silane - based self - assembled monolayers 
 ( sams ) with long organic 
 chains play an important role in surface chemistry as they establish 
 physically , thermally , and chemically 
 stable films on substrates such as silicon 
 oxide and glass .
in addition , varying the functionality of the organic 
 chain offers the potential to tailor surface properties in a controllable 
 fashion . while there are many options 
 to deposit a sam from solution or to chemically modify a preformed 
 sam in solution ( see , for example , refs ( 610 ) ) , direct gas - phase modification 
 is much more limited . in organic chemistry , c  c 
 bond - forming reactions that occur entirely in the gas phase and without 
 an added catalyst are extremely rare , thus there are few options for 
 direct c  c bond gas - phase surface modification . despite the 
 limited reactions in the gas phase , solventless reactions are attractive 
 as they are potentially less disruptive to film integrity than dipping 
 a preformed sam into a reaction solution .
immersion can reduce the 
 quality of the resultant films , and the penetration and homogeneity 
 are compromised if a solution can not wet all parts of a ( sub)microstructure 
 substrate , for example , in the case of microelectromechanical systems 
 ( mems ) , due to microscopic gas bubbles . on the other hand , vapor - phase 
 deposition has been used to form sams on silica and graphite , and we have recently shown that high - quality 
 vinyl - terminated films can be achieved by this approach . the present study aimed to chemically modify 
 the sam in a second gas - phase process by a c  c bond - forming 
 reaction . in this context ,
hexafluoroacetone azine ( hfaa ) 1 emerged as an appropriate reagent as it has been reported to generate 
 cyclopropanes 2 with olefins such as but-2-ene without 
 any catalyst or coadditive on heating as illustrated in figure 1 .
the reaction also gave rise to some nitrogen - containing cycloadducts 
 when performed in the gas phase and does not appear to proceed via 
 a discrete ( cf3)2c : carbene , but rather the 
 cyclopropane arises from decomposition and nitrogen ( n2 ) extrusion after the addition of 1 to the olefin .
clearly , 
 if 1 is activated and long - lived enough in the vapor 
 phase , then it may react directly in the gas phase with the olefinic 
 residues of the vinyl - terminated silanes of preformed sams the 
 goal of the present study was to demonstrate the potential 
 of vinyl - terminated films for direct chemical modification in the 
 gas phase .
such an approach offers cleaner reaction conditions than 
 solution - based techniques and would be preferable in many high - end 
 applications .
in addition , the selectivity of the reaction would have 
 a clear advantage over the gas - phase plasma treatment of hydrocarbons 
 ( e.g. , ref ( 17 ) ) . in 
 the present study ,
fluorine was employed as an xps marker to track 
 the c  c bond - forming reaction , rather than a substituent to 
 modify surface properties per se .
commercially available n - type , one - side - polished 
 silicon ( 100 ) wafers ( wacker chemie ag , munich , germany ) were used 
 as substrates .
vinyl - terminated trichlorosilanes with alkyl 
 chain lengths of 11 carbon atoms ( 10-undecenyltrichlorosilane , ch2=ch(ch2)9sicl3 ) and 15 carbon atoms ( 14-pentadecenyltrichlorosilane , ch2=ch(ch2)13sicl3 ) , referred to as molecules 3 and 4 ( figure 2 ) , were prepared according to the 
 literature .
the purity of the newly synthesized 
 compounds was similar to values reported in the literature ( see si ) .
the methyl - terminated 
 species ( referred to as 5 , 6 , and 7 , see figure 2 ) were obtained from 
 alfa aesar ( lancashire , u.k . ) : decyltrichlorosilane 
 ( ch3(ch2)9sicl3 - 97% ) , 
 dodecyltrichlorosilane ( ch3(ch2)11sicl3 - 95% ) , and octadecyltrichlorosilane ( ch3(ch2)17sicl3 - 95% ) . hplc - grade 
 toluene ( 99.9% )
hplc - grade 
 dichloromethane was obtained from vwr international ( leicestershire , 
 u.k . ) . hydrogen peroxide ( 30% ) ,
ammonium hydroxide ( 35% ) , sulfuric 
 acid ( 95% ) , and hydrochloric acid ( 36% ) were all obtained from fisher 
 scientific ( leicestershire , u.k . ) .
all experimental steps were performed 
 in a fume hood under an inert 
 atmosphere of nitrogen or argon using oven - dried glassware .
the silicon substrates 
 were cleaned before the deposition of the self - assembling monolayers 
 as described in ref ( 14 ) .
once 
 precleaned , the substrates were immediately coated with an organic 
 ( silane ) film . at the outset , we 
 prepared good - quality methyl- and vinyl - terminated sams on silicon 
 oxide substrates .
the methyl - terminated 
 sams served as nonreactive reference substrates , devoid of olefinic 
 groups .
freshly prepared , sam - coated silicon substrates were placed in 
 a schott duran bottle ( 100 ml capacity ) equipped with a ptfe sealed 
 cap .
the bottle was purged three times with nitrogen before a certain 
 amount of reactive intermediate precursor material hfaa ( boiling point : 
 6768 c ) was added to the bottle in a separate , smaller , 
 open - topped vessel ( see figure 3 ) .
schematic of 
 the setup used for the chemical gas - phase modification 
 of the sams .
there was no direct contact 
 between the liquid hfaa 1 reservoir and the organically 
 precoated substrates during the reaction . 

the vessel was held at fixed temperatures ( 80 , 120 , and 160 c ) 
 and for defined periods of time ( 10 , 20 , and 30 min and 1 , 2 , 5 , and 
 48 h ) . after each reaction
, the samples were sonicated ( 15 min ) sequentially 
 in toluene , dichloromethane , and then deionized water .
the chemically 
 modified silicon substrates were analyzed using contact angle measurements 
 to determine their hydrophilicity / hydrophobicity , ellipsometry to 
 determine the thickness of the ( modified ) coating , x - ray photoelectron 
 spectroscopy ( xps ) to determine the chemical composition of the ( modified ) 
 coating , and atomic force microscopy ( afm ) to investigate the homogeneity 
 and roughness of the ( modified ) coating .
water contact angles ( di 
 water ) were measured with a g10 goniometer microscope ( krss 
 gmbh , hamburg , germany ) under ambient conditions at room temperature . 

all 
 reported values are the average of three measurements taken from different 
 places on the surface .
the thickness of the 
 sams was measured 
 with an m-2000di spectroscopic ellipsometer ( j. a. woollam co. , inc . , 

thickness values were extracted from fits to the data taken 
 at 45 , 50 , 55 , 60 , 65 , and 70 over wavelengths from 200 to 1000 
 nm .
the sample surface was modeled as a si substrate with an oxide 
 layer and a cauchy layer .
the thickness of the silicon oxide after 
 the oxidative cleaning treatment was 16  1  ( average of 
 three samples ) .
the thickness of the monolayer films was calculated 
 with an index of refraction of 1.45 .
film thickness 
 values are averages of at least three measurements . the observed variability 
 of the thickness of organic films prepared under identical conditions 
 was 2  , i.e. , the variation of chemically identical 
 films which were prepared at different times ( not the same  batch  ) . 
 in some cases , we prepared a batch of samples ( 10 to 15 ) with the 
 c11 vinyl - terminated film .
we determined the film thickness 
 for several samples ( typically three to five ) rather than for each 
 sample individually .
values between 14.5 and 15.5  are reported 
 as 15  ( mean value ) for all samples from the same batch 
 in the following .
the reproducibility ( standard deviation ) of the 
 thickness for a single film was 0.2  ( measured on at 
 least three different spots on the same surface ) .
x - ray photoelectron spectroscopy spectra were obtained 
 using a vg sigma probe ( vg scientific ltd . , u.k . ) and al k 
 radiation ( 1486.6 ev ) . during the analysis , the pressure in the test 
 chamber was kept at around 1  10 torr .
the 
 xps spectra were corrected for charging by referencing the aliphatic 
 c 1s peak of hydrocarbon to 285.0 ev .
elemental compositions of the 
 various surfaces were determined from the area under individual elemental 
 peaks using sensitivity factors provided with the software as well 
 as taking the transmission function of the analyzer into account . 

casaxps ( casa software ltd . , u.k . ) was used for the analysis .
quantitative data is 
 reported assuming a model where all constituents are homogeneously 
 and isotropically distributed in the analyzed volume .
the atomic force microscope 
 used was a picospm ii ( molecular imaging , usa ) with an interchangeable 
 nose scanner .
images were recorded under 
 ambient conditions at a scanning speed of 0.8 lines / s and 
 with a nominal contact force on the order of 10 nn .
hexafluoroacetone azine ( 1.0 g , 3.1 mmol , 1 equiv ) and 1-decene 
 ( 0.9 g , 6.2 mmol , 2 equiv ) were placed in an autoclave and held at 
 160 c for 48 h. the reaction mixture was purified by silica 
 gel chromatography ( hexane ) to give the  criss - cross  
 adduct shown in figure 4 as a colorless oil 
 ( 278 mg , 15% ) .
h nmr ( 300 mhz , cdcl3 ) : h 3.623.48 ( 2h , m , ch(ch2)7ch3 ) , 2.75 ( 2h , dd , j 13.8 , 
 6.1 hz , ( cf3)2ch2 10a ) , 2.45 ( 2h , dd , j 13.8 , 9.7 hz , ( cf3)2cch2 10b ) , 1.891.75 ( 2h , m , 
 ch2(ch2)6ch3 8a ) , 1.371.14 ( 26h , m , ch2(ch2)6ch3 8b ) , 
 0.88 ( 6h , t , j 6.5 hz , ch3 ) .
c nmr ( 75 mhz , cdcl3 ) : c 123.7 ( q , j 2.8 hz , cf3 12a ) , 123.5 ( q , j 2.9 hz , cf3 12b ) , 69.0 ( m , ( cf3)2c ) , 56.6 ( ch : c9 ) , 41.1 ( ( cf3)2cch2 : c10 ) , 33.0 ( ch2(ch2)6ch3 : c8 ) , 32.0 , 29.7 , 
 29.6 , 29.3 , 25.9 , 22.8 ( ch2 , c2c7 ) , 
 14.2 ( ch3 ) .
f nmr ( 282 mhz , 
 cdcl3 ) : f 67.6 ( 6f , q , j 18.0 , 8.6 hz , cf3 a ) , 73.6 
 ( 6f , q , j 18.0 , 8.6 hz , cf3 b ) .
hrmsm / z ( ci ) : 
 calculated for c26h41f12n2 [ m + h ] , 609.3073 ; found , 609.3064 .
the xps survey spectra 
 of all samples with methyl- and vinyl - terminated sams ( not shown ) 
 showed the presence of si ( 2s 150.4 ev , 2p 99.2 ev ) , c ( 1s 285.0 ev ) , 
 and o ( 1s 533.0 ev ) as expected . the spectra distinguish between the 
 bulk si ( si 2s 150.4 ev , si 2p 99.2 ev ) and the silicon oxide ( si 
 2s 154.3 ev , si 2p 102.8 ev ) .
contact angle measurements and the film thickness determination 
 with ellipsometry confirmed densely packed monolayers . the results 
 are summarized in tables 1 and 2 . in the first step ,
the best preparation conditions for the reaction 
 between hfaa and the vinyl - terminated films were determined by testing 
 a range of different temperatures ( 80 , 120 , and 160 c ) at several 
 defined periods ( 10 , 20 , and 30 min and 1 , 2 , 5 , and 48 h ) .
in addition , 
 different amounts of hfaa ( 50 , 100 , 150 , 200 , 300 , and 400 l ) 
 were employed for modification .
the water contact angle values as 
 well as the film thicknesses , before and after the reaction of the 
 vinyl - terminated sams , are listed in table 1 .
if the evaluation indicated a significant decrease ( > 5  ) 
 in film thickness after the reaction , then we interpreted this as 
 an indication of degradation .
those thickness values are reported 
 as < 10  and were not investigated further . for the vinyl - terminated 
 surfaces derived from 3 and 4 , organic fluorine 
 on the surface became obvious in the xps 

xps survey scans of vinyl - terminated sams prepared from 3 after the reaction ( 20 min ) with 1 , performed 
 at 80 , 
 120 , and 160 c .
samples that reacted 
 for times greater than 1 h showed the strongest f signal by xps .
xps scans 
 of vinyl - terminated sams prepared from 3 after the reaction 
 with 1 , at 160 c for various 
 time periods .
the results reported 
 in table 1 and the 
 xps spectra in figure 6 indicate that degradation 
 of the vinyl - terminated sams occurs for reaction periods of more than 
 30 min .
therefore , different amounts of hfaa were tested in combination 
 with a reaction period of 20 min and a temperature of 160 c . 

xps results obtained from reactions of sams derived from 3 and treated for 20 min at 160 c , varying the levels 
 of 1 . although a reservoir 
 volume of 400 l of hfaa resulted in 
 a higher f signal than a volume of 200 l , table 1 indicates that 400 l also caused a slight decrease 
 in both the film thickness and contact angle value .
since a volume of 300 l appeared 
 to give very similar results to that of 200 l , without a further 
 increase in the contact angle value , we decided to use a volume of 
 200 l as the optimum for the preparation of nondegraded modified 
 films .
on the basis of the results above a temperature of 160 
 c , 
 a reaction period of 20 min and a volume of 200 l of hfaa gave 
 the optimum conditions for the modification of the vinyl - terminated 
 sams .
the f 1s single - region scan of the films treated with hfaa under 
 these conditions showed one symmetric peak ( not shown ) . in the c 1s 
 single -region scan of hfaa - treated vinyl - terminated films ,
afm images of the vinyl - terminated 
 films before and after modification with carbene showed very smooth 
 surfaces with rms values for roughness of 75120 pm 
 averaged over an area of 5 m  5 m , in agreement 
 with monolayer coverage . to explore 
 the degree of selective reactivity of hfaa with the c = c double 
 bond versus direct ch2 insertion , several experiments were 
 also performed with methyl - terminated sams of 5 and 7 as an unreactive control .
a batch of c10-methyl - terminated 
 films ( 5 ) was prepared , and film thickness values were 
 determined in selected cases .
the water contact angle values as well 
 as the film thicknesses , before and after exposure of the methyl - terminated 
 sams to hfaa , are listed in table 2 .
methyl - terminated 
 samples exposed to hfaa under the optimum preparation 
 conditions were further investigated with xps .
after their exposure 
 to hfaa , no changes were observed , consistent with their lack of reactivity 
 ( see figure 9 ) .
xps survey scans of methyl - terminated 
 sams prepared from 5 , 6 , and 7 after the reaction with 
 hfaa .
the spectra showed the presence of si , c , and o. no f signal 
 was observed ( reaction conditions : 20 min , 160 c , 200 l 
 hfaa ) .
a control 
 vapor - phase reaction between hfaa and 1-decene was performed under 
 similar reaction conditions to those described above with the vinyl - terminated 
 sams . in the reaction mixture ,
the methyl - terminated surfaces prepared from 57 remained unmodified for reaction periods of less than 5 
 h. this is obvious by the absence of any signal in the xps spectra 
 corresponding to the presence of fluorine on the surface .
this lack 
 of reaction was also confirmed by film thickness measurements with 
 ellipsometry and water contact angle measurements both before and 
 after the reaction of the precoated substrates .
the contact angle 
 values as well as the film thicknesses remained approximately constant 
 before and after exposure to hfaa ( table 2 ) .
for the vinyl - terminated sams , the optimum reaction conditions 
 produced surfaces with slightly higher contact angles and similar 
 film thickness values to those of the unmodified vinyl - terminated 
 sams ( table 1 ) .
the reactivity of hfaa and 
 the intermediates formed during reaction and the mechanism of the 
 reaction with a double bond have been a source of discussion for some 
 time .
figure 10 illustrates 
 a putative mechanism for the formation of bis(trifluoromethyl)methylene - terminated 
 sams in the vapor phase .
putative minimal mechanism for the formation of 
 bis(trifluoromethyl)methylene - terminated sams in the vapor phase .
the mechanism is obtained from 
 previous studies on the addition 
 of 1 to organic olefins in reactions 
 carried out in sealed tubes . in those reactions ,
cyclopropanes were 
 formed , as were dimeric adducts where two molecules of the olefin 
 substrate had reacted with 1 to generate nitrogen - containing 
 heterocyclic products .
we did not find any xps evidence for nitrogen - containing 
 products in the surface reactions between vinyl - terminated sams of 3 and 4 and azine 1 for reaction 
 periods of less than 30 min . only after 1 h of reaction with vinyl - terminated 
 films was a small amount of nitrogen detected by xps ( see si ) .
the 
 presence of nitrogen might suggest the formation of the criss - cross 
 adduct , but this was apparent only when the sam had undergone degradation , 
 and presumably the steric constraints of the surface became less of 
 a constraint .
the control vapor - phase reaction between hfaa 
 and 1-decene showed 
 the criss - cross adduct and some unidentified compounds in the reaction 
 mixture .
since reaction on the surface has unique conformational constraints 
 in bringing two olefin groups together , the preferred mode of breakdown 
 of the initial adduct appears to result in products where nitrogen 
 ( n2 ) is extruded and the c3 fragment adds to 
 the double bond ( see figure 10 ) .
the 
 fact that the xps f 1s single - region scan of the films treated 
 with hfaa showed one symmetric peak suggests a single fluorine environment 
 on the surface , which is assigned to the cf3 group .
the 
 carbon region has four different species ( figure 8) and is consistent with a bis(trifluoromethyl)methylene cyclopropyl 
 motif as illustrated in figure 11 .
the 
 carbon peak , showing the largest shift toward higher binding 
 energies from the region of 282.5288.5 ev , was assigned to 
 carbon c1 ( 288.1 ev ) .
schematic of the proposed 
 reaction product of the vinyl - terminated 
 sams with hfaa , a bis(trifluoromethyl)methylene cyclopropyl motif . 

the xps c 1s scans indicate four different carbon species . according to the proposed structure
, this corresponds 
 to the c(cf3)2 species . 
 a second carbon peak , 
 with a binding energy of 286.6 ev ,
was assigned to the c2 
 carbon species ( two carbons of the cyclopropane ring ) .
the most intense 
 signal , c3 , was assigned to the carbons from the alkyl chain ( 285.0 
 ev ) .
the xps spectrum in figure 8 indicates 
 a close to 2:1 ratio of the carbon c2 and c1 species . because of the 
 low intensity of the c1 and c2 signals ,
note , however , that these signals should be viewed in combination 
 with the clearly observable cf3 signal .
the correlation of the three carbon signals supports the 
 assignment and the quantification of the ratios .
the ratios are consistent 
 with a predominant cyclopropanation product at the chain terminus . 

other c3 addition products may also be relevant as illustrated 
 in the box in figure 10 ; however , they fit 
 much less well into the integrated carbon ratios from xps as their 
 carbon c2 to c1 ratio is 1:1 . the corresponding ratios obtained from 
 the c11-vinyl 3 and the c15-vinyl 4 sams based on the c 1s and f 1s single - region scans
figure 8 also demonstrates that the experimentally 
 observed ratio of the aliphatic c 1s to the cf3 signal is as high as 20:1 , while the theoretical ratio is 
 7:1 if all vinyl groups of the sam derived from 3 react . on the basis of the intensity of the f 1s signal and the 
 sum of intensities of the c 1s signals and taking into account the 
 attenuation of the latter , the reaction conversions of hfaa with c11-vinyl 3 and c15-vinyl 4
can be estimated to be 27 and 23% , respectively . for temperatures 
 above 120 c , surface degradation competed 
 with chemical modification over the time period of 30 min to 48 h , 
 as was evident from a reduced carbon signal in xps ( see figure 6 ) , a reduced film thickness , and the decrease in 
 water contact angle values ( see table 1 ) . a 
 comparison of the vinyl- and the methyl - terminated films ( tables 1 and 2 ) suggests that entire 
 surfactant molecules are most likely removed , i.e. , cleavage at the 
 si  o interface occurs because otherwise the methyl - terminated 
 films should also show signs of degradation under these conditions .
the f signals in the survey scans of the vinyl - terminated films 
 indicate that some f - containing species are present , which might be 
 expected to lead to an increase in the contact angle ( figure 6 ) . however
the vinyl - terminated films are less well packed than their methyl - terminated 
 counterparts , which makes it easier for 
 hydroxyl or potentially fluoride species to access the surfactant 
 substrate interface and to attack the si  o bond .
a possible 
 route for degradation is then via cleavage of the si  o bond .
c addition reaction is competing 
 with the removal of entire molecules , which is reflected by the increasing 
 f signal but decreasing c signal in the xps survey spectra for periods 
 of between 30 min and 5 h ( figure 6 ) . from 
 a certain point on , however , all remaining surfactants are modified 
 and further removal of surfactants will then lead to a decrease in 
 the f signal ( figure 6 , spectrum for 48 h ) . 

this is also supported by the appearance of a nitrogen signal in the 
 xps spectra for periods of between 1 and 5 h ( see si ) , indicating 
 the establishment of the criss - cross product followed by a decrease 
 in the n signal if modified molecules are removed by further degradation . 

it has been suggested that similar films decompose via c  c 
 bond cleavage ; however , this is inconsistent 
 with our observations . elucidating the mechanism of degradation
we observed indications of degradation in methyl - terminated films 
 only after a reaction period of 5 h ( see table 2 ) . at this time
this 
 observation is consistent with that reported in ref ( 23 ) that hfaa is stable at 
 elevated temperatures .
however , after 5 h at 160 c hfaa may 
 start to decompose , and the various species formed from its decomposition 
 may initiate sam degradation . after this time , heating of the methyl - terminated 
 films to 160 c led to a degradation in control experiments . for a reaction period of 20 min
there was no degradation if less 
 than 300 l of hfaa was used , while the presence of fluorine 
 atoms at the surface was clearly evident from xps . at a constant reaction 
 time of 20 min and a temperature of 160 c the amount of hfaa 1 precursor in the reservoir evaporated completely from the 
 liquid reservoir .
there is a clear correlation between the amount 
 of hfaa 1 precursor in the vapor phase and the level 
 of conversion . at this relatively short reaction time
the resulting films were smooth 
 when analyzed by afm ( the rms roughness varied from 75 to 
 120 pm over a scan area of 5 m  5 m ) and 
 showed no significant change in the film thickness and a slight increase 
 in the contact angle value , supporting the integrity of high - quality 
 films .
the modified films are likely to be slightly more disordered 
 than the vinyl - terminated layers due to the bulky bis(trifluoromethyl)cyclopropane 
 group . with a reaction yield of around 25% ,
this indicates that the processes that occur 
 are complex and depend on several parameters such as exposure dose 
 and temperature .
it appears that the chemical modification is determined 
 by an activation energy and the kinetics of the competing reactions 
 that are involved .
in this article , we 
 describe a gas - phase surface modification reaction 
 that is performed by heating in standard glassware under standard 
 conditions , and there is no need for a mixed monolayer to ensure that 
 the functional groups are particularly exposed .
the 
 reaction of hfaa with terminal olefins generates a bis(trifluoromethyl)cyclopropyl 
 product , which constitutes a rare c  c bond - forming reaction 
 in the gas phase .
. a future challenge will be to extend 
 the range of precursors that can generate suitable reactive species 
 for such modification .
it is also a challenge to offset the chemically 
 induced thermal degradation of the surface layer for longer reaction 
 periods .
 vinyl - terminated self - assembled monolayers 
 ( sams ) on silicon oxide 
 substrates were chemically modified by the addition of a bis(trifluoromethyl)methylene 
 group in a rare gas - phase c  c bond - forming reaction to directly 
 generate films carrying terminal cf3 groups . 
 the vinyl - terminated 
 films were treated with hexafluoroacetone azine ( hfaa ) for modification . 
 
 the films were characterized with ellipsometry , contact angle measurements , 
 atomic force microscopy ( afm ) , and x - ray photoelectron spectroscopy 
 ( xps ) . in this study , we find that for optimized conditions clean 
 reactions occur on a surface between sams with terminal olefins and 
 hfaa , and the product is consistent with bis(trifluoromethyl)cyclopropanation 
 formation after nitrogen extrusion . 
Introduction
Experimental
Section
Results
Discussion
Conclusions