Fourier-Imaging of Single Self-Assembled CdSe Nanoplatelet Chains and Clusters Reveals out-of-Plane Dipole Contribution

Abstract

Fluorescent semiconductor nanoplatelets (epitaxial quantum wells) can be synthesized with excellent monodispersity and self-assembled in highly-ordered structures. Modifications of their electronic and luminescence properties when stacked, due to strong mechanical, electronic or optical interactions between them, have been the topic of intense recent discussions. In this paper, we use Fourier imaging to measure the different dipole components of various nanoplatelet assemblies. By comparing different measurement conditions and corroborating them with polarimetric analysis, we confirm an excellent precision on the dipole components. For single nanoplatelets, only in-plane dipoles (parallel to the platelet plane) are evidenced. For clusters of 2-10 platelets and chains of 30-300 platelets, on the other hand, a clear out-of-plane dipole component is demonstrated. Its contribution becomes more significant as the number of platelets is increased. We review possible explanations and suggest that the added out-of-plane dipole can be induced by strain-induced nanoplatelet deformations.Semiconductor nanoplatelets (NPLs) 1 are quasi-two-dimensional atomically flat nanoemitters with intense, spectrally monodisperse 2 and anisotropic 3-6 emission and potential applications in light-emitting devices 7 , photovoltaic cells 8 , field effect transistors 9 and lasers 10 . Fluorescence studies usually consider, as much as possible, isolated emitters, either by spectroscopy of dilute solutions or by single-molecule microscopy. Interactions between compact assemblies of stacked emitters are however receiving increased attention. In optoelectronics, for instance, these interactions must be understood as they can either be detrimental to a device's efficiency or allow new energy transfer strategies 8,[11][12] . Semiconductor platelets are an ideal model system for studying such interactions because they have a pronounced tendency to spontaneous cofacial stacking [13][14] and because, when stacked, their interactions should be strong as they present high absorption cross-section [15][16] , low Stokes shift and large in-plane dipoles 3,[5][6] which can be aligned parallel at very short distance (2-5 nm) from each other [17][18] . For instance, a second emission peak appears at low temperatures for stacked NPLs and was tentatively attributed to phonon coupling 13 , p-state

Manipulation of the Optical Properties of Colloidal 2D CdSe Nanoplatelets

“…Self‐assembly‐mediated aggregation describes the process in which individual building blocks spontaneously aggregate and these blocks are packed into ordered superstructures on a macroscopic scale. Due to the unique properties of a single CdSe NPL, [ 133 ] including narrow emission linewidth, small Stokes shift, high PLQY, and unity in‐plane transition dipole moment (TDM), [ 137 ] the CdSe NPL superlattice becomes an ideal platform for investigating the underlying collective optical properties that may not appear in bulk counterparts, isotropic nanostructures, or NPL solid‐state ensembles with random dipole orientations. Moreover, the close distance (2–5 nm) between adjacent NPLs in assembled cofacial stacks triggers strong electronic coupling and ultrafast exciton transfer over a long range, which could be exploited to improve efficiencies in diverse advanced optoelectronic devices.…”

“…[ 51 ] Then, the presence of out‐of‐plane dipole components in NPL aggregates was also confirmed via Fourier‐imaging techniques. [ 137 ] Previous study of intrinsic TDMs in ML CdSe NPL film via 2D k‐space spectra characterization has manifested the 3D isotropic orientation of absorption dipoles and 2D in‐plane distribution of emission dipoles. [ 146 ] Based on their interpretations of experimental results, the effects of anisotropy of the electronic Bloch states account for the presence of anisotropic 2D emission dipoles.…”

Tailoring Experimental Configurations to Probe Transition Dipoles of Fluorescent Nanoemitters by Polarimetry or Fourier Imaging with Enhanced Sensitivity

“…For example, in a recent paper, we used Fourier imaging combined with polarimetry to probe quantitatively the three dipole components (η x , η y , and η z ) in stacked chains of nanoplatelets. 11 The dipole components are then obtained by fitting the Fourier image with the theoretical radiation patterns, which again take into account the experimental conditions (substrate, objective, etc.). 15 Combining Fourier imaging and polarization analysis would be quite challenging for a single nanoemitter, but it has been performed for various ensembles of dipoles, for instance, on a layer of nanoplatelets to analyze the ratio between in-plane and out-of-plane dipoles (which determines its efficiency as a lightemitting device) 17 or on rare-earth ions to compare the contributions of electric and magnetic dipoles.…”

“…Rectangular nanoplatelets were shown to behave as a sum of two orthogonal dipoles of different oscillator strengths 9 while three orthogonal dipoles were necessary to model a chain of selfassembled nanoplatelets 11 . For 2D sheets of semiconductors, in-plane dipoles were evidenced with MoS2 12 , MoSe2 and WSe2 13 while layered InSe flakes displayed a single out-of-plane dipole 13 .…”

“…A first range of methods used polarimetric analysis of the luminescence, either by decomposition into several polarization components 20,21 or by using a rotating polarizer to measure the degree of polarization 3,4,22 . A second class of methods uses the radiation pattern (angular distribution of the emission) measured by Fourier (back focal plane) imaging 9,11,23,24 or indirectly by defocused imaging 19,[25][26][27] . A widespread variant used polarized Fourier imaging to distinguish in-plane and out-of-plane dipole contributions 7,8,[12][13][14]28 .…”