Nanoscale Tomography Based in Electrostatic Force Microscopy

Resumen

In various research fields ranging from material sciences, microelectronics to life sciences, there is an increasing need for nanoscale characterization techniques. Achieving detailed 3D maps of the structural properties of materials in a non-destructive and label-free technique is of dire necessity. For example, Nanocomposites, which consist of nanostructures in their bulk matrix to improve the matrix efficiency, have successfully incorporated into material science applications in the last two decades. Silver nanoparticle nanocomposites especially have a barrage of applications to their credit, ranging from solar cell applications, touch screens, and LEDs to flexible wearable devices. Understanding these nanocomposites' subsurface features or tomography could help us understand their properties, interpreting/analyzing them based on their parametric dependence, which would later aid us in tuning them for our desired applications. The demand for nanoscale tomographic characterization has given rise to the development of different techniques and methods, mainly based on Electron, X-Ray and Optical Microscopies. Each of the techniques can provide us with the sub-surface information required, but all present certain limitations. For example, Electron Microscopy methods require extensive sample preparation, and so the sample is altered or destroyed in the characterization process. Fluorescence microscopy and two-photon microscopy require a fluorescence tag or two-photon dyes to be tagged/attached to identify the dispersed particles in the matrix/composites. For this reason, additional nanotomographic microscopy techniques are still being investigated. Among them, nanotomographic Scanning Probe Microscopy (SPM) techniques have emerged with a great potential in recent years. Scanning Probe Microscopies is the family of microscopies that scans the surface using a nanometric probe. The acquired data is used to reconstruct the samples' physical properties in nanometric resolution. (e.g., topography). Since the measurements could be carried out in non-contact mode, studying tomography has made them a better contender. SPM also possess numerous advantages compared to the existing techniques like of being a) non-invasive, b) non-destructive, c) requiring relatively minimal sample preparation, d) can be extended into any environment (inert, ambient vacuum) and e) can also be measured in air, water, or any biological medium. Among them, Electrostatic Force Microscopy is a technique where a voltage bias is applied between the probe and the sample, and the electric force felt by the tip is measured. When the probe/tip is moved along the surface, the cantilever and probe deflect to the electrical properties of the localized point beneath them. This force exerted on the tip (or) the change in capacitive force is measured and recorded. This capacitive force is a long-range force, (i.e.) the force might not pertain only to the surface but also corresponds to the entities below the surface. This quality has been utilized to study the tomography (sub-surface characteristics), which has been successfully studied in recent investigations such as studying the compositional modifications below the organic layers, imaging below the organic layers, imaging of carbon nanotubes, graphene networks, nanoparticles inside the polymeric nanocomposites. Along with the qualitative analysis of EFM images, recent advances proved that nanotomography quantitative information could be obtained such as the depth of carbon nanotubes in polymeric films, the dielectric properties of buried water in nanochannels. In this study, the best parameters have been identified using numerical methods for experimental analysis. In the continued efforts to this objective, we theoretically analyze the use of Scanning Dielectric Microscopy (SDM) to non-destructively investigate the nanocomposites. There is a particular focus on the attainable spatial resolution and the possibility to identify the capacitive coupling between neighboring nanowires. The preliminary study demonstrates that the nanowires spread function consists of a modified Lorentzian with a cubic decay. Following that, we have shown that the spatial resolution can be defined with the help of this function and noted how the different system parameters like tip-sample distance, the radius of the probe, depth of the nanowires influences it. We also found that nanowires with a diameter of 50 nm with a spatial resolution below 100 nm are easily achievable for shallowly buried nanowires. The spatial resolution increases when a system parameter decreases the maximum of the nanowire spread function or increases their widths. These results are tested with multiple nanowire numerical and theoretical calculations. These same calculations and procedures are used to demonstrate the sensitivity of the capacitive coupling between neighboring nanowires. This result is especially relevant for separations below the diameter of the nanowires. These findings are of utmost relevance since the electric interaction between nanowires largely determines nanowire nanocomposites' macroscopic electrical properties. Present results prove that that SDM could impact the development of novel materials for applications in fields such as wearable electronics, solar cell technologies, and photonics and can be a valuable non-destructive subsurface characterization technique for nanowire nanocomposites. We also show that Scanning Dielectric Microscopy (SDM) can image the subsurface nanowires and also map the depth of the nanowires distribution within the nanocomposites non-destructively. To achieve it, we incorporate the sub-surface imaging capabilities of Electrostatic Force Microscopy(EFM) along with its quantitative analysis with the help of finite element numerical calculations. To test them in an application, the three-dimensional spatial distribution of ~50 nm diameter silver nanowires in ~100 nm-250 nm thick gelatin films is determined. The advantage of using a gelatin matrix presents a relatively low dielectric constant, εr~5, as well as, in humid ambient conditions, where its dielectric constant rises up to εr ~14, and so characterization is done both in dry ambient conditions. The results show that SDM can optimize these materials' properties for applications in fields such as wearable electronics, solar cell technologies, or printable electronics and can be a valuable non-destructive subsurface characterization technique for nanowire-based nanocomposite materials. Identifying the nanowires' depth in the composites could provide the option of three-dimensional reconstruction of the material, which opens the door for far more applications in different fields. We believe that the thorough subsurface investigations carried out in this thesis, would play an indispensable part in the advancement of nanoscale tomography studies.