AVS2009 Session EM-WeM: Organic & Molecular Electronics

Increasing complexity in bottom-up molecular designs of amorphous structures with multiple relaxation modes demand an integrated and cognitive design approach, where chemical synthesis is guided by both analytical tools and theoretical simulations. This is true of organic second-order nonlinear optical (NLO) materials, which are being actively pursued for applications in photonic devices. For practical applications, NLO materials must have both high macroscopic EO activity and thermal stability. High EO activity can be achieved by acentrically ordering a system containing a high density of high dipole chromophores via electric field poling at elevated temperatures. Thermal stability requires the system to have internal constraints to prevent collapse of the acentric order at operating temperatures. Recent efforts for achieving both requirements have focused on dendrons capable of self-assembly through arene-perfluoroarene (Ar^H-Ar^F) quadrupolar interactions within self-assembled glassy chromophore systems, which provided excellent EO activity above 300pm/V and good thermal stability.

In this study, nanoscale scanning probe based thermo-mechanical analyses, intrinsic friction microscopy (IFM) and shear-modulation force microscopy yield direct insight into the molecular enthalpic and entropic relaxation modes of these materials. Ar^H-Ar^F interactions of dendritic moieties for coarse self assembly are found to impose three phase relaxation regimes with two transition temperatures, T₁ and T₂. Energetic analyses based on IFM identify increasing temporal stability with increasing arene size for the low temperature regime. Electric field poling efficiency is found to be inversely proportional to entropic cooperative contributions. Based on a molecular dynamic simulation, activation energies are tied primarily to interactions between chromophore (dipole), dendritic (quadrupole) moieties and combinations thereof below the incipient glass transition temperature. Above T₁, molecular mobility becomes increasingly cooperative. Sufficient mobility exists in the region of T₁<T<T₂ to allow for chromophore acentric electric field alignment, as non-covalent interactions associated with stabilization of the system below T₁ are in competition with melt-like effects. Further, cooperativity increases with increasing arene size, and accounts for approximately 80% of the observed apparent activation energy above T₂. Although beneficial to temporal stability with increased operating temperatures, cooperativity was found to lower the poling efficiency. Future synthesis efforts therefore must balance cooperativity to obtain satisfactory results in both stability and efficiency.

We investigated the effect of crystallinity and grain boundaries on the conductivity of Langmuir-Blodgett oligothiophene monolayers using Current-Sensing Atomic Force Microscopy (CS-AFM).

We used the AFM tip as a tool to inject charges and manipulate the crystalline monolayer. We found that passing electrical current locally from the conductive AFM tip led to reversible charging of the native SiO2 layer supporting the film as far as microns away from the charge injection point. This effect, due to charge spreading through the crystalline monolayer, was used to image conduction pathways and study the effect of grain boundaries on the resistivity of the monolayer.

In addition, we found that scanning manipulation at loads in the order of 100nN lead to a 5 fold decrease of the monolayer conductivity in CS-AFM. Subsequent molecular resolution AFM revealed that the degree of crystalline order in manipulated regions of the monolayer had strongly decreased, offering a direct proof of the correlation between order and conductivity in organic monolayers.

We have studied (single) octanethiol molecules adsorbed on a Pt-modified Ge(001) surface, using scanning tunneling microscopy/spectroscopy.

On a clean Ge(001) surface we deposited a submonolayer amount of platinum by evaporation. Patches of dimerized atomic chains form via self-organization on the surface during the subsequent annealing-step. We have decorated the Pt-modified Ge(001) surface with octanethiol molecules. STM at 77 K revealed that at low coverage the molecules selectively adsorb on the Pt chains and not on the underlying terrace. The molecules lay down on the Pt chains, in contrast to SAM's, where they stand upwards.

In order to distinguish between the octane-tail and the thiol-head of the molecule, we have performed STS above the different regions of the molecule, again at 77 K. We observed that the I(V) spectra recorded above the thiol-head were conspicuously different from the spectra recorded above the octane-tail. From that we could determine the molecular orientation of the adsorbent.

In addition, we have measured current-time traces on the adsorbed octanethiol molecules. Throughout these experiments we turned off the feedback-loop of the STM. Then each measurable rearrangement or conformational change of the molecule is reflected in the I(t) traces. During these measurements we occasionally found a sudden dramatic increase in current from 1 nA (set point current) to values between 10-15 nA. The residence times at this high current varied between 10-40 seconds. In most cases the current jumps back to its original set point value of 1 nA within the open-loop measurements, which typically last for 50-100 seconds. STM images recorded after the open-loop experiments revealed that the octanethiol molecules remained at their original position.

From these observations we concluded that during the I(t) measurements the molecule wagged its tail upward, thereby making contact with the tip of the STM. Hence, we measured electron transport through the molecule instead of electron tunneling from tip to molecule. The derived single molecule resistance, 100-150 MΩ, is in accordance with literature.

We have investigated the selective deposition of copper on alkanethiolate self-assembled monolayers (SAMs) using electroless deposition. This work has important applications in molecular and organic electronics, sensing, biotechnology, photonics and other technologies. We demonstrate that useful deposition rates can be obtained by “self-seeding” - the immersion of the SAM in a solution containing Cu²⁺ ions prior to addition of the reducing agent, formaldehyde. The selectivity of the deposition is achieved by increasing the bath temperature to 45 ºC. At this temperature Cu deposition ceases on –CH₃ terminated SAMs but continues on –COOH terminated SAMs. Finally, and perhaps most importantly, copper penetration through SAMs can be prevented by the addition of adenine to the bath. The addition of adenine also leads to smooth film morphologies. Each of these effects is explained by the formation of complexes with the SAM terminal group. Copper-terminal group complexes lead to useful deposition rates and control of the selectivity of the deposition. The formation of adenine-terminal group complexes prevents Cu penetration through the monolayer. Similarly, we observe that strongly adherent nickel films can be selectively deposited on functionalized SAMs through careful control of the bath conditions, especially temperature and pH.

A polymer light emitting device (PLED) with an active layer consisting of CdSe/ZnS or CdS/ZnS core/shell quantum dots (QDs), and an electron transport/passivation layer consisting of 5 nm ZnO nanoparticles has been studied. The complete device consists of a glass substrate with the following layers: glass/ITO/PEDOT-PSS/poly-TPD/QDs/ZnO/Al-top-contact. The PEDOT-PSS, poly-TPD, QD, and ZnO layers were all deposited from solutions using spin casting, while the ITO layer was sputter deposited and the Al contact was deposited by thermal evaporation through a shadow mask. Thus the device is predominantly a solution-processed QD-PLED with simple vacuum processing instead of thermal evaporation of multiple organic small molecule layers. The emitted color was tuned from blue (CdS/ZnS) to green or red (CdSe/ZnS) by adjusting the composition and size of the QDs. Use of the ZnO layer reduced the injection voltage for a brightness of ~200 cd/m² from ~11 to ~2.6 V (green light), and improved the stability with time. For example, an unencapsulated QD-PLED with a ZnO nanoparticle layer exhibited a stable ~200 cd/m² brightness for 18 hrs in ambient air, while a comparable device without the ZnO layer degraded from 80 to 6 cd/m² in 3 hrs under the same conditions. The reasons for these improvements will be discussed.

One of the critical issues for applications of flexible organic thin film transistors (OTFTs) for flexible electronic systems is the electrical stabilities of the OTFT devices, including variation of the current on/off ratio (I_on/I_off), leakage current, threshold voltage, and hysteresis, under repetitive mechanical deformation. In particular, repetitive mechanical deformation accelerates the degradation of device performance at the ambient environment. In this work, electrical stabilities of the pentacene organic thin film transistors (OTFTs) employing various passivation layers were investigated under mechanical cyclic bending. Flexible bottom-gated pentacene-based OTFTs fabricated on flexible polyimide substrate with poly-4-vinyl phenol (PVP) dielectric as a gate dielectric were passivated by the solution-processed, evaporated, and plasma-deposited organic layers. For cyclic bending experiment of flexible OTFTs, the devices were cyclically bent up to 10⁵ times with 5 mm bending radius. In the most of the devices after 10⁵ times of bending cycles, the off-current of the OTFT with no passivation layers was quickly increased due to increases in the conductivity of the pentacene caused by doping effects from O₂ and H₂O in the atmosphere, which leads to decrease in the I_on/I_off and increase in the hysteresis. With passivation layers, however, the electrical stabilities of the OTFTs were improved significantly. In particular, the OTFTs with plasma-deposited organic layers showed the best electrical stabilities up to the bending cycles of 10⁵ times compared to the devices with the solution-processed or thermal-evaporated organic layer. Changes in electrical properties of cyclically bent OTFTs with different organic passivation layers will be compared and discussed in detail.