Home Page Researchers Roi Baer

Roi Baer

Institute of Chemistry
Faculty of Science
The Hebrew University, Jerusalem 91904, Israel
Tel: +972-2-6586114; Fax: +972-2-6513742
E-mail: roi.baer@huji.ac.il
Website: www.fh.huji.ac.il/~roib


Our group is basicaly involved in two domains of electronic structure. One domain is the development and application of density functional theory to study systems with relevance to molecular electronics and photovoltaics. For these applications, density functional theory needs substantial revisions and new ideas and we have developed a method that enables to treat many of these issues within DFT. Within this domain our major acievements in recent years are (details of our theory and applications appeared in a recent review, with Dr. E. Livshits from my group and Prod. U. Salzner from Bilkent Univ. in Turkey (R. Baer, E. Livshits, and U. Salzner, Annu Rev Phys Chem 61, 85 (2010)):

Fundamental gaps of finite systems from the eigenvalues of a generalized Kohn-Sham method:
We present a broadly-applicable, physically-motivated first-principles approach to determining the fundamental gap of finite systems. The approach is based on using a range-separated hybrid functional within the generalized Kohn-Sham approach to density functional theory. Its key element is the choice of a range-separation parameter such that Koopmans' theorem for both neutral and anion is obeyed as closely as possible. We demonstrated the validity, accuracy, and advantages of this approach on first, second and third row atoms, the oligoacene family of molecules, and a set of hydrogen-passivated silicon nanocrystals. This extends the quantitative usage of density functional theory to an area long believed to be outside its reach

Figure 1: The ionization potential and electron affinity as frontier orbital energies in our DFT approach, compared to the GW quasiparticle energies: from molecules to hdrogented silicon nanocrsytals

With Ms. Tamar Stein and Dr. Eisenberg from my group and Prof. Leeor Kronik from the Weizmann Institute. (T. Stein, H. Eisenberg, L. Kronik, and R. Baer, Phys. Rev. Lett. 105, 266802 (2010)).

Reliable Prediction of Charge Transfer Excitations in Molecular Complexes  Using Time-Dependent Density Functional Theory
We show how charge transfer excitations at molecular complexes can be calculated quantitatively using time-dependent density functional theory. Predictive power is obtained from range-separated hybrid functionals using nonempirical tuning of the range-splitting parameter. Excellent performance of this approach is obtained for a series of complexes composed of various aromatic donors and the tetracyanoethylene acceptor, paving the way to systematic nonempirical quantitative studies of charge-transfer excitations in real systems. With Ms. Tamar Stein from my group and Prof. Leeor Kronik from the Weizmann Institute. (T. Stein, L. Kronik, and R. Baer, J. Am. Chem. Soc. 131, 2818 (2009); J. Chem. Phys. 131, 244119 (2009)).

Communication- Tailoring the optical gap in light-harvesting molecules:
Systematically varying the optical gap that is associated with charge-transfer excitations is an important step in the design of light-harvesting molecules. So far the guidance that time-dependent density functional theory could give in this process was limited by the traditional functionals' inability to describe charge-transfer excitations. We show that a nonempirical range-separated hybrid approach allows to reliably predict charge-transfer excitations for molecules of practically relevant complexity. Calculated absorption energies agree with measured ones. We predict from theory that by varying the number of thiophenes in donor-acceptor-donor molecules, the energy of the lowest optical absorption can be tuned to the lower end of the visible spectrum. Saturation sets in at about five thiophene rings. Here Ms. Tamar Stein and myself participated in the research of Prof. Dr. Stephan Kummel from the Physics Deprtment in Bayreuth Univ in Gemrany (A. Karolewski, T. Stein, R. Baer, and S. Kummel, J. Chem. Phys. 134, 151101 (2011))

Fundamental and excitation gaps in molecules of relevance for organic photovoltaics from an optimally tuned range-separated hybrid functional:
The fundamental and optical gaps of relevant molecular systems are of primary importance for organic-based photovoltaics. Unfortunately, whereas optical gaps are accessible with time-dependent density functional theory (DFT), the highest-occupied – lowest-unoccupied eigenvalue gaps resulting from DFT calculations with semi-local or hybrid functionals routinely and severely underestimate the fundamental gaps of gas-phase organic molecules. Here, we show that a range-separated hybrid functional, optimally tuned so as to obey Koopmans' theorem, provides fundamental gaps that are very close to benchmark results obtained from many-body perturbation theory in the GW approximation. We then show that using this functional does not compromise the possibility of obtaining reliable optical gaps from time-dependent DFT. We therefore suggest optimally tuned range-separated hybrid functionals as a practical and accurate tool for DFT-based predictions of photovoltaically relevant and other molecular systems. With Sivan Refaely-Abramson from Prof. Leeor Kronik's group at the Weizmann Inst. (S. Refaely-Abramson, R. Baer, and L. Kronik, Phys. Rev. B 84, 075144 (2011)).

The second main domain of our work concenrs carrier multiplication in nanocrsytals and nanotubes. This work is done in collaboration with Prof. Eran Rabani from Tel Aviv University:

Can impact excitation explain efficient carrier multiplication in carbon nanotube photodiodes?
We address recent experiments (Science 325, 1367 (2009)) reporting on highly efficient multiplication of electron-hole pairs in carbon nanotube photodiodes at photon energies near the carrier multiplication threshold (twice the quasi-particle band gap). This result is surprising in light of recent experimental and theoretical work on multiexciton generation in other confined materials, such as semiconducting nanocrystals. We propose a detailed mechanism based on carrier dynamics and impact excitation resulting in highly efficient multiplication of electron-hole pairs. We discuss the important time and energy scales of the problem and provide analysis of the role of temperature and the length of the diode.

Baer_1

Figure 2: The band structure of a 19,0 zigzag carbon nanotube (left) and the nanotube laying on an Silicon oxide surface as it is exposed to strong gating, causing an effective diode to form. As light is absorbed the formed charge carriers are accelrated by the diode field. This allows the charge carriers to resist deaccelration forces (caused by phonons) and enhances the rate of biexciton creation, causing enhancement of the photocurrent.

We have recently published a review on the subject (E. Rabani and R. Baer, Chem. Phys. Lett. 496, 227 (2010)).

List of publications in Nanoscience and Nanotechnology (2008-2011)

In peer-reviewed journals:
    • S. Refaely-Abramson, R. Baer, and L. Kronik, "Fundamental and excitation gaps in molecules of relevance for organic photovoltaics from an optimally tuned range-separated hybrid functional", Phys. Rev. B 84, 075144 (2011).
    • N. Kuritz, T. Stein, R. Baer, and L. Kronik, "Charge-Transfer-Like pi->pi* Excitations in Time-Dependent Density Functional Theory: A Conundrum and Its Solution", J. Chem. Theor. Comp 7, 2408-2415 (2011).
    • A. Karolewski, T. Stein, R. Baer, and S. Kummel, "Communication: Tailoring the optical gap in light-harvesting molecules", J. Chem. Phys. 134, 151101-4 (2011).
    • T. Stein, H. Eisenberg, L. Kronik, and R. Baer, "Fundamental gaps in finite systems from the eigenvalues of a generalized Kohn-Sham method", Phys. Rev. Lett. 105, 266802 (2010).
    • E. Rabani and R. Baer, "Theory of multiexciton generation in semiconductor nanocrystals", Chem. Phys. Lett. 496, 227-235 (2010).
    • R. Baer, E. Livshits, and U. Salzner, "Tuned Range-Separated Hybrids in Density Functional Theory", Annu Rev Phys Chem 61, 85-109 (2010).
    • R. Baer, "Ground-State Degeneracies Leave Recognizable Topological Scars in the Electronic Density", Phys. Rev. Lett. 104, 073001 (2010).
    • R. Baer and E. Rabani, "Can impact excitation explain efficient carrier multiplication in carbon nanotube photodiodes?" Nano Lett. ASAP http://pubs.acs.org/doi/pdf/10.1021/nl100639h (2010).
    • E. Rabani and R. Baer, "Theory of multiexciton generation in semiconductor nanocrystals", Chem. Phys. Lett. 496, 227-235 (2010).
    • K. Paul, S. Adhikari, M. Baer, and R. Baer, "Photodissociation by an intense pulsed photonic Fock state", Phys. Rev. A 81, 013412 (2010).
    • R. Baer, E. Livshits, and U. Salzner, ""Tuned" Range-separated hybrids in density functional theory", Ann. Rev. Phys. Chem. 61, 85-109 (2010).
    • R. Baer, "Ground-State Degeneracies Leave Recognizable Topological Scars in the Electronic Density", Phys. Rev. Lett. 104, 073001 (2010).
    • R. Baer and E. Rabani, "Can impact excitation explain efficient carrier multiplication in carbon nanotube photodiodes?" Nano Lett. ASAP http://pubs.acs.org/doi/pdf/10.1021/nl100639h (2010).
    • E. Rabani and R. Baer, "Theory of multiexciton generation in semiconductor nanocrystals", Chem. Phys. Lett. 496, 227-235 (2010).
    • A. K. Paul, S. Adhikari, M. Baer, and R. Baer, "H2 + photodissociation by an intense pulsed photonic Fock state", Phys. Rev. A 81, 013412 (2010).
    • T. Stein, L. Kronik, and R. Baer, "Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Using Time-Dependent Density Functional Theory", J. Am. Chem. Soc. 131, 2818-2820 (2009).
    • A. K. Paul, S. Adhikari, D. Mukhopadhyay, G. J. Halasz, A. Vibok, R. Baer, and M. Baer, "Photodissociation of H2+ upon Exposure to an Intense Pulsed Photonic Fock State", J. Phys. Chem. A 113, 7331-7337 (2009).
    • E. Livshits, R. Baer, and R. Kosloff, "Deleterious Effects of Long-Range Self-Repulsion on the Density Functional Description of O-2 Sticking on Aluminum", J. Phys. Chem. A 113, 7521-7527 (2009).
    • H. R. Eisenberg and R. Baer, "A new generalized Kohn-Sham method for fundamental band-gaps in solids", Phys. Chem. Chem. Phys. 11, 4674-4680 (2009).
    • R. Baer, E. Livshits, and U. Salzner, ""Tuned" Range-separated hybrids in density functional theory", Ann. Rev. Phys. Chem. in press (2009).
    • R. Baer, "Prevalence of the adiabatic exchange-correlation potential approximation in time-dependent density functional theory",Theochem-Journal ofMolecularStructure doi:10.1016, j.theochem.2009.04.018 (2009).
    • J. Andzelm, B. C. Rinderspacher, A. Rawlett, J. Dougherty, R. Baer, and N. Govind, "Performance of DFT methods in the calculation of optical spectra of TCF-chromophores", J. Chem. Theo. Comp. 5, 2835 (2009).

    Significant publications in the past five years

    • T. Stein, H. Eisenberg, L. Kronik, and R. Baer, "Fundamental gaps in finite systems from the eigenvalues of a generalized Kohn-Sham method", Phys. Rev. Lett. 105, 266802 (2010).

    • R. Baer and E. Rabani, "Can impact excitation explain efficient carrier multiplication in carbon nanotube photodiodes?" Nano Lett. ASAP DOI: 10.1021/nl100639h  (http://pubs.acs.org/doi/pdf/10.1021/nl100639h) (2010).

    • T. Stein, L. Kronik, and R. Baer, "Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Using Time-Dependent Density Functional Theory", J. Am. Chem. Soc. 131, 2818-2820 (2009).

    • E. Rabani and R. Baer, "Distribution of Multiexciton Generation Rates in CdSe and InAs Nanocrystals", Nano Lett. 8, 4488-4492 (2008).

    Cooperation with other universities in Israel:

    • Professor Eran Rabani, Dept of Chemistry Tel Aviv University
    • Professor Leeor Kronik, Dept of Materials and Interfaces, Weizmann Institute of Science, Rehovot
    • Professor Dr. Stephan Kummel, Dept of Physics, Bauyreuth Univ. Germany
    • Professor Daniel Neuhauser, Dept of Chemistry and Biochemistry, University of California Los Angeles

    Students, postdocs and researchers (include those graduated in 2007-2009):

    Postdocs: Dr. Helen Eisenberg, Dr. Esther Livshits
    PhD Students: Tamar Stein, Adva Baratz, Omri Buchman, Yael Cytter, Shlomit Jacobi.
    .

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