High Spatial and Time Resolution Characterization Technologies
Since atomic and nanoscale interactions that govern the macroscopic behavior of materials originate from a length scale that can be accessed using the atomic force microscope AFM, I have invested a consistent effort towards mastering AFM techniques with the ambition of merging high spatial and temporal resolution. In principle, the versatility of the instrument arises from the fact that single atoms and nanostructures can be probed with a nanoscopic tip and with high precision by monitoring and controlling the structure onto which the tip is mounted; typically a micro-cantilever. In particular, when the cantilever is vibrated, the rich dynamics arising from the non-linear forces have led to the branching of AFM into several modes of operation. Nevertheless, a general trend in the AFM community can be identified in the attempt to extend its capabilities towards extracting more quantitative information and increasing sensitivity, and throughput. The interest in extracting quantitative information about interaction forces is clearly related to identifying and decoupling chemical, mechanical and other properties that are characteristic of the material. Phase imaging is particularly interesting in dynamic AFM since it provides compositional or chemical contrast with, arguably, the highest resolution. Phase contrast was put in firm grounds when Cleveland et al. showed that, through the phase signal, the energy principle provides quantifiable results about the energy dissipated per cycle without putting any restrictions on the nature of dissipative forces. In this way, and while phase imaging was already broadly used in AFM, it became possible to relate phase shift contrast to variations in dissipation quantitatively. As already predicted by Cleveland et al., different dissipative processes have later been shown to control the dynamics. Some methods to decouple the different dissipative processes have also been proposed where the behavior of amplitude and phase curves is monitored as the cantilever approaches the sample. In general, it was shown with relative success that sample parameters can be quantified with the use of force models in combination with experimental data, sometimes with the use of the energy dissipation principle or even by proposing and increasing the number of experimental observables or constraints. Another way of dealing with energy dissipation is to define effective damping coefficients where these typically involve viscous forces only or, at best, assume that dissipative forces are odd functions of velocity and conservative forces are even. In summary however, while it provides quantitative information, the energy dissipation approach alone is limited in that it does not provide direct information about the unrestricted force distance or magnitude dependencies. In this respect, we have focused on force reconstruction where no restriction is placed in terms of the nature of conservative or dissipative forces. Two approaches for force recovery have been recently proposed in both the steady and transient states in what we call single cycle measurements. The instantaneous force is accurately reconstructed thus capturing the details of conservative and dissipative interactions. These include a broad range of phenomena from the formation and rupture of bonds to the local detection and probing of water molecules. With single cycle measurements, we add high temporal resolution (possibly microsecond range) to the impressive spatial resolution of AFM, to study kinetic processes moving towards the objective of merging spatial and temporal resolution as illustrated in Figure 3. These recent advances in atomic force microscopy AFM indicate that the instrument is reaching a new state of maturity in terms of quantification of material properties of complex systems, i.e. young modulus, viscoelasticity, sample-deformation, etc., and even atoms via the selection of suitable models when imaging under appropriate environments. This is in contrast with the nineties and early twenty first century when reports were vastly based on contrast directly arising from variations in the experimental observables. On the other hand, arguably, these developments are mostly led by advances in the understanding of the instrument rather than on the understanding of nanoscale forces and phenomena. In particular, the availability and success of contact mechanics models has led to the vast majority of works dealing with quantification to focus on contact (repulsive) forces and the related Young Modulus, viscoelasticity and mechanical deformation. The reactivity and chemistry of the surface however depends on longer range forces that are prevalent in ambient conditions and that are of paramount importance for the development of functional surfaces, nanofabrication and the appropriate understanding of nanoscale contacts for the general nanotechnology applications. My understanding is that suitable models and appropriate parameterization of the prevalent long range forces, with the possible exception of adhesion and work of adhesion, are lacking and therefore cannot be employed to quantify relevant properties. With the lack of experimental data and models, most groups turn to molecular dynamics. Nevertheless, such approach typically leads to controversy because of lack of reproducibility and lack of sufficient experimental evidence to support predictions. The consistent attempt to fill this gap is illustrated in the Figure. We have succeeded defining and employing metrics to parameterize long range forces and experimental observables in dynamic atomic force microscopy (dAFM). We have employed statistical classification models to derive predictive relationships between these metrics and further establish simple functional relationships between computationally low and high cost metrics. Our results for example indicate that the presence of plateaus in the attractive part of the force, which can be of more than one nm in range, are a general characteristic of surfaces exposed to the ambient environment. These plateaus were first predicted by our group in 2012, but we lacked direct experimental evidence to establish conclusive predictions and parameterize the phenomena. We have further demonstrated that these plateaus are responsible for distinctive features that are size dependent, i.e. sub 10 nm, thus providing a method for rapid discrimination between sharp and blunt tips.
Functional Coating for the Energy and Health Sector
The Figure illustrates the importance of recognizing the wider implication that few molecular layers of water have in substantially modifying chemical and physical properties on the surface. These properties ultimately dictate the way particles and other molecular structures interact with each other on the surface. In the energy field, for example, we have employed our understanding to develop large scale process to functionalize the surfaces of different photovoltaic modules with the final intent of reducing the energy necessary to keep the modules clean from the sand naturally accumulating on the modules. This has huge consequence for the large scale penetration of photovoltaic technologies in arid desert areas. It is through the understanding of the solid/liquid interface in the nanoscale that we have demonstrated novel functionalization scheme for the development of antibacterial coatings.
Advanced Composites for the Aviation Industry
The Figure llustrates the investigation of both thermal and electrical transport phenomena by means of advanced high spatial resolution characterization techniques to develop composites with enhanced properties. In the context of the aviation industry, for example, we successfully proposed a framework to achieve high electrical conductivity in aligned carbon nanotube polymer composites. The local understanding of the interaction between the polymer matrix and the carbon nanotubes allowed finding the optimal structural compromise to guarantee the enhancement of both mechanical and electrical properties. In the context of energy storage application we have looked at the effect of surface transport properties on the performance of carbon plastic electrodes for flow battery applications. In all of these applications the role of surface interaction is the single most important factor to develop more efficient solutions.
Nanostructures for Energy Harvesting
The Figure illustrates the investigation of thermal transport properties over length scale from nanometers upward in nanostructured materials for thermoelectric applications. A variation of a pump-probe like technique that I implemented was paramount in gaining an in-depth understanding of the role that grain boundaries have in decoupling thermal from electrical transport phenomena. Furthermore the role that thermal cycle plays in modifying the grain boundaries density and consequently the thermal transport behavior has been investigated in order to engineer strategies to counteract this process.
Light Matter Interaction for Novel Solar Applications
Finally illustrates the effort carried out in developing a novel solar power generation system that better utilizes the full spectrum of the solar radiation. In this context we have successfully demonstrated how spectral splitting represents a valid alternative to multi-junction solar cells for broadband light-to-electricity conversion. While this concept has existed for decades, its adoption at the industrial scale is still stifled by high manufacturing costs and inability to scale to large areas. We have recently reported the experimental validation of a novel design that could allow the widespread adoption of spectral splitting as a low-cost approach to ultrahigh efficiency photovoltaic conversion. Our system consists of a prismatic lens that can be manufactured using the same methods employed for conventional CPV optic production, and pair of inexpensive CuInGaSe2 (CIGS) solar cells of different band gaps. We demonstrate a large improvement in cell efficiency under the splitter and show how this can lead to substantial increases in system output at competitive cost using existing technologies.