Kwansei Gakuin University
Report of Research Outcome
2023/03/13

To President
Department： Science and Technology
Position： Postdoctoral fellow
Name： Li W ang
I report the outcome of the research as follows.
□Sabbatical leave with grantName of theFund/Prog ra m

□KGU Joint Research□Sabbatical leave with no grant□Individual Special Research⊠Postdoctoral f ellow※ Please report by desi gnated f orm as f or “Inte rnational Research
Collaboration”.

ResearchThe me

Elementary carrier dynamics of blue-emissive semiconductor quantum
dots controlled by interfacial engineering

ResearchTamai Lab.

Site/VenueResearchperiod2022/04/01

2023/03/31 （ 12～month）◆ Summary of the research outcome （ appr ox . 2 ,5 00 wo r ds）
Please write down the outcomes in detail regarding the research theme above.
Colloidal quantum dots (QDs) exhibit unique size-tunable optoelectronic properties due tostrong quantu m conf inement ef f ects, making th em as potential candida tes f or optoelectronicdevices. For applications such as lasers, displays, and nanosensors that require hightransition strength with a narrow ba ndwidth, t he spectroscopy and d yna mics of QDs aredetermined by size-dist ribution, exciton-phono n coupling, and thermal active e nerg y. Thesef actors can be studied through te mpe rature-dep endent photolu minescence (PL) experiments.
Recently, there has bee n growing interest in heavy metal -f ree QDs as a means to reduce theirenviron me ntal impact. ZnSe with 2.7 eV bulk bandgap is a suitable choice f or violet-bluenano-e mitters. In this study, ZnSe QDs (426 n m) prepa red b y a hot -i njection method w ere
dispersed in a poly met hyl methac rylate (P MMA) mat rix or drop -casted on a qua rt z plate f ortempe ratu re-de pendenc e experime nts. PL spectra of QD f ilms were measured in a Montanacryostation (f rom 295 to 5 K) with ~400 mW /cm 2 365 n m CW or UV f s excitation, and detected ina trans mission mode b y a Princeton Spec-10 C CD. Moreover, ZnSe/Zn S core/shell (C/S) QDs
were dissolved in isopentane (OD 3 0 0 n m = 0 .15, co re QD 397 n m). Te mpe rature depen dent PL ofC/S QDs were measured by Fluorolog-3, and te mperature was controlled by UnispeKs (>80 K).
10.0
350

375
400
425
Wavelength / nm

450c

3.14
140

3.12

130

3.10

120

3.08
3.06
100

150
200
250
Temperature / K

110
300

Relative Intensity (a.u.)

0.5b

FWHM / meV

123K
143K
183K
203K
223K
243K
263K
283K
293K

Energy / eV

Relative Intensity / a.u.

a 1.0

1.0300 250

Temperature / K
200

150

0.8
0.6
0.4

4050

60

70

80

901/kBT (eV-1)
Figure 1. (a) Tem perature dependent PL spectra, (b) PL peak energy and FW HM, and (c) PL
intensity of ZnSe/ZnS core/shell QDs with PL peak at 406 nm in isopentane.

100ZnSe/ZnS C/S QDs with 1S peak at 397 n m w ere washed with hexane/ acetone mixture (1:4), an dthen the de position w as dissolved in isopentane (QY, 18 %). Te mperature-depe ndent PL
spectra, peak energy, and the f ull width at half -maxi mu m (FW HM) of C/S QDs in isopentane areshown in Figure 1. As the tempe ratu re increases , the PL intensity decreases along with a
red-shif ted emission en ergy and a broad ened FW HM. The redshif t is well fitted to the Varshni
relation 𝐸𝐸𝑔𝑔 (𝑇𝑇) = 𝐸𝐸𝑔𝑔 (0) − 𝛼𝛼 𝑇𝑇 2 ⁄(𝑇𝑇 + 𝛽𝛽), where E g (0 ) i s the ene rgy gap at 0 K, α is the te mp eratu recoeff icient, and β has a value close to the Debye tempe rature of the material. [ 1 ] The broadeningof the FW HM can be we ll explained by exciton coupling with acoustic and LO pho nons, which
can be described by the f ollowing equation
where Γ i n h𝛤𝛤(𝑇𝑇) = 𝛤𝛤inh + 𝜎𝜎𝑇𝑇 + 𝛤𝛤LO /(𝑒𝑒 𝐸𝐸LO⁄𝑘𝑘𝐵𝐵𝑇𝑇−1 ),represents the te mpera ture-indepe ndent inho mogeneous distribution resulting f rom the size
f luctuations. σ is the exciton-acoustic phonon coupling coef f icient and Γ L O represents t hestrength of exciton-LO phonon coupling. E L O is theCore426 C/S397
LO-ph onon ene rg y and k B is the Boltzma nn con stant.
The te mperat ure -depe ndent PL intensity wasE g (0)eV2.74
3.15
analyze d to reve al the nonradiative processes. TheαmeV/ K
0.110.61
integrated PL intensity was plotted as a f unction ofβK
340
3321/k B T, as shown in Fig. 1(c). It is evident tha t th e PL
intensity re mains almost constant up to 170 K andmeV105
129Γinhthen rapidly decreases. Considering the thermall yσμeV/K
7.6
18activated carrier escape (with activation energy E a ),the te mperature depend ence of PL intensity can be ΓLO
meV
1753
describedby
𝐼𝐼_PL (𝑇𝑇) = 𝐼𝐼_0/(1 + 𝑎𝑎𝑎𝑎^(−𝐸𝐸_a ⁄ELOmeV2039(𝑘𝑘_𝐵𝐵 𝑇𝑇)) ),where I P L (T) is the inte grated PL intensit ymeV33103Eaat temp eratu re T and I 0 i s the integrated PL inte nsity
at 0 K. Th e experi ment data are well f itted with theconstants listed in Table 1. The activation energy E a corresponds to the exciton binding
energ y. Fo r small sizes, the binding energ y is larger. Th e results will be f urther discussed interms of quantum conf inement ef f ects.
Reference:[1] Valerini et al., Anni, Phy s . Rev . B 2005, 71, 235409.

Conference:

1.
R. Takeuchi, D. Eguchi, N. Ta mai, L. W ang, “Localized Surf ace Plasmo n Resonance inGold Nano rods Unde r Ul trahigh Pressure ”, The 103rd CSJ Annual Meeti ng, K205-2 vn-03
Publication:

1.
S. Jung, L. W ang, H. Su giya ma, H . Uekusa, T. K ataya ma, K. Ka mad a, T. Ha mura, and N.
Ta mai, “Intra molecular Singlet Fission in Penta cene Oligo mers via an I ntermediate State” (J.
PhChA iti
Deadline： W ithin two months af ter f inishing the research pe riod.
Sabbatical leave with grant: Sub mit this report t o President with conf irmation by the dean ofschool you belong to.
※ Postdoctoral f ellow is required to sub mit this report with conf irmation by the de an of graduate

school bef ore the end of emplo yment pe riod.
W here to submit： Orga nization f or Research a nd Develop ment and Ou treach (N UC)◆ W e put this report on t he web of KGU. If there is any proble m abo ut it because of diff iculties on
your research, please le t us know.
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