Metal-coated Tip Research Articles

Mechanical energy dissipation of an oscillating cantilever close to a conductive substrate partly covered with thin mica films evaluated by frequency modulation atomic force microscopy

Mechanical energy stored in an oscillating cantilever in frequency modulation atomic force microscopy (FM-AFM) was dissipated through nonconservative interactions between a sample and a tip on the cantilever. The energy dissipation (D dis) was measured using FM-AFM with a metal-coated tip for a metal-coated Si substrate partly covered with thin mica films. At tip–sample separations where electrostatic force was dominant under a bias voltage, Joule heat was generated owing to the tip oscillation, responsible for D dis. From analysis of D dis and the frequency shift of the cantilever, electric resistance responsible for the Joule heat was estimated to be of the order of GΩ. The great values of the resistance were discussed in the terms of surface scattering of charges moved by the oscillating tip and the dielectric energy loss in the mica films. Measurement of the energy dissipation exhibited potential to probe the local surface electronic properties in non-contact.

Anisotropic Piezoelectric Response from InGaN Nanowires with Spatially Modulated Composition and Topography over a Textured Si(100) Substrate.

An anisotropic piezoelectric response is demonstrated from InGaN nanowires (NWs) over a pyramid-textured Si(100) substrate with an interfacet composition and topography modulation induced by stationary molecular beam epitaxy growth conditions, taking advantage of the unidirectional source beam flux. The variations of InGaN NWs between the pyramid facets are verified in terms of morphology, element distribution, and crystalline properties. The piezoelectric response is investigated by electrical atomic force microscopy (AFM) with a statistic analyzing method. Representative pyramids from the ensemble, on top of which InGaN NWs grown with a substrate held at an oblique angle, were characterized for understanding and confirming the degree of anisotropy. The positive deviated oscillation of the peak force error is identified as a measure of the effective AFM tip/NW interaction with respect to the electrical contact and mechanical deformation. The Schottky contact between the metal-coated AFM tip and the NWs on the different facets reveals distinctions consistent with the interfacet composition variation. The interfacet variation of the piezoelectric response of the InGaN NWs is first evaluated by electrical AFM under zero bias. The average current monotonically depends on the scan frequency, which determines the average peak force error, that is, mechanical deformation, with a facet characteristic slope. A piezoelectric nanogenerator device is fabricated out of a sample with an ensemble of pyramids, which exhibits anisotropic output under periodic directional pressing. This work provides a universal strategy for the synthesis of composite semiconductor materials with an anisotropic piezoelectric response.

Partially Metal-Coated Tips for Near-Field Nanospectroscopy

Scanning probes with functional optical responses are key components of scanning near-field optical microscopes. For nanospectroscopy performed at IR and terahertz (THz) frequencies, one major challenge is that the commonly used metal-coated silicon tips yield nonadjustable coupling efficiency across the spectrum, which greatly limits the signal-to-noise ratio. Here, we test the possibility of a generic design scheme for wavelength-selective tip enhancement via finite-element numerical modeling. We employ a $\mathrm{Si}$-based tip with various gold-coating lengths on the top, yielding a customizable near-field field strength at the tip apex. Calculations show a wavelength-dependent enhancement factor of the metal-coated tip due to the geometrical antenna resonances, which can be precisely tuned throughout a broad spectral range from visible to terahertz frequencies by adjusting the length of the metal coating. By changing the coating pattern into a chiral helical structure on an achiral tip, we also demonstrate the usefulness of coating-length effect in designing high-performance enantiomeric near-field scanning. Our methods and findings offer interesting perspectives for developing near-field optical probes, pushing the detection and resolution limits of tip-enhanced near-field detections, such as fluorescence, Raman, IR, and THz nanospectroscopies.

A dual-use probe for nano-metric photoelectric characterization using a confined light field generated by photonic crystals in the cantilever

We propose a plasmonic atomic force microscopy (AFM) probe, which takes a part of the laser beam for monitoring cantilever deflection as the excitation light source. Photonic crystal cavities are integrated near the cantilever’s free end where the laser spot locates. The transmitted light excites surface plasmon polaritons on the metal-coated tip and induces a confined hot-spot at the tip apex. Numerical simulations demonstrate that the plasmonic probe can couple a tilted, linearly polarized beam efficiently and yield a remarkable local electromagnetic enhancement with the intensity being around 21 times stronger than that of the original probe. For demonstration, we employ the plasmonic probe in electrostatic force microscopy and scanning Kelvin probe microscopy to study the impact of local light field on the photoelectric characteristics of SiO2 and Au nanoparticles. Compared with the original probe, obvious differences are observed in the electrostatic force gradients on SiO2 nanoparticles and in the surface potentials of Au nanoparticles. The plasmonic probe can enable AFM as a powerful tool for simultaneous optical, mechanical and electrical characterizations.

Revealing nanoscale chemical heterogeneity at the surface of water-based coatings prepared from urethane–acrylic hybrids by photo-induced force microscopy

The surface chemical composition of water-based coatings prepared from urethane–acrylic hybrids or blends is investigated using infrared photo-induced force microscopy. By monitoring the interaction between the optically driven molecular dipole and its mirror image in a metal-coated tip, detailed information of the chemical composition at the coating surface down to 20 nm local resolution in all three dimensions is obtained. The film matrix of the coatings is determined to be the polyurethane while the polyacrylate particles are dispersed in the film. Phase separation of the two materials is found to be at a much smaller length scale in hybrid coatings compared to that in blends. This is because the acrylic monomers are introduced during the dispersion of the polyurethane in the synthesis of the hybrid particles, in contrast to the simple mixing of the two polymer particles in the case of the blends. Photo-induced force microscopy proved to be a powerful tool in the identification and visualization of different chemical species of coating surfaces at the nanoscale.

Stable contrast mode on TiO2(110) surface with metal-coated tips using AFM

We investigated a method to obtain a stable contrast mode on the TiO2(110) surface. The stable contrast rate is approximately 95% with a W-coated Si cantilever, which demonstrates that a stable tip apex plays an important role to obtain the real geometry of the surface during atomic force microscopy measurement. Information related to surface structure and tunnelling current on the TiO2(110) surface can be obtained by the W-coated Si cantilever. It is possible to investigate the electronic structure and surface potential on the TiO2(110) surface with atomic resolution. In particular, the proposed method could be widely applied to investigate the catalytic activity and the mechanism of a catalytic reaction by a metal-coated tip in the future.

High optical resonance sensitivity to its environment of a fibered bowtie nano-aperture antenna

In this paper, we present a numerical study based on 3D FDTD (finite-difference time-domain) simulations that demonstrate the high sensitivity of the optical response of a bowtie nano-aperture antenna (BNA), engraved at the apex of a metal-coated tip, to the distance variation between it and a given substrate. This study mainly discussed the case of the collection mode regime of the BNA and considered the case of two different substrates (high and low refractive index). The coupling between the substrate and the BNA may greatly affect the properties of the optical resonance of the nano-antenna. A blueshift of the resonance wavelength, as large as \(\frac{\varDelta \lambda }{\lambda }=0.3\), is obtained when the tip moves away over only 10 nm from the substrate (InP) interface. These results open the way to the design of stand-alone optical near-field probes that allow faithful interaction control with a given sample as a function of the distance between them.

Tip Enhanced Laser Ablation Sample Transfer for Mass Spectrometry

ABSTRACTMass spectrometry is one of the primary analysis techniques for biological analysis but there are technological barriers in sampling scale that must be overcome for it to be used to its full potential on the size scale of single cells. Current mass spectrometry imaging methods are limited in spatial resolution when analyzing large biomolecules. The goal of this project is to use atomic force microscope (AFM) tip enhanced laser ablation to remove material from cells and tissue and capture it for subsequent mass spectrometry analysis. The laser ablation sample transfer system uses an AFM stage to hold the metal-coated tip at a distance of approximately 10 nm from a sample surface. The metal tip acts as an antenna for the electromagnetic radiation and enables the ablation of the sample with a spot size much smaller than a laser focused with a conventional lens system. A pulsed nanosecond UV or visible wavelength laser is focused onto the gold-coated silicon tip at an angle nearly parallel with the surface, which results in the removal of material from a spot between 500 nm and 1 µm in diameter and 200 and 500 nm deep. This corresponds to a few picograms of ablated material, which can be captured on a metal surface for MALDI analysis. We have used this approach to transfer small peptides and proteins from a thin film for analysis by mass spectrometry as a first step toward high spatial resolution imaging.

Pb(Zr,Ti)O3 recording media for probe data storage devices prepared by rf magnetron sputtering

Pb(Zr,Ti)O3 (PZT) thin films for ferroelectric probe data storage technology were studied. (001)-oriented PZT thin films were deposited on SrRuO3/SrTiO3 substrates by rf magnetron sputtering. Dc voltage was applied on the films using a metal-coated tip and the poling region was observed by scanning nonlinear dielectric microscopy (SNDM). The contrasts in the positive and negative poling regions in the SNDM images obtained were improved by using the PZT films after ion-beam irradiation. This suggests that a surface layer of a few nanometers in thickness was formed on the as-grown PZT film and the polarization was not invertible in the surface layer. The deposition condition was examined with focus on deposition temperature. Nanosized domain dots were successfully formed on a PZT film deposited at 550 °C.

Fiber-integrated optical nano-tweezer based on a bowtie-aperture nano-antenna at the apex of a SNOM tip

We propose a new concept of fiber-integrated optical nano-tweezer on the basis of a single bowtie-aperture nano-antenna (BNA) fabricated at the apex of a metal-coated SNOM tip. We demonstrate 3D optical trapping of 0.5 micrometer latex beads with input power which does not exceed 1 mW. Optical forces induced by the BNA on tip are then analyzed numerically. They are found to be 10(3) times larger than the optical forces of a circular aperture of the same area. Such a fiber nanostructure provides a new path for manipulating nano-objects in a compact, flexible and versatile architecture and should thus open promising perspectives in physical, chemical and biomedical domains.

Spontaneous nonparametric down-conversion of light

The spontaneous down-conversion of laser light in a dielectric plate and/or metal-dielectric interface is considered. It is found that strong enhancement of the process takes place near the critical intensity of excitation. The enhancement of the efficiency takes place if the amplitude of oscillations of the eikonal coincides with the wavelength of emission. The down-conversion in metal-coated SNOM tip may be used for the generation of completely polarization- entangled photon pairs.

Modal dispersion due to photon–plasmon coupling in a SNOM tip

The mesoscopic effect of spectral modulation is used to experimentally investigate the possible effect of a metal-coated SNOM tip on the intermodal dispersion of an optical fiber. Highly regular modulation registered in a fixed spectral region of the broadband light transmitted by a 200nm SNOM tip yields the value of optical path difference (OPD) in the two-mode model. To investigate the origin of this OPD a series of spectral measurements accompanied with a gradual reduction of the multimode fiber tail length has been performed. The results indicate that the observed OPD consists of two clearly distinguishable contributions: the OPD in the non-coated multimode fiber and the OPD in the metal-coated SNOM tip. The first one is a result of the inherent modal dispersion of an optical fiber. A numerical consideration shows that the second one can be attributed to a mode-dependent contribution of surface plasmon polaritons: one of the two modes couples to plasmons stronger than the other one, resulting in a remarkably slower propagation. This leads to a completely different value of intermodal dispersion, which actually changes its sign in the SNOM tip. Proposed is a logical explanation for such kind of a sign-switching behavior.

Simultaneous near field imaging of electric and magnetic field in photonic crystal nanocavities

The insertion of a metal-coated tip on the surface of a photonic crystal microcavity is used for simultaneous near field imaging of electric and magnetic fields in photonic crystal nanocavities, via the radiative emission of embedded semiconductor quantum dots (QD). The photoluminescence intensity map directly gives the electric field distribution, to which the electric dipole of the QD is coupled. The magnetic field generates, via Faraday's law, a circular current in the apex of the metallized probe that can be schematized as a ring. The resulting magnetic perturbation of the photonic modes induces a blue shift, which can be used to map the magnetic field, within a single near-field scan.

Near-field and far-field analysis of an azimuthally polarized slow Bloch mode microlaser

We report on the near- and far-field investigation of the slow Bloch modes associated with the Γ point of the Brillouin zone, for a honeycomb lattice photonic crystal, using near-field scanning optical microscopy (NSOM) and infra-red CCD camera. The array of doughnut-shaped monopolar mode (mode M) inside each unit cell, predicted previously by numerical simulation, is experimentally observed in the near-field by means of a metal-coated NSOM tip. In far-field, we detect the azimuthal polarization of the doughnut laser beam due to destructive and constructive interference of the mode radiating from the surface (mode TEM(01*)). A divergence of 2° for the laser beam and a mode size of (12.8 ± 1) µm for the slow Bloch mode at the surface of the crystal are also estimated.

Superfocussing in a metal-coated tetrahedral tip by dimensional reduction of surface-to edge-plasmon modes

Metal-coated dielectric tetrahedral tips (T-tip) have long been considered to be interesting structures for the confinement of light to nanoscopic dimensions, and in particular as probes for scanning near-field optical microscopy. Numerical investigations using the Finite-Difference Time-Domain (FDTD) method are used to explore the operation of a T-tip in extraction mode. A dipole source in close proximity to the apex excites the tip, revealing the field evolution in the tip, the resulting edge and face modes on the metal-coated surfaces, and the coupling from these modes into highly directional radiation into the dielectric interior of the tip. These results are the starting point for illumination-mode numerical investigations by a Volume Integral equation method, which compute the field distribution that develops in a T-tip when a Gaussian beam is incident into the tip, and which show that a highly confined electric field is produced at the apex of the tip. The process of light confinement can be considered as a superfocussing effect, because the intensity of the tightly confined light spot is significantly higher than that of the focussed yet much wider incident beam. The mechanism of superfocussing can be considered as a dimensional reduction of surface plasmon modes, where an edge plasmon is the most important link between the waveguide-modes inside the tip and the confined near field at the apex.

Offset-apertured near-field scanning optical microscope probes

Near-field scanning optical microscope (NSOM) probe designs consisting of a subwavelength aperture offset of either a metallic or metal-coated dielectric cantilevered tip are investigated using finite-difference time-domain calculations. The offset aperture and metal-coated dielectric tip couple surface plasmons that illuminate the tip apex, which results in a single-lobed probing optical spot having a full-width half maximum (FWHM) similar to the apex diameter. Since the surface plasmons converge at the apex, an offset-apertured probe promises significantly higher throughput light intensities than an apertured NSOM having a comparable spot FWHM.

Temporal Fluctuation of Tip-Enhanced Raman Spectra of Adenine Molecules

Temporal fluctuations of spectral shapes have been observed in tip-enhanced Raman scattering (TERS) spectra of adenine molecules. The TERS spectra of a nanocrystalline cluster of adenine molecules showed distinct temporal changes including fluctuations of peak frequencies and peak intensities and extraordinary enhancements of several peaks. The fluctuating TERS signals originate primarily from the molecules in the first layer of the 5 nm thick adenine nanocrystalline cluster. The possible origin of the temporal changes is attributed to the changes of molecular orientation under the metal-coated tip, which is responsible for changing the adsorption angle and the angle of local incident polarization for the molecules adsorbed to the tip. This result suggests the importance of sample−tip interaction at molecular level in the interpretation of TERS spectra and indicates the possibility of the single molecule sensitivity of TERS spectroscopy of DNA base molecules.

The strong localization of surface plasmon polariton on a metal-coated tip of optical fiber

It is shown that during the propagation of surface plasmon polariton (SPP) on a metal-coated tip of an optical fiber its wavelength essentially decreases and wave fields anomalously increase. Dependence of a degree of localization of SPP on sharpness of the structure and width of the metal layer is defined. The received results can be used to increase the resolution of scanning optical microscope.

Atomic force microscopy based, multiphoton, photoelectron emission imaging

Images of photoelectron emission from metallic surfaces were obtained with a modified atomic force microscope operating in air. Illumination of the samples was achieved in the near field of a metal-coated microcantilever tip, placed in the beam of a femtosecond pulsed laser that is incident at a grazing angle with respect to the sample surface. Photoelectron currents were measured through the tip with a prototype amplifier. The power law dependence of average photocurrent on light intensity is compatible with multiphoton photoelectric effect and the work function of the metal covering a particular area on the two-metal patterned samples used.

Use of dielectrophoresis in the fabrication of an atomic force microscope tip with a carbonnanotube: experimental investigation

We have developed a method for fabricating a carbon nanotube (CNT) tip for an atomicforce microscope (AFM). To attach a CNT to the tip apex, we used dielectrophoresis(DEP) with a non-uniform electric field. After inserting a drop of CNT solution andapplying an AC electric field between a metal-coated AFM tip and an electrode plate,CNTs were deposited directly on the tip so that they protruded from the tip. We fabricatedtips with individual multi-walled carbon nanotubes and found the experimental conditionsthat gave high fabrication yields. From AFM measurements of the nanoscale anodizedaluminium oxide (AAO) structure, we have shown that a CNT tip assembledusing DEP can produce high-resolution images and have a good wear resistance.