Electronically reprinted from March 2016

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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 is important and one needs a laser spot size commensurate with the spatially varying structure to be analyzed. Con- Intensit y 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 choice of excitation wavelength directly affects the spatial resolution of micro- Raman measurements. For example, Intensit y 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. Spectra of this with commercial polymers, even those that are colorless sample were acquired using 532-, 638-, and 785-nm excita- and transparent. The polymers themselves are very often tion and are shown in Figure 1. Clearly, the polystyrene transparent with absorption because of electronic transi- Raman bands can be observed in all of the spectra and the tions generally occurring in the ultraviolet region of the photoluminescence does not preclude one from obtaining a spectrum. Polymers that are colored often appear that way good Raman spectrum using any of these excitation wave- because of dyes or colorants added to the polymer in the lengths. However, you can see that 532-nm excitation yields manufacturing process. One might expect the colorless and the greatest photoluminescent background and accompany- transparent commercial polymers to yield a photolumines- ing noise. Close examination of the spectra reveals greater cence free Raman spectrum without any significant back- noise in the Raman component of the 532-nm excited spec- ground. However, that is very often not the case. You may trum than is observed when using either 638-nm or 785-nm try to obtain a Raman spectrum using 532-nm excitation excitation. The spectral noise demonstrates the essential of a colorless, transparent plastic bottle that had contained problem associated with the acquisition of a Raman spec- a beverage or other commercial product and you will very trum over a wavelength region where photoluminescence likely generate a strong photoluminescence in the region also occurs. If the emission is too strong, the Raman bands where you expect to detect Raman scattering. Moreover, you may be lost in the noise and simply not observed in the pho- may find while observing the spectrum in real time display toluminescence spectrum. that the photoluminescent background diminishes over We stated earlier that the traditional approach to avoid- time with continued illumination. This familiar phenom- ing photoluminescence once it has been detected with one enon is termed photobleaching and has been used extensively excitation wavelength is to change to another at a longer in the past decades by Raman spectroscopists to obtain a wavelength. The reasoning behind that protocol is based Raman spectrum with a good signal-to-noise ratio, far bet- on the premise that one may be moving out of an absorp- ter than the one that existed upon initial illumination. The tion band or excitation manifold of the material. Of course, justification for using photobleaching is the premise that the that does not always work because by changing to a longer photoluminescence is not from the polymer itself but from excitation wavelength one may “move out” of one excita- additives or minority components that are still present from tion manifold only to “move in” to another from perhaps the polymerization process or were added for other reasons the same or a different chemical compound in the sample. such as stabilization or brightening. If the Raman spectrum The move to longer excitation wavelength is effective with of the bulk polymer remains essentially the same over the our sample of polystyrene pipe. You see that using 638-nm time of the photobleaching and the measurement, then the excitation has moved our Raman spectrum into the long assumption that the photobleaching is the result of photoly- wavelength tail of the photoluminescence. Consequently, photoluminescence or Raman scat- tering from the glass will be detected and the proper choice of alternative substrate can be just as critical as that of the excitation wavelength because of the spectroscopic properties of the Raman scattering glass microscope slide. Photoluminescence In the case of the polystyrene

Intensit y pipe, we saw the advantage of using 785-nm laser excitation to eliminate Photoluminescence the photoluminescence generated when using 532-nm light. There are many such samples, particularly in the biosciences, for which 785-nm 500 1000 1500 2000 2500 3000 3500 excitation produces the least amount Raman shift (cm-1) of photoluminescence. However, we must remember that the laser light Figure 3: Raman spectra of glass microscope slide obtained using excitation wavelengths of 532 will penetrate transparent or translu- nm (red spectrum), 638 nm (blue spectrum), and 785 nm (black spectrum). cent samples, particularly if they are thin, and thereby potentially generate the photoluminescence is much little or no photoluminescence, and Raman scattering and photolumines- weaker, there is less of a background, thereby reduces the detector noise as- cence from the underlying substrate. and the signal-to-noise of the Raman sociated with it, will always produce This is where the spectroscopic proper- component of the spectrum is much superior results to methods involving ties of the glass slide become impor- better. Finally, using 785-nm excita- spectral subtraction of the photolu- tant. Spectra of a glass microscope slide tion generates no discernible pho- minescent component or other data acquired using 532-, 638-, and 785-nm toluminescence. Consequently, the treatments that still retain the high excitation are shown in Figure 3. The Raman spectrum has no background background detector noise of the spectrum acquired using 532-nm exci- and a very good signal-to-noise ratio. original spectral measurements. tation consists of broad Raman bands

I stated above that the selection of from SiO2 with minimal photolumi- a longer wavelength was intended to Raman Scattering and nescence beyond approximately 3200 “move out” of an excitation manifold. Photoluminescence from Glass cm-1. A spectrum from the same glass Actually, one might say that there are The most commonly used substrate slide and acquired using 638-nm exci- two moves occurring when chang- when performing micro-Raman spec- tation consists of very strong photolu- ing to a longer excitation wavelength. troscopy is the glass microscope slide. minescence throughout and peaking There is the move of the laser excita- The chemical composition and optical at approximately 3000 cm-1. A sense tion wavelength out of the photolu- spectroscopic properties of the glass of how strong the photoluminescence minescence excitation manifold and slide are of course not relevant when it is can be ascertained by the weakness the move of the Raman scattering to functions merely as a support to hold of the Raman bands relative to that of the photoluminescence “wavelength a sample in position that is opaque. the photoluminescence. Finally, a spec- tail” or even entirely out of the spec- However, its spectroscopic properties trum of the glass slide acquired using tral region of the emission. Figure 2 are important if the sample is transpar- 785-nm excitation consists of a very shows the same spectra from Figure ent to the excitation wavelength. If the strong and broad photoluminescence 1 now plotted on an absolute wave- transparent sample is thick enough peaking at approximately 1400 cm-1 length scale. Here, you can clearly see such that the glass slide is completely and completely obscuring the finger- how the Raman spectrum moves to out of focus, the spectral contribution print region of the Raman spectrum. the photoluminescence wavelength from the glass slide may be minimal. Here again, the Raman bands of glass tail when using 638-nm excitation That is because the laser power density can barely be detected above the strong and then completely out of the emis- at the glass slide, which is well below photoluminescence generated by the sion region when using 785-nm the focal plane, will be low. Further- illumination with 785-nm light. In this excitation. The take-home message more, the microscope objective will trend, we encounter the exact opposite here is that by selecting an excitation collect few photons that originate out wavelength dependent response that wavelength that generates little or of focus depending upon the physical we observed for the commercial poly- no photoluminescence the signal-to- optics and light transmission of the styrene pipe; the photoluminescence noise of the Raman spectrum can be sample itself. However, when analyzing from glass varies and grows stronger greatly improved. The selection of an thin transparent samples such as poly- with increasing excitation wavelength! excitation wavelength that generates mer films or many biological samples, Of course, we want to choose the excitation wavelength based upon the best response from the sample and not from the substrate. Clearly, then, we want to find an alternative to the glass microscope slide. If a transpar- ent substrate is needed, commercially available fused quartz substrates meet that need very nicely. Raman spectra of

fused quartz obtained using 532-, 638-, Intensit y and 785-nm excitation are shown in Figure 4. All three spectra appear the same consisting of strong Raman scat- tering only at Raman shifts less than 500 cm-1 and there is no photolumines- 500 1000 1500 2000 2500 3000 3500 cence. Therefore, if a transparent sub- Raman shift (cm-1) strate is needed, fused quartz should be chosen instead of the glass microscope Figure 4: Raman spectra of fused quartz obtained using excitation wavelengths of 532 nm (red slide for micro-Raman analyses of spectrum), 638 nm (blue spectrum), and 785 nm (black spectrum). biological samples, thin polymer films, or any other transparent sample whose Raman scattering is weak relative to larly when comparing them to those help the reader note the differences in that of glass. Don’t forget that the glass acquired with other excitation wave- relative intensities of the Raman bands yields significant Raman scattering lengths or Raman spectra of the same for the various excitation wavelengths. out to approximately 1200 cm-1, so the material published in the literature. Note how different the overall spectra fused quartz may still offer some ad- Pentacene is a dark colored organic appear dependent upon the excitation vantages over a glass microscope slide compound showing great promise for wavelength. Nevertheless, careful ex- even when using 532-nm excitation. its use in molecular electronic devices, amination of the spectra reveals that And certainly fused quartz is the only particularly organic field effect transis- the same Raman bands are present in sensible choice when a transparent tors. Attempts to understand the con- all of the spectra. What varies with substrate is needed and one is using duction mechanism and the role that excitation wavelength are the relative 785-nm excitation. chemical bonding and crystal structure intensities of the Raman bands. As we play in that process can be assisted by said earlier, those vibrational modes Resonance Raman the use of resonance Raman spectros- coupled to the electronic transition Spectroscopy of Pentacene copy. In particular, Franck-Condon will yield enhanced Raman scattering. Resonance Raman spectroscopy can processes can be probed through the Therefore, the correct identification provide significantly enhanced sig- resonance Raman effect (5). Absorp- of the vibrational modes and assign- nals over normal Raman scattering tion in pentacene occurs broadly be- ment of the Raman bands that undergo from the same sample. The resonance tween 500 and 725 nm with maxima enhancement can provide insight enhancement is achieved when using at approximately 550, 575, 625, and 675 into the chemical bonds involved in an excitation wavelength that is in nm (6). Consequently, one can couple an electronic transition at a specific resonance with an electronic transition in to these electronic transitions to wavelength. Much can be said about of the compound. Those vibrational generate resonance-enhanced Raman the chemical bonding and photophys- modes that are coupled to this elec- spectra using the appropriate excita- ics of pentacene, as revealed in Figure tronic transition will be resonantly tion wavelengths. 5. However, that detail is beyond the enhanced and their Raman bands We have probed individual grains scope of this publication. The impor- will appear much stronger relative to of pentacene obtaining micro-Raman tant lesson here is to recognize and un- those bands of vibrational modes not spectra from single locations on a given derstand the importance of excitation coupled to the electronic transition. grain using different excitation wave- wavelength when the potential exists Consequently, Raman spectra obtained lengths at each location. The variation for resonance Raman effects. from one compound and even a single in the Raman spectra from a single sample can appear different depending location acquired using different exci- Resonance Raman upon whether the laser excitation is in tation wavelengths reveals the coupling Spectroscopy of 2D Crystals resonance with an electronic transi- between the phonons and electronic Two-dimensional (2D) crystals con- tion of the material. This is a very im- transitions. Raman spectra acquired stitute another class of materials for portant point. Raman spectroscopists using 473-, 532-, 633-, and 785-nm which the selection of excitation wave- must be aware of the potential for reso- excitation are shown in Figure 5. Some length is important. Two-dimensional nance effects in their spectra particu- of the band positions are labeled to crystals are not just small portions of first-order Raman bands listed above

are for bulk MoS2. However, these bands shift to different energies pro-

λex: 473 nm gressively as the structure changes from approximately six layers down 1373 1598 2 1177 to a single monolayer. The E 2g band 1159 λex: 532 nm has been observed to shift progres- sively from 32 cm-1 in the bulk to 23 -1

Intensit y cm in the single trilayer (17). The λ : 633 nm ex term trilayer derives from the fact that

266 MoS2 is not a planar compound, that 446 501 753 996 is, the Mo and S atoms are not all in

λex: 785 nm the same plane. So, a plane of S atoms can be envisioned on the surface with 200 400 600 800 1000 1200 1400 1600 1800 Raman shift (cm-1) a plane of Mo atoms above that plane and another plane of S atoms above Figure 5: Raman spectra acquired from a single location on an individual grain of pentacene the Mo one, hence the term trilayer. using 473-, 532-, 633-, and 785-nm excitation. Also, the band positions and respec- 1 tive separations of the E 2g and A1g the bulk materials carrying the same eV, but it also manifests fine structure bands show progressive changes from 1 properties as those of the three dimen- with narrow absorption peaks at 1.9 eV the bulk to a single layer. The E 2g (383 sional kind. Rather, their physical, elec- (653 nm) and 2.1 eV (590 nm) related cm-1) band shifts to higher wavenum- -1 tronic, and spectral characteristics can to d-to-d orbital transitions split by bers and the A1g (408 cm ) band shifts be significantly different. Specifically, spin-orbit coupling and designated A1 to lower wavenumbers in progressing

MoS2 in its bulk form is an indirect and B1 excitons, respectively (9,16). from the bulk to a single trilayer cor- bandgap semiconductor, whereas in Consequently, one can observe an ex- responding to a band separation of 25 its monolayer to few-layer forms it be- citation wavelength dependence of the cm-1 to 19 cm-1, respectively (18). comes a direct bandgap semiconductor first-order Raman band intensities as The changes of Raman band posi- (7). Splendiani and coworkers (8) have well as the appearance of Raman bands tion with the number of molecular studied exfoliated MoS2 monolayer and assigned to harmonic and combination layers described above are most read- few-layer flakes by photoluminescence modes when the laser excitation is of a ily observed when using 532-nm or and have observed direct excitonic wavelength that couples into these ex- shorter wavelength excitation. The transitions. Exciting with 532-nm laser citonic transitions (9,13,15,16). Raman spectra obtained when using light, they observed broad photolumi- Just as the monolayer to few-layer 632.8-nm light to excite few-layer nescence centered at 627 and 677 nm, 2D crystal structure of MoS2 affects MoS2 appear quite different because of whereas these emissions are completely the electronic structure, particularly coupling with the aforementioned A1 absent when illuminating bulk MoS2. the nature of the bandgap, the vibra- excitonic transition and the subsequent The room temperature bandgap of tional modes are also affected. The resonance enhancement. The activ- bulk MoS2 is approximately 1.7 eV (corresponding to 730 nm) with com- plex excitonic features between 1.7 and 2.5 eV (9). 462 The crystalline structure of 2H- Si MoS2 belongs to the D6h crystal class, 417 and factor group analysis predicts one 642 λ : 633 nm 178 383 ex A1g, one E1g, and two E2g Raman active modes (9–15). The symmetry assign- 598 ments and corresponding Raman band Intensit y positions for bulk hexagonal MoS2 are: 408 -1 1 -1 383 E1g (286 cm ), E 2g (383 cm ), A1g (408 λ : 532 nm -1 2 -1 ex cm ), and E 2g (32 cm ). Furthermore, Si it is important to remember that some visible wavelengths of the laser light 100 200 300 400 500 600 700 800 900 used to excite Raman scattering cor- Raman shift (cm-1) respond to energies of MoS2 electronic transitions. The absorption spectrum Figure 6: Raman spectra of few-layer MoS2 flake taken at the same location with different of MoS2 reflects the band gap of 1.7 excitation wavelengths. The band at 520 cm-1 corresponds to the substrate Si. ity of the harmonic and combination Raman signal itself. The problem is that (9) A.M. Stacy and D.T. Hodul, J. Phys. modes yields a richer spectrum and the the background photoluminescence Chem. Solids 46, 405–409 (1985). determination of the number of trilay- can be so great that the noise gener- (10) J.R. Ferraro, Appl. Spec. 29, 418–421 ers present is determined from both ated by this signal is on the order of or (1975). changes in band shape and relative even greater than the Raman signal (11) S. Jimenez Sandoval, D. Yang, R.F. intensities (19,20). alone, thus obscuring the Raman com- Frindt, and J.C. Irwin, Phys. Rev. B 44, 3955–3962 (1991). Exfoliated few-layer flakes of MoS2 ponent. The selection of an excitation are typically heterogeneous, and so the wavelength that generates little or no (12) T.J. Wieting and J.L. Verble, Phys. Raman and photoluminescence spectra photoluminescence, and thereby re- Rev. B 3, 4286–4292 (1971). tend to spatially vary on the flake. Nev- duces the detector noise associated with (13) J.M. Chen and C.S. Wang, Solid State ertheless, there are spectra that can be it, will always produce superior results. Commun. 14, 857–860 (1974). considered representative of few-layer Resonance Raman spectroscopy can (14) S. Sugai and T. Ueda, Phys. Rev. B 26, 6554–6558 (1982). MoS2 and they are shown in Figure 6. provide significantly enhanced signals These spectra were acquired at the same over normal Raman scattering from the (15) D.O. Dumcenco, K.Y. Chen, Y.P. Wang, Y.S. Huang, and K.K. Tiong, J. location from the interior of a MoS2 same sample. Raman scattering from flake, but with different excitation wave- those vibrational modes coupled to the Alloys Compounds 506, 940–943 lengths. That spectrum acquired with electronic transition is enhanced when (2010). 532-nm excitation is typical of those using the appropriate excitation wave- (16) B.C. Windom, W.G. Sawyer, and reported in the literature; it consists pri- length. Consequently, Raman spectra D.W. Hahn, Tribol. Lett. 42, 301–310 1 -1 (2011). marily of the E 2g band at 383 cm and obtained from one compound and even -1 (17) Y. Zhao, X. Luo, H. Li, J. Zhang, P.T. the A1g band at 408 cm as well as the a single sample can appear different Si substrate Raman band at 520 cm-1. In depending upon whether the laser exci- Araujo, C.K. Gan, J. Wu, H. Zhang, contrast, the spectrum generated with tation is in resonance with an electronic S.Y. Quek, M.S. Dresselhaus, and Q. 632.8-nm excitation is striking insofar transition of the material. Xiong, Nano Lett. 13, 1007–1015 as it consists of the same first order (2013). bands and those due to harmonics and References (18) C. Lee, H. Yan L.E. Brus, T.F. Heinz, combination modes. Remember that the (1) P.X. Zhang, I.V. Mitchell, P.J. Schultz, J. Hone, and S. Ryu, ACS Nano 4, and D.J. Lockwood, J. Raman Spec. 2695–2700 (2010). absorption spectrum of MoS2 contains fine structure with narrow absorption 25, 515–520 (1994). (19) K. Golasa, M. Grzeszczyk, R. Bozek, peaks at 1.9 eV (653 nm) and 2.1 eV (590 (2) A.C. deWilton, M. Simard-Norman- P. Leszczynski, A. Wysmolek, M. Po- nm) related to d-to-d orbital transitions din, and P.T.T. Wong, J. Electrochem. temski, and A. Babinski, Solid State split by spin-orbit coupling and desig- Soc. 133, 988–993 (1986). Commun. 197, 53–56 (2014). nated A1 and B1 excitons, respectively (3) J.P. Lavine and D.D. Tuschel, Mat. (20) B. Chakrabory, H.S.S. Ramakrishna (9,16). Consequently, the 632.8-nm Res. Soc. Symp. Proc. 588, 149–154 Matte, A.K. Sood, and C.N.R. Rao, J. excitation couples into the A1 transition (2000). Raman Spectrosc. 44, 92–96 (2013). producing a resonance enhancement of (4) D.D. Tuschel and J.P. Lavine, Mat. all of the additional modes observed in Res. Soc. Symp. Proc. 438, 143–148 Figure 6. All of the bands in the spec- (1997). David Tuschel is a trum excited with 632.8-nm light can be (5) R. He, N.G. Tassi, G.B. Blanchet, and Raman applications man- accounted for and have previously been A. Pinczuk, Phys. Rev. B 83, 115452 ager at Horiba Scientific, assigned (9,13,16). (2011). in Edison, New Jersey, (6) A. Rao, M.W.B. Wilson, S. Albert-Sei- where he works with Conclusions fried, R. Di Pietro, and R.H. Friend, Fran Adar. David is shar- The selection of excitation wavelength Phys. Rev. B 84, 195411 (2011). ing authorship of this for Raman spectroscopy has been ex- (7) K.F. Mak, C. Lee, J. Hone, J. Shan, column with Fran. He can be reached at: amined within the context of photolu- and T.F. Heinz, Phys. Rev. Lett. 105, [email protected] minescence that obscures the Raman 136805 (2010). bands and resonance enhancement of (8) A. Splendiani, L. Sun, Y. Zhang, T. Raman signal strength. Photolumi- Li, J. Kim, C.-Y. Chim, G. Galli, and For more information on nescence from either the sample or the F. Wang, Nano Lett. 10, 1271–1275 this topic, please visit: substrate produces detector noise on (2010). www.spectroscopyonline.com the order of or even greater than the

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