Yingfeng Guan

Pulsed laser dewetting of patterned thin metal films: A means of directed assembly

Thin nickel films were patterned into various shapes and treated with a series of laser pulses. The edges and vertices of the patterned shapes act as programable instabilities, which enable directed assembly via dewetting when the laser energy density is above the melting threshold. The pattern formations were monitored as a function of laser pulse and the retraction process was attributed liquid dewetting and a subsequent resolidification. The calculated retraction velocity (83m∕s) and liquid lifetime (12.3ns) were consistent with the measured nickel retraction distances. The vertices of the shapes had an initially larger retraction velocity which was attributed to an additional in-plane curvature.

Pulsed laser dewetting of nickel catalyst for carbon nanofiber growth

We present a pulsed laser dewetting technique that produces single nickel catalyst particles from lithographically patterned disks for subsequent carbon nanofiber growth through plasma enhanced chemical vapor deposition. Unlike the case for standard heat treated Ni catalyst disks, for which multiple nickel particles and consequently multiple carbon nanofibers (CNFs) are observed, single vertically aligned CNFs could be obtained from the laser dewetted catalyst. Different laser dewetting parameters were tested in this study, such as the laser energy density and the laser processing time measured by the total number of laser pulses. Various nickel disk radii and thicknesses were attempted and the resultant number of carbon nanofibers was found to be a function of the initial disk dimension and the number of laser pulses.

Nanoscale lithography via electron beam induced deposition

We demonstrate the resolution and characteristics of a nanolithography process utilizing electron beam induced deposition (EBID) of W(CO)(6) and C(10)H(8) to define the imaging and masking layers. Lines and dot matrices were defined/written with various electron beam doses onto both polymethylmethacrylate (PMMA) coated silicon substrates (PMMA-Si) and bare silicon substrates (Si). The selectivity of the W(CO)(x) for the PMMA dry development process (no measurable etching) and the silicon ( approximately 18:1) reactive ion etch was very good. C(10)H(8) directly patterned on Si also provided good selectivity for the silicon etch process, 21:1. The pattern transfer of the EBID material patterns into the silicon had high fidelity. The resolution scaled with exposure dose and was correlated with the EBID broadening/scattering via a Monte Carlo simulation. Using the bi-layer approach, imaging layers on PMMA-Si, a silicon nanowire resolution of 13.5 nm and linewidth of 24.5 nm are demonstrated. Furthermore, using the single-layer approach, EBID directly on Si, a silicon nanowire resolution of 33 nm is demonstrated.

Synthesis of aligned nanoparticles on laser-generated templates

A fast and simple technique that produces self-aligned gold nanoparticles on a silicon substrate, covering a large area, is described. This technique involves three consecutive steps: first, the substrate is laser-irradiated to produce a periodic nanorippled structure; second, a thin film of gold is grown using ion-beam sputter deposition; and third, a thermal treatment is conducted to produce the formation and self-alignment of nanoparticles. The nanoparticles form along strips parallel to the nanoripple lines. The strip spacing equals the nanoripple spacing and the strip width depends on the angle of deposition and the divergence of the ion beam. The nanoparticle diameter is a function of the annealing temperature and time. It was found that deposition of the film at a very shallow grazing angle (0°) induces, upon thermal annealing in air at 800 °C, the formation of single nanoparticle rows aligned along the ripple ridges. In order to obtain the single-particle lines, a beam collimator, aimed at reducing the angular spread of the incident ion beam, was employed during deposition. This technique is general and could be used in a large number of substrate/film combinations. Studies of other substrates would provide optimum conditions to obtain similar results, as laser-induced periodic structures are a fairly universal phenomenon.

Non-lithographic organization of nickel catalyst for carbon nanofiber synthesis on laser-induced periodic surface structures

We present a non-lithographic technique that produces organized nanoscale nickel catalyst for carbon nanofiber growth on a silicon substrate. This technique involves three consecutive steps: first, the substrate is laser-irradiated to produce a periodic nanorippled structure; second, a thin film of nickel is deposited using glancing-angle ion-beam sputter deposition, followed by heat treatment, and third, a catalytic dc plasma-enhanced chemical vapor deposition (PECVD) process is conducted to produce the vertically aligned carbon nanofibers (VACNF). The nickel catalyst is distributed along the laser-induced periodic surface structure (LIPSS) and the Ni particle dimension varies as a function of the location on the LIPSS and is correlated to the nanoripple dimensions. The glancing angle, the distance between the ion beam collimators and the total deposition time all play important roles in determining the final catalyst size and subsequent carbon nanofiber properties. Due to the gradual aspect ratio change of the nanoripples across the sample, Ni catalyst nanoparticles of different dimensions were obtained. After the prescribed three minute PECVD growth, it was observed that, in order for the carbon nanofibers to survive, the nickel catalyst dimension should be larger than a critical value of ~19 nm, below which the Ni is insufficient to sustain carbon nanofiber growth.

Generation and manipulation of nanostructures by pulsed-laser ablation

Laser-induced surface structuring of silicon was studied using fluences close to the metering threshold and He gas background atmosphere. The effects of an initial surface microstructured region and of light polarization on the evolution of the surface topography were investigated. The microstructured surface topology consisted of an array of microholes surround by microcones of 2-3 micrometers tip-diameter and over 20 micrometers high. Pulsed laser irradiation of laser- microstructured silicon induces the formation of nanostructures. Nanocolumns having a diameter of 100 to 200 nm and reaching a height of up to 3 micrometers upon cumulative laser pulses grow on top of every microcone. The mechanisms of nanocolumn origin and growth are analyzed. Periodic undulations approximately 10 nm-high are formed when flat silicon substrates are irradiated with polarized laser light. These periodic structures have a wavelength that is a function of the light wavelength and the angle of incidence of the laser beam. At a slightly higher laser fluence, approximately 30 nm-diameter nanoparticles form on the surface of laser irradiated flat silicon specimens. Linear arrays of silicon nanoparticles with fairly uniform size that extend up to a millimeter are formed if the irradiation is performed using polarized light or the irradiated area contains a microstructured region. These nanostructures are analyzed within the frame of the theory of laser induced surface periodic structures.

Laser-promoted nanostructure evolution and nanoparticle alignment

A cone microstructure has been used as a template to generate nanotips and to promote nanoparticle alignment. A quasi-periodic array of nanotips is produced when the laser-induced cone microstructure is subject to chemical etching due to tapering of the cone tips. Nanoparticles can be produced on the surface of a silicon specimen by irradiating it in the presence of an inert gas atmosphere. The backscattered material that is re-deposited on the substrate, upon irradiation at fluences close to the melting threshold, is composed of a thin film intermixed with extremely small nanoparticles. Further irradiation promotes film clustering and nanoparticle formation. In the presence of cones, the nanoparticles become aligned into straight and long (~1 mm) lines whose spacing is close to the laser wavelength. This result suggested an ordering mechanism similar to that occurring for laser-induced periodic surface structures. The relation between nanoparticle line spacing and angle of incidence of the radiation supported this similarity. Nanoparticle ordering also was promoted by laser-enhanced chemical vapor deposition (LCVD) using polarized light, when a laser-induced periodic surface nanostructure was present in the substrate.