Surface Science 630 (2014) 356–360
Contents lists available at ScienceDirect
Surface Science
journal homepage: www.elsevier.com/locate/susc
Mapping spin distributions in electron acceptor molecules adsorbed on
nanostructured graphene by the Kondo effect
Manuela Garnica a,b, Fabián Calleja b, Amadeo L. Vázquez de Parga a,b,⁎, Rodolfo Miranda a,b
a
b
Departamento de Física de la Materia Condensada and IFIMAC, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia), Cantoblanco, 28049 Madrid, Spain
a r t i c l e
i n f o
Available online 1 August 2014
Keywords:
Magnetic molecules
Graphene
LT-STM/STS
Kondo effect
a b s t r a c t
Electron acceptor molecules adsorbed on nanostructured graphene grown on Ru(0001) were investigated by low
temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS). Our experiments reveal a considerable charge transfer from the substrate to the single molecules leading to the partial occupation of the LUMO
of the neutral molecules. The nanostructured graphene modulates the hybridization between the transferred unpaired electron and the ruthenium conduction electrons leading to the appearance of a Kondo effect. Spatially resolved LT-STS allows the high resolution mapping of the spin distribution of the charge transferred and a
characteristic inelastic Kondo features associated to specific vibrational modes.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Being able to detect and visualize the spin distribution at surfaces
with atomic detail allows us to explore a rich variety of magnetic properties. Recent developments in low temperature scanning tunneling
microscopy (LT-STM) and spectroscopy (STS) have permitted the interrogation of spin-related phenomena down to the atomic scale. Among
them, the study of Kondo impurities at surfaces has attracted a widespread interest. The Kondo effect was discovered in the 1930s [1] and
explained initially in the 1960s [2]. In 1964, J. Kondo gave the first explanation for the anomalous increase of the resistivity of some, in principle
pure, bulk materials at low temperatures. His explanations were based
on a spin-flip scattering process due to the presence of magnetic impurities in the metallic samples. In late 1990s, several groups performed
the first experiments where single magnetic adatoms were deposited
on non-magnetic surfaces, e.g., Ce on Ag(111) [3], Co on Au(111) [4]
or Mn on Al2O3 [5]. To explain the behavior of these systems, the theoretical model by P.W. Anderson in 1961 for a single magnetic impurity
diluted in a metal can be applied [6,7]. The model considers the hybridization between the conduction electrons of the metal and the unpaired
electron state in the impurity, dismissing all the others electronic states
of the impurity. In this situation, the electrons from the STM tip have
two possible channels to tunnel into the sample. They can tunnel directly into the empty states of the sample or indirectly via a spin-flip process
into the localized impurity state. Therefore, when Kondo screening
exists, the STS spectrum presents a resonance at εF spatially localized
at the magnetic impurity. This resonance is the result of the quantum
⁎ Corresponding author.
E-mail address: [email protected] (A.L. Vázquez de Parga).
http://dx.doi.org/10.1016/j.susc.2014.07.028
0039-6028/© 2014 Elsevier B.V. All rights reserved.
interference of the discrete state of the impurity and the continuum of
the conduction electrons in the metal and can be described by the
Fano equation [8], as was demonstrated by V. Madhavan et al. [4]. In
this context, the tunneling conductance is given by: dI/dV ∝ (ε + q) /
(1 + ε) where q is the so-called factor form, ε = (eV − ε0) / Γ is a reduced energy, ε0 is the energy shift of the resonance from the Fermi
level and Γ is the half-width at half-maximum of the resonance given
by [9,10]: 2Γ = [(αkBT)2 + (2kBTK)2]1/2. Then, the half-width at halfmaximum of the Kondo resonance presents a dependence of the temperature and defines the characteristic Kondo temperature, Tk. The
Kondo temperature reflects the hybridization between the magnetic
impurity and the substrate and, in general, due to the reduced coupling
strength between the impurity and the conduction electrons on a metallic surface, Tk is usually lower for a magnetic impurity deposited on
a surface than for one embedded in the bulk.
In this paper we demonstrate that STS permits the high-resolution
mapping of the spin distribution of the charge transferred from a
graphene substrate into adsorbed, isolated electron-acceptor molecules.
We illustrate this principle using two related electron-acceptor
molecules: namely 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and
2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane (F4-TCNQ)
adsorbed on graphene grown on Ru(0001). The molecules are nonmagnetic in the gas phase, but, upon adsorption, they accept charge
from graphene, creating an unpaired electron ground state [11] and developing magnetic moments. The decoupling of the molecular orbitals
from the electron sea of the metallic substrate facilitated by the
graphene monolayer allows us to image the spatial distribution of the
Kondo resonance and the corresponding spin distribution with much
higher resolution than in the case of direct molecular adsorption on
metals [12,13]. The spatial maps of the Kondo resonances obtained by
local Scanning Tunneling Spectroscopy (STS) at low temperature are
M. Garnica et al. / Surface Science 630 (2014) 356–360
compared to the charge distribution and show that the spin distribution
might be delocalized all over a given molecular orbital or localized in
certain bonds depending on the details of the charge being transferred
from the graphene substrate to the molecule. The singly charged molecules also show characteristic inelastic Kondo features associated to
specific vibrational modes.
2. Methods
The experiments were done in an ultra high vacuum (UHV) chamber
with a base pressure of 1 × 10−10 mbar, equipped with standard
facilities for surface preparation, low energy electron diffraction, Auger
spectroscopy and a low temperature scanning tunneling microscope
(LT-STM) working at 4.6 K. The substrate is a single crystal of ruthenium
exposing the (0001) surface, which was cleaned in UHV by ion
sputtering and annealing to 1400 K. Following this treatment, the Ru surface was exposed to an oxygen partial pressure of 6 × 10−7 mbar for
2 min (32 L) and flashed at 1400 K to remove the oxygen from the surface producing an atomically clean and ordered surface. The graphene
samples are then grown by thermal decomposition of ethylene. The Ru
crystal, kept at 1150 K in UHV, was exposed to a partial pressure of ethylene of 1 × 10−8 mbar for 10 min (48 L). The TCNQ and F4-TCNQ molecules were evaporated at 353 K from the evaporator over the
graphene/Ru(0001) sample held at room temperature. High resolution
STM images were acquired in a constant-current mode at 4.6 K and all
the data were analyzed with WsXM software [14].
357
3. Results and discussion
Graphene grown on metallic substrates is an efficient way to
produce large graphene samples with a high structural quality; however, the properties of the graphene layer can be strongly modified depending the metallic substrate used. This is the case of graphene
grown on Ru(0001) where the registry between carbon and ruthenium
atoms changes, going from no interaction in the high areas to strong interaction in the low ones producing the appearance of a moiré superstructure [15]. This modulation in the chemical bonding between
graphene and ruthenium within the moiré unit cell modifies the surface
dipole that has a direct influence on the surface potential which is
0.25 eV lower in the low areas of the moiré pattern [16,17]. In-situ
STM imaging of graphene monolayers on Ru(0001) reveals an ordered
triangular array of bumps with a periodicity of almost 3 nm [18–21].
Fig. 1a and b show representative STM images recorded at 4.6 K after
the deposition of TCNQ and F4-TCNQ molecules on graphene/
Ru(0001) at room temperature. At this particular bias voltage, one can
distinguish both the characteristics protrusions of the moiré superstructure and the molecules. At 300 K the surface diffusion of molecules is
large and it is necessary to cool the sample down to 4.6 K and use low
tunneling currents (below 40 pA) for the STM/STS measurements to
image individual molecules. In both cases, the molecules lie planar
and present a spatial selectivity in the adsorption. As it was previously
studied for other molecules in the same substrate [22–26], this behavior
has its origin in the nanostructured graphene surface which forces the
molecules to adsorb in the lower areas of moiré pattern, where the
Fig. 1. STM images of early stages coverage of (a) TCNQ (10 × 10 nm2, Vb = −1 V and It = 50 pA) and (b) F4-TCNQ (10 × 10 nm2, Vb = −1 V and It = 10 pA at 4.7 K. (c) and (d) STS data
measured on graphene (red curve) and on a molecule (black curve) of the corresponding images (a) and (b). Three features, two at negative bias voltage (occupied states) and one at
positive bias voltages (empty states) marked with black arrows, not present in the curve measured on graphene can be observed. The bottom panels of (a) and (b) show the chemical
structure (white = hydrogen atoms, cyan = carbon atoms, dark blue = nitrogen atoms, yellow = fluorine atoms) and calculated LUMO for the neutral gas-phase TCNQ and F4-TCNQ
molecules respectively.
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M. Garnica et al. / Surface Science 630 (2014) 356–360
lower surface potential allows an effective charge transfer and a stronger interaction between the molecules and the substrate in these areas.
Taking advantage of the high intramolecular resolution facilitated by
decoupling from the electron sea of the Ru substrate provided by the
graphene layer, one can easily distinguish the molecules exclusively
on the basis of the frontier molecular orbital shape. In particular, the topographic images were recorded at − 1 V, i.e. imaging the occupied
states, and the intramolecular resolution of TCNQ and F4-TCNQ resembles the lowest unoccupied molecular orbital (LUMO) of the neutral
molecules in gas-phase (see Fig. 1a and b). The observation of this orbital below the Fermi level is a clear indication of the effectiveness of the
charge transfer from graphene to the isolated molecules which lead to
the partial occupation of the LUMO of the neutral molecule [11]. A
close inspection of the Fig. 1a also reveals the coexistence of isolated
TCNQ molecules with dimers and chains of TCNQ molecules. Previous
studies show that the origin of this interaction can be understood by a
combination of hydrogen bonds and delocalization of charge across a
new spatially-extended intermolecular orbital [26]. On the other hand,
for F4-TCNQ the presence of the fluorine atoms prevents the formation
of hydrogen bonds, and, accordingly, the molecules appear always
isolated.
A definite experimental evidence of the charge transfer between the
molecules and graphene is given by recording the spatially resolved STS
in single molecules; see Fig. 1c and d. In both cases, the spectrum taken
on the graphene surface (red curve) is essentially featureless when
measured using the low tunneling currents needed to simultaneously
image the individual molecules. It only presents the already known
asymmetry between the occupied and empty density of states when
moving from the ripples to the low areas [19]. On the contrary, the spectrum measured on the molecules (black curve) shows three clear features marked with black vertical arrows. For negative bias voltages,
there are two peaks at − 2.0 V and − 0.86 V and for positive bias
voltages; it presents higher intensity in the differential tunneling conductance around +1.0 V than the spectra measured in graphene. As it
was previously discussed for TCNQ [26], due to the transfer of one
electron to the F4-TCNQ molecules from the graphene/Ru(0001) surface
to the LUMO of the neutral molecule, this orbital splits in two: single occupied molecular orbital (somo) and single Unoccupied Molecular
Orbital (SUMO). Therefore, the peaks of STS spectra can be identified
with the resonance through the highest occupied molecular orbital
(HOMO) (−2.0 eV), SOMO (−0.86 eV) and SUMO (1 eV).
The localization of this unpaired electron in the SOMO gives rise to
the development of a magnetic moment in the molecules of the order
of 0.4 μB [26]. As mentioned in the introduction, an experimental proof
of the presence of a magnetic moment in a single molecule or adatom
adsorbed on a non-magnetic metallic surface is the appearance of a
Kondo resonance. Fig. 2 shows the STS experiments performed on a
single TCNQ molecule adsorbed on graphene/Ru(0001) close to the
Fermi level. The dI/dV spectra measured with the STM tip held on different locations in the TCNQ molecule show a prominent peak which is absent in the graphene surface (Fig. 2c). We identify it as a Kondo
resonance due to the interplay between the conduction electrons of
graphene/Ru(0001) and the magnetic moment associated with the
TCNQ molecules.
The inset of Fig. 2c shows the spatial map of the Kondo resonance.
First principle calculations [26] indicate that the unpaired electron
added to TCNQ, as well as the calculated spin distribution, is delocalized
all over the SOMO orbital. The map of the Kondo resonance mirrors indeed the shape of the orbital (and the topographic image of adsorbed
TCNQ). Moreover, the intensity of the Kondo peak exhibits a strong dependence on the STM tip location within the TCNQ molecules. The blue
and red spectra were fitted to the Fano expression. The resulting fitting
Fig. 2. Inelastic Kondo effect. (a) Topographic STM image (2.4 × 2.4 nm2, Vb = −0.1 V, It = 30 pA) of a TCNQ molecule on graphene/Ru(0001) with a TCNQ model superimposed. The color
circles indicate the spatial location where the spectra shown in (c) have been recorded. (b) dI/dV map at Vb = −45 mV acquired simultaneously with (a). A TCNQ model is also
superimposed. (c) dI/dV spectra measured with the STM tip held on graphene and on different locations within the TCNQ molecule where Kondo resonance is observed (Vmod = 8.5 mV
RMS and ν = 703 Hz). The solid lines represent the Fano fitting to the experimental data (Blue: q = 40, ε0 = 1.2 mV and Γ = 5 ± 1 mV. Red: q = 10, ε0 = 1.2 mV and Γ = 5 ± 1 mV).
The black arrows indicate the inelastic Kondo features. The inset shows a Kondo maps (dI/dV map at Vb = 1 mV) (adapted from [26]).
M. Garnica et al. / Surface Science 630 (2014) 356–360
parameters are q = 40, ε0 = 1.2 meV and Γ = 5 ± 1 mV for the blue
curve and q = 10, ε0 = 1.2 meV and Γ = 5 ± 1 mV for the red one.
These large values of q are expected for molecules weakly interacting
with the substrate. The experiments have been performed at 4.6 K and
therefore, we expect to be in the T«TK regime. In this situation one can
obtain an estimation of the Kondo temperature from the width of the
peak (TK = Γ / kB). The estimated Kondo temperature is 58 K which is
on the range expected for a weakly interacting molecule. Nevertheless,
it should be mentioned that a complete analysis of the width of the resonance with the temperature would be necessary to obtain an accurate
value of TK. However, for single molecules, these experiments cannot
easily be performed due to the high mobility of the isolated TCNQ molecules when the temperature increases.
The spectrum measured with the STM tip held above the center of
the molecule (green spectra of Fig. 2c) shows that the Kondo resonance almost disappears due to the node in the wavefunction of
the SOMO orbital, while two side bands around ± 45 mV come up.
The origin of these peaks is an interplane molecular vibration of the
central ring of the TCNQ molecule [27,28], which is strongly coupled
to the SOMO. The dI/dV map of Fig. 2b shows the spatial distribution
of the peak at − 45 mV. The maximum of the signal corresponds exactly to the area of the TCNQ molecule around the central ring. The
inelastic Kondo effect was predicted in quantum dots coupled to vibrational modes [29] and has also been observed in electron transport studies through C 60 molecule in the Kondo regime, using a
mechanically controllable break junction [30]. For a bias voltage
higher than a quantum vibrational mode of the molecule, a new
tunneling channel is opened. Then, an electron can tunnel through
the molecule, exciting its internal states and decaying in the impurity ground state. The result is the excitation of the vibrational mode of
the molecule in addition to a spin-flip process. The effect in the DOS
is the appearance of two symmetric peaks at the vibrational mode
359
energy superimposed to the characteristic steps of the ordinary inelastic tunneling processes. Similar results were reported for TCNQTFF layers on Au(111) [31]. In this case, one electron is donated
from TTF to TCNQ giving rise to a Kondo effect due to the interaction
between the unpaired electron from TCNQ and the conduction electrons of the Au substrate.
F4-TCNQ is a slightly stronger acceptor than TCNQ in which the hydrogen atoms of the periphery are substituted by fluorine. Adsorption
on graphene/Ru(0001) takes place in the same low region of the
moiré as for TCNQ. Upon adsorption, F4-TCNQ accepts almost exactly
one electron per molecule from the graphene substrate, as demonstrated by the spectrum in Fig. 1d. The added charge, however, is not distributed homogeneously over the molecule, but rather concentrated in its
dicyanomethylene (C(CN2)) terminations. Fig. 3a reproduces the
topographic image of an isolated adsorbed molecule superimposed
with a scheme of the molecule. Fig. 3c shows the corresponding differential conductance spectra at different locations of F4-TCNQ molecules
adsorbed on graphene/Ru(0001) at low coverage. In this case, a sharp
resonance appears at the Fermi level in the dicyanomethylene
(C(CN2)) terminations of the molecules. This peak can be fitted perfectly
to a Fano line shape (q = −8, ε0 = 2 meV and Γ = 6 ± 1 mV) leading to
a TK = 69.6 K. This Kondo temperature is similar to TK for individual
TCNQ on graphene/Ru(0001) indicating a similar coupling between
the molecules and graphene in both cases. The inset in Fig. 3c reproduces the spatial map of the Kondo resonance which now is strongly localized in the C(CN2) ends and looks rather different from the orbital
shape and the topographic image shown in Fig. 3a. Thus, the localized
character of the charge transfer results in a localized spin distribution.
As in the case of TCNQ, when the tip is located on the center of the
F4-TCNQ molecules, the Kondo resonance is gone and two symmetric
bands appear at ± 50 mV. We attribute these peaks to an inelastic
Kondo effect due to an interplane molecular vibration of the central
Fig. 3. Inelastic Kondo effect. (a) Topographic STM image (2.05 × 2.05 nm2, Vb = −0.05 V, It = 10 pA) of a F4-TCNQ molecule on graphene/Ru(0001) with a F4-TCNQ model superimposed.
The color circles indicate the spatial location where the spectra shown in (c) have been recorded. (b) dI/dV map at Vb = −50 mV acquired simultaneously with (a). A F4-TCNQ model is
also superimposed. (c) dI/dV spectra measured with the STM tip held on graphene and on different locations within the F4-TCNQ molecule (Vmod = 8.5 mV RMS and ν = 703 Hz). The solid
line represent the Fano fitting to the experimental data (q = −8, ε0 = 2 mV and Γ = 6 ± 1 mV). The inset shows a Kondo maps (dI/dV map at Vb = 1 mV).
360
M. Garnica et al. / Surface Science 630 (2014) 356–360
ring of the molecule. The substitution of hydrogen atoms by heavier
fluorine atoms modifies the vibrational assignment of the in-plane normal modes of the molecule and the energy associated to these inelastic
peaks is slightly higher than in the case of TCNQ molecules on graphene/
Ru(0001). The vibrational features are also localized on the center of the
F4-TCNQ molecules as can be seen in the Fig. 3b; however, they correspond to a different vibrational mode of the radical anion [32].
4. Conclusions
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
In summary, we demonstrate that mapping the spin distribution in
singly-charged, purely organic molecules adsorbed on graphene/
Ru(0001) can be done by imaging the spatial distribution of the
Kondo resonance by dI/dV maps obtained with LT-STS. The nanostructured graphene substrate acts as a decoupling layer which also allows
the charge transfer between the surface and the molecules, converting,
thus, the molecules into isolated magnetic impurities on top of a nonmagnetic metallic surface. Besides, the molecules exhibit two symmetric side peaks due to the convolution of the Kondo resonance with the
step-like features characteristic of an inelastic tunneling process. The
mapping of the inelastic Kondo features reveals the spatial origin of
these molecular vibrational modes.
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
Acknowledgments
Financial support from projects CONSOLIDER-INGENIO 2010C-0725200 on Molecular Nanoscience, FIS-2010-18847 and Comunidad de
Madrid through the program NANOBIO-MAGNET S2009/MAT1726 is
gratefully acknowledged.
[24]
[25]
[26]
[27]
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