Optical Properties of SnO2 QDs-Muy Bueno-Paper India.
Optical Properties of SnO2 QDs-Muy Bueno-Paper India.
Optical Properties of SnO2 QDs-Muy Bueno-Paper India.
a r t i c l e i n f o a b s t r a c t
Article history: SnO2 quantum dots (QDs) were controllably synthesized by laser ablation in liquid (LAL). The HRTEM
Received 26 January 2012 image shows that the diameters of the SnO2 nanoparticles fall into a small range of 1–5 nm, with the
In final form 23 March 2012 majority being less than the exciton Bohr radius of SnO2 (2.7 nm). The Selected Area Electron Diffraction
Available online 30 March 2012
(SAED) pattern of SnO2 QDs shows tetragonal crystalline structure. The photoluminescence (PL) spectrum
of such SnO2 QDs exhibits blue and green emission peaks at 445 nm and 540 nm respectively. These QDs
have potential future applications in optoelectronics, biosensor and other modern technologies.
Ó 2012 Elsevier B.V. All rights reserved.
⇑ Corresponding author. Fax: +91 532 2545341. Figure 1a shows UV–Visible absorption spectrum of colloidal
E-mail address: maksingh4@gmail.com (M.K. Singh). solution obtained by pulsed laser ablation of tin in double
0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cplett.2012.03.084
88 M.K. Singh et al. / Chemical Physics Letters 536 (2012) 87–91
Figure 1. (a) UV–Visible absorption spectrum of SnO2 quantum dots and (b) plot of (ahm)2 versus hm for SnO2 quantum dots synthesized by pulsed laser ablation method.
deionized water. SnO2 exciton broad band starts with a hump at ðahmÞ1=n ¼ Bðhm Eg Þ ð2Þ
300 nm because of near band edge absorption of electron in con-
duction band and exciton broad band further increases up to where B is a constant, a is absorption coefficient, Eg is the band gap,
200 nm. Stronger exciton effect is an important character of quan- hm is the photon energy and n is a value that depends on the nature
tum confinement in nano-semiconductors as carriers are confined of the transition. In this case, n is equal to 1/2 for the direct allowed
in a very small region which makes the electron and hole to move transition. The band gap can be estimated from a plot of (ahm)2 ver-
only in a potential well. It can enhance the coupling interaction sus photon energy. The estimated band gap energy is 4.37 eV for
with each other and the exciton becomes stronger, probability of synthesized sample Figure 1b. The increase of the band gap of
binding increases, more exciton absorption peak may appear when SnO2 is due to the smaller size of the particles. By applying Eq. (1)
the particle size decreases. Therefore the exciton absorption is ob- the radius (R) of SnO2 QDs found to be 1.55 nm. This study shows
served and it shows blue shift near band edge absorption relative that the quantum confinement effect of SnO2 is only at particle size
to the bulk exciton absorption (345 nm or 3.6 eV) [19]. This blue below the Bohr radius (2.7 nm).
shift represents the quantum confinement property of nanoparti-
cles. In the quantum confinement range, the band gap of the parti- 3.2. Transmission electron microscopy
cle increases as the particle size decreases resulting in the shift of
absorption edge to lower wavelength [28]. The morphology, particle size and structure of the synthesized
When the dimensions of nanocrystalline particles approach the sample were characterized by TEM and HRTEM. The TEM images
exciton Bohr radius (aB), a blueshift in energy is observed due to are shown in Figure 2a and b which reveals that the product is
the quantum confinement phenomenon. The effective mass model composed of homogeneous ultrafine self assembled nanoparticles
is commonly used to study the size dependence of optical proper- with a diameter below 5 nm. The SAED ring pattern in Figure 2c
ties of QD systems. In this approach the exciton is treated analo- for SnO2 QDs shows that the particles are well crystallized [31].
gously to a hydrogen atom, but is limited by spatial confinement. The diameter of the diffraction ring in SAED pattern is proportional
Therefore the energy of the system is obtained by solving the prop- to (h2 + k2 + l2)2 where (hkl) are the Miller indices of the planes cor-
er Schrodinger equation. In this manner two regimes are defined responding to the ring, counting from the center. First, second and
due to the coupling of motion of the electron and the hole in the third ring correspond to (1 1 1), (2 0 0) and (2 2 0) planes respec-
exciton: weak and strong confinements. In the former the particle tively. Figure 2d is the HRTEM image of the SnO2 QDs, which
size is larger than aB, and the electron and the hole are treated as a clearly reveals the lattice fringes with a d-spacing of about
correlated pair. The blue shift of the band gap energy is described 0.176 nm, corresponding to (2 1 1) planes of SnO2 (JCPDS No. 41-
by the following equation: 1445). Highly ordered lattice fringes, even in the surface region,
are evidence of the crystallinity of the particles.
2
Eeff p2 =2lR2 1:8e2 =eR
g ¼ Eg þ h ð1Þ From the data obtained by TEM images, the particle size distri-
bution graph shown in Figure 2e, and the mean size of the particles
h the plank constant, l is the effective
where R is the particle radius, can be determined. It can be seen that the diameter (2R) of the
reduced mass, e is the charge of the electron, Eg is the bulk band gap quantum dot is from 1 nm to 5 nm, and the median diameter (taken
energy (3.60 eV), static dielectric constant e of approximately 14 as average particle diameter) is about 2.5 nm. The radius (R) of QDs
and Eeff calculated from absorption data is 1.55 nm. Both the results show
g is the effective band gap energy. As the effective mass of
the electrons is much smaller than that of the holes (me = 0.27me), that the size of QDs is less than the Bohr radius (R = 2.7 nm).
since me mh (me and mh are the electron and hole effective Therefore, the SnO2 nanoparticles are called SnO2 quantum dots
masses respectively). The charge carrier confinement mainly affects (QDs).
the energy level of the electrons [29,30].
The size dependence of the band gap energies of the quantum- 3.3. FTIR spectra
confined SnO2 particles agrees very well with the confinement re-
gime. The band gap energy Eg for SnO2 nanoparticles can also be The FTIR spectrum is shown in Figure 3 for SnO2 nanoparticles
determined by extrapolation to the zero absorption coefficients, synthesized by LAL in water. The spectrum was recorded of a film
which are calculated using the following equation: coated on a glass substrate by suspending the solution drop by
M.K. Singh et al. / Chemical Physics Letters 536 (2012) 87–91 89
Figure 2. TEM images of the SnO2 quantum dots: (a and b) TEM images at 20 nm and 5 nm scale respectively (c) HRTEM image (d) SAED pattern (e) particle size histogram
curve of SnO2 QDs.
drop at 80 °C for evaporating the water. FTIR is recorded in the not directly recombining to a hole in the valance band from con-
range of 1000–400 cm1 for confirmation of SnO2. A band appears duction band [34].
in the range of 400–700 cm1 is assigned to Sn–O antisymmetric Generally, defects such as oxygen vacancies are known to be the
vibration [32,33]. most common defects in oxides and usually act as radiative centers
in luminescence processes. Among the oxygen vacancies in the
oxide, only the single ionized oxygen vacancy (V 0o ) state observed
3.4. Photoluminescence spectrum by EPR is paramagnetic, and it is expected that most oxygen vacan-
cies will be in their paramagnetic V 0o state under flat-band condi-
The photoluminescence (PL) emission spectrum for SnO2 quan- tions [34]. V 0o is assumed to be the recombination center for the
tum dots synthesized by PLA in water at an excitation wavelength luminescence emission, which has an effective monovalent posi-
of 271 nm is shown in Figure 4. The peak shows three individual tive charge with respect to the regular O2 site. After such a recom-
peaks at 409 nm, 447 nm and 540 nm respectively indicating the bination the effectively neutral center V 0o (neutral charge vacancy
blue and green emissions. These three peaks are smaller than for oxygen) will be formed, whose energy is very close to the
the band gap of SnO2 which is 4.37 eV therefore the electron is conduction band edge due to the correlation energy of the two
90 M.K. Singh et al. / Chemical Physics Letters 536 (2012) 87–91
4. Conclusions
Acknowledgments
References
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