Optical properties of zinc selenide clusters from first-principles calculations
Sachin P. Nanavati,
1,2
V. Sundararajan,
1
Shailaja Mahamuni,
3
Vijay Kumar,
4
and S. V. Ghaisas
2,
*
1
Centre for Development of Advanced Computing (C-DAC), Pune University Campus, Pune 411 007, India
2
Department of Electronic Science, University of Pune, Pune 411 007, India
3
Department of Physics, University of Pune, Pune 411 007, India
4
Dr. Vijay Kumar Foundation, 1969, Sector 4, Gurgaon 122 001, India
Received 17 August 2009; published 14 December 2009
The optical properties of bare and passivated Zn
n
Se
n
n =1–13 clusters have been studied within the
framework of time-dependent local density approximation. The atomic structure of the clusters has been
obtained using projector augmented wave pseudopotential method, with generalized gradient approximation
for the exchange-correlation energy. The small clusters with n up to 5 have two-dimensional 2D structure and
for larger sizes, cagelike 3D structures become favorable. At n = 13, the clusters start getting an atom inside the
cage to attain bulklike local structure. For the bare clusters, the highest occupied molecular orbital HOMO
and lowest unoccupied molecular orbital LUMO gap increases from a small value for ZnSe dimer and
beyond n = 3, the variation is small. On the other hand, the HOMO-LUMO gap of the clusters passivated with
partially charged hydrogen atom decreases nearly monotonically with increasing size, though the value remains
higher compared with that of the bare clusters even for the case of n =13. Further, the optical absorption spectra
and the corresponding optical gap have been calculated and a decreasing trend as a function of the increasing
cluster size has been obtained. This compares well with the experimental results available on larger clusters in
the literature though the calculated values underestimate the optical absorption gap as expected within the local
density approximation framework.
DOI: 10.1103/PhysRevB.80.245417 PACS numbers: 61.46.Df, 71.15.Mb, 71.15.Dx, 73.22.-f
I. INTRODUCTION
Quantum dots QDs of semiconductors have been attract-
ing great attention in recent years due to their possible appli-
cations in nanotechnology and miniature devices such as op-
toelectronics, solar cells, and light-emitting diodes LEDs.
1
The optical properties of QDs and small clusters are size and
shape dependent, besides composition, and therefore, it is
possible to tailor these nanomaterials with desired properties.
This aspect has led to a large number of studies on II–VI
compound clusters, where different colors have been ob-
tained from the same material by changing size and shape,
that affect the highest occupied molecular orbital HOMO—
lowest unoccupied molecular orbital LUMO gap due to
quantum confinement of electrons. A promising application
of such QDs is in their usage as a fluorescent marker to
provide information about a biological state or event.
2
They
have remarkable advantages as compared to the traditional
organic dyes, viz., broad absorption with narrow and sym-
metric photoluminescence PL spectra, high resistance to
photobleaching and exceptional resistance to photo and
chemical degradation.
3
They can be observed and tracked
over an extended period of time up to few hours in vivo
imaging and diagnostics of live cells.
4
Therefore, developing
an understanding of the properties of such systems is an area
of wide interest, not only in semiconductor physics, but also
from biological systems point of view.
Among the II–VI compound semiconductors, QDs of
ZnSe have several technological advantages and have been
actively studied. This environment friendly material is one of
the leading candidates for the fabrication of blue LEDs and
laser diodes.
5
Experimentally, highly monodisperse ZnSe
nanoparticles have been prepared through chemical route
treatment.
5,6
Recently, ZnSe/ZnS core/shell nanoparticles
have shown a spectacular increase in the PL quantum effi-
ciency from 2% to 42%, in their spectra.
7
From a theoretical point of view, most studies have been
done on small clusters because one needs to find the ground
state structure of clusters that are often very different from
the bulk. As the size grows, it becomes challenging to find
the lowest energy structure of clusters and so far, most of the
first principles studies on semiconductor clusters are on sys-
tems having up to about 50 atoms. In the case of ZnSe, small
Zn
n
Se
n
n =1–9 clusters have been studied earlier by
Matxain et al.,
8
who calculated the ground state geometries
and cohesive energies using B3LYP gradient-corrected
density-functional method with GAUSSIAN98 package. Degl-
mann et al.,
9
have obtained the atomic structures of ZnSe
clusters up to heptamers n =7. Recently, Goswami et al.
10,11
have calculated the structural and electronic properties of
unpassivated, Zn
m
Se
n
m + n 200 clusters using density
functional tight-binding DFTB method within the local
density approximation LDA. The excitation spectrum was
calculated using time-dependent TD density functional re-
sponse theory DFRT within the tight-binding approach.
They considered initial structure as spherical parts of either
zinc blende or wurtzite crystal structure. The stoichiometric
Zn
n
Se
n
clusters were also passivated by terminating the dan-
gling bonds on the surface by either -H or -OH. However, as
mentioned before, the ground-state structure of small clusters
is expected to be different from the bulk. Therefore, determi-
nation of the atomic structures using a parameter free, first
principles method is important for understanding the proper-
ties of semiconductor clusters. In the present work, we in-
vestigate the structure of small bare as well as surface pas-
sivated Zn
n
Se
n
n =1–13 clusters with partially charged
hydrogen and report their optical absorption spectra.
PHYSICAL REVIEW B 80, 245417 2009
1098-0121/2009/8024/2454179 ©2009 The American Physical Society 245417-1