Enhanced Photocarrier Generation with Selectable Wavelengths by M-Decorated-CuInS2 Nanocrystals (M = Au and Pt) Synthesized in a Single Surfactant Process on MoS2 Bilayers

Synthesis and hybridization of CuInS2 nanocrystals for emerging applications

Copper indium sulfide (CuInS2) is a ternary A(I)B(III)X(VI)2-type semiconductor featuring a direct bandgap with a high absorption coefficient. In attempts to explore their practical applications, nanoscale CuInS2 has been synthesized with crystal sizes down to the quantum confinement regime. The merits of CuInS2 nanocrystals (NCs) include wide emission tunability, a large Stokes shift, long decay time, and eco-friendliness, making them promising candidates in photoelectronics and photovoltaics. Over the past two decades, advances in wet-chemistry synthesis have achieved rational control over cation-anion reactivity during the preparation of colloidal CuInS2 NCs and post-synthesis cation exchange. The precise nano-synthesis coupled with a series of hybridization strategies has given birth to a library of CuInS2 NCs with highly customizable photophysical properties. This review article focuses on the recent development of CuInS2 NCs enabled by advanced synthetic and hybridization techniques. We show that the state-of-the-art CuInS2 NCs play significant roles in optoelectronic and biomedical applications.

Embedded Integration of Sb2Se3 Film by Low-Temperature Plasma-Assisted Chemical Vapor Reaction with Polycrystalline Si Transistor for High-Performance Flexible Visible-to-Near-Infrared Photodetector

Flexible optoelectronics have garnered considerable interest for applications such as optical communication, motion capture, biosignal detection, and night vision. Transition-metal dichalcogenides are widely used as flexible photodetectors owing to their outstanding electrical and optical properties and high flexibility. Herein, a two-dimensional (2D) Sb₂Se₃ film-based one transistor-one resistor (1T1R) flexible photodetector with high photosensing current and detection ranges from visible to near-infrared was developed. The flexible 1T1R was fabricated using an efficient field-effect transistor platform with the 2D Sb₂Se₃ film directly deposited on the sensing region using a low-temperature plasma-assisted chemical vapor reaction. The photodetector could achieve a maximum I_photo/I_dark of 15,000 under white light with a power density of 26 mW/cm², in which the photodetector showed quick rising and falling response times of 0.16 and 0.28 s, respectively. The 2D Sb₂Se₃ film exhibits broadband absorption in the visible and IR regions, yielding an excellent photoresponse under laser illumination with different wavelengths. To investigate the flexibility and stability of the 1T1R photodetector, the photoresponses were measured under different bending cycles and curvatures, which maintained its functions and exhibited high stability under convex and concave bending at a curvature radius of 20 mm.

Rich Landscape of Colloidal Semiconductor-Metal Hybrid Nanostructures: Synthesis, Synergetic Characteristics, and Emerging Applications

Nanochemistry provides powerful synthetic tools allowing one to combine different materials on a single nanostructure, thus unfolding numerous possibilities to tailor their properties toward diverse functionalities. Herein, we review the progress in the field of semiconductor-metal hybrid nanoparticles (HNPs) focusing on metal-chalcogenides-metal combined systems. The fundamental principles of their synthesis are discussed, leading to a myriad of possible hybrid architectures including Janus zero-dimensional quantum dot-based systems and anisotropic quasi 1D nanorods and quasi-2D platelets. The properties of HNPs are described with particular focus on emergent synergetic characteristics. Of these, the light-induced charge-separation effect across the semiconductor-metal nanojunction is of particular interest as a basis for the utilization of HNPs in photocatalytic applications. The extensive studies on the charge-separation behavior and its dependence on the HNPs structural characteristics, environmental and chemical conditions, and light excitation regime are surveyed. Combining the advanced synthetic control with the charge-separation effect has led to demonstration of various applications of HNPs in different fields. A particular promise lies in their functionality as photocatalysts for a variety of uses, including solar-to-fuel conversion, as a new type of photoinitiator for photopolymerization and 3D printing, and in novel chemical and biomedical uses.

Synergy of MoO2 with Pt as Unilateral Dual Cocatalyst for Improving Photocatalytic Hydrogen Evolution over g-C3 N4

Pt is usually used as cocatalyst for g-C₃ N₄ to produce H₂ by photocatalytic splitting of water. However, the photocatalytic performance is still limited by the fast recombination of photo-generated electrons and holes, as well as the poor absorption of visible light. In this work, MoO₂ /g-C₃ N₄ composites were prepared, in which MoO₂ synergetic with Pt photo-deposited during H₂ evolution reaction worked as unilateral dual cocatalyst to improve the photocatalytic activity. Within 4 hours of irradiation, the hydrogen production rate of MoO₂ -Pt dual cocatalyst modified g-C₃ N₄ reached 3804.89 μmol/g/h, which was 120.18 times of that of pure g-C₃ N₄ (GCN, 31.66 μmol/g/h), 10.98 times of that of MoO₂ modified g-C₃ N₄ (346.39 μmol/g/h), and 9.18 times of that of Pt modified g-C₃ N₄ (413.64 μmol/g/h). Characterization results demonstrate that the deficient MoO₂ not only promoted visible light absorption of g-C₃ N₄ , but also worked as a "electron pool" to capture and transfer electrons to Pt.

High-Performance MoS2 Photodetectors Prepared Using a Patterned Gallium Nitride Substrate

Strain-adjusting the band gap of MoS₂ using patterned substrates to improve the photoelectric performance of MoS₂ has gradually become a research hotspot in recent years. However, there are still difficulties in obtaining high-quality two-dimensional materials and preparing photodetectors on patterned substrates. To overcome this, a continuous multilayer MoS₂ film was transferred to a patterned gallium nitride substrate (PGS) for the fabrication of photodetectors, and density functional theory calculations showed that the band gap of the MoS₂ film increased and that the electron effective mass decreased due to the introduction of PGS. In addition, finite difference time domain simulation showed that the electric field in the MoS₂ area on the PGS is enhanced compared with that on the flat gallium nitride substrate due to the enhanced light scattering effect of the PGS. The photoresponse of the MoS₂/PGS photodetector at 460 nm was also enhanced, with I_ph increasing by 5 times, R increasing by 2 times, NEP decreasing to 3.88 × 10^-13 W/Hz^1/2, and D* increasing to 5.6 × 10⁸ Jones. Our research has important guiding significance in adjusting the band gap of MoS₂ and enhancing the photoelectric performance of MoS₂ photodetectors.

Wrapping Plasmonic Silver Nanoparticles inside One-Dimensional Nanoscrolls of Transition-Metal Dichalcogenides for Enhanced Photoresponse

The low light absorption of transition-metal dichalcogenide (TMDC) nanosheets hinders their application as high-performance optoelectronic devices. Rolling them up into one-dimensional (1D) nanoscrolls and decorating them with plasmonic nanoparticles (NPs) are both effective strategies for enhancing their performance. When these two approaches are combined, in this work, the light-matter interaction in TMDC nanosheets is greatly improved by encapsulating silver nanoparticles (Ag NPs) in TMDC nanoscrolls. After the silver nitrate (AgNO₃) solution was spin-coated on monolayer (1L) MoS₂ and WS₂ nanosheets grown by chemical vapor deposition, Ag NPs were homogeneously formed to obtain MoS₂-Ag and WS₂-Ag nanosheets due to the TMDC-assisted spontaneous reduction, and their size and density can be well controlled by tuning the concentration of the AgNO₃ solution. By the simple placement of alkaline droplets on MoS₂-Ag or WS₂-Ag hybrid nanosheets, MoS₂-Ag or WS₂-Ag nanoscrolls with large sizes were obtained in large area. The obtained hybrid nanoscrolls exhibited up to 500 times increased photosensitivities compared with 1L MoS₂ nanosheets, arising from the localized surface plasmon resonance effect of Ag NPs and the scrolled-nanosheet structure. Our work provides a reliable method for the facile and large-area preparation of NP/nanosheet hybrid nanoscrolls and demonstrates their great potential for high-performance optoelectronic devices.

Highly Enhanced Light-Matter Interaction in MXene Quantum Dots-Monolayer WS2 Heterostructure

Since the Ti₃ C₂ was discovered in 2011, the family of MXenes has attracted much attention. MXenes offer great potential in the tuning of many fundamental properties by the synthesis of new structures. The synthesis methods of MXene mainly require steps including immersing a MAX phase in hydrofluoric acid (HF) and processing at high temperatures. However, the HF may be hard to acquire in many countries and processing at high temperatures may cause risk issues. In this article, a simple and cost-effective synthesis of Ti₃ C₂ T_x quantum dots (QDs) via chemical solution method that follows the long-time magnetic stirring process-initiated etching of Al atoms from commercial Ti₃ AlC₂ powder at room temperature is introduced. With WS₂ monolayer sitting over the MXenes QD arrays, a higher level of photoluminescence (PL) enhancement is found in the heterostructure with increasing laser power at room temperature and a few novel quasi-particles species in the heterostructure at -190 °C. The observations show that the possible plasmonic behavior initiated by QD arrays and the suspension state of WS₂ may coplay the roles to trigger multiple quasi-particles species. This study can be an important benchmark for the extensive understanding of quasi-particles species, and their dynamics.

Emerging Light-Emitting Materials for Photonic Integration

The arrival of the information explosion era is urging the development of large-bandwidth high-data-rate optical interconnection technology. Up to now, the biggest stumbling block in optical interconnections has been the lack of efficient light sources despite significant progress that has been made in germanium-on-silicon (Ge-on-Si) and III-V-on-silicon (III-V-on-Si) lasers. 2D materials and metal halide perovskites have attracted much attention in recent years, and exhibit distinctive advantages in the application of on-chip light emitters. Herein, this Progress Report reviews the recent progress made in light-emitting materials with a focus on new materials, i.e., 2D materials and metal halide perovskites. The report briefly introduces the current status of Ge-on-Si and III-V-on-Si lasers and discusses the advances of 2D and perovskite light-emitting materials for photonic integration, including their optical properties, preparation methods, as well as the light sources based on these materials. Finally, challenges and perspectives of these emerging materials on the way to the efficient light sources are discussed.

Synthesis of Ultrahigh-Quality Monolayer Molybdenum Disulfide through In Situ Defect Healing with Thiol Molecules

Monolayer transition metal dichalcogenides are 2D materials with many potential applications. Chemical vapor deposition (CVD) is a promising method to synthesize these materials. However, CVD-grown materials generally have poorer quality than mechanically exfoliated ones and contain more defects due to the difficulties in controlling precursors' distribution and concentration during growth where solid precursors are used. Here, thiol is proposed to be used as a liquid precursor for CVD growth of high quality and uniform 2D MoS₂ . Atomic-resolved structure characterizations indicate that the concentration of sulfur vacancies in the MoS₂ grown from thiol is the lowest among all reported CVD samples. Low temperature spectroscopic characterization further reveals the ultrahigh optical quality of the grown MoS₂ . Density functional theory simulations indicate that thiol molecules could interact with sulfur vacancies in MoS₂ and repair these defects during the growth of MoS₂ , resulting in high-quality MoS₂ . This work provides a facile and controllable method for the growth of high-quality 2D materials with ultralow sulfur vacancies and high optical quality, which will benefit their optoelectronic applications.

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

Hybrid nanostructures, composed of multi-component crystals of various shapes, sizes and compositions are much sought-after functional materials. Pairing the ability to tune each material separately and controllably combine two (or more) domains with defined spatial orientation results in new properties. In this review, we discuss the various synthetic mechanisms for the formation of hybrid nanostructures of various complexities containing at least one metal/semiconductor interface, with a focus on colloidal chemistry. Different synthetic approaches, alongside the underlying kinetic and thermodynamic principles are discussed, and future advancement prospects are evaluated. Furthermore, the proved unique properties are reviewed with emphasis on the connection between the synthetic method and the resulting physical, chemical and optical properties with applications in fields such as photocatalysis.

Ultra-sensitive solution-processed broadband photodetectors based on vertical field-effect transistor

In the past few decades, great attention has been paid to the development of IV-VI semiconductor colloidal quantum dots, such as PbSe, PbS and PbSSe, in infrared (IR) photodetectors due to their high photosensitivity, solution-processing and low cost fabrication. IR photodetectors based on field-effect transistors (FETs) showed high detectivity since the transconductance can magnify the drain-source current under certain applied gate voltages. However, traditional lateral FETs usually suffer from low photosensitivity and slow responsivity, which restricts their widespread commercial applications. In this work, therefore, novel vertical FET (VFET) based photodetectors are presented, in which the active layer is sandwiched between porous source electrode and planar drain electrode, resulting to ultrashort channel length. In this way, enhanced photoresponsivity and specific detectivity of 291 A W-1 and 1.84 × 1014 Jones, respectively, can be obtained at low drain-source voltage (V DS) of -1 V and gate voltage (V g) of -2 V under 100 μW cm-2 illumination intensity, which was better than that of the traditional lateral FET based photodetectors. Therefore, it is promising to fabricate broadband photodetectors with high performance and good stability by this easy approach.

Spatial Surface Charge Engineering for Electrochemical Electrodes

We introduce a novel concept for the design of functional surfaces of materials: Spatial surface charge engineering. We exploit the concept for an all-solid-state, epitaxial InN/InGaN-on-Si reference electrode to replace the inconvenient liquid-filled reference electrodes, such as Ag/AgCl. Reference electrodes are universal components of electrochemical sensors, ubiquitous in electrochemistry to set a constant potential. For subtle interrelation of structure design, surface morphology and the unique surface charge properties of InGaN, the reference electrode has less than 10 mV/decade sensitivity over a wide concentration range, evaluated for KCl aqueous solutions and less than 2 mV/hour long-time drift over 12 hours. Key is a nanoscale charge balanced surface for the right InGaN composition, InN amount and InGaN surface morphology, depending on growth conditions and layer thickness, which is underpinned by the surface potential measured by Kelvin probe force microscopy. When paired with the InN/InGaN quantum dot sensing electrode with super-Nernstian sensitivity, where only structure design and surface morphology are changed, this completes an all-InGaN-based electrochemical sensor with unprecedented performance.

Atomically Sharp Dual Grain Boundaries in 2D WS2 Bilayers

It is shown that tilt grain boundaries (GBs) in bilayer 2D crystals of the transition metal dichalcogenide WS₂ can be atomically sharp, where top and bottom layer GBs are located within sub-nanometer distances of each other. This expands the current knowledge of GBs in 2D bilayer crystals, beyond the established large overlapping GB types typically formed in chemical vapor deposition growth, to now include atomically sharp dual bilayer GBs. By using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging, different atomic structures in the dual GBs are distinguished considering bilayers with a 3R (AB stacking)/2H (AA' stacking) interface as well as bilayers with 2H/2H boundaries. An in situ heating holder is used in ADF-STEM and the GBs are stable to at least 800 °C, with negligible thermally induced reconstructions observed. Normal dislocation cores are seen in one WS₂ layer, but the second WS₂ layer has different dislocation structures not seen in freestanding monolayers, which have metal-rich clusters to accommodate the stacking mismatch of the 2H:3R interface. These results reveal the competition between maintaining van der Waals bilayer stacking uniformity and dislocation cores required to stitch tilted bilayer GBs together.

Origin of Extended UV Stability of 2D Atomic Layer Titania-Based Perovskite Solar Cells Unveiled by Ultrafast Spectroscopy

The inherent instability of UV-induced degradation in TiO₂-based perovskite solar cells was largely improved by replacing the anatase-phase compact TiO₂ layer with an atomic sheet transport layer (ASTL) of two-dimensional (2D) Ti_1-δO₂. The vital role of microscopic carrier dynamics that govern the UV stability of perovskite solar cells was comprehensively examined in this work by performing time-resolved pump-probe spectroscopy. In conventional perovskite solar cells, the presence of a UV-active oxygen vacancy in compact TiO₂ prohibits current generation by heavily trapping electrons after UV degradation. Conversely, the dominant vacancy type in the 2D Ti_1-δO₂ ASTL is a titanium vacancy, which is a shallow acceptor and is not UV-sensitive. Therefore, it significantly suppresses carrier recombination and extends UV stability in perovskite solar cells with a 2D Ti_1-δO₂ ASTL. Other carrier dynamics, such as electron diffusion, electron injection, and hot hole transfer processes, were found to be less affected by UV irradiation. Quantitative pump-probe data clearly show a correlation between the carrier dynamics and UV aging of perovskite solar cells, thus providing a profound insight into the factors driving UV-induced degradation in perovskite solar cells and the origin of its performance.

High-Uniformity Planar Mini-Chip-Scale Packaged LEDs with Quantum Dot Converter for White Light Source

This study proposes a novel direct-lit mini-chip-scale packaged light-emitting diode (mini-CSPLED) backlight unit (BLU) that used quantum dot (QD) film, diffusion plate, and two prism films to improve brightness uniformity. Three different luminous intensity units, 120° mini-CSPLED, 150° mini-CSPLED, and 180° mini-CSPLED with different emission angle structures were fabricated using a CSP process. In terms of component characteristics, although the 180° mini-CSPLED light output power is about loss 4% (at 10 mA) compared with 150° mini-CSPLED, it has a large emission angle that forms a planar light source that contributes to improving the BLU brightness uniformity and reduced quantity of LEDs at the same area. In terms of BLU analysis, the blue mini-CSPLEDs with different emission angles excite the different QD film thicknesses; the chromaticity coordinates conversion to the white light region. The BLU brightness increases as the QD film thickness increases from 60, 90, and 150 μm. This result can achieve a brightness uniformity of 86% in a 180° mini-CSPLED BLU + 150-μm-thick QD films as compared to the 120° mini-CSPLED BLU and 150° mini-CSPLED BLU.

Integration of Environmental Friendly Perovskites for High-efficiency White Light-emitting Diodes

Perovskite quantum dots (QDs) have been widely used in white light-emitting diodes (WLEDs), due to their high quantum yield (QY), tunable bandgap, and simple preparation. However, the red-emitting perovskite QDs are usually containing iodine (I), which is not stable under continuous light irradiation. Herein, perovskite-based WLED is fabricated by lead-free bismuth (Bi)-doped inorganic perovskites Cs₂SnCl₆ and less-lead Mn-doped CsPbCl₃ QDs, which emits white light with color coordinates of (0.334, 0.297). The Bi-doped Cs₂SnCl₆ and Mn-doped CsPbCl₃ QDs both show excellent stability when kept in the ambient air. As benefits from this desired characteristic, the as-prepared WLED shows excellent stability along with operating time. These results can promote the application of inorganic perovskite QDs in the field of WLEDs.

Electronic Properties of Armchair Black Phosphorene Nanoribbons Edge-Modified by Transition Elements V, Cr, and Mn

The structural, electrical, and magnetic properties of armchair black phosphorene nanoribbons (APNRs) edge-functionalized by transitional metal (TM) elements V, Cr, and Mn were studied by the density functional theory combined with the non-equilibrium Green's function. Spin-polarized edge states introduce great varieties to the electronic structures of TM-APNRs. For APNRs with Mn-stitched edge, their band structures exhibit half-semiconductor electrical properties in the ferromagnetic state. A transverse electric field can then make the Mn-APNRs metallic by shifting the conduction bands of edge states via the Stark effect. The Mn/Cr-APNR heterojunction may be used to fabricate spin p-n diode where strong rectification acts only on one spin.

Using Different Ions to Tune Graphene Stack Structures from Sheet- to Onion-Like During Plasma Exfoliation, with Supercapacitor Applications

In this article, we report a facile and simple approach for tuning graphene nanosheet structures (GNS) with different ions in the electrolytes through cathodic plasma exfoliation process in electrochemical reactions. We obtained sheet- and onion-like GNS when aqueous electrolyte NaOH and H₂SO₄, respectively, were present during plasma exfoliation in the electrochemical reactions, as evidenced from scanning electron microscopy and transmission electron microscopy images. Moreover, the onion-like GNS exhibited a specific surface area of 464 m² g^-1 and a supercapacitive performance of 67.1 F g^-1, measured at a scan rate of 5 mV s^-1 in 1 M NaCl; these values were much higher than those (72 m² g^-1 and 21.6 F g^-1, respectively) of the sheet-like GNS. This new approach for efficiently generating tunable stacked graphene structures with different ions, in the cathodic plasma exfoliation process, has promising potentials for use in energy storage devices.