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Journal ArticleDOI

Fine Structure of Nearly Isotropic Bright Excitons in InP/ZnSe Colloidal Quantum Dots

19 Sep 2019-Journal of Physical Chemistry Letters (American Chemical Society)-Vol. 10, Iss: 18, pp 5468-5475

TL;DR: It is argued that excitons in InP-based QDs are nearly isotropic because the particular ratio of light and heavy hole masses in In P makes the exciton fine structure insensitive to shape anisotropy.
Abstract: The fine structure of exciton states in colloidal quantum dots (QDs) results from the compound effect of anisotropy and electron-hole exchange. By means of single-dot photoluminescence spectroscopy, we show that the emission of photoexcited InP/ZnSe QDs originates from radiative recombination of such fine structure exciton states. Depending on the excitation power, we identify a bright exciton doublet, a trion singlet, and a biexciton doublet line that all show pronounced polarization. Fluorescence line narrowing spectra of an ensemble of InP/ZnSe QDs in magnetic fields demonstrate that the bright exciton effectively consists of three states. The Zeeman splitting of these states is well described by an isotropic exciton model, where the fine structure is dominated by electron-hole exchange and shape anisotropy leads to only a minor splitting of the F = 1 triplet. We argue that excitons in InP-based QDs are nearly isotropic because the particular ratio of light and heavy hole masses in InP makes the exciton fine structure insensitive to shape anisotropy.
Topics: Biexciton (71%), Trion (60%), Exciton (59%), Photoluminescence (56%), Spontaneous emission (52%)

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Citation for final published version:
Brodu, Annalisa, Chandrasekaran, Vigneshwaran, Scarpelli, Lorenzo, Buhot, Jonathan, Masia,
Francesco, Ballottin, Mariana V., Severijnen, Marion, Tessier, Mickael D., Dupont, Dorian,
Rabouw, Freddy T., Christianen, Peter C. M., de Mello Donega, Celso, Vanmaekelbergh, Daniel,
Langbein, Wolfgang and Hens, Zeger 2019. Fine structure of nearly isotropic bright excitons in
InP/ZnSe colloidal quantum dots. Journal of Physical Chemistry Letters 10 (18) , pp. 5468-5475.
10.1021/acs.jpclett.9b01824 file
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Fine Structure of Nearly Isotropic Bright
Excitons in InP/ZnSe Colloidal Quantum Dots
Annalisa Brodu,
,
Vigneshwar a n Chandrasekaran,
,,
Lorenzo Scarpelli,
§
Jonathan Buhot,
k
Francesco Masia,
§
Mariana V. Ballottin,
k
Marion Severijnen,
k
Micka¨el D. Tessier,
,
Dorian Dup on t,
,
Freddy T. Rabouw,
Peter C.M.
Christianen,
k
Celso de Mello Donega,
Dani¨el Vanmaekelbergh,
,
Wolfgang
Langbein,
,§
and Zeger Hens
,,
Debye Institute for Nanomaterials Science, Utrecht University, The Netherlands
Physics and Chemistry of Nanostructures, Ghent University, Ghent, Belgium
Center for Nano and Biophotonics, Ghent University, Belgium
§School of Physics and Astronomy, Cardiff University, Cardiff, United Kingdom
kHigh Field Magnet Laboratory, HFML-EMFL, Radboud University, The Netherlands
Contributed equally to this work
E-mail: d.vanmaekelbergh@uu.nl; langbeinww@cardiff.ac.uk; zeger.hens@ugent.be
Abstract
The fine structure of exciton states in colloidal quantum dots (QDs) results from
the compound effect of anisotropy and electron-hole exchange. By means of single-dot
photoluminescence spectroscopy, we show that the emission of photo-excited InP/ZnSe
QDs originates from radiative recombination of such fine-structure exciton states. De-
pending on the excitation power, we identify a bright exciton doublet, a trion singlet
and a biexciton doublet line that all show a pronounced polarization. Fluorescence line
1

narrowing spectra of an ensemble of InP/ZnSe QDs in magnetic fields demonstrate th at
the bri ght exciton effectively consists of three states. The Zeeman splitting of these
states is well described by an isotropic exciton model, wh er e the fine structure is dom-
inated by electron-hole exchange and shape anisotropy only leads to a minor splitting
of the F = 1 triplet. We argue that e xc i t ons in InP-based QDs are nearly isotropic
because the partic ul ar ratio of light and heavy hole masses in InP makes the exciton
fine stru ct u re insensitive to shape anisotropy.
2

Colloidal quantum dots (QDs) are quasi-spherical semiconductor nanocrystals in which
an electron-hole pair is con n e d in a volume with dimensions sm a l l er than the exciton Bohr
radius of t he corresponding bulk material. Under such strong confinement conditions, the
conduction- and valence-b a n d edge are reduced to a set of quantized eigenstates th a t describe
electron and hole motion. While these states can be calculated using different theoretical
frameworks,
1–3
a multi-band effective mass appr oximation has the advantage of providing
analytical expressions in which semiconductors are characterized by a limited set of param-
eters which are often known for the corresponding bulk material and the QD diam et er
is impl em ented as a continuously changing variable.
2
In the case of zinc blende or wurtzite
semiconductors, which include II-VI and III-V materials such as CdSe, CdTe an d InP, th i s
approach leads t o a two-fold degenerate lowest conductio n -b a n d state and a fourfold d eg en -
erate upper valence-band state.
2
As highlighted in Figure 1a-b, th ese degeneracies reflect the
angular momentum of the Bloch states that make up the electron states at the edge of the
conduction band (s = 1/2) and the valence band (j = 3/2), respectively.
2
The eigenstates of
electron-hole pairs or excitons are then conveniently expressed using direct products of the
two different co n d u ct i on - b an d (electron) and four different valence-ba n d (hole) states as a
basis, see Figure 1c.
Quantum dots of b o t h wurtz i te and zinc blend e semicon d uc to r s have a set of fine st r u ct u r e
eigenstates with different eigenenergies that can be described by linear combinations of the
8 direct pr oduct exciton states. In the case of spherical zinc blende QDs, for example, the
electron-hole exchange interaction splits the exciton levels in an opticall y dark low energy
quintuplet and a high energy, bright trip l et that are exciton eigenstates with total an gu l a r
moment F = j + s of 2 and 1, r espectively (see Figure 1d). A fur t h er splitting of the exciton
levels is obtained for QD s with an internal symmetry axi s or quantization axis z, which can be
the c-axis for QDs wit h the wurtzite structure or the rotation axis for QDs with a spheroidal
shape. In such cases, 5 different fine structure levels ar e obtained, each characterized by a
given projection |F
z
| along the quantization axis. Continuing with the example of a zinc
3

Figure 1: (a) Outl i n e of the edges of (b l u e, VB) the valence-band and (red, CB) the conduc-
tion of a zin c blende semiconduct or around the center of the Brillouin zone, showing the (lh)
light hole, (hh) heavy hole and (so) split off VB. (b) Representation of the CB and VB Bloch
states at the Γ point, highli g hting the degeneracy of both states in relation to thei r s = 1/2
and j = 3/2 angular momentum. Bright col o rs represent occupied states, semi-transparent
colors em p ty states. (c) Overview of the 8 exciton states obtained as direct products of a CB
electron and a VB hole state. The direct product states are labeled using th e z-component of
the total angular momentum F of the exciton. Transitions from the ground states to states
boxed with a dashed line a r e spin forbidd en . Again, bright colors represent o cc u p i ed states
and semi-transparent colors empty states. (d) Exciton eigenenergi es calcu l a t ed as a func-
tion of the anisotropy splitting energy using an exchange splitting parameter η = 2 meV.
States are labelled by me an s of the angular momentum projection quantum number F
z
along
the quantization axis. Full lines represent bright states, dashed lines dark states. Note that
exchange couples the two F
z
= 0 states to yield a bright (singlet) state an d a dark (tri p l et )
state. In the isotropic case, only the three F = 1 states are bright, whereas the five F = 2
states are dark as indic at ed by the filled and open circles.
blende QD, shape anisotropy thus makes the subset of heavy hole or light hole excitons the
lowest energy states, a situation typically seen with self-assembled QDs.
4
In either case, a
dark exciton ground st at e is obtained, in combination with a twofold deg en er at e bright state
at slightly higher energy, an energy difference often described as the dark-bright splitting.
For CdSe-based colloidal QDs, multiple studies have shown that the multi-band effec-
tive mass description of exciton sta t es agrees with the experimental characteristics of these
states. First, the ob s ervation that CdSe QDs exhibit l o n ge r radiative lifetimes at cryogenic
temperatures, was assigned to the presence of a lowest energy dark exciton state.
5
Next, a
4

Figures (6)
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References
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TL;DR: The observed shortening of the luminescence decay time in CdSe nanoncrystals in a magnetic field is in excellent agreement with the theory, giving further support to the validity of the model.
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"Fine Structure of Nearly Isotropic ..." refers background in this paper

  • ...diagonal in the give basis.(4) For the Fz = +1 states, we thus have:...

    [...]


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Abstract: ▪ Abstract We review the rapid progress made in our understanding of the crystal properties of semiconductors and nanocrystals focussing on theoretical results obtained within the multiband effective mass approximation. A comparison with experiment shows these results are valid for nanocrystals down 22–26 A in diameter. The effect of the electron-hole Coulomb interaction on the optical spectra is analyzed. A theory of the quantum–size levels in wide gap (CdSe) and narrow gap semiconductors (InAs) is presented that describes the absorption spectra of these semiconductors well. A great enhancement of the electron-hole exchange interaction leads to the formation of the optically forbidden Dark Exciton in nanocrystals, which strongly affects their photoluminescence. A theory of the band-edge exciton fine structure is presented and applied to the study of the PL in CdSe nanocrystals. The effect of doping on nanocrystal spectra is considered. The enhancement of the short–range spin-spin interaction in Mn-doped ...

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TL;DR: The band edge exciton structure is calculated, including the effects of the electron-hole exchange interaction and a nonspherical shape, in CdSe quantum dots to show the importance of exciton spin dynamics in the recombination mechanism.
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15 Jun 1996-Physical Review B
Abstract: We use photoluminescence excitation and fluorescence line narrowing spectroscopies to examine structure observed in the band-edge absorption feature of CdSe quantum dots. We study eight samples ranging from \ensuremath{\sim}15 to \ensuremath{\sim}50 \AA{} in radius to probe the size dependence of this structure. We compare our results with recent theories, which predict band-edge exciton splittings in CdSe dots due to their internal crystal structure, nonspherical shape, and the exchange interaction between the electron and hole. We find reasonable agreement between our data and theory, supporting the observation of exciton fine structure. \textcopyright{} 1996 The American Physical Society.

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"Fine Structure of Nearly Isotropic ..." refers background in this paper

  • ...Lineshape of the PL spectrum of the QD at cryogenic temperature is often a sharp ZPL superimposed by the broadband acoustic phonons.(2) The intensity of the broadband reflects the thermal distribution of phonons, described by the phonon population factor:...

    [...]


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