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Page 6 of 15 Keeney et al. Microstructures 2023;3:2023041 https://dx.doi.org/10.20517/microstructures.2023.41

substrate were 0.09 nm (SD = ±0.07 nm) and 0.12 nm (SD = ±0.10 nm) for NGO underneath the 7.9 nm film
and 5.6 nm film, respectively. Nanoscale electromechanical properties of the ultrathin films were evaluated
TM
by PFM using an Asylum Research MFP-3D AFM in contact mode in conjunction with a HVA220
Amplifier and a cantilever scan angle of 90°. The Dual AC (alternating current) Resonance Tracking PFM
(DART-PFM) mode was utilized to amplify both the vertical and lateral piezo signals. Apex Super Sharp
conductive diamond probes AD-2.8-SS (boron doped, Au reflex coated, < 5 nm tip radius, 75 kHz resonant
frequency) were purchased from Adama Innovations (c/o Republic of Work, 12 South Mall, Cork, T12
RD43, Ireland) and used to image piezoelectric domains over 1 µm × 1 µm scan areas. Imaging of the same
sample area by vertical PFM (Vert PFM) and lateral PFM (Lat PFM) enables corresponding measurements
of the normal component of the piezoresponse vector (out-of-plane direction) and the transversal
[42]
component of the in-plane piezoresponse (perpendicular to the cantilever axis), respectively . The Lat
PFM measurements were conducted by a comparable method to the Vert PFM measurements, except the
“InFast” option was set to “Lateral” as opposed to “AC Deflection”, respectively. The drive frequencies were
operated near contact resonance for lateral (480 to 520 kHz) and vertical (250 to 280 kHz) modes. PFM
measurements were operated at AC drive amplitudes (V ) of 4.0 V and below.
AC
AC
RESULTS AND DISCUSSION
Characterization of structure, surface morphology, and surface piezo-configurations of as-prepared
ultrathin B6TFMO films
Two different thicknesses of B6TFMO films were synthesized on (001)-orientated NGO substrates by
pulsing either 75 injections or 100 injections of each precursor during the DLI-CVD process. The pseudo-
cubic lattice parameter (a ′) of the perovskite-type layer in B6TFMO is 3.89 Å and a ′ is 3.858 Å for NGO;
p
p
therefore, the lattice mismatch = -0.82% . Out-of-plane XRD profiles, as shown in Figure 1B, are consistent
[27]
with epitaxial films of an orthorhombic Aurivillius phase with five perovskite layers per half unit cell (m = 5)
interleaved by bismuth oxide interfaces, as depicted schematically in Figure 1A . Both B6TFMO films
[43]
demonstrate a preference for orientating along the c-axis of the structure, with only (00l) Bragg reflections
visible in the diffractograms. The broadening of the peaks [e.g., FWHM values for the (0010), (0012), and
(0020) peaks are 1.721°, 0.400°, and 2.114° and are 1.534°, 0.389° and 1.186° for B6TFMO deposited with 75
injections and 100 injections, respectively] is due to grain size effects and due to the fact that the X-ray beam
is scattering from only three, or less, half-unit cell repeat entities . Comparative XRD patterns obtained
[44]
from B6TFMO films deposited with 75 injections and 100 injections demonstrate that the intensities of the
film deposited using 75 injections [e.g., the log intensity of the (0010), (0012), and (0020) peaks are: 0.461,
10
0.616, and 0.130 counts] are lower than the intensities of the film deposited using 100 injections [e.g., the
log intensity of the (0010), (0012), and (0020) peaks are: 0.717, 0.951, and 0.371 counts], corresponding to a
10
decreased film thickness for the film deposited using 75 injections. HR-TEM imaging [Figure 1C] confirms
that the films have an m = 5 Aurivillius phase structure, displaying some regions with out-of-phase
boundary defects and associated stacking faults (indicated by the blue arrows), typically observed for layered
Aurivillius materials. This structural disorder within the samples would also contribute to peak broadening
observed in the XRD profiles and might explain why there are additional variations in peak broadening
between the 7.9 nm and 5.6 nm B6TFMO films [e.g., the FWHM value for the (008) reflection of the 7.9 nm
film (1.079°) indicates that it is broader compared to the 5.6 nm film (0.970°)]. Contributions to the
broadening of the asymmetric peaks may also result from segregation of secondary phases or crystal
twinning, which will be discussed later. HR-TEM and XRR [Figure 1D] and [Supplementary Table 1]
measurements yield average thickness values of 7.9 nm and 5.6 nm for B6TFMO films deposited using 100
injections and 75 injections, respectively. The former equates to a thickness of approximately 1.5 unit cells,
and the latter equates to a thickness of approximately 1 unit cell of the m = 5 Aurivillius phase. These
thickness measurements are consistent with previous TEM measurements of B6TFMO films prepared on
different substrates under similar growth DLI-CVD conditions [27,28] .