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Cite this: DOI: 10.1039/d1nr08227b

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Switchable molecular functionalization of an STM
tip: from a Yu–Shiba–Rusinov Tip to a Kondo tip†
Cosme G. Ayani, a,b Fabian Calleja,*b Ivan M. Ibarburu,a,b Pablo Casado Aguilar,a,b
Nana K. M. Nazriq,c Toyo K. Yamada, c,d Manuela Garnica, b,f
Amadeo L. Vázquez de Pargaa,b,e,f and Rodolfo Mirandaa,b,e,f
In this work we fabricate and characterize a functionalized superconducting (SC) Nb tip of a scanning tunnelling microscope (STM). The tip is functionalized with a Tetracyanoquinodimethane molecule (TCNQ)
that accepts charge from the tip and develops a magnetic moment. As a consequence, in scanning tunnelling spectroscopy (STS), sharp, bias symmetric sub-gap states identiﬁed as Yu–Shiba–Rusinov (YSR)
bound states appear against the featureless density of states of a metallic graphene on Ir(111) sample.
Although the coupling regime of the magnetic impurity with the SC tip depends on the initial absorption
conﬁguration of the molecule, the interaction strength between the SC tip and the charged TCNQ mole-

Received 14th December 2021,
Accepted 29th September 2022

cule can be reversibly controlled by tuning the tip–sample distance. The controlled transition from one
coupling regime to the other allows us to verify the relation between the energy scales of the two com-

DOI: 10.1039/d1nr08227b

peting many-body eﬀects for the functionalized tip. Quenching the SC state of the Nb tip with a magnetic

rsc.li/nanoscaleﬁeld switches abruptly from a tip dominated by the YSR bound states to a Kondo tip.

Introduction
One of the main drawbacks of STM and STS is often the lackof detailed knowledge of the atomic and electronic structure of
the tip of the microscope. In this sense, STM tips were firstfunctionalized with bulk materials in order to increase theenergy resolution or to open the possibility to investigate themagnetic properties of surfaces.1–5 For magnetic measurements, it is crucial to control precisely the exact shape, “chemical identity” and magnetic state of the last few atoms at theapex of the tip and, accordingly, a controlled, well-characterized functionalization of the tip is of paramount importance.
This is one of the reasons why in recent years a great eﬀort has
been devoted to functionalizing STM tips with either nanoparticles, nanostructures or even molecules and adatoms. For
example, CO-functionalized tips have increased the spatiala

Departamento de Física de la Materia Condensada, Universidad Autónoma de
Madrid, Cantoblanco 28049, Madrid, Spainb
IMDEA-Nanociencia, Calle Faraday 9, Cantoblanco 28049, Madrid, Spain.
E-mail: fabian.calleja@imdea.org
c
Department of Materials Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba
263-8522, Japand
Molecular Chirality Research center, Chiba University, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japane
Condensed Matter Physics Center (IFIMAC), Cantoblanco 28049, Madrid, SpainfInstituto ‘Nicolás Cabrera’, Universidad Autónoma de Madrid, 28049 Madrid, Spain† Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d1nr08227b

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resolution in both STM and non-contact atomic force
microscopy (AFM),6 reversible functionalization of standard W
tips with a nanosized superconducting cluster at the apex has
increased the energy resolution,7 and functionalizing either ametallic tip with a magnetic molecule8 or a superconducting
tip with a magnetic atom9–12 has provided new ways to probe
locally the exchange field through the splitting of the Kondo
resonance8 or to quantify the spin polarization of magnetic
samples through the changes in the intensity of the 100% spinpolarized YSR sub-gap states.9 The latter two alternative
approaches to functionalize the tip, that rely on the development of either a Kondo resonance8,13–20 or sub-gap YSR
states,9–12,21–29 respectively, could be combined in a switchable
single probe, as we describe in the present article.
The Kondo eﬀect8,13–20,30–34 predicts a many body groundstate below a certain temperature, TK, in which the spin of themagnetic impurity in a metal is screened by the antiferromagnetic interaction between the impurity spin and the conduction electrons of the metal host.32–34 The Kondo eﬀect has
been extensively investigated by depositing on metal surfaces,
magnetic adatoms,13 molecular radicals16 (either organic or inorganic) or even non-magnetic molecules.15 The hallmark of
the Kondo eﬀect in the local density of states (LDOS) is a zerobias resonance (ZBR) with linewidth proportional to 2kBTK atzero temperature produced by the spin flip processes between
the conduction electrons and the magnetic impurity.16,33 ThisZBR can be directly probed by STS and, as Fano showed, theresulting STS spectrum can be explained as the interference

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Nanoscaleproduced by the two available tunnel paths for a tunnellingelectron, one directly into the sample and the other indirectly
via the ZBR.14,35
Magnetic impurities on superconductors were theoretically
predicted in the 1960s and first observed with STM in 1997,24
the impurities produce an exchange scattering potential on
the surface of the superconductor, which misorients the spinsin the Cooper pairs. In the limit of large impurity density, thelocalized spins of the impurities produce a gradual reductionof the superconducting gap with concentration. However, inthe dilute limit, the pair-breaking potential produces boundstates within the gap and localized at the impurity sites, called
YSR bound states.27
In the case of a spin 12 impurity, the YSR bound states giverise to a pair of sub-gap resonances symmetric in energy withrespect to the Fermi level, easily detected by STS.25,36 The subgap resonances reflect a quasiparticle excitation from theground state to the first excited state via the particle or thehole component of the YSR states. The energy position of the
sub-gap YSR resonances, EBS, depends on the strength of theexchange interaction of the magnetic impurity, J:
EBS ¼Δ1  α21 þ α2

Where α = πρJS/2, with ρ being the density of states at the Fermi
level in the normal state, J the magnetic exchange and S thespin of the magnetic impurity. For weak coupling, i.e. negligible
J (α ≈ 0), EBS ≈ Δ and positive, while for strong coupling, large J,
EBS ≈ −Δ (negative). Notice that there is a quantum phase transition at α2 = 1, where EBS = 0. The spectral weight of the particleand hole-like excitation is influenced by the Coulomb potential(which breaks particle hole symmetry), and by asymmetries in
the normal state conductance of the superconductor, althoughthese contributions will be neglected in the following.27
The strength of the exchange interaction determines inwhich of the two possible magnetic ground states the system isfound: (i) If the screening energy scale (kBTK) overcomes thepairing energy of the Cooper pairs (Δ), the impurity spin iscompletely Kondo screened, the ground state is a singlet (S =0) and a Kondo resonance is expected to be found outside theexcitation gap and the system is said to be in the strong coupling regime;24 (ii) On the contrary, if kBTK < Δ, the screening of
the local spin is incomplete, the ground state has s = 1/2 andthe system is said to be in the free spin case or weak coupling
regime.26,37,38 Finally, a quantum phase (QP) transition from
one to the other ground state occurs when kBTK ∼ Δ, and it isobserved in tunnelling spectroscopy by the inversion of therelative spectral weight of the electron and hole components ofthe bound state as they cross the Fermi level.24,26,36,37
As already stated these two many body eﬀects, the Kondo
resonance and the YSR states, result from the same exchange
coupling strength, J, and, in fact, their energies are connectedby a universal relation.30,31 In principle, one should expect toobserve the transition from one eﬀect to the other by quenching the superconducting state of the host either by means oftemperature or of magnetic field. In this way, the competition

Nanoscalebetween both phenomena may be studied25–29 corroborating
for diﬀerent systems the predicted relation between theirenergy scales.30,31
In this work, we manipulate vertically a purely organic and,initially, non-magnetic Tetracyanoquinodimethane molecule(TCNQ) from a graphene/Ir(111) substrate onto a superconducting Nb STM tip. Upon adsorption on the STM tip, themolecule charges and develops a magnetic moment, which
produces two bias-symmetric intra gap states in STS that areassigned to a pair of YSR bound states. The initial interaction
strength between the magnetic impurity and the SC tip isdetermined by the adsorption configuration of the molecule
onto the tip apex. We are able, however, to change continuously the strength of the exchange interaction, J, by controllingthe distance between the functionalized-tip and the sample.
This allows us to explore the quantum phase transition
between two diﬀerent magnetic ground states (free spin andKondo screened, respectively) related to the weak (kBTK < Δ)
and strong (kBTK > Δ) coupling of the magnetic impurity to theSC tip. We also verify the relation between the energy scale ofthe two many body eﬀects, EBS (energy position of larger
bound state) and kBTK, respectively, showing that the results
obtained with our functionalized tips are consistent with theuniversal relation predicted by Matsuura30,31 that connects therelative strength of both spin-dependent scattering processes.
Finally, we demonstrate the in situ reversible switching from a
YSR tip to a Kondo tip by quenching the superconductivity of
the Nb tip with an external magnetic field.

Results & discussion
The SC tips used to perform these experiments were produced
by electrochemically etching a Nb wire (see ESI†). The resulting
STM tips have an average gap at 1.2 K of Δ = 1.05 mV,
Fig. S1(a),† and are fully capable of attaining atomic resolution
as shown in Fig. S1(b)† where both the atomic periodicity andthe standing waves of Cu (111) are resolved simultaneously.
The tips have been characterized by their temperature andmagnetic field dependence yielding a critical temperature andcritical magnetic field of 8.4 K and 1.2 T (Fig. S1(c) and (d)†),respectively. These values diﬀer from their bulk counterparts,
especially in the case of the critical magnetic field due to tip
size and geometry.38–40
TCNQ molecules were deposited onto graphene/Ir(111)(hereafter called for short gr/Ir). The two distinctive characteristics of gr/Ir are the preservation of the Dirac cone of graphene with a slight p-doping that leaves the Fermi levelaround 110 meV below the Dirac point,41,42–44 and the incommensurate (9.3 × 9.3) moiré pattern produced by the mismatch
between the two atomic lattices.45 Once the molecules areabsorbed, as reported previously, the graphene sheet eﬀectively decouples the TCNQ molecules from the metal
underneath.46,47 The presence of the gr/Ir moiré pattern does
not aﬀect the molecule organization, as they show no preferential absorption sites within the moiré unit cell. The molecules

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Nanoscaleform close packed, well-organized islands which are stabilizedby hydrogen bonds between the neighbouring molecules andare bonded to the surface by π–π* interaction.48 Althoughthese molecules are well-known electron acceptors,48,49 STS
measurements reveal a negligible charge transfer from the gr/
Ir substrate to the molecules, consistent with the above mentioned slight p-doping of the graphene layer. Finally, the molecular coverage of the surface is kept intentionally below themonolayer, as shown in Fig. 1(a), for two reasons. First, tofacilitate the vertical manipulation of the molecular species
onto the apex of the SC Nb tip. Second, to use the patches ofgr/Ir not covered with TCNQ molecules to perform all STS
measurements, as explained in the following.
In order to manipulate vertically a molecule onto the tip
apex and functionalize the Nb tip, the edge of the molecularislands was scanned at high speed between 75 and 110 nm s−1
with currents above 1 nA while the sample was kept at 1.2 K. In
these conditions, with our typical feedback loop bandwidth ofthe order of 200 Hz, the interaction between the tip and themolecules at the edge of the island is large enough to produce
the tip functionalization. When a change in relative tip–
sample distance is observed, the scanning is stopped and thetip is moved to a TCNQ-free area of the gr/Ir substrate, where
STS is performed (see section S2 in ESI†).
While the dI/dV spectrum recorded with the clean tip onlyshows its superconducting gap, the molecular functionali-

Paperzation of the tip results in the appearance of sub-gap states(Fig. 1(b)). The process is reversible. If the functionalized tip ispulsed at +2 V on the clean gr/Ir patches (bias voltage applied
to the sample), the TCNQ molecule is released from the tip
apex and the SC gap is recovered in the tunnelling spectrum(see Fig. S2(a) and (b)†). The manipulation procedure is completely reproducible: every tip successfully functionalized witha TCNQ molecule shows sub-gap states. Such states may varyslightly in relative intensity or position inside the SC gap, but
all of them can be classified into the three types of spectra
depicted in Fig. 1(c). All spectra are taken against clean, metallic gr/Ir surfaces with a constant LDOS in the region exploredwhich, accordingly, do not contribute to the recorded spectra.
In Fig. 1(c), we show the STS spectra of diﬀerent TCNQfunctionalized tips. They resemble the shape expected for the
YSR bound states due to a spin 12 impurity on a superconducting substrate, with asymmetric intensities for the electron-likeand hole-like resonances at positive and negative voltages,
respectively. As it will be confirmed by the results presented
further up on the article, we can make the following assignments: the blue spectrum corresponds to the STS of a magnetic impurity weakly coupled to a superconductor, with themagnetic impurity in a free spin ground state (S = 1/2), thesuperconductor ground state fully paired and the sub-gap state
unoccupied, (EBS > 0).25–29,40 The red spectrum corresponds tothe one of a magnetic impurity in the strong coupling (or com-

Fig. 1 (a) STM topographic image (V = 2 V, I = 0.1 nA, T = 1.2 K) showing the surface of gr/Ir(111) partially covered with TCNQ molecules. The moiré
pattern periodicity of the substrate (blue dashed rhombus) can be resolved through the TCNQ layer. The molecular unit cell is indicated by thegreen dotted line. (b) Diﬀerential conductance spectra recorded on a clean area of gr/Ir before (black) and after (violet) manipulation of a TCNQ
molecule onto the apex of the Nb STM tip. Stabilization parameters V = 15 mV, I = 0.5 nA, Vmod = 150 µV and T = 1.2 K. (c) dI/dV spectra of three
representative TCNQ-functionalized tips. As indicated by the colour code at the right of the image and the asymmetry of the peaks, they represent
the two diﬀerent coupling regimes and the QP transition discussed in the text. STS parameters: V = 15 mV, I = 0.5 nA, Vmod = 100 µV and T = 1.2 K.

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pletely Kondo screened) regime, with the superconductor
ground state involving an unpaired electron screening themagnetic impurity (S = 0) and the bound state occupied (EBS <0).25–29,40 The spectrum in the center ( purple) with a peak atzero energy would correspond to a magnetic impurity in the
QP transition between the two coupling regimes25–29,40 that
occur at a critical strength of the exchange coupling, Jcr, where
the ground and excited state reverse their role.
TCNQ is a strong electron acceptor, well known to accept
charge transferred from metal surfaces with low work function49 or even from donor molecular species.50 Although a
TCNQ molecule physisorbed on the surface of gr/Ir shows nocharging of the lowest unoccupied molecular orbital(LUMO),48 we propose that the TCNQ acquires charge from thesuperconducting Nb tip once it is manipulated onto its apex.
Furthermore, it is known that individual TCNQ molecules thathave accepted enough charge from the substrate as to half fill
their LUMO orbital can acquire a substantial net magnetic
moment.51 As a consequence, a Kondo resonance appeared atzero bias in the STS measurements with normal tips.15,50,51
In order to demonstrate that the TCNQ molecule transferred to the Nb tip has acquired a net magnetic moment and,thus, behaves as a magnetic impurity, we quench the SC stateof the Nb tip by applying a magnetic field perpendicular to thesample surface (and therefore parallel to the tip axis). Fig. 2
shows the result for two tips functionalized with TCNQ molecules in diﬀerent coupling regimes. In both cases, when themagnetic field is increased above the critical field of the SC tip(B = 1.5 T), a broader ZBP remains indeed at zero energy.
To prove that these zero bias resonances observed after
quenching the SC state are indeed Kondo resonances, we performed temperature dependent experiments for two functionalized tips whose molecules are in the strong coupling regime
and QP transition, respectively. Fig. S5† shows the dependenceof the intrinsic half width at half maximum (HWHM) withtemperature for the Kondo resonances of the two functionalized tips. In both cases the sets of data follow the expected
temperature dependence for a Kondo resonance in the Fermi
liquid theory with free α fit.14,16–18 The results are in good
agreement with those of the magnetic field experiments as the
Kondo temperature of the molecule in the strong coupling
regime (23.8 K) is higher than that of the molecule in the QP
transition (10.8 K). These results confirm that the moleculecharges up and develops a magnetic moment when verticallymanipulated from the surface of gr/Ir onto the SC Nb tip apex.
In order to quantify the strength of the Kondo coupling, thecorresponding spectra obtained after quenching the superconductivity of the tip with a magnetic field at 1.2 K (Fig. 2) are fittedwith the Fano function taking into account both thermal andmodulation broadenings, and the obtained intrinsic width isthen corrected by subtracting the corresponding Zeeman splitting for the case of a spin 12 magnetic impurity (see Fig. S4 of
ESI†).52 As it may already be observed in Fig. 2, the Kondo resonance produced by the molecule in the strong coupling regime
has a larger intrinsic HWHM (2.9 meV) than the TCNQ moleculein the quantum phase transition (1.1 meV).

NanoscaleNanoscaleFig. 2 Two sets of experiments performed with diﬀerent TCNQ-functionalized Nb tips on bare gr/Ir substrates. The magnetic ﬁeld is ramped
above the critical ﬁeld of the Nb tip (1.2 T) to quench its SC state. The
quenching unveils the Kondo resonance at the Fermi level demonstrating that the TCNQ molecules have acquired a magnetic moment. (a)Evolution in the case of a TCNQ molecule in the strong coupling regime(EBS < 0). (b) In this case the molecule is initially in the quantum phasetransition (EBS ≈ 0). Stabilization parameters are V = 15 mV, I = 0.5 nA,
Vmod = 150 µV and T = 1.2 K.

Likewise, the energy of the magnetically induced bound
YSR states (EBS) can be obtained by fitting the original dI/dV
spectra at B = 0 T with the Cauchy function.36 For a moleculein the strong coupling regime, the larger bound state is below
the Fermi level (EBS < 0) while for a TCNQ molecule in the
weak coupling regime it is above the Fermi level (EBS > 0). For
the same functionalized tips (same molecule and configuration), the energy scale of the Kondo screening channel (kBTK)
can be obtained from the Fano fit of the spectra upon quenching the SC state of the tip. As a one to one correspondence has
been stablished between the position of the YSR bound states
and the linewidth of the Kondo resonance for a same molecule, the competition between both many body eﬀects can bestudied by plotting the energy value of the magneticallyinduced bound states against the energy of the screening
channel (EBS/Δ vs. kBTK/Δ) as shown in Fig. S6†25–29 andcompare it to the universal relation introduced by
Matsuura.30,31
Matsuura’s universal relation describes the competition
between the Kondo screening and the induced YSR boundstates. When a magnetic impurity is placed on a superconductor, the spin of the magnetic impurity becomes underscreened as there is a decrease in the number of itinerant electrons that can participate in the Kondo cloud. He predicted

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Nanoscalethat the weaker the Kondo screening, the more strongly boundwill the sub-gap states should be. If the energy value of the
sub-gap states (EBS) is plotted against the energy of the Kondo
screening channel (kBTK) the data should follow the relation
predicted by Matsuura (green dashed line show on
Fig. S6†).30,31 Therefore, it would be very interesting to develop
a method to modify the coupling regime of the magnetic
impurity, in order to access whole spectrum of the interaction
strength of a magnetic impurity.
We have no control over the coupling regime establishedwhen the molecule is manipulated onto the tip apex, as it
depends on the initial adsorption configuration of the molecule on the tip. Statistical analysis of 15 diﬀerent functionalized tips show that it is more probable that the molecule ends
in the QP transition regime with a 55% chance, followed witha 27% probability of ending in the weak coupling regime stateand only a 18% of ending in the strong coupling regime.
Interestingly, once the Nb tip is functionalized with the TCNQ
molecule, the coupling strength between the magnetic impurity and the STM tip can be modified in a reproducible andreversible manner (see section S7 in the ESI†) by changing thetip-sample distance (Z). It has been proved before36,37,53–56 that
applying a mechanical force on the molecules on SC substrates, i.e. decreasing the tip–sample distance, the interaction
strength between the magnetic moment of molecules and theSC substrates can be modified. In our case, the molecule isattached to the SC tip and the forces applied to the moleculecan be as well modified by changing the tip–sample distance,
similarly as in atomic force spectroscopy experiments performed in non-contact AFM.57–60 The relative tip–sample distance is controlled by applying a well-defined ΔZ oﬀset after
disabling the feedback loop and before the spectrum isrecorded in order to observe the sub-gap states.
Fig. 3(a) shows a representative example of the shift inenergy of the sub-gap states as the functionalized SC Nb tip ismoved closer to the gr/Ir sample. Upon decreasing the tip–
sample distance, the sub-gap states emerge. As the tip
approaches the sample, at −300 pm, the YSR states start toshift towards the Fermi energy (Fig. 3(a)), indicating an
increase in the strength of the coupling between the molecularspin and the SC tip. The relative spectral weight of the boundstates indicates that the impurity is initially in the weak coupling regime (see Fig. 3(b)). Both YSR peaks meet at zero bias
for relative Zoﬀset = −460 pm and remain at this position for atleast another 140 pm. The current versus relative Zoﬀset
acquired with the same functionalized tip for this same distance range is shown in Fig. 3(c). A loss of the initial exponential dependence of the current coinciding with the emergenceof the sub-gap states at −300 pm is observed. Although thequantum of conductance G0 is never reached, this indicatesthat the forces between tip, molecule and substrate play a relevant role here. Note that the total change in distance is 900
pm. For comparison, in atomic force spectroscopy experiments, where the tip–sample distance is normally varied in
the order of hundreds of pm, the frequency shift minimum isoften crossed marking the change of force regime from attrac-

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Fig. 3 Dependence of the STS spectra with the relative tip–sample distance. (a) Normalized dI/dV intensity (green-yellow colour scale) as afunction of bias voltage (horizontal axis) and relative tip–sample distance (vertical axis) for a TCNQ-functionalized tip and a gr/Ir sample.
The relative distance is controlled by applying a certain Z oﬀset (negativevalues correspond to shorter distances) after opening the feedback loop
prior to each STS acquisition, while maintaining the same stabilization
parameters (V = 15 mV, I = 0.5 nA, Vmod = 100 µV) throughout all theprocess. (b) Three representative dI/dV spectra extracted from thestacked plot in (a). (c) I vs. relative Zoﬀset curve acquired with the samefunctionalized tip, covering the same 900 pm distance as in (a). The
dashed green line indicates the quantum of conductance, G0.
Stabilization parameters, V = 15 mV, I = 0.5 nA.

tive to repulsive.56–60 We would like to highlight the fact thatthe process is completely reversible, as demonstrated in panels
A and B of Fig. S7 in the ESI,† where several reversibility tests
are presented.
Fig. 4(a) shows another experiment where the coupling
regime of the magnetic impurity is modified by changing thetip–sample distance with a diﬀerent TCNQ-functionalized Nb
tip. In this case, when the tip–sample distance is reduced, thetransition occurs from a molecule at the quantum phase transition to the strong coupling regime, as the coupling of themagnetic impurity to the superconductor increases. The tip isapproached towards the sample a total distance of 200 pm(Fig. 4(c)), however, in the normalized dI/dV intensity maps
only the section of the experiment where the bound states
split is shown. The two bound states split at ΔZ = −176 pm
and shift away from the Fermi energy for the next 6 pm. The
relative spectral weight of the bound states indicates that the
impurity ends up in the strong coupling regime. As in the pre-NanoscaleView Article Online

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Fig. 4 Dependence of the STS spectra with the relative tip–sample distance. (a) Normalized dI/dV intensity (green-yellow colour scale) as afunction of bias voltage (horizontal axis) and relative tip–sample distance (vertical axis) for a TCNQ-functionalized tip and a gr/Ir sample. In
this case the transition is from a molecule in the quantum phase transition to a strongly coupled molecule. Stabilization parameters throughout all the process (V = 15 mV, I = 500 pA, Vmod = 100 µV). (b) Three
representative dI/dV spectra extracted from the stacked plot in (a). (c)Long range I vs. ΔZ curve acquired with the same functionalized tip, the16 pm z-range shown in (a) is highlighted in blue. The dashed green line
indicates the quantum of conductance, G0. Stabilization parameters, V =15 mV, I = 500 pA.

vious example, the shift occurs well below the quantum of conductance, although a small bump can be observed in thecorresponding region of the I(z) curve shown in panel c. The
reversibility of this process can be checked in panels C and D
of Fig. S7† in the SM. We would like to note that the same
experiments have been carried out with clean, not functionalized, Nb tips as control experiments. The results shown in
Fig. S3 of the ESI,† demonstrate that no sub gap states appear
throughout the experiment in a total length of 1010 pm.
Finally, the possibility to control the interaction strength of amagnetic impurity manipulated onto a SC tip may open a new
way to validate Matsuura’s universal relation,30,31 as in principle the whole spectrum of the interaction strength of a magnetic molecule or adatom could be accessed by manipulating
it onto a SC tip apex and performing spectroscopy experiments
while changing the tip–sample distance (ΔZ) (see section S6 of
ESI† for a more detailed explanation).
In conclusion, we show that by manipulating an initiallyuncharged TCNQ molecule from the surface of gr/Ir to the

NanoscaleNanoscale
apex of a SC Nb tip the molecule charges and develops a magnetic moment. The exchange scattering potential produced byits spin results in the appearance of two symmetric sub-gap
states. When an external magnetic field is applied to quench
the superconductivity of the tip a Kondo resonance is recovered, demonstrating that the molecule has indeed developed amagnetic moment and that the sub-gap states observed are
Yu–Shiba–Rushinov bound states.25,26 The fact that the magnetic impurity is located in the SC tip apex may be used totune continuously the exchange interaction, J, by controllingthe distance between the functionalized-tip and the sample. In
this work we have explored the transition between from theweak coupling regime to the QP transition and from the QP
transition to the strong coupling regime. Also verifying therelation between the energy scale of YSR states, EBS, andKondo screening, kBTK, consistent in this case with the universal relation predicted by Matsuura30,31 that connects the relative strength of both spin-dependent scattering processes.61–66
This experimental approach could be used to enhance thesensitivity to the local spin texture by combining the Kondo9(to probe the magnetic exchange field) and YSR10–13 (to probe
spin polarization) regimes in the same probe, with the added
value of being able to control the interaction strength between
tip and molecule. A variety of magnetic systems can be investigated in this way, from adsorbed atoms and molecules tocrystal surfaces, complementing other well-established spinpolarized scanning probe techniques.

Methods
STM experiments
All experiments have been carried out in a UHV chamber witha base pressure of 5 × 10−11 mbar equipped with a Joule
Thompson STM (JT-STM) and facilities for tip and sample
preparation. The STM is fitted with a superconducting coilthat can provide a magnetic field up to 3 T perpendicular tothe plane of the sample and therefore parallel to the tip axis.
All STM/STS data were measured with tip and sample thermalized at 1.2 K. In the case of our experimental setup the tunnelling voltage is applied always to the sample.
Sample preparation
The Ir(111) crystal was Ar+ sputtered and flash annealed in
vacuum until a (1 × 1) LEED pattern and no traces of contaminants were observed. The graphene sample on Iridium wereprepared via chemical vapour deposition (CVD) by exposing aclean Ir(111) crystal to a partial pressure of ethylene gas C2H4
(Pethylene = 5 × 10−8 mbar) while the sample is held at T =1453 K for 10 minutes. TCNQ molecules are then thermally deposited from a quartz crucible heated to 353 K onto the cleansurface of gr/Ir held at room temperature. The coverage is kept
below the monolayer by evaporating for less than 20 min.
Subsequently, the sample was introduced into the STM andcooled to 1.2 K for inspection.

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Spectra normalization

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All dI/dV spectra shown in the main manuscript and in the
ESI† have been normalized in order to compare the diﬀerent
spectra. The STS spectra has been normalized by dividing theexperimental data by the average value of the 10 first and last
(total of 20 points) diﬀerential conductance values of thecorresponding spectrum. In this way the edges of all spectra
are always in the vicinity of the same dI/dV values.

Conﬂicts of interest
There are no conflicts of interest to declare.

Acknowledgements
Work partially supported by the Ministerio de Ciencia,
Innovación y Universidades projects PGC2018-093291-B-I00,
PGC2018-097028-A-I00, PGC2018-098613-B-C21 and PID2019109525RB-I00 and the Comunidad de Madrid (CM) projects
Nanomag COST-CM, P2018/NMAT-4321 and NMAT2D-CM
P2018/NMT-4511. IMDEA Nanociencia acknowledges support
from the ‘Severo Ochoa’ Programme for Centres of Excellencein R&D (MINECO, Grant SEV-2016-0686). IFIMAC acknowledges financial support from the Spanish Ministry of Scienceand Innovation, through the “María de Maeztu” Programmefor Units of Excellence in R&D (CEX2018-000805-M). M. G.
has received financial support through the “Ramón y Cajal”
Fellowship program (RYC2020-029317-I. T. K. Yamada andN. K. M. Nazriq have also recieved support from JSPS KAKENHI
Grant Numbers 17K19023 and Murata Science Foundation.

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