Electrostatic Field Driven Alignment of Organic Oligomers on ZnO Surfaces

1.4 MB·English

by  F. Della Sala

Electrostatic Field Driven Alignment of Organic Oligomers on ZnO Surfaces
F. Della Sala
National Nanotechnology Laboratory, Istituto Nanoscienze-CNR, Via per Arnesano, I-73100 Lecce, Italy
Centre for Biomolecular Nanotechnologies, IIT, Arnesano, Italy and
IRIS Adlershof, Humboldt-Universita¨t zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany
S. Blumstengel and F. Henneberger
Institut fu¨r Physik, Humboldt-Universita¨t zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany
(Dated: January 24, 2011)
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1 We study the physisorption of organic oligomers on the ZnO(10¯10) surface using first-principles
0
density-functional theory and non-empirical embedding methods. We find that both in-plane loca-
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tionandorientationofthemoleculesarecompletelydeterminedbythecouplingoftheirquadrupole

n momentstotheperiodicdipolarelectricfieldpresentatthesemiconductorsurface. Theadsorption
a is associated with theformation of a molecular dipole moment perpendicular to the surface, which
J bears an unexpected linear relation to the molecule-substrate interaction energy. Long oligomers

1 suchassexiphenylbecomewell-aligned withstabilization energiesofseveral100meValongrowsof
2 positive electric field,in full agreement with recent experiments. These findingsdefinea new route
towardstherealizationofhighly-orderedself-assembledarraysofoligomers/polymersonZnO(10¯10)

] and similar surfaces.
i
c
s PACSnumbers: ValidPACSappear here
-
l
r
t Hybrid structures made of conjugated organic oligomer. Fig. 1a and b depict the configuration ex-
m
molecules andinorganicsemiconductorsexhibit anenor- amined. The originof the reference coordinate systemis
.
t mousapplicationpotentialastheycombinethefavorable locatedatthecenterofasurfaceZn-Obond,thez-andy-
a
featuresofboth componentsina single new material[1]. axis point along the surface normal and the polar [0001]
m
However, interfacing of organic molecules with the typ- direction, respectively. The position of the molecule is
-
d ically highly reactive semiconductor is a complex issue. denotedbythecoordinatesofitscenterofmass. Wecon-
n Ruptureandfragmentationarefrequentlyobservedlead- sider a clean, non reconstructed surface optimized using
o ing to ill-defined interfaces [2]. On the other hand, the density functional theory (DFT) as described in Ref. 8.
c
electronic structure of the semiconductor surface might Ourgoalistheconstructionoftheground-statepotential
[
be exploited for developing novel strategies of molecular energysurface(PES)ofthe molecule-semiconductorsys-

1 aggregation. InthisLetter,wedemonstratethattheelec- tem. In a first step, we set the center of the molecule on
v
trostatic interaction between the semiconductor and the topoftheZn-Obond(x=y=0)withitslongaxisaligned
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π-electronsystemgivesindeedrisetotheself-assemblage inx-directionandthemolecularplaneparalleltothesur-
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0 of stable and highly ordered monolayers for a wide class face. Theinteractionenergyofthisarrangementkeeping
4 of conjugated organic molecules. both the molecular and ZnO(10¯10)surface configuration
1. The specific surface under consideration is the non- frozen is plotted versus distance from surface (z) in Fig.
0 polar(10¯10)crystalplaneofZnO.ThechemistryofZnO 1c. The curves are computed at the PBE level [9] and
1 surfaces, see e.g. Ref. 3, has been largely investigated with dispersion correction (PBE+D) [10], using two dif-
1
in the context of catalysis [4] and, more recently, much ferent computational methods: a periodic pseudopoten-
:
v attention is paid to the linkage with organic dyes and tialplane-wave(PW)approach[11]andtheperiodicelec-
Xi polymers,driven,e.g.,byphotovoltaicapplications[5,6]. trostaticembeddedclustermethod(PEECM)[11,12],as
In particular, it has been found experimentally that p- implemented in the TURBOMOLE [13] program.
r
a sexiphenyl (6P) absorbs flat on the ZnO(10¯10)surface Figure 1c shows that the PEECM results agree very
with the long axis of the molecule perpendicular to the well with the PW ones. The practical advantage of
polar [0001] direction [7]. In this study, the hybrid in- PEECM lies in the fact that it considers only a single
terface has been formed entirely under ultra-high vac- molecule in interaction with the surface. Unlike the PW
uum conditions suggesting that intrinsic features of the method, where a whole periodic organic monolayer (of
semiconductor-molecule system are behind that type of hypothetical structure) has to be treated, PEECM de-
aggregation. The theoretical analysis presented below fines thus a cost effective way to tackle the initial ad-
not only confirms this conjecture but reveals systematic sorption step of the molecule. As expected from previ-
tendencies common to all oligomers that can be used to ousstudies,seee.g. Ref. 14,thePBEfunctionalleadsto
engineer the growth of inorganic/organicstructures. weak binding, while the dispersion correction increases
In order to establish a proper and efficient methodical the binding energy and reduces the molecule-substrate
framework,westartwithbiphenyl(2P)asashortmodel distance. In order to verify the accuracy of PBE+D for
2
a) As a next step, we now consider the change of the
0.4 c)
molecule-substrate interaction energy (∆E) when the
x V] 0.2 molecule is translated along y-direction. As displayed in
e Fig. 2a, the computations performed again in different
y y [ 0.0 approximationscommonlyrevealadistinctminimumfor
g
ner-0.2 y ≈3.7˚A,i.e.,whenthecenterofthemoleculeiscloseto
E
n a positionatopa Znatom. Interestingly,the relativeen-
o-0.4
cti ergiesofthePBE(PW)approacharepracticallynotmod-
a PBE(PEECM)
z nter-0.6 PPBBEE(+PDW(P)EECM) ifiedbyinclusionofthedispersion. Inordertosavecom-
o Zn y I PBE+D(PW) putation time for the examination of the complete PES
-0.8
HF(PEECM) below, we performed the same procedure but modeling
MP2(PEECM)
-1.0 the ZnO(10¯10)surface only by point-charges with values
3 4 5 6
b) z [A] +qand−qatthelatticepositionsoftheZnandOatoms,
respectively. We call this method QM/PPC (quantum
mechanics/periodicpoint-charges),becausethemolecule
FIG. 1: a) Top and b) side view of 2P on the
ZnO(10¯10)surface (dotted rectangle: surface unit cell). c) is treatedquantum mechanicallyatPBE level,while the
Molecule-substrate interaction energy for 2P verus vertical ZnOsurfaceisclassicallydescribed. Hence,onlytheelec-
distance z (x=y=0), as computed by different theoretical trostatic interaction between the molecule and the sub-
methods (see text). strateisconsideredinthisapproach. Forq=1.2,excellent
agreement with the PBE(PW) result is indeed achieved.
ThisvalueqisveryclosetowhatisfoundintheMulliken
ZnO surfaces, we performed reference MP2 calculations population analysis of the ZnO(10¯10)surface [8]. There-
within the PEECM scheme[11]. Figure 1c exposes that fore, we conclude that exchange-correlationforces deter-
PBE+DisquitefarfromMP2andthuscannotbesafely mine the absolute energy (see Fig. 1c), but the energy
used for ZnO surfaces. The MP2 predicts an interaction variation when moving the molecule within the surface
energyof370meVwithanequilibriumdistancez0 ≈3.5 plane is completely dominated by the electrostatic cou-
˚A. This molecule-substrate distance will be used in all pling. Exchange-correlation effects vary on the atomic
the following calculations. length scale, but are averaged out as the molecule is
larger than the ZnO(10¯10)unit cell.
a)
0.10 The alternating point charges which characterize the
V] PBE(PW)
gy [e 0.05 PPBBEE(+QDM(P/WPP)C) ZF~n.OA(n10i¯1m0p)osurtrafanctecocrnesaetqeueanpceeroiofdtihcisdifipeolldarisetlhecattriict gfieenld-
er 0.00
n erates in turn an induced dipole moment ~µ in the 2P
E
nt. -0.05 molecule. For symmetry reasons, Fx is negligible, while
I-0.10 F and F reach values of several V/nm. The resultant
y z
0 1 2 3 4 5 µ and µ are plotted versus y-positionin Fig. 2b. Over
y [A] y z
c) the length a of the unit cell, they change signwith a rel-
µ
0.4 µz ative of shift of a/4. This behavior reflects the dipolar
D] 0.2 y Fz character of F~, as illustrated in Fig. 2c,d. The electric
e [ 0 field is largely inhomogeneous, but sufficiently far from
ol d)
Dip-0.2 the surface where the molecule is located, it has oscillat-
ing character. When y ≈ 2.5˚A, µ reaches its negative
-0.4 b) Fy y
maximum. At this position, as schematized in Fig. 2d,
0 1 2 3 4 5 the molecule experiences a negative electric field overal-
y [A]
most its whole size, whereas the average of F is almost
z
zero and thus µ ≈0.
FIG. 2: a) Molecule-substrate interaction energy versus y- z
position measured relativetoy=0(x=0,z=z0),bydiffer- The knowledge gained above enables us now to search
enttheoreticalmethods. ThelengthoftheZnO(10¯10)surface
fortheglobalminimumofthePESbychangingtheresid-
unit cell in this direction is 5.19˚A. b) Induced molecular
ual degrees of freedom[15] - translation of the molecule
dipole moment, components µy and µz (µx = 0) computed
inQM/PPC.TheelectrostaticfieldoftheZnO(10¯10)surface, alongthex-directionandrotationaroundthez-axis. The
from PW/PBE calculations, is illustrated (color online) in QM/PPC approach makes it possible to scan a set of
c) (z-component) and d) (y-component). The colormap cov- 1500 different molecular configurations. The results are
ers therange from -5V/nm (intenseblue) to 5V/nm (intense condensed in Fig. 3. The QM/PPC interaction energy
red). Also shown is the molecule position at minimum µy (relative to the isolated molecule) is represented in Fig.
(y=2.5˚A).
3a as a function of the rotation angle θ for the whole set
3
ofx-andy-positionssampledovertheZnO(10¯10)surface accounting for the molecular polarization created by the
unit cell. field. Eq. (1) is valid for a large class of planar, sym-
metric oligomers characterized by vanishing off-diagonal
0.10 a) b) c) elements of Mij and αij. The ZnO(10¯10)surface peri-
odic dipolar electric field seen by the molecule can be
V] 0.05 approximated by (see Fig 2c,d and Ref. 11)
e
nergy [ 0.00 Fy(y,z)≈Ae−kzcos(ky) , Fz(y,z)≈−Ae−kzsin(ky)
E
n -0.05
ctio with k =2π/a so that its norm(F ≈Ae−kz) is indepen-
a
er-0.10 dent on y and
nt
I
-0.15 dFy dFz
θ=90 ≈kFz , ≈−kFz. (2)
dy dz
-0.20
0 30 60 90 120150180 -0.2 0 0.2 -0.5 0 0.5
θ [degree] µ [D] µ[D]
z y Using that µi =αiiFi and inserting (2) in (1), we obtain
FIG. 3: Adsorption scenario of 2P on the ZnO(10¯10)surface α F2
2 yy
ascomputedbytheQM/PPCmethod. a)Molecule-substrate ∆E ≈−Bµz −Cµz − 2 (3)
interaction energy versus rotation angle θ for all x- and y-
positions counted. θ = 0: long molecular axis along x- withB =k(M −M )/2α andC =(α −α )/2α2 .
yy zz zz yy zz zz
direction (see Fig 1a). In b) and c), the interaction energy
isplottedversusthez-andy-component,respectively,of the
induced dipole moment
molecule Myy Mzz αxx αzz B C
◦ a.u. a.u. a.u. a.u. eV/D eV/D2
The absolute minimum is found at θ = 90 (long 2P
2P -46.2 -57.3 217.4 68.2 0.558 0.068
axis ky). However, there is also a second minimum at
6P -135.7 -168.4 1257.6 190.5 0.589 0.062
θ = 0, which corresponds to the one in Fig. 2a. The
energy difference between the two minima is only 20 5A -82.6 -101.4 645.2 115.2 0.557 0.084
meV and hence within the numerical error range. We 5PV -145.3 -180.1 2052.9 207.8 0.574 0.090
conclude that the 2P molecule can be arranged on the
ZnO(10¯10)surface in two different cross-aligned orienta- TABLEI:PBEquadrupolemoments(Myy,Mzz),polarizabil-
tionswhichmakesthe formationofawellorderedmono- ities (αxx,αzz) as well as B and C coefficients (see text) for
different molecules.
layer rather questionable.
Thecentralquestiontobeansweredisaboutthemech-
anism controlling the alignment of the molecule. The Table I compiles the values of the relevant param-
interplay between the surface electrostatic field, the in- eters for 2P and three other representative molecules
duced dipole moment, and the interaction energy be- (6P,5A=pentacene,5PV=penta-phenylene-vinylene),all
comes evident from Fig. 3b and c. There is a distinct widely used in organic opto-electronics. Though the
linear relation between the ∆E and µz. The energy is anisotropy of α can be quite significant, the term
mimimized if and only if the dipole moment along z is quadratic in µ in (3), originating from the induction
z
maximized. On the other hand, as seen in Fig. 3c, it energy, is negligible against the linear term (i.e., the
holds µy ≈ 0 at the energy minimum, but there is no quadrupolar contribution) for weak fields. Hence, the
direct relation like in the case of µz. analytical model fully recovers the numerical results of
The above findings become more transparent in an Fig. 3b. A linear fit to the data in Fig. 3b for 2P pro-
analytical model gathering the leading features of the vides a slope of-0.48eV/D in verygoodagreementwith
molecule-substrate electrostatics. The energy of a the value expected from Table I.
molecule with zero static dipole but finite quadrupole The analytical treatment suggests that the electro-
moment Mij in a weak but non-uniform electric field static scenario found for 2P is of general validity. In-
(Fx ≈0) is [16] deed,thefullnumericalanalysisconfirmsthisfor6P,6A,
1 dF and5PV.Aglobaloptimizationformoleculesofthissize
i 2
∆E ≈− M +α F , (1)
2 (cid:18) ii dr ii i (cid:19) cannotbe carriedoutusing first-principlesmethods only
iX=y,z i and, usually, semi-empirical approximations have to be
where α is the molecule’s polarizability tensor. This employed [17]. The QM/PPC approach is instead non-
ij
expression is derived from perturbation theory: the first empirical because the only parameter (q) is fixed from
term represents the electrostatic interaction between the a first-principles (PBE/PW) calculations. We thus were
external non-uniform perturbing field and the unper- able to perform the same global scan of the PES as for
turbedmolecule,thesecondonetheinductionenergy[16] 2P.Fig. 4ademonstratesthatthelinearrelationbetween
4
interaction energy and vertical dipole moment is a com- adsorptionof typical oligomers. When the molecules ex-
mon feature for this class of non-polar molecules. Even, hibitanaxiallyorientedπ-electronsystem,awell-defined
the slope of the curves is almost the same (-0.46 ÷ -0.58 molecular alignment, stabilized by energies larger than
eV/D), consistentwith the similar values ofB inTab. I. 100meVagainstreorientation,isestablished,asobserved
experimentally for 6P [7]. The electrostatic coupling is
]
V 0.2 characterized by a linear relation between the molecule-
e a)
y [ 0 substrate interaction energy and the induced vertical
g
r moleculardipolemoment,whichcanbeemployedtopre-
e
n-0.2
E dict and/or to design the molecular orientation on the
on -0.4 6P surface. Moreover,this dipole momentis directly associ-
racti-0.6 55PAV ated with workfunction changes [18], and thus provides
Inte -0.4 -0.2 0 µ0[.2D] 0.4 0.6 0.8 1 aortgoaonlicf/oorregnagniinceehryibnrgidthsetreuncetrugryesle[v7e].l aFdijnuastllmy,enwteonfointe-
z
0 that the single-molecule adsorption described above will
b) 6P be perpetuated and will result in molecular assemblies
] 5PV
V
e 5A reflecting the topology of the surface field. Although
[
gy -0.2 the induced dipole moment is modified by depolariza-
er tion effects [18, 19], this energy scale is certainly signif-
n
E
n icantly smaller than the electrostatic molecule-substrate
ctio-0.4 couplingcontrollingthealignmentonthesurface. There-
era ∆y fore, we believe that our findings define a route towards
nt the realizationof highly-orderedself-assembledarraysof
I
-0.6 oligomer/polymerson ZnO(10¯10)and similar surfaces.
0 30 60 90 120 150 180
θ [degree]
We thank R. Ahlrichs for providing us with the TUR-
BOMOLE program package, M. Sierka, G. Heimel and
FIG. 4: Interaction energy for 6P, 5A and 5PV on
ZnO(10¯10)inQM/PPC.a)Linearrelationbetweenmolecule- I. Ciofini for discussions. This work is partially funded
by the ERC Starting Grant FP7 Project DEDOM (no.
substrate interaction energy and induced vertical dipole mo-
ment. b) Interaction energy versus rotation angle, only the 207441).
minimum values from all x- and y-positions scanned are
shown. Inset (color-online): Orientation of 6P at the global
minimum shown on a colormap of the z-component of the
surface electric field.
[1] S.Blumstengel, S.Sadofev, andF.Henneberger,NewJ.
Althoughthelinearenergy-dipolerelationholdsforall Phys. 10, 065010 (2008).
these molecules, the specific alignment on the substrate [2] R. Lin et al., J. Chem. Phys.117, 321 (2002).
canbesubstantiallydifferent. ThisisdocumentedinFig. [3] C. W¨oll, Prog. Surf. Sci. 82, 55 (2007).
4b, where the energy is plotted as a function of θ at x- [4] C. Catlow et al., J. Comput. Chem. 29 (2008).
[5] M. Law et al., Nat.Mater. 4, 455 (2005).
and y-positions with minimized energy. In contrast to
[6] S. Dag and L.-W. Wang, Nano Letters 8, 4185 (2008).
2P, the PES of both 6P and 5PV exhibits deep global
[7] S. Blumstengel et al., Phys. Chem. Chem. Phys. 12,
minima at θ =0, clearly separated by 140 meV and 330
11642 (2010).
meV, respectively, from other arrangements. Not only [8] F.Labat,I.Ciofini,andC.Adamo,J.Chem.Phys.131,
the orientation but also the lateral position of the ad- 044708 (2009).
sorption site is uniquely defined, with y ≈ 3.7 ˚A and [9] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev.
x = 0 for both 5PV and 6P. Thus, as illustrated in the Lett. 77, 3865 (1996).
[10] S. Grimme, J. Comput. Chem. 27, 1787 (2006).
inset for 6P, the energy is minimized when the long axis
[11] SeesupplementarymaterialXXXXforcomputationalde-
of the molecule matches with the lines of largest posi-
tails and a simple model for thesurface periodic dipolar
tive F , where the electrostatic coupling and thus µ are
z z electric field.
maximized. The longer the molecule, the more stable [12] A.M. Burow et al., J. Chem. Phys.130, 174710 (2009).
the alignment. The PES of 5A is instead less deep and [13] TURBOMOLE V6.1, University of Karlsruhe,
structured. In contrast to 6P/5PV, no preferred orien- www.turbomole.com.
tation can be thus anticipated here. This finding can be [14] M. Rohlfing and T. Bredow, Phys. Rev. Lett. 101,
266106 (2008).
rationalizedbythe factthat 5Ahas no carbon atoms ex-
[15] Bendingandinter-ringtorsion ortiltwithrespecttothe
actly on the long molecular axiswhichcanbemosteasily
surface plane do certainly increase the total energy and
polarized by the electric field, as it is for 6P/5PV.
can be thussafely ignored.
In conclusion, we found that the periodic dipolar elec- [16] A. Buckingham et al., Chem. Rev. 88, 963 (1988).
tric field ofthe ZnO(10¯10)surface plays a key role in the [17] J. D.Gale and A. L. Rohl, Mol. Sim. 29 (2003).
5
[18] A.Natan et al., Adv.Mater. 19, 4103 (2007). Sala, Phys. Rev.B 80, 153101 (2009).
[19] M. Piacenza, S. D’Agostino, E. Fabiano, and F. Della