D. Grozea

Vacuum-assisted aerosol deposition of a low-dielectric-constant periodic mesoporous organosilica film.

Adv. Mater. 2010, 22, 99–102 2010 WILEY-VCH Verlag Gm Since the conceptual and experimental breakthrough in 1992 on a soft-templating method to synthesize periodic mesoporous silica materials, their synthesis has evolved to the level of being able to adroitly control composition, pore size, mesopore architecture, and morphology. A significant advance in controlling the composition of periodic mesoporous silica materials is the successful integration of bridging organic groups into the siliceous pore wall to create periodic mesoporous organosilicas (PMOs). The enabling power of PMOs to fuse organic synthesis and inorganic materials chemistry has created opportunities for application in numerous areas, encompassing catalysis, separation, drug delivery, photonics, and nanoelectronics. Most PMOs synthesized to date are powders but many technologically relevant applications, such as sensors, membranes, displays, optical coatings, and microelectromechanical systems, require thin-film morphology because of the ease of materials property characterization and device fabrication on thin films. A quintessential example of the need for thin-film PMOs is their application as a lowdielectric-constant (low-k) insulating material on microelectronic chips in the semiconductor industry. The synthesis of PMO thin films has been achieved primarily through evaporation-induced self-assembly liquid-phase delivery techniques, such as spin-coating and dip-coating. However, the best-controlled and preferred techniques for fabricating insulating dielectrics in the semiconductor industry are vaporphase delivery techniques, such as chemical vapor deposition. An aerosol-deposition strategy for fabricating mesostructured thin films combines the advantages of vapor-phase delivery and evaporation-induced self-assembly and, at the same time, circumvents the difficulty of vaporizing low-volatility templates. Here, we report a vacuum-assisted aerosol deposition evaporation-induced self-assembly (VAAD-EISA) approach for fabricating highly ordered PMO thin films. The aerosol is generated by atomizing a solution of PMO precursors, surfactant, acid catalyst, and water in a chosen organic solvent. During aerosol transport, vacuum assists the evaporation of the solvent to induce self-assembly of themesostructure inside sub-micrometer aerosol droplets at room temperature. The aerosol is then forced through a specially designed converging nozzle to facilitate its deposition on a spinning substrate to form thin films with uniform surface coverage. After aging at room temperature, the as-synthesized films are heated under a nitrogen flow to remove templates. The structural, dielectric, and mechanical characterization of the thin films are subsequently carried out, and the results are reported herein. Figure 1 shows the VAAD system. It consists of an atomizer (TSI Model 3076), a transport tube, a deposition chamber with a spinning stage and a vacuum system. To achieve thin-film deposition, a de Laval nozzle design is used in order to accelerate the aerosol flow at the nozzle exit. A higher aerosol flow rate and a shorter nozzle-to-substrate distance (usually 0.5–1mm) significantly increase the deposition rate and decrease the deposition time from a few minutes to 10–20 seconds. The solution preparation is based on the previous spin-coating procedure with an increased amount of ethanol so as to extend the window of optimum deposition time.

Fullerene-organic nanocomposite: A flexible material platform for organic light-emitting diodes

CuPc:C60 organic-nanocarbon composite coated metals (Au, Ag, and ITO) are found to form efficient hole injection anode structures for organic light-emitting diodes (OLEDs). A significant increase (∼ two times ) in current efficiency has been observed in OLEDs when the nanocomposite anode structures are used to replace the conventional CuPc/indium tin oxide hole injection structure. Moreover, the composite anode structures enable the use of simple metal electrodes for efficient and stable OLEDs. The composite provides, through a controlled variation in the C60 concentration, a flexible material platform in regulating the hole injection and transport through the various layers in an OLED.

Interaction between organic semiconductors and LiF dopant

X-ray photoemission spectroscopy and optical absorption have been used to study the interaction between LiF dopant and various organic semiconductor hosts including N,N′-diphenyl-N,N′-bis (3-methylphenyl)-1, 1′-biphenyl-4, 4′-diamine (TPD), N,N′-bis (1-naphthyl)-N,N′-diphenyl-1, 1′-biphenyl-4, 4′-diamine (NPB), tris (8-hydroxyquinolinato) aluminum (Alq3), C60, and copper phthalacyanine (CuPc). It was found that there is a charge transfer between host and dopant in the Alq3–LiF and C60–LiF systems, while no such charge transfer was observed for the TPD–LiF, NPB–LiF, and CuPc–LiF systems. The experimental data show that F− anion acts as an n-type donor—donating electron charge to the electron transport molecules.

Water Repellent Periodic Mesoporous Organosilicas

This paper demonstrates for the first time thermally induced gradual hydrophobization, monitored quantitatively by ellipsometric porosimetry, of four prototypical periodic mesoporous organosilicas (PMOs) that are tailored through materials chemistry for use as low-dielectric-constant (low k) materials in microprocessors. Theoretical aspects of this quantification are briefly discussed. A comparison of structural, mechanical, dielectric, and hydrophobic properties of ethane, methane, ethene, and 3-ring PMOs is made. Particularly, ethane, methane, and 3-ring PMOs show impressive water repellency at post-treatment temperatures as low as 350 °C, with corresponding Youngs modulus values greater than 10 GPa and k values smaller than 2, a figure of merit that satisfies the technological requirements of future generation microchips.

Fullerene-doped hole transport molecular films for organic light-emitting diodes

C60-doped N,N′-bis(l-naphthyl)-N, N′-diphenyl-1, 1′-biphenyl-4, 4′-diamine(NPB) film is studied as hole injection layer between indium tin oxide (ITO) and NPB. The doped films on ITO substrates were found to be thermally stable after being annealed at temperatures up to 120°C. This was attributed to a strong interaction between NPB and C60 providing a dipole force crosslinking NPB molecules, similar to a crosslinked thermoset polymer networks. Furthermore, the C60-doped NPB p-type hole injection layers yield devices having better efficiency and low driving voltage as compared with standard devices with CuPc as the hole injection layers.

Enhanced thermal stability in organic light-emitting diodes through nanocomposite buffer layers at the anode/organic interface

The effect of doped buffer layers at the anode/organic interface in small molecule organic light-emitting diodes was investigated. Appropriate doping of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) and Cu-phthalacyanine (CuPc) layers using LiF or C60 molecules leads to improved interfacial morphology and thermal stability for both standard indium tin oxide or metals anodes, such as Au and Ag. Graded interfaces remain stable at temperatures well above the hole transport layer (i.e., NPB) glass transition temperature.

Organic light-emitting devices with silicon anodes

Both n and p-type Si(100) surfaces, treated by various methods, have been investigated as electrodes for hole injection in organic light-emitting devices. It was found that the Fermi level of silicon dictates the device characteristics. The surface Fermi level, which can be varied by doping and surface states, has been found to be a good reference level for controlling the hole injection barrier.

Passivation effect of Al∕LiF electrode on C60 diodes

The current–voltage characteristics, the temperature dependence of the dark conductivity, and the effect of oxygen exposure of C60 sandwich diodes are compared with Al and Al∕LiF as electrodes. It appears that a thin LiF interlayer can help to preserve the space-charge limited conduction in C60 diodes under exposure to air, by considerably suppressing the oxygen diffusion into the C60 film and reaction at the Al∕C60 interface.

Oxidation of LiF –Coated Metal Surfaces Multilayer Cathode Structures as Used for Organic Optoelectronics

X-ray photoelectron spectroscopy was used to study the growth of oxides on the surface of Al and Mg films with and without a thin LiF coating under ambient conditions. At thicknesses typically used in optoelectronic device cathodes, LiF does not completely cover the surface, likely forming islands on the metal surface. On Al, 10 A LiF (61% coverage) is sufficient to significantly decrease oxidation. The passivation of Al surfaces is enhanced due to a diffusion dominated oxidation mechanism, with metal ions diffusing through the LiF islands. LiF coated Mg, on the other hand, shows preferential oxidation to form MgCO 3 on the surface. These changes in the oxidation of the surface due to the introduction of a LiF layer can be used to explain the recent results for organic light-emitting devices. Bulk lattice constants can be used as a guide to predicting oxidation resistance, with matching interlayers providing better resistance in devices than nonmatching ones.

Superluminescent organic light-emitting diode with a novel anode structure

A novel anode structure comprising a nanocomposite and metal, has enabled highly efficient and stable superluminescent organic light-emitting diodes (SOLED). For C545T singlet green emitter, SOLED can reach 33 cd/A at 1000 cd/m2, doubled the efficiency as comparing to conventional devices with ITO/CuPc structure. More importantly, the SOLED can still hold 24 cd/A at 50000 cd/m2, indicating a highly efficient hole injection capability at ultra-high brightness. In addition, the simulated electronluminescent spectra with angle dependence, agree with experimental results. It is expected that SOLED might find wide applications, not only in display but in general lighting or ultra-high brightness application, by replacing the problematic ITO anode.