Negative differential resistance and resistive switching behaviors in Cu2S
Xiaohua Liu, Matthew T. Mayer, and Dunwei Wanga?
Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon Street, Chestnut Hill,
Massachusetts 02467, USA
?Received 24 March 2010; accepted 10 May 2010; published online 1 June 2010?
Two-terminal devices of Cu2S/ZnO core/shell nanowires were fabricated and measured. Forward
bias sweeping produced a rectified I-V characteristic of a diode, with turn-on voltages varying from
150 to 300 mV. The turn-on voltages depended on the rate at which the bias was varied. When the
bias scan was reversed, a resistive switching ?RS? behavior was observed.Alow-resistance state was
measured, and the diode characteristic diminished. At ?50 to ?150 mV, negative differential
resistance ?NDR? was observed, after which the diode behavior was restored. This phenomenon was
explained using the diffusion of Cu+within Cu2S. ZnO acted to limit RS to the positive bias range
and NDR to the negative bias range. © 2010 American Institute of Physics.
Cu2S is a cation-deficient p-type semiconductor with a
band gap of 1.2 eV. It can potentially be used to interface
with n-type semiconductors such as CdS or ZnO for efficient
solar energy conversion.1,2The high mobility Cu+exhibits,
however, has been proven tremendously cumbersome for this
application, which has led to the abandonment of Cu2S as a
photovoltaic material over two decades ago.3Recent ad-
vancement in the synthesis of its nanostructures has renewed
the interest in Cu2S.4–7We have shown that the high mobility
of Cu+vacancies in Cu2S is the driving force in the unique
growth of regularly aligned Cu2S nanowires,6opening up
opportunities to rejuvenate research on Cu2S for solar energy
harvesting. Nevertheless, to reach this goal it is essential to
understand the properties of Cu+in Cu2S ?Ref. 8? and par-
ticularly at the interface of the p/n junction. This paper re-
ports our latest discovery toward this end. We found that
when interfaced with ZnO, Cu2S exhibits obvious negative
differential resistance ?NDR?. A resistive switching ?RS?
characteristic is also observed on this device. We show that
these observations can be accounted for by the high mobility
of Cu+. Our results shed light on the nature of the p/n junc-
tion consisting of Cu2S and highlight the necessity to control
the cation diffusion in Cu2S for energy-related applications.
The synthesis of vertically aligned Cu2S nanowires has
been described elsewhere.6Briefly, a Cu foil ?Sigma-
Aldrich? was electrochemically polished then placed in a
chemical vapor deposition chamber, where a mixture of H2S
?10 SCCM ?SCCM denotes cubic centimeter per minute
at STP??, N2?160 SCCM, saturated with H2O? and O2
?80 SCCM? flowed over the surface of the Cu foil. The re-
action took place at room temperature and lasted for 5 h.
Vertical Cu2S nanowires, 100 nm wide and 2 ?m long, were
obtained. The resulting material was transferred to an atomic
layer deposition ?ALD, Savannah 100, Cambridge Nanotech?
chamber for ZnO deposition. For this purpose, diethyl zinc
?ZnEt2? ?Sigma-Aldrich? and H2O served as the precursors,
and the reaction took place at 100 °C.9Thin ZnO films
??50 nm? were grown for imaging purposes; and thicker
ZnO films ??200 nm? were used for the measurements. The
electrical measurement was conducted on a probe station
?Cascade Microtech, M150 Measurement Platform?. The Cu
support on which Cu2S was grown served as one contact and
a tungsten or gold tip ?radius?2.4 ?m? gently touching the
ZnO from the top was the other contact ?Fig. 1?a??. The data
was collected using a source meter ?Keithley 2400? con-
trolled by a computer through a LABVIEW program. The elec-
trode contacting ZnO was grounded for all data presented
here. Control experiments, where ZnO was absent, were per-
formed in the same fashion except that the top contact was
made to the Cu2S nanowires directly.
Also shown in Fig. 1 is a typical I-V plot ?Fig. 1?c??.
More detailed data are plotted in Fig. 2. There are three
important features we wish to emphasize. First, the forward
scan produced an I-V plot characteristic to that of p/n diodes.
The turn-on voltage, however, depended on the rate at which
the voltage was varied. Second, the reverse scan yielded a
low-resistance state, which extended to voltages in the nega-
tive range. The I-V curve resulting from the reverse scan was
typified by a good linearity in the low voltage region, mani-
festing the behaviors of a resistor. Third, a NDR behavior
was observed in the negative voltage range. The exact volt-
ages at which the NDR appeared varied with the voltage scan
rates. The NDR diminished at extremely slow scan rates such
as 2 mV/s ?Fig. 3?a??.
RS and NDR have been reported earlier in a number of
systems, including TiO2,10,11NiO,12ZnO/SiOx,13Co3O4,14
and Cu2S ?Refs. 15 and 16?. Compared with the existing
reports, our result is unique in following two aspects: the I-V
characteristics and the governing mechanism. As described
above, a typical I-V curve measured in our experiments in-
cludes two nonlinear components, a diode and a RS, both of
which are well defined and highly reproducible. The intro-
duction of ZnO to form a p/n junction with Cu2S permits us
to restrict the NDR peak to the negative bias range and the
RS behavior to the positive bias range. These features and
the low operating potentials make it possible to utilize the
reported structure for future device fabrications, promising
high reliability, low energy consumption, and low heat dissi-
a?Electronic mail: firstname.lastname@example.org. Tel.: 617-552-3121.
APPLIED PHYSICS LETTERS 96, 223103 ?2010?
0003-6951/2010/96?22?/223103/3/$30.00 © 2010 American Institute of Physics
We suggest that the key to understanding the observed
NDR and RS lies in the high mobility of Cu+.6,17Without
external bias, the Cu2S/ZnO interface represents that of a p/n
junction. The initial forward scan indeed generates an I-V
curve resembling that of a diode. The forward bias, however,
has an additional effect to the interface—it drives Cu+to
accumulate in Cu2S near the Cu2S/ZnO interface. In effect,
this accumulation decreases the p-doping level of Cu2S,
which weakens the junction as the forward bias is increased
or held for longer durations. The weakening effect is mani-
fested as the abrupt turn-on of the forward I-V curve ?Fig.
1?c??. When the forward bias is reduced ?backward scan from
positive to negative?, the weakened p/n junction fails to be
reinstalled immediately, maintaining the low-resistance even
below the original turn-on voltage. The low-resistance state
is responsible for the linearity of the I-V curve in the low
bias range.After the bias is swept into the negative range, the
bias polarity is reversed, and the accumulated Cu+is gradu-
ally removed from the junction area, allowing the p/n junc-
tion to reestablish, upon the completion of which the resis-
tance increases to limit backward current flow. The
reinstallment of the diode behavior produces the NDR as
observed in the negative bias region ?Fig. 1?c??.
The hypothesis is supported by the dependence of the
I-V plot on the voltage scan rates. As shown in Fig. 2, when
the scan rate is fast ?2500 mV/s?, the diode was turned on at
a large bias ?0.3 V?. This is because at a fast scan rate, the
Cu+accumulation lags behind the rapidly increasing bias,
and thus a larger bias is reached before the turn-on state is
achieved. Conversely, at slower scan rates, the turn-on volt-
ages were reduced consistently, and the reduction in the
turn-on voltage depended on the scan rate in a monotonic
fashion. Similarly, the height and the position of the NDR
peaks varied with the scan rate as well. The highest and the
most negative NDR peak was observed when a 2500 mV/s
scan rate was used ?the fastest scan rate tested in our experi-
ments?. This phenomenon is related to the rate at which the
accumulated Cu+is dissipated. The dissipation of Cu+leads
to the reinstallment of the Cu2S/ZnO diode, which is the
primary reason for the NDR behavior. Consistent with this
hypothesis, the NDR peak was not observed at the extremely
slow scan rate of 2 mV/s because at such a rate the accumu-
lated Cu+is provided enough time to dissipate before reach-
ing the NDR region ?Fig. 3?a??. Note that at this slow scan
rate, the RS behavior is still pronounced. This observation is
in line with the hypothesis that Cu+is the primary cause for
the NDR and RS. We note that the ON/OFF switching is
highly reversible, which we attribute to the reversibility of
Cu+diffusion under small biases.
ZnO plays an important role in the results reported here.
As shown in Fig. 3, the incorporation of ZnO introduces a
diode to the device, which helps suppress the current in the
reverse bias region. The suppressed current makes it possible
to distinguish the different features of the I-V curves in the
forward and reverse bias regions. Although similar NDR and
RS behaviors were observed in the control experiments
where ZnO was absent ?Fig. 3?c??, the nature of the proper-
ties is different. When Cu2S is contacted by a metal directly,
the Cu+accumulation and dissipation changes the Schottky
barrier height, which is manifested as the conductance
change. The lack of rectification of the I-V curve, however,
can make the NDR difficult to observe ?Fig. 3?c??. While it is
FIG. 1. ?Color online? ?a? A schematic illustration of the ZnO/Cu2S nano-
wires and measurement setup. ?b?Atransmission electron microscopy image
showing the contact between the sample and a probe. The insets show the
side view ?upper? and the cross section ?lower? of the core/shell structure.
Thicker ZnO coating ??200 nm? was used in the measurements. ?c? A
typical I-V curve showing both the NDR and RS features.
FIG. 2. I-V plots at various scan rates. With a larger scan rate, the
ZnO/Cu2S diode was turned on at a larger bias when scan from “?” to “+”
?denoted “n”? and a larger NDR peak appeared in the reverse “p” scan.
Inset: semilog scale plot.
223103-2Liu, Mayer, and WangAppl. Phys. Lett. 96, 223103 ?2010?
important to have a diode to rectify the current, the exact
nature of the diode plays an insignificant role. When ZnO
was replaced by CdS, another n-type semiconductor, compa-
rable RS and NDR were measured.
The switching of the resistance as the polarity of the
voltage varies has been the topic of intense research lately.
The mechanism researchers employ to explain the observa-
tion usually involves charge trap/detrap, ion/defect diffusion,
and/or redox reaction. In particular, the formation and disso-
lution of highly conductive metal filaments has been adopted
to explain the RS phenomena based on Cu2S materials.16The
governing mechanism that depends on ionic accumulation
we proposed here is distinct from the existing models but is
based on similar considerations of the ionic nature of the
active material, Cu2S. While further studies will be needed to
compare these models to understand their distinctions and
relations, the result presented here sheds light on the detri-
mental effect caused by the high mobility of Cu+in utilizing
Cu2S for solar energy harvesting. For this purpose, it will be
of paramount significance to understand and, more impor-
tantly, to control the interface Cu2S forms with the n-type
In conclusion, we observed resistance switching and
NDR in Cu2S nanowire-based devices. These behaviors were
explained using the high ionic mobility of Cu+in Cu2S. The
high reproducibility and low operating potential may be use-
ful for memristor device fabrications. The result also pro-
vides insight to the Cu2S-based p/n junctions, paving the way
for efficient solar energy harvesting using low-cost materials.
The work was supported by Boston College. The authors
thank S. Shepard and G. McMahon for their technical assis-
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FIG. 3. ?Color online? ?a? NDR peak diminishes at slow scan rate of 2 mV/s.
??b? and ?c?? Comparison of I-V plots with or without ZnO.
223103-3Liu, Mayer, and WangAppl. Phys. Lett. 96, 223103 ?2010?