when deposited on various 
 substrate surfaces , rod - like , -conjugated , 
 small organic molecules are well - known for their tendency to form 
 highly anisotropic crystal shapes , which are frequently called fibers 
 or needles . whereas the epitaxial growth has been studied on various substrates , 
 in particular anisotropic surfaces seem favorable to conserve the 
 highly anistropic morphology and optical properties , for example , 
 polarized emission or adsorption provided by a parallel molecular 
 orientation obtained by self - assembly . consequently , 
 cu(110 ) , tio2(110 ) and muscovite mica(001 ) are frequently chosen as a proper fundament to study the epitaxial 
 growth of rod - like small molecules . in this paper , the epitaxial 
 growth of 2,2:6,2-ternaphthalene 
 ( nnn ) on muscovite mica(001 ) is reported . as indicated in figure 1a ,
the molecule is built from three naphthalene 
 units , which are linked together by c 
the coexistence of needle - like structures 
 and island - like crystallites is verified .
structural analysis reveals 
 two different crystal orientations . whereas island - like structures 
 are built up by upright standing molecules orientated with a ( 001 ) 
 contact plane relative to the muscovite mica substrate ( see figure 1b ) ,
needles consist of nnn molecules with a ( 111 ) 
 lying orientation ( see figure 1c ) .
both crystal 
 configurations provide a well - defined azimuthal alignment , which is 
 discussed based on force field simulations and a recently reported 
 growth model .
the azimuthal alignment 
 of island like structures is explained by  ledge directed  

( b ) a side view of nnn molecules packed in the observed crystal 
 structure . each unit cell houses two nnn molecules .
molecules are 
 approximately standing on the s ( 001 ) contact plane , which is indicated 
 in blue .
( c ) a side view along the long molecular axis visualizing 
 the edge - on / flat - on herringbone stacking of nnn .
the blue area represents 
 the orientation of the b ( 111 ) contact plane where molecules are aligned 
 in almost in lying configuration .
this all - aromatic compound could be obtained in high yield by coupling 
 2 equiv of 2-naphthaleneboronic acid ( 1 ) with 1 equiv of 2,6-dibromonaphthalene 
 ( 2 ) , as described in the supporting information .
the final product , 2,2:6,2-ternaphthalene 
 ( nnn ) , was obtained as a colorless product , which appears to be highly 
 insoluble in common solvents and could only be recrystallized from 
 1,2,4-trichlorobenzene ( colorless platelets ) .
the material was checked 
 with gas chromatography and mass spectroscopy and found to be > 99% 
 pure before thermal sublimation .
all samples have 
 been fabricated 
 on muscovite mica(001 ) substrates ( spi , structure probe , inc . ) .
muscovite 
 mica is a representative of sheet silicate minerals and provides a 
 layered structure of aluminum silicate sheets weakly bound by layers 
 of potassium ions .
each layer is characterized by a high symmetry 
 direction identified by parallel aligned surface grooves . between 
 the individual sheets , the high symmetry direction alternates by 120 
 leading to a periodic  stacking sequence 
 along   direction . immediately after 
 cleaving , the mica substrates were transferred to the hot wall epitaxy 
 ( hwe ) chamber .
the hwe technique was applied for the deposition 
 of the organic material , which allows us to perform the growth process 
 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 compared with , for example , molecular beam epitaxy .
the source material nnn was purified twice by 
 thermal sublimation before filling it into the quartz tube of the 
 hwe reactor .
muscovite mica substrates were transferred into the deposition 
 chamber via a load lock and subsequently preheated at the deposition 
 temperature ( 80 c ) for 30 min to ensure a stable temperature 
 during the whole deposition process .
the deposition was performed 
 thereafter under a base pressure of 9  10 mbar .
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 10  10 m images have been acquired at scan speeds of 46 m / s 
 using sic tips ( masch , hq : nsc15/al bs ) exhibiting a cone angle 
 of 40. nominal values for resonance frequency and tip radius 
 are 325 khz and 10 nm , respectively .
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
. please note that the monochromator is transparent 
 for  , /2 , /3 , etc .
, so despite the weak intensity 
 of the bremsspektrum , it can give clear bragg peaks due to the scatting 
 on the single crystalline mica substrate .
specular scans were performed 
 in bragg  brentano configuration by varying the z - component of the scattering vector q. consequently 
 it is possible to detect lattice planes that are parallel to the sample 
 surface .
pole 
 figures were acquired by measuring at a constant length of q and only varying its direction .
the unit cell parameters of nnn , 
 which were used for analysis , are defined by a = 
 8.148  0.005  , b = 5.978  0.005 
  , c = 19.45  0.2  , and  
 = 94.6  0.2 describing a monoclinic lattice ( p21/a ) . the 
 unit houses two nnn molecules in planar configuration .
the van der waals ( vdw ) interaction 
 between the organic molecule and the dielectric substrate is modeled 
 by lennard - jones - type potentials .
corresponding parameters are taken 
 from the universal force field implemented 
 in a matlab program .
the molecules and substrates are assumed to be 
 rigid where the internal structure of an isolated nnn molecules is 
 determined from the crystal structure .
simulations were performed for the adsorption of a single nnn molecule 
 as well as a crystal stack . by assuming that a single nnn molecule 
 prefers to lie flat on the surface , the energy minimization procedure 
 is simplified in the following way : we consider only four molecular 
 degrees of freedom , the x- , y-
, 
 and z - positions of the molecular center of mass and 
 the angle . the angle  defines the azimuthal molecular 
 alignment and is probed by rotating the nnn molecule around the z - axis ( surface normal ) . we perform a grid - based optimization 
 to search for the best molecular adsorption geometry using a grid 
 of 81  81 points for the lateral position .
the adsorption angle was tested 
 with a step size of  = 1. the surface structure 
 of the substrate has been assumed to be the same as in the bulk where 
 the corrugation is about 0.2  . the substrate surface is assumed 
 to be terminated by the tetrahedral layer of muscovite mica .
for the 
 simulation of the 7  2 nnn stack , the molecular packing has 
 been deduced from the crystal structure of a ( 111 ) orientated nnn crystallite . due to the presence of flat 
 and edge - on molecules ,
the adsorption distance has been optimized , 
 yielding a distance of 1.6  of the lowest h atom of edge - on 
 nnn molecules to the substrate surface . because energetic minima are 
 significantly narrow for the molecular stack , an angular resolution 
 of  = 0.5 has been chosen for the calculations .
in a first 
 step , nnn was deposited by hot wall epitaxy ( hwe ) on muscovite mica(001 ) . 
 whereas the substrate temperature was kept constant at 80 c , 
 the deposition time was continuously increased .
scanning force microscopy 
 was chosen to study the sample morphology versus deposition time , 
 and obtained images ( 10  10 m ) are depicted 
 in figure 2 . as the maximum height scale z0 significantly changes with increasing deposition 
 time , the corresponding values are indicated for each sample .
the 
 morphology of all samples is dominated by the presence of several 
 micrometer long needle like structures that are aligned along multiple 
 orientations . as exemplified by the cross - section ( 1 ) ,
these fibers 
 reach height levels up to 200 nm and are characterized by 
 similar dimensions in width .
a more detailed analysis is provided 
 in figure 2a , plotting the mean needle height 
 versus growth time . as indicated by the solid line
, the growth rate 
 can be approximated by a linear fit , yielding a slope of 17.8 
  1.2 nmmin .
interestingly , the fibers 
 surface coverage is approximately constant for the whole sample series 
 yielding a value of 12.5% .
the sfm analysis reveals 
 that flat islands start to nucleate at the side walls of the fibers 
 and continuously fill the substrate surface between the fibers with 
 increasing deposition time .
the latter observation is underlined by 
 analyzing the islands surface coverage , which is depicted 
 in figure 2b as a function of growth time . 
 the solid line , which represents a guide for the eye , indicates an 
 asymptotic approach to 85% of the surface area .
again , a representative 
 sample position has been chosen to deduce a cross - section ( 2 ) , which 
 is presented in the bottom right of figure 2 . as reported for other rod - like molecules , a steplike morphology with height levels in the range of 1.82 
 nm
the obtained value 
 corresponds to approximately one monolayer of upright standing nnn 
 molecules .
contrarily , the step size of the boundary , defined by the 
 islands and the substrate surface , is significantly larger reaching 
 values in the range of 15 nm . these steps are further characterized 
 by straight extensions , which suggest the formation of well - defined 
 crystal facets . scanning force microscopy ( sfm ) images showing the sample 
 morphology 
 versus deposition time .
all samples are dominated by needle - like structures . 
 with increasing growth time , island - like structures start to nucleate 
 at the needle side walls covering continuously the substrate surface 
 between the fibers .
exemplary cross sections for both morphologies 
 are indicated in the bottom part of the figure .
as indicated in part 
 a , the height of the fiber like structures linearly increases with 
 growth time reaching values of some 100 nm .
contrarily , the surface 
 coverage by needles stays approximately constant ( part b ) .
step heights 
 in the center part of the islands approximately correspond to a monolayer 
 of upright standing molecules . in a next step ,
x - ray diffraction ( xrd ) has been chosen to 
 study 
 the structural properties of the organic crystallites . in order to 
 obtain sufficient diffraction intensity , a sample with 216 nm high 
 nnn fibers has been selected .
figure 3a reports 
 the acquired specular xrd diffraction pattern , which is dominated 
 by a series of { 00n } diffraction peaks .
these peaks 
 are characteristic for island - shaped crystal morphologies , built up 
 by approximately standing nnn molecules , and are consequently abbreviated 
 by s - orientations ( q001 = 0.324  ) .
arrows in the upper part of figure 3a indicate the positions of ( 00.2n ) diffraction 
 peaks stemming from the muscovite mica(001 ) substrate .
additionally , 
 a diffraction peak arises at qz = 1.36  , which correlates with ( 111 ) 
 orientated nnn crystallites , abbreviated as b orientation .
the orientation 
 is characteristic for a nearly flat lying molecular configuration 
 and thus explains the presence of needle - like crystallites as revealed 
 by sfm analysis .
a more detailed analysis is reported in the supporting information . in order to analyze 
 the azimuthal alignment of the nnn
crystallites 
 relative to the muscovite mica(001 ) substrate , xrd - pf measurements 
 have been performed and are reported in figure 3b . for a profound analysis , xrd - pf have been acquired with a maximum 
 sensitivity to diffraction intensities stemming from the scattering 
 at { 202 } and { 201 } netplanes and are depicted in the bottom leftand 
 right part of figure 3b .
the diffraction 
 intensities show a distinct azimuthal distribution , 
 which underlines a well - defined epitaxial relationship to the muscovite 
 mica substrate .
the obtained symmetry of the diffraction intensities 
 further underlines the presence of an  terminated muscovite 
 mica surface , which is characterized by a mirror plane along the [ 110]mica orientation .
the orientation of the substrate has been 
 determined from diffraction patterns stemming from muscovite mica 
 ( 001 ) and are indicated by black solid circles .
xrd - pf patterns of 
 the organic crystallites can be constructed by mirror operation from 
 the top hemisphere , sketched by a gray shaded sector . moreover , the 
 xrd - pf patterns reveal a 2-fold rotational symmetry , which can be 
 understood by an approximately equivalent adsorption energy for 180 
 turned organic crystallites .
consequently , discrimination between 
 both crystal alignments has been omitted and simulated 2-fold symmetric 
 diffraction spots are labeled identically .
based on these geometrical 
 considerations , only the diffraction spots of a single quadrant have 
 to be analyzed and labeled . ( a ) specular x - ray diffraction ( xrd ) spectrum 
 of nnn on muscovite 
 mica(001 ) .
scanning force microscopy images revealed a needle height 
 of 216 nm for the chosen sample .
the spectrum is dominated by a series 
 of s ( 001 ) diffractions peaks , which are representative for island - like 
 morphologies .
( b ) xrd pole figure ( xrd - pf ) 
 analysis of { 202 } and { 201 } diffraction peaks , providing information 
 about the azimuthal crystal orientation . as indicated by the simulated 
 pole distribution ( bottom ) ,
all diffraction spots can be explained 
 by the presence of three differently aligned ( 001 ) crystallites , labeled 
 as s13 ( red ) .
the mirror symmetry plane of the 
 muscovite mica ( 001 ) surface is indicated by a horizontal line , which 
 explains the presence of mirrored s13 * crystallites ( blue ) with a ( 001 ) 
 contact plane .
moreover , xrd - pf reveal a well - defined azimuthal orientation 
 of b1 ( 111 ) crystallites ( red circles ) .
again mirror symmetric 
 crystallites b1 * ( 111 ) are indicated by blue symbols .
diffraction 
 intensities from the muscovite mica substrate are indicated by black 
 solid filled circles .
diffraction intensities that are characteristic for s - orientated 
 crystallites are located at  = 63 ( 74 ) in the left 
 ( right ) xrd - pf , and their azimuthal distribution can be explained 
 by the presence of three crystal orientations labeled as s13 .
consistently , both xrd - pf measurements hint the strongest diffraction 
 intensities originating from s1 crystallites .
diffraction 
 spots , which can be attributed to b * ( 111)/b 
 ( 111 ) crystallites are expected to appear at   51 
 for both diffraction geometries and are marked by blue / red filled 
 circles . as each quadrant reveals the presence of one diffraction 
 spot , the azimuthal alignment of b orientated fibers
again , mirror symmetric crystals 
 are labeled as b1 * and are characterized by a ( 111 ) contact plane .
based on the simulated xrd - pf diffraction peaks , the long needle 
 axis ( lna ) and long molecular axis ( lma ) orientations of b orientated 
 fibers have been deduced and are presented in figure 4 by solid filled arrows . whereas the lna coincides with the 
 [ 110 ] orientation , the lma can be approximated by the alignment 
 of [ 101 ] relative to the muscovite mica substrate .
mirror symmetry 
 of the muscovite mica substrate leads to the generation of two energetically 
 equivalent crystallites .
fibers that 
 are built up by b1 * ( 111 ) orientated crystallites ( blue ) are aligned with their 
 lna ( lma ) 59.5 ( 49 ) relative to the muscovite 
 mica substrate s [ 110]mica crystallographic 
 orientation .
contrarily , their mirror symmetric twins ( b1 ) can be constructed by flipping the b1 * crystallites upside down ( red arrows ) , 
 azimuthally aligned with their lna ( lma ) 59.5 ( 49 ) 
 relative to [ 110]mica . in order to verify 
 the lna alignment , which has been constructed based on xrd - pf measurements
, 
 optical microscopy has been chosen , and an image of a representative 
 sample area is depicted in figure 4 .
the sample 
 morphology is dominated by fibers that are aligned in a v - shaped , 
 herringbone fashion . as indicated by large red and blue arrows , which 
 represent the expected b1 and b1 * lna orientations deduced by xrd - pf analysis
nevertheless , additional needle orientations , which are present in 
 minor fraction , can be observed and are marked by small red ( b2 ) and blue ( b1 * ) arrows , respectively . in order to gain better statistics , 
 a microscopy image of a larger sample area has been chosen to perform 
 a fast fourier transformation ( fft ) and the obtained pattern is depicted 
 beside .
the fft is dominated by two stripes , which are characterized 
 by an enclosing angle that perfectly reflects the lna orientation 
 of b1 and b*1 crystallites ( indicated by red 
 and blue arrows ) . long needle axis ( lna ) ( left ) and long molecular axis 
 ( lma ) ( right ) 
 orientations of b1 ( red solid filled arrows ) and mirror 
 symmetric b1 * ( blue solid filled arrows ) crystallites , deduced by x - ray pole figure 
 ( xrd - pf ) measurements . in the outer ring of the lna polar plot , additionally 
 the [ 110 ] crystallographic orientations of s13 ( red ) and
it is visualized that 
 the [ 110 ] crystallographic direction of s1 crystallites 
 provides the same azimuthal orientation as the lna of b1 fibers .
below , the obtained lna orientations are verified by the 
 observed sample morphology using optical microscopy . beside b1 and b1
* fibers additionally minor fractions of approximately horizontally 
 aligned crystallites are observed ( marked by small red and blue arrows ) . 
 by using fast fourier transformation ( fft )
( depicted beside ) , the 
 dominant fraction of b1 and b1 * crystal orientations is further underlined . in a next step
in particular , the azimuthal alignment 
 of their [ 110 ] directions has been deduced and is depicted 
 in the outer ring of figure 4 ( lna ) . because 
 s1 and s1 * crystallites represent the major fraction , they are indicated 
 by large arrows approximately aligned 60 relative to [ 110]mica .
the azimuthal orientation perfectly coincides with the 
 lna of b1 and b1 * crystallites , which already suggests an epitaxial 
 relationship of both crystal types . in order to analyze the latter 
 observation in more detail ,
the sfm image , which 
 is shown in figure 5 , is dominated by two approximately 
 vertically aligned fibers . between 
 fibers
, the presence of an s orientated island can be observed , which 
 is terminated in the bottom part of the image by a sharp l - shaped 
 boundary .
the observed boundary shape perfectly correlates with the 
 expected angle between the   and [ 110 ] crystallographic 
 orientations of an ( 001 ) orientated crystallite .
the generation 
 of the observed crystal shape can be understood by the formation of 
 (11n ) and (11n ) side facets , which represent low index planes for n = 1 , 0 , or 1 .
an extracted cross - section of the observed 
 fiber is presented in 
 the left part of figure 5 . the observed fiber 

is terminated in the right part by a flat plane , which is aligned 
 parallel to the substrate at a height of approximately 100 nm .
contrarily , 
 the left side of the fiber shows a constantly decreasing height level . 

the slope of the side facet approximately correlates with a 25 nm 
 decrease in height along 100 nm of the needle width ( 14 ) . 

the lna of b * type crystallites is defined by their [ 110 ] orientation 
 and consequently all crystallographic planes (11n ) are aligned parallel to it . because the angular tilt of 13.8 
 between the low index planes ( 110 ) and ( 111 ) 
 perfectly correlates with the observed sfm analysis , the theoretically 
 expected cross - section of a b1 * fiber has been modeled and is depicted below 
 the experimental data . although , the observed steep height decrease 
 at the fiber side walls is below the resolution limit of the sfm , 
 a termination of the fibers by ( 001 ) and ( 001 ) facets can be 
 assumed .
scanning force microscopy ( sfm ) image 
 showing vertically aligned 
 fibers and s orientated islands in between after the deposition of 
 60 min nnn at 80 c substrate temperature .
the island s 
 boundaries correlate with the geometrical alignment of   and [ 110 ] 
 orientations . the extracted fiber cross - section ( top , left ) can be 
 explained by the formation of ( 111 ) , ( 110 ) 
 and (001 ) facets . based on the observed crystal shapes , their 
 crystallographic orientations relative to each other has been deduced 
 and is visualized by a 3d model below .
the epitaxial relationship 
 between fibers and islands is consistent with the structural analysis 
 and can be explained by a nucleation of nnn molecules at the fiber 
 side walls , also called  ledge directed epitaxy  .
based on the latter analysis , 
 a three - dimensional model of both 
 crystal types has been generated and is depicted in the bottom part 
 of figure 5 . as indicated by the xrd - pf analysis , 
 which is presented in figure 4 ( lna ) , b1 * and s1 * crystallites shared 
 the same azimuth for their crystallographic [110 ] orientations . 
 in that way
, the tilt angle of standing nnn molecules within the s - type 
 crystallite approximately correlates with the tilt angle of the fiber 
 ( 001 ) low energy plane .
analogous observations were demonstrated for 
 6 t fibers and are explained by the nucleation of islands at the sidewalls 
 of already existing needles .
moreover , the latter 
 picture is perfectly consistent with the sfm analysis presented in 
 figure 2 , which reports a continuously increasing 
 island coverage for longer deposition times .
such epitaxial alignment 
 based on a geometrical fit between nucleating crystallites and already 
 existing topographic features on the substrate is called 
ledge 
 directed epitaxy  . based on the latter 
 analysis together with xrd - pf data presented in figure 4 ( lna ) , it can be concluded that the minor fraction of s23 crystallites has most likely nucleated at b - type 
 fibers , which are orientated approximately 30 tilted relative 
 to [ 110]mica of the muscovite mica substrate . 

this conclusion is further consistent with the microscopy image , presented 
 in figure 4 , revealing the presence of a minor 
 fraction of such fibers ( below the detection limit of xrd - pf measurements ) , 
 which are subsequently labeled by b2 and b2*.
in order to understand the observed growth behavior of b1 and b2 fibers on muscovite mica(001 ) , the left part of 
 figure 6 depicts a planar nnn molecule in gas 
 phase .
analogous to 6 t , a planar molecule 
 is characterized by a mirror plane h , which is aligned in the plane of the naphthalene rings , and a 2-fold 
 rotational axis , which is aligned normal to h .
, it should be assumed that single nnn molecules tend 
 to adsorb lying flat on the muscovite mica substrate in order to maximize 
 their contact area . in that way ,
h is orientated parallel to the substrate , and the molecule becomes 
 chiral when adsorbed on an arbitrary surface .
molecules that are intrinsically 
 achiral but obtain a form of 2d chirality when adsorbed on a substrate 
 surface are also called prochiral .
analogous 
 to 6 t , two mirror symmetric nnn enantiomers ( sketched 
 as red and blue molecules ) can adsorb on the muscovite mica surface , 
 which can not be brought into congruence by translation and rotation . taking a top view of the molecular stacking at the contact plane 
 of b - type crystallites , which is depicted in the right panel of figure 6
, reveals that ( 111 ) orientated fibers are built 
 up by an alternating assembly of only one enantiomer ( red ) and edge - on 
 nnn molecules .
contrarily , their twin crystallites b * ( 111 ) 
 are built up by the mirrored molecular configuration ( blue ) only . 

the latter observation further explains the consistent choice of a 
 red and blue color code for molecules and crystallites . beside a real 
 space model for b1 crystallites , which is deduced by xrd - pf 
 analysis ,
the right part of figure 6 further 
 includes the proposed geometry of b2 fibers .
based on a 
 growth model that has been deduced for 6 t fibers , it is assumed that two needle orientations , for example , 
 b1 and b2 * can originate from one molecular adsorption site .
the existence 
 of these two lna orientations is explained by a mirror symmetric molecular 
 stacking during crystal nucleation .
interestingly , epitaxially grown 
 6 t on muscovite mica showed a comparable fraction of both stacking 
 types , which explains the observation of four lna orientations . the 
 latter phenomenon is further explained by force field simulations 
 for both crystal types , yielding a similar adsorption energy with 
 a deviation of some millielectronvolts / molecule .
contrarily , xrd - pf analysis revealed that nnn fibers are 
 dominantly present only in one configuration , which reduces the observed 
 lna orientations to two ( see the fft in figure 4 ) .
consequently , it can be stated that both crystal types seem to 
 significantly differ concerning their adsorption energy , which should 
 be investigated and underlined by the discussion of force field simulations 
 within the next paragraphs .
molecular symmetry of a planar nnn in gas phase 
 and when adsorbed 
 flat on an arbitrary surface . due to
the presence of a mirror symmetry 
 plane h parallel to the naphthalene 
 rings and a 2-fold rotational axis , aligned normal to it , the nnn 
 molecule in gas phase can be described by the c2h point group .
contrarily , when adsorbed 
 flat on a substrate surface , nnn can form two mirror symmetric enantiomers 
 ( sketched by red and blue molecules ) , which follow c2 symmetry .
the right panel depicts a real space model 
 of the discussed crystal orientations ( top view ) .
the molecular alignment 
 of nnn within the surface unit cell has been deduced from its bulk 
 structure , oriented with a ( 111)/(111 ) contact 
 plane for b / b * crystallites .
the orientation of the long molecular 
 axis ( lma ) or long needle axis ( lna ) is indicated by blue or red arrows . 

taking a closer look at the molecular stacking at the contact plane 
 of b - type crystallites reveals that ( 111 ) orientated crystals are 
 alternately assembled by red enantiomers and edge - on nnn molecules . 

contrarily , their mirror symmetric twins b * only consist of blue molecular 
 configurations . the real space image of b1 ( b1 * ) and b2 * ( b2 )
the adsorption energy as a function of the long molecular axis 
 ( lma ) orientation (  ) is depicted in the top panel in terms 
 of a polar plot .
red and blue arrows indicate the molecular orientation 
 deduced by experiments . at the indicated positions , force field simulations 
 reveal a broad maximum for the corresponding enantiomer .
contrarily , 
 the adsorption site seems less favorable for the mirror symmetric 
 molecule ( e  20 mev ) . a real space 
 model that sketches the lateral position for the molecular adsorption 
 angle at  = 57
simulations 
 reveal a preferred alignment of the terminating naphthalene units 
 in the surface corrugations of the muscovite mica substrate ( indicated 
 by horizontal lines ) .
below , an analogous analysis has been done for 
 a 7  2 molecular stack representative for the contact plane 
 of a b ( 111 ) crystallite .
simulations reveal a strongly pronounced 
 adsorption maximum at the experimentally observed adsorption angle 
 ( red arrow ) and a significantly different adsorption energy for a 
 b * crystal with opposite stacking sequence ( blue arrow , e  300 mev / molecule ) . a real space model of the 
 optimized adsorption position
is depicted in the right panel . by comparing 
 the alignment of the nnn molecules with the muscovite mica unit cell
, 
 an approximately periodic alignment can be recognized along mica/[110]mica for an / terminated 
 surface . in a first step ,
the optimal adsorption 
 energy of a single nnn 
 molecule has been deduced based on force - field simulations by selecting 
 the most favorable adsorption site for each angle . the angle 
  characterizes the azimuthal alignment of the lma relative 
 to mirror symmetry plane of the muscovite mica substrate surface . 

the molecules are assumed to adsorb flat on the substrate and the 
 adsorption energy , ead , is defined as 
 the difference between the energies of the isolated subsystems and 
 the energy of the combined system . therefore , maxima in the ead versus  curves evidence the favorable 
 adsorption geometries . to increase the readability ,
ead curves are presented in figure 7 in terms of a polar plot , where ead = e0  ead .
the parameter e0 represents 
 the adsorption energy at the least favorable angle  , yielding 
 a value of e0 = 2.49 ev for the 
 isolated molecules .
because nnn molecules can adsorb either in their 
 left- or right - handed configuration , simulations have been performed 
 for both enantiomers and are color coded by red- and blue - filled curves .
simulations yield , for both molecular configurations , multiple 
 adsorption maxima , which are located for the blue marked enantiomer 
 at max , blue = 42 , 57 , 102 , 161 , and 178. 
 due to 2-fold rotational symmetry of the nnn molecule , identical values 
 are obtained for ead(+180 ) . 

moreover , optimized adsorption positions for the red molecular type 
 are found at max , red = max , blue due to mirror symmetry of the substrate surface .
experimentally 
 obtained adsorption angles are further indicated by a blue ( red ) arrow 
 at  = 49 ( 131 ) .
both adsorption angles correlate 
 with the broadest maxima obtained by simulations and are importantly 
 consistent with the simulations of the corresponding enantiomer .
the 
 fact that beside experimentally observed adsorption geometries force 
 field calculations also yield additional maxima is attributed to the 
 usage of empirical potentials , which in some cases may yield the wrong 
 energetic ordering of competing structure solutions .
nevertheless , it has to be underlined that simulations 
 indicate a significant less favorable adsorption site for the mirror 
 symmetric molecular configuration ( 20 mev ) .
in general , adsorption 
 energies for both molecular configurations significantly differ , which 
 in further consequence leads to a nonequal distribution or even breakup 
 of both enantiomers depending on the adsorption angle .
contrarily , 
 simulations as well as experimental data that are reported for 6 t indicate a significantly lower energetic splitting 
 between both molecular configurations , which may result from a higher 
 symmetry of the molecule .
the latter statement can be understood by 
 the fact that thiophene molecules with an odd ring number , for example , 
 quinquethiophene or septithiophene , are characterized by a mirror 
 symmetry plane when adsorbed on a surface and consequently do not 
 show a prochiral character .
contrarily , the asymmetric alignment of 
 the c  c bond between two naphthalene units of nnn inevitably 
 leads to a prochiral behavior when adsorbed on a substrate surface 
 and consequently plays an essential role concerning the energetic 
 separation of both enantiomers at a defined adsorption angle .
the experimentally confirmed adsorption position of nnn molecules 
 is further depicted in the right part of figure 7 . for both molecules , an adsorption angle  = 
analogous to calculations for p-6p and 6 t , nnn molecules tend to align their rings in the 
 surface corrugations , which are indicated by vertical solid lines . in order to study the adsorption energetics of a b - type crystallite
, 
 a 7  2 molecular stack has been deduced from a ( 111 ) orientated 
 crystallite .
analogous to the force field simulations of an isolated 
 molecule , the adsorption energy ead for the stack has been probed depending on the molecular orientation 
  and the adsorption energy at the least favorable adsorption 
 angle is given by e0 = 0.41 ev / molecule . 
 because the curve calculated for the a b * ( 111 ) 

stack follows the same behavior as discussed for a single molecule 
 ( mirror symmetric ) , only the results for a b contact plane are depicted 
 in order to increase readability .
interestingly , simulations reveal 
 that not only the number of energetically favorable adsorption sites 
 decreases but also the angular acceptance , which becomes visible by 
 well pronounced peaks .
simulations further indicate the presence of 
 two adsorption maxima , which are located at  = 12 and 
 48. again , the experimentally obtained adsorption angle 
 for b1 crystallites is indicated by a red arrow and underlines 
 a nearly perfect agreement .
moreover , it can be recognized that the 
 adsorption energy for a b2 crystal ( at + 48 ) becomes 
 even more unfavorable than that for a single molecule , which is manifested 
 by a much lower value of ead in 
 the range of some 100 mev / molecule .
consequently , force field 
 simulations not only reflect the experimental 
 observations but also explain the dominant fraction of b1 crystallites by a preferred nucleation of their stacking sequence 
 in contrast to b2 crystallites
. the observed behavior can 
 be even better understood by analyzing the real space model of the 
 simulated adsorption position at  = 48. besides 
 the molecular alignment , also the surface unit cells of the muscovite 
 mica and b crystal have been indicated .
obviously , the unit vector 
 [ 110 ] of the nnn crystal stack , which also defines its lna , 
 tends to align parallel to one surface unit vector of the muscovite 
 mica crystal , which is defined by the mica, ( [ 110]mica, ) orientation for an  
 (  ) terminated surface .
the epitaxial growth of ternaphtalene 
 ( nnn ) on muscovite mica(001 ) has been investigated by combining structural 
 ( xrd - pf ) and morphological ( sfm ) methods .
consistently , both methods 
 reveal the formation of s ( 001 ) orientated nnn island - like structures 
 which have nucleated at the sidewalls of b ( 111 ) orientated fibers . 

it is demonstrated that the latter nnn crystal types tend to align 
 along two dominant directions , which leads to the formation of a v - shaped 
 sample fiber morphology .
because the tilt angle of nnn molecules within 
 s - orientated crystallites correlates with the tilt angle of the fiber 
 side facets , the island nucleation is explained by  ledge directed 
 epitaxy  .
based on this growth model , it can be understood 
 that both crystal types provide a well - defined azimuthal orientation 
 relative to the muscovite mica substrate . by use of force field 
 simulations , the growth of the fibers is further analyzed .
the epitaxial 
 growth of sexithiophene ( 6 t ) on muscovite mica showed the formation 
 of four well - defined fiber orientations , which can be explained by 
 mirror symmetry of the muscovite mica substrate and two differently 
 stacked 6 t crystallites , which can nucleate at a molecular adsorption 
 position .
contrarily , experimental investigations 
 indicate that nnn crystallites tend to stack in a single configuration , 
 which explains the dominant formation of only two fiber orientations . 
 based on force field simulations ,
the latter observation is further 
 investigated and explained by significantly different adsorption energies 
 of both crystal types .
it is further demonstrated that the observed 
 behavior results from an interplay of the molecular adsorption and 
 lattice match .
 the 
 morphology and structure of 2,2:6,2-ternaphthalene 
 ( nnn ) deposited on muscovite mica(001 ) substrates was investigated 
 by scanning force microscopy ( sfm ) and specular x - ray diffraction 
 measurements . 
 consistently , both methods reveal the coexistence of 
 needle - like structures with a { 111 } contact plane and { 001 } orientated 
 island - like crystallites , which are built up by almost upright standing 
 nnn molecules . 
 both orientations are characterized by a well - defined 
 azimuthal alignment relative to the substrate surface , which is analyzed 
 by x - ray diffraction pole figure ( xrd - pf ) measurements . 
 based on xrd - pf 
 and sfm analysis , the azimuthal alignment of { 001 } orientated crystallites 
 is explained by ledge - directed epitaxy along the fibers sidewalls . 
 
 these fibers are found to orient along two dominant directions , which 
 is verified and explained by a doubling of the energetically preferred 
 molecular adsorption site by mirror symmetry of the substrate surface . 
 
 the experimental findings are confirmed by force - field simulations 
 and are discussed based on a recently reported growth model . 
Introduction
Experimental Section
Experimental Results
Discussion