Chemical Coding at the Atomic Scale: Designing Hybrid and Quantum Nanostructures for Applications in Optical Biosensing, Light Harvesting, Chiral Catalysis, and More

The wet-chemical synthesis of two-dimensional (2D) lead chalcogenide semiconductors PbX (X = S, Se, Te) yields photoluminescent materials with strong excitonic contribution at room temperature. They act as model systems for efficient charge carrier multiplication and hold potential as intriguing candidates for fiber-based photonic quantum applications. However, synthetic access to the third family member, 2D PbTe, remains elusive due to a challenging precursor chemistry. Here, we report a direct synthesis for 2D PbTe nanoplatelets (NPLs) with tunable photoluminescence (PL, 910 – 1460 nm (1.36 – 0.85 eV), PLQY 1 - 15 %), based on aminophosphine precursor chemistry. Ex-situ transamination of tris(dimethylamino)phosphine telluride with octylamine is confirmed by 31P NMR and yields a reactive tellurium precursor for the formation of 2D PbTe NPLs at temperatures as low as 0 °C. Our results expand and complete the row of lead chalcogenide-based 2D NPLs, opening up new ways for further pushing the optical properties of 2D NPLs into the infrared and toward technologically relevant wavelengths. (1) Biesterfeld, Vochezer, Kögel, Zaluzhnyy, Rosebrock, Klepzig, Leis, Seitz, Meyer, Lauth "Solving the Synthetic Riddle of Colloidal 2D PbTe Nanoplatelets with Tunable Near-Infrared Emission" arXiv:2406.09223, 2024. (currently in revision)

Integrated sensor arrays enhance sensing performance and offer an efficient method for comprehensive health monitoring allowing the detection of various health issues in a non-invasive manner, for example via the use of breath analysis.[1,2] For this application, gold nanoparticle (GNP) based chemiresistors gained scientific interest, as they are promising gas sensors with tunable chemical selectivity, rapid response and recovery times, high sensitivity, and low power consumption.[3] In applications like breath analysis, many different factors have an impact on the sensor responses, for example humidity fluctuations. However, only very limited information exists about the impact of the humidity on the response characteristics. Hence, the first part of this study focuses on the impact of humidity changes on the sensor responses to different analyte vapors. In order to adjust the sensors selectivity and to boost the sensitivity, the interfacial properties of the GNP-based thin film transducers are tuned by employing different thiol/dithiol mixtures for their layer-by-layer assembly.[4] When exposed to analyte vapors using humidified carrier gas, the sensitivity of these films increases significantly. The functionalized monothiols play a significant role in tuning the selectivity of the films, with hydrophobic monothiols showing higher to nonpolar analytes and hydrophilic monothiols showing higher sensitivity to polar analytes. Microgravimetric measurements reveal that the presence of humidity generally leads to enhanced uptake of analyte molecules within the GNP films. However, this effect depends on the polarity of the analyte and the thin films interfacial properties. Further, for real world applications it is important, that these sensors are able to discriminate between analytes and, if possible, to provide qualitative and quantitative information about analytes in mixtures. Therefore, the second part of this work focuses on the examination of binary analyte mixtures. Mixtures of toluene and 1-propanol vapor were selected as model systems. We demonstrate that it is possible to discriminate between different analytes and compositions by using kinetic and resistive features of the sensor responses. Even isomers like 1-propanol and 2-propanol can be discriminated using these features. References [1] Tai, H., Wang, S., Duan, Z.., Jiang, Y., Sensors & Actuators: B. Chemical 2020, 318, 128104. [2] Kim, S., Choi, S., Jang, J., Cho, H., Kim, I., Acc. Chem. Res. 2017, 50(7), 1587–1596. [3] Schlicke, H., Bittinger, S., Noei, H. and Vossmeyer, T., ACS Appl. Nano Mater. 2021, 4, 10399-10408. [4] Liu, C. Y., Bittinger, S. C., Bose, A., Meyer, A., Schlicke, H., Vossmeyer, T., Adv. Mater. Interfaces 2024, 2301058.

Traditional metal oxide (MOX) gas sensors find widespread applications due to their high sensitivity and inexpensive fabrication. However, such resistive sensing elements require high operating temperature (> 250 °C), are difficult to integrate, and feature only limited chemical selectivity. Addressing these problems, photoactivated resistive MOX sensors, which can be operated at room temperature, are currently gaining increasing attention. In this work, we present the application of titania nanocrystal thin films as photoactivated chemiresistors for the detection of volatile organic compounds (VOCs). The films' structural properties are characterized via XRD, AFM, SEM, and TEM. Further, the photocurrent and its perturbation by the photoactivated reaction with VOCs is studied via in situ charge transport measurements. To correlate the response of the photocurrent with the amount of adsorbed analyte molecules, these measurements are combined with microgravimetric measurements. As part of these investigations, we also study the reactivity of different crystal facets by using titania nanocrystals of various shapes for sensor fabrication. First results reveal that these photoactivated sensors are highly selective towards alcohols. This pronounced selectivity is attributed to the selective binding of alcohols to Ti3+ species at the nanocrystals' surface and their subsequent oxidation to aldehydes or ketones. In addition, tuning the response kinetics of the sensors by varying the irradiance of the UV light source enables the operation of a single sensing element as virtual sensor arrays. Further, the influence of humidity on the gas sensing mechanism is investigated to evaluate the performance of these sensors under ambient conditions.

Monolayered $WS_2$, among other transition metal dichalcogenides (TMDCs), shows remarkably high nonlinear susceptibility (NLS) owing to its lack of inversion symmetry.[1] The NLS can be further increased by strain,[2] while defective crystal structures allow symmetry breakage also in few-layered $WS_2$.[3] In recent years the wet chemical TMDC synthesis showed to yield predominantly monolayered semiconducting $MoS_2$[4,5,6], $WS_2$[5,7], $Mo_{1-x}W_xS_2$[8] in colloidal solution and optical properties (steady state, ultrafast absorption[4], photoluminescence[7]) have been studied. Now we utilized a femtosecond-laser-pulsed confocal mirror microscope to explore the non-linear optical response of colloidal $WS_2$. This work includes one- and two-photon photoluminescence and efficient second harmonic generation of colloidal $WS_2$, while thorough analysis via HR-TEM, Raman spectroscopy, and steady-state absorption spectroscopy confirms the synthesis of primarily monolayered semiconducting NSs with a narrow size distribution (5 25 nm)[8]. The exceptionally high NLS of colloidal WS2 expands the portfolio of potential applications for colloidal TMDCs as functional 2D semiconductors. [1] Wang, Y. et al. Opt. Mater. Express 2019, 9,1136-1149. [2] Khan, A.R. et al. ACS Nano. 2020 14 (11), 15806-15815. [3] Liu, J. et al. J. Mater. Chem. C. 2017, 5, 11239-11245. [4] Niebur, A.; Söll, A. et al. Nanoscale, 2023,15, 5679-5688. [5] Kapuria, N. et al. J. Mater. Chem. A, 2023, 11, 11341-11353. [6] Schiettecatte, P. et al. 2023, 39, (18), 6568-6579. [7] Frauendorf, A.P.; Niebur, A. et al. J. Phys. Chem. C. 2021, 125, (34), 18841-18848. [8] Fröhlich, M. et al. Phys. Chem. Chem. Phys., 2024,26, 13271-13278. [9] Zhou, P. et al. Cryst. Growth Des. 2021, 21 (3), 1451-1460.

Lead halide perovskite nanocrystals (NC) have a great prospect in next generation optoelectronic applications due to their high photoluminescence quantum yield (PLQY), defect tolerance and their easy modification by anion exchange. Especially, the PLQY is of interest when it comes to building light emitting devices (LED). Therefore, it is important to know the absolute band edge positions. A method to study the electronic structure is Spectroelectrochemistry (SEC). SEC combines the advantages of optical spectroscopy (probing the optical bandgap) and electrochemistry (probing the electrical band gap). Hence, it is a powerful tool to investigate the electronic structure depending on the NC surface. In our work, we use CsPbBr$_3$ NCs and modify their organic ligand shell, introducing DDABr, lecithin or cinnamic acid derivatives.[2,3] We are able to show by SEC that altering the organic shell results in changes in the NCs oxidation potential. Thus, their valence band changes either due to the dipole moment introduced by the ligand or the binding motive to the NC. Comparing all of these systems, we aim to derive a systematic approach for choosing the ligands on the NCs depending on the hole or electron transport layer of the LED and therefore, improving their external quantum efficiency. [1] Mulder, J.T., du Fossé, I. et al., ACS Energy Letters 6, 2519–2525 (2021). [2] Dai, J., Roshan, H. et al., ACS Appl. Mater. Interfaces 16, 9, 11627–11636 (2024). [3] Kroupa, D., Vörös, M., Brawand, N. et al., Nat Commun 8, 15257 (2017).

Preparing aerogels in two dimensions (2D) is an attractive yet challenging task that can be achieved through phase-boundary gelation at the liquid-liquid interface. Understanding the direct influence of experimental parameters on the formation, geometry, and properties of these structures offers opportunities to uncover fundamental features that affect the gelation process. The flat structure of 2D aerogels allows for easy geometric analysis and determination of fractal features, with no overlapping components to complicate the analysis. Therefore, objective is to investigate in detail the parameters affecting the gelation mechanism and to determine the direct effect of the structural change on the mechanical and electrical properties of the 2D mesh networks. The detailed investigation will be performed by applying Graph Theory and Node-based Multifractal Ananlysis to 2D gold aerogels, which will quantify the morphology of the composite network to provide multiple descriptors to numerically compare the effects of different synthesis conditions. With this information, the underlying aggregation mechanisms can be studied in more detail. In addition, the determined structure-property relationships provide the necessary insight for precise control of the mechanical and electrical properties of the 2D metal aerogels required for specific applications.

Superparamagnetic octapod iron oxide nanoparticles hold significant potential for medical imaging techniques such as magnetic particle imaging (MPI) and magnetic resonance tomography (MRI), as well as for magnetic hyperthermia and targeted drug delivery. These nanoparticles are favored due to their low toxicity and enhanced magnetic properties derived from their unique star-like shape. To better control their shape and crystalline structure, a comprehensive understanding of the synthesis mechanism is essential. In this study, thorough investigations of both the liquid reaction mixture and the gas phase during synthesis were conducted. Viscosimetry, Small-Angle X-ray Scattering (SAXS), Wide-Angle X-ray Scattering (WAXS), and Transmission Electron Microscopy (TEM) were utilized to analyze the liquid aliquots. Gas chromatography was employed to examine the gas phase. Initial findings suggest a complex and dynamic synthesis process involving oleic acid. Changes in the reaction mixture and the composition of released gases provide new insights into the mechanism. These observations highlight the need for further research to fully elucidate the synthesis pathway.

Smartphones, computers, televisions, radios, power chargers - transistors can be found in almost every electronic device. Nowadays, such electronic components are expected to become smaller, flexible, and more powerful. However, a high-performance, flexible, thin-film transistor technology is still missing. Semiconductor nanocrystals (NCs) have the potential to achieve a breakthrough in transistor performance, as they are processed solution-based and can overcome the intrinsic low carrier mobility of organic semiconductors. In the last decades, different functional NCs have been successfully fabricated. It is possible to precisely control the physical and electronic properties of NCs via parameters such as size, shape and composition. Using them in different transistor architectures is promising, as studies with CdSe NCs have shown recently.[1] In this work, we strive to use different NCs for transistors, which allow to easily realize high-performance devices without nanostructuring. Therefore, M(Ag,Cu)InE(S,Se)2 NCs are synthesized to replace the widely studied but more toxic Cd- or Pb-containing materials. To improve the electronic communication, the insulating organic ligands necessary for the synthesis are exchanged with inorganic ligands in simple solution-based phase transition ligand exchanges. The materials are then characterized in terms of their film-forming properties and electronic parameters. [1] Fan, X., J. Phys. Chem. Lett., 2019, 10(14), 4025–4031.

The family of copper chalcogenides is currently of significant interest in optoelectronic research due to their unique combination of semiconducting and metallic properties. These materials exhibit very strong ang highly tuneable surface plasmon bands in the near-infrared (NIR) region and excitonic absorption in the visible range. This dual behavior allows for a wide range of customizable combinations of plasmonic and excitonic properties within a single monolithic system, enabling adjustable nonlinear coupling. Controlling the size and shape of nanocrystals enables precise tailoring of these properties. To this end, we employ a one-pot hot injection based colloidal synthesis method to produce well-defined 2D copper sulfide (CuS) and copper selenide (CuSe) nanocrystals. We can adjust the growth conditions by carefully manipulating synthesis conditions to achieve resulting nanocrystals with specific geometries. The primary objective of our research is to demonstrate broad optical and plasmon tunability in the NIR region by systematically varying the size, shape, thickness of the nanocrystals.

Dominik A. Rudolph, Dr. A. Antanovich, V. Adolfs, Dr. S. Spelthann, D. H. Chau, F. Li, S. L. Hachmeister, F. Engelhardt, Dr. H. H. Johannes, Dr. M. Steinke, Prof. Dr. S. Häußler Prof. Dr. D. Ristau, Prof. Dr. H. Menzel, Jun.-Prof. Dr. Jannika Lauth Colloidally synthesized cadmium chalcogenide-based 2D nanoplatelets (NPLs), especially core-crown heterostructures, display a high photoluminescence quantum yield and large absorption and gain cross sections [1] as well as low Auger recombination rates. [2] This makes these colloidal nanoemitters promising for optical applications e.g., in lasing and LEDs. [3] Due to an anisotropic emission pattern of the NPLs along their lateral surface normal, many current applications, which rely on a flat-on orientation of the NPLs in deposited thin films, are limited in making full use of the light-matter interactions in the nanostructures, e.g., light redirection. [4] Here, we explore several approaches to incorporate bright CdSe/CdS core-crown NPLs into optical applications. The first method is based on colloidally dispersed NPLs inside a micrometer-scaled core of hollow core glass fibers. To achieve fiber-guided emission, a high refractive index solvent is used. Then, by utilizing a nanosecond pulsed pump laser, we can then observe ASE of the NPLs. Further modifications of our setup to enable lasing are currently being examined. As a second approach, we encapsulate NPLs in PMMA fibers by stable-jet electrospinning. Here, the NPLs align perpendicular to the fiber axis during the fiber formation, which corresponds to the emission axis of these NPLs [5]. This allows scalable light-matter interactions while minimizing negative influences on emission, like Förster Resonance Energy Transfer and is also investigated towards ASE and lasing. Finally, the different types of cadmium chalcogenide NPLs are combined with polystyrene beads to be utilized in fluorescence tag coding of biological protein targets. Here, the narrow emission pattern of NPLs allows a greater usage of the UV/vis spectrum and therefore a higher number of encoded target molecules, compared to conventional fluorophores or spherical nanoparticles. Literature: [1] A. Yeltik et al., J. Phys. Chem. C 2015, 119, 47, 26768–26775. [2] A. Poloyitsyn et al., Chem. Mater. 2017, 29, 13, 5671–5680. [3] Q. Zhang et al., ACS Photonics 2023, 10, 5, 1397-1404. [4] F. Li et al., Langmuir 2022, 38, 37, 11149–11159. [5] X. Liu et al., Macromol. Mater. Eng. 2023, 308, 2300027.

The presentation discusses the application of colloidal metasurfaces to enhance plasmonic photodetection and photocatalysis through innovative bottom-up techniques. By focusing on plasmonic charge transfer mechanisms, our study advances the utility of plasmon-photon hybridization, utilizing self-assembled plasmonic metasurfaces consisting of 1D nanoiparticle chains. These structures, strategically deployed, enable effective charge injection into adjacent semiconductor layers, thus presenting a scalable and economically viable method for photodetection, as highlighted in our recent publication [Adv. Funct. Mater., 2021, 2011099]. Furthermore, we explore the integration of soft-lithography with convective self-assembly, enhancing the implementation of large-scale, cost-efficient printing of plasmonic metasurfaces. This approach significantly optimizes charge transfer efficiency, resulting in amplified photocatalytic processes, as detailed in our subsequent work [Adv. Funct. Mater., 2021, 2105054]. Together, these methodologies pave the way for significant advancements in energy harvesting and optoelectronic applications, showcasing the potential of colloidal systems in next-generation technologies.

Tin(II)selenide (SnSe) and tin(II)sulfide (SnS) are promising materials due to their optoelectronic properties and their low toxicity. They have great potential for applications in solar cells because of a high absorption coefficient, high electrical conductivity and because the energies of the direct bandgaps coincide with the spectral range required for these applications. Moreover, there is an increasing interest in the formation of two-dimensional (2D) heterostructured nanosheets, where many different possible material combinations allow for extending the range of properties in 2D systems. We report on the synthesis of SnSe/SnS nanosheets. Starting from SnSe nanosheets, which we synthesized using a one-pot reaction technique with tin(II)chloride, hexamethyldisilazane and selen precursor in oleylamine, we produced two-dimensional heterostructures by continuously adding various amounts of sulfur precursor to a hot solution of the SnSe nanosheets. The structure of the final heterostructure can be controlled by the variation of the precursors. We can therefore control the number and position of the crowns in a core/crown heterostructure system and synthesize different kinds of heterostructures, for example SnSe wires with a SnS tip. The SnSe/SnS nanosheets were investigated via TEM, AFM and optical methods, e.g., Raman spectroscopy. EDX measurements were utilized to determine the elemental composition and the crystal structure of the nanosheets.

Despite recent progress, surface-enhanced Raman spectroscopy (SERS) still faces challenges in achieving high sensitivity and uniform Raman signals across a large area. Our research shows a significant 43-fold increase in the SERS signal by using the directional self-assembly of plasmonic nanoparticles in lattice structures without relying on the photoluminescence contribution of Rhodamine 6G. We have specifically selected the lattice constant for an off-resonant case that matches the lattice resonance and super-radiant plasmon mode along the particle chain. Our approach is supported by electromagnetic simulations, where we analyzed the radiative components of the plasmon modes by varying the particle size while keeping the lattice periodicity constant. Additionally, we performed polarization-dependent SERS measurements by employing different SERS excitation wavelengths. We have created an efficient SERS substrate by working with the self-assembled plasmonic particle lattice, which produces a much stronger signal with 73% less surface coverage compared to randomly distributed particles. Controlling the particle size while maintaining constant periodicity enables us to adjust the SERS enhancement factor, beneficial for various sensing applications. Using this colloidal method, we were able to produce highly sensitive, uniform, and polarization-dependent SERS substrates in a cost-effective and scalable manner.

The arrangement of nanoparticles into large ordered structures has attracted great attention in the nanosciences due to their special electronic and photonic properties. Modifying the surface of the particles with different ligands opens up a wide range of possibilities for linking nanoparticles and exploring their orientation and cross-links. In this work, we utilise our recently developed two-dimensional gelation of hedgehog-like gold particles, which exhibit pronounced stochastic branching and intense optical activity. We show that the two-dimensional Au hedgehog aerogels can be prospectively used in electrocatalysis and as substrates for surface-enhanced Raman spectroscopy (SERS).

The adsorption of carbon-conjugated molecules represents an established route to tuning the electronic and optical properties of transition-metal dichalcogenide (TMDC) monolayers. Here, we demonstrate from first principles that such a functionalization with prototypical compounds pyrene and tetracene can also enhance the magnitude of selected plasmon resonances in a MoS2 single sheet without significantly altering their energy and dispersion. Our proof-of-principle results indicate that such a magnification can be achieved by the proper alignment of the molecules with respect to the direction of the transferred momentum. The distinct signatures in the loss function of the interface compared to those of its constituents suggest not only the presence of non-negligible interactions between them but also the possibility of using electron energy loss spectroscopy to detect the presence and the orientation of molecular adsorbates on TMDCs.

Magnetic nanoparticles are of significant interest for research and technological applications due to their fascinating magnetic properties, including blocking temperature, coercivity, saturation magnetization, magnetic domain size, and exchange coupling effects. To achieve these properties, a new synthetic route was developed based on the thermal decomposition of iron oleate. The oleate was synthesized using a novel precursor system - Fe(II)CO3 and Fe2(III)(CO3)3. This method allows for synthesizing highly uniform nanocrystals (NCc) in various well-defined sizes and shapes on a gram scale. Variations in morphologies and sizes were obtained by using different total concentrations of the reaction solution, different reaction times, and temperatures. The octopod-star shape in a size range of 20-80 nm is kinetically favored and formed shortly after nucleation. Under thermodynamic control, the octopods underwent metamorphism and gradually smoothed, resulting in cubes with slightly truncated corners and possible sizes of 20-60 nm. The hydrophobic iron oxide NCs can be transferred individually or clustered into water and encapsulated with a polystyrene shell using an established seeded emulsion polymerization technique, providing biocompatibility and water solubility. Magneto-fluorescent or magneto-plasmonic hybrids can be produced by attaching other particle types, such as quantum dots and gold NCs, on the surface of these iron oxide clusters. The polystyrene shell prevents fluorescence quenching or disturbance of the plasmonic behavior, allowing for their use in multimodal bio-imaging. It was also possible to synthesize a homogeneous magneto-rheological nanocomposite by controlled dispersing of the hybrid micellar-encapsulated SPIONs in a poly(ethylene oxide) matrix. The crosslinking of the polymer shell by covalent bonds provides maximum stability and prevents phase separation, enabling a responsive polymeric nanocomposite that can adapt to different environments.

S. Westendorf Tübingen/Germany, Prof. Dr. M. Scheele, Tübingen/Germany, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 16, 72076 Tübingen, Germany Potential Modulated Absorption Spectroscopy (EMAS) is a spectro-electrochemical technique, which measures the changes in absorption upon applying different potentials, subsequently oxidizing or reducing the material. Generally, electrochemical methods can provide information about the exact band edge positions vs. the vacuum level, which is crucial when designing, for example, (opto-)electronic devices in which the electronical match of the different material layers is important. However, these techniques merely show at which potentials current is flowing and cannot distinguish between trap states and band edges of the material. This differentiation is possible by combining electrochemistry and optical spectroscopy in our EMAS setup.[1,2] By determination of the potential at which a bleach occurs (the absorption decreases), the band edges or energy levels involved in an optical transition can be identified. Therefore, from the measurement data we can gain information about the electronic structure of the sample material in reference to the vacuum level or to the ferrocene/ferrocenium (Fc/Fc+) couple. By utilizing Hydrogen doped Indium Oxide (IHO) as substrate material [3], the spectral range of the setup can be enhanced into the NIR region and due to the lock-in amplifier, the setup has an excellent sensitivity which allows us to study nanoparticle thin films.[1] By investigating the electrochemical oxidation and reduction of PbS Quantum Dots, this presentation details the unique advantages of EMAS in general and the usage of IHO as substrate material for measurements in the NIR. Literature: [1] Wurst, Kai M., et al. "Electronic Structure of Colloidal 2H‐MoS2 Mono and Bilayers Determined by Spectroelectrochemistry." Small 19.23 (2023): 2207101. [2] Spittel, Daniel, et al. "Absolute energy level positions in CdSe nanostructures from potential-modulated absorption spectroscopy (EMAS)." ACS nano 11.12 (2017): 12174-12184. [3] Yang, Erqi, and Bin Hu. "Fabrication of High Transmittance and High Mobility Transparent Conductive Oxide Films: Hydrogen-doped Indium Oxide." Journal of Physics: Conference Series. Vol. 2510. No. 1. IOP Publishing, 2023.

With the rapid development of electric vehicles, portable electronic devices, drones, electric tools, and the Internet of Things (IoT) in enhancing human smart living, the demand for energy storage devices is increasing day by day. Zinc-ion batteries, with their high safety, environmental friendliness, low manufacturing cost, and long cycle life, are emerging as promising candidates to complement or even replace lithium-ion batteries. However, zinc-ion batteries often suffer from significant performance degradation due to hydrogen evolution reactions between the zinc anode and aqueous electrolytes, or severe dendrite formation during electrochemical reactions. In this study, we have designed a 2D gold nanoparticle aerogel on the anode surface, which serves as an effective protective interfacial layer between the anode and electrolyte, efficiently inhibiting the uncontrolled growth of dendrites on the anode. Additionally, we have investigated its specific role and mechanisms in electrochemical reactions.