Low-Dimensional Polyoxometalate Molecules/Tantalum Oxide Hybrids for Non-Volatile Capacitive Memories

Abstract

Transition-metal-oxide hybrids composed of high surface-to-volume ratio Ta2O5 matrices and a molecular analogue of transition metal oxides, tungsten polyoxometalates ([PW12O40](3-)), are introduced herein as a charge storage medium in molecular nonvolatile capacitive memory cells. The polyoxometalate molecules are electrostatically self-assembled on a low-dimensional Ta2O5 matrix, functionalized with an aminosilane molecule with primary amines as the anchoring moiety. The charge trapping sites are located onto the metal framework of the electron-accepting molecular entities as well as on the molecule/oxide interfaces which can immobilize negatively charged mobile oxygen vacancies. The memory characteristics of this novel nanocomposite were tested using no blocking oxide for extraction of structure-specific characteristics. The film was formed on top of the 3.1 nm-thick SiO2/n-Si(001) substrates and has been found to serve as both SiO2/Si interface states' reducer (i.e., quality enhancer) and electron storage medium. The device with the polyoxometalates sandwiched between two Ta2O5 films results in enhanced internal scattering of carriers. Thanks to this, it exhibits a significantly larger memory window than the one containing the plain hybrid and comparable retention time, resulting in a memory window of 4.0 V for the write state and a retention time around 10(4) s without blocking medium. Differential distance of molecular trapping centers from the cell's gate and electronic coupling to the space charge region of the underlying Si substrate were identified as critical parameters for enhanced electron trapping for the first time in such devices. Implementing a numerical electrostatic model incorporating structural and electronic characteristics of the molecular nodes derived from scanning probe and spectroscopic characterization, we are able to interpret the hybrid's electrical response and gain some insight into the electrostatics of the trapping medium.

Polyoxometalates‐Modulated Reduced Graphene Oxide Flash Memory with Ambipolar Trapping as Bidirectional Artificial Synapse

“…POMs are 0D n‐type semiconducting materials in which the discrete and anionic polynuclear molecular cluster are comprised by corner‐ and edge‐sharing octahedral M O6 (M = V, Nb, Mo, W) units and the metal normally exists in their highest oxidation state with either d0 or d1 electronic configuration (inset of Figure f) . In this work, Keggin type [PW 12 O 40 ] 3− (PW) were selected to function as both catalyst for in situ photoreduction of the GO into RGO under UV illumination and electron trapping sites due its ability to accept electrons without structural alteration, delocalised exchanged electrons for charge confinement, and multiple redox states . The characteristic to accept electrons of PW molecule induces energy level realignment between the PW/RGO interface and subsequent ambipolar charge trapping capability.…”

Controlled Nonvolatile Transition in Polyoxometalates‐Graphene Oxide Hybrid Memristive Devices

“…Noteworthy, the POMs we used here are nanometer‐size and anionic polynuclear molecular cluster exhibiting substantial variety charge and framework structures. Particularly, POMs preserve strong electron‐accepting ability without structural alteration, enabling them to play vital roles in charge trapping elements in electronic devices …”

Hybrid molecular solid state memories and electronic nanodevices based on polyoxometalates and other molecules of reversible redox activity

“…In fact, depending on the feature dimension which is parallel to the rough surface, the subsequently deposited molecular epilayer can be substrate-determined or structural unit self-determined. This dependency is quantified and described by the value of the fractal dimension, which means that fractal dimension controls the impact of roughness on the variability of the molecular epilayer features and, hence, final device performance [58,61].…”

“…Electrical characterization techniques are used for both the evaluation of transport and charging mechanisms of an electronic device performing under voltage application and its evaluation as a memory cell. They are core measurements in understanding the device physics, extracting modeling parameters [63] and getting real-operation feedback towards better engineering and re-design of devices [64].…”

“…The technique applied in this work involves application of programming pulses of different widths (durations) to the gate of the device and measurement of the flat band voltage shift on the capacitive core when compared to the "fresh" state of the device (see In the black curve we observe a memory window saturation for +14 V pulses, while in the pink curve case we have a continuously expanded memory window. This difference accounts on the presence of either a discrete trapping layer (first case) or an extended spatial distribution of trapping centers (second case) [61].…”