This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/copyright

Author's personal copy

Low-dimensional charge transport of the ferroicNH2(C2H5)4CoCl2Br2 nanocrystals

S.W. Tkaczyk a,n, I.V. Kityk b,d, V. Rudyk c, V. Kapustianyk c

a Institute of Physics, J.Dlugosz University of Czestochowa, Al. Armii Krajowej 13/15, Czestochowa, Polandb Electrical Engineering Department, Czestochowa Technical University, Al. Armii Krajowej 17/19, Czestochowa, Polandc Physical Department, Lviv State University, Lviv, Ukrained Physical Department, College of Science, King Saud University, P.O. Box 2455, Rijadh 11451, Saudi Arabia

a r t i c l e i n f o

Article history:

Received 15 September 2009

Received in revised form

5 April 2010

Accepted 7 April 2010Available online 11 April 2010

Keywords:

Nanocrystallites

Ferroic

a b s t r a c t

DC-conductivity of the nanocomposites consisting of ferroic nanocrystals NH2(C2H5)4CoCl2Br2 (TEA-

CCB) incorporated into the polymer PMMA matrices is investigated. The investigations are performed

for different number of nanocrystallite chromophores and thickness of the samples. The thickness of the

nanocomposite films was varied within 0.44–0.48 mm. The measurements were done at different

temperatures and different applied voltages. As a dominant mechanism thermoemission was

considered, which is determined by temperature of the samples.. The gold electrodes were used as

principle electrodes. The observed phenomena were explained within a framework of hopping between

the trapping levels and possible contribution of self-trapped excitons.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Recently there has been a reanimation of interest regardinginvestigations of complex ferroic nanocrystallites (NC) incorpo-rated into polymer matrices [1–3]. Among the ferroic materialsparticular interest present NH2(C2H5)4CoCl2Br2 (TEA-CCB) singlecrystals. The former investigations were mainly devoted to opticalproperties, i.e. absorption, birefringence and dielectric relaxationprocesses [4]. The TEA-CCB possess interesting nonlinear opticaleffects with phase transition temperature below ambient tem-perature (T1¼249 K and T2¼224 K). Below T2 these crystals areferroelectrics. Due to relatively high hygroscopicity we haveincorporated the investigated nanocrystallites into the PMMAmatrix [4]. The dc-conductivity measurements were performed inthe films of PMMA with thickness varying within 0.3–1 mm. It wasestablished that all the physical features are crucially dependenton the content of the incorporated nanocrystallites. The content ofthe nanoparticles (NP) was varied within 3–7% in weighting unitswith respect to the PMMA mass. For dc-conductivity measure-ments we have used gold and aluminum films as electrodes. Theperformed investigations allowed to establish that injection of theelectrons from electrode into the investigated material has athermoemission origin and depends on the temperature of theexperiment, the electric field strength and the value of the contactpotential barrier on the border contact electrode-investigatedmaterial. The electron charge transfer was monitored through the

trapping states intra the energy gap (two- and three-dimensionalhopping). The value of dc-conductivity for the PMMA with theembedded NP is enhanced at least two orders with respect to thedc conductivity of the pure PMMA matrix.

Usually the polycrystalline low dimensional structures arecharacterized by a large number of defects which substantiallyinfluence on the electrical conductivity. Varying the defectconcentration one can receive materials with the desirablemechanisms of the electron transport and the current–voltagefeatures. For the structures with the low degree of long-rangeordering we deal with the high number of the trapping states.

ARTICLE IN PRESS

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/physe

Physica E

Fig. 1. The size distribution of the nanoparticles (crystallites) obtained by TEM

microscopy versus their sizes.

1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.physe.2010.04.008

n Corresponding author.

E-mail address: stkaczyk2007@wp.pl (S.W. Tkaczyk).

Physica E 42 (2010) 2124–2130

Author's personal copyARTICLE IN PRESS

Fermi energy level for such kinds of materials is situated in themiddle of the energy gap and the broadening of the correspondingquasi bands is determined by the concentration of the defects. Thepolycrystalline materials are convenient materials for explorationof the hopping conductivity in the large range of temperature and

electric field strengths [5]. Deviations from classical behavior ofelectron transport for materials with low degree of orderingallowed to propose hopping models with the varied range by Mottet al. in 1968 [6,7]. There exist some difficulties with thedetermination of electron transport through the volume of theinvestigated material only following the dependence ln s¼ f(T�1/4).This fact cannot be changed even assuming that we deal with thehomogenous space distribution of the trapping levels intra thematerial’s volume and with the linear changes of the activationenergy.

When distinguishing two- and three-dimensional hopping, thefollowing properties of materials play a major role:

1) The varied density of the trapping levels versus temperature;2) The thickness of the film;3) Occurrence of structural non-homogeneities of materials;4) Multi-photon processes and5) Columbic interactions during electron transport (the restric-

tion of the currents by the space charges).Fig. 2. General geometry of the samples. 1 – substrate (BK-7 glass), 2 – top

electrode (aluminum), 3 – film PMMA+TEA-CCB and 4 – bottom electrode (gold).

Fig. 3. Current–voltage dependences for the pure PMMA, Au–Al; (a) T¼30 K and (b) T¼180 K.

Fig. 4. Current–voltage dependences for PMMA possessing 3%TEA-CCB for Au(+) and Au(�): T¼30 K (a) and T¼180 K (b).

S.W. Tkaczyk et al. / Physica E 42 (2010) 2124–2130 2125

Author's personal copyARTICLE IN PRESS

Fig. 5. Current–voltage dependences for PMMA+5%TEA-CCB for Au(+) and Au(�): (a) T¼30 K and (b) T¼180 K.

Fig. 6. Current–voltage dependence for the PMMA+7% TEA-CCB for Au(+) and Au(�) at (a) T¼30 K and (b) T¼180 K.

Fig. 7. Current–voltage dependence of PMMA with the different content of TEA-CCB at T¼30 K; Au–Al. Au(+) (a) dependence I¼ f(U) and (b) dependence log I¼ f(U).

S.W. Tkaczyk et al. / Physica E 42 (2010) 2124–21302126

Author's personal copyARTICLE IN PRESS

Fig. 8. Current–voltage dependence of PMMA with the different content of TEA-CCB at T¼200 K; Au–Al. Au(+) (a) dependence I¼ f(U) and (b) dependence log I¼ f(U).

Fig. 9. Dependence ln I¼ f(1/kT) for the pure PMMA; Au–Al, d¼0.48 mm (a) Au(+) and (b) Au(�).

Fig. 10. Dependence ln I¼ f(T�1/3) (a) and ln I¼ f(T�1/4) and (b) for PMMA+3% TEA-CCB, Au–Al. Au(�), d¼0.44 mm.

S.W. Tkaczyk et al. / Physica E 42 (2010) 2124–2130 2127

Author's personal copyARTICLE IN PRESS

In the case when the contact of electrode with the investigatedmaterial is not perfectly injected, than the flowing current ismonitored through the surface (contact) phenomena andvolume’s effect (Schottky effect, Poole–Frenkel effect) [8–12].

2. Experimental

The thin films of the PMMA with the embedded TEA-CCB NCwere prepared by spin coating method. As an organic solvent wehave used tetrahydrofuran (THF). The nanocrystallite structure ofthe TEA-CCB was obtained by solvation and incorporation of theTEA-CCB into the solution solvent–non-solvent with the simulta-neous ultrasound application. The thickness control was per-formed using interferometric microscope MII-4. The sizedistribution of the nanoparticles (crystallites) obtained by TEMmicroscopy versus their sizes is presented in the Fig. 1.

The solvent mixture of PMMA with THF and TEA-CCB wasdeposited on the BK-7 glass substrate with previously evaporatedgold electrodes. Afterwards the thin layer of polymer was etchedto purify electrical contact. Finally the top Al electrodes weredeposited. In such a way we have received 25 identical measuredsegments (see Fig. 2).

The thickness of the investigated polycrystalline films wasvaried within the 0.44–0.46 mm.

The dc measurements consisted in the measurements of thecurrent flowing through the PMMA+TEACCB film between thegold and Al electrodes. The measurements were done for differentelectrode polarization at applied voltages 0–20 V and tempera-tures 20–325 K. The measurements were conducted inhelium cryostat and the dc-experimental technique is describedin Ref. [13].

3. Results and discussion

Following the obtained dc-conductivity results for the sampleswith different NC content one can conclude that for the electrontransport are responsible at least two mechanisms: two- andthree dimensional hopping and some contribution is caused byPoole–Frenkel mechanism. Generally the dc-conductivity dependson the applied electric strength field, temperature and electrode’spolarization. The shape of the current–voltage dependences

presented in Figs. 3–6 indicates non-Ohmic features of theconductivity.

The Figs. 7 and 8 present current–voltage dependences forsemi-logarithmic scale to present an influence of TEA-CCB inPMMA on the dc-conductivity. The corresponding dependencesare given in Figs. 7 and 8 at temperatures T¼30 and 200 K.

The studied ferroic nanocomposites possess substantial ad-vantageous with respect to the other nanocrystals embedded intothe PMMA, for example In2O3 [14], oxide nanocrystals [15] andeven borates [16] due to the existence of large number of local

Fig. 11. Dependence log I¼ f(U�1/2) for PMMA+TEA-CCB, (a) 5% TEA-CCB, Au(�), d¼0.48 mm and (b) 7% TEA-CCB Au(�), d¼0.44 mm.

Fig. 12. Dependence Fowler–Nordheim for pure PMMA AT polarization Au(�),

d¼0.48 mm.

S.W. Tkaczyk et al. / Physica E 42 (2010) 2124–21302128

Author's personal copyARTICLE IN PRESS

polarized states originating from the ferroic states which inter-acting with the partial nano-confined bands [17] form a goodopportunity for operation by the charge transfer transport.

Following Fig. 9 it seems that the activation energy isindependent on the electric field which is in contradiction withthe principles of Poole–Frenkel mechanism. However, moredetailed evaluations have shown that the activation energydetermined from Fig. 9 was equal to 0.08 eV, at the same timefollowing Fig. 9 these values were varied within the0.016–0.036 eV, which agrees well with the Poole–Frenkel model.

Following dependence ln I¼ f(kT)�1 we have determined thevalues of activation energies for the case of the dc-conductivityfor different voltages of polarizing the samples (see Fig. 9). At lowtemperatures the carrier transport is performed through thejumping between the localized states about the potential barriersmodified by the external dc electric field following the Poole–Frenkel mechanisms linear parts of the corresponding depen-dences for the higher voltages correspond to the manifestation ofthe Poole–Frenkel effect Fig. 11, [18–20]. As a support for two-and three-dimensional hopping may serve linear features

ln I¼ f(T�1/3) and ln I¼ f(T�1/4) (see Fig. 10). Fig. 10 shows thatthe transport is nearly temperature independent since the datafalls horizontally with the temperature axis. This may be due toeither the material being in critical regime of M–I transition orderto normal tunneling transport or other effects.

It is necessary to emphasize that the features for bothelectrode charge injection polarizations and for the differentcontent of TEA-CCB in PMMA matrix (Fig. 12) have taken a placethrough the field emission and thermoemission for the purePMMA. However, principally different mechanism exists due tothermoemission for the PMMA doped by different content of TEA-CCB (Figs. 13 and 14). One can see that the value of electric fieldstrength is higher than critical electric field (about 3�105 V/cm).This appearance may be explained by use of the tips possessinglarge local electric field strengths. The presented dependence iscalled the Fowler–Nodheim curve. One can also expect that somerole may play self-trapped excitons [20].

It is necessary to add that during investigation of such kinds ofprocesses principal role begin to play phonon sub-systemsubstantially modifying the obtained experimental dependences.

Fig. 13. Fowler–Nordheim dependence for PMMA+3% TEA-CCB, d¼0.44 mm, T¼30 and 180 K (a) Au(�) and (b) Au(+).

Fig. 14. Fowler–Nordheim dependences for PMMA+7% TEA-CCB, d¼0.44 mm, T¼30 and 180 K (a) Au(�) and (b) Au(+).

S.W. Tkaczyk et al. / Physica E 42 (2010) 2124–2130 2129

Author's personal copyARTICLE IN PRESS

We have performed the spectrophotometry control of thetrapping levels following Cu and Co absorption lines. It wasestablished that this deviation does not exceed 2.4%. Moreoverdue to the existence of some ferroic states these trapping levelsare partially charged.

Following Figs. 12 and 13 thermoemission is supposed to bethe conduction mechanism. And for the thermionic emissionthese dependences should be linear [21]. For this reason we haveindicated the linear region in Fig. 11.

Our evaluations of the charge concentration following theobtained Fowler–Nodheim dependences have given the valueswhich have varied from 1020 up to 1023 cm�3.

For NP content varying from 3% in weighting units up to 7% inweighting units.

4. Conclusions

1) There exist different mechanisms of dc-conductivity in thecrystalline alloys TEA-CCB embedded in PMMA investigated inthe wide temperature range and electric field strengths.Injection of the carriers from electrode into the investigatedmaterial has taken a place by field emission (tunneling) for thepure PMMA (the phenomenon typical for large field strengthsand low temperatures) together with thermoemission. At lowelectric field strengths with the increasing temperature thereappears a process of thermoemission from electrodes into thematerial’s volume. The similar influence has a doping of thePMMA by addition of the TEA-CCB NP.

2) Two- and three-dimensional hopping is a prevailing mechan-ism for charge transport through the volume of investigatedmaterials. This mechanism is particularly obvious at lowtemperatures and low electric strengths (low activationenergy).

3) During the processes of the charge transport transfer throughthe volume of the investigated material there occurs severalinfluence of the Poole–Frenkel mechanisms.

References

[1] G. Lach, Ł. Laskowski, I.V. Kityk, V. Kapustianyk, V. Rudyk, Ya. Shchur,S. Tkaczyk, J. Swiatek, M. Piasecki, J. Non-Cryst. Solids 353 (2007) 4353.

[2] V. Kapustianyk, Ya. Shchur, I. Kityk, V. RudyG.Lac, L. Laskowski, S. Tkaczyk,J. Swiatek, V. Davydov, J. Phys.: Condens. Matter 20 (2008) 365215 7pp.

[3] K. Ozga, M. Piasecki, S. Tkaczyk, B. Kapustianyk, P. Bragiel, A.H. Reshak,M.G. Brik, I.V. Kityk, Physica B 403 (2008) 2561.

[4] M. Piasecki, P. Bragiel, S. Tkaczyk, I.V. Kityk, J. Ebothe, V. Kapustianyk, M.Patryka, V. /Rudyk, K. Nouneh, A.H. Reshak, Mater. Lett. 62 (2008) 2088.

[5] S.W. Tkaczyk, J. Phys. D: Appl. Phys. 41 (2008) 8pp.[6] Stanis"aw W, Tkaczyk, Nonlinear Optics, Quantum Optics, X, 2007, pp. 1–10.[7] N.F. Mott, Philos. Mag. 24 (1971) 911.[8] Swiatek, J. Phys. Status Solidi (a) 38 (1976) 285.[9] S.W. Tkaczyk, Syn. Met. 109 (2000) 249.

[10] F. Gutmann, L.E. Lyons, in: Organic Semiconductors, Wiley, New York,London, Sydney, 1967.

[11] D.A. Seanor (Ed.), Electrical Properties of Polymers, Academic Press,New York, London, Paris, Sydney, 1992.

[12] M.A. Lampert, P. Mark, in: Current Injection in Solids, Academic Press,New York, London, 1970.

[13] S.W. Tkaczyk, SPIE—Int. Soc. Opt. Eng. 3725 (1998) 232.[14] I.V. Kityk, J. Ebothe, Q. Liu, Z. Sun, J. Fang, Nanotechnology 17 (2006)

1871.[15] S. Tkaczyk, M. Galceran, S. Kret, M.C. Pujol, M. Aguilo, F. Diaz, A.H. Reshak, I.V.

Kityk, Acta Mater. 56 (2008) 5677.[16] A. Majchrowski, I.V. Kityk, T. Łukasiewicz, A. Mefleh, S. Benet, Opt. Mater. 15

(1) (2000) 51.[17] I.V. Kityk, A. Kassiba, K.J. Plucinski, Phys. Lett. 265A (2000) 403.[18] S.W. Tkaczyk, J. Swiatek, Syn. Met. 109 (2000) 255.[19] W. Hwang, C.K. Kao, in: Electrical Transport in Solids, Pergamon Press,

Oxford, New York, Toronto, Paris, Sydney, Frankfurt, 1981.[20] M.I. Miah, Mater. Chem. Phys. 116 (2009) 327.[21] S. Lyshevski, in: Nano and Molecular Electronic (Handbook), Taylor and

Francis Group, 2007, p. 1–12.

S.W. Tkaczyk et al. / Physica E 42 (2010) 2124–21302130