# Probing charge redistribution at the interface of self-assembled
cyclo-$P_{\rm 5}$ pentamers on Ag(111)

Outhmane Chahib  1 Yulin Yin  2 Jung-Ching Liu  1 Chao Li  1 Thilo Glatzel  1
Feng Ding  2 Qinghong Yuan  3 Ernst Meyer1,∗ & Rémy Pawlak1,∗

Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel,
Switzerland

Faculty of Materials Science and Engineering/Institute of Technology for
Carbon Neutrality, Shenzhen Institute of Advanced Technology, Chinese Academy
of Sciences, Shenzhen 518055, China

State Key Laboratory of Precision Spectroscopy School of Physics and
Electronic Science, East China Normal University, 500 Dongchuan Road, Shanghai
200241, China

## Abstract

Phosphorus pentamer (cyclo-$P_{\rm 5}^{-}$) ions are unstable in nature but
can be synthesized at the Ag(111) surface. Unlike monolayer black phosphorous,
little is known about their electronic properties when in contact with metal
electrodes, although this is crucial for future applications. Here we
characterize the atomic structure of cyclo-$P_{\rm 5}$ assembled on Ag(111)
using atomic force microscopy with functionalized tips and density functional
theory. Combining force and tunneling spectroscopy, we find that a strong
charge transfer induces an inward dipole moment at the cyclo-$P_{\rm 5}$/Ag
interface as well as the formation of an interface state. We probe the image
potential states by field-effect resonant tunneling and quantify the increase
of the local change of work function of 0.46 eV at the cyclo-$P_{\rm 5}$
assembly. Our results suggest that the high-quality of the cyclo-$P_{\rm
5}$/Ag interface might serve as a prototypical system for electric contacts in
phosphorus-based semiconductor devices.

### Keywords:

cyclo-$P_{\rm 5}^{-}$ pentamer, work function, atomic force microscopy,
scanning tunneling microscopy, ﬁeld-emission resonance spectroscopy, density
functional theory

## Introduction

Elemental phosphorus (P) is not only ubiquitous in human life, it is also one
of the most fascinating areas of chemistry as it can exist in a large
diversity of allotropes,[1, 2, 3] in various cluster configurations [4, 5] or
in organic compounds.[6] The phosphorous polymorphism is even multiplied on
the atomic-scale when using a surface to constrain the reaction in two
dimensions (2D). As in the field of on-surface chemistry producing complex
nanographene structures in ultra-high vacuum (UHV),[7, 8] surface-assisted
phosphorus reactions on metals have synthesized blue phosphorus,[9] P
chains[10] or even planar cyclo-$P_{\rm 5}$ rings.[11, 12] Since then,
phosphorus allotropes have emerged as a promising one-atom thick 2D material
beyond graphene, due to its moderate direct band gap (0.3 to 2.0 eV)[13]
suitable for nanoelectronics and nanophotonics applications.[14, 15] However,
allotropic configurations, their atomic buckling, defects or potential alloy
formation can be detrimental for the semiconducting character. In addition,
the interaction of 2D materials with delocalized electrons of a metal, as well
as the dynamical charge transfer between the two media, are key factors in
fostering new gate-tunable functionalities such as superconductivity.[16, 17]
Experimental study of these aspects at the fundamental level is therefore
essential for future quantum applications where metallic electrical contacts
are required.[18]

Low-temperature scanning probe microscopy is an incontrovertible tool for
assessing atomic structures in contact to metals and characterizing their
electronic properties with high spectral resolution in UHV. Atomic force
microscopy (AFM) with functionalized tips[19, 20] has opened new avenues into
the real-space imaging with improved lateral and vertical resolution of
aromatic molecules, cyclo-carbon[21] and monoelemental 2D materials,[22, 23]
while charge distributions and work function changes at the nanometer scale
are also accessible using Kelvin probe force microscopy (KPFM).[24, 25, 26,
27] The investigation of the local density of states (LDOS) of 2D materials
near the Fermi level is readily achieved by means of scanning tunneling
microscopy and spectroscopy (STM/STS). Tunneling spectroscopy can also probe
the IPS of 2D synthetic materials such as graphene,[28] germanene[29] or
borophene.[30, 31] Quantifying these Stark-shifted unoccupied states lying
below the vacuum level give not only access to the fundamental physical
processes involved in charge carrier dynamics but also to quantify local
modulations of the work function at the interface between 2D materials and
metals.

By applying this in-situ methodology, we determine here the structure of
phosphorus chains and self-assembled cyclo-$P_{\rm 5}$ pentamers on Ag(111)
using low-temperature (4.5 K) AFM imaging with CO-terminated tips. KPFM
spectroscopic measurements indicates the formation of an inwards dipole moment
at the $P_{\rm 5}$/Ag interface, which results from the charge transfer from
the Ag substrate to the network as confirmed by DFT calculations. This charge
transfer leads to a complex charge redistribution and the formation of an
interfacial hybridized state (IS). Through field-emission resonance tunneling
(FERT) and STS spectroscopy, we determined the energy position of the IS state
and the series of IPS at the cyclo-$P_{\rm 5}$ assembly as compared to
pristine Ag, confirming an increase of the local work function of $\sim$ 0.46
eV. Given the strong interest in tailoring the physical characteristics of
monoelemental 2D materials contacted to a metal, we think that the
cyclo-$P_{\rm 5}$/Ag interface might serve as a model system for future
devices involving electric contacts.

## Atomic-scale imaging of phosphorus chains and cyclo-$P_{\rm 5}$ pentamers

Phosphorus atoms were sublimed in UHV onto the Ag(111) substrate kept at about
150 °C (see Methods). Figure 1a shows an STM overview image of the resulting
structures for a relative coverage of less than 0.3 monolayer (ML). Extended
1D chains aligned along the $[1\bar{1}0]$ directions of Ag(111) (marked 1 in
Fig. 1a) coexist with domains of $P_{\rm 5}$ molecules (2), as recently
reported in references.[11, 10] The inset of Fig. 1a further shows a STM image
of the double and triple chains, that depends on the P deposition rate.[11]
Each chain configuration has a relative STM height of 1.6 Å, and a width of 11
Å, 17 Å, and 26 Å for the single, double and triple chains, respectively.

To precisely determine their atomic configurations, we employed AFM imaging
with CO-terminated tips (see Methods, Figs. 1c-d).[19] A common AFM contrast
is observed for all configurations assigned to an armchair structure of the
chain, which resembles that of hydrocarbon chains.[32] The relaxed structure
of the triple chain configuration calculated from DFT is shown in Fig. 1e.
Phosphorus atoms colored in orange sit on bridge sites of the Ag lattice
(gray) and are aligned along one $[1\bar{1}0]$ direction in accordance with
the experimental data. Based on the DFT coordinates we simulated the AFM image
(see Methods, Fig.1f). The excellent agreement with the experimental image of
Fig. 1d confirms the armchair structure of the P chains on Ag(111).

Increasing the P coverage to about 0.4 ML while keeping the substrate at 150°C
leads to the formation of large islands of cyclo-$P_{\rm 5}$ pentamers
relative to the chains (Fig. 2a).[12] In Fig. 2b, the close-up STM image
reveals the structure of the self-assembled domains consisting of an hexagonal
lattice with parameters $a_{1}$ = $b_{1}$ = 7.3 Å . Each bright protrusion
corresponds to one cyclo-$P_{\rm 5}$ molecule as schematized by the black
dashed pentagons. Domains of cyclo-$P_{\rm 5}$ rings also exhibit a
superstructure characterized by stripes separated by $\approx$ 3.8 nm (i.e. 6
$P_{\rm 5}$ rows) as shown by black dotted lines in Fig. 2a. These lines are
rotated by 19° as compared to the $[1\bar{1}0]$ directions of the Ag(111)
substrate, which agrees with previous experimental works[11] as well as the
relaxed structure obtained by DFT calculations (Fig. 2d).[12] A deeper
insights into the chemical structure of the cyclo-$P_{\rm 5}$ molecules is
provided by the AFM image of Fig. 2c. The P-P bond length within the pentagon
extracted by AFM is about 2.2 Å , which is comparable to the value of 2.185 Å
obtained by DFT. For comparison, we also simulated the AFM image based on the
DFT coordinates, allowing us to confirm the exact position and structure of
the $P_{\rm 5}$ molecules in their self-assembly in registry with the Ag(111).

To accurately quantify the atomic corrugation within the cyclo-$P_{\rm 5}$
structure, we acquired a series of site-dependent $\Delta f(Z)$ spectroscopic
curves (Fig. 2f), at the locations marked in the inset AFM image. The black
and gray curves were obtained on Ag and between two pentamers, respectively.
On top of neighboring atoms of a cyclo-$P_{\rm 5}$ (orange and brown curve),
the spectra exhibits a characteristic dip arising from the interaction between
the front-end oxygen atom of the CO-terminated tip with the phosphorus atom.
The dashed vertical lines indicate the $Z$ position of their bottoms and is
the signature of the relative atomic $Z$ height.[23] The difference $\Delta Z$
of $\approx$ 20-30 pm thus represents the intrinsic atomic corrugation within
the cyclo-$P_{\rm 5}$ pentagonal structure,[5] which is comparable with atomic
corrugations in graphene[33] or planar molecules.[34] Thus, this confirms the
planarity of the cyclo-$P_{\rm 5}$ structure,[5] as reflected in the constant-
height AFM image of Fig. 2e.

## Charge distribution at the cyclo-$P_{\rm 5}$/Ag interface

The binding energy of $P_{5}$ pentamers has been calculated by DFT to be
strong on Ag(111), allowing the stabilization of the cyclo-$P_{\rm 5}$
structure through a charge transfer.[12] To provide insights into the charge
distribution at the cyclo-$P_{\rm 5}$/Ag interface, we performed force versus
voltage spectroscopic measurements (see Methods). Experimentally, the
frequency shift $\Delta f$ as a function of the sample bias $V_{\rm s}$ is
measured at a constant tip height $Z$, providing in the $\Delta f(V)$ curve a
parabola due to the electric force acting between tip and sample. The voltage
$V^{*}$ at top of the parabola represents the local contact potential
difference (LCPD) between tip and sample, which allows one to image charge
distributions and work function changes with nanoscale resolution.[24, 25, 26,
27] Figure 3a shows a $\Delta f(V)$ cross-section acquired across a $P_{\rm
5}$ domain (see STM inset of Fig. 3a). Single $\Delta f(V)$ point-spectra on
top of the $P_{\rm 5}$ network (orange) and on Ag(111) (black ) are plotted in
Fig. 3b, respectively. Dashed lines in both figures refers to the $V^{*}$
position. The LCPD value systematically shifts towards positive values
($\Delta V^{*}$ $\approx$ 0.22 V) for the pentamer assembly as compared to the
pristine Ag substrate. This indicates the accumulation of charges at the
$P_{\rm 5}$ network as compared to the Ag substrate.

To better rationalize this, we calculated the charge redistribution at the
cyclo-$P_{\rm 5}$/Ag interface (see Methods), whose top and side views of
isosurfaces of electron accumulation (blue, +13$\times$10-3 e/Å3) and
depletion (red, -13$\times$10-3 e/Å3) are displayed in Fig. 3c. An electron
transfer from the Ag(111) substrate to the P atoms of the pentamers ( is
observed as a charge accumulation located at the cyclo-$P_{\rm 5}$ ring (red).
In the $P_{5}$/Ag gap (marked by white and black dashed lines in the side view
of Fig. 3c), charge accumulation/depletion layers emerges below each
cyclo-$P_{\rm 5}$ structure, which supports the formation of an hybridized
state.[30, 31] We emphasize that such an interface state is not restricted to
2D Xenes on metals since it is well-established in organic/metal systems.[35]

Between cyclo-$P_{\rm 5}$ rings, we note the absence of in-plane charge
redistribution. Considering that the last Ag layer is depleted (red) while
each cyclo-$P_{\rm 5}$ has an excess of charges (blue), the $P_{5}$ assembly
can be approximated to a lattice of surface dipole moments of $D$ = 1.42 Debye
pointing towards the substrate (see arrow in Fig. 3c). This observation is
consistent with an increase of the LWF at the $P_{5}$/Ag interface induced by
an inwards dipole moment as schematized in Fig. 3d, which is in agreement with
the increase of the LCPD in force spectroscopy. It is important to mention
here that the LCPD value has a strong distance-dependence on metal substrate,
which is good indicate of the local work function changes at the atomic scale,
but prevents quantitative determination.[36, 37] Indeed, the $\Delta V^{*}$
cannot directly account for the difference of work function $\Delta\phi$ =
$\phi_{P_{5}/Ag}$ \- $\phi_{Ag}$ shown in Fig. 3d, due to averaging effects of
the electrostatic interactions between tip and sample. Last, the presence of
an interfacial state can alter the amount of charge transfer away from an
integer number compared to those with weaker interactions or adsorbed on
insulating layers. Indeed, the Bader charge analysis show an accumulation of
electrons on P atoms (-0.115 e) and an electron depletion (+0.057 e) of the
depleted Ag layer. Thus, we conclude that the cyclo-$P_{\rm 5}$ does not have
a pure anionic character for the $P_{5}$ molecule when adsorbed on Ag(111)
(i.e. cyclo-$P_{\rm 5}^{-}$), as expected by theory for its gas-phase
counterpart.

## Interface state and work function of the cyclo-$P_{\rm 5}$ assembly

To shed more lights into the electronic properties at the $P_{\rm 5}$/Ag
interface, we next performed differential conductance measurements (dI/dV)
across one $P_{5}$ domain (see Methods). Figure 4a shows the typical dI/dV
point-spectra spectra of the network (orange) as compared to Ag(111) (black).
We assign the gap of the $P_{5}$ assembly to about 0.9-1.0 eV (dashed lines)
similar to Ref.[11] The spectra also shows a strong resonance at 2.5 V, which
we attribute to tunneling into the interface state ($IS$), respectively. dI/dV
maps (Fig. 4b) further reveal the density of states at the valence band at
$V_{\rm s}$ = -0.5 V. This atomic feature evolves to a stripe pattern at
$V_{\rm s}$ = +2.5 V (Fig. 4c), revealing the spatial modulation of the IS
state (Fig. 4c) similar to the superstructure shown in STM topographic image
of Fig. 2a. This state, which derived from the occupied Shockley state of the
clean Ag(111) surface, is upshifted by more than 2 eV and becomes unoccupied
by the presence of the $P_{5}$ assembly.[35]

To quantify the local change of work function (LWF), we acquired field-effect
resonant tunneling (FERT) spectra in order to probe IPS between the
cyclo-$P_{\rm 5}$ assembly and silver.[28, 29, 30, 31] Experimentally, FERT
spectra (also called dZ/dV spectroscopy) are obtained by sweeping the sample
voltage $V_{\rm s}$ while measuring dI/dV at constant-current by adjusting the
tip height $Z$ using the STM feedback loop (see Methods). From a quasi-
classical approximation (Fig. 4d), tunneling resonances spectrum occurs when
the Fermi level of the tip aligns with the Stark-shifted IPS states and follow
the equation :

$eV_{\rm n}=\phi+\left(\frac{3n\pi\hbar eE}{\sqrt{2m}}\right)$ (1)

where $V_{\rm n}$ is the sample voltage for the nth IPS, $\phi$ is the work
function of the sample, $m$ is the free electron mass and $E$ is the electric
field.

Figure 4d shows the series of IPS states obtained above the cyclo-$P_{\rm 5}$
self-assembly as compared to Ag(111), respectively. The resonance at $V_{\rm
s}$ = 2.2 eV of the orange spectrum, which is absent for the Ag one (black),
corresponds to the IS state. The peaks noted $n$ = $1$ to $7$ of the black
spectra are the IPS states of the pristine Ag substrate. On the $P_{5}$
assembly, IPS states are clearly shifted to higher voltage when increasing the
electric field (i.e. $V_{\rm s}$), which is the signature of the change of
LWF.[30, 31]

A quantitative estimation of the LWF can be obtained from Eq. 1. In Fig. 4e,
we plot the voltage position $V_{\rm n}$ of the IPS states as a function of
$n^{2/3}$ for both the cyclo-$P_{\rm 5}$ network (orange squares) and the
Ag(111) substrate (black triangles). By fitting the linear progression of each
datasets, we extract the LWF value corresponding to the $y$-intercepts to
$\phi_{\rm Ag}$ = 4.49 eV and $\phi_{\rm P_{\rm 5}}$ = 4.95 eV. Considering
that our experimental estimate of $\phi_{\rm Ag}$ is in excellent agreement
with that obtained by ultraviolet photoelectron spectroscopy (UPS),[38] we
confirm the strong increase of LWF of $\Delta\phi$ = 0.46 eV induced by the
cyclo-$P_{\rm 5}$ assembly adsorbed on Ag(111). Altogether, the observation of
an IS and the shift of the IPS resonances in tunneling spectroscopy point to a
charge transfer from the Ag substrate to the cyclo-$P_{\rm 5}$ network and the
creation of a strong inwards electric dipole at the interface.

## Summary and outlook

In summary, we synthesized phosphorus chains and cyclo-$P_{\rm 5}$ pentamers
by depositing phosphorus atoms on atomically flat Ag(111) in ultra-high
vacuum. Using low-temperature AFM with CO-terminated tips, armchair $P$ chains
and the planar cyclo-$P_{\rm 5}$ rings are resolved with atomic precision.
Flat-lying cyclo-$P_{\rm 5}$ pentamers self-assemble into an extended
hexagonal assembly in registry with the Ag substrate. DFT calculations support
a substantial charge transfer from the Ag substrate to $P_{\rm 5}$ pentamers,
which results in a complex charge redistribution at the $P_{\rm 5}$/Ag
interface and the emergence of an interface state. Using force spectroscopic
measurements, the inward surface dipole moment induced by this charge transfer
is confirmed as an increase of the LCPD value at the $P_{\rm 5}$ assembly in
comparison to the pristine Ag. This corresponds to an increase of LWF at the
$P_{5}$ network as compared to the bare metal substrate. We corroborated these
measurement with FERT spectroscopy allowing us to quantify the LWF increase of
0.46 eV at the $P_{\rm 5}$/Ag interface. By exploring the fundamental
characteristics of the prototypical cyclo-$P_{\rm 5}$/metal interface, our
results not only underline the importance of scanning probe microscopy
(applicable to other emerging 2D materials and related quantum materials) to
study structural and electronic properties at the atomic scale, but also
provides new insights for improved performances of phosphorus-based devices.

## Methods

Sample preparation
The Ag(111) substrate purchased from Mateck GmbH was sputtered by Ar+ ions and
annealed at 500 °C to eliminate any surface contaminations. Phosphorus atoms
were sublimed by heating up a black phosphorus crystal contained in a Knudsen
cell in ultra high vacuum (UHV). The P flux was estimated using a quartz
microbalance. To obtain the phosphorus chains and $P_{5}$ domains, we annealed
the Ag(111) substrate during deposition at temperatures described in Ref. [9]

STM experiments
The STM experiments were conducted at a temperature of 4.8 K using an Omicron
GmbH low-temperature STM/AFM system operated with Nanonis RC5 electronics.
Differential conductance spectroscopy dI/dV(V) spectra were acquired with the
lock-in amplifier technique using a modulation of 610 Hz and a modulation
amplitude of 10 meV. All voltages refer to the sample bias $V_{\rm s}$ with
respect to the tip. For field-emission resonance tunneling spectroscopy
(FERT), the lock-in amplifier generates a 15-30 mV (RMS) bias modulation at
650 Hz. The FERT spectra is obtained by recording the differential conductance
data while the sample bias is swept with a closed feedback loop (setpoints:
$I_{\rm t}$ = 1 pA, $V_{\rm s}$ = 500 mV).

AFM experiments
AFM measurements were performed with commercially available tuning-fork
sensors in the qPlus configuration[39] equipped with a tungsten tip ($f_{0}$ =
26 kHz, Q = 10000 to 25 000, nominal spring constant $k$ = 1800, N.m-1,
oscillation amplitude A $\approx$ 50 pm. Constant-height AFM images were
obtained using tips terminated with a single carbon monoxide (CO) in the non-
contact mode (frequency-modulated AFM–FMAFM) at zero voltage.[19, 40] CO
molecules were adsorbed on the sample maintained at low temperature below 20
K. Before its functionalization, the apex was sharpened by gentle indentations
into the silver surface. A single CO molecule was carefully attached to the
tip following the procedure of reference.[41] Simulations of the AFM images
based on the DFT coordinates were carried out using the probe-particle
model.[42] Site-dependent $\Delta f(Z)$ spectroscopic measurements to
determine the atomic buckling of phosphorus pentamers were obtained with CO-
terminated tips. The $\Delta f(V)$ cross-section of 1$\times$85 pixels2 was
acquired with Ag-coated metallic tips (tunneling setpoints: $I_{\rm t}$ = 1
pA, $V_{\rm s}$ = 800 mV, $Z_{\rm offset}$ = +80 pm).

DFT calculations
All density functional theory calculations were carried out in the Vienna ab
initio simulation package (VASP)[43] with projector augmented wave (PAW)[44,
45] method. The generalized gradient approximation (GGA) in the framework of
Perdew-Burke-Ernzerhof (PBE)[46] was chosen with the plane-wave cutoff energy
set at 400 eV for all calculations. The DFT-D3[47] method of Grimme was
employed to describe the van der Waals (vdW) interactions. The geometries of
the structures were relaxed until the force on each atom was less than 0.02 eV
Å-1, and the energy convergence criterion of 1$\times$10-4 eV was met. The
Brillouin zone was sampled using Gamma k-mesh with a separation criterion of
0.03. Metal slabs with 3 atomic layers was adopted as the substrate and the
bottom layer was fixed to simulate the bulk. The vacuum spacing between
neighboring images was set at least 15 Åalong the non-periodic directions to
avoid a periodic interaction.

## Data availability

The data that supports the findings of this study are available within the
paper or its Supplementary Information. All STM/AFM images are raw data. The
raw data of spectroscopic measurements are available from the repository
ZENODO (link will be updated).

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## Acknowledgments

E.M. and R.P. acknowledge funding from the Swiss Nanoscience Institute (SNI),
the European Research Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (ULTRADISS grant agreement No 834402 and
supports as a part of NCCR SPIN, a National Centre of Competence (or
Excellence) in Research, funded by the SNF (grant number 51NF40-180604). E.M.,
T.G. and S.-X.L. acknowledge the Sinergia Project funded by the SNF
(CRSII5_213533). E.M., T.G. and R.P. acknowledge the SNF grant
(200020_188445). T.G. acknowledges the FET-Open program (Q-AFM grant agreement
No 828966) of the European Commission. J.-C.L. acknowledges funding from the
European Union’s Horizon 2020 research and innovation programme under the
Marie Sklodowska-Curie grant agreement number 847471. C.L. and E.M.
acknowledges the Georg H. Endress Foundation.

## Author information

Authors and Affiliations
Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel,
Switzerland
Outhmane Chahib, Chao Li, Jung-Ching Liu, Thilo Glatzel, Ernst Meyer & Rémy
Pawlak

State Key Laboratory of Precision Spectroscopy School of Physics and
Electronic Science, East China Normal University, 500 Dongchuan Road, Shanghai
200241, China
Yulin Yin & Qinghong Yuan,

Faculty of Materials Science and Engineering/Institute of Technology for
Carbon Neutrality, Shenzhen Institute of Advanced Technology, Chinese Academy
of Sciences, Shenzhen 518055, China
Feng Ding

Contributions
R.P. and E.M. conceived the experiments. O.C. and R.P. performed the STM/AFM
measurements. Y.Y. F.D. and Q.Y. performed DFT calculations. O.C. and R.P.
analyzed the data. R.P. wrote the manuscript. All authors discussed on the
results and revised the manuscript.

Corresponding authors
Correspondence to<EMAIL_ADDRESS>or<EMAIL_ADDRESS>
## Ethics declarations

Competing interests
The authors declare no competing interests.

Fig. 1: Atomic structure of phosphorus chains on Ag(111). a, STM topographic
image after sublimation of phosphorus atoms on Ag(111) leading to P chains (1)
and cyclo-$P_{\rm 5}$ domains (2), ($I_{\rm T}$ = 1 pA, $V$ = 0.15 mV). The
inset shows a STM image of the single, double and triple chains, respectively.
b-d, Series of AFM images with CO-terminated tip revealing the armchair
structure of single, double and triple P chains, ($f_{\rm 0}$ = 26 kHz, $A$ =
50 pm). Scale bars are 1 nm. e, Atomic configurations of the triple armchair
chains obtained by DFT calculations. Phosphorus and silver atoms are shown in
orang and gray, respectively. f, Corresponding AFM simulation using the DFT
coordinates.  Fig. 2: Atomic structure of the self-assembled cyclo-$P_{\rm 5}$
molecules. a, STM image of the self-assembled pentamers on Ag(111), ($I_{\rm
T}$ = 1 pA, $V$ = 0.15 mV). Islands systematically shows a superlattice of
bright lines rotated by 19° with respect to the $[1\bar{1}0]$ directions of
Ag(111). b, Close-up STM topography showing the $P_{\rm 5}$ molecules depicted
by dashed pentagons. c, Corresponding AFM image revealing the $P_{\rm 5}$
chemical structure, ($f_{\rm 0}$ = 26 kHz, $A$ = 50 pm). d, Atomic
configurations of the pentamer assembly on Ag(111) obtained by DFT. Phosphorus
and silver atoms are shown in orange and gray, respectively. e, Corresponding
AFM simulation using the DFT coordinates. f, Site-dependent $\Delta f(Z)$
spectroscopic curves acquired at one P atoms of a $P_{\rm 5}$ molecule
(orange), between two $P_{\rm 5}$ molecules (brown) and on Ag(111) (black),
respectively. The local minima of the $\Delta f(Z)$ curves indicate the
relative height of the phosphorus atoms. Fig. 3: Charge redistribution at the
cyclo-$P_{\rm 5}$/Ag(111) interface. a, Frequency shift $\Delta f$ as a
function of sample bias voltage $V_{\rm s}$, measured across a pentamer domain
shown in the STM image (top), (parameters : $f_{\rm 0}$ = 26 kHz, $A$ = 80
pm). b, Single $\Delta f(V)$ curves at the pentamer assembly (orange) as
compared to the Ag(111) (black). Dashed lines mark the top of the parabola
allowing to extract a LCPD shift $\Delta V^{*}$ = 0.22 V. c, Top and side
views of the charge redistribution between pentamers and Ag(111). Blue areas
show electron accumulation, red areas electron depletion. The isosurface level
of the plot is set to $\pm$ 13$\times$10-3 e/Å3. d, Schematic illustration of
the charge redistribution at the $P_{\rm 5}$/Ag(111) interface leading to an
inward surface dipole ($D$) moment and a local work function change
($\phi_{\rm P_{5}/Ag}$). The cyclo-$P_{\rm 5}$ layer is colored in orange.
$\Delta V^{*}$ refers to the LCPD change.  Fig. 4: Tunneling spectroscopy of
the $P_{\rm 5}$/Ag interface. a, dI/dV point-spectra acquired above the
$P_{\rm 5}$ assembly (orange) and on Ag(111) (black), where precise locations
are shown in the STM inset. (parameters: $I_{\rm t}$ = 1 pA, $V_{\rm s}$ = 500
mV, $A_{\rm mod}$ = 10 mV, $f$ = 511 Hz). b, dI/dV maps at $V_{\rm s}$ = -1.25
and 2.5 V corresponding to the valence band energy and the IS interface state
, respectively. c, STM topographic image of three $P_{\rm 5}$ domains and the
corresponding dI/dV maps of the IS modulation. d, Scheme of the band alignment
and the formation of Stark-shifted IPS (orange lines). e, Field-effect
resonance tunneling (FERT cross-section acquired across the $P_{\rm 5}$
assembly along the dashed line in a, (Set-points: $I_{\rm t}$ = 1 pA, $V_{\rm
s}$ = 500 mV, $A_{\rm mod}$ = 35 mV, $f$ = 511 Hz). f, Single FERT spectra of
the $P_{\rm 5}$ assembly and the Ag(111) substrate, showing the series of nth
IPS. g, Extracted IPS peak voltages as a function of $n^{\rm 2/3}$.