Browsing by Subject "Photoluminescence spectroscopy"

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Open Access
Microcavity coupled interlayer excitons in MOSE2-WSE2 heterostructures
(2024-08) Atalay, Şeyma Esra
After the discovery of graphene, two-dimensional (2D) materials gained immense attention due to their exceptional mechanical, optical, and electronic properties. One of the well-known family of 2D materials is transition metal dichalcogenides (TMDs). Their electronic bandgap makes a transition from indirect to direct when the monolayer limit is reached, making them an excellent medium for studying many-body interactions in condensed matter and light-matter interactions. Also, stacking monolayer TMDs on top of each other enables the creation of heterostructures (HS). The vertical van der Waals heterostructures made from monolayer TMDs can host two types of excitons: one is the intralayer excitons consisting of the strongly Coulomb-bound electron-hole pair within the same material, and the other one is the interlayer exciton made up by spatially separated electrons and holes located in different layers. Intralayer excitons in TMDs exhibit higher oscillator strengths due to spatial confinement. Furthermore, coupling intra- and interlayer excitons with optical cavities allows for observing anticrossing phenomena between excitons and cavity photons. Consequently, TMDs and their HSs provide a versatile platform for investigating exciton-polariton formation and their associated photophysical properties. This thesis presents a detailed study of the room- and low-temperature photoluminescence (PL) spectroscopy of interlayer excitons (IEX) in near-hexagonal MoSe2-WSe2 HSs. The studied hexagonal boron nitride (hBN) encapsulated heterostructures were fabricated using a combination of three methods: (i) mechanical exfoliation for cleaving the monolayers from bulk material, (ii) dry transfer technique to stack them vertically to achieve the heterostructure and (iii) edge identification method to adjust the twist angle between monolayers during stacking. Fabry-Pérot planar microcavities were fabricated for both room and low-temperature studies with different cavity modes using plasma-enhanced chemical vapor deposition with the aim of studying the interlayer exciton-polaritons. Fabricated cavities were further characterized by transmission electron microscopy and focused ion beam lithography for imaging the alternating layers of distributed Bragg mirrors (DBRs) and their thicknesses. The PL of IEXs in MoSe2-WSe2 heterostructures were measured both at room and low temperatures. Low-temperature PL measurements were conducted using a closed-cycle cryostat integrated into a home-built micro-PL setup. The results indicate that our heterostructures exhibit a near-hexagonal structure. The intensities of the spin-triplet and the spin-singlet states of the IEXs are prominent in the energy range of approximately 1.35-1.42 eV. The observed splitting between the spin-triplet and spin-singlet IEXs was in the range of 27-34 meV. Further investigations into the temperature and pump power dependence of the IEXs revealed that the intensity of the spin-triplet IEX is highly sensitive to temperature variations between 3.5 K and 50 K, becoming less intense at higher temperatures. Although the intensity of the spin-singlet IEX also decreased, it remained detectable at 50 K. With increasing pump power, a blueshift of 15 meV was observed for the spin-triplet IEX, while the spin-singlet IEX exhibited a blueshift of only 3 meV in the same pump power range. This indicates that the density of the spin-triplet state IEX increases more significantly with higher pump power than the spin-singlet state IEX. Additionally, the lifetime of the spin-singlet IEX was measured using Time-Resolved Photoluminescence (TRPL) spectroscopy. The fast and slow decay components were in the range of a few nanoseconds and a few tens of nanoseconds, respectively. Moreover, an approximately 9 meV splitting on the spin-singlet IEX PL emission was observed in one of the studied emitters, which has the same resonance wavelength as the FP cavity. This splitting might be attributed to the interlayer exciton-polariton formation at k|| equals zero. However, the whole angle-resolved PL spectrum should be measured to ensure this emission belongs to the polariton formation.
Open Access
Synthesis of ultra-small Si / Ge semiconductor nano-particles using electrochemistry
(Elsevier, 2012) Alkis, S.; Ghaffari, M.; Okyay, Ali Kemal
In this paper, we describe the formation of colloidal Si/Ge semiconductor nano-particles by electrochemical etching of Ge quantum dots (GEDOT), Silicon-Germanium graded layers (GRADE) and Silicon-Germanium multi-quantum well (MQW) structures which are prepared on Silicon wafers using low pressure chemical vapor deposition (LPCVD) technique. The formation of Si/Ge nano-particles is verified by transmission electron microscope (TEM) images and photoluminescence (PL) measurements. The Si/Ge nano-particles obtained from GEDOT and GRADE structures, gave blue emissions, upon 250 nm, and 300 nm UV excitations. However, the nano-particles obtained from the MQW structure did exhibit various color emissions (orange, blue, green and red) upon excitation with 250 nm, 360 nm, 380 nm and 400 nm wavelength light.
Open Access
Time resolved photoluminescence study of magnetic CdSe/CdMnS/CdS core/multi-shell nanoplatelets
(SPIE, 2017) Murphy, J. R.; Delikanlı, Savaş; Zhang, T.; Scrace, T. A.; Zhang, P.; Norden, T.; Thomay, T.; Cartwright, A. N.; Demir, Himli Volkan; Petrou, A.
Colloidal semiconductor nanoplatelets (NPLs) are quasi 2D-nanostructures that are grown and processed inexpensively using a solution based method and thus have recently attracted considerable attention. We observe two features in the photoluminescence spectrum, suggesting two possible recombination channels. Their intensity ratio varies with temperature and two distinct temperature regions are identified; a low temperature region (10K < T < 90K) and a high temperature region (90K < T < 200K). This ratio increases with increasing temperature, suggesting that one recombination channel involves holes that are weakly localized with a localization energy of 0.043meV. A possible origin of these localized states are energy-variations in the xy-plane of the nanoplatelet. The presence of positive photoluminescence circular polarization in the magnetically-doped core/multi-shell NPLs indicates a hole-dopant exchange interaction and therefore the incorporated magnetic Manganese ions act as a marker that determines the location of the localized hole states.1 Time-resolved measurements show two distinct timescales (τfast and τslow) that can be modeled using a rate equation model. We identify these timescales as closely related to the corresponding recombination times for the channels. The stronger hole localization of one of these channels leads to a decreased electron-hole wave function overlap and thus a decreased oscillator strength and an increased lifetime. We show that we can model and understand the magnetic interaction of doped 2D-colloidal nanoplatelets which opens a pathway to solution processable spin controllable light sources. Copyright © 2017 SPIE.
Open Access
Time-resolved photoluminescence study of CdSe/CdMnS/CdS core/multi-shell nanoplatelets
(American Institute of Physics Inc., 2016) Murphy, J. R.; Delikanli S.; Scrace, T.; Zhang, P.; Norden, T.; Thomay, T.; Cartwright, A. N.; Demir, Hilmi Volkan; Petrou, A.
We used photoluminescence spectroscopy to resolve two emission features in CdSe/CdMnS/CdS and CdSe/CdS core/multi-shell nanoplatelet heterostructures. The photoluminescence from the magnetic sample has a positive circular polarization with a maximum centered at the position of the lower energy feature. The higher energy feature has a corresponding signature in the absorption spectrum; this is not the case for the low-energy feature. We have also studied the temporal evolution of these features using a pulsed-excitation/time-resolved photoluminescence technique to investigate their corresponding recombination channels. A model was used to analyze the temporal dynamics of the photoluminescence which yielded two distinct timescales associated with these recombination channels. The above results indicate that the low-energy feature is associated with recombination of electrons with holes localized at the core/shell interfaces; the high-energy feature, on the other hand, is excitonic in nature with the holes confined within the CdSe cores.