Infrared vibrational nanocrystallography and nanoimaging

Abstract

Molecular solids and polymers can form low-symmetry crystal structures that exhibit anisotropic electron and ion mobility in engineered devices or biological systems. The distribution of molecular orientation and disorder then controls the macroscopic material response, yet it is difficult to image with conventional techniques on the nanoscale. We demonstrated a new form of optical nanocrystallography that combines scattering-type scanning near-field optical microscopy with both optical antenna and tip-selective infrared vibrational spectroscopy. From the symmetry-selective probing of molecular bond orientation with nanometer spatial resolution, we determined crystalline phases and orientation in aggregates and films of the organic electronic material perylenetetracarboxylic dianhydride. Mapping disorder within and between individual nanoscale domains, the correlative hybrid imaging of nanoscale heterogeneity provides insight into defect formation and propagation during growth in functional molecular solids.

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Infrared vibrational nano-crystallography
and nano-imaging
Eric A. Muller,
1
Benjamin Pollard,
1
Hans A. Bechtel,
2
Peter van Blerkom,
1
Markus B. Raschke
1
*
Molecular solids and polymers can form low-symmetry crystal structures that exhibit anisotropic electron and ion
mobility in engineered devices or biological systems. The distribution of molecular orientation and disorder then
controls the macroscopic material response, yet it is difficult to image with conventional techniques on the nano-
scale. We demonstrated a new form of optical nano-crystallography that combines scattering-type scanning near-
field optical microscopy with both optical antenna and tip-selective infrared vibrational spectroscopy. From the
symmetry-selective probing of molecular bond orientation with nanometer spatial resolution, we determined crys-
talline phases and orientation in aggregates and films of the organic electronic material perylenetetracarboxylic
dianhydride. Mapping disorder within and between individual nanoscale domains, the correlative hybrid imaging
of nanoscale heterogeneity provides insight into defect formation and propagation during growth in functional
molecular solids.
INTRODUCTION
Heterogeneous molecular spatial distribution, morphology, and struc-
ture define coupling and photophysical properties of organic
electronic materials and biological systems. Directional bonding and
intermolecular interactions lead to anisotropic ion, electron, and hole
mobilities. In molecular devices, charge mobility is typically confined
to one or two dimensions and has a high sensitivity to crystallinity,
defects, and nanoscale orientation. Mobility enhancements of over two
orders of magnitude have been demonstrated by, for example, im-
proving long-range orientational ordering in spin-cast conductive
polymers (1,2) and engineering oriented growth of crystals within
field-effect transistors and organic light-emitting diodes (3,4).
Optical crystallography measures crystal axis orientation through
the anisotropic response of electronic or vibrational modes on the
basis of the projection of transition dipoles along the electric field vectors
of incident radiation (5). However, complex soft matter has significant
heterogeneity on the nanometer length scale, and conventional tech-
niques typically measure only a diffraction-limited response that is
spatially averaged over a large sample area.
Superresolution methods can determine molecular orientation in
materials but require suitable optical response or exogenous fluorescent
probes (6–8). High-spatial resolution crystallography is, in principle,
possible using electron or x-ray methods, but restrictions abound be-
cause of the low-scattering cross section of organic materials, requirements
for sufficient crystallinity, special sample preparation requirements, and
sample damage (9–12).
Vibrational spectroscopy can be used for nondestructive optical
crystallography across a wide range of inorganic, organic, and bio-
logical systems, additionally providing information on crystalline
phase, chemical identity, and local chemical environment (13–16). Se-
lective probing of characteristic vibrational modes can directly measure
molecular orientation and crystallinity. Infrared (IR) optical crystallog-
raphy uses an anisotropic material response proportional to the pro-
jection of vibrational transition dipole. Symmetry selectivity is achieved
through choice of polarization and kvector of incident and emitted ra-
diation, although it is limited in momentum space compared to x-ray
techniques and optical spatial resolution. Here, we use scattering-type
scanning near-field optical microscopy (s-SNOM) with combined tip-
and vibrational-mode polarization and spectral selectivity for optical nano-
crystallography, as shown in Fig. 1A. In s-SNOM, the metallized tip of
an atomic force microscope (AFM) localizes and enhances the optical
field linearly polarized parallel with respect to the tip axis. The resulting
anisotropic linear and nonlinear optical response of the coupled tip and
sample has already been applied to phonon selection rules in solids and
broken symmetries in ferroelectrics (17–19).
We extended this approach to IR nano-crystallography of molecular
and soft matter using linearly polarized near-field excitation with se-
lection rules for vibrational modes, simultaneously providing chemical
and polymorph identification and mapping heterogeneity in the local
environment. We measured crystalline orientation with subdomain
spatial resolution in films and aggregates of the molecular semi-
conductor perylenetetracarboxylic dianhydride (PTCDA). We performed
hybrid correlative imaging by combining optical and topographic
information to determine the characteristic length scales of order. We
observed orientational defects with implications toward understand-
ing growth mechanisms and enabling directed design of molecular
devices.
PTCDA has attracted interest in organic electronics because of its
high charge carrier mobility, crystallinity, and chemical stability (20).
Good device performance can be obtained from high-purity thin films
with well-ordered morphology and large crystalline domains, typically
forming in one of two polymorphs, aor b, with both monoclinic
structures containing stacked sheets of PTCDA arranged in a herring-
bone pattern, as shown in Fig. 1B. However, the charge carrier mobil-
ity of PTCDA is anisotropic and is significantly higher along the
direction of pstacking (21). Even in devices with high crystallinity
and micrometer grain size, misorientation of crystalline domains with
respect to larger device architecture can decrease performance (3,22).
In vacuum-deposited films, PTCDA crystals can be grown with the
aaxis oriented along the surface normal, favoring pstacking (Fig. 1C).
However, the morphology is sensitive to growth conditions (23–25),
substrate interactions (26), surface polymorphs distinct from the bulk
phase (27,28), minority phases (29), and defects. Although the structure
1
Department of Physics, Department of Chemistry, and JILA, University of Colorado,
Boulder, CO 80309, USA.
2
Advanced Light Source Division, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720, USA.
*Corresponding author. Email: markus.raschke@colorado.edu
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in PTCDA and related molecular materials has been investigated at
length scales ranging from molecular adsorption conformation to en-
semble-averaged crystallinity, a unified understanding of defects and
domain structure remains elusive.
EXPERIMENT
We used the combination of both narrowband IR s-SNOM based on
tunable QCLs and broadband synchrotron IR nanospectroscopy
(SINS) (19,30,31). Figure 1A shows the IR s-SNOM setup. Polarized
IR light from a tunable laser or synchrotron is focused onto a metal-
lized AFM tip operated in intermittent contact mode (see Materials
and Methods). Tip-scattered near-field signal is combined with light
from a reference arm and interferometrically amplified and detected,
measuring the complete IR optical response through the frequency-
dependent optical amplitude ANFð
nÞand phase FNFð
nÞ(31–33).
The frequency-dependent optical response for linearly polarized
near-field excitation of characteristic modes with known transition di-
poles is a general measure of molecular orientation relative to the
s-SNOM tip and substrate. We implemented tip-enhanced IR optical
nano-crystallography by selectively detecting only the material response
along the tip axis, combining vertically polarized incident light, aniso-
tropic field enhancement by the tip, and surface optical selection rules
to measure the projection of vibrational dipoles along the tip axis. The
selected orthogonal vibrational normal modes we probed have
transition dipoles parallel to the primary molecular axes, thus reflect-
ing the crystal axes for monoclinic PTCDA crystals (for details, see the
Supplementary Materials).
Figure 1 (C and D) shows the AFM topography of drop-cast PTCDA
aggregates and vacuum-deposited films, respectively. AFM topography
across the surface shows well-separated regions containing clusters of
PTCDA with a typical grain size in the range of 0.5 to 2 mm. The AFM
height image of vacuum-deposited films in Fig. 1D shows islands with
an average height of ~80 nm relative to the Au substrate. Island
growth is observed by AFM tapping phase imaging, which identifies
the islands and the spacing between them via differences in the tapping
phase (see the Supplementary Materials) (34,35). The topography is
consistent with previous observations of PTCDA films grown under
similar conditions, producing a modified Stranski-Krastanov growth and
appearing as three-dimensional (3D) islands for films greater than a few
nanometers thick.
RESULTS
Figure 2A shows far-field Fourier transform IR spectroscopy (FTIR)
spectra of PTCDA crystallites over two spectral regions of interest, 700 to
900 cm
−1
and 1600 to 1900 cm
−1
, with the full spectra shown in the Sup-
plementary Materials. The low-wavenumber region contains three
prominent peaks assigned to C–H out-of-plane bending modes, where-
as the carbonyl stretching region contains four overlapping peaks as-
signed to the anhydride groups. The anhydride modes are split into
four peaks in the carbonyl region due to dipole-dipole coupling (36).
Although broad line shapes prevent distinguishing between aand b
Detector
AFM
Lock-in
A
C
Synchrotron
QCL
D
0
80
Height (nm)
0
400
Height (nm)
O
O
O
O
O
O
B
Substrate
b
ac
θ
z
x
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
b
c
a
Fig. 1. IRnano-crystallography. (A)Schematic of IR s-SNOM using both synchrotron and
quantum cascade laser (QCL) radiation. (B) Lewis structure (top) and crystal structure
(bottom) of PTCDA in the bphase. (Bottom right) Crystal axes (red) oriented relative to
the substrate, with qindicating the angle between the aaxis of the crystal and the zaxis
of the surface normal. (C) AFM height of aggregates, depicted schematically (bottom). Scale
bar, 500 nm. (D) AFM height of vacuum-deposited film, with schematic (bottom) showing
growth of molecules with the aaxis parallel to the zaxis (blue) and defects with the aaxis
perpendicular to the zaxis (red and green). Scale bar, 200 nm.
ΦNF(ν) (deg) ΦNF(ν) (deg)
Wavenumber ν (cm–1)
Transmission (%)
A
B
500
Height (nm)
0
50
100
0
50
100
800 1700 1800900
D
SINS
Fit
SINS
Fit
FTIR
Fit
OO
OO
O
O
a
b
c
a
bc
0
C550
Height (nm)
0
O
O
O
O
OO
O
O
O
O
O
O
700
100
80
60
40
a
a
a
0
90
θ (deg)
Fig. 2. Broadband IR s-SNOM of large crystallites. (A) Far-field transmission
spectrum of dispersed PTCDA particles, fit to calculated reflectance using the Lorentz
model. Inset: Schematic of C–H out-of-plane vibrations and C=O stretch of the anhy-
dride. (B) SINS spectra (red) collected at two different nanoscale locations with
(green) fit to spherical dipole model with Lorentz oscillators. (Cand D) Molecular
orientation is measured from SINS spectra in a spatial map across large crystals of
PTCDA. Point spectra (circles) show fits of point spectra for q, represented on a false-
color scale. Scale bars, 500 nm.
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phases, the splitting is consistent with the typical herringbone crystal
structure and indicates high crystallinity of the powdered sample (37).
We fit the far-field transmission spectrum to quantify the peak po-
sition and oscillator strength (see the Supplementary Materials). The
measured transmission of each peak is proportional to the vibrational
oscillator strength for an ensemble measurement of isotropically dis-
persed particles. We found good agreement between measured and
previously reported oscillator strengths (37).
Figure 2B shows SINS point spectra of FNF ð
nÞcollected from dif-
ferent drop-cast PTCDA crystallites. As in the far-field spectrum (Fig. 2A),
three C–H bending modes are observed in the low-frequency region, and
four C=O stretching modes appear in the high-frequency region. The
C–H and C=O modes alternate in peak height in s-SNOM spectra from
different spatial regions containing subensembles with distinct orienta-
tions. Whereas the far-field spectra represent an isotropic ensemble-
averaged sample response, near-field F
NF
(C–H) and F
NF
(C=O) mea-
sure anisotropic linear dichroism of an oriented nanoscale subensemble.
To determine the local crystallographic orientation, we calculated
the near-field optical response of the coupled tip and sample system.
The frequency-dependent IR dielectric constant Dð
nÞof PTCDA can
be expressed as the sum of individual molecular vibrations using a
Lorentz model for vibrational oscillators. The transition dipole of
the C–H modes is parallel to the aaxis, whereas the transition dipole
of the C=O modes is along the b−cplane of monoclinic PTCDA
crystals and nearly isotropic in two dimensions. We include the Lorentz
oscillator model in the well-known spherical-dipole model for near-field
scattering (see the Supplementary Materials), which approximates the
AFM tip as a polarizable sphere (38–41). The optical near-field main-
tains the polarization of incident light oriented parallel to or perpendic-
ular to the tip axis (42), particularly in the strongly localized and
enhanced region at the tip apex that dominates the signal. We then used
the combined Lorentz oscillator and spherical dipole model to quanti-
tatively calculate the IR s-SNOM signal as a function of molecular and
crystalline orientation.
Figure 2B also shows the resulting fits of the model to the s-SNOM
scattered signal from two representative PTCDA crystallites. The
measured spectra quantify the projection of each mode along the tip
axis and are insensitive to molecular rotation about the tip axis. We
fit each spectrum to determine the projection angle qof the crystalline
aaxis along the surface normal z,asshowninFig.1B.Theangleqis
the only free fit parameter, whereas the oscillator strength and line
widths of each normal mode are left as constants. The resulting values
for qin Fig. 2B are 70 ± 10° and 20 ± 10°, respectively.
From repeated measurements on a spatial grid and analysis from
above, Fig. 2 (C and D) shows the molecular orientation measured
across two PTCDA crystallites. Color scale indicates the orientation
at each location in the image, with blue corresponding to q=0°and
red corresponding to q= 90°, with the aaxis parallel to the substrate.
These spectrally resolved maps identify large oriented regions with the a
axis nearly parallel to the substrate across micrometer-sized crystallites
and preferentially ordered in stacked aggregates. The spectral features
and orientation-dependent enhancement are also consistent across all vi-
brational modes in the spectral range of SINS measurements (700 to
900 cm
−1
) and are relatively insensitive to topographic variations or
eigenmodes appearing due to sample geometry. Measured angles of
q= 60° to 90° across most of the crystallite demonstrate quantitative
and robust measurement of orientation.
We extended the IR optical nano-crystallography to high-resolution
single-wavelength imaging of orientational ordering in device-relevant
thin films. Near-field FNF ð
nÞquantitatively measures the orientation q
by the relative contribution of C–H versus C=O peaks. Close agree-
ment between calculated fits and broadband SINS spectra across all
modes in PTCDA aggregates with different orientation establishes that
F
NF
at a selected frequency resonant with only one mode can be used
to determine qinthecaseofpolycrystallinemolecularsemiconductors
with uniform density. Further, near-field FNFð
nÞis also independent of
laser intensity, enabling quantitative comparison between measure-
ments using different IR sources.
Figure 3A shows the AFM topography at the edge of a vacuum-
deposited ~80-nm-thick film with the expected morphology of 3D
islands across most of the continuous films. Figure 3B shows the cor-
responding s-SNOM imaging resonant with the carbonyl mode at
1777 cm
−1
,F
NF
(C=O). F
NF
(C=O) measures the projection of the
C=O transition dipole along the tip axis. The resonant F
NF
(C=O)
image shows significant spatial variation and several distinct regions
across the image, as opposed to F
NF
at nonresonant frequencies, which
shows no optical contrast across the sample (see the Supplemen-
tary Materials).
Several distinct regions are seen in the F
NF
(C=O) image. The Au
substrate is nonresonant and servesasareferenceregion,withF
NF
(C=O) near 0°, whereas F
NF
(C=O) = 10° to 30° is observed across
most of the PTCDA. Near the film edge, regions with F
NF
(C=O) ≥
70° are observed. Comparing the resonant FNF ð
nÞacross the PTCDA
to the calculated s-SNOM signal with the spherical dipole model, we
can assign the orientation distribution across most of the sample area.
Away from the edge, q≤20°, whereas the region of higher FNFð
nÞcor-
responds to a larger q= 50 ± 10°, with uncertainties calculated from the
95% confidence limit of F
NF
.
Correlation analysis (Fig. 3C) of topography and F
NF
(C=O) pro-
vides for assignment of spatial regions with distinct crystal orientation.
RegionsoflowestheightalsohaveF
NF
(C=O) ~ 0°, indicating non-
resonant response, which can be assigned to the exposed Au substrate.
Higher topography corresponds to the thin film with 3D islands. Most
of the population of PTCDA regions has a low measured F
NF
(C=O)
and q, whereas smaller populations are observed to exhibit a larger
F
NF
(C=O) and q.
Figure 3D shows the resulting orientational map. Most of the image
(green) has a height near the average film thickness and q= 10° to 20°
measured by F
NF
(C=O). However, near the edge, a region is per-
turbed with both higher topography and greater q(red). Although
much of the uplifted region near the edge has greater q,oneofthe
tallest island features (light blue) exhibits low q, similar to the response
of the unperturbed region (green). At the edge of the PTCDA region, a
small region (yellow) of low height yet with large qis also observed as a
spatially distinct subensemble that is unobservable from either height
or qalone.
We next analyzed a small region of PTCDA in a central region of
the overall film to study the intrinsic heterogeneity in the vacuum-
deposited film. Figure 4 (A and B) shows a high-resolution AFM
and s-SNOM image of a small region containing the orientational de-
fects. The AFM height shows morphological features associated with
the 3D island growth of PTCDA. The F
NF
(C=O) image that is res-
onant with the carbonyl mode shows that, across most of the image,
the molecular orientation is relatively flat, as expected for vacuum-
deposited growth on Au. Across most of the image, a weak correlation
is observed between features observed in AFM height and in F
NF
(C=O), whereas, in two distinct regions with higher F
NF
(C=O)
(Fig. 4B), the aaxis is oriented significantly away from the surface
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normal. In particular, the defect in the lower right portion of the
image has a maximum F
NF
(C=O) of 85° localized to a region of
~100 nm as well as an extended region with F
NF
(C=O) near 45°.
From the corresponding line scan (Fig. 4C) of the location indicated
by the dotted lines in Fig. 4 (A and B), we observed regions of high
F
NF
(C=O) in regions with both low and high topography. Spatial
variations in F
NF
(C=O) appear uncorrelated with height and extend
across and beyond the edge of the 3D islands.
Figure 4D shows the radial correlation functions from AFM and
F
NF
(C=O) signals. We analyzed the AFM tapping phase, which was
found to be a sensitive measure of islands and the regions between
islands, which may be due to differences in sample viscoelasticity, ad-
sorbed water, or geometrical effects (34,35). Periodicity in the AFM
tapping phase signals indicates well-separated island features with
200-nm spacing, as measured by the location of the first maximum in
the radial correlation function. However, the radial correlation
function of the optical phase decays monotonically with no periodic
recurrence of correlation, supporting the finding that the observed qis
uncorrelated with the morphological features or edge effects. Further,
the correlation length F
NF
(C=O)iscomparabletoorgreaterthanthe
spacing of morphological features. The long radial correlation of the
F
NF
(C=O) agrees with the conclusions from the line scan in Fig. 4C
and supports the idea that the orientational defects are not confined
within a single morphological feature and may, in fact, extend across
multiple island features.
DISCUSSION
Our IR s-SNOM optical nano-crystallography and nano-imaging show
the presence of well-oriented PTCDA in macroscopically disordered
drop-cast crystallites. Conversely, vacuum-deposited PTCDA is mac-
roscopically ordered in the crystalline thin film, with molecules pri-
marily oriented with q≃10° to 20°, in close agreement with previous
ensemble-averaged measurements (26). However, we found nano-
scale spatial variation of molecular orientation in otherwise morpho-
logically well-ordered regions of the vacuum-deposited thin film. Line
scans and correlation analysis of F
NF
(C=O) suggest that defects
and crystalline domains may not be confined to individual morpho-
logical features.
The observed orientational radial correlation length across the
PTCDA sample can be expected to occur during growth by vacuum
deposition. Thin-film growth of different morphologies with varying
crystallinity, polymorphs, and even amorphous growth can occur as a
function of growth rate and temperature. Roughness of the substrate
may also play a role, which, for our template-stripped Au, has a
measured root mean square roughness of 0.5 nm, yet with many
features as high as 5 nm and rare pit defects. Defect propagation during
vacuum deposition is expected to be influenced by both the density of
seed crystals and the molecular surface diffusion rate across morpho-
logical features.
IR vibrational nano-crystallography extends related tip-enhanced
nanoscale crystallography efforts to molecular materials. Imaging ori-
entation of inorganic crystals has been demonstrated via IR s-SNOM
using crystallographic anisotropy of the phonon response (17), al-
though quantitative determination of orientation angle and structural
information were inaccessible. Symmetry selectivity of nonlinear re-
sponse has also been used to image domain order in ferroelectrics (18).
0
80
100
Height (nm)Φ(1777 cm–1
) (deg)
0
A
B
C
0
100
Height (nm)
y (µm)
x (µm)
D
a
bc
3D island
O
O
O
O
O
O
a
b
c
Low
O
O
O
O
O
O
a
b
c
High
a
bc
High
Au
Substrate
Φ(1777 cm–1
) (deg)
Height (nm)
0
max
2
1
0
1
100 0
Counts
120
0
80 60 40 20
20
40
60
80
20
40
60
80
20
40
60
0.5
0.5
1.5
2.5
Fig. 3. Correlative analysis of defects. AFM topography of vacuum-deposited PTCDA near the film edge (A) and corresponding near-field F
NF
(1777 cm
−1
)measuredon
resonance with the carbonyl mode (B). Scale bars, 500 nm. (C) Histogram constructed from the images (A and B) showing correlation between AFM height and F
NF
(1777 cm
−1
).
Distinct subpopulations are identified by their statistical correlation and labeled by the colored borders. (D) Spatial map reconstructed from the correlated populations identified in
(C) overlaid on AFM height image. Most of the population (green) has the aaxis of PTCDA parallel to the surface normal, with smaller populations uplifted with the aaxis parallel
to the surface normal (light blue), uplifted regions with the aaxis oriented away from the surface normal (red), and regions with less coverage and aaxis oriented away from the
surface normal (yellow). The substrate is shaded dark blue.
70
90
Height (nm)
5
85
A
0 200 400
Distance (nm)
s-SNOM phase
AFM phase
30
40
50
60
70
80
90
Height (nm)
0 200 400
Position (nm)
B
D
C
Φ(1777 cm
–1
) (deg)
Φ(1777 cm
–1
) (deg)
585
Φ
80 45
Fig. 4. Subdomain nanoscale orientation. Topography (A)andF
NF
(C=O)
images (B) of PTCDA. Scale bars, 100 nm. (C) Line cut of AFM height and F(
n)
along the dashed transect indicated in (A) and (B), showing that molecular orien-
tation is uncorrelated with topographic features. (D) Radial correlation function
(red) of the AFM tapping phase, indicating the presence of 3D islands with
characteristic size and spacing and F(
n) (blue), which shows an extended corre-
lation length of crystallite orientation. Cor. coeff., correlation coefficient; a.u., ar-
bitrary units.
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The polarizability tensor has also been used in tip-enhanced Raman spec-
troscopy to measure orientation of inorganic microcrystals (19).
With the combination of selected wavelength imaging and broad-
band IR spectroscopy, the approach of s-SNOM–based optical crystal-
lography can be generalized and extended to a much wider range of
materials. The low photon energy of IR light and lower-field enhance-
ment in IR s-SNOM provide minimally invasive imaging, suitable for
organic materials even under ambient and device-relevant conditions.
Individual-wavelength IR s-SNOM enables rapid imaging of FNFð
nÞ
simultaneously with AFM information under the conditions of spa-
tially invariant chemical identity and density. These methods can be
expanded to image more complex and multicomponent materials. IR
s-SNOM nano-crystallography measured using both horizontal and
vertical polarized optical fields can measure molecular orientation
through linear dichroism of a single vibrational mode even with varying
molecular concentration and oscillator density. Hyperspectral imaging
of several vibrational modes with different point group symmetries can
uniquely reflect molecular orientation while also measuring chemical
identity.
Broadband IR–nano-crystallography based on hyperspectral imaging
can be improved with faster spectral acquisition times using sources of
adequate average power and a carefully selected frequency bandwidth.
By overcoming these technical limitations, our methods and analysis
are generalizable to imaging orientational heterogeneity in virtually
any soft matter, multicomponent, or partially ordered system.
CONCLUSIONS
In summary, we implemented IR s-SNOM for optical nano-crystallography
using tip-enhanced spectroscopy of molecular vibrations with IR se-
lection to quantify the molecular and crystalline orientation while
providing simultaneous spectral sensitivity for chemical identification
and measurement of crystalline phases and variation in the local chem-
ical environment. We measured the heterogeneity of polycrystalline
PTCDA on the natural length scales of disorder, including orientation
distribution and correlation across domain boundaries and morpho-
logical features. We observed both the correlation between nanoscale
crystallinity and associated morphological features and the local vari-
ation in molecular orientation that is uncorrelated with morphological
features. We directly measured the molecular-level structure and inter-
actions that govern macroscopic properties, adding unique insight into
crystalline growth and defect propagation in device-relevant films.
MATERIALS AND METHODS
In the IR s-SNOM apparatus, light from a tunable laser or synchrotron
radiation was focused onto a metallized AFM tip operated in intermittent
contact mode n
0
~ 250 kHz. Near-field signal was detected at a higher
harmonic of the tapping frequency using lock-in demodulation. In both
continuous-wave and broadband implementations, the tip-scattered
near-field signal was combined with light from a reference arm and in-
terferometrically detected with a HgCdTe photovoltaic detector.
SINS was performed using a Bruker Innova AFM modified for
s-SNOM. Point spectra were measured interferometrically with a modi-
fied FTIR (Nicolet 6700), which was collected as asymmetric interfero-
grams. Interferograms were Fourier-transformed to produce near-field
ANFð
nÞand FNFð
nÞspectra. A single interferogram at 8 cm
−1
was
collected within 2 s, and spectra were averaged over typically 60 to
1000 repeated scans and referenced to a nonresonant bare Au sub-
strate. For spatially resolved spectroscopic imaging, faster resolution
imaging was performed at a lower resolution of 16 cm
−1
.
Selected, fixed-wavelength imaging was performed using a tunable
QCL (Daylight Solutions Inc.) with an Anasys nanoIR2-s AFM. Fixed-
wavelength imaging within the range (1650 to 1850 cm
−1
) measures
ANFð
nÞand FNFð
nÞsimultaneously with AFM topographic information.
PTCDA films and aggregates were prepared from sublimation-
purified ≥99% pure powder (Sigma-Aldrich). Transition dipole strength
of vibrational modes was determined from FTIR of crystallites dispersed
in KBr powder, with similar results observed for drop-cast crystallites.
Aggregated crystallite samples were prepared drop-cast from a low-
solubility suspension in dichloromethane solvent on substrates of fresh-
ly prepared template-stripped Au substrates with a root mean square
roughness of 0.5 nm. Vacuum-deposited films were prepared by organ-
ic molecular beam deposition from a Knudsen cell held at 630 K at a
rate of ~2 nm/min, producing films with a nominal thickness of 80 nm.
Film morphology was characterized with tapping-mode AFM, and
far-field FTIR spectra were collected in transmission mode (Nicolet
6700; 2-cm
−1
resolution) as well as a far-field transmission and reflec-
tance FTIR microscope (Continuum) with numerical aperture of 0.65
using a 50-mm × 50-mm aperture.
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/2/10/e1601006/DC1
Description of modeling and calculations for IR vibrational nano-crystallography
Description of nonresonant signal and signal penetration depth
fig. S1. Experiment and calculated near- and far-field spectra.
fig. S2. Orientation dependence of near-field signal.
fig. S3. Nonresonant near-field signal.
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Acknowledgments: We thank J. Atkin, O. Khatib, and M. Martin for the stimulating
collaborations and discussions. Funding: We acknowledge funding from the NSF
(grant CHE1306398). We also acknowledge partial support provided by NSF Materials
Research Science and Engineering Center grant DMR-1420736 and NSF Science and
Technology Center on Real-Time Functional Imaging under DMR-1548924. The Advanced
Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences,
of the U.S. Department of Energy under contract no. DE-AC02-05CH11231. Publication of
this article was funded by the University of Colorado Boulder Libraries Open Access Fund.
Author contributions: E.A.M. and M.B.R. conceived the study; E.A.M., B.P., H.A.B., and P.v.B.
performed the experiments; E.A.M. and M.B.R. wrote the manuscript; and all authors discussed
and edited the manuscript. Competing interests: The authors declare that they have
no competing interests. Data and materials availability: All data needed to evaluate the
conclusions in the paper are present in the paper and/or the Supplementary Materials.
Additional data related to this paper may be requested from the authors.
Submitted 5 May 2016
Accepted 18 August 2016
Published 7 October 2016
10.1126/sciadv.1601006
Citation: E. A. Muller, B. Pollard, H. A. Bechtel, P. van Blerkom, M. B. Raschke, Infrared
vibrational nano-crystallography and nano-imaging. Sci. Adv. 2, e1601006 (2016).
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doi: 10.1126/sciadv.1601006
2016, 2:.Sci Adv
Blerkom and Markus B. Raschke (October 7, 2016)
Eric A. Muller, Benjamin Pollard, Hans A. Bechtel, Peter van
Infrared vibrational nanocrystallography and nanoimaging
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