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Optische und elektronische Eigenschaften von Halbleiter Nanopartikeln

From spontaneous emission and its decay we extract energies of ground and excited states, energy transfer processes, radiative and non-radiative lifetimes, excitonic effects and their interaction with phonons or many-particle Coulomb interactions and the influence of external electric fields. We strive for highly-sensitive detection on a single nanoobject and single photon level. Non-linear optical experiments let us also study, for instance, the two-photon absorption cross sections and spectra of these nanoobjects.


Riccardo Scott

Dr. Alexander Achtstein

Linear Properties

Directed emission of CdSe nanoplatelets originating from strongly anisotropic 2D electronic structure


Intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy- efficient display technology. Here, we propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters.

Scott, R. and Heckmann, J and Achtstein, A. W. et al. Nat. Nano (2017).

Excited Exciton State Luminescence in CdSe Nanoplatelets: Lateral Confinement and an LO-Phonon Bottleneck


We evidence excited state emission from p states well below ground state saturation in CdSe nanoplatelets. Size-dependent exciton ground and excited state energies and population dynamics are determined by four independent methods: time-resolved PL, time-integrated PL, rate equation modeling, and Hartree renormalized k · p calculations—all in very good agreement. The ground state–excited state energy spacing strongly increases with the lateral platelet quantization. Depending on its detuning to the LO phonon energy, the PL decay of CdSe platelets is governed by a size tunable LO phonon bottleneck, related to the low exciton-phonon coupling, very large oscillator strength, and energy spacing of both states. This control of the dynamics by the lateral size on the one hand and of optical transition energy by their thickness on the other is, for instance, ideal to tune lasing properties CdSe platelets. 

Achtstein, A. W. and Scott, R. et al. Physical Review Letters116(11), 116802 (2016)

Thickness and Lateral Size Dependency of the Intrinsic Absorption in CdSe nanoparticles


For many optical investigations the knowledge of linear absorption cross sections or intrinsic absorption coefficients is essential to determine the absolute particle concentrations of nanoparticles in colloidal solutions.

We derive intrinsic absorption coefficients μi from inductively coupled plasma−atomic emission spectroscopy (ICP-AES) measurements and find good agreement with a theoretical approach by a continuum absorption Lorentz local field model. Nanoplatelets’ intrinsic absorption coefficients μi are strongly thickness and aspect ratio dependent and generally larger than in dots and rods. This gives possibility to tune their absorption properties by the lateral size and thickness and makes platelets more efficient absorbers with higher light−matter interaction.

Achtstein, A. W. et al. J. Phys. Chem. C119(34), pp 20156–20161 (2015)

Two-Dimensional Colloidal CdSe platelets / Electronic Structure and Exciton−Phonon Interaction


The electronic structure of ultrathin zinc-blende two-dimensional (2D)-CdSe nanosheets was studied theoretically, by Hartreerenormalized k·p calculations including Coulomb interaction, and experimentally, by temperature-dependent, time-resolved photoluminescence measurements. The 2D-heavy hole exciton states show a strong influence of vertical confinement and dielectric screening. A very weak coupling to phonons results in a low phonon-contribution to the homogeneous line-broadening so that 2D-nanoplatelets exhibit much narrower ensemble absorption and emission linewidths compared to colloidal CdSe dots or rods.

[2] Achtstein, A. W et al. (2012), Nano Letters, 12(6), 3151–3157

Self-Assembly of CdSe Nanoplatelets into Stacks of Controlled Size Induced by Ligand Exchange


In this paper we present a simple method for the preparation of highly stable colloidal solutions of individual nanoplatelets (NPls) with increased fluorescence quantum yield and a versatile procedure of NPls self-assembly into stacks of controlled size. Dynamic light scattering technique has been demonstrated to be simple and accurate method for in situ studies of the growth kinetics of NPls aggregates. The self-assembly method introduced in this work is based on the exchange of ligands on the surface of CdSe nanoplatelets. Hexadecylphosphonic acid allows control of the average size (length) of NPls stacks in a broad range by varying its concentration and reaction time. The main mechanism governing controlled formation of NPls stacks is based on strong van der Waals interaction between rigid brushes of alkyl chains on the surface of neighboring NPls. The interaction strength and, consequently, the length and colloidal stability of stacks have been shown to be dependent on type and concentration of different ligands.

[4] Antanovich, A. et al., J. Phys. Chem. C, 120 (10), pp. 5764–5775 (2016)



CdSe Nanocrystals in external electric fields

Time-Resolved Stark Spectroscopy in CdSe Nanoplatelets: Exciton Binding Energy, Polarizability and Field-Dependent Radiative Rates


We present a study of the application potential of CdSe nanoplatelets, as field controlled emitters. Their emission can be changed by 28 % upon application of electrical fields up to 175 kV/cm, a very high modulation depth for field controlled nanoemitters. From our experimental results we estimate the exciton binding energy in 5.5 ML CdSe nanoplatelets to be  EB=170 meV; CdSe NPLs exhibit highly robust excitons which are stable even at room temperature. This opens up the possibility to tune the emission and recombination dynamics efficiently by external fields. With our developed field-dependent population model treating all occurring field-dependent effects in a global analysis we are able to quantify e.g. the ground state exciton transition dipole moment (3.0 x 10-29 Cm) and its polarizability, which determine the radiative rate, as well as the (static) exciton polarizability (8.6 x 10-8 eV cm2 / kV2), all in good agreement with theory. Our results show that an efficient field control over the exciton recombination dynamics, emission linewidth and emission energy in these nanoparticles is feasible and opens up application potential as field-controlled emitters.

[5] Scott, R., Achtstein, A. W. et al., Nano Letters (2016)

Electroabsorption of CdSe Nanocrystals


A change in the optical properties in response to an electric field is employed in many photonic components such as electro-optical modulators or switches - essential to the world of modern communication. We experimentally demonstrate that the exposure of the nanoplatelets to a dc electric field leads to a drastically reduced absorption efficiency, found to be more than 10 times the change in the absorption experienced by quantum dots. These are results of the quantum-confined Stark and Franz_Keldysh effects.

[6] Achtstein, A. W., et al., ACS Nano 8(8), 7678–7686 (2014)

Two-Photon Absorption (TPA)


The understanding of the spectral dependence of the two-photon absorption is of strong interest for the design of nanocrystals with optimized two-photon absorption properties for bioimaging, phototherapy and nonlinear optoelectronics applications.

Directed Two-Photon Absorption in CdSe Nanoplatelets Revealed by k‑Space Spectroscopy


We show that two-photon absorption (TPA) is highly anisotropic in CdSe nanoplatelets. This promotes them as a new class of directional two-photon absorbers with large cross-sections. In contrast to isotropic one-photon absorption, TPA is shown to be directional. We recover the internal transition dipole distribution and find that this directionality arises from the intrinsic directionality of the underlying Bloch and envelope functions of the states involved. We note that the photo-emission from the CdSe platelets is highly anisotropic following either one- or two-photon excitation. Given the directionality and high TPA cross-section of these platelets, they may, for example, find employment as efficient logic AND elements in integrated photonic devices, or directional photon converters.

Heckmann, J. and Scott, R. and Achtstein, A. W. et al., Nano Lett. (2017).

Anisotropic confinement / Tayloring two-photon absorption spectra in CdS nanorods


Absolute two-photon absorption cross sections are obtained by means of a new reference dyebased method. The two-photon spectrum features of rods strongly differ from those of dots, due to the distinct energy structure of quasi-1D systems. The transversal confinement is found to dominate the energy of the absorption maxima while the longitudinal one dominates their absorption intensity. This suggests two-photon transition energy and intensity can be controlled independently in nanorods.

[4] Achtstein, A. W. et al. (2013), J. Phys. Chem. C, 117(48), 25756−25760

[5] Achtstein, A. W. et al. (2015), J. Phys. Chem. C, 119(2), 1260–1267

Record Two-photon absorption cross sections in 2D CdSe nanoplatelets / The largest ever reported for (colloidal) semiconductor nanocrystals


2D nanoplatelets combine large particle volumes with ultra strong confinement. In contrast to weakly confined nanocrystals, the TPA cross sections of CdSe nanoplatelets scale superlinearly with volume (V2) and show ten times more efficient two-photon absorption than nanorods or dots. This unexpectedly strong shape dependence goes well beyond the effect of local fields. Electronic confinement as well as local field effects favour the platelets and make them unique two-photon absorbers with outstanding cross sections of up to 107 GM. These mechanisms are possibly transferable to other material systems.

Scott, R., et al., Nano Lett. 15(8), 4985–4992 (2015)

Type II / Hetero -Nanocrystals


Beyond the possibilities to alter physical properties by engineered size, composition, shape and dimensionality hetero nanoparticles offer a further possibility to tune their properties by engineering their built-in type I or II transition. Type-II heteronanoplatelets obtained by colloidal synthesis (CdSe–CdTe and inverted CdTe–CdSe heteronanoplatelets) exhibit efficient spatially indirect radiative exciton recombination with a quantum yield as high as 23%.

Recombination dynamics in CdSe–CdTe Type II hetero nanoplatelets / Type II excitons for lasing applications


From a kinetic analysis we deduce the temperature dependence of the non-radiative and radiative lifetimes of hetero CdSe-CdTe nanoplates. Compared to core only platelets we observe a significant prolongation of the radiative lifetime in type II platelets by two orders in magnitude while the quantum yield is barely affected. We conclude that the observed biexponential PL decay behavior in hetero platelets is predominately due to spatially indirect excitons being present at the hetero junction and not ionized e–h pair recombination. This suggest that lasing applications might strongly benefit from a broad gain spectrum generated by the excitonic type II transition.

[7] R. Scott et al. (2016), Phys. Chem. Chem. Phys. (tba)



Scott, R., Achtstein, A. W., Prudnikau, A. V, Antanovich, A., Siebbeles, L. D. A., Artemyev, M., & Woggon, U. (2016). Time-Resolved Stark Spectroscopy in CdSe Nanoplatelets: Exciton Binding Energy, Polarizability, and Field-Dependent Radiative Rates. Nano Letters. doi:10.1021/acs.nanolett.6b03244

Achtstein, A. W., Scott, R., Kickhöfel, S., Jagsch, S. T., Christodoulou, S., Bertrand, G. H. V., … Woggon, U. (2016). p -State Luminescence in CdSe Nanoplatelets: Role of Lateral Confinement and a Longitudinal Optical Phonon Bottleneck. Physical Review Letters, 116(11), 116802. doi:10.1103/PhysRevLett.116.116802

Scott, R., Kickhöfel, S., Schoeps, O., Antanovich, A., Prudnikau, A., Chuvilin, A., … Achtstein, A. W. (2016). Temperature dependent radiative and non-radiative recombination dynamics in CdSe–CdTe and CdTe–CdSe type II hetero nanoplatelets. Phys. Chem. Chem. Phys., 18(4), 3197–3203. doi:10.1039/C5CP06623A

Antanovich, A., Prudnikau, A., Matsukovich, A., Achtstein, A., & Artemyev, M. (2016). Self-Assembly of CdSe Nanoplatelets into Stacks of Controlled Size Induced by Ligand Exchange. Journal of Physical Chemistry C, 120(10), 5764–5775. doi:10.1021/acs.jpcc.5b12139


Achtstein, A. W., Antanovich, A., Prudnikau, A., Scott, R., Woggon, U., & Artemyev, M. (2015). Linear Absorption in CdSe Nanoplates: Thickness and Lateral Size Dependency of the Intrinsic Absorption. The Journal of Physical Chemistry C, 119(34), 20156–20161. doi:10.1021/acs.jpcc.5b06208

Scott, R., Achtstein, A. W., Prudnikau, A., Antanovich, A., Christodoulou, S., Moreels, I., … Woggon, U. (2015). Two Photon Absorption in II-VI Semiconductors: The Influence of Dimensionality and Size. Nano Letters, 15, 4985–4992. doi:10.1021/acs.nanolett.5b00966

Achtstein, A. W., Ballester, A., Movilla, J. L., Hennig, J., Climente, J. I., Prudnikau, A., … Woggon, U. (2015). One- and Two-Photon Absorption in CdS Nanodots and Wires: The Role of Dimensionality in the One- and Two-Photon Luminescence Excitation Spectrum. The Journal of Physical Chemistry C, 119(2), 1260–1267. doi:10.1021/jp511346w

Antanovich, A., Prudnikau, A., Melnikau, D., Rakovich, Y., Chuvilin, A., Woggon, U., … Artemyev, M. (2015). Colloidal synthesis and optical properties of type-II СdSe-CdTe and inverted CdTe-CdSe core-wings heteronanoplatelets. Nanoscale, 8084–8092. doi:10.1039/C4NR07134D


Achtstein, A. W., Prudnikau, A. V, Ermolenko, M. V, Gurinovich, L. I., Gaponenko, S. V, Woggon, U., … Rukhlenko, I. D. (2014). Electroabsorption by 0D, 1D, and 2D Nanocrystals : A Comparative Study of CdSe Colloidal Quantum Dots , Nanorods, and Nanoplatelets. ACS Nano, 8(8), 7678–7686. doi:10.1021/nn503745u


Achtstein, A. W., Hennig, J., Prudnikau, A., Artemyev, M. V, & Woggon, U. (2013). Linear and Two-Photon Absorption in Zero- and One-Dimensional CdS Nanocrystals: In fl uence of Size and Shape. The Journal of Physical Chemistry C, 117, 25756−25760. doi:dx.doi.org/10.1021/jp407453e


Achtstein, A. W., Schliwa, A., Prudnikau, A., Hardzei, M., Artemyev, M. V, Thomsen, C., & Woggon, U. (2012). Electronic structure and exciton-phonon interaction in two-dimensional colloidal CdSe nanosheets. Nano Letters, 12(6), 3151–7. doi:10.1021/nl301071n

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