Nanoscience represents the promise of a revolution in the way we use materials for technological applications. This is due to the decrease in length scales (which permits an ongoing miniaturization of the devices), but mostly to the new physical phenomena that emerge when these scales approach the nanometer (which can be used to design new devices based on new functioning principles). However, understanding and controlling materials properties at these scales, with the necessary atomic-level control, are tasks that cannot be achieved only on the basis of experimental effort.
A theoretical description and prediction of the behavior of nanostructured matter is at the heart of most of the advances in this area. At the same time, computer simulations have advanced as an alternative way to attack these problems, which complements both the experimental effort and the standard, often oversimplified theoretical models and frameworks. In simulations, one solves the basic equations governing the behavior of matter, by using numerical algorithms that are implemented in computer codes. The level of detail in the simulation can be tuned as needed for the particular problem under consideration, by choosing the appropriate model (governing equations) to solve, and the appropriate level of accuracy in the simulation. On the most basic level are the so-called first-principles or ab-initio simulations, in which one solves the fundamental equations without using parameters or fits to experimental data. Although these simulations are of great complexity and computational cost, both the development of efficient methods and algorithms and the increase of computing power of nowadays computers has brought the limits of the system sizes that can be solved to those relevant for nanoscience.
These are precisely the main focus areas of this Research Group: the development of efficient methods for atomistic simulations in nanostructured systems, which can take full advantage of modern multiprocessor computer architectures, and their application to selected problems in nanoscience and nanotechnology.
The broad objective of this Research Line is to advance in the ability to predict and explain the behavior of nanoscale systems, using theory and simulation methods and tools. This has two aspects, which are intertwined. The first one is the active development of theoretical models and computational algorithms for atomistic simulations in nanosystems, including the implementation of these in computer codes that become a reference and basic research tool for other research groups worldwide. The second one is the application to particular problems in nanoscience and nanotechnology, with a very special focus in the direct collaboration with experimental groups. A non-exhaustive list of areas of interest is: (i) Electronic transport in nanoscale devices, with the focus on applications for nanoelectronics and on the simulation of STM images and STS spectra, among others. (ii) Nanostructures on surfaces, and interactions of molecules and adsorbates on surfaces. (iii) Electronicic and ionic dynamical processes in nanostructured materials, with a specific interest in devices for energy applications, such as solar cells.
The specific scientific goals of the line are of two kinds: methodological and applied. We classify them within the following four thematic scientific objectives:
1. Methodological developments: As in the past, one of our main methodological objectives will be to develop methods, algorithms and codes for atomistic simulations. These include (but are not restricted to) the SIESTA (see www.uam.es/siesta), and TranSIESTA codes. SIESTA is a multi-purpose first principles method and code, based on Density Functional Theory, which can be used to describe the atomic and electronic properties of systems with up to several thousands of atoms (therefore falling right in the nanoscale regime). TranSIESTA is an extension of SIESTA that allows the study of electronic transport phenomena in nanoscale devices. Both codes are among the most important and widely used ones of their kind by the academic community, with more than 3000 registered users worldwide.
2. Understanding the processes involving electronic transport in nanoelectronic devices and in STM experiments. We aim at being able to explain and predict, preferably from first-principles, the processes that govern electronic transport in these systems, including effects such as Negative Differential Resistance (NDR), inelastic processes (involving atomic vibrations or other sources of inelastic scattering, both in the weak and strong coupling regimes), Kondo physics, etc. An important field of application will be the prediction and interpretation of experimental data from Scanning Tunnel Microscopy (STM) and Spectroscopy (STS).
3. Understanding the interaction of molecules with surfaces. This is an ubiquitous issue in nanoscicence, where the interaction of molecules with surfaces of nanostructured materials is used for functionalization, to produce new physical and chemical effects, or to stabilize the nanostructures (among many other applications). The size of the molecules (often too large for standard techniques) and the complexity of the surfaces of nanostructured materials, together with the subtle physics and chemistry of the molecule-surface interactions, make these studies very difficult and highly non-trivial.
4. Understanding dynamical electronic and ionic processes in nanostructures for energy applications. This is one of the most promising areas of application of nanoscience, but many fundamental problems still need to be solved to provide practical solutions. We aim at providing basic understanding of the processes involved in devices like solar cells (such as electron excitation and transfer), electrochemical devices and fuel cells, which can help in designing new and more efficient nanomaterials and nanostructures for these important applications.