Controlling chemical reactions beyond thermally activated reaction schemes can open alternative reaction channels, and thus lead to new final products. Herein, we show for tetrahydrofuran (THF) cleavage on Si(001), the surface analogue of an SN2 reaction, that excitation by electrons from the tip of a scanning tunneling microscope (STM) not only opens new reaction channels, but that different final products can be selectively addressed by the type of excitation: Above a threshold voltage of 2.5 V, direct excitation by electron transfer into the antibonding C−O orbital of the THF molecules induces ether cleavage of the datively bonded intermediate of THF on Si(001). Below the threshold, ether cleavage is induced by multiple excitation of vibrational modes. In both modes of excitation, additional final configurations were observed when compared to the thermally activated reaction. The branching ratios of the final configurations are different for the two different excitation mechanisms, which in turn can be controlled by the applied sample bias.
Tip-induced hopping
of the ethyl fragment (−C2H5) of diethyl
ether molecules reacted on Si(001) was
shown to be a field-driven process. Although the hopping rate increases
continuously with increasing bias voltage, it remains constant when
varying the tunneling current. No hopping events are observed at 50
K. The process is thus concluded to be thermally activated with the
respective energy barrier being reduced by the applied electric field.
At a positive sample bias, the field in the tunneling gap is strong
enough to effectively depolarize and thus weaken the covalent Si–C
bond. The effect of this depolarization on the hopping barrier is
quantified and compared to the strength of the electric field.
Controlling chemicalr eactions beyond thermally activated reaction schemes can open alternative reaction channels,a nd thus lead to new final products.H erein, we show for tetrahydrofuran (THF) cleavage on Si(001), the surface analogue of an S N 2reaction, that excitation by electrons from the tip of as canning tunneling microscope (STM) not only opens new reaction channels,b ut that different final products can be selectively addressed by the type of excitation: Above athreshold voltage of 2.5 V, direct excitation by electron transfer into the antibonding CÀOo rbital of the THF molecules induces ether cleavage of the datively bonded intermediate of THF on Si(001). Belowt he threshold, ether cleavage is induced by multiple excitation of vibrational modes. In both modes of excitation, additional final configurations were observed when compared to the thermally activated reaction. The branching ratios of the final configurations are different for the two different excitation mechanisms,which in turn can be controlled by the applied sample bias.
Atomic-scale chemical modification of surface-adsorbed ethyl groups on Si(001) was induced and studied by means of scanning tunneling microscopy. Tunneling at sample bias >+1.5 V leads to tip-induced C–H cleavage of a β-hydrogen of the covalently bound ethyl configuration. The reaction is characterized by the formation of an additional Si–H and a Si–C bond. The reaction probability shows a linear dependence on the tunneling current at 300 K; the reaction is largely suppressed at 50 K. The observed tip-induced surface reaction at room temperature is thus attributed to a one-electron excitation in combination with thermal activation.
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