PbSe Structures Research Articles

Identifying the mechanistic insights into PbSeO3 decomposition, during milling, to give way to PbSe: An experimental and theoretical approach

This study has been designed with the aim to understand the stability of the as-milled PbSeO3 reaction product, which is considered the determining step associated with the mechanochemical synthesis of PbSe nanostructures. Herein, a detailed discussion of the transition pathway from PbSeO3 to PbSe nanostructures is proposed. Hence, the decomposition process of PbSeO3, during milling, is studied by X-ray diffraction. According to the Rietveld refinement, the largest PbSeO3 concentration is obtained after 2 h of milling; after that, it decreases as the milling time is increased. In addition, the chemical composition changes are studied via density functional theory (DFT) calculations because the changes have an impact on the electronic properties, especially the bandgap value (Eg) associated with the presence of oxygen. Furthermore, the DFT calculations are also used to understand the transition state (TS) -via the diffusion of O-atoms by the atomic layers- from PbSeO3 to PbSe. After the oxygen removal, an energetically expensive process of spatial rearrangement (∼26.97 kJ·mol−1) took place to obtain the ground state structure of PbSe. Therefore, this work provides a comprehensive understanding of the decomposition mechanism of PbSeO3 that allows to tune the electronic properties of devices based on PbSe nanostructures, which are concerned with surface chemical composition changes.

Strategies for boosting thermoelectric performance of PbSe: A review

• General physical properties of typical IV-VI compounds are summarized. • Phase diagram, crystal and electrical band structures of PbSe are presented. • Recent ways of adjusting charge and phonon transport processes in PbSe are updated. • Typical preparation methods for PbSe and their scalability are discussed. • Key challenges and future research directions of PbSe are discussed. Thermoelectric materials enable the direct conversion between waste heat and electric energy, playing an important role in alleviating energy crisis. Many excellent thermoelectrics developed so far contain expensive and scarce Te element, largely limiting their applications. Therefore, exploring Te-free compounds with extraordinary thermoelectric performance becomes a vital topic in thermoelectric community in recent years. PbSe is an ideal candidate that meets above criteria and has advanced rapidly in the last decade with reported peak ZT s close to 2.0. Herein we review the recent research progress of PbSe-based thermoelectric materials. This review article starts with a general introduction of the properties of IV-VI semiconductors as advanced thermoelectric materials by comparing their cost, crustal abundance, mechanical strength and chemical bonding. Following that, phase diagram, crystal and electronic band structures of PbSe are comprehensively summarized. Then we discuss how the frequently used dopants regulate its carrier concentrations. Subsequently, electronic band structure engineering (including resonant levels, band flattening, band convergence, band inversion, etc. ) and microstructural architecturing (including atomic arrangements, dislocation arrays, nanoscale precipitates, etc. ) approaches and their impacts on charge and phonon transport properties of PbSe are elaborated. Finally, we summarize the typical production processes of PbSe and comment on their scalability. The future directions for how to further improve the thermoelectric properties of PbSe and promote its applications are discussed at the end of this article.

Strong Valence Band Convergence to Enhance Thermoelectric Performance in PbSe with Two Chemically Independent Controls.

We present an effective approach to favorably modify the electronic structure of PbSe using Ag doping coupled with SrSe or BaSe alloying. The Ag 4d states make a contribution to in the top of the heavy hole valence band and raise its energy. The Sr and Ba atoms diminish the contribution of Pb 6s2 states and decrease the energy of the light hole valence band. This electronic structure modification increases the density-of-states effective mass, and strongly enhances the thermoelectric performance. Moreover, the Ag-rich nanoscale precipitates, discordant Ag atoms, and Pb/Sr, Pb/Ba point defects in the PbSe matrix work together to reduce the lattice thermal conductivity, resulting a record high average ZTavg of around 0.86 over 400-923 K.

Strong Valence Band Convergence to Enhance Thermoelectric Performance in PbSe with Two Chemically Independent Controls

AbstractWe present an effective approach to favorably modify the electronic structure of PbSe using Ag doping coupled with SrSe or BaSe alloying. The Ag 4d states make a contribution to in the top of the heavy hole valence band and raise its energy. The Sr and Ba atoms diminish the contribution of Pb 6s2 states and decrease the energy of the light hole valence band. This electronic structure modification increases the density‐of‐states effective mass, and strongly enhances the thermoelectric performance. Moreover, the Ag‐rich nanoscale precipitates, discordant Ag atoms, and Pb/Sr, Pb/Ba point defects in the PbSe matrix work together to reduce the lattice thermal conductivity, resulting a record high average ZTavg of around 0.86 over 400–923 K.

Aqueous-Phase Synthesis of Size-Tunable PbSe Nanocubes at Room Temperature for Optical Property Characterization.

A simple aqueous-phase synthesis of PbSe nanocubes with tunable sizes has been developed by first preparing a Na2 SeSO3 stock solution through dissolution of selenium powder in a solution of Na2 SO3 at 90-100 °C for 30 min, and adding part of this solution to a mixture of lead acetate and acetic acid at room temperature with stirring for only 5-8 min to complete the nanocrystal growth. Adjusting the volume of acetic acid and Na2 SeSO3 solution added enabled the size of the nanocrystals to be tuned, with average edge lengths of 13 to 121 nm attained. Changes in solution color revealed very different crystal growth rates for the 13 and 121 nm nanocubes. The PbSe cubes exhibit size-dependent absorption bands in the ultraviolet and visible-light region; the band positions show progressive redshifts with increasing particle size. Slight photocatalytic activity upon 532 nm laser irradiation of the nanocubes suggests the presence of higher energy levels in the band structure of PbSe. The synthetic conditions can be easily scaled up to obtain a large quantity of PbSe nanocubes for applications.

Emphanitic anharmonicity in PbSe at high temperature and anomalous electronic properties in the PbQ(Q=S,Se,Te) system

The temperature dependence of the local structure of PbSe has been investigated using pair distribution function (PDF) analysis of x-ray and neutron powder diffraction data and density functional theory (DFT) calculations. Observation of non-Gaussian PDF peaks at high temperature indicates the presence of significant anharmonicity, which can be modeled as Pb off-centering along [100] directions that grows on warming similar to the behavior seen in PbTe and PbS and sometimes called emphanisis. Interestingly, the emphanitic response is smaller in PbSe than in both PbS and PbTe indicating a non-monotonic response with chalcogen atomic number in the PbQ (Q=S, Se, Te) series. The DFT calculations indicate a correlation between band gap and the amplitude of [100] dipolar distortion, suggesting that emphanisis may be behind the anomalous composition and temperature dependencies of the band gaps in this series.

Lead Selenide Nanostructures Self-Assembled across Multiple Length Scales and Dimensions

A self-assembly approach to lead selenide (PbSe) structures that have organized across multiple length scales and multiple dimensions has been achieved. These structures consist of angstrom-scale 0D PbSe crystals, synthesized via a hot solution process, which have stacked into 1D nanorods via aligned dipoles. These 1D nanorods have arranged into nanoscale 2D sheets via directional short-ranged attraction. The nanoscale 2D sheets then further aligned into larger 2D microscale planes. In this study, the authors have characterized the PbSe structures via normal and cryo-TEM and EDX showing that this multiscale multidimensional self-assembled alignment is not due to drying effects. These PbSe structures hold promise for applications in advanced materials—particularly electronic technologies, where alignment can aid in device performance.

Electronic band structure of zinc blende CdSe and rock salt PbSe semiconductors with silicene-type honeycomb geometry

We report on the electronic band structure of 2D CdSe and PbSe semiconductors that have a silicene-type honeycomb geometry. Atomistic tight-binding calculations are performed on several model systems that bear a strong resemblance to the silicene-type honeycomb structures that were recently obtained by nanocrystal self-assembly. The calculated band structures are compared both to those of 2D quantum wells and graphene-type honeycomb structures. It is found that in silicene type CdSe honeycomb structures, the lowest electron conduction bands (derived from S-type nanocrystal wave functions) form a Dirac-type dispersion, very similar as in graphene. The P-type bands are usually more complex. However, when the hybridization between S- and P-type bands increases, a second Dirac cone and a genuine non-trivial flat band is observed, similar as in the case of graphene-type honeycomb structures of CdSe. There is a strong non-trivial gap between the first and second valence band, hosting the quantum spin Hall effect. Silicene-type PbSe structures show Dirac features in their bands, which however can be clouded due to the multi-valley character of PbSe.

Structure determination of the intermediate phase of PbSe using experiments and calculations

The pressure-induced structure transitions of PbSe were investigated at pressures up to 50 GPa in diamond anvil cells using synchrotron powder and single-crystal X-ray diffraction methods. A phase transition from the initial NaCl-type structure to an intermediate phase was observed at 4.8 GPa, followed by another phase transition to a CsCl-type structure at 19.5 GPa, which is in agreement with previous reports. In addition, the NaCl-type, Pnma, Cmcm, and CsCl-type structures of PbSe were investigated using density functional theory calculations. Based on the experiments and calculations, the intermediate phase has Pnma symmetry rather than Cmcm symmetry, as has been previously suggested. Some of the diffraction peaks of the Pnma phase were broad in the high-pressure experiments, but were determined to be single diffraction peaks rather than the overlap of multiple peaks by different experimental methods. The Pnma and Cmcm structures of PbSe were compared. It was found that two structures were very similar, and it was difficult to distinguish one power diffraction pattern from the other.

Structures and optical absorptions of PbSe clusters from ab initio calculations

Based on the low-lying structures of (PbSe)n (n = 1-10) clusters identified with a first-principles molecular dynamics approach, two growth patterns with distinct structure and energy evolutions were predicted for the even-n and odd-n clusters, respectively. Moreover, the clusters favor a simple cubic and bulk-like growth pattern, unlike the extensively studied II-VI clusters whose structural diversity has been well established. The overlap between 6p of Pb and 4p of Se makes not only the ordered and bulk-like structures but also a stable building block of (PbSe)4. The high stability of (PbSe)4 is recognized in terms of its binding energy, HOMO-LUMO gap, appearance in the structures of larger-size clusters, as well as its appearance in the fragmentation products of PbSe clusters. The geometrical and electronic structures of the PbSe clusters were further studied within the density functional theory framework including spin-orbital (SO) coupling. We found that SO coupling does not change the relative stability of the clusters but reduces their binding energy significantly. Particularly, the SO effect has a great impact on the UV-vis spectra of the clusters, which were simulated with time-dependent density functional theory at SO level of zeroth-order regular approximation.

Direct laser writing of three-dimensional narrow bandgap and high refractive-index PbSe structures in a solution

Three-dimensional (3D) micro/nano structures made of narrow electronic bandgap semiconductor materials have important applications in a wide range of disciplines. Direct laser writing (DLW) provides the unparalleled advantage to fabricate 3D arbitrary geometric structures at the micro and nano meter scale. The fabrication of 3D structures within bulk narrow electronic bandgap semiconductor materials by DLW is challenged for the top-down strategy due to their narrow bandgap and high refractive index. Here, we report on the bottom-up strategy for the fabrication of 3D micro/nano structures made from PbSe with an electronic bandgap as narrow as 0.27 eV and a refractive index as high as 4.82 in a solution.

Investigation of the valence band structure of PbSe by optical and transport measurement

ABSTRACTWe investigated the valence band structure of PbSe by a combined study of the optical and transport properties of p-type Pb1-xNaxSe, with Na concentrations ranging from 0 – 4%, yielding carrier densities in a wide range of 1018 – 1020 cm−3. Room temperature infrared reflectivity studies showed that the susceptibility (or conductivity) effective mass m* increases from ∼ 0.06mo to ∼ 0.5mo on increasing Na content from 0.08% to 3%. The Seebeck coefficient scales with doping in the whole temperature range, yielding lower values for higher Na contents, while the Hall coefficient increases on heating from room temperature showing a peak close to 650 K. The room temperature Pisarenko plot is well described by the simple parabolic band model up to ∼ 1·1020 cm−3. In order to describe the behaviour in the whole concentration range, the application of the two band model, i.e. light hole and heavy hole, was used giving density of states effective masses 0.28mo and 2.5mo for the two bands respectively.

Electronic structure and transport properties of doped PbSe

Understanding the electronic structure and transport properties of doped PbSe for its thermoelectric applications is an urgent need. Using a first-principles approach, we first explore the band structures of PbSe doped with a series of impurities, including cation-site substitutional impurities (Na, K, Rb; Mg, Ca, Sr; Cu, Ag, Au; Zn, Cd, Hg; Ga, In, Tl; Ge, Sn; As, Sb, Bi) and anion-site substitutional impurities (P, As, Sb; O, S, Te). Then we calculate the density of states (DOS) difference between the doped samples and pure host sample, which is a useful quantity to recognize the possibility of improving transport properties. The exhibited chemical trends and the nature of the impurity states are well explained with a simplified linear combination of atomic orbitals (LCAO) picture. Finally, we calculate the transport properties of these doped systems within the framework of Boltzmann theory and constant relaxation time approximation. Typical competing behavior between the electrical conductivity and Seebeck coefficient is exhibited, and a significant enhancement of thermoelectric power factor is found in the cation-site Au-doped $p$-type samples, and cation-site As-doped $n$-type samples.

Synthesis and Characterization of Lead Selenide Nanocrystal Quantum Dots and Wires

Lead chalcogenide nanocrystalline materials offer possibilities of improving the efficiency of various optoelectric/thermoelectric applications, especially in solar cells, by generating more carriers with incoming photons, or by extending the bandgap toward the infra-red region. In this work, we suggest the synthetic approach of creating extended PbSe structures which shows better performances when incorporated into an electric device. Firstly, we synthesized monodisperse cubic-structured single-crystalline lead selenide nanocrystal quantum dots using lead acetate and oleic acid in non-coordinating solvent without additional surfactants. Also, single-crystal cubic PbSe nanowires were synthesized in a mixture of surfactants such as trioctylphosphine and phenyl ether. Morphologies of wires and dots were precisely controlled via reaction temperature and the surface ligands. Phenyl ether was found to facilitate the oriented attachment. Further, current-voltage characteristics of drop-casted 2D arrays of nanocrystalline materials were examined.

Synthesis and characterization of [Pb{Se2P(OiPr)2}2]n pseudo polymorphs: Polymeric, single source precursor enabling preparation of shape-controlled lead selenide structures

Novel lead pseudo polymorphs supported by diselenophosphate (dsep) ligands, [Pb{Se(2)P(O(i)Pr)(2)}(2)](n) (1alpha and 1beta), were synthesized and structurally characterized. The two structures differed in the binding modes of the dsep ligand. Each repeating unit in 1alpha was composed of a lead atom coordinated by two dsep ligands, one in a chelating mode and the other in a bridging-dangling mode. By contrast, the dsep ligands in 1beta adopted a bimetallic-biconnective (mu(1)-S, mu(1)-S) binding pattern. The bridging-dangling mode observed in 1alpha is the first example of this mode for the dsep ligand. [Pb{Se(2)P(O(i)Pr)(2)}(2)](n) could be successfully utilized as a single source precursor (SSP) for growing lead selenide (PbSe) nanostructures with different shapes via the solvothermal process. The morphologies of the as-grown PbSe structures were controlled by capping agents, poly(vinyl pyrrolidone) (PVP) and ethylenediamine, both bind on the {200} plane and enhance the growth rate of the <111> direction of the PbSe crystals, resulting in the formation of cubes and eight-horned rod dendrites, respectively. Raman spectroscopy was utilized to investigate the phonon vibration behaviour of the as-grown eight-horned rod dendrites. All peaks in the Raman spectra can be attributed to longitudinal (LO) and transverse (TO) optical phonon modes of the PbSe rod dendrites.

Controllable Synthesis of PbSe Nanostructures and Growth Mechanisms

Octopod-like PbSe structures with eight symmetric arms of 1.0−3.0 µm in length have been hydrothermally synthesized without any surfactant. Keeping the other conditions the same, only by changing the molar ratio of NaOH/Pb(NO3)2 from 40 to 5, quasi-spherical nanoparticles were obtained. As poly(vinyl pyrrolidone) (PVP) was added, single-crystal PbSe nanosheets of 20−60 nm in thickness and 0.5−5.0 µm in-plane sizes can be formed. Similarly, by changing the molar ratio of NaOH/Pb(NO3)2 from 40 to 5, the nanocubes of ∼70 nm in edge length were formed. The possible growth mechanisms for the nanosheets and octopod-like structures of PbSe are discussed on the basis of transmission electron microscopy and scanning electron microscopy analysis from the time-dependent experiments.

Selected-control solution-phase route to multiple-dendritic and cuboidal structures of PbSe

Well-crystalline PbSe multiple-dendritic hierarchical structures have been prepared through a facile hydrothermal process in an alkaline glycerol/water solution system using SeO 2 as selenium source and hydrous hydrazine as reducing agent at 160 °C for 12 h. The obtained products were characterized by X-ray powder diffraction, field emission scanning electron microscopy and transmission electron microscopy, which showed that the obtained products were face-centered cubic PbSe multiple-dendritic superstructures with length of each dendrite ranging from 1.0 to 1.5 μm. Additionally, cuboidal PbSe microcrystals with different concave faces can be obtained through a similar process except for using Se powders instead of SeO 2 as selenium source and without hydrous hydrazine. The edge lengths of these cuboidal microcrystals range from 1.0 to 2.5 μm observed by field emission scanning electron microscopy. The influencing factors for the formation of the two kinds of PbSe microstructures were discussed and the possible growth mechanisms were proposed from the point of crystallographic and kinetic views. The studies on the corresponding photoluminescence (PL) properties of the two kinds of PbSe structures are also carried out.

Induced Branching in Confined PbSe Nanowires

The synthesis of narrow diameter (<10 nm) straight and branched PbSe nanowires (NWs) using a seeded solution approach is described. Solution-based PbSe NWs are obtained by injecting a solution consisting of trioctylphosphine selenide (TOPSe) and Au/Bi core/shell nanoparticles (NPs) into a mixture composed primarily of a mild coordinating solvent, a fatty acid, and a Pb precursor at moderate temperatures. The resulting NWs have diameters between 5 and 10 nm and lengths ranging from 1 to 5 μm. High-resolution transmission electron microscopy (TEM) reveals that the NWs exhibit a high degree of crystallinity and grow along the 〈100〉 directions of the lattice. By varying the initial reaction conditions, in particular the Pb to Se precursor ratio, branched NWs can be obtained. The growth mechanism appears similar to the case of analogous (branched) CdSe NWs, although the underlying rock salt structure of PbSe leads to right angles and t-shapes as opposed to CdSe, where v-shape, y-shape, and tripod morphologies ...

Pseudopotential band calculations along a symmetry axis: Central potential

A method for band-structure calculations along a symmetry axis is introduced. It entails a cylindrical-coordinate multipole expansion of the wavefunction and an exact reduction of the 3D Schrodinger equation into a set of 1D wave equations for the multipoles. Group-theoretic considerations and energy arguments imply good convergence, regardless of the unit-cell extension along the symmetry direction. Calculations of a test case, the band structure of PbSe along the (111) direction without spin-orbit coupling, demonstrate the method's good convergence. The method is expected to be particularly useful for structures highly anisotropic along a symmetry axis, e.g., superlattices.

The Electronic Structure of PbSe and Its Interaction with O2 Adsorbate Layers

Clean and O2 exposed surfaces of PbSe have been studied by UV and X-ray photoelectron spectroscopy. The O2 reaction was found to be quite slow. Considerations regarding relative peak intensities of the core levels indicated a diffusion of oxygen into the lattice. The sticking coefficient of O2 was estimated to be of the order of 10-10-10-11.