Experimental observation of exciton splitting and relaxation dynamics from PbS quantum dots in a glass matrix

Abstract

Multiple optical transitions from PbS quantum dots (QDs) in glass matrix are observed. Energy separations between them amount up to ∼200 meV. Instead of being due to a size distribution of QDs, they are found to be related to a splitting of the lowest exciton levels. Systematic analysis of the relaxation dynamics reveals the lifetimes of the split states ranging from sub-100-ns to ∼μs. Moreover, we observe excited and "intrinsic" states having a sub-100-ps and ∼ns lifetime, respectively. The behavior of the split structure can be modeled by a phonon-assisted relaxation mechanism. This investigation offers a conclusive interpretation for the wide distribution of experimentally observed nonequilibrium carrier lifetimes in PbS QDs.Lead salt (e.g., PbS) quantum dots (QDs) have been widely investigated for applications in optoelectronic devices [1,2]. The QDs are either colloidal or embedded into a glass matrix. The latter provides substantial advantages in terms of longterm stability [3]. In order to evaluate and improve the properties of materials or devices, the QD size and its distribution as well as the excitonic lifetime are major issues, which should be accurately characterized, e.g., directly by optical methods including steady-state (SS) or transient absorption and photoluminescence (PL) spectroscopy at room temperature [4].However, in rock salt PbS QDs the dimension of the excitonic manifold is theoretically 64 because both the valence-band maximum and the conduction-band minimum originate from the 8-fold (including spin) L valleys in the first Brillouin zone of bulk PbS. The degeneracy of these L valleys becomes lifted due to, e.g., intervalley coupling, interband coupling, and electron-hole Coulomb and exchange interaction [5]. Another mechanism potentially acting into this direction is external forces such as pressure as well as a special chemical surrounding, e.g., in a matrix. The split structure makes the optical transition in QDs complex and affects the characterization of QD features including the lowest excitonic transition energy (for simplicity called E g ) and subsequently the size and distribution of QDs as well as the nonequilibrium carrier recombination mechanism. In spite of the fact that asymmetric-or multi-PL (or absorption) peaks from these kinds of QDs have been observed even at room temperature, no direct reports involve these split states. Typically extrinsic assumptions are made including size distribution (e.g., multimodal QDs) [6], temperature/excitation-induced broadening [7], or defect states [8,9].In this Rapid Communication, PbS QDs embedded in a borosilicate glass matrix are prepared and characterized by SS-or transient-PL spectroscopy at low excitation densities and temperatures. A three-transition structure from the QDs is observed with an energy separation up to 200 meV, in dependence of growth conditions including annealing. The * yue@mbi-berlin.de intrinsic mechanism for this three-transition structure that seems to be sensitive to the host matrix, e.g., with borosilicate b...

Emission Kinetics from PbSe Quantum Dots in Glass Matrix at High Excitation Levels

“…Even more important, the integral intensity of the fast component depends in a superlinear way on excitation, as shown in Figure d, which shows a threshold‐like excitation value at 20 μJ · cm −2 ; and the fast component shows no dependencies of excitation, and even temperature, therefore, the fast component with a typical decay time of ∼40 ps could account for amplified spontaneous emission (ASE). This is consistent with the observation of stimulated emission in PbS:Glass material system …”

Epitaxial Dimers and Auger-Assisted Detrapping in PbS Quantum Dot Solids

“…Similar sub-band emission has been observed in lead chalcogenide QDs in a variety of chemical environments. [41][42][43][44] Chuang et al identified this state as the likely origin of the large open-circuit voltage deficit in PbS QD photovoltaics. 3 Although the trap state is lower in energy by an amount ($170 meV) many times greater than the available thermal energy at 300 K ($25 meV), the room-temperature emission spectrum is nonetheless dominated by the band edge.…”

Semiconductor Quantum Dots‐Embedded Inorganic Glasses: Fabrication, Luminescent Properties, and Potential Applications

“…[70] Instead of being due to the size difference of QDs, dual-band emission with energy separations of 200 meV was also found to be related to a splitting of the lowest exciton levels in PbS QDsdoped borosilicate glass with a rather narrow QD size distribution. [71] Large full-width at half-maximum (FWHM) of these dual-band PL (≈600 nm) has potentials for broad-band infrared light sources. [70,71] As for QDs-doped fiber amplifier (QDFA), both theoretical simulation and experimental exploration were carried out.…”

“…[71] Large full-width at half-maximum (FWHM) of these dual-band PL (≈600 nm) has potentials for broad-band infrared light sources. [70,71] As for QDs-doped fiber amplifier (QDFA), both theoretical simulation and experimental exploration were carried out. Wundke et al reported the optical gain at 1.3 µm of PbS QDs-embedded glass for the first time.…”