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Materials Today � Volume 18, Number 4 �May 2015 RESEARCH

Nanoparticle plasmonics: going practicalwith transition metal nitridesUrcan Guler, Vladimir M. Shalaev* and Alexandra Boltasseva*

School of Electrical & Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA

Promising designs and experimental realizations of devices with unusual properties in the field of

plasmonics have attracted a great deal of attention over the past few decades. However, the high

expectations for realized technology products have not been met so far. The main complication is the

absence of robust, high performance, low cost plasmonic materials that can be easily integrated into

already established technologies such as microelectronics. This review provides a brief discussion on

alternative plasmonic materials for localized surface plasmon applications and focuses on transition

metal nitrides, in particular, titanium nitride, which has recently been shown to be a high performance

refractory plasmonic material that could replace and even outperform gold in various plasmonic

devices. As a material compatible with biological environments and the semiconductor industry,

titanium nitride possesses superior properties compared to noble metals such as high temperature

durability, chemical stability, corrosion resistance, low cost and mechanical hardness.

IntroductionPlasmonics has attracted great attention over the last few decades

due to the alluring physical mechanisms arising from the interac-

tion of light with resonant structures at the nanometer scale. The

unique characteristics of plasmonic materials stem from resonant

oscillations of free electrons as a response to electromagnetic

waves, known as surface plasmons (SPs). SPs have been in the

limelight for researchers working in many interdisciplinary areas

involving photonics, electronics, mechanics, chemistry, biology,

nanofabrication, microscopy, condensed matter, quantum phys-

ics, and many others [1–11]. These coupled oscillations can also

propagate along an interface (called surface plasmon polaritons

(SPPs) in this case), giving us the prospect of nanoscale chip level

interconnects, or they can exist as localized surface plasmon

resonances (LSPR), enabling ultra-small optical nanoantennas,

optical detectors, advanced sensors, data storage, and energy

harvesting devices [12,13]. Intense work has been carried out in

the field of plasmonics with a high level of participation from

researchers with diverse backgrounds which has resulted in a

*Corresponding authors. Shalaev, V.M. (shalaev@purdue.edu),

Boltasseva, A. (aeb@purdue.edu)

1369-7021/Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://c

comprehensive understanding of various physical phenomena

at the nanoscale, and many application demonstrations which

were considered as breakthroughs in their fields [14]. However,

many achievements in the laboratory environment could not be

transferred into real applications, mostly due to the limited num-

ber of materials employed in the research [15]. Noble metals, the

traditional plasmonic component, caused various problems that

are to be discussed briefly, later in this review.

The strong plasmonic response from noble metals in the visible

region of the electromagnetic spectrum has been the main reason

for the frequent use of these materials in past studies. In his

pioneering work, Michael Faraday reported the color changes he

observed from metal nanoparticles, mainly gold (Au) and silver

(Ag), dispersed in a dielectric body [16]. Another determinative

reason for the wide use of metal nanoparticles is certainly the well-

developed colloidal assembly methods, which have been studied

extensively over the years [17,18]. Due to these two fundamental

advantages, natural selection in the field of plasmonics resulted in

the frequent use of Au and Ag in research activities. However,

problems from a wide range of disciplines awaiting a solution from

the field of plasmonics cannot be solved with a limited pool of

materials. In order to provide solutions to problems with different

reativecommons.org/licenses/by-nc-nd/3.0/). http://dx.doi.org/10.1016/j.mattod.2014.10.039

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RESEARCH:Review

requirements such as operation at specific windows of the electro-

magnetic spectrum and different ambient conditions, alternative

materials that provide the ‘golden mean’ are needed [19,20].

Plasmonic resonances of nanostructured noble metals naturally

occur at the shorter wavelength region of the visible window.

Although resonances at visible wavelengths have been a great

advantage for earlier studies [16], adapting these materials to appli-

cations that require operation at longer wavelengths has been a

problem. A case in point is the integration of Au nanostructures to

biomedical applications that require resonances in the near infrared

window of the electromagnetic spectrum due to the lower attenua-

tion of light through biological tissues at these wavelengths [21].

Hirsch et al. provided a remarkable solution to this problem by

modifying the geometry of metal nanoparticles to core-shells, com-

monly referred to as nanoshells, thereby red-shifting the resonance

peak of Au nanostructures to the near infrared window [22,23].

Engineered shapes of metal nanoparticles for efficient operation in

the biological transparency window have been extended to several

different geometries such as nanorods, triangles, dimers, and even

more complicated structures like multi-shells (Fig. 1(a–d)), and

nanostars (Fig. 1(e)) [24,25]. However, plasmonic nanoparticles

FIGURE 1

Matching the spectral position of plasmonic resonances to windows for

specific applications is a complicated problem. Alloyed metals and modified

structures have been used in order to engineer resonance frequencies. (a)Schematic representation of multi-walled nanoshell fabrication [47]. (b–d)

TEM images of Au/Ag alloy nanoshell structures with single, double, and

triple walls; respectively. Reprinted with permission from [47]. Copyright

(2004) American Chemical Society. (e) SEM image of star-shaped Aunanoparticles exhibiting plasmonic resonance in the near-infrared window

where attenuation of light through biological tissue is lower. Reprinted with

permission from [25]. Copyright (2006) American Chemical Society. (f ) TEM

image of reduced graphene oxide coated Au nanorod with enhancedphotothermal effect. Reprinted with permission from [40]. Copyright (2013)

American Chemical Society.

228

with smaller sizes and simpler shapes, like spheres, while exhibiting

resonances in the near infrared window, would be more desirable for

many practical reasons. Similarly, many applications requiring

operation in the mid-infrared and far-infrared windows suffer from

a spectral mismatch between the plasmonic resonances of metal

nanostructures and the operating wavelengths of devices. Apart

from the spectral mismatch of resonance positions, metals often

suffer from high losses due to interband transitions that are spec-

trally located very close to the resonance regions. In addition to

interband losses, carrier concentration and mobility have to be

taken into account when a material is considered for a specific

application. Although high carrier concentrations are required

for a plasmonic response, very large values are not always desirable

[26]. At frequencies lower than the interband transitions, Bouillard

et al. experimentally showed that losses arising from free electron

scattering can also be a factor affecting the performance of a

plasmonic structure depending on the nature of its interaction with

light [27]. West et al. compared alternative materials from an optical

point of view where dielectric permittivities were used for calcula-

tion of quality factors depending on specific applications [19].

For most of the applications, physical and chemical properties are

equally important as the optical properties of the plasmonic mate-

rial in use. A textbook example for a plasmonic material that has

outstanding optical performance, but severely limiting chemical

characteristics and challenging fabrication is Ag [28,29]. There is still

a great ongoing effort in the plasmonics community for obtaining

stable Ag nanoparticles [30]. Similarly, other metals such as copper,

aluminum and alkali metals exhibit strong plasmonic responses,

but suffer from chemical instabilities [31–34]. Gold is a plasmonic

material with proven chemical stability and consequently has been

a favorable choice for most applications where the plasmon reso-

nances of the material match with the spectral regions of interest.

However, the thermal stability of nanostructured Au is considered

poor for heating applications at elevated temperatures. Although it

has been shown that thermal stability of these nanostructures can be

increased by use of relatively thick encapsulating silica shells [35],

such geometric modifications would highly affect the optical prop-

erties, which is not desired for many applications. Tittl et al. showed

that it is possible to locally probe chemical reactions in real time

with ultra-thin shell-isolated Au nanoparticles [36]. Tunability with

metal nanostructures was achieved by coupling to phase changing

materials [37]. Graphene, as a material with very interesting physical

properties, attracted attention in the plasmonics community and

has been used in order to solve known problems with noble metals.

Emani et al. demonstrated electrical tunability of the plasmon

resonances in the infrared region with Au antennae fabricated over

a thin graphene sheet [38]. Liu et al. used a single layer graphene in

order to increase the chemical stability of Ag nanoparticles [39]. In a

more recent study, Lim et al. demonstrated that Au nanoshells and

nanorods coated with an additional thin layer of graphene oxide

provide an enhanced photothermal effect (Fig. 1(f)) [40]. Figure 1

highlights the methods proposed in order to solve known problems

with noble metals.

Alternative plasmonic materials for LSPR applicationsDue to the wide range of parameters for separate applications, it

is impossible to address all the requirements by use of noble

metals; therefore, alternative materials are needed to optimize

Materials Today � Volume 18, Number 4 �May 2015 RESEARCH

FIGURE 2

Comparison of selected transition metal nitrides with plasmonic and

refractory metals. TiN and ZrN have optical properties similar to Au and

melting points similar to refractory metals. The low melting point and

softness of plasmonic metals cause problems under harsh operationalconditions. On the other side, refractory metals lack plasmonic properties in

the visible range and exhibit poor resonances in the near infrared.

Transition metal nitrides provide the two desired properties: high quality

plasmonic resonances in the visible region and refractory properties.Reprinted with permission from [55]. Copyright 2014, AAAS.

RESEARCH:Review

the performance [20]. The first approach to the problem would be

to combine the materials that are already available, and obtain

engineered optical properties. Indeed, in a very early example, the

Lycurgus Cup, a Ag–Au alloy of nanoparticles was used in order to

obtain colored glass [41]. Recently, there has been a tremendous

effort on band engineering of metals in the plasmonics commu-

nity. Blaber et al. showed how plasma frequency varies with the

composition of intermetallic compounds and proposed KAu as an

infrared plasmonic material [42]. Bobb et al. used Au–Cd alloy

nanoparticles to obtain enhanced performance at a desired wave-

length region in expense of degraded performance at other wave-

lengths [43]. As a part of the search for better performance, Blaber

et al. further investigated the properties of liquid metals and alloys

[44]. Strohfeldt et al. demonstrated a long-term stable palladium–

nickel plasmonic hydrogen sensing system [45]. Wang et al. dem-

onstrated a reconfigurable terahertz device based on liquid metals

[46]. In a previous work by Sun et al., two different approaches

were combined and multishell nanoparticles of metal alloys were

investigated [47]. Wu et al. demonstrated broad tunability with

GaMg alloy nanoparticles [48]. However, adjusting the optical

properties of metals via alloying and band-structure engineering

has limitations, resulting in marginal improvements thus far

[42,43].

Regarding tunability, doped semiconductor nanocrystals pro-

vide exciting results in the infrared window. Luther et al. demon-

strated the tunability of the localized surface plasmon resonance

in the near infrared window by using Cu2�xS nanoparticles with

varying Cu vacancies due to oxygen exposure [49]. Kanehara et al.

reported tunability of the plasmonic resonance peak of indium tin

oxide nanoparticles by changing the tin concentration [50]. Two

years later, Garcia et al. showed that the plasmonic resonance of

tin-doped indium oxide nanocrystals can be reversibly tuned [51].

Meanwhile, Buonsanti et al. realized tunability with aluminum-

doped zinc oxide nanocrystals at longer wavelengths [52]. Ko et al.

studied the effects of doping to the carrier concentration and

mobility for solid nanocrystals [53]. Recently, Kim et al. investi-

gated the lithographic fabrication of transparent conductive oxi-

des and demonstrated tunability by varying the doping and

annealing parameters [54].

Another set of materials exhibiting plasmonic resonances are

the transition metal nitrides. They are known for their refractory

properties, meaning chemically stable at temperatures above

2000 8C, and metallic behavior. Dielectric permittivities of titani-

um nitride (TiN) and zirconium nitride (ZrN) have a zero cross-

over wavelength in the visible range, very similar to Au, making

them plasmonic in the visible and near infrared range. Addition-

ally, their bulk melting points are close to refractory metals such as

tungsten, molybdenum and tantalum, making them good candi-

dates for high temperature applications (Fig. 2) [55]. In fact, their

metal-like appearance combined with many other favorable phys-

ical properties attracted attention decades ago [56,57]. Among

these materials, TiN has been of higher interest due to potential

applications in the microelectronics industry [58]. Optical prop-

erties of TiN thin films were studied via SPP experiments by

Hibbins et al. with grating coupling and Chen et al. with the

Kretschmann method [59,60]. Recently, it has been shown that

epitaxially grown TiN exhibits better plasmonic properties and

enables exciting applications such as hyperbolic metamaterials

and plasmonic interconnects [61,62]. Optical properties of TiN

nanoparticles were studied numerically by Quinten in 2001 where

extinction peaks similar to Au, but with broader widths, were

reported [63]. Later in 2004, Reinholdt et al. studied the optical

properties of TiN nanoparticles fabricated via a laser ablation

method and demonstrated their plasmonic behavior at slightly

longer wavelengths [64]. In 2010, Cortie et al. examined the

optical properties of TiN semi-shell structures and reported reso-

nance peaks in the near infrared region for these geometries [65].

In 2012, we computationally showed that TiN can be a better choice

than Au for specific applications [66]. Recently, we have demon-

strated that lithographically fabricated TiN nanoparticles provide

better heating efficiencies in the biological transparency window

when compared to Au nanoparticles with identical geometries [67].

In this review, we focus on plasmonic TiN nanoparticles from

different aspects including fabrication, characterization, physical

properties and potential applications. In the next section, we will

characterize the physical properties of TiN and the potential of the

material for practical applications based on the previously reported

work.

TiN nanoparticles for LSPR applicationsOwing to its physical properties suitable for harsh environments,

TiN has been of interest as a refractory material in a variety of

applications for decades. It is usually mentioned as a hard material

with a high melting point, chemical durability and yet good

conductivity [68]. Among many others, TiN stands out by means

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of two important properties; bio-compatibility and CMOS com-

patibility [69,70]. In addition, it has been considered as a protec-

tive coating in high temperature environments such as supersonic

jets [71], and harsh mechanical applications such as milling [72].

Lavrenko et al. demonstrated the chemical resistance of TiN

powders in biochemical environments [73]. As a non-stoichiomet-

ric material, TiN has been studied extensively in terms of adjust-

able properties with varying deposition parameters. Perry and

Schoenes investigated color variations in TiN films where the

dependence on composition and lattice parameters were demon-

strated [74]. It has also been demonstrated several times that

varying the nitrogen content can be used in order to achieve

the desired physical properties with TiN [75,76]. Figure 3 shows

the metallic luster, and adjustable optical properties of TiN, along

with a Au thin film as reference. Featuring all of the above physical

properties, as well as optical properties comparable to popular

plasmonic materials, would decidedly put TiN forward as a favor-

able material for plasmonic applications.

A precise comparison of the attainable performance with differ-

ent materials for plasmonic applications can be demanding. Due

to the wide range of parameters affecting the performance of a

material, separate quality factors that reflect the priorities of

specific application are required. A nice example of such an

approach was demonstrated by Garcia et al. in which the authors

defined a compound figure of merit based on two dominating

mechanisms, thermal heating and two-photon absorption, for

photonic switching applications with metal nanocomposites

[77]. For a comparison of different materials in the scope of LSPR

applications, a simpler approach is to consider small particles in

the quasistatic regime where the propagation/retardation effects

are not important, and the quality factors are dependent only on

the complex permittivities [78,79]. Although this approach is

intended for nanostructured systems such as fractal nanocompo-

sites where very small features are present, it has also been used in

order to compare performances of alternative materials for LSPR

applications in the quasistatic regime [19,80]. Figure 4 (a,b) illus-

trates a plasmonic nanoparticle under illumination with a wave-

length corresponding to the dipolar resonance. Figure 4(c) shows

the quality factors calculated for Au, TiN and ZrN in the quasistatic

regime for LSPR applications; QLSPR ¼ �e0=e00, where the complex

dielectric permittivities used in these calculations are given in

Fig. 4(d,e) [66]. A brief overview of the figure is enough to conclude

that Au outperforms both metal nitrides over the visible and near

infrared regions of the electromagnetic spectrum. However, the

use of very small particles requires the consideration of additional

FIGURE 3

Images of gold, and titanium nitride thin films deposited at 400 and 800 8C.Adjustable optical properties and metallic luster are notable properties of

titanium nitride.

230

mechanisms such as reduced electron mean free path and in-

creased contribution from surface chemistry of plasmonic material

[81]. In addition, the elements used in practical LSPR applications

are not always limited to nanoparticles with dimensions very small

compared to wavelength of incident light. In 2012, we showed

that using quasistatic approximations when comparing alterna-

tive plasmonic materials may lead to misleading results especially

for particle dimensions larger than a few tens of nanometers, and

the Mie scattering theory is indeed a very useful tool for the

performance comparison of alternative materials for specific appli-

cations [66]. Although the theory was originally developed for

spherical particles, it has now been modified for a variety of cases

including spheroids, multi-shell geometries, particles standing

over planar substrate, dimers, arrays of particles, aggregates and

so on [82–87]. Scattering, absorption or extinction efficiencies

calculated from Mie coefficients can easily be considered as a

figure of merit when comparing different materials for specific

applications. Scattering efficiency is a useful tool for far field

enhancement applications while absorption efficiency is directly

applicable to applications aiming at local heating of small volumes

in the vicinity of a particle. In 1981, Messinger et al. calculated the

local field at the surface of spherical particles which is a more

realistic approach to near field enhancement applications [88].

Using near field intensity efficiencies, the comparison of Au and

TiN spherical particles gives a more complicated picture where

both materials can provide better performances in different spec-

tral regions (Fig. 4(f,g)) [66]. First, the peak position for TiN is

found to be red-shifted compared to Au and located in the impor-

tant region widely known as a biological transparency window.

When the peak positions for both materials are considered, Au

provides a larger maximum enhancement, which makes it the

material of choice for shorter wavelengths of the visible window.

However, when we consider longer wavelengths, the tail of the TiN

resonance peak elongates in to yet another important region, the

telecommunication window. Overall, one can conclude that al-

though it is not as good of a resonator as Au, TiN provides

comparable or even better field enhancement in the near infrared

region where the two very important spectral windows are present.

Another important class of LSPR applications is based on local

heating of a confined volume via absorption of electromagnetic

energy by plasmonic nanoparticles. Thus, the absorption efficien-

cy of nanoparticles can be considered as a quality factor for these

applications. Figure 4(h,i) shows that the peak values of the Au and

TiN absorption efficiencies are indeed very similar [67]. A spectral

match of the peak position for small, spherical TiN nanoparticles

to the biological transparency window can be a significant advan-

tage of TiN for therapeutic applications. Owing to its high melting

point, energy harvesting applications that typically require elevat-

ed operation temperatures can be realized with TiN. Bearing in

mind many other superior physical properties as well as CMOS and

bio-compatibility of the material, we believe TiN is a valuable

element of future plasmonic applications.

Fabrication and characterization of plasmonic TiNnanoparticlesThe wide use of TiN in industry for various applications ranging

from electronics to machine tooling has resulted in a rich docu-

mentation of fabrication methods and parameters. So far, the

Materials Today � Volume 18, Number 4 �May 2015 RESEARCH

FIGURE 4

(a) Illustration of a spherical nanoparticle polarized under illumination at dipolar resonance wavelength. (b) Illustration of near field distribution at resonance

wavelength. Arrows show scattered electric field. (c) Quality factors in the quasistatic regime showing the oscillator quality for Au, TiN and ZrN nanoparticles

[66]. (d) Real and (e) imaginary part of dielectric permittivities used in quality factor calculations [66]. Performance metrics for localized surface plasmons hasto be specific to application area. (f,g) Near field intensity efficiencies, showing the field enhancement at the surface of plasmonic particle, calculated for

spherical Au and TiN nanoparticles based on Mie coefficients. Reprinted with permission from [66]. Copyright 2012, Springer. (h,i) Absorption efficiency,

which is an efficient indicator for electromagnetic-to-heat energy conversion, for spherical Au and TiN nanoparticles with varying radii. Reprinted with

permission from [67]. Copyright (2013) American Chemical Society.

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RESEARCH Materials Today � Volume 18, Number 4 �May 2015

FIGURE 5

(a) TiN nanoparticles produced with laser ablation method from a pressed

powder target. Cubic shapes of particles reveal ionic and covalent bonding

[64]. (b) Particles with sizes below 10 nm exhibited LSPR peaks around1.7 eV. Reprinted with permission from [64]. Copyright 2004, Springer.

RESEARCH:Review

material properties of interest for these applications were mainly

mechanical strength, chemical inertness and high temperature

durability with some interest on the optical properties

[58,74,76,89,90]. We believe that the existence of such a deep

knowledge will be very helpful in producing TiN nanoparticles

with the desired plasmonic properties. Protective coating applica-

tions are mainly focused on chemical vapor deposition, physical

vapor deposition and thermal spray methods which can be used to

obtain TiN thin films of optimized properties specific to a given

application. Naik et al. recently studied the optical properties

of TiN thin films with varying fabrication parameters and investi-

gated possible application areas in the field of plasmonics and

metamaterials [20,91,92]. We have recently shown that, with a

top-down approach, nanoparticles of plasmonic TiN can be fabri-

cated with electron beam lithography (EBL) followed by a lift-off

process by using optimized thin film deposition for desired plas-

monic properties [67].

Although lithographic techniques can be used for many appli-

cations, a bottom-up approach would be favorable, or indispen-

sible for many cases, where nanoparticles in a solution are

required. Therefore, the fabrication of TiN powders with optimized

plasmonic properties becomes a very important and exciting

challenge. A wide variety of methods leading to TiN powders have

been reported to date. Among several different approaches direct

nitridation of Ti, ammonolysis of TiO2, vapor synthesis, reduc-

tion–nitridation, urea route, and benzene thermal route can be

listed as common methods [93–97]. Nitridation of TiO2 seems

particularly interesting due to the fact that TiO2 itself is a widely

studied material with well understood fabrication methods. The

possibility of combining interesting TiO2 research with plasmonic

TiN would be very exciting [98,99]. Although the optical proper-

ties of TiN powders obtained via these methods have not been

investigated, they can be optimized in order to give nanoparticles

with the desired plasmonic properties. In an effort to obtain

plasmonic TiN nanoparticles, the main focus would be to reduce

Ti vacancies which act as electron traps and increase the size of

possible grains within the particle. In 2001, Patsalas and Logothe-

tidis showed that in a nanocrystalline TiN thin film, the electron

mean free path scales with the grain size, suggesting that point

defects are mainly located around the grain boundaries [90]. For

the case of nanoparticles, dimensions would typically be on the

order of the grain sizes and a polycrystalline structure would not be

a severe problem. Another promising way of obtaining plasmonic

TiN particles is the laser ablation method. Using a TiN thin film

with optimized plasmonic properties as a source, one can obtain

nanoparticles by ablating the target. Indeed, Reinholdt et al. used

the laser ablation technique in their work where the first experi-

mental data on plasmonic TiN particles were reported (Fig. 5) [64].

The target used in this first demonstration was prepared from

pressed TiN powder and the resulting nanoparticles showed iden-

tical lattice structures with the source. Reported particle sizes were

below 10 nm with a broad dispersion and extinction peaks around

1.7 eV were obtained. Possible problems that may arise with the

laser ablation method are the tough nature of TiN which may push

the limits of the method, and the lack of control over the nano-

particle dimension with small dispersion. Considering the fact

that laser ablation has been widely studied and the level of

knowledge on the method is increasing steadily [100], the method

232

can be considered as one potential way of reaching the goal of

plasmonic TiN powders. Indeed, the method is known to be

advantageous for biological applications due to ligand-free nano-

particle surfaces [101]. Takada et al. used a Ti target in a liquid

nitrogen environment in order to avoid problems that arise from

the hard TiN source. However, the resulting particle sizes were

around 800 nm which is far larger than dimensions typically used

in plasmonics applications [102]. Similarly, mechanical ball mill-

ing can be considered as a potential method [103]; however precise

size control would not be easy to achieve and contaminations from

the milling equipment, which is a common problem of this

technique, would require additional sample preparation steps.

Undoubtedly, the optimization of fabrication techniques for

desired material properties requires extensive characterization of

samples, and for the case of plasmonics, optical characterization

will be the fundamental step. In addition, electron microscopy

would be very useful in order to analyze the compound character-

istics of nanoparticles. With the advances in transmission electron

microscopy (TEM) instrumentation, several exciting studies have

been reported in the field of plasmonics. Electron energy loss

spectroscopy (EELS) seems particularly important since it allows

elemental analysis of individual nanoparticles as well as resolved

bulk and surface plasmon resonances when improved with mono-

chromators [7,104]. Another interesting electron beam excitation

technique is the cathodoluminescence spectroscopy which can be

used for thicker samples when compared to EELS and provides

Materials Today � Volume 18, Number 4 �May 2015 RESEARCH

FIGURE 6

Plasmonic photothermal therapy explained in a nutshell. Plasmonicnanoparticles are injected into the body where they accumulate in tumor

tissue. Nanoparticles are illuminated with a near infrared laser in the

biological transparency window. The enhanced absorption cross section of

the plasmonic nanoparticles results in a concentrated heating effect thatablates the tumor region with minimal damage to healthy tissue [6].

Photographs of tumor bearing mice before and after treatment can be

seen in the inset. Nanoparticles should be cleared from the body aftertreatment. Biodegradable Au nanovesicles are recently proposed for

improved clearance. Reprinted with permission from [123]. Copyright 2013,

Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

RESEARCH:Review

information such as polarization and angular emission profile

[105]. Recent advances in TEM related methods may lead to very

exciting results which will likely change the future of nanoparticle

plasmonics. Considering the fact that the chemistry of nanoparti-

cle synthesis is still not well understood and very exciting results

have already been demonstrated with a lack of knowledge of

growth mechanisms, gaining knowledge on details may result

in significant improvements in the field. In this direction, recent-

ly, Goris et al. succeeded in 3D elemental mapping of bimetallic

nanocrystals with atomic scale resolution [106]. In an earlier study,

Zheng et al. achieved the observation of individual platinum

nanoparticle dynamic growth with sub-nanometer resolution by

using in situ TEM methods [107]. Another spectroscopic technique

that would typically be very effective on TiN powders is X-ray

photoelectron spectroscopy (XPS). Indeed, this method has been

used for TiN thin film characterization with significant contribu-

tion to the understanding of growth mechanisms [108,109]. Sev-

eral other important results reported with XPS can be listed as the

study on the oxidation chemistry of TiN by Saha and Tompkins

[110], and the spectra for an oxygen free single crystalline TiN film

by Jaeger and Patscheider [111]. In addition to elemental compo-

sition, XPS allows the observation of surface plasmon peaks [112],

which would be the principle evidence of the plasmonic behavior

of TiN particles.

ApplicationsThe field of plasmonics has provided many exciting results which

have led to high expectations for the soon-to-come real-life applica-

tions [13,14]. However, integrating the new ideas into current

industrial standards has been a compelling challenge [15], mainly

due to high cost and the difficulties in adapting plasmonic materials

into semiconductor fabrication processes. Therefore, materials al-

ready included in current fabrication processes and exhibiting

plasmonic behavior can be considered as critical factors for the

future of the field. TiN, which has been widely used in microelec-

tronics industry [58], is a promising material due to additional

reasons such as a high melting point, resistance to corrosion, bio-

compatibility and others [68]. LSPR applications can be considered

mainly under two titles based on scattering and absorption. In this

review, we discuss the advantages of TiN plasmonic components in

exemplary applications based on absorption of electromagnetic

energy in order to provide an insight for the future. Applications

based on scattering mechanisms shall be considered in a later work

as new developments emerge in the field.

Strong field enhancements provided by metallic structures lead to

various effects such as enhanced light–matter interactions, enhanced

thermal transfer, and extremely high local temperatures. Elevated

temperatures within confined volumes introduce new challenges to

plasmonic applications mostly due to the material softness and

melting point depression in nanostructured materials. Transition

metal nitrides provide refractory properties along with reasonably

good plasmonic properties in the visible and near infrared ranges

[55]. This unique combination of material properties offers the

potential to advance several technologies such as plasmonic pho-

to-thermal therapy [22,23], solar/thermophotovoltaics (S/TPV)

[113], heat assisted magnetic recording (HAMR) [114], solar thermo-

electric generators (STEG) [115], plasmon-mediated photocatalysis

[116–118], and plasmon assisted chemical vapor deposition [119].

Therapeutic applicationsResonant plasmonic nanoparticles can be used for the efficient

collection of electromagnetic energy and heating of a confined

volume in the vicinity of particle [120]. Nanoparticles delivered

to a tumor region can be heated via laser light with a wavelength

in the near infrared region where the attenuation of light through

biological tissue is low. Au nanostructures have been successfully

used in the experiments; however, the spectral mismatch of the

spherical Au nanoparticles and the biological transparency window

resulted in a need for relatively larger dimensions and more compli-

cated nanostructures such as nanoshells and multishells [6,121,122].

Nanoparticle size is a critical parameter affecting vital steps like

cellular uptake and clearance from the body after treatment. Huang

et al. used biodegradable gold nanovesicles in order to improve the

clearance of nanoparticles [123]. In their work, Au nanoparticles

with sizes around 26 nm were closely packed into vesicular assem-

blies of total sizes around 200 nm which can be dissociated back into

smaller nanoparticles after treatment for easier clearance. Enhanced

plasmonic coupling between closely packed nanoparticles resulted

in a redshift and broadening of the resonance, enabling optical

excitation in the biological transparency window. Figure 6 sum-

marizes the plasmonic photothermal therapy method with the

additional biodegradability feature of vesicular assemblies. Consid-

ering the complex conditions in the body, controlled use of the

assembly approach needs further examination. For example, in an

effort to reduce the particle dimensions and increase tumor uptake,

Goodman et al. recently examined in vivo hollow Au nanoshells and

observed instabilities due to fragmentation [124]. Nanorods can be

used as a simpler-geometry solution where the spectral mismatch

problem is solved via higher aspect ratios that redshift the plasmonic

resonance into the biological transparency window. However, sur-

factants inherited from nanoparticle synthesis introduce toxicity

problems that limit the use of nanorods for biomedical applications

[125]. Moreover, it was reported that nanoparticles with higher

aspect ratio are likely to suppress cellular uptake [126].

233

RESEARCH Materials Today � Volume 18, Number 4 �May 2015

RESEARCH:Review

Smaller particle dimensions with simpler structures would elim-

inate complexities due to nanoparticle geometry. Therefore, a bio-

compatible material with a dipolar plasmonic peak located near

the biological transparency window would increase the effective-

ness of the method. Recently, we have shown that lithographically

fabricated TiN nanoparticles provide dipolar resonances exactly in

the desired range for therapeutic applications. Furthermore, it was

also shown that TiN nanoparticles act more efficient than Au

nanoparticles of identical geometries when excited with 800 nm

laser light (Fig. 7) [67]. The resonance peak of TiN nanoparticles

was also found to be broader than Au nanoparticles. These results

show that plasmonic TiN nanoparticles with a simple spherical

shape and no size restrictions can be used for photothermal

therapy. The fact that TiN is already being used as a coating for

biomedical implants, and many other biological applications,

makes it a very interesting alternative to Au nanostructures. Pro-

duction of colloidal plasmonic TiN nanoparticles with optimized

optical properties remains as an exciting, promissory research

opportunity.

Solar/thermophotovoltaicsLight harvesting applications are progressively becoming more

important due to the steady increase in the need for alternative

energy sources. Photovoltaics has been intensely studied for the

last few decades and has proven to be an essential upcoming

energy supply. However, the technology is still not mature enough

for unquestioned investments. Some of the problems in the field

arise from the light conversion efficiencies and long term stability

of the semiconductor devices [127]. Absorption of light with

energies higher than the bandgap of the semiconductor device

is known to result in heating, which in turn causes efficiency

degradation. In addition to long-term stability reduction due to

the over-heating of semiconductors, environmental conditions

FIGURE 7

(a) Schematic of the experimental setup used for comparison of heating

efficiencies from identical TiN and Au nanoparticle arrays [67]. (b)

Temperature increase of sapphire substrate in time due to heating fromoptically excited nanoparticle arrays. Reprinted with permission from [67].

Copyright (2013) American Chemical Society.

234

such as humidity and UV radiation are known to cause degrada-

tion in device performance. The solar/thermophotovoltaic ap-

proach can be a single solution for many problems arising from

direct exposure of semiconductor components. In a solar-thermo-

photovoltaic device, a perfect absorber designed for broad absorp-

tion of solar radiation can be used to heat an intermediate layer to

elevated temperatures. In a thermophotovoltaic device, the emit-

ter can be heated via chemical, nuclear, or waste heat sources

[128]. When combined with a solar absorber, the system can be

used in a hybrid mode and can be called a solar/thermophoto-

voltaic device (Fig. 8). The heated body is expected to re-radiate

light in the infrared region of the spectrum following Planck’s law

[129]. This broad emission of light can be engineered with opti-

mized surface designs for a selective emission of light with a

narrow energy peak just above the bandgap of semiconductor

[130]. As a result of spectrally matched emission, high conversion

efficiencies can be obtained with longer lifetimes. However, high

temperature operation in such devices would bring new challenges

in material degradation. For the absorber component, the use of

metals with high melting points have been proposed [131]. For

emitters, one-dimensional photonic crystal structures were

designed [130]. However, the experimental demonstration seems

to be rather difficult due to the lack of materials exhibiting all of

the desired physical properties.

After several decades, reported results in the field are mostly

theoretical discussions. Recently, Lenert et al. demonstrated a full

FIGURE 8

Schematic illustration of thermophotovoltaic systems. Heat input can besupplied from a variety of sources including solar, chemical, nuclear and

waste heat, or any combination of these. Blackbody emission of the heated

body is spectrally engineered with a selective emitter for an optimized

coupling to the photovoltaic cell.

Materials Today � Volume 18, Number 4 �May 2015 RESEARCH

RESEARCH:Review

solar-thermophotovoltaic device with a record efficiency of 3.2%

[132]. In this pioneering work, the main limiting factors were the

spectral selectivity of the absorber and the relatively low opera-

tional temperatures due to material limitations. Plasmonic mate-

rials with refractory properties can solve both problems and

significantly push the experimental limits. We have shown that

broad absorption in the visible and near infrared regions can be

realized with a metamaterial design consisting of patterned rect-

angular rings which lead to an impedance match and reduced

reflection (Fig. 9(a)) [133]. In addition, owing to its adjustable

optical properties, TiN can be engineered to optimize the imped-

ance matching and can be used with a variety of perfect absorber

designs for efficient collection of energy from the sun. Recently,

Molesky et al. have proposed designs for selective emitters where

high melting point materials, including TiN, have been considered

[134]. TiN, with a melting point around 2900 8C, is again a very

promising plasmonic material. Fig. 9(b) shows the absorption

spectrum of an intact TiN absorber before and after annealing

at 800 8C. Indeed, the strength of TiN nanostructures has been

further tested and compared to Au when illuminated with a pulsed

laser for 5 seconds at 550 nm where both absorbers have high

absorbance. Scanning electron microscope images after test veri-

fies that TiN nanostructures can stand against high intensity

pulses where Au nanostructures fail (Fig. 9(c,d)) [133]. Durability

under high intensity laser illumination is inherently a desired

feature for nonlinear plasmonics [135].

Heat assisted magnetic recordingTip-based plasmonics is one broad application area where metallic

nanoparticles have been found useful. Apart from research appli-

cations such as near-field scanning optical microscopes (NSOM) or

other local field enhanced signal measurements, some commercial

applications like HAMR are also considered feasible. In magnetic

FIGURE 9

(a) Metamaterial perfect absorber design with high melting point materials.

(b) Measured absorbance of TiN perfect absorber before and afterannealing at 800 8C. (c) Au nanostructures illuminated with laser pulses

were highly damaged where (d) TiN nanostructures survived from laser

pulses under identical conditions. The scale bars in the figure are 400 nm.Reproduced with permission from [133]. Copyright 2014, John Wiley &

Sons, Inc.

recording technology, achieving higher data storage densities

requires the reduction of bit sizes, which brings new challenges

such as the loss of data in very small grains due to thermal

instabilities. Materials with higher magnetic anisotropy can be

used as a solution; however this would bring the need for higher

field strengths. A promising solution to the problem is to locally

heat the material to high temperatures and lower the coercivity for

a short amount of time during the writing process. The required

higher coercivity values that are needed for stable data storage can

be achieved by cooling the confined volume back to its initial

temperature [136]. Plasmonic nanoparticle-based tips have the

advantage of sub-wavelength focusing of electromagnetic energy

with high field strength and heating efficiencies in a confined

volume. Figure 10(a–c) illustrates an optically excited plasmonic

near field transducer (NFT) locally heating a bit-patterned layer,

the operation principles of HAMR, and the spatial distribution of

temperature and magnetic field on the medium. In 2009, Chall-

ener et al. integrated an NFT into a magnetic recording head and

demonstrated sub-wavelength recording on a high coercivity me-

dium at an operation wavelength of 830 nm [114]. However, long-

term stability of the system remains a problem due to tough

operation conditions. In their work, Challener et al. reported

operation temperatures above 350 8C. Thermal load on the NFT

can be reduced via alternative designs [137], however the long

term stability remains a challenge. For a long lifetime and reliabil-

ity, the nanostructured near-field transducer shall exhibit chemi-

cal stability and corrosion resistance [138,139], and the search for

alternative plasmonic materials become inevitable for HAMR

technology [140]. As we have been discussing through this review,

TiN is known to be corrosion resistant and durable at high tem-

peratures. It has been recently shown that optical properties of the

material slightly change with increasing temperatures up to 375 8C[141]. In addition, we have shown that the optical performance of

FIGURE 10

(a) Illustration of an optically excited plasmonic near field transducer (NFT)locally heating a bit-patterned medium. (b) A diagram of HAMR write

process. Reproduced with permission from [139]. Copyright 2008, IEEE. (c)

Magnetic field (dashed line) and temperature (solid line) distribution alongthe down track position for a HAMR design. Reproduced with permission

from [142]. Copyright 2012, The Optical Society.

235

RESEARCH Materials Today � Volume 18, Number 4 �May 2015

RESEARCH:Review

TiN nanoparticles is indeed comparable to identical Au nanopar-

ticles in the near infrared region where diode laser technology is

well established [143]. Therefore, near-field transducer designs

based on TiN nanostructures would bring the technology one step

closer to real life products.

OutlookResearch activities on the exciting field of plasmonics are steadily

getting more attention from researchers working across a broad

spectrum of science and technology. Accordingly, research

expenses are increasing continuously and the level of expectations

on practical results are getting higher. As the research area gets

more interdisciplinary, the integration of plasmonic components

to different technology standards becomes a necessity. A limited

pool of plasmonic materials would make it almost impossible to

satisfy fundamentally different requirements for each application

environment. Although Au and Ag have been very useful during

the initial research period in the ideal world of laboratory experi-

ments, alternative plasmonic materials are now essential require-

ments for day-to-day applications. TiN stands as a very promising

refractory plasmonic material for some critical applications that

require tough operating conditions. The high temperature dura-

bility, chemical stability, corrosion resistance and mechanical

strength of TiN are considered as great advantages over metals.

Furthermore, biological and CMOS compatibility of the material

makes it much easier to work with. Indeed, TiN has been already

employed in both biomedical and microelectronics technologies

and previous experience on material processing has been gained.

With the recent demonstrations of high optical efficiencies for

absorption based applications, TiN and other transition metal

nitrides become true plasmonic materials that can be employed

in technology products with minimal modification of production

chains.

AcknowledgementsThe authors acknowledge generous support from the following

grants: ARO grant 57981-PH (W911NF-11-1-0359), ONR-MURI

grant N00014-10-1-0942, NSF grant DMR-1120923. We are

thankful to Dr. Gururaj Naik, Dr. Alex Kildishev, Dr. Wei Li, Justus

C. Ndukaife, Nathaniel Kinsey, Clayton Devault for useful

discussions and help in preparation of this manuscript.

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