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.
We studied the photoluminescence (PL)) from CdSe/CdMnS/CdS core/multi-shell colloidal nanoplatelets, a versatile platform to study the interplay of optical properties and nanomagnetism. The photoluminescence (PL) exhibits σ+ polarization in the applied magnetic field. Our measurement detects the presence of even a single magnetic monolayer shell. The PLL consists of a higher and a lower energy component; the latter exhibits a circular polarization peak. The time-resolved PL (trPL) shows a red shift as function of time delay. At early (later) times the trPL spectra coincide with the high (low) energy PL component. A model is proposed to interpret these results.
B. Barman, Y. Tsai, T. Scrace, J. Murphy, A. Cartwright, J. Pientka, I. Zutic, B. McCombe, A. Petrou, I. Sellers, R. Oszwaldowski, A. Petukhov, W. C. Fan, W. C. Chou, C. S. Yang
We used time resolved photoluminescence (TRPL) spectroscopy to compare the properties of magnetic polarons in
two related, spatially indirect, II-VI epitaxially grown quantum dot systems. In sample A (ZnMnTe/ZnSe), the photoexcited
holes are confined in the magnetic ZnMnTe quantum dots (QDs), while the electrons remain in the surrounding
non-magnetic ZnSe matrix. In sample B (ZnTe/ZnMnSe) on the other hand, the holes are confined in the non-magnetic
ZnTe QDs and the electrons move in the magnetic ZnMnSe matrix. The magnetic polaron formation energies, EMP , in
these samples were measured from the temporal red-shift of the excitonic emission peak. The magnetic polarons in the
two samples exhibit distinct characteristics. In sample A, the magnetic polaron is strongly bound with EMP=35 meV.
Furthermore, EMP has unconventionally weak dependence of on both temperature T and magnetic field Bappl . In
contrast, magnetic polarons in sample B show conventional characteristics with EMP decreasing with increasing
temperature and increasing external magnetic field. We attribute the difference in magnetic polaron properties between
the two types of QDs to the difference in the location of the Mn ions in the respective structures.
Optomechanical actuation was achieved reversibly using highly oriented pyrolytic graphite intercalated with bromine (1.9 mol% Br2). White light from a 150 W tungsten-halogen lamp was used for optomechanical switching. The displacement was approximately 4 micrometers and occurred only along the c-axis of the graphite. The rise and fall times were approximately 15 s. The origin of the optomechanical effect is the reversible exfoliation of the near surface region of the intercalated graphite.
We present a photoluminescence and Raman study of a GaAs/AlAs multiple quantum well structure doped n-type in the AlAs barriers. Electrons transfer from the silicon donor states associated with the AlAs X-valley minima to the GaAs wells where they form a dense quasi two-dimensional electron gas. At zero magnetic field the luminescence spectrum is broad and featureless lacking any sharp excitonic features characteristic of undoped wells. In the presence of a magnetic field applied parallel to the growth axis the luminescence exhibits a number of distinct lines that are due to interband transitions between conduction and valence band Landau levels. The Raman spectra were excited in resonance with the e3h3 exciton. At zero field a feature associated with the e1 yields e2 intersubband transition is observed. When an external magnetic field is applied, this feature evolves into a combination mode between the e1 yields e2 transition and the electron cyclotron resonance.
A Nd:YAG LASER operating at 1.064 micrometers was used to deposit CdS thin films on glass and GaAs (100) substrates. Depending on the substrate temperature, the as-deposited films were either cubic or hexagonal dominant. At a substrate temperature of approximately 200 degree(s)C, predominantly cubic structured films were grown while at approximately 400 degree(s)C the hexagonal phase was formed. The films were in the stoichiometric ratio of 1:1 for Cd:S. The surface of the films were optically smooth. Optical transmission revealed the room temperature absorption edge for the cubic films was close to 515 nm, and that of the hexagonal films was approximately 500 nm. At a temperature of 10 K, the absorption edge of the cubic film was 502 nm. Raman spectrum studies revealed the locations of the first two longitudinal optical (LO) modes for the cubic films were close to that of the hexagonal films.
The results of magnetoreflection, photoluminescence and photoluminescence
excitation experiments are reported in GaAs-AlAs multiple quantum wells of
different well widths, demonstrating the influence of the X-band in the AlAs on
the electron levels in GaAs. Evidence is presented for the existence of an
exciton formed from a delocalized electron, and for the conduction of electrons
from narrow wells to wide wells via the X-band of AlAs.
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