Characterization and processability of molecular-based nanoparticles and 2D crystals by scanning probe microscopy

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The development of new and pioneering multifunctional nanodevices relies on the possibility of the miniaturization and the assembly of nanometric building blocks with specific configurations. The size, shape and properties of these nanocomponents as well as the possibility of their integration and combination are crucial in order to fabricate clever devices that interface small assemblies of nanoobjects with macro(nano)scaled electrodes. The challenge then is twofold: first, the manipulation and characterization of novel materials at the nanoscale and second, their organization onto surfaces and/or their assembly to create heterostructured multifunctional materials. The two aspects of this challenge will be covered in this thesis by the manipulation, organization and characterization of two types of novel nanomaterials: twodimensional (2D) atomic crystals and molecular-based nanoparticles. This manuscript is divided in two main parts: The first part deals with the manipulation of the two kinds of afore mentioned nanomaterials. The aim of this section is to efficiently produce and transfer onto surfaces these nanoobjects in controllable ways, and one step forward, their manipulation to generate heterostructures. This part comprises three chapters: The first chapter (chapter 1) is a general overview of micro and nanolithography. Three types of alternative lithographic methods will be explained as they will be used in chapters 2 and 3: Local oxidation nanolithography (LON) performed with atomic force microscopy (AFM), dip-pen nanolithography (DPN) and soft lithography. In chapter 2, a feasible route for the creation of heterostructured materials will be proposed which involves an accurate chemical manipulation of atomic monolayers of metallic transition metal dichalcogenides (TMDCs) by means of LON. In this line, a new AFM-LON mode, coined as static-tip LON, will be presented that will allow the creation of reproducible oxidation of very low-profile motifs on monolayers of TaS2. This general method will be also applied to other three types of metallic TMDCs: TaSe2, NbS2 and NbSe2. A detailed study of the oxidation growth processes on each material will be presented. Besides, a new method for the micromechanical exfoliation of atomically thin layers of 2D layered materials is developed which tries to overcome the limitations of the Scotch Tape method for exfoliating metallic TMDCs. This new method it will allow the transference of the delaminated thin patches to a variety of substrates, as well. In chapter 3, the three lithographic methods introduced in chapter 1, will be used for the accurate and reproducible organization of magnetic Prusian blue analogue nanoparticles (PBA-NPs) which are an important class of molecular-based materials exhibiting tunable magnetic properties. This tunability together with an easy chemical synthesis makes PBA-NPs good candidates for their integration into new electronic or spintronic devices. As a previous step, the development of proper techniques for their structuration on surfaces with control over their positioning, and organization is a crucial issue. This will be achievable by applying hybrid nanoscale patterning strategies to fabricate nanopatterns onto functionalized surfaces combining top down with bottom up approaches. The organization will be performed onto very specific positions of the surface by means of LON and DPN, while large area patternings of several cm2 will be achieved via soft lithography. Detailed and dedicated studies of the three methods and their results will be presented. For the proper assembling and integration of these molecular-based NPs on devices, the maintenance and manipulation of their properties once organized onto the surface is crucial. The second part of the thesis is then devoted to the magnetic characterization and tuning of the above mentioned PBA-NPs by low temperature magnetic force microscopy. This part is also divided in three chapters. Chapter 4 is a brief introduction of the characterization of magnetic NPs on surfaces in general by means of very different magnetic characterization techniques, and in particular it will be introduced the magnetic force microscopy (MFM) as a powerful tool to characterize the PBA-NPs at low temperature. In chapter 5, a detailed study of the size-dependence and temperature-dependence of the magnetic properties of single and isolated PBA-NPs will be presented. As well, the different magnetization reversal mechanisms of the NPs derived from the different sizes will be studied by variable field MFM at 4.2 K. The calibration of the magnetic tip as well as other issues related with the measurements at low temperature will be also discussed. The in-situ functionalization of the magnetic tip by the attachment of a unique NP onto it will permit the assignment of the magnetization directions of the magnetic moments of individual NPs. Finally, in chapter 6, it will be presented the magnetic characterization of ordered arrays of PBA-NPs organized by soft lithography. The magnetic interaction between the closed packed NPs and the influence of the organization in the reversibility of the individual NPs inside the patterned lines will be studied. This will be possible by working at low temperatures and with variable applied field.
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