In our group we investigate phenomena emerging in novel solid state materials with dimensions in the nanometer range. Such materials are generated by colloidal synthesis, chemical vapor deposition, and electron-beam lithography. We characterize them by means of optoelectronic transport measurements, spectroscopical methods, and electron microscopy. Of special interest are the electrical properties of colloidal nanomaterials, carbon nanotubes, and composites.

The head of the group, Christian Klinke, is also faculty member at the Chemistry Department of the Swansea University.

UPCOMING EVENTS

November 2024 WE-Heraeus-Seminar on "Uniting today’s nanotechnology for advancing tomorrow’s semiconductor physics" (Physikzentrum Bad Honnef, Germany).

NEWS

Follow us on Twitter: Klinke Lab , on BlueSky: Klinke Lab, and on Mastodon: Klinke Lab.

04/02/2024 We published a new paper in Small Science.
The surface ligands in colloidal metal halide perovskites influence not only their intrinsic optoelectronic properties but also their interaction with other materials and molecules. Donor–acceptor interactions of CsPbBr₃ perovskite nanocrystals with TiO₂ nanoparticles and nanotubes are explored by replacing long-chain oleylamine ligands with short-chain butylamines. Through postsynthesis ligand exchange, the nanocrystals are functionalized with butylamine ligands while their intrinsic properties are maintained. In solution, butylamine-capped nanocrystals exhibit reduced photoluminescence intensity with increasing TiO₂ concentration but without any change in photoluminescence lifetime. Intriguingly, the Stern–Volmer plot depicts different slopes at low and high TiO₂ concentrations, suggesting donor-acceptor interaction through mixed static photoluminescence quenching and quenching sphere of action mechanism . Oleylamine-capped nanocrystals in solution, on the other hand, show no interaction with TiO₂, as indicated by consistent photoluminescence intensities and lifetimes before and after TiO₂ addition. In films, both types exhibit decreased photoluminescence lifetime with TiO₂, indicating enhanced donor–acceptor interaction, which is discussed in terms of electron transfer. TiO₂ nanotubes enhance nonradiative recombination more in butylamine-capped CsPbBr₃ perovskite nanocrystals, emphasizing the role of ligand chain length.
Reference: Syed Abdul Basit Shah et al., Small Science (2024) online.

11/01/2023 Fugui He joined us as a DFG Post-Doc fellow. A warm welcome!

07/03/2023 We published a new paper in ACS Appl. Nano Mater.
Doping quasi-two-dimensional semiconductor nanoplatelets (NPLs) which possess atomically exact thicknesses has attracted intense research interests, because it determines their unique optoelectronic properties. Dopants in ultrathin NPLs tend to diffuse to the surface due to the self-purification effect, which can result in reduced optical performance such as shortened photoluminescence (PL) lifetimes, low PL quantum yields (PLQYs), and broadening of spectral linewidth. To address these issues, an effective way is to overgrow the NPLs with a semiconductor shell to locate dopants away from the surface. In contrast to Cd-based core/shell NPLs, heavy-metal-free core/shell NPLs made of Zn-chalcogenides have hardly been explored. Here, we synthesized colloidal ZnSe:Mn/ZnS core/shell NPLs for the first time via a heat-up method. A combination of zinc diethyldithiocarbamate and ZnCl2 yielded smooth and homogeneous shells on pregrown ZnSe:Mn NPLs. The resulting ZnSe:Mn/ZnS core/shell NPLs exhibit noticeably improved optical properties compared to the ZnSe:Mn core-only NPLs. In particular, the Mn2+ PLQY is enhanced by more than one order of magnitude upon deposition of ZnS shells, which can be attributed to an increase in the Mn2+ internal quantum efficiency. We systematically investigated both the matrix- and dopant-related PL kinetics as well as the PLQYs. Further, using a descriptive mathematical model, we recognized the dominant role of the Mn2+ nonradiative relaxation channels in the energy-transfer route from ZnSe absorption to the luminescence of Mn2+ ions. Our findings can contribute to a better understanding and to applications of heavy-metal-free core/shell NPLs with superior fluorescence, photostability, and low toxicity, for example, in UV-light-converting devices, light-emitting diodes, imaging, and bio-labeling.
Reference: Liwei Dai et al., ACS Appl. Nano Mater. 6 (2023) 11124.

06/03/2023 We published a new paper in Neural Networks.
We explore different strategies to integrate prior domain knowledge into the design of graph neural networks (GNN). Our study is supported by a use-case of estimating the potential energy of chemical systems (molecules and crystals) represented as graphs. We integrate two elements of domain knowledge into the design of the GNN to constrain and regularise its learning, towards higher accuracy and generalisation. First, knowledge on the existence of different types of relations/graph edges (e.g. chemical bonds in our case study) between nodes of the graph is used to modulate their interactions. We formulate and compare two strategies, namely specialised message production and specialised update of internal states. Second, knowledge of the relevance of some physical quantities is used to constrain the learnt features towards a higher physical relevance using a simple multi-task learning (MTL) paradigm. We explore the potential of MTL to better capture the underlying mechanisms behind the studied phenomenon. We demonstrate the general applicability of our two knowledge integrations by applying them to three architectures that rely on different mechanisms to propagate information between nodes and to update node states. Our implementations are made publicly available. To support these experiments, we release three new datasets of out-of-equilibrium molecules and crystals of various complexities.
Reference: Jay Morgan et al., Neural Networks 165 (2023) 938.

06/20/2023 We published a new paper in the Journal of Materials Chemistry C.
Nanostructured lead halide perovskites are an attractive group of materials for highly efficient optoelectronic applications. Considering the wide variety in shape, size and crystal structures (layered, non-layered, hollow, double), many different excited-state population and relaxation processes can be present, such as exciton formation, free charge carrier generation, exciton-exciton annihilation, and Auger recombination. Here, we present the synthesis of nanostructures with different morphology to improve the optical properties of methylammonium lead iodide and to investigate the corresponding charge-carrier dynamics and recombination processes. Excitation intensity dependent time-resolved photoluminescence (PL) measurements are performed to highlight the PL decay behaviour of nanosheets, nanoplatelets, and nanostripes. In all three cases a faster decay rate is observed with increasing excitation fluence. This behaviour can be attributed to the occurrence of bimolecular recombination of electrons and holes in nanosheets and exciton-exciton annihilation in nanostripes with high carrier densities. Performing these measurements at low excitation intensities revealed unusual long charge-carrier lifetimes of a few microseconds. Additionally, the nanostripes showed the best results in terms of photoluminescence quantum yield of up to 75 % and the nanosheets demonstrate improved stability. The nanoplatelets are considered as intermediate structures between nanosheets and nanostripes – balancing quantum yield and stability.
Reference: Eugen Klein et al., J. Mater. Chem. C 11 (2023) 9495.

04/01/2023 Brindhu Malani Satya Seelan joined us as Henriette-Herz Humboldt fellow. A warm welcome!

03/01/2023 Together with the group of Zhongwu Wang (Cornell), we published a new paper in JACS.
Diffusion-mediated assembly of octahedral PbS nanocrystals (NCs) in a confined antisolvent environment displays a primary burst nucleation and Ostwald ripening growth of rhombic bcc supercrystals, followed by a secondary seed-based nucleation and oriented attachment growth of triangle fcc supercrystals. As the diffusion proceeds from ethanol across a sharp interface into NC-suspended toluene, a burst nucleation of supercrystal seeds occurs, and such supercrystals are quickly developed into rhombic grains that have a bcc structure. At a critical size of 10 μm, an Ostwald ripening event appears to guide the supercrystal growth. Upon grain growth above 30 μm, the fcc supercrystals start a nucleation at two symmetrical tips of individual rhombic crystals. Such fcc supercrystals are developed with a triangle shape, and two triangles are combined with one bcc rhombus in-between to form a butterfly-like bowtie stacking structure. The fcc triangle wings grow larger at a reduction of bcc rhombus cores. As the bcc cores gradually fade, such butterfly-like bowtie crystals aggregate and undergo an oriented attachment process, leading to the formation of freestanding 3D triangle crystals that have a single fcc lattice. Analysis of experimental observations and defined diffusion parameters reveals that fast solvent diffusion and high-NC concentration promote the growth of rhombic bcc supercrystals, while slow solvent diffusion and low-NC concentration accelerate the development of triangle fcc supercrystals. Upon succeeding in designable growth of 3D fcc supercrystals, this study provides designing principles for controlled fabrication of supercrystals with desired superlattices for additional engineering and applications.
Reference: Xin Huang et al., J. Am. Soc. Chem. 145 (2023) 4500.

02/04/2023 The registration for our ...
... Bad Honnef Physics School on "Exciting nanostructures: Characterizing advanced confined systems" is open now!

12/30/2022 We published a new paper in Nanoscale.
Non-toxicity and stability make two-dimensional (2D) bismuth halide perovskites better alternatives to lead-based ones for optoelectronic applications and catalysis. In this work, we synthesize sub-micron size colloidal quasi-2D Cs3Bi2I9 perovskite nanosheets and study their generation and relaxation of charge carriers. Steady-state absorption spectroscopy reveals an indirect bandgap of 2.07 eV, which is supported by the band structure calculated using density functional theory. The nanosheets show no detectable photoluminescence at room temperature at near bandgap excitation which is attributed to the indirect bandgap. However, cathodoluminescence spanning a broad range from 500 nm to 750 nm with an asymmetric and Stokes-shifted emission is observed, indicating the phonon- and trap-assisted recombination of charge carriers. We study the ultrafast charge carrier dynamics in Cs3Bi2I9 nanosheets using a femtosecond transient absorption spectroscopy. The samples are excited with photon energies higher than their bandgap, and the results are interpreted in terms of hot carrier generation (<1 ps), thermalization with local phonons (~1 ps), and cooling (>30 ps). Further, a relatively slow relaxation of excitons (≳3 ns) at the band edge suggests the formation of stable polarons which decay nonradiatively by releasing phonons.
Reference: Sushant Ghimire et al., Nanoscale 15 (2023) 2096.

11/22/2022 We published a new paper in the Journal of Physical Chemistry C.
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.
Reference: Fagui He et al., J. Phys. Chem. C 126 (2022) 20498.

10/01/2022 Ronja Piehler joined our group as PhD student. A warm welcome!

06/13/2022 Christian Klinke has been appointed Henriette Herz Scout by the Humboldt Foundation.

06/01/2022 Urvi Parekh joined our group as PhD student. A warm welcome!

04/13/2022 We published a new paper in ACS Appl. Mater. Interfaces.
Mn²⁺-doped semiconductor nanocrystals with tuned location and concentration of Mn²⁺ ions can yield diverse coupling regimes, which can highly influence their optical properties such as emission wavelength and photoluminescence (PL) lifetime. However, investigation on the relationship between the Mn²⁺ concentration and the optical properties is still challenging because of the complex interactions of Mn²⁺ ions and the host and between the Mn²⁺ ions. Here, atomically flat ZnS nanoplatelets (NPLs) with uniform thickness were chosen as matrixes for Mn²⁺ doping. Using time-resolved (TR) PL spectroscopy and density functional theory (DFT) calculations, a connection between coupling and PL kinetics of Mn²⁺ ions was established. Moreover, it is found that the Mn²⁺ ions residing on the surface of a nanostructure produce emissive states and interfere with the change of properties by Mn²⁺–Mn²⁺ coupling. In a configuration with suppressed surface contribution to the optical response, we show the underlying physical reasons for double and triple exponential decay by DFT methods. We believe that the presented doping strategy and simulation methodology of the Mn²⁺-doped ZnS (ZnS:Mn) system is a universal platform to study dopant location- and concentration-dependent properties also in other semiconductors.
Reference: Liwei Dai et al., ACS Appl. Mater. Interfaces 14 (2022) 18806.

04/01/2022 Christian Klinke joined the Associate Editor team of Nanoscale and Nanoscale Advances.

02/11/2022 We published a new paper in ACS Energy Letters.
We report a structural reconstruction-induced high photoluminescence quantum yield of 25% in colloidal two-dimensional tin iodide nanosheets that are synthesized by a hot-injection method. The as-synthesized red-colored nanosheets of octylammonium tin iodide perovskites at room temperature transform to white hexagonal nanosheets upon washing or exposure to light. This structural change increases the bandgap from 2.0 to 3.0 eV, inducing a large Stokes shift and a broadband emission. Further, a long photoluminescence lifetime of about 1 μs is measured for the nanosheets. Such long-lived broad and intense photoluminescence with a large Stokes shift is anticipated to originate from tin iodide clusters that are formed during the structural reconstruction. The stereoactive 5s² lone pair of tin(II) ions perturbs the excited state geometry of the white hexagonal nanosheets and facilitates the formation of self-trapped excitons. Such broadband and intensely emitting metal halide nanosheets are promising for white light-emitting diodes.
Reference: Sushant Ghimire et al., ACS Energy Lett 7 (2022) 975.

10/22/2021 We published a new review paper in Aggregate.
Atomically precise nanoclusters (NCs) with fascinating physicochemical characteristics different from their nanoparticles (NPs) counterparts have gained increasing attention in diverse fields of applications. The foremost outcome of such NC-based applications is leading to transform them into devices. In fact, there are already some reports on the development of NC-based devices. For instance, NCs exhibit their potential in solar cells, showing high light-harvesting efficiency comparable to traditional semiconductor solar cells. Further, recent progress in characterizing Au NCs films and micro-crystals shows semiconductor-like properties such as field effect and photoresponse. These successes indicate that metal NCs possess a high potential for application in multidisciplinary areas for advancing the development in fundamental and practical purposes. However, no such comprehensive review is available to highlight recent advances and new applicable devices based on noble metal NCs. Herein, we reviewed the recent development in this area, including synthesis challenges of metal NCs and related applications of NC-sensitized solar cells, strain sensors, chemo-/biosensors, transistors, floating memory, and other devices. Furthermore, the future opportunities such as modifying synthetic methods to make other metal NCs, enhancing the efficiency of solar cells, and exploring more NC-based devices alternative to semiconductors are pointed out. We hope that rapidly increasing interest in NC-based devices will stimulate the research in this area and inspire the advances in combined devices accordingly.
Reference: Lizhen Chen et al., Aggregate 3 (2022) e132.

10/08/2021 We published a new review paper in Accounts of Chemical Research.
Due 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.
Reference: Ziyi Hu et al., Acc. Chem. Res. 54 (2021) 3792.

09/01/2021 Katja Schulz joined our group as Technical Assistant. Welcome!

06/29/2021 We published a new review paper in Nanoscale.
Halide perovskites are promising materials for light-emitting and light-harvesting applications. In this context, two-dimensional perovskites such as nanoplatelets or Ruddlesden-Popper and Dion-Jacobson layered structures are important because of their structural flexibility, electronic confinement, and better stability. This review article brings forth an extensive overview of the recent developments of two-dimensional halide perovskites both in the colloidal and non-colloidal forms. We outline the strategy to synthesize and control the shape and discuss different crystalline phases and optoelectronic properties. We review the applications of two-dimensional perovskites in solar cells, light-emitting diodes, lasers, photodetectors, and photocatalysis. Besides, we also emphasize the moisture, thermal, and photostability of these materials in comparison to their three-dimensional analogs.
Reference: Sushant Ghimire and Christian Klinke, Nanoscale 13 (2021) 12394.

03/12/2021 We published a new paper in the Journal of Physical Chemisty C.
We investigated the colloidal synthesis of Bi₂Se₃ nanosheets in ethylene glycol with the goal of increasing their lateral dimensions while keeping their thickness within a few nanometers. The influence of proton-donating sulfuric acid was found to be a triggering factor that promoted the lateral growth of up to 10 μm, whereas the thickness remained in the range 10–12 nm. The lateral size distribution was further optimized by size-selective precipitation enabling individual contacting of the nanosheets by electron-beam lithography. Electrical characterization of individually contacted nanosheets revealed a metal-like temperature dependence of the resistivity and values of the specific conductivity in the range of 470 S/cm at room temperature and 880 S/cm at 5.5 K, which is attributed to the surface-induced off-stoichiometry. The presented colloidal nanosheets can potentially serve as a platform for further studies on topologically nontrivial surface states and arising quantum phenomena in two-dimensional systems.
Reference: Christoph Bauer et al., J. Phys. Chem. C 125 (2021) 6442.

03/09/2021 We published a new paper in the Journal of the Americal Chemical Society.
Crystallization and growth of anisotropic nanocrystals (NCs) into distinct superlattices were studied in real time, yielding kinetic details and designer parameters for scale-up fabrication of functional materials. Using octahedral PbS NC blocks, we discovered that NC assembly involves a primary lamellar ordering of NC-detached Pb(OA)₂ molecules on the front-spreading solvent surfaces. Upon a spontaneous increase of NC concentration during solvent processing, PbS NCs preferentially self-assembled into an orientation-disordered face-centered cubic (fcc) superlattice, which subsequently transformed into a body-centered cubic (bcc) superlattice with single NC-orientational ordering across individual domains. Unlike the deformation-based transformation route claimed previously, this solid–solid phase transformation involved a hidden intermediate formation of a lamellar-confined liquid interface at cost of the disassembly (melting) of small fcc grains. Such highly condensed and liquidized NCs recrystallized into the stable bcc phase with an energy reduction of 1.16 k_BT. This energy-favorable and high NC-fraction-driven bcc phase grew as a 2D film at a propagation rate of 0.74 μm/min, smaller than the 1.23 μm/min observed in the early nucleated fcc phase under a dilute NC environment. Taking such insights and defined parameters, we designed experiments to manipulate the NC assembly pathway and achieved scalable fabrication of a large/single bcc supercrystal with coherent ordering of NC translation and atomic plane orientation. This study not only provides a design avenue for controllable fabrication of a large supercrystal with desired superlattices for application but also sheds new light on the nature of crystal nucleation/growth and phase transformation by extending the lengths from the nanoscale into the atomic scale, molecular scale, and microscale levels.
Reference: Xin Huang et al., J. Am. Chem. Soc. 143 (2021) 4234.

01/13/2021 We published a new review paper in Nano Select.
During the last few decades, noble metal nanoclusters (NCs) have become an exciting building block in the field of nanoscience. With their ultrasmall size that ranges between 1 and 2 nm, NCs fill the gap between atoms and nanoparticles (NPs), and they show significantly different physicochemical properties compared to their bulk counterparts, such as molecule‐like HOMO‐LUMO discrete electronic transitions, photoluminescence, etc. These properties made NCs potential candidates in various applications, including catalysis, chemical/bioimaging, biomedicine, sensing, and energy conversion. Controlling the size of NPs, which usually exhibit a degree of polydispersity, has been a significant challenge for nano‐scientists. However, metal NCs with atomic precision pave the way to accurately fabricate NPs based on an atom‐by‐atom assembly. This Perspective is directed to the community of nano‐scientists interested in the field of NCs and summarizes the most commonly used synthetic routes of atomically precise metal NCs. Moreover, this Perspective provides an understanding of the different techniques used to control the size of metal NCs with insights on switching the surface ligands from phosphine to thiol. This Perspective also explains the role of physicochemical parameters in different synthetic routes such as high‐temperature route, CO‐directed route, solid‐state route, ligand‐exchange‐induced size/structure transformation (LEIST), etc. We finally give a brief outlook on future challenges of currently used synthetic routes with some suggestions to improve them.
Reference: Mustafa Gharib et al., Nano Select 2 (2021) 831.

01/02/2021 We published a new paper in Chemistry of Materials.
Manganese (Mn)-doped ZnS nanocrystals (NCs) have been extensively explored for optical applications with the advantages of low toxicity, large Stokes shifts, and enhanced thermal and environmental stability. Although numerous studies on Mn-doped ZnS dots, rods, and wires have been reported, the literature related to Mn-doped ZnS nanoplatelets (ZnS:Mn NPLs) is scarce. Here, we present the first example of direct doping of Mn²⁺ ions into ZnS NPLs via the nucleation-doping strategy. The resulting ZnS:Mn NPLs exhibit Mn luminescence, indicative for successful doping of the host ZnS NPLs with Mn²⁺ ions. The energy transfer from the ZnS NPLs to the Mn²⁺ ions was observed by employing spectroscopic methods. Furthermore, the impact of the Mn concentration on the optical properties of ZnS:Mn NPLs was systematically investigated. As a result of Mn–Mn interaction, tunable Mn emission and shortened photoluminescence (PL) lifetime decay were observed and rationalized by means of electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). Finally, we show that the initially low dopant PL quantum yield (QY) of ZnS:Mn NPLs can be dramatically enhanced by passivating the surface trap states of the samples. The presented synthetic strategy of ZnS:Mn NPLs opens a new way to synthesize further doped systems of two-dimensional (2D) NPLs.
Reference: Liwei Dai et al., Chem. Mater 33 (2021) 275.

11/01/2020 Sushant Ghimire joined our group as Post-Doc. Welcome!

05/18/2020 We published a new paper in Advanced Materials Interfaces.
Tin telluride is a narrow‐gap semiconductor with promising properties for infrared (IR) optical applications and topological insulators. A convenient colloidal synthesis of quasi‐2D SnTe nanocrystals through the hot‐injection method in a nonpolar solvent is reported. By introducing the halide alkane 1‐bromotetradecane as well as oleic acid and trioctylphosphine, the thickness of 2D SnTe nanostripes can be tuned down to 30 nm, while the lateral dimensional can reach 6 µm. The obtained SnTe nanostripes are single crystalline with a rock‐salt crystal structure. The absorption spectra demonstrate pronounced absorption features in the IR range revealing the effect of quantum confinement in such structures.
Reference: Fu Li et al., Adv. Mater. Interfaces 7 (2020) 2000410.

04/08/2020 We organize the first virtual conference on solution-based optoelectronic nanomaterials: NANOPLUS on April 30, 2020.

03/16/2020 We published a new paper in Advanced Functional Materials.
In the realm of colloidal nanostructures, with their immense capacity for shape and dimensionality control, the form is undoubtedly a driving factor for the tunability of optical and electrical properties in semiconducting or metallic materials. However, influencing the fundamental properties is still challenging and requires sophisticated surface or dimensionality manipulation. Such a modification is presented for the example of colloidal lead‐sulfide nanowires. It is shown that the electrical properties of lead sulfide nanostructures can be altered from semiconducting to metallic with indications of superconductivity, by exploiting the flexibility of the colloidal synthesis to sculpt the crystal and to form different surface facets. A particular morphology of lead sulfide nanowires is prepared through the formation of {111} surface facets, which shows metallic and superconducting properties in contrast to other forms of this semiconducting crystal, which contain other surface facets ({100} and {110}). This effect, which is investigated with several experimental and theoretical approaches, is attributed to the presence of lead‐rich {111} facets. The insights promote new strategies for tuning the properties of crystals and new applications for lead sulfide nanostructures.
Reference: Mohammad Mehdi Ramin Moayed et al., Adv. Func. Mater. 30 (2020) 1910503.

03/05/2020 We published a new paper in Nanoscale.
Tin sulfide promises very interesting properties such as a high optical absorption coefficient and a small band gap, while being less toxic compared to other metal chalcogenides. However, the limitations in growing atomically thin structures of tin sulfide hinder the experimental realization of these properties. Due to the flexibility of the colloidal synthesis, it is possible to synthesize very thin and at the same time large nanosheets. Electrical transport measurements show that these nanosheets can function as field-effect transistors with high on/off ratios at low temperatures and p-type behavior. The temperature dependency of the charge transport reveals that defects in the crystal are responsible for the formation of holes as majority carriers. During illumination with circularly polarized light, these crystals generate a helicity dependent photocurrent at zero-volt bias, since their symmetry is broken by asymmetric interfaces (substrate and vacuum). Further, the observed circular photogalvanic effect shows a pronounced in-plane anisotropy, with a higher photocurrent along the armchair direction, originating from the higher absorption coefficient in this direction. Our new insights show the potential of tin sulfide for new functionalities in electronics and optoelectronics, for instance as polarization sensors.
Reference: Mohammad Mehdi Ramin Moayed et al., Nanoscale 12 (2020) 6256.

11/18/2019 We published a new paper in Advanced Optical Materials.
Colloidal quantum dots assembled into quantum dot solids usually suffer from poor conductivity. The most common charge transport mechanism through the solid is hopping transport where the hopping probability depends on the barrier type (stabilizing/connecting ligand molecule) and the interparticle distance. It is demonstrated that the electronic structure of the ligand molecule strongly alters the transport behavior through CuInSe₂ quantum dot solids. Transport measurements and optical‐pump terahertz‐probe experiments after a ligand exchange to fully conjugated molecules show an increase of the conductivity by orders of magnitude, as well as a change of the hopping transport mechanism. This change is not due to a reduced interparticle distance, but the electronic structure: the obtained frequency‐dependent complex conductivities point toward an efficient hole transport enabled by an alignment of the quantum dot valence bands and ligand states.
Reference: Friederieke Gorris et al., Adv. Opt. Mater. 8 (2020) 1901058.

10/28/2019 We published a new paper in Nanoscale.
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 V⁻¹ s⁻¹ for PbS nanosheets with thicknesses ranging from 4 to 16 nm. Results underpin the suitability of colloidal 2D PbS nanosheets for optoelectronic applications.
Reference: Jannika Lauth et al., Nanoscale 11 (2019) 21576.

10/14/2018 Fa-Gui He joined our group as Post-Doc.  Welcome!

09/20/2019 Fu Li defended her PhD thesis and graduated. Congratulations! 

09/20/2019 Sascha Kull defended his PhD thesis and graduated. Congratulations! 

09/20/2019 Eugen Klein defended his PhD thesis and graduated. Congratulations! 

07/26/2019 We published a new paper in Chemistry of Materials.
We present a colloidal synthesis strategy to obtain single-crystalline PbS nanorings. By controlling the ripening process in the presence of halide ions, a transformation of initial PbS nanosheets to frame-like structures and finally to nanorings was achieved. We found that the competing ligands oleic acid, oleate and halide ions play an important role in the formation of these nanostructures. Therefore, we propose a formation mechanism based on a thermally induced ripening of crystal facets dependent on the surface passivation. With this method, it became possible to synthesize colloidal nanorings of cubic crystal phase galena PbS. The synthesis was followed via TEM and the products are characterized by XRD, AFM and STEM tomography. Control of the initial nanoframe morphology allows adjusting the later nanoring dimensions.
Reference: Sascha Kull et al., Chem. Mater. 31 (2019) 5646.

07/17/2019 Frauke Gerdes defended her PhD thesis and graduated. Congratulations! 

06/18/2019 We published a new paper in the Journal of Physical Chemistry Letters.
Ultrathin semiconductor nanocrystals (NCs) with at least one dimension below their exciton Bohr radius receive a rapidly increasing attention due to their unique physicochemical properties. These superior properties highly depend on the shape and crystal phase of semiconductor NCs. Here, we demonstrate not only the synthesis of well-defined ultrathin ZnS nanoplatelets (NPLs) with excitonic absorption and emission, but also the shape/phase transformation between wurtzite (WZ) NPLs and zinc blende (ZB) nanorods (NRs). UV–vis absorption spectra of WZ-ZnS NPLs clearly exhibit a sharp excitonic peak that is not observed in ZB-ZnS NRs. Besides, the photoluminescence characterization shows that WZ-ZnS NPLs have a narrow excitonic emission peak, while ZB-ZnS NRs exhibit a broad collective emission band consisting of four emission peaks. The appearance of excitonic features in the absorption spectra of ZnS NPLs is explained by interband electronic transitions, which is simulated in the framework of atomic effective pseudopotentials (AEP).
Reference: Liwei Dai et al., J. Phys. Chem. Lett. 10 (2019) 3828.

06/14/2019 Michael Galchenko defended his PhD thesis and graduated. Congratulations! 

06/07/2019 We published a new paper in ACS Nano.
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 µm, 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≤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%. These large, tunable 2D nanosheets could serve as convenient platforms for future high efficiency optoelectronic devices.
Reference: Eugen Klein et al., ACS Nano 13 (2019) 6955.

05/01/2019 We started our activities at the University of Rostock.

04/12/2019 Leonard Heymann defended his PhD thesis and graduated. Congratulations! 

04/01/2019 We published a new paper in Advanced Materials.
Quantum‐confined Au nanoclusters exhibit molecule‐like properties, including atomic precision and discrete energy levels. The electrical conductivity of Au nanocluster films can vary by several orders of magnitude and is determined by the strength of the electronic coupling between the individual nanoclusters in the film. Similar to quantum‐confined, semiconducting quantum dots, the electrical coupling in films is dependent on the size and structure of the Au core and the length and conjugation of the organic ligands surrounding it. Unlike quantum dots, however, semiconducting transport has not been reported in Au nanocluster films. Here, it is demonstrated that through a simple yet careful choice of cluster size and organic ligands, stable Au nanocluster films can electronically couple and become semiconducting, exhibiting electric field effect and photoconductivity. The molecule‐like nature of the Au nanoclusters is evidenced by a hopping transport mechanism reminiscent of doped, disordered organic semiconductor films. These results demonstrate the potential of metal nanoclusters as a solution‐processed material for semiconducting devices. 
Reference: Michael Galchanko et al., Adv. Mater. 31 (2019) 1900684.

02/14/2019 We published a new paper in Journal of Physical Chemistry Letters.
The colloidal synthesis of large, thin two-dimensional (2D) nanosheets is fascinating but challenging, since the growth along the lateral and vertical dimensions needs 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 μm. The synthesis uses trioctylphosphine-S (TOP-S) as sulfur source and oleic acid (with or without trioctylphosphine, 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 tin(II) sulfide (SnS) nanosheets (NSs) with tunable edge faceting. Furthermore, electronic transport measurements show strong dependency on the crystallographic directions confirming structural anisotropy. 
Reference: Fu Li et al., J. Phy. Chem. Lett. 10 (2019) 993.

01/04/2019 We published a new paper in Nanoscale.
Colloidal approaches allow for the synthesis of Au nanoclusters (NCs) with atomic precision and sizes ranging from a few to hundreds of atoms. In most of the cases, these processes involve a common strategy of thiol etching of initially polydisperse Au nanoparticles into atomically precise NCs, resulting in the release of Au-thiolate complexes as byproducts. To the best of our knowledge, neither the removal of these byproducts nor the mass spectra in the relevant mass region were shown in previous studies. A thorough analysis of inorganic byproducts in the synthesis of Au25 NCs reveals that published protocols lead to Au25 NCs in vanishingly small quantities compared to their byproducts. Three purification methods are presented to separate byproducts from the desired Au25 NCs which are proposed to be applicable to other promising Au NC systems. Additionally, critical factors for a successful synthesis of Au25 NCs are identified and discussed including the role of residual water. An important finding is that the etching duration is very critical and must be monitored by UV-Vis spectroscopy resulting in synthetic yields as high as 40%. 
Reference: Michael Galchenko et al., Nanoscale 11 (2019) 1988.

12/14/2018 We published a new paper in ACS Omega. 
Copolymerization of melamine with 2,4,6-triaminopyrimidine (TAP) in an electrochemically induced polymerization process leads to the formation of molecular doped poly(triazine imide) (PTI). The polymerization is based on the electrolysis of water and evolving radicals during this process. The incorporation of TAP is shown by techniques such as elemental analysis, Fourier transform infrared and NMR spectroscopies, and powder X-ray diffraction, and it is shown that the carbon content can be tuned by the variation of the molar ratio of the two precursors. This incorporation of TAP directly influences the electronic structure of PTI and as a result, a red shift can be observed in UV–vis spectroscopy. The smaller band gap and the increased absorption in the visible range lead to improved photocatalytic properties. In dye degradation experiments, it was possible to observe an increase of the rate of the degradation of methylene blue by a factor of 4 in comparison to undoped PTI or 7 if compared to melon.
Reference: Leonard Heymann et al., ACS Omega 3 (2018) 17042.

11/12/2018 We published a new paper in Nanoscale.
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.
Reference: Eugen Klein et al., Nanoscale 10 (2018) 21197.

10/12/2018 We published a new paper in Nanoscale.
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.
Reference: Rostyslav Lesyuk et al., Nanoscale 10 (2018) 20640.

08/17/2018 Mehdi Ramin defended his PhD thesis and graduated. Congratulations!

08/10/2018 We published a new paper in the Journal of Material Chemistry C.
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.
Reference: Fu Li et al., J. Mater. Chem. C 6 (2018) 9410.

04/06/2018 We published a new paper in ACS Omega.
Poly (triazine imide) (PTI) is a material belonging to the group of carbon nitrides and has shown to have competitive properties compared to melon or g-C3N4, especially in photocatalysis. As most of the carbon nitrides PTI is usually synthesized by thermal or hydrothermal approaches. We present and discuss an alternative synthesis for PTI which exhibits a pH dependent solubility in aqueous solutions. This synthesis is based on the formation of radicals during electrolysis of an aqueous melamine solution, coupling of resulting melamine radicals and the final formation of PTI. We applied different characterization techniques to identify PTI as the product of this reaction and report the first liquid state NMR experiments on a triazine-based carbon nitride. We show that PTI has a relatively high specific surface area and a pH dependent adsorption of charged molecules. This tunable adsorption has a significant influence on the photocatalytic properties of PTI which we investigated in dye degradation experiments.
Reference: Leonard Heymann et al., ACS Omega 3 (2018) 3892. 

03/28/2018 We published a new review article in Zeitschrift für Physikalische Chemie.
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.
Reference: Frauke Gerdes et al., Z. Phys. Chem. 232 (2018) 1267. 

03/27/2018 We published a new article in Advanced Functional Materials.
Colloidally synthesized nanomaterials are among the promising candidates for future electronic devices due to their simplicity and the inexpensiveness of their production. Specifically, colloidal nanosheets are of great interest since they are conveniently producible through the colloidal approach while having the advantages of two-dimensionality. In order to employ these materials, according transistor behavior should be adjustable and of high performance. We show that the transistor performance of colloidal lead sulfide nanosheets is tunable by altering the surface passivation, the contact metal, or by exposing them to air. We found that adding halide ions to the synthesis leads to an improvement of the conductivity, the field-effect mobility, and the on/off ratio of these transistors by passivating their surface defects. As a possible solution for the post-Moore era, realizing new high quality semiconductors such as colloidal materials is crucial. In this respect, our results can provide new insights which helps to accelerate their optimization for potential applications.
Reference: Mohammad Mehdi Ramin Moayed et al., Adv. Funct. Mater. 28 (2018) 1706815. 

02/09/2017 We published a paper in Nanoscale.
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.
Reference: Eugen Klein et al., Nanoscale 10 (2018) 4442. 

01/01/2018 Irina Nefedova joined our group as Post-Doc. She will strengthen our efforts in advanced spectroscopy of colloidal nanomaterials. 

10/13/2017 We published a review article in the Journal of Materials Chemistry.
We modeled a prototype of a photovoltaic window, a passive source of clean energy, using a Monte Carlo ray-tracing method. We considered different geometries, material properties, and edge solar cells to determine the optimal conditions and possible electrical power yield. The modeled photovoltaic window prototype was based on colloidal luminescent low-toxic I–III–VI quantum dots (core/shell CuInS2/ZnS nanocrystals) with large Stokes shifts, high quantum yields, and tunable spectral properties. We also showed the influence of the quantum dot absorption/emission spectra on the resulting spectrum of transmitted light using a chromaticity diagram.
Reference: Rostyslav Lesyuk et al., J. Mater. Chem. C 5 (2017) 11790. 

11/01/2017 Andrés Black joined our group as Post-Doc. He will investigate the opto-electronic properties of two-dimensional colloidal nanomaterials. 

10/15/2017 Liwei Dai joined our group as PhD student. He will investigate the growth of non-toxic two-dimensional colloidal nanocrystals. 

10/13/2017 We published a review article in EPL (Europhys. Lett.)
Colloidal nanoparticles developed as interesting objects to establish two- or threedimensional super-structures with properties not known from conventional bulk materials. Beyond, the properties can be tuned and quantum effects can be exploited. This allows understanding electronic and optoelectronic transport phenomena and developing corresponding devices. The state-of-the-art in this field will be reviewed and possible challenges and prospects will be identified.
Reference: Christian Klinke, EPL (Europhys. Lett.) 119 (2017) 36002. 

09/15/2017 Thomas Bielewicz defended his PhD thesis and graduated. Congratulations! 

07/21/2017 Our WEH Physics Summer School "Exciting nanostructures" finished very successfully! 

07/17/2017 Radio broadcast on our metal nanoparticle transistor concept appeared in Deutschlandfunk (MP3). 

07/14/2017 We published a new paper in Science Advances.
Single-electron transistors would represent an approach to developing less power–consuming microelectronic devices if room temperature operation and industry-compatible fabrication were possible. We present a concept based on stripes of small, self-assembled, colloidal, metal nanoparticles on a back-gate device architecture, which leads to well-defined and well-controllable transistor characteristics. This Coulomb transistor has three main advantages. By using the scalable Langmuir-Blodgett method, we combine high-quality chemically synthesized metal nanoparticles with standard lithography techniques. The resulting transistors show on/off ratios above 90%, reliable and sinusoidal Coulomb oscillations, and room temperature operation. Furthermore, this concept allows for versatile tuning of the device properties such as Coulomb energy gap and threshold voltage, as well as period, position, and strength of the oscillations.
Reference: Svenja Willing et al., Science Advances 3 (2017) e1603191. 

07/12/2017 We published a new paper in Nano Letters.
Ultrathin two-dimensional nanosheets raise a rapidly increasing interest due to their unique dimensionality-dependent properties. Most of the two-dimensional materials are obtained by exfoliation of layered bulk materials or are grown on substrates by vapor deposition methods. To produce free-standing nanosheets, solution-based colloidal methods are emerging as promising routes. In this work, we demonstrate ultrathin CdSe nanosheets with controllable size, shape and phase. The key of our approach is the use of halogenated alkanes as additives in a hot-injection synthesis. Increasing concentrations of bromoalkanes can tune the shape from sexangular to quadrangular to triangular and the phase from zinc blende to wurtzite. Geometry and crystal structure evolution of the nanosheets take place in the presence of halide ions, acting as cadmium complexing agents and as surface X-type ligands, according to mass spectrometry and X-ray photoelectron spectroscopies. Our experimental findings show that the degree of these changes depends on the molecular structure of the halogen alkanes and the type of halogen atom.
Reference: Frauke Gerdes et al., Nano Lett. 17 (2017) 4165. 

06/16/2017 Mirjam Volkmann defended her PhD thesis and graduated. Congratulations! 

06/07/2017 We published a new paper in Nature Communications.
Employing the spin degree of freedom of charge carriers offers the possibility to extend the functionality of conventional electronic devices, while colloidal chemistry can be used to synthesize inexpensive and tuneable nanomaterials. In order to benefit from both concepts, Rashba spin-orbit interaction has been investigated in colloidal lead sulphide nanosheets by electrical measurements on the circular photo-galvanic effect. Lead sulphide nanosheets possess rock salt crystal structure, which is centrosymmetric. The symmetry can be broken by quantum confinement, asymmetric vertical interfaces and a gate electric field leading to Rashba-type band splitting in momentum space at the M points, which results in an unconventional selection mechanism for the excitation of the carriers. The effect, which is supported by simulations of the band structure using density functional theory, can be tuned by the gate electric field and by the thickness of the sheets. Spin-related electrical transport phenomena in colloidal materials open a promising pathway towards future inexpensive spintronic devices.
Reference: Mohammad Mehdi Ramin Moayed et al., Nature Commun. 8 (2017) 15721. 

03/01/2017 We published a new paper in the Journal of Computational Chemistry.
Because of their potential for chemical functionalization, carbon nanotubes are promising candidates for the development of devices such as nanoscale sensors or transistors with novel gating mechanisms. However, the mechanisms underlying the property changes due to thier functionalization still remain subject to debate. Our goal is to reliably model one possible mechanism for such chemical gating: adsorption directly on the nanotubes. Within a Kohn–Sham density functional theory framework, such systems would ideally be described using periodic boundary conditions. Truncating the tube and saturating the edges in practice often offers a broader selection of approximate exchange–correlation functionals and analysis methods. By comparing the two approaches systematically for NH3 and NO2 adsorbates on semiconducting and metallic carbon nanotubes, we find that while structural properties are less sensitive to the details of the model, local properties of the adsorbate may be as sensitive to truncation as they are to the choice of exchange–correlation functional, and are similarly challenging to compute as adsorption energies. This suggests that these adsorbate effects are nonlocal.
Reference: Lynn Gross et al., J. Comput. Chem. 38 (2017) 861. 

02/28/2017 The registration for our WEH Physics Summer School "Exciting nanostructures" is closed now. We are happy about the numerous high-quality applications. 

01/01/2017 The Special Issue on "Hierarchical Colloidal Nanostructures" appeared in Zeitschrift für Physikalische Chemie with an Editorial of the Guest Editor Christian Klinke. 

12/19/2016 We published a new paper in Chemistry of Materials. 
There is a strong interest to attach nanoparticles non-covalently to one-dimensional systems like boron nitride nanotubes to form composites. The combination of those materials might be used for catalysis, in solar cells, or for water splitting. Additionally, the fundamental aspect of charge transfer between the components can be studied in such systems. We report on the synthesis and characterization of nanocomposites based on semiconductor nanoparticles attached directly and non-covalently to boron nitride nanotubes. Boron nitride nanotubes were simply integrated into the colloidal synthesis of the corresponding nanoparticles. With PbSe, CdSe, and ZnO nanoparticles a wide range of semiconductor bandgaps from the near infrared to the ultra violet range was covered. A high surface coverage of the boron nitride nanotubes with these semiconducting nanoparticles was achieved, while it was found that a similar in-situ approach with metallic nanoparticles does not lead to proper attachment. To emphasize the new possibilities that boron nitride nanotubes offer as a support material for semiconductor nanoparticles we investigated the fluorescence of BN-CdSe composites. In contrast to CdSe nanoparticles attached to carbon nanotubes, where the fluorescence is quenched, particles attached to boron nitride nanotubes remain fluorescent. With our versatile approaches we expand the library of BN-nanoparticle composites that present an interesting, electronically non-interacting complement to the widely applied carbon nanotube-nanoparticle composite materials.
Reference: Mirjam Volkmann et al., Chem. Mater. 20 (2017) 726. 

09/06/2016 The registration for our WEH Physics Summer School "Exciting nanostructures" is open now!!! 

08/01/2016 Rostyslav Lesyuk joined our group as Post-Doc. He will investigate the electrical properties of colloidal nanocrystals. 

07/26/2016 We published a new paper in Nanotechnology. 
Two-dimensional colloidal nanosheets represent very attractive optoelectronic materials. They combine good lateral conductivity with solution-processability and geometry-tunable electronic properties. In the case of PbS nanosheets, so far synthesis has been 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 170 °C a coligand is not even necessary and the only ligand, oleic acid, controls the anisotropic growth of the two-dimensional structures. Also, using thiourea as a sulfide source, nanosheets with lateral dimensions of over 10 μm are possible.
Reference: Thomas Bielewicz et al., Nanotechnolody 27 (2016) 355602. 

05/22/2016 We published a new paper in Nanoscale. 
Metallic nanoparticles offer possibilities to build basic electric devices with new functionality and improved performance. Due to the small volume and the resulting low self-capacitance, each single nanoparticle exhibits a high charging energy. Thus, a Coulomb-energy gap emerges during transport experiments that can be shifted by electric fields, allowing for charge transport whenever energy levels of neighboring particles match. Hence, the state of the device changes sequentially between conducting and non-conducting instead of just one transition from conducting to pinch-off as in semiconductors. To exploit this behavior for field-effect transistors, it is necessary to use uniform nanoparticles in ordered arrays separated by well-defined tunnel barriers. In this work, CoPt nanoparticles with a narrow size distribution are synthesized by colloidal chemistry. These particles are deposited via the scalable Langmuir-Blodgett technique as ordered, homogeneous monolayers onto Si/SiO2 substrates with pre-patterned gold electrodes. The resulting nanoparticle arrays are limited to stripes of adjustable lengths and widths. In such a defined channel with a limited number of conduction paths the current can be controlled precisely by a gate voltage.
Reference: Hauke Lehmann et al., Nanoscale 8 (2016) 14384. 

03/31/2016 We published a new paper in Chemistry of Materials. 
The most prevalent image of the morphology of Au-CdSe hybrid nanoparticles is that of dumbbells or matchsticks with CdSe nanoparticles acting as seed material onto which spherical Au dots are deposited. Based on a system with only three reaction components, CdSe seeds, n-dodecyltrimethylammonium bromide-complexed AuCl3, and dodecanethiol, we demonstrate how the morphology of the Au deposits on the semiconductor nanoparticles, either in form of dots on the corners or in the form of a shell around the nanoparticle surface can be determined by controlling the oxidation state of the metal precursor. Furthermore, we apply X-Ray Photoelectron Spectroscopy to show that the resultant deposits are composed of partially oxidized Au, corresponding to a Au-Se compound regardless the deposit morphology. To obtain a detailed characterization of the hybrid nanoparticles with different morphologies and to gain mechanistic insights into the deposition process, (cryogenic) high-resolution transmission electron microscopy, mass spectrometry, cyclic voltammetry and computational simulations have been performed. Our results emphasize that the knowledge of the surface chemistry of the seed particles as well as a defined picture of the metal precursors are necessary to understand hetero-deposition processes.
Reference: Leonor de la Cueva et al., Chem. Mater. 28 (2016) 2704. 

03/15/2016 We published a new paper in Nano Letters. 
Anisotropic nanocrystal assembled supercrystals with open superlattices (SLs) manifest novel and unique properties, but poor understanding of the nucleation/growth mechanisms limits their design and fabrication for practical applications. Using highly anisotropic Pt3Ni octahedral nanocrystals, we have grown large single supercrystals with an open body-centered-cubic (bcc) superlattice that has a low filling factor of 26.8%. Synchrotron-based X-ray structural reconstruction fully revealed the coherence of translational and orientational orderings, and determined that the constituent octahedra arrange themselves with the vertex-to-vertex and face-to-face configurations along the SL[100] and SL[111] directions, respectively. The large face-to-face separation and flexible vertex-to-vertex elastic contact provided the rattle space and supporting axis for local rotations of Pt3Ni octahedra within the bcc superlattice. Development of orientational disordering along with robust preservation of translational ordering during the heating process of a supercrystal in the oleic acid wetting environment confirmed the dominance of rotational entropy of hard octahedra in the formation of the open bcc superlattice. Ultimate achievement of dynamic equilibrium between the vertex-oriented elastic repulsions and the face-oriented attractions of surface-coating ligands governs the structural and mechanical stability of the supercrystal. This discovery provides significant insights into the design and control of geometrical shapes for the fabrication of highly anisotropic nanocrystals into desired open superlattices with tailored optical and electronic properties.
Reference: Ruipeng Li et al., Nano Lett. 16 (2016) 2792. 

03/01/2016 Gabriele Selvaggio joined our group as Master student. He will work on new two-dimensional nanomaterials. 

02/21/2016 We published a new paper in Chemistry of Materials. 
The rise of two-dimensional (2D) graphene-cognated crystals with non-zero band gaps like transition metal dichalcogenides has led to a rapidly increasing interest in their dimensionality-dependent anisotropic properties, which bear high potential for ultrathin electronics. 2D crystals of the III-VI metal chalcogenide InSe represent a new kind of material class predestined for the use in optoelectronic applications as highly responsive photodetectors and field-effect transistors. We present a solution-processible method for 2D ultrathin InSe nanosheets with a detailed characterization of the sheet formation by a lamellar ligand templated growth. Optical and electrical transport properties, as well as in depth analysis of the crystal structure and stoichiometry of the colloidal nanosheets by electron and atomic force microscopy, X-ray photoelectron spectroscopy and scattering methods complete this comprehensive study on a wet-chemical alternative to produce ultrathin InSe nanosheets.
Reference: Jannika Lauth et al., Chem. Mater. 28 (2016) 1728. 

02/01/2016 Michael Galchenko joined our group as PhD student. He will support our efforts in the frame of the Center of Excellence CUI. 

02/01/2016 Sascha Kull joined our group as PhD student. He will work on the synthesis of complex colloidal nanomaterials. 

01/05/2016 We published a new paper in Langmuir. 
Metallodielectric nanostructured core–shell–shell particles are particularly desirable for enabling novel types of optical components, including narrow-band absorbers, narrow-band photodetectors, and thermal emitters, as well as new types of sensors and catalysts. In the paper, we present a facile approach for the preparation of submicron SiO2@Pt@SiO2 core–shell–shell particles. 
Reference: Alexey Petrov et al., Langmuir 32 (2016) 848. 

01/01/2016 Christian Saggau joined our group as PhD student. He will work on the electronical transport through colloidal nanomaterials. 

12/17/2015 Angelique Rieckmann defended her Master thesis and graduated. Congratulations! 

12/09/2015 We published a new paper in Chemistry of Materials. 
We demonstrate that the shape of colloidal lead sulfide nanostructures can be tuned from spheres to stripes to sheets by means of the precursor concentrations, the concentration of a chloroalkane coligand and the synthesis temperature. All final structures still possess at least one dimension in confinement. The structures cover all dimensionalities from 0D to 3D. Additionally, the effect of temperature on the shape and thickness of PbS nanosheets is shown and electrical transport measurements complement the findings. 
Reference: Thomas Bielewicz et al., Chem. Mater. 27 (2015) 8248. 

10/05/2015 Fu Li joined our group as PhD student. She will work on the synthesis and electronic properties of nanopartilce films. 

09/26/2015 Svenja Willing defended her Master thesis and graduated. Congratulations! 

09/04/2015 We published a new paper in the Journal of Physical Chemistry C. 
ZnO is a promising catalyst for hydrogen and oxygen production. Nevertheless, there are some limitations to the efficiency. Once in contact with the electrolyte, an extraction barrier for electrons is formed. The purpose of this work was to create an extraction site for electrons by synthesizing small pyramidally shaped ZnO nanocrystals, which consist of a predefined site for the growth of a gold particle at the tip. Photoelectrochemical deposition of gold on ZnO was performed to yield the hybrid structure. Photoluminescence studies of the relative change of intensities of band gap versus defect state relaxation showed electron transfer from the conduction band of ZnO to Au. Using cyclic voltammetry, Au-mediated charge extraction from Au–ZnO hybrids was demonstrated, which circumvents the electron extraction barrier in ZnO. 
Reference: Alina Chanaewa et al., J. Phys. Chem. C 119 (2015) 21704. 

08/01/2015 Our DPG Physics School "Physical properties of nanoparticles: Characterization and applications" finished very successfully! 

07/16/2015 Two new videos related to our work can be viewed on science-media.org: 
Animation of a setup for the synthesis of colloidal nanoparticles
Synthesis of colloidal PbS nanosheets 

06/09/2015 We published a new paper in ACS Nano. 
Thin films prepared of semiconductor nanoparticles are promising for low-cost electronic applications such as transistors and solar cells. One hurdle for their breakthrough is their notoriously low conductivity. To address this, we precisely decorate CdSe nanoparticles with platinum domains of one to three nanometers in diameter by a facile and robust seeded growth method. We demonstrate the transition from semiconductor to metal dominated conduction in monolayered films. By adjusting the platinum content in such solution-processable hybrid, oligomeric nanoparticles the dark currents through deposited arrays become tunable while maintaining electronic confinement and photoconductivity. Comprehensive electrical measurements allow determining the reigning charge transport mechanisms. 
Reference: Michaela Meyns et al., ACS Nano 9 (2015) 6077. 

05/20/2015 We published a new paper in ACS Applied Materials & Interfaces. 
We developed highly sensitive and fast photodetector devices with CdSe quantum nanowires as active elements exploiting the advantages of electro- and wet-chemical routes. Bismuth nanoparticles electrochemically synthesized directly onto interdigitating platinum electrodes serve as catalysts in the following solution–liquid–solid synthesis of quantum nanowires directly on immersed substrates under mild conditions at low temperature. This fast and simple preparation process leads to a photodetector device with a film of nanowires of limited thickness bridging the electrode gaps, in which a high fraction of individual nanowires are electrically contacted and can be exposed to light at the same time. The remarkable combination of a high sensitivity and a fast response is attributed to depletion regions inside the nanowires, tunnel–junction barriers between nanowires, and Schottky contacts at the electrodes, where all of these features are strongly influenced by the number of photogenerated charge carriers.
Reference: Alexander Littig et al., ACS Appl. Mater. Interfaces 7 (2015) 12184. 

05/11/2015 Angelique Rieckmann joined our group as Master student. She will work on the synthesis of advanced two-dimensional nanosheets. 

05/08/2015 Sedat Dogan defended his PhD thesis and graduated. Congratulations! 

05/04/2015 Sascha Kull joined our group as Master student. He will work on the synthesis of advanced colloidal nanomaterials. 

03/01/2015 Svenja Willing joined our group as Master student. She will work on the electrical transport through super-structural nanoparticle films. 

02/10/2015 We published a new paper in Nanoscale. 
Solution-processable, two-dimensional semiconductors are promising optoelectronic materials which could find application in low-cost solar cells. Lead sulfide nanocrystals raised attention since the effective band gap can be adapted over a wide range by electronic confinement and observed multi-exciton generation promises higher efficiencies. We report on the influence of the contact metal work function on the properties of transistors based on individual two-dimensional lead sulfide nanosheets. Furthermore, we demonstrate that asymmetrically contacted nanosheets show photovoltaic effect and that the nanosheets’ height has a decisive impact on the device performance. The results underline the high hopes put on such materials.
Reference: Sedat Dogan et al., Nanoscale 7 (2015) 4875. 

01/15/2015 Leonard Heymann joined our group as PhD student. He will work on the growth of advanced tubular nanostructures.

01/01/2015 We published a new paper in Zeitschrift für Physikalische Chemie. 
Colloidal lead sulfide is a versatile material with great opportunities to tune the bandgap by electronic confinement and to adapt the optical and electrical properties to the target application. We present a new and simple synthetic route to control size and shape of PbS nanoparticles. Increasing concentrations of explicitly added acetic acid are used to tune the shape of PbS nanoparticles from quasi-spherical particles via octahedrons to six-armed stars. The presence of acetate changes the intrinsic surface energies of the different crystal facets and enables the growth along the 100 direction. Furthermore, the presence of 1,2-dichloroethane alters the reaction kinetics,which results in smaller nanoparticles with a narrower size distribution.
Reference: Frauke Gerdes et al., Z. Phys. Chem. 229 (2015) 139. 

11/03/2014 Eugen Klein joined our group as PhD student. He will work on the growth of PbS nanostructures investigated by X-ray methods. 

10/22/2014 We published a new paper in Small. 
Two-dimensional, solution-processable semiconductor materials are anticipated to be used in low-cost electronic applications, such as transistors and solar cells. We show that the height of lead sulfide nanosheets can be tuned by the variation of the ligand concentrations. As a consequence of the adjustability of the nanosheets' height between 4 to more than 20 nm charge carriers are in confinement, which has a decisive impact on their electronic properties. This is demonstrated by their use as conduction channel in a field-effect transistor. The experiments show that the performance in terms of current, On/Off ratio, and sub-threshold swing is tunable over a large range.
Reference: Thomas Bielewicz et al., Small 11 (2015) 826. 

10/14/2014 Registration for the DPG Physics School "Physical properties of nanoparticles: Characterization and applications" in 2015 is open! 

08/01/2014 Mehdi Ramin joined our group as PhD student. He will investigate the electrical transport through two-dimensional nanomaterials. 

08/01/2014 Frauke Gerdes joined our group as PhD student. She will work on the synthesis of sheet-like colloidal nanostructures. 

05/30/2014 Sedat Dogan won the second poster prize on the NANAX 6 conference. Congratulations! 

05/16/2014 Michaela Meyns defended her PhD thesis and graduated. Congratulations! 

04/30/2014 We published a new paper in Nature Communications. 
Semiconductor nanocrystals are promising for use in cheap and highly efficient solar cells. A high efficiency can be achieved by carrier multiplication (CM), which yields multiple electron-hole pairs for a single absorbed photon. Lead chalcogenide nanocrystals are of specific interest, since their band gap can be tuned to be optimal to exploit CM in solar cells. Interestingly, for a given photon energy CM is more efficient in bulk PbS and PbSe, which has been attributed to the higher density of states. Unfortunately, these bulk materials are not useful for solar cells due to their low band gap. Here we demonstrate that two-dimensional PbS nanosheets combine the band gap of a confined system with the high CM efficiency of bulk. Interestingly, in thin PbS nanosheets virtually the entire excess photon energy above the CM threshold is used for CM, in contrast to quantum dots, nanorods and bulk lead chalcogenide materials.
Reference: Michiel Aerts et al., Nature Comm. 5 (2014) 3789. 

04/30/2014 We published a review paper in the Bunsenmagazin. 
Reference: Christian Klinke, Bunsenmagazin (2014) 133. 

04/08/2014 We published a new paper in Nanoscale. 
Halides cap and stabilize colloidal semiconductor nanocrystal surfaces allowing for nanocrystal surface interactions that may improve the performance of thin film devices such as photo-detectors and/or solar cells. Current ways to introduce halide anions as ligands on the surface of nanocrystals produced by the hot injection method are based on post-synthetic treatments. In this work, we explore the possibility to introduce Cl in the nanocrystal ligand shell in-situ during the synthesis. With this aim, the effect of 1,2-dichloroethane (DCE) in the synthesis of CdSe rod-like nanocrystals produced under different Cd/Se precursor molar ratios has been studied. We report on a double role of DCE depending on the Cd/Se precursor molar ratio (either under cadmium or selenium precursor excess). According to mass spectrometry (ESI-TOF) and nuclear magnetic resonance (1H-NMR), under Se precursor (Se dissolved in trioctylphosphine, TOP) excess conditions ethane-1,2-diylbis(trioctylphosphonium)dichloride is released as by-product of the reaction between DCE and TOP. According to XPS studies chlorine gets incorporated into the CdSe ligand shell, promoting re-shaping of rod-like nanocrystals into pyramidal-like ones.
Reference: Cristina Palencia et al., Nanoscale 6 (2014) 6812. 

03/20/2014 First announcement for the DPG Physics School "Physical properties of nanoparticles: Characterization and applications" in 2015. 

01/13/2014 Magdalena Olesinska joined our group as PhD student. She will work on the synthesis of sheet-like colloidal nanostructures. 

01/08/2014 We published a new paper in Chemistry of Materials. 
Halogen compounds are capable of playing an important role in the manipulation of nanoparticle shapes and properties. In a new approach, we examined the shape evolution of CdSe nanorods to hexagonal pyramids in a hot-injection synthesis under the influence of halogenated additives in the form of organic chlorine, bromine and iodine compounds. This shape evolution is explained as a result of X-type ligand coordination to sloped and flat Cd-rich facets and an equilibrium shape strongly influenced by halides. Synchrotron XPS measure-ments and TXRF results show that the shape evolution is accompanied by a modification in the chemical composition of the ligand sphere. Our experimental results suggest that the molecular structure of the halogenated compound is related to the degree of the effect on both rod growth and further shape evolution. This presents a new degree of freedom in nanoparticle shape control and highlights the role of additives in nanoparticle synthesis and their possible in situ formation of ligands.
Reference: Michaela Meyns et al., Chem. Mater. 26 (2014) 1813. 

01/07/2014 We published a new paper in Advanced Functional Materials. 
A facile and safe ligand exchange method for readily synthesized CuInSe₂ (CIS) and CuIn_1-xGa_xSe₂(CIGS) nanocrystals from oleylamine to 1-ethyl-5-thiotetrazole, preserving the colloidal stability of the chalcopyrite structure, is presented. 1-Ethyl-5-thiotetrazole as thermally degradable ligand is adapted for the first time for trigonal pyramidal CIS, elongated CIS and CIGS nanocrystals. Exchanged nanocrystal solutions are processed onto gold electrodes yielding ordered thin films. These films are thermally annealed to completely remove 1-ethyl-5-thiotetrazol leaving individual closely assembled nanocrystals with virtually bare surfaces. The conductivity of trigonal pyramidal CIS increases by four orders of magnitude for ligand-free nanocrystal films. Elongated CIS nanocrystal films show a three orders of magnitude conductivity increase and CIGS NC films exhibit improved conductivity by two orders of magnitude. This approach for the first time offers the possibility to address chalcopyrite materials’ electrical properties in a virtually ligand-free state.
Reference: Jannika Lauth et al., Adv. Funct. Mater. 24 (2014) 1081. 

01/02/2014 Leonard Heymann joined our group as Master student. He will investigate the CVD growth of advanced tubular nanostructures. 

12/02/2013 Eugen Klein joined our group as Master student. He will work on the growth of PbS nanostructures at high concentrations. 

11/01/2013 Frauke Gerdes joined our group as Master student. She will investigate the growth of new PbS nanostructures. 

07/22/2013 We published a new paper in Journal of Applied Physics. 
We demonstrate that by means of a local top-gate current oscillations can be observed in extended, monolayered films assembled from monodisperse metal nanocrystals—realizing transistor function. The oscillations in this metal-based system are due to the occurrence of a Coulomb energy gap in the nanocrystals which is tunable via the nanocrystal size. The nanocrystal assembly by the Langmuir-Blodgett method yields homogeneous monolayered films over vast areas. The dielectric oxide layer protects the metal nanocrystal field-effect transistors from oxidation and leads to stable function for months. The transistor function can be reached due to the high monodispersity of the nanocrystals and the high super-crystallinity of the assembled films. Due to the fact that the film consists of only one monolayer of nanocrystals and all nanocrystals are simultaneously in the state of Coulomb blockade the energy levels can be influenced efficiently (limited screening).
Reference: Yuxue Cai et al., J. Appl. Phys. 114 (2013) 034311. 

04/02/2013 Prof. Christian Klinke obtained a Heisenberg fellowship of the German Funding Agency DFG. 

04/02/2013 Hauke Lehmann joined our group as Post-Doc. He will investigate the electrical transport through colloidal nanocrystals. 

02/08/2013 We published a new paper in ACS Nano. 
The performance of devices based on semiconductor nanocrystals improves both with stronger interface interactions among nanocrystals and between nanocrystals and solid electrode surfaces. The combination of X-ray photoelectron spectroscopy (XPS) and solid 31P CP/MAS NMR (cross-polarization/magic angle spinning nuclear magnetic resonance) shows that the selective substitution of long organic chains by chlorine atomic ligands during the colloidal synthesis by the hot injection method promotes the adsorption of CdSe nanocrystals to carbon sp2 surfaces, leading to the formation of well-ordered nanocrystal monolayers on graphitic materials.
Reference: Fabiola Iacono et al., ACS Nano 7 (2013) 2559. 

02/08/2013 We published a new paper in Nano Letters. 
Assemblies of 3.5 nm PbS nanoparticles nucleate in three dominant superlattice polymorphs: amorphous, body-centered-cubic (bcc) and face-centered-cubic (fcc) phase. This superlattice relationship can be controlled by the inter-nanoparticle distance without changing the nanoparticles size. Upon increase of inter-nanoparticle distance, the packing density decreases, and the capping molecules at the nanoparticle surfaces change in structure and accordingly modify the surface energy. The driving force for nanoparticle assembly develops from an entropic maximization to a reduction of total free energy through multiple interactions between surface molecules and nanoparticles and resulting variation of surface molecules. Upon long-term aging and additional thermal treatment, fcc undergoes a tetragonal distortion and subsequently transforms to bcc phase, and simultaneously, the nanoparticles embedded in supercrystals reduce surface energy primarily in {200} facets. Linking molecule-nanoparticle interactions with a series of changes of packing density and surface lattice spacings of nanoparticles allows for an interpretation of principles governing the nucleation, structure stability, and transformation of PbS nanoparticle-assembled supercrystals.
Reference: Zhongwu Wang et al., Nano Lett. 13 (2013) 1303. 

02/01/2013 Vera Paulava joined our group as PhD student. She will investigate the growth of colloidal nanostructures by means of X-ray scattering methods. 

10/19/2012 The research of Prof. Christian Klinke will be supported by an ERC Starting Grant 2012. 

10/18/2012 We published a new paper in Nanotechnology. 
The frictional properties of individual multiwalled boron nitride nanotubes (BN-NTs) and deposited on a silicon substrate are investigated using an atomic force microscope tip sliding along (longitudinal sliding) and across (transverse sliding) the tube's principal axis. Because of the tube's transverse deformations during the tip sliding, a larger friction coefficient is found for the transverse sliding as compared to the longitudinal sliding. We show that the friction anisotropy in BN-NTs, defined as the ratio between transverse and longitudinal friction forces per unit area, increases with the nanotube–substrate contact area. Larger contact area denotes stronger surface adhesion, resulting in a longitudinal friction coefficient closer to the value expected in the absence of transverse deformations.
Reference: Hsiang-Chih Chiu et al., Nanotech. 23 (2012) 455706. 

09/28/2012 Annette Wurl and Yuxue Cai defended their PhD theses and graduated. Congratulations! 

09/10/2012 We published a new paper in Journal of Physical Chemistry C. 
The supramolecular interaction between individual single-walled carbon nanotubes and a functional organic material based on tetrathiafulvalene is investigated by means of electric transport measurements in a field-effect transistor configuration as well as by NIR absorption spectroscopy. The results clearly point to a charge-transfer interaction in which the adsorbed molecule serves as an electron acceptor for the nanotubes through its pyrene units. Exposure to iodine vapors enhances this effect. The comparison with pristine carbon nanotube field-effect transistor devices demonstrates the possibility to exploit charge-transfer interactions taking place in supramolecular assemblies in which a mediator unit is used to transduce and enhance an external signal.
Reference: Annette Wurl et al., J. Chem. Phys. C 116 (2012) 20062. 

09/07/2012 We published a new paper in Applied Physics Letters. 
We report on measurements of the radial modulus of boron nitride nanotubes (BN-NTs) with various sizes and thicknesses. These BN-NTs are radially much stiffer than previously reported thinner and smaller BN-NTs. We show the key role of the morphology of the nanotubes in determining their radial rigidity; in particular, we find that the external and internal radii have a stronger influence on the radial modulus than the nanotube’s thickness.
Reference: Hsiang-Chih Chiu et al., Appl. Phys. Lett. 101 (2012) 103109. 

08/12/2012 We published a new paper in Applied Physics Letters. 
Two-dimensional materials are considered for future quantum devices and are usually produced by extensive methods like molecular beam epitaxy. We report on the fabrication of field–effect transistors using individual ultra–thin lead sulfide nanostructures with lateral dimensions in the micrometer range and a height of a few nanometers as conductive channel produced by a comparatively fast, inexpensive, and scalable colloidal chemistry approach. Contacted with gold electrodes, such devices exhibit p–type behavior and temperature–dependent photoconductivity. Trap states play a crucial role in the conduction mechanism. The performance of the transistors is among the ones of the best devices based on colloidal nanostructures.
Reference: Sedat Dogan et al., Appl. Phys. Lett. 101 (2012) 073102. 

04/30/2012 Thomas Bielewicz joined our group as PhD student. He will work on the synthesis of sheet-like colloidal nanostructures. 

04/16/2012 Mirjam Volkmann joined our group as PhD student. She will work on the synthesis of hybrids materials composed of inorganic nanoparticles and nanotubes. 

04/02/2012 Cristina Palencia joined our group as Post-Doc for three months. In collaboration with Beatriz H. Juarez we will work on the synthesis of size-tunable, pyramidal CdSe nanoparticles and their attachment to carbon nanotubes. 

03/07/2012 We published a new paper in Advanced Materials. 
Recently, we found that a larger friction coefficient is present when the tip of an AFM is sliding perpendicular to the axis of a carbon nanotube, as compared with sliding along the tube axis (Nature Mater. 8 (2009) 876). This behavior is explained by a deformation, like a lateral swaying (or a "hindered rolling") of the tube during the transverse sliding, which produces additional friction dissipation. This soft deformation mode is absent, or partially absent, when the tip slides along the nanotube axis, thus, for the longitudinal sliding the friction force arises mostly only from sliding the hard nano-contact between the tip and the tube. The ratio between transverse and longitudinal friction per unit area is called friction anisotropy. Here, we show how structural defects, surface chemistry and possibly chirality can couple the transverse and longitudinal sliding, modulating nanotubes frictional properties. A simple analytical model has been developed to compute the amount of coupling, between the transverse and longitudinal sliding, the "intrinsic" hard contact sliding shear strength, and the soft "hindered rolling" shear strength. This model captures very well the observed experimental behavior indicating for all the nanotubes a common supralinear decrease of the coupling with increasing ratio between transverse and longitudinal friction. In the experiments, the amount of structural defects in the CNTs is controlled by using different growth methods, whereas the surface chemistry is controlled with after-growth chemical functionalization. 
Reference: Hsiang-Chih Chiu et al., Adv. Mater. 24 (2012) 2879. 

12/05/2011 We published a new paper in the Journal of Physical Chemistry C. 
We report on the fabrication of carbon nanotube arrays with spatial dimensions that are suitable for application in polymer-based solar cells. We demonstrate that nanotubes can be grown on ITO covered glass at temperatures below 600 °C by static pressure chemical vapor deposition (CVD). Using short growth times, we were able to obtain nanotube arrays with a rather uniform length limited to about 200 nm. With a plasma-enhanced CVD process, we were also able to produce wall-like carbon nanostructures (multilayered graphene sheets) with controllable height on ITO. Both types of carbon nanostructures were investigated in test solar cells, in order to explore their suitability for application in organic photovoltaics. 
Reference: Holger Borchert et al., J. Phys. Chem. C 116 (2012) 412. 

11/11/2011 We published a new paper in Nanoscale. 
In this work we study the charge transfer in individual double-walled carbon nanotubes highly covered with uniform ZnO nanoparticles. The composite material was used as conductive channel in a field-effect transistor device and the electrical photo-response was analysed under various conditions. By means of the transfer characteristics we could elucidate the mechanism of charge transfer from non-covalently attached semiconducting nanoparticles to carbon nanotubes. The role of positive charges remaining on the nanoparticles is discussed in terms of a gating effect. 
Reference: Alina Chanaewa et al., Nanoscale 4 (2012) 251. 

10/07/2011 We published a new paper in ACS Nano. 
We present the full thermoelectric characterization of nanostructured bulk PbTe and PbTe–PbSe samples fabricated from colloidal core–shell nanoparticles followed by spark plasma sintering. An unusually large thermopower is found in both materials, and the possibility of energy filtering as opposed to grain boundary scattering as an explanation is discussed. A decreased Debye temperature and an increased molar specific heat are in accordance with recent predictions for nanostructured materials. On the basis of these results we propose suitable core–shell material combinations for future thermoelectric materials of large electric conductivities in combination with an increased thermopower by energy filtering. 
Reference: Marcus Scheele et al., ACS Nano 5 (2011) 8541. 

09/01/2011 Mirjam Volkmann joined our group as diploma student. She will work on the synthesis of hybrids materials composed of inorganic nanoparticles and nanotubes. 

08/01/2011 Thomas Bielewicz joined our group as diploma student. He will work on the synthesis of sheet-like colloidal nanostructures. 

12/22/2010 We published a new paper in ACS Nano. 
We report about the electrical transport through monolayers of monodisperse cobalt−platinum nanoparticles. Upon annealing we observe an increase of conductivity over more than 4 orders of magnitude. A first attempt of explanation of this unanticipated effect, a nanoparticle displacement, could not be confirmed for annealing temperatures below 400 °C. A second approach, a carbonization of the ligands, however, could be confirmed by Raman spectroscopy. The simple thermal treatment allows tuning essential properties of electronic devices based on nanoparticles by the manipulation of the interparticle coupling, namely the electrical conductivity, the Coulomb blockade characteristic, and the activation energy of the system.
Reference: Yuxue Cai et al., ACS Nano 5 (2011) 67. 

10/26/2010 We published a new paper in Journal of Materials Chemistry. 
We report the growth of an unstable shell-like gold structure around dihexagonal pyramidal CdSe nanocrystals in organic solution and the structural transformation to spherical domains by two means: (i) electron beam irradiation (in situ) and (ii) addition of a strong reducing agent during synthesis. By varying the conditions of gold deposition, such as ligands present or the geometry of the CdSe nanocrystals, we were able to tune the gold domain size between 1.4 nm and 3.9 nm and gain important information on the role of surface chemistry in heteronanoparticle synthesis and seed reactivity, both of which are crucial points regarding the chemical design of new materials for photocatalysis and optoelectronic applications.
Reference: Michaela Meyns et al., J. Mater. Chem. 20 (2010) 10602. 

09/01/2010 Sedat Dogan joined our group as PhD student. He will work on the electrical characterization of two-dimensional colloidal nanostructures. 

07/30/2010 We published a new paper in Science. 
Controlling anisotropy is a key concept in the generation of complex functionality in advanced materials. For this concept, oriented attachment of nanocrystal building blocks, a self-assembly of particles into larger single-crystalline objects, is one of the most promising approaches in nanotechnology. We report here the two-dimensional oriented attachment of lead sulfide (PbS) nanocrystals into ultrathin single-crystal sheets with dimensions on the micrometer scale. We found that this process is initiated by cosolvents, which alter nucleation and growth rates during the primary nanocrystal formation, and is finally driven by dense packing of oleic acid ligands on {100} facets of PbS. The obtained nanosheets can be readily integrated in a photodetector device without further treatment. More information in English and German. 
Reference: Constanze Schliehe et al., Science 329 (2010) 550. 

06/25/2010 We published a new paper in ACS Nano. 
We report a solution-processed, ligand-supported synthesis of 15−20 nm thick Sb(2−x)BixTe3 nanoplatelets. After complete ligand removal by a facile NH3-based etching procedure, the platelets are spark plasma sintered to a p-type nanostructured bulk material with preserved crystal grain sizes. Due to this nanostructure, the total thermal conductivity is reduced by 60% in combination with a reduction in electric conductivity of as low as 20% as compared to the bulk material demonstrating the feasibility of the phonon-glass electron-crystal concept. An enhancement in the dimensionless thermoelectric figure of merit of up to 15% over state-of-the-art bulk materials is achieved, meanwhile, shifting the maximum to significantly higher temperatures.
Reference: Marcus Scheele et al., ACS Nano 4 (2010) 4283. 

04/01/2010 Michaela Meyns joined our group as PhD student. She will work on combined semiconductor-metal nanoparticles. 

03/01/2010 We published a new paper in ACS Nano. 
The formation of monodisperse, tunable sized, alloyed nanoparticles of Ni, Co, or Fe with Pt and pure Pt nanoparticles attached to carbon nanotubes has been investigated. Following homogeneous nucleation, nanoparticles attach directly to non-functionalized singlewall and multiwall carbon nanotubes during nanoparticle synthesis as a function of ligand nature and the nanoparticle work function. Raman spectroscopy reveals that the sp2 hybridization of the carbon lattice is not modified by the attachment. In order to better understand the interaction between the directly attached nanoparticles and the non-functionalized carbon nanotubes we employed first-principles calculations on model systems of small Pt clusters and both zig-zag and armchair singlewall carbon nanotubes. The detailed comprehension of such systems is of major importance since they find applications in catalysis and energy storage.
Reference: Beate Ritz et al., ACS Nano 4 (2010) 2438. 

11/18/2009 Timm Reumann joined our group as diploma student. He will work on theoretical aspects of organic molecules adsorbed on carbon nanotubes. 

10/07/2009 We published a new paper in Advanced Functional Materials. 
A novel synthesis for near monodisperse, sub-10 nm Bi2Te3 nanoparticles is reported. A new reduction route to bismuth nanoparticles is described, which are then applied as starting materials in the formation of rhombohedral Bi2Te3 nanoparticles. After ligand removal by a novel hydrazine hydrate etching procedure, the nanoparticle powder is spark plasma sintered to a pellet with preserved crystal grain sizes. Unlike previous works on the properties of Bi2Te3 nanoparticles, the full thermoelectric characterization of such sintered pellets shows a highly reduced thermal conductivity and the same electric conductivity as bulk n-type Bi2Te3.
Reference: Marcus Scheele et al., Adv. Func. Mater. 19 (2009) 3476. 

09/27/2009 Beatriz H. Juarez and Christian Klinke received the German Nanotech Prize 2009 (Nanowissenschaftspreis) at the Schlosshotel Cecilienhof in Potsdam. The prize is offered by the Arbeitsgemeinschaft der Nanotechnologie-Kompetenzzentren Deutschlands (AGeNT-D) and the Federal Ministry of Education and Research (BMBF). 

09/21/2009 Francisco Martinez joined our group for three months as exchange PhD student from the Jose Dobado group in Granada, Spain. He will work on theoretical aspects of combined nanoparticle-nanotube systems. 

09/13/2009 We published a new paper in Nature Materials. 
We present a combined theoretical and experimental study of the frictional forces encountered by a nano-size tip sliding on top of a supported multiwall carbon nanotube along a direction parallel or transverse to the nanotube axis. Surprisingly, we find a higher friction coefficient in the transverse direction compared to the parallel direction. This behavior is explained by a simulation showing that transverse friction elicits a soft "hindered rolling" of the tube and a frictional dissipation that is absent, or partially absent for chiral nanotubes, when the tip slides parallel to the nanotube axis.
Reference: Marcel Lucas et al., Nature Mater. 8 (2009) 876. 

09/04/2009 Michaela Meyns joined our group as diploma student. She will work on combined semiconductor-metal nanoparticles. 

07/03/2009 Successful participation in the science initiative called "Landesexzellenzinitiative". We are part of the cluster project Nano spintronics and a graduate school C1 chemistry.