Chapter 6: ZnO Nanowire Heterostructures: Photoresponse and Photoluminescence
6.2. Au decorated Al doped ZnO Nanowire Heterostructures
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.
6.2.2. Photocurrent and Photoresponse Studies
The dark current–voltage characteristic of the Au NPs deposited Al:ZnO NWs is shown in Fig. 6.17. Compared to the dark current of the 3% Al:ZnO NWs (110 nA), the Au/Al:ZnO NWs shows a reduction in the dark current (70 nA). The observed change in the dark current is due to the upward band bending at the interface between Au and ZnO.
The upward energy band bending occurs at the interface due to the differences in work function of ZnO (4.65 eV) and Au (5.47 eV), as explained in details in the section 6.1.1.4.
-12 -8 -4 0 4 8 12
-400 -200 0 200
400 3% Al:ZnO NWs 3% Al:ZnO/Au NWs
Dark Current (nA)
Bias Voltage (Volt)
Figure 6.17: Dark current–voltage characteristic of the 3% Al:ZnO nanowires after the Au nanoparticles decoration.
The photoresponse of the Al:ZnO NWs measured under the excitation of 365 nm UV light is shown in Fig. 6.18. The Al:ZnO/Au NWs shows a maximum PC of 21.0µA, leading to a photosensitivity of 300. Therefore, the heterostructure shows a significant improvement in the UV photosensitivity with an enhancement factor of six with respect to 3% Al:ZnO NWs. The changes in the PC and photoresponse parameters are tabulated in Table 6.5. For comparison, the results of the 3% doped Al:ZnO NWs are also tabulated. At the same time, the photoresponse and reset times are also significantly reduced, which results in an ultra fast photodetection. The obtained photoresponse and reset times are 0.10 and 0.11s, respectively. After the Au NPs decoration, the photoresponse and reset times are about one order of magnitude faster, which is very significant. Therefore, the Au NPs decorated Al:ZnO NWs are found to be most appropriate for the fabrication of ultra fast UV photodetectors.
0 4 8 12 16 20
0.1 1 10
Photocurrent (A)
Time (sec)
UV ON UV OFF
Figure 6.18: Photoresponse of the 3% Al:ZnO nanowires after the surface decoration with Au nanoparticles.
Table 6.5: Photocurrent, photoresponse and photoluminescence parameters of the Al:ZnO/Au nanowire heterostructure.
ZnO NWs Dark current
(nA)
Photoc- -urrent (µA)
Photo sensiti vity
PC Response
time (s)
PC Reset
time (s)
PL Enhancement Factor UV peak UV-to-
494 peak
3%Al:ZnO 110 5.2 47 0.90 1.10 1 0.94
Al:ZnO/Au 70 21.0 300 0.10 0.11 4 7.5
The improvement in the photoresponse and photosensitivity for the Al:ZnO/Au NWs can be explained as follows. In the Al:ZnO/Au NWs, the decoration of Au NPs induces rough surface morphology resulting in interfacial states between Au and ZnO. These rough surface induced traps can capture the carriers (photogenerated) resulting in an ultrafast photoresponse and reset times. The enhancement in photosensitivity in the Al:ZnO/Au NWs could be explained on the basis of interband transition and surface plasmon assisted interfacial electrons transfer to the conduction band of ZnO. From the absorption study of the Au NPs [Fig. 6.5], it is known that the electrons in the Fermi level of Au NPs are excited by incident light in the UV–violet region due to interband transition and in the green region due to the surface plasmon resonance. 245 In the present case, when the Al:ZnO/Au NWs were illuminated with UV light, the excited energetic electrons in the Au NPs can escape from the surface of the NPs and transfer to the conduction band of ZnO through the interface. Under external bias, these electrons along with photogenerated electrons (due to band edge absorption of ZnO) contributed to the
current conduction process, resulting in the enhanced photocurrent. Therefore, we believe that the enhanced photocurrent in the present case is due to the increase in electron density in the conduction band of ZnO by band edge absorption of ZnO and plasmon assisted interfacial electron transport from Au NPs.
6.2.3. Photoluminescence Studies
The PL spectrum of the Al:ZnO/Au NWs is shown in Fig. 6.19. The UV peak intensity is dramatically enhanced and green emission peak is significantly reduced in the doped heterostructure. Here a four–fold enhancement in the UV peak intensity is obtained from the Al:ZnO//Au NWs, while the 1st green emission intensity reduced to half and the 2nd green emission intensity is one third of its initial value. Compared to the as–grown 3%
Al:ZnO case, the UV peak is enhanced by a factor of four, while the UV–to–green emission ratio shows more than seven times improvement. The observed improvement in the PL spectrum is due to the surface plasmon assisted enhanced radiative recombination in the Al:ZnO heterostructures.
350 400 450 500 550 600 650
0 10 20 30 40 50 60
494 545
PL Intensity (a. u.)
Wavelength (nm) Experimental Gaussian Fit Component 378
Figure 6.19: PL spectrum of the 3% Al doped nanowires after the decoration with Au nanoparticles.