Transient Nonlinear Spectroscopy Of Single Quantum Dots

Author : Aaron Jacob Goodman
Publisher :
ISBN 13 :
Total Pages : 164 pages
Book Rating : 4.:/5 (15 download)

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Download or read book Charge and Energy Transport in 0D/2D Systems Probed Using Linear and Nonlinear Spectroscopy written by Aaron Jacob Goodman and published by . This book was released on 2018 with total page 164 pages. Available in PDF, EPUB and Kindle. Book excerpt: Low-dimensional nanostructured semiconductors are promising technologies for next generation optoelectronics. Colloidal quantum dots (QDs) have been applied in many light emitting applications such as ambient lighting downconverters, LEDs, and displays. Proof-of-concept transistors, phototransistors and LEDs have been demonstrated using two dimensional atomically thin transition metal dichalcogenides (TMDs). To realize efficient next generation optoelectronics using these materials it is crucial to understand the dynamics and transport of energy and charge in these materials and hybrid structures built from them. The first Chapter of this thesis briefly motivates the technological importance of QDs and TMDs. The remainder explores exciton dynamics and transport in these low-dimensional semiconductor systems. In Chapter 2, low temperature spectroscopy and time-resolved photoluminescence spectroscopy are used to probe the complex energy landscape seen by excitons in acid-treated MoS2. We show that deeply trapped "dark" exciton states are responsible for the surprisingly long lifetime of band-edge photoluminescence in acid-treated single-layer MoS2 . Temperature-dependent transient photoluminescence spectroscopy reveals an exponential tail of long-lived states extending hundreds of meV into the band gap. These subband states, which are characterized by a 4 [mu]s radiative lifetime, quickly capture and store photogenerated excitons before subsequent thermalization up to the band edge where fast radiative recombination occurs. By intentionally saturating these trap states, we are able to measure the "true" 150 ps radiative lifetime of the band-edge exciton at 77 K, which extrapolates to ~600 ps at room temperature. These experiments reveal the dominant role of dark exciton states in acid-treated MoS2 , and suggest that excitons spend > 95% of their lifetime at room temperature in trap states below the band edge. We hypothesize that these states are associated with native structural defects, which are not introduced by the superacid treatment; rather, the superacid treatment dramatically reduces nonradiative recombination through these states, extending the exciton lifetime and increasing the likelihood of eventual radiative recombination. In the second half of Chapter 2, we study exciton diffusive transport in MoS2 using time-resolved diffusion imaging. We also probe exciton-exciton dynamics and elucidate the role that exciton traps play in both exciton transport and dynamics. Atomically thin semiconductors such as monolayer MoS2 and WS2 exhibit nonlinear exciton-exciton annihilation at notably low excitation densities (below ~10 excitons/pm 2 in MoS2 ). We show that the density threshold at which annihilation occurs can be tuned by two orders of magnitude by varying the refractive index of the underlying supporting substrate. Using spatially-resolved transient photoluminescence spectroscopy in conjunction with numerical simulations, we find that this behavior arises from screening interactions between trapped and mobile exciton pairs. We measure the effective exciton diffusion coefficient in superacid-treated MoS2 to be D = 0.06 ± 0.01 cm2/s, corresponding to a diffusion length of LD = 350 nm for an exciton lifetime of [tau] = 20 ns. Exciton-exciton annihilation limits the overall efficiency of 2D semiconductor devices operating at high exciton densities. The ability to tune these interactions via the dielectric environment is an important step toward more efficient optoelectronic technologies featuring atomically thin materials. In Chapter 3 we investigate coupled QD/TMD hybrid structures. In the first half of Chapter 3, we investigate dipole-dipole coupling in the regime that the QD and TMD exhibit weak electronic coupling by engineering the interface to be insulating. We report highly efficient F6rster resonant energy transfer from cadmium selenide (CdSe) quantum dots to monolayer and few-layer molybdenum disulfide (MoS2). The quenching of the donor quantum dot photoluminescence increases as the MoS 2 flake thickness decreases with the highest efficiency (> 95%) observed for monolayer MoS2. This counterintuitive result (that deviates from the predictions of F6rster theory) arises from reduced dielectric screening in thin layer semiconductors having unusually large permittivity and a strong in-plane transition dipole moment, as found in MoS2 In the second half of Chapter 3, we investigate QD/TMD hybrid structures in the strongly coupled regime. We demonstrate tunable electronic coupling between CdSe QDs and monolayer WS2 using variable length alkanethiol ligands on the QD surface. Using femtosecond time-resolved second harmonic generation (SHG) microscopy, we show that electron transfer from photoexcited CdSe QDs to single-layer WS 2 occurs on ultrafast (10 fs - 1 ps) timescales. Moreover, charge transfer excites coherent acoustic phonons in the donor QDs, which modulate the SHG response of the underlying WS2 layer on picosecond timescales. These results reveal surprisingly strong electronic coupling at the QD/TMD interface and demonstrate the usefulness of time-resolved SHG for exploring ultrafast electronic-vibrational dynamics in TMD heterostructures. In Chapter 4, we improve the sensitivity of traditional SHG spectroscopy by adding an additional field. We demonstrate the dramatic enhancement of weak second-order nonlinear optical signals via stimulated sum and difference frequency generation. We present a conceptual framework to quantitatively describe the interaction and show that the process is highly sensitive to the relative optical phase of the stimulating field. To emphasize the utility of the technique, we demonstrate stimulated enhancement of second harmonic generation (SHG) from bovine collagen-I fibrils. Using a stimulating pulse fluence of only 3 nJ=cm2 , we obtain an SHG enhancement > 104 relative to the spontaneous signal. We discuss the conceptual differences between optical heterodyning of SHG signals and our presented "stimulation" framework. In the second half of Chapter 4, we use the additional field to resolve the SHG signal phase revealing the sign and magnitude of the sample nonlinear susceptibility. We perform phase-resolved SHG imaging of polycrystalline MoS2 flakes and determine the absolute grain orientations using the phase information.