in recent years a lot of effort has been 
 undertaken in order to 
 prepare and investigate ordered structures of -conjugated organic 
 molecules on nanoscopic and mesoscopic scales .
the interest of both 
 fundamental and applied science was mainly driven by the goal to understand 
 and improve the properties of materials used for organic electronics 
 ( e.g. , light emitting diodes , field effect transistors and solar cells ) and to establish novel systems 
 for optical applications ( e.g. , waveguiding and lasing ) . frequently small rodlike molecules like oligothiophenes ,
oligoacenes 
 and oligophenylenes have been selected as model systems . in the group 
 of oligothiophenes sexithiophene ( 6 t )
has shown interesting high quality 
 properties like a charge carrier mobility of up to 0.1 cm/(v s ) .
consequently there has been an interest 
 in understanding the growth mechanism of such molecules on well - defined 
 model substrates like single crystalline cu , sheet silicates or patterned sio2 .
the study in this paper 
 aims at providing a conclusive picture 
 of the formation of 6 t nanostructures by an in - depth analysis of the 
 growth on an alkali - halide single crystal , namely potassium chloride 
 ( kcl ) .
kcl exhibits a face centered 
 cubic crystal structure with a lattice constant of 6.36  .
the unit cell of the ( 100 ) surface can be described 
 by the 2d - space group p4 mm as shown 
 in figure 1 .
importantly , this high symmetry 
 surface exhibits rotation centers of order 2 and 4 as well as mirror 
 and gliding planes .
it is expected that the symmetry of the substrate 
 will be reflected in the multiplicity of epitaxial alignments of the 
 deposited molecules .
( a ) the ( 100 ) surface 
 of the kcl crystal , with the white rectangle 
 illustrating the surface unit cell .
( b ) the p4 mm symmetry of the surface unit cell including 
 the respective symmetry elements .
the hot wall epitaxy ( hwe ) technique 
 was applied for the deposition of the organic material , which allows 
 the growth process to be performed close to thermodynamic equilibrium , 
 and in further consequence relatively high vapor pressure of the organic 
 deposit in the substrate region can be achieved .
therefore the requirements 
 concerning vacuum conditions are reduced as compared to , e.g. , molecular 
 beam epitaxy .
the source material 6 t 
 was purified twice by thermal sublimation before filling it into the 
 quartz tube of the hwe reactor .
kcl substrates were transferred into 
 the deposition chamber via a load lock and subsequently preheated 
 at the deposition temperature ( 60135 c ) for 30 min to 
 clean the surface from adsorbed species and to ensure a stable temperature 
 during the whole deposition process .
the deposition was performed 
 thereafter under a base pressure of 9  10 mbar at a nominal deposition rate of 0.4 nm / min .
x - ray diffraction ( xrd ) measurements 
 were carried out on a philips xpert x - ray diffractometer using 
 cr k radiation (  = 2.29  ) and a secondary graphite 
 monochromator .
brentano 
 configuration by varying the z - component of the scattering 
 vector q. consequently it is possible to detect lattice 
 planes which are parallel to the sample surface .
pole figures are acquired by measuring at a 
 constant length of q and only varying its direction . 

based on both 
 the observed bragg peaks of the specular scan and the direction of 
 the poles ( net - plane normals ) within the pole figures , the involved 
 crystallographic phases as well as the azimuthal alignment of the 
 crystallites with respect to the substrate can be identified .
optical microscope images 
 have been acquired by a nikon labophot 2a microscope in combination 
 with a nikon type 115 digital camera .
scanning force microscopy 
 ( sfm ) studies of the deposited organic films were performed using 
 a digital instruments dimension 3100 in the tapping mode .
the images 
 have been acquired at scan speeds of 46 m / s using sic 
 tips exhibiting a cone angle of 40. nominal values for resonance 
 frequency and tip radius are 325 khz and 10 nm respectively .
in this paper we reported 
 on epitaxial growth of 6 t on kcl ( 100 ) , 
 which was examined by crystallographic and morphological investigations . by means of sfm analysis different morphologies
furthermore , it was 
 found that the area covered by islands is declining with increasing 
 substrate temperature , whereas the sample area covered by needles 
 is constant over the investigated temperature range ( 60135 
 c ) .
consequently we can draw the conclusion that needle - like 
 structures are energetically more stable as compared to flat islands 
 formed of standing 6 t molecules on kcl ( 100 ) .
additionally , different 
 activation energies for the growth of nanoneedles in height and width 
 have been deduced from sfm analysis , namely , 0.26  0.02 ev and 
 0.15  0.04 ev , respectively . by means of xrd specular 

scans it was possible to determine the 
 contact planes of the structures on the surface .
needles with { 010 } 
 and { 411 } and islands with the { 100 } facet parallel to the 
 substrate surface have been detected . utilizing xrd pole figures and 
 fft analysis of optical microscope images we observed that there is 
 a well - defined azimuthal alignment of the structures on the surface 
 in addition to the stacking in growth direction . notably structures 
 with { 100 } and
{ 411 } contact planes are in perfect azimuthal 
 alignment to each other , which is explained by ledge directed epitaxy 
 of the islands on the sidewalls of already existing needles .
the orientation of the crystallographic structures which are present 
 on the substrate surface fulfills the growth model for rodlike molecules 
 on sheet silicate substrates developed by simbrunner et al .
therein it is explained that the adsorption 
 of a single molecule dominates in a first step and the nucleation 
 of organic crystallites which follows thereafter only leads to a slight 
 reorientation of the lmas .
in particular rotational and mirror symmetries 
 have to be reflected by the formed needle orientations .
the latter 
 considerations are substantiated by the results presented in this 
 paper . the 4-fold rotational symmetry and the mirror symmetry of the 
 kcl ( 100 ) substrate surface leads to eight distinct growth directions 
 of each crystal type .
moreover two different crystal contact planes , 
 namely , { 010 } and { 411 } , are characterized by an analogous 
 lma orientation as similarly observed for p - hexaphenyl 
 on kcl ( 100 ) .
this hints that the adsorption 
 geometry of single molecules seems to be decisive for the organic 
 crystal nucleation as expected by the growth model .
the fact that 
 one adsorption geometry can lead to different crystal contact planes 
 may be explained by polymorphs of the first monolayer , that are both 
 based on the same azimuthal orientation of the lma on the substrate 
 surface .
the argument is supported by the fact that monolayer polymorphs 
 of 6 t have already been reported on ag ( 001 ) and ag ( 110 ) . concluding the 
 latter discussion it can be stated that the azimuthal 
 alignment of the organic crystals and molecules is basically determined 
 by a two step process .
the initial phase is dominated by the adsorption 
 of single molecules in their energetically preferred adsorption geometry 
 on the substrate surface .
this adsorption process is mainly determined 
 by an interplay between substrate geometry , molecular geometry and 
 importantly symmetry properties . in the second step , where the organic 
 crystal nucleation takes place ,
only a slight azimuthal realignment 
 of the molecules can be caused in order to optimize the lattice match 
 at the interface .
 the morphology and structure of sexithiophene deposited 
 on kcl 
 ( 100 ) substrates was investigated by scanning force microscopy and 
 specular x - ray diffraction measurements . 
 two different needle - like 
 structures with { 010 } and { 411 } contact planes have been observed 
 as well as islands of almost upright standing sexithiophene molecules 
 with a { 100 } contact plane . 
 furthermore an azimuthal alignment of 
 all three crystal orientations was observed by x - ray diffraction pole 
 figure measurements , and the growth directions reflect the 4-fold 
 rotational symmetry of the substrate surface . 
 in addition the analysis 
 of crystals with { 411 } and { 100 } contact planes unveiled that 
 they share a common crystallographic direction which is explained 
 by ledge directed epitaxy . 
Introduction
Experimental Procedures
Conclusion and Discussion