Ultrafast Electronic Relaxation Dynamics in Layered Iodide Semiconductors: A Comparative Study of Colloidal BiI3and PbI2Nanoparticles

Synthesis and Properties of Stable Amino Metal Halide Molecular Clusters in the Solid State

Nanosized molecular clusters (MCs) composed of PbBr₂ and neutral ligand butylamine (BTYA) with unique optical properties in solution and solid states have been synthesized using ligand-assisted reprecipitation and spin-coating, separately. The studies of their optical properties using ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) show the first electronic absorption and PL band of the MCs at 401 and 411 nm, respectively, for the solution and solid state samples that exhibit good stability under ambient conditions. Low-temperature PL spectra below 30 K show vibronic peaks indicative of a single size or a very narrow size distribution of the MCs. On the basis of Raman, X-ray diffraction, and transmission electron microscopy measurements, a layered structural model is proposed for the MCs with a BTYA ligand capping on the surface of the corner-shared tilted [PbBr₆]^4- octahedral framework. The stable and retained structure of MCs in the solid state is promising for photonics applications.

Orientation-Controlled Large-Area Epitaxial PbI2 Thin Films with Tunable Optical Properties

Lead iodide (PbI₂) as a layered material has emerged as an excellent candidate for optoelectronics in the visible and ultraviolet regime. Micrometer-sized flakes synthesized by mechanical exfoliation from bulk crystals or by physical vapor deposition have shown a plethora of applications from low-threshold lasing at room temperature to high-performance photodetectors with large responsivity and faster response. However, large-area centimeter-sized growth of epitaxial thin films of PbI₂ with well-controlled orientation has been challenging. Additionally, the nature of grain boundaries in epitaxial thin films of PbI₂ remains elusive. Here, we use mica as a model substrate to unravel the growth mechanism of large-area epitaxial PbI₂ thin films. The partial growth leading to uncoalesced domains reveals the existence of inversion domain boundaries in epitaxial PbI₂ thin films on mica. Combining the experimental results with first-principles calculations, we also develop an understanding of the thermodynamic and kinetic factors that govern the growth mechanism, which paves the way for the synthesis of high-quality large-area PbI₂ on other substrates and heterostructures of PbI₂ on single-crystalline graphene. The ability to reproducibly synthesize high-quality large-area thin films with precise control over orientation and tunable optical properties could open up unique and hitherto unavailable opportunities for the use of PbI₂ and its heterostructures in optoelectronics, twistronics, substrate engineering, and strain engineering.

A facile synthesis of Au-nanoparticles decorated PbI2 single crystalline nanosheets for optoelectronic device applications

This research communication presents a rapid and facile microwave-assisted synthesis of single crystalline nanosheets (SCNSs) of hexagonal lead iodide (PbI₂) decorated with Au nanoparticles, a potential optoelectronics material. Homogeneous low dimensional AuNP decoration in PbI₂ resulted in a new absorption band at ~604 nm and a shift in band gap from 3.23 to 3.00 eV. The significant enhancement of photoluminescent (PL) intensity observed in the AuNP-PbI₂ SCNSs is attributed to the coupling of the localized surface plasmon resonanzce of AuNP leading to improved excitation and emission rates of PbI₂-SCNSs in the region of the localized electromagnetic field. The Au-PbI₂ SCNSs display a compelling increment in photoconductivity, and its fabricated photodetector showed a stable and switchable photo-response. Due to ease of synthesis and enhanced photoconductivity along with appealing PL features, Au-PbI₂ SCNS has the potential to be used as a material of choice when fabricating an optoelectronic devices of high performance.

Insights into the formation mechanism of two-dimensional lead halide nanostructures

We present a colloidal synthesis strategy for lead halide nanosheets with a thickness of far below 100 nm. Due to the layered structure and the synthesis parameters the crystals of PbI₂ are initially composed of many polytypes. We propose a mechanism which gives insight into the chemical process of the PbI₂ formation. Further, we found that the crystal structure changes with increasing reaction temperature or by performing the synthesis for longer time periods changing for the final 2H structure. In addition, we demonstrate a route to prepare nanosheets of lead bromide as well as lead chloride in a similar way. Lead halides can be used as a detector material for high-energy photons including gamma and X-rays.

Single and twinned plates of 2D layered BiI3 for use as nanoscale pressure sensors

Single and twinned plates of 2D layered BiI₃ have been found to be piezoelectric and can be used as a nanoscale pressure sensor.

Tailoring the structural, morphological, optical and dielectric properties of lead iodide through Nd3+ doping

Hexagonal single crystal nanosheets of Nd³⁺ doped PbI₂ were effortlessly synthesized via microwave-assisted technique under a power of 700 W and in a duration of 15 minutes with a homogeneous morphology. X-ray diffraction, energy dispersive X-ray spectroscope, scanning electron microscope, FT-Raman, UV-Visible, photoluminescence and dielectric measurement were employed to study the product. High purity, single phase and presence of Nd³⁺ doping was confirmed. SEM study confirm the formation of nanorods and single crystal nanosheets of very few nanometers in size. Robust vibrational analysis has been carried out and the observed bands are assigned to the vibration modes of E₂¹, A₁¹, A₁², 2E₂¹ and 2E₁¹, respectively. These bands are red-shifted when compare to the corresponding bulk values which indicate relaxed nanostructure formation and occurrence of confinement effect. The thickness of the synthesized single crystal nanosheets are found to be in the range of ~20 to 30 nm. The energy band gap was calculated and found to be 3.35, 3.34, 3.42 and 3.39 eV for pure, 1, 3 and 5% Nd³⁺ doped lead iodide, respectively. The clear blue luminescence has been observed at 440 nm and 466 nm when excited at 250 nm and 280 nm respectively. Dielectric and ac electrical conductivity was also measured and discussed.

Quantum well effect in bulk PbI(2) crystals revealed by the anisotropy of photoluminescence and Raman spectra

On subjecting a bulk 2H-PbI(2) crystal to vacuum annealing at 500 K followed by a sudden cooling at liquid nitrogen temperature stacking faults are generated that separate distinct layers of nanometric thickness in which different numbers of I-Pb-I atomic layers are bundled together. Such structures, containing two, three, four, five etc I-Pb-I atomic layers, behave as quantum wells of different widths. The signature of such a transformation is given by a shift towards higher energies of the fundamental absorption edge, which is experimentally revealed by specific anisotropies in the photoluminescence and Raman spectra. The quantum confining effect is made visible by specific variations of a wide extra-excitonic band (G) at 2.06 eV that originates in the radiative recombination of carriers (electrons and holes), trapped on the surface defects. The excitation spectrum of the G band, with p polarized exciting light, reveals a fine structure comprised of narrow bands at 2.75, 2.64, 2.59 and 2.56 eV, which are associated with the PbI(2) quantum wells formed from two, three, four and five I-Pb-I atomic layers of 0.7 nm thickness. Regardless of the polarization state of the laser exciting light of 514.5 nm (2.41 eV), which is close to the band gap energy of PbI(2) (2.52 eV), the Raman scattering on bulk as-grown PbI(2) crystals has the character of a resonant process. For p polarized exciting light, the Raman scattering process on vacuum annealed PbI(2) becomes non-resonant. This originates from the quantum well structures generated inside the crystal, whose band gap energies are higher than the energy of the exciting light.