Defects of monolayer PbI2: a computational study

PbI₂ is highly desired for optoelectronics, particularly as a crucial precursor in the preparation of hybrid perovskites. Multifarious defects of PbI₂ have been frequently observed in experiments; however, the origin and impact on its intrinsic properties are not clear currently. Hereby, a systematic study on the defects in PbI₂ was performed by first principles calculations. Likely defect structures in both 1T and 1H phases were predicted, and the corresponding formation energies at the neutral and charged states based on the pre-estimated chemical potential of I/Pb were both calculated. At the neutral states, the formation energy of V_I is relatively low, which is consistent with the experimental observation. However, at the charged states, V_Pb seems to be dominant regardless of the chemical potential and contributes more to the conductivity. Compared with the two-dimensional transition metal dichalcogenides, the relatively low formation energy of PbI₂ indicates its flexibility and the weak Pb-I interaction. Our DFT calculations bring new insights into understanding the influence of defects on its opto-electrical performance and instruct to suppress the formation of those defects that are deleterious to its performance.

Effect of electron-phonon coupling on the transport properties of monolayers of ZrS2, BiI3 and PbI2: a thermoelectric perspective

Thermoelectric materials are used for the conversion of waste heat to electrical energy. The transport coefficients that determine their thermoelectric properties depend on the band structure and the relaxation time of the charge carriers. Both of these are significantly affected by electron-phonon coupling. In this work, using a combination of density functional theory and semiclassical Boltzmann transport theory, we have studied the effect of electron-phonon coupling in monolayers of ZrS2, BiI3 and PbI2. Our results show that in these ionic materials charge carriers are primarily scattered by the optical modes that strongly couple with them. From our study it is conclusively shown that neglecting the contributions of optical modes to electron-phonon coupling in these low-dimensional ionic solids, as is usually done in the computation of relaxation time from deformation theory, results in severe overestimation of the relaxation time. In particular, neglecting the scattering of the optical phonons results in about two orders of magnitude overestimation of relaxation times in these materials. Moreover, we also find that the renormalization of the band structure not only results in the reduction of band gaps but also changes the band dispersion, which strongly affect different transport properties like the electrical conductivity and Seebeck coefficient. Amongst these three materials, we observe that carrier relaxation time due to electron-phonon scattering is reduced as the ionicity of the material decreases.

Atomic structure and defect dynamics of monolayer lead iodide nanodisks with epitaxial alignment on graphene

Lead Iodide (PbI2) is a large bandgap 2D layered material that has potential for semiconductor applications. However, atomic level study of PbI2 monolayer has been limited due to challenges in obtaining thin crystals. Here, we use liquid exfoliation to produce monolayer PbI2 nanodisks (30-40 nm in diameter and > 99% monolayer purity) and deposit them onto suspended graphene supports to enable atomic structure study of PbI2. Strong epitaxial alignment of PbI2 monolayers with the underlying graphene lattice occurs, leading to a phase shift from the 1 T to 1 H structure to increase the level of commensuration in the two lattice spacings. The fundamental point vacancy and nanopore structures in PbI2 monolayers are directly imaged, showing rapid vacancy migration and self-healing. These results provide a detailed insight into the atomic structure of monolayer PbI2, and the impact of the strong van der Waals interaction with graphene, which has importance for future applications in optoelectronics.

Achieving highly uniform two-dimensional PbI2 flakes for photodetectors via space confined physical vapor deposition

Two-dimensional (2D) PbI₂ flakes have been attracting intensive attention as one potential candidate for the modern optoelectronics. However, suffered from the instability of kinetics-driven growth, the fabricated 2D PbI₂ flakes have a wide dimensional distribution even under the same conditions. Herein, a novel facile space confined physical vapor deposition (PVD) process is provided to synthesize uniform triangle PbI₂ flakes with high quality. The confined space provides a relatively stable growth environment that renders more control on the growth kinetics, leading to highly uniform triangle PbI₂ flakes with the average size of 5 µm and thickness of 17 nm. Moreover, as-fabricated PbI₂-based photodetectors show promising stable and flexible optoelectronic performances to 470 nm light, including high responsivity (0.72 A W^-1), large on/off ratio up to 900, fast photoresponse speed (rise time of 13.5 ms and decay time of 20 ms) and high detectivity (1.04 × 10¹⁰ Jones). The well-controllable growth of the uniform triangle PbI₂ flakes and the detailed exploration of their optoelectronic properties are particularly valuable for their further practical applications.

Hydrogenation-driven phase transition in single-layer TiSe2

First-principles calculations based on density-functional theory are used to investigate the effects of hydrogenation on the structural, vibrational, thermal and electronic properties of the charge density wave (CDW) phase of single-layer TiSe₂. It is found that hydrogenation of single-layer TiSe₂ is possible through adsorption of a H atom on each Se site. Our total energy and phonon calculations reveal that a structural phase transition occurs from the CDW phase to the T _d phase upon full hydrogenation. Fully hydrogenated TiSe₂ presents a direct gap semiconducting behavior with a band gap of 119 meV. Full hydrogenation also leads to a significant decrease in the heat capacity of single-layer TiSe₂.

Characterization of highly crystalline lead iodide nanosheets prepared by room-temperature solution processing

Two-dimensional (2D) semiconducting materials are particularly appealing for many applications. Although theory predicts a large number of 2D materials, experimentally only a few of these materials have been identified and characterized comprehensively in the ultrathin limit. Lead iodide, which belongs to the transition metal halides family and has a direct bandgap in the visible spectrum, has been known for a long time and has been well characterized in its bulk form. Nevertheless, studies of this material in the nanometer thickness regime are rather scarce. In this article we demonstrate an easy way to synthesize ultrathin, highly crystalline flakes of PbI₂ by precipitation from a solution in water. We thoroughly characterize the produced thin flakes with different techniques ranging from optical and Raman spectroscopy to temperature-dependent photoluminescence and electron microscopy. We compare the results to ab initio calculations of the band structure of the material. Finally, we fabricate photodetectors based on PbI₂ and study their optoelectronic properties.