báo cáo hóa học:" Temperature Dependence of Photoelectrical Properties of Single Selenium Nanowires" doc

NANO EXPRESS
Temperature Dependence of Photoelectrical Properties of Single
Selenium Nanowires
Zhi-Min Liao
•
Chong Hou
•
Li-Ping Liu
•
Da-Peng Yu
Received: 22 February 2010 / Accepted: 16 March 2010 /Published online: 28 March 2010
Ó The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract Influence of temperature on photoconductivity
of single Se nanowires has been studied. Time response of
photocurrent at both room temperature and low tempera-
ture suggests that the trap states play an important role in
the photoelectrical process. Further investigations about
light intensity dependence on photocurrent at different
temperatures reveal that the trap states significantly affect
the carrier generation and recombination. This work may
be valuable for improving the device optoelectronic per-
formances by understanding the photoelectrical properties.
Keywords Se nanowires Á Trap states Á
Photoconductivity Á Temperature effects
Semiconductor nanowires have great potential applications
as the building blocks for nanoscale electronic and opto-
electronic devices. Selenium (Se) is an important semi-
conductor, and Se nanowires have attracted enormous
attentions due to their unique physical properties, such as
high photoconductivity, large piezoelectric, and thermo-
electric effects [1–14]. In particular, photodetectors and

photoelectrical switchers based on individual Se nanowires
have been fabricated [10–14]. In order to further improve
the performance of the Se nanowires optoelectronic devi-
ces, well understanding of the photoelectrical properties is
desirable. Measurement of the temperature dependence of
photoconductivity is an efficient method to study the
photoelectrical properties because it can yield more infor-
mation about the carrier generation and recombination.
However, the detailed temperature effects on photocon-
ductivity of single Se nanowires are not yet reported. In
this work, we study the photoconductivity of single Se
nanowires by measuring the time response on photocurrent
and the light intensity dependence on photocurrent at dif-
ferent temperatures. It is found that the trap states are
sensitive to the temperature.
The Se nanowires were grown by solution method as
described previously [9]. Briefly, solid Se spheres were first
fabricated by the dismutation of Na
2
SeSO
3
solution. And
then, the Se spheres were dispersed in ethanol to finally
form the solid Se nanowires. Field-emission scanning
electron microscopy (SEM, FEI DB 235) was used to
image the Se nanowires. The SEM image shown in Fig. 1a
reveals the nanowires having lengths of about several
micrometers with dendritic structures. The Se nanowires
were characterized through Raman spectroscopy and pho-
toluminescence (PL) spectroscopy using a Renishaw inVia

Raman-PL microscope with a 514 nm laser excitation.
Figure 1b shows the Raman spectra taken on the Se
nanowires. The sole peak is centered at 238 cm
-1
sug-
gesting that the selenium nanowires are with the trigonal
phase [10]. A typical PL spectrum of the Se nanowires is
shown in Fig. 1c. The PL spectrum is dominated by a peak
centered at 706 nm corresponding to 1.76 eV, which is
well consistent with the band-gap of trigonal-Se [10].
In order to perform the measurement of photoelectrical
properties of individual Se nanowires, the Se nanowires
were mechanically transferred onto the SiO
2
(500 nm)/Si
Z M. Liao (&) Á C. Hou Á D P. Yu (&)
State Key Laboratory for Mesoscopic Physics, Department
of Physics, Peking University, Beijing 100871,
People’s Republic of China
e-mail:
D P. Yu
e-mail:
L P. Liu
Department of Chemistry, TsingHua University, Beijing,
People’s Republic of China
123
Nanoscale Res Lett (2010) 5:926–929
DOI 10.1007/s11671-010-9585-2
substrate. Ti/Au (10/70 nm) electrodes contacting onto
individual Se nanowires were fabricated by electron beam

lithography, metal sputtering deposition, and lift-off. SEM
image of the fabricated two-terminal nanowire device is
shown in Fig. 1d. Conductance measurements were carried
out using a Keithley 4200 Semiconductor Characterization
System and the samples were placed on a Janis micro-
cryostat under vacuum (*10
-6
Torr). For photoconduc-
tivity measurements, illumination was provided by an Ar
ion laser with 514 nm wavelength guided by a Renishaw
Raman microscope.
Figure 2a shows the time course of the photocurrent
measured at 300 K and with constant 0.1 V bias voltage as
the laser illumination was turned on and off. The photo-
current rapidly increased upon light exposure and then
reduced to a constant, as seen the sharp peaks in Fig. 2a.
When illumination was removed, the current quickly
jumped down to the level of initial dark current value. The
photocurrent–time characterization suggest the existence of
trap states in the Se nanowires. Our previous work also
indicated that the surface-absorbed oxygen molecules on
the Se nanowire can capture electrons and induce the
p-type conductivity. [14] At room temperatures and dark
conditions, the trap states are mostly not occupied due to
the thermal activation. At the beginning of the illumination,
a mass of electron–hole pairs are generated resulting in the
sudden jump of current in Fig. 2a. The photo-generated
electron–hole pairs upset the original carrier balance and
will fill in the trap states. The filling of the trap states leads
to the decay of the photocurrent in Fig. 2a. The rebalance

of the carrier concentration makes the photocurrent toward
saturation. Figure 2a also shows that the current is con-
tinued to increase slightly after the illumination was turned
off, which is attributed to the fact that the carriers are
returned slowly from the trap states with the assistant of
thermal activation.
Figure 2b shows the time course of photocurrent mea-
sured at 100 K and 0.2 V bias. The photocurrent has a
quick response to the laser illumination switching. No
transient decay of photocurrent was observed after the
device was exposed under illumination. At low tempera-
tures and at dark, the carriers captured in the trap states are
almost frozen. Under illumination, the carriers generated
from the trap states and the bandgap contribute to the
photoconductivity. The dynamic balance between the car-
rier generation and recombination results in the relatively
steady photocurrent but with some noise. After turning off
the illumination, the current declines quickly, and then the
dark current slowly decreases with a long relaxation time,
as shown in Fig. 2b. The relaxation of the dark current is
due to the recapture of the carrier by the trap states.
Figure 3a shows the time response of a single Se
nanowire device at different excitation intensities measured
at 10 K and 0.5 V bias voltage. As the illumination
intensity was varied from 0 to 2.3 9 10
4
mW/mm
2
, the
photocurrent was increased from 0.03 to 22 nA. The light

100 200 300 400
Intensity (a.u.)
Raman shift (cm
-1
)
(b)
500 600 700 800 900
Intensity (a.u.)
Wavelen
g
th (nm)
(c)
(a)
(d)
Fig. 1 a SEM image of the Se
nanowires. b Raman spectrum
of the Se nanowires
corresponding to trigonal Se
structure. c PL spectrum of the
Se nanowires showing 1.76 eV
(706 nm) band-gap of trigonal
Se. d SEM of single Se
nanowire two-terminal device
Nanoscale Res Lett (2010) 5:926–929 927
123
intensity dependences of the photocurrent at different
temperatures are shown in Fig. 3b with log–log scales. It is
interesting to notice the photocurrent toward the similar
values at high light intensities for the different tempera-
tures. At high illumination intensities, the number of

photogenerated carrier is overwhelmingly larger than the
thermal activated carriers. Therefore, the current is gov-
erned by the light intensity but not the temperature. The
dependence of photocurrent (I
ph
) on laser intensity (P) can
be well fitted by a power law, I
ph
µ P
a
, where exponent a
can help to reveal the dynamics of carrier generation and
recombination. Fitting the power law dependence to the
experimental data gives a = 0.64, 0.49, and 0.07 at tem-
peratures of 10, 200, and 300 K, respectively. The power
law dependence can be further analyzed by inspecting the
variation of the density of free carriers (N) in the nanowire
[15]
dN
dt
¼ F ÀCNþ N
trap
ÀÁ
N; ð1Þ
where, F is the photon absorption rate and is proportional
to the illumination intensity P, C is the probability of a
charge to be captured, and N
trap
is the density of trapped
carriers. Under steady-state conditions, (dN/dt) = 0 and we

can obtain that
N þN
trap
ÀÁ
N / P: ð2Þ
Assuming the photocurrent I
ph
proportional to the free
carrier density N and considering two extreme conditions,
if N
trap
[[ N, then N µ P and thus I
ph
µ P;butif
N
trap
\\ N, then N µ P
0.5
and thus I
ph
µ P
0.5
. The expo-
nent a = 1 and a = 0.5 are corresponding to the mono-
molecular recombination and bimolecular recombination,
respectively [15]. For our experimental results, at 10 K,
a = 0.64, a signature of existence of both monomolecular
recombination and bimolecular recombination. At low
temperatures, the free carriers are not thermally activated,
and therefore, the response is dominated by the trap states.

At elevated temperature of 200 K, a = 0.49, which is
characteristic of bimolecular recombination. The free car-
riers increase as a result of increased thermal activation,
which induces the transition from monomolecular to
bimolecular recombination. However, the exponent a of
the photocurrent dependence on the excitation intensity
changes to 0.07 at 300 K, which may be due to the fact that
the trap states become recombination centers. At 300 K,
the trap states are thermally activated and act as recombi-
nation centers under illumination, leading to the weak light
intensity dependence of photocurrent.
In summary, the temperature effects on photoelectrical
properties of single Se nanowires have been investigated.
0.2
0.6
1.0
1.4
Current (nA)
Time (S)
300 K
0.1 V Bias
(a)
0 500 1000 1500 2000
0 250 500 750 1000
0.0
0.2
0.4
0.6
Current (nA)
Time (S)

100 K
0.2 V Bias
(b)
Fig. 2 Photocurrent versus time curves a at 300 K and 0.1 V bias
voltage, b at 100 K and 0.2 V bias
0 500 1000 1500 2000
0
5
10
15
20
25
Current (nA)
Time (S)
10 K
0.5 V Bias
10
100
1000
5000
18500
mW/mm
2
(a)
10
0
10
1
10
2

10
3
10
4
10
-10
10
-9
10
-8
300 K
200 K
10 K
Current (A)
Laser Intensity (mW/mm
2
)
0.5 V Bias
(b)
Fig. 3 a Photoelectrical response of a single Se nanowire device
under laser illumination of varying intensities at 10 K and 0.5 V bias.
b Photocurrent as a function of illumination intensity for 514 nm laser
excitation measured at 0.5 V bias and at different temperatures
928 Nanoscale Res Lett (2010) 5:926–929
123
The light intensity dependence of photocurrent suggests
that the Se nanowire photoconductivity is dominated by
trap states at low temperatures, while a weak dependence
of photocurrent on incident light intensity was observed at
room temperature.

Acknowledgments We thank Prof. Yadong Li of Tsinghua Uni-
versity for supplying the Se nanowires. This work was supported by
NSFC (No. 10804002), MOST (Nos. 2007CB936202, 2009CB62
3703), and the Research Fund for the Doctoral Program of Higher
Education (RFDP).
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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