A new approach to vibrational sum frequency generation spectroscopy using near infrared pulse shaping

Chowdhury, Azhad U.; Watson, Brianna R.; Ma, Ying‐Zhong; Sacci, Robert L.; Lutterman, Daniel A.; Calhoun, Tessa R.; Doughty, Benjamin

doi:10.1063/1.5084971

Cited by 23 publications

(24 citation statements)

References 41 publications

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“…The decreasing spectral separation for transitions in larger particles is expected due to the convergence of the spacing between the levels, so while providing less insight into optical transitions here, this result serves to validate our measurements in terms of the limitations of the experiment. These spectral signatures might be better resolved in the future through upgrades to the experiment to utilize narrow band light as the up-conversion pulse , or by probing even fewer QDs in the excitation volume to reduce heterogeneous broadening.…”

Section: Resultsmentioning

confidence: 99%

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

2019

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Understanding and controlling the electronic structure of nanomaterials is the key to tailoring their use in a wide range of practical applications. Despite this need, many important electronic states are invisible to conventional optical measurements and are typically identified indirectly based on their inferred impact on luminescence properties. This is especially common and important in the study of nanomaterial surfaces and their associated defects. Surface trap states play a crucial role in photophysical processes yet remain remarkably poorly understood. Here we demonstrate for the first time that broadband electronic sum frequency generation (eSFG) microspectroscopy can directly map the optically bright and dark states of nanoparticles, including the elusive below gap states. This new approach is applied to model cadmium selenide (CdSe) quantum dots (QDs), where the energies of surface trap states have eluded direct optical characterization for decades. Our eSFG measurements show clear signatures of electronic transitions both above the band gap, which we assign to previously reported one- and two-photon transitions associated with the CdSe core, as well as broad spectral signatures below the band gap that are attributed to surface states. In addition to the core states, this analysis reveals two distinct distributions of below gap states, providing the first direct optical measurement of both shallow and deep surface states on this system. Finally, chemical modification of the surfaces via oxidation results in the relative increase in the signals originating from the surface states. Overall, our eSFG experiments provide an avenue to directly map the entirety of the QD core and surface electronic structure, which is expected to open up opportunities to study how these materials are grown in situ and how surface states can be controlled to tune functionality.

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Section: Resultsmentioning

confidence: 99%

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

2019

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“…SFG experiments were performed using an instrument detailed elsewhere. , Briefly, a broad-band mid-infrared (IR) light centered near 2900 cm –1 was colinearly combined with a narrow-band near-infrared light (NIR ∼1 nm, full width at half max at ∼800 nm) and focused on the liquid sample at a 60° angle with respect to the surface normal. After passing through the oil phase, the incident angles at the oil–aqueous interface were ∼37°. , Polarizations for both IR and NIR beams were varied with half-waveplates after being polarization purified using the appropriate polarizers.…”

Section: Methodsmentioning

confidence: 99%

Nanoparticle-Induced Disorder at Complex Liquid–Liquid Interfaces: Effects of Curvature and Compositional Synergy on Functional Surfaces

Basham

Premadasa

et al. 2021

ACS Nano

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The self-assembly of surfactant monolayers at interfaces plays a sweeping role in tasks ranging from household cleaning to the regulation of the respiratory system. The synergy between different nanoscale species at an interface can yield assemblies with exceptional properties, which enhance or modulate their function. However, understanding the mechanisms underlying coassembly, as well as the effects of intermolecular interactions at an interface, remains an emerging and challenging field of study. Herein, we study the interactions of gold nanoparticles striped with hydrophobic and hydrophilic ligands with phospholipids at a liquid–liquid interface and the resulting surface-bound complexes. We show that these nanoparticles, which are themselves minimally surface active, have a direct concentration-dependent effect on the rapid reduction of tension for assembling phospholipids at the interface, implying molecular coassembly. Through the use of sum frequency generation vibrational spectroscopy, we reveal that nanoparticles impart structural disorder to the lipid molecular layers, which is related to the increased volumes that amphiphiles can sample at the curved surface of a particle. The results strongly suggest that hydrophobic and electrostatic attractions imparted by nanoparticle functionalization drive lipid–nanoparticle complex assembly at the interface, which synergistically aids lipid adsorption even when lipids and nanoparticles approach the interface from opposite phases. The use of tensiometric and spectroscopic analyses reveals a physical picture of the system at the nanoscale, allowing for a quantitative analysis of the intermolecular behavior that can be extended to other systems.

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“…Approximately 2.4 W of the remaining laser output was directed to a 4f-pulse shaper equipped with a spatial light modulator to generate narrow-band NIR laser pulses used for upconversion. 26,28 A dichroic mirror was used to combine the IR and NIR beams in a collinear geometry. The light was then focused on the sample at an angle of 60°with respect to the surface normal.…”

Section: ■ Materials and Methodsmentioning

confidence: 99%

Mapping the Interfacial Chemistry and Structure of Partially Fluorinated Bottlebrush Polymers and Their Linear Analogues

Chowdhury¹,

Chang²,

et al. 2020

Langmuir

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Polymer interfaces are key to a range of applications including membranes for chemical separations, hydrophobic coatings, and passivating layers for antifouling. While important, challenges remain in probing the interfacial monolayer where the molecular ordering and orientation can change depending on the chemical makeup or processing conditions. In this work, we leverage surface specific vibrational sum frequency generation (SFG) and the associated dependence on molecular symmetry to elucidate the ordering and orientations of key functional groups for poly(2,2,2-trifluoroethyl methacrylate) bottlebrush polymers and their linear polymer analogues. These measurements were framed by atomistic molecular dynamic simulations to provide a complementary physical picture of the gas−polymer interface. Simulations and SFG measurements show that methacrylate backbones are buried beneath a layer of trifluoroethyl containing side groups that result in structurally similar interfaces regardless of the polymer molecular weight or architecture. The average orientational angles of the trifluoroethyl containing side groups differ depending on polymer linear and bottlebrush architectures, suggesting that the surface groups can reorient via available rotational degrees of freedom. Results show that the surfaces of the bottlebrush and linear polymer samples do not strongly depend on molecular weight or architecture. As such, one cannot rely on increasing the molecular weight or altering the architecture to tune surface properties. This insight into the polymer interfacial structure is expected to advance the design of new material interfaces with tailored chemical/functional properties.

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A new approach to vibrational sum frequency generation spectroscopy using near infrared pulse shaping

Cited by 23 publications

References 41 publications

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

Nanoparticle-Induced Disorder at Complex Liquid–Liquid Interfaces: Effects of Curvature and Compositional Synergy on Functional Surfaces

Mapping the Interfacial Chemistry and Structure of Partially Fluorinated Bottlebrush Polymers and Their Linear Analogues

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