Data Storage

Optical data storage and rewritable DVDs

Several Tellurides alloys containing Ge and Sb exhibit a pronounced contrast in the optical absorption of the crystalline and the amorphous state. This phenomenon is the basis for their application in optical data storage e.g. in rewritable DVDs. These two phases also present a profound change in electrical properties 'such as the resistivity change' which is one of the crucial features that would be used in phase change random access memories, a very promising candidate for future non-volatile memories.

Fig.1 Setup to measure electrical switching in PCRAM's

To understand the change in optical properties upon amorphization of the phase change materials (PCM), experimentalists in Germany [1] wanted to correlate a macroscopic measurement like the optical properties with microscopic details like the local atomic structure. Using ab initio calculations on structural models with different local order, theoreticians in Palaiseau node [2,3] proved that the optical contrast between the two phases is due to the change of the number of first neighbours of Ge atom and that it should be possible to adjust the optical contrast by changing the composition of the PCM. These results provide a fundamentally new insight in the physics of the optical absorption of amorphous materials and in addition they represent important contributions to a systematic material optimization of phase change alloys.

This example illustrates how first principles calculations can provide an ideal complementary tool in technological issues.

[1] A. Kolobov, P. Fons, A. Frenkel, A. Ankudinov, J. Tominaga, and T. Uruga, Nature Materials 3, 703 (2004).

[2] W. Welnic, A. Pamungkas, R. Detemple, C. Steimer, S. Blügel, and M. Wuttig, Nature Materials 5, 56 (2006).

[3] W. Welnic, S. Botti, L. Reining and M. Wuttig, Optical contrast in phase change materials, submitted.

Green Fluorescent Protein

Not surprisingly, the theoretical understanding of biophysical processes is a very active field of research. However, and in spite of the large amount of experimental work in photo-active molecules, the theoretical description of the interaction of these molecules with external time-dependent fields is very much in its infancy. Many biological processes rely on a subtle interplay between optical absorption in the photo-active center and its coupling to internal vibrational modes and to the environment (hosting protein and solvent). The most famous of these processes is vision, that is triggered by a photo-isomerization mechanism. Another paradigmatic case is the green fluorescent protein (GFP). This molecule has become a unique tool in various kinds of biomedical research due to its fluorescence and inertness when attached to other proteins.

Several members of the ETFS are currently involved in the study of the processes of light absorption and luminescence of the GFP and its mutants. It is hoped that a better understanding of the basic processes will allow for the design of novel mutants of the GFP or of other completely new chromophores with enhanced properties.

The study of the optical properties of biochromophores has developed into an important and active field of research. The rationale is clear, as the absorption and emission of light by biomolecules are at the center of crucial biophysical processes -- such as vision or photosynthesis, and has led to various important technological applications. Among photo-active proteins, and due to its unique photophysical properties, the family composed by the green fluorescent protein (GFP) and its mutants has attracted a considerable amount of attention during the last decade. These molecules have been used as important and versatile fluorescent markers, with widespread applications in the field of biotechnology. One of the originalities of the GFP resides in the fact that the chromophore responsible for the photophysics of the protein, 4-(p)-hydroxybenzylidene-imidazolidin-5-one, is completely generated by an autocatalytic, post-translational cyclization and oxidation of the --Ser66--Tyr66--Gly67-- triad, without the need of any external cofactor. Thus, all the information needed to synthesize the biochromophore is encoded in the corresponding gene. Furthermore, the GFP can be easily attached to other proteins without changing its own absorption properties. This unique characteristic, due to the protective cage-like secondary structure of the protein, makes the GFP an ideal candidate for a biological marker.

With the widespread use of the GFP, there has been an increasing demand for the ability to visualize different proteins in vivo that require multicolor mode imaging. This has triggered intensive research aimed at the development of GFP-mutant forms with different optical responses. A mutant of the chromophore of particular interest is the Y66H variant, in which Tyr66 is mutated to His. The resultant protein exhibits fluorescence shifted to the blue range, and is for that reasonoften referred to as the blue emission variant of the GFP, or the blue fluorescent protein.

Calculations on the chromophores of the green fluorescent protein, the blue fluorescent mutant (Y66H), and the photoactive yellow protein have appeared recently. Some of these carried out by members of the proponent team. To illustrate the usefulness of these calculations, we take the case of the GFP. The main excitation peaks, calculated using time-dependent density functional theory (TDDFT), for the neutral and anionic forms are at 3.01 and 2.67 eV, respectively, in really good agreement with the measured excitation energies, located at 3.05 and 2.63 eV. Furthermore, the comparison to the experimental spectrum allows for the clear assignment of the measured peaks to either the neutral or anionic forms of the GFP. Finally, from the calculations it is possible to extract a 4:1 ratio for the concentration of the neutral/anionic forms in vivo, which is very close to the estimated experimental ratio of 80% neutral and 20% anionic.

Note that the process of light absorption and emission is quantum mechanical, and it is not therefore, accessible using the classical mechanics techniques usually applied in biochemistry. However, the size of the problem (that involves thousands of atoms), and its complexity forbid the use of pure quantum mechanical approaches. Therefore, the solution of such problems requires a mixture of approaches.

• T. Wilson and J. W. Hastings, Annu. Rev. Cell Dev. Biol. 14 197-230 (1998).
• V. Pieribone and D. F. Gruber, Alone in the Dark: the Revolutionary Science of Biofluorescence (The Belknap Press of Harvard University Press, Cambridge, Massachussetts).
• M.A.L. Marques, et al, Phys. Rev. Lett. 90, 258101 (2003); X. Lopez, et al, J. Am. Chem. Soc. 127, 12329-12337 (2005).

Nano Electronics

Quantum Transport

Since the introduction of the integrated circuit or chip in the sixties, the power of microprocessors has always grown together with the integration scale, following what is called the Moore's law. If such a miniaturization rate will continue, modern electronics will attain its ultimate limit, the molecular and atomic scale, around the year 2015. As a consequence, Electronics at the Nanoscale, namely Nanoelectronics, represents the next years' technological challenge. A bottom-up process instead of a top-down as followed so far, will allow the realization of Electronic Devices made out of Molecules and Nanostructures. And this is boosted not only by the need for shorter integration scales, and device miniaturization, but also by the expectation that unusual quantum effects are going to be observed due to quantum phenomena effects.

Beside the experimental efforts to synthesize nanoelectronic devices, quantum transport theory has the formidable task to understand and to model the mechanisms behind these phenomena and to predict them from a first principles approach.

Several years ago, important progresses have been accomplished in the theory of quantum transport thank to the setup of two frameworks: the Landauer-Buttiker (LB) and Kubo-Greenwood formalisms. These two formalisms rely on theories able to provide the electronic structure of the nanodevices. And these can be the semi-empirical Tight-Binding (TB); or fully ab initio Density-Functional Theory (DFT).

The Landauer-Buttiker on the top of Density-Functional Theory, today to be considered the state-of-the-art, has demonstrated its ability to describe small bias coherent transport in nanojunctions. These approaches were successful in accounting for the contact resistance and conductance degrading mechanisms induced by impurities, defects and non-commensurability patterns in the conductor region.

The theoretical effort and trend today is to move toward theories able to account for non-coherent and dissipative effects due to electron-phonon and electron-electron scattering mechanisms inside the conductor and for non-linear response, far from equilibrium, finite-bias transport. Along these directions, the two major research lines are Time-Dependent Density-Functional Theory (TDDFT) [Runge, E.K.U. Gross, W. Kohn]; and Non-Equilibrium Green's Function (NEGF) theory [Schwinger, Baym, Kadanoff, Keldysh]. Both NEGF and also TDDFT [G. Stefanucci, C.-O. Almbladh] are in principle correct frameworks to address the above objections.

Researchers at the Rhone-Alpes associated ETSF node have demonstrated [Darancet et al], in the framework of NEGF, that a GW approximation on the Self-Energy can introduce diffusion and loss-of-coherence effects due to the electron-electron scattering and giving rise to reduction of conductance and appearance of resistance inside the conductor.

Gold Monoatomic Nanowire:
Differential conductance vs applied voltage

Together with the result of another theory group in Copenhaguen who introduced electron-phonon scattering effects through a self-consistent Born approximation (SCBA), their calculated conductance characteristics as a function of the applied voltage for a gold monoatomic nanowire fully explains all the features present in the experimental measures of Agrait et al.

P. Darancet, A. Ferretti, D. Mayou and V. Olevano, Phys. Rev. B (2007).
T. Frederiksen, M. Brandbyge, N. Lorente, and A.-P. Jauho, Phys. Rev. Lett., 93, 256601 (2004).
G. Stefanucci and C.-O. Almbladh, Europhysics Letters 67, 14 (2004).
N. Agrait, C. Untiedt, G. Rubio-Bollinger and S. Vieira, Phys. Rev. Lett., 88, 216803 (2002).

Solar Cells

Silicon solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 30% or higher with multiple-junction research lab cells. Solar cell energy conversion efficiencies for commercially available crystalline Si solar cells are around 14-16%. The highest efficiency cells have not always been the most economical - for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering about four times the electrical power.

Organic solar cells and polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes. Energy conversion efficiencies achieved to date using conductive polymers are as low as 4-5% for the best cells to date. However, these cells could be beneficial for some applications where mechanical flexibility and disposability are important.

Ternary I-III-VI compounds, members of the chalcopyrite semiconductor family, are promising solutions for the production of economically competitive photovoltaic energy. For example, Cu(In,Ga)(S,Se)₂ compounds reach conversion efficiencies of 22.5%. However, indium is rather rare in the earth crust. This problem limits strongly the expectation of maximum production of solar modules based on this material. As a consequence, it is very important to investigate the possibility to replace In with other more abundant elements, without losing the electronic properties that make this compound so attractive.

The challenges for material science related to the development of solar cells are on two levels: to study new materials created in a controlled way and to characterize the material in detail, especially with spectroscopic methods. In view of that, numerical simulations of structural, electronic and optical properties are extremely valuable in the design of advanced materials for photovoltaics.

[1] G. D. Scholes and G. Rumbles, Nature Materials 5, 683 (2006).
[2] J-M Raulot, C. Domain, J-F. Guillemoles, Phys. Rev. B 71, 035203 (2005).
[3] P. Peumans, S. Uchida, and S. R. Forrest, Nature 425, 158 (2003).
[4] W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science 295, 2425 (2002).

Surface Reconstruction Puzzle

We study the two lowest-energy isomers of the Ge(111)-(2x1) surface, by a state-of-the-art first-principles calculation of their optical spectra, including the electron-hole interaction effects. A comparison of our results with the available experimental data suggests that, at difference with the Silicon case, the stablest isomer differs from the standard buckled Pandey chains reconstruction. This conclusion is supported by accurate total-energy results.

Fig.1 Two possible reconstructions

Fig.2 Experiment compared with the BSE results of both reconstructions.

1. We have studied the two lowest-energy isomers of the Ge(111)-(2x1) surface, by a state-of-the-art parameter-free calculation of their total energy and optical spectra, including the electron-hole interaction effects.
2. The two isomers yield a surface geometry which differs only starting from the third atomic layer below the uppermost one, hence experimental probes like STM can hardly be employed to discriminate between the two isomers.
3. Optical properties deduced from electronic structure results obtained at the one-particle level - i.e. neglecting the electron-hole interaction effects - cannot be used to discriminate between the two isomers.
4. A comparison of our results with the available experimental data suggests that, at difference with the Silicon case, the stablest isomer differs from the standard buckled Pandey chains reconstruction.
5. The upper panel in Fig.2 is for the positively buckled (i.e., the traditional) Pandey chain while the lower panel is for the negatively buckled chain. In both panels, the full (dashed) curves include (neglect) excitonic effects.
6. In conclusion, the ground-state geometry of Ge(111)(2x1) is found to correspond to Pandey-like chains with a buckling angle in the opposite direction with respect to that of the commonly assumed geometry. This work has been published in Phys. Rev. Lett. 85, 5440 (2000)
7. Our conclusions have been later confirmed by low-temperature STM experiments: see R.M. Feeenstra, G. Meyer, F. Moresco and K.H. Rieder, Phys. Rev. B 64, 081306 (2001).