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T. Leoni, T. Lelaidier, A. Thomas, A. Ranguis, O. Siri, C. Attaccalite and C. Becker
Chemical reactions by means of an STM:
a new route to synthesize Tetraazapentacene
Unsubstituted Tetraazapentacene (TAP) is a molecule isostructural analog of pentacene, resistant to photo-oxydation with a strong internal dipole.
Different theoretical studies highlight its remarkable characteristics for applications in organic electronics. Moreover the absence of substituent in the molecule favours an optimal arrangement in the solid state. However there is an open challenge on TAP, how can we synthesize this molecule? Here we propose a new route to synthesize TAP by using the electric field generated by an Scanning Tunneling Microscope (STM).
Introduction
Experimental setup
Molecule identification
Conclusions
Commercial
STM/nc-AFM 5 K – 78 K
The DHTAP molecule was deposited on a Au(111) surface. The Au(111) single crystal was prepared by repeated cycles of sputtering and annealing at 700 K. DHTAP was thermo-evaporated from a quartz crucible heated to 500 K and deposited on the Au (111) surface cooled down at 250 K.
From DHTAP to TAP
We start from the DHTAP molecule,
that is commercal available.
Here the reaction path to the TAP
TAP
DHTAP
STM can induce formation or dissociation of chemical bond. These capabilities of the STM have been exploited in particularly to induce dehydrogenation, tautomerization or hydrogenation of single isolated molecule ii and more rarely on single molecule embedded in self-assembled layer
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DFT Calculations
Experiments
Structures
Synthesis of a Tetra-azapentacene is possible on surface at single molecule level and at larger area by scanning the surface with high electric field.
The electric field could be used as a non local tool to induce chemical reaction at mesoscopic scale
Molecules can be identified by comparing STM images with DFT calculations and studying the gap reduction
In order to identify the observed species, we have undertaken DFT calculations of various two-dimensional crystal composed of two layers of molecules derived from DHTAP. The results allowed us to identify the different species.
A second clue to recognize the molecules was the gap reduction of 0.9 eV, reproduced by means of GW calculations.
Molecule switching
Image STM showing the successive switching of a molecule of DHTAP to MHTAP molecule and then to the TAP molecule. Each switching is induced by the same one second voltage pulse performed at 4.1V.
By means of the STM bias it is possible to align the HOMO level of the molecule with the Fermi level of the substrate. Electrons occupying the HOMO have the possibility to cross the tunnel barrier to populate the first empty states of the substrate.
When this happens the DHTAP molecule is temporarily positively charged. A good pathway to restore its electronic neutrality is to evacuate its positive charge by dehydrogenation.
Vacuum
First Layer
Vbias =0
µtip
µAu(111)
HOMO
DHTAP
0.82eV
eVbias = eVseuil
µtip
µAu(111)
DHTAP+
ε =0.83eV
We have investigated the synthesis of tetraazapentacene (TPA), a very promising analog of pentacene in organic electronics, using a scanning tunneling microscope. In dihydrotetraazapentacene (DHTPA) molecular bilayers deposited on Au(111), we have induced single or double dehydrogenations by applying high electric field between a STM tip and the substrate. The reaction products have been identified by comparison of Density Functional Theory calculations with experimental images of the molecular orbitals and scanning tunneling spectroscopy. The present results point the way to a synthesis of well-ordered layers of a compound which cannot be obtained by conventional organic chemistry.