Direct spectroscopic monitoring of conductance switching in polythiophene memory devices

a b s t r a c t A large number of nonvolatile memory devices have been reported with both inorganic and organic components, and many of these involve changes in device resistance between a high conductivity "ON" state and a low conductivity "OFF" state. The mechanism of memory action in many of these devices is uncertain, and may be based on many phenomena, including redox reactions, metal filament formation, charge storage in "floating gates", and redistribution of oxide vacancies. We report here a Raman spectroscopic probe of organic polymer memory devices which permits direct monitoring of the doping state and conductivity of polythiophene in a 3-terminal device. The polymer conductance is controlled by voltage pulses between the source and gate electrodes in FET geometry, while the conductance state is read out by a separate circuit between source and drain. The conductance was directly correlated with the Raman determination of the density of polarons in the polymer film, which was shown to control both the "electroforming" process and the conductance switching in working memory devices. The polymer conductance change requires a redox counter-reaction at the gate electrode, and atmospheric effects on performance indicate that water and oxygen reduction are involved. The observations are consistent with a redox process between the gate and source electrodes which modulates the polaron concentration and source-drain conductivity. This mechanism provides a framework for optimization of the device by changing its composition and geometry, particularly the identity of the redox counter-reaction and control of ion mobility

Direct spectroscopic monitoring of conductance switching in polythiophene memory devices

A large number of nonvolatile memory devices have been reported with both inorganic and organic components, and many of these involve changes in device resistance between a high conductivity "ON" state and a low conductivity "OFF" state. The mechanism of memory action in many of these devices is uncertain, and may be based on many phenomena, including redox reactions, metal filament formation, charge storage in "floating gates", and redistribution of oxide vacancies. We report here a Raman spectroscopic probe of organic polymer memory devices which permits direct monitoring of the doping state and conductivity of polythiophene in a 3-terminal device. The polymer conductance is controlled by voltage pulses between the source and gate electrodes in FET geometry, while the conductance state is read out by a separate circuit between source and drain. The conductance was directly correlated with the Raman determination of the density of polarons in the polymer film, which was shown to control both the "electroforming" process and the conductance switching in working memory devices. The polymer conductance change requires a redox counter-reaction at the gate electrode, and atmospheric effects on performance indicate that water and oxygen reduction are involved. The observations are consistent with a redox process between the gate and source electrodes which modulates the polaron concentration and source-drain conductivity. This mechanism provides a framework for optimization of the device by changing its composition and geometry, particularly the identity of the redox counter-reaction and control of ion mobility. \ua9 2012 Elsevier Ltd. All rights reserved.Peer reviewed: YesNRC publication: Ye

Label-free impedimetric immunosensor for point-of-care detection of COVID-19 antibodies

Abstract The COVID-19 pandemic has posed enormous challenges for existing diagnostic tools to detect and monitor pathogens. Therefore, there is a need to develop point-of-care (POC) devices to perform fast, accurate, and accessible diagnostic methods to detect infections and monitor immune responses. Devices most amenable to miniaturization and suitable for POC applications are biosensors based on electrochemical detection. We have developed an impedimetric immunosensor based on an interdigitated microelectrode array (IMA) to detect and monitor SARS-CoV-2 antibodies in human serum. Conjugation chemistry was applied to functionalize and covalently immobilize the spike protein (S-protein) of SARS-CoV-2 on the surface of the IMA to serve as the recognition layer and specifically bind anti-spike antibodies. Antibodies bound to the S-proteins in the recognition layer result in an increase in capacitance and a consequent change in the impedance of the system. The impedimetric immunosensor is label-free and uses non-Faradaic impedance with low nonperturbing AC voltage for detection. The sensitivity of a capacitive immunosensor can be enhanced by simply tuning the ionic strength of the sample solution. The device exhibits an LOD of 0.4 BAU/ml, as determined from the standard curve using WHO IS for anti-SARS-CoV-2 immunoglobulins; this LOD is similar to the corresponding LODs reported for all validated and established commercial assays, which range from 0.41 to 4.81 BAU/ml. The proof-of-concept biosensor has been demonstrated to detect anti-spike antibodies in sera from patients infected with COVID-19 within 1 h. Photolithographically microfabricated interdigitated microelectrode array sensor chips & label-free impedimetric detection of COVID-19 antibody