Thin Film of Conjugated Schiff-Base as Ultrahigh Density Data Storage Material
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such
as
cyanuric acid
melamine-7,7,8,8,tetracyanoquinodimethane
(TCNQ)
complex or Bis[2-butene-2,3-dithiolato(2-)-S,S']nickel (BBDN)-TCNQ complex, achieving data storage density of 4x l0 7bits/cm 2 [4,5]. In our laboratory, thin films from a complex of an electron acceptor, m-nitrobenzal malononitrile and a donor, 1,2-benzenediamine, were used as recording media [7,8]. Marks of 1.3nm in size were obtained with STM under ambient conditions. I-V characteristics of the film suggested that the recording mechanism be due to the conductance change of the films from insulating to conducing. In order to further clarify the mechanism and functionality process of such kind of recording media, we prepared a conjugated schiff-base N-(3-nitrobenzylidene)-p-phenylenediamine (NBPDA) with both electron donor and acceptor group located in a single molecule at opposite sites simultaneously. Recording process was successfully carried out to make marks of 1.4nm in diameter by applying voltage pulses between the STM tip and the substrate film. NBPDA crystal and film structure were characterized by IR, UV-Vis, XRD, STM and verified by DFT quantum chemical calculation. The possible mechanism of the recording process were tentatively discussed. EXPERIMENTS Materials and Thin Film Preparation To whom the correspondence should be addressed. 891
Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
N-(3-nitrobenzylidene)-p-phenylenediamine (NBPDA) was prepared by condensation reaction of equal molar amounts of 3-nitrobenzaldehyde with 1,4-benzenediamine, according to literature [9]. Thus, a solution of 0.1mmol 1,4-benzene-diamine in 10ml ethyl acetate was added to a solution of 0.1mmol 3-nitrobenzaldehyde in 10ml ethyl acetate. After standing at 0°C for 6 days, deep red crystals were obtained with melting point 146-147°C. Ultrathin films of NBPDA were obtained by a thermal vacuum deposition method. The NBPDA crystals were placed in a glass crucible and heated under 9X 1004Pa and deposited on a substrate kept at room temperature. Recording Experiments by STM Experiments were performed with a CSTM-9100 STM in a constant height mode. Ultrathin films were deposited on freshly cleaved highly ordered pyrolytic graphite (HOPG). The STM tips were 0.25mm in diameter Pt/Ir (80/20) wires snipped with a wire cutter. The recording experiments were carried out by applying voltage pulses between the tip and the substrate. Strcture Characterizations and Quantum Calculation Crystal structure analysis data were collected with an ENTSF-NONLUS CAD-4 diffractometer. FT-IR spectrum for NBPDA crystal pilled with KBr, and micro FT-IR spectrum for NBPDA thin film on HOPG substrate were both taken by a Nicolet Magna-IR 750. UV-Vis spectra were taken by a Shimadzo UV 250. The crystallinity of the thin film was examined by using a Riguku max/2400 X-ray diffractometer employing Ni-filtered CuKa radiation. Sample film was deposited on a glass substrate. Quantum chemistry calculations were performed based on density functional theory (DFT) using Ams
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