Chapter 4: Optical Absorption and Photoluminescence Studies
4.1. As–grown ZnO Nanowires, Nanorods and Nanoribbons
In this section, we present the absorption and PL properties of ZnO NWs, nanoribbons and NRs which were grown by TVD method using the nano powder source. For the UV–
Vis absorption studies, the specular reflectance spectra of the samples were measured at an incident angle 45° in the range 200–900 nm. Thereafter the absorption coefficient for each wavelength is calculated according to the method proposed by Rusli et al.205 as described in the Chapter–2. The room temperature PL spectra of the ZnO NWs,
nanoribbons and NRs were measured under the excitation at 325 nm from a steady–state He–Cd laser.
4.1.1. Specular Reflection Studies
Figure 4.1 shows the UV–Vis specular reflectance spectra of the as–grown ZnO NWs (grown at 750°C), nanoribbons and NRs, respectively. When the incident photon energy is larger than the direct band gap energy of ZnO (λ<367 nm), the reflectance is related with band–gap absorption, leading to a flat and very low reflectance. When the incident photon energy is below the band–gap energy of ZnO (λ>367 nm), observed reflectance characteristics can be correlated with the structural properties instead of band–gap absorption. All the samples exhibits significantly low reflectance over a wide range of wavelengths. In addition, the reflectance spectra reveal interference oscillations. The observed oscillation is due to multiple internal reflections between air–NWs and NWs–
substrate interface.226 The periodicity of the reflectance oscillation is inversely proportional to the wavelength of the incident light. The as–grown NWs shows a maximum reflectance of ~16% in the range 200–900 nm. While for the nanoribbons and NRs, the maximum reflectance is 23% and 28%, respectively.
200 400 600 800
0 5 10 15 20 25 30
Nanowires Nanoribbons Nanorods
Reflectance (%)
Wavelength (nm)
Figure 4.1: UV–Vis specular reflectance spectra of the as–grown ZnO nanowires,
nanoribbons and nanorods synthesized from ZnO nano powder source.
4.1.2. Optical Absorption Studies
The variation in absorption coefficient of the ZnO nanostructures calculated from their corresponding reflectance spectrum is shown in Fig. 4.2. The UV–Vis absorption spectra
of the ZnO nanostructures show a high absorption in the UV region with a sharp peak at 367 nm. This absorption peak is due to the corresponding band–to–band transition in ZnO. The weak absorption in the visible region of 400–550 nm is due to the absorption by the surface defects present on the as–grown ZnO nanostructure. The NWs shows comparatively higher band edge absorption and very low absorption in the visible region.
Whereas, lower band edge absorption and comparatively higher visible absorption found in the nanoribbons and NRs indicate presence of high density of surface defect states.
300 400 500 600 700
Nanowires Nanoribbons Nanorods
Absorption Coeff. (cm-1 )
Wavelength (nm) 367
Figure 4.2: UV–Vis absorption spectra of the as–grown ZnO nanowires, nanoribbons and nanorods, calculated from corresponding reflectance spectra.
4.1.3. Photoluminescence Studies
The room temperature PL spectra of the ZnO NWs (grown at 750°C), nanoribbons and NRs are shown in Fig. 4.3, which are measured under identical conditions. ZnO NWs exhibit strong near band edge (NBE) UV emission at 380 nm and a broad green emission peak [Fig. 4.3(a)]. On the other hand, nanoribbons and NRs show relatively weak UV emission peak with nearly equal intensities at ~380 nm and an intense broad green emission band at ~527 nm [Fig. 4.3(b–c)]. The PL spectra of the ZnO nanostructures are fully consistent with their corresponding absorption spectra. The exact positions and other parameters of the constitute PL peaks are obtained from the Gaussian multi–peak fitting to the experimental data points, and these are tabulated in Table 4.1. The Gaussian fitting reveals that the broad green emission band constitutes of two Gaussian peaks, one at ~500 nm and other at ~545 nm, as shown by the fitted line. The NBE emission is due to the free excitonic recombinationand green emission at ~500 nm is attributed to the recombination of photo–generated holes with the electrons belonging to oxygen vacancy
states on the surface.122 And the other green emission band at ~545 nm is due to presence of deep interstitial oxygen inside the nanostructure.117 With increase in growth temperature, intensity of the 1st green emission is increased by 3.12 times for the nanoribbons and 4.08 times for the NRs, as compared to the peak intensity of the NWs.
Whereas, intensity of ~545 nm peak is increased by 3.44 times for the nanoribbons and then decreased to 2.17 times for the NRs. The intensity ratio of 1st to 2nd green emissions is calculated to be 0.89, 0.82 and 2.29 for the NWs, nanoribbons and NRs, respectively.
Since the ZnO NRs and nanoribbons are grown at relatively higher temperatures where oxygen vapor pressure is relatively low compared to the low temperature growth region this results in the formation of large number of oxygen vacancy related defect states. As a result, stronger green emission is observed from the NRs/nanoribbons. The PL results
360 420 480 540 600
0 5 10 15
420 480 540 600
0 3 6
420 480 540 600
0 5 10 15
500
545 380
Wavelength (nm) 495
(c) (b)
545
378
Expt. Data Gausssian Fitting
Component (a)
Intensity (arb. units)
547 500
380
Figure 4.3: Room temperature PL spectra of the as–grown ZnO: (a) nanowires, (b) nanoribbons and (c) nanorods synthesized from ZnO nano powder source. Solid lines are the fitted line with Gaussian function to the experimental data.
on the ZnO NWs, NRs and nanoribbons are consistent with the earlier reports,112,227,228
which show relatively weak UV emission in the nanostructures grown at relatively higher temperature. FWHM of the above mentioned PL peaks are marginally constant for the NWs, nanoribbons and NRs.
Table 4.1: Summary of the Gaussian fitted parameters for the PL spectra of the ZnO nanowires, nanoribbons and nanorods.
Sample
Peak I Peak II Peak III
Centre (nm) Ampl. (count) FWHM (nm) Centre (nm) Ampl. (count) FWHM (nm) Centre (nm) Ampl. (count) FWHM (nm)
Nanowires 380 6.1 13.6 500 3.1 76.8 545 3.4 103.9 Nanoribbons 380 3.2 15.4 500 9.5 70.5 547 11.7 128.2 Nanorods 378 3.4 13.0 495 12.3 78.0 545 5.4 116.5
Peak
identity Free exciton Oxygen vacancy defects
Deep interstitial oxygen defects
4.1.4. Time–resolved Photoluminescence Studies
The steady–state PL spectra indicate the presence of defects in the as–grown ZnO nanostructures, which are essentially radiative trap centre and these affect the intrinsic PL decay behavior. In order to study the PL decay dynamics of the green emission from the ZnO nanostructures, we measured TRPL decay at 500 nm using a pulsed 375 nm
5 10 15 20 25 5 10 15 20 25
(b)
Time (ns)
PL Intensity (arb. unit)
Expt. Data Bi-exponential fit Emission at 500nm
(a)
Expt. Data Bi-exponential fit Emission at 500nm
PL Intensity (arb. unit)
Time (ns)
Figure 4.4: PL decay profile of the as–grown ZnO: (a) nanowires and (b) nanorods. The PL decay was measured at emission wavelength of 500 nm.
laser excitation and the results are shown in Fig. 4.4. The as–grown ZnO NWs (grown at 750°C) and NRs show a bi–exponential decay in the TRPL data. The observed decay can be fitted with a bi–exponential decay equation
⁄ ⁄ ………. (4.1)
where τ1 and τ2 are the decay time constants. The fitted parameters are tabulated in Table 4.2. The NWs show τ1=0.75 ns and τ2=23.8 ns, while the NRs shows τ1=0.58 ns and τ2=38.2 ns. Two different time constants are associated with the two different defects states having different radiative decay channels and this is consistent with the steady state PL spectra described in the previous subsection. The relative amplitude ratios of the decay time constants are found to be 322 and 809 for the ZnO NWs and NRs, respectively. From the ratio of their relative amplitude, τ1 can be attributed to decay time constant of 1st green emission (~500 nm) and τ2 can be attributed to decay time constant of 2nd green emission (~545 nm). Our results are consistent with previous reports on green emission from ZnO NPs, thin films and doped NWs with decay time constants in µs range.229-231 However, in the present case due to the presence of high density of trap centers, it is in the range of ns.
Table 4.2: Summary of the bi–exponential fitting parameters for the PL decay in ZnO nanowires and nanorods.
Sample
PL decay time constants at emission
wavelength 500 nm (ns) Amplitude Ratio (A1/A2)
τ1 τ2
Nanowires 0.75±0.08 23.8±0.37 322
Nanorods 0.58±0.08 38.2±0.32 809