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Selecting an Excitation Wavelength for Raman Spectroscopy

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Selecting an Excitation Wavelength for Raman Spectroscopy Electronically reprinted from March 2016 ® Molecular Spectroscopy Workbench Selecting an Excitation Wavelength for Raman Spectroscopy Were it not for the problem of photoluminescence, only one laser excitation wavelength would be neces- sary to perform Raman spectroscopy. Here, we examine the problem of photoluminescence from the material being analyzed and the substrate on which it is supported. We describe how to select an excita- tion wavelength that does not generate photoluminescence, reduces the noise level, and yields a Raman spectrum with a superior signal-to-noise ratio. Furthermore, we discuss the phenomenon of resonance Raman spectroscopy and the effect that laser excitation wavelength has on the Raman spectrum. David Tuschel ne of the most frequent questions that I hear from Another consideration when selecting an excitation people new to Raman spectroscopy is, “What laser wavelength can be the variation of the optical density of Oexcitation wavelength do I need?” Of course, the the material as a function of wavelength. If the material answer to that question is that it depends entirely upon the is transparent, then the depth of focus and focal volume materials one wishes to analyze. The Raman scattering of the laser beam will be dictated by the numerical aper- cross-section of the material is important and so too are its ture of the lens, the wavelength of the laser light, and the physical and optical properties. For example, if the sample real component of the sample’s refractive index at that is transparent to the excitation wavelength and thin enough, wavelength. However, if the sample is not transparent one can expect a spectral contribution from the substrate on (that is, the imaginary component of the refractive index which the sample is mounted or positioned. And that spec- of the sample is nonzero), then the depth of light penetra- tral contribution can be either Raman scattering or photolu- tion will be dictated not by the physical optics but by the minescence. absorptivity of the sample at that wavelength. These cir- Let’s work through some of the considerations relevant to cumstances have allowed many spectroscopists to perform choosing a laser excitation wavelength for Raman spectros- depth profiling of materials such as semiconductors by copy. To begin with, one should be aware that the Raman changing the excitation wavelength. In general, the longer scattering strength is proportional to the fourth power of the the excitation wavelength the deeper into the sample the 4 excitation frequency, νexc. Consequently, one can expect to light penetrates. The variation of depth penetration in obtain a much stronger Raman signal from a given sample semiconductors afforded by the range of commercially when using a higher excitation frequency. The frequency available visible wavelength lasers conveniently matches of the light is inversely proportional to the wavelength, and the depths to which certain microelectronic devices have so all other things being equal, the shorter excitation wave- been fabricated. The variation of depth penetration in the length will yield a stronger Raman signal. That is one of the visible region has allowed Raman spectroscopists to per- reasons why, when given a choice, Raman spectroscopists form depth profiles in ion implanted Si merely by chang- prefer shorter excitation wavelengths. ing the excitation wavelength (1–4). cal microscopes. The need to analyze samples whose chemical composition or solid state structure varies on a mi- crometer spatial scale is driving the use of micro-Raman spectroscopy. Here, spatial resolution of the measurement y is important and one needs a laser spot size commensurate with the spatially varying structure to be analyzed. Con- Intensit sequently, when selecting an excitation wavelength you should know that the size of the focused laser beam is dif- fraction limited and dependent upon the laser wavelength. The Airy disk di- 500 1000 1500 2000 2500 3000 3500 ameter (the ideal laser spot size [D ]) -1 Airy Raman shift (cm ) and spatial resolution (ρ) for the micro- Raman configuration are given by the Figure 1: Raman spectra of commercial polystyrene pipe obtained using excitation wavelengths following expressions: of 532 nm (red spectrum), 638 nm (blue spectrum), and 785 nm (black spectrum). DAiry = 1.22 λ/NA [1] ρ = 0.61 λ/NA [2] where λ is the wavelength of light and NA is the numerical aperture of the microscope objective. Therefore, the y choice of excitation wavelength directly affects the spatial resolution of micro- Raman measurements. For example, Intensit the diffraction limited spatial resolu- tions for excitation at 532 nm and 785 nm are 360 nm and 530 nm, respec- tively. Of course, these values represent the ideal and actual spatial resolution 550600 650700 750800 850900 95010001050 will depend on the quality and align- Wavelength (nm) ment of your optics. Figure 2: Raman spectra of commercial polystyrene pipe obtained using excitation wavelengths The Photoluminescent of 532 nm (red spectrum), 638 nm (blue spectrum), and 785 nm (black spectrum). Background Anyone having had any experience In some structures, it is essential signal from the very thin strained Si. with Raman spectroscopy will tell to control the depth of penetration to Consequently, the strained Si Raman you that fluorescence is the nemesis constrain the analysis to a thin film at scattering is buried in the substrate of Raman spectroscopists. Even if the the surface. This is particularly true signal at 520.7 cm-1. To resolve the primary substance in the sample does when analyzing strained Si. A common strained Si signal from that of the sub- not itself emit, even trace impurities structure is to have a thin strained Si strate Si one needs to limit the depth can cause enough photoluminescence layer grown on a SiGe layer that is on a of penetration of the laser light. There- to overwhelm the Raman signal. The strain-free Si substrate. If the excitation fore, most analyses of thin strained principal reason for this problem is wavelength is too long, the laser light Si structures built on a Si substrate that emission is a one photon process will penetrate through the strained Si require excitations wavelengths in the whereas Raman scattering is a two and SiGe to the strain-free Si substrate. violet region or shorter. photon process; that is, photolumines- The deeper the penetration of the laser Lateral spatial resolution is an- cence has a much higher probability light, the greater the fractional contri- other consideration when selecting of occurring than does Raman scat- bution of the substrate Si to the overall an excitation wavelength. Increas- tering. Related to that fact, you have Raman signal and spectrum. The re- ingly, Raman spectroscopy is being perhaps heard the oft given explana- sult is that the much stronger substrate done on the micrometer scale using tion of the weak Raman effect and how Si signal overwhelms the much weaker Raman spectrometers coupled to opti- in general only one Raman photon is generated for every 106 to 109 photons incident upon the sis of minority components would seem reasonable. There is sample. Therefore, the presence of a fluorophore with even a bit of craft involved in photobleaching and only laboratory extremely low quantum efficiency can produce an emission experience will teach you the right conditions (laser power that overwhelms the Raman signal. density, duration of laser exposure) for successfully photo- You may be asking yourself why a simple background bleaching a polymer or other sample to remove background subtraction of the photoluminescent component wouldn’t photoluminescence without altering or completely degrad- be sufficient to reveal the remaining Raman spectrum if the ing the material of interest. One must apply photobleaching Raman and photoluminescent signals are superimposed. with great care to be sure that the Raman spectrum that you The problem is that the background photoluminescence acquire is that of the original polymer or bulk material and can be so great that the noise generated by this signal is on not that of a laser-induced photolysis product. the order of or even greater than the Raman signal alone. Photobleaching may not always be a reasonable or accept- Consequently, software treatment of the data or any other able procedure for acquiring a Raman spectrum. There are experimental mechanism that does not eliminate the pho- times when even continued laser illumination at low power toluminescent background from the raw signal generally density over even tens of minutes will not remove the photo- does not produce results as good as those for which no luminescent background. If that is the case, you need to try a photoluminescent background is present. To avoid the pho- different excitation wavelength. The thinking behind select- toluminescence background and the noise that it produces ing a different excitation wavelength is to use one for which one should identify an excitation wavelength that does not the absorptivity is much lower and thereby yields much less induce photoluminescence in the sample either from the photoluminescence. Think of this as trying to move your principal component or even trace impurities. That is why Raman laser source out of the absorption spectrum or ex- old school Raman spectroscopists always want as many laser citation profile that is the source of the photoluminescence wavelengths as possible available to them when working of the sample. The default selection is to a longer excitation with a variety of materials. wavelength. The assumption is that there will be less absorp- Many samples that appear transparent will neverthe- tion if any at all and therefore generate weaker or no photo- less yield a photoluminescent background, sometimes so luminescence. strong that nothing but photoluminescence is observed in A sample of commercially available polystyrene pipe the Raman spectrum. One finds that this is often the case demonstrates this concept fairly effectively.
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