OPTICAL CHARACTERIZATION OF SIZE-TUNABLE LUMINESCENT
PbS COLLOIDAL NANOCRYSTALS
MITIOGLU Anatolie
Institute of Applied Physics, Academy of Sciences of Moldova
anatol.mitioglu@phys.asm.md
Reviewer: KULYUK L. cor. mem.
Keywords: nanocrystal, colloidal solution, exciton, photoluminescence, absorption.
Semiconductor nanocrystal colloids are the focus of a steadily increasing number of investigations in
the pursuit and exploration of their unique optical and electronic properties due to quantum confinement of the
exciton. The synthesis of these “quantum dots” has evolved to routinely produce particles with final size
dispersions about 10% and encompasses production of a wide variety of materials within the groups II-VI, III-V,
and IV-VI semiconductor nanocrystals (NCs). PbS is currently available within this classification of materials
and yet is a highly attractive material to pursue. With a Bohr radius of 20 nm [1], its nanocrystals are in the limit
of strong confinement, and with a small band gap energy (in bulk phase) of 0.41 eV (at 300 K) it can exhibit size
tunable optical properties that span from the visible through the near-infrared (IR).
In this paper the synthesis details for the production of size-tunable PbS colloidal nanocrystals with
narrow size dispersions via a reaction of organo-metallic reagents nucleated and grown in an organic
coordinating solvent are reported. The details of characterization by photoluminescence (PL), optical absorption
and dynamic light scattering (DLS) are presented as well.
PbS quantum dots were synthesized using the modified versions of the method described in [2]. The
solutions of PbS nanoparticles exhibit clear exciton peaks in absorption spectra and bright band-edge
luminescence at room temperature located in the near-IR spectral range. The spectral PL peak of PbS
nanoparticles shifts from about 900 to 1500 nm as the synthesis temperature is increased that indicates the
growth of PbS NCs (Fig. 1). The position of the absorption spectra shoulder shows a similar trend. The
bandwidth of the luminescence spectra is comparable to that of the absorption bands. This fact suggests that both
spectral bandwidths are determined by the particle-size distribution.
The band gap values (ΔE) of small sized particles can be estimated by using the theoretical equation
derived by Wang [3]. The calculated band gap values are in good agreement with our observed near-IR emission
peaks.
1.7 1.6 1.5 1.4
1.3
1.2
1.1
h, eV
1
0.20
0.08
0.15
0.06
1.4 1.3 1.2
1.1
1
0.9
h, eV
0.8
0.10
0.5
0.10
0.2
0.05
0.1
0.0
700
excitonic
excitonic
absorption luminescence
0.06
0.04
0.04
IPL, arb. un
0.3
-1
excitonic
luminescence
, cm
-1
, cm
excitonic
absorption
IPL, arb. un
0.08
0.4
0.02
0.02
800
900
1000
1100
1200
0.00
1300
nm
0.00
0.00
900 1000 1100 1200 1300 1400 1500 1600
nm
a)
b)
FIG. 1. Room temperature PL and absorption spectra of PbS NCs synthesized at a) T=80 0C and b) T=150 0C.
References:
1. F. W. Wise, Accounts of Chemical Research, 33, 773-780, 2000.
2. C. Paquet et al., Advanced Functional Materials, 16, 1892-1896, 2006.
3. Y. Wang et al., J. Chem. Phys., 87, 7315-7322, 1987.