Ultrathin Anodic Aluminum Oxide Research Articles

Investigation of magnetic hysteresis in biased Ta/Pt/Co/FeMn/Ta antidots: Influence of structural dimensions

There exists a controversy in the literature concerning the values of coercive and bias fields in antidots magnetic structures formed by a hexagonal network of nanoholes. The coercive fields (HC) and the exchange bias fields (∣HEXC∣) for antidots (deposited on ultrathin anodic aluminum oxide, namely, AAO) are either increased or diminished by comparison with the same magnetic nanostructures grown on continuous substrates (namely, CML). We propose to elucidate these debates by showing the importance of the easy axis of the magnetization, the direction of the applied magnetic field, the thicknesses of the layers, and the 3D-topology of nanoholes, as well as the magnetic and thermal history of the magnetic measurements. Here, biased Ta(5 nm)/Pt(5 nm)/Co(0.6 nm)/Fe50Mn50(X)/Ta(5 nm) antidots are investigated by extraordinary Hall effect measurements at 5 K, where X varies in the (0–5.5) nm range. The substrate consists in a hexagonal array of holes, described by the pair of (p,d) values, respectively, the period as the distance from center to center of two consecutive holes and the hole diameter. The dimensions of antidots are (p≈100 and d≈40 nm) for X=(2–5.5) nm, (p≈150 and d≈60 nm) for X=3.5 nm, and (p≈100 and d≈60 nm) for X=0. A continuous stack using Si/SiO2(100 nm) is used for comparison. HC and ∣HEXC∣ gradually increase when X is enhanced for both substrates, with nevertheless a weak decrease at high X for the continuous system. Perpendicular magnetic anisotropy is only observed for both unbiased samples, the X=2 nm continuous sample, and both X=5 nm samples that have undergone field cooling treatment from 500 to 5 K under −2 T. Usually, HC(AAO)>HC(CML), ∣HEXC(AAO)∣>∣HEXC(CML)∣, and ∣HA(AAO)∣<∣HA(CML)∣ (HA designating the anisotropy field). However, for certain conditions, as, for instance, for FC-procedures starting from high temperatures and/or strong magnetic field, other situations might be observed. A discussion pertaining to the amplitudes of HC, ∣HEXC∣ and the anisotropy field (∣HA∣) of continuous and discontinuous samples is given for our experimental results as well as for published data in the literature, in the light of structural characteristics (wedge-to-wedge distance, porosity, or coverage ratio). Such biased perpendicular antidots might be particularly used in specific nanomaterials devoted to spintronics.

Large-scale plasmonic nanodisk array as a biosensing platform fabricated by transfer nanoprinting.

Surface plasmon resonance based on nanostructures has been a powerful analytical tool in rapid detection and analysis of biomolecules. However, the fabrication of nanostructure sensors, such as electron beam lithography and focused ion beam milling, has inherent defects as manufacturing cost, complex process flow, and small fabrication area. In this paper, using the transfer nanoprinting approach based on an ultrathin anodic aluminum oxide membrane, a centimeter-scale ordered periodic Ag-ZnS bilayer nanodisk on Au film with a low cost and simple process is fabricated. A surface plasmon polariton Bloch mode from nanodisk arrays is experimentally demonstrated at normal incident of light. The plasmonic platform exhibits an ideal refractive index bulk sensitivity of up to 438nm/RIU. Furthermore, by using a polyelectrolyte bilayer with well-defined thickness, the surface sensitivity of the biosensing platform is also investigated. The large-scale plasmonic bilayer nanoparticle biosensing platform has broad application prospects in development of low-cost and high-performance biosensing chips.

Gas-Flow-Assisted Wrinkle-Free Transfer of a Centimeter-Scale Ultrathin Alumina Membrane onto Arbitrary Substrates.

The transfer of an ultrathin membrane onto arbitrary substrates is important in different practical fields. Conventional wet-transfer methods inevitably induce wrinkle defects as a result of the large contact angle of the trapped droplet between the membrane and the substrate. Here, we demonstrate a gas flow-assisted method (GFAM) to transfer centimeter (cm)-scale ultrathin membranes onto arbitrary substrates (including a curved substrate) without wrinkles. GFAM makes use of contact angle hysteresis to bulge the trapped droplet between the substrate and the ultrathin membrane and simultaneously stretch the ultrathin membrane during rapid dewetting driven by gas flow. Moreover, GFAM can be easily fulfilled by using compressed air for seconds. Compared with conventional hydrophilic treatments or organic liquid wetting, this method has no durability concern and does not change the surface nature of substrates. Taking a widely used ultrathin anodic aluminum oxide (AAO) membrane as an example, we successfully demonstrate the application of a large-area wrinkle-free ultrathin AAO membrane to defect-free ordered nanostructure array fabrication and investigate the micro-scale details of macro-scale wrinkles generated by the conventional ways. In addition, its corresponding superiority over the defective counterpart is further studied in optical sensing. This method is highly valuable for promoting the simplicity of large-area ultrathin membrane transfer in practice.

A novel WOx-based memristor with a Ti nano-island array

A novel WOx-based memristor with Ti nano-island arrays was developed in this study. A 100 nm thick Pt bottom electrode was deposited, and an ultrathin anodic aluminum oxide (AAO) template was used to obtain the Ti nano-island arrays. The WOx/Ti nano-island (NI)/Pt devices were prepared by magnetron sputtering using a WOy (y=2.7–2.9) target to obtain a 30 nm deposition layer of WOx. Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and semiconductor analysis confirmed that the WOx-based memristor could be successfully produced with a highly ordered Ti nano-island array. The size and density of the NIs could be controlled by selecting the aperture of the AAO template. The oxygen interaction between the Ti nano-islands and WOx led to the creation of oxygen vacancies in their vicinity, resulting in the formation of an oxygen vacancy conductive filament (CF) without the need for a forming process. NIs tend to strengthen the electric field along the bias voltage direction, fostering the generation of the CF. The absolute values of VSET and VRESET were about 0.19 V and 0.69 V, respectively. Compared with the devices without the Ti NI arrays, the variation coefficients of VSET and VRESET of the WOx/Ti NI/Pt devices were reduced by about 84.9% and 83.7%, respectively. With smaller and more concentrated VSET and VRESET, the WOx/Ti NIs/Pt device showed a significantly improved switching stability.

Morphology-dependent high antireflective surfaces via anodic aluminum oxide nanostructures

High density honeycombed nano-pattern of anodic aluminum oxide (AAO) is of great use to various high-performance optoelectronic devices due to its non-lithographic and excellent sub-wavelength structure (SWS) anti-reflection (AR) properties. In this paper, we introduce an ultra-thin AAO based bilayer polydimethylsiloxane (PDMS) scheme to simply replicate and obtain regular triangle-like surface nano-pillar array with a period of 125 nm, which shows a 5% to 27% AR enhancement under the light wavelength ranging from 250 nm to 850 nm. In order to investigate the intrinsic property of AAO nano-structure based light extraction properties, gallium nitride (GaN) light-emitting models are built. Compared to the bare GaN, the maximum AR enhancements of low ordered AAO array, regular array and high ordered AAO array are respectively 20.2%, 22.0% and 23.5%. Grain size effect is found to further improve the AR performance of the high ordered AAO array and the effective refractive index based thin film model demonstrates a success in AAO pattern-dependent surface AR analysis. Besides, by tailoring the side-wall profile, a further AR enhancement (e.g. 39.1%) can be obtained because of a smoothly changing of the refractive index along the height direction.

Comprehensive Study of Au Nano-Mesh as a Catalyst in the Fabrication of Silicon Nanowires Arrays by Metal-Assisted Chemical Etching

Silicon nanowires (SiNWs) arrays have become one of low-dimensional structural nanomaterials for the preparation of high-performance optoelectronic devices with the advantages of highly efficient light trapping effect, carrier multiplication, and adjustable optical bandgap. The controlled growth of SiNWs determines their electrical and optical properties. The morphology of silicon nanowires fabricated by conventional metal-assisted chemical etching (MACE) involving the Ag-based etching process cannot be precisely controlled. Ultra-thin anodic aluminum oxide (AAO) is one of the new-pattern nanostructure assembly systems for the synthesis of nanomaterials. The synthesized nanostructure arrays can be tuned to exhibit different optical and electrical properties in a certain wavelength range by adjusting the AAO membrane parameters. In this paper, we demonstrate an ultra-thin Au nano-meshes array from a single hexagonal AAO membrane as a replication master instead of conventional Ag particles as etching catalyst. The extended ordered silicon nanowires arrays are fabricated by the selective chemical dissolution of nanoscale noble metal meshes that exhibit excellent anti-reflection performance in broadband wavelengths and a wide incidence angle.

Large‐Scale Plasmonic Nanodisk Structures for a High Sensitivity Biosensing Platform Fabricated by Transfer Nanoprinting

AbstractSurface plasmon resonance in plasmonic nanostructures has become a powerful analytical tool in ultrasensitive label‐free biomolecule sensing. However, the fabrication of plasmonic nanostructure sensors relies on lithography‐based top‐down nanofabrication approaches, which have inherent shortcomings, such as high manufacturing cost, time‐consuming fabrication, and a small fabrication footprint. In particular, the small footprint of fabricated plasmonic nanostructures has significantly restrained the study of their angle‐dependent sensitivity. Here, a centimeter‐scale, high sensitivity sensing platform with low cost and high yield is experimentally demonstrated, fabricated through a transfer nanoprinting approach based on an ultrathin anodic aluminum oxide membrane. Two angle‐dependent resonant modes from the plasmonic nanodisk structure are observed at nonzero incidence angles. The dependence of the bulk sensitivity and surface sensitivity on the incidence angle under these two modes is systematically investigated. By functionalizing the plasmonic nanostructure, monitoring of binding of protein at dilute concentration is performed in real time at different incidence angles. The specific label‐free lowest detection limit is found to be 1.8 nanomolar for the longer wavelength resonant mode at a 10° incidence angle. This plasmonic nanostructure sensing platform should hold promise in the development of low cost and high throughput biosensing chips.

Preparation of Anodic Aluminum Oxide Masks with Size-Controlled Pores for 2D Plasmonic Nanodot Arrays

Anodic aluminum oxide (AAO) layers with nano-sized pores have been used as shadow masks for the fabrication of two-dimensional (2D) metal nanodot arrays (NDAs). However, the localized surface plasmon resonance (LSPR) of size-controlled NDAs fabricated using AAO masks has not been much studied. In this study, we report on the improved preparation method and utilization of an ultrathin AAO mask for the fabrication of 2D plasmonic metal NDAs. The greatest challenge in preparing an AAO mask is to control the pore diameter and to make it reproducible. AAO masks with size-controlled pores were reproducibly prepared using a two-step chemical wet etching method. Ag NDAs with different dot diameters (42, 60, and 80 nm) and Ag, Cu, and Au NDAs with dot a diameter of 80 ± 5 nm were fabricated on indium tin oxide glass substrates using AAO masks. The wavelengths corresponding to LSPR of 2D metal NDAs were investigated using ultraviolet–visible spectroscopy. Our results show that AAO masks with tunable pores can be used as shadow masks for the fabrication of 2D plasmonic NDAs.

Preparation of highly-ordered lanthanum hexaboride nanotube arrays and optimizing its field emission property by ion bombardment post-treatment

Large scale, highly-ordered and vertically aligned lanthanum hexaboride (LaB6) nanotube arrays were prepared by DC magnetron sputtering deposition LaB6 on ultra-thin anodic aluminum oxide (AAO) substrates. The LaB6 nanotube arrays that were 210 nm in diameter and 620 nm in height haven’t collapsed and aggregated after the AAO templates have been removed by wet etching. After that, LaB6 nanotube arrays were bombarded by low energy Ar ion, which was used to further optimize its field emission property. X-ray diffraction measurement shows that LaB6 nanotube arrays are uniform polycrystalline phase without preferred orientation. SEM analysis shows that ion bombardment is an effective method for adjusting the micro-morphology of nanotube arrays. The cap-end nanotube arrays are turned into open-end, and the B/La atoms on its surface are decreased after ion bombardment. The formation of the open-end nanotube arrays with sharp edges and the enrichment of La atoms on the surface play an important role in enhancing the field emission property. The open-ended LaB6 nanotube arrays with sharp edges have the lowest turn-on field (6.8 V µm−1) and the highest current density (2.3 mA cm−2 at 11 V µm−1).

Template-assisted electrodeposition of Ni and Ni/Au nanowires on planar and curved substrates

We present a variant of the template-assisted electrodeposition method that enables the synthesis of large arrays of nanowires (NWs) on flat and curved substrates. This method uses ultra-thin (50 nm–10 μm) anodic aluminum oxide membranes as a template. We have developed a procedure that uses a two-polymer protective layer to transfer these templates onto almost any surface. We have applied this technique to the fabrication of large arrays of Ni and segmented composition Ni/Au NWs on silicon wafers, Cu tapes, and thin (0.2 mm) Cu wires. In all cases, a complete coverage with NWs is achieved. The magnetic properties of these samples show an accentuated in-plane anisotropy which is affected by the form of the substrate (flat or curve) and the length of the NWs. Unlike current lithography techniques, the fabrication method proposed here allows the integration of complex nanostructures into devices, which can be fabricated on unconventional surfaces.

Boosting the Photoluminescence of Monolayer MoS2 on High‐Density Nanodimer Arrays with Sub‐10 nm Gap

AbstractPatterned plasmonic nanodimers are fabricated exploiting an ultrathin porous anodic aluminum oxide membrane as a mask during angle‐resolved shadow deposition. The fabricated nanodimer arrays exhibit consistent sub‐10 nm gaps and a high particle density up to 1.0 × 1010 cm−2 over a large area. The ultrasmall dimer gaps provide highly confined electromagnetic fields, which strongly enhance the photoluminescence (PL) emission and Raman scattering from the surrounding monolayer molybdenum disulphide (MoS2). The ensemble PL intensity from MoS2/dimers is enhanced by up to a factor of ≈160 by resonant excitation of the dimer modes. Anisotropic polarization‐dependent characteristics of PL and Raman from the MoS2/dimers confirm that the dominant enhancement originates from the dimer configuration. These experiments demonstrate a facile approach for the fabrication of low‐cost high‐performance 2D material‐based optoelectronic devices.

Controlled Patterning of Plasmonic Dimers by Using an Ultrathin Nanoporous Alumina Membrane as a Shadow Mask.

We report on design and fabrication of patterned plasmonic dimer arrays by using an ultrathin anodic aluminum oxide (AAO) membrane as a shadow mask. This strategy allows for controllable fabrication of plasmonic dimers where the location, size, and orientation of each particle in the dimer pairs can be independently tuned. Particularly, plasmonic dimers with ultrasmall nanogaps down to the sub-10 nm scale as well as a large dimer density up to 1.0 × 1010 cm-2 are fabricated over a centimeter-sized area. The plasmonic dimers exhibit significant surface-enhanced Raman scattering (SERS) enhancement with a polarization-dependent behavior, which is well interpreted by finite-difference time-domain (FDTD) simulations. Our results reveal a facile approach for controllable fabrication of large-area dimer arrays, which is of fundamental interest for plasmon-based applications in surface-enhanced spectroscopy, biochemical sensing, and optoelectronics.

Patterned FePt nanostructures using ultrathin self-organized templates

Patterned magnetic thin films are both scientifically interesting and technologically useful. Ultrathin self-organized anodic aluminum oxide (AAO) template can be used to fabricate large area nanodot and antidot arrays. The magnetic properties of these nanostructures may be tuned by the morphology of the AAO template, which in turn can be controlled by synthetic parameters. In this work, ultrathin AAO templates were used as etching masks for the fabrication of both FePt nanodot and antidot arrays with high areal density. The perpendicular magnetic anisotropy of L10 FePt thin films are preserved in the nanostructures.

Facile design of ultra-thin anodic aluminum oxide membranes for the fabrication of plasmonic nanoarrays

Ultra-thin anodic aluminum oxide (AAO) membranes are efficient templates for the fabrication of patterned nanostructures. Herein, a three-step etching method to control the morphology of AAO is described. The morphological evolution of the AAO during phosphoric acid etching is systematically investigated and a nonlinear growth mechanism during unsteady-state anodization is revealed. The thickness of the AAO can be quantitatively controlled from ∼100 nm to several micrometers while maintaining the tunablity of the pore diameter. The AAO membranes are robust and readily transferable to different types of substrates to prepare patterned plasmonic nanoarrays such as nanoislands, nanoclusters, ultra-small nanodots, and core–satellite superstructures. The localized surface plasmon resonance from these nanostructures can be easily tuned by adjusting the morphology of the AAO template. The custom AAO template provides a platform for the fabrication of low-cost and large-scale functional nanoarrays suitable for fundamental studies as well as applications including biochemical sensing, imaging, photocatalysis, and photovoltaics.

Size correlation of optical and SERS properties for highly ordered Au nanocone arrays with sub-100 nm feature size

Using ultrathin anodic aluminum oxide membranes as evaporation masks, highly ordered Au nanocone arrays with sub-100 nm feature size were fabricated on glass substrates. The size of Au nanocones was adjusted by the easily controllable nanopores’ dimension. Influences of Au nanocone size on the optical property and SERS activity were characterized by extinction and Raman spectra, respectively. For one thing, the spectral position of plasmon resonances was seen to slightly blue-shift with increasing the nanocone size, which coincided well with the result of finite-difference-time-domain simulation. For another, glass substrates patterned by Au nanocone arrays exhibited a high surface enhanced Raman scattering sensitivity to Rhodamine 6G. Compared with the bulk sample, the estimated enhancement factor was boosted from 8 × 106 to 1.79 × 107 as the diameter of Au nanocone increased from 36 to 77 nm. Such highly ordered nanocone arrays with tunable and uniform size have great potential for various optical or spectrographic applications.

Ultrathin AAO Membrane as a Generic Template for Sub-100 nm Nanostructure Fabrication

Anodic aluminum oxide (AAO) templates are emerging as a platform for simple, cost-effective, high-throughput top-down nanofabrication of regular arrays of nanostructures. Thus far, however, AAO pattern transfer has largely been restricted to smooth and chemically inert surfaces, mostly Silicon substrates. Here, we present a more generalizable strategy for preparing free-standing through-hole ultrathin alumina membranes (UTAMs) and transferring them to both smooth and rough substrates, thereby enabling the fabrication of centimeter-scale arrays of nanostructures with sub-100 nm feature diameters on almost arbitrary substrates. To validate the utility of our procedures, we transferred UTAMs to surfaces relevant for photocatalytic applications and prepared plasmonic photocathodes consisting of dense arrays of size-controlled sub-100 nm Au and Ni nanodots on top of chemically noninert NiOx thin films. To demonstrate the functionality of the fabricated structures, we used a plasmonic photocathode consisting of...

CoPt Antidot Arrays Fabricated With Dry-Etching Using AAO Templates

We demonstrate a technique to fabricate large-area, highly ordered CoPt antidot arrays with perpendicular orientation. Using ultrathin anodic aluminum oxide (AAO) templates as a dry etching mask, the pattern of the pore array was transferred to the film without deteriorating the magnetic properties, in particular coercivity and preferential perpendicular orientation. The size, spacing, and density of the nonmagnetic nano-columns within the CoPt antidot arrays can be adjusted by changing the fabrication parameters of the AAO templates. The nano-columns can serve as pinning sites, impeding the magnetic domain wall motion, which can influence the magnetic properties of the film. The magnetic properties of the antidot arrays depend sensitively on the antidot morphology. The coercivity (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of the CoPt antidot array with a pore density of 2.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> fabricated using the AAO template anodized at 20 V is increased from 9.2 to 12.5 kOe due to the existence of such pinning sites.

Plasmonic silicon solar cell based on silver nanoparticles using ultra-thin anodic aluminum oxide template

This study fabricated a plasmonic silicon solar cell covered with silver (Ag) nanoparticles (NPs) using an ultra-thin anodic aluminum oxide (AAO) template as a deposition mask. An ultra-thin AAO template of approximately 200nm was produced using a single-step anodization process in which an Al substrate was etched with phosphoric acid (H3PO4) for 3min. We then used scanning electron microscopy (SEM) to examine the thickness and density of the AAO as a function of anodization duration, the results of which were confirmed by optical transmission measurement. The photovoltaic performance of the resulting silicon solar cell with Ag NPs was characterized according to photovoltaic current-voltage and external quantum efficiency. The inclusion of Ag NPs resulted in a 32.92% increase in conversion efficiency, compared with reference solar cells produced without Ag NPs.

Templated deposition of multiscale periodic metallic nanodot arrays with sub-10 nm gaps on rigid and flexible substrates

Multiscale metallic nanostrucutures, which support hybrid coupling of plasmon resonances, are essential for the engineering of plasmonic devices. The fabrication of large area periodic multiscale structures still remains a challenge, considering the cost and efficiency. In this work, highly ordered multiscale Ag nanoarrays with lateral dimensions of up to 6 mm × 6 mm have been successfully fabricated on both rigid silicon and flexible polydimethylsiloxane (PDMS) substrate by thermal evaporation using ultrathin anodic aluminum oxide films as masks. Owing to the peculiarities of thermal evaporation and the variance of substrate surface energy, the unit cell of the periodic arrays consist of a core-satellite structure on silicon and randomly distributed child particles on PDMS, with gaps as small as 10 nm. The flexible Ag nanoarrays on PDMS demonstrate a broadband extraordinary optical transmission with an enhancement up to 2.7 times when normalized to the exposed area. Moreover, the transmission and diffraction properties can readily be controlled by stretching the PDMS. These tunable optical properties support the multiscale Ag nanoarrays to be applied in some optical and optoelectronic devices.

Synthesis of zinc oxide nanotubes within ultrathin anodic aluminum oxide membrane by sol–gel method

In this paper we report a simple method that enables the easy fabrication of ultrathin porous anodic aluminum oxide (AAO) membranes from two-step anodization in oxalic acid solution. ZnO nanotubes have been synthesized by injecting sol–gel into the pores of the ultrathin AAO membrane. The structures and morphologies of the ultrathin AAO membrane and ZnO nanotubes were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscope. The thickness of the as-prepared AAO membranes is about 1μm with highly ordered pores inside, and the ZnO nanotubes are straight with uniform thickness along the length and with diameter equal to those of nanopores.