4-1 Introduction
Currently, in order to improve the sensitivity of gas sensors, functionalizing
semiconductor NWs. 10,11 The metal particles on the surface will results in the formation of a localized Schottky junction, which creates a charge depletion region in
NWs in the
intermediate in chemical combustion reactions which reduce the current through the s and then enhance the sensitivity. In this lette
of SnO2 NWs with Au-particles decorated can be enhanced. All our results can be rpreted based on the formation of
-2 Results and Discussion
A scanning electron microscopy (SEM) image of the sample is shown in the inset of Fig. 4-1. The distance of the c o terminals is 10 μm, and the diameters of the three NWs are respectively 137, 295, and 479 nm. The dark current versus bias (I-V) measurement is routinely checked to ensure Ohmic contact between nanw
s shown in Fig. 4-2, the diameter of the Au-particles are about 10 nm, and the n of the Au-particles does not cover the whole surface of the NWs.
Thus 4
hannel between tw
ires and metal electrodes as shown in Fig. 4-1.
A
random distributio
the decorated Au-particles will not screen the incident light, and the excess Au-decoration will not produce a new conductive path on the surface and the substrate. As shown in Fig. 1, the I-V curve of the NWs with the Au-particles is a little higher than that of the NWs with Au-particle. The dark current doesn’t decrease by the expansion of SCRs after the Au-decoration because of the large density of conduction electrons in Au-nanoparticles. Figure 4-3 shows the results of photoresponse under different excitation intensity performed on SnO2 nanowires in ambient air. It is obviously that the PC increases with increasing the light intensity from 2.5 W/m2 to 1410 W/m2. Quite interestingly, when the nanowires are decorated
ith Au-particles, the measured PC can be enhanced by up to about 110%. It will be t mechanism that causes the PC enhancement after Au-p
of the SnO2 nanowires, the free electrons are accumulated on the surface.14 Consequently, the existence of upward band
w
intriguing to know the exac
articles decoration. For the pristine SnO2 nanowires studied here, the measured PC has an extremely high PC gain. It’s value can reach up to about 900.
The high gain of SnO2 is attributed to the presence of oxygen vacancies. Due to the existence of oxygen vacancies on the surface
-bending forms a low-conductivity depletion region at the surface referred to space charge regions (SCRs). After the electron-hole pairs are photogenerated, the photoinduced holes can migrate to the surface by the strong electric field. As a result, a conductive volume increment is produced. And the spatial separation of electrons and holes also reduce the electron-hole recombination rate, and therefore, the electron lifetime increases and the photoresponse is enhanced. According to the simulation of Garrido et al,14 the PC induced by the modulation of surface SCRs causes Γ
following the inverse law with excitation intensity, i.e., Γ I , and the exponent ∝ −κ κ is between 0.5 and 0.9. As shown in Fig. 4-4, the gain logarithmic plot in the intensity ranging from 2.5 W/m2 to 1410 W/m2 shows a clear power law κ~0.6, which is in good agreement with the decoration prediction.
n the vicinity f the metallic particles. The formation of the Schottky barrier on the surface will enha
Let us now try to understand the origin of the PC enhancement after the Au decoration. It is known that depositing the metallic particles on the surface of a semiconductor, such as SnO2,10,11 results in a localized Schottky barrier i
o
nce the surface electric field and increase the width and height of space charge region as shown in Fig. 4-5. This is due to the fact that the work function of SnO2
nanowires 4.7 eV, is smaller than that of Au cluster, 5.1 eV. The increase of the barrier height of the SCRs on the surface will enhance the spatial separation effect in illuminating process, and the electron lifetime is increased. In turn, the measured PC is enhanced. The exponent κ of inverse power law now changes from 0.6 to 0.65 as shown in Fig. 4-5 due to the enhancement of the SCRs on the surface of SnO2
nanowires. This behavior can be simulated according to the following equations.
SCRs inside a semiconductor produce a variation of the conductive volume when carriers are photogenerated, and the Δi can be expressed as
( )
is the dopping level,
d
kTq
VT = , and is Richardson constant =1.2x106 A/m2K2.15 Figure 4-6 shows the
simu
A *
lation results, which indicate the gain and the slope κ become larger with the increasing barrier height. This result is consistent with the Schottky junction model proposed as described above.11
Finally, as shown in Fig. 4-7, the decay time, that describes the photocurrent recovers to the dark current after turning off the incident light, increases form 112 s to 207 s, when the SnO2 nanowires are decorated with Au particles. The behavior is consistent with enhancement of surface electric field caused by Au particles. Because the increased surface electric field will enhance the spatial separation of photoexcited electrons and holes, it becomes more difficult to return to the original states, and hence the decay time is increased.
ig. 4-2: Scanning electron microscope (SEM) image of the Au-decorated SnO2
anowires.
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
-5x10-8 0 5x10-8 1x10-7
Fig. 4-1: I-V characteristics of SnO2 nanowires with and without Au-nanoparticles in ambient air.
-1x10-7
Current (A)
Bias (V)
pristine
Au-decoration
10μm
F n
Fig. 4-3: Photoresponse of SnO2 nanowires by UV illumination under different xcitation intensity.
0 200 400 600 800
e
1x10
-72x10
-73x10
-7Pristine
Au-decoration
Current (A)
Time (s)
Fig. 4-4: Power dependence of pristine and Au-decorated SnO2 nanowires.