Effects of metal layer morphology to silicon nanostructure formation in metal-assisted etching

This study presents a rapid and simple approach for creating silicon nanostructures using metal-assisted etching. The thickness of the metal layer was found to be a key process parameter affecting the surface morphology of silicon nanostructures. Au and Ag layers with a thickness of 3 nm, 5 nm, and 10 nm were used to study the effects of metal catalyst thickness on silicon nanostructure morphology. The experimental results show that the surface morphology of metal has a significant influence on the silicon nanostructure morphology, such that the silicon nanostructures transform from porous silicon surfaces into filament nanostructures or silicon nanowire with increasing thicknesses of both the Au and Ag metal layers.     

Using optical vortex to control the chirality of twisted metal nanostructures

We discovered for the first time that light can twist metal to control the chirality of metal nanostructures (hereafter, chiral metal nanoneedles). The helicity of optical vortices is transferred to the constituent elements of the irradiated material (mostly melted material), resulting in the formation of chiral metal nanoneedles. The chirality of these nanoneedles could be controlled by just changing the sign of the helicity of the optical vortex. The tip curvature of these chiral nanoneedles was measured to be <40 nm, which is less than 1/25th of the laser wavelength (1064 nm). Such chiral metal nanoneedles will enable us to selectively distinguish the chirality and optical activity of molecules and chemical composites on a nanoscale and they will provide chiral selectivity for nanoscale imaging systems (e.g., atomic force microscopes), chemical reactions on plasmonic nanostructures, and planar metamaterials.     

Preparation and photoluminescence of ZnO with nanostructure by hollow-cathode discharge

Without the use of a metal catalyst in the process, ZnO with nanostructures was successfully prepared on Si (100) substrate by simple chemical vapor-deposition method. In our work, Ar was used as the plasma forming gas, O₂ was the reactive gas and metal zinc powder (99.99% purity) vaporized by cylinder hollow-cathode discharge (HCD) acted as the zinc source. The crystal structures of the as-synthesized ZnO nanostructures were characterized by X-ray diffraction (XRD); the ZnO sample growing on the wall of the crucible showed a ‘comb-like’ nanostructure, while the other one at the bottom of the crucible showed a ‘rod-like’ structure, which can be attributed to the difference of the oxygen content. The measurement on the photoluminescence (PL) performance of the ZnO nanostructures was carried out at room temperature. The results indicated that the ‘comb-shape’ ZnO nanomaterial possessed a remarkably strong ultraviolet emission peak centered at 388 nm, while ZnO nanorods, except better ultraviolet emission, also had relatively strong blue-green emission ranging from 470 to 600 nm due to the existence of oxygen vacancies. The growth mechanism of ZnO with nanostructures is also discussed in this paper.     

Assembly of aligned linear metallic patterns on silicon

In order to harness the potential of block copolymers to produce nanoscale structures that can be integrated with existing silicon-based technologies, there is a need for compatible chemistries. Block copolymer nanostructures can form a wide variety of two-dimensional patterns, and can be controlled to present long-range order. Here we use the acid-responsive nature of self-assembled monolayers of aligned, horizontal block copolymer cylinders for metal loading with simple aqueous solutions of anionic metal complexes, followed by brief plasma treatment to simultaneously remove the block copolymer and produce metallic nanostructures. Aligned lines of metal with widths on the order of 10 nm and less are efficiently produced by means of this approach on Si(100) interfaces. The method is highly versatile because the chemistry to manipulate nanowire composition, structure and choice of semiconductor is under the control of the user.     

Preparation and photoluminescence of ZnO with nanostructure by hollow-cathode discharge

Without the use of a metal catalyst in the process, ZnO with nanostructures was successfully prepared on Si (100) substrate by simple chemical vapor-deposition method. In our work, Ar was used as the plasma forming gas, O₂ was the reactive gas and metal zinc powder (99.99% purity) vaporized by cylinder hollow-cathode discharge (HCD) acted as the zinc source. The crystal structures of the as-synthesized ZnO nanostructures were characterized by X-ray diffraction (XRD); the ZnO sample growing on the wall of the crucible showed a ‘comb-like’ nanostructure, while the other one at the bottom of the crucible showed a ‘rod-like’ structure, which can be attributed to the difference of the oxygen content. The measurement on the photoluminescence (PL) performance of the ZnO nanostructures was carried out at room temperature. The results indicated that the ‘comb-shape’ ZnO nanomaterial possessed a remarkably strong ultraviolet emission peak centered at 388 nm, while ZnO nanorods, except better ultraviolet emission, also had relatively strong blue-green emission ranging from 470 to 600 nm due to the existence of oxygen vacancies. The growth mechanism of ZnO with nanostructures is also discussed in this paper.     

OWL-Based Nanomasks for Preparing Graphene Ribbons with Sub-10 nm Gaps

We report a simple and highly efficient method for creating graphene nanostructures with gaps that can be controlled on the sub-10 nm length scale by utilizing etch masks comprised of electrochemically synthesized multisegmented metal nanowires. This method involves depositing striped nanowires with Au and Ni segments on a graphene-coated substrate, chemically etching the Ni segments, and using a reactive ion etch to remove the graphene not protected by the remaining Au segments. Graphene nanoribbons with gaps as small as 6 nm are fabricated and characterized with atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. The high level of control afforded by electrochemical synthesis of the nanowires allows us to specify the dimensions of the nanoribbon, as well as the number, location, and size of nanogaps within the nanoribbon. In addition, the generality of this technique is demonstrated by creating silicon nanostructures with nanogaps.     

Metal nanostructures fabricated by selective metal nanoscale etch method

A new method for fabricating metal nanostructures, called ‘the selective metal nanoscale etch method (SMNEM)’, was developed. The SMNEM consists of a galvanic displacement and selective etching process. The process was found to be simple and produced a uniform surface with a self-controlled etch rate of 32.2 ± 2.1 nm per cycle at a temperature and immersion time of 75°C and 3 min, respectively. Since it is a wet chemical process, SMNEM provides high throughput and low temperature etching which is compatible with conventional semiconductor processes. Various metal nanostructures, such as nanostairs, nanogratings, and nanowires were produced using SMNEM.     

Low cost fabrication of the large-area anti-reflection films from polymer by nanoimprint/hot-embossing technology

In this paper, we present a low cost approach to produce large-area polymer sheets with sub-wavelength nanostructures. The fabricated polymer films would have great potentials to attach to optical or solar-cell-related consumer products when anti-reflection/anti-glaring is mandatory. We employed a special electron cyclotron resonance (ECR) plasma process to fabricate the SWSs with large area directly on silicon substrates. Homogeneously distributed nanotips on the full 4 inch silicon substrate were fabricated by using gas mixtures of silane, methane, argon and hydrogen. An Ni-Co metal mold with a hardness of 550 HV was produced through the replication of the Si mold by electroplating. Afterwards, the molding process was applied to manufacture the nanostructures in PMMA plates in large volume. The nanostructures in PMMA plates with aspect ratios of 4 and diameters of 150?nm were fabricated. The fabricated PMMA sheets could generate the gradient of the refractive indices, absorb the light and greatly reduce the reflectivity. Compared with the PMMA without SWSs, the reflectivity of PMMA with SWSs decreased dramatically from 4.25% to 0.5% at the wavelength of light from 400 to 800?nm.     

Solution phase reduction to Fe–Ni alloy nanostructures with tunable shape and size

γ-(Fe,Ni) alloy nanostructures were prepared by solution phase reduction in the system of surfactant-cyclohexane-water, which were characterized by means of transmission electron microscopy (TEM), selected-area electron diffraction pattern (SAED), powder X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). By modifying the micelles system, such as changing the kinds of surfactant and the concentrations of metal ions, the alloy nanostructures can be tunable in size and shape. Only spheres with diameter of ca. 70 nm and the mixture of spheres and sheets of the nanoalloys were obtained without and in the presence of cyclohexane, respectively. The sphere size can be tuned from 70 to 25 nm in diameter by changing the concentrations of metal ions. The lower concentrations of metal ions were useful for preparing tinier particles; the higher concentrations of metal ions promoted the forming of rods-like nanoparticles. The values of remnant magnetization Mr and covercivity Hc increased with the alloy diameters decreasing.     

Induction of zinc particles on the morphology and photoluminescent property of globular Zn/ZnO core/shell nanorod heterojunction array architectures

Zn/ZnO metal/semiconductor nanostructures were successfully synthesised by a facile zinc-rich chemistry liquid-phase approach with zinc microspheres as sacrificial templates at ambient temperature. A series of globular Zn/ZnO core/shell structures and hollow microsphere architectures self-assembled by Zn/ZnO nanorod heterojunction arrays were obtained by controlling the amount of zinc particles. The structure, morphology, composition and optical properties of the products have been characterised by X-ray diffraction, scanning electron microscopy, Raman spectroscopy and photoluminescent spectroscopy. A possible growth mechanism of the Zn/ZnO nanostructures has been proposed based on the structural analysis. The growth mechanism of Zn/ZnO hollow microspheres is ascribed to Kirkendall effect. A new strong blue emission at 440 nm and a green emission around 500 nm with an enhancement over one order of magnitude compared with the pure ZnO sample have been observed. These emission bands are attributed to two kinds of mechanisms that have been discussed in detail.     

Catalyst-free growth of pyramidal zinc sulfide nanostructure arrays on single walled carbon nanotubes

Uniform and ordered pyramidal zinc sulfide (ZnS) nanostructure arrays have been fabricated on the single walled carbon nanotube (SWNT) films by chemical vapor deposition without using any metal catalyst. Each ZnS pyramid has a 100 nm-sized base, a uniform length of 600 nm, and a sharp tip of 10 nm. The control of interspatial distance between ZnS nanostructures was achieved by creation of selective growth on the SWNTs in voids with the assistance of a close-packed silica particle monolayer as a template. Furthermore, this kind of morphology control of nanostructure arrays can play an important role for potential applications, such as high efficiency of field emission because of the strong correlation between shapes and functionalities of nanostructures.     

Scanning Nanowelding Lithography for Rewritable One‐Step Patterning of Sub‐50 nm High‐Aspect‐Ratio Metal Nanostructures

The development of a new nanolithographic strategy, named scanning nanowelding lithography (SNWL), for the one‐step fabrication of arbitrary high‐aspect‐ratio nanostructures of metal is reported in this study. Different from conventional pattern transfer and additive printing strategies which require subtraction or addition of materials, SNWL makes use of a sharp scanning tip to reshape metal thin films or existing nanostructures into desirable high‐aspect‐ratio patterns, through a cold‐welding effect of metal at the nanoscale. As a consequence, SNWL can easily fabricate, in one step and at ambient conditions, sub‐50 nm metal nanowalls with remarkable aspect ratio >5, which are found to be strong waveguide of light. More importantly, SNWL outweighs the existing strategies in terms of the unique ability to erase the as‐made nanostructures and rewrite them into other shapes and orientations on‐demand. Taking advantages of the serial and rewriting capabilities of SNWL, the smart information storage–erasure of Morse codes is demonstrated. SNWL is a promising method to construct arbitrary high‐aspect‐ratio nanostructure arrays that are highly desirable for biological, medical, optical, electronic, and information applications.     

Interface controlled growth of nanostructures in discontinuous Ag and Au thin films fabricated by ion beam sputter deposition for plasmonic applications

The growth of discontinuous thin films of Ag and Au by low energy ion beam sputter deposition is reported. The study focuses on the role of the film?Csubstrate in determining the shape and size of nanostructures achieved in such films. Ag films were deposited using Ar ion energy of 150?eV while the Au films were deposited with Ar ion energies of 250?C450?eV. Three types of interfaces were investigated in this study. The first set of film?Csubstrate interfaces consisted of Ag and Au films grown on borosilicate glass and carbon coated Cu grids used as substrates. The second set of films was metallic bilayers in which one of the metals (Ag or Au) was grown on a continuous film of the other metal (Au or Ag). The third set of interfaces comprised of discontinuous Ag and Au films deposited on different dielectrics such as SiO₂, TiO₂ and ZrO₂. In each case, a rich variety of nanostructures including self organized arrays of nanoparticles, nanoclusters and nanoneedles have been achieved. The role of the film?Csubstrate interface is discussed within the framework of existing theories of thin film nucleation and growth. Interfacial nanostructuring of thin films is demonstrated to be a viable technique to realize a variety of nanostructures. The use of interfacial nanostructuring for plasmonic applications is demonstrated. It is shown that the surface Plasmon resonance of the metal nanostructures can be tuned over a wide range of wavelengths from 400 to 700?nm by controlling the film?Csubstrate interface.     

Low-dimensional, hinged bar-code metal oxide layers and free-standing, ordered organic nanostructures from turbostratic vanadium oxide

Both low-dimensional bar-coded metal oxide layers, which exhibit molecular hinging, and free-standing organic nanostructures can be obtained from unique nanofibers of vanadium oxide (VO(x)). The nanofibers are successfully synthesized by a simple chemical route using an ethanolic solution of vanadium pentoxide xerogel and dodecanethiol resulting in a double bilayered laminar turbostratic structure. The formation of vanadium oxide nanofibers is observed after hydrothermal treatment of the thiol-intercalated xerogel, resulting in typical lengths in the range 2-6 microm and widths of about 50-500 nm. We observe concomitant hinging of the flexible nanofiber lamina at periodic hinge points in the final product on both the nanoscale and molecular level. Bar-coded nanofibers comprise alternating segments of organic-inorganic (thiols-VO(x)) material and are amenable to segmented, localized metal nanoparticle docking. Under certain conditions free-standing bilayered organic nanostructures are realized.     

Formation of Au nanostructures through an alumina mask by laser-assisted deposition

We report a new method to produce ordered arrays of metal nanostructures on substrates. The method employs a through-hole nanoporous alumina membrane as a mask that is attached onto the substrate, silicon in this study. The material of deposition, Au in this study, was provided by pulsed laser ablation of a target gold. At an early stage of the deposition, a significant portion of Au penetrated the alumina through-holes and formed an ordered nanodot array on the silicon surface. At the later stage, the through-hole deposition was blocked by the growth of Au film on the top surface of the alumina, so that the heights of the Au nanodots were limited to about 10 nm under current experimental conditions. Subsequent attempts to clean up the top surface of the alumina with a lower power laser illumination resulted in the formation of new nanostructures around the alumina pores, nanospheres, or nanorings, depending on the fluence of the laser and the duration of the cleanup. We will discuss the underlying mechanism of the formation of these nanostructures.     

Rapid and Low‐Cost Prototyping of 3D Nanostructures with Multi‐Layer Hydrogen Silsesquioxane Scaffolds

A layer‐by‐layer (LBL) method can generate or approximate any three‐dimensional (3D) structure, and has been the approach for the manufacturing of complementary metal‐oxide‐semiconductor (CMOS) devices. However, its high cost precludes the fabrication of anything other than CMOS‐compatible devices, and general 3D nanostructures have been difficult to prototype in academia and small businesses, due to the lack of expensive facility and state‐of‐the‐art tools. It is proposed and demonstrated that a novel process that can rapidly fabricate high‐resolution three‐dimensional (3D) nanostructures at low cost, without requiring specialized equipment. An individual layer is realized through electron‐beam lithography patterning of hydrogen silsesquioxane (HSQ) resist, followed by planarization via spinning SU‐8 resist and etch‐back. A 4‐layer silicon inverse woodpile photonic crystal with a period of 650 nm and a 7‐layer HSQ scaffold with a period of 300 nm are demonstrated. This process provides a versatile and accessible solution to the fabrication of highly complex 3D nanostructures.     

In situ fabrication of meso-tetrakis(4-sulfonatophenyl)porphyrin nanostructures with excitonic absorption on glass substrate

In this article, we report in situ fabrication of meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) nanostructures with excitonic absorption on glass substrate. The exposure of TPPS thin film coated on a glass plate to HCl vapor resulted in the formation of nanostructures of TPPS. The formed TPPS nanostructures on glass plate were characterized by UV-vis, steady state emission spectral techniques, atomic force microscopy, and high resolution transmission electron microscopy (HR-TEM). A new sharp and intense absorption band (excitonic band) at 490 nm was observed for TPPS nanostructures on glass plate. Protonation of tertiary nitrogen atoms of TPPS ring by HCl molecules leads to the formation of nanostructures of J-aggregates of TPPS on glass surface. The height of the TPPS nanostructures was found to be 50-170 nm with an average width of 100 nm. HR-TEM images showed that the formed TPPS J-aggregates consist of tiny nanorods. The size of the nanostructures was tuned successfully by varying the concentration of TPPS used for thin film preparation.     

Self-catalytic growth and photoluminescence properties of ZnS nanostructures

Mass production of uniform wurtzite ZnS nanostructures has been achieved by a H₂-assisted thermal evaporation technique. X-ray diffraction (XRD) analyses, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) observations show that the ZnS nanostructures consist of nanobelts, nanosheets with a hexagonal wurtzite structure. The as-synthesized nanobelts have a length of several tens of micrometers and a width of several hundreds of nanometers. Self-catalytic vapor-liquid-solid (VLS) growth and vapor-solid (VS) growth are proposed for the formation of the ZnS nanostructures because neither a metal catalyst nor a template was introduced in the synthesis process. Room-temperature photoluminescence measurement indicates that the synthesized ZnS nanostructures have a strong emission band at a wavelength of 443 nm, which may be attributed to the presence of various surface states.     

Self-organized 2D periodic arrays of nanostructures in silicon by nanosecond laser irradiation

We report a phenomenon of spontaneous formation of self-organized 2D periodic arrays of nanostructures (protrusions) by directly exposing a silicon surface to multiple nanosecond laser pulses. These self-organized 2D periodic nanostructures are produced toward the edge as an annular region around the circular laser spot. The heights of these nanostructures are around 500?nm with tip diameter ~100?nm. The period of the nanostructures is about 1064?nm, the wavelength of the incident radiation. In the central region of the laser spot, nanostructures are destroyed because of the higher laser intensity (due to the Gaussian shape of the laser beam) and accumulation of large number of laser pulses. Optical diffraction from these nanostructures indicates a threefold symmetry, which is in accordance with the observed morphological symmetries of these nanostructures.     

Novel fabrication method of diverse one-dimensional Pt/ZnO hybrid nanostructures and its sensor application

A novel low-temperature, solution-phase method for the facile fabrication of a variety of one-dimensional (1D) metal/metal oxide hybrid nanostructures has been developed. This method is based on the wet chemical synthesis of metal oxide nanowires, followed by the surface coating of metal nanoparticles on metal oxide nanowire templates via reduction of metal ions along with controlled etching of metal oxide nanowires at the core, all in a low-temperature liquid environment. As a proof-of-concept, we applied this method to the fabrication of various 1D Pt/ZnO hybrid nanostructures including Pt nanoparticle-coated ZnO nanowires/nanotubes and Pt nanotubes on silicon and polymer substrates. The diverse morphology tuning is attributed to the control of pH in the solution with different metal precursor concentrations and amounts of reducing agent. The change of morphology, crystalline structure, and composition of various 1D Pt/ZnO hybrid nanostructures was observed by SEM, TEM (HRTEM), XRD and ICP-AES, respectively. Further, we have demonstrated a highly sensitive strain sensor (gauge factor = 15) with a Pt nanotube film fabricated by the developed method on a flexible polymer substrate.