UV-Vis optical fiber assisted spectroscopy in thin films and solutions

Transcription

1 UV-Vis optical fiber assisted spectroscopy in thin films and solutions Description UV-Visible absorption and transmission spectra provide fundamental information for all experiments related to the attenuation of a beam of light after it passes through a sample (absorption and transmission). Both approaches are suited for thin films measurements but the first type is mainly devoted to the measurement of solutions. They serve to identify the excited species in the sample, the presence of specific chromophores and their potential interactions, to verify the sample purity, to identify the bathochromic and hypsochromic shift effect. Spectra usually obey the golden rules given below. Goal To familiarize the student with an optical (fiber) spectrometer, to introduce the concepts of absorption and transmission spectra, to introduce the concept of reference and dark spectra, and to illustrate the Golden Rules. Introduction This exercise involves recording of absorption and transmission spectra and verification of the Golden Rules. The reason for the Golden Rules may be found in the reference text. These rules are: 1. The intensity of the spectra is proportional to the number of the active molecules (concentration), directly for absorption and inversely for transmission spectra. 2. The shape of the absorption/transmission spectrum does not change when the concentration of the sample is changed. 3. The intensity of the spectra is proportional to the length of the optical path (thickness of the thin film), directly for absorption and inversely for transmission spectra. 4. The shape of the absorption/transmission spectrum changes when the thickness of the film is changed. 5. When absorption maximum (λmax) of a compound shifts to longer wavelength, it is known as bathochromic shift or red shift. The effect is due to a heteroatom-carbon double bond or a negatively charged heteroatom which delocalizes more effectively than the lone pair of electrons, when situated in the vicinity of a carbon-carbon double bond (conjugation is favorized). 6. When absorption maximum (λmax) of a compound shifts to shorter wavelength, it is known as hypsochromic shift or blue shift. The effect is due to the presence of a group which causes removal of conjugation or by change of solvent. Safety issues High voltage is produced by the lamp power supplier; normally, students will have no need to have access to this circuitry. The samples used in the exercise are not toxic but wearing disposable gloves during the experiment is recommended. Materials Samples set 1: Thin films of Rhodamine 6G coated on the top of ultrathin glass cover slips ( 0.17 mm thickness) 1

2 1A 0.2M Rhodamine 6G (R6G) in MeOH glass cover slip 0.17 mm thickness d1 1B 40 mm Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d1 1C 20 mm Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d1 1D 10 mm Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d1 1E glass cover slip 0.17 mm thickness Samples set 2: 2A c2m Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d1 (recipe R2)d1 2B c2m Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d2 (recipe R1)d2 2C c1m Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d2 (recipe R2)d3 2D glass cover slip 0.17 mm thickness Samples set 3: Blank 1 mm quartz cuvette empty 3A Acetone Blank 1 mm quartz cuvette with pure ethanol 3B 1mM 3-benzophenone in ethanol 3C 1 mm 3-benzophenone in ethanol with 0.01N HCl ph 3 3D 1 mm 3-benzophenone in ethanol with 0.01N NaOH ph 11 UV-Vis optical fibers of 400 µm and 200 µm diameters, absorption spectra of deuterium and tungsten lamps, absorption spectra of samples 1D(Rh6G), 3A(acetone), 3B(3-benzophenone), and transmission spectra of samples 1D(Rh6G). Procedures Taking an Absorption and a Transmission Spectrum in thin films Rules 1 and 2 1. Turn on the light source and the computer 2. Configure the experimental setup connecting the light source and other sampling optics with spectrometer and computer. If follow the previous steps and start SpectraSuite application, the spectrometer is already acquiring data in Scope Mode. Even with no light in the spectrometer, SpectraSuite should display a dynamic trace in the bottom of the graph window. If the light is allowed into the spectrometer, the graph trace should rise with increasing light intensity. This indicates that the software and hardware are correctly installed. How can you check this if our light source has constant power? Compare the graph traces recorded with the optical fiber end oriented to the day light or any other light source before and after placing in the light path a neutral density filter or your own hand. 3. Disconnect the optical fiber from the light source. Then insert a visit card in front of the lamp exit having both the deuterium lamp and tungsten lamp on. Note the color of the light. 2

3 Repeat the same procedure with the deuterium lamp turned off and tungsten lamp turned off, respectively. Note the color of the light in either case. How this color changes when the card contains a chromophore (fluorophore) absorbing in UV range? Place a quartz cuvette filled in with acetone (sample 3A) in the light source path and observe the color of the light spot on the visit card, then replace the sample 3A and observe the color of the light spot on the visit card. Note the light spot color in each case. Reconnect the optical fiber to the light source and click again on the Scope Mode icon in the graph toolbar. Calibrate the correct integration time (IT) so that the maximum intensity to be around counts or a little bit less. Visualize the spectrum of each deuterium and tungsten lamp, then the entire light source spectrum. Save the spectra as text file and OOI Binary Format. 4. Use the set 1, where samples 1A 1D are Rhodamine 6G thin films of decreased concentrations from 0.2M to 10mM (in methanol, MeOH) coated on ultrathin glass cover plate. Sample 1E (blank sample) is similar to samples 1A-1D but without Rhodamine 6G. 5. Insert the sample 1E (blank) and recalibrate the integration time (IT) so that the maximum intensity of the signal to be counts or a little bit less. Set the number of scan to average and the boxcar width so that the signal-to-noise ratio and the spectral resolution of the spectrum requirements to be fulfilled. 6. Store a reference spectrum and save it as a reference spectrum in OOI Binary Format on the disk. 7. Turn off the lamp to store a dark spectrum and save it as a dark spectrum in OOI Binary Format on the disk. If the light source is built in with a triggering system, it is recommended to turn off the shutter instead the lamp. 8. Turn on the lamp/shutter and replace sample 1E with sample 1A. 9. Start an absorption measurement to record the spectrum under the same conditions. Select File/New/Absorption and set the same acquisition parameters. Load first the stored/saved Reference spectrum, then load the stored/saved Dark spectrum and press Finish. Save the absorption spectrum of the sample 1A as a processed spectrum in OOI Binary Format but also in Text file Format on the disk. Same conditions mean same acquisition settings. There are several reasons for doing this (e.g., potential artifacts); do you know them? Lower or higher integration times (ITs) affect the position of the graph trace: upper or downer than the baseline position. Scan to average changes affect the signal-to-noise ratio of the spectrum and boxcar width changes will bring losses in the spectral resolution. 10. Replace sample 1A with sample 1B and record the spectrum under the same conditions. Save the absorption spectrum of the sample 1B as a processed spectrum in OOI Binary Format but also in Text file Format on the disk. 11. Compare then the spectra of the samples 1A and 1B. How do they differ? (Golden Rule 1) 12. Apply the same procedure for the samples 1C and 1D. Save the absorption spectra of the samples 1C and 1D as processed spectra in OOI Binary Format but also in Text file 3

4 Format on the disk. Compare their spectra with those of the samples 1A and 1B. How do they differ? (Golden Rule 1 and Golden Rule 2). Taking a Transmission Spectrum in thin films Rules 3 and 4 1. Use the set 2, where samples 2A 2C are Rhodamine 6G thin films of c2m and c1m concentrations (in methanol, MeOH), coated at different rotation speed on ultrathin glass cover plate (R2 and R1 recipes). Samples 2D (the corresponding blank sample) is similar to samples 2A - 2C but without Rhodamine 6G. 2. Insert the sample 2D (blank) and recalibrate the integration time (IT) so that the maximum intensity of the signal to be counts or a little bit less. Set the number of scan to average and the boxcar width so that the signal-to-noise ratio and the spectral resolution of the spectrum to be fulfilled. 3. Store a reference spectrum and save it as a reference spectrum in OOI Binary Format on the disk. 4. Turn off the shutter to store a dark spectrum and save it as a dark spectrum in OOI Binary Format on the disk. 5. Turn on the shutter and replace sample 2D with sample 2A. 6. Start a transmission measurement to record the spectrum under the same conditions. Select File/New/Transmission and set the same acquisition parameters. Load first the stored/saved Reference spectrum, then load the stored/saved Dark spectrum and press Finish. Save the transmission spectrum of sample 2A as a processed spectrum in OOI Binary Format but also in Text file Format. 7. Repeat the same procedure to record spectra of the samples 2B and 2C using as blanks the samples 2D. Save the transmission spectra of samples 2B and 2C as processed spectra in OOI Binary Format but also in Text file Format. Compare the recorded spectra of the samples 2A-2C. How do their intensities differ? (Golden Rule 3) Do the shape of these spectra changes and if yes, how? (Golden Rule 4) Taking an Absorption Spectrum in solutions - Bathochromic and Hypsochromic Shift Effect Rules 5 and 6 1. Use the set 3, where the sample 3A is pure acetone, 3B is 1mM 3-benzophenone in ethanol, 3C is 1 mm 3-benzophenone in ethanol with 0.01N HCl ph 3 and 3D is 1 mm 3-benzophenone in ethanol with 0.01N NaOH ph 11. For the acetone (3A) the used blank is an empty 1 mm quartz cuvette, whereas for the rest of the samples was used a 1 mm quartz cuvette with pure ethanol. 2. Insert an empty 1 mm quartz cuvette as blank and recalibrate the integration time (IT) so that the maximum intensity of the signal to be counts or a little bit less. Set the number of scan to average and the boxcar width so that the signal-to-noise ratio and the spectral resolution of the spectrum to be fulfilled. 3. Store a reference spectrum and save it as a reference spectrum in OOI Binary Format on the disk. 4. Turn off the shutter to store a dark spectrum and save it as a dark spectrum in OOI Binary Format on the disk. 4

5 5. Turn on the shutter and replace the blank with sample 3A. 6. Start an absorption measurement to record the spectrum under the same conditions. Select File/New/Absorption and set the same acquisition parameters. Load first the stored/saved Reference spectrum, then load the stored/saved Dark spectrum and press Finish. Save the absorption spectrum of the sample 3A as a processed spectrum in OOI Binary Format but also in Text file Format on the disk. 7. Insert a 1 mm quartz cuvette with pure ethanol as new blank and follow again the procedure described by steps Turn on the shutter and replace the new blank with sample 3B. 9. Start an absorption measurement to record the spectrum under the same conditions. Save the absorption spectrum of the sample 3B as a processed spectrum in OOI Binary Format but also in Text file Format on the disk. Compare the spectra of the samples 3A and 3B. How do they differ? (Golden Rule 5) 10. Replace sample 3B with sample 3C and record the spectrum under the same conditions. Save the absorption spectrum of the sample 3C as a processed spectrum in OOI Binary Format but also in Text file Format on the disk. Compare then the spectra of the samples 3B and 3C. How do they differ? (Golden Rule 5) How can be explained this redshift? 11. Apply the same procedure for the sample 3D. Save the absorption spectrum of the sample 3D as a processed spectrum in OOI Binary Format but also in Text file Format on the disk. Compare its spectrum with that of the sample 3B. How do they differ? (Golden Rule 6). How can be explained this blueshift? 12. Plot the spectra of the samples 3A-3D in the same graph and illustrate by colored graph traces and arrows the effect of Golden Rules 5 and 6. Which is the chromophore responsible for the absorption peaks? Assign the observed peaks to the subsequent electronic transitions. Processing Mode Processing mode function includes all of the modes necessary to conduct experiments, in our case Scope, Absorbance and Transmission modes. Select Processing Processing Mode Scope/Absorbance/Transmission to switch the current spectral window into desired mode. These modes are also available from the graph s toolbar by click on the corresponding icons: S, A and T. Scope The signal graphed in Scope mode is the raw voltage coming out of the A/D converter. The spectral view mode provides complete control of signal processing functions before taking absorbance, transmission and other measurements. This mode reflects the intensity of the light source, the response of the detector, and the spectral characteristics of the sample. Use Scope mode when configuring the setup, adjusting the integration time, and taking a reference and a dark scan. A Dark spectrum is a spectrum taken in the absence of light with the light path blocked. Select File/Store/Store Dark Spectrum to store a dark spectrum or click the specific icon in the graph toolbar, then save the spectrum as Dark Spectrum. 5

6 A Reference spectrum is a spectrum taken with the light source on and a blank in the sampling region (or there is no sample present). Select File/Store/Store Reference Spectrum to store a reference spectrum or click the specific icon in the graph toolbar, then save the spectrum as Reference Spectrum. SpectraSuite application Tools related to the data being acquired and displayed in SpectraSuite: Integration Time (IT) tool that specifies the integration time of the spectrophotometer, which is analogue to the shutter speed of a camera. The higher the integration time, the longer the detector monitors the incoming photons. The Scope mode intensity is can be adjusted by increasing the IT value if the signal is too low or decreasing the IT value if the signal is too high. The integration time has to be adjusted so that the greatest amount of light that is anticipated for the application causes a signal of about 85% of the spectrometer s capability counts for a total counts in our case. The integration time specified controls enabled spectrometer channels in the active spectral window. Scan to average tool that specifies the number of discrete spectral acquisitions that the device driver accumulates before SpectraSuite receives a spectrum. The higher the value, the better the signal-to-noise ratio (S:N). The S:N will improve by the square root of the number of scans averaged. Boxcar width tool that sets the boxcar smoothing width, a technique that averages across spectral data. This technique averages a group of adjacent detector elements. A value of 5, for example, averages each data point with 5 points to its left and 5 points to its right. The greatest this value, the smoother the data and the higher the signal-to-noise ratio. If the value entered is too high, a loss in spectral resolution will result. The S:N will improve by the square root of the number of pixels averaged. NOTE!!! For each measurement, you must first take (store&save) a reference and dark spectrum in Scope mode before the experiment mode icon (A, T) on the toolbar becomes active. After you take a reference and a dark spectrum, you can take as many measurement scans as you needed. However, if you change any sampling variable (integration time, averaging, smoothing, fiber size, etc.), you must store a new reference and a dark spectrum. Absorbance Experiments Absorbance spectra are a measure of how much light a sample absorbs. SpectraSuite software calculates absorbance (A λ) using the following equation: where: S λ = Sample intensity at wavelength λ D λ = Dark intensity at wavelength λ R λ = Reference intensity at wavelength λ. A λ = log 10 ( S λ D λ R λ D λ ), The concentration of a specie in solution directly affects the absorbance of the solutions. For most samples, absorbance relates linearly to the concentration of the substance. This relationship, known as Lambert- Beer s law, is expressed as: A λ = ε λ cl, where: A λ = Absorbance at wavelength λ ε λ = Extinction coefficient of the absorbing species at wavelength λ c = Concentration of the absorbing species l = Optical path length of the absorption. Typical setup: 6

7 The light source (far right) sends light via an input fiber into a cuvette in a cuvette holder (bottom center). The light interacts with the sample. The output fiber carries light from the sample to the spectrometer (top center) connected to the computer (far left). Transmission Experiments Transmission spectra are a measure of how much light a sample transmits. Transmission is the percentage of energy passing through a sample relative to the amount that passes through the reference. Transmission mode can also display the portion of light reflected from a sample, since transmission and reflection measurements use the same mathematical calculations. It is expressed as a percentage (%T λ) relative to a standard substance (such as air or a glass cover plate). SpectraSuite software calculates transmission (%T λ) using the following equation: where: S λ = Sample intensity at wavelength λ D λ = Dark intensity at wavelength λ R λ = Reference intensity at wavelength λ. %T λ = S λ D λ R λ D λ 100%, Common transmission applications include measuring light through solutions, optical filters, optical coatings and other optical elements (such as lenses and fibers). Typical setup for absorbance/transmission measurements: The light source (LS, far right) sends light via an input fiber(iof) into a cuvette (film) in a cuvette(film) holder (SH, bottom center). The light interacts with the sample. The output fiber(oof) carries light from the sample to the spectrometer (S, top center) connected to the computer (PC, far left). Deuterium lamp spectrum 7

8 Tungsten lamp spectrum 3A Sample (Acetone) spectrum 1D (Rh6G) absorption spectrum 8

9 Acidified (3-benzophenone) in ethanol spectrum um Transmission spectra of samples 1D (Rh6G). 9