[en] In the scientific literature colloidal nanocrystals are presented as promising materials for multiple applications, in areas covering optoelectronics, photovoltaics, spintronics, catalysis, and bio-medicine. On the marked are, however, only a very limited number of examples found, indeed implementing colloidal nanocrystals. Thus the scope of this thesis was to modify nanocrystals and to tune their properties to fulfill specific demands. While some modifications could be achieved by post synthetic treatments, one key problem of colloidal nanocrystals, hampering there widespread application is the toxicity of their constituents. To develop nanocrystals from non-toxic materials has been a major goal of this thesis as well. Roughly, the results in this thesis could be subdivided into three parts: (i) the development of ion exchange methods to tailor the properties of metallic and metal-oxide based nanocrystal heterostructures, (ii), the synthesis of semiconductor nanocrystals from non-toxic materials, and (iii) the characterization of the nanocrystals by measurements of their morphology, chemical composition, magnetic-, optical-, and electronic properties. In detail, the thesis is subdivided into an introductory chapter, 4 chapters reporting on scientific results, a chapter reporting the used methods, and the conclusions. The 4 chapters devoted to the scientific results correspond to manuscripts, which are either currently in preparation, or have been published in highly ranked scientific journals such as NanoLetters (chapter 2), ACS Nano (chapter 4), or JACS (chapter 5). Thus, these chapters provide also an extra introduction and conclusion section, as well as separate reference lists. Chapter 2 describes a cation exchange process which is used to tune and improve the magnetic properties of different iron-oxide based colloidal nanocrystal-heterostructures. The superparamagnetic blocking temperature, magnetic remanence, and coercivity is tuned by replacing Fe2+ by Co2+, which results in an enhancement of the magnetocrystalline anisotropy. As a result the complex ferri-magnetic properties of the nanocrystals become detectable at room temperature, whereas they were greatly restricted to cryogenic temperatures before the cation exchange. The improvements achieved by the cation exchange widens the applicability of the iron-oxide nanocrystals for spin based magneto-electronics applications. A related post synthetic treatment to the iron exchange is the galvanic exchange, applied in chapter 3 to transform Sn nanocrystals into Ag-Sn intermetallic alloys. These alloys are of high interest for catalytic applications and batteries. The special case of Sn nanocrystals appeared to be highly interesting due to the metal/metal-oxide core/shell nature of these nanocrystals. The naturally formed SnO2 shell, which spontaneously forms as soon the nanocrystals are exposed to air, plays a decisive role in the galvanic exchange process. While it appears to be permeable for Ag ions, enabling the desired galvanic transformation of the nanocrystal core to an AgSn alloy, it effectively protects the nanocrystals core from other metals, including nobel metals. These processes were evidenced in this work in detail by in-situ experiments, performed by synchrotron X-ray diffraction and proven by transmission electron microscopy. That the ion exchange can be used also for direct synthesis of chalcogenide semiconductor nanocrystals is shown in chapter 4. In this case the cation exchange reaction has been used for the in-situ synthesis of highly reactive metal precursors, which subsequently react with chalcogenides to form 2-4 nm small nanocrystals. Encouraging results were obtained for silver chalcogenides, representing 'green' alternatives to the commonly used infrared nanocrystals based on semiconductors containing toxic elements such as Pb and Hg. In this chapter only my own contribution to the work is described, namely the synthesis strategy, because further details of these developments were worked out by another PhD student. Thus the full story of the Ag-chalcogenide nanocrystals is presented in Appendix A. Chapter 5 of the thesis introduces another possibility to obtain non-toxic nanocrystals. It describes a procedure to transform powders of archetypical organic pigments into colloidal solutions of semiconductor nanocrystals. These nanocrystals are synthesized with controlled sizes and shapes. They are eventually covered with smart ligands, providing rapid charge separation, and exhibit emission in the visible and near-infrared spectral region. Based on them, photodetectors with responsivities up to 0.9 A/W, humidity sensors with a dynamic range of 7 orders of magnitude, and field effect transistors are demonstrated, fabricated by drop-casting or paint-brushing. These results show up an enormous potential of these colloidal pigment nanocrystals for the development of an environmentally-friendly, biocompatible, and low-cost electronics. (author)