Surface Science 678 (2018) 163–168

Contents lists available at ScienceDirect

Surface Science
journal homepage: www.elsevier.com/locate/susc

Long starphene single molecule NOR Boolean logic gate
a,⁎

b

c

d

T
c

W.H. Soe , C. Manzano , P. de Mendoza , P.R. McGonigal , A.M. Echavarren , C. Joachim

a,e

a

CEMES-CNRS, 29 rue J. Marvig, Toulouse Cedex 31055, France
IMRE, A*STAR (Agency for Science, Technology and Research), Research Link 117602, Singapore
Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, Tarragona 43007, Spain
d
Department of Chemistry, Durham University, South Road Durham, DH1 3LE, United Kingdom
e
International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
b
c

A R T I C LE I N FO

A B S T R A C T

Keywords:
Molecular electronics
Single molecule computing
Logical Operation
STM

Using a low temperature scanning tunneling microscope (LT-UHV-STM), local electronic tunneling spectroscopy
and differential conductance mapping are performed to investigate how by extending one phenyl more each
branch of the conjugated board of a trinaphthylene starphene molecule, the corresponding longer trianthracene
starphene new molecule is functioning like a NOR Boolean logic gate according to a Quantum Hamiltonian
Computing (QHC) design. Here the STM tip is used to manipulate single Au atoms one at a time for contacting a
trianthracene molecule. Each Au atom is acting like a classical digital input on the molecule encoding for a
logical “0'' when the atom is not interacting with the trianthracene input branch and for a logical “1'' when
interacting. The inputs are converted in quantum information inside the trianthracene molecule and the logical
output status available on the output branch. QHC is demonstrated to be robust since quantum information
transfer can be used on the long range along the trianthracene for the NOR logic gate to function properly as
compared to the shorter trinaphthylene molecule.

1. Introduction
The new molecular electronic approach called QHC (standing for
Quantum Hamiltonian Computing) [1] has been recently proposed and
demonstrated experimentally in which the complex functionality of a
Boolean logic gate is fully implemented inside a single molecule
without integrating or mimicking standard electronic components like
rectifiers[2,3], switches [4,5], transistors [6] or even qubit-like chemical groups [7] along the molecular structure [8]. QHC is based on the
phenomenon that the eigenstates of a well-designed quantum system
can be shifted in energy relative to reference energy to conform to a
Boolean truth table by locally perturbing this quantum system, where
the local perturbations serve as logical inputs [1]. The advantage of this
approach is that the logical output can be measured locally at any
position on the quantum system where the energy shifted quantum
eigenstates have a large electronic probability density. In the absence of
a planar technology to perform such measurements with a picometer
precision [9], a low temperature Scanning Tunneling Microscope (LTUHV-STM) was used first to map the electronic probability density of
the molecular electronic states involved and then to perform tunneling
spectroscopy. Applied to molecular electronic circuit design, the QHC
approach offers the possibility to measure the logical output of a logic

⁎

Corresponding author.
E-mail address: we-hyo.soe@cemes.fr (W.H. Soe).

https://doi.org/10.1016/j.susc.2018.04.020
Received 1 December 2017; Received in revised form 23 April 2018; Accepted 27 April 2018

Available online 30 April 2018
0039-6028/ © 2018 Elsevier B.V. All rights reserved.

gate without passing a current through the complete molecular structure of the circuit. This is an advantage with respect to other approaches trying to use a molecule per replicate of classical electronic
components [10,11] in which, by increasing the complexity of the intramolecular circuit [12], the molecular system size would rapidly increase and consequently the tunneling intensity through the circuit
would decay exponentially [13].
A QHC molecule able to function like a NOR Boolean logic gate has
been demonstrated experimentally using a single trinaphthylene molecule [14]. In these experiments, the tip of a LT-UHV-STM was used to
manipulate and contact single Au atoms to each of the two input
branches of the ``Y” shaped trinaphthylene. Here, each Au atom functions as a classical digital input. This classical binary information (set
by the interaction (or absence) of a single Au atom with the π system of
the Y molecule) is converted into quantum information [14] by trinaphthylene's electronic structure itself to perform the expected NOR
digital logic operation. On a standard Slater atomic orbitals basis set
decomposition of the molecular orbitals which are impacted by the
single Au atom electronic interactions [15,16], the amount of quantum
information distributed along the molecule can be estimated theoretically by following how this decomposition is changing as compared to
the same decomposition on the sole molecule. The logical output status

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to deposition of the molecules and the extraction and manipulation of
atoms, this crystal was cleaned via sputtering and annealing cycles. The
trianthracene molecules were sublimed from a quartz crucible using
evaporation parameters set to produce sub-monolayer molecular coverage on the Au(111) surface. After deposition, the Au(111) sample was
pre-cooled using liquid helium and transferred to the low temperature
STM chamber where a base pressure below 10−10 mbar and a cryogenic
working temperature of ∼7 K are normally maintained. Tunneling
spectra and differential conductance maps were recorded using a
standard lock-in spectroscopy method.
3. Results and discussion
LT-UHV-STM constant current images recorded after adsorption of
the trianthracene molecules on Au(111) show that some molecules
adsorbed at step edges while others are isolated on the Au(111) surface
terraces. Imaged by the LT-UHV-STM, an isolated molecule reveals its Y
shape with equal length branches. As presented in the image series in
the Fig. 1, different sub-molecular features are observed upon varying
the imaging bias voltage. To identify those features, two non-averaged
dI/dV tunneling spectra recorded on a single trianthracene molecule
are presented in Fig. 2a. For the dI/dV spectrum recorded at the end of
the Y output branch two resonances can be clearly identified at
−1.06 V and −1.33 V bias voltages, respectively. For the dI/dV spectrum recorded exactly at the phenyl node in between the 2 input
branches, two other resonances can be identified at + 1.63 V
and + 2.2 V bias voltages. These two resonances can also be observed
by positioning the tip apex at the Y output branch end. In Fig. 2a, the
separation between the first negative and the first positive resonances is
about 2.69 V and corresponds to the energy separation between the
ground and the first reduced states of the trianthracene molecule on Au
(111). By comparison, this energy separation was 3.4 eV for the shorter
trinaphthylene homologue deposited on Au(111) [14].
To identify the main molecular orbital (MO) contributions to those 4
low lying electronic resonances, constant current differential conductance dI/dV maps were recorded to measure the detailed spatial
distribution of the Y molecule electronic eigenstates as presented in
Fig. 2b. This mapping is also essential to determine where to position
the single Au atom at the end of the Y input branches, to set the logical
inputs and the tip apex at the output branch end with respect to molecular skeleton, and to enhance the signal-to-noise ratio during the
measurement of the logical output of the molecule logic gate. The
identification of trianthracene's electronic resonances and their localization relative to the trianthracene structure had been performed experimentally by Guillermet et al. [17] by comparing LT-UHV-STM and
LT-UHV-NC-AFM images of trianthracene molecules physisorbed on a
NaCl/Cu(111) surface. On such a surface, planar conjugated molecules
are known to be very well decoupled electronically from the underlying
metallic surface. This electronic decoupling in conjunction with crossexamination of experimental imaging and STM image calculations
permitted an unambiguous identification of the main molecular orbital
contributions to at least the first negative and positive electronic resonances of such molecule in a STM tunnel junction.
For an isolated trianthracene molecule adsorbed on Au(111), the
Fig. 2b dI/dV maps recorded at −1.06 V and + 1.63 Vs are similar to
the ones recorded for trianthracene on NaCl/Cu(111), corresponding to
the molecule's first negative and positive resonances, already confirming that the trianthracene molecules on Au(111) are physisorbed.
According to the mono-electronic Elastic Scattering Quantum Chemistry (ESQC) dI/dV STM image calculations [17], these dI/dV maps at
−1.06 V and 1.63 V can be identified to electronic resonances whose
main MO contributors captured experimentally by the STM are the
Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied
Molecular Orbital (LUMO) molecular orbitals of the trianthracene
molecule, respectively. When physisorbed on Au(111), the HOMO and
LUMO can be used to describe the virtual first oxidized and first

Fig. 1. LT-UHV-STM constant current topographic images of a ‘Y’ shape trianthracene molecule physisorbed on a Au(111) terrace taken at different bias
voltages. Some intramolecular features are clearly seen at −1.3 V. The trianthracene molecular structure is represented on scale with the STM images recorded at + 1.6 V. (image size: 2.5 × 2.5 nm).

of this gate is obtained by performing a local scanning tunneling
spectroscopy (STS) measurement on the Y molecule's output branch to
measure how the ground state of the trinaphthylene molecule is shifted
in energy due to the Au atoms classical logical inputs relative to a
Boolean truth table [14].
To explore the concept further, we have studied how the π system
length of a Y shaped molecule maintained the functioning of the NOR
QHC logic gate by using a trianthracene molecule (anthra[2,3-j]heptaphene) (see Fig. 1) which is also a symmetric Y shaped molecule but
longer since it has one phenyl more per branch. This molecule was
synthesized and ultra-purified in Tarragona for UHV sublimation purpose. Single gold atoms were also manipulated with the STM tip to
coordinate them at each of the 2 input branches of this Y molecule. The
effects of each logical Au input on the low lying electronic states of this
new Y molecule were measured by STM mapping the differential conductance in real space and also by recording dI/dV spectra at the end of
the molecule's output branch. These dI/dV maps were recorded at
voltages corresponding to the main electronic resonances of the trianthracene's tunneling spectra. In this article, we also discuss how the Y
structure of the trianthracene molecule permits a moderation of the
decay with length of the quantum information distribution when one
Au atom is coordinated at one end of the trianthracene as compared to
the initial trinaphthylene. In a trianthracene molecule, the Au atom
electronic interaction can be detected at the other end of the transmission path, defined from the input to the output branch, showing no
significant decay of the ground state shift induced by the coordination
of one Au Atom in spite of the extended transmission path as compared
to the shorter trinaphthylene. This has interesting implications for the
future design of QHC logic gates more complex than a NOR gate.

2. Experiments
All experiments, molecule deposition, imaging, atom manipulation
and electronic spectroscopy measurements, were carried out using a
gold crystal with (111) surface plane orientation as the substrate. Prior
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Fig. 2. (a) LT-UHV-STM dI/dV electronic tunneling spectra recorded at the two indicated
locations on a single Y trianthracene molecule
and (b) the corresponding constant current dI/
dV maps recorded exactly at the 4 tunneling
resonance maxima determined in (a): −1.33 V
for HOMO-1, −1.06 for HOMO, + 1.63 for
LUMO and + 2.2 V for LUMO + 1. (image size:
2.5 × 2.5 nm).

reduced forms of the trianthracene molecule giving rise to the −1.06 V
and + 1.63 V tunneling electronic resonances. This comes from the fact
that a given dI/dV molecule electronic resonance in a tunneling junction must be first interpreted by decomposing the corresponding molecule resonating multi-electron quantum state in a superposition of
Slater determinants and each determinant built up for example on a MO
basis set. For example, this is the case here for the first resonance below
the Fermi level of the trianthracene molecule. For such a closed shell
molecule, the electronic ground state is essentially described by a single
Slater determinant built up by filling all the mono-electronic levels up
to 2 electrons on its HOMO. In this case, the detected resonance at
−1.06 V is coming from a single hole transfer process through the
molecule well described using a virtual quantum state described in first
approximation by a single Slater determinant with one electron less in
its HOMO. It is mainly the overlap between this HOMO and the atomic
orbital pointer like quantum state of the tip apex end atom which is
governing the characteristics width of this resonance and its spatial
distribution along the trianthracene molecule. More elaborate configuration interaction (CI) calculations are now in developments to describe better such tunneling process involving a multi-determinant decomposition to span the CI active space required to describe a tunneling
process [18].
Following this interpretation, the two other resonances at −1.33 V
and + 2.2 V can be attributed in first approximation to the HOMO-1
and LUMO + 1 MOs as also confirmed by ESQC calculations [17]. The
HOMO dI/dV map in Fig. 2b clearly indicates the location of the tunnel
junction conductance maxima, which is where the Au atoms should be
positioned in order to ensure the maximum electronic interaction with
the Y input branches. The cross examination between dI/dV maps and
the corresponding trianthracene NC-AFM images reported by Guillermet et al. [17] enables the identification of the spatial relationship
between differential conductance lobes captured in the dI/dV maps
relative to the molecule's structure, hence facilitating the identification

of the exact site for the gold-aromatic ring coordination that takes place
after the manipulated atom is brought into interaction with the selected
input branch, as well as the optimum location for the STM tip apex that
records the dI/dV spectra and characterizes the logical output.
To set the logical inputs in a trianthracene molecule, Au atoms were
produced by gently indenting the STM tip apex into the Au(111) surface
[19]. Following this procedure, single Au atoms can be found around
the indented area. Some of them were manipulated by STM so as to
interact with an isolated trianthracene molecule and act as the logical
inputs. In keeping with the previously established logical inputs on
trinaphthylene [14], a ``1'' logical input is performed on trianthracene
when a single Au atom is manipulated towards one of its two input
branches and coordinated to the corresponding terminal aromatic ring.
An electronic re-organization of the trianthracene electronic structure
results from such electronic interaction producing a shift in energy of
trianthracene's ground state (imaged like its main HOMO contribution
to the tunnel junction conductance). Aside from the (0,0) logical input
configuration for a non-coordinated Y molecule, the three other (0,1),
(1,0) and (1,1) logical input configurations were constructed by coordinating an Au atom each to one or two of the input branches. The
corresponding configuration and STM constant current images are
presented in Fig. 3. It is also possible to coordinate single Au atoms to
any of the two aromatic rings located between the center of the molecule and the end of each branch. However, such Au-trianthracene
complexes are very unstable and prone to movement during imaging or
while recording the dI/dV spectra. This arises from the fact that an Au
atom coordinated to the terminal aryl group of a branch induces a local
deformation of this aromatic ring – the atom coordinates between two
carbon atoms whose two corresponding terminal hydrogen atoms are
pointing upwards with respect to the plane of the substrate [14]. An
aryl group within a Y branch is less deformable resulting in a rather
unstable coordination. Such intermediate complexes were not considered further in this investigation.
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Fig. 3. Topographic images and dI/dV maps of all Au-trianthracene input conformations investigated. Each input conformation is indicated in parenthesis where, for example, (1,1)
indicates that one atom was input at the position α and another one at β as indicated on the molecule models by solid
circles representing the Au atoms (STM image size:
2.5 × 2.5 nm).

broader. At those negative voltages, the two native electronic resonances of the bare trianthracene molecule were better resolved
(Fig. 4) as compared to those in the spectra in Fig. 2. After one Au input,
the first −1.172 V resonance for the (0,1) or (1,0) logical input configuration is shifted down by −99 mV with respect to the bare trianthracene −1.073 V resonance. When a second Au atom is coordinated
to trianthracene setting the (1,1) logical input, the first −1.138 V resonance is also shifted down relative to bare trianthracene's −1.073 V
resonance, but only by 65 mV. Notice that the lock-in detection technique used in those measurements is indicating that in average the
−1.172 V and −1.138 V resonances are not so well distinguishable as
compared to the −1.073 V reference one for the logical output determination.
According to QHC theory, the logical output status of a QHC logic
gate can be obtained by a spectral or a current intensity measurement at
the output branch [14]. With a physisorbed molecule and with the tip
apex kept at large distance from the surface, the dI/dV measurements
used are characteristic of a spectral output. To identify the molecular

In Fig. 3, topography STM images clearly indicate the position of the
Au atoms coordinated at the end of the trianthracene input branches. As
expected, significant changes are clearly visible in the internal contrast
of the dI/dV maps from those of bare trianthracene on account of Autrianthracene electronic interactions, as shown in Fig. 3. A higher differential conductance is observed at the end of the Y output branch
corresponding to the logical inputs configuration (0,1) and (1,1).
As presented in Fig. 4, the three dI/dV spectra recorded for bare
trianthracene and its metal-molecule conformations corresponding
(0,0), (0,1) and (1,1) were recorded by positioning the STM tip apex at
one of the conductance maxima at the end of the Y output branch
(observed in the dI/dV maps shown in Fig. 3). In the −2.0 V to + 2.0 V
bias voltage range, it is always possible to capture the four electronic
resonances of the Au-trianthracene complexes, as already observed in
Fig. 2 for the bare trianthracene. To track better the trianthracene's
QHC logic gate functioning, we decided to focus only on the negative
voltage resonances because they are very sharp and easily distinguished
from each other, while the resonances at positive voltages are much

Fig. 4. dI/dV spectra of the constructed Au-trianthracene
complexes corresponding to all (α,β) input conformations.
The spectra were taken by positioning the STM tip over the
output branch as shown in the sketch. Trianthracene's resonance centered at −1073 mV corresponding to the molecule's HOMO and the output i.e. 1, produced by the (0,0) input
is selected as the molecule QHC energy reference.
Accordingly, spectral shifts for the other inputs are also tabulated in a Boolean truth table showing that the molecule's
output conforms with a NOR logic gate output.

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Fig. 5. For comparison, the dI/dV spectra of the constructed
Au-trinaphthylene complexes corresponding to all (α,β) input
conformations. The spectra were taken by positioning the STM
tip over the output branch as shown in the sketch. The short
trinaphthylene's resonance centered at −1570 mV corresponding to the molecule's HOMO and to the logical output
“1” of the (0,0) input configuration is selected as the molecule
QHC reference. Accordingly spectral shifts for the other inputs
are also tabulated in a Boolean truth table showing that the
molecule's output conforms with a NOR logic gate output.
Notice that as compared to Fig 4, the (1,1) first resonance in
Fig. 5 is pushed further down in energy as compared to the
(1,0) peak because the Au-Au distance is still small on the
trinaphthylene molecule as compared to the Fig. 5 Au-Au
larger distance for also the (1,1) logical input. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article).

organization through the whole Y topology is different. In this case, the
decay of the shift resulting from an increase of the transmission path
length is very slow because of the central node which plays the role of a
``stub'' as in a standard microwave circuit. This is a very interesting
effect to consider in the design of more complex QHC logic gate molecules whose lateral spatial extension will be necessary, even if in QHC
this extension would be very moderate [1] as compared to a standard
molecular electronic intramolecular circuit design proposal [10–12].
Further investigations with larger molecules are still necessary to confirm the moderate shift decay of the Y topology experimentally.

logic gate output status, the HOMO like resonance centered at
−1.073 V was used as a reference for the logical output ``1''. Any input
configuration giving rise to a resonance at the same voltage value also
corresponds to ``1'' and resonances shifted by a voltage offset with respect to this reference or the absence of a resonance corresponds to a
``0'' logical output. Accordingly, the trianthracene molecule functions
like a NOR gate but with a narrower margin for the different output
resonances than for the shorter trinaphthylene molecule (See Fig. 5).
4. Conclusion

Acknowledgments

When a single Au atom is coordinated to one of the input branch of
any of those Y molecules i.e. trianthracene or trinaphthylene, a shift in
the molecules’ ground state takes place as presented in Figs. 4 and 5.
This shift was detected by recording the position in voltage (in energy
relative to the Fermi energy) of the corresponding electronic resonances. For a conjugated molecule, this change in the electronic
spectrum is distributed all along the molecule skeleton and is better
detected at a conductance maximum as indicated by the Fig. 3 dI/dV
maps. Interestingly and as reported in Fig. 4, the effects of the electronic interaction between the Au atom and the terminal aromatic ring
of one input branch can be detected rather far away in distance from
the Au-molecule coordination site, actually at the other end of the
molecule at the last phenyl ring of the output branch. As indicated by
the QHC theory, there is a quantum information conversion from the
classical input event, through the manipulation of the Au atom that is in
contact with the input aromatic ring, and the resulting change in the
π electronic structures of the molecule. Then, the information about the
event of an Au atom interacting with a terminal phenyl group is distributed over the electronic states of the molecule. The number of
π electronic states involved can be evaluated in detail by considering
the change in the composition of the low lying in energy π electronic
states decomposed on a standard Slater's orbital basis set.
For a linear system like a long oligo-acene molecule, the orbital
weight distribution of the perturbation induced by a classical logic
input at one end of the molecule will decay very fast along the molecular skeleton and with the length of the molecule. Almost no ground
state shift will be detectable at the opposite end of an oligo-acene of at
least 10 linearly fused phenyl rings. For the trianthracene and trinaphthylene molecules, a comparison between their respective (0,0) to
(0,1) resonance shift indicates only a small change, ΔV: 99 mV for trianthracene (Fig. 4) and ΔV: 94 mV for trinaphthylene (Fig. 5). Intuitively, an exponential decay is expected since the transmission path
for trianthracene is longer by two phenyls as compared to trinaphthylene's. Indeed this phenomenon can be found from calculations using a
simple linear tight binding (TB) model with alternant couplings along
the TB chain. However the transmission of quantum information for a Y
shape TB model featuring the alternant electronic coupling

We acknowledge financial support from Singapore's Agency of
Science, Technology and Research (A*STAR) for the Visiting
Investigatorship Program (Phase III): AtomTech Project, the AtMol
European Commission integrated project and the WPI MANA-NIMS
program.
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