pentacene was deposited on a muscovite mica surface by physical vapor deposition from a knudsen cell in an ultrahigh vacuum chamber .
the cell temperature was about 500 k to realize a deposition rate of 1 monolayer / min .
the mica samples ( 10  10  0.01 mm ) were attached to a steel plate via tantalum wires .
the steel plate was heated resistively , and its temperature was controlled by a ni 
, the temperature of the mica sample could be varied between 100 and 1000 k. for a quantitative determination of the deposited material , a quartz microbalance was used , which was located next to the mica substrate .
the mica substrate was cleaved with adhesive tape prior to installation into the vacuum chamber and subsequently gently sputtered by argon ions .
ten minutes of sputtering with 500 ev ar ions at an argon pressure of 5  10 mbar was sufficient to change the 5a film morphology from needle - like islands , composed of lying molecules , to compact islands composed of standing molecules .
the surface chemical composition was analyzed by auger electron spectroscopy and x - ray photoelectron spectroscopy .
thermal desorption spectroscopy was applied to determine the thermal stability of the pentacene film and the sticking coefficient .
ex situ atomic force microscopy ( nanosurf , easyscan2 ) was used to analyze the film morphology .
 organic thin films have attracted considerable interest due to their applicability in organic electronics . 
 the classical scenario for thin film nucleation is the diffusion - limited aggregation ( dla ) . 
 recently , it has been shown that organic thin film growth is better described by attachment - limited aggregation ( ala ) . however , in both cases , an unusual relationship between the island density and the substrate temperature was observed . 
 here , we present an aggregation model that goes beyond the classical dla or ala models to explain this behavior . 
 we propose that the ( hot ) molecules impinging on the surface can not immediately equilibrate to the substrate temperature but remain in a hot precursor state . in this state 
 , the molecules can migrate considerable distances before attaching to a stable or unstable island . 
 this results in a significantly smaller island density than expected by assuming fast equilibration and random diffusion . 
 we have applied our model to pentacene film growth on amorphous muscovite mica . 
Experimental Methods