Gap-mode enhancement on MoS 2 probed by functionalized tipenhanced

Transcription

1 Gap-mode enhancement on MoS 2 probed by functionalized tipenhanced Raman spectroscopy Abdulrahman M. Alajlan, 1,2,a Dmitri V. Voronine, 1,3,a Alexander M. Sinyukov, 1 Zhenrong Zhang, 3 Alexei V. Sokolov, 1,3 and Marlan O. Scully 1,3,4 1 Texas A&M University, College Station, Texas 77843, USA 2 National Center for Applied Physics, KACST, Riyadh 11442, Saudi Arabia 3 Baylor University, Waco, Texas 76798, USA 4 Princeton University, Princeton, New Jersey 08544, USA Abstract Surface enhancement of molecular spectroscopic signals has been widely used for sensing and nanoscale imaging. Because of the weak electromagnetic enhancement of Raman signals on semiconductors, it is motivating but challenging to study the electromagnetic effect separately from the chemical effects. We report tip-enhanced Raman scattering (TERS) measurements on Au and bulk MoS 2 substrates using a metallic tip functionalized with copper phthalocyanine (CuPc) molecules, and demonstrate similar gap-mode enhancement on both substrates. We compare the experimental results with theoretical calculations which confirm the gap-mode enhancement on MoS 2 using a well-established electrostatic model. The functionalized tip approach allows for suppressing the background, and is ideal for separating electromagnetic and chemical enhancement mechanisms on various substrates. Our results may find a wide range of applications in MoS 2 -based devices, sensors, and metal-free nanoscale bio-imaging. a E mail: almqtar@tamu.ed;dmitri.voronine@gmail.com 1

2 Tip-enhanced Raman scattering (TERS) is a spectroscopic technique that combines surfaceenhanced Raman scattering (SERS) with scanning probe microscopy, e.g. atomic force microscopy (AFM) or scanning tunneling microscopy (STM), providing chemical and topographic information with a high spatial resolution. 1-4 The strongest SERS signals are typically attributed to so-called hot spots, i. e. localized electromagnetic fields in nanoscale gap junctions between metallic nanoparticles. 5,6 Similarly, a sharp metallic tip can generate a hot spot when placed in proximity of a metallic surface, resulting in the electromagnetic near-field enhancement at the tip apex. 4,7,8 This gap-mode TERS has been intensively used for nanoscale imaging and single molecule detection. 9,10 Au and Ag are the conventional metallic gap-mode TERS substrates which, in some cases, may impose disadvantages such as spectral background, molecular deformation, and metal-catalyzed side reactions. 11,12 Recent developments used Au nanoparticles on dielectric substrates to circumvent limitations of the metallic substrates, 13,14 some of which cannot be easily overcome due to the metallic nature of the nanoparticles. Recent studies demonstrated SERS enhancement on semiconducting two-dimensional (2D) materials which depends on the molecule-substrate coupling via the chemical enhancement mechanism. 11,15 In particular, graphene has been extensively studied due to its interesting electric and chemical properties, leading to chemical enhancement of Raman signals termed grapheneenhanced Raman scattering (GERS). 16,17 TERS on 2D materials, however, is yet to be demonstrated although the corresponding theoretical analysis has been developed. 18 Here, we demonstrate gap-mode TERS on bulk molybdenum disulfide (MoS 2 ) substrate, which is an emerging layered transition metal dichalcogenide material with promising optoelectronic applications

3 Previous studies used an alternative gap-mode TERS approach by functionalizing the tip with probe molecules instead of depositing them onto metallic substrates. 22,23 This approach may be used to design reference materials for the comparison of different TERS substrates based on the functionalized tip measurements under identical conditions such as molecular concentration and orientation. 24,25 Here, we compare the relative TERS enhancement on Au and MoS 2 substrates using a Au nanosphere tip functionalized with CuPc molecules and investigate the efficiency of MoS 2 as a substrate for gap-mode TERS. Fig. 1a shows the sketch of the functionalized TERS process. Tip-enhanced Raman spectra were obtained using ihr550 spectrometer (Horiba) and Synapse CCD detector. Briefly, a 532 nm excitation laser (5W Verdi, Coherent Inc.) was focused by an aspheric lens (N.A. = 0.45) on the xyz-controlled sample stage (5 mw at the sample) in the atomic force microscope (AFM) system (M4000, Nanonics Imaging, Ltd). A single Au-nanosphere (100 nm radius) affixed to the tipapex of a SiO 2 cantilever (Nanocs, Inc) was functionalized with CuPc molecules by scanning the surface of bulk MoS 2 covered by vapor-deposited CuPc (scanning area = 9 µm 2, thickness of CuPc < 10 nm) using the tapping-mode AFM (tip-oscillation frequency ~ 30 khz, acquisition time = 5 s). TERS measurements were performed by approaching the CuPc-functionalized tip to three different substrates: Au foil (Goodfellow, 99.95%, thickness = 0.05 mm), bulk MoS 2 (SPI Supplies, thickness ~ 1 mm), and SiO 2. The average tip-substrate distance was 10 nm ± 2 nm. The average distance is controlled by the oscillating amplitude of the tip used in the tappingmode AFM, and was the same in all measurements. The multilayer MoS 2 substrate was mechanically exfoliated to expose a clean surface before the measurements. Due to the surface inhomogeneity, we performed measurements at several spatial locations on MoS 2, half of which 3

4 showed strong TERS enhancement results presented below whereas the other half showed smaller enhancements. FIG. 1. (a) Sketch of the functionalized tip-enhanced Raman scattering (TERS) process. An enlarged view shows the chemical structure of the copper phthalocyanine (CuPc). (b) TERS spectra of CuPc in the vicinity of Au, MoS 2, and SiO 2 substrates, and in air. The spectra are vertically shifted for convenience. Fig. 1b shows TERS spectra of CuPc obtained with a Au nanosphere tip in the vicinity of three different substrates: Au, MoS 2, and SiO 2, and in air where the spectrum was measured from the tip itself. All three substrates show the same order of magnitude signals as a result of the relatively large tip-substrate distance. The similar enhancement observed on the MoS 2 and Au substrates indicates the tip-substrate coupling and gap-mode enhancement on MoS 2 in the TERS measurements. We were able to obtain similar results with a different tip shape and size to confirm the results with a different tip geometry. It should be noted that the tip itself can have surface plasmons leading to an enhanced local field in the absence of any substrate. Therefore, TERS signals on SiO 2 are similar to the signals of the tip itself in air in the absence of any substrate, see Fig. 1b. 4

5 In order to examine the nature of the electromagnetic gap-mode enhancement separately from the chemical effects, we analyzed the TERS signals of the strongest CuPc transitions. Fig. 2b shows the relative enhancement on Au (circles) and MoS 2 (triangles), i. e. the ratio of the CuPc Raman intensity with the tip in the vicinity of the Au and MoS 2 substrates to that of SiO 2. The low frequency modes at 581 and 668 cm -1 show larger relative enhancement due to the selection rules. 26,27 A detailed discussion of the enhancements of different frequency modes and the corresponding mode assignment was previously reported. 11,28 The difference between the relative enhancement on Au and MoS 2 for different CuPc modes is nearly the same, which is in contrast to the previously reported different enhancement of low and high frequency modes on graphene compared to MoS 2 due to the chemical mechanism in SERS measurements. 11 The mode-independent difference in the relative enhancement indicates that the same electromagnetic mechanism dominates on both substrates, i.e. the enhancement due to the chemical effects plays a minor role. Negligible chemical contribution is expected because of the relatively large average tip-substrate distance of ~ 10 nm during the tapping-mode AFM. However, the chemical interactions such as charge transfer are expected for smaller sub-nm gaps. 29,30 5

6 FIG. 2. Relative enhancement of the CuPc TERS signals on Au (circles) and MoS 2 (triangles) substrates. The error bar of the relative enhancement is estimated to be ± 0.25 for both substrates. Different transitions show similar relative enhancement difference Δ which indicates similar gap-mode electromagnetic enhancement mechanism on Au and MoS 2. In order to compare our experimental results with the well-established theory, we performed calculations based on the electrostatic model where the metallic tip is modeled by a point dipole induced at z = a + d, where a is the tip curvature radius, and d is the tip-substrate distance. The interaction of the dipole and its image at z = a d results in an enhanced local field in the nanoscale gap junction, as shown in Fig. 3a. The TERS enhancement is determined by calculating the ratio of the local field s magnitude to that of the far field. That ratio is denoted by and derived with a dipole polarized normal to the surface. 31 In order to generalize the expression for κ, we added a constant γ to the denominator of the last term and set it to be 1 when the dipole is normal to the surface, or 1 2 when it is parallel to the surface. 32 Hence, the resulting expression is 1 2 α α 4, 6

7 where α is the dipole polarizability and is the coefficient associated with the image dipole. By solving the Laplace equation with proper boundary conditions, one finds α a ε 1 ε 2 and β ε 1 / ε 1, where ε and ε are the dielectric functions of the tip and the substrate, respectively. We used previously reported values of the dielectric functions of Au and MoS 2. 34,35 Because the SERS enhancement is proportional to the fourth power of the local field, 5,6 the relative enhancement is calculated by taking the fourth power of the field enhancement on Au and MoS 2 to that on SiO 2, i.e. the relative enhancement = κ κ, where i corresponds to Au or MoS 2 substrates. The calculation of the relative enhancement factors of the functionalized TERS is simplified compared to the conventional TERS due to the cancellation of the illumination area effects, i. e. the same number of molecules contribute to both near field and far field on any substrate. This functionalized tip approach therefore also eliminates the far field background of the conventional TERS. Fig. 3b shows the calculated relative enhancement on Au and MoS 2, for both polarizations vs. the tip-substrate distance. The decrease of the relative enhancement with the distance is slower compared to the conventional TERS enhancement. 33,36 The experimentally observed relative enhancement on Au agrees with the calculated relative enhancement, whereas the measurements on MoS 2 show a range of enhancement values due to the uneven distribution of charge carriers on the surface, 37,38 and the spatial dependence of the tip-sample interactions at the polycrystalline bulk MoS 2 edges and grain boundaries. 30 7

8 FIG. 3. (a) Gap-mode enhancement for a metallic nanosphere tip in the vicinity of a substrate. Two spheres represent the induced dipole of the tip and its image in the substrate. (b) Calculated relative enhancement on Au (solid) and MoS 2 (dashed) substrates vs. tip-substrate distance for the perpendicular E (thick line) and parallel E ll (thin line) components of the field. Highlighted bar shows the average tipsubstrate distance in the experiment. 8

9 In summary, we demonstrated electromagnetic gap-mode enhancement on MoS 2 in TERS measurements with a Au nanosphere tip functionalized with CuPc molecules. This functionalized TERS approach allows for the suppression of the conventional TERS background and direct comparison of the enhancement on various substrates under highly reproducible conditions. Our results show that MoS 2 can generate large relative electromagnetic gap-mode enhancement comparable to that of conventional plasmonic substrates, which could be used as a substrate for sensing and nanoscale bio-imaging applications. Acknowledgments We acknowledge the support of the National Science Foundation Grants No. EEC (MIRTHE ERC), No. PHY , No. PHY (INSPIRE CREATIV), No. PHY , and CHE , the Office of Naval Research, and the Robert A. Welch Foundation (Awards A-1261 and A-1547). 9

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12 Intensity (counts/s) Au substrate (a) Raman shift (cm -1 ) (b) Au MoS 2 SiO 2 air

13 Relative enhancement Raman shift (cm -1 )

14 (a) (b) E E Au d Au substrate MoS 2