Electron Transfer through Organic Monolayers Directly Bonded to Silicon Probed by Current Sensing Atomic Force Microscopy:  Effect of Chain Length and Applied Force

Abstract

Electron transfer through organic monolayers directly bonded to a silicon surface has been investigated by using a current sensing atomic force microscope (CSAFM). The research system employs a platinum-coated CSAFM tip in point contact at a confined nanometer size with a set of alkyl monolayers of various chain lengths, which are covalently bonded directly to an n-type Si(111) by immersing a H-terminated silicon substrate in neat CH 2 dCH-(CH 2 ) n-3 CH 3 (n ) 12, 14, 16, and 18) under heating. The current flows were analyzed with a modified electron-tunneling model and showed strong force and chain length dependences. Increasing the applied force resulted in a negative shift of the current-voltage (I-V) curves. This observation was mainly explained by the amplified contact area and shortened tunneling distance. In addition, the I-V curves showed chain length dependence, from which the bias-dependent electron tunneling coefficient, , was analyzed.

Length‐Dependent Conductance of Molecular Wires and Contact Resistance in Metal–Molecule–Metal Junctions

“…The different contact types also affect the characteristics of the I-V curves; for example, carbon nanotube devices have shown different I-V characteristics at negative and positive bias depending on the contacting metal. [47] All these facts indicate that the contact plays a key role in the determination of the electron-transport mechanism, [12] and is easily influenced by many other stochastic factors. One major influence comes Molecular wires are covalently bonded to gold electrodes-to form metal-molecule-metal junctions-by functionalizing each end with a ÀSH group.…”

“…[48] The effect of the loading force between the tip and the self-assembled monolayer (SAM) surface in STM or C-AFM measurements is also important; [9,12,14,15] for example, loose physical contacts, induced by slight tip compression, can impede direct electron tunneling and lead to dielectric breakdown. [12,[49][50][51] Considering the above factors, the data of the contact resistance of molecular junctions obtained from different experimental approaches are rarely consistent. Therefore, the contact between organic molecules and electrodes is an important issue that needs clarification.…”

Surface nanopatterning through styrene adsorption on Si(100)

“…6 However, since the surface reactivity is essentially ruled by the presence of dangling bonds at surface, most experiments were done using the hydrogen terminated Si substrates. [7][8][9][10] Such surface passivation at the same time prevents spurious oxidation processes and induces a barrier for chemisorption, so that further dynamical mechanisms are required to promote the reaction. This was the case, e.g.…”

Monolayer formation of hydrocarbons with various reactive groups via photochemical reaction on Si(111)-H surface

“…On the other hand, the covalent bond formation on Si-H surface inhibits the oxidation of Si and adds the new functionality [2]. The modification of Si without oxidized layer of SiO 2 can be utilized for the molecular level electrical circuit [3].…”