A Raman-atomic force microscope for apertureless-near-field spectroscopy and optical trapping

Transcription

1 REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 73, NUMBER 3 MARCH 2002 A Raman-atomic force microscope for apertureless-near-field spectroscopy and optical trapping Mark S. Anderson a) and William T. Pike Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, M/S , Pasadena, California Received 3 August 2001; accepted for publication 3 December 2001 An instrument that combines the analytical power of Raman spectroscopy with the spatial resolution of the Atomic Force Microscope AFM is presented. This instrument is capable of resolving 50 nm scale spectral features or better by using surface enhanced Raman scattering at the AFM tip. The localized spectrochemical information allows the interpretation of the concurrently acquired friction or phase contrast AFM images. This instrument has a unique combination of features including side illumination of the tip sample interface that permits opaque samples. As a result of precise focusing of a laser at the AFM tip sample interface this instrument is also capable of laser beam profiling and studying optical trapping at the probe tip. Applications of this versatile instrument include chemical analysis of nanometer scale phenomena, chemical separation, and the potential for targeted single molecule spectroscopy American Institute of Physics. DOI: / I. INTRODUCTION The revolutionary development of the scanning tunneling microscope 1 has given rise to an entire family of scanning probe microscopes SPM. These atomic resolution instruments are differentiated by how the probe tip interacts with the scanned surface. 2 The atomic force microscope AFM is the most utilized member of the SPM family. 3 6 With an AFM, a tip mounted on a microfabricated cantilever is scanned over the substrate and the interaction between the tip and the substrate is detected by monitoring the deflection of the cantilever. In addition to high-resolution topography, a variety of other signals are available from the AFM tip. These include local friction, phase shift contrast of the oscillating tip, magnetic force, and chemical force. There is increasing effort to acquire more chemical information in conjunction with SPMs atomic-resolution topographic images. 7,8 This is driven by a broad technological need to chemically analyze nanometer scale phenomena and devices. Molecular spectroscopy at the SPM probe tip provides detailed chemical information and is currently being developed. Such efforts are faced with the challenge of coupling incident optical radiation into an area far below the radiation s wavelength. For example, research groups working with near-field scanning optical microscopy NSOM are moving in a variety of directions to channel light to the sample, either through subwavelength apertures or solid immersion lenses Others are working to combine the AFM with photothermal detection using infrared spectroscopy A promising approach to add the desired spectrochemical information is to exploit the enhancement of the electric field in proximity to an illuminated STM tip. 15,16 This apertureless near-field technique has been further motivated by developments in surface enhanced Raman scattering SERS a Author to whom correspondence should be addressed; electronic mail: mark.s.anderson@jpl.nasa.gov spectroscopy. The SERS effect has been used to provide enhancement of the Raman signal in proximity to small noble metal particles on substrates. 17,18 SERS amplifies the normally weak Raman signal up to and has led to Raman spectroscopy on single molecules. 19,20 Recently it has also been demonstrated that an AFM tip, suitably coated with metal grains, can locally enhance the Raman spectra in proximity to the AFM tip The large enhancements and short range of the SERS effect provides the potential for high spatial resolution and increased sensitivity of Raman spectroscopy. This feasibility work has led us to build a fully integrated Raman-atomic force microscope RAFM system. The instrument described here combines a multimode AFM and a near infrared Raman microprobe. In the parlance of analytical instruments this is a hyphenated technique, a Raman-AFM or RAFM. The resulting versatile instrument provides a unique multifunctional platform that provides: a simultaneous AFM imaging and Raman spectroscopy at the same location; b targeted SERS with better than 50 nanometer nm spatial resolution of Raman spectral features; c side illumination permitting opaque samples; d lateral force frictional or phase contrast AFM imaging; e the ability to profile the Raman laser beam using photoinduced deformation of the AFM cantilever; and f the use of the illuminated AFM tip for optical trapping. The last two functions were not originally part of the instrument design but are a consequence of the instruments ability to precisely focus a laser at an AFM tip sample interface. II. PRINCIPLE OF OPERATION AND THEORY This instrument development started with the conjecture that the AFM tip could be used to place SERS-active silver grains on a surface for Raman enhancement at a very localized target area. 21 Figure 1 illustrates the basic concept. The development of a RAFM depends on harnessing the complex interaction between the incident radiation, the AFM tip, and /2002/73(3)/1198/6/$ American Institute of Physics

2 Rev. Sci. Instrum., Vol. 73, No. 3, March 2002 Raman-AFM 1199 FIG. 1. The Raman-atomic force microscope RAFM instrument provides high-resolution AFM image and targets the local enhancement of a Raman signal at the special AFM tip. Areas of interest revealed by the extraordinary magnification of the AFM tip can then be analyzed for chemical identification. This development opens the possibility of targeting and obtaining the spectrum of a single molecule. the underlying sample surface. A general theoretical approach would consider the field enhancement for the laser illuminated tip/surface geometry with arbitrary dielectric properties. 26 This requires the solution to the self-consistent electrodynamics problem. There are analytical solutions for simple tip geometry and numerical methods have been proposed and applied. 27 To summarize the results from these theoretical considerations, there are two enhancement effects due to the metallization of the tip. There is a purely geometric effect that is a nonresonant contribution from the enhancement from electric field near the metal surface that is best with the high aspect particles aligned to the incident electric field. The second enhancement effect is due to the resonant excitation of local surface plasmons in the tip, an effect that would give rise to infinite gain the absence of plasmon damping. The SERS effect has been explained as having both resonant and nonresonant field enhancement due to the metallization of the tip as well as an additional chemical enhancement that helps explain the extraordinary amplification of the Raman signals. It has been postulated that the chemical enhancement occurs when the optical excitation induces a charge transfer from the metal particles into an unoccupied orbital of the analyzed molecules, or from the occupied orbital of the molecule into the metallic conduction band. 28 Experimentally, SERS is usually measured on a dielectric substrate covered with metallic nanoparticles or a solution with a metallic colloidal suspension. The SERS enhancement is maximized when the metal grains are smaller than the incident laser wavelength, the metal has the optical properties to generate surface plasmons, and the analyte molecules have matching optical properties to couple to the plasma field. The greatest enhancements are observed with silver, gold, and copper with grain diameters between 10 and 200 nm. Recently, single molecules of rhodamine-6g absorbed on so-called hot silver grains have been detected. 19 These hot grains have intrinsic enhancement of the Raman signal that can be 10 6 to 10 7 higher than the ensemble averaged particles. Only a fraction of the particles have very large enhancements. SERS active AFM tips ideally should have the properties of hot SERS particles. In this work, the SERS-AFM tips are fabricated by simple plasma deposition of gold or silver grain size of 40 nm onto the silicon tip. In order to produce a maximally enhanced spectroscopy at probe tip it would be ideal to have one of the hot particles on the tip of the AFM. This would exploit electric field enhancement, plasmon resonance enhancement, and the socalled chemical enhancement effect. Another approach is to completely metallize the tip to produce a sharp metallic probe that provides mainly field enhancement with possible chemical enhancement. Milner and Richards have modeled the field enhancement in the latter approach as a function of laser incidence angle. 27 Their results show that if the enhancement is the result of sharp probe geometry alone, then there is only moderate field enhancement when the probe is illuminated by light incident normal to the sample, that is bottom illuminated through a clear sample substrate. However, it has been shown experimentally that there is significant enhancement with this normal incidence, through-sample illumination. 21,22 As an explanation, there may be grains on the tips with a higher curvature than the tip itself and/or there is a chemical enhancement effect occurring. This has significance for optimal tip design. If very high enhancements enabling single molecule spectroscopy are to be achieved then hot metal particles at the end of the AFM tip should be pursued. This may pose tip fabrication difficulties for consistently enhancing tips. However, if more consistent enhancement is the goal, sharp metallic tips may be the best choice. Milner and Richards 27 show for the sharp metallic tip case that the illumination off normal incidence side illumination is required. In addition, side illumination is likely to be optimum for chemical enhancement as the incident radiation can be aligned with the charge-transfer state of the molecules. Above all, side illumination has the advantage of permitting opaque samples. For all these reasons this side-illuminating configuration was chosen for our instrument. In addition to enhanced spectroscopic detection provided by this instrument configuration, the field enhancement at the probe tip has potential applications in optical trapping studies. The use of a laser beam to control the motion of neutral atoms, laser cooling, and the optical trapping of particles has been widely investigated and applied to biological research. 29,30 An illuminated probe tip for optical trapping has the advantage of significantly reduced trapping volumes, larger trapping forces due to the larger field gradients, and reduction of the external illumination power from the locally enhanced field. Novotny recently proposed a theory of optical trapping at the probe tip for similar laser conditions that are used in the instrument described in this article. 31 Although not tested extensively in this study, this instrument may be very useful for studying these applications. Optical trapping with the subsequent analysis by the RAFM would add chemical separation to the analysis. Chiu and Zare have shown that conventional optical trapping not using a probe can measurably change diffusion rates in solution. 32 A fascinating potential application would be to perform localized optical chromatography near the probe tip that can use the enhanced spectroscopy as a detector. The instrument pre-

3 1200 Rev. Sci. Instrum., Vol. 73, No. 3, March 2002 M. S. Anderson and W. T. Pike FIG. 2. A labeled photo of the Raman-atomic force microscope. The Raman microprobe objective focuses the Raman laser and allows a side view with respect to the tip. The pick off mirror allows a top down view of the AFM tip and sample with a side mounded optical microscope not shown. sented provides a flexible platform for studying these applications. III. INSTRUMENT A. Description, schematic The RAFM system described here is the integration of commercial AFM and Raman systems. The AFM uses a Digital Instruments Santa Barbara, CA Nanoscope 3 controller with a modified D3000 head and custom-built stage. The Raman microprobe system is a Kaiser Holoprobe Kaiser Optical Instrument Systems, Ann Arbor, MI attached to an optical microscope with a modified microscope stage to accommodate the AFM head. The laser excitation wavelength is 785 nm. The modified stages allow AFM operation under the Raman microprobe objective 10 and provide side illumination of the AFM tip. This arrangement, shown in Fig. 2, allows the AFM to scan a sample while side illuminated with the Raman microprobe beam. The Raman beam size is approximately 2 4 m and intensity is approximately 10 mw depending on focusing. The angle of incidence of the beam relative to the surface is grazing at approximately 3 and can be adjusted to accommodate other angles. The system is also equipped with dual optical microscope viewing. The Raman microprobe objective allows side viewing of the AFM tip and sample. The D3000 AFM head has a pick-off mirror that allows top down viewing of a sample with a side mounted video microscope. This dual optical microscope allows the Raman microprobe beam to be readily focused at the tip sample interface. B. Preparation of SERS active AFM tips The SERS active AFM tips for this study are prepared by plasma sputtering silver or gold onto the conventional silicon AFM tips TESP, Digital Instruments, Santa Barbara, CA. This is a stochastic process and yields only a fraction of the tips with the proper alignment of metal grains on the apex of the tip to provide localized SERS. A Hummer 6.2 sputtering system Springfield, VA was used with either gold or silver targets. The data presented used silver deposition with 150 s in argon with 15-mA current. AFM analysis of a silicon test FIG. 3. A side view from the optical microscope of the RAFM instrument showing a laser illuminated diamond particle on the SERS-AFM tip. The SERS spectrum is compared to the conventional Raman spectrum dashed line of the diamond particle on glass examined using the same instrument conditions. surface similarly coated as the silicon tips reveals a silver grain structure with an average of 40-nm grain size. As a test of the tip for SERS activity, a diamond particle 1 m, Mant, USA on a tip is shown in Fig. 3. This clearly demonstrates SERS activity when compared with a conventional Raman spectrum of a particle on a glass slide. The actual enhancement factor is difficult to determine because the signal is from an unknown volume in proximity to the diamond particle. IV. EXPERIMENT A. Scanning inside the laser beam and beam profiling The system was first tested to determine if the AFM could effectively scan while the tip sample interface was illuminated with the Raman laser. When silicon-nitride type Digital Instruments, model NP cantilevers were used the laser beam heating of the AFM tip-cantilever was detrimental and interfered with proper engagement of the scan. When silicon cantilevers were used, the AFM tip could successfully scan through the Raman laser beam. Interestingly, the photothermal deflection of the AFM tip-cantilever could be measured in the lateral force or phase contrast modes as the AFM tip passes through the laser beam. The lateral force signal is generally used for making friction measurements on a surface by causing torsion in the cantilever. In this case the lateral deflection signal was caused by the laser impinging the cantilever and could be used to image the laser beam position. Figure 4 shows the lateral deflection signal as the AFM passes through the Raman laser beam while scanning a glass microscope slide. This ability to profile the beam may be a useful method for monitoring the laser beam intensity profiles for other applications. B. AFM targeted surface enhanced Raman spectroscopy The ability of this instrument to provide spatially localized SERS was tested with diamond particle imbedded in a glass microscope slide. 33 The diamond particles were located optically with the optical microscopes side and top viewing and were translated under the AFM tip using the manual

4 Rev. Sci. Instrum., Vol. 73, No. 3, March 2002 Raman-AFM 1201 FIG. 4. A lateral deflection friction scan of a m area on a glass surface. This photothermal deformation information allows the AFM to be engaged precisely inside the Raman laser. This could be a generally useful method for profiling laser beam intensity. X-Y-Z stage. The AFM was typically operated in tapping mode to locate the area of interest. The scanning was then changed to contact mode for the local spectroscopic measurements. In contact mode the lateral friction force measurements could be simultaneously acquired along with the topography. The friction contrast was useful when measuring the diamond glass interface because the topography alone could not necessarily discern the edge. The Raman instrument was typically set for a 20-s integration time with the cosmic ray filter turned off. The 1332 cm 1 diamond peak was monitored. Figure 5 shows the AFM friction image of the edge of a diamond particle and the locations of the targeted SERS spectra. The diamond signal is clearly enhanced on the particle than off the particle 250 nm away. This was observed when the tip was positioned at various orientations around the perimeter of the diamond particle. Figure 6 shows the signal as the AFM probe is translated across a diamond glass boundary. In this case the background signal from the FIG. 5. The top spectrum shows the local SERS surface enhanced Raman scattering on a diamond particle. The bottom spectrum was acquired when the AFM tip was approximately 250 nm away from the diamond particle on the glass substrate. The locations are shown on the AFM friction force image. This was repeatable when the tip was positioned around the perimeter of the particle on and off the diamond. Note that the lateral force, friction image provides chemical contrast at nanometer resolution. The Raman spectra allow the chemical functionality to be extrapolated to this scale. FIG. 6. The SERS-active AFM tip was translated across the diamond glass interface. The Raman signal was plotted against the distance from the edge. In this typical case, there is a conventional nonenhanced Raman signal background that is approximately seven times the enhanced signal. The resolution is approximately 50 nm or better. conventional Raman scattering is larger than the enhanced signal. The enhanced signal is 15% of the total signal in this case. It is possible to find a location where the background signal is smaller. However, a typical analysis would likely have some background component. The spatial resolution of the spectral discrimination is estimated to be better than 50 nm. The resolution is further analyzed in the Discussion section. The SERS-active tips has a limited life and this was evident after scanning large rough areas, the tips failed to produce any measurable enhancement. There was sometimes a problem with keeping the Raman laser focused on the tip sample interface. However, a drift in focus was evident in a steady change in the Raman signal from the silicon tip at 520 wave numbers. The silicon in the tip could therefore provide an internal standard. C. Manipulation and optical trapping with the illuminated AFM tip An unattached diamond particle with a Raman laser illuminated gold-coated tip was picked up from a glass surface possibly aided by optical trapping. It should be noted that there are other adhesive mechanisms that could cause the tip to pick up a particle and when the laser was turned off the particle remained on the tip. Subsequently the enhanced Raman spectrum of the removed particle was measured. A similar sized diamond particle on a glass substrate directly illuminated without the AFM tip was used for comparison. Figure 3 shows the illuminated diamond on the AFM tip and its surface enhanced Raman spectrum compared to a diamond on a quartz sidle of similar size. This may be a useful method of separating material from the surface for subsequent analysis. The experiments studying laser trapping at a probe tip should ideally be performed in solution because the reduction of the capillary forces. Although not tested extensively here, this instrument configuration is a useful platform

5 1202 Rev. Sci. Instrum., Vol. 73, No. 3, March 2002 M. S. Anderson and W. T. Pike for testing optical manipulation and related phenomena with probe tips. Diamond particles were chosen for the evaluation of this RAFM instrument because of the characteristic Raman line and because they would provide a hard discrete edge. We observed that the AFM did not image the unattached diamond particles as well as expected based on our previous experience with imaging particles unattached to a surface. This led us to imbed the diamond on the glass as described above. It was surmised that the illuminated tip could be acting like optical tweezers by increasing the particle interaction with the illuminated tip. This qualitative observation is supported in the literature where similarly illuminated SPM tips and particles have been evaluated for their trapping potential. 31 As previously mentioned, a phase contrast was observed when operating in tapping mode as the tip was scanned through an illuminated region on a glass surface. This could be from the tip surface affinity changing as a result of optical trapping of the tip to the surface. In support of this hypothesis, Nonnenmacher 34 has measured a contact potential difference between an AFM tip and gold surface as a function of laser wavelength. In that study the change in contact potential difference was attributed to local heating or to a change in the surface dipole moment caused by optical excitation of the sample and/or tip. V. DISCUSSION A. Resolution potential The determination of the spatial resolution is a fairly complicated issue in near-field imaging measurements. Zenhausern has reported apertureless near-field optical spectroscopy with 3 nm resolution with an STM. Apertureless-nearfield Raman spectroscopy using bottom illumination and transparent substrates have reported between 50 and 100 nm resolution In this work we are achieving comparable resolution with an important distinction. We are not using an extremely SERS active dye, such as Rhodamine 6G. Diamond was chosen for its hardness and its distinct Raman line, not its SERS activity. Here we only attempt to estimate the resolution and leave to future work actual improvement of the tips for the ultimate spatial resolution. Typically the resolution is determined by measuring the full width at half maximum of an observed structure 35,36 or measuring signal transition form translation over an edge feature. 37,38 The diamond on glass sample was selected because it provided a discrete edge. However, to prevent the diamond from moving it was imbedded in the glass. This may have made the glass diamond transition less abrupt and may have limited the apparent resolution. An accurate measurement of the resolution of the spectral discrimination requires a sample with a very discrete edge. In the future other chemical edges with spectroscopic contrast will be examined. The results presented here show an 50 nm resolution or better for targeting a specific region. An optimized SERS-AFM tip could further improve the resolution. Of course, when used in conjunction with friction or lateral force imaging of the AFM, the resolution can be an order of magnitude better at subnanometer resolution. This is an important practical consideration because the high spatial resolution nm of targeted SERS may provide key information for interpreting the subnanometer chemical contrast of friction, phase contrast, or force modulation mode AFM. B. Component optimization In this work we produce the tips using a stochastic process that yields 5% 10% of the tips with the right qualities. We speculate that silicon is not the best tip material due to its optical absorption at the Raman excitation wavelength of 785 nm. Apparently this is overcome somewhat by a relatively thick metal coating that may isolate the SERS active metal grain from the silicon substrate. However, the literature indicates that the most active SERS substrates are transparent dielectrics with relatively isolated silver grains or clusters. 39 Therefore improvements are expected if transparent to the excitation wavelength dielectric AFM tips are used. There are other important factors that would allow tuning of the SERS effect. This includes adjusting the tip grain size, aspect ratio, and the angle of laser incidence. In order to control these properties there would need to be a more elaborate tip fabrication process. Electron beam techniques may be one way to fabricate such tips. With a properly plasmon tuned tip the SERS enhancement could improve considerably and consequently the potential spatial resolution of the spectral features. An experimental improvement would be to use oscillation of he tip on and off the sample and thereby remove conventional Raman signal from adjacent illuminated regions. Because of the long integration times required by the charge coupled device detector, low frequency tapping of the surface would be required. This is used in the so-called lift mode that allows low frequency tapping below the resonance frequency of the cantilever. A planned modification is the fabrication of a liquid cell. A liquid cell would be useful for optical trapping studies and for the analysis of biological samples. VI. FUTURE APPLICATIONS This combined Raman-AFM instrument has demonstrated the ability to simultaneously provide high-resolution topography, friction contrast, and phase contrast while scanning inside the laser beam of a Raman microprobe. We believe this combined information alone has considerable analytical utility. The further ability of the AFM tip to target the SERS effect is a significant development in analytical microscopy and has the potential for targeted single molecule spectroscopy. The unique side illumination provides the flexibility to probe samples that are on opaque substrates. This instrument also has use in studying the ability for the AFM tip to function as optical tweezers and to profile laser beam intensity. In addition, this system has relevance for in situ instruments for studying planetary surfaces. For this application it should be noted that there are space flight qualified AFM, Raman, and optical microscope instruments. This instrument serves as a test bed that demonstrates that an AFM and Raman spectrometer can be constructed around a central optical microscope. The advantage of this arrangement is that a

6 Rev. Sci. Instrum., Vol. 73, No. 3, March 2002 Raman-AFM 1203 sample, planetary soil, for example, can be translated using a common stage for examination with three instruments that may work independently or together. ACKNOWLEDGMENTS The work described in this article was carried out at the Jet Propulsion Laboratory, California Institute of Technology, through an agreement with the National Aeronautics and Space Administration. The authors would like to thank Andre Yavrouian, Mike Hecht, and the entire MECA team for their support and encouragement. The authors want to give special thanks to James Cutts for his early support of this project. 1 G. Binning, H. Rohrer, C. Gerber, and E. Weibel, Phys. Rev. Lett. 49, H. K. Wickramasinghe, Acta Mater. 48, G. Binning, C. F. Quate, and C. Gerber, Phys. Rev. Lett. 56, D. Rugar and P. Hansma, Phys. Today 43, D. Sarid, Scanning Force Microscopy, with Applications to Electric, Magnetic, and Atomic Forces, revised ed. Oxford University, New York, D. Sarid, Exploring Scanning Probe Microscopy with a Mathematica Wiley, New York, A. Noy et al., JACS 117, Recent review of chemical AFM, R. McKendry et al., Jpn. J. Appl. Phys. Part. 1 38, B. G. Levi, Phys. Today H. F. Hamann, A. Gallagher, and D. J. Nesbitt, Appl. Phys. Lett. 73, L. P. Ghislain and V. B. Elings, Appl. Phys. Lett. 72, M. S. Anderson, NASA Tech. Briefs 21, A. Hammiche et al., Appl. Spectrosc. 53, M. S. Anderson, Appl. Spectrosc F. Zenhausern, M. P. Oboyle, and H. K. Wickramasinghe, Appl. Phys. Lett. 65, K. M. Berland and H. K. Wickramasinghe, Biophys. J. 74, A M. Fleishmann, P. J. Hendra, and A. J. McQuillan, Chem. Phys. Lett. 26, A. Campion and P. Kambhampati, Chem. Soc. Rev. 27, S. M. Nie and S. R. Emery, Science 275, K. Kneipp et al., J. Raman Spectrosc. 29, M. S. Anderson, Appl. Phys. Lett. 76, R. M. Stockle, Y. D. Suh, V. Deckert, and R. Zenobi, Chem. Phys. Lett. 318, N. Hayazawa, Y. Inouye, Z. Sekkat, and S. Kawata, Chem. Phys. Lett. 335, B. Pettinger, G. Picardi, R. Schuster, and G. Ertl, Electrochemistry 68, L. T. Nieman, G. M. Krampert, and R. E. Martinez, Rev. Sci. Instrum. 72, J. Jersch, F. Demming, L. J. Hildenhagen, and K. Dickmann, Appl. Phys. A: Mater. Sci. Process. 66, R. G. Milner and D. Richards, J. Microsc.-Oxford 202, B. J. Kennedy, S. Spaeth, M. Dickey, and K. T. Carron, J. Chem. B 103, A. Ashkin and J. M. Dziedzic, Science 235, S. Chu, Science 253, L. Novotny, R. X. Bian, and X. S. Xie, Phys. Rev. Lett. 79, D. T. Chiu and R. N. Zare, J. Am. Chem. Soc. 118, Diamond particles 0 2 m, Mant, USA were spread on a microscope slide and heated to a dull red for 1 min. This slightly imbedded the diamond into glass. The excess was washed off with water and dichloromethane. 34 M. Nonnenmacher and H. Wickramasinghe, Ultramicroscopy 42, M. Radmacher, P. E. Hillner, and P. K. Hansma, Rev. Sci. Instrum. 65, C. Mihalcea, W. Scholz, S. Werner, S. Munstyer, E. Oesterschulze, and R. Kassing, Appl. Phys. Lett. 68, H. Muramatsu, N. Chiba, K. Homma, K. Nakajima, S. Ohta, A. Kusumi, and M. Fujihira, Appl. Phys. Lett. 66, H. Zhou, A. Midha, G. Mills, L. Donaldson, and J. M. R. Weaver, Appl. Phys. Lett. 75, K. L. Norrod et al., Appl. Spectrosc. 51,