Solid-State Electrochemical Process and Performance Optimization of Memristive Materials and Devices

As an emerging technology, memristors are nanoionic-based electrochemical systems that retains their resistance state based on the history of the applied voltage/current. They can be used for on-chip memory and storage, biologically inspired computing, and in-memory computing. However, the underlying physicochemical processes of memristors still need deeper understanding for the optimization of the device properties to meet the practical application requirements. Herein, we review recent progress in understanding the memristive mechanisms and influential factors for the optimization of memristive switching performances. We first describe the working mechanisms of memristors, including the dynamic processes of active metal ions, native oxygen ions and other active ions in ECM cells, VCM devices and ion gel-based devices, and the switching mechanisms in organic devices, along with discussions on the influential factors of the device performances. The optimization of device properties by electrode/interface engineering, types/configurations of dielectric materials and bias scheme is then illustrated. Finally, we discuss the current challenges and the future development of the memristor.

Ion Gated Synaptic Transistors Based on 2D van der Waals Crystals with Tunable Diffusive Dynamics

Neuromorphic computing represents an innovative technology that can perform intelligent and energy-efficient computation, whereas construction of neuromorphic systems requires biorealistic synaptic elements with rich dynamics that can be tuned based on a robust mechanism. Here, an ionic-gating-modulated synaptic transistor based on layered crystals of transitional metal dichalcogenides and phosphorus trichalcogenides is demonstrated, which produce a diversity of short-term and long-term plasticity including excitatory postsynaptic current, paired pulse facilitation, spiking-rate-dependent plasticity, dynamic filtering, etc., with remarkable linearity and ultralow energy consumption of ≈30 fJ per spike. Detailed transmission electron microscopy characterization and ab initio calculation reveal two-stage ionic gating effects, namely, surface adsorption and internal intercalation in the channel medium, causing different poststimulation diffusive dynamics and thus accounting for the observed short-term and long-term plasticity, respectively. The synaptic activity can therefore be effectively manipulated by tailoring the ionic gating and consequent diffusion dynamics with varied thickness and structure of the van der Waals material as well as the number, duration, rate, and polarity of gate stimulations, making the present synaptic transistors intriguing candidates for low-power neuromorphic systems.

Electronic structure of VxTi1-xSe2 in wide concentration region (0.06 ≤ x ≤ 0.9)

An experimental study of the electronic structure of V_xTi_1-xSe₂ system in a wide range of vanadium concentrations (x = 0.06-0.9) using x-ray photoelectron spectroscopy and resonant photoelectron spectroscopy has been performed. The partial charge transfer from the VSe₂ to TiSe₂ structural fragments is experimentally observed, and the most part of the charge is localized on the vanadium atoms in the VSe₂ structural fragments.

Chemical bond in FexTiSe2 intercalation compounds: dramatic influence of Fe concentration

iThe changes in the electronic structure and nature of the chemical bonds due to the ordering of the intercalated atoms inside the van der Waals gap were observed for the first time.

Structural Chemistry and Metamagnetism of an Homologous Series of Layered Manganese Oxysulfides

An homologous series of layered oxysulfides Sr2MnO2Cu(2m-delta)S(m+1) with metamagnetic properties is described. Sr2MnO2Cu(2-delta)S2 (m = 1), Sr2MnO2Cu(4-delta)S3 (m = 2) and Sr2MnO2Cu(6-delta)S4 (m = 3), consist of MnO2 sheets separated from antifluorite-type copper sulfide layers of variable thickness by Sr(2+) ions. All three compounds show substantial and similar copper deficiencies (delta approximately equal to 0.5) in the copper sulfide layers, and single-crystal X-ray and powder neutron diffraction measurements show that the copper ions in the m = 2 and m = 3 compounds are crystallographically disordered, consistent with the possibility of high two-dimensional copper ion mobility. Magnetic susceptibility measurements show high-temperature Curie-Weiss behavior with magnetic moments consistent with high spin manganese ions which have been oxidized to the (2+delta)+ state in order to maintain a full Cu-3d/S-3p valence band, and the compounds are correspondingly p-type semiconductors with resistivities around 25 Omega cm at 295 K. Positive Weiss temperatures indicate net ferromagnetic interactions between moments. Accordingly, magnetic susceptibility measurements and low-temperature powder neutron diffraction measurements show that the moments within a MnO(2) sheet couple ferromagnetically and that weaker antiferromagnetic coupling between sheets leads to A-type antiferromagnets in zero applied magnetic field. Sr2MnO2Cu(5.5)S4 and Sr2MnO2Cu(3.5)S3 are metamagnets which may be driven into the fully ordered ferromagnetic state below 25 K by the application of fields of 0.06 and 1.3 T respectively. The relationships between the compositions, structures, and physical properties of these compounds, and the prospects for chemical control of the properties, are discussed.

Ultrasound-assisted cracking process to prepare MoS2 nanorods

A simple ultrasound-assisted cracking process was applied to prepare high-crystallinity MoS2 nanorods using MoS2 micron particles as raw materials. The products were characterized by various techniques, including XRD, SEM, TEM and HRTEM. Systematic studies showed that acid treatment and ultrasound irradiation are both prerequisites for the preparation of ideal MoS2 nanorods. TEM images indicate that these nanorods have uniform morphology, with an average diameter of approximately 150 nm and length up to several microm. Based on the controlled experiments and observation on morphological changes, it is proposed that the activated layered MoS2 first exfoliate into sheets and further crack into smaller nanorods due to the strong mechanical agitation, shear forces, and micro-jets created by the ultrasound irradiation.

Studies on the Synthetic System of V/Ag/S Cluster Compounds and Structural Characterizations of V(2)AgS(4), V(2)Ag(2)S(4), and V(2)O(2)(&mgr;-S)(2) Complexes

The reaction system (NH(4))(3)VS(4)/Ag(PPh(3))(2)Cl/R(2)dtcNa in CH(3)CN was studied to afford two V(2)Ag(1)(-)(2)S(4) cubanelike cluster compounds and a trinuclear V(3)O(3)S(2) complex. (Et(4)N)[V(2)AgS(4)(Me(2)dtc)(2)(PPh(3))].(1)/(2)CH(3)CN ([Et(4)N]1.(1)/(2)CH(3)CN) crystallizes in the monoclinic space group P2(1)/c with a = 10.525(3) Å, b = 16.340(8) Å, c = 26.834(8) Å, beta = 101.28(2) degrees, and Z = 4. Complex (Et(4)N)(2)[V(2)Ag(2)S(4)(CS(3))(2)(PPh(3))(2)] ([Et(4)N](2)2) crystallizes in the monoclinic space group C2/c with a = 21.212(5) Å, b = 20.100(4) Å, c = 15.039(2) Å, beta = 102.72(2) degrees, and Z = 4. A stepwise aggregate process including a dinuclear V(2)Y(2)(&mgr;-S)(2) (Y = S, O) intermediate was suggested to explain the formation of 1, 2, and trinuclear anion [V(3)O(&mgr;-O)(2)(&mgr;-S)(2)(Et(2)dtc)(3)](-). Structural features of these complexes show that the V(2)Y(2)(&mgr;-S)(2) moiety can be seen as an independent unit to combine with other metal ion(s). A complex containing the [V(2)O(2)(&mgr;-S)(2)(Et(2)dtc)(2)](2)(-) (3) cluster anion was separated from a reaction system of (NH(4))(3)VS(4)/PPh(3)/Et(2)dtcNa, and its solvate complex {(Et(4)N)(3)Na[V(2)O(2)(&mgr;-S)(2)(Et(2)dtc)(2)](2)}(2).(1)/(2)CH(3)OH.H(2)O crystallizes in orthorhombic space group Pnn2 with a = 31.599(1) Å, b = 17.228(5) Å, c = 14.104(7) Å, and Z = 2. Infrared frequencies at 844-970 cm(-1) were associated with a V=O stretching vibration. The V=O additional coordination to the other metal ion gives rise to the red shift of the frequency. Proton and (51)V NMR spectra exhibit a paramagnetic V(IV) center existing in the trinuclear complex (Et(4)N)[V(3)O(&mgr;-O)(2)(&mgr;-S)(2)(Et(2)dtc)(3)] ([Et(4)N]4) and d(1)-d(1) coupling of V-V of the V(2)Y(2)(&mgr;-S)(2) moiety for these complexes. Two sets of the (31)P signals for 2at 16.6-19.3 ppm with equal intensity are attributed to (31)P-(107)Ag coupling and may well hide two V atoms in a similar environment. Cyclic voltammetries show some similar electrochemical behaviors between 3 and 4 indicated by a common couple at -0.7 V/-0.65 V and an oxidation peak at +0.7 to +0.6 V which are proposed to be due to redox in the V(2)O(2)(&mgr;-S)(2) moiety.

New Layered Zirconium Tellurides: Zr(0.30)ZrTe(2), Zr(0.29)Zr(2)Te(2)As, and NaZr(2)Te(2)As

The synthesis and crystal structure determinations of Zr(0.30)ZrTe(2) and M(x)Zr(2)Te(2)As (M = Zr, Na) compounds are reported. The structure of Zr(0.30)ZrTe(2) was refined in the hexagonal space group P6(3)mc (No. 186, Z = 2) with lattice parameters a = 3.9840(3) Å and c = 13.366(3) Å; Zr(0.29)Zr(2)Te(2)As was refined in the rhombohedral space group R&thremacr;m (No. 166, Z = 3) with lattice parameters a = 3.9329(4) Å and c = 29.564(5) Å. Zr(0.30)ZrTe(2) and Zr(0.29)Zr(2)Te(2)As have close structural similarities to Zr(2)Se(3) and Ta(2)S(2)C, respectively, and are built up by stacking hexagonal layers with [Zr(0.30)-Te-Zr-Te] and [Zr(0.29)-Te-Zr-As-Zr-Te] sequences. Four-probe resistivity measurements (77-300 K) show both Zr(0.30)ZrTe(2) and Zr(0.29)Zr(2)Te(2)As to be metallic (Zr(0.29)Zr(2)Te(2)As: 8.9 x 10(-)(5) Omega cm at 273 K). Both compounds exhibit structures wherein Zr atoms are included between layers (ZrTe(2) and Zr(2)Te(2)As) by partially filling trigonal antiprismatic holes. The replacement of the included Zr ions in Zr(0.29)Zr(2)Te(2)As by Na ions has been demonstrated. Powder diffraction data showed that NaZr(2)Te(2)As is isostructural with Zr(0.29)Zr(2)Te(2)As. By use of Rietveld refinements, sodium ions were found to reside in the trigonal antiprismatic sites between the layers. Extended Hückel band calculations on the [Zr(2)Te(2)As](1.16)(-) layer indicate that it should be a metallic conductor and that the [Zr(2)Te(2)As] layer can bear a greater negative charge than has so far been observed. We suggest that the [Zr(2)Te(2)As] layered compounds may offer new opportunities as electron-donating hosts.