ZnO nanowires were synthesized on a large scale by a modified carbothermal reduction method. Vaporized zinc and oxygen react at atmospheric pressure and result in a cotton-like product consisting of ZnO nanowires. The nanowires were processed into free-standing thin sheets, which can be cut by a blade into any shape [16
]. The optical photograph in shows a strap-shaped nanowire sheet held by a tweezer. Scanning electron microscopy (SEM) observation reveals that these nanowires interdigitate to form a felt-like film (). Transmission electron microscopy (TEM) images of the nanowires are shown in and . The high-resolution TEM image in reveals that the nanowires grow along the [1
] direction, as the distance between the (0001) planes is 0.52 nm. The nanowires are 20–60 nm in thickness and tens of microns in length. The high surface-to-volume ratio ensures a large surface adsorbance of gaseous molecules.
(a) An optical photograph of a strap-shaped ZnO nanowire film; (b) SEM image of the nanowire film; (c,d) TEM images of the nanowires.
shows the room-temperature photoluminescence (PL) spectrum of the ZnO nanowires, at the excitation wavelength of 325 nm. The insert is a PL excitation (PLE) spectrum taken for the emission at 570 nm. The PLE spectrum indicates that these nanowires absorb UV light with a wavelength below 378 nm, whereas they are transparent to visible light. In the PL spectrum the sharp peak at 379 nm is ascribed to the near-band-edge emission of ZnO. The broad emission band in the visible range is related to some crystal defects as luminescent centres. In our method, the nanowires were grown quickly by vapour-phase reaction at low temperature, below 200 °C, and crystal defects are expected. Oxygen vacancies and antisite oxygen give contributions to the green PL emission around 520 nm; zinc vacancies and interstitial oxygen contribute to the blue PL emission (450–470 nm) [17
Room-temperature PL spectrum of the ZnO nanowire film. The insert is a PLE spectrum.
We made devices based on the strap-shaped nanowire sheets (width 2 mm). To make electrode contacts, silver paste made by mixing Ag nanopowder and the viscous PDMS liquid was painted onto a nanowire film strap at two ends, leaving a 10 mm gap between the two electrodes. The whole device, with a Ag–ZnO–Ag structure, was embedded in PDMS for measurement. The PDMS liquid wets the ZnO nanowires well, such that after curing the interspaces between nanowires were filled with PDMS polymer, making the paper-like nanowire sheet more translucent. For comparison, we made two devices without embedding in PDMS. For these two devices we deposited metal electrodes (Ti or Ag) onto the strap-shaped ZnO nanowire films (2 mm width) by e-beam evaporation through a shadow mask, in order to rule out the influence of PDMS on the UV-detection performance. The gap between two opposite electrodes is 10 mm, which is the same as that of the device in PDMS. If Ag–PDMS paste were applied onto the ZnO nanowire film, the liquid PDMS would slowly spread out of the agglomeration of Ag nanoparticles and infiltrate the nanowire film due to capillarity. shows the current–voltage (I–V
) curves of the device in PDMS, measured under UV-light illumination (312 nm, 30 mW·cm−2
) and in the dark. The I–V
curves of two other devices measured in air are plotted in . ZnO is naturally n-type with an electron affinity of 4.35 eV; the work functions of Ag and Ti are 4.26 eV and 4.33 eV, respectively. Under UV illumination Ti shows good ohmic contact on ZnO due to the extremely low barrier at the Ti/ZnO junction. The vacuum deposited Ag also shows nearly ohmic contact on ZnO (). However, for the contact of Ag paste on ZnO, the I–V
curves measured under UV illumination and in the dark show rectifying features. Presumably, the Ag nanoparticles that we used to make the Ag–PDMS paste were spontaneously oxidized due to ageing. The surface Ag2
O layer, a narrow bandgap semiconductor, added an extra barrier for electrons flowing between Ag nanoparticles and ZnO nanowires, resulting in the nonlinear I–V
curves in . The I–V
curve of the device in PDMS measured in the dark, plotted in the insert in , shows that at a forward bias of 8 V the current is only 0.11 µA, whereas under UV light illumination the current reaches 405 µA at 8 V, ~3650 times larger. However, for the two devices measured in air, the current values at 8 V are around 85 µA for the device with Ti electrodes, and 78 µA for the device with Ag electrodes. Previously, we studied the UV photoconduction of ZnO nanowire films with different PL properties, i.e., defect contents. The UV photocurrent of those ZnO nanowire films (2 mm wide and 10 mm long) with vacuum deposited Ag electrodes was in the range of 13–90 μA at 8 V when measured in air [10
]. In this work, the UV photocurrent of the device with Ag paste electrodes, embedded in PDMS, exceeds 400 μA at 8 V. Even at a reverse bias of −8 V, the current is 160 μA, much higher than that of the device without PDMS. In the dark, the current values of the device in PDMS are 110 nA at 8 V and 18 nA at −8 V. However, the dark current of the ZnO nanowire films exposed to air is extremely low, about 0.1 nA at 8 V (see the insert in ), indicating considerable adsorbance of oxygen on the nanowire surfaces. Hence, both the UV photoconduction and dark conduction of the nanowire film can be enhanced by PDMS coating. The responsivity of the device, defined as the photocurrent per unit of incident optical power, is determined by the UV photoconductivity of the ZnO nanowires. From the I–V
curves we can deduce that the PDMS coating over ZnO nanowires results in an approximately fivefold enhancement in the responsivity of the device.
I–V curves of ZnO nanowire films under UV illumination and in the dark. (a) The device in PDMS with Ag–PDMS paste electrodes; (b) two devices in air, with vacuum-deposited metal electrodes (Ti or Ag).
shows the UV-photoresponse characteristics of two devices based on ZnO-nanowire sheets with the same dimensions. The device with vacuum-deposited Ti electrodes was exposed to air, and the other one with Ag paste electrodes was embedded in PDMS. Both were measured at 8 V with the UV light switched on and off periodically. and show the UV photoresponse characteristics of the devices in air and in PDMS, respectively. The current–time curves depicting the UV photoresponse are further plotted in and on a logarithmic scale. The maximum photocurrent of the device in air is 86 μA, while that of the device in PDMS reaches 476 µA. The PDMS coating over ZnO nanowires leads to an enhancement of the UV photosensitivity and prolongs the decay time. The rise of the photocurrent and the dark current decay of the two devices are compared in and , respectively, with the current normalized to the maximum. Upon the switching on of the UV light, the current through the nanowire film in PDMS rises fast with a speed equivalent to that in air. Within one second the current reaches half of the maximum seen upon UV illumination. We define the dark current decay time as the time taken for the current to decay to 10% of the peak value. For the nanowire film in air the decay time is about 7 s, as shown in , whereas for that in PDMS the decay time is 29 s, i.e., about four times longer. Previous reports on ZnO nanowires that had gone through a lithography process involving polymers for making the electrodes [18
], demonstrate a slow UV photoresponse compared to those that had never been in contact with polymers [7
]. Surface contaminations from photoresist or electron-beam-resist polymers are probably responsible for the reduced response speed. The photoresponse speed of our ZnO nanowire film in PDMS is equivalent or even faster than for those contaminated by polymers from lithography processes, suggesting that ZnO nanowires in PDMS may be adequate for applications that do not require particularly high speeds.
Figure 4 (a,b) UV photoresponse characteristics of ZnO nanowire sheets in air and in PDMS, respectively. The electrode of the device in air is vacuum-deposited Ti, and that of the device in PDMS is Ag–PDMS paste. The bias was set at 8 V for both devices; (more ...)
The photoresponse of ZnO in air is known to be governed by the adsorption (in the dark) and desorption (under UV illumination) of oxygen molecules. In the dark, oxygen molecules adsorbed on the surface of ZnO nanowires capture free electrons of the n-type semiconductor: O2(g) + e
− → O2
–(ad). This produces a depletion layer near the nanowire surface, resulting in an upward band bending near the surface. Due to the large surface-to-volume ratio, the adsorption of O2 significantly reduces the conductivity of the nanowires. UV-light illumination with a photon energy higher than E
g generates electron–hole pairs in the ZnO. Holes migrate to the surface along the potential slope created by the band bending and recombine with O2-trapped electrons, thus releasing oxygen from the surface: O2
–(ad) + h
+ → O2(g). The heavily populated electrons in the conduction band enhance the conductivity of the ZnO nanowires.
The UV photoresponse of ZnO nanowires in PDMS can be explained with the help of . After polymerization, the PDMS molecule chains of CH3
, where n
is the number of repeating monomer [SiO(CH3
] units, form a network polymer structure that is highly permeable. PDMS has a high oxygen permeability due to the large free volume from the flexibility of the siloxane (–SiO–) linkages; the oxygen concentration in PDMS has been reported as 2 mM [23
]. The diffusion coefficient of oxygen in PDMS is reported as 3.55 × 10−5
]. Therefore, entangled PDMS molecule chains and dissociated oxygen molecules surround the ZnO nanowires, as illustrated in . The interaction between ZnO and the cross-linked PDMS is believed to be of the van der Waals type. Charge interaction between the two materials is unlikely to occur, based on the fact that the PL properties of ZnO nanowires in air and embedded in PDMS are very similar. In the dark the current of the nanowire film in PDMS reached 110 nA at 8 V as shown in , whereas the dark current of such a nanowire film exposed to air was only 0.1 nA at 8 V, as reported in our previous work [10
]. This means that the nanowires in PDMS have a smaller surface available for the absorbtion of oxygen molecules. In fact, when a ZnO nanowire is embedded in cross-linked PDMS, it is conceivable that only the nanowire surface areas that are not in contact with PDMS adsorb oxygen molecules diffusing through the rubbery polymer (). Upon UV illumination the holes in the valence band combine with O2
-trapped electrons, releasing the surface O2
and enhancing the photoconduction (). The desorbed O2
diffuse into PDMS molecule networks, and are slowly re-adsorbed onto the ZnO nanowires after the UV light is switched off. PDMS molecule chains hamper the migration of oxygen molecules towards the ZnO nanowire surface, leading to the slow decay of the dark current. It is worth mentioning that the UV photoresponse speed of ZnO nanowires in an oxygen atmosphere is also dependent on the gas pressure, i.e., oxygen concentration: The lower the pressure the slower the UV photoresponse speed [4
]. A number of silicone and polysulphone polymers have been studied for gas separation membranes [25
]. Hence, we speculate that ZnO nanowires embedded in a gas-permeable polymer with higher oxygen permeability would have a faster UV photoresponse. The UV penetration depth in ZnO is less than 100 nm, whereas the thickness of the nanowire film is several micrometers, indicating that the contribution to the photoconduction of a nanowire sheet comes mostly from the top layers. With PDMS (refractive index ≈ 1.5) filling the interspaces between nanowires, the paper-like ZnO nanowire film becomes more translucent to the naked eye, facilitating UV-light scattering to a deeper level in the nanowire sheet. Therefore more nanowires will receive UV photons, leading to the enhanced responsivity of the nanowire sheet in PDMS.
Figure 5 (a) Illustration of a ZnO nanowire in cross-linked PDMS. The blue lines represent PDMS molecule chains; (b) Illustration of a nanowire in PDMS without UV illumination, with oxygen adsorbed on parts of the surface. The colour gradient across the cross-section (more ...)