Optical nanocrystallography with tip-enhanced phonon Raman spectroscopy.

Abstract

Conventional phonon Raman spectroscopy is a powerful experimental technique for the study of crystalline solids that allows crystallography, phase and domain identification on length scales down to approximately 1 microm. Here we demonstrate the extension of tip-enhanced Raman spectroscopy to optical crystallography on the nanoscale by identifying intrinsic ferroelectric domains of individual BaTiO(3) nanocrystals through selective probing of different transverse optical phonon modes in the system. The technique is generally applicable for most crystal classes, and for example, structural inhomogeneities, phase transitions, ferroic order and related finite-size effects occurring on nanometre length scales can be studied with simultaneous symmetry selectivity, nanoscale sensitivity and chemical specificity.

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Optical nanocrystallography with tip-enhanced
phonon Raman spectroscopy
Samuel Berweger1, Catalin C. Neacsu1, Yuanbing Mao2, Hongjun Zhou2, Stanislaus S. Wong2,3
and Markus B. Raschke1*
Conventional phonon Raman spectroscopy is a powerful experi-
mental technique for the study of crystalline solids1–5that
allows crystallography, phase and domain identification6,7on
length scales down to ?1 mm. Here we demonstrate the exten-
sion of tip-enhanced Raman spectroscopy to optical crystallo-
graphy on the nanoscale by identifying intrinsic ferroelectric
domains of individual BaTiO3nanocrystals through selective
probing of different transverse optical phonon modes in the
system. The technique is generally applicable for most crystal
classes, and for example, structural inhomogeneities, phase
transitions, ferroic order and related finite-size effects occur-
ring on nanometre length scales can be studied with simul-
taneous symmetry selectivity, nanoscale sensitivity and
chemical specificity.
Taking advantage of the local field enhancement provided by the
nanoscopic apex of a plasmonic scanning probe tip, tip-enhanced
Raman spectroscopy (TERS) achieves spatial resolution in the nano-
metre range. TERS of molecular adsorbates8and carbon nanotubes9
with sensitivity down to the single molecule level10–12has been
demonstrated. Although previously used in a specific tip-enhanced
geometry to maximize the near-field contrast13,14, the full potential
offered by the symmetry selectivity of the Raman response has not
yet been explored15. Here we derive and experimentally demonstrate
the tensor-based phonon Raman selection rules that will enable
TERS to determine crystallographic orientation in general, and
the ferroelectric order in particular, of crystalline solids, with nano-
metre spatial resolution.
Without loss of generality we use tetragonal BaTiO3as a specific
example. A variety of potential nanoscale devices are based on the
ferroelectric properties of BaTiO3, including piezoelectric actuators
and non-volatile memory16. In addition, although it has long
been considered a prototypical displacive ferroelectric17, recent
experiments have indicated a more complex mechanism with order–
disorder contributions17,18. This makes a fundamental understanding
of its nanoscale ferroelectric order and domain behaviour desirable.
Perovskite BaTiO3has a paraelectric pseudocubic phase above a
Curie temperature of TC¼ 1208C. Below TCits non-centrosym-
metric tetragonal 4mm phase is characterized by the intrinsic for-
mation of ferroelectric domains polarized along the crystallographic
c-axis.Inkeeping withconventional definitions,domainsformed par-
allel to the length of the rod are referred to as c-domains (see Fig. 1),
and those perpendicular as ax0- and ay0-domains, that is, oriented in
the sample plane and along the surface normal, respectively.
Figure 2a shows a TERS spectrum acquired for 60 s on top of a
BaTiO3nanorod (unpolarized detection, oblique rod orientation).
The two dominant peaks can be assigned to the A1transverse
optical (TO) mode at 516 cm21, the E longitudinal optical (LO)
mode at 715 cm21, and the spectrally unresolved A1LO and E TO
modes at 727 cm21and 487 cm21, respectively19. The Raman-
active LO modes are characteristic of the tetragonal phase20, and
their observation thus reflects the ferroelectric state. A tip–sample
distance dependence of the Raman response is shown in Fig. 2b,
with spectra acquired for 10 s each. For the tip within 30 nm of
the nanorod and correlated with the apex radius, a rise in the
near-field signal above the far-field response is seen. A lateral line
scan exhibiting the enhanced Raman signal across a BaTiO3
nanorod is shown in Fig. 2c, together with the corresponding topo-
graphy. Changes in the LO peak intensity across the rod can be
attributed to fluctuations in the near-field k-vector distribution
from the tip near the edges of the rod. The underlying Raman tip
enhancement I ?1042105can be derived from the distance
dependence and lateral scan (see Supplementary Information).
These results demonstrate the capability of TERS for imaging
crystalline nanostructures based on the optical phonon response.
With the Raman tensor reflecting the crystal symmetry, selective
probing of specific phonon modes in certain scattering geometries
then allows for determination of the crystal orientation and
associated ferroelectric order. Specifically, the induced Raman
polarization of the optical phonon mode n is given by
Prad
l
(i,j,k,l ¼ x,y,z) with Raman tensor xjk,n, inci-
dent light Elinc, and tip scattering tensors Fijand F0
the TERS geometry. For polar phonon modes the relationship
between the phonon polarization direction, the phonon wavevector
q, and its momentum conservation with incident and scattered
wavevector (q ¼ ki2 ks) then dictatesthe excitation of the spectrally
distinct LO and TO A1and E modes.
In TERS these fundamental Raman selection rules are super-
imposed by the additional considerations of the polarization-
selective enhancement and scattering by the tip of both incident
radiation and induced Raman polarization, due to both the optical
antenna geometry and excitation of an axial plasmon resonance13.
This can be expressed by the effective (empirical) scattering
tensors Fijand F0
ing efficiency and local field enhancement, respectively, associated
with the broken symmetry of the tip in its axial direction (1mm
symmetry). In general, with Fp’F0
elements (with s and p referring to the polarization direction of
Raman scattered and incident light, as shown in Fig. 1a) the
details of which depend on the nature of the tip plasmon excitation,
the amount of Raman shift or the degree of depolarization due to tip
inhomogeneities21–23(see Supplementary Information).
To determine the ferroelectric order in BaTiO3we considered a
nanorod oriented parallel to the incident light as shown in Fig. 1.
From the symmetry of the Raman tensor (see Supplementary
i;n/ Fijxjk;nF0
klEinc
klreflecting
kl, describing the spatial anisotropy in tip scatter-
p.Fs’F0
sfor the tensor
1Department of Chemistry and Department of Physics, University of Washington, Seattle, Washington 98195, USA,
State University of New York at Stony Brook, Stony Brook, New York 11794-3400, USA,
Brookhaven National Laboratory, Upton, New York 11973, USA. *e-mail: raschke@chem.washington.edu
2Department of Chemistry,
3Condensed Matter and Material Sciences Department,
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Information) it follows that probing the c-domain in the absence of
a polarization contribution along the z-axis, the E modes do not
contribute to the Raman signal. In contrast, for the ax0- and ay0-
domain configurations, with either the incident or scattered light
polarized along the respective ferroelectric polarization directions,
both E(x) and E(y) TO modes contribute. The A1mode may be
observed in all domain orientations, as the diagonal A1tensor
allows the strongly enhanced pinpoutcombination for all cases.
Specifically, for the c- and a-domains, as a result of the phonon pro-
pagating parallel or perpendicular with respect to the phonon polar-
ization direction, either the A1LO or A1TO mode is excited,
respectively. With the A1TO mode being strongest, followed by
the E TO mode, this provides the imaging contrast to distinguish
the c- and a-domains in the experiment.
Owing to the limited signal-to-noise ratio resulting in impracti-
cally long scan times (sample drift), we performed spectrally inte-
grated detection for sample scanning. The associated overall E LO
and A1TO Raman signal intensity variations as dictated by the
selection rules provided the contrast between the different ferroelec-
tric domains. Figure 3 shows the shear-force topography (Fig. 3a) of
an individual BaTiO3nanorod on a gold substrate together with the
TERS signal acquired for 10 ms per pixel with incident p-polariz-
ation (Fig. 3b). In the TERS image a region of ?60 nm ?400 nm
associated with an enhanced near-field Raman signal can be
discerned. The overall signal trend across the domain can be
attributed to variations in the far-field illumination conditions of
the tip apex due to the rod being scanned under the tip, as is
evident from the concurrent spatial fluorescence variations of the
gold surface adjacent to the rod. Under the given nanorod orien-
tation and TERS geometry a low signal intensity for the c-domain
is expected due to the weak A1LO mode and the absence of the E
mode, in contrast to a larger signal for the a-domains arising predo-
minantly from the strong A1TO mode. Under the backscattering
geometry, a further spectroscopic distinction of the a-domain into
ax0 and ay0 is not readily possible, as this would manifest itself
only in a difference in the respective A1TO Raman amplitudes.
However, with the domain boundaries known to be oriented 458
with respect to the domain orientations24, the observation of the
surface domain boundary parallel to z suggests an ay0-domain.
This allows for the domain assignment as shown in Fig. 3d.
This assignment is corroborated by the cross-section profiles for
the topography and TERS signal shown in Fig. 3c. At the left
domain boundary the TERS contrast coincides with a ?3 nm topo-
graphic height variation. Related topographic features in single-
crystal bulk BaTiO3associated with ferroelectric domain boundaries
have been observed previously25,26.
The localized regions of strong optical signal on the gold sub-
strate itself, as seen in Fig. 3, are due to nanometre-scale height
y'
z'
x'
Au tip
APD
Polarizer
CCD
Spectrometer
Shear-force
AFM
Au substrate
BaTiO3
s
p
70°
45 nm
200 nm
Ba
O
Ti
ki
ks
q
y
z
x
c
s
i
Figure 1 | Experimental setup and crystal symmetry. a, Schematic of the experimental TERS setup for backscattering Raman imaging of nanocrystals based
on phonon Raman selection rules (see Methods). b, Shear-force topography of a typical BaTiO3nanorod with a height of ?45 nm. c, Ferroelectric
displacement (c-domain) and geometry illustrating the phonon Raman excitation process with optical wavevectors (ki, ks) and phonon wavevector q.
The coordinates x0, y0, z0and x, y, z refer to the laboratory and sample frame, respectively.
900700 500
x' (nm)
300100
800
700
600
500
400
x'
50 nm
0
1
2
y' (nm)
0
80
40
ωph (cm−1)
ωph (cm−1)
ωph (cm−1)
500700600 800
120
y'
Raman intensity (a.u.)
500 700900
ELO
ETO
A1LO
A1TO
Figure 2 | Phonon TERS of BaTiO3. a, TERS spectrum acquired on a BaTiO3nanorod with phonon mode assignment (fluorescence background subtracted).
b, Corresponding tip–sample distance dependence. c, Spectrally resolved line scan across a BaTiO3nanorod. (The abrupt signal change at x0¼580 nm is due
to an acquisition artifact).
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variations inherent to the evaporated gold substrate that lead to an
increase and associated spatial variation in the intrinsic tip-
fluoresence contributing to the signal background27. Conversely,
the apparent overall signal decrease with the tip positioned above
the c-domain of the BaTiO3is due to the weak near-field Raman
response from the A1LO mode and the decrease in tip-enhanced
fluorescence background. However, despite the short range of the
evanescent tip–sample coupling the TERS process nevertheless
benefits from the Fresnel factors of the Au substrate providing
additional far-field enhancement.
Previously, the use of birefringence in the infrared spectral range
has allowed for near-field imaging of ferroelectric domains in bulk
BaTiO3(ref. 28). Similarly, piezoelectric force microscopy can
identify ferroelectric domains. However, with its ability to probe
multiple tensors simultaneously, phonon Raman scattering using
TERS provides an enhanced versatility, allowing crystal symmetry
properties to be probed in addition to its a priori capability of deter-
mining the chemical identity and structural phase of the material.
The power of the symmetry sensitivity of the Raman response
becomes particularly evident when considering that the lattice con-
stants differ between the cubic paraelectric phase and the tetragonal
ferroelectric phase by only 0.044 Å, making their distinction diffi-
cult even with high-resolution transmission electron microscopy
in combination with electron diffraction techniques29,30.
Ferroelectric domains arise from an interplay between the
depolarization energy lowered by the formation of domains and
the increased energy associated with domain walls24. Our results
show that, despite crystal sizes of only several hundreds of nano-
metres, and as such comparable to or smaller than typical
domains found for bulk dielectrics, these nanocrystals are not
necessarily single domain. Although a-domains have previously
been artificially induced in chemically synthesized BaTiO3nano-
rods30, our study provides evidence of spontaneous domain for-
mation in such structures. This may indicate finite size effects on
ferroelectric ordering due to the large surface-to-volume ratio by,
for example, pinning due to surface defects, the possible influence
of adsorbates, and the influence of the surface free energy in
general and its spatial variation with crystal shape and size.
Methods
A TERS setup based on a scattering-type scanning near-field optical microscope
(s-SNOM) was used, as shown schematically in Fig. 1a. Side-on tip illumination with
near–grazing angle (708) was provided by a HeNe laser (l ¼ 632.8 nm), with a
maximum fluence of ?2.1 ?104W cm22at the focus of a long working distance
microscope objective (Nikon, NA¼ 0.35, working distance ¼ 20.5 mm).
Electrochemically etched gold tips with tip radii of ?10–20 nm were used as
described previously31, and were mounted on the quartz tuning fork of a shear-force
atomic force microscope (AFM). The optical signal was spectrally filtered with a
notch filter (cutoff ? 400 cm21) and detected using either an avalanche
photodiode (APD), or spectrally resolved using an imaging spectrograph coupled
to a N2(l)-cooled charge-coupled device (CCD) (Acton Research SpectraPro 500i).
The spectral resolution (slit width) of the imaging spectrograph was set to ?40 cm21
to maximize the signal-to-noise ratio while still maintaining the capability to
distinguish the TO and LO modes.
The BaTiO3nanocrystals were synthesized using a molten salt process. BaC2O4
was mixed with TiO2, NaCl and a nonionic surfactant, and heated to 820 8C for
3.5 h as described in ref. 29. The resulting single crystalline BaTiO3nanorods
were characterized by transmission electron microscopy and dispersed onto an
evaporated gold substrate by spin coating from ethanol. Figure 1b shows the shear-
force atomic force microscopy topography of a BaTiO3nanocrystal with typical
dimensions of ?1.4 mm ?200 nm ?45 nm.
Received 24 December 2008; accepted 19 June 2009;
published online 26 July 2009
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Acknowledgements
S. Berweger acknowledges support from the University of Washington Center for
Nanotechnology with funding from NSF-IGERT. Funding from the National Science
Foundation (NSF CAREER grant CHE 0748226) is gratefully acknowledged.
Author contributions
S.B., C.C.N., and M.B.R. conceived the experiments. S.B. and C.C.N. carried out the
experiments. S.B. performed the data analysis. Y.M., H.Z. and S.S.W. synthesized the
sample materials. S.B. wrote the manuscript with contributions from C.C.N. and M.B.R.
Additional information
Supplementary information accompanies this paper at www.nature.com/
naturenanotechnology. Reprints and permission information is available online at http://npg.
nature.com/reprintsandpermissions/. Correspondence and requests for materials should be
addressed to M.B.R.
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