herein, a modified molten salt-assisted exfoliation (MMSAE) strategy is proposed to prepare high-quality 2D materials with large sizes and controllable thickness. The MMSAE approach involves using high-frequency disturbance to create a shear force that peels off the layers of the intercalated material (e.g., graphene, MoS₂, WS₂, and clays), resulting in the successful and efficient exfoliation of 2D material. Taking h-BN for example, the MMSAE approach achieves the preparation of 2D h-BN with an average lateral size of 6.30 µm, thickness of 2.34 nm, and a high aspect ratio of 2692.

A new method is established to characterize the transverse (in-plane) piezostrain of ferroelectric/piezoelectric thin films by the inverse magnetoelectric coupling effect. The in-plane piezoelectric coefficient d ₃₁ and piezoelectric strain S ₃₁ of PZT thin films are measured by this method for demonstration. Moreover, it has a high strain sensitivity about 0.005%.

Piezoelectric response of the ferroelectric/piezoelectric thin films is crucial for actuation of micro-electromechanical systems (MEMS) devices as well as strain-mediated electronic devices. Among many piezoelectric modes, longitudinal and transverse piezostrain is the most widely used. Various methods have been well established for quantitative characterization of the longitudinal piezostrain, such as laser interferometer and piezoelectric force microscopy. However, it is still a great challenge to characterize transverse piezostrain. Herein, a new method is established to characterize the transverse (in-plane) piezostrain of ferroelectric/piezoelectric thin films by the inverse magnetoelectric (ME) coupling effect. A ferromagnetic thin film is patterned onto the ferroelectric thin film to form an artificial multiferroic heterostructure. The transverse piezostrain can transfer from the ferroelectric layer to the ferromagnetic layer and shift its ferromagnetic resonance field through the ME coupling effect. By comparing with a control sample of “bulk piezoceramic/ferromagnetic thin film” whose in-plane piezostrain can be easily measured, the in-plane piezostrain and piezoelectric coefficient of the ferroelectric thin films are then estimated. The in-plane piezoelectric coefficient d ₃₁ and piezoelectric strain S ₃₁ of Pb(Zr,Ti)O₃ (PZT) thin films are measured by this method. This new method will promote to fully understand the piezoelectric properties of the ferroelectric/piezoelectric thin films.

Two types of 1D grain boundaries, that is, 4|4P and 4|4E mirror twin boundaries (MTBs), are synthesized in a charge-density-wave metal monolayer NbSe₂. Both the confined electronic states and the charge density modulations at the MTBs are captured via scanning tunneling microscopy measurements. These results pave the way for the grain boundary engineering of the functionality.

Abstract

1D grain boundaries in transition metal dichalcogenides (TMDs) are ideal for investigating the collective electron behavior in confined systems. However, clear identification of atomic structures at the grain boundaries, as well as precise characterization of the electronic ground states, have largely been elusive. Here, direct evidence for the confined electronic states and the charge density modulations at mirror twin boundaries (MTBs) of monolayer NbSe₂, a representative charge-density-wave (CDW) metal, is provided. The scanning tunneling microscopy (STM) measurements, accompanied by the first-principles calculations, reveal that there are two types of MTBs in monolayer NbSe₂, both of which exhibit band bending effect and 1D boundary states. Moreover, the intrinsic CDW signatures of monolayer NbSe₂ are dramatically suppressed as approaching an isolated MTB but can be either enhanced or suppressed in the MTB-constituted confined wedges. Such a phenomenon can be well explained by the MTB-CDW interference interactions. The results reveal the underlying physics of the confined electrons at MTBs of CDW metals, paving the way for the grain boundary engineering of the functionality.

Bayesian optimization (BO) is successfully applied to the dual-layer oxide thin film transistors (OS TFTs) design by managing two OS layers as inputs and field effect mobility and threshold voltage as outputs. BO produces TFTs with high field-effect mobility (36 cm² V^-1 s^-1) and a proper operation range. Also, machine learning provides valuable process optimization guidelines using a figure of merit.

Abstract

Machine learning (ML) provides temporal advantage and performance improvement in practical electronic device design by adaptive learning. Herein, Bayesian optimization (BO) is successfully applied to the design of optimal dual-layer oxide semiconductor thin film transistors (OS TFTs). This approach effectively manages the complex correlation and interdependency between two oxide semiconductor layers, resulting in the efficient design of experiment (DoE) and reducing the trial-and-error. Considering field effect mobility (𝜇) and threshold voltage (V _th) simultaneously, the dual-layer structure designed by the BO model allows to produce OS TFTs with remarkable electrical performance while significantly saving an amount of experimental trial (only 15 data sets are required). The optimized dual-layer OS TFTs achieve the enhanced field effect mobility of 36.1 cm² V⁻¹ s⁻¹ and show good stability under bias stress with negligible difference in its threshold voltage compared to conventional IGZO TFTs. Moreover, the BO algorithm is successfully customized to the individual preferences by applying the weight factors assigned to both field effect mobility (𝜇) and threshold voltage (V _th).

Two 2D hybrid metal halides, (C₇H₇N₂)₂PbCl₄ and (C₇H₇N₂)₂PbBr₄, with comparable initial structures exhibit distinct excitonic emissions at ambient conditions and under high pressure due to varied structural stiffness, Coulomb force, and octahedral distortion.

Abstract

Two dimensional (2D) hybrid metal halides (HMHs) usually exhibit free excitonic (FE) emission, and self-trapped excitonic (STE) emission can also be achieved by adopting appropriate halogens and organic cations. Recently, significant efforts have been made to modulate and then clarify the transformation and connection between these two types of excitonic emissions in 2D HMHs. In this study, intriguing pressure-tuned transitions between FE emission and STE emission are observed in 2D (C₇H₇N₂)₂PbCl₄. In contrast, only FE emissions with tunable emission energies are observed in 2D (C₇H₇N₂)₂PbBr₄ which possesses a similar structure with (C₇H₇N₂)₂PbCl₄ under compression. Such distinct halide-dependent optical responses under pressure are experimentally revealed to arise from the intricate interplay among several factors in these HMHs, including the stiffness of the structure, the Coulomb force between the organic cations and the inorganic octahedra, and the magnitude of inorganic octahedral distortion. These high-pressure optical explorations can unravel the underlying interrelationship between the crystal structure and excitonic emission in 2D HMHs.

A tough hydrogel with high water retention capacity was prepared via solvent exchange involving a deep eutectic solvent and the construction of dual networks. Utilizing the pH-induced shape memory and tunable fluorescence characteristics, the hydrogel can be used as a dual-encryption platform with a high security level and the advantages of rewritability, reprogrammability, and reusability.

Abstract

Information encryption platforms with reliable encryption performance, excellent mechanical performance, and high water retention capacity are highly desired. In this study, a tough double-network hydrogel is designed using the first network of a polyion complex containing lanthanide complexes via one-pot polymerization and the second network of a poly (N-hydroxyethyl acrylamide) (PHEAA) obtained by deep eutectic solvent (DES)-assisted introduction and subsequent photopolymerization. In this system, the pH-induced shape memory function and pH-/wavelength-dependent fluorescence allow the use of the prepared hydrogel as a dual-encryption platform. Owing to its high response reversibility, the hydrogel-based platform exhibits both a high security level and the advantages of rewritability, reprogrammability, and reusability. Additionally, the excellent mechanical properties and water retention capacity owing to the solvent exchange process involving the low-volatility solvent DES and the resulting introduction of the second network of PHEAA offer high practical application value for the hydrogel-based dual encryption platform, demonstrating its potential for information security protection.

An amorphous tellurium phase is synthesized and its electrochemical performance is compared with that of the crystalline phase using operando transmission electron microscopy. The different lithiation/delithiation behaviors and mechanisms of the two phases are revealed, and the structural order and disorder effects on their battery performance are explained. Insights for the design of novel tellurium-based electrode materials are provided.

Abstract

Lithium-ion batteries (LIBs) using tellurium (Te) as electrode material are appealing because of their high capacities, conductivities, and lithium-ion diffusivity relative to those of silicon. However, crystalline Te electrode suffers from mechanical instability and poor cyclability during Li⁺ insertion and extraction. Moreover, the reaction mechanisms governing Te electrode during the electrochemical charge and discharge are poorly understood. Here, an amorphous Te phase is deliberately conducted and the results of comparative operando experiments on the crystalline and amorphous Te phases are reported. The lithiation of the crystalline Te phase results in grains with concomitant pulverization. On the lithiation-induced volumetric expansion and aggregation of the intrinsic stress, the Te crystalline phase undergoes bending, fracture, and finally collapse. In addition to the Li-rich phase (Li₂Te), a new Li-deficient phase (LiTe₃) that may be associated with incomplete lithiation owing to the poor ion conductivity of pulverized lithiation product is also detected. However, the amorphous Te specimens show promising lithiation/delithiation properties, particularly no pulverization behavior or structural damage, suggesting better capacity and reversibility. The different performances of crystalline and amorphous Te can be ascribed to the ordered and disordered structures. The findings will serve as a reference for the design of Te-containing LIBs.

To improve the stability of III-V quantum dots-based infrared photodetector, this work addresses the issue of zinc ion migration, decreasing device performance. Using a hybrid NiO_X/poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine (PTAA) hole transport layer and inversion of the device structure, this work achieves photodetectors with a quantum efficiency (EQE) of 53% at 970 nm and maintained 95% of their initial performance after 19 h of continuous illuminated operation.

Abstract

III-V colloidal quantum dots (CQDs) are of interest in infrared photodetection, and recent developments in CQDs synthesis and surface engineering have improved performance. Here this work investigates photodetector stability, finding that the diffusion of zinc ions from charge transport layers (CTLs) into the CQDs active layer increases trap density therein, leading to rapid and irreversible performance loss during operation. In an effort to prevent this, this work introduces organic blocking layers between the CQDs and ZnO layers; but these negatively impact device performance. The device is then, allowing to use a C60:BCP as top electron-transport layer (ETL) for good morphology and process compatibility, and selecting NiO_X as the bottom hole-transport layer (HTL). The first round of NiO_X-based devices show efficient light response but suffer from high leakage current and a low open-circuit voltage (Voc) due to pinholes. This work introduces poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) with NiO_X NC to form a hybrid HTL, an addition that reduces pinhole formation, interfacial trap density, and bimolecular recombination, enhancing carrier harvesting. The photodetectors achieve 53% external quantum efficiency (EQE) at 970 nm at 1 V applied bias, and they maintain 95% of initial performance after 19 h of continuous illuminated operation. The photodetectors retain over 80% of performance after 80 days of shelf storage.

npj 2D Materials and Applications, Published online: 21 November 2023; doi:10.1038/s41699-023-00435-8

Nature Communications, Published online: 21 November 2023; doi:10.1038/s41467-023-43472-z

Light: Science & Applications, Published online: 21 November 2023; doi:10.1038/s41377-023-01319-8

Nature Communications, Published online: 18 November 2023; doi:10.1038/s41467-023-43323-x

A novel InAs_0.85P_0.15 homojunction detector with λ _c = 2.6 μm is achieved through lattice and bandgap engineering with well-designed InAs _y P_{1- y} metamorphic buffers. Such an InAsP metamorphic detector with excellent photodetection capabilities is expected to find applications in the implementation of large-format focal plane arrays for extended short-wavelength infrared imaging.

Abstract

Short-wavelength infrared (SWIR) photodetectors are of great interest owing to their unique advantages of SWIR imaging such as better penetration ability and improved sensitivity that allow high-resolution imaging. Commercially, extended In _x Ga_{1- x} As heterojunction detectors with cut-off wavelengths beyond λ _c = 1.7 µm are incorporated into SWIR imagers. However, their large dark current and limited cut-off wavelength tunability prevent their widespread use in SWIR imaging. Herein, a novel InAs_0.85P_0.15 homojunction detector with λ _c = 2.6 µm is achieved through lattice and bandgap engineering with well-designed InAs _y P_{1- y} metamorphic buffers. Compared to conventional In_0.83Ga_0.17As/InP detectors with a lattice mismatch of f = 2.0%, the InAs_0.85P_0.15 detector with f = 2.7% exhibits full strain relaxation and lower surface roughness (2.4 nm due to its superior crystallinity). Photocarrier generation in the InAsP detector is more efficiently supported by a smaller entropy for electron–hole pair formation. The lower trap density and longer carrier lifetime of the InAsP detector decrease the dark current, leading to high uniform detectivities of ≈1.0 × 10⁹ cm Hz^1/2 W⁻¹ over a wider voltage range at 300 K. Such an InAsP metamorphic detector with excellent photodetection capabilities is expected to find applications in the implementation of large-format focal plane arrays for extended SWIR imaging.

Highlights

• The (001)-oriented ferromagnetic La_0.67Sr_0.33MnO₃ films are stuck onto the (011)-oriented ferroelectric single-crystal 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ substrate with 0° and 45° twist angle.

• By applying a 7.2 kV cm⁻¹ electric field, the coexistence of uniaxial and fourfold in-plane magnetic anisotropy is observed in 45° Sample, while a typical uniaxial anisotropy is found in 0° Sample.

Abstract

Manipulating strain mode and degree that can be applied to epitaxial complex oxide thin films have been a cornerstone of strain engineering. In recent years, lift-off and transfer technology of the epitaxial oxide thin films have been developed that enabled the integration of heterostructures without the limitation of material types and crystal orientations. Moreover, twisted integration would provide a more interesting strategy in artificial magnetoelectric heterostructures. A specific twist angle between the ferroelectric and ferromagnetic oxide layers corresponds to the distinct strain regulation modes in the magnetoelectric coupling process, which could provide some insight in to the physical phenomena. In this work, the La_0.67Sr_0.33MnO₃ (001)/0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (011) (LSMO/PMN-PT) heterostructures with 45º and 0º twist angles were assembled via water-etching and transfer process. The transferred LSMO films exhibit a fourfold magnetic anisotropy with easy axis along LSMO < 110 >. A coexistence of uniaxial and fourfold magnetic anisotropy with LSMO [110] easy axis is observed for the 45° Sample by applying a 7.2 kV cm⁻¹ electrical field, significantly different from a uniaxial anisotropy with LSMO [100] easy axis for the 0° Sample. The fitting of the ferromagnetic resonance field reveals that the strain coupling generated by the 45° twist angle causes different lattice distortion of LSMO, thereby enhancing both the fourfold and uniaxial anisotropy. This work confirms the twisting degrees of freedom for magnetoelectric coupling and opens opportunities for fabricating artificial magnetoelectric heterostructures.

Nature, Published online: 17 November 2023; doi:10.1038/d41586-023-03558-6

Nature Communications, Published online: 17 November 2023; doi:10.1038/s41467-023-43357-1