Nanoscale oxide patterning with electron-solid-gas reactions

A novel nanoscale oxide patterning technique based on electron beam induced transformation in a gas environment is demonstrated. Localized phase transformation is induced in the surface region of a substrate, resulting in the formation of an oxide pattern that is embedded in the surface. The composition of the transformed region is determined only by the substrate and gas composition. The spatial resolution of the technique is about 15 nm.     

Direct patterning of three-dimensional periodic nanostructures by surface-plasmon-assisted nanolithography

The technical applications of three-dimensional (3D) nanostructures demand a fabrication technique that is convenient and yet offers design flexibility. We describe here a nanofabrication technique called surface-plasmon-assisted three-dimensional nanolithography (3D-SPAN). By utilizing optical near-field interference patterns generated by surface plasmons (SPs), we have fabricated different 2D/3D periodic polymeric nanostructures in a typical photolithography setup. We have also shown here that the nanostructures fabricated by 3D-SPAN can be controlled easily in terms of size, layout, and defects by designing the SPAN mask. Because of its design flexibility and fabrication convenience, 3D-SPAN could be used to develop "photonics on a chip", where signal processing is carried out by photons instead of electrons and be extended to other applications of direct-writing 3D functional nanostructures.     

Nanoscale patterning on insulating substrates by critical energy electron beam lithography

This Letter describes a method to generate nanometer scale patterns on insulating substrates and wide band gap materials using critical energy electron beam lithography. By operating at the critical energy (E2) where a charge balance between incoming and outgoing electrons leaves the surface neutral, charge-induced pattern distortions typically seen in e-beam lithography on insulators were practically eliminated. This removes the need for conductive dissipation layers or differentially pumped e-beam columns with sophisticated gas delivery systems to control charging effects. Using a "scan square" method to find the critical energy, sub-100 nm features in 65 nm thick poly(methyl methacrylate) on glass were achieved at area doses as low as 10 microC/cm2 at E2 = 1.3 keV. This method has potential applications in high-density biochips, flexible electronics, and optoelectronics and may improve the fidelity of low voltage e-beam lithography for parallel microcolumn arrays.     

Nanoscale patterning of Zr-Al-Cu-Ni metallic glass thin films deposited by magnetron sputtering

Completely glassy thin films of Zr-Al-Cu-Ni exhibiting a large super-cooled liquid region (deltaTx = 95 K), very smooth surface (Ra = 0.65 nm), and an extremely high value of Vicker's hardness (Hv = 940), as compared to bulk Zr-Al-Cu-Ni metallic glass, were deposited by radiofrequency magnetron sputtering. Nanoscale patterning ability of Zr-Al-Cu-Ni metallic glass thin films was demonstrated by a focused ion beam etching. The capability to write nanometer-scale patterns (line width approximately 12 nm) opens up a variety of possibilities for fabricating nanomolds for imprint lithography, and a wide range of two- or three-dimensional components for future nanoelectromechanical systems.     

Nanoscale patterning of gold nanoparticles using an atomic force microscope

We report on the use of the tip of an atomic force microscope to remove selectively, and subsequently to deposit, nanoparticles of gold passivated with tri-n-octylphosphine oxide (TOPO)/octadecylamine. The study has revealed a minimum feature size of 50 nm in the removal experiment, while lines of 180 nm could be drawn with the gold nanoparticles, limited by the quality of the substrate surface.     

Nanoscale programmable sequence-specific patterning of DNA scaffolds using RecA protein

Molecular self-assembly inherent to many biological molecules, in conjunction with suitable molecular scaffolds to facilitate programmable positioning of nanoscale objects, offers a promising approach for the integration of functional nanoscale complexes into macroscopic host devices. Here, we report the use of the protein RecA as a means of highly efficient programmable patterning of double-stranded (ds)DNA molecules with molecular-scale precision at specific locations along the DNA strand. RecA proteins form nucleoprotein filaments with single-stranded (ss)DNA molecules, which are chosen to be of sequence homologous to the desired binding region on the dsDNA scaffold. We show that the patterning yield can be in excess of 85% and we demonstrate that concurrent patterning of multiple locations on the same dsDNA scaffold can be achieved with separation between the assembled nucleoprotein filaments of less than 4?nm. This is an important prerequisite for this programmable and flexible DNA scaffold patterning technique to be employed in molecular-?and nanoscale assembly applications.     

Engineering magnetic nanostructures with inverse hysteresis loops

Top-down lithography techniques allow the fabrication of nanostructured elements with novel spin configurations,which provide a new route to engineer and manipulate the magnetic response of sensors and electronic devices and understand the role of fundamental interactions in materials science.In this study, shallow nanostructure-pattemed thin films were designed to present inverse magnetization curves,i.e.,an anomalous magnetic mechanism characterized by a negative coercivity and negative remanence.This procedure involved a method for manipulating the spin configuration that yielded a negative coercivity after the patterning of a single material layer.Patterned NiFe thin films with trench depths between 15％-25％ of the total film thickness exhibited inverse hysteresis loops for a wide angular range of the applied field and the trench axis.A model based on two exchange-coupled subsystems accounts for the experimental results and thus predicts the conditions for the appearance of this magnetic behavior.The findings of the study not only advance our understanding of patterning effects and confined magnetic systems but also enable the local design and control of the magnetic response of thin materials with potential use in sensor engineering.     

Nanoscale magnetic regions formed in GaN implanted with Mn

Platelet structures with diameters less than 250 A and hexagonal symmetry were formed in GaN by high dose Mn+ ion implantation and annealing at 700-1000 degrees C. Selected-area diffraction pattern analysis indicates that these regions are GaxMn1-xN with a different lattice constant to the host GaN. The presence of the GaMnN corresponds to ferromagnetic behavior of the samples with a Curie temperature of approximately 250 K.     

Nanoscale patterning of colloidal quantum dots on transparent and metallic planar surfaces

The patterning of colloidal quantum dots with nanometer resolution is essential for their application in photonics and plasmonics. Several patterning approaches, such as the use of polymer composites, molecular lock-and-key methods, inkjet printing and microcontact printing of quantum dots have been recently developed. Herein, we present a simple method of patterning colloidal quantum dots for photonic nanostructures such as straight lines, rings and dot patterns either on transparent or metallic substrates. Sub-10?nm width of the patterned line could be achieved with a well-defined sidewall profile. Using this method, we demonstrate a surface plasmon launcher from a quantum dot cluster in the visible spectrum.     

Nanoscale control of Ag nanostructures for plasmonic fluorescence enhancement of near-infrared dyes

Potential utilization of proteins for early detection and diagnosis of various diseases has drawn considerable interest in the development of protein-based detection techniques. Metal induced fluorescence enhancement offers the possibility of increasing the sensitivity of protein detection in clinical applications. We report the use of tunable plasmonic silver nanostructures for the fluorescence enhancement of a near-infrared (NIR) dye (Alexa Fluor 790). Extensive fluorescence enhancement of ～2 orders of magnitude is obtained by the nanoscale control of the Ag nanostructure dimensions and interparticle distance. These Ag nanostructures also enhanced fluorescence from a dye with very high quantum yield (7.8 fold for Alexa Fluor 488, quantum efficiency (Qy) = 0.92). A combination of greatly enhanced excitation and an increased radiative decay rate, leading to an associated enhancement of the quantum efficiency leads to the large enhancement. These results show the potential of Ag nanostructures as metal induced fluorescence enhancement (MIFE) substrates for dyes in the NIR “biological window” as well as the visible region. Ag nanostructured arrays fabricated by colloidal lithography thus show great potential for NIR dye-based biosensing applications.      

Direct sub-micrometer patterning of nanostructured conducting polymer films via a low-energy infrared laser

Despite the many attractive properties of conjugated polymers, their practical applications are often limited by the lack of a simple, scalable, and nondisruptive patterning method. Here, a direct, scalable, high-resolution patterning technique for conducting polymers is demonstrated that does not involve photoresists, masks, or postprocessing treatment. Complex, well-defined patterns down to sub-micrometer scales can be created from nanofibrous films of a wide variety of conducting polymers by photothermally welding the nanofibers using a low-energy infrared laser. The welding depth, structural robustness, and optical properties of the films are readily controlled. In addition, the electrical properties such as conductivity can be precisely tuned over a 7-order of magnitude range, while maintaining the characteristic tunable electronic properties in the nonwelded polyaniline regions.     

On calculating the magnetic state of nanostructures

We review some of our recent work on first principles calculations of the magnetic structure of surface and bulk nanostructures. The calculations are based on implementation of relativistic density functional theory within state of the art surface embedding and order-N multiple scattering Green’s function methods. First principles spin-dynamics and the constrained local moment approximation are reviewed as they relate to optimization of moment configurations in highly inhomogeneous materials such as surface and bulk nanostructures. Results are present for three prototypical nanostructures – short Co-chains adjacent to a Pt{1 1 1}-surface step-edge, a Cr-trimer on the Au{1 1 1}-surface, and Fe-chains and impurities in Cu – that illustrate the need to treat the underlying electronic interactions on a fully self-consistent basis in which the very different energy scales appropriate to exchange coupling and magneto-crystalline anisotropy are treated on an equal footing.     

Selective sensitivity of ellipsometry to magnetic nanostructures

Magneto-optic (MO) ellipsometry of ferromagnetic materials is extremely sensitive to ultra-thin films, multilayers, and nanostructures. It gives a possibility to measure all components of the magnetization vector in the frame of the magneto-optic vector magnetometry and enables us to separate magnetic contributions from different depths and materials in nanostructures, which is reviewed in this article. The method is based on ellipsometric separation using the selective MO Kerr effect. The figure of merit used to quantify the ellipsometric selectivity to magnetic nanostructures is defined on the basis of linear matrix algebra. We show that the method can be also used to separate MO contributions from areas of the same ferromagnetic materials deposited on different buffer layers. The method is demonstrated using both: (i) modeling of the MO ellipsometry response and (ii) MO measurement of ultra-thin Co islands epitaxially grown on self-organized gold islands on Mo/Al₂O₃ buffer layer prepared using the molecular beam epitaxy at elevated temperatures. The system is studied using longitudinal (in-plane) and polar (perpendicular) MO Kerr effects.     

Synthesis of Fe-group metal oxide nanostructures by thermal oxidation and their magnetic properties

In this paper, we review the preparation of Fe-group metal oxide nanostructures by the thermal oxidation method developed in our lab. By this method, we have prepared several kinds of nanostructures, including nanowires and nanoleaves. The magnetic properties of these nanostructures have also been studied. By carefully controlling the reacting time, temperature, and humidity, we have prepared alpha-Fe2O3, gamma-Fe2O3, Fe3O4, and Co3O4 nanowires and alpha-Fe2O3 nanoleaves by heating the substrates in proper atmosphere. The alpha-Fe2O3 and Co3O4 nanowires are produced by directly oxygenating pure metal at 550 to approximately 650 degrees C and 480-520 degrees C, separately. The gamma-Fe2O3 and Fe3O4 nanowires are produced by reducing as-prepared alpha-Fe2O3 nanowires in a mixture of N2 and H2. The nanowires are about 10-20 microm, with diameter of about 20 to approximately 100 nm. Most of the nanowire arrays are grown vertically from the surface of the substrate at a high surface density (10(8)-10(9) cm(-2)). Compared with the nanowires prepared by hydrothermal process and template method, Most of our nanowires are structurally uniform and single crystallites. The magnetic properties of these nanostructures are also studied, and demonstrate some novel properties.     

One-pot reaction to synthesize PEG-coated hollow magnetite nanostructures with excellent magnetic properties

We first demonstrate a simple "one-pot" method to synthesis uniform Fe3O4 hollow microspheres in the presence of PEG in ethylene glycol by using urea to control their morphologies. The interior cavity of the hollow spheres can be tunable by reaction time. The Lamer model was used to explain the formation of magnetite hollow spherical structures based on the experimental observations. The obtained hollow Fe3O4 microspheres showing superparamagnetism with a high saturation magnetization of ca. 86.4 emu/g, and also had an enrichment surface of -OH groups, which will be favorable to the further modification with other biomedical molecules.     

Extraordinarily complex crystal structure with mesoscopic patterning in barium at high pressure

Elemental barium adopts a series of high-pressure phases with such complex crystal structures that some of them have eluded structure determination for many years. Using single-crystal synchrotron X-ray diffraction and new data analysis strategies, we have now solved the most complex of these crystal structures, that of phase Ba-IVc at 19?GPa. It is a commensurate host-guest structure with 768 atoms in the representative unit, where the relative alignment of the guest-atom chains can be represented as a two-dimensional pattern with interlocking S-shaped 12-chain motifs repeating regularly in one direction and repeating with constrained disorder in the other. The existence of such patterning on the nanometre scale points at medium-range interactions that are not fully screened by the itinerant electrons in this metal. On the basis of first-principles electronic structure calculations, pseudopotential theory and an analysis of the lattice periodicities and interatomic distances, we rationalize why the Ba phases with the common densely packed crystal structures become energetically unfavourable in comparison with the complex-structured Ba-IVc phase, and what the role of the well-known pressure-induced s-d electronic transfer is.     

Designed synthesis and magnetic properties of Co hierarchical nanostructures

Size and shape controlled fabrication of magnetic Co microsphere, nanoribbon, nanochain and rose-like microarchitecture has been successfully realized via a simple hydrothermal route. X-ray diffraction analysis suggests that Co hierarchical nanostructures are identified as hexagonal phase. Magnetic hysteresis measurements demonstrate that the obtained different Co hierarchical structures show structure-dependent magnetic properties. Saturation magnetization (M_S) found for Co spherical flowers and spherical powders are larger than Co nanoribbons, smaller than sphere-rebuilt micro particles or chain-like structures. Chain-like and nanorribon structures have abnormally large coercivity (H_C). H_C values of Co nanoribbons and one dimensional chains become as large as 256 Oe and 316 Oe.     

Structure and magnetic properties of Fe films with pyramid-like nanostructures deposited by oblique target direct current magnetron sputtering

Fe thin films were deposited by oblique target direct current magnetron sputtering on Si (100) and (111) substrates. The structure, surface morphology and magnetic properties of the thin films were characterized using X-ray diffraction, field emission scanning electron microscopy, and superconducting quantum interference device magnetometer, respectively. The results reveal that the structure of the as-deposited Fe thin films is body-centered cubic with the preferential [110] crystalline orientation. A pyramid-like nanostructure with sharp tip was formed on the surfaces of Fe thin films under appropriate sputtering power. Formation of the pyramid-like nanostructure is mainly owed to the enhancement of atomic mobility and the bombardment effect with increasing of sputtering power. Meanwhile, the crystalline orientation of Si substrate and the intrinsic stress in the films are expected to have little contribution to the formation of the pyramid-like nanostructure. The magnetic anisotropy was found in the as-deposited Fe thin films, and varies with the thickness of the Fe thin films. As the film thickness increases from 604 to 1,786 nm, the magnetic anisotropy field and the uniaxial anisotropy constant increase from 3.8 to 5.6 kOe, and from 0.4 × 10⁶ to 1.1 × 10⁶ erg/cm³, respectively, which indicates that besides magnetocrystalline anisotropy, stress induced anisotropy and shape anisotropy also exist in the as-deposited Fe thin films.