Giant Change in Electrical Resistivity Induced by Moderate Pressure in Pt(bqd)2 – First Candidate Material for an Organic Piezoelectronic Transistor (OPET)

The piezoelectronic transistor (PET) has been proposed to overcome the voltage and clock speed limitations of conventional field-effect transistors (FET). In a PET, voltage is transduced to stress, which leads to an insulator-metal transition in a piezo-resistive (PR) element. Although the simulated switching speeds are promising, the viable candidates proposed so far for the PR layer are rare earth compounds that require several GPa of pressure (P) to metalize, necessitating breakthroughs in transduction mechanism scaling and processing. Here, a PR candidate that metalizes in the 0–300 MPa range – the transition metal complex platinum benzoquinonedioximato (Pt(bqd)2) is demonstrated. Such electrical sensitivity to the application of P arises when the material is grown as a thin film with the preferred needle orientation perpendicular to the substrate. As evidence, a combination of hydrostatic and uniaxial pressure studies is provided. The former studies are produced on the compressed powder pellet in a specially developed piston-cylinder cell (P-C cell) under variable temperatures (T) and P. The latter is via thin film deposition and uniaxial resistivity (ρ) measurements and these revealed the high potential of this material for the PET concept.</p

The Effect of Pressure on Halogen Bonding in 4-Iodobenzonitrile

The crystal structure of 4-iodobenzonitrile, which is monoclinic (space group I2/a) under ambient conditions, contains chains of molecules linked through C&#8801;N&#183;&#183;&#183;I halogen-bonds. The chains interact through CH&#183;&#183;&#183;I, CH&#183;&#183;&#183;N and &#960;-stacking contacts. The crystal structure remains in the same phase up to 5.0 GPa, the b axis compressing by 3.3%, and the a and c axes by 12.3 and 10.9 %. Since the chains are exactly aligned with the crystallographic b axis these data characterise the compressibility of the I&#183;&#183;&#183;N interaction relative to the inter-chain interactions, and indicate that the halogen bond is the most robust intermolecular interaction in the structure, shortening from 3.168(4) at ambient pressure to 2.840(1) &#197; at 5.0 GPa. The &#960;∙∙∙&#960; contacts are most sensitive to pressure, and in one case the perpendicular stacking distance shortens from 3.6420(8) to 3.139(4) &#197;. Packing energy calculations (PIXEL) indicate that the &#960;∙∙∙&#960; interactions have been distorted into a destabilising region of their potentials at 5.0 GPa. The structure undergoes a transition to a triclinic ( P 1 &#175; ) phase at 5.5 GPa. Over the course of the transition, the initially colourless and transparent crystal darkens on account of formation of microscopic cracks. The resistance drops by 10% and the optical transmittance drops by almost two orders of magnitude. The I&#183;&#183;&#183;N bond increases in length to 2.928(10) &#197; and become less linear [&lt;C&#8722;I∙∙∙N = 166.2(5)&#176;]; the energy stabilises by 2.5 kJ mol&#8722;1 and the mixed C-I/I..N stretching frequency observed by Raman spectroscopy increases from 249 to 252 cm&#8722;1. The driving force of the transition is shown to be relief of strain built-up in the &#960;∙∙∙&#960; interactions rather than minimisation of the molar volume. The triclinic phase persists up to 8.1 GPa

Instrumentation development to study candidate materials for an organic piezoelectronic transistor

High pressure (HP) is a powerful tool which is used to modify the material’s physical properties. The work described in this thesis is dedicated to the development of new or adaptation of the existing HP instrumentation which is capable of producing in situ conductivity (γ) measurements on the test materials to identify the most promising candidates for the organic piezoelectronic transistor (OPET). OPET is a concept of a new transistor which overcomes limitations of the currently employed transistor technology because it utilizes piezoelectric transduction rather than the electric field to propagate digital logic signals. This, in turn, implies small driving voltages, higher processing speeds and denser integration/scaling capabilities. The OPET device concept utilizes a piezoelectric actuator which, when the voltage (V) is applied to it, expands and, as a result, uniaxially compresses the thin layer of piezoresistive (PR) material within the rigid system. The employed PR materials need to have high pressure-dependent resistivity (ρ) to turn from the insulator/semiconductor into the conductor and, therefore, to pass the electric signal further within the ambient and 3 gigapascals (GPa) (pressure within the suitable range for OPET application). Among a wide variety of materials, organics were selected as PRs due to the high interest which they attracted in the recent decades by the worldwide multidisciplinary research in the electronic materials and because molecular organics are much more compressible than inorganic lattices. Although the OPET device concept implies the uniaxial compression, on the initial project stages it is rationally and economically viable to first characterise the PRs in the single crystal or the compressed powder form before their deposition into thin films. The characterisation implies the variable pressure (P) and variable temperature (T) ρ studies which resulted in necessity in developing double-layer autofrettaged piston-cylinder cell (PCC). The PCC is capable of reaching 3 GPa and is equipped with the feedthrough plug which introduces the probe wires into the HP environment to monitor sample resistance (R) and P changes in situ. To achieve P beyond the 3 GPa the DAC of the Merrill Bassett type was adapted for the electric γ measurements. DAC is equipped with 0.8 mm in diameter diamond culets and the NiCrAl seats to allow safe exploitation up to the 10 GPa to characterise those PR materials which failed to metallise within the desired P range but still are having a good ρ - P tendency which might find an application in the future OPET devices when better performance piezoelectric actuators will be made. Designs of both: PCC and DAC were analytically verified and validated using finite element analysis (FEA) as well as experimentally tested to indeed survive the P extremes with no yielding in the employed materials. Both cells were made to fit the required sample geometry with the necessary optimal probe contact separation which is the important prerequisite for the precise R into ρ conversion. In the case of the DAC the special sample loading techniques, gasket (a mechanical seal and a sample chamber between two opposed diamonds) preparation and insulation, as well as the gold sputtering of the probe contacts procedures, were implemented to achieve experimental success. Another HP cell which is reported in this thesis is the uniaxial high-pressure cell (UHPC). It was designed to produce both: static and dynamic P experiments to mimic the OPET device concept to study those PR materials which were deposited into the thin films. The performance of the above-mentioned P cells within this project is illustrated in the form of the project related outcomes. Among selected for the study materials, the hydrostatic HP measurements were performed on the platinum and iridium complexes with organic ligands, Magnus salts (organic-inorganic hybrids) as well as on the gold dithiolene complexes. The achieved results showed that some candidate materials indeed are promising for an application in the OPET device due to high P dependence on the electronic properties within the above-mentioned P range. For instance, the gold radical with (4-(4-chlorophenyl)-1,3-dithiolene) ligand and the Pt(bqd)2 materials were found to undergo 3 and 7 orders of magnitude change in ρ respectively between ambient pressure (Pamb) and 2 GPa at room T. The latter material was also deposited into the thin film form and exposed to the uniaxial HP. The produced measurements showed that the sample R gradually decreases from 600000 Ohm (Ω) at Pamb to 35 Ω at 0.11 GPa and values stay consistent between P cycles

Giant Change in Electrical Resistivity Induced by Moderate Pressure in Pt(bqd)2 – First Candidate Material for an Organic Piezoelectronic Transistor (OPET)

Abstract The piezoelectronic transistor (PET) has been proposed to overcome the voltage and clock speed limitations of conventional field‐effect transistors (FET). In a PET, voltage is transduced to stress, which leads to an insulator‐metal transition in a piezo‐resistive (PR) element. Although the simulated switching speeds are promising, the viable candidates proposed so far for the PR layer are rare earth compounds that require several GPa of pressure (P) to metalize, necessitating breakthroughs in transduction mechanism scaling and processing. Here, a PR candidate that metalizes in the 0–300 MPa range – the transition metal complex platinum benzoquinonedioximato (Pt(bqd)2) is demonstrated. Such electrical sensitivity to the application of P arises when the material is grown as a thin film with the preferred needle orientation perpendicular to the substrate. As evidence, a combination of hydrostatic and uniaxial pressure studies is provided. The former studies are produced on the compressed powder pellet in a specially developed piston‐cylinder cell (P‐C cell) under variable temperatures (T) and P. The latter is via thin film deposition and uniaxial resistivity (ρ) measurements and these revealed the high potential of this material for the PET concept