Nanostructure influence on the photorefractivity and
photoconductivity of the organic systems
N.V. Kamanina*, S.A.Serov, N.A.Shurpo, Yu.A.Zubtsova, A.V.Shmidt
Vavilov State Optical Institute, 12 Birzhevaya Line, St. Petersburg, 199034, Russia,
*e-mail: nvkamanina@mail.ru
A.V. Prokhorenkov, E.A.Tsareva
State Electrotechnical University, St. Petersburg, 197376, Russia
ABSTRACT
Study and optimization of new 3D materials useful for optoelectronic application have been
considered. The increase of the photorefractive parameters and photoconductive
characteristics of the organics, including conjugated polymers, electrooptic monomers and
liquid crystals doped with nanoobjects have been revealed. The second harmonic of
nanosecond pulsed Nd-laser at wavelength of 532 nm has been used. The energy
density has been chosen in the range of 0.1-0.5 J×cm-2. The amplitude-phase thin
gratings have been recorded at spatial frequency of 90-100 and 150 mm-1 under
Raman-Nath diffraction conditions.
It has been testified that that the nonlinear optical characteristics (nonlinear refraction n2 and
cubic nonlinearity χ(3)) of the organics thin films sensitized with fullerenes, nanotubes or
quantum dots can be increased up to 2-3 orders of magnitude in comparison with the same
parameters for pure organics and balk materials traditionally used for nonlinear optics.
1. Introduction
Conjugated polymers, monomers, and liquid crystals are of good model and
promising media to test new properties via nanoobjects sensitization [1-4]. Both
fullerenes and quantum dots with high electron affinity energy as well as the carbon
nanotubes with large charge of odd electrons from the core of nanotubes, can be
considered as effective nanoobjects to dope the media mentioned above. The
incorporation of new sensitizers in the organic matrixes is perspective way to increase
their charge carrier mobility [5] and local volume polarizability [6] saving good
resolution estimated via record of thin amplitude-phase holographic gratings. As a
result of this process, the increase in dark and photoconductivity can be observed and
the correlation between the photoconductive and the photorefractive parameters can
be found. The current paper is devoted to investigation of these features as
characteristic of nanostructured conjugated organic systems.
2. Experimental conditions
The experiments have been performed with thin films of conjugated monomer,
polymer and liquid crystals sensitized with fullerenes, nanotubes or quantum dots. 2cyclooctylamino-5-nitropyridine (COANP), polyimide (PI), N-(4-nitrophenyl)-(L)prolinol (NPP), nematic liquid crystal (NLC) have been considered as organic
matrixes. Fullerenes concentration was varied in the range of 0.1-1.0 wt.%. The
concentration of carbon nanotubes was in the range of 0.01-0.1 wt.%; the ones of
quantum dots was close to 0.003 wt.%. The films have been developed using
centrifuge deposition. The thickness of the films was 2-5 micrometers. The LC cell
thickness was 5-10 micrometers. The nanostructuref LC films have been placed onto
glass substrates covered with transparent conducting layers based on ITO contacts.
The nanostructured monomer or polymer films have been deposited on the substrate
2-45
with ITO contact. For the electric measurements, gold contact has been put to the
polymer upper side. The bias voltage applied to the photosensitive polymer layers has
been varied from 0 to 50 V. The current–voltage characteristics have been measured
under the illumination conditions from dark to light. Voltmeter-electrometer В7—30
and Characteriscope—Z, type TR-4805 has been used for these photoconductive
experiments. The photorefractive characteristics have been studied using four-wave
mixing technique analogous to paper [7]. The second harmonic of pulsed Nd-laser at
wave length of 532 nm has been used. The laser energy density has been chosen in the
range of 0.1-0.5 J×cm-2. The nanosecond laser regime (the pulse width was 10-20 ns)
has been applied. The amplitude-phase thin gratings have been recorded under
Raman-Nath diffraction conditions at spatial frequency of 90-150 mm-1.
3. Results and discussion
Table 1 presents the current-voltage characteristics of some pure and nanosensitized
compounds. The current–voltage characteristics of pure and fullerene-containing
COANP, PIs, and NPP were measured for the samples with various concentrations of
fullerene C70 or C60 additives in the organic matrixes under the conditions of dark and
light illumination. In Table 1 the basic accent is given on current–voltage
characteristics under light irradiation.
Table 1
Bias
volta
Pure PI
ge,
V
Current, A, under light irradiation
PI+0.2
wt.% C70
Pure
COANP
COANP+
1.0 wt.% C70
Pure NPP
NPP+
1.0 wt.%
C60
0
2.7710-12
4.3310-12
5
2.9810-11
5.010-11
0.510-12
0.810-12
0.1410-11
0.410-9
10
6.9610-11
1.110-10
0.610-12
0.810-12
0.5410-11
0.1710-8
15
1.0110-10
2.310-10
0.1610-11
0.210-11
0.5710-11
0.2510-8
20
1.4410-10
3.710-10
0.2610-11
0.3610-11
0.610-11
0.310-8
30
2.510-10
8.010-10
0.310-11
0.2110-10
0.810-11
0.3710-8
40
3.810-10
1.410-9
0.4210-11
0.4510-10
0.210-10
0.4810-8
50
5.710-10
2.410-9
0.4610-11
0.810-10
0.3510-10
0.710-8
One can see from the Table 1 that the increase in the current response can be observed
after fullerene sensitization at the same bias voltage. Two orders of magnitude
difference in the current has been observed. This fact is connected with the charge
transfer complex (СTС) formation between donor part of the organic molecule and
fullerene. To support the CTC formation, the mass-spectrometry analysis has been
made. The mass spectroscopy data point to the CTC formation between fullerene and
triphenylamine and between fullerene and the HN group in the C70-PI and C70COANP systems, respectively [8]. Photoconductive measuring shows that an increase
in photoconductivity by at least two orders of magnitude is observed for all fullerenedoped films. Therefore, photoconductivity and mass spectrometry data confirm the
complex formation in the studied structures. Moreover, some photorefractive
parameter, namely two photon absorption coefficient increases in the fullerene-doped
COANP and PI [8].
2-46
It should be noticed that for PI-C70 systems СTС formation can be predicted to be
increased by irradiation of laser beam at 532 nm. Really, the laser irradiation at 532
nm excites CTC between fullerene and triphenylamine (donor fragment of PI
molecule), for which the following condition is met:
hCTCID–EA–W
(1)
Here hCTC is the photon energy exciting CTC, ID is the donor ionization potential, EA
is the electron affinity of the acceptor, and W was the polarization energy of bonds.
For the C70-PI system hCTC was about 2.35 eV at ID=6.5 eV (for triphenylamine),
EA=2.65 eV (for fullerene), and W1.5 eV, that is close to the laser wavelength of
532 nm. It should be mentioned, that for fullerene-doped PI irradiating laser energy
density Win of 0.5-0.6 Jcm-2 corresponds to 350-400 kcalmol-1 at molecular mass
of PI units of 750. The rotation threshold is 100-700 kcalmol-1 [9] This effect is
likely to result in a better arrangement of PI donor fragment (triphenylamine) and
fullerene planes, and hence in efficient CT between them [10].
Table 2 presents the current-voltage characteristics of pure and nanotubes-sensitized
PI compounds. It should be mentioned, that this fact can be explained by the
additional complex mechanisms responsible for the electron moving in the conjugated
organics with nanotubes. The additional odd electron from the core of the nanotubes
should be taken into consideration.
Table 2
Type of the PI system
Contacts polarity
Bias voltage, V
Current, A
Pure PIs
Au (-), ITO (+)
Au (+), ITO (-)
0.5
1
10
0.5
1
10
0.5-1
2.210-13
610-13
5.810-11
1.110-12
1.110-12
610-11
Close to 10-4 - 10-3
Pure PIs
Au (+), ITO (-)
PIs with carbon
nanotubes
PIs with carbon
nanotubes
Au (-), ITO (+)
0.5-1
Close to 10-4 - 10-3
The data of Table 2 testifies that the drastic increase in the current response can be
observed after carbon nanotubes sensitization of PI structures. More than six orders of
magnitude difference in the current has been observed at the same value of bias
voltage.
Some procedure to estimate the charge currier mobility for the pure, fullerene- and
nanotubes-doped PI has been made. It should be noticed that for PI conjugated
systems we have drown the attention mostly on the role of charge carrier mobility
rather than on their concentration. This is explained by the fact that in this type of
organics the conductivity activation energy coincides with the mobility activation
energy; plotted in the double logarithmic scale, the values of conductivity and
mobility (measured at equal temperatures and electric field strengths) fit to the same
straight line inclined at 45° relative to the coordinate axes. The charge carrier mobility
has been estimated via the Child–Langmuir current–voltage relationship by the
method proposed in paper [11]. The charge carrier mobility of pure and fullerenedoped samples have been estimated via following equation:
2-47
µ=1013Id3/εV2,
(2)
where I is the density of the current, V –is value of bias voltage, d – is thickness of the
films, ε is dielectric permittivity.
The results of these calculations showed that the introduction of fullerenes leads to a
tenfold increase in the mobility. Under these conditions, the carrier mobility in a
fullerene-doped PI film has been ~0.310–4 cm2V-1s-1, while the carrier mobility of
pure PI films ranges has been placed in the interval from 10–7 to 0.510–5 cm2V-1s-1.
The values of mobility have been estimated at a bias voltage of 10 V, a film thickness
of d = 2 µm, a dielectric constant of ε ~ 3.3, a fullerene concentration of about 0.2
wt % C70. Using this method the charge carrier mobility of the nanotubes-doped PI
films has been found and placed in the range of 0.510–4 0.9510–4 cm2V-1s-1. Thus,
at smaller nanoobjects concentration, the charge carrier mobility has been found a
little bit larger for nanotube-doped structures in comparison with fullerene-doped ones.
It should be noticed that the photoconductive parameters coincide with the
photorefractive ones. Really, for polyimide matrixes the basic results regarded to the
laser-induced change of refractive index, n, under condition of different nanoobjects
sensitization and at varied spatial frequency, Λ, are shown in Table 3.
Table 3
Structure
Pure PI
PI +quantum
dots based on
CdSe(ZnS)
PI+С70
PI +nanotubes
n
Nanoobjects
content,wt%
0
0.003
Energy density, Jcm-2
0.6
0.2-0.3
Λ
mm-1
90
100
10-4-10-5
2.010-3
0.2
0.1
0.5-0.6
0.3-0.5
90
100-150
4.6810-3
5.5-5.710-3
Ref.
[12]
current
[12]
[12]
From the laser-induced changes of the refractive index, the nonlinear refraction n2 and
the third order susceptibility (3) were determined via procedure [13]. They were,
respectively: ~10-7 cm2kW-1 and ~10-9 esu for thin conjugated films of the
nanoobjects-doped organic structures; ~10-6 cm2kW-1 and ~10-8 esu for thin films of
the nanoobjects-doped polymer-dispersed liquid crystals. The model explaining
improving of nonlinear optical and of dynamicс features of the nanoobjects-doped
organic structures via increase in dipole moment and local volume polarizability were
early presented in the paper [6].
It should be mentioned that nonlinear optical parameters of nanoobjects-doped
structures are larger than those obtained for traditional nonlinear inorganic systems
and for pure organics. Moreover, the data testify that nonlinear characteristics of the
materials studied are close to those for Si-based structures and larger than the ones for
classical inorganic widely used crystal of LiNbO3, that provoke the organics
structures with nanoobjects to be used in extended area of optical and non-linear
optical applications.
4. Conclusions
Influence of the nanoobjects based on fullerenes, carbon nanotubes and quantum dots
on photorefractive and photoconductive parameters of the PI, COANP, NPP, liquid
crystals, materials have been observed and discussed. The correlation between
2-48
photorefractivity and photoconductivity has been considered as the feature of
nanoobjects-sensitized organics. As the results of investigations, the area of
applications of the materials can be found in the optoelectronics and laser optics, for
example, for holography, for telecommunication systems, for gas storage and solar
energy accumulation, as well as in nonlinear optical field and search for 3D-media.
5. Acknowledgement
The authors would like to thank their colleagues, namely, Dr. V.I.Studeonov,
P.Ya.Vasilyev (Vavilov State Optical Institute) and Prof. N.M.Shmidt (Ioffe PhysicalTechnical Institute, St.-Petersburg) for their help in this study. The work is supported
by Russian Foundation for Basic Researches (grant #10-03-00916, 2010-2012) and by
Vavilov State Optical Institute (grant named “Perspectiva”, 2010). It should be noticed
that the students, namely, Alexey Prokhorenkov and Ekaterina Tsareva have been
involved in this study.
References
[1] McBranch DW, Maniloff ES, Vacar D, Heeger AJ: “Ultrafast holography and
transient-absorption spectroscopy in charge-transfer polymers”. Proceed. SPIE
1997 3142, 161.
[2] Silence SM, Walsh CA, Scott JC, Moerner WE: “C60 sensitization of a
photorefractive polymer”. Appl.Phys.Lett. 1992 61 (25) 2967.
[3] Xiaowen Jiang, Yuezhen Bin, Masaru Matsuo: “Electrical and mechanical
properties of polyimide–carbon nanotubes composites fabricated by in situ
polymerization”. Journal: Polymer 2005 46 (18) 7418.
[4] Robertson J: “Realistic applications of CNTs”. Materialtoday 2004 Oct. 46.
[5] Mikhailova MM, Kosyreva MM, Kamanina NV: “On the increase in the charge
carrier mobility in fullerene-containing conjugated organic systems”. Tech.Phys.
Lett. 2002 28 (6) 450.
[6] Kamanina NV: “Fullerene-dispersed liquid crystal structure: dynamic
characteristics and self-organization processes”. Physics-Uspekhi 2005 48 (4)
419.
[7] Kamanina NV, Vasilenko NA: “Influence of operating conditions and of interface
properties on dynamic characteristics of liquid-crystal spatial light modulators”.
Opt. Quantum Electron. 1997 29 (1) 1.
[8] Kamanina NV, Plekhanov AI: “Mechanisms of optical limiting in fullerenedoped -conjugated organic structures demonstrated with polyimide and COANP
molecules”. Optics and Spectroscopy 2002 93 (3) 408.
[9] Bershtein V , Egorov V , DSC in Physical Chemistry of Polymers, Chemistry:
Leningrad 1990.
[10] Cherkasov YA, Kamanina NV , Alexandrova EL , Berendyaev VI, Vasilenko
NA , Kotov BV: “Polyimides: New properties of xerographic, thermoplastic, and
liquid-crystal structures”. Proceed. SPIE 1998 3471 254.
[11] Gutman F, Lyons LE, Organic Semiconductors, Wiley: New York, 1967.
[12] Kamanina NV, Uskokovic DP: “Refractive Index of Organic Systems Doped
with Nano-Objects”. Materials and Manufacturing Processes 2008 23 552.
[13] Akhmanov SA, Nikitin SYu, Physical Optics, Moscow University Press:
Moscow, 1997.
2-49