ZnO–CuO core–shell nanorods and CuO-nanoparticle–ZnO-nanorod integrated structures

ZnO–CuO core–shell nanorods and CuO-nanoparticle–ZnO-nanorod integrated structures were synthesized for the first time by a two-stage solution process. Scanning electron microscopy and high-resolution transmission electron microscopy show that the diameter and the length of the nanorods are around 60 and 800 nm, respectively. The morphologies of outer CuO could be varied from nanoparticles to nanoshells by adjusting the solvent and dipping processes of copper (II) nitrate solution. The CuO nanoparticles are single-crystalline or highly textured structures with size of around 30 nm. The CuO shell with thickness of around 10 nm is constructed of nanocrystals with sizes in the range of 3–10 nm embedded in an amorphous matrix. Room-temperature cathodoluminescence measurements of the CuO–ZnO nanocomposites exhibit relatively sharp ultraviolet emissions at 380 nm as well as broad green and yellow emissions at 500 and 585 nm. The p-CuO/n-ZnO one-dimensional nanocomposites are promising for optoelectronic nanodevice applications.     

Plasmon coupling in binary metal core–satellite assemblies

Controlled plasmon coupling is observed in nanoparticle assemblies composed of 20 nm silver ‘satellite’ nanoparticles tethered by reconfigurable duplex DNA linkers to a 50 nm gold ‘core’ particle. The assemblies incorporate silver nanoparticle–oligonucleotide conjugates prepared using a new conjugation method in which the recognition strand is anchored by a 10 base pair, double strand spacer that presents adjacent 3’- and 5’-thiols to the silver surface. Reconfiguration of the DNA linkers from a compact to an extended state results in decreased core–satellite coupling and a blue-shift in the gold core plasmon resonance. The structural basis for the observed resonance modulation is investigated through simulation of the scattering spectra of binary assemblies with various core–satellite separations. Additional simulations of core–satellite assemblies composed of gold satellite particles bound to silver cores and of assemblies composed entirely of silver particles are used to clarify the dependence of the coupling response on the composition of the components and their distribution within the assembly. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Material processing of dielectrics with temporally asymmetric shaped femtosecond laser pulses on the nanometer scale

Laser material processing of dielectrics with temporally asymmetric femtosecond laser pulses of identical fluence, spectrum, and statistical pulse duration is investigated experimentally. To that end single shot structures at the surface of fused silica as a function of fluence and pulse shape are analyzed with the help of scanning electron microscopy. Structures for the bandwidth limited pulses show the known expansion in structure size with increasing laser fluence approaching the diffraction limit, which is 1.4 μm for the 0.5NA microscope objective used. In contrast, structures from the asymmetric pulses are remarkably stable with respect to variations in laser fluence and stay below 300 nm despite doubling the fluence. Different thresholds for surface material modification with respect to an asymmetric pulse and its time reversed counterpart are attributed to control of different ionization processes.     

Boundary layer-assisted chemical bath deposition of well-aligned ZnO rods on Si by a one-step method

ZnO seed layers and well-aligned ZnO single-crystalline micro/nanorods were synthesized on bare Si in one step without the assistance of catalysts by chemical bath deposition. Scanning electron microscopy (SEM) images and X-ray diffraction patterns show that the alignment of ZnO rods on Si(100) could be adjusted by varying the substrates’ angles of incline, the reaction temperature, and the precursor concentration. Transmission electron microscopy cross-sectional images demonstrate that a polycrystalline seed layer with (0002) preferred orientation was formed between the well-aligned rods and Si substrate placed vertically while a randomly oriented layer was formed between the randomly aligned rods and Si substrate placed horizontally. The formation of seed layers and alignment of as-synthesized ZnO rods were attributed to the assistance of boundary layers in a chemical bath deposition system.     

High repetition rate femtosecond laser nano-machining of thin films

Thin film laser micromachining has been utilized for repairing semiconductor masks, creating solar cells and fabricating MEMS devices. A unique high repetition rate femtosecond fiber laser system capable of variable repetition rates from 200 KHz to 25 MHz along with helium gas assist was used to study the effect of pulse repetition rate and pulse energy on femtosecond laser machining of gold-coated silicon wafer. It was seen that high repetition rates lead to smaller craters with uniform line width. Craters created at 13 MHz pulse repetition rate with 2.042 J/cm² beam energy fluence measured 110 nm in width and had a heat affected zone of 0.79 μm. It was found that pulse repetition rate only played a significant role in the size of the heat affected zone in the lower pulse energy ranges. In the future, a 1 W laser system will be acquired to find the optimal repetition rate that would create the minimal feature size with the least heat affected zone. Using this kind of setup along with techniques such as radial polarization and a different gas assist may enable us to create sub 100 nm feature size with good quality.     

Inverse pulsed laser deposition

Since the advent of pulsed laser deposition (PLD), several different target-substrate arrangements have been proposed. Besides the most common on-axis PLD, several off-axis geometries were studied, mainly to protect the substrate from the agglomerated species (clusters, droplets, particulates) of the plasma plume, which are detrimental to the homogeneity of films. Recently we introduced a novel geometry, termed inverse pulsed laser deposition (IPLD), in which the substrate is placed parallel to and slightly above the target plane. In this paper we summarize our results on this new geometry, and show how it can extend the perspectives of pulsed laser deposition, e.g., by improving the surface morphology of the films. Effects of ambient pressure are presented and exemplified on metallic and compound IPLD films, including Ti, CN _x , and Ti-oxides. AFM topographic images are used to prove that under optimized conditions IPLD is capable of growing compact and smooth films that are superior to PLD ones. A special—but easy-to-implement—IPLD arrangement is also introduced that considerably improves the homogeneity of IPLD films. In this geometry, the properties (e.g., deposition rate and roughness) of the films grown in the 1–25 Pa pressure domain are examined.     

Highly effective three-dimensional large-scale microfabrication using a continuous scanning method

Effective scanning is an important issue in two-photon induced stereolithography (TPS) for the reduction of the processing time required to fabricate three-dimensional (3D) nano/microstructures. In large-scale TPS based on a stage-scanning system, the large processing time due to the intrinsic slow response in the stage scanning system is a major obstacle to its use as a practical nanofabrication process. To overcome this drawback, we propose a continuous scanning method (CSM) for a stage-scanning system. In CSM, a pattern is generated with continuous movement of the stage during laser beam exposure. A stable fabrication window (SFW), which enables uniform motion of the stage without errors considering the stage characteristics, was obtained from fundamental experiments. Within the condition of a SFW, 2D and 3D microstructures having nanoscale details were fabricated with a whole scale range of several hundred micrometers using CSM.     

Spatial characterization of the laser-induced plasma plumes generated by IR CO2 pulsed laser on carbon targets

We have used ferrocene and paraffin wax as novel precursor and solvent for the growth of iron oxide nanoparticles. The proposed method of growth has several advantages over existing methods of growth using iron pentacarbonyl a precursor. Highly crystalline and monodispersed particles are obtained which assemble in two- and three-dimensional hexagonal closed packed superlattices. Growth kinetics has been studied by varying concentration of the precursor and time of growth. A phenomenological model has been proposed to explain the growth kinetics.     

Coherent linking of periodic nano-ripples on a ZnO crystal surface induced by femtosecond laser pulses

We demonstrate the coherent linking of periodic nano-ripples formed on the surface of ZnO crystals induced by femtosecond laser pulses. By adjusting the distance between two laser scanning zones, the periodic nano-ripples induced by two separated laser writing processes can be coherently linked and the ZnO nanograting with much longer grooves is therefore produced. The length limitation of this kind of nanograting previously set by the laser focus size is thus overcome. The micro-Raman mapping technique is used to evaluate the quality of coherent linking, and the underlying physics is discussed. The demonstrated scheme is promising for producing large-size self-organized nanogratings induced by femtosecond laser pulses.     

Femtosecond-laser nanofabrication onto silicon surface with near-field localization generated by plasmon polaritons in gold nanoparticles with oblique irradiation

Nanohole fabrication process with gold nanoparticles irradiated by femtosecond laser at different incident angles is investigated. Nanoparticles with diameter of 200 nm and laser irradiation with center wavelength of 800 nm are used in the present study. The analysis of the electromagnetic field distribution in the near-field zone of the particle is made by simulations based on finite-differential time domain (FDTD) method. It is shown that when gold nanoparticle is irradiated by laser pulse surface plasmon excitation can be induced, and associated with it, high-intensity near field is produced in a limited area around the particle. It is found that the change of the irradiation conditions by means of irradiation from various incident directions gives a possibility of laser nanoprocessing with tunable characteristics. Our results show that enhanced optical intensity is able to be induced on the substrate surface regardless of incident direction of the laser due to the image charge interaction with the substrate. Furthermore, the use of p-polarized laser irradiation at a certain angle gives a minimum of the spatial dimensions of the enhanced zone on the substrate which is about two times smaller than that obtained at normal incidence.     

Dye-sensitized solar cells based on TiO2 nanotube/porous layer mixed morphology

Titania porous layer has been fabricated on titania nanotubes for dye sensitized solar cells and the photovoltaic performance of solar cells with mixed morphology has been investigated. The porous layer results in a similar improvement in the short circuit current density to conventional TiCl₄ treatment, although the mechanisms responsible for the observed increase in the efficiency are different. This enables further improvements of the photovoltaic performance by combining the TiCl₄ treatment and porous layer deposition, so that the efficiency in the case of ∼5 μm long tubes increases on average from ∼1.6 to ∼2.2%.     

Self-guided glass drilling by femtosecond laser pulses

Straight through-holes of high aspect ratio have been fabricated in glass by femtosecond laser pulses, utilizing unique characteristics of ultrafast lasers such as volumetric multi-photon absorption and nonlinear self-focusing. In this study, interestingly, the drilling process was initiated and progressed in a self-regulated manner, while the laser focus was fixed through the specimen at the neighborhood of the rear surface that was in contact with liquid during the entire drilling process. The deposition of laser energy along the nonlinearly extended focal range and the guided drilling along the pre-defined region are explained based on time-resolved optical transmission and emission measurements.     

Nanofoaming dynamics in biopolymers by femtosecond laser irradiation

We have recently shown that irradiation of self-standing films of the biopolymers collagen and gelatine with single femtosecond laser pulses produces a nanofoaming layer with regular bubble size which can be controlled by wavelength selection. Following these initial studies, here we report on the temporal evolution of the foaming effect by measurements in situ and in real time of the change in the transmittance of a cw probe HeNe laser through the irradiated films. Self standing films of the biopolymers were irradiated with 90 fs laser pulses at 800, 400, and 266 nm. For fluences below and above the modification threshold a permanent attenuation of the transmission occurs (increasing with fluence). The initial decay of the transmission is fast (around few tens of ns), and is followed by dynamics in the longer timescale (micro and milliseconds). The temporal evolution of the transmission measured upon fs laser irradiation is similar with that determined in the irradiation of the biopolymer films at 248 nm with 25 ns laser pulses. The method allows separating in time the different processes occurring after irradiation that lead to a permanent nanofoaming structure, while the results allow us to understand the mechanisms of femtosecond laser processing of the biopolymers and their interest in biomedical applications.     

Nanolithography using high transmission nanoscale ridge aperture probe

Nanoscale ridge apertures provide a highly confined radiation spot with a high transmission efficiency when used in the near field approach. The radiation confinement and enhancement is due to the electric–magnetic field concentrated in the gap between the ridges. This paper reports the experimental demonstration of radiation enhancement using such antenna apertures and lithography of nanometer size structures. The process utilizes a NSOM (near field scanning optical microscopy) probe with a ridge aperture at the tip, and it combines the nonlinear two photon effect from femtosecond laser irradiation to achieve sub-diffraction limit lithography resolution.     

Nanoscale laser processing and diagnostics

The article summarizes research activities of the Laser Thermal Laboratory on pulsed nanosecond and femtosecond laser-based processing of materials and diagnostics at the nanoscale using optical-near-field processing. Both apertureless and apertured near-field probes can deliver highly confined irradiation at sufficiently high intensities to impart morphological and structural changes in materials at the nanometric level. Processing examples include nanoscale selective subtractive (ablation), additive (chemical vapor deposition), crystallization, and electric, magnetic activation. In the context of nanoscale diagnostics, optical-near-field-ablation-induced plasma emission was utilized for chemical species analysis by laser-induced breakdown spectroscopy. Furthermore, optical-near-field irradiation greatly improved sensitivity and reliability of electrical conductance atomic force microscopy enabling characterization of electron tunneling through the oxide shell on silicon nanowires. Efficient in-situ monitoring greatly benefits optical-near-field processing. Due to close proximity of the probe tip with respect to the sample under processing, frequent degradation of the probe end occurs leading to unstable processing conditions. Optical-fiber-based probes have been coupled to a dual-beam (scanning electron microscopy and focused ion beam) system in order to achieve in-situ monitoring and probe repair.     

Preparation of conventional thermoelectric nanopowders by pulsed laser fracture in water: application to the fabrication of a pn hetero-junction

The technique of laser fracture in a liquid medium has been applied to the synthesis of n-type (Bi_0.95Sb_0.05)₂ (Te_0.95Se_0.05)₃ and p-type (Bi_0.2Sb_0.8)₂Te₃ semiconducting nanopowders which are the best conventional materials currently used for thermoelectric applications at ambient temperature. The nanopowders have been prepared with a high yield in an especially built-up cell. Laser fracture in water of micronsized powders has been applied, using a nanosecond Nd:YAG laser working at 532 nm. The obtained powders have been characterized by scanning and transmission electron microscopy and by X-ray diffraction. The mean diameter is about 10 nm and the phase of the initial powders is kept. To test the potentiality of these nanosized materials, we have shown the feasibility to produce a pn hetero-junction.     

Laser-assisted local patterning of ZnO-based spots for mirror-less lasing

ZnO is known as one of the best materials for the implementation of the random lasing effect, associated with mirror-less laser emission in a simultaneously amplifying and highly scattering medium. Normally, the fabrication of this medium requires a rather complicated procedure of deposition and thermal treatment of ZnO-based films on some specific substrates, yielding wurtzite-orientation ZnO nanocrystals. We demonstrate a rapid synthesis of highly efficient ZnO-based random lasing spots on a piece of Zn by employing the phenomenon of laser-induced air breakdown. Being ignited near the surface of a Zn target, plasma of the air breakdown serves as a local reactor to locally transform its properties and thus form a film of well-packed 20–40 nm ZnO nanospheres. Exhibiting extremely high amplification and scattering, this medium is capable of generating the random lasing effect within the exciton-based photoluminescent band.     

Simple synthetic route for hydroxyapatite colloidal nanoparticles via a Nd:YAG laser ablation in liquid medium

Pulsed laser ablation (PLA) in liquid medium was successfully employed to synthesize hydroxyapatite (HAp) colloidal nanoparticles. The crystalline phase, particle morphology, size distribution and microstructure of the HAp nanoparticles were investigated in detail. The obtained HAp nanoparticles had spherical shape with sizes ranging from 5 to 20 nm. The laser ablation and the nanoparticle forming process were studied in terms of the explosive ejection mechanism by investigating the change of the surface morphology on target. The stoichiometry and bonding properties were studied by using XPS, FT-IR and Raman spectroscopy. A molar ratio of Ca/P of the prepared HAp nanoparticles was more stoichiometric than the value reported in the case of ablation in vacuum.     

Geometrical dependence of optical negative index meta-materials at 1.55 μm

We have studied the geometry dependency of fishnet-like negative refractive index meta-materials (NIMs), and developed a process to fabricate such NIMs using nanoimprint lithography (NIL) in a controlled way for it to achieve negative refractive index in the desired frequency range. As an example, we fabricated a fishnet structure with a minimum negative refractive index of −1.7 at 1560 nm, which was only 10 nm off the targeted wavelength of 1550 nm.     

Submicrometer-MOS capacitor with ultra high capacitance biased by Au nanoelectrodes

The ultimate limits of size of the current metal-oxide-semiconductor capacitors can be overcome by preparation of three-dimensional devices that can vertically be biased using one-dimensional metal nanostructures. Here, we present a general and efficient approach to the assembly and integration of Au nanocrystals into functional nanoelectrodes of three-dimensional submicrometer-MOS (0.35 μm²) capacitors, presenting an ultra high capacitance (24±1 pF). The Au nanocrystals were directly produced into a nanoporous template of anodized aluminum oxide that was evaluated, and the electrical characterization of this device corroborates the formation of the MOS capacitor. Flat band voltage is independent of sweep voltage range, and negligible hysteresis of capacitance-voltage curves is observed when sweep voltage ranges from positive to negative and turned again to positive bias. In addition, experimental results match theoretical analysis and indicate the presence of free surface charges stored in the Au nanostructures. The demonstrated ability to control the assembling of the nanocrystals and the results of electrical characterization indicate that the embedded Au nanoelectrodes have a high potential for memory applications based on three-dimensional devices.