Charge transport in phenazine-fused triphenylene discotic mesogens doped with CdS nanowires

Article abstract of DOI:10.1039/d0nj03290e

Herein, we report the synthesis of oleylamine-capped CdS nanowires (NWs) and the dispersion of a small optimized amount of these NWs in the Col_hphase of a recently synthesized phenazine-fused-triphenylene discotic liquid crystal (PFT DLC) to un

Full text of DOI:10.1039/d0nj03290e

View Article Online
View Journal
NJC
New Journal of Chemistry
Accepted Manuscript
A journal for new directions in chemistry
This article can be cited before page numbers have been issued, to do this please use: A. Shah, B.
Duponchel, A. Gowda, S. Kumar, M. Becuwe, C. Davoisne, C. Legrand, R. Douali and D. P. Singh, New J.
Chem., 2020, DOI: 10.1039/D0NJ03290E.
Volume 42
This is an Accepted Manuscript, which has been through the
Number
6
21 March 2018
Pages 3963-4776
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
New Journal of Chemistry
rsc.li/njc
A journal for new directions in chemistry
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 1144-2546
PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
rsc.li/njc

Page 1 of 8
New Journal of Chemistry
View Article Online
DOI: 10.1039/D0NJ03290E
1
2
3
4
5
6
7
8
9
Charge transport in phenazine–fused triphenylene
discotic mesogens doped with CdS nanowires†
Asmita Shah,a Benoît Duponchel,b Ashwathanarayana Gowda,c Sandeep Kumar,∗c,d
Matthieu Becuwe,e, f,g Carine Davoisne,e, f,g Christian Legrand,a Redouane Douali,a and
Dharmendra Pratap Singh∗a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Herein, we report the synthesis of oleylamine capped CdS nanowires (NWs) and we have dis-
persed a small optimized amount of these NWs in the Colh phase of a recently synthesized
phenazine-fused-triphenylene discotic liquid crystal (PFT DLC) to understand the temperature-
dependent charge transport via time-of-flight technic. X-ray diffraction study reveals the reduction
in core−core separation; however, upon doping CdS NWs, we observed a trivial decrease in hole
mobility. For the PFT-DLC and its composite, the hole mobility was found to be in the range of
0.07 − 0.43 × 10−3 cm2V−1s−1 following the inverse-power law as a function of temperature. This
could be due to the large size of CdS NWs which eventually disturb the Colh packing and hence
the charge carrier mobility. Besides, a nanocomposite device consisting of the heterojunction of
ITO-PFTDLC/CdS-Au has been constructed and I−V characteristics are carried out which obey
the Ohm’s law under a limiting case of Poole-Frenkel conduction mechanism.
1 Introduction
aligns homeotropically (i.e. molecular orientation parallel to the
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
substrate) under a slow cooling from the isotropic phase, but
ideal planar or homeotropic alignment of discotic mesogens is
an unsolved challenge to the scientific community; however few
methods have been proposed to align the discogens in a preferred
orientation in which authors have used thiol-on-gold monolayers
functionalized by COOH- and CH3- to achieve homeotropic align-
ment.5 As the orientation of discogen is decisive for the electronic
and opto-electronic applications; therefore, few other approaches
have also been made in which the homeotropic self-alignment of
discogens via nanoporous polymer film is a remarkable piece of
work.6 Among all discogens, the triphenylene core based discotic
mesogens have proven their ponderability for the modern opto-
electronic devices due to the high charge carrier mobility of order
Discotic liquid crystals (DLCs), in which mesophase is formed by
the disc shape molecules, have emerged as a new class of or-
ganic semiconductors during the last decade.1,2 Due to quasi-
one-dimensional charge and energy transport, discotic mesogens
are potential candidates that can be utilized in organic electronic
devices.3 The chemical structure of these discogens allows to ma-
nipulate their electronic properties by changing the composition
of π-conjugated cores and the peripheral flexible chains which,
sometimes, hinder the charge transport because of their thermal
instability. In particular, these molecules exhibit self-organisation
via π-π stacking and commonly turn into a hexagonal or rect-
angular columnar (Colh or Colr) phase.4 Often the Colh phase
≈ 10−2 cm2V−1s−1 7–11
,
but the electrical conductivity remains
limited to 10−7−10−9 Sm−1 9
. It is previously shown that the dis-
aUniv. Littoral Côte d’Opale, UR 4476, UDSMM, Unité de Dynamique et Structure des
Matériaux Moléculaires, F-62228 Calais, France. Fax: 03 21 46 36 42; Tel: 03 21 46
57 58; E-mail: Dharmendra.Singh@univ-littoral.fr
bUniv. Littoral Côte d’Opale, UR 4476, UDSMM, Unité de Dynamique et Structure des
Matériaux Moléculaires, F-59140 Dunkerque, France.
cRaman Research Institute, Bangalore, 560080-India.
d Department of Chemistry, Nitte Meenakshi Institute of Technology (NMIT), Yelahanka,
Bangalore, India E-mail: skumar.sandeep@gmail.com
eLaboratoire de Réactivité et Chimie des Solides (LRCS), UMR CNRS 7314, Université
de Picardie Jules Verne (UPJV), Amiens, France.
f Institut de Chimie de Picardie (ICP), FR CNRS 3085, 80039 Amiens, France.
gRéseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, 80039
Amiens, France.
† Electronic Supplementary Information (ESI) available: [details of any supplemen-
tary information available should be included here]. See DOI: 10.1039/b000000x/
persion of a small amount of various metallic and semiconduct-
ing NPs significantly enhances the conductivity of the host disco-
gens.12,13 In the geometrical arrangement of discogens, the space
between hexagonally arranged columns is filled by the aliphatic
chains which are thermally unstable. In the columnar phase,
quasi-one-dimensional charge transport, along the columns, takes
place via hopping process; wherein, the intra-columnar core-core
separation (hopping distance) plays a crucial role in obtaining a
precise value of mobility. The electronic transport parameters of
discogens preponderatingly depend on the degree of orientation,
composition of donor-acceptor moities, core-core separation and
1–7 | 1

New Journal of Chemistry
Page 2 of 8
View Article Online
DOI: 10.1039/D0NJ03290E
1
2
3
4
5
6
7
8
energy gap compatibility with the incident light of a particular
wavelength. In spite of having tremendous opportunities to in-
vestigate the charge transport mechanism in a guest-host system
of the columnar phase in the light of structure-property relation-
ship, the scant literature is available so far. It is essential to study
the charge carrier mobility as a function of temperature and ex-
ternal applied field for the applications in electronic devices. In
a mesophase, it is usually observed that the value of charge mo-
bility is invariant of temperature and applied electric field due to
their debilitated dipole moment.14 This type of charge transport
mechanism is specifically observed in the liquid crystalline mate-
rials and is well explained by the Gaussian disorder model with a
narrow distribution of density of states.15 In such a case, density
of states is shallow and charges are easily thermalized near the
conduction band for a given temperature range above the room
temperature, leading to temperature and field invariant carrier
mobilities.16 However, few reports also demonstrate the variation
in charge mobility with the change in temperature. Funahashi et
al.17 have shown that the hole and electron mobilities decrease
with decreasing temperature and electric field in the smectic E
phase. Interestingly, this nature found to be completely different
above 0 ◦C which has shown a temperature and field-independent
mobilities. This behavior could be explained via the Gaussian dis-
order model15 or unifield theory18 that are already reported for
amorphous organic semiconductors.19 Kato et al. have shown the
temperature-dependent mobility for the guanine–oligothiophene
conjugated columnar liquid-crystalline phase20, but the physical
mechanism behind this character is still missing. In contrast to
aforementioned temperature-invariant charge carrier mobility in
the triphenylene based discotic LCs, we have recently reported an
increase in the hole mobility with increasing temperature for a
phenazine fused triphenylene supramolecular system. This be-
havior is similar to that of the single crystalline organic semi-
conductors. For the semiconducting devices, the tunability of
transport properties with increasing temperature is a zestful task.
Therefore, previously described temperature-dependent charge
carrier mobility in a phenazine fused triphenylene discotic sys-
tem has been performed in the presence of semiconducting CdS
nanowires. Cadmium sulfide (CdS) exhibits a metastable cu-
bic zinc blende structure that could be converted to a more sta-
ble hexagonal wurtzite structure via temperature treatment.21,22
CdS nanomaterials have been utilized as one-dimensional semi-
conductor nanostructures in smart optoelectronic applications
coupled with liquid crystalline molecules.23,24 Controllable cou-
pling between the exciton of CdS and the localized π−electrons
of the liquid crystalline system could lead to new features for op-
toelectronic devices. Besides their use in thermotropic LCs, they
have also been investigated with lyotropic LCs.25,26 In spite of
showing tremendous technical applications of discotic LC/CdS
nanocomposites, limited investigations have been done so far.
In contrast to Cd based QDs, CdS NWs have been selected in
the present study due to their large lateral size along which LC
molecules could be aligned. The size of Cd based QDs lies in the
range of ≤ 10 nm and as a dopant, the condition of homogenous
distribution and formation of self-assembly is essential. The main
drawback of these QDs is to obtain their stable self-assembly.
In this article, CdS nanowires (NWs) are synthesized and their
optimized concentration is mixed with the phenazine fused triph-
enylene discogens to tailor the orientation of discogen molecules
along the NWs and studying their charge transport proper-
ties by controlling the core-core hopping distance for the ITO-
PFTDLC/CdS-Au heterojunction. The distinctive evolution of
charge carrier mobility of discotic mesogens as a function of tem-
perature has been preponderantly discussed.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2 Experimental details
2.1 Synthesis of phenazine–fused triphenylene discotic liq-
uid crystal (PFT DLC)
In this study, recently synthesized phenazine–fused triphenylene
discotic liquid crystal (PFT DLC) was taken as a host matrix.
The synthesis process of the host PFT DLC matrix is shown in
ESI Scheme 1.9 The condensation of 4,5-dibromobenzene-1,2-
diamine with 3,6,7,10,11-pentakis(alkyloxy)triphenylene-1,2-
diones in presence of glacial acetic acid in toluene (7:3) under re-
flux condition to afford intermediate 9,10-dibromo-2,3,6,14,15-
pentaalkoxyphenanthro phenazine compound and its reaction
with 1-butanethiol in presence of cesium carbonate under re-
flux condition produces the desired extended phenazine fused
triphenylene mesogen. The PFT discotic mesogen exhibits stable
hexagonal columnar (Colh) phase upon cooling from the isotropic
phase. The chemical structure of the PFT DLC is depicted in Fig.
1.
Fig. 1 Chemical structure of the phenazine fused triphenylene (PFT)
discotic mesogen, where R = C12H25 ; R1= C4H9.
2.2 Synthesis of CdS nanowires (NWs)
The CdS NWs were synthesized via low-temperature reaction us-
ing oleylamine as surface-capping ligands as reported by Son et
al.27. 1.5 mmol of CdCl2 complex was added to 10 ml of octy-
lamine and the mixture was stirred at 120 ◦C for 2 hrs. Sulfur
octylamine complex was additionally prepared and injected into
the CdCl2 at room temperature. This mixture was slowly heated
up to 50 ◦C and kept at this temperature for next 48 hrs. After
the completion of the reaction, nanowires were precipitated out
of the solution by adding excess ethanol, followed by the repe-
tition of centrifugation, washing with ethanol several times and
dried under vacuum.
2 |
1–7

Page 3 of 8
1
New Journal of Chemistry
View Article Online
DOI: 10.1039/D0NJ03290E
2.3 Characterization and instrumentation
attraction with the neighboring NWs and in the absence of a cap-
ping ligands, they tend to form aggregates. The XRD spectrum
of CdS NWs is shown in Fig. 2(c) that exhibits three diffrac-
tion peaks corresponding to (100), (002), and (101) planes at
25.82, 26.89 and 29.12◦, which is analogous to the hexagonal
wurtzite structure of CdS (ICDD, File No. 41–1049).26 General
Structure Analysis System (GSAS) package and its graphical in-
terface EXPGUI28 has been used for Rietveld refinement and ap-
propriately extracted the structural parameters of CdS. In the re-
finement process, initially, lattice parameter, symmetry, positions
of Cd atom site metal ions and S atom metal ions have been as-
signed (adopted from29). The peaks profile was fitted by Gaus-
sian/Cauchy Pseudo-Voigt model. The result of Rietveld fitting
reveals that the prepared CdS NWs have hexagonal wurtzite crys-
tal structure with P63mc (space group no. 186) symmetry (lat-
tice parameter a=b= 3.99 (2) Å and c=6.62 (1) Å) which are in
good agreement with the reported CdS structural information.29
The bond length between Cd-S found to be 2.5163 (5) Å, 2.4347
(11) Å and 2.4379 (11) Å (Fig. S1, ESI). Figures 2(d) and (e) de-
pict the polarized optical micrographs (POMs) of the pristine PFT
DLC and PFT/CdS composite, respectively. The POM analysis is a
fundamental tool that describes the molecular ordering of a sys-
tem. For the PFT DLC, we observed the rectilinear defect texture
at 100◦C which delineates the typical signature of the hexago-
nal columnar mesophase; however, the change in POM pattern
and larger domain size in the PFT/CdS composite apprises the
self-assembling nature of the PFT molecules along the orienta-
tion of CdS NWs. The surface topography of the PFT/CdS com-
posite was investigated by atomic force microscopy (AFM) which
is shown in Fig. 2(f). It is evident that NWs evolve the long
range orientations along c-axis on the substrate because of their
wide-size in length and the PFT DLC molecules are preferentially
stacked around them. We have not done any additonal treatment
for the alignment of NWs; therefore, some zig-zag oriented NWs
are additionally observed. The perfect alignment of CdS NWs is
a separate subject to investigate.30 The maximum vertical sur-
face roughness for the PFT/CdS composite is found to be ≈ 80
nm (shown in Fig. S2). The observed topographical geometry
depicts a good distribution of π-stacked columns along the NWs.
This assembly encouraged us to measure the core-core separation
and subsequent effect on charge-carrier mobility.
2
The phase identification of CdS NWs was done using the X-ray
diffraction (XRD). The microstructural and structural investiga-
tions were performed using a Tecnai F20-STWIN through bright
field imaging, high resolution transmission electron microscopy
(HRTEM) and selected area electron diffraction (SAED). The
surface topography of the PFT/CdS composite was obtained by
means of a Bruker (ex Veeco) Multimode Atomic Force Micro-
scope, equipped with a Nanoscope IIIa electronic controller. AFM
observation was conducted in the tapping mode for the surface
area of 1×1 µm2 in ambient air at room temperature. The
mesophase structure along with the supramolecular self-assembly
of the PFT DLC and PFT/CdS composite was analyzed by the
X-ray diffraction (PANalytical X-ray diffractometer, Netherlands).
For recording the X-ray diffractograms, PFT DLC and PFT/CdS
composite were filled in Lindemann capillaries and placed in a
custom-made hotstage inside the instrument. Charge carrier mo-
bility of PFT DLC and PFT/CdS composite was investigated by the
time of flight (TOF) technique. In this technique, a laser pulse of
355 nm, having 5 ns pulse width, was made to incident on the
PFT DLC and PFT/CdS composite, filled in the homeotropic cells
of thickness 9.2 µm (AWAT, Poland), by using Nd:YAG laser with a
laser spot of 8 mm in diameter. The induced charge displacement
was observed via a transient photocurrent curve that was mea-
sured by a digital oscilloscope (Keysight, DSOX 13022T) coupled
to a current amplifier (Keithley 428). The charge carrier mobility
was obtained by an output transient photocurrent curve. Under
the application of external voltage (70−120 V), we obtained a
non-dispersive transient photocurrent curves (shown in Fig. 4a).
The transit time (τ) of charge carriers (holes) was obtained by
the transient photocurrent curve and the mobility (µ) was cal-
culated by µ = d2/τ.V, where τ is the transit time obtained by
photocurrent curves, V is the applied voltage and d is the ITO
cell thickness. Polarized optical microscopy was performed on
LEICA DMRXP to analyze the homogenous distribution of NWs
in the PFT DLC. Room temperature current-voltage (I−V) char-
acteristics of the PFT DLC and PFT/CdS composite in the form
of ITO-DLC-Au heterojunctions were carried out using Keysight
B2902, a precision Source/Measure Unit (SMU). The differential
scanning calorimetry (DSC) was performed using TA Instruments
Q1000 with a rate of 2 ◦C/min in the heating/cooling cycles and
the information is tabulated in Table 1.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The typical◦XRD patterns of the PFT DLC and PFT/CdS com-
posite at 100 C during heating cycle in the columnar phase are
shown in Fig. 3. The PFT DLC compound has shown an increas-
ing order of diffraction angle, the d-spacings of the first reflection
3 RESULTS AND DISCUSSION
The microstructure of CdS NWs was investigated by TEM which
is shown in Fig.2(a). The TEM image of CdS renders the agglom-
erated crystalline NWs of thickness 1−2 nm and length up to 500
nm as indicated with blue arrows. The stacking scale of NWs can
be up to 30 nm. The agglomerates possess an inside porosity of
the size in the range of 3−10 nm as highlighted with green arrows
in Fig.2(a) and the corresponding SAED pattern is depicted in
Fig.2(b). The SAED pattern is composed of thin and well-defined
rings indicating the polycrystalline nature of CdS and crystallites
are randomly oriented which is in agreement with the observed
microstructure. The SAED pattern indexing is analogous to the
hexagonal phase of CdS. These NWs render strong van der Waals
(lowest angle and highest intensity) to the second one is in the ra-
√
tio of 1:1/ 3:1/2 which represents a two-dimensional hexagonal
lattice. A broad peak nearly at 20◦ in the wide angle regime cor-
responds to the liquid like packing of the molten aliphatic chains.
In the small angle region, three diffraction peaks having the d-
spacing values of 24.58, 14.19, 12.28 Å were observed that cor-
respond to the (100), (110) and (200) planes, respectively show-
ing a signature of hexagonal columnar phase. After the addition
of CdS NWs to PFT matrix, feeble reduction in the d-spacing val-
ues has been observed suggesting the supression of geometry by
large size CdS NWs (data presented in Table S1, ESI). Here, we
1–7 | 3

New Journal of Chemistry
Page 4 of 8
View Article Online
DOI: 10.1039/D0NJ03290E
1
2
3
4
Phase sequence
Heating cycle
Compound
Cooling cycle
5
6
7
PFT DLC
Cr 55.1 (36.60) Colh 172 (6.49) I
I 170.3 (-6.24) Colh 17.4 (-38.83) Cr
PFT/CdS composite Cr 37.1 (9.96) Colh 157.3 (4.70) I I 170.3 (-10.23) Colh 17.3 (-21.70) Cr
8
9
Table 1 Phase transition temperatures (in oC) and the corresponding enthalpy changes (in kJmol−1). Peak temperatures/oC (enthalpy/kJmol−1) ob-
tained by the second heating and first cooling at the rate of 2 oC/min; Cr= crystalline phase; Colh = Columnar hexagonal phase; I = Isotropic liquid
phase.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
are mainly interested in determining the change in the core-core
separation (hopping distance) as it plays a vital role in the charge
transport. In the triphenylene based discogens, charge carrier
transport is assigned by one or two-dimensional hopping within a
narrow Gaussian distribution of localized states of width equiva-
lent to kT.31 Lesser hopping distance yields higher value of charge
carrier mobility and vice-versa. The core-core separations for the
PFT DLC and PFT/CdS composite were found to be 3.55 and 3.50
Å, respectively indicating to obtain a higher charge mobility in
the composite. Consequently, the molecular orientation depen-
dent charge carrier mobility has been analyzed via ToF which is
presented in the next section.
range correlation34 and thermally activated Gaussian function of
hopping sites.35
Interestingly, in our study, we found that the hole mobil-
ity increases with increasing temperature, similar to the single-
crystalline materials, following an inverse power law-9
µh(T) = µoTn
(1)
where, n is the exponent. The value of n for the PFT DLC and
PFT/CdS composite was found to be > 3 and ≈ 2.4, respectively
under the application of electric field of ≈ 10 × 104 Vcm−1
.
The Holstein small polaron theory is valid for the low-
temperature range and within a nonadiabatic small polaron limit.
These two conditions are not fulfilled in the present investigation;
therefore, we overlooked the possibility of electron-phonon cou-
pling in the investigated compounds. The relative permittivity,
(Fig. 6), of the PFT DLC and PFT/CdS NWs composite is almost
temperature independent in the Colh phase region which confirms
the absence of electron-phonon interactions and charge transfer
mechanisms in the composite. The temperature dependence of
hole mobility is nearly consistent with thermally activated and at-
tenuated delocalized charge carriers resulting from an energetic
or positional disorder. The dispersion of CdS NWs causes delo-
calization of the charge carriers under which the hole mobility
increases with increasing temperature; however, the overall hole
mobility of the PFT/CdS NWs composite is lesser as compared to
pristine PFT DLC which might be attributed to disordering of the
system.
The transient photocurrent curves for the PFT DLC and
PFT/CdS composite at 100 ◦C under the application external ap-
plied voltage of 100 V are presented in Fig. 4(a), whereas Fig.
4(b) shows the temperature dependence of hole mobility (µh) in
the Colh phase. For the pristine PFT DLC, we observed a plateau
level in the photocurrent curve, which corresponds to a classical
non-dispersive transient nature. In the PFT/CdS composite, the
transient photocurrent curve becomes slightly dispersive and the
transit time shifts to the longer side that might be due to the for-
mation of CdS surface traps. For a disorder-free single crystalline
semiconductor, the charge carrier mobility is proportional to D/T
(i.e. D/µ = kT/q); where D is diffusion coefficient, T is temper-
ature, k is Boltzmann’s constant and q is the electronic charge.19
Following to the degenerate state in a disordered semiconduc-
tor, the ratio D/µ becomes a function of the carrier concentration
in the the generalized Einstein relation; according to which the
mobility can also be a function of energy by assuming the hop-
ping probability. In contrast to single crystalline semiconductors,
the charge carrier mobility in the triphenylene based discogens is
temperature invariant within the DLC mesophase;14,16,32 never-
theless, a weak temperature dependence of the mobility has also
been explained by introducing the theory of Holstein polaron and
disordering dynamics of the system.33 The Holstein polarons are
the small composite particles that carry a cloud of phonons and
play an important role in the determination of charge carrier mo-
bility in organic semiconducting devices. The temperature depen-
dence of charge carrier mobility follows two characteristics: First,
the thermally activated system in which mobility shows an in-
creasing trend and second, a power law where mobility decreases
with increasing temperature. In the previously reported articles,
the decreasing positive charge mobility with increasing tempera-
ture has been assigned due to the decreased intracolumnar short-
The current−voltage (I−V) characteristic in the nano-
dimensional thin films of the PFT and PFT/CdS NWs compos-
ite, in the hexagonal columnar discotic phase homeotrop-
ically aligned between ITO and Au electrode surfaces
(ITO−PFT/CdS−Au heterojunction), has been studied ex-
perimentally. The I−V curves are presented in Fig. 5; whereas
inset illustrates the schematic depiction of the device. The
thicknesses of ITO, PFT/CdS and Au were determined using a
Taylor Hobson thickness profilometer and found to be ≈ 200,
450 and 150 nm (Fig. S3, ESI).
The Poole-Frenkel conduction mechanism is most commonly
observed in the DLCs.36. It can be written as follows-
ꢀ
ꢁ
V
d
−Et
kT
J = e nµ. exp
(2)
where, J is the current density, n is the charge carrier density,
e is the elementary charge, µ is the charge carrier mobility, V is
4 |
1–7

Page 5 of 8
New Journal of Chemistry
View Article Online
DOI: 10.1039/D0NJ03290E
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 2 (a) Transmission electron microscopic (TEM) image of CdS nanowires, (b) selected area electron diffraction pattern, (c) X-ray diffraction spectrum
of CdS nanowires, (d) polarized optical micrograph (POM) of the PFT DLC, (e) polarized optical micrograph of the PFT/CdS composite in Colh phase
(at 100 ◦C), and (f) 3D AFM image of PFT DLC/CdS NWs composite. In Fig.2(b), blue and green arrows represent the lateral size distribution and
porous points of CdS, respectively. The scale bar is 100 µm in both the POM images. The inset of (c) represents the fitting of experimental XRD data
in the Gaussian/Cauchy Pseudo-Voigt model.
the equation, J = neµ.V/d. The linear fitting equations for the
PFT DLC and PFT/CdS composite were obtained to be I = 0.058
V and I = 0.078 V with the values of resistances of 17.24 and
12.82 Ω, respectively.
The PFT DLC has low intrinsic carrier concentration due to the
large bandgap ∼ 2.86 eV which makes it a good insulator. Simi-
lar to organic insulators, PFT DLC and PFT/CdS NWs composite
exhibit the Ohmic behavior for the fields of 0.1 to 1 × 104 Vcm−1
and the enhanced value of conduction current in the composite
is attributed to extrinsic impurities because of the CdS NWs. The
CdS NWs act as n-type impurity in the PFT DLC matrix. Besides, it
is also observed that the role of carrier concentration is minimal.
The number of charge carriers (estimated by σ=nqµ), at 100 ◦C,
in the PFT DLC and PFT/CdS NWs composite was found to be ≈ 8
× 1015
m
−3 and 7 × 1015
m
−3, respectively which is exceptionally
low as compared to other organic materials like napthalene. I−V
measurement was restricted within the lower voltages to avoid
the space charge effect.
Fig. 3 X-ray diffraction spectrum of the PFT DLC/CdS NWs composite
whereas the inset represents the XRD of the pristine PFT DLC at 100 ◦C.
Relative dielectric permittivity versus temperature curves for
the PFT DLC and PFT/CdS composite are plotted in Fig. 6. It
is observed that the relative permittivity of the PFT DLC and
PFT/CdS composite is almost temperature invariant; however, the
presence of CdS NWs reduces the relative permittivity of the pris-
tine PFT matrix. The expression for the dielectric permittivity can
be written as follow-37
the applied voltage, d is the sample thickness and Et is the trap
energy level.
In our investigation, the I−V curves follow Ohm’s law, a lim-
iting case of the Poole-Frenkel conduction mechanism under the
condition of exp[-Et/kT] ≈ 1, at low voltage in accordance with
1–7 | 5

New Journal of Chemistry
Page 6 of 8
View Article Online
DOI: 10.1039/D0NJ03290E
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 5 Current−Voltage curves of the PFT and PFT/CdS composite
recorded at room temperature. The inset represents the schematic of
ITO-PFT/CdS-Au heterojunction. Error bars depict the 5% uncertainty
in experimental data whereas solid lines show the linear fitting of I−V
curves of PFT DLC and PFT/CdS composite using equation, J = neµ.V/d.
Fig. 4 (a) Transient photocurrent curves for the PFT DLC and PFT/CdS
composite at 100 ◦C under the application external applied voltage of 100
V. τ1 and τ2 are the transit times for positive charge carriers in the PFT
DLC and PFT/CdS composite, respectively. (b) Variation of hole mobility
as a function of temperature. The dotted lines represent the power law
fitting of the experimental data by using equation 1.
Fig. 6 Dielectric permittivity versus temperature curves of the PFT DLC
and PFT/CdS composite at 3 kHz.
into the phenazine-fused-triphenylene discotic liquid crystal (PFT-
DLC) to understand the temperature-dependent charge transport
via structure-property relationship. PFT DLC molecules align
along the long axis of NWs and thus the Colh system has been
changed. The presence of NWs reduces the core-core distance
within a column which theoretically renders higher mobility in
the composite, but surprisingly, we observed a trivial decrease in
the hole mobility. This might be related to the decreased molecu-
lar ordering of the Colh phase. The temperature-dependent hole
mobility fits with inverse power law and is nearly consistent with
thermally activated and attenuated delocalized charge carriers re-
sulting from an energetic or positional disorder. The I−V charac-
teristics of the PFT DLC and PFT/CdS NWs composite were exam-
ined by fabricating a device consisting of the ITO−PFT/CdS−Au
heterojunction. The I−V curves obey the Ohm’s law under a lim-
iting case of, exp[-Et/kT] ≈ 1, Poole-Frenkel conduction mecha-
nism. The high hole mobility coupled with fair conductivity of
PFT/CdS NWs composite would be suitable materials for opto-
δε
ε∗ = ε(∞)+
(3)
1+(iωτ)1−α
where, δε is the relaxation strength, ε(∞) is the high frequency
limit of dielectric permittivity, ω=2.π.f, τ is the relaxation time
and α is the distribution parameter.
As the dielectric function is a combination static part, ε(∞),
and contribution of the dynamics of dipoles, δε/[1+(iωτ)1−α ].
The enhancement or reduction in the relative dielectric permit-
tivity takes place due to the effective contribution of the dipolar
dynamics. In the case of PFT/CdS composite, the relative per-
mittivity has been decreased as compared to the pristine system
because of the destructive dipolar correlation between the PFT
molecules and CdS.
4 Conclusions
In summary, oleylamine capped CdS nanowires (NWs) were syn-
thesized and an optimized amount of the NWs has been dispersed
6 |
1–7

Page 7 of 8
1
New Journal of Chemistry
View Article Online
DOI: 10.1039/D0NJ03290E
electronic applications.
15 H. Bässler, physica status solidi (b), 1993, 175, 15–56.
2
16 T. Kreouzis, K. J. Donovan, N. Boden, R. J. Bushby, O. R. Loz-
3
4
5
6
7
8
9
Electronic supplementary material
man and Q. Liu, The Journal of Chemical Physics, 2001, 114,
1797–1802.
The 3D crystal structure of CdS is shown as Fig. S1 whereas
the average roughness profile of the PFT/CdS composite is rep-
resented in Fig. S2. The thickness profilometric image of the
ITO-PFTDLC/CdS-Au heterojunction is depicted in Fig. S3 and
X-ray indexing data of the PFT DLC and PFT/CdS composite are
tabulated in Table S1; whereas, transit times at different temper-
atures are presented in Table S2. A description of the methods to
calculate the hole mobility is also presented.
17 M. Funahashi, F. Zhang, N. Tamaoki and J.-i. Hanna,
ChemPhysChem, 2008, 9, 1465–1473.
18 V. M. Kenkre, J. D. Andersen, D. H. Dunlap and C. B. Duke,
Physical Review Letters, 1989, 62, 1165–1168.
19 C. Liu, K. Huang, W.-T. Park, M. Li, T. Yang, X. Liu, L. Liang,
T. Minari and Y.-Y. Noh, Materials Horizons, 2017, 4, 608–
618.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
20 K. P. Gan, M. Yoshio, Y. Sugihara and T. Kato, Chemical Sci-
ence, 2018, 9, 576–585.
Conflicts of interest
“There are no conflicts to declare”.
21 W. Jia and E. P. Douglas, Journal of Materials Chemistry, 2004,
14, 744–751.
Acknowledgements
22 I.-H. Choi and P. Y. Yu, physica status solidi (b), 2005, 242,
1610–1616.
DPS sincerely thanks to the UDSMM, ULCO for financial assis-
tance. Prof. A. Hadj Sahraoui, the Director of UDSMM is grate-
fully acknowledged for his kind support and encouragement. We
sincerely thanks to Mr. Parthiban Palani for his help in the XRD
calculations.
23 L.-s. Li, J. Walda, L. Manna and A. P. Alivisatos, Nano Letters,
2002, 2, 557–560.
24 K.-J. Wu, K.-C. Chu, C.-Y. Chao, Y.-F. Chen, C.-W. Lai, C.-C.
Kang, C.-Y. Chen and P.-T. Chou, Nano Letters, 2007, 7, 1908–
1913.
Notes and references
25 N. Huang, C. Kan and S. Radiman, Applied Physics A, 2003,
76, 555–559.
1 S. Sergeyev, W. Pisula and Y. H. Geerts, Chemical Society Re-
views, 2007, 36, 1902–1929.
26 A. B. Shivanandareddy, M. Kumar and S. Kumar, Journal of
Molecular Liquids, 2018, 264, 352–357.
2 J. De, I. Bala, S. P. Gupta, U. K. Pandey and S. K. Pal, Journal
of the American Chemical Society, 2019, 141, 18799–18805.
3 C. Zou, J. Wang, M. Wang, Y. Wu, K. Gu, Z. Shen, G. Xiong,
H. Yang, L. Jiang and T. Ikeda, Small, 2018, 14, 1800557.
4 S. Kumar, Chemical Society Reviews, 2006, 35, 83–109.
5 Z. H. Al-Lawati, R. J. Bushby and S. D. Evans, The Journal of
Physical Chemistry C, 2013, 117, 7533–7539.
27 J. S. Son, K. Park, S. G. Kwon, J. Yang, M. K. Choi, J. Kim,
J. H. Yu, J. Joo and T. Hyeon, Small, 2012, 8, 2394–2402.
28 B. H. Toby, Journal of Applied Crystallography, 2001, 34, 210–
213.
29 S. I. Sadovnikov, A. A. Rempel and A. I. Gusev, Nanostructured
Lead, Cadmium, and Silver Sulfides: Structure, Nonstoichiome-
try and Properties, Springer, 2017.
6 J. A. M. Lugger, D. J. Mulder, S. Bhattacharjee and R. P. Si-
jbesma, ACS Nano, 2018, 12, 6714–6724.
30 J. Schneider, W. Zhang, A. K. Srivastava, V. G. Chigrinov, H.-S.
Kwok and A. L. Rogach, Nano Letters, 2017, 17, 3133–3138.
31 H. Iino, J.-i. Hanna, R. J. Bushby, B. Movaghar and B. J.
Whitaker, Journal of Applied Physics, 2006, 100, 043716.
32 D. Adam, F. Closs, T. Frey, D. Funhoff, D. Haarer, P. Schuh-
macher and K. Siemensmeyer, Phys. Rev. Lett., 1993, 70, 457–
460.
7 B. R. Kaafarani, Chemistry of Materials, 2011, 23, 378–396.
8 T. Wöhrle, I. Wurzbach, J. Kirres, A. Kostidou, N. Kapernaum,
J. Litterscheidt, J. C. Haenle, P. Staffeld, A. Baro, F. Giessel-
mann and S. Laschat, Chemical Reviews, 2016, 116, 1139–
1241.
9 A. Gowda, L. Jacob, D. P. Singh, R. Douali and S. Kumar,
ChemistrySelect, 2018, 3, 6551–6560.
33 V. Duzhko, A. Semyonov, R. J. Twieg and K. D. Singer, Physical
Review B, 2006, 73, 064201.
10 I. Bala, W.-Y. Yang, S. P. Gupta, J. De, R. A. K. Yadav, D. P.
Singh, D. K. Dubey, J.-H. Jou, R. Douali and S. K. Pal, Journal
of Materials Chemistry C, 2019, 7, 5724–5738.
34 D. Adam, P. Schuhmacher, J. Simmerer, L. Häussling,
K. Siemensmeyer, K. H. Etzbachi, H. Ringsdorf and D. Haarer,
Nature, 1994, 371, 141–143.
11 I. Bala, N. Singh, R. A. K. Yadav, J. De, S. P. Gupta, D. P.
Singh, D. K. Dubey, J.-H. Jou, R. Douali and S. K. Pal, Journal
of Materials Chemistry C, 2020.
35 A. Ohno and J. Hanna, Applied Physics Letters, 2003, 82, 751–
753.
12 A. Gowda and S. Kumar, Materials, 2018, 11, 382.
13 P. Mahesh, A. Shah, K. Swamynathan, D. P. Singh, R. Douali
and S. Kumar, Journal of Materials Chemistry C, 2020, 8,
9252–9261.
36 K. J. Donovan, K. Scott, M. Somerton, J. Preece and M. Man-
ickam, Chemical Physics, 2006, 322, 471–476.
37 R. Manohar, A. K. Srivastava, P. K. Tripathi and D. P. Singh,
Journal of Materials Science, 2011, 46, 5969–5976.
14 H. Iino, J.-i. Hanna and D. Haarer, Physical Review B, 2005,
72, 193203.
1–7 | 7

New Journal of Chemistry
Page 8 of 8
View Article Online
DOI: 10.1039/D0NJ03290E
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
351x281mm (150 x 150 DPI)