Metal Nanotips Research Articles

Field-induced evolution of metallic nano-tips in indium tin oxide-tris-(8-hydroxyquinoline) aluminum-aluminum device

The effect of electric field and temperature on bistable characteristics of indium tin oxide/tris-(8-hydroxyquinoline) aluminum (Alq3)/aluminum (Al) device has been investigated. The switching time, during which the device turns from a high resistive (OFF) state to a low resistive (ON) state, increases with decreasing field and temperature. The observed phenomena are accounted for by a phenomenological model that metallic nano-tips may evolve under the influence of the electric field and temperature.

Synthesis and attachment of silver nanowires on atomic force microscopy cantilevers for tip‐enhanced Raman spectroscopy

Surface‐enhanced Raman spectroscopy is based on the absorption of light by nanometer‐sized metal particles, resulting in large enhancement of the Raman signal. By replacing the metal particles by a metallic nanotip, the enhancement can be localized. The resulting tip‐enhanced Raman spectroscopy is capable of measuring Raman spectra with high spatial resolution, effectively overcoming the diffraction limit. A successful tip‐enhanced Raman spectroscopy experiment depends heavily on the ability to fabricate tips of a definite metal with the appropriate shape and size, which is still a challenging process. We have prepared silver nanowires with a diameter of 200–300 nm by templated electrochemical deposition and attached them onto atomic force microscope cantilevers by focused electron beam induced deposition. We found that they produce a reproducible enhancement of the Raman signal intensity. Other metals and smaller nanostructures might also be produced, suggesting an interesting development potential for these novel nanoprobes. Copyright © 2011 John Wiley & Sons, Ltd.

Five picocoulomb electron bunch generation by ultrafast laser-induced field emission from metallic nano-tip arrays

Laser-induced field emission from metallic field emitter array cathodes excited by femtosecond near infrared laser pulses is explored. When 50 fs laser pulses irradiated a 1.2 × 105-tip emitter array under a DC field emission bias, electron bunches with bunch charge up to 5.2 pC were observed. The variation of the bunch charge at different laser intensities and polarizations indicated that electrons were produced from the field emitters by a photofield emission process. The result demonstrates the feasibility of metallic field emitter array cathodes for high-charge short-pulse electron source applications.

Homogeneity improvement of field emission beam from metallic nano-tip arrays by noble-gas conditioning

The authors explored control of field-emission electron beam homogeneity emitted from metallic nano-tips fabricated by molding. Improved beam homogeneity and increased maximum bunch charge were observed after DC operation of cathodes in low-pressure argon gas environment. They also observed average current per tip of 4.5 μA from a 1 × 104 emitter array device after conditioning in neon gas. Evolution of Fowler-Nordheim parameters as a result of noble-gas conditioning was discussed.

Sub-nanosecond switching and acceleration to relativistic energies of field emission electron bunches from metallic nano-tips

The authors explore the generation of short electron bunches from a field-emitter array cathode consisting of nanometer-sized metallic tips that is compatible with an acceleration electric field above 10 MV/m. Sub-nanosecond field emission electron bunches were generated by applying fast electrical pulses to an on-chip electron extraction gate electrode of the cathode. The subsequent acceleration of the field emission electron bunches to 5 MeV was demonstrated using the combined diode-RF cavity SwissFEL electron gun test facility.

Tip-Enhanced Strong-Field Photoemission

Nonlinear photoelectron emission from metallic nanotips is explored in the strong-field regime. The passage between the multiphoton and the optical field emission regimes is clearly identified. The experimental observations are in agreement with a quantum mechanical strong-field model.

Trapping and Manipulation of Isolated Atoms Using Nanoscale Plasmonic Structures

We propose and analyze a scheme to interface individual neutral atoms with nanoscale solid-state systems. The interface is enabled by optically trapping the atom via the strong near-field generated by a sharp metallic nanotip. We show that under realistic conditions, a neutral atom can be trapped with position uncertainties of just a few nanometers, and within tens of nanometers of other surfaces. Simultaneously, the guided surface plasmon modes of the nanotip allow the atom to be optically manipulated, or for fluorescence photons to be collected, with very high efficiency. Finally, we analyze the surface forces, heating and decoherence rates acting on the trapped atom.

Nanoscale Mode Selector in Silicon Waveguide for on Chip Nanofocusing Applications

We demonstrate a nanoscale mode selector supporting the propagation of the first antisymmetric mode of a silicon waveguide. The mode selector is based on embedding a short section of PhC into the waveguide. On the basis of the difference in k-vector distribution between orthogonal waveguide modes, the PhC can be designed to have a band gap for the fundamental mode, while allowing the transmission of the first antisymmetric mode. The device was tested by directly measuring the modal content before and after the PhC section using a near field scanning optical microscope. Extinction ratio was estimated to be approximately 23 dB. Finally, we provide numerical simulations demonstrating strong coupling of the antisymmetric mode to metallic nanotips. On the basis of the results, we believe that the mode selector may become an important building block in the realization of on chip nanofocusing devices.

Surface-enhanced Raman scattering on periodic metal nanotips with tunablesharpness

This paper reports on a scalable bottom-up technology for producing periodic goldnanotips with tunable sharpness as surface-enhanced Raman scattering (SERS)substrates. Inverted silicon pyramidal pits, which are templated from non-close-packedcolloidal crystals prepared by a spin-coating technology, are used as structuraltemplates to replicate arrays of polymer nanopyramids with nanoscale sharp tips.The deposition of a thin layer of gold on the polymer nanopyramids leads tothe formation of SERS-active substrates with a high enhancement factor (up to108). The thickness of the deposited metal determines the sharpness of the nanotips and theresulting Raman enhancement factor. Finite-element electromagnetic modeling shows thatthe nanotips can significantly enhance the local electromagnetic field and the sharpness ofnanotips greatly affects the SERS enhancement.

Subnanometric Near-Field Raman Investigation in the Vicinity of a Metallic Nanostructure

We present a near-field Raman investigation in the subnanometric vicinity of a metallic nanotip, where the tip-sample distance is precisely controlled by our newly developed time-gated illumination technique. Using this scheme on an isolated carbon nanotube, we have profiled the spatial decay of evanescent light. We also investigated extremely short-ranged chemical and mechanical interactions between the metal on the tip apex and the molecules of an adenine sample, which are observable only within the subnanometric vicinity of the tip. The results show a near-field Raman investigation with an accuracy of better than a few angstroms. Further, this shows strong promise for superhigh resolution in optical microscopy based on this technique.

Static and optical field enhancement in metallic nanotips studied by two-photon photoemission microscopy and spectroscopy excited by picosecond laser pulses

The authors studied the photoemission from molybdenum nanotip arrays with controlled apex curvatures by photoelectron microscopy and spectroscopy excited by 10 ps visible laser pulses under dc electric field. While two-photon photoemission microscopy demonstrates the enhancement of photoabsorption at the sharp tip apex, spatially resolved analysis of the photoelectron energy revealed anomalous energy distribution that is ascribed to the dc field enhancement and resultant barrier reduction at the tip apex. The results show the applicability of photoelectron microscopy and spectroscopy to study the distribution of dc and optical electric field enhancement in field-emitter arrays.

Fabrication of high aspect ratio metal nanotips by nanosecond pulse laser melting

The authors have developed an approach to fabricate sharp and high aspect ratio metaltips using nanosecond pulse laser melting. A quartz wafer covered with a thin chromium(Cr) film was placed on top of a second wafer with a sub-micrometer gap between themand the Cr film facing the second wafer. Then an excimer laser pulse (308 nmwavelength, 20 ns pulse duration) was shone from the back of the quartz waferand melted the Cr film momentarily (several hundred nanoseconds). It is foundthat the molten Cr films can self-form discrete metal pillars connecting the twowafers. After separating the two wafers, nanotips were formed at the broken pillarnecks. The sharpest tip achieved has an apex diameter 10 nm and height 180 nm.The self-formation of Cr pillars between the two wafers was attributed to theattractive electrostatic force caused by the work function difference of two wafersthat were in close proximity. This technique could be extended to other metals,and a periodic uniform tip array could be obtained by pre-patterning the metalinto identical isolated mesas and precisely controlling the gap between the twowafers.

Light Confinement at Ultrasharp Metallic Tips

In this article, two alternative experimental strategies for reaching light confinement at metallic nanotips are discussed. The first approach utilizes optical field enhancement to localize nonlinear optical signals at the tip end. The second approach employs surface plasmon polaritons propagating coherently along the tip shaft and converging at its apex.

Ultrafast electron emission from metallic nanotip arrays induced by near infrared femtosecond laser pulses

The authors explore the impact of femtosecond light pulses on the field-emission properties of single-gate molybdenum field-emitter arrays with nanometer scale tip apex. Despite the small fraction of the emission area, we observed a single-photon photoelectric current from the emitter tips on top of the dc field-emission current under the irradiation of 50fs laser pulses at a wavelength of 800nm with an external quantum efficiency up to ∼2×10−7 and the emitter tip quantum efficiency of ∼10−2. The result indicates that metallic field-emitter arrays are promising for applications that require high-brightness short electron beams.

Plasmonic Enhancement of Fluorescence and Raman Scattering by Metal Nanotips

We report modifications to the optical properties of fluorophores in the vicinity of noble metal nanotips. The fluorescence from small clusters of quantum dots has been imaged using an apertureless scanning near-field optical microscope. When a sharp gold tip is brought close to the sample surface, a strong distance-dependent enhancement of the quantum dot fluorescence is observed, leading to a simultaneous increase in optical resolution. These results are consistent with simulations of the electric field and fluorescence enhancement near plasmonic nanostructures. Highly ordered periodic arrays of silver nanotips have been fabricated by nanosphere lithography. Using fluorescence lifetime imaging microscopy, we have created high-resolution spatial maps of the lifetime components of vicinal fluorophores; these show an order of magnitude increase in decay rate from a localized volume around the nanotips, resulting in a commensurate enhancement in the fluorescence emission intensity. Spatial maps of the Raman scattering signal from molecules on the nanotips shows an enhancement of more than five orders of magnitude.

Ultrafast optical excitations of metallic nanostructures: from light confinement to a novel electron source

Combining ultrafast coherent spectroscopy with nano-optical microscopy techniques offers a wealth of new possibilities for exploring the structure and function of nanostructures. In this paper, we describe newly developed nano-optical methods based on short-pulse laser sources with durations in the 10 fs regime. These techniques are used to unravel some of the intricate dynamics of elementary excitations in metallic nanostructures. Specifically, we explore light localization and storage in plasmonic crystals, demonstrate field enhancement and second harmonic generation from metallic nanotips and describe a novel nanometre-sized source of electron pulses. The rapid progress in this area offers exciting new prospects for probing and controlling electron dynamics in metallic nanostructures with femtosecond temporal and nanometre spatial resolution.

Plasmon-polaritons on the surface of a pseudosphere

Plasmon-polariton behavior on the surfaces of constant negative curvature is considered. Analytical solutions for the eigenfrequencies and eigenfunctions are found, which describe resonant behavior of metal nanotips and nanoholes. These resonances may be used to improve performance of metal tips and nanoapertures in various scanning probe microscopy and surface-enhanced spectroscopy techniques.

Femtosecond laser photoelectron projection microscopy of organic nanocomplexes

Organic nanocomplexes obtained by applying an organic dye (Coumarin 153) solution onto the surface of a metal nanotip were studied by femtosecond laser photoelectron projection microscopy. The tip was a 100-nm-diameter capillary covered with a thin nickel layer. The image of a through hole of the capillary was a convenient reference for interpretation of the results. The spatial resolution achieved in these experiments was equal to about 5 nm.

Tungsten pedestal structure for nanotriode devices

The nanotriode is a vacuum nanoelectronic device with a turn-on voltage for a field emission of ∼8 V, in which electrons are emitted from a metal nanotip with a radius ∼1 nm in an integrated vacuum chamber, over a distance of ∼100 nm. However, surface breakdown of the gate-cathode dielectric within the device chamber occurs ∼2 V outside of its operating range, limiting the reliable range of field-emission observation. To increase this surface breakdown voltage, while maintaining the operating voltage range, a pedestal structure has been developed that can be incorporated into the nanotriode. This structure has a thicker dielectric layer, while maintaining a gate-cathode separation similar to that used previously. Measurements on vacuum-encapsulated two-terminal devices show field emission in the same voltage range and with similar characteristics to those previously observed; in consequence of the pedestal, higher surface breakdown voltages are achieved and the significance of the leakage current is reduced.

Quantum-mechanical theory of field electron emission under axially symmetric forces

Field emission from a metal nanotip is studied, because of its relevance to the modelling of the Fresnel projection microscope. This study includes the development of a new numerical approach to the axially symmetric Schrödinger equations.