SLIDE 1 Key roles of metallo-organic complexes: from photovoltaics materials to enzymatic structures
- P. Giannozzi
- Dip. Chimica Fisica Ambiente, Universit`
a di Udine, Italy, and IOM-Democritos, Trieste
ISM Montelibretti, 12 Novembre 2013 Work done in collaboration with a lot of people (see next slide)
– Typeset by FoilT EX –
SLIDE 2 About this talk
The two subjects presented here:
- 1. Hybrid heterostructures for photovoltaic applications
in collaboration with: G. Mattioli, P. Alippi, F. Filippone, A. Amore Bonapasta (ISM); M.I. Saba, G. Malloci, C. Melis, A.Mattoni, (IOM Cagliari) S. Ben Dkhil,
- A. Thakur, M. Gaceur, O. Margeat, A. K. Diallo, Ch. Videlot-Ackermann, J.
Ackermann (CNRS Marseille)
- 2. Metal-induced aggregation processes in β-amyloids peptides
in collaboration with K. Jansen (DESY), G. La Penna (ICCOM), V. Minicozzi,
- S. Morante, G. C. Rossi, F. Stellato (Roma II)
are quite different but they have something more in common than Zn atoms, DFT simulations, and a collaboration with people from Rome:
- both are joint experimental and theoretical investigations, and
- on the theory side, in both cases complementary theoretical techniques: classical
- r tight-binding MD + first-principle DFT, have been used.
SLIDE 3 New hybrid materials for solar cells
Hybrid photovoltaic cells: organic molecule or π−conjugated polymer acting as dye (light absorber) and electron donor, on inorganic substrate acting as acceptor. Hold great promises for the realization of cheap and high-yield solar cells. Good dye and donor candidates: (on the right) polymers such as P3HT, poly(3-hexylthiophene-2,5-diyl); Phtalocyanines (Pc) (on the left, ZnPc) Good substrate candidate: metal oxide nanoparticles, typically TiO2, with ZnO emerging as alternative material (both are cheap and nontoxic). ZnO is a high mobility wide gap (3.4 eV) material with wurtzite
- structure. On the right, the (1010) surface of ZnO,
the most common surface in ZnO nanoparticles
SLIDE 4 Model systems
In the past, both P3HT/ZnO and ZnPc/ZnO hybrid systems have been proposed and studied. In this work, the idea is to increase the efficiency of such systems by introducing ternary heterostructures such as P3HT/ZnPc/ZnO. Hopefully, they may provide better efficiency via
- Increased optical absorption over a wider spectrum, and
- Reduced electron-hole recombination
Problems for a first-principle theoretical approach:
- Very large supercells (hundreds of atoms) even for simplest model structures
(few layers of a surface, or a very small nanoparticle): big calculations!
- Hard problem in a Density-Functional Theory (DFT) framework, due to
– Long-range dispersion (van der Waals) interactions – Strongly correlated 3d states in Zn (correct energy level alignement is crucial) – Need for reliable (or not too wrong) excited states: band gap, optical spectra
SLIDE 5 Theoretical Methods
Theoretical solutions adopted:
- Model Potential Molecular Dynamics allows relatively quick selection of
potentially stable structures, followed by Density-Functional Theory refinements
- Usage of advanced DFT functionals:
– DFT+U corrects the worst failures of DFT in correlated materials – vdw-DF allows to include van der Waals forces – tests with hybrid functionals to gain confidence in the results
- Usage of Time-Dependent DF(P)T for calculation of optical spectra (good for
molecules, much less so for solids) DFT calculations performed on HPC machines (mostly on the SP at Cineca) using the parallel algorithms of the QUANTUM Espresso distribution.
SLIDE 6
Model P3HT/ZnPc/ZnO: structure, stability
ZnPc on (1010) ZnO surface forms stable layer (Eb = 2.2 eV/molecule) 8-unit P3HT binds with Eb = 0.6 eV/unit to ZnPc/ZnO (vs 0.4 eV/unit to ZnO)
SLIDE 7
Electronic states, energies
CS (charge-separated) states: e− is in ZnO CBM (Conduction Band Minimum), h+ is in molecular HOMO. The ZnPc layer raises P3HT LUMO to a more favorable position for e− transfer to ZnPc and ZnO, improving charge separation at interface
SLIDE 8
Electronic states, localization in space
Electron-hole recombination made less likely by ZnPc layer: e− and h+ densities in charge-separated state are more spacially separated and have smaller overlap
SLIDE 9 Simulated TD-DFPT optical spectra
- A. ZnPc/ZnO absorption: split Q-bands
at 1.7 and 1.9 eV, Soret band at 3.1 eV. B. P3HT/ZnPc/ZnO: superposition
ZnPc/ZnO peaks and
the blue-shifted (2.3 eV) peak of P3HT.
- C. 4-unit P3HT on ZnO: absorption
peak at 2.15 eV. (Contribution from ZnO substrate is subtracted out)
SLIDE 10
Experiments: optical spectra, ZnPc on ZnO
ZnPc on glass: two peaks (Q bands) at 622 nm and 711 nm ZnPc on ZnO: additional peaks due to molecule-substrate interactions appear at 674 nm (blue arrow) and at 742 nm (light blue arrow)
SLIDE 11 Experiments: optical spectra, P3HT/ZnPc/ZnO
ZnPc film thickness: black dots 4 nm, blue dots 15 nm. Up: The spectrum
- f P3HT/ZnPc/ZnO exhibits absorption peaks of P3HT and of ZnPc, plus the
new optical features of ZnPc/ZnO interface. Down: External Quantum Efficiency (EQE) shows that the new band at 674 nm contributes additional photocurrent.
SLIDE 12
Experiments: current density-voltage curves
Measured performances: Voc Jsc PCE no ZnPc 0.71 0.17 0.06 4 nm ZnPc 0.61 0.26 0.09 15 nm ZnPc 0.60 0.07 0.07 Open-circuit voltage Voc in V, short-circuit density current Jsc in mA/cm2, Power Conversion Efficiency (PCE) in %
SLIDE 13 Experiments: transient open circuit voltage decay
Blue: P3HT/ZnPc/ZnO, Red: P3HT/ZnO. Illumination is suppressed with circuit
- pen (no current flowing) and the decay time of carriers is measured.
Carrier lifetime as a function of the open circuit voltage, in the region Voc < 0.48 V, is a measure of recombination in the heterostructure region, showing improved lifetime for P3HT/ZnPc/ZnO.
SLIDE 14 Discussion and conclusions (1)
Theoretical predictions on the ternary P3HT/ZnPc/ZnO system:
- The system is thermodynamically stable
- Light absorption from both P3HT and ZnPc covering a wide spectrum
- Increased charge separation due to ZnPc layer reduces recombination
- The P3HT HOMO is shifted by the ZnPc layer to higher energies, leading to a
reduction of Voc of ∼ 0.1 V. Experimental data on actual samples, produced and measured at CNRS Marseille, confirm all of the above findings.
SLIDE 15 Aggregation of peptides induced by metal ions
Very nasty degenerative illnesses are caused by aggregation of naturally present proteins or peptides into toxic amyloid fibrils and plaques In Alzheimer disease, the main components of plaques are β-amyloids peptides (Aβ): chains
- f 39 to 43 aminoacids, obtained by cleavage
- f a precursor protein
(in the figure: Aβ40 peptide in water) There is experimental evidence that transition metal ions Cu, Zn, Fe play a role in the processes of Aβ aggregation and plaque formation The details of the metal-Aβ binding are thus subject of intense study
SLIDE 16 β-amyloids binding with Cu and Zn: state of the art
- The structure of Aβ binding with Cu is relatively well characterized,
with Cu having a stable intra-peptide coordination
- Aβ binding with Zn is not as clear. Competing structural models
– from XAS: inter-peptide Zn2+ bridge between three or more histidines (His) belonging to different peptides. Rather peculiar and infrequent: hallmark of peptide aggregation? – from NMR: intra-peptide binding to three His and either the N-terminus or a residue (Glu11)
- Competition for peptide binding between Cu and Zn ions likely
Goal of this work: to find, using numerical simulations, realistic configurations for Aβ chains coordinated by Zn2+, fitting XAS results
SLIDE 17 Simulation procedure
- Initial configurations generated with graphical tools (VMD),
- ptimized with Amber force fields and Monte Carlo Random Walk
- Selected configurations truncated (aminoacids 1-10 removed),
- ptimized, set into an orthorhombic cell filled with water molecules,
thermalized with classical MD, optimized with Tight-Binding MD
- Finally, first-principle (i.e. from electronic structure) Car-Parrinello
Molecular Dynamics runs are performed to check the stability and refine the structure of the various binding configurations The last step is by far the most time-consuming, requiring parallel execution on big computer facilities, including the BG/P
(courtesy of DEISA DECI and of John von Neumann Institute for Computing)
SLIDE 18 Choosing the starting configurations
Four good starting models (generated for Aβ16) compatible with XAS data (many more turned out to be bad and were discarded):
Zn bound to four histidines
Zn bound to three histidines
to four histidines
to three peptides
SLIDE 19 Car-Parrinello Molecular Dynamics
Introduce fictitious dynamics on the electronic orbitals φv: L = µ
φv( r)|2d r + 1 2
MI∇2
(µ = fictitious electronic mass), subject to orthonormality constraints
- n the orbitals, implemented via Lagrange multipliers Λij. The above
Lagrangian generates the following equations of motion: µ¨ φi = −δE δφi +
Λijφj MI ¨
RIE[φ, R]
(nuclear motion is classical). These equations can be integrated (i.e. solved) for both electrons and nuclei using classical Molecular Dynamics algorithms. The combined electronic and nuclear dynamics keeps electrons close to the ground state.
SLIDE 20 Technical details
- Perdew-Burke-Erzerhof (PBE) exchange-correlation functionals
- Ultrasoft (Vanderbilt) pseudopotentials with 25 Ry (orbitals) or
250 Ry (charge density) kinetic energy cutoff for plane waves
– S1: 1351 atoms, 21.291×35.193×23.241˚ A3 – S2: 1204 atoms, 21.976×27.947×22.881˚ A3 – S3: 1349 atoms, cell as S1 – S4: 2347 atoms, 32.083×31.01×23.14˚ A3: Tight-Binding MD only, too big for Car-Parrinello MD
- At least 3.6 ps simulation time
SLIDE 21
Results
Structures for S1, S2, S3 models before and after CP-MD. Note the fourth His leaving the Zn site in S1, while in S3 model Zn keeps a stable fourfold coordination
SLIDE 22 Results: a nice cover picture...
PG, K. Jansen, G. La Penna,
- V. Minicozzi, S. Morante, G. C.
Rossi, F. Stellato, Metallomics 4, 156-165 (2012) (S4 model)
SLIDE 23
More serious results: simulated XAS spectra
SLIDE 24
Similar structures in other biological systems?
Comparison of the Zna site in S4 model (green: fit to XAS) with the Zn site in the reduced bovine superoxide-dismutase (SOD) enzyme Only residues involved in binding with Zn are shown
SLIDE 25 Discussion and conclusions
- XAS yields information on short-range structure (up to 5÷6˚
A) only
- First-principle techniques can take into account both the peculiar
chemical binding of metals with peptides and the electrostatic interactions between peptides
- Structures (S3 and S4) in which Zn is bound in a stable ways to
four His have been identified...
- ...but their structure is not trivial, requiring a second Zn and/or a
third peptide chain; in the simpler S1 structure Zn loses a His and a satisfactory XAS fit is not obtained
- S4 model reminiscent of the Zn site of bovine SOD
