In this work, we conducted nanoscale imaging of both the Raman peaks and electron Raman scattering to study the strain and doping of graphene wrinkles and compared the varying Raman spectral characteristics to flat graphene using tip-enhanced Raman spectroscopy (TERS). We found that on the wrinkle, anisotropic strain and non-uniform doping was present. We also found that the G band peaks were broader on the flat graphene as compared to on the wrinkle and attribute it to the plasmon-mediated excitation that opens other possible paths for electron-hole recombination due to the high electric field confinement underneath the tip apex. A broad background continuum that we surmise is electronic Raman scattering originating from both the graphene monolayer and the Au(111) substrate was observed and when imaged has a strong correlation to the TERS images of the Raman peaks.
In order to characterize nanomaterial-based devices, such as transistors, in working conditions (e.g. in ambient), we are constantly developing and improving our tip-enhanced Raman spectroscopy (TERS) system to probe our samples with both high chemical sensitivity and high spatial resolution. We have achieved the detection of temperature at nanoscale volumes using our technique called tip-enhanced THz-Raman spectroscopy (TE-THzRS) and have achieved sub-nanometer spatial resolution through our environment stable TERS system. We have also probed nanometer scale strain variations in monolayer graphene membranes using TERS. Now, aside from studying the strain distribution in graphene wrinkles, we are also studying carrier doping, one aspect of graphene’s electronic properties, through TERS.
In this work, we studied the Raman spectra characteristics, strain distribution and carrier concentration of monolayer graphene wrinkles on a Au(111) crystal facet using tip-enhanced Raman spectroscopy (TERS) in ambient. G-band splitting was observed in certain areas of the wrinkle indicating the presence of anisotropic strain. A broad background was also observed in the near-field spectra that is hypothesized to be electronic Raman scattering originating from the graphene monolayer and the Au(111) substrate. Hence, careful spectral analysis must be done since this broad background potentially contains information on the sample.
Tip-enhanced Raman spectroscopy (TERS) done in ambient conditions opens the door to characterize the as-fabricated properties of nanodevices in their operating environment with both high spatial resolution and high chemical sensitivity. With sub-nanometer resolution now achievable using our TERS system in ambient, we can image nanoscale strain variations in graphene and study the strain distribution in such local domains. The effects of high photon confinement are also investigated, whose immediate manifestation is the plasmonic activation of certain Raman modes. This leads to the question of how to analyze strain at the near-field, which is quite relevant today as technology continuous to grow ever smaller.
We present a tip-enhanced Raman spectroscopy (TERS) system that permits efficient illumination and detection of
optical properties in the visible range to obtain high contrasts Raman signals from strained silicon (ε-Si) assembled on
silicon germanium substrate using an edge filter. The cut-off investigation in a depolarized TERS configuration. We overshadow wavelength of the edge filter is tuned by changing the angle
of incident beam to deliver high incident power and effectively collect scattered near-field signals for nanoscopic
strong far-field background signals from Raman
active materials by utilizing the results obtained from depolarized surface-enhanced Raman scattering experiments in
conjunction with silicon Raman tensor calculation to quantify which polarizer, analyzer and sample azimuth combination
gives the minimum far-field signals. Here, we utilize the s-polarization instead of p-polarized light in conjunction with
polarization properties of ε-Si to obtain a high contrast Raman signal. We found that for imaging Raman active and bulk
crystalline materials such as silicon, background signal suppression (s-illumination) is more important than the field
enhancement with strong far-field signal levels (p-polarization). The utilization of an edge filter for shorter collection
time, depolarized configuration for higher contrast and tip heating for higher resolution are discussed.
Vibrational spectroscopy, including Raman spectroscopy can be used for identifying molecular species, which is
not possible by a scanning probe microscopy or an electron microscopy. Moreover, vibrational spectra contain structural
information, such as intermolecular interactions, molecular orientations, and symmetry distortions of each species.
Therefore, Raman spectroscopy is a powerful tool for studying the chemical composition of matter.
By employing Tip-enhanced Raman spectroscopy (TERS), we can perform Raman spectroscopy with nano-scale
spatial resolution. Our approach relies on the enhanced filed near a laser irradiated metal tip which works as the Raman
excitation source. We have investigated nano-composite materials by TERS. Near-field Raman spectra revealed the
nano-scale properties of molecules encapsulated in single-wall carbon nanotubes (SWNT). The enhanced field act on
encapsulated molecules through the wall of SWNT to extract chemical information inside. &bgr;-carotene which has strong
Raman intensities under visible light illuminations is used as an encapsulated molecule. The advantage of Raman
spectroscopy is that the information of both SWNT and &bgr;-carotene can be obtained at the same time. So, it is possible to
discuss the interaction between SWNT and the encapsulated molecules. Near-field Raman spectra measured at several
different positions on SWNT bundle show that &bgr;-carotenes inside the tube are not uniformly distributed. We also find
that the filling rates and the peak positions of the radial breathing mode of SWNT are linearly correlated.
Strained silicon (ε-Si), the fundamental material of integrated circuit, is finding tremendous attention not only
because it boosts the speed but also reduces the power consumptions of electronic devices. Carrier mobility in a ε-Si thin
layer is enhanced compared to unstrained layers. However, strain distribution in ε-Si layers is inhomogeneous in the
nano-scale, which can degrade performance of electronic devices. Raman spectroscopy can be used to study strain
fluctuations in silicon because the optical phonons in Raman spectra are strongly influenced by strain. Though silicon are
Raman active devices, the Raman efficiency of a nanometer layer of strained silicon is extremely weak and is often
eclipsed under the Raman scattering of underlying buffer substrates. Here, we utilized surface enhancement in Raman
scattering to overcome weak emission problems and to suppress averaging effect. Thin ε-Si layers were covered with
thin silver layer to invoke surface enhanced Raman spectroscopy. This technique is promising but it lacks the spatial
resolution in the nano-scale due to diffraction limit from the probing light. In order to achieve nano-scale spectroscopy,
point-surface-enhancement was used, rather than a large surface enhancement. We used a silver-coated sharp tip, just
like SERS, but only the sample region very close to the tip apex is characterized. This technique, known as the tipenhanced
Raman spectroscopy, provides nanometric resolution in our measurement. For further improvement of SNR,
we introduce several approaches mainly for the suppression of background signals arising from crystalline bulk materials.
The characterization techniques describe above is applicable to other nano-materials.
Nanoscale characterization of strained silicon is essential for developing reliable next generation integrated circuits. Vibration mode of Si-Si in strained silicon was selectively enhanced to be observed by surface enhanced Raman spectroscopy technique. Covering the silver island film on a strained silicon layer Raman signal from the strained silicon can be detected with a high sensitivity against the overwhelming background signal from the underlying silicon layer. This technique allowed us for micro-Raman spectroscopy on strained silicon, and is straightforward to nano-Raman spectroscopy by tip-enhanced Raman microscope in which a sharpened metallic tip is used instead. We observe localized strains in strained silicon by tip-enhanced near-field Raman spectroscope in reflection-mode. The tip-enhanced Raman spectra show that the Raman frequency and intensity of strained silicon were different within a crosshatch pattern induced by lattice-mismatch. Micro Raman measurements, however, show only uniform features because of averaging effect due to the diffraction limit of light.
Optical microscopy that can visualize the molecular vibration with a nanometric spatial resolution has been realized by a combination of near-field optics and coherent anti-Stokes Raman scattering (CARS) spectroscopy. A metallic probe with a sharp tip is used to strongly enhance optical near-field in the local vicinity of the tip owing to the excitation of local surface plasmon polariton. CARS signals of molecules in the local area can be strongly induced by the plasmonic field. We have visualized DNA molecules and single-walled carbon nanotubes (SWNTs) with a spatial resolution far beyond the diffraction limit by the tip-enhanced near-field CARS microscopy.
A metallic nano-probe has locally induced coherent anti-Stokes Raman scattering (CARS) of adenine molecules in a nanometric DNA network structure. The excitation fields and CARS polarization are enhanced by the tip apex of the nano-probe through the excitation of local surface plasmons. Owing to the third-order nonlinearity, the excitation of the CARS polarization is extremely confined to the end of the tip apex, resulting in the spatial resolution far beyond the diffraction limit of light. Our CARS microscope using a silver-coated probe visualized the DNA network structure at a specific vibrational frequency (~1337 cm-1) of adenine molecules with a spatial resolution of ~15 nm and sufficient sensitivity.
A light microscope capable to show images of molecules in nanometer scale has been a dream of scientists, which, however, is difficult due to the strict limitation of spatial resolution due to the wave nature of light. While there have been attempts to overcome the diffraction limit by using nonlinear response of materials, near-field optical microscopy could provide better detecting accuracy. In this paper, we present molecular distribution nano-imaging colored by Raman-scattering spectral shifting, which is probed with a metallic tip. The metallic probe tip has been used to enhance the optical field only in the vicinity of probe tip. The effect is similar to the one seen in the detection of molecules on the metal-island film, known as surface-enhanced Raman spectroscopy (SERS), while in this case a single metallic tip works for the field enhancement in nanometer scale.
KEYWORDS: Near field scanning optical microscopy, Luminescence, Raman scattering, Near field, Light scattering, Stray light, Photons, Near field optics, Objectives, Rhodamine
We have developed a NSOM which has a metallic probe tip and a highly focused evanescent light field spot. Evanescent illumination effectively rejects the background light, e.g. the stray light from the shaft of the probe. By suppressing the stray light and utilizing the field enhancement generated by the metallic probe, a sudden increment of the fluorescence was observed in the near-field region. We have used this for near-field Raman scattering detection of molecules vibrations with the aid of surface enhanced Raman scattering. One specific stokes-Raman-shifted lines was observed by near-field excitation together with several other lines that were excited by the far-field light.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.