TIREMISU

TIme REsolved MIcroscopy of SUrfaces

Dr. Roberto Otero & Dr. José María Gallego

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The TIREMISU system at LASUAM is devoted to the study of molecular self-assembly on solid surfaces in Ultra-High Vacuum conditions by means of variable-temperature Scanning Tunneling Microscopy.

The study of the adsorption and self-assembly processes of molecular species on solid surfaces is relevant for a large number of technical applications such as the design of organic electronic and opto-electronic devices, the improvement of nano-mechanical biological and chemical sensors, the development of enantio-specific heterogeneous catalysts or the functionalization of solid surfaces for implant applications.

An atomic-level knowledge on the processes that govern the adsorption of organic molecules on solid surfaces, and the self-assembly of the adsorbed chemical species, is needed for all these applications. At TIREMISU we aim at understanding all these processes by:

• Depositing organic molecules on solid surfaces under Ultra-High Vacuum conditions, to ensure cleanliness and lack of any pollutant
• Studying the self-assembly process by time-resolved, variable temperature STM experiments that allow us to study the diffusion mechanisms of organic adsorbates
• Studying the final geometry of the resulting self-assembled monolayers by acquiring molecularly resolved STM images
• Characterizing the chemical reactivity of the self-assembled monolayers by depositing a second molecular species on the self-assembled monolayer

Experimental Setup

The TIREMISU system is an UHV chamber provided with a Variable Temperature Fast-Scanning STM; a Low-Energy Electron Diffraction (LEED) system to check the crystalline quality of the sample that also enables us to carry out Video-LEED experiments, thereby precisely determining the atomic structure of the sample; Auger-electron spectroscopy to ensure the cleanliness of the surface; a plasma-source for depositing atomic nitrogen; an evaporator specifically designed to work with organic molecules; and several facilities for sample preparation and assessment of the background pressure.

It will soon count with a Surface Magneto-Optical Kerr Effect (SMOKE) set-up that will open the possibility of investigating surface magnetic properties.

The STM is a commercial design from the German company SPECS . It is based upon the original design by Erik Laegsgaard, Ivan Stensgaard and Flemming Besenbacher, from Aarhus University in Denmark , with whom Dr. Roberto Otero carried out a post-doctoral stay. Its compact design leads to a high mechanical stability, allowing a very fast scan (record at 30 frames/second) that, combined with the variable-temperature feature, makes it very well suited for the study of adsorbates' dynamical processes (such as diffusion, rotation and conformational changes).

Description of the STM design
Collection of STM movies recorded with the Aarhus STM

Previous Research

The senior staff of TIREMISU has a combined background in Surface Physic, covering topics as diverse as metal-on-metal growth, magnetism in nanostructures, electronic structure of nanostructures, STM studies of surface diffusion and molecular self-assembly on solid surfaces.

In particular, during the last 3 years Dr. José María Gallego has been studying the growth of iron and copper nitrides on Cu(100). Magnetic Tunnel Junctions (MTJ) based on epitaxial Fe4N/Cu3N/Fe4N trilayers had been proposed as ideal candidates for the development of new memory elements (M-RAMs). The main results of these investigations can be summarized as follows:

• We have been capable of growing single-crystal, single-phase, magnetic and smooth ?'-Fe4N on a Cu(100) substrate up to 300 monolayers (ML)
• By means of a slow iron deposition rate, we have grown self-organized iron nitride nano-dots on Cu(100). The magnetic properties of such array of dots are currently under investigation.
• Under similar conditions, for coverages above 1 ML, it has been shown that the surface reconstructs to a new structure of p4g symmetry. Comparison with theoretical calculations (with the SIESTA code) allowed us to determine that the driving force for such a reconstruction is magnetic in nature.

On the other hand, Dr. Roberto Otero has spent the last 3 years in the group of Prof. Flemming Besenbacher, studying the diffusion and self-assembly of organic molecules adsorbed on solid surfaces. Some highlights of his research can be described as follows:

• The dependence of the diffusion coefficient of individual Violet Lander molecules (VL, C108H104) adsorbed on a Cu(110) substrate on the molecular orientation with respect to the substrate has been studied by means of a combination of STM imaging, STM manipulation and STM movies
• The DNA base guanine (G) deposited on a Au(111) substrate was found to self-assemble into a hydrogen-bonded network of G-quartets with the same structure as those found in G-quadruplex DNA in solution. Comparison with theoretical calculations allowed us to identify a non-additive behavour of the hydrogen bonds that contributes to the stability of the G-quartets.
• VL molecules, originally designed to act as molecular wires in molecular electronic devices, were forced to assemble in long 1-D structures by depositing them on a chemically templated, O-Cu nanopattern
• The step-edge reconstructions created by adsorption of VL molecules were characterized by STM manipulations