Remote Excitation of Tip-Enhanced Photoluminescence with a Parallel AgNW Coupler.

Tip-enhanced photoluminescence (TEPL) microscopy allows for the correlation of scanning probe microscopic images and photoluminescent spectra at the nanoscale level in a similar way to tip-enhanced Raman scattering (TERS) microscopy. However, due to the higher cross-section of fluorescence compared to Raman scattering, the diffraction-limited background signal generated by far-field excitation is a limiting factor in the achievable spatial resolution of TEPL. Here, we demonstrate a way to overcome this drawback by using remote excitation TEPL (RE-TEPL). With this approach, the excitation and detection positions are spatially separated, minimizing the far-field contribution. Two probe designs are evaluated, both experimentally and via simulations. The first system consists of gold nanoparticles (AuNPs) through photoinduced deposition on a silver nanowire (AgNW), and the second system consists of two offset parallel AgNWs. This latter coupler system shows a higher coupling efficiency and is used to successfully demonstrate RE-TEPL spectral mapping on a MoSe2/WSe2 lateral heterostructure to reveal spatial heterogeneity at the heterojunction.

Liquid-phase photo-induced covalent modification (PICM) of single-layer graphene by short-chain fatty acids.

We report an efficient photo-induced covalent modification (PICM) of graphene by short-chain fatty acids (SCFAs) with an alkyl chain at the liquid-solid interface for spatially resolved chemical functionalization of graphene. Light irradiation on monolayer graphene under an aqueous solution of the SCFAs with an alkyl chain efficiently introduces sp3-hybridized defects, where the reaction rates of PICM are significantly higher than those in pure water. Raman and IR spectroscopy revealed that a high density of methyl, methoxy, and acetate groups is covalently attached to the graphene surface while it was partially oxidized by other oxygen-containing functional groups, such as OH and COOH. A greater downshift of the G-band in Raman spectra was observed upon the PICM with longer alkyl chains, suggesting that the charge doping effect can be controlled by the alkyl chain length of the SCFAs. The systematic research and exploration of covalent modification in SCFAs provide new insight and a potentially facile method for bandgap engineering of graphene.

All-Optical and One-Color Rewritable Chemical Patterning on Pristine Graphene under Water.

We report a facile all-optical method for spatially resolved and reversible chemical modification of a graphene monolayer. A tightly focused laser on graphene under water introduces an sp3-type chemical defect by photo-oxidation. The sp3-type defects can be reversibly restored to sp2 carbon centers by the same laser with higher intensity. The photoreduction occurs due to laser-induced local heating on the graphene. These optical methods combined with a laser direct writing technique allow photowriting and erasing of a well-defined chemical pattern on a graphene canvas with a spatial resolution of about 300 nm. The pattern is visualized by Raman mapping with the same excitation laser, enabling an optical read-out of the chemical information on the graphene. Here, we successfully demonstrate all-optical Write/Read-out/Erase of chemical functionalization patterns on graphene by simply adjusting the one-color laser intensity. The all-optical method enables flexible and efficient tailoring of physicochemical properties in nanoscale for future applications.

Li@C60 thin films: characterization and nonlinear optical properties.

Organic materials have attracted considerable attention in nonlinear optical (NLO) applications as they have several advantages over inorganic materials, including high NLO response, and fast response time as well as low-cost and easy fabrication. Lithium-containing C60 (Li@C60) is promising for NLO over other organic materials because of its strong NLO response proven by theoretical and experimental studies. However, the low purity of Li@C60 has been a bottleneck for applications in the fields of solar cells, electronics and optics. In 2010, highly purified Li@C60 was finally obtained, encouraging further studies. In this study, we demonstrate a facile method to fabricate thin films of Li@C60 and their strong NLO potential for high harmonic generation by showing its comparatively strong emission of degenerate-six-wave mixing, a fifth-order NLO effect.

Label-free visualization of heterogeneities and defects in metal-organic frameworks using nonlinear optics.

Defects influence the properties of metal-organic frameworks (MOFs), such as their storage amount and the diffusion kinetics of gas molecules. However, the spatial distribution of defects is still poorly understood due to a lack of visualization methods. Here, we present a new method using nonlinear optics (NLO) that allows the visualization of defects within MOFs.

Controlled Fabrication of Optical Signal Input/Output Sites on Plasmonic Nanowires.

Silver nanowires have attracted considerable attention as subdiffraction limited diameter waveguides in a variety of applications including cell endoscopy and photonic integrated circuitry. Optical signal transport occurs by coupling light into propagating surface plasmons, which scatter back into light further along the wire. However, these interconversions only occur efficiently at wire ends, or at defects along the wire, which are not controlled during synthesis. Here, we overcome this limitation, demonstrating the visible laser light-induced fabrication of gold nanostructures at desired positions on silver nanowires, and their utility as efficient in/out coupling points for light. The gold nanostructures grow via plasmon-induced reduction of Au(III) and are shown to be excellent "hotspots" for surface-enhanced Raman scattering.

Facilitating Tip-Enhanced Raman Scattering on Dielectric Substrates via Electrical Cutting of Silver Nanowire Probes.

TERS is a powerful tool for nanoscale optical characterization of surfaces. However, even after 20 years of development, the parameters for optimal TERS tips are still up for debate. As a result, routine measurements on bulk or dielectric substrates remain exceptionally challenging. Herein we help to alleviate this by using electrical cutting to strategically modify silver nanowire TERS probes. Following cutting, the tips present a large, spherical apex and are often nanostructured with numerous nanoparticles, which we argue improve light collection and optical coupling. This doubles TERS signals on a highly enhancing, gap-mode substrate compared to our standard nanowire tips while maintaining a high reproducibility and resolution. More interestingly, on a dielectric substrate (graphene on SiO2) the tips give ∼7× higher signals than our standard tips. Further investigations point to the nonlocal nature of the enhancement using standard, smooth TERS probes without gap-mode, making such nanostructuring highly beneficial in these cases.

Clinical evaluation of a computer-aided diagnosis (CAD) prototype for the detection of pulmonary embolism.

RATIONALE AND OBJECTIVES: To evaluate the performance of a prototype computer-aided diagnosis (CAD) tool using artificial intelligence techniques for the detection of pulmonary embolism (PE) and the possible benefit for general radiologists. MATERIALS AND METHODS: Forty multidetector row computed tomography datasets (16/64- channel scanner) using 100 kVp, 100 mAs effective/slice, and 1-mm axial reformats in a low-frequency reconstruction kernel were evaluated. A total of 80 mL iodinated contrast material was injected at a flow rate of 5 mL/seconds. Primarily, six general radiologists marked any PE using a commercially available lung evaluation software with simultaneous, automatic processing by CAD in the background. An expert panel consisting of two chest radiologists analyzed all PE marks from the readers and CAD, also searching for additional finding primarily missed by both, forming the ground truth. RESULTS: The ground truth consisted of 212 emboli. Of these, 65 (31%) were centrally and 147 (69%) were peripherally located. The readers detected 157/212 emboli (74%) leading to a sensitivity of 97% (63/65) for central and 70% (103/147) for peripheral emboli with 9 false-positive findings. CAD detected 168/212 emboli (79%), reaching a sensitivity of 74% for central (48/65) and 82%(120/147) for peripheral emboli. A total of 154 CAD candidates were considered as false positives, yielding an average of 3.85 false positives/case. CONCLUSIONS: The CAD software showed a sensitivity comparable to that of the general radiologists, but with more false positives. CAD detection of findings incremental to the radiologists suggests benefit when used as a second reader. Future versions of CAD have the potential to further increase clinical benefit by improving sensitivity and reducing false marks.