Jiuxiang Dai

The unprecedented thermo-enhanced second harmonic generation (SHG) response is achieved in an organic configurationally-locked polyene nonlinear optical (NLO) crystal, ascribed to the decrease of torsion degree of the π-conjugated bridge induced by the molecular dynamic motions. This work paves a new pathway to develop smart photoelectronics devices.

Abstract

To modulate nonlinear optical (NLO) effects of crystalline material holds great application potential in the photoelectronic and optical fields. Organic configurationally-locked polyene represents an exciting NLO family with large second harmonic generation (SHG) effects, whereas it is a huge blank to switch and modulate their NLO property through external stimuli. For the first time, here present unusual thermo-enhanced SHG activities are presented in a polyene-based NLO compound, 2-{3-[2-(4-pyrrolidinphenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene}malononitrile (1), giving a record-high magnitude of SHG enhancement up to ≈170% during its isomorphic phase transition. Theoretical analysis discloses this behavior stems from the reduced degree of torsion in the π-conjugated structures in 1, as verified by dihedral angles between its pyrrolidine and phenyl planes. As the first study on thermo-enhanced SHG properties of organic crystals, this work affords a new avenue of modulating physical properties to fabricate high-performance photoelectronic and optical devices.

Nature Communications, Published online: 25 November 2024; doi:10.1038/s41467-024-54660-w

The unconventional anomalous Hall effect (AHE) in Cr2Te3, characterized by a sign change and humps and dips near the coercive field in its hysteresis, is attributed to an intrinsic mechanism dictated by self-intercalated Cr atoms with spin canting, closely linked to multiple band anti-crossings near the Fermi level.

Abstract

Covalent 2D magnets such as Cr₂Te₃, which feature self-intercalated magnetic cations located between monolayers of transition-metal dichalcogenide material, offer a unique platform for controlling magnetic order and spin texture, enabling new potential applications for spintronic devices. Here, it is demonstrated that the unconventional anomalous Hall effect (AHE) in Cr₂Te₃, characterized by additional humps and dips near the coercive field in AHE hysteresis, originates from an intrinsic mechanism dictated by the self-intercalation. This mechanism is distinctly different from previously proposed mechanisms such as topological Hall effect, or two-channel AHE arising from spatial inhomogeneities. Crucially, multiple Weyl-like nodes emerge in the electronic band structure due to strong spin-orbit coupling, whose positions relative to the Fermi level is sensitively modulated by the canting angles of the self-intercalated Cr cations. These nodes contribute strongly to the Berry curvature and AHE conductivity. This component competes with the contribution from bands that are less affected by the self-intercalation, resulting in a sign change in AHE with temperature and the emergence of additional humps and dips. The findings provide compelling evidence for the intrinsic origin of the unconventional AHE in Cr₂Te₃ and further establish self-intercalation as a control knob for engineering AHE in complex magnets.

Maximizing electromechanical energy conversion in 2D materials requires structural manipulation with extra lattice extension for better power generation performance. Here, a way of extending lattices in a favorable direction using the strain and doping engineering approaches is proposed. Applying two approaches for the optimization of oxygen-doping level in tensile-strain-applied flat α-In₂Se₃ nanosheets produces a remarkable energy-harvesting result with a record power density of 420 µW cm⁻².

Abstract

With the emergence of electromechanical devices, considerable efforts have been devoted to improving the piezoelectricity of 2D materials. Herein, an anion-doping approach is proposed as an effective way to enhance the piezoelectricity of α-In₂Se₃ nanosheets, which has a rare asymmetric structure in both the in-plane and out-of-plane directions. As the O₂ plasma treatment gradually substitutes selenium with oxygen, it changes the crystal structure, creating a larger lattice distortion and, thus, an extended dipole moment. Prior to the O₂ treatment, the lattice extension is deliberately maximized in the lateral direction by imposing in situ tensile strain during the exfoliation process for preparing the nanosheets. Combining doping and strain engineering substantially enhances the piezoelectric coefficient and electromechanical energy conversion. As a result, the optimal harvester with a 0.9% in situ strain and 10 min plasma exposure achieves the highest piezoelectric energy harvesting values of ≈13.5 nA and ≈420 µW cm⁻² under bending operation, outperforming all previously reported 2D materials. Theoretical estimation of the structural changes and polarization with gradual oxygen substitution supports the observed dependence of the electromechanical performance.

In₂Se₃/InSe vertical heterojunctions are prepared with specific phases by optimizing the growth conditions using 2D InSe as the seeding material. Meanwhile, the microstructural mechanism for the temperature-dependent phase evolution of β-In₂Se₃, 3R α-In₂Se₃, and 2H α-In₂Se3 is revealed by using the state-of-the-art Cs-STEM technique. It has been found that β-In₂Se₃ is the intermediate state for the phase transition between different In₂Se₃ polymorphs.

Abstract

The polymorphic nature of In₂Se₃ leads to excellent phase-dependent physical properties including ferroelectricity, photoelectricity, and especially the intriguing phase change ability, making the precise phase modulation of In₂Se₃ of fundamental importance but very challenging. Here, the growth of In₂Se₃ with desired-phase is realized by temperature-controlled selenization of van der Waals (vdW) layered bulk γ-InSe. Detailed results of Raman spectroscopy, scanning electron microscopy (SEM), and state-of-the-art spherical aberration-corrected transmission electron microscopy (Cs-TEM) clearly and consistently show that β-In₂Se₃, 3R α-In₂Se₃, and 2H α-In₂Se₃ can be perfectly obtained at ≈270, ≈300, and ≈600 °C, respectively. Further comprehensive atomic imaging analyses confirm that the seeding material, InSe, plays a critical role in the low-temperature epitaxial growth of vdW-layered In₂Se₃, and, more interestingly, β-In₂Se₃ acts as an intermediate phase between 3R and 2H α-In₂Se₃ transitions. This investigation not only provides a simple yet versatile strategy for the phase modulation of In₂Se₃, but also sheds light on the temperature-dependent phase evolution of In₂Se₃.

Author(s): Jonas von Milczewski, Xin Chen, Atac Imamoglu, and Richard Schmidt

We study a mechanism to induce superconductivity in atomically thin semiconductors where excitons mediate an effective attraction between electrons. Our model includes interaction effects beyond the paradigm of phonon-mediated superconductivity and connects to the well-established limits of Bose and…

[Phys. Rev. Lett. 133, 226903] Published Wed Nov 27, 2024

The study explores the effect of chromium oxychloride (CrOCl) spin states on bilayer graphene (BLG) using capacitance measurements. A unique hysteretic behavior in charging states is observed, influenced by magnetization history, not electrical gating. This behavior can be electrically controlled. First-principles calculations show it results from charge transfer during CrOCl's phase transition, offering new insights for 2D device design.

Abstract

Anti-ferromagnetic insulator chromium oxychloride (CrOCl) has shown peculiar charge transfer and correlation-enhanced emerging properties when interfaced with other van der Waals conductive channels. However, the influence of its spin states to the channel material remains largely unknown. Here, this issue is addressed by directly measuring the density of states in bilayer graphene (BLG) interfaced with CrOCl via a high-precision capacitance measurement technique and a surprising hysteretic behavior in the charging states of the heterostructure is observed. Such hysteretic behavior depends only on the history of magnetization, but not on the history of electrical gating; it can also be turned off electrically, providing a synergetic control of these non-volatile states. First-principles calculations attribute this observation to magnetic field-controlled charge transfer between BLG and CrOCl during the phase transition of CrOCl from antiferromagnetic (AFM) to ferrimagnetic-like (FiM) states. This magnetic-electrical synergetic control mechanism broadens the scope of proximity effects and opens new possibilities for the design of advanced 2D heterostructures and devices.

Nature Materials, Published online: 26 November 2024; doi:10.1038/s41563-024-02051-3

Nature Materials, Published online: 26 November 2024; doi:10.1038/s41563-024-02014-8

Direct detection of diffusing hydrocarbons on a graphene surface is shown. These hydrocarbons are impeded by e-beam deposited diffusion barriers. Purely indirect hydrocarbon deposition is also shown illustrating that diffusion after dissociation can be a significant factor in the deposition process. Evidence is presented for an unexplained sudden drop in hydrocarbon concentration and we discuss possible causes.

Abstract

Researchers working with thin samples, such as monolayer graphene, are consistently struggling against contamination. Indeed, the problem of hydrocarbon contamination is known from the earliest days of electron microscopy and efforts to reduce this problem are ubiquitous to almost all high-vacuum experiments. Accurate knowledge of the behavior of such contamination is essential for electron beam (e-beam) based atomic fabrication, where it is aspired to select and control matter on an atom-by-atom basis. Here, the vexing question of hydrocarbon contamination on graphene is taken up. Image intensity is used to directly reveal the presence of diffusing hydrocarbons on ostensibly clean graphene. These diffusing hydrocarbons are previously inferred but not directly observed. Surprising dynamic variations of the concentration of these hydrocarbons impels questions about their origin. Here, some possible explanations are presented and some tentative conclusions are drawn. This work updates the conceptual model of “clean graphene” and offers refinements to the description of e-beam induced hydrocarbon deposition.

Wafer-scale and uniform multi-layer MoS₂ can be prepared by sulfurizing amorphous MoS₂ films. The MoS₂ film can act as light absorption layer in vertical device architecture and exhibit photovoltaic responses. With separate light absorption (multi-layer MoS₂) and carrier transport (mono-layer graphene) layers in planar device architecture, high responsivity and short response times can be achieved.

Abstract

Wafer-scale and layered MoS₂ films are grown by sulfurizing amorphous MoS₂ films deposited on sapphire substrates by using a radio-frequency sputtering system. To verify the layer numbers of the multi-layer MoS₂ films, atomic layer etchings are adopted. Wafer-scale MoS₂ film growth with good layer number uniformity up to 30 is observed. A vertical device with 20-layer MoS₂ embedded in between Al (bottom) and Au (top) electrodes is fabricated. With different work functions of the metal electrodes, photo-excited electrons and holes in the MoS₂ layer can be separated and form photovoltaic responses. With the insertion of 5 nm MoO₃ carrier transport layer between the MoS₂ layer and the top Au electrode, enhanced photovoltaic responses are observed for the device. By using graphene as the carrier transport layer and MoS₂ as the light absorption layer, avalanche photocurrents are observed for planar MoS₂/graphene photoconductive devices. With the assist of the higher electron density in multi-layer MoS₂, an easier compensation in the loss of photo-excited electrons and therefore, charge neutrality in the MoS₂ layer can be maintained. Significant reduction in the rise/fall times from >100 ms. to <10 ms. is also observed for the planar photodetector with 10-layer MoS₂ absorption layer.

Publication date: March–April 2025

Source: Materials Today, Volume 83

Author(s): Yafang Li, Lin Wang, Yu Ouyang, Dexiang Li, Yuting Yan, Kai Dai, Liyan Shang, Jinzhong Zhang, Liangqing Zhu, Yawei Li, Zhigao Hu

Nature Reviews Methods Primers, Published online: 28 November 2024; doi:10.1038/s43586-024-00361-z

Gold-assisted exfoliation allows separating large-area (mm² to cm²) monolayers of 2D materials (such as MoS₂) from layered bulk crystals. The excellent electronic quality of these 2D films makes them suitable for a wide range of applications, including transistors, memristors, optoelectronics & photovoltaics, sensors, quantum and van der Waals heterojunction devices.

Abstract

Semiconductor transition metal dichalcogenides (TMDs), such as MoS₂, are currently regarded as key-enabling materials for sub-1 nm channel transistors, beyond-complementary metal–oxide–semiconductor electronic and optoelectronic devices and sensors. Owing to this wide application potential, several bottom-up and top-down synthesis approaches for these materials have been explored so far. Despite the huge progresses in scalable deposition methods (such as chemical vapor deposition, metal–organic chemical vapor deposition), exfoliated layers from bulk crystals still represent the benchmark for record electronic properties of TMDs. Among exfoliation approaches, metal-assisted mechanical exfoliation emerges as the most effective method to separate large-area (mm² to cm²) single-crystalline monolayer membranes of TMDs (and many other 2D materials) from the parent bulk crystals. This paper reviews the state-of-the-art in this field, from current understanding of MoS₂ exfoliation mechanisms on Au (considered as a model system), to the main device applications of as-exfoliated and transferred large-area MoS₂ membranes (including Au/MoS₂/Au memristors, MoS₂ photodetectors, and field-effect transistors). Perspectives of this method in the realization of arrays of 2D heterojunction devices, including Moirè superlattice devices, and open challenges for its widespread application are finally discussed.

Starting from a single-layer NbS₂ on graphene, new 2D materials Nb_5/3S₃-2D and Nb₂S₃-2D are synthesized via sulfur-deficient annealing or Nb deposition at elevated temperatures. These covalently bound materials exhibit unique structures influenced by surface effects, differing from expected bulk stacking sequences.

Abstract

Starting from a single layer of NbS₂ grown on graphene by molecular beam epitaxy, the single unit cell thick 2D materials Nb_5/3S₃-2D and Nb₂S₃-2D are created using two different pathways. Either annealing under sulfur-deficient conditions at progressively higher temperatures or deposition of increasing amounts of Nb at elevated temperature result in phase-pure Nb_5/3S₃-2D followed by Nb₂S₃-2D. The materials are characterized by scanning tunneling microscopy, scanning tunneling spectroscopy, and X-ray photoemission spectroscopy. The experimental assessment combined with systematic density functional theory calculations reveals their structure. The 2D materials are covalently bound without any van der Waals gap. Their stacking sequence and structure are at variance with expectations based on corresponding bulk materials highlighting the importance of surface and interface effects in structure formation.

A Carbon–Oxygen radical assisted low-temperature growth strategy is proposed, which affords defect-free, wrinkle-free, and single-crystalline graphene films. A deep insight into the methanol precursor fueled growth process is made via DFT calculations, unveiling the methanol dissociation pathway and the roles of intermediate Carbon–Oxygen radicals in carbon attaching and assembling to graphene lattice without defect formation.

Abstract

Low-temperature chemical vapor deposition growth of graphene films is a long-term pursuit in the graphene synthesis field because of the low energy consumption, short heating-cooling process and low wrinkle density of as-obtained films. However, insufficient energy supply at low temperature (below 850 °C) usually leads to the difficulty in carbon source dissociation, graphene growth, and defect healing. Herein, a Carbon–Oxygen (C─O) radical assisted strategy is proposed for low-temperature growth of defect-free, wrinkle-free, and single-crystalline graphene films by using methanol precursor. We provide a deep insight into the growth process fueled by methanol precursor, unveiling the dissociation pathway of methanol and the roles of intermediate C─O radicals in carbon attaching and assembling to graphene lattice without defect formation. This method shows promising prospects in the cost-effective production of high-quality graphene films and provides inspiration for growing other 2D materials.