-
Dynamics of tungsten hexacarbonyl, dicobalt octacarbonyl and
theirfragments adsorbed on silica surfaces
Kaliappan Muthukumar, Roser Valent́ı, and Harald O.
JeschkeInstitut für Theoretische Physik, Goethe-Universität
Frankfurt, Max-von-Laue-Straße 1, 60438 Frankfurt am
Main,Germany
(Dated: 2 January 2019)
Tungsten and cobalt carbonyls adsorbed on a substrate are
typical starting points for the electron beaminduced deposition of
tungsten or cobalt based metallic nanostructures. We employ first
principles moleculardynamics simulations to investigate the
dynamics and vibrational spectra of W(CO)6 and W(CO)5 as well
asCo2(CO)8 and Co(CO)4 precursor molecules on fully and partially
hydroxylated silica surfaces. Such surfacesresemble the initial
conditions of electron beam induced growth processes. We find that
both W(CO)6 andCo2(CO)8 are stable at room temperature and mobile
on a silica surface saturated with hydroxyl groups (OH),moving up
to half an Angström per picosecond. In contrast, chemisorbed
W(CO)5 or Co(CO)4 ions at roomtemperature do not change their
binding site. These results contribute to gaining fundamental
insight intohow the molecules behave in the simulated time window
of 20 ps and our determined vibrational spectra of allspecies
provide signatures for experimentally distinguishing the form in
which precursors cover a substrate.
PACS numbers: 68.43.-h,68.43.Fg,71.15.Mb,71.15.Nc
I. INTRODUCTION
The study of the energetics and dynamics of individualmolecules
adsorbed on a substrate is relevant for severalprominent fields of
research such as molecular electron-ics1,2, molecular magnetism3
and catalysis4. The pos-sibility to manipulate individual molecules
adsorbed onsurfaces using the tip of a scanning tunneling
microscope(STM) leads to an increasing need for theoretical
in-formation of the geometries as well as adsorption anddesorption
mechanisms and reaction pathways of adsor-bates5. STM probes can
not only be used to arrangecomplex molecules on surfaces but also
to measure theirvibrational spectra6. Molecules adsorbed on
insulatingsurfaces which due to insufficient conductivity are
hardto study by STM techniques have been investigated suc-cessfully
using non-contact atomic force microscopy7.
Here, we will study molecules and substrates that areimportant
in the context of electron beam induced de-position (EBID) of
organometallic precursor molecules.This is a widely used method to
grow size- and shape-controlled nanometer-sized structures.8–14 The
obtainedEBID deposits nevertheless possess a significant
percent-age of organic contaminants mainly from carbon and oxy-gen,
lowering the conductivity of these deposits and thuslimiting the
possible applications of EBID.15–21 Severalpre- and
post-fabrication approaches have been employedto remove these
contaminants, but reproducibility is stillan issue. First
principles calculations can provide a de-tailed description of the
microscopic behavior of the de-posits and are often used to improve
the quality of thedeposition processes. Important progress has been
done,for instance, on atomic layer deposition22–26 and chemi-cal
vapor deposition27,28 processes.
Recently29–31, in an attempt to understand theEBID growth
process, we analyzed by means of den-sity functional theory (DFT)
calculations the inter-
action of precursors like W(CO)6, Co2(CO)8 andCH3(C5H5)Pt[CH3]3
on two different SiO2 surfaces (fullyand partially hydroxylated) as
a representative for un-treated and pretreated EBID surfaces
(Hereafter, sur-faces corresponds to SiO2 surfaces unless otherwise
men-tioned). These studies illustrate the preferred orientationof
the adsorbate and the nature of the interaction be-tween the
precursor molecules and the SiO2 substrates.Further, interesting
phenomena such as the spontaneousfragmentation of the carbonyl
precursors (W(CO)6 toW(CO)5 and Co2(CO)8 to two Co(CO)4 molecules)
onthe partially hydroxylated surfaces that correspond topre-treated
surfaces were observed.30,31
It has recently been reported that the surface resi-dence time
of an organometallic precursor should be suf-ficiently long
(lasting from microseconds to milliseconds)to have an efficient
deposition yield.17 In order to under-stand these observations and
to improve the conditionsfor the adhesion of precursor molecules to
the substrate,it is essential to have knowledge on the behavior of
theprecursor molecules and their fragments on the surfaceSiO2
substrates. Theoretical and experimental studiesfocussing on the
adsorption process of free CO on vari-ous surfaces and several
molecules on different SiO2 sub-strates have been reported.32–35
However, little is knownon the dynamics of W(CO)6 and Co2(CO)8
precursorsand their fragments on SiO2 surfaces. Therefore, in
thiswork we use first principles molecular dynamics simula-tions to
investigate the nature of W(CO)6 and Co2(CO)8molecules adsorption
on fully and partially hydroxylatedSiO2 surfaces and provide
quantitative microscopic in-sight into the stability of these
fragmented precursors onthese surfaces and on their vibrational
spectra.
arX
iv:1
306.
2567
v4 [
cond
-mat
.mtr
l-sc
i] 1
5 A
pr 2
014
-
2
FIG. 1. (Color online) Adsorption of W(CO)6 on fully
hydroxylated (FOH, upper panel) and partially hydroxylated
(POH,lower panel) substrates. The illustrated snapshots are the
configurations taken at every 5 ps interval. Color code : Green -
Si,Red - O, light Blue - W, Gray - C and Magenta - H throughout
this manuscript. M1,M2 and M3 are the labels for specificligands to
identify the changes in 2D view and the snapshots have slightly
different orientations (note the coordinate axis) tobetter display
the changes happening to the system
II. COMPUTATIONAL DETAILS
Ab initio molecular dynamics (MD) simulations wereperformed in
the framework of density functional the-ory (DFT). We employed the
projector augmented wave(PAW) basis36,37 within the generalized
gradient approx-imation in the parametrization of Perdew, Burke
andErnzerhof (PBE) as implemented in VASP37–40. Spinpolarized
calculations including the corrections for longrange van der Waals
interactions [37,38] were used inall calculations. All ions were
relaxed using the conju-gate gradient scheme until the forces were
reduced by. 0.01 eV/Å with a plane wave energy cut-off of 400
eV.The Brillouin zone for the substrate-precursor complexwas
sampled at the gamma point only.
The structures considered as initial configurations forthe MD
simulations are shown in Figs. 1 and 5 (see thet = 0 ps snapshot).
MD simulations were performedfor 20 ps on a canonical ensemble at a
finite tempera-ture of T = 298 K using the Nose-Hoover
thermostat.41
The temperature was chosen in accordance with the re-ported
experimental results where the largest temper-ature rise during
W(CO)6 deposition was 1
◦C when a1.42 nA electron beam was used for depositing
tungstennanostructures.42,43 The Verlet algorithm in its
velocity
form with a time step of ∆t = 1 fs was used to inte-grate the
equations of motion. We also performed MDsimulations for the
precursor molecules (W(CO)6 andCo2(CO)8) in the gas phase by
placing the molecules ina cubic box of a = b = c = 30 Å using
similar parametersused for the substrate-precursor complex. The
length ofthe simulation for the molecule was limited to 10 ps.
The power spectrum I(ω) provides information aboutthe
distribution of the vibrational energy of the system.It can be
computed by a Fourier transformation of the ve-locity
autocorrelation function obtained from the veloci-ties vi(t) of all
atoms in the course of the MD trajectory.The method we use is
described in the Appendix. Ini-tially, we have compared the
vibrations of the gas phaseprecursor molecules obtained by the
finite displacementmethod with the calculated power spectrum
(Vibrationsthrough the finite displacement method were computedfor
the precursor molecules W(CO)6 and Co2(CO)8 inthe gas phase using
Turbomole 6.0.44–46 The geometrieswere optimized using triple ζ
basis sets for all elementsand we used an effective core potential
(60 core electrons)for W in the case of W(CO)6.
47). The vibrations com-puted by these two methods agree with
each other andare explained in Section III C. Therefore, vibrations
forthe SiO2 substrate and the complex precursor molecule-
-
3
−5
−4
−3
−2
−1
0
1
2
3
4
5
6
0 5 10 15 20time (ps)
CO
M c
oord
inat
e (Å
)
W(CO)6 on FOH − SiO2(b)
xCOMyCOMzCOM
FIG. 2. (Color online) Schematic illustration of the movementof
W(CO)6 on a fully hydroxylated surface. In the top panel,the
polygon of the oxygen atoms of W(CO)6 is shown, withone facet
always shown in red in order to visualize the rota-tions of the
molecule. In the lower panel the time evolutionof the three
center-of-mass (COM) coordinates of W(CO)6 isshown. FOH in the
figure corresponds to the fully hydroxy-lated surface
substrate have been evaluated only from the power spec-trum so
as to reduce the computational effort in comput-ing the Hessian
matrix.
FIG. 3. (Color online) DFT optimized structure of W(CO)5.In the
gas phase it possess a C4v symmetric structure. Thetrigonal
bipyramidal structure (middle panel) is 0.643 eV lessstable than
the square pyramidal structure (left and rightpanels)48 but it is
stabilized by surface-molecule interactionon partially
hydroxylated.
FIG. 4. (Color online) Variations in the structural features
ofW(CO)6 on SiO2 surfaces on fully hydroxylated (left panel)and
partially hydroxylated (right panel). Bold (red) letterscorrespond
to WC and normal letters (blue) for CO bond.The values shown in
this figure are the standard deviation ofthe respective values in
the initial structure.
III. RESULTS
A. Dynamics of W(CO)6 on SiO2 surfaces
Previous DFT calculations30 indicate that W(CO)6interacts
through weak physisorption with surface hy-droxyls on the fully
hydroxylated surface and by strongchemisorption on the Si sites
available on partially hy-droxylated surfaces with substantial
changes in the struc-ture and electronic properties. The most
stable configu-ration of W(CO)6 on fully hydroxylated has an
adsorp-tion energy of -0.498 eV while the fragment W(CO)5 to-gether
with a free CO ligand stabilize with an energy-1.262 eV on a
partially hydroxylated surface. In Fig. 1we show five snapshots in
5 ps intervals of the MD sim-ulations of W(CO)6 on a fully
hydroxylated surface (up-
-
4
FIG. 5. (Color online) Adsorption of Co2(CO)8 on fully
hydroxylated (top) and partially hydroxylated (POH) substrates.
Theillustrated snapshots are the configurations taken at every 5ps
interval. S1 and S2 in partially hydroxylated case denotes thetwo
bonding sites on the surface where Co(CO)4 fragments are bonded.
Color code: Blue - Co, throughout this manuscript.The orientation
of the configuration is denoted by the coordinate system on the
bottom left hand side of each panel.
−4
−3
−2
−1
0
1
2
3
0 5 10 15 20time (ps)
CO
M c
oord
inat
e (Å
)
Co2(CO)8 on FOH − SiO2
xCOMyCOMzCOM
FIG. 6. (Color online) Evolution of the three
center-of-masscoordinates of Co2(CO)8 adsorbed on a fully
hydroxylatedsurface during the 20 ps trajectory.
per panel) and of the fragment W(CO)5 together with afree CO
ligand on a partially hydroxylated surface (lowerpanel).
Analysis of the trajectory in the fully hydroxylatedcase
indicates that the W(CO)6 molecule exhibits a con-siderable drift
and moves around the initial binding sites
0
50
100
150
200
pow
er
spectr
um
(norm
aliz
ed)
gas phase W(CO)6
0 500 1000 1500 2000
frequency (cm−1
)
IR Raman neither
FIG. 7. (Color online) Calculated vibrational spectrum of thegas
phase of W(CO)6 molecules by Fourier transforming thevelocity
autocorrelation function obtained from the MD tra-jectory (upper
panel) and by the finite displacement method(lower panel).
in a fully hydroxylated surface. In order to visualizethis
drifting, Fig. 2 shows a schematic depiction of theW(CO)6
displacement together with the center-of-mass(COM) analysis. The
calculated drifting distance on thesurface after 20 ps is ca. 5 Å.
These simulations illus-trate that the undissociated W(CO)6
molecule changes
-
5
0
20
40
60
pow
er
spectr
um
(norm
aliz
ed)
W(CO)6 on SiO2
Raman (Ref. [47])
IR (Ref. [46])
0 500 1000 1500 2000
frequency (cm−1
)
Γ (Ref. [49]) M
FIG. 8. (Color online) Comparison of the calculated
powerspectrum for W(CO)6 adsorbed on fully hydroxylated withthe
experimentally measured infrared and Raman spec-trum53,54 for the
SiO2 substrate. In the lower panel, thevibrational frequences of β
cristobalite at Γ and M pointsin the Brillouin zone as calculated
in Ref.56 are shown.
the orientation considerably but does not desorb awayfrom the
fully hydroxylated surface.
However, a chemisorbed W(CO)5 molecule which isformed by the
release of a CO ligand from W(CO)6(Fig. 1 lower panel) remains
localized on its binding siteon a partially hydroxylated surface.
The dissociated COligand doesn’t recombine with the parent moiety
and thevacant site on W remains empty and is not filled by sur-face
hydroxyls as has been suggested as a possibility forMo(CO)5.
49 It should be noted that W(CO)5 in the gasphase is stable on a
square pyramidal structure and dif-ferent conformations are
possible through the pathwayshown in Fig. 3 involving a trigonal
bipyramidal transi-tion state.50 Analysis of the adsorbed W(CO)5
structure(compare the transition state on Fig. 3 with the
config-uration at 0 ps in Fig. 1 lower panel) indicates that
theW(CO)5 molecule is stabilized in a trigonal bipyramidalstructure
on partially hydroxylated surfaces. The stabi-lization of such a
transient intermediate has been pro-posed for Cr(CO)5 and the
present work supports sucha proposal also for W(CO)5.
51 This stabilization, shouldhave an impact over the kinetics of
further release of intramolecular CO ligands.
Evaluation of the changes in the WC and CO bondlengths of the
adsorbates W(CO)6 and W(CO)5 (seeFig. 4) during the first 20 ps of
the trajectory show de-viations from the initial configuration of
the order of 1-2% and might be due to thermal fluctuations. We
willanalyze these deviations in more detail in Section III C.
0
20
40
60
(a)W(CO)6 on FOH − SiO2W(CO)5 on POH − SiO2
0
100
200
pow
er
spectr
um
(norm
aliz
ed)
(b) physisorbed W(CO)6 only
IR
Raman other
0
100
200
pow
er
spectr
um
(norm
aliz
ed)
(c) chemisorbed W(CO)5 only
0 500 1000 1500 2000frequency (cm
−1)
IR/Raman
0 500 1000 1500 2000frequency (cm
−1)
Raman other
FIG. 9. (Color online) (a) Comparison of the total powerspectrum
for (i) W(CO)6 on a fully hydroxylated surfaceand (ii) W(CO)5 on a
partially hydroxylated surface. (b)Power spectrum of the
physisorbed molecule W(CO)6 onlyin case (i) compared to the
vibrational modes of W(CO)6 inthe gas phase. The modes are
classified in Raman, infraredand other. (c) Power spectrum of the
chemisorbed moleculeW(CO)5 only in case (ii) compared to the
vibrational modesof W(CO)5 in the gas phase.
The electronic structure of the final configurations of
thecomplex molecule-substrate on both fully and
partiallyhydroxylated cases shows only minor variations with
re-spect to the initial configurations (results not shown).
B. Dynamics of Co2(CO)8 on SiO2 surfaces
We now proceed with the adsorption scenario ofCo2(CO)8
interacting with SiO2 surfaces. In Ref.
31 aweak bonding of Co2(CO)8 on fully hydroxylated sur-faces was
observed, similar to the case of W(CO)6, withan adsorption energy
of -0.76 eV. In contrast, Co2(CO)8molecules on a partially
hydroxylated surface fragmentinto two Co(CO)4 moieties rather than
eliminating aCO ligand as in the case of W(CO)6. The fragmented
-
6
0
50
100
150
200pow
er
spectr
um
(norm
aliz
ed) gas phase Co2(CO)8
0 500 1000 1500 2000frequency (cm
−1)
IR/Raman Raman
FIG. 10. (Color online) Calculated vibrational spectrum ofthe
gas phase of Co2(CO)8 molecules by fourier transformingthe
velocities obtained from the MD trajectory (upper panel)and by the
finite displacement method (lower panel).
Co(CO)4 moieties exhibit a strong chemisorption on par-tially
hydroxylated surfaces with an adsorption energyof -1.77 eV.31
Considering the above configurations asthe initial setting for our
MD simulations, Fig. 5 showssnapshots in 5 ps intervals of Co2(CO)8
adsorbed on afully hydroxylated surface (upper panel) and of
Co(CO)4fragments adsorbed on a partially hydroxylated surface(lower
panel).
We observe a significant drift of Co2(CO)8 on the
fullyhydroxylated surface but as this is a larger molecule
thanW(CO)6, the displacement is less pronounced than inW(CO)6. This
can be seen by comparing Fig. 6, wherethe COM movement of Co2(CO)8
on a fully hydroxylatedsurface is depicted, with Fig. 2 (b) which
is analogous toW(CO)6. The calculated drifting radius of
Co2(CO)8within 20 ps is about 4 Å.
We now investigate the adsorption of the Co(CO)4species on
partially hydroxylated surfaces (Fig. 5 lowerpanel). We would like
to note that, unlike in the caseof W(CO)5, the Co(CO)4 species in
the gas phase pos-sesses a tetrahedral structure which remains
stable witha slight distortion on the partially hydroxylated
surfaces(see t = 0 ps in Fig. 5 lower panel). During the
MDsimulations, the fragmented Co(CO)4 species are local-ized on the
surface but we observe severe changes in theorientation of CO
ligands. In particular, we find that theCo(CO)4 fragment bonded to
the S2 site on the partiallyhydroxylated surface as shown in Fig. 5
(lower panel)shows bonding of the Co atom to surface oxygen as
timeevolves (compare t = 0 ps and t = 5 ps snapshots). Asimilar
situation was suggested by Rao et al.52 during theadsorption of
Co2(CO)8 on dehydroxylated MgO and onSiO2 surfaces. However, within
our simulation window,we only observe this effect for the Co(CO)4
fragmentbonded to the S2 site but not for the fragment bonded tothe
S1 site (see Fig. 5 (lower panel)) of the partially hy-
0
20
40
60
(a)Co2(CO)8 on FOH − SiO2Co(CO)4 on POH − SiO2
0
100
200
pow
er
spectr
um
(norm
aliz
ed)
(b) physisorbed Co2(CO)8 only
IR/Raman Raman
0
100
200
pow
er
spectr
um
(norm
aliz
ed)
(c) chemisorbed Co(CO)4 (S1 site)chemisorbed Co(CO)4 (S2
site)
0 500 1000 1500 2000frequency (cm
−1)
IR/Raman
0 500 1000 1500 2000frequency (cm
−1)
Raman neither
FIG. 11. (Color online) (a) Comparison of the total
powerspectrum for (i) Co2(CO)8 on a fully hydroxylated surfaceand
(ii) the two Co(CO)4 fragments on a partially hydroxy-lated
surface. (b) Power spectrum of the physisorbed moleculeCo2(CO)8 in
case (i) compared to the vibrational modesof Co2(CO)8 in the gas
phase. (c) Power spectrum of thechemisorbed Co(CO)4 fragments in
case (ii) compared to thevibrational modes of Co(CO)4 in the gas
phase.
droxylated SiO2 surfaces. Such a metal-substrate bondmight be
due to activation of the substrate surface bydehydroxylation.
C. Vibrations of W(CO)6 and Co2(CO)8 on a SiO2surface
1. W(CO)6 adsorbed on SiO2 surface
We first start with the analysis of the vibrational spec-trum of
W(CO)6 in the gas phase and the modes arisingdue to SiO2 surfaces.
We observe W−C stretching modesand (OC)−W−(CO) bending modes at
300-600 cm−1,and C−O stretching modes at 1900-2150 cm−1 for thegas
phase W(CO)6. The bending vibrations that involveW−(CO) atoms are
seen at 60-100 cm−1. All these nor-
-
7
mal modes are seen in both upper (power spectrum) andlower panel
(finite displacement method) of Fig. 7. Also,in Fig. 8, we compare
the power spectrum calculatedfor W(CO)6 adsorbed on a fully
hydroxylated surfacewith experimental infrared and Raman spectra
measuredfor SiO2 substrates so as to analyze the vibrations
ofSiO2.
53,54 We observe a good overall agreement betweenthe calculated
SiO2 modes (Infrared and Raman activepeaks at 100-200, 400-450,
700-800 and 1000-1100 cm−1)and the experimental spectrum.55,56
Apart from SiO2modes, the additional modes in Fig. 8 are due to the
sur-face adsorbed W(CO)6 molecule (compare with Fig. 7).This
analysis indicates that the adopted methodology tocalculate the
power spectrum could be used to describethe vibrations of W(CO)6
adsorbed on SiO2 surfaces.
In Fig. 9 we present the total power spectrum ofW(CO)6 adsorbed
on fully and partially hydroxylatedsurfaces (Fig. 9(a)) and only
the vibrations of ph-ysisorbed W(CO)6 and chemisorbed W(CO)5
extractedfrom it which are compared with the vibrations (obtainedby
finite displacement method, shown as vertical bars) ofisolated gas
phase structures (Fig. 9(b) and (c)). FromFig. 9 (b), we observe
only minor changes in the vibra-tional modes of W(CO)6. In
particular, the CO fre-quencies observed between 1900 and 2110 cm−1
(two ofwhich are Raman active) for the gas phase of W(CO)6have also
been observed upon its bonding to fully hy-droxylated surfaces
without being perturbed indicatingthat the molecule retains its
gas-phase characteristics.These features are indeed similar to
W(CO)6 adsorbedon hydroxylated alumina (γ−Al2O3) surfaces where
allthe molecular vibrations are retained57,58 indicating theweak
interaction between W(CO)6 and the substrate.
From the analysis of the chemisorbed W(CO)6 frag-ments on
partially hydroxylated surfaces (see Fig. 9(c))we find, in addition
to the shifts of several peaks, two newmodes occurring at 700-800
cm−1 and 1400-1600 cm−1.Peaks at 700-800 cm−1 which correspond to
the Ramanvibrations of β-cristobalite55, are now seen, because
ofprecursor bonding to the surface. Also, the peaks at1400-1600
cm−1 appear as a result of oxygen (of CO)interaction with substrate
Si atoms.58,59 Due to this in-teraction the bonding between
tungsten and the carbonylin W(CO)6 gets affected leading to a
considerable split ofpeaks in the regions that correspond to the
vibrations ofC−O (1900-2100 cm−1) and W−C (300-600 cm−1) bondsof
W(CO)6 respectively.
2. Co2(CO)8 adsorbed on SiO2 surface
There have been some studies aimed at understand-ing the
vibrations of Co2(CO)8 adsorbed on SiO2. How-ever they are either
limited to the carbonyl region ofthe spectrum52,60 or they observe
immediate conversionof Co2(CO)8 to subcarbonyl species.
61,62 We first showin Fig. 10 the analysis of the vibrational
spectrum ofCo2(CO)8 in the gas phase from power spectrum and
the finite displacement method and both methods agreewith each
other and the existing reports.63 The two setsof vibrations in the
region 1850-1900 and 2100-2000 cm−1
are assigned to the stretching modes of the bridging andterminal
CO ligands and the Co-Co stretching mode inCo2(CO)8 is observed
between 200-250 cm
−1.We now investigate the vibrations of Co2(CO)8 ad-
sorbed on SiO2 by analyzing the power spectrum. InFig. 11 (a) we
show the power spectrum of Co2(CO)8on a fully hydroxylated surface
compared to the powerspectrum of Co(CO)4 fragments on a partially
hydroxy-lated surface. The extracted vibrations of
physisorbedCo2(CO)8 and chemisorbed Co(CO)4 from the totalpower
spectrum are compared with the vibrations of theisolated molecules
and are shown in Fig. 11 (b) and (c) re-spectively. The results
illustrate that Co2(CO)8 on fullyhydroxylated surface preserves the
gas-phase moleculecharacteristics (cf. Fig. 10 and Fig. 11
(b)).
In the case of Co(CO)4 on partially hydroxylated sur-faces, the
overall agreement of the power spectrum withthat of the gas phase
Co(CO)4 moiety is relatively poorbecause the Co(CO)4 moieties
exhibit strong structuraldistortions upon bonding to the SiO2
substrate which isabsent in the gas phase computations. In the
power spec-trum, we find that the modes in the region 220-250
cm−1
(Co-Co stretching) and the mode at approx. 1850 cm−1 (bridging
carbonyls) disappear owing to the fact thatthe Co2(CO)8 molecule
fragments on partially hydrox-ylated surfaces. Further, two new
modes in the 600-800 cm−1 and 1400-1600 cm−1 regions appear in this
case(cf. Fig. 11 (a) and Fig. 11 (c)) illustrating the interac-tion
of surface Si atoms with the carbonyls through theoxygen atom of
the CO. However, this situation is onlyobserved for one of the
Co(CO)4 moieties indicating thatthe fragmented species behave
differently. This is a con-sequence of the interaction of surface
oxygen atoms withthe Co atom observed for the fragment bonded to
the S2site (see Fig. 5).
D. Discussion
Investigation of surface adsorption and residence-timeof the
precursors on the surface is necessary for improvingthe
understanding of deposition processes. The presentstudy elucidates
the vibrational footprints of precursorswhich interact through weak
physisorption with fully hy-droxylated surfaces and by strong
chemisorption withpartially hydroxylated surfaces.
The authors of Ref.64 found that physisorbed W(CO)6molecules on
TiO2 surfaces completely desorb when thesystem is cooled down to
room temperature. Our studyfinds no desorption of W(CO)6 molecules
on fully hydrox-ylated surfaces in the considered length of
simulation.Also, Co2(CO)8 molecules on fully hydroxylated
surfaceswere reported to fragment spontaneously61,65 but, in
ourcalculations on fully hydroxylated surfaces we didn’t ob-serve
any tendency to fragmentation or an indication of
-
8
chemisorption. Other effects (probably extrinsic) may
beresponsible for the experimental observations.
The bond variations of W(CO)6 and Co2(CO)8 ad-sorbed on fully
hydroxylated surfaces indicate uniformfluctuations (W−C and C−O
bonds) on either side (i.e,oriented towards the surface and the
vacuum) of themolecule. On the contrary, the relative bond values
ofW(CO)5 on partially hydroxylated surfaces show non-uniform
variations indicating that certain bonds (i.e.,W−C bonds oriented
towards the vacuum and the CObonds towards the substrate)
experience larger changesthan others. Thus, for favorable
conditions, the bond be-tween C and O for the CO bonded to the
surface mightcleave leaving the surface Si atoms terminated with
oxy-gens. This situation is not observed in the present
MDsimulations but the higher ratio of carbon contaminationwith
respect to oxygen (before atmospheric air exposure)in the
EBID-obtained samples might provide an evidencefor such a
fact.17–21 Similarly, we found in the case ofCo2(CO)8 on partially
hydroxylated surfaces, that sur-face oxygen atoms are involved in
bonding to the disso-ciated surface species. This suggests that the
removal ofoxygen components from the deposits might not be aneasy
task to achieve. Presence of oxygen contaminationis also expected
to occur as a result of exposure of EBIDdeposits to the air. Thus
the composition of the EBIDdeposits are determined to a large
extent by the numberof available active Si sites (alternatively the
degree of hy-droxylation), the frequency of these precursor
moleculesapproaching such a site and the exposure time to the
en-vironments. Furthermore, the surface defects observedin SiO2
during the interaction of Co(CO)4 with partiallyhydroxylated
surfaces might also act as an active sitefor activating the
approaching precursor molecule thatcould account for the facile
dissociation of Co2(CO)8 onSiO2.
31,66
On partially hydroxylated surfaces our results for bothW(CO)6
and Co2(CO)8 show that the fragmented speciesremain localized, thus
blocking active sites on the sur-face. Therefore, further
deposition should occur on thedeposited layers and this may be
likely the reason for theincrease in height of the deposits as the
irradiation timeincreases during EBID experiments.67
IV. CONCLUSIONS
The purpose of this work was to model by means ofab initio
molecular dynamics simulations the dynamicsof two precursor
molecules adsorbed on fully and par-tially hydroxylated SiO2
surfaces in order to achievea better understanding of the
microscopics of electron-beam induced deposition of nanostructures.
Our resultsreveal that W(CO)6 and Co2(CO)8 molecules preservetheir
gas-phase bonding characteristics on fully hydrox-ylated surfaces.
Apart from a considerable drift, onlyminor variations in the
structure and vibrations is ob-served. Therefore spontaneous
dissociation of these pre-
cursor molecules will not be possible on fully hydroxy-lated
surfaces, unless some surface active sites are createdby external
forces.
For the case of W(CO)6 and Co2(CO)8 on partiallyhydroxylated
surfaces, the fragmented species retain thechemisorbed character on
the surface and we do not ob-serve any reformation of the parent
precursor moiety,but instead a slight tendency towards
fragmentation. Wealso observe a smaller weakening of the
surface-orientedCO bonds compared to vacuum-oriented CO bonds
inW(CO)6 and Co2(CO)8 on partially hydroxylated sur-faces.
Therefore, conditions that favor the formation ofactive sites (in
this case surface Si atoms) are needed inorder to have high
efficiency in fragmentation and im-prove the metal content of the
deposit.
The calculated vibrational spectra of these carbonylmolecule/
SiO2 substrate systems show clear fingerprintsto be detected
experimentally. We propose therefore theconsideration of such
simulations as a route to experi-mentally distinguish the form in
which precursors covera substrate.
Finally, while the present simulations provide insightson the
surface-precursor interaction, the investigation ofother processes
like surface-electron, precursor-electronand deposit-molecule
interaction remains a challenge forfuture work.
V. ACKNOWLEDGMENTS
We would like to thank the Beilstein-Institut, Frank-furt/Main,
Germany, within the research collaborationNanoBiC for financial
support. The generous allotmentof computer time by CSC-Frankfurt
and LOEWE-CSCis also gratefully acknowledged.
Appendix A: Estimation of the power spectrum
We use the power spectrum for analysing the vibra-tional
characteristics of precursor molecules in the gasphase and for the
entire system of precursors chemi- orphysisorbed on a silica
substrate68. The power spectrumis defined as69,70
I(ω) =1
2π
∫ ∞−∞
C(τ)e−iωτdτ , (A1)
where C(τ) is the velocity autocorrelation function
C(τ) = 〈v(0)v(τ)〉 = limT→∞
[1
T N
∫ T0
N∑l=1
vl(t)vl(t+ τ)dt
].
(A2)vl(t) represents the velocity of atom l at time t for all
Natoms of the system. As the calculation of the time aver-ages 〈. .
. 〉 of Eq. (A2) is quite inefficient computationally,we use the
Wiener-Khinchin theorem which guarantees
-
9
that the power spectrum can also be calculated by in-dividually
Fourier-transforming the velocities vl(t) andsumming the squares of
the result:
I(ω) =1
N
N∑l=1
∣∣∣∣[ 12π∫ ∞−∞
vl(t)e−iωtdt
]∣∣∣∣2 . (A3)Finite trajectories, calculated with a finite time
step ∆t,only yield estimates to the power spectrum. Instead ofthe
Fourier integral in Eq. (A3), discrete Fourier sumshave to be
calculated according to
V lµk =
J−1∑j=0
vlµj e2πijkJ , (A4)
where the vlµj stand for the µ component (µ ∈ {x, y, z}) ofthe
velocity of atom l at time step j, and J is the numberof time steps
of the trajectory. The so-called periodogramestimate for the power
spectrum is then defined for J/2+1 frequencies71:
P lµ(0) = P lµ(f0) =1
J2|V lµ0 |2 ,
P lµ(fk) =1
J2
[|V lµk |
2 + |V lµJ−k|2],
P lµ(fc) = Plµ(fJ/2) =
1
J2|V lµJ/2|
2 .
(A5)
The highest frequency fc =1
2∆t is called Nyquist fre-quency and is determined by the time
step ∆t of the MDcalculation. Thus, the finite time estimate of the
powerspectrum can finally be written as
IJ(ω) =1
N
N∑l=1
3∑µ=1
P lµ(ω) . (A6)
The function IJ(ω) will approach the true power spec-trum I(ω)
of the system in the limit J → ∞, i. e. inthe limit of infinitely
long trajectories. The power spec-trum estimates now provide us
with information aboutthe distribution of the vibrational energy of
a moleculeor a solid over the frequencies. Power spectra at
differenttemperatures may be used to investigate the differencesin
the population of vibrational modes. The integral overthe power
spectrum corresponds to the kinetic energy ofthe system.
1G. Joachim, J. K. Gimzewski, and A. Aviram, Nature 408,
541(2000).
2J. R. Heath, Annu. Rev. Mater. Res. 39, 1 (2009).3J. S. Miller
and A. J. Epstein, MRS Bulletin 25, 21 (2000).4F. Besenbacher, J.
V. Lauritsen, and S. Wendt, Nano Today 2,30 (2007).
5S.-W. Hla and K.-H. Rieder, Annu. Rev. Phys. Chem. 54,
307(2003).
6R. Otero, F. Rosei, and F. Besenbacher, Annu. Rev. Phys.
Chem.57, 497 (2006).
7A. Schwarz, D. Z. Gao, K. Lämmle, J. Grenz, M. B. Watkins,
A.L. Shluger, and R. Wiesendanger, J. Phys. Chem. C 117,
1105(2013).
8S. Frabboni, G. C. Gazzadi, L. Felisari, and A. Spessot,
Appl.Phys. Lett. 88, 213116 (2006).
9S. Kim, D. Kulkarni, K. Rykaczewski, M. Henry, V. Tsukruk,and
A. Fedorov, IEEE Trans. Nanotechnol. 11, 1223 (2012).
10J. D. Wnuk, S. G. Rosenberg, J. M. Gorham, W. F. van Dorp,C.
W. Hagen, and D. H. Fairbrother, Surf. Sci. 605, 257 (2011).
11S. J. Randolph, J. D. Fowlkes, and P. D. Rack, Crc. Cr.
Sol.State. 31, 55 (2006).
12N. S.-Cividjian, C. W. Hagen, and P. Kruit, J. Appl. Phys.
98,084905 (2005).
13F. Porrati, R. Sachser, and M. Huth, Nanotechnology 20,
195301(2009).
14I. Utke, P. Hoffmann, and J. Melngailis, J. Vac. Sci. Technol.
B.26, 1197 (2008).
15K. Muthukumar, R. Valent́ı, H. O. Jeschke, New J. Phys.
14,113028 (2012).
16F. Salvat-Pujol, H. O. Jeschke, R. Valent́ı, Beilstein J.
Nanotech-nol. 4, 781 (2013).
17M. Huth, F. Porrati, C. Schwalb, M. Winhold, R. Sachser,
M.Dukic, J. Adams, and G. Fantner, Beilstein J. Nanotechnol. 3,597
(2012).
18A. Botman, M. Hesselberth, and J. Mulders, Microelectron.
Eng.85, 1139 (2008).
19J. Li, M. Toth, V. Tileli, K. A. Dunn, C. J. Lobo, and B.
L.Thiel, Appl. Phys. Lett. 93, 023130 (2008).
20J. D. Barry, M. Ervin, J. Molstad, A. Wickenden, T.
Brintlinger,P. Hoffman, and J. Meingailis, J. Vac. Sci. Technol. B
24, 3165(2006).
21M. Huth, D. Klingenberger, C. Grimm, F. Porrati, and
R.Sachser, New J. Phys. 11, 033032 (2009).
22Y. Widjaja and C. B. Musgrave, Appl. Phys. Lett. 80,
3304(2002).
23S. M. George, Chem. Rev. 110, 111 (2010).24L. Jeloaica, A.
Esteve, M. D. Rouhani, and D. Esteve, Appl.
Phys. Lett. 83, 542 (2003).25T. Aaltonen, A. Rahtu, M. Ritala,
and M. Leskel, Electrochem.
Solid-State. Lett. 6, C130 (2003).26Y. Xu and C. B. Musgrave,
Chem. Mater. 16, 646 (2004).27A. Y. Timoshkin, H. F. Bettinger, and
H. F. Schaefer, J. Phys.
Chem. A 105, 3240 (2001).28A. Y. Timoshkin, H. F. Bettinger, and
H. F. Schaefer, J. Phys.
Chem. A 105, 3249 (2001).29J. Shen, K. Muthukumar, H. O.
Jeschke, and R. Valent́ı, New.
J. Phys. 14, 073040 (2012).30K. Muthukumar, I. Opahle, J. Shen,
H. O. Jeschke, and R. Va-
lent́ı, Phys. Rev. B 84, 205442 (2011).31K. Muthukumar, H. O.
Jeschke, R. Valent́ı, E. Begun, J.
Schwenk, F. Porrati, and M. Huth, Beilstein J. Nanotechnol.
3,546 (2012).
32P. Philipsen, E. Van Lenthe, J. Snijders, and E. Baerends,
Phys.Rev. B 56, 13556 (1997).
33H. Steininger, S. Lehwald, and H. Ibach, Surf. Sci. 123,
264(1982).
34S. Y. Chin, C. T. Williams, and M. D. Amiridis, J. Phys.
Chem.B 110, 871 (2006).
35M. L. Smith, N. Kumar, and J. J. Spivey, J. Phys. Chem. C
116,7931 (2012).
36P. E. Blöchl, Phys. Rev. B 50, 17953 (1994).37G. Kresse and
D. Joubert, Phys. Rev. B 59, 1758 (1999).38G. Kresse and J. Hafner,
Phys. Rev. B 47, 558 (1993).39G. Kresse G and J. Furthmüller,
Phys. Rev. B 54, 11169 (1996).40G. Kresse G and J. Furthmüller,
Comput. Mater. Sci. 6, 15
(1996).41J. Hafner, J. Comput. Chem. 29, 2044 (2008).42Z-Q. Liu,
K. Mitsuishi, and K. Furuya, Jpn. J. Appl. Phys. 46,
6254 (2007).43M. Weber, M. Rudolph, J. Kretz, and H.W.P. Koops,
J. Vac.
Sci. Technol. B 13, 461 (1995).44O. Treutler and R. Ahlrichs, J.
Chem. Phys. 102, 346 (1995).
-
10
45P. Deglmann, K. May, F. Furche, and R. Ahlrichs, Chem.
Phys.Lett. 384, 103 (2004).
46K. Eichkorn, O. Treutler, H.Öhm, M. Häser, and R.
Ahlrichs,Chem. Phys. Lett. 242, 652 (1995).
47K. Eichkorn, F. Weigend, O. Treutler, and R. Ahlrichs,
Theor.Chem. Acc. 97, 1432 (1997).
48Y.-I. Ishikawa, private communication.49K. P. Reddy and T. L.
Brown, J. Am. Chem. Soc. 117, 2845
(2005).50Y.-I. Ishikawa and K. Kawakami, J. Phys. Chem. A 111,
9940
(2007).51E. Kochanski, ed., ”Photoprocesses in transition metal
com-
plexes, biosystems and other molecules, experiment and the-ory
(NATO Science Series C),” (Kluwer Academic Publishers)(1991).
52K. M. Rao, G. Spoto, and A. Zecchina, J. Catal. 113, 466
(1988).53K. Finnie, J. Thompson, and R. Withers, J. Phys. Chem.
Solids55, 23 (1994).
54J. B. Bates, J. Chem. Phys. 57, 4042 (1972).55Y. Liang, C. R.
Miranda, and S. Scandolo, J. Chem. Phys. 125,
194524 (2006).56S. Coh, D. Vanderbilt, Phys. Rev. B 78, 054117
(2008).57R. F. Howe, Inorg. Chem. 15, 486 (1976).58A. Zecchina, E.
Escalona Platero, and C. Otero Arean, Inorg.
Chem. 27, 102 (1988).59M. Suvanto and T. A. Pakkanen, J. Mol.
Catal. A: Chem. 138,
211 (1999).60K. M. Rao, G. Spoto, E. Guglielminotti, and A.
Zecchina, J.
Chem. Soc. Faraday. Trans. 184, 2195 (1988).
61R. L. Schneider, R. F. Howe, and K. L. Watters, Inorg.
Chem.23, 4593 (1984).
62S. Suvanto, T. A. Pakkanen and L. Backman, Appl. Catal. A:Gen.
177, 25 (1999).
63P. J. Kenny, B. R. King and H. F. Scheaefer, Inorg. Chem.
900,5 (2001).
64B. Domenichini and J. Prunier and M. Petukov and Z. Li and
P.J. Moller and S. Bourgeois, J. Electron Spectrosc. Relat.
Phenom.163, 19 (2008).
65A. Zecchina and C. O. Aran, surfaces: IR Studies,” Catal.
Rev.35, 261 (1993).
66R. Cordoba, J. Sese, M. R. Ibarra, J. M. De Teresa, Appl.
Surf.Sci., 263, 242 (2012).
67M. Winhold, C. H. Schwalb, F. Porrati, R. Sachser, A. S.
Fran-gakis, B. Kmpken, A. Terfort, N. Auner, and M. Huth, ACSNano
5, 9675 (2011).
68H. O. Jeschke, ”Theory for optically created nonequilibrium
incovalent solids,” PhD thesis, Freie Universität Berlin
(2000).
69X. Y. Chang, T. D. Sewell, L. M. Raff, D. L. Thompson, J.
Chem.Phys. 97, 7354 (1992).
70D. W. Noid , M. L. Koszykowski, R. A. Marcus, J. Chem.
Phys.67, 404 (1997).
71”W. H. Press, S. A. Teukolsky, W. T. Vetterling, B.
Flanery,Numerical recipes: the art of scientific computing, Third
Ed.,”Cambridge University Press (2007).
Dynamics of tungsten hexacarbonyl, dicobalt octacarbonyl and
their fragments adsorbed on silica surfacesAbstractI IntroductionII
Computational detailsIII ResultsA Dynamics of W(CO)6 on SiO2
surfacesB Dynamics of Co2(CO)8 on SiO2 surfacesC Vibrations of
W(CO)6 and Co2(CO)8 on a SiO2 surface 1 W(CO)6 adsorbed on SiO2
surface2 Co2(CO)8 adsorbed on SiO2 surface
D Discussion
IV ConclusionsV AcknowledgmentsA Estimation of the power
spectrum
