From c561cd27239ede72090bfd8f8cc785c52ae48673 Mon Sep 17 00:00:00 2001 From: Blaise Thompson Date: Fri, 13 Apr 2018 18:16:17 -0500 Subject: 2018-04-13 18:16 --- introduction/chapter.tex | 96 ++++++++++++++++++++++++++++++++---------------- 1 file changed, 65 insertions(+), 31 deletions(-) (limited to 'introduction/chapter.tex') diff --git a/introduction/chapter.tex b/introduction/chapter.tex index 18ffd06..fe43cb4 100644 --- a/introduction/chapter.tex +++ b/introduction/chapter.tex @@ -25,9 +25,9 @@ dimensionality and selection rules. % With the advent of ultrafast lasers, CMDS can resolve dynamics in excited states and the coupling between them. \cite{RentzepisPM1970a} % -CMDS is most often performed in the time domain, where multiple broadband pulses are scanned to -collect a multidimensional interferogram. \cite{MukamelShaul2009a, GallagherSarahM1998a} % -% BJT: scanned in WHAT? delay? time? +CMDS is most often performed in the time domain, where multiple broadband pulses are scanned in +time (phase) to collect a multidimensional interferogram. \cite{MukamelShaul2009a, + GallagherSarahM1998a} % This technique is fast and robust---it has even been performed on a single shot. \cite{HarelElad2010a} % However time-domain CMDS has some fundamental limitations: @@ -132,7 +132,6 @@ decreased acquisition times by up to two orders of magnitude. % Like any analytical technique, MR-CMDS is subject to artifacts: features of the data that are caused by instrumental imperfections or limitations, and do not reflect the intrinsic material response that is of interest. % -% JCW: HOW THE EXPERIMENT WAS DONE, NOT WHAT IT IS HOPING TO MEASURE For example, consider well-known artifacts such as absorptive effects \cite{CarlsonRogerJohn1989a}, pulse effects \cite{SpencerAustinP2015a}, and window contributions \cite{MurdochKiethM2000a}. % Since MR-CMDS is a very active experiment, with many moving motors, an active approach to artifact @@ -154,13 +153,13 @@ Resonant responses are impulsive, like a hammer hitting a bell. % The impulsive limit is particularly well suited for describing time domain experiments. % In the driven limit, pulses are narrow in frequency and long in time compared to material response. % -Resonant responses are driven, like a jiggling jello dessert sitting on a washing machine. % +Resonant responses are driven, like jiggling jello dessert sitting on a washing machine. % The expected spectrum in both of these limits can be computed analytically. % Things get more complicated in the mixed domain, where pulses have similar bandwidth as the material response. % Experiments in this domain are a practical necessity as CMDS addresses systems with very fast -dephasing times. \cite{SmallwoodChristopherL2016a, PerlikVaclav2017a} % BJT: connect bw and - % dephasing +dephasing times. \cite{SmallwoodChristopherL2016a, PerlikVaclav2017a} +% BJT: connect bw and dephasing At the same time, the marginal resolution in frequency \emph{and} time that the mixed domain possess promises huge potential in pathway resolution and decongestion. \cite{PakoulevAndreiV2009a} % @@ -170,34 +169,69 @@ An intuitive description of mixed-domain experiments is given. % False signatures of material correlation are discussed, and strategies for resolving true material correlation are defined. % -In \hyperref[prt:applications]{Part III: Applications}, three projects in which MR-CMDS was used to +In \hyperref[prt:applications]{Part III: Applications}, four projects in which MR-CMDS was used to answer chemical questions in materials systems are described. % These chapters do not directly address improvements to the MR-CMDS methodology, but instead serve as case studies in the potential of MR-CMDS and the utility of the improvements described in \autoref{prt:development}. % -[PARAGRAPH ABOUT PbSe QUANTUM DOTS] - -%Chapter \ref{cha:pbx} describes a series of experiments performed on PbSe quantum dots. % -%Quantum dots are an excellent [STARTING SAMPLE... BEGINNING] -%PbSe quantum dots are useful because [...] % -%We learned [...] % - -[PARAGRAPH ABOUT MOS2] - -%Chapter [...] describes an experiment performed on MoS2. -%MoS2 is useful because [...] % -%We learned [...] % - -[PARAGRAPH ABOUT PEDOT:PSS] - -%Chapter [...] describes an experiment performed on PEDOT:PSS. -%useful because... -%we learned... - -% BJT: consider getting rid of the following paragraph -% if it remains, it needs to address a more 'broader impacts approach' rather than simply -% re-summarizing +In \autoref{cha:pss}, we employ transient grating MR-CMDS to interrogate the photophysics of +lead selenide (PbSe) quantum dots (QDs). % +PbSe QDs are an interesting semiconductor system with many appealing properties for basic method +development work. % +They are easy to synthesize, store and prepare in the solution phase, and they have bright and +relatively narrow band-edge excitons which are easy to interrogate using MR-CMDS. % +In \autoref{cha:pss}, we describe a simple approach to extracting the quantitative third-order +susceptibility of PbSe quantum dots using MR-CMDS. % +Using a simple approach of standard dilutions, we define this susceptibility in ratio to the known +well-quantified susceptibility of our solvent and cuvette windows. % +A few-parameter model is employed to extract this ratio. % +We are optimistic that this approach will be generally applicable, making it simple to perform +quantitative solution-phase MR-CMDS. % + +In \autoref{cha:psg} we continue to investigate PbSe QDs. % +Here we combine transient grating and transient absorption MR-CMDS to learn more about the +nonlinear spectrum near the band edge, around the 1S exciton. % +By combining both methods with the information from \autoref{cha:pss}, we are able to extract the +complete amplitude and phase of the non-linear susceptibility. % +We develop a simulation that relates the microscopic physics of PbSe electronic states to transient +grating and transient absorption spectra, and fit our model to both spectra simultaneously. % +Our model reveals the presence of continuum transitions, mostly invisible in typical transient +absorption experiments. % +We show that our model is able to describe spectra from two different syntheses with two different +sizes of quantum dot. % + +In \autoref{cha:mx2} we report the first MR-CMDS study performed on a molybdenum disulfide thin +film. % +MoS\textsubscript{2} is a member of a class of materials called transition metal dichacolgenides +which have recently attracted a large amount of attention for their unique photophysical +properties. % +These thin films have relatively low optical density and are highly scattering, making them +particularly challenging for MR-CMDS experiments. % +We employed several strategies to overcome these challenges, and performed three dimensional +frequency-frequency-delay transient grating spectroscopy to understand the basic coupling and +dynamics of MoS\textsubscript{2}. % +We show that the band-edge excitons of MoS\textsubscript{2} are not easily resolved, and the +dynamics of MoS\textsubscript{2} are fast. % +We develop a picture of MoS\textsubscript{2} electronic states that is consistent with our +results. % + +In \autoref{cha:pps} we use MR-CMDS to interrogate the dynamics of electronic states of +(PEDOT:PSS). % +PEDOT:PSS is a transparent, electrically conductive polymer. % +The exact origin of the conductivity is not well understood, so it is unclear how to improve the +conductivity or synthesize other conductive polymers. % +We performed photon echo experiments on PEDOT:PSS, directly interrogating the electronic states +that are responsible for conductivity in the polymer. % +Using a sophisticated model extended from the work in \autoref{cha:mix}, we constrain the pure and +ensemble dephasing lifetimes of PEDOT:PSS. % +These lifetimes can be directly related to the homogeneous and inhomogeneous broadening parameters +in PEDOT:PSS. % +Amazingly, we find that PEDOT:PSS has very broad homogeneous \emph{and} inhomogeneous +linewidths. % +We cannot constrain either quantity, but we can put lower limits on both. % +This basic information is complementary to other experiments in the ongoing effort to fully +understand PEDOT:PSS. % Despite challenges in software, hardware, and theory MR-CMDS is a crucial tool in the hands of scientists. % @@ -206,4 +240,4 @@ hardware development. % PyCMDS has enabled new experiments, and has made data collection faster and more artifact-free. % WrightTools has trivialized data processing, tightening the loop between idea and execution. % Theory can be used to guide experimental insight in the promising, if challenging, mixed domain. % -Applications of these ideas in three materials are presented. % +Applications of these ideas in three material systems are presented. % -- cgit v1.2.3