Colloidal Semiconductor Nanoparticles as Functional Materials: Design, Assembly and Applications

This work summarizes results of about ten years of the author’s own research activities in the field of colloidal synthesis of semiconductor nanoparticles, their postsynthetic chemical modification, assembly, and applications. I attempted to provide a concise yet comprehensive overview presenting my own results as a part of the knowledge framework created in close collaboration with many colleagues from all over the world. This habilitation thesis consists of an introduction, explaining the motivation of the research accomplished, followed by a main part which briefly presents key achievements of the author with links to appropriate annexes, i.e. original published articles in peer review journals which are attached to this cumulative script, and completed by conclusions

Precise Engineering of Nanocrystal Shells via Colloidal Atomic Layer Deposition

We present a general strategy for a facile synthesis of complex multifunctional nanoscale materials via colloidal atomic layer deposition (c-ALD). The c-ALD technique is based on self-limiting half-reactions of ionic precursors on the surface of a nanocrystal (NC) occurring at room temperature. Using this technique, uniform layers of CdS and ZnS semiconductor shells were epitaxially grown on CdSe semiconductor cores with different shell combinations, leading to the precise control of the optical properties of these heterostructures. All core-shell multicomponent nanoparticles preserve narrow size distributions, phase crystallinity, and shape homogeneity of the initial NCs. Furthermore, we attempted to extend the toolbox of the c-ALD to combine materials with intrinsically different properties, such as Au/CdS core/shell structures with substantial lattice mismatch. The results presented in this work demonstrate great opportunities for creating functional materials with programmable properties for electronics and optoelectronics

Self-Supported Three-Dimensional Quantum Dot Aerogels as a Promising Photocatalyst for CO2 Reduction

With the merits of quantum dots (QDs) (e.g., high molar extinction coefficient, strong visible light absorption, large specific surface area, and abundant functional surface active sites) and aerogels (e.g., self-supported architectures, porous network), semiconductor QD aerogels show great prospect in photocatalytic applications. However, typical gelation methods rely on oxidative treatments of QDs. Moreover, the remaining organic ligands (e.g., mercaptoacids) are still present on the surface of gels. Both these factors inhibit the activity of such photocatalysts, hampering their widespread use. Herein, we present a facile 3D assembly of II–VI semiconductor QDs capped with inorganic (NH4)2S ligands into aerogels using H2O as a dispersion solvent. Without any sacrificial agents, the resulting CdSe QD aerogels achieve a high CO generation rate of 15 μmol g–1 h–1, which is 12-fold higher than that of pristine-aggregated QD powders. Our work not only provides a facile strategy to fabricate QD aerogels but also offers a platform for designing advanced aerogel-based photocatalysts

Lasing by Template-Assisted Self-Assembled Quantum Dots

Miniaturized laser sources with low threshold power are required for integrated photonic devices. Photostable core/shell nanocrystals are well suited as gain material and their laser properties can be exploited by direct patterning as distributed feedback (DFB) lasers. Here, the 2nd-order DFB resonators tuned to the photoluminescence wavelength of the QDs are used. Soft lithography based on template-assisted colloidal self-assembly enables pattern resolution in the subwavelength range. Combined with the directional Langmuir–Blodgett arrangement, control of the waveguide layer thickness is further achieved. It is shown that a lasing threshold of 5.5 mJ cm−2 is reached by a direct printing method, which can be further reduced by a factor of ten (0.6 mJ cm−2) at an optimal waveguide thickness. Moreover, it is discussed how one can adjust the DFB geometries to any working wavelength. This colloidal approach offers prospects for applications in bioimaging, biomedical sensing, anti-counterfeiting, or displays

fully solution processed conductive films based on colloidal copper selenide nanosheets for flexible electronics

A novel colloidal synthesis of copper selenide nanosheets (NSs) with lateral dimensions of up to 3 μm is developed. This material is used for the fabrication of flexible conductive films prepared via simple drop-casting of the NS dispersions without any additional treatment. The electrical performance of these coatings is benchmarked against copper selenide spherical nanocrystals (SNCs) in order to demonstrate the advantage of 2D morphology of the NSs for flexible electronics. In this contest, Cu2−xSe SNC films exhibit higher conductivity but lower reproducibility due to the formation of cracks leading to discontinuous films. Furthermore, the electrical properties of the films deposited on different flexible substrates following their bending, stretching and folding are studied. A comparison of Cu2−xSe SNC and CuSe NS films reveals an increased stability of the CuSe NS films under mechanical stress applied to the samples and their improved long-term stability in air

3D Assembly of All-Inorganic Colloidal Nanocrystals into Gels and Aerogels

We report on an efficient assembly approach to a variety of electrostatically stabilized all-inorganic semiconductor nanocrystals (NCs) via their linking with appropriate ions into multibranched gel networks. These all-inorganic non-ordered 3D assemblies can combine strong interparticle coupling which facilitates charge transport between the NCs with their diverse morphology, composition, size, and functional capping ligands. Moreover, the resulting dry gels (aerogels) are highly porous monolithic structures, which preserve the quantum confinement of their building blocks. The inorganic semiconductor aerogel made of 4.5 nm CdSe colloidal NCs, capped with iodide ions and bridged with Cd2+ ions, exhibited a surface area as high as 146 m2/g

Heterostructured Bismuth Telluride Selenide Nanosheets for Enhanced Thermoelectric Performance

The n-type semiconductor system Bi2Te3Bi2Se3 is known as a low-temperature thermoelectric material with a potentially high efficiency. Herein, a facile approach is reported to synthesize core/shell heterostructured Bi2Te2Se/Bi2Te3 nanosheets (NSs) with lateral dimensions of 1-3 mu m and thickness of about 50nm. Bi2Te3 and Bi2Se3, as well as heterostructured Bi2Te2Se/Bi2Te3 NSs are obtained via colloidal synthesis. Heterostructured NSs show an inhomogeneous distribution of the chalcogen atoms forming selenium and tellurium-rich layers across the NS thickness, resulting in a core/shell structure. Detailed morphological studies reveal that these structures contain nanosized pores. These features contribute to the overall thermoelectric properties of the material, inducing strong phonon scattering at grain boundaries in compacted solids. NSs are processed into nanostructured bulks through spark plasma sintering of dry powders to form a thermoelectric material with high power factor. Electrical characterization of our materials reveals a strong anisotropic behavior in consolidated pellets. It is further demonstrated that by simple thermal annealing, core/shell structure can be controllably transformed into alloyed one. Using this approach pellets with Bi2Te2.55Se0.45 composition are obtained, which exhibit low thermal conductivity and high power factor for in-plane direction with zT of 1.34 at 400K

Absolute photoluminescence quantum yields of IR26 and IR-emissive Cd₁₋ₓHgₓTe and PbS quantum dots: method- and material-inherent challenges

Bright emitters with photoluminescence in the spectral region of 800–1600 nm are increasingly important as optical reporters for molecular imaging, sensing, and telecommunication and as active components in electrooptical and photovoltaic devices. Their rational design is directly linked to suitable methods for the characterization of their signal-relevant properties, especially their photoluminescence quantum yield (Φf ). Aiming at the development of bright semiconductor nanocrystals with emission >1000 nm, we designed a new NIR/IR integrating sphere setup for the wavelength region of 600–1600 nm. We assessed the performance of this setup by acquiring the corrected emission spectra and Φf of the organic dyes |trybe, IR140, and IR26 and several infrared (IR)-emissive Cd₁₋ₓHgₓTe and PbS semiconductor nanocrystals and comparing them to data obtained with two independently calibrated fluorescence instruments absolutely or relative to previously evaluated reference dyes. Our results highlight special challenges of photoluminescence studies in the IR ranging from solvent absorption to the lack of spectral and intensity standards together with quantum dot-specific challenges like photobrightening and photodarkening and the size-dependent air stability and photostability of differently sized oleate-capped PbS colloids. These effects can be representative of lead chalcogenides. Moreover, we redetermined the Φf of IR26, the most frequently used IR reference dye, to 1.1 × 10⁻³ in 1,2-dichloroethane DCE with a thorough sample reabsorption and solvent absorption correction. Our results indicate the need for a critical reevaluation of Φf values of IR-emissive nanomaterials and offer guidelines for improved Φf measurements

Colloidal PbS nanoplatelets synthesized via cation exchange for electronic applications

In this work, we present a new synthetic approach to colloidal PbS nanoplatelets (NPLs) utilizing a cation exchange (CE) strategy starting from CuS NPLs synthesized via the hot-injection method. Whereas the thickness of the resulting CuS NPLs was fixed at approx. 5 nm, the lateral size could be tuned by varying the reaction conditions, such as time from 6 to 16 h, the reaction temperature (120 °C, 140 °C), and the amount of copper precursor. In a second step, Cu+ cations were replaced with Pb2+ ions within the crystal lattice via CE. While the shape and the size of parental CuS platelets were preserved, the crystal structure was rearranged from hexagonal covellite to PbS galena, accompanied by the fragmentation of the monocrystalline phase into polycrystalline one. Afterwards a halide mediated ligand exchange (LE) was carried out in order to remove insulating oleic acid residues from the PbS NPL surface and to form stable dispersions in polar organic solvents enabling thin-film fabrication. Both CE and LE processes were monitored by several characterization techniques. Furthermore, we measured the electrical conductivity of the resulting PbS NPL-based films before and after LE and compared the processing in ambient to inert atmosphere. Finally, we fabricated field-effect transistors with an on/off ratio of up to 60 and linear charge carrier mobility for holes of 0.02 cm2 V−1 s−1