We report the first time nanoscale, tip enhanced Raman scattering (TERS) imaging of the SeMoS Janus monolayers crystals both as-grown on gold foil and transferred from the growth substrate to the gold-coated silicon wafers. Due to the preferential enhancement of the out-of-plane modes in the gap-mode of TERS, the TERS spectra of SeMoS differ from the conventional Raman spectra reported earlier [1 , 2]. The A11and A12 out-of-plane modes are shown to be the first and the second strongest Raman peaks in TERS, while in conventional Raman spectroscopy the A12 mode is extremely weak. Interestingly, the red shift of the spectral position of A12 mode correlates with a decrease of the contact potential difference in Kelvin probe force microscopy (KPFM) images. While the TERS maps mostly show the Raman spectra characteristic to the high quality SeMoS Janus monolayers, we observed in some cases narrow, below 20-30 nm, areas that featured a peak at 406 cm-1 which has been proved to be the A’ band of MoS2. The ability to detect the nanoscale imperfections in Janus monolayer crystals is a mandatory condition for optimizing their synthetic routes. TERS imaging cross-correlated with KPFM measurements demonstrate the applicability for the nanoscale assessment of the structural homogeneity of both the as-grown and transferred SeMoS Janus monolayer crystals.
References
1. Z. Gan, I. Paradisanos, A. Estrada-Real et.al. ADVANCED MATERIALS 2022 34, 2205226
2. Marko M. Petrić, Malte Kremser , Matteo Barbone et.al. PHYSICAL REVIEW B 103, 035414 (2021)
Tip enhanced Raman Scattering (TERS) imaging is gaining more and more popularity for the nanoscale spectroscopic characterization of novel 2D semiconductors and their vertical and lateral heterostructures. Gap mode TERS imaging when a thin sample is sandwiched between the TERS active SPM probe and plasmonic substrate like silver or gold, is the most popular and advantageous option due to strong enhancement of optical electric field in the tip-substrate cavity. Despite multiple examples of successful application of the gap-mode TERS imaging of nanoscale heterogeneities in 2D semiconductors such as transition metal dichalcogenides (TMD), the exact physics of complex interaction between the plasmons in the tip-substrate junction and excitons in TMDs remains poorly understood. One of the possible approaches towards probing complex resonant phenomena occurring in gap-mode TERS experimental conditions is performing TERS imaging with varied excitation wavelength [1]. I’ll demonstrate that the TERS spectra of WS2-WSe2 vertical heterostructures obtained with 785 nm, 632.8 nm, 671 nm and 593.8 nm excitation may differ significantly, the intensity of characteristic Raman bands of WSe2 at around 250-260 cm-1 being extremely low in case of 632.8 nm excitation but rising as we move to shorter or longer excitation wavelengths. I’ll discuss the advantages of concurrent two-color excitation TERS imaging as well as interesting counterintuitive variations of the intensity of TERS bands of the monolayer WS2 as the function of the excitation wavelength. Finally, I’ll demonstrate that the ratio of the intensity of the in-plane E2g and the out-of-plane A1g modes in the above- and below- the-band-gap non resonant TERS spectra of bilayer WS2 on silver inverses as we move from 532 nm to 785 nm excitation.
Références
[1] Andrey Krayev, Peng Chen, Humberto Terrones, Xidong Duan, Zhengwei Zhang, and Xiangfeng Duan The Journal of Physical Chemistry C 2022, 126, 11, 5218-522
DOI: 10.1021/acs.jpcc.1c10469
Despite the pandemic, recent two years showed significant progress in application of Tip Enhanced Raman Spectroscopy (TERS) and tip enhanced Photoluminescence (TEPL) to nanoscale spectroscopic characterization of lateral and vertical heterostructures of transition metal dichalcogenides (TMDCs).
In this talk I'll discuss how TERS imaging revealed existence of the alloyed transition area at the junction of MoS2-WS2 heteromonolayers, it's width varying greatly along the junction line from over 500nm to less then 25nm ( pixel limited resolution in those experiments).
Next, I'll demonstrate how ultra low frequency TERS imaging that can directly probe interlayer phonons in twisted vertical heterobilayers of WS2 and WSe2 can sense the layer decoupling in the nanobubbles formed within these heterostructures. In addition, I'll demonstrate that the Stokes/ anti-Stokes ratio of the first and second order modes in TERS spectra of WSe2 and WS2-WSe2 heterostructures can be dramatically different even when those bands are separated by less than 10 cm-1 in spectral space. Possible physical mechanisms of such dramatic difference will be discussed.
Finally, I'll demonstrate a crucial importance of varied excitation wavelengths for TERS characterization of TMDCs and their vertical heterostructures. In case of the CVD-grown WS2-WSe2 vertical heterostructures, with 785 nm excitation the characteristic Raman bands of both constituents can be clearly seen, while TERS spectra of the same crystal obtained with the same TERS probe but with 638nm excitation showed greatly suppressed intensity of the WSe2 bands. In addition, I'll show that the TERS spectra of the monolayer WS2 collected with 671 nm excitation, contain a resonant band at 324 cm-1, which is completely absent in TERS spectra of the same crystal collected with 638 or 785 nm excitation, which assumes the presence of the resonance at this wavelength.
Raman microscopy proved to be an extremely useful technique for characterization of 2D materials such as graphene, transition metal dichalcogenides (TMDs), black phosphorous, etc. Unfortunately, natural spatial resolution of confocal Raman microscopy, which is limited by the wavelength of the laser used (400-800 nm), is not sufficient for mapping heterogeneities and defects in these materials with characteristic dimensions of few – to few tens of nanometers.
Tip Enhanced Raman Spectroscopy (TERS) provides dramatically improved spatial resolution of Raman maps, down to few nanometers, and in addition provides dramatic enhancement of the Raman signal. Since TERS is a relatively new technique, peculiarities of the near-field Raman response of many 2D materials still remain to be discovered and explained.
Interesting unexpected effect was observed in the course of TERS characterization of WSe2 and MoSe2 exfoliated to gold and chromium. It is well known that TERS produces the strongest enhancement in so-called gap mode, when a thin sample is sandwiched between a plasmonic tip and a plasmonic substrate, usually silver or gold. To our surprise, TERS spectra of both WSe2 and MoSe2 crystals exfoliated to chromium showed very similar intensities of characteristic Raman bands compared to samples exfoliated to gold, although the background spectra obtained from the bare metal areas were much weaker for chromium compared to gold. Calculations of the optical field intensity of a TERS probe over gold and chromium surface confirmed that we indeed observed a gap mode TERS response on non-plasmonic chromium substrate. This important observation expands the choice of substrates suitable for high quality TERS characterization of 2D materials.
Another interesting phenomenon discovered in the course of TERS imaging of WSe2 and MoSe2 deposited on metallic substrates was the appearance of new Raman peak in WSe2 deposited on silver at 295-297 cm-1 and an intense peak in TERS spectra of MoSe2 at 335cm-1, which is either absent or much less pronounced in conventional confocal Raman spectra of this material. We’ll discuss possible nature of these unexpected peaks.
2D materials such as graphene and its derivatives and broad class of transition metal dichalcogenides attracted significant attention of the research community during last decade. Tip enhanced optical spectroscopy ( TEOS) that includes tip enhanced Raman spectroscopy (TERS) and tip enhanced photoluminescence (TEPL) allows characterization of defects and inhomogeneities in these materials at nanometer scale, something conventional confocal Raman or photoluminescence microscopy can not do.
We observed that the gap mode TERS and TEPL reponse in grpahene, graphene oxide and TMDCs gets significantly enhanced over the wrinkles in 2D sheets. Despite similarity in behavior, the nature of this increased intensity is different for graphene and TMDCs. In case of 2D carbon, D,G,2D modes, all in-plain vibrations, got enhanced over wrinkles due to increased coupling of the optical electric field normal to the sample plain and the vertically aligned portions of 2D sheet in the wrinkles. In case of TMDCs such as WS2, or others, mechanical strain in wrinkles results in funneling of defects and excitons into those areas, which leads to increased concentration of defect bound excitons that demonstrate strongly enhanced and significantly red-shifted PL response, which should be expected taking into account that the binding energy of defect-bound excitons is lower compared to free excitons in 2D materials.
Different nature of increased TEOS response in wrinkles of 2D sheets of carbon and TMDCs is further supported by the fact that TERS signal of flat graphene transferred to gold is negligible, since the in-plain modes do not couple to the electric field in the tip-substrate gap, while the TERS signal of flat sheets of TMDCs on gold or silver is very strong, specifically for the out-of-plain modes, which will be illustrated with examples of TERS maps of mono-to few-layer sheets of WS2, MoS2, and MoSe2.
During last 15 years significant attention of the research community was devoted to 2D materials, first-carbon based and recently- broad class of 2D semiconductors such as transition metal dichalcogenides ( TMDC), black phosphorous etc. Amazing wealth of physical and optoelectronic phenomena in TMDC make them an extremely attractive object of research both from the fundamental and applied points of view.
Raman and photoluminescence spectroscopy proved to be powerful tools for characterization of 2D semiconductors. Unfortunately, spatial resolution of these techniques, on the order of few hundreds of nanometers is not sufficient to address important heterogeneities in these materials. STEM, to the contrary, provides true atomic resolution and allows addressing defects at single atom scale, but lacks to great extent correlation with physical properties of the materials in question.
Luckily, recent advances in tip enhanced Raman spectroscopy ( TERS) and tip enhanced photoluminescence ( TEPL) and cross-correlation of these near-filed spectroscopic data with other properties probed by scanning probe microscopy, provide scientific community with a powerful and relatively easy-to-use characterization method that address the properties of 2D materials at proper scale.
We’ll demonstrate application of cross-correlated TERS, TEPL, Kelvin probe microscopy, photocurrent mapping, friction etc for characterization of grain boundaries, physical defects, nanoscale doping heterogeneities in TMDCs and exciton population heterogeneities in the vicinity of the crystal edges in 2D semiconductors.
We report results of TERS characterization of graphene oxide and the 2D semiconductors, MoS2 and WS2. The gap mode TERS signal of these 2D materials becomes dramatically enhanced over wrinkles and creases, as well as over nanopatterns imprinted into flakes using a sharp diamond probe. The resonant Raman signal of MoS2 contains additional peaks normally forbidden by selection rules. TERS maps of few-layer-flakes of this 2D semiconductor show that the spatial distribution of Raman intensity across the flake varies for different peaks, providing interesting insights into the structure of such 2D semiconductors with 10-20 nm spatial resolution.
We report successful chemically specific high pixel density, high speed (less than 10 minutes per map) TERS imaging of graphene oxide, carbon nanotubes of different chirality, fullerenes and self-assembled layers of organic molecules in both the single-component and complex samples. The spatial resolution routinely obtained in such chemically specific TERS maps is in the 15 - 20 nm range, with the best resolution achieved being 7 nm. The ease of use of the TERS imaging system and high speed TERS imaging capability enabled by advanced hardware and software move TERS closer to become a real life analytical method for chemically specific imaging at the nano scale.
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