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Probing the Chemistry of Alumina Atomic Layer Deposition
UsingOperando Surface-Enhanced Raman SpectroscopySicelo S.
Masango,† Ryan A. Hackler,† Anne-Isabelle Henry,† Michael O.
McAnally,† George C. Schatz,†
Peter C. Stair,†,‡ and Richard P. Van Duyne*,†
†Department of Chemistry and Center for Catalysis and Surface
Science, Northwestern University, Evanston, Illinois 60208,
UnitedStates‡Chemical Sciences and Engineering Division, Argonne
National Laboratory, Argonne, Illinois 60439, United States
*S Supporting Information
ABSTRACT: This work demonstrates for the first time
thecapability of measuring surface vibrational spectra
foradsorbates during atomic layer deposition (ALD) reactionsusing
operando surface-enhanced Raman spectroscopy (SERS).We use SERS to
study alumina ALD growth at 55 °C on baresilver film-over
nanosphere (AgFON) substrates as well asAgFONs functionalized with
thiol self-assembled monolayers(SAMs). On bare AgFONs, we observe
the growth of Al−Cstretches, symmetric C−H and asymmetric C−H
stretchesduring the trimethylaluminum (TMA) dose half-cycle,
andtheir subsequent decay after dosing with H2O. Al−C and C−H
vibrational modes decay in intensity with time even without
H2Oexposure providing evidence that residual H2O in the ALD chamber
reacts with −CH3 groups on AgFONs. The observed Al−Cstretches are
attributed to TMA dimeric species on the AgFON surface in agreement
with density functional theory (DFT)studies. We observe Al−C
stretches and no thiol vibrational frequency shifts after dosing
TMA on AgFONs functionalized withtoluenethiol and benzenethiol
SAMs. Conversely, we observe thiol vibrational frequency shifts and
no Al−C stretches forAgFONs functionalized with 4-mercaptobenzoic
acid and 4-mercaptophenol SAMs. Lack of observed Al−C stretches
forCOOH- and OH-terminated SAMs is explained by the spacing of
Al−(CH3)x groups from the SERS substrate. TMA penetratesthrough
SAMs and reacts directly with Ag for benzenethiol and toluenethiol
SAMs and selectively reacts with the −COOH and−OH groups for
4-mercaptobenzoic acid and 4-mercaptophenol SAMs, respectively. The
high sensitivity and chemical specificityof SERS provides valuable
information about the location of ALD deposits with respect to the
enhancing substrate. Thisinformation can be used to evaluate the
efficacy of SAMs in blocking or allowing ALD deposition on metal
surfaces. The abilityto probe ALD reactions using SERS under
realistic reaction conditions will lead to a better understanding
of the mechanisms ofALD reactions.
■ INTRODUCTIONAtomic layer deposition (ALD) relies on sequential
self-limitingbinary reactions between gaseous precursor molecules
and asubstrate to deposit thin films in a layer-by-layer
fashion.1−4
ALD provides angstrom-level precision over film thickness.Unlike
films deposited by line-of-sight techniques, ALD filmsare highly
uniform and can be deposited on most substratesregardless of
whether they are flat, porous, or have complexthree-dimensional
topologies. A wide range of materials can bedeposited by ALD
including metal oxides, metal nitrides, metalsulfides, pure metal
thin films, and others. Recent applicationsof ALD extend beyond
thin films and include deposition ofmetal nanoparticles such as
Pd,5−8 Pt,9−11 Ag,12 Ru,13 Ru−Pt,14and Pt−Pd15,16 alloys for
applications in catalysis.The nucleation and growth of ALD
materials is highly
dependent on the surface chemistry involved.17,18 A
deepunderstanding of ALD reactions at the molecular level is
crucialto optimize ALD processes and grow materials
efficiently.Operando spectroscopic techniques, in which reactions
are
studied in situ under temperatures and pressures employed inALD
reactions,19,20 can provide valuable insight towardunderstanding
mechanistic details of ALD processes. Most insitu characterization
techniques that have been used toinvestigate the mechanisms of ALD
reactions either do notprovide detailed chemical structural
information or areperformed under conditions that differ from
realistic ALDconditions.18 Examples include quartz crystal
microbalance(QCM),21 quadrupole mass spectrometry (QMS),22
Fouriertransform infrared (FTIR) spectroscopy,23 and
spectroscopicellipsometry.24 QCM studies are used to monitor mass
gains onthe surface during ALD deposition and determine the
growthrate per cycle and film thickness as a function of the number
ofcycles. A major limitation of QCM measurements is that theydo not
provide any chemically specific information.18 QMS is
Received: November 24, 2015Revised: January 28, 2016
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used to detect products that desorb into the gas phase duringALD
reactions, which helps to identify possible mechanisms ofALD
reactions. However, QMS does not provide directmolecular-level
information about adsorbed surface species. Inaddition, some of the
gas-phase species generated during ALDprocesses may not be stable,
and many undergo furtherconversion in the gas phase, upon collision
with the walls of thereactor, or in the ionizer of the mass
spectrometer beforedetection.18 Spectroscopic ellipsometry provides
informationabout the thickness, growth rate, optical properties,
crystallinephase, and material composition of ALD films but does
notdirectly probe the presence of chemical bonds at surfaces.24
X-ray photoelectron spectroscopy (XPS) is another techniqueoften
used to study ALD reactions but is usually performed inultrahigh
vacuum (UHV), an environment that is different fromtypical
conditions at which ALD reactions are performed.25,26
As a result, the data collected may not be representative of
theproper surface species during the ALD process. Operando
XPSsetups typically require the use of a specially
constructedelectrostatic lens system and a differential pumping
stagebetween the sample cell and the electron energy analyzer
tominimize the scattering of photoelectrons by
gas-phasemolecules.27 Additionally, a synchrotron radiation source
thatcan deliver a much higher photon flux than a conventional X-ray
source is often used to increase the total photoelectronsignal,
although Tao and co-workers recently designed and builtan operando
XPS setup that uses a benchtop Al Kα X-raysource.28−31 Compared to
a synchrotron-based setup, alaboratory-based operando XPS
instrument has limited spectralresolution and surface sensitivity
because of its X-ray sourceintensity and fixed photon energy, which
affects the accuracy ofthe quantitative analysis.32 FTIR provides
molecular-levelinformation about adsorbed surface species and does
notneed a UHV environment.23,33 FTIR readily detects
vibrationalmodes in the ∼1000−3000 cm−1 region but has
difficultymeasuring low frequency vibrations (∼100−1000
cm−1).34Raman spectroscopy is a complementary technique to FTIR
that can probe both low frequency and high frequencymolecular
vibrations. However, Raman scattering has inherentlylow sensitivity
because of small Raman scattering cross sections(∼10−30 cm2
steradian−1 molecule−1).35 Surface-enhancedRaman spectroscopy
(SERS) overcomes the sensitivitylimitations of normal Raman
scattering.36−38 The enhancementof Raman signals is attributed to
the excitation of localizedsurface plasmon resonances (LSPRs) that
result in enhancedelectromagnetic fields around noble metal
nanostructures suchas Ag, Au, and Cu.39,40 The high sensitivity and
distancedependence of SERS38 should make it possible to evaluate
thelocation of ALD deposits with respect to the enhancingsubstrate.
To the best of our knowledge, in situ studies that useSERS to
monitor ALD reactions do not exist in the literature.Most of the
studies published so far use SERS to monitorcatalytic reactions
occurring on thermally robust SERSsubstrates.41,42 In contrast, our
study focuses on demonstratingthat operando SERS can be used to
monitor adsorbed surfacespecies during the ALD process itself,
which results in animproved understanding of the surface chemistry
involved.Using in situ QCM and ex situ LSPR spectroscopy, it
was
demonstrated that ALD Al2O3 grows on bare Ag surfaces with
agrowth rate of ∼1.1 Å/cycle and induces a redshift in the LSPRpeak
position due to a change in refractive index.43 Thedeposition of
Al2O3 by ALD on hydroxylated surfaces is themodel ALD process and
has been the subject of extensive
investigations.2,3,44,45 A few studies have investigated
thegrowth of ALD Al2O3 on metallic substrates.
46,47 ALD Al2O3thin films have been shown to protect metal
nanoparticles fromsintering at elevated temperatures.48,49 This
offers both apromising route to stabilize metal nanoparticles,
which haveapplications in catalysis, and in situ monitoring of
catalyticreactions using operando surface-enhanced optical
spectros-copies.In this work, we demonstrate, for the first time,
that SERS
can be used to probe ALD reactions in situ with high
sensitivity.We use SERS to monitor the surface chemistry of Al2O3
ALDat 55 °C on Ar plasma-cleaned silver-film-over nanosphere(AgFON)
substrates. We also use SERS to monitor ALD Al2O3growth on AgFONs
functionalized with thiol self-assembledmonolayers (SAMs). Thiol
SAMs are ideal molecular systemsto use because they form
well-defined and well-orderedstructures on metal surfaces that have
been extensively studiedin the literature.50 SAMs provide a
well-characterized startingsurface that can be used as an internal
standard against whichspectral changes after ALD growth can be
measured. Further,thiol SAMs on AgFONs are known to displace
carboncontamination and offer an alternative way to clean
SERSsubstrates.51,52 They also preserve the enhancing nature ofSERS
substrates and eliminate the need for plasma cleaning,which reduces
the SERS signal.51 Using operando SERS, weinvestigate the
nucleation and growth of ALD Al2O3 aroundSAMs possessing different
terminal groups. The use of SAMspossessing different terminal
functional groups provides a wayto tailor the surface and direct
ALD Al2O3 growth to achievearea-selective ALD.53,54
■ EXPERIMENTAL SECTIONFabrication of AgFON SERS Substrates.
AgFONs were
fabricated on polished 25 mm silicon wafers according to
astandard procedure described in previous publications.55
Siliconwafers were cleaned by immersion in piranha solution (3:1
byvolume H2SO4/30% H2O2) for 1 h. Caution: Piranha isextremely
corrosive, and appropriate personal protectiveequipment should be
used during handling. Clean siliconwafers were thoroughly rinsed
with deionized (DI) water. Thewafers were then sonicated for 1 h in
5:1:1 by volume H2O/NH4OH/30% H2O2 followed by rinsing with DI
water. Silicananospheres (390 nm, Bangs Laboratories) were diluted
to 5%silica by volume. The solvent was replaced twice with
Millipore(Milli-Q, 18.2 MΩ·cm−1) H2O by a conventional
centrifuga-tion/supernatant removal procedure, followed by
sonication for1 h. The solvent-replaced nanosphere solution (10−12
μL) wasdrop-coated and distributed homogeneously across theprepared
silicon wafer surface. The solvent was then allowedto evaporate
under ambient conditions and spheres assembledin a hexagonal
close-packed array as verified by SEMmeasurements. Ag films (200
nm) were deposited at a rate of2 Å/s under vacuum (6 × 10−6 Torr)
over the nanospheresusing a home-built thermal vapor deposition
system. Thesubstrates were spun during deposition while the
metalthickness and deposition rate were measured by a 6
MHzgold-plated QCM (Sigma Instruments, Fort Collins, CO).AgFONs,
which were not plasma-cleaned, were incubated in 1mM ethanolic
solutions of benzenethiol, toluenethiol, 4-mercaptobenzoic acid, or
4-mercaptophenol (Sigma-Aldrich)for a minimum of 4 h. The
extinction spectra of AgFONs weremeasured using a fiber-optic
coupled halogen light source(World Precision Instruments) and
UV/vis spectrometer (SD
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2000, Ocean Optics) in specular reflectance mode. A Ag mirrorwas
used as a spectral reference and was fabricated bydepositing 200 nm
of Ag film on cleaned glass using thermalvapor
deposition.Surface-Enhanced Raman Spectroscopy. A 532 nm
continuous wave (CW) laser (Innovative Photonic Solutions)was
used for all spectroscopic measurements. Laser light wasdirected
using protected silver mirrors to a 3 mm right-angleprism and then
focused using a visible achromatic doublet lens(1 in. diameter, 4
in. focal length) through a quartz window to aplasmonic substrate
placed inside the ALD reactor. The spotsize radius measured at the
sample was ∼124 μm and ∼57 μmfor the 532 and 633 nm lasers,
respectively, using a scanningknife-edge technique. Raman scattered
light was collected in a180° backscattering geometry and focused
onto a 0.3 mimaging spectrograph (Acton SpectraPro 2300i) using a
visibleachromatic doublet lens (1 in. diameter, 4 in. focal
length).Scattered light was dispersed (1200 grooves/mm grating,
500nm blaze) onto a liquid N2-cooled CCD detector
(PrincetonInstruments, Model 7509-0001). SER spectra were
collectedwith 1−7 mW of laser power (Paq), 1−10 s of acquisition
time(taq), and 1−10 accumulations each, depending on the
systeminvestigated. No background contribution or SERS
signalattenuation was observed from the quartz window (Figure
S1).Computational Modeling. Electronic structure calcula-
tions presented in this work have been performed with
theamsterdam density functional (ADF) computational
chemistrypackage.56 Full geometry optimization, frequency, and
polar-izability calculations for isolated monomer and dimer
TMAcomplexes were completed using the Becke−Perdew
(BP86)generalized gradient approximation (GGA) exchange
correla-tion functional and a triple-ζ polarized (TZP) Slater
orbitalbasis set.Static Raman polarizabilities (ω = 0) were
calculated in the
RESPONSE package by two-point numerical differentiationusing the
RAMANRANGE keyword. Raman scatteringintensities were determined by
the scattering factor 45α̅j′ +7γj̅′, where α̅j′ and γj̅′ are the
isotropic and anisotropicpolarizability tensors with respect to the
jth vibrational mode.The Raman intensity for each vibrational mode
were broadenedto a Lorentzian line shape with full-width at
half-maximum(fwhm) of 10 cm−1 for comparison to experimental
data.Atomic Layer Deposition. ALD was performed in a home-
built viscous flow reactor that has been described previouslyand
is shown in Figure 1.12,21 SERS substrates were mountedon a movable
sample holder, placed inside the ALD chamberunder vacuum (∼0.05
Torr), and heated to ∼55 °C to preservetheir fine nanostructure.
SER spectra were taken before andafter dosing 60 sccm of
trimethylaluminum (TMA) and 60sccm H2O using ultrahigh purity (UHP)
N2 as the carrier gas.For one-half-cycle of Al2O3, TMA was dosed on
the SERSsubstrate for 10 or 30 s unless otherwise stated, and
SERspectra were acquired while purging with N2. The second
half-cycle involved dosing H2O for 60 s and thereafter acquiringSER
spectra during N2 purging. Thermocouples were placedabove and below
the sample compartment to ensure uniformheating around the sample.
Variacs were used to manuallyadjust the voltage supplied to heating
tapes to keep thetemperature at 55 ± 2 °C throughout
experiments.
■ RESULTSSEM and LSPR Characterization of AgFONs. Reactive
ion etching (RIE) using Ar plasma was used to remove carbon
contamination on AgFONs, decreasing the background of SERspectra
prior to operando monitoring of ALD reactions, andultimately
improve the sensitivity of the AgFONs (Figure S2).To ensure that
the AgFON structure was not altered duringRIE, scanning electron
microscopy (SEM) was used to observethe Ag film morphology. Figure
2 shows SEM images ofAgFONs after RIE exposure times of 25 s
(Figure 2A,B) and300 s (Figure 2C,D). The top-down views (Figures
2A,C)clearly show that the Ag film undergoes dramatic
structuralchanges upon long (300 s) Ar plasma exposure, with
theappearance of a vitreous-like, smoothened film (Figure 2C).
Asimilar trend was observed on the side observation of
thesubstrates (Figures 2B,D). It is also important to notice that
themetal pillars that act as electromagnetic hot spots55,57 in
bareAgFONs are maintained after 25 s of Ar plasma exposure. Thisis
consistent with the high intensity of the SER spectra (FigureS2C).
On the basis of these observations, 25 s of Ar plasmacleaning was
chosen because it had no adverse effects on thepillar-like
structure of the AgFONs while still removingbackground signal due
to carbonaceous species. Additionally,Figure S2D shows that the
LSPR of the AgFONs wasoptimized for 532 nm laser excitation, with a
reflectanceminimum indicative of the LSPR at 534 nm.
Reaction of TMA with Bare AgFON. Figure 3 shows SERspectra
acquired before and after two cycles of ALD Al2O3growth on an Ar
plasma-cleaned AgFON at 55 °C. In Figure3A, only peaks below 900
cm−1 are shown because Ar plasmaremoves most of the surface
contaminants observed in thatregion. Two peaks at 583 and 671 cm−1
are observed afterdosing TMA for 10 s. TMA dosing for 5−10 s was
long enoughto saturate the surface as shown in Figure S3. Figure 3B
showsSER difference spectra for the first two ALD cycles. The
583and 671 cm−1 peaks decay after dosing H2O for 60 s andreappear
after 10 s of TMA dosing in the second cycle. The 583and 671 cm−1
peaks are assigned to the symmetric andasymmetric Al-CH3 stretching
modes,
58 respectively (see Table1 and Table S1).Figure 4 shows both
the recorded SER (A) and difference
spectra (B) in the C−H stretching region. A peak at 2892
cm−1appears after 30 s of TMA exposure. This peak decays after 60
sof H2O dosing and reappears after the second TMA half-cycle.The
difference spectra in Figure 4B clearly show the spectralchanges
before and after ALD Al2O3 growth. Additionalpositive peaks at 2822
and 2921 cm−1 are seen after the first
Figure 1. Experimental schematic for operando ALD-SERS reactor.
IF= interference filter, M = mirror, FM = flip mirror, ND =
neutraldensity filter.
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TMA half-cycle. All positive peaks observed after TMAexposure
(red curves, Figures 3B and 4B) switch and becomenegative after
dosing H2O, indicative of loss of surface-boundAl−(CH3)x species
(blue curves, Figures 3B and 4B). TMAdosing in the second cycle
restores the positive peaks, however,with reduced signal intensity
because of the distance depend-ence of the electromagnetic
enhancement in SERS (red curves,Figures 3B, 4B). The 2822 and 2892
cm−1 peaks are assigned totwo different symmetric C−H stretches of
Al−(CH3)x surfacespecies.72 The 2921 cm−1 peak is assigned to the
asymmetricC−H stretch.59−62 The observation of surface-bound
speciesfrom ALD precursors shows that operando SERS is capable
ofmonitoring ALD half-reactions and can provide valuable
insight
into the mechanistic details of the surface chemistry of
ALDprocesses.
Reaction of TMA with Toluenethiol and Benzenethiol.The
nucleation and growth of ALD Al2O3 on AgFONsfunctionalized with
thiol SAMs was also investigated at 55 °C.Figure 5 shows the first
ALD Al2O3 cycle performed on atoluenethiol (TT)-functionalized
AgFON. TMA was dosed onthe surface for 10 min to ensure complete
saturation. Tenminutes of TMA exposure was long enough to allow TMA
tocompletely react with Ag (Figure S9). In Figure 5A, a peak at585
cm−1 (symmetric Al−CH3 stretch) is observed after thefirst TMA
half-cycle. A slight reduction in TT peak intensitiesafter TMA
exposure is also seen. The 585 cm−1 peak disappearsafter dosing H2O
(see Figure S10 for difference spectra). On abenzenethiol
(BT)-functionalized AgFON, peaks at 583 cm−1
(symmetric Al−CH3 stretch) and 1211 cm−1 (symmetric Al−CH3
stretch + CH3 bend)
23 appear and decay after TMA andH2O exposures, respectively
(see Figures S11 and S12). Figure5B−D and Table S2 show the absence
of vibrational frequencyshifts of TT and BT SAMs after dosing TMA
on the surface.Figure 5B−D and Table S3 shows that the 622 (CC
bend),1081 (CCC in-plane bend + C−S stretch), and 1381 cm−1
(CCstretch)67,73 peaks of TT decrease in intensity after
TMAexposure. The fwhm of the peaks is roughly the same beforeand
after TMA exposure (see Table S3).
Reaction of TMA with Mercaptobenzoic Acid andMercaptophenol. The
reaction of TMA with 4-mercapto-benzoic acid (MBA) and
4-mercaptophenol (MP) on AgFONswas also investigated because the
−COOH and −OH groupscould react directly with TMA.74,75 Figure 6
shows the firstALD cycle of Al2O3 performed on MBA-functionalized
AgFONat 55 °C. After 10 min of TMA exposure, thiol peak shifts
from845 to 849 cm−1 (COO− out-of-plane bend)63 and 1080−1082cm−1
(CCC ring deformation and C−S stretch),65 and theappearance of a
broad shoulder peak at ∼1397 cm−1 (COO−stretch)71 is seen. On
MP-functionalized AgFON, thiol peakshifts from 1074 to 1075 cm−1
(CCC ring deformation and C−S stretch),65 1290−1276 cm−1 (C−OH
stretch + ringbreathing),66 and 1577−1584 cm−1 (CC stretch and
OHbend)65,66 are observed (Figure S13). In addition to thiol
peakshifts, the peak intensities generally decreased after
TMAexposure (except for the 845 cm−1 peak of MBA) for bothMBA and
MP SAMs (Table S3). Table S3 also shows dramaticchanges in the fwhm
of MBA peaks after TMA exposure; thefwhm of the 845 cm−1 peak
increases from 33 to 52 cm−1. ForMP, the fwhm of the 1290 cm−1 peak
increases from 86 to 113cm−1. The fwhm of the 1577 cm−1 peak
decreases from 45 to37 cm−1 after TMA exposure.
C−H Stretching Region of Functionalized AgFONs.Figure 7 displays
SER and difference spectra of TT- and MBA-functionalized AgFONs in
the C−H stretching region. On aTT-functionalized AgFON, the 2822
and 2892 cm−1 peaks,assigned to two different symmetric C−H
stretches of Al−CH3surface species, are observed after TMA exposure
(Figure7A,B). They decay after H2O exposure and reappear after
TMAexposure in the second cycle (Figure 7B). On an
MBA-functionalized AgFON, the symmetric C−H stretch of TMAappears
at 2898 cm−1 after TMA exposure (Figures 7C,D).Similarly to a bare
AgFON, the TMA symmetric C−H stretchpeak decays after H2O exposure
and reappears after TMAexposure in the second cycle (Figure
7D).
Figure 2. SEM images of AgFONs after 25 s (A, B) and 300 s (C,
D)of Ar plasma cleaning. Top-down view is shown in A and C and
sideview is displayed in B and D.
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■ DISCUSSIONReaction of TMA with Bare AgFON. The 583
(symmetric
Al−CH3 stretch) and 671 cm−1 (asymmetric Al−CH3 stretch)modes
shown in Figure 3 and Table 1 match well with thevibrational
frequencies of dimeric TMA calculated by DFT (seeFigure S5 and
Table 1). In the first ALD cycle, we propose thatTMA reacts
directly with Ag and decomposes into Al atomsand Al−(CH3)x surface
species. The Al−CH3 stretches
observed at 583 and 671 cm−1 (Figure 3B) become negativeafter
H2O exposure because H2O replaces Al−(CH3)x surfacespecies with
Al−(OH)x species. They reappear after TMAdosing because TMA reacts
with Al−(OH)x species andreplaces them with Al−(CH3)x species.
Previous DFT studiessuggest that TMA reacts directly with metals
such as Pd, Pt,and Ir by dissociative chemisorption to form Al−CH3
and Alsurface species, which transform to Al−(OH)x surface
species
Figure 3. (A) SER spectra of a 25 s Ar plasma-cleaned AgFON
before ALD (a) and after 10 s of TMA (1st cycle) (b), 60 s of H2O
(1st cycle) (c),10 s of TMA (2nd cycle) (d), and 60 s of H2O (2nd
cycle) (e). (B) Difference SER spectra for the first 2 ALD cycles.
SER spectra were acquiredwith λexc = 532 nm, Pexc = 2 mW, taq = 5 s
and 10 accumulations each. Data are shifted on y-axis for
clarity.
Table 1. Vibrational Modes and Corresponding Assignments*
peak positions (cm−1)
TMA thiol SAMs
expt calcd mon calcd dim TT BT MBA MP assignment23,58−71
583 572 ν(Al−CH3)symm622 γ(CC)
671 671 ν(Al−CH3)asymm691 β(CCC) + ν(C−S)
845 γ(COO−)1000 β(CCC)1022 β(C−H)ring
1081 1072 1080 1074 β(CCC) + ν(C−S)1211 1190 1190 ν(Al−C)asymm,
bending CH3
1290 ν(C−OH) + ring breathing1381 ν(CC)
1397 ν(COO−)1432 ν(CC)
1577 ν(CC) + β(OH)1599 ν(CC)
2822 ν(C−H)symm2865 ν(C−H)symm (CH3 group)
2892 2905 ν(CbridgeH)symm2917 ν(C−H)asymm (CH3 group)
2919 2919 ν(C−H)symm2921 2996, 3022 2960, 3008, 3032
ν(C−H)asymm
3051 3058 3055 3055 ν(C−H)ring*Experimental and DFT-calculated
vibrational frequencies and peak assignments. TMA =
trimethylaluminum, mon = monomer, dim = dimer, TT =toluenethiol,
MBA = 4-mercaptobenzoic acid, BT = benzenethiol, MP =
4-mercaptophenol. γ, β, and ν indicate the out-of-plane bending,
in-planebending, and stretching modes, respectively.
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after H2O exposure.46 Nucleation of TMA could also occur on
hydroxyl (−OH) groups adsorbed on a very thin layer of oxideon
the AgFON surface.43 The percentage of oxygen atoms inthe volume
sampled by XPS was calculated to be ∼8% for both0 and 25 s Ar
plasma-cleaned AgFONs (Figure S14). If
nucleation of TMA occurs on −OH groups, then Al−Ostretches could
be observed by SERS but they were notobserved in our studies even
with an optimized AgFON for thelow frequency Al−O stretching region
(Figure S2D). Ramanpeaks from Al2O3 compounds are typically
observed in the
Figure 4. SER spectra (A) of a 25 s Ar plasma-cleaned AgFON
depicting the C−H stretching region before ALD (a) and after 30 s
of TMA (1stcycle) (b), 60 s of H2O (1st cycle) (c), 30 s of TMA
(2nd cycle) (d), and 60 s of H2O (2nd cycle) (e). Difference
spectra (B) showing C−Hstretching region after 2 ALD Al2O3 cycles.
SER spectra were acquired with λexc = 532 nm, Pexc = 5 mW, taq = 10
s and 10 accumulations each.
Figure 5. (A) SER spectra of a AgFON functionalized with
toluenethiol (TT) before and after 1 ALD Al2O3 cycle. (B−D) SER and
differencespectra showing no thiol peak shifts before and after ALD
Al2O3. The red curves in B−D represent Lorentzian peak fits. SER
spectra were acquiredwith λexc = 532 nm, Pexc = 1 mW, taq = 2 s,
and 10 accumulations each.
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∼700−1000 cm−1 region.76 The scarcity of SERS studies thatreport
the observation of Al−O vibrations is attributed to thelow Raman
cross sections of amorphous Al2O3 as reported byStair and
co-workers.76
The TMA C−H stretches positioned at 2822, 2892, and2921 cm−1 in
Figure 4B become negative after H2O exposurebecause H2O replaces
(CH3)x surface species with (OH)xspecies. In the second cycle, TMA
replaces (OH)x specieswith (CH3)x species. Figure S6 shows gradual
decay over timeof the TMA symmetric C−H stretch at 2892 cm−1
duringconstant 5 mW of laser irradiation after TMA exposure.
Thepeak intensity decreases to 50 and 15% of the initial
intensityafter 240 and 780 s, respectively. A control experiment
shownin Figure S7 provides evidence that the decay of the
C−Hstretches over time was not caused by laser irradiation but
wasdue to the reaction of residual H2O inside the ALD chamberwith
adsorbed Al−(CH3)x surface species. A power dependencestudy of the
2892 cm−1 peak is displayed in Figure S8 andshows increasing SERS
intensity when the laser power wasincreased from 0.2 to 7 mW. The
study was performed veryquickly for each laser power using 3 s
acquisition time and 10accumulations to minimize the reaction of
TMA with residualH2O.Reaction of TMA with Toluenethiol and
Benzenethiol.
The toluenethiol SAM does not prevent TMA from reactingwith the
AgFON since the symmetric Al−CH3 stretch of TMA(585 cm−1) is
observed (Figure 5A). We propose that TMA
penetrates through the monolayer and reacts with Ag as shownin
Scheme 1. It is also possible that TMA displaces somesurface
toluenethiol molecules and reacts with the AgFON asindicated by the
slight reduction in toluenethiol SERS signalafter TMA exposure
(Figures 5A−D and Table S2). Anotherpossible explanation for the
decrease of the toluenethiol signalis that TMA changes the spatial
orientation of toluenethiolmolecules. The toluenethiol molecules
are initially orientedalmost perpendicular to the AgFON surface.77
When TMAreacts with the AgFON surface, it changes the orientation
ofthe toluenethiol molecules to a more tilted position. Thechange
in molecular orientation lowers the Raman scatteringcross-section
and affects the SERS signal as seen previously forother molecular
systems.78
As with the reaction of TMA with an unfunctionalizedAgFON, the
Al−CH3 peak at 585 cm−1 (Figure 5A) and TMAC−H symmetric stretches
at 2822 and 2892 cm−1 (Figures 7A,7B) decay after H2O exposure
because H2O reacts with Al−(CH3)x groups to produce Al−(OH)x
species. Figure 5B−Dand Table S2 show that toluenethiol vibrational
modes do notshift after TMA exposure. Lack of vibrational frequency
shiftsindicate that toluenethiol molecules are not
significantlyperturbed by TMA and this supports the claim that
TMAreacts directly with the Ag surface. In addition, the fwhm
oftoluenethiol peaks does not change significantly after
TMAexposure (Table S2), which might further indicate that TMAdoes
not react directly with toluenethiol but with the Ag
Figure 6. (A) SER spectra of the first ALD Al2O3 cycle deposited
on a AgFON functionalized with 4-mercaptobenzoic acid (MBA). (B−D)
SER anddifference spectra of MBA showing thiol peak shifts observed
after 10 min of TMA exposure. The red curves in B−D are Lorentzian
peak fits.Spectra were acquired with λexc = 532 nm, Pexc = 1 mW,
taq = 2 s, and 10 accumulations each.
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surface. We observed that the fwhm of vibrational modes ofSAMs
with nonreactive groups to TMA did not changesignificantly after
TMA exposure unlike SAMs with reactivegroups as we discuss in the
following section. In similar fashion,the symmetric Al−CH3 stretch
at 583 cm−1 and the Al−CH3deformation mode at 1211 cm−1 were
observed after TMAexposure on benzenethiol-functionalized AgFON and
no thiolvibrational frequency shifts were observed (Figures S11,
S12and Table S2). We believe that TMA also reacts with Ag
ratherthan the benzenethiol molecules similar to the
TT-function-alized AgFON.Reaction of TMA with Mercaptobenzoic Acid
and
Mercaptophenol. On MBA-functionalized AgFON, vibra-tional
frequency shifts are seen after TMA exposure as depictedin Figure
6B−D. The 4 cm−1 shift of the COO− bendingvibration63 at 845 cm−1
is due to complexation with TMA asillustrated in Scheme 1. A
shoulder also appears at 1397 cm−1,which is attributed to the
replacement of −COO−H groupswith −COO−Al− species after TMA
exposure. This assign-ment is consistent with literature studies
that reported strongerCOO− stretches of adsorbed MBA at ∼1400 cm−1
in its
deprotonated form than the protonated form.71 Chabal and
co-workers observed a 1476 cm−1 band after TMA exposure
onCOOH-terminated SAMs on Si and assigned it to the COstretch mode
found in acid salt structures. They also observedcomplete
disappearance of the CO stretch mode at 1718cm−1 after TMA
exposure.80 Unfortunately, CO stretches ofMBA are usually very weak
in SERS.63 The 2 cm−1 peak shift ofthe 1080 cm−1 peak (CCC ring
deformation mode and C−Sstretch)65 (Figure 6C) and the shifted
position of the TMA C−H stretch at 2898 cm−1 (Figure 7C,D) are
explained by thedirect reaction of TMA with the −COOH groups of
MBA,which perturbs MBA molecules and changes their
vibrationalfrequencies. Similarly to a bare AgFON and
TT-functionalizedAgFON, the TMA C−H stretch at 2898 cm−1 (Figure
7D)decays after H2O exposure because H2O reacts with Al−(CH3)x
surface groups and replaces them with Al−(OH)xgroups. TMA
regenerates Al−(CH3)x species in the secondTMA half-cycle.For the
MP-functionalized AgFON, vibrational frequency
shifts of MP peaks were seen after TMA exposure (FiguresS13A−D
and Table S3). The CCC ring deformation and C−S
Figure 7. SER spectra of a TT-functionalized AgFON (A) and
MBA-functionalized AgFON (C) showing the evolution of C−H peaks
before (a)and after 10 min of TMA exposure (first cycle) (b), 60 s
of H2O (first cycle) (c), 10 min of TMA (second cycle) (d), and 60
s of H2O (secondcycle) (e). Difference spectra of the first 2 ALD
Al2O3 cycles on a TT-functionalized AgFON (B) and
MBA-functionalized AgFON (D). Spectra in Awere obtained with λexc =
532 nm, Pexc = 1 mW, taq = 10 s and 10 accumulations. Spectra in C
were acquired with λexc = 532 nm, Pexc = 5 mW, taq = 4s and 10
accumulations each.
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stretch mode shifted from 1074 to 1075 cm−1.65 The
C−Ostretch64−66 shifted from 1290 to 1276 cm−1 after the reactionof
TMA with the −COH group. The CC stretch and OHbending mode65,66
shifted from 1577 to 1584 cm−1 as a resultof TMA reacting with the
−OH group (Figures S13B−D).Vibrational frequency shifts of SAMs
suggest that TMA
selectively reacts with the −COOH and −OH groups of SAMson
AgFON. The fwhm of MBA and MP peaks associated withthe −COOH and
−OH groups increases significantly (TableS3) after TMA exposure,
which suggests that TMA reactsdirectly with the −COOH and −OH
groups of the SAMs.Changes in the peak intensities after TMA
exposure did notreveal significant differences in the reactivity of
TMA with thetypes of SAMs investigated. The observation of the
symmetricAl-CH3 stretch on TT- and BT-functionalized and not onMBA-
and MP-functionalized AgFONs is consistent with thedistance
dependence of SERS in which vibrational modes ofmolecules closer to
the plasmonic substrate show strongersignals than those spaced
further away.38 Furthermore, theTMA C−H stretches for
MBA-functionalized AgFONs (Figure7C,D) are ∼7× weaker than TMA C−H
stretches for TT-functionalized AgFONs (Figures 7A,B). This could
beexplained by the larger distance of the TMA−CH3 groupsfrom the
AgFON surface for MBA-functionalized AgFONsthan TT-functionalized
AgFONs as illustrated in Scheme 1. It isalso possible that there is
a difference in the amount of TMAdeposited on MBA- and
TT-functionalized AgFONs. Forexample, if the amount of TMA
deposited on MBA-functionalized SAMs is higher than on
TT-functionalizedAgFONs, then the TMA −CH3 groups will be much
fartheraway from the surface and would appear much weaker
forMBA-functionalized AgFONs. At present, we assume that theamount
of TMA deposited on MBA- and TT-functionalizedAgFONs is the same
(∼1.1 Å/cycle).The S:Ag atomic ratio, determined by XPS,
provides
information about the relative packing density of SAMs onAgFONs.
Figure S15 shows S 2p and Ag 3d peaks for all SAMs;the calculated
S:Ag ratio increases from 0.0575 to 0.0824 in thefollowing order:
TT < BT < MP < MBA. This indicates a lowerpacking density
of TT and BT SAMs on AgFONs than MP and
MBA SAMs. This is consistent with the work of Lee and co-workers
who observed a lower packing density of BT SAMscompared to MP and
MBA SAMs on Au.66 These studiessupport our claim that TMA is able
to penetrate through lessdensely packed, unreactive SAMs and react
directly with theAgFON surface.
Comparison with Other Studies. Several studies haveused SAMs as
enablers of area-selective ALD, a process inwhich ALD material is
deposited only where desired. On SiO2surfaces, Chen et al. showed
that to completely block HfO2ALD, long (>12 carbon units),
linear, densely packed,hydrophobic alkyltrichlorosilane SAMs are
necessary.53,81
Using in situ IR spectroscopy, Chabal and co-workers
observeddisappearance of CO stretches and no perturbation of
alkylchain modes and concluded that COOH-terminated SAMs onSi were
effective in blocking TMA from reacting with Si.80 Tothe best of
our knowledge, studies that investigate ALDblocking mechanisms
using SAMs have been extensivelystudied on Au but not as thoroughly
on Ag. Preiner et al.showed that SAMs with hydrophobic tail groups
such as 1-dodecanethiol on Au act as ALD resists and block Al2O3
growthwhile SAMs with hydrophilic groups such as MBA allow
fastergrowth of Al2O3.
82 Our study on CH3-terminated SAMs isconsistent with Hooper et
al., who studied the interaction ofvapor-deposited Al atoms with
CH3-terminated SAMs on Auand observed Al−S species by XPS to
indicate penetration of Alatoms through the SAM to the Au/SAM
interface.83
Furthermore, lack of major perturbations in the SAM IRspectra
indicated that no chemical interaction occurredbetween Al and the
SAM.83 On COOH-terminated SAMs onAu, Fisher et al. concluded that
the −COOH group reacts withAl to prevent Al atoms from penetrating
into the monolayer.XPS data indicated that Al atoms bind with both
oxygen atomsof the SAM to form metal−organic species. IR spectra
showeda 14 cm−1 shift and 25% drop in intensity of the SAM’s
COstretch after Al deposition to indicate direct reaction of
Alatoms with the −COOH group.74 Similarly, they observedsignificant
perturbation of the C−O stretch (a decrease inintensity and 35 cm−1
shift) of OH-terminated SAMs on Au
Scheme 1. Proposed Scheme for the Reaction of TMA with AgFONs
Functionalized with Toluenethiol and 4-MercaptobenzoicAcid
SAMsa
aThe structure of dimeric TMA contains bridging pentacoordinated
carbon atoms as determined by gas-phase electron
diffraction.69,79.
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using IR spectroscopy and concluded that deposited Alselectively
reacts with −OH groups to form −OAl groups.75These observations are
consistent with our studies in which
we observe vibrational frequency shifts of OH- and
COOH-terminated SAMs after TMA exposure. Finding the ideal SAMon
AgFON to completely block ALD growth is beyond thescope of this
investigation. Rather, the focus of this study is todemonstrate
that operando SERS provides unique informationabout where ALD
deposition occurs on SAM-functionalizedsurfaces, information that
is difficult to obtain from techniquessuch as FTIR, QCM, QMS, and
spectroscopic ellipsometry.SERS provides valuable information about
surface-boundmolecular species and on where the ALD deposition
occurson the surface under realistic ALD working conditions.
Ourstudy reveals that TMA forms dimeric species on the AgFONsurface
and this information is difficult to obtain from othertechniques.
The rich structural information provided by SERScan be used to
provide insight about the surface chemistry ofALD reactions.
■ CONCLUSIONSThis work demonstrates the capability of operando
SERS tomeasure surface vibrational spectra of adsorbates during
ALDreactions. On bare AgFONs, Al−C, symmetric C−H, andasymmetric
C−H stretches were observed after TMA exposureat 55 °C, and they
decayed after dosing H2O. Residual H2O inthe ALD chamber reacted
with −CH3 groups on AgFONs asevidenced by the decay of the Al−C and
C−H stretches evenbefore H2O exposure. DFT studies revealed that
the observedAl−C stretches were consistent with TMA dimeric species
onthe AgFON surface. On thiol SAM-functionalized AgFONs,Al−C
stretches and no thiol vibrational frequency shifts wereseen after
TMA exposure in the case of toluenethiol andbenzenethiol SAMs.
Thiol vibrational frequency shifts and noAl−C stretches were
observed after TMA exposure onAgFONs functionalized with
4-mercaptobenzoic acid and 4-mercaptophenol SAMs. For COOH- and
OH-terminatedSAMs, the absence of observable Al−C stretches was due
totheir larger distance away from the enhancing SERS
substrate.Operando SERS revealed that TMA selectively reacted with
theAgFON surface for benzenethiol and toluenethiol SAMs andwith the
−COOH and −OH groups for 4-mercaptobenzoicacid and 4-mercaptophenol
SAMs, respectively. This shows thatSAMs can be used to tailor the
surface and direct ALD Al2O3growth to achieve area-selective ALD.
The surface sensitivityand rich structural information provided by
operando SERS willallow for the probing of the deposition
mechanisms of otherALD reactions such as metal ALD processes under
realisticreaction conditions. Knowledge of the surface chemistry
ofALD reactions will help in the selection of better precursorsand
in the optimization of ALD processes.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting
Information is available free of charge on theACS Publications
website at DOI: 10.1021/acs.jpcc.5b11487.
Extinction spectra of AgFONs, SER spectra acquiredwith and
without the quartz window, SER spectra ofAgFONs before and after Ar
plasma cleaning, DFT-calculated Raman spectra of TMA, SER spectra
showingreaction of surface species with residual H2O, laser
powerdependence study, SER spectra of benzenethiol and 4-
mercaptophenol before and after ALD, and XP spectra(PDF)
■ AUTHOR INFORMATIONCorresponding Author*E-mail:
[email protected] ContributionsThe manuscript was
written through contributions of allauthors. All authors have given
approval to the final version ofthe manuscript.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSWe acknowledge Dr. Jon Dieringer, Dr. Bogdan
Negru, Dr.Neil Schweitzer, Dr. Dragos Seghete, Dr. Nathan
Greeneltch,Stephanie Zaleski, and Naihao Chiang for experimental
help,data analysis, and valuable discussions. We gratefully
acknowl-edge financial support from the Northwestern
UniversityInstitute for Catalysis in Energy Processes (ICEP). ICEP
isfunded through the US Department of Energy, Office of BasicEnergy
Science (Award No. DE-FG02-03-ER15457). M.O.M.acknowledges support
from the National Science FoundationGraduate Fellowship Research
Program under Grant No. DGE-0824162. This work made use of the EPIC
facility (NUANCECenter-Northwestern University), which has received
supportfrom the MRSEC program (NSF DMR-1121262) at theMaterials
Research Center, the International Institute forNanotechnology
(IIN), and the State of Illinois, through theIIN.
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