Lynette Keeney

Nanoscale Ferroelectric and Piezoelectric Properties of Sb2S3 Nanowire Arrays

We report the first observation of piezoelectricity and ferroelectricity in individual Sb(2)S(3) nanowires embedded in anodic alumina templates. Switching spectroscopy-piezoresponse force microscopy (SS-PFM) measurements demonstrate that individual, c-axis-oriented Sb(2)S(3) nanowires exhibit ferroelectric as well as piezoelectric switching behavior. Sb(2)S(3) nanowires with nominal diameters of 200 and 100 nm showed d(33(eff)) values around 2 pm V(-1), while the piezo coefficient obtained for 50 nm diameter nanowires was relatively low at around 0.8 pm V(-1). A spontaneous polarization (P(s)) of approximately 1.8 μC cm(-2) was observed in the 200 and 100 nm Sb(2)S(3) nanowires, which is a 100% enhancement when compared to bulk Sb(2)S(3) and is probably due to the defect-free, single-crystalline nature of the nanowires synthesized. The 180° ferroelectric monodomains observed in Sb(2)S(3) nanowires were due to uniform polarization alignment along the polar c-axis.

Room temperature ferroelectric and magnetic investigations and detailed phase analysis of Aurivillius phase Bi5Ti3Fe0.7Co0.3O15 thin films

Aurivillius phase Bi5Ti3Fe0.7Co0.3O15 (BTF7C3O) thin films on α-quartz substrates were fabricated by a chemical solution deposition method and the room temperature ferroelectric and magnetic properties of this candidate multiferroic were compared with those of thin films of Mn3+ substituted, Bi5Ti3Fe0.7Mn0.3O15 (BTF7M3O). Vertical and lateral piezoresponse force microscopy (PFM) measurements of the films conclusively demonstrate that BTF7C3O and BTF7M3O thin films are piezoelectric and ferroelectric at room temperature, with the major polarization vector in the lateral plane of the films. No net magnetization was observed for the in-plane superconducting quantum interference device (SQUID) magnetometry measurements of BTF7M3O thin films. In contrast, SQUID measurements of the BTF7C3O films clearly demonstrated ferromagnetic behavior, with a remanent magnetization, Br, of 6.37 emu/cm3 (or 804 memu/g), remanent moment = 4.99 × 10−5 emu. The BTF7C3O films were scrutinized by x-ray diffraction, high resolutio...

The structural and piezoresponse properties of c-axis-oriented Aurivillius phase Bi5Ti3FeO15 thin films deposited by atomic vapor deposition

The deposition by atomic vapor deposition of highly c-axis-oriented Aurivillius phase Bi5Ti3FeO15 (BTFO) thin films on (100) Si substrates is reported. Partially crystallized BTFO films with c-axis perpendicular to the substrate surface were first deposited at 610 °C (8% excess Bi), and subsequently annealed at 820 °C to get stoichiometric composition. After annealing, the films were highly c-axis-oriented, showing only (00l) peaks in x-ray diffraction (XRD), up to (0024). Transmission electron microscopy (TEM) confirms the BTFO film has a clear layered structure, and the bismuth oxide layer interleaves the four-block pseudoperovskite layer, indicating the n = 4 Aurivillius phase structure. Piezoresponse force microscopy measurements indicate strong in-plane piezoelectric response, consistent with the c-axis layered structure, shown by XRD and TEM.

Flexible-CMOS and biocompatible piezoelectric AlN material for MEMS applications

The development of a CMOS compatible flexible piezoelectric material is desired for numerous applications and in particular for biomedical MEMS devices. Aluminum nitride (AlN) is the most commonly used CMOS compatible piezoelectric material, which is typically deposited on Si in order to enhance the c-axis (002) crystal orientation which gives AlN its high piezoelectric properties. This paper reports on the successful deposition of AlN on polyimide (PI-2611) material. The AlN deposited has a FWHM (002) value of 5.1 and a piezoelectric d33 value of 1.12 pm V 1 , and SEM images show high quality columnar grains. The highly crystalline AlN material is due to the semi-crystalline properties of the polyimide film used. Cytotoxicity testing showed the AlN/polyimide material to be non-toxic to 3T3 cells and primary neurons. Surface properties of the AlN/polyimide film were evaluated as they have a significant effect on the adhesion of cells to the film. The results show neurons adhering to the AlN surface. The results of this paper show the characterization of a new flexible-CMOS and biocompatible AlN/polyimide material for MEMS devices with improved crystallinity and piezoelectric properties. (Some figures may appear in colour only in the online journal)

Piezoresponse force microscopy investigations of Aurivillius phase thin films

The sol-gel synthesis and characterization of n≥3 Aurivillius phase thin films deposited on Pt/Ti/SiO2–Si substrates is described. The number of perovskite layers, n, was increased by inserting BiFeO3 into three layered Aurivillius phase Bi4Ti3O12 to form compounds such as Bi5FeTi3O15 (n=4). 30% of the Fe3+ ions in Bi5FeTi3O15 were substituted with Mn3+ ions to form the structure Bi5Ti3Fe0.7Mn0.3O15. The electromechanical responses of the materials were investigated using piezoresponse force microscopy and the results are discussed in relation to the crystallinity of the films as measured by x-ray diffraction.

Room temperature electromechanical and magnetic investigations of ferroelectric Aurivillius phase Bi5Ti3(FexMn1−x)O15 (x = 1 and 0.7) chemical solution deposited thin films

Aurivillius phase thin films of Bi5Ti3(FexMn1−x)O15 with x = 1 (Bi5Ti3FeO15) and 0.7 (Bi5Ti3Fe0.7Mn0.3O15) on SiO2-Si(100) and Pt/Ti/SiO2-Si substrates were fabricated by chemical solution deposition. The method was optimized in order to suppress formation of pyrochlore phase Bi2Ti2O7 and improve crystallinity. The structural properties of the films were examined by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. Optimum crystallinity and pyrochlore phase suppression was achieved by the addition of 15 to 25 mol. % excess bismuth to the sols. Based on this study, 17.5 mol. % excess bismuth was used in the preparation of Bi2Ti2O7-free films of Bi5Ti3FeO15 on SrTiO3(100) and NdGaO3(001) substrates, confirming the suppression of pyrochlore phase using this excess of bismuth. Thirty percent of the Fe3+ ions in Bi5Ti3FeO15 was substituted with Mn3+ ions to form Bi2Ti2O7-free thin films of Bi5Ti3Fe0.7Mn0.3O15 on Pt/Ti/SiO2-Si, SiO2-Si(100), SrTiO3(100), and NdGaO3(001) substrates. B...

Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics

Oxide materials which exhibit both ferroelectricity and ferromagnetism are of great interest for sensors and memory applications. Layered bismuth titanates with an Aurivillius structure, (BiFeO<inf>3</inf>)nBi<inf>4</inf>Ti<inf>3</inf>O<inf>12</inf>, can possess ferroelectric and ferromagnetic order parameters simultaneously. It has recently been demonstrated that one such example, Bi<inf>5</inf>Fe<inf>0.5</inf>Co<inf>0.5</inf>Ti<inf>3</inf>O<inf>15</inf>, where n = 1 with half the Fe³⁺ sites substituted by Co³⁺ ions, exhibits both ferroelectric and ferromagnetic properties at room temperature. Here we report the fabrication of highly-oriented polycrystalline ceramics of this material, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets. Electron backscatter images showed that there is a secondary phase within the ceramic matrix which is rich in cobalt and iron, hence this secondary phase could contribute in the main phase ferromagnetic property. The concentration of the secondary phase obtained from secondary electron microscopy is estimated at less than 2.5 %, below the detection limit of XRD. TEM was used to identify the crystallographic structure of the secondary phase, which was shown to be cobalt ferrite, CoFe<inf>2</inf>O<inf>4</inf>. It is inferred from the data that the resultant ferromagnetic response identified using VSM measurements was due to the presence of the minor secondary phase. The Remanent magnetization at room temperature was M<inf>r</inf> ≈ 76 memu/g which dropped down to almost zero (M<inf>r</inf> ≈ 0.8 memu/g) at 460 °C, far lower than the anticipated for CoFe<inf>2</inf>O<inf>4</inf>.

Absence of Evidence ≠ Evidence of Absence: Statistical Analysis of Inclusions in Multiferroic Thin Films

Assertions that a new material may offer particularly advantageous properties should always be subjected to careful critical evaluation, especially when those properties can be affected by the presence of inclusions at trace level. This is particularly important for claims relating to new multiferroic compounds, which can easily be confounded by unobserved second phase magnetic inclusions. We demonstrate an original methodology for the detection, localization and quantification of second phase inclusions in thin Aurivillius type films. Additionally, we develop a dedicated statistical model and demonstrate its application to the analysis of Bi6Ti2.8Fe1.52Mn0.68O18 (B6TFMO) thin films, that makes it possible to put a high, defined confidence level (e.g. 99.5%) to the statement of ‘new single phase multiferroic materials’. While our methodology has been specifically developed for magnetic inclusions, it can easily be adapted to any other material system that can be affected by low level inclusions.

Atomic layer deposition of Cu with a carbene-stabilized Cu(I) silylamide

The metal–organic Cu(I) complex 1,3-diisopropyl-imidazolin-2-ylidene copper hexamethyl disilazide has been tested as a novel oxygen-free precursor for atomic layer deposition of Cu with molecular hydrogen. Being a strong Lewis base, the carbene stabilizes the metal centre to form a monomeric compound that can be vaporised and transported without visible degradation. A significant substrate dependence of the growth process not only with respect to the film material but also to the structure of the films was observed. On Pd surfaces continuous films are grown and no phase boundary can be observed between the Cu film and the Pd, while island growth is observed on Ru substrates, which as a consequence requires thicker films in order to achieve a fully coalesced layer. Island growth is also observed for ultra-thin (<10 nm) Pd layers on Si substrates. Possible explanations for the different growth modes observed are discussed.

Atomic vapor deposition of bismuth titanate thin films

C-axis oriented ferroelectric bismuth titanate (Bi4Ti3O12) thin films were grown on (001) strontium titanate (SrTiO3) substrates by atomic vapour deposition technique. Ferroelectric properties of thin films are greatly affected by the presence of various kinds of defects. Detailed x-ray diffraction data (XRD) and transmission electron microscopy (TEM) analysis showed presence of out-of-phase boundaries (OPBs). These OPBs originate at atomic steps on the SrTiO3 substrate surface. It is found that the OPB density changes appreciably with the amount of titanium injected during growth of the thin films.