surfaces of rutile 
 tio2 have for many years been used 
 as model systems to explore the physics and chemistry associated with 
 the varied applications of the material . of particular 
 interest has been the surface science associated with light harvesting 
 processes such as photocatalysis and dye - sensitized photovoltaics .
applications in the field of molecular electronics have also been 
 suggested , with tio2 being employed as a dielectric substrate . 

this has motivated studies of metal wire growth as interconnects using 
 physical vapor deposition .
moreover , electron - induced 
 surface modification of tio2(110 ) is of interest 
 as a means of patterning the substrate for wire formation or to template 
 an array of functional molecules .
there is a related interest in the 
 generation of memristor elements on tio2 . in this work
we examine the potential of electron 
 beam - induced 
 surface activation ( ebisa ) to deposit 
 metallic structures , including wires , on rutile tio2(110 ) 
 in ultrahigh vacuum ( uhv ) .
the latter environment is essential for 
 the analysis methods employed as well as to avoid contamination from 
 residual gases .
ebisa is a technique that 
 is part of a more general approach termed focused electron beam - induced 
 processing ( febip ) , which also includes the related technique of electron 
 beam - induced deposition ( ebid ) . a step - by - step explanation of the recently 
 developed ebisa process 
 is given in figure 1 .
the substrate ( a ) is 
 first irradiated with energetic electrons , resulting in local activation 
 of the surface by electron - stimulated desorption of o atoms ( b ) . in a second step
, precursor molecules are decomposed upon contact 
 with the activated area , resulting in a deposition of nonvolatile 
 material ( c , d ) . in addition
, the initial deposit grows in size because 
 of autocatalytic decomposition of precursor molecules ( e , f )
. the autocatalytic growth has been demonstrated 
 for iron pentacarbonyl , fe(co)5 , on siox in uhv and recently also for co2(co)8 , indicating 
 the general nature of this process for metalcarbonyls .
moreover , additional insight into the mechanism of 
 the ebisa process has been achieved using local auger electron spectroscopy 
 ( aes ) to gain chemical information and scanning tunneling microscopy 
 ( stm ) to provide atomic scale images .
electron beam - induced surface activation 
 ( ebisa ) and the secondary 
 growth process .
the pristine ( 110 ) surface of a tio2 ( rutile ) 
 crystal ( a ) is exposed to a focused 15 kev electron beam ( b ) , leading 
 to the localized release of oxygen via electron - stimulated desorption 
 processes .
the activated surface mediates the dissociation of fe(co)5 molecules ( c ) into volatile co molecules and a primary deposit 
 of iron atoms ( d ) . further supply of fe(co)5 increases 
 the deposit size via an autocatalytic decomposition reaction ( e ) . 
 under the given reaction conditions , the growth process produces very 
 pure , cubic iron crystallites ( f ) .
the instrument includes an electron 
 column for scanning electron microscopy ( sem ) with spatial resolution 
 better than 3 nm .
this column is also used in combination with a hemispherical 
 electron energy analyzer for local auger electron spectroscopy ( aes ) 
 and scanning auger microscopy ( sam ) with a spatial resolution better 
 than 10 nm . for aes ,
a beam energy of 15 kev and a current of 3 na 
 were used , while all electron exposures for sem and lithography were 
 done at an electron beam energy of 15 kev and a probe current of 400 
 pa .
the lithographic processes were controlled via a homemade lithography 
 application based on labview 8.6 ( national instruments ) and a high - speed 
 dac pcie - card ( m2i.6021-exp , spectrum gmbh , germany ) . for line exposures 
 ( single pixel width ) ,
the line dose dl is calculated 
 as dl = ipe  tdwell/x , where ipe is the beam current , tdwell the dwell time at every position , and 
 x the step size along the line .
the square patterns 
 were exposed in a single sweep , meander - like pattern at a step size 
 of 6 nm with the area dose da being defined 
 as da = ipe  tdwell/(x ) .
the corresponding area dose da for a given line dose dl can 
 be estimated by dividing dl by the step 
 size .
the backscattered electron ( bse ) exit areas and coefficient 
 were 
 simulated with the program casino v2.42 .
the diameter corresponds to an area of which 99% of bses are emitted . 
 for the simulations , a primary electron ( pe ) number of 10 was applied .
an stm scanner 
 can be inserted between the pole piece of the sem 
 column and the sample for in situ stm measurements .
w tips were used in this work , negatively 
 biased relative to the sample .
st micrographs were acquired using 
 matrix v3.0 ( omicron ) and evaluated using the latest versions of wsxm ( nanotec electronica , madrid , spain ) .
rutile tio2(110 ) samples ( pikem and crystec ) were prepared 
 by repeated cycles of ar sputtering and annealing to 11601180 
 k until a sharp ( 1  1 ) low - energy electron diffraction ( leed ) 
 pattern was obtained with no detectable contaminants in aes .
the purity 
 of the fe(co)5 precursor gas ( acros organics , 99.5% ) was 
 confirmed using a mass spectrometer .
the precursor gas was dosed through 
 a nozzle to about 12 mm from the tio2(110 ) surface at an 
 estimated local pressure of 9  10 mbar , based on simulations with the software gis simulator ( version 
 1.5 ) .
the first 
 goal of this work was to extend the use of the ebisa 
 technique , which thus far has only been reported for silica surfaces , 
 to a different oxide material , namely a tio2(110)-(1  
 1 ) surface .
for that purpose , line patterns and square patterns ( 2 
  2 m ) were irradiated with a focused electron 
 beam .
fe(co)5 was subsequently introduced to allow growth 
 of iron structures at the irradiated positions .
the gas was pumped 
 off after 270 min , corresponding to an exposure of about 1.1  
 10 langmuir ( 1 langmuir is 10 torr 
 s ) .
figure 2 shows sem images of the resulting 
 fe line deposits ( panels a 
the images show a strong increase in the amount of deposited 
 iron with electron dose .
aes measurements 24 h after fabrication ( see 
 figure s1 in supporting information ) indicate 
 the purity of the iron structures is > 90 atom % , which is similar 
 to that achieved on sio2 substrates . those iron deposits on sio2 exhibit 
 a low room - temperature resistivity of 88  cm as well 
 as ferromagnetic behavior .
these properties 
 can be expected to be similar on tio2 , potentially making 
 the corresponding iron deposits suitable interconnects .
line ( a  d ) 
 and 2  2 m square deposits 
 ( e  h ) fabricated by ebisa on tio2(110 ) using fe(co)5 as a precursor ( approximately 1.1  10 langmuir ) . 

primary electron ( pe ) doses are indicated on the top right of each 
 panel . the line width ( fwhm , sem intensity ) increases with pe dose 
 range from approximately 150 nm ( a ) to 2.6 m ( d ) .
the central 
 line is a result of activation by pes , while the fringe is caused 
 by bse proximity effects .
for the square fields , deposit density increases 
 from a loose grain assembly at 0.27 c / cm via a close - packed 
 assembly at 0.55 c / cm to a fused grain deposit of near 
 ideal geometry at 1.1 c / cm . at even higher doses , proximity 
 effects also cause a fringe growth ( 3.3 c / cm ) .
note that 
 scattered iron clusters are found all over the surface without electron 
 exposure , a fact which is attributed to active defects created during 
 sample preparation . at higher sem magnification 

the density of the cubes 
 within the line and square deposits varies with the applied electron 
 dose , whereas the size of the individual cubes at a given gas exposure 
 seems to be similar .
with increasing gas exposure the size of the 
 clusters increases because of autocatalytic growth . for increased 
 electron doses
, the cubes start to merge into continuous patches , 
 as can be seen for the 1.1 c / cm square deposit ( figure 2 g ) or the line deposits ( figure 2a , b ) .
higher electron doses mark the start of pronounced proximity 
 effects , i.e. , unintended deposits near the irradiated patterns due 
 to electron scattering .
pronounced proximity effects are present at the 3.3 c / cm square irradiation doses ( figure 2h ) and 
 the higher line doses ( figure 2c , d ) . for ebisa , 
 all of the observed proximity effects can be attributed solely to 
 bse and associated secondary electrons ( seii ) .
proximity effects from forward scattered electrons 
 ( fse ) are not observed in ebisa because of the absence of structure 
 growth during the electron beam exposure step .
the observation of homogeneously scattered cubes 
 in the nonirradiated 
 surface regions at low electron 
 irradiation doses ( figure 2a , b , e ) points to 
 the presence of nucleation points on the tio2(110 ) surface 
 that were not induced by the electron beam ( see also supporting information , figure s2 ) .
the defects are oxygen 
 vacancies , which typically are present at an initial concentration 
 of 5% ml ( 1 monolayer is the number of surface unit cells ) .
the vacancies 
 react with water in the residual vacuum to form the second major form 
 of defects , bridging hydroxyls ( one water molecule reacts with one 
 vacancy to produce two hydroxyls ) . turning to the ebisa mechanism
, we can first look at the effect 
 of the 15 kev electron beam on the reduction state of the surface . 

figure 3a shows auger electron spectra for 
 nonirradiated ( red line ,  pristine  ) and strongly irradiated 
 ( blue line ,  reduced  ) areas of a freshly prepared tio2(110 ) surface .
the most prominent features are the tilmm and tilmv peaks at 383 and 413419 ev , 
 respectively , and the okll peaks at 491 and 511 ev .
it 
 is immediately apparent that the oxygen signal for the strongly irradiated 
 surface ( blue ) is considerably smaller than that for the pristine 
 surface ( red ) ; the ti signals show the opposite trend , albeit not 
 as pronounced .
this behavior evidences the electron beam - induced desorption 
 of oxygen from the surface and is in line with that expected from 
 electron - stimulated desorption ( esd ) results from tio2 at 
 lower electron beam energies .
in addition , the change 
 of the peak shape of the tilmv signal allows for insight 
 into the chemical state of the ti atoms of the surface . as discussed 
 by nishigaki and gpel ,
the lmv auger peak consists of two components : 
 413 ev for ti and 419 ev for ti . in 
 a simplified picture , for ti , in the complete absence 
 of a ti 3d valence electron ,
the valence electron in the lmv auger 
 process stems from an interatomic transition of an o 2p electron with 
 a higher binding energy ; on the contrary , for ti , 
 at least a partial electron charge remains in the ti 3d level ( at 
 lower binding energy than the o 2p level ) and thus participates in 
 an intra - atomic auger transition at higher kinetic energy .
therefore , 
 the observed intensity shift from the low- to the high - kinetic energy 
 contribution of the tilmv peak is a direct indication of 
 a reduction of the ti by the electron beam .
the evolution 
 of the lmv peak shape upon increasing electron irradiation is illustrated 
 in figure 3b from top ( pristine , red ) to bottom 
 ( reduced , blue ) .
( a ) ae spectra of pristine ( red ) and electron irradiated , 
 reduced 
 ( blue)tio2 samples in nonderivative mode .
the loss of oxygen 
 kll intensity is a clear indication of oxygen loss , i.e. , esd , while 
 the intensity shift of the titanium lmv peak from lower- to higher - kinetic 
 energy is an indication of a shift from fully oxidized ti toward more reduced states like ti or even lower .
the 
 gradual shift of the peak shape with increasing electron dose is shown 
 from top to bottom in ( b ) .
( c ) quantitative evaluation of the normalized 
 tilmm ( black ) and okll ( orange ) peak areas and 
 apparent o : ti ratio ( green ) versus applied electron dose .
the apparent 
 o : ti ratio ( green ) is set to 2:1 ( tio2 ) for very low electron 
 doses , i.e. , a pristine substrate .
electron stimulated desorption 
 causes a loss of oxygen atoms , i.e. , signal intensity , which simultaneously 
 leads to an increase in the relative intensity of the tilmm signal . in figure 3c , the normalized ( ipristine = 1 ) peak 
 areas of the tilmm ( black ) 
 and okll ( orange ) signals and the o : ti peak ratio ( green ) 
 are plotted versus the primary electron dose ( note the logarithmic 
 scale ) .
the apparent o : ti peak ratio is normalized so that the value 
 for the pristine surface is set to 2 , i.e. , tio2.0 . at 
 doses
lower than 0.1 c / cm , the peak ratio does not change , 
 i.e. , the applied dose is not sufficient to induce a reduction of 
 the surface detectable in aes . after crossing the 0.1 c / cm threshold , the oxygen intensity and the o : ti ratio drop
rapidly 
 until a fairly stable lower limit at 0.9 is approached at 
 doses larger than 10 c / cm .
the corresponding 
 relative oxygen intensity is 0.6 of the initial value of the pristine 
 surface . the obtained o : ti ratio of 0.9 indicates an average surface 
 composition of tio0.9 .
note that the chemical composition 
 of the surface is likely to be nonuniform after the reduction process , 
 so the detected auger signal is a convolution of different ti oxidation 
 states and chemical surrounding .
in addition , decreased damping due 
 to the loss of oxygen atoms from the topmost layers has to be taken 
 into consideration when assessing the tilmm intensity . 
 nevertheless , the o : ti ratio serves as an indicator for the degree 
 of electron beam - induced reduction of the surface .
the auger 
 spectra indicate that above 0.1 c / cm the 
 stoichiometry of the surface changes gradually from tio2 to tio0.9 at 10 c / cm . as for 
 the accompanying morphology change ,
prior to 
 electron beam exposure , the stm image shows   direction bright 
 rows separated by 0.65 nm that correspond to ti atoms .
a line exposure corresponding to an electron area dose of approximately 
 60100 c / cm ( electron energy , 15 kev ; beam current , 
 400 pa ; step size , 12 nm ; dwell time , 150 ms ; line dose , 50 c / cm ) 
 results in an o : ti ratio of 1.11.2 .
the exposed region can 
 be clearly identified in the image in figure 4 as an area with a disordered appearance and a strongly increased 
 number of protrusions ( bright spots ) arranged within a vertical line 
 with a width of approximately 20  5 nm ( indicated by dotted 
 yellow lines ) ; this width is 7 times larger than the nominal 
 beam diameter of 3 nm ( gauss profile , 20/80 criterion ) , indicating 
 the well - known broadening due to se and bse proximity effects .
the 
 apparent surface roughness ( rms ) of the exposed area ( 0.12 nm ) in 
 stm is double that of the nonirradiated surface .
the left - hand side 
 of the image in figure 4 resembles the as - prepared 
 surface , with the characteristic   direction ti rows visible ( see 
 also supporting information , figure s3 ) . 
 however , there are additional defects in the form of isolated protrusions 
 and in - row depressions ( white arrows ) that can be attributed to proximity 
 effects , as the density is higher than on the as - prepared surface . 

apart from the increase in mean roughness , the electron beam - induced 
 disorder does not cause a significant increase in average apparent 
 height .
high resolution st micrograph ( i = 280 
 pa , u = 1.2 v , moderately high - pass filtered ) of 
 an electron 
 beamline irradiation ( 50 c / cm ) , marked by the large arrow and 
 yellow lines .
the surface on the left shows only minor damage ; the 
 typical row pattern of the tio2(110 ) surface is preserved . 

defects include ( 1  2 ) reconstructed sections ( small arrows ) , 
 dark defects in the row structure , and isolated protrusions . on the 
 right ,
a disordered section running top to bottom was identified as 
 the result of a line irradiation with a primary electron dose of 50 
 c / cm . the ( 1  1 ) structure is barely retained in the 
 highly damaged area ; no characteristic defects can be identified in 
 the irradiated area .
the loss of long- and short - range order corresponds 
 to an electron - induced surface amorphization .
the center of the sem image in figure 5a 
 clearly shows a significant electron beam - induced darkening . in previous 
 studies we showed that this darkening of irradiated areas is an intrinsic 
 feature of electron beamed oxide surfaces and is not indicative of 
 contamination .
these results indicate 
 that the modifications to the tio2(110 ) morphology and 
 subsequent reaction with fe(co)5 are a result of electron 
 beam - induced oxygen loss .
sem scan ( a ) and stm topography ( b ) of the tio2(110 ) 
 surface after line irradiation with 50 c / cm and after initial 
 fe growth from fe(co)5 ( c ) .
the strongly contrast - enhanced scanning electron 
 micrograph ( a ) shows a dark feature at the position of the line irradiation , 
 conventionally attributed to carbon deposition from residual gases . 

stm reveals a high degree of disorder at the line position , while 
 the surrounding surface remains fairly unaffected by the irradiation . 
 upon exposure to about 10 langmuir of fe(co)5 , clusters 
 form predominantly at the irradiated surface area ( c ) .
cluster formation 
 next to the line exposure is attributed to defects induced by backscattered 
 electrons , i.e. , common proximity effects .
panels b and c in figure 5 show the 
 stm 
 images of the irradiated 50 c / cm line pattern before and after 
 fe(co)5 dosing ( total exposure : 10 langmuir ) , respectively . 
 after the gas exposure , the formation of iron deposits in the form 
 of clusters is observed , primarily in the regions irradiated with 
 electrons ( indicated by yellow lines ) .
larger clusters appear as slightly 
 asymmetric protrusions in the image , a consequence of the applied 
 scanning speed .
these clusters have a typical diameter of 3.2  
 0.3 nm , including tip convolution , and an apparent height of 1.2  
 0.1 nm ( see supporting information , figure 
 s2 ) .
in addition to the clusters observed within the yellow lines in 
 figure 5 , some deposits are also found in nonirradiated 
 areas to both sides of the irradiated line .
these clusters originate 
 from growth due to the intrinsic surface defects in tio2(110 ) ( see above ) , as well as from growth due to bse proximity effect - induced 
 defects .
the bse exit range of tio2 , here defined as the 
 distance from the point of impact that includes 99% of all emitted 
 bses ( r99 ) , was extracted from mc simulations 
 and amounts to about 1050 nm . all of the scanned area in figure 5c is well within this exit range .
the bses ( e > 50 ev by definition ) are able to induce esd of oxygen 
 atoms within the knotek  feibelman framework ( emin , esd = 34 ev ) , thus creating isolated defects .
 electron 
 beam - induced surface activation ( ebisa ) has been used 
 to grow wires of iron on rutile tio2(110)-(1  1 ) 
 in ultrahigh vacuum . 
 the wires have a width down to 20 nm 
 and hence have potential utility as interconnects on this dielectric 
 substrate . 
 wire formation was achieved using an electron beam from 
 a scanning electron microscope to activate the surface , which was 
 subsequently exposed to fe(co)5 . on the basis of scanning 
 tunneling microscopy and auger electron spectroscopy measurements , 
 
 the activation mechanism involves electron beam - induced surface reduction 
 and restructuring . 
Introduction
Experimental Section
Results and Discussion