Bulge deformation measurement and elastic modulus analysis of nanoporous alumina membrane using time sequence speckle pattern interferometry

Nanoporous alumina membranes containing parallel regular pores of uniform size and normal to substrate surface were fabricated by anodically oxidizing high purity aluminum foils in acid solutions. The continuous out-of-plane displacement and current load of the porous membranes were obtained through bulge test combining time sequence speckle pattern interferometry (TSSPI) observation system. Then the deformation of whole field under different loads was deduced through point-to-point scan analysis and the elastic modulus was calculated through an analytical model. Measurement of mechanical properties indicates unusual mechanical behavior of these anodic alumina films compared with bulk alumina materials or dense alumina films. This method is useful and convenient for mechanical test on membranes with such structure and bring further understanding on connecting of micro–meso structure and mechanical properties.     

Random and localized resistive switching observation in Pt/NiO/Pt

Resistive memory switching devices based on transition metal oxides are now emerging as a candidate for nonvolatile memories. To visualize nano‐sized (10 nm to 30 nm in diameter) conducting filamentary paths in the surface of NiO thin films during repetitive switching, current sensing–atomic force microscopy and ultra‐thin (<5 nm) Pt films as top electrodes were used. Some areas (or spots), which were assumed to be the beginning of the conducting filaments, appeared (formation) and disappeared (rupture) in a localized and random fashion during the switching and are thought to contribute to resistive memory switching. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

多孔铝镶嵌8-羟基喹啉铝荧光光谱研究

利用多孔铝非常高的孔隙率,将8羟基喹啉铝(Alq3)镶嵌到多孔铝中,得到多孔铝Alq3镶嵌膜.研究了镶嵌膜的荧光光谱,并与Alq3在溶液状态下的荧光光谱进行比较,发现其荧光光谱与Alq3在乙醇溶液中的光谱相似,呈现单分子的发光特征,并且光谱线形更加对称.实验表明,多孔介质有机镶嵌膜有可能成为进一步发展Alq3在电致发光器件方面应用的新途径. 关键词： 多孔铝 8-羟基喹啉铝 光致发光光谱     

Laser damage mechanism of porous Al2O3 films prepared by a two-step anodization method

Pure aluminum films were deposited on a B270 glass by electron beam evaporation technique. These aluminum films, which were used as anode, were put in sulfuric acid and oxalic acid to prepare porous alumina films using a two-step anodization method. The microstructure and laser damage characteristics of the alumina films were then tested. Results show that the microstructure of the alumina films formed in sulfuric acid and oxalic acid were vertical (cylindrical) pores with different diameters. The laser-damaged spot of the porous films was formed by innumerable small damaged pits with no mutual influence. Films prepared in different acids reveal different damage characteristics and reflect different mechanisms.     

含分枝铜纳米线多孔铝膜的偏振特性

利用二次阳极氧化法制备了具有分枝结构的多孔铝(PAA)模板,并以交流电化学沉积的方法在该模板中合成了分枝状的铜纳米线。用SEM观察样品的形貌结构,用分光光度计测量了样品的透射光谱和偏振光谱。实验结果表明,含分枝状铜纳米线结构的多孔铝膜在近红外光区具有较高的透射率,且得到了14～22 dB的消光比。这种微偏振器件制备方法简单、效率高、造价低,在光电集成领域有着广泛的应用前景。     

多孔氧化铝薄膜的制备和光学特性研究

采用阳极氧化法制备了二维有序纳米孔氧化铝膜.研究了工艺参数对多孔薄膜有序性、孔径、膜厚度等的影响，测量了多孔氧化铝有序膜的光透过、光吸收和光发射等光学特性.结果表明，在波长360 nm附近多孔氧化铝有序膜的光透过谱线和光吸收谱线发生突变，波长大于360 nm时，光透过增强；波长小于360 nm时，光吸收增强.多孔氧化铝有序膜的光致发光强度和峰位与激发光波长有关，光致发光谱范围在340~600 nm.     

Materials,technologies, and circuit concepts for nanocrossbar-based bipolar RRAM

The paper reports on the characterization of bipolar resistive switching materials and their integration into nanocrossbar structures, as well as on different memory operation schemes in terms of memory density and the challenging problem of sneak paths. TiO₂, WO₃, GeSe, SiO₂ and MSQ thin films were integrated into nanojunctions of 100×100 nm². The variation between inert Pt and Cu or Ag top electrodes leads to valence change (VCM) switching or electrochemical metallization (ECM) switching and has significant impact on the resistive properties. All materials showed promising characteristics with switching speeds down to 10 ns, multilevel switching, good endurance and retention. Nanoimprint lithography was found to be a suitable tool for processing crossbar arrays down to a feature size of 50 nm and 3D stacking was demonstrated. The inherent occurrence of current sneak paths in passive crossbar arrays can be circumvented by the implementation of complementary resistive switching (CRS) cells. The comparison with other operation schemes shows that the CRS concept dramatically increases the addressable memory size to about 10¹⁰ bit.     

Cherenkov effect as a probe of photonic nanostructures

Electron energy-loss spectroscopy (EELS) is shown to be an excellent source of information both on photonic crystal bands and on radiation modes of complex nanostructures. Good agreement is reported between measurements and parameter-free calculations of EELS in porous alumina films, where Cherenkov radiation is scattered by the pores to yield a strong 8.3-eV (7-eV) feature for 120-keV (200-keV) electrons. The latter is related to the bands of two-dimensional photonic crystals formed by air cylinders in an alumina matrix with similar near-range ordering. Finally, the band structure is proved to be directly mapped by angle-resolved EELS.     

Conduction switching in aluminum nitride thin films containing Al nanocrystals

Conduction switching, i.e., a sharp change in the conduction from a lower-conductance state to a higher-conductance state or vice versa in aluminum nitride thin films embedded with Al nanocrystals (nc–Al) has been observed in the ramped-voltage and ramped-current current–voltage (I–V) measurements and the time-domain current measurement as well. Each state is well defined and its I–V characteristic follows a power law. It is observed that the conductance decreases (or increases) with charging (or discharging) in the nc–Al. It is shown that the conduction switching is due to the charging and discharging in the nc–Al at certain strategic sites. With the connecting (or breaking) of some conductive tunneling paths formed by the uncharged nc–Al due to the discharging (or charging) in the nc–Al at the strategic sites, a conduction switching occurs.     

Iron films deposited on porous alumina substrates

Iron films were deposited on porous alumina substrates using an arc plasma gun. The pore sizes (120 – 250 nm) of the substrates were controlled by changing the temperature during the anodic oxidation of aluminum plates. Iron atoms penetrated into pores with diameters of less than 160 nm, and were stabilized by forming γ-Fe, whereas α-Fe was produced as a flat plane covering the pores. For porous alumina substrates with pore sizes larger than 200 nm, the deposited iron films contained many defects and the resulting α-Fe had smaller hyperfine magnetic fields. In addition, only a very small amount of γ-Fe was obtained. It was demonstrated that the composition and structure of an iron film can be affected by the surface morphology of the porous alumina substrate on which the film is grown.     

Effect of varying pore size of AAO films on refractive index and birefringence measured by prism coupling technique

Anodic aluminum oxide (AAO) films with different pore sizes were prepared to modulate the effective refractive index and birefringence. To investigate the relationship between the refractive index and the pore size of the AAO film, optical constants were obtained using a prism coupler with various lasers. With experimental results, the dispersion curve of alumina itself without pores was extracted using a theoretical anisotropic model. We demonstrated that AAO films could offer a wide range of refractive index and birefringence values for optical device applications. Furthermore, index profiles as a function of the thickness of the AAO films were obtained by inverse Wentzel-Kramer-Brillouin approximation to examine the optical homogeneity.     

Nonvolatile unipolar resistive switching in ultrathin films of graphene and carbon nanotubes

We report unipolar resistive switching in ultrathin films of chemically produced graphene (reduced graphene oxide) and multiwalled carbon nanotubes. The two-terminal devices with yield >99% are made at room temperature by forming continuous films of graphene of thickness ∼20 nm on indium tin oxide coated glass electrode, followed by metal (Au or Al) deposition on the film. These memory devices are nonvolatile, rewritable with ON/OFF ratios up to ∼ 10⁵ and switching times up to 10 μs. The devices made of MWNT films are rewritable with ON/OFF ratios up to ∼400. The resistive switching mechanism is proposed to be nanogap formation and filamentary conduction paths.     

Photonic stop bands in quasi-random nanoporous anodic alumina structures

The existence of photonic stop bands in the self-assembled arrangement of pores in porous anodic alumina structures is investigated by means of rigorous 2D finite-difference time-domain calculations. Self-assembled porous anodic alumina shows a random distribution of domains, each of them with a very definite triangular pattern, constituting a quasi-random structure. The observed stop bands are similar to those of photonic quasicrystals or random structures. As the pores of nanoporous anodic alumina can be infiltrated with noble metals, nonlinear or active media, it makes this material very attractive and cost-effective for applications including inhibition of spontaneous emission, random lasing, LEDs and biosensors.     

Formation process of a strong water-repellent alumina surface by the sol-gel method

A novel strong water-repellent alumina thin film is fabricated by chemically adsorbing stearic acid (STA) layer onto the porous and roughened aluminum film coated with polyethyleneimine (PEI). The formation process and the structure of the strong water-repellent alumina film are investigated by means of contact angle measurement and atomic force microscope (AFM). Results show that the water contact angles for the alumina films increase with the increase of the immersion time in the boiling water, and meanwhile, the roughness of the alumina films increases with the dissolution of the boehmite in the boiling water. Finally, the strong water-repellent film with a high water contact angle of 139.1° is obtained when the alumina films have distinct roughened morphology with some papillary peaks and porous structure. Moreover, both the roughened structure and the hydrophobic materials of the STA endow the alumina films with the strong water-repellence.     

Rectifying resistance switching behaviors of SnO2 microsphere films modulated by top electrodes

Based on the bipolar resistive switching (RS) characteristics of SnO₂ films, we have fabricated a new prototypical device with sandwiched structure of Metal/SnO₂/fluorine-doped tin oxide (FTO). The SnO₂ microspheres film was grown on FTO glass by template-free hydrothermal synthesis, which was evaporated with various commonly used electrodes such as aluminium (Al), silver (Ag), and gold (Au), respectively. Typical self-rectifying resistance switching behaviors were observed for the RS devices with Al and Au electrodes. However, no obvious rectifying resistance switching behavior was observed for the RS device with Ag electrode. Above results were interpreted by considering the different interface barriers between SnO₂ and top metal electrodes. Our current studies pave the ways for modulating the self-rectifying resistance switching properties of resistive memory devices by choosing suitable metal electrodes.     

Coercive field behavior of permalloy antidot arrays based on self-assembled template fabrication

High-density magnetic antidot arrays have been fabricated by deposition of Fe₂₀Ni₈₀ thin films on self-assembled nanoporous alumina membranes (NAM) with high-order hexagonal symmetry. The magnetic properties induced by the size and the geometry configuration of the holes introduced in a Fe₂₀Ni₈₀ thin film are discussed based on hysteresis loops measured as a function of temperature. The precursor NAMs have pore diameters ranging between 35 and 95 nm (55 and 75 nm after the film deposition) and a lattice parameter of 105 nm. An enormous increase of coercitivity, as compared with the corresponding continuous films, was observed for temperatures between 2 and 300 K. This effect depends on the size and surface density of holes in the Fe₂₀Ni₈₀ antidot arrays. Rutherford backscattering spectrometry (RBS) measurements were performed in order to better clarify the magnetic material that was eventually deposited within the NAM pores.     

Ferroelectric domain structure of the BiFeO3 film grown on different substrates

Domain structure of BiFeO₃ (BFO) films grown on different substrates, with a conductive La_0.7Sr_0.3MnO₃ underlayer, has been experimentally studied. Two oppositely orientated polarizations, along the long body diagonal to the perovskite unit cell of BFO, are detected in the BFO films on the (0 0 1)-oriented NdGaO₃. Electric pulses applied in the [0 0 1] direction produce a polarization switching, resulting in the domain structure characterized by the 109° domain walls. Contrary to the BFO films on NdGaO₃, the BFO films on SrTiO₃ (0 0 1) exhibit a much complex domain structure. Both 71° and 109° domain walls are possible with a uniform polarization component pointing to the bottom electrode.     

Analysis of the resistive switching behaviors of vanadium oxide thin film

We demonstrate the polarization of resistive switching for a Cu/VOx/Cu memory cell.The switching behaviors of Cu/VOx/Cu cell are tested by using a semiconductor device analyzer(Agilent B1500A),and the relative micro-analysis of I-V characteristics of VOx/Cu is characterized by using a conductive atomic force microscope(CAFM).The I-V test results indicate that both the forming and the reversible resistive switching between low resistance state(LRS) and high resistance state(HRS) can be observed under either positive or negative sweep.The CAFM images for LRS and HRS directly exhibit evidence for the formation and rupture of filaments based on positive or negative voltage.The Cu/VOx/Cu sandwiched structure exhibits reversible resistive switching behavior and shows potential applications in the next generation of nonvolatile memory.     

Ferromagnetic resonance and magnetization in permalloy films with nanostructured antidot arrays of variable size

We present the results of structural and magnetic characterization of Permalloy (Fe₂₀Ni₈₀) films deposited by sputtering on self-organized nanoporous alumina previously treated with phosphoric acid to vary the pore diameters. SEM and AFM images of the top film surfaces show a triangular array of pores with diameters similar to the untreated porous alumina. However, the underlying pore enlargement is evidenced by the magnetic study. Indeed magnetization measurements reveal a decrease in the easy-plane anisotropy energy with elapsing time of chemical etching. Consistent with these results, ferromagnetic resonance measurements perpendicular to the film indicate a systematic reduction of the resonance field which can be directly related to an increase in the pore diameter. The effect of lateral confinement (due to the pores) in the spin-wave resonance is evidenced in multiple absorption lines when the applied field is in the film plane and perpendicular to it. This contrasts with the results for the continuous test film.