Showing papers in "Applied Physics A in 2011"

Resistance Switching Memories Are Memristors

Abstract: All 2-terminal non-volatile memory devices based on resistance switching are memristors, regardless of the device material and physical operating mechanisms. They all exhibit a distinctive “fingerprint” characterized by a pinched hysteresis loop confined to the first and the third quadrants of the v–i plane whose contour shape in general changes with both the amplitude and frequency of any periodic “sine-wave-like” input voltage source, or current source. In particular, the pinched hysteresis loop shrinks and tends to a straight line as frequency increases. Though numerous examples of voltage vs. current pinched hysteresis loops have been published in many unrelated fields, such as biology, chemistry, physics, etc., and observed from many unrelated phenomena, such as gas discharge arcs, mercury lamps, power conversion devices, earthquake conductance variations, etc., we restrict our examples in this tutorial to solid-state and/or nano devices where copious examples of published pinched hysteresis loops abound. In particular, we sampled arbitrarily, one example from each year between the years 2000 and 2010, to demonstrate that the memristor is a device that does not depend on any particular material, or physical mechanism. For example, we have shown that spin-transfer magnetic tunnel junctions are examples of memristors. We have also demonstrated that both bipolar and unipolar resistance switching devices are memristors.

Synaptic behaviors and modeling of a metal oxide memristive device

Abstract: Nanoscale memristive devices using tungsten oxide as the switching layer have been fabricated and characterized. The devices show the characteristics of a flux-controlled memristor such that the conductance change is governed by the history of the applied voltage signals, leading to synaptic behaviors including long-term potentiation and depression. The memristive behavior is attributed to the migration of oxygen vacancies upon bias which modulates the interplay between Schottky barrier emission and tunneling at the WOX/electrode interface. A physical model incorporating ion drift and diffusion effects using an internal state variable representing the area of the conductive region has been proposed to explain the observed memristive behaviors. A SPICE model has been further developed that can be directly incorporated into existing circuit simulators. This type of device can be fabricated with low-temperature processes and has potential applications in synaptic computations and as analog circuit components.

Perfect metamaterial absorber based on a split-ring-cross resonator

Abstract: In this paper, we present a polarization-insensitive metamaterial (MM) absorber which is composed of the dielectric substrate sandwiched with split-ring-cross resonator (SRCR) and continuous metal film. The MM absorber is not limited by the quarter-wavelength thickness and can achieve near-unity absorbance by properly assembling the sandwiched structure. Microwave experiments demonstrate the maximum absorptivity to be about 99% around 10.91 GHz for incident wave with different polarizations. The surface currents distributions of the resonance structure are discussed to look into the resonance mechanism. Importantly, our absorber is only 0.4 mm thick, and numerical simulations confirm that the MM absorber could achieve very high absorptivity at wide angles of incidence for both transverse electric (TE) wave and transverse magnetic (TM) wave. The sandwiched structure is also suitable for designing of a THz and even higher frequency MM absorber, and simulations demonstrate the absorption of 99% at 1.105 THz.

Nucleation of electroactive β-phase poly(vinilidene fluoride) with CoFe2O4 and NiFe2O4 nanofillers : a new method for the preparation of multiferroic nanocomposites

Abstract: Multiferroic and magnetoelectric materials show enormous potential for technological developments. Multiferroic composites are more attractive for applications due to their enhanced properties with respect to single-phase multiferroic materials. In this paper we report on the nucleation of the electroactive β-phase of poly(vinylidene fluoride), PVDF, by the addition of CoFe2O4 and NiFe2O4 nanoparticles in order to prepare poly(vinylidene fluoride)/ferrite nanocomposite for multiferroic and magnetoelectric applications,. The dispersed ferrite nanofiller particles strongly enhance the nucleation of the β-phase of the polymer matrix. In this way, magnetoelectric polymer nanocomposites can be processed avoiding the usual α- to β-phase transformation by stretching of the polymer matrix.

Frictional behavior of oxide graphene nanosheets as water-base lubricant additive

Abstract: Oxide graphene (GO) nanosheets were prepared by modified Hummers and Offeman methods. The products were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectrometry (FTIR), and thermogravimetric analysis (TGA). The tribological properties of GO nanosheets as water-base lubricant additive were investigated using a UMT-2 ball-plate tribotester. By the addition of GO nanosheets in pure water, the antiwear ability was improved and the friction coefficient was decreased. The water with GO nanosheets showed better tribological properties than the water with oxide multiwall carbon nanotubes (CNTs-COOH). It is concluded that the formation of a thin physical tribofilms on the substrate can explain the good friction and wear properties of GO nanosheets.

Metal/TiO2 interfaces for memristive switches

Abstract: The interfaces between metal electrodes and the oxide in TiO2-based memristive switches play a key role in the switching as well as in the I–V characteristics of the devices in different resistance states. We demonstrate here that the work function of the metal electrode has a surprisingly minor effect in determining the electronic barrier at the interface. In contrast, Ti oxides can be readily reduced by most electrode metals. The amount of oxygen vacancies created by these chemical reactions essentially determines the electronic barrier at the device interfaces.

Zinc oxide films prepared by sol―gel spin coating technique

Abstract: Zinc oxide (ZnO) thin films and micro- and nanostructures are very promising candidates for novel applications in emerging thin-film transistors, solar cells, sensors and optoelectronic devices. In this paper, a low-cost sol–gel spin coating technique was used to fabricate ZnO films on glass substrates. The sol–gel fabrication process of the ZnO films is described. The influence of precursor concentration on the material properties of the ZnO films was investigated. Atomic force microscopy and X-ray diffractometry were employed to examine the structural properties of the ZnO films. The optical properties of the ZnO films were characterized with ultraviolet–visible spectroscopy. The experimental results reveal that the precursor concentration in the sol–gel spin coating process exerts a strong influence on the properties of the ZnO films. The effects of the precursor concentration are discussed.

Burstein–Moss shift and room temperature near-band-edge luminescence in lithium-doped zinc oxide

Abstract: Nanopowders of pure and lithium-doped semiconducting ZnO (Zn1−x Li x O, where x= 0, 0.01, 0.03, 0.06, 0.09 and 0.15 in atomic percent (at.%)) are prepared by PEG-assisted low-temperature hydrothermal method. The average crystallite size is calculated using Debye–Scherrer formula and corrected for strain-induced broadening by Williamson–Hall (W–H) plot. The peak shift in XRD and the lattice constant of ZnO as a function of unit cell composition are predicted by Vegard’s law. The evolution of ZnO nanostructures from rod-shaped to particle nature is observed from TEM images and the influence of dopant on the morphology is investigated. The optical absorption measurement marks an indication that the incorporation of lithium ion into the lattice of ZnO widens the optical band gap energy from ∼2.60 to ∼3.20 eV. The near band edge (NBE) emission peak centered at ∼3.10 eV is considered to be the dominant emission peak in the PL spectra. Blue emission peak is not observed in doped ZnO, thus promoting defect-free nanoparticles. The Burstein–Moss shift serves as a qualitative tool to analyze the widening of the optical band gap and to study the shape of the NBE luminescence in doped ZnO nanopowders. FT-IR spectra are used to identify the strong metal–oxide (Zn–O) interaction.

Time-resolved imaging of hydrogel printing via laser-induced forward transfer

Abstract: In this work, the printing mechanism of an alginate-based hydrogel via laser-induced forward transfer (LIFT) is investigated by spatial and temporal high-resolved stroboscopic imaging. First, the generation of the liquid jet is studied at two different laser fluences in a process without collector slide. Furthermore, the impingement process onto the collector slide at the same fluence levels is observed. With the help of these images the development of the jet is explained. Besides the influences of the collector slide as well as the applied laser fluence on the transfer are demonstrated.

Space-charge relaxation and electrical conduction in K0.5Na0.5NbO3 at high temperatures

Abstract: Sodium potassium niobate K0.5Na0.5NbO3(KNN) ceramic was synthesized by a solid-state technique. The X-ray diffraction of the sample at room temperature showed a monoclinic phase. The real part (e′) and imaginary part (e″) of dielectric permittivity of the sample were measured in a frequency range from 40 Hz to 1 MHz and in a temperature range from 350 to 850 K. The e′ deviated from Curie–Weiss law above 702 K, due to additional dielectric contributions resulting from universal dielectric response and thermally activated space charges at high temperatures. This anomaly arose from a Debye dielectric dispersion that slowed down following an Arrhenius law. We have established a link between the dielectric relaxation and the conductivity.

Bonding of glass with femtosecond laser pulses at high repetition rates

Abstract: We report on the welding of fused silica with ultrashort laser pulses at high repetition rates. Femtosecond laser pulses were focused at the interface of two optically contacted fused silica samples. Due to the nonlinear absorption in the focal volume and heat accumulation of successive pulses, the laser acts as a localized heat source at the focus position. Here, we analyze the influence of the laser and processing parameters on the amount of molten material. Moreover, we determine the achievable breaking stress by a three point bending test. With optimized parameters up to 75% of the breaking stress of the bulk material have been obtained.

Ultra-short pulse laser ablation of copper, silver and tungsten: experimental data and two-temperature model simulations

Abstract: Experimental results of femtosecond laser ablation of the metals copper, silver and tungsten are compared to simulations based on the two-temperature model. The comparison provides new information about the laser-heating process: For the noble metals (Cu, Ag), the energy transport via ballistic electrons must be included, while this effect is negligible for a transition metal (W). The comparison provides values for the range of ballistic electrons in the noble metals. The model calculation is also employed to investigate the dependence of the threshold fluence and melting depth on pulse duration. It is observed that for pulses shorter than approximately 1 ps the threshold fluence and melting depth are independent of the pulse duration, while they increase as τ 0.47 and τ 0.51, respectively, for pulses longer than ∼40 ps, in good agreement with approximate analytical expressions predicting a $\sqrt{\tau}$ dependence.

Fabrication of superhydrophobic nanostructured surface on aluminum alloy

Abstract: A superhydrophobic surface was prepared by consecutive immersion in boiling water and sputtering of polytetrafluoroethylene (PTFE or Teflon®) on the surface of an aluminum alloy substrate. Immersion in boiling water was used to create a micro-nanostructure on the alloy substrate. Then, the rough surface was coated with RF-sputtered Teflon film. The immersion time in boiling water plays an important role in surface morphology and water repellency of the deposited Teflon coating. Scanning electron microscopy images showed a “flower-like” structure in first few minutes of immersion. And as the immersion time lengthened, a “cornflake” structure appeared. FTIR analyses of Teflon-like coating deposited on water treated aluminum alloy surfaces showed fluorinated groups, which effectively reduce surface energy. The Teflon-like coating deposited on a rough surface achieved with five-minute immersion in boiling water provided a high static contact angle (∼164°) and low contact angle hysteresis (∼4°).

The Matrix-Assisted Pulsed Laser Evaporation (MAPLE) process: origins and future directions

Abstract: This review discusses the origins and evolution of the MAPLE technique as it started as an alternative to spray coating of thin films for chemical vapor sensors. It describes its numerous applications for the deposition of thin films of polymeric, organic and biomaterials for various applications. This is followed by an overview of several new variations of the MAPLE technique. This review concludes with an outlook on the future of this highly versatile and successful vapor deposition process.

A new model of dispersion for metals leading to a more accurate modeling of plasmonic structures using the FDTD method

Abstract: We present FDTD simulations results obtained using the Drude Critical points model. This model enables spectroscopic studies of metallic structures over wider wavelength ranges than usually used, and it facilitates the study of structures made of several metals.

Feedback write scheme for memristive switching devices

Abstract: In nanoscale memristive switching devices, the statistical distribution of resistance values and other relevant parameters for device operation often exhibits a lognormal distribution, causing large fluctuations of memristive analog state variables after each switching event, which may be problematic for digital nonvolatile memory applications. The state variable w in such devices has been proposed to be the length of an undoped semiconductor region along the thickness of the thin film that acts as a tunnel barrier for electronic transport across it. The dynamical behavior of w is governed by the drift diffusion of ionized dopants such as oxygen vacancies. Making an analogy to scanning tunneling microscopes (STM), a closed-loop write scheme using current feedback is proposed to switch the memristive devices in a controlled manner. An integrated closed-loop current driver circuit for switching a bipolar memristive device is designed and simulated. The estimated upper limit of the feedback loop bandwidth is in the order of 100 MHz. We applied a SPICE model built upon the TiO2 memristive switching dynamics to simulate the single-device write operation and found the closed-loop write scheme caused a narrowing of the statistical distribution of the state variable w.

Room-temperature ferromagnetism in EDTA capped Cr-doped ZnS nanoparticles

Abstract: Cr-doped ZnS nanoparticles with Cr concentrations of 0.5, 1, 2 and 3 atm.% were successfully synthesized by the chemical co-precipitation method using ethylenediaminetetraacetic acid (EDTA) as the capping agent. The structural, optical and magnetic properties of the prepared samples were studied. Energy dispersive spectroscopy (EDS) measurements showed the presence of Cr in the Cr-doped ZnS. No mixed phase was observed from X-ray diffraction (XRD) studies and all the peaks were indexed to the cubic phase of ZnS. The average diameter of the particles was in the range of 6–10 nm, and it was confirmed by TEM studies. The magnetic behavior of the nanoparticles for different chromium concentrations was investigated by magnetism measurements using a vibrating sample magnetometer (VSM). The nanoparticles with lower Cr concentration exhibited strong ferromagnetism, where as in samples of higher Cr concentrations the ferromagnetism suppressed. The electron paramagnetic resonance (EPR) spectra of the nanocrystals showed the resonance of electron centers with a g-value of 1.989. The signal intensity and linewidth of the EPR signal increased with increasing Cr content. FTIR studies indicated that the nanoparticles were sterically stabilized by EDTA.

Identifying firing mammalian neurons in networks with high-resolution multi-transistor array (MTA)

Abstract: The electrical activity of a network of mammalian neurons is mapped with a Multi-Transistor Array (MTA) fabricated with extended CMOS technology. The spatial resolution is 7.4 μm on an area of 1 mm2 at a sampling frequency of 6 kHz for a complete readout of 16,384 sensor transistors. Action potentials give rise to extracellular voltages with amplitudes in a range of 500 μV. On the basis of the high resolution in space and time, correlation algorithms are used to identify single action potentials with amplitudes as low as about 200μV, and to assign the signals to the activity of individual neurons even in a dense network.

Self-assembly and photoluminescence of molybdenum oxide nanoparticles

Abstract: We report on the synthesis of self-assembled hillock shaped MoO3 nanoparticles on thin films exhibiting intense photoluminescence (PL) by RF magnetron sputtering and subsequent oxidation. MoO3 nanocrystals of size ∼29 nm are self-assembled into uniform nanoparticles with diameter ∼174 nm. The mechanism of the intense PL behaviour from MoO3 nanoparticles is investigated and systematically discussed. The films exhibit two bands; a near-band-edge UV emission and a defect related deep level visible emission. The enhancement in PL intensity with annealing is not only by the improvement in crystallinity and grain size but also by the increase in the rms surface roughness and porosity of the films. The PL intensity is thermally activated with activation energy 1.07 and 0.87 eV respectively for the UV and visible emissions. The UV band exhibits a blue shift according to the band gap with increasing post-annealing temperatures, which suggests the possibility to tune the UV photoluminescence band by varying the oxidation temperature.

Effect of nanostructures on evaporation and explosive boiling of thin liquid films: a molecular dynamics study

Abstract: Molecular dynamics simulations have been employed to investigate the boiling phenomena of thin liquid films adsorbed on a nanostructured solid surface. The molecular system was comprised of the following: solid platinum wall, liquid argon, and argon vapor. A few layers of the liquid argon were placed on the nanoposts decorated solid surface. The nanoposts height was varied keeping the liquid film thickness constant to capture three scenarios: (i) liquid-film thickness is higher than the height of the nanoposts, (ii) liquid-film and nanoposts are of same height, and (iii) liquid-film thickness is less than the height of the nanoposts. The rest of the simulation box was filled with argon vapor. The simulation was started from its initial configuration, and once the equilibrium of the three phase system was established, the wall was suddenly heated to a higher temperature which resembles an ultrafast laser heating. Two different jump temperatures were selected: a few degrees above the boiling point to initiate normal evaporation and far above the critical point to initiate explosive boiling. Simulation results indicate nanostructures play a significant role in both cases: Argon responds very quickly for the nanostructured surface, the transition from liquid to vapor becomes more gradual, and the evaporation rate increases with the nanoposts height.

Fabrication of optically transparent chitin nanocomposites

Abstract: This paper demonstrates the preparation of chitin nanofibers from crab shells using a simple mechanical treatment. The nanofibers are small enough to retain the transparency of neat acrylic resin. Possessing hydroxyl and amine/N-acetyl functionalities, water suspension of chitin nanofibers was vacuum-filtered 9 times faster than cellulose nanofibers to prepare a nanofiber sheet of 90 mm in diameter. This is a prominent advantage of chitin nanofibers over cellulose nanofibers in terms of commercial application. Interestingly, chitin acrylic resin films exhibited much higher transparency than cellulose acrylic resin films owing to the close affinity between less hydrophilic chitin and hydrophobic resin. Furthermore, the incorporation of chitin nanofibers contributes to the significant improvement of the thermal expansion and mechanical properties of the neat acrylic resin. The properties of high light transmittance and low thermal expansion make chitin nanocomposites promising candidates for the substrate in a continuous roll-to-roll process in the manufacturing of various optoelectronic devices such as flat panel displays, bendable displays, and solar cells.

Laser ablation of thin molybdenum films on transparent substrates at low fluences

Abstract: The selective structuring of thin molybdenum (Mo) films is a major challenge for the monolithic interconnection of CIS thin film solar cells during their production. Here we present the structuring of ca. 0.5 μm thin molybdenum films on glass substrates with picosecond laser pulses (pulse duration 10 ps, wavelength 1064 nm) without any visible thermal effect on both, the remaining film and the substrate material. When the molybdenum film is irradiated from the transparent substrate side with a fluence level below 1 J/cm2 a “lift-off” process is initiated, which seems to be induced by a direct effect in the removed molybdenum film. At that fluence level, the energy input per ablated volume of ca. 30 J/mm3 is much less than would be needed for a thermodynamic heating, melting and vaporization of the complete film with ca. 78 J/mm3. Therefore we conclude that the molybdenum is only evaporated partially. Parts of the ablated Mo-film can be found as structurally intact debris. We assume that partial melting and vaporization with high-pressure formation play an important role during that picosecond laser ablation without thermal side effects. Due to its remarkable physical nature we called that process “directly induced laser ablation”.

The role of asymmetric excitation in self-organized nanostructure formation upon femtosecond laser ablation

Abstract: The strong influence of laser polarization on the orientation and shape of femtosecond-laser-induced self-organized nanostructures (‘ripples’, LIPSS) still constitutes an open question, taking into account that the laser electric field is present only at the first step of electronic excitation. Based on the explanation of similar structures generated during ion sputtering, we present a theoretical model indicating a possible explanation for this phenomenon. Our model shows that a directional asymmetry in the pattern can result from a spatial asymmetry of the initial excitation, induced e.g. by a corresponding distribution of excited-electron kinetic energies. Numerical simulation of this model yields typical patterns which are compared to experimental observations under appropriate conditions.

Optical metamaterial absorber based on leaf-shaped cells

Abstract: We present the model of an infrared metamaterial absorber composed of metallic leaf-shaped cells, dielectric substrate, and continuous metallic film. Numerical simulation confirms an absorptivity of 99.3% at the infrared frequency of 126.7 THz with this metamaterial model. The proposed metamaterial absorber could be fabricated with an electrochemical deposition technique. Our simulated results show the absorption feature of this metamaterial absorber could be well manipulated with different incident angles and radiation modes. The optical metamaterial absorber proposed in this paper has potential applications such as infrared imaging devices, thermal bolometers, wavelength-selective radiators, and optical bistable switches.

Synthesis and characterization of nitrogen-rich carbon nitride nanobelts by pyrolysis of melamine

Abstract: Nitrogen-rich g-C3N4 nanobelts were successfully synthesized via pyrolysis of melamine (C3N6H6). The as-synthesized nanobelts are structurally uniform, and each belt is uniform in width and thickness along its length direction. The typical width is in the range of 600 nm and 1.5 μm, respectively. The typical length of belts is in the range of several hundreds of micrometers; some of them even have lengths on the order of millimeters. The structure and morphology were researched by XRD, SEM, TEM, CEA, XPS, FTIR, and TG measurements. The theoretical FTIR spectra are calculated to compare with experiment value. It is found that the NH2 edges in the nanobelts are precisely for the reason of rich nitrogen. The photoluminescence (PL) spectrum was carried out. Two peaks at 443 and 500 nm were observed in the spectrum. In addition, a possible growth mechanism is also inferred by principle of VLS.

Formation of femtosecond laser-induced nanogratings at high repetition rates

Abstract: We investigate the formation of self-organized nanostructures inside the bulk of fused silica induced by ultrafast laser pulses at high repetition rates. Three distinct stages of the evolution from random nanostructures to periodic nanogratings are identified. In addition, for the first time nanograting formation in fused silica is observed at repetition rates as high as 9.4 MHz, thereby significantly extending the parameter window for nanograting inscription. Throughout the whole parameter range, a strong dependence of the grating period on the number of incident pulses was revealed.

Multi-nozzle electrohydrodynamic inkjet printing of silver colloidal solution for the fabrication of electrically functional microstructures

Abstract: In this paper, multi-nozzle electrohydrodynamic (EHD) inkjet printing of a colloidal solution containing silver nanoparticles in a fully controlled fashion is reported. For minimizing interaction, i.e. cross-talk, between neighboring jets, the distance between the nozzles was optimized numerically by investigating the magnitude of the electric field strength around the tip of each nozzle. A multi-nozzle EHD inkjet printing head consisting of three nozzles was fabricated and successfully tested by simultaneously printing electrically conductive lines of a colloidal solution containing silver nanoparticles onto a glass substrate. The printed results show electrical resistivity of 5.05×10−8 Ω m, which is almost three times larger than that of bulk silver. These conductive microtracks demonstrate the feasibility of the multi-nozzle EHD inkjet printing process for industrial fabrication of microelectronic devices.

Femtosecond laser drilling of high-aspect ratio microchannels in glass

Abstract: A femtosecond laser is used to fabricate microchannels with high aspect ratios by laser direct ablation. Drilling both in air and in water is investigated. It is found that at low pulse energy, drilling in water can generate channels with high aspect ratios. However, at high pulse energy, water-assisted drilling stops working and only very shallow holes can be obtained. The reason for this is presented. On the contrary, the aspect ratio of holes drilled in air increases significantly at high pulse energy. The effects of writing speed and repeated fabrication are also investigated, and an optimum writing speed is determined for fixed laser parameters.

Applications of the matrix-assisted pulsed laser evaporation method for the deposition of organic, biological and nanoparticle thin films: a review

Abstract: The matrix-assisted pulsed laser evaporation (MAPLE) technique offers an efficient mechanism to transfer soft materials from the condensed to the vapor phase, preserving the versatility, ease of use and high deposition rates of the pulsed laser deposition (PLD) technique. The materials of interest (polymers, biological cells, proteins, …) are diluted in a volatile solvent. Then the solution is frozen and irradiated with a pulsed laser beam. Here, important results of MAPLE deposition of polymer, biomaterials and nanoparticle films are summarized. Finally, the MAPLE mechanism is discussed. A review of experimental and theoretical works points out that the simple model of individual molecule evaporation must be abandoned. Solute concentration, solubility, evaporation temperature of solvents, laser pulse power density and laser penetration depth emerge as important parameters to explain the morphology of the MAPLE-deposited films.

Influence of processing time on nanoparticle generation during picosecond-pulsed fundamental and second harmonic laser ablation of metals in tetrahydrofuran

Abstract: The influence of fundamental and second harmonic wavelength on ablation efficiency and nanoparticle properties is studied during picosecond laser ablation of silver, zinc, and magnesium in polymer-doped tetrahydrofuran. Laser ablation in stationary liquid involves simultaneously the fabrication of nanoparticles by ablation of the target material and fragmentation of dispersed nanoparticles by post irradiation. The ratio in which the laser pulse energy contributes to these processes depends on laser wavelength and colloidal properties. For plasmon absorbers (silver), using the second harmonic wavelength leads to a decrease of the nanoparticle productivity over process time along with exponential decrease in particle diameter, while using the fundamental wavelength results in a constant ablation rate and linear decrease in particle diameter. For colloids made of materials without plasmon absorption (zinc, magnesium), laser scattering is the colloidal property that limits nanoparticle productivity by Mie-scattering of dispersed nanoparticle clusters.