during the last few years , the use of 
 hydrogen - bonded pigments 
 has attracted significant interest in organic electronics , due to 
 their promising charge transport mobilities . apart from being 
 biodegradable and nontoxic , an important characteristic of this material 
 class is the typically high environmental stability and 
 their ability to form hydrogen bonds , which are strong intermolecular 
 interactions compared to van der waals interactions . in addition , 
 hydrogen bonds are directional interactions . as a consequence , the 
 molecular arrangement within the crystalline structure deviates significantly 
 from the known molecular packing motifs of aromatic hydrocarbons . for example , the common organic pigment quinacridone 
 is known to form ( at least ) three significantly different types of 
 crystal structures , which are characterized by different hydrogen - bonded 
 networks . however , for hydrogen - bonded 
 pigments , the occurrence of surface - induced phases ( specific polymorphs 
 within thin films mediated by the presence of a substrate surface 
 during the crystallization ) is not reported . such surface - induced 
 phases are frequently observed for rod - like conjugated molecules , 
 where a strong influence to application relevant physical properties 
 is observed . in the field of organic 
 electronics , the structural characteristics 
 of aromatic molecules such as pentacene in thin films have been intensively 
 investigated as organic electronic devices are typically based on 
 such architectures . likewise , scientific interest has recently increased for thin films 
 of hydrogen - bonded pigments . the film formation and crystallization 
 at surfaces was studied for indigo and quinacridone , 
 where , in addition , the electronic structure was investigated . for these systems , stronger intermolecular interactions due to hydrogen 
 bond formation and the presence of stronger bonds toward the substrate 
 result in lower diffusion rates and smaller crystallites . nevertheless , the preferred molecular orientation 
 can be influenced by substrate surface engineering . in this work , the structural properties of thin films 
 of 6,6-dibromoindigo , 
 commonly known as tyrian purple , are studied . the crystal bulk cell 
 is well - known , and in addition , a new crystalline 
 phase of tyrian purple within thin films has been reported , but not yet characterized . its crystallographic 
 properties are investigated as a function of temperature and film 
 thickness , from the sub - monolayer regime up to thick ( multilayer ) 
 films . our study confirms the existence of this yet uncharacterized 
 polymorph of tyrian purple and provides a crystal structure solution ; 
 its thermodynamic stability is also explored . tyrian purple powder was synthesized following the 
 previously reported 
 procedure and purified by temperature 
 gradient sublimation . a first set of thin films was deposited by hot 
 wall epitaxy ( hwe ) using a base reactor pressure of 1  10 mbar . pristine , polyethylene - coated 
 glass , as well as thermally oxidized 
 silicon wafers with an oxide thickness of 150 nm served as substrates 
 and were preheated to the desired growth temperature for 30 min , in 
 order to ensure a stable substrate temperature during the deposition 
 process . the material was deposited for 60 min using a source / wall 
 temperature of 240 c/260 c . a second set of samples 
 was prepared by physical vapor deposition 
 ( pvd ) , in order to have better control over the film thickness , especially 
 in the monolayer regime . tyrian purple powder was purchased from endotherm 
 ltd . and used without any further purification . a steel knudsen cell 
 and a base pressure of 3  10 mbar were used . plasma - etched and chemically cleaned silicon substrates ( 150 nm thermal 
 oxide ) were used . the chemical cleaning procedure involved sonication 
 in acetone for 10 min and subsequent rinsing with isopropanol . the 
 plasma cleaned substrates were prepared using a femto plasma cleaner 
 by diener electronic after the chemical cleaning procedure . the oxygen 
 plasma was ignited for 42 s. the plasma etching changes the surface 
 energy of the substrate from 61 mn / m after the chemical cleaning to 
 73 mn / m after plasma etching , where the difference could be assigned 
 solely to changes in the polar part of the surface energy . films were 
 deposited with a deposition rate of 0.18 nm / min with final nominal 
 thicknesses between 0.6 and 9 nm , as determined with a quartz crystal 
 microbalance ; the substrates were kept at room temperature . atomic force microscopy ( afm ) studies were performed using a digital 
 instruments dimension 3100 in tapping mode . the images were acquired 
 at a scan speed of 5 m / s using sic tips exhibiting a cone angle 
 of 40. nominal values for the resonance frequency and the tip 
 radius were 325 khz and 10 nm , respectively . specular x - ray 
 scattering studies were performed with a panalytical 
 empyrean system using k radiation from a sealed 
 cu - tube . at the primary side , a multilayer x - ray mirror was used to 
 generate a monochromatic (  = 1.54  ) and parallel beam 
 with a height of 100 m ; at the secondary side , a 100 m 
 receiving slit and a 0.02 rad soller slit were used in combination 
 with a panalytical pixcel 3d detector acting as a point detector . 
 the x - ray diffraction ( xrd ) as well as the x - ray reflectivity ( xrr ) data are plotted as a function of the scattering vector qz via qz = 4/ sin  ; 
  being the wavelength and  half of the scattering angle 
 2. the x - ray reflectivity data were fitted using the software stochfit(27 ) to determine an electron 
 density profile across the thin film . the error of the peak position 
 is determined from the experimental shifts of the critical angles 
 to qz =  
 0.002  . the size of the crystallites was 
 determined by the scherrer equation using the full width at half - maximum 
 of the bragg peaks . in - situ temperature studies were performed by 
 means of a dhs900 heating stage ( anton paar ltd . ) . the experiments 
 were performed under inert conditions using a dynamic flow of helium . , the samples were heated to the next temperature 
 with a heating rate of 50 c / min . grazing incidence x - ray 
 diffraction ( gixd ) was performed at the 
 beamline id10 ( esrf grenoble , france ) using a pseudo z - axis geometry . a wavelength of 0.564 the incidence 
 angle was chosen between 0.075 and 0.09 , which is slightly 
 below the critical angle of the substrate , in order to minimize the 
 signal from the substrate and to enhance the diffracted intensity 
 due to the evanescent wave . the diffracted signal was recorded by 
 a pilatus 1 m detector mounted on a goniometer . the typical illumination 
 time was 30 s. the diffraction pattern was transformed into reciprocal 
 space using the software library xrayutilities . the calculation of the peak positions and structure 
 factors was performed using the custom - made software pygid . the molecular packing within 
 the experimentally determined unit 
 cell has been calculated by theoretical modeling , where a combination 
 of molecular dynamics ( md ) and density functional theory ( dft ) was 
 applied . md simulations were carried out using the lammps code in combination with the charmm general force 
 field v. 2b7 . in a first step , several 
 hundred trial structures were created by placing one molecule randomly 
 into a slightly expanded unit cell . during the subsequent md run , 
 the system was allowed to relax while the unit - cell was shrunk continuously 
 to its experimental size . the most promising structures were further 
 refined using dft geometry optimizations as implemented in the castep 
 program . ultrasoft pseudopotentials were 
 used in combination with a plane wave cutoff energy of 280 ev . pack 
 grid with a density of 0.05  was used for choosing the k - points . figure 1 depicts 
 afm micrographs of tyrian purple thin films grown on polyethylene 
 surfaces , which were prepared at substrate temperatures of 50 , 100 , 
 and 150 c . the formation of elongated islands together with 
 elevated hillocks is observed . the latter exhibit heights of up to 
 400 nm , whereas the elongated islands have a maximum height of 80 
 nm . the lateral size of the islands increases with growth temperature : 
 at low temperatures the length of the islands is below 1 m , 
 the size of the islands then increases to several micrometers in length 
 while remaining less than 1 m in width for a growth temperature 
 of 100 c ; at 150 c the overall size of the islands is 
 further increased . interestingly , ridge like structures with a typical 
 height of several nanometers are found within the islands . partly , 
 the heights of the ridges are irregular ( cf . figure 1d ) and in some cases terrace - like structures 
 with step heights of 1.2 nm ( about the length of a single molecule ) , 
 but also of about 4 nm are found ( cf . figure 1e ) . atomic force microscopy ( afm ) of tyrian purple 
 films grown on polyethylene 
 substrates at substrate temperatures of 50 c ( a ) , 100 c 
 ( b ) , and 150 c ( c ) together with the corresponding line profiles 
 ( d , e ) taken along the white lines in b and c. specular x - ray diffraction was performed on these three films 
 grown 
 at different temperatures ; the results are depicted in figure 2 . in the case of the films 
 prepared at 150 c , bragg peaks from two different interplanar 
 distances and their higher - order reflections are observed . one peak 
 series ( labeled as 00l in figure 2 ) is observed up to sixth order , with the first order 
 peak corresponding to an interplanar distance of 12.1  . this 
 value is in good agreement with the interplanar distance of the 001 
 peak ( d001 = 12.2  ) taken from the 
 known bulk crystal structure with a unit cell of a = 11.611(12 )  , b = 4.842(2 )  , c = 12.609(16 )  , and  = 104.42(6) ; the observed intensities of the 00l peak series agree well with the respective calculated intensities 
 for the known bulk phase . therefore , we conclude that the known bulk 
 structure of tyrian purple is present within the sample with a preferred 
 orientation of the crystallites ; i.e. , their ( 001 ) planes are parallel 
 to the substrate surface ( 001 fiber texture ) . the preferred orientation 
 reveals that the molecules are standing at the substrate surface , 
 however , with a tilt angle of 40 with respect to the 
 surface normal ( the van der waals length of the molecule is 17.8  ) . note that epitaxial growth of tyrian purple on polyethylene single 
 crystals results in a different orientation with the ( 010 ) plane of 
 tyrian purple parallel to ( 110 ) of polyethylene . specular x - ray diffraction results for tyrian purple films grown 
 on polyethylene at different substrate temperatures ( see the corresponding 
 morphologies in figure 1 ) . the arrows indicate the 00l peak series of 
 the surface - induced phase ( sip ) . the first peak in the other series of peaks corresponds to 
 an interplanar 
 distance of 15.2  , where higher - order reflections are also observed 
 ( labeled as surface induced phase ( sip ) in figure 2 ; arrows indicate the peak positions ) and 
 can not be explained by any known polymorph . this peak series is also 
 indexed by 00l , since the observed interplanar distance 
 of the bragg peaks is comparable to the bulk structure . the distance 
 of 15.2  suggests that the molecules are similarly oriented 
 in this unknown polymorph , however , with a smaller tilt angle to the 
 surface normal of about 25. similar features of polymorphism 
 are found in thin films of other rod - like conjugated molecules where 
 the polymorph with the smaller tilt angle is found to be a sip . in the following discussion we will use the term sip for the unknown 
 phase and give further evidence justifying the presence of a surface - induced 
 phase in tyrian purple films . the films prepared at a lower 
 substrate temperature of 100 c 
 show both crystallographic phases , however , with strongly reduced 
 intensity of the bulk crystal structure relative to the sip . in contrast , 
 at a substrate temperature of 50 c only bragg peaks assigned 
 to the surface - induced polymorph are present . as an aside we note 
 that this temperature dependent polymorphism of tyrian purple is also 
 observed in thin films prepared on thermally oxidized silicon as well 
 as on glass surfaces ( not shown ) . in - situ temperature studies 
 were performed to investigate the thermodynamic 
 stability of crystalline tyrian purple . in figure 3 , such a temperature study of a film grown 
 on a polyethylene surface at a substrate temperature of 100 c 
 is shown . starting from room temperature , the dominant diffraction 
 peak arises due to the sip , and only weak traces of the bulk phase 
 are present . with increasing temperature , at a temperature of 210 c , a 
 sudden change in the diffraction pattern takes place : the 001 bulk 
 phase peak increases considerably in intensity , which is accompanied 
 by a sudden drop in intensity of the 001 sip peak . this phase transition 
 is also accompanied by a drastic change of the peak width . subsequent 
 cooling to room temperature did not result in a reversible behavior 
 of the phase transition , and the bulk phase remains the only one present . the irreversible nature of the phase transition reveals that the temperature 
 dependent phase behavior of tyrian purple is not enantiotropic . additionally , 
 the irreversibility of the phase transition shows that the sip is 
 a metastable phase . temperature dependent specular x - ray diffraction measurements 
 on 
 a tyrian purple film grown on polyethylene at a substrate temperature 
 of 100 c . the measurements are performed for a full temperature 
 cycle at discrete temperature steps by first heating up from room 
 temperature to 210 c and subsequent cooling down to room temperature 
 ( from top to bottom ) . heating samples beyond 210 c reveals a rather constant 
 peak 
 intensity of the bulk phase up to a temperature of 250 c ; at 
 higher temperatures the bragg peak of the bulk phase disappears . cooling 
 back to room temperature is not accompanied by the reappearance of 
 any bragg peaks as the molecular material has sublimated from the 
 substrate surface . the next step of the experimental investigations 
 was performed 
 on tyrian purple films prepared at room temperature by pvd . figure 4 shows x - ray reflectivity 
 ( xrr ) measurements of samples prepared with different thicknesses . the nominal film thickness of the films varies from a sub - monolayer 
 coverage of 0.6 nm to a monolayer coverage of 1.5 nm up to multilayer 
 coverages of up to 9 nm . the x - ray reflectivity curves show critical 
 angles between 0.193 ( qz = 0.027  ) and 0.203 ( qz = 0.029  ) , which is close to the expected critical angle of amorphous silicon 
 dioxide ( c = 0.216 , qz = 0.031  ) and that of crystalline tyrian purple ( c = 0.204 , qz = 0.029  ) . in the case of films grown on chemically cleaned surfaces ( figure 4a ) , first , a distinct 
 minimum is formed at qz  0.04  and second , a bragg peak 
 at qz  0.45  develops . the peak position of the bragg peak shifts 
 with the nominal film thickness from 0.456  ( nominal thickness 0.6 nm ) to 0.422  ( 9.0 
 nm ) ; detailed values are given in table 1 . thin films prepared on plasma - etched silicon 
 oxide surfaces show a comparable tendency of peak shift as a function 
 of nominal film thickness ( figure 4b ) . in general , the position of the bragg peak clearly 
 confirms that the sip is present within the thin films . nevertheless , 
 the interplanar distance varies between 14.9 and 13.8  for crystallites 
 grown on chemically cleaned silicon oxide and 15.1  to 14.7 
  on the plasma - etched silicon oxide surface . this might suggest 
 that in the very low thickness regime , a slight variation of the molecular 
 packing occurs . another possible explanation for this peak shift could 
 be a coherent superposition of x - ray reflectivity from a molecular 
 underlayer with x - ray diffraction due to three - dimensional crystallites , 
 as has already been observed for the initial growth of pentacene in 
 thin films . the presence of bragg peaks 
 for nominally sub - monolayer thick films reveals that island formation 
 dominates the film growth . estimated crystallite sizes , calculated 
 on the basis of the peak width , are given in table 1 . x - ray reflectivity of 
 tyrian purple films of different nominal 
 thicknesses prepared on silicon oxide ( a ) and on plasma - etched silicon 
 oxide ( b ) . for ultrathin films no reliable 
 values fits of the x - ray reflectivity curves 
 were performed to obtain 
 total electron density profiles across the thin film . figure 5a shows the fitted curves superimposed 
 on the experimental data , the electron density profiles obtained from 
 the fits are depicted in figure 5b . oscillations of the electron density are due to 
 the regular crystalline packing across the layers . a slight reduction 
 of the electron density is observed at the substrate / organic interface 
 around z  1 nm . it seems that the order at 
 the interface is better developed for the chemically cleaned surface , 
 since the oscillations are sustained down to the silicon oxide interface . the continuous reduction of the electron density at z > 7 nm appears due to surface roughness . ( a ) comparison of the 
 experimental data with the theoretical fit 
 for films of 9 nm nominal thickness prepared on chemically cleaned 
 ( top ) and plasma - etched ( bottom ) silicon oxide . ( b ) electron densities 
 as a function of the surface normal coordinate z , 
 as deduced from the fits . d denotes the periodic oscillations of the 
 electron density across the 001 planes ; the chemical structure of 
 tyrian purple is shown as an inset . the curves are vertically shifted 
 for clarity ( a , b ) . after the detailed investigations 
 of the out - of - plane order by 
 x - ray reflectivity and specular x - ray diffraction , the in - plane order 
 of the films is studied by grazing incidence x - ray diffraction ( gixd ) . characteristic gixd patterns are given in figure 6 for films of 9 nm nominal thickness prepared 
 on chemically cleaned ( figure 6a ) and plasma - etched silicon oxide surfaces ( figure 6b ) . reciprocal space maps 
 calculated from grazing incidence diffraction 
 data of nominally 9 nm thick tyrian purple films grown on chemically 
 cleaned ( a ) and plasma - etched silicon oxide ( b ) . calculated positions 
 of the bragg peaks are given by black points . the areas of the circles 
 in ( b ) are proportional to the calculated structure factors of the 
 solved crystal structure of the surface - induced phase . strong bragg peaks reveal crystallographic in - plane 
 order within 
 the thin films . rods with distinct bragg peaks along qz are found at qxy values of 1.08 , 1.63 , and 1.91  . the indexation of bragg peaks corresponding to the 
 surface - induced structure is performed on the basis of assigning the 
 peaks observed in the specular scan ( qxy = 0 ) as 00l as described above ( i.e. , the 001 plane 
 is parallel to the substrate surface ) ; as a consequence , the crystallographic 
 axes a and b are parallel to the 
 substrate surface . in a subsequent step , the bragg peaks along qxy = 1.08  are assigned to the 0  1l peak series , peaks along qxy = 1.63  to 10l and peaks along qxy = 1.91  to 11l . the lattice 
 constants of a triclinic unit cell can be determined by using three 
 clearly defined qxy values 
 and three qz values together 
 with their corresponding miller indices . analytical expressions are 
 derived to calculate the lattice constants for a triclinic unit cell 
 with the ab - plane parallel to the substrate surface 
 ( see eqs 1 and 2).12whereandby 
 solving the first set of eq 1 the unit cell parameters which 
 lie in - plane , namely a , b , and  can be determined . qxy values from three independent 
 bragg peak series ( here 01l , 10l and 11l ) are required . solving the 
 second set of eqs 2 leads 
 to parameters  and  , using the relationsand the lattice 
 constant c can be calculated 
 from   usingthe most convenient 
 method to solve eqs 1 and 2 is by calculating the inverse matrices . in the case of the film prepared on the chemically cleaned silicon 
 oxide ( figure 6a ) , the following values were used for the calculation of a , b , and  : qxy = 1.08  , qxy = 1.63  , and qxy = 1.91  with miller indices 01l , 10l , and 11l , respectively . to determine  ,  , and c , three peaks with different qz values are taken : qz = 0.32  ( 101 ) , qz = 0.75  ( 102 ) , 
 and qz = 1.01  ( 012 ) ; the resulting lattice constants are given 
 in table 2 . the volume 
 of the crystallographic unit cell is 328.9  , which 
 is slightly smaller than the volume of one tyrian purple molecule 
 in the bulk crystal structure ( 340.9  ) . this shows that the unit cell contains only 
 one molecule . in the case of the 
 film prepared on the plasma - etched surface ( figure 6b ) , the following 
 peaks and q values were used to calculate the unit 
 cell : 011 ( qxy = 1.05 
  / qz = 0.50 ) , 102 ( qxy = 1.64  / qz = 0.78  ) , 110 ( qxy = 1.90  / qz = 
 0.15  ) , which results in slightly different 
 lattice constants and a slightly enlarged volume of the unit cell 
 compared with the structure found on chemically cleaned substrates 
 ( cf . table 2 ) . on the basis of the experimentally determined crystallographic 
 unit cell of the surface - induced phase of tyrian purple on a plasma - etched 
 silicon oxide surface , molecular dynamics ( md ) simulations were performed 
 to determine the molecular packing and give a complete crystal structure 
 solution . initial starting structures were generated by expansion 
 of the unit cell volume by 30% and the random alignment of a single 
 molecule . a short md run of 10 ps was performed at 150 k , during which 
 the unit cell was shrunk back to its experimental dimensions . the 
 resulting structures were sorted in groups based on the simulated 
 powder diffraction pattern and the total energy . the molecule was 
 allowed to bend during the simulation , but due to the structure of 
 the chemical bonds within the molecule the best solutions were straight 
 again . the best structures were used as a starting point for further 
 md simulations in which the initial volume of the unit cell was expanded 
 only by 5% and the alignment of the single tyrian purple molecule 
 rocked randomly by 0.3 degrees along cartesian directions . the structures 
 with the lowest energies were selected and optimized with density 
 functional theory . all selected solutions pack in an identical way 
 as could be expected from basic packing consideration of a single 
 molecule with the given size of the crystallographic unit cell . the 
 molecules form stacks with their aromatic planes along the a - axis , hydrogen bonding along the b - axis 
 and a slight tilt of the long molecular axis relative to the pole 
 of the ab - plane . the final structure was selected 
 on the basis of the experimental intensities by calculating structure 
 factors and comparing them to the experimental peak intensities . please 
 note that due to experimental limitations this comparison is not fully 
 correct since experimental geometry factors , e.g. lorentz and polarization 
 factors , sample absorption , area factor , etc . , are not taken into 
 account . in the case of the 9 nm film prepared 
 on plasma - etched silicon 
 oxide , the crystal structure with the best agreement is compared to 
 the experimental diffraction data in figure 6b . bragg peaks are represented by circles 
 where the exact position is given by their centers and the structure 
 factors by their areas . overall , we find an excellent agreement for 
 the bragg peaks along qxy = 1.05  and qxy = 1.64  where the intensity 
 variation of the peaks matches the calculated structure factors . the 
 solution of the surface - induced phase of tyrian purple is provided 
 as ccdc 1449758 . the molecular arrangement in relation to 
 the crystallographic unit 
 cell is depicted in figure 7a . tyrian purple forms molecular -stacks with the aromatic 
 planes closely packed parallel to each other . within one stack the 
 average distance between the aromatic planes of neighboring molecules 
 is 3.27  . hydrogen bonds connect two stacks along the b - axis , where a single molecule of one stack is connected 
 laterally to a single molecule in the neighboring stack . in total , 
 four hydrogen bonds are formed by a single molecule , they all have 
 an equal distance of 2.80  between the nitrogen and oxygen atoms . 
 along the c - axis , the stacks are separated by layers 
 of bromine atoms with an intermolecular distance of 3.90  . since 
 the shortest distance between neighboring bromine atoms is larger 
 than the sum of the van der waals radii ( 3.7  ) , it can be concluded 
 that halogen interactions are not decisive for the molecular packing . ( a ) molecular packing of tyrian purple 
 molecules in the surface - induced 
 crystal phase , as viewed along the b - axis . ( b ) the 
 bulk structure of tyrian purple viewed along the b - axis with the depiction of two consecutively arranged molecular 
 stacks , the stack behind is displayed with light gray atoms . the 
 present molecular packing is similar to that of the bulk structure 
 of tyrian purple , as depicted in figure 7b . there , likewise , molecular stacks are 
 formed with an average distance between the aromatic planes of 3.45 
  . however , molecules in neighboring stacks are tilted in opposite 
 directions so that a single molecule of one stack is connected to 
 two molecules of a neighboring stack . even in that case , one molecule 
 forms four hydrogen bonds with a hydrogen bond distance of 2.95  . the crystallization of the molecule tyrian purple was investigated 
 in thin films on the weakly interacting surfaces polyethylene and 
 silicon oxide . the variation of thin film preparation conditions such 
 as substrate temperature and film thickness , combined with x - ray diffraction 
 studies , reveals two key results . first , a surface - induced polymorph 
 with a previously unknown crystal structure is found when the thin 
 film preparation is performed at a substrate temperature of 50 c , 
 while at elevated substrate temperatures of 150 c the known 
 bulk phase of tyrian purple forms ( cf . figure 2 ) . second , we observe a non - negligible dependence 
 of the crystal structure on the type of silicon oxide surface employed 
 ( cf . the formation 
 of the new polymorph of tyrian purple in thin films 
 is in analogy to other aromatic molecules , where surface - induced phases 
 are clearly assigned . rod - like , rigid 
 molecules like pentacene and its derivatives , -sexithiophene , diindenoperylene , but also rod - like molecules with flexible ( alkyl- ) 
 side chains like dioctyl - terthiophene ( dott ) or dioctyloxy - benzothienobenzothiophene 
 ( c8o - btbt - oc8 ) crystallize 
 with their long molecular axes approximately perpendicular to the 
 substrate surface together with an overlap of the aromatic units parallel 
 to the substrate surface . moreover , a strong interplay between growth 
 kinetics and sip formation has been clearly demonstrated . we observe that the formation of the tyrian 
 purple sip is favored at low temperatures , analogously to pentacene , but in contrast to the sips of p - sexiphenyl and -sexithiophene , which appear at elevated temperatures . in - line with our observations , surface - induced phases often show 
 a metastable character as observed by solvent treatment , thermal annealing , 
 and aging experiments . our growth studies reveal 
 that tyrian purple shows island growth 
 ( volmer  weber type ) , since already at sub - monolayer coverage , 
 the 001 bragg peak is observed in specular x - ray diffraction , which 
 reveals three - dimensional growth . this is in clear contrast to the 
 film growth of other organic electronic molecules on silicon dioxide 
 surfaces . there , the formation of an initial closed first monolayer 
 is often observed where the molecular packing may deviate slightly 
 from the molecular packing of the subsequently formed three - dimensional 
 crystallites . the gixd patterns of the surface - induced phase prepared on 
 the 
 chemically cleaned and the plasma - etched surfaces are slightly shifted 
 with respect to each other ( cf . figure 6 ) . determination of the crystallographic unit cells 
 by indexing the reciprocal space maps gives slightly different values 
 for the lattice constants ( cf . surprisingly , no differences are observed for the 
 lattice constant a , some variations are observed 
 for the lattice constants b and c. comparing this result to the molecular packing within the surface - induced 
 polymorph ( figure 7a ) reveals that the  stacking of the tyrian 
 purple molecules forms along the a - axis and therefore 
 does not vary . this means that the formation of stacks of parallel - arranged 
 tyrian purple molecules is only weakly influenced by different substrates . however , since the lattice parameter b varies considerably 
 and the hydrogen bonds are directed along this axis , this suggests 
 that the different hydrogen bonding geometries of neighboring molecular 
 stacks might be responsible for the observed variation in the molecular 
 packing . note that the main difference between the bulk and the surface - induced 
 structure of tyrian purple is , indeed , the hydrogen bonding geometry 
 between neighboring stacks . in the surface - induced phase two neighboring 
 stacks are oriented parallel to one another , while in the bulk phase 
 one stack is tilted with respect to its neighboring stack ( cf . comparably small 
 variations of the molecular packing have also been reported recently 
 for tips - pentacene , where , additionally , a marked influence on the 
 performance of organic electronic devices was found . to summarize , a surface - induced polymorph 
 of tyrian purple was 
 found via physical vapor deposition on weakly interacting surfaces , 
 namely , silicon oxide and polyethylene . this phase forms at substrate 
 temperatures below 50 c , while at a substrate temperature of 
 100 c the well - known bulk structure of tyrian purple forms . heating experiments show that the surface - induced phase undergoes 
 a solid - state phase transition to the bulk phase at 210 c . the 
 irreversibility of this transition reveals the metastable character 
 of the surface - induced phase . the formation of the surface - induced 
 phase is studied at two different 
 silicon oxide surfaces : the first is a plasma - etched surface with 
 a higher polar part of the surface energy than the second , chemically 
 cleaned , surface . in both cases , a closed monolayer of up - right standing 
 molecules does not form and instead the formation of three - dimensional 
 islands is already observed in the nominal sub - monolayer regime . volmer weber 
 type growth behavior is identified by the appearance of bragg peaks 
 at nominal coverages in the sub - monolayer regime . calculated electron 
 densities from xrr investigations reveal reduced electron densities 
 close to the substrate / organic interface and also a highly rough thin 
 film surface due to the presence of islands . slight variations of 
 the bragg peak positions are observed as a function of the crystallite 
 size and the substrate type . the crystal structure of the surface - induced 
 phase is solved for 
 a thin film with a nominal thickness of 9 nm grown on a plasma - etched 
 silicon oxide surface . grazing incidence x - ray diffraction patterns 
 were successfully indexed , and the lattice constants of a triclinic 
 unit cell were deduced using analytical expressions . these unit cell 
 dimensions were used as input for the determination of the molecular 
 packing by combining molecular dynamics simulations with density functional 
 theory calculations . it is found that the packing motif of this novel 
 polymorph is comparable to the bulk phase ; tyrian purple molecules 
 stack with their aromatic planes parallel to each other while the 
 molecular stacks are interconnected by hydrogen bonds . the main difference 
 between the two polymorphs is the nature of the hydrogen bond network 
 between the neighboring stacks . for the surface - induced phase grown 
 on two different surfaces , we observed small packing differences which 
 are also due to slightly varying hydrogen bond geometries , as variations 
 of the unit cell parameter along the direction of the hydrogen bonds 
 are experimentally observed .
the 
 appearance of surface - induced phases of molecular crystals 
 is a frequently observed phenomenon in organic electronics . 
 however , 
 despite their fundamental importance , the origin of such phases is 
 not yet fully resolved . 
 the organic molecule 6,6-dibromoindigo 
 ( tyrian purple ) forms two polymorphs within thin films . at growth 
 temperatures of 150 c , the well - known bulk structure forms , 
 while at a substrate temperature of 50 c , a surface - induced 
 phase is observed instead . in the present work , the crystal structure 
 of the surface - induced polymorph 
 is solved by a combined experimental 
 and theoretical approach using grazing incidence x - ray diffraction 
 and molecular dynamics simulations . 
 a comparison of both phases reveals 
 that  stacking and hydrogen bonds are common 
 motifs for the intermolecular packing . 
 in - situ temperature studies 
 reveal a phase transition from the surface - induced phase to the bulk 
 phase at a temperature of 210 c ; the irreversibility of the 
 transition indicates that the surface - induced phase is metastable . 
 the crystallization behavior is investigated ex - situ starting from 
 the sub - monolayer regime up to a nominal thickness of 9 nm using two 
 different silicon oxide surfaces ; island formation is observed together 
 with a slight variation of the crystal structure . 
 this work shows 
 that surface - induced phases not only appear for compounds with weak , 
 isotropic van der waals bonds , but also for molecules exhibiting strong 
 and highly directional hydrogen bonds .