Nanostructured surfaces
The origin of STM-contrast anomalies of n-alkanes on graphite
Kutsenko V.Ye., Vasko A.A., Marchenko А.A.
Institute of Physics of the NAS of Ukraine. Avenue Nauky, 46, Kyiv-03039, Ukraine. 
E-mail: v.e.kutsenko@gmail.com

It is well known that ordered monolayers of n-alkanes (CnH2n+2, hereafter Cn) possess a “rectangular” lamella-furrow structure on the graphite surface. Distinctive features of these structures are: (i) main axes of molecules are aligned on <100> direction of (0001) basic face of graphite, (ii) alkyl chains are parallel to the (0001) face and (iii) CH2-groups are located above centers of graphite elementary cell. 

Fig. 1. STM-image of (a) – C50, (b) – C60, (c) – C48, (d) – C34 on graphite. 

Paradigm describing packaging of n-alkanes is based on the use of a “conventional” STM-contrast (e.g. contrast of C50 monolayer Fig. 1.a). This contrast is characterized by the fact that differential dI/dz-signal from lamellae is higher than the one from furrows. Also it accepts that alkyl chains are rigid and a mismatch between periods of chain (Talk ~ 0.251nm) and graphite (Tgr ~ 0.246nm) is negligible. 
We have obtained STM-images of long-chain n-alkanes with distinctive  peculiarities of STM-contrast such as monotonous modulation of brightness along alkyl chains (Fig. 1.b-d), increased brightness of the terminal CH3-groups (Fig. 1.b), sharp brightness jumps (Fig. 1.c), absence of furrows between lamellae (Fig. 1.d). Such peculiarities demonstrate that, in fact, alkyl skeletons are more likely to be a set of CH2-nodes, linked by elastic springs, so molecules can stretch, contract, bend and rotate. Moreover, graphite substrate imposes its own period on monolayer causing molecules compression and, therefore, STM-contrast anomalies listed above.