UV-Vis-NIR
spectroscopy fornanomaterials research
Essential
Knowledge
Briefings

1st Edition, 2021

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UV-Vis-NIR spectroscopy for nanomaterials research© 2021 John Wiley & Sons Ltd, The Atrium, Southern Gate,
Chichester, West Sussex PO19 8SQ, UK
Microscopy EKB Series Editor: Dr Julian Heath
Spectroscopy and Separations EKB Series Editor:
Dr Christina Poggel

UV-Vis-NIR spectroscopy for nanomaterials research

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CONTENTS
4	INTRODUCTION TO
NANOMATERIALS
7	CHARACTERIZATION WITH UV-VISNIR - THEORY AND BACKGROUND
17	CHARACTERIZATION WITH UV-VIS
NIR – IN PRACTICE
21 CASE STUDIES
27	COMMON CHALLENGESIN NANOMATERIAL
CHARACTERIZATION
33	WHAT’S NEXT IN NANOMATERIALRESEARCH
36 FURTHER INFORMATION

About Essential Knowledge Briefings
Essential Knowledge Briefings, published by John Wiley &Sons, comprise a series of short guides to the latest techniques,appli­cations and equipment used in analytical science. Revisedand updated annually, EKBs are an essential resource for scientists working in both academia and industry looking to update
their understanding of key developments within each specialty.
Free to download in a range of electronic formats, the EKB
range is available at www.essentialknowledgebriefings.com

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UV-Vis-NIR spectroscopy for nanomaterials research

INTRODUCTION TO NANOMATERIALS
By their very nature, nanomaterials are difficult to see.
With at least one of their dimensions less than 100 nanometers(one billionth of a meter) in size, they encompass everythingfrom spherical particles to long nanotubes to flat sheets with athickness of a single atom or molecule (see Table 1).
Their tiny size makes them interesting, by giving them awhole host of useful properties, from great strength, to highelectrical conductivity to bright fluorescence to efficient catalysis. These properties are a consequence of nanomaterials
having a large surface area relative to their size and being
small enough to be influenced by quantum effects, leading tothem being utilized in areas as diverse as computing, solar
cells, batteries and chemical manufacturing.
Type ofnanomaterial

Examples1

Uses

Relevant opticalproperties

Metal
nanoparticles

Gold nanorods,
silver nanowires,
titanium dioxide
nanoparticles

Photocatalysts,
Absorption,
medical imaging
scatteringand treatment, sunprotection, sensing,displays

Semiconducting
nanomaterials

Quantum dots,
tungstendisulfide,copper(I) oxide

Photocatalysts, solar
cells, computing,photodetectors,
displays

Absorption,
fluorescence,reflection

Carbon-based
nanomaterials

Carbonnanotubes,
graphene

Composites,
batteries,
computing, sensing

Absorption,
scattering

2D materials

Graphene,molybdenum
diselenide,
MXene

Photocatalysts,
computing,composites, sensing,electromagnetic
shieldingAbsorption,
scattering

1. Some nanomaterials straddle different types e.g. graphene is both a carbon-based material and a 2D material.

Table 1: Different types of nanomaterials

UV-Vis-NIR spectroscopy for nanomaterials research

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Nanomaterial analysis techniques
Nanomaterials may be difficult to see, but their interaction
with light can reveal a great deal of information about them,which is why UV-Vis-NIR spectroscopy has become an important technique for characterizing them.
In UV-Vis-NIR spectroscopy, a beam of light is shone ata sample and a detector placed behind the sample measures
the intensity of the light transmitted through it. The difference between the intensity of the original light beam and thatmeasured by the detector indicates how much of the light isabsorbed by the sample. By doing this for individual wavelengths of light that can stretch from the ultraviolet (UV)
across the visible to the near-infrared (NIR), an absorption
spectrum for the sample can be built up, showing those wavelengths of light that are most effectively absorbed.
Not only does this reveal the optical properties of thesample, which is important for nanomaterials designed to interact with light, such as photocatalysts, photodetectors andsolar cell materials. It can also reveal various other propertiesof the nanomaterials, such as their size, shape, compositionand electrical properties, all of which can influence their absorption of light.
This is more difficult than it sounds, because as well as
absorbing incoming light, some nanomaterials can reflect andscatter it, which also determines how much light reaches thedetector. The effect of this reflection and scattering thus needs
to be taken into account when calculating the absorption
spectrum, which can be done with various accessories available for modern UV-Vis-NIR spectrophotometers. As an add-

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UV-Vis-NIR spectroscopy for nanomaterials researched bonus, this information on reflection and scattering canalso be useful in its own right.
With the ability to produce all this information, UV-VisNIR spectroscopy has joined transmission electron microscopy and inductively coupled plasma-mass spectrometry in becoming a standard and widely used technique for characterizing nanomaterials, as this technology briefing reveals.

UV-Vis-NIR spectroscopy for nanomaterials research

CHARACTERIZATION WITH UV-VIS NIR- THEORY AND BACKGROUND

7

UV-Vis-NIR spectroscopy
Like most forms of spectroscopy, UV-Vis-NIR spectroscopy
monitors the interaction between materials and electromagnetic radiation, but with a focus on the wavelengths eitherside of visible light. As its name suggests, in practice thesewavelengths can stretch from the ultraviolet (200–400nm)
through the visible light spectrum (400–800nm) to the NIR(800–2500nm).
All non-transparent materials will absorb, scatter and reflect specific visible wavelengths, as this determines theircolor, as well as certain UV and NIR wavelengths. As such,
UV-Vis-NIR spectroscopy can be used to determine the optical properties of materials, in terms of what specific wavelengths they absorb, scatter and reflect. But it can also be used
to probe various other properties that influence a material’s
optical properties, such as its structure, composition and electronic properties.
UV-Vis-NIR spectroscopy works by shining specificwavelengths of light at a material and recording the intensityof the light that reaches a detector behind it, known as transmittance. Any loss of intensity recorded by the detector, compared with the original beam, means that some of the light atthat specific wavelength has been absorbed, reflected or scattered by the sample, collectively known as extinction.
For many non-reflective samples, any light that doesn’treach the detector can be assumed to be absorbed by the sample, and thus the transmittance is directly related to the ab-

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UV-Vis-NIR spectroscopy for nanomaterials research
Detector

Incident
light IO

Reflected
light IR

Backscattered
light IBSSAMPLE

Absorbed light IA
Transmitted
light IT

Forward scattered
light IFS

Figure 1: Absorption, reflection, scattering and transmission in UV-Vis-NIR spectroscopy

sorbance. Doing this for a range of different wavelengths allows an absorption spectrum to be built up, which reveals
those wavelengths of light that are most effectively absorbedby the sample.
For samples that can reflect or scatter light, however,such as smooth films or milky solutions, the same assumption
can’t be made, as some of the light will have been reflected or
scattered rather than absorbed (see Figure 1). This might not
matter, as the extinction will often vary with wavelength in asimilar way to absorption, producing a similar spectrum. In
many cases, however, scientists will want to discover exactly
how much light is absorbed, reflected or scattered by a sample, which means finding a way to determine the contributionof each to the measured extinction.
Nanomaterial characterization
UV-Vis-NIR spectroscopy is commonly used to probe a wide
variety of materials, both solid and liquid, from thin films to oil
shale to DNA. More recently, it has proved to be a useful technique for studying various nanomaterials, including metal nanoparticles, semiconducting nanomaterials such as quantum

UV-Vis-NIR spectroscopy for nanomaterials research

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dots, carbon nanotubes, and two-dimensional (2D) materials
such as graphene. These nanomaterials are usually dispersed insome form of solvent, often just water, as they tend to be easier
to analyze as dispersions than as concentrated powders.
As with other materials, UV-Vis-NIR spectroscopy is notjust used to probe the optical properties of nanomaterials, butalso to characterize attributes that influence their opticalproperties, which for nanomaterials include size, shape andstability. A related form of optical spectroscopy known as fluorescence spectroscopy can reveal further information about
nanomaterials. Rather than record the wavelengths absorbed
by a nanomaterial, this uses a similar set-up to analyze thefluorescence emitted by certain nanomaterials as a result of
their absorption of the original light.
A nanomaterial’s electrons are responsible for its absorption of light at UV and visible wavelengths, because the energy
in the light excites the electrons up to higher energy levels. In
some nanomaterials, this can be the case for NIR wavelengths
as well, but these wavelengths can also be absorbed by inducing
vibrations in the bonds between the component atoms, causing
the nanomaterial to heat up. Because bonds between different
atoms absorb at different NIR wavelengths, this can reveal information about the composition of a nanomaterial.
Metal nanoparticles
In metal nanoparticles, UV and visible light interacts with asea of electrons at their surface, which are no longer boundto an atom and so are free to roam over the nanoparticles.
These surface electrons all tend to resonate at a set frequen-

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UV-Vis-NIR spectroscopy for nanomaterials researchcy, producing what is known as a localized surface plasmon
resonance (LSPR), which absorbs light with similar frequencies and scatters light at different frequencies. As a consequence, metal nanoparticles are being developed for applications ranging from photocatalysis, where the absorptionof light powers their catalytic abilities, to transparent electrodes that can absorb and reflect light at specific wavelengths, for use in smart windows and touchscreen displays.
The frequency of a nanoparticle’s LSPR is determined byvarious factors, including what the nanoparticle is made of,and its size and shape. These factors can be revealed by measuring the wavelengths of light absorbed by the nanoparticles
in a sample (because the frequency and wavelength of lightare inversely related), but only if the optical properties for that
nanoparticle have already been determined. Modifying thesefactors also offers a useful way to produce nanomaterials withdesired optical properties.
The absorption spectrum for a metal nanoparticle resembles a bell-shaped curve, rising to a peak of maximum
absorption (see Figure 2). If this maximum absorption is
known for a specific metal nanoparticle of a certain size,
then UV-Vis-NIR spectroscopy can determine whether
metal nanoparticles of the same type in a different sample
are larger or smaller from the position of the peak. This isbecause smaller metal nanoparticles absorb light at shorter
wavelengths.
UV-Vis-NIR spectroscopy can determine various otherproperties of a solution of metal nanoparticles as well. One is
the size distribution of the nanoparticles, because a narrow

UV-Vis-NIR spectroscopy for nanomaterials research

112

Optical density (cm-1)

1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
400

500

600
Wavelength (nm)

700

800

Figure 2: Absorption spectrum for 50 nm gold nanospheres

absorption peak means that all the nanoparticles in the solution are of a similar size, as they absorb a small set of wavelengths. Whereas a broad peak means a greater size range,
because the nanoparticles absorb a larger set of wavelengths.
It can determine the concentration of nanoparticles ina solvent, because a higher concentration means more nanoparticles and thus more absorption of light, producing alarger peak. It can also determine nanoparticle aggregation, because this reduces and broadens the peak, due tothe fall in the number of distinct nanoparticles (see Figure3). Finally, it can determine whether any molecules bind to
the surface of the nanoparticles, because this changes the
LSPR and thus the wavelengths absorbed, which allows
metal nanoparticles to act as sensors for molecules thatbind to them.
Researchers have synthesized metal nanoparticles frommany different metallic materials, ranging from nanoparticles
of pure gold and silver, which shine different colors depend-

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UV-Vis-NIR spectroscopy for nanomaterials research
0 min
2 min
4 min
6 min
8 min
10 min25 minExtinction (a.u.)

1.0

0.5

0.0
300

400

500
600
Wavelength (nm)

700

800

Figure 3: Absorption spectrum for silver nanoparticles as they transition from a welldispersed state to an aggregated state following the addition of a concentrated salt solution

ing on their size, to complex iron- and titanium-based nanoparticles, which are being developed as photocatalysts. But
they all possess metallic properties and thus absorb light via
their LSPRs.
Semiconducting nanomaterials
Other metal-containing nanomaterials, however, behave as
semiconductors rather than metals. These include the semiconductor nanocrystals known as quantum dots, made frommaterials such as cadmium selenide and zinc sulfide, andcertain 2D materials, like tungsten disulfide, which consistof a single layer of molecules. Although they also absorb
light via their electrons being excited up to higher energy
levels, the process works in a different way, because it doesn’t
involve LSPRs.
Rather, the energy levels that the electrons in semiconducting materials can adopt have a large gap in them, be-UV-Vis-NIR spectroscopy for nanomaterials research

13

Normalised UV
absorbance/PL intensity

tween the levels making up the lower valence band and thelevels making up the higher conduction band. This is known
as the band gap. Because shorter wavelengths of light possess more energy, wavelengths below a certain point can givethe electrons in semiconducting materials sufficient energy
to cross this gap, exciting them from the valence band to the
conduction band.
By monitoring the absorption of light at different wavelengths, UV-Vis-NIR spectroscopy can determine the point
at which the wavelengths have sufficient energy to excite theelectrons from the valence band to the conduction band.
This is reflected as a clear peak in the absorption spectrum,
known as the exciton peak (see Figure 4). Because more energy is required to excite electrons over larger band gaps, thewavelength of the exciton peak can be used to determine thesize of the band gap. This knowledge can be useful when developing semiconductor nanoparticles as replacements forthe silicon in conventional solar cells, for example.

400

500

600
Wavelength (nm)Figure 4: Absorption spectrum for cadmium selenide quantum dots

700

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UV-Vis-NIR spectroscopy for nanomaterials research

For quantum dots, the size of this band gap depends on
the size of the quantum dots, which usually vary between
2nm and 10nm, with smaller dots having a larger band gap.
This means smaller dots absorb light of shorter wavelengths,
allowing UV-Vis-NIR spectroscopy to be used to determine
their size. As with metal nanoparticles, UV-Vis-NIR spectroscopy can also determine the size distribution of a sample ofquantum dots, as well as the concentration, aggregation andthe presence of any binding molecules.
In quantum dots, the excitation to the conduction bandis unstable and the electrons quickly fall back down to thevalence band, releasing the excess energy as a photon. Thismeans that quantum dots are fluorescent, with the wavelengths they emit depending on the size of their band gap
and thus on their physical size, although their fluorescenceis always at longer wavelengths (and thus a lower energy)
than the excitation light. This is because the electrons first
drop down through the various energy levels that make up
the conduction band by losing energy as vibrations, before
crossing the band gap by releasing a photon. Thus, they havealready lost some of the excitation energy before emitting
the fluorescence.
Because different size quantum dots fluoresce different colors, they are being developed for various imaging
and display applications. They can also be analyzed by fluorescence spectroscopy, as the wavelengths of light theyemit provide additional information about their opticalproperties and can be used to determine their size andband gap.

UV-Vis-NIR spectroscopy for nanomaterials research

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Carbon-based nanomaterials
Carbon-based nanomaterials such as carbon nanotubes andgraphene tend to be less straightforward to analyse with UVVis-NIR spectroscopy, because they have a greater tendency
to scatter and reflect light rather than absorb it. But by utilizing the accessories that are available with modern UV-VisNIR spectrophotometers for distinguishing between absorbed, scattered and reflected light (see Problems andsolutions), their optical properties can still be studied.
Other characterization techniques
Although UV-Vis-NIR spectroscopy can be used to characterize various properties of nanomaterials, researchers often
just use it for probing their optical properties, relying on othertechniques to determine properties such as size, especially for
novel nanomaterials. One reason for this is that UV-Vis-NIR
spectroscopy can only determine properties such as size andaggregation by comparing the absorption spectra with thespectra of an identical nanoparticle of known size.
Techniques such as transmission electron microscopy(TEM) can determine the size of a nanoparticle without
knowing anything about it. However, TEM is more complex
to conduct than UV-Vis-NIR spectroscopy, which requires
minimal sample preparation and is entirely non-destructive.
In addition to TEM, UV-Vis-NIR spectroscopy is often
combined with inductively coupled plasma mass spectrometry (ICP-MS) for determining the elemental composition of
nanoparticles. ICP-MS works by using an electrically charged
plasma to break the nanoparticles in a sample down into their

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UV-Vis-NIR spectroscopy for nanomaterials researchcomponent elements, which are then identified by mass spectrometry, and is sensitive enough to analyze individual nanoparticles (see the related publication Nanoparticle Analysis
Using the Sensitivity of ICP-MS for more details).

UV-Vis-NIR spectroscopy for nanomaterials research

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CHARACTERIZATION WITH UV-VIS NIR– IN PRACTICEUV-Vis-NIR spectrophotometer
A standard UV-Vis-NIR spectrophotometer consists of a lightsource, a monochromator or diffraction grating for isolating asingle wavelength from the wide range produced by the lightsource, optics for directing the beam of light to a sample held
in a sample compartment, and a detector behind the sample(see Figure 5).
Several light sources are often used to cover all the required wavelengths. A deuterium lamp can be used to produce UV wavelengths, while a tungsten or halogen lamp canbe used for visible wavelengths. Increasingly, these are beingLightsourceMonochromator(double)Sample compartment
Detectors

Figure 5: Schematic of a UV-Vis-NIR spectrophotometer

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UV-Vis-NIR spectroscopy for nanomaterials research

replaced by xenon lamps, which are more long-lasting andable to cover all wavelengths.
Different instruments are designed to study different
wavelength ranges. Some just study UV and visible wavelengths, from around 200nm to 900nm, but others extend
into the NIR. For example, Agilent produces a range of spectrophotometers under its Cary brand, all start at UV wavelengths, but the wavelengths they extend to vary from 900nm,
just past visible wavelengths, to 3300nm, deep into NIR.
The detector works by using a semiconducting material
such as indium gallium arsenide (InGaAs) to convert the lightthat hits it into an electric current, with the size of the current
proportional to the intensity of the light.
Optical design
The beam from the light source is usually split and sent along
two different paths to produce a reference signal (see Figure 5).
This can be done using a rotating disc with transparent andmirrored sections that alternate between letting the light beampass straight through to the sample and reflecting it so that it
travels along a different path to the detector. This double-beamapproach accounts for any variation in the source intensity orsource over time and any changes in temperature or other environmental conditions. It achieves this by subtracting any signal
produced by the reference beam from the beam that interacts
with the sample. Spectrophotometers that employ this approach are known as double beam spectrophotometers.
Nanomaterials are normally analyzed when dispersed ina solvent held in a glass or quartz cuvette with a volume of

UV-Vis-NIR spectroscopy for nanomaterials research

19

between 100µL and 3.5mL. But this introduces the problem
that both the solvent and cuvette will absorb certain wavelengths of light, and this obviously needs to be accounted forwhen producing the absorption spectrum for a nanomaterial.
In modern spectrophotometers, this is done by first taking areference measurement for the solvent and cuvette, without
the sample, and then subtracting this reference measurementfrom any subsequent sample measurements.
Sample handling
The choice of cuvette and solvent is important because bothwill have absorption cut-off wavelengths, below which thesolvent or cuvette will absorb all the light, making them useless for assessing absorption by the sample. For water, thiswavelength is 180nm, which means all but the shortest UV
wavelengths will pass through, but for acetone it is a more
problematic 329nm, which is bordering on visible wavelengths. Similarly, the absorption cut-off wavelength for glass
is around 300nm, but for quartz is 160nm, making quartz cuvettes more suitable when studying nanomaterials that absorb
at UV wavelengths.
Of course, many materials don’t just absorb light at UV,
visible and NIR wavelengths, but also reflect and scatter it,
which will obviously affect the transmittance and thus the calculated absorbance. This can be important when studying nanomaterials, as nanomaterials suspended in water are oftenvery effective at scattering light, although smaller nanoparticles tend to scatter less.
Fortunately, there are several accessories available for

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UV-Vis-NIR spectroscopy for nanomaterials research

UV-Vis-NIR spectrophotometers, including specular reflection and diffuse reflection accessories, that can determine thedegree of reflection and scattering. Not only do these accessories allow a more accurate measure of absorption by a sample,
but the degree of reflection and scattering can also be useful
information in its own right, such as when analyzing thinfilms or developing materials for smart windows. These accessories will be discussed in more detail in the Common challenges in nanomaterial characterization chapter.

UV-Vis-NIR spectroscopy for nanomaterials research

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Case study 1: Splitting water with graphitic
carbon nitride (Dong-Liang Peng, Xiamen
University, China)
Electric cars may finally be starting to challenge the dominance of petrol-powered cars, but that doesn’t mean researchers have stopped trying to improve their performance. One way
they’re doing this is by developing better batteries that can store
more charge, allowing electric cars to travel for longer between
charges. Another is by developing fuel cells that generate electricity from a clean fuel such as hydrogen, which may one day
replace batteries.
Novel nanomaterials are central to both efforts, and researchers such as Dong-Liang Peng, professor in the College of Materials
at Xiamen University, are busy developing those nanomaterials
and then characterizing them with techniques such as UV-VisNIR spectroscopy.
The cleanest way to produce the hydrogen for fuel cells is bysplitting water. This can be done via electrolysis, which involves
passing an electric current through the water, but an even cleaner
way is to use a photocatalyst that can split water powered purely
by sunlight. One nanomaterial that has shown a lot of potential
as a photocatalyst for hydrogen production is graphitic carbon
nitride (g-C3N4), which is a semiconductor.
When sunlight hits g-C3N4, it excites the material’s electrons tothe conduction band, where they can provide the energy for splitting water. The problem is that the excited electrons tend to fallback down to the valence band quite quickly, often before they
get a chance to split any water. So Peng and his team have been

22

UV-Vis-NIR spectroscopy for nanomaterials researchexperimenting with combing g-C3N4 with other nanomaterials toprevent this from happening.
In recent work, they tried combining g-C3N4 with molybdenum diselenide (MoSe2), which is a type of two-dimensional material known as a transition metal dichalcogenide. They
characterized the resulting nanocomposite using a variety oftechniques, including TEM, energy dispersive X-ray spectroscopy
and UV-Vis spectroscopy.
With UV-Vis spectroscopy, they were able to determine thatthe nanocomposite had the same band gap as g-C3N4, and thus
could also split water, but that it was much better at absorbing
wavelengths above 420nm, meaning in the visible spectrum. As aconsequence, it proved to be a much more effective photocatalyst,
able to produce hydrogen from water at a faster rate than g-C3N4
on its own.
Zeng D, Wu P, Ong W-J, et al. Construction of network-like andflower-like 2H-MoSe2 nanostructures coupled with porous g-C3N4
for noble-metal-free photocatalytic H2 evolution under visible
light. Appl Catal, B 2018;233:26–34. (https://doi.org/10.1016/j.apcatb.2018.03.102)

UV-Vis-NIR spectroscopy for nanomaterials research

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Case study 2: Optical properties of copperoxide nanoparticles (Kathryn Knowles,
University of Rochester, US)
A whole range of factors can influence a nanomaterial’s opticalproperties, including its composition, size, shape and electronic
properties. Using UV-Vis-NIR spectroscopy, Kathryn Knowles
and her colleagues at the University of Rochester have recently
shown that surface oxidation states can also be added to this list,at least for metal nanoparticles.
Knowles and her team were investigating nanoparticles ofcopper(I) oxide (Cu2O), which have proven to be effective photocatalysts for various organic reactions. These include the reduction of carbon dioxide to carbon monoxide, which is the firststep in converting carbon dioxide into useful products such asfuels. Nanoparticles of metallic copper can also make effective
photocatalysts, but they work in a different way, because theirabsorption of light is mediated by an LSPR. In contrast, Cu2O
nanoparticles are semiconductors and don’t possess an LSPR.
But copper nanoparticles and Cu2O nanoparticles can transform into each other, via oxidation and reduction. This process
can also produce hybrid versions, such as nanoparticles with apure copper core covered in a layer of Cu2O. Knowles and herteam were interested in discovering what happens to the LSPR
during this oxidation and reduction.
So they synthesized several forms of copper nanoparticle andprobed their optical properties with UV-Vis-NIR spectroscopy.
This involved synthesizing Cu2O nanoparticles and then removing all the oxidized copper species from their surface by expos-

24

UV-Vis-NIR spectroscopy for nanomaterials researching them to nitrogen, and synthesizing copper nanoparticles andthen exposing them to air to form a surface layer of Cu2O containing oxidized copper species.
Removing all the oxidized copper species from the surface of
the Cu2O nanoparticles led to the formation of an LSPR, because
the surface was now covered in unoxidized copper. This LSPR
absorbed light at similar wavelengths to the LSPR on metallic
copper nanoparticles. Copper nanoparticles with a covering of
Cu2O and oxidized copper species also possessed an LSPR, but
this LSPR absorbed longer, lower-energy wavelengths than the
LSPR on metallic copper nanoparticles.
This shows that the surface oxidation state of copper nanoparticles can have a strong influence on their optical properties,
potentially offering a novel way to tune their optical and catalytic properties.
Tariq M, Koch MD, Andrews JW and Knowles KE. Correlation between surface chemistry and optical properties in colloidal Cu2O
nanoparticles. J Phys Chem C 2020;124:4810−4819. (https://doi.
org/10.1021/acs.jpcc.9b10753)

UV-Vis-NIR spectroscopy for nanomaterials research

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Case study 3: Semiconducting nanoparticles
for transparent electrodes (Enrico Della
Gaspera, RMIT University, Australia)
In general, metal-containing nanomaterials can either behave
like metals, in which case they possess an LSPR, or behave likesemiconductors, in which case they don’t. But by doping metal-containing nanoparticles with elements such as gallium andlanthanum, scientists have managed to develop metal-containing nanoparticles that behave like semiconductors and also possess an LSPR, although one that absorbs light at NIR wavelengths
rather than visible wavelengths.
Enrico Della Gaspera and his colleagues at RMIT University are developing such nanoparticles as the next generation oftransparent electrodes. At the moment, transparent electrodes
are mainly used in touchscreen displays, but they could alsofind use in smart windows and solar cells. Indium tin oxide is
the most widely used material for the transparent electrodes intouchscreens, but the search is on for alternative materials thatare cheaper and utilize more abundant elements than indium.
Such materials obviously need to be electrically conductive andtransparent to visible light, but for applications such as smart widows they also ideally need to reflect or absorb light at NIR wavelengths. Hence the interest in developing semiconducting nanoparticles with an LSPR that interacts with NIR wavelengths.
Della Gaspera and his colleagues have developed several such
nanoparticles, including zinc oxide (ZnO) nanocrystals dopedwith gallium, barium stannate (BaSnO3) nanocrystals dopedwith lanthanum and tin oxide (SnO2) nanocrystals doped with

26

UV-Vis-NIR spectroscopy for nanomaterials researchfluorine. The doping is what gives these nanocrystals their LSPRs,
because the dopants leave behind excess electrons after they bindwith the elements making up the nanocrystals. These excess electrons then form the LSPR.
After synthesizing the nanocrystals, Della Gaspera and his colleagues assess their optical properties with UV-Vis-NIR spectroscopy. For example, with the fluorine-doped SnO2 nanocrystals, theyused UV-Vis-NIR spectroscopy to determine that the nanocrystals
had an absorption peak at a wavelength of around 275nm, due totheir band gap, but also possessed an LSPR that absorbed at NIR
wavelengths above around 1000nm. They further confirmed that
this LSPR wasn’t present in undoped SnO2 nanocrystals and thatits strength increased with greater fluorine doping.
Kendall O, Wainer P, Barrow S, et al. Fluorine-doped tin oxide
colloidal nanocrystals. Nanomaterials 2020;10:863. (https://doi.
org/10.3390/nano10050863)

UV-Vis-NIR spectroscopy for nanomaterials research

COMMON CHALLENGES IN
NANOMATERIAL CHARACTERIZATION

27

Reflection and scattering
Operating as standard, UV-Vis-NIR spectroscopy determines
the extinction at different wavelengths, meaning the combination of absorption, reflection and scattering that prevents
light from passing through a nanomaterial sample to a detector behind it (see Figure 1). But scientists often want to distinguish the specific contributions of absorption, reflection andscattering, in order to obtain a full characterization of a nanomaterial’s optical properties.
To make matters more complicated, this reflection and scattering
can come in several different forms (see Figure 6). Materials withflat, shiny surfaces, such as mirrors, display specular reflection, in
which the beam of light is reflected at the same angle that it hits

1. Absolute specularreflection

2. Diffuse scattering

3. Glossy scattering

4. Direct transmission

5. Scattered transmission

6. Partial scatteredtransmission

Figure 6: Different forms of reflection, scattering and transmission

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UV-Vis-NIR spectroscopy for nanomaterials research

the surface. Materials with rougher surfaces, such as paper, display diffuse scattering, in which the light is reflected at many different angles. There is also glossy scattering, as displayed by thesurface of a polished car, in which the light is reflected at a narrower range of angles than in diffuse scattering.
This kind of reflection and scattering happens when thelight hits the surface of a material and is reflected or scattered
back, but scattering can also occur when light exits a material
after travelling through, known as forward scattering. If the nanomaterial sample is transparent at a specific wavelength, thenthe light will pass straight through without any scattering at all,allowing it to reach the detector. Even here, the angle that thelight exits the sample will be different from the angle that it enters, because of the refraction caused by the light travellingthrough the sample at a slightly different speed than through air.
For materials that are translucent rather than transparent
at specific wavelengths, such as a leaf, the light is scattered as
it travels through the material, emerging at a wide range ofdifferent angles. This is known as scattered transmission andwill result in some of the light being scattered away from thedetector. There is also partial scattered transmission, where
the light emerges at a narrow range of angles.
Specular reflection accessories and diffuse reflectanceaccessories
Various accessories are available for UV-Vis-NIR spectrophotometers to measure reflected and scattered light. To usethem, they must be fitted into the sample compartment. Specular reflection accessories are placed in front of the sample

UV-Vis-NIR spectroscopy for nanomaterials research29

and are designed tovary the angle of the
Incoming lightincoming light (seeFigure 7). They then Rotational
sample stage
collect the resulting
specular reflection
and use a system of
mirrors to direct it
Translation stage
around the sample tothe detector.
Diffuse reflec- Figure 7: Specular reflection accessory
tance accessories possess their own detector, located within
an integrating sphere, and can detect both diffuse and specular reflection, and scattered transmission (see Figure 8). An
integrating sphere is a sphere with a reflective inner coating,such that light entering the sphere is repeatedly reflected until
it encounters a detector on the inner surface.

Figure 8: Integrating sphere from a diffuse
reflectance accessory, with detector at bottom

The sample and sphere are placed in various arrangements to detect different forms of reflection and scattering(see Figure 9). To detect specular and diffuse reflection, thesample is placed against a hole in the side of the sphere andopposite a second hole through which light is shone so that it

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UV-Vis-NIR spectroscopy for nanomaterials research

Transmission

Center

Refl ection
Schematics showing transmitted and refl ected light. Diffuse R (red), specular (blue)and diffuse T (green).
Figure 9: Different arrangements of sample and integrating sphere for measuringreflection and scattering

hits the sample. All the light reflected and backscattered by
the sample is then directed towards the detector.
To detect the direct and scattered transmission, the sample is placed in front of the sphere, so that it covers the hole.
This ensures all the transmitted and forward scattered lightpasses through the hole and into the sphere, where it is directed towards the detector. Alternatively, the sample can beplaced at the center of the sphere, with light shone at it throughthe hole. In this case, all the light that is reflected, scattered ortransmitted by the sample is directed towards the detector.
Universal measurement spectrophotometer
Recent advances in technology have allowed Agilent to develop a
new system that can embrace broad measurement types. Knownas the universal measurement spectrophotometer (UMS), this

UV-Vis-NIR spectroscopy for nanomaterials research

31

single system can distinguish between reflected, scattered andtransmitted light. By mounting the sample in the center of thesample compartment and utilizing a detector that can be moved
all around the sample, the UMS can automatically measure lightreflected or scattered at any angle (see Figure 9).
By performing multiple measurements, the UMS can replace all other accessories in most cases. Being able to use asingle accessory is convenient, but also minimizes error as the
sample is in a constant position, so measurements are always
through the same point in the sample. For collecting scattered
light over a wide arc, however, a diffuse reflectance accessory
is still recommended.
With angular control over light collection, the UMS canprovide more insight into the optical properties of a sample,

Figure 10: Agilent’s Cary 7000 UMS

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UV-Vis-NIR spectroscopy for nanomaterials researchby recording exactly how much light is reflected at each specific angle, rather than simply measuring all the diffuse scattering or scattered transmission simultaneously. It is well-suited for analyzing solid samples, such as deposits of nanoparticles
on glass or thin films, but isn’t routinely used to analyze dispersions of nanoparticles in a liquid solvent.
The type of accessory employed and how it is used depends on the optical properties being probed. Determining
separate and individual contributions from reflection, scattering, transmission and absorption has traditionally required
various specular reflection accessories and a diffuse reflectance accessory with the sample placed both in front of theintegrating sphere and behind it. Alternatively, this can all bedone with the UMS. When angular information relating todiffuse and transmissive scattering is not required, absorptance can be accurately determined by placing the samples inside the integrating sphere to measure all reflected,
scattered and transmitted light in a single collect.

UV-Vis-NIR spectroscopy for nanomaterials research

WHAT’S NEXT IN NANOMATERIALRESEARCH

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Nanomaterials research
UV-Vis-NIR spectroscopy has become an important component of the suite of analytical techniques that researchers use
for characterizing newly developed nanomaterials. Not only
is UV-Vis-NIR spectroscopy the only practical option forprobing their optical properties, but the interaction between
materials and light can reveal a whole lot of additional information about the materials. Unlike other commonly usedcharacterization techniques, such as TEM or ICP-MS, UVVis-NIR spectroscopy is also entirely non-destructive, able toanalyze nanomaterials without destroying them or needingthem to be modified in any way.
As a consequence, UV-Vis-NIR spectroscopy is playing acentral role in the most exciting and active areas of nanomaterials research. These include the creation of stacks, or heterostructures, of different 2D materials, such as graphene andtransition metal dichalcogenides like tungsten disulfide.
Whereas stacks of graphene layers produce graphite andstacks of tungsten disulfide layers produce bulk tungsten disulfide, stacks containing layers of graphene and tungsten disulfide simply don’t exist in nature and could thus possess
some novel and interesting properties.
They also include nanocrystals of a mineral known asperovskite, which encompasses a family of materials comprising an organic cation, an inorganic cation and a halide anion
such as iodide, bromide or chloride. Certain perovskites have
proven to be very effective absorbers and emitters of light,

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UV-Vis-NIR spectroscopy for nanomaterials researchand so are being explored for use in solar cells and light-emitting diodes. As an added bonus, unlike the silicon that forms
the basis of conventional solar cells, perovskite nanocrystals
can be processed in solution, which offers the possibility of
producing solar cells very cheaply by printing.
As with perovskite nanocrystals, many of these nanomaterials are being developed for applications that take advantage of their optical properties, such as photocatalysts, photodetectors and optical sensors for use in electronic devices
such as mobile phones. But they are also being developed fora range of other applications, including as novel battery materials and as materials for novel forms of computing, such as
spintronics. In this case, researchers just want to provide a fullcharacterization of the properties of these novel nanomaterials, including their optical properties.
Advances in UV-Vis-NIR spectroscopy
Recent advances in the technology for UV-Vis-NIR spectroscopy is helping in this regard. The latest xenon light sources,
diffraction gratings and detectors are all enhancing the sensitivity and versatility of the technique. The light sources are
generating a wider range of wavelengths, the grating technology provides better wavelength selectivity in smaller andsmaller packages, and the detectors are able to detect wider
dynamic ranges. As an added bonus, some of these advances
are also reducing the cost of ownership; the latest xenon lightsources, for example, are more robust than previous versions,meaning they last for longer.
In addition to increasing the sensitivity and versatility,

UV-Vis-NIR spectroscopy for nanomaterials research

35

manufacturers are also looking to make their UV-Vis-NIR
spectrophotometers easier to use, by automating as much asof the analytical process as possible. The ideal is for the instrument to accurately and reliably determine all the required optical information about a sample at the push of a button.
The UMS moves towards that goal by allowing the transmittance, reflection and scattering of different wavelengths bya sample to be distinguished and determined in a single automated operation, albeit one comprising lots of individual
measurements. As does the data analysis software that comeswith the latest instruments, which can automatically calculate
the absorption spectrum from the data generated on thetransmittance, reflection and scattering of different wavelengths, however determined.
As the range of nanomaterials being developed and theirpotential applications continue to expand, these kinds oftechnological advances will ensure UV-Vis-NIR spectroscopy
continues to be an essential technique for characterizing nanomaterials. Even if they remain exceedingly difficult to see.

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UV-Vis-NIR spectroscopy for nanomaterials research

FURTHER INFORMATION
C&EN webinar. Advancing Nanomaterial Research with
Optical Spectroscopy. Focus - Wet Chemical Synthesis of
Semiconductor Nanomaterials. https://connect.acspubs.org/
CENWebinar_Agilent_6_16_20?partnerref=Agilent
Mourdikoudis S, Pallares RM and Thanh NTK. Characterization techniques for nanoparticles: comparison and complementarity upon studying nanoparticle properties. Nanoscale 2018;10:12871. (https://doi.org/10.1039/C8NR02278J)
Nair AK, Mayeen A, Shaji LK, et al. Optical Characterization of Nanomaterials. In: Characterization of Nanomaterials: Advances and Key Technologies. Edited by Bhagyaraj SM,
Oluwafemi OS, Kalarikkal N and Thomas S. Duxford: Woodhead Publishing, 2018. (https://www.sciencedirect.com/
book/9780081019733/characterization-of-nanomaterials)

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July 12, 2021
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