opening - Jan-Michael Rost, mpipks and the scientific coordinators
Two dimensional heterostructures (chair: David J. Norris)
09:30 - 10:00 Thomas Heine (Universität Leipzig)
From layered materials to two-dimensional crystals and back
Layered materials have been studied in detail for decades. Many of them are semiconductors with properties that are not particularly interesting for application. For example, they are photocatalysts with too narrow band gaps which are unsuitable for water splitting. Thinning them down to the monolayer generally increases the band gap, which overcomes aforementioned shortcoming. We demonstrate this interesting effect for a couple of noble metal chalcogenide and chalcophosphide materials. On the other hand, now the materials density is very low, so the total amount of harvested light is insufficient for application. Increasing the layer density would result in restacking, thus in loss of the interesting properties of the monolayers. We propose a method to form heterostructures composed of 2D crystals and surfactant, where the properties of the monolayers are maintained in the bulk system.
10:00 - 10:30 Ermin Malic (Chalmers University of Technology)
Dark exciton dynamics in atomically thin materials
Monolayers of semiconducting transition metal dichalcogenides (TMDs) build a new class of atomically thin two-dimensional materials. They exhibit a remarkably strong Coulomb interaction giving rise to the formation of tightly bound excitons. In addition to regular bright excitonic states, there is also a variety of dark states that cannot be accessed by light due to the required momentum-transfer or spin-flip. To model the exciton physics in TMD monolayers, we apply a microscopic approach combining semiconductor Bloch equations with the Wannier equation providing access to time- and momentum-resolved optical response and the non-equilibrium dynamics in TMDs. In this talk, we review our recent work focusing on: (i) Uncovering the full exciton landscape including bright as well as momentum- and/or spin-forbidden dark excitonic states. (ii) Demonstrating signatures of dark excitons by probing the intra-excitonic 1s-2p transition. (iii) Activating momentum-forbidden dark excitonic states via efficient coupling with molecules and suggesting a novel dark-exciton-based sensing mechanism for molecules. (iv) Providing a fully quantum mechanical description of formation, thermalization, and photoemission of interlayer excitons in TMD-based heterostructures.
Two dimensional heterostructures (chair: Vollrath Martin Axt)
11:00 - 11:30 Fabian Mooshammer (Universität Regensburg)
Electron-hole dynamics in van der Waals heterostructures: from interlayer excitons to switchable polaritons
Semiconducting van der Waals-layered materials - such as black phosphorus (BP) and the transition metal dichalcogenides (TMDCs) - have emerged as a versatile platform for optics and nanophotonics$^1$, because their bandgaps span a wide range from the near infrared into the visible. Furthermore, these materials host a plethora of interesting quasiparticles ranging from confined polaritons$^{2,3}$ to strongly bound excitons$^4$. Here, I will present two examples of how the unique optical and electronic properties of the individual materials can further be tailored by combining different constituents and forming van der Waals heterostructures: In a heterostructure composed of two monolayer TMDCs, electron and hole can be spatially separated into different layers and form a so-called interlayer exciton. Typically, the particles are assumed to stay in the $K$ valleys of the monolayer Brillouin zone. However, combining photoluminescence spectroscopy and first-principle calculations we have detected a novel momentum-space indirect exciton in a MoS$_2$/WSe$_2$ heterostructure$^5$. The hole of this new quasiparticle resides at the $\Gamma$ point, whereas the electron is located in the $K$ valley. Consequently, the interlayer exciton is only partially charge-separated and hence strongly bound. By varying the relative orientation of the individual monolayers - a degree of freedom exclusive to van der Waals heterostructures - we can control the exciton's emission energy, which is a desirable property for potential photonic applications. In a black phosphorus (BP)/SiO$_2$ heterostructure, surface plasmon polaritons in the mid-infrared can be activated within $\approx$50 fs by ultrafast near-infrared photo-doping of the BP layer. These modes strongly couple to surface phonons in the surrounding SiO$_2$ layers and form a novel hybrid polariton. This new excitation within the heterostructure features exceptional properties such as constant energy and momentum throughout the entire lifetime of the photo-excited carriers. Employing time-resolved scattering-type scanning near-field optical microscopy, we directly trace these polaritons in energy, space and time including the first ultrafast real-space snapshots of a switchable surface polariton mode$^6$. The exceptional switching contrast and the constant wavelength make it a promising candidate for ultrafast nanophotonic devices. References 1. F. Xia et al. Nat. Photon. 8, 899-907 (2014) 2. D. N. Basov, M. M. Fogler & F. J García de Abajo, Science 354, 195 (2016). 3. T. Low et al., Nat. Mater. 16, 182–194 (2016). 4. A. Chernikov et al., Phys. Rev. Lett. 113, 076802 (2014) 5. J. Kunstmann, F. Mooshammer et al., in press, arXiv:1803.04936 6. M. A. Huber, F. Mooshammer et al., Nat. Nanotech. 12, 207-2011 (2016)
11:30 - 12:00 Eric Stinaff (Ohio University)
Opto-electronic studies of self-contacted 2D materials
12:00 - 12:30 Kristian Sommer Thygesen (Technical University of Denmark)
High-throughput computational discovery of new 2D materials
Many-body dynamics in nanoscale systems (chair: Eric Stinaff)
14:00 - 14:30 Vollrath Martin Axt (Universität Bayreuth)
Phonon impact on the dynamics of quantum dot cavity systems in the limit of simultaneous strong coupling and strong driving
The impact of longitudinal acoustic phonons on the laser-driven dynamics of a quantum dot embedded in a micro cavity is analyzed in the limit of strong cavity-dot coupling and a laser driving of comparable strength accounting also for cavity losses. In order to obtain conclusive results in this situation where there is no obvious small parameter, the study makes use of a path integral approach that allows for a numerical complete treatment without introducing approximations to the model. Due to the strong driving high photon numbers in the cavity are reached and thus we accounted for 41 states of the corresponding Jaynes-Cummings ladder. The numerical complete treatment of a quantum dissipative system with such a high number of system states has become feasible due to a reformulation of a state-of-the-art path-integral algorithm which in our case reduced the numerical effort by more than 15 orders of magnitude. While without phonons cavity feeding is only efficient in a small range of laser-cavity detunings phonons effectively eliminate the photon blockade and lead over a wide detuning range to a strongly enhanced generation of cavity photons that is almost independent of the detuning. Furthermore, in the strong-driving, strong-coupling limit, the photon statistics is highly non-Poissonian. While without phonons a double-peaked structure in the photon distribution is predicted, phonons make the photon statistics thermal-like with very high effective temperatures ~47000 K, even for low phonon temperatures ∼4 K.
14:30 - 15:00 David J. Norris (ETH Zurich)
Plasmon-exciton polaritons in semiconductor nanoplatelets
When an ensemble of colloidal semiconductor nanocrystals is strongly coupled to a plasmonic resonance, hybrid plasmon–exciton states known as polaritons can form. Excitons that are isoenergetic with the plasmon are split into an upper and lower polariton due to mixing with the electromagnetic field. These states represent light–matter quasiparticles that can potentially lead to novel optoelectronic devices. To observe strong coupling, the nanocrystals should exhibit narrow optical transitions with large dipole moments. These should then be coupled to electromagnetic modes confined to small volumes. Here, we will discuss the observation of room-temperature strong coupling for CdSe nanoplatelets placed on high-quality silver hole arrays. We observe Rabi splittings of ~150 meV along with emission from the lowest polariton state. The influence of experimental parameters on the size of the polariton splitting will be examined.
15:00 - 15:15 Lucas Vazquez Besteiro (University of Electronic Science and Technology of China)
Plasmonic nanoparticles in near-field interaction: energy conversion and coherent plasmon transfer
Plasmonic Nanoparticles in Near-Field Interaction: Energy Conversion and Coherent Plasmon Transfer Lucas V. Besteiro(1,2), Xiang-Tian Kong(1,3), Alexander O. Govorov(3) (1) Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China (2) Centre Énergie Matériaux et Télécommunications, Institut National de la Recherche Scientifique, Varennes, Québec J3X 1S2, Canada (3) Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, United States Metal nanoparticles support collective oscillation modes of their quasi-free carriers, known as plasmonic resonances. Their excitation induces a greatly enhanced electric field inside and around the nanoparticle, which can in turn modify or drive other effects of interest in different fields of nanotechnology. We call hot spots to the regions where the field enhancement is highly concentrated, arising either from the nanoparticle having sharp geometrical features or from the near-field interaction of more than one nanoparticle. In this talk I will discuss the relevance of hot spots in two phenomena: 1) Plasmonic generation of hot electrons, in the context of a phenomenological theoretical framework that allows the modeling of this effect in systems with interparticle interaction [1-3] and nanoparticles with arbitrary geometries [4]. 2) Coherent low-loss energy transfer along a heterogeneous chain of metal nanoparticles, where the plasmonic excitation is transferred from neighboring particles through their near-field interaction. Alternating the nanoparticle materials in the array is crucial to reduce energy loss [5]. [1] H. Harutyunyan et al., Nature Nanotech. 10, 770 (2015) [2] G.V. Hartland et al., ACS Energy Letters 2, 1641 (2017) [3] L.V. Besteiro et al., ACS Photonics 4, 2759 (2017) [4] X.-T. Kong et al., Adv. Opt. Mat. 5, 1600594 (2017) [5] E.-M. Roller et al., Nature Physics 13, 761 (2017)
15:15 - 15:30 Eduard Zenkevich (National Technical University of Belarus)
Processes of interface ligand exchange and non-FRET photoluminescence quenching in self-assembled nanocomposites "CdSe/ZnS quantum dot – functionalized dye molecule"
E. Zenkevich1, A. Stupak2, T. Blaudeck3, D. Kowerko3, S. Krause4, C. von Borczyskowski3 1Department of Information Technologies and Robotics National Technical University of Belarus Prospect Nezavisimosti 65, 220013 Minsk, Belarus 2B.I. Stepanov Institute of Physics National Academy of Sciences of Belarus Prospect Nezavisimosti 70, 220072 Minsk, Belarus 3Institute of Physics and Center for Nanostructured Materials and Analytics Technische Universität Chemnitz Reichenhainer Str. 70, D-09107 Chemnitz, Germany 4Department of Chemistry, University of Copenhagen Universitätsparken 5, 2100 Copenhagen, Demark Organic-inorganic nanostructures based on colloidal semiconducting quantum dots (QDs) in combination with organic dye molecules are of special interest with respect to nanodevices, sensor technology and photovoltaics. We have shown that the attachment of one or few dye molecules (tetrapyridyl substituted porphyrins or perylene bisimides) via suitable anchor groups to the surface of QDs (CdSe or CdSe/ZnS of various sizes) is followed by QD photopluminescence (PL) quenching and PL decay times shortening. In this report, we present a detailed comparison of static and dynamic PL quenching via spectral intensities and PL decays. We were able to separate FRET (leading to 10-14 % of the total quenching effeiciency) and non-FRET processes quantitatively comparing QD donor PL quenching and porphyrin acceptor fluorescence enhancement. The FRET related part is in agreement with the Foerster-type model while non-FRET is related to the replacement of several ligands by the spacious and chemically differently bonding dye molecules. Based on bulk and single nanoobjects detection results, we have shown that non-FRET QD PL quenching results in an increase of the relative contribution of intrinsic weakly radiative QD states. In terms of single QD spectroscopy, the dye attachment increases the probabilities of dark and dim states. The modification and/or creation of dye induced surface traps is due to formation of additional and/or new Cd2+ dangling bonds at the QD surface because of dye-induced ligand depletion. The obtained results demonstrate that on the basis of the combination of steady-state and time-resolved measurements for bulk and single QD-dye nanoassemblies, it is in principle feasible to follow non-radiative pathways from near-band energy states to intra-gap states thus investigating microscopic features of surface related energy distributions and decay channels.
cpqc18 colloquium (chair: Inti Sodemann, mpipks)
Steven Erwin (Naval Research Laboratory)
The brief but fascinating history of silicon spin chains
Thirty years ago, scientists first observed that when a small amount of gold is deposited on the surface of silicon, both the gold and silicon atoms automatically organize themselves into parallel linear rows, so-called "atom chains", with nearly perfect structural order. This observation marked the beginning of a new research direction in which theoretical predictions about "physics in one dimension" could now be investigated experimentally using the standard tools of surface science such as scanning tunneling microscopy, x-ray diffraction, and photoemission. A particularly striking discovery, first reported ten years ago, was that at low temperature the silicon chains can develop local magnetic moments, which form regular highly ordered, periodic patterns. These "silicon spin chains" have now become the main focus of this research field. This talk will describe three recent theoretical and experimental aspects of silicon spin chains: the possibility of magnetic ordering in these one-dimensional systems; the prospects for using surface chemistry to tailor spin chains by creating or destroying individual spins; and the properties and dynamics of solitons in the spin chains.
poster flash session (chairs: Alexander Eychmüller and Andreas Knorr)
1 slide, 2 minutes per poster
Dark excitons (chair: Anna Rodina)
09:30 - 10:00 Alexander Högele (Ludwig-Maximilians-Universität München)
Signatures of dark excitons in atomically thin semiconductors
The optical properties of layered transition metal dichalcogenides are dominated by excitonic effects. While the photophysics of bright excitons with dipole-allowed optical transitions have been meanwhile well established, the role of dark excitons remains in part unexplored. In my presentation, I will discuss signatures of dark excitons in the photoluminescence spectra of monolayer, bilayer and hetero-bilayer systems, and how their notion complements our understanding of optical excitations in atomically thin semiconductors.
10:00 - 10:30 Malte Selig (Technische Universität Berlin)
Impact of dark states on excitonic spectra of transition metal dichalcogenides
In atomically thin transition metal dichalcogenides, due to a reduced screening of the Coulomb interaction strongly bound electron hole pairs (excitons) dominate the optical spectra. Besides bright excitons, dark exciton states are formed by electrons and holes with opposite spin, or by excitons with non-vanishing center of mass momentum well above the lightcone, including intervalley excitons. Evaluating the excitonic spectrum it turns out, that in tungsten based materials some of these dark states are energetically located below the optical active states. Here, we present an excitonic theory within the Heisenberg equation of motion framework. We investigate the impact of exciton phonon interaction on the excitonic linewidth and lineshape [1,2] as well as the phonon mediated exciton formation and thermalization of exciton densities. It turns out, that dark states as a ground state in tungsten based TMDs strongly influence the luminescence yield and the radiative lifetime of excitons[3]. Our theoretical results explain several recent experimental results. [1] M. Selig et al., Nat. Commun. 7, 13279 (2016) [2] D. Christiansen et al., Phys. Rev. Lett. 119, 187402 (2017) [3] M. Selig et al., arXiv:1703.03317 (2017) [*] in cooperation with Dominik Christiansen, Florian Katsch, Samuel Brem, Gunnar Berghäuser, Ermin Malic and Andreas Knorr
Nanocrystals! (chair: Hilmi Volkan Demir)
11:00 - 11:30 Andrey Rogach (City University of Hong Kong)
Synthesis of light-emitting perovskite nanocrystals and their application in optoelectronic devices
Chemically synthesized lead halide perovskite nanocrystals have emerged as a new class of efficient light emitting materials. High emission quantum yield, easily tuned emission colors, and high color purity make this class of materials particular attractive for light-emitting devices (LEDs) and display applications. I will review our recent synthetic strategies leading to highly luminescent CH3NH3PbX3 and CsPbX3 (X = Cl, Br or I) perovskite nanocrystals of different sizes and shapes, focusing on the elucidation of mechanism of their nucleation and growth, as well as methods to improve their stability in humid/aqueous environment. I will further consider the interface engineering of perovskite nanocrystal based hybrid films, which allows us to use them as phosphors in down-conversion LEDs, and in charge injection electroluminescent devices.
11:30 - 12:00 Tobias Kraus (Leibniz Institut für neue Materialien)
Hybrid nanostrucures: colloidal and electronic properties
12:00 - 12:30 Andreas Riedinger (Max-Planck-Institut für Polymerforschung)
How do semiconductor nanoplatelets form?
Colloidal nanoplatelets — quasi-two-dimensional sheets of semiconductor exhibiting efficient, spectrally pure fluorescence — form when liquid-phase syntheses of spherical quantum dots are modified. Despite intense interest in their properties, the mechanism behind their anisotropic shape and precise atomic-scale thickness remained unclear, and even counterintuitive when the underlying crystal structure is isotropic. Proposed explanations include fusion of preformed nanoclusters within molecular templates exhibiting layered structures. We experimentally disprove this mechanism by synthesizing CdSe nanoplatelets in isotropic melts containing only cadmium carboxylate and chalcogen, a result incompatible with previous mechanisms. Based on our experimental results we develop a theoretical model in which intrinsic kinetic instabilities in growth kinetics cause enhanced growth rates on narrow surface facets. This ultimately leads to the formation of nanoplatelets. Additionally, our model predicts lateral Ostwald ripening within each thickness population and complete absence of growth in thickness on nanoplatelets with large surface areas. We show that thicker populations evolve from thinner nanoplatelets with small surface areas, provided through lateral size distribution broadening induced by lateral Ostwald ripening. Furthermore, we identified the reactive organoselenide (sulfide) species, formed through the reaction of a cadmium carboxylates decomposition product and elemental Se (S). Based on this knowledge we synthesize organoselenide compounds with tailored reactivity and demonstrate reactivity-controlled thickness dependencies in the synthesis of CdSe nanoplatelets. We anticipate that our experimental results and the theoretical understanding for controlling the shape at the nanoscale will lead to broader libraries of quasi-two-dimensional materials.
Nanocrystals! (chair: Andrey Rogach)
14:00 - 14:30 Alexander Efros (Naval Research Laboratory)
Electronic and optical properties of lead halide perovskite nanocrystals
The growing attention to perovskite nanocrystals is connected with their unusual and potentially useful electronic and optical properties. I will discuss the bulk energy band structure of CsPbX$_3$ (X = I, Cl, and Br) perovskites and show that all of them have the band edge at R-point of the Brillouin zone. To describe electronic and optical properties of perovskite nanocrystals we have derived the four band effective mass Hamiltonian, which describes the electronic properties of electron and holes near the band edge. Using this Hamiltonian we calculate the lowest quantum confined levels of electrons and holes and the spectra of the allowed optical transitions. The calculations takes into account the cubic shape of the perovskite nanocrystals, that results into inhomogeneous electric field of emitted and absorb photons. The symmetry of the ground exciton state has been analyzed and the radiative decay time has been calculated. The results of our theoretical calculations have explained 200 ps radiative decay time and polarization properties measured in single CsPb(BrCl$_2$) quantum dot experiments.[1] [1} M. A. Becker, R. Vaxenburg, G. Nedelcu, P. C. Sercel, A. Shabaev, M. J. Mehl, J. G. Michopoulos, S. G. Lambrakos, N. Bernstein, J. L. Lyons, T. Stöferle, R. F. Mahrt, M. V. Kovalenko, D. J. Norris, G. Rainò, and Al. L. Efros, Nature, 5 5 3, 1 8 9 (2018)
14:30 - 15:00 Ulrike Woggon (Technische Universität Berlin)
Excitons in CdSe colloidal quantum wells
Intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy efficient display technology. In this contribution we report about a monolayer of aligned CdSe nanoplatelets that exhibits strongly anisotropic and directed photoluminescence. We briefly introduce at first the electronic properties, exciton dynamics, electric-field controlled emission and two-photon absoprtion of these nanoplatelets, a new class of colloidal quantum wells with finite size. The directionality of emission and absorption is investigated using high-resolution two-dimensional k-space spectroscopy. The obtained k-space distribution reveals the underlying internal transition dipole distribution, both in one-photon- and two-photon absorption processes. The observed directed emission and directed two-photon absorption is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment [1]. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment [1]. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. These optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters for photonic and nonlinear applications, e.g. as directional gain media and efficient two-photon imaging. [1] R. Scott, J. Heckmann, J. Prudnikau A. Antanovich A. Mikhailov, N. Owschimikow, M.V. Artemyev, J. Climente, U. Woggon, N. Grofle, A.A. Achtstein Directed emission of CdSe nanoplatelets originating from strongly anisotropic 2D electronic structure. Nature Nanotechnology 12, S.1155-1160 (2017).
15:00 - 15:30 Anna Rodina (Ioffe Institute, St. Petersburg)
Circular polarized emission in magnetic fields from neutral and charged excitons in ensemble of colloidal nanoсrystals
We present a review of our past [1-6] and new results on the magneto-optical studies of charged and neutral excitons in ensemble of colloidal nanocrystals (NCs): bare and core/shell spherical quantum dots, dot-in-rod nanocrystals, bare and core/shell nanoplatelets. The influence of the magnetic field on the time-resolved dynamics of photoluminescence (PL) allows us to distinguish between the recombination originating from neutral and changed excitons. The sign of the circular polarization of the PL induced in magnetic field allows us to distinguish between the recombination originating from negative or positive trions. The theoretical analysis of the magnetic field dependence of the degree of circular polarization (DCP) allows us to determine the effective g-factor controlling the Zeeman splitting of the spin sublevels in external magnetic as well as an energy of the exchange interaction between exciton and dangling bond spins at the NC surfaces. It allows us also to extract an information about the shape and preferable orientation of the NCs in the ensemble. The time-resolved studies of the DCP dynamics allows us to analysis the spin relaxation processes. Importantly, the ensemble studies along with the theoretical modeling give access to the information about the spin relaxation rate and Zeeman splitting in individual NC depending on their orientation in ensemble with respect to the magnetic field direction. [1] D.R. Yakovlev and A.V. Rodina, Spin dynamics of charged and neutral excitons in colloidal nanocrystals (review), Journal of electronic materials, published online 13 March 2018. [2] E. V. Shornikova, L. Biadala, D. R. Yakovlev, D. H. Feng, V. F. Sapega, N. Flipo, A. A. Golovatenko, M. A. Semina, A. V. Rodina, A. A. Mitioglu, M. V. Ballottin, P. C. M. Christianen, Y. G. Kusrayev, M. Nasilowski, B. Dubertret, M. Bayer, Electron and hole g-factors and spin dynamics of negatively charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells. Nano Letters 18, 373 (2018). [3] B. Siebers, L. Biadala, D. R. Yakovlev, A. V. Rodina, T. Aubert, Z. Hens, and M. Bayer, Exciton spin dynamics and photoluminescence polarization of CdSe/CdS dot-in-rod nanocrystals in high magnetic fields. Phys. Rev. B. 91, 155304 (2015). [4] F. Liu, A. V. Rodina, D. R. Yakovlev, A. Greilich, A. A. Golovatenko, A. S. Susha, A. L. Rogach, Yu. G. Kusrayev, M. Bayer, Exciton spin dynamics of colloidal CdTe nanocrystals in magnetic fields, Phys. Rev. B 89, 115306 (2014). [5] C. Javaux, B. Mahler, B. Dubertret, A. Shabaev, A. V. Rodina, Al. L. Efros, D.R. Yakovlev, F. Liu, M. Bayer, G. Camps, L. Biadala, S. Buil, X. Quelin, and J.-P. Hermier, Thermal activation of non-radiative Auger recombination in charged colloidal nanocrystals, Nature Nanotechnology 8, 206 (2013). [6] F. Liu, L. Biadala, A. V. Rodina, D. R. Yakovlev, D. Dunker, C. Javaux, J. P. Hermier, Al. L. Efros, B. Dubertret, M. Bayer, Spin dynamics of negatively charged excitons in CdSe/CdS colloidal nanocrystals, Phys. Rev. B 88, 035302 (2013).
Application of nanocrystals (chair: Elena Shornikova)
16:30 - 17:00 Victor Klimov (Los Alamos National Laboratory)
Recent advances in quantum dot lasing: from zero-threshold optical gain to light amplification with electrical pumping
Chemically synthesized quantum dots (QDs) can potentially enable new classes of highly flexible, spectrally tunable lasers processible from solutions. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage. A major complication is fast nonradiative Auger recombination of gain-active multicarrier species such as biexcitons. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of "designer" QDs with a radially graded composition that show a considerable suppression of Auger decay. Using these novel QDs, we have been able to realize optical gain with electrical pumping [1], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrate the viability of a "zero-threshold-optical-gain" concept using not neutral but negatively charged particles wherein the pre-existing electrons completely block ground-state absorption [2]. Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices. 1.	Lim, J., Park, Y.-S., Klimov, V. I., Nat. Mater. 17, 42 (2018) 2.	Wu, K., Park, Y.-S., Lim, J., Klimov, V. I., Nat. Nanotechnol. 12, 1140 (2017)
17:00 - 17:30 Hilmi Volkan Demir (Nanyang Technological University & Bilkent University)
Atomically flat nanocrystals for high-performance lasing and energy transfer
In this talk, we will present all-colloidal lasers developed by incorporating atomically-flat nanocrystals, also known as nanoplatelets, as the optical gain medium in fully colloidal cavities. As an extreme case of solution-processed tightly-confined quasi-2D colloids, we will also discuss that the heterostructures of these nanoplatelets uniquely offer record high optical gain coefficients and ultra-low threshold stimulated emission. In addition, we will show that controlled stacking of the nanoplatelets provides us with the ability to further tune their excitonic properties. Given the recent accelerating progress in nanocyrstal optoelectronics, we find that solution-processed quantum materials hold great promise to challenge their epitaxial counterparts in the near future.
poster session (focus on odd poster numbers)
Optical activity and chirality in nanomaterials (chair: Dmitri V. Talapin)
09:30 - 10:00 Anvar Baimuratov (ITMO University)
Optical activity of semiconductor nanocrystals and structures based on them
10:00 - 10:30 Gil Markovich (Tel Aviv University)
Spontaneous and directed symmetry breaking in the synthesis of chiral nanocrystals
We prepare rod shaped nanocrystals made of terbium phosphate hydrate, which crystallizes in a chiral space group. Under certain synthesis conditions, the nanocrystals grow with a large, random enantiomeric excess. The enantiomeric excess is probed by circularly polarized emission of europium dopant ions within the nanocrystals. In addition, on using chiral molecules (tartaric acid) which form a strong complex with the lanthanide ions, the handedness of the nanocrystals could be controlled and it was possible to obtain pure left- or right-handed nanocrystal samples. The mechanism of the nanocrystal symmetry breaking will be discussed.
Optics and photonics of 2D materials (chair: Garnett W. Bryant)
11:00 - 11:30 Alexey Chernikov (Universität Regensburg)
Exciton physics of semiconducting 2D nanomaterials
The consequences of Coulomb interaction of charge carriers have been in the focus of the solid-state research for many decades. In semiconductor nanostructures, in particular, they remain of paramount importance for the fundamental understanding of electrical and optical excitations with major implications for basic science and technology. Recently, a number of nanomaterials, such as van der Waals monolayers and heterostructures as well as organic-inorganic perovskite hybrids were found to exhibit a number of highly intriguing phenomena, including large exciton binding energies on the order of 0.5 eV, efficient light-matter coupling and spin-valley locking. In these systems, the Coulomb interaction governs both the fundamental physics and can be harnessed to manipulate key properties and their response to external fields. In this talk I will address the topics of manipulation and control of the exciton states in two-dimensional nanomaterials from the experimental perspective. I will discuss both the intriguing ground state properties of these systems and show how the excitations can be externally tuned with high precision using electric and optical injection. It will include unique strategies to engineer and design the electronic and excitonic states through customizing the dielectric environment. I will further address recent studies of exciton transport in van der Waals materials, accessible at ambient conditions, exhibiting highly non-linear behavior and memory effects. Finally, I will discuss the consequences and opportunities of the studied physics for future research.
11:30 - 12:00 Atac Imamoglu (ETH Zurich)
Photonics with two-dimensional materials
12:00 - 12:30 Doris Reiter (Westfälische Wilhelms-Universität Münster)
Microscopic simulation of higher harmonic generation in two-dimensional semiconductors
Semiconducting monolayers of transition metal dichalcogenides (TMDCs) promise to revolutionize semiconductor devices which are used in computer and communication technology. To address these monolayers one can utilize laser light, which induces a strong optical response. To tailor the optical activity of the monolayer, the semiconductor can be embedded into a nano-photonic structure or combined with plamonic nanoparticles. We here present a new, versatile simulation method combining the Bloch equations describing the carrier dynamics with a finite-difference time-domain approach to Maxwell's equations for the light field dynamics [1]. Excitonic effects are taken on board by calculating the Coulomb interaction dynamically on the Hartree-Fock level. To describe the second harmonic generation we further introduce a permanent dipole. With this, we calculate the linear and nonlinear optical response of a TMDC monolayer embedded in a nano-cavity consisting of a Bragg mirror. We show how the second harmonic can be greatly enhanced by the photonic structure [2]. Our simulation method can be readily extended to sophiosticated geometries, including plasmonics structures, and complements the fast-paced experimental development of devices with 2D materials. [1] S. Guazzotti, A. Pusch, D. E. Reiter, and O. Hess, PRB 94, 115303 (2016) [2] S. Guazzotti, A. Pusch, D. E. Reiter, and O. Hess, submitted (2018)
Engineering of nanocrystals and their properties (chair: Gil Markovich)
14:00 - 14:30 Mikhail Artemyev (Belarussian State University)
Engineering and optical properties of self-assembled CdSe nanoplatelets
Individual colloidal CdSe nanopatelets show unique optical properties due to 2D nature of energy levels, large surface area and quantized number of atomic layers. CdSe nanopatelets possess ca. order of magnitude larger linear and non-linear two-photon absorption coefficients as compared to other types of quantum-sized CdSe nanocrystals (1D nanorods and 0D quantum dots). Enhanced two-photon absorption and photoluminescence excitation coefficients make colloidal nanoplatelets perspective material for two-photon fluorescent labels for biomedical applications. Large surface area accompanied with extremely small thickness results in strong sensitivity of optical properties of CdSe nanoplatelets to their surface properties. Here, we demonstrate that the surface ligand exchange can produce a spectral shift of optical transitions up to 240 meV due to the ligand-induced distortion of crystalline structure of CdSe core. Additionally, the ligand exchange can be used to prepare 1D ensemble (stacks) of CdSe nanoplatelets with controlled length. Utilization of Langmuir or Langmuir-Blodgett techniques allowed prepare another type of spatially-organized structures, the monolayers of laterally oriented CdSe nanoplatelets which demonstrated for the first time the spatially-directed emission. Surface ligand exchange activates also CdSe nanoplatelets toward electrostatic or chemical deposition on different substrates to produce optically and electrically active structures. This work was supported by Belarusian republican foundation for fundamental research, grant X17KIG-004.
14:30 - 15:00 Dmitri V. Talapin (University of Chicago)
Engineering functionality in colloidal semiconductor nanomaterials
The synthesis of inorganic nanomaterials has seen impressive developments, both in the fundamental understanding of nucleation, growth and surface chemistry of inorganic phases, and in the ability to prepare functional materials with precisely engineered optical and electronic properties. However, the lack of atomic precision in nanomaterial synthesis restricts our ability to harness all the power of this broad and diverse class of matter. Heterogeneity introduces broadening of the absorption and emission spectra, reduces charge carrier mobility in nanocrystal solids, and generally restricts our ability to engineer nanomaterials. I will discuss a new approach for colloidal synthesis of nanomaterials with minimal, ideally no, size distribution. The concept is inspired by gas-phase atomic layer deposition (ALD) widely used in microelectronics. Our studies show that the ALD concept can be successfully implemented in solution and, when applied to nanomaterials, enables layer-by-layer growth of crystalline lattices with close-to-atomic precision. I will also discuss recent advances in the surface chemistry of semiconductor nanostructures. Molecular inorganic species can be designed to electronically couple individual nanostructures into nanocomposite materials with high electron mobility. By making surface ligands photochemically active, we introduce a general approach for photoresist-free, direct optical lithography of functional inorganic nanomaterials (DOLFIN). Examples of patterned materials include metals, semiconductors, oxides, and magnetic and rare earth compositions. No organic impurities are present in the patterned layers, which helps achieve good electronic and optical properties. The ability to directly pattern all-inorganic layers using a light exposure dose comparable to that of organic photoresists opens up a host of new opportunities for additive nanomanufacturing.
15:00 - 15:30 Kirill Bolotin (Freie Universität Berlin)
Excitons in 2D semiconductors "talking" to their environment
group photo (to be published on the workshop web page)
Aerogels (chair: Lucas Vazquez Besteiro)
16:30 - 17:00 Markus Niederberger (ETH Zurich)
Nanoparticle assembly and processing: films, foams and aerogels
17:00 - 17:30 Nadja-Carola Bigall (Leibniz Universität Hannover)
Nanocrystals - defined building blocks for macroscopic aerogels with nanoscopic properties
workshop dinner at the restaurant Palastecke on Schloßstr. 2, phone: +49-351-48417330
Applications of nanomaterials (chair: Steffen Michaelis de Vasconcellos)
09:30 - 10:00 Rodion Belosludov (Tohoku University)
Functional nanomaterials for energy and medical applications: theoretical aspects
In order to design materials with novel composition and desirable characteristics it is important to have a good understanding of the atomic-scale chemical and physical properties of materials. Computer simulation has become an indispensable research tool because often experiments require theoretical interpretation and computing power has grown rapidly in the last decade. We try to estimate the physicochemical properties of various nanostructures in order to accelerate the realization of novel materials, hand-in-hand with experiment and propose these materials for energy and medical applications. It has been shown experimentally that nano-sized titania (TiO2) particles enhance the burn rate of a composite propellant composition through catalyzing the decomposition of ammonium perchlorate (AP). Our calculations show that adsorption energy of AP on the TiO2 clusters are larger as compared with AP adsorption on TiO2 surface that can be associated both with the structural reconstruction due to interaction with AP and the large ratio of (001) face in anatase TiO2 clusters. The effects of transition metals doping and oxygen vacancy defects on the catalytic activity of TiO2 clusters were also studied. It has been found that the Fe and Co doping stabilize the small cluster of TiO2 anatase with size of 1 nm and the formation of metal cluster on TiO2 surface is not energetically preferable. Moreover, the anatase TiO2 clusters with oxygen vacancy show higher catalytic activity toward AP adsorption. Recently, we have presented a conceptual design for functional 3D porphyrin-based nanostructures, which would bridge the gap between the well-known fullerenes and nanotubes and a new class of the functional nanomaterials, which may potentially be used as analogues of quantum dots for solar cells and imaging/ bioimaging applications [1]. In collaboration with experimentalists, we have shown that the concept using a designable regular MOF material could be applicable to a highly stable, selective adsorption system [2]. A detailed study of zinc oxide clusters containing up to almost 200 ZnO pairs was conducted. Inspired by the magic numbers found in the mass spectra, the row of nested shell clusters was constructed. The proposed cluster structures explained the experimentally observed sequence of the magic numbers 12, 60, and 168, predicted the composition and the structure for larger yet unexplored magic clusters, and provided the origin of ZnO tetrapods [3]. REFERENCES 1.	R. V. Belosludov et al. Phys. Chem. Chem. Phys. 18 (2016) 13503. 2.	R. Matsuda et al. Nature 436 238 (2005); H. Sato et al. Science 343 (2014) 167. 3.	A. Dmytruk et al. RSC Advances 7 (2017) 21933-21942.
10:00 - 10:30 Thomas J. Schmidt (Paul Scherrer Institute)
Application of noble-metal aerogels in polymer electrolyte fuel cells
State-of-the-art polymer electrolyte fuel cells (PEFCs) require large amounts of carbon-supported platinum nanoparticle (Pt/C) catalysts (~ 0.4 mgPt/cm2geometric) to account for the large overpotential of the oxygen reduction reaction (ORR). [1] These excessive Pt-loadings that impede widespread commercialization of PEFCs can be mitigated by increasing the catalysts' ORR activity, e.g. by alloying platinum with other metals like Ni, Cu and Co, to form materials which show up to one order of magnitude higher mass-specific activity than commercial Pt/C catalysts. On the other hand, state-of-the art carbon-supported materials suffer from significant carbon and Pt corrosion during the normal operation of PEFCs, gradually compromising their performance. To partially overcome these stability issues, a lot of research effort is dedicated to the development of unsupported ORR catalysts. Among these materials, bimetallic alloy aerogels consisting of nanoparticles interconnected to nanochains [2] present an interesting option, since their extended 3D structure should facilitate transfer to actual PEFC cathodes. In this talk, after having elucidated the high electrocatalytic activity of noble metal aerogels towards the ORR in model studies [3], the successful transfer of these catalysts into technical PEFC cathode environment will be presented demonstrating the practical applicability of these systems. References: [1] A. Rabis, P. Rodriguez, T.J. Schmidt, ACS Catalysis 2 (2013) 768-800. [2] W. Liu, et al., Acc. Chem. Res. 48 (2015) 154-162 [3] S. Henning, et al., J. Electrochem. Soc. 163 (2016) F1-F6
Ultra-fast dynamics, plasmons and hot electrons (chair: Guido Burkard)
11:00 - 11:30 Gary Wiederrecht (Argonne National Laboratory)
Ultrafast nanophotonics for light harvesting, hot electron, and quantum optics applications
Understanding ultrafast photoprocesses in nanostructures is important for realizing new opportunities related to technologies for light harvesting, energy conversion, single photon emission for quantum optics applications. In this talk I discuss our work on characterizing several ultrafast optical phenomena that are specifically enhanced in nanoscale structures. I begin with describing the decay of plasmons into hot "nonthermal" plasmonic carriers and their potential importance for increasing efficiencies of photovoltaic and photocatalytic processes. This is followed by our efforts to spectroscopically characterize hot electrons on an ultrafast timescale. Insights for extracting hot carriers are given. This is followed by introducing new biomimetic nanostructures for light harvesting, which offer opportunities for tuning energy transport through structural control of biomimetic assemblies. This work focuses on chromophore-doped peptide amphiphiles to create varied and potentially self-healing structures for transporting energy. I close with describing recent studies of ultrafast biexciton formation in semiconductor nanoplatelets, and the production of correlated photon pairs of interest for quantum entanglement and quantum optics. [1] M. E. Sykes, J. W. Stewart, G. M. Akselrod, X.-T. Kong, Z. Wang, D. J. Gosztola, A. B. F. Martinson, D. Rosenmann, M. H. Mikkelsen, A. O. Govorov, G. P. Wiederrecht, Nature Comm. 2017, 8, 986. [2] L. A. Solomon, M. E. Sykes, Y. A. Wu, R. D. Schaller, G. P. Wiederrecht, and H. C. Fry, ACS Nano 2017, 11, 9112. [3] X. Ma, B. T. Diroll, I. Fedin, W. Cho, R. D. Schaller, D. V. Talapin, S. K. Gray, G. P. Wiederrecht, D. J. Gosztola, ACS Nano 2017, 11, 9119.
11:30 - 12:00 Garnett Bryant (National Institute of Standards and Technology)
Atomic-scale solid-state many-body systems: the birth of quantum plasmons, "nano"hybrids, and entanglement generation at the atomic scale
12:00 - 12:30 Stephen Gray (Argonne National Laboratory)
Inferring hot carrier distributions from pump-probe measurements
An approach for more directly inferring hot carrier distributions as a function of time from pump-probe experimental data is discussed. The approach requires prior knowledge of band structure information and involves a double inversion procedure. We apply the method to ultrafast measurements on a 30nm thick gold film and show early-time quantum features.
Strain in 2D materials (chair: Gary Wiederrecht)
14:00 - 14:30 Guido Burkard (Universität Konstanz)
Spin and valley dynamics in nanostructured and strained 2D materials
The semiconducting monolayer transition-metal dichalcogenides (TMDCs) are interesting for their electronic and opto-electronic properties. The lack of inversion symmetry in the monolayer TMDCs leads to their optical bandgap, as well as interesting spin-orbit effects both in the valence and conduction band. The intrinsic bandgap of the TMDCs allows for the localization of carriers in quantum dots by either external electric fields [1] or strain [2]. Based on our earlier effective-mass model, we describe the spin and valley dynamics of single electrons localized in single and double quantum dots [1,3,4]. The application of mechanical strain in the TMDCs leads to the appearance of a fictitious gauge field and allows for the tuning of the electronic band structure [2] which can be used to form quantum dots on patterned substrates. [1] A. Kormányos, V. Zólyomi, N. D. Drummond, and G. Burkard, Phys. Rev. X 4, 011034 (2014). [2] A. J. Pearce, E. Mariani, and G. Burkard, Phys. Rev. B 94, 155416 (2016). [3] M. Brooks and G. Burkard, Phys. Rev. B 95, 245411 (2017). [4] A. David, G. Burkard, and A. Kormányos, arXiv: 1802.09789.
14:30 - 15:00 Tilmann Kuhn (Westfälische Wilhelms-Universität Münster)
Control of carrier capture processes into localized states in 2D materials
Two-dimensional (2D) materials such as monolayers of transition metal dichalcogenides (TMDCs) have attracted wide attention due to their interesting optical and electronic properties. Strain distributions in these materials may lead to the formation of localized states, which can be understood as a quantum dot (QD) embedded in the 2D material [1]. Such localized QD states have been shown to act as efficient single-photon emitters, which makes them interesting candidates in the field of quantum information and communication. In order to populate the bound QD states, carriers are often excited in the 2D regions of the structure; from there they travel towards the QD and are captured by releasing their energy to phonons. In this talk I will present results of our recent studies of phonon-mediated capture processes from a wave packet traveling in a MoSe2 monolayer into the localized states of an embedded QD potential. We are particularly interested in the spatio-temporal dynamics of both the carriers in the unbound 2D states and of the captured carriers in the bound QD states. It turns out that the carrier capture typically leads to the build-up of coherent superpositions of the bound states associated with non-trivial spatio-temporal dynamics of the carrier distribution inside the QD. We find that both the occupations of the bound states as well as the coherences among these states sensitively depend on the geometrical details, in particular on the relative orientation of the initial wave packet and the QD potential, which opens up the possibility of a spatial control of the capture dynamics. Technically our calculations are based on a Lindblad single-particle approach, which combines computational affordability and the ability of catching most of the relevant quantum features of the carrier capture, such as the locality of the capture process and the possibility of capture into coherent superposition states, as has been shown by comparison with a full quantum kinetic modeling of carrier capture in a 1D-0D system [3]. [1] J. Kern et al., Adv. Mater., 28, 7101 (2016) [2] R. Rosati, F. Lengers, D. E. Reiter, and T. Kuhn, arXiv:1802.01409 (2018) [3] R. Rosati, D. E. Reiter, and T. Kuhn, Phys. Rev. B 95, 165302 (2017)
15:00 - 15:30 Steffen Michaelis de Vasconcellos (Westfälische Wilhelms-Universität Münster)
Strain-induced quantum emitters in layered semiconductors
The availability of efficient and robust single-photon emitters in large numbers and their integration in photonic devices constitutes one of the most stringent roadblocks for the practical implementation of quantum communications and optical quantum computing. Recently, we have found single-photon emitters in monolayer WSe2 [1]. The material belongs to the class of atomically thin transition metal dichalcogenides which are a promising new material class for novel opto-electronics devices. We show, that the quantum light sources in this two-dimensional material are induced by local strain in the crystal and demonstrate deterministic positioning of the emitters on the nanoscale using a prepatterned substrate [2]. The atomically thin semiconductor conforms to the nanostructures on the substrate which induces local strain due to the deformation of the monolayer. The created single-photon emitters are localized to a region smaller than 140 nm. Finally, we present single-photon emission from the layered semiconductor GaSe and provide evidence that the incorporated non-classical light sources are also strain-induced [3]. By placing GaSe crystals with a thickness below 100 nm on Si3N4 rib or slot waveguides a modified mode structure efficient for light coupling is created. Using optical excitation from within the waveguide, we find nonclassicality of generated photons routed on the photonic chip [4]. [1] P. Tonndorf, R. Schmidt, R. Schneider, J. Kern, M. Buscema, G. A. Steele, A. Castellanos-Gomez, H. S. J. van der Zant, S. Michaelis de Vasconcellos, and R. Bratschitsch, Optica 2, 347 (2015). [2] J. Kern, I. Niehues, P. Tonndorf, R. Schmidt, D. Wigger, R. Schneider, T. Stiehm, S. Michaelis de Vasconcellos, D. Reiter, T. Kuhn, and R. Bratschitsch, Advanced Mater. 28, 7101 (2016). [3] P. Tonndorf, S. Schwarz, J. Kern, I. Niehues, O. Del Pozo-Zamudio, A. I. Dmitriev, A. P. Bakhtinov, D. N. Borisenko, N. N. Kolesnikov, A. I. Tartakovskii, S. Michaelis de Vasconcellos, and R. Bratschitsch, 2D Mater. 4, 21010 (2017). [4] P. Tonndorf, O. Del Pozo-Zamudio, N. Gruhler, J. Kern, R. Schmidt, A. I. Dmitriev, A. P. Bakhtinov, A. I. Tartakovskii, W. Pernice, S. Michaelis De Vasconcellos, and R. Bratschitsch, Nano Lett. 17, 5446–5451 (2017).
Excitons, spins and plasmons (chair: Stephen Gray)
16:30 - 17:00 Alexander Steinhoff (Universität Bremen)
Excitons versus electron-hole plasma in monolayer transition metal dichalcogenide semiconductors
When electron-hole pairs are excited in a semiconductor, it is a priori not clear if subsequent relaxation leads to a gas of bound excitons or to an interacting plasma of unbound electrons and holes. Usually, the exciton phase is associated with low temperatures. In atomically thin transition metal dichalcogenide semiconductors, excitons are particularly important even at room temperature due to strong Coulomb interaction and a large exciton density of states. Using state-of-the-art many-body theory, we show that the thermodynamic fission-fusion balance of excitons and electron-hole plasma can be efficiently tuned via the dielectric environment as well as charge carrier doping. We observe entropy ionization of excitons at low excitation densities and a Mott transition to a fully ionized plasma at high densities between 3x10$^{12}$ cm$^{-2}$ and 1x10$^{13}$ cm$^{-2}$ depending on experimental parameters. Below the Mott transition, excitons become dominant with maximal fractions of excitons between 70 % and more than 99.9 %. Moreover, we find that excitonic screening, although two orders of magnitude less efficient than free-carrier screening at comparable excitation densities, plays an important role in the description of the exciton-plasma balance. We propose the observation of these effects by studying exciton satellites in photoemission and tunneling spectroscopy, which are sensitive to the single-particle spectral functions, thus containing information about the degree of exciton fission and the extent of exciton wave functions in reciprocal space. Photoemission spectroscopy presents a direct solid-state counterpart of high-energy collider experiments on the induced fission of composite particles.
17:00 - 17:15 Elena Shornikova (Technische Universität Dortmund)
Surface spins in colloidal CdSe nanoplatelets revealed by circular polarization of emission in strong magnetic fields
Surface of colloidal nanocrystals is an abrupt interface between the material, e.g. crystalline CdSe, and surrounding media. It is normally passivated by surfactant molecules (ligands), which can cause charge transfer from the surface atom. On the other hand, the absence of ligands results in dangling bonds, i.e. unpassivated atoms with excess electrons. In both cases, electron configuration of surface atoms is very different from the bulk. These atoms might act as localized spins, similar to Mn impurities in diluted magnetic semiconductors. This can play a crucial role in properties of small colloidal nanocrystal, where significant amount of atoms are located on surface. Here we report a magneto-optical study of exciton interaction with surface spins in colloidal two-dimensional CdSe nanoplatelets (NPLs) with 4.5 CdSe monolayer thickness (1.2 nm) and comparatively large lateral dimensions of the order of 10 nm. In these nanocrystals, 40% of Cd atoms are located on surface. The large exchange interaction of excitons with surface spins causes giant Zeeman splitting of exciton energy levels. The sign of exchange interaction is positive for NPLs synthesized in argon atmosphere and negative for NPLs prepared in ambient conditions. This results in different signs of circular polarization degree of photoluminescence in applied magnetic field.
17:15 - 17:30 Alexandra Crai (Imperial College London)
Linear and non-linear effects in the optically driven dynamics of an interacting electron gas in a nanosphere
Plasmonic nanostructures provide a suitable environment for light-matter interaction on the nanoscale. With recent experiments investigating smaller and smaller nanoparticles, quantum effects come into play and a microscopic description of the electron structure is required. In this work, we study the optically generated many-particle dynamics using the density matrix formalism providing a quantum picture of the optical response of a nanosphere. Considering a nanoparticle, discrete electronic states will form and the optical excitation will induce non-equilibrium dynamics of the excited electrons. Our model describes a metal nanosphere with analytic wave functions obeying the stationary Schrödinger equation with a hard-wall potential. The Hamiltonian takes into account both the Coulomb interaction between electrons and the interaction with an external electric field in dipole approximation. Due to its many-body nature, the Coulomb interaction leads to an infinite hierarchy of equations of motion. This is truncated on the mean-field (Hartree-Fock) level. The dynamics is calculated via the Heisenberg equations of motion between the Hamiltonian and the density matrix operator. We first analyse the linear optical response by exciting the partially filled few-electron system from the ground state with a weak short pulse and we calculate the total induced macroscopic polarisation from the light-induced coherences between states in the density matrix. The excited electrons will interact strongly via Coulomb interaction giving rise to new eigenstates of the system and a change in its optical response. We find that the Coulomb interaction is an essential factor in obtaining a collective response of the interacting electron-system and thus towards the microscopic description for the formation of plasmons. In the non-linear regime, the occupations of the system exhibit Rabi-like oscillations between the interacting eigenstates. Nevertheless, the dynamics following the excitation with a strong continuous pulse deviates considerably from the expected behaviour of Rabi oscillations in a quantum system. Higher harmonics of the Rabi frequency and a dynamical shift of the resonance is observed in the strongly driven electron system. We attribute these non-linear effects to the change in occupations leading to a change in the Coulomb interaction which in turn dynamically shifts the resonance frequency. Our approach provides an ideal platform to study non-linear and quantum plasmonic, fitting in between two limiting cases. One is the simplified one- or two-electron picture used in the description of Rabi oscillations and the other is the many-electrons picture common to describe plasmons within the RPA.
poster session (focus on even poster numbers)
Colloidal nanocrystals for the infrared, and novel nanostructures (chair: Mikhail Artemyev)
09:30 - 10:00 Philippe Guyot-Sionnest (The University of Chicago)
Auger processes in colloidal quantum dots and mid-infrared applications
Within the field of colloidal quantum dots and this workshop, multicarrier interactions and Auger processes remain a central research topic. The effect of carriers on the PL efficiency and stimulated emission (1,2) is relevant for light emitting devices while making small quantum dots with strong confinement but slow Auger is a basic challenge. A number of structures have shown slower Auger, and they all strongly reduce the confinement energy of one of the carriers in the region of wavefunction overlap. These include CdSe/CdS giant shells, (3,4) alloyed shells, (5) and type II structures that allow the smallest sizes. (6) It is also interesting to compare the Auger rates in CQDs with those of bulk materials. (7) Typically, very large Auger coefficients are associated with small gaps. It therefore came as a surprise when the biexciton lifetimes in small gap HgTe CQD (0.5 eV) were found to be similar to wide gap CdSe CQD (2.3 eV). (8) This suggests again that the confinement energy is a strong effect. It is also beneficial for applications since Auger is a fundamental limit to the performance of infrared devices based on bulk HgCdTe materials. These basic studies inform the development of CQD infrared devices for which there is a growing interest. Steady progress in CQD synthesis and device structure (9) has led to the demonstration of thermal imaging and detectivity approaching standard semiconductor detectors such as InSb (>50k$ camera) but with simple drop-cast PV devices. (10) (1) Light emission and amplification in charged CdSe quantum dots, C Wang, BL Wehrenberg, CY Woo, P Guyot-Sionnest, The Journal of Physical Chemistry B 108 (26), 9027-9031 2004. (2) Towards zero-threshold optical gain using charged semiconductor quantum dots, K Wu, YS Park, J Lim, VI Klimov Nature nanotechnology 12 (12), 1140 2017. (3) Towards non-blinking colloidal quantum dots, B. Mahler, P. Spinicelli, S. Buil, X. Quelin, JP.Hermier, B. Dubertret, Nature Mat. 7 (8) Pages: 659-664, 2008. (4) Suppressed auger recombination in "giant" nanocrystals boosts optical gain performance, F García-Santamaría, Y Chen, J Vela, RD Schaller, JA Hollingsworth, VI. Klimov, Nano letters 9 (10), 3482-3488 2009. (5) Suppression of Auger Processes in Confined Structures, G. E. Cragg and Al. L. Efros, Nano Lett., 2010, 10 (1), 313–317, 2009. (6) Small bright charged colloidal quantum dots, W Qin, H Liu, P Guyot-Sionnest, ACS nano 8 (1), 283-291, 2013 (7) Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals, I. Robel, R. Gresback, U. Kortshagen, RD. Schaller, VI. Klimov, Phys. Rev. Lett. 102, 177404, 2009. (8) Slow Auger Relaxation in HgTe Colloidal Quantum Dots, C Melnychuk, P Guyot-Sionnest, The journal of physical chemistry letters 9 (9), 2208-2211, 2018 (9) Fast and Sensitive Colloidal Quantum Dot Mid-Wave Infrared Photodetectors, MM Ackerman, X Tang, P Guyot-Sionnest, ACS nano 2018 (10) Thermal Imaging with Plasmon Resonance Enhanced HgTe Colloidal Quantum Dots Photovoltaic Devices, X Tang, MM Ackerman, P Guyot-Sionnest, ACS nano 2018
10:00 - 10:30 Libo Ma (IFW Dresden)
Photon-plasmon coupling in optoplasmonic microtube cavities
Microtubular cavities, which are self-assembled from prestrained nanomembranes, can support whispering-gallery mode (WGM) resonances in the rolled-up dielectric nanomembranes. Owing to the ultra-thin cavity wall, optical evanescent field greatly extend out of cavity surfaces, allowing for efficient interactions with the surrounding media. By coating plasmonic nanosturctures onto the microcavity surfaces, photon-plasmon coupling were studied both theoretically and experimentally relying on the interaction of WGM resonant light and localized surface plasmons, which leads to many novel phenomena on optical tuning and potential applications. As a novel platform, these optoplasmonic microcavities imply promising applications such as enhanced light-matter interactions, optical tuning, and opto-chemical sensing with on-chip integration.
11:00 - 11:30 Vladimir M. Fomin (IFW Dresden)
Topology- and geometry-driven effects in nanoarchitectures
An overview is given about some of topological effects, owing to special geometries in real space, implemented by the high-tech self-organization techniques to fabricate micro- and nanoarchitectures, e.g., nanostructured microtubes, microhelices and their arrays [1]. Combination of a geometric potential and an inhomogeneous twist renders an observation of the topology-driven effects in the electron ground-state energy in Möbius rings at the microscale into the area of experimental verification [2]. The rolled-up conical-shape asymmetric microcavities provide a background to realize the spin–orbit interaction of light for the analysis of topological effects in the course of a non-Abelian evolution of light [3]. Robustness of the topology-induced geometric phase of light opens novel ways of manipulating photons and thus implies promising perspectives of applications in on-chip quantum devices. Advances in the high-tech roll-up fabrication methods have provided qualitatively novel curved superconductor micro- and nanoarchitectures, Phonon spectra in multishell nanostructured microtubes imply that the number of shells as the key geometric characteristic is an important control parameter of the phonon dispersion along with the structure dimensions and acoustic impedance mismatch between the constituent layers [4]. Theoretical analysis performed on a large interval of wave vectors and in a wide spectrum of frequencies implies, in accord with experiment, a prominent effect of the number of layers on the phonon energy dispersion and group velocity as well as on the phonon transport. An effective approach to manage the thermal conductivity of Si thin-film-based nanoarchitectures has been realized through the formation of radial and planar Si/SiO$_x$ hybrid nanomembrane superlattices [5]. In superconductor helical microcoils, the distribution and number of vortices in a quasi-stationary pattern can be controlled by the helical radius, pitch distance and stripe width [6]. In the helical microcoils, quasi-degeneracy of vortex patterns, which emerges under the condition that the total number of vortices is incommensurable with the number of half-turns, opens up new possibilities for bifurcations and the related control of the vortex transport. In summary, rolled-up nanoarchitectures show fascinating potential in tailoring the electronic, optical and phonon properties because of topology and geometry-controlled effects. Acknowledgments: The present work has been supported by the by the European COST Action # CA16218 and the German Research Foundation (DFG) grant # FO 956/4-1. Fruitful collaboration with A. A. Balandin, D. Bürger, A. Cocemasov, D. Grimm, E. A. Levchenko, G. Li, S. Li, S. Lösch, L. Ma, A. Mavrokefalos, D. L. Nika, R. O. Rezaev, S. Singh, O. G. Schmidt, and F. Zhu is gratefully acknowledged. [1] V. M. Fomin, in: A. Sidorenko (Ed.), Functional Nanostructures and Metamaterials: From Superconducting Qubits to Self-Organized Nanostructures (Springer, Berlin – Heidelberg, 2018). [2] V. M. Fomin (Ed.), Physics of Quantum Rings, 2nd edition (Springer, Berlin–Heidelberg, 2018). [3] L. B. Ma, S. L. Li, V. M. Fomin, M. Hentschel, J. B. Götte, Y. Yin, M. R. Jorgensen, and O. G. Schmidt. Nature Communications (2016), 7, 10983. [4] V. M. Fomin and A. A. Balandin. Appl. Sci. (2015), 5, 728. [5] G. Li, M. Yarali, A. Cocemasov, S. Baunack, D. L. Nika, V. M. Fomin, S. Singh, T. Gemming, F. Zhu, A. Mavrokefalos, and O. G. Schmidt. ACS Nano (2017), 11, 8215. [6] V. M. Fomin, R. O. Rezaev, E. A. Levchenko, D. Grimm and O. G. Schmidt. Journal of Physics: Condensed Matter (2017) 29, 395301.
11:30 - 11:45 Thomas Brumme (Universität Leipzig)
Electrochemical doping of HfNCl: spin-orbit coupling, spin texture, and Ising superconductivity
In recent years it became possible to dope few-layer materials in the order of $10^{14}$ charge carriers $\mathrm{cm}^{-2}$ using ionic-liquid based field-effect transistors. This allows for the exploration of the semiconducting, metallic, superconducting, and charge-density-wave regimes in reduced dimensionality. Especially, the transition-metal dichalcogenides (TMDs) have received a lot of attention as possible candidates to build valleytronic devices in which the valley degree of freedom is used to store and process information. Furthermore, as the spin is polarized perpendicularly to the layer for charge carriers in the spin-orbit split band extrema, TMDs show so-called Ising superconductivity:[1-3] electrons with opposite out-of-plane spins in opposite K and K′ valleys form singlet Cooper pairs, thus increasing the upper critical field that is needed to destroy the superconducting state. The related transition-metal chloronitride HfNCl is usually not considered in this context, as the bands are not spin-orbit split due to the presence of inversion symmetry. Yet, in a field-effect setup the asymmetric external electric field leads to a breaking of the inversion symmetry. We calculate within density-functional theory[4,5] the band structure and spin-orbit splitting for field-effect doped HfNCl. We show how the external electric field leads to changes in the spin texture in the conduction band minimum and estimate the in-plane, upper critical field $H_{c2}^{||}$. [1] J. M. Lu, O. Zheliuk, I. Leermakers, N. F. Q. Yuan, U. Zeitler, K. T. Law & J. T. Ye, Science 350, 1353 (2015) [2] X. Xi, Z. Wang, W. Zhao, J.-H. Park, K. T. Law, H. Berger, L. Forró, J. Shan & K. F. Mak, Nat. Phys. 12, 139 (2016) [3] S. C. de la Barrera, M. R. Sinko, D. P. Gopalan, N. Sivadas, K. L. Seyler, K. Watanabe, T. Taniguchi, A. W. Tsen, X. Xu, D. Xiao & B. M. Hunt, Natu. Commun. 9, 1427 (2018) [4] T. Brumme, M. Calandra & F. Mauri, Phys. Rev. B 89, 245406 (2014) [5] T. Brumme, M. Calandra & F. Mauri, Phys. Rev. B 91, 155436 (2015)
11:45 - 12:00 Dirk Dorfs (Leibniz Universität Hannover)
Alternative plasmonic materials – colloid chemical synthesis, characterization and properties
The most famous classes of colloidal plasmonic nanoparticles are still Ag and Au nanostructures. Nevertheless, there are plenty of alternative materials other than elemental metals, which also show plasmonic properties. In this talk, several recently investigated alternative plasmonic materials ranging from degenerately doped semiconductors (e.g. Cu$_{2-x}$Se[1-5] to metallic compounds (e.g. Cu$_{1.1}$S[2] and various nickel sulfides[6-7]) in form of colloidal nanocrystals will be discussed concerning their colloid chemical synthesis and optical properties. It will be shown that the localized surface plasmon resonances (LSPRs) of these particles are capable of covering a wide spectral range (NIR and visible spectral range). Also, multicomponent nanocrystals composed of a combination of these alternative plasmonic materials and traditional metal domains will be presented and discussed.[6-8] 1. Wolf, A.; Hartling, T.; Hinrichs, D.; Dorfs, D., ChemPhysChem 2016, 17 (5), 717-723. 2. Wolf, A.; Kodanek, T.; Dorfs, D., Nanoscale 2015, 7 (46), 19519-19527. 3. Dilena, E.; Dorfs, D.; George, C.; Miszta, K.; Povia, M.; Genovese, A.; Casu, A.; Prato, M.; Manna, L., J. Mater. Chem. 2012, 22 (26), 13023-13031. 4. Scotognella, F.; Della Valle, G.; Kandada, A. R. S.; Dorfs, D.; Zavelani-Rossi, M.; Conforti, M.; Miszta, K.; Comin, A.; Korobcheyskaya, K.; Lanzani, G.; Manna, L.; Tassone, F., Nano Lett. 2011, 11 (11), 4711-4717. 5. Dorfs, D.; Hartling, T.; Miszta, K.; Bigall, N. C.; Kim, M. R.; Genovese, A.; Falqui, A.; Povia, M.; Manna, L., J. Am. Chem. Soc. 2011, 133 (29), 11175-11180. 6. Himstedt, R.; Hinrichs, D.; Dorfs, D., Z. Phys. Chem., 2018, published online, DOI: 10.1515/zpch-2018-1165 7. Himstedt, R.; Rusch, P.; Hinrichs, D.; Kodanek, T.; Lauth, J.; Kinge, S.; Siebbeles, L. D. A.; Dorfs, D., Chem. Mater. 2017, 29 (17), 7371-7377. 8. Wolf, A.; Hinrichs, D.; Sann, J.; Miethe, J. F.; Bigall, N. C.; Dorfs, D., J. Phys. Chem. C 2016, 120 (38), 21925-21931.