Fabrication of FeSi and Fe3Si compounds by electron beam induced mixing of [Fe/Si]2 and [Fe3/Si]2 multilayers grown by focused electron beam induced deposition

Fe-Si binary compounds have been fabricated by focused electron beam induced deposition by the alternating use of iron pentacarbonyl, Fe(CO)5, and neopentasilane, Si5H12 as precursor gases. The fabrication procedure consisted in preparing multilayer structures which were treated by low-energy electron irradiation and annealing to induce atomic species intermixing. In this way we are able to fabricate FeSi and Fe3Si binary compounds from [Fe=Si]2 and [Fe3=Si]2 multilayers, as shown by transmission electron microscopy investigations. This fabrication procedure is useful to obtain nanostructured binary alloys from precursors which compete for adsorption sites during growth and, therefore, cannot be used simultaneously

Tuning electrical properties of hierarchically assembled Al-doped ZnO nanoforests by room temperature Pulsed Laser Deposition

Large surface area, 3D structured transparent electrodes with effective light management capability may represent a key component in the development of new generation optoelectronic and energy harvesting devices. We present an approach to obtain forest-like nanoporous/hierarchical Al-doped ZnO conducting layers with tunable transparency and light scattering properties, by means of room temperature Pulsed Laser Deposition in a mixed Ar:O2 atmosphere. The composition of the background atmosphere during deposition can be varied to modify stoichiometry-related defects, and therefore achieve control of electrical and optical properties, while the total background pressure controls the material morphology at the nano- and mesoscale and thus the light scattering properties. This approach allows to tune electrical resistivity over a very wide range (10^-1 - 10^6 Ohm*cm), both in the in-plane and cross-plane directions. Optical transparency and haze can also be tuned by varying the stoichiometry and thickness of the nano-forests

Image charge screening: a new approach to enhance magnetic ordering temperatures

We have tested the concept of image charge screening as a new approach to enhance magnetic ordering temperatures and superexchange interactions in ultra thin films. Using a 3 monolayer NiO(100) film grown on Ag(100) and an identically thin film on MgO(100) as model systems, we observed that the Neel temperature of the NiO film on the highly polarizable metal substrate is 390 K while that of the film on the poorly polarizable insulator substrate is below 40 K. This demonstrates that screening by highly polarizable media may point to a practical way towards designing strongly correlated oxide nanostructures with greatly improved magnetic properties.Comment: 5 pages, 4 figure

Growth and characterization of EBID-fabricated suspended nanostructures

While the standard beam-induced deposition approach is to grow deposits on a substrate, deposition of self-supporting suspended structures can be obtained by slowly moving the beam laterally from an elevated edge, in order to grow an overhanging deposit behind the beam path. An example of nanofabrication by lateral-EBID is given in fig. 1, where a suspended nanowire has been deposited across two vertical pillars. The structure is obtained (fig. 1c) by moving the electron beam from the top of left pillar toward the right one with a scan speed of 33 nm/s, at steps of 5 nm, under a Pt-metallorganic gas flow. As indicated by the lateral dimensions in fig. 1a and fig. 1b, an advantage of this deposition method is the high lateral resolution because of the lack of secondary emissions from the substrate, that enlarge the deposit footprint well above the beam spotsize.The growth mechanism at the basis of lateral deposition, as proposed by Liu and co-workers, is depicted in fig. 2, where a gaussian electron beam, positioned over a thin edge and then moved to the right, is considered. On the first spot, the beam generates a gaussian deposit above (A) and below (B) the edge (because of the deep electron penetration), having a volume and gaussian width which are proportional to the beam dwell time and also depend on the beam shape. When the beam is shifted laterally it falls on a point of the deposit that may be above (O1a), at the same level of (O2), or below (O3b) the edge, depending on the amplitude of the shift, and thus give rise to upward (A1B1), parallel (A2B2) or downward (A3B3) growth, respectively. If the lateral shift is much larger than the deposit width, lateral growth doesn’t take place. The process is ruled by the interplay of several parameters, such as the deposition rate, the beam dwell time, the lateral shift amplitude and the beam shape. In most studies, these are often resumed into the beam shift speed which is also a reasonable parameter for a given beam shape and gas flux. A major capability offered by lateral deposition is the three-dimensional (3D) nanofabrication. Matsui and co-workers demonstrated the potentiality of the technique with several examples of both functional (nanocoils, electrical circuit elements, nanogrippers) and artwork shapes (nanoglass) produced with IBID of carbon precursor. In the case of EBID, Ooi and co-workers presented the fabrication of tools and probes based on suspended nanowires for the manipulation and observation, with SNOM microscopy, of DNA fibers. The structures were realized by lateral-EBID of carbon-contamination gas. Another noteworthy example are the 10 nm-size nanotweezers, with a gap of 25 nm, fabricated at the ends of conventional Si microtweezers by lateral-EBID of C contamination by Boggild et al. Several works were devoted to the study of the growth mechanism and material properties. EBID of 3D freestanding nanostructures from a Cu precursor was explored by Utke et al.. Suspended horizontal nanorods were used as a support for the growth of vertical pillars, and it was found that the reduced thermal conductivity with respect to a bulk substrate, resulted in a thermal decomposition of the precursor with higher crystallinity of the Cu deposit. Fujita et al. fabricated 5 nm-width suspended nanowires by lateral-EBID of C contamination with a SEM. They compared the suspended growth with the one of vertical pillars and concluded that the higher resolution in the former was due to the reduced secondary electrons generation within the structure. Liu and co-workers studied the lateral deposition of W precursors with TEM high energy electrons (200keV), deriving the growth model presented in fig. 2. Lateral EBID has been also investigated on bulk substrates. By varying the lateral scan speed, the inclination of pillars deposited from Au and Mo precursors [ ] and the periodicity of arch-like structures grown from Cu precursor [ ] have been studied. The geometry of suspended depositions, grown from a gold precursor in an environmental SEM, has also been explored [ ].In the following paragraphs we will focus on lateral EBID of suspended nanostructures from Pt-metallorganic and TEOS precursors, performed with an SEM. We will cover the growth mode as a function of e-beam parameters, the characterization (structure and composition) and sculpting by TEM, and, limited to the Pt structures, the thermal processing, the electrical characterization, and the structural modifications under high electrical current densities

Structural evolution and graphitization of metallorganic-Pt suspended nanowires under high-current-density electrical test

We present a real-time investigation of the dramatic structural evolution occurring in metallorganic-Pt suspended nanowires (SNWs) (20 nm size) under high-current-density electrical test. SNWs are fabricated by electron beam-induced deposition and consist of Pt nanograins (2-3 nm) embedded in a carbonaceous matrix. As current increases, the Pt-C granular material transforms into Pt-depleted, graphitized C with a two-stage process. First, Pt coalescence into big grains (10-15 nm) is observed, then, for current density approaching 10(7) A/cm(2), grains are depleted by Pt electro- and thermomigration, leaving a graphitized C matrix. The graphitic-C wire eventually breaks forming a nanosize gap

In-depth structural characterisation of the bct-hcp phase transition in Co epitaxial films

In-depth structural evolution of Co layers grown on Fe(001) has been investigated by modulated electron emission measurements during both film growth, by thermal evaporation, and erosion by mild ion sputtering. Co growth results in an epitaxial body-centered tetragonal (bct) phase which progressively turns to hexagonal close packed (hcp) in the 10–35 monolayers (ML) range. Erosion of a 40 ML thick film, performed under sputtering conditions such as to preserve structural order, shows that the bct/hcp interface is located significantly deeper than expected from measurements during growth. It is concluded that the transition to the hcp phase in the growing film begins on top of the bct layer and afterwards extends both upward and downward, reducing the thickness of the underlying bct layer

Fabrication of 5 nm gap pillar-electrodes by electron-beam Pt deposition

Using a focused ion beam (FIB)-scanning electron microscope (SEM) workstation, free-standing nanoelectrodes were grown by SEM-assisted Pt deposition between FIB-patterned Au pads. Two pillar electrodes were first grown with opposite-tilted geometries up to a spacing of 120 nm. By SEM scanning over the pillar tips, under a precursor gas flow, gap reduction down to 5 nm was monitored in live imaging mode. As shown by transmission electron microscopy (TEM) analysis, the deposit consisted of Pt crystallites embedded in amorphous- C. Local annealing by high-current TEM irradiation increased the size of the Pt grains, which produced clear diffraction rings. The annealing procedure did not affect the overall shape of the tips, indicating good mechanical stability of the pillars. We show how this FIB-SEM approach is suitable to fabricate multielectrode nanostructures by depositing a third pillar electrode below the gap of the tilted electrodes

Il FIB come strumento nella sintesi, preparazione e caratterizzazione di materiali e dispositivi

In questo capitolo vengono descritte alcune tra le metodologie di analisi e fabbricazione disponibili su una classe relativamente recente di microscopi a scansione come il Focusd Ion Beam (FIB) e gli strumenti a doppia colonna FIB+SEM. Sul versante analisi verranno descritti metodi per l’osservazione in pianta e sezione trasversale con i microscopi a scansione e metodi per la preparazione di campioni da aree selezionate per la microscopia elettronica in trasmissione in sezione trasversale. Per quanto concerne la fabbricazione saranno riportati esempi relativi alla nano-strutturazione di superfici, la modifica di punte per microscopia a sonda e la deposizione assistita da fascio (sia ionico che elettronico)