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Register a new userAdaptive tip-enhanced nano-spectroscopy
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Tip-enhanced nano-spectroscopy and -imaging, such as tip-enhanced photoluminescence (TEPL), tip-enhanced Raman spectroscopy (TERS), and others, have become indispensable from materials science to single molecule studies. However, the techniques suffer from inconsistent performance due to lack of nanoscale control of tip apex structure, which often leads to irreproducible spectral, spatial, and polarization resolved imaging. Instead of refining tip-fabrication to resolve this problem, we pursue the inverse approach of optimizing the nano-optical vector-field at the tip apex via adaptive optics. Specifically, we demonstrate dynamic wavefront shaping of the excitation field to effectively couple light to the tip and adaptively control for enhanced sensitivity and polarization-controlled TEPL and TERS, with performance exceeding what can be achieved by conventional tip-fabrication and optimal excitation polarization. Employing a sequence feedback algorithm, we achieve 4.4$times$10$^4$-fold TEPL enhancement of a WSe$_2$ monolayer which is >2$times$ larger than the normal TEPL intensity without wavefront shaping, as well as the largest plasmon-enhanced PL intensity of a transition metal dichalcogenide (TMD) monolayer reported to date. In addition, with dynamical near-field polarization control in TERS, we demonstrate the investigation of conformational heterogeneity of brilliant cresyl blue (BCB) molecules as well as the controllable observation of IR-active modes due to a large gradient field effect. Adaptive tip-enhanced spectroscopy and imaging thus provides for a new systematic approach towards computational nanoscopy making optical nano-imaging more robust, versatile, and widely deployable.
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Many classes of two-dimensional (2D) materials have emerged as potential platforms for novel electronic and optical devices. However, the physical properties are strongly influenced by nanoscale heterogeneities in the form of edges, grain boundaries, and nucleation sites. Using combined tip-enhanced Raman scattering (TERS) and photoluminescence (TEPL) nano-spectroscopy and -imaging, we study the associated effects on the excitonic properties in monolayer WSe2 grown by physical vapor deposition (PVD). With <15 nm spatial resolution we resolve nonlocal nanoscale correlations of PL spectral intensity and shifts with crystal edges and internal twin boundaries associated with the expected exciton diffusion length. Through an active atomic force tip interaction we can control the crystal strain on the nanoscale, and tune the local bandgap in reversible (up to 24 meV shift) and irreversible (up to 48 meV shift) fashion. This allows us to distinguish the effect of strain from the dominant influence of defects on the PL modification at the different structural heterogeneities. Hybrid nano-optical and nano-mechanical imaging and spectroscopy thus enables the systematic study of the coupling of structural and mechanical degrees of freedom to the nanoscale electronic and optical properties in layered 2D materials.
Raman intensity of Rhodamine B (RhB) is enhanced by inserting a thin high k{appa} dielectric layer which reduces the surface plasmon damping at the gold-graphene interface. The results indicate that the Raman intensity increases sharply by plasmonic resonance enhancement while maintaining efficient fluorescence quenching with optimized dielectric layer thickness.
We study diffusion of heat in an aqueous suspension of disc shaped nanoparticles of Laponite, which has finite elasticity and paste-like consistency, by using the Mach-Zehnder interferometer. We estimate the thermal diffusivity of the suspension by comparing the experimentally obtained temperature distribution to that with analytical solution. We observe that despite highly constrained Brownian diffusivity of particles owing to its soft glassy nature, suspensions at very small concentrations of Laponite demonstrates significant enhancement in thermal diffusivity. We correlate the observed enhancement with the possible microstructures of the Laponite suspension.
Incorporation of magnetism in graphene based compounds holds great promise for potential spintronic applications. By optimizing point defects and high edge density of defects, we report many-fold increase in the ferromagnetic saturation moment in lacey reduced graphene oxide nanoribbons (LRGONR) as compared to other graphene derivatives. The samples were synthesized using chemical unzipping methodology. Detailed structural and morphological characterizations are discussed that include XRD, Raman, SEM, HRTEM and XPS measurements. Brilluoin function analysis to magnetization data reflects best fit for J = 7/2 with a saturation moment of 1.1 emu/g. The microscopic origin of magnetization in LRGONR is assigned to high edge defect density which has also been correlated to microstructure.
We demonstrate a high-performance apertureless near-field probe made of a tapered metal tip with a set of periodic shallow grooves near the apex. The spontaneous emission from a single emitter near the tip is investigated systematically for the side-illumination tip enhanced spectroscopy (TES). In contrast with the bare tapered metal tip in conventional side-illumination TES, the corrugated probe not only enhances strongly local excitation field but also concentrates the emission directivity, which leads to high collection efficiency and signal-to-noise ratio. In particular, we propose an asymmetric TES tip based on two coupling nanorods with different length at the apex to realize unidirectional enhanced emission rate from a single emitter. Interestingly, we find that the radiation pattern is sensitive to the emission wavelength and the emitter positions respective to the apex, which can result in an increase of signal-to-noise ratio by suppressing undesired signal. The proposed asymmetrical corrugated probe opens up a broad range of practical applications, e.g. increasing the detection efficiency of tip enhanced spectroscopy at the single-molecule level.
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