Jiuxiang Dai

Heterogeneities such as defects, impurities, edges, interfaces, and disorder often define and affect the coherence, interaction, entanglement, and topology of quantum states. Their roles in quantum materials are examined at the atomistic level by revealing the different degrees of freedom of heterogeneities, understanding their effects on the host material, and controlling heterogeneities to create new materials with tailored quantum functions.

Abstract

Quantum materials are usually heterogeneous, with structural defects, impurities, surfaces, edges, interfaces, and disorder. These heterogeneities are sometimes viewed as liabilities within conventional systems; however, their electronic and magnetic structures often define and affect the quantum phenomena such as coherence, interaction, entanglement, and topological effects in the host system. Therefore, a critical need is to understand the roles of heterogeneities in order to endow materials with new quantum functions for energy and quantum information science applications. In this article, several representative examples are reviewed on the recent progress in connecting the heterogeneities to the quantum behaviors of real materials. Specifically, three intertwined topic areas are assessed: i) Reveal the structural, electronic, magnetic, vibrational, and optical degrees of freedom of heterogeneities. ii) Understand the effect of heterogeneities on the behaviors of quantum states in host material systems. iii) Control heterogeneities for new quantum functions. This progress is achieved by establishing the atomistic-level structure-property relationships associated with heterogeneities in quantum materials. The understanding of the interactions between electronic, magnetic, photonic, and vibrational states of heterogeneities enables the design of new quantum materials, including topological matter and quantum light emitters based on heterogenous 2D materials.

A 7.18 meV near-intrinsic exciton linewidth in monolayer WS₂ is achieved at room temperature by coupling it with a moderate-refractive-index hydrogenated silicon nanosphere in water. The dielectric nanosphere is designed to boost the dynamic competition between WS₂ exciton and trion decay channels and rebuild the excitonic relaxation processes with suppressed exciton nonradiative recombination. A tunable exciton linewidth is also demonstrated.

Abstract

The homogeneous exciton linewidth, which captures the coherent quantum dynamics of an excitonic state, is a vital parameter in exploring light–matter interactions in 2D transition metal dichalcogenides (TMDs). An efficient control of the exciton linewidth is of great significance, and in particular of its intrinsic linewidth, which determines the minimum timescale for the coherent manipulation of excitons. However, such a control is rarely achieved in TMDs at room temperature (RT). While the intrinsic A exciton linewidth is down to 7 meV in monolayer WS₂, the reported RT linewidth is typically a few tens of meV due to inevitable homogeneous and inhomogeneous broadening effects. Here, it is shown that a 7.18 meV near-intrinsic linewidth can be observed at RT when monolayer WS₂ is coupled with a moderate-refractive-index hydrogenated silicon nanosphere in water. By boosting the dynamic competition between exciton and trion decay channels in WS₂ through the nanosphere-supported Mie resonances, the coherent linewidth can be tuned from 35 down to 7.18 meV. Such modulation of exciton linewidth and its associated mechanism are robust even in presence of defects, easing the sample quality requirement and providing new opportunities for TMD-based nanophotonics and optoelectronics.

Nature Communications, Published online: 16 February 2022; doi:10.1038/s41467-022-28542-y

The interplay between reduced dimensionality and interactions in monolayer transition metal dichalcogenides has been of great research interest. Here the authors report an insulating dimer ground state in 1T-IrTe2, driven by the combined effect of the charge density wave instability and local atomic bond formation.

Weyl semimetals are topological materials with promising electronic and optical properties. This work thoroughly characterizes the nonlinear optical properties of the Weyl semimetal niobium phosphide by using third-harmonic generation and nondegenerate pump–probe spectroscopy. A measured third-harmonic generation efficiency of 10⁻⁴% and an ultrafast change in reflectivity of close to 1% paves the way for the coming Weyl-semimetal-based nanophotonics.

Abstract

Since their experimental discovery in 2015, Weyl semimetals have generated a large amount of attention due their intriguing physical properties that arise from their linear electron dispersion relation and topological surface states. In particular, in the field of nonlinear (NL) optics and light harvesting, Weyl semimetals have shown outstanding performances and achieved record NL conversion coefficients. In this context, the first steps toward Weyl semimetal nanophotonics are performed here by thoroughly characterizing the linear and NL optical behavior of epitaxially grown niobium phosphide (NbP) thin films, covering the visible to the near-infrared regime of the electromagnetic spectrum. Despite the measured high linear absorption, third-harmonic generation studies demonstrate high conversion efficiencies up to 10⁻⁴% that can be attributed to the topological electron states at the surface of the material. Furthermore, nondegenerate pump–probe measurements with sub-10 fs pulses reveal a maximum modulation depth of ≈1%, completely decaying within 100 fs and therefore suggesting the possibility of developing all-optical switching devices based on NbP. Altogether, this work reveals the promising NL optical properties of Weyl semimetal thin films, which outperform bulk crystals of the same material, laying the grounds for nanoscale applications, enabled by top-down nanostructuring, such as light-harvesting, on-chip frequency conversion, and all-optical processing.

MoS₂ Devices

In article number 2106411, Wen-Wei Wu and co-workers reveal the direct observation of an MoS₂ device under biasing via powerful in situ transmission electron microscopy (TEM). During in situ TEM biasing, the MoS₂ is etched vertically and horizontally; the former is dominated by knock-on damage, while the latter involves atomic migration induced by Joule heating. Also, the long cracks that form by thermal stress, which are discovered in both in situ and ex situ biasing at 10 V, are discussed in this research. It is believed that insights of material damage can push the limit of material properties and broaden the range of MoS₂-based device applications.

Monodisperse CaZnOS semiconductor crystals with various doping ions (such as Mn²⁺, Cu²⁺, Pb²⁺, Bi³⁺, Ln³⁺ = 10 lanthanide ions) are successfully synthesized by the molten salt-shielded method. The doped CaZnOS crystals can realize various colors of mechanoluminescence and photoluminescence, resulting in the potential applications of piezo-photonics, stress sensors, anti-counterfeiting, and artificial skin as well as X-ray scintillators.

Abstract

CaZnOS-based semiconductors are the only series of material system discovered that can simultaneously realize a large number of dopant elements to directly fulfill the highly efficient full-spectrum functionality from ultraviolet to near-infrared under the same force/pressure. Nevertheless, owing to the high agglomeration of the high temperature solid phase manufacturing process, which is unable to control the crystal morphology, the application progress is limited. Here, the authors report first that CaZnOS-based fine monodisperse semiconductor crystals with various doping ions are successfully synthesized by a molten salt shielded method in an air environment. This method does not require inert gas ventilation, and therefore can greatly reduce the synthesis cost and more importantly improve the fine control of the crystal morphology, along with the crystals’ dispersibility and stability. These doped semiconductors can not only realize different colors of mechanical-to-optical energy conversion, but also can achieve multicolor luminescence under low-dose X-ray irradiation, moreover their intensities are comparable to the commercial NaI:Tl. They can pave the way to the new fields of advanced optoelectronic applications, such as piezophotonic systems, mechanical energy conversion and harvesting devices, intelligent sensors, and artificial skin as well as X-ray applications.

The performances of 2D photodetectors are largely limited by their poor light absorption and small detection range. A strain-plasmonic coupled 2D photodetector is developed by mechanically integrating monolayer MoS₂ on top of prefabricated Au nanoparticle arrays. The large tensile strain can greatly reduce the MoS₂ bandgap for broadband photodetection, and the nanoparticles can significantly enhance the light absorption.

Abstract

2D Semiconductors are promising in the development of next-generation photodetectors. However, the performances of 2D photodetectors are largely limited by their poor light absorption (due to ultrathin thickness) and small detection range (due to large bandgap). To overcome the limitations, a strain-plasmonic coupled 2D photodetector is designed by mechanically integrating monolayer MoS₂ on top of prefabricated Au nanoparticle arrays. Within this structure, the large biaxial tensile strain can greatly reduce the MoS₂ bandgap for broadband photodetection, and at the same time, the nanoparticles can significantly enhance the light intensity around MoS₂ with much improved light absorption. Together, the strain-plasmonic coupled photodetector can broaden the detection range by 60 nm and increase the signal-to-noise ratio by 650%, representing the ultimate optimization of detection range and detection intensity at the same time. The strain-plasmonic coupling effect is further systematically characterized and confirmed by using Raman and photoluminescence spectrophotometry. Furthermore, the existence of built-in potential and photo-switching behavior is demonstrated between the strained and unstrained region, constructing a self-powered homojunction photodetector. This approach provides a simple strategy to couple strain effect and plasmonic effect, which can provide a new strategy for designing high-performance and broadband 2D optoelectronic devices.

This review summarizes the recent progress in the rational design and fabrication of heterojunction nanomedicine. The catalytic mechanisms and properties of applied heterojunction systems are also discussed in relation to biomedical applications, especially cancer treatment and sterilization. This review concludes with a summary of the challenges and perspectives on future directions in this evolving field of research.

Abstract

Exogenous stimulation catalytic therapy has received enormous attention as it holds great promise to address global medical issues. However, the therapeutic effect of catalytic therapy is seriously restricted by the fast charge recombination and the limited utilization of exogenous stimulation by catalysts. In the past few decades, many strategies have been developed to overcome the above serious drawbacks, among which heterojunctions are the most widely used and promising strategy. This review attempts to summarize the recent progress in the rational design and fabrication of heterojunction nanomedicine, such as semiconductor–semiconductor heterojunctions (including type I, type II, type III, PN, and Z–scheme junctions) and semiconductor–metal heterojunctions (including Schottky, Ohmic, and localized surface plasmon resonance–mediated junctions). The catalytic mechanisms and properties of the above junction systems are also discussed in relation to biomedical applications, especially cancer treatment and sterilization. This review concludes with a summary of the challenges and some perspectives on future directions in this exciting and still evolving field of research.

Energy transport processes are critical for the efficiency of many optoelectronic applications. The energy transport in technologically promising transition metal dichalcogenides is determined by exciton diffusion, which strongly depends on the underlying excitonic and phononic dispersion. Based on a fully microscopic theory we demonstrate that the valley-exchange interaction leads to an enhanced exciton diffusion due to the emergence of a linear excitonic dispersion and the resulting decreased exciton-phonon scattering. Interestingly, we find that the application of a uniaxial strain can drastically boost the diffusion speed and even give rise to a pronounced anisotropic diffusion, which persists up to room temperature. We reveal that this behaviour originates from the highly anisotropic exciton dispersion in the presence of strain, displaying parabolic and linear behaviour perpendicular and parallel to the strain direction, respectively. Our work demonstrates the possibility t...

The self-limiting nature of atomic layer deposition (ALD) makes it an appealing option for growing single layers of two-dimensional van der Waals (2D-VDW) materials. In this paper it is demonstrated that a single layer of a 2D-VDW form of SiO 2 can be grown by ALD on Au and Pd polycrystalline foils and epitaxial films. The silica was deposited by two cycles of bis(diethylamino) silane and oxygen plasma exposure at 525 K. Initial deposition produced a three-dimensionally disordered silica layer; however, subsequent annealing above 950 K drove a structural rearrangement resulting in 2D-VDW. The annealing could be performed at ambient pressure. Surface spectra recorded after annealing indicated that the two ALD cycles yielded close to the silica coverage obtained for 2D-VDW silica prepared by precision SiO deposition in ultra-high vacuum (UHV). Analysis of ALD-grown 2D-VDW silica on a Pd(111) film revealed the co-existence of amorphous and incommensurate crystalline 2D ph...

Electronic structure of XY haeckelite compounds (X = B, Al, Ga, In, or Tl; Y = N, P, As, or Sb) is studied from the first principles. The calculations reveal these compounds possess fascinating electronic properties with nontrivial band topologies relevant to future optoelectronic and quantum information technologies.

Abstract

The family of III–V element compounds (i.e., XY compounds; X = B, Al, Ga, In, or Tl; Y = N, P, As, or Sb) have been intensively investigated for several decades because of their enormous applications for many optoelectronic devices. Here, by employing first-principles calculations, the electronic structures of bulk XY haeckelite compounds are examined. It is identified that InSb (TlN and TlP) is Dirac semimetal (are strong topological insulators). The other fifteen XY compounds are semiconducting. The effect of biaxial and uniaxial tensile and compressive strains on the electronic structures are studied. These materials offer diverse topological orders. The semiconducting band gaps are mainly found between the bonding and antibonding states of the mixed X(p)–Y(p) orbitals at the top of the valence band and the bottom of the conduction bands, respectively. The topological insulating nature of the XY compounds is explained based on the degenerate p _x  + p _y orbitals and their orbital energies relative to the p _z orbitals near the Fermi energy. The nontrivial band topologies of TlN and TlP are confirmed by calculating the Z ₂ (1;000) index, surface states, and Wilson loop calculations. The bands split into two branches by including spin-orbit interaction. The results demonstrate that haeckelite compounds are fascinating materials with broad potential applications in optoelectronics and possessing the possibility of hosting emergent physical phenomena.

In this work, novel van der Waals MoS₂/PdSe₂ heterostructures are constructed for high-performance broadband polarized photodetection, which show a high responsivity of 6.9 A W⁻¹, specific detectivity of 6.3 × 10¹⁰ Jones, a rapid response speed of 0.378/0.708 s at 830 nm, and a good polarization sensitivity with dichroic ratio about 1.34.

Abstract

Recently, group-X transition metal dichalcogenides with tunable distinct optical and superior electrical properties have displayed profound potential in various optoelectronic devices. In this work, novel van der Waals (vdWs) heterostructures composed of monolayer MoS₂ and few-layer PdSe₂ grown by chemical vapor deposition are employed in photodetectors for broadband and polarized photodetection. Excitingly, optoelectronic measurements show that the MoS₂/PdSe₂ photodetector is sensitive to near-infrared light with responsivity as high as 6.9 A W⁻¹, specific detectivity up to 6.3 × 10¹⁰ Jones, and a rapid response speed of 0.378/0.708 s at 830 nm. This is mainly attributed to the enhanced light absorption and the type-II band alignment of the heterostructure. Under illumination, not only the intralayer excitations but also the interlayer excitations contribute to the carrier's generation in relevant layers, which enables the broadband photoresponse. In addition, this photodetector also exhibits a good polarization sensitivity with dichroic ratio about 1.34. All these results demonstrate that the MoS₂/PdSe₂ vdWs heterostructures are suitable for the realization of next-generation high-performance broad and polarized photodetectors.

Combining indium antimonide (InSb) nanowires with materials of dissimilar properties, has enabled the engineering of exotic materials, such as topological superconductors. This work explores potential applications of the material combination of InSb with cadmium telluride in the form of core–shell nanowire heterostructures. These heterostructures are studied in terms of growth, epitaxy, electronic structure of their interface, and electric transport properties.

Abstract

Indium antimonide (InSb) nanowires are used as building blocks for quantum devices because of their unique properties, that is, strong spin-orbit interaction and large Landé g-factor. Integrating InSb nanowires with other materials could potentially unfold novel devices with distinctive functionality. A prominent example is the combination of InSb nanowires with superconductors for the emerging topological particles research. Here, the combination of the II–VI cadmium telluride (CdTe) with the III–V InSb in the form of core–shell (InSb–CdTe) nanowires is investigated and potential applications based on the electronic structure of the InSb–CdTe interface and the epitaxy of CdTe on the InSb nanowires are explored. The electronic structure of the InSb–CdTe interface using density functional theory is determined and a type-I band alignment is extracted with a small conduction band offset ( ⩽0.3 eV). These results indicate the potential application of these shells for surface passivation or as tunnel barriers in combination with superconductors. In terms of structural quality, it is demonstrated that the lattice-matched CdTe can be grown epitaxially on the InSb nanowires without interfacial strain or defects. These shells do not introduce disorder to the InSb nanowires as indicated by the comparable field-effect mobility measured for both uncapped and CdTe-capped nanowires.