Ink Formulation and Printing Parameters for Inkjet Printing of Two Dimensional Materials: A Mini Review

Abstract

Inkjet printing of two-dimensional (2D) material has been a center of interest for wearable electronics and has become a promising platform for next-generation technologies. Despite the enormous progress made in printed 2D materials, there are still challenges in finding the optimal printing conditions involving the ink formulation and printing parameters. Adequate ink formulation and printing parameters for target 2D materials rely on empirical studies and repeated trials. Therefore, it is essential to compile promising strategies for ink formulation and printing parameters. In this context, this review discusses the optimal ink formulations to prepare stable ink and steady ink jetting and then explores the critical printing parameters for fabricating printed 2D materials of a high quality. The summary and future prospects for inkjet-printed 2D materials are also addressed.

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nanomaterials
Review
Ink Formulation and Printing Parameters for Inkjet Printing of
Two Dimensional Materials: A Mini Review
Ho-Young Jun 1, Se-Jung Kim 2and Chang-Ho Choi 1,3,*


Citation: Jun, H.-Y.; Kim, S.-J.; Choi,
C.-H. Ink Formulation and Printing
Parameters for Inkjet Printing of Two
Dimensional Materials: A Mini
Review. Nanomaterials 2021,11, 3441.
https://doi.org/10.3390/nano11123441
Academic Editor: Wolfgang Heiss
Received: 24 November 2021
Accepted: 16 December 2021
Published: 19 December 2021
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1Department of Chemical Engineering, Gyeongsang National University, Jinju 52828, Korea;
jhy1848@gnu.ac.kr
2School of Chemical Engineering, Jeonbuk National University, Jeonju 54896, Korea; sejung.kim@jbnu.ac.kr
3Department of Materials Engineering and Convergence Technology, Gyeongsang National University,
Jinju 52828, Korea
*Correspondence: ch_choi@gnu.ac.kr; Tel.: +82-55-772-1781
Abstract:
Inkjet printing of two-dimensional (2D) material has been a center of interest for wearable
electronics and has become a promising platform for next-generation technologies. Despite the enor-
mous progress made in printed 2D materials, there are still challenges in finding the optimal printing
conditions involving the ink formulation and printing parameters. Adequate ink formulation and
printing parameters for target 2D materials rely on empirical studies and repeated trials. Therefore,
it is essential to compile promising strategies for ink formulation and printing parameters. In this
context, this review discusses the optimal ink formulations to prepare stable ink and steady ink
jetting and then explores the critical printing parameters for fabricating printed 2D materials of a
high quality. The summary and future prospects for inkjet-printed 2D materials are also addressed.
Keywords: 2D materials; inkjet printing; ink formulation
1. Introduction
Since the discovery of graphene, two-dimensional (2D) layered materials have at-
tracted great attention in various research fields due to their large surface area and unique
quantum confinement effect [
1
]. The unique properties of 2D materials provide abundant
opportunities for next-generation applications and technologies [
2
]. In order to turn these
opportunities into the real, scalable production of 2D nanosheets (NSs), it should be accom-
panied by the advancement of deposition techniques. Liquid phase exfoliation (LPE) has
proved its ability to produce scalable and high-yield 2D NS dispersion. When it comes to
promising deposition techniques, several candidates have been introduced, including spin
coating [
3
,
4
], spray coating [
5
,
6
], inkjet printing [
7
–
9
], and screen printing [
10
,
11
]. Among
them, inkjet printing is a high-volume and low-cost manufacturing process, allowing
complex and large-area patterning of 2D NSs, and has been applied to various fields such
as electronics, bio, and optics based on inorganic or organic materials [
12
–
14
]. Based on the
development of various 2D NS inks, inkjet-printed 2D NSs have been flourishing in many
research fields [15,16].
Graphene, transition metal dichalcogenides (TMD), boron nitride (BN), black phospho-
rus (BP), and MXenes are common 2D NSs for inkjet printing [
17
–
21
]. These 2D NSs possess
different intrinsic properties and thus correspondingly demand different ink formulation
strategies [
15
]. The viscosity, surface tension, and concentration are critical factors to be
considered in designing the ink formulation of 2D NSs [
16
,
22
]. For most cases, these factors
interactively determine the ink stability and ink-jetting dynamics. Once stable droplets are
discharged from nozzles, they encounter diffusion and evaporation that simultaneously
occur on the substrate. The diffusion and evaporation should be tailored to accomplish
uniform printing patterns [
23
]. Like the ink formulation, the solvent’s viscosity and surface
tension play a vital role in tuning the diffusion and evaporation rates. Additionally, some
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Nanomaterials 2021,11, 3441 2 of 21
operating parameters of inkjet printing also affect the quality of printed 2D NSs. Overall,
the 2D NS-inkjet printing is not straightforward, and all these parameters should be consid-
ered interactively. Some review articles have recently been published, providing general
information and insights ranging from ink formulation to the application of printed 2D
NSs [
15
,
16
,
24
]. In addition to such comprehensive reviews, a mini-review article focusing
on optimizing the printing conditions for both ink formulation and the printing process
is also worthy of being reported. This article is composed of several sections. The first
section deals with ink formulation strategies, starting with exfoliation and then moving
on to formulation. The second begins with the working principles of inkjet printing and
takes a significant portion to discuss the operating conditions of inkjet printing to achieve
high-quality printed 2D NSs. A brief explanation of the applications of printed 2D NSs
is assigned to the third one. Lastly, a summary and future respective are presented. This
review will offer sound guidance for relevant researchers seeking adequate ink formulation
and printing parameters for their target 2D NSs.
2. Ink Formulation of 2D Materials
The first step for an ink formulation of 2D materials is to exfoliate the bulk 2D materials
into thin-layered 2D nanosheets (NSs). There have been many exfoliation methods designed
to obtain 2D NSs with high quality, high yield, and excellent processibility for target
applications [
25
]. Since the 2D material ink should be formulated into a colloidal dispersion,
liquid-phase exfoliation (LPE) is generally used to produce 2D NSs [
1
]. The physical
properties of 2D materials are diverse, depending on their structure and composition.
This means that each 2D material needs its optimal exfoliation conditions involving an
exfoliation medium, additives, and exfoliation equipment parameters [
16
]. Once the 2D
NSs are prepared by exfoliation, the following step is to formulate the ink using the
exfoliated 2D NSs. There are also many factors to be considered in an ink formulation to
ensure stable ink jetting, printing resolution, and printing pattern quality. Two different ink
formulation approaches have been reported: direct ink formulation and solvent exchange
ink formulation. As the names indicate, the direct ink formulation is to directly utilize
the colloidal dispersion of the exfoliated 2D NSs as an ink, while in the solvent exchange
formulation, the exfoliation medium is exchanged with new solvents to make the ink more
suitable for printing. This section discusses the ink formulation of 2D materials, covering
the exfoliation methods and two ink formulation approaches with representative works.
2.1. Exfoliation of Bulk 2D Materials into 2D Nanosheets
A wide range of exfoliation methods has been developed for the scalable production of
2D materials [
26
]. Among these, LPE is the most suitable for preparing a 2D NS dispersion
for printing ink. In LPE, cavitation induced by ultrasonic-wave or high shear stress by
a shear rotor is the driving force to delaminate bulk-layered materials into mono- and
few-layer NSs with higher exfoliation yield [27]. Although shear stress created by a shear
rotor is more efficient in improving the exfoliation yield and scalable production, 2D NSs
exfoliated by cavitation are generally employed for ink formulation. This may be attributed
to the operational simplicity and applicability of an ultrasonic bath or probe sonic in
ordinary laboratories. Additionally, the ink for inkjet printing permits a low exfoliation
yield because it requires relatively low viscosity and low concentration, compared to other
printing methods such as screen printing [28,29].
It is generally preferred to use wider and thinner 2D NSs for inkjet printing. Key
exfoliation conditions in ultrasonics include sonication power and treatment time. The
selection of the solvent and additives is also a major determinant for yielding wider and
thinner 2D NSs. Different 2D materials demand different optimal exfoliation conditions.
All these complexities lead researchers to conduct exfoliation in various conditions, mostly
relying on empirical studies. Table 1summarizes the exfoliation conditions and their
corresponding results represented by the dimension of exfoliated 2D NSs. A solvent of
N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) are common exfoliation

Nanomaterials 2021,11, 3441 3 of 21
solvents regardless of the type of 2D materials. The surface tension of organic solvents
and their Hansen solubility parameters (HSPs) define whether there are intermolecular
interactions between the 2D materials and the solvents [
30
,
31
]. High boiling point organic
solvents such as NMP and DMF have HSPs and surface tension optimized for exfoliation,
and thus these solvents can efficiently produce 2D NSs without additives [
16
]. All of the
NMP-exfoliated 2D NSs present the thickness within the few-layer range (less than ten lay-
ers) without the aid of additives (Table 1). However, the high boiling point solvents induce
NS agglomeration on the substrate after printing and take a long time for evaporation,
making the ink unfeasible for inkjet printing [23]. In an effort to overcome the drawbacks
of high-boiling point solvents, alcohol solvents with a low boiling point are frequently used
for exfoliation. Although the alcohol solvents are more affordable for printing and drying,
they show the mismatch of surface tension and HSPs, causing inferior exfoliation efficiency
and low dispersion stability. Yao. et al. recently reported that adding water to ethanol
helped alleviate the mismatch issue and successfully obtained MoS
2
NSs with a thickness
range of 1.2~8.5 nm [
32
]. Alternatively, various surfactants can be added to exfoliation
solvents to induce an electrostatic or steric hindrance that consequently improves the
exfoliation efficiency and dispersion stability. Surfactants used for exfoliation are divided
into ionic and non-ionic surfactants. Sodium cholate (SC), carboxymethylcellulose (CMC),
and sodium deoxycholate (SDC) are representative ionic surfactants [
33
,
34
]. The ionic
surfactants interact with the 2D material in water to balance the vdW forces in the layer to
aid the exfoliation and prevent re-agglomeration of the exfoliated 2D material. However,
these ionic surfactants remain after printing and cause the degradation of printed 2D NSs,
requiring additional processes for removing the surfactants. Typical non-ionic surfactants
include polymers such as ethyl cellulose (EC) and polyvinylpyrrolidone (PVP). The poly-
mers are attached to 2D materials to provide a physical separation between the NS layers
and enhance the dispersion stability [
35
–
38
]. In particular, EC, used in the coatings industry
for decades, is mildly sonicated in exfoliation solvents to facilitate the deep insertion of
EC molecules into the 2D materials’ layers, preventing the aggregation of 2D materials
during exfoliation [
39
]. In addition to the improved ink dispersibility, it also improves the
ink-jetting stability of low-boiling solvents with low viscosity that suffer from unstable ink
jetting (see more details in Section 3.2). The polymer addition increases the viscosity of the
ink, allowing stable jetting. Thanks to these merits, cases of using non-ionic surfactants are
more prevalent than ionic surfactants [40,41].
Ultrasonic equipment that creates cavitation can also affect exfoliation performance,
particularly the dimension of 2D NSs. A bath sonicator has an ultrasonic transducer
at the bottom of a bath, such that the ultrasonic waves need to transfer through water
in the bath. On the other hand, a probe sonicator has a relatively short transfer length
of ultrasonic waves, with the result that cavitation is performed more vigorously in a
probe sonicator than the bath counterpart. That is why a probe sonicator usually takes a
much shorter exfoliation time than a bath sonicator. As shown in Table 1, graphene was
produced following 72 h exfoliation in a bath sonicator, much longer than the 1.5 h in a
probe sonicator. However, such high cavitation with a probe sonicator usually produces
2D NSs with a small lateral size, less than 100 nm in diameter, and a relatively wider
lateral size range. A bath sonicator can produce 2D NSs with a larger size in a narrow
size distribution, but excessive sonication treatment may create significant defects on 2D
NSs [
42
]. Concerning the optimal exfoliation conditions of 2D materials, it is currently
difficult to make a concluding mark because exfoliation conditions reported are different
in each study. Therefore, more systematic and comprehensive exfoliation experiments are
urgent to enlighten the optimal exfoliation conditions for ink formulation.

Nanomaterials 2021,11, 3441 4 of 21
Table 1. Two-dimensional materials produced by LPE.
Materials
Exfoliation 2D Materials
Ref.
Solvent Surfactant and
Binders Sonication Time (h) Thickness Lateral Size
Graphene DMF EC Bath 40 - 100–500 nm [43]
Graphene IPA PVP Bath 12 <10 nm 200 nm [44]
Graphene IPA PVP Bath 12 <5.9 nm 196 nm [29]
Graphene NMP Bath (20W) 9 Single 300 nm [45]
Graphene NMP CMC Bath 9 6 nm 121 nm [34]
Graphene Water PS1 salt Bath (300W) 72 <10 layer 400 nm [7]
Graphene Cyclohexanone EC
Probe (120W)
7 < 1 nm 30–100 nm [46]
Graphene Ethanol EC Probe (50W) 1.5 <2 nm 50 nm [40]
Graphene NMP
Probe (120W)
7 <10 layer 35–600 nm [17]
Graphene
BN
NMP
IPA
Probe (120W)
1.5 <8 nm 195 nm
450 nm [47]
BP NMP
Probe (120W)
1 3.6 nm 234 nm [22]
BP NMP Probe 12 3.37 nm 80.46 nm [20]
MoS2DMF EC Bath 48 <7 nm 40–100 nm [36]
MoS2Ethanol, water PVP Bath 48 - 100–200 nm [48]
MoS2Ethanol, water Bath (with
grinding) 2 1.2–8.5 nm 20–60 nm [32]
MoS2Ethanol EC Shear mixer 2 <6nm <100 nm [49]
2.2. Directing Ink Formulation
The dispersion of exfoliated 2D NSs can be directly transferred to the ink formula-
tion step. This direct ink formulation approach was widely adopted at an early stage of
2D NS-inkjet printing. After exfoliation, thin-layered 2D NSs coexist with unexfoliated
thick flakes in the dispersion. The remaining unexfoliated contents are removed by cen-
trifugation, leaving a supernatant of thin-layered 2D NSs to proceed to inkjet printing.
Centrifugation is a representative technique for classifying 2D materials into various lateral
sizes and thicknesses [
15
]. During centrifugation, 2D material flakes are precipitated at
different RPMs, depending on their dimensions [
50
]. Larger or thicker flakes tend to settle
because of their high mass ratio. Since the precipitation of 2D flakes is closely related to
the centrifugation RPMs, the stepwise increase in centrifugation rate enables the fine size
classification of 2D flake sediments (Figure 1a). The size selection by centrifugation also
tailors the concentrations of 2D NSs in the dispersion: the higher the RPM of centrifugation,
the lower the concentration of 2D NSs with smaller and thinner dimensions. Ding. et al.
observed that the concentration of graphene, MoS
2
, and BN NSs decreased at a higher
centrifugation rate where thicker and larger flakes likely precipitated at increased cen-
trifugation rate
(Figure 1b,c)
[
51
]. A statistical TEM analysis confirms the inverse-linear
relationship between 2D NS size and centrifugation rate (Figure 1d) [
50
]. Based on these
results, it is essential to find an appropriate centrifugation rate that satisfies both the con-
centration and dimension of 2D NSs for printing ink. It is particularly critical for the direct
ink formulation approach because the supernatant obtained after centrifugation is directly
used as printing ink.

Nanomaterials 2021,11, 3441 5 of 21
Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 21
Figure 1. (a) Schematic of the size selection by liquid cascade centrifugation. Reprinted with permission from Ref.
[52]
.
Copyright 2016 MyJoVE. (b) Photographs of aqueous dispersions of graphene, MoS
2
, and BN with different concentrations
after centrifugation at 500–10,000 rpm for 30 min; (c) Lambert–Beer plots for graphene, MoS
2
, and BN; dependence of 2D
material concentration on centrifugation speed for the supernatant. Reprinted with permission from Ref.
[51]
. Copyright
2018, IOP Publishing. (d) Mean nanosheet length (L), versus centrifugation rate (ω). Reprinted with permission from Ref.
[50]
. Copyright 2013 IOP Publishing.
As one of the pioneering works in direct ink formulation, Torrisi. et al. exfoliated
graphite flakes in NMP for 9 h using a sonic bath, followed by centrifugation at 10,000
rpm for 1 h to remove large flakes >1 μm in lateral size [45]. Given the optimal exfoliation
and centrifugation conditions, they prepared graphene ink primarily consisting of single,
bilayer, and few-layer graphene with a lateral size of 300~1000 nm. As mentioned
previously, NMP is not a suitable ink solvent, such that ethylene glycol was added to
increase the viscosity of the ink. Modifying the NMP-based graphene ink with the
additive successfully led to the formation of stable ink jetting but failed to form a uniform
pattern due to the mismatch between the surface tension of the ink and substrate. To
address the surface tension issue, the authors pretreated the substrate with
hexamethyldisilazane (HMDS) and improved the printing pattern quality. Nonetheless,
the NMP-based graphene ink could not pattern a fine straight line of around 100 μm on
the HMDS-treated substrate. The printed graphene pattern reached a maximum electrical
conductivity of ≈ 10
2
S/m (Figure 2j), which was much lower than that of a typical graphene
pattern recently developed (≈ 30
3
S/m) [53]. This study proves that the proper selection of
an ink solvent plays a critical role in determining the quality of a printing pattern and the
resulting electrical conductivity. However, the authors demonstrated the potential of
large-area fabrication of graphene devices by pioneering graphene ink formulation early
in this research field.
Figure 1.
(
a
) Schematic of the size selection by liquid cascade centrifugation. Reprinted with permission from Ref. [
52
].
Copyright 2016 MyJoVE. (
b
) Photographs of aqueous dispersions of graphene, MoS
2
, and BN with different concentrations
after centrifugation at 500–10,000 rpm for 30 min; (
c
) Lambert–Beer plots for graphene, MoS
2
, and BN; dependence of 2D
material concentration on centrifugation speed for the supernatant. Reprinted with permission from Ref. [
51
]. Copyright
2018, IOP Publishing. (
d
) Mean nanosheet length (L), versus centrifugation rate (
ω
). Reprinted with permission from
Ref. [50]. Copyright 2013 IOP Publishing.
As one of the pioneering works in direct ink formulation, Torrisi. et al. exfoliated
graphite flakes in NMP for 9 h using a sonic bath, followed by centrifugation at 10,000 rpm
for 1 h to remove large flakes >1
µ
m in lateral size [
45
]. Given the optimal exfoliation and
centrifugation conditions, they prepared graphene ink primarily consisting of single, bi-
layer, and few-layer graphene with a lateral size of 300~1000 nm. As mentioned previously,
NMP is not a suitable ink solvent, such that ethylene glycol was added to increase the
viscosity of the ink. Modifying the NMP-based graphene ink with the additive successfully
led to the formation of stable ink jetting but failed to form a uniform pattern due to the
mismatch between the surface tension of the ink and substrate. To address the surface
tension issue, the authors pretreated the substrate with hexamethyldisilazane (HMDS)
and improved the printing pattern quality. Nonetheless, the NMP-based graphene ink
could not pattern a fine straight line of around 100
µ
m on the HMDS-treated substrate.
The printed graphene pattern reached a maximum electrical conductivity of
≈
10
2
S/m
(Figure 2j),
which was much lower than that of a typical graphene pattern recently devel-
oped (
≈
30
3
S/m) [
53
]. This study proves that the proper selection of an ink solvent plays
a critical role in determining the quality of a printing pattern and the resulting electrical
conductivity. However, the authors demonstrated the potential of large-area fabrication of
graphene devices by pioneering graphene ink formulation early in this research field.
An ink solution prepared by dissolving PVP in isopropyl alcohol (IPA), a low boiling
point alcohol solvent, could be a promising approach to overcome the problems arising
from NMP. Juntunen et al. exfoliated bulk graphite flakes dispersed in IPA/PVP mixture

Nanomaterials 2021,11, 3441 6 of 21
using a bath sonicator for 12 h [
44
]. PVP in the dispersion prevented the graphene NSs
from aggregation and settlement, yielding a homogeneous graphene dispersion
(Figure 3a)
.
Fifty percent of the graphene NSs had a thickness of < 10 nm, and the average lateral
size was around 200 nm (Figure 3b). The supernatant was stable and met the demands
of surface tension and viscosity, and thus it could be directly used as an ink without
the aid of any additives. The IPA/PVP-based graphene ink was printed on a flexible
substrate for thermoelectric applications (Figure 3c). The printed graphene film showed
excellent thermoelectric properties and even retained the functions against mechanical
deformation tests for 10,000 bending cycles. However, the removal of PVP after printing
was necessary for the excellent thermoelectric properties, implying that a trade-off between
optimal ink stability and device performance dictated by the PVP concentration exists.
Therefore, systematic experiments concerning the effects of PVP on ink stability and device
performance should be carried out to find an optimal trade-off (Figure 3e,f).
Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 21
Figure 2. Dark-field optical micrograph of inkjet-printed drops on (a) plasma-cleaned, (b) pristine, and (c) HMDS-treated
substrate. Scale is 20 μm. Optical micrograph of inkjet-printed stripes on (d) pristine, (e) O
2
-treated and (f) HMDS-treated
substrates. AFM images of (d–i), respectively. (j) Conductivity (σ) as a function of thickness for HMDS-coated, O
2
-plasma-
treated and pristine substrates. Reprinted with permission from Ref.
[45]
. Copyright 2012 American Chemical Society.
An ink solution prepared by dissolving PVP in isopropyl alcohol (IPA), a low boiling
point alcohol solvent, could be a promising approach to overcome the problems arising
from NMP. Juntunen et al. exfoliated bulk graphite flakes dispersed in IPA/PVP mixture
using a bath sonicator for 12 h [44]. PVP in the dispersion prevented the graphene NSs
from aggregation and settlement, yielding a homogeneous graphene dispersion (Figure
3a). Fifty percent of the graphene NSs had a thickness of < 10 nm, and the average lateral
size was around 200 nm (Figure 3b). The supernatant was stable and met the demands of
surface tension and viscosity, and thus it could be directly used as an ink without the aid
of any additives. The IPA/PVP-based graphene ink was printed on a flexible substrate for
thermoelectric applications (Figure 3c). The printed graphene film showed excellent
thermoelectric properties and even retained the functions against mechanical deformation
tests for 10,000 bending cycles. However, the removal of PVP after printing was necessary
for the excellent thermoelectric properties, implying that a trade-off between optimal ink
stability and device performance dictated by the PVP concentration exists. Therefore,
systematic experiments concerning the effects of PVP on ink stability and device
performance should be carried out to find an optimal trade-off (Figure 3e,f).
Figure 2.
Dark-field optical micrograph of inkjet-printed drops on (
a
) plasma-cleaned, (
b
) pristine, and (
c
) HMDS-treated
substrate. Scale is 20
µ
m. Optical micrograph of inkjet-printed stripes on (
d
) pristine, (
e
) O
2
-treated and (
f
) HMDS-treated
substrates. AFM images of (
d
–
i
), respectively. (
j
) Conductivity (
σ
) as a function of thickness for HMDS-coated, O
2
-plasma-
treated and pristine substrates. Reprinted with permission from Ref. [45]. Copyright 2012 American Chemical Society.
Besides the graphene ink, MoS
2
ink was also prepared by the direct ink formulation
method. Yao. et al. selected a mixture of ethanol and water solvent as an exfoliation
medium to avoid using NMP but suffered from a low MoS
2
concentration of ~0.3 mg/mL,
insufficient for inkjet printing [
32
]. By grinding bulk MoS
2
flakes, followed by exfoliation
in the solvent, the authors prepared a high concentration of MoS
2
ink (26.7
±
0.7 mg/mL).
Glycerol was added to the ink to meet the viscosity and surface tension for its stable jetting.
Although the combined exfoliation strategy yielded a relatively small lateral size, ranging
from 20 to 60 nm, a printed MoS
2
sensing device was fabricated, capable of detecting NH
3
at several ppm levels for NH
3
. This study offers a promising way for how to increase the
concentration of 2D materials, as a solvent bringing low exfoliation efficiency is used as an
exfoliation medium.
The direct ink formulation can simplify the process by sharing the same solvent for the
exfoliation and printing. However, it is likely a challenge to achieve an appropriate balance
between exfoliation efficiency and printing stability. In addition, pre-or post-treatment
has been occasionally required to enhance the performance of both the printing and the
resulting devices.

Nanomaterials 2021,11, 3441 7 of 21
Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 21
Figure 3. Deposition and characterization of the graphene thin films: (a) deposition and annealing scheme of the exfoliated
few-layer graphene flakes suspended in an IPA/PVP solution; (b) AFM thickness distribution of graphene flakes after
annealing. (c) Photographs of an inkjet-printed device consisting of 20 silver and graphene legs bent (above) and as is
(below); thermoelectric and transport characterization of graphene films: (d) thickness, (e) resistivity as a function of ink
PVP concentration pre- and post-annealing; (f) post-annealing charge concentration with band overlap energy δE = 28.1 ±
2.3 meV as illustrated in the inset for two parabolic bands. Reprinted with permission from Ref.
[44]
, Copyright 2018 John
Wiley and Sons.
Besides the graphene ink, MoS
2
ink was also prepared by the direct ink formulation
method. Yao. et al. selected a mixture of ethanol and water solvent as an exfoliation
medium to avoid using NMP but suffered from a low MoS
2
concentration of ~0.3 mg/mL,
insufficient for inkjet printing [32]. By grinding bulk MoS
2
flakes, followed by exfoliation
in the solvent, the authors prepared a high concentration of MoS
2
ink (26.7 ± 0.7 mg/mL).
Glycerol was added to the ink to meet the viscosity and surface tension for its stable
jetting. Although the combined exfoliation strategy yielded a relatively small lateral size,
ranging from 20 to 60 nm, a printed MoS
2
sensing device was fabricated, capable of
detecting NH
3
at several ppm levels for NH
3
. This study offers a promising way for how
to increase the concentration of 2D materials, as a solvent bringing low exfoliation
efficiency is used as an exfoliation medium.
The direct ink formulation can simplify the process by sharing the same solvent for
the exfoliation and printing. However, it is likely a challenge to achieve an appropriate
balance between exfoliation efficiency and printing stability. In addition, pre-or post-
treatment has been occasionally required to enhance the performance of both the printing
and the resulting devices.
2.3. Solvent Exchange Ink Formulation
Solvent exchange ink formulation separates the exfoliation process and the ink
formulation process (Figure 4a). This implies that the whole ink formulation procedures
become more time-consuming and complicated than the direct ink formulation in which
Figure 3.
Deposition and characterization of the graphene thin films: (
a
) deposition and annealing scheme of the exfoliated
few-layer graphene flakes suspended in an IPA/PVP solution; (
b
) AFM thickness distribution of graphene flakes after
annealing. (
c
) Photographs of an inkjet-printed device consisting of 20 silver and graphene legs bent (above) and as
is (below); thermoelectric and transport characterization of graphene films: (
d
) thickness, (
e
) resistivity as a function
of ink PVP concentration pre- and post-annealing; (
f
) post-annealing charge concentration with band overlap energy
δE = 28.1 ±2.3 meV
as illustrated in the inset for two parabolic bands. Reprinted with permission from Ref. [
44
], Copyright
2018 John Wiley and Sons.
2.3. Solvent Exchange Ink Formulation
Solvent exchange ink formulation separates the exfoliation process and the ink for-
mulation process (Figure 4a). This implies that the whole ink formulation procedures
become more time-consuming and complicated than the direct ink formulation in which
the exfoliation and ink formulation can proceed in a continuous process. The main advan-
tages of a solvent exchange formulation are to employ high-boiling point solvents such as
NMP as an exfoliation medium to ensure the exfoliation efficiency and to control the ink
concentration to meet an adequate printing condition, regardless of the exfoliation yield.
A general strategy of solvent exchange ink formulation is as follows (Figure 4). The 2D
materials are exfoliated in a high-boiling point solvent. Then, the exfoliated 2D NSs are
collected by centrifugation at high RPMs. Lastly, the collected 2D NSs are re-dispersed in
new inkjet-friendly solvents.
For instance, Jun. et al. exfoliated bulk BP flakes in NMP using a sonic probe. NMP
was removed by a solvent exchange process, and the exfoliated BP NSs were replenished
with 2-methoxyethanol (2-ME) solvent, an inkjet-friendly solvent [
22
]. While exchanging
the solvent, the exfoliated BP NSs did not aggregate and remained in stable colloids with
the average lateral size and thickness of 234 nm and 3.6 nm, respectively
(Figure 5a).
Interestingly, the authors claimed that 2-ME-based BP NSs ink showed a very stable ink
jetting, free of additives and enabling a line pattern of 97
µ
m in width as well as complex
printing features (Figure 5b,c). They also developed the printed BP pattern into a multi-
inverse structure diode to demonstrate its potential for electronic devices (Figure 5d).

Nanomaterials 2021,11, 3441 8 of 21
Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 21
the exfoliation and ink formulation can proceed in a continuous process. The main
advantages of a solvent exchange formulation are to employ high-boiling point solvents
such as NMP as an exfoliation medium to ensure the exfoliation efficiency and to control
the ink concentration to meet an adequate printing condition, regardless of the exfoliation
yield. A general strategy of solvent exchange ink formulation is as follows (Figure 4). The
2D materials are exfoliated in a high-boiling point solvent. Then, the exfoliated 2D NSs
are collected by centrifugation at high RPMs. Lastly, the collected 2D NSs are re-dispersed
in new inkjet-friendly solvents.
Figure 4. Schematic of ink formulation: (a) direct ink formulation and (b) solvent exchange ink formulation.
For instance, Jun. et al. exfoliated bulk BP flakes in NMP using a sonic probe. NMP
was removed by a solvent exchange process, and the exfoliated BP NSs were replenished
with 2-methoxyethanol (2-ME) solvent, an inkjet-friendly solvent [22]. While exchanging
the solvent, the exfoliated BP NSs did not aggregate and remained in stable colloids with
the average lateral size and thickness of 234 nm and 3.6 nm, respectively (Figure 5a).
Interestingly, the authors claimed that 2-ME-based BP NSs ink showed a very stable ink
jetting, free of additives and enabling a line pattern of 97 μm in width as well as complex
printing features (Figure 5b,c). They also developed the printed BP pattern into a multi-
inverse structure diode to demonstrate its potential for electronic devices (Figure 5d).
Figure 4. Schematic of ink formulation: (a) direct ink formulation and (b) solvent exchange ink formulation.
Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 21
Figure 5. (a) Statistical dimensional analysis of exfoliated BP NSs; characterizations of the printed BP films upon printing
repetition (N): (b) patterned BP film on PET substrate (100 mm × 40 mm); (c) SEM image of printing line at droplet spacing
of 25 μm and 5 N; (d) diode characteristics of the single- and multi-structured BP film. Reprinted with permission from
Ref.
[22]
. Copyright 2021 John Wiley and Sons. (e) Photograph of the MoS
2
/EC ink; (f) inkjet-printed MoS
2
/EC lines on
glass (top) and polyimide (bottom). The width of the printed lines can be tuned with the number of rows of droplets per
line, as indicated by the red circles. The scale bar is 100 μm; (g) average height measured by profilometry from the as-
printed (initial), thermally annealed (TA), and photonically annealed (PA) MoS
2
/EC lines. Inset: height profiles from the
initial (black), TA (red, 177.65 ± 14.52 nm), and PA (blue, 124.68 ± 49.63 nm) lines after 7 printing passes, illustrating larger
standard deviation in thickness and morphological roughness after PA; (h) bending test over 500 cycles showing invariant
sensitivity. Inset: photograph of the flexible MoS
2
-Gr device. The scale bar is 3 mm. Reprinted with permission from Ref.
[49]
. Copyright 2019 American Chemical Society.
Seo. et al. exfoliated bulk MoS
2
flakes in a mixture of ethanol and EC using a sonic
bath to produce MoS
2
NSs with a thickness of <6 nm and a lateral size of <100 nm. Then,
MoS
2
NSs were re-dispersed in cyclohexanone/terpineol (C/T) mixture by a solvent
exchange process (Figure 5e) [49]. The C/T solvent has been widely used to improve the
dispersibility and printing stability of 2D NSs [40,43,54]. While cyclohexanone provides a
stable dispersion of MoS
2
NSs, terpineol plays multiplying effects in improving the
surface tension and viscosity for durable printing. The authors determined an optimal ink
concentration by exploring a wide range of ink concentrations and their impact on
printing. The printability of the ink was demonstrated by a successful line patterning of
100 μm in width, along with a proportional increase of the thickness over the printing
pass, ~100 nm increment at each pass (Figure 5f,g). The authors also investigated the
annealing effects on the thickness reduction and found that thermal- and photo-annealing
were similar in reducing the thickness. Printed MoS
2
patterns were fabricated for a
photodetector on flexible substrates that exhibited stable photosensitivity even against the
bending test of 500 cycles (Figure 5h).
With the advantages of the solvent exchange process brought by separating
exfoliation and ink formulation, the efficient ink design is affordable to meet the strict
requirements of inkjet printing ink. Therefore, this approach has been increasingly
adopted to formulate various 2D-based inks in diversified printing research fields.
3. Inkjet Printing of 2D Ink
The previous section dealt with the ink formulation process. The following section
discusses some critical parameters determining the quality of printing patterns and the
performance of the corresponding devices. Primarily, four essential metrics, including the
Figure 5.
(
a
) Statistical dimensional analysis of exfoliated BP NSs; characterizations of the printed BP films upon printing
repetition (N): (
b
) patterned BP film on PET substrate (100 mm
×
40 mm); (
c
) SEM image of printing line at droplet spacing
of 25
µ
m and 5 N; (
d
) diode characteristics of the single- and multi-structured BP film. Reprinted with permission from
Ref. [
22
]. Copyright 2021 John Wiley and Sons. (
e
) Photograph of the MoS
2
/EC ink; (
f
) inkjet-printed MoS
2
/EC lines
on glass (top) and polyimide (bottom). The width of the printed lines can be tuned with the number of rows of droplets
per line, as indicated by the red circles. The scale bar is 100
µ
m; (
g
) average height measured by profilometry from the
as-printed (initial), thermally annealed (TA), and photonically annealed (PA) MoS
2
/EC lines. Inset: height profiles from
the initial (black), TA (red, 177.65
±
14.52 nm), and PA (blue, 124.68
±
49.63 nm) lines after 7 printing passes, illustrating
larger standard deviation in thickness and morphological roughness after PA; (
h
) bending test over 500 cycles showing
invariant sensitivity. Inset: photograph of the flexible MoS
2
-Gr device. The scale bar is 3 mm. Reprinted with permission
from Ref. [49]. Copyright 2019 American Chemical Society.

Nanomaterials 2021,11, 3441 9 of 21
Seo. et al. exfoliated bulk MoS
2
flakes in a mixture of ethanol and EC using a sonic
bath to produce MoS
2
NSs with a thickness of <6 nm and a lateral size of <100 nm. Then,
MoS
2
NSs were re-dispersed in cyclohexanone/terpineol (C/T) mixture by a solvent
exchange process (Figure 5e) [
49
]. The C/T solvent has been widely used to improve the
dispersibility and printing stability of 2D NSs [
40
,
43
,
54
]. While cyclohexanone provides
a stable dispersion of MoS
2
NSs, terpineol plays multiplying effects in improving the
surface tension and viscosity for durable printing. The authors determined an optimal ink
concentration by exploring a wide range of ink concentrations and their impact on printing.
The printability of the ink was demonstrated by a successful line patterning of 100
µ
m in
width, along with a proportional increase of the thickness over the printing pass, ~100 nm
increment at each pass (Figure 5f,g). The authors also investigated the annealing effects
on the thickness reduction and found that thermal- and photo-annealing were similar
in reducing the thickness. Printed MoS
2
patterns were fabricated for a photodetector on
flexible substrates that exhibited stable photosensitivity even against the bending test of
500 cycles (Figure 5h).
With the advantages of the solvent exchange process brought by separating exfoliation
and ink formulation, the efficient ink design is affordable to meet the strict requirements of
inkjet printing ink. Therefore, this approach has been increasingly adopted to formulate
various 2D-based inks in diversified printing research fields.
3. Inkjet Printing of 2D Ink
The previous section dealt with the ink formulation process. The following section
discusses some critical parameters determining the quality of printing patterns and the
performance of the corresponding devices. Primarily, four essential metrics, including
the Ohnesorge number, coffee ring effect, droplet spacing, and percolation network, are
addressed in each subsection, exemplifying some representative works.
3.1. Mechanism of Ink Jet Printing
Inkjet printing can be divided into two categories based on operating mechanism:
continuous and drop-on-demand (DOD) (Figure 6a,b) [
16
]. In continuous printing, a
continuous ink-jetting stream is ejected from a nozzle but is immediately turned into a dis-
continuous phase by the stream’s surface tension, called Plateau–Rayleigh instability [
55
].
Although continuous printing allows high-speed operation, its inherent complexities aris-
ing from strict operational requirements limit its utilization to specific applications such as
label patterning. In contrast, DOD is a process in which ink droplets are ejected only on
demand via a piezoelectric or thermal response. In the piezoelectric one, a voltage pulse
stimulant induces the deformation of the piezoelectric material, leading to the formation
and release of ink droplets. In the thermal inkjet process, the ink quickly heats up, creating
air bubbles for the release of ink droplets. A DOD inkjet printing has been the primary
technique due to the merits of precisely controlling the droplet size. Additionally, while
the thermal DOD process generally requires volatile solvents such as water or alcohol,
the piezoelectric one can afford low volatile solvents because its operation relies on the
fluctuation of the piezoelectric material. The precise control in discharge rate and size of
a droplet is also available by simply tuning the operating voltage. Four critical metrics
that DOD printer users generally face during 2D NS-inkjet printing are discussed in the
following sections.

Nanomaterials 2021,11, 3441 10 of 21
Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 21
Ohnesorge number, coffee ring effect, droplet spacing, and percolation network, are
addressed in each subsection, exemplifying some representative works.
3.1. Mechanism of Ink Jet Printing
Inkjet printing can be divided into two categories based on operating mechanism:
continuous and drop-on-demand (DOD) (Figure 6a,b) [16]. In continuous printing, a
continuous ink-jetting stream is ejected from a nozzle but is immediately turned into a
discontinuous phase by the stream’s surface tension, called Plateau–Rayleigh instability
[55]. Although continuous printing allows high-speed operation, its inherent complexities
arising from strict operational requirements limit its utilization to specific applications
such as label patterning. In contrast, DOD is a process in which ink droplets are ejected
only on demand via a piezoelectric or thermal response. In the piezoelectric one, a voltage
pulse stimulant induces the deformation of the piezoelectric material, leading to the
formation and release of ink droplets. In the thermal inkjet process, the ink quickly heats
up, creating air bubbles for the release of ink droplets. A DOD inkjet printing has been the
primary technique due to the merits of precisely controlling the droplet size. Additionally,
while the thermal DOD process generally requires volatile solvents such as water or
alcohol, the piezoelectric one can afford low volatile solvents because its operation relies
on the fluctuation of the piezoelectric material. The precise control in discharge rate and
size of a droplet is also available by simply tuning the operating voltage. Four critical
metrics that DOD printer users generally face during 2D NS-inkjet printing are discussed
in the following sections.
Figure 6. Schematics of DoD inkjet printing with (a) piezoelectric and (b) thermal head. Representative photo sequence of
drop formation for fluids with (c) Z = 4.08 and (d) Z = 13.68 at a constant driving voltage of 25 V. Reprinted with permission
from Ref.
[56]
. Copyright 2009 American Chemical Society. (e) Graph showing the change in viscosity of the five solvents
with the addition of EC and area of recommended viscosity values for inkjet printing. Inset shows successful drop
formation and ejection from the printer nozzle with minimal satellite droplets. Reprinted with permission from Ref.
[41]
.
Copyright 2016 IOP Publishing. (f) Graphene line obtained with a water based ink with Z ≈ 20 and excess surfactant which
aggregates in the centre of the dots. Reprinted with permission from Ref. [7]. Copyright 2017 Springer Nature.
3.2. Ohnesorge Number
To accomplish a stable jetting discharge in a DOD inkjet printing, several vital
parameters should be considered. The ink’s surface tension and viscosity play a critical
role in determining the droplet size and behavior. The nozzle size of an inkjet cartridge
also has an impact on the proper ejection of ink droplets. If these parameters are not
correctly adjusted, ink jetting may be impossible or satellite droplets may be generated
[56]. Because satellite droplets are likely to land on untargeted areas of the substrate, they
should be suppressed. The Reynolds number and Weber number can be the metrics to
characterize droplet dynamics by correlating all these parameters [57]. The inverse
Figure 6.
Schematics of DoD inkjet printing with (
a
) piezoelectric and (
b
) thermal head. Representative photo sequence of
drop formation for fluids with (
c
) Z = 4.08 and (
d
) Z = 13.68 at a constant driving voltage of 25 V. Reprinted with permission
from Ref. [
56
]. Copyright 2009 American Chemical Society. (
e
) Graph showing the change in viscosity of the five solvents
with the addition of EC and area of recommended viscosity values for inkjet printing. Inset shows successful drop formation
and ejection from the printer nozzle with minimal satellite droplets. Reprinted with permission from Ref. [
41
]. Copyright
2016 IOP Publishing. (
f
) Graphene line obtained with a water based ink with Z
≈
20 and excess surfactant which aggregates
in the centre of the dots. Reprinted with permission from Ref. [7]. Copyright 2017 Springer Nature.
3.2. Ohnesorge Number
To accomplish a stable jetting discharge in a DOD inkjet printing, several vital pa-
rameters should be considered. The ink’s surface tension and viscosity play a critical role
in determining the droplet size and behavior. The nozzle size of an inkjet cartridge also
has an impact on the proper ejection of ink droplets. If these parameters are not correctly
adjusted, ink jetting may be impossible or satellite droplets may be generated [
56
]. Because
satellite droplets are likely to land on untargeted areas of the substrate, they should be
suppressed. The Reynolds number and Weber number can be the metrics to characterize
droplet dynamics by correlating all these parameters [
57
]. The inverse Ohnesorge num-
ber, derived by integrating the Reynolds number and the Weber number, becomes more
relevant to predict droplet dynamics [58].
Z=
1
Oh
=
√γρa
η(1)
where
γ
,
ρ
, and
η
denote the surface tension (mN m
−1
), density (g cm
−3
) and viscosity
(mPa s) of an ink, respectively, and
a
is the diameter of a jetting nozzle. It has been known
that if the ink has a Z value of 1 < Z < 14, stable droplets can be steadily formed. At high
viscosity (Z < 1), the ink cannot discharge from the nozzle, and at low viscosity (Z > 14),
it likely forms satellite droplets. Figure 6c shows a stable droplet jet at Z = 4.08, and a
satellite droplet at Z = 17.32 (Figure 6d). Therefore, the Ohnesorge number offers guidance
on the parameter range for steady ink jetting as well as the prediction of printing pattern
quality of DOD inkjet printing [
56
,
58
]. Among the parameters of the Ohnesorge number,
the viscosity is the most influential in the ink rheology, and by adding polymer surfactants
such as EC, it can be easily tailored. Michel. et al. reported that the printable viscosity range
was achieved after adding EC to various solvents such as C/T, IPA, NMP, DMF, and N,N-
Dimethylacetamide (DMA), demonstrating EC’s versatile role in adjusting the viscosity for
printing [
41
]. This surfactant also disrupts agglomeration and sedimentation of 2D NSs in
ink, preventing nozzle clogging. In this context, an optimal surfactant amount should be
determined in each printing trial because its overuse has a detrimental effect on the printing.
For example, the polymer surfactant inhibits the conductivity of printed graphene patterns.

Nanomaterials 2021,11, 3441 11 of 21
Although high-temperature annealing enables the removal of the surfactants, their residue
is still present in the printing pattern. Moreover, some 2D materials including BP NSs
quickly degrade via oxidation in atmospheric conditions [
20
]. Because high-temperature
annealing accelerates oxidation, surfactant addition should be avoided during BP ink
formulation. Excessive use of the surfactant also causes aggregation of 2D NSs on the
substrate after printing. In Figure 6f, a printed graphene line was initially intended using a
water-based graphene ink modified with surfactant. The remained surfactant interrupted
the uniform distribution of graphene and instead led to graphene’s aggregation to form an
array of irregular dot patterns [
7
]. These results indicate that polymer surfactants should
be carefully manipulated, taking into account the type of 2D materials, applied substrate,
and target patterning.
3.3. Coffee Ring Effect
Once an ink droplet is stably jetted from a nozzle, it settles on the substrate and
proceeds to spreading and drying. The spreading of ink over a substrate is defined by
wetting, which can be explained by Young’s equation:
γsv =γsi +γiv cos θ(2)
where
γsv
,
γiv
and
γsi
represent the respective interfacial tensions between the substrate
(s), the vapor (v), and the ink (i), and
θ
is the formed contact angle. The spreading
and drying rate of the droplet depends on the surface energy difference between the
droplet and substrate, depicted by the wettability. It is generally known that adequate
wettability can be achieved when the surface tension of the ink is 7–10 mN m
−1
lower
than the substrate’s surface energy [
16
]. There are many practical substrates used in
printing industries, including Si/SiO
2
and glass. Recently, wearable substrates such as
polyimide (PI), polyethylene terephthalate (PET), Kapton, cotton, and paper are also
increasingly applied [
17
,
29
,
32
,
37
,
59
,
60
]. Because these substrates have a different intrinsic
surface energy, the surface tension of the ink should be properly engineered to ensure the
appropriate wettability. However, as discussed previously, an ink solvent is concerned with
the exfoliation efficiency, ink stability, and steady ink jetting, such that simply engineering
ink toward the proper wettability is not straightforward. A more feasible way to achieve
good wettability is through the surface treatment of the substrate. The most common
surface treatment utilizes O
2
plasma or UV/O
3
, improving the wettability for lower surface
energy [
61
,
62
]. These surface treatments allow large-area surface modification in a simple
manner, along with time-dependent wettability control by forming hydroxyl groups on
the surface of the substrate. Park. et al. revealed that O
2
plasma treatment time changed
the diameter of a printed silver dot (Figure 7a) [
62
]. The diameter of a silver dot reached
up to 90
µ
m from 40
µ
m after plasma treatment for 90 s. Functionalizing the surface with
HMDS molecules can lower the wettability. During HMDS treatment, a hydroxyl group
(
−
OH) is replaced with a silane group (
−
Si(CH
3
)
3
), resulting in higher surface energy. This
treatment is frequently beneficial, particularly for the direct printing of an NMP-based 2D
NS ink [
45
]. These indicate that adjusting the surface wettability into higher or lower one
exclusively depends on the type of applied ink. Nonetheless, fine control of the surface
wettability through the treatment offers the potential to diversify ink solvents, which would
expand inkjet printing toward various 2D materials.

Nanomaterials 2021,11, 3441 12 of 21
Nanomaterials 2021, 11, x FOR PEER REVIEW 12 of 21
common surface treatment utilizes O
2
plasma or UV/O
3
, improving the wettability for
lower surface energy [61,62]. These surface treatments allow large-area surface
modification in a simple manner, along with time-dependent wettability control by
forming hydroxyl groups on the surface of the substrate. Park. et al. revealed that O
2
plasma treatment time changed the diameter of a printed silver dot (Figure 7a) [62]. The
diameter of a silver dot reached up to 90 μm from 40 μm after plasma treatment for 90 s.
Functionalizing the surface with HMDS molecules can lower the wettability. During
HMDS treatment, a hydroxyl group (−OH) is replaced with a silane group (−Si(CH
3
)
3
),
resulting in higher surface energy. This treatment is frequently beneficial, particularly for
the direct printing of an NMP-based 2D NS ink [45]. These indicate that adjusting the
surface wettability into higher or lower one exclusively depends on the type of applied
ink. Nonetheless, fine control of the surface wettability through the treatment offers the
potential to diversify ink solvents, which would expand inkjet printing toward various
2D materials.
Figure 7. (a) Variation in inkjet-printed dot diameter as a function of O
2
or Ar plasma treatment time. Reprinted with
permission from Ref.
[62]
. Copyright 2013 Elsevier. (b) Schematic drying process showing CRE formation; (c) inverted
optical micrographs of dried inkjet-printed droplets on clean glass: common solution-processed 2D crystal dispersions.
Formulated inks via solvent exchange in IPA or binary solvents of IPA/ethanol (10 volume %), IPA/2-butanol (10 and 20
volume %), and IPA/t-butanol (10 volume %). Reprinted with permission from Ref.
[23]
, Copyright 2020 American
Association for the Advancement of Science. (d) SEM image map for printed droplets in rows with increasing substrate
temperature and in columns from left to right with increasing mean GO flake size. The dotted line indicates the parameter
space where the coffee ring is fully suppressed. All scale bars are 100 μm. Reprinted with permission from Ref.
[42]
.
Copyright 2017 John Wiley and Sons.
Following decent contact of an ink droplet to the substrate, an ink droplet diffuses
and evaporates over the substrate simultaneously. Due to the low ink viscosity, printed
2D NSs tend to concentrate on the pattern edges upon evaporation, leading to the
formation of a coffee ring and subsequently a concave pattern (Figure 7b) [23]. The coffee
ring is triggered as the solvent evaporates more rapidly at the edge of the droplet than the
central area, creating a contact line pinned at the edge. The capillary flow also promotes
the spreading of the solvent toward the edge, accelerating the coffee ring formation
(Figure 7b). A non-uniform pattern degrades device performances significantly and
disrupts the reproducible and scalable fabrication of printing patterns. Therefore, effort
has been made to suppress coffee ring formation. Representative of this, Hu. Et al.
Figure 7.
(
a
) Variation in inkjet-printed dot diameter as a function of O
2
or Ar plasma treatment time. Reprinted with
permission from Ref. [
62
]. Copyright 2013 Elsevier. (
b
) Schematic drying process showing CRE formation; (
c
) inverted
optical micrographs of dried inkjet-printed droplets on clean glass: common solution-processed 2D crystal dispersions.
Formulated inks via solvent exchange in IPA or binary solvents of IPA/ethanol (10 volume %), IPA/2-butanol (10 and
20 volume %), and IPA/t-butanol (10 volume %). Reprinted with permission from Ref. [
23
], Copyright 2020 American
Association for the Advancement of Science. (
d
) SEM image map for printed droplets in rows with increasing substrate
temperature and in columns from left to right with increasing mean GO flake size. The dotted line indicates the parameter
space where the coffee ring is fully suppressed. All scale bars are 100
µ
m. Reprinted with permission from Ref. [
42
].
Copyright 2017 John Wiley and Sons.
Following decent contact of an ink droplet to the substrate, an ink droplet diffuses and
evaporates over the substrate simultaneously. Due to the low ink viscosity, printed 2D NSs
tend to concentrate on the pattern edges upon evaporation, leading to the formation of a
coffee ring and subsequently a concave pattern (Figure 7b) [
23
]. The coffee ring is triggered
as the solvent evaporates more rapidly at the edge of the droplet than the central area,
creating a contact line pinned at the edge. The capillary flow also promotes the spreading
of the solvent toward the edge, accelerating the coffee ring formation
(Figure 7b).
A non-
uniform pattern degrades device performances significantly and disrupts the reproducible
and scalable fabrication of printing patterns. Therefore, effort has been made to suppress
coffee ring formation. Representative of this, Hu. Et al. harnessed the Marangoni effect to
relieve the coffee ring effect [
23
]. The Marangoni flow involves the mass transfer driven by
a surface tension gradient of solvents in a solvent mixture, such that adequately tuning
a surface tension gradient by mixing two solvents could manipulate the flow dynamics
of the solvents and therefore the particle position on the substrate as well. Inspired
by this mechanism, the authors prepared a series of mixtures comprising two solvents
with different surface tensions and investigated the coffee ring formation of the mixtures
(Figure 7c)
. They found that a mixture of IPA and 2-butanol is the best formulation to
suppress the coffee ring formation. In this system, a uniform 2D NS pattern was printed,
although different 2D NSs demanded different combinations of solvents.
He. et al. claimed that tailoring the size of 2D NSs and the substrate temperature
could address the coffee ring issue [
42
]. The comparison of printed graphene oxide
(GO) morphology acquired from various experimental conditions suggested elevated
temperature is generally beneficial for weakening the coffee ring effect (Figure 7d). When

Nanomaterials 2021,11, 3441 13 of 21
it comes to the size effect of GO, using larger GO was preferred to suppress the coffee ring
formation, and this size effect became more pronounced at elevated temperatures.
Recently, the ink concentration was reported to be an effective parameter in controlling
the coffee ring effect. Jun. et al. varied the concentration of BP NSs in ink in a wide
range and found that a concentration over 1 mg/mL effectively weakened the coffee ring
effect [
22
]. However, it is unclear whether the concentration is solely an influential factor
for coffee ring inhibition. It is also uncertain that this concentration effect still holds for 2D
NSs other than BP NSs.
3.4. Droplet Spacing
Vital parameters involving DOD inkjet printing are substrate temperature and droplet
spacing that define the morphology of printed patterns. Figure 8a is a schematic diagram
to show how ink droplets complete a line pattern. Ink droplets ejected from a nozzle
sequentially combine with the neighboring one to constitute a line pattern. The droplet
spacing refers to the distance between ink droplets sitting on the substrate (Figure 8b). Since
droplet spacing significantly influences the morphology of printing patterns such as line
width, thickness, and uniformity, it should be explored to find the optimum [
20
]. Figure 8c
exhibits the line width variation of printed BP NSs with respect to droplet spacing [
22
]. At
a droplet spacing of less than 20
µ
m, droplets began to overlap considerably and eventually
formed a broad line up to 280
µ
m at a 10
µ
m droplet spacing. On the other hand, when
droplet spacing was 25
µ
m, the printing line width was reduced almost one-third to 97
µ
m
and remained consistent beyond that. This reflects the existence of a droplet spacing
threshold that limits the line width control. Hu. et al. studied the correlating effects of
droplet spacing and substrate temperature on the morphology of printed BP patterns [
20
].
A narrow droplet spacing caused the droplets to overlap significantly, resulting in a broader
line (stacked coins in Figure 8d). A drop spacing from 75
µ
m through 85
µ
m led to the
formation of scalloped lines due to insufficient ink to merge, and a drop spacing beyond
that resulted in isolated droplets that were too distant to merge. In this experiment, an
optimal droplet spacing was revealed to range from 35
µ
m to 65
µ
m within which a uniform
line pattern was printed. Because the substrate temperature affects the evaporation rate
of the solvent and behaviors of printed 2D NSs, printing morphology could vary with
the temperature applied. As shown in Figure 8e, both the morphology uniformity and
optimal droplet spacing range were changed, compared to those at a constant substrate
temperature (Figure 6d).
3.5. Percolation Network
The percolation theory describes a state of connectivity between particles and can
be divided into three phases of the state (Figure 9a). In an isolation state, particles are
separated with little contact. Then, more particles participate in the connection and form
into clusters, called percolation clusters. Although the percolation clusters are established
through the linking of many particles, they are still insufficient to form a network structure
across the entire surface. By engaging more particles with the percolation clusters, the
percolation networks are completed [
63
,
64
]. Many materials, such as carbon nanotubes,
graphene, and nanocomposites, have been tested to attempt to reach the percolation
networks via inkjet printing [
65
,
66
]. The percolation theory can also be applied to assess
the connectivity of printed 2D NSs. Particularly, the electrical conductivity of printed
2D NSs is a reflection of the percolation state. The electrical conductivity of printed 2D
NSs rapidly increases at the entry of the percolation networks, so-called the percolation
threshold. Figure 9b displays that printing repetition (N) could control the progress of the
percolation process for printed MoS
2
NSs. At a low N value between 3~5, MoS
2
NSs were
within isolation or percolation clusters, and thus the electrical conductivity was inferior.
As printing repetition approached 15, MoS
2
NSs were deposited in a high population,
enough to fulfill the percolation threshold and permitted electrons to flow through the
percolation networks. Once the percolation threshold was met, additional printing passes

Nanomaterials 2021,11, 3441 14 of 21
no longer helped to increase the electrical conductivity. It is reasoned that larger 2D NSs
are pursued to establish the percolation threshold for the least number of printing passes.
Larger 2D NSs are also beneficial for high electrical conductivity because electrons can
flow better in a low density of grain boundaries. That is why an optimal exfoliation
condition should be explored to produce larger and thinner 2D NSs during the exfoliation
process. As demonstrated by the printed MoS
2
NSs, increasing the printing passes is
the most straightforward way to achieve the percolation threshold of 2D NSs. Another
example belongs to printed BP NSs whose percolation network was formed by repeating
the printing pass up to 8 times (Figure 9c). The morphology of printed BP NSs taken at
different printing repetitions corresponded well to the electrical conductivity results. It
seems that printed BP NSs featured percolation clusters at 5 N and reached the percolation
network at around 10 N. Although printing repetition offers the most straightforward
way to produce the percolation networks, it is achieved by compensating printing time
and efficiency. Therefore, ink concentration and droplet spacing should also be tailored
concurrently to accomplish the percolation network efficiently.
Nanomaterials 2021, 11, x FOR PEER REVIEW 14 of 21
Figure 8. (a) Schematic of 2D material patterning process and (b) droplet spacing. Characterizations of the printed BP films
upon printing repetition (N): (c) printing line width variation with respect to droplet spacing. Reprinted with permission
from Ref.
[22]
. Copyright 2021 John Wiley and Sons. (d) Optimization of BP printing conditions. a BP printed on Si/SiO
2
at 60 °C showing the effect of droplet spacing on line morphology, photos taken by printer fiducial camera, scale bar, 100
μm; (e) effect of droplet spacing and printing temperature on the roughness along line edges, the roughness from uniform
to stacked coins is defined as negative. Reprinted with permission from Ref.
[20]
. Copyright 2017 Springer Nature.
3.5. Percolation Network
The percolation theory describes a state of connectivity between particles and can be
divided into three phases of the state (Figure 9a). In an isolation state, particles are
separated with little contact. Then, more particles participate in the connection and form
into clusters, called percolation clusters. Although the percolation clusters are established
through the linking of many particles, they are still insufficient to form a network
structure across the entire surface. By engaging more particles with the percolation
clusters, the percolation networks are completed [63,64]. Many materials, such as carbon
nanotubes, graphene, and nanocomposites, have been tested to attempt to reach the
percolation networks via inkjet printing [65,66]. The percolation theory can also be applied
to assess the connectivity of printed 2D NSs. Particularly, the electrical conductivity of
printed 2D NSs is a reflection of the percolation state. The electrical conductivity of printed
2D NSs rapidly increases at the entry of the percolation networks, so-called the percolation
threshold. Figure 9b displays that printing repetition (N) could control the progress of the
percolation process for printed MoS
2
NSs. At a low N value between 3~5, MoS
2
NSs were
within isolation or percolation clusters, and thus the electrical conductivity was inferior.
As printing repetition approached 15, MoS
2
NSs were deposited in a high population,
enough to fulfill the percolation threshold and permitted electrons to flow through the
percolation networks. Once the percolation threshold was met, additional printing passes
no longer helped to increase the electrical conductivity. It is reasoned that larger 2D NSs
are pursued to establish the percolation threshold for the least number of printing passes.
Larger 2D NSs are also beneficial for high electrical conductivity because electrons can
flow better in a low density of grain boundaries. That is why an optimal exfoliation
condition should be explored to produce larger and thinner 2D NSs during the exfoliation
process. As demonstrated by the printed MoS
2
NSs, increasing the printing passes is the
Figure 8.
(
a
) Schematic of 2D material patterning process and (
b
) droplet spacing. Characterizations of the printed BP films
upon printing repetition (N): (
c
) printing line width variation with respect to droplet spacing. Reprinted with permission
from Ref. [
22
]. Copyright 2021 John Wiley and Sons. (
d
) Optimization of BP printing conditions. a BP printed on Si/SiO
2
at
60
◦
C showing the effect of droplet spacing on line morphology, photos taken by printer fiducial camera, scale bar, 100
µ
m;
(
e
) effect of droplet spacing and printing temperature on the roughness along line edges, the roughness from uniform to
stacked coins is defined as negative. Reprinted with permission from Ref. [20]. Copyright 2017 Springer Nature.

Nanomaterials 2021,11, 3441 15 of 21
Nanomaterials 2021, 11, x FOR PEER REVIEW 15 of 21
most straightforward way to achieve the percolation threshold of 2D NSs. Another
example belongs to printed BP NSs whose percolation network was formed by repeating
the printing pass up to 8 times (Figure 9c). The morphology of printed BP NSs taken at
different printing repetitions corresponded well to the electrical conductivity results. It
seems that printed BP NSs featured percolation clusters at 5 N and reached the percolation
network at around 10 N. Although printing repetition offers the most straightforward
way to produce the percolation networks, it is achieved by compensating printing time
and efficiency. Therefore, ink concentration and droplet spacing should also be tailored
concurrently to accomplish the percolation network efficiently.
Figure 9. (a) Schematic of percolation theory: isolation flakes, percolation clusters, percolation networks. (b) Optical
micrograph of the printed MoS
2
transistors on Si/SiO
2
wafers with different printing passes N; conductivity variation of
the printed BP films with respect to N. Reprinted with permission from Ref.
[36]
. Copyright 2014 John Wiley and Sons.
(c) SEM images of BP films printed at 5 N, 10 N, 15 N, and 20 N (Scale bar = 10 μm); conductivity variation of the printed
BP films with respect to N. Reprinted with permission from Ref.
[22]
. Copyright 2021 John Wiley and Sons.
4. Application
The primary goal of this review is to account for ink formulation and printing
parameters that affect printing pattern quality and printing efficiency. Several review
articles have already been published to explain the applications of printed 2D NSs in
detail. Therefore, this section summarizes the application trends of printed 2D NSs
without addressing a specific work. As outlined in Table 2, a variety of 2D NS inks have
been formulated for inkjet printing. The vast majority of printed 2D NSs belong to
graphene and GO due to their unique properties and excellent processibility. The high
intrinsic electrical conductivity of graphene is attractive in some research fields, including
printed electrodes and microsupercapacitors. Graphene also features excellent thermal
conductivity, finding applications for thermal treatment. Although GO is inferior to
graphene in thermal and electrical conductivity, its excellent processibility makes GO ink
formulation more feasible for various applications. In particular, GO could be reduced to
Figure 9.
(
a
) Schematic of percolation theory: isolation flakes, percolation clusters, percolation networks. (
b
) Optical
micrograph of the printed MoS
2
transistors on Si/SiO
2
wafers with different printing passes N; conductivity variation of
the printed BP films with respect to N. Reprinted with permission from Ref. [
36
]. Copyright 2014 John Wiley and Sons.
(
c
) SEM images of BP films printed at 5 N, 10 N, 15 N, and 20 N (Scale bar = 10
µ
m); conductivity variation of the printed BP
films with respect to N. Reprinted with permission from Ref. [22]. Copyright 2021 John Wiley and Sons.
4. Application
The primary goal of this review is to account for ink formulation and printing parame-
ters that affect printing pattern quality and printing efficiency. Several review articles have
already been published to explain the applications of printed 2D NSs in detail. Therefore,
this section summarizes the application trends of printed 2D NSs without addressing a
specific work. As outlined in Table 2, a variety of 2D NS inks have been formulated for
inkjet printing. The vast majority of printed 2D NSs belong to graphene and GO due
to their unique properties and excellent processibility. The high intrinsic electrical con-
ductivity of graphene is attractive in some research fields, including printed electrodes
and microsupercapacitors. Graphene also features excellent thermal conductivity, finding
applications for thermal treatment. Although GO is inferior to graphene in thermal and
electrical conductivity, its excellent processibility makes GO ink formulation more feasible
for various applications. In particular, GO could be reduced to recover the intrinsic proper-
ties of graphene to the extent of being applicable for chemical sensors and photodetectors.
Following graphene and its derivatives, various 2D materials have emerged for inkjet
printing applications, and ink formulation strategies have correspondingly diversified to
accommodate them. The successful settlement of these materials in inkjet printing has
led printed 2D NSs to expand their applications to the semiconductor area that graphene
families are hardly used due to an almost zero band gap. Particularly, BP has a tunable
band gap depending on its thickness, and thus printed BP NSs were employed for diode
and photodetector. Other semiconducting 2D NSs such as MoS
2
and WS
2
could also
share similar applications with BP. BN, an intrinsically insulating material, has been fre-
quently printed for some applications. Since MXene appeared as a promising alternative to

Nanomaterials 2021,11, 3441 16 of 21
graphene, printed MXene has recently attracted considerable interest from many research
fields. The represented applications of printed MXene include microsupercapacitors and
energy storage devices, such as rechargeable batteries. The advantage of MXene over
graphene is its potential for scalable production and improved processibility, although the
exfoliation process of MXene generally involves harsh environments. Hybrid inks formu-
lated by mixing 2D NSs with some foreign materials have been printed to harness either
the synergetic effects or multiplying effects, consequently improving the performance of
the target devices.
With a wide range of 2D NSs available for ink formulation and the scalable patterning
of inkjet printing, printed 2D NSs become the center of electronics and miniaturized
energy storage devices. The post-treatment process to decompose additives after printing
has also advanced to fabricate printed 2D NSs for wearable electronics. The fabrication
strategies for flexible and stretchable electronics have been driven by the huge demand for
wearable, intelligent, and integrated electronics systems. Wearable electronic applications
require functional materials and manufacturing strategies. The inkjet printing process
and 2D material inks are new trends in the field of printing electronics for flexible and
stretchable electronics [
67
,
68
]. Based on the enormous progress made so far, it is believed
that printed 2D NSs will offer a promising platform for manufacturing next-generation
wearable devices, sensors, biodevices, and energy conversion devices.
Table 2. Two-dimensional material inks and their printable applications.
Materials
Exfoliation Ink Formulation
Substrate Application Ref.
Solvent Surfactant
and Binders Solvent Surfactant
and Binders
2D Material inks
Graphene
Cyclohexanone
EC
Cyclohexanone
EC Si/SiO2, PI,
PET Conductive ink [46]
Graphene DMF C/T EC Kapton,
Glass
Microsupercapacitors
[43]
Graphene DMF EC Terpineol/
ethanol
Kapton,
Glass
Microsupercapacitors
[37]
Graphene Ethanol EC C/T EC Si/SiO2Conductive ink [40]
Graphene IPA PVP IPA PVP Si/SiO2Thermoelectrics [44]
Graphene IPA PVP IPA PVP Glass Solar cells [29]
Graphene NMP NMP Ethylene
glycol Si/SiO2Conductive ink [45]
Graphene,
h-BN
NMP
Water CMC Ethanol
Water Textile Conductive ink [34]
Graphene,
MoS2
IPA, C/T,
DMA, DMF,
NMP
EC NMP EC Si/SiO2, PET Conductive ink [41]
Graphene
BN
NMP
IPA
NMP
IPA PET Capacitors [47]
Graphene,
MoS2
IPA, C/T,
NMP C/T EC PET, Pi Photodetectors [60]
Graphene,
MoS2NMP NMP Glass Photodetectors [69]
Graphene,
MoS2NMP NMP PET Conductive ink [17]
Graphene,
WS2Water PS1 salt Water
Triton x-100,
Propylene
glycol
Paper Photodetectors [70]
Graphene,
WS2, MoS2,
BN
Water PS1 salt Water
Triton x-100,
Propylene
glycol
Si/SiO2, PI,
Quartz, PET
Photodetectors,
memory [7]

Nanomaterials 2021,11, 3441 17 of 21
Table 2. Cont.
Materials
Exfoliation Ink Formulation
Substrate Application Ref.
Solvent Surfactant
and Binders Solvent Surfactant
and Binders
BP NMP 2ME Si/SiO2Diode [22]
BP NMP, CHP,
IPA IPA 2-butanol Si/SiO2,
Glass, PET Photonic device [20]
MoS2Water/IPA PVP Water/IPA Propylene
glycol PI
Microsupercapacitors
[18]
MoS2DMF PVP IPA PVP Si/SiO2CMOS logic [71]
MoS2DMF EC
DMF/
Terpineol/
Ethanol
Si/SiO2Photodetectors [36]
MoS2Ethanol EC C/T Glass, PI Photodetectors [49]
MoS2Ethanol,
water PVP
Ethanol/water
/
n-propanol
Glycerol/ethylene
glycol paper Conductive ink [48]
MoS2Ethanol,
water Ethanol/water Glycerol Si/SiO2Gas sensor [32]
MXene Water NMP Paper Energy Storage
Devices [72]
MXene Water DMSO PET Electromagnetic
shielding [59]
MXene Water, NMP Water, NMP
DMSO/DMF/
Ethanol
PET, Glass,
Kapton
Microsupercapacitors
[9]
MXene IPA IPA Si/SiO2, PET Laser [21]
MXene, GO
Water Water
Triton x-100,
Propylene
glycol
Si/SiO2, PI
Microsupercapacitors
[73]
WS2C/T EC C/T EC Si/SiO2Photodiode [54]
rGO Water PVA Water Glycerol/
Triton x-100
Si/SiO2,
Cotton fabric
Wearable
application [74]
GO Water Ethylene
glycol Water Ethylene
glycol Glass H2O2sensor [38]
GO Water Water Si/SiO2Conductive ink [42]
Hybrid ink
Graphene/Ag
C/T EC C/T EC Si/SiO2Conductive ink [75]
MXene/
PEDOT:PSS
Water Water Ethylene
glycol PET
Microsupercapacitors
[76]
Mxene/Ag NMP Water Ethylene
glycol PET, PEN Touchless sensor [77]
MXene/GO
Water Water Nafion
polymer
Glass, gold
foil
Hydrogen
peroxide sensor [78]
5. Summary and Future Prospects
This review underscores the optimizing conditions for ink formulation and printing
parameters for inkjet printing of 2D NSs by compiling the strategies that have been sug-
gested over the years. Table 3lists the relationships to correlate the printing strategies
with the vital printing parameters that determine the printing quality of 2D NSs. The
exfoliation process correlates with the dimension of exfoliated 2D NSs, ink concentration,
and solvent and additives. Droplet stability is primarily determined by ink concentration,
solvent, and additives. The coffee ring effect and percolation network involve nearly all
of the printing strategies. These relationships indicate that a printing strategy interplays
with several printing parameters, such that it should be tailored interactively. For ink
formulation, exfoliation of bulk 2D materials and ink stability should be first considered.
Using a suitable exfoliation solvent could improve the exfoliation efficiency, but it could
compromise the ink stability and ink jetting. Adding surfactants to ink solvent could

Nanomaterials 2021,11, 3441 18 of 21
improve the ink stability, but the surfactants may adversely affect the quality of printed
2D NSs. In this regard, the most efficient way for ink formulation is to employ the solvent
exchange ink formulation, with which the efficient exfoliation of 2D NSs and stable ink
dispersion could be achieved.
Table 3. Factors for 2D material printing performance and their correlation with each parameter.
Printing
Metrics
Dimension
of 2D NSs
Ink
Concentration
Solvent and
Additive
Substrate
Treatment
Substrate
Temperature
Droplet
Spacing
Printing
Repetition
Exfoliation 4 4 4
Droplet
stability 4 4
Coffee ring
effect 4 4 4 4 4 4
Percolation
network 4 4 4 4 4 4 4
In recent years, the applications of printed 2D NSs have extended to various research
fields, including optoelectronics, photonics, sensors, and energy storage, with significant
advances. However, there remain challenges to achieving the commercial viability of
printed 2D NS devices. First, it is necessary to improve the exfoliation yield of 2D NSs
to supply high-quality 2D NS ink at a low cost. Because the exfoliation yield by LPE
is typically below 10%, other exfoliation methods should be introduced to improve the
exfoliation yield. In the centrifugation process, it is difficult to precisely classify the
thickness and size of 2D NSs by their weight, and many thick flakes are still dispersed
in printing ink. Therefore, a methodology to precisely assort the exfoliated 2D NSs for
thickness and lateral size is urgent in order to manufacture high-performing printed 2D NS
devices. These challenges are in effect as long as 2D NSs-based applications are associated.
Thus, numerous efforts have increasingly been devoted to resolving the issues in a 2D
material research community. The ink formulations and printing parameters suggested so
far are based on empirical studies. Fundamental studies should be conducted to deliver
more scientific insight into the ink formulation and printing parameters. Lastly, printed
2D NSs are mainly dedicated to graphene or GO, and thus diversifying 2D NSs for inkjet
printing is required to heighten the potential of printed 2D NSs. It seems that establishing
the commercial viability of printed 2D NSs is a long way from now. However, once these
hurdles are removed, commercial viability may be realized.
Author Contributions:
H.-Y.J. and C.-H.C. wrote the paper. H.-Y.J. collected all the cited references.
S.-J.K. advised on the writing of the paper. All authors have read and agreed to the published version
of the manuscript.
Funding:
This work was supported by the National Research Foundation of Korea (NRF) grant
funded by the Korea government (No.2019R1I1A3A01058865) and Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded by the Ministry of Education
(No.2021R1A6A3A01086737).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Nicolosi, V.; Chhowalla, M.; Kanatzidis, M.G.; Strano, M.S.; Coleman, J.N. Liquid Exfoliation of Layered Materials. Science
2013
,
340, 1226419. [CrossRef]
2.
Khan, K.; Tareen, A.K.; Aslam, M.; Wang, R.; Zhang, Y.; Mahmood, A.; Ouyang, Z.; Zhang, H.; Guo, Z. Recent developments in
emerging two-dimensional materials and their applications. J. Mater. Chem. C 2020,8, 387–440. [CrossRef]

Nanomaterials 2021,11, 3441 19 of 21
3.
Li, Z.; Lv, Y.; Ren, L.; Li, J.; Kong, L.; Zeng, Y.; Tao, Q.; Wu, R.; Ma, H.; Zhao, B.; et al. Efficient strain modulation of 2D materials
via polymer encapsulation. Nat. Commun. 2020,11, 1151. [CrossRef]
4. Yang, S.; Jiang, C.; Wei, S.-h. Gas sensing in 2D materials. Appl. Phys. Rev. 2017,4, 021304. [CrossRef]
5.
Bonaccorso, F.; Lombardo, A.; Hasan, T.; Sun, Z.; Colombo, L.; Ferrari, A.C. Production and processing of graphene and 2d
crystals. Mater. Today 2012,15, 564–589. [CrossRef]
6.
Cataldi, P.; Bayer, I.S.; Bonaccorso, F.; Pellegrini, V.; Athanassiou, A.; Cingolani, R. Foldable Conductive Cellulose Fiber Networks
Modified by Graphene Nanoplatelet-Bio-Based Composites. Adv. Electron. Mater. 2015,1, 1500224. [CrossRef]
7.
McManus, D.; Vranic, S.; Withers, F.; Sanchez-Romaguera, V.; Macucci, M.; Yang, H.; Sorrentino, R.; Parvez, K.; Son, S.-K.;
Iannaccone, G.; et al. Water-based and biocompatible 2D crystal inks for all-inkjet-printed heterostructures. Nat. Nanotechnol.
2017,12, 343–350. [CrossRef]
8.
Conti, S.; Pimpolari, L.; Calabrese, G.; Worsley, R.; Majee, S.; Polyushkin, D.K.; Paur, M.; Pace, S.; Keum, D.H.; Fabbri, F.; et al.
Low-voltage 2D materials-based printed field-effect transistors for integrated digital and analog electronics on paper. Nat.
Commun. 2020,11, 3566. [CrossRef]
9.
Zhang, C.; McKeon, L.; Kremer, M.P.; Park, S.-H.; Ronan, O.; Seral-Ascaso, A.; Barwich, S.; Coileáin, C.Ó.; McEvoy, N.;
Nerl, H.C.; et al.
Additive-free MXene inks and direct printing of micro-supercapacitors. Nat. Commun.
2019
,10, 1795. [CrossRef]
10.
Arapov, K.; Rubingh, E.; Abbel, R.; Laven, J.; de With, G.; Friedrich, H. Conductive Screen Printing Inks by Gelation of Graphene
Dispersions. Adv. Funct. Mater. 2016,26, 586–593. [CrossRef]
11.
Xu, Y.; Schwab, M.G.; Strudwick, A.J.; Hennig, I.; Feng, X.; Wu, Z.; Müllen, K. Screen-Printable Thin Film Supercapacitor Device
Utilizing Graphene/Polyaniline Inks. Adv. Energy Mater. 2013,3, 1035–1040. [CrossRef]
12.
Singh, M.; Haverinen, H.M.; Dhagat, P.; Jabbour, G.E. Inkjet Printing—Process and Its Applications. Adv. Mater.
2010
,22, 673–685.
[CrossRef]
13. Calvert, P. Inkjet Printing for Materials and Devices. Chem. Mater. 2001,13, 3299–3305. [CrossRef]
14.
Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.y.; Haas, S.; Chiba, R.; Kumai, R.; Hasegawa, T. Inkjet printing of
single-crystal films. Nature 2011,475, 364–367. [CrossRef]
15.
Bonaccorso, F.; Bartolotta, A.; Coleman, J.N.; Backes, C. 2D-Crystal-Based Functional Inks. Adv. Mater.
2016
,28, 6136–6166.
[CrossRef] [PubMed]
16.
Hu, G.; Kang, J.; Ng, L.W.T.; Zhu, X.; Howe, R.C.T.; Jones, C.G.; Hersam, M.C.; Hasan, T. Functional inks and printing of
two-dimensional materials. Chem. Soc. Rev. 2018,47, 3265–3300. [CrossRef] [PubMed]
17.
Finn, D.J.; Lotya, M.; Cunningham, G.; Smith, R.J.; McCloskey, D.; Donegan, J.F.; Coleman, J.N. Inkjet deposition of liquid-
exfoliated graphene and MoS2nanosheets for printed device applications. J. Mater. Chem. C 2014,2, 925–932. [CrossRef]
18.
Li, B.; Liang, X.; Li, G.; Shao, F.; Xia, T.; Xu, S.; Hu, N.; Su, Y.; Yang, Z.; Zhang, Y. Inkjet-Printed Ultrathin MoS2-Based Electrodes
for Flexible In-Plane Microsupercapacitors. ACS Appl. Mater. Interfaces 2020,12, 39444–39454. [CrossRef]
19.
Casiraghi, C.; Macucci, M.; Parvez, K.; Worsley, R.; Shin, Y.; Bronte, F.; Borri, C.; Paggi, M.; Fiori, G. Inkjet printed 2D-crystal
based strain gauges on paper. Carbon 2018,129, 462–467. [CrossRef]
20.
Hu, G.; Albrow-Owen, T.; Jin, X.; Ali, A.; Hu, Y.; Howe, R.C.T.; Shehzad, K.; Yang, Z.; Zhu, X.; Woodward, R.I.; et al. Black
phosphorus ink formulation for inkjet printing of optoelectronics and photonics. Nat. Commun. 2017,8, 278. [CrossRef]
21.
Jiang, X.; Li, W.; Hai, T.; Yue, R.; Chen, Z.; Lao, C.; Ge, Y.; Xie, G.; Wen, Q.; Zhang, H. Inkjet-printed MXene micro-scale devices for
integrated broadband ultrafast photonics. npj 2d Mater. Appl. 2019,3, 34. [CrossRef]
22.
Jun, H.Y.; Ryu, S.O.; Kim, S.H.; Kim, J.Y.; Chang, C.-H.; Ryu, S.O.; Choi, C.-H. Inkjet Printing of Few-Layer Enriched Black
Phosphorus Nanosheets for Electronic Devices. Adv. Electron. Mater. 2021,7, 2100577. [CrossRef]
23.
Hu, G.; Yang, L.; Yang, Z.; Wang, Y.; Jin, X.; Dai, J.; Wu, Q.; Liu, S.; Zhu, X.; Wang, X.; et al. A general ink formulation of 2D
crystals for wafer-scale inkjet printing. Sci. Adv. 2020,6, eaba5029. [CrossRef] [PubMed]
24.
Li, J.; Lemme, M.C.; Östling, M. Inkjet Printing of 2D Layered Materials. ChemPhysChem
2014
,15, 3427–3434. [CrossRef] [PubMed]
25.
Xu, Y.; Cao, H.; Xue, Y.; Li, B.; Cai, W. Liquid-Phase Exfoliation of Graphene: An Overview on Exfoliation Media, Techniques, and
Challenges. Nanomaterials 2018,8, 942. [CrossRef]
26.
Cai, X.; Luo, Y.; Liu, B.; Cheng, H.-M. Preparation of 2D material dispersions and their applications. Chem. Soc. Rev.
2018
,
47, 6224–6266. [CrossRef]
27.
Jawaid, A.; Nepal, D.; Park, K.; Jespersen, M.; Qualley, A.; Mirau, P.; Drummy, L.F.; Vaia, R.A. Mechanism for Liquid Phase
Exfoliation of MoS2.Chem. Mater. 2016,28, 337–348. [CrossRef]
28.
Kang, J.; Wood, J.D.; Wells, S.A.; Lee, J.-H.; Liu, X.; Chen, K.-S.; Hersam, M.C. Solvent Exfoliation of Electronic-Grade, Two-
Dimensional Black Phosphorus. ACS Nano 2015,9, 3596–3604. [CrossRef]
29.
Dodoo-Arhin, D.; Howe, R.C.T.; Hu, G.; Zhang, Y.; Hiralal, P.; Bello, A.; Amaratunga, G.; Hasan, T. Inkjet-printed graphene
electrodes for dye-sensitized solar cells. Carbon 2016,105, 33–41. [CrossRef]
30.
Cunningham, G.; Lotya, M.; Cucinotta, C.S.; Sanvito, S.; Bergin, S.D.; Menzel, R.; Shaffer, M.S.P.; Coleman, J.N. Solvent Exfoliation
of Transition Metal Dichalcogenides: Dispersibility of Exfoliated Nanosheets Varies Only Weakly between Compounds. ACS
Nano 2012,6, 3468–3480. [CrossRef]
31. Coleman, J.N. Liquid Exfoliation of Defect-Free Graphene. Acc. Chem. Res. 2013,46, 14–22. [CrossRef]
32.
Yao, Y.; Tolentino, L.; Yang, Z.; Song, X.; Zhang, W.; Chen, Y.; Wong, C.-P. High-Concentration Aqueous Dispersions of MoS
2
.Adv.
Funct. Mater. 2013,23, 3577–3583. [CrossRef]

Nanomaterials 2021,11, 3441 20 of 21
33.
Howe, R.C.T.; Woodward, R.I.; Hu, G.; Yang, Z.; Kelleher, E.J.R.; Hasan, T. Surfactant-aided exfoliation of molybdenum disulfide
for ultrafast pulse generation through edge-state saturable absorption. Phys. Status Solidi B 2016,253, 911–917. [CrossRef]
34.
Carey, T.; Cacovich, S.; Divitini, G.; Ren, J.; Mansouri, A.; Kim, J.M.; Wang, C.; Ducati, C.; Sordan, R.; Torrisi, F. Fully inkjet-printed
two-dimensional material field-effect heterojunctions for wearable and textile electronics. Nat. Commun.
2017
,8, 1202. [CrossRef]
35.
Guardia, L.; Fernández-Merino, M.J.; Paredes, J.I.; Solís-Fernández, P.; Villar-Rodil, S.; Martínez-Alonso, A.; Tascón, J.M.D.
High-throughput production of pristine graphene in an aqueous dispersion assisted by non-ionic surfactants. Carbon
2011
,
49, 1653–1662. [CrossRef]
36.
Li, J.; Naiini, M.M.; Vaziri, S.; Lemme, M.C.; Östling, M. Inkjet Printing of MoS2. Adv. Funct. Mater.
2014
,24, 6524–6531. [CrossRef]
37.
Li, J.; Ye, F.; Vaziri, S.; Muhammed, M.; Lemme, M.C.; Östling, M. Efficient Inkjet Printing of Graphene. Adv. Mater.
2013
,
25, 3985–3992. [CrossRef]
38.
Sui, Y.; Hess-Dunning, A.; Wei, P.; Pentzer, E.; Sankaran, R.M.; Zorman, C.A. Electrically Conductive, Reduced Graphene Oxide
Structures Fabricated by Inkjet Printing and Low Temperature Plasma Reduction. Adv. Mater. Technol.
2019
,4, 1900834. [CrossRef]
39.
Liang, Y.T.; Hersam, M.C. Highly Concentrated Graphene Solutions via Polymer Enhanced Solvent Exfoliation and Iterative
Solvent Exchange. J. Am. Chem. Soc. 2010,132, 17661–17663. [CrossRef] [PubMed]
40.
Secor, E.B.; Prabhumirashi, P.L.; Puntambekar, K.; Geier, M.L.; Hersam, M.C. Inkjet Printing of High Conductivity, Flexible
Graphene Patterns. J. Phys. Chem. Lett. 2013,4, 1347–1351. [CrossRef]
41.
Michel, M.; Desai, J.A.; Biswas, C.; Kaul, A.B. Engineering chemically exfoliated dispersions of two-dimensional graphite and
molybdenum disulphide for ink-jet printing. Nanotechnology 2016,27, 485602. [CrossRef]
42.
He, P.; Derby, B. Controlling Coffee Ring Formation during Drying of Inkjet Printed 2D Inks. Adv. Mater. Interfaces
2017
,
4, 1700944. [CrossRef]
43.
Li, J.; Sollami Delekta, S.; Zhang, P.; Yang, S.; Lohe, M.R.; Zhuang, X.; Feng, X.; Östling, M. Scalable Fabrication and Integration of
Graphene Microsupercapacitors through Full Inkjet Printing. ACS Nano 2017,11, 8249–8256. [CrossRef] [PubMed]
44.
Juntunen, T.; Jussila, H.; Ruoho, M.; Liu, S.; Hu, G.; Albrow-Owen, T.; Ng, L.W.T.; Howe, R.C.T.; Hasan, T.; Sun, Z.; et al. Inkjet
Printed Large-Area Flexible Few-Layer Graphene Thermoelectrics. Adv. Funct. Mater. 2018,28, 1800480. [CrossRef]
45.
Torrisi, F.; Hasan, T.; Wu, W.; Sun, Z.; Lombardo, A.; Kulmala, T.S.; Hsieh, G.-W.; Jung, S.; Bonaccorso, F.; Paul, P.J.; et al.
Inkjet-Printed Graphene Electronics. ACS Nano 2012,6, 2992–3006. [CrossRef] [PubMed]
46.
Gao, Y.; Shi, W.; Wang, W.; Leng, Y.; Zhao, Y. Inkjet Printing Patterns of Highly Conductive Pristine Graphene on Flexible
Substrates. Ind. Eng. Chem. Res. 2014,53, 16777–16784. [CrossRef]
47.
Kelly, A.G.; Finn, D.; Harvey, A.; Hallam, T.; Coleman, J.N. All-printed capacitors from graphene-BN-graphene nanosheet
heterostructures. Appl. Phys. Lett. 2016,109, 023107. [CrossRef]
48.
Jiang, Z.; Chen, L.; Chen, J.-J.; Wang, Y.; Xu, Z.-q.; Sowade, E.; Baumann, R.R.; Sheremet, E.; Rodriguez, R.D.; Feng, Z.-S.
All-inkjet-printed MoS
2
field-effect transistors on paper for low-cost and flexible electronics. Appl. Nanosci.
2020
,10, 3649–3658.
[CrossRef]
49.
Seo, J.-W.T.; Zhu, J.; Sangwan, V.K.; Secor, E.B.; Wallace, S.G.; Hersam, M.C. Fully Inkjet-Printed, Mechanically Flexible MoS
2
Nanosheet Photodetectors. ACS Appl. Mater. Interfaces 2019,11, 5675–5681. [CrossRef]
50.
Lotya, M.; Rakovich, A.; Donegan, J.F.; Coleman, J.N. Measuring the lateral size of liquid-exfoliated nanosheets with dynamic
light scattering. Nanotechnology 2013,24, 265703. [CrossRef]
51.
Ding, J.; Zhao, H.; Zheng, Y.; Wang, Q.; Chen, H.; Dou, H.; Yu, H. Efficient exfoliation of layered materials by waste liquor.
Nanotechnology 2018,29, 095603. [CrossRef]
52.
Backes, C.; Hanlon, D.; Szydlowska, B.M.; Harvey, A.; Smith, R.J.; Higgins, T.M.; Coleman, J.N. Preparation of Liquid-exfoliated
Transition Metal Dichalcogenide Nanosheets with Controlled Size and Thickness: A State of the Art Protocol. JoVE
2016
,
118, e54806. [CrossRef]
53.
Majee, S.; Liu, C.; Wu, B.; Zhang, S.L.; Zhang, Z.B. Ink-jet printed highly conductive pristine graphene patterns achieved with
water-based ink and aqueous doping processing. Carbon 2017,114, 77–83. [CrossRef]
54.
Desai, J.A.; Adhikari, N.; Kaul, A.B. Chemical exfoliation efficacy of semiconducting WS
2
and its use in an additively manufactured
heterostructure graphene–WS2–graphene photodiode. RSC Adv. 2019,9, 25805–25816. [CrossRef]
55.
Hon, K.K.B.; Li, L.; Hutchings, I.M. Direct writing technology—Advances and developments. CIRP Ann.
2008
,57, 601–620.
[CrossRef]
56.
Jang, D.; Kim, D.; Moon, J. Influence of Fluid Physical Properties on Ink-Jet Printability. Langmuir
2009
,25, 2629–2635. [CrossRef]
57.
Nayak, L.; Mohanty, S.; Nayak, S.K.; Ramadoss, A. A review on inkjet printing of nanoparticle inks for flexible electronics. J.
Mater. Chem. C 2019,7, 8771–8795. [CrossRef]
58.
Derby, B. Inkjet Printing of Functional and Structural Materials: Fluid Property Requirements, Feature Stability, and Resolution.
Annu. Rev. Mater. Res. 2010,40, 395–414. [CrossRef]
59.
Vural, M.; Pena-Francesch, A.; Bars-Pomes, J.; Jung, H.; Gudapati, H.; Hatter, C.B.; Allen, B.D.; Anasori, B.; Ozbolat, I.T.;
Gogotsi, Y.; et al. Inkjet Printing of Self-Assembled 2D Titanium Carbide and Protein Electrodes for Stimuli-Responsive Electro-
magnetic Shielding. Adv. Funct. Mater. 2018,28, 1801972. [CrossRef]
60. Hossain, R.F.; Deaguero, I.G.; Boland, T.; Kaul, A.B. Biocompatible, large-format, inkjet printed heterostructure MoS2-graphene
photodetectors on conformable substrates. npj 2d Mater. Appl. 2017,1, 28. [CrossRef]

Nanomaterials 2021,11, 3441 21 of 21
61.
Farina, F.E.; Binti Azmi, W.S.; Harafuji, K. Ultraviolet-ozone anode surface treatment and its effect on organic solar cells. Thin
Solid Film. 2017,623, 72–83. [CrossRef]
62.
Park, H.Y.; Kang, B.J.; Lee, D.; Oh, J.H. Control of surface wettability for inkjet printing by combining hydrophobic coating and
plasma treatment. Thin Solid Film. 2013,546, 162–166. [CrossRef]
63.
Chalker, J.T.; Coddington, P.D. Percolation, quantum tunnelling and the integer Hall effect. J. Phys. C Solid State Phys.
1988
,
21, 2665–2679. [CrossRef]
64. Kirkpatrick, S. Percolation and Conduction. Rev. Mod. Phys. 1973,45, 574–588. [CrossRef]
65.
Khan, T.; Irfan, M.S.; Ali, M.; Dong, Y.; Ramakrishna, S.; Umer, R. Insights to low electrical percolation thresholds of carbon-based
polypropylene nanocomposites. Carbon 2021,176, 602–631. [CrossRef]
66.
Bauhofer, W.; Kovacs, J.Z. A review and analysis of electrical percolation in carbon nanotube polymer composites. Compos. Sci.
Technol. 2009,69, 1486–1498. [CrossRef]
67.
Huang, Q.; Zhu, Y. Printing Conductive Nanomaterials for Flexible and Stretchable Electronics: A Review of Materials, Processes,
and Applications. Adv. Mater. Technol. 2019,4, 1800546. [CrossRef]
68.
Kamyshny, A.; Magdassi, S. Conductive nanomaterials for 2D and 3D printed flexible electronics. Chem. Soc. Rev.
2019
,
48, 1712–1740. [CrossRef] [PubMed]
69.
Kelly, A.G.; Murphy, C.; Vega-Mayoral, V.; Harvey, A.; Esmaeily, A.S.; Hallam, T.; McCloskey, D.; Coleman, J.N. Tuneable
photoconductivity and mobility enhancement in printed MoS2/graphene composites. 2d Mater. 2017,4, 041006. [CrossRef]
70.
Leng, T.; Parvez, K.; Pan, K.; Ali, J.; McManus, D.; Novoselov, K.S.; Casiraghi, C.; Hu, Z. Printed graphene/WS2 battery-free
wireless photosensor on papers. 2d Mater. 2020,7, 024004. [CrossRef]
71.
Carey, T.; Arbab, A.; Anzi, L.; Bristow, H.; Hui, F.; Bohm, S.; Wyatt-Moon, G.; Flewitt, A.; Wadsworth, A.; Gasparini, N.; et al.
Inkjet Printed Circuits with 2D Semiconductor Inks for High-Performance Electronics. Adv. Electron. Mater.
2021
,7, 2100112.
[CrossRef]
72.
Wen, D.; Wang, X.; Liu, L.; Hu, C.; Sun, C.; Wu, Y.; Zhao, Y.; Zhang, J.; Liu, X.; Ying, G. Inkjet Printing Transparent and Conductive
MXene (Ti
3
C
2
T
x
) Films: A Strategy for Flexible Energy Storage Devices. ACS Appl. Mater. Interfaces
2021
,13, 17766–17780.
[CrossRef] [PubMed]
73.
Wang, Y.; Mehrali, M.; Zhang, Y.-Z.; Timmerman, M.A.; Boukamp, B.A.; Xu, P.-Y.; ten Elshof, J.E. Tunable capacitance in
all-inkjet-printed nanosheet heterostructures. Energy Storage Mater. 2021,36, 318–325. [CrossRef]
74.
Karim, N.; Afroj, S.; Malandraki, A.; Butterworth, S.; Beach, C.; Rigout, M.; Novoselov, K.S.; Casson, A.J.; Yeates, S.G. All
inkjet-printed graphene-based conductive patterns for wearable e-textile applications. J. Mater. Chem. C
2017
,5, 11640–11648.
[CrossRef]
75.
Wang, G.; Wang, Z.; Liu, Z.; Xue, J.; Xin, G.; Yu, Q.; Lian, J.; Chen, M.Y. Annealed graphene sheets decorated with silver
nanoparticles for inkjet printing. Chem. Eng. J. 2015,260, 582–589. [CrossRef]
76.
Ma, J.; Zheng, S.; Cao, Y.; Zhu, Y.; Das, P.; Wang, H.; Liu, Y.; Wang, J.; Chi, L.; Liu, S.; et al. Micro-Supercapacitors: Aqueous
MXene/PH1000 Hybrid Inks for Inkjet-Printing Micro-Supercapacitors with Unprecedented Volumetric Capacitance and Modular
Self-Powered Microelectronics (Adv. Energy Mater. 23/2021). Adv. Energy Mater. 2021,11, 2170088. [CrossRef]
77.
Li, N.; Jiang, Y.; Xiao, Y.; Meng, B.; Xing, C.; Zhang, H.; Peng, Z. A fully inkjet-printed transparent humidity sensor based on a
Ti3C2/Ag hybrid for touchless sensing of finger motion. Nanoscale 2019,11, 21522–21531. [CrossRef]
78.
Zheng, J.; Diao, J.; Jin, Y.; Ding, A.; Wang, B.; Wu, L.; Weng, B.; Chen, J. An Inkjet Printed Ti
3
C
2
-GO Electrode for the
Electrochemical Sensing of Hydrogen Peroxide. J. Electrochem. Soc. 2018,165, B227–B231. [CrossRef]