Strong Quantum Confinement Effect Research Articles

Telecom-band multiwavelength vertical emitting quantum well nanowire laser arrays

Highly integrated optoelectronic and photonic systems underpin the development of next-generation advanced optical and quantum communication technologies, which require compact, multiwavelength laser sources at the telecom band. Here, we report on-substrate vertical emitting lasing from ordered InGaAs/InP multi-quantum well core–shell nanowire array epitaxially grown on InP substrate by selective area epitaxy. To reduce optical loss and tailor the cavity mode, a new nanowire facet engineering approach has been developed to achieve controlled quantum well nanowire dimensions with uniform morphology and high crystal quality. Owing to the strong quantum confinement effect of InGaAs quantum wells and the successful formation of a vertical Fabry–Pérot cavity between the top nanowire facet and bottom nanowire/SiO2 mask interface, stimulated emissions of the EH11a/b mode from single vertical nanowires from an on-substrate nanowire array have been demonstrated with a lasing threshold of ~28.2 μJ cm−2 per pulse and a high characteristic temperature of ~128 K. By fine-tuning the In composition of the quantum wells, room temperature, single-mode lasing is achieved in the vertical direction across a broad near-infrared spectral range, spanning from 940 nm to the telecommunication O and C bands. Our research indicates that through a carefully designed facet engineering strategy, highly ordered, uniform nanowire arrays with precise dimension control can be achieved to simultaneously deliver thousands of nanolasers with multiple wavelengths on the same substrate, paving a promising and scalable pathway towards future advanced optoelectronic and photonic systems.

Intrinsic performance limits of extremely scaled field-effect transistors based on MX2 (M = {Zr, Hf}, X = {S, Se}) nanoribbons

We investigate the MX2 (M = {Hf, Zr}, X = {S, Se}) transition metal dichalcogenides patterned into armchair (AC) and zigzag (ZZ) nanoribbons (NRs) as potential channel materials in future logic field-effect devices. Ab initio quantum transport simulations are employed to assess the electronic, transport, and ballistic field-effect transistor (FET) properties of devices with such quasi-one-dimensional channels. We report a non-monotonic scaling behavior of MX2NR properties due to strong quantum confinement effects, which is reflected in a strong dependence of the ON-state current (ION) of MX2NR FETs on the nanoribbon configuration. The ∼2 nm-wide HfSe2 and ZrSe2 AC-PFETs have the highest ION of up to 2.6 mA/μm at 10 nA/μm OFF-state current. Surprisingly, MX2NR ZZ-NFETs exhibit a current increase of up to 70% when channel width is scaled down, with ION reaching 2.2 mA/μm in ∼2 nm-wide devices. The high ON-state performance is a direct consequence of high carrier injection velocity, which is explained by analyzing the band structure, transmission, and density of states. We demonstrate that nanostructured MX2 materials can be promising candidates for future logic transistors based on multi-nanowire architectures.

Highly efficient AlGaN-based deep-ultraviolet light-emitting diodes: from bandgap engineering to device craft

AlGaN-based light-emitting diodes (LEDs) operating in the deep-ultraviolet (DUV) spectral range (210–280 nm) have demonstrated potential applications in physical sterilization. However, the poor external quantum efficiency (EQE) hinders further advances in the emission performance of AlGaN-based DUV LEDs. Here, we demonstrate the performance of 270-nm AlGaN-based DUV LEDs beyond the state-of-the-art by exploiting the innovative combination of bandgap engineering and device craft. By adopting tailored multiple quantum wells (MQWs), a reflective Al reflector, a low-optical-loss tunneling junction (TJ) and a dielectric SiO2 insertion structure (IS-SiO2), outstanding light output powers (LOPs) of 140.1 mW are achieved in our DUV LEDs at 850 mA. The EQEs of our DUV LEDs are 4.5 times greater than those of their conventional counterparts. This comprehensive approach overcomes the major difficulties commonly faced in the pursuit of high-performance AlGaN-based DUV LEDs, such as strong quantum-confined Stark effect (QCSE), severe optical absorption in the p-electrode/ohmic contact layer and poor transverse magnetic (TM)-polarized light extraction. Furthermore, the on-wafer electroluminescence characterization validated the scalability of our DUV LEDs to larger production scales. Our work is promising for the development of highly efficient AlGaN-based DUV LEDs.

Size dependent dual functionality of CeO2 quantum dots: A correlation among parameters for hydrogen gas sensor and pollutant remediation

The metal oxide-based nanostructures of variable size and shape are found effective in optimizing the gas sensing ability and pollutant degradation. The size induced lattice strain and large band gap in 3nm CeO2 quantum dots evolved the ability towards hydrogen gas sensing and dye degradation compared to nanopebbles and nanoparticles of sizes 15 ± 3, and 30 ± 12 nm. The smaller CeO2 quantum dots than Debye length was found underlying reason for nearly four times sensor response and selectivity towards reducing hydrogen gases than the oxidizing gases at 1–10 ppm level. The lattice strain calculated by Rietveld refinement and W–H analysis was found in-line with the size of CeO2 nanostructures. The enhancement in lattice strain and optical band gap (2.66, 2.78, and 2.89 eV) with decrease in size are found critical for determining the overall efficiency of CeO2 nanostructures for photocatalytic activity, attributed to the strong quantum confinement effect. The higher catalytic activity of 98 % was achieved CeO2 quantum dots in comparison to the 95 % and 94 % obtained for CeO2 nanopebbles and nanoparticles. The impact of change in degradation efficacy and gas sensing ability of different CeO2 nanomaterials is discussed in detail. This work offers a novel and simplistic method to produce CeO2 quantum dots as an efficient sensor for selective detection of H2 gas and photocatalyst. The correlation between size, Debye length, band gap, and lattice strain gives an insight for understanding the underlying detection mechanism for selective detection of reducing gas molecules and efficient pollutant remediation.

Ultrasmall 2D Sn‐Doped MAPbBr3 Nanoplatelets Enable Bright Pure‐Blue Emission

AbstractSynthesis of perovskites that exhibit pure‐blue emission with high photoluminescence quantum yield (PLQY) in both nanocrystal solutions and nanocrystal‐only films presents a significant challenge. In this work, a room‐temperature method is developed to synthesize ultrasmall, monodispersed, Sn‐doped methylammonium lead bromide (MAPb1‐xSnxBr3) perovskite nanoplatelets (NPLs) in which the strong quantum confinement effect endows pure blue emission (460 nm) and a high quantum yield (87%). Post‐treatment using n‐hexylammonium bromide (HABr) repaired surface defects and thus substantially increased the stability and PLQY (80%) of the NPL films. Concurrently, high‐precision patterned films (200‐µm linewidth) are successfully fabricated by using cost‐effective spray‐coating technology. This research provides a novel perspective for the preparation of high PLQY, highly stable, and easily processable perovskite nanomaterials.

Heterointerface Effects on Carrier Dynamics in Colloidal Quantum Dots and Their Application to Light-Emitting Diodes.

Colloidal quantum dots (QDs) are promising candidates for next-generation display technology because of their unique optical properties and have already appeared in the market as a high-end product. On the basis of their extraordinary properties, QD emissions with a given chemical composition can be tailored in a wide spectral window due to quantum size effects, which constitutes a key advantage of QDs in the display field. Specifically, investigations of structure-dependent and composition-dependent characterizations outside the quantum confinement effect have become an important part of practical applications. Therefore, from the perspective of designing nanostructures with well-defined heterointerfaces, strong quantum confinement effects with effective carrier confinement are desirable. Our results show that the photoluminescence (PL) intensity of CdSe/CdZnS core-shell QDs was enhanced 5.7 times compared with that of the CdSe core QDs. Supplementary analytical techniques involving transmission electron microscopy revealed the heterointerface configuration and composition distribution of the core and shell materials. The effects of the heterointerface on carrier dynamics in core-shell QDs were revealed by monitoring wavelength-dependent time-resolved PL. To further develop the QD light-emitting diodes (QD-LEDs), we produced an all-solution processed inverted QD-LEDs using CdSe/CdZnS core-shell QDs as the emitter. The electroluminescence spectrum of deep-red emissive QD-LEDs with CIE chromaticity coordinates of (0.68, 0.32) exhibited a peak at 638 nm.

Wet Chemical Synthesis of Ultrathin γ-Ga2O3 Quantum Wires Enabling Far-UVC Photodetection with Ultrahigh Selectivity and Sensitivity.

As compared to solar-blind ultraviolet (UV) photodetectors (PDs), far-UVC PDs not only show some irreplaceable advantages but also are more challenging to be developed. To solve this challenge, we report herein a soft template-assisted solvothermal route to synthesize ultrathin γ-Ga2O3 quantum wires (UQWs) with diameters down to 1-2 nm. These UQWs all exhibit a cluster-like absorption feature with a strong peak located between 190 and 230 nm and an edge below 250 nm, allowing highly selective absorption to far-UVC light. Notably, their normalized absorption coefficients were experimentally and theoretically confirmed to increase obviously with decreasing their diameters. Self-powered photoelectrochemical-type PDs based on Ga2O3 QWs of 1.7 nm diameter were therefore fabricated, exhibiting an excellent far-UVC detection performance with an unprecedented ultrahigh spectral selectivity (R210 nm/R250 nm = 452). As a proof of concept, this paper offers a new idea for developing ultrawide bandgap semiconductor materials and devices by leveraging a strong quantum confinement effect.

Klein tunneling degradation and enhanced Fabry-Pérot interference in graphene/h-BN moiré-superlattice devices

Hexagonal boron-nitride (h-BN) provides an ideal substrate for supporting graphene devices to achieve fascinating transport properties, such as Klein tunneling, electron optics and other novel quantum transport phenomena. However, depositing graphene on h-BN creates moiré superlattices, whose electronic properties can be significantly manipulated by controlling the lattice alignment between layers. In this work, the effects of these moiré structures on the transport properties of graphene are investigated using atomistic simulations. At large misalignment angles (leading to small moiré cells), the transport properties (most remarkably, Klein tunneling) of pristine graphene devices are conserved. On the other hand, in the nearly aligned cases, the moiré interaction induces stronger effects, significantly affecting electron transport in graphene. In particular, Klein tunneling is significantly degraded. In contrast, strong Fabry-Pérot interference (accordingly, strong quantum confinement) effects and non-linear I-V characteristics are observed. P-N interface smoothness engineering is also considered, suggesting as a potential way to improve these transport features in graphene/h-BN devices.

Observation of enhanced WSe2 exciton–exciton annihilation in WSe2/Gr/hBN heterostructure

Due to strong quantum confinement effects and novel physical properties, two-dimensional transition metal dichalcogenides (TMDCs) as well as their heterostructures provide an attractive platform for studying excitonic effects and many-body interactions. However, manipulation on the excitonic effect in TMDCs remains challenge owing to the complex interplay of various factors. In this Letter, we report large exciton peak redshift and enhanced exciton–exciton annihilation in WSe2/Gr/hBN heterostructures investigated with static and transient optical spectroscopy. The pronounced redshift of exciton energy in the triple layer heterostructure arises from the charge transfer effect between graphene and WSe2, which leads to the reduction of the WSe2 exciton binding energy significantly due to the Coulomb screening effect. As a result, the reduced exciton binding energy increases the exciton delocalization in the WSe2 layer, leading to an increased probability of exciton–exciton collisions, which results in fast exciton annihilation rate. This study demonstrates the impact of graphene layer on exciton energy as well as the relaxation dynamics in WSe2/Gr/hBN heterostructures, which provides insights into the understanding of quasiparticle physics and many-body interactions in 2D materials.

Size-dependent linear and nonlinear optical responses of silicon clusters.

Owing to strong quantum confinement effects and no periodic constraints, the geometric and electronic structures of silicon clusters differ from those of crystalline silicon. Although previous studies have elucidated the optical properties of silicon clusters, some issues remain unresolved. To address these, this study examined the size-dependent linear and nonlinear optical responses of silicon clusters through first-principles calculations. Silicon clusters exhibited lone-pair-electron-dominated optical response behaviors. With the investigated size range, the orientationally average polarizability (αave) and second-order hyperpolarizability (γave) increased with cluster size. However, αave and γave per atom exhibited no evident size-dependent trends owing to co-modulation of the lone-pair-number-to-atomic-number ratio and geometry. αave and γave were notably sensitive to the nuclear binding strength of lone-pair electrons. Thus, the nonlinear optical effects of silicon clusters are superior to those of phosphorus and sulfur clusters. This investigation offers valuable insights into the optical responses of atomic-precision clusters.

Chemical activity, oxidation resistance and infrared transparency of (Ti3C2)n quantum dots induced by quantum confinement effect

MXene-derived quantum dots (QDs) not only inherit excellent physicochemical properties from its parement bulk phase, but also exhibit a unique set of optical and electronic properties. Herein, the electronic and optical properties of (Ti3C2)n QDs are systematically investigated using density function theory. It is found that binding energy and hardness gradually go up with the decrease of the size of (Ti3C2)n QDs, indicating that (Ti3C2)n QDs are unstable. The origin of the reduced stability is the increase of the proportion of unsaturated atoms. In contrast, the activity of (Ti3C2)n QDs, i.e., averaged energy of d-states of Ti atom (〈εd〉) and averaged energy of p-states of C atom (〈εp〉), is linearly correlated with their coordination numbers. However, VIP and VEA analyses show that (Ti3C2)n QDs are more easier to obtain electrons, suggesting that the oxidation properties of (Ti3C2)n QDs are gradually improved. It is found that HOMO-LUMO gap gradually increases as the size of (Ti3C2)n QDs decreases, indicating strong quantum confinement effect. Finally, it is observed that quantum confinement effect has significant influence on the optical properties of (Ti3C2)n QDs. For example, the spectra exhibit discrete sharp spectral peaks, absorption threshold blue shift, low reflectivity and low absorption coefficient etc., as the size decreases. The spectrum peaks in high energy region are mainly determined by the inter-band transition. These findings will be useful to get an insight into designing functionalized quantum dots based on Ti3C2.

(Invited) Interlayer Excitons in Two-Dimensional Perovskite/Monolayer Transition Metal Dichalcogenide Heterostructures

The emergence of two-dimensional (2D) materials has intrigued a great deal of research on novel physical phenomena and various functional applications due to their particular crystal structures and reduced dimensionality. Unlike 3D materials, bulks of 2D materials can be easily thinned to atomic thickness by mechanical exfoliation and atomically thin 2D materials can be arbitrarily stacked to form vertical van der Waals (vdW) heterostructures, which could inherit the unique characteristics of the constituent layers and even exhibit new properties not possessed by them. Particularly, when type-Ⅱ vdW heterostructures are excited, the positive and the negative charges would reside in the different layers after the charge transfer but be limited within a short distance due to the strong quantum confinement effect of 2D materials, leading to strong Coulomb interaction and the formation of interlayer excitons (IXs). IXs are generally equipped with orientated dipole moment and long lifetime, making them ideal media for the future interconnects between optical transmission and electronic computation. Currently, studies on IXs are mainly focused on the vdW heterostructures formed by monolayers of transition-metal dichalcogenides (TMDs). Here, I will first talk about vdW heterostructures formed by 2D perovskites and TMDs for studying IXs. Stacking different kinds of 2D perovskites and TMDs, formation of IXs is confirmed by excitation-power-, temperature-, electric-field-dependent and time-resolved photoluminescence (PL) studies. Notably, robust IX emission can be observed regardless of the stacking sequence and geometric alignment of the constituent layers, showing great advantages over the TMD/TMD vdW heterostructures, which require special twist-angle and thermal annealing. Then, I would like to give a brief introduction on widely tuning the IX emission energy by changing the layer number of the 2D perovskite or the TMD, which shed light on the application of 2D perovskite/TMD vdW heterostructures in broad-spectrum optoelectronics. Furthermore, I would like to next talk about how the selection of organic chains in 2D perovskites influences the properties of the IXs. By using chiral 2D perovskites, the IX emission shows substantial circular polarization and the polarization direction is only related to the chirality of the molecules regardless of the excitation scheme or any other external field, which could open up new passages for controlling valley- or spin-polarization of IXs. By introducing molecules with different dielectric constants, the dielectric screening strength in the vdW heterostructure is changed and hence the binding energy of the IXs is also modified, which offers great opportunities for exploiting tunable excitonic devices and studying exciton condensation. Finally, I will also introduce IX as a non-destructive tool for probing the local phase transition at the surface of the 2D perovskite (BA)2PbI4 flakes. By spatially PL mapping of the (BA)2PbI4/WSe2 heterostructure, two different IX species can be observed to distinguish the low-temperature and the high-temperature phase respectively.

Designed Polymer Ligands for Perovskite Quantum Dots and Their Block Copolymer Nanocomposites

AbstractPerovskite quantum dots (QDs) have efficient optical absorption and emission in the visible range, and show a strong quantum confinement effect and high external quantum efficiency. They have been at the forefront of next‐generation photovoltaics and optoelectronics applications. However, two major challenges associated with perovskites and their nanomaterials are poor stability (such as against moisture and polar solvents), as well as the lack of efficient nanopatterning methods. In this work, a promising approach is provided to address both of those major challenges by molecular engineering and integration of QDs with block copolymers (BCP). The BCP thermoplastic elastomers not only substantially improve the stability of perovskite QDs by encapsulating them in a highly stable and soft matrix, but also enable molecular‐level control of the alignment and assembly of perovskite QDs in the microphase‐separated BCP matrix. It is demonstrated that designing and synthesis of compatible polymer ligands for perovskite QDs is key to enabling their selective and strong interaction with the BCP matrix. The structure and molecular weight of the BCP also play an important role in the interfacial structure and optical properties of the QDs‐BCP nanocomposites. Such soft and flexible optical nanocomposites have potential applications in flexible optoelectronics, optical storage, and displays.

Corrosion-induced growth control of lead halide perovskite nanoparticles

Ultrasmall CsPbBr3 perovskite nanoparticles (NPs) have gained great attention as a potential blue-cyan emissive material due to their strong quantum confinement effects. However, the preparation of these ultrasmall perovskite NPs remains a challenging task due to their high ionicity and low formation energy. In this study, we propose and demonstrate a corrosion-growth synthesis strategy to achieve size-adjustable, highly-confined CsPbBr3 NPs. Initially, we obtain seed nanoplates (NPLs) through a controlled nucleation process. Subsequently, ethylenediamine (EDA), serving as a polar solvent, controls the corrosion process of unstable NPLs. Interestingly, these corroded NPLs then act as monomers, supplying for the growth of the remaining, stable NPLs. Concurrently, these robust NPLs undergo passivation via the chelation of EDA. By increasing the dosage of EDA, the three dimensions of CsPbBr3 exhibit distinct growth patterns, leading to shape transformation. During the entire process, the absorption and photoluminescence (PL) of the NPLs are predominantly influenced by their thickness, which continues to grow through a modified Ostwald ripening process. Consequently, we achieve thickness-controlled NPs whose PL wavelength can be adjusted from 435 nm to 510 nm, with steps as fine as 1 nm. These NPs exhibit remarkable stability and high PL quantum yield (QYs) due to the passivation by EDA and the strong Mn-ligand interaction.

Phase distribution management for high-efficiency and bright blue perovskite light-emitting diodes

Quasi-two-dimensional (quasi-2D) perovskite materials have attracted significant attention for application in light-emitting diodes (PeLEDs) due to their unique optical characteristics and extraordinary performance. The inherent multi-quantum well structure will generate a strong quantum confinement effect which is beneficial for the blue emission. However, the efficiency and stability of the quasi-2D blue PeLEDs lag behind their red and green counterparts, which prevents the further commercial application of the PeLEDs. The performance of quasi-2D blue PeLEDs was limited on account of the inefficient phase distribution management, which causes an inefficient energy transfer and severe non-radiative recombination. Herein, we employ guanidine thiocyanate (GASCN) as the pre-deposited film at the bottom of the perovskite film to manage the phase distribution of the PBA2Csn−1PbnBr3n+1 quasi-2D perovskite film (where n is the number of [PbBr6]4- sheets, and PBA is phenylbutylammonium). The pre-deposited GASCN can not only inhibit the small n-phase (i.e., n = 1: PBA2PbBr4, and n = 2: PBA2CsPb2Br7) but also avoid the undesired emission redshift from the over-growth of the large n-phase as well as passivate defects of the quasi-2D perovskite film, which accelerates energy transfer efficiently, strengthens the carrier transport, and enhances the luminous efficiency. As a result, the optimized device demonstrated the highest external quantum efficiency (EQE) of 16.40% and a maximum luminance of 8290 cd m−2. This strategy provides a new pathway to accurately manage phase distribution of quasi-2D perovskite achieving the high efficiency in blue PeLEDs.

A New Metal-Free Molecular Perovskite-Related Single Crystal with Quantum Wire Structure for High-Performance X-Ray Detection.

The family of metal-free molecular perovskites, an emerging novel class of eco-friendly semiconductor, welcomes a new member with a unique 1D hexagonal perovskite structure. Lowering dimensionality at molecular level is a facile strategy for crystal structure conversion, optoelectronic property regulation, and device performance optimization. Herein, the study reports the design, synthesis, packing structure, and photophysical properties of the 1D metal-free molecular perovskite-related single crystal, rac-3APD-NH4I3(rac-3APD= racemic-3-Aminopiperidinium), that features a quantum wire structure formed by infinite chains of face-sharing NH4I6 octahedra, enabling strong quantum confinement with strongly self-trapped excited (STE) states to give efficient warm orange emission with a photoluminescence quantum yield (PLQY) as high as ≈41.6%. The study accordingly unveils its photoexcited carrier dynamics: rac-3APD-NH4I3 relaxes to STE state with a short lifetime of 10ps but decays to ground state by emitting photons with a relatively longer lifetime of 560ps. Additionally, strong quantum confinement effect is conducive to charge transport along the octahedral channels that enables the co-planar single-crystal X-ray detectors to achieve a sensitivity as high as 1556 µC Gyair -1 cm-2. This work demonstrates the first case of photoluminescence mechanism and photophysical dynamics of 1D metal-free perovskite-related semiconductor, as well as the promise for high-performance X-ray detector.

Distinguishing metal halide molecular clusters from perovskite magic sized clusters in the synthesis of metal halide perovskite nanostructures

AbstractBackgroundResearch into perovskite nanocrystals (PNCs) has uncovered interesting properties compared to their bulk counterparts, including tunable optical properties due to size‐dependent quantum confinement effect (QCE). More recently, smaller PNCs with even stronger QCE have been discovered, such as perovskite magic sized clusters (PMSCs) and ligand passivated PbX2 metal halide molecular clusters (MHMCs) analogous to perovskites.ObjectiveThis review aims to present recent data comparing and contrasting the optical and structural properties of PQDs, PMSCs, and MHMCs, where CsPbBr3 PQDs have first excitonic absorption around 520 nm, the corresponding PMSCS have absorption around 420 nm, and ligand passivated MHMCs absorb around 400 nm.ResultsCompared to normal perovskite quantum dots (PQDs), these clusters exhibit both a much bluer optical absorption and emission and larger surface‐to‐volume (S/V) ratio. Due to their larger S/V ratio, the clusters tend to have more surface defects that require more effective passivation for stability.ConclusionRecent study of novel clusters has led to better understanding of their properties. The sharper optical bands of clusters indicate relatively narrow or single size distribution, which, in conjunction with their blue absorption and emission, makes them potentially attractive for applications in fields such as blue single photon emission.

Tungsten oxide nanostructures peculiarity and photocatalytic activity for the efficient elimination of the organic pollutant.

The WO3 nanostructures were synthesized by a simple hydrothermal route in the presence of C14TAB and gemini-based twin-tail surfactant. The impact of using these special shape and size directing agents for the synthesis of nanostructures was observed in the form of different shapes and sizes. The WO3 web of chains type nanostructure was obtained using C14TAB in comparison to the cube-shaped nanoparticles through twin-tail surfactant. On contrary, the twin-tail surfactant provides sustainable and controlled growth of cube shape nanoparticles of size ~ 15nm nearly half of the size ~ 35nm obtained using conventional surfactant C14TAB, respectively. For the detailed structural features, the Williamson-Hall analysis method was implemented to find out the crystalline size and lattice strain of the prepared nanostructures. Owing to the strong quantum confinement effect, the WO3 cube-shaped nanoparticles with an optical band gap of 2.69eV of the prepared nanoparticles showed excellent photocatalytic efficacy toward organic pollutant (fast green FCF) compared to the web of chain nanostructures with an optical band gap of 2.66eV. The ability of the prepared systems to decompose the organic pollutant (fast green FCF) in water was tested under visible light irradiations. The percentage degradation was found to be 94% and 86% for WO3 cube-shaped nanoparticles and WO3 web of chains, respectively. The simplicity of the fabrication method and the high photocatalytic performance of the systems can be promising in environmental applications to treat water pollution.

Surface-Stabilized CsPbI3 Nanocrystals with Tailored Organic Polymer Ligand Binding.

Perovskite nanocrystals (NCs) exhibit attractive photophysical properties by combining the excellent optoelectronic properties of bulk perovskites with the strong quantum confinement effect at the nanoscale. However, CsPbI3 NCs easily transform into a non-perovskite phase because of the ionic lattice and dynamic ligand binding. Herein, stable black-phase CsPbI3 NCs capped with a new organic ligand, HO-PS-N3 (HOPS), which consists of a polystyrene segment with hydroxyl and azide end groups, are reported. This organic polymer ligand passivated the surface defects and enhanced the stability of CsPbI3 NCs by exposing the linking hydrophobic polystyrene segment. Consequently, the optimized CsPbI3 NCs exhibit significantly improved resistance to moisture or light and maintained 70 % of the original luminous intensity after immersion in water for two months. The theoretical results revealed that the binding energy of the HOPS ligand on the surface of the CsPbI3 NCs is higher than that of the commonly used oleic acid, alleviating the defects-induced degradation during purification. Thus, surface-stabilized CsPbI3 NCs are beneficial for a broad range of optoelectronic applications.

Two-dimensional metal halide perovskites and their heterostructures: from synthesis to applications

Abstract Size- and shape-dependent unique properties of the metal halide perovskite nanocrystals make them promising building blocks for constructing various electronic and optoelectronic devices. These unique properties together with their easy colloidal synthesis render them efficient nanoscale functional components for multiple applications ranging from light emission devices to energy conversion and storage devices. Recently, two-dimensional (2D) metal halide perovskites in the form of nanosheets (NSs) or nanoplatelets (NPls) are being intensively studied due to their promising 2D geometry which is more compatible with the conventional electronic and optoelectronic device structures where film-like components are usually employed. In particular, 2D perovskites exhibit unique thickness-dependent properties due to the strong quantum confinement effect, while enabling the bandgap tuning in a wide spectral range. In this review the synthesis procedures of 2D perovskite nanostructures will be summarized, while the application-related properties together with the corresponding applications will be extensively discussed. In addition, perovskite nanocrystals/2D material heterostructures will be reviewed in detail. Finally, the wide application range of the 2D perovskite-based structures developed to date, including pure perovskites and their heterostructures, will be presented while the improved synergetic properties of the multifunctional materials will be discussed in a comprehensive way.