the lanthanide double - decker 
 molecules , and in particular the terbium 
 double - decker molecule ( bis(phthalocyaninato)terbium(iii ) , tbpc2 , figure 1 ) , 
 have attracted great interest due to their single - molecule magnet 
 behavior . this property makes them promising 
 candidates for those applications in spintronics and quantum computing , 
 where the spin functionality is retained by individual single molecules . substrate interaction , the molecular 
 structure , the large spin magnetic moment of the lanthanide center , 
 as well as the property as single - molecule magnets , are preserved ; 
 the latter property , however , was found to suffer from a reduction 
 of the magnetic hysteresis on noble and ferromagnetic metals . recent 
 successful experiments to switch the electronic and nuclear spin state 
 at the level of single individual tbpc2 molecules adsorbed 
 on surfaces have fueled attempts to enhance the magnetic bistability of surface - adsorbed 
 tbpc2 molecules by tailoring the molecule a prerequisite for optimizing the magnetic behavior ( achieving 
 long spin lifetimes ) is a weak electronic coupling between the molecule 
 and substrate . while lattice formation of tbpc2 on au(111 ) substrate interaction , a significant enhancement of the magnetic bistability 
 has been recently achieved by chemical grafting of tbpc2 derivatives on doped semiconductor surfaces . previous attempts to electronically decouple 
 similar functional molecules from the substrate include thin oxide or cloride layers , 
 which have been found to lower the electronic interaction between 
 molecules and metal substrates . moreover , decreased electronic coupling 
 of molecules and substrates has been reported for multilayer coverages 
 of ( metal)organic functional molecules such as porphyrins , phthalocyanines , and c60 on different surfaces . the 
 bottom phthalocyanine ligand is displayed in transparency 
 for clarity ( top view ) . here , we investigate an alternative strategy for lowering 
 the coupling 
 of tbpc2 molecules to a metal template , utilizing a compact 
 ordered monolayer of tbpc2 , as a spacer layer , on top of 
 which additional tbpc2 molecules are adsorbed . their topographical 
 and electronic properties are investigated by scanning tunneling microscopy 
 ( stm ) and - spectroscopy . while the structural and electronic properties 
 of tbpc2 have been studied by stm on different substrates 
 including au(111 ) , cu(111 ) , co(111 ) , ir(111 ) , highly ordered pyrolytic graphite , and h si(001 ) , reports on bilayer tbpc2 are still scarce . our results 
 indicate that the tbpc2 molecules of the second layer lie 
 in a flat manner on the spacer layer and form a square - like packing 
 similar to the underlying monolayer but are weakly coupled to the 
 au(111 ) substrate . the single - crystal au(111 ) 
 substrate , exhibiting the well - known 
 22  3 herringbone reconstruction , was prepared by repeated 
 cycles of 0.6 kev ar ion bombardment and annealing at 
 about 700 k. a neutral form of tbpc2 was obtained by modified 
 protocol based on templating reactions starting from a mixture of 
 the phthalonitrile precursor o - dicyanobenzene and 
 the terbium acetylacetinate tb(acac)nh2o , in the presence of a strong base ( e.g. , dbu ) and high - boiling 
 solvents , such as hexanol . deep green crystals 
 of the products can be produced by using slow diffusion of ch2cl2 into c2h2cl4 solution of the pristine material . the tbpc2 crystallized 
 in the space group ( -phase ) in the same way as reported by takeya 
 et al . tbpc2 was thermally 
 evaporated from a quartz crucible at a source temperature of 700 k 
 and a pressure of less than 1  10 mbar onto 
 the au(111 ) substrate held at room temperature , after thorough degassing 
 the molecules for > 12 h at 363 k 
 and < 1 h at 473 
 k. stm experiments were performed at 5 k and a base pressure below 
 5  10 mbar employing electrochemically 
 etched w tips deoxidized by annealing above 1100 k in vacuum . the differential tunneling 
 conductance , di / dv , signal was obtained 
 from the first harmonic current signal detected by lock - in technique 
 with sinusoidal modulation of the sample bias at 704 hz and 10 mv 
 zero - to - peak . di / dv spectra were 
 recorded at constant - height conditions after deactivating the stm 
 feedback at a set point of + 1 v and 50 pa with z - offset 
 85 pm and sweep time of 40 s. impurity and tip effects were 
 minimized by multiple tip forming ( controlled indentation ) on the 
 pristine au substrate between the experiments , resulting in au - coated 
 tips . reliable tip performance was established by accurately reproducing 
 the characteristic conductance signature of the au(111 ) surface state 
 well - known in the literature . the tbpc2 molecule is a double - decker complex , where 
 a single tb ion is sandwiched by two phthalocyanine ligands ; 
 the two ligands are oppositely domed ( concave and convex ) and rotated 
 by 45 with respect to each other ( figure 1 ) . on a au(111 ) 
 surface , tbpc2 adsorbs flat , i.e. , with the ligand planes 
 parallel to the substrate surface  as previously reported for 
 individual molecules as well as ordered molecular film.figure 2a shows an stm image obtained after depositing tbpc2 molecules on au(111 ) with approximately 80% of coverage . the dark 
 area of the image corresponds to the bare substrate ; the au112 
  ( herringbone ) direction is indicated by an arrow . the large 
 bright ( orange ) region , labeled 1 , represents an ordered film of tbpc2 molecules lying directly on au(111 ) and denoted herein as 
 first - layer film . molecules of the first - layer film form a square - like 
 packing ( about 1.4  1.4 nm ) as reported earlier ; respective lattice vectors are labeled   and . on top of the first - layer film , additional 
 ( bright ) protrusions are imaged by stm ( figure 2a ) , exhibiting size and shape very similar 
 to that of first - layer tbpc2 molecules . apparently , additional 
 tbpc2 molecules lie on top of the first - layer film , either 
 as individual molecules ( labeled 2 ) or as part of a molecular film 
 ( labeled 3 ) . ( a ) stm image of tbpc2 molecules on au(111 ) 
 at a coverage 
 of approximately 80% ( 25  27 nm , 40 pa , + 0.9 v ) ; 
 arrow marks au112 direction ; inset : stm image 
 of a single isolated tbpc2 molecule on au(111 ) , 3  
 3 nm , 50 pa , + 0.9 v ; labels : see text . ( b ) topographic 
 stm height profiles ; left : profile across the film of 1st- and 2nd - layer 
 tbpc2 molecules along the dashed line in ( a ) ; right : profile 
 across a single tbpc2 molecule on au(111 ) along the dashed 
 line in the inset of ( a ) . in order to unambiguously corroborate this attribution of 
 type 
 2 and type 3 molecules , figure 2b , left , shows a representative topographical profile across the 
 film along the dashed line in figure 2a . the profile shows that molecules in the first - layer 
 film ( type 1 ) have an almost identical height of 0.4 nm compared to 
 individual single tbpc2 molecules on au(111 ) . the latter 
 is shown in the inset of figure 2a ( type 4 ) ; the respective profile of a single tbpc2 molecule ( dashed line in the inset of figure 2a ) is shown in figure 2b , right . in contrast , type 2 and type 3 
 molecules lying on top of the first - layer film exhibit a height of 
 about 0.7 nm with respect to the au(111 ) substrate . this value is 
 slightly lower than the expected value of 0.8 nm , corresponding to 
 two times the first - layer height . we rule out 
 that type 2 and type 3 molecules are triple deckers or phthalocyanines 
 formed accidentally during evaporation : a comparison with the literature 
 shows that triple - decker phthalocyanines exhibit significantly lower 
 stm topographic height of about 0.55 nm on ag(100 ) and au(111 ) . furthermore , 
 in contrast to the eight - lobe appearance of tbpc2 ( see 
 below ) , phthalocyanine molecules ( 5 , figure 2a ) exhibit typical four - lobe appearance in 
 topographic stm images with a height of only 0.14 nm on au(111 ) . based on these findings , we attribute type 2 and type 3 to intact 
 tbpc2 molecules lying on top of the first layer , i.e. , 
 representing second - layer tbpc2 molecules . figure 3a juxtaposes 
 typical topographic stm images of single isolated tbpc2 molecules adsorbed on the au(111 ) surface ( left ) as well as on top 
 of the first - layer film ( right ) with increased magnification ; both 
 images are displayed with the same lateral scale of 2.5 nm and topographical z - scale of 0.4 nm . obviously , the internal structure imaged 
 by stm ( i.e. , the intramolecular contrast ) appears to be very similar 
 in both images , leading to a characteristic eight - lobe appearance . 
 the lobes are labeled 18 in the figure . as shown earlier , the stm topographic appearance arises predominantly 
 from the density of states of the upper phthalocyanine ligand of tbpc2 . notice that the eight lobes are not equispaced but rather 
 form four pairs with closer spacing ( 12 , 34 , 56 , 
 and 78 ) . a close inspection of figure 3a reveals that the lateral diameter of isolated 
 single tbpc2 molecules , measured as full width at half - maximum 
 of the topographic profile , is about 10% smaller in the case of the 
 second layer than in the first layer . as a guide to the eye for comparing 
 the lateral sizes of molecules , figure 3a , right , displays a chemical structure model of tbpc2 ( top view ) with van der waals atomic radii and scaled 1:1 
 with respect to the stm images . ( a ) typical stm images of isolated single 
 tbpc2 molecules ; 
 left : on au(111 ) ( type 4 ) , 2.5  2.5 nm , 50 pa , + 0.9 
 v ; middle : adsorbed on top of a compact first - layer film of tbpc2 ( type 2 ) , 2.5  2.5 nm , 40 pa , + 0.9 v ; right : 
 chemical structure model showing van der waals atomic radii of tbpc2 as guide to the eye . ( b ) histogram of azimuthal orientation 
 angles measured on different isolated single tbpc2 molecules 
 ( type 2 ) lying on top of the first - layer film ; a gaussian distribution 
 is shown in red ( r - square : 0.85 ) ; inset : definition 
 of the azimuthal angle used for obtaining histogram ( see text for 
 details ) . the second - layer tbpc2 molecules are found to lie on 
 top of first - layer molecules with their centers aligned ( figure 2a ) ; a similar behavior 
 has been reported for lutetium double - decker molecules . a detailed 
 topographic analysis of our stm images reveals that the isolated single 
 tbpc2 molecules of the second layer ( type 2 ) exhibit different 
 azimuthal orientations relative to the underlying first molecular 
 layer . we have determined the values of the azimuthal angles of more 
 than 65 type 2 molecules . here , the azimuthal angle is defined as 
 the angle between the high - symmetry axes of second- and first - layer 
 molecules , as depicted in the inset of figure 3b . a rather high variation of angles between 
 28 and 45 is found , as illustrated by the histogram of figure 3b . the reported data 
 are fitted with a gaussian distribution ( red line in figure 3b ) . the maximum of the distribution 
 lies at 36.5. in contrast , tbpc2 molecules within 
 compact films of the second layer ( type 3 ) are found to exhibit almost 
 identical azimuthal orientation of about 45. a comparison 
 between compact films of tbpc2 molecules 
 in the second and first layer is shown in figure 4a and b , respectively . the images have been 
 recorded at slightly different bias voltages of + 0.7 and + 0.9 v , respectively , 
 which correspond to the energies of strong unoccupied molecular orbital 
 states of the second- and first - layer tbpc2 molecules ( discussed 
 in detail below , see figure 5 ) . as clearly seen in figure 4 , the first and second layers exhibit very similar 
 packing structures , both characterized by a square - like unit cell 
 of approximately 1.4  1.4 nm . the lattice vectors 
 are labeled a1,2 and b1,2 , respectively . a schematic model of the tbpc2 molecule stm images ( 4.5  4.5 nm ) of the ( a ) 
 2nd tbpc2 layer ( 40 pa , + 0.7 v ) and ( b ) 1st tbpc2 layer 
 ( 40 pa , + 0.9 v ) ; the au112 direction is indicated 
 in both images ; the 1st- and 2nd - layer molecules are ordered with 
 the same square - like pattern with lattice vectors ,   ( , ) indicated by arrows . schematic models 
 of the tbpc2 molecules are shown as a guide to the eye . experimental di / dv curves of 
 tbpc2 in the 1st layer ( a ) and 2nd layer film ( b ) and isolated 
 molecule ( c ) . prominent features are labeled i , ii , iii , k , and m 
 ( see text ) . the reduced height observed 
 for second - layer molecules may point 
 to a changed electronic coupling compared to the first layer . to investigate 
 the electronic coupling , we have studied the frontier - orbital related 
 electronic properties of first- and second - layer tbpc2 molecules 
 by spatially resolved scanning tunneling spectroscopy . figure 5 displays typical energy spectra 
 of the differential tunnel conductance ( di / dv ) recorded on first- and second - layer molecules , respectively 
 ( with the stm tip placed over one of the eight - lobe features ) . the 
 curve of figure 5a 
 shows a typical spectrum of tbpc2 within the first - layer 
 film ( type 1 ) . it exhibits pronounced peaks , consistent with the literature , 
 at about 0.9 ev ( i ) , + 0.8 ev ( ii ) , and 0.2 ev ( iii ) . peaks similar to i and ii have been previously assigned to the highest 
 occupied and lowest unoccupied molecular orbitals , respectively , and a peak similar to iii has been attributed to the singly occupied 
 molecular orbital of tbpc2 . the latter has been reported to exhibit a rather pronounced variation 
 of its energy between about 0.1 and 0.5 ev . here , 
 we have observed a similar large energy variation of peak iii ( see supporting information ) consistent with ref ( 13 ) , where the energy variation 
 has been attributed to a varying molecule substrate interaction 
 within the first - layer film due to a partial screening effect of the 
 surrounding molecules . in figure 5a , close to the fermi level a sharp zero - bias peak 
 is observed , labeled k. it originates from the kondo resonance of an unpaired electron ( s = 1/2 ) sitting in the 
 -orbital of the bottom phthalocyanine ligand , which interacts 
 with the electron system of the au(111 ) substrate . in addition , a broad peak , labeled m , is observed close 
 to + 0.1 ev that was previously assigned to the electronic contribution 
 of the metal substrate  in particular to hybrid 
 electronic states of first - layer tbpc2 molecules and au(111 ) . the curve of figure 5b displays a typical di / dv spectrum 
 of second - layer molecules in films ( type 3 ) . intriguingly , no conductance 
 features are observed close to the fermi level , which is in marked 
 contrast to the first - layer molecules ( compare curves a and b of figure 5 ) . in other words , 
 neither the kondo resonance ( k ) nor a hybrid electronic state ( m ) 
 with strong contributions of both the substrate and molecule are found 
 in the di / dv spectrum of second - layer 
 molecules . similarly , the spectra of isolated single molecules of 
 the second layer also lack both k and m , as clearly shown in figure 5c . since k and m , 
 both , originate from the electronic interaction of tbpc2 molecules and au(111 ) substrate , their observed absence in figure 5b indicates that 
 such interactions are negligible for second - layer tbpc2 molecules . notice that we have intentionally placed the stm tip 
 over the ligand position for comparing the local spectroscopic features 
 of first- and second - layer molecules because k is well - known to relate 
 to an unpaired -electron spin located on the phthalocyanine 
 ligand , rather than the metallic center . while k is 
 of magnetic origin ( many - body spin scattering ) , in contrast , m originates from the hybridization of substrate 
 and first - layer molecular electronic states . as shown previously , the observed absence of k may indicate only 
 a different interaction of the ligands of first- and second - layer 
 molecules since they are known to participate deeply to the magnetic 
 properties of tbpc2 . however , the additionally 
 observed absence of m indicates a decreased electronic coupling of 
 second - layer molecules with the substrate . our finding is in line 
 with earlier results on multilayer iron - phthalocyanine molecules on 
 ag(111 ) , where the vanishing of the electronic contribution from the 
 substrate was reported to occur already in the third phthalocyanine 
 layer . in addition to the lack 
 of substrate - related peaks close to ef , the spectra of second - layer molecules are 
 clearly distinguished by the different energies of their frontier - orbital - related 
 peaks . the compact second layer of tbpc2 ( type 3 , curve 
 b ) exhibits a pronounced conductance peak at 1.15 ev together 
 with two peaks located almost symmetrically above and below ef at 0.65 ev . the symmetric peaks apparently 
 correspond to the highest occupied and lowest unoccupied electronic 
 levels . similar energies have been earlier reported for a bilayer 
 of lutetium double - decker molecules on highly oriented pyrolytic graphite . intriguingly , the symmetry of occupied and unoccupied 
 level energies is absent in the case of isolated single molecules 
 ( type 2 ) as shown in curve c. compared to curve b , the whole spectrum 
 of curve c appears to be shifted by about 0.25 ev toward the positive 
 side . apparently , this is a consequence of the lack of second - layer 
 molecule  molecule interactions . finally , we remark that the 
 peak energies observed in various different second - layer tbpc2 molecules of both type 2 and type 3 have shown only variation 
 smaller than 0.1 ev . this is in marked contrast to the more 
 pronounced variation of more than 0.2 ev observed for peak iii 
 of first - layer molecules ( see supporting information ) . obviously , the electronic level energies of the second - layer molecules 
 are better defined . in conclusion , we present 
 experimental results obtained by scanning 
 tunneling microscopy and spectroscopy on the structural and electronic 
 properties of second - layer tbpc2 molecules , i.e. , tbpc2 molecules adsorbed on top of a compact monolayer of tbpc2 on au(111 ) . our results indicate that the first monolayer 
 of tbpc2 decreases the electronic coupling of second - layer 
 molecules with the au(111 ) substrate . this is evidenced by the observed 
 ( i ) absence of electronic states originating from molecule substrate 
 interactions close to the fermi level and ( ii ) negligible variation 
 of the energy of frontier - orbital - related states . in this respect , second - layer tbpc2 molecules exhibit frontier - orbital electronic 
 properties similar to the gas phase .
we report a low - temperature scanning 
 tunneling microscopy and spectroscopy 
 study of the structural and electronic properties of a bilayer of 
 terbium double - decker ( bis(phthalocyaninato)terbium(iii ) , tbpc2 ) molecules on au(111 ) at 5 k. the tbpc2 molecules 
 are found to adsorb flat on top of a first compact tbpc2 monolayer on au(111 ) , forming a square - like packing similar to the 
 underlying first layer . 
 their frontier - orbital electronic structure , 
 measured by tunneling conductance spectroscopy , clearly differs from 
 that of the underlying first monolayer . 
 our results of second - layer 
 molecules indicate the absence of , both , hybrid molecule 
 substrate 
 electronic states close to the fermi level and a zero - bias kondo resonance . 
 we attribute these findings to a decreased electronic coupling with 
 the au(111 ) substrate .