an extended -conjugated system 
 is generally considered to 
 be a prerequisite for high electron mobility and , consequently , a 
 good performance in organic thin film devices . pentacene and rubrene 
 are typical examples of this class of molecules . from the point of 
 view of mesomeric structures , 5,12-dihydro - quino[2,3-b]acridine-7,14-dione shortly called quinacridone ( qa)is 
 a molecule with limited intramolecular -conjugation , as resonance 
 contributions of enol / imine mesomers are small . advances that this view may require 
 revision : in their recent publications they illustrate the potential 
 of quinacridone as functional material in organic field effect transistors 
 ( ofets ) and photovoltaics ( pv ) . the authors were able to achieve 
 field effect mobilities larger than 0.1 cm/(vs ) and photocurrents in the ma / cm range under simulated solar illumination . the surprisingly high 
 mobility is attributed to strong intermolecular interactions via hydrogen 
 bonds reinforcing -stacking and crystalline ordering . quinacridone is a widely used organic pigment ( violet 19 , c20h12n2o2 , see inset of figure 2 for its structural formula ) , which is industrially 
 produced in large amounts and at low costs . it is perhaps best - known as constituting the magenta 
 colorant for printer toners . in contrast to the well - known 
 pentacene , its intramolecular conjugation is broken . besides van der 
 waals interactions , intermolecular hydrogen bonds are most crucial 
 for its crystalline structure . the polymorphs of the bulk material 
 can be divided into two groups . in the  and  phase , 
 quinacridone forms flat rows of parallelly arranged molecules . this 
 allows each quinacridone molecule to form two hydrogen bonds with 
 both adjacent neighbors in the row . in contrast , the  polymorph exhibits 
 a criss - cross arrangement so that each molecule is connected by one 
 hydrogen bond to each of its four neighbors ( for details on the bulk 
 polymorphs , see refs ( 79 ) ) . predict that the hole mobility differs 
 by a factor 100 between the  ( 4.44 
  10 cm/(v s ) ) and the  
 ( 4.18  10 cm/(v s ) ) polymorph . in epitaxial thin films , one also has 
 to consider the orientation 
 of the crystallites , i.e. , which lattice plane is parallel to the 
 substrate surface . depending on the crystallographic alignment , the 
 long axis of rodlike molecules will adopt an upright standing or a 
 flat - lying geometry on the surface . since the highest mobility is 
 generally obtained along the -stacking direction , i.e. , perpendicular 
 to the long axis of the molecules , the orientation of the molecules 
 on the substrate surface is essential for the functionality of a thin 
 film device . for an ofet , where the current 
 flows parallel to the surface , an upright standing geometry is favored . in contrast , for an oled or photovoltaic device where the charge transport 
 and the emitted or incident light are directed perpendicular to the 
 substrate surface , flat - lying molecules are preferred . accordingly , 
 gowacki et al . have prepared quinacridone films with a  standing 
 up configuration on an aluminum oxide surface passivated by 
 tetratetracontane to fabricate their ofets . on the other hand , adsorption of rodlike molecules on clean metal 
 surfaces generally adopt a flat - lying adsorption geometry due to the 
 strong interaction of the molecules with the metal substrate . this 
 is also the case for hydrogen - bonded molecules like 4-[trans-2-(pyrid-4-ylvinyl)]benzoic acid ( pvba ) and 4-[(pyrid-4-ylethynyl)]benzoic 
 acid ( peba ) on ag(111 ) or indigo on cu(111 ) as well as for the adsorption of quinacridone 
 on ag(111 ) as will be reported below . finally , we want to stress 
 the importance of 2d - chirality in the 
 context of the adsorption of molecules ( like quinacridone ) on surfaces . although chirality is a central and powerful concept in chemistry 
 ever since its introduction by pasteur , the notion of  surface 
 chirality  and its fascinating consequences were explored only 
 recently . for a 2d , prochiral molecule with a 3d inversion 
 center like indigo or quinacridone , which is inclined with respect 
 to a high symmetry direction of the surface , the 2d - mirror operation 
 about this axis will leave the adsorption configuration unchanged 
 but switch the handedness of the molecule on the surface . therefore , 
 the two adsorption configurations depicted in figure 1 are the energetically equivalent ones 
 for the two different 2d - enantiomers . however , in its ground state , each of the two enantiomers adopts 
 only one of the two mirror symmetric adsorption geometries , in particular 
 either  or +  according to its handedness . in 
 the following , we will take advantage of this conjunction to differentiate 
 between the two enantiomers of quinacridone molecules adsorbed on 
 the ag(111 ) surface . schematic illustrating the relationship between the adsorption 
 geometry of a quinacridone molecule and its 2d - chirality . in this 
 example , the mirror plane is parallel to 112 
 symmetry axis and normal to the surface . the atoms of the quinacridone 
 are color coded as follows : carbon = gray , hydrogen = red , nitrogen 
 = white , and oxygen = blue . the experiments were carried out in 
 an ultrahigh - vacuum system 
 housing an omicron vt afm / stm , an omicron low - energy electron diffraction 
 optics ( leed ) , and a focus photoelectron emission microscope ( peem ) . the ag(111 ) single crystal was prepared by several cycles 
 of sputtering with ar ions ( 900 v , 4 a / cm for 60 min ) and subsequent annealing at 660 k. the quinacridone 
 was purchased from tci and purified by double temperature gradient 
 sublimation in a vacuum of 5  10 mbar . the 
 quinacridone was deposited while monitoring the photoelectron emission 
 yield in the peem . during deposition , the quartz crucible with the 
 quinacridone powder was held at 553 k while the sample was at room 
 temperature . a hg lamp ( 4.9 ev ) was used as excitation source for 
 the photoelectrons . a typical transient of the electron yield as a 
 function of the deposition time is shown in figure 2 . we assume that the maximum of the electron 
 yield corresponds to the completion of the first monolayer . in order 
 to prepare a single monolayer of quinacridone , the deposition was 
 stopped either after reaching the maximum of the electron yield or 
 after the deposition of an equivalent of 45 times this amount . in the second case , the sample was heated to 550570 k to desorb 
 the excess molecules in the multilayer . the stm and leed measurements 
 were carried out in the same vacuum 
 vessel without breaking the vacuum . for the particular stm setup , the bias voltage is applied 
 to the tip while the sample is grounded . if not stated otherwise , 
 only a background subtraction and an optimization of the contrast 
 were applied to the stm data . in order to minimize the electron beam 
 induced damage during leed measurements , the sample was moved to a 
  fresh  position every time a leed image was acquired . although the leed pattern usually vanished after a couple of seconds 
 when exposing the organic thin film to the electron beam , we were 
 not able to detect any damaged molecules by stm . in the same way we 
 can exclude that there is a significant decomposition of the molecules 
 due to the light exposure in the peem . to monitor the growth of quinacridone , 
 sequences of peem images ( movies ) were recorded while the ag(111 ) 
 was exposed to quinacridone . during the deposition , no characteristic 
 features with length scales larger than 0.1 m were observed ; 
 i.e. , the local electron yield was spatially homogeneous within the 
 field of view varying between 80 and 145 m . even after deposition 
 of more than 10 monolayers of quinacridone for other organic 
 materials like -sexithiophene ( 6t),p - sexiphenyl ( 6p ) , and perylene-3,4,9,10-tetracarboxylic 
 dianhydride ( ptcda ) , stranski krastanov 
 growth is typically observed ; i.e. , after formation of a wetting layer 
 the growth of 3d islands sets in . as in these cases the ionization 
 potential , i.e. , the binding energy of the homo level with respect 
 to the vacuum level , is higher than the maximum photon energy supplied 
 by the hg lamp ; 3d crystalline structures have a lower emission yield 
 in the peem than the adjacent wetting layer and thus appear darker 
 than the surrounding wetting layer . for the case of quinacridone films , , we do not expect any photoelectron 
 emission from a thick layer of quinacridone if a hg lamp is used for 
 photoelectron excitation . indeed , the electron yield depicted in figure 2 for the bare ag(111 ) surface ( deposition time t = 0 s ) is higher than for the surface covered with about 
 45 monolayer of quinacridone ( deposition time t = 1150 s ) . since for ag(111 ) work function values  in the 
 range between 4.3 and 4.8 ev are reported , the pristine ag(111 ) surface 
 should ( and actually does ) appear bright in the peem images . obviously , the growth transient depicted in figure 2 does not show a monotonous decrease of the electron yield 
 but a maximum which can be attributed to the completion of the first 
 monolayer of quinacridone . the physical background of this feature 
 is the formation of an interface dipole during the deposition of the 
 first monolayer . subsequent layers deposited on 
 top of this monolayer do not alter the metal  organic interface 
 any further but lead to an attenuation of the electron yield due to 
 inelastic scattering of the photoelectrons excited at the interface 
 upon traversing the quinacridone layer . as a consequence representative 
 peem images ( top ) and transient of the electron 
 yield acquired during deposition of quinacridone on the ag(111 ) surface 
 ( buttom ) . the white line shows the transient of the mean electron 
 yield ( averaged over the entire field of view of 80 m ) . the 
 dark ribbon behind the white line arises from vertical  error 
 bars , each of which is actually a complete histogram of the 
 electron yield sampled over the entire field of view . the color ranges 
 from white ( low probability for the given electron yield ) to black 
 ( high probability ) . in other words , the width of the dark ribbon is 
 a measure for the variation of the electron yield across the peem 
 image recorded at a given time t. in total , the ribbon 
 shows the histograms of 702 images recorded during the growth experiment . 
 the time interval between subsequent images is 2 s. the inset shows 
 a structural formula of the quinacridone molecule . the sample surface 
 shown in figure 3 was prepared by thermal evaporation 
 of quinacridone while the sample was held at room temperature . once 
 the maximum of the electron yield was reached in the peem transient 
 ( cf . therefore , the coverage should be just above one monolayer 
 ( 1 ml ) . indeed , the stm micrograph in figure 3 shows a close - packed layer of quinacridone molecules . the molecules 
 form extended quasi - one - dimensional rows of molecules segmented into 
 stacks of a few nanometers length . the molecular rows have distinct 
 orientations inclined by about 11 with respect to one 
 of the three equivalent 112 crystallographic 
 axes of the substrate . therefore , the two mirror domains combined 
 with the three rotationally equivalent 112 
 axes of the substrate give rise to a total of six possible orientations 
 of the rows . the angle between two neighboring , mirror symmetric orientations 
 is 2  11 = 22 , as indicated in figure 3 . this is consistent with the leed results discussed later 
 on , which reveal that the molecular rows are aligned 10.9 
 with respect to the 112 directions of the substrate 
 ( see figure 6 ) . stm image of a close - packed 
 monolayer of quinacridone on ag(111 ) 
 deposited while the sample was held at room temperature . the tunneling parameters 
 were u = 2 v and i = 2 pa . the white 
 arrow at the bottom marks the position of the cross section displayed 
 in the inset . the dashed lines indicate the expected step height of 
 the ag(111 ) substrate . along the line profile indicated in figure 3 , two steps can be identified . their height corresponds almost 
 exactly 
 to 0.24 nm , which is the expected step height on the ag(111 ) surface . as no steps with other heights were observed , the coverage of quinacridone 
 corresponds to just a single monolayer as expected from the peem transient 
 in figure 2 . excess molecules which might be 
 present in the second layer on top of the quinacridone monolayer can not 
 be imaged . because of their small concentration , these molecules might 
 diffuse as a 2d molecular gas on top of the first layer , but the stm 
 is too slow to image them . the occasional  spikes  in 
 the topography support this hypothesis : each time a molecule in the 
 second layer diffuses through the tunneling gap , the associated increase 
 of the tunneling current is counteracted by a small temporary retraction 
 of the tip . because of the high mobility of the molecules at room 
 temperature , these intermittent events lead to  spikes  
 in the topographic images obtained in constant current mode . figure 4 shows an stm image with molecular 
 resolution of the monolayer of quinacridone prepared at room temperature . 
 from this kind of image we are able to extract the spacing between 
 neighboring rows of molecules to be b = ( 2.03  
 0.02 ) nm . the indicated error corresponds to the standard deviation 
 of the mean value and does not include any systematic error due to , 
 e.g. , a poorly calibrated scanner . for the latter , a relative error 
 of 5% was estimated and has to be added to the statistical error . 
 within each stack , the molecules are oriented parallel to each other 
 and approximately perpendicular to the row direction . the so - determined 
 unit cell would be a primitive one with orthogonal unit cell vectors . the distance between adjacent molecules within the same stack is about a = ( 0.70  0.01 ) nm . this value is in good agreement 
 with the spacing of hydrogen - bonded quinacridone molecules along the 
 molecular rows in the  or  polymorphs . the same value 
 is also reported by trixler and co - workers for the spacing between 
 parallel quinacridone molecules within the quinacridone  nanowires as can be seen in the overview image in figure 3 , the bright parts ( stacks ) along the molecular 
 rows are separated by short darker sections ( defects ) . to avoid systematic 
 errors , we have extracted the intermolecular distance only from molecules 
 located within the same stacks . at higher magnification , the details 
 of the molecular arrangement can be resolved . an example is shown 
 in figure 4 together with a network of solid 
 and dotted lines highlighting the positions and relative arrangement 
 of the quinacridone molecules . thick 
 solid or dashed lines enclose stacks in neighboring rows where the 
 molecules are in perfect registry parallel and perpendicular to the 
 row direction . these different sets of domains ( solid and dashed lines ) , 
 however , form two antiphase domains with a lateral shift of half a 
 lattice spacing along the row direction . the antiphase boundaries 
 are introduced by the  defects  between the stacks or 
 domains and are thus enclosed partly by solid and dotted thick lines . the defects contain either one or two quinacridone molecules ( marked 
 by crosses in figure 4 ) which appear more  fuzzy  
 than those in the neighboring stacks . clearly , each defect introduces 
 a  phase shift  of  ( half a unit cell ) between 
 the adjacent stacks along the row direction . the apparently regular 
 arrangement of the defects in figure 4 is accidental : 
 a statistical analysis of the larger scale images ( see figure 3 ) reveals that the spacing between two defects along 
 the rows follows a random distribution , characterized by an exponentially 
 decaying pair distribution function with a mean spacing ( stack width ) 
 of about 45 molecules . stm image of a molecularly resolved monolayer 
 of quinacridone on 
 ag(111 ) at room temperature . the stm was operated with a bias voltage of u = 0.5 v and a current set point of i = 5 pa . the 
 thin ( solid and dashed ) lines mark the distance between molecules 
 with a regular spacing . and form spacings that are 1.5 times 
 wider than the regular ones . an explanation of the structures as revealed by stm involves 
 both 
 the hydrogen bonding and the 2d - chirality of the quinacridone molecules 
 and is illustrated by the model in figure 5 . the tendency to aggregate in stacks of parallel molecules with 
 an intermolecular spacing of about 0.7 nm can be attributed to the 
 strong interaction mediated by two c = oh  n 
 hydrogen bonds between neighboring molecules with the same handedness 
 in a row . the same bonding motif is also dominating the  and 
  polymorphs in the bulk . on the other hand , the azimuthal orientation 
 of the molecules on the substrate , and , hence , the ( perpendicular ) 
 orientation of the molecular rows , is determined by the interaction 
 of the quinacridone molecules with the ag(111 ) substrate , i.e. , the 
 corrugation of the molecule ( see figure 1 ) , quinacridone molecules with different azimuthal orientation 
 can be associated with different 2d - enantiomers , albeit our room temperature 
 stm data do not allow to determine the handedness , directly . consequently , 
 the two mirror domains composed of rows of molecules oriented + 11 
 and 11 with respect to the ag112 
 direction ( see figure 3 ) can be attributed 
 to two almost enantiopure phases with opposite handedness . the  defects  
 in figure 4 could thus be due to a residual 
 amount ( of about 18% ) of molecules with the  wrong handedness , which were trapped during the growth and coalescence of 
 the enantiopure stacks . as can be noticed in figure 4 , molecules in stacks on neighboring rows can have only two 
 possible relative alignments : they are either aligned ( in - phase ) or 
 shifted by half a unit cell spacing . the existence of only two antiphase 
 domains along the rows suggests that the structure of the quinacridone 
 domains is commensurate along this direction . in fact , the above 
 growth scenario is very similar to the one observed 
 for indigo on cu(111 ) by villagomez et al . in this case , the azimuthal orientation of the isolated indigo enantiomers 
 is inclined by about + 20 and 20 with respect to 
 the cu110 direction , respectively . upon 2d 
 condensation , the indigo molecules were found to self - assemble into 
 mirror domains composed of essentially enantiopure molecular rows 
 oriented + 9.5 and 9.5 with respect to the 110 
 direction . were also observed along the rows which could be identified as indigo 
 molecules with the  wrong  handedness . these defects 
 could be eliminated almost completely by annealing the sample at 100 
 c for 1 h. with their low - temperature stm villagomez et al . , 
 the authors could actually determine the 2d - chirality from the asymmetry 
 of the shape of the stm image for each individual indigo enantiomer . in order to further quantify the superstructure of the quinacridone 
 adlayer and to explore its suspected commensurability , figure 6a shows the characteristic leed 
 pattern obtained from the same sample which was previously characterized 
 by stm ( see figures 3 and 4 ) . a major problem with leed is that very low electron beam 
 energies have to be used to acquire diffraction patterns of large 
 organic molecules with correspondingly large unit cells . these slow 
 electrons can interact very efficiently with the molecules , so that 
 the risk of beam damage is very high . for the particular system of 
 quinacridone on silver , the leed pattern was only visible for a couple 
 of seconds before it faded away . therefore , we had to change the position 
 of the sample right before acquisition of a single leed pattern . in 
 addition , we did not align the sample in the usual way so that the 
 ( 00 ) spot is back - reflected into the electron gun . because of this 
 intentional misalignment , we were able to image the diffraction spots 
 in close proximity to the specular ( 00 ) spot , but it also resulted 
 in a geometrical distortion of the diffraction pattern . structure model 
 of the quinacridone monolayer on ag(111 ) as deposited 
 at room temperature . the hexagonal mesh represents the ag(111 ) lattice . 
 at each intersection of the gray lines the solid 
 and dotted lines separate neighboring quinacridone molecules as in 
 the overlay in the stm image of figure 4 . while 
 the molecules in the stacks all have the same handedness , a minority 
 of the molecules with the handedness forms defects 
 which introduce antiphase domains with a phase shift corresponding 
 to half an intermolecular spacing along the row direction . ( a ) leed pattern of a monolayer of quinacridone deposited 
 on the 
 ag(111 ) surface at room temperature . the sample was intentionally misaligned 
 so that the electron gun did not hide the diffraction spots around 
 the ( 00 ) diffraction beam . ( b ) simulated leed pattern on the basis 
 of the superstructure given by the matrix in eq 1 . besides the ( 00 ) spot , the ( 10 ) 
 and the ( 01 ) spot of the silver 
 substrate can be identified in the leed pattern in figure 6a . this allows determining the symmetry axes of 
 the substrate , i.e. , the 112 directions ( indicated 
 by the dotted lines in figure 6 ) and calibrating 
 the scale of the observed pattern . the most prominent 
 features induced by the quinacridone adlayer are the two rings centered 
 around the ( 00 ) spot with radii a * and b*. the inner ring consists of 12 individual spots . these 12 spots 
 form six pairs which are azimuthally centered in the middle of the 
 60 sector defined by the ( 00 ) , ( 10 ) , and ( 01 ) substrate peaks ; 
 i.e. , each pair is split symmetrically around the 110 
 direction of the substrate . the radius b * of the 
 inner ring corresponds to a distance b = ( 2.00  
 0.17 ) nm in real space , which exactly matches the spacing between 
 neighboring rows of quinacridone molecules ( see figures 3 and 4 ) . as in the following , the experimental 
 error was always estimated from the width of the diffraction spots . any distortion of the pattern , which is obviously present in figure 6a , is neglected . in agreement with the stm findings 
 concerning the relative orientations of the different rows of molecules 
 ( figure 3 ) , the splitting between the two spots 
 forming a pair with respect to the 110 direction 
 is about 2025. this is fully consistent with the 
 stm observation showing molecular rows with an inclination of 11 
 with respect to the 112 direction . the 
 second ring with radius a * , marked with a 
 dash - dotted line in figure 6 , also consists 
 of 2  6 = 12 intense spots , but their azimuthal position is 
 rotated by 90 with respect to the inner ring . in addition , six 
 additional very faint spots are located just slightly outside this 
 ring . the radius a * of the second ring can be attributed 
 to the measured spacing a between neighboring molecules 
 in a close - packed stack . here we find a real space value of a = ( 0.72  0.05 ) nm . upon closer inspection of the 
 leed pattern in figure 6a , one can identify 
 rather diffuse rings with radii 2a * and 3a * which are concentric around the central ( 00 ) spot . an 
 important detail for the structural analysis is that the ring with 
 radius 3a * does not intersect the first - order diffraction 
 spots of the substrate but has a radius which is about 7% larger than 
 the length of the { 10 } reciprocal lattice vectors of the substrate . 
 taking into account the so - derived values for a and b , the orientation with respect to the symmetry directions 
 of the substrate as well as the commensurability along the a - direction one arrives at a superstructure unit cell which 
 can be expressed in matrix notation as1for the two mirror domains , respectively . the half integer values in the second row ( i.e. , along the b - direction ) suggest that the superstructure is actually 
  high - order commensurate  with a commensurate coincidence 
 lattice with twice the lattice constant b and two 
 inequivalent quinacridone molecules within the commensurate supercell . 
 the primitive unit cell given by eq 1 is also 
 the basis for the model drawn in figure 5 . 
 within each close - packed stack all the molecules have the same orientation 
 and , therefore , the same handedness on the surface . the fact that 
 there are two antiphase domains with a relative shift of a/2 along the row direction and that no moir pattern was observed 
 in the stm images is consistent with a simple commensurability of 
 the structure along the a - direction ; i.e. , all the 
 molecules within the stacks are located on equivalent adsorption sites 
 on the ag(111 ) surface . for the model depicted in figure 5 we assumed that the center of the molecule is located 
 above a 2-fold bridge position of the substrate . in this case , also 
 the two outer aromatic rings are located very close to bridge sites . moreover , the molecules in different antiphase domains and in neighboring 
 rows would also rest on bridge sites , albeit nonequivalent ones . a 
 similar arrangement , in which all the molecules are located on a single 
 type of lattice sites , although this might be a good argument in favor of this particular 
 adsorption geometry , the experimental data do not allow to determine 
 the absolute positions of the molecules . the matrix is rather 
 speculative for the b direction 
 ( i.e. , the second row of the superstructure matrix ) as neither stm 
 nor the leed measurements at low electron energies are free of distortions . however , in the case of more complicated modes of epitaxy like point - on - line 
 or point - on - point structures , moir 
 patterns are usually observed in the stm images , which can help to 
 track down the structure . in our case moreover , changing the values of the bottom row of the 
 matrix , i.e. , ( 1.5 , 6 ) or ( 1.5 , 7.5 ) , by 10% ( which 
 is an upper limit for the experimental uncertainty ) would fundamentally 
 change the symmetry of the unit cell , which would then be no longer 
 perfectly rectangular . in addition , the arguments concerning a possible 
 single adsorption site for all quinacridone molecules would no longer 
 be valid . although there is a good general agreement concerning 
 the main 
 features and symmetries between the simulated and the measured leed 
 pattern ( see figure 6a ) , the simulation predicts 
 many more spots . to make the simulation more accurate , the structure 
 factor of the quinacridone molecule in addition , the stm data 
 show that the periodicity along the rows is quite often interrupted 
 by  defects  introducing antiphase boundaries between 
 consecutive stacks of molecules . for a realistic description of the 
 experimental leed pattern the structural 
 properties of a quinacridone monolayer adsorbed on the ag(111 ) surface 
 change completely if the sample is heated up to 550 k. this temperature 
 corresponds to the one used in the evaporator during the preparation 
 of the quinacridone thin film . therefore , we assume that excess molecules 
 in the second and higher layers gain enough thermal energy so that 
 they can desorb from the sample , whereas the quinacridone molecules 
 in the first monolayer in direct contact with the ag(111 ) substrate 
 remain on the surface . indeed , it is quite common that the desorption 
 of the molecules in the first monolayer of an organic film desorb 
 at significantly higher temperatures than those in the multilayer . one can take advantage of this effect to prepare 
 well - defined molecular monolayers : at first , several layers of the 
 organic molecule are deposited , and subsequently all but the first 
 molecular layer are desorbed by heating the sample appropriately . 
 in the present case , quinacridone monolayers which were prepared by 
 depositing a single layer of molecules and subsequently heated ( see 
 figure 9a ) and those which were prepared by 
 multilayer desorption ( see figures 7 and 8) exhibit the same structure . it is surprising 
 that the thermally treated samples have a completely different structure 
 ( see figures 7 and 8) as compared to those deposited at room temperature without further 
 heating ( see figure 3 ) . the domains are often 
 larger than 50 nm in both lateral dimensions although the structures 
 may also be interrupted by discommensuration lines as seen , for instance , 
 in figure 8 . accordingly , the leed pattern 
 shown in figure 9a exhibits much sharper spots 
 than the nonannealed samples ( figure 6a ) . the 
 stm images in figures 7 and 8 reveal rows of parallely stacked quinacridone molecules . the row direction now coincides with the 110 
 symmetry axis of the substrate . while in the nonannealed case all 
 molecules were uniformly aligned along the rows ( a  a stacking ) , 
 the molecules in the annealed monolayer exhibit a kind of a b 
 stacking . depending on the resolution of the stm tip ( and the tunneling 
 parameters ) , we can recognize two inequivalent molecules ( a and b 
 in figure 7 ) . along the molecular rows ( a - direction ) , the molecules are alternately shifted left 
 ( a ) and right ( b ) in the direction of the b - axis 
 of the unit cell indicated in figure 7 . the 
 stm image in figure 8 further reveals that 
 their are two possible , mirror symmetric arrangements of the molecules 
 for a given row direction , since the orientation of the b - axis is not perpendicular to the row direction . stm image of a quinacridone 
 monolayer on ag(111 ) , prepared by multilayer 
 desorption : a thick film was deposited while the sample surface was 
 held at room temperature . after deposition , the temperature of the 
 sample was raised up to about 570 k , and then the heating was stopped . the aim of this procedure was to desorb all molecules in excess of 
 one monolayer . the stacking direction of the molecules now coincides 
 with the 110 symmetry axis of the substrate . the image has a size of 20  20 nm and was acquired 
 with a bias voltage of u = 300 mv and a constant 
 tunneling current of i = 30 pa . the inset shows an 
 averaged pattern obtained with the  auto mesh average algorithm of the wsxm software from an 
 stm image similar to the one shown here . stm image of a quinacridone monolayer on ag(111 ) prepared by multilayer 
 desorption ( see text and figure 7 ) . the image 
 has a size of 20  20 nm and was acquired with a 
 bias voltage of u = 1.0 v and a current set 
 point of i = 25 pa . the mirror axis ( dashed 
 line ) is parallel to the row direction ( thick arrows ) and coincides 
 with the 110-direction of the substrate . we measured the distance between equivalent molecules , 
 i.e. , from 
 one  a  molecule to the next  a  molecule 
 along the row direction , to be a = ( 1.46  0.01 ) 
 nm . the distance between adjacent  a  and  b  
 molecules is just half of this value , namely 0.73 nm , consistent with 
 the width of a single quinacridone molecule . for the spacing between 
 equivalent molecules in neighboring rows , we find a value of b = ( 1.69  0.01 ) nm . the angle between the two unit 
 cell vectors a and b is  = ( 110  1) ( see inset in figure 7 ) . because of the dimer stacking ab  ab  , 
 the unit cell is no longer primitive but contains at least two molecules . in fact , a closer inspection of figure 8 reveals that the orientation of the molecules in the two 
 mirror domains differs by ( 44  2). therefore , the long 
 axis of the molecules is rotated by ( [ 90 + 22 ]  2) = ( 112 
  2) with respect to the direction of the rows , which coincides 
 with the b - direction within the experimental error . the pattern 
 looks much simpler than the one of the as - prepared surface ( see figure 6a ) . the inner structure around the ( 00 ) spot is 
 just a simple hexagon . the distance b * of these six 
 spots nearest to the ( 00 ) spot corresponds to a distance in real space 
 of b = ( 1.66  0.22 ) nm , which agrees well with 
 the value for the spacing between neighboring rows as extracted from 
 the stm images . in addition , the six spots are located on the 112 
 symmetry axes of the substrate . this confirms that the rows are aligned 
 along the 110-directions of the substrate because 
 this arrangement gives rise to a periodicity b along 
 the orthogonal 112-directions in the diffraction 
 pattern . ( a ) leed pattern of quinacridone on ag(111 ) after deposition of 
 slightly more than one monolayer at room temperature and subsequent 
 annealing at 550 k. the leed pattern was acquired with an electron 
 energy of 30 ev . the sample is intentionally misaligned so that the 
 electron gun does not hide the diffraction spots around the ( 00 ) beam . the simulation is based 
 on the commensurate unit cells given in eq 2 and takes into account the 6-fold symmetry of the substrate . the 
 kinematic simulation assumes point scatterers at the centers of the 
 molecules . outside the inner hexagon the faint spots on the 
 sides of this hexagon occur in pairs centered around the 110-directions 
 at a distance 2a * from the ( 00 ) spot , which corresponds 
 to a spacing of a/2 = ( 0.74  0.04 ) nm . within 
 the experimental error this value is identical to the spacing between 
 neighboring a and b molecules as deduced from the stm images in figure 7 . the two spots in each pair are split azimuthally 
 by 23 with respect to the 110 
 symmetry axes of the substrate . the leed pattern can be explained 
 by a reciprocal unit cell and its mirror image indicated by the solid 
 and dashed lines in figure 9 , respectively . 
 in total , there are six possible configurations for the unit cell : 
 three rotationally equivalent domains for each of the two mirror symmetric 
 superstructures . transformation into real space yields the two characteristic 
 matrices2for the two 
 mirror domains . these suggested 
 unit cells are also the basis of the kinematic leed simulations shown 
 in figure 9b , which are in very good agreement 
 with the measured leed pattern in figure 9a . note that the first - order superstructure spot ( 10)s along 
 the a * direction ( and all other odd multiples ) are 
 absent in the leed pattern , such that only every other spot with a 
 spacing of 2a * can be seen . this is due to the fact 
 that the superstructure unit cell is not a primitive one but contains 
 a second molecule at a position a/2 + b , i.e. , halfway along the molecular rows and shifted 
 by a fraction  along the b - axis . as a result , 
 the structure factor ( which is included in the kinematic simulations ) 
 leads to an extinction of all the odd - numbered diffraction spots along 
 the a*-axis , independent of the relative shift . 
 however , other ( off - axis ) diffraction spots containing an odd multiple 
 of a * , like the ( 11)s spot , may 
 have a nonvanishing intensity depending on the value of . in 
 fact , the kinematic structure factor for the ( 11)s spot 
 varies as sin( ) . this means that the ( 11)s spot is fully extinct for a perfectly uniform stacking (  
 = 0 ) but acquires maximum intensity for a centered c(2  2 ) type 
 arrangement (  = 1/2 ) . in the latter case , the odd - numbered 
 spots along the b*-axis would carry zero intensity . since the ( 11)s spot ( which is located next to a ( 01)s spot of another rotational domain ) could not be detected 
 with a measurable intensity and since the ( 01)s spot is 
 very bright , we can conclude that the relative shift  along 
 the b - axis between molecules a and b in the unit 
 cell is closer to 0 than to 0.5 . because of the different appearance 
 of the two inequivalent molecules a and b in the stm images ( see figure 7 ) , their relative shift  is hard to judge 
 but should be in the range between 0.1 and 0.2 , in accordance with 
 the leed simulation . model of the annealed monolayer of quinacridone on ag(111 ) 
 as described 
 in the text . the labeling of the molecules accounts for the inequivalent 
 sites in the unit cell (  a  and  b  ) and 
 the different 2d - handedness of the molecules on the surface (  r  
 and  l  ) . on the basis of the experimental findings , we propose a structure 
 model for the annealed quinacridone monolayer on ag(111 ) , which is 
 illustrated in figure 10 . the model is based 
 on the unit cells in eq 2 with two flat - lying 
 molecules aligned along the b - axis and shifted by 
 a certain fraction  along this axis . in order to arrange the 
 molecules within the unit cell , we tried to optimize the number of 
 hydrogen bonds , especially those between the oxygen and the nh group 
 of adjacent molecules . in doing so , we realized that the periodic 
 indentation of the molecules along the rows is incompatible with a 
 enantiopure stacking . this would rather lead to uniform , quasi - one - dimensional 
 rows oriented perpendicular to the molecular long axis , as observed 
 in the nonannealed monolayer phase ( see figure 5 ) . likewise , stacking molecules with alternating handedness would 
 also favor straight rows of molecules but with their long axis inclined 
 against the direction normal to the rows . in fact , the periodic indentation 
 together with the observed inclination of the molecules can best be 
 accounted for by a combination of the two limiting cases , i.e. , a 
 sequence of homochiral dimers with alternating handedness . as a result , al  bl  
 as shown in figure 10 is obtained where  a  
 and b  refer to the two inequivalent lattice sites 
 and  r  ( right ) and  l  ( left ) denote 
 the two different 2d - chiral enantiomers . we want to emphasize that 
 ( i ) we have no experimental proof for this peculiar stacking sequence , 
 apart from the intuitive arguments concerning the lateral interaction 
 strength , and ( ii ) we do not know the exact position and orientation 
 of the two molecules within the unit cell , except that the molecules 
 should be ( nearly ) parallel and aligned , within a few degrees , along 
 the b - axis ( 9 with respect to the nearest 
 110 axis of the substrate ) . the biggest uncertainty 
 is the amount of the offset along the b - axis between 
 molecules a and b. varying this parameter (  ) , we can make the 
 stacking such that the hydrogen bonds between neighboring molecules 
 either form straight lines ( as chosen in figure 10 ) or zigzag chains across the molecules . clearly , submolecularly 
 resolved stm images and/or theoretical calculations are needed to 
 clarify these points . a direct consequence of the suggested 
  ar  br  al  bl stacking is that the actual periodicity along the rows ( a - direction ) is increased by a factor of 2 , so that the actual superstructure 
 unit cells taking into account the 2d - chirality of the molecules should 
 be given by3for the two 
 mirror domains . each unit cell 
 thus contains four molecules ( two entantiomers with opposite handedness 
 for each of the two sites a and b ) . the fact that neither the stm 
 images nor the leed patterns are sensitive to the doubling of the 
 periodicity from a to 2a along the 
 molecular rows is easily explained : the doubling of the periodicity 
 is solely due to the different handedness of adjacent quinacridone 
 dimers . our stm studies were conducted at room temperature , and the 
 images do not allow to resolve the fine contrast between different 
 enantiomers . as far as the leed measurements are concerned , the atom 
 factors for the backscattering of 30 ev electrons for the elements 
 carbon , nitrogen , and oxygen are almost the same , while the contribution of the hydrogen atoms can be totally 
 neglected . in addition , the symmetry of the quinacridone molecules 
 reduces the effect of the 2d - chirality even further : the contribution 
 to the molecular structure factor of 18 carbon atoms , which do not 
 change their relative positions upon mirroring the quinacridone molecule 
 along its long axis , clearly outweigh the contribution of the two 
 carbon , two oxygen , and two nitrogen atoms , which actually exchange 
 positions . therefore , the diffraction contrast between the two enantiomers 
 of the quinacridone molecules adsorbed on the surface is likely too 
 small to provide enough intensity at the half - integer positions along 
 the a*-direction in the leed pattern . the 
 structures of the first layer of quinacridone can best be compared 
 to the  phase discussed by paulus and co - workers 
 in ref ( 9 ) . table 1 shows our data vis -  - vis the ( 111 ) 
 plane of this bulk polymorph . only the  polymorph 
 exhibits a primitive unit cell . for the other polymorphs , the unit 
 cell contains two molecules which are tilted with respect to each 
 other . in the  polymorphs , each molecule interacts with four 
 of its neighbors via hydrogen bonds in a nonplanar geometry . in the 
 case of the  and the  polymorph , neighboring molecular rows do not share the same plane but are tilted 
 or vertically shifted , depending on the crystal plane that is used 
 to cut through the crystal . a rather flat configuration is obtained 
 for the low index ( 112 ) plane of the  polymorph . therefore , we compare the quasi - two - dimensional monolayer structures 
 of quinacridone on ag(111 ) with this particular plane . z represents 
 the number of molecules per unit cell given by the length of the vectors a = |a| and b = 
 |b| and the angle (  ) between the 
 two . the final column contains the calculated area per molecule , i.e. , a = cos ab / z . for the as - prepared and 
 the annealed monolayer of quinacridone 
 on ag(111 ) , an intermolecular spacing of about 0.7 nm is found along 
 the stacking direction . this is quite similar to the distance between 
 adjacent molecules along the   direction of the  polymorph , which amounts to 0.69 nm . along this direction parallel 
 aligned molecules form hydrogen bonded stacks and can thus maximize 
 their binding energy . whereas also the direction and the length 
 of the second unit cell 
 vector b of the annealed phase matches almost 
 perfectly the values of projected bulk unit cell , there are significant 
 differences between the as - prepared structure of quinacridone and 
 its  polymorph . as quantified in table 1 , the annealed monolayer of quinacridone is considerably 
 denser than the as - prepared monolayer at room temperature . in the 
 early stage of deposition , the substrate surface will impose a particular 
 adsorption geometry ( with a mirror symmetric orientation for the two 
 2d - chiral enantiomers ) . other quinacridone molecules will preferentially 
 attach via strong hydrogen bonds in a parallel configuration , thus 
 forming straight enantiopure stacks which assemble into extended domains 
 of molecular rows . although the packing density in the nonannealed 
 phase is rather low , this structure is stable for days . therefore , 
 it is likely that the energy barrier required to initiate the transition 
 from the nonannealed phase into the more densely packed heterochiral 
 phase is considerably higher than the thermal energy available at 
 room temperature . in fact , the temperature required to trigger the 
 transformation is only slightly lower than the sublimation temperature 
 of the  quinacridone bulk phase . this makes sense 
 if one considers that the transition from the homochiral phase into 
 the heterochiral phase upon annealing requires the breaking of the 
 strong hydrogen bonds between molecules originally arranged in homochiral 
 stacks . the growth 
 and the monolayer stuctures of quinacridone on ag(111 ) 
 were investigated by peem , stm , and leed . the quinacridone molecule 
 is prochiral , since it acquires a handedness when adsorbed flat on 
 a surface . for room temperature deposition a   superstructure 
 and its 
 mirror domains are 
 obtained . the structure is characterized by stacks of quinacridone 
 molecules of a few nanometers length , which assemble into quasi - one - dimensional 
 molecular rows . the individual stacks and the row direction is oriented 
 at + 10.9 and 10.9 with respect to the 112 
 symmetry axis of the ag(111 ) surface . the perpendicular orientation 
 of the molecules in the stacks and along the rows is a strong indication 
 that these are composed of molecules with the same handedness . ( probably molecules 
 with the  wrong  handedness ) , which introduce a large 
 number of antiphase boundaries . upon annealing at high temperature , 
 the monolayer structure changes 
 completely . from the stm and leed data a well - ordered   superstructure , 
 containing two 
 molecules 
 per unit cell , and its mirror image are inferred . in the annealed phase the quinacridone molecules also form extended rows , but the 
 molecules are no longer oriented perpendicular to the row direction . in addition , every second molecule is shifted along the b - axis of the superstructure unit cell , such that rows are periodically 
 indented . instead , we propose a stacking sequence of homochiral pairs 
 of molecules with alternating handedness . taking this heterochiral ordering into account would double the periodicity along the rows 
 ( a - direction ) , leading to a   superstructure ( with four atoms 
 per unit 
 cell ) and the corresponding mirror image . the observed monolayer 
 structures are the result of two competing 
 effects : ( i ) the interaction between the quinacridone molecules , in 
 particular , the strong hydrogen bonds connecting neighboring molecules 
 in the stacks , and ( ii ) the 2d - chirality of the adsorbed quinacridone 
 molecules which determines the arrangement of the molecules within 
 these stacks . as a result , two very distinct structures can be stabilized 
 on the ag(111 ) surface for which the particular stacking sequence 
 is intimately linked with the different chiral ordering . finally , 
 the interaction between the quinacridone molecules and the ag(111 ) 
 surface is responsible for the flat - lying geometry of the molecules 
 as well as the particular epitaxial relationships of the monolayer 
 structures , namely the specific orientations of the molecular rows 
 and the commensurability of the superstructures . in order to 
 transform the ( metastable ) nonannealed homochiral phase 
 into the denser heterochiral phase , the former has to be annealed 
 at very high temperature , as the chiral reodering requires the breaking 
 of the strong hydrogen bonds .