Photoexcitation of PbS nanosheets leads to highly mobile charge carriers and stable excitons

Colloidal 2D Lead Chalcogenide Nanocrystals: Synthetic Strategies, Optical Properties, and Applications

Lead chalcogenide nanocrystals (NCs) are an emerging class of photoactive materials that have become a versatile tool for fabricating new generation photonics devices operating in the near-IR spectral range. NCs are presented in a wide variety of forms and sizes, each of which has its own unique features. Here, we discuss colloidal lead chalcogenide NCs in which one dimension is much smaller than the others, i.e., two-dimensional (2D) NCs. The purpose of this review is to present a complete picture of today's progress on such materials. The topic is quite complicated, as a variety of synthetic approaches result in NCs with different thicknesses and lateral sizes, which dramatically change the NCs photophysical properties. The recent advances highlighted in this review demonstrate lead chalcogenide 2D NCs as promising materials for breakthrough developments. We summarized and organized the known data, including theoretical works, to highlight the most important 2D NC features and give the basis for their interpretation.

Single-particle properties of topological Wannier excitons in bismuth chalcogenide nanosheets

We analyze the topology, dispersion, and optical selection rules of bulk Wannier excitons in nanosheets of Bi₂Se₃, a topological insulator in the family of the bismuth chalcogenides. Our main finding is that excitons also inherit the topology of the electronic bands, quantified by the skyrmion winding numbers of the constituent electron and hole pseudospins as a function of the total exciton momentum. The excitonic bands are found to be strongly indirect due to the band inversion of the underlying single-particle model. At zero total momentum, we predict that the s-wave and d-wave states of two exciton families are selectively bright under left- or right-circularly polarized light. We furthermore show that every s-wave exciton state consists of a quartet with a degenerate and quadratically dispersing nonchiral doublet, and a chiral doublet with one linearly dispersing mode as in transition metal dichalcogenides. Finally, we discuss the potential existence of topological edge states of chiral excitons arising from the bulk-boundary correspondence.

THz mobility and polarizability: impact of transformation and dephasing on the spectral response of excitons in a 2D semiconductor

We introduce a response theory based transformation for excitonic polarizability into mobility, which allows an in-depth analysis of optical pump-THz probe conductivity experiments, and compare the results with those of a conventional oscillator model. THz spectroscopy is of high interest e.g. for investigations in high bandwidth and low noise nanoelectronics or solar energy harvesting nanomaterials. In contrast to simple ω scaling of estimated static polarizability, suggested in the literature, an appropriate transformation of the spectral response into mobility can be achieved in principle forward and backward due to the presence of dephasing, as we show for the exemplary system of CdSe nanoplatelets. Common analysis approaches capture the excitonic properties only under specific conditions, and do not apply in many cases. We demonstrate that a thermal distribution of excitons and transitions between higher states in general have to be considered and that dephasing has to be taken into account for a proper transformation at all temperatures. The presented in-depth understanding of the exciton mobility in nanoparticles can help improve e.g. solar hydrogen generation, charge extraction efficiencies of solar cells, or light emission performance of LEDs.

Carrier confinement and interband optical transitions in lead chalcogenide quantum wells, nanosheets, and nanoplatelets

Analytic equation for energy dispersion of electronic states in lead chalcogenide nanosheets is derived within an effective mass model. Selection rules for interband optical transitions are analyzed and expressions for interband optical matrix elements are obtained. It is shown that the main effect of the lateral confinement in nanoplatelets can be accounted for in terms of the quantized in-plane wave vector.

Exciton States and Optical Absorption in CdSe and PbS Nanoplatelets

The exciton states and their influence on the optical absorption spectrum of CdSe and PbS nanoplatelets (NPLs) are considered theoretically in this paper. The problem is discussed in cases of strong, intermediate, and weak size quantization regimes of charge carrier motion in NPLs. For each size quantization regime, the corresponding potential that adequately describes the electron-hole interaction in this mode of space quantization of charge carriers is chosen. The single-particle energy spectra and corresponding wave functions for strong intermediate and weak size quantization regimes have been revealed. The dependence of material parameters on the number of monolayers in the sample has been considered. The related selection rules and the dependence of the absorption coefficient on the frequency and polarization direction of the incident light wave were obtained. The interband transition threshold energy dependencies were obtained for each size quantization regime. The effect of dielectric coefficient mismatch and different models of electron-hole interaction potentials have been studied in CdSe and PbS NPLs. It is also shown that with an increase in the linear dimensions of the structure, the threshold frequency of absorption decreases. The binding energies and absorption coefficient results for NPL with different thicknesses agree with the experimental data. The values of the absorption exciton peaks measured experimentally are close to our calculated values for CdSe and PbS samples.

Dielectrically Confined Stable Excitons in Few-Atom-Thick PbS Nanosheets

Two-dimensional colloidal PbS nanosheets exhibit more than one order of magnitude larger exciton binding energy than their bulk counterpart, making it possible to generate stable excitons at room temperature. It is experimentally revealed that the binding energy of the exciton increases from 26 to 68 meV as the thickness of the PbS nanosheet decreases from 4.7 to 1.2 nm. The dielectric confinement of the exciton plays a critical role in the binding-energy enhancement. The large binding energy results in a fast thickness-dependent exciton radiative recombination rate, confirmed experimentally.

Colloidal 2D PbSe nanoplatelets with efficient emission reaching the telecom O-, E- and S-band

Colloidal two-dimensional (2D) lead chalcogenide nanoplatelets (NPLs) represent highly interesting materials for near- and short wave-infrared applications including innovative glass fiber optics exhibiting negligible attenuation. In this work, we demonstrate a direct synthesis route for 2D PbSe NPLs with cubic rock salt crystal structure at low reaction temperatures of 0 °C and room temperature. A lateral size tuning of the PbSe NPLs by controlling the temperature and by adding small amounts of octylamine to the reaction leads to excitonic absorption features in the range of 1.55-1.24 eV (800-1000 nm) and narrow photoluminescence (PL) reaching the telecom O-, E- and S-band (1.38-0.86 eV, 900-1450 nm). The PL quantum yield of the as-synthesized PbSe NPLs is more than doubled by a postsynthetic treatment with CdCl₂ (e.g. from 14.7% to 37.4% for NPLs emitting at 980 nm with a FWHM of 214 meV). An analysis of the slightly asymmetric PL line shape of the PbSe NPLs and their characterization by ultrafast transient absorption and time-resolved PL spectroscopy reveal a surface trap related PL contribution which is successfully reduced by the CdCl₂ treatment from 40% down to 15%. Our results open up new pathways for a direct synthesis and straightforward incorporation of colloidal PbSe NPLs as efficient infrared emitters at technologically relevant telecom wavelengths.

Population dependence of THz charge carrier mobility and non-Drude-like behavior in short semiconductor nanowires

We investigate THz radiation absorption by charge carriers, focusing on the mobility in nanorods and wires. We show that for short rods the mobility is limited by the high spacing of the charge carrier energy levels, while for longer wires (greater 25 nm) finite dephasing results in considerably higher low frequency mobility. Analyzing the length, temperature and population dependence, we demonstrate that, apart from the temperature dependent dephasing, the mobility becomes strongly charge carrier population dependent. The latter results in no simple linear relationship between carrier density and conductivity. Additionally their thermal distribution determines the mobility, measured in experiments. We further show that Drude or Plasmon models apply only for long wires at elevated temperatures, while for short length quantization results in considerable alterations. In contrast to those phenomenological models, i.e. a negative imaginary part of the frequency-dependent conductivity in a nanosystem can be understood microscopically. Based on the results, we develop guidelines to analyze 1D terahertz conductivity spectra. Our approach provides also a new tool to optimize the mobility by nanowire length as well as to analyze the dephasing, not by conventional wave mixing techniques, but by coherent optical pump-THz probe spectroscopy.

Colloidal Two-Dimensional Metal Chalcogenides: Realization and Application of the Structural Anisotropy

ConspectusDue to the spatial confinement, two-dimensional metal chalcogenides display an extraordinary optical response and carrier transport ability. Solution-based synthesis techniques such as colloidal hot injection and ion exchange provide a cost-effective way to fabricate such low-dimensional semiconducting nanocrystals. Over the years, developments in colloidal chemistry made it possible to synthesize various kinds of ultrathin colloidal nanoplatelets, including wurtzite- and zinc blende-type CdSe, rock salt PbS, black phosphorus-like SnX (X = S or Se), hexagonal copper sulfides, selenides, and even transition metal dichalcogenides like MoS₂. By altering experimental conditions and applying capping ligands with specific functional groups, it is possible to accurately tune the dimensionality, geometry, and consequently the optical properties of these colloidal metal chalcogenide crystals. Here, we review recent progress in the syntheses of two-dimensional colloidal metal chalcogenides (CMCs) and property characterizations based on optical spectroscopy or device-related measurements. The discoveries shine a light on their huge prospect for applications in areas such as photovoltaics, optoelectronics, and spintronics. In specific, the formation mechanisms of two-dimensional CMCs are discussed. The growth of colloidal nanocrystals into a two-dimensional shape is found to require either an intrinsic structural asymmetry or the assist of coexisted ligand molecules, which act as lamellar double-layer templates or "facet" the crystals via selective adsorption. By performing optical characterizations and especially ultrafast spectroscopic measurements on these two-dimensional CMCs, their unique electronic and excitonic features are revealed. A strong dependence of optical transition energies linked to both interband and inter-subband processes on the crystal geometry can be verified, highlighting a tremendous confinement effect in such nanocrystals. With the self-assembly of two-dimensional nanocrystals or coupling of different phases by growing heterostructures, unconventional optical performances such as charge transfer state generation or efficient Förster resonance energy transfer are discovered. The growth of large-scale individualized PbS and SnS nanosheets can be realized by facile hot injection techniques, which gives the opportunity to investigate the charge carrier behavior within a single nanocrystal. According to the results of the device-based measurements on these individualized crystals, structure asymmetry-induced anisotropic electrical responses and Rashba effects caused by a splitting of spin-resolved bands in the momentum space due to strong spin-orbit-coupling are demonstrated. It is foreseen that such geometry-controlled, large-scale two-dimensional CMCs can be the ideal materials used for designing high-efficiency photonics and electronics.

Prospects of Coupled Organic-Inorganic Nanostructures for Charge and Energy Transfer Applications

We review the field of organic-inorganic nanocomposites with a focus on materials that exhibit a significant degree of electronic coupling across the hybrid interface. These nanocomposites undergo a variety of charge and energy transfer processes, enabling optoelectronic applications in devices which exploit singlet fission, triplet energy harvesting, photon upconversion or hot charge carrier transfer. We discuss the physical chemistry of the most common organic and inorganic components. Based on those we derive synthesis and assembly strategies and design criteria on material and device level with a focus on photovoltaics, spin memories or optical upconverters. We conclude that future research in the field should be directed towards an improved understanding of the binding motif and molecular orientation at the hybrid interface.

Probing Excitons in Ultrathin PbS Nanoplatelets with Enhanced Near-Infrared Emission

Colloidal PbS nanoplatelets (NPLs) are highly interesting materials for near-infrared optoelectronic applications. We use ultrafast transient optical absorption spectroscopy to study the characteristics and dynamics of photoexcited excitons in ultrathin PbS NPLs with a cubic crystal structure. NPLs are synthesized at near room temperature from lead oleate and thiourea precursors; they show an optical absorption onset at 680 nm (1.8 eV) and photoluminescence at 720 nm (1.7 eV). By postsynthetically treating PbS NPLs with CdCl₂, their photoluminescence quantum yield is strongly enhanced from 1.4% to 19.4%. The surface treatment leads to an increased lead to sulfur ratio in the structures and associated reduced nonradiative recombination. Additionally, exciton-phonon interactions in pristine and CdCl₂ treated NPLs at frequencies of 1.96 and 2.04 THz are apparent from coherent oscillations in the transient absorption spectra. This study is an important step forward in unraveling and controlling the optical properties of IV-VI semiconductor NPLs.