Photoconductivity in air

Figure 6.5(a) shows the temporal evolutions of the electric current for the NW/QD hybrid sam-ple and a pure ZnO nanowire samsam-ple for reference. The reference samsam-ple has been thermally treated with the same conditions as used in the assembly of the NW/QD hybrid sample. The laser intensity is 1.0 mW/cm². The bias voltage applied on the nanowires is 1 V. When laser irradiation starts, the current of the hybrid sample rapidly increases from the initial 28μA to 200μA in the first 6 minutes. After 30 minutes of irradiation, it reaches a value around 250 μA. Then the current gradually approaches saturation. When the laser is blocked, the current drops immediately with a rate slowing down over time. It takes more than 6 hours for it to return completely to the original dark value. In comparison, the current enhancement in the bare ZnO nanowires under irradiation is very weak. The current climbs from 36μA to 56μA in 2 hours and restores back to the original value in darkness. The above observations indicate that the CdSe QDs in the hybrid sample play a crucial role in its strong PC enhancement.

The decay kinetics of the photoconductivity in darkness can be well described by a stretched

6 Oxygen-controlled Photoconductivity in ZnO Nanowires Functionalized with Colloidal CdSe Quantum Dots

Figure 6.5: (a) Photoresponse of the current of the ZnO-NW/CdSe-QD hybrid sample and a bare ZnO NW reference sample measured in air as a function of time. (b) Decay kinetics of the current of the NW/QD hybrid sample in darkness fitted with the stretched exponential function Equ. 6.1.

exponential function with the form:

I = Iin f +(I₀−Iin f)exp[−(t/τ)^α] (6.1)

whereI₀andIin f are the steady-state current reached under irradiation and the final dark current after decay. τ is the decay time constant, describing the decay speed. α is the stretching exponent. The zero point of timetis set at the termination of laser irradiation. Figure 6.5(b) shows the fitting result of the current decay in air up to 5.5 h with the parametersτ = 0.5 h andα= 0.43. It can be seen that the experimental result shows an overall excellent agreement with the model in the whole time period.

To further examine the effect of the QDs in the photoconductivity enhancement, new measure-ments are performed using a laser emitting at 532 nm. Figure 6.6 shows the measured current evolutions of the NW/QD sample compared with the results obtained using the argon ion laser with the same photon flux. As shown in the inset, the 532 nm laser locates spectrally at the absorption tail of the QDs, implying that it can only excite a small part of the QDs. The cur-rent evolution kinetics in both cases is very similar to that observed in Fig. 6.5(a). However, the steady current reached under the 532 nm irradiation is only 75 μA compared to 160 μA reached under the argon ion laser. This observation is consistent with the above speculation, confirming that the CdSe QDs are strongly involved in the PC enhancement of the NW/QD hybrid sample.

To interpret the observed PC enhancement in the NW/QD sample, the energy band alignment of the hybrid structure has to be considered. It is known that the size-dependence of the band gap is one of the main interesting features of semiconductor QDs. In the frame of the effective mass approximation [45, 162], the band edge positions (Ecb for the conduction band and Evb

72

6.2 Photoconductivity of the ZnO-nanowire/CdSe-quantum-dot hybrid structure

Figure 6.6: Photoresponse of the current of the ZnO-NW/CdSe-QD hybrid sample versus time under different laser irradiation.

for the valence band) can be approximately determined by Ecb = E_cb,bulk+ ∆E m^∗_h

m^∗_h+m^∗_e (6.2a)

Evb = Evb,bulk−∆E m^∗_e

m^∗_h+m^∗_e (6.2b)

with

∆E = E_g,QD−E_g,bulk

whereEg,bulk,Ecb,bulkandEvb,bulkare the band gap energy, the conduction band and the valence band positions of the bulk semiconductor material, respectively. Eg,QDis the band gap energy of the QDs determined from the spectral position of their first absorption peak. m^∗_e andm^∗_hare the effective masses of electrons and holes, respectively. For CdSe, the following parameters are taken [162]. Eg,bulk is around 1.74 eV at room temperature. The bulk band edges locate at E_cb,bulk≈ −4.3 eV andE_vb,bulk≈ −6.04 eV in the absolute vacuum scale. The effective masses of electrons and holes are 0.13m0 and 0.45m0, respectively. The first absorption peak of the CdSe QDs in the hybrid sample is measured to be 505 nm, corresponding to a band gap of 2.45 eV. From Equ. 6.2a, the absolute energy positions of their conduction band and valence band are determined to be -3.75 eV and -6.20 eV, respectively. For the band positions of ZnO nanowires, bulk values are taken. The conduction band is located at around -4.19 eV [163].

Using the band gap of 3.37 eV of ZnO [164], its valence band position is determined to be -7.56 eV.

Figure 6.7 shows a diagram of the energy band alignment inside the NW/QD hybrid struc-ture in the absolute vacuum scale. When electrons in the CdSe QDs are photoexcited, they can transfer into the conduction band of ZnO nanowires lying at a lower energy position, which increases the concentration of the conduction electrons in the nanowires and therefore increases the conductivity. This charge transfer may be a factor that accounts for the observed PC enhancement of the hybrid sample. However, such a process is not complete yet. After electron transfer, it is supposed to be a mechanism for the charged QDs to be neutralized and

6 Oxygen-controlled Photoconductivity in ZnO Nanowires Functionalized with Colloidal CdSe Quantum Dots

Figure 6.7: Diagram of energy levels of ZnO nanowires and CdSe QDs in absolute vacuum scale determined from the literature values [162, 163].

go back to their original states. In addition, when laser is blocked in the photoconductivity measurements, the currents reduce and return to the original values, indicating that the added conduction electrons during the irradiation stage get trapped again. Moreover, the slow PC rise and decay kinetics (in order of hours) suggests that it may be related to some slow chemical effects.

Previous studies have shown that the gas environment has significant impact on the electrical conductivity of ZnO materials, which is related to the chemisorption of gas molecules on their surfaces [95, 165, 166]. Such effect can be more significant on nanostructured materials due to their large surface-to-volume ratios. In addition, as shown in Fig. 6.5(b), the decay kinetics in air follows accurately a stretched exponential function, which is a typical behavior for the reduction of the conductivity induced by surface adsorption of gas molecules [167,168]. Thus, the influence of gas environment on the photoconductivity of the NW/QD hybrid structure has to be examined.