MOLECULAR MATERIALS AS COMPONENTS OF ELECTRONIC DEVICES – Research

  1. Molecular memory cells based on tungsten polyoxometalates and high-k dielectrics

Polyoxometalates are complex inorganic anions that may be considered as the molecular analog of transition metals oxides. In this application we developed a hybrid molecular/semiconductor capacitor type memory cell based on the redox properties of a Keggin type Tungsten polyoxometalate. The molecular layer is the charging part of the structure. A thin silicon bottom oxide is for the control of the cell (write/erase operations)   and a high-k tantalum oxide serves as a gate. The performance of the cell depends on the quality of the gate oxide. The resulting non-volatile memories are the first documented CMOS-compatible long-term-retention molecular capacitive cell of its kind, implementing inherent structure-emerging heat management. Great potential emerges for numerous energy-inspired innovations, enabling functional oxide-molecular hybrids exploitation as high-end non-volatile memory products.

  1. Balliou, A. et al. Size-dependent single electron transfer and semi-metal-to-insulator transitions in molecular metal oxide electronics. Nanotechnology 29, (2018).
  2. Balliou, A. et al. Low-Dimensional Polyoxometalate Molecules/Tantalum Oxide Hybrids for Non-Volatile Capacitive Memories. ACS Appl. Mater. Interfaces 8, (2016).

 

2. Electronic properties of phthalocyanine/gold nanoparticle networks

We studied the electronic transport properties of hybrid networks based on semiconducting conjugated oligomers and metal nanoparticles (NPs). Spherical gold (Au) NPs were linked by means of copper 3-diethylamino-1-propylsulphonamide sulfonic acid substituted phthalocyanine (CuPcSu) molecules to form a network confined between Au nanodistant electrodes. It was possible to discriminate between various transport mechanisms typical for such structures (i.e. tunnelling and hopping), to evaluate conduction thresholds and to reveal charging effects involving few electrons, at lower temperatures. The interpretation was assisted by AFM, FE-SEM and TEM imaging techniques.

  1. Balliou, A. et al. Probing the localization of charge and the extent of disorder through electronic transport on Au nanoparticle-copper phthalocyanine multijunction networks. Phys. Status Solidi Basic Res. 253, (2016).
  1. Carbon Nanotube Schottky type Photodetectors for UV applications

 Multiple wall carbon nanotubes (MWCNT) present advantages for optoelectronic applications such as the large effective photo-collector surface as well as the possibility to tune their band gap and absorbance through the growth parameters. We demonstrate a hybrid MWCNT/Si3N4/n-Si photodetector based on ordered MWCNTs and evaluate its performance in the UV, visual and near IR spectrum (200-1000nm).

 

             The principal objective – and challenge –  of this work is to realize novel  photodetectors based on dense arrays  of MWCNTs deposited directly on a Silicon substrate,  To this end it is necessary to match the high temperature (~800 0C)  carbon nanotube formation process with the other processes required for device fabrication. Since a Fe catalyst (Ferrocene) is required during the growth stage, the presence of a diffusion barrier between the MWCNT layer and Si is mandatory and to this end we used Si3N4. In sequence it will be provided evidence that the  silicon nitride interlayer (a) acts as a metal diffusion barrier (b) its resistivity decreases due to the the presence of metal atoms (reduces to 109 Ohm.cm compared to  1014 Ohm.cm when non-contaminated) thus contributing to charge transport through trap assisted tunneling and (c) due to its large bandgap (5.2eV) renders the device sensitive in the UV region. It will be discussed that the MWCNT layer acts as an effective  photon absorber and electron-hole generation takes place in the substrate including the silicon nitride layer.

  1. Kyriakis, A. et al. A UV photodetector based on ordered free standing MWCNT. in Journal of Instrumentation (2020). doi:10.1088/1748-0221/15/01/C01015
  2. Filatzikioti, A. et al. Carbon nanotube Schottky type photodetectors for UV applications. Solid State Electron. 151, 27–35 (2019).