Scanning Near-field Optical Microscopy Measurements Research Articles

Dynamic-mode AFM using the piezoelectric cantilever: investigations of local optical and electrical properties

We demonstrated applications of a microfabricated cantilever with a lead zirconate titanate (PZT) piezoelectric thin film as an integrated deflection sensor to dynamic-mode atomic force microscopy (AFM) and to the related techniques including scanning near-field optical microscopy (SNOM) and Kelvin force microscopy (KFM). In the SNOM measurement, the evanescent light scattered by the tip apex was transmitted through the pyramidal hollow tip of the cantilever and it was detected by a photodetector placed in a confocal arrangement. Furthermore, local surface potential of a ferroelectric film was mapped by KFM using the frequency modulation (FM) detection method. Local poled ferroelectric domains made by the AFM tip were imaged by both SNOM and KFM.

Fabrication and characterization of a silicon cantilever probe with an integrated quartz-glass (fused-silica) tip for scanning near-field optical microscopy

A cantilever-based probe is introduced for use in scanning near-field optical microscopy (SNOM) combined with scanning atomic-force microscopy (AFM). The probes consist of silicon cantilevers with integrated 25-mum-high fused-silica tips. The probes are batch fabricated by microfabrication technology. Transmission electron microscopy reveals that the transparent quartz tips are completely covered with an opaque aluminum layer before the SNOM measurement. Static and dynamic AFM imaging was performed. SNOM imaging in transmission mode of single fluorescent molecules shows an optical resolution better than 32 nm.

Near-field scanning optical microscopy studies of electronic and photonic materials and devices

Probing optical properties of materials and optical characterization of crystallographic defects at the nanometer scale have been inaccessible until recently due to the diffraction limit of light. With the invention of near-field scanning optical microscopy (NSOM), resolution at the 50–100 nm level using visible or near infrared light is now practical. In addition to describing the NSOM technique, this review focuses on the application of NSOM to the characterization of electronic and photonic materials and devices, with particular emphasis on defects. The unique capability of NSOM to simultaneously measure surface topography and local optoelectronic properties, thereby eliminating the need to perform cross correlation analysis on results obtained using different techniques, is particularly useful in this area. Several examples are discussed. By performing near-field photocurrent (NPC) measurements, NSOM is used to probe electrical activities associated with individual threading dislocations and dislocation networks in strain relaxed, compositionally graded GeSi films. The non-destructive nature of NSOM helps elucidate how microstructural defects in the SrTiO3 bicrystal substrates affect YBa2Cu3O7 film growth and GBJJ performance. Characterization of III–V and II–VI semiconductors, guantum dots grown by strain epitaxy, laser diodes, waveguides, and photonic crystals is also included. The advantages and disadvantages of NSOM in each application will be outlined. Throughout the review, emphasis is placed on how NSOM complements existing materials characterization techniques, as well as how quantitative results can be obtained from NSOM measurements.

Probing the spatial dependence of the emission spectrum of single human retinal lipofuscin granules using near-field scanning optical microscopy.

The emission spectra of single lipofuscin granules are examined using spectrally resolved confocal microscopy and near-field scanning optical microscopy (NSOM). The emission spectrum varies among the granules examined revealing that individual granules are characterized by different distributions of fluorophores. The range of spectra observed is consistent with in vivo spectra of human retinal pigment epithelium cells. NSOM measurements reveal that the shape of the spectrum does not vary with position within the emissive regions of single lipofuscin granules. These results suggest that the relative distribution of fluorophores within the emissive regions of an individual granule is homogeneous on the spatial scale approximately 150 nm.

Near-field scanning optical microscopy cross-sectional measurements of crystalline GaAs solar cells

Near-field scanning optical microscopy (NSOM) was used to study cleaved edges of GaAs solar cell devices. Using visible light for excitation, the NSOM acquired spatially resolved traces of the photocurrent response across the various layers in the device. For excitation energies well above the band gap, carrier recombination at the cleaved surface had a strong influence on the photocurrent signal. Decreasing the excitation energy, which increased the optical penetration depth, allowed the effects of surface recombination to be separated from collection by the pn junction. Using this approach, the NSOM measurements directly observed the effects of a buried minority carrier reflector/passivation layer.

Near-field scanning optical microscopy as a simultaneous probe of fields and band structure of photonic crystals: A computational study

We demonstrate the feasibility of employing near-field scanning optical microscopy (NSOM) imaging to simultaneously obtain both the eigenfield distribution and the band-structure information of a photonic crystal. We introduce the NSOM measurement configuration required and simulate the imaging process, with both the tip and the sample included, using three-dimensional finite-difference time-domain calculations. Both the field-pattern and the frequency–wave-vector relations of photonic crystal eigenmodes are revealed by analyzing simulated images.

Submicron Structure in l- α-Dipalmitoylphosphatidylcholine Monolayers and Bilayers Probed with Confocal, Atomic Force, and Near-Field Microscopy

Langmuir-Blodgett (LB) monolayers and bilayers of l- α-dipalmitoylphosphatidylcholine (DPPC), fluorescently doped with 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (diIC 18), are studied by confocal microscopy, atomic force microscopy (AFM), and near-field scanning optical microscopy (NSOM). Beyond the resolution limit of confocal microscopy, both AFM and NSOM measurements of mica-supported lipid monolayers reveal small domains on the submicron scale. In the NSOM studies, simultaneous high-resolution fluorescence and topography measurements of these structures confirm that they arise from coexisting liquid condensed (LC) and liquid expanded (LE) lipid phases, and not defects in the monolayer. AFM studies of bilayers formed by a combination of LB dipping and Langmuir-Schaefer monolayer transfer exhibit complex surface topographies that reflect a convolution of the phase structure present in each of the individual monolayers. NSOM fluorescence measurements, however, are able to resolve the underlying lipid domains from each side of the bilayer and show that they are qualitatively similar to those observed in the monolayers. The observation of the small lipid domains in these bilayers is beyond the spatial resolving power of confocal microscopy and is complicated in the topography measurements taken with AFM, illustrating the utility of NSOM for these types of studies. The data suggest that the small LC and LE lipid domains are formed after lipid transfer to the substrate through a dewetting mechanism. The possible extension of these measurements to probing for lipid phase domains in natural biomembranes is discussed.

Direct Observation of DPPC Phase Domain Motion on Mica Surfaces under Conditions of High Relative Humidity

Langmuir−Blodgett monolayers of L-α-dipalmitoylphosphatidylcholine (DPPC) doped with 1,1‘-dioctadecyl-3,3,3‘,3‘-tetramethylindocarbocyanine perchlorate (diIC18) are studied under conditions of high relative humidity (RH). Monolayers transferred to a freshly cleaved mica surface in the liquid expanded (LE)−liquid condensed (LC) phase coexistence region of the pressure isotherm are studied with tapping-mode atomic force microscopy (TM-AFM), near-field scanning optical microscopy (NSOM), and confocal microscopy. As the RH is increased above 65%, the small LE and LC domains become mobile on the surface and aggregate to form larger domains of like phase. Analysis of the AFM images confirms that this process does not reflect phase transitions in the monolayer at increased RH but is driven by the energetically favorable decrease in the interfacial line energy of the lipid domains upon aggregation. NSOM measurements of the monolayer confirm that the evolution in the monolayer structure at high RH does not involve...

Contrast enhancement using polarization-modulation scanning near-field optical microscopy (PM-SNOM)

Polarization contrast scanning near-field optical microscopy (SNOM) provides information on the orientation of molecules and molecular aggregates on surfaces. Other than in transmission or fluorescence SNOM, the control of polarization demands for SNOM probes having extinction ratios better than 20 : 1 in at least two perpendicular polarization directions. Most common SNOM probes consist of tapered and metal-coated fiber tips. While the birefringence of fibers can be compensated, the tapering often exhibits depolarizing effects that make good qualitative or even quantitative measurements difficult. In an attempt to improve the quality of the apertures, we have modified metal-coated fiber probes with a focused ion beam (FIB). As an example of investigations where a very good polarization control is needed, we present and discuss polarization-modulation SNOM (PM-SNOM) measurements on dye crystals. The modulation of the direction of linearly polarized light and the use of lock-in techniques allows the simultaneous detection of absorption together with magnitude and orientation of optical anisotropy.

Assessment of NSOM resolution on III-V semiconductor thin films

Emission-mode aperture near-field scanning optical microscopy (NSOM) is applied to semiconductor thin films for resolution assessment and artifact investigation purposes. We have used GaAs thin films deposited on GaP(111) substrates with A and B polarities by metal organic vapour phase epitaxy (MOVPE). Optical discrimination of GaAs islands on GaP(111) has been accomplished with a lateral resolution better than 20 nm (λ/35) in the transmission mode. These samples have proven valuable for inspection of contrast mechanisms, based on discrimination of materials having different refractive index and absorption coefficients, as well as investigation of topography and shadowing artifacts in optical near-field imaging. Since its invention in 1984 [1] the near-field scanning optical microscope (NSOM) has become a widely adopted tool for optical investigation in the submicron range, well beyond the diffraction limit. Nowadays, near-field microscopy is applied to a variety of issues: nanospectroscopy [2], nanowriting [3], the study of biological samples [4], polymers [5], and magnetic samples [6], among others. Different working configurations have been demonstrated [7], but the most commonly used is the emission-mode aperture NSOM, as also witnessed by commercially available instruments. In the emission mode the near-field is generated by means of a tapered, metalcoated optical fiber with an aperture on the free edge in the range from 20 to 200 nm in diameter, coupled to a light source (usually a laser beam). The light scattered by the sample is collected afterwards in the far field, by means of a microscope objective, and detected by a photomultiplier (PMT). The tip is held at a constant distance from the sample by shear-force detection and stabilization [8] in order to maintain the sensor in the near-field region and to avoid damaging it. Lateral resolution of the aperture NSOM appears to be limited by the effective size of the aperture, which is determined by the penetration depth of the metal defining the aperture itself, which is ≈ 15 nm in the visible range. With the invention of the ∗ Present address: Dipartimento di Fisica, Universita di Firenze, Largo E. Fermi 2, I-50125 Firenze, Italy apertureless NSOM [9] the resolution limit has been pushed further by another order of magnitude. A resolution of few nm has been demonstrated; atomic resolution is theoretically predicted. In this configuration the subwavelength illuminating source is reduced to extreme limits, with a few dipoles located at the apex of a very sharp AFM or STM tip. When these dipoles are irradiated by an external light source they act as scattering centers and generate the near field that is used to probe the sample surface. Thus the mechanisms responsible here for enhanced resolution are of “particle imaging particle” kind [9, 10]. This excellent resolution is reached at the expenses of a more complex and critical experimental setup, as witnessed by the fact that at present, to our knowledge, only a few apertureless NSOM prototypes are operating. In this article we illustrate aperture NSOM measurements, on semiconductor thin films, that we have used for resolution calibration purposes and for evaluation of contrast mechanisms. The remarkable finding is that even in aperture NSOM with wide apertures the resolution can be higher than the aperture diameter for some genuinely optical processes resembling apertureless scattering. 1 Materials and methods The samples used here are heterolayers of a III-V semiconductor deposited on a different III-V semiconductor monocrystal. The study of features during the epitaxial growth of semiconductors is one of the most promising application of NSOM. There is a great interest in determining the growth mechanism and in growing reproducible quantum structures. In addition to the morphological analysis, NSOM is able to perform structural analysis by traditional absorption and photoluminescence characterization at a nanoscale range. NSOM is thus becoming a powerful tool for the investigation of homogeneity properties, strain distribution, and the optical behaviour of single defects like dislocations or stacking faults. The applicability of this technique to morphological studies has already been demonstrated, although continuous research efforts are increasing image resolution and improving electronic control. As regards the structural analysis, more improvements are still necessary to achieve

Spatial resolution of localized photoluminescence by near-field scanning optical microscopy

A numerical model describing the spatial resolution limitations on localized band edge photoluminescence (PL) and trap-based luminescence, obtained through near-field scanning optical microscopy (NSOM), is presented. Calculations were carried out for NSOM measurements performed in the tip illumination mode. Variations in surface recombination velocity, probe aperture, and the physical structure on the achievable spatial resolution were investigated. These calculations indicate that the drift of photogenerated minority carriers to the surface narrows their lateral distribution, leading to improved spatial resolution. There is a weak dependence on the spatial resolution of the photoluminescence on the tip aperture. An order of magnitude reduction of the illuminating tip aperture resulted in only a minor improvement in the lateral spatial resolution of the emitted PL. The spatial resolution of the localized trap luminescence is presented within the context of typical material structures.

A novel probe for near field optical microscopy based on luminescent silicon

A porous silicon (PS) based tip has been developed as a small light emitting probe for scanning near field optical microscopy (SNOM). Another possible application is photoresist patterning on the micrometer and nanometer scale, where dimensions ∼ 100 nm are required for extending conventional lithographic processes. In principal light emission can be excited optically in fluorescence as well as electrically by an applied bias voltage. We show the mechanical fabrication of a luminescent probe at the apex of a conventional STM tip and its use to record initial NFO images of micron-sized test structures. The sensor has been characterized by life time measurements and shows its stability over more than thousand minutes. Its quantum efficiency has been measured to range from 0.025 to 0.14 depending on the excitation photon rate and is found to increase with small size of the particles. The emission power with the present set-up is 7 × 10 −11 W at 630 nm under an Ar + laser (488) nm excitation at room temperature. Additionally high frequency plasma etching has been used to fabricate conventional Si atomic force microscopy (AFM) cantilevers with a PS luminescent tip which can be used simultaneously for AFM and SNOM measurements. We present design studies for the fabrication of integrated tips suitable for simultaneous AFM, STM and SNOM measurements.