Facet‐Dependent Optical Properties of Semiconductor Nanocrystals

Recent observations of facet‐dependent electrical conductivity and photocatalytic activity of various semiconductor crystals are presented. Then, the discovery of facet‐dependent surface plasmon resonance absorption of metal–Cu₂O core–shell nanocrystals with tunable sizes and shapes is discussed. The Cu₂O shells also exhibit a facet‐specific optical absorption feature. The facet‐dependent electrical conductivity, photocatalytic activity, and optical properties are related phenomena, resulting from the presence of an ultrathin surface layer with different band structures and thus varying degrees of band bending for the {100}, {110}, and {111} faces of Cu₂O to absorb light of somewhat different wavelengths. Recently, it is shown that the light absorption and photoluminescence properties of pure Cu₂O cubes, octahedra, and rhombic dodecahedra also display size and facet effects because of their tunable band gaps. A modified band diagram of Cu₂O can be constructed to incorporate these optical effects. Literature also provides examples of facet‐dependent optical behaviors of semiconductor nanostructures, indicating that optical properties of nanoscale semiconductor materials are intrinsically facet‐dependent. Some applications of semiconductor optical size and facet effects are considered.     

Merits and Challenges of Ruddlesden–Popper Soft Halide Perovskites in Electro‐Optics and Optoelectronics

Following the rejuvenation of 3D organic–inorganic hybrid perovskites, like CH₃NH₃PbI₃, (quasi)‐2D Ruddlesden–Popper soft halide perovskites R₂A_n?1Pb_nX_3n+1 have recently become another focus in the optoelectronic and photovoltaic device community. Although quasi‐2D perovskites were first introduced to stabilize optoelectronic/photovoltaic devices against moisture, more interesting properties and device applications, such as solar cells, light‐emitting diodes, white‐light emitters, lasers, and polaritonic emission, have followed. While delicate engineering design has pushed the performance of various devices forward remarkably, understanding of the fundamental properties, especially the charge‐transfer process, electron–phonon interactions, and the growth mechanism in (quasi)‐2D halide perovskites, remains limited and even controversial. Here, after reviewing the current understanding and the nexus between optoelectronic/photovoltaic properties of 2D and 3D halide perovskites, the growth mechanisms, charge‐transfer processes, vibrational properties, and electron–phonon interactions of soft halide perovskites, mainly in quasi‐2D systems, are discussed. It is suggested that single‐crystal‐based studies are needed to deepen the understanding of the aforementioned fundamental properties, and will eventually contribute to device performance.     

Hybridizing Plasmonic Materials with 2D‐Transition Metal Dichalcogenides toward Functional Applications

Recently, 2D transition metal dichalcogenides (TMDs) have become intriguing materials in the versatile field of photonics and optoelectronics because of their strong light–matter interaction that stems from the atomic layer thickness, broadband optical response, controllable optoelectronic properties, and high nonlinearity, as well as compatibility. Nevertheless, the low optical cross‐section of 2D‐TMDs inhibits the light–matter interaction, resulting in lower quantum yield. Therefore, hybridizing the 2D‐TMDs with plasmonic nanomaterials has become one of the promising strategies to boost the optical absorption of thin 2D‐TMDs. The appeal of plasmonics is based on their capability to localize and enhance the electromagnetic field and increase the optical path length of light by scattering and injecting hot electrons to TMDs. In this regard, recent achievements with respect to hybridization of the plasmonic effect in 2D‐TMDs systems and its augmented optical and optoelectronic properties are reviewed. The phenomenon of plasmon‐enhanced interaction in 2D‐TMDs is briefly described and state‐of‐the‐art hybrid device applications are comprehensively discussed. Finally, an outlook on future applications of these hybrid devices is provided.     

Tailoring MoS2 Valley‐Polarized Photoluminescence with Super Chiral Near‐Field

Transition metal dichalcogenides with intrinsic spin–valley degrees of freedom hold great potentials for applications in spintronic and valleytronic devices. MoS₂ monolayer possesses two inequivalent valleys in the Brillouin zone, with each valley coupling selectively with circularly polarized photons. The degree of valley polarization (DVP) is a parameter to characterize the purity of valley‐polarized photoluminescence (PL) of MoS₂ monolayer. Usually, the detected values of DVP in MoS₂ monolayer show achiral property under optical excitation of opposite helicities due to reciprocal phonon‐assisted intervalley scattering process. Here, it is reported that valley‐polarized PL of MoS₂ can be tailored through near‐field interaction with plasmonic chiral metasurface. The resonant field of the chiral metasurface couples with valley‐polarized excitons, and tailors the measured PL spectra in the far‐field, resulting in observation of chiral DVP of MoS₂‐metasurface under opposite helicities excitations. Valley‐contrast PL in the chiral heterostructure is also observed when illuminated by linearly polarized light. The manipulation of valley‐polarized PL in 2D materials using chiral metasurface represents a viable route toward valley‐polaritonic devices.     

Functional Single‐Walled Carbon Nanotubes and Nanoengineered Networks for Organic‐ and Perovskite‐Solar‐Cell Applications

Carbon nanotubes have a variety of remarkable electronic and mechanical properties that, in principle, lend them to promising optoelectronic applications. However, the field has been plagued by heterogeneity in the distributions of synthesized tubes and uncontrolled bundling, both of which have prevented nanotubes from reaching their full potential. Here, a variety of recently demonstrated solution‐processing avenues is presented, which may combat these challenges through manipulation of nanoscale structures. Recent advances in polymer‐wrapping of single‐walled carbon nanotubes (SWNTs) are shown, along with how the resulting nanostructures can selectively disperse tubes while also exploiting the favorable properties of the polymer, such as light‐harvesting ability. New methods to controllably form nanoengineered SWNT networks with controlled nanotube placement are discussed. These nanoengineered networks decrease bundling, lower the percolation threshold, and enable a strong enhancement in charge conductivity compared to random networks, making them potentially attractive for optoelectronic applications. Finally, SWNT applications, to date, in organic and perovskite photovoltaics are reviewed, and insights as to how the aforementioned recent advancements can lead to improved device performance provided.     

Electronic Property Modification of Single‐Walled Carbon Nanotubes by Encapsulation of Sulfur‐Terminated Graphene Nanoribbons

The use of carbon nanotubes (CNTs) as cylindrical reactor vessels has become a viable means for synthesizing graphene nanoribbons (GNRs). While previous studies demonstrated that the size and edge structure of the as‐produced GNRs are strongly dependent on the diameter of the tubes and the nature of the precursor, the atomic interactions between GNRs and surrounding CNTs and their effect on the electronic properties of the overall system are not well understood. Here, it is shown that the functional terminations of the GNR edges can have a strong influence on the electronic structure of the system. Analysis of SWCNTs before and after the insertion of sulfur‐terminated GNRs suggests a metallization of the majority of semiconducting SWCNTs. This is indicated by changes in the radial breathing modes and the D and G band Raman features, as well as UV–vis–NIR absorption spectra. The variation in resonance conditions of the nanotubes following GNR insertion make direct (n,m) assignment by Raman spectroscopy difficult. Thus, density functional theory calculations of representative GNR/SWCNT systems are performed. The results confirm significant changes in the band structure, including the development of a metallic state in the semiconducting SWCNTs due to sulfur/tube interactions. The GNR‐induced metallization of semiconducting SWCNTs may offer a means of controlling the electronic properties of bulk CNT samples and eliminate the need for a physical separation of semiconducting and metallic tubes.     

Highly‐Anisotropic Dion–Jacobson Hybrid Perovskite by Tailoring Diamine into CsPbBr3 for Polarization‐Sensitive Photodetection

2D materials with inherent attributes of structural anisotropy have been well applied in the field of polarization‐sensitive photodetection. However, to explore new 2D members with strong polarized‐light responses still remains a challenge. Herein, by alloying diamine molecule into the 3D prototype of CsPbBr₃, a new Dion–Jacobson (DJ) type 2D perovskite of (HDA)CsPb₂Br₇ ( 1 , where HDA²⁺ is 1,6‐hexamethylenediammonium), containing both inorganic Cs metal and organic cations is designed. The natural anisotropy characteristics of 1 are solidly elucidated by analyzing crystal structure, electric conductivity, and optical properties. Strikingly, distinct polarization‐sensitive responses are observed in 1 , owing to its strong anisotropy of optical absorption (the ratio of α_c/α_b ≈ 2.2). Consequently, crystal‐based detectors of 1 exhibit fascinating photo‐activities to polarized‐light, including high detectivity (1.5 × 10⁹ Jones), large dichroism ratio (I_ph^c/I_ph^b ≈ 1.6) and fast responding rate (200 µs). All these polarization‐sensitive performances along with intriguing phase stability make 1 a potential candidate for polarized‐light detection. This work paves a pathway toward new functionalities of DJ‐type 2D hybrid perovskites for their future optoelectronic device applications.     

Wrap‐Around Core–Shell Heterostructures of Layered Crystals

Engineered heterostructures create new functionality by integrating dissimilar materials. Combining different 2D crystals naturally produces two distinct classes of heterostructures, vertical van der Waals (vdW) stacks or 2D sheets bonded laterally by covalent line interfaces. When joining thicker layered crystals, the arising structural and topological conflicts can result in more complex geometries. Phase separation during one‐pot synthesis of layered tin chalcogenides spontaneously creates core–shell structures in which large orthorhombic SnS crystals are enclosed in a wrap‐around shell of trigonal SnS₂, forcing the coexistence of parallel vdW layering along with unconventional, orthogonally layered core–shell interfaces. Measurements of the optoelectronic properties establish anisotropic carrier separation near type II core–shell interfaces and extended long‐wavelength light harvesting via spatially indirect interfacial absorption, making multifunctional layered core–shell structures attractive for energy‐conversion applications.     

Self‐Confined Growth of Ultrathin 2D Nonlayered Wide‐Bandgap Semiconductor CuBr Flakes

2D planar structures of nonlayered wide‐bandgap semiconductors enable distinguished electronic properties, desirable short wavelength emission, and facile construction of 2D heterojunction without lattice match. However, the growth of ultrathin 2D nonlayered materials is limited by their strong covalent bonded nature. Herein, the synthesis of ultrathin 2D nonlayered CuBr nanosheets with a thickness of about 0.91 nm and an edge size of 45 µm via a controllable self‐confined chemical vapor deposition method is described. The enhanced spin‐triplet exciton (Z_f, 2.98 eV) luminescence and polarization‐enhanced second‐harmonic generation based on the 2D CuBr flakes demonstrate the potential of short‐wavelength luminescent applications. Solar‐blind and self‐driven ultraviolet (UV) photodetectors based on the as‐synthesized 2D CuBr flakes exhibit a high photoresponsivity of 3.17 A W^?1, an external quantum efficiency of 1126%, and a detectivity (D*) of 1.4 × 10¹¹ Jones, accompanied by a fast rise time of 32 ms and a decay time of 48 ms. The unique nonlayered structure and novel optical properties of the 2D CuBr flakes, together with their controllable growth, make them a highly promising candidate for future applications in short‐wavelength light‐emitting devices, nonlinear optical devices, and UV photodetectors.     

Light‐Induced Motion of Microengines Based on Microarrays of TiO2 Nanotubes

An electrochemical approach for manufacturing light‐driven nanostructured titanium dioxide (TiO₂) microengines with controlled spatial architecture for improved performance is reported. The microengines based on microscale arrays of TiO₂ nanotubes with variable (50–120 nm) inner diameter show a quasi‐ordered arrangement of nanotubes, being the smallest tubular entities for catalytic microengines reported to date. The nanotubes exhibit well defined crystalline phases depending upon the postfabrication annealing conditions that determine the microengines' efficiency. When exposed to UV‐light, the microarrays of TiO₂ nanotubes exhibiting conical internal shapes show directed motion in confined space, both in the presence and absence of hydrogen peroxide. In the former case, two different motion patterns related to diffusiophoresis and localized nanobubble generation inside of the tubes due to the photocatalytic decomposition of H₂O₂ are disclosed. Controlled pick‐up, transport, and release of individual and agglomerated particles are demonstrated using the UV light irradiation of microengines. The obtained results show that light‐driven microengines based on microarrays of TiO₂ nanotubes represent a promising platform for controlled micro/nanoscale sample transportation in fluids as well as for environmental applications, in particular, for the enhanced photocatalytic degradation of organic pollutants due to the improved intermixing taking place during the motion of TiO₂ microengines.     

Phase‐Change Hyperbolic Heterostructures for Nanopolaritonics: A Case Study of hBN/VO2

Unlike conventional plasmonic media, polaritonic van der Waals (vdW) materials hold promise for active control of light–matter interactions. The dispersion relations of elementary excitations such as phonons and plasmons can be tuned in layered vdW systems via stacking using functional substrates. In this work, infrared nanoimaging and nanospectroscopy of hyperbolic phonon polaritons are demonstrated in a novel vdW heterostructure combining hexagonal boron nitride (hBN) and vanadium dioxide (VO₂). It is observed that the insulator‐to‐metal transition in VO₂ has a profound impact on the polaritons in the proximal hBN layer. In effect, the real‐space propagation of hyperbolic polaritons and their spectroscopic resonances can be actively controlled by temperature. This tunability originates from the effective change in local dielectric properties of the VO₂ sublayer in the course of the temperature‐tuned insulator‐to‐metal phase transition. The high susceptibility of polaritons to electronic phase transitions opens new possibilities for applications of vdW materials in combination with strongly correlated quantum materials.     

Flexible and High‐Performance All‐2D Photodetector for Wearable Devices

Emerging novel applications at the forefront of innovation horizon raise new requirements including good flexibility and unprecedented properties for the photoelectronic industry. On account of diversity in transport and photoelectric properties, 2D layered materials have proven as competent building blocks toward next‐generation photodetectors. Herein, an all‐2D Bi₂Te₃‐SnS‐Bi₂Te₃ photodetector is fabricated with pulsed‐laser deposition. It is sensitive to broadband wavelength from ultraviolet (370 nm) to near‐infrared (808 nm). In addition, it exhibits great durability to bend, with intact photoresponse after 100 bend cycles. Upon 370 nm illumination, it achieves a high responsivity of 115 A W^?1, a large external quantum efficiency of 3.9 × 10⁴%, and a superior detectivity of 4.1 × 10¹¹ Jones. They are among the best figures‐of‐merit of state‐of‐the‐art 2D photodetectors. The synergistic effect of SnS's strong light–matter interaction, efficient carrier separation of Bi₂Te₃–SnS interface, expedite carrier injection across Bi₂Te₃–SnS interface, and excellent carrier collection of Bi₂Te₃ topological insulator electrodes accounts for the superior photodetection properties. In summary, this work depicts a facile all‐in‐one fabrication strategy toward a Bi₂Te₃‐SnS‐Bi₂Te₃ photodetector. More importantly, it reveals a novel all‐2D concept for construction of flexible, broadband, and high‐performance photoelectronic devices by integrating 2D layered metallic electrodes and 2D layered semiconducting channels.     

Tuning Excitonic Properties of Monolayer MoS2 with Microsphere Cavity by High‐Throughput Chemical Vapor Deposition Method

Tuning the optical properties of 2D direct bandgap semiconductors is crucial for applications in photonic light source, optical communication, and sensing. In this work, the excitonic properties of molybdenum disulphide (MoS₂) are successfully tuned by directly depositing it onto silica microsphere resonators using chemical vapor deposition method. Multiple whispering gallery mode (WGM) peaks in the emission wavelength range of ≈650–750 nm are observed under continuous wave excitation at room temperature. Time‐resolved photoluminescence (TRPL) and femtosecond transient absorption (TA) spectroscopy are conducted to study light‐matter interaction dynamics of the MoS₂ microcavities. TRPL study suggests radiative recombination rate of carrier‐phonon scattering and interband transition processes in MoS₂ is enhanced by a factor of ≈1.65 due to Purcell effect in microcavities. TA spectroscopy study shows modulation of the interband transition process mainly occurs at PB‐A band with an estimated F ≈ 1.60. Furthermore, refractive index sensing utilizing WGM peaks of MoS₂ is established with sensitivity up to ≈150 nm per refractive index unit. The present work provides a large‐scale and straightforward method for coupling atomically thin 2D gain media with cavities for high‐performance optoelectronic devices and sensors.     

In‐Plane Isotropic/Anisotropic 2D van der Waals Heterostructures for Future Devices

Mono‐ to few‐layers of 2D semiconducting materials have uniquely inherent optical, electronic, and magnetic properties that make them ideal for probing fundamental scientific phenomena up to the 2D quantum limit and exploring their emerging technological applications. This Review focuses on the fundamental optoelectronic studies and potential applications of in‐plane isotropic/anisotropic 2D semiconducting heterostructures. Strong light–matter interaction, reduced dimensionality, and dielectric screening in mono‐ to few‐layers of 2D semiconducting materials result in strong many‐body interactions, leading to the formation of robust quasiparticles such as excitons, trions, and biexcitons. An in‐plane isotropic nature leads to the quasi‐2D particles, whereas, an anisotropic nature leads to quasi‐1D particles. Hence, in‐plane isotropic/anisotropic 2D heterostructures lead to the formation of quasi‐1D/2D particle systems allowing for the manipulation of high binding energy quasi‐1D particle populations for use in a wide variety of applications. This Review emphasizes an exciting 1D–2D particles dynamic in such heterostructures and their potential for high‐performance photoemitters and exciton–polariton lasers. Moreover, their scopes are also broadened in thermoelectricity, piezoelectricity, photostriction, energy storage, hydrogen evolution reactions, and chemical sensor fields. The unique in‐plane isotropic/anisotropic 2D heterostructures may open the possibility of engineering smart devices in the nanodomain with complex opto‐electromechanical functions.     

On‐Chip Rolling Design for Controllable Strain Engineering and Enhanced Photon–Phonon Interaction in Graphene

On‐chip strain engineering is highly demanded in 2D materials as an effective route for tuning their extraordinary properties and integrating consistent functionalities toward various applications. Herein, rolling technique is proposed for strain engineering in monolayer graphene grown on a germanium substrate, where compressive or tensile strain could be acquired, depending on the designed layer stressors. Unusual compressive strains up to 0.30% are achieved in the rolled‐up graphene tubular structures. The subsequent phonon hardening under compressive loading is observed through strain‐induced Raman G band splitting, while distinct blueshifts of characteristic peaks (G⁺, G^?, or 2D) can be well regulated on an asymmetric tubular structure with a strain variation. In addition, due to the strong confinement of the local electromagnetic field under 3D tubular geometry, the photon–phonon interaction is highly strengthened, and thus, the Raman scattering of graphene in rolled‐up tubes is enhanced. Such an on‐chip rolling approach leads to a superior strain tuning method in 2D materials and could improve their light–matter interaction in a tubular configuration, which may hold great capability in 2D materials integration for on‐chip applications such as in mechanics, electronics, and photonics.     

Free‐Standing Polyurethane Nanofiber/Nets Air Filters for Effective PM Capture

The filtration performance and light transmittance of nanofiber air filters are restricted by their thick fiber diameter, large pore size, and substrate dependence, which can be solved by constructing substrate‐free fibrous membranes with true nanoscale diameters and ultrathin thicknesses, however, it has proven to be extremely challenging. Herein, a roust approach is presented to create free‐standing polyurethane (PU) nanofiber/nets air filters composed of bonded nanofibers and 2D nanonets for particular matter (PM) capture via combining electrospinning/netting technique and facile peel off process from designed substrates. This strategy causes widely distributed Steiner‐tree structured nanonets with diameters of ≈20 nm and bonded scaffold nanofibers to assemble into ultrathin membranes with small pore size, high porosity, and robust mechanical strength on a large scale based on ionic liquid inspiration and surface structure optimization of receiver substrates. As a consequence, the resulting free‐standing PU nanofiber/nets filters exhibit high PM_1–0.5 removal efficiency of >99.00% and PM_2.5–1 removal efficiency of >99.73%, maintaining high light transmittance of ≈70% and low pressure drop of 28 Pa; even achieve >99.97% removal efficiency with ≈40% transmittance for PM_0.3 filtration, and robust purification capacity for real smoke PM_2.5, making them promising high‐efficiency and transparent filtration materials for various filtration and separation applications.     

Highly Selective Dispersion of Carbon Nanotubes by Using Poly(phenyleneethynylene)‐Guided Supermolecular Assembly

Isolation of single‐walled carbon nanotubes (SWNTs) with specific chirality and diameters is critical for achieving optimum performance of SWNTs in various applications. A water‐soluble π‐conjugated polymer, poly[(m‐phenyleneethynylene)‐alt‐(p‐phenyleneethynylene)], 3 , is found to exhibit high selectivity in dispersing SWNT (6,5). The polymer's ability to sort out SWNT (6,5) appears to be related to the carbon–carbon triple bond, whose free rotation allows a unique assembly of chromophores in a helical conformation. The observation is consistently supported by fluorescence, Raman, and UV‐vis‐NIR absorption spectra. The intriguing selectivity of 3 to SWNT (6,5), however, is not observed for the vinylene analogue polymer 1 , showing that the carbon–carbon triple bond could play a unique role in sorting out a specific SWNT. The observed selectivity from 3 could be attributed to a combination of the helical cavity size restrain and electronic interaction associated with the local chromophore arrangement. This strategy could be expanded for efficient SWNT sorting when the helical conformation is further finely tuned.     

3D Aerogel of Graphitic Carbon Nitride Modified with Perylene Imide and Graphene Oxide for Highly Efficient Nitric Oxide Removal under Visible Light

3D materials are considered promising for photocatalytic applications in air purification because of their large surface areas, controllability, and recyclability. Here, a series of aerogels consisting of graphitic‐carbon nitride (g‐C₃N₄) modified with a perylene imide (PI) and graphene oxide (GO) are prepared for nitric oxide (NO) removal under visible‐light irradiation. All of the photocatalysts exhibit excellent activity in NO removal because of the strong light absorption and good planarity of PI–g‐C₃N₄ coupled with the favorable charge transport properties of GO, which slow the recombination of electron–hole pairs. The aerogel containing thiophene displays the most efficient NO removal of the aerogel series, with a removal ratio of up to 66%. Density functional theory calculations are conducted to explain this result and recycling experiments are carried out to verify the stability and recyclability of these photocatalysts.     

A Dual‐Function Na2SO4 Template Directed Formation of Cathode Materials with a High Content of Sulfur Nanodots for Lithium–Sulfur Batteries

The sulfur content in carbon–sulfur hybrid using the melt‐diffusion method is normally lower than 70 wt%, which greatly decreases the energy density of the cathode in lithium–sulfur (Li‐S) batteries. Here, a scalable method inspired by the commercialized production of Na₂S is used to prepare a hierarchical porous carbon–sulfur hybrid (denoted HPC‐S) with high sulfur content (≈85 wt%). The HPC‐S is characterized by the structure of sulfur nanodots naturally embedded in a 3D carbon network. The strategy uses Na₂SO₄ as the starting material, which serves not only as the sulfur precursor but also as a salt template for the formation of the 3D carbon network. The HPC‐S cathode with such a high sulfur content shows excellent rate performance and cycling stability in Li–S batteries because of the sulfur nanoparticles, the unique carbon framework, and the strong interaction between them. The production method can also be readily scaled up and used in practical Li–S battery applications.     

Light‐Switchable Oxygen Vacancies in Ultrafine Bi5O7Br Nanotubes for Boosting Solar‐Driven Nitrogen Fixation in Pure Water

Solar‐driven reduction of dinitrogen (N₂) to ammonia (NH₃) is severely hampered by the kinetically complex and energetically challenging multielectron reaction. Oxygen vacancies (OVs) with abundant localized electrons on the surface of bismuth oxybromide‐based semiconductors are demonstrated to have the ability to capture and activate N₂, providing an alternative pathway to overcome such limitations. However, bismuth oxybromide materials are susceptible to photocorrosion, and the surface OVs are easily oxidized and therefore lose their activities. For realistic photocatalytic N₂ fixation, fabricating and enhancing the stability of sustainable OVs on semiconductors is indispensable. This study shows the first synthesis of self‐assembled 5 nm diameter Bi₅O₇Br nanotubes with strong nanotube structure, suitable absorption edge, and many exposed surface sites, which are favorable for furnishing sufficient visible light‐induced OVs to realize excellent and stable photoreduction of atmospheric N₂ into NH₃ in pure water. The NH₃ generation rate is as high as 1.38 mmol h^?1 g^?1, accompanied by an apparent quantum efficiency over 2.3% at 420 nm. The results presented herein provide new insights into rational design and engineering for the creation of highly active catalysts with light‐switchable OVs toward efficient, stable, and sustainable visible light N₂ fixation in mild conditions.