The growth and assembly of organic molecules and inorganic 2D materials on graphene for van der Waals heterostructures

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The growth and assembly of organic molecules and inorganic 2D materials on graphene for
van der Waals heterostructures
Akinola D. Oyedele1,2, Christopher M. Rouleau2, David B. Geohegan2, Kai Xiao2,1*
1Bredesen Center for Interdisciplinary and Graduate Education, University of Tennessee,
Knoxville, Tennessee 37996, United States
2Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831, United States
Abstract
The unique properties of graphene have made it a promising material for integration in future
electronic applications. The idealized surface of graphene, atomically-flat and without dangling
bonds, offers the opportunity to understand the assembly of organic and inorganic molecules to
form a wide range of ordered architectures and functional graphene-based heterostructures. In
this review, we summarize recent progress in the growth of hierarchical nanostructures on
graphene. The self-assembly of organic molecules and inorganic two-dimensional (2D) layers on
graphene for the construction of various types of heterostructures are highlighted. Van der Waals
interactions between the assembled molecules and graphene are shown to allow the formation of
highly-ordered structures with preferred molecular orientations and stacking configurations that
circumvent the strict lattice-matching requirements in traditional epitaxial growth. Finally, we
briefly discuss representative applications of graphene-based heterostructures in electronic and
optoelectronics.
*Corresponding author. Tel: 865-574-7690. E-mail: xiaok@ornl.gov (Kai Xiao)

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Table of contents
1. Introduction
2. Synthesis and processing of graphene as substrates for vdW heterostructures
2.1. Top-down approach
2.2. Bottom-up approach
3. Molecular assembly of organic molecules on graphene
3.1. Metal-phthalocyanines (MPcs) assembly on epitaxial graphene with weakly and strongly
interacting substrates
3.2. MPcs assembly on CVD-grown graphene
3.3. Assembly of macromolecular covalent organic frameworks (COFs) on graphene
4. Graphene – 2D transition metal chalcogenide (TMC) heterostructures
4.1. 2D TMCs growth on CVD-graphene
4.2. 2D TMCs growth on epitaxial graphene (EG)
5. Applications of graphene-based heterostructures
6. Summary
Acknowledgements
References

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1 Introduction
The realization of graphene by Geim and Novoselov in 2004 inspired fundamental studies of
physics and quantum phenomena arising from atomically-thin materials [1, 2]. These materials
are broadly classified as “two-dimensional (2D) materials” due to the confinement of charge
carriers in a quasi-atomic x-y plane. Single-layer graphene is composed of carbon atoms packed
hexagonally in a 2D sheet, and can be derived from graphite wherein the sheets of graphene are
stacked in layers that are bound by weak van der Waals (vdW) forces. Within the sheet,
however, the C-C interactions are governed by strong covalent forces and are characterized by
sp2-hybridization. Charge transport in graphene is characterized as ballistic, with nearly massless
Dirac fermions traveling at ~ 1/300 of the speed of light [3], leading to electron mobilities in
excess of 105 cm2 V-1s-1 at room temperature [4, 5]. Graphene also displays excellent mechanical
properties; it is strong, light, and can undergo mechanical stress and strain, making it favorable
for flexible electronic and wearable applications [6].
Like graphene, other 2D materials have been isolated successfully from their layered bulk
counterparts that similarly have strong in-plane covalent bonds but weak interlayer vdW forces
[7]. Layered 2D materials include metals, insulators, semiconductors, superconductors, and
magnets that all offer new promise for wide-ranging applications that include transistors,
memory devices, and light-emitting diodes (LEDs) [8-13]. Prominent in the emerging class of
atomically-thin 2D materials are the transition metal chalcogenides (TMCs), with chemical
formulas of MX or MX2, where M represents the transition metal (e.g., Mo, W, Pd) and X
represents the chalcogenide (e.g., S, Se, Te), with MoS2, WSe2, InSe, and GaSe as key examples
[14-18].

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Apart from quasi-2D morphologies, nanomaterials can be envisioned as three-
dimensional (3D), quasi-one-dimensional (1D), and quasi-zero-dimensional (0D) structures [19].
While all materials are strictly 3D, quasi-1D nanostructures include nanowires, nanotubes,
nanoribbons, and organic polymers with high aspect ratios in predominantly one dimension [20,
21]. Quantum dots, fullerenes, and some small organic molecules, on the other hand, are
examples of 0D nanomaterials [22-24]. Since any surface free of dangling bonds will interact
with another through vdW forces, 2D materials can be integrated into vdW heterostructures with
both 2D and nD (n = 0, 1, and 3) materials to create mixed dimensional vdW heterostructures
with a variety of new functionalities [19]. Fig. 1, for example, schematically illustrates graphene-
based organic and inorganic heterostructures. Recently, such heterostructures have emerged to
find broader applications in field-effect transistors, photodetectors, photovoltaics, and biosensors
[25, 26].
Graphene has generated interest as an organic and inorganic 2D heterostructure template
to align the molecular orientation and stacking. Comparing to the chemically-reactive substrates
such as SiO2, sapphire, and mica, the surface of graphene is atomically-flat and lack of dangling
bonds, resulting in a sharp, distinct interface to interact solely by vdW forces with the overlayer,
thus circumventing the need for rigid lattice matching. In vdW epitaxy, heterostructures with
both organic molecules and inorganic 2D materials can form in a variety of orientations, but are
often found in preferred orientations that minimize the energy resulting from the atomistic
interactions between the atoms in each layer. For organic materials, the orientation of the
molecules is often important in determining the direction of charge transport, which can be
optimized to be primarily horizontal or vertical to the substrate, as in transistor or solar cell
applications, respectively. With graphene,  interactions tend to assemble planar organic

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molecules in face-on orientations (e.g., flat as in Fig. 1) that are desirable for organic solar cells
[27, 28]. The high conductivity of graphene also allows it to be used as an electrode for the fast
transport of charges, which, when coupled with its high optical transparency, makes graphene a
good transparent conductive electrode in photovoltaic devices [29]. For inorganic 2D materials,
graphene offers an atomically-smooth surface to grow large-area, monocrystalline vdW epitaxial
layers for hybrid heterostructures with improved photoresponsivity and photodetection[30], [32].
In addition, inert graphene can also be utilized to improve doping efficiency in 2D TMC
materials for tuning their electronic properties [31]. In summary, graphene represents an
excellent template for the growth of a wide range of materials for vdW heteroepitaxial structures.
In this Review, we present an overview of graphene heterostructures and their applications.
We examine the role of graphene as a template to assemble both organic semiconductor–
graphene heterostructures as well as inorganic 2D TMC–graphene heterostructures. We then
briefly highlight the applications of these heterostructures in solar cells, field-effect transistors,
and photodetectors.
2 Synthesis and processing of graphene as substrates for vdW heterostructures
Graphene that is used for the growth of vdW heterostructures can be synthesized in various
ways, on different substrates. The synthesis method affects the size and quality of graphene,
while the type of substrate influences the growth and properties of molecular assemblies. There
are two broad categories for graphene synthesis approaches: ‘top-down’ as in mechanical
exfoliation and ‘bottom-up’ as in chemical vapor deposition [33].
2.1 Top-down

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In the top-down approach, graphene sheets are produced by mechanical or chemical
exfoliation of graphite. Mechanical exfoliation involves the use of tapes or adhesives to carefully
remove a few-layers or a single-layer of graphene from graphite, the ease of which is facilitated
by the comparatively weak vdW forces in the layered material (Fig. 2a). The advantage of this
method is that pristine graphene sheets can be achieved inexpensively. However, the resulting
graphene flakes are very small, and are randomly distributed across the substrate with varying
thicknesses, making it challenging to scale this process for mass assembly of such structures.
Also, this approach requires an additional step to identify the layer number of relevant crystals
before further processing. Chemical exfoliation, on the other hand, uses a colloidal suspension to
separate constituent graphene sheets in two steps [34]. First, the interlayer forces are weakened
by increasing the interlayer spacing using intercalated atoms or defect introduction. Second,
graphene is exfoliated by rapid heating or sonication. Although this process is inexpensive and
fast, it can be prone to contamination from the solution used during exfoliation. Like
mechanically-exfoliated graphene, the number of layers of chemically-exfoliated graphene
cannot be predicted. For graphene to be used in commercial applications, not only are large-area
sheets of graphene desired, but a scalable synthesis method as well.
2.2 Bottom-up
To achieve large-area, deterministically-controlled graphene layers, catalytic metallic or
semi-insulating substrates are used as reactive surfaces to form graphene sheets through vapor
deposition, surface segregation, or thermal decomposition. This approach, unlike exfoliation,
results in a deterministic growth of graphene sheets in a layer-by-layer fashion. On metallic
substrates, such as Cu, Pt, Ni, Ru and Ir, graphene can be synthesized via chemical vapor
deposition (CVD) from carbon-containing precursors and/or by surface segregation of carbon

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dissolved in the metals (Fig. 2b-c) [35-39]. The solubility of carbon in the metal determines the
dominant mechanism for the growth process. For example, due to the low solubility of carbon in
copper, CVD growth of graphene on copper foil is thought to be nearly entirely surface-
mediated, which facilitates control of the layer number. By comparison, because of the high
solubility of carbon in nickel, dissolution and precipitation leads to surface segregation in
different grains of the metal film, making it much more difficult to control the thickness
uniformity of graphene. Also, the crystalline morphology of the metal substrates also impacts
the quality of the graphene sheets. Polycrystalline metal substrates result in graphene domains
with multiple crystal orientations, which can impact the quality and performance of resulting
graphene devices due to the presence of grain boundaries [40]. However, on monocrystalline
metals, large-area single-crystal domains of graphene can be formed.
Although CVD synthesis on metal substrates has proven to be a prominent process for
producing large-area graphene, the requirement to transfer the graphene to a suitable dielectric
substrate impacts its integrity, properties, and performance. For example, during transfer to an
insulating substrate such as SiO2, not only can strain and structural defects be introduced by the
host substrate, but the surface and grain boundaries of graphene are prone to contamination with
residues from polymers used in the transfer process. Any of these problems affect the growth and
properties of subsequent molecules on graphene. CVD graphene also suffers from the lack of a
preferred orientation among the graphene crystals, resulting in the formation of grain boundaries
during large-area synthesis that can misalign the orientation of organic or inorganic molecules
that grow subsequently [41]. Consequently, epitaxial graphene (EG) growth, with a preferential
orientation across large-area insulating substrates is desirable as a starting point to grow vdW
heterostructures.

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To this end, epitaxial graphene (EG) can be synthesized on single crystal SiC(0001) through
thermal decomposition, in which the top-most Si atoms are desorbed and the remaining C-atoms
reorganize to form EG (Fig. 2d-e) [33]. The semi-insulating nature of SiC also eliminates the
need for subsequent graphene transfer, thus ensuring that the pristine properties of the graphene
sheet are maintained. The simplicity of the thermal decomposition process allows for
reproducible control of EG growth, reliably achieving a uniform number of layers. Also, due to
the epitaxial relationship with the substrate, large-area graphene with a preferred azimuthal
crystal orientation can be synthesized. This leads to large-area structural coherence, which is
important for molecular assembly. Here, it is important to point out that EG growth is not limited
to SiC, as there have been several reports that claim EG growth on metallic substrates, including
Ir(111) that weakly perturbs the inherent electronic properties of graphene, and Ru(0001) or
Ni(111)) where the inherent electronic properties of graphene are perturbed significantly [37, 39,
42, 43] (Fig. 2f, g). In summary, the quality and properties of the substrates used in graphene
synthesis have a significant impact on both the molecular assembly of graphene itself as well as
graphene-based vdW heterostructure. As we describe below, most studies have focused on clean,
well-defined material systems where the energetics are dominated by molecule-molecule and
molecule-graphene interactions, instead of by defects or contaminants.
3 Molecular assembly of organic molecules on graphene
The electronic properties of organic small molecule materials depend greatly on the
molecular orientation. For organic photovoltaics, it is desirable for planar molecules to have a
face-on orientation with respect to the substrate, as shown in Fig. 1, in order to enhance the out-
of-plane charge transport. However, charge transport in organic field effect transistors (OFETs)
is optimized when the molecular orientation of the molecules is edge-on with respect to the

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substrate, like books on a shelf. It is challenging to grow well-ordered and smooth films with a
controlled orientation on conventional substrates such as metals and oxides. Recent work and
MD simulations reveal that the assembly of organic molecules into an ordered structure is driven
by molecule-molecule interactions such as covalent bonds, hydrogen bonds, van der Waals
interactions (i.e., the vdW interaction in nonpolar systems and vdW and Coulomb interactions in
polar systems), and molecular-substrate interactions, whereas the substrate governs the molecule
orientation, and its influence on the assembly process is indirect and mediated [44, 45]. The
weak intermolecular interactions in organic thin films facilitate these molecule-substrate
interactions to mediate the molecular orientation and packing, leading to high degrees of order
and structural diversity [46]. The quality of the first few layers of molecules is critically
important because the charge transport process is confined to the first two or three monolayers in
an OFET channel [47]. Therefore, the surface properties of the substrate used for the deposition
of organic semiconductors have a significant impact on the molecular orientation and film
morphology, and consequently, the performance of the semiconductor device.
Graphene has proven to be a reliable atomistic platform to enable the formation of well-
defined interfaces with organic materials, thereby reducing the density of interfacial defects that
act as charge traps or scattering centers in high-performance organic electronic devices [48-50].
Due to its strength, flexibility, transparency, and conductivity, graphene is ideal for transparent
conducting electrodes in OPVs and OLEDs. In addition, graphene has been demonstrated as a
selective interfacial layer for the extraction of electrons or holes from OPVs [51]. Recent work
also shows that vdW epitaxy can result in the growth of large, highly crystalline organic
semiconductor domains on graphene with improved device performance [52]. In the following

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sections, we review the assembly of organic molecules on graphene to form organic/graphene
heterostructures.
Metal phthalocyanines (MPcs) represent a particularly appealing class of organic
semiconductor molecules, since they consist of planar organic frameworks containing atomic
metal centers such as Cu, Sn, Zn, etc. [53] (see schematic in Fig. 3a). This planar morphology
and tendency to assemble by  stacking has made MPc molecules ideal model systems to
demonstrate that graphene can serve as a template for growing high quality, ultra-flat organic
films with a well-defined orientation. In addition, MPcs bring the capability to couple metal
centers with different functionality to specific sites on the graphene lattice. Understanding and
controlling the crystallization of these molecules on graphene is important to create the
functionalization that is desired. Below, we discuss how the assembly processes of metal-Pcs
vary depending on the type of graphene employed, namely epitaxial and non-epitaxial graphene.
3.1 Metal-phthalocyanines (MPcs) assembly on epitaxial graphene, including weakly and
strongly interacting substrates
Graphene on Ni(111), Ru(0001), or Ir(111) represents opposite extremes of the graphene–
metal interaction: a strong interaction with a strong modification of the free-standing graphene
band structure is observed on Ni or Ru, while a weak interaction with an almost unperturbed
Dirac cone is present if graphene is grown on Ir [50]. The assembly process of phthalocyanine
molecules is sensitive to the interaction between the epitaxial graphene and the supported metals.
When graphene interacts strongly with its substrate, such as for graphene/Ru(0001), the
diffusion and adsorption of MPc molecules are constrained spatially by the corrugations in the
surface morphology that results from lattice mismatch. This leads to a Moiré pattern between the
graphene and metal lattices represented by a Kagome lattice where it is found that the MPc

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molecules only reside on the face centered cubic (fcc) and hexagonal close packed (hcp) sites of
the Moiré unit cell [54, 55] (Fig. 3b-c). Therefore, the inhomogeneous adsorption landscape of
the Moiré superlattice between graphene and metals provides a unique opportunity for guiding
molecular organization. Another example of Pc molecular assembly influenced by strongly
interacting substrates is for graphene on Ni substrates. FePc molecules were observed to interact
with the Ni substrate through the graphene by the emergence of an interface state with a binding
energy of ~0.3 eV, and the quenching of the Gr–Ni π−d hybrid state at the K point of the
Brillouin zone [55, 56].
On the other hand, for MPc assembly on epitaxial graphene that weakly interacts with
metals, such as Ir(111), orientations are obtained that are similar to those obtained on graphite.
For example, graphene on Ir(111) also displays a Moiré pattern, but with significantly smaller
corrugation than that for Ru(0001) (Fig. 3d-f). CoPc self-assembles on graphene/Ir(111) at room
temperature, forming large, well-ordered domains with a nearly square lattice. In contrast to
previous studies of strong interacting-graphene/Ru (0001) with more strongly corrugated Moiré
structures, the Moiré pattern of graphene/Ir(111) does not affect the adsorption structure, and its
hexagonal symmetry is not translated to the CoPc layer [57]. Betti, et al. studied the adsorption
of MPc layers (M = Fe, Co, Cu) assembled on graphene/Ir(111) by temperature-programmed X-
ray photoemission spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS)
[58]. The balance between interaction forces among the MPc molecules and the underlying
graphene gives rise to molecular layers that are flat and weakly interacting with the underlying
graphene. As deduced by NEXAFS characterization, further MPc layers stack face-on with
respect to the first layer, up to a few nanometers in thickness. Furthermore, the FePc, CoPc, and

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CuPc multilayers have comparable desorption temperatures, compatible with molecule–molecule
interactions dominated by vdW forces between the π-conjugated macrocycles.
3.2 MPcs assembly on CVD-grown graphene
The current advances in CVD growth and transfer of graphene offer tremendous opportunities
for molecular assembly to form organic/graphene heterostructures. Although CVD growth on Cu
can provide a uniform and large area single layer graphene film, the as–grown graphene usually
has a multigrain structure with grain boundaries that may affect the assembled structures of
adsorbed molecules.
Well-aligned CuPc molecules in thin film form are very important for optimizing charge
transport in electronic devices, but it is hard to control the molecular orientation on silicon, for
instance, when thermal evaporation is utilized [47]. However, CuPc nanowires, with π-π stacking
along the wire direction as a result of the strong intermolecular coupling between the packed
molecules in 1D systems, have been synthesized on silicon by a vapor phase transport method
[59]. Although the nanowires exhibited a mobility over 20 times greater than that of a vapor-
deposited thin film, it was difficult to grow high density arrays of vertically-oriented single
crystalline nanowires on typical substrates [59]. Graphene, on the other hand, allowed Zhang et
al. to demonstrate that it can be used to grow vertically-oriented CuPc nanowires via vdW
epitaxy through a physical vapor transport method [60]. They observed a clear transition from a
2D to 3D growth mode, which was illustrated and explained in terms of the crucial role of
intrinsic (competition between intermolecular interactions and substrate-molecule interactions)
and extrinsic factors (fast deposition and ballistic mass transport). Furthermore, the structural
superiority of vertically-oriented nanowires over a conventional bilayer thin film system was
evidenced by a 32% improvement of the photoconversion efficiency (PCE). This efficiency

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increase is largely attributed to the enhanced interfacial area produced by the vertical nanowires,
creating more available sites for exciton dissociation. Two other recent studies realized vertical
organic nanowire arrays on graphene, and further illustrated the advantage of having high
quality, intimate, interfacial contact and directional transport in solar cell applications and
vertical conducting devices, respectively. [61, 62]
In contrast to work involving growth of vertically-oriented CuPc nanowire on graphene,
Xiao et al. recently evaporated CuPc to form a thin film directly on CVD graphene to control the
growth and orientation of CuPc molecules over the mesoscopic length scales required for organic
electronics [27] (see Fig. 4a-g). They found that both CVD graphene grown on Cu and graphene
transferred onto silicon are shown to orient and pack CuPc molecules with a face-on orientation
in thin films. More specifically, X-ray scattering and pole figure measurements show that CuPc
molecules on graphene initially exhibit a face-on orientation at room temperature, but following
nucleation into islands and coalescence by edge growth into a continuous film, the molecules
start to tilt up to an edge-on orientation. Growth at higher temperatures, however, was able to
overcome this orientation change, and a fully face-on molecular orientation was maintained. As
shown in Fig. 4b, CuPc molecules in this case prefer to reside along the wrinkles and grain
boundaries of the graphene at the initial growth stage, and then form large 2D strip-like crystals
aligned along the graphene lattice with an ordered superstructure that maintains the face-on
orientation throughout the growth process. Such order is maintained by intermolecular vdW
forces and the molecule-substrate interactions associated with a small amount of charge transfer
from graphene to the CuPc molecules. In addition, CuPc molecules at high temperature were
shown to grow preferentially on single-layer graphene compared to few-layer graphene due to
the charge transfer and induced interfacial dipole interactions (Fig. 4g). This work demonstrated

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that transferred CVD graphene enables the growth of crystalline and well-aligned molecular
superstructures for enhanced charge and exciton transport in nanostructured heterojunction
organic photovoltaics.
It is well-known that molecular orientation is an important but challenging topic in
traditional surface enhanced Raman scattering (SERS) studies. However, as discussed above,
engineering the molecular orientation on graphene is convenient, and therefore graphene can
serve as an effective SERS substrate for aligned CuPc molecules. In addition, since it is easy to
combine graphene with traditional metallic substrates, one can take advantage of both metal and
2D substrates for SERS. The mechanism for enhancement is best illustrated by Ling et al.,
wherein they found that strong charge transfer interaction between graphene and aligned CuPc
molecules can induce an increase in the electron transition probability, and consequently, a
significant enhancement of the Raman signal [63, 64] (see Fig. 4h). Consequently, graphene
surfaces represent a key discovery in SERS, and have proven to be part of an attractive signal
enhancement technique that yields clean, uniform, and repeatable Raman-enhanced signals.
Transferring CVD graphene with PMMA usually leaves polymer residues on its surface or at
grain boundaries, and the influence of these residues on the assembly of organic molecules on
graphene is crucial for electronic devices. Shi et al. investigated the assembly of CuPc and
pentacene molecules on PMMA contaminated graphene, [65] and they found that the grain size
and morphologies of the films was governed by the density of the PMMA residues. However, the
molecular orientation of CuPc and pentacene on graphene responded differently to the density of
PMMA residues. The orientation of CuPc molecules on graphene was not affected by PMMA
residues – i.e., the molecules always took a face-on orientation with the graphene regardless of
the annealing temperature of graphene prior to CuPc deposition. However, the orientation of

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pentacene film was sensitive to PMMA residues – i.e., pentacene adopted an edge-on orientation
on as-transferred graphene, whereas it switched to face-on orientation mode when the PMMA
residues were essentially removed. This discrepancy indicates a relatively stronger molecule-
substrate interaction in the case of CuPc-graphene than in the case of pentacene-graphene [66].
3.3 Assembly of macromolecular covalent organic frameworks (COFs) on graphene
Macromolecules, such as covalent organic frameworks (COFs), can also be assembled on
graphene to form oriented, well-aligned multilayer films as shown in Fig. 5a. The framework of
some COFs can be made up of π-stacking conjugated building blocks that create porous
networks with electronically coupled channels [67]. COFs containing MPcs have been reported
recently, and the NiPc-based COFs were the first crystalline porous frameworks to exhibit high
charge carrier mobilities [68]. However, they are usually synthesized as insoluble and
unprocessable powders, with no control over the orientation of the pore system, and thus have
limited uses in optoelectronic device applications. Recently, Dichtel and colleagues [69] reported
the synthesis of oriented, 2D COF films on graphene as illustrated in Fig. 5b. Although
crystalline COF films are formed with their aromatic groups stacked on graphene when graphene
is supported by either Cu, SiC or SiO2, the film thickness and uniformity is strongly affected by
the quality of the graphene support. The graphene supported films were confirmed by
synchrotron x-ray diffraction to be oriented, with the plane of the 2D COF parallel to the plane of
the graphene, thus indicating that the presence of graphene facilitated controlled synthesis of the
COFs films with pores aligned in the same direction. Interestingly, COF films do not grow
epitaxially on graphene synthesized with SiC, and this suggests that matching the COF lattice
size and symmetry to the underlying graphene is not necessary to obtain a crystalline film. The
crystallinity and alignment of COF films on transparent graphene/SiO2 substrate provides a

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means to organize functional π-electron systems within optoelectronic devices. Sun et al.[70]
recently demonstrated a vertical FET based on a COFTFPy-PPDA graphene heterostructure. As
shown in Fig. 5c, the molecular orientation of the COFTFPy-PPDA film on graphene exhibits a
periodically eclipsed π-stacking column in 2D COFs. The heterostructure shows ambipolar
charge carrier behavior with an on/off ratio >105 and high on-current density >4.1 A cm–2. The
ability to synthesize vertically aligned open pores in COF films, combined with the possibility of
preparing graphene on a variety of substrates may enable the integration of charge-transporting
COFs and conducting graphene into ideal bulk heterojunctions for photovoltaics because
anisotropic exciton and charge transport are expected to proceed along the π-stacked directions.
This work also represents a possible way to confine molecular assembly to two dimensions, such
as surface-confined polymerization, to synthesize 2D polymers on graphene.
The utility of a graphene layer for facilitating planar adsorption of organic semiconductor
molecules for well-oriented superstructures has been demonstrated repeatedly, indicating that it
can be used to direct molecular adsorption to form well-defined graphene/organic
heterostructures, and therefore promote carrier transport in a certain orientation. This suggests
that using graphene may allow for predictable molecular assembly on a wide range of surfaces
for optoelectronic applications.
4 Graphene – 2D transition metal chalcogenide (TMC) heterostructures
In addition to serving as a platform for growing well-aligned organic molecules, graphene is
also a good template for the assembly of inorganic 2D materials. The semi-metallic property of
graphene limits its optoelectronic device applications because it lacks a bandgap, resulting in a
low current on/off ratio that is important for transistor logic/switching applications. Recently, 2D
TMCs have emerged as complementary 2D materials for graphene in optoelectronic applications.

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For example, 2D TMC semiconductors exhibit high carrier mobility, high on-off current ratios,
strong light-matter interactions, and excellent mechanical properties that are well-suited for
future low-power electronics, flexible electronics and logic applications. Although 2D TMC
semiconductors have demonstrated remarkable properties, a unique opportunity lies in
combining them with graphene to form graphene-based 2D vdW heterostructures, which not
only retains the properties of the individual constituent materials, but manifests entirely new
functionalities [25, 71]. For example, the grain boundaries due to randomly-distributed domains
in 2D materials, which are typically formed on SiO2 during CVD synthesis, are detrimental to
electrical properties, leading to low carrier mobility and therefore slow response times (generally
in ms) in photosensitive devices [72]. Graphene provides an excellent template for growing large
area 2D materials through vdW epitaxy, and consequently, when integrated with graphene, the
photoresponse properties of TMCs might be improved significantly because graphene offers
ultrafast photoresponse due to its extremely high carrier mobility. In addition, the well-defined
atomic interfaces in TMC/graphene heterostructures also eliminate the presence of charge traps,
thereby improving the transport properties of the 2D material and the associated device
operation. Furthermore, recent work indicates that graphene can improve the doping efficiency
of 2D TMCs [31].
Common 2D layered materials include the metal monochalcogenides, (e.g., GaSe, InSe) and
metal dichalcogenides (e.g., MoS2 and WSe2). In this section, we will focus on two typical
TMCs, namely GaSe and MoS2. Gallium selenide (GaSe) is a layered metal monochalcogenide,
and has been used widely in optoelectronics, nonlinear optics, and studies in terahertz irradiation.
GaSe exhibits a direct-to-indirect bandgap transition and a progressive bandgap shift when it is
thinned from bulk crystals to flakes having less than 7 layers [73]. Similarly, MoS2 is a layered

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TMC semiconductor which possesses a relatively high mobility (~200 cm2V-1s-1), and has a layer
dependent bandgap with a crossover from indirect (1.2 eV) to direct (1.9 eV) at the bulk-to-
monolayer transition [9, 14]. Below we discuss how the growth process of GaSe and MoS2 vary
depending on the type of graphene employed: non-epitaxial CVD graphene and epitaxial
graphene grown on SiC. We also discuss the impact of graphene templates on the enhancement
of substitutional doping in TMCs.
4.1 2D TMCs growth on CVD-graphene
Chemical vapor deposition (CVD) of graphene on Cu foil allows for the growth of large
crystalline domains, and the ability to uniformly cover large areas due to the aforementioned
self-limiting growth behavior. Thus, CVD-grown graphene have been envisioned as a promising
template for the assembly of 2D TMC materials for various large-area electronic applications.
Although 2D GaSe has been grown on SiO2 through a vapor transport method, the flakes were
randomly oriented on the amorphous SiO2 surface, leading to the formation of grain boundaries
as the flakes merged to form larger domains. Grain boundaries limit the charge carrier mobility,
and thus, large-area monocrystalline GaSe with a preferred orientation is needed for device
applications. Li et al. recently demonstrated the epitaxial growth of monocrystalline, 2D GaSe
with a preferential orientation on CVD-grown graphene [74]. The graphene used was
synthesized by CVD on Cu foil, and then transferred onto a SiO2/Si substrate. Although high
quality GaSe monolayers with preferred orientation were grown epitaxially on graphene despite
the large lattice mismatch (~58%), graphene grown by CVD is usually polycrystalline and
composed of randomly oriented grains with limited size. Consequently, GaSe epilayers start to
nucleate and grow predominantly from random wrinkles and grain boundaries in the graphene,
forming large, irregularly-shaped, single crystal GaSe domains through the coalescence of

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aligned triangular edges in the most energetically favored stacking as shown in Fig. 6a-b. The
electron diffraction of GaSe on graphene revealed that the preferred lattice rotation angle
between GaSe and graphene layers was ~10.5°, which was consistent with the theoretically
calculated orientation (10.9°) that produces the smallest supercell with the lowest strain and
strongest binding energy (Fig. 6c-d). These energetically preferred orientations appeared to be
correlated with the growth of high quality, large single-crystal GaSe domains on graphene.
Strong charge transfer between GaSe and graphene was evidenced by an enhanced in-plane
Raman mode and strong photoluminescence quenching. However, only ~50% of the GaSe
crystals grown on transferred CVD graphene adopted these most energetically favored stacking
configurations. The reason for the misalignment of some crystal grains appeared to originate
from random wrinkles and grain boundaries in the graphene that served as preferred nucleation
sites, which impacted the stacking and orientation of subsequent GaSe crystal domains. As
shown in Fig. 6e-f, vdW epitaxy of MoS2 with a preferred orientation on transferred CVD
graphene was found to follow a similar trend [75, 76].
To avoid such complications that often result from graphene transfer to a desired substrate,
pristine CVD graphene without transfer is required. However, the temperatures employed in the
CVD growth of 2D TMC materials on Cu or Ni substrates is sufficiently high ro result in
chemical reactions with sulfur or selenium, making this approach impractical. Instead, more inert
metal substrates such as Au or Pt foil are desirable. Shi et al. recently reported the growth of
MoS2 and graphene on Au foil in a two-step CVD process [77]. Briefly, graphene was grown on
Au using CH4 at ambient pressure, and then MoS2 was grown sequentially on the graphene at
low pressure. However, the MoS2 grains were randomly oriented with respect to underlying
graphene, due either to the polycrystalline domains of the as-grown graphene or mechanical

20
strain between the materials. Similarly, Cai et al. grew monolayer MoS2 on pristine CVD-grown
graphene that was synthesized on Pt foil [78]. The ~28% lattice mismatch between MoS2 and
graphene was relaxed through the weak vdW force between the heterolayers, resulting in
relatively large-scale monolayer MoS2 domains with randomized crystallographic orientations
that mimic the polycrystalline nature of the underlying CVD graphene. However, unlike TMCs
on transferred graphene, MoS2 grown on pristine graphene exhibits a strict epitaxial alignment
with the graphene without lattice rotation. The interlayer coupling and strain effects in such
MoS2/graphene heterostructures were also found to impact their electronic properties
significantly, ultimately resulting in metal-like behavior. While these are significant advances in
the pursuit of well-aligned large area 2D materials on the graphene, it is still necessary to
develop methods to suppress other orientations resulting from grain boundaries, defects, strain,
and wrinkles inherited from the graphene template.
4.2 2D TMCs growth on epitaxial graphene
Epitaxial graphene (EG) provides a pathway to achieve structural coherence and uniform
coverage at long-length scales because of its monocrystalline properties and minimal
perturbations from its supporting substrate. As such, EG allows for the direct growth of large-
area 2D TMCs with well-defined orientation and high crystallinity. Also, due to the weakly
interacting nature of the graphene template, epitaxial graphene provides tunable substrate
coupling that may permit the growth of strain-free TMCs. Epitaxial graphene grown on SiC by
the graphitization of hexagonal SiC crystals during annealing at high temperature results in
graphene layers on semi-insulating SiC [79]. This eliminates the need to transfer graphene to
another substrate, which usually weakens the integrity of the graphene layer due to polymeric
contaminates and structural defects. As shown in Fig. 7a, Lin et al. utilized EG on SiC as the

21
growth template for different TMCs including MoS2, and the resultant interfaces were atomically
sharp [41]. They determined the influence of the underlying graphene on the nucleation and
growth process, especially at SiC step edges where the presence of strain promotes vertical
growth. Hydrogenation of SiC was observed to passivate the graphene/SiC layer and relax the
residual strain. Although they observed epitaxial growth of TMCs on EG, there were no
discussions of the relative crystal orientations in this work. However, in later work it was
demonstrated clearly that WSe2 is strictly aligned with graphene without lattice rotation between
two layers (Fig. 7b) [80]. They highlighted the influence of the substrate on the consistent
rotational angles between the heterolayers. Similarly, Liu et al. observed two types of azimuthal
registrations for the growth of CVD MoS2 on EG, with one more predominant than the 30°
relative crystal orientation [81] (see Fig. 7c-d). From Raman spectroscopy, it can be deduced
that the MoS2 grown on EG has lower tensile strain and stronger charge transfer in comparison to
MoS2 on SiO2. The lower tensile strain can be explained by the fact that while MoS2/SiO2 is
under tensile strain [82], MoS2/EG is strain-free as confirmed by X-ray scattering. The growth of
strain-free TMCs with strong charge transfer and preferred crystal orientation is interesting for
probing the fundamental properties of TMCs and scaling their application for commercial
purposes. In addition, EG can play a prominent role in realizing heterogeneous TMC/TMC
vertical structures [80].
We have thus far highlighted the importance of the graphene template in the growth of
2D TMCs. In addition, the inert surface of graphene can improve the ease of doping 2D TMCs,
which may provide a route to functionalizing their properties. For example, Se-doping of MoS2
has been an effective way to engineer its optical bandgap [83], while W-doping exhibits a p-type
transport property complementary to the n-type behavior of undoped-MoSe2 [84-86]. Also,

22
doping of MoS2 with Mn may lead to measurable magnetism at the 2D material limit [87].
Efforts on doping MoS2 with Mn on reactive substrates (e.g. SiO2 and sapphire) have been
unsuccessful. However, Robinson, et al., [31] recently observed that the incorporation of Mn
dopants into the lattice of 2D MoS2 requires an inert, sp2 substrate – i.e., graphene (see Fig. 8a).
They found that due to the surface reactivity of traditional substrates, Mn dopant atoms react
more strongly with the substrate terminating atoms instead of being incorporated in the MoS2
layer, resulting in the formation of Mn-Si bonds which are more thermodynamically favorable
than Mn-S bonds. This highlights the importance of the substrate surface chemistry on the
doping process of TMCs with non-TMC like elements. Mn doping of monolayer MoS2 was
achieved with vapor-phase precursors during the MoS2 synthesis, resulting in a doping
concentration of ~2% (Fig. 8b-c). An enhanced charge transfer between Mn-doped MoS2 and the
graphene substrate completely-quenched the PL signal from MoS2, and modified the electronic
structure of MoS2 by shifting the valence band edge by 0.15 eV (Fig. 8d-e). The successful
doping of MoS2 on graphene over reactive substrates (SiO2 and sapphire) highlights the
importance of the choice of substrate in the growth, properties, and functionalization of 2D
TMCs.
5 Applications of graphene-based heterostructures
Due to the atomically-flat nature of graphene, its ability to align crystals and molecules
preferentially, and its excellent electrical properties, graphene heterostructures have found
broader applications in organic electronics, field-effect transistors, photodetectors, and
biochemical sensing [25, 26]. For organic solar cells, graphene templates can help induce a
preferential face-on molecular alignment in which exciton diffusion and charge transport are
optimized. The use of graphene instead of reactive substrates has shown an increase in power

23
conversion efficiency in PV cells [88]. In addition, graphene can serve as a transparent
conductive electrode (TCE) in organic electronics because of its very high charge conductivity,
low absorption (~2.3%), excellent stability in air, and adaptability to flexible substrates [6, 29].
For example, graphene has been demonstrated as a TCE for constructing highly efficient hybrid
heterojunction organic-silicon solar cells, and has been responsible for the suppression of charge
recombination and improvement in light harvesting capability [89]. The high electrical
conductivity of graphene makes it a great candidate for electrical contacts for FETs. Replacing
traditional metal contacts such as Au or Pd in FET devices with graphene has shown
improvement in device performance because of the formation of an Ohmic contact at the
semiconductor interface [90, 91]. Also, MoS2/graphene heterostructures have shown high
photoresponsivity, greater than 107 A/W, which is interesting for photodetector applications [92].
This result is attributed to the presence of a perpendicular electric field across the heterojunction,
which allows for fast transport of photoelectrons from the MoS2 layer to the underlying
graphene. In addition, Ugeda et al. have shown that MoSe2/graphene heterostructures have
strong many-electron interactions, which result in ~51% reduction of the exciton binding energy
without significantly changing the optical gap. The main reason is that increased free-carrier
screening in graphene plays a large role in determining the MoSe2 exciton binding energy [93].
Furthermore, the flexible nature of graphene makes graphene heterostructures attractive for
wearable optoelectronic applications. Also, due to the sensitivity of the electronic, optical, and
mechanical properties of graphene to their surrounding environment, graphene-based
heterostructures can be particularly useful as biochemical sensors [94, 95]. For example, rapid,
sensitive, and reliable biochemical sensing have been developed for detection of DNA, proteins,
antibodies, and enzymes [96-100]. By using the heterostructures of few-layer BP-

24
graphene/TMCs, Wu et al. doubled the sensor sensitivity when compared to conventional sensors
based on single Ag films [101]. These graphene-based biosensors thus exhibit promising
applications in chemical examination, medical diagnosis, and biological detection.
6 Summary
The atomically flat and chemically-inert nature of graphene was shown to provide excellent
surfaces for the growth and assembly of both organic and inorganic molecules into highly
ordered vdW heterostructures, where vdW interactions relax strict lattice-matching requirements
typically found in epitaxial growth. The nature and quality of the substrate on which graphene is
grown is important in determining both the structure and the properties of the resulting graphene-
heterostructure. Therefore, epitaxially-grown graphene on flat, semi-insulating substrates such as
SiC are desirable because it removes the need to transfer graphene, which can lead to
contamination, defects, and impurities. The ability to utilize underlying substrates to vary the
adsorption strength of molecules through different interfacial electron density of states was
shown to allow for the preferential molecular assembly and alignment of molecules on graphene.
This is important for applications. For example, graphene tends to orient CuPc molecules in the
face-on orientation that is favorable for the out-of-plane charge transport necessary for organic
solar cells. Similarly, graphene templates allow for the growth and alignment of COFs pore
systems to optimize their electrical properties. In addition, graphene is an excellent template for
the assembly of inorganic 2D TMCs that tend to self-assemble in energetically preferred
orientations without significant strain. The growth of large-area, monocrystalline, strain-free
TMCs, with strong charge transfer and preferred crystal orientation is interesting for applications
in commercial electronics. Due to the chemical inertness of its surface, graphene also allows an
extra degree of freedom to alter the electronic functionality of TMCs via external dopants.

25
Overall, graphene makes an excellent template for the growth of both organic and inorganic 2D
heterostructures, with a wide-range of future electronic applications in organic photovoltaics,
field-effect transistors, and photodetectors.
Acknowledgements
This research was supported by the Center for Nanophase Materials Sciences (CNMS), which is
a DOE Office of Science User Facility. A. O. acknowledges fellowship support from the
UT/ORNL Bredesen Center for Interdisciplinary Research and Graduate Education. Synthesis of
inorganic 2D materials was supported by the Division of Materials Science and Engineering,
Basic Energy Sciences, DOE Office of Science.

26
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33
Figure 1. Schematic illustrations of prototypical organic molecules (Copper phthalocyanine
(CuPc)) and inorganic 2D materials (such as GaSe and MoS2) on graphene for vdW
heterostructures.

34
Figure 2. Graphene synthesized with different methods (a) An optical image of mechanically-
exfoliated graphene on SiO2 substrate [102]. (b) large area graphene grown on Cu by CVD
[103]. (c) ADF-STEM image showing the atomic structure of CVD-graphene with hexagonal
lattice [104]. (d) AFM image of epitaxial graphene grown on SiC substrate. Inset: LEED pattern
showing the structural coherence between SiC(0001) substrate (blue arrows) and the graphene
lattice (red arrows) [105]. (e) Atomic-resolution STM image graphene on SiC overlaid with the
structural model [106]. (f) SEM image of epitaxial graphene on Ru(0001) surface. Inset:
corresponding scanning Auger microscopy image [38]. (g) Atomically resolved STM image of
the Moiré pattern in epitaxial graphene on Ru, also revealing the graphene periodicity. The high
(H), intermediate (I), and low (L) regions correspond to different alignments between the
graphene and the Ru [43].

35
Figure 3. Organic molecule assembly on epitaxial graphene. (a) Molecular structure of metal
phthalocyanines (MPc) where M = Cr, Mn, Fe, Co, Ni, Cu and Zn [107]. (b) STM image of FePc
molecules on graphene/Ru(0001) surface showing site-selective assembly. The molecules
occupy the fcc sites of the Moiré pattern. The top site of the Moiré is brighter than the low-lying
area. (c) As the coverage is increased, the molecules begin to occupy the HCP sites while the top
sites still remain occupied [108]. (d) Schematic illustration of the structure of CoPc on
graphene/Ir(111) surface forming regular superstructures. (e) STM image of the surface after the
deposition of a monolayer of CoPc. (f) In gap STM image overlaid by the molecule
backbone [57].

36
Figure 4. Organic molecules assembly on transferred CVD graphene. (a) Schematic of CuPc
molecule on graphene with face-on and edge-on orientations. (b) STM image of face-on CuPc
molecules on graphene. Bottom-left corner inset is a high-magnification STM image, while the
top-right corner shows a schematic of the orientation. (d-f) SEM of CuPc films deposited at 130
°C with different thickness on graphene (2 nm, 20 nm, and 30 nm, respectively). Inset: XRD
pole figure indicating a face-on orientation. (g) SEM images CuPc on graphene showing
selective growth on single layer graphene (SLG) [27]. (h) Raman enhancement of the CuPc
molecule on graphene substrate compared to other substrates (h-BN, MoS2,and SiO2/Si) [64].

37
Figure 5. Large COF molecules assembly on graphene (a) Schematic stack of MPc 2D sheets
and microporous channels in NiPc COF. (Color codes: Pc unit: sky blue; Ni green, N violet, C
gray, O red, B orange, H white) [68]. (b) 2D COF films on graphene surfaces using a
solvothermal condensation process [69]. (c) Schematic of COFTFPy-PPDA film on graphene/SiO2
surface showing a periodically eclipsed π-stacking column.

38
Figure 6. 2D TMC grown on CVD graphene or transferred CVD graphene (a) SEM image of
monolayer GaSe on graphene at the initial growth stage. Inset is an enlarged image showing
GaSe starting to grow from random wrinkles or grain boundaries. (b) SEM image of large area
monolayer GaSe grown on CVD graphene. (c) Atomic resolution ADF-STEM images of 1L
GaSe on graphene. Inset: SAED pattern showing the relative orientation between GaSe and
graphene. (d) Schematic illustrations of vdW heterostructure constructed by 1L GaSe epitaxy on
1L graphene with interlayer lattice rotations of 10.9° and 19.1°, respectively [74]. (e) SEM

39
image of a MoS2 on graphene. Inset: Selected-area diffraction. (f) Different grain structure of
graphene [76].

40
Figure 7. 2D TMC monolayer grown on epitaxial graphene. (a) AFM image of monolayer
MoS2 grown on EG/SiC. Inset: Cross-sectional HRTEM of MoS2/EG [41]. (b) Monolayer WSe2
epitaxially grown on EG/SiC. There is no lattice rotation between WSe2 and the underlying
graphene as indicated by the inset electron diffraction image [80], [109]. (c) MoS2 grown on EG
showing two predominant orientations 30° relative to each other. (d) Schematic illustration of the
MoS2/EG heterostructure with MoS2 lattice aligned with that of EG [81].

41
Figure 8. The effect of graphene on the 2D TMC doping. (a) Synthesis of Mn-doped MoS2 using
graphene as a substrate. (b) Optical image of Mn-doped monolayer MoS2 grown on graphene (c)
TEM shows incorporated Mn within the MoS2 matrix with a resultant doping concentration of
~2%. (d) PL of monolayer MoS2 indicating that the addition of Mn leads to enhanced
nonradiative recombination in Mn-doped MoS2. (e) XPS confirming that the valence band
maximum (VBM) shifts from 0.76 eV for pristine MoS2 to 0.61 eV for Mn-doped MoS2 [31].

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