Suchand Sangeeth - Academia.edu (original) (raw)

Papers by Suchand Sangeeth

Advanced materials (Deerfield Beach, Fla.), 2016

Monolayer graphene is used by C. A. Nijhuis and co-workers as the bottom electrode to fabricate s... more Monolayer graphene is used by C. A. Nijhuis and co-workers as the bottom electrode to fabricate stable and high-quality tunnel junctions based on self-assembled monolayers (SAMs), as described on page 631. The SAMs are formed on graphene via non-covalent interactions without altering the structure of the graphene. This work paves the way to new types of molecular electronic junctions based on 2D materials.

Advanced materials (Deerfield Beach, Fla.), Jan 30, 2015

Monolayer graphene is used as the bottom electrode to fabricate stable and high-quality self-asse... more Monolayer graphene is used as the bottom electrode to fabricate stable and high-quality self-assembled monolayer (SAM) based tunnel junctions. The SAMs are formed on graphene via noncovalent bonds without altering the structure of the graphene. This work paves the way to new types of molecular electronic junctions based on 2D materials.

Nanoscale, 2015

This paper describes a method to fabricate a microfluidic top-electrode that can be utilized to g... more This paper describes a method to fabricate a microfluidic top-electrode that can be utilized to generate arrays of self-assembled monolayer (SAM)-based junctions. The top-electrodes consist of a liquid-metal of GaOx/EGaIn mechanically stabilized in microchannels and through-holes in polydimethylsiloxane (PDMS); these top-electrodes form molecular junctions by directly placing them onto the SAM supported by template-stripped (TS) Ag or Au bottom-electrodes. Unlike conventional techniques to form multiple junctions, our method does not require lithography to pattern the bottom-electrode and is compatible with TS bottom-electrodes, which are ultra-flat with large grains, free from potential contamination of photoresist residues, and do not have electrode-edges where the molecules are unable to pack well. We formed tunneling junctions with n-alkanethiolate SAMs in yields of ∼80%, with good reproducibility and electrical stability. Temperature dependent J(V) measurements indicated that the mechanism of charge transport across the junction is coherent tunneling. To demonstrate the usefulness of these junctions, we formed molecular diodes based on SAMs with Fc head groups. These junctions rectify currents with a rectification ratio R of 45. These molecular diodes were incorporated in simple electronic circuitry to demonstrate molecular diode-based Boolean logic.

Advanced materials (Deerfield Beach, Fla.), Jan 28, 2015

The dielectric response and electrical properties of junctions based on self-assembled monolayers... more The dielectric response and electrical properties of junctions based on self-assembled monolayers (SAMs) of the form S(CH2 )11 X can be controlled by changing the polarizability of X (here X = H, F, Cl, Br, or I). A 1000-fold increase in the tunneling rate and a four-fold increase of the dielectric constant (ε r ) with increasing polarizability of X are found.

Nano letters, Jan 4, 2015

Defects in self-assembled monolayers (SAMs) based junctions cause the largest deviation between p... more Defects in self-assembled monolayers (SAMs) based junctions cause the largest deviation between predicted and measured values of the tunnelling current. We report the remarkable, seemingly counter-intuitive finding that shorter, less-ordered SAMs provide, unlike taller crystalline-like SAMs, higher quality tunnelling barriers on defective substrates, which points to self-repair of liquid-like SAMs on defects. The molecular dynamics show that short-chain molecules can more easily rotate into low-density boundary regions and smoothen out defects than thick solid-like SAMs. Our findings point to an attractive means of removing their deleterious effects simply by using flexible molecules.

Journal of the American Chemical Society, Jan 13, 2015

Odd-even effects in molecular junctions with self-assembled monolayers (SAMs) of n-alkanethiolate... more Odd-even effects in molecular junctions with self-assembled monolayers (SAMs) of n-alkanethiolates have been rarely observed. It is challenging to pinpoint the origin of odd-even effects and address the following question: are the odd-even effects an interface effect, caused by the intrinsic properties of the SAMs, or a combination of both? This paper describes the odd-even effects in SAM-based tunnel junctions of the form Ag(A-TS)-SCn//GaOx/EGaIn junctions with a large range of molecular lengths (n = 2 to 18) that are characterized by both AC and DC methods along with a detailed statistical analysis of the data. This combination of techniques allowed us to separate interface effects from the contributions of the SAMs and to show that the odd-even effect observed in the value of J obtained by DC-methods are caused by the intrinsic properties of the SAMs. Impedance spectroscopy (an AC technique) allowed us to analyze the SAM resistance (RSAM), SAM capacitance (CSAM), and contact resi...

Nanoscale, 2015

It is challenging to quantify the contact resistance and to determine the nature of the molecule-... more It is challenging to quantify the contact resistance and to determine the nature of the molecule-electrode contacts in molecular two-terminal junctions. Here we show that potentiodynamic and temperature dependent impedance measurements give insights into the nature of the SAM-electrode interface and other bottlenecks of charge transport (the capacitance of the SAM (C(SAM)) and the resistance of the SAM (R(SAM))), unlike DC methods, independently of each other. We found that the resistance of the top-electrode-SAM contact for junctions with the form of Ag(TS)-SC(n)//GaO(x)/EGaIn with n = 10, 12, 14, 16 or 18 is bias and temperature independent and hence Ohmic (non-rectifying) in nature, and is orders of magnitude smaller than R(SAM). The C(SAM) and R(SAM) are independent of the temperature, indicating that the mechanism of charge transport in these SAM-based junctions is coherent tunneling and the charge carrier trapping at the interfaces is negligible.

The Journal of Physical Chemistry C, 2015

Nature Communications, 2014

Layer-by-layer-stacked chemical vapour deposition (CVD) graphene films find applications as trans... more Layer-by-layer-stacked chemical vapour deposition (CVD) graphene films find applications as transparent and conductive electrodes in solar cells, organic light-emitting diodes and touch panels. Common to lamellar-type systems with anisotropic electron delocalization, the plane-to-plane (vertical) conductivity in such systems is several orders lower than its in-plane conductivity. The poor electronic coupling between the planes is due to the presence of transfer process organic residues and trapped air pocket in wrinkles. Here we show the plane-to-plane tunnelling conductivity of stacked CVD graphene layers can be improved significantly by inserting 1-pyrenebutyric acid N-hydroxysuccinimide ester between the graphene layers. The six orders of magnitude increase in plane-to-plane conductivity is due to hole doping, orbital hybridization, planarization and the exclusion of polymer residues. Our results highlight the importance of interfacial modification for enhancing the performance of LBL-stacked CVD graphene films, which should be applicable to other types of stacked two-dimensional films.

Journal of the American Chemical Society, 2014

The electrical characteristics of molecular tunnel junctions are normally determined by DC method... more The electrical characteristics of molecular tunnel junctions are normally determined by DC methods. Using these methods it is difficult to discriminate the contribution of each component of the junctions, e.g., the molecule-electrode contacts, protective layer (if present), or the SAM, to the electrical characteristics of the junctions. Here we show that frequency-dependent AC measurements, impedance spectroscopy, make it possible to separate the contribution of each component from each other. We studied junctions that consist of self-assembled monolayers (SAMs) of n-alkanethiolates (S(CH2)(n-1)CH3 ≡ SC(n) with n = 8, 10, 12, or 14) of the form Ag(TS)-SC(n)//GaO(x)/EGaIn (a protective thin (~0.7 nm) layer of GaO(x) forms spontaneously on the surface of EGaIn). The impedance data were fitted to an equivalent circuit consisting of a series resistor (R(S), which includes the SAM-electrode contact resistance), the capacitance of the SAM (C(SAM)), and the resistance of the SAM (R(SAM)). A plot of R(SAM) vs n(C) yielded a tunneling decay constant β of 1.03 ± 0.04 n(C)(-1), which is similar to values determined by DC methods. The value of C(SAM) is similar to previously reported values, and R(S) (2.9-3.6 × 10(-2) Ω·cm(2)) is dominated by the SAM-top contact resistance (and not by the conductive layer of GaO(x)) and independent of n(C). Using the values of R(SAM), we estimated the resistance per molecule r as a function of n(C), which are similar to values obtained by single molecule experiments. Thus, impedance measurements give detailed information regarding the electrical characteristics of the individual components of SAM-based junctions.

Advanced Functional Materials, 2014

A wide variation in the charge transport properties of poly(3,4-ethylenedioxythiophene)-poly(styr... more A wide variation in the charge transport properties of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) films is attributed to the degree of phase-segregation of the excess insulating polyanion. The results indicate that the charge transport in PEDOT-PSS can vary from hopping to critical regime of the metal-insulator transition, depending on the subtle details of morphology. The extent of electrical-connectivity in films, directly obtained from a temperature dependent high-frequency transport study, indicates various limiting factors to the transport, which are correlated with the phase separation process. The low temperature magnetotransport further supports this morphology-dependent transport scenario.

The electrical and optical response of a field-effect device comprising a network of semiconducto... more The electrical and optical response of a field-effect device comprising a network of semiconductor-enriched single-wall carbon nanotubes, gated with sodium chloride solution is investigated. Field-effect is demonstrated in a device that uses facile fabrication techniques along with a small-ion as the gate electrolyte - and this is accomplished as a result of the semiconductor enhancement of the tubes. The optical transparency and electrical resistance of the device are modulated with gate voltage. A time-response study of the modulation of optical transparency and electrical resistance upon application of gate voltage suggests the percolative charge transport in the network. Also the ac response in the network is investigated as a function of frequency and temperature down to 5 K. An empirical relation between onset frequency and temperature is determined.

Journal of Physics D: Applied Physics, 2014

Electric field activated charge transport is studied in the metal/polymer/metal device structure ... more Electric field activated charge transport is studied in the metal/polymer/metal device structure of electropolymerized polypyrrole down to 10 K with varying carrier density and disorder. Disorder induced nonlinear behaviour is observed in polypyrrole devices grown at room temperature which is correlated to delocalization of states. The slope parameter of currentvoltage characteristics (in log-log scale) increases as the temperature decreases, which indicates the onset of stronger field dependence. The field dependence of mobility becomes dominant as the carrier density decreases. The sharp dip in differential conductance indicates the localization of carriers at low temperatures which reduces the effective number of carriers involved in the transport.

ABSTRACT The small signal ac response is measured across the source-drain terminals of organic fi... more ABSTRACT The small signal ac response is measured across the source-drain terminals of organic field-effect transistors (OFET) under dc bias to obtain the equivalent circuit parameters of poly (2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and poly(3-hexyl thiophene) (P3HT) based devices. The numerically simulated response based on these parameters is in good agreement with the experimental data for PBTTT-FET except at low frequencies, while the P3HT-FET data show significant deviations. This indicates that the interface with the metal electrode is rather complex for the latter, involving additional circuit elements arising from contact impedance or charge injection processes. Such an investigation can help in identifying the operational bottlenecks and to improve the performance of OFETs.

Journal of Physics: Condensed Matter, 2009

A wide variation in the charge transport properties of poly(3,4-ethylenedioxythiophene)poly(styre... more A wide variation in the charge transport properties of poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT-PSS) films is attributed to the degree of phase-segregation of the excess insulating polyanion. The results indicate that the charge transport in PEDOT-PSS can vary from hopping to critical regime of the metal-insulator transition, depending on the subtle details of morphology. The extent of electrical-connectivity in films, directly obtained from a temperaturedependent high-frequency transport study, indicates various limiting factors to the transport, which are correlated with the phase separation process. The low temperature magnetotransport further supports this morphology-dependent transport scenario.

Journal of Physics D: Applied Physics, 2011

ABSTRACT Current–voltage (I–V) and impedance measurements were carried out in doped poly(3-methyl... more ABSTRACT Current–voltage (I–V) and impedance measurements were carried out in doped poly(3-methylthiophene) devices by varying the carrier density. As the carrier concentration reduces the I–V characteristics indicate that the conduction mechanism is limited by metal–polymer interface, as also observed in impedance data. The temperature dependence of I–V in moderately doped samples shows a trap-controlled space-charge-limited conduction (SCLC); whereas in lightly doped devices injection-limited conduction is observed at lower bias and SCLC at higher voltages. The carrier density-dependent quasi-Fermi level adjustment and trap-limited transport could explain this variation in conduction mechanism. Capacitance measurements at lower frequencies and higher bias voltages show a sign change in values due to the significant variations in the relaxation behaviour for lightly and moderately doped samples. The electrical hysteresis increases as carrier density is reduced due to the time scales involved in the de-trapping of carriers.

Journal of Physics D: Applied Physics, 2010

The current density-voltage (J-V) characteristics of poly(3-methylthiophene) devices show a negat... more The current density-voltage (J-V) characteristics of poly(3-methylthiophene) devices show a negative differential resistance (NDR) at room temperature with a large peak to valley current ratio (~507). This NDR can be tuned by two orders of magnitude by controlling the carrier density due to the variation of the space-charge region in the device. The temperature and scan rate dependent J-V measurements infer that the NDR is mainly driven by the trapping and de-trapping of carriers. The photo-generation of carriers is observed to reduce the NDR effect.

Journal of Nanoscience and Nanotechnology, 2009

The electrical and optical response of a field-effect device comprising a network of semiconducto... more The electrical and optical response of a field-effect device comprising a network of semiconductor-enriched single-wall carbon nanotubes, gated with sodium chloride solution is investigated. Field-effect is demonstrated in a device that uses facile fabrication techniques along with a small-ion as the gate electrolyte -and this is accomplished as a result of the semiconductor enhancement of the tubes. The optical transparency and electrical resistance of the device are modulated with gate voltage. A time-response study of the modulation of optical transparency and electrical resistance upon application of gate voltage suggests the percolative charge transport in the network.

Advanced materials (Deerfield Beach, Fla.), 2016

Monolayer graphene is used by C. A. Nijhuis and co-workers as the bottom electrode to fabricate s... more Monolayer graphene is used by C. A. Nijhuis and co-workers as the bottom electrode to fabricate stable and high-quality tunnel junctions based on self-assembled monolayers (SAMs), as described on page 631. The SAMs are formed on graphene via non-covalent interactions without altering the structure of the graphene. This work paves the way to new types of molecular electronic junctions based on 2D materials.

Advanced materials (Deerfield Beach, Fla.), Jan 30, 2015

Monolayer graphene is used as the bottom electrode to fabricate stable and high-quality self-asse... more Monolayer graphene is used as the bottom electrode to fabricate stable and high-quality self-assembled monolayer (SAM) based tunnel junctions. The SAMs are formed on graphene via noncovalent bonds without altering the structure of the graphene. This work paves the way to new types of molecular electronic junctions based on 2D materials.

Nanoscale, 2015

This paper describes a method to fabricate a microfluidic top-electrode that can be utilized to g... more This paper describes a method to fabricate a microfluidic top-electrode that can be utilized to generate arrays of self-assembled monolayer (SAM)-based junctions. The top-electrodes consist of a liquid-metal of GaOx/EGaIn mechanically stabilized in microchannels and through-holes in polydimethylsiloxane (PDMS); these top-electrodes form molecular junctions by directly placing them onto the SAM supported by template-stripped (TS) Ag or Au bottom-electrodes. Unlike conventional techniques to form multiple junctions, our method does not require lithography to pattern the bottom-electrode and is compatible with TS bottom-electrodes, which are ultra-flat with large grains, free from potential contamination of photoresist residues, and do not have electrode-edges where the molecules are unable to pack well. We formed tunneling junctions with n-alkanethiolate SAMs in yields of ∼80%, with good reproducibility and electrical stability. Temperature dependent J(V) measurements indicated that the mechanism of charge transport across the junction is coherent tunneling. To demonstrate the usefulness of these junctions, we formed molecular diodes based on SAMs with Fc head groups. These junctions rectify currents with a rectification ratio R of 45. These molecular diodes were incorporated in simple electronic circuitry to demonstrate molecular diode-based Boolean logic.

Advanced materials (Deerfield Beach, Fla.), Jan 28, 2015

The dielectric response and electrical properties of junctions based on self-assembled monolayers... more The dielectric response and electrical properties of junctions based on self-assembled monolayers (SAMs) of the form S(CH2 )11 X can be controlled by changing the polarizability of X (here X = H, F, Cl, Br, or I). A 1000-fold increase in the tunneling rate and a four-fold increase of the dielectric constant (ε r ) with increasing polarizability of X are found.

Nano letters, Jan 4, 2015

Defects in self-assembled monolayers (SAMs) based junctions cause the largest deviation between p... more Defects in self-assembled monolayers (SAMs) based junctions cause the largest deviation between predicted and measured values of the tunnelling current. We report the remarkable, seemingly counter-intuitive finding that shorter, less-ordered SAMs provide, unlike taller crystalline-like SAMs, higher quality tunnelling barriers on defective substrates, which points to self-repair of liquid-like SAMs on defects. The molecular dynamics show that short-chain molecules can more easily rotate into low-density boundary regions and smoothen out defects than thick solid-like SAMs. Our findings point to an attractive means of removing their deleterious effects simply by using flexible molecules.

Journal of the American Chemical Society, Jan 13, 2015

Odd-even effects in molecular junctions with self-assembled monolayers (SAMs) of n-alkanethiolate... more Odd-even effects in molecular junctions with self-assembled monolayers (SAMs) of n-alkanethiolates have been rarely observed. It is challenging to pinpoint the origin of odd-even effects and address the following question: are the odd-even effects an interface effect, caused by the intrinsic properties of the SAMs, or a combination of both? This paper describes the odd-even effects in SAM-based tunnel junctions of the form Ag(A-TS)-SCn//GaOx/EGaIn junctions with a large range of molecular lengths (n = 2 to 18) that are characterized by both AC and DC methods along with a detailed statistical analysis of the data. This combination of techniques allowed us to separate interface effects from the contributions of the SAMs and to show that the odd-even effect observed in the value of J obtained by DC-methods are caused by the intrinsic properties of the SAMs. Impedance spectroscopy (an AC technique) allowed us to analyze the SAM resistance (RSAM), SAM capacitance (CSAM), and contact resi...

Nanoscale, 2015

It is challenging to quantify the contact resistance and to determine the nature of the molecule-... more It is challenging to quantify the contact resistance and to determine the nature of the molecule-electrode contacts in molecular two-terminal junctions. Here we show that potentiodynamic and temperature dependent impedance measurements give insights into the nature of the SAM-electrode interface and other bottlenecks of charge transport (the capacitance of the SAM (C(SAM)) and the resistance of the SAM (R(SAM))), unlike DC methods, independently of each other. We found that the resistance of the top-electrode-SAM contact for junctions with the form of Ag(TS)-SC(n)//GaO(x)/EGaIn with n = 10, 12, 14, 16 or 18 is bias and temperature independent and hence Ohmic (non-rectifying) in nature, and is orders of magnitude smaller than R(SAM). The C(SAM) and R(SAM) are independent of the temperature, indicating that the mechanism of charge transport in these SAM-based junctions is coherent tunneling and the charge carrier trapping at the interfaces is negligible.

The Journal of Physical Chemistry C, 2015

Nature Communications, 2014

Layer-by-layer-stacked chemical vapour deposition (CVD) graphene films find applications as trans... more Layer-by-layer-stacked chemical vapour deposition (CVD) graphene films find applications as transparent and conductive electrodes in solar cells, organic light-emitting diodes and touch panels. Common to lamellar-type systems with anisotropic electron delocalization, the plane-to-plane (vertical) conductivity in such systems is several orders lower than its in-plane conductivity. The poor electronic coupling between the planes is due to the presence of transfer process organic residues and trapped air pocket in wrinkles. Here we show the plane-to-plane tunnelling conductivity of stacked CVD graphene layers can be improved significantly by inserting 1-pyrenebutyric acid N-hydroxysuccinimide ester between the graphene layers. The six orders of magnitude increase in plane-to-plane conductivity is due to hole doping, orbital hybridization, planarization and the exclusion of polymer residues. Our results highlight the importance of interfacial modification for enhancing the performance of LBL-stacked CVD graphene films, which should be applicable to other types of stacked two-dimensional films.

Journal of the American Chemical Society, 2014

The electrical characteristics of molecular tunnel junctions are normally determined by DC method... more The electrical characteristics of molecular tunnel junctions are normally determined by DC methods. Using these methods it is difficult to discriminate the contribution of each component of the junctions, e.g., the molecule-electrode contacts, protective layer (if present), or the SAM, to the electrical characteristics of the junctions. Here we show that frequency-dependent AC measurements, impedance spectroscopy, make it possible to separate the contribution of each component from each other. We studied junctions that consist of self-assembled monolayers (SAMs) of n-alkanethiolates (S(CH2)(n-1)CH3 ≡ SC(n) with n = 8, 10, 12, or 14) of the form Ag(TS)-SC(n)//GaO(x)/EGaIn (a protective thin (~0.7 nm) layer of GaO(x) forms spontaneously on the surface of EGaIn). The impedance data were fitted to an equivalent circuit consisting of a series resistor (R(S), which includes the SAM-electrode contact resistance), the capacitance of the SAM (C(SAM)), and the resistance of the SAM (R(SAM)). A plot of R(SAM) vs n(C) yielded a tunneling decay constant β of 1.03 ± 0.04 n(C)(-1), which is similar to values determined by DC methods. The value of C(SAM) is similar to previously reported values, and R(S) (2.9-3.6 × 10(-2) Ω·cm(2)) is dominated by the SAM-top contact resistance (and not by the conductive layer of GaO(x)) and independent of n(C). Using the values of R(SAM), we estimated the resistance per molecule r as a function of n(C), which are similar to values obtained by single molecule experiments. Thus, impedance measurements give detailed information regarding the electrical characteristics of the individual components of SAM-based junctions.

Advanced Functional Materials, 2014

A wide variation in the charge transport properties of poly(3,4-ethylenedioxythiophene)-poly(styr... more A wide variation in the charge transport properties of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) films is attributed to the degree of phase-segregation of the excess insulating polyanion. The results indicate that the charge transport in PEDOT-PSS can vary from hopping to critical regime of the metal-insulator transition, depending on the subtle details of morphology. The extent of electrical-connectivity in films, directly obtained from a temperature dependent high-frequency transport study, indicates various limiting factors to the transport, which are correlated with the phase separation process. The low temperature magnetotransport further supports this morphology-dependent transport scenario.

The electrical and optical response of a field-effect device comprising a network of semiconducto... more The electrical and optical response of a field-effect device comprising a network of semiconductor-enriched single-wall carbon nanotubes, gated with sodium chloride solution is investigated. Field-effect is demonstrated in a device that uses facile fabrication techniques along with a small-ion as the gate electrolyte - and this is accomplished as a result of the semiconductor enhancement of the tubes. The optical transparency and electrical resistance of the device are modulated with gate voltage. A time-response study of the modulation of optical transparency and electrical resistance upon application of gate voltage suggests the percolative charge transport in the network. Also the ac response in the network is investigated as a function of frequency and temperature down to 5 K. An empirical relation between onset frequency and temperature is determined.

Journal of Physics D: Applied Physics, 2014

Electric field activated charge transport is studied in the metal/polymer/metal device structure ... more Electric field activated charge transport is studied in the metal/polymer/metal device structure of electropolymerized polypyrrole down to 10 K with varying carrier density and disorder. Disorder induced nonlinear behaviour is observed in polypyrrole devices grown at room temperature which is correlated to delocalization of states. The slope parameter of currentvoltage characteristics (in log-log scale) increases as the temperature decreases, which indicates the onset of stronger field dependence. The field dependence of mobility becomes dominant as the carrier density decreases. The sharp dip in differential conductance indicates the localization of carriers at low temperatures which reduces the effective number of carriers involved in the transport.

ABSTRACT The small signal ac response is measured across the source-drain terminals of organic fi... more ABSTRACT The small signal ac response is measured across the source-drain terminals of organic field-effect transistors (OFET) under dc bias to obtain the equivalent circuit parameters of poly (2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and poly(3-hexyl thiophene) (P3HT) based devices. The numerically simulated response based on these parameters is in good agreement with the experimental data for PBTTT-FET except at low frequencies, while the P3HT-FET data show significant deviations. This indicates that the interface with the metal electrode is rather complex for the latter, involving additional circuit elements arising from contact impedance or charge injection processes. Such an investigation can help in identifying the operational bottlenecks and to improve the performance of OFETs.

Journal of Physics: Condensed Matter, 2009

A wide variation in the charge transport properties of poly(3,4-ethylenedioxythiophene)poly(styre... more A wide variation in the charge transport properties of poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT-PSS) films is attributed to the degree of phase-segregation of the excess insulating polyanion. The results indicate that the charge transport in PEDOT-PSS can vary from hopping to critical regime of the metal-insulator transition, depending on the subtle details of morphology. The extent of electrical-connectivity in films, directly obtained from a temperaturedependent high-frequency transport study, indicates various limiting factors to the transport, which are correlated with the phase separation process. The low temperature magnetotransport further supports this morphology-dependent transport scenario.

Journal of Physics D: Applied Physics, 2011

ABSTRACT Current–voltage (I–V) and impedance measurements were carried out in doped poly(3-methyl... more ABSTRACT Current–voltage (I–V) and impedance measurements were carried out in doped poly(3-methylthiophene) devices by varying the carrier density. As the carrier concentration reduces the I–V characteristics indicate that the conduction mechanism is limited by metal–polymer interface, as also observed in impedance data. The temperature dependence of I–V in moderately doped samples shows a trap-controlled space-charge-limited conduction (SCLC); whereas in lightly doped devices injection-limited conduction is observed at lower bias and SCLC at higher voltages. The carrier density-dependent quasi-Fermi level adjustment and trap-limited transport could explain this variation in conduction mechanism. Capacitance measurements at lower frequencies and higher bias voltages show a sign change in values due to the significant variations in the relaxation behaviour for lightly and moderately doped samples. The electrical hysteresis increases as carrier density is reduced due to the time scales involved in the de-trapping of carriers.

Journal of Physics D: Applied Physics, 2010

The current density-voltage (J-V) characteristics of poly(3-methylthiophene) devices show a negat... more The current density-voltage (J-V) characteristics of poly(3-methylthiophene) devices show a negative differential resistance (NDR) at room temperature with a large peak to valley current ratio (~507). This NDR can be tuned by two orders of magnitude by controlling the carrier density due to the variation of the space-charge region in the device. The temperature and scan rate dependent J-V measurements infer that the NDR is mainly driven by the trapping and de-trapping of carriers. The photo-generation of carriers is observed to reduce the NDR effect.

Journal of Nanoscience and Nanotechnology, 2009

The electrical and optical response of a field-effect device comprising a network of semiconducto... more The electrical and optical response of a field-effect device comprising a network of semiconductor-enriched single-wall carbon nanotubes, gated with sodium chloride solution is investigated. Field-effect is demonstrated in a device that uses facile fabrication techniques along with a small-ion as the gate electrolyte -and this is accomplished as a result of the semiconductor enhancement of the tubes. The optical transparency and electrical resistance of the device are modulated with gate voltage. A time-response study of the modulation of optical transparency and electrical resistance upon application of gate voltage suggests the percolative charge transport in the network.