No abstract

Light-induced orientation of electron spins in the negatively charged InP quantum dots is found to persist longer than 100 s. We have proved experimentally that the long-lived orientation is due to slow relaxation of the electron spins rather than to the dynamic nuclear polarization effects.

Articles you may be interested inQuasi-Fermi level splitting and sub-bandgap absorptivity from semiconductor photoluminescence Observation of room temperature optical absorption in InP/GaAs type-II ultrathin quantum wells and quantum dots

We report a two-photon responsive drug delivery system (DDS) namely, p-hydroxyphenacyl-naphthalene-chlorambucil (pHP-Naph-Cbl) having two-photon absorption (TPA) cross-section of ≥ 20 GM in the phototherapeutic window (700 nm). Our DDS exhibited...

We study optical transitions from a periodic array of InP/InAs/InP core-multishell nanowires ͑CMNs͒ having a wurtzite crystal structure by using photoluminescence ͑PL͒ and PL excitation ͑PLE͒ spectroscopy. Observing a large Stokes shift between PL and PLE spectra, a blueshift of the PL peak with a cube-root dependence on the excitation power and a slow and nonexponential decay of PL with an effective decay time of 16 ns suggest a type-II band alignment. Band-offset calculation based on the "model-solid theory" of Van Nanometer-scale semiconductor heterostructures such as quantum dots, quantum wires, and quantum wells ͑QWs͒ have been interesting research targets due to their unique size-dependent electronic and optical properties associated with the lower dimensionality and quantum confinement effects. There have been revived interests in semiconductor nanowires ͑NWs͒ due to recent success in growth and fabrication of a regular array of core-shell and core-multishell NWs ͑CMNs͒.1,2 Semiconductor NWs can be used as building blocks of sophisticated nanoscale electronic and photonic devices.3 Much effort has been devoted so far to the growth and fabrication of CMNs and the periodic arrays of such structures.1,2 However, very little is known about the electronic structure and optical properties of this new class of technologically important structures.In this letter we report optical studies on the periodic array of InP/InAs/InP CMNs. Both InP and InAs have a wurtzite crystal structure in the NW form, 4-6 although bulk InP and InAs crystallize to a zinc-blende structure. Moreover, in our CMN sample a thin InAs layer is surrounded by thick InP layers from all sides and the InP layers act like a mold. As a first approximation, we may consider that the InAs lattice experiences a three-dimensional compressive strain and attains the size of the InP lattice in all directions unlike InAs/InP QWs, where the InAs lattice is compressed in the crystal growth plane and is elongated in the growth direction. There have been a few studies on ultrathin InAs/ InP QWs having zinc-blende structure. 7 These experimental results and theoretical calculations using an envelopefunction scheme with effective-mass approximation and empirical tight-binding model 8 seem to show type-I direct transitions in zinc-blende InAs/InP QWs, although some calculations also predict type-II behavior.9 In contrast, the electronic structure and optical properties of wurtzite InP and InAs are almost unknown. 5,10 Due to the differences in crystal structure and strain, the insights gathered from the studies on zinc-blende InAs/InP QWs cannot be applied directly to wurtzite InP/InAs/InP CMNs.We study the InP/InAs/InP CMNs using time-resolved ͑TR͒ and spectrally resolved ͑SR͒ photoluminescence ͑PL͒ and PL excitation ͑PLE͒ measurements. A large Stokes shift between PL and PLE spectra was observed, which along with the absence of strong PLE peak suggests type-II radiative recombination. With increasing excitation power ͑P͒ the PL peaks show a blueshift with a cu...

Semiconductor nanostructures with near-unity photoluminescence quantum yields (PLQYs) are imperative for light-emitting diodes and display devices. A PLQY of 99.7 ± 0.3% has been obtained by stabilizing 91% Sn2+ in the Dion–Jacobson (8N8)­SnBr4 (8N8-DJ) perovskite with 1,8-diaminooctane (8N8) spacer. The PLQY is favored by a longer spacer molecule and out-of-plane octahedral tilting. The PLQY shows one-month ambient stability under high relative humidity (RH) and temperature. With n-octylamine (8N) spacer, Ruddlesden–Popper (8N)2SnBr4 (8N-RP) also shows PLQY of 91.7 ± 0.6%, but it has poor ambient stability. The 5–300 K PL experiments decipher the self-trapped excitons (STEs) where the self-trapping depth is 25.6 ± 0.4 meV below the conduction band because of strong carrier–phonon coupling. The microsecond long-lived STE dominates over the band edge (BE) peaks at lower excitation wavelengths and higher temperatures. The higher PLQY and stability of 8N8-DJ are due to the stronger interaction between SnBr6 4– octahedra and 8N8 spacer, leading to a rigid structure.

Experimental investigation of nuclear spin effects on the electron spin polarization in singly negatively charged InP quantum dots is reported. Pump-probe photoluminescence measurements of electron spin relaxation in the microsecond timescale are used to estimate the time-period TN of the Larmor precession of nuclear spins in the hyperfine field of electrons. We find TN to be ∼ 1 µs at T ≈ 5 K, under the vanishing external magnetic field. From the time-integrated measurements of electron spin polarization as a function of a longitudinally applied magnetic field at T ≈ 5 K, we find that the Overhauser field appearing due to the dynamic nuclear polarization increases linearly with the excitation power, though its magnitude remains smaller than 10 mT up to the highest excitation power (50 mW) used in these experiments. The effective magnetic field of the frozen fluctuations of nuclear spins is found to be 15 mT, independent of the excitation power.

The spin dephasing relaxation in single-electron-doped InP quantum dots was studied by means of Hanle measurements. When an InP quantum dot is doped with one electron on average, a narrow Lorentzian dip with half-width of 4.6 mT appeared and was superposed on two Lorentzians with half-widths of 1.54 and 128 mT in the Hanle curve. The half-widths 1.54 T, 128 mT, and 4.6 mT are ascribed to spin-dephasing relaxation of holes, electron-hole pairs, and doped electrons consistuting negative trions, respectively. The corresponding spin coherence time of the doped electrons at 5 K is 1.7 ns, which is determined by the frozen fluctuation of nuclear spins in the quantum dots. With increase of temperature, the spin-dephasing rate of the doped electrons increases.