the chemical versatility 
 as well as relative abundance of carbon - based 
 molecules makes them promising candidates for integration into next - generation 
 devices such as molecular machines , organic field - effect 
 transistors , or light - emitting diodes . however , 
 technological applications require controllable , scalable , and cost - efficient 
 techniques before implementation into industrially manufactured devices 
 becomes available . toward this end ,
the exploitation of self - assembly 
 of organic nanostructures on surfaces is one of the major challenges 
 in the field of nanotechnologies . in this respect ,
the surface - confined 
 ullmann reaction is a powerful bottom - up approach that exploits covalent 
 bonding of halogenated precursors on top of metallic substrates .
the covalently bonded nanostructures exhibit superior thermal and 
 mechanical stability , as well as improved charge transport properties 
 that are mandatory in the field of electronics . in addition , both the geometry and electronic properties , including 
 the band gap , can be tailored by a judicious choice of monomers . to date , a wide variety of these structures have been reported , 
 including well - defined clusters , one - dimensional 
 wires , two - dimensional networks , and a large 
 panel of width - controlled graphene nanoribbons ( gnrs ) . however , practical applications are so far limited by the use of 
 a metallic substrate , requisite to catalyze the reaction .
further 
 processing is thus required to transfer the synthesized structures 
 onto device - ready semiconductor surfaces which enable back - gating 
 and minimize leak currents as well as substrate hybridization effects .
recent experiments performed by local probe 
 techniques indicate that the synthesis of covalently bonded nanostructures 
 on insulator surfaces , transition - metal oxides , and also passivated semiconductors may 
 be feasible , but in particular the electronic properties of the synthesized 
 structures have not been established yet . toward this end
, photoemission 
 spectroscopy is known as a powerful 
 technique to investigate electronic states in self - assembled organic 
 layers on surfaces .
it is commonly used to probe the energies of molecular 
 levels , thereby allowing tracing such properties as molecule 
moreover , the angular 
 dependence of the photoemission intensity contains rich information 
 about orbital topologies . using inverse 
 fourier transformation ,
these orbitals can be recovered , including their phase which is supposed to be 
 lost during the photoemission process .
band dispersions were also reported for molecular layers and crystals 
 as the signature of weak electronic coupling mediated either directly 
 by intermolecular interations or indirectly 
 by the substrate .
direct evidence of 
 conjugation associated with covalent bonding was reported for bottom - up - synthesized 
 carbonaceous materials such as 2d covalent networks and graphene nanorribons .
more 
 recently , the complete valence band structure of poly(p - phenylene ) ( ppp ) has been measured both on cu(110 ) and on au(788 ) .
we note that 
 the long - range ordering and uniaxial alignment , requisite for angle - resolved 
 photoemission measurements of one - dimensional systems , is frequently 
 achieved by exploiting inherently anisotropic substrates such as high - index 
 surfaces .
herein 
 we report the first spectroscopic study of ordered arrays 
 of ppp chains directly synthesized on top of a titanium dioxide surface . 

the growth of 4,4-dibromoterphenyl ( dbtp ) precursors is followed 
 by low - energy electron diffraction ( leed ) , scanning tunneling microscopy 
 ( stm ) , and x - ray photoemission spectroscopy ( xps ) as a function of 
 the substrate temperature .
leed and stm clearly resolve two distinct 
 and well - ordered structural phases with a transition temperature between 
 the phases of about 475 k. this structural transition coincides with 
 a shift of both c 1s and br 3d core levels , as observed in xps .
similar 
 shifts are often considered as the fingerprint of the dehalogenation 
 of precursors .
successful polymerization is established by angle - resolved 
 photoemission ( arpes ) , resolving the transition from a single , well - defined 
 highest occupied molecular orbital ( homo ) , corresponding to a supramolecular 
 phase , to a continuous valence band dispersion associated with the 
 ppp polymer .
the rutile - tio2(110 ) 1  1 substrate ( figure 1a  c ) consists 
 mainly of alternating rows of 
 5-fold - coordinated titanium ( 5f - ti ) and 2-fold- or 3-fold - coordinated 
 oxygen atoms ( 2f - o/3f - o ) , running along the   direction . due to its rectangular unit cell ( 2.95  
 6.49  ) , the surface
this inherent anisotropy is further enhanced by the 2f - o 
 atoms protruding out of the surface plane , framing quasi - one - dimensional 
 trenches of 5-fold - coordinated 5f - ti atoms as depicted in the model 
 in figure 1c .
the stm 
 image ( figure 1b ) of 
 the surface is , at the given tunneling conditions , dominated by electronic 
 rather than geometrical effects ; hence , bright rows are assigned to 
 5f - ti atoms and dark spaces in between to 2f - o atoms . moreover , after reduction of the crystal by several sputtering 
 and annealing cycles , the surface exhibits defects , in particular 
 oxygen vacancies ( ovs , green arrows in figure 1b , c ) , which are known to be involved in several 
 catalytic properties of titanium dioxide such as water dissociation . structural characterization of supramolecular and polymeric 
 phases . 

leed patterns ( a , d , g ) , room temperature stm images ( b , e , h ) , and 
 ball models ( c , f , i ) associated with the clean rutile - tio2(110 ) substrate ( a  c ) , the supramolecular layer ( d  f ) , 
 and the polymeric layer ( g  i ) .
( d ) and ( e ) were measured after 
 saturation of the surface with dbtp deposited at room temperature , 
 ( g ) was measured after saturation with the substrate held at 575 k , 
 and ( h ) was measured after semisaturation with the surface held at 
 575 k. for clarity , hydrogen atoms have been omitted in the ball models . 
 in ( b ) and ( c ) , green arrows indicate some oxygen vacancies .
the white 
 arrows in ( h ) indicate the polymeric chains . upon evaporation of dbtp on top of the surface held at room 
 temperature 
 ( rt ) ,
several additional stripes , highlighted by red arrows in figure 1d , appear in the 
 leed image .
the separation between these stripes indicates a superperiodicity 
 along the   direction of 2.1  0.2 nm . with saturation of 
 the surface ( see figure s1 , supporting 
 information ) , the stm images exhibit well - defined rows running along 
 the   direction ( figure 1e ) .
the distance between these rows is 6.5  0.1  , 
 equal to the lattice parameter of the clean substrate in the [ 110 ] 
 direction .
higher resolution data ( figure 1e , bottom , and figure s2 ) have revealed an intrachain superperiodicity of 1.8  
 0.2 nm along the   axis , close to the value deduced in leed .
all these considerations 
 strongly suggest a one - dimensional arrangement of the dbtp molecules 
 into supramolecular chains , as schematically depicted in figure 1e .
this model can 
 readily be rationalized considering density functional theory ( dft ) 
 calculations of the free - standing molecules ( figures s3 and s4 and note 1 , supporting information ) .
geometry optimization 
 leads to a molecule s length ( br to br ) of about 15.2  . 
 considering two molecules facing each other along their main axis , 
 the van der waals interactions stabilize the intermolecular br 
therefore , the periodicity of a one - dimensional 
 dbtp supramolecular chain is about 1.9 nm , in agreement with the values 
 experimentally deduced from leed and stm .
note that , in this model , 
 the lack of a phase relation between two adjacent chains explains 
 the apparition of a striped pattern instead of well - defined diffraction 
 spots in leed ( figure s5 and note 2 , supporting 
 information ) . postannealing the sample prepared at room temperature 
 leads to 
 a clear transition characterized in leed by the rapid disappearance 
 of the supramolecular features around 470 k ( figure s7 , supporting information ) .
only the characteristic spots 
 of the substrate remain , and no signature of any ordered molecular 
 layer is observed in leed .
however , stm images ( figure s8 ) show some short chainlike features oriented along 
 the   direction and a multitude of disordered protrusions .
while 
 it is appealing to attribute the chainlike features to polymerized 
 molecules , this assignment remains ambiguous without further characterization . 
 to improve the overall order of the surface
, we proceeded to evaporate 
 the precursors on the surface kept at 575 k. this procedure leads 
 to the appearance of two well - defined stripes in leed images , indicated 
 by red arrows in figure 1 g .
the corresponding lattice parameter along the   direction 
 is about 4.2  0.2  .
this value matches the theoretical 
 inter - phenyl distance of 4.23  obtained by dft for a ppp chain , as well as that of 4.4  measured experimentally 
 on cu(110 ) .
stm images of a nonsaturated 
 layer ( figure s1 ) reveal several chains 
 running along the   direction , marked by white arrows in figure 1h ( see also figure s8b ) . in this case , the bare tio2 surface is also readily identified by the familiar striped pattern 
 and the characteristic defects . in the [ 110 ]
direction , surface 
 and molecular chains show the same perpendicular spacing of 6.5  
 0.2  as the bare substrate and the supramolecular layer .
an 
 intrachain periodicity of 4.0  0.3  is deduced from high - resolution 
 stm images , which is in complete agreement with the formation of ppp 
 chains running along oxygen rows , as depicted in figure 1i .
once again , the lack of 
 a phase relation between two adjacent polymers is responsible for 
 the striped pattern observed in leed ( figure s6 ) .
however , we note that , up to now , for both supramolecular and 
 polymeric phases , it has not been possible to determine with accuracy 
 the absolute positions of the chains with respect to the substrate .
xps analysis is intensively 
 used to follow the evolution of core levels in several chemical reactions 
 that occur on top of surfaces . especially on metals ,
the ullmann 
 coupling was recently studied both theoretically and experimentally , revealing an organometallic 
 intermediate that appears close to room temperature on copper and 
 silver substrates . in our case , both c 1s and br 3d core levels were 
 followed as a function of the sample temperature , to characterize 
 the chemical process of the reaction on titanium dioxide .
first , 
 dbtp was evaporated on the sample kept at low temperature ( lt , 80 
 k ) to characterize a multilayer of intact ( nondebrominated ) molecules 
 ( figure 2a , bottom ) . 
 in the br 3d region ,
two contributions associated with br 3d3/2 and br 3d5/2 are resolved with binding energies ( bes ) 
 of 70.6 and 71.6 ev , respectively .
likewise , the spectrum in the c 
 1s region is fitted by two contributions at 284.7 and 285.3 ev , respectively 
 associated with the active and hydrogen - passivated carbon atoms , in 
 agreement with the literature .
after 1 monolayer ( ml ) deposition at room temperature , both of the 
 c 1s and br 3d features remain globally unchanged compared to lt multilayers , 
 and no significant shifts were observed ( figure 2a , middle ) .
this result suggests that the 
 molecules are still intact on the surface , in agreement with the model 
 proposed in figure 1f .
however , for high - temperature depositions , a substantial shift 
 of both br 3d and c 1s core levels , respectively by 1.4 and 0.2 ev 
 toward lower be , are observed .
the shift of the br 3d doublet is attributed 
 to the scission of the c  br bond and the formation of br  ti 
 species .
its magnitude of 1.4 ev is comparable to the theoretically 
 predicted value of 1.61.9 ev core level shift on noble - metal 
 surfaces .
the evolution of the carbon 
 line shape is in agreement with the formation of a polymeric phase , 
 i.e. , a c  c homocoupling of the dehalogenated precursors .
x - ray photoemission spectroscopy : ( a ) c 1s and br 3d core 
 level 
 spectra measured after a multilayer deposition of dbtp with the sample 
 kept at 80 k ( bottom ) , after 1 ml deposition at rt ( middle ) , and after 
 1 ml deposition at 520 k ( top ) ; ( b ) evolution of c 1s and br 3d core 
 levels as a function of temperature , measured after deposition of 
 1 ml at rt ; ( c ) evolution of the integrated c 1s signal as a function 
 of temperature , extracted from ( b ) .
a precise determination of the activation temperature for 
 the high - temperature 
 ( ht ) phase formation is obtained by following the xps spectra during 
 the annealing process of a sample prepared initially at rt ( figure 2b ) .
the line shape 
 of the c 1s and br 3d core levels remains unchanged until 450 k. then 
 a clear shift accompanied by a substantial loss in photoemission intensity 
 takes place within 50 k until about 500 k. the obtained energy shifts 
 are identical to those discussed above , but the reduction in intensity 
 is about 50% throughout the transition as depicted in figure 2c .
this overall decline is 
 distinct from previous experiments reported on metals suggesting that , 
 due to the weakly interacting nature of the molecular precursor on 
 the substrate , a large part of the molecules are desorbed during the 
 annealing process .
since the intensity decrease coincides with the 
 onset of the core level shifts , we can conclude that the activation 
 energy of c  br scission on tio2 is on the same order 
 as the binding energy of dbpt molecules in the rt phase .
then , by 
 increasing the temperature past 575 k , bromine is gradually desorbed 
 from the surface .
we notice that xps data of a postannealed rt phase 
 and a ht - deposited phase are indistinguishable within our experimental 
 resolution ; i.e. , dehalogenation and c  c coupling occur in 
 both cases , even if the global order is improved after a high - temperature 
 deposition ( figure s8 , supporting information ) . 

furthermore , we exclude the possibility that the clusters observed 
 with stm after polymerization are due to br atoms remaining on the 
 surface since these features are visible also in stm data taken on 
 samples that showed no trace of the br 3d core level in xps after 
 annealing at higher temperature . in summary
however , in contrast to noble - metal - catalyzed 
 reactions reported previously , a substantial loss of molecular precursors 
 due to desorption from the surface is observed .
the structural and chemical characterization 
 of the rt and ht phases offers compelling evidence for the polymerization 
 reaction .
however , measurement of electronic dispersion throughout 
 the formed ppp oligomers is still lacking .
in many ways the band structure 
 of the polymer is its key property and may be considered as its defining 
 fingerprint . in particular , a large precursor s orbital overlap 
 associated with covalent bonding induces generally highly dispersive 
 bands , while a small dispersion is rather 
 a sign of weak interactions . to probe the electronic 
 levels in the different structural and chemical phases
, we directly 
 mapped the electron dispersion with arpes along the   direction , 
 parallel to the oxygen rows .
the comparison between measurements of the 
 pristine substrate with the rt and ht molecular layers enables us 
 to determine the features originating from molecules . the clean 
 substrate ( figure 3a , d , gray trace in ( d ) ) is characterized by broad features appearing 
 between 3.3 and 9 ev below the fermi energy , which correspond to the 
 valence bulk bands ( bvbs ) with a predominantly o2p character .
the arpes map also shows a characteristic defect 
 state ( ds ) close to 1 ev , indicated in figure 3d by the red arrow .
the origin of this feature 
 is still under debate and has been assigned both to o vacancies and to interstitial ti atoms .
after deposition of 1 ml of dbtp at room temperature , a 
 well - defined molecular state appears just over the bvb , around 2.9 
 ev , as depicted in figure 3b , d ( blue trace in ( d ) ) .
such levels have already been evidenced 
 for a large variety of organic molecules on metallic surfaces and are usually associated with their  homo state .
the photoemission 
 intensity of this feature is modulated along the   direction , 
 with two maxima appearing at 0.6 and 1.4  .
this spectral weight distribution , already observed for similar 
 molecules such as sexiphenyl , is intrinsically related 
 to the topology of the homo state and its fourier transform .
thus , the main contribution at 1.4  is 
 directly related to the inter - phenyl distance ( 4.4  ) , while 
 that at 0.6  is due to the natural twisted 
 conformation of the molecule ( see also figure s3 , supporting information ) . furthermore , 
 a strong suppression of the defect state is also observed and will 
 be discussed in the following section .
then depositing dbtp at high 
 temperature leads to the complete disappearance of the single molecular 
 level , replacing it by a strongly dispersive band , as presented in figure 3c , d ( green trace 
 in ( d ) ) .
the top of the band is located at 2.06 ev , shifted 
 toward the fermi energy by 0.9 ev when compared to the supramolecular 
 phase .
this shift is fully compatible with the homo / lumo ( lowest unoccupied 
 molecular orbital ) gap reduction expected for the formation of covalent 
 c  c bonds between the dehalogenated precursors .
the band apex is reached at 1.45  , in agreement with the polymer periodicity of 4.2 
  0.2  deduced by leed . from the parabolic fit , we determine 
 an effective mass of 0.17m0 , with m0 being the free electron mass .
another important 
 quantity is the group velocity , which reaches a value of 9.2  
 10 ms in the linear part of 
 the band , around 1 ev below the maximum .
such characteristics 
 are in perfect agreement with previous works reported on ppp oligomers , reflecting the large overlap between the precursor s molecular 
 orbitals , and are more generally expected for graphene - based materials 
 such as nanoribbons .
angle - resolved photoemission 
 spectroscopy : ( a  c ) experimental 
 raw arpes intensity maps acquired on the pristine surface ( a ) , after 
 1 ml dbtp deposition at room temperature ( b ) , and after 1 ml deposition 
 with the sample kept at 575 k ( c ) ; ( d ) energy dispersion curves taken 
 at k = 1.44  from the three previous maps ; ( e , f ) theoretical arpes intensity 
 distributions calculated within a tight - binding model for a terphenyl 
 molecule ( e ) and an infinite poly(p - phenylene ) chain 
 ( f ) .
the energy scale ( e ) is normalized by the resonance 
 integral ( t ) between the pz orbitals of two neighboring carbon atoms .
a simple tight - binding ( tb ) model allows us to qualitatively 
 reproduce 
 the main characteristics of the supramolecular and polymeric features 
 observed in arpes .
the theoretical signature of the homo of a terphenyl 
 molecule is depicted in figure 3e .
the state is characterized by two maxima appearing at 0.6 
 and 1.4  , in agreement with experimental 
 observations .
considering an infinite polymer chain , we observe that 
 the discrete molecular states transform into a dispersive band , as 
 depicted in figure 3f . for an inter - phenyl distance of 4.4  , the top of this band 
 is found at 1.43  .
quantitatively , considering 
 a resonance integral ( t ) between the two pz orbitals of neighboring c atoms equal to 2.7 ev 
 ( expected for graphene - like materials ) , these calculations lead to an effective mass of 0.14m0 , close to that deduced experimentally . therefore , 
 we consider the observed band as the spectroscopic fingerprint of 
 a successful polymerization of dbpt molecules into an ordered array 
 of long ppp chains .
arpes also provides 
 important information about the mechanism responsible for the dehalogenation 
 reaction occurring on titanium dioxide . as stated previously , for 
 the clean surface , the reduction of the substrate by sputtering / annealing 
 cycles generates defects , leading to an excess of charge that originates 
 the ti 3d ds , which is visible in photoemission around 1.0 ev below 
 the fermi energy ( red arrow in figure 3d ) .
after rt deposition , this peak almost vanishes , 
 but is fully recovered for the polymeric phase . in a recent study , 
 kolmer et al
. demonstrated on the ( 011 ) surface that the reaction 
 is catalyzed by hydroxyl groups .
however , 
 our stm images of the clean surface suggest only a negligible amount 
 of hydroxyl groups prior to molecule deposition . although a small amount of hydroxyl groups may be created 
 during thermal evaporation of dbtp onto the surface , a temperature 
 of more than 575 k during molecule deposition for the polymerized 
 ht phase is sufficient to induce diffusion of h from the surface hydroxyl 
 groups into the bulk of tio2(110 )
. detailed information on the rates of surface hydroxyl group annihilation 
 and creation are thus necessary to draw conclusions about their role 
 in the present polymerization reaction , but other factors appear to 
 play a role as well . in particular , the observed attenuation of the 
 ds peak in our arpes data was shown previously to not be related to 
 the formation of oh groups . moreover , 
 several other studies reported that the ds is preserved after thin - layer 
 depositions of different organic molecules such as phthalocyanines or perylenetetracarboxylic 
 dianhydride .
thus , a direct screening 
 of the ds by an organic layer seems very unlikely .
therefore , another 
 interpretation of the arpes data is the direct interaction of halogen 
 atoms with ovs , leading to the disappearance of the ds .
however , in the present case a 
 question arises about whether the quenching of the ds is due to a 
 minute amount of br atoms that were directly evaporated from the molecular 
 source or whether br - terminated molecules are interacting directly 
 with the ds .
unfortunately , the resolution of our laboratory xps source 
 does not allow deciding between these two scenarios .
nonetheless , 
 we can conclude that upon heating the sample during polymerization 
 of dbtp a re - evaporation of br correlates with the restoration of 
 the ds .
here , we have presented 
 a spectroscopic study of an in situ polymerization 
 of dbtp precursors into ordered arrays of poly(p - phenylene ) 
 nanowires on top of the tio2(110 ) surface .
the signatures 
 of the dbtp supramolecular assembly as well as ppp oligomers were 
 characterized by leed , stm , xps , arpes , and tb calculations . in particular , 
 leed and stm data resolved long organic wires perfectly aligned along 
 the oxygen rows in the   crystal direction . collecting temperature 
 dependent xps spectra ,
we have traced the c  br bond scission 
 temperature of the dbpt precursors which is accompanied by the onset 
 of the polymerization process to 475  25 k. angle - resolved photoemission 
 spectroscopy was used to map a strongly dispersive -conjugated 
 band upon formation of long ppp chains .
analysis of the data has revealed 
 an effective mass of 0.17m0 at 
 the top of the valence band as well as a group velocity of up to 9.2 
  10 m / s in the linear part of the dispersion .
the 
 latter reflects the large orbital overlap originating from covalent 
 coupling of the precursors and generally expected for conjugated systems . 

therefore , the presented on - surface bottom - up synthesis opens up the 
 prospect of the fabrication of an atomically controlled nanostructure , 
 such as graphene nanoribbons , on top of transition - metal oxides .
this 
 paves a new pathway toward the integration of these structures into 
 multifunctional electronic devices that will take advantage of both 
 molecular and substrate properties .
experiments were carried out in ultra - high - vacuum ( uhv ) systems 
 at a base pressure of 10 mbar .
commercial tio2(110 ) single crystals ( cyrstek ) were prepared by repeated 
 cycles of sputtering ( ar , 1 kev ) and annealing ( 900 k ) . 
 while still considered as a semiconductor , optical inspection of the crystals revealed a dark blue color indicating 
 a certain degree of reduction of the tio2 bulk .
4,4-dibromoterphenyl 
 molecules ( sigma - aldrich ) were initially degassed thoroughly for several 
 hours / days by heating the source to temperatures slightly below 350 
 k under uhv conditions and subsequently sublimed using a knudsen cell 
 heated to about 375 k , to obtain a rate close to 0.33 ml / min . for 
 both rt and ht depositions , the sample saturates at 1 ml , leading 
 to a self - limited growth ( figure s1 , supporting 
 information ) .
scanning tunneling microscopy was carried out at rt 
 using either an omicron vt - stm or a specs aarhus scanning tunneling 
 micrscope .
stm images were recorded at constant tunneling current 
 ( 30 pa ) and constant bias voltage ( applied to the sample ) between 
 1.8 and 2.5 v. xps experiments were performed 
 using a phoibos photoelectron 
 spectrometer equipped with an al k x - ray source ( 16 ma , 12.5 
 kv ) as the incident photon radiation .
the overall resolution of the 
 instrument is approximately 0.9 ev . during the temperature - dependent 
 measurement
photoemission measurements were performed using a 
 phoibos 150 specs high - resolution hemispherical electron analyzer . 

the sample was cooled to 150 k. he i ( h = 
 21.2 ev ) radiation was provided by a high - intensity uvs-300 specs 
 discharge lamp coupled to a tmm-302 specs monochromator .
theoretical 
 arpes intensity distributions were obtained 
 by taking the square modulus of the fourier transform of the molecular 
 orbitals calculated for both terphenyl and long ppp chains within 
 the tight - binding model , according to the method presented in refs ( 41 ) and ( 42 ) .
 surface - confined dehalogenation reactions 
 are versatile bottom - up 
 approaches for the synthesis of carbon - based nanostructures with predefined 
 chemical properties . however , for devices generally requiring low - conductivity 
 substrates , potential applications are so far severely hampered by 
 the necessity of a metallic surface to catalyze the reactions . in 
 this work 
 we report the synthesis of ordered arrays of poly(p - phenylene ) chains on the surface of semiconducting tio2(110 ) via a dehalogenative homocoupling of 4,4-dibromoterphenyl 
 precursors . 
 the supramolecular phase is clearly distinguished from 
 the polymeric one using low - energy electron diffraction and scanning 
 tunneling microscopy as the substrate temperature used for deposition 
 is varied . 
 x - ray photoelectron spectroscopy of c 1s and br 3d core 
 levels traces the temperature of the onset of dehalogenation to around 
 475 k. moreover , angle - resolved photoemission spectroscopy and tight - binding 
 calculations identify a highly dispersive band characteristic of a 
 substantial overlap between the precursor s  states 
 along the polymer , considered as the fingerprint of a successful polymerization . 
 
 thus , these results establish the first spectroscopic evidence that 
 atomically precise carbon - based nanostructures can readily be synthesized 
 on top of a transition - metal oxide surface , opening the prospect for 
 the bottom - up production of novel molecule  
 semiconductor devices . 
Introduction
Results and Discussion
Conclusion
Methods