single-molecule tip-enhanced raman spectroscopy€¦ · dx.doi.org/10.1021/jp209982h | j. phys....

Published: December 09, 2011

r 2011 American Chemical Society 478 dx.doi.org/10.1021/jp209982h | J. Phys. Chem. C 2012, 116, 478–483

ARTICLE

pubs.acs.org/JPCC

Single-Molecule Tip-Enhanced Raman SpectroscopyMatthew D. Sonntag, Jordan M. Klingsporn, Luis K. Garibay,† John M. Roberts, Jon A. Dieringer,Tamar Seideman, Karl A. Scheidt, Lasse Jensen,‡ George C. Schatz, and Richard P. Van Duyne*

Northwestern University, Department of Chemistry, 2145 Sheridan Road, Evanston, Illinois 60208, United States

bS Supporting Information

1. INTRODUCTION

Since its development just over 10 years ago, tip-enhancedRaman spectroscopy (TERS) has emerged as a promising ap-proach to obtain chemical information on the nanometer lengthscale.1�3 Excitation of the localized surface plasmon resonance(LSPR) of a Ag or Au tip under side illumination leads to bothfield localization on the order of tens of nanometers and Ramanenhancement in the 106 to 109 range.4�6 These attributes allowTERS to overcome the diffraction limited spatial resolution andlow sensitivity of normal Raman spectroscopy. The promise ofTERS lies in its ability to provide a combination of spectroscopicand spatially resolved chemical information from molecules on asurface. Recently, TERS has been successfully demonstrated in avariety of areas of nanoscale analysis including quantum dots,7

carbon nanotubes,8�10 DNA bases,11�13 viruses,14 and so on.TERS not only provides an excellent tool for nanoscale vibra-tional spectroscopy but also allows for more fundamentalresearch into increasing its sensitivity to the ultimate level�single-molecule detection. Here we demonstrate single-moleculeTERS (SMTERS) by using the isotopologue approach to verifyunambiguously single-molecule detection via frequency ratherthan intensity fluctuations.15�17

To date, several papers have suggested that SMTERS is plau-sible based on a statistical analysis of the fluctuations in both peakintensity and spectral position. One interpretation of the fluctua-tions in peak intensity suggested that the signal was indicative of aPoisson distribution of intensities, with the isolated peaks corre-sponding to the number of molecules probed.5 However, theRaman spectra presented differ from previous literature reports,

and newmodes were observed whose origin was not identified.18

Additionally, it has been pointed out that small sample sizes(∼100 events) are not sufficient to define a Poisson distribution.19

Other reports pointing toward SMTERS have presented analysesof intensity fluctuations without involving Poissonian statisticsbut cite discrete signal loss events, a broad intensity distribution,and fluctuations in the peak location.6,20,21 A slightly differentmethod, fishing mode TERS (FM-TERS), has recently claimedsingle-molecule observation based on changes in line shape andpeak location that were correlated with jumps in conductancethrough the tip�sample junction.21 However, these fluctuationsof intensity and peak location, by themselves, do not rigorouslydemonstrate single-molecule behavior.22 With respect to TERS,the enhanced field in the tip�sample junction depends sensitivelyon the position of the molecule relative to the tip apex, and suchvariations would make quantization of intensities impossible andfluctuations in intensity common.23 The most convincing demon-stration of SMTERS was reported in ultrahigh vacuum (UHV)by imaging single molecules with a STM before collecting theirRaman spectra.24 Under ambient conditions, the water meniscusformed between the tip and sample makes molecular resolution im-possible due to surface diffusion. In our judgment, rigorous valida-tion of SMTERS in ambient requires an alternative approach.

The approach we are examining here to establish SMTERSunambiguously is an adaption of the bianalyte or isotopologue

Received: October 17, 2011Revised: November 18, 2011

ABSTRACT: An existence proof for single-molecule tip-enhanced Raman spectroscopy (SMTERS) is given using thefrequency domain approach involving the two isotopologues ofRhodamine 6G (R6G) that were previously employed for single-molecule surface-enhanced Raman spectroscopy (SMSERS).A combination of experimental and theoretical studies pro-vides a detailed view of the isotopic response of R6G�d0 andR6G�d4 in the 600 � 800 cm�1 region. The single-moleculenature of the TERS experiment is confirmed through two linesof evidence. First, the vibrational signature of only one iso-topologue at a time was observed from multiple TER spectra.Second, the spectral wandering of the 610 cm�1 mode of R6G�d0 was less than (4 cm�1, which in turn is less than the 10 cm�1

isotopic shift so that no confusion in assignment resulted. As a consequence, the total TERS enhancement factor can now beaccurately established as EFTERS = 1.0 � 1013 because only one molecule at a time is measured. Furthermore, EFTERS can bepartitioned into an electromagnetic contribution of 106 and a molecule-localized resonance Raman contribution of 107.

479 dx.doi.org/10.1021/jp209982h |J. Phys. Chem. C 2012, 116, 478–483

The Journal of Physical Chemistry C ARTICLE

technique introduced to prove the existence of single-moleculesurface-enhanced Raman spectroscopy (SMSERS).19,25,26 The bia-nalyte method involves the competition of two different analytemolecules for the adsorption site (viz. hot spot) that generatesSMSERS. As a consequence, the interpretation of bianalyte datamustcontend with the different Raman cross sections, electronic absorp-tion spectra, and surface binding affinities of the analytes. Such com-plications are eliminated by using two isotopologues as the analytes.

Application of the isotopologue approach to SMTERS relieson the ability to identify uniquely each isotopologue by distinctvibrational bands. To accomplish this, the isotope shift in theselected vibrational band must be larger than the frequencyexcursions caused by spectral wandering. Under ambient condi-tions, a meniscus of water forms between the tip and underlyingsurface, allowing for transport of molecules into and out of theenhancing region of the tip. With sufficiently low surface con-centrations of both molecules, it is possible to observe only onemolecule at a time moving through the electromagnetic hot spotdefined by the tip�sample junction. As the surface concentrationof molecules decreases, the number of molecules moving throughthe hot spot will decrease and observation of the distinct vibra-tional features of each isotopologue will be possible.

We characterize the vibrations of both R6G�d0 and R6G�d4with ensemble-averaged TERS in combination with TDDFT cal-culations. Using this analysis, we determine which vibrations areaffected by isotopic substitution and therefore allow for unam-biguous identification of each isotopologue.

2. EXPERIMENTAL SECTION

Syntheses. The syntheses of both R6G�d0 and R6G�d4 arebased on conditions given by Zhang27 and have been reportedelsewhere.15 Standard solutions (10�4 to 10�7 M in EtOH) ofR6G�d0 and R6G�d4 were created and analyzed by UV�visabsorbance spectroscopy to quantify concentration. The spectro-photometer consisted of a white light source (F�O Lite, WorldPrecision Industries) fiber-coupled to a cuvette holder (CUV,Ocean Optics) with the output fiber-coupled to a visible lightspectrometer (SD2000, Ocean Optics).Sample Preparation I (TERS Characterization of Isotopo-

logues). Smooth silver filmswere prepared by electron beamdeposi-tion (AXXIS,Kurt J. Lesker) of 200nmof silver at a rate of 2Å/s ontoa glass slide. These Ag films were incubated in 3� 10�4 M ethanolicsolutions of either R6G�d0 or R6G�d4 for at least 4 h and rinsedwith ethanol prior to use to achieve monolayer coverage.Sample Preparation II (LowAdsorbate Coverage).Ag films

prepared as described above were used in SMTERS experiments.The Ag films were incubated in an equimolar ethanolic R6G�d0and R6G�d4 (5� 10�7 M each, total 1� 10�6 dye) and rinsedthoroughly with ethanol prior to use to achieve submonolayercoverage.Tip Preparation. The Ag tips used in this experiment were

prepared through electrochemical etching similar to the methoddescribed by Zhang et al.6 In brief, a mixture of perchloric acid(70%, Aldrich) and ethanol in a volume ratio of 1:4 was used asthe etching solution. A platinum ring with diameter 20 mm wasused as the negative electrode and positioned at the surface ofthe etching solution. The silver wire (99.99%, Aldrich) diameter0.25 mm was employed as the positive electrode, and a constantvoltage of 1.6 V was applied. The circuit was manually disconnectedafter drop off of the lower part of the wire. The tips were rinsedwith Milli-Q water, followed by ethanol after etching.

Instrumentation. TER spectra were collected on a home-built microscope. A schematic of the optical microscope is shownin Figure 1. In brief, a 532 nm laser (Spectra Physics Excelsior,100 mW) was fiber-coupled to the optical microscope via asingle-mode optical fiber. The laser light was passed through afilter to remove Raman light generated by the fiber (MaxLinelaser line 532, Semrock) and polarizer to achieve the desired ppolarization. The incident light was focused onto the tip samplejunction through an aspheric lens (f = 13.86 mm, NA = 0.18,Geltech Aspheric Lens) at an angle of 55� relative to the tip axis.Inelastic scattered light was collected through the same lens,filtered to remove residual laser light (RazorEdge long-pass 532,Semrock), and fiber-coupled to a 1/3-m spectrometer (SP2300,Princeton Instruments). The Raman light was dispersed by a1200 groove/mm grating and collected on a thermoelectricallycooled CCD (PIXIS 400, Princeton Instruments). The tip ap-proach and tunneling parameters were operated by a commercialSTM system (Molecular Imaging) controlled by RHK electro-nics. TERS characterization of individual isotopologues wasconducted on Ag films. The experimental conditions are: λex =532 nm, incident power (Iex) = 0.5�0.7 mW, taq = 3� 10 s, bias(V) = 50�500 mV, and tunneling current (Ic) = 3�5 nA.Computational Modeling. The electronic structure calcula-

tions presented in this work have been performed using theNWChem program package.28 The ground-state equilibriumgeometries and normal modes of R6G-d0 and R6G-d4 weredetermined using the B3LYP functional and a 6-311G* basis set.A scaling of the frequencies by 0.98 was introduced to account formissing anharmonicity in the simulations. To simulate theresonance Raman scattering (RRS) spectra, we used Heller’stime-dependent theory wherein29,30

ααβ ¼ ∑nμ0nα μ

0nβ � i

Z ∞

0Æf jinðtÞæeiðEL þ νi0Þt � Γnt dt

Figure 1. Schematic of the experimental setup consisting of a home-built optical microscope and a commercial STM.



where EL is the energy of the incident light, n is the electronicstate, μ0n is the electronic transition dipole moment, νi0 is thevibrational energy of state |f>, and |in(t)> is the wavepacketcorresponding to the time-dependent nuclear wave function ofelectronic state n. A homogeneous broadening, Γn, of 500 cm

�1,was used in all simulations. The overlap between the initial andfinal wavepacket was obtained analytically using the independentmode displaced harmonic oscillator (IMDHO) method. Thismodel accounts for vibronic coupling effects, but solvent effectsin the calculations were not included. The dimensionless dis-placements were obtained by finite differentiation of the excited-state energy in the Franck�Condon region. A solvent shift wasapplied to the excitation energy so as to match the experimentalresult.

3. RESULTS AND DISCUSSION

The visible absorption spectrum as well as the structure of theisotopologues are given in Figure 2. Both the line shape and theabsorbance maxima are identical, indicating that isotopic sub-stitution does not perturb the electronic structure of the mole-cule. The absorbance spectra exhibit a major peak at 527 nm.Laser excitation at 532 nm provides an additional resonant en-hancement. Therefore, the measurements are more rigorouslytermed tip-enhanced resonance Raman spectroscopy (TERRS);however, in agreement with previous literature, we will refer tothis study as TERS.

The ensemble-averaged TER spectra of a monolayer ofR6G�d4 and R6G�d0 with both the tip engaged and retractedon a Ag film are shown in Figure 3A,C, respectively. Each iso-topologue spectrum was taken on different days and thus utilizeddifferent tips, accounting for the differences in the intensity of theTER signal. The experimental spectrum of the isotopologuesmatches well with previous literature reports.31 Several subtledifferences in both peak intensity and frequency are observedupon isotopic substitution, the most prominent of which are

discussed in what follows. For example, an additional peakappears in the R6G�d4 spectrum at ∼1350 cm�1, which is notpresent in R6G�d0 spectrum. However, this peak is oftenobscured due to the presence of two other peaks in the regionof 1330�1360 cm�1. A more convenient point of contrast is the610 cm�1 mode in R6G�d0, which shifts to ∼600 cm�1 in theR6G�d4 spectrum. When the tip is retracted several micro-meters from the surface, no far field spectrum is observed despitelonger acquisition times (1 min).

An estimate of the relative enhancement factor can beachieved by assuming that the far field signal is below the noise.The laser spot size is ∼12 μm2 and (assuming monolayercoverage) the packing density is 5 � 1013 molecules/cm2. Thelocalization length associated with the tip-enhanced field can beestimated by L = (2Rd)1/2 where R is the radius of curvature ofthe tip and d is the tip�sample separation.32 Here R is∼160 nmas estimated from SEM and d is 1 nm, leading to a field local-ization length of ∼18 nm. The peak intensity for the 600 cm�1

mode in the near field is 80 ADU mW�1 s�1, whereas the peakintensity in the far field with the tip retracted is below the noiselevel (∼1 ADU mW�1 s�1), leading to a relative enhancementfactor of 1.0 � 106.

In the original SMSERS reports, the enhancement factornecessary to reach single-molecule detection was ∼1014.33,34

For Rhodamine 6G, the use of excitation at the molecular reso-nance (λex = 532 nm) provides a resonance Raman contributionleading to an enhancement on the order of 107.35 To reach theappropriate level of enhancement, an electromagnetic enhancement

Figure 2. Visible absorption spectra and structure of (A) R6G-d4 and(B) R6G-d0. No perturbation of the electronic structure is observedupon deuteration of the molecule.

Figure 3. Experimental ensemble TER and simulated normal Ramanspectra of R6G�d4 on (A) silver film engaged and retracted (B) gas-phase TDDFT analysis; R6G�d0 on (C) silver film engaged andretracted (D) gas-phase TDDFT analysis. TER spectra acquired withλex = 532 nm, taq≈ 3 s, and Iex≈ 330 μW. Note that the ensemble TERspectra of each individual isotopologue were taken with different tips.



of roughly 107 is necessary for single-molecule detection; how-ever, it has been reported that single-molecule detection forresonant dye molecules can be achieved with an electromagneticenhancement of ∼106.24 Our calculated relative enhancementfactor reflects the fact that we are within the necessary totalenhancement necessary for single-molecule detection. A similarcalculation of the enhancement factor using the integrated peakintensities would yield a more meaningful answer. However, thelack of signal in the far field spectrum makes this difficult toaccomplish and in any case would yield a higher enhancementfactor.

The simulated Raman spectrum of both isotopologues ispresented in Figure 3B,D, where we use time-dependent densityfunctional theory (TDDFT). Small shifts are observed between theexperimental and theoretical vibrational frequencies (<10 cm�1).As noted above, the TDDFT simulations were performed for anisolated molecule, neglecting solvent effects. Therefore, thesediscrepancies could be attributed to interactions between themolecules and the silver surface as well as intermolecular

interactions. However, the roughly 10 cm�1 shift of the 610 cm�1

peak upon deuteration is seen in both experiment and simulationand allows for unequivocal identification of the isotopicallylabeled probe molecules. Because of its proximity to other peaks,the presence or absence of the 1350 cm�1 resonance is notalways evident. Therefore, the vibrational resonance at 600 cm�1

is used for differentiating spectra in the subsequent SMTERSmeasurements.

Prominent changes in the vibrational modes upon isotopicsubstitution were analyzed by comparing experimental data tovibrational mode simulations. Analysis of the simulation resultsshows that vibrational resonances with energies that do not shiftsignificantly upon isotopic substitution do not involve the mo-tion of protons on the isolated ring of the molecule. Those reso-nances in the spectrum for which a frequency shift is observed doinvolve motion of the protons on the pendent benzene ring, asexpected. Results of the ensemble TERS and simulated Ramanmeasurements were used in the analysis of SMTERS events.

Figure 4A shows a waterfall plot of the Raman scattering as afunction of time. Large intensity fluctuations of the signal are evi-dent throughout the spectrum. Fluctuations in the frequency of

Figure 4. (A) Time series waterfall plot of spectra taken continuouslywith a single tip. The false color represents signal intensity where red ishighest and blue is lowest. (B) Three time slices extracted from timeseries waterfall plot shown in panel A, where (a) is R6G�d4, (b) is bothisotopologues, (c) is R6G�d0, and (d) corresponds to retracted tip.Spectra acquired with λex = 532 nm, taq = 10 s, and Iex ≈ 520 μW andtunneling conditions of 3 nA and 500 mV.

Figure 5. (A) Zoom of the low wavenumber region of the experimentalspectra presented in Figure 4 for (a) R6G�d4, (b) both isotopologues,(c) R6G�d0, and (d) tip retracted. Spectra acquired with λex = 532 nm,taq = 10 s, and Iex ≈ 520 μW and tunneling conditions of 3 nA and500 mV. (B) Fits of the individual data points to Lorentzian line shapes,further illustrating the frequency shift between the isotopologues. (C) Zoomof the low-wavenumber region of the simulated spectra shown in Figure 3 for(a) R6G�d4 and (b) R6G�d0.



the resonances are somewhat more subtle but are especiallypronounced in the 600 cm�1 regime. Throughout the acquisi-tion, switching behavior is observed in which the spectrum shiftsbetween the vibrational signal of a single isotopologue and that ofboth isotopologues. In Figure 4B, three time slices are extractedfrom the waterfall plot shown in Figure 4A, demonstratingspectra featuring distinctly character from (a) R6G�d4, (b) bothisotopologues, (c) R6G�d, and (d) tip retracted.

Figure 5A shows the details of the modes in the 600 cm�1

region along with the 772 cm�1 mode for spectra shown inFigure 4B. The individual data points and a Lorentzian fit to thedata are shown in Figure 5B. The shift in peak location is clearlydistinguishable between the two isotopologues. No frequencyshifts are observed for the 772 cm�1 mode, indicating that thefluctuations observed in the 600 cm�1 region are due to thepresence of one or the other isotopologues. TDDFT simulationsreveal that the source of the frequency shift of the 610 cm�1

resonance is due to vibration of the hydrogens on the pendantphenyl ring of the molecule. The vibrations responsible for

the 772 cm�1 resonance arise from the hydrogens on the threefused aromatic rings.

Figure 6A shows in greater detail the frequency and intensityfluctuations of the 600 cm�1 region. In addition to the switchingbehavior previously discussed, spectral wandering was also ob-served. The last 100 s of the acquisition show purely R6G�d0behavior. Figure 6B shows the Raman shift versus time for the610 cm�1 mode of R6G�d0 during that time period. With onemolecule in the hot spot, the spectral position of the Raman peakcan fluctuate due to variations in the orientation of the moleculewith respect to the tip apex, laser polarization, and so on.Compared with bulk Raman spectra, the peak fluctuations willbe much larger when detecting signal from single molecules.Fluctuations in peak position <5 cm�1 were observed, muchsmaller than the frequency shift of the resonance upon isotopicsubstitution. This demonstrates that the frequency shift from610 to 600 cm�1 cannot be explained through spectral wander-ing. Similar changes in peak position (<5 cm�1) were observedfor experiments conducted with one isotopologue present at lowcoverage. Figure 6C demonstrates that the fluctuations in TERSintensity are dramatic, spanning roughly an order of magnitude.This variation in intensity is too large to observe a quantized (viz.Poisson) distribution of intensities stemming from n= 1, 2, 3, andso onmolecules, as previously discussed.23,36 The degree of spec-tral wandering is measured by a fit to the histogram shown inFigure 6D. The full width half-maximum (fwhm) of the R6G�d0is 6.0 cm�1, whereas the fwhm of the ensemble spectrum is9.8 cm�1. These results indicate that the ensemble-averagedspectrum is a super position of the single-molecule states.

No evidence of tip degradation over the course of an experi-ment was observable; however, a reduction in the total enhance-ment was observed over a period of 24 h, most likely due tooxidation of the tip. Tips used in these experiments were usedwithin several hours of being etched.

4. CONCLUSIONS

The data presented here strongly support the existence ofsingle-molecule TERS using an isotopologue pair of Rhodamine6Gmolecules. First, observation of the vibrational signature fromeither one isotopologue or the other was observed on multipleoccasions. Second, spectral wandering of the resonances asso-ciated with isotopic substitution display behavior in accordancewith single to few molecules. Furthermore, the frequency shiftsobserved in the experimental TER spectra upon isotopic sub-stitution are understood using TDDFT simulations. A total en-hancement of the Raman signal was calculated to be on the orderof 1013 through a combination of electromagnetic and resonanceRaman enhancement. This application of the isotopologue exis-tence proof to TERS further demonstrates its generality in single-molecule spectroscopy. The high signal-to-noise ratio and shortcollection times indicate that TERS mapping of individual nano-structures could be possible in addition to real-time TERS imag-ing of chemical reactions. Another exciting avenue for future re-search, which we plan to explore, is measurement of the dynamicsof R6G as it diffuses in and out of the hot spot, as defined by thetip sample junction.

’ASSOCIATED CONTENT

bS Supporting Information. Images of the normal modesof Rhodamine 6G of the 615 and 772 cm�1 vibrations and

Figure 6. (A) Time series waterfall plot illustrating change in vibra-tional character. The false color represents signal intensity where red ishighest and blue is lowest. The system changes from R6G�d4 to bothand finally to R6G�d0 in 300 s. (B) Plot of the time evolution of thespectral wandering present in R6G�d0 610 cm�1 mode (blue data).Black line corresponds to normal Raman scattering of 1028.3 cm�1

mode of cyclohexane at similar S/N as the time evolution data. (C)Histogram of the peak intensity of the R6G�d0 610 cm�1 mode forsingle-molecule events, illustrating the large intensity variations ob-served at different times. (D) Histogram of the low-frequency peaklocation of R6G�d0 exhibiting single-molecule character, demonstrat-ing the bandwidth of the spectral wandering; the fwhm of the Gaussianfit is 6.0 cm�1.



complete ref 28. This material is available free of charge via theInternet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

Present Addresses†Department of Chemistry, AndrewsUniversity, Berrien Springs,Michigan 49104, United States.‡Department of Chemistry, The Pennsylvania State University,University Park, Pennsylvania 16802, United States.

’ACKNOWLEDGMENT

This work was supported by the National Science Founda-tion (CHE-0802913, CHE-0911145, CHE-0955689, and DMR-1121262), AFOSR/DARPA Project BAA07-61 (FA9550-08-1-0221), and the Department of Energy Basic Energy Sciences(DE-SC0001785 and DE-FG02-033R15475).

’REFERENCES

(1) Anderson, M. S. Appl. Phys. Lett. 2000, 76, 3130.(2) Hayazawa, N.; Inouye, Y.; Sekkat, Z.; Kawata, S. Opt. Commun.

2000, 183, 333.(3) Stockle, R. M.; Suh, Y. D.; Deckert, V.; Zenobi, R. Chem. Phys.

Lett. 2000, 318, 131.(4) Domke, K. F.; Zhang, D.; Pettinger, B. J. Am. Chem. Soc. 2006,

128, 14721.(5) Neacsu, C. C.; Dreyer, J.; Behr, N.; Raschke, M. B. Phys. Rev. B:

Condens. Matter Mater. Phys. 2006, 73, 193406/1.(6) Zhang, W.; Yeo, B. S.; Schmid, T.; Zenobi, R. J. Phys. Chem. C

2007, 111, 1733.(7) Ogawa, Y.; Toizumi, T.; Minami, F.; Baranov, A. V. Phys. Rev. B:

Condens. Matter Mater. Phys. 2011, 83, 081302/1.(8) Hartschuh, A.; Sanchez, E. J.; Xie, X. S.; Novotny, L. Phys. Rev.

Lett. 2003, 90, 095503/1.(9) Saito, Y.; Hayazawa, N.; Kataura, H.; Murakami, T.; Tsukagoshi,

K.; Inouye, Y.; Kawata, S. Chem. Phys. Lett. 2005, 410, 136.(10) Chan, K. L. A.; Kazarian, S. G. Nanotechnology 2010, 21,

445704/1.(11) Watanabe, H.; Ishida, Y.; Hayazawa, N.; Inouye, Y.; Kawata, S.

Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 155418/1.(12) Ichimura, T.; Watanabe, H.; Morita, Y.; Verma, P.; Kawata, S.;

Inouye, Y. J. Phys. Chem. C 2007, 111, 9460.(13) Zhang, D.; Domke, K. F.; Pettinger, B. ChemPhysChem 2010,

11, 1662.(14) Cialla, D.; Deckert-Gaudig, T.; Budich, C.; Laue, M.; Moeller,

R.; Naumann, D.; Deckert, V.; Popp, J. J. Raman Spectrosc. 2009, 40, 240.(15) Dieringer, J. A.; Lettan, R. B., II; Scheidt, K. A.; VanDuyne, R. P.

J. Am. Chem. Soc. 2007, 129, 16249.(16) Blackie, E.; Le Ru, E. C.; Meyer, M.; Timmer, M.; Burkett, B.;

Northcote, P.; Etchegoin, P. G. Phys. Chem. Chem. Phys. 2008, 10, 4147.(17) Kleinman, S. L.; Ringe, E.; Valley, N.; Wustholz, K. L.; Phillips,

E.; Scheidt, K. A.; Schatz, G. C.; VanDuyne, R. P. J. Am. Chem. Soc. 2011,133, 4115.(18) Pettinger, B.; Ren, B.; Picardi, G.; Schuster, R.; Ertl, G. Phys.

Rev. Lett. 2004, 92, 096101/1.(19) Le Ru, E. C.; Meyer, M.; Etchegoin, P. G. J. Phys. Chem. B 2006,

110, 1944.(20) Hayazawa, N.;Watanabe, H.; Saito, Y.; Kawata, S. J. Chem. Phys.

2006, 125, 244706/1.(21) Liu, Z.; Ding, S.-Y.; Chen, Z.-B.; Wang, X.; Tian, J.-H.; Anema,

J. R.; Zhou, X.-S.; Wu, D.-Y.; Mao, B.-W.; Xu, X.; Ren, B.; Tian, Z.-Q.Nat. Commun. 2011, 2, 1310/1.

(22) Andersen, P. C.; Jacobson, M. L.; Rowlen, K. L. J. Phys. Chem. B2004, 108, 2148.

(23) Domke, K. F.; Zhang, D.; Pettinger, B. J. Phys. Chem. C 2007,111, 8611.

(24) Steidtner, J.; Pettinger, B. Phys. Rev. Lett. 2008, 100, 236101/1.(25) Etchegoin, P. G.; Meyer, M.; Blackie, E.; Le Ru, E. C. Anal.

Chem. 2007, 79, 8411.(26) Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. J. Phys.

Chem. C 2007, 111, 13794.(27) Zhang, D.; Xie, Y.; Deb, S. K.; Davison, V. J.; Ben-Amotz, D.

Anal. Chem. 2005, 77, 3563.(28) Bylaska, E. J., et al. NWChem, A Computational Chemistry

Package for Parallel Computers, version 5.1; Pacific Northwest NationalLaboratory: Richland, WA, 2007 (a modified version).

(29) Tannor, D. J.; Heller, E. J. J. Chem. Phys. 1982, 77, 202.(30) Silverstein, D. W.; Jensen, L. J. Chem. Theory Comput. 2010,

6, 2845.(31) Hildebrandt, P.; Stockburger, M. J. Phys. Chem. 1984, 88, 5935.(32) Pettinger, B.; Picardi, G.; Schuster, R.; Ertl, G. Single Mol. 2002,

3, 285.(33) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.;

Dasari, R. R.; Feld, M. S. Phys. Rev. Lett. 1997, 78, 1667.(34) Nie, S.; Emory, S. R. Science 1997, 275, 1102.(35) Shim, S.; Stuart, C. M.; Mathies, R. A. ChemPhysChem 2008,

9, 697.(36) Le Ru, E. C.; Etchegoin, P. G.; Meyer, M. J. Chem. Phys. 2006,

125, 204701/1.

single-molecule tip-enhanced raman spectroscopy€¦ · dx.doi.org/10.1021/jp209982h | j. phys....

Documents