F. Donati

Thermoelectric properties of Bi–Te films with controlled structure and morphology

A study of the thermoelectric transport properties of Bi–Te thin films with different structures and morphologies is here presented. Films were grown by pulsed laser deposition (PLD), which permits to control the composition, phase and crystallinity of the deposited material, and the morphology at the micrometer/nanometer scale. The carrier density and mobility at room temperature and the in plane electrical resistivity and Seebeck coefficient in the temperature range 300–400 K have been measured both for films characterized by a compact morphology and by the presence of different phases (Bi2Te3, BiTe, and Bi4Te3) and for Bi2Te3 films with different morphologies at the micrometer/nanometer scale (from a compact structure to a less connected assembly of randomly oriented crystalline grains). The correlation among thermoelectric and structural properties has been investigated, showing the potential of PLD to produce n-type Bi–Te thin films with desired properties for peculiar applications. Films with a laye...

Bulk Cr tips for scanning tunneling microscopy and spin-polarized scanning tunneling microscopy

A simple, reliable method for the preparation of bulk Cr tips for scanning tunneling microscopy (STM) is proposed and its potentialities in performing high-quality and high-resolution STM and spin-polarized STM (SP-STM) are investigated. Cr tips show atomic resolution on ordered surfaces. Contrary to what happens with conventional W tips, rest atoms of the Si(111)-7×7 reconstruction can be routinely observed, probably due to a different electronic structure of the tip apex. SP-STM measurements of the Cr(001) surface showing magnetic contrast are reported. Our results reveal that the peculiar properties of these tips can be suited in a number of STM experimental situations.

Three-dimensional approach to scanning tunneling spectroscopy and application to Shockley states

The problem of the interpretation of scanning tunneling spectroscopy (STS) data is analytically solved using a three-dimensional (3D) transfer Hamiltonian approach. We present an analytical model capable of including both the electronic structure of the sample and the symmetry of the tip states (s, pz, dz2, ...) and we discuss the role of these 3D aspects in tunneling. We applied this model to the case of Shockley states. This system, allowing a full analytical treatment, led us to a detailed simulation and comprehension of the tunneling process. A procedure for the recovery of the sample local density of states from STS measurements is then proposed and applied to both the simulated and the experimental STS data of Shockley states. Comparing this approach with other methods proposed in the literature, the importance of considering the 3D aspects in treating and interpreting STS data is demonstrated.