Molecular layer epitaxy method and compositions

Details Molecular layer epitaxy method and compositions Molecular layer epitaxy method and compositions

2001-10-01

2004-08-31

Truong, Duc (Department: 1711)

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C428S338000, C428S332000, 43, 43, 43, C117S084000, C117S099000

Reexamination Certificate

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06783849

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to molecular monolayers compositions and methods of forming the same.
References
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BACKGROUND OF THE INVENTION
The interest in two dimensional (2D) materials results from the fact that optoelectronics and molecular electronics have become frontier areas of material science (Ulman, 1991). Multilayered organic structures have recently received theoretical. (Lam, et al., 1991) and experimental (So, et al., 1991; Forrest, et al., 1994; Haskal, et al., 1994; Ohmori, et al., 1993; Yoshimura, et al., 1991; Yoshimura, et al., 1992; Donovan, et al, 1994; Donovan, et al., 1993) treatment. Novel and applicable photophysical properties of organic superlattices have been predicted, including enhancement of optical nonlinearities (Lam, et al., 1991; Zakhidov and Yoshino, 1994) and photoelectric transformations (Zakhidov and Yoshino, 1994). Techniques such as organic molecular beam deposition (OMBD) (So, et al., 1991; Forrest, et al., 1994; Haskal, et al., 1994; Ohmori, et al, 1993) have already proved the capability of growing ultrathin layers having the quality of inorganic quantum well (QW) structures.
A number of interesting optical and photophysical phenomena have already been found in OMBD derived organic QW, including the observation of exciton confinement in photoluminescence (PL) (So, et al., 1991; Forrest, et al., 1994) and electroluminescence (EL) and electric field modulation of PL (Ohmori, et al., 1993). Preparation of crystalline thin organic films by the OMBD relies on the bonding of molecular layers via weak van der Waals foxrces to achieve and preserve quasi-epitaxial structures (Forrest, et al., 1994). Thus, perfect monolayers without step edges cannot be achieved and the lower limit is an average of three “monomolecular” layers. A solution to these limitations can be found in another ultrahigh vacuum (UHV) technique: molecular layer deposition (MLD) (Yoshimura, et al., 1991; Yoshimura, et al., 1992). MLD demonstrated the construction of quantum dots and quantum wires and the potential use of functionalized organic precursors to form alternating multilayered structures. This approach is similar to (inorganic) atomic layer deposition (ALD) (Pessa, et al., 1981) and the solution analog-molecular self-assembly (MSA) (Ulman, 1991).
A solution derived method to produce 2D layered structures, the Langmuir-Blodgett (LB) technique, yields films exhibiting anisotropic electron transport (Donovan, et al., 1994) and tunneling (Donovan, et al., 1993), again suggesting QW behavior. While the LB method is useful in achieving 2D multilayered physiadsorbed structures, LB films suffer from low chemical and thermal stability and cannot incorporate large chromophores without phase-segregation and micro-crystal formation. Alternatively, the trichlorosilane-based MSA approach
1
provides the advantages of strong chemiadsorption through Si—O bonds, chemical and thermal stability, and the ability to form noncentrosymmetric structures (Yitzchaik, et al., 1993; Li, et al, 1990). Vapor phase growth techniques (Kubono, et al., 1994) such as vapor deposition polymerization (VDP) of thin, films was recently demonstrated for aromatic polymers such as polyimides (Maruo, et al., 1993), polyamides (Takahashi, et al., 1991), polyureas (Wang, et al., 1933), and polyether-amines (Tatsuura, et al., 1992). In the VDP process, two types of monomers are evaporated onto the surface of a substrate in a vacuum chamber. Condensation or addition polymerization, then takes place between deposited monomers to produce thin polymeric films. Thin polymer films of high quality and uniformity can be fabricated by this process (Maruo, et al., 1993; Takahashi, et al., 1991). Thermally stable piezo- and pyro-electrical properties were found in poled samples (Wang, et al., 1993). Moreover, electric field assisted VDP (in situ poling of hyperpolarizable monomers) was employed to fabricate electro-optic polymer waveguides (Tatsuura, et al., 1992).
SUMMARY OF THE INVENTION
In one aspect, the invention includes a method of forming a multilayered structure composed of two or more discrete monomolecular layers, where at least one layer is composed of molecules of a selected polycyclic aromatic compound having a defined axis oriented substantially upright with respect to the plane of the monolayer, e.g., normal to the plane, or within up to 54° of normal. The said axis is typically the molecule's z axis, namely, the longest axis of the molecule. The method includes depositing molecules of a selected aromatic compound, preferably a polycyclic aromatic compound, having a defined axis with a chemically reactive group at each axial end, by vapor phase deposition, onto a substrate having surface-reactive sites capable of reacting with the chemically reactive group in the selected compound. The deposition step is carried under conditions which allow chemiadsorption of the selected compound in a molecular monolayer, by covalent coupling of one end of the compound to the substrate, and sublimation of non-covalently bonded compounds from the surface. There is formed by the deposition step a monomolecular layer of the selected compound. For some applications of the multilayered structures of the invention it may be preferable that the monolayer formed by the said deposition is characterized by in-plane compound ordering. These steps are carried out one or more times, where the monomolecular layer formed at each deposition cycle forms a ne

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