Organic Electronics 14 (2013) 554–559

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Organic Electronics

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Linearly polarized emission from PTCDI-C8 one-dimensionalmicrostructures

1566-1199/$ - see front matter � 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2012.12.012

⇑ Corresponding author. Fax: +91 495 2287250.E-mail address: sivaji@nitc.ac.in (V.S. Reddy).

M. Yoosuf Ameen a, T. Abhijith a, Susmita De b, S.K. Ray c, V.S. Reddy a,⇑a Organic and Nano Electronics Laboratory, Department of Physics, National Institute of Technology, Calicut 673 601, Kerala, Indiab Department of Chemistry, National Institute of Technology, Calicut 673 601, Kerala, Indiac Department of Physics and Meteorology, IIT Kharagpur, Kharagpur 721 302, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 October 2012Accepted 11 December 2012Available online 25 December 2012

Keywords:Organic semiconductorOne-dimensional microstructureLinearly polarized emissionStructural properties

Linearly polarized emission has been observed from a crystalline one-dimensional (1D)microstructure fabricated from N,N0-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8)molecules via solution phase self-assembly. Rotating microscopy imaging of a 1D micro-structure under crossed polarization was performed for the investigation of polarizedemission. The anisotropy birefringence was maximum only when the 1D microstructurewas aligned 45� to the direction of the polarizer and it was minimum when aligned parallelto the polarizer implying that the transmission axis of the 1D microstructure is perpendic-ular to its p–p stacking direction. A model has been proposed to explain linearly polarizedemission from the microstructure.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Organic semiconductor micro/nanostructures haveattracted great research interest as interconnects and func-tional units for the development of miniaturized optoelec-tronic devices. Their unique optoelectronic properties,attractive mechanical and chemical properties, good pro-cessability, and high photoluminescence (PL) efficiencymake them complementary to their inorganic counterparts[1,2]. Among these organic micro/nanostructures, one-dimensional (1D) structures have been gaining specialattention, because of their crystalline nature, better stabil-ities and charge transport properties, which make thempromising candidates as building blocks for devices suchas field effect transistors [3], color tunable displays [4],chemical sensors [5], optical waveguides [6], light emittingdiodes [7], solar cells [8] and lasers [9].

The 1D structure of organic semiconductors usuallyexhibits enhanced optical properties beyond those of bulkand thin film materials. Unlike the inorganic semiconduc-

tor 1D structure, these enhancements in optical propertiesare not originating from quantum confinement size effect[10], but are due to the nature, geometry and dynamicsof excitons in the 1D structure [10,11]. Absorption, photo-luminescence (PL) and other photo-physical properties arecontrolled by exciton delocalization within the self-assem-bly and geometrical arrangement of the building blocks inthe 1D organic semiconductor structures [12].

p–p stacking is an effective way to assemble 1D struc-ture from rigid, disc-shaped aromatic molecules [8,13]. Re-cently, some works have given clear evidence for theimproved electron transport pathways of 1D structurealong its long axis [8,10]. But a big hurdle for the research-ers in this area is that the fabrication of well-definedorganic semiconductor 1D structures with controllable sizeand morphology is not as advanced as for their inorganiccounterparts. Solution based and surface supported self-assembly techniques are convenient methods to synthe-size 1D organic semiconductor structures. Perylenederivatives have long been used extensively in organicphotovoltaics and transistor devices due to their high elec-tron accepting and transporting ability [8,14,15]. Such anefficient conductivity is due to p-electron delocalization

M. Yoosuf Ameen et al. / Organic Electronics 14 (2013) 554–559 555

between the self-assembled molecules [16,17]. The strongp–p interaction between the planar aromatic cores andtheir non collapsible structure make perylene derivativesideal candidates for growth of 1D micro/nanostructures[18].

The 1D structure formed by self-assembly of planar aro-matic molecules of perylene derivatives usually exhibitsuniaxial optical properties along the p–p stacking direction[19]. Structural properties of 1D structures of perylenederivatives have also been reported [20,21]. However, nomodel has been proposed to correlate structural propertiesof 1D structures with their uniaxial optical properties. Theuniaxial optical property along with the uniaxial conduc-tivity may set a strong foundation for a new class of opticalsensors or switches in response to polarized light.

2. Experimental

1D microstructures and nanostructures were fabricatedfrom commercially available PTCDI-C8 powder (Sigma Al-drich, 98%) by solution phase self-assembly. First, an opa-que yellow colored solution of PTCDI-C8 was prepared bystirring it in ‘good solvent’ chloroform (1 mg/ml) for 6 hat room temperature. Then equal amount of ‘poor solvent’methanol was added to the above solution, to induce theself-assembly by p–p interaction of PTCDI-C8 molecules.Immediately after mixing the methanol, complete forma-tion of 1D microstructures were observed within few min-utes. Nanostructures were fabricated by following thesame procedure except that the opaque yellow coloredsolution of PTCDI-C8 in chloroform was heat treated at50 �C for 1 h before adding methanol. The microstructuresand nanostructures thus formed are transferred and castonto a glass surface by pipetting for structural and opticalcharacterization. Variable pressure field emission scanningelectron microscope (FE-SEM) (HITACHI, SU6600) and highresolution transmission electron microscope (HR-TEM)(JEOL JEM 2100) were used to determine the shape, sizeand morphology of the obtained 1D structures. Philips X-Pert PRO MRD X-ray diffractometer with Cu Ka radiation(k = 1.5418 Å) was used to investigate their crystallinity.NIKON-Eclipse E400 POL microscope under cross polariza-tion state was used to investigate the uniaxial optical prop-erty. The geometry of a single PTCDI-C8 molecule wasoptimized at HF/6-31G� level of theory using Gaussian-09program package.

3. Results and discussion

Fig. 1a shows the FE-SEM image of 1D structures syn-thesized from the opaque yellow colored solution ofPTCDI-C8 prepared at room temperature and Fig. 1b showsits magnified image. Both the images revealed well definedgrass like microstructures with 50–70 lm length and 1–3lm width. When the opaque yellow colored solution ofPTCDI-C8 in chloroform (1 mg/ml) was heat treated at50 �C for 1 h, it has totally changed to a yellow transparentsolution. It is mainly due to the increased solubility ofPTCDI-C8 molecules in chloroform, i.e. the solution becamesuper saturated after the heat treatment. Immediately

after the temperature treatment equal amount of metha-nol was added to induce self-assembly resulting in the for-mation of nanostructures. Fig. 1c shows the FE-SEM imageof a bundle of nanoribbons and Fig. 1d shows the HR-TEMimage of a single nanoribbon. Both the FE-SEM and HR-TEM studies revealed that the nanoribbons have severalmicrometers length and 100–150 nm width.

Fig. 2 shows the X-ray diffraction (XRD) patterns of thePTCDI-C8 microstructures and nanostructures deposited onglass substrate and the inset shows the zoom-in pattern ofthe microstructures between 2h = 4� and 25�. From the fig-ure it is clear that the microstructure exhibits multiple dis-tinct diffraction peaks at 2h values 4.29�, 10.97�, 13.07�,19.71� and 23.65� corresponding to d-spacings 20.60 Å,8.07 Å, 6.77 Å, 4.50 Å and 3.76 Å, respectively. The wholespectrum is dominated by a sharp peak at 2h = 4.29�, whichis a typical characteristic of a columnar stacked phase. Thed-spacing of 20.60 Å deduced from this peak position cor-responds to the inter-columnar distance along the c-axis.The considerably smaller value of inter-columnar distanceas compared to the length of the extended PTCDI-C8 mole-cule (29.7 Å, obtained from theoretical calculations as dis-cussed in the following paragraph) may be due to tworeasons: interdigitation between alkyl chains of adjacentmolecular layers or a certain tilt of the PTCDI core. The dif-fraction peaks observed for the d-spacings 4.50 Å and8.07 Å correspond to the lattice parameters a (along thelength) and b (along the width), respectively. The d-spacingcorresponding to the cofacial intermolecular stackingalong the a-axis is also observed at 3.76 Å, which is a typ-ical intermolecular distance of perylene molecule along thep–p stacking direction.

Fig. 3a shows the optimized geometry of a single PTCDI-C8 molecule at HF/6-31G� level of theory using Gaussian-09 program package [22]. This molecule contains an aro-matic core and two alkyl side chains oriented at an angleof 38.3� with respect to the aromatic core in trans-configu-ration. The distance between the two terminal C-atoms inthe plane of the aromatic core is 14.3 Å and the distancefrom this C-atom to the farthest H-atom in the alkyl sidechain is 9.8 Å. These geometrical parameters are used forthe subsequent calculation of the length of the molecule,which was found to be 29.7 Å. The unit cell of PTCDI-C8

crystal drawn using experimentally obtained cell parame-ters and the optimized molecular geometry is shown inFig. 3b. Projections of c–a and c–b planes are shown inFig. 3c and d, respectively. Thus the PTCDI-C8 moleculesexhibit slipped p–p stacking along a-axis with the shortestvertical distance between the aromatic cores of neighbor-ing PTCDI-C8 molecules at 3.76 Å. As a result, the overlapof the molecular p orbitals, which is crucial for the chargetransport, is expected to be larger along the p–p stackingdirection i.e., along the a-axis of the microstructure. Theseresults are in good agreement with the results reported byother researchers [20,23,24]. For millimeter long needles ofPTCDI-C8 Briseno et al. have reported the lattice parame-ters a = 4.68 Å, b = 8.50 Å, c = 19.72 Å and the shortest dis-tance between the aromatic planes of neighboring PTCDI-C8 molecules as 3.24 Å. The nanostructures also exhibitedmultiple distinct diffraction peaks almost at the same posi-

Fig. 1. (a) FE-SEM image of PTCDI-C8 microstructures. (b) Magnified FE-SEM image of microstructures. (c) FE-SEM image of PTCDI-C8 nanostructures. (d)HR-TEM image of a single nanoribbon.

Fig. 2. XRD pattern of PTCDI-C8 1D microstructures and nanostructures.Inset: the zoom-in pattern for the microstructures between 2h = 4� and25�.

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tions. This indicates that the molecular packing of PTCDI-C8 nanostructure is similar to that of the microstructure.

The XRD results of PTCDI-C8 microstructures clearlyindicate the strong crystalline nature due to the p–p stack-ing of the molecules along its long axis. Crystalline 1D self-assembly of PTCDI-like molecules usually exhibits uniaxialoptical property along the p–p stacking direction which issimilar to the uniaxial columnar packing of discotic liquidcrystal molecules. Fig. 4a shows the cross-polarizedmicroscopy image of the PTCDI-C8 microstructures cast

on to a glass substrate, whereas Fig. 4b shows the imageof the same area after 45� rotation of the sample. In boththe images some microstructures are bright and some oth-ers are dark. This observation indicates that transition di-pole moments of different microstructures are orientedin different directions as the microstructures are randomlydistributed on the surface. The images shown in Fig. 4a andb are complementary to each other in terms of emissionintensity of individual microstructures i.e., microstructureswhich show diminished emission in Fig. 4a are bright inFig. 4b and vice versa.

Fig. 5 shows successive rotating microscopy images of asingle PTCDI-C8 microstructure, which has been placed be-tween a pair of crossed polarizers. When the microstruc-ture was nearly parallel to the transmission axis of eitherpolarizers (microstructure oriented at h = 0� or 90� with re-spect to the polarizers), no light can propagate through themicrostructure, and it appears dark. Whereas, maximumtransmission of light was observed when the microstruc-ture was aligned nearly 45� to the transmission axis ofthe polarizers. These results indicate that the transmissionaxis of PTCDI-C8 1D microstructure is perpendicular to itsp–p stacking direction. The linearly polarized emissionfrom PTCDI-C8 1D structures can be explained with thehelp of working principle of wire-grid polarizer. Thewire-grid polarizer is a device containing parallel conduct-ing wires, which facilitate the 1D movement of electronsalong the long axis of the wires. Fig. 6a shows the sche-matic diagram of wire-grid polarizer, the input electricfield vibrations can be resolved into two orthogonal com-ponents, one chosen parallel to the wires (y-component)and the other perpendicular to the wires (x-component).

Fig. 3. (a) Optimized geometry of a single PTCDI-C8 molecule at HF/6-31G� level. (b) Unit cell of PTCDI-C8 crystal drawn using experimentallyobtained cell parameters. (c) Projection of c–a plane. (d) Projection of c–bplane.

Fig. 5. A single microstructure under cross-polarized microscope: suc-cessive rotation of the sample showed alternate appearance of maximumbirefringence as the microstructure was aligned 45� to either of thepolarizers. The crossed polarizers are indicated as arrows.

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The y-component of light drives the electron clouds alongthe long axis of wires, so this energy is completely utilizedfor accelerating the electrons. The strong electric fieldabsorption by the parallel wires is resulting in the negligi-ble amount of y-component transmission. But the x-com-

Fig. 4. Cross-polarized microscopy images of PTCDI-C8 microstructures of the srotation of the sample.

ponent of the field is unaltered and it will propagatethrough the grid. Therefore the transmission axis of thewire-grid polarizer is perpendicular to the wires (x-direc-tion) [25]. Similarly, the PTCDI-C8 1D structure containsmolecular arrays oriented parallel to its length, which areanalogous to parallel wires of the wire-grid polarizer asshown in Fig. 6b. From the XRD results it is clear that theoverlapping of the p-orbitals is larger along the p–p stack-ing direction. Therefore, the electric field vibrations alongthe p–p stacking direction will be completely utilized foraccelerating the electrons. Hence, only those vibrationsperpendicular to the p–p stacking direction of 1D structurewill effectively propagate without any considerable de-crease in intensity as shown in Fig. 6c. Thus the parallelarrangement of the PTCDI-C8 molecules within 1D micro-structure is exactly similar to the wire-grid polarizer.

Similar to other PTCDI-like 1D structure, microstructureof PTCDI-C8 molecules shows uniaxial optical propertyalong the p–p stacking direction leading to linearly polar-ized emission. The linearly polarized emission from themicrostructures mainly depends on the molecular packingwithin the 1D microstructure. The above observations indi-

ame area of the sample, (a) taken before rotation and (b) taken after 45�

Fig. 6. Schematic diagram showing: (a) linearly polarized emission froma wire-grid polarizer, (b) p–p stacking of PTCDI-C8 molecules in a 1Dmicrostructure and (c) linearly polarized emission from PTCDI-C8 1Dmicrostructure. Green arrow indicates the transition dipole momentdirection. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

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cate that the transition dipole moment to the lowest exci-tonic state of the molecular self-assembly is orientednearly along the long axis of the microstructure, thoughthe transition dipole of an individual molecule is com-pletely in plane. Since the molecular arrangement withinthe 1D nanostructure is similar to that of the microstruc-ture, we expect that the nanostructure will also show uni-axial optical property along the p–p stacking direction.

4. Summary

In summary, we have fabricated 1D microstructuresand nanostructures of PTCDI-C8 molecules by solutionphase self-assembly. The X-ray diffraction studies revealedthat the PTCDI-C8 molecules exhibit slipped p–p stackingin the [100] direction (a-axis) which results in larger over-lap of the molecular p orbitals (and hence better chargetransport) along the same direction. This molecular pack-ing within PTCDI-C8 1D microstructure results in stronganisotropy of the refractive index and leads to birefrin-gence and linearly polarized emission as revealed bycross-polarized microscopy images. Thus, the linear polar-izability, high crystallinity (assured from XRD results) andextremely high thermal stability and photostability ofPTCDI-C8 microstructures make them potentially ideal forthe next generation optical sensors or switches in response

to polarized light and for many orientation-sensitive appli-cations, such as polarized light emitting diodes and flat pa-nel displays.

Acknowledgements

Authors acknowledge Dr. A. Dhar (Dept. of Physics andMeteorology, IIT Kharagpur) for providing the XRD pat-terns of microstructures. Authors also acknowledge NITCalicut for partially supporting this work under FRG grant.

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