ARTICLESPUBLISHED ONLINE: 20 JANUARY 2013 | DOI: 10.1038/NMAT3547
Site- and orbital-dependent charge donation andspin manipulation in electron-doped metalphthalocyaninesCornelius Krull1, Roberto Robles2, Aitor Mugarza1* and Pietro Gambardella1,3
Chemical doping offers promise as a means of tailoring the electrical characteristics of organic molecular compounds.However, unlike for inorganic semiconductors used in electronics applications, controlling the influence of dopants inmolecular complexes is complicated by the presence of multiple doping sites, electron acceptor levels, and intramolecularcorrelation effects. Here we use scanning tunnelling microscopy to analyse the position of individual Li dopants within Cu-and Ni-phthalocyanine molecules in contact with a metal substrate, and probe the charge transfer process with unprecedentedspatial resolution. We show that individual phthalocyanine molecules can host at least three distinct stable doping sites and upto six dopant atoms, and that the ligand and metal orbitals can be selectively charged by modifying the configuration of the Licomplexes. Li manipulation reveals that charge transfer is determined solely by dopants embedded in the molecules, whereasthe magnitude of the conductance gap is sensitive to the molecule–dopant separation. As a result of the strong spin–chargecorrelation in confined molecular orbitals, alkali atoms provide an effective way for tuning the molecular spin without resortingto magnetic dopants.
Electron doping of organic semiconductors constitutes aneffective strategy for enhancing the performances of organicoptoelectronic devices1 and fabricating materials with exotic
electronic properties2, such as organic superconductors3,4 andMott–Hubbard antiferromagnets5. In many cases, doping isachieved by the intercalation or co-deposition of alkali metalswith molecular complexes, which can lead to greatly increasedconductivity6–8, better carrier injection efficiency from metalelectrodes9 and improved electroluminescent properties10. Thefamily of metal-phthalocyanines (MPc), a class of macro-cyclicplanar polyconjugated molecules, represents one of the bestcandidates to study the effects of chemical doping on organicsystems and metal–organic contacts11–22. The great chemicalstability and distinctive optical and electrical properties makethem equally appreciated for technological applications, includingorganic field effect transistors23,24, light emitting devices10,25 andphotovoltaic cells26, as well as for fundamental studies11–22.
Recent experiments reveal that multiple electron doping ofMPc with alkali atoms leads to striking insulator–metal–insulatortransitions as the molecular orbitals involved in the conductiongo from being entirely empty to completely filled19,20. Moreover,the deposition of alkali species has been shown to improve boththe spin injection efficiency27 and spin coupling28 between MPcand ferromagnetic metal layers. Photoemission and electron energyloss studies show that the filling pattern of MPc orbitals witheither π- or d-character depends on the dopant concentration aswell as on the central metal ion, highlighting the complexity ofthe doping process in metal–organic compounds11–17. Despite theincreasing attention devoted to such systems, however, there is stillno detailed description of the doping mechanisms at the singlemolecule level. Likewise, the interplay between dopants, molecules,
1Catalan Institute of Nanotechnology (ICN), UAB Campus, E-08193 Barcelona, Spain, 2Centre d’Investigacions en Nanociència i Nanotecnologia CIN2(ICN-CSIC), UAB Campus, E-08193 Barcelona, Spain, 3Institució Catalana de Recerca i Estudis Avançats (ICREA) and Departament de Física, UniversitatAutonoma de Barcelona, E-08193 Barcelona, Spain. *e-mail: [email protected].
and the conduction electron bath of a metal interface, whichdetermines the low energy spectrum of the molecules and theirconductivity, remains an open issue. As ametallic substrate can playthe dual role of charge donor towards the molecules and acceptorfor the dopants, this interplay needs to be addressed in detail.
Here, by performing scanning tunnelling microscopy (STM)and spectroscopy experiments and ab initio electronic structurecalculations, we investigate the charge transfer process in controlleddoping sequences of CuPc and NiPc deposited on an Ag(100)substrate. We identify distinct doping sites, unequivocally relatingthe dopant position to the filling of specific molecular orbitals, andshow that, depending on the doping configuration, a molecularcharge (Q) and spin (S) of Q= 1 and 2 electrons, and S= 0,1/2,can be obtained for a single donor atom. By manipulating Li atomsone-by-one, we show that a single molecule can accomodate up tosix dopants. However, in spite of the fact that the doubly degeneratelowest occupied molecular orbital of CuPc and NiPc can be filledwith up to four electrons in thickMPc films19,20, we find that chargetransfer is limited to Q = 2 due to the presence of the metallicsubstrate. The latter acts as an electron sink to doped molecules,whereas it acts as an electron donor to undoped species. Finally, weshow that long-range electrostatic interactions can also affect theelectronic structure of MPc, as Li adatoms located as far away as3 nm from the centre of the molecules modify the gap between thehighest occupied and lowest unoccupiedmolecular orbitals.
Formation of LixMPc complexesCuPc and NiPc molecules were deposited on a clean Ag(100)surface by thermal evaporation in ultra-high-vacuum. Doping wasperformed by dosing submonolayer amounts of Li at 5 K onthe same surface. Figure 1a shows an STM image of Li atoms
Figure 1 | Formation of LixCuPc complexes. a, STM topographic image of CuPc molecules and Li atoms co-deposited on Ag(100). The image size is19.3 nm× 19.3 nm. Tunnelling bias conditions: Vbias=−0.3 V, It=0.17 nA. Green and red labels indicate doped molecules that formed spontaneously afterLi deposition. b–d, Illustrative diagrams (left) and close up STM images (right) of different doping configurations: b, LiCuPc-LA , c, LiCuPc-LB , and d,LiCuPc-M.
co-adsorbed with CuPc. The undoped molecules exhibit a typicalfour-leaf structure originating from the four isoindole ligandsthat form the aromatic Pc macrocycle. Each ‘leaf’ presents acharacteristic asymmetric intensity due to the rotation of the ligandaxes relative to the high symmetry directions of the substrate,which gives rise to a chiral adsorption geometry29,30. The centreappears as a depression owing to the planar symmetry of theCu2+ d-states near the Fermi level (EF), which have a very lowtunnelling probability to the STM tip. Figure 1a reveals the presenceof several molecules with distinct topographic features comparedto CuPc, which we attribute to the spontaneous uptake of Liatoms. The most common complexes, constituting more than75% of the total number of doped species, are those labelledLiCuPc-LA and LiCuPc-M. The first type presents a brightdouble-lobe structure in correspondence of a benzene ring and adark centre. The LiCuPc-M type, on the other hand, presents abright centre and four symmetric ligands. Themodified topographyof the molecules can be directly associated to the position ofthe Li dopants on ligand and metal sites, respectively, as shownin the diagrams in Fig. 1b,c. Moreover, by manipulating the Liatom of a LiCuPc-LA complex with the tip of the STM, wefind that a third stable doping site exists next to the aza-Natoms, which we label as LiCuPc-LB (Fig. 1c). These experimentalfindings are corroborated by density functional theory (DFT)calculations of several LiCuPc and LiNiPc bonding configurations(Fig. 2 and Supplementary Information). The configurations withthe lowest adsorption energy of Li correspond to LA, LB, andM. Furthermore, total energy calculations and simulations ofthe STM topography suggest that the Li atom in the LiCuPc-LAcomplex is located between the benzene ring and the Agsurface rather than on top (Fig. 2 and Supplementary Fig. S3).Other configurations have also been observed in rare instances(Supplementary Fig. S1).
Selective charge and spin doping of molecular orbitalsThe adsorption and electronic structure of CuPc and NiPc onAg(100) has been extensively characterized in previous studies29–32.
This allows us to focus on the effects of doping and, in particular,to correlate the dopant position to the charge transfer anddistribution within the molecules. We performed differentialconductance (dI/dV ) measurements to probe the electronicstructure of the different complexes, namely the local densityof states at the position of the STM tip (see Methods formore details). We recall that undoped CuPc on Ag(100) havea triplet ground state given by the coupling of the Cu spin(SM) to a ligand spin (SL) induced by charge transfer fromthe substrate, which resides in a 2eg orbital14. We refer tothis state as having charge Q = 1 (relative to gas phase CuPc)and total spin S = SM + SL = 1 (Fig. 3). The dI/dV spectraof CuPc reflect the single occupancy of the 2eg orbital as oneconductance peak below EF , labelled ‘2eg’ in Fig. 3a, and itsunoccupied counterpart ‘2eg + U’ above EF, separated by theCoulomb energy U that corresponds to the energy cost of placingtwo electrons in the same orbital. Spatial maps of the dI/dVintensity, reported in Fig. 3b, confirm that the 2eg and 2eg+U stateshave overlapping intensity and identical symmetry, as expected.At low temperature, the 2eg ligand spin is screened by Agelectrons through the Kondo effect, which gives rise to the sharpresonance observed at EF.
The doped species present a very different electronic structurecompared to CuPc. We discuss first the case of Li attached to ligandsites, taking LiCuPc-LA as a representative example. Interestingly,all the L-type configurations present very similar dI/dV spectra,leading to the conclusion, also supported by theory, that the chargetransfer process is approximately the same in all the cases whereLi dopants interact primarily with the aromatic Pc ring (Fig. 2cand Supplementary Fig. S1). The dI/dV spectrum of LiCuPc-LAreveals a multiple peaked structure below EF, labelled α2eg , and ashallow peak above EF, labelled β2eg . Unlike the 2eg orbital in CuPc,spatial maps of the occupied α2eg and unoccupied β2eg features showtwo non-overlapping intensity profiles, each oriented along a singleligand axis (Fig. 3c). According to their symmetry, these orthogonalmolecular orbitals originate from the splitting of the doublydegenerate 2eg orbital of CuPc induced by Li. DFT calculations of
Figure 2 | Calculated electronic structure of LixMPc complexes. a, Relaxed geometry calculated for different bonding configurations of LiCuPc. b, Totalenergy difference relative to the minimum energy configuration for the different configurations. c, Differential charge of the molecules and of thetransition-metal ions relative to the gas-phase. d, Magnetic moment of the transition-metal ions. e, Projected density of states (PDOS) of the undopedspecies, LA , and M configurations. Black solid and dashed lines represent respectively majority and minority states projected on the molecule. Colouredlines represent projections to different d states of the metal ion.
the charge distribution confirm this assignment (SupplementaryFig. S2). The equally spaced multiple peaks observed for the α2egresonance correspond to vibronic excitations (see SupplementaryInformation), indicating partial decoupling of the doped complexfrom the substrate33. The vanishing intensity of the α2eg peaksapproaching EF and the absence of its Coulomb pair above EFindicate that the α2eg orbital is fully occupied. With two electronsoccupying one of the formerly degenerate 2eg states of CuPc,LiCuPc-LA is thus doubly charged. Moreover, the pairing of theelectron spins in the α2eg orbital leads to SL= 0, in agreement withthe disappearance of the Kondo resonance at EF (Fig. 3a). DFTcalculations, on the other hand, show that the Cu spin remainsunperturbed, as charge transfer is restricted to the α2eg orbital(Supplementary Table SI). Therefore, Li doping at ligand sites leadsto aQ= 2, S= 1/2 configuration.
LiCuPc-M differs from both CuPc and LiCuPc-LA. For thiscomplex, a single peak is observed above EF (Fig. 3a), which we
assign to an unoccupied 2eg resonance because of its four-foldsymmetry about the ligand axes (Fig. 3d), similar to that of theunoccupied 2eg states of CuPc. The higher intensity at the centreis due to the presence of the Li atom at this site. The absence ofoccupied 2eg peaks indicates that Li dopants at M-sites inhibitelectron transfer from the substrate to ligand orbitals, leading toSL = 0. Although STM provides no access to the Cu states near EF(ref. 30), our DFT calculations clearly show that Li transfers oneelectron to the b1g orbital, the molecular state that originates fromthe hybridization of the Cu dx2−y2 and N 2p states, as shown by thespin-resolved projected density of states of LiCuPc-M comparedto CuPc and LiCuPc-LA (Fig. 2e). This leads to a nonmagneticcomplex with Q= 1 and SM= SL= 0. Note that the larger chargetransfer obtained by DFT (Fig. 2c and Supplementary Table SI)corresponds to an excess of charge in the 2eg states arising from thewell-known problem of treating correlation in delocalized orbitalswith this method34. For the same reason, the splitting (U ) of
Figure 3 | Selective orbital doping of CuPc. a, dI/dV spectra taken on the benzene ring of CuPc, LiCuPc-LA , and LiCuPc-M. The setpoints areVbias=−1 V, I= 1 nA for LiCuPc-LA and LiCuPc-M and Vbias=−2 V, I= 3 nA for CuPc. A vertical offset has been added for clarity. b–d, Topographicimages and dI/dV maps of CuPc (b), LiCuPc-LA (c), and LiCuPc-M (d) at the energies of the 2eg-related orbitals. Dashed arrows in the schematic orbitalfilling diagram correspond to the spin of pristine molecules, whereas solid arrows correspond to charge/spin transferred from substrate and dopant.
the singly and doubly occupied 2eg states cannot be reproducedwith accuracy by DFT.
Changing Cu for Ni and Li for KMPc with different metal ions are believed to behave differentlyon doping20. Here we investigate this effect by replacing CuPc byNiPc. This choice is motivated by the fact that, aside from thesinglet ground state of the Ni2+ ion, the electronic structure aswell as the substrate-induced charge transfer for the two species arevery similar31. From the analysis of the STM images, we observethat the L-type position of the Li dopants is the same in NiPcand CuPc. However, M-type species are rare and appear fuzzyat the centre (Supplementary Fig. S1d), suggesting an unstablebonding configuration between Li and Ni atoms that can beeasily perturbed by the STM tip. This fact can be qualitativelyexplained by the smaller electronegativity of Ni compared to Cuand is reflected also by the much larger total energy computedfor LiNiPc-M relative to the L-species (Fig. 2b) as well as by thehigher energy of the b1g states of NiPc relative to CuPc (Fig. 2e).Figure 4 shows the dI/dV spectra and maps of NiPc, LiNiPc-LA,and Li2NiPc-LA. The undoped molecule presents a spectrum thatis very similar to CuPc, with a singly occupied 2eg orbital below EF,Kondo resonance, and unoccupied 2eg+U state. Charge transferin LiNiPc-LA, in contrast, differs significantly from LiCuPc-LAdespite the equivalent position of the Li donor. The two sharp peaksthat appear on either side of EF (Fig. 4a) both have the symmetry ofthe α2eg state (Fig. 4b). The orthogonal β2eg state appears at higherpositive bias and is thus fully unoccupied. Therefore, LiNiPc-LAhas charge Q= 1 as opposed to Q= 2 as for LiCuPc-LA, where theα2eg orbital is doubly occupied. This effect is attributed to the smallerelectron affinity of the NiPc adsorbate, which is also reflected bythe smaller uptake of charge estimated by DFT for NiPc relative toCuPc (Supplementary Table SI). The absence of a Kondo resonancein LiNiPc-LA related to the unpaired α2eg electron is consistent withthe sharpness of theα2eg peaks and the presence of vibronic satellites,indicating electronic decoupling from the substrate.
A Q = 2 state can be induced by doping LiNiPc-LA withan additional Li atom. The Li2NiPc-LA complex has a fullyoccupied α2eg orbital and a remarkably similar dI/dV spectrumcompared to that of singly doped LiCuPc-LA (Fig. 4a,c). Thisresults in an equivalent electronic configuration of the Pc ringfor these two species despite the fact that Li donors supply mostof the excess charge in Li2NiPc-LA, whereas the Ag substratecontributes almost one electron to LiCuPc-LA. Interestingly, thesame charge state can be reached by doping NiPc with a singleK atom (Fig. 4a). This agrees with the intuitive concept that theheavy alkali metal atoms are more effective electron donors, evenin the presence of the charge reservoir represented by the metallicsubstrate. Keeping in mind that Ni is diamagnetic in NiPc and thatcharge transfer to the metal orbitals is negligible for the L-typecomplexes (Fig. 2c), the correlated charge and spin configurationsof LiNiPc-LA and Li2NiPc-LA correspond to Q= 1, S= 1/2, andQ= 2, S= 0, respectively.
Doping limit of single MPcAn important issue to optimize the doping process of organicsemiconductors and metal–organic interfaces is the maximumnumber of dopants that can be hosted by a single molecule versustheir combined effect in terms of the amount of transferred chargeand perturbation of the orbital energies. Evidently, the answerto this question depends on the type and size of each molecule.Nevertheless, the results presented here are relevant for the MPcfamily and reveal general trends for metal–organic interfaces thatmay be extrapolated to other systems. As seen in Figs 1a and 4c,ligand sites can host more than one Li dopant. However, owingto the low coverage of Li required by this study, the spontaneousformation of higher-doped species is extremely unlikely. We thusresorted to the STM manipulation of Li adsorbates to inducethe formation of LixCuPc complexes atom-by-atom. A similarstrategy, consisting in dragging the molecules over the dopantsrather than vice versa, has been employed to investigate K-doped C60 complexes35. The complete manipulation sequence
Figure 4 | Doping of NiPc. a, dI/dV spectra taken on the benzene ring of NiPc and LixNiPc-LA complexes with x= 1,2. A vertical offset has been added forclarity. The spectra of LiCuPc-LA and KNiPc-LA are shown for comparison with Li2NiPc-LA . b,c, Topographic image and dI/dV maps of LiNiPc-LA (b),Li2NiPc-LA (c). Note that a Li atom is adsorbed close to LiNiPc-LA in b, which does not affect the conductance maps. Solid arrows in the schematicorbital filling diagram correspond to charge/spin transferred from substrate and dopant.
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Figure 5 | Atom-by-atom doping of CuPc. a, STM images showing the addition of up to 5 Li atoms to the same CuPc molecule. Size of images of complexesup to 1 Li: 4.7 nm×4.7 nm; size of images of complexes with more than 1 Li: 3 nm×3 nm. Each image is taken after a manipulation event where anadditional Li atom is successfully bonded to the CuPc molecule (see Supplementary Information for more details). b, Corresponding dI/dV spectra of theLixCuPc complexes with x=0–6. The spectra are taken on the bright benzene ring of each doped molecule, with setpoint Vb=−1 V, I= 1 nA. c, Position ofthe α2eg (green circles) and β2eg (open blue squares) orbitals as a function of the number of Li dopants as determined by the onset of the peaks, indicatedby ticks in b, and corresponding energy gap (black circles in lower graph). The red line is a linear fit to the data. The error bar at x= 3 corresponds to gapvalues obtained for different dopant configurations (see Supplementary Fig. S5).
of LixCuPc is reported in the Supplementary Fig. S5. Figure 5ashows that CuPc with up to six Li dopants forms stable LixCuPccomplexes. We observe that the first complex formed in thisway corresponds to LiCuPc-LA, where the Li atom is attachedto the benzene ring lying along the lateral approach direction ofthe STM tip. The second one corresponds to Li2CuPc-LA, withthe additional Li atom occupying the benzene site opposite tothe first one. This seems to be the most stable double dopingconfiguration, as we never observed complexes with Li atomsoccupying adjacent benzene sites. Although the position of theadditional dopants cannot be determined with high accuracy, also
the third and subsequent dopants seem to prefer either benzeneor pyrrole sites lying on the same axis and avoid empty ringsadjacent to filled ones.
Figure 5b shows the dI/dV spectra as a function of the numberof Li atoms. Remarkably, the absence of peaks crossing the Fermilevel beyond x = 1 indicates that the charge transferred to themolecules saturates at Q= 2. This state, as mentioned before, isalready reached in LiCuPc-LA with the filling of the α2eg orbitalwith electrons coming from the substrate and a single Li dopant.The interaction with additional Li atoms affects only the energyof the α2eg orbital, increasing the α2eg–β2eg energy gap (Fig. 5c)
Figure 6 | Long-range interactions between dopants and molecules.a, dI/dV spectra of a LiNiPc-LA complex with a second Li atom placed at adistance r= 1.2 and 1.7 Å from the centre of the molecule (inset). Greenand blue arrows indicate the position of the α2eg and α2eg +U orbitals,respectively. b, Energy of the occupied α2eg (green circles) and unoccupiedα2eg +U (open blue circles) orbitals as a function of r and correspondingenergy gap (black circles in lower graph). The red line is a 1/r fit to the data.
and inhibiting further charge transfer to the β2eg state. The energyshifts of the α2eg orbital are linear with x whereas the β2eg energyremains approximately constant, which suggests that theQ=2 statewith all the dopants aligned along the same ligand axis is furtherstabilized by electrostatic interactions. As a consequence, we finda 100% increase of the gap, from 0.55 to 1.17 eV, as measuredusing the onset of the peaks (Fig. 5c). Such strong variations dueto local electrostatic interactions could play an important role indefining the homogeneity of the energy gap of doped organicsemiconductor interfaces. Moreover, these results highlight thedifferences and similarities between the doping ofmolecular crystalsand metal–organic interfaces. On the one hand, the (lifting ofdegeneration and corresponding) energy gap opening at Q= 2 canexplain the phase separation betweenK2MPc andK4MPc complexesobserved in bulk-like films of FePc (ref. 36), CuPc (ref. 12) andZnPc (ref. 13) suggesting a stabilization plateau at Q= 2 commonto MPc interfaced with metals as well as homogenous crystallinestructures. On the other hand, the stabilization of the Q= 2 stateseems to be much more significant at the metallic interface. Herethe impossibility to induce higher charge states contrasts with themeasurements of MPc bulk crystals, where a complete filling of thetwo-fold degenerate 2eg orbital with Q= 4 occurs at high dopingconcentration11. A key element for such difference is the role ofthe metallic substrate, which seems to act as a charge sink limitingthe MPc uptake of electrons to Q= 2. The tendency of groupingdopants along one ligand axis observed in Fig. 5a is consistent withthe inhibition of charge transfer to the β2eg orbital, which is locatedon the opposite axis.
Long-range interactions between dopants and moleculesElectron doping of organic semiconductor complexes is usuallyachieved by co-deposition or postdeposition of alkali metalswithout accurate control of the final position of the dopantatoms. This can result in local stoichiometry variations, posing theproblem of how the electronic structure of a single molecule isaffected by dopants that might, or might not, be attached to it.In general, we observe no uptake of charge unless the Li atomsare directly attached to the molecules. This explains why alkalimetal levels of the order of one dopant per molecule or largerare required to induce appreciable conductivity changes in organicsemiconductors3–8,20. Despite the absence of charging, however, wereport sizeable electrostatic effects on the conductance gap induced
by Li atoms in the vicinity of MPc. Figure 6 shows the result of amanipulation experiment where a Li atom is gradually approachedto a LiNiPc-LA complex from a distance r = 3 nm to contact(see inset). The conductance gap of LiNiPc-LA corresponds to theseparation between the singly (α2eg) and doubly (α2eg+U ) occupiedstates, indicated by arrows in Fig. 6a. We find that the gap increasesas the Li atom approaches the molecule (Fig. 6b), indicating thatthe excess charge induced by the Li atom to its surroundings affectsthe Coulomb repulsion of the molecular orbitals. Note that onlythe energy of the doubly occupied α2eg +U orbital shifts in anappreciable way, suggesting that the higher charge state is moresensitive to Coulomb interactions. The spatial dependence is in linewith a 1/r decay of the energy gap on a length scale of about 3 nm,which is similar to that reported for electrostatic interactions of Liadatoms on Ag(100) (ref. 37), Cs superlattices on graphene38, andthe lateral extension of charge densities induced by positive chargeson metallic surfaces39. Such variations of the molecular orbitalenergies induced by proximity may be critical to reliably controlelectron injection and transport atmetal–organic interfaces1,7,9.
OutlookUnderstanding the interaction between molecules, dopants, andmetallic interfaces with sub-molecular accuracy is key to thepreparation ofmore efficientmaterials for organic optoelectronics1.Our results, which are free of the averaging effects typical of pho-toemission and transport studies, provide a detailed understandingof chemically doped metal–organic complexes. The competitionbetween π- and d-like charge transfer channels and strong electronconfinement make the doping of metal–organic molecules far morecomplicated than for purely organic compounds35. However, asshown here, this complexity can be turned to an advantage andexploited to control the magnetism of the molecules, as the spin ofeither metal or ligand sites can be switched on or off by selectivelydoping different orbitals. Because the implementation of molecularspintronics relies on the ability to inject andmanipulate charges andspins in metal–organic heterojunctions40, doped MPc provide anexcellent example for promoting the use of doping beyond the realmof optoelectronics. Finally, the detection of single dopant propertiesin individual molecules and the ability to manipulate such dopantsmay be also foreseen as a first step for the realization of organicdevices with magnetic and optoelectronic properties governed byultimate discretization effects41.
MethodsSample preparation and STM measurements. CuPc and NiPc moleculeswere evaporated in ultrahigh-vacuum on single crystal Ag(100). The substratewas prepared by repeated sputter–anneal cycles using Ar+ ions at an energyof 700 eV and annealing to 800K. The molecules were deposited at a rate of∼0.05monolayersmin−1 with the sample kept at room temperature, afterdegassing the 99% pure powder material (Sigma Aldrich) to 500K for 24 h. Thebase pressure during evaporation was below 5×10−10 mbar. In a second step, thealkali metals were deposited in situ at 4.8 K from SAES Getters sources. The STMmeasurements were carried out at a base temperature of 4.8 K. dI/dV spectrawere measured using the lock-in technique, by modulating the bias voltage at afrequency of 3 kHz with rms amplitude of 3mV. The dI/dV spectra are shownas measured, apart from that of NiPc (Fig. 4a), for which it was necessary tosubtract a background spectrum acquired on the bare Ag surface with the sametip and feedback conditions to remove electronic features due to the substrate andtip. The dI/dV maps were measured at constant current using 10mV rms biasmodulation. The manipulation of Li atoms was carried out by placing the STM tipon top of a single atom, lowering the tunnelling resistance down to 114–500 k�(Vb =−40 to −100mV, I = 200–350 nA), and laterally moving the tip at a speedof 20 Å s−1 to the target site.
Ab initio calculations. The electronic structure of doped and undoped CuPcand NiPc molecules adsorbed on Ag(100) was calculated using the VASPimplementation of DFT in the projector augmented plane-wave scheme. We usedthe local density approximation42,43, which has been shown to give reliable resultsfor MPc adsorbed on metals30,31. We used a plane-wave cutoff of 300 eV. Thecalculated slab include five Ag atomic layers intercalated by seven vacuum layers
NATURE MATERIALS DOI: 10.1038/NMAT3547 ARTICLESin the vertical direction, and a 7×7 lateral supercell. The positions of all atoms inthe molecule and the first three Ag layers were relaxed vertically and laterally untilforces were smaller than 0.05 eVÅ−1. Charge transfer and local magnetic momentswere calculated using a Bader charge analysis44.
Received 10 September 2012; accepted 11 December 2012;published online 20 January 2013; corrected online 11 March 2013
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AcknowledgementsWe thank N. Lorente and D. Sánchez-Portal for useful discussions. This work wassupported by the European Research Council (StG 203239 NOMAD), Ministeriode Economía y Competitividad (MAT2010-15659), and Agència de Gestió d’AjutsUniversitaris i de Recerca (2009 SGR 695). A.M. acknlowledges the Spanish Ministerio deCiencia e Innovación for a Ramon y Cajal Fellowship.
Author contributionsC.K., A.M. and P.G. planned the experiment; C.K. and A.M. performed themeasurements; C.K., A.M. and P.G. analysed the data and wrote the manuscript. R.R.performed the DFT calculations. All authors discussed the results and commentedon the manuscript.
Additional informationSupplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints. Correspondenceand requests for materials should be addressed to A.M.
Competing financial interestsThe authors declare no competing financial interests.
In the version of this Article originally published online, at the start of the second paragraph of the section ‘Doping limit of single MPc’, the sentence describing a figure that “shows the dI/dV spectra as a function of the number of Li atoms” should have referred to Figure 5b rather than 4b. This error has been corrected in all versions of the Article.
Site- and orbital-dependent charge donation and spin manipulation in electron-doped metal phthalocyaninesCornelius Krull, Roberto Robles, Aitor Mugarza and Pietro Gambardella
Nature Materials http://dx.doi.org/10.1038/nmat3547 (2013); published online 20 January 2013; corrected online 11 March 2013.
