Electrical conduction mechanisms and dielectric constantsof nanostructured methyl violet 2B thin films

H. M. Zeyada • M. M. Makhlouf

Received: 24 August 2014 / Accepted: 23 February 2015 / Published online: 4 March 2015

� Springer-Verlag Berlin Heidelberg 2015

Abstract The uniform thin films of methyl violet 2B,

MV2B, with thicknesses ranged from 96 to 300 nm, have

been successfully prepared by spin coating technique.

X-ray diffraction showed that the powder and pristine thin

film of MV2B have amorphous structure. The amorphous

pristine films become polymorphous nanocrystallites after

annealing at 433 K. The electrical properties of MV2B thin

films have been studied. There are a number of operational

environments where the performance of MV2B thin films

is likely to be affected significantly on their electrical

properties and dielectric constants such as the differences

of film thicknesses, temperatures and frequencies. It was

found that the DC conductivity of MV2B films increases

with increasing temperature. The extrinsic conduction

mechanism is operating in temperature range of

288–360 K with activation energy of 0.16 eV, and the

conduction in extrinsic region is explained via applying

Mott model for variable range hopping. The intrinsic

conduction mechanism is operating in temperatures

[360 K with activation energy of 0.91 eV. The conduction

in intrinsic region is explained by applying band to band

transitions theory. The AC electrical conductivity and di-

electric relaxation of MV2B thin films in the temperature

range 365–473 K and in frequency range 0.1–100 kHz has

been also studied. It has been shown that theoretical curves

generated from correlated barrier hopping, CBH, model

gives the best fitting with experimental results. Analysis of

these results proved that conduction occurs by phonon-

assisted hopping between localized states and it is per-

formed by bipolaron hopping mechanism. The temperature

and frequency dependence of both the real and imaginary

parts of dielectric constant have been investigated.

1 Introduction

Organic semiconductors have been extensively studied in

the last two decades due to their unique electronic, optical

and photoelectric properties compared with conventional

inorganic semiconductors [1–4]. One of the main advan-

tages of organic semiconductors is their easy and low cost

in fabrication; therefore, they posses applications in elec-

tronics and optoelectronic devices. Organic semiconduc-

tors can open up avenues for a variety of industrial

applications with large quantities by simple techniques [5,

6]. However, most of fundamental electrical properties for

organic semiconductors have not yet sufficiently been

clarified since most traditional measuring techniques de-

veloped for inorganic semiconductors are not necessarily

applicable to them. In order to improve the performance of

the organic devices, it is necessary to obtain a deep insight

into electronic processes in the organic semiconductors and

junction properties of organic semiconductors/metal

interfaces.

Methyl violets are organic compounds belonging to the

triphenylmethane family that is mainly used as aromatic

dyes [7]. Methyl violets are mixtures of tetramethyl, pen-

tamethyl and hexamethyl pararosanilines, and by blending

H. M. Zeyada � M. M. Makhlouf

Department of Physics, Faculty of Science, University of

Damietta, New Damietta 34517, Egypt

M. M. Makhlouf (&)

Department of Physics, Faculty of Applied Medical Sciences at

Turabah, Taif University, Taif 21995, Saudi Arabia

e-mail: m_makhlof@hotmail.com; m.m.makhlouf@hotmail.com

M. M. Makhlouf

Department of Physics, Demiatta Cancer Institute,

Damietta, Egypt

123

Appl. Phys. A (2015) 119:1109–1118

DOI 10.1007/s00339-015-9076-5

the different versions depending on the amount of attached

methyl groups to the amine functional group results in

different types of methyl violet dyes, where tetramethyl

(four methyls) is known as methyl violet 2B, pentamethyl

(five methyls) is known as methyl violet 6B and hexam-

ethyl (six methyls) is known as methyl violet 10B [8, 9].

Methyl violet 2B (MV2B) which is used in the present

work has molecular formula C24H28N3Cl and its molecular

structure is shown in Fig. 1. The skeleton of MV2B has an

extended p-conjugated system due to the delocalization of

electrons in the benzene ring, methyl and amine functional

groups and that leading to a wide range of high optical

absorption in visible spectrum. The spectral characteristics

and nonlinear optical properties of MV2B dissolved in

ethanol and in a dye-doped polymer film have been studied

[10]. The linear optical constants, optical dispersion pa-

rameters and dielectric constant of MV2B thin films de-

posited by the spin coating technique also were measured

[11]. All of these studies showed that MV2B has remark-

able optical absorption in the visible region of spectrum

and low optical band gap recommending it as optical

limiting material in optoelectronic devices. Due to the

richness in its properties, it has assumed a peculiar role in

different fields of disciplines such as applications in a dye

sensitized solar cell, DSSC, [12], solar cells for energy

conversion [13] display devices [14], photo-resistors [15],

nonlinear optical devices [10] and gas sensors [16].

Thin films of dyes have been extensively prepared

adopting many techniques including spin coating [11],

thermal evaporation [17], magnetron sputtering [18], sol–

gel [19], chemical vapor deposition [20] and spray py-

rolysis [21]. Application of any technique for thin film

formation depends on molecular size of the dye; spin

coating, sol–gel and spray pyrolysis techniques are applied

for large size molecules other techniques are applied for

small size molecules. With these techniques, thin films of

different thicknesses can be successfully prepared, surface

morphologies and crystallite sizes, electrical and optical

properties depend on applied processing parameters. The

parameters controlling the properties of thin films are as

follows: structure [22, 23], composition [24, 25], film

thickness [26, 27], faults probability [28] and the presence

of impurities [29].

Annealing and substrate temperatures are considered as

processing variables that are used to influence structural

parameters such as: volume fraction of crystallized and

second phase, grain size and its shape, and inter-particle

spacing. Irradiation by ionizing particles such as electrons

or ions and electromagnetic waves such as X and c-rays,laser and UV irradiation may introduce structural defects or

induce phase transformation in the materials depending on

energy of the incident radiation.

In a previous work [11], we investigated the effect of

annealing temperatures on optical constants of MV2B thin

films manufactured by the spin coating technique. The

energy band gap of 1.82 eV, absorption characteristic and

dispersion parameters of MV2B films recommended it for

applications in semiconductors devices applications. In the

present paper, we report on the studies of electrical trans-

port mechanisms, thermoelectric power measurements and

dielectric constants of MV2B thin films prepared by the

spin coating technique and also the influence of different

processing parameters such as film thicknesses, frequencies

and temperatures on the electrical parameters and dielectric

constants. These studies are capable of providing consid-

erable information in order to improve the performance of

organic devices, and it may also be beneficial for the fab-

rication of photoelectric organic devices.

2 Experimental details

2.1 Materials

A green powder of MV2B (IUPAC name: N-(4-(bis(4-

(dimethylamino) phenyl) methylene) cyclohexa-2,5-dien-

1-ylidene) methane aminium chloride) was purchased from

Sigma-Aldrich Co. (USA) and was used in as-received

condition without any further purification. Spectroscopic-

grade ethanol was chosen as a solvent because it possesses

a good solubility for MV2B.

2.2 Thin films preparation

The as-received MV2B dye in a powder form with dif-

ferent weights of 40, 50, 75, 100 and 175 mg individually

was dissolved in 5 mL absolute ethanol and filtered.

Cleaned ordinary glass slides were used as substrates for

depositing films for XRD analysis. The spin coating

NCH3H3C

NH2N

CH3

H3CCl

Fig. 1 Molecular structure of methyl violet 2B (MV2B)

1110 H. M. Zeyada, M. M. Makhlouf

123

technique had been adopted for depositing uniform thin

films onto flat glass substrates. The speed of spin coating

machine was adjusted to 2800 rpm. A drop from each cast

was dropped onto the rotating substrate to form films of

different thickness. The films were dried for 48 h in dark

and at room temperature which results in a film of uniform

thickness. The thickness of deposited films was determined

accurately using an interferometer technique [30]. The

obtained thicknesses were 96, 125, 145, 152 and 300 nm.

The Ohmic contacts at the two ends of MV2B thin films

that used in electrical measurements were provided under

vacuum by thermal evaporation of pure gold using

molybdenum boat.

2.3 Experimental techniques

The structural analysis of MV2B in powder form, pristine

and annealed thin films were analyzed by XRD system

(modelX’ Pert Pro, PhilipsCo.) equippedwithCu target. The

filteredCuKa radiation (k = 1.5408 A)was used. TheX-ray

tube voltage and current were 40 kV and 30 mA, respec-

tively. The transport properties includemeasurements of DC

and AC electrical resistivity, q, of MV2B thin films with

different thicknesses. These measurements were carried out

by adopting the two-point probe technique. The dark elec-

trical resistivity, q, can be calculated from the relation:

q ¼ Rwd

Lð1Þ

where L, w, d and R are the film length, width, thickness

and electrical resistance between the two Au electrodes,

respectively. The resistance of the MV2B films as a func-

tion of temperature was measured in the temperature range

303–408 K using high impedance Keithley 617 pro-

grammable electrometer.

The thermoelectric power, S, of MV2B thin films was

measured by depositing masked rectangular thin films of it

onto clean optical flat ordinary glass substrates. The sample

had a dimension &3 9 0.5 cm2. The contacts were made

by evaporating thick pure Cu electrodes on the ends of

MV2B film. A temperature gradient was produced by

heating one end of the film. The resulting potential dif-

ference, DV, was measured by sensitive digital voltmeter

(Keithley Model 182). The temperature difference, DT, wasmeasured by NiCr-NiAl thermocouple. The thermoelectric

power (Seebeck coefficient) was measured by using the

differential technique based on the following equation [31]:

S ¼ DVDT

ð2Þ

The temperatures T1 and T2 of the two ends were increased

by using two different high power resistances R1 and R2 as

a heat source and a heat sink across the thin film under test.

Samples of sandwich structure Au/MV2B/Au are used

for SCLC measurements (cross-plane measurements). The

gold electrodes are considered as Ohmic contact. The

thickness of MV2B layer is 300 nm and that of gold

electrodes is about 100 nm. The active area of the device is

4 9 10-6 m2. The current was measured by high impe-

dance Keithley 617 programmable electrometer. The in-

dependent stabilized DC power supply of the Keithley 617

electrometer was used as a power supply. All measure-

ments were taken in dark at different temperatures in am-

bient atmosphere.

The AC measurements aimed to investigate the AC

conductivity, rAC, of MV2B films as a function of fre-

quency in the range 1–700 kHz, as well as, in the tem-

perature range 300–393 K. The thickness of MV2B layer is

300 nm, and all measurements are taken under dark inside

a tubular furnace. The AC measurements were taken using

Stanford LCR meter model SR720. This LCR meter can

measure the capacitance, C, the resistance, R, the dissipa-

tion factor, tand, (displayed as D) and the quality factor, Q,

in both of parallel and series modes. The bridge SR720 also

measures the impedance, Z, of the device under test by

measuring the voltage across the device and the current

through it. The ratio of voltage to current is equal to the

complex impedance.

3 Results and discussion

3.1 X-ray diffraction analysis

Figure 3 shows the XRD pattern of MV2B in powder form,

pristine film and annealed film at 433 K with a soaking

time of 1 h. The films are of thickness 300 nm. It is ob-

served that the powder form and pristine films of MV2B

have a halo in the 2h range of 13�–37�, indicating their

amorphous structure. Annealing of MV2B film at 433 K

with soaking time of 1 h results in partial transformation of

amorphous MV2B into nanocrystallites structure; this is

confirmed by presence of a major peak at 2h of 31.67� as inFig. 2c. Figure 2 shows that the amorphous structure of the

pristine thin film (Fig. 2b) crystallizes into a single

nanocrystallites phase (Fig. 2c). The results of FTIR

spectroscopy indicate no change in molecular bonds of

MV2B upon deposition or thermal annealing [11]; this

indicates that polymorphous nanocrystallization occurred,

where atoms in the disordered state jump to crystal front

and those in clusters change their orientation to match the

growing crystal and deposit onto the crystal front. The

formation of nanocrystallites structure by annealing indi-

cates a decrease in structural disorder in annealed film in

comparison with that in pristine film.

Electrical conduction mechanisms and dielectric constants 1111

123

3.2 DC electrical measurements

The DC electrical conductivity, rDC, for planar samples of

MV2B has been calculated by using the following equation

[32]:

rDC ¼ r0 exp�Et

kBT

� �ð3Þ

where r0 is the pre-exponential factors, Et is the thermal

activation energy for this process, T is the temperature

expressed in Kelvin and kB is the Boltzmann constant. The

plot of log (rDC) versus reciprocal temperature (1000/T) in

the temperature range 288–470 K for planar samples with

different film thickness in the range 96–152 nm is shown in

Fig. 3, for a fixed temperature log rDC increases as film

thickness increases. A linear relationship between log rDCand 1000/T is observed; the linear relationship consists of

two segments depending on temperature; region, I, lies in

the temperature range from 288 to 360 K, where the ex-

trinsic conduction prevails, and region, II, is in the tem-

perature range from 360 to 470 K where the intrinsic

conduction prevails. The extrinsic and intrinsic activation

energy, DEex and DEin, respectively, are calculated from

the slope of straight lines for each region, and ro is ob-

tained from the intercept of the straight line with ordinate

axis. The values of activation energies and the pre-expo-

nential factor are listed in Table 1. The calculated average

activation energy in extrinsic region is 0.16 eV and in in-

trinsic region is 0.91 eV. The obtained activation energy in

intrinsic region is half the value of onset energy gap ob-

tained from optical measurements of MV2B [11]. For each

conduction region, the conduction is through a thermally

activated process and can be explained by considering the

contribution from extrinsic and intrinsic mechanisms [33].

In the low temperature region; the conduction is due to

intermolecular conduction process and is done if the in-

termolecular orbital overlap in the organic compound and it

is caused by transfer of charge carriers between molecules.

At high temperature region, intrinsic conduction is due to

electron transport from HOMO to LUMO orbital. The

electrical conduction in the extrinsic region can be ana-

lyzed by applying Mott model for variable range hopping

in localized states near Fermi level [34]. In this model, the

conductivity as a function of temperature for three-di-

mensional hopping is given by:

r ¼ roffiffiffiv

pexp � 1

v1=4

� �; ro ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið9=8pÞ18:1

p e2

jTtpha

ð4Þ

and

v ¼ NF

No

; No �18:1a3

jTð5Þ

where NF is the density of states at Fermi level, vph is the

frequency of phonons assisted with the hopping process

and a is the inverse of Bohr radius. Figure 4 illustrates the

relation between Ln rT0.5 and T-0.25 in temperature region,

I. Straight lines are obtained indicating that the operating

0

50

100

150

200

250

0.0

10 20 30 40 50 60

(c)

(b)

Powder Pristine film Annealed film at 423K

(2

(a)

Inte

nsity

(cou

nts/

Sec.

)

(

Fig. 2 XRD patterns for MV2B: (a) powder, (b) pristine thin film

and (c) annealed thin film at 433 K for 1 h

2.4 2.6 2.8 3.0 3.2 3.4

e-17

e-16

e-15

e-14

e-13

e-12

e-11

e-10

e-9 d,nm 96 125 145 152Average value

E1 = 0.16 eVE2 = 0.91eV

log

DC,

-1 . cm

-1)

1000/T ,K-1

(I)

(II)

Fig. 3 DC conductivity of Au/MV2B/Au in planar configuration as a

function of reciprocal temperature for different film thickness

Table 1 Activation energies of DC conductivity for planar samples

with different film thickness

Film thickness (nm) Activation energy (eV)

DEex DEin

96 0.187 0.894

125 0.186 0.901

145 0.204 0.909

152 0.213 0.916

1112 H. M. Zeyada, M. M. Makhlouf

123

conduction mechanism is a variable range hopping in lo-

calized states near Fermi level. The values of parameters

such as NF, Ro(hopping distance) and Df (average hoppingenergy between two hopping states) have been evaluated

using the following formulae [34]:

Df ¼ 3

4pR3oNF

ð6Þ

Ro ¼9

8pakBTNF

ð7Þ

together with slopes obtained from Fig. 4 and taking into

account that phonon energy is 100 meV, Bohr radius is

1 nm and the measurements are taken at 300 K. The ob-

tained results are presented in Table 2. Results shown in

Table 2 indicate that NF increases and Ro and Df decrease

with increasing film thickness (Table 3).

3.3 Thermoelectric power measurements

Hot probe test showed that MV2B films exhibit p-type

semiconductor conductivity. Results of determination of

Seebeck coefficient, S, as a function of inverse temperature

for pristine MV2B films of four different thicknesses are

depicted in Fig. 5. S decreases as film thickness and tem-

perature are increased; a linear relationship between S and

1000/T for all thickness values in the investigated tem-

perature range suggests that Seebeck coefficient obeys the

relation [35]:

S ¼ � kB

e

� �ES

kBT� ckB þ 1

� �ð8Þ

where Es is the thermal activation energy and c is the

temperature coefficient of thermal expansion. Es is

determined from the slope of straight lines, and it takes a

value of 0.67 eV.

The polaron activation energy, Ep, is deduced according

to [35] as:

Ep ¼ Et � ES ð9Þ

The calculated polaron activation energy is 0.24 eV.

3.4 AC electrical measurements

The AC electrical measurements were taken on thin film

sample in the sandwich structure with active area

0.15 9 1.8 cm2 and thickness of 300 nm. Temperature and

0.232 0.233 0.234 0.235 0.236 0.237 0.238 0.239 0.240-14.4

-14.2

-14.0

-13.8

-13.6

-13.4

-13.2

-13.0

-12.8

-12.6

-12.4

-12.2

-12.0 d, nm 96 nm 125 nm 145 nm 152 nm

T-1/4 , k-1/4

ln (

DCT1/

2 , -1cm

-1k1/

2 )

Fig. 4 Dependence of ln rDC T1/2 on T-1/4 for MV2B thin films with

different thickness in the extrinsic conduction region

Table 2 Dependence of Mott parameters on film thickness for the

pristine films of MV2B

d (nm) NF (cm-3 eV-1) Ro (cm) Df (meV)

96 5.37 9 1018 3.19 9 10-6 1.26

125 7.52 9 1018 2.93 9 10-6 1.16

145 1.02 9 1019 2.71 9 10-6 1.08

152 1.39 9 1019 2.51 9 10-6 1.00

Table 3 Dependence of electrical, thermal and polaron activation

energies on film thickness of MV2B

d (nm) DEt (eV) DES (eV) EP (eV)

96 0.87 0.63 0.24

125 0.88 0.67 0.21

145 0.90 0.68 0.22

152 0.91 0.71 0.20

2.45 2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85

e-16

e-15

e-14

e-13

e-12

e-11

average ( )S(

V/K

)

1000/T , K-1

DC

, -1

cm-1

100

150

200

250

300

350

400

450 d [ = nm ]

96 125 145 155 average (S)

Fig. 5 Dependence of electrical conductivity and thermoelectric

power coefficient on temperature for MV2B films of different

thickness

Electrical conduction mechanisms and dielectric constants 1113

123

frequency dependence of the electrical properties have

been studied.

AC electrical conductivity measurements of semicon-

ductors have been extensively used to understand the elec-

tronic transport mechanisms and are a powerful tool for

obtaining information about the defect states in amorphous

semiconductors [36]. Various models such as quantum

mechanical tunneling (QMT) model [37] and correlated

barrier hopping (CBH) model [36] have been proposed to

explain the AC conduction mechanisms. The total conduc-

tivity rtotal of many semiconductors [36] over a wide range

of frequencies, x, and temperature, T, can be written as:

rtotal ¼ rAC þ rDC ð10Þ

where rAC is the AC conductivity and rDC is the DC

conductivity. This equation is valid only when the AC and

DC conductions arise from completely separate mechan-

isms, but when x ? 0, the AC conductivity represents the

DC conductivity.

Figure 6 shows the AC conductivity as a function of the

reciprocal temperature at nine fixed frequencies for MV2B

sandwich film with a thickness of 300 nm. A linear rela-

tionship is obtained between ln rAC and 1000/T, it consists

of two segments intersecting at a transition temperature, Tt,

depending on temperature and frequency. It is evident that

the conductivity increases with increasing both frequency

and temperature, such a trend indicates that MV2B films

behave as a semiconductor and the temperature depen-

dence of AC conductivity is also represented by Eq. 3. The

values of activation energies and transition temperatures

were calculated for different frequencies and are listed in

Table 4. The activation energies decrease and transition

temperature increases with increasing frequency; that is

because increasing the applied frequency enhances the

electronic jumps between the localized states [38]. The

small values of activation energy confirm that hopping

conduction is the dominant current transport mechanism.

The frequency dependence of rAC (x) at various tem-

peratures is shown in Fig. 7. It is evident from these curves

that AC conductivity has a frequency dependence given by

Eqs. 11 and 12. The frequency dependence of the real part

of the total AC conductivity, rAC, [39] can be given by:

rACðxÞ ¼ eoxe2ðxÞ ð11Þ

rAC ¼ A0xs ð12Þ

where A0is a constant depending on temperature, x is the

angular frequency (x = 2v where v is the frequency), e0 isthe permittivity of free space, e2 is the imaginary part of the

dielectric constant and s is an exponent, generally less than

or equal to unity. This exponent, s, has been used fre-

quently to characterize the AC electrical conduction in

different semiconductor materials. It is clear from Fig. 7

that the variation in the logarithmic AC conductivity is

almost linear with the variation in logarithmic frequency

and rAC (x) increases with increasing frequency and

temperature. The frequency exponent s was obtained from

the slope of ln rA versus ln x and is plotted as a function of

temperature in Fig. 8. The exponent s has temperature

dependence and generally it decreases as the temperature

increases. The observed frequency dependence reveals that

the responsible mechanism for AC conduction could be

due to hopping [36]. The AC conductivity, having fre-

quency dependence (rAC � xs) with s\ 1, has been ob-

served in organic semiconductors thin films [40]. We

invoke the correlated barrier hopping CBH model to ex-

plain the observed behavior in Fig. 8. In the CBH model,

the temperature dependent exponent sis predicted where s

decreases with increasing temperature and it approaches

unity as temperature tends to zero K, in marked contrast to

the quantum mechanical tunneling QMT [36] or simple

hopping over a barrier HOB mechanism [41].

The AC conductivity in the CBH model [36, 41] is given

by:

rACðxÞ ¼ ðp3=24Þn0N2e1eoxR6x ð13Þ

where N is the concentration of sites in pairs, n0 is the

number of electrons that hop over the barrier, e1 is the realpart of the dielectric constant and Rx is the hopping dis-

tance which is given by:

Rx ¼ n0e2=pe1eo½WM þ kBT lnðxsoÞ� ð14Þ

kB is Boltzmann constant, e is the electron charge, WM is

the maximum barrier height and the frequency exponent s

for this model is given by:

2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4

e-29

e-27

e-25

e-23

e-21

e-19

e-17

e-15

Tt

DC 1kHz 5KHz 10KHz 30KHz 50KHz 100KHz 300KHz 500KHz700KHz

1000/T , K-1

AC

,DC ,

-1cm

-1

(I)(II)

Tt

Tt

Fig. 6 Temperature dependence of the AC conductivity at different

frequencies for MV2B thin films

1114 H. M. Zeyada, M. M. Makhlouf

123

s ¼ 1� 6kBT=WM ð15Þ

The temperature dependence on s shown in Fig. 8 is con-

sistent with Eq. 15 indicating the dominance of CBH

model as electrical transport mechanism for MV2B thin

films. According to the CBH model [36], the theoretical

AC electrical conductivity can be calculated by using

Eq. 13 over all the range of the temperatures and

frequencies used in this study. Fitting is made at constant

frequency of 1 kHz, and the same values are used for the

other frequencies. Figure 8 shows that there is agreement

between experimental and theoretical results of electrical

conductivity. This agreement is satisfied only when the

number of polaron, the values of density of states N and the

relaxation time so are 2, 5.51 9 1028 m-3 eV-1 and

1.3 9 10-14 s, respectively. The solid line is the best fit

predicted by CBH model as shown in Fig. 8. As it is clear,

the fitting is reasonably good, indicating that the conduc-

tion is through bipolaron hopping mechanism.

3.5 Dielectric properties

Studies of dielectric properties are important to understand

the nature and the origin of dielectric constants, which in

turn may be useful in the determination of structure and

defects in solids. The dielectric behavior of thin film de-

vices depends not only on their material properties, but also

on the substrate used for fabrication and the type of the

metal electrodes.

Figure 9 shows the capacitance–temperature, C–T,

characteristics of MV2B sandwich thin film with thickness

of 900 nm at various constant frequencies ranging from 1

to 700 kHz. It can be seen that as the temperature in-

creases, the capacitance increases nonlinearly. It is also

observed that the rate of increase in the capacitance in the

low temperature region,\353 K, is lower than that for the

high temperature region, [353 K. Inspection of Fig. 9

shows that for constant temperature the capacity of MV2B

films decreases as the applied frequency is increased. Such

a behavior of the capacitance of MV2B thin film is the

ordinary one for the semiconductor materials [33, 41].

The dielectric loss, in which a part of the energy of an

electric filed is dissipated as heat in the dielectric, is com-

prised of two parts: the first part, which arises due to elec-

trodes resistance, is frequency independent. This can be

minimized using electrodes of highly conducting metal

(Ohmic electrodes). The second part is a property of the

material itself, which is frequency dependent. The dielectric

permittivity, e, is a complex function expressed as

e ¼ e1 þ ie2. The dissipation factor, tan d = e2e1where e2 is a

measure of the energy required for molecular motion and e1is related to the capacitive nature of the material and is a

measure of the stored energy in the material by polarization

Table 4 DC and AC conductivity parameters for MV2B sandwiched film with thickness of 300 nm

kHz 0 (DC) 1 5 10 30 50 100 300 500 700

DE1 0.16 0.09 0.077 0.072 0.056 0.043 0.032 0.018 0.015 0.002

DE2, (eV) 0.92 0.875 0.871 0.863 0.851 0.84 0.8 0.743 0.65 0.62

Tt, (K) 346 353 357 359 365.8 365.8 365.8 365.8 365.8 365.8

8 9 10 11 12 13 14 15 16

-23

-22

-21

-20

-19

-18

-17

-16

-15

-14

TA,K 303 313 323 333 343 353 363 373 383

ln(

AC ,

-1cm

-1)

ln( , rad/s )

Fig. 7 Frequency dependence of the AC conductivity at various

temperatures

310 320 330 340 350 360 370 380 3900.70

0.75

0.80

0.85

0.90

0.95

T , K

Freq

uenc

y ex

pone

nt ,

S

Expermintal Theoretical fit

Fig. 8 Temperature dependence of the frequency exponent, s

Electrical conduction mechanisms and dielectric constants 1115

123

[42]. Figure 10 shows the loss tangent–temperature char-

acteristics of MV2B thin film of thickness 300 nm at var-

ious constant frequencies ranges from 1 to 700 kHz. It can

be seen that the loss tangent, tand, increases with increasingthe temperature and the rate of its increase with tan d is

approximately constant and have a small value at low

temperature region\353 K. For temperatures[353 K, tandincreases nonlinearly with temperature and for a constant

temperature it decreases with increasing frequency [33].

Temperature and frequency dependence of dielectric

constant, e1, are studied for film sample of thickness

300 nm in the temperature range 300–393 K and frequency

range of 1–700 kHz, such a dependence is shown in

Fig. 11. It is observed that e1 increases with increasing

temperature at different constant frequencies. The variation

of e1 with the temperature is related to the charge carriers

which at low temperatures some of them can orient

themselves to the direction of the applied field; therefore,

they possess a weak contribution to the polarization and the

dielectric constant e1. As the temperature increases, a lot of

the bound charge carriers get enough thermal excitation

energy to be able to obey the change in the external field

more easily. This in turn enhances their contribution to the

polarization leading to an increase of the dielectric constant

e1 of the sample [43]. Inspection of Fig. 11 shows that at a

fixed temperature, e1 decreases with increasing the fre-

quency, and this is a result of the dipoles which will no

longer be able to rotate sufficiently rapidly with increasing

frequency, so that, their oscillation will lag behind that of

the field [43]. As the frequency is further increased, the

dipole will be completely unable to follow the field and the

orientation stopped, so e1 decreases at a high frequency

approaching a constant value due to the interfacial

polarization.

Temperature dependence of the dielectric loss e2 for

MV2B sandwich thin film with thickness of 300 nm at

different but constant frequencies in the range 1–700 kHz

is shown in Fig. 12. It is observed that dielectric loss e2increases as the temperature increases for the considered

frequencies; at low temperatures up to 353 K, this increase

is linear and the variation of e2 with the frequency is very

small. For temperatures[353 K, e2 increases nonlinearly.

This behavior can be clarified by plotting ln e2 versus ln xfor various temperatures as shown in Fig. 13. It is observed

that a series of straight lines with different slopes are ob-

tained, e2 decreases as the frequency increases and it in-

creases with the temperature increase, such a behavior has

been realized in many other organic [44–46] and inorganic

semiconductors [47]. The frequency dependence of the

320 340 360 380 4001.6x10-12

2.0x10-12

2.4x10-12

2.8x10-12

3.2x10-12

3.6x10-12

4.0x10-12

4.4x10-12

1kHz 5kHz 10kHz 30kHz 50kHz 100kHz 300kHz 500kHz 700kHz

C ,

F

T , K

Fig. 9 Capacitance dependence on temperature at different

frequencies

320 340 360 380

0.1

0.2

0.3

0.4

1kHz 5kHz 10kHz 30kHz 50kHz 100kHz 300kHz 500kHz 700kHz

tan

T , K

Fig. 10 Loss tangent dependence on temperature at different fre-

quencies for MV2B thin film of thickness 900 nm

320 340 360 380

5

6

7

8

9

10

11

12

13 1kHz 5kHz 10kHz 30kHz 50kHz 100kHz 300kHz 500kHz 700kHz

1

T , K

Fig. 11 Temperature dependence of dielectric constant, e1, at differ-ent frequencies for MV2B thin film

1116 H. M. Zeyada, M. M. Makhlouf

123

dielectric loss, e2, is given by Giuntini model [48] and can

be written by the following expression:

e2 ¼ Yxm ð16Þ

where Y is a constant and m is an exponent given by:

m ¼ �4kBT=WM ð17Þ

To estimate a value for the parameter m, a plot of ln e2versus ln x in the temperature range 303–393 K is illus-

trated in Fig. 13, straight lines are obtained each of them

represent a definite temperature in the temperature range

303–393 K. The power m is calculated from the slopes of

the curves, and it was found that the values of m are

negative and linear with an increase in temperature as

shown in Fig. 14; this is in agreement with the prediction

of the theory [48]. The parameter m confirms the observed

variations as a function of the measuring temperature. In

fact, this result is satisfying if we consider the empirical

law rAC / xs [49]. It is obvious that if s is temperature

dependent, m should consequently depend on temperature.

The obtained experimental results confirm the values of

s obtained during studies on conductivity. Indeed, if we

consider, the value of m = 0.028 at T = 303 K, the ob-

tained value of WM is &3.7 eV for the investigated films.

Such a value of WM is the fundamental energy gap of

MV2B films [11], and it is consistent with the theory of

hopping of charge carriers over a potential barrier as sug-

gested by Elliott [36].

Comparative study of these figures indicates that e1 ande2 increase with the increase of temperature (the rate of

increase varies with the different frequencies), and they

decrease with increasing frequency. This type of behavior

has also been reported in organic films [33, 39].

4 Conclusions

Spin coating technique has been successfully applied to

deposit uniform methyl violet 2B thin films. X-ray

diffraction analysis showed that MV2B in a powder form

and pristine thin films has an amorphous structure. An-

nealing MV2B thin films at 433 K decreased the structure

disorder of these films and partially transformed the

amorphous structure of pristine film into a polymorphous

nanocrystallites structure.

The dark electrical resistivity of MV2B films decreases

nonlinearly with increasing film thickness. The transport

electrical properties of MV2B thin films have been inves-

tigated. Analysis of results of DC conductivity reveals the

presence of two conduction mechanisms: extrinsic

mechanism with average activation energy of 0.16 eV and

320 340 360 380

1

2

3

4

5 1kHz 5kHz 10kHz 30kHz 50kHz 100kHz 300kHz 500kHz 700kHz

T , K

2

Fig. 12 Temperature dependence of dielectric loss, e2, at differentfrequencies

8 9 10 11 12 13 14 15 16 17-1.8-1.6-1.4-1.2-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.01.21.41.61.8

ln ( , rad/s)

ln 2

TA, K 303 313 323 333 343 353 363 373 383 393

Fig. 13 Frequency dependence of dielectric loss, e2, at different

temperatures

300 320 340 360 380 400-0.18

-0.16

-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

m

T , K

Fig. 14 Temperature dependence of parameter m

Electrical conduction mechanisms and dielectric constants 1117

123

intrinsic mechanism with activation energy of 0.91 eV.

The conduction in extrinsic region is explained via apply-

ing Mott model for variable range hopping. The operating

conduction mechanism is a variable range hopping in lo-

calized states near Fermi level. The values of parameters

such density of localized states near Fermi level, hopping

distance and average hopping energy between two hopping

states have been evaluated as 5.4 9 1018 cm-1 eV-1,

3.2 9 10-6 cm and 1.26 meV, respectively, the values of

these parameters are influenced by annealing temperatures.

In intrinsic region, the conduction is by electron transitions

from HOMO to LUMO orbital with activation energy of

0.91 eV. Thermoelectric power measurements showed that

MV2B is p-type semiconductor and the polaron activation

energy is 0.26 eV.

Analysis of AC conductivity data on the basis of corre-

lated barrier hopping (CBH) model predicts that bipolaron

hopping mechanism is the dominant conduction mechan-

ism. The deduced values of density of states and relaxation

time are 5.5 9 1028 m-3 eV-1 and 1.3 9 10-14 s, respec-

tively. The values of real part of dielectric function, e1, anddielectric loss, e2, increase with increasing temperature, and

they decrease with increasing frequency. Analysis of data of

e2 showed that the maximum barrier height is 3.7 eV.

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