Tuning of electron tunneling: A case study using BODIPY molecular layers

Article abstract of DOI:10.1039/c9cp05918k

Redox active π-conjugated organic molecules have shown the potential to be used as electronic components such as diode and memory elements. Here, we demonstrate that using simple surface chemistry, rectification characteristics can be tuned to reproducibl

Full text of DOI:10.1039/c9cp05918k

View Article Online
View Journal
PCCP
Physical Chemistry Chemical Physics
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: N. Shivran, S. P.
Koiry, C. Majumder, A. K. Chauhan, D. K. Aswal, S. Chattopadhyay and S. Mula, Phys. Chem. Chem. Phys.,
2020, DOI: 10.1039/C9CP05918K.
Volume 19
This is an Accepted Manuscript, which has been through the
Number
January 2017
Pages 1-896
1
7
Royal Society of Chemistry peer review process and has been
accepted for publication.
PCCP
Physical Chemistry Chemical Physics
rsc.li/pccp
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1463-9076
PAPER
H.-P. Loock et al.
Determination of the thermal, oxidative and photochemical
degradation rates of scintillator liquid by fluorescence EEM
spectroscopy
rsc.li/pccp

Please do not adjust margins
Physical Chemistry Chemical Physics
Page 1 of 7
View Article Online
DOI: 10.1039/C9CP05918K
ARTICLE
Tuning of electron tunneling: A case study using BODIPY
molecular layers
Neelam Shivran,a Shankar P. Koiry,*b,c Chiranjib Majumder,c,d Anil K. Chauhan,b,c Dinesh K. Aswal,c,e
Received 00th January 20xx,
Accepted 00th January 20xx
Subrata Chattopadhyay a and Soumyaditya Mula*a,c
DOI: 10.1039/x0xx00000x
Redox active π-conjugated organic molecules have shown the potential to be used as electronic components such as diode
and memory elements. Here we demonstrate that using simple surface chemistry, rectification characteristics can be tuned
to reproducible negative differential resistance (NDR) with a very high peak-to-valley ratio (PVR) upto 1000 in 2,6-Diethyl-
4,4-difluoro-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indecene (BODIPY) grafted on Si. This change in properties is
related to oxidation and reduction of BODIPY which results change in resonant to non-resonant tunneling of electrons under
bias. This has been explained by the ab initio molecular-orbital theoretical calculations.
shown in σ–π systems which have high rectification ratio (RR)
[3a, 3b, 4]. On the other hand, the σ–π–σ structures have shown
the negative differential resistance (NDR) characteristic i.e.
decreasing current with increasing voltage in particular voltage
ranges with high peak to valley ratio (PVR) which has profound
potential in the realization of logic devices and memory circuits
[3a, 5].
For designing new molecules, it is prerequisite to get into
the details of electron transports through such molecular
structures. Since the size of the molecules is in the range of 0.7-
3 nm, the charge transport through molecules and across
molecule/electrode interfaces cannot be explained solely from
classical semiconductor physics. At that nm range, electron
transport is better understood by quantum mechanical
tunneling. Thus the detailed understanding of charge transport
is essential for developing next-generation molecular
electronics.
Introduction
Two decades ago, the electrical characterization of organic
molecules was started in a quest for tackling anticipated problems
arising from the size reduction of Si-based transistors, which has
been approaching fast to its physical limit [1]. However, the
semiconductor industries find ways to invent new technologies and
advance so much that many such technologies are waiting at our
doorsteps, such as artificial intelligence and the Internet of Things
(IoT) [2]. This advancement brings other challenges, for example,
materials and devices required for IoT applications should be suitable
for flexible substrates, extremely low powered, and capable of
energy generation or scavenging. Thus, the arrival of newer
technologies makes the studies of organic molecules for electronic
applications even more relevant. Because electronic properties of
organic molecules are studied only to make electronic devices faster,
smarter, portable, flexible, and low powered.
Although, this research area is in developing state, different
theories and experimental evidences have been proposed. The
most widely acceptable explanation for rectification
characteristics is resonant tunneling of electron through
molecular energy levels of π moiety in σ–π systems under
forward bias. In reverse bias tunneling is non-resonant which
results nonlinear current-voltage (J-V) characteristics. For NDR
behaviours, two tunneling mechanisms are involved. During the
current rise, the electron undergoes resonant tunneling
through molecular energy levels of π moiety as explained for
diode, and after further increase in voltage, the energy levels
get shifted which causes non-resonant tunneling leading to
decrease in current with increase in voltage. Thus, J-V
characteristics of σ–π–σ structures show current peak in J-V
measurements.
Initially, molecules grafted to silicon wafers, known as hybrid
nanoelectronics have been studied to develop various
electronics components e.g. molecular diodes, molecular
resonant tunnel diodes, molecular memory, molecular
transistors etc. [3]. In view of the device applications in hybrid
nanoelectronics, molecules grafted on Si should have
reproducible electronics properties at room temperature.
The recent studies have almost identified which molecular
structures would be useful for
a particular electronic
application. For example, the diode characteristics have been
a. Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai-400085, India.
b. Technical Physics Division, Bhabha Atomic Research Centre, Mumbai-400085,
India.
c. Homi Bhabha National Institute, Anushakti Nagar, Mumbai 400094, India.
d. Chemistry Division, Bhabha Atomic Research Centre, Mumbai-400085, India.
e. National Physical Laboratory, New Delhi, 110012, India.
Recently, various organic molecules such as conjugated
oligomers, redox-active-self-assembled monolayers, and hybrid
materials also has shown NDR property [6-7]. To explain the
observed NDR, several proposition have been given such as
† E-mail: spkoiry@barc.gov.in (SPK) and smula@barc.gov.in (SM)
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please do not adjust margins
Physical Chemistry Chemical Physics
Page 2 of 7
ARTICLE
Journal Name
formation of radical anion or cation by charging and Synthetic procedures
View Article Online
Synthesis of 4-(4'-penentenyloxy)benzaldehDyOdeI: (120).1.0A3m9/Cix9tCuPr0e5o9f184K-
hydroxybenzaldehyde (4.0 g, 32.7 mmol), 5-bromo-1-pentene
(4.7 mL, 39.3 mmol), K2CO3 (5.52 g, 40 mmol) and Bu4NI (10
mol%) in acetone (100 mL) was refluxed. After completion of
the reaction (cf. TLC ~ 16 h) the mixture was filtered,
concentrated in vacuum, the residue taken in Et2O (40 mL) and
washed with H2O (2 × 10 mL) and brine (1 × 20 mL), dried and
concentrated in vacuo. The residue was purified by column
chromatography (silica gel, 5% EtOAc/hexane) to give pure 2
(5.7 g, 91%) as viscous liquid. IR  3019, 1687 cm-1; 1H NMR (200
MHz, CDCl3, 25 oC, TMS):  1.28-1.42 (m, 2H), 2.32 (t, J = 6.8 Hz,
2H), 4.01 (t, J = 6.2 Hz, 2H), 6.97 (d, J = 9.5 Hz, 2H), 7.80 (d, J =
conformational changes [8]. Theoretical studies showed that
the origin of NDR with hysteresis is associated with the
polarization response, i.e., combined effects of charging and
conformational change [9]. These theoretical studies predict
that the molecules having redox properties and bias-induced
conformational changes are potential candidate for NDR effect
with hysteresis. It could be envisioned that the bias induced
conformational changes are difficult to accommodate in a
chemical bonded molecular layer. However, the change of
conformation would be easier to apprehend for the molecular
layers which are formed by weak physical interactions. This
strategy is explored here using bilayer of BODIPY (4,4-Difluoro-
4-bora-3a,4a-diaza-s-indacene) [10] molecules in which one
layer has σ–π structures grafted on Si and other layer (only π
moiety), formed by van der Waals force [11a].
The BODIPY class of compounds have robust reversible
redox properties [12]. These properties can be tuned by varying
different substituent on the dye chromophore to use in OLEDs,
solar cell etc. [13]. This reversible redox property of BODIPY
dyes and their robust stability under ambient conditions could
be very useful for their use in hybrid nanoelectronics which still
remains unprecedented. In this report, we discussed
rectification property of a tailor made BODIPY dye (σ–π moiety)
grafted on Si and how this property can be tuned to NDR
property by forming bi-layer with another BODIPY molecule
(only π moiety).
o
9.5 Hz, 2H), 9.85 (s, 1H); 13C{1H} NMR (50 MHz, CDCl3, 25 C,
TMS)  13.6, 22.2, 25.3, 28.7, 31.2, 68.1, 114.4, 129.6, 131.5,
163.9, 190.1; Anal. Calcd. for C12H14O2: C, 75.76; H, 7.42%.
Found: C, 75.44; H, 7.16%.
Synthesis of 2,6-Diethyl-4,4-difluoro-1,3,5,7-tetramethyl-8-(4’-(4”-
penteneoxyphenyl)-4-bora-3a,4a-diaza-s-indecene (BODIPY-C5).
A
mixture of 2,4-dimethyl-3-ethyl-1H-pyrrole (1) (1.00 g, 8.13
mmol), 4-(4'-penentenyloxy)benzaldehyde (2) (0.70 g, 3.7
o
mmol) and TFA (1 drop) in CH2Cl2 (30 mL) was stirred at 25 C
for 12 h. DDQ (0.84 g, 3.7 mmol) was added to the resulting
deep color solution and stirring continued for 4 h. The mixture
was treated with Et3N (3.1 mL) and stirred for 10 min. Finally,
BF3.Et2O (2.8 mL, 22.2 mmol) was added into the mixture and
the solution stirred at room temperature for 12 h. The resulting
dark mixture was washed with aqueous saturated NaHCO3 (50
mL), H2O (50 mL) and brine (50 mL) and dried. Removal of
solvent in vacuum followed by column chromatography of the
residue (silica gel, hexane-EtOAc) furnished BODIPY-C5 (0.24 g,
13.7%), which was recrystallized from CH2Cl2/cyclohexane to
afford red square shaped crystals. Mp: 177-178 °C; IR: 2964,
2880, 1608, 1643 cm-1; 1H NMR (200 MHz, CDCl3, 25 oC, TMS): δ
0.98 (t, J = 7.6 Hz, 6H), 1.34 (s, 6H), 1.93 (t, J = 7.7 Hz, 2H), 2.16–
2.36 (m, 6H), 2.53 (s, 6H), 4.03 (t, J = 6.5 Hz, 2H), 5.00–5.12 (m,
2H), 5.89-5.99 (m, 1H), 7.01 (d, J = 6.7, 2H), 7.13 (d, J = 6.7, 2H);
Experimental
General methods
For synthesis of BODIPY-C5, the chemicals and spectroscopic
grade solvents were purchased from Aldrich, Merck or Sigma
and were used without any further purification. PM567 was
purchased from Aldrich and was used without any further
purification. The Fourier transform infrared spectra (FTIR) were
1
recorded with a Nicolet 430 FT-IR spectrophotometer. The H
NMR and 13C NMR spectra were recorded with a Bruker 200 FT-
spectrometer in CDCl3. The mass spectra (70 eV) were recorded
with a MD-80 Fission instrument. The surface morphology of
mono/bilayer was characterized by AFM imaging (Multiview
4000, Nanonics) and molecular mass by SIMS (BARC make,
Kore’s Technology software). The current voltage
o
13C{1H} NMR (50 MHz, CDCl3, 25 C, TMS): δ11.7, 12.4, 14.5,
17.0, 28.3, 30.0, 67.2, 114.9, 115.2, 127.6, 129.3, 131.1, 132.5,
137.5, 138.4, 140.3, 153.3, 159.4; EI-MS (m/z): 464.0 (M+); Anal.
Calcd. For C28H35BF2N2O: C, 72.42; H, 7.60; N, 6.03%. Found: C,
72.05; H, 7.91; N, 6.28%.
characteristics were recorded in
a dark box using a
Measurement of current-voltage characteristics
potentiostat/galvanostat system (model: Autolab PGSTAT 30).
The thickness of the films was measured using an ellipsometer
(Sentech: model SE400adv); For measurement, a Si/organic
layer/Air model was designed. Before deposition of monolayers
on Si, we measured n and k values for the substrate. After
organic layer deposition, we used measured n, k value of the
substrate and assumed n = 1.5 and k = 0 for organic. With these
input values, model was fitted to obtained thickness of the
organic layer.
To measure current–voltage (J–V) characteristics,
a
metal/monolayer or bilayer/ Si(n++) structure was completed by
using a very small drop of liquid mercury as a counter electrode.
The J–V’s were recorded at room temperature by scanning the
bias in the sequence -1.8 V → 0 V → +1.8 V→ 0 V → -1.8 V at a
scan speed was 5 mV/s in a dark box using HP4140 (pA meter–
dc voltage source). Room temperature J-V characteristic
recorded for un-deposited Si surface is shown in Figure 1.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 7
Physical Chemistry Chemical Physics
Please do not adjust margins
Journal Name
ARTICLE
View Article Online
DOI: 10.1039/C9CP05918K
Scheme 1. Synthesis of BODIPY-C5 and chemical structure
ofPM567.a) TFA, CH2Cl2, 25 oC, 4 h; b) DDQ, 25 oC, 4 h; c) TEA,
BF3.Et2O, 25 oC, 12 h.
At first, 4-(4'-penentenyloxy)benzaldehyde (2) was
Figure 1. Room temperature J-V characteristic recorded for un-
deposited Si surface by scanning the bias in the sequence -1.5 V →
0 V → +1.5 V→ 0 V → -1.5 V at a scan speed of 5 mV/s.
synthesized
by
base-catalyzed
alkylation
of4-
hydroxybenzaldehyde with 5-bromo-1-pentene. Then, TFA-
catalyzed condensation of 2 with kryptopyrrole (1) furnished
the dipyrromethane which was subjected to oxidation with
DDQ, followed by complexation with BF3 in the presence of Et3N
as the base to afford BODIPY-C5(Scheme 1).The structure of
BODIPY-C5was confirmed by analysing its NMR spectra in CDCl3.
For example, 1H NMR spectrum (Figure S3(a)) showed two
doublets for aromatic protonswith expected two
protonsintegration for each signal.The vinyl group of the
pentenoxylateral chain resonated as two different multiplets at
5.95 (one proton) and 5.05 ppm (two protons).Diagnostic triplet
for the pentenoxy methylene group appeared at 4.03 ppm. Four
benzylic methyl groups attached to the BODIPY core resonated
as two singlets at 2.53 and 1.34 ppmwith the expected six
protons integration for each signal.Another diagnostic tripletfor
the terminal methyl groups of the ethyl chains of the BODIPY
core appeared at 0.98 ppm with six protons integration.The
carbon spectrum showed six signals for the alkyl atoms,
expected 11 signals for aromatic and alkene atoms, and one
diagnostic signal for pentenoxygroup (Figure S3(b)).
Theoretical computation
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energies for neutral, and
at different oxidation state of BODIPY-C5 and PM567:BODIPY-
C5 molecules respectively were calculatedtheoretically. The
geometries and total energies of molecules were optimized
under the density functional theory using the linear
combination of atomic orbital (LCAO) approach using General
Atomic and Molecular Electronic Structure System (GAMESS). A
standard 6-31G+(d,p) basis set was employed for this purpose.
The exchange correlation energy was calculated using B3LYP
functional. This functional uses part of Hartree-Fock exchange
and Becke’s exchange functional, and the Lee-Yang-Parr
correlation functional. For the computation, we have used PC
and two supercomputers, namely, Ameya and Ajeya. Ameya is
128-proceesor ANUPUM supercomputer built on 64 dual Xeon
servers (2.4 GHz) as compute node, interconnected by a high-
speed (300 MBps) communication network. The 512-processors
ANUPAM-ameya supercomputer is built using 256 dual Xeon
servers as a compute node. Each server is based on dual Xeon,
3.6 GHz processors. The inter-communication network is
Gigabit Ethernet with a node-to-node communication speed of
1 Gbps. The performance of ANUPAM-ameya is 1.73 Teraflops.
The open source Linux operating system is used on each parallel
processing node in these two supercomputers.
Preparation of the Si(n++)-BODIPYs assemblies
BODIPY bilayer on H-terminated Si(n++) substrates was
deposited in two steps (Scheme 2) (see supplementary
information for details). In the first step, monolayer of BODIPY-
C5 was electrografted to H-terminated Si(n++) (Figure S2). The
electrografting mechanism of BODIPY-C5 to H-Si(n++) is based
on formation of Si-radicals on application of negative potential,
which reacts with C=C group of the molecules to form Si–C bond
(Figure S1) [14]. Electrografted BODIPY-C5/Si(n++) was sonicated
in dichloromethane, acetone and methanol for 10 min to
remove any physisorbed molecules. In the second step, PM567
monolayer was self-assembled on electrografted BODIPY-
C5/Si(n++) by dipping it into dichloromethane solution of PM567
for 24 h in inert atmosphere. It is proposed that PM567 through
its F atoms form hydrogen bonds, i.e., B–F…H–C with surface
terminating CH3 groups of BODIPY-C5/Si(n++) monolayer
(Scheme 2) [11a].
Results and discussion
Design and synthesis
2,6-Diethyl-4,4-difluro-1,3,5,7-tetramethyl-8-(4'-(4"-
pentenoxyphenyl)-4-bora-3a,4a-diaza-s-indecene
(BODIPY-
C5)(Scheme 1) was designed first in order to graft it on Si(n++)
surface. BODIPY-C5 has a C-5 alkyl chain terminated with C=C
group, which is essential for its electrografting to Si(n++) [3a, 4].
For the bi-layer formation, another BODIPY dye, PM567
(Scheme 1) was chosen because of its planner structure which
can easily intercalate into the self-assembled mono layer of
BODIPY-C5.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Physical Chemistry Chemical Physics
Page 4 of 7
ARTICLE
Journal Name
with the peaks due to the BODIPYfragments (Figure 2(b)). Also,
View Article Online
DOI: 10.1039/C9CP05918K
the SIMS data shown inthe Figure 3(a) shows fragmented boron
species B10 and B11 of BODIPY and, as expected, the number
of boron atoms in bilayer is nearly double than that of
monolayer.Thus, the SIMS data confirmed deposition of their
respective layers on the Si wafers.
Scheme 2. Pictorial representation of two steps deposition process
of BODIPY on Si(n++). Step 1: Electrografting of BODIPY-C5
monolayer on H-Si(n++); Step 2: formation of bilayer by self-
assembly of PM567 onto BODIPY-C5/Si(n++).
Hydrogen bonding ability of the covalently bound fluorine
atomsisquestioned previously [11b]. However, distinct C–H…F
hydrogen bond interactions were reported to stabilize the
layered crystal structure of Fuoroaromatics [11c],and to form
robust supramolecular assemblies of organic molecules [11d]
including BODIPYs [11a].In the present case also, C–H…F
hydrogen bond interactions helps to form bilayer BODIPY.
In bilayer, as the surface is not terminated by F atoms,
possibility of multilayer formation does not exist. This in fact has
been experimentally confirmed by dipping the BODIPY-C5
monolayer in PM567 solution for several days, as well as by
increasing its concentration. In all these experiments, no
increase in the film thickness was found.
Figure 3. (a) Time-of-flight secondary ion mass spectra of positive
secondary ions desorbed from monolayer and bilayer by mono-
isotopic 69Ga projectile.(b) Typical fast scan (100V/s) CVs recorded
using 0.1M TBAP in dichloromethane with BODIPY monolayer or
bilayer as working electrodes. Inset shows the magnified version of
the oxidation peaks after subtraction of the background. (c) and (d)
The noncontact atomic force microscope images recorded for
monolayer and bilayer, respectively.
Characterization of modified surfaces
The electrografting of the BODIPY-C5 monolayer on Si(n++)
and the formation of PM567:BODIPY-C5/Si(n++) bilayer was
experimentally confirmed by several techniques, such as,
secondary ion mass-spectrometry (SIMS), electrochemical
characterization, ellipsometry and atomic force microscopy
(AFM).
The SIMS data of the monolayer of BODIPY-C5 showed
peaks at m/z 325 and 281 amu due to the BODIPY fragments
(Figure 2(a)). Interestingly, the SIMS data of the bilayer
PM567:BODIPY-C5/Si(n++) showed an additional mass peak at
m/z 317 amu, accounting for the [M-1]+ peak for PM567 along
Figure 3(b) shows the fast scan (100 V/s) cyclic
voltammograms (CVs) recorded for BODIPY-C5/Si(n++) or bilayer
PM567:BODIPY-C5/Si(n++) as working electrode (surface area =
0.025 cm2) and 0.1 M TBAP as electrolyte with a Pt counter-
electrode and a Ag/AgCl reference electrode. As shown in the
inset of Figure 3(b), monolayer exhibits a single peak at 0.77 V,
indicating only one oxidation for BODIPY-C5 molecules.
Whereas, bilayer exhibits two closely spaced peaks at 0.73 V
and 0.83 V, indicating that bilayer undergoes a double
oxidation.
Both mono- and bilayers exhibit granular surface
morphology with an average grain size of 8 and 14 nm,
respectively (Figure 3(c) and 3(d)). The thickness of the mono-
and multilayer, as measured by ellipsometery, was found to be
respectively, 1.1±0.2 and 1.9±0.2 nm, which are in agreement
with corresponding theoretical lengths of BODIPY-C5 (1.2 nm)
and PM567:BODIPY-C5 (1.86 nm) molecules.
Figure 2. TOF-SIMS of positive secondary ions desorbed from
monolayer and bilayer by mono-isotopic 69Ga projectile. (a) BODIPY-
C5/Si(n++) monolayer; (b) PM567:BODIPY-C5/Si(n++) bilayer. (insets:
enlarged plots.)
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 7
Physical Chemistry Chemical Physics
Please do not adjust margins
Journal Name
ARTICLE
J-V characteristics
method, as implemented in GAMESS software to explained the
observed rectification and NDR behaviors of monolayer and
View Article Online
DOI: 10.1039/C9CP05918K
As shown in Figure 4, the J-V characteristics of monolayer
and bilayer are remarkably different. The monolayer exhibits a
rectification behavior, which is in accordance with that
predicted for σ-π molecular rectifiers [3a, 3b]. The rectification
ratio (RR) i.e. ratio of current at -1.5 V (in absolute value) and
the current at 1.5 V (RR = | J-1.5 V | / J1.5 V) was ~64. The RR value
measured for 50 samples are varied in the range of 40-70, and
more than 90% samples showed rectification behaviours. It is
important to mention that the J–Vs of these diodes exhibit
hysteresis which may be due to formation of electric dipole
under high-applied electric field. RR decreases with repeated
measurements but the initial value of RR was restored by
keeping the structure is at zero voltage for about 10 min. Similar
observation was also reported previously [15]. On the other
hand, bilayer exhibited a strong NDR effect in concomitant with
a pronounced hysteresis. The room temperature PVR values
measured for 80 samples. Around 50% of samples show PVR
values between 670 and 1000, which are the very high values as
compared to reported values till date for the molecules
covalently grafted on Si [3d, 5]. Among the rest, the values are
lying between 100-600 (~30%) and 50-100 (~20%). However,
during the experiment, we also encountered no results in a few
samples, which are because of the device got short-circuited
due to improper sample preparation. It may be noted that NDR
is obtained only in the first bias scan and in the subsequent bias
scans NDR disappears as the current remains low. The NDR
effect reappears after the electrodes are short-circuited which
is discussed vide infra.
bilayers, respectively [16]. Optimization of the geometries and
total energies of molecules were done by the density functional
theory calculations using the linear combination of atomic
orbital (LCAO) approach with a standard 6-31G+(d,p) basis set.
The exchange correlation energy was calculated using B3LYP
functional, which consists of Hartree-Fock exchange, Becke’s
exchange and Lee-Yang-Parr correlation functional [16].
Theoretically, it was found that BODIPY-C5 undergoes for a
single oxidation; while PM567:BODIPY-C5 undergoes for double
oxidation. This inference is in agreement with the experimental
results presented in the Figure 3(b). Bilayer undergoing double
oxidation is expected as it has two BODIPY layers. Theoretically
calculated spatial orientation of HOMO’s of neutral and, after Ist
and IInd oxidations of PM567:BODIPY-C5 are shown in Figure
5(a). It is seen that in the neutral state the HOMO lies on the
PM567:BODIPY-C5 moiety and after Ist oxidation though it more
or less remains on PM567:BODIPY-C5 moiety but with a slight
conformational change. However, after IInd oxidation, there is
huge conformational change as the HOMO largely shifts to the
alkyl-chain and this could be attributed to slippage or rotation
of PM567:BODIPY-C5 interface, which is weakly bonded though
B–F…H–C type hydrogen bonds.
One of the best ways to understand the electron transport,
is to draw the schematic energy level diagrams of the
Hg/PM567:BODIPY-C5/Si(n++) structure and to correlate with
observed J-V characteristics (Figure 5(b)). To draw the
schematic, we use theoretically calculated HOMO (-5.25, 5.26
eV) and LUMO (-2.01, -2.07 eV) energy levels of neutral
PM567:BODIPY-C5, and the standard Fermi levels of Hg and Si.
For neutral state, doubly degenerate and singly occupied
HOMO levels indicate the possibility of double oxidation
PM567:BODIPY-C5, which is in agreement with experimental
data presented in Figure. 1 (a). As shown in the Figure 5 (b), the
Figure 4. (a) Room temperature J-V characteristic recorded for
BODIPY monolayer and bilayer by scanning the bias in the sequence
-1.8 V → 0 V→ +1.8 V→ 0 V → -1.8 V at a scan speed was 5 mV/s.
Inset shows the schematic of the structures employed for the
measurements.
Theoretical explanation
One way to visualize the cause of such J-V characteristics is
to draw an energy levels diagram with respect to vacuum, and
to depict electron transport across the energy levels under
applied electric filed. For this purpose, the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energies of BODIPY-C5 and PM567:BODIPY-C5
molecules were calculated using molecular orbital theory
Figure 5.(a) Thoretically calculated geometrical orientation of
HOMO of PM567:BODIPY-C5 under different oxidation states. (b)
Schematic representation of the energy level diagrams for Hg/
PM567:BODIPY-C5/Si(n++) device at different applied bias.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Physical Chemistry Chemical Physics
Page 6 of 7
ARTICLE
Journal Name
degenerate HOMO and LUMO levels of PM567:BODIPY-C5 are
off-resonance with the Fermi level of the Hg electrode at zero
bias i.e. V=0. For an applied positive bias (V>0), a rise in current
in the J-V (see Figure 4) is attributed to non-resonant tunneling.
A sharp rise in current above > 0.8V as per the J-V curve (Figure
5 (b) would be attributed to the alignment of the Fermi level of
Hg with HOMO energy levels. The alignment results in resonant
tunneling through HOMO. Even if the bilayer undergoes Ist
oxidation, the current keeps rising sharply because the
oxidation does not bring any significant changes in the
conformation (Figure 5(a)). Moreover, the LUMOs of oxidized
molecules come under the applied potential window and
consequently would also take part in electron transport
enhancing current further (Figure 5(b)). However, at further
higher bias, the current drops sharply, which could explain from
IInd oxidation of the bilayer. The doubly oxidized bilayer
undergoes a large conformational change, see Figure 5(a).
Consequently, the HOMO and LUMO levels are shifted and
become off-resonance to the Fermi level of Hg. As a result, the
current now can pass only through the direct tunneling process,
which manifests as the NDR effect in J-V measurements. We
also observed that the current always remains low in the
subsequent J-V cycles. However, if the electrodes are short-
circuited, the neutral state of the PM567:BODIPY-C5 is
regained, and hence, the NDR effect reappears. Thus it is
inferred that the conformational changes achieved after IInd
oxidation are quite stable.
On the other hand, for negative bias on Hg electrode i.e.
V<0, the current remains low as a very high bias is required to
obtain a resonance between the Hg Fermi level and LUMO of
the molecule. Moreover, the molecule does not undergo for any
reduction, and therefore, no additional feature other than non-
resonant tunneling current, appears in the negative bias. In the
case of monolayer i.e. Hg/BODIPY-C5/Si(n++) structure,
observed rectification behavior is similar to those of σ-π
molecular diodes, which arises because of resonant tunneling
through the HOMO of the π group [17]. Since BODIPY-C5
molecule does not undergo IInd oxidation and major
conformational changes do not occur, therefore no NDR effect
is observed in this case.
Conflicts of interest
There are no conflicts to declare.
View Article Online
DOI: 10.1039/C9CP05918K
Acknowledgements
We thank Dr. K. G. Bhusan for conducting the SIMS
measurements.
Notes and references
1
a) M. -Y. Li, S. -K. Su, H. -S. P. Wong and L. -J. Li, Nature, 2019,
567, 169-170; b) M. C. Petty in Molecular Electronics: From
Principles to Practice, Wiley, 2008; c) P. T. Mathew and F.
Fang, Engineering, 2018, 4, 760-771.
2
3
https://eps.ieee.org/images/files/Roadmap/SIA-SRC-Vision-
Report-3.30.17.pdf
a) D. K. Aswal, S. Lenfant, D. Guerin, J. V. Yakhmi and D.
Vuillaume, Anal. Chim. Acta., 2006, 568, 84-108; b) A.
Salomon, D. Cahen, S. Lindsay, J. Tomfohr, V. B. Engelkes and
C. D. Frisbie, Adv. Mater.,2003, 15, 1881-1890; c) M. Laurans,
K. Dalla Francesca, F. Volatron, G. Izzet, D. Guerin, D.
Vuillaume, S. Lenfant and A. Proust, Nanoscale, 2018, 10,
17156-17165; d) T. Rakshit, G. -C. Liang, A. W. Ghosh and S.
Datta, Nano Lett., 2004,4, 1803-1807.
4
5
6
K. Garg, C. Majumder, S. K. Gupta, D. K. Aswal, S. K. Nayak and
S. Chattopadhyay, Chem. Sci., 2016, 7, 1548-1557.
K. Garg, C. Majumder, S. K. Gupta, D. K. Aswal, S. K. Nayak and
S. Chattopadhyay, RSC Adv., 2015, 5, 50234-50244.
a) Q. Tang, H. K. Moon, Y. Lee, S. M. Yoon, H. J. Song, H. Lim
and H. C. Choi, J. Am. Chem. Soc., 2007, 129, 11018-11019; b)
R. A. Wassel, G. M. Credo, R. R. Fuierer, D. L. Feldheim and C.
B. Gorman, J. Am. Chem. Soc., 2004, 126, 295-300; c) R.
Khurana, J. Mohanty, N. Padma, N. Barooah and A. C.
Bhasikuttan, Chem. Eur. J., 2019, 10.1002/chem.201902641;
d) P. Chal, A. Shit, D. Levy, S. Das, S. Mondal and A. K. Nandi,
Colloids Surf. A: Physicochemical and Engineering Aspects,
2019, 577, 480-492.
a) J. Chen, M. A. Reed, A. M. Rawlett and J. M. Tour, Science,
1999, 286, 1550; b) Z. J. Donhauser, B. A. Mantooth, K. F. Kelly,
L. A. Bumm, J. D. Monnell, J. J. Stapleton, D. W. Price, A. M.
Rawlett, D. L. Allara, J. M. Tour and P. S. Weiss, Science, 2001,
292, 2303; c) J. L. Pitters and R. A. Wolkow, Nano Lett., 2006,
6, 390-397; d) N. P. Guisinger, M. E. Greene, R. Basu, A. S.
Baluch and M. C. Hersam, Nano Lett., 2004, 4, 55-59; e) Y.
Selzer, A. Salomon, J. Ghabboun and D. Cahen, Angew. Chem.
Int. Ed., 2002, 41, 827-830; f) A. Salomon, R. Arad-Yellin, A.
Shanzer, A. Karton and D. Cahen, J. Am. Chem. Soc., 2004, 126,
11648-11657.
7
Conclusions
In conclusion, we have demonstrated the strategy to tune the
electrical behavior of BODIPY dye grafted on Si. The tailor-made
BODIPY-C5 moiety grafted on Si showed rectification property
while BODIPY bilayers grafted on Si showed room temperature
NDR effect with PVR upto 1000. Our studies show that
rectification characteristics are due to resonant tunneling.
However, the NDR behaviour is associated with bias induced
conformational changes of the bilayer molecules by which the
resonant tunneling is changed to non-resonant tunneling.
These results are important for the development of resonant
tunnel diodes for molecules-on-Si hybrid nanoelectronics and
single molecular based futuristic electronics.
8
9
a) J. M. Seminario, A. G. Zacarias and J. M. Tour, J. Am. Chem.
Soc., 2000, 122, 3015-3020; b) Y. Karzazi, J. Cornil and J. L. Br
das, Nanotechnology, 2003, 14, 165-171; c) E. G. Emberly, G.
Kirczenow, Phys. Rev. B, 2001, 64, 125318.
M. Galperin, M. A. Ratner and A. Nitzan, Nano Lett., 2005, 5,
125-130.
10 a) J. Bañuelos, Chem. Rec. 2016, 16, 335-348; b) H. Lu, J. Mack,
Y. Yang and Z. Shen, Chem. Soc. Rev., 2014, 43,4778-4823.
11 a) S. Cherumukkil, B. Vedhanarayanan, G. Das, V. K. Praveen
and A. Ajayaghosh, Bull. Chem. Soc.Jpn,2018, 91, 100-120; b)
J. D. Dunitz and R. Taylor, Chem.–Eur. J., 1997, 3, 89–98; c) T.
S. Thakur, M. T. Kirchner, D. Blaser, R. Boese and G. R.
Desiraju, CrystEngComm, 2010, 12, 2079–2085; d) E. A.
Meyer, R. K. Castellano and F. Diederich, Angew. Chem., Int.
Ed., 2003, 42, 1210–1250.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 7
Physical Chemistry Chemical Physics
Please do not adjust margins
Journal Name
ARTICLE
12 a) S. Thakare, P. Stachelek, S. Mula, A. B. More, S.
Chattopadhyay, A. K. Ray, N. Sekar, R. Ziessel and A. Harriman,
Chem. Eur. J., 2016, 22, 14356-14366; b) S. Mula, K. Elliott, A.
Harriman and R. Ziessel, J. Phys. Chem. A, 2010,114, 10515-
10522.
View Article Online
DOI: 10.1039/C9CP05918K
13 A. Bessette and G. S. Hanan, Chem. Soc. Rev., 2014, 43, 3342-
3405.
14 D. K. Aswal, S. P. Koiry, B. Jousselme, S. K. Gupta, S. Palacin
and J. V. Yakhmi, Physica E: Low-dimensional Systems and
Nanostructures, 2009, 41, 325-344.
15 R. M. Metzger, J. W. Baldwin, W. J. Shumate, I. R. Peterson, P.
Mani, G. J. Mankey, T. Morris, G. Szulczewski, S. Bosi, M.
Prato, A. Comito and Y. Rubin, J. Phys. Chem. B, 2003, 107,
1021-1027.
16 M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S.
Gordon, J. H. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S.
Su, T. L. Windus, M. Dupuis and J. A. Montgomery Jr, J.
Comput. Chem.,1993, 14, 1347-1363.
17 S. Lenfant, C. Krzeminski, C. Delerue, G. Allan and D.
Vuillaume, Nano Lett., 2003, 3, 741-746.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins