Selective-Area, Water-Free Atomic Layer Deposition of Metal Oxides
on Graphene Defects
Michael F. Mazza, Miguel Cab
á
n-Acevedo, Harold J. Fu, Madeline C. Meier, Annelise C. Thompson,
Zachary P. Ifkovits, Azhar I. Carim, and Nathan S. Lewis
*
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Supporting Information
ABSTRACT:
Passivating defective regions on monolayer gra-
phene with metal oxides remains an active area of research for
graphene device integration. To e
ff
ectively passivate these regions,
a water-free atomic layer deposition (ALD) recipe was developed
and yielded selective-area ALD (sa-ALD) of mixed-metal oxides
onto line defects in monolayer graphene. The anisotropically
deposited
fi
lm targeted high-energy defect sites that were formed
during synthesis or transfer of the graphene layer. The passivating
layer exceeded 10 nm thickness with minimal deposition onto the
basal plane of graphene. The mixed-metal oxide
fi
lm was of
comparable quality to
fi
lms deposited using nonselective water-
based ALD methods, as shown by X-ray photoelectron spectroscopy. The development of sa-ALD techniques to target defect
regions on the graphene sheet, while keeping the basal plane intact, will provide a new mechanism to passivate graphene defects and
modify the electronic and physical properties of graphene.
KEYWORDS:
atomic layer deposition, graphene, defect driven reactivity, defect passivation, device integration
M
onolayer graphene consists of an atomically
fl
at surface
of sp
2
-hybridized carbon atoms arranged in a hexagonal
lattice.
1
Graphene has remarkable tensile strength, electrical
conductivity, and optical transmissibility.
2
−
4
Although these
properties are present at microscopic scales, monolayer
graphene grown by large-area chemical vapor deposition
(CVD) possesses a high density of atomic vacancies (point
defects) as well as line defects that result in grain sizes on the
order of 50
μ
m.
5
−
7
These defects are generated during
synthesis
8
and have deleterious e
ff
ects on the properties of the
resulting graphene.
9
Commonly used transfer mechanisms,
such as polymer-assisted wet transfer, can also induce folds,
cracks, and tears in the graphene sheet that can lead to further
reductions in the material quality.
10
Nanoscale electrochemical
methods have been utilized to probe isolated pristine regions
of the basal plane of an otherwise polycrystalline graphene
sample.
11
To take advantage of the properties of graphene on
the macroscale, methods need to be developed to selectively
terminate the defects in the transferred material with minimal
impact to the basal plane.
Atomic layer deposition (ALD) is a low-cost and scalable
method for the deposition of metals, metal nitrides, and metal
oxides.
12
−
14
ALD exploits self-limiting chemical reactions on
the surface of the substrate and results in highly conformal thin
fi
lms with precise control over the resulting
fi
lm thickness.
15
Typically, ALD of metal oxides has been performed by
introducing a metal
−
organic precursor molecule along with a
counter-reactant, often water or ozone, to decompose the
precursor and produce a surface-terminated thin
fi
lm. In
contrast, selective-area ALD (sa-ALD) has been used to
generate anisotropic and patterned depositions for device
integration and catalytic layers.
16
−
18
Sa-ALD has been
achieved by using self-assembled monolayers and chemical
functional groups to block the ALD precursors from adsorbing
onto speci
fi
c regions of the substrate.
19
Patterned deposition
has also been obtained using heterogeneous substrates or
catalytic surface reactions, which rely on di
ff
erential reactivity
of the surfaces toward precursor molecules.
20
,
21
Additionally,
platinum metal has been deposited via ALD onto line defects
of graphene, taking advantage of the high energy defects on the
basal plane to enable selective-area deposition.
22
Expanding
the library of materials that can be selectively deposited on
graphene would allow for greater versatility when integrating a
monolayer of graphene into a device.
We report herein sa-ALD of a mixed-metal oxide, aluminum
titanium oxide (Al
x
Ti
y
O
z
), onto line defects of CVD-grown
monolayer graphene through a water-free deposition recipe.
Received:
September 20, 2021
Revised:
November 9, 2021
Accepted:
November 11, 2021
Published:
November 19, 2021
Letter
pubs.acs.org/materialsau
© 2021 The Authors. Published by
American Chemical Society
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Water-free deposition was achieved by using a precursor
containing a metal
−
oxygen bond and a separate precursor as
the counter-reactant to facilitate decomposition. In this system,
titanium(IV) isopropoxide (TTIP) is used as the oxygen-
containing precursor, and trimethyl aluminum (TMA) is used
as the counter reactant. This technique prevents super-
saturation of the graphene basal plane with a strong oxidant,
and instead relies on the inherent reactivity di
ff
erences
between the defective regions of the graphene sheet and the
inert basal plane.
Figure 1
shows scanning electron microscopy (SEM) images
of CVD-synthesized monolayer graphene on Si after 200 ALD
cycles of the water-free deposition recipe at 110
°
C. The metal
oxide was preferentially deposited in linear networks, with as-
deposited regions appearing bright relative to the inert
graphene sheet. Additional island deposits can be assigned to
either point defects or random nucleation sites such as dust or
polymer residue resulting from the wet transfer. The selective
deposition chemistry occurred across wide areas on the
graphene sheet (
Figure 1
b).
Layered materials, such as graphene and molybdenum
disul
fi
de, lack dangling bonds on the basal plane. Con-
sequently, ALD
fi
lm growth occurs preferentially at step edges
and defect sites.
23
,
24
The dangling bonds or otherwise
functionalized species at these grain boundaries, folds, cracks,
and line defects are substantially more reactive than the
pristine basal plane. When a strong oxidant is removed from
the ALD process, the basal plane is not supersaturated with
water molecules or hydroxyl groups. Without an oxidant to
generate new reactive sites, the only reactive sites are found
along the grain boundaries and defects in the graphene
monolayer. The deposition chemistry should therefore become
increasingly favorable on defect sites, creating the conditions
necessary for anisotropic nucleation. Indeed, the use of a
water-based Al
2
O
3
recipe consistently resulted in a non-
conformal but isotropic
fi
lm (Supporting Information,
Figure
S1
). Moreover, the TiO
2
recipe with TTIP and water initially
nucleated at grain boundaries, but the growth rate was
extremely low and proceeded with less selectivity than the
water-free recipe. Atomic-
force microscopy (AFM) was
performed in addition to SEM to evaluate the
fi
lm morphology
and distribution after 200 ALD cycles (
Figure 2
). Appreciable
oxide deposition occurred at line defects, resulting in
fi
lms at
or approaching 10 nm thick (
Figure 2
b). Assuming this upper
maximum, the growth rate on defect regions on the graphene
sheet was near 0.5 Å cycle
−
1
, well within the expected range
based on reported growth rates of 1.33 and 0.11 Å cycle
−
1
at
110
°
C for the water-based TMA and TTIP recipes,
respectively.
25
,
26
An estimate of the average
fi
lm thickness
was determined by measuring the
fi
lm thickness at 10 spots
across the AFM map (
Figure S6
). The average
fi
lm thickness
was calculated to be 8
±
2 nm.
Selective-area ALD re
fl
ects a competition between intrinsic
di
ff
erences in the substrate reactivity and the inherent self-
reinforcing nucleation chemistry of island
fi
lm growth.
27
The
degree of selectivity in an ALD process is therefore determined
by the thickness of the
fi
lm in deposited regions relative to
deposition on the pristine basal plane. The >10 nm thick
fi
lms
reported herein can be selectively deposited on defects,
Figure 1.
(A) High-resolution SEM images of a CVD-grown
monolayer graphene sheet patterned with 200 cycles of the water-
free ALD recipe. (B) Wide-area SEM image showing point defects,
line defects, and tears in the graphene sheet patterned over large areas.
Figure 2.
(A) Atomic orce microscopy images showing selective
metal oxide deposition onto line defects. Scale bar 500 nm. (B)
Topographic pro
fi
le of the white trace in (a).
ACS Materials Au
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ACSMater.Au
2022, 2, 74
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generating a robust passivating layer on defective regions on
the graphene surface. This process can be developed to
maximize the anisotropic
fi
lm thickness, as both the SEM data
in
Figure 1
and the AFM data in
Figure 2
do not show the
fi
lm
converging into a conformal layer. Kelvin Probe Force
Microscopy (KPFM) data revealed the surface potential and
adhesion force maps of the same region (
Figure S2
). This sa-
ALD technique is strongly dependent on the deposition
temperature, with the metal
−
organic precursor condensing
onto the basal plane at low temperatures whereas the
precursors more readily reacted with the basal plane at higher
temperatures (
Figure S3
).
Figure 3
shows high-resolution X-ray photoelectron
spectroscopic (XPS) data for the Ti 2p and Al 2p peaks for
fi
lms produced by the water-based and water-free ALD recipes.
As a reference, standard water-based ALD recipes were used to
deposit Al
2
O
3
and TiO
2
onto graphene monolayers. The Ti 2p
data indicate that titanium dioxide was deposited through both
the water-based and water-free recipe. A small peak shift
observed in the
fi
lm produced by the water-free recipe (459
eV) can be attributed to the high concentration of Al
3+
in the
mixed-metal oxide
fi
lm, because the more electronegative Al
3+
metal cation will increase the relative concentration of Ti
4+
in
the
fi
lm and shift the Ti 2p peak to higher binding energy.
28
The 7:1 ratio of aluminum to titanium is in good agreement
with the growth rate of each precursor when reacted with water
at 110
°
C, with titanium oxide expected to grow 10 times
slower than aluminum oxide. This deposition system can be
modi
fi
ed by adjusting the recipe. By modifying the number of
TTIP or TMA pulses in each step, the ratio of aluminum to
titanium can be modi
fi
ed, as has been shown previously for
mixed-metal oxides.
29
The TTIP/TMA pulse produced Al
2
O
3
with a peak binding energy of (74.6 eV), whereas TMA/H
2
O
yielded alumina
fi
lms with a shoulder near 73 eV and a
substantially broadened full emission. This shoulder at lower
binding energy is consistent with incomplete reactions and
impurities in the
fi
lm, and the broadened full emission can
include incorporation of hydroxl species due to the low
deposition temperature.
25
A water-free ALD recipe has been developed that allows for
selective targeting of the high energy defect sites in monolayer
graphene. The resulting mixed-metal oxide forms a robust
fi
lm
at or exceeding 10 nm thick without appreciable deposition on
the inert basal plane. The deposition does not require surface
processing or high temperature, and the stoichiometry of the
fi
lm can be modi
fi
ed by changing the precursor pulse times.
The deposition system discussed herein provides a highly
controllable synthetic route for defect-selective passivation of
graphene and is of particular importance in electrochemical
systems where defect regions commonly lead to device
failure.
30
Future work will explore the electronic and chemical
properties of the passivated graphene
fi
lm and develop new
water-free recipes to expand the library of mixed-metal oxides
that can be selectively deposited.
■
ASSOCIATED CONTENT
*
s
ı
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsmaterialsau.1c00049
.
Detailed materials and methods section, KPFM mapping
data, additional SEM images to elucidate the temper-
ature dependence of the reaction, and comparison to
thermal ALD recipes (
PDF
)
■
AUTHOR INFORMATION
Corresponding Author
Nathan S. Lewis
−
127-72, 210 Noyes Laboratory, Division of
Chemistry and Chemical Engineering and Beckman Institute,
California Institute of Technology, Pasadena, California
91125, United States;
orcid.org/0000-0001-5245-0538
;
Phone: (626) 395-6335; Email:
nslewis@caltech.edu
Authors
Michael F. Mazza
−
127-72, 210 Noyes Laboratory, Division
of Chemistry and Chemical Engineering, California Institute
of Technology, Pasadena, California 91125, United States;
orcid.org/0000-0003-3995-3100
Miguel Cabán-Acevedo
−
127-72, 210 Noyes Laboratory,
Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125, United
States;
orcid.org/0000-0003-0054-8044
Figure 3.
High-resolution XPS data of the as-deposited metal-oxide
fi
lms on graphene. (A) Titanium 2p region for both water-based
controls and the water-free deposition scheme. (B) Aluminum 2p
region for both water-based controls and the water-free deposition
scheme.
ACS Materials Au
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Letter
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ACSMater.Au
2022, 2, 74
−
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76
Harold J. Fu
−
127-72, 210 Noyes Laboratory, Division of
Chemistry and Chemical Engineering, California Institute of
Technology, Pasadena, California 91125, United States
Madeline C. Meier
−
127-72, 210 Noyes Laboratory, Division
of Chemistry and Chemical Engineering, California Institute
of Technology, Pasadena, California 91125, United States;
orcid.org/0000-0003-1608-0810
Annelise C. Thompson
−
127-72, 210 Noyes Laboratory,
Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125, United
States;
orcid.org/0000-0003-2414-7050
Zachary P. Ifkovits
−
127-72, 210 Noyes Laboratory, Division
of Chemistry and Chemical Engineering, California Institute
of Technology, Pasadena, California 91125, United States
Azhar I. Carim
−
127-72, 210 Noyes Laboratory, Division of
Chemistry and Chemical Engineering, California Institute of
Technology, Pasadena, California 91125, United States;
orcid.org/0000-0003-3630-6872
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsmaterialsau.1c00049
Notes
The authors declare no competing
fi
nancial interest.
■
ACKNOWLEDGMENTS
M.F.M. and N.S.L. acknowledge support from the U.S.
Department of Energy under Award DE-FG02-04ER15483.
M.C. acknowledges support from the Ford Foundation under
the Postdoctoral Scholar Fellowship program. M.C. acknowl-
edges support from the National Science Foundation CCI
Solar Fuels Program under Grant No. CHE-1305124. A.C.T.
and M.C.M. acknowledge support from National Science
Foundation graduate research fellowships. M.C.M. also
acknowledges the Resnick Sustainability Institute at Caltech
for fellowship support. Research was in part carried out at the
Molecular Materials Research Center in the Beckman Institute
at the California Institute of Technology.
■
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