Chemical Physics Letters 582 (2013) 90–94

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Chemical Physics Letters

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The transition behavior of FePc on Ag(110)

0009-2614/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cplett.2013.07.023

⇑ Corresponding author. Fax: +86 571 87951328.E-mail addresses: phybao@zju.edu.cn, phybao@dial.zju.edu.cn (S. Bao).

Y.L. Cai a, R.A. Rehman a,b, Wu Ke a, Han-Jie Zhang a, Pimo He a, Shining Bao a,⇑a Physics Department, Zhejiang University, Hangzhou 310027, People’s Republic of Chinab Physics Department, Forman Christian College University, Lahore, Pakistan

a r t i c l e i n f o

Article history:Received 27 March 2013In final form 12 July 2013Available online 18 July 2013

a b s t r a c t

The transition behavior of FePc on Ag(110) has been investigated by room temperature scanning tunnel-ing microscopy (STM) and density functional theory (DFT) simulation. After FePc molecules deposited onAg(110) surface, two adsorption structures were observed. The pathway between these different config-urations was investigated by DFT simulation. The transition of electronic structures of FePc/Ag(110)interface and change in work function during the transformation were investigated by the calculateddensity of states (DOS) together with the application of induced density of interface states (IDIS).

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Organic semiconductors are treated as a promising candidate forsubstitution of silicon in industrialization of nanometer sized elec-tronic components. Among such a class of material, the metalphthalocyanine (MPc) is a typical representative because of itsextraordinary electronic properties brought by the delocalized p-electron [1]. Varieties of systems in which MPcs adsorbed onto no-ble metals have been studied by all sorts of techniques includinglow energy electron diffraction (LEED) [2], photoelectron spectros-copy (PES) [2], scanning tunneling microscopy (STM) [3,4], normal-incidence X-ray standing-wave (NIXSW) spectroscopy [5] and den-sity functional theory [6]. Therein FePc/Ag(110) system was inten-sively investigated concerning the self-assembly and interactionmechanisms. Palmgren et al. firstly carried out their investigationon such a system by LEED and synchrotron based X-ray photoelec-tron spectroscopy (XPS) [7]. The structural and self-assembly prop-erties of FePc sublimated onto Ag(110) were obtained by Casarinet al. based on STM data and DFT simulations [8]. Recently Qianet al. in our group reported the work on electronic structures ofFePc/Ag(110) by angle resolved ultra-violet photoelectron spec-troscopy (ARUPS) [9]. Since two adsorption structures were ob-served after FePc molecules deposited on Ag(110) surface, thedifference in adsorption structures and the transition behavior ofadsorption structure are interesting. In this Letter, we show theadsorption structures differing from the ones observed by Casarinet al. [8] with STM, and the pathway between these different config-urations was investigated by DFT simulation. A transition processand the transition barrier between the configurations were ob-served by DFT, the transition of the structure and the interface elec-tronic properties were determined by nudged elastic band (NEB)

method and the recently developed induced density of interfacestates (IDIS) model [10].

2. Experimental and computational details

FePc (Sigma Aldrich, 99%) were degassed overnight in the MBEchamber with pressure carefully monitored. A tantalum boat wasused to deposit FePc onto Ag(110) at 540 K with a deposition rateof 0.1 ML/min. The substrate was in situ prepared by several cyclesof argon ion sputtering (1000 eV � 1 h) and annealing at 820 K,while kept at room temperature during sublimation of FePcs.STM measurements were performed in an Omicron VT-SPM sys-tem with a base pressure of 2 � 10�10 mbar under ambient condi-tions. The details about the experimental system can be foundelsewhere [11].

The first principle calculations were performed within the DFTpackage DMOL3 [12,13] and CASTEP [14] integrated in the commercialsoftware Materials Studio of Accelrys Inc. Exchange–correlation ef-fects were modeled by Perdew–Wang (PW91) [15,16] functional.The DND basis was employed to expand the wavefunctions, andDFT semi-core pseudopotentials (DSPPs) [17] were used to acceler-ate the simulations. A Fermi smearing of 0.1 eV was adopted tominimize the errors in the Hellmann–Feynman forces from theentropic contribution to the electronic free energy [18]. The con-vergence thresholds for energy, energy gradient and displacementconvergence were chosen to be 2 � 10�5 hartree, 4 � 10�3 hartree/Å and 5 � 10�3 Å, respectively. Periodic boundary conditions wereapplied to the supercell in which an FePc molecule adsorbed onto afour-layers Ag(110) slab separated by 18Å vacuum. The first twolayers and FePc molecule were fully relaxed.

3. Results and discussion

As shown in Figure 1(a), when the coverage of the FePc mole-cules on Ag(110) surface was near monolayer, two types of 2D

Y.L. Cai et al. / Chemical Physics Letters 582 (2013) 90–94 91

lattices whose coexistence and universalities could also be found inFigure 1(b) formed in the regions A (B) and C. The molecules in do-main A are arranged in ½2 �21� and ½2 �2 �3� directions, so that the lat-tice vectors aA and bA from the quasi-quadratic periodic structureare along ½2 �21� and ½2�2 �3� azimuths respectively. Taking into ac-count the relationship between the unit cell of the adsorbate andsubstrate, the analysis yields the following commensurate super-structure matrix:

aA

bA

� �¼

4 22 �3

� �ab

� �

Here vectors a and b are primitive lattice vectors of Ag(110) surfacealong the ½1 �10� and [001] azimuths.

The molecules in domain B are arranged in ½2 �2 �1� and ½2 �23�directions, so that the lattice vectors aB and bB from the quasi-qua-dratic periodic structure are along ½2 �21� and ½2 �2 �3� azimuthsrespectively. Taking into account the relationship between the unitcell of the adsorbate and substrate, the analysis yields the follow-ing commensurate superstructure matrix:

aB

bB

� �¼

2 34 �2

� �ab

� �

In domains A and B, FePc molecule axes enclose angles of�(30� ± 2�) and 30� ± 2� respect to the ½1 �10� azimuth. Dependenton the superstructure matrix and the orientation of FePc moleculeaxes respect to the substrate in domains A and B, the local adsorp-tion structures of FePc molecule are equivalent. The adsorptionstructure in domain C is different from that in domain A or B.

The molecules in domain C are arranged in ½1 �10� and [001]directions, so that the lattice vectors aC and bC from the quasi-qua-dratic periodic structure are along ½1 �10� and [001] azimuthsrespectively. Taking into account the relationship between the unitcell of the adsorbate and substrate, the analysis yields the follow-ing commensurate superstructure matrix:

aC

bC

� �¼

5 00 4

� �ab

� �

In domain C, FePc molecule axis encloses an angle of 45� ± 2� re-spect to the ½1 �10� azimuth which is different from that in domain Aor B.

As a comparison, the commensurate superstructure matricesfor the coexisting c(10 � 4) and p(10 � 4) structures observed byCasarin et al. are deduced [8]. The c(10 � 4) structure, though con-sisting of 45� ± 2�-configuration molecules too, possess a unit cell

Figure 1. (a) Constant-current STM image of the two types of 2D lattices (V = �0.929 V(V = �0.987 V, I = 0.079 nA).

which obviously is different from the one in domain C of our case.Its superstructure matrix is as follow:

acð10�4Þ

bcð10�4Þ

� �¼

5 20 4

� �ab

� �

with ac(10�4) and bc(10�4) are the lattice vectors of the c(10 � 4) unitcell. Similarly, p(10 � 4) structure, though also consisting of±(30� ± 2�)-configuration molecules as observed in domain A andB of our case, possess a distinct unit cell incorporating two FePcmolecules in 30� ± 2� and �(30� ± 2�) configuration respectively.Its superstructure matrix can be written as:

apð10�4Þ

bpð10�4Þ

� �¼

10 00 4

� �ab

� �

with ap(10�4) and bp(10�4) are the lattice vectors of the p(10 � 4) unitcell. Analysis based on the superstructure matrices leads to the fol-lowing conclusion: despite of the difference in the unit cell shape,structure C and the c(10 � 4), p(10 � 4) structures are under thesame coverage. Nonetheless, for structure A or B, the coverage in-creases. Therefore, the distinct ways for unit cell construction areconsidered to be a consequence of the coverage which is higher inour case.

In order to further investigate the adsorption structure of FePcmolecule on Ag(110) substrate, the density functional theory(DFT) calculations were carried out. During the DFT calculationsperformed for an isolated FePc molecule adsorption, four adsorp-tion structures with high symmetry of the FePc on Ag(110) surfaceare selected as the initial structures. At the initial stage, the mole-cule adsorbed on top site, short bridge (SB), long bridge (LB) andhollow site. Here the site is distinguished by the position of thecentral iron atom of the molecule on the substrate. A generalizedexpression for determination of the adsorption energy Ead can bewritten as:

Ead ¼ EðFePc=AgÞ � EðFePcÞ � EðAgÞ

EðFePcÞ, EðAgÞ are energies of the adsorbate and substrate, andEðFePc=AgÞ is the total energy of the substrate–adsorbate system.

During the process of structural optimization, all atoms, exceptthe bottom two Ag layers, were fully relaxed. The DFT calculationresults of an isolated FePc molecule on Ag(110) surface are listedin Table 1 indicating that the molecules prefer to adsorb on topand short-bridge sites in flat-lying geometries. When the moleculeadsorbs on top site, the molecular axis is in a direction with an an-gle of 45� respect to the ½1 �10� azimuth. When the molecule ad-

, I = 0.081 nA) and (b) constant-current STM image of 2D lattices of type A and C

Table 1Simulated Fe-substrate distances and adsorption energy obtained from DFT calculation.

Top SB LB Hollow

45� hFe(Å) 2.677 Unstable 2.668 2.934EAdsop(eV) �3.085 �2.233 �2.288

30� ± 2� hFe(Å) Unstable 2.631 2.893 2.877EAdsop(eV) �2.976 �2.242 �2.208

92 Y.L. Cai et al. / Chemical Physics Letters 582 (2013) 90–94

sorbs on short-bridge site, the molecular axis is in a direction withan angle of 30� respect to the ½1 �10� azimuth. The separations be-tween the molecules and substrate are 2.677 and 2.631 Å in thecase on top and short-bridge sites respectively. With the adsorp-tion of FePc molecules on these sites, the separations betweenthe molecules and substrate are shorter than those in other cases.

Based on the STM image, the axis of FePc molecule in region A orB takes an angle of 30� respect to the ½1 �10� azimuth, so the mole-cule adsorbs on shot-bridge site, and for the FePc molecule in re-gion C, its axis is in a direction forming an angle of 45� respect tothe ½1 �10� azimuth, so the molecule adsorbs on top site. The sketchof the adsorption structures of FePc molecules with respect to thesubstrate are shown in Figure 2.

It had been reached a consensus that, generally, for sparsely ad-sorbed MPc (M are transition metals) molecules on noble metals,the Pc macrocycles would rotate to a specific symmetry directionwith respect to the metal substrates regardless of the central metalions [19,20]. Such a universality revealed that it was the interac-tion of aza-N(Na) and substrate atoms which played the decisiverole in determining the adsorption azimuth [21]. However, whenthe coverage increased or decreased, the interaction between themore crowded packed molecules started to make effect not onlyin the 2D-lattice construction but also the individual molecular ori-entation. As a consequence, a coverage-driven transition was trig-gered. In our case, region A, B pose a packing density of 0.535 nm�2,while region C has a lower one, 0.431 nm�2. Such a transition ofadsorption structure occurs when the coverage increases ordecreases.

To clarify such a transition process, the DFT transition statesearch (TS search) method [22] was employed to construct thetransition pathways which were subsequently optimized by

Figure 2. Sketch of the adsorption structures of FePc molecules on Ag(110).

nudged elastic band (NEB) method [23]. The coverage decreasesin the edge of domain A. Figure 3 shows a schematic diagram ofthe unit cell transition process composed of the optimized tracksfor each molecule. In the transition process, the FePc moleculesshift from shot-bridge site to top site simultaneously rotate themolecular axes from 30� to 45� respect to the ½1 �10� azimuth. Theenergy plot in Figure 4 depicts the corresponding adsorption en-ergy change in the transition process. An energy barrier of0.236 eV is found in the transition process. A minimum in adsorp-tion energy is found between TS1 and TS2 denoting the existenceof a sub-stable state between the structures in region A and C.

Figure 3. The transition from the adsorption structure in region A to that in regionC.

Figure 4. The adsorption energy during the transition process.

Figure 5. Calculated DOS for bare FePc molecule and PDOS for FePc on Ag(110)posing top-45� configuration and short bridge-30� configuration. The vacuum levelis taken as the origin. EM

F represents the Fermi level of the clean Ag(110) substrate,while CNL1, E1

F and CNL2, E2F depict the charge neutrality levels and calculated Fermi

levels of the interaction systems in top-45� configuration and short bridge-30�configuration, respectively.

Y.L. Cai et al. / Chemical Physics Letters 582 (2013) 90–94 93

The structure of the sub-stable state is shown in the region T of Fig-ure 1(a).

According to a series of work by Flores et al. [10], the work func-tion change could be attributed to three factors: (1) charge trans-fer; (2) the pillow effect; (3) orientation of intrinsic moleculardipoles. Herein, such a model was adopted to calculate the workfunction change with the third effect neglected. Two adsorptionsystems observed in region A (or B) and C in Figure 1(a) were mod-eled to clarify the effect on the electronic structures resulted fromthe transition. An emphasis needed here is that because of theband gap problem of DFT/GGA method for organic semiconductors,GGA+U [24] scheme was employed to overcome it. During the cal-culation of density of states (DOS), a 3 � 3 � 1 k-points grid wasobtained from the Monkhorst–Pack scheme [25], and the Hubbardpotentials for Ag atom and Fe atom were set to be 2 eV and 2.5 eV.

Eqs (1) and (2) are adopted to calculate the work functionchange resulted from charge transfer. And Eq. (3) determines theinterface slope parameter quantifying the pinning of Fermi levelto the charge neutrality level (CNL) [10]. Herein, EM

F representsthe Fermi level of the metal substrate, D(EF) denotes the states den-sity of Fermi level of the interaction system (practically CNL), dmeans the adsorption height of the organic molecule, and A isthe adsorption area of the unit cell:

EF � CNL ¼ SðEMF � CNLÞ ð1Þ

DIDIS ¼ ð1� SÞðEMF � CNLÞ ð2Þ

S ¼ dEF

dEMF

¼ 11þ 4pe2DðEFÞd=A

ð3Þ

The Hirshfeld dipole analysis [26], seeing Eq. (4), is adopted toobtain the dipole moment change of the atoms in the first two lay-ers of substrate caused by the pillow effect.

lk ¼X

i

Xi;kqi ð4Þ

Xi,k denotes the k coordinate of atom i relative to a chosen origin.Once obtained the dipole moment change, it is straight to get thepotential change DP resulted from it. As such potential change isscreened by S, the practical work function change related to it isSDP.

Taken both effects into consideration, the Fermi level of the sys-tem and total work function change are as follows:

EF ¼ CNLþ SðEMF � CNLÞ þ SDP ð5Þ

DT ¼ SDP þ ð1� SÞðCNL� EMF Þ ð6Þ

In our approaches, for domain C in Figure 1(a) (top-45� config-uration), the specific values are: EM

F for Ag(110) surface deter-mined by our previous UPS experiment is 4.23 eV below thevacuum level [9]; molecule–substrate distance d = 2.677 Å; theadsorption area A = 231.813 Å2; the density at EF, D(EF) = 3.45/eV;S = 0.12; ðCNL� EM

F Þ ¼ 0:221 eV; DP = 0.399 eV and DT = 0.243 eV.Figure 5 plots the calculated DOS for bare FePc molecule and pro-jected density of states (PDOS) of FePc on Ag(110) in the top-45�configuration showing the alignment of the energy levels. Someexpectable broadenings as a result of the interaction between themolecule and substrate are presented at the HOMO and LUMOpeaks. The vacuum level is taken as the origin, the HOMO is locatedbelow the LUMO with a band gap of 2.931 eV, while the CNL1 isdetermined to be 1.129 eV lower than the LUMO. The calculatedFermi level of the system E1

F is 0.243 eV above the clean Ag(110)surface Fermi level EM

F denoting a decrease of work function.While for domain A or B in Figure 1(a) (short bridge-30� config-

uration), d = 2.631 Å; A = 185.466 Å2; D(EF) = 3.27/eV; S = 0.11;

ðCNL� EMF Þ ¼ 0:205 eV; DP = 0.385 eV and DT = 0.225 eV. Com-

pared to the PDOS for top-45� configuration, there are the similarbroadenings of the HOMO and LUMO peaks with respect to theDOS of bare molecule, however, some minor intensity variationsin the range of energy gap are also observable. The CNL2 is deter-mined to be 1.145 eV lower than the LUMO. Notably, the calculatedFermi level of the system E2

F locates 0.225 eV above the cleanAg(110) surface Fermi level EM

F , slightly lower than the above men-tioned 0.243 eV work function change for the top-45� configura-tion. Such a difference indicates the variations of the electronicstructures brought about by the coverage-driven transition pro-cess. The change in work function corresponding to both configu-rations conforms to our previous UPS data, a change in workfunction of 0.240 eV, very well [9].

4. Conclusion

When FePc molecules adsorb on Ag(110) surface, the moleculesprefer to adsorb on top and short-bridge sites in flat-lying geome-tries, the molecular axes are in directions with angles of 45� and30� respect to the ½1 �10� azimuth. The separations between themolecules and substrate are 2.677 and 2.631 Å. A transition ofadsorption structure occurs when the coverage increases or de-creases. Change in work function of 0.018 eV is found in the tran-sition of adsorption structure.

Acknowledgments

This work was supported by the National Science Foundation ofChina (Grant No. 10974172 and 11074214) and the FundamentalResearch Funds for the Central Universities. We also gratefullyacknowledge the support from the Ministry of Science and Tech-nology of China (Grant No. 2011CB921903) and the Scientific Re-search Fund of Zhejiang Provincial Education Department (GrantNo. Y201121234).

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