The Perspective focuses on the investigation of an unresolved conflict in semiconductor colloidal quantum dots (CQDs) research, concerning the influence of the immediate surrounding on the optical ...properties of the materials. Today’s advanced synthetic colloidal procedures offer formation of a high-quality inorganic crystallite, capped with various organic/inorganic molecular ligands. The Perspective aims to clarify whether exciton recombination processes in CQDs are influenced by the type of crystallite–ligand bonding and, moreover, whether these excitonic processes experience direct coupling to the ligands’ vibrational modes. Most ligands used have redox characteristics whose functional groups are added on to the CQDs’ surface via coordination, covalent or ionic bonding. The surface–ligand bonding introduces electronic states either above or below the intraband/interband energy gap, resulting in electronic passivation or in creation of trapping states that affect intraband and interband relaxation processes. Furthermore, crystalline electronic states may have a direct coupling to molecular vibrational states via direct overlap of electronic wave functions or through a long-range energy-transfer process. Also, photoejected carriers resulting from an Auger process or ionization processes may diffuse temporarily onto a ligand site. These scenarios are discussed in the current publication with supporting theoretical and experimental observations.
Building devices from colloidal quantum dots Kagan, Cherie R.; Lifshitz, Efrat; Sargent, Edward H. ...
Science (American Association for the Advancement of Science),
08/2016, Letnik:
353, Številka:
6302
Journal Article
Recenzirano
The continued growth of mobile and interactive computing requires devices manufactured with low-cost processes, compatible with large-area and flexible form factors, and with additional ...functionality. We review recent advances in the design of electronic and optoelectronic devices that use colloidal semiconductor quantum dots (QDs). The properties of materials assembled of QDs may be tailored not only by the atomic composition but also by the size, shape, and surface functionalization of the individual QDs and by the communication among these QDs. The chemical and physical properties of QD surfaces and the interfaces in QD devices are of particular importance, and these enable the solution-based fabrication of low-cost, large-area, flexible, and functional devices. We discuss challenges that must be addressed in the move to solution-processed functional optoelectronic nanomaterials.
Lead halide perovskites show excellent optoelectronic properties but are unsatisfactory in terms of stability and toxicity. Herein, bismuth (Bi)‐doped lead‐free inorganic perovskites Cs2SnCl6:Bi are ...reported as blue emissive phosphors. Upon Bi doping, the originally nonluminous Cs2SnCl6 exhibits a highly efficient deep‐blue emission at 455 nm, with a Stokes shift of 106 nm and a high photoluminescence quantum yield (PLQY) close to 80%. Hybrid density functional theory calculations suggest the preferred formation of BiSn+VCl defect complex, which is believed to be responsible for the optical absorption and the associated blue emission. The Cs2SnCl6:Bi also shows impressive thermal and water stability due to its inorganic nature and the formation of protective BiOCl layer. White light‐emitting diodes (LEDs) are constructed using Cs2SnCl6:Bi and commercial yellow phosphors combined with commercial UV LED chips, giving the Commission Internationale de I'Eclairage (CIE) color coordinates of (0.36, 0.37). This work represents a significant step toward the realization of highly efficient, stable, and environmentally benign next‐generation solid‐state lighting.
Upon Bi doping, the nonluminous Cs2SnCl6 exhibits a blue emission centered at 455 nm with a photoluminescence quantum yield close to 80%. Hybrid density functional theory calculations clarify that the electronic transition between the defect band consisting of the pseudoclosed Bi 6s2 orbitals hybridized with Cl 3p and the conduction band explains the defect absorption and the associated strong emission.
Scintillators are widely utilized for radiation detections in many fields, such as nondestructive inspection, medical imaging, and space exploration. Lead halide perovskite scintillators have ...recently received extensive research attention owing to their tunable emission wavelength, low detection limit, and ease of fabrication. However, the low light yields toward X‐ray irradiation and the lead toxicity of these perovskites severely restricts their practical application. A novel lead‐free halide is presented, namely Rb2CuBr3, as a scintillator with exceptionally high light yield. Rb2CuBr3 exhibits a 1D crystal structure and enjoys strong carrier confinement and near‐unity photoluminescence quantum yield (98.6%) in violet emission. The high photoluminescence quantum yield combined with negligible self‐absorption from self‐trapped exciton emission and strong X‐ray absorption capability enables a record high light yield of ≈91056 photons per MeV among perovskite and relative scintillators. Overall, Rb2CuBr3 provides nontoxicity, high radioluminescence intensity, and good stability, thus laying good foundations for potential application in low‐dose radiography.
A new lead‐free halide Rb2CuBr3 scintillator with 1D crystal structure is presented. It exhibits self‐trapped exciton emission with a large Stokes shift (0.91 eV). Thus, it has near‐unity photoluminescence quantum yield (98.6%) and a high radioluminescence light yield of ≈91 056 photons per MeV.
This study depicts the influence of the Rashba effect on the band-edge exciton processes in all-inorganic CsPbBr3 perovskite single colloidal nanocrystal (NC). The study is based on magneto-optical ...measurements carried out at cryogenic temperatures under various magnetic field strengths in which discrete excitonic transitions were detected by linearly and circularly polarized measurements. Interestingly, the experiments show a nonlinear energy splitting between polarized transitions versus magnetic field strength, indicating a crossover between a Rashba effect (at the lowest fields) to a Zeeman effect at fields above 4 T. We postulate that the Rashba effect emanates from a lattice distortion induced by the Cs+ motion degree of freedom or due to a surface effect in nanoscale NCs. The unusual magneto-optical properties shown here underscore the importance of the Rashba effect in the implementation of such perovskite materials in various optical and spin-based devices.
A theoretical study of the positive and negative trion channels in the nonradiative Auger recombination of band-edge biexcitons (BXs) in CdSe/CdS core/shell nanocrystals (NCs) is presented. The ...theory takes into account the BX fine-structure produced by NC asymmetry and hole−hole exchange interaction. The calculations show that growth of CdS shell upon CdSe core suppresses the rate of the Auger recombination via negative trion channel, while the more efficient Auger recombination via positive trion channel shows much weaker dependence on the shell thickness. The demonstrated oscillatory dependence of the BX Auger rate on the core and shell sizes is explained qualitatively in terms of overlap of the ground and excited carrier wave functions. The calculations show that raise of temperature accelerates the Auger recombination in CdSe/CdS NCs due to reduction of the bulk energy gaps of CdSe and CdS.
Tunable sources of X-ray radiation are widely used for imaging and spectroscopy in fundamental science, medicine and industry. The growing demand for highly tunable, high-brightness laboratory-scale ...X-ray sources motivates research into new fundamental mechanisms of X-ray generation. Here, we demonstrate the ability of van der Waals materials to serve as a platform for tunable X-ray generation when irradiated by moderately relativistic electrons available, for example, from a transmission electron microscope. The radiation spectrum can be precisely controlled by tuning the acceleration voltage of the incident electrons, as well as by our proposed approach: adjusting the lattice structure of the van der Waals material. We present experimental results for both methods, observing the energy tunability of X-ray radiation from the van der Waals materials WSe2, CrPS4, MnPS3, FePS3, CoPS3 and NiPS3. Our findings demonstrate the concept of material design at the atomic level, using van der Waals heterostructures and other atomic superlattices, for exploring novel phenomena of X-ray physics.Tunable X-ray generation, from ultrathin van der Waals materials impacted by relativistic electrons, is demonstrated.
Semiconductor colloidal quantum dots (CQDs) have attracted vast scientific and technological interest throughout the past three decades, due to the unique tuneability of their optoelectronic ...properties by variation of size and composition. However, the nanoscale size brings about a large surface-to-bulk volume ratio, where exterior surfaces have a pronounced influence on the chemical stability and on the physical properties of the semiconductor. Therefore, numerous approaches have been developed to gain efficient surface passivation, including a coverage by organic or inorganic molecular surfactants as well as the formation of core/shell heterostructures (a semiconductor core epitaxially covered by another semiconductor shell). This review focuses on special designs of core/shell heterostructures from the IV-VI and II-VI semiconductor compounds, and on synthetic approaches and characterization of the optical properties. Experimental observations revealed the formation of core/shell structures with type-I or quasi-type-II band alignment between the core and shell constituents. Theoretical calculations of the electronic band structures, which were also confirmed by experimental work, exposed surplus electronic tuning (beyond the radial diameter) with adaptation of the composition and control of the interface properties. The studies also considered strain effects that are created between two different semiconductors. It was disclosed experimentally and theoretically that the strain can be released
via
the formation of alloys at the core-shell interface. Overall, the core/shell and core/alloyed-shell heterostructures showed enhancement in luminescence quantum efficiency with respect to that of pure cores, extended lifetime, uniformity in size and in many cases good chemical sustainability under ambient conditions.
Core/shell heterostructures provide controlled optical properties, tuneable electronic structure, and chemical stability due to an appropriate interface design.
In this work, a novel dynamic cation exchange driven by an anion exchange procedure was used for incorporating Ni dopants into the Pb-site of CsPbBr3 and CsPb(BrCl)3 perovskite nanocrystals under ...ambient conditions. Ni doping has significant merits in tuning the optical properties and introducing magnetic functionality. Moreover, Ni doping provides chemical and photochemical stability to a host phase with consequent importance for solar cell applications. The present work describes a thorough investigation of the Ni incorporation mechanism via structural, compositional, and optical characterizations. The results indicated the essential need for cation–anion coexchange, enabling control of the Ni concentration from <1% to about 12%, with uniform distribution across the host lattice. In addition, the doping improved the photoluminescence quantum yields beyond those of the undoped nanocrystals. The observations were corroborated by the density functional theory (DFT), confirming that the incorporation of Ni is energetically favorable.
The full blossoming of quantum technologies requires the availability of easy-to-prepare materials where quantum coherences can be effectively initiated, controlled, and exploited, preferably at ...ambient conditions. Solid-state multilayers of colloidally grown quantum dots (QDs) are highly promising for this task because of the possibility of assembling networks of electronically coupled QDs through the modulation of sizes, inter-dot linkers, and distances. To usefully probe coherence in these materials, the dynamical characterization of their collective quantum mechanically coupled states is needed. Here, we explore by two-dimensional electronic spectroscopy the coherent dynamics of solid-state multilayers of electronically coupled colloidally grown CdSe QDs and complement it by detailed computations. The time evolution of a coherent superposition of states delocalized over more than one QD was captured at ambient conditions. We thus provide important evidence for inter-dot coherences in such solid-state materials, opening up new avenues for the effective application of these materials in quantum technologies.
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