Ultrathin-film differential-thermal-analysis apparatus with simultaneous photoemission measurements

Abstract

We developed an apparatus of differential thermal analysis (DTA) capable of simultaneous surface specific ultraviolet (UV) photoemission measurements to investigate thin-film phase transitions. The apparatus was installed in a vacuum chamber of 10−6 Torr range for thermal isolation and the measurements of UV photoemission. As a sample substrate, we used a thin (10 μm) copper sheet supported by two wires for optimal thermal resistivity. The performance of the apparatus was examined using a 650-Å-thick pentacontane (n-C50H102) film, which may exhibit a unique monolayer phase transition known as surface freezing. We observed two anomalies of DTA curve around the bulk melting temperature, one of which is apparently due to the bulk melting. Since the temperature dependence of the surface specific UV photoemission measurements showed corresponding changes in photoemission current, we could conclude that the other phase transition peak originates from the surface freezing effect. This demonstrates that our DTA-UV apparatus is sufficiently sensitive to examine such monolayer phase transitions.

Crystallization and Aggregation Processes of Vacuum-Evaporated TPD Films

“…In the history of OLED development, T g has been used as an index of heat resistance of the material, 17 however, these values are mostly measured from powder samples. Although some trials have been done to measure T g for film samples, 18 at present the data is insufficient. The physical significance of the T g value on vacuumevaporated films is not well understood.…”

Experimental Observations of Surface Freezing

“…The fact that the substance is "self-distilled" in a dry environment in this case strongly suggests that surface freezing is an intrinsic property of n-alkanes, not of impurities. The study of n-pentacontane on copper substrates by Yamamoto et al 52 carried out under vacuum (ultra-low pressure 10 −7 mm Hg) is particularly convincing in this regard.…”

“…52,59,60 These measurements were carried out with much thicker films (d = 65 nm). 52 When the thickness exceeds the range of surface forces, the two interfaces of the film are acting independently, with the two surfaces separated by bulk layer of the liquid (the thick film approximation). A DTA peak arising from bulk freezing was indeed detected together with the one arising from the surface freezing.…”