Pressure-dependent optical behaviors of colloidal CdSe nanoplatelets

Ultrafast dynamics under high-pressure

High-pressure is a mechanical method to regulate the structure and internal interaction of materials. Therefore, observation of properties' change can be realized in a relatively pure environment. Furthermore, high-pressure affects the delocalization of wavefunction among materials' atoms and thus their dynamics process. Dynamics results are essential data for understanding the physical and chemical characteristics, which is valuable for materials application and development. Ultrafast spectroscopy is a powerful tool to investigate dynamics process and becoming a necessary characterization method for materials investigation. The combination of high-pressure with ultrafast spectroscopy in the nanocosecond∼femtosecond scale enables us to investigate the influence of the enhanced interaction between particles on the physical and chemical properties of materials, such as energy transfer, charge transfer, Auger recombination, etc. Base on this point of view, this review summarizes recent progress in the ultrafast dynamics under high-pressure for various materials, in which new phenomena and new mechanisms are observed. In this review, we describe in detail the principles ofin situhigh pressure ultrafast dynamics probing technology and its field of application. On this basis, the progress of the study of dynamic processes under high-pressure in different material systems is summarized. An outlook onin situhigh-pressure ultrafast dynamics research is also provided.

2D II-VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

The field of colloidal synthesis of semiconductors emerged 40 years ago and has reached a certain level of maturity thanks to the use of nanocrystals as phosphors in commercial displays. In particular, II-VI semiconductors based on cadmium, zinc, or mercury chalcogenides can now be synthesized with tailored shapes, composition by alloying, and even as nanocrystal heterostructures. Fifteen years ago, II-VI semiconductor nanoplatelets injected new ideas into this field. Indeed, despite the emergence of other promising semiconductors such as halide perovskites or 2D transition metal dichalcogenides, colloidal II-VI semiconductor nanoplatelets remain among the narrowest room-temperature emitters that can be synthesized over a wide spectral range, and they exhibit good material stability over time. Such nanoplatelets are scientifically and technologically interesting because they exhibit optical features and production advantages at the intersection of those expected from colloidal quantum dots and epitaxial quantum wells. In organic solvents, gram-scale syntheses can produce nanoparticles with the same thicknesses and optical properties without inhomogeneous broadening. In such nanoplatelets, quantum confinement is limited to one dimension, defined at the atomic scale, which allows them to be treated as quantum wells. In this review, we discuss the synthetic developments, spectroscopic properties, and applications of such nanoplatelets. Covering growth mechanisms, we explain how a thorough understanding of nanoplatelet growth has enabled the development of nanoplatelets and heterostructured nanoplatelets with multiple emission colors, spatially localized excitations, narrow emission, and high quantum yields over a wide spectral range. Moreover, nanoplatelets, with their large lateral extension and their thin short axis and low dielectric surroundings, can support one or several electron-hole pairs with large exciton binding energies. Thus, we also discuss how the relaxation processes and lifetime of the carriers and excitons are modified in nanoplatelets compared to both spherical quantum dots and epitaxial quantum wells. Finally, we explore how nanoplatelets, with their strong and narrow emission, can be considered as ideal candidates for pure-color light emitting diodes (LEDs), strong gain media for lasers, or for use in luminescent light concentrators.

Tailoring Optical Properties of Luminescent Semiconducting Nanocrystals through Hydrostatic, Anisotropic Static, and Dynamic Pressures

Luminescent semiconductor nanocrystals are a fascinating class of materials because of their size-dependent emissions. Numerous past studies have demonstrated that semiconductor nanoparticles with radii smaller than their Bohr radius experience quantum confinement and thus size-dependent emissions. Exerting pressure on these nanoparticles represents an additional, more dynamic, strategy to alter their size and shift their emission. The application of pressure results in the lattices becoming strained and the electronic structure altered. In this Minireview, colloidal semiconductor nanocrystals are first introduced. The effects of uniform hydrostatic pressure on the optical properties of metal halide perovskite (ABX₃ ), II-VI, III-V, and IV-VI semiconductor nanocrystals are then examined. The optical properties of semiconductor nanocrystals under static and dynamic anisotropic pressure are then summarized. Finally, future research directions and applications utilizing the pressure-dependent optical properties of semiconductor nanocrystals are discussed.

Piezochromic luminescence in all-inorganic core-shell InP/ZnS nanocrystals via pressure-modulated strain engineering

Piezochromic materials alter their photoluminescent (PL) colors in response to the action of external force. Such materials have attracted much attention owing to their promising applications in pressure-sensing, optoelectronic memory and anticounterfeiting. However, almost all the reported piezochromic materials were limited to the organic matters or compounds containing organic components. Here we present piezochromic materials and pressure-induced optical response based on all-inorganic core/shell InP/ZnS nanocrystals (NCs). The InP/ZnS NCs exhibit noticeable PL color changes, shifting from orange (2.08 eV) to green (2.25 eV), with the PL intensity showing slight enhancement below an applied pressure of 2.5 GPa. Further compressing to fluorescence quenching produces an ultrabroad energy tenability range up to 400 meV. Structural and time-resolved PL lifetime studies, together with first-principle calculations, reveal the weakening of strain-induced defect states in the low pressure regime, which contributes to effective excition recombination, thus ensuring high fluorescence emission of InP/ZnS NCs. This work provides a promising strategy to prepare piezochromic materials of all-inorganic semiconductors, thereby greatly increasing the choice of materials for new applications.

Pressure Induced Nanoparticle Phase Behavior, Property, and Applications

Nanoparticle (NP) high pressure behavior has been extensively studied over the years. In this review, we summarize recent progress on the studies of pressure induced NP phase behavior, property, and applications. This review starts with a brief overview of high pressure characterization techniques, coupled with synchrotron X-ray scattering, Raman, fluorescence, and absorption. Then, we survey the pressure induced phase transition of NP atomic crystal structure including size dependent phase transition, amorphization, and threshold pressures using several typical NP material systems as examples. Next, we discuss the pressure induced phase transition of NP mesoscale structures including topics on pressure induced interparticle separation distance, NP coupling, and NP coalescence. Pressure induced new properties and applications in different NP systems are highlighted. Finally, outlooks with future directions are discussed.

Semiconductor Nanoplatelet Excimers

Excimers, a portmanteau of "excited dimer", are transient species that are formed from the electronic interaction of a fluorophore in the excited state with a neighbor in the ground state, which have found extensive use as laser gain media. Although common in molecular fluorophores, this work presents evidence for the formation of excimers in a new class of materials: atomically precise two-dimensional semiconductor nanoplatelets. Colloidal nanoplatelets of CdSe display two-color photoluminescence resolved at low temperatures with one band attributed to band-edge fluorescence and a second, red band attributed to excimer fluorescence. Previously reasonable explanations for two-color fluorescence, such as charging, are shown to be inconsistent with additional evidence. As with excimers in other materials systems, excimer emission is increased by increasing nanoplatelet concentration and the degree of cofacial stacking. Consistent with their promise as low-threshold gain media, amplified spontaneous emission emerges from the excimer emission line.

Pressure induced photoluminescence modulation in a wide range and synthesis of monodispersed ternary AgCuS nanocrystal based on Ag2S nanocrystals

Binary Ag₂S nanocrystals (NCs) have many potential optical applications because of their low toxicity, narrow direct band gaps and near-infrared photoluminescence (PL) with high emission efficiency. However, due to its small exciton Bohr radius (2.2 nm), the PL spectra of Ag₂S NCs can only be modulated below ∼1200 nm with increasing particle size. Meanwhile, ternary silver copper chalcogenides (AgCuX, X = S, Se) have also attracted increased attention in recent years. Temperature-dependent structural phase transformation leads electrical transport to exhibit fascinating transitions between p and n type conduction, which makes AgCuS and AgCuSe ideal materials for diode or transistor devices. Nevertheless, the traditional method to synthesize these materials is mainly through melting the mixture of Ag, Cu and S/Se powder under extremely high reaction temperatures (973-1373 K) and long reaction time, forming a bulk product. Therefore, the synthesis of high quality monodispersed and size-tunable AgCuS or AgCuSe NCs is still a challenge. To address these issues, in this paper, we report using Ag₂S NCs as a template, a method to synthesize monodispersed and size-tunable β-AgCuS NCs via ion exchange and diffusion processes. Similarly, monodispersed β-AgCuSe NCs were also synthesized by this simple and reproducible strategy. This synthetic method opens new avenues for investigating the size-, morphology- and temperature-dependent phase transitions of these ternary AgCuS and AgCuSe materials. Thus, the corresponding electrical transport between p and n type conduction can be studied in the future. Furthermore, we systematically investigated the pressure-dependent PL properties and band gap modulation of monodispersed Ag₂S NCs using in situ high pressure PL and absorption spectroscopy. We found that the PL peak of 6.0 nm for Ag₂S NCs could be easily adjusted from ∼1200 to 1900 nm with increasing pressure from 0 to 5.1 GPa, which greatly extends the wavelength range of the PL peak beyond that of other approaches.

High-Pressure-Induced Comminution and Recrystallization of CH3 NH3 PbBr3 Nanocrystals as Large Thin Nanoplates

High pressure (HP) can drive the direct sintering of nanoparticle assemblies for Ag/Au, CdSe/PbS nanocrystals (NCs). Instead of direct sintering for the conventional nanocrystals, this study experimentally observes for the first time high-pressure-induced comminution and recrystallization of organic-inorganic hybrid perovskite nanocrystals into highly luminescent nanoplates with a shorter carrier lifetime. Such novel pressure response is attributed to the unique structural nature of hybrid perovskites under high pressure: during the drastic cubic-orthorhombic structural transformation at ≈2 GPa, (301) the crystal plane fully occupied by organic molecules possesses a higher surface energy, triggering the comminution of nanocrystals into nanoslices along such crystal plane. Beyond bulk perovskites, in which pressure-induced modifications on crystal structures and functional properties will disappear after pressure release, the pressure-formed variants, i.e., large (≈100 nm) and thin (<10 nm) perovskite nanoplates, are retained and these exhibit simultaneous photoluminescence emission enhancing (a 15-fold enhancement in the photoluminescence) and carrier lifetime shortening (from ≈18.3 ± 0.8 to ≈7.6 ± 0.5 ns) after releasing of pressure from 11 GPa. This pressure-induced comminution of hybrid perovskite NCs and a subsequent amorphization-recrystallization treatment offer the possibilities of engineering the advanced hybrid perovskites with specific properties.

Synthesis of Quantum Dot-ZnS Nanosheet Inorganic Assembly with Low Thermal Fluorescent Quenching for LED Application

In this report, to tackle the thermal fluorescent quenching issue of II-VI semiconductor quantum dots (QDs), which hinders their on-chip packaging application to light-emitting diodes (LEDs), a QD-ZnS nanosheet inorganic assembly monolith (QD-ZnS NIAM) is developed through chemisorption of QDs on the surface of two-dimensional (2D) ZnS nanosheets and subsequent assembly of the nanosheets into a compact inorganic monolith. The QD-ZnS NIAM could reduce the thermal fluorescent quenching of QDs effectively, possibly due to fewer thermally induced permanent trap states and decreased Förster resonance energy transfer (FRET) among QDs when compared with those in a reference QD composite thin film. We have demonstrated that the QD-ZnS NIAM enables QDs to be directly packaged on-chip in LEDs with over 90% of their initial luminance being retained at above 85 °C, showing advantage in LED application in comparison with conventional QD composite film.

Environmental effects on the natural vibrations of nanoplatelets: a high pressure study

Resonant acoustic modes from ultrathin CdS colloidal nanoplatelets (NPLs) are probed under high pressure using low frequency Raman spectroscopy. In particular we focus on the characterization of the recently evidenced mass load effect that is responsible for a significant downshift of the NPL breathing frequency due to the inert mass of organic ligands. We show that a key parameter in the observation of the mass effect is whether the surrounding medium is able to support THz acoustic wave propagation, at a frequency close to that of the inorganic vibrating core. At low pressures, surface organic molecules show a single particle-like behavior and a strong mass effect is observed. Upon pressure loading the ligands are compacted together with the surrounding medium and slowly turned into a solid medium that supports THz acoustic phonons. We observe a continuous transition towards a fully embedded NPL with a frequency close to that of a freely vibrating slab and a progressive loss of the mass effect. The quality factor of the detected vibration significantly decreases as a result of the appearance of a "phonon-like" behavior of the environment at the origin of damping and energy dissipation.