††thanks: NEXT Co-spokesperson.††thanks: Corresponding Author.††thanks: NEXT
Co-spokesperson.††thanks: Deceased.

#  ${\rm Ba^{2+}}$ ion trapping by organic submonolayer: towards an ultra-low
background neutrinoless double beta decay detector

P. Herrero-Gómez  Centro de Física de Materiales (CSIC-UPV/EHU), San Sebastián
/ Donostia, E-20018, Spain  Donostia International Physics Center DIPC, San
Sebastián / Donostia, E-20018, Spain  J.P. Calupitan  Centro de Física de
Materiales (CSIC-UPV/EHU), San Sebastián / Donostia, E-20018, Spain  M. Ilyn
Centro de Física de Materiales (CSIC-UPV/EHU), San Sebastián / Donostia,
E-20018, Spain  A. Berdonces-Layunta  Centro de Física de Materiales (CSIC-
UPV/EHU), San Sebastián / Donostia, E-20018, Spain  Donostia International
Physics Center DIPC, San Sebastián / Donostia, E-20018, Spain  T. Wang  Centro
de Física de Materiales (CSIC-UPV/EHU), San Sebastián / Donostia, E-20018,
Spain  Donostia International Physics Center DIPC, San Sebastián / Donostia,
E-20018, Spain  D. G. de Oteyza  Centro de Física de Materiales (CSIC-
UPV/EHU), San Sebastián / Donostia, E-20018, Spain  Donostia International
Physics Center DIPC, San Sebastián / Donostia, E-20018, Spain  M. Corso
Centro de Física de Materiales (CSIC-UPV/EHU), San Sebastián / Donostia,
E-20018, Spain  Donostia International Physics Center DIPC, San Sebastián /
Donostia, E-20018, Spain  R. González-Moreno  Donostia International Physics
Center DIPC, San Sebastián / Donostia, E-20018, Spain  I. Rivilla  Donostia
International Physics Center DIPC, San Sebastián / Donostia, E-20018, Spain
Ikerbasque, Basque Foundation for Science, Bilbao, E-48009, Spain  B. Aparicio
Department of Organic Chemistry I, University of the Basque Country (UPV/EHU),
Centro de Innovación en Química Avanzada (ORFEO-CINQA), San Sebastián /
Donostia, E-20018, Spain  A.I. Aranburu  Department of Applied Chemistry,
University of the Basque Country (UPV/EHU), San Sebastián / Donostia, E-20018,
Spain  Z. Freixa  Department of Applied Chemistry, University of the Basque
Country (UPV/EHU), San Sebastián / Donostia, E-20018, Spain  Ikerbasque,
Basque Foundation for Science, Bilbao, E-48009, Spain  F. Monrabal  Donostia
International Physics Center DIPC, San Sebastián / Donostia, E-20018, Spain
Ikerbasque, Basque Foundation for Science, Bilbao, E-48009, Spain  F.P. Cossío
Donostia International Physics Center DIPC, San Sebastián / Donostia, E-20018,
Spain  Department of Organic Chemistry I, University of the Basque Country
(UPV/EHU), Centro de Innovación en Química Avanzada (ORFEO-CINQA), San
Sebastián / Donostia, E-20018, Spain  J.J. Gómez-Cadenas  Donostia
International Physics Center DIPC, San Sebastián / Donostia, E-20018, Spain
Ikerbasque, Basque Foundation for Science, Bilbao, E-48009, Spain  C. Rogero
Centro de Física de Materiales (CSIC-UPV/EHU), San Sebastián / Donostia,
E-20018, Spain  Donostia International Physics Center DIPC, San Sebastián /
Donostia, E-20018, Spain  C. Adams  Argonne National Laboratory, Argonne, IL
60439, USA  H. Almazán  Department of Physics, Harvard University, Cambridge,
MA 02138, USA  V. Álvarez  Instituto de Instrumentación para Imagen Molecular
I3M (CSIC-UPV), Valencia, E-46022, Spain  L. Arazi  Unit of Nuclear
Engineering, Faculty of Engineering Sciences, Ben-Gurion University of the
Negev, Beer-Sheva, 8410501, Israel  I.J. Arnquist  Pacific Northwest National
Laboratory (PNNL), Richland, WA 99352, USA  S. Ayet  II. Physikalisches
Institut, Justus-Liebig-Universitat Giessen, Giessen, Germany  C.D.R. Azevedo
Institute of Nanostructures, Nanomodelling and Nanofabrication (i3N),
Universidade de Aveiro, Campus de Santiago, Aveiro, 3810-193, Portugal  K.
Bailey  Argonne National Laboratory, Argonne, IL 60439, USA  F. Ballester
Instituto de Instrumentación para Imagen Molecular I3M (CSIC-UPV), Valencia,
E-46022, Spain  J.M. Benlloch-Rodríguez  Donostia International Physics Center
DIPC, San Sebastián / Donostia, E-20018, Spain  F.I.G.M. Borges  LIP,
Department of Physics, University of Coimbra, Coimbra, 3004-516, Portugal  S.
Bounasser  Department of Physics, Harvard University, Cambridge, MA 02138, USA
N. Byrnes  Department of Physics, University of Texas at Arlington, Arlington,
TX 76019, USA  S. Cárcel  Instituto de Física Corpuscular (IFIC), CSIC &
Universitat de València, Paterna, E-46980, Spain  J.V. Carrión  Instituto de
Física Corpuscular (IFIC), CSIC & Universitat de València, Paterna, E-46980,
Spain  S. Cebrián  Centro de Astropartículas y Física de Altas Energías
(CAPA), Universidad de Zaragoza, Zaragoza, E-50009, Spain  E. Church  Pacific
Northwest National Laboratory (PNNL), Richland, WA 99352, USA  C.A.N. Conde
LIP, Department of Physics, University of Coimbra, Coimbra, 3004-516, Portugal
T. Contreras  Department of Physics, Harvard University, Cambridge, MA 02138,
USA  A.A. Denisenko  Department of Chemistry and Biochemistry, University of
Texas at Arlington, Arlington, TX 76019, USA  G. Díaz  Instituto Gallego de
Física de Altas Energías, Univ. de Santiago de Compostela, Campus sur,
Santiago de Compostela, E-15782, Spain  J. Díaz  Instituto de Física
Corpuscular (IFIC), CSIC & Universitat de València, Paterna, E-46980, Spain
T. Dickel  II. Physikalisches Institut, Justus-Liebig-Universitat Giessen,
Giessen, Germany  J. Escada  LIP, Department of Physics, University of
Coimbra, Coimbra, 3004-516, Portugal  R. Esteve  Instituto de Instrumentación
para Imagen Molecular I3M (CSIC-UPV), Valencia, E-46022, Spain  A. Fahs
Department of Physics, Harvard University, Cambridge, MA 02138, USA  R. Felkai
Unit of Nuclear Engineering, Faculty of Engineering Sciences, Ben-Gurion
University of the Negev, Beer-Sheva, 8410501, Israel  L.M.P. Fernandes
LIBPhys, Physics Department, University of Coimbra, Coimbra, 3004-516,
Portugal  P. Ferrario  Donostia International Physics Center DIPC, San
Sebastián / Donostia, E-20018, Spain  Ikerbasque, Basque Foundation for
Science, Bilbao, E-48009, Spain  A.L. Ferreira  Institute of Nanostructures,
Nanomodelling and Nanofabrication (i3N), Universidade de Aveiro, Campus de
Santiago, Aveiro, 3810-193, Portugal  F.W. Foss  Department of Chemistry and
Biochemistry, University of Texas at Arlington, Arlington, TX 76019, USA
E.D.C. Freitas  LIBPhys, Physics Department, University of Coimbra, Coimbra,
3004-516, Portugal  J. Generowicz  Donostia International Physics Center DIPC,
San Sebastián / Donostia, E-20018, Spain  A. Goldschmidt  Lawrence Berkeley
National Laboratory (LBNL), Berkeley, CA 94720, USA  D. González-Díaz
Instituto Gallego de Física de Altas Energías, Univ. de Santiago de
Compostela, Campus sur, Santiago de Compostela, E-15782, Spain  R. Guenette
Department of Physics, Harvard University, Cambridge, MA 02138, USA  R.M.
Gutiérrez  Centro de Investigación en Ciencias Básicas y Aplicadas,
Universidad Antonio Nariño, Sede Circunvalar, Bogotá, Colombia  J. Haefner
Department of Physics, Harvard University, Cambridge, MA 02138, USA  K. Hafidi
Argonne National Laboratory, Argonne, IL 60439, USA  J. Hauptman  Department
of Physics and Astronomy, Iowa State University, Ames, IA 50011-3160, USA
C.A.O. Henriques  LIBPhys, Physics Department, University of Coimbra, Coimbra,
3004-516, Portugal  J.A. Hernando Morata  Instituto Gallego de Física de Altas
Energías, Univ. de Santiago de Compostela, Campus sur, Santiago de Compostela,
E-15782, Spain  V. Herrero  Instituto de Instrumentación para Imagen Molecular
I3M (CSIC-UPV), Valencia, E-46022, Spain  J. Ho  Department of Physics,
Harvard University, Cambridge, MA 02138, USA  Y. Ifergan  Unit of Nuclear
Engineering, Faculty of Engineering Sciences, Ben-Gurion University of the
Negev, Beer-Sheva, 8410501, Israel  B.J.P. Jones  Department of Physics,
University of Texas at Arlington, Arlington, TX 76019, USA  M. Kekic
Instituto Gallego de Física de Altas Energías, Univ. de Santiago de
Compostela, Campus sur, Santiago de Compostela, E-15782, Spain  L. Labarga
Departamento de Física Teórica, Universidad Autónoma de Madrid, Campus de
Cantoblanco, Madrid, E-28049, Spain  A. Laing  Department of Physics,
University of Texas at Arlington, Arlington, TX 76019, USA  L. Larizgoitia
Donostia International Physics Center DIPC, San Sebastián / Donostia, E-20018,
Spain  P. Lebrun  Fermi National Accelerator Laboratory, Batavia, IL 60510,
USA  D. Lopez Gutierrez  Department of Physics, Harvard University, Cambridge,
MA 02138, USA  N. López-March  Instituto de Física Corpuscular (IFIC), CSIC &
Universitat de València, Paterna, E-46980, Spain  M. Losada  Centro de
Investigación en Ciencias Básicas y Aplicadas, Universidad Antonio Nariño,
Sede Circunvalar, Bogotá, Colombia  R.D.P. Mano  LIBPhys, Physics Department,
University of Coimbra, Coimbra, 3004-516, Portugal  J. Martín-Albo  Instituto
de Física Corpuscular (IFIC), CSIC & Universitat de València, Paterna,
E-46980, Spain  A. Martínez  Instituto de Física Corpuscular (IFIC), CSIC &
Universitat de València, Paterna, E-46980, Spain  G. Martínez-Lema  Unit of
Nuclear Engineering, Faculty of Engineering Sciences, Ben-Gurion University of
the Negev, Beer-Sheva, 8410501, Israel  M. Martínez-Vara  Donostia
International Physics Center DIPC, San Sebastián / Donostia, E-20018, Spain
Instituto de Física Corpuscular (IFIC), CSIC & Universitat de València,
Paterna, E-46980, Spain  A.D. McDonald  Department of Physics, University of
Texas at Arlington, Arlington, TX 76019, USA  Z.E. Meziani  Argonne National
Laboratory, Argonne, IL 60439, USA  K. Mistry  Department of Physics,
University of Texas at Arlington, Arlington, TX 76019, USA  C.M.B. Monteiro
LIBPhys, Physics Department, University of Coimbra, Coimbra, 3004-516,
Portugal  F.J. Mora  Instituto de Instrumentación para Imagen Molecular I3M
(CSIC-UPV), Valencia, E-46022, Spain  J. Muñoz Vidal  Instituto de Física
Corpuscular (IFIC), CSIC & Universitat de València, Paterna, E-46980, Spain
K. Navarro  Department of Physics, University of Texas at Arlington,
Arlington, TX 76019, USA  P. Novella  Instituto de Física Corpuscular (IFIC),
CSIC & Universitat de València, Paterna, E-46980, Spain  D.R. Nygren
Department of Physics, University of Texas at Arlington, Arlington, TX 76019,
USA  E. Oblak  Donostia International Physics Center DIPC, San Sebastián /
Donostia, E-20018, Spain  M. Odriozola-Gimeno  Donostia International Physics
Center DIPC, San Sebastián / Donostia, E-20018, Spain  B. Palmeiro  Instituto
Gallego de Física de Altas Energías, Univ. de Santiago de Compostela, Campus
sur, Santiago de Compostela, E-15782, Spain  Instituto de Física Corpuscular
(IFIC), CSIC & Universitat de València, Paterna, E-46980, Spain  A. Para
Fermi National Accelerator Laboratory, Batavia, IL 60510, USA  J. Pérez
Laboratorio Subterráneo de Canfranc, Canfranc Estación, E-22880, Spain  M.
Querol  Instituto de Física Corpuscular (IFIC), CSIC & Universitat de
València, Paterna, E-46980, Spain  A. Raymond  Department of Physics,
University of Texas at Arlington, Arlington, TX 76019, USA  A.B. Redwine  Unit
of Nuclear Engineering, Faculty of Engineering Sciences, Ben-Gurion University
of the Negev, Beer-Sheva, 8410501, Israel  J. Renner  Instituto Gallego de
Física de Altas Energías, Univ. de Santiago de Compostela, Campus sur,
Santiago de Compostela, E-15782, Spain  L. Ripoll  Escola Politècnica
Superior, Universitat de Girona, Girona, E-17071, Spain  Y. Rodríguez García
Centro de Investigación en Ciencias Básicas y Aplicadas, Universidad Antonio
Nariño, Sede Circunvalar, Bogotá, Colombia  J. Rodríguez  Instituto de
Instrumentación para Imagen Molecular I3M (CSIC-UPV), Valencia, E-46022, Spain
L. Rogers  Department of Physics, University of Texas at Arlington, Arlington,
TX 76019, USA  B. Romeo  Donostia International Physics Center DIPC, San
Sebastián / Donostia, E-20018, Spain  Laboratorio Subterráneo de Canfranc,
Canfranc Estación, E-22880, Spain  C. Romo-Luque  Instituto de Física
Corpuscular (IFIC), CSIC & Universitat de València, Paterna, E-46980, Spain
F.P. Santos  LIP, Department of Physics, University of Coimbra, Coimbra,
3004-516, Portugal  J.M.F. dos Santos  LIBPhys, Physics Department, University
of Coimbra, Coimbra, 3004-516, Portugal  A. Simón  Unit of Nuclear
Engineering, Faculty of Engineering Sciences, Ben-Gurion University of the
Negev, Beer-Sheva, 8410501, Israel  M. Sorel  Instituto de Física Corpuscular
(IFIC), CSIC & Universitat de València, Paterna, E-46980, Spain  C. Stanford
Department of Physics, Harvard University, Cambridge, MA 02138, USA  J.M.R.
Teixeira  LIBPhys, Physics Department, University of Coimbra, Coimbra,
3004-516, Portugal  P. Thapa  Department of Chemistry and Biochemistry,
University of Texas at Arlington, Arlington, TX 76019, USA  J.F. Toledo
Instituto de Instrumentación para Imagen Molecular I3M (CSIC-UPV), Valencia,
E-46022, Spain  J. Torrent  Donostia International Physics Center DIPC, San
Sebastián / Donostia, E-20018, Spain  A. Usón  Instituto de Física Corpuscular
(IFIC), CSIC & Universitat de València, Paterna, E-46980, Spain  J.F.C.A.
Veloso  Institute of Nanostructures, Nanomodelling and Nanofabrication (i3N),
Universidade de Aveiro, Campus de Santiago, Aveiro, 3810-193, Portugal  T.T.
Vuong  Department of Chemistry and Biochemistry, University of Texas at
Arlington, Arlington, TX 76019, USA  R. Webb  Department of Physics and
Astronomy, Texas A&M University, College Station, TX 77843-4242, USA  J.T.
White  Department of Physics and Astronomy, Texas A&M University, College
Station, TX 77843-4242, USA  K. Woodruff  Department of Physics, University of
Texas at Arlington, Arlington, TX 76019, USA  N. Yahlali  Instituto de Física
Corpuscular (IFIC), CSIC & Universitat de València, Paterna, E-46980, Spain

###### Abstract

If neutrinos are their own antiparticles Majorana (1937), the otherwise-
forbidden nuclear reaction known as neutrinoless double beta decay
($\beta\beta 0\nu$) can occur, with a characteristic lifetime which is
expected to be very long, making the suppression of backgrounds a daunting
task. It has been shown that detecting (“tagging”) the ${\rm Ba^{2+}}$
dication produced in the double beta decay
${}^{136}\mathrm{Xe}\rightarrow{}^{136}{\rm Ba^{2+}}+2e+(2\nu)$ in a high
pressure gas experiment, could lead to a virtually background free experiment
Nygren (2015); Jones  _et al._ (2016); McDonald  _et al._ (2018); Rivilla  _et
al._ (2020). To identify these ${\rm Ba^{2+}}$, chemical sensors are being
explored as a key tool by the NEXT collaboration Thapa  _et al._ (2019);
Rivilla  _et al._ (2020); Thapa  _et al._ (2021). Although used in many
fields, the application of such chemosensors to the field of particle physics
is totally novel and requires experimental demonstration of their suitability
in the ultra-dry environment of a xenon gas chamber. Here we use a combination
of complementary surface science techniques to unambiguously show that ${\rm
Ba^{2+}}$ ions can be trapped (chelated) in vacuum by an organic molecule, the
so-called fluorescent bicolour indicator (FBI) Rivilla  _et al._ (2020) (one
of the chemosensors developed by NEXT), immobilized on a surface. We unravel
the ion capture mechanism once the molecules are immobilised on Au(111)
surface and explain the origin of the emission fluorescence shift associated
to the trapping of different ions. Moreover, we prove that chelation also
takes place on a technologically relevant substrate, as such, demonstrating
the feasibility of using FBI indicators as building blocks of a ${\rm
Ba^{2+}}$ detector.

The rare neutrinoless double beta ($\beta\beta 0\nu$) decay,
$(Z,A)\rightarrow(Z+2,A)+2\ e^{-}$, can occur if and only neutrinos are
Majorana particles Majorana (1937), i.e., identical to their antiparticles. An
unambiguous observation of such a decay would have deep implications in
particle physics and cosmology Sakharov (1967); Fukugita and Yanagida (1986);
Gell-Mann  _et al._ (1979); Yanagida (1979); Mohapatra and Senjanovic (1980).
The conventional double beta decay ($\beta\beta 2\nu$), in which two neutrinos
are emitted in addition to the electrons, occurs in a handful of isotopes,
some of which also offer the necessary features (a reasonable isotopic
abundance, a decay energy sufficiently high, etc.) to be used as
sources/targets in experiments seeking to observe the $\beta\beta 0\nu$ decay.
Examples of such isotopes which have been used for large-scale searches are
${}^{76}\rm Ge$, ${}^{128}\rm Te$, and ${}^{136}\rm Xe$. All attempts to
detect a signal so far have not succeeded, and experimental bounds in excess
of $10^{26}$ years have been set for the most sensitive searches based on
${}^{136}\rm Xe$ and ${}^{76}\rm Ge$ Gando  _et al._ (2016); Agostini  _et
al._ (2018). The field is currently aiming to increase the sensitivity by at
least one, and eventually two or more orders of magnitude Gomez-Cadenas
(2019). This, in turn, implies large exposures, measured in ton-years, and
even more importantly, a greatly enhanced capability to suppress radioactive
backgrounds.

In a high pressure xenon gas time projection chamber (HPXe-TPC) such as those
being developed by the NEXT experiment Martín-Albo  _et al._ (2016); Adams
_et al._ (2020), the double beta decay of ${}^{136}\rm Xe$ will create a
136${\rm Ba^{2+}}$ dication and two electrons. The decay mode with two
neutrinos, $\beta\beta 2\nu$, has been observed in xenon with a lifetime of
the order of $2\times 10^{21}$ years Ackerman  _et al._ (2011). The signal is
the same as that of $\beta\beta 0\nu$, except for the total energy of the
electrons, which is a continuous distribution for the $\beta\beta 2\nu$ case
and spikes around the decay energy, Qββ (about 2.45 MeV in the case of
${}^{136}\rm Xe$) for $\beta\beta 0\nu$ Rivilla  _et al._ (2020). The
excellent energy resolution of NEXT allows suppressing the contamination of
$\beta\beta 2\nu$ to $\beta\beta 0\nu$ by at least nine orders of magnitude
Álvarez (2013); Renner  _et al._ (2019); Rivilla  _et al._ (2020), and thus,
$\beta\beta 2\nu$ is not a significant background for NEXT up to lifetimes of
$\beta\beta 0\nu$ in the order of $10^{30}$ years.

In the TPC, the electric field that drifts the ionization electrons from the
two emitted electrons towards the detector anode, will cause the ${\rm
Ba^{2+}}$ dication to drift towards the cathode Bainglass  _et al._ (2018).
Thus, it is feasible to trigger on interesting events (e.g, those with
sufficiently high energy). The barycenter of the electron tracks can be used
to predict the impact position of the ion, or alternatively, ion transport
devices, known as RF carpets could be used to direct the ion to a specific
region in the cathode Jones  _et al._ (2021). Furthermore, it is possible to
correlate the arrival time of the dication to the cathode with the electrons
detected in the anode. Such coincidence could lead to a virtually background
free experiment.

Although the barium tagging concept was proposed more than three decades ago
Moe (1991), a practical way to detect ${\rm Ba^{2+}}$ in situ in a HPXe-TPC
was only conceived recently Nygren (2015); Jones  _et al._ (2016). The idea
relies on the capability of fluorescent molecules of changing their optical
properties upon detecting target analytes val ; Wolfbeis (2005). An initial
proof of concept McDonald  _et al._ (2018) resolved individual ${\rm Ba^{2+}}$
dications in aqueous solution using Fluo-3, a well known commercial indicator.
A suitable barium detector in NEXT requires a functionalised surface that must
include a monolayer of the molecular sensor and must efficiently operate in a
noble gas atmosphere. Over the last three years an intense R&D program to
develop chemosensors able to form a supramolecular complex with ${\rm
Ba^{2+}}$ in dry medium has been carried out Thapa  _et al._ (2019); Rivilla
_et al._ (2020); Thapa  _et al._ (2021). To date, no experiments have been
conducted in which the processes of chelation and detection occur fully under
such conditions.

One of the sensors developed by NEXT, the so-called Fluorescent Bicolor
Indicator (FBI) Rivilla  _et al._ (2020) combines an enhanced fluorescence
with a shift of the emission spectrum (about 61 nm towards the blue) when the
indicator is complexed with ${\rm Ba^{2+}}$. This is due to the specific
molecular design of the fluorescent indicator having a crown ether connected
to a benzo[a]imidazo[5,1,2-cd]indolizine fluorophore by a phenyl group. The
benzoimidazoindolizine group has been shown to have highly tunable bright
emission Stasyuk  _et al._ (2014); Lévesque  _et al._ (2017) while the crown
ether is capable of interacting with ${\rm Ba^{2+}}$ ions val ; Maleknia and
Brodbelt (2002). In the presence of a ${\rm Ba^{2+}}$ ion, calculations
predict that coordination happens between the cation, one nitrogen atom of the
fluorophore, the crown ether and the para-substituted phenyl ring, causing a
torsion-induced decoupling between non-coplanar components of the fluorophore
with respect to the non-chelated molecule. This induces a large change in the
electronic properties of the dye, causing a blue-shift in emission, which can
be used to filter the signal of chelated indicators.

In this paper we combine two highly sensitive surface techniques: X-ray
Photoemission Spectroscopy (XPS) and Scanning Tunnelling Microscopy and
Spectroscopy (STM/STS), to prove how different ions interact with FBI
molecules deposited on suitable substrates. We demonstrate that only ${\rm
Ba^{2+}}$ ions induce molecular structural changes, modifying the electronic
structure in ways that would affect the fluorescence emission at suitable
surfaces. Coordination with crown-ether happens entirely in ultra-high vacuum
(UHV) (see model in Figure 1), which ensures that chemical, structural and
electronic changes happen in the absence of solvents, air molecules or
spurious contaminants. This is, therefore, a crucial step toward the
development of a ${\rm Ba^{2+}}$ detector.

All the indicators developed by the NEXT experiment are based on crown ethers.
Because of their capability to capture a variety of guest species, including
metal cations, protonated species and neutral and ionic molecules Dobler
(1981), crown ethers Stoddart (1991) have been extensively used to recognise
and trap metal or molecular ions More  _et al._ (1999), Maleknia and Brodbelt
(2002). However, they have been poorly studied in solid state. Few examples
can be found in the literature where self-assembled monolayers of crown ether
derivatives have been grown and used on surfaces. Moreover, in all previous
studies, either the growth or ion trapping or both have taken place in
solution Yoshimoto  _et al._ (2003), Flink  _et al._ (1999), Inokuchi  _et
al._ (2015). There are only two works, as far as we know, where crown ethers
were deposited under UHV conditions Feng  _et al._ (2018) and their metal
trapping capability was proven also under UHV Stredansky  _et al._ (2019).
Thus, in addition to the progress relevant for a future $\beta\beta 0\nu$
experiment, the work presented here advances substantially the understanding
of the physico-chemical properties of crown ethers immobilised on solid
surfaces.

Figure 1:  Model of the FBI molecules before and after chelation and schematic
representation of the experiment we have carried out: FBI molecules were
sublimated on a Au(111) surface, chelated in-situ and characterized inside the
UHV chamber.

##  I Molecular sublimation in vacuum

In order to characterize the ion trapping capability of the FBI molecules,
first it is mandatory to deposit these FBI molecules on a surface by
sublimation in UHV and characterize them structural and chemically. Thus, we
chose a gold substrate, the Au(111) face (single crystal cut to present (111)
close-packed planes parallel to the surface), because it is a well known noble
metal surface where very low substrate-molecule interaction was expected. To
determine their chemical composition, we used XPS. It is worth mentioning
that, since XPS sensitivity is limited to a depth of a few nanometers,
molecular coverage below 1 monolayer (ML) were always used in this study to
ensure access to the substrate core levels for calibration.

Figure 2a shows the XPS spectra of the three molecular core levels of FBI,
i.e. O 1s, N 1s and C 1s. The spectra were measured for 0.6 ML deposited on
Au(111) at room temperature (RT). The C 1s core level can be fitted using two
components, one centered at around 284.7 eV and a second and more intense one
at 286.3 eV. The former component corresponds to C-C bonds whereas the
component at higher binding energy (BE) includes contributions from C-O and
C-N bonds, with their relative intensities in agreement with having intact
molecules. In the N 1s region, around 400.4 eV, a faint peak is visible. The
position of the maximum is compatible with the enamine-imine groups of the
molecular composition. Finally, the O 1s core level presents a single
component peak centered at 533.0 eV, which is compatible with previous reports
on closely related crown ether groups Stredansky  _et al._ (2019). The ratios
between the core levels, C/O = 6.2, C/N = 10.3, are in agreement with having
molecules of $\mathrm{C_{31}N_{3}O_{5}H_{35}}$ stoichiometry on the surface.

In addition to the XPS analysis, we have confirmed the intact sublimation of
the molecules by STM. Deposition of a submonolayer of FBI resulted mostly in
disordered islands, coexisting with isolated molecules (Figure 3a). STM images
of the single molecules (inset in Figure 3a) show two main triangular-like
lobes. Based on the molecular structure, one of them corresponds to the
fluorophore and the other to the crown-ether moieties bonded to the phenyl
ring. The apparent height of both is different, with one of them appearing
lower and flatter than the other. In order to correlate the apparent molecular
shape and the molecular structure, bond-resolved STM images were taken. For
that, the STM tip was functionalized with CO molecules operating in the
repulsive tip-sample interaction regime, a method extensively used in STM
field for characterizing planar organic molecules Gross (2011); Gross  _et
al._ (2018). Application of this measurement mode to the FBI results in the
images displayed in Figure 3b. On the right, the internal bonding structure of
the fluorophore is approximately resolved, while the aza-crown ether regions
is not well resolved. Due to its larger conformational flexibility and its
non-planarity, the tip stability is not good enough and, as a consequence, the
contrast observed in the aza-crown ether part is an artefact related to
excessively strong interactions with the flexible CO tip-apex. Moll  _et al._
(2010); Hapala  _et al._ (2014). To confirm the imaging signature of the
fluorophore, we also sublimated two FBI derivatives specifically synthesized
without the aza-crown ether component. Figures 3c and d show the bond-
resolving images of FBI derivatives sublimated on Au(111) surface: the
fluorophore (Figure 3c) and the fluorophore with the phenyl ring but without
the aza-crown ether (Figure 3d). The absence of the aza-crown ether allowed
the molecules on the surface to adopt a planar, rigid conformation. This
clearly improved the clarity of the images, with the carbon framework of the
molecule well visualised. In turn, this supports our previous assignment of
the different parts when measuring the original FBI molecules and confirms
their unchanged chemical structure upon sublimation.

Figure 2:  XPS demonstration of the chemical changes induced by the molecular
chelation: (a) O 1s, N 1s and C 1s core level spectra measured after
sublimation of 0.6 ML of FBI deposition on Au(111); (b) O 1s, N 1s and C 1s
core level spectra measured on the previous sample after chelation with 0.80
${\rm Ba^{2+}}$ions per FBI molecule; (c) O 1s, N 1s and C 1s core level
spectra measured after 0.6 ML of FBI deposited on Au(111) (green spectra) and
after chelation with $\rm Na^{+}$ (2.60 $\rm Na^{+}$ions per FBI molecule)
(orange line). For the three panels, dots correspond to raw values and solid
lines to fitted curves (the fitting procedure is discussed in the Methods
section). (d) Evolution of O 1s core level as a function of the ${\rm
Ba^{2+}}$deposition on 0.9 ML of FBI on Au(111). The spectra were manually
shifted in the y-axis to better show the evolution. The O 1s core level region
is displayed in (c) and (d) after subtraction of the contribution from Au 4p
3/2, in order to emphasize the spectral changes in the series; (e) Au 4p 3/2
and O 1s shown previous to any data treatment. Figure 3:  (a) Large scale STM
image ($50\times 50$ nm2) of around 0.4 ML of FBI molecules deposited on
Au(111) surface and measured at 4 K. Red squares show isolated molecules.
Inset: constant current zoom STM image of a FBI molecule (I = 60 pA / U = 1.4
V) deposited on Au(111). (b-d) Bond-resolved STM image measured with a CO-
functionalized probe (constant height, U = 5 mV) of individual FBI molecules:
(b) benzo[a]imidazo[5,1,2-cd]indolizine fluorophore with phenyl ring and aza-
crown ether (exactly the same molecule shown in the inset in (a)); (c)
benzo[a]imidazo[5,1,2-cd]indolizine fluorophore; (d)
benzo[a]imidazo[5,1,2-cd]indolizine fluorophore with phenyl ring but without
the crown ether (d). For clarity, the same image is shown twice, one with and
one without the molecular model superimposed to guide the eyes.

##  II Chemical demonstration of chelation

Once confirmed the presence of intact FBI molecule on the surface, BaCl2 was
sublimated to test the molecular chelation. Prior to the sublimation on the
FBI, BaCl2 was sublimated on clean Au(111) surface to confirm its
stoichiometry. By comparing the core level intensities of Ba 3d 5/2 and Cl 2p
core levels (taking into account the corresponding sensitivity factor of each
element), the ratio Ba:Cl was 1:2 as expected. Surprisingly, when BaCl2 was
deposited on FBI-functionalised Au(111), this ratio was around
1:1.5$\pm{0.2}$, meaning that some chlorine atoms desorbed when they reached
the sample. From simulations Rivilla  _et al._ (2020), the chloride ions are
expected to behave as passive spectators in the chelation, which is consistent
with their desorption when the FBI molecule trapped the ${\rm Ba^{2+}}$.
Because of this lack of stoichiometry, we refrain from using ${\rm Ba^{2+}}$
ML to refer to the amount of sublimated molecules and, instead, we will refer
the ${\rm Ba^{2+}}$ ions per FBI molecule. The ${\rm Ba^{2+}}$ dose was
estimated using the ratio between XPS intensities of the Ba 3d 5/2 and N 1s
core levels divided by the corresponding sensitivity factor, and always taking
into account that there are 3 N atoms per molecule.

After sublimation of 0.80 ${\rm Ba^{2+}}$ ions per FBI molecule, the core
levels slightly shift, which indicates changes in the chemical environment.
Figure 2b shows the O 1s, N 1s, and C 1s, whose shifts toward higher BE are
distinguished, mainly on O 1s. The O 1s core level exhibits an upward chemical
shift of 0.5 eV (0.4eV for N 1s). By curve fitting deconvolution (discussed in
the Methods section) we determine that this shift is not just a doping shift,
but a real chemical change, inferred by the growth of a new component. This is
confirmed by the evolution of the O 1s for very low ${\rm Ba^{2+}}$ doses on
the surface. Figure 2d shows the evolution of the O 1s for incremental amounts
of ${\rm Ba^{2+}}$ ions from 0 to almost 0.5 ${\rm Ba^{2+}}$ ions per FBI
molecule, i.e. up to about 2 FBI molecules per 1 ${\rm Ba^{2+}}$. Once BaCl2
is added to the sample, in the O 1s core level a second component grows at
higher binding energy, which induces an increase of the peak width. At the
latest stage of barium addition, the FWHM increases by 9% with respect to the
initial state, which indicates that an extra component appears. We
deconvoluted the peaks after ${\rm Ba^{2+}}$ addition using two components,
one at 532.95 eV (FWHM = 2.20 eV), corresponding to the FBI molecules and
another at around 533.9 eV, consistent with the molecules undergoing a
chemical change upon chelation, as previously observed for crown ether
chelation with Na Stredansky  _et al._ (2019). It is important to mention the
difficulty of quantifying correctly the variation of one component with
respect to the other for O 1s. This is due to the close proximity of the Au 4p
core level right tail, Figure 2d, with a contribution in the O 1s region.

To evaluate whether the nature of the ions has any influence on the oxygen
core level shift for the FBI molecules, we tested the chelation with $\rm
Na^{+}$. Thus, Figure 2c shows the O 1s, N 1s and C 1s core levels measured on
0.6 ML of FBI on Au(111) before and after the sublimation of 2.60 $\rm Na^{+}$
per FBI. The maximum of the O 1s core level shifts again for $\rm
Na^{+}$-chelated FBI molecules 0.5 eV toward higher BE, confirming that O 1s
shift can be considered as a fingerprint of the chelation. Moreover, there is
no apparent shift on the N 1s (nor C 1s) core level, which reveals that N 1s
is not playing any role in the chelation process. According to theoretical
calculations, upon chelation with ${\rm Ba^{2+}}$, FBI undergoes a structural
torsion generated by the interaction between the ion, the iminic N atom of the
benzo[a]imidazo[5,1,2-cd]indolizine moiety and the phenyl group of the unbound
fluorophore Rivilla  _et al._ (2020). On the contrary, in the case of $\rm
Na^{+}$ calculations do not show such an important molecular distortion
because of the smaller size of the ion. Thus, this difference between ${\rm
Ba^{2+}}$ and $\rm Na^{+}$-chelation can be interpreted as a first proof that
the molecular conformation of FBI and the cation-phenyl interaction varies
depending on the nature of the trapped ions.

##  III Molecular structural rearrangement induced by chelation

Figure 4:  (a) Schematic diagram of metal intercalation. (b-d) STM images of
(b) $\rm Na^{+}$-complexed (U = 0.9 V, I = 60 pA), (c) native (U = 1.4 V, I =
60 pA), and (d) ${\rm Ba^{2+}}$-complexed (U = -1.8 V, I = 20 pA) FBI. (Scale
bars = 0.3 nm) (e) STS spectra of native (green), $\rm Na^{+}$-complexed
(red), and ${\rm Ba^{2+}}$-complexed (blue) FBI.

In order to visualize the aforementioned structural changes undergone by the
FBI molecules upon chelation, STM experiments were, again, carried out. Figure
4a shows the schematic representation of the FBI molecules before and after
chelation with $\rm Na^{+}$ and ${\rm Ba^{2+}}$, and Figure 4b-d displays
representative STM images next to each of the cases. In all of them, the
fluorophore and the aza-crown ether region are distinguished. The $\rm
Na^{+}$-chelated FBI image remains almost unchanged compared to the non-
chelated FBI, while the FBI chelated with ${\rm Ba^{2+}}$ has a more complex
shape. Although the interpretation of constant current mode STM images is
always difficult to obtain because of the contrast determined by a convolution
of topographical and electronic contributions, here we can directly relate the
images differences with the expected conformational changes, since the images
were measured at bias voltages well inside the molecular gap, i.e., with no
variation in the electronic structure. Taking into account that the three STM
images are plotted with a common colour code, it can be directly observed that
the apparent heights of the crown ether follow the expected trend (higher for
${\rm Ba^{2+}}$ chelated molecule). Indeed, the apparent height for each
molecule, as measured on the highest point of the crown, is 2.5$\pm$ 0.1 Å,
2.1$\pm$ 0.1 Å, and 2.9$\pm$ 0.1 Å, for the pristine FBI and the $\rm
Na^{+}$\- and ${\rm Ba^{2+}}$-chelated molecules respectively. Thus,
complexation of crown ethers with alkali metals like $\rm Na^{+}$ causes the
oxygen atoms to point to the center, forcing the ring to adopt a flatter
conformation relative to the native molecule. Instead, FBI molecule adopts a
more three-dimensional conformation upon chelation with ${\rm Ba^{2+}}$.

Species | Band gap / eV (nm) |  $\lambda^{\mathrm{abs}}_{\mathrm{max}}$ / nm |  $\lambda^{\mathrm{emi}}_{\mathrm{max}}$/nm
---|---|---|---
FBI | 3.17 (391.1) | 432.5 | 489
FBI-$\rm Na^{+}$ | 3.31 (374.6) | 430 | 489
FBI-${\rm Ba^{2+}}$ | 3.63 (341.6) | 420.5 | 428
Table 1: Band gap values measured by STS on Au(111) vs. absorption
($\lambda^{\mathrm{abs}}_{\mathrm{max}}$) and fluorescence emission
($\lambda^{\mathrm{emi}}_{\mathrm{max}}$) spectral peaks measured in solution.

As we mentioned before, the distortion of the molecule upon metal coordination
has an important impact on the fluorescence response. The lowest fluorescence
emission peak of the non-chelated FBI molecule comes from the de-excitation of
electrons from the Lowest Unoccupied Molecular Orbital (LUMO) to the Highest
Occupied Molecular Orbital (HOMO). The torsion of the phenyl group decreases
the effective conjugation, thus increases the symmetry allowed
$\pi\rightarrow\pi^{*}$ gap, resulting in an increase of the HOMO-LUMO gap,
and therefore introducing a blue shift of the fluorescent emission. In order
to check that, we use scanning tunneling spectroscopy (STS) to scan the local
density of states of the system. In this way, it is possible to detect the
changes in the HOMO-LUMO gap which are associated with the changes in the
fluorescence emission measured in solution. Figure 4e shows the associated STS
spectroscopy measured for the three molecules. The FBI molecule deposited on
Au(111) has an HOMO-LUMO gap of 3.17 eV. Complexation with $\rm Na^{+}$
slightly changes this band gap, while complexation with ${\rm Ba^{2+}}$
increased it to 3.63 eV. The absence of substantial frontier molecular orbital
gap upon complexation with $\rm Na^{+}$ is in line with other dyes containing
the same crown ether, whose emission profiles do not change upon $\rm Na^{+}$
complexation Ast  _et al._ (2011) and it is supported by the fact that no
change in the molecular conformation does not alter the molecular emission.
Meanwhile, the conformational change upon complexation with ${\rm Ba^{2+}}$
causes a decrease in the extent of $\pi$-conjugation, which no longer extends
into the phenyl ring linking the fluorophore and the crown, consequently
increasing the bandgap. The measured HOMO-LUMO gap values are in agreement
with the $\lambda^{\mathrm{emi}}_{\mathrm{max}}$ spectra measured in solution,
as it is summarized in Table 1. The three molecules present similar
$\lambda^{\mathrm{abs}}_{\mathrm{max}}$ associated to the transition of
electrons from HOMO to the LUMO+n states, while the fluorescence emmision,
associated to the LUMO to HOMO transition is comparable for non-chelated and
$\rm Na^{+}$-chelated molecules and shifted by 0.36 eV photon energy
(489$\rightarrow$428 nm) for ${\rm Ba^{2+}}$ chelated molecules.
Unfortunately, because of the metallic character of the substrate we can not
measure the fluorescence emission (the excitation energy is dumped to the
metal without further emission).

##  IV Chelation tested on different surface support

Figure 5:  O 1s core level evolution upon chelation measured on two
submonolayer FBI-functionalized surfaces: (a) chelation on Cu(111) sublimating
BaCl2 and NaCl (inset); (b) chelation on ITO. In this case, the substrate
component (${\mathrm{O}_{\mathrm{ITO}}}$) was subtracted to emphasize the O 1s
contribution coming from FBI (${\mathrm{O}_{\mathrm{FBI}}}$). The entire
spectra are shown inserted in (b). Dot spectra correspond to raw values and
solid lines to fitted curves.

Finally, we have tested that chelation takes place also on other surfaces, in
particular Cu(111) and Indium Tin Oxide (ITO). We chose Cu(111) because it is
a more reactive substrate. This enabled us to study whether the molecule-
substrate interaction could alter the chelation response. On the other hand,
ITO was selected as a promising candidate for the potential implementation of
a barium tagging detector on a xenon-based TPC Rivilla  _et al._ (2020). As
degenerated semiconductor, ITO is transparent, which would allow the direct
detection of fluorescence in transmission. Furthermore, its conductivity is
high enough to guide the ${\rm Ba^{2+}}$ ions towards the sensor surface
facilitating their capture by FBI, but potentially low enough to avoid the
fluorescence quenching that is expected at conductive surfaces.

As previously discussed, the O 1s shift is a fingerprint of chelation. Figure
5 shows the O 1s core level of FBI on Cu(111) and ITO. In both cases, upon
chelation with ${\rm Ba^{2+}}$ ($\phi=10$ for 0.6 ML of FBI on Cu(111) and
$\phi=17.5$ for 0.7 ML of FBI on ITO) we observed the chemical shift on the O
1s towards higher BE, indicating that chelation is happening. In the case of
Cu(111), the shift is of about 0.7 eV, while for ITO is around 0.9 eV. As far
as ion capture is concerned, FBI chelation of $\rm Na^{+}$ on Cu(111) was also
tested and, again, O 1s shows the expected upwards shift associated with
chelation (inset in Figure 5a). These results ensure that chelation is
independent from the choice of substrate.

The exact values of the O 1s core level shift are different for the three
substrates. In photoemission, the absolute core level shift values depends on
many factors such as the substrate, the molecular coverage, the presence of
defects or other molecules. Therefore, what is important here is that the
direction of the shifts is the same in the three cases, and that the magnitude
of the shift is similar.

Notice that in the case of Cu(111), when the FBI-functionalized surface is
exposed to BaCl2, the residual contamination partially oxidizes the surface.
For this reason, the core level has a smaller component at lower BE, around
531 eV, associated to Cu2O Zhu  _et al._ (2013). Moreover, because of the
presence of oxygen in the ITO structure, the analysis of the O 1s core level
shifts required a subtraction of the core level measured on the bare ITO to
enhance the FBI and ${\rm Ba^{2+}}$ chelated FBI contributions
(${\mathrm{O}_{\mathrm{FBI}}}$). The result of the subtraction is shown in
Figure 5 b), and the inset gathers the original normalized spectra, including
the contribution to the O 1s coming from the ITO
(${\mathrm{O}_{\mathrm{ITO}}}$).

##  V Conclusions

The demonstration of chelation in vacuum of FBI indicator by ${\rm Ba^{2+}}$
ions once submonolayers of molecules are deposited on suitable surfaces is a
major step towards the development of a sensor capable of single barium
tagging detection, with immediate application to a new, ultra-low background
xenon-based $\beta\beta 0\nu$ detection experiment. Chemical and
conformational changes occur upon chelation, independently of the substrate
where molecules were deposited. These changes are furthermore in agreement
with the calculations for the behaviour of free standing molecules, thus
providing considerable freedom to choose the substrate. Moreover, the measured
variation in the molecular HOMO-LUMO gap is perfectly consistent with the
observed bicolour property of the sensor for ${\rm Ba^{2+}}$ ions compared to
the absence of shift observed and predicted for other ions, such as $\rm
Na^{+}$.

Furthermore, this study has also important implications beyond neutrino
particle physics field. The capability of aza-crown ethers to interact with
many different ions is very important in applications such as drug carriers
Uchegbu and Vyas (1998) or photo-switching devices Malval  _et al._ (2002);
Uda  _et al._ (2005).

##  VI Methods

The experiments were performed in two different UHV chambers, one for XPS
experiments and the other for STM experiments. Both chambers have a base
pressure of $1\times 10^{-10}$ mbar. Prior to deposition, the Au(111) and
Cu(111) and Indium Tin Oxide (ITO) surfaces were cleaned via cycles of Ar
sputtering and annealing to 500 °C and their cleanliness was checked by XPS
prior to molecular deposition.

Molecular evaporation Pure FBI, BaCl2 and NaCl powders were evaporated from
homemade Knudsen cell evaporators. To avoid cross-talking between FBI and the
salts molecules and to exclude any possibility of chelation of the molecule
inside the cell, the FBI and BaCl2 (NaCl) evaporators were located in two
separated parts of the UHV chambers. To ensure sublimation reproducibility,
the same molecular evaporator cells (containing the FBI, BaCl2 and NaCl) were
used in both STM and XPS chambers (removed from one chamber and installed in
the other).

FBI molecules were synthesized using the procedure described in Rivilla  _et
al._ (2020), while BaCl2 and NaCl were commercially (Sigma-Aldrich) available.
The molecules were used after degassing in UHV. The molecular evaporation rate
was monitorized using a quartz microbalance and the amount of molecules on the
surfaces was afterwards quantified by analyzing the relative intensity of the
core level peaks for the experiments done in XPS as well as with the
percenteage of covered surface in STM.

Adsoption and emission spectra UV-vis spectra were acquired on a Shimatzu
UV-2600 Spectrophotometer. Emission spectra were acquired on an Agilent Cary
Eclipse Fluorescence Spectrophotometer. Excitation and emission monochromator
bandwidth was fixed at 5 nm. Spectra were recorded at $5\times 10^{-5}$ M
solutions of FBI and equimolecular quantities of the corresponing perchlorate
cation salt for the chelated species. Emission spectra were acquired using 250
nm light.

STM experiments STM experiments were performed with a commercial Scienta-
Omicron ultra-high vacuum LT-STM at 4.3 K. A W-tip was used. Topography images
were measured using constant current mode, while for bond resolution images
constant height mode was used, Figure 3 b-d. During the STM experiments, the
tip was functionalized with CO for bond resolution STM by exposing the sample
to low pressure (approximately $1\times 10^{-8}$ mbar) of CO whilst the sample
was held below 7 K. CO molecules were trapped by the tip from their adsorption
sites via scanning over them or by applying $\sim$ 2 V bias voltage pulses.

XPS experiments The XPS measurements were carried out using a Phoibos-100
electron analyzer (SPECS GmbH), using a non monochromatic Al K$\alpha$ photon
source of 1486.6 eV. The spectra were calibrated to the substrate main core
level (Au 4f, Cu 2p, and In 3d respectively).

The evaporation thicknesses were estimated using the attenuation of the most
intense substrate core levels, i.e. Au 4f, Cu 2p and In 3d for Au(111),
Cu(111) and ITO respectively. The calculations followed the guidelines
provided in ref. Powell (2020). For this purpose, we estimated the electron
Effective Attenuation Length (EAL) through the FBI layers for electrons with
kinetic energies of 1402.6, 1041.6 and 554.61 eV (Au 4f 5/2, In 3d 5/2 and Cu
2p 3/2). The resulting EALs are 3.87, 3.05 and 1.85 nm respectively. The
thickness of the FBI samples was then estimated using the intensity of clean
substrate core level as reference and the attenuated intensity after the
evaporation. To correct for systematic uncertainties affecting different sets
of data, the intensities of all spectra were rescaled to the main substrate
core levels (Au 4f, Cu 2p and In 3d respectively). The core level peak area
were numerically integrated between fixed local minima. This yields an error
in the stoichiometry ratios of 7%.

The stoichiometry of the FBI (BaCl2) was calculating following the expression
$R(A/B)=(I_{A}/I_{B}\cdot S_{B}/S_{A})$ where A and B are the different
molecular elements, $I_{X}$ is the integrated area under the main core level
and $S_{X}$ is the corresponding sensitivity factor of this core level.

The amount of ${\rm Ba^{2+}}$($\rm Na^{+}$) per FBI molecule were estimated by
computing the ratio $\phi=I_{Ba}/(I_{N}/3)\cdot S_{N}/S_{Ba}$, where $I_{Ba}$,
$I_{N}$ are total areas of Ba 3d 5/2 and N 1s core levels respectively, and
$S_{Ba}=25.84$, $S_{N}=1.80$ are the corresponding atomic sensitivity factors
Scofield (1976). The factor 3 responds to stoichiometric reasons, considering
3 N atoms per molecule.

XPS fitting The spectra fitting was performed using custom-made software
written in Python, using the lmfit library 111https://lmfit.github.io/lmfit-
py/. An example of fit can be seen in Figure 6. The core levels were modeled
as a pseudo-Voigt function (Lorentzian to Gaussian ratio of 0.6) convoluted
with a Shirley type background. The error in the estimation of the position of
the maxima derived from fitting is 50 meV. However, we only consider as
significant shifts higher than 100 meV, which is the energy step we use in the
adquisition. The goodness of fit is given by a $\chi^{2}$ parameter of about
0.05 counts (in A.U.) when the data are normalized to a maximum intensity of
1. Here $\chi^{2}=\sum_{i}(y_{i}-Y_{i})^{2}/Y_{i}$, where $y_{i}$ and $Y_{i}$
are the data intensity values and the expected values from the fit
respectively.

Figure 6:  Detail of fitting components (dotted lines) for O 1s in FBI 0.6 ML
(green) and FBI 0.6 ML + BaCl2 $\phi$ = 0.8 (blue) from Figure 2 (a) and (b).
Circles and solid lines represent raw data and best fit respectively. The
green component was fitted to the unchelated FBI data (green circles) at
532.95 $\pm$ 0.05 eV, with width 2.16 $\pm$ 0.04 eV at FWHM. This component
was then fixed in position and width for the chelated FBI data (blue dots). An
additional component was fitted to the chelated FBI CL, shown as blue filled
area, at 533.91 $\pm$ 0.05 eV, with 1.41 $\pm$ 0.05. The background of each
curve is shown as dashed lines in green and blue respectively. Inset: same
blue curve and components with its background subtracted.

##  VII Acknowledgment

This material is based upon work supported by the following agencies and
institutions: the European Research Council (ERC) under ERC-2020-SyG 951281;
the MCIN/AEI/10.13039/501100011033 of Spain and ERDF A way of making Europe
under grants PID2020-114252GB-I00, PID2019-107338RB-C63, PID2019-104772GB-I00,
PID2019-111281GB-I00 and RTI2018-095979, the Severo Ochoa Program grant
CEX2018-000867-S; the Basque Government (GV/EJ) under grants IT-1346-19,
IT-1255-19, and IT-1180-19.

The NEXT Collaboration also acknowledges support from the following agencies
and institutions: the European Union’s Framework Programme for Research and
Innovation Horizon 2020 (2014–2020) under Grant Agreement No. 957202-HIDDEN;
the MCIN/AEI/10.13039/501100011033 of Spain under the María de Maeztu Program
grant MDM-2016-0692; the Generalitat Valenciana of Spain under grants
PROMETEO/2021/087 and CIDEGENT/2019/049; the Portuguese FCT under project
UID/FIS/04559/2020 to fund the activities of LIBPhys-UC; the Pazy Foundation
(Israel) under grants 877040 and 877041; the US Department of Energy under
contracts number DE-AC02-06CH11357 (Argonne National Laboratory), DE-
AC02-07CH11359 (Fermi National Accelerator Laboratory), DE-FG02-13ER42020
(Texas A&M), DE-SC0019054 (Texas Arlington) and DE-SC0019223 (Texas
Arlington); the US National Science Foundation under award number NSF CHE
2004111; the Robert A. Welch Foundation under award number Y-2031-20200401.
DGD acknowledges support from the Ramón y Cajal program (Spain) under contract
number RYC-2015-18820. Finally, we are grateful to the Laboratorio Subterráneo
de Canfranc for hosting and supporting the NEXT experiment.

##  VIII Author contributions

J.J.G.-C., F.P.C. and C.R. conceived the project. C.R. designed and
coordinated the experiments and data analysis. P.H.G. and M.I. performed and
analysed the XPS experiments. J.P.C., A.B.L., T.W., D.G.O. and M.C. performed
the STM experiments. I.R., B.A., A.A. and Z.F. carried out the chemical
synthesis. R.G.M. I.R., B.A., A.A. and Z.F. performed the characterisation in
solution of fluorescence studies. P.H.G., J.P.C., J.J.G.-C., F.P.C., and C.R.
wrote the manuscript. F.R. contributed to the revision of the manuscript. The
NEXT collaboration assisted with editing and revision of the manuscript.

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|  Nima Abdi received her B.Sc. in Electrical Engineering from Qatar
University in 2020 and is currently pursuing an M.Sc. in Data Science and
Engineering at Hamad Bin Khalifa University (HBKU). Her research focus is on
the application of Artificial Intelligence on smart grid security,
specifically the physical layer.
---|---
|  Abdullatif Albaseer (Member, IEEE) received an M.Sc. degree in computer
networks from King Fahd University of Petroleum and Minerals, Dhahran, Saudi
Arabia, in 2017 and a Ph.D. degree in computer science and engineering from
Hamad Bin Khalifa University, Doha, Qatar, in 2022. He is a Postdoctoral
Research Fellow with the Smart Cities and IoT Lab at Hamad Bin Khalifa
University. He has authored and co-authored more than twenty conference and
journal papers in IEEE ICC, IEEE Globecom, IEEE CCNC, IEEE WCNC and IEEE
Transactions. He also has six US patents in the area of the wireless network
edge. His current research interests include AI for Networking, AI for
Cybersecurity, Distributed AI, and Edge Intelligence.
---|---
|  Mohamed Abdallah (Senior Member, IEEE) received his B.Sc. degree from Cairo
University, Giza, Egypt, in 1996, and his M.Sc. and Ph.D. degrees from the
University of Maryland at College Park, College Park, MD, USA, in 2001 and
2006, respectively.,From 2006 to 2016, he held academic and research positions
with Cairo University and Texas A & M University in Qatar, Doha, Qatar. He is
currently a Founding Faculty Member with the rank of Associate Professor with
the College of Science and Engineering, Hamad Bin Khalifa University, Doha. He
has published more than 150 journals and conferences and four book chapters
and co-invented four patents. His current research interests include wireless
networks, wireless security, smart grids, optical wireless communication, and
blockchain applications for emerging networks. He is a recipient of the
Research Fellow Excellence Award at Texas A& M University in Qatar in 2016,
the Best Paper Award in multiple IEEE conferences, including IEEE BlackSeaCom
2019 and the IEEE First Workshop on Smart Grid and Renewable Energy in 2015,
and the Nortel Networks Industrial Fellowship for five consecutive years,
1999–2003. His professional activities include an Associate Editor of the IEEE
Transactions on Communications and the IEEE Open Access Journal of
Communications, the Track Co-Chair of the IEEE VTC Fall 2019 Conference, the
Technical Program Chair of the 10th International Conference on Cognitive
Radio-Oriented Wireless Networks, and a technical program committee member of
several major IEEE conferences.
---|---