sms spectral diffusion

7/29/2019 SMS Spectral Diffusion

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PL spectroscopy of single CdSe nanocrystalline quantum dots

Empedocles et al., Phys. Rev. Lett. 77, 3873 (1996)

Two kinds of CdSe nanocrystals were used: "Standard" (crystal structure: wurtzite,

slightly prolate, 3 different sizes: 3.9, 4.5 and 5.0 nm) and "overcoated" (ZnS capping

layer, size=4.3 nm, higher QYield ~ 50%)

NCs dispersed in 50 nm PMMA film, lateral dispersion


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A) PL Image of single 4.5 nm NCs at 10 K. White dotted lines represent the center of theentrance slit. Note that all NCs are not equally bright.B) Spectra of a few NCs collected after closing the entrance slit to 0.125 mm. Only thoseparticles lying between the dotted lines in (A) were measured. Note that the general

nature of the spectra (one main peak, and a vibronic side band) is same, but not theirposition.


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(B) (C) (D) are comparison of

single particle and ensemblespectra for 3 differently sizedNCs. Note that vibronicsidebands cannot be seen inensemble spectrum.

(A) Comparison of a single NCspectrum with that offluorescence line narrowedspectrum. Single NC spectrumis an order of magnitude

narrower.(B) The histogram of PL peakpositions for 513 individual NCreproduces the ensemble Plspectra, suggesting that

ensemble spectrum isinhomogeneously broadened.


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Power dependence of Linewidth

314 W/cm2, t = 60s 150 W/cm2, t = 60s 65 W/cm2, t = 60s

t = 30s Averaged over 34core-shell 56.5 NCs

Saturation is best seenwhen not averaged over

several particles.

PL linewidth broadenswith increasing excitation

power/intensity, andeventually saturates.


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t = 0.1 s

t = 10 s

200 W/cm2

0.48 meV

2 meV

85 W/cm2

Same sample!!Averaged over 40 NCs

Acquisition time dependence of Linewidth

PL linewidth broadening with increasing integration (exposure) time. Onceagain saturation is observed at higher time scales.


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t = 30 s 85 W/cm2

10 K

40 KPL linewidths of single core-shell dots werestudied at 514 nm and 573 nm excitations.Integration time, temp., and number of

excitations were kept constant.

For 126 dots studied, 573 nm (lower energy)excitation resulted in 25% decrease inaverage linewidth

Linewidth saturation at higher energy densities!!


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Surface charge : unsaturated dangling bonds, distortions of ionic lattice. Net chargesetup a local electric field that can induce Stark shiftof emission energies.

It is a fluid situation that can change as excess excitation energy is dumped onto it.

The amount of excess excitation energy released as the exciton relaxes to itsemitting state (internal conversion).

Could be energy released following a single excitation (excitation energydependence) OR cumulative release over several excitation events (time and powerdependence).

Cannot be non-radiative contribution (0.25 eV vs. 2.4 eV)

Cannot even be a local heating effect. Heat dissipation happens over 10-12 s.

At high energy densities, the charges can effectively sample all possible trap sitesleading to a saturation of linewidth.

What is changing: NC or its surroundings?


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Single charge in a double well trap potential

In absence of excess excitationenergy, i.e, lower powers and/orsmall exposure times, the charge islocalized in a particular trap site.Correspondingly the field it creates

is not time dependent. Depending onwhich site traps the charge, one getseither spectrum (B) or (D).With excess excitation energy, thecharge can move between traps and

the resultant spectrum is a linearcombination of (B) and (D). Withpoor spectral resolution, this will beseen as an effective broadening. Butwith high spectral resolution it ispossible to see individual peaks.


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Higher resolution spectra (0-0 line), for t = 60 s


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Linewidth in the limit ofvanishing energy density

25 dots

34 dots

Electrical and physicalscreening of local electric field

in core-shell NCs


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Large spectral diffusion

Spectral shifts happening over 10s of meV and on timescales from sec to minutes

4 nm core-only NC, tinteg= 60s, 2.5 kW/cm2

80 meV shifts over 16 min.

DecreasedPL intensity andincreasedvibronic (phonon)coupling in red shifted spectra.

Similarity with single NC Starkspectroscopy


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Stark spectroscopy

Why should a NC PL spectrum respond to external electric field? Charge traps at the surface, very localized, dipole character : Linear stark effect Electron and hole wavefunctions, delocalized and therefore polarizable : Quadratic

7.9 nm core-shell NC, 25 W/cm2

Red shift associated with decreased PLintensity and increased phonon coupling

Both

linear and quadraticresponses


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Linear and Quadratic Stark responses

Stark response from 4 different 5.8 nm single NC particles

Response averaged over for 54 single NC particles is all quadratic! Stark shift observed in ensemble measurement is also quadratic

Average dipolar contribution in an ensemble goes to 0

Is this dipole related to transition dipole moment?


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Polarizable wavefunctions and Frolich coupling

on-overlapping

External electric field can separateelectron and hole wavefunctions:quantum confined stark effect (QCSE)

Red shift is a result of apparentreduction in quantum confinement due toe- and h+ wavefunctions pulled apart.

Reduced PL intensity is a result ofreduced e- and h+ wavefunction overlap.

What sort of electric field produces largespectral diffusion?~ an electron on the surface of the NC

Type II nanocrystals are another way

Will you expect identical phonon coupling

across different NC particles?


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This is a good comparison!

Spectral diffusion Stark shifts

Correlation of phonon coupling andspectral shift are identical in Starkspectroscopy and spectral diffusion

experiments. This suggests that alocal field is responsible for theobserved spectral diffusion.

What is the source of local field?

Spectral Jitter (Gaussian)

slowly varying surface charge density

Spectral Jump (Lorentzian)rapidly occurring polarization changedue to a reversible expulsion of acharge carrier (Auger photoionization)


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No effect on the transition dipole moment (orientation)


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Effect of spectral diffusion on Stark shift

Before the shift:= 80 D, = 3.3105 3

After the shift: = 33 D, = 3.5105 3

tinteg= 30 s


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1. For epifluorescence and confocal microscopy, referMethods of single-molecule fluorescence spectroscopy

and microscopy, Moerner and Fromm, Rev. Sci. Intrum. 74, 3597 (2003).

2. Empedocles et al., Photoluminescence spectroscopy of single CdSe nanocrystallite quantum dots, Phys. Rev.

Lett. 77, 3783 (1996).

3. Empedocles et al., Spectroscopy of single CdSe nanocrystallites, Acc. Chem. Res. 32, 389 (1999).

4. Empedocles et al., Photoluminescence from single semiconductor nanostructures, Adv. Mater. 11, 1243(1999). Section 5 onwards

5. Empedocles et al., Influence of spectral diffusion on the lineshapes of single CdSe nanocrystallite quantum

dots, J. Phys. Chem. B 103, 1826 (1999).

** Ref. 3 and 4 are reviews based mainly on results reported in Ref. 2 and 5.

6. Neuhauser et al., Correlation between fluorescence intermittency and spectral diffusion in single

semiconductor q-dots, Phys. Rev. Lett. 85, 3301 (2000).

References

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