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51, 1253
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Liquid metals: an ideal platform for the synthesis
of two-dimensional materials†
Patjaree Aukarasereenont, a Abigail Goff, b Chung Kim Nguyen,b
Chris F. McConville, c Aaron Elbourne, a Ali Zavabeti d and
Torben Daeneke *b
The surfaces of liquid metals can serve as a platform to synthesise two-dimensional materials. By
exploiting the self-limiting Cabrera-Mott oxidation reaction that takes place at the surface of liquid
metals exposed to ambient air, an ultrathin oxide layer can be synthesised and isolated. Several synthesis
approaches based on this phenomenon have been developed in recent years, resulting in a diverse
family of functional 2D materials that covers a significant fraction of the periodic table. These
Received 20th December 2021
straightforward and inherently scalable techniques may enable the fabrication of novel devices and thus
DOI: 10.1039/d1cs01166a
harbour significant application potential. This review provides a brief introduction to liquid metals and
rsc.li/chem-soc-rev
their alloys, followed by detailed guidance on each developed synthesis technique, post-growth
processing methods, integration processes, as well as potential applications of the developed materials.
Key learning points
(1) Theoretical concepts behind the oxide formation on liquid metal surface and brief overview of metal selection for liquid metal alloy synthesis.
(2) Walkthrough of each developed liquid metal-based synthesis approach for two-dimensional materials synthesis and available post-growth processing
methods used to convert oxide materials derived from liquid metals to other compounds.
(3) Integration of two-dimensional materials synthesized via liquid metal-based approaches.
(4) Current and possible future applications of the synthesized materials.
(5) Prospects and challenges of the liquid metal-based synthesis platform.
1. Introduction
Two-dimensional (2D) metal oxides are an exciting class of lowdimensional materials that have unique characteristics and represent promising candidates for a wide range of device applications in
electronics, optoelectronics, sensing, and energy storage.1–3 To date,
several approaches have been employed to obtain these materials,
including mechanical exfoliation, vapor phase techniques, and
solution-based synthesis. However, these methods possess limitations and are complex to scale and optimise, hence alternative
approaches should be considered. Recently, a simple synthesis
a
School of Science, RMIT University, Melbourne, VIC, 3001, Australia
School of Engineering, RMIT University, Melbourne, VIC, 3001, Australia.
E-mail: Torben.Daeneke@rmit.edu.au
c
Institute for Frontier Materials, Deakin University, Geelong, VIC, 3216, Australia
d
Department of Chemical Engineering, The University of Melbourne, Parkville, VIC,
3010, Australia
† Electronic supplementary information (ESI) available: A demonstration of some
synthesis techniques on a SiO2 substrate and TEM grid as well as cleaning process
is available in Video. S1. See DOI: 10.1039/d1cs01166a
b
This journal is © The Royal Society of Chemistry 2022
technique involving liquid metals has been demonstrated to effectively isolate 2D metal oxides.4 The term liquid metals, for the
purpose of this review, covers metals and metal alloys present in
liquid form below approximately 350 1C which includes several posttransition elements, group 12 elements, and their alloys (see
Fig. 1a). Following the Cabrera–Mott oxidation model depicted in
Fig. 1b,5 an ultrathin oxide layer forms on liquid metal surfaces
once it is exposed to ambient atmosphere. This oxide skin is weakly
attached to its parent metal, resulting in an ease of exfoliation onto
desired substrates. Other 2D material compounds are also accessible by performing subsequent post-processing on the obtained
oxides.6,7 The use of liquid metals and liquid metal alloys as
reaction platforms for the synthesis of 2D metal oxides is gaining
popularity due to the relative simplicity of the process and the ability
to create large, ultra-thin materials with minimal defects.8 Furthermore, liquid metal-based techniques enable the synthesis of a range
of materials including naturally stratified, non-layered and amorphous compounds, thereby significantly expanding the available
compound family of 2D materials. The synthesised materials can
then be exploited in a variety of applications. This tutorial review
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Chem Soc Rev
Fig. 1 (a) Selected liquid metal base elements with their atomic number and melting temperature. (b) Cabrera–Mott oxidation occurs at the surface of
liquid metals in the presence of oxygen.5 Electrons from the metal tunnel through the growing oxide shell, resulting in a self-generated electric field
called the Mott field. This field promotes the diffusion of metal and oxygen ions into the oxide shell, leading to oxide growth. As the oxide becomes
thicker, the field strength reduces until ion diffusion ceases. Note: (b) Reproduced from ref. 9 and 29 with permission from r 2020 Elsevier Inc. and
r The Royal Society of Chemistry 2021, respectively.
introduces liquid metal-based synthesis techniques that have been
developed to date as a guide for researchers looking for alternative
and novel synthesis methods for creating 2D materials. The fundamentals of liquid metals will be discussed first, followed by a
walkthrough of each developed technique. Furthermore, postprocessing methods that can be applied to convert oxides into other
compounds such as phosphates, sulphides, and nitrides will be
discussed. The assembly of liquid metal derived 2D materials into
heterostructures for potential device applications will then be
reviewed. Following this, we present recently demonstrated as well
Patjaree Aukarasereenont is a PhD
candidate at RMIT University in
Melbourne, Australia, and a
member of the ARC Centre of
Excellence in Future Low-Energy
Electronics Technologies (FLEET).
Prior
to
commencing
her
candidature in 2020, she received
her BEng (Mechanical Engineering)
from Kasetsart University, Thailand,
and MEng (Micro-nano Engineering)
from RMIT University, Melbourne,
Australia. Her PhD research focuses
Patjaree Aukarasereenont
on the fabrication of transparent
and flexible electronics based on 2D materials synthesised via liquid
metal chemistry.
1254 | Chem. Soc. Rev., 2022, 51, 1253–1276
as possible future applications of liquid metal-based 2D materials.
Finally, the conclusion section will discuss the challenges and future
aspects of liquid metal-based synthesis methods.
2. Fundamentals of liquid metals
In contrast to other liquids, liquid metals exhibit metallic
bonding inside their bulk phase, which is characterised by
highly delocalised valence electrons that are responsible for the
electronic properties of molten metals.9,10 The loss of valence
Abigail Goff
Abigail Goff received her Bachelor of
Science and Honours Degree in
Chemistry in 2018/2019 from
Swinburne University of Technology
in Melbourne Australia. She is
currently
undertaking
her
PhD at the Royal Melbourne
Institute of Technology in Chemical
engineering, examining the use of
liquid metals as reaction platforms
for the growth and exfoliation of
self-limiting 2D metal oxides for
use in low-energy logic-based
devices.
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Tutorial Review
electrons results in the formation of metal ions that are
arranged without long-range order, which is contrary to the
regular atomic arrangements observed in solid metals. Strong
coulombic forces maintain the local charge neutrality within
the assembly of ions and electrons, while both charged entities
remain fluid. The formation of partially or fully isolated particles such as free ions or electrons is unfavourable and therefore
coulombic forces result in a well-defined boundary on the
surface of liquid metals.10,11 The surface of liquid metals is
frequently described as atomically flat due to the high surface
tension, resulting in an ideal growth environment for creating
ultrathin 2D materials. One can also consider alloying two or
more liquid metal elements for synergic enhancement of
physical and chemical properties of the alloy mixture as well
as the exploration of more materials. Metal solubility, which is
exponentially dependent on temperature in the dilute solution
regime, needs to be taken into account when it comes to
producing alloys.12 Other material properties contributing to
the solubility include: the atomic diameter; crystal structures;
valency; as well as electronegativity; which should all be as
similar as possible for the constituent metals.8
2.1
Surface tension
The high surface tension of liquid metals is one of their key
attributes and can be calculated from the difference between the
heat of evaporation and the total pairwise atomic interactions inside
the bulk or through modelling of atomic interactions.13–15 Experimentally, it can be measured from the contact angle between a
droplet and a surface16,17 via the pendant drop methods18,19 and
can be modulated through electrochemical methods.20 However,
Chung Kim Nguyen received his
MEng
degree
in
Chemical
Engineering from Ho Chi Minh
City University of Technology
(HCMUT) in 2014. He is currently
a PhD candidate in the School of
Engineering, RMIT University. His
research interests mainly focus on
the chemistry of liquid metals and
2D materials synthesis for use in
electronic applications.
Dr Aaron Elbourne is a Research
Fellow within the School of Science
at RMIT University, Melbourne,
Australia. He is currently the
recipient of a Jack Brockhoff
Foundation Early Career Medical
Research Fellowship and an
Australian
Research
Council
Discovery Early Career Researcher
Award (DECRA). He obtained his
PhD in Chemistry in 2017 from
The University of Newcastle,
Australia under the supervision of
Aaron Elbourne
Professor Erica J. Wanless. He began
his postdoctoral fellowship in February of 2017. His research interests
involve high-resolution atomic force microscopy, ion adsorption, solid–
liquid interfaces, bio-interfaces, nanomaterials, liquid metals, and
antimicrobial technologies.
Dr Ali Zavabeti is a McKenzie
Fellow at the University of
Melbourne,
department
of
chemical
engineering.
He
obtained his BEng (Hons) and
MEngSc (Res) from Monash
University before receiving his
PhD
award
from
RMIT
University in Australia. He
worked as a research fellow for
the ARC Centre of Excellence in
Future Low-Energy Electronics
Technologies (FLEET) before
Ali Zavabeti
joining
the
University
of
Melbourne. His primary research focus is on surface and
interfacial sciences, nanoelectronics, two-dimensional oxides and
liquid metals.
Dr Torben Daeneke received his PhD
in Chemistry from Monash
University, Australia in 2012. After
graduating he held postdoctoral
appointments at the CSIRO and at
RMIT
University
(Australia).
In 2015 he received an RMIT
Vice Chancellor’s postdoctoral
fellowship. In 2018 he joined
RMIT’s School of Engineering as a
faculty member and is now an
Associate Professor. Dr Daeneke
has authored over 100 peerTorben Daeneke
reviewed journal articles in areas
such as materials for electronic devices, the chemistry of liquid metals,
catalysis and 2D materials. In recent years he has developed novel
techniques for the synthesis of functional materials using liquid metal
solvents.
Chung Kim Nguyen
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Tutorial Review
measuring the contact angle of liquid metal droplets with nonreactive surfaces was found to significantly underestimate the surface tension due to the formation of an oxide layer in air.21 The
lower apparent surface tension arises due to surface oxidation and
results in a higher wetting ability. This indicates significant interactions between the native oxide and solid interfaces, suggesting
that selective transfer of surface oxides onto different substrates may
be possible.
In liquid alloys, surface layering may also occur, which is
limited to a few atomic diameters on the surface.11,22,23 Surface
layering occurs on molten alloys as a response to the abrupt
termination of the metallic phase and serves to reduce surface
tension, causing the metal with the lower surface tension to
dominate the interface.24 This phenomenon has been observed
in a number of binary alloys, including eutectic gallium indium
(EGaIn) where, after in situ sputtering in vacuum, the top
surface became significantly indium enriched, even though it
is the minority component of the bulk alloy.25,26 Interestingly,
the oxide layer that forms on the surface of EGaIn is found to be
composed of gallium oxide.27,28 The preferential growth of
gallium oxide is due to the more favourable Gibbs free energy
of oxide formation for gallium when compared with indium. By
exploiting this concept, elements with thermodynamically
more favourable oxide formations can be alloyed into
gallium-based liquid metals to produce oxides of the dissolved
elements.4,29 Thus, adding metallic solutes to a molten metal
can substantially modify interfacial layering and the composition of the surface oxide.
2.2
Chem Soc Rev
work, indicating the absence of covalent bonds.34 The nonpolar
nature of liquid metals also limits non-covalent interactions,
and a combination of these two effects results in the weak
attachment of native surface oxides to their molten parent
metal.34,35
The formation of preferential surface oxides from binary
liquid metal alloys may be predicted using a combination of
thermodynamic equations.29 In the selected alloy system at
temperatures ranging from 298 to 773 K, the enthalpies of
formation (DHf) of solute metal (m), metal solvent (s), and each
possible metal oxide (mo) were first calculated using the
following expression;
DHf ¼ DGf þ TDS
1256 | Chem. Soc. Rev., 2022, 51, 1253–1276
(1)
where DGf is the Gibbs free energy of formation under standard
state conditions (kJ mol1), T is the temperature (K), and S1 is the
entropy under standard state conditions (kJ K1). The obtained
enthalpies of formation of the solute metal (DHf(m)) and metal
solvent (DHf(s)) were multiplied by their corresponding alloy ratio
[m] and [s], extracted from the phase diagrams, respectively. The
summation of both values then resulted in the enthalpy of alloy
formation (DHa) as shown below.
DHa ¼ DHf ðmÞ ½m þ DHf ðsÞ ½s
(2)
Subsequently, the enthalpy of combustion (DHcomb) for each
possible metal oxide produced from the alloy system can be
calculated with the following equation:
DHcomb = ADHf(mo) (BDHf(O2)
Surface oxidation
Surface oxides are known to form spontaneously on the surface
of liquid metals in the presence of ambient atmosphere, as well
as other oxygen containing environments.26 The surface oxide
growth is a self-limiting reaction at low to moderate temperatures and can be described by the Cabrera–Mott process.5 The
theory developed by Cabrera and Mott describes initially rapid
growth kinetics where a few monolayers of oxide form nearly
instantaneously upon exposure to molecular oxygen.5 After the
formation of an initial oxide layer, electrons from the metallic
bulk ionise surface adsorbed oxygen via a tunnelling process,
resulting in the formation of an electric field gradient across
the oxide layer. This electric field (Mott field), which is defined
by the work function of the metal and the ionised oxygen
species, drives ion migration into the oxide layer and thus
promotes growth. As the oxide layer becomes thicker, the field
strength is reduced, resulting in self-termination of the process
and a final oxide thickness of a few nanometres. The oxide
properties, such as thickness and oxidation kinetics, can be
controlled by reducing the oxygen partial pressure or via surface functionalisation.30–32
The interfacial force between the oxide skin and its parent
liquid metal is weak, allowing the straightforward isolation of
this surface oxide as well as the exploitation of these naturally
occurring ultrathin interfacial compounds.8,33 A minimum of
the electron density distribution at the boundary between the
liquid Ga surface and the oxide has been observed in previous
CDHa)
(3)
where A, B, and C represent the stoichiometric coefficients for the
metal oxide, oxygen, and the metal or solvent based on which metal
oxide DHcomb is being calculated, respectively. Finally, by replacing
DHf with DHcomb and rearranging eqn (1), the Gibbs free energy of
combustion (DGalloy) for each oxide can be determined before
comparing the values to identify the one that will preferentially
form. Note that the entropy is no longer under standard state
conditions. The prediction utilising these equations helps minimising trial and error and allows for the selection of the ideal metal
solvent. Recent examples of this approach include Sb, which when
alloyed with Ga, leads to the formation of Ga2O3. However, if alloyed
with Bi, the predominant oxide formed is Sb2O3.36 Eqn (3) does not
take into account the effect of O2 concentration or possible formation of mixed oxides, as is seen when an alloy of In–Sn is used to
print 2D indium-tin oxide (ITO).37 As such, a deeper understanding
of the thermodynamic properties of liquid metal alloy interfaces
should be developed.
3. Synthesis methods
3.1
Liquid metal alloy synthesis
Although low melting point metals can be utilised as unitary
element, alloying with other metals allows us to expand our
knowledge of liquid metal chemistry further and to isolate a
wider range of 2D metal oxides. Liquid metals with low melting
points can act as solvents into which other higher melting
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temperature metals may be alloyed. The solubility of liquid
metals can be estimated prior to the alloying process using
published solubility parameters and phase diagrams of the
relevant system. Either employing induction furnaces and inert
atmosphere arc systems or mechanical grinding with a mortar
and pestle in an inert atmosphere has been found to be
effective when it comes to producing alloys.4,38,39 For many
metal systems, the grinding process removes any pre-existing
oxide skins and facilitates wetting between the solid solute and
the liquid solvent metal, while also increasing the surface area.
3.2 Two-dimensional metal oxides from liquid metal-based
synthesis techniques
Two-dimensional (2D) metal oxides can be isolated through various
liquid metal-based techniques, including printing (touch,4
squeeze,6,37 or blade print40–45 techniques), liquid phase growth
and exfoliation (including sonication46 and bubbling4,47,48 methods), and surface templated growth of materials on liquid metal
surfaces.49,50 The liquid metal printing processes may also be used
to assist the synthesis of heterostructures which will be discussed
later in this review.51,52
Other 2D compounds may also be synthesised as the smooth
surface of the liquid allows for the synthesis of low-defect
materials.53 Their high conductivity, non-polar nature, and
the existence of a large number of free electrons and ions
within the bulk are also useful as the liquid metal can be used
to reduce precursor materials to form ultrathin surface
skins,49,50 and the weak forces between the bulk liquid and
the solid surface materials allow for these skins to be removed
with ease.8
The resulting 2D materials are shown to be ultra-thin (o5 nm)
with high consistency in thicknesses47,54,55 and laterally large reaching centimetre scale,6,56 making these techniques ideal alternatives
to more established epitaxial approaches such as chemical vapor
deposition (CVD),57 and mechanical exfoliation.58 The liquid metalbased methods also offer simple and fast synthesis routes for
fabricating large area 2D materials, while most conventional methods require longer growth times and relatively high operating
temperatures.59–61 Furthermore, most liquid metal-based syntheses
are substrate independent,37,45 which is a remarkable advantage
over conventional growth methods that require lattice-matched
substrates.62,63 As such, by using the synthesis approaches presented herein, a wide range of 2D metal oxides and other 2D
materials including some that would otherwise be impossible to
synthesise, due to their non-layered natures, are suddenly quite
realisable.8 A video showing the different deposition techniques in
detail is available as digital ESI,† online (Vid. S1).
3.2.1 Touch and squeeze printing. The oxides grown via
the Cabrera–Mott process are easily removed by a process
known as touch printing. This is carried out by gently touching
the surface of a liquid metal droplet with an appropriate
substrate allowing the oxide to be transferred (Fig. 2a). As the
adhesive forces between the liquid and oxide are weak, transfer
of the 2D sheet from the bulk liquid metal is readily possible.4,8
In the touch-printing process, a droplet of liquid metal is
placed on a glass slide on a hotplate and is heated to its
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Tutorial Review
melting point. Non-uniform pre-existing oxides that could also
contain contamination will initially dominate the surface and
should be removed, allowing for a fresh and clean oxide layer to
grow. A substrate that has been pre-heated on the hotplate is
then brought into contact with the surface of the oxide-coated
liquid metal before being lifted off smoothly.4 Substrate preheating is required, since pressing a cold substrate against a
liquid metal may freeze the alloy, reducing the flexibility
required during transfer. Fig. 2(b–e) depict a touch-printed
2D Bi2O3 nanosheet that was isolated from molten Bi and
transferred onto substrates for different characterisations.64
During touch printing, liquid metals should rest on a steady
surface to avoid wrinkles in the 2D skin that can form due to
vibrations and shaking of the liquid metal surface. Synthesis
can also be achieved in reverse, where the liquid metal dropped
from a syringe, touches a substrate lying underneath. Previous
work has demonstrated this technique to synthesise 2D bGa2O3 using liquid EGaIn.65
In a similar way, squeezing the droplet between two substrates allows the oxide skin to spread out and attach to both
substrates (Fig. 2f).6,37 A substrate is placed on a hotplate and
warmed to the melting point of the chosen liquid metal. A
droplet of liquid metal (smaller than that is used in touchprinting) is then placed on the substrate, allowed to fully
oxidise before a second warmed substrate is placed on top
and gently pressed downward, allowing the oxides to be transferred onto both substrates. During the squeeze-printing process, the oxide skin may form cracks, however, any gaps in the
surface skin are filled due to the near instantaneous CabreraMott re-oxidation.5,66 As such, the self-healing of the interfacial
oxide enables the growth of larger sheets when compared to the
touch printing method. The method results in two laterally
large 2D metal oxide flakes that are attached to both substrates
via van der Waals forces.6,37,52 Examples of ultrathin ITO37 and
Ga2O352 derived from a liquid In-Sn alloy and molten Ga,
respectively, are presented in Fig. 2(g–j). The remaining bulk
liquid metal forms spherical droplets after the substrates are
separated due to the high surface tension of the liquid metal.
These residual droplets are easily removed, as shown in Vid. S1
(ESI†).6,37
Work has previously been done to examine the surface properties of EGaIn, specifically the elasticity of the oxide skin, whose
critical surface stress was found to be B0.5 N m1.26 As the touchprinting process requires a simple touching motion of the substrate
to the liquid metal, the force applied should remain within the
elastic region to retain the droplet shape and avoid damaging the
2D flakes. This method results in the formation of a 2D sheet of
similar size to that of the droplet used. The squeeze printing
method, however, utilises the critical stress and the ability of the
liquid metal to rapidly re-oxidise upon fracturing and allows for a
much larger flake in comparison to the size of the droplet used. In
one such endeavours, the ideal force applied to squeeze-print
centimetre-sized ITO from a 9 mg droplet of In–Sn alloy was
monitored and found to be 2.4 0.5 N.37
In some circumstances, the liquid metal may freeze on the
substrate after printing, resulting in a metallic residue which
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Fig. 2 (a) Touch-printing process. A metal is heated to its melting temperature and an oxide skin rapidly forms on the droplet surface following
the Cabrera–Mott mechanism. This oxide layer is subsequently exfoliated
by gently touching the surface with a substrate. Ultrathin Bi2O3 sheets
exfoliated from liquid Bi via the touch-printing method. (b) AFM image with
the inset shows the step-height profile, (c) low-, (d) high-resolution TEM
images and (e) the corresponding fast Fourier transform (FFT) image.64
(f) Squeeze-printing process. A relatively small liquid metal droplet is
placed between two substrates. Force is applied to squeeze the molten
metal resulting in ultrathin oxide sheets being obtained on both substrates.
(g) AFM morphology and (h) low-resolution TEM image of monolayer ITO
obtained from liquid In–Sn alloy utilising the squeeze-printing technique.37
(i) An AFM image of squeeze-printed 2D Ga2O3 isolated from liquid Ga and
(j) the height profiles collected at 56 locations from 5 different Ga2O3
sheets showing high consistency in layer thicknesses due to the Cabrera–
Mott reaction.52 Note: (b–e) Reproduced from ref. 64 with permission
from r The Royal Society of Chemistry 2018. (g and h) Reproduced from
ref. 37 with permission from r Springer Nature Limited 2020. (i and j)
Reproduced from ref. 52 with permission from r 2020 Wiley-VCH GmbH.
should be removed via a cleaning procedure using warm
alcohol and a cotton bud, which was found to be highly
1258 | Chem. Soc. Rev., 2022, 51, 1253–1276
Chem Soc Rev
effective (see Vid. S1, ESI†).6,7 Alternatively, metal inclusions
can be removed via selective etching. Iodide–triiodide (I/I3)
solutions have been found to effectively remove indium inclusions when 2D ITO was etched overnight.37 When touchprinting is used, cleaning may be avoided if the substrate is
heated on a second hotplate to a higher temperature to prevent
freezing upon contact with the liquid metal.
Certain liquid metal and liquid metal alloys may also
respond differently to the O2 concentration. For example, liquid
tin oxidises rapidly in ambient conditions to form multiple
layers of oxide consisting of a thin SnO sheet that is covered
with SnO2 scales.67 To avoid thicker flakes or mixed oxides,
reactions using liquid metal such as Sn, Bi, and Zn should be
performed in an environment where the O2 level is well controlled. Improved outcomes have been reported when employing a graphite nozzle to effectively remove and trap pre-existing
surface impurities that may be present. The nozzle is used to
press downward onto the liquid metal, resulting in a small
droplet that emerges on its surface. This limited area was then
exposed to a dilute O2 in N2 mixture from which high quality 2D
SnO layers were easily exfoliated via touch-printing.68
The touch and squeeze printing methods have been used
extensively in the fabrication of various 2D metal oxides showing the versatility of the method. Oxides of high melting
temperature metals such as Gd, Al, and Hf have been produced
at near room temperature after alloying with liquid Ga and
subsequent touch-printing.4 Centimetre-sized two-dimensional
sheets of ITO have also been squeeze-printed from In–Sn alloy
which were then incorporated into capacitive touch screens
(Fig. 2g and h).37
3.2.2 Blade coating and roll transfer. Blade coating is
another alternative to the touch- and squeeze-printing
techniques.40,44 Several studies have reported different ways to
perform this method, including brushing,41 tape printing,42,43 and
roll transfer.45 Nevertheless, they share a similarity in concept
whereby a liquid metal is being moved across the substrate and
leaves behind an oxide skin which is only weakly attached to the
parent liquid metal. The initial step is similar to the first two
methods described, placing a metal pellet on a substrate before
applying heat to melt it and allow an oxide layer to form. The next
step of moving the liquid metal across the substrate is implemented
differently for each approach. For the blade coating process, any
clean simple tool such as a piece of polydimethylsiloxane (PDMS)
can be utilised as a ‘squeegee’ to scrape the droplet along the
surface of the substrate (see Fig. 3a).40,44,69
However, one should always be aware of the potential for
introducing undesired cracks into the synthesised 2D sheets,
which has been observed in previous work.40 Brushing, on the
other hand, is carried out by placing the substrate with fully
oxidised liquid metal on it facing down, followed by gently
brushing on to another substrate.41 Ultrathin Ga2O3 sheets
synthesised via the blade coating44 and brushing41 are presented in Fig. 3b and c, respectively.
A combination of synthesis and transfer has been proposed
via tape-printing (Fig. 3d) where the liquid metal is first
dropped on a tape surface and subsequently slid on PDMS
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Fig. 3 (a) Blade-coating technique. A liquid metal droplet is scraped with
a ‘squeegee’ across a substrate allowing an oxide layer to attach to the
surface. (b) AFM morphology of Ga2O3 nanosheet synthesised from liquid
Ga through blade-coating (the inset is the height profile).44 (c) An optical
microscopy image of 2D Ga2O3 as a result of ‘brushing’ liquid Ga across a
SiO2 substrate.41 (d) Tape-printing method. A liquid metal is placed on a
tape which is subsequently slid across a PDMS surface to print an oxide
layer. The PDMS is then used as a stamp to transfer the 2D oxide onto
another substrate. (e) SEM image and (f) AFM image with the step height
profile inset of a tape-printed Ga2O3 nanosheet. (g) High-resolution TEM
image with the corresponding SAED pattern of the synthesised Ga2O3.43
Note: (b) Reproduced from ref. 44 with permission. (c) Reproduced from
ref. 41 with permission from r 2019 American Chemical Society. (e and f)
Reproduced from ref. 43 with permission from r 2021 American
Chemical Society.
leaving a trace of oxide. Next, by utilising PDMS as a stamp, the
oxide is transferred onto a desired substrate.42,43 Fig. 3(e–g)
elucidate characterisations of 2D tape-printed Ga2O3 sheet.43 As
for the roll transfer method, a glass pipette is used to roll a
liquid metal droplet across the surface of a substrate imitating
the act of writing with a pen as depicted in Fig. 4a.45 This
technique was utilised to synthesise 2D b-TeO2 from molten Se–
Te chalcogen mixture shown in Fig. 4(b–f).45 For all of these
methods, any liquid metal residue could subsequently be
removed either physically with tweezers, or by a suitable cleaning process as mentioned in the previous section. Alternatively,
it was reported that an iodide/triiodide (I/I3) solution could
be used as an etchant to clean 2D ITO deposited on both glass
and quartz.37 These versatile methods are beneficial in order to
explore various 2D materials as well as their different structures
and compounds. For example, crystalline Ga2O3 can be synthesised at 200 1C 44 or by further annealing amorphous Ga2O3
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obtained at room temperature.41,43 Oxide conversions (see the
section on Post-processing) also allow synthesis of other compounds e.g. nitrides and sulphides, where some materials are
non-layered and thus not accessible through other more conventional deposition methods.40,42
These dry printing processes may also be used to print
multiple layers and heterostructures where an initial printed
2D metal oxide is followed by a second print of a second layer.
For instance, double-layer ITO37 and b-TeO245 (Fig. 5j) have
been produced through multi squeeze-printing and roll transfer, respectively. In the latter, printing of the oxide onto a
variety of substrates was demonstrated in Fig. 5(a–i), indicating
the versatility of the method. A heterostructure made of individual SnO and In2O3 layers has also been created via a double
touch printing process, creating semiconductor junctions that
can be applied in photodetection applications.51
3.2.3 Liquid phase synthesis of 2D metal oxides, including
sonication and bubbling. In spite of scalable and high-quality
2D sheets being reliably obtained on a substrate from the
aforementioned methods, in some applications, a different
approach is required. For example in catalysis, batteries, certain sensors, and reinforcing polymer fillers, larger quantities
of nanomaterials in liquid suspension that can be further
processed are preferred. Therefore, solution-based synthesis
methods have also been introduced to create novel 2D materials from liquid metal and liquid metal alloys.
As a process, sonication can be used to create nanoparticles
composed of a metal core and an ultrathin oxide shell.46 In this
process, bulk droplets are submerged in water and are then
heated and sonicated to create a dispersion of liquid metal
micro- and nano-droplets that subsequently react to form a
hydrated oxide skin, as elucidated in Fig. 6a. If these nanoparticles are allowed to age in the aqueous solution (for
example overnight), it has been shown that for sonicated EGaIn
nanoparticles, the oxidation by water will dominate and the
Ga2O3 oxide skin will be converted into GaOOH.70 Process
parameters such as the sonication source (probe30 vs. bath71),
probe amplitude, process times30,72 and temperature46,73 may
all be varied to alter the shape,46,72 size,46 and thickness30,46 of
the produced nano-structures. The incorporation of compounds such as lysozyme72 or different thiols30 may also be
used to stabilise and control the different materials produced.
Increasing the sonication time was found to lead to cracking of
the surface oxides followed by delamination into suspension,
allowing for the production of oxide sheets that may be dropcast onto substrates or used in colloidal suspension. This
process is self-repeating, since once the initial oxide layer
detaches from the liquid metal surface, a new oxide skin
forms.46 Fig. 6(b–d) shows an example of Ga2O3 nanoflakes
obtained from liquid EGaIn submerged in DI water followed by
sonication.46
2D metal oxides can also be produced using a bubbling
technique, where a pool of liquid metal or liquid metal alloy is
submerged under a suitable solvent solution before the air is
injected into the bulk liquid metal to create a stream of gas
bubbles.4 The gas bubbles expand within the bulk metal and
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Fig. 4 (a) Roll-transfer method. A glass pipette or a glass rod is utilised to roll a liquid metal droplet across a substrate surface mimicking handwriting,
resulting in a 2D oxide sheet being deposited onto the substrate. (b) An optical microscopy image of ultrathin 2D b-TeO2 printed from liquid Se–Te alloy
onto a SiO2 substrate via the roll-transfer approach. (c) Low-resolution TEM image, (d) high-resolution TEM image, and (e) selected area diffraction
pattern of the material synthesised onto a TEM grid. (f) An AFM image of the exfoliated 2D b-TeO2.45 Reproduced from ref. 45 with permission from
r Springer Nature Limited 2021.
Fig. 5 (a–i) Printed 2D b-TeO2 on a variety of substrates where X indicates the location of the flake and I–IX represent quartz, glass, 100 nm Pt on Si, Si,
300 nm Al2O3 on Si, sapphire, GaAs, mica, and 300 nm SiO2 on Si substrates, respectively. (j) The result from multi-printing of 2D b-TeO2 on 300 nm SiO2
via roll-transfer.45 Reproduced from ref. 45 with permission from r Springer Nature Limited 2021.
the O2 reacts at the bubble-liquid metal interface to form oxide
skins following the Cabrera–Mott process.5 These bubbles, and
the oxides grown at their interface, rise into the solution where
the solvent collects the oxide,74 producing 2D sheets with
lateral dimensions in the nano-48 and micro-47 meter regime.
The bulk liquid metal and larger particulates are then allowed
to fall out of suspension before the dispersed 2D sheets can be
removed and drop-cast onto substrates or are otherwise
utilised.4 Fig. 6e depicts the process of liquid metal bubbling
where Fig. 6(f–g) present ultrathin Ga2O3 and HfO2 synthesised
from this method. The solution chosen to suspend the droplet
may also be tailored. For example, 2D TiO2 and CoO were
synthesised from liquid Ga alloyed with Ti47 and Co,48 respectively. Both alloys were suspended in hydrochloric acid which
dissolves Ga2O3 that may form rapidly on the alloy surface,
enabling TiO2 and CoO to grow. Recent works also demonstrated the use of polyol solvents and molten salt as a dispersion solvent which enables working with higher melting point
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liquid metals and liquid metal alloys (above 100 1C) such as
elemental Sn and Sn–Bi.75,76
3.2.4 Liquid metal as a redox reaction platform for 2D
metal oxide self-deposition. Liquid metals may also be used as
redox reaction platforms where precursor materials react with
the liquid metal and are subsequently reduced to form a
surface skin as depicted in Fig. 7a.49,50,77 This method is ideal
for 2D metal oxides and other 2D materials, whose precursor
metal solubility in liquid metal is low. Therefore, instead of
alloying the precursor metal with a liquid metal, a precursor
compound is solubilised in a traditional solvent which is then
brought into contact with the bulk liquid metal. The liquid
metal then reduces the solubilised precursor at its surface, and
the resulting ultrathin sheets may be removed via sonication
for drop-casting. Prior to suspension in precursor solution,
EGaIn, the most prevalent selected liquid metal alloy used in
many published works, is usually washed with HCl to dissolve
the native oxide skin and create a surface potential at the
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Fig. 6 (a) Sonication process where liquid metal is placed in water or another solution, resulting in nanomaterials in liquid suspension. (b) SEM image,
(c) HRTEM micrograph, and (d) AFM image with the inset featuring height profile of Ga2O3 nanoflakes.46 (e) Bubbling technique. Pressurised air is injected
into the liquid metal ‘pool’ submerged in a selected solution, creating air bubbles in which metal oxide is formed at the bubble surfaces. The oxides
accumulate in the solution and can be further processed. AFM images and TEM characterisation including SAED pattern and HRTEM images of (f) Ga2O3
and (g) HfO2 derived from the bubbling method.4 Note: (b–d) Reproduced from ref. 46 with permission from r 2017 WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim. (f and g) Reproduced from ref. 4 with permission.
electrical double layer (EDL). The surface electrons within
EGaIn, and the ions within the EDL are then able to reduce
the precursor material to form a surface skin at the liquid metal
interface. This self-deposition method has been successful in
creating materials such as hydrated MnO2 monolayers,49 2D
MoS2,50 and MoO2 layers.77
Hydrated MnO2 monolayers, for example, can be grown
utilising aqueous solutions of KMnO4. If a droplet of EGaIn is
then immersed in this solution, the KMnO4 is reduced to
MnO2. When the sample was left to develop and age for a
week, the weakly attached sheet of 2D oxide could be removed
by gently shaking the vial leading to self-detachment. Alternatively, the EGaIn droplet could be sonicated in different concentrations of KMnO4 to form particles, upon which 2D MnO2
flakes formed in a process similar to that shown in sonication
and bubbling based oxide synthesis (Fig. 7a).49 The oxides
grown via a galvanic replacement reactions detach from the
liquid metal bulk and become suspended in the permanganate
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solution. This can subsequently be drop cast onto substrates
for analysis or device fabrication. The flakes synthesised using
the sonication method were found to be more rigid and
contained minimal wrinkling, unlike the single monolayer
grown in the week-long experiment which was only B 1 nm
thick, porous, and slightly wrinkled. The wrinkling effect was
attributed to the self-delamination process and minimal sheet
thickness.49 To obtain 2D MoS2, an aqueous (NH4)2MoS4
solution was used to react with liquid EGaIn. The alloy immediately reduced the precursor at its surface to form a MoS2 layer
that encompassed the droplet. The droplet was then washed
multiple times to remove the precursor solution and prevent
further growth. It was then dried before the 2D sheet was
extracted using a piezoelectric nano-positioning setup as shown
in Fig. 7a, which aids in the touch printing of the sulphide
sheet onto a substrate (Fig. 7b and c).50 Immersing an EGaIn
droplet in a Na2MoO4 solution provided a uniform layer of
H2MoO3 on the surface of the alloy through a similar reaction
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Fig. 7 (a) Schematic illustrating the synthesis of 2D MoS2 exploiting a
redox reaction that occurs on the liquid EGaIn surface which is submerged
in an aqueous solution of (NH4)2MoS4. The MoS2 nanosheet that formed
on the EGaIn surface is then exfoliated via other liquid metal synthesis
techniques such as sonication or touch-printing. (b) AFM morphology and
(c) HRTEM image of the synthesised 2D MoS2 nanosheet.50 Reproduced
from ref. 50 with permission from r 2020 Wiley-VCH GmbH.
concept.77 The piezo-stage was then employed to touch-print
the H2MoO3 layer onto a substrate. Subsequent thermal annealing or exposure to a laser can then help converting the material
to crystalline MoO2, and selected area patterns can be created
directly in specific areas of the materials.
In the case of synthesising MoS2, the [MoS4]2 replaces the
anions within the EDL and reacts with H+ ions to form MoS3
before being reduced to MoS2.50 In the case of MnO2, MnO4
ions are reduced by the Ga at the surface of EGaIn to hydrated
MnO2 and Ga3+ to form [Ga(OH)4] ions that are soluble,
preventing the formation of a Ga2O3 surface skin.49 In most
cases the liquid metal functions both as a smooth template and
as a reducing agent and hence is not a catalyst. As such, the
molten metal is usually oxidised, leading to the release of ions
into the solvent.
The EDL of the liquid metal droplet provides an ideal
reaction environment for the reduction of the precursor material to a 2D surface skin. The droplet itself provides an ultrasmooth platform for this growth process to take place and
results in the desired material. The length of time in suspension, sonication time, as well as concentration of the precursor
solution all affect the thickness, porosity, and size of the
materials produced and can be tailored to suit the needs of
the experiment. For example, MnO2 tends to be used in dye
degradation and photocatalysis49 where a high surface area is
desired. MoS2 on the other hand, is a transition metal dichalcogenide (TMD) that is used in optoelectronics50 and therefore,
a single complete flake may be more desirable for device
applications.
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Another self-deposition method used involves suspending a
droplet of liquid metal (Ga or Galinstan) from a syringe as the
working electrode in a three-electrode setup. This type of setup
was used to synthesise GaOOH nanoparticles. The potential
was varied to allow for crystal formation and subsequent
ejection from the liquid metal into a sodium nitrate/sodium
hydroxide solution of varying pH, which aided in the tailoring
of particle size and shape.78 This method differs from the
previous two examples as the source of the synthesised material
is the liquid metal and not the solution it was suspended in. It
also produces crystalline oxyhydroxide nanoparticles and not
2D metal oxide sheets in contrast to other synthesised materials
discussed here, however, the approach may useful for the
synthesis of other 2D materials in the future.26 More recently,
an electrochemically actuated liquid metal pump setup has
been demonstrated for the synthesis of colloidal bismuth
oxides from dilute gallium bismuth alloys. The induced surface
flow has been shown to effectively delaminate the formed
surface compounds, enabling the production of larger quantities of material.79
3.3
2D materials beyond oxides
The self-deposition method discussed above allows for the
selection of different precursor materials, which enables the
synthesis of different materials at the bulk liquid metal interface, including two-dimensional metal oxides,49 sulphides,50
hydroxides,78 and other 2D materials. Various 2D materials can
also be grown directly from the liquid metal. One such
approach involves altering the reaction environment, which
enables the synthesis of materials such as metal sulphides
when using an anoxic H2S environment (Fig. 8a–c),80 or metal
nitrides in the presence of NH3 (Fig. 8d–f).81 Alternatively, other
2D materials may be synthesised by first printing a 2D metal
oxide and then converting it to other materials via any number
of post- processing steps. These methods will be discussed
further in the following sections of this tutorial review.7,55,56
4. Post-growth processing
Since liquid metals can be used as a platform to produce 2D
metal oxides, they also provide a synthesis route to access
pnictogenide and chalcogenide compounds via post-growth
processing treatments. Metal pnictogenides (nitrides, phosphides, arsenides and antimonides) and metal chalcogenides
(sulphides, selenides and tellurides) are important members of
the larger family of 2D materials. These materials are expected
to play a significant role in the future of high-performance
electronic and optoelectronic devices, with their high carrier
mobilities, tunable bandgaps, and many exotic properties –
such as giant valley splitting, topological states and giant
piezoelectricity – making them highly versatile for a range of
device applications.82–92 The approaches developed here have
enabled the synthesis of numerous ultrathin semiconductors
featuring tenuity down to a few unit cells while also featuring
lateral dimensions reaching centimetres. Furthermore, these
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Fig. 8 (a) Touch-printing of 2D SnS from liquid Sn in the presence of
50 ppm H2S gas at 350 1C with (b) dark field TEM image and (c) AFM image
(the inset featuring the height profile corresponds to the yellow line).80
(d) The synthesis of GaN nanosheets from liquid Ga exposed to an NH3
environment. (e) AFM image with the corresponding thickness profile and
(f) low-resolution TEM image of the synthesised 2D GaN.81 Note: (a–c)
Reproduced from ref. 80 with permission. (d–f) Reproduced from ref. 81
with permission from r 2018 American Chemical Society.
methods provide access to a range of 2D materials that do not
naturally crystallise in a stratified structure and are thus
inaccessible via conventional methods.6,7,40,42,43,54–56,93
4.1
Vapor phase anion exchange
Conventional 2D metal oxides derived from the liquid metal-based
methods can be converted into desired 2D crystals via hightemperature tube furnace-based conversions.6,40,42,43,54–56,93 The
setup consists of three main components: a temperature programmable tubular furnace, a quartz process tube with two ceramic heat
isolation blocks, and a carrier gas controller resembling conventional chemical vapor deposition equipment. In the first step,
2D metal oxides prepared on preferred substrates are placed
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upside-down on a ceramic boat to avoid the accumulation of
unwanted compounds on the synthesised nanosheets, while precursor materials are placed upstream with respect to the oxide
samples.6,40,54,55 The reaction tube is then evacuated or purged with
an inert gas flow to remove oxygen and moisture.6,40,43,54–56,93 A
schematic of the vapor phase anion exchange set-up is illustrated in
Fig. 9a. In order to achieve uniform 2D structures with minimal
cracks, the heating rate is kept at B10 1C min1 and the system
allowed to cool naturally to room temperature after the
reaction.6,43,55,93 In most cases, nitrogen (N2) or argon (Ar) – with
a flow rate of 50–100 sccm – is utilised as a carrier gas.6,40,43,55,56,93
The transformation into another 2D compound occurs when the 2D
metal oxide reacts at high temperatures with the chosen vaporised
species. The reaction involves the anion exchange and the removal
of oxygen species via the vapor phase. The process is also associated
with the re-crystallisation of the nanosheets.6,54 Following synthesis,
it is recommended that the 2D materials are kept in an inert
atmosphere for prolonged storage in order to avoid reoxidation.6,43,55
To date, it has been reported that several metal sulphides have
been synthesised using this technique.40,43,55,56,93 For example,
gallium(II) sulphide (GaS) was successfully synthesised at a relatively
low temperature using sulphur powder as a precursor.40 This
approach involved an intermediate step of chlorination of 2D
Ga2O3 to ease the subsequent sulphurisation. As-synthesised
Ga2O3 layers were first exposed to fuming hydrochloric acid (HCl)
to convert inert gallium oxide into more reactive gallium chloride,
followed by the reaction with sulphur vapor at 300 1C for 90 min.40
Similarly, In2S3 could be synthesised by employing a bromide
intermediate. Recently, direct sulphurisation of Ga2O3 films has
been demonstrated without the halogenation step, revealing a
temperature-dependent stoichiometry of gallium sulphide for this
reaction. It was also reported that a reaction at 600 1C resulted in
gallium(III) sulphide (Ga2S3)55 while one carried out at 800 1C
resulted in gallium(II) sulphide (GaS) being formed.43 The direct
sulphurisation process can be extended to produce 2D bismuth
sulphide (Bi2S3) and 2D indium sulphide (In2S3).56,93 Importantly,
by controlling the synthesis temperature, either ordered vacancies
or random vacancies in 2D In2S3 nanosheets can be achieved
resulting in remarkably different electronic characteristics of the
final material.56
The chemical vapor deposition route can also be used to
create 2D III-V semiconductors. Ultrathin gallium nitride (GaN)
nanosheets were obtained via an ammonolysis reaction conducted at 800 1C as presented in Fig. 9(b–i).6,42,43 In earlier
work, ammonia was generated from the decomposition of urea
(CH4N2O) and then reacted with 2D Ga2O3 samples.6 However,
later studies directly employed NH3 as the reagent.42,43 In the
case of indium nitride (InN), 2D indium oxide (In2O3)
nanosheets were treated with hydrobromic acid (HBr) fumes
before the nitration process and N2/H2 (1%) mixture was used
as the carrier gas.6 The use of anhydrous phosphoric acid
(H3PO4) as a source material encouraged the growth of piezoelectric 2D gallium phosphate (GaPO4). This phosphatisation
process was carried out at 300 1C.54 Interestingly, by
altering the precursors, different 2D phosphorous compounds
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Fig. 9 (a) A schematic of the vapor phase anion exchange process carried out in a tubular furnace. The 2D metal oxide reacts with the precursor material
at high temperature resulting in an anion exchange reaction and subsequent oxygen removal. Optical microscopy image, AFM images and XPS spectra of
2D Ga2O3 (b–e, respectively) and 2D GaN (f–i, respectively) indicate a successful conversion with the ammonolysis reaction at 800 1C.6 Note:
(b–i) Reproduced from ref. 6 with permission from r 2018 American Chemical Society.
can be synthesised. In another study, sodium hypophosphite
(NaH2PO2) was employed to transform thin Ga2O3 layers into
2D semiconducting gallium phosphide (GaP) at 800 1C in argon
atmosphere.43
Despite being an efficient method of producing large-scale
2D metal pnictogenides and chalcogenides, the furnace-based
methods still require relatively high-temperature processing,
which prohibits the use of many substrates. In addition, gas
phase conversions are often considered more hazardous and
limit the choices of available precursors.
4.2
Solution-based method
In order to develop a well-controlled and less toxic pathway for
post-growth processing of 2D metal oxides, a wet chemical
approach has been investigated where In2O3 was converted to
In2O3xSx (Fig. 10a–d).7 A solution containing [S3] radicals
was prepared by mixing sulphur powder (S), sodium sulphide
(Na2S), and 15-crown-5 ether (15C5) in dimethylformamide
(DMF) under the exclusion of oxygen and water.7,94 The 2D
In2O3 nanosheets were printed using molten indium metal and
then immersed in the [S3] solution. The reaction was carried
out at 150 1C in a nitrogen atmosphere inside a glovebox.7 The
use of the highly reactive trisulphur radical anion [S3] was to
enable sulphur insertion into the 2D metal oxides and partial
oxide anion replacement at comparatively mild reaction conditions. While complete sulphurisation was not achieved, metal
oxysulphides are increasingly being investigated for electronic
applications and catalysis.95–97 This study not only highlights
the compositional tunability of these 2D indium oxysulphide
nanosheets, but also reveals a possible synthesis route for other
2D metal oxysulphide materials, including bismuth oxysulphide and tin oxysulphide. Other reactive species may also be
used to significantly expand the library of 2D materials that are
accessible via this furnace-free method, particularly if reactive
pnictogen and chalcogen species are utilised.
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5. Integration of 2D materials for
device applications
Stacking 2D material layers has been widely used to fabricate
functional heterostructures where individual layers are held
together by van der Waals (vdW) interactions.98–103 Over the
past decade significant effort has been dedicated to study vdW
heterostructures which have been demonstrated to exhibit
unique physics and harbour exceptional properties which are
not present in individual 2D materials.104–108 Integrating 2D
materials derived from liquid metals into such heterostructures
holds significant potential for studying new physical phenomena and enabling their practical application in devices that can
be produced at scale due to the possibility of growing large
lateral area 2D materials using liquid metals. Consequently,
several assembly techniques have been established to fabricate
such hetero-structures including direct synthesis atop of previously deposited 2D materials and deterministic transfer
methods.109
Liquid metal-based synthesis methods offer an alternative
platform to assemble 2D materials as well as integrate them
into useful devices. By way of example, a 2D SnO/In2O3 vdW
heterostructure photodetector was successfully developed
through the liquid metal touch printing method.51 An ultrathin
p-type SnO sheet was exfoliated from liquid Sn onto a SiO2
substrate, followed by the printing an n-type In2O3 nanosheet
from molten In using the same procedure as illustrated in
Fig. 11a. The method resulted in overlapping regions of SnO
and In2O3 (see Fig. 11b), which was utilised to fabricate a
photodetector – as shown in Fig. 15a–c in the later section.
This p–n heterojunction was found to lead to a device featuring
an impressive performance, clearly demonstrating that liquid
metal-based synthesis methods are a promising alternative to
conventional methods for creating 2D vdW heterostructures.
The large-scale synthesis and relative ease of fabrication are
clear advantages. In addition, the thicknesses of these
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Fig. 10 (a) A schematic of a solution-based post-processing method utilised to synthesise 2D In2O3xSx. Ultrathin In2O3 obtained from squeeze-printing
was submersed in a polysulphide solution allowing the sulphur insertion reaction to take place. (b) Optical image, (c) AFM image with the height profile
inset, (d) low-resolution and high-resolution TEM images as well as indexed SAED pattern of the 2D In2O3xSx.7 Note: Reproduced and edited from ref. 7
with permission from r The Royal Society of Chemistry 2021.
synthesised materials are remarkably thin (only a few atomic
layers) and reproducible when compared to those obtained via
conventional methods.61,110–113 Challenges do, however, arise
due to the manual nature of the process with variable flake
sizes and inaccurate spatial deposition limiting this approach
to only 2–3 subsequent layers. Future automation of the process
may aid in rendering this approach more practical.
As an alternative, several deterministic transfer techniques
have been reported where a typical stage set-up equipped with
an optical microscope is required to assist with the alignment
and transfer.114 PDMS is commonly used as a viscoelastic
stamp to support the 2D materials while being transferred.
Amorphous polymer membranes such as polymethyl methacrylate (PMMA), polycarbonate (PC), and poly–propylene carbonate (PPC) are often chosen for an optional sacrificial layer.114
A transfer method has been developed and used to fabricate
WS2/Ga2O3 heterostructures, where the amorphous 2D Ga2O3
glass is evaluated as a passivation layer for photonic
structures.52,115 Squeeze printing was carried out on a PDMS
stamp coated with PPC resulting in an ultrathin and laterally
large-area Ga2O3 sheet. The 2D Ga2O3 on the stamp was then
aligned and brought into contact with the exfoliated WS2 on a
SiO2 substrate using the transfer stage. After the surfaces were
touched, the substrate was heated to release the Ga2O3 on PPC
onto the SiO2 wafer to encapsulate the WS2. The sacrificial
polymer was subsequently removed by washing in acetone,
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isopropyl alcohol and ethanol. A brief schematic describing the
transfer technique as well as an optical image of the WS2/Ga2O3
heterostructure are presented in Fig. 11c and d, respectively. This
study highlights the fact that Ga2O3 can serve as an alternative to
hBN – which is currently the most common insulating material for
2D systems. One of the main advantages of using 2D Ga2O3 as an
insulating material is the reproducible thickness of 2D Ga2O3
determined by the Cabrera–Mott oxidation process, overcoming a
key limitation associated with the mechanical exfoliation process
used for h-BN. Furthermore, the squeeze-printing synthesis process
enables laterally large nanosheets to be isolated, that routinely reach
centimetres, providing a pathway towards up-scaling and integrated
device fabrication.
In order to apply this transfer technique to other 2D materials derived from a liquid metal, alternative transfer methods or
sacrificial polymers should be considered. The relatively low
melting point of Ga (29.76 1C) is beneficial when working with
the temperature-sensitive PPC. Different sacrificial layers featuring higher temperature tolerance exceeding that of the target
metal can also be considered. Alternatively, sheet pick-up
methods may be employed where the 2D material is grown
on a weakly interacting substrate and then delaminated using a
PPC/PDMS stamp. One example of this approach includes the
transfer of 2D TeO2 from a Si substrate to a SiO2 surface.45
Further work should be dedicated to developing these transfer
techniques in order to provide access to high quality, laterally large
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Fig. 11 (a) Fabrication of a SnO/In2O3 heterostructure via a double touch-printing approach with (b) a corresponding optical image.51 (c) A deterministic
transfer method using a PDMS stamp coated with a sacrificial layer of PPC to transfer 2D Ga2O3 onto monolayer WS2 for encapsulation purposes.
(d) Optical image of the WS2/Ga2O3 heterostructure.52 Note: (a and b) Reproduced from ref. 51 with permission from r 2019 WILEY-VCH Verlag
GmbH & Co. KGaA, Weinheim. (c and d) Reproduced from ref. 52 with permission from r 2020 Wiley-VCH GmbH.
heterostructures and multilayers. One interesting observation is the
presence of vdW gaps between bilayer structures that are obtained
via either direct or deterministic transfer approaches, as depicted in
Fig. 12. This gap showcases the fact that the grown nano-sheets,
even when not naturally stratified, maintain their two-dimensional
nature when stacked. Furthermore, these vdW gaps may provide an
opportunity to modify a number of the material properties through
intercalation.116
Fig. 12 Cross-sectional TEM images of (a) squeeze-printed monolayer and (b) bilayer ITO obtained from multi-printing.37 The images with high
magnification in the insets show a distinct difference between monolayer and bilayer ITO with an observed vdW gap. (c) AFM image of 2D Ga2O3
transferred onto a SiO2 substrate with the describe technique utilising a PPC/PDMS stamp.52 In comparison to the as-synthesised Ga2O3 shown in Fig. 2i
and d Ga2O3 after being transferred presented a pronounce vdW gap. Note: (a and b) Reproduced from ref. 37 with permission from r Springer Nature
Limited 2020. (c) Reproduced from ref. 52 with permission from r 2020 Wiley-VCH GmbH.
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6. Recent applications of 2D materials
from liquid metal-based synthesis
2D materials synthesised via liquid metal-based techniques are
increasingly being used to create functional devices and enable
a whole range of new applications. The following section
reviews these emerging fields of research while also highlighting several areas where 2D materials could fill critical
capability gaps.
6.1
Field-effect transistors (FETs)
Field-effect transistors (FETs) are among the basic building
blocks of electronic and optoelectronic devices. In essence,
FETs fulfil the function of a switch, where adjusting the
potential on one electrode, known as a gate, regulates current
flow through the semiconducting layer located between a
source and a drain electrode. 2D metal oxides are attractive
for this application due to their robust chemical nature, high
carrier mobilities and commonly wide bandgaps. A wide variety
of liquid metal-derived 2D metal oxides and 2D materials have
been synthesised and investigated as channel materials for
FETs. Touch-printing of Ga2O3 followed by sulphurisation was
demonstrated in an earlier reported study.40 This process
produced GaS, which was then utilised as a semiconducting
channel for FETs featured an average electron mobility and on/
off ratio of 0.2 cm2 V1 s1 and 150, respectively. The comparatively poor device performance of these early examples could be
attributed to the intrinsically unfavourable electronic properties of GaS. However, the same approach was repeated in later
work using elemental In to grow the 2D In2O3 which was then
converted into In2S3. Here the best performing In2S3-FET
exhibited an impressive electron mobility of 58 cm2 V1 s1
with an on/off ratio approaching 104.56 More recently,
In2O3xSx was successfully synthesised from liquid indium
using squeeze printing, followed by a solvothermal sulphur
insertion.7 The isolated 2D material was then employed in FETs
featuring a maximum electron mobility of 44 cm2 V1 s1.
Similarly, squeeze-printed Zn-doped In2O3 (IZO) obtained from
an In–Zn alloy and employed in a FET featured an electron
mobility of 87 cm2 V1 s1 (see Fig. 13a–c).117 These studies
highlight that liquid metal derived nanosheets possess electronic properties that are defined by the fundamental nature of
the synthesised material. While being comparatively simple,
the printing methods can produce excellent semiconductors
that may find application in practical devices. While many
n-type metal oxides have been reported, there is generally a
lack of high mobility p-type oxide materials. This results from
the fact that most metal oxides feature valence band edges
dominated by highly localised O 2p orbitals and this in turn
leads to relatively low mobilities.118
Nevertheless, the discovery of high mobility p-type oxides
could significantly advance the broader field of transparent
electronics, enabling p–n junctions, as well as photovoltaic
research where degenerately doped p-type oxides could provide
improved ohmic contact to light absorbers.118 As such, liquid
metal-based methods have recently been used to investigate
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p-type 2D metal oxides. The touch-printing method was
exploited to exfoliate SnO from liquid Sn, resulting in an FET
mobility 0.7 cm2 V1 s1.119 The isolated 2D p-type SnO was
also integrated into a p-n heterojunction for photodetection
applications, as previously mentioned (see Integration of 2D
materials section). Another study also reported the use of 2D
SnO as a semiconducting material for thin-film transistors
(TFT) with a channel thickness of B1 nm.120 The device
exhibited p-type characteristics with the mobility and on/off
ratio of 0.47 cm2 V1 s1 and 106, respectively, confirming the
findings of earlier work. The performance data extracted from
these liquid metal-based SnO devices are competitive with 2D
SnO FETs obtained via a conventional method.110,111 Furthermore, a low-power complementary inverter was also fabricated
using p-SnO/n-SnO2 and p-SnO/n-In2O3 where all the materials
used were derived from liquid metal touch-printing.120 It was
observed that the p-SnO/n-In2O3 inverter provided lower power
dissipation compared to the p-SnO/n-SnO2 device.120 Recent
work also successfully isolated a novel p-type b-TeO2 from Se–
Te chalcogen mixture through roll-transfer technique.45 This
indicates that these methods produce materials that are not
only compatible with metallic systems, but may also be applied
to low melting mixtures of non-metals. The TeO2-FET featured
a very high hole mobility of 232 cm2 V1 s1 and an on/off ratio
that exceeded 106 (Fig. 13d–f). The reason for this high performance is the structure of the valence band edge of TeO2, which
is dominated by dispersed hybridised Te 5p–O 2p states.45,121
FET performances extracted from selected 2D materials-based
devices, where the active materials were derived from the liquid
metal-based syntheses, are shown in Table 1.
Most reported FETs in this section were fabricated using a
back-gated configuration, where a conductive p-type doped Si/
SiO2 wafer is used as substrate. While this configuration is well
suited for material characterisation purposes, the joint gate
electrode prohibits the independent switching of multiple
devices on a single wafer. To further improve device performance, and offer the opportunity to create integrated circuits, it
is necessary to adjust the device design to enable multiple gates
and optimise the electrode materials. This allows for improved
contacts, as well as introduce doping in order to enable a wider
variety of applications. Encapsulating 2D materials to protect
them from further fabrication processes is equally important.
Although hexagonal boron nitride (hBN) is widely used as a
protective layer, it has limitations in terms of the size and
quality of the flakes of material.122 Recent work has proposed
the use of large-area 2D Ga2O3 nanosheet derived from liquid
Ga via squeeze printing as an alternative passivation
material.52,115 The transfer method described earlier in the
Integration of 2D materials section was utilised to transfer
the 2D Ga2O3 onto exfoliated WS2. Results have shown that
2D Ga2O3 works well as a protective layer and could enhance
optical performances, as shown in Fig. 14(a–c), rendering it a
desirable alternative to hBN.52,115 Importantly, the 2D Ga2O3
can be used as a protective layer onto which thicker oxide layers
were grown via conventional vapor phase deposition methods
(Fig. 14d–f).52 As such, this technique may be well suited for
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Fig. 13 Electronic applications. (a) AFM image of IZO with corresponding height profile inset. (b) Gate voltage and drain current transfer curve and
(c) n-type I–V characteristic curves of the IZO-based FET.117 (d) SEM image of the FET with 2D b-TeO2 as a channel material. (e) Transfer and
(f) characteristic curves (VD = 1 V) revealing p-type behaviour of the 2D b-TeO2 FET.45 (g) An optical microscopy image of ultrathin a-Sb2O3. (h) I–V
curve obtained from conductive AFM tunnelling current through a point on the material indicating a competitive breakdown voltage. The inset shows
data fitted utilising the Schottky emission model, which revealed the maximum dielectric constant of 84 for a 3 nm thick a-Sb2O3. (i) Dielectric constantfrequency characteristics of a-Sb2O3 measured from a Au/a-Sb2O3/p-GaAs device (inset) with a maximum extracted dielectric constant of 80.36 Note:
(a–c) Reproduced from ref. 117 with permission from r 2021 American Chemical Society. (d–f) Reproduced from ref. 45 with permission from
r Springer Nature Limited 2021. (g–i) Reproduced from ref. 36 with permission from r 2021 American Chemical Society.
creating top-gate structures for liquid metal-derived 2D semiconductors. Ultrathin a-Sb2O3 presented in Fig. 13g was also
recently reported as a promising high dielectric constant (k)
gate oxide material.36 This material was isolated via touch-print
from a liquid Bi–Sb alloy. Conductive AFM analysis combined
with electron energy loss spectroscopy revealed a maximum
dielectric constant of between 80 to 84 (Fig. 13h–i).36 The
material was then utilised as a top-gate dielectric for FETs.
The a-Sb2O3 gate oxide demonstrated low leakage currents
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confirming the excellent dielectric properties and insulating
capability of this material.
6.2
Photodetection
Wide bandgap 2D metal oxides also find roles in photodetection and with the aid of the liquid metal-based syntheses,
several materials have been studied in this application space.
Very recent work has successfully developed a visible-blind
photodetector exploiting 2D ZnO exfoliated from liquid Zn
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Table 1
Selected 2D materials derived from the liquid metals and their application as semiconducting transistor channel materials
Synthesis
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Material
Blade coating
GaS
Touch-printing
SnO
Touch + sulphurisation
Ga2S3
Touch + sulphurisation
In2S3
Touch + sulphurisation
Bi2S3
Squeeze-printing
IZO
Roll-transfer
b-TeO2
Touch-printing
SnO
Squeeze + solution-based oxygen replacement In2O3xSx
Squeeze + ammonolysis
GaN
Surface-confined nitridation
GaN
Thickness (nm) Bandgap (eV) Charge carrier Mobility (cm2 V1 s1) On/off Ref.
1.5
0.6
2
3.7
2.5
1.59
1.5
1
2.2
1.3
4.1
3.02
4.2
2.1
2.3
2.3
3.51
3.7
—
3.33
3.5
—
Electron
Hole
Hole
Electron
Hole
Electron
Hole
Hole
Electron
Electron
Electron
mFET = 0.2
mFET = 0.7
mFET = 3.5
mFET = 58
mFET = 12.4
mFET = 87
mFET = up to 232
mFET = 0.47
mFET = up to 44
mHall = 21.5
mFET = 160
150
20
102
104
103
105
106
106
60
—
106
40
119
55
56
93
117
45
120
7
6
81
Fig. 14 PL images of (a) an as-exfoliated monolayer WS2 and (b) WS2 capped with ultrathin Ga2O3 under large Gaussian laser spot excitation (25 mm)
showing a homogeneous PL texture. The scale bar size is 5 mm. (c) PL spectra of the as-exfoliated WS2 and WS2/Ga2O3 heterostructure with laser intensity
of 34 mW mm2 at cryogenic temperature.52 (d) PL spectra of the as-exfoliated WS2 as well as WS2/Al2O3 heterostructure where Al2O3 was directly
deposited onto the monolayer via electron beam evaporation with high electron beam energy. As a result of the deposition, the exciton PL was
significantly suppressed. (e) Schematic of a monolayer WS2 capped with a protective layer of hBN (top) and Ga2O3 (bottom) prior to the direct electron
beam evaporation of Al2O3 and (f) corresponding PL spectra.52 The results indicated that 2D Ga2O3 outperformed hBN in protecting the monolayer WS2.
Note: Reproduced from ref. 52 with permission from r 2020 Wiley-VCH GmbH.
using a touch-printing method.123 At a wavelength of 365 nm,
this device exhibited high figures of merit and the material was
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also explored in an artificial optoelectronic synapse in which
the promising results could pave the way to neuromorphic
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applications. A printed solar-blind photodetector was developed on a Ga2O3/p+–Si heterojunction (Fig. 15d–f).44 Crystalline
Ga2O3 was synthesised via a blade coating method at 200 1C.
The device showed maximum responsivity (R) and greatest
quantum efficiency (EQE) of 44.6 A W1 and 2.2 104% at
the wavelength of 254 nm, respectively. A 2D MoS2/GaN van der
Waals heterostructure photodetector was fabricated and possessed EQE of 764% and R of 328 A W1 under 532 nm light
illumination (Fig. 15g–i).42 The device was produced using a
tape-printing approach to synthesise Ga2O3 followed by exposure to NH3 gas at 800 1C in a tube furnace to acquire GaN on
SiO2/Si substrate. Subsequently, p-MoS2 was exfoliated from a
bulk crystal using scotch tape, which was then transferred onto
PDMS before being deposited onto the 2D GaN, creating a
Chem Soc Rev
heterostructure system with photosensitivity in the visible
region. Using GaS 2D sheet synthesised from molten Ga
followed by sulphurisation was also explored in a series of
phototransistors, where high on/off ratios of 170 and a photoresponsivity of 6.4 A W1 were observed.40
6.3
Piezotronics and 2D flexible devices
2D piezoelectric materials have the ability to convert mechanical force into electricity and vice versa, which makes them both
useful and intriguing for applications in the area of piezotronics. Although it is a well-known piezoelectric material,
GaPO4 has not been intensively investigated in terms of its
structure and electronic properties.124,125 With the liquid metal
based touch-printing, followed by phosphatisation, ultrathin
Fig. 15 Photodetection applications. (a) The ID–VD curves of the 2D p-SnO/n-In2O3 heterostructure operating under dark and light illumination at
wavelengths of 280, 365, and 455 nm. The inset illustrates the device which was fabricated via multiple touch-printing as described in the previous
section. (b) Photoresponsivity (R) at VD = 20 V of the individual SnO and In2O3 devices and the SnO/In2O3 heterostructure under three different
illumination wavelengths (280, 365, and 455 nm). (c) The photocurrent responses as a function of time measured from the SnO/In2O3 p–n
heterojunction at VD = 20 V under the three illumination wavelengths.51 (d) A schematic of the Ga2O3/Si heterostructure photodetector. (e) The
I–V properties of the device excited with 254 nm and 365 nm (the inset) UV light sources and under dark condition suggesting no response to 365 nm
wavelength. (f) Photoresponsivity and the efficiency of external quantum (EQE) under 254 nm UV light at different voltages.44 (g) Schematic representing
a photodetector fabricated on 2D MoS2/GaN heterojunction. (h) The I–V characteristics showing the device provided good response to light illumination
(532 nm and 365 nm). (i) The time-resolved photoresponse of the device under 365 nm light illumination revealing impressive switching behavior.42 Note:
(a–c) Reproduced from ref. 51 with permission from r 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d–f) Reproduced from ref. 44 with
permission. (g–i) Reproduced from ref. 42 with permission from r 2020 American Chemical Society.
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GaPO4 sheets were readily obtained.54 The material provides
relatively high piezoelectricity and experimental values agreed
well with theory, indicating its potential use in 2D piezotronics.
Among group IV monochalcogenides, SnS has been theoretically predicted to possess a high piezoelectric coefficient.126 An
experiment was carried out exposing liquid Sn in anoxic atmosphere (H2S and N2) to form a sulphide skin which was
subsequently exfoliated onto a flexible substrate.80 The SnSbased piezoelectric nanogenerator was then fabricated and
showed a large average voltage peak output of B150 mV at
0.7% strain. The 2D SnS featured a piezoelectric coefficient d11
of 26 pm V1. Another example is PbO, which represents a
promising alternative to lead zirconate titanate (PZT) as a
piezoelectric material. The oxide layer formed on liquid
Pb and was printed onto a substrate.127 The subsequent fabricated device revealed vertical piezoelectric coefficient d33 of
29.6 pm V1 which is competitive with other 2D materials used
in piezotronic systems.
Several piezoelectric devices are, by necessity, constructed
on flexible substrates. However, the wider field of flexible
electronics may well benefit more broadly from the use of
liquid metal-derived 2D materials. The capability to deposit
large area, 2D nanosheets that exceed several centimetres in
size while still featuring excellent electronic properties is a
crucial advantage of these techniques. Furthermore, the capability to grow 2D nanosheets at comparatively low temperatures enables the use of a wider range of flexible substrates. In a
recent study large area 2D ITO was printed directly onto flexible
polyimide substrates, introducing a new flexible conductive
material with extraordinary transparency of 99.7% for a single
layer.37 The same study also demonstrated the use of liquid
metal printed 2D ITO in a capacitive touch screen prototype as
shown in Fig. 16(a and b).37 Overall, these early prototype
device structures indicate that liquid metal based techniques
offer significant opportunities for transparent and flexible
electronics.
6.4
Gas sensors
Electronic gas sensors typically rely on semiconducting materials that can be doped via surface adsorbed gaseous species, and
the use of ultrathin materials has become increasingly prevalent due to their intrinsically high surface area to volume ratio.
As such, it is not surprising that liquid metal derived compounds have found multiple applications in gas sensing. For
example, 2D Ga2S3 has been synthesised by converting Ga2O3
derived from squeeze printing in a tube furnace at 600 1C using
sulphur powder as a reagent.55 The material exhibited p-type
characteristics and was applied to fabricate a series of NO2 gas
sensors (Fig. 16c and d). A decrease in the device resistance was
observed since the NO2 gas molecules act as electron donors,
resulting in charge transfer to the 2D Ga2S3, after surface
adsorption. The device showed excellent performance with a
NO2 detection limit of 10.7 ppm at room temperature. The
fastest response and recovery times of 215 and 185 s, respectively, where achieved when operating at elevated temperatures
(150 1C). Similarly p-type SnO has been synthesised using liquid
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Fig. 16 (a) An image and schematic of the 2D ITO capacitive touch screen
(left) and test pattern used for touch screen characterisation measuring the
response to a touch by a metal pin (right). (b) Capacitive response maps
showing high sensitivity of the fabricated capacitive touch screen.37
(c) A schematic illustration of the 2D Ga2S3-based gas sensor. (d) Normalised response factor of the gas sensor. In the presence of 10.7 ppm of NO2
gas at 150 1C, a response and recovery time of 215 s and 185 s,
respectively, with a normalised response factor of 8.5% were obtained.55
(e) A schematic of a 2D SnO2 memtransistor with a cross-sectional TEM
image of the ultrathin SnO2. (f) Simulated pattern recognition accuracy for
a 8 8 pixel image in ideal numeric training (blue line) and the 2D SnO2based memtransistor (VG from 2 to 10 V). The accuracy of the fabricated
memtransistor was found to be B92.25%. The inset shows a three-layer
neural network including 8 8 pre-neurons, 36 hidden neurons, and 10
output neurons, for pattern recognition simulation.129 Note: (a and b)
Reproduced from ref. 37 with permission from r Springer Nature Limited
2020. (c and d) Reproduced from ref. 55 with permission from r 2019
American Chemical Society. (e and f) Reproduced from ref. 129 with
permission from r 2021 American Chemical Society.
metal techniques, enabling the fabrication of an alternative
NO2 gas sensor.119 Liquid metal-based methods have also been
applied to synthesise Bi2Te3, a topological insulator, highlighting the remarkable versatility of these methods.128 In this case,
the Bi2Te3 produced was also used for NO2 gas sensing applications, revealing an outstanding detection limit of 0.90 ppm and
excellent selectivity when operating at near room temperature.
6.5
Other applications
Two-dimensional metal oxides and other 2D materials derived
from liquid metals are extremely versatile and are increasingly
utilised in a wide array of applications in addition to those
mentioned above. For example, a 2D SnO-based lateral
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memristor was recently demonstrated using an adapted touchprinting method involving liquid Sn.68 In this work, molten Sn
was extruded through a graphite nozzle in an oxygen-free
environment resulting in a clean area that was then exposed
to diluted O2 containing carrier gas. A flexible substrate with
pre-patterned electrodes was then brought into contact to
exfoliate the SnO from the liquid parent metal. After several
mechanical bending tests, the device displayed high stability
and durability as well as consistent memristive switching
behavior.68 A 2D-SnO2 memtransistor that could be utilised in
neuromorphic computing was also recently fabricated via a
touch-printing method and is shown in Fig. 16(e and f).129
Impressive resistive switching results were obtained, while a
high recognition accuracy of 92.25% indicated this approach to
be a promising development in neuromorphic device fabrication. Another work also reported the isolation of SnOx
nanosheets through a bubbling synthesis, which was then
utilised for electrocatalytic CO2 reduction.75
Recent work also demonstrated the fabrication of antimicrobial fabrics with the aid of the reduction reaction method.130
The galvanic replacement occurs when a fabric coated with Ga
particles is immersed in a CuSO4 precursor solution resulting
in the growth of Cu crystals which strongly attached to the
fabric. The coated fabric showed effective responses to contaminations and was reusable multiple times causing less Ga
consumption. The exceptional results have shown potential in
creating antibacterial, antifungal, and antiviral surfaces for
applications such as face masks. This highlights that the
strategies involving the deposition of liquid metal derived
2D materials for healthcare products should be explored,
potentially expanding the application range of developed
compounds.
Other potential applications for the materials discussed in
this tutorial review include photocatalysis,49 biosensors and131
spintronics,132 where the developed liquid metal-based
approaches could provide a pathway to new 2D materials that
may be suitable for these applications. Overall, liquid metal
derived 2D materials hold the potential to supplement the
already existing family of nanoscale materials while offering
the capability to create 2D materials with reliable thickness and
extraordinarily large lateral dimensions. As such, these methods may facilitate future upscaling and device manufacturing
as well as contributing new materials for fundamental research.
7. Conclusions and outlook
As research into 2D materials is expanding, new synthesis
routes need to be developed in order to overcome current
limitations associated with lateral sizes, substrate specific
deposition and inconsistent layer thicknesses. By exploiting
oxidation phenomena that take place at the surface of liquid
metals, new synthesis approaches have been developed that
overcome many of these challenges. Over the past few years,
several liquid metal-based methods have been developed which
are presented in this tutorial review. While the majority of the
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work has focused on two-dimensional oxides, 2D materials
beyond oxides have also been obtained either by directly
introducing the liquid metal to a tailored anoxic environment
or by conducting subsequent post-growth processing on previously isolated oxide sheets. The methods developed and
described have been shown to successfully provide a wide range
of large area, high crystalline quality, and uniform 2D materials. However, despite this rapid early progress, liquid metal
chemistry is still in its infancy, requiring further research while
offering significant potential for future discoveries.
To date, limited thermodynamic data is available for mixed
oxides and more complex alloy systems, limiting the accuracy
of predictive equations for preferential oxide growth. The
physicochemical environment of the metal–air interface also
remains poorly understood and further theoretical and experimental studies are required in order to support future progress.
The development of fundamental knowledge in the surface
chemistry and physics of liquid metals and liquid metal alloys
is also needed. Investigation of properties such as surface
wettability and interactions with various substrates as well as
the mechanical properties of various interfacial oxides will be
required to further improve these synthesis methods and
ultimately reveal the key process parameters required to enable
large scale automated 2D layer production. Developing a deeper
knowledge will also lead to a more detailed understanding of
the formations of defects, pinholes, metal inclusions, and
wrinkles, and assist in either minimising their occurrence or
controlling their formation specifically to exploit these particular features.
Future work should also focus on exploring different 2D
materials, particularly those that are not experimentally accessible via other growth techniques. Gaining full control over a
material’s crystal structure, thickness, and composition may be
possible by precisely controlling liquid metal alloy compositions, temperature, reaction times and the composition of the
oxidising environment. Post-growth processing techniques
have also been shown to be a powerful tool to enhance properties and broaden the scope of liquid metal chemistry. Future
work should include mechanisms and pathways of introducing
dopants into semiconducting 2D materials. One intriguing
strategy could involve the chemical intercalation of multilayer
structures.
The assembly of 2D layers to obtain high-quality heterostructures is also crucial to unlock the full potential of these
materials. Further work should focus on developing a wider
array of transfer techniques that work at different temperatures
and facilitate reliable stacking of a vast array of liquid metal
derived nanosheets. Different temperature-sensitive sacrificial
polymers could be considered and ideally, contact-free transfer
methods would be developed that minimise contamination.
Addressing these challenges as well as pursuing the successful integration of 2D materials into functional devices, will
provide new pathways for many applications, including applications in transparent, flexible, and wearable electronics.
Further applications are expected to be developed within the
areas of catalysis; however, the vast array of accessible 2D
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materials that can already be synthesised is expected to enable
further applications in many different areas.
Conflicts of interest
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There are no conflicts to declare.
Acknowledgements
P. A., T. D. and C. K. N. acknowledge funding received from the
Australian Research Council (ARC) through the DECRA scheme
(DE190100100). P. A. and A. G. acknowledge additional scholarship support from the ARC Centre of Excellence FLEET
(CE170100039). A. E. acknowledges funding received from the
Jack Brockhoff Foundation (JBF grant no. 4655-2019-AE). A. Z.
acknowledges the University of Melbourne for support through
the McKenzie postdoctoral fellowship program.
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