Showcasing research from the Yale Faboratory, School ofEngineering and Applied Science, Yale University, United States.

As featured in:Volume 11 Number 38 14 October 2019 Pages 17521–17948

Oxide rupture-induced conductivity in liquid metal nanoparticles
by laser and thermal sintering

Nanoscale
rsc.li/nanoscale

Gallium-based liquid metals have shown great potential insoft and stretchable electronics. When ﬁrst printed, ﬁlms ofliquid metal nanoparticles are non-conductive. We present
two thermally involved sintering methods – laser and thermalsintering – to achieve electrical conductivity. Our work reveals
the oxide rupture-induced mechanisms and shows that
thermal and laser sintering under different conditions generatehighly conductive liquid, solid or solid-liquid ﬁlms with tunablemorphologies and properties that can be used in soft robotics,
wearable electronics, or broad semiconductor applications.

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Oxide rupture-induced conductivity in liquid metal
nanoparticles by laser and thermal sintering†Cite this: Nanoscale, 2019, 11, 17615
Shanliangzi Liu, a,b Serrae N. Reed,b Matthew J. Higgins,c Michael S. Titusc andRebecca Kramer-Bottiglio*b
Metallic inks with superior conductivity and printability are necessary for high-throughput manufacturingof printed electronics. In particular, gallium-based liquid metal inks have shown great potential in creating
soft, ﬂexible and stretchable electronics. Despite their metallic composition, as-printed liquid metal nanoparticle ﬁlms are non-conductive due to the surrounding metal oxide shells which are primarily Ga2O3, awide-bandgap semiconductor. Hence, these ﬁlms require a sintering process to recover their conductivity. For conventional solid metallic nanoparticles, thermal and laser processing are two commonly used
sintering methods, and the sintering mechanism is well understood. Nevertheless, laser sintering of liquid
metal nanoparticles was only recently demonstrated, and to date, the eﬀect of thermal sintering has
rarely been investigated. Here, eutectic gallium–indium nanoparticle ﬁlms are processed separately bylaser or thermal sintering in an ambient environment. Laser and thermally sintered ﬁlms are comparedwith respect to electrical conductivity, surface morphology and elemental composition, crystallinity andsurface composition. Both methods impart thermal energy to the ﬁlms and generate thermal stress in theparticles, resulting in rupture of the gallium oxide shells and achieving electrical conductivity across theﬁlm. For laser sintering, extensive oxide rupture allows liquid metal cores to ﬂow out and coalesce into
conductive pathways. For thermal sintering, due to less thermal stress and more oxidation, the oxideshells only rupture locally and extensive phase segregation occurs, leading to non-liquid particle ﬁlms at
Received 7th May 2019,
Accepted 30th June 2019room temperature. Electrical conductivity is instead attributed to segregated metal layers and gallium

DOI: 10.1039/c9nr03903aoxide which becomes crystalline and conductive at high temperatures. This comprehensive comparison
conﬁrms the necessity of oxidation suppression and signiﬁcant thermal stress via instantaneous laser

rsc.li/nanoscale

irradiation to achieve conductive patterns in liquid form.

1.

Introduction

Recent advances in the field of soft robotics and wearables
have increased the demand for printed soft and flexible
electronics.1–5 Additive manufacturing techniques, suchas inkjet printing,6–9 spray printing10–12 and aerosol jet
printing9,13–15 are all promising alternatives to conventionallithography-based methods.3 These techniques may be applicable to roll-to-roll production, paving the way for future mass
manufacturing.16–18 Applying these printing techniques to
produce soft and flexible electronics requires functional inksaSchool of Mechanical Engineering, Purdue University, West Lafayette,
Indiana 47907, USAb
School of Engineering and Applied Science, Yale University, New Haven,
Connecticut 06511, USA. E-mail: rebecca.kramer@yale.edu
cSchool of Materials Engineering, Purdue University, West Lafayette, Indiana 47907,
USA† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9nr03903a

This journal is © The Royal Society of Chemistry 2019with satisfactory conductivity and printabiliy.19–22 The
main choices of conductive materials include metal
nanoparticles,23,24 carbon-based materials25 such as carbon
nanotubes26,27 and graphene,28,29 and conductive polymers.30,31
With superior electrical conductivity, metal nanoparticles
are optimal for utilization in conductive inks. Recent applications of metallic nanoparticles in printed flexible electronics
include sensors,32,33 transistors34,35 and light-emitting
devices,36,37 etc.
Two types of metallic nanoparticles are often used for softand flexible electronics: conventional solid metal nanoparticles and gallium-based liquid metal nanoparticles. Withhigh conductivity (6.3 × 107 S m−1) and oxidation stability,
silver has emerged as the most widely used material in flexible
electronics.33,35,38 However, due to its high cost, copper39,40(5.96 × 107 S m−1), aluminum41,42 (3.78 × 107 S m−1) andnickel43,44 (1.43 × 107 S m−1) nanoparticles have been utilized
as a replacement. The main challenge in implementation ofthese alternatives is in overcoming the issue of oxidation inambient conditions, which leads to low electrical conductivity;

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otherwise, an inert environment would be necessary for manufacturing. Therefore, a layer of metal or other capping agent(thiol, polymer) is often coated on the surface of the nanoparticles to minimize the penetration of oxygen.45,46
Additionally, a proper dispersing agent is imperative toprevent particle aggregation and sedimentation.47
In recent years, gallium-based liquid metals have received
tremendous attention in the field of soft and flexible electronics. In particular, eutectic gallium–indium alloy (eGaIn;
75.5 wt% Ga, 24.5 wt% In) has a combination of high electricalconductivity (3.4 × 106 S m−1), low melting temperature(15.5 °C), low viscosity (2 mPa s),48 negligible vapor pressure49
and intrinsic softness. One of the most unique properties ofgallium-based liquid metals is that the surfaces rapidly reactwith oxygen to form a thin passivating oxide skin (∼1–3 nm
in ambient environment;50–52 ∼0.7 nm under vacuum
conditions53,54) primarily composed of Ga2O3. The oxide
growth has also been found to be accelerated under elevatedtemperature.55 Previously, Hohman et al. demonstrated theability to create stable liquid metal nanoparticle suspensionsin ethanol via ultrasonication.56 During the sonication
process, gallium oxide continuously forms, fractures, andreforms, resulting in smaller nanoparticles. Dispersing agentsor other chemical additives are not necessary as the oxide prevents the particles from coalescing. Deposition of liquid metal
nanoparticle dispersions using inkjet printing57 and spray
printing techniques11 has been presented in our previous
work.
Both liquid metal and conventional solid metal nanoparticles require an additional sintering step to recover metallic conductivity after deposition. For conventional metallic
nanoparticles, the presence of insulating stabilizing agents or
chemical additives around the particles prevents direct contacts between particles and results in low electrical conductivity. Therefore, sintering needs to be employed to decompose
the organic stabiliser of the ink to then achieve a continuous
percolating network.22,58 The conventional method of sintering metallic nanoparticles is thermal sintering.20,59 Exposureof printed patterns to elevated temperatures removes organic
additives and enhances self-diﬀusion of surface atoms ofnanoparticles, resulting in initial neck formations between
adjacent nanoparticles and percolation networks below
melting temperature. However, thermal sintering is usually
inapplicable to flexible substrates due to high applied
temperatures. Selective laser sintering is a promisingalternative.23,39 The particles near the surface absorb theenergy of the focused laser beam, generating localized heat by
the induced photothermal eﬀect and leading to rapid sintering. Heat transferred from the top sinters the underlying
nanoparticles without damaging the substrate. The surface
morphology of laser and thermal sintered metallic nanoparticles look very similar.39
For liquid metal nanoparticles, although there is no chemical agent, the oxide skin ( primarily Ga2O3) prevents the particles from spontaneously coalescing. The estimated resistivityof Ga2O3 surrounding bulk eGaIn is around 106 Ω cm,60 which

17616 | Nanoscale, 2019, 11, 17615–17629

Nanoscaleis about 10 orders of magnitude less conductive than eGaIn
itself (29.4 × 10−6 Ω cm), and electrically insulates the particles
from their neighbors. This contact resistance leads to electricallosses and renders the particle film non-conductive. The oxideskin needs to be ruptured in order to allow liquid metal cores
flow out and coalesce into conductive pathways. Previous work
using liquid metal particles has relied on mechanical sintering
methods,57 which put a lower bound on both the size of theparticles and softness of the substrate that may be utilized.11
The resolution of the mechanically sintered patterns was also
limited.11,61,62
Recently, we showed that laser sintering of liquid metal
nanoparticles results in conductive patterns, and that the laser
sintering technique is fast, scalable, and boundless.63 We
hypothesized that the observed coalescing phenomenon canbe attributed to ablation (i.e., vaporization) of the metal oxideshell induced by the heated liquid metal core, or to therupture of the metal oxide shell due to thermal expansion of
the liquid metal core relative to the shell. Our previous work
also found that the uncoalesced particles underneath the coalesced particles on the top surface of the film were conductive
up to an eﬀective sintering depth.63 However, the reason forthis phenomenon was unclear, and the sintering mechanisms,
structure and properties of the resulting films were not fully
studied yet. Furthermore, Cutinho et al.55 demonstrated thatby simply applying heat to liquid metal microparticles, ruptureof the oxide shell and phase segregation occurred due tothermal expansion and surface oxidation. Changes in surface
roughness, surface composition and oxidation were also
observed.55 The published work implies that, as two thermally
involved methods, laser and thermal sintering both involve
rupturing of the gallium oxide shell, but the resulting film
morphology and properties may be diﬀerent.
While laser sintering of liquid metal nanoparticles has
been demonstrated in our previous work,63,64 there has notbeen any work done to investigate the changes in electricalconductivity of liquid metal nanoparticle films during thermalsintering. Furthermore, the comparison between these two
methods in terms of mechanism, resulting film structure andproperties has never been studied. A comprehensive examination and evaluation is required to help us further confirm the
hypothesis for the laser sintering mechanism, understand thethermal characteristics of liquid metal nanoparticles andexploit these sintering methods to create electronics withdesired properties.
In this work, we compare two thermally involved sintering
methods by thermal heating liquid metal nanoparticles in an
enclosed furnace at varying temperatures, and selectively laser
irradiating liquid metal nanoparticles using various laser parameters, all in ambient environment. Both methods result in
conductivity of the liquid metal nanoparticle films, but theworking mechanisms and resulting film structure and properties are diﬀerent. Simultaneous diﬀerential scanning calorimetry–thermo gravimetricanalysis (DSC–TGA) measurements
are performed to examine the thermal characteristics of liquid
metal nanoparticle film under diﬀerent temperatures. The

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thermal and laser sintered films are compared with respect toelectrical conductivity, surface morphology and elemental
composition, crystallinity and surface composition. Moreover,
the observation from previous work63 that uncoalesced particles below the laser sintered trace on the top surface become
conductive is explained. Finally, high-resolution laser sinteredconductive patterns are demonstrated.

2

Results and discussion

2.1

Preparation and processing

Liquid metal nanoparticle inks were created by sonicating
bulk eutectic gallium–indium alloy (eGaIn, 75.5% Ga–24.5%
In) in ethanol for 120 min, resulting in particles with average
diameters of 220 nm as characterized by Lear et al.65
Subsequently, the nanoparticle inks were deposited onto substrates by spray printing, creating a non-conductive nanoparticle film. For laser sintering, we used a UV laser micromachining system (Protolaser U4, LPKF) with a wavelength of
355 nm, a pulse duration of 900 ns and a beam diameter of
15 μm, operated in ambient environment. Thermal sintering
processes were conducted in an enclosed furnace (Thermo
Fisher Scientific) in ambient condition. The nanoparticle films
were deposited on ceramic substrates, placed in the furnace
for 30 min at a certain temperature and then cooled downin air.
2.2

Thermal characteristics under diﬀerent temperatures

Prior to comparing the two thermally involved sintering processes, we performed simultaneous DSC–TGA measurements
in both air and inert atmosphere (argon gas) to examine
the thermal characteristics of liquid metal nanoparticles byheating up from 200 °C to 900 °C at a heating rate of 10 °C
min−1 (Fig. 1). The black curve is the weight gain percentage,
and the red curve is the changes in heat flow withtemperature.
In air environment, the weight of the nanoparticles stays
constant, or shows a very slight increase at temperatures below
300 °C. The evident weight gain of the nanoparticles begin toshow at ∼340 °C where the prominent increase in both TGA
and DSC curves are observed, indicating the beginning of oxidation. The fastest reaction occurs at ∼463 °C where the slopeof the TGA curve reaches maximum, while the first DSC peakis detected, possibly corresponding to the exothermic oxidation reaction. The weight of the nanoparticles continues toincrease rapidly until ∼486 °C in the TGA curve where thesecond DSC peak is detected. The following slope decrease of
the TGA curve indicates the decrease of the oxidation rate. We
speculate that the second DSC peak corresponds to crystallization of the nanoparticles oxide shells from amorphous Ga2O3
to β-Ga2O3. The crystallization enhances structural order andpacking density of the oxides of the nanoparticles near thesurface, making further oxidation of the particles deeper inthe film slower. The total weight gain of the nanoparticles
after the thermal heating process is ∼12.65%.

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Fig. 1 Simultaneous DSC–TGA measurements of liquid metal nanoparticles heated up to 900 °C from 200 °C at a heating rate of
10 °C min−1. The black curve is the weight gain percentage, and the red
curve is the changes in heat ﬂow.

In order to prove our speculation, we further performed
TGA and DSC analysis in an argon gas atmosphere by eliminating the oxidation eﬀect. The weight of the nanoparticles stays
constant throughout the heating process, which indicates nooxidation. Therefore, the small DSC peak at ∼510 °C likely
corresponds to crystallization of the Ga2O3 shells of the nanoparticles. The crystallization temperature in argon is ∼24 °C
higher than in air. Variations in the crystallization temperatures in diﬀerent heating atmosphere have been observed inother materials, which could be due to changes in chemistry,
residual stresses, activation energy etc.66,67 Moreover, exothermic energy release (area under the heat flow curve) for thiscrystallization peak is much lower in argon than in air, likely
due to the fact that the Ga2O3 layer is much thinner since oxidation is inhibited.
2.3

Electrical properties

We measured resistance values via four-terminal resistancesensing on laser and thermal sintered dogbone patterns(7 × 1.4 mm). For the laser process, pre-designed dogbone patterns were directly sintered on liquid metal nanoparticle films
deposited on polydimethylsiloxane (PDMS) substrates. We
varied laser fluence (optical energy delivered per unit area,
1.52, 1.89, 2.31, 3.11, 4.4, 6.01, 8.49 J cm−2) and beam pulse
overlap (the percentage amount of overlap between the diameters of two consecutive pulses, 40, 50, 60, 70, 80, 90%). The
beam pulse overlap is a function of the laser beam diameter,
scanning speed and pulse frequency. Detailed informationand the calculation of the selected laser sintering parameters
are included in the ESI.†

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NanoscaleFig. 3 The changes of resistance with varying heating temperatures ofliquid metal nanoparticle ﬁlms. Error bars represent 95% conﬁdence
interval. Particle ﬁlms heated at 300 °C and 400 °C are not conductive.
Fig. 2 A 3D bar graph showing the changes of resistance with varying laser ﬂuence and pulse overlap of spray printed liquid metal
nanoparticle ﬁlms. The z-axis is log scale. Diﬀerent colors represent
resistance in diﬀerent ranges. Error bars represent 95% conﬁdence interval. At high ﬂuence and overlap (black), liquid metal nanoparticles are
ablated.

Fig. 2 shows the change in measured resistance withvarying laser fluence and pulse overlap. As laser fluence or
pulse overlap increases, resistance values decrease, since moreparticles are coalesced as more energy is absorbed by the particles during sintering. At a low laser fluence with low pulse
overlap, the employed laser energy is insuﬃcient to sinterliquid metal nanoparticles into conductive patterns, and themeasured resistance values are ∼103 Ω to ∼106 Ω (transparentbars). Sintered films with resistance values lower than 100 Ωstart to appear when pulse overlap is increased to 90% or 80%
at a fluence of 1.52 J cm−2 or 1.89 J cm−2 (green bars). At intermediate laser fluences (3.11 J cm−2 and 4.4 J cm−2), low resistance values below 10 Ω are attained when moderate pulse
overlap (60%–80%) is applied (red bars). High laser fluencewith high pulse overlap causes ablation of liquid metal nanoparticle films (black).
Laser sintered liquid metal nanoparticle films with distinct
resistance values can be achieved by tuning laser parameters.
Although we have shown this capability using an ytterbium
pulsed fiber laser in previous work,63 the current UV micromachining system certainly has more advantages. With theability to tune not only laser fluence, but also scanning speed(330–1800 mm s−1) and pulse frequency (160–260 kHz) in awide range, we are able to attain conductive patterns more
rapidly with less energy applied, and an extensive range ofresistance values for diﬀerent potential applications.
For the thermal sintering process, PDMS was used asa mask to pattern dogbone shapes of the same sizes(7 × 1.4 mm) onto a ceramic substrate before spray printing
and heating the liquid metal nanoparticle film in the furnace.

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Details about the fabrication process are included in theExperimental section. We varied thermal heating temperaturesfrom 300 °C to 900 °C at 100 °C increments. At each temperature, resistance values of ten samples were measured and averaged. Fig. 3 shows the change of resistance with varying
heating temperatures. Liquid metal nanoparticle films heatedat 300 °C and 400 °C do not show any conductivity. The particle films become conductive after heating at 500 °C, and theresistance value is very low (2.23 ± 0.56 Ω, mean ± 95% confidence interval). At 600 °C, the resistance value drops to0.41 ± 0.33 Ω and only slightly changes from 700 to 900 °C.
The plot indicates that liquid metal nanoparticle films canattain conductivity via thermal treatment above 500 °C. Fromdimensions of the dogbone pattern (length/width = 5) and thesample thickness (∼10 μm), resistivity of the thermal sinteredfilms is calculated to be in the range of 8 × 10−5–4.5 × 10−4 Ω cm.
However, dissimilar to laser sintering, the resistance values
are unable to be tuned by varying heating temperature,
which implies a diﬀerent mechanism. To further investigate
the cause of conductivity during thermal treatment, liquid
metal nanoparticle film structure and composition arestudied.
2.4 Eﬀect of laser parameters and heating temperatures on
surface morphology and elemental composition
Liquid metal nanoparticle films processed at diﬀerent laserparameters (fluence and pulse overlap), and heating temperatures are studied. Fig. 4 illustrates the mechanism of laser andthermal sintering liquid metal nanoparticle films. Bothmethods impart thermal energy to the films and generate
thermal stress in the particles, leading to rupture of thegallium oxide shells. However, due to diﬀerent extents ofsurface oxidation and oxide rupture, the resulting surface morphologies are diﬀerent. Typical processes that the particles
undergo include thermal expansion, anisotropic contraction,
extensive oxide rupture or phase segregation and local oxide

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Fig. 4 Schematic demonstrating the mechanism of laser and thermal sintering liquid metal nanoparticle ﬁlms. Typical processes that the particles
experience during laser and thermal sintering are illustrated in a–d. The surface morphologies of laser and thermal sintered ﬁlms are illustrated ine(i–iii) and f(i–iii), respectively. The direction of the arrows represent the increase of laser energy (blue) and heating temperature (green). At ﬁrst,
both laser and thermal processes induce thermal expansion of the liquid metal core relative to the oxide shell (a). As the oxide grows thicker withincrease of energy, anisotropic contraction of the particles occurs during cooling (b). Dimple-like surface deformations are observed on the particles(e(i) and f(i)). Partial sintering of some adjacent particles are observed on the laser sintered ﬁlm at low laser energy (e(i)). As laser energy increases,
laser irradiation causes extensive rupture of the oxide skin of the particles (c), allowing liquid cores to ﬂow out and merge into continuous structures(e(ii)). When laser energy further increases, due to phase segregation and local oxide rupture (d), two types of surface textures are observed on thelaser coalesced ﬁlm (e(iii)): large nanoridges and small, bright nanoparticles. Similar textures are observed on the thermal sintered ﬁlm as temperature increases (f(ii)). When heating temperature is very high, extreme particle morphology is observed (f(iii)).

rupture, as illustrated in Fig. 4a–d. The surface morphologies
of laser sintered films with increasing laser energy andthermal sintered films with increasing heating temperature
are illustrated in Fig. 4e(i–iii) and f(i–iii), respectively.
Extensive oxide rupture (Fig. 4c) only occurs in the laser sintering process where as oxide ruptures, liquid metal cores flowout and coalesce into conductive pathways. The scanning electron microscopy (SEM) images of laser and thermal sinteredliquid metal nanoparticles films are presented in Fig. 5–7. For
some selected sintering parameters, energy dispersive X-ray
spectroscopy (EDS) elemental mappings are included to identify the spatial distribution of elements.
2.4.1 Eﬀect of low laser energy and heating temperature.
Prior to examining conductive structures achieved by laser andthermal sintering, we compare the morphologies of laser andthermal processed liquid metal nanoparticle films at low
fluence (1.52 J cm−2, 70%) and temperature (300 °C) where thefilms are still not conductive. The resistance of the laser sintered film (Fig. 5a) is around ∼106 Ω, while the thermal heatedfilm (Fig. 5b) is completely non-conductive. As shown in
Fig. 4e(i), f(i) and 5, morphologies of the films processed at

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these two conditions appear very similar. Most of the particles
are still isolated, and no visible continuous pathways areformed to create electrical connections. Moreover, dimple-likesurface deformations are observed on the particles. We postulate that as the oxide skin grows thicker during heating, diﬀerences in thickness, hence thermal expansion and elasticity ofthe oxide (Fig. 4a), lead to anisotropic contraction of the particles during the subsequent cooling process (Fig. 4b). Cutinho
et al. presented a similar phenomenon on liquid metal microparticles heated at 300 °C.55
For the laser sintered film (Fig. 4e(i) and 5a), the particles
are bigger and the film is more compact. It suggests that
despite insuﬃcient laser energy overall to sinter all the particles, the very high peak energy focused on the small areas
may generate enough thermal stress in the particles to breakthe oxide and coalesce some adjacent particles. The reduced
contact resistance among the particles contributes to theslightly conductive reading of the laser sintered film. These
results indicate that the eﬀects of laser and thermal sinteringon the particle films are similar when oxide rupturing and oxidation are not prominent.

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Fig. 5 Surface morphology comparison of non-conductive liquid metal
nanoparticle ﬁlms after laser irradiation and thermal heating at low laser
ﬂuence (1.52 J cm−2, 70%) (a) and low heating temperature (300 °C) (b).

2.4.2 Eﬀect of diﬀerent laser parameters. In contrast to theconventional solid metallic nanoparticles, surface morphologies of laser and thermal sintered liquid metal nanoparticle films diverge at the appearance of electrical conductivity. As shown in Fig. 6, when supplied laser energy increases(in the direction of the arrow), the sintered films start tobecome more conductive, and surface morphologies change
significantly. At 1.89 J cm−2 laser fluence and 80% pulse
overlap, the resistance of the sintered pattern is 46.35 Ω.
Comparing to Fig. 5a where most of the particles are disconnected, the morphology shown in Fig. 6a exhibits a partiallysintered structure with much larger coalesced particles in
micron scale. When laser fluence is further increased to
3.11 J cm−2 with a pulse overlap of 50%, the resistance isreduced to 13.84 Ω. As seen in Fig. 6b, a fully connected liquid
metal network is formed, suggesting that stress generated by
the photothermal eﬀect of the laser beam is suﬃcient to cause
extensive rupture of the oxide skin of the particles (Fig. 4c),allowing liquid cores to flow out and merge into continuous
structures (Fig. 4e(ii)). As laser fluence is further increased to
4.4 J cm−2 with 80% overlap, the resistance drops to 1.86 Ω.
The SEM image in Fig. 6c indicates that as more laser energy
is delivered during sintering, more particles are coalesced

17620 | Nanoscale, 2019, 11, 17615–17629

Nanoscaleacross the film, forming large protruding networks due to highsurface tension of liquid metal.
While the sintered structures seen in Fig. 5a and 6a, b are
generally smooth, non-uniform surface textures begin toappear at high laser fluence, as illustrated in Fig. 4e(iii).
Fig. 6d shows a detailed view of the textures appearing on
the surface of the coalesced network in Fig. 6c. Two types of
surface textures are observed: large nanoridges with lengthscales on the order of hundreds of nanometers and small,bright particles with lengths on the order of a few tens ofnanometers (inset). The elemental mapping in Fig. 6e showsthat gallium and indium are well distributed over the entire
selected region, while oxygen is more concentrated in theareas with nanoridges. When laser energy is high, the oxideskin is more permeable to oxygen due to thermal expansion,
causing it to become thicker and more brittle. We hypothesizethat during cooling, rather than undergoing surface deformations at low laser energy, the oxide skin ruptures locally,
allowing liquid metal cores or the underlying gallium layer toflow out. Then, the released metal quickly forms an oxide skinon the surface which stabilizes them in the shape of nanoridges (Fig. 4d). This hypothesis is supported by the elemental
mapping as shown in Fig. 6e, which shows a higher oxygen
concentration in the regions with nanoridges as induced by
growth of the new oxide layer on the nanoridges over the previous oxide layer underneath. The small, bright nanoparticles
are too small to be detected by EDS.
It is known that gallium is easier to oxidize thanindium.68,69 In addition, previous X-ray reflectivity studies byRegan et al.70 concluded that there is a segregated indium
layer underneath the surface oxide, consistent with the Gibbs
adsorption rule.71 At some specific regions where moregallium is oxidized on the surface, more indium from the corecan be segregated to the surface in order to keep the core at
the eutectic composition. Therefore, we speculate that thesmall, bright nanoparticles (Fig. 6d inset) have more indium
content that is released as the oxide ruptures during cooling.
Liquid cores of the coalesced structures after laser sintering
were confirmed by scratching the sintered films using tweezers. The SEM images in Fig. S1 (ESI†) show that after
mechanically breaking the oxide, the coalesced network reflows
into a complete liquid trace.
2.4.3 Eﬀect
of
diﬀerent
heating
temperatures.
Interestingly, the surface textures observed from the laser
sintered samples at high energy appear similar to the two tiersof texture on liquid metal microparticles thermally sintered at
500 °C presented by Cutinho et al.55 This suggests that these
two thermally involved sintering methods are both associatedwith oxide rupturing due to surface oxidation and thermal
expansion. We further performed thermal treatment on liquid
metal nanoparticle films at varying temperatures to examine if
electrical conductivity is related to particle coalescence as well.
Fig. 7 shows the SEM images of thermal sintered liquid metal
nanoparticle films from 500 °C to 900 °C at increments of
100 °C. The EDS elemental mapping of the film heated at
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Fig. 6 SEM and EDS analyses of laser sintered liquid metal nanoparticle ﬁlms, indicating changes in surface morphology and elemental composition
at varying ﬂuence and pulse overlap. (a)–(c) SEM images of laser sintered nanoparticle ﬁlms at 1.89 J cm−2, 80% (46.35 Ω); 3.11 J cm−2, 50%(13.84 Ω); 4.4 J cm−2, 80% (1.86 Ω). The direction of the arrow represents the increase of laser energy. (d) The detailed view of the textures appearing
on the surface of the coalesced network in (c). (e) EDS mapping images of liquid metal nanoparticle ﬁlms sintered at 4.4 J cm−2, 80% (1.86 Ω).
Oxygen is more concentrated in the areas with nanoridges.

bution. Furthermore, we scratched the thermal sintered films
using tweezers to test the cores of the particles at room temperature. Thermal sintered particles at 500 °C coalesce into aliquid trace which is not fully continuous (Fig. S2, ESI†), indicating a small amount of liquid left in the cores. Above 500 °C,
the particle cores are not liquid anymore. Our findings suggestthat as we expect, oxidation is much more severe in thermal
than laser sintering due to longer exposure to hightemperature.
The nanoparticle films thermally sintered at 500 °C appear
similar to the microparticles studied by Cutinho et al.55 The
parent particles are covered with rough nanoridges and small,bright nanoparticles with average diameters of 50 nm (Fig. 4f(ii)and 7a). As shown in Fig. S3 (ESI†), the small, bright nanoparticles are well distributed over the entire surface of the film.
The sizes of these nanoparticles are ∼4–5 times larger than theones on the laser sintered films shown in Fig. 6d (inset) andcan be identified by EDS. The Ga and Ga–In overlay maps in
Fig. 7a show that these bright nanoparticles on the parent particles indeed have higher indium content as hypothesized. We
speculate that compared to laser sintering, longer exposure toelevated temperatures (30 min) during thermal sintering
process induces a higher degree of oxidation, hence a thicker
indium segregated layer at the interface, leading to larger andThis journal is © The Royal Society of Chemistry 2019

more extensively distributed indium-enriched particles as theoxide ruptures upon cooling (Fig. 4d). At 600 °C (Fig. 7b), clusters of bright nanoparticles are present at specific regions. The
size of a single nanoparticle in the clusters is about the same
as 500 °C (∼50 nm). The regions with the clusters are more oxidized and have a higher content of indium, as shown in the
EDS results in Fig. S4 (ESI†). We hypothesize that due to ahigher degree of oxidation, the isolated bright nanoparticles
may have formed at first, and then was quickly displaced by
Ga2O3 72,73 while the adjacent nanoparticles are fused into
clusters.
2.4.4 Extreme particle morphology at high heating temperatures. Thermal heating liquid metal nanoparticle films at
high temperatures (700 °C–900 °C) induces extreme particle
morphology changes, as shown in Fig. 7c–g. We note thatsimilar particle morphology was not observed by Cutinho
et al.,55 and the discrepancy is probably due to diﬀerent particle sizes (∼220 nm vs. 1–5 μm 55), hence diﬀerent extent ofoxidation and crystallization at high temperature.
At 700 °C (Fig. 7c), bright clusters with bigger nanoparticles(∼73 nm) appear on the surface, and nanorods with average
diameters of ∼25 nm and lengths of 30–150 nm are present. At
800 °C (Fig. 7d), the single nanoparticles in the bright clusters(∼153 nm) are about twice as big as the particles that appeared

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NanoscaleFig. 7 SEM and EDS analyses of thermal sintered liquid metal nanoparticle ﬁlms, indicating changes in surface morphology and elemental composition at varying heating temperatures. (a) EDS mapping images of liquid metal nanoparticle ﬁlms sintered at 500 °C. (b)–(e) SEM images of thermalsintered liquid nanoparticle ﬁlms at 600 °C, 700 °C, 800 °C and 900 °C. The direction of the arrow represents the increase of heating temperature.
(f )–(g) The detailed view of the particles and nanowires shown in (e).

at 700 °C. The EDS spectra in Fig. S5 (ESI†) show that the clusters are highly oxidized. A few polygonal facets and nanowires
with average diameters of ∼23 nm and lengths of 50–500 nm
are present, as illustrated in Fig. 4f(iii). At 900 °C (Fig. 7e),bright clusters are not observed, whereas a significant amountof polygonal facets are present, implying higher extent of oxidation and crystallization. Bright clusters have likely formed at
the early stage and grown into facets or fallen oﬀ the surface,
as implied in Fig. S6 (ESI†). The observed nanowires have thesame average diameters as those appeared at 800 °C, withlengths ranging from 300 nm to 1.7 μm.
In order to study composition of the nanowires, we separated single nanowires from the particles by sonicating asample thermally sintered at 900 °C in isopropyl alcohol for a
few minutes and then drop casted onto a copper grid for
imaging. The retention of the nanowires morphology after
ultrasonication was confirmed by SEM. The EDS spectra andmappings from the high-angle annular dark-field scanning

17622 | Nanoscale, 2019, 11, 17615–17629

transmission electron microscopy (HAADF-STEM) image
demonstrate that the nanowries are primarily composed of Ga
and O, with a negligible amount of In (Fig. S7, ESI†).
The detailed view in Fig. 7f demonstrates that the nanowires grow out of the facet islands that sprouted from thesurface of the particles, resembling urchin-like nanostructures.
As shown in the detailed view of the nanowire (Fig. 7g), facets
seem to grow on the sides, and no particle is observed on thetip, indicating the inapplicability of the well-known vapor–liquid–solid (VLS) mechanism.74,75 Previously, researchers
have synthesized gallium oxide nanowires with similar
structures by heating metallic gallium droplets in air at
750–1000 °C.76,77 The gallium oxide nanospheres formed on
the molten gallium droplets act as nuclei at the early stageof nanowire formation. The underlying gallium provides
necessary feeding materials that can react with oxygen in air
and take place on these nuclei to form nanowires. The mechanism was described as self-catalytic growth.

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Nanoscale

To observe the nanowire formation at the early stage andbetter understand the growth process, liquid metal nanoparticle films were thermally sintered at 800 °C or 900 °C for
only 5 minutes. At 800 °C (Fig. S8a, ESI†), only a few nanowires
are observed with lengths of 10–300 nm and a few facets arepresent. The nanowires are grown out of the small, bright
nanoparticles on the parent particles. At 900 °C (Fig. S8b,
ESI†), both the density and lengths of nanowires on thesurface increase (50 nm–1 μm). Bright nanoparticles havelikely formed on the parent particles at first, and then oxidized
and crystallized into facets where the nanowires are nucleated.
The facets on the sides of the nanowires are visually distinct.
These findings indicate that at high temperature, the surfaceof the nanoparticle film is considerably oxidized and crystallized. The facets are acting as a self-catalyst for the growth ofnanowires. In the nanowires, oxygen is from air, while galliumis from the parent particles. The nucleation and growth rate of
the nanowires are increased with heating temperature andtime. In the most extreme case, we thermally sintered the particles at 900 °C for 12 h. As shown in Fig. S9 (ESI†), the density
and lengths of nanowires are significantly increased.
It is evident that laser sintered liquid metal nanoparticle
films are conductive because as the oxide ruptures, liquidcores of the nanoparticles flow out and coalesce into a continuous liquid network, forming electrical pathways. Unlike
laser sintered particles, despite getting more compact, thermalsintered particles are not coalesced and no percolation pathways are observed. Hence, liquid metal nanoparticles cannot
be actually “sintered” in a similar way to conventional metallic
nanoparticles at elevated temperature due to extensive phasesegregation and severe surface oxidation. It is worth notingthat particle cores are no longer liquid above 500 °C, whichPaper

indicates that the particles are substantially oxidized, hence
the low resistance of thermal sintered films may be attributed
to gallium or indium oxide. Ramana et al.78 demonstrated that
resistivity of a sputter deposited gallium oxide film significantly drops when it changes from amorphous at room temperature to crystalline structure at high temperature, and resistivity is dependent on crystallinity. Furthermore, researcherspresented that resistivity of a deposited indium oxide film also
decreases as film crystallinity increases.79,80 Therefore, tofurther investigate the source of conductivity, we examined the
crystallinity of gallium and indium oxide of laser and thermalsintered liquid metal nanoparticle films.
2.5

Crystalline phases

X-ray diﬀraction (XRD) analysis was conducted to identify crystalline phases of laser (Fig. 8a) and thermal (Fig. 8b) sinteredliquid metal nanoparticle films. Researchers have found that it
is significantly easier to form an amorphous gallium oxide on adisordered structure like liquid metal.70,81 The diﬀraction
pattern at the bottom of Fig. 8b confirms that as-printed liquid
metal nanoparticles are completely amorphous. As shown in
Fig. 8a, at varying laser parameters, none of the samples show
any crystalline peaks, indicating that all the laser sintered nanoparticle films stay amorphous. This data is in accordance with
our previous investigation that after laser sintering, the coalesced structures are still at the eutectic composition.
Moreover, since the detection depth of XRD is beyond the
top sintered region (∼800 nm),63 the uncoalesced particles(including their oxide layers) underneath are amorphous aswell. In previous work, we noticed that the uncoalesced particles underneath the coalesced particles on the top surface ofthe film were conductive up to an eﬀective sintering depth.63

Fig. 8 X-ray diﬀraction patterns of (a) laser sintered liquid metal nanoparticle ﬁlms at varying laser ﬂuence and pulse overlap; (b) thermal sinteredliquid metal nanoparticle ﬁlms at varying temperatures and as-printed liquid metal nanoparticle ﬁlms. The eﬀect of applied laser energy and heating
temperature on the evolution of the patterns can be seen.

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Hence, the conductivity of the uncoalesced particles is not
attributed to the crystallinity change of the surface oxide.
The cross-section images in Fig. S10 (ESI†) show that comparing to non-conductive as-printed liquid metal nanoparticles(Fig. S10a and b, ESI†), the uncoalesced particles underneaththe top coalesced regions of the laser sintered samples are
more closely packed due to partial sintering from the instantaneous heating with the laser beam (Fig. S10c–f, ESI†). The
partial sintering phenomenon is more evident on the particle
films sintered at high laser energy 4.4 J cm−2, 80% (Fig. S10e
and f, ESI†). In this case, contact resistance among the particles is reduced, hence the surface oxide has less eﬀect on theoverall conductivity of the film.
For thermal sintering, no crystalline peaks are observed in
the films heated up to 300 °C, suggesting no crystallization orphase segregation at this point. Diﬀraction peaks start toappear when heating to 400 °C, indicating the beginning ofoxidation and phase segregation, which is in accordance withthe simultaneous DSC–TGA measurements (Fig. 1). The identified peaks (101), (002), (110) at 2θ values of 32.95, 36.27 and39.18 correspond to tetragonal indium (space group I4/mmm,
PDF #00-005-0642). At 500 °C, the intensities of these indium
peaks increase while the peak widths are reduced. This data
confirms our hypothesis that as temperature increases, surface
oxidation is accelerated, leading to more indium from theeutectic core segregated to the interface and eventually formed
into indium-enriched bright nanoparticles as the oxide ruptures (Fig. 7a). Moreover, peaks located at 30.32, 31.72, 35.22
and 38.34 corresponding to monoclinic β-Ga2O3 (space groupC2/m, PDF #01-087-1901) (400), (20-2), (111) and (31-1) start toget pronounced at 500 °C, which is close to the temperature of
the crystallization peak observed in the DSC curve (Fig. 1). The
intensities of these peaks gradually increase and become
sharper as temperature increases, which indicates high crystalline structure. When maximum heating temperature (900 °C)
is reached, these peaks exhibit distinctively high intensities,
and additional diﬀraction peaks of β-Ga2O3 (110), (11-1) and(401) located at 30.46, 33.44 and 37.44 are evident. The relative peak intensity is consistent with the reported pattern(PDF #01-087-1901).
Our findings agree with the literature that Ga2O3 films deposited at high temperatures (>500 °C) become crystalline, andthe crystallinity of Ga2O3 is enhanced with temperature due toimproved structural quality and packing density.78,82,83 The
amorphous nature of Ga2O3 on thermal sintered films below
500 °C and laser sintered films can be attributed to insuﬃcient thermal energy to promote structural order. The demonstrated crystallinity of gallium oxide supports our conjecturethat conductivity of the thermal sintered films above 500 °C is
in large part attributed to gallium oxide. Segregated indium
may contribute to the conductivity as well, whereas indium
oxide and gallium peaks are not observed, likely because ofpoor crystallinity or small quantity. In addition, the XRD
pattern of the nanoparticle films sintered at 900 °C for 12 h(Fig. S11, ESI†) shows that while the intensities of (20-2) and(111) are still high, the intensity of (400) increases signifi-

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Nanoscalecantly, which may indicate that the observed nanowires have apreferred growing direction.
2.6

Surface composition analysis

We also performed an X-ray photoelectron spectroscopic (XPS)
analysis to characterize the surface composition of liquid
metal nanoparticle films after laser and thermal sintering.
Since the maximum detection depth of XPS is ∼10 nm, all thesignals are within single nanoparticles and representing composition at the top surface. The core level XPS spectra ofgallium and indium are shown in Fig. 9a, b, d and e (Ga 3d,
In 3d) and Fig. S12† (Ga 2p) in the ESI.† In order to quantify
surface composition, relative concentration of gallium oxide(Ga2O3) vs. elemental gallium (Ga) and total gallium (Ga + Ga2O3)
vs. total indium (In + In2O3) are calculated from scaled peakarea ratios and plotted against laser parameters and heating
temperature as shown in Fig. 9c and f. With the accessibility tomeasure both Ga and Ga2O3 concomitantly, Ga 3d region was
selected to obtain the Ga/Ga2O3 ratio in order to investigate
the extent of surface oxidation at diﬀerent conditions. To
make a direct comparison, the spectra shown in Fig. 9a and d
are normalized to the maximum peak intensity. The total Ga is
obtained from Ga 2p spectra (Fig. S12, ESI†) where Ga andGa2O3 are indistinguishable. The total indium is obtained
from In 3d spectra (Fig. 9b and e) by adding up both In andIn2O3 signals.
The relative peak areas (Fig. 9a) and the calculated ratios(Fig. 9c) of Ga/Ga2O3 show that as laser energy increases,
signals from elemental Ga decrease relative to Ga2O3, indicating that gallium on the surface is more oxidized. Furthermore,as shown in Fig. 9b, at the lowest laser energy, indium peaks
are positioned at ∼450.5 eV (In 3d3) and ∼443.9 eV (In 3d5),
which correspond to elemental In. However, with increasing
laser energy, while elemental In peaks are still visible, thepeaks positioned at ∼452.9 eV and ∼445.6 eV corresponding toIn2O3 start to appear and dominate at the highest selected
laser energy. As seen in the plot of concentrations (Fig. 9c), at
first, the amount of total In decreases relative to total Ga (redcurve) since more gallium is oxidized on the surface (black
curve). However, as applied laser energy further increases,
total In concentration increases (red curve). These results
further verify our previous speculation that as the surface ismore oxidized, more indium is segregated to the interface,
eventually forming into indium-enriched bright nanoparticles.
These particles quickly oxidize and form thin indium oxide on
the surface.
On the other hand, as indicated by the appearance ofvarious surface features at varying temperatures, the surface
composition change of thermal sintered liquid metal nanoparticle films is very complicated. Similar to the compositionchange of laser sintered film at low energy, as the film isheated up to 400 °C, elemental Ga decreases relative to Ga2O3
due to more surface oxidation (Fig. 9d and f ). In the meantime, elemental In peaks are replaced by In2O3 peaks (Fig. 9e),indicating the oxidation of In on the surface. The concentration of total indium signals (In2O3 in this case) increases

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Paper

Fig. 9 XPS analysis of laser and thermal sintered liquid metal nanoparticle ﬁlms. (a) Ga 3d, (b) In 3d core level spectra of liquid metal nanoparticle
ﬁlms sintered at varying laser parameters. (c) The ratio of elemental gallium (Ga)/gallium oxide (Ga2O3) and total indium (In + In2O3)/gallium(Ga + Ga2O3) at varying laser parameters. (d) Ga 3d, (e) In 3d core level spectra of liquid metal nanoparticle ﬁlms at diﬀerent heating temperatures.
(f ) The ratio of elemental gallium(Ga)/gallium oxide (Ga2O3) and total indium (In + In2O3)/total gallium (Ga + Ga2O3) at varying heating temperatures.
To make a direct comparison, the spectra shown in (a), (d) are normalized to the maximum peak intensity.

and hits the maximum at 500 °C (Fig. 9f ). This data confirms
the previous conclusion that indium starts to segregate to theinterface at 400 °C and release as small, bright nanoparticles
on the surface at 500 °C.
At 600 °C, the ratios of total In/total Ga and Ga/Ga2O3
decrease (Fig. 9f ). We speculate that due to a higher degree ofoxidation, the isolated small, bright nanoparticles are displaced by Ga2O3 while the adjacent nanoparticles are fused
into clusters and less distributed across the surface. Therefore,
surface areas covered with Ga2O3 dominate, contributing tomore gallium signals. At 700 °C, the concentrations of total
In/total Ga, and Ga/Ga2O3 increase again (Fig. 9f), which implies
that more bright nanoparticles are sintered into clusters andcover the surface. As the surface becomes more crystallized,
metallic gallium starts to be driven towards the surface to takeplace on the nuclei and form nanorods, thus contributing toThis journal is © The Royal Society of Chemistry 2019

more elemental Ga signals relative to Ga2O3. As temperature isfurther increased to 800 °C and 900 °C, almost all the elemental Ga is completely oxidized both in the form of particles andnanowires, so the surface is mostly covered by Ga2O3. These
results indicate that in thermal sintered films, metallic
gallium and indium oxide may also contribute to the conductivity. At high heating temperature (800–900 °C), when thesurface is mostly oxidized, conductivity is primarily induced by
crystalline Ga2O3.
2.7

High-resolution selective laser sintering

This comprehensive comparison confirms that owing toinstantaneous irradiation of the laser sintering process, severeoxidation is avoided and large thermal stress is generated.
Therefore, oxide rupturing followed by particle coalescence,and resulting conductive patterns in liquid form can be

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Fig. 10 (a) Demonstration of laser sintered complex conductive patterns: Purdue train, Yale seal and Faboratory gear. Scale bars are 100 μm
in length. (b) Optical microscope image showing the minimum width ofa single laser sintered trace. (c) Photographs of laser sintered liquid
metal patterns acting as circuits to connect LED.

achieved. Previously, we have shown the ability to make flexible, wearable and multilayer circuits using selective laser sintering.63 However, the minimum width of the traces was∼200 μm, which limits packing density of electronic devices.
Here, for the first time we demonstrate the ability to createhigh-resolution liquid metal patterns by selective laser sintering using a UV laser micromachining system. Fig. 10a shows
examples of very intricate patterns in the form of the Purdue
University train, Yale University seal and Faboratory lab gear.
These patterns can be processed to have both raster-based
regions achieved by scanning a pulsed laser spot over an areaand vector-based regions achieved by drawing a single trace.
The optical microscope image in Fig. 10b shows that the sintered single trace width is as small as 37 μm. As shown in
Fig. 10c, these high-resolution traces are conductive and canbe employed as electronic circuits to connect LED. Achieving
liquid metal patterns with resolution comparable to this isalmost unfeasible using other patterning methods including
injection into microchannels73 and masked deposition.84
Furthermore, the sintering time for each pattern is less than
10 s. The capability of creating precise and high-resolution
liquid metal patterns enables miniaturization of soft electronic
components that can be implemented to realize small scaleand high-density electronics for wearable and soft robotic
systems.

3. Conclusions
This work demonstrates two sintering techniques to sinternon-conductive liquid metal nanoparticle films into conductive patterns. The films are sintered either by selective laser
sintering or furnace heating in ambient environment. The
thermal characteristics of liquid metal nanoparticle films are
examined by simultaneous DSC–TGA measurements. Structure
and properties of laser and thermal sintered films, including
electrical conductivity (4-terminal sensing), surface mor-

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Nanoscalephology and elemental composition (SEM/EDS), crystallinity(XRD) and surface composition (XPS) are compared.
These two thermally involved methods both induce rupturing of the gallium oxide shells, and eventually achieve electrical conductivity of the films. When the supplied laser energy
or heating temperature is low, the films remain non-conductive, and dimple-like surface deformations are observed on
the particles due to anisotropic contraction upon cooling.
Nevertheless, the surface morphology diverges as laser energy
or heating temperature is further increased. Instantaneous
heating with a laser beam generates significant thermal stressin the particles that ruptures oxide skin of the particles extensively, allowing liquid cores to flow out and merge into continuous conductive structures. Despite more oxidation andlocalized phase segregation at high laser energy, the sinteredfilms stay liquid at all times and the gallium oxide remains
amorphous. Conductivity of the uncoalesced particles underneath the top coalesced region is attributed to particle partial
sintering and compact packing. On the contrary, in thermalsintering process, particles cannot be coalesced due to much
more severe oxidation and less thermal stress. Instead, local
oxide rupture and extensive phase segregation occur, resulting
in the depletion of liquid cores above 500 °C. Electrical conductivity of the film is caused by crystalline indium andgallium oxide, as well as segregated metal layers. Both sintering methods induce similar surface textures and corresponding surface composition change. Moreover, thermal sintering liquid metal at high temperature induces extreme particle morphology changes.
This comprehensive comparison confirms that oxidation
suppression and large thermal stress are indispensable for
instantaneous laser irradiating to prevent extensive phase segregation, promote oxide rupturing followed by particle coalescence, and ultimately achieve conductive patterns in liquidform. The laser sintered liquid metal nanoparticle film exhibits superior electrical conductivity and softness. The selective
laser sintering process of liquid metal nanoparticles has
merits including selective high-resolution patterning (no maskneeded), fast sintering speed, and compatibility with soft substrates, which can be employed to create patterns with tunable
resistances for soft and flexible electronics with small scaleand high density.
For the thermal sintering process, formation of conductive
heterogeneous structures including surface textured particles
and metal oxide nanowires, have potential applications in the
broader semiconductor industry for nanoscale optoelectronic
devices, gas sensors or photocatalysis. The thermal sinteredliquid metal nanoparticle film presented in this work is notsuitable for soft and flexible electronics due to its resulting
solid form and strong adhesion to the substrate. However, wefound that when the printed film is suﬃciently thick, thermalsintering at high temperature forms a very dense oxide layeron the top surface, preventing further oxygen penetration andcoalescing the particles underneath. The resulting composite
film can be transferred to a soft substrate and applied to thefield of soft electronics. Future eﬀorts will focus on combining

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NanoscalePaperlaser and laser sintering processes to create high-density integrated soft electronic devices for soft robotics applications.

high-resolution patterns were designed in CorelDraw, exported
as DXF files and then imported into the software of the laser
system.

4.

Experimental section

4.4

4.1

Preparation of liquid metal dispersion

The liquid metal dispersion was made by depositing
362 ± 5 mg of eutectic gallium–indium alloy (75.5% Ga, 24.5%
In, 495425, Sigma Aldrich) into the bottom of a 3 dram glassvial (03-339-10C, Fisher Scientific). 4 mL of ethanol (V1001,
Koptec) was then added to the vial by a micropipette(BPP1000, Lagnet BioPette Plus). The top of the vial wascovered with Parafilm (52858-076, VWR) to prevent entry of
foreign particles into the sample. The tip of the sonicator
probe (Q700 with 1/8″ microtip probe, QSonica) was thenpunched through the Parafilm and positioned approximately
1 mm from the bottom of the glass vial. A water bath held at
6 °C was raised to immerse the vial for keeping the sample
cool. The sample was sonicated at an amplitude of 36 μm(30% setting) for 120 min. Just prior to spray printing, eachsample was mixed vigorously using a vortex mixer(VortexGenie) for 3 min to ensure uniform dispersion.
4.2

Spray printing of liquid metal dispersion

The nanoparticle ink was then sprayed onto substrate using acustomized spray printer. Compressed air was blown over asyringe needle while ink was dispensed at a fixed rate (0.22mL min−1). The velocity of the printer stage was 7 mm s−1.
Details are described in our previous work.63 Samples for laser
and thermal sintering were fabricated by depositing liquid
metal nanoparticle inks onto PDMS and patterned ceramic
substrates, respectively. PDMS (Polydimethylsiloxane, Dow
Corning, Sylgard 184) substrates were fabricated by mixing andthen defoaming the elastomer base and the curing agent in a
10 : 1 ratio using a Thinky Mixer (ARE-310, Planetary
Centrifugal Mixer) for 30 s each, spin coated onto a glass slide(48382-180, VWR) at 200 rpm for 60 s using a spin coater(G3-8, SCS), and cured in an incubator at 60 °C for 3 h. The
patterend ceramic substrates were fabricated by spin coating
PDMS at 500 rpm for 60 s, cutting dogbone patterns using an
IR laser (Universal Laser Systems VLS 2.30, 30 W, 10.6 μm CO2;
vector mode, 50% power, 50% speed) and then peeling oﬀ thedogbone shapes before spray printing. Ceramic (nonporousalumina, 8462K25, McMaster-Carr) substrates were cleaned byrinsing with ethanol, isopropanol and distilled water before
printing. After spray deposition, the rest of PDMS masks were
peeled oﬀ before furnace heating.
4.3

Laser sintering

Samples for laser sintering were sintered using a UV laser
micromachining system (Protolaser U4, LPKF) with a wavelength of 355 nm, a pulse duration of 900 ns and a beam diameter of 15 μm, operated in ambient environment. The laser
fluence was varied between 1.52 to 8.49 J cm−2, and the beampulse overlap was varied between 40 to 90%. The dogbone andThis journal is © The Royal Society of Chemistry 2019

Furnace heating

Thermal sintering process was conducted in an enclosed
furnace (Thermo Fisher Scientific) in ambient condition. The
nanoparticle films were deposited onto patterned ceramic substrates, placed in the furnace for 30 min at a certain temperature and then cooled down in air.
4.5

Material characterization

Simultaneous diﬀerential scanning calorimetry–thermal gravimetric analysis (DSC–TGA) measurements were collected on a
Setaram Setsys Evolution. The furnace chamber was purged
twice ( pulling vacuum and backfilling with ultra-high purityAr) prior to running experiment. The sample chamber wasthen held at ambient temperature for three additional minutes
to avoid thermogravimetric drift during temperature ramp up.
The TGA balance was tared immediately prior to the experiment. Eutectic gallium–indium alloy nanoparticles were
heated in a 90 μL alumina crucible to 1000 °C at 10 °C min−1
in a carrying gas atmosphere of either compressed air or ultrahigh purity Ar, flowing at 20 mL min−1. An empty alumina crucible was used as the reference. Mass and heat flow data werecollected for each experiment, and the baseline signal was subtracted from an empty sample experiment run immediately
prior.
The electrical resistance measurements were made via
4-terminal resistance methods using a digital multimeter(5492B, BK Precision). For laser sintered samples, drops ofliquid metal were used to interface between the measurement
probes and the surface of the sintered pattern to enable goodelectrical contact without physically disturbing the surface.
The values reported in this paper were calculated from triplicate measurements. After resistance measurements, threeexperimental samples at each processing parameter wereselected and used for further characterization.
SEM images were taken by a Hitachi SU8230 UHR Cold
Field Emission Scanning Electron Microscope. Surface morphology of these samples were checked under SEM to confirm
good condition (no scratch or break) before conducting XRD
and XPS characterization. Elemental composition and mappings were obtained by using energy dispersive X-ray spectroscopy (Flat-Quad, Bruker). XRD patterns were collectedusing a Rigaku Smartlab X-Ray Diﬀractometer with Cu Kα radiation (8.04 keV, 1.5406 Å). XPS spectra were collected by use ofa monochromatic 1486.7 eV Al Kα X-ray source on PHI
VersaProbe II X-ray photoelectron spectrometer with 0.47 eV
system resolution. The energy scale has been calibrated by use
of Cu 2p3/2 (932.67 eV) and Au 4f7/2 (84.00 eV) peaks on aclean copper plate and a clean gold foil. Survey spectra werecollected at a constant pass energy of 187.85 eV. The peak positions were corrected with carbon peak C 1s at 284.8 eV. The
high-resolution spectra C 1s, O 1s, Ga 3d, Ga 2p, and In 3dwere collected at a constant pass energy of 23.5 eV. Samples

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for XPS measurements sat in a desiccator under vacuum overnight and then were blown with dry nitrogen before being
loaded into the XPS chamber. The data in Fig. 8 and 9 werechecked for repeatability and selected as the representative
data.

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Conﬂicts of interest
There are no conflicts to declare.

Acknowledgements
We thank R. Adam Bilodeau for his help with optical images,
Dr Michelle C. Yuen and Shaotang Wang for their valuablecomments on earlier versions of the paper, Dr Min Li for training on the XRD, SEM/EDS and XPS at Yale West Campus
Materials Characterization Core and his advises on data analysis, and Pengzi liu for her help with the TEM experiment.
This work was supported by the National Science Foundation(CAREER Award 1454284).

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