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Transcript
Materialanalytik
Praktikum
UV-VIS Absorption
Spectroscopy
B507
Stand: 06.10.2014
The objective of this experiment is to gain hands-on experience with
(1) Calibration of the UV-VIS spectrometer
(2) Optical characterization of thin film with UV-VIS spectrometer
(3) Spectroscopy analysis of metal nanoclusters as well as metal/polymer nanocomposites in
comparison to bulk material
Contents
1.
Introduction..................................................................................................................................... 2
2.
Experimental ................................................................................................................................... 3
2.1
UV/VIS Spectroscopy ............................................................................................................... 3
2.2
Procedures............................................................................................................................... 5
2.2.1
Removal of background................................................................................................... 5
2.2.2
Measurement .................................................................................................................. 5
2.2.3
Analysis of the Data ......................................................................................................... 5
3.
Questions......................................................................................................................................... 5
4.
References ....................................................................................................................................... 6
1
B507: UV-VIS Absorption Spectroscopy
1. Introduction
Wonderful "stained glass" remaining to ancient Egyptian and Roman civilizations or those
found in archaic cathedrals and mosques are the examples implying antique synthesis and
application of nanoparticles. Nevertheless, nano revolution started after discovery of
scattering from individual gold "nano" colloids by Richard Adolf Zsigmondy, the Nobel
laureate in chemistry in 1925. The fast progress in the field of nano-optics provided us a
deeper understanding of the correlation between material properties (e.g. size, shape)
surrounding environment and the observed color of a metal suspension. An understanding
of the optical properties of noble metal nanoparticles can enable us to realize some new
functional system, in particular in nano-scale. For instance, the tunable optical properties of
nanostructures can be applied as materials for surface-enhanced spectroscopy, optical
filters, plasmonic devices and sensors amongst others.
One of the most interesting properties of noble metal nanoparticles arises from their ability
to support a localized surface plasmon resonance (LSPR). The LSPR results when the incident
photon frequency is resonant with the collective oscillation of the conduction electrons of
the nanoparticles as schematically shown in Figure 1.
Figure 1: Schematic of localized surface plasmon resonance (LSPR) where the free conduction
electrons in the metal nanoparticle are driven into oscillation due to strong coupling with incident
light [1].
One of the simplest theoretical approaches for modeling of the optical properties of small
object is the Mie theory. According to his theory, the extinction of spherical object at desire
frequency (λ) can be calculated by the following equation [2]:
(1)
where E(λ) is the extinction (sum of absorption and scattering), NA is the number of particles ,
a is the radius of the metallic nano-sphere, εenv. is the dielectric constant of the environment
2
B507: UV-VIS Absorption Spectroscopy
surrounding the metallic nano-sphere (assumed to be a positive, real number and
wavelength independent), λ is the wavelength of the incident wave, x is the shape factor,
εm1 and εm2 are the real and imaginary part of the dielectric function of metal, particle.
Please note that the shape factor is assumed to be 2 in the case of spherical particles. Based
on Mie theory the LSPR spectrum of an isolated metallic nano-sphere embedded in an
external dielectric medium will depend on the nanoparticles radius a, the nanoparticle
intrinsic optical property (εm1 and εm2), and the environment’s dielectric constant (εenv.).
Furthermore, the plasmon absorptions wavelength depends also on the number of the
particles. Variation of each of the parameters (Equation 1) will result in a change of the
sample absorption and ultimately the color as shown in Figure 2.
Figure 2: Demonstration of different color through using of composite (gold-silicon dioxide) with
variety of metal filling factor [3].
2. Experimental
2.1 UV/VIS Spectroscopy
UV-VIS spectroscopy is a useful analytical tool to characterize the optical properties (i.e.
absorption, transmission, and reflectivity) of a variety of technologically important materials,
coatings, thin films and solutions. When the electromagnetic waves interact with a colored
substance, a characteristic portion of the mixed wavelengths is absorbed while the
complementary color appears in the transmission mode. This relationship is demonstrated
by the color wheel shown on Figure 3A. It can be clearly seen that the complementary colors
are utterly opposite each other. For example, absorption of 420-430 nm light gives rise to
yellow colored substance in the transmission mode.
The UV-VIS spectral range is approximately 190 to 750 nm, as defined by the working range
of typical commercial UV-VIS spectrophotometers. In general specific regions of the
electromagnetic spectrum are absorbed by exciting specific types of molecular and atomic
motion to higher energy levels. The absorption of microwave radiation bases on excitation of
molecular rotational motion, while Infrared absorption is generally due to vibrational
motions of molecules. Absorption of visible and ultraviolet (UV) radiation is associated with
electronic excitation of molecules following the absorption of light in the UV-VIS spectral
range.
3
B507: UV-VIS Absorption Spectroscopy
Figure 3: A) color wheel. B) Sketch of a single-beam UV/VIS spectrophotometer. C) Sketch of a dualbeam UV/VIS spectrophotometer. D) Image of the UV/VIS/NIR Spectrometer Lambda900 from
PerkinElmer.
A spectrophotometer can be either single beam or double beam. In a single beam
instrument, Figure 3B, all of the light passes through the sample cell. However, in a doublebeam instrument, Figure 3C, the light is split into two beams before it reaches the sample.
One beam is used as the reference; the other beam passes through the sample. Some
double-beam instruments have two detectors (photodiodes), and the sample and reference
beam are measured at the same time. The detector alternates between measuring the
sample beam and the reference beam. Both techniques are seen in Figure 3 (B, C)
respectively. The instrument measures the intensity of light passing through a sample (I),
and compares it to the intensity of incident light (Io). The ratio I/I0 is called the
transmittance, and are usually expressed as a percentage (by multiplying the numeric value of
the ratio by 100). The absorbance, A, is based on the transmittance: A = −log (%T).
In this experiment, a UV/Vis Spectrometer (Lambda900 of PerkinElmer), will be used to
measure the plasmon resonance of metallic nanoparticles embedded in a dielectric matrix
(i.e. nanocomposite), which lies in the visible frequency regime. Figure 3D shows the real
image of the UV/VIS/NIR Spectrometer (Lambda900, PerkinElmer) which will be used in this
experiment.
4
B507: UV-VIS Absorption Spectroscopy
2.2 Procedures
2.2.1 Removal of background
In order to omit the effects of the substrate on the measurements, the substrate needs to be
initially measured. Before that, select the wavelength range that are intended to be spanned
(visible: 300-750 nm). Then, select the type of the measurement (e.g. absorbance (A)). Place
the substrate (e.g. glass) in the sample compartment and hit “auto-zero”. The software run
twice and record the absorption spectra of the sample.
2.2.2 Measurement
Right after the background removal, the experiment can be conducted. Place the sample
CAREFULLY in the compartment (not to scratch the surface) and hit “start”. The default
setting is set in a way that only one round the sample will be measured. To ensure the
accuracy, run the experiment twice. When the experiment is done, save the sample in the
local disk and name it.
2.2.3 Analysis of the Data
In this experiment, spectra of nanocomposite (Silver or Gold nanoparticles embedded in
various dielectrics (e.g. polymer, SiO2, TiO2, etc.)) will be measured. The variables can be the
following: type of the metal or dielectric, filling factor (volume fraction) of the metals. To
analyze the data, do the following:
• Draw the spectra (Absorbance vs. Wavelength)
• Determine the position of the localized plasmon resonance (LPR)
• Discuss how the filling factor change the LPR peak
• Discuss the influence of the type of metal on the resonance peak (LPR)
3. Questions
1. Name some other applications of UV-VIS spectrometer which is not mentioned in this
instruction?
2. What does blue- and red-shift mean?
3. How does the particle size change plasmon resonance peak position?
4. Do only metals support (sustain) surface plasmon resonance? Why?
5
B507: UV-VIS Absorption Spectroscopy
4. References
[1] J. L. Hammond, N. Bhalla, S. D. Rafiee and P. Estrela, "Localized Surface Plasmon Resonance as a
Biosensing Platform for Developing Countries," Biosensors, vol. 4, no. 2, pp. 172-188, 2014.
[2] J. M. K. ,. Z. A. Matthew D. Sonntag, B. Sharma, L. Ruvuna and R. Van Duyne, "Molecular
plasmonics for nanoscale spectroscopy," Chemical Society Reviews, vol. 43, pp. 1230-1247, 2014.
[3] M. K. Hedayati, S. Fahr, C. Etrich, F. Faupel, C. Rockstuhl and M. Elbahri, "The hybrid concept for
realization of an ultra-thin plasmonic metamaterial antireflection coating and plasmonic
rainbow," Nanoscale, vol. 6, no. 11, pp. 6037-6045, 2014.
6