We present spatially-, temporally- and polarization-resolved dual photoluminescence/linear dichroism microscopy experiments that investigate the correlation between long-range order and the nature of exciton states in solution-processed phthalocyanine thin films. The influence of grain boundaries and disorder is absent in these films because typical grain sizes are 3 orders of magnitude larger than focused excitation beam diameters. These experiments reveal the existence of a delocalized singlet exciton, polarized along the high mobility axis in this quasi-1D electronic system. The strong delocalized π orbitals overlap, controlled by the molecular stacking along the high mobility axis, is responsible for breaking the radiative recombination selection rules. Using our linear dichroism scanning microscopy setup, we further established that a rotation of molecules (i.e., a structural phase transition) that occurs above 100 K prevents the observation of this exciton at room temperature.
Using time-resolved photoluminescence spectroscopy, we studied the electronic levels of semiconductor nanocrystals ͑NCs͒ with small spin-orbit coupling such as CdS. Low temperature radiative lifetimes indicate that the lowest-energy transition changes from orbital allowed to orbital forbidden with decreasing NC size. Our results are well explained by a size-dependent hierarchy of s-and p-orbital hole levels that is in agreement with theoretical predictions. Around the critical NC radius of ϳ2 nm we observe an anticrossing of s-and p-orbital hole levels and large changes in transition rates.Size tunability of optical properties and flexibility of surface chemistry make semiconductor nanocrystals ͑NCs͒ ͑Refs. 1 and 2͒ attractive fluorophores for a wide range of applications including solid-state lighting, 3,4 lasing, 5,6 and fluorescent labeling. 7,8 High emission efficiencies with quantum yields larger than 50% were demonstrated, 9-11 despite the optically passive character of the lowest-energy transition in many NCs. 12-20 The high emission yields at room temperature ͑RT͒ are obtained because of thermally populated excited transitions that are characterized by high oscillator strengths. Understanding such important optical properties such as radiative lifetimes requires a detailed knowledge of the electronic level structure. Extensive studies of CdSe NCs led to the concept of dark and bright excitons 12-14 that allowed explaining radiative lifetime measurements. 20-23 Significantly less experimental studies exist on radiative lifetimes and the electronic level structure of NCs made of other semiconductors, including CdS. 15-17 Besides the applicationrelevant emission in the blue and UV spectral range, 24-26 the importance of CdS NCs arises from the distinct differences in the electronic band structure between CdS and CdSe. Specifically, CdS is a representative of semiconductors ͑e.g., sulphides and phosphides͒ with very small spin-orbit coupling ⌬ SO in contrast to CdSe which has a large spin-orbit coupling.In this Brief Report, we discuss radiative lifetimes and the electronic level structure of CdS NCs ͑as an example of a small ⌬ SO material system͒ using time-resolved photoluminescence ͑PL͒ measurements. We determined temperatureand size-dependent radiative lifetimes of CdS NCs and analyzed them with a model that takes into account two lowestenergy transitions between s-orbital electron and s-or p-orbital hole levels. We show that the radiative lifetime in certain NCs is accelerated at low temperatures ͑LTs͒. Moreover, we demonstrate experimental evidence that the orbital symmetry of the hole ground state in CdS NCs depends on the NC size, a phenomenon that has only been predicted theoretically. 27,28 Around the critical NC size, our measurements indicate an anticrossing of the s-and p-orbital hole levels that is accompanied by significant changes in the transition rates. Our work highlights the interesting regime of intermediate CdS sizes and bridges the gap between a few reports on small and large CdS NCs. [...
Exploration of optical properties of organic crystalline semiconductors thin films is challenging due to submicron grain sizes and the presence of numerous structural defects, disorder and grain boundaries. Here we report on the results of combined linear dichroism (LD)/ polarization-resolved photoluminescence (PL) scanning microscopy experiments that simultaneously probe the excitonic radiative recombination and the molecular ordering in solution-processed metal-free phthalocyanine crystalline thin films with macroscopic grain sizes. LD/PL images reveal the relative orientation of the singlet exciton transition dipoles at the grain boundaries and the presence of a localized electronic state that acts like a barrier for exciton diffusion across the grain boundary. We also show how this energy barrier can be entirely eliminated through the optimization of deposition parameters that results in films with large grain sizes and small-angle boundaries. These studies open an avenue for exploring the influence of long-range order on exciton diffusion and carrier transport.
The origins of spin exchange in crystalline thin films of Copper Octabutoxy Phthalocyanine (Cu-OBPc) are investigated using Magnetic Circular Dichroism (MCD) spectroscopy. These studies are made possible by a solution deposition technique which produces highly ordered films with macroscopic grain sizes suitable for optical studies. For temperatures lower than 2 K, the contribution of a specific state in the valence band manifold originating from the hybridized lone pair in nitrogen orbitals of the Phthalocyanine ring, bears the Brillouin-like signature of an exchange interaction with the localized d-shell Cu spins. A comprehensive MCD spectral analysis coupled with a molecular field model of a σπ − d exchange analogous to sp-d interactions in Diluted Magnetic Semiconductors (DMS) renders an enhanced Zeeman splitting and a modified g-factor of −4 for the electrons that mediate the interaction. These studies define an experimental tool for identifying electronic states involved in spin-dependent exchange interactions in organic materials.
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