Memory and Photovoltaic Elements in Organic Field Effect Transistors with Donor/Acceptor Planar-Hetero Junction Interfaces

Interfacial charge transfer at organic/organic planar-hetero junctions allows access to device structures that create new opportunities for flexible electronic devices. Fundamental characteristics of a pentacene/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) interface are explored via a comprehensive study of charge transfer between these two materials using the field-effect transistor (FET) geometry both in the dark and under illumination. Organic memory elements in a field effect transistor are demonstrated for a device fabricated with a pentacene/PCBM interface. Electric field induced charge transfer at the interface, in the dark, induced a nonvolatile memory effect with a large hysteresis characterized by a memory window of 43 V in the transfer characteristics. A photoinduced threshold voltage shift induced by exciton dissociation at the interface, in the absence of a gate electric field, is consistent with the formation of the photoinduced conducting channel in pentacene

Signature Vibrational Bands for Defects in CVD Single-Layer Graphene by Surface-Enhanced Raman Spectroscopy

We report the observation of signature vibrational bands in the frequency region between 900 and 1600 cm^–1 for defects in single-layer graphene (SLG) using surface Raman spectroscopy in ultrahigh vacuum. Vapor deposition of Ag leads to the formation of surface nanoparticles that migrate to defects in the SLG, leading to surface-enhanced Raman scattering (SERS) of the graphene G and 2D bands as well as new vibrational modes ascribed to native defects. Many of the new spectral bands of these native defects are similar, although not identical, to those predicted previously for −C₂ defects. These new bands are observed in addition to bands more commonly observed for defective graphene that are attributed to the D, G*, D+G, and 2D′ modes. The defects observed in these SLG films are not believed to result from the Ag deposition process but are postulated to be formed during the graphene CVD growth process. These defects are then made visible by postdeposition of Ag due to SERS

Environmentally Assisted Cracking in Silicon Nitride Barrier Films on Poly(ethylene terephthalate) Substrates

A singular critical onset strain value has been used to characterize the strain limits of barrier films used in flexible electronics. However, such metrics do not account for time-dependent or environmentally assisted cracking, which can be critical in determining the overall reliability of these thin-film coatings. In this work, the time-dependent channel crack growth behavior of silicon nitride barrier films on poly­(ethylene terephthalate) (PET) substrates is investigated in dry and humid environments by tensile tests with in situ optical microscopy and numerical models. The results reveal the occurrence of environmentally assisted crack growth at strains well below the critical onset crack strain and in the absence of polymer-relaxation-assisted, time-dependent crack growth. The crack growth rates in laboratory air are about 1 order of magnitude larger than those tested in dry environments (dry air or dry nitrogen). In laboratory air, crack growth rates increase from ∼200 nm/s to 60 μm/s for applied stress intensity factors, <i>K</i>, ranging from 1.0 to 1.4 MPa·m^1/2, below the measured fracture toughness <i>K</i>_c of 1.8 MPa·m^1/2. The crack growth rates in dry environments were also strongly dependent on the prior storage of the specimens, with larger rates for specimens exposed to laboratory air (and therefore moisture) prior to testing compared to specimens stored in a dry environment. This behavior is attributed to moisture-assisted cracking, with a measured power law exponent of ∼22 in laboratory air. This study also reveals that much larger densities of channel cracks develop in the humid environment, suggesting an easier initiation of channel cracks in the presence of water vapor. The results obtained in this work are critical to address the time-dependent and environmental reliability issues of thin brittle barriers on PET substrates for flexible electronics applications

Thermal Conductance across Phosphonic Acid Molecules and Interfaces: Ballistic versus Diffusive Vibrational Transport in Molecular Monolayers

The influence of planar organic linkers on thermal boundary conductance across hybrid interfaces has focused on the organic/inorganic interaction energy rather than on vibrational mechanisms in the molecule. As a result, research into interfacial transport at planar organic monolayer junctions has treated molecular systems as thermally ballistic. We show that thermal conductance in phosphonic acid (PA) molecules is ballistic, and the thermal boundary conductance across metal/PA/sapphire interfaces is driven by the same phononic processes as those across metal/sapphire interfaces without PAs, with one exception. We find a more than 40% reduction in conductance across henicosa­fluoro­dodecyl­phosphonic acid (F21PA) interfaces, independent of metal contact, despite similarities in structure, composition, and terminal group to the variety of other PAs studied. Our results suggest diffusive scattering of thermal vibrations in F21PA, demonstrating a clear path toward modification of interfacial thermal transport based on knowledge of ballistic and diffusive scattering in single monolayer molecular interfacial films

Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: An <i>in-Situ</i> Study

We report mechanical behavior and strain rate dependence of recoverability and energy dissipation in vertically aligned carbon nanotube (VACNT) bundles subjected to quasi-static uniaxial compression. We observe three distinct regimes in their stress–strain curves for all explored strain rates from 4 × 10^–2 down to 4 × 10^–4 /sec: (1) a short initial elastic section followed by (2) a sloped plateau with characteristic wavy features corresponding to buckle formation and (3) densification characterized by rapid stress increase. Load–unload cycles reveal a stiffer response and virtually 100% recoverability at faster strain rates of 0.04/sec, while the response is more compliant at slower rates, characterized by permanent localized buckling and significantly reduced recoverability. We propose that it is the kinetics of attractive adhesive interactions between the individual carbon nanotubes within the VACNT matrix that governs morphology evolution and ensuing recoverability. In addition, we report a 6-fold increase in elastic modulus and gradual decrease in recoverability (down to 50%) when VACNT bundles are unloaded from postdensification stage as compared with predensification. Finally, we demonstrate energy dissipation capability, as revealed by hysteresis in load–unload cycles. These findings, together with high thermal and electrical conductivities, position VACNTs in the “unattained-as-of-to-date-space” in the material property landscape

Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: An <i>in-Situ</i> Study

We report mechanical behavior and strain rate dependence of recoverability and energy dissipation in vertically aligned carbon nanotube (VACNT) bundles subjected to quasi-static uniaxial compression. We observe three distinct regimes in their stress–strain curves for all explored strain rates from 4 × 10^–2 down to 4 × 10^–4 /sec: (1) a short initial elastic section followed by (2) a sloped plateau with characteristic wavy features corresponding to buckle formation and (3) densification characterized by rapid stress increase. Load–unload cycles reveal a stiffer response and virtually 100% recoverability at faster strain rates of 0.04/sec, while the response is more compliant at slower rates, characterized by permanent localized buckling and significantly reduced recoverability. We propose that it is the kinetics of attractive adhesive interactions between the individual carbon nanotubes within the VACNT matrix that governs morphology evolution and ensuing recoverability. In addition, we report a 6-fold increase in elastic modulus and gradual decrease in recoverability (down to 50%) when VACNT bundles are unloaded from postdensification stage as compared with predensification. Finally, we demonstrate energy dissipation capability, as revealed by hysteresis in load–unload cycles. These findings, together with high thermal and electrical conductivities, position VACNTs in the “unattained-as-of-to-date-space” in the material property landscape

Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: An <i>in-Situ</i> Study

We report mechanical behavior and strain rate dependence of recoverability and energy dissipation in vertically aligned carbon nanotube (VACNT) bundles subjected to quasi-static uniaxial compression. We observe three distinct regimes in their stress–strain curves for all explored strain rates from 4 × 10^–2 down to 4 × 10^–4 /sec: (1) a short initial elastic section followed by (2) a sloped plateau with characteristic wavy features corresponding to buckle formation and (3) densification characterized by rapid stress increase. Load–unload cycles reveal a stiffer response and virtually 100% recoverability at faster strain rates of 0.04/sec, while the response is more compliant at slower rates, characterized by permanent localized buckling and significantly reduced recoverability. We propose that it is the kinetics of attractive adhesive interactions between the individual carbon nanotubes within the VACNT matrix that governs morphology evolution and ensuing recoverability. In addition, we report a 6-fold increase in elastic modulus and gradual decrease in recoverability (down to 50%) when VACNT bundles are unloaded from postdensification stage as compared with predensification. Finally, we demonstrate energy dissipation capability, as revealed by hysteresis in load–unload cycles. These findings, together with high thermal and electrical conductivities, position VACNTs in the “unattained-as-of-to-date-space” in the material property landscape

Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: An <i>in-Situ</i> Study

We report mechanical behavior and strain rate dependence of recoverability and energy dissipation in vertically aligned carbon nanotube (VACNT) bundles subjected to quasi-static uniaxial compression. We observe three distinct regimes in their stress–strain curves for all explored strain rates from 4 × 10^–2 down to 4 × 10^–4 /sec: (1) a short initial elastic section followed by (2) a sloped plateau with characteristic wavy features corresponding to buckle formation and (3) densification characterized by rapid stress increase. Load–unload cycles reveal a stiffer response and virtually 100% recoverability at faster strain rates of 0.04/sec, while the response is more compliant at slower rates, characterized by permanent localized buckling and significantly reduced recoverability. We propose that it is the kinetics of attractive adhesive interactions between the individual carbon nanotubes within the VACNT matrix that governs morphology evolution and ensuing recoverability. In addition, we report a 6-fold increase in elastic modulus and gradual decrease in recoverability (down to 50%) when VACNT bundles are unloaded from postdensification stage as compared with predensification. Finally, we demonstrate energy dissipation capability, as revealed by hysteresis in load–unload cycles. These findings, together with high thermal and electrical conductivities, position VACNTs in the “unattained-as-of-to-date-space” in the material property landscape

Elastomer–Polymer Semiconductor Blends for High-Performance Stretchable Charge Transport Networks

An inverse relationship between mechanical ductility and mobility/molecular ordering in conjugated polymer systems was determined definitively through systematic interrogation of poly­(3-hexylthiophene) (P3HT) films with varied degrees of molecular ordering and associated charge transport performance. The dilemma, whereby molecular ordering required for efficient charge transport conclusively undermines the applicability of these materials for stretchable, flexible device applications, was resolved using a polymer blend approach. Specifically, the molecular interactions between dissimilar polymer materials advantageously induced semiconducting polymer ordering into efficient π–π stacked fibrillar networks. Changes in the molecular environment surrounding the conjugated polymer during the elastomer curing process further facilitated development of high mobility networked semiconductor pathways. A processed P3HT: poly­(dimethylsiloxane) (PDMS) composite afforded a semiconducting film that exhibits superior ductility and notable mobility versus the single-component polymer semiconductor counterpart

Facile Formation of Graphene P–N Junctions Using Self-Assembled Monolayers

Monolithic and patterned aminopropyltriethoxysilane (APTES) layers are used to create n-doped graphene, graphene p–n junctions, and FET devices containing p–n junctions in the device channel through transfer of CVD graphene onto APTES coated substrates. APTES doping is shown to not result in introduction of defects. <i>I</i>–<i>V</i> measurements of FET devices containing patterned APTES layers show it is possible to control the position of the two current minima (two Dirac points) in the ambipolar p–n junction