Viet Cuong Nguyen

Direct Observation of Indium Conductive Filaments in Transparent, Flexible, and Transferable Resistive Switching Memory

Electronics with multifunctionalities such as transparency, portability, and flexibility are anticipated for future circuitry development. Flexible memory is one of the indispensable elements in a hybrid electronic integrated circuit as the information storage device. Herein, we demonstrate a transparent, flexible, and transferable hexagonal boron nitride (hBN)-based resistive switching memory with indium tin oxide (ITO) and graphene electrodes on soft polydimethylsiloxane (PDMS) substrate. The ITO/hBN/graphene/PDMS memory device not only exhibits excellent performance in terms of optical transmittance (∼85% in the visible wavelength), ON/OFF ratio (∼480), retention time (∼5 × 104 s) but also shows robust flexibility under bending conditions and stable operation on arbitrary substrates. More importantly, direct observation of indium filaments in an ITO/hBN/graphene device is found via ex situ transmission electron microscopy, which provides critical insight on the complex resistive switching mechanisms.

Abnormal Poisson's ratio and Linear Compressibility in Perovskite Materials

Materials with unusual properties, such as negative thermal expansion ( NTE ), [ 1 ] negative linear compressibility ( NLC ), [ 2 ] and a negative Poisson’s ratio ( NPR ) [ 3 ] have attracted considerable attention recently not only because of the underlying basic physics but also for the potential applications in tailoring thermal/mechanical related properties. The Poisson’s effect is a fundamental mechanical phenomenon that refers to a material’s deformation in an orthogonal direction to an uniaxial external force. Poisson’s Ratio ( PR ) ( νi j = −ε ( j )/ε (i ) ) , is defi ned as the negative ratio between the transverse strain ε ( j ) and longitudinal strain ε ( i ). Modern materials’ PR remains a stirring area of research since the original publication by Siméon Denis Poisson 200 years ago. [ 4 ] PR was predicted to vary from −1.0 to 0.5 in the isotropic elastic theory, [ 5 ] but practically, a vast majority of materials become thinner laterally when stretched longitudinally giving a positive Poisson’s ratio ( PPR ). Negative Poisson’s ratio ( NPR ) has been observed in a few materials such as SiO 2 , [ 6 ] polymers and composites. [ 7–9 ] Driven by a variety of potential applications of NPR and its mechanically coupled properties such as fracture toughness and acoustic performance, many theoretical studies were carried out. The theoretical limits of PR have been described mathematically for different crystal symmetries. [ 10–14 ] The studies based on the single crystal materials’ elastic stiffness data predicted that NPR could occur in many cubic metallic alloys, paratellurite, and the zeolite mineral natrolite, along two specifi ed orthogonal directions. For example, ( [110] , [ 110 ]) ), [ 3 , 15 , 16 ] where [110] is the load direction and [ 110 ] is the transverse direction exhibiting the Poisson’s effect. Some of these predictions have been observed experimentally in cubic iron-gallium and iron-aluminum alloys. [ 17 ] Furthermore, a recent statistical study showed that many materials, from organics to inorganics, possess not only NPRs but also unusual PPRs larger than the isotropic upper limit of 0.5 along specifi c directions. [ 18 ]

Direct Observation of Conducting Filaments in Tungsten Oxide Based Transparent Resistive Switching Memory

Transparent nonvolatile memory has great potential in integrated transparent electronics. Here, we present highly transparent resistive switching memory using stoichiometric WO3 film produced by cathodic electrodeposition with indium tin oxide electrodes. The memory device demonstrates good optical transmittance, excellent operative uniformity, low operating voltages (+0.25 V/-0.42 V), and long retention time (>104 s). Conductive atomic force microscopy, ex situ transmission electron microscopy, and X-ray photoelectron spectroscopy experiments directly confirm that the resistive switching effects occur due to the electric field-induced formation and annihilation of the tungsten-rich conductive channel between two electrodes. Information on the physical and chemical nature of conductive filaments offers insightful design strategies for resistive switching memories with excellent performances. Moreover, we demonstrate the promising applicability of the cathodic electrodeposition method for future resistive memory devices.

Resistive Switching Memory Phenomena in PEDOT PSS: Coexistence of Switchable Diode Effect and Write Once Read Many Memory

We study resistive switching memory phenomena in conducting polymer PEDOT PSS. In the same film, there are two types of memory behavior coexisting; namely, the switchable diode effect and write once read many memory. This is the first report on switchable diode phenomenon based on conducting organic materials. The effect was explained as charge trapping of PEDOT PSS film and movement of proton. The same PEDOT PSS device also exhibits write once read many memory (WORM) phenomenon which arises due to redox reaction that reduces PEDOT PSS and renders it non-conducting. The revelation of these two types of memory phenomena in PEDOT PSS highlights the remarkable versatility of this conducting conjugated polymer.

Optically readout write once read many memory with single active organic layer

An optically readable write once read many memory (WORM) in Ag/Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH PPV)/ITO is demonstrated in this work. Utilising characteristics of the organic light emitting diode structure of Ag/MEH PPV/ITO and electrochemical metallization of Ag, a WORM with light emitting capability can be realised. The simple fabrication process and multifunction capability of the device can be useful for future wearable optoelectronics and photomemory applications, where fast and parallel readout can be achieved by photons.

Localized Charge Transfer in Two-Dimensional Molybdenum Trioxide

Molybdenum trioxide is an interesting inorganic system in which the empty 4d states have potential to hold extra electrons and therefore can change states from insulating opaque (MoO3) to colored semimetallic (HxMoO3). Here, we characterize the local electrogeneration and charge transfer of the synthetic layered two-dimensional 2D MoO3-II (a polymorph of MoO3 and analogous to α-MoO3) in response to two different redox couples, i.e., [Ru(NH3)6]3+ and [Fe(CN)6]3- by scanning electrochemical microscopy (SECM). We identify the reduction of [Ru(NH3)6]3+ to [Ru(NH3)6]2+ at the microelectrode that leads to the reduction of MoO3-II to conducting blue-colored molybdenum bronze HxMoO3. It is recognized that the dominant conduction of the charges occurred preferentially at the edges active sites of the sheets, as edges of the sheets are found to be more conducting. This yields positive feedback current when approaching the microelectrode toward 2D MoO3-II-coated electrode. In contrast, the [Fe(CN)6]4-, which is reduced from [Fe(CN)6]3-, is found unfavorable to reduce MoO3-II due to its higher redox potential, thus showing a negative feedback current. The charge transfer on MoO3-II is further studied as a function of applied potential. The results shed light on the charge transfer behavior on the surface of MoO3-II coatings and opens the possibility of locally tuning of their oxidation states.

Coexistence of Write Once Read Many Memory and Memristor in blend of Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate and Polyvinyl Alcohol

In this work, the coexistence of Write Once Read Many Memory (WORM) and memristor can be achieved in a single device of Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) and Polyvinyl Alcohol (PVA) blend organic memory system. In memristor mode, the bistable resistance states of the device can be cycled for more than 1000 times. Once a large negative bias of −8V was applied to the device, it was switched to permanent high resistance state that cannot be restored back to lower resistance states. The mechanism of the memristor effect can be attributed to the charge trapping behaviour in PVA while the WORM effect can be explained as the electrochemical characteristic of PEDOT: PSS which harnesses the percolative conduction pathways. The results may facilitate multipurpose memory device with active tunability.

Enhance total transmission in stacking meta material and positive index material

Meta material has been demonstrated and applied in fabricating novel structures such as super lens, invisible cloaking and perfect absorber. The existence of a meta material layer in a stack consists of positive index material can result in a zero refractive index stack, which is also interesting in both physics and technology points of view. The stacking of negative index material and positive index material, where the impedances of negative index slab and positive index slab are matched, was manipulated so that the favorable outcome will be a zero equivalent index material with equivalent impedance different from zero. The reflection coefficient was zero at 90 degree incident angle, hence no reflection, and the calculated transmission is unity. Simulation demonstrates that there is no phase change and the wave propagates smoothly through the stack and the interfaces between slabs. Another kind of perfect absorber is also proposed by composing the 3-slab structure into the diffraction multi slits. Simulations show there are no back-scattering wave and transmitted wave.