arXiv subjects

Eugen Klein

Publications and source records attributed to Eugen Klein.

17 recordsLinked to original sources

arXiv2025

Unlocking klockmannite: formation of colloidal quasi-2D CuSe nanocrystals and photo-physical properties arising from crystal anisotropy

Copper selenide is an exceptional quasi-layered monolithic material that exhibits both semiconducting and metallic properties in adjacent visible and near-infrared (NIR) spectral ranges. Here we introduce a thiol-free colloidal synthesis for generating quasi-2D klockmannite copper selenide nanocrystals via hot injection method, achieving shape control by tuning the injection temperature and precursor concentrations without any additional ligands. This approach produces large klockmannite nanosheets with lateral sizes from 200 nm to several micrometres, as well as uniform triangular nanoplatelets with sizes of 12-25 nm that are monocrystalline and display strong NIR plasmonic absorption. The spectral features of the anisotropic klockmannite phase in the NIR have been analysed using complex-scaled discrete dipole approximation (CSDDA) calculations, which reveal pronounced optical anisotropy and the emergence of hyperbolic regime. The combined effect of propagating and evanescent fields is regarded as the underlying reason of such modes in the hyperbolic domain. Finally, the ultrafast photophysical behaviour of the material in klockmannite phase is examined, including hot-hole cooling, trapping, and coherent phonons generation. Our findings emphasize the important role of the intrinsic crystal anisotropy in governing the physical properties of nanoscale klockmannite.

cond-mat.mtrl-sci↗

arXiv2025

Colloidal quasi-2D Cs2AgBiBr6 double perovskite nanosheets: synthesis and application as high-performance photodetectors

The search for non-toxic lead-free halide perovskites that can compete with the lead-based counterparts has led to the emergence of double perovskites as potential candidates. Among many options, Cs2AgBiBr6 stands out as one of the most suitable eco-friendly materials for numerous optoelectronic applications. In this study, quasi-2D Cs2AgBiBr6 nanosheets (NSs) were prepared via the low-temperature injection colloidal synthesis and used to fabricate high-performance photodetectors in a transport-layer-free architecture. The reaction temperature and ligands played vital roles in the structural purity, shape, and size of the synthesized Cs2AgBiBr6 NSs. The fabricated NSs disclosed lateral sizes of up to 1.4 um and are only a few nanometers thick. The high-performance photodetectors fabricated using the Cs2AgBiBr6 NSs yielded a high detectivity (D) of 1.15*10^12 Jones, responsivity (R) of 121 mA/W, a notable on/off ratio of 2.39*10^4, and a fast rise and decay time of 857 and 829 us, respectively. The device demonstrates remarkable stability. Basically, it sustains its entire photocurrent after storage in ambient conditions for 80 days. This work showcases a pathway for the colloidal synthesis of quasi-2D Cs2AgBiBr6 lead-free double perovskite NSs with suitable properties for high-performance photodetection and other optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Boosting Photodetection via Plasmonic Coupling in Quasi-2D Mixed-n Ruddlesden-Popper Perovskite Nanostripes

Quasi-2D metal halide perovskites have emerged as a promising material for photodetection due to excellent optoelectronic properties, simple synthesis, and robust stability. Albeit, developing high-performance photodetectors based on low-dimensional quasi-2D metal halide perovskite nanoparticles remains challenging due to quantum and dielectric confinement effects. Several approaches have been employed to improve efficiency, with plasmonic nanostructures being among the most effective ones. The resonant energy transfer and coupling between plasmons and excitons play a vital role in enhancing device performance. Here, we demonstrate enhanced photodetection of quasi-2D perovskite nanostripes resulting from the incorporation of octadecanethiol (ODT) functionalized Ag nanostructure arrays (ANA). Using colloidal lithography, ANA were fabricated. Reflectance spectroscopy and finite element method (FEM) simulations show that ANA supports localised surface plasmon resonance (LSPR) modes that spectrally coincide with the absorption and emission band of the perovskite. This spectral overlap enables interesting coupling interactions between the excitons and plasmons. The ODT-functionalized ANA photodetectors exhibit weak to intermediate coupling, resulting in a photocurrent enhancement factor of 838 %. They achieve photoresponsivities of up to 70.41 mA W^-1, detectivities of 1.48*10^11 Jones and external quantum efficiencies of 21.55 %, which are approximately 10 times higher than those of the reference photodetector. We present an approach to optimize the plasmon-exciton coupling and non-radiative energy transfer for developing high-performance plasmonic-perovskite hybrid photodetectors.

cond-mat.mtrl-sci↗

arXiv2025

Chasing Charge Carriers: Diffusion Dynamics in Mixed-n Quasi-Two-Dimensional Colloidal MAPbBr3 Perovskites

In optoelectronic applications, metal halide perovskites (MHPs) are compelling materials because of their highly tuneable and intensely competitive optical properties. Colloidal synthesis enables the controlled formation of various morphologies of MHP nanocrystals, all with different carrier properties and, hence, different optical and carrier transport behaviours. We characterized three different methylammonium lead tribromide perovskite (MAPbBr3) morphologies: nanoplatelets (NPLs), nanosheets (NSs), and nanostripes (NSTs) synthesized by hot-injection synthesis protocols with slightly different parameters. A fluorescence imaging microscope (FLIM) for time- and space-resolved measurements of the carrier migration was employed to quantify the charge carriers' migration process upon photoexcitation. The results are rationalized in the two-dimensional diffusion model framework, considering funnelling and trapping processes in mixed-n colloidal MHPs. Subdiffusion mode was found to prevail in the nanocrystals, whereby the highest carrier diffusivity was found for bulk-like NSTs, followed by layered NSs and a film of NPLs. These findings provide a better understanding of optoelectronic processes in perovskites relevant to photovoltaic and light-emitting devices.

cond-mat.mtrl-sci↗

arXiv2024

Towards high-performance photodetectors based on quasi-2D Ruddlesden-Popper mixed-n perovskite nanomaterials

The excellent optoelectronic properties, straightforward synthesis, and robust material stability of quasi-2D metal halide perovskites have made them a hot research topic for optoelectronic devices. In layered organic-inorganic perovskites, the optoelectronic properties are greatly influenced by morphology and quantum confinement, which are crucial for their photodetection performance and offer an excellent tunability tool. Here we investigated the optoelectronic properties and photodetection performance of quasi-2D methylammonium lead bromide perovskites with three different morphologies nanoplatelets, nanosheets, and nanostripes produced by colloidal hot injection technique. The structural and optical characterization reveals the mixed-n phases with varying extents of confinement. Our study shows that nanostripes with the weak confinement effect exhibit superior photodetection performance. They demonstrate highest photoresponsivity, EQE, and detectivity of 183 mA/W, 56 % and 2.9*10^11 Jones respectively. They also show the highest on/off ratio and fastest photoresponse (tau_r=3.4 ms and tau_f=4.3 ms) in comparison to nanoplatelets and nanosheets. Our study will aid in realizing an inexpensive high-performance photodetector based on nanoparticles of quasi-2D perovskites.

cond-mat.mtrl-sci↗

arXiv2024

Colloidal quasi-2D Methylammonium Lead Bromide Perovskite Nanostructures with Tunable Shape and High Chemical Stability

Control over the lateral dimensions of colloidal nanostructures is a complex task which requires a deep understanding of the formation mechanism and reactivity in the corresponding systems. As a result, it provides a well-founded insight to the physical and chemical properties of these materials. In this work, we demonstrate the preparation of quasi-2D methylammonium lead bromide nanostripes and discuss the influence of some specific parameters on the morphology and stability of this material. The variation in the amount of the main ligand dodecylamine gives a large range of structures beginning with 3D brick-like particles at low concentrations, nanostripes at elevated and ultimately nanosheets at large concentrations. The amount of the co-ligand trioctylphosphine can alter the width of the nanostripe shape to a certain degree. The thickness can be adjusted by the amount of the second precursor methylammonium bromide. Additionally, insights are given for the suggested formation mechanism of these anisotropic structures as well as for stability against moisture at ambient conditions in comparison with differently synthesized nanosheet samples.

cond-mat.mtrl-sci↗

arXiv2024

Understanding the Optoelectronic Processes in Colloidal 2D Multi-Layered MAPbBr3 Perovskite Nanosheets: Funneling, Recombination and Self-Trapped Excitons

Quasi two-dimensional (2D) colloidal synthesis made quantum confinement readily accessible in perovskites, generating additional momentum in perovskite LED research and lasing. Ultrathin perovskite layers exhibit high exciton binding energies and beneficial charge transport properties interesting for solar cells. In 2D perovskites, the combination of layers with different thickness helps to direct charge carriers in a targeted manner toward thicker layers with a smaller bandgap. However, detailed knowledge about the mechanisms by which excitons and charge carriers funnel and recombine in these structures is lacking. Here, we characterize colloidal 2D methylammonium lead bromide (MAPbBr3) Ruddlesden-Popper perovskites with a broad combination of layers (n = 3 to 10, and bulk fractions with n > 10) in one stack by femtosecond transient absorption spectroscopy and time-resolved photoluminescence, which gives comprehensive insights into the complexity of funneling and recombination processes. We find that after photoexcitation second- and third-order processes dominate in MAPbBr3 nanosheets, which indicates exciton-exciton annihilation (EEA) and Auger recombination. Long-lived excitons in thin layers (e.g., n = 5, Eb = 136 meV) funnel into high n with t = 10-50 ps, which decreases their exciton binding energy below kB T = 26 meV ( T = 300K) and leads to radiative recombination. Parallel and consecutive funneling compete with exciton trapping processes, making funneling an excellent tool to overcome exciton self-trapping when high-quality n-n interfaces are present. Free charge carriers in high n regions on the other hand facilitate radiative recombination and EEA is bypassed, which is desirable for LED and lasing applications.

cond-mat.mtrl-sci↗

arXiv2023

Template Mediated Formation of Colloidal Two-Dimensional Tin Telluride Nanosheets and the Role of the Ligands

We report the colloidal synthesis of 2D SnTe nanosheets through precursor hot-injection in a nonpolar solvent. During the reaction, an important intermediate - Sn-template - is formed which defines the confined growth of SnTe. This "flake-like" structure gives the first evidence for the possible 2D morphology formation prior to the anion precursor injection (TOP-Te). Additionally, we explore the role of each ligand in the reaction process. Thus, we explain the formation and morphology evolution of 2D SnTe nanostructures from a mechanism perspective as well as the role of each ligand on the molecular scale. The interplay of ligands provides the necessary conditions for the realization of stable low-dimensional SnTe nanomaterials with tunable size and shape.

cond-mat.mtrl-sci↗

arXiv2019

Photoexcitation of PbS Nanosheets Leads to Highly Mobile Charge Carriers and Stable Excitons

Solution-processable two-dimensional (2D) semiconductors with chemically tunable thickness and associated tunable band gaps are highly promising materials for ultrathin optoelectronics. Here, the properties of free charge carriers and excitons in 2D PbS nanosheets of different thickness are investigated by means of optical pump-terahertz probe spectroscopy. By analyzing the frequency-dependent THz response, a large quantum yield of excitons is found. The scattering time of free charge carriers increases with nanosheet thickness, which is ascribed to reduced effects of surface defects and ligands in thicker nanosheets. The data discussed provide values for the DC mobility in the range 550 - 1000 cm2/Vs for PbS nanosheets with thicknesses ranging from 4 to 16 nm. Results underpin the suitability of colloidal 2D PbS nanosheets for optoelectronic applications.

physics.chem-ph↗

arXiv2019

Micron-size two-dimensional methylammonium lead halide perovskites

Hybrid lead halide perovskites with 2D stacking structures have recently emerged as promising materials for optoelectronic applications. We report a method for growing 2D nanosheets of hybrid lead halide perovskites (I, Br and Cl), with tunable lateral sizes ranging from 0.05 to 8 microns, and a structure consisting of n stacked monolayers separated by long alkylamines, tunable from bulk down to n=1. The key to obtaining such a wide range of perovskite properties hinged on utilizing the respective lead halide nanosheets as precursors in a hot-injection synthesis that afforded careful control over all process parameters. The layered, quantum confined (n small than 4) nanosheets were comprised of major and minor fractions with differing n. Energy funneling from low to high n (high to low energy) regions within a single sheet, mediated by the length of the ligands between stacks, produced photoluminescent quantum yields as high as 49 percent. These large, tunable 2D nanosheets could serve as convenient platforms for future high efficiency optoelectronic devices.

physics.app-ph↗

arXiv2019

In-plane anisotropic faceting of ultralarge and thin single-crystalline colloidal SnS nanosheets

The colloidal synthesis of large thin two-dimensional (2D) nanosheets is fascinating but challenging, since the growth along the lateral and vertical dimensions need to be controlled independently. In-plane anisotropy in 2D nanosheets is attracting more attention as well. We present a new synthesis for large colloidal single-crystalline SnS nanosheets with the thicknesses down to 7 nm and lateral sizes up to 8 um. The synthesis uses trioctylphosphine-S (TOP-S) as sulfur source and oleic acid (with or without TOP) as ligands. Upon adjusting the capping ligand amount, the growth direction can be switched between anisotropic directions (armchair and zigzag) and isotropic directions ("ladder" directions), leading to an edge-morphology anisotropy. This is the first report on solution-phase synthesis of large thin SnS NSs with tunable edge faceting. Furthermore, electronic transport measurements show strong dependency on the crystallographic directions confirming structural anisotropy.

cond-mat.mtrl-sci↗

arXiv2018

Colloidal Lead Iodide Nanorings

Colloidal chemistry of nanomaterials experienced a tremendous development in the last decades. In the course of the journey 0D nanoparticles, 1D nanowires, and 2D nanosheets have been synthesized. They have in common to possess a simple topology. We present a colloidal synthesis strategy for lead iodide nanorings, with a non-trivial topology. First, two-dimensional structures were synthesized in nonanoic acid as the sole solvent. Subsequently, they underwent an etching process in the presence of trioctylphosphine, which determines the size of the hole in the ring structure. We propose a mechanism for the formation of lead iodide nanosheets which also explains the etching of the two-dimensional structures starting from the inside, leading to nanorings. In addition, we demonstrate a possible application of the as-prepared nanorings in photodetectors. These devices are characterized by a fast response, high gain values, and a linear relation between photocurrent and incident light power intensity over a large range. The synthesis approach allows for inexpensive large-scale production of nanorings with tunable properties.

cond-mat.mtrl-sci↗

arXiv2018

Copper sulfide nanosheets with shape-tunable plasmonic properties in the NIR

2D copper sulfide nanocrystals are promising building blocks for plasmonic materials in the near-infrared (NIR) spectral region. We demonstrate precise shape and size control (hexagonal/triangle) of colloidal plasmonic copper sulfide (covellite) nano-prisms simply by tuning the precursors concentration without introduction of additional ligands. The ultra-thin 2D nanocrystals possess sizes between 13 and 100 nm and triangular or hexangular shapes. We also demonstrate CuS nanosheets (NSs) with lateral sizes up to 2 microns using a syringe pump. Based on the experimental findings and DFT simulations we propose a qualitative and quantitative mechanism for the formation of different shapes. The analysis of the spectral features in the NIR of synthesized CuS nanocrystals has been performed in respect to the shape and the size of particles by the discrete dipole approximation method and the Drude-Sommerfeld theory.

cond-mat.mtrl-sci↗

arXiv2018

Colloidal Tin Sulfide Nanosheets: Formation Mechanism, Ligand-mediated Shape Tuning and Photo-detection

Colloidal materials of tin(II) sulfide (SnS), as a layered semiconductor with a narrow band gap, are emerging as a potential alternative to the more toxic metal chalcogenides (PbS, PbSe, CdS, CdSe) for various applications such as electronic and optoelectronic devices. We describe a new and simple pathway to produce colloidal SnS nanosheets with large lateral sizes and controllable thickness, as well as single-crystallinity. The synthesis of the nanosheets is achieved by employing tin(II) acetate as tin precursor instead of harmful precursors such as bis[bis(trimethylsilyl)amino] tin(II) and halogen-involved precursors like tin chloride, which limits the large-scale production. We successfully tuned the morphology between squared nanosheets with lateral dimensions from 150 to about 500 nm and a thickness from 24 to 29 nm, and hexagonal nanosheets with lateral sizes from 230 to 1680 nm and heights ranging from 16 to 50 nm by varying the ligands oleic acid and trioctylphosphine. The formation mechanism of both shapes has been investigated in depth, which is also supported by DFT simulations. The optoelectronic measurements show their relatively high conductivity with a pronounced sensitivity to light, which is promising in terms of photo-switching, photo-sensing, and photovoltaic applications also due to their reduced toxicity.

cond-mat.mtrl-sci↗

arXiv2018

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.

cond-mat.mtrl-sci↗

arXiv2018

Halogens in the synthesis of colloidal semiconductor nanocrystals

In this review, we highlight the role of halogenated compounds in the colloidal synthesis of nanostructured semiconductors. Halogen-containing metallic salts used as precursors and halogenated hydrocarbons used as ligands allow stabilizing different shapes and crystal phases, and enable the formation of colloidal systems with different dimensionality. We summarize recent reports on the tremendous influence of these compounds on the physical properties of nanocrystals, like field-effect mobility and solar cell performance and outline main analytical methods for the nanocrystal surface control.

cond-mat.mtrl-sci↗

arXiv2016

New ways to synthesize lead sulfide nanosheets - substituted alkanes direct the growth of 2D nanostructures

Two-dimensional colloidal nanosheets represent very attractive optoelectronic materials. They combine good lateral conductivity with solution-processability and geometry-tunable electronic properties. In case of PbS nanosheets, so far the synthesis was driven by the addition of chloroalkanes as coligands. Here, we demonstrate how to synthesize two-dimensional lead sulfide nanostructures using other halogen alkanes and primary amines. Further, we show that at a reaction temperature of 170C a coligand is not even necessary, and the only ligand, oleic acid, controls the anisotropic growth of the two-dimensional structures and using thiourea as sulfide source, nanosheets with lateral dimensions of over 10 microns are possible.

cond-mat.mtrl-sci↗