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Journal of Crystal Growth 275 (2005) e1779–e1786

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Molecular organic crystalline matrix for hybridorganic–inorganic (nano) composite materials

A. Stanculescua,�, L. Tuguleab, H.V. Alexandrub, F. Stanculescub, M. Socola

aNational Institute of Materials Physics, 105bis Atomistilor Street, PO Box MG-7, 077125 Magurele-Bucharest, RomaniabFaculty of Physics, University of Bucharest, 077125 Magurele-Bucharest, Romania

Available online 16 December 2004

Abstract

Metal-doped benzil crystals have been grown by thermal gradient solidification in a vertical transparent growth

configuration to investigate the effect of metallic guest on the ordered organic host. We have identified the conditions

for growing homogeneous, optically good crystals of benzil doped with sodium and silver, limiting the effect of

supercooling, low thermal conductivity and anisotropy of the growth speed (temperature gradient at the liquid–solid

interface: 10–25 1C, moving speed of the growth interface 2.0mm/h). The nature and concentration of the dopant are

parameters affecting, through the growth process, the crystalline perfection and the optical properties of the organic

matrix. Bulk optical characterisation, by spectrophotometrical methods, has offered details on some intrinsic properties

of the system metal particles/benzil crystalline matrix. Analytical processing of the experimental data emphasised that

benzil is a wide optical band gap organic semiconductor Eg ¼ 2:65 eV: We also have investigated the effect of sodiumand silver on the properties of benzil crystal as potential transparent semiconductor matrix for (nano)composite metal/

molecular organic material. With the increase of sodium concentration from c ¼ 1 to 6wt%, a small narrowing of the

band gap has been remarked. The same behaviour has been found for benzil doped with silver ðc ¼ 2wt%Þ compared to

pure benzil.

r 2004 Elsevier B.V. All rights reserved.

PACS: 42.70.Jk; 42.70.Nq; 61.66.Hq; 61.72.Ss; 61.82.Pv; 78.40.Me; 81.10.Fq

Keywords: A1. Characterization; A1. Doping; A1. Impurities; A2. Bridgman technique; B1. Organic compounds; B2. Semiconducting

materials

e front matter r 2004 Elsevier B.V. All rights reserve

ysgro.2004.11.210

ng author. Tel.: +4021 493 0047; fax:

.

ss: sanca@infim.ro (A. Stanculescu).

1. Introduction

Composite materials, in which metallic particlesare dispersed in a dielectric matrix, were inten-sively studied because of the possibility to tailorprecisely their optical and electrical properties by a

d.

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A. Stanculescu et al. / Journal of Crystal Growth 275 (2005) e1779–e1786e1780

proper choice of the system components, metaland matrix and varying the metal content. In thiscase, the dielectric matrix plays only a passive roleas mechanical support for the metallic particles.On the contrary, only little attention has been paidto metal particles embedded in a semiconductormatrix.Although the possibility to tailor make semi-

conductor molecular solids with specific physicalproperties through guest particles embedded in thecrystalline lattice seems very attractive, somecomplications may appear from host crystallinestructure and dopant impurities sites.Till now, a special attention was paid to the

molecular crystals doped with rare earth metallicions as materials for luminescent and laserapplications [1,2]. Only a few investigations havebeen made on the effect of other metallic dopantson the properties of crystalline benzil, like thesystem benzil/Cd2þ [1].Organic solid-state properties strongly depend

on crystalline perfection and chemical defects. Togrow organic crystals maintaining a reasonableratio between cost and desired properties stillremains a challenge. Growth of large, structurallygood organic crystals is very important for afundamental understanding of phenomena takingplace in this special class of materials combiningspecific optical and electronical properties of bothcomponents and the development of some newhybrid organic–inorganic materials for targetapplications.The major problems for large-scale applications

of crystalline matrices of aromatic derivativematerials are connected with the requirements ofcrystal growth (low melting point, supercooling,low thermal conductivity [3,4]) and of crystalprocessing (weak mechanical properties [5] causedby weak bonding forces between molecules).Doping of these materials also raises difficultiesconcerning the homogeneous distribution of theguest atoms during the crystal growth, requiringspecific solutions.Benzil is an uniaxial crystal that belongs to the

space-group D43 or D63; isomorphic with a quartz

(previous work has been reported on metal atomsdiffusion and the growth of microstructure inquartz [6]) being labelled as ‘‘organic quartz’’ [7,8].

This explains the interest in studying benzil asmatrix for (nano)composite and the effect of guestmetal atoms on its properties.In this paper we present a study on the influence

of monovalent alkali (sodium) and non-alkali(silver) metallic dopants on the intrinsic propertiesof benzil ðC6H5COCOC6H5Þ crystalline matrix.

2. Experimental methods

We have obtained pure and doped crystals ofbenzil (known also as diphenyl-a;b-diketone,diphenylethanedione, bibenzoyl) from the melt,in vertical Bridgman–Stockbarger configuration,previously used to grow meta-dinitrobenzene pureand doped crystals [9–11], with a transparentfurnace for the direct (real-time) observation andcontrol of the growth.Because the purity of the starting material is

very important for the crystal growth process,benzil Aldrich Inc. (purity 98wt%) has beenpurified using repeated (6–10 passes) molten zonerefining method, benzil being chemically stable atthe melting point 95 1C [12].We have analysed the influence of inorganic

dopants (silver: c ¼ 2wt% and sodium: c ¼ 1 and6wt%) on the growth process and on the proper-ties of the organic crystalline matrix. Generally, itis difficult to obtain organic crystal homoge-neously doped because it is necessary to assureand maintain conditions close to the stationarygrowth by rigorously controlling the temperatureand the moving speed of the solid–liquid interface.For organic crystal growth, temperature has two

counteracting actions and its control remains veryimportant. Low thermal gradients are required toprevent generation of mechanical defects andcracking of the grown crystal during a plasticdeformation, but at the same time steep gradientsare necessary to counteract the supercooling effect.To control the freezing process for every moltenmixture benzil-dopant we have selected the ade-quate thermal gradient at the growth (solid–liquid)interface, taking into account these two factorswith counteracting effects.Pure and doped benzil have been crystallised

in specific experimental conditions (Table 1).

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Table 1

Experimental parameters for melt crystallisation of benzil/dopant system

Molten mixture composition Hot zone temperature Liquid–solid interface gradient Moving speed of the growth interface

(1C) (1C) (mm/h)

Benzil pure 108 13 1.9–2.0

Benzil /silver ðc ¼ 2wt%Þ 112 17 1.7–1.9

Benzil/sodium ðc ¼ 1wt%Þ 106 11 1.9–2.0

Benzil/sodium ðc ¼ 6wt%Þ 95 10 1.7–1.8

A. Stanculescu et al. / Journal of Crystal Growth 275 (2005) e1779–e1786 e1781

Correlating the thermal regime, moving speed ofthe ampoule, low thermal conductivity of organicmaterials (7:38� 10�4 Wcm�1 �C�1 for moltenbenzil [13]) and the anisotropic growth speed wetry to annihilate the supercooling effect, controlthe freezing mechanism of the molten mixture andthe shape of the crystal–melt interface with the aimto avoid the growth of nonhomogeneous organiccrystals favoured by the Bridgman–Stockbargermethod [14].Benzil crystals have been doped with alkali

metals by melting the mixture benzil/sodium undervacuum in a closed system (ampoule). We havechosen as source for doping with Ag, a silvercomplex, ½AgðNH3Þ2Cl obtained in a two stagesprocess following the chemical reactions givenbelow, starting from silver nitrate [29]:

AgNO3 þHCl! AgCl # þHNO3; (1)

AgClþ 2NH3ðexcessÞ ! ½AgðNH3Þ2Cl: (2)

The ampoule containing benzil and½AgðNH3Þ2Cl was evacuated, sealed under va-cuum and then heated till the charge wascompletely molten.To grow doped organic crystals matrices we

have established a thermal gradient of 10–25 1C atthe solid–liquid interface to avoid the suddenfreezing of the melt and the generation of amaterial with a glassy structure and used a slowmoving speed (2mm/h) of this interface to assurethe evacuation of the melting latent heat, controlthe crystallisation process and so, the crystalperfection. With the increase of the impuritycontent in benzil for a given dopant, we have todecrease the moving speed of the solid–liquidinterface (1.7mm/h) to control the crystal growth

process and the incorporation of the guest metalatoms in the organic host.Cutting and polishing of generally soft organic

molecular crystals need special techniques [5], likegrown ingots cut with a wire saw machine andmechanically polishing of the wafers with amixture of ethyleneglycol–alumina powder and awool tissue on a glass optically flat [10].To analyse the effect of the variations in the

parameters of the growth process on the intrinsicproperties of the system metal/crystalline organicmatrix, we have used bulk sample transmissionmeasurements realized with computer assisteddouble beam Perkin-Elmer Lambda 2S Spectro-photometer.

3. Results and discussions

The shape, volume and bonding interactions ofthe guest dopant entities determine the incorpora-tion of a certain dopant in the lattice of the hostcrystal [15]. If the dopant is not satisfying in benzil,the condition necessary for solubility in solid phase(matching from the point of view of the geome-trical dimension, corresponding to a geometricalsimilarity [16,17] between guest and host latticeentities) it remains as microinclusions of a distinctphase on which the light could be scattered givingthe milky aspect of the sample. The overlappingfactor (unoverlapped volume/overlapped volume[18]) between the two components of the system(host/dopant) give the first evaluation of thedoping level and a lower value of this factor canfavour only low doping level.Comparing the diameter of the free space in

benzil lattice, evaluated at 2.9 A from themolecular structure and the crystalline lattice

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400 500 600 700 800

0.0

0.5

1.0

1.5

2.0

2.5

3.0

benzil pure

2.5

3.0

3.5

4.0

Wavelength (nm)

Ab

so

rba

nce

(a

.u.)

benzil: 2 wt% Ag

2.58

2.61

2.64

380 400 4203.3

3.6

3.9

Fig. 1. Absorbance spectra of bulk sample of benzil doped with

Ag (2wt%) compared with pure benzil (narrow peaks spectrum

in detail).

A. Stanculescu et al. / Journal of Crystal Growth 275 (2005) e1779–e1786e1782

parameters [19] with atomic/ionic diameter ofsodium (3.80/1.9 A) and silver (2.88/2.52 A) wehave deduced that these atoms could penetratethrough benzil lattice and can be localised ininterstitial positions, the incorporation of metallicatoms being facilitated by the local deformation inthe weakly van der Waals bonded organic solid. Itis unlikely for Ag and Na atoms to occupysubstitutional position in benzil lattice as aconsequence of different geometrical dimensionsbetween guest metal atoms and host latticemolecules.From geometrical considerations, Na atom

could be incorporated in benzil in interstitialposition with difficulties, by local deformationperturbing the lattice and generating cracks,because the atomic diameter is greater than thediameter of free space in benzil unit cell, but Agatom, could be incorporated more easier ininterstitial position because the atomic diameteris comparable with the free space diameter. Naþ

and Agþ ions with ionic diameters smaller than thefree space diameter also could be interstitiallyincorporated. The 13.75 A spacing between mole-cular planes on the direction [0001], which is largecompared to the Na and Ag atomic and ionicdiameters, provides a natural path for metal atommotion in the crystal.No chemical reaction is involved in the incor-

poration of Ag in benzil lattice, because Ag has avery low reactivity and is very unlikely to reactwith H or O atoms. Non-alkali metals (Ag) couldform only weak DþA� complex with aromaticcompounds because the donor, Ag, has largeionisation energy (7.57 eV) [20] and the chargetransfer process could be possible permitting thetransfer of electron from metal to organic withoutrearrangement in molecular structure [21].For Na atoms, things are more complicated

because of their high reactivity. The interactionbetween alkali and alkali-earth metal atoms (Li[22], Na, K, Ca) and organic molecules consists ina chemical reaction to form an organometalliccomplex or a charge transfer generating anion–ca-tion pairs. The generation of benzil–sodiumorganometallic complex has a very low probabilityto take place because the hydrogen in benzil hasnot a too strong acid character to be substituted

directly with alkali metal atoms and the workunder vacuum in a closed system reduces thepossibility of sodium oxides, Na2O and/or Na2O2formation.The charge transfer interaction is caused by a

nonbonding force, as van der Waals type, and adative bonding force [21] corresponding to astructure of the complex in which two donoratoms of Na could give each the outer 3s electronto two oxygen atoms of the two acceptor carbonylgroups in benzil (having a strong electron affinity).The isolated metal atom is stabilized by thesurrounding organic molecules and because theionisation energy (for most alkali metal atoms) islow, of the order of 4–6 eV [23], the electrontransfer from the guest (Na atom) to the host(benzil) is allowed, without the formation of acovalent bound. The colour change from clearyellow to red-brown with the increase of Naconcentration from 1 to 6wt%, could be explainby a DþA� complex formation with the aromaticcompound or by the light scattering on thenonhomogeneities of the sample.UV–VIS spectra of bulk pure and doped benzil

samples have a specific shape and an obviousnarrow peaks structure (Figs. 1–3) at wavelengthsshorter than 450 nm. The shift of the fundamentalabsorption edge through longer wavelength bydoping suggests the absorption of light on anenergetic level within the band gap. The differences

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400 500 600 700 8000

2

4

6

8

10

3

2

1

1 - benzil pure2 - benzil: 1 wt% Na3 - benzil: 6 wt% Na

0.0

0.5

1.0

1.5

2.0

2.51

2

Abs

orba

nce

(a.u

.)

Wavelength (nm)

Fig. 2. Absorbance spectra of bulk samples of benzil doped

with Na ðc ¼ 1wt%Þ (2) and Na ðc ¼ 6wt%Þ (3) compared with

pure benzil (1).

380 390 400 410 420 430

2.60

2.62

2.64

Wavelength (nm)

Abs

orba

nce

(a.u

.)

0

5

10

151

1

1 - benzil pure2 - benzil: 1 wt% Na3 - benzil: 6 wt% Na

2 3

Fig. 3. Detail with the absorbance spectra of bulk samples of

benzil doped with Na ðc ¼ 1wt%Þ (2) and ðc ¼ 6wt%Þ (3)

compared with pure benzil (1) between 375–435nm.

A. Stanculescu et al. / Journal of Crystal Growth 275 (2005) e1779–e1786 e1783

between the intensities of absorption are attributedto different thickness of the wafers.The UV–VIS spectrum of pure benzil conserves

its general pattern also for bulk samplesðthickness ¼ 1:9mmÞ of benzil doped with Ag ðc ¼

2wt%Þ; but some detail changes can be remarked:the narrow peaks keep their positions but theirintensities change compared with those for purebenzil sample ðthickness ¼ 1:6mmÞ (Fig. 1). In thiscase, the dominant weak character of van derWaals bonding of the organic molecules is not

modified because Ag does not interact with benzilmolecules and the structure of the fundamentalabsorption edge is not affected preserving thenarrow peaks structure. The peak situated at380 nm is correlated with some particularities ofthe benzil molecular configuration and it isassigned to the absorption of the dicarbonylgroups [24,29].The doping with Na (Figs. 2 and 3) induced

significant changes in the shape of the UV–VISspectrum of the bulk sample. Instead of a narrowpeaks spectrum appears a large, structured band,both for sample with low doping level (c ¼ 1wt%;thickness ¼ 2mm) and sample with high dopinglevel (c ¼ 6wt%; thickness ¼ 3.3mm), whichcould be associated with the scattering of lighton the nonhomogeneities of the bulk Na-dopedbenzil sample or with changes in the nature of theforces acting between the two components of thesystem, guest (Na)/host (benzil). Beside the weakvan der Waals, stronger interaction also couldexist between organic molecule and metal atoms.Information deduced from the transmission

data near the fundamental absorption edge hasbeen correlated with some intrinsic properties ofthe crystalline organic compound, as the opticalband gap. Experimental absorption data havebeen processed using an adequate formula ob-tained by superimposing a linear function and apower function (linear-power model):

a ¼ a þ bðEg � cÞd þ mE (3)

or

a ¼ a þ bðc � lgÞdþ ml; (4)

where a is the absorption coefficient, c is the bandgap energy Eg in Eq. (3) respectively the edge ofthe fundamental absorption lg in (4), d is acoefficient depending on the light absorptionmechanism and the line ða þ mEÞ; respectively,ða þ mlÞ is associated with all the other parasiticalprocesses affecting the band to band absorption,as scattering of the light on the nonhomogeneitiesof the bulk sample, having a slow variation withthe energy (the base line).We have evaluated the optical band gap energy

as a parameter characterising the organic solid-state light absorption process and determined the

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shift of the edge of the fundamental absorption.For benzil we have obtained a wide optical bandgap energy Eg ¼ 2:65 eV indicating a semiconduc-tor behaviour of this organic crystalline compound(Fig. 4). Pure, high-quality benzil crystallinematrix with low concentration of defects do nothave gap states and a narrowing of the opticalband gap of pure benzil crystalline sample couldbe associated with some physical defects. In pure,defects free crystalline materials, such gap energylevel are introduced by a controlled doping of thehost material, being a way to modify the band gapenergy. Band gap states can be also introducedduring the process of growing doped crystals, asstructural defects, or by chemical reaction with, orcharge transfer from, metal atoms [25].We have also investigated the effect of metallic

dopant on the band gap energy comparing theresults obtained on doped with those obtained onundoped benzil crystals. From geometrical con-sideration it is very difficult to obtain good benzilcrystal doped with Na, because the large radiusmetal atoms disturb the organic crystal lattice andthe effect of impurities is hidden by structuralimperfections. Part of the imperfections is presentas macroscopic or microscopic fissures, crystalliteinclusions, grain boundaries and dislocations.Using function (3), we have identified no signifi-cant shift of the energetic threshold of thefundamental absorption by doping (Fig. 4): from

1.0

1.2

1.4

1.6

1.8

Abs

orpt

ion

α (a

.u.)

2.6 2.7 2.8

4

6

8

10

α-α o

(a.

u.)

3

2

1

Photon Energy (eV)

Fitting of experimental data

1: Benzil + 6 wt% Na2: Benzil pure3: Benzil + 1 wt% Na

2.5

Fig. 4. Experimental data fitting for bulk samples of pure and

Na-doped benzil using function (3); (ao is the base line).

Eg ¼ 2:67 eV for low ðc ¼ 1wt%Þ to Eg ¼ 2:61 eVfor high ðc ¼ 6wt%Þ concentration of sodium. Inbenzil doped with Na, these physical defects areintroducing levels in the optical band gap of thehost, which can trap the 3s electrons of sodiumguest atoms. Responsible for the generation ofnew electronic states in the organic semiconductorband gap could be also a chemical reaction takingplace at high concentration of alkali metal atomsor a charge transfer from the metal atom (Na) tothe lowest unoccupied molecular orbital of benzil[22,26]. As we have discussed above, at highconcentration of Na, the charge transfer betweenNa and benzil produces the gap states. In Na-doped crystals with physical defects, impuritiesmigrate and concentrate at these defects (grainsboundaries, dislocations). In these conditions, theoptical transition from the valence to the conduc-tion band is weakly affected and we can remark asmall variation in the optical band gap of theorganic crystalline host.For benzil doped with Ag we cannot obtain a

good fitting of the experimental data usingfunction (3). A small shifting of the fundamentalabsorption edge through longer wavelength and anapparently narrowing of the band gap energy,Eg ¼ 2:52 eV; compared to pure benzil, Eg ¼

2:56 eV; has been obtained with the fitting function(4) (Fig. 5), suggesting the existence of someenergetic levels in the band gap. Some differences

5

1.0

1.5

Abs

orpt

ion

(a.u

.)

Wavelength (nm)460 480 500

2.5

3.0

3.5

Fitting of experimental data

1 Benzil pure2 Ag-doped Benzil

2

1

Fig. 5. Experimental data fitting for bulk sample of benzil

doped with Ag (2wt%) using function (4).

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must be emphasised compared to Na, for benzilcrystal matrix doped with Ag as a consequence ofthe low reactivity of the non-alkali metal atoms.So, in an optical transition from the valence to theconduction band are involved energetic unoccu-pied levels in the band gap due to metallic Agrather than to the products of a chemical reaction[27,28] suggesting an apparently narrowing of theband gap. The high first ionisation energy for Agatoms is favourable to clusters generation [20] andnot to incorporation of isolated atoms. Ag clustersdispersed in semiconductor transparent benzilcrystalline matrix can reduce the optical bandgap energy and generate a small red shift of thefundamental absorption edge.

4. Conclusions

We have grown pure and metal doped with Na(c ¼ 1 and 6wt%), and silver ðc ¼ 2wt%Þ benzilcrystalline matrices and we have analysed theinfluence of the guest, different types of metallicdopants, on the growth process (temperaturegradient at the liquid–solid interface: 10–25 1Cand moving speed of the growth interface:1.7–2.0mm/h) and on the optical properties ofthe guest/host system.Experimental absorption data have been pro-

cessed using a formula obtained superimposing alinear and a power function. We have evaluatedthe optical band gap energy and determined theshift of the fundamental absorption edge asparameters characterising the light absorptionprocess in organic solid state and the propertiesof the organic matrix based hybrid material.We also have investigated the effect of the guest

on the wide band gap semiconductor behaviour ofbenzil crystalline matrix ðEg ¼ 2:65 eVÞ; compar-ing the results obtained on metal doped benzilcrystals with those obtained on undoped benzilcrystals.For pure and doped with Ag ðc ¼ 2wt%Þ benzil

samples the shape of the fundamental absorptionedge is conserved and it is a characteristic of thesolids with relatively narrow energetic bands andweak intermolecular van der Waals bonds. Thedoping with sodium (c ¼ 1 and 6wt%) induced

some changes in the UV–VIS spectrum of the bulkpure benzil sample by replacing the narrow peaksspectrum with a large, structured spectral band.The two peaks of the absorption edge are hiddenby the scattering of light on the nonhomogeneitiesof the bulk sodium-doped benzil sample.An apparent narrowing of the band gap of the

host material has been obtained for benzil dopedwith 2wt% silver indicating energetic levels inband gap, associated with defects attributed tosilver clusters with low reactivity.For high reactivity alkali metal atoms (sodium)

the absorption properties can be slightly changedmodifying the metal content from c ¼ 1 to 6wt%.The changes in the optical behaviour we haveremarked for high concentration of sodium aresuggesting that the alkali metal atoms havegenerated a significant density of band gap activestates to be involved in optical absorptionmechanism.The optical properties of the guest/host system

(metal/organic molecular crystal) could be variedby a proper choice of the nature (sodium, silver)and concentration of the guest. Doping organicsemiconductor matrices with alkali or non-alkalimetallic guest could be a method to introduce newenergetic states in the gap, modify the band gapenergy and so, the optical and electrical propertiesof molecular organic solids.

Acknowledgements

This investigation has been supported finan-cially by The Ministry of Education and Researchthrough National Plan for Research, Developmentand Innovation-CERES Programme, Contract no.15/2001.

References

[1] W.S. Wang, M.D. Aggarwal, J. Crystal Growth 128 (1993)

891.

[2] M.A. Noginov, M. Curley, N. Noginova, W.S. Wang,

M.D. Aggarwal, Appl. Opt. 37 (1998) 5737.

[3] G.F. Reynolds, in: D. Fox, M.M. Labes, A. Weissberger

(Eds.), Physics and Chemistry of the Organic Solid State,

vol. 1, Interscience, New York, 1963, p. 267.

ARTICLE IN PRESS

A. Stanculescu et al. / Journal of Crystal Growth 275 (2005) e1779–e1786e1786

[4] A. Perigaud, Bull. Soc. Sci. Bretagne 48 (Hors Serie) (1973)

57.

[5] N. Karl, High purity organic molecular crystals, in: H.C.

Freyhardt (Ed.), Crystals: Growth, Properties and Appli-

cations, vol. 4, Springer, Berlin, 1980, p. 1.

[6] B. Iliescu, I. Enculescu, F. Vasiliu, M. Secu, J. Crystal

Growth 198–199 (1999) 507.

[7] D.R. Moore, V.J. Tekippe, A.K. Ramdas, J.C. Toledano,

Phys. Rev. B 27 (1983) 7676.

[8] R. Claus, H.H. Hacker, H.W. Schrotter, J. Brandmuller,

Phys. Rev. 187 (1969) 1128.

[9] A. Stanculescu, Al. Popina, SPIE Proc. 2700 (1996) 93.

[10] A. Stanculescu, F. Stanculescu, H. Alexandru, J. Crystal

Growth 198/199 (1999) 572.

[11] A. Stanculescu, F. Stanculescu, SPIE Proc. 4068 (2000) 97.

[12] E. Schudt, G. Weitz, in: Landolt-Bornstein: Numerical

data and functional relationships, in: K.-H. Hellwege

(Ed.), Science and Technology, New Series, Group III:

Crystal and Solid State Physics, vol. 5, Structure Data of

Organic Crystals, Part b, K.-H. Hellwege, A.M. Hellwege

(Eds.), Springer, Berlin, Heidelberg, New York, 1971,

p. 755.

[13] C.W. Lang, C.R. Song, J. Crystal Growth 180 (1997) 127.

[14] D. Aggarwal, W.S. Wang, M. Tambwe, J. Crystal Growth

128 (1993) 891.

[15] H. Lee, A.J. Pearlstein, J. Crystal growth 218 (2000)

334.

[16] A.I. Kitaigorodski, Molecular Crystals and Molecules,

Academic Press, New York, 1973.

[17] A.I. Kitaigorodski, J. Chem. Phys. 63 (1966) 9.

[18] P.J. Halfpenny, R.I. Ristic, E.E.A. Shepherd, J.N. Sher-

wood, J. Crystal Growth 128 (2) (1993) 970.

[19] C.J. Brown, R. Sadanaga, Acta Cryst. 18 (1965) 158.

[20] Y. Hirose, A. Kahn, Appl. Phys. Lett. 68 (1996) 217.

[21] Y. Okamoto, W. Brenner, Organic Semiconductor, Rein-

hold Publishing Corporation, New York, 1964, p. 63.

[22] G. Parthasarathy, C. Shen, A. Kahn, S.R. Forrest, J. Appl.

Phys. 89 (2001) 4986.

[23] Chogfei Shen, A. Kahn, Org. Electron. 2 (2000) 89.

[24] N.J. Leonard, E.R. Blout, J. Am. Chem. Soc. 72 (1950)

484.

[25] N. Koch, C. Chan, A. Kahn, Phys. Rev. B 67 (2003)

195330.

[26] C. Shen, A. Kahn, Org. Electron. 2 (2001) 89.

[27] Y. Hirose, A. Kahn, V. Aristov, P. Soukiassian, V.

Bulovic, S.R. Forrest, Phys. Rev. 54 (1996) 13748.

[28] I.C. Hill, A. Rajagopal, A. Kahn, J. Appl. Phys. 84 (1998)

3236.

[29] A. Stanculescu, S. Antohe, H.V. Alexandru, L. Tugulea,

F. Stanculescu, M. Socol, Synthetic Met. 147 (2004) 215.