James Robert Matthews

Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles

Injection of photoexcited electrons from colloidal PbS quantum dots into TiO(2) nanoparticles is investigated. The electron affinity and ionization potential of PbS quantum dots, inferred from cyclic voltammetry measurements, show strong size dependence due to quantum confinement. On the basis of the measured energy levels, photoexcited electrons should transfer efficiently from the quantum dots into TiO(2) only for quantum-dot diameter below approximately 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of PbS quantum dots coupled to titanium dioxide nanoparticles are consistent with electron transfer for small quantum dots. The measured electron transfer time is surprisingly slow ( approximately 100 ns), and implications of this for future photovoltaics will be discussed. Initial results obtained from solar cells sensitized with PbS quantum dots are presented.

Significance of the double-layer capacitor effect in polar rubbery dielectrics and exceptionally stable low-voltage high transconductance organic transistors.

Both high gain and transconductance at low operating voltages are essential for practical applications of organic field-effect transistors (OFETs). Here, we describe the significance of the double-layer capacitance effect in polar rubbery dielectrics, even when present in a very low ion concentration and conductivity. We observed that this effect can greatly enhance the OFET transconductance when driven at low voltages. Specifically, when the polar elastomer poly(vinylidene fluoride-co-hexafluoropropylene) (e-PVDF-HFP) was used as the dielectric layer, despite a thickness of several micrometers, we obtained a transconductance per channel width 30 times higher than that measured for the same organic semiconductors fabricated on a semicrystalline PVDF-HFP with a similar thickness. After a series of detailed experimental investigations, we attribute the above observation to the double-layer capacitance effect, even though the ionic conductivity is as low as 10–10 S/cm. Different from previously reported OFETs with double-layer capacitance effects, our devices showed unprecedented high bias-stress stability in air and even in water.

Thermal conduction effects impacting morphology during synthesis of columnar nanostructured TiO2 thin films

The aerosol chemical vapor deposition (ACVD) process allows for the synthesis of nanostructured films with well tuned morphologies that can be controlled based on the desired functionality and application. A robust understanding of the process parameters that result in desired features of the film is elucidated. One dimensional TiO2 nanostructured columns that have superior properties for solar energy harvesting and conversion applications were deposited on tin doped indium oxide (ITO) substrates. The sintering of the deposited particles was a key factor in the growth of the 1D structure with desired crystal planes. By ensuring that the sintering rate is faster than the arrival rate of deposited particles; a 1D columnar structure could be obtained. The sintering rate was controlled by the temperature and depositing particle size. As the columns grew in length, the increased thermal conduction resistance resulted in a drop in temperature and subsequently a slowing of the sintering process in upper regions of the film. This led to growth of branched structures rather than continued growth in a preferred direction. The growth of the branched structure could be overcome by enhancing the sintering rate by increasing the substrate temperature or reducing the depositing particle size (by lowering the feed rate of the precursor). The phenomenon was also confirmed by using different deposition substrates, such as FTO and glass. Dye sensitized solar cell performance efficiencies with different column lengths of 2 and 7 µm were determined to be 1.8 and 2.7% respectively.

Deformable Organic Nanowire Field‐Effect Transistors

Deformable electronic devices that are impervious to mechanical influence when mounted on surfaces of dynamically changing soft matters have great potential for next-generation implantable bioelectronic devices. Here, deformable field-effect transistors (FETs) composed of single organic nanowires (NWs) as the semiconductor are presented. The NWs are composed of fused thiophene diketopyrrolopyrrole based polymer semiconductor and high-molecular-weight polyethylene oxide as both the molecular binder and deformability enhancer. The obtained transistors show high field-effect mobility >8 cm2 V-1 s-1 with poly(vinylidenefluoride-co-trifluoroethylene) polymer dielectric and can easily be deformed by applied strains (both 100% tensile and compressive strains). The electrical reliability and mechanical durability of the NWs can be significantly enhanced by forming serpentine-like structures of the NWs. Remarkably, the fully deformable NW FETs withstand 3D volume changes (>1700% and reverting back to original state) of a rubber balloon with constant current output, on the surface of which it is attached. The deformable transistors can robustly operate without noticeable degradation on a mechanically dynamic soft matter surface, e.g., a pulsating balloon (pulse rate: 40 min-1 (0.67 Hz) and 40% volume expansion) that mimics a beating heart, which underscores its potential for future biomedical applications.

Development of organic semiconducting technology to realize low driving voltages

Corning has developed three generations of polymeric organic semiconducting (OSC) materials, each with progressively improved electronic performance and processability. These materials possess excellent solubility, mobility and stability. Stanford University has developed a new polymer dielectric material based on a fluoroelastomer. Combined with Comings OSC polymers, this enables easy to fabricate transistors with high transconductance, low driving voltage and excellent device stability, even in water.