Schematic illustration of growth and in-situ observation chamber. TTF and TCNQ molecular fluxes were evaporated from individual crucibles. A QCM was placed on the TCNQ flux side. Thin film electrodes of Cr/Pt formed on the glass substrate were connected to a voltage supply and protection resistance (Rp). The top of the chamber has a large quartz window specially designed for this experiment. From the ambient atmosphere side, crystal growth was observed by a digital microscope that has a working distance of 25 mm. Infrared and ultraviolet wavelengths of the epi-illumination light were cut by optical filters so that they did not affect crystal growth of TTF-TCNQ.
Plots showing growth conditions: (a) TCNQ flux rate monitored by the QCM; (b) temperature of TTF crucible; and (c) substrate temperature. Before starting growth, the substrate temperature was kept above 44 °C to prevent contamination. After the TCNQ deposition rate had stabilized, the TTF crucible temperature was increased to approximately 62.4 °C, while the substrate temperature was rapidly reduced to 43.2 °C. This gives quasi-thermal equilibrium conditions suitable for field-assisted growth; they were empirically optimized. After 30 min, the substrate temperature was gradually reduced toward off-thermal equilibrium.
Ex-situ optical micrograph of TTF-TCNQ crystals grown on Pt thin film electrodes. The viewing side is the substrate surface in this micrograph, namely, the observation configuration is different from Figs. 1 and 4 . In the lower half of this micrograph, the electrode surface is bare Pt, whereas in the upper half of the Pt electrode, the top surface is covered with a thin Al layer (as illustrated in the inset). After crystal growth, the lower half of the electrode is fully covered with grown TTF-TCNQ crystals. In contrast, few crystals grow on the top surface in the upper half of the electrode. Preferential TTF-TCNQ growth occurs on the side surface (i.e., bare Pt surface) in the Al/Pt region relative to that from the top surface. An Al or Cr thin cap layer on the top surface of a Pt electrode suppresses TTF-TCNQ growth on the top surface. This micrograph clearly shows that different metal surfaces have different nucleation TTF-TCNQ probabilities.
Optical micrographs near the tip of the designed electrode and calculated electric field distribution. (a) Optical micrograph of designed electrode. (b) Electric-field distribution calculated by finite element method. (c)–(f) Time variations of in-situ observation images during growth. Arrows indicate TTF-TCNQ crystals. Insets: magnified view of the TTF-TCNQ crystals after proper image processing.
Time evolutions of length of grown crystals in (a) AHF, (b) ALF, (c) CLF, and (d) CHF regions. The length of different crystals observed in one in-situ observation is shown in different marks. These regions clearly have different incubation times and initial growth rates.
Schematic illustration of vapor phase growth process of TTF-TCNQ crystal under an electric field. TTF and TCNQ molecular fluxes are uniform over the observation area. Under quasi-thermal equilibrium conditions, TTF and TCNQ thermally diffuse on the substrate surface and temporarily form a TTF-TCNQ charge-transfer complex and dissociate into TTF+1 and TCNQ−1. Charged TCNQ molecules drift along the electric field and are concentrated near the anode, which contribute to the electric-field-induced selective growth. In contrast, the contribution of charged TTF is lower than that of TCNQ because of the high desorption probability from the substrate surface.
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