In Situ Observations of the Growth Mode of Vacuum-Deposited
α‑Sexithiophene
T. L. Derrien,*A. E. Lauritzen, P. Kaienburg, J. F. M. Hardigree, C. Nicklin, and M. Riede*
Cite This: J. Phys. Chem. C 2020, 124, 11863−11869
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ABSTRACT: The real-time morphological evolution of vacuum-deposited α-sexithiophene (α-6T) on a weakly interacting (glass)
substrate at ambient temperature is reported. In situ grazing-incidence small-angle X-ray scattering (GISAXS) enabled the
observation of nanoscale aggregates, while in situ grazing-incidence wide-angle scattering (GIWAXS) allowed the study of the
molecular-scale morphology. The in situ GISAXS measurements revealed that the α-6T growth proceeds via a Stranski−Krastanov
mode, whereby 2−4 complete monolayers are deposited, followed by subsequent layers formed via island growth. In situ GIWAXS
also showed the evolution of the polymorph composition during the thin-film growth. Initially, the disordered β-phase and the low-
temperature (LT)-phase are deposited in nearly equal proportion until a thickness of 8 nm, whereby the LT-phase begins to
dominate until a final α-6T thickness of 50 nm where the scattering intensity of the LT-phase is more than double that of the β-
phase. The change in the polymorph composition coincided with an increase in the LT-phase d-spacing, indicating a lattice strain
relief as the thin film moves from surface to bulk-mediated growth. The GISAXS findings were confirmed through direct imaging
using ex situ atomic force microscopy (AFM) at various thicknesses, revealing the existence of both initial the initial and intermediate
monolayers and final island morphologies. The findings reveal the real-time morphological evolution of α-6T across both the
molecular scale and the nanoscale and highlight the role of strain in polymorph growth. Due to the importance of the thin-film
microstructure in device performance, it is expected that these results will aid in the development of structure−property relationships
necessary to realize the full potential of organic electronics.
■INTRODUCTION
Small-molecule organic semiconductors (OSCs) have emerged
as interesting materials for a variety of applications including
organic field-effect transistors (OFETs),
1,2
organic light-
emitting diodes (OLEDs),
3
and organic photovoltaics
(OPVs).
4−6
In addition to low-cost and environmentally-
friendly processing techniques, the diverse range of molecules
available allow tailored structures to be created that can
optimize a variety of physical parameters.
7,8
However, OPV
and OFET devices are still outperformed by their inorganic
counterparts, in part due to the microstructural complexity of
OSC thin films. Such thin-film morphologies are typically
amorphous or polycrystalline with several crystalline poly-
morph components affecting numerous factors, such as
crystalline orientation, domain size, and purity, that influence
the optical and electronic properties.
9−16
This complicates
theoretical predictions of performance and necessitates de-
tailed studies across multiple length scales, targeting both bulk
and interfaces to understand and engineer the structures that
influence the properties and realize the potential of OSCs.
α-Sexithiophene (α-6T) is an OSC material that has been
extensively studied for use in OLEDs,
17
OFETs,
18
and
OPVs
19−21
due to its combination of high carrier mobility
and appropriate optical gap. Several polymorphs of α-6T have
been shown to exist in thin films.
22−26
The selection of the
appropriate polymorph or combination of polymorphs is
crucial to device performance, as each polymorph displays
distinct properties. For example, the high-temperature (HT)
and low-temperature (LT) phases of α-6T show differing
absorption strengths (under normal incidence).
27,28
While the
HT and the LT phases exist both in bulk and thin films,
another α-6T polymorph is the kinetically favored and
disordered β-phase, which is unique to thin films.
24,29
α-6T
polymorph control in thin films has been shown to be
dependent on several parameters such as illumination
conditions,
27
substrate temperature,
26
deposition rate,
30
post-
deposition annealing,
31
or vacuum incubation.
19
Here, we used several methods to characterize the real-time
growth of α-6T on a commercially relevant glass substrate to
characterize the growth mode across several length scales from
several angstroms to ≈200 nm. In situ grazing-incidence small-
angle X-ray scattering (GISAXS) was used to characterize the
nanoscale structure of the growing films, while in situ grazing-
incidence wide-angle X-ray scattering (GIWAXS) was used to
characterize the molecular- scale microstructure. The mor-
phologies observed via X-ray scattering were confirmed
through direct surface topography imaging by atomic force
microscopy (AFM), revealing a Stanski−Krastanov growth
mode. These observations of both the final and intermediate α-
Received: January 16, 2020
Revised: May 9, 2020
Published: May 12, 2020
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6T growth stages provide important insight into the structural
control of OSC thin films.
■METHODS
Grazing-Incidence Small-Angle X-ray Scattering (GI-
SAXS). GISAXS measurements used the Diamond Light
Source surface diffraction beamline (I07). In situ experiments
were performed using the purpose-built MINERVA sample
chamber
32
that consists of dual low-temperature thermal
evaporation sources whose deposition rates are monitored by
quartz crystal microbalances (QCMs). Prior to α-6T
evaporation, the Corning Eagle XG substrates (manufactured
with an root mean square (RMS) of 5 Å, low roughness
confirmed by the lack of discernible diffuse scattering) were
cleaned in an ultrasonic bath for 10 min using 2.5% Hellmanex
(non-etching) solution, deionized water, acetone, and finally
isopropanol. The cleaned substrate was mounted, and the
chamber was pumped down until a base pressure of 10−7mbar
was achieved. α-6T was deposited onto the substrate at a rate
of 0.3−0.45 Å/s, as monitored by the QCMs, which were
calibrated using spectroscopic ellipsometry using film thick-
nesses of 5, 10, and 50 nm. In situ GISAXS measurements were
recorded every 10 s using an exposure time of 5 s with an X-ray
beam energy of 10.0 keV and an incidence angle of 0.1°(below
the critical angle of the glass substrate αc≈0.14°). No beam
damage was detected for thick films, as verified by comparing
the images of the X-ray illuminated section with an off-beam
section after the deposition. Images were collected using a
Pilatus 2M detector at a sample to detector distance of 3.02 m
that was calibrated using a silver behenate standard (AgBeh).
Data reduction was performed in DAWN.
33
Grazing-Incidence Wide-Angle X-ray Scattering (GI-
WAXS). In situ GIWAXS measurements were recorded using
the previously described MINERVA chamber and deposition
conditions on glass substrates with a beam energy of 20.0 keV
and an incidence angle of 0.05°(αc≈0.07°). Images with a 1 s
exposure time were collected every 10 s using a Pilatus 2M
detector at a sample to detector distance of 41.8 cm that had
been calibrated using AgBeh.
Atomic Force Microscopy (AFM). The AFM images were
collected using a Veeco Multimode V in tapping mode
mounted with Bruker SCANASYST-AIR-HR tips. A scanning
rate of 0.5 Hz was used, and the images were analyzed using
Gwyddion.
34
The samples were prepared in the same manner
as the in situ samples, but the sample shutter was closed at
QCM readings corresponding to 2.5, 3.5, 4.5, and 5.5 nm.
■RESULTS
To obtain a complete understanding of the growth mode of α-
6T, we employed several characterization methods capable of
probing the thin-film microstructure across broad length scales.
In situ GISAXS was used to track the real-time morphology of
nanoscale aggregates, and these findings were confirmed via
AFM imaging. The evolution of the molecular structure of the
α-6T film growth was then monitored in real-time via in situ
GIWAXS, which enabled the tracking of the polymorph
composition during growth.
Selected detector images from the in situ GISAXS are shown
in Figure 1A. The images are taken from time points
corresponding to multiples of approximately 0.7-1.3 mono-
layers (MLs). The number of MLs deposited was calculated
using the recorded QCM thicknesses and assuming an LT-
phase structure (2.2 nm).
24
This assumption leads to only
approximate ML multiples, due to variation in the α-6T
polymorph composition, but is convenient for discussing the
resulting small-angle scattering. From these detector images, it
can be seen that off-specular scattering is strongest at
incomplete ML multiples (Figure 1A; 0.7, 1.5, and 2.6 MLs)
and weakest at complete or near-complete MLs (Figure 1A;
1.1 and 1.9 MLs). This is a result of strong scattering from
island-like morphologies for unfilled MLs and weak scattering
from the smooth, complete, ML surface
3535
(Figure 1B). Such
strong oscillations in the scattering intensity only occurred
during these early stages of the α-6T deposition. This can be
seen in the real-time intensity variation of the scattering in the
in-plane direction shown in Figure 2 and in a video of the
detector images of the full 50 nm thin-film deposition
(available in the electronic Supporting Information). The
data in Figure 2, composed of stacked linecuts at q⊥= 0.095
Å−1, are indicative of the changing morphology of the growing
α-6T film. The appearance and disappearance of the off-
specular scattering can be clearly seen in the early time points
corresponding to thicknesses of up to 2.6 MLs. The absolute
peak intensity was calculated by fitting a Lorentizian shape on
a Gaussian background in the linecut data and was plotted
against film thickness, as shown in Figure 3A. The first two
minima indicated by the dashed lines correspond to the nearly
complete monolayer coverage shown in Figure 1, at 1.1 and 1.9
ML coverage. Upon further α-6T deposition, or after 2 MLs
have been deposited, the degree of oscillation in the off-
specular scattering intensity is reduced and completely absent
after 4 MLs (8.4 nm) corresponding to a loss of the layerwise
growth and the presence of α-6T islands.
In addition to the intensity variations, Figure 2 shows the
narrowing and broadening of the peaks occurring during these
oscillations. Shifts in the peak positions (dashed lines in Figure
2A) are due to the varying correlation length resulting from the
changing separation of the growing islands. This disparity can
be clearly seen in the sample linecuts and fits for the peaks
corresponding to 0.7 and 1.5 MLs shown in Figure 2B, which
highlight the variations seen from the first two scattering peaks
seen in the time series. The peak positions from the fits were
used to calculate the characteristic length scale of the average
Figure 1. (A) Two-dimensional (2D) in situ GISAXS images obtained
at various α-6T ML coverages showing the variation in scattering
during the course of the deposition. At 0.7 ML (1.7 nm), 1.5 ML (3.3
nm), and 2.6 ML (5.7 nm) coverages, strong scattering peaks are
observed, while at 1.1 ML (2.4 nm) and 1.9 ML (4.1 nm) coverages,
the peaks disappear. (B) Schematic of the growth mode elucidated
from the detector images.
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separation of the islands, or the correlation length (Lc), via Lc=
2π/q∥, where q∥is the peak position in q-space (as confirmed
via simulation using BornAgain,
36
Figure S1). The correlation
length of the growing islands is plotted in Figure 3B, which
indicates that the islands formed on top of the first monolayer
grow with a larger correlation length than those grown on the
glass substrate. The correlation length is significantly increased
near complete ML coverage. This can be attributed to the fact
that near complete ML coverage, the scattering signal is the
result of the gaps in the film, not the island separation. The
diffuse scattering close to the specular beam (at q∥=0Å
−1),
calculated from a 5 ×5 (0.0015 Å−1×0.015 Å−1) pixel region
centered at q⊥= 0.11 Å−1, was also used to monitor the
progression of the film growth (Figure 3C), revealing a period
of layerwise oscillation. The oscillation in the peak intensity,
correlation length, and diffuse scattering near the specular
beam were all reduced after 2 MLs were deposited and absent
after 4 MLs, indicating that after this point, the growth of the
α-6T film proceeds via the formation of adsorbate islands. The
decreased correlation length observed indicates that in the
latter stages of the film growth the islands formed were more
closely spaced together. Collectively, the GISAXS data show
that α-6T growth by vacuum deposition on glass proceeds via a
Stranski−Krastanov (SK)
37
growth mode where beyond a
critical wetting layer thickness of two monolayers, α-6T
deposition progresses via island growth (Figure 1B). The
observation of SK growth, which depends on lattice
interactions (strain) between the surface and the adsorbate,
demonstrates the templating potential of the thin-film
substrate. Though SK growth modes for α-6T have been
observed in situ on metal substrates using photoelectron
emission microscopy (PEEM) and reflectance difference
spectroscopy (RDS),
38,39
they have never been expressly
captured on weakly interacting, device-relevant substrates such
as the glass substrates used here. Furthermore, the in situ
GISAXS employed here enabled the calculation of morpho-
logical parameters, with nm resolution, of the growing film not
accessible with other techniques.
Direct imaging of the various growth stages via AFM of ex
situ-prepared samples was consistent with the analysis of the
GISAXS data. We were able to image the early stages of α-6T
growth by stopping the deposition at thicknesses of 2.5 nm
(Figure 4A), 3.5 nm (Figure 4B), 4.5 nm (Figure 4C), and 5.5
nm (Figure 4D), as measured by QCM. Additionally, Figure
4E shows the surface structure of the 50 nm α-6T film, where
the island morphology is clearly visible. Watershed analysis of
the islands of the 50 nm-thick film indicated a mean island size
of 53 ±15 nm. The size of the islands was also estimated using
the off-specular GISAXS peaks according to the Scherrer
equation,
40
Dhkl =2πK/FWHM, where Dhkl is the average grain
size, taken here as the size of the islands, and Kis a shape
constant, 0.9 for spherical grains. Due to instrumental
broadening and detector resolution considerations, Scherrer
analysis can be used to calculate a lower limit grain size.
41
For
the 50 nm-thick film, Scherrer analysis returned a lower limit
grain size of 41 nm, which is within the standard deviation of
theAFMmeasurement.Theimageseriesshowsthe
progression from near monolayer coverage (Figure 4A), to
island formation (Figure 4B), growth (Figure 4C), coalescence
(Figure 4D), and repeated island formation (Figure 4E) of the
α-6T film. This was also observed through the increase of RMS
roughness, as calculated from the AFM images (Table 1),
where at thicknesses below 2 MLs (2.5, 3.5, and 4.5 nm), the
Figure 2. Real-time GISAXS data of α-6T deposited on glass at room
temperature. (A) Surface plot of real-time α-6T GISAXS scans
measured up to 20 nm thickness showing variation in off-specular
scattering intensity and peak positions. The surface plot is a
composite of linecuts taken at q⊥= 0.095 Å−1. The peaks indicated
with dashed lines occur between the deposition of the first (0.7 ML)
and second monolayers (1.5 ML), as shown in Figure 1A. The
linecuts taken from the detector images for these peaks are shown in
(B), along with the corresponding fits.
Figure 3. Evolution of the (A) peak scattering intensity, (B)
correlation length, and (C) intensity at q∥=0Å
−1(calculated from a
5×5 pixel region centered at q⊥= 0.11 Å−1) of the α-6T film during
growth as measured by GISAXS. Dashed lines correspond to the
deposition of one and two monolayers.
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J. Phys. Chem. C 2020, 124, 11863−11869
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RMS roughness remains approximately constant, after which it
quickly increases as a result of island formation (5.5 and 50
nm). The ex situ AFM observations further confirm the SK
growth mode observed in situ above. Additional image analysis
was conducted to extract an approximate correlation length of
the islands (Figure S2). The results shown in Table 1
correspond with the correlation lengths extracted from the
GISAXS data.
The crystalline structure of the growing α-6T film was
monitored by real-time in situ GIWAXS measurements. By
performing these time-resolved measurements under the same
conditions as the above GISAXS experiments, we were able to
track the evolution of the different α-6T polymorphs
throughout the deposition process. Figure 5A, composed of a
series of linecuts taken from the in-plane direction, shows the
evolution of the β- and the LT-phase peaks during the film
growth. The heatmap clearly shows the inverse relation
between the intensity of the βand LT peaks at the beginning
and the end of the evaporation. At the beginning of the thin-
film deposition, the βpeak at q∥= 1.38 Å−1intensity was
stronger in intensity than the (020) LT peak (q∥≈1.6 Å−1).
After the full 50 nm film growth, the intensity of the βpeak
was notably less intense than its maximum, while the (020) LT
peak intensity was strongest. Additionally, a weak LT (011)
peak at q∥= 1.32 Å−1was detected, which was not present in
the early stages of the α-6T deposition. A timelapse of the full
scattering images is available in the electronic Supporting
Information. The peaks were fitted as described above to
calculate the scattering intensity and the full width half-
maximum (FWHM). The size of the islands was estimated
according to the Scherrer equation. Using this equation, the
grain size, i.e., the diameter of the growing polymorph grains,
can be plotted against the film thickness. The results, plotted in
Figure 5B, show that after an initial increase in the grain size
during the early stages of the film growth, the domains of both
the β- and LT-phases contracted in size until a film thickness of
≈19 nm where the LT-phase reached an equilibrium grain size
value of ≈15 nm and the β-phase continued to shrink to a final
grain size of ≈10 nm until the deposition was stopped. Finally,
the integrated scattering intensities (ISI) of the βpeak and the
LT (020) peak were normalized to the maximum value (LT
ISI at the end of the film growth). The normalized ISI can then
be used to compare the scattering intensity of the α-6T-phases
throughout the film growth (Figure 5C).
From the ISI data, three regions can be distinguished: region
(1), where the β- and LT-phase ISI increase quickly, and the β-
phase ISI dominates; (2), where the rate of ISI increase slows
for both the β- and the LT-phases; and (3), where the LT-
phase ISI again increases while the β-phase ISI remains
relatively constant. The first region of rapid ISI increase occurs
up to about 4.8 nm (2 ML), roughly corresponding to the
region of monolayer growth. The ISI then slows at the
beginning of region 2, which occurs at the onset of island
growth up to a thickness of approximately 8 nm. Region 3 then
begins at a film thickness of ≈8 nm, and it is at this point that
the ISI of the LT-phase continues to increase while the β-phase
ISI stagnates until the deposition is stopped at 50 nm and the
ISI of the LT-phase is more than twice that of the β-phase.
Analyses of the β- and LT-phase peak positions (Figures S2
and 6, respectively) revealed that the start of region 3
corresponded with a sudden increase in the LT-phase (020)
peak spacing, as seen in Figure 6, near a thickness of 8 nm.
This abrupt shift in d-spacing is indicative of strain relief, which
accompanied the transition from monolayer to island growth
associated with the SK growth mode. However, the GISAXS
data indicated that the island growth occurred from a thickness
of approximately 4 nm, which corresponds to the start of
region 2. This finding suggests that although the island growth
found in the GISAXS and AFM data occurs at smaller
thicknesses, the full extent of the lattice strain induced by the
glass surface is not relieved until a thickness of 8 nm. Indeed,
this is the thickness in which the off-specular peak intensities
completely vanish. In the intermediate growth region (2), the
growth of two polymorphs stagnates until the lattice strain is
relaxed, at which point the LT-phase grows notably quicker
than the β-phase. In contrast to the relaxing of the α-6T LT-
phase, the d-spacing of the β-phase peak did not undergo any
substantial shifts following the growth mode (Figure S3) shift.
The in situ GIWAXS results obtained here closely mirror
those reported by Lorch et al.,
26
who reported the growth of
competing α-6T crystal phases on SiO2substrates at both 308
and 373 K. However, our ambient temperature measurements
on glass more closely resemble the growth they reported for
higher substrate temperatures (373 K), where the β-phase
dominated in the early stages of growth, after which the LT-
phase began to dominate. They reported that at high
temperature, at thicknesses above 7 nm, the LT-phase takes
over the β-phase, nearly equal to the 8 nm thickness where we
observed the β- to LT-phase transition. Conversely, on SiO2at
308 K, a temperature similar to our ambient measurements on
glass substrates, their results showed that the β-phase
dominates throughout the deposition process. Furthermore,
though the measurements by Lorch et al. show a larger LT-
phase d-spacing for the films grown at high temperature, the
Figure 4. AFM images of vacuum-deposited α-6T films at (A) 2.5 (≈1.1 ML), (B) 3.5 (≈1.6 ML), (C) 4.5 (≈2.0 ML), (D) 5.5 (≈2.5 ML), and
(E) 50 nm thickness.
Table 1. RMS Roughness and Correlation Length
Calculated from Figure 4
sample thickness (nm) RMS roughness (nm) correlation length (nm)
2.5 1.2 157
3.5 1.2 132
4.5 1.5 142
5.5 3.2 150
50 8.2 98
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real-time GIWAXS results at both high and low temperatures
do not indicate surface strain such as we have observed on
glass at ambient temperature. This disparity highlights the
strain-mediated templating potential of the substrate on which
α-6T is deposited, a key consideration in device fabrication,
given the distinct physical properties, such as absorption
27
displayed by each α-6T polymorph.
■CONCLUSIONS
Through the use of real-time in situ X-ray scattering
experiments, we were able to directly probe the growth
mode of α-6T on glass at room temperature from the
molecular scale to ≈200 nm. We employed in situ GISAXS to
probe the formation and morphology of nanoscale aggregates,
and in situ GIWAXS to probe the evolution of the crystalline
structure of the growing films, both in real time. The GISAXS
data revealed that α-6T films grow according to a Stranski−
Krastanov growth mode where two-dimensional (2D) layer-
by-layer growth transitioned to three-dimensional (3D) island
growth. The shift from 2D to 3D island growth was found to
occur after two monolayers were deposited. The presence of
both monolayer and island morphologies was confirmed
through direct imaging via AFM. In situ GIWAXS measure-
ments revealed the changing crystalline structure of the thin
film, where initially more of the α-6T β-phase formed on the
glass surface, but when the film thickness exceeded ≈8 nm, the
LT-phase began to dominate. Taken together, the results
present a multi length scale characterization of the evolution of
vacuum-deposited α-6T films on weakly interacting substrates,
revealing a growth mode where both the molecular scale and
the nanoscale morphology undergo a transition from surface to
bulk-mediated growth. In addition to emphasizing the
capability of the combination of in situ GIWAXS and GISAXS
to characterize the thin-film growth, the findings highlight the
importance of the surface in controlling the morphology and
crystalline structure of vacuum-deposited films. Further in situ
studies on additional OSC-relevant substrates and the
characterization of device parameters will aid in the develop-
ment of structure−property relationships necessary to improve
performance.
■ASSOCIATED CONTENT
*
sıSupporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jpcc.0c00447.
Simulation model and parameters; comparison of
experimental and simulated offspecular scattering
peaks; real time evolution of the α-6T β-phase peak
Figure 5. Real-time GIWAXS data of α-6T deposited on glass at
room temperature. (A) Real-time GIWAXS scans of vacuum-
deposited α-6T showing the evolution of the LT- and β-phases
during the film growth. (B) Real-time evolution of the α-6T β-phase
and LT-phase (020) grain sizes. (C) Normalized integrated scattering
intensity, with dashed lines indicating the regions of rapid β-phase
growth (1), slowed growth (2), and LT-phase dominated growth (3).
Figure 6. Real-time evolution of the α-6T (020)-LT peak position
(black) and d-spacing during film deposition.
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position and d-spacing during film deposition; AFM
(PDF)
Real-time GISAXS timelapse (AVI)
Real-time GIWAXS timelapse (AVI)
■AUTHOR INFORMATION
Corresponding Authors
T. L. Derrien −Diamond Light Source, Didcot, Oxfordshire
OX11 0DE, United Kingdom; orcid.org/0000-0001-7875-
2278; Email: thomas.derrien@diamond.ac.uk
M. Riede −Clarendon Laboratory, Department of Physics,
University of Oxford, Oxford, Oxfordshire OX1 3PU, United
Kingdom; orcid.org/0000-0002-5399-5510;
Email: moritz.riede@physics.ox.ac.uk
Authors
A. E. Lauritzen −Clarendon Laboratory, Department of Physics,
University of Oxford, Oxford, Oxfordshire OX1 3PU, United
Kingdom
P. Kaienburg −Clarendon Laboratory, Department of Physics,
University of Oxford, Oxford, Oxfordshire OX1 3PU, United
Kingdom
J. F. M. Hardigree −Clarendon Laboratory, Department of
Physics, University of Oxford, Oxford, Oxfordshire OX1 3PU,
United Kingdom
C. Nicklin −Diamond Light Source, Didcot, Oxfordshire OX11
0DE, United Kingdom
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jpcc.0c00447
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
T.L.D. prepared the original manuscript of the draft with all
authors contributing to the reviewing and editing process.
T.L.D., A.E.L., P.K., and J.F.M. performed the experiments and
T.L.D. analyzed the data. The project was conceived by T.L.D.,
A.E.L., C.N., and M.R., with C.N. and M.R. providing
supervision. T.L.D., P.K., C.N., and M.R. acknowledge funding
for this work from the UKRI-GCRF grant Synchrotron
Techniques for African Research and Technology (START)
ST/R002754/1. A.E.L. thanks EPSRC for funding through the
Doctoral Training Partnership (EP/N509711/1) as well as
STFC and the ISIS Neutron and Muon facility and project
(1948713). Access to Diamond beamtime at I07 was provided
under experiment Nos. SI20426-1 and NT24871-1. The
authors thank A. Warne, J. Rawle, H. Hussain, and F. Carla
(Diamond Light Source) for their assistance with beamline
instrumentation. They are grateful to J. Naylor, D. Wicks, and
A. Dorman of K. J. Lesker Ltd. for generously providing
deposition control and evaporation sources along with
technical support for MINERVA, which was the result of an
STFC CLASP project (ST/L006294/1).
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