Local polarization dynamics in ferroelectric materials

Rep. Prog. Phys. 73 (2010) 056502
defect and decipher associated mechanisms. This provides a
crucial missing step between the macroscopic statistical theories,
and atomistic-scale electron microscopy and density functional
studies of phase transitions in solids. Given the ubiquity
of electromechanical phenomena in solids, including energy
storage materials (e.g. Li-ion batteries), Jahn–Teller solids,
etc this combined approach can be expected to be applicable
well beyond the ferroelectric and multiferroic materials studied
to date.
6. Advanced topics in PFM of ferroelectrics
6.1. Polarization mediated surface chemistry
The surface of ferroelectric (and other polar) materials is
characterized by the presence of bound polarization charge,
screening charge and band bending, which may be used
to enable a broad range of applications including 2DEGbased
devices, molecular assembly, adsorption, chemical
reactions, etc. Domain decoration of ferroelectric surfaces
was first demonstrated in the 1950s and 1960s [397–399].
Domain-specific surface chemistry has been demonstrated
by friction force microscopy, and continues to be a topic
of interest today. The combination of PFM-based domain
patterning and domain specific chemical reactivity opens a
natural pathway for the fabrication of ferroelectric based device
structures.
Notably, photochemical deposition has been demonstrated
by Giocondi et al [45, 400], Jones et al [401–403], Dunn
et al [48, 404] and Liu et al [405] and has been used
to fabricate nanowires [50, 51] and complex nanostructures
[46, 49]. Recently, adsorption of charged polystyrene [406]
and the assembly of virus particles [407] and DNA [408]have
all been demonstrated.
6.2. PFM in a liquid environment
6.2.1. Imaging in a liquid environment. Two of the most
detrimental effects on resolution in PFM arise from capillary
forces due to the presence of a water layer on the surface
and non-local electrostatic interactions between the sample
and the conductive tip and cantilever beam. The surface
water layer leads to the presence of liquid necks when the
AFM tip contacts the sample surface, increasing the effective
tip–sample contact area and thus reducing the resolution (see
section 3.2.5). Non-local electrostatic forces can lead to longrange
tip–sample interactions that can preclude high resolution
studies of the local piezoelectric response [409]. By imaging
electromechanical response in a liquid environment, i.e. by
controlling the dielectric constant of the imaging media, it
would be possible to eliminate both the capillary and the longrange
electrostatic forces. Liquid PFM was demonstrated
in 2006 and showed an order of magnitude improvement in
resolution for a bulk ceramic PZT sample [410]. The ability to
image electromechanical coupling in a liquid environment may
open the door to studies of soft and biological piezoelectrically
active materials, the potential for which has been suggested
and the challenges outlined [111, 411]. Recently, PFM has
S V Kalinin et al
been demonstrated in a tapping or intermittent contact mode
in liquid, which may assist in the imaging of electromechanical
coupling in biological systems [412].
6.2.2. Polarization switching in solution. As previously
mentioned, one limitation of imaging through a top electrode is
that the domain structure and nucleation sites cannot be related
to surface topography without removing the top electrode.
Using a tip as a top electrode allows the domain dynamics
to be studied locally. In ambient, an applied dc voltage of
sufficient magnitude will result in the nucleation of a domain
directly under the tip. In de-ionized water, however, the
application of a dc voltage will result in electrochemical
reactions. Switching in solvents of low to intermediate
conductivity combined with liquid PFM allows the spatial
extent of the applied field to be controlled independently of
the local piezoresponse measurement [413]. Through the
choice of solvent, it has been shown that it is possible to
nucleate a single domain, switch an entire sample surface
and even partially switch a large region (figure 61), much
like the step-by-step switching studies described earlier
(section 4.5). This allows an additional tool for nucleation site
visualization. Future progress in this area requires specially
fabricated shielded probes (figure 62) [414, 415] similar
to those produced for scanning electrochemical microscopy
[416, 417].
6.3. PFM and transport measurements
The interest in combined PFM–conductivity measurements
stems both from the non-volatile memory applications and
potential for electroresitive memory devices. The early
work of Gruverman et al explored the relationship between
domain dynamics and conductivity at interfaces in thin films
[418]. The combination of local electromechanical and
conductivity measurements has shown a relation between
current and pinning [261] at the bicrystal grain boundary in
BFO. These studies have addressed the intrinsic conductivity
mediated by the structural defects only weakly affected by
polarization.
Following early work on ferroelectric diodes, much
attention has been devoted to electroresistances in ferroelectric
heterostructures. An extensive review of this area has recently
been provided by Watanabe [419]. The work by Rodriguez
Contreras [420] has sparked an extensive search for theoretical
mechanisms [42, 421] and experimental demonstrations
of electroresistance in conductor–ferroelectric–conductor
junctions. However, in many cases, the presence of extended
defects and oxygen vacancy accumulation has precluded
identification of polarization mediated transport mechanisms.
The use of the PFM approach allowed localization of
field within small defect free regions, allowing the direct
unambiguous probing of polarization-controlled tunneling
into ferroelectric surface [308, 422]. In parallel, increased
conductivity at domain walls in bismuth ferrite is due to
structurally driven changes in the local potential and bandgap
at a domain wall [267]. This increased conductivity likely
plays a role in the observed relaxation of ferroelastically
60