We report a simple, solution-based, postsynthetic colloidal, atomic layer deposition (PS-cALD) process to engineer stepwise the surface stoichiometry and therefore the electronic properties of lead ...chalcogenide nanocrystal (NC) thin films integrated in devices. We found that unlike chalcogen-enriched NC surfaces that are structurally, optically, and electronically unstable, lead chloride treatment creates a well-passivated shell that stabilizes the NCs. Using PS-cALD of lead chalcogenide NC thin films we demonstrate high electron field-effect mobilities of ∼4.5 cm2/(V s).
The absorption and photoluminescence, both steady‐state and time‐resolved, of CsPbX3 (X = Cl, Br, I) nanocrystals are reported at temperatures ranging from 3 to 300 K. These measurements offer a ...unique window into the fundamental properties of this class of materials which is considered promising for light‐emitting and detection devices. The bandgaps are shown to increase from low to high temperature, and none of the examined cesium‐based perovskite nanocrystals exhibit a bandgap discontinuity in this temperature range suggesting constant crystal phase. Time‐resolved measurements show that the radiative lifetime of the band‐edge emission depends strongly on the halide ion and increases with heating. The increasing lifetime at higher temperatures is attributed primarily to free carriers produced from exciton fission, corroborated by the prevalence of excitonic character in absorption. The results particularly highlight many of the similarities in physical properties, such as low exciton binding energy and long lifetime, between CsPbI3 and hybrid organic–inorganic plumbotrihalide perovskites.
Temperature‐dependent optical studies of CsPbX3 (X = Cl, Br, I) nanocrystals in the bulk‐like size regime demonstrate the bright nature of the lowest emitting state of this class of materials. Low‐temperature absorption measurements directly reveal the composition‐dependent exciton binding energy. Dynamic measurements strongly suggest increased lifetimes at higher temperatures are due to exciton fission.
Although surface ligands of colloidal nanocrystals are known to adjust the absolute energy levels of valence and conduction bands of semiconductor nanocrystals, they typically have only minor ...influence on the band gap or effective masses. This changes in nanoplatelets. Ligand exchange of CdSe colloidal nanoplatelets induces large (up to 300 meV) bathochromic shifts of both interband and intersubband transitions. Here, three families of ligandshalides, thiolates, and phosphonatesare used to tune interband transitions, reflecting electron and hole confinement, across visible wavelengths and intersubband transitions, reflecting electron confinement, across the near-infrared spectral window. Careful examination shows that delocalization from expansion of the nanoplatelet short axis, which was reported previously, cannot alone explain observed red shifts. Instead, comparison of intersubband, interband, and hole energy levels shows that ligand head group chemistry confers specific, idiosyncratic adjustments of the contribution of conduction and valence bands to the observed bathochromic shifts. Phosphonate ligands show the largest band gap reductions but the smallest red shift of intersubband transition energies; halide-exchanged samples displayed smaller reductions in band gap but large red shifts of intersubband transitions; thiolates fall in between. A related specificity is observed in hole states, which implicates ligand-responsive valence band curvature as an additional contribution driving optical changes. For nanoplatelets, surface ligand chemistry offers not only a tool to adjust the absolute energy level of conduction and valence bands but also an alternative route to preferential electron or hole band engineering that is normally achieved with inorganic shells.
The optical Stark effect is a universal response of the electronic structure to incident light. In semiconductors, particularly nanomaterials, the optical Stark effect achieved with sub-band gap ...photons can drive large, narrowband, and potentially ultrafast changes in the absorption or reflection at the band gap through excitation of virtual excitons. Rapid optical modulation using the optical Stark effect is ultimately constrained, however, by the generation of long-lived excitons through multiphoton absorption. This work compares the modulation achievable using the optical Stark effect on CdSe nanoplatelets with several different pump photon energies, from the visible to mid-infrared. Despite expected lower efficiencies for spectrally-remote pump energies, infrared pump pulses can ultimately drive larger sub-picosecond optical Stark shifts of virtual excitons without creation of real excitons. The CdSe nanoplatelets show subpicosecond shifts of the lowest excitonic resonance of up to 22 meV, resulting in change in absorption as large as 0.32 OD (49% increase in transmission), with a long-lived offset from real excitons less than 1% of the peak signal.
Intraband relaxation in all‐inorganic cesium lead tribromide (CsPbBr3) and hybrid organic–inorganic formamidinium lead tribromide (FAPbBr3) nanocrystals is experimentally investigated for a range of ...particle sizes, excitation energies, sample temperatures, and excitation fluences. Hot carriers in CsPbBr3 nanocrystals consistently exhibit slower cooling than FAPbBr3 nanocrystals in the single electron–hole pair per nanocrystal regime. In both compositions, long‐lived hot carriers (>3 ps) are only observed at excitation densities corresponding to production of multiple electron–hole pairs per nanocrystal—and concomitant Auger recombination. These presented results are distinct from previous reports in bulk hybrid perovskite materials that convey persistent hot carriers at low excitation fluences. Time‐resolved photoluminescence confirms the rapid cooling of carriers in the low‐fluence (single electron–hole pair per nanocrystal) regime. Intraband relaxation processes, as a function of excitation energy, size, and temperature are broadly consistent with other nanocrystalline semiconductor materials.
Intraband cooling in all‐inorganic CsPbBr3 and hybrid organic–inorganic FAPbBr3 nanocrystals reveal longer cooling times in CsPbBr3 at low fluence. Persistent hot carriers beyond 2 ps are not observed in either set of nanocrystals except in the multiple electron–hole pairs per nanocrystal regime.
Colloidal quantum wells, also called nanoplatelets, are nanoscopic materials displaying quantum confinement in two dimensions. Unlike colloidal quantum dots, colloidal quantum well ensembles have no ...inhomogeneous broadening due to an atomically-precise definition of the short axis, a fact which results in much narrower ensemble absorption and emission. Thus, colloidal quantum wells can translate many advantages of colloidal nanocrsytals or other solution-processable materials, such as scalable synthesis and substrate-agnostic deposition (particularly compared to epitaxial quantum wells), without sacrificing material uniformity. Due to very narrow photoluminescnece peaks, these materials have found a home in applications involving light emission, such as downconversion enhancement films, light-emitting diodes, and lasers, in which they represent some of the best performers among solution-cast materials. As argued in this review, the full spectrum of epitaxial quantum well devices offers a roadmap to other potential applications, such as detection, electronics, electro-optics, non-linear optics, or intersubband devices, in which only nascent efforts have been made.
Colloidal quantum wells, or nanoplatelets, are a promising class of solution-processable two-dimensional materials with properties well-suited for diverse optoelectronic devices.
Colloidal quantum wells, or nanoplatelets, exhibit large, circularly polarized optical Stark effects under sub-band-gap femtosecond illumination. The optical Stark effect is measured for CdSe ...colloidal quantum wells of several thicknesses and separately as a measure of pump photon energy, pump fluence, and temperature. These measurements show that optical Stark effects in colloidal quantum wells shift the absorption features up to 5 meV, at the intensities up to 2.9 GW·cm–2 and large detuning (>400 meV) of the pump photon energy from the band edge absorption. Optical Stark shifts are underpinned by large transition dipoles of the colloidal quantum wells (μ = 15–23 D), which are larger than those of any reported colloidal quantum dots or epitaxial quantum wells. The rapid (<500 fs), narrow band blue shift of the excitonic features under circular excitation indicates the viability of these materials beyond light emission such as spintronics or all-optical switching.
Intraband relaxation in polycrystalline films of hybrid perovskites methylammonium lead tribromide and methylammonium lead triiodide are studied by transient absorption spectroscopy from 80 K to >350 ...K. This temperature range spans the transitions of these materials from the high-temperature cubic phases, intermediate tetragonal phases, and low-temperature orthorhombic phases. The organic cation undergoes a distinct transition from an ordered lattice in the orthorhombic phase to a plastic crystal in cubic and tetragonal phases, which reportedly influences many optoelectronic properties. The much larger exciton binding energy of orthorhombic MAPbI3 (compared to cubic or tetragonal phases) or MAPbBr3 substantially changes the transient spectral responses of the materials by reducing the number of free carriers. However, for these measurements at low fluences, both MAPbBr3 and MAPbI3 exhibit subpicosecond intraband relaxation over the entire temperature range studied. Intraband relaxation becomes somewhat faster at higher temperatures, but freezing of organic cations are not accompanied by a discontinuity of the intraband relaxation time. These results suggest that configurational freedom of organic cations does not screen carriers from electron–phonon coupling.
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.
Recent synthetic developments have generated intense interest in the use of cesium lead halide perovskite nanocrystals for light‐emitting applications. This work presents the photoluminescence (PL) ...of cesium lead halide perovskite nanocrystals with tunable halide composition recorded as function of temperature from 80 to 550 K. CsPbBr3 nanocrystals show the highest resilience to temperature while chloride‐containing samples show relatively poorer preservation of photoluminescence at elevated temperatures. Thermal cycling experiments show that PL loss of CsPbBr3 is largely reversible at temperatures below 450 K, but shows irreversible degradation at higher temperatures. Time‐resolved measurements of CsPbX3 samples show an increase in the PL lifetime with temperature elevation, consistent with exciton fission to form free carriers, followed by a decrease in the apparent PL lifetime due to trapping. PL persistence measurements and time‐resolved spectroscopies implicate thermally assisted trapping, most likely to halogen vacancy traps, as the mechanism of reversible PL loss.
High‐temperature photoluminescence properties of CsPbX3 nanocrystals are important to the viability of this family of emitters in lighting and display applications. The persistence of photoluminescence at temperatures above 400 K depends strongly on the halide composition, and photoluminescence loss occurs through thermally assisted trapping to halogen vacancies.
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