Temperature Dependence of Emission Line Widths from

Nov 29, 2017 - As the temperature decreases, broad emission from the surface becomes prominent. This surface emission also exhibits vibronic contribut...
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Temperature Dependence of Emission Line Widths from Semiconductor Nanocrystals Reveals Vibronic Contributions to Line Broadening Processes Timothy G. Mack, Lakshay Jethi, and Patanjali Kambhampati* Department of Chemistry, McGill University, Montreal, Quebec H3A 0B8, Canada S Supporting Information *

ABSTRACT: The emission line widths of semiconductor nanocrystals yield insight into the factors that give rise to their electronic structure, thereby providing a path for utilizing nanocrystals in light emissive applications. Experiment and theory in conjunction reveal the contributions to line broadening to the core and surface emission bands. As nanocrystals become small, broad emission from the surface becomes prominent. In the case of the core emission, we reveal previously unobserved vibronic contributions in addition to the already well-known electronic structure of the band-edge exciton. As the temperature decreases, broad emission from the surface becomes prominent. This surface emission also exhibits vibronic contributions albeit more strongly. Analysis of the surface emission reveals the existence of a previously unobserved electronic structure of the surface in complete parallel to that of the core. The surface is characterized by a bright and dark state as well as a spectrum of bright states.

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py.21,22 Resonance Raman measurements as well as femtosecond pump−probe spectroscopy are also used to provide ensemble measurements of exciton−phonon coupling strengths.23−25 In addition to emission from the core band, NCs also exhibit emission from the surface which is broadened and red-shifted with respect to that from the core.26−28 Our group has reported on these phenomena in detail as well having provided an initial model which rationalized the observed phenomena of those experiments in light of a microscopic theory based upon charge transfer.29−31 This surface emission seems to be general to nanocrystals and is more prominent at lower temperatures and for smaller sizes.11,26,28 The surface emission may also be controlled by the surface chemistry.32−34 While understand of emission from the core is now sufficiently mature as to enable applications, emission from the surface is now just seeing its emergence. The combination of the band-edge and surface emissions yield a “white-light” spectrum.28 Recent examples of exploiting the surface include white light generation and optical thermometry.35−37 However, the underlying physics of this surface emissive feature remain poorly understood. Here we report on the temperature dependence of emission line widths for both core and surface emission bands of CdSe NCs. The line widths show a temperature dependence with a characteristic high-temperature slope and low-temperature

INTRODUCTION Colloidal semiconductor nanocrystals (NCs) have attracted significant attention due to their potential use in solar cells,1 light-emitting diodes2 (LEDs), and fluorescent imaging probes.3 These applications exploit the size-dependent energetics of excitons confined in the NC core.4 Advances in synthetic methodology over the past decade have produced samples with near-unity quantum yields and narrow (1000 cm−1) show negligible line width temperature dependence between 10 and 300 K. It is well-known that the chemical nature of the surface also plays a crucial role in determining emissive properties in addition to imparting colloidal stability.32,33 Having described the thermodynamics in broad terms, Figure 6 reveals the vibronic-coupling dependence upon material parameters. The relative contributions of ligand (Figure 6a) and size (Figure 6b) parameters to the coupling are qualitatively differentiated. Figure 6a shows a set of tetradecylphosphonic acid (TDPA)capped white-light NCs that have been ligand-exchanged with adamantanethiol (ADMT) and dodecanethiol (DDT). The slopes are provided in Table 2. The native phosphonic acidcapped sample possessed a significantly higher slope than the thiol exchanged samples. The difference in slopes can be understood in terms of ligand dependent exciton−phonon coupling strengths. Both the TDPA and thiol-capped samples possess lower surface line width slopes than the amine-capped samples studied in Figures 2−4. We note that the surface emission line width decreases with the extent of hole delocalization, which is consistent with previously published work.32,58 Figure 6b shows the line width plots of two different sized TDPA-capped white-light CdSe NC samples (0.9 and 1.5 nm). It is observed that the surface line width is greater for the smaller size than in the case of the larger dot for all

Figure 6. Temperature dependence of the line width of the surface emission of CdSe NCs for various ligands and radii. (a) Temperature dependence of the line width of TDPA-capped CdSe NCs ligand exchanged with two different thiols. DDT = dodecanethiol, ADMT = adamantanethiol, and TDPA = tetradecylphosphonic acid. (b) Temperature dependence of the line width data for TDPA-capped CdSe NCs of two different radii.

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DOI: 10.1021/acs.jpcc.7b09903 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Table 2. Temperature Dependence of the Surface Emission Line Width

Details of model calculations and additional data (PDF)

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slope (dΓ/dT)/kB nanocrystal type ligand Figure 6a CdSe−TDPA(R = 0.85 nm)a CdSe−ADMTb(R = 1.0 nm)a CdSe−DDTb(R = 1.1 nm)a Figure 6b CdSe−TDPA(R = 0.9 nm)a CdSe−TDPA(R = 1.5 nm)a

core

surface

2.7 ± 0.3 − −

7.3 ± 0.4 5.0 ± 0.5 3.3 ± 0.4

2.5 ± 0.4 1.8 ± 0.2

8.9 ± 0.4 8.1 ± 0.6

Corresponding Author

*E-mail [email protected] (P.K.). ORCID

Patanjali Kambhampati: 0000-0003-0146-3544 Notes

The authors declare no competing financial interest.

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a Size obtained from band-edge absorbance.65 bCore emission below the limit of detection.

ACKNOWLEDGMENTS T.G.M. thanks Hydro Quebec for a scholarship. PD Dr. Tobias Kipp is thanked for helpful discussions. Financial support from NSERC (fund 206254), FQRNT, and McGill University is acknowledged.

temperatures. However, the values of dΓ/dT are comparable, with a value of 8.9 ± 0.4 for the 0.9 nm sample and 8.1 ± 0.6 for the 1.5 nm sample. The size dependence of the exciton−phonon coupling for TDPA samples is not as pronounced as the changes observed in the case of ligand substitution with thiols, even when the greater effective radius caused by hole delocalization is considered.32 Moreover, the amine passivated sample in Figure 4 possesses a significantly higher value of dΓ/dT than the phosphonic acid or thiol passivated samples. This suggests that exciton−phonon coupling is more sensitive to changes in surface energy than to variations in size. Possibly, this may also be attributable to differences in surface binding motifs, whereby amines are formally dative (L-type) ligands, whereas phosphonates and thiolates are formally anionic ligands (Xtype).59 This is turn may affect the extent of charge localization at the NC surface.

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CONCLUSION The temperature dependence of the line width of the core and surface emission in CdSe NCs as a function of temperature can be calculated in a straightforward manner by considering vibronic contributions. We employed a model based on an excitonic linear coupling to a single internal vibrational mode corresponding to the LO phonon of CdSe. The core line width data are well described by a low exciton−phonon coupling strength, while the surface line width suggests the presence of multiple surface states with varying electron−phonon coupling strengths. The surface low-temperature data also revealed the presence of a surface bright−dark splitting analogous to the core. The exciton−phonon coupling strength was shown to be largely ligand dependent, while size dependence was less appreciable. On the basis of the observed temperature dependence of the surface emission, we propose that the surface emission in CdSe NCs is caused by exciton selftrapping, whose broadness is mainly attributable to large exciton−phonon coupling and whose relative population with respect to the core emission may be dictated by semiclassical charge transfer kinetics.60 This phenomenon does not appear to be exclusive to II−VI semiconductor nanocrystals studied in this work and could be broadly applicable to other semiconductoring materials of interest including broadband emitting perovskites.61−64

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AUTHOR INFORMATION

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09903. G

DOI: 10.1021/acs.jpcc.7b09903 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b09903 J. Phys. Chem. C XXXX, XXX, XXX−XXX