Nano News: 02/16/15

Autonomous atom assembly of nanostructures using a scanning tunneling microscope

NIST researchers have demonstrated the autonomous computer-controlled assembly of atoms into perfect nanostructures using a low temperature scanning tunneling microscope. The results, published in an invited article in the ("Autonomous assembly of atomically perfect nanostructures using a scanning tunneling microscope"), show the construction without human intervention of quantum confined two-dimensional nanostructures using single atoms or single molecules on a copper surface. Lund

Automated assembly of individual cobalt atoms on an atomically flat copper surface into simple geometric shapes, a square, a triangle, and a circle. From left to right, each figure shows the configuration after each atom move. Image size 15 nm × 15 nm. Center: Perfect assembly of the NIST logo after four steps of automated assembly. Image size 40 nm × 17 nm. All images are shown in colored 3D top view with light shadowing with a height range of ∼100 pm. A major goal of nanotechnology is to develop so-called “bottom up” technologies to arrange matter at will by placing atoms exactly where one wants them in order to build nanostructures with specific properties or function. The researchers, led by Robert Celotta and Joseph Stroscio from the CNST, have demonstrated the first steps towards achieving that capability using the atom manipulation mode of a scanning tunneling microscope (STM) in combination with autonomous motion algorithms. The team, which includes Stephen Balakirsky (previously in EL and now at Georgia Tech), Aaron Fein (PML), Frank Hess (previously in the CNST), and Gregory Rutter (previously in the CNST and now at Intel), used autonomous algorithms to manipulate single atoms and molecules, much like the algorithms for “hands-free” car driving. The system works by first scanning the locations of available atoms on the surface. It then specifies the desired coordinates of atoms of a nanostructure, and autonomously calculates and directs the trajectories for the STM probe tip to move all the atoms to their desired locations. The team was able to demonstrate that it could autonomously construct cobalt atoms into nanostructures that confine the quantum properties of the copper’s surface electrons. It then used the STM to measure those properties. In addition to demonstrating the construction of nanostructures made out of atoms, they demonstrated that it was possible to construct nanoscale lattices made of carbon monoxide molecules and to tailor-make interacting quantum dots formed from vacancies in the carbon monoxide lattices. The researchers believe that an approach based on autonomous construction of atoms and molecules using this technique could be the foundation for an easily accessed toolkit for producing tailored quantum states with applications in quantum information processing and nanophotonics.

Buckybowls for molecular circuits

Corannulene is a carbon molecule with a unique shape (similar to the better known fullerene) and promising properties. A team of scientists from SISSA and the University of Zurich carried out computer simulations of the molecule’s properties and discovered that it might help overcome the difficulties building molecular circuits (i.e., of the size of molecules). The study has just been published in ("Buckybowl superatom states: a unique route for electron transport?").

A unique paradigm for intermolecular charge transport mediated by diffuse atomic-like orbital (SAMOs), typically present in conjugated hollow shaped molecules, is investigated for C20H10 molecular fragments by means of G0W0 theory. Imagine taking a fullerene (C60) and cutting it in half like a melon. What you get is a corannulene (C20H10), a molecule that, according to a just-published study conducted with SISSA’s collaboration, could be an important component of future “molecular circuits”, that is, circuits miniaturized to the size of molecules, to be used for various kinds of electronic devices (transistors, diodes, etc.). Fullerene is a very popular molecule: also called buckybowl, it is formed of carbon atoms arranged in a hexagonal network shaped like a hollow sphere. It is an intensely studied material that displays interesting properties in different fields. Even though C60 is known to contain “empty states” (of a very special nature known as buckybowl superatom states, BSS) capable of accepting electrons, these states are found at very high energies, a feature that makes them difficult to exploit in electronic devices. The electrons in electronic circuits have to be able to travel easily. “In fullerene the energy levels of the BSS type capable of accommodating ‘travelling electrons’ are difficult to achieve energetically”, explains Layla Martin-Samos, researcher at Democritos IOM-CNR and SISSA and among the authors of the study published in Physical Chemistry Chemical Physics. “Corannullene, on the other hand, seems to be much better suited to the purpose, as demonstrated by our calculations”. Martin-Samos and colleagues had already studied the optical properties of this molecule. “This time instead we focused on its electronic properties with special emphasis on the study of BSS”. The observations - theoretical and based on computer simulations – of Martin-Samos and colleagues show that BSS in corannulene are found at much lower energy levels compared to fullerene and can therefore be more easily accessed. “This makes the material an excellent prospective candidate for the construction of electronic circuits” continues Martin-Samos. “In fact if we put corannulene molecules next to one another in a row, the electrons will flow easily from one to the next, forming a sort of tunnel which makes up the circuit”. “Our work not only uncovered the potential of this molecule, but it also served as a guide for the subsequent experimental analysis, by indicating where and what to look at and reducing the time and cost of the experiments. The investigators have recently finished collecting the experimental data and are now going to start their analysis to verify experimentally what we observed in our simulation. We’re keeping our fingers crossed: who knows, in a few months’ time we might be celebrating”.

Discovery of new ferroelectric silicate materials

Barium titanate (BaTiO3) and lead zirconium titanate (Pb(Zr,Ti)O3) are well known ferroelectric materials that are used for fabricating capacitors and actuators. Their crystal structures belong to the perovskite-type oxide group (ABO3) with a BO6 octahedral coordination. However, new and important ferroelectric compounds with other structural groups have not been found during the last decade. Now, Mitsuru Itoh and Hiroki Taniguchi (present address:Nagoya University) and their colleagues have succeeded in the synthesis of new ferroelectric silicates with a tetrahedral coordination ("Ferroelectricity Driven by Twisting of Silicate Tetrahedral Chains"). The mechanism for the evolution of ferroelectricity was studied by both experiments and theoretical calculations.

Structure of Bi2SiO5 at 773 and 300 K. Single crystalline Bi2SiO5 was fabricated by a melting and solidification process via an intermediate glassy state. Dielectric measurements, Raman spectroscopy, X-ray structural analysis, transmission electron microscopy, and first principle calculations were conducted for single crystals. The refined crystal structure and calculated electronic and phonon structures consistently explained the ferroelectricity of this material appearing below 673 K. The origin of ferroelectricity of Bi2SiO5 was attributed to the twisting of the silicate tetrahedral chains. This new finding may trigger materials research of new ferroelectrics with other structural groups and coordination numbers of 4, 5, and 7.