Article Cite This: J. Phys. Chem. C 2018, 122, 2753−2760
pubs.acs.org/JPCC
Probing Interaction between Individual Submonolayer Nanoislands and Bulk MoS2 Using Ambient TERS Blake Birmingham,†,‡,§ Zachary Liege,†,‡,§ Nick Larson,† Weigang Lu,† Kenneth T. Park,† Ho Wai Howard Lee,†,‡,§ Dmitri V. Voronine,*,‡,§,∥ Marlan O. Scully,†,‡,§ and Zhenrong Zhang*,†,‡ †
Department of Physics and ‡Baylor Research and Innovation Collaborative, Baylor University, Waco, Texas 76706, United States Institute for Quantum Science and Engineering, Texas A&M University, College Station, Texas 77843, United States ∥ Department of Physics, University of South Florida, Tampa, Florida 33620, United States §
S Supporting Information *
ABSTRACT: Tip-enhanced Raman spectroscopy (TERS) has shown that detecting single molecules with a high spatial resolution is possible in ultrahigh vacuum (UHV) at low temperature with plasmonic metallic substrates. It is still challenging to probe interactions of molecules with semiconductors, which is important in biosensing, photovoltaics, and many other applications. Here we demonstrate that in ambient conditions it is possible to obtain Raman signals from submonolayer molecular islands on bulk MoS2 using TERS. Analysis of relative Raman signal intensity ratio and Raman spectral peak position from spatial TERS mapping showed differences in the adsorbate− adsorbate and adsorbate−substrate interactions on Au and MoS2 substrates. The Raman transition which involves the vibration of the metal center of the CuPc molecule experienced a change in the relative Raman signal intensity ratio due to the differences in the molecule−substrate charge transfer interaction. In comparison to the other vibrational modes, the vibrational modes of the surface charge transfer interacting moieties involving the metal center experienced the smallest shift in the Raman spectral peak position on both Au and MoS2 substrates. Further, the distributions of the peak position and relative intensity were narrower at the center of the island with respect to the isolated molecules due to the adsorbate−adsorbate interaction. This study shows the contribution of charge transfer between molecules and MoS2 in the TERS spectra.
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INTRODUCTION
MoS2 and graphene, are imaged in ambient conditions with atomic force TERS systems.7−9 While plasmonic noble metal substrates such as Au and Ag offer the highest spatial confinement of Raman enhancement due to the tip−sample gap plasmonic mode,10,11 there is a need for imaging molecules on nonmetallic substrates in various applications. SERS of molecular films was observed on nonmetallic semiconducting substrates. Substrates, such as MoS2 and graphene, exhibit chemical enhancement of Raman scattering.12−14 This enhancement has been further amplified by TERS.15,16 However, submonolayer molecule TERS resolution was not previously demonstrated on a bulk semiconducting substrate such as MoS2. It is important to push the limits of detection using TERS on both conducting and semiconducting substrates in ambient conditions for studying molecule−molecule interaction and organic molecule−substrate interaction. MoS2 has a wide range of applications from photovoltaics, energy storage, and optoelectronics to catalysis.17−19 MoS2 has
Imaging molecules with both topographic and chemical information is important in many fields such as catalysis, biosensing, photovoltaics, and material science. Recently, tipenhanced Raman spectroscopy (TERS) was developed into an astounding tool for studying interaction of individual molecules with surfaces.1−4 The technique has proven that scanning probe microscopy capable of obtaining topographic images with high spatial resolution coupled with Raman spectroscopy can be used to achieve subnanometer scale Raman signals. Individual molecules, intermolecular interactions, and even intramolecular excitations were imaged with chemical vibrational information on single crystal Au and Ag surfaces in ultrahigh vacuum (UHV) at cryogenic temperatures.5,6 Nanoscale chemical mapping with TERS is not technically limited to low temperatures. Imaging of submonolayer molecular domains was conducted on Au single crystal at room temperature.1 While experiments conducted in UHV allow control of surface roughness and adsorbate selection, TERS has been proven to be robust even in ambient conditions. TERS and tip-enhanced photoluminescence of carbon nanotubes and monolayer two-dimensional materials, © 2018 American Chemical Society
Received: November 11, 2017 Revised: January 12, 2018 Published: January 13, 2018 2753
DOI: 10.1021/acs.jpcc.7b11178 J. Phys. Chem. C 2018, 122, 2753−2760
Article
The Journal of Physical Chemistry C
Figure 1. STM images of (a) isolated CuPc molecules on bulk MoS2 after 3 s CuPc deposition, (b) CuPc island on bulk MoS2 after 30 s CuPc deposition, (c) isolated CuPc molecules on bulk MoS2 after 30 s CuPc deposition, and (d) surface of Au foil. Additional examples STM images of isolated CuPc molecules and of MoS2 substrate are provided in Figure S1 (see Supporting Information).
a 660 nm CW laser. The excitation energy is in resonance with the Q-band electronic transition of CuPc.27
been used as an industrial catalyst for hydrodesulfurization and is a promising catalyst for hydrogen evolution.20,21 While these catalytic reactions have been extensively studied, the specifics of the structure and mechanism of activation sites in reactions are not fully understood. TERS possesses the capability to provide new insights into adsorbate interaction and reaction on bulk MoS2. MoS2 is a two-dimensional transitional metal dichalcogenide with a hexagonal arrangement of Mo and S atoms covalently bonded into single layers of S−Mo−S. The interactions between the layers are via weak van der Waals forces. The structural dependence of the photoluminesence of monolayer MoS2 was measured with 20 nm resolution on Au substrates using TERS.22 However, bulk MoS2, consisting of many layers up to several millimeters thick, forms a weak tip−sample gap mode in TERS.23 It has not been shown whether a weak gap mode TERS can resolve submonolayer molecules on a bulk semiconducting substrate. Copper phthalocyanine (CuPc) is an organic molecule applicable to photovoltaics, light-emitting diodes, organic field effect transistors, and optoelectronic devices, etc.24−26 Here, we utilize CuPc as a probe molecule to study the molecule− molecule and molecule−substrate interactions on bulk MoS2. CuPc has a large resonance Raman scattering cross section, and surface coverage of the molecule can be well-controlled to the submonolayer level using physical vapor deposition.
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RESULTS Figure 1a shows STM images obtained on MoS2 surface after 3 s CuPc evaporation. Isolated CuPc molecules are observed randomly distributed on the flat MoS2 surface. The small bright features are CuPc molecules. The arrow marks one of the isolated CuPc molecules in the STM image. Inset in Figure 1a is an atomically resolved STM image of the MoS2 surface, which shows the individual sulfur atoms. The lattice constant of the MoS2 surface is 3.2 Å as expected for bulk MoS2.28 The circle marks a single CuPc molecule, which has an appearance of a dark hole in the center of the bright ring. The dark hole at the center is consistent with the reported appearance of CuPc molecules in STM images.29 The large dark spots visible in the Figure 1a are defects in the MoS2 surface. The defects were prevalent on the MoS2 surface and ranged from 0.5 to 2 nm deep, from about half of a monolayer to several layers deep.30 The RMS roughness of the MoS2 substrate obtained from large STM images is 0.4 nm (Figure S1b), which shows that the MoS2 surface is atomically flat in most of the areas on the surface. The CuPc molecular density was approximately 15 molecules/900 nm2 as determined from the STM images taken from different areas on the surface. The 30 s CuPc evaporation onto bulk MoS2 produced a nonuniform submonolayer coverage of molecules. There are regions of self-assembled molecular islands (Figure 1b) and regions of isolated molecules on the surfaces (Figure 1c). Figure 1b is taken on a CuPc island, where CuPc molecules self-assembled into an ordered structure. The patterns of bright and dark rows correspond to the close-packed CuPc molecules (marked by small circles) with a rectangular unit cell of 14.1 Å × 13.1 Å. The size of the unit cell is consistent with the closepacked monolayer CuPc structure reported previously.29,31−33 The formation of islands is a result of the attractive interaction between the molecules. Figure 1c is an STM image taken from a different position on the same surface as Figure 1b. This image shows isolated individual molecules, which appear similar to those in the STM images taken on the low coverage surface (Figure S1a). The presence of both isolated and assembled molecules indicates that islands of assembled molecules on the surface are surrounded by areas with isolated molecules. The interface between the ordered and disordered areas tend to consist of randomly oriented loose packed clusters of molecules.34−37 The STM image taken on the Au foil after the 3 s CuPc deposition is shown in Figure 1d. The measured root-meansquare surface roughness of the Au foil is 1.5 nm. As seen from
EXPERIMENTAL SECTION
Bulk MoS2 (SPI Supplies) was mechanically exfoliated to remove surface impurities and adsorbates. Au foil (Goodfellow, 99%) was sonicated in three consecutive cycles of ethanol, acetone, and ethanol to remove adsorbates. Copper phthalocyanine (CuPc) molecules were then thermally deposited onto the bulk MoS2 and Au foil via physical vapor deposition in high vacuum using a low-temperature effusion cell (UMC Corp.). To keep the coverage comparable, CuPc was evaporated onto Au foil and MoS2 substrate simultaneously. The coverage was controlled via temperature and deposition time. After deposition, the samples were taken out from the evaporation chamber and loaded into the ultrahigh-vacuum (UHV) scanning tunneling microscope (STM, SPECS) chamber. The coverage and the distribution of the molecules were determined from the STM images. The images were taken on MoS2 at bias voltages around 1.20 V and tunneling currents around 0.10 nA. The samples were independently imaged using an atomic force microscope (AFM, OmegaScopeR, AIST) combined with a Raman spectrometer (LabRam-EVO, Horiba Scientific). The tips used for TERS were Au tips with the apex curvature radius of ∼10 - 20 nm (Horiba Scientific) and was excited with 2754
DOI: 10.1021/acs.jpcc.7b11178 J. Phys. Chem. C 2018, 122, 2753−2760
Article
The Journal of Physical Chemistry C
resolution, i.e., the excitation field confinement of the tipenhanced signal, is at least 10 nm. In addition to the overall integrated intensity variation, the Raman signal of different pixels on the TERS map exhibits variations in the relative ratio of different excited vibrational modes. For example, in spectrum 2 of Figure 2b the ratio of the intensity of the 749 cm−1 peak over the 1530 cm−1 is 2.1. However, in spectrum 3, the 749/1530 ratio is 1.1. Furthermore, there are shifts in the positions of several vibrational modes. An observable difference in the center of the 1530 cm−1 peak between spectra 2 and 3 is indicated by the vertical dotted line in Figure 2c. These relative peak ratio and peak position shifts are discussed in detail in Figures 4 and 5. Figure 3a is a TERS map resolving a self-assembled island of CuPc molecules on the bulk MoS2 surface after a 30 s CuPc
the image, the surface features on the Au foil were too high and irregular to resolve single molecules with a height of ∼3 Å via STM. It is known that metal phthalocyanine is mobile on single crystal Au(111) at room temperature.34 However, the irregular structures on the Au foil surface serve as defects. Any longrange mobility of the molecules is likely to be hindered by defects on the surface. Therefore, it is reasonable to assume that the molecular distribution on Au foil after 3 s evaporation is similar to that on MoS2 with coverage as low as approximately 15 molecules/900 nm2. The CuPc would appear as isolated molecules on the surface of the Au foil. Because of the roughness of the Au foil, the molecules are likely to be in a wide array of orientations with different tilts and rotations. Figure 2a shows a representative map of the tip-enhanced Raman signal taken on the surface of Au foil after 3 s CuPc
Figure 2. (a) TERS map of the isolated CuPc Raman signal on Au surface. (b) Raman spectra from the adjacent tip positions separated by 10 nm taken from the TERS map of CuPc on Au surface and their Lorentzian peak fits (black). (c) Zoom-in spectra showing the peak position shift of the 1530 cm−1 mode.
deposition. The map is composed of pixels taken as the tip is raster scanned across the surface in 10 nm increments. At each pixel, a Raman spectrum is taken with the tip in contact with the surface,
