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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