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| Figure 1: Chemical structure of the organic semiconductor pentacene. | |
While the positions of the diffraction peaks recorded on the image plate
provide information on the lattice geometry of the crystalline film, the
brightness of the diffraction peaks contains information on the molecular
orientation and packing within the lattice unit cell. In the case of a 2D
crystal (e.g., a monolayer), discrete peaks are only observed "in-plane" (the
qxy-direction in Figs. 2 and 3A). In the out of plane direction (the
qZ-axis in
The molecular packing that best explains the observed intensity profiles was
extracted by numerical optimization of the differences between theoretical and
observed diffraction intensities; the results not only reproduced the three
visible intensity profiles with good accuracy, but also verified the
experimentally observed absence of measurable diffraction intensity in the two
first order in-plane diffraction profiles (labeled {01L} and {10L} in Fig 3A).
The packing motif of two pentacene molecules in the unit cell from the
calculation procedure is shown in Fig. 4. Similar to previous reports
investigating thicker pentacene films (20-50 nm), the pentacene molecules in
the first monolayer form a so-called herringbone pattern (upper left panel in
Fig. 4). However, in contrast to the thicker multilayer pentacene films, the
molecules in the first monolayer stand exactly upright on the SiO2
surface. To compare the electronic properties of the monolayer arrangement with
vertical pentacene molecules to the packing motif in bulk pentacene single
crystals, DFT calculations were performed (for details refer to first reference
below). The results of these calculations suggest that the upright
configuration in conjunction with the slightly smaller unit cell in the
pentacene monolayer (compared to bulk pentacene) lead to the high mobility
values observed in pentacene films.
Primary Citation
Precise Structure of Pentacene Monolayers on Amorphous Silicon Oxide and
Relation to Charge Transport, S.C.B Mannsfeld, A. Virkar, C. Reese, M.F. Toney,
Z. Bao, Adv. Mat. 21, 2294 (2009).
Thin film structure of tetraceno[2,3-b]thiophene characterized by grazing
incidence X-ray scattering and near-edge X-ray absorption fine structure
analysis, Q. Yuan, S.C.B. Mannsfeld, M.L. Tang, M.F. Toney, J. Lüning, Z. Bao,
J. Am. Chem. Soc. 130, 3502 (2008).
Microstructure of Oligofluorene Asymmetric Derivatives in Organic Thin Film
Transistors, Q. Yuan, S.C.B. Mannsfeld, M.L. Tang, M. Roberts, M.F. Toney, D.M.
DeLongchamp, Z. Bao, Chem. Mater. 20, 2763 (2008).
Pentacene sub-monolayer films were prepared by vacuum-deposition onto
SiO2 substrates at 60°C, which are typical deposition
parameters we use to produce high-performance TFTs. Grazing incidence X-ray
diffraction (GIXD) with synchrotron light offers the unique opportunity to
study the diffraction from weakly scattering, ultra-thin films such as a single
molecular layer of pentacene. The scattering geometry of the GIXD experiment
performed on SSRL's Beam Line 11-3 is depicted in Fig. 2. The incidence angle
a
of the incident beam k0 was set to 0.1°. The diffraction
intensity was detected on a 2D image plate (MAR-345, distance L=402 mm).
Figure 2:
Geometry of the GIXD setup. The incident beam k0 makes an
angle a with the sample surface; the scattered beam
k is registered by an image plate detector at a distance L. The in-plane
component of the momentum transfer vector q is qxy and
the vertical component qZ.
Fig. 3A), there
is no limiting Bragg condition (because the film is a single
layer thick) which leads to continuous scattering profiles (Bragg rods). These
Bragg rod intensity profiles as obtained from GIXD on a single monolayer of
pentacene were used to extract the alignment of the pentacene molecules.
Figure 3:
GIXD diffraction data and fitting results. A, Portion of the diffraction
image of a pentacene sub-monolayer showing the {11L}, {02L}, and {12L} Bragg
rods (the {01L} and {10L} rods being extinct). B, Integrated intensity
values Ihk(qZ) for the first five Bragg rods, obtained
by integration across the Bragg rods at different out-of-plane momentum
transfer values qZ. The corresponding best-fit calculated
Ihk(qZ) values are plotted as lines.
In summary, this work shows that it is possible to determine the exact
structure in a single molecule-thick layer of organic semiconductor molecules
using a combination of synchrotron-based X-ray diffraction and crystallographic
refinement calculations. The monolayer structure we obtain explains the
previously poorly understood excellent transport in polycrystalline pentacene
films. The differences in the packing motifs of a pentacene monolayer and
thicker pentacene films underline the importance of probing the relevant
physical properties (e.g. structure) exactly at the location relevant to the
properties that one desires to explain.
Figure 4:
Alignment of pentacene molecules in the first monolayer as obtained from the
intensity fitting calculations. The two molecules in the unit cell are oriented
vertically to the substrate surface and form a herringbone-pattern with an
herringbone angle (angle enclosed by the normals to the molecular planes) of
52.7°.
Further Reading
SSRL is supported by the Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program, and the National Institute of General Medical Sciences.