Low bias negative differential resistance with dual-peaks based on octacene molecular device

Based on non-equilibrium Green's function and density functional theory, a first-principles study of the transport properties of a molecular device is performed. This device is composed of two octacene molecules separated by a ethyl barrier, which are then linked to Ag leads through thiolates. The device shows low bias negative differential resistance effect with dual-peaks, which may be useful for designing molecular devices with low power-dissipation and multi-function in the future.     

Influence of oxygen ion drift on a negative difference behavior in a reset process of bipolar resistive switching

We present the oxygen ion drift-based resistive switching features of TiO_x/TiO_y bi-layer homo-junctions. The TiO_x layer in this bi-layer configuration was designed to have a stoichiometric chemical composition of TiO₂, while the TiO_y layer was designed to have a non-stoichiometric chemical composition. X-ray photoelectron spectroscopy measurements were carried out before and after electro-forming to determine the role of non-lattice oxygen content. Variation of the oxygen ion content in the TiO₂ layers resulted in changes in the on/off ratio and increased the non-lattice oxygen content. A possible switching mechanism based on oxygen ion content is discussed.     

Observation of room temperature negative differential resistance in solution synthesized ZnO nanorod

We report the observation of negative differential resistance (NDR) in solution synthesized ZnO nanorod. The ZnO nanorod was fabricated as a two terminal planar device using lithographically patterned Au electrodes. The measured current–voltage response of the device has shown a negative differential resistance behavior. The peak-to-valley current ratio of the NDR is found to be greater than 4. The mechanism of this observed NDR effect has been explained based on charge trapping and de-trapping at the nanoscale contacts. It is the first observation of negative differential resistance effect in solution synthesized ZnO nanorod.     

Thickness dependence of the resistive switching behavior of nonvolatile memory device structures based on undoped ZnO films

The resistive switching behavior of Al/ZnO/Al layered memory device structures was investigated in connection with varying ZnO layer thickness and related changes in crystallinity and concentration of oxygen-related defects. It was observed that, with increasing thickness, the crystallinity of the ZnO layer was improved and the concentration of oxygen-related defects within the layer increased. While the device showed unipolar switching characteristics, the current-voltage hysteresis was dependent on the thickness of the ZnO layer. In particular, the set voltage gradually increased with increasing layer thickness in the high resistive state whereas the reset voltage remained almost constant in the low resistive state. The observed operation characteristics of the device structures in relation to the crystallinity and oxygen-related defect concentration of the ZnO layer suggest that extended defects such as grain boundaries and dislocations play important roles in determining device performances.     

Negative differential resistance in polymer molecular devices modulated with molecular length

We study the electronic transport properties of polymer molecular devices by applying first-principles method. The results show that the electronic transport properties depend on molecular length. Negative differential resistance can be observed and can be modulated with molecular length.     

Multi-polar resistance switching and memory effect in copper phthalocyanine junctions

Copper phthalocyanine junctions, fabricated by magnetron sputtering and evaporating methods, show multi-polar （unipolar and bipolar） resistance switching and the memory effect. The multi-polar resistance switching has not been observed simultaneously in one organic material before. With both electrodes being cobalt, the unipolar resistance switching is universal. The high resistance state is switched to the low resistance state when the bias reaches the set voltage. Generally, the set voltage increases with the thickness of copper phthalocyanine and decreases with increasing dwell time of bias. Moreover, the low resistance state could be switched to the high resistance state by absorbing the phonon energy. The stability of the low resistance state could be tuned by different electrodes. In Au/copper phthalocyanine/Co system, the low resistance state is far more stable, and the bipolar resistance switching is found. Temperature dependence of electrical transport measurements demonstrates that there are no obvious differences in the electrical transport mechanism before and after the resistance switching. They fit quite well with Mott variable range hopping theory. The effect of A1203 on the resistance switching is excluded by control experiments. The holes trapping and detrapping in copper phthalocyanine layer are responsible for the resistance switching, and the interfacial effect between electrodes and copper phthalocyanine layer affects the memory effect.     

Resistive switching functional quantum-dot light-emitting diodes

This study demonstrates quantum-dot light-emitting diodes (QD-LEDs) with a function of resistive switching memory, capable of on/off operation at the same driving current depending on reset/set state. The QD-LEDs were fabricated by spin-coating process and experienced two different annealing conditions, which yielded defective or less-defective V₂O_5–x layer. One of the annealing conditions produced QD-LEDs with the unusual electrical behaviors of negative differential resistance (NDR), capacitance oscillation, and voltage–current hysteresis curves, signifying so-called resistive switching characteristics. X-ray and ultraviolet photoelectron spectroscopies were used to examine the chemical state of the differently annealed V₂O_5–x layers. The less stoichiometric V₂O_5–x layer was found to be responsible for the resistive switching behaviors of the NDR and the low and high resistance states (LRS and HRS, respectively). We discuss the LRS/HRS of V₂O_5–x for resistive switching in terms of a conductive filament effect, induced by microstructural changes caused by oxygen drift and vacancy annihilation processes in the high defect density V₂O_5–x layer.     

Engineering of carbon-based superlight spin filter with negative differential resistance

Based on density functional theory and non-equilibrium Green's function, we investigate the edge hydrogenation and oxidation effects on the spin transport of devices consisting of a zigzag C₂N nanoribbon (ZC₂NNR) embedded in zigzag graphene nanoribbons in parallel (P) and antiparallel (AP) spin configurations. The results show that device with edge hydrogenation exhibits dual spin filtering effect in AP spin configuration and obvious negative differential resistance in both P and AP spin configuration. By substituting oxygen for hydrogen as passivation atoms of ZC₂NNR, the spin filtering efficiency is as high as 100% in the P spin configuration, and the negative differential resistance is largely enhanced with a peak to valley ratio in excess of 4×10³. Our theoretical studies suggest that zigzag C₂N nanoribbon modulated by edge substitution has great potential in the design of future multifunctional spin devices.     

Strong n-type molecule as low bias negative differential resistance device predicted by first-principles study

A first-principles study of the transport properties of two thiolated pentacenes sandwiching ethyl is performed. The thiolated pentacene molecule shows strong n-type characteristics when contact Ag lead because of low work function about metal Ag. A strong negative differential resistance (NDR) effect with large peak-to-valley ratio of 758% is present under low bias. Our investigations indicate that strong n- or p-type molecules can be used as low bias molecular NDR devices and that the molecular NDR effect based on molecular-level leaving not on molecular-level crossing has no hysteresis.     

Bipolar resistance switching in the fully transparent BaSnO_3-based memory device

The fully transparent indium-tin-oxide/BaSnO3/F-doped SnO2 devices that show a stable bipolar resistance switching effect are successfully fabricated. In addition to the transmittance being above 87% for visible light, an initial forming process is unnecessary for the production of transparent memory. Fittings to the current-voltage curves reveal the interfacial conduction in the devices. The first-principles calculation indicates that the oxygen vacancies in cubic BaSnO3 will form the defective energy level below the bottom of conduction band. The field-induced resistance change can be explained based on the change of the interracial Schottky barrier, due to the migration of oxygen vacancies in the vicinity of the interface. This work presents a candidate material BaSnO3 for the application of resistive random access memory to transparent electronics.     

Mechanism analysis of switching direction transformation in an Er2O3 based RRAM device

The resistive random access memory (RRAM) based on resistive switching effect has considered to be the most advanced next generation memory, in which the switching direction determines the order of reading-writing. In this work, the rare-earth metal Er₂O₃ was used as functional layer, and Ag and indium-tin-oxide (ITO) are selected as top and bottom electrode to fabricate resistive switching device. Further, it is observed that the switching direction and memory window of resistive switching device can be regulated by exchanging top and bottom electrode. Moreover, the complementary switching memory behavior in Ag/Er₂O₃/ITO/Er₂O₃/Ag structure was also observed. Through mechanism analysis, it is expected that the barrier changes and metal-ions oxidation-reduction should be responsible for the conversion of switching direction and regulation of memory window. This work opens up a way to the development of next generation new concept memory.     

Large negative differential resistance in graphene nanoribbon superlattices

A graphene nanoribbon superlattice with a large negative differential resistance (NDR) is proposed. Our results show that the peak-to-valley ratio (PVR) of the graphene superlattices can reach 21 at room temperature with bias voltages between 90–220 mV, which is quite large compared with the one of traditional graphene-based devices. It is found that the NDR is strongly influenced by the thicknesses of the potential barrier. Therefore, the NDR effect can be optimized by designing a proper barrier thickness. The large NDR effect can be attributed to the splitting of the gap in transmission spectrum (segment of Wannier–Stark ladder) with larger thicknesses of barrier when the applied voltage increases.     

Modulation of rectification and negative differential resistance in graphene nanoribbon by nitrogen doping

By applying the nonequilibrium Green?s function formalism combined with density functional theory, we have investigated the electronic transport properties of two nitrogen-doped armchair graphene nanoribbon-based junctions M1 and M2. In the left part of M1 and M2, nitrogen atoms are doped at two edges of the nanoribbon. In the right part, nitrogen atoms are doped at one edge and at the center for M1 and M2, respectively. Obvious rectifying and negative differential resistance behaviors are found, which are strongly dependent on the doping position. The maximum rectification and peak-to-valley ratios are up to the order of 10⁴ in M2.     

Carbon doping induced giant low bias negative differential resistance in boron nitride nanoribbon

By applying nonequilibrium Green's function combined with density functional theory, we investigated the electronic transport properties of carbon-doped armchair boron nitride nanoribbons. Obvious negative differential resistance (NDR) behavior with giant peak-to-valley ratio up to the order of ${10}^4\text{–}{10}^6$ is found by tuning the doping position and concentration. Especially, with the reduction of doping concentration, NDR peak position can enter into mV bias range and even can be expected lower than mV bias. The negative differential resistance behavior is explained by the evolution of the transmission spectra and band structures with applied bias.     

Large negative differential resistance behavior in arsenene nanoribbons induced by vacant defects

We have studied the electronic structures of arsenene nanoribbons with different edge passivations by employing first-principle calculations. Furthermore, the effects of the defect in different positions on the transport properties of arsenene nanoribbons are also investigated. We find that the band structures of arsenene nanoribbons are sensitive to the edge passivation. The current-voltage characteristics of unpassivated and O-passivated zigzag arsenene nanoribbons exhibit a negative differential resistance behavior, while such a peculiar phenomenon has not emerged in the unpassivated and O-passivated armchair arsenene nanoribbons. The vacant defects on both top and bottom edges in unpassivated armchair arsenene nanoribbon can make its current-voltage characteristic also present a negative differential resistance behavior. After expanding the areas of the top and bottom defects in unpassivated armchair arsenene nanoribbon, the peak-to-valley ratio of the negative differential resistance behavior can be enlarged obviously, which opens another way for the application of arsenene-based devices with a high switching ratio.     

Physical mechanism of negative differential conductance in substrateless metallic carbon nanotubes

The existence of pronounced negative differential conductance at room temperature in suspended metallic carbon nanotubes was recently proven. We investigate here the physical nature of this phenomenon, which is of considerable importance for high-frequency devices, such as oscillators working up to few hundreds of GHz. Besides previous explanations, we find a new physical mechanism that explains the negative differential conductivity at room temperature. The entire suspended metallic carbon nanotube behaves as a very large quantum well, the negative differential conductance occurring due to the depletion of carriers on high-energy resonant levels.     

Improved resistive switching performance in Cu-cation migrated MoS2 based ReRAM device incorporated with tungsten nitride bottom electrode

The resistive switching characteristics of sputtered deposited molybdenum disulphide (MoS₂) thin film has been investigated in Cu/MoS₂/W₂N stack configuration for Resistive Random Access Memory (ReRAM) application. The benefits of incorporating tungsten nitride (W₂N) as a bottom electrode material were demonstrate by stability in operating voltages, good endurance (10³ cycles) and long non-volatile retention (10³?s) characteristics. Resistive switching properties in Cu/MoS₂/W₂N structure are induced by the formation/disruption of Cu conducting filaments in MoS₂ thin film. Ohmic law and space charge limited current (SCLC) are observed as dominant conduction mechanism in low resistance state (LRS) and high resistance state (HRS) respectively. This study suggests the application of MoS₂ thin films with W₂N bottom electrode for next generation non-volatile ReRAM application.     

Bipolar tri-state resistive switching characteristics in Ti/CeO_x/Pt memory device

Highly repeatable multilevel bipolar resistive switching in Ti/Ce Ox/Pt nonvolatile memory device has been demonstrated. X-ray diffraction studies of Ce O2 films reveal the formation of weak polycrystalline structure. The observed good memory performance, including stable cycling endurance and long data retention times（〉10^4s） with an acceptable resistance ratio（～10^2）, enables the device for its applications in future non-volatile resistive random access memories（RRAMs）. Based on the unique distribution characteristics of oxygen vacancies in Ce Ox films, the possible mechanism of multilevel resistive switching in Ce Ox RRAM devices has been discussed. The conduction mechanism in low resistance state is found to be Ohmic due to conductive filamentary paths, while that in the high resistance state was identified as Ohmic for low applied voltages and a space-charge-limited conduction dominated by Schottky emission at high applied voltages.     

Large negative differential resistance and rectifying performance modulated by contact sites in fused thiophene trimmer-based molecular devices

By applying density functional theory with non-equilibrium Green?s function formalism, we have carried out a theoretical study of the electron transport in fused thiophene trimmer-based molecular devices with ethylene connections at three different sites. The simulation results indicate that the electronic transport properties strongly depend on the contact sites. Negative differential resistance and rectifying behaviors occur simultaneously in the current–voltage curves when ethylene connects the fused thiophene trimer at one second-nearest site and one third-nearest site. A larger negative differential resistance occurs only when ethylene connects the fused thiophene trimer at two second-nearest sites.