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Cite This: J. Phys. Chem. Lett. 2017, 8, 5366-5371

High Hole-Mobility Molecular Layer Made from Strong Electron
Acceptor Molecules with Metal Adatoms
Hiroyuki Yamane*,†,‡,§ and Nobuhiro Kosugi†,‡†

Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki 444-8585, JapanSOKENDAI (The Graduate University for Advanced Studies), Myodaiji, Okazaki 444-8585, Japan‡S Supporting Information*

ABSTRACT: The electronic structure of 7,7,8,8-tetracyanoquinodimethane(TCNQ) and 2,3,5,6-tetraﬂuoro-TCNQ (F4TCNQ) monolayers on Au(111)
has been investigated by means of angle-resolved photoemission spectroscopy(ARPES) with synchrotron radiation. In contrast to the physisorbed TCNQ/
Au(111) interface, the high-resolution core-level photoemission spectra andthe low-energy electron diﬀraction at the F4TCNQ/Au(111) interface show
evidence for the strong charge transfer (CT) from Au to F4TCNQ and for the
Au atom segregation from the underlying Au(111) surface, suggesting a
possible origin of the spontaneous formation of the two-dimensional
F4TCNQ-Au network. The ARPES experiment reveals a low hole-injection
barrier and large band dispersion in the CT-induced topmost valence level ofthe F4TCNQ-Au network with 260 meV bandwidth due to the adatommediated intermolecular interaction. These results indicate that strong
electron acceptor molecules with metal adatoms can form high hole-mobility molecular layers by controlling the molecule−metal ordered structure and their CT interaction.

Tdistortion16−18 and the 2D network formation,19−22 depending
on metal atoms and underlying surfaces; especially, theF4TCNQ molecules on an Au(111) surface show both
evidence for the strong molecular distortion and for the
spontaneous formation of the 2D network via the cyano groupwith the Au adatom segregated from the underlying Au(111)
surface as demonstrated using scanning tunneling microscopy(STM).23,24
Molecule−metal coordination systems have a large degree of
freedom for the design of the 2D network structure and its
functionalities. In order to understand characteristics of the 2D
TCNQ-metal network systematically, we have studied thegeometric and electronic structures of the F4TCNQ/Au(111)and TCNQ/Au(111) interfaces as a benchmark by using lowenergy electron diﬀraction (LEED) and angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation.
We have succeeded in the observation of large in-plane band
dispersion for the F4TCNQ-derived electronic states, which areformed just below the Fermi level (EF) by the strong CT fromthe Au atom to the F4TCNQ molecule. The observed
electronic interaction in the F4TCNQ-Au network satisﬁes
the high hole concentration (n) due to the low hole-injection
barrier and also the high hole mobility (μ) due to the large
band dispersion in the conductivity formula σ = neμ.

wo-dimensional (2D) atomic/molecular networks, e.g.,graphene and supramolecular macrocyclic systems, andtheir layer-by-layer assemblies have attracted much attention forpotential applications in (opto)electronics and energy storage
because of their low-dimensional electron systems andnanoporous structures.1−4 In order to fabricate functional
molecular layers based on the selective assembly of the 2D
network, the ordered structure of molecules, which dominates
anisotropic physical and chemical properties, has to be designedby considering the molecular symmetry and the intermolecularinteraction such as van der Waals, electrostatic, exchange,polarization, and charge-transfer (CT) interactions. Indeed, alarge variety of molecular systems such as metal−organic
frameworks (MOF) and covalent organic frameworks (COF)
have been designed, and their 2D thin-ﬁlm fabrications havebeen developed as a challenging issue.
Here, we shed light on the CT interaction in archetypicalorganic semiconductors. 7,7,8,8-Tetracyanoquinodimethane(TCNQ) and 2,3,5,6-tetraﬂuoro-TCNQ (F4TCNQ) are wellknown as electron acceptor molecules with large electronaﬃnity and have been anticipated for the CT complex incombination with electron donor molecules,5,6 for the p-type
doping of molecules,7−9 and for the control of the interfacial
charge-injection barrier with respect to the highest occupied
molecular orbital (HOMO) and/or the lowest unoccupied
molecular orbital (LUMO).10−15 The cyano group in TCNQ
derivatives plays a crucial role in the strong electron-accepting
ability in the CT interaction and often induces the intra- and/orinter-molecular structural relaxations such as the molecular© XXXX American Chemical Society

Received: September 8, 2017
Accepted: October 18, 2017
Published: October 18, 2017
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Letter

The Journal of Physical Chemistry Lettersthe formation of the F4TCNQ-Au network based on the Auatom segregation from the Au(111) surface by the strong CT
interaction between F4TCNQ and Au.24 As illustrated in Figure
1c, 1-F4TCNQ molecule and 1-Au adatom exist in the obliqueunit cell of the F4TCNQ-Au network, where F4TCNQ
molecules are 2-fold coordinated with Au adatoms.
In addition to the ordered structure, the CT interaction
introduces the signiﬁcant change in the work function (Φ) andthe valence electronic structure. Figure 2a shows the normal-

As mentioned above, the cyano group in TCNQ derivatives
plays a crucial role in the CT interaction. The strong CT
interaction at the F4TCNQ/Au(111) interface is clearly
detected by the high-resolution N 1s photoemission spectra.
As shown in Figure 1a, a neutral N 1s peak (labeled N0)

Figure 1. Chemical interaction and ordered structure at the F4TCNQ/
Au(111) and TCNQ/Au(111) interfaces at 20 K. (a) High-resolution
N 1s photoemission spectra at 20 K obtained by hv = 500 eV at thenormal emission. The chemical structure of F4TCNQ (R = F) and
TCNQ (R = H) is given in the inset. (b) LEED image obtained by theelectron energy of 40 eV. (c) Ordered structure in the real spacedetermined from the LEED and previous STM data.24,26 The
segregated Au adatoms and the bulk-surface Au atoms are indicatedby the yellow and white circles, respectively.

Figure 2. Electronic structure of the 1 ML ﬁlms of F4TCNQ and
TCNQ on Au(111) at T = 20 K. (a) Normal-emission ARPES spectraof the clean Au(111) surface (gray shadow) and the 1 ML ﬁlms on
Au(111) (red curve) measured using hv = 45 eV. Left panel shows thesecondary-electron cutoﬀ. (b) E−k∥ map of the Shockley state. The
white and black dashed lines represent the parabolic ﬁtting to theobserved dispersion for the 1 ML ﬁlm and the clean Au(111) surface,respectively.

appears at the binding energy (Eb) of 398.5 eV for the 1 ML
(monolayer) TCNQ on Au(111), indicating the physisorptive
interaction at the TCNQ/Au(111) interface. On the other
hand, the 1 ML F4TCNQ on Au(111) shows a predominant N
1s peak at Eb = 397.6 eV, which is lower than the N0 peak inenergy and is ascribed to the negatively charged N 1s (labeledN−) due to the CT interaction at the interface. The N− peak isobservable also for the 2 ML F4TCNQ on Au(111) together
with the N0 peak, corresponding well with the concept of theextended space charge region for the F4TCNQ multilayer on
Au(111).25 The strong CT interaction in general contributes tothe ordered structure of the molecules. The LEED images in
Figure 1b exhibit that the F4TCNQ and TCNQ molecules on
Au(111) form the diﬀerent ordered structures to each otherwith the commensurate 15 23 structure for the F4TCNQ/−0.5
Au(111) interface and the incommensurate 3.5
2.5 4.5
structure for the TCNQ/Au(111) interface as shown in Figure
1c, as previously reported.23−26 We found that the LEED imageof the F4TCNQ/Au(111) interface does not show the wellknown 22 × √3 herringbone pattern of the Au(111) surface
just around the (0,0) spot, which is clearly observable at thephysisorbed TCNQ/Au(111) interface. Furthermore, we
observed that the Au 4f7/2 photoemission line shape for the
F4TCNQ/Au(111) interface is slightly broader than that for
the TCNQ/Au(111) interface because of the appearance of the
new peak component at the low Eb side (Figure S1 in the
Supporting Information). The observed Au 4f7/2 photoemission
line shape can be reproduced by considering the surface corelevel shift due to the presence of bulk, surface, and adatom Au
sites. These results support the validity of the present sample
preparation in accordance with the earlier STM observation ofemission ARPES spectra [emission angle (θ) of 0 ∼ ±7°
integrated] measured for the F4TCNQ/Au(111) and TCNQ/
Au(111) interfaces. The secondary-electron cutoﬀ for the
TCNQ/Au(111) interface, shown in the left panel of Figure 2a,
shifts to the higher Eb side from that for the clean Au(111)
surface. This higher-Eb shift indicates the decrease in Φ upon
the adsorption of TCNQ due to the Pauli repulsion (i.e.,
exchange interaction) of surface electrons at the interface (ΔΦ= −0.34 eV). The opposite trend is observed for the F4TCNQ/
Au(111) interface, giving the shift in the secondary-electron
cutoﬀ to the lower Eb side upon adsorption of F4TCNQ, i.e.,
the increase in Φ, due to CT from Au to F4TCNQ (ΔΦ =+0.27 eV), which overwhelms the contribution of the Pauli
repulsion.
At the valence-band region for the F4TCNQ/Au(111)interface (right panel of Figure 2a), the broad F 2p peak
appears at Eb = 10.4 eV, and the CT-induced valence peaks
appear just below EF; Eb = 0.5 eV (labeled CT1) and 1.5 eV(labeled CT2), which can be ascribed to the ﬁlled LUMO due
to CT and the former HOMO, respectively.11,24 Furthermore,
the Shockley-type surface state of Au(111) (labeled S) is almost
quenched at the F4TCNQ/Au(111) interface because of thesigniﬁcant electronic modiﬁcation by F4TCNQ as discussed
later. On the other hand, the TCNQ-derived peak is notobservable just below EF, and the Shockley state is weakly
modiﬁed with the shift to the lower Eb side at the TCNQ/
Au(111) interface. It has been understood that the modiﬁcation

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The Journal of Physical Chemistry Letters

Figure 3. θ-Dependent ARPES spectra along the Γ̅ -K̅ -M̅ and Γ̅ -M̅ directions at T = 20 K: (a) clean Au(111) surface, (b) TCNQ monolayer on
Au(111), and (c) F4TCNQ monolayer on Au(111), shown with 1° step. The nondispersive HOMO-derived peak for TCNQ/Au(111) and thedispersive CT-derived peaks for F4TCNQ/Au(111) are indicated by the red dots.

with θ, which is direct evidence for the delocalization ofmolecular electronic states. As shown in Figure 3c, the CTinduced peaks are getting visibly clearer at the higher θ (k∥)
region, which is the reﬂection of the orbital-speciﬁc spatial
electron distribution in ARPES.22 The CT1 peak appears at Eb= 0.41 eV when θ = 27°. With increasing θ, the CT1 peak shifts
to the higher Eb side and turns back at around θ = 32°. The
inﬂection point is also found at various θ such as 38°.
Moreover, the dispersive behavior for the CT-induced peaks isobservable not only at 20 K but also at 300 K (Figure S2).
The LEED analysis for the F4TCNQ/Au interface using
LEEDpat 4.2,31 shown in Figure 4a, yields two domains of the
F4TCNQ-Au network structure that are rotated with respect toof the Shockley state by adsorbates is derived from therearrangement of surface electron systems by complex
interfacial interactions.27−29 The lateral energy versus wavevector (E−k∥) map of the Shockley state is shown in Figure 2b
as obtained using the relation ofk = [2m0(hv − E b − Φ)]1/2 sin θ /ℏ

where m0 is the free-electron mass. The Shockley state at the
TCNQ/Au(111) interface shows a weak modiﬁcation with theupshift by ΔEb = 170 meV from that at the clean Au(111)
surface, which is in general ascribed to the Pauli repulsion of
surface electron systems. At the F4TCNQ/Au(111) interface,
the bottom tail of the Shockley state is barely observable at Eb =0 eV and k∥ = 0 Å−1; in other words, the large upshift by morethan 465 meV is induced as a result of the signiﬁcant electronic
modiﬁcation due to the strong CT interaction at the interface.
As demonstrated previously,27,28 the observed shift in the
Shockley state enables us to determine the interfacialadsorption energies, only 18.0 meV/Å2 (= 0.106ΔEb) for the
TCNQ/Au(111) interface, suggesting again the weak physisorptive interaction,27 and more than 49.3 meV/Å2 for the
F4TCNQ/Au(111) interface, which involves various interfacial
electronic interactions.
In order to discuss the interfacial electronic interaction in
more detail, the θ-dependent ARPES spectra along the highsymmetry Γ̅ -K̅ -M̅ and Γ̅ -M̅ directions are shown in Figure 3.
The clean Au(111) surface at 20 K shows the well-known
electronic band dispersion for the Shockley state (S) around θ= 0°, sp band around θ = 20°, and 5d band in full-θ region. At
the TCNQ/Au(111) interface at 20 K, the HOMO-derived
peak is observable behind the Au 5d band at the constant Eb
position with θ (Eb = 2.6 eV), and a quite weak signal from thedeeper lying TCNQ-derived electronic state (HOMO−1) canbe seen at Eb = 4.0 eV at θ = 20−30°. The energy separation
between HOMO and HOMO−1 by 1.4 eV corresponds wellwith the electronic structure of the TCNQ thick ﬁlm.30 The
nondispersive HOMO and HOMO−1 peaks indicate thelocalized molecular electronic states at the TCNQ/Au(111)interface, while the CT-induced peaks (CT1 and CT2) at the
F4TCNQ/Au(111) interface at 20 K show a large dispersion

Figure 4. Valence-band dispersion at the F4TCNQ/Au(111) interface.
(a) Simulated LEED pattern, considering the (5, 2/1, 3)
commensurate structure. The white and colored circles represent thediﬀraction spots from the substrate’s and overlayer’s lattices,
respectively. (b) Surface Brillouin zone (SBZ) determined from the
LEED pattern. The black and red/blue hexagons represent thesubstrate’s SBZ and the example of the overlayer’s SBZ, respectively.
(c) Valence-band dispersion at 20 K obtained from the ARPES spectra
along the Γ̅ -M̅ direction. The eye-catching curves for the experimental
dispersion are given as the blue dashed curve.
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Letter

The Journal of Physical Chemistry Letterssigniﬁcant lateral intermolecular interaction in the TCNQ-Mn
network. The metal−atom segregation from the underlying
substrate might be one of promising methods to fabricate
functional molecule−metal coordination networks, where theelectron aﬃnity and the functional group in the molecule play a
crucial role, as demonstrated in the present work. Considering
the large degree of freedom for the design of the 2D network
structure, the present observation for the delocalized CT bandin the F4TCNQ-Au/Au(111) system suggests possible
realization of highly eﬃcient molecular spintronic devices bychoosing organic radicals or magnetic atoms in 2D molecule−metal hybrid networks, under adequate control of the network
structure to delocalize molecular electronic states.
Finally, we brieﬂy discuss the largely dispersive band at kΓM =0∼ ± 0.3 Å−1 (labeled X in Figure 4c), which is also observablein the raw ARPES spectra in Figure 3c as a quite weak signal atθ = 0−10°. If band X originates from a genuine molecularelectronic state of the F4TCNQ-Au network, it should appear
with the same k∥ periodicity for the CT1 and CT2 bands.
Because band X appears only around the Γ̅ point, band X canbe ascribed to the replica of the Au sp band due to the surface
Umklapp scattering by the overlayer’s lattice.38 Further
investigation using the low-hv beam in ARPES is necessary
for more conﬁdent assignment of band X.
In conclusion, the geometric and electronic structures of the
F4TCNQ-Au network, which is spontaneously formed at the
F4TCNQ/Au(111) interface, have been studied by means of
LEED and ARPES with synchrotron radiation in comparison
with the physisorbed TCNQ/Au(111) interface. The highresolution core-level photoemission spectra and the LEED
image of the F4TCNQ/Au(111) interface comprehensively
support the earlier ﬁnding of the F4TCNQ-Au network
formation,24 originating from the Au atom segregation due tothe CT interaction. Through the strong electron-accepting
molecular functionality of F4TCNQ, the CT interaction from
Au to F4TCNQ introduces the ﬁlling of LUMO, and the CTinduced electronic states are of temperature-independent
delocalized character due to the stable adatom-mediatedintermolecular interaction, as revealed from the ARPES
experiments. The resultant ﬁlled-LUMO band has thecharacteristics of (i) the low hole-injection barrier by thethreshold Eb of 0.18 eV and (ii) the large bandwidth by 260 ±10 meV with mh* = 0.46m0 at the Γ̅ point, both of which satisfy
the conductivity formula σ = neμ as the high hole-mobility 2D
molecular layer. All of the above characteristics are notobservable at the TCNQ/Au(111) interface. The present
ﬁnding can be a benchmark for the development of novel
functional molecular materials based on molecule−metal
coordination networks.

the reciprocal unit cell of the 6-fold symmetric Au(111) surface.
The multidomain overlayer structure introduces the intricate
surface Brillouin zone (SBZ) as displayed in Figure 4b;
therefore, it is in principle diﬃcult to characterize the observed
band dispersion for the multidomain ﬁlms precisely. Remarkably, the E−k∥ map of the F4TCNQ/Au(111) interface at 20 K
along the high-symmetry Γ̅ -M̅ direction (kΓM), shown in Figure
4c, reveals the simple dispersion curves with a single
component for the CT-induced states, probably due to theequivalent scanned region in two overlayer’s SBZs as well as thematrix element eﬀect in ARPES. Indeed, the CT1 and CT2 band
dispersion curves get complex if the E−k∥ measurement is not
performed along the high-symmetry directions (Figure S3).
The intensity of CT1 is weaker than that of CT2 at the lowkΓM region (top panel of Figure 4c), and their relative intensity
gets ﬂipped at the higher kΓM region (bottom panel of Figure4c), indicating diﬀerent spatial electron distributions between
the ﬁlled LUMO (CT1) and the former HOMO (CT2). Fromthe line proﬁle analysis for the CT1-band dispersion at kΓM =0−0.6 Å−1, the dispersion center, bandwidth, and dispersion
periodicity are determined as 0.57 ± 0.01 eV, 260 ± 10 meV,and 0.54 ± 0.02 Å−1, respectively. The threshold Eb position for
the CT1 band is determined to be 0.18 eV. The same dispersion
periodicity is determined from the CT2 band dispersion with
the narrower bandwidth of 180 ± 10 meV. These dispersion
curves give the ﬁrst zone boundary at kΓM = 0.27 ± 0.02 Å−1, in
good agreement with the SBZ boundary determined from
LEED as shown in Figure 4b; therefore, the observed
dispersion can be ascribed to the F4TCNQ-Au network
structure and its electronic interaction.
The observed bandwidth for CT1 by 260 ± 10 meV iscomparable to the previously observed lateral band dispersions
using ARPES for ﬂat-lying organic monolayers on noble metal
surfaces, which are categorized as the substrate-mediated band
dispersion, e.g., 250 meV for pentacene on Cu(110),32,33 90
meV for iron and cobalt phthalocyanine (FePc and CoPc) on
Au(110),34 230 meV for perylene-3,4,9,10-tetracarboxylic
dianhydride (PTCDA) on Ag(110),35 >500 meV for PTCDA
on Cu(100),36 150 meV for naphthalene-1,4,5,8-tetracarboxylic
dianhydride (NTCDA) on Ag(110),37 and 200 meV for
NTCDA on Cu(100).37 The strong CT interaction between
F4TCNQ and Au could play a main role in the present large
band dispersion and also introduce the decrease in the holeinjection barrier at the upper branch of the ﬁlled-LUMO band
(CT1). The free-electron-like parabolic ﬁtting to the observed
dispersion around the Γ̅ point enables us to determine the holeeﬀective mass (mh*) as 0.46m0 for CT1 and 0.71m0 for CT2,
both of which are heavier than that for Au sp band (0.27m0)(Figure S4). According to the Bader analysis in the electron
density distribution, 0.9 electrons contribute to the CT
interaction in the F4TCNQ-Au/Au(111) system and 0.29 and0.61 electrons transfer from the Au(111) surface and thesegregated Au adatom, respectively.24 The heavier mh* for CT1
with respect to the Au sp band thus suggests that the observed
dispersion mainly originates not from the “pure” substratemediated intermolecular interaction but from the adatommediated intermolecular interaction, which is invisible at the
TCNQ/Au(111) interface. It was reported that a TCNQ-Mn
network ﬁlm on Ag(001), prepared by the vacuum deposition
for both TCNQ and Mn, does not show the band dispersion
for molecular electronic states, although the CT-derived peaks
are formed just below EF.22 It was inferred that the lateral
distance is too large in the ordered structure to mediate a■

EXPERIMENTAL METHODS
The present ARPES experiment was performed on a highly
brilliant in-vacuum undulator beamline BL6U of the UVSORIII Synchrotron in the Institute for Molecular Science (Okazaki,
Japan). The BL6U covers the photon energy from 40 to 800 eVwith a photon ﬂux of ca. 1011 photons/sec under the top-up
operation. The ARPES endstation at BL6U consists of themeasurement, preparation, and load-lock chambers in UHV.
The clean Au(111) substrate was obtained by repeated cycles
of Ar+ sputtering and the subsequent annealing at 700 K, as
conﬁrmed by LEED with a microchannel-plate (MCP) detector
and by the Shockley state in ARPES. In order to obtain theordered monolayers of F4TCNQ and TCNQ on Au(111), the5369

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thin multilayer sample with the layer thickness of ca. 50 Å washeated at above the multilayer desorption temperature of 380
K, which is slightly lower than the desorption temperature of
the F4TCNQ and TCNQ monolayers on Au(111), as
conﬁrmed by ARPES (Figure S5). The lateral ordered structureof organic layers was conﬁrmed by MCP-LEED.
The ARPES spectra were acquired from the energy versus
angle image by using the MCP detector of a hemisphericalelectron energy analyzer (MB Scientiﬁc A-1). All ARPES
spectra shown in this Letter were measured at 20 K by using
the liquid He cryostat (Janis ST400-UHV) in the six-axes (x, y,z, polar, and azimuth) manipulator. For the secondary-electroncutoﬀ (i.e., work function) measurement in ARPES, thenegative bias of −5.000 V was applied to the sample toobserve photoelectrons close to the zero-kinetic energy. The Eb
position for the valence-band and core-level ARPES spectra wascalibrated by measuring the EF edge and the Au 4f peak of theunderlying Au(111) substrate, respectively.

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ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpclett.7b02390.
Extra experimental data and corresponding discussion;
high-resolution Au 4f7/2 photoemission spectra, temperature dependence of ARPES, azimuthal-angle dependence of the CT-band dispersion, ﬁtting analysis of theCT-band dispersion, and annealing-temperature dependence of ARPES (PDF)■

AUTHOR INFORMATION

Corresponding Author*E-mail: yamane@spring8.or.jp.
ORCID

Hiroyuki Yamane: 0000-0002-8023-7918
Present Address§

H.Y.: RIKEN Spring-8 Center, Sayo, Hyogo 679−5148, Japan.

Notes

The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS
The authors thank the staﬀ of the UVSOR-III Synchrotron for
their kind support in the present ARPES experiment. This work
is supported by Grant-in-Aid for Scientiﬁc Research on
Innovative Areas (π-System Figuration: Control of Electron
and Structural Dynamism for Innovative Functions)
JP17H05167 and Grant-in-Aid for Scientiﬁc Research (C)JP17K05767 from Japan Society for the Promotion of Science(JSPS) and Grant for Basic Science Research Projects from theSumitomo Foundation 150301.

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