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- 实验方案
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Direct Observation of Core-Shell Structures in Individual Lead Titanate Ferroelectric Nanostructures by Tip-Enhanced Refractive Index Mapping
摘要： Ferroelectrics undergo a size-driven phase transition at the nanoscale below which the spontaneous polarization, their defining property, irrevocably ceases. This threshold often referred to as the superparaelectric limit has tremendous technological relevance in an era of progressing integration. Just as the balance of short-range elastic and long-range electrostatic ordering in bulk, the critical size depends on temperature. Room-temperature tip-enhanced Raman spectroscopy (TERS) imaging of individual lead titanate (PbTiO3) nanoislands is reported with a spatial resolution of ≈3 nm. Monitoring the spectral shift of the gold-tip enhanced luminescence, which depends on the local refractive index, images grains composing the nanoislands. The wavelength of the enhanced luminescence shifts between the grains and their boundaries indicating the predicted core–shell structure of ferroelectric and paraelectric phase. The shear force configuration rules out the distance dependence of capacitive plasmonic coupling between tip and substrate as the origin of the observed shift. As the reported temperature-changes in nonresonant TERS do not account for noticeable thermal effects, the underlying, even though weak, tip-enhanced Raman spectrum of the grain core reflects PbTiO3 close to the ferroelectric-to-paraelectric phase transition which is primarily related to the finite size of the grains.
关键词： ferroelectric,tip-enhanced Raman spectroscopy,core–shell structure,local refractive index imaging,lead titanate
更新于2025-09-23 15:21:01
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Study of the Molecular Bending in Azobenzene Self-Assembled Monolayers Observed by Tip-Enhanced Raman Spectroscopy in Scanning Tunneling Mode
摘要： Tip-enhanced Raman Spectroscopy (TERS) is capable of amplifying the extremely weak Raman response of azobenzene self-assembled monolayers (SAMs), thus allowing for the chemical characterization of the sample surface at the nanoscale. Recently, we introduced a physical model describing the TERS intensity of azobenzene SAMs probed in a scanning tunneling mode configuration (STM-TERS), that takes into account the molecular bending induced by the high electric field inside the tunneling junction. The model predicts quite well the experimental variation in the TERS intensity of a hexil azobenzene SAM (AzoC6) on gold polycrystalline film (111) with changing the electric field in the gap between the tip and the substrate. Nevertheless, a disagreement between the model and the experiment has been observed while studying, in the same conditions, the TERS intensity of undecyl azobenzene (AzoC11) SAM formed by molecules featuring an alkyl chain nearly two times longer with respect to the previous case. In this work we extend the molecular bending model through considering an additional bending mechanism due to the mechanical interaction between the tip and the SAM, occurring when the tip-to-sample distance is shorter than the molecular length. The extended model is able to describe well the TERS intensity behavior with changing either the bias voltage or the tip-to-sample distance for both AzoC6 and AzoC11. Eventually, we determine quantitatively the difference in elastic properties of the two molecules physically accounting for the difference in the TERS intensity behavior of the two SAMs.
关键词： STM-TERS,self-assembled monolayers,SAMs,Tip-enhanced Raman Spectroscopy,TERS,molecular bending,azobenzene
更新于2025-09-19 17:13:59
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Visualization of subnanometric phonon modes in a plasmonic nano-cavity via ambient tip-enhanced Raman spectroscopy
摘要： Phonons provide information on the physicochemical properties of a crystalline lattice from the material’s vibrational spectrum. Optical phonons, in particular, can be probed at both micrometre and nanometre scales using light-based techniques, such as, micro-Raman and tip-enhanced Raman spectroscopy (TERS), respectively. Selection rules, however, govern the accessibility of the phonons and, hence, the information that can be extracted about the sample. Herein, we simultaneously observe both allowed and forbidden optical phonon modes of defect-free areas in monolayer graphene to study nanometre scale strain variations and plasmonic activation of the Raman peaks, respectively, using our home-built TERS system in ambient. Through TERS imaging, strain variations and nanometre-sized domains down to 5 nm were visualised with a spatial resolution of 0.7 nm. Moreover, such subnanometric con?nement was found to activate not only the D and D’ forbidden phonon modes but also their D + D’ combination mode. With our TERS in ambient system, the full phonon characterisation of defect-free graphene and other 2D nanomaterials is now possible, which will be useful for subnanometre strain analysis and exploring the inherent properties of defect-free materials.
关键词： graphene,plasmonic nano-cavity,tip-enhanced Raman spectroscopy,phonon modes,strain analysis
更新于2025-09-19 17:13:59
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Direct molecular-level near-field plasmon and temperature assessment in a single plasmonic hotspot
摘要： Tip-enhanced Raman spectroscopy (TERS) is currently widely recognized as an essential but still emergent technique for exploring the nanoscale. However, our lack of comprehension of crucial parameters still limits its potential as a user-friendly analytical tool. The tip’s surface plasmon resonance, heating due to near-field temperature rise, and spatial resolution are undoubtedly three challenging experimental parameters to unravel. However, they are also the most fundamentally relevant parameters to explore, because they ultimately influence the state of the investigated molecule and consequently the probed signal. Here we propose a straightforward and purely experimental method to access quantitative information of the plasmon resonance and near-field temperature experienced exclusively by the molecules directly contributing to the TERS signal. The detailed near-field optical response, both at the molecular level and as a function of time, is evaluated using standard TERS experimental equipment by simultaneously probing the Stokes and anti-Stokes spectral intensities. Self-assembled 16-mercaptohexadodecanoic acid monolayers covalently bond to an ultra-flat gold surface were used as a demonstrator. Observation of blinking lines in the spectra also provides crucial information on the lateral resolution and indication of atomic-scale thermally induced morphological changes of the tip during the experiment. This study provides access to unprecedented molecular-level information on physical parameters that crucially affect experiments under TERS conditions. The study thereby improves the usability of TERS in day-to-day operation. The obtained information is of central importance for any experimental plasmonic investigation and for the application of TERS in the field of nanoscale thermometry.
关键词： surface plasmon resonance,nanoscale thermometry,Tip-enhanced Raman spectroscopy (TERS),spatial resolution,near-field temperature rise
更新于2025-09-19 17:13:59
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Grating-assisted coupling enhancing plasmonic tip nanofocusing illuminated via radial vector beam
摘要： Tip-enhanced Raman spectroscopy (TERS) is a very useful method to achieve label-free and super-resolution imaging, and the plasmonic tip nanofocusing plays a decisive role for TERS performance. Here, we present a method to enhance the nanofocusing characteristic of a plasmonic tip integrated in a grating near the tip apex. Simulation results show that the grating near the tip apex can significantly improve the electric field intensity of the nanofocusing field compared with a conventional bare tip, under axial excitation of a tightly focused radial vector beam. The electric field enhancement characteristic is quantified in relation with the groove number of grating, excitation wavelength, period of grating, and numerical aperture of the micro-objective (MO). These simulation results could be a good reference to fabricate a plasmonic tip for TERS applications, which is an effective way to promote the development of tip-enhanced near-field optical microscopy.
关键词： tip-enhanced Raman spectroscopy,radial vector beam,plasmonic tip nanofocusing,surface plasmon polariton
更新于2025-09-16 10:30:52
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Selective excitation of one among the three peaks of tip‐enhanced Raman spectroscopy by a shaped ultrafast laser pulse
摘要： There are generally several neighboring Raman peaks that are excited simultaneously in tip‐enhanced Raman spectroscopy (TERS). A method to selectively excite a specific Raman peak and simultaneously depress the others is required for suppressing noise and improving detection sensitivity and accuracy. In this study, we demonstrate a scheme for selective excitation of one Raman peak among three excited states in TERS by properly shaping the pump and Stokes femtosecond laser pulses. The shaped pump and Stokes pulses are transformed into time domain, discretized and reconstructed; they are then used to irradiate the TERS structure obliquely. The impulse responses at five probe points of the TERS structure are calculated followed by a Fourier transform. In these cases, the probability of one selectively excited Raman transition increases by several hundreds to tens of thousands in amplitude, whereas the others are both depressed to zero. The probe points at different positions in the tip apex do not affect the enhanced and selective excitation of one Raman transition among the three excited states.
关键词： tip‐enhanced Raman spectroscopy,femtosecond laser pulse shaping,selective excitation
更新于2025-09-16 10:30:52
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Excitation-Tunable Tip-Enhanced Raman Spectroscopy
摘要： Tip-enhanced Raman spectroscopy is a powerful tool to investigate chemical composition, for obtaining molecular information, and for recording images with a spatial resolution on the nanometer scale. However, it typically has been limited to a fixed excitation wavelength. We demonstrate excitation-dependent hyperspectral imaging by implementing a wavelength tunable laser to our TERS setup. Varying the excitation wavelength during TERS experiments is a key to perform spatially resolved resonant Raman scattering (RRS) with nanometer resolution, which enables mapping of transition centers and to study for example the quantum properties of electrons and phonons. To present the application potential and to verify the setup, we recorded excitation-dependent hyperspectral nanoimages of a densely packed film of carbon nanotubes (CNTs) on an Au surface and use the spectral position and intensity of the radial breathing modes for a unique assignment of the CNTs. We succeeded in identifying and imaging at least nine different tube species. The nanoimages revealed the exact position and the distribution of certain CNTs inside the film. e-TERS will have manifold application in nanoimaging, for chemical analysis and electronic studies on the nanometer scale, making it highly interesting in fields ranging from biomedicine and chemistry to material science.
关键词： nanoimaging,Tip-enhanced Raman spectroscopy,resonant Raman scattering,carbon nanotubes,hyperspectral imaging
更新于2025-09-09 09:28:46
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Toward high-contrast AFM-TERS imaging: nano-antenna-mediated remote-excitation on sharp-tip silver nanowire probes
摘要： The tip-enhanced Raman spectroscopy (TERS) imaging technique is designed to provide correlated morphological and chemical information with a nanoscale spatial resolution by utilizing the plasmonic resonance supported by metallic nanostructures at the tip apex of a scanning probe. However, limited by the scattering cross-sections of these nanostructures, only a small fraction of the incident light can be coupled to the plasmonic resonance to generate Raman signals. The uncoupled light then directly excites background spectra with a diffraction-limited resolution, which becomes the background noise that often blurs the TERS image. Here, we demonstrate how this problem can be solved by physically separating the light excitation region from the Raman signal generation region on the scanning probe. The remote-excitation TERS (RE-TERS) probe, which can be fabricated with a facile, robust and reproducible method, utilizes silver nanoparticles as nano-antennas to mediate the coupling of free-space excitation light to propagating surface plasmon polaritons (SPPs) in a sharp-tip silver nanowire to excite Raman signals remotely. With this RE-TERS probe, a 10 nm spatial resolution was demonstrated on a single-walled carbon nanotube (SW-CNT) sample, and the strain distribution in a monolayer molybdenum disulfide (MoS2) was mapped.
关键词： plasmonic antenna,self-assembly,high spatial resolution,Tip-enhanced Raman spectroscopy imaging,stress-mapping,remote-excitation
更新于2025-09-09 09:28:46