Energetics at the Surface: Direct Optical Mapping of Core and Surface

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Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots using Broadband Electronic Sum Frequency Generation Microspectroscopy Brianna Renee Watson, Benjamin Doughty, and Tessa R. Calhoun Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02201 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 1, 2019

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Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots using Broadband Electronic Sum Frequency Generation Microspectroscopy Brianna R. Watson1, †, Benjamin Doughty2*, Tessa R. Calhoun1* 1

Department of Chemistry, University of Tennessee, Knoxville, Tennessee

2 Chemical

Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee

†

Current Address: Program in Cellular and Molecular Medicine, Boston Children’s Hospital, Boston, Massachusetts

Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Keywords: nonlinear microscopy, surface chemistry, nanomaterials, trap states, defects

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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 bandgap 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 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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Introduction Understanding and controlling nanomaterial surfaces is a well-known barrier to developing the next generation of these materials for practical applications.1-5 Such applications are wide ranging and include catalytic transformations,6-10 chemical sensors,11-13 and biological imaging.14, 15

Of relevance to the work presented here are semiconducting quantum dots (QDs) that through a

size-tunable electronic structure have proven to be advantageous in both light absorbing and light emitting applications.2, 3, 16-31 The tunable optical properties are generally considered to be a bulk characteristic arising from confinement in the core of the nanoparticle; however, uncoordinated surface atoms and related point defects can result in states that are optically dark and serve as charge trapping sites.22, 32-34 The presence and characteristics of these defects are typically inferred by measuring the photoluminescence (PL) spectrum/quantum yield or excited state dynamics, which are influenced by surface functionalization and termination due to the aforementioned surface trapping.26, 33, 35-45 In fact, the presence of only a handful of defects at the surfaces of QDs can strongly suppress quantum yields by opening up new nonradiative relaxation pathways.10, 46-52 An atomistic understanding of both the characteristics and origins of these surface trap states is an active field of research, where a large diversity of types and sources have been reported that strongly depend on the nanoparticle’s composition, crystal structure, surface passivation, and growth conditions.34, 38, 46, 52-56 When considering the large number of different samples that have been studied and the diversity in methodologies applied to elucidate the effect of their surface chemistry and associated surface state properties, a large breadth of work emerges proposing states with ranging energies and mechanisms of action.39-44, 54, 56, 57 As such, understanding the nature of these states represents a critical aspect to the continued development of new technologies based on these materials, and specifically includes the direct measurement of the surface states and extraction of their associated energies unambiguously. A major challenge in probing surface states is that they are optically dark; transitions from the valence band directly to these states are forbidden within the dipole approximation.33 As such, directly probing these states using traditional absorption linear spectroscopies is generally not possible hindering optical characterization. Emission from these states can provide key information on the exit channel for relaxation but is often weak or nonexistent at ambient conditions. Additionally, while powerful and readily available, PL methods are inherently indirect probes of surface states as their energies are inferred by measuring emitted intensity and spectra 3 ACS Paragon Plus Environment

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(or the lack there of), which represents the excited state exit channel after photoexcitation and other relaxation processes, including charge trapping. Only somewhat recently has research been directed at elucidating the energetics of QD interfaces using a variety of methods such nonlinear spectroscopies,58-61 conductance spectroscopy,62 and scanning tunneling microscopy.53, 63 Despite these advances, there remains ambiguities in the literature regarding the energies of these surface localized states with proposed “shallow” and “deep” surface states expected to lie energetically anywhere between 10 meV and 300 meV below the band edge, respectively.39, 41, 62 Clearly, new methods capable of directly interrogating the surface electronic structure of nanomaterials under a wide variety of different experimental conditions are essential to elucidate the energies of these fundamentally important states. To address this challenge and provide an avenue to directly probe these interfacial states, we have developed a novel low-cost broadband electronic sum frequency generation (eSFG) microspectroscopy platform that is capable of directly probing the complete optical electronic structure of a small ensemble of CdSe QDs, including their surface states and other one photon forbidden transitions. eSFG is considered to be a surface specific nonlinear optical technique that is pictorially described in Figure 1. In eSFG experiments, two incident laser pulses are focused onto a sample of interest; the first is broadband white light spanning the visible spectral range, which induces a polarization in the sample (straight, colored arrows). A second non-resonant laser pulse (straight, black arrows) interacts with the sample to induce a second-order polarization that oscillates Figure 1: Energy level diagram illustrating resonant enhancement of eSFG by various transitions. Weak or forbidden transitions (purple, such as two photon transitions) are probed equally as well as core transitions (black). Below gap transitions arising from surface states (blue) are also directly probed, which is not possible using linear spectroscopies

and therefore radiates light at the sum and difference frequencies of the incident light fields (wavy arrows point back to the ground state in Figure 1). Since the overall


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incident field interactions are even-order, only molecules or atoms in non-centrosymmetric and/or anisotropic environments (e.g., the non-centrosymmetric crystal structure of the CdSe wurtzite core and the CdSe surface) can generate appreciable coherent signals. In instances where the wavelength of either one of the incident fields or the radiated sum-frequency light is resonant with electronic transitions from the ground to the excited state, there is an additional enhancement of the radiated light. In our system, the broadband visible light is designed to be resonant with onephoton allowed core transitions and the dark surface states. Resonance enhancement with the sumfrequency light (two-photon enhancement) occurs for higher lying two-photon transitions. An important advantage of eSFG, is that this resonant enhancement has been shown to also apply to states that are dipole forbidden.64 This provides an avenue to generate an optical spectrum analogous to that obtained with traditional UV-Vis spectroscopy but uniquely sensitive to surface species and transitions that are one-photon forbidden. As we will show below, eSFG allows us to directly measure the energies of the optically allowed one-photon transitions of the core (traditionally measured via PL and UV-Vis), two-photon core transitions, and forbidden transitions to both core and surface states in a single experiment at ambient conditions. Despite its potential, there are limited examples of eSFG being utilized in the literature.6571

None of these existing studies have investigated the electronic structure of nanomaterials despite

the importance of their surfaces. A recent SFG study probed the ligand vibrational modes on nanoparticles surfaces in the presence of a pulse resonant with the QD exciton transitions but was not able to extract any electronic structure information beyond the first exciton.72 The limited use of eSFG is in part due to challenges in the typical implementation of second order spectroscopies that rely on expensive low-repetition rate (kHz) amplified laser systems to produce high peak powers. While these instruments provide light fields capable of producing both the white light and up-conversion pulses necessary eSFG measurements, they also make signal averaging time prohibitive in many cases since the incident lasers must be attenuated and loosely focused to avoid sample damage. This limits the spectral and chemical fidelity one can achieve in an experiment by averaging for very long times over an ensemble of local chemical environments. As an alternative to amplifier-based systems, our approach couples a low cost, high-repetition rate (MHz) femtosecond oscillator with a microscopic platform.73 The instrument is shown in Figure 2 and described in detail in the Supporting Information and elsewhere.73 Briefly, the output of a Ti:sapphire oscillator split into two excitation paths. The pulse widths in both paths are minimized 5 ACS Paragon Plus Environment

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with prism compression lines. In order to generate the broadband visible light, one path is focused into a photonic crystal fiber (PCF) to produce supercontinuum. A temporal delay line is included in the 800 nm up-conversion beam path in order to scan over the temporally chirped visible bandwidth that is used to encode the spectral response (Figure S3).73 The two excitation beams are introduced into a microscope objective in a total internal reflection (TIR) geometry. Both of the excitation beams were introduced with p-polarization in order to ensure complete total internal reflection. We have also previously shown that this improves the intensity of the eSFG light collected in our microscope.73

Figure 2: Experimental schematic of the eSFG microspectroscopy platform used in this work and described in the Supporting Information. WP – half waveplate, TFP – thin film polarizer, PO – pickoff mirror, Obj1 – focusing objective, Obj2 – collection objective, DC1 – dichroic optic, Pol – GlanLaser polarizer, AWP – achromatic half waveplate, DC2 – optional dichroic/filters for epi-imaging, RC – reflective collimator, FO – fiber optic bundle.

In TIR microscopy, a wide field of the sample is illuminated by the incident beams which are brought in at an angle such that they totally internally reflect off of the sample coverslip due to the change in the media index of refraction. This generates an evanescent field that penetrates a few hundred nanometers into the sample, and this compression of the field in the z-direction in TIR is sufficient to enable nonlinear microscopy.74-76 The dramatic increase in repetition rate relative to 6 ACS Paragon Plus Environment

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amplified systems coupled with access to the complete visible spectral region via the PCF supercontinuum allows us to probe both a wide range of transitions above and below the bandgap of CdSe QDs in only a few minutes. Results and Discussion Three different sizes of CdSe QDs were studied with diameters of 4.6 nm, 5.6 nm and 6.9 nm. The UV-Vis spectra for each (Figure 3) was fit to a series of Gaussian peaks to estimate the energies one-photon allowed transitions arising from the core of the QDs. These values match well with the literature values that are provided in Tables 1, S1 and S2. Given the wurtzite crystal structure of the CdSe QDs used in this work, eSFG signals are expected to originate from both the core and the surface species. Given the enhancement of transitions in eSFG is dependent on more than just the dipole strength,64 the intensity of the peaks arising from the core and surface species as well as one- vs. two-photon transitions will differ from other, more conventional spectroscopies. This becomes immediately apparent when the eSFG spectra presented in Figure 4 are considered.

Figure 3: Fitted UV-Vis linear absorption spectra of 4.6, 5.6, and 6.9 nm CdSe QDs. The gray line is the data, the black line is the fit, and the colored peaks indicate the individual peaks comprising the fit. The energies of each of these peaks is shown in Tables 1, S1 and S2.

Figure 4 shows the eSFG spectra for each of the different QD sizes. The top spectra are those from QDs immediately following solvent evaporation after the dilute samples were dropcast onto the microscope coverslip. The bottom spectra were collected on the same samples >24 hours later allowing the surfaces of the QDs to oxidize. The raw data in each spectrum is shown with a gray line, and to estimate the spectral band position, we fit the data to an incoherent sum of Gaussian functions, which is represented by a black line. The individual peaks comprising this fit are shown as the colored lines below the data and fits. For each particle size, a plurality of sharp peaks are readily observed above and below the band gap that change with oxidation in ambient conditions. 7 ACS Paragon Plus Environment

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The function used to fit the data incorporates a few approximations to those typically employed in second order nonlinear spectroscopies.65 First, the nonresonant contribution is assumed to be negligible. It was calculated in a recent 2-color SFG work that the nonresonant contribution from a QD sample was 1- to 2-orders of magnitude smaller in amplitude than the contribution from the resonant vibrational modes being probed.72 The approximation to neglect the nonresonant contribution is even more robust in eSFG because of the much larger susceptibilities associated with electronic transitions in comparison to vibrational ones.77 This approximation is also consistent with previous work using both SHG and eSFG spectroscopies.58, 65

A second approximation in our fitting function is the omission of interference between eSFG

resonances.69 While this approximation is more likely to affect our fits, it was necessary to reduce the number of fitting parameters in order to avoid overfitting the data. Specifically, additional resonances were only added to the fit if they reliably lowered the reduced 𝜒2 value. Our methodology and approximations were validated by the resulting fits yielding excellent agreement with previous work as will be discussed below.

Figure 4: eSFG spectra from the three different sized CdSe QD samples studied here. Fresh and oxidized samples are shown at the same sample location as collected on different days. The solid black line is a fit to the data (grey), whereas colored Gaussians are the individual components used in the fit. Dashed components represent features below the bandgap as described in the text and the dashed vertical line in each spectrum represents the bandgap energy.

Turning to previous literature that mapped the electronic structure of CdSe QDs using low temperature one and two photon PLE measurements, we can readily assign the fit bands to discrete transitions; a summary of this information is contained in Tables 1, S1, and S2. It is worth noting 8 ACS Paragon Plus Environment

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that we are comparing eSFG micro-spectra collected at ambient conditions (at room temperature, in air) with photoluminescence excitation (PLE) measurements made at cryogenic temperatures, which was necessary to resolve the closely spaced features. In fact, this comparison shows that by probing a micro-ensemble of CdSe QDs using eSFG we limit the inhomogeneous broadening arising from a large distribution of particles and can quickly perform measurements at ambient conditions. Core Transitions The eSFG spectrum for a freshly cast sample of 4.6 nm QDs is shown in the upper left of Figure 4 and the peak wavelengths extracted from the fit are shown in Table 1. Starting with the bands energetically at and above the band edge, the characteristic first exciton peak corresponding to the 1S3/2 -1Se transition appears at 598 nm in the eSFG spectrum, in excellent agreement with our UV-Vis measurements, which measured it at 595 nm. Using the 595 nm transition wavelength, we extrapolate from previously published PLE studies the relative energies for higher lying states that are both one- and two-photon allowed.78, 79 It should be noted that the data set in the literature for the two-photon PLE measurements does not encompass the QD sizes we investigated greatly increasing the uncertainties in the energies we extrapolated for these transitions.79 From energies reported from these PLE experiments, the next highest lying state is be the one-photon forbidden 1P3/2-1Se, expected near 590 nm.79 We tentatively assign this state to our eSFG peak at 581 nm that is not observed in our UV-Vis spectra, whereas Schmidt et al. noted that although their model extracted a resolved energy for this transition, they were unable to separate it from the 1S3/2 - 1Se experimentally.79 The next feature in our eSFG spectrum at 568 nm agrees well with a peak in our UV-Vis at 565 and the optically allowed 2S3/2-1Se.78 The two-photon 2P5/2-1Se was also detected near this energy at 570 nm.79 Next, the somewhat broad feature at 557 nm in the eSFG spectrum is assigned to the unresolved set of 2P3/2-1Se, and 1P1/2-1Se transitions expected at ~550 nm. These are not observed in linear spectra, supporting their assignment to one-photon forbidden transitions. The peak at 538 nm in the eSFG spectrum can be assigned to either the optically allowed 1S1/2-1Se or 1P3/2 – 1Se states, the former is a weak optical transition78 and the latter forbidden79 – thus explaining their absence in our UV-Vis spectrum. Finally, the peak at ~522 nm is attributed to the 1P3/2-1Pe transition, in accordance with previous work that shows is appearance at ~525 nm78 although the 1S3/2-1Pe transition was also detected in a similar energetic position.79 We also 9 ACS Paragon Plus Environment

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detected a peak at 505 nm in our UV-Vis spectrum which is energetically consistent with a 2S1/21Se transition,78 but this feature was out of range for our eSFG measurements. peak color

eSFG fresh  (nm)

navy

eSFG oxidized  (nm)

UV-Vis  (nm)

1p PLE  (nm)a

505

500 525

2p PLE  (nm)b

assignment

2S1/2-1Se (e) 1P3/2-1Pe (d) magenta 522 1S3/2-1Pe 540 1S1/2-1Se (c) purple 538 538 545 2P5/2-1Se 550 2P3/2-1Se blue 557 550 1P1/2-1Se 570 1P5/2-1Se green 568 568 565 570 572 2S3/2-1Se (b) olive 581 582 590 1P3/2-1Se orange 598 599 595 595 595 1S3/2-1Se (a) red 619 637 surface states dark red 658 689 surface states Table 1: Transition wavelengths (nm) obtained from the eSFG spectra of fresh and oxidized 4.6 nm CdSe QDs are plotted here and compared to literature values from one- (1p) and two-photon (2p) PLE studies and their corresponding assignments. aEstimated wavelengths extracted from Figure 4 in Ref. 75. bEstimated wavelengths extracted from Figure 3 in Ref. 76 521 525

Using the same analysis, features in the 5.6 QDs can be assigned to core transitions (Figure 4 top, middle and Table S1). Briefly, the peak found at 619 nm for the freshly prepared 5.6 nm QDs is in excellent agreement with the observed first excitonic transition observed by our UV-Vis at 619 nm and the expected literature value for 1S3/2-1S3.78 Here this state is unresolved from the neighboring 1P3/2-1Se transition expected at ~614 nm.79 The feature at 606 nm in the eSFG spectra is attributed to an unresolved mixture of the one-photon 2S3/2-1Se and two-photon 1P5/2-1Se states. The peak at 588 nm observed with eSFG agrees with the presence of the two-photon transitions to 2P3/2-1Se and 2P5/2-1Se states expected at 588 nm and 580 nm.79 The feature at 569 nm agrees with previous PLE measurements showing a feature at 570 nm corresponding to a 1S1/2-1Se transition, which again is expected to be weak and is indeed not observed in our UV-Vis spectrum.78 There are also the two-photon transitions 1P1/2-1Se, and 1S3/2-1Pe nearby at 575 and 560 nm, respectively.79 The highest energy peak we could detect in this eSFG spectrum is at 545 nm and can be assigned to the 1P3/2-1Pe which is observed at 540 nm in our UV-Vis and ~555 nm in the previous PLE studies.78, 79


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The spectral congestion should increase as the QD gets larger and the splitting between levels decreases. This is apparent in the 6.9 nm QDs (Figure 4 top, right and Table S2), where far fewer states are clearly resolved. Adding more resonances to the fitting function can reproduce some more subtle aspects of the data such as weak shoulders, etc. that appear to correspond to states anticipated by previous work; as the addition of these resonances did not improve the quality of the fit, they were removed to prevent overfitting the data. As such, the eSFG spectra for the 6.9 nm QDs were fit to broad features that represent the convolution of several closely spaced transitions. Attempts to present assignments for these features can be found in the SI (Table S2); however, we note additional uncertainty in these assignments due to the limited ability to resolve individual peaks. 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 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 pulse80, 81 or by probing even fewer QDs in the excitation volume to reduce heterogeneous broadening. Surface Transitions While the eSFG spectra clearly provide insight into the QD core transitions the true strength of this measurement comes in its detection of features below the bandgap. While the UVVis spectra are devoid of features here, this mid-band gap energy range contains the surface states that are forbidden via one photon optical transitions.33,

38

For CdSe QDs, it has been well

established that these states arise from trapped holes on under-coordinated Se atoms at the surface.34, 82 In the eSFG data presented here, a wide distribution of overlapping features below the bandgap are observed for all QD samples measured. For the two smaller sizes, 4.6 nm and 5.6 nm, two different distributions are necessary to fit the data in this region. While the eSFG spectra for the 6.9 nm QDs can be fit with only a single distribution in the below gap region, we believe that this is largely limited by the energetic range we could detect with our instrument. These surface states are qualitatively found to have much broader distributions than those from the core transitions. Due to both the number of QDs generating this signal and the approximations of our fitting function; however, we are unable to definitively conclude the origin of the broad surface state widths. For the systems with two different distributions, one is relatively “shallow” (~100 11 ACS Paragon Plus Environment

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meV below the band edge) while the other distribution lies deeper within the gap (>200 meV from the edge). While it might appear that there is additional structure in the below gap energetic region than captured with our fit, the inclusion of more resonances in the fit was not found to quantifiably improve the fit quality. The energies of surface states lying several hundred eV below the band edge were detected in wurtzite CdSe QDs using conductance spectroscopy.62 In contrast, explanation for other experimental PL observations have relied on shallow trap states lying closer to the conduction band.39, 41 The presence of two different distributions of surface trap states for CdSe QDs has been previously shown in theoretical studies,56 and experimentally, two different surface trapping sites have also been elucidated on zinc-blende CdSe QDs.46, 52 While it is well known that there is a greater diversity of different Se species on wurtzite surfaces,83,

84

even some studies in these

systems have invoked models including both deep and shallow surface trap states to explain different measurements.42-44 Our eSFG spectra, however, represent the first direct optical measurements of shallow surface states and the first measurements to verify the distinct and simultaneous presence of surface states in both energetic regions. This confirms the descriptions and measurements made in the CdSe literature, by simply showing that both shallow and deep trapping descriptions are correct and that differences in say, PL yield as mediated by shallow traps, are simply due to measuring the exit channel emission, and not the energies of the surface states via direct optical excitation. In order to study these surface states in more detail, the surface of the QDs was modified. Exposure of CdSe QDs to air during over a 24-72 hour timescale is known to alter the surface chemistry with water and oxygen in the air to form new surface defects and their resulting states.23, 48, 85, 86

For all the sizes, prolonged exposure to air caused changes to band transitions as well as

mid-band peaks (Figure 4 bottom). In all cases, oxidation substantially reduces the net eSFG signal, presumably due to the formation of a centrosymmetric oxide layer on the surface. This was previously imaged for the case of ZnO nanowires using near-field second harmonic generation (SHG) microscopy and showed a corresponding drop in radiated intensity even from the core transitions.87 When scaling the oxidized eSFG data to the most prominent features in their freshly prepared precursors we find that in all cases, the relative abundance of below gaps states qualitatively increases (Figure 4 plots the fresh and oxidized data on this relative scale). This makes 12 ACS Paragon Plus Environment

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sense since the oxides are considered surface trapping sites88 – as such, increasing the number of oxides should, and does, increase the relative number of traps energetically below the bandgap. Notably, the measured band positions do not alter substantially on oxidation suggesting that the size of the core remains largely unchanged (i.e., not etched). In contrast, some peaks are observed to disappear or broaden enough that they are no longer resolved. This may be an indicator of the surface character of the various transitions since those would be most impacted by surface modification as suggested by previous work.58 While two-photon resonances are also likely to contribute to the overall broad features underlying the entire spectra, they are as sensitive to oxidation (i.e., bands corresponding to well-defined transitions are still observed) and therefore cannot be the sole source of eSFG signal in the below gap region. For the surface states in the gap, those shallower in energy are qualitatively seen to increase in relative intensity after oxidation more than the deep traps. Studies on zinc-blende CdSe QDs have also concluded that surface sites with dissimilar coordination impact trapping to varying extents.34, 46, 52 Our work then extends the important finding of different surface state behavior to wurtzite CdSe QDs. At this point, we have focused our analysis of the eSFG spectra primarily on the energetic positions of each of the peaks. This has provided the ability to determine spectral band positions and probe surface states at ambient conditions in excellent agreement with previous work at cryogenic temperatures. In contrast, there is less information that we can extract from the absolute amplitudes of these features, which can differ from sample to sample. This arises from the wellknown interference of overlapping peaks in the eSFG spectrum, faster electronic dephasing at lower wavelengths, and the dependence of eSFG amplitudes on the number density and associated orientations for the few QDs in the focal volume.58, 89-91 For instance, it is well-known that SFG spectra are composed of interferences from light originating from different resonances having unique phase factors that spectrally overlap to yield asymmetric line shapes.65, 92 This can shift the apparent position of the resonance (though here we do not see a strong effect from this in our work) and alter the associated spectral amplitude from positive signed peaks to dips in the spectrum. Fresnel factors can further alter the relative amplitudes of the excitonic peaks in our eSFG spectra but are also unaccounted for in our analysis due to missing information about the real and imaginary parks of the bulk and interfacial indices of refraction.93 In parallel, transitions to higherlying states must contend with ultrafast dephasing of the electronic coherence, which can reduce the overall signal from those transitions.

58, 91, 94

This is seen in our work at lower wavelengths,


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where signals start to drop off even though there should be as much (or more) enhancement due to the double resonance effect as compared to energetically lower lying states. This means peaks at higher energies (lower wavelengths) will always appear weaker than those at lower energy. Finally, in the limit that one can probe an individual QD, the different amplitudes measured for any given sample can be related to the orientation of the CdSe core at the substrate.58, 89-91 Since we are probing many thousands of randomly orientated QDs, such information is still only an average and does not describe a physically meaningful orientation. As such, at this time we cannot extract quantitative information from the amplitudes of the peaks in isolation – however, given that sample oxidation does not change the QD orientations/number density and interferences between signals in the eSFG spectrum we can compare relative changes in fresh and oxidized samples. Future experiments could explore the azimuthal angle dependence of the eSFG signal as others have done on macroscopic surfaces70 to gain additional insight into the core and surface states. Conclusion As the field of nanoparticle surface science continues to seek new ways to exploit their complex interfaces for practical devices and applications, it is becoming more evident that methods uniquely capable of directly probing the complete electronic structures are necessary. The broadband eSFG microspectroscopy platform presented here is uniquely capable of probing both optically bright and dark states in model CdSe QDs in addition to states associated with their surfaces. The electronic transitions observed in eSFG spectra from three different sizes of CdSe QDs were compared to previous high resolution PLE measurements at cryogenic temperatures, showing that eSFG was able to map both core and surface electronic transitions at ambient conditions. Further, the nascent surface trap states were chemically modified via atmospheric oxidation confirming the technique’s surface sensitivity to transitions outside of those typically measured in spectroscopy. It is expected that these approaches can be improved upon by reducing the number of species being probed in the microscope, imaging individual QDs, spectrally narrowing the up-conversion pulses, while temporally compressing the broadband white light output. This could allow for the fast and quantitative survey of a broad range of nanomaterials in technologically relevant conditions and devices. Through an understanding of the fundamental energetics of both core and surface states in nanomaterial and thin film samples, many fundamental 14 ACS Paragon Plus Environment

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gaps in connecting the chemistry-electronic structure relationship can be elucidated and used to improve and advance existing technologies.

Supporting Information Description Supporting information contains experimental details and associated characterization, representative raw experimental eSFG data, and tables summarizing fitting results for larger QD samples.

Corresponding Author Tessa R. Calhoun, Email: [email protected] Benjamin Doughty, Email: [email protected]

Author Contributions B.R.W., B.D. and T.R.C. participated in the design, analysis and execution of all measurements contained in this work. All authors contributed to writing the manuscript and have given approval to the final version.

Acknowledgements B.R.W. and T.R.C. were supported by the University of Tennessee and the Science Alliance Joint Directed Research & Development Program. B.D. was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy.


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Energetics at the Surface: Direct Optical Mapping of Core and Surface

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