Film Growth, by Seeded Supersonic Beams, of Thiophene-Based Oligomers with Controlled Optical, Structural and Morphologi
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INTRODUCTION Thiophene based oligomers are of increasing interest as model system for their optical, electronic and electro -optical properties and as prototype conjugated systems for technological applications [1]. Designing and synthesizing the right molecules is far from being enough for controlling the properties of the desired material[2]. Characteristic and quality of films are strongly affected by degree of order-disorder and by orientation of the backbone of the oligomers that is by how molecules assemble to form the solid [1]. Standard deposition methods show limitation in controlling morphology and structure as well as in producing reliable active layers suitable for devices. The urge for better quality films is pushing to explore growth methods that would guarantee better control on deposition parameters . Vacuum deposition has already allowed several groups to investigate growth under better-characterized conditions improving films properties. There is also a growing awareness of the need to study and control the role of the filmsubstrate interface. In particular it has become possible to grow thin oligothiophene films (less than about 50 nm) oriented and ordered on crystalline substrates (Ag(111), HOPG) [3] and single crystals with a thickness in the range 20 ÷ 100 µm obtainet by growing them for several days [4]. The weak interaction between the molecules and the influence of the substrate control the growth proces s so that classical chemical vapor deposition (where the internal energy of the molecules is relatively large and their orientation is casual) gives rises to films of poor quality [5,6]. The lack of control of amorphouscrystalline character, morphology, thickness and interfaces is pushing in the direction of exploring growth methods which would guarantee better control on deposition parameters . W e have developed a method under ultra high vacuum conditions based on seeding the oligomers in a supersonic molecular beam. In this way we could achieve an
unprecedented control on the properties (energy, momentum, etc.) of the molecules which could be characterized during the deposition process [7]. During the supersonic expansion the oligomer internal degrees of freedom undergo a strong cooling process while the kinetic energy can be increased up to several eV by varying the degree of seeding. Furthermore the strong anisotropy of the oligothiophenes (similarto “stripes”) can be exploited to control the alignment of their backbone along the axis of the beam during the expansion [7,8]. The fine-tuning of the beam’s parameters allows overcoming some of the difficulties present in the growth of oligothiophenes with other methods including standard vacuum deposition.
EXPERIMENTAL The experimental set-up consists of three differentially pumped chambers separated by two skimmers. The hyperthermal supersonic beam source is housed in the first chamber, while the deposition is performed in the third one under ultra high vacuum condition. The source is derived by a previous one designed for producing hype
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