organic thin films have 
 attracted considerable interest in the 
 recent past , most notably due to new manufacturing steps and the possibility 
 to create ultrathin and lightweight devices with extreme bending stability , that are very promising in the field of organic electronics . through observing current and projected environmental 
 problems , the aspect of biodegradability has progressively risen in 
 importance and
might now be the most crucial argument in pursuing 
 research in the field of organic semiconductors .
quinacridone 
 ( qa , c20h12n2o2 ) and other 
 high - performance organic pigments have particularly been in the focus 
 of attention , attributable to the formation of intermolecular hydrogen 
 bonds .
quinacridone , also known as linear 
 trans - quinacridone , has a molecular mass 
 of 312.32 amu and was first synthesized 1935 by liebermann et al . the discovery of its polymorphism and more efficient 
 synthesis methods years later consequently led to simpler synthesis routes and 
 has provoked considerable interest in the scientific community ever 
 since .
current literature research 
 unveils a formidable amount of works elucidating how adding various 
 kinds of solubilizing groups to the quinacridone molecule gave rise 
 to a number of new applications , most notably as photodetectors and both donor and acceptor molecules in organic 
 solar cells .
their general insolubility , in turn , makes physical 
 vapor deposition the method of choice for manufacturing thin layers , 
 which is in the focus of the present publication .
it is well - known 
 that preparation parameters such as substrate temperature , substrate 
 conditions , base pressure , and deposition rate determine the morphology 
 and hence the electronic structure and optical properties of thin 
 films grown by vapor deposition . despite recent reports of air - stable 
 quinacridone field - effect transistors with relatively high carrier 
 mobilities of 0.2 cm/(v s )
, there is still a lack of knowledge concerning the kinetics of vacuum 
 deposition and film formation on industrially relevant silicon dioxide 
 substrates .
comparable investigations on the kinetics of adsorption , 
 layer growth and desorption exist for a number of organic molecules 
 ( e.g. , pentacene on sio2 and 
 rubicene on sio2 ) , while the 
 focus has only recently shifted to h - bonded semiconductors . in this work
, we focus on the growth and desorption behavior 
 of 
 quinacridone on / from sio2 substrates under ultrahigh vacuum 
 conditions and report multiple collision - induced thermal decomposition 
 processes both within the knudsen evaporation cells and on the substrate 
 surface .
thermally induced cracking leads to a formation of multiple , 
 typically undefined fragmentation products that may influence , e.g. , 
 transistor characteristics .
it is therefore of high importance to 
 study molecular decomposition and the underlying driving forces on 
 a chemical level and also to evaluate its occurrence and magnitude 
 upon variation of the deposition methods . complementing the in situ 
 methods of thermal desorption spectroscopy ( tds ) and auger electron 
 spectroscopy ( aes ) , ex - situ analysis by atomic force microscopy ( afm ) , 
 specular x - ray diffraction ( xrd ) , grazing incidence x - ray diffraction 
 ( gixd ) , and raman spectroscopy were used to address this issue .
for all of our experiments sublimation 
 purified quinacridone , provided 
 by tokyo chemical industry with a purity of > 99% , was used .
typically , 
 the material was deposited , after proper outgassing , i.e. , the evaporation 
 of smaller and more volatile impurity molecules during the heating 
 process , via physical vapor deposition from a stainless steel knudsen 
 cell heated to about 620 k. the inner diameter of the cell was 7 mm , 
 the length was 20 mm , and the effusion hole diameter was 1 mm ( see 
 insert in figure 1 ) . 
 for several experiments it was necessary to switch to a glass knudsen 
 cell or other glass evaporation sources , explained in more detail 
 below . as substrate material ,
silicon dioxide with a nominal thickness 
 of 150 nm , that has been thermally grown on 0.67 mm thick si(001 ) 
 wafers ( siegert wafer gmbh ) , was used .
the 1  1 cm large substrates were mounted onto a stainless steel heating plate 
 via four tantalum clamps .
attached to the steel plate are two tantalum 
 wires for resistive heating and a ni / nicr thermocouple to complete 
 the feedback loop , necessary for thermal desorption measurements . 

the deposition process was conducted 
 with a typical deposition rate of 105 ng/(mincm ) , 
 equivalent to 0.7 nm / min and controlled by a quartz microbalance , 
 which could be placed intermittently at the sample position .
deposition 
 and in situ characterization were carried out in an ultrahigh vacuum 
 system housing a pfeiffer qms 200 mass spectrometer and a staib instruments 
 auger electron analyzer .
the base pressure of the unbaked uhv chamber 
 was typically 6  10 mbar .
furthermore , the 
 whole sample stage was ln2 cooled to 200 k during film 
 deposition , if not stated otherwise .
an analysis of the chemical 
 composition of the untreated substrate 
 surface via aes revealed silicon and oxygen peaks originating from 
 the substrate , with a slight carbon contamination on top .
an auger analysis of vapor deposited quinacridone 
 films showed the expected carbon , nitrogen and oxygen signals .
unfortunately , 
 it was not possible to perform continuous auger surveillance during 
 different stages of the film growth process , due to the destructive 
 behavior of the electron beam on the organic films . after material 
 deposition the samples were placed in front of a quadrupole mass spectrometer 
 ( qms ) tuned to typical cracking masses of quinacridone , i.e. , either 
 to 128 or 76 amu .
continuous heating with a heating rate of 1 k / s 
 subsequently caused material desorption and made for a convenient 
 way to study film characteristics , growth , and desorption behavior 
 as well as desorption order and activation energies for desorption . in this context
, we have to emphasize that the 
 sample temperature , as measured at the back side of the silicon wafer , 
 is not the same as on the sio2 surface , due to the bad 
 heat conductivity in silicon and sio2 , as well as due to 
 a thermal bridge at the interface between the si wafer and the stainless 
 steel heating plate .
however , a temperature correction can be made 
 by comparing the multilayer desorption peak from the sample with that 
 from the metal clamps , the temperature of which can be considered 
 to be equal to the measured temperature .
more details on the conducted 
 temperature correction can be found elsewhere . when analyzing td spectra of organic molecules with a qms , 
 cracking of the molecules in the ionization chamber of the spectrometer 
 is unavoidable .
the measured cracking masses should 
 not be mixed up with possible decomposition products entering the 
 ionization chamber of the qms after desorption from the surface .
ex - situ studies of film morphologies and structures were done utilizing 
 an atomic force microscope ( nanosurf easyscan 2 ) in tapping mode employing 
 ppp - nclr-50 silicon tips from nanosensors .
specular x - ray diffraction 
 ( sxrd ) measurements have been performed on a panalytical empyrean 
 diffractometer fitted with a cu sealed tube and a multilayer mirror 
 (  = 1.54  ) on the primary side .
the secondary side was 
 equipped with a slit system and intensities were recorded using a 
 panalytical pixcel detector in 1d mode .
grazing incidence x - ray diffraction 
 ( gixd ) measurements were conducted at the kmc-2 beamline at bessy 
 ii ( berlin , germany ) using x - rays with a wavelength of 1.00  
 and a 2d cross - wire detector ( bruker ) .
an incident angle of i = 0.13 was chosen 
 to enhance the scattered intensities of the adsorbate .
the micro - raman experiments were performed with 
 a hecd - laser excitation wavelength of 325 nm on a labram hr-800 ( horiba 
 jobin yvon ) raman - spectrometer using gratings of 2400 lines / mm and 
 providing a spectral resolution of 3.92 cm.the 
 detection system was a liquid - nitrogen - cooled ccd spectrometer .
initially , 
 thermal desorption experiments were carried out after physical vapor 
 deposition of quinacridone on carbon covered sio2 from 
 a metal knudsen cell .
the thermal desorption spectra of a 4.25 nm 
 thick quinacridone film yielded a single desorption peak at about 
 500 k ( denoted  , see red line in figure 1 ) when the qms was tuned to a cracking mass 
 of 128 amu .
the cracking fragment at 128 amu was chosen because it 
 is very prominent in the quinacridone cracking pattern and showed a reasonably good signal - to - noise 
 ratio .
experimental limitations did not allow detecting the main mass 
 of quinacridone or any other fragments higher than 200 amu .
if there 
 were only a single molecular species present on the surface , as one 
 might assume from the red curve in figure 1 , then the choice of cracking mass should 
 have no significant influence and spectra for different cracking masses 
 should be comparable
. however , surprisingly , when tuning the qms to 
 the cracking mass 76 amu , a second peak at about 420 k ( denoted -peak , 
 black line in figure 1 ) appeared , in addition to the expected peak at 500 k. this points 
 toward the presence of a second , weakly bonded type of molecules with 
 a different cracking pattern and , hence , a different desorption behavior . 

indeed , a full mass scan between 35 and 150 amu during desorption 
 of the  and -peak , respectively , clearly shows different 
 cracking patterns ( figure 2 ) .
therefore , we can deduce the existence of two distinct 
 and chemically different thin - film species on the surface , despite 
 the fact that only purified quinacridone was present in the knudsen 
 cell .
the -peak and its cracking pattern are in good agreement 
 with literature data of quinacridone , and we therefore assume that it corresponds to thermal desorption 
 of pure quinacridone .
this assumption is supported by several spectroscopic 
 analyses , as will be shown below . combining the cracking pattern of 
 the -peak with specular x - ray diffraction , raman spectroscopy 
 and gixd allowed us to match this peak with indigo ( in , c16h10n2o2 ) . indeed ,
multiple spectroscopic observations of a film corresponding 
 to the -peak agree reasonably well with data from an earlier 
 publication describing the growth and desorption behavior of pure 
 indigo films that were grown on sio2 under identical conditions . however , the spectroscopic data hint at the 
 existence of an additional type of molecules in this film , which we 
 could identify to be carbazole ( ca , c12h9n ) .
thermal desorption spectra for cracking masses of 76 amu 
 ( black ) 
 and 128 amu ( red ) of a 4.25 nm thick film which was grown on carbon 
 covered silicon dioxide by evaporation of quinacridone from a stainless 
 steel cell .
adsorption temperature tad = 200 k , deposition rate a = 0.7 nm / min , heating 
 rate  = 1 k / s . the small peaks at 310 , 340 , and 380 k stem 
 from desorption from the ta mounting clamps . at this point
, the question arises whether indigo is generated 
 on the silicon dioxide surface during heating of a pure quinacridone 
 film or whether this type of molecules is already effusing from the 
 stainless steel knudsen cell . to address this issue ,
we have performed 
 the following experiments : after the growth of a thin film with the 
 described physical vapor deposition from the stainless steel knudsen 
 cell , a second sample holder with an identical si / sio2 wafer , 
 held at room temperature , was put in front of the primary sample . 

heating this primary sample to 425 k exclusively caused the molecules 
 corresponding to the -peak to desorb while the more strongly 
 bonded quinacridone remained . the employed geometry ( 1.0 cm distance 
 between the two 1 cm large samples ,
assuming a cosine 
 desorption distribution ) allowed about 1/3 of the desorbed molecules 
 to readsorb on the stationary sample , which turned out to be a convenient 
 way to create films with solely weakly bonded indigo and/or other 
 decomposition molecules ( -peak ) .
after breaking the vacuum 
 this stationary sample was installed on the heatable sample holder 
 and a tds was performed as soon as uhv conditions were reached .
the 
 desorption spectrum of this film was similar in shape to the original -peak , 
 demonstrating that this molecular species remains unchanged during 
 the described readsorption and subsequent desorption . applying 
 a similar experimental procedure ,
material corresponding 
 to the -peak was deposited on the stationary sample and a tds 
 was performed after venting and re - evacuation . in this case , solely 
 the -peak appeared in the desorption spectrum , confirming that 
 the -peak is not a result of quinacridone decomposition during 
 sample heating . thus , the indigo observed in the desorption spectrum 
 ( figure 1 ) must have 
 originated from the stainless steel knudsen cell .
indeed , a full mass 
 spectrum between 35 and 200 amu of the deposition flux leaving the 
 metal knudsen cell ( see figure 3 ) confirmed the existence of prominent cracking fragments 
 from both indigo ( -peak ) and quinacridone ( -peak ) molecules . 

thermal decomposition ( pyrolysis ) , dissociation and isomerization 
 processes involving collisions of organic molecules with hot surfaces 
 are commonly observed and well characterized in the field of organic 
 chemistry .
apparently , quinacridone seems 
 to be particularly prone to such decomposition processes , due to its 
 high sublimation temperature . however , 
 only sparse reports exist involving cracking and/or restructuring 
 of chemical bonds with the subsequent formation of new and stable 
 molecules for quinacridone - based and other h - bonded materials .
haucke 
 et al . described a smooth and homogeneous 
 transition from indigo to epindolidione in the vapor phase , if the 
 former is heated to 733 k under vacuum conditions .
berg et al . observed decreasing mobilities for quinacridone 
 organic field - effect transistors grown in vacuum after repeated temperature 
 gradient sublimation preparation circles . after having verified 
 that a decomposition process takes place 
 in the stainless steel knudsen cell the question arises as to the 
 quantitative ratio of the effusing molecules , corresponding to the 
 - and -peak .
despite the fact that both species exhibit 
 common cracking masses ( e.g. , 76 amu ) , one has to be aware that the 
 intensities of the different desorption peaks depend on the detailed 
 measurement conditions and no statements can be made on the actual 
 distribution and relative amount of the molecules on the samples . 
 to address this issue , we have performed afm measurements on films 
 prepared with the method described above , consisting either of material 
 corresponding to the  or -peak , respectively .
evaluation 
 of the cross sections of the island - like films allowed a quantitative 
 determination of the effusion rates . a flux ratio of 63  5% 
 indigo to 37  5% quinacridone was obtained .
a detailed description 
 of the calibration procedure for quinacridone decomposition in the 
 stainless steel knudsen cell will be given in a future publication . 

all given mean thickness values in this publication correspond to 
 the total amount of adsorbed material , hence including both molecules . 

the molecular ratio given above should therefore be taken into account 
 if the two types of molecules were to be analyzed separately .
accumulated 
 mass spectra ( cracking spectra ) of the desorption flux 
 in the range of 350450 k ( -peak ) and 470530 
 k ( -peak ) .
a 3 nm thick film , prepared 
 by evaporation of quinacridone from a stainless steel knudsen cell 
 was used for this experiment .
heating rate : 1 k / s , adsorption temperature : 
 200 k. cracking spectrum of the stainless steel knudsen 
 cell deposition 
 flux .
knudsen cell temperature : 650 k. in a first attempt to quantify and describe 
 the chemical cracking process and to confirm the coexistence of different 
 types of molecules on the surface , despite the fact that only purified 
 quinacridone is present in the knudsen cell , the deposition process 
 was repeated with various knudsen cells and different evaporation 
 sources . in order to check the purity of the quinacridone source material 
 and to , therefore , rule out possible contaminations ,
a 18 nm thick 
 quinacridone film was grown on a silicon dioxide sample via langmuir 
 evaporation from an open quartz glass tube ( see inset in figure 4 ) in a separate vacuum 
 chamber .
the sample was then again installed in the uhv chamber and 
 analyzed via thermal desorption .
as shown in figure 4 , only the -peak appears in the desorption 
 spectrum , indicating that the employed material is indeed pure and 
 that no material decomposition occurs in that case . in the case of 
 langmuir evaporation ( or free evaporation ) the sublimed molecules can be deposited at the substrate surface 
 without any further collision in between .
contrary , in the evaporation 
 from a knudsen cell the sublimed molecules hit many times the inner 
 cell wall before they leave the small effusion hole . for our particular 
 stainless steel knudsen cell dimensions , a distance of 5 cm between 
 sample and knudsen cell orifice and a typical deposition flux of 105 
 ng/(min cm )
we calculate that a sublimed particle hits 
 the inner walls about 400 times before escaping through the effusion 
 hole .
this corroborates our assumption of a thermally activated decomposition 
 process within the metal knudsen cell . as a next step ,
it comes 
 to answer the question whether the decomposition of quinacridone occurs 
 due to a possible catalytic behavior of stainless steel or whether 
 it can be reproduced in other knudsen - like evaporation cells . to this 
 end
this cell 
 featured a diameter of about 10 mm with a 3 mm wide nozzle .
wiring 
 of the resistive heating filament ( 0.5 mm thick ta - wires ) around the 
 cell was done in different ways , either by focusing on the nozzle 
 area ( inset in figure 5a ) or on the backside of the cell ( inset in figure 5b ) . to discuss differences in the deposition 
 behavior of this glass cell
compared to a typical metal knudsen cell , 
 one needs to first recall the physical processes happening within 
 such a cell .
clearly , the limited wiring of the heating filament , 
 combined with the poor heat conductivity of quartz glass lead to a 
 strongly varying temperature distribution along the long cell axis , 
 contrary to the required uniform heating of an ideal knudsen source . 

in the first case , the nozzle area is , due to its smaller cross - section , 
 significantly hotter than the residual cell walls . due to the missing 
 wiring at the backside we can , in turn , assume , that the back wall , 
 where all the material leaving a knudsen cell in a straight path typically 
 comes from , is significantly colder in comparison and does therefore 
 not contribute to any material deposition whatsoever .
instead , all 
 molecules that eventually leave the cell in a direction where they 
 can reach the sample surface need to have at least undergone one collision 
 under low angle with the higher temperature cell wall in the nozzle 
 area .
actually , the temperature of the nozzle is much higher than 
 needed to just evaporate the material .
therefore , the thermally activated 
 cracking process of quinacridone molecules can be expected to occur 
 more likely in such an excessively hot nozzle .
indeed , deposition 
 from a glass knudsen cell featuring dense wiring near the nozzle area 
 resulted in the complete cracking of every single quinacridone molecule , 
 hence only the -peak being visible in the tds spectrum ( figure 5a ) . on the contrary , 

the usage of a glass knudsen cell with a colder nozzle and a heated 
 backside again showed partial cracking , comparable to results from 
 a metal cell , and different molecular species were once again present 
 on the sample ( figure 5b ) .
thermal desorption spectrum for an 18 nm thick quinacridone film 
 deposited from a glass evaporation cell ( langmuir evaporation ) .
substrate 
 temperature : 300 k. thermal desorption spectra 
 for quinacridone films deposited from 
 a glass knudsen - type cell with dense wiring near the nozzle ( a ) and 
 after removal of said wiring and increased heating at the backside 
 ( b ) .
the nominal film thicknesses are 5 nm in plot ( a ) and 2.5 nm 
 in plot ( b ) .
adsorption temperature : 200 k. in order to take 
 a closer look at the morphologies of the obtained films and for a 
 possible morphological confirmation of the separate types of molecules 
 on the surface , we used ex - situ atomic force microscopy .
all images 
 were taken in tapping mode to not damage the sensitive organic films 
 and plotted using derived data . in order to distinguish quinacridone 
 islands from islands consisting of other molecules ,
it was necessary 
 to create samples with either just quinacridone or just the decomposed 
 molecules present . samples with no quinacridone molecules adsorbed , 
 i.e. , samples with indigo and possible other decomposition products , 
 were prepared by using the glass knudsen cell described above with 
 enhanced heating filament wiring near its opening ( figure 5a ) .
additionally , similar films 
 were prepared via partial material desorption ( -peak ) from 
 one sio2 sample and subsequent adsorption onto another 
 sample , as described above . both methods produced comparable films 
 and will therefore not be distinguished in the following .
moreover , 
 all samples above a certain thickness that were produced by either 
 of these methods displayed a deep blue color visible to the bare eye .
pure quinacridone samples ( with a bright pink color ) could be manufactured 
 by using the original metal knudsen cell and heating the substrate 
 to 425 k during the adsorption process , resulting in an immediate 
 desorption of the weakly bonded smaller molecules .
figure 6 shows afm images of three 
 thick films on silicon dioxide with either all types of molecules 
 ( a ) , pure quinacridone ( b ) or solely indigo and other decomposed molecules 
 ( c ) being present .
all films did not show significant dewetting during 
 venting ( confirmed by tds measurements ) and ostwald ripening or other 
 morphology changes after storage under atmospheric conditions for 
 at least 90 days , independent of molecule type and thickness .
a comparison 
 of surface morphologies for films of 5 , 60 , and 120 nm mean film thickness , 
 as measured by afm immediately after exposure to air and after 24 
 h , showed no changes in island size , shape , or number .
this suggests 
 that molecules within multilayer islands are immobile on the surface 
 and show a stable configuration once they are incorporated into their 
 respective bulk crystal structures .
figure 6a shows 
 a 42.5 nm thick film that was deposited from the metal knudsen cell . 

therefore , multiple types of molecules have been adsorbed and subsist 
 on the surface .
a multitude of islands is apparent , either round or 
 slightly elongated , with heights up to 200 nm ( cross - section figure 6b ) .
samples grown 
 from the special glass deposition cell or via substrate heating , as 
 depicted in figures 6c - e , show similarly shaped islands with comparable mean heights . 

interestingly , for the pure films we could not observe the elongated 
 islands as found for the mixed film .
initially we hoped to be able 
 to correlate the two different island morphologies to the different 
 types of molecules . while this might still be true , the different 
 experimental conditions for the individual film fabrication ( e.g. , 
 425 k substrate temperature during quinacridone deposition , high nozzle 
 temperature during indigo film preparation ) might be responsible for 
 this failure .
( a ) afm micrograph ( 8 m  8 m ) of a 
 quinacridone 
 film containing additional cracking molecules on silicon dioxide deposited 
 from a metal knudsen cell at 200 k substrate temperature ; nominal 
 thickness : 42.5 nm
( b ) cross section 
 along the line marked by the black arrow in ( a ) .
( c ) afm micrograph 
 ( 8 m  8 m ) of a 85 nm thick film of the smaller 
 cracking molecules , assumed to be mainly indigo , on silicon dioxide . 
 the film was deposited from a glass knudsen cell with enhanced heating 
 near the opening ; substrate deposition temperature : 200 k , deposition 
 rate : 0.7 nm / min .
( d ) cross section along the line marked by the black 
 arrow in ( c ) .
( e ) afm micrograph ( 8 m  8 m ) of 
 a quinacridone film on silicon dioxide deposited from a metal knudsen 
 cell at a substrate temperature of 425 k. for this film , material 
 equivalent to a nominal thickness of 170 nm was deposited onto the 
 sample , but partial desorption caused the resulting layer to be significantly 
 thinner ( estimated thickness : 63 nm ) ; deposition rate : 0.7 nm / min . 

( f ) cross section along the line marked by the black arrow in ( e ) .
quinacridone 
 is a molecule with a planar conformation that crystallizes within 
 the space group p21/c with two molecules per unit cell . in the literature , up to seven 
 different polymorph structures of linear trans -
however , only the  and -polymorphs ( not to be mixed 
 with the  and  desorption peaks ) are commercially important 
 due to -quinacridone transitioning either fully or partially 
 into -quinacridone at elevated temperatures . from the crystal bulk structure
it is apparent , that each molecule 
 is connected to two neighboring molecules in the  and -phase 
 and to four neighboring molecules in the -phase via intermolecular 
 hydrogen bonds between the carbonyl and imino groups .
these comparatively strong bonds allow for a high thermal 
 stability and a high melting point .
full crystallographic data of 
 the bulk structure has only sparsely been published due to difficulties 
 of crystal growth in solution and due to the typically insufficient 
 quality of vapor deposited crystals . in our own experiments we have investigated 
 the crystallographic properties of two differently prepared films 
 grown on sio2 with a thickness of 63 nm ( sample 1 ) and 
 85 nm ( sample 2 ) , respectively , by using both specular and grazing - incidence 
 x - ray diffraction .
a specular scan of the first sample , grown from 
 a metal knudsen cell at a substrate temperature of 425 k , showed multiple 
 out - of - plane reflections for scattering vectors qz between 0.3 and 2.2  ( figure 7 ) .
a comparison 
 with calculated diffraction data from powder cell measurements matches the peaks at 0.42 and 0.84  with 
 ( 002 ) and ( 004 ) orientations of the quinacridone -polymorph 
 structure and peaks at 0.44 and 0.89  with 
 ( 001 ) and ( 002 ) crystal orientations of the -polymorph . therefore , 
 quinacridone molecules seem to exclusively arrange in crystallographic 
 orientations where the ( 00l ) planes are parallel to the substrate .
interestingly , 
 there exist clear additional reflections which could not be attributed 
 to any quinacridone net planes .
namely , reflections at 1.38 , 1.42 , 
 and 1.63  are indications of the presence 
 of p - sexiphenyl ( 6p , c36h26 ) and/or other oligophenylenes . however , there is no evidence 
 of such molecules in the thermal desorption or mass spectra .
we believe 
 that the occurrence of 6p is due to additional thermal decomposition 
 and reaction of the impinging molecules at the relatively high substrate 
 temperature of 425 k. during the growth process of this sample quinacridone 
 molecules as well as all the described decomposition products in the 
 knudsen cell ( indigo , carbazole ) are present in the deposition flux . 

however , the latter can not form a stable phase at a substrate temperature 
 of 425 k and either instantly desorb or undergo further thermal decomposition . 
 from a chemical point of view
, both the break - off of co and nh3 molecules is quite likely , leading to a possible explanation 
 of the formation of oligophenylenes from indigo , carbazole , and quinacridone 
 molecules .
the newly formed molecules have to be thermally stable 
 on the surface at elevated temperatures , restricting the possible 
 oligophenylenes to p - sexiphenyl and larger molecules .
conducted similar experiments 
 with 105 nm thick quinacridone films grown by vacuum sublimation on 
 sio2 substrates .
xrd peaks corresponding to -(001 ) , 
 -(002 ) , -(002 ) , and -(004 ) orientations were 
 found , in accordance with our own experiments . in both phases molecules 
 are orientated almost perpendicular to the substrate with their long 
 axis tilted by only 10 and 20 with respect to the surface 
 normal .
furthermore , it was shown that the relative peak distribution 
 changes with increasing substrate temperature for the simple reason 
 that the -phase is metastable and that it can transform into 
 a -polymorph at elevated temperatures .
the -polymorph was found to be favored for thin films with 
 11 nm nominal thickness
. therefore , we assign  to an interface - near 
 substrate induced thin film phase while the -polymorph is dominant 
 within the bulk phase . additionally , sytnyk et al . recorded xrd patterns of quinacridone micro- and nanocrystals 
 synthesized at various temperatures , and observed molecular arrangements 
 that correspond to the  and -polymorph throughout the 
 measured temperature range .
the second film ( sample 2 ) was prepared 
 by using the special glass 
 cell mentioned above with increased heating near the nozzle for the 
 deposition process where desorption hinted at indigo .
indeed , a specular 
 xrd scan ( shown in figure 7 ) clearly links the peaks at 0.76 and 1.87  to indigo corresponding to orientations where the ( 100 ) and ( 210 ) 
 planes , respectively , are parallel to the substrate .
however , for this film again a special reflection appears 
 in the xrd ( 0.66  ) which can not be attributed 
 to indigo .
extensive literature search showed that this reflection 
 could be matched with a multitude of possible quinacridone cracking 
 products , of which the ( 020 ) orientation of carbazole seems most likely if 
 we take into account the entirety of the available spectroscopic data .
specular 
 x - ray diffraction pattern for a bare si / sio2 substrate 
 ( black ) , a 63 nm thick quinacridone film with additional 
 6p reflections ( sample 1 , pink , compare to figure 6e ) and for a 85 nm thick film consisting 
 of indigo and carbazole ( sample 2 , blue , compare to figure 6c ) .
data are vertically shifted 
 for better visibility . preferred orientation of quinacridone molecules packing within 
 the - and -polymorph with the ( 00l)-planes parallel 
 to the substrate . in order to provide support for the conclusions drawn from 
 the 
 specular x - ray diffraction data and to obtain additional information 
 on the in - plane molecule arrangement
we have carried out grazing incidence 
 x - ray diffraction at bessy ii on the same samples as described before .
figure 9 shows two - dimensional 
 gixd images with an incidence wavelength of 1  and under an 
 incidence angle of 0.13. numerous in - plane and out - of - plane 
 reflections confirm the existence of a variety of molecular species 
 and additionally crystalline phases with different crystallographic 
 orientations .
the superimposed white lines represent the highest intensity 
 reflections of quinacridone and p - sexiphenyl ( top ) as well as indigo 
 and carbazole ( bottom ) taken from literature values ( csd - codes qnacrd06 , 
 qnacrd07 , zzzntq01 , indigo01 , indigo02 , and crbzol01 ) .
2d - gixd patterns of a 
 63 nm thick quinacridone film ( sample 1 , 
 top ) and a 85 nm thick film solely consisting of cracking molecules 
 ( sample 2 , bottom ) .
the white lines depict debye  scherrer rings 
 of selected net planes of quinacridone ( solid ) , p - sexiphenyl ( dashed ) , 
 indigo ( solid ) , and carbazole ( dashed ) ; for detailed information see table 1 . to interpret the 
 results and features observed so far and for more information about 
 the chemical composition of the thin organic films , raman spectroscopy 
 was employed on the two samples that were used for afm , specular xrd 
 and gixd investigations before ( compare figures 6 , 7 , and 9 ) .
both samples were irradiated with monochromatic 
 light with a wavelength of 325 nm yielding a multitude of scattering 
 peaks , as shown in figure 10 .
we stress that the absolute peak intensities of the raman 
 modes in figure 10 are arbitrary values that depend on the molecular orientations within 
 the samples .
we did not elaborate this finding in further detail , 
 as our focus is here on a qualitative sample analysis .
the quinacridone 
 sample which was grown at a high substrate temperature ( sample 1 , 
 pink ) showed strong features at wavenumbers between 1550 and 1650 
 cm and certain peaks in the area of 1350 cm that agree reasonably well with our own powder measurements 
 and literature values of quinacridone ( see figure 11 and table 2 ) .
differences between 
 thin film and powder measurements may , in this case , be explained 
 by ( a ) , the presence or dominant behavior of different polymorphs 
 or ( b ) , an increased amount of intermolecular hydrogen bonds within 
 the thin film compared to powder samples .
additional peaks at 1221 
 and 1279 cm agree very well with p - sexiphenyl 
 raman excitations .
the second sample 
 ( sample 2 , blue ) shows a thoroughly different fingerprint with multiple 
 excitations , due to no quinacridone but rather indigo and possible 
 additional cracking molecules being present .
features at 1227 , 1370 , 
 1474 , 1565 , 1571 , and 1610 cm as well as additional 
 smaller peaks , can be matched with raman excitations of indigo powder 
 ( see figure 11 and table 2 ) .
certain peaks , 
 most notably at 662 , 1029 , and 1122 cm , are neither 
 originating from quinacridone , nor from indigo vibrations , again corroborating 
 our claim of the existence of at least one additional cracking molecule . 

raman spectra of a bare si / sio2 substrate ( black ) , 
 a 
 63 nm thick quinacridone film ( sample 1 , pink ) , and a 85 nm thick 
 film consisting of indigo and carbazole ( sample 2 , blue ) upon irradiation 
 with a wavelength of 325 nm .
raman spectra of quinacridone 
 ( pink ) and indigo ( blue ) powder samples 
 upon irradiation with a wavelength of 325 nm .
in this work , we describe 
 some idiosyncrasies in the context of 
 the preparation of thin quinacridone films upon employing knudsen 
 cell deposition in ultrahigh vacuum .
since quinacridone exhibits a 
 comparably high sublimation temperature , due to the existing hydrogen 
 bonds , this causes partial decomposition in the knudsen cell during 
 evaporation , which impedes the formation of pure quinacridone films . 

we have used a variety of spectroscopies to unravel the thermally 
 activated fragmentation and decomposition in the knudsen cell by analyzing 
 the effusion flux and the composition and structure of the deposited 
 film . by using thermal desorption spectroscopy
we could unambiguously 
 demonstrate that at least two different species were deposited on 
 a silicon dioxide substrate upon evaporation of quinacridone from 
 a stainless steel knudsen cell .
in addition to quinacridone which 
 desorbed at about 500 k ( -peak ) , a second desorption peak ( -peak ) 
 appeared already at about 420 k , which could be attributed to indigo . 
 with special experimental methods we were able to prepare films which 
 exclusively consist of molecules corresponding to either the -peak 
 or the -peak , respectively .
this was realized by desorbing 
 and readsorbing molecules corresponding to the  or the -peak 
 onto a second sample positioned opposite to the primary sample . additionally , 
 it was possible to prepare similar films with only one molecular species 
 on the surface by either using a special knudsen - like glass cell or 
 by quinacridone deposition at elevated substrate temperatures .
the 
 latter films were subsequently analyzed by atomic force microscopy , 
 specular x - ray diffraction , grazing - incidence x - ray diffraction , and 
 raman spectroscopy .
however , additional decomposition 
 products were found which could not be observed in the desorption 
 spectra . in the film corresponding to the -peak carbazole
was 
 detected in addition to indigo , while the film corresponding to the 
 -peak displayed p - sexiphenyl , most likely 
 created through further thermal decomposition processes at the hot 
 substrate surface , in addition to quinacridone .
pure quinacridone 
 films could be prepared only upon using a langmuir evaporation source 
 ( free evaporation ) .
all films were stable and showed no significant 
 morphology changes during venting and after storage in air for at 
 least 90 days .
our study highlights that these findings are of relevance 
 for choosing the proper deposition techniques for organic molecules 
 with high sublimation temperature , in particular for hydrogen - bonded 
 molecules , which attain increasing importance in the context of organic 
 electronics .
 thin 
 films of quinacridone deposited by physical vapor deposition 
 on silicon dioxide were investigated by thermal desorption spectroscopy 
 ( tds ) , mass spectrometry ( ms ) , atomic force microscopy ( afm ) , specular 
 and grazing incidence x - ray diffraction ( xrd , gixd ) , and raman spectroscopy . 
 using a stainless steel knudsen cell did not allow the preparation 
 of a pure quinacridone film . 
 tds and ms unambiguously showed that 
 in addition to quinacridone , desorbing at about 500 k ( -peak ) , 
 significant amounts of indigo desorbed at about 420 k ( -peak ) . 
 
 the existence of these two species on the surface was verified by 
 xrd , gixd , and raman spectroscopy . 
 the latter spectroscopies revealed 
 that additional species are contained in the films , not detected by 
 tds . in the film mainly composed of indigo a species 
 was identified 
 which we tentatively attribute to carbazole . 
 the film consisting of 
 mainly quinacridone contained in addition p - sexiphenyl . 
 the reason 
 for the various decomposition species effusing from the metal knudsen 
 cell is the comparably high sublimation temperature of the hydrogen 
 bonded quinacridone . with special experimental methods and by using 
 glass knudsen - type cells we were able to prepare films which exclusively 
 consist of molecules either corresponding to the -peak or the 
 -peak . 
 these findings are of relevance for choosing the proper 
 deposition techniques in the preparation of quinacridone films in 
 the context of organic electronic devices . 
Introduction
Experimental Setup
Results and Discussion
Conclusions