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

Abstract

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.

In Situ Spectroscopic Probing of Polarity and Molecular Configuration at Aerosol Particle Surfaces

“…47,57,84−86 On the other hand, the development of electronic SFG and SHG spectroscopies has been left behind, even with tremendous effort. 70 However, an electronically sum frequency scattering (ESFS) technique capable of probing electronic characters of surfaces has yet to be developed.…”

“…Only a few techniques are available for investigating liquid surfaces, − not to mention the surfaces of liquid droplets. Second harmonic generation (SHG) and sum frequency generation (SFG) are two of the proven surface-sensitive methods to investigate liquid surfaces. ,− As a spectroscopic technique, SFG is anticipated to provide vibrational structures, ,− ,− as well as electronic characters, ,− both of which are essential to understanding chemical and physical properties of surfaces. Several groups demonstrated the applications of the SHG and SFG techniques in atmospheric science. ,− These pioneering works have shown cases of the powerful surface-specific techniques.…”

“…Several groups demonstrated the applications of the SHG and SFG techniques in atmospheric science. ,− These pioneering works have shown cases of the powerful surface-specific techniques. More than two decades ago, second harmonic scattering (SHS) was developed to extend the applications of the second-order nonlinear optical techniques from planar surfaces to curved surfaces. − Later, vibrationally sum frequency scattering (VSFS) was further developed to characterize vibrational chemical structures of micrometer to a few nanometer particle surfaces suspended in liquids. ,,− On the other hand, the development of electronic SFG and SHG spectroscopies has been left behind, even with tremendous effort. ,− For example, Shen et al showed interfacial electronic structure of molecules by tuning laser wavelengths . Eisenthal et al measured surface polarity at the air/water interface and interfacial electronic transition of a dye on TiO 2 surfaces .…”

“…Xiong et al measured interfacial electronic spectra of different polymer interfaces . Calhoun et al investigated the surface electronic structure of quantum dots using broadband ESFG microspectroscopy . However, an electronically sum frequency scattering (ESFS) technique capable of probing electronic characters of surfaces has yet to be developed.…”

Interface-Specific Two-Dimensional Electronic Sum Frequency Generation Spectroscopy

“…It has been well-known that a chirped light in 2D-ES experiments leads to distortion of 2D spectra. , Thus, it is critical to obtain a broadband laser with least chirp and high pulse energy for genuine 2D-ESFG spectra. Previous experiments of surface and interfacial electronic spectra such as ESFG relied upon scanning the narrow band input pulses. − Recently, a white light continuum was used to generate ESFG signal. ,− However, the white light is not only rather weak but also severely chirped in time. The chirped white light continuum is not suitable for implementing a 2D-ESFG technique.…”