We describe an apertureless near-field Raman spectroscopy setup that has successfully produced substantial enhancements for a wide variety of samples and achieved a high contrast. The tremendous potential of tip-enhanced Raman spectroscopy (TERS) for nanoscale chemical characterization has been demonstrated by various groups by measuring organic dyes, biological molecules, single-walled carbon nanotubes and silicon. Keys to rapid advances in the application of TERS to pressing scientific problems include the optimization of the method to achieve greater reproducibility and greater enhancement factors if possible, but more importantly, greater imaging contrast. Using a side-illumination geometry, we demonstrate reproducible enhancements of the Raman signal per volume on the order of 10 3 -10 4 using silver-and gold-coated tips on various molecular, polymeric and semiconducting materials as well as on carbon nanotubes. We have experimentally verified localization of the enhancement to a depth of ∼20 nm. Most importantly, optimization of the polarization geometry makes possible a contrast between the near-field and far-field signals of 900% in the case of silicon -a level that makes the technique attractive for various applications.

We have demonstrated that scanning nano-Raman spectroscopy (SNRS), generally known as tip-enhanced Raman spectroscopy (TERS), with side illumination optics can be effectively used for analysis of siliconbased structures at the nanoscale. Even though the side illumination optics has disadvantages such as difficulties in optical alignment and shadowing by the tip, it has the critical advantage that it may be used for the analysis of nontransparent samples. A key criterion for making SNRS effective for imaging Si samples is the optimization of the contrast between near-field and far-field (background) Raman signals. This has been achieved by optimizing the beam polarization, resulting in an order of magnitude improvement in the contrast. We estimate the lateral resolution of our Raman images to be ∼ 20 nm.

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Developing high-color-quality white light-emitting diodes (LEDs) is crucial for energy-efficient light bulbs and modern flat panel displays. Creating the luminescent phosphors that enable these advanced lighting technologies requires stable photoluminescence under varying temperatures. In this study, we examine Ba2Ca2B4O10 substituted with Ce3+, which emits an efficient blue-cyan light (λem ≈ 455 nm and a quantum yield of 74%) that is required for high color rendering lighting. Synchrotron powder X-ray diffraction and optical spectroscopy reveal that the broad emission band is attributed to Ce3+ ions occupying two crystallographically independent Ba2+ positions in the host crystal structure. However, temperature-dependent luminescence measurements unveil a surprising phenomenon: a significant blue shift (from 460 to 415 nm) accompanied by a drastic narrowing of the total emission bandwidth (from 140 nm, 6900 cm–1 to 58 nm, and 3200 cm–1). This extreme optical response arises from two simultaneous thermal quenching mechanismssite preferential quenching and high thermal expansion (αV ≈ 5.39 × 10–5 K–1). Consequently, the phosphor experiences a chromatic shift that transforms a fabricated prototype light bulb’s perceived color from functional white light to an undesirable yellow-green hue. These findings underscore the considerable impact of chromatic instabilities in phosphors and the effects they can have on the performance of LED lighting.

The local electric field enhancement in the vicinity of a metal-coated or metal tip is a significant factor in the performance of apertureless near-field optical microscopy and spectroscopy techniques. Enhancement, which is related to the generation of localized surface plasmons in the metal tip, can be maximized when the plasmons resonate at the probing wavelength. Thus the resonance frequencies of the tip apex are crucial to near-field optics. However, it remains a challenge to measure the optical properties of the apex of a tip with a radius much smaller than the wavelength of light. A dark-field scattering spectroscopy method is presented in combination with a side-illumination nano-Raman spectrometer to experimentally determine the optical properties of the tip. The dependence of the optical resonance on the metal deposited is shown for silver-and gold-coated tungsten tips as well as gold-coated silicon nitride tips. The enhancement for Si using gold-coated silicon nitride tips is somewhat larger for a wavelength of 647 nm than for a wavelength of 514.5 nm. The former is closer to the plasmon resonance observed for this tip at ~680 nm.

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