since the first observations 
 by gomberg , stable 
 free radicals are desirable spin standards , polarizing agents , and 
 building blocks of carbon - based magnetic systems . purely 
 hydrocarbon stable radicals are gaining ground as versatile model 
 systems for the development of novel -magnetic systems along with other benzenoid compounds 
 like graphene flakes , carbon nanotubes , fullerenes , and -conjugated 
 polymers . recently , the real - space study 
 of single unpaired electron spins on the atomic scale has become a 
 reality with the help of electron - transport experiments in surface - supported 
 d metal impurities and metalorganic molecules inside the tunnel junction of a scanning tunneling microscope ( stm ) . such experiments utilize the kondo effect , that is , the screening of a localized electron spin by delocalized 
 conduction electrons at sufficiently low temperatures , as experimental 
 proof of the spin system maintaining its unpaired electron . here we report on a low - temperature stm study of a new type of 
 kondo system based on metal - free stable hydrocarbon  radicals , 
 bdpa ( ,-bisdiphenylene--phenylallyl ) ( figure 1a ) , that organize 
 by self - assembly into different 1-d nanostructures on a au(111 ) surface 
 while maintaining their spin-1/2 state . our results based on scanning 
 tunneling spectroscopy ( sts ) reveal the characteristic properties 
 of the surface kondo effect for bdpa / au(111 ) with submolecular resolution 
 for isolated bdpa monomers and dimers . compared with the monomer with 
 a kondo temperature , tk , close to 50 k , 
 an increase in tk is observed for dimers , 
 suggesting a stabilization of the kondo state by the radical  radical 
 interaction . bdpa recrystallized 
 in benzene was thermally evaporated in ultrahigh 
 vacuum ( uhv ) from a quartz crucible at 383 k after thorough degassing 
 at 373 k. the single - crystal au(111 ) surface was prepared by repeated 
 cycles of 0.5 kev ar bombardment and annealing at 720 
 k. stm and sts  experiments were carried out at 57 k employing 
 electrochemically etched w tips deoxidized by annealing in uhv . the 
 di / dv signal was obtained from the 
 first - harmonic current signal detected by lock - in technique ( 0.5 to 
 2 khz ; 520 mv sinusoidal peak - to - peak voltage ; average of 
 10 single spectra ) . impurity and tip effects were minimized by careful 
 sample preparation and multiple tip formings between the di / dv experiments , resulting in au - coated 
 stm tips . reliable tip  performance was established by accurately 
 reproducing the di / dv signature 
 of the au(111 ) surface state from literature . di / dv spectra were recorded under 
 constant - height conditions , the spectroscopic images ( di / dv maps ) were simultaneously recorded with constant - current 
 stm topographic imaging . ( a ) dft - calculated isodensity representation 
 of the singly occupied 
 molecular orbital ( somo ) of the stable  radical bdpa ( c33h21 ) from ref ( 19 ) . ( b ) stm topographic image of 0.2 monolayers 
 of bdpa grown at 300 k on the au(111 ) surface ( + 0.5 v , 50 pa , 7 k ) ; 
 scale bar = 1 nm . ( c , d ) spectroscopic di / dv maps at different electron energies ( 6  6 nm ) ; overlaid contour lines ( dashed ) mark areas covered by bdpa 
 molecules as extracted from stm topographs ; arrows indicate a surface 
 defect . deposition of submonolayer 
 coverages of bdpa onto au(111 ) at 300 k leads to the self - assembly 
 of regular radical clusters . details of the structural and electronic 
 properties of bdpa clusters on au(111 ) have been recently reported 
 by our group . the stm topograph of figure 1b shows a 15  15 nm frame of the 
 sample surface . single isolated bdpa monomers are rarely observed on the sample surface 
 due to an increased surface mobility . dimers are more stable due to 
 pinning at surface defect sites and constitute 
 the smallest bdpa clusters . larger clusters are 1-d chains up to several 
 nanometers long that start to grow at trimers with a characteristic 
 three - fold symmetric arrangement acting as nucleation centers for 
 the chain growth . the resonance energies of 
 bdpa s frontier molecular orbitals ( mos ) have been investigated 
 in detail by our group in ref ( 19 ) . figure 1c shows as an example the 
 di / dv map at the energy of the highest 
 occupied molecular orbital ( homo ) . here we focus on the energy range 
 of 100 mev close to ef , where kondo 
 features are typically expected . at 100 mev , bdpa is almost 
 invisible in the di / dv map against 
 the conductance background of the au(111 ) surface , where only the standing - wave pattern of the 2-d surface 
 state of au(111 ) is observed ( figure 1d ) . in 
 addition to mo resonances , typical di / dv spectra of bdpa / au(111 ) exhibit a narrow conductance minimum ( dip ) 
 close to ef , whose amplitude is significantly 
 larger than the modulation (  ripples  ) of the surface - state 
 band by the herringbone reconstruction of au(111 ) . figure 2a shows this conductance 
 minimum recorded with the stm tip over a single isolated bdpa monomer 
 ( red curve ) and a bdpa dimer ( black curve ) . the tip position is marked 
 by a circle (  ) in the insets showing the respective stm topographic 
 images . the maximum amplitude of the conductance dip is observed whenever 
 the stm tip is near the rim of bdpa . amplitudes of up to 20% of the 
 zero - bias conductance of the pristine substrate are observed with 
 our best stm tips ( figure 2a ) . over intramolecular 
 positions the conductance dip decreases ( figure 2b ) . a detailed analysis of our sts results on the conductance minimum 
 confirms a number of characteristic properties of the transmission 
 kondo effect for bdpa / au(111 ) : ( i ) energetic 
 position : the conductance minimum is centered at an energy offset , 
 |e| = e  ef , of a few millielectronvolts around ef ( figure 2a ) . the width 
 of the signal is larger than the offset , 2 > e , so that it spans over both the filled and empty state 
 regimes , that is , below and above ef . 
 according to fermi - liquid theory , the asymmetry is determined by the alignment of the mo 
 level that is singly occupied ( somo ) relative to ef and the coulomb charging energy u that 
 separates somo from the singly unoccupied level ( sumo ) . for bdpa / au(111 ) , 
 the somo was found to lie several hundred millielectronvolts below ef , and bdpa exhibits 
 a large u on the order of 1.4 ev . the large u value is responsible for the 
 high stability of the radical state and strongly reduces electron 
 tunneling for the somo / sumo levels in stm experiments ( consistent 
 with the small q value obtained from the fano fits 
 below ) . we observe e < 0 for monomers and e > 0 for dimers 
 ( table 1 ) , suggesting that e is affected by the presence of neighboring bdpa radicals . ( ii ) spectral shape : the spectral shape of the conductance minimum 
 of figure 2a is well - fitted by the fano function ( compare experimental spectra with numerical 
 fits shown as black and red solid lines in figure 2a ) . the values of the fit parameters are listed in table 1 . in the fano picture , the shape of the resonance 
 is described by the asymmetry parameter , q. the numerical 
 value of q depends on the quantum mechanical phase 
 difference between different coherent spin - conserving tunneling paths 
 of electrons between stm tip and kondo system and determines whether a dip ( reduced conductance ) or peak ( increased 
 conductance ) is observed . for bdpa / au(111 ) , we obtain an almost constant 
 value of q  0.2 for monomers and 
 dimers . the small absolute value 
 of q indicates that electrons tunnel predominantly 
 between the tip and the continuum of substrate states rather than 
 the molecular impurity state . ( iii ) temperature dependence : 
 their lower surface mobility compared 
 with monomers facilitates the recording of di / dv spectra of bdpa dimers up to 50 k ( figure 2c ) . with increasing temperature , the signal width , 2 , 
 increases and the amplitude decreases simultaneously , as expected 
 for surface kondo systems . numerical values of 
  determined by fano fits of each spectrum are plotted against 
 the substrate temperature in figure 2d for 
 the bdpa dimer . we have further analyzed the temperature broadening 
 based on a fermi liquid description put forward by nagaoka et al . the least - squares fit based on this formalism , 
  = 2((kbt ) + 2(kbtk ) ) , is shown as a dashed curve 
 in figure 2d and yields a nominal value of tk = 54  5 k for the bdpa dimer . this value 
 is consistent with the requirement of fermi liquid theory for the 
 existence of kondo physics that the width 
 of the resonance , 2 , has to be much smaller than the spectral 
 energy of the bare impurity level ( see above ) . ( iv ) vibrational 
 satellites : the conductance minimum is accompanied 
 by two broad shoulders symmetrically offset above and below the kondo 
 dip ( marked by arrows in figure 2a ) . this is common for molecule - based kondo systems , where 
 the impurity orbital is spatially extended and thus sensitive to geometric 
 changes of the molecular backbone . the shoulders 
 in figure 2a range from 25 to 90 mev 
 above and below the kondo dip . this is significantly larger than the 
 expected thermal broadening of a single vibrational mode and points 
 to the excitation of a number of different vibrational modes with 
 wave numbers between 200 and 750 cm . most 
 likely , collective backbone vibrations of bdpa are involved because 
 plain c  h vibrations typically exhibit wave numbers higher 
 than 1000 cm . this interpretation is supported 
 by density functional theory calculations of bdpa in the gas phase 
 performed previously by our group . the 
 vertical bars in the top part of figure 2a 
 mark the calculated energies of collective backbone vibrations ; they 
 are symmetrically aligned above and below the kondo minimum of the 
 dimer ( marked by dashed line ) . indeed , a number of calculated energies 
 lie within the broad symmetric shoulders of the spectrum . ( a ) di / dv spectrum 
 of bdpa monomer ( red ) and a dimer ( black ) recorded 
 at 5 k over positions marked by circles in the stm topographic insets ; 
 the feedback loop has been opened at + 1 v , 50 pa , and a z - offset of 70 pm ; the zero - bias conductance of pristine au(111 ) was 
 calibrated to 1.3 ns ; solid lines are numerical fano fits of the kondo 
 antiresonance ( fit parameters see table 1 ) ; 
 arrows mark broad shoulders attributed to inelastic excitations ( details 
 see text ) ; and vertical bars indicate energies of possible collective 
 vibrational modes obtained from dft calculations relative to the kondo 
 dip of the dimer marked by the dashed line . ( b ) di / dv kondo spectra over different positions (  ) 
 of a bdpa dimer ; scale bar = 0.2 nm . ( c ) di / dv kondo spectra of the bdpa dimer at different temperatures ; 
 the feedback loop has been opened at + 1 v , 50 pa , and a z - offset of 70 pm . ( d ) temperature - dependent broadening of the kondo 
 signal for the bdpa dimer recorded over similar stm tip position close 
 to the rim of bdpa ; the dashed line represents best - fit based on the 
 formalism proposed by nagaoka et al . fano fit parameters 
 of the bdpa 
 monomer ( average of four ) and dimer ( average of three ) adsorbed on 
 au(111 ) at 5 k ; asymmetry parameter q , energy offset 
 e , and half - width . in figure 2b , the inelastic excitations 
 are not as clearly distinguishable from the kondo signal as in figure 2a , which hampers the accurate determination of the 
 kondo parameters with respect to the precise location over the bdpa 
 molecule . we attribute the spectral deviations to different tip structures , 
 varying adsorption sites , excitation of lateral motion ( diffusion ) 
 and noise in the di / dv signal . nevertheless , 
 figure 2b evidences considerable variations 
 of the kondo amplitude over different positions of the radical . the 
 kondo amplitude of bdpa reaches maximum values close to the rim compared 
 with decreased values over the radical , similar to metal  organic 
 adsorbates with significant spin contributions from the hydrocarbon 
 backbone . the spatial distribution of the kondo amplitude 
 over the molecule is commonly assigned to the distribution of the 
 unpaired spin in the impurity orbital . in that respect , the existence 
 of a kondo signal indicates that the bdpa radicals remain magnetic 
 after adsorption on au(111 ) . our recent electron spin resonance ( esr ) 
 experiments on samples with bdpa monolayer coverage on au(111)/mica 
 substrates have yielded g = 1.96 at 7 k. this value is in good agreement with the isotropic g factor of g = 2.008 of the bdpa crystalline 
 bulk phase at 4 k and g = 2.0026 at room temperature . the observed esr activity of monolayer bdpa / au(111 ) 
 together with the small g shift compared with bulk 
 phase suggest that a possible charge transfer from the au substrate 
 to the adsorbed bdpa is small . this is corroborated by the standing 
 wave pattern of the au(111 ) surface state shown in figure 1d , where we find no evidence of an enhanced electrostatic 
 scattering potential at the bdpa radicals that would indicate a significant 
 charge transfer . the above results corroborate 
 that the bdpa radicals preserve the initial radical spin-1/2 state 
 upon adsorption on the au(111 ) surface . figure 3 illustrates 
 the results of distance - dependent sts 
 measurements along a specific high - symmetry direction of a bdpa dimer . sts spectra were recorded with the stm tip over positions at increasing 
 lateral distance , r , from the center of bdpa , as 
 marked by the circles in figure 3a . the respective 
 spectra shown in figure 3b exhibit the characteristic kondo dip even over the 
 pristine au(111 ) surface at large lateral distance of up to 1.5 nm 
 from the center of the radical . outside of bdpa the signal amplitude 
 decreases monotonically according to 1/r . the 1/r dependence suggests that 2-d electronic states contribute to kondo 
 screening . in the present case , this points 
 to the surface - state band of au(111 ) , which acts as a 2-d electron 
 gas . our experimental results indicate that the two bdpas of 
 a dimer are individual spin-1/2 kondo systems . nevertheless , their 
 spins appear to be correlated through the surface - state : the two kondo 
 screening clouds are so large that they may readily overlap ; that 
 is , the two bdpas of the dimer share part of a common kondo cloud . this seems to stabilize the individual screened states , as evidenced 
 by the higher tk of dimers compared with 
 monomers . ( a ) stm topography 
 of bdpa dimer recorded at + 1 v and 50 pa ; scale bar = 1 nm . ( b ) di / dv kondo spectra recorded at different 
 tip positions marked by the circles in panel a ; the feedback loop 
 has been opened at + 1 v , 50 pa , and a z - offset of 
 70 pm . ( c ) decay of the kondo amplitude with increasing lateral distance , r , from the center of bdpa ; dashed line is a fit of 1/r . in 
 conclusion , our experimental stm and sts results obtained at 
 submolecular resolution from monomers and dimers of stable hydrocarbon 
  radicals on au(111 ) provide conclusive evidence of establishing 
 bdpa / au(111 ) as a novel kondo system , where a radical state in gas - phase 
 is preserved after adsorption on a metal surface .
stable hydrocarbon radicals are utilized 
 as spin standards and 
 prototype metal - free molecular magnets able to withstand ambient conditions . 
 our study presents experimental results obtained with submolecular 
 resolution by scanning tunneling microscopy and spectroscopy from 
 monomers and dimers of stable hydrocarbon  radicals adsorbed 
 on the au(111 ) surface at 750 k. we provide conclusive evidence 
 of the preservation of the radical spin-1/2 state , aiming to establish 
 ,-bisdiphenylene--phenylallyl ( bdpa ) on au(111 ) 
 as a novel kondo system , where the impurity spin is localized in a 
 metal - free  molecular orbital of a neutral radical state in 
 gas phase preserved on a metal support .