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Printed Electronics
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Metallic Nanoparticle Inks for 3D Printing of Electronics

Hong Wei Tan, Jia An, Chee Kai Chua, and Tuan Tran*

Although 3D printed electronics has the

The three-dimensional (3D) printed electronics additive manufacturing potentials to allow low-cost rapid proto-
industry sector has grown substantially in the past few years, and there typing of functional electronics on different
is increasing demand for different types of metallic nanoparticle inks in substrates for various applications, the inks
used in 3D printing of electronics are hin-
electronics printing for various applications. Metallic nanoparticle inks are
dering the progression of the 3D printed
commonly used for fabricating conductive tracks and patterns due to their electronics due to their high price tags and
relatively high electrical conductivity as compared to other types of inks, and low electrical performances. The inks are
they can be further categorized into single-element metallic nanoparticle one of the most critical components in the
inks, alloy metallic nanoparticle inks, metallic oxide nanoparticle inks, 3D printing of electronics, in which the final
electrical and mechanical properties of the
and core–shell bimetallic nanoparticle (BNP) inks. It is critical to gain a
printed patterns are affected by the mate-
deep understanding of the metallic nanoparticle inks used in 3D printed rial properties of the inks. Therefore, it is
electronics as the material properties of these inks can directly affect the critical to gain a deep understanding of inks
final electrical and mechanical properties of the printed patterns. This review used in 3D printed electronics. Different
presents an overview of the available metallic nanoparticle inks used for 3D types of inks have different specialized
printing of electronics, and critically reviews the strengths and weaknesses properties and functionalities, and some
of the available inks in the market are
of each type of ink. Finally, the challenges of metallic nanoparticle inks in 3D
metallic nanoparticle inks, metallic-organic
printed electronics are also discussed along with the future outlook for 3D decomposition inks, conductive polymers
printed electronics. inks, dielectric inks, semiconductor inks,
carbon nanotubes inks, and graphene inks
(see Figure 1).
1. Introduction Metallic nanoparticle inks are readily obtainable from the
commercial market and are commonly used for fabricating
3D printed electronics have captured much attention in the conductive tracks and patterns due to their relatively higher
recent years, owing to their success in allowing on-demand electrical conductivity as compared to other types of inks. These
fabrication of highly-customizable electronics on wide varie- metallic nanoparticle inks are made up of electrically conduc-
ties of substrates (for instance, polymer films, papers, and tex- tive metallic nanoparticles suspended in liquid mediums, with
tiles) and conformal surfaces. 3D printing of electronics is an particle size ranging from 1 to 100 nm. However, there are
interdisciplinary field which integrates materials science with also diverse types of metallic nanoparticle inks in the market
engineering principles, in which machines precisely deposit (for instance, silver nanoparticle inks, gold nanoparticle inks,
conductive inks onto substrates (for instance, direct-writable and copper nanoparticle inks). Metallic nanoparticle inks of
printing technologies such as aerosol jet and inkjet printers[1]). different material compositions can directly influence the final
Current conventional electronics fabrication methods comprise electrical, material, and mechanical properties of the printed
of a series of additive and subtractive manufacturing tech- patterns. This paper presents an overview of the different types
niques which are complicated, time-consuming, uneconomical, of metallic nanoparticle inks used for the 3D printing of elec-
and environmentally unfriendly. Therefore, 3D printing of elec- tronics. The strengths and weaknesses of each type of ink are
tronics aims to mitigate these shortfalls and at the same time, critically reviewed. The challenges and potentials of metallic
allowing low-cost fabrication of electronics and helps to increase nanoparticle inks in 3D printed electronics are also discussed.
designs freedom by allowing electrical circuits and components
to be directly fabricated onto various types of substrates.[2–14]
2. Metallic Nanoparticle Inks
H. W. Tan, Dr. J. An, Prof. C. K. Chua, Prof. T. Tran
Singapore Centre for 3D Printing Metallic nanoparticle inks are suspensions of metallic nano­
School of Mechanical and Aerospace Engineering particles in liquid mediums (see Figure 2a), in which individual
Nanyang Technological University metallic nanoparticle is encapsulated in a layer of insulating
50 Nanyang Ave, Singapore 639798, Singapore organic additives and stabilizing agents (see Figure 2b). The
E-mail: ttran@ntu.edu.sg
encapsulating organic additives and stabilizing agents help pre-
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/aelm.201800831.
vent the metallic nanoparticles from agglomeration, but at the
same time, also impede the flow of electrons from particles to
DOI: 10.1002/aelm.201800831 particles. Therefore, sintering processes are required to remove

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Chee Kai Chua is a professor

of Nanyang Technological
University, Singapore and
the Executive Director of
the Singapore Centre for
3D Printing (SC3DP). He
is an active contributor to
the additive manufacturing
(AM) field for over 28 years.
His research interests are
Figure 1. Different types of inks used in 3D printed electronics.
powder bed fusion processes,
bioprinting, 4D printing, and
these liquid mediums and encapsulating organic additives food printing.
from the wet as-deposited printed patterns so that the metallic
nanoparticles can form contacting points with neighboring
Tuan Tran is a Nanyang
particles to conduct electricity (see Figure 2c). The printed pat-
Assistant Professor at
terns mainly consist of metallic nanoparticles after the decom-
Nanyang Technological
position of liquid mediums and organic additives. Therefore,
University, Singapore. He
the material composition of the metallic nanoparticles largely
received his B.Sc. degree
determines the electrical, mechanical, and material properties
in Engineering Mechanics
of the printed patterns.
from the Hanoi University
of Science, Vietnam in
2004. He then pursued
2.1. Parameters of Metallic Nanoparticles
his graduate study, which
focused on thin-film flows
Although the electrical, material and mechanical properties of
and turbulent frictional
the printed patterns and the required sintering temperatures are
drag at the University of Illinois at Urbana-Champaign,
primarily influenced by the material compositions of metallic
USA. Upon completion of his Ph.D. degree in 2010, he
nanoparticle inks, other parameters can also affect these prop-
worked at the Physics of Fluids Group at the University
erties in one way or another. The parameters of metallic nano-
of Twente, The Netherlands, as a post-doctoral researcher.
particle inks can be categorized into metallic nanoparticles and
His research interests are drop-surface interactions and
liquid medium (see Figure 3). Metallic nanoparticles can be
their applications including 3D printing and thermal
further classified according to their material compositions, par-
management.
ticle size, particle shape, and particle concentration; and liquid
medium can also be further classified according to the carriers,
dispersants and additives.[15] The particle size can impact the
melting point of the metallic nanoparticles, in which nano­ catalytic properties of metallic nanoparticles.[21,22] The solid
particles have significantly lower melting points than their bulk loading of the metallic nanoparticle inks can also influence
counterparts. For instance, 5 nm diameter spherical gold nano­ the electrical conductivity of the printed patterns directly.[23,24]
particles have a melting temperature of less than 300 °C as Furthermore, the decomposition temperatures of the organic
compared to bulk gold’s melting temperature of 1064 °C.[16–20] additives and stabilizers in the inks may also affect the required
This unique phenomenon allows the sintering of nano­particles sintering temperatures,[21,22] as these organic additives and sta-
at a much lower temperature.[18–20] In addition, the particle bilizers are required to be removed before coalescence of the
shape may also influence the optical, electrical, magnetic, and metallic nanoparticles can take place.

Figure 2. a) Metallic nanoparticles suspended in a liquid medium, b) organic additives prevent nanoparticles contact with each other, and c) initial
contact with neighboring metallic nanoparticles.

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Figure 3. Parameters of metallic nanoparticle inks.[15]

2.2. Choice of Metallic Nanoparticles possess good electrical and material properties with significant
low cost. Finally, materials with unique electrical, material, and
There are several important factors to consider for deciding magnetic properties may also be a factor of consideration for
the choice of metallic nanoparticles for the ink: electrical the choice of material composition.
conductivity, oxidation stability, costs, and the desired elec-
trical and magnetic properties (see Figure 4). However,
individual application’s requirements primarily determine these 2.3. Types of Metallic Nanoparticle Inks
factors. 3D printed electronics applications generally require
the printed conductive tracks to have minimum electrical These metallic nanoparticle inks can be further catego-
resistance to reduce Joule heating effects and promote efficiency. rized according to the material compositions of the metallic
Therefore, it is logical to choose materials with better elec- nanoparticles. Hence, the metallic nanoparticle inks are catego-
trical conductivity as a choice of material composition for rized into four main categories: single element metallic nanopar-
metallic nanoparticle inks. For instance, silver, gold, and ticle inks, alloy metallic nanoparticle inks, metallic oxides
copper are usually preferred as a material choice for metallic nanoparticle inks, and core–shell bimetallic nanoparticle inks.
nanoparticle inks due to their superior electrical conduc- On the one hand, single element metallic nanoparticle inks
tivity. Apart from electrical conductivity, the material com- are the most commonly used and widely available metallic
position of metallic nanoparticles should also possess good nanoparticle inks in the market. Silver nanoparticle inks and
oxidation stability. At elevated temperatures, some metals gold nanoparticle inks are one of the few single element metallic
tend to form metal oxides easily when exposed to air and nanoparticle inks used in the 3D printing of electronics due to
these types of metals are not ideal for formulating metallic their superb electrical conductivity and oxidation stability. On
nanoparticle inks. For instance, copper nanoparticles rapidly the other hand, the other three types of inks are less commonly
oxidize in the atmosphere at elevated temperatures, and these used as they are not widely available in the market. However,
undesirable copper oxides (CuOs) are found to be significantly some of these types of inks possess very interesting proper-
less conductive than pure copper nanoparticles.[19] Although ties and have the potential to revolutionize 3D printing of
good electrical and material properties are crucial, their mate- electronics. The next section will discuss each type of metallic
rial costs cannot be neglected too. As 3D printed electronics nanoparticles in greater detail.
aim to decrease fabrication costs, the material costs must
not be too high. For example, highly conductive gold nano-
particle inks exhibit excellent electrical properties and oxida- 3. Single Element Metallic Nanoparticle Inks
tion stability, but the price is too high for cost-effectiveness
in mass productions of cheap 3D printed electronics. There- 3.1. Silver Nanoparticle Inks
fore, ongoing research has been conducted to search for suit-
able materials for formulating metallic nanoparticle inks that Silver has been used as money for centuries, and its silvery luster
makes it a perfect metal candidate for making jewelry. Silver
is commonly known as a precious metal associated with hefty
price tags. Apart from silver’s monetary values and ornamental
functions, silver is a unique metal that is very pertinent in many
industrial applications: electrical, optical, and biological areas.
Silver is extensively used in electronics as excellent electrical
conductors, and its unique optical properties also extended its
uses in photovoltaic applications such as solar cells fabrications.
Figure 4. Choice of metallic nanoparticles for metallic nanoparticle inks. Silver has remarkable antibacterial properties and is also being

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used in medical applications. Among all metals, silver has the 3.2. Copper Nanoparticle Inks
highest thermal and electrical conductivity.[21] It has a thermal
conductivity of 427 W m−1 K−1[25] at 298.15 K, an electrical con- Copper is a ductile and malleable metal with excellent electrical
ductivity of 62.9 × 106 S m−1[25] at 293.15 K and a melting tem- and thermal conductivity, which is generally used in the manu-
perature of 1234.93 K.[26] Despite these advantages, silver is facture of electronics and wirings. Copper has reduced electro-
prone to tarnish when exposed to hydrogen sulfide in air. migration effects as compared to noble metals[28] and is also
Silver nanoparticle inks are still extensively used for 3D significantly cheaper as compared to silver and gold.[63] Copper
printed electronics applications[4] due to their superior elec- has an electrical conductivity of 58.8 × 106 S m−1[25] at 293.15 K,
trical conductivity and oxidation stability, despite their high a thermal conductivity of 397 W m−1 K−1[25] at 298.15 K and
costs.[21,27,28] These conductive silver nanoparticle inks are a melting temperature of 1357.77 K.[26] Although copper is
available commercially in the market. Some of the major silver only 6% electrically less conductive than silver,[64] it is almost
nanoparticle inks manufacturers[29] include Advanced Nano Prod- 100 times cheaper than silver. Therefore, many research inter-
ucts Co. Ltd.,[30] Applied Nanotech Inc.,[31] Clariant International ests are looking into the usage of copper nanoparticle inks for
Ltd,[32,33] Creative Materials,[34,35] Dupont,[36] Harima Chemical 3D printed electronics applications in recent years. However,
Co.,[37] Nova-Centrix.[38–40] Paru,[41] PV Nanocell,[42] Sigma-Aldrich the propensity of copper nanoparticles oxidizing in a noninert
Corporation,[43] Sun Chemical Corporation,[44,45] UT Dots Inc.,[46] environment[28,64–67] prevents extensive usage of pure copper
and Xerox.[47] nanoparticles in 3D printed electronics applications. The sin-
Most silver nanoparticle inks can be deposited onto sub- tering of pure copper nanoparticles in the ambient environ-
strates by inkjet and aerosol jet printers, and they typically ment at high elevated temperatures also expedites the oxidation
require sintering temperatures ranging from 100 to 300 °C process.[67]
through thermal sintering in the ambient environment.[48,49] As oxidation takes place, thin coats of copper oxides (CuO)
Silver nanoparticle inks can be used in many applications such are formed on copper nanoparticles surfaces to achieve ther-
as the fabrications of conductive patterns, strain gauges,[5,50] modynamic stability.[68] These undesirable copper oxides are
patch antennas,[51] 3D antennas,[52] radio-frequency identifica- found to be significantly less conductive than pure copper nan-
tion (RFID) tags,[53] and many more. However, the hefty price oparticles,[19] and they also have a substantially higher melting
tags of silver nanoparticle inks result in difficulties achieving point (Tm, CuO = 1603.15 K) than bulk copper too.[69] The elec-
cost-effectiveness in industrial applications.[28] Therefore, one trical resistivity of bulk copper and bulk CuO are 1.72 and
of the primary motivations is to find other alternatives which 5.1 × 107 µΩ cm, respectively.[70] Hence, viable electrons paths
have lower costs and exhibit suitable electrical properties. among the copper nanoparticles are significantly reduced[68]
Silver nanoparticles also display unique optical, plasmonic, and lead to a decrease in electrical conductivity.[65] Also, it is
and antibacterial properties.[21,54] It is interesting to note that the also technically challenging to form highly conductive patterns
optical, electrical, and chemical properties of silver nanoparti- by thermally sintering copper nanoparticles with oxides shells
cles are also tunable, by merely modifying the particles’ shapes, in the ambient environment.[63] As compared to silver and gold,
sizes, structures, and other parameters.[55] The optical proper- copper’s relatively higher melting point requires higher input
ties of the spherical silver nanoparticles are directly dependent amount of energy to coalesce copper nanoparticles to form con-
on their diameters. The extinction spectra of spherical silver ductive patterns.[66] Consequently, solving these challenges will
nanoparticles with diameters ranging from 10 to 100 nm shows unleash great potentials for fabricating cheaper electronics and
that the spherical silver nanoparticles with diameters lesser enjoying significant cost savings, as compared to using silver or
than 50 nm have peak absorptance of light wavelengths at gold nanoparticle inks.
around 400 nm.[56] As the diameters of the spherical silver nan- Nevertheless, there is literature indicating successful sin-
oparticles increase, the peaks broaden and shift toward longer tering of pure copper nanoparticles. Park et al.[28] formulated
wavelengths. These nanoparticles absorb the light at high effi- conductive copper nanoink for inkjet printing on glass sub-
ciencies when they are exposed to the wavelengths of the light strates, thermally sintered the printed patterns in a vacuum
that corresponds to their peaks. Thus, these unique properties chamber under 325 °C for an hour and successfully achieved
of silver can be further exploited to improve the qualities of sheet resistivity of 17.2 µΩ cm. Despite the successful fabrica-
the end-products of many printed electronics applications and tion of conductive copper patterns, this complex and time-con-
as well as considering other interesting sintering techniques suming method required a vacuum chamber[67] and involved
(such as intense pulse light (IPL) sintering[57,58] and ultraviolet sintering at high temperatures that might not be feasible for
(UV) sintering[59]) to sinter the deposited silver nanoparticle use with temperature-sensitive substrates.[58,63] Apart from sin-
inks. Apart from the commonly used spherical silver nano­ tering the copper nanoparticle inks in a vacuum chamber, the
particles, nanosilver with unique geometries such as nanowires copper nanoparticle inks can also be thermally sintered in inert
and nanoplates are also of great interests to researchers. Silver gas (for instance, argon gas[70] and nitrogen gas[71]) or reducing
nanowires also can display up to 90% optical transparency gas conditions. There is difficulty in translating this time-con-
and can be used to fabricate transparent electrodes.[21] How- suming thermal sintering method into the electronic industry,
ever, silver nanowires have high tendency to clog up the inkjet which requires high-speed fabrications for mass production in
printers’ nozzles due to their geometries.[60,61] Silver nanoplates the ambient environment. Literature also has shown that IPL
can form very dense microstructures due to their flat geome- sintering[72–75] and laser sintering[72,76] techniques are able to
tries, and hence allowing the printed patterns to have very good sinter copper nanoparticle inks at high speed in ambient con-
electrical conductance.[21,62] ditions with low complexity, while maintaining high electrical

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conductance within the printed patterns. Niittynen et al.[72] preferred in certain applications due to their unique properties.
sintered copper nanoparticle inks with both lPL and laser sin- Tunable optical and electrical properties made gold nanoparti-
tering techniques. He demonstrated that both techniques could cles to be one of the most unique metallic nanoparticles. These
achieve an electrical conductivity of more than 20% of bulk properties can be fine-tuned to each specific application needs,
copper. However, the sintered copper nanoparticle inks are still by directly modifying its parameters: compositions, sizes,
vulnerable to oxidation and applications of protective layers shapes, structures, and surface chemistry.[54,89,90] Gold nano­
are needed to prevent further oxidation and extend product particles also exhibit interesting phenomenon like surface
life-time.[77] plasmon resonance effects,[91] which can be further exploited
Copper nanoparticle inks are also commercially available for the sintering process through photonic means (for instance,
via chemicals manufacturers like Applied Nanotech Inc.,[78] IPL and laser sintering techniques). Gold is highly resistant
and Nanoshel.[79] For instance, aerosol jet printable copper to oxidation, and it is very suitable for use in applications
nanoparticle ink from Applied Nanotech Inc. (ANI Cu-OC70)[78] which require the electrodes to be resistant to oxidation due to
has particle size ranging from 20 to 100 nm, and it can achieve external stimuli (for instance, electrochemical sensors[92] and
electrical resistivity between 20 and 50 µΩ cm. ANI Cu-OC70 wearable devices, which may experience high salt concentra-
can be either be thermal sintered at above 350 °C in a reducing tion from sweat[88]). For printed organic thin film transistors
atmosphere (hydrogen, H2 = 4% in nitrogen, N2) for 20 min- (OTFTs) applications, OTFTs require very tight tolerances on
utes or sintered through the IPL sintering technique. Some their electrical resistances as slight variations may cause sig-
applications with copper nanoparticle inks include conductive nificant declines in their electrical performances.[88] Therefore,
tracks, electrodes[75] and RFID tags.[80] operationally stable gold nanoparticle inks are preferred for fab-
ricating source and drain electrodes in OTFTs. Wu et al.[93] dem-
onstrated the use of gold nanoparticle inks for the fabrication of
3.3. Gold Nanoparticle Inks highly conductive electrodes for OTFTs as most p-type organic
semiconductors could form superb ohmic contacts with gold.
Gold is a precious metal that has always captivated mankind Due to gold’s good electrical conductivity, gold nanoparticle inks
for thousands of years for its highly lustrous appearances. Over can also be used to fabricate conductive patterns, electrodes
the centuries, gold is also used as a common form of currency arrays,[92,94] microelectrode arrays (MEAs) for biosensing appli-
and in ornamental jewelry. Gold is a highly ductile and mal- cations[88] and interdigitated electrodes (IDEs).[88] It is also inter-
leable noble metal, which can resist corrosion and oxidation esting to look into other shapes of gold nanoparticles other than
and does not discolor or tarnish. Gold has a thermal conduc- spherical particles for printed electronics applications, such as
tivity of 314 W m−1 K−1[25] at 298.15 K, an electrical conductivity gold nanorods[95] and nanowires.[96] For instance, mechanically
of 41.0 × 106 S m−1[25] at 293.15 K and a melting temperature flexible ultrathin gold nanowires have high optical transparency
of 1337.33 K.[26] Gold is also commonly used in the electronics and are highly resistant to oxidation and corrosion. They can
industries for gold plating on other metallic conductors like be utilized to fabricate flexible transparent electrodes[97] and
copper and silver, to help prevent corrosions. However, it has flexible wearable sensors.[96] However, gold nanoparticle inks
an exorbitant price tag as compared to other metals. typically require sintering temperatures of more than 190 °C,
The first usage of gold nanoparticles could be traced back which can limit the use of temperature-sensitive substrates
from the 4th century A.D. of the late Roman empire and was such as polyethylene terephthalate (PET) for printed electronics
demonstrated in the famous “Lycurgus” Cup. Romans artisans applications.[88] Gold nanoparticle inks are commercially avail-
embedded gold and silver nanoparticles into molten glass, cre- able via chemicals manufacturer like UT Dots Inc.[98]
ating a cup that was able to give different colors depending on
the directions of the incident light.[81] In 1857, Faraday demon-
strated that by merely changing the particles size, it could alter 3.4. Aluminum Nanoparticle Inks
the color of the gold colloidal solutions.[81,82] Thus, implying
that gold nanoparticles can have tunable optical properties. Due to its low cost and good mechanical properties, aluminum
To date, gold nanoparticles are generally used in electronics, is used extensively in many industries such as aerospace, auto-
medical, biological, chemical, and engineering applications mobiles, and even food and beverages industries. Aluminum
areas. In electronics, gold nanoparticles are used for the fabri- is lightweight and highly corrosion resistant due to the thin
cations of conductive tracks.[83] In the medical field, gold nano- films of protective aluminum oxides formed on its surfaces.
particles are adopted to be used as mechanisms for therapeutic Aluminum has an electrical conductivity of 35.5 × 106 S m−1[25]
agent delivery,[84] diagnostics applications,[85] and photodynamic at 293.15 K, a thermal conductivity of 238 W m−1 K−1[25] at
therapy.[83,86] Gold nanoparticles are even being exploited to be 298.15 K and a melting temperature at 933.47 K.[26] Its electrical
used as chemical sensors and optical probes in transmission conductivity is ≈0.6 times of copper’s electrical conductivity.[99]
electrons microscopy.[87] With the low material costs and compatible electrical properties
Literature also indicated strong interests in using gold of aluminum, aluminum nanoparticle inks are also synthesized
nanoparticle inks for printed electronics applications due to for printed electronics applications. Conductive aluminum
its good thermal stability, oxidation stability, and electrical nanoparticle inks (Al-IS1000[100] and Al-PS1000[101]) are com-
conductivity.[66,88] However, gold nanoparticle inks are too expen- mercially available from Applied Nanotech Inc. From the man-
sive for cost-effective large-scale fabrications of cheap printed ufacturer’s datasheet,[100] Al-IS1000 is an aerosol jet printable
electronics. Nonetheless, gold nanoparticle inks are highly aluminum nanoparticle ink, which has a viscosity ranging from

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80 to 120 cP. Al-IS1000 is formulated for fabricating conduc- electrical conductivity of 14.3 × 106 S m−1.[26] Nickel’s melting
tive patterns on silicon substrates, particularly for photovoltaic temperature is at 1728 K.[26]
applications, as the ink has low contact resistivity on silicon Nickel’s good electrical conductivity and ferromagnetic
substrates. In addition, Al-IS1000 is glass frit free, has passi- properties generate new interests in formulating printable
vation layer diffusion properties and can form highly uniform nickel nanoparticle inks for printed electronics applications.
back surface field layer. The printed aluminum film is said to Tseng and Chen[107] studied the rheological and suspen-
be less than 10 mΩ sq−1 if sintered at the recommended tem- sion structures properties of dispersed nickel nanoparticles
peratures ranging from 700 to 900 °C for less than a minute in α-terpineol solvents and expected the printed patterns
in air.[100] However, the high sintering temperatures restrict of these nickel nanoparticle inks to be conductive if printed
the usages of many temperature-sensitive substrates and may on non-conductive substrates. Park and Kim[108] formulated
only allow substrates like glass or silicon to be used. This is their nickel nanoparticle inks, and they sintered the printed
also one of the main reasons why aluminum nanoparticle inks patterns using the flashlight sintering technique. They were
are not broadly adopted for 3D printed electronics applications. able to achieve a sheet resistance of 0.347 Ω sq−1, which was
To date, there is few research in literature stating the usage of suitable for printed electronics applications. Nickel is highly
conductive aluminum nanoparticle inks for printed electronics corrosion resistant and can be used as a barrier against oxi-
applications. Khorramdel et al.[102] and Platt el al.[103] both dis- dation.[26] Therefore, nickel nanoparticle inks can be used as
cussed the use of aerosol jet printable aluminum nanoparticle a passivation layer to protect the underlying printed patterns
inks, Al-IS1000, for fabrication of conductive tracks on silicon (e.g., printed with copper and silver nanoparticle inks) from
substrates. oxidation and corrosion for long-term operation. This can be
done by merely depositing nickel nanoparticle inks over the
underlying printed patterns. Furthermore, nickel nanoparticle
3.5. Cobalt Nanoparticle Inks inks are a lot cheaper than silver, gold, and platinum inks.
However, nickel nanoparticle inks tend to agglomerate and
Cobalt is ferromagnetic, corrosion resistant, and commonly clog the inkjet printheads easily. Nickel nanoparticle inks are
used for alloying. Cobalt also has good thermal and electrical also commercially available in the markets, but only from a
conductivity, with a thermal conductivity of 100.0 W m−1 K−1 few vendors, such as Applied Nanotech Inc.[109,110] and Nano
and electrical conductivity of 17.0 × 106 S m−1.[26] Cobalt’s Dimension.[111] Applied Nanotech Inc.’s nickel nanoparticle
melting temperature is at 1768 K.[26] Cobalt’s high perme- inks: Ni-IJ70-30[109] and Ni-OC70,[110] are inkjet and aerosol jet
ability and permittivity properties generate much research printable respectively. Their nickel nanoparticle inks can either
interests to formulate conductive cobalt nanoparticle inks be sintered through thermal sintering at temperatures more
for printed electronics applications, specifically to applica- than 350 °C in a reducing environment (H2 = 4% in N2) for
tions which require interactions with electromagnetic waves 20 min or IPL sintering with the use of a xenon arc-discharge
and high frequencies. However, cobalt nanoparticle inks for system. The sintered nickel nanoparticle inks can obtain an
printed electronics applications are not available commer- electrical resistivity ranging from 20 to 50 µΩ cm, which are
cially. Nevertheless, some researchers formulate printable applicable for fabricating conductive tracks and patterns. Pas-
cobalt nanoparticle inks in-house with commercially available quarelli et al.[112] also reported depositing nickel nanoparticle
cobalt nanoparticles. Nelo et al.[104,105] formulated cobalt nano- inks onto glass and zinc oxide with an inkjet printer to fab-
particle inks for screen printing applications and the printed ricate electrode contacts for solar cell applications. Possible
patterns only required 10 min of sintering at 110 °C. Their applications with nickel nanoparticle inks may include tem-
cobalt nanoparticle inks had relative permeability and mag- perature sensors, inductors and microheaters.
netic loss tangent values ranging between 1.5 and 3 and 0.01
and 0.06, respectively, in the 45 MHz–10 GHz band.[104] These
cobalt nanoparticle inks allow more explorations of printed 3.7. Palladium Nanoparticle Inks
electronics applications such as fabrications of radio frequency
absorbers, antennas, magnetic sensors, filters, resonators, and Palladium, a precious metal with silvery appearance, is com-
phase shifters.[104–106] monly used in ornamental jewelry, dentistry for dental fillings
and catalytic converters for automobiles. Palladium is also
used in the fabrications of ceramic capacitors in the electronics
3.6. Nickel Nanoparticle Inks industry. Chemical vapor deposition (CVD), sputtering and
electroplating are some of the conventional methods to fabri-
Similar to cobalt, nickel is a strong and corrosion resistant cate palladium structures.[113] Similar to all precious metals,
metal that able to withstand high temperatures. Nickel is the palladium also has an expensive price tag. Palladium has a
fifth most abundant metal on earth that commonly plays sup- melting point of 1828 K, a thermal conductivity of 72 W m−1 K−1
porting roles for strengthening and stabilizing other metals. It and electrical conductivity of 9.5 × 106 S m−1.[26]
is commonly used in the plating industries, and it is also used Due to palladium’s unique reactions with hydrogen gas
for manufacturing superalloys and coins. Nickel metal is also and its electrocatalytic properties,[114] researchers are inter-
ferromagnetic and commonly used for manufacturing perma- ested in synthesizing palladium nanoparticle ink for printed
nent magnets. Nickel has good thermal and electrical conduc- electronics applications, particularly in fabricating electro-
tivity, with a thermal conductivity of 91.0 W m−1 K−1[26] and chemical sensors. However, inkjet and aerosol jet printable

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palladium nanoparticle inks are not widely available in the 3.9. Tin Nanoparticle inks
market. Chemical manufacturers like SunChemical[115] and
American Elements[116] do sell screen printable palladium inks Since the bronze age, archaeological records have revealed that
for printed electronics applications. Liu et al.[113] filed a patent people from ancient civilizations started using bronze for the
on the formulation of palladium nanoparticle inks. Their for- creation of harder and durable tools and weapons.[26] Bronze is
mulated palladium nanoparticle inks require a sintering tem- an alloy that primarily consists of copper and tin, and hence
perature ranging between 180 and 250 °C for the coalescence the earliest usage of tin is probably during the bronze age. In
of the palladium nanoparticles. Tseng et al. also looked into today’s world, tin is commonly used for the manufacture of tin
the deposition of palladium nanoparticle ink onto PET sub- cans for the canning of processed food, whereby steel cans are
strates through inkjet printing. The as-deposited palladium plated with a thin layer of tin. Tin is corrosion resistant, and
nanoparticles facilitate as a catalyst for electroless nickel therefore tin is usually used to plate other types of metals that
plating at low operating temperatures.[117] Literature[118–120] also are prone to corrosions.[26] Tin has a melting point of 505.08 K,
showed that palladium nanoparticle inks were commonly used a thermal conductivity of 67 W m−1 K−1 and electrical conduc-
as a catalyst for electroless plating of copper on polymer sub- tivity of 8.7 × 106 S m−1.[26]
strates for fabricating flexible electronics. Qin et al.[114] dem- Although tin has a much poorer electrical conductivity as
onstrated the fabrication of pH sensors with their synthesized compared to silver and gold, it has significantly lower melting
palladium precursor ink and thus implying that palladium temperature and costs. Therefore, tin nanoparticles are ideal
nanoparticle inks also have the potential to be used for fabri- candidates for formulating low-cost conductive inks that are
cating pH sensors. Possible applications with palladium nano­ used for printed electronics applications. However, there are no
particle inks may include electrode materials for fuel cells, commercially available tin nanoparticle inks. Jo et al.[124] synthe-
pH sensors, conductive tracks, thin film transistors, gas sen- sized tin nanoparticle inks for printed electronics applications
sors, biosensors, and electrochemical sensors and monitoring and their conductive patterns were able to achieve an electrical
systems.[113,114] resistivity of 64.27 µΩ cm (six times higher than bulk tin) after
60 min of thermal sintering at 250 °C. The electrical resistivity
of the printed patterns remained unchanged after 1 month and
3.8. Platinum Nanoparticle Inks demonstrated the reliability of tin nanoparticle inks. Although
the electrical resistivity of the tin nanoparticle inks are relatively
Platinum, one of the rarest noble metals, is corrosion resistant, higher than silver and gold nanoparticle inks, there are still
malleable, ductile, highly dense, and does not tarnish. Platinum potentials in using tin nanoparticle inks for printed electronics
metal is extensively used in engineering industries for applica- applications considering tin’s price and melting temperature.
tions like catalytic converters and electrical contacts. Due to its
properties and rarity, platinum is also classified as a precious
metal. It is commonly used for ornamental jewelry and is 4. Alloy Metallic Nanoparticle Inks
highly sought after for bullion investments. Therefore, hefty
price tags are always tagged with platinum metal too. Platinum 4.1. Copper–Nickel Alloy Nanoparticle Inks
has a thermal conductivity of 71.6 W m−1 K−1 at 298.15 K,
electrical conductivity of 9.1 × 106 S m−1[25] at 293.15 K and Copper–nickel alloy nanoparticle inks are receiving many
melting temperature at 2041.4 K.[26] Due to platinum’s unique interests recently for printed electronics applications due
properties, there are much research interests to explore printable to their unique electrical properties. The nickel content in
platinum nanoparticle inks for printed electronics applications. copper–nickel alloys helps to enhance the tensile strength, hot
Although platinum nanoparticle inks are not as accessible strength and proof strength, but it also results in a decrease in
as silver nanoparticle inks or gold nanoparticle inks, manu- the electrical and thermal conductivity.[125] Copper–nickel alloy
facturers such as DuPont,[121] Fraunhofer IKTS,[122] LCC nanoparticle inks are also commercially available in the mar-
AkKoLab,[122] and UT Dots Inc[98] do also manufacture these kets, but only from a few vendors, such as Applied Nanotech
platinum nanoparticle inks. Most platinum nanoparticle inks Inc.[126] Applied Nanotech Inc.’s copper–nickel alloy nano­
can be deposited through aerosol jet or inkjet printing tech- particle inks: CuNi-OC5050[126] and CuNi-IJ5050[127] are aerosol
nologies.[98] The platinum nanoparticle inks typically require jet and inkjet printable respectively. Their copper–nickel alloy
sintering temperatures ranging from 200 to 250 °C through nanoparticle inks can either be sintered through thermal sin-
thermal sintering.[98] However, the sintering temperatures are tering at temperatures more than 350 °C in a reducing environ-
too high for the sintering of the platinum nanoparticle inks on ment (H2 = 4% in N2) for 20 min or IPL sintering with the use
temperature-sensitive substrates. In literature, Kim and Park of a xenon arc-discharge system.[126] CuNi-OC5050 can achieve
used platinum nanoparticle ink to fabricate dye-sensitized solar a sheet resistance of 200–300 mΩ sq−1 for 2 µm thin film
cell (DSSC) counter electrodes.[123] Dupont’s conductive plat- according to the manufacturer.[126] Applied Nanotech Inc. also
inum ink, BQ321, can also be used for printing of high activity does sell copper–nickel alloy nanoparticle inks (CuNi-IJ5545)
electrodes in polymer thick film (PTF) sensors and biosen- with 55/45 CuNi alloy ratio, which is similar to Constantan.[127]
sors.[121] Platinum nanoparticle inks can also be utilized for fab- Constantan comprises 55% copper and 45% nickel, and it is
ricating microelectromechanical systems (MEMS) gas sensors the most widely used material for fabrication of strain gauges
on thin ceramic substrates[123] and as well as electrodes for fuel conventionally. Constantan possesses desirable properties like
cells applications. good fatigue life, high strain sensitivity, and relatively high

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elongation capability.[128] Hence, the copper–nickel alloy nano- 5.3. Indium Tin Oxide Nanoparticle Inks
particle inks are favorable to be used in sensing applications
such as strain gauges and thermocouples.[126] ITO is generally used for fabricating transparent electrodes in
liquid crystal display, organic light emission display, and touch
panels due to its high optical transmittance, electrical conduc-
5. Metallic Oxides Nanoparticle Inks tion, chemical inertness, and good substrate adhesion.[136–139]
There is also much interest to look into the use of inkjet printers
Metallic oxides nanoparticle inks, apart from single element for fabricating ITO thin films, as inkjet printing potentially offers
metallic nanoparticle inks, are also gaining popularity in 3D better cost-effectiveness, material savings, direct patterning, and
printed electronics applications over time due to their supe- printing efficiency as compared to current conventional fabrica-
rior oxidation stability[129] and hence, allowing the sintering of tion techniques (for example, CVD, magnetron sputtering, and
printed patterns in an ambient environment without the need spin-coating). There were several research teams[136–140] demon-
for vacuum or inert gas conditions. Some of the commonly strating the use of self-formulated ITO nanoparticle inks for fab-
used metallic oxides nanoparticle inks for various functional ricating transparent electrodes through inkjet printing. Hwang
printed electronics applications include copper oxide, iron oxide, et al.[140] inkjet-printed 580 nm thick transparent electrically con-
indium tin oxide (ITO), and zinc oxide (ZnO) nanoparticle inks. ductive films with ITO nanoparticle inks, and achieved a sheet
resistance of 517 Ω sq−1 and optical transmittance of 87% after
sintering them at 400 °C. In addition, they also sandwiched
5.1. Copper Oxide Nanoparticle Inks silver lines grid in between two layers of ITO thin films to fur-
ther reduce the sheet resistance to 3.4 Ω sq−1, but the optical
Although copper is significantly cheaper than silver, copper oxi- transmittance was mildly reduced to 82% for the exchange of
dizes readily in air to form copper oxides which made formu- better electrical conductivity. However, ITO nanoparticle inks
lation and sintering of copper nanoparticle ink expensive and require high sintering temperatures which are not suitable for
complicated. Hence, reducible copper oxide nanoparticle inks cheap temperature sensitive substrates.
are one of the solutions to copper’s oxidation and cost problems.
These copper oxide nanoparticles are significantly cheaper than
pure copper nanoparticles and easier to manufacture without 5.4. Zinc Oxide Nanoparticle Inks
worrying about oxidation. Reducible copper oxide nanopar-
ticle inks comprise copper oxides nanoparticles and a reducer There are strong motivations to look for alternative materials
agent. Deposited reducible copper oxide nanoparticle inks are to replace ITO due to the scarcity and high costs of indium,
typically sintered by photonic sintering techniques in ambient although ITO is an ideal material for fabricating transparent
conditions, in which the copper oxides nanoparticles are con- electrodes. Furthermore, ITO nanoparticle inks also require
verted back into copper through a chemical reduction process. high sintering temperatures for well-optimized properties.
However, protective coatings should also be applied to the sin- Doped zinc oxide (ZnO) is one of the potential materials to
tered printed patterns to prevent any potential oxidations.[77] replace ITO due to its superior material properties and low
Copper oxide nanoparticle inks are also commercially available costs. ZnO is an n-type semiconducting material that has
via chemicals manufacturers like Nanoshel[130] and NovaCen- a wide bandgap of 3.36 eV and high optical transmittance,
trix.[131,132] For instance, inkjet printable copper oxide nano- making it a potential material for fabricating transparent
particle ink from NovaCentrix (Metalon ICI–002HV)[131] has electrodes.[141,142] In addition, ZnO is also suitable for opto-
copper oxide nanoparticles with particle size ranging from electronics, bioimaging, sensing, electronics, photovoltaics,
110 to 130 nm, and it can achieve electrical resistivity between and sensing applications due to their unique piezoelectrical,
7.5 and 10.8 µΩ cm (≈4.5x bulk resistivity of copper). However, luminescent, and pyroelectrical properties.[142] Zinc oxide
Metalon ICI–002HV can only be sintered via IPL sintering tech- nanoparticle inks are also commercially available via chemicals
nique. Paquet et al.[133] demonstrated the use of IPL sintering manufacturer like Genes’Ink.[143] For instance, the inkjet print-
technique on inkjet printed copper nanoparticle ink (NovaCen- able zinc oxide nanoparticle ink from Genes’Ink (Helios′Ink
trix Metalon ICI–002HV) and were able to achieve an electrical H-SZ01034 semiconductive ink[144]) can achieve electrical con-
resistivity of 9 µΩ cm. ductivity between 10−8 and 10−7 S cm−1 before illumination.
This ink is compatible with most flexible substrates, ITO and
silver nanowires layers.
5.2. Iron Oxide Nanoparticle Inks

Iron oxide nanoparticle inks recently gained many interests 6. Core–Shell Bimetallic Nanoparticle Inks
among researchers for their magnetic properties (high perme-
ability and relatively high saturation magnetization)[134] and are In recent years, there is much on-going research exploring
further explored as functional printable magnetic inks for core–shell bimetallic nanoparticle (BNP) inks for low-cost and
printable electronics applications such as inductors and radio high-performance 3D printed electronics applications.[64] Core–
frequency devices.[135] Vaseem et al.[135] formulated an iron oxide shell BNP consist of a core and a shell of two different ele-
nanoparticle-based magnetic ink for the fabrication of a low-cost ments, in which one type of metal atoms encircle the inner core
tunable frequency printed patch antenna. that is made up of another type of metal atoms. Noble metals

Adv. Electron. Mater. 2019, 1800831 1800831 (8 of 20) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.advancedsciencenews.com
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with good oxidation stability are usually used as outer shells than silver nanoparticle inks due to their reduced silver load-
in core–shell BNP to protect the inner core from oxidations. ings. However, these Cu–Ag core–shell BNP inks are not avail-
Examples of core–shell BNP include silver–gold (Ag–Au),[64] able for sale commercially. Pajor-Świerzy et al.[152] reported on
gold–palladium (Au–Pd),[64] tin–silver (Sn–Ag),[145] and copper– the synthesis of a low-cost conductive Cu–Ag core–shell nano-
silver (Cu–Ag)[146,147] core–shell BNP. particle ink for printed electronics applications. The Cu–Ag
Core–shell BNP are attractive to researchers due to the pres- core–shell BNP were made up of 1 µm diameter copper core
ence of additional unique properties that are not found in mon- coated with 20 nm thick silver shells. Their ink was reported
ometallic nanoparticles. Core–shells BNP not only can give a to achieve 16% bulk conductivity of copper after 15 min of
combination of material, electrical, catalytic, optical, and pho- thermal sintering at 250 °C. Grouchko et al.[153] also demon-
tocatalytic properties from two different metals, but also new strated that 2 nm of silver layer was only required to coat the
properties arising from the synergy between both metals.[148,149] 40 nm copper nanoparticles to prevent oxidation and maintain
Core–shell BNP also allow the tuning of the nanoparticles the essential electrical properties of copper. These findings
properties, in which specific desired properties are amplified showed that the silver load could be significantly reduced with
and undesired properties are minimized. These properties can the use of Cu–Ag core–shell BNP, as only thin shells of silver
be altered by merely modifying the core-to-shell ratio of the were needed to coat over the copper nanoparticles. Lee et al.[146]
core–shell BNP or changing its constituting materials.[150] With also synthesized Cu–Ag core–shell BNP inks for printed elec-
core–shell BNP, it is also possible to reduce the expensive noble tronics applications. Their results indicated that the Cu–Ag
metal load in nanoparticles by substituting the inner core with core–shell BNP have better electrical properties and oxidation
cheaper metal alternatives. Costly noble metals are coated over stability as compared to copper nanoparticles under ambient
cheaper metal alternatives (such as copper) to reduce the usage conditions, achieving 12 µΩ cm at 350 °C.
of costly monometallic noble monometallic nanoparticles for
additional cost savings while maintaining similar electrical con-
ductivities, oxidation stabilities, and particle size.[150] However, 6.2. Cu–Ni Core–Shell Bimetallic Nanoparticle Inks
synthesizing these core–shell BNP may be complicated and
time-consuming. As silver is an expensive material, some researchers have
been looking into other metals such as nickel to replace the
silver shells in Cu–Ag core–shell bimetallic nanoparticle inks.
6.1. Cu–Ag Core–Shell Bimetallic Nanoparticle Inks Kim et al.[156] explored the use of highly oxidation-resistant
copper–nickel (Cu–Ni) core–shell BNP inks for fabricating
On the one hand, silver nanoparticle inks are too costly to be printed flexible electrodes. They achieved a reasonably low
used for large-scale fabrications of printed electronics despite sheet resistance of 1.3 Ω sq−1 under the intense pulse light sin-
their superior electrical and material properties. Silver is tering process. Their research also showed that the printed elec-
≈100 times more expensive than copper,[151] and silver is only trodes still maintained their electrical properties after 30 days
6% more electrically conductive than copper. On the other under 85% relative humidity at 85 °C. Hence, Cu–Ni core–shell
hand, copper has excellent electrical properties, but copper BNP inks can be further explored as an alternative conductive
nanoparticles are highly susceptible to oxidation in ambient ink to the expensive silver nanoparticle inks in the future for
environment at elevated temperatures. These copper oxides cost-effective printed electronics applications.
are undesirable for printed electronics applications due to
their poor electrical conductivity and the need for higher sin-
tering temperatures. Thus, copper nanoparticles are not very 7. Comparison of Different Metallic Nanoparticle
ideal for printed electronics applications. To simultaneously
Inks used in 3D Printed Electronics
solve both cost issue in silver and oxidation issue in copper,
it is particularly attractive and appealing to look into Cu–Ag Table 1 presents and compares the advantages and disadvan-
core–shell BNP inks for 3D printed electronics applications. tages of different metallic nanoparticle inks used in 3D printed
The copper core is coated with a thin layer of silver shell in electronics. From the table, it can be deduced that silver nano­
Cu–Ag core–shell BNP to help solve this oxidation problem of particle inks are still most suitable for fabricating highly
copper nanoparticles. As silver has high oxidation resistance, electrically conductive patterns due to their better electrical con-
the silver shell can protect the copper nanoparticles from oxi- ductivity and oxidation stability, despite their high costs. Fur-
dation. Hence, with Cu–Ag core–shells BNP, it is possible to thermore, silver nanoparticle inks are widely available in the
significantly reduce the silver load in nanoparticles to enjoy commercial market, and extensive research has been done by
better cost savings while maintaining similar electrical con- various manufacturers in the past few years to formulate silver
ductivities.[146] There is literature[146,147,152–155] demonstrating nanoparticle inks for optimized depositions through the aerosol
successful uses of Cu–Ag core–shell BNP inks for 3D printed jet and inkjet printers.
electronics applications. These Cu–Ag core–shell BNP inks It is also evident that silver and copper have the best bulk
demonstrate good electromigration and oxidation resistance electrical conductivity among all metals. Although they are
properties. As silver is coated on the surface of the copper core, highly suitable for formulating highly conductive metallic
the required sintering temperatures of Cu–Ag core–shell BNP nanoparticle inks, their melting temperatures are considerably
inks are lower as compared to copper nanoparticle inks.[155] high also. It is interesting to note that tin has a low melting
Cu–Ag core–shell BNP inks are expected to have lower costs temperature of 232 °C and low material cost. Hence, it can be

Adv. Electron. Mater. 2019, 1800831 1800831 (9 of 20) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
 Table 1. Comparison of different metallic nanoparticle inks used in 3D printed electronics.

Types of metallic Bulk properties Advantages Disadvantages Applications Applicable sintering Reported Reported electrical Inks Commercially
nanoparticle ink techniques thermal sintering properties manufacturers available inks
temperatures references
Single Silver • E lectrical •  xcellent
E • Expensive • C
 onductive • T hermal • S
 intering • 3 µΩ cm • A dvanced Nano •  TDAgTE
U
element nanoparticle conductivity: 62.9 electrical • D ifficulties in tracks and sintering[48,49,157] temperatures after Products Co. (20–60 wt% Ag
metallic inks × 106 S m−1[25] conductivity, thermal achieving patterns • Laser ranging 10 min Ltd.[30] loading), UT Dots

Adv. Electron. Mater. 2019, 1800831

nanoparticle • Thermal conductivity, and oxida- cost • Strain sintering[158] from 100 to of thermal • Applied Inc.[157]
www.advancedsciencenews.com

inks conductivity: tion stability[21,27,28] effectiveness gauges[5,50] • Intense pulse 300 °C[31,48,49,157] sintering Nanotech, • UTDAgPA,
427 W m−1 K−1[25] • Unique optical, plas- in industrial • Patch light (IPL) at 200 °C Inc.[31] UT Dots Inc.[98]
• Melting point: monic, applications antennas[51] sintering[57,58] (UTDAgTE)[157] • Clariant • UTDAgIJ,
1234.93 K[26] and antibacterial • Prone • 3D antennas[52] • Infrared (IR) • 8 µΩ cm International UT Dots Inc.[98]
properties[21,54] to tarnish • RFID tags[53] sintering[159] after 60 min of Ltd[32,33] • UTDAgX,
• Tunable optical, when • Ultraviolet (UV) thermal • Creative UT Dots Inc.[98]
electrical, and chemical exposed to sintering[59] sintering Materials[34,35] • Ag-IJ10
properties[55] hydrogen • Microwave at 140 °C • Dupont[29,36] Nanosilver
• Exhibits surface plasmon sulfide sintering[160] (UTDAgTE)[157] • Harima Ink (30–50 wt%
resonance (SPR) effects • Plasma • 10–50 µΩ cm Chemical Co.[37] Ag loading),
• Can be used in ambient sintering[161] after 30 min • Nova-Cen- Applied
environment • Electrical of thermal trix[29,38–40] Nanotech, Inc.[31]
• Widely available in the sintering[162] sintering at 150 °C • Paru[29,41] • Metalon®
market (Ag-IJ10) [31] • PV Nanocell[29,42] JS-A221AE (50
• 9.1 µΩ cm • Sigma-Aldrich wt% Ag loading),
after 60 min Corporation[29,43] Nova-Centrix[163]
of thermal • Sun Chemical
sintering Corporation[44,45]

1800831 (10 of 20)

at 200 °C • UT Dots Inc[46]
(Metalon • Xerox[29,47]
JS-A221AE) [163]
Copper • Electrical •  xcellent
E • E
 asily • Conductive • T hermal • T
 hermal • 5 –7 µΩ cm • A pplied • C u-IJ70
nanoparticle Conductivity: 58.8 electrical and thermal oxidizes in air, tracks sintering[78] sintered (Cu-IJ70)[164] Nanotech, Nanocopper
inks × 106 S m−1[25] conductivity[64] which and patterns • Intense pulse at above 350 °C • 20–50 µΩ cm Inc.[78] Ink (30–50 wt%
• Thermal • Reduced in turn • Electrodes[75] light (IPL) in a reducing (Cu-OC70)[78] • Nanoshel[79] Cu loading),
conductivity: electromigration significantly • RFID tags[80] sintering[72–75,78,164] atmosphere • Achieve • Niittynen Applied
397 W m−1 K−1[25] effects reduces the • Laser (H2 = 4% in N2) an electrical et al.[72] Nanotech,
• Melting point: • Cheaper than noble electrical sintering[72,76] for 20 min[78] conductivity Inc.[164]
1357.77 K[26] metals conductivity of more than 20% • Cu-OC70
• Exhibits of the printed of bulk copper[72] Nanocopper
surface patterns.[28,64–67] Ink (50 wt%
plasmon • High melting Cu loading),
resonance point Applied
(SPR) effects Nanotech,
Inc.[78]
• NS6130-04-476
Copper
Nanoparticle

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advelectronicmat.de

Ink, Nanoshel[79]
 Table 1. Continued.

Types of metallic Bulk properties Advantages Disadvantages Applications Applicable sintering Reported Reported electrical Inks Commercially
nanoparticle ink techniques thermal sintering properties manufacturers available inks
temperatures references
Gold • Electrical • E xcellent •  ery
V • C onductive • Thermal • T ypically • 2 -7 x of bulk • UT Dots • U TDAuTE – Gold
nanoparticle conductivity: electrical expensive tracks sintering [88,165] require gold resistivity Inc.[98] nanoink
inks 41.0 × 106 S conductivity, thermal • High melting and patterns sintering depending for aerosol-jet
m−1[25] conductivity, and oxida- point • OTFTs[93] temperatures on temperature printing using
• Thermal tion stability[66,88] • Electrochemical of more than and time ultrasonic

Adv. Electron. Mater. 2019, 1800831

www.advancedsciencenews.com

conductivity: • Tarnish sensors and 190 °C[88] (UTDAuTE)[165] atomizer

314 W m−1 K−1[25] and corrosion resistant electrodes[92] • Sintering (20–60 wt%
• Melting point: • Does not discolor • MEAs[88] temperatures Au loading),
1337.33 K[26] • Tunable optical • IDEs[88] of 200–400 °C UT Dots
and electrical • Electrodes arrays (UTDAuTE)[165] Inc.[98,165]
properties[54,89,90] • Flexible • UTDAuIJ,
• Exhibits surface transparent UT Dots Inc.[98]
plasmon resonance (SPR) electrodes[97] • UTDAuX,
effects[91] • Flexible wearable UT Dots Inc.[98]
sensors[96]
Aluminum • Electrical • L ow cost •  igh sintering
H • C
 onductive • Thermal • L ow-temperature • <10 mΩ sq−1*[100] • A
 pplied • A
 l-IS1000
nanoparticle conductivity: 35.5 • Very good temperatures patterns sintering[100] sintering: • 8 0 mΩ sq−1 Nanotech, aluminum ink
inks × 106 S m−1[25] electrical and thermal required on silicon 550 °C[100] at both 550 Inc.[100] (Applied
• Thermal conductivity for printed substrates, • High- and 600 °C Nanotech, Inc.)[100]
conductivity: • Corrosion resistant conductive particularly temperature for 4 min[103]
238 W m−1 K−1[25] • Low contact resistivity on patterns for photovoltaic sintering: • 20 mΩ sq−1
• Melting point: silicon substrates[100] applications 700–900 °C at 800 °C
933.47 K[26] • Passivation layer diffusion for <1 min for 4 min[103]

1800831 (11 of 20)

properties[100] in air[100]
• Can form highly uniform
back
surface field (BSF)
layer[100]
Cobalt • Electrical • H ave • Much lower elec- •  adiofrequency • T
R  hermal • T
 hermal • R
 elative • N
 elo • Not commercially
nanoparticle conductivity: 17.0 ferromagnetic properties trical conductivity absorbers sintering[104,105] sintering at 350 °C permeability et al.[104,105] available
inks × 106 S m−1[26] • Good electrical as compared • Antennas for 10 min[104,105] and magnetic loss
• Thermal and thermal conductivity to silver • Magnetic tangent values
conductivity: • Corrosion resistant[26] and gold sensors ranging between 1.5
100 W m−1 K−1[26] • High permeability • Filters and 3 and 0.01 and
• Melting point: • Required low • Resonators 0.06, respectively, in
1768 K[26] sintering temperatures • Phase the 45 MHz–10 GHz
shifters[104–106] band[104]
• Antenna minia-
turization[104,105]
• Antenna
substrates[104,105]
• Magnetic
sensors or

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

sensing[104,105]
www.advelectronicmat.de
 Table 1. Continued.

Types of metallic Bulk properties Advantages Disadvantages Applications Applicable sintering Reported Reported electrical Inks Commercially
nanoparticle ink techniques thermal sintering properties manufacturers available inks
temperatures references
Nickel • Electrical • H ave • Much lower • P assivation • Thermal • T
 hermal • 20–50 • A pplied • Ni-IJ70-30
nanoparticle conductivity: 14.3 ferromagnetic electrical layer for under- sinteirng[109,110] sintering at tem- µΩ cm[109,110] Nanotech, (30% Ni
inks × 106 S m−1[26] properties conductivity as lying silver and • Intense pulse peratures more • 76.34 µΩ cm[108] Inc.[109,110] loading),
• Thermal • Good electrical compared to copper light (IPL) than 350 °C • 0.347 Ω sq−1[108] • Nano Applied
conductivity: and thermal conductivity silver conductive sintering[108–110] in a reducing Dimensions[111] Nanotech Inc.[109]

Adv. Electron. Mater. 2019, 1800831

www.advancedsciencenews.com

91.0 W m−1 • Corrosion and gold patterns environment • Pasquarelli • Ni-OC70

K−1[26] resistant • Tend to clog • Electrode (H2 = 4% in N2) et al.[112] (60% Ni
• Melting point: • Cheap inkjet contacts for for 20 min[109,110] • Tseng et al.[107] loading), Applied
1728 K−1[26] • Passivation printheads solar cell • Park and Nanotech
properties due to agglom- applications Kim[108] Inc.)[110]
eration[111] • Conductive
tracks and
patterns
• Temperature
sensors
• Inductors
• Microheaters
Palladium • Electrical • G ood electrical and • Very expensive • E lectrode • Thermal • T hermal •Not • G
 went Group, • C2031105P2 –
nanoparticle conductivity: 9.5 thermal conductivity • M uch lower materials for sintering[113,115] sintering tem- available SunChem- Screen Printable
inks × 106 S m−1[26] • Unique reactions with electrical fuel cells perature ranging ical[115] Palladium Ink
• Thermal hydrogen gas[114] conductivity as • pH sensors between 180 and • American Paste (67–68% Pd
conductivity: • Have compared to • Conductive 250 °C[113] Elements[116] loading at 750 °C),

1800831 (12 of 20)

72 W m−1 K−1[26] electrocatalytic silver and gold tracks • Above 850 °C • Liu et al.[113] Gwent Group,
• Melting point: properties • High melting •Thin film for at least • Qin et al.[114] SunChemical[115]
1828 K[26] • Unique material’s point transistors 10 min • PD-M-01-INK –
properties • Gas sensors (C2031105P2 – Screen Printable
for electrochemical • Biosensors Screen Printable Palladium
sensors applications[113] • Electrochemical Palladium Ink Ink, American
sensors Paste)[115] Elements[116]
• Monitoring
systems[113,114]
Electrical • Corrosion resistant • Very expensive • D ye-sensitized • Thermal • T hermal sin- • S heet resistivity: • DuPont[121] • D uPont
Platinum conductivity: • Does not tarnish • V ery high solar cell sintering[121,166] tering at 130 °C 5–10 Ω sq−1 • LCC BQ321 – Screen
nanoparticle 9.1 × 106 S m−1[25] • Good electrical melting (DSSC) counter for 5–10 min mil−1 (DuPont AkKoLab[122] Printable
inks Thermal and thermal conductivity temperature electrodes (DuPont BQ321)[121] • UT Dots Inc[98] Platinum Ink,
conductivity: • Unique material’s • Much lower • High activity BQ321)[121] • ≈40 µΩ cm at • Kim et al.[123] DuPont[121]
71.6 W m−1 K−1[26] properties electrical electrodes in • Thermal sin- 300 °C for 30 min • UTDPtTE
Melting point: for sensors, solar cells conductivity as polymer thick tering at 300 °C (UTDPtTE)[166] (10-15%
2041.4 K[26] and biosensors compared to film (PTF) for 30 min Pt loading),
applications silver sensors (UTDPtTE)[166] UT Dots Inc.[166]
• Inert and gold and biosensors • UTDPtX, UT
• Good oxidation stability • Ceramic MEMS Dots Inc[98]
• Inkjet and aerosol jet gas sensors

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advelectronicmat.de

printable[98] • Electrodes
 Table 1. Continued.

Types of metallic Bulk properties Advantages Disadvantages Applications Applicable sintering Reported Reported electrical Inks Commercially
nanoparticle ink techniques thermal sintering properties manufacturers available inks
temperatures references
• N
 ot
Tin • Electrical • G ood electrical and • M
 uch lower • C
 onductive • T
 hermal • T
 hermal sin- • 64.27 µΩ cm After • Jo et al.[124]
commercially
nanoparticle conductivity: 8.7 thermal conductivity electrical tracks and sintering[124] tering at 250 °C 60 min of thermal
available
inks × 106 S m−1[26] • Corrosion conductivity as patterns[124] for 60 min[124] sintering at 250 °C
• Thermal resistant compared to (approximately six

Adv. Electron. Mater. 2019, 1800831

www.advancedsciencenews.com

conductivity: • Low melting silver and gold times higher than

67 W m−1 K−1[26] temperature bulk tin)[124]
• Melting point: • Cheaper cost[26]
505.08 K[26]

• C uNi-OC5050
Alloy metallic Copper– • Not available • High tensile strength • M
 uch lower • C
 onductive • T hermal • Thermal • 200–300 mΩ sq−1 • A
 pplied
(Available from
nanoparticle nickel alloy • Unique material’s electrical tracks and sintering[126] sintered at above for 2 µm film[126] Nanotech,
10 to 50 wt%
inks nanoparticle properties for strain conductivity as patterns[126] • Intense pulse light 350 °C in a Inc.[126]
loading), Applied
inks gauges compared • Strain (IPL) sintering[126] reducing
Nanotech,
and thermocouples to silver Gauges[126] atmosphere
Inc.[126]
applications[126] and gold • Thermo- (H2 = 4% in N2)
• CuNi-IJ5545,
• Inkjet and aerosol jet couples[126] for 20 min[126]
Applied
printable
Nanotech,
Inc.[127]
• CuNi-IJ5050,
Applied
Nanotech,
Inc.[127]

1800831 (13 of 20)

• M etalon ICI–
Metal oxides Copper • Electrical • Significantly cheaper • Required • C
 onductive • I ntense pulse • T
 hermal • 7.5–10.8 µΩ cm • NovaCen-
002HV (16%
nanoparticle oxide-based resistivity: than silver and copper photonic tracks and light (IPL) sintering not (Metalon trix[131,132]
CuO loading),
inks nanoparticle 5.1 × 107 µΩ cm nanoparticle inks sintering tech- patterns sintering[131,132] applicable[131,132] ICI–002HV)[131] • Nanoshel[130]
Nova-Centrix[131]
inks (CuO)[70] • Good electrical niques • 9 µΩ cm (Metalon • Paquet et al.[133]
• Metalon
• Melting point: conductivity to sinter ICI–002HV)[133]
ICI–003 (10%
1603.15 K • Able to sinter in ambient • 13–15 µΩ cm
CuO loading),
(CuO)[69] conditions (Metalon
Nova-Centrix[132]
ICI–003)[132]
• NS6130-10-1304,
• 65–95 mΩ sq−1
Nanoshel[130]
(NS6130-10-
1304)[130]

• N
 ot commer-
Iron oxide • Not available • G
 ood magnetic • L ow electrical • I nductors • Thermal drying[135] • Drying at 80 °C • S
 aturation • V
 aseem
cially available
nanoparticle properties[134] conductivity[134] • Radio frequency for 30 min[135] magnetization et al.[135]
inks devices of ≈12.4 under an
• Patch applied field
antennas[135] of 1 kOe[135]

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advelectronicmat.de
 Table 1. Continued.

Types of metallic Bulk properties Advantages Disadvantages Applications Applicable sintering Reported Reported electrical Inks Commercially
nanoparticle ink techniques thermal sintering properties manufacturers available inks
temperatures references

Indium • Not available • H igh optical • H igh sintering • Transparent • T

 hermal • T
 hermal • S heet resistance of • Hong et al.[137] • N
 ot commer-
tin oxide transmittance temperatures electrodes sintering[137,138] sintering at 2.19 × 10−3 Ω sq−1 • Y  u et al.[138] cially available
nanoparticle • Good electrical required for • Microwave 600 °C and optical trans- • Hwang
inks conduction optimized sintering[140] for 60 min[137] mittance of 78.6%, et al.[140]
• Chemical inertness transparency after 60 min of

Adv. Electron. Mater. 2019, 1800831

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• Good substrate and electrical thermal sintering

adhesion[136–139] conductivity at 600 °C[137]
• Expensive[136–139] • 50–300 Ω sq−1[138]
• Sheet resistance
of 517 Ω sq−1 and
optical transmit-
tance of 87%
after microwave
sintering them at
400 °C[140]

Zinc oxide • Not available • Low costs • L ow electrical • T

 ransparent • Thermal • T
 hermally dried • 10−8–10−7 S cm−1 • Genes’Ink[144] • Helioś Ink
nanoparticle • n-type semiconducting conductivity electrodes[141,142] drying[141,144] from 25 to (H-SZ01034, • Suganthi • H-SZ01034,
inks material • Optoelec- 600 °C for 15–60 Genes’Ink)[144] et al.[141] Genes’Ink
[141]
• Wide bandgap of tronics, bioim- min semiconductive
3.36 eV aging, sensing, ink[144]
• High optical electronics,
transmittance photovoltaics,

1800831 (14 of 20)

and sensing
applications[142]

Core–shell Cu–Ag • Not available • B etter electrical proper- • Not widely • Conductive • T
 hermal • Thermal sin- • A chieve 16% bulk • Pajor-S′wierzy • N
 ot commer-
bimetallic Core–shell ties and oxidation available in tracks and sintering[146,152,153] tering at 250 °C conductivity of et al.[152] cially available
nanoparticle bimetallic stability as compared commercial patterns for 15 min[152] copper after 15 • Grouchko
inks nanoparticle to copper nanopar- markets min of thermal sin- et al.[153]
inks ticles under ambient • Synthesis of tering at 250 °C[152] • Lee et al.[146]
conditions Cu–Ag core– • Achieving
• Potentially lower mate- shell BNP inks 12 µΩ cm at
rial costs as compared may be com- 350 °C[146]
to silver nanoparticle plicated and
inks due to reduced time-consuming
silver loading[146]
• Lower initial sintering
temperature as compare
to copper nanoparticle
inks[155]

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www.advancedsciencenews.com
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explored as an option for formulating low-cost conductive inks

Commercially
that only require low sintering temperatures.

available inks

cially available
 ot commer-
8. Challenges
• N In spite of increasing demand for metallic nanoparticle inks,
manufacturers

 chieved a reason- • Kim et al.[156]

references

several challenges are required to be addressed to allow broader

Inks

adoptions of metallic nanoparticle inks for 3D printed elec-

tronics applications.
resistance of 1.3 Ω
Reported electrical

8.1. High Costs of Silver Nanoparticle Inks

ably low sheet
properties

sq−1[156]

High costs of the commonly-used silver nanoparticle inks pre-

sent a challenge for achieving cost effectiveness in industrial
• A

applications, but on-demand printing techniques such as inkjet

and aerosol jet printing can help reduce material wastage to
thermal sintering
temperatures

applicable[156]

offset the high costs of silver nanoparticle inks. There is also

sintering not
Reported

much on-going research looking into finding cheaper alterna-

• I ntense pulse light • Thermal

tives to silver nanoparticle inks such as copper nanoparticles

and copper oxide inks.
Applicable sintering

(IPL) sintering[156]
techniques

8.2. High Sintering Temperatures

Metallic nanoparticle inks typically require sintering tempera-

tures above 100 °C to become electrically conductive in the
thermal sintering process, and hence limiting the use of cheap
electrodes[156]
Applications

temperature-sensitive substrates such as paper and common

• Conductive
tracks and

poly­mer foils for fabricating low-cost electronics. However, exten-

patterns
• Flexible

sive research is being conducted in exploring alternative sintering

techniques (for example, IPL and laser sintering) for low-temper-
ature selective sintering to allow the use of temperature-sensitive
time-consuming
Disadvantages

shell BNP inks

substrates without severe damages or degradations.

may be com-
Cu–Ni core–

plicated and
nanoparticle inks or Cu– • Synthesis of
commercial
available in
• Not widely

markets

8.3. Stretchability Issues

Ag core–shell bimetallic

Metallic nanoparticle inks are not suitable for fabricating stretch-

as compared to silver
• Lower material costs

able electronics due to their low stretchability characteristics,

oxidation-resistant

nanoparticle inks
Advantages

but they can be used for fabricating flexible electronics. On the

one hand, soft substrates are highly stretchable and deformable.
On the other hand, the sintered metallic nanoparticle inks are
 ighly

brittle and have low fracture strain.[167] Therefore, the primary

• H

technical challenge lies in the mismatch of the mechanical prop-

erties of the stretchable soft substrates and the sintered metallic
Bulk properties

• Not available

nanoparticle inks where they adhere to each other. Hence, dif-

ferent types of inks such as conductive polymers are investi-
Dependent on sintering temperatures.

gated to use for stretchable electronics applications.

nanoparticle
core–shell

8.4. Geometric Repeatability

bimetallic
Types of metallic
nanoparticle ink
Table 1. Continued.

Cu–Ni

inks

3D printing of electronics with metallic nanoparticle inks inevi-

tably face technical challenges in achieving void-free geometri-
cally-uniform structures with smooth surface morphologies and
well-defined edges. The coffee-ring effect is especially pertinent
in inkjet-deposited metallic nanoparticle inks droplets, in which
*

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www.advelectronicmat.de

Figure 5. Ideal essential properties of inks, printers, and sintering process for progress in 3D printed electronics.

nanoparticles accumulate along the edges of the droplets.[168] for printing with a specific printer on a particular substrate.
Hence, 3D printed electrical components with consistent elec- Therefore, there is a need for greater cooperation efforts among
trical properties and low tolerances may not be repeatable in inks and machines manufacturers to share their expertise.
mass productions, as morphology and geometry of the compo- Nano Dimension[169] is one good example. They formulate
nents play significant roles on their mechanical and electrical and customize inks specifically for their inkjet printers (Drag-
properties.[4] Therefore, more research efforts are required to onFly 2020 Pro 3D Printer) for better optimization and print
solve these technical challenges to improve product reliability quality.[170] To further expedite the progression of 3D printed
so that the electronic industry can accept printed electronics. electronics in revolutionizing industrial applications, research
efforts should also focus on optimizing the synergy between
printing and sintering processes. These processes are interde-
9. Outlooks and Potentials pendent with each other, and it is not possible to advance the
3D printed electronics technologies forward without concur-
The research trend is currently going toward fabricating printed rent research efforts (see Figure 5.). An ideal ink for electronics
stretchable electronics and sensors on soft substrates for wear- printing should possess these qualities: cost-effective, low sin-
able electronics and healthcare monitoring applications. How- tering temperatures, favorable electrical, and material proper-
ever, metallic nanoparticle inks are not highly stretchable for ties, optimized for printers, and environmentally friendly. In
these applications, and many researchers have been looking addition, an ideal printer should also have high printing reso-
into other novel materials for fabricating stretchable electronics lution, fast printing speed, on-demand non-contact printing to
(for example, graphene, graphene oxides, and conductive poly- minimize contaminations, and ease of modifications and scal-
mers). Nevertheless, metallic nanoparticle inks still play an ability of the printed patterns. At the same time, the sintering
essential role in fabricating highly conductive patterns for cir- process should also possess high sintering speed, be compat-
cuitries on rigid and flexible substrates. ible with the inks, gives excellent sintering properties and not
Apart from metallic nanoparticle inks, electrically conductive damage the sintered patterns and substrates.
hybrid composite inks are also gaining significant attentions
for various applications due to their ability to build conductive
3D structures.[1] Jo et al.[1] formulated a type of 3D printable 10. Conclusion
electrically conductive hybrid composite ink, which comprised
of carboxyl-terminated silver nanoparticles, silver flakes, 3D printing of electronics shows great potentials in on-
amine-functionalized carbon nanotubes, and thermoplastic demand fabrications of customizable electronics, reducing
poly­styrene–polyisoprene–polystyrene (SIS) triblock copoly- the time taken for prototyping and exploring of highly innova-
mer. Their material can achieve electrical conductivity of
­ tive applications. This disruptive technology also aims to reduce
22939 S cm−1 and only require a sintering temperature of wastage, time bottlenecks, costs, and increase efficiencies as
80 °C. This type of hybrid composite inks can allow researchers compared to the conventional ways of fabricating electronics.
to explore the potentials of printing electrically conductive The emergence of various metallic nanoparticle inks in the
3D structures, especially in antenna fabrications, to allow full market for 3D printed electronics has accentuated the growing
optimizations of the available spaces. 3D printed electronics sector in the additive manufacturing
Although there are many types of metallic nanoparticle inks industry. Increasing adoptions of 3D printing by the elec-
in the market, they are not fully optimized and customized tronics industry for fabricating electronics also increase the

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