Chapter 6: ZnO Nanowire Heterostructures: Photoresponse and Photoluminescence
6.1. Metal nanoparticles decorated ZnO Nanowire Heterostructures
6.1.2. ZnO/Ti Nanowire Heterostructure
Figure 6.12: (a) Dark I–V characteristics of the ZnO/Ti nanowire heterostructures with different deposition time of Ti nanoparticles. (b) Schematic of the energy band diagrams with band alignment for ZnO/Ti NWs heterostructure.
will contribute to the current conduction process resulting in a higher dark current. As the thickness of the Ti layer increased, more number of electrons can flow from Ti to the ZnO causing gradual increase of dark current with increase in Ti coverage. Therefore, the band bending at metal–ZnO interface can explain the observed changes in the dark current satisfactorily.
Table 6.4: Photocurrent and photoresponse parameters of the as–grown and ZnO/Ti nanowire heterostructures.
Sample Name
Dark Current
(nA)
Photoc- -urrent
(µA)
Photosensitivity PC Growth time constants
(s)
PC Decay time constants
(s)
τ1 τ2 τ3 τ4
Ti_0s 11.5 4.72 414 17.3 218.8 19.3 316.0
Ti_60s 25.4 9.33 367 4.71 53.0 9.1 65.0
Ti_290s 211.0 65.50 310 1.4 43.9 4.7 219.5
The measured PC spectra for the ZnO/Ti heterostructures are shown in Fig. 6.13, along with that of the as–grown ZnO NWs. For the ZnO/Ti NWs, the PC gradually increases with increase in Ti NPs coverage. The PC in the UV region dramatically enhanced, while in the visible region it increases slightly with higher Ti coverage. The Ti_290s sample gives a PC of 65.5 µA for the excitation at 369 nm. Although the obtained PC from ZnO/Ti heterostructures are quite high, but due to higher dark current the photosensitivity values are relatively low. In the ZnO/Ti heterostructures, due to the
formation of Ohmic contact, a large numbers of electrons transfer from Ti to conduction band of ZnO and it increases the electron density. When the sample is excited with UV light it gives a very high PC. With the higher thickness of Ti coating, more numbers of Ti NPs are attached on the surface of the ZnO NWs, which increases the electron density in the conduction band of ZnO. As a result, PC increases with the increase in Ti coverage.
300 400 500 600 700
0 10 20 30 40 50 60 70
Photocurrent (A)
Wavelength (nm)
Ti_0S Ti_60 Ti_290s
367
Figure 6.13: Photocurrent spectra of the ZnO/Ti nanowire heterostructures with different deposition time of Ti nanoparticles.
In order to study the effect of Ti NPs decoration on the photoresponse, we measured the photocurrent growth and decay under the excitation at 365 nm, which is shown in Fig.
6.14. In this case, we also observed faster photoresponse time. The growth time constants are significantly reduced and show a gradual decrement with the increase in Ti coverage.
The Ti_290s sample shows fastest photoresponse with growth and decay time constants of 1.4, 43.9s and 4.7, 219.5s, respectively. These data demonstrate a significant improvement in the photoresponse process in heterostructures.
6.1.2.3. Photoluminescence Studies
As compared to the pristine ZnO NWs, the room temperature PL spectra of the ZnO/Ti heterostructures show enhancement in the UV as well as green emissions intensities, as shown in Fig. 6.15(a–d). The change in enhancement factors for various PL peaks with the Ti deposition time is shown in Fig. 6.15(e and f). In this case, enhancement factors of UV, 1st green, 2nd green emission and ratio of UV to 1st green emission gradually increase with the increase in Ti NPs deposition time. The obtained PL spectra of the
0.0 1.5 3.0 4.5 6.0
0 3 6 9
0 600 1200 1800 2400
0 20 40 60
Exp. Data Bi-exponential Fit
UV ON UV OFF
(a)
(b)
Photocurrent (A)
(c)
Time (sec)
Figure 6.14: Photoresponse behaviour of the: (a) Ti_0s, (b) Ti_60s and (c) Ti_290s nanowire heterostructures, measured at a bias voltage of 3 V and under the illumination of 365 nm UV light.
ZnO/Ti heterostructures are consistent with the corresponding PC spectra. The observed enhancement on the ZnO/Ti heterostructures is similar to the previous work by Liao et al..250 They prepared Ti doped ZnO NWs by Ti plasma immersion ion implantation method and shown that UV PL intensity as well as the transport properties could be tuned by energy controlled Ti plasma immersion ion implantation.
The observed changes in the PL spectra from the ZnO/Ti heterostructures can be explained as follows. Due to the formation of Ohmic type contacts at the interface, ZnO/Ti heterostructures will facilitate the electrons transferring process from Ti to ZnO and electrons accumulation on the interface will increase substantially. Therefore, even if the photon generated electron–hole pairs are separated at the interface, the radiative recombination probability of electrons on the conduction band and in the defect energy
Figure 6.15: The room temperature PL spectra of the: (a) Ti_0s, (b) Ti_60s, (c) Ti_145s, and (d) Ti_290s NWs. Insets show the corresponding PL spectra in magnified scale. (e and f) Variations of enhancement factors of UV and green emissions with different deposition time of Ti nanoparticles.
band with holes in the valence band will increase dramatically due to replenishment of electrons from Ti. As a result, both the UV and green emissions intensities increase with Ti coverage. The low temperature PL data of the polymer coated ZnO NWs measured by Liu et al.108 and Zn coated ZnO NWs measured by Fang et al.115 showed a significant improvement in the intensity of donar bound exciton (DX)/surface exciton (SX) related emission. Since these emissions are associated with the excitons trapped on the surface, means after coating number of excitons and thus exciton related recombination increases.
The PL decay profile of the green emission of ZnO/Ti heterostructures was studied further. The decay rate is measured at 500 nm, which is shown in Fig. 6.16. The calculation of individual decay time constants reveals no significant changes in the recombination time with Ti decoration. Therefore, Ti NPs decoration on the ZnO NWs does not affect the decay mechanism of the green emission.
10 20 30 40 50 10 20 30 40 50 10 20 30 40 50
PL Intensity (arb. unit)
Time (ns)
Experimental Bi-exponential Fit
(a) (b)
Time (ns)
(c)
Time (ns)
Figure 6.16: Green PL decay profile of the: (a) Ti_0s, (b) Ti_60s and (c) Ti_290s nanowire heterostructures.