時間帯 16:10~17:40
講師 Prof. Norbert Koch
(Humboldt-Universität zu Berlin, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH )

Electronic properties of electronic materials: interfaces & doping


Electronic and opto-electronic devices used in information, communication, energy conversion, and energy storage technologies rely on a precise control of the charge density distribution, which is the key parameter for a wide range of electronic and optical processes in devices. The charge density and its energy spectrum in electronic materials determine elemental parameters and functions, such as the Fermi level position, type and mobility of charge carriers, interfacial energy level alignment, carrier injection and extraction at contacts, and the characteristics of excitations. The primary conventional method to control the charge density in electronic materials is doping, as already employed by Shockley, Bardeen, and Brattain in the first transistors in the 1940s. However, established electronic materials and doping concepts, e.g., the statistical incorporation of dopant atoms in a covalent lattice, will soon reach fundamental limits. The anticipated route beyond this deadlock is the use of new electronic materials and combinations thereof, where tuning quantum confinement, dimensionality, and the charge density enable new device concepts. In this contribution, at first the fundamental differences in the electronic properties of selected advanced and emerging electronic materials are contrasted, i.e., organic semiconductors, metal halide perovskites, and two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers. Next, considerations of how the energy levels differ in practical thin films from those in single crystals will provide the basis for discussing fundamental interfacial phenomena in hybrid heterostructures comprising dissimilar material classes. Modern approaches to tune the interfacial charge density re-arrangement, and concomitantly the energy level alignment, will then be introduced and their impact on interface functionality in devices exemplified. Most of these approaches are based on employing very strong molecular electron donor or acceptor molecules as interlayers, and photochromic molecular switches even facilitate operando optical control over electrical device characteristics, i.e., multifunctionality.

会場 自然科学系総合研究棟2 マルチメディア講義室

2018/2/19 (月)

時間帯 13:00~
講師 Dr. Jürgen Müller
(Associate Professor, Algebra and Number Theory, Wuppertal University, Germany)

On numbers and secrets


Data security has become a most important issue: How can information be protected from unauthorised access? And how can mathematical number theory be utilised to achieve this? The aim of this talk is to introduce a few of the (simpler) ideas playing a role here.

会場 理学部 2号館 209

2017/7/21 (金)

時間帯 18:30~20:00
講師 Dr. Ipek Tuvay
(Assistant Professor of Mathematics, Mimar Sinan Fine Arts University, Turkey)

Fusion systems and Scott modules


In the modular representation theory of finite groups there is an important notion called "a Scott module" after L.L.Scott. This is an indecomposable kG-module which has a G-stable k-basis and it has the trivial kG-module in its socle, where G is a finite group, k is an algebraically closed field of prime characteristic p, and kG is the group algebra of G over k. In this lecture we will be discussing (so-called) fusion systems and the Scott modules for kind of particular but interesting cases.
1. J.L.Alperin. Local representation theory. Cambridge University Press, 1986.
2. H.Nagao and Y.Tsushima. Representations of Finite Groups. Academic Press, 1989.

会場 理学部 2号館 208

2016/4/28 (木)

時間帯 16:10-17:40
講師 Dr. Carlo Requião da Cunha
(Assistant Professor of Physics, Univ. Federal do Rio Grande do Sul, Brazil)

Nanostructuring Oxide Semiconductors for Solar Cell Applications


According to the American Energy Information Administration (EIA), the global demand for energy is expected to grow 56 % by 2040. Nonetheless, 80 % of the world energy is expected to keep coming from fossil fuels. These are pollutant sources that not only increase an ongoing problematic global warming but are also often extracted from regions of high political instabilities. One possible candidate that can remedy this problem is the solar cell. These are semiconductor devices that use a nearly inexhaustible and green energy supply. Nevertheless, the detailed balance maximum efficiency for solar cells is limited to approximately 46 %.

In our group we have been investigating both theoretically [*1][*2] and experimentally[*3][*4] nanostructured materials that might be able to circumvent this limit with multiple carrier generation. Our main effort is on the preparation and characterization of oxide semiconductors by low-cost processes such as the low-temperature sol-gel chemistry.

In this seminar, the thermodynamics of P-N solar cells will be discussed and then some theoretical results from Monte Carlo simulations on gel structures will be shown. Our results suggest that there is a disorder threshold that separates fracton and phonon states. Moreover, the nanoparticles that compose the gel structure can be tuned to form either Zipf scale-free or Poisson networks depending on the synthesis temperature and concentration of precursors.

Experimentally, we have been synthesizing monolithic high-surface area tin oxide and tungsten trioxide aerogels by a sol-gel technique.[*3} A full characterization with SEM/TEM, BET/BJH, XRD, UV/Vis, FTIR, PL, PIXE and Thermogravimetry was performed. Our results indicate that the sol-gel synthesis inadvertently produces materials with oxygen deficiency. These defects produces a Burstein-Moss shift in oxide semiconductors, which is readily detectable by optical spectroscopy and severly changes its electrical properties.[*4]

Along this seminar all techniques will be thoroughly discussed and experimental results will be compared to theoretical models.

[*1] E. L. da Rocha and C. R. da Cunha, Chaos, Solitons & Fractals 44 (2011) 241.
[*2] R. V. Morales, C. R. da Cunha and C. R. Rambo, Physica A 406 (2014) 131.
[*3] C. R. da Cunha, G. H. Toffolo, C. E. I. dos Santos and R. P. Pezzi, J. Non-Cryst. Sol. 380 (2013) 48.
[*4] C. R. da Cunha, F. D. da Silva and R. Morales, MRS Proc. 1731 (2015).

会場 自然科学系総合研究棟2 マルチメディア講義室

2016/1/13 (水)

時間帯 19:30~21:00
講師 Dr. Kevin Nute
(Professor of Architecture, University of Oregon, USA)

The Wilderness in the Metropolis
Natural Change in Urban Buildings


Over the past three decades the human benefits of contact with nature have been widely documented, but most building occupants do not have immediate access to natural outdoor spaces, particularly in large cities. This illustrated talk showed how the movements of the weather can be used to bring both nature and sensory stimulation to the occupants of urban buildings while also raising awareness of under-utilized sustainable practices.

会場 工学部15号棟110番講義室