Hybrid perovskites formed of both organic and inorganic materials in the same crystal unit provide new functionalities otherwise not possible in either organic and inorganic components alone. Due to the ionic nature of these materials, electrons and phonons are strongly coupled and coherent phonons could be excited with ultrashort laser pulses. In this study, using transient absorption spectroscopy measurements, we show that these coherent phonons could be used to manipulate charge transfer in a two-dimensional double perovskite. Findings of this study may serve as a new degree of freedom to consider when designing materials for optoelectronic applications where charge transfer kinetics are crucial.
Superfluorescence is a quantum optical phenomenon in which an initially excited ensemble of incoherent dipoles first acquire macroscopic coherence and then collectively recombine and radiate a burst of photons. This process is a symmetry breaking macroscopic quantum phase transition similar to superconductivity and Bose-Einstein condensation. Since quantum coherence is extremely fragile at high temperatures, similar to other macroscopic quantum phase transitions, superfluorescence has been almost always observed at cryogenic temperatures. In this presentation I will first present our results on room temperature superfluorescence in lead-halide perovskites and then present the mechanism that enables this exotic quantum phase transition at room temperature.
KEYWORDS: Quantum wells, Perovskite, Nanostructures, Thin films, Crystals, X-rays, X-ray characterization, Thin film solar cells, Thin film devices, Solar energy
Controlling the formation, size distribution and orientation of quantum wells (QWs) in layered hybrid perovskite (LHP) thin films is foundational to their optoelectronic device applications. These applications require exquisite control of energy and charge transport which tend to be highly anisotropic in low-dimensional phases in LHP thin films. Here, we combine a powerful suite of multimodal in situ characterizations to elucidate the precise solution-to-solid conversion of the sol into the LHP thin film. We identify, for the first time, the presence of oriented colloidal transient nanostructures during spin coating well before the onset of crystallization of phases.
In this work, we demonstrated that through the addition of large organic cation PEABr, the quality of cesium lead halide thin films has been enhanced by controlling the crystallization process and thus the domain size and morphology. As a result, by increasing the PEABr concentration to 40%, the perovskite thin films with a small domain size, highly oriented crystal structure and low defect density demonstrated a lowest amplified spontaneous emission threshold of 7 µJ/cm2. However, further increasing PEABr concentration will result in a slower energy transfer, which limits the population inversion process and thus showed a higher ASE threshold. Using this strategy, by tuning the halide ratio, the ASE emissions were observed over the broad visible range (from 490 nm to 680 nm), demonstrating its wide wavelength-tunability.
Quasi-2D Ruddlesden-Popper halide perovskites with a large exciton binding energy, self-assembled quantum wells and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi-2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower-dimensional nanosheets (narrower bandgap domains) to three-dimensional nanocrystals (wide bandgap domains). Here, the quality of quasi-2D perovskite (PEA)2(FA)3Pb4Br13 is controlled by solution engineering. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were conducted to study the crystal orientation, and transient absorption spectroscopy (TAS) measurements were conducted to study the charge carrier dynamics. Our data show that the highly oriented 2D crystals have a faster energy transfer to the low bandgap regime (< 0.5 ps) compared to less oriented films. High-performance LEDs can be realized with these 2D films. Finally, amplified spontaneous emission is achieved with an ultralow threshold of 4.16 μJ/cm2 and distributed feedback lasers are realized.
Our results show that it is important to control the morphology and spatial domain distribution of the quasi-2D films to achieve efficient energy transfer, which is a critical requirement for optoelectronic devices.
Shape memory alloys (SMAs) are a unique class of smart materials and they were employed in various applications in engineering, biomedical, and aerospace technologies. Here, we report an advanced, efficient, and low-cost direct imprinting method with low environmental impact to create thermally controllable surface patterns. Patterned microindents were generated on Ni50Ti50 (at. %) SMAs using an Nd:YAG laser with 1064 nm wavelength at 10 Hz. Laser pulses at selected fluences were focused on the NiTi surface and generated pressure pulses of up to a few GPa. Optical microscope images showed that surface patterns with tailorable sizes can be obtained. The depth of the patterns increases with laser power and irradiation time. Upon heating, the depth profile of SMA surfaces changed where the maximum depth recovery ratio of 30 % was observed. Recovery ratio decreased and saturated at about 15 % when the amount of time and thus the indent depth was increased. Laser-induced shock wave propagation inside the material was simulated and showed a good agreement with the experimental results. The stress wave closely followed the rise time of the laser pulse to its peak value and initial decay. Rapid attenuation and dispersion of the stress wave were observed.
An advanced direct imprinting method with low cost, quick, and less environmental impact to create thermally controllable surface pattern using the laser pulses is reported. Patterned micro indents were generated on Ni50Ti50 shape memory alloys (SMA) using an Nd:YAG laser operating at 1064 nm combined with suitable transparent overlay, a sacrificial layer of graphite, and copper grid. Laser pulses at different energy densities which generates pressure pulses up to 10 GPa on the surface was focused through the confinement medium, ablating the copper grid to create plasma and transferring the grid pattern onto the NiTi surface. Scanning electron microscope (SEM) and optical microscope images of square pattern with different sizes were studied. One dimensional profile analysis shows that the depth of the patterned sample initially increase linearly with the laser energy until 125 mJ/pulse where the plasma further absorbs and reflects the laser beam. In addition, light the microscope image show that the surface of NiTi alloy was damaged due to the high power laser energy which removes the graphite layer.
The surfaces of Ni50Ti50 shape memory alloys (SMAs) were patterned by laser scribing. This method is more simplistic and efficient than traditional indentation techniques, and has also shown to be an effective method in patterning these materials. Different laser energy densities ranging from 5 mJ/pulse to 56 mJ/pulse were used to observe recovery on SMA surface. The temperature dependent heat profiles of the NiTi surfaces after laser scribing at 56 mJ/pulse show the partially-recovered indents, which indicate a "shape memory effect (SME)" Experimental data is in good agreement with theoretical simulation of laser induced shock wave propagation inside NiTi SMAs. Stress wave closely followed the rise time of the laser pulse to its peak values and initial decay. Further investigations are underway to improve the SME such that the indents are recovered to a greater extent.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.