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MINERVA: A facility to study Microstructure and INterface Evolution in Realtime under VAcuum


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A sample environment to enable real-time X-ray scattering measurements to be recorded during the growth of materials by thermal evaporation in vacuum is presented. The in situ capabilities include studying microstructure development with time or during exposure to different environmental conditions, such as temperature and gas pressure. The chamber provides internal slits and a beam stop, to reduce the background scattering from the X-rays passing through the entrance and exit windows, together with highly controllable flux rates of the evaporants. Initial experiments demonstrate some of the possibilities by monitoring the growth of bathophenanthroline (BPhen), a common molecule used in organic solar cells and organic light emitting diodes, including the development of the microstructure with time and depth within the film. The results show how BPhen nanocrystal structures coarsen at room temperature under vacuum, highlighting the importance of using real time measurements to understand the as-deposited pristine film structure and its development with time. More generally, this sample environment is versatile and can be used for investigation of structure-property relationships in a wide range of vacuum deposited materials and their applications in, for example, optoelectronic devices and energy storage.
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MINERVA: A facility to study Microstructure and INterface Evolution in Realtime
under VAcuum
Chris Nicklin, Josue Martinez-Hardigree, Adam Warne, Stephen Green, Martin Burt, John Naylor, Adam
Dorman, Dean Wicks, Salahud Din, and Moritz Riede
Citation: Review of Scientific Instruments 88, 103901 (2017); doi: 10.1063/1.4989761
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Published by the American Institute of Physics
MINERVA: A facility to study Microstructure and INterface Evolution
in Realtime under VAcuum
Chris Nicklin,1,a) Josue Martinez-Hardigree,2,b) Adam Warne,1Stephen Green,1
Martin Burt,1John Naylor,3Adam Dorman,3Dean Wicks,3Salahud Din,3and Moritz Riede2
1Diamond Light Source, Ltd., Harwell Science and Innovation Campus, Chilton OX11 0DE, United Kingdom
2Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom
3Kurt J. Lesker Company, Hastings TN35 4NR, United Kingdom
(Received 12 June 2017; accepted 11 September 2017; published online 2 October 2017)
A sample environment to enable real-time X-ray scattering measurements to be recorded during the
growth of materials by thermal evaporation in vacuum is presented. The in situ capabilities include
studying microstructure development with time or during exposure to different environmental con-
ditions, such as temperature and gas pressure. The chamber provides internal slits and a beam stop,
to reduce the background scattering from the X-rays passing through the entrance and exit windows,
together with highly controllable flux rates of the evaporants. Initial experiments demonstrate some of
the possibilities by monitoring the growth of bathophenanthroline (BPhen), a common molecule used
in organic solar cells and organic light emitting diodes, including the development of the microstructure
with time and depth within the film. The results show how BPhen nanocrystal structures coarsen at room
temperature under vacuum, highlighting the importance of using real time measurements to understand
the as-deposited pristine film structure and its development with time. More generally, this sample
environment is versatile and can be used for investigation of structure-property relationships in a wide
range of vacuum deposited materials and their applications in, for example, optoelectronic devices
and energy storage. 2017 Author(s). All article content, except where otherwise noted, is licensed
under a Creative Commons Attribution (CC BY) license (
A key focus of materials science is the study of
structure-property relationships, in which macroscopic proper-
ties often arise hierarchically from atomic interactions through
mesoscale ordering. Common examples include the depen-
dence of mechanical strength of a polycrystalline metal on its
constituent grain size1or the modification of catalytic activ-
ity with film thickness or surface area.2Similar relationships
frequently play a role in photovoltaic devices based on emerg-
ing energy materials, such as charge-transfer dyes, organic
semiconductors, and organometal halide perovskites. Struc-
turally dependent parameters such as charge carrier mobility,
electronic band structure, or molecular orientation have been
shown to have a direct influence on power conversion effi-
ciency.3,4Several fabrication techniques have been used to
explore the effect of processing on structure-property rela-
tionships, including solution-based slot-die coating and ink-
jet printing,5as well as vacuum deposition,6the latter an
increasingly popular tool as a method for fabricating large
area electronic devices with fine control over nanoscale thick-
ness, deposition rate, and composition. There are two main
drawbacks of investigating post-deposition films: (a) it is chal-
lenging to identify interface effects during the growth and
resolve variations in microstructure at different depths inside
the film, though both can be critical for the performance of
b); Joint first author.
devices made from such films; (b) that many emerging materi-
als are susceptible to degradation or morphological evolution
from ambient moisture or oxygen79or even illumination.10
As a consequence, ex situ investigations in which samples must
be transferred through air, as is the case with many systems
for atomic force microscopy (AFM), scanning and transmis-
sion electron microscopy (SEM, TEM), or laboratory based
X-ray scattering, may create the additional challenge of relat-
ing measurements taken hours or days after fabrication to the
structures captured in encapsulated, device-processed materi-
als. In situ techniques are needed to capture film growth as it
proceeds during real-world device processing.
The high flux of X-rays available at a synchrotron source
means that high quality, low noise structural data can be col-
lected very quickly. This has been recognised in the grow-
ing application of synchrotron radiation to real-time mea-
surements of vacuum-deposited thin films fabricated with a
wide range of deposition techniques including sputtering,1114
chemical vapour deposition,15 molecular beam epitaxy,16,17
and organic vapor-jet deposition.18 Such scattering measure-
ments are complementary to local structure measurements
with AFM, as the elongated footprint of the X-ray beam at
grazing angles probes the statistical average structure over a
large area. Information such as the degree of crystallinity of
the film, surface roughness, and domain size in polycrystalline
regions and evidence of lattice strain or preferred orientation
can be probed with high spatial and time resolution. Moreover,
these parameters can be probed as a function of growth con-
ditions such as substrate temperature, deposition rate, or the
0034-6748/2017/88(10)/103901/10 88, 103901-1 ©Author(s) 2017
103901-2 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
pre-deposition of a buffer or templating layer. It is also possi-
ble to monitor the development of the film structure with time,
with exposure to a specific gas or in an atmospheric pressure
of air (due to the penetrating nature of X-rays).
Research into emerging energy materials is progress-
ing across several materials and device architectures, from
organic bulk heterojunctions to planar perovskite absorbers,
and with many solution-based fabrication techniques spanning
spin-casting,19 blade-coating,2022 and ink-jet printing.5How-
ever, vacuum deposition is increasingly popular as a scalable
method for fabricating large area electronic devices with fine
control over nanoscale thickness, deposition rate, and ambi-
ent composition.6Here we present an environmental chamber
specifically designed and optimised for the grazing incidence
wide angle X-ray scattering (GIWAXS) or grazing incidence
small angle X-ray scattering (GISAXS) study of in situ vac-
uum deposited layers on the synchrotron beamline I07,23 at
Diamond Light Source.
GIWAXS and GISAXS are techniques where an X-ray
beam hits a sample at grazing angles, close to the critical angle
for total external reflection (0.1for most organic materials
at typical X-ray energies of 12.5 keV). The scattered X-rays
are collected using a large area two-dimensional detector to
image the full scattering pattern in both the in-plane and out-
of-plane directions (perpendicular and parallel to the sample
normal, respectively). The implementation available at beam-
line I07 uses a Pilatus P2M detector mounted at a distance
of 300 mm for GIWAXS (or up to 3 m for GISAXS) from
the sample to give a potential angular collection range of up
to 40. The use of a flat detector introduces some geometri-
cal distortions at such small distances that must be corrected
to remap the pixels to the true momentum transfer values (qxy
and qz). Many sample environments exist that have been devel-
oped to study pre-grown samples and optimise the signal to
noise for weakly scattering samples such as for organic photo-
voltaic (OPV) bulk heterojunction (BHJ) structures. They can
generally incorporate a flowing gas to reduce air scattering
or prevent oxidation/degradation of the interface during the
X-ray measurements. We utilise similar design principles for
our vacuum system, by incorporating elements such as inter-
nal pre-sample slits and a post-sample beam stop, to reduce
the background caused by the incoming or exiting X-ray beam
hitting the window material of the sample environment.
An additional constraint in the design is the requirement
for the evaporation sources (to vacuum deposit the materi-
als) to point upwards. Two sources need to be incorporated,
to enable studies of application relevant interfaces as well as
the growth of binary mixtures. In order to provide a versa-
tile and accurate system, the design also includes two Quartz
Crystal Microbalances (QCMs) for independent monitoring of
the evaporating fluxes, allowing co-evaporation of materials.
Other requirements are a sample shutter and a sample transfer
system to limit the changeover time between experiments as
well as a heatable substrate stage. To undertake the measure-
ments, it is necessary to have accurate control over the sample
motion, both the translations and rotations about the sample
position. Beamline I0723 is a beamline for grazing incidence
and surface diffraction studies, which uses a high precision
hexapod for sample alignment and a large diffractometer for
complex multicircle positioning. In the GIWAXS studies, the
P2M detector is fixed to the diffractometer detector circles,
and the sample does not need the full range of motions pro-
vided by the diffractometer. Therefore in order to produce a
stable platform for the GIWAXS studies, a frame that fits onto
the diffractometer base has been implemented with all sample
movement then provided via the hexapod. During GISAXS
measurements, the same sample manipulation is used, and
the P2M detector sits on an independent positioning platform
downstream of the diffractometer.
A. General arrangement
The general arrangement of the chamber is shown in
Fig. 1, whilst a cut-through view of the chamber is shown in
Fig. 2. The MINERVA (Microstructure and INterface Evolu-
tion in Realtime under VAcuum) system is designed to meet
the design criteria and consists of four different sections:
(1) The lower deposition chamber contains the evaporation
sources and QCMs. MINERVA’s overall arrangement is
dictated by the requirement for the evaporation sources
to be upward pointing, to enable a constant evaporation
rate through material covering the base of the crucible.
They also operate at an optimum throw distance of
230 mm between the source and the sample position,
ensuring an even flux over the sample area. This cham-
ber also houses two QCMs to independently measure
the flux rates of the two materials and a baffle between
the deposition sources to ensure no cross talk in the
FIG. 1. Overview of the design of the MINERVA chamber. It consists of
four modules: the deposition chamber houses the low temperature evapora-
tion (LTE) sources and quartz crystal microbalances (QCMs); the scattering
chamber with beryllium windows and slits; the sample manipulator using an
external hexapod to allow accurate positioning of the sample; the vacuum
component chamber with all pumps, gauges, and valves.
103901-3 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
FIG. 2. Cut-through of the MINERVAchamber looking from the front (access
ports), showing key internal components and the path of the X-ray beam.
flux calibrations. A separate ring baffle at the top of
the base chamber protects all components above from
being coated with an evaporating material, except for
the substrate.
(2) The central scattering chamber houses the X-ray trans-
parent beryllium entrance and exit windows. It also
contains a set of pre-sample slits to define the X-ray
beam and a post-sample beam stop, so that any unscat-
tered beam does not add to the background levels (see
Sec. III C). This chamber also provides an auto-
mated sample shutter which prevents deposition onto
the sample during initial evaporator configuration and
(3) The upper sample manipulator consists of a hexapod
external to the vacuum, the motions of which are fed
into the vacuum chamber via a flexible vacuum bellows
and feedthrough. The stage, incorporating the sample
heater assembly, reaches down into the centre of the
scattering chamber.
(4) The vacuum component chamber contains the pump-
ing and ancillary services (venting, gauges), mounted
on a multiport vessel that is at the rear of the system
when it is mounted on the diffractometer available at
beamline I07.23 As GIWAXS/GISAXS measurements
do not require substantial movement of the diffractome-
ter, this allows short, simple cable and pipework man-
agement to common electrical controller and backing
pump locations.
MINERVA is built in a very modular way, to enable flex-
ibility in the studies for which it can be used. It allows, for
example, replacement of the bottom chamber with another
design that may contain different types of vacuum deposi-
tion sources. In addition, it would be possible to combine
the X-ray scattering measurements with other related tech-
niques by modifying the upper scattering chamber. The current
implementation includes four flanged ports with the two
smaller ports optically centred on the substrate position. These
are currently used for video monitoring of the sample during
alignment and deposition but could be used for introducing var-
ious kinds of probes to the sample. Although the system in its
current configuration does not have optical viewports set at 74
for in situ spectroscopic ellipsometry, a technique which has
been successfully combined with GIXD measurements,24 the
available ports could accommodate fibre-optic feedthroughs
for complementary optical techniques such as in situ differ-
ential reflectance.25,26 In addition, connections for enabling
electrical measurements of films in organic field-effect tran-
sistors (OFETs) during or after deposition27,28 can be easily
integrated using the current flange ports.
B. Deposition chamber
The lower deposition chamber contains two low tempera-
ture evaporator (LTE) sources (Kurt J. Lesker Company) with
individual flip shutters and power supplied through a common
electrical feedthrough flange. Their temperature range of up to
600 C is sufficient for depositing the organic materials used
in OPVs. The sources are mounted directly on the base flange
with a baffle placed between them to remove any temperature
cross talk or cross contamination of the sources and to ensure
that the flux rates correspond only to the specific source. This
design allows very accurate studies of two component material
systems where the relative composition of the two components
can affect both the structure and the photovoltaic efficiency of
the sample. Separate QCMs record the flux rates of the evapo-
rants, enabling complex heterojunctions to be produced. This
may include growing a single component wetting layer before
a two-component material is deposited as required, for exam-
ple, in many BHJ structures of OPVs. The flux rates can be
calibrated before the deposition, as the sample is protected by
a pneumatically operated shutter. This acts off one of the ports
in the central scattering chamber, and the pneumatic actuator
rotates to either place a steel plate in the path of the evap-
orating materials (but not blocking the X-ray path to enable
alignment of a clean sample) or completely removing it out
of the way. This shutter can be actuated remotely allowing the
initial stage dynamics (e.g., nucleation, wetting behaviour, or
strain development) to be monitored during the deposition.
C. Scattering chamber: X-ray background reduction
The quality of the GIWAXS/GISAXS measurement is
critically dependent on the signal to noise ratio, as there are
very many weak signals that are important to fully understand
the structure of the developing interface. The measurements
typically use an X-ray incidence angle very close to the criti-
cal angle for total external reflection (typically 0.1for most
organic materials at X-ray energies of 12.5 keV). A well
defined X-ray beam is therefore required and I07 has beam
parameters of 80 µm vertically by 120 µm horizontally. The
vertical size will flood the sample at 0.1when the sample has a
diameter of 10 mm, but even then a strong reflected beam can
lead to background scattering. Much of the background scat-
tering originates from the intense X-ray beam hitting either
the entrance or exit window of the sample environment. In
103901-4 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
a vacuum chamber, these windows are generally made from
polycrystalline beryllium that leads to some powder diffraction
but also has a grain structure leading to small angle scattering
close to the direct beam. In order to reduce the amount of
this scattering reaching the detector, we borrow ideas from
non-vacuum GIWAXS sample environments where the beam
definition slits are after the entrance window and an internal
beam stop is located to prevent the direct beam or reflected
beam from hitting the exit window. This combination blocks
the majority of the unwanted scattering that would reach the
detector, cleaning up the signal significantly. It is important
that the slits are high precision to define an accurate beam
size and the beam stop can be reliably moved and relocated in
order to enable initial alignment that is usually performed on
an attenuated direct X-ray beam. This implementation makes
use of in-vacuum stages incorporating SmarACT-precision lin-
ear drives (travel range of 10 mm and nm precision) in both
the slits and the beam stops. Figure 3shows the slit arrange-
ment that uses four separate linear drives, each attached to
one blade of the slits. These are software coupled to allow
the gap and position of the slit to be set directly. The slits
are mounted within a custom designed housing that splits to
allow easy maintenance of the individual slit pairs. They join
together using an O-ring mechanism that is compressed by
a series of bolts through the external part of the housing. The
slits mount directly onto the flange of the chamber, and a small
Be window is mounted on the upstream side as indicated in
Fig. 2. This combination serves two purposes: to define a small
beam onto the sample and to block the front window X-ray
The beam stop assembly is constructed from two Smar-
ACT stages mounted perpendicular to each other to provide an
XY motion with a travel range of ±10 mm. The beam stop (see
Fig. 4) mounts directly onto these stages and is designed so
that it nominally blocks the beam at the centre of travel in both
directions. The beam stop is made from 3 mm tungsten in order
to ensure blocking of the intense direct beam when required
FIG. 3. Cut-through of the internal slit assembly. The slits are highlighted in
yellow and allow the vertical and horizontal widths of the beam to be adjusted.
FIG. 4. The internal tungsten beam stop assembly is mounted after the sample
with respect to the incoming X-ray beam. It allows cutting out the intense
direct beam or the direct and reflected beam without blocking too much of the
scattered signal.
and is arrow shaped to allow selective blocking of the direct
beam or the direct and reflected beam without shadowing too
much of the GIWAXS scattering.
D. Sample manipulator: Sample motion and heating
There are limited requirements for sample motion in
the GIWAXS/GISAXS measurements; there must be vertical
motion to position the sample accurately in the X-ray beam
and angular motion of a few degrees to flatten the sample in
two orthogonal directions and to set the beam incidence angle.
We implement all of these motions using a hexapod directly
attached to the sample through a vacuum feedthrough and a
flexible edge welded bellows. An internal tube mounts off the
flange and passes down to the sample heater assembly with
the cabling routed axially along the hexapod to prevent cable
snagging. In these experiments, there is no requirement for
azimuthal rotation although this could be added using either
an internal rotation stage or a differentially pumped rotary
seal attached to the bellows. A hexapod provides a number of
advantages for the sample positioning, the most significant of
which is the ability to define the rotational pivot point, allow-
ing optimal alignment even if the beam height entering the
chamber is varied. In addition, the hexapod allows translation
across the sample surface during the measurements which can
be important when determining the role of beam damage on
structural degradation. It should be noted that the hexapod local
Cartesian axes must be correctly aligned parallel (and perpen-
dicular) to the beam direction to ensure accurate calibration of
the incidence angle.
The design of the sample heater is shown in Fig. 5. It uses
resistive heating from a Nichrome wire threaded through a
machinable ceramic assembly, allowing sample temperatures
of up to 600 C. The Nichrome wire is used to provide com-
patibility with heating in a non-vacuum environment that may
be required for certain sample processing or to study the effects
of gas exposure on sample structure. The heater is controlled
via a Eurotherm 2504 controller using feedback from the
K-type thermocouple mounted as close to the sample position
as possible. Samples are mounted on Omicron flag type sample
103901-5 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
FIG. 5. Cut-through of the sample heater assembly. The upper section (green)
shows the substrate coupling that attaches to the alignment hexapod, whilst
the pink section and purple fixtures indicate the spring loaded sample clips
that hold the transferable sample holders rigidly during measurement. The
blue section is the machinable ceramic heater, housing a Nichrome heating
wire to enable heating in oxygen or ambient atmospheres. The transferable
sample plate and sample are shown in red.
plates that can be inserted either individually through the upper
access door or by replacing the upper door (see Fig. 1) with a
customised cluster flange that contains a sample transfer wob-
ble stick, a sample carousel for storage of up to twelve samples,
and viewing windows to enable simple sample transfers. This
reduces the time between experiments significantly, and the
only extended changeovers occur when evaporant materials
need to be exchanged.
E. Vacuum component chamber: Vacuum control
The base vacuum required during deposition, in a produc-
tion relevant process, is <1×10 6mbar so it is not necessary
to bakeout the chamber. This also enables a number of differ-
ent flange sealing techniques to be used. Where compatibility
with other ultrahigh vacuum (UHV) beamline equipment is
required, conflat flanges are used where a knife edge cuts into
a copper gasket, but in other places O-rings are used with com-
pression fittings. The chamber is pumped via a combination
of a backing line scroll pump and a 330 l/s turbomolecular
pump (TMP), whilst pressure measurement is provided via a
combination of a Pirani gauge for low vacuum and an Inverted
Magnetron Gauge (IMG) for the higher vacuum. Initial pump-
down is achieved by closing a pneumatic valve between the
backing pump and the turbomolecular pump and opening a
bypass valve to pump the main chamber. At a pressure of
10 2mbar, the bypass line valve is automatically closed and
the valve to the TMP backing line is opened. The main valve
between the TMP and the chamber is then opened, allowing
the vacuum to rapidly reach the required level. The ultimate
vacuum achieved is 5×10 8mbar after 2 h of pumping,
which is more than adequate for deposition of these molecular
Control of the vacuum system is undertaken using a pro-
grammable logic controller (PLC) that allows the sequence
above to be performed automatically at the touch of a button
on a pressure sensitive screen, whilst monitoring the vac-
uum interlocks for safety and equipment protection. The PLC
controls electronically actuated valves to ensure that a consis-
tent and well defined sequence of events is used for different
operations. These include pumpdown as described above and
venting, opening up control of the vacuum system to the
Initial functional tests included test pumpdown and leak
test to ensure that a useable vacuum could be reached as
quickly as possible. A pressure of 1×10 6mbar is achieved
after 20 min reaching 1×10 7mbar within an hour. Tests of
the sample heater showed rapid heat up and cooldown possi-
bilities, requiring approximately 20 min to stabilise once the
correct control parameters had been established. Test deposi-
tions were made with the sample held at room temperature,
where the evaporation rates measured using the QCM’s were
calibrated for samples deposited for a known amount of time
and subsequently measured ex situ using laboratory based
X-ray diffraction (see S1 in the supplementary material). The
film thickness for the sample grown at room temperature is in
excellent agreement with the expected thickness, confirming
that the flux rates from the QCMs are accurate. The alignment
procedure was simulated prior to first experiments, and the
range of motion of the samples was checked to establish the
parameters such as the pivot point and angular ranges for depth
profiling measurements. It is clear that the design goal of ±5
was easily achieved and more than enough to enable sample
alignment and setting of the incidence X-ray angle.
After installation on the beamline, initial X-ray alignment
tests were carried out. The sample was moved out of the beam
path by retracting the hexapod to its lowest position, and checks
were made that the direct beam passed through the chamber
and hit the detector at an appropriate position. The in-vacuum
pre-sample slits were aligned around the direct beam, and they
were calibrated to ensure that when fully closed there was not
any transmission of the X-rays. The beam size was then set to
be 100 µm×100 µm. The beam stop was aligned by driving it
to its lowest level vertically and then scanning in the horizontal
direction to find the position at which the direct beam was
blocked. The vertical motion was then scanned to find the
point of the beam stop so that it could be aligned appropriately
and block just the direct beam. Tests were then made on the
reproducibility of the positioning and the ability of the beam
stop to block the full X-ray beam when all attenuation had
been removed. The slit and beam stop assembly were shown
to behave very well, limiting the background scatter on the
The capabilities of the MINERVA chamber extend beyond
probing growth dynamics during in situ deposition, enabling
also the observation of freshly deposited films exposed to
external stimuli. Such investigations provide potentially criti-
cal information otherwise inaccessible in laboratory settings,
where thin films must be transferred to an inert ambient-
pressure glove box or to air. These experimental limitations
preclude the observation of thin film evolution from its pris-
tine, as-deposited state when exposed to air or analyte-carrying
gases. Such studies have become increasingly important
103901-6 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
in the context of solvent annealing microstructural trans-
formations,29 organic semiconductor (OSC)-based sensors,30
and OPV stability.31 One frequently used molecule is
bathophenanthroline (BPhen), which is typically used as a
buffer or doped electron transport layer between the active
layer organic absorber/emitter and the evaporated metal cath-
ode, composed of materials such as aluminium. Although
ubiquitous in a number of organic light emitting diode (OLED)
and OPV architectures, few if any investigations of the micro-
structure of vacuum-deposited BPhen exist. We have inves-
tigated thin films of BPhen on silica, which is known to
transform from a glass-like organic layer into a more opaque,
translucent scattering film over the course of an hour.
Sublimation-purified BPhen (Luminescence Technology
Corp., Taiwan) was evaporated onto glass substrates (Thin
Film Devices, Inc., USA) that were cleaned following a stan-
dard procedure of sonication successively in 2.5% Hellmanex
(Sigma Aldrich) in deionised water, deionised water, acetone,
and 2-propanol and subsequently dried in a light stream of N2,
followed by surface cleaning under ultraviolet (UV) ozone for
10 min prior to loading in the chamber. The X-ray energy
was tuned to 12.5 keV (wavelength λ= 0.992 Å), and the
substrate was aligned using optical methods to accurately ori-
ent the sample along the direction of the beam. The reflected
spot position was recorded as the sample was translated across
the X-ray beam to level it in the orthogonal direction. To
monitor the surface of the film while limiting the scattering
from the bulk, the substrate was rotated such that the X-ray
incidence angle was Θ=0.071, well below both the glass
critical angle Θc,glass =0.152and the BPhen critical angle
Θc,Bphen =0.127. Although the intensity at this shallow graz-
ing angle is relatively low, we can achieve significant depth
sensitivity, with a minimum effective penetration depth32 given
by z0=1
2reπ ρ, where reis the Thomson scattering length and
ρis the electron density of the compound. Assuming vacuum
deposited Bphen is crystalline, with an orthorhombic crys-
tal structure33 (a=7.253 Å, b=10.810 Å, c=21.14 Å), the
penetration depth z0=116 Å which is slightly larger than that
reported for crystalline P3HT of 85 Å.32 As a result, changes
in the depth dependence of the crystallographic features with
increasing film thickness can be monitored closely in films
relevant for optoelectronic devices.
The aim of the study was to investigate BPhen using the
parameter space accessible with MINERVA, initially by mon-
itoring the growth of a 40 nm thin film of BPhen and observing
its evolution while under vacuum. Figure 6(a) shows the real-
time deposition rate and thickness measured during growth
with 2D GIWAXS images recorded at key points shown in
Figs. 6(b) and 6(c). The appearance of powder rings in films
of thickness greater than 10 nm indicates an isotropically grow-
ing film with negligible preferential vertical ordering. Using
these images as a guide, radial regions of interest were defined
and applied to all images acquired during in situ deposition,
resulting in the plots shown in Fig. 7. These profiles show that
in-plane ordering occurs in films of thickness as low as 2 nm,
as evidenced by the appearance of two closely spaced peaks
at qxy = 0.61 Å 1and 0.62 Å 1. These features correspond to
spacings of 10.13–10.30 Å, which are relatively close to the
previously reported values for the [010] spacing of 10.4 Åin
solution-growth BPhen crystals.34 Notably, both these peaks
shift to slightly higher qvalues with increasing thickness, sug-
gesting that BPhen may exhibit closer in-plane packing when
grown on itself as compared to on bare glass. As the film grows
to nearly 40 nm thickness, new scattering features are observed
at qxy = 1.11 Å 1and 1.19 Å 1, respectively, indicating the
concomitant development of in-plane scattering features.
After deposition, images were recorded while the X-ray
incidence angle was varied from 0.073to 0.153(0.01incre-
ment) to evaluate any depth-dependence in the crystallinity of
the sample. Radial regions of interest identical to those shown
in Fig. 7were applied to the images, with results plotted in
Fig. 8. The large change in penetration depth between the ini-
tial and final incidence angles provides information about the
internal microstructure as a function of depth from the surface.
FIG. 6. In situ monitoring of BPhen grown on glass.
(a) Recorded quartz crystal monitor (QCM) readout
during BPhen deposition (blue) indicating a rate of
0.250.3 Ås1. Green step function denotes 10 s detec-
tor exposure time and shaded regions with arrows indicate
the upper 11.6 nm probed in a given image based on the
penetration depth into the film at 12.5 keV. Lower panels
show GIWAXS images monitoring the 001 features after
total growth time and film thickness of (b) 700 s, 17.5 nm
and (c) 1470 s, 39 nm.
103901-7 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
FIG. 7. Monitoring growth of BPhen
grown on glass at rates of 0.250.3
Ås1. (a) Radial regions of interest
(i-v) and (b) profiles from 2D detec-
tor images. Linear colorbar denotes
the total film thickness at which each
image was acquired during deposition
[cf. Fig. 6(a)].
Several of the higher order qpeaks, corresponding to the 020
and 002 features at 1.21 Å 1, are clearly resolved at the highest
penetration depth. A striking contrast with Fig. 7is that the in-
plane low-q features no longer appear as two separate peaks but
are convoluted, with only a small shoulder near 0.62 Å 1. It is
the superposition of the q-shifted peaks of the various 11.6 nm
slices of the film observed in Fig. 7which results in this smear-
ing of a single peak with only a shoulder at 0.62 Å 1. By
comparison, the same range along qzreveals a more complex
depth-dependence of the features; upper layers show a single
FIG. 8. Angle of incidence dependent region of interest profiles for a 40 nm BPhen film on glass. The variation in penetration depth ranges from 11.6 nm at
Θ=0.071to the full 40 nm film thickness at Θ=0.15, with the full film probed at Θ=0.13.
103901-8 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
broad peak with larger lattice spacing, while the layers in the
lower half of the film exhibit two separate peaks, with lattice
spacings larger and smaller, respectively, than at the surface.
The larger spacing at shallower angles suggests a degree of
surface relaxation in BPhen, a phenomenon observed in other
crystalline organic semiconductors.35 Although the evapora-
tion rate varied only 10% (see Fig. 6), we cannot entirely
rule out that the slightly higher evaporation rate (0.28 Ås 1vs
0.25 Ås 1) during the first few nm of the deposition could lead
to a kinetically constrained lower layer at the glass interface
(with a different microstructure), as could the OSC “wetting
layer” polymorphism observed in systems such as pentacene.36
The temperature- and rate-dependence of such trapping, as
well the influence of surface energy on BPhen arrangement,
will be the subject of future studies with MINERVA.
The observed surface relaxation in the post-deposition
scan of the BPhen film, coupled with the material’s relatively
low sublimation temperature of 140 C, suggests that some
of the rapid crystallisation seen in air may already occur to
a limited degree under vacuum. To probe this behaviour, the
thin film was maintained at 24 C under vacuum in the depo-
sition chamber (P=6×107mbar) and monitored at various
time steps. Comparison of the radial profiles (Fig. S2 in the
supplementary material) demonstrates a clear increase in scat-
tering intensity for all of the scattering features, consistent with
increased crystallisation of the film. Notably, comparison of
the qzfeatures near the 001 peak indicates a slight relaxation
within the upper part of the film, with the lattice spacing in
this layer (corresponding to dark lines) shifting by +0.05 Å,
while the layers near the glass substrate remain unchanged.
Additionally, we can more clearly resolve the peak splitting in
the 002 features for the film near the glass interface, as well as
the emergence of a higher order powder ring at 1.69 Å 1.
Comparison of the radial profiles for samples kept under
vacuum reveals that the film undergoes greater out-of-plane
reorganisation than in-plane, with the upper surface showing
an increase of nearly 0.08 Å between initial deposition and
the scan 2 h later. In addition to the changes with time, all
samples show a small but systematic decrease in the lattice
constant as the greater depth of the film is probed, as shown in
Figs. 9(a) and 9(b). This change becomes significantly larger
at an incidence angle of 0.13, where the X-ray penetration
depth is equal to the film thickness. At these higher angles, the
out-of-plane layer spacing (derived from the [001] peak posi-
tion) diverges into two separate peaks. The larger dimension
corresponds to a lattice spacing of 10.39 Å, very close to the
10.40 Å value found by Mazumdar et al.,34 but smaller than
10.81 Å identified by Ceolin et al.33 The smaller of the two lat-
tice constants is 9.97 Å, considerably smaller than previously
reported values. Given that this large difference in qclose to
the specular beam arises when we reach the critical angle of
the substrate, one possibility is that this distortion is due to
refraction at the vacuum/film interface.37,38 However, in that
case, qzwould be expected to be of order 0.005 Å 1and so
does not account for the large separation of the two qzvalues.
Alternatively, the presence of more than one peak when the
sample is fully illuminated is most likely due to the presence
of several close families of the BPhen crystal structure with
different out-of-plane strain in the layers close to the interface,
a hypothesis supported by the additional peaks near the [002]
position (Figs. S2 and S3 in the supplementary material) which
do not exhibit any peak splitting above the critical angle of the
glass. Together with the absence of any change in qxy with
increasing time as seen in Fig. 9(a), these data suggest that the
structure of nanocrystalline BPhen may accommodate greater
reordering along the out-of-plane [00l] direction than along
other planes, likely arising from the steric hindrance imposed
by its two phenyl rings.
One key aspect of the MINERVA chamber is the abil-
ity to track the evolution of crystalline features with time,
either during growth or under vacuum. Figure 10 shows such
an evolution for the [001] peak, with initial time points corre-
sponding to monitoring the upper surface during in situ growth
and the final three-time points when kept under vacuum, post-
deposition. In the case of the [001] peak, the relaxation of
the upper surface with time can be compared against the
steady-state growth of the film. Of note is the appearance
of the [001] peak during the last few minutes of the film
growth. This increased scatter from the [001] peak might arise
from preferential out-of-plane growth, but it is also likely to
arise from an increase in film roughness which results in a
lower film density and hence a greater effective X-ray penetra-
tion depth probing out-of-plane scatter. Closer examination of
Fig. 7reveals that the film scatters highly in the in-plane
FIG. 9. Comparison of [010] and [001] evolution under vacuum at room temperature. Different symbols represent the time elapsed following the final deposition
as indicated in the legend. (a) Radial RoI along qxy and (b) along qz, showing the presence of a layer with different out-of-plane ordering near the substrate
interface. In-plane alignment does not change significantly while holding the sample under vacuum, while vertical ordering exhibits a greater change near the
free surface. The shaded region indicates the angle at which the penetration depth of the X-rays exceeds the sample thickness.
103901-9 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
FIG. 10. Time series comparison of the [001] peak evolution. Earlier, closely
spaced time points correspond to in situ measurements of the growing upper
surface. The final three time points relate to the post-growth measurements
illustrated in Fig. 9(b). In all cases, the angle of incidence was Θ=0.07.
direction at low thickness, while it scatters more in the out-
of-plane direction at higher thickness, suggesting that accu-
mulation of orientational disorder during film growth is partly
responsible for the observed coarsening of the film surface
along the [001] direction.
A new facility (MINERVA, a facility to study Microstruc-
ture and INterface Evolution in Realtime under VAcuum) for
use on the synchrotron beamline I07 at Diamond Light Source
has been built for in situ monitoring of GIWAXS/GISAXS
measurements during growth of vacuum deposited materials
or modification of the environment in which the thin films are
studied. The chamber allows co-evaporation of two materials
and is configured to reduce the X-ray background, essen-
tial to monitor the scattering from, e.g., weakly scattering
organic molecules. We have successfully demonstrated the
parameter space accessible by MINERVA to observe the in
situ nucleation and growth of a nanocrystalline BPhen thin
film and observed how the nanocrystals coarsen with time at
room temperature under vacuum. Our measurements reveal
that BPhen, a common electrode interface material, under-
goes minor but continuous surface relaxation relative to the
bulk of the film when not covered by metal even in vacuum.
Although a simple model system, these findings underscore
the need to employ in situ methods to probe the effect of
thin film fabrication conditions on the OSC microstructure,
not only for the fabrication of high performance and stable
devices but also to understand the degree to which measure-
ments carried out even under vacuum (e.g., XPS/UPS39) on
samples at long times after deposition can be related to the
rapidly processed, device-relevant thin films used in large-
scale production facilities. Furthermore, MINERVA opens the
door for a wide variety of investigations of structure-property
relationships involving vacuum deposited materials and their
applications in optoelectronic and energy-related devices.
See supplementary material for calibration data used to
determine the accuracy of the flux rates. Also included are
data showing how the BPhen crystal structure develops as a
function of film thickness and with time in vacuum.
C.N., M.R., and J.M.H. were responsible for the initial
design concept and overall management of the project. M.B.,
S.G., J.N., A.D., D.W., S.D., and A.W. detailed the design
of the components and were responsible for assembly. All
authors contributed to writing the manuscript under the coordi-
nation of C.N. We gratefully acknowledge funding to develop
the chamber through the STFC Challenge Led Applied Sys-
tems Programme (CLASP) under Grant Nos. ST/L006294/1,
ST/L006219/1 and access to Diamond beamtime at I07 under
experiments Nos. SI14220-1 and SI15207-1. M.R. gratefully
acknowledges funding through an EU FP7 Marie Curie Career
Integration Grant (No. PCIG14-GA-2013-630864). We thank
Simon Lay, Hugo Shiers, and Matthew Barnes (Diamond Light
Source) for their work implementing the vacuum control and
1F. Knudsen, J. Am. Ceram. Soc. 42, 376 (1959).
2A. Wittstock, V. Zielasek, J. Biener, C. Friend, and M. Baeumer, Science
327, 319 (2010).
3C. Poelking, M. Tietze, C. Elschner, S. Olthof, D. Hertel, B. Baumeier,
F. W¨
urthner, K. Meerholz, K. Leo, and D. Andrienko, Nat. Mater. 14, 434
4M. Schwarze, W. Tress, B. Beyer, F. Gao, R. Scholz, C. Poelking,
K. Ortstein, A. G¨
unther, D. Kasemann, D. Andrienko, and K. Leo, Science
352, 1446 (2016).
5M. Jørgensen, K. Norrman, S. Gevorgyan, T. Tromholt, B. Andreasen, and
F. Krebs, Adv. Mater. 24, 580 (2012).
6M. Riede, T. Mueller, W. Tress, R. Schueppel, and K. Leo, Nanotechnology
19, 424001 (2008).
7Y. Han, S. Meyer, Y. Dkhissi, K. Weber, J. Pringle, U. Bach, L. Spiccia, and
Y.-B. Cheng, J. Mater. Chem. A 3, 8139 (2015).
8D. Mastrogiovanni, J. Mayer, A. Wan, A. Vishnyakov, A. Neimark,
V. Podzorov, L. Feldman, and E. Garfunkel, Sci. Rep. 4, 4753 (2014).
9M. Hermenau, M. Riede, K. Leo, S. Gevorgyan, F. Krebs, and K. Norrman,
Sol. Energy Mater. Sol. Cells 95, 1268 (2011).
10A. Distler, T. Sauermann, H.-J. Egelhaaf, S. Rodman, D. Waller,
K.-S. Cheon, M. Lee, and D. Guldi, Adv. Energy Mater. 4, 1300693
11B. Krause, S. Darma, M. Kaufholz, H.-H. Gr¨
afe, S. Ulrich, M. Mantilla,
R. Weigel, S. Rembold, and T. Baumbach, J. Synchrotron Radiat. 19, 216
12R. D ¨
ohrmann, S. Botta, A. Buffet, G. Santoro, K. Schlage, M. Schwartzkopf,
S. Bommel, J. Risch, R. Mannweiler, S. Brunner, E. Metwalli, P.
uller-Buschbaum, and S. Roth, Rev. Sci. Instrum. 84, 43901 (2013).
13S. Couet, T. Diederich, K. Schlage, and R. R¨
ohlsberger, Rev. Sci. Instrum.
79, 93908 (2008).
14S. Kowarik, J. Phys. Condens. Matter. 29, 43003 (2017).
15D. Fong and C. Thompson, Annu. Rev. Mater. Res. 36, 431 (2006).
16K. Ritley, B. Krause, F. Schreiber, and H. Dosch, Rev. Sci. Instrum. 72,
1453 (2001).
17B. Jenichen, W. Braun, V. Kaganer, A. Shtukenberg, L. D¨
C.-G. Schulz, K. Ploog, and A. Erko, Rev. Sci. Instrum. 74, 1267 (2003).
18A. Amassian, T. Desai, S. Kowarik, S. Hong, A. Woll, G. Malliaras,
F. Schreiber, and J. Engstrom, J. Chem. Phys. 130, 124701 (2009).
19K. Chou, B. Yan, R. Li, E. Li, K. Zhao, D. Anjum, S. Alvarez, R. Gassaway,
A. Biocca, S. Thoroddsen, A. Hexemer, and A. Amassian, Adv. Mater. 25,
1923 (2013).
20M. Sanyal, B. Schmidt-Hansberg, M. Klein, A. Colsmann, C. Munuera,
A. Vorobiev, U. Lemmer, W. Schabel, H. Dosch, and E. Barrena, Adv.
Energy Mater. 1, 363 (2011).
21F. Bokel, S. Engmann, A. Herzing, B. Collins, H. Ro, D. DeLongchamp,
L. Richter, E. Schaible, and A. Hexemer, Chem. Mater. 29, 2283
103901-10 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
22H. Ro, J. Downing, S. Engmann, A. Herzing, D. DeLongchamp, L. Richter,
S. Mukherjee, H. Ade, M. Abdelsamie, L. Jagadamma, A. Amassian, Y. Liu,
and H. Yan, Energy Environ. Sci. 9, 2835 (2016).
23C. Nicklin, T. Arnold, J. Rawle, and A. Warne, J. Synchrotron Radiat. 23,
1245 (2016).
24V. K¨
orstgens, J. Wiedersich, R. Meier, J. Perlich, S. Roth, R. Gehrke, and
P. M¨
uller-Buschbaum, Anal. Bioanal. Chem. 396, 139 (2010).
25H. Proehl, R. Nitsche, T. Dienel, K. Leo, and T. Fritz, Phys. Rev. B 71,
165207 (2005).
26R. Forker, M. Gruenewald, and T. Fritz, Annu. Rep. Sect. C Phys. Chem.
108, 34 (2012).
27M. Huss-Hansen, A. Lauritzen, O. Bikondoa, M. Torkkeli, L. Tavares,
M. Knaapila, and J. Kjelstrup-Hansen, Org. Electron. 49, 375 (2017).
28F. Liscio, C. Albonetti, K. Broch, A. Shehu, S. Quiroga, L. Ferlauto,
C. Frank, S. Kowarik, R. Nervo, A. Gerlach, S. Milita, F. Schreiber, and
F. Biscarini, ACS Nano 7, 1257 (2013).
29G. Purdum, N. Yao, A. Woll, T. Gessner, R. Weitz, and Y.-L. Loo, Adv.
Funct. Mater. 26, 2357 (2016).
30C. Zhang, P. Chen, and W. Hu, Chem. Soc. Rev. 44, 2087 (2015).
31F. Anger, T. Breuer, A. Ruff, M. Klues, A. Gerlach, R. Scholz,
S. Ludwigs, G. Witte, and F. Schreiber, J. Phys. Chem. C 120, 5515
32T. Schuettfort, L. Thomsen, and C. McNeill, J. Am. Chem. Soc. 135, 1092
33R. Ceolin, M. Mariaud, P. Levillain, and N. Rodier, Acta. Cryst. B 35, 1630
34P. Mazumdar, D. Das, G. Sahoo, G. Salgado-Mor´
an, and A. Misra, Phys.
Chem. Chem. Phys. 16, 6283 (2014).
35H. Morisaki, T. Koretsune, C. Hotta, J. Takeya, T. Kimura, and
Y. Wakabayashi, Nat. Commun. 5, 5400 (2014).
36T. Kakudate, N. Yoshimoto, and Y. Saito, Appl. Phys. Lett. 90, 81903
37X. Lu, K. Yager, D. Johnston, C. Black, and B. Ocko, J. Appl. Crystallogr.
46, 165 (2013).
38M. Tate, V. Urade, J. Kowalski, T. Wei, B. Hamilton, B. Eggiman, and
H. Hillhouse, J. Phys. Chem. B 110, 9882 (2006).
39S. Olthof, J. Meiss, B. L ¨
ussem, M. Riede, and K. Leo, Thin Solid Films
519, 1872 (2011).
... Beyond protection of the sample, vacuum environments with X-ray windows are used to study molecular beam deposition via thermal deposition in situ and in real time. 3,17,18 In this example, two cylinder shaped 100 mm diameter vacuum chambers with either a Kapton or Be window were used. Inside these vacuum chambers, a 30 nm PTCDI-C 8 film on a silicon substrate with native oxide is placed and we perform grazing incidence x-ray diffraction (GIXD) with an incident angle of 0.12 • , which is below the critical angle of silicon (0.14 • ) but above the critical angle of PTCDI-C 8 (0.10 • ) so that we selectively measure the scattering from the crystalline thin film. ...
... To completely eliminate all beryllium rings including those of the entry window, one can, for example, design a vacuum chamber with a beryllium window far away from the sample or a window followed by in vacuum slits. 17,19,20 Both the above solutions however have drawbacks compared to radial collimators in that beam-scrapers need to be very close to a beryllium window enhancing collision risks, require a larger vacuum chamber, or have the added complexity of in-vacuum precision movements. It should also be stressed that these options are not usable for a range of sample environments due to physical constraints or the experimental geometry, as in the case of liquid cells or a catalytic reactor. ...
We demonstrate the use of a 3D printed radial collimator in X-ray powder diffraction and surface sensitive grazing incidence X-ray diffraction. We find a significant improvement in the overall signal to background ratio of up to 100 and a suppression of more than a factor 3 · 10⁵ for undesirable Bragg reflections generated by the X-ray “transparent” windows of the sample environment. The background reduction and the removal of the high intensity signals from the windows, which limit the detector’s dynamic range, enable significantly higher sensitivity in experiments within sample environments such as vacuum chambers and gas- or liquid-cells. Details of the additively manufactured steel collimator geometry, alignment strategies using X-ray fluorescence, and data analysis are also briefly discussed. The flexibility and affordability of 3D prints enable designs optimized for specific detectors and sample environments, without compromising the degrees of freedom of the diffractometer.
... To investigate the evolution of C 60 , we employed a recently developed multi-source deposition chamber at the Diamond Light Source that enables grazing-incidence X-ray scattering (GIXS) measurements of growing molecular thin films during thermal deposition. 28 Capturing the growth dynamics of organic molecular films at industrially relevant deposition rates (0.1-1 Å /s) 29 presents various technical challenges. The low scattering density of organic materials necessitates longer exposure times than that of films of a similar thickness of high-Z atomic species to achieve similar contrast. ...
... Simple reflectivity models with constant roughness r C60/air ¼ 0.2 nm for the C 60 layer and glass roughness r glass/C60 ¼ 1.0 nm ( Fig. S2 in the supplementary material) are only able to match the oscillation period obtained at q z ¼ 0.138 Å À1 , but a full description of the measured offspecular reflectivity using an adjustment of the model by Woll et al. 18,19 is required to fully capture the growth behavior of the film. As a consequence, the reported film thickness is based on the response of a calibrated quartz crystal monitor (QCM) within the deposition chamber 28 and does not account for changes in thickness which may arise from differences in the sticking coefficient of C 60 at room temperature within the first few layers of the film. For a comprehensive investigation of the sticking coefficient and thermally assisted dewetting of C 60 and its implications on observed film growth, the reader is referred to a recent study that makes use of in-situ x-ray reflectivity to quantify dewetting and upward mass transport by monitoring the specular signal from film deposition up to 60 min post-deposition. ...
Full-text available
We report an in-situ study of stacking fault evolution in C60 thin films using grazing-incidence x-ray scattering. A Williamson-Hall analysis of the main scattering features during growth of a 15 nm film on glass indicates lattice strain as high as 6% in the first 5 nm of the film, with a decrease to 2% beyond 8 nm thickness. Deformation stacking faults along the {220} plane are found to occur with 68% probability and closely linked to the formation of a nanocrystalline powder-like film. Our findings, which capture monolayer-resolution growth, are consistent with previous work on crystalline and powder C60 films, and provide a crystallographic context for the real-time study of organic semiconductor thin films.
... In situ GIXS measurements were performed at the Diamond Light Source surface diffraction beamline (I07) [28] using an energy of 20 keV in the purpose-built MINERVA sample chamber [29]. ZnPc evaporation in MINERVA was achieved with a low-temperature thermal evaporation source at a rate of 0.26 ± 0.03 Å/s as monitored by a water cooled quartz crystal microbalance (QCM), previously calibrated using ellipsometry. ...
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We report on the characterization of the growth of vacuum-deposited zinc phthalocyanine (ZnPc) thin films on glass through a combination of in situ grazing incidence x-ray scattering, x-ray reflectivity, and atomic force microscopy. We found that the growth at room temperature proceeds via the formation of two structurally unique substrate-induced interfacial layers, followed by the growth of the γ-ZnPc polymorph thereafter (thickness ≈1.0 nm). As the growth of the bulk γ-ZnPc progresses, a substantial out-of-plane lattice strain (≈15% relative to γ-ZnPc powder) is continually relaxed during the thin film growth. The rate of strain relaxation was slowed after a thickness of ≈13 nm, corresponding to the transition from layer growth to island growth. The findings reveal the real-time microstructural evolution of ZnPc and highlight the importance of substrate-induced strain on thin film growth.
... The samples are probed while inside a vacuum deposition chamber at a pressure of around 10 À3 mbar with the MINERVA setup. 38 The sample-to-detector distance was 42.1 cm as determined via AgBeh calibration. Images are converted to 2D reciprocal space using the DAWN software package with an applied polarisation and solid angle correction. ...
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We use the electron spin as a probe to gain insight into the mechanism of molecular doping in a p-doped zinc phthalocyanine host across a broad range of temperatures (80–280 K) and doping concentrations (0–5 wt% of F6-TCNNQ). Electron paramagnetic resonance (EPR) spectroscopy discloses the presence of two main paramagnetic species distinguished by two different g-tensors, which are assigned based on density functional theory calculations to the formation of a positive polaron on the host and a radical anion on the dopant. Close inspection of the EPR spectra shows that radical anions on the dopants couple in an antiferromagnetic manner at device-relevant doping concentrations, thereby suggesting the presence of dopant clustering, and that positive polarons on the molecular host move by polaron hopping with an activation energy of 5 meV. This activation energy is substantially smaller than that inferred from electrical conductivity measurements (∼233 meV), as the latter also includes a (major) contribution from charge-transfer state dissociation. It emerges from this study that probing the electron spin can provide rich information on the nature and dynamics of charge carriers generated upon doping molecular semiconductors, which could serve as a basis for the design of the next generation of dopant and host materials.
... Grazing-incidence wide-angle x-ray scattering (GIWAXS) studies are carried out at the Surface and Interface Diffraction beamline (I07) at the Diamond Light Source (DLS) using a beam energy of 20 keV (0.62 Å) and a Pilatus2M area detector. The samples are probed while inside a vacuum deposition chamber at a pressure of around 10 À3 mbar with the MINERVA setup 37 . The sample-to-detector distance was 42.1 cm as determined via AgBeh calibration. ...
Full-text available
Simultaneous control over both the energy levels and Fermi level, a key breakthrough for inorganic electronics, has yet to be shown for organic semiconductors. Here, energy level tuning and molecular doping are combined to demonstrate controlled shifts in ionisation potential and Fermi level of an organic thin film. This is achieved by p-doping a blend of two host molecules, zinc phthalocyanine and its eight-times fluorinated derivative, with tunable energy levels based on mixing ratio. The doping efficiency is found to depend on host mixing ratio, which is explained using a statistical model that includes both shifts of the host's ionisation potentials and, importantly, the electron affinity of the dopant. Therefore, the energy level tuning effect has a crucial impact on the molecular doping process. The practice of comparing host and dopant energy levels must consider the long-range electrostatic shifts to consistently explain the doping mechanism in organic semiconductors.
... Grazing-incidence wide-angle x-ray scattering (GIWAXS) studies were carried out at the Surface and Interface Diffraction beamline (IO7) at the Diamond Light Source (DLS) using a beam energy of 20 keV (0.62 Å) and a Pilatus2M area detector. The samples were probed while inside a vacuum deposition chamber at a pressure of around 10 −3 mbar with the MINERVA setup as described by Nicklin et al. 15 The sample-to-detector distance was 42.1 cm as determined via AgBeh calibration. Images were converted to 2D reciprocal space using the DAWN software package 16 with an applied solid angle correction. ...
Full-text available
We report on a method for fabricating balanced hole and electron transport in ambipolar organic field-effect transistors (OFETs) based on the co-evaporation of zinc-phthalocyanine (ZnPc) and its fluorinated derivative (F 8 ZnPc). The semiconducting behaviour of the OFET can be tuned continuously from unipolar p-type, with a hole mobility in the range of (1.7 ± 0.1) × 10 ⁻⁴ cm ² /Vs, to unipolar n-type, with an electron mobility of (1.0 ± 0.1) × 10 ⁻⁴ cm ² /Vs. Devices of the pristine ZnPc and F 8 ZnPc show a current on/off ratio of 10 ⁵ . By co-evaporating the p-type ZnPc with the n-type F 8 ZnPc, we fabricate ambipolar transistors and complementary-like voltage inverters. For the ambipolar devices, the optimum balance between the hole and electron mobilities is found for the blend of 1:1.5 weight ratio with hole and electron mobilities of (8.3 ± 0.2) × 10 ⁻⁷ cm ² /Vs and (5.5 ± 0.1) × 10 ⁻⁷ cm ² /Vs, respectively. Finally we demonstrate application of the ambipolar devices in a complementary-like voltage inverter circuit with the performance comparable to an inverter based on separate ZnPc and F 8 ZnPc OFETs.
... Hence, the formation and growth processes are difficult to study in particular in the case of amorphous and nanocrystalline films. Many specialized sputtering chambers have been developed for in situ investigations of film growth at synchrotron sources using both small-angle X-ray scattering (SAXS) and X-ray diffraction (Schroeder et al., 2015;Walter et al., 2015;Payne et al., 1992;Matz et al., 2001;Williams et al., 1992;Lee et al., 2000;Couet et al., 2008;Renaud et al., 2009;Folkman et al., 2013;Kaufholz et al., 2015;Nicklin et al., 2017). So far, to our knowledge, none of the reported in situ filmgrowth studies have addressed the local atomic structure. ...
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Characterization of local order in thin films is challenging with pair distribution function (PDF) analysis because of the minute mass of the scattering material. Here, it is demonstrated that reliable high-energy grazing-incidence total X-ray scattering data can be obtained in situ during thin-film deposition by radio-frequency magnetron sputtering. A benchmark system of Pt was investigated in a novel sputtering chamber mounted on beamline P07-EH2 at the PETRA III synchrotron. Robust and high-quality PDFs can be obtained from films as thin as 3 nm and atomistic modelling of the PDFs with a time resolution of 0.5 s is possible. In this way, it was found that a polycrystalline Pt thin film deposits with random orientation at 8 W and 2 × 10 ⁻² mbar at room temperature. From the PDF it was found that the coherent-scattering domains grow with time. While the first layers are formed with a small tensile strain this relaxes towards the bulk value with increasing film thickness.
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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 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 monolayers and intermediate 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 thin film microstructure in device performance, it is expected that these results will aid in the development of the structure-property relationships necessary to realise the full potential of organic electronics.
The quenching of excitons in organic solar cells can play a significant role in limiting their power conversion efficiency (PCE). In this article, we investigate the effect of a thin layer of hexapropyltruxene inserted at the interface between the electron donor boron subphthalocyanine chloride (SubPc) and its underlying hole contact in planar heterojunction solar cells. We find that a 3.8 nm hexapropyltruxene interlayer between the molybdenum oxide (MoOx) hole contact and SubPc is sufficient to improve PCE in SubPc/C60 fullerene solar cells from 2.6 % to 3.0 %, a ∼20 % performance improvement. While the absorption stays roughly the same, the comparison of external and internal quantum efficiencies reveals a significant increase in SubPc's contribution to the current for light with wavelengths between 520 and 600 nm. Microstructure and surface morphology assessed with in-situ Grazing-Incidence Wide-Angle X-Ray Scattering (GIWAXS) and Atomic Force Microscopy (AFM), are evaluated alongside in-situ spectroscopic ellipsometry, and photoluminescence measurements. The microstructural investigations demonstrate changes to the surface and bulk of SubPc grown atop a hexapropyltruxene interlayer indicating that the latter acts as a template layer in a similar way as MoOx. However, the improvement in PCE is found to be mainly via reduced exciton quenching at the MoOx contact with the insertion of the hexapropyltruxene layer.
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Thin-film growth is important for novel functional materials and new generations of devices. The non-equilibrium growth physics involved is very challenging, because the energy landscape for atomic scale processes is determined by many parameters, such as the diffusion and Ehrlich-Schwoebel barriers. We review the in situ real-time techniques of x-ray diffraction (XRD), x-ray growth oscillations and diffuse x-ray scattering (GISAXS) for the determination of structure and morphology on length scales from Å to µm. We give examples of time resolved growth experiments mainly from molecular thin film growth, but also highlight growth of inorganic materials using molecular beam epitaxy (MBE) and electrochemical deposition from liquids. We discuss how scaling parameters of rate equation models and fundamental energy barriers in kinetic Monte Carlo methods can be determined from fits of the real-time x-ray data.
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Beamline I07 at Diamond Light Source is dedicated to the study of the structure of surfaces and interfaces for a wide range of sample types, from soft matter to ultrahigh vacuum. The beamline operates in the energy range 8–30 keV and has two endstations. The first houses a 2+3 diffractometer, which acts as a versatile platform for grazing-incidence techniques including surface X-ray diffraction, grazing-incidence small- (and wide-) angle X-ray scattering, X-ray reflectivity and grazing-incidence X-ray diffraction. A method for deflecting the X-rays (a double-crystal deflector) has been designed and incorporated into this endstation, extending the surfaces that can be studied to include structures formed on liquid surfaces or at liquid–liquid interfaces. The second experimental hutch contains a similar diffractometer with a large environmental chamber mounted on it, dedicated to in situ ultrahigh-vacuum studies. It houses a range of complementary surface science equipment including a scanning tunnelling microscope, low-energy electron diffraction and X-ray photoelectron spectroscopy ensuring that correlations between the different techniques can be performed on the same sample, in the same chamber. This endstation allows accurate determination of well ordered structures, measurement of growth behaviour during molecular beam epitaxy and has also been used to measure coherent X-ray diffraction from nanoparticles during alloying.
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The stability of encapsulated planar-structured CH3NH3PbI3 (MAPbI3) perovskite solar cells (PSCs) was investigated under various simulated environmental conditions. The tests were performed under approximately one sun (100 mW/cm2) illumination, varying temperature (up to 85 °C cell temperature) and humidity (up to 80%). The application of advanced sealing techniques improved the device stability, but all devices showed significant degradation after prolonged aging at high temperature and humidity. The degradation mechanism was studied by post-mortem analysis of the disassembled cells using SEM and XRD. This revealed that the degradation was mainly due to the decomposition of MAPbI3, as a result of reaction with H2O, and the subsequent reaction of hydroiodic acid, formed during MAPbI3 decomposition, with the silver back contact electrode layer.
We report on microstructural durability of 5,5′-bis(naphth-2-yl)-2,2′-bithiophene (NaT2) in organic field effect transistors (OFETs) in operando monitored by grazing-incidence X-ray diffraction (GIXRD). NaT2 maintains its monoclinic bulk motif in operating OFETs with Å, Å, Å and . Crystallites appear as a mosaic of single crystals reaching through the whole 50 nm thick active layer. The lattice parameters variation (<1%) falls within the statistical error of structure refinement when the OFET gate voltage is varied from 0 V to −40 V; or when the OFET is continuously cycled within this voltage interval over more than 10 h period. Within the first few cycles, both the hole mobility and threshold voltage are changing but then reach stable levels with an average mobility of and an average threshold voltage of V, both varying less than 4% for the remainder of the 10 h period. This demonstrates crystalline stability of NaT2 in operating OFETs.
The evolution of the morphology of a high-efficiency, blade-coated, organic-photovoltaic (OPV) active layer containing the low band-gap polymer poly[(4,8-bis[5-(2-ethylhexyl)thiophene-2-yl]benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene))-2,6-diyl] (PBDTTT-C-T) is examined by in situ X-ray scattering. In situ studies enable real-time characterization of the effect of the processing additive 1,8-diiodoocatane (DIO) on the active layer morphology. In the presence of DIO, X-ray scattering indicates that domain structure radically changes and increases in purity, while X-ray diffraction reveals little change in crystallinity/local order. The solidification behavior of this active layer differs dramatically from those that strongly crystallize such as poly(3-hexylthiophene) and small molecule - containing systems, exposing significant diversity in the solidification routes relevant to high-efficiency polymer-fullerene OPV processing. In the presence of DIO, we find quantitative agreement between the evolution of the phase structure revealed by small angle X-ray scattering and the binodal phase structure of a simple Flory-Huggins model.
Solution processing via roll-to-roll (R2R) coating promises a low cost, low thermal budget, sustainable revolution for the production of solar cells. Poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′′′-di(2-octyldodecyl)-2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5-diyl)], PffBT4T-2OD, has recently been shown to achieve high power conversion efficiency (>10%) paired with multiple acceptors when thick films are spun-coat from hot solutions. We present detailed morphology studies of PffBT4T-2OD based bulk heterojunction films deposited by the volume manufacturing compatible techniques of blade-coating and slot-die coating. Significant aspects of the film morphology, the average crystal domain orientation and the distribution of the characteristic phase separation length scales, are remarkably different when deposited by the scalable techniques vs. spun-coat. Yet, we find that optimized blade-coated devices achieve PCE > 9.5%, nearly the same as spun-coat. These results challenge some widely accepted propositions regarding what is an optimal BHJ morphology and suggest the hypothesis that diversity in the morphology that supports high performance may be a characteristic of manufacturable systems, those that maintain performance when coated thicker than ≈200 nm. In situ measurements reveal the key differences in the solidification routes for spin- and blade-coating leading to the distinct film structures.
Organic solar cells tuned by blending Electrical engineers can finetune the energetics of rigid photovoltaics and transistors by blending different semiconducting materials. However, it's hard to apply this tuning protocol to the flexible class of carbon-based semiconductors. Schwarze et al. now show that continuous band energy tuning is indeed possible by varying the blend ratios of certain organic phthalocyanines and their fluorinated or chlorinated derivatives (see the Perspective by Ueno). They demonstrated the effect, which they attribute to quadrupolar interactions, in model solar cells. Science , this issue p. 1446 ; see also p. 1395
We report on the oxidation potential of partially fluorinated (C42F14H14, F14-RUB) and perfluorinated rubrene (C42F28, PF-RUB) studied by cyclic voltammetry (CV) in solution as well as by spectroscopic ellipsometry and near edge X-ray absorption fine structure (NEXAFS) spectroscopy in thin films in combination with density functional theory computations. Due to their different electronic structure, the fluorinated derivatives have a higher oxidation potential and are more stable than rubrene (C42H28, RUB).
Though charge transport is sensitive to subtle changes in the packing motifs of molecular semiconductors, research addressing how intermolecular packing influences electrical properties has largely been carried out on single-crystals, as opposed to the more technologically relevant thin-film transistors (TFTs). Here, independent and reversible access to the monoclinic and triclinic crystal structures of a core-chlorinated naphthalene tetracarboxylic diimide (NTCDI-1) is demonstrated in polycrystalline thin films via post-deposition annealing. Time-resolved measurements of these transitions via UV–visible spectroscopy and grazing-incidence X-ray diffraction indicate that the polymorphic transformations follow second-order Avrami kinetics, suggestive of 2D growth after initial nucleation. Thin-film transistors comprising triclinic NTCDI-1 consistently outperform those comprising its monoclinic counterpart. This behavior contrasts that of single-crystal transistors in which devices comprising monoclinic crystals are consistently superior to devices with triclinic crystals. This difference is attributed to more uniform in-plane charge transport in polycrystalline thin films having the triclinic compared to the monoclinic polymorph. As the mobility of TFTs is a reflection of ensemble-average charge transport across crystalline grains having different molecular orientations, this study suggests that among different polymorphs of a particular molecular semiconductor, those with smaller in-plane anisotropy are more beneficial for efficient lateral charge transport in polycrystalline devices.
Organic field-effect transistors (OFETs) are one of the key components of modern organic electronics. While the past several decades have witnessed huge successes in high-performance OFETs, their sophisticated functionalization with regard to the responses towards external stimulations has also aroused increasing attention and become an important field of general concern. This is promoted by the inherent merits of organic semiconductors, including considerable variety in molecular design, low cost, light weight, mechanical flexibility, and solution processability, as well as by the intrinsic advantages of OFETs including multiparameter accessibility and ease of large-scale manufacturing, which provide OFETs with great potential as portable yet reliable sensors offering high sensitivity, selectivity, and expeditious responses. With special emphases on the works achieved since 2009, this tutorial review focuses on OFET-based gas sensors. The working principles of this type of gas sensors are discussed in detail, the state-of-the-art protocols developed for high-performance gas sensing are highlighted, and the advanced gas discrimination systems in terms of sensory arrays of OFETs are also introduced. This tutorial review intends to provide readers with a deep understanding for the future design of high-quality OFET gas sensors for potential uses.