Covalent Anchoring of Re₆Seⁱ₈ Cluster Cores Monolayers on Modified n- and p-Type Si(111) Surfaces: Effect of Coverage on Electronic Properties

Abstract

The electronic properties of redox-active transition metal clusters (Re 6 Se 8 ) covalently immobilized on modified Si(111) surfaces through linear alkyl spacers have been studied as a function of the cluster coverage (1 × 10 13 -6 × 10 13 cm -2 ). The latter is controlled by using Si(111)/H surfaces modified by dense mixed alkyl/ acid-terminated monolayers with variable fraction of the acid grafting sites from 5 to 100% in solution. Quantitative X-ray photoemission analysis, spectroscopic ellipsometry, and scanning tunnelling microscopy indicate a covalent attachment of a submonolayer to densely packed monolayer of Re 6 Se 8 clusters, while the vibrational Raman signature confirms the cluster integrity within the monolayer. Electrical band gaps as deduced from scanning tunnelling spectroscopy have been obtained for low Re 6 Se 8 cluster coverage. Using ultraviolet photoemission spectroscopy, electronic properties such as ionization potential changes and energy level alignments at organic/inorganic interfaces are studied. We show that the lowest unoccupied molecular orbital of the Re 6 Se 8 cluster is close to the bottom of the Si conduction band. At high cluster coverage, this affects the current-voltage characteristics measured using a weakly interacting top mercury contact onto the organic monolayer/silicon junctions. Indeed, on n-type silicon, the high level current at low bias and the shape of the conductance G(V) curve indicate a Schottky barrier height lowering. On the other hand, the current-voltage characteristics are the same for both acid-terminated and low coverage Re 6 Se 8 cluster junctions at low bias; the high Schottky barrier height limits the current at low bias. When the forward bias increases, the current is tunnelling limited. As expected from the band alignment deduced from photoemission data, the opposite behavior is obtained on p-type silicon.

Red-NIR luminescence of Mo6 monolayered assembly directly anchored on Au(001)

“…These nanometric grains are attributed to individual [Mo 6 Br 8 (NCS) 6 ] 2− grafted cluster as already observed in similar systems prepared on Si(111) surfaces. 23,37 Preliminary scanning tunneling spectroscopy (STS) measurements on the obtained [Mo 6 Br 8 (NCS) 6 ] 2− self-assembled monolayer demonstrate an electronic bandgap of 3.5 eV with a system Fermi level located in the middle of the bandgap ( Figure S6, ESI † ).…”

Temperature dependence of current density and admittance in metal-insulator-semiconductor junctions with molecular insulator

“…The alkyl molecular coverage (R ML ) and unintentional silicon oxidation were characterized by XPS after a few minutes exposure to the ambient, 30 inelastic mean free path value k OML ¼ 3.5 nm (for Si2p photoelectrons) typical of a dense molecular layer. 32,33 C. Spectroscopic ellipsometry SE experiments were performed in the range from 1.0 to 4.7 eV, at an incidence angle of 70 , using a Horiba (UVI-SEL) ellipsometer and analyzed with a multilayer model.…”

“…7-8 ml of alkene reactant, previously deoxygenated at 100 C for 2 h. Dodecyl monolayers were prepared from the photochemical reaction at 300 nm for 3 h of Si(111):H with 1-dodecene (puriss, >99%, Fluka). 29,30 The covalently modified surface was rinsed with distilled CH 2 Cl 2 , then dipped in hot acetic acid at 65 C( 2Â 20 min), 31 and dried under an Ar stream before characterization by XPS, SE, and electrical transport measurements.…”

Entropy effects in the collective dynamic behavior of alkyl monolayers tethered to Si(111)

“…Covalent immobilization of mixed alkyl/acid OML Covalent OML grafting was performed on hydrogen-terminated Si(111):H surfaces using linear alkene molecules with a UV-assisted liquid phase process [40,64]. A low-doped n-type Si (phosphorus doped, 1-10 Ω•cm resistivity, Siltronix) was chosen to obtain rectifying junctions.…”