HYBRID ORGANIC-INORGANIC INTERFACES
People
Valentina De Renzi
Hybrid organic/inorganic systems are ubiquitous in nanoscience and nanotechnology. Indeed, hybrid interfaces represent the core of many novel devices, spanning from molecular electronics and nano-sensors, to third-generation dye-sensitized solar cell (DSSC). The understanding of the structural, electronic and magnetic properties of these interfaces is therefore a key issue in nanoscience. The charge transport properties of such hybrid systems are determined to a large extent by energy level alignment across the interface. A detailed comprehension of the mechanisms of energy level alignment is therefore fundamental for an effective tailoring and optimization of potentially novel devices. We addressed these issues investigating the electronic properties of model systems, such as in particular thiol-based self-assembled monolayers (SAM)[4,7], by means of X-ray and Ultraviolet Photoemission Spectroscopies (XPS and UPS), in strict collaboration with theoretical groups. More recently, we have driven our attention to the characterization and understanding of dye/metal-oxide interfaces, which lies at the core of third generation DSSC systems.
Our further interest regards the investigation of self-assembling at surfaces. In any attempt to engineer hybrid interfaces, an important goal is to gain control on non-covalent interaction mechanisms, with particular regards to Hydrogen bonding. In most cases, the formation of H-bond can be inferred from the evaluation of inter-molecular distances, carried out by structural probes. On the other hand, vibrational spectroscopy in the terahertz (0-100 cm-1) region, can provide a direct characterization of the intermolecular bond, as recently shown in the case of 3D systems.
High-Resolution Electron Energy Loss Spectroscopy (HREELS) represent a promising tool for investigating this aspect in 2D systems, thanks to its high surface sensitivity and ability to detect very low energy vibrational modes. As a model system, we studied Cysteine SAM on Au(111), proving that the low-energy collective modes are indeed extremely sensitive to the molecular local environment, and that their frequency variations can be used to characterize H-bond networking[6].
Besides SAMs on metal surfaces, our interest has been also recently devoted to investigate the functionalization of silicon surface with multifunctional moieties. In particular, we studied N-allylurea adsorption on the Si(111)-(7x7) surface by means of combined XPS, UPS and HREELS measurements, and demonstrated that different adsorption mechanisms take place at this interface, competing with and mutually influencing each other [1].
Furthermore, part of our activity is also devoted, in collaboration with the Nano-magnetism activity of S3 (see here for more details) to investigate the structural, electronic and magnetic properties of single-molecule magnets on metal surfaces[2,3,5].
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Fig 1: Schematic representation of the electrostatic potential at the metalmolecule interface (panel a) [4,7] and at the metal-oxidedyeelectrolite interface in DSSC (panel b). Ultraviolet Photoemission Spectroscopy investigations are able to provide a quantitative evaluation of the quantities which determine energy level alignment across these interfaces, i.e., the dipole layer potential ΔΦ= ???/?0 (where ? is the molecular layer density and ?? is the component perpendicular to the surface of the molecular dipole moment), the work function change ΔWF, the band bending potential Vbb. Note that the sign of the dipole layer potential ΔΦ depends on the orientation of the molecular dipoles ? and is therefore strongly affected by the adsorption geometry.
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Fig 2: We investigated the structural and electronic properties of dimethyl-disulfide (DMDS) and methylthiol (MT) adsorbed on the Au(111) surface. Dosing DMDS at low temperature results in a weakly-bound layer of intact DMDS molecules. By increasing the temperature above 250 K, DMDS dissociate and a chemisorbed MT layer is formed. These ?ndings provided us with a unique route for investigating the evolution of the interface electronic properties upon switching from weak-adsorption to chemisorption by simply increasing the temperature. In the above figure, UPS spectra of the DMDS/Au(111) surface are reported as a function of annealing temperature. The vertical lines indicate the position of a molecular state localized on the methyl group (M-state), whose binding energy shifts with annealing temperature. In the inset, the temperature evolution of the M-state binding energy (?lled squares – right axis) and the corresponding work function variation (empty squares – left axis) are compared, demonstrating that the M-state energy is affected by the work function change induced by the chemisorption process [4,7].
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Fig 3: Among fundamental amino acids, Cysteine (Cys) is currently attracting special attention as it provides the molecular hook between several biomolecules and metal surfaces and in the study of enantioselectivity processes at surfaces. We have investigated the Cys/Au(111) interface as a model system in which H-bond molecular interactions could plays a fundamental role. The vibrational properties of the system has been studied on passing from the quite heterogeneous layer deposited at RT to the more homogeneous and ordered interface obtained for a compact layer at 330 K (HT). An extremely low-frequency feature (H mode) is observed in the HT spectrum (blue curve) at 74 cm-1, and assigned to a collective mode of a highly organized H-bonding network. Most interestingly, the H peak is absent in the RT spectrum (green curve), showing that it is extremely sensitive to the heterogeneity and disorder of the adlayer. Moreover, in the 0.5 ML spectrum (red curve), the H mode frequency is red-shifted by 19 cm-1 with respects to the 1.0 ML spectrum. This shift is attributed to the differences in the packing and/or relative orientation of adjacent molecules for different coverage. [6]
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Publications
Competing Pathways in N-allylurea Adsorption on Si(111)-(7x7)
V. De Renzi, G. Arnaud, and U. del Pennino
submitted to J. Phys. Chem. C (2011)
Self-Assembled Monolayer of Cr(7)Ni Molecular Nanomagnets by Sublimation
A. Ghirri, V. Corradini, V. Bellini, R. Biagi, U. del Pennino, V. De Renzi, J. C. Cezar, C. A. Muryn, G. A. Timco, R. E. P. Winpenny, and M. Affronte
ACS NANO 5, 7090 (2011)
Addressing the magnetic properties of sub-monolayers of single-molecule magnets by X-ray magnetic circular dichroism
F. Moro, V. Corradini, M. Evangelisti, R. Biagi, V. De Renzi, U. del Pennino, J. C. Cezar, R. Inglis, C. J. Milios, and E. K. Brechin
NANOSCALE 2, 2698 (2010)
Understanding the electronic properties of molecule/metal junctions: The case study of thiols on gold
V. De Renzi
Surf. Sci. 603, 1518-1525 (2009)
Grafting derivatives of Mn-6 single-molecule magnets with high anisotropy energy barrier on Au(111) surface
F. Moro, V. Corradini, M. Evangelisti, V. De Renzi, R. Biagi, U. del Pennino, C. J. Milios, L. F. Jones, and E. K. Brechin
J. Phys. Chem. B 112, 9729 (2008)
Very low energy vibrational modes as a fingerprint of H-bond network formation: L-cysteine on Au(111)
V. De Renzi, L. Lavagnino, V. Corradini, R. Biagi, M. Canepa, and U. del Pennino
J. Phys. Chem. C 112, 14439 (2008)
Metal work-function changes induced by organic adsorbates: A combined experimental and theoretical study
V. De Renzi, R. Rousseau, D. Marchetto, R. Biagi, S. Scandolo, U. del Pennino
Phys. Rev. Lett. 95, 046804 (2005)