C.V. Cojocaru

Complex oxide nanostructures by pulsed laser deposition through nanostencils

We achieved parallel nanoscale patterning of ferroelectric complex oxides by pulsed laser deposition through a nanostencil (i.e., through a pattern of apertures in a thin free-standing membrane). Ordered arrays of nanostructured barium titanate (BaTiO3) were obtained onto different substrates in a single deposition step, at room temperature, replicating accurately the aperture patterns in the stencil membrane. After a postdeposition annealing treatment, x-ray diffraction pattern showed a nanocrystalline BaTiO3 structure close to the perovskite cubic phase with grains 30–35nm in size. Their local ferroelectric properties were detected using piezoresponse force microscopy.

Towards ferroelectric and multiferroic nanostructures and their characterisation

We summarise here our efforts toward the fabrication and characterisation of ferroelectric and multiferroic films and structures. First, we discuss the challenges related with the fabrication and characterisation of nanostructures of functional complex oxides. In particular, to demonstrate the functionality of our films and especially of our structures, we briefly describe atomic force microscopy techniques tailored for local electrical or magnetic characterisation. Piezoresponse Force Microscopy and Magnetic Force Microscopy enable the characterisation of piezoelectric, ferroelectric and magnetic properties at the nanoscale. We then report the fabrication of various functional oxide films by Pulsed Laser Deposition (PLD), in particular the deposition of the conducting oxide electrode SrRuO3 at room temperature. We also describe the fabrication of ferroelectric BaTiO3 and BiFeO3, both in the form of film and mesoscopic (sub-micron size) islands. The formation of ferroelectric structures of arbitrary shape at controlled location was also achieved by nanostencilling, i.e., using a shadow-mask with nanoscale features. Finally, the successful synthesis of Bi2FeCrO6 films by pulsed laser deposition is then detailed; this is a new multiferroic material predicted by ab-initio calculations. The Bi2FeCrO6 films have the correct cationic stoichiometry throughout their thickness and their crystal structure is found to be very similar to that of BiFeO3. Bi2FeCrO6 films exhibit good piezoelectric and ferroelectric properties at room temperature, a property that was not predicted. Magnetic Force Microscopy reveals the presence of magnetic domains and confirms the macroscopic magnetic measurements showing that the Bi2FeCrO6 films do exhibit a saturation magnetisation about one order of magnitude higher than that of BiFeO3 films having the same thickness.

PIEZORESPONSE FORCE MICROSCOPY OF PLD-GROWN MULTIFERROIC BiFeO3 FILMS AND MESOSTRUCTURES

ABSTRACT Bismuth ferrite BiFeO3 is a multiferroic material, exhibiting both electric and magnetic ordering at room temperature, properties which make it a promising material for use in novel device fabrication. We report here the successful fabrication of BiFeO3 thin films and of isolated micron-sized structures by pulsed laser deposition. We present the structure and morphology of the films as well as their local ferroelectric properties investigated by piezoresponse force microscopy, a unique tool for the study of the basic piezoelectric & ferroelectric phenomena at the nanometer scale (ferroelectric domains imaging and local switching). In addition, preliminary magnetic measurements demonstrate the coexistence at room temperature of weak ferromagnetism and of ferroelectricity.

Characterization of individual multifunctional nanoobjects with restricted geometry

Anisotropic nanostructures such as nanowires (NWs)/nanotubes and nanometer-sized islands are the subject of intense research efforts due to several interesting phenomena observed to occur in confined geometries with a high aspect ratio and/or reduced size. Understanding and controlling the mechanical and electromechanical behaviors of these nanostructures are crucial for the practical implementation of such building blocks as active or passive components in nanodevices. Recent progresses toward the characterization of individual one-dimensional (NWs and nanotubes) and two-dimensional (islands) functional nanoscale objects of piezoelectric and ferroelectric (BaTiO3, Bi4Ti3O12 (BiT), and NaNbO3) and multiferroics (BiFeO3 and Bi2FeCrO6 (BFCO)) are presented. These nanoobjects have been characterized using scanning probe microscopy-based techniques, combined with various modulation schemes for enhanced sensing capabilities. The most remarkable results include: ferroelectricity of NaNbO3/Nb2O5 nanotube heterostructures, negative piezoelectric coefficient in highly anisotropic BiT mesoscopic rods, and preservation of the multiferroic character of epitaxial BFCO nanoislands.

Modified Stranski?Krastanov growth in Ge/Si heterostructures via nanostenciled pulsed laser deposition

The combination of nanostenciling with pulsed laser deposition (PLD) provides a flexible, fast approach for patterning the growth of Ge on Si. Within each stencilled site, the morphological evolution of the Ge structures with deposition follows a modified Stranski-Krastanov (SK) growth mode. By systematically varying the PLD parameters (laser repetition rate and number of pulses) on two different substrate orientations (111 and 100), we have observed corresponding changes in growth morphology, strain and elemental composition using scanning electron microscopy, atomic force microscopy and μ-Raman spectroscopy. The growth behaviour is well predicted within a classical SK scheme, although the Si(100) growth exhibits significant relaxation and ripening with increasing coverage. Other novel aspects of the growth include the increased thickness of the wetting layer and the kinetic control of Si/Ge intermixing via the PLD repetition rate.

Ferroelectric mesoscopic structures by room-temperature PLD

Usually ferroelectric thin films are deposited by pulsed laser deposition (PLD) at elevated substrate temperatures, in order to obtain a good crystalline quality. Here we report the fabrication of ferroelectric BaTiO3 and BiFeO3 structures by room-temperature PLD combined with post-deposition annealing, and their ferroelectric characterization. A method to control the position of the deposited structures is also presented.

Patterning of Functional Materials by Pulsed Laser Deposition through Nanostencils

We present how various features drawn in a miniature shadow-mask (nanostencil) can be efficiently transferred to a surface in the form of 3D nanostructures of metals (Pt, Cr), semiconductors (Ge) or complex oxides (e.g. BaTiO3) by room temperature pulsed laser deposition (PLD) and stenciling. Using the proposed method, there is no aggressive interaction with the substrate, but selective deposition of the material by simply interposing a sieve with apertures down to 100 nm between the deposition source and the substrate. Nanostenciling allows organizing the structures in given architectures, with high accuracy, while reducing drastically the number of processes present in resist-based lithography. The material deposited through the stencil mask conserves the desired functionality even at the level of the individual nanostructures. The patterning process is simple and rapid since it is not implying additional processing steps to the deposition process; it is also parallel, resist-less and without interfering with the structures natural growth dynamics. Nanostenciling can be performed in high or ultra high vacuum and is suitable for parallel prototyping of fragile or functionalized surfaces.