Download Positive Temperature Coefficient of Resistance and Bistable

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Positive Temperature Coefficient of Resistance and Bistable
Conduction in Lead Selenide Quantum Dot Thin Films
Tyler N. Otto and Dong Yu*
Department of Physics, University of CaliforniaDavis, 1 Shields Ave, Davis, California 95616, United States
ABSTRACT: We report the observation of a positive temperature
coefficient of resistance in strongly coupled quantum dot (QD) arrays.
Conductivities of lead selenide (PbSe) QD thin films treated with 1,2ethanedithiol increase when cooled from room temperature to 78 K,
consistent with bulk PbSe crystals and indicating bandlike transport.
Small angle X-ray scattering and infrared absorption spectroscopy results
confirm a very strong electronic coupling among QDs. These QD thin
films also exhibit bistable conduction above 160 K. As the electric field
reaches a threshold, on the order of 0.1 V/μm, the film conductance
increases abruptly by up to 10 orders of magnitude and remains high
until the bias voltage is reduced to about zero volts. This bistability is likely induced either by filament formation or by a traprelated electronic switching.
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INTRODUCTION
Colloidal semiconductor quantum dots (QDs) have optoelectronic properties that greatly differ from their bulk counterparts. Both n-type and p-type bulk lead chalcogenide crystals
exhibit unusual positive temperature coefficients of resistance
(resistance increases with increasing temperature).1 On the
other hand, colloidal lead selenide (PbSe) QDs show negative
temperature coefficients of resistance in almost all reports to
date.2−5 This difference is likely caused by a weak electronic
coupling among QDs, where the low temperature conduction
behaviors can be described very well by the variable range
hopping mechanism.5−7 Improving the coupling, while
maintaining quantum confinement, can lead to more efficient
carrier extraction and better electronic devices based upon
QDs.
QDs have also recently been studied as potential building
blocks for electrically driven memory devices,8−12 which take
advantage of the low fabrication cost and tunable electronic
properties of QDs. The investigation of the memory effect can
also shed light on the charge transport mechanism in QD thin
films. Bistable conduction of QD thin films is typically achieved
by embedding the QDs in a polymer matrix, where the
application of a voltage pulse changes the conductivity of the
system between two metastable conductive states.8−10,13 The
bistability in the QD/polymer system has been attributed to an
electric field induced charge transfer from QDs to the
conducting polymers. Recently, there have been reports of
bistable conduction of pure QD thin films.9,12,13 However, the
memory switching mechanism of devices composed solely of
QDs is not well understood.
Here, we report a positive temperature coefficient of
resistance in strongly coupled PbSe QD thin films. While the
QDs in the films treated with 1,2-ethanedithiol (EDT) still
show clear quantum confined absorption peaks, conductivity
increases with decreasing temperature, as in the bulk PbSe. In
© 2013 American Chemical Society
addition, we have observed a drastic, repeatable, and rapid
increase in the conductivity of QDs by up to 10 orders of
magnitude within 1 ms, under a small threshold field on the
order of 0.1 V/μm.
■
EXPERIMENTAL DETAILS
PbSe QDs were synthesized using standard air-free techniques
as described elsewhere.14 The QDs were transferred and stored
in a N2 glovebox (O2 < 10 ppm, H2O < 0.1 ppm) where all
EDT treatment and subsequent electrical measurements were
performed. Planar devices with bottom contact to QDs were
made on 300 nm SiO2 covered Si wafer previously patterned
with interdigitated Cr/Au (5/25 nm) electrodes by photolithography (Figure 1a). The electrodes consisted of 350 pairs
of 10 μm wide, 7 mm long Au pads with 10 μm gap. The QDs
were dispersed in a 9:1 mixture of hexane/octane and passed
through a 0.2 μm PTFE filter before being drop cast on the
prepatterned substrate. The film was then placed in 0.1 M EDT
in anhydrous acetonitrile for 2−3 min and was subsequently
dried in N2 without rinsing.
Small angle X-ray scattering (SAXS) and X-ray diffraction
measurements were performed with a Bruker D8 Discover Xray diffractometer. Absorption spectra were taken with a
Nicolet 6700 Fourier transform infrared (FTIR) spectrometer.
Atomic force microscopic (AFM) images were measured with a
Veeco Dimension 3100 system. Electrical characterization was
made with a NI data acquisition system and a Stanford
Research Systems model SR570 current preamplifier. The QD
devices were transferred into a cryostat without exposure to air
and placed under vacuum with a base pressure of ∼20 mTorr
Received: July 11, 2012
Revised: January 22, 2013
Published: January 25, 2013
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Figure 1. (a) Schematic of a planar device with a QD thin film on top of prepatterned interdigitated electrodes. (b) Absorption spectra of PbSe QDs
with different diameters. (c) X-ray diffraction spectrum of PbSe QDs.
spacing within a couple angstroms after treatment, consistent
with the length of an EDT molecule.16 The SAXS result
confirms that the compact EDT ligands have successfully
replaced the bulky oleic acid ligands and the QDs are in near
direct contact with each other after EDT treatment.
We then took the absorption spectra of the QD thin films
before and after EDT treatment. After EDT treatment, QDs
maintained the discrete peaks indicating the QDs were still
quantum confined. The slight increase in the peak width can be
attributed to the electron enhanced coupling.17 The first
exciton absorption peak of the QD thin films red-shifted by 61
meV after EDT treatment, while maintaining discrete energy
levels (Figure 2b). The red-shift in our films was significantly
larger than that of thicker spin-coated films (27−32 meV) or
films created by the layer-by-layer method (23 meV).3,16 The
red-shift is caused by the increase in electronic coupling
between QDs or by other effects, such as the change in the
dielectric environment. Liu et al. has observed a 22 meV redshift due to noncoupling effect,16 from which we can estimate
our coupling energy, hΓ, is approximately 39 meV, corresponding a charge transfer rate, Γ, of 9.4 × 1012 s−1. This coupling
energy is significantly higher than the previously reported 10
meV,16 indicating that our QDs are very strongly coupled. This
large shift in the absorption spectrum is consistent with the
small interdot distance indicated by the large SAXS shift. A
large red-shift in the absorption spectrum has also been
observed previously in thiocyanate treated CdSe QDs
exhibiting bandlike transport.17
2. Charge Transport. We now present the electronic
characterization of the PbSe QD thin films. The QD thin films
exhibited a bistable conduction detailed in the later section and
here we first focus on their charge transport behavior in the
conductive state. The as-deposited films were completely
insulating beyond the detection limit of our setup (σ < 10−11
S/cm). The EDT treatment led to an increase in film
conductivity of at least 10 orders of magnitude (σ ∼ 10−1 S/
cm) after the film conductance was turned on by applying an
overthreshold bias voltage. Our film conductivity was 1−2
orders of magnitude higher than that in earlier reports,3,16
indicating a stronger electronic coupling between neighboring
QDs and consistent with our larger absorption peak shift and
small interdot spacing. We prepared our QD films by drop
casting instead of a layer-by-layer method or spin-casting as in
previous reports. It should be noted that the electrical
properties of QD thin films are sensitive to the details of the
film preparation, as the charge transfer rate exponentially
depends on the interdot distance and is strongly influenced by
the surface traps. This sensitivity has been noted by other
groups as well.5,14
We want to make two notes regarding to the electrical
measurements. (1) The interdigitated electrode structure
for temperature dependent measurements. To ensure an inert
and water free environment, the device was transferred into the
cryostat inside the glovebox. The cryostat was then sealed and
transferred out of the glovebox and placed under vacuum for
characterization.
■
RESULTS AND DISCUSSION
1. X-ray Diffraction and Spectroscopy Characterization. The QD sizes were determined from the first exciton
peak positions following the literature (Figure 1b).15 The first
exciton absorption peaks of the PbSe QDs were narrow (<40
meV), corresponding to a size distribution of <10% for all three
samples (Figure 1b). XRD measurements confirmed the rock
salt structure of the PbSe QDs (Figure 1c). The final film
thickness after EDT treatment was typically around 40 nm as
measured by AFM. The entire film had a RMS roughness of
about 15 nm, corresponding to 2−3 monolayers of QDs.
In order to determine the interdot spacing, we performed
SAXS measurements before and after EDT treatment. The QD
samples were exposed to air during the SAXS measurements,
and we observed that the results slowly changed over hours due
to oxidation. The following data was taken immediately after
the samples were removed from the glovebox, and before a
significant change occurred. SAXS measurements of the QD
thin films before EDT treatment exhibited clear superlattice
peaks indicating the film had a long-range order. After EDT
treatment, these peaks were greatly diminished and broadened,
signaling that the film transitioned to a more disordered
structure. The scattering intensity for spherical particles of
uniform radius R is proportional to the form factor F(q) = (4/
3)πR3{3[sin(qR) − qR cos(qR)]/(qR)3}, where q = (4π sin θ)/
λ . We extracted R by setting the derivative of F(q) to zero and
plugging in θ at maximum scattering intensity. By analyzing the
SAXS data this way, the interdot spacing was found to decrease
by approximately 1.6 nm (Figure 2a), bringing the interdot
Figure 2. (a) SAXS data showing a 1.6 nm reduction in interdot
spacing. (b) Spectral shift of 3.9 nm diameter PbSe QDs after EDT
treatment. Upper blue line, EDT treated thin film; middle black line, as
deposited thin film; bottom red line, QD solution.
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CdSe QDs.17 The bandlike transport in our QD films is
consistent with our other observations: near direct contact
between neighboring QDs confirmed by SAXS and strong
electronic coupling indicated by the absorption peak red-shift.
The observed positive temperature coefficient of resistance
further suggests that strong electronic coupling lead to
extended electronic states and electronic band formation. The
40% resistivity increase in our PbSe QD thin films from 78 to
300 K is significantly smaller than the 10-fold increase in bulk
PbSe, indicating the carrier scattering at the boundary of the
QDs still play an important role in the QD thin film.
The other possibility is that the films have already undergone
a metal−insulator transition at room temperature.18 This would
explain the absence of a persistent gate response. However,
given the positive temperature coefficient of bulk PbSe, along
with the observations of bandlike transport in QD thin films by
other groups,17,19 we believe it is more likely that the observed
positive temperature coefficient in PbSe QD thin films is caused
by a bandlike transport.
3. Bistable Conduction. At room temperature, the EDT
treated films showed a strong hysteresis in their I−V curves
(Figure 4). Upon reaching a threshold voltage (Vth), the film
allowed us to greatly increase the electrode area and thus
overall conductance; however, it prevented us from making 4point measurements. Though the measurements were made in
2-point devices, our linear I−V curves (even at low temperatures) and high conductivity in the conductive state strongly
indicate ignorable contact barriers. (2) These films only showed
temporary gate response as in earlier reports.3 In our films, the
gate response decayed on the order of seconds, preventing us
from separating the current generated by the gate pulse from
the temporary increase in carrier concentration. This decay is
not understood, but it has been attributed to a slow screening
of the applied gate field by trapped carriers.3 The instability in
gate response prohibits us from extracting the field effect
mobilities.
To better understand the charge transport mechanism, we
cooled a device incorporating a QD thin film to approximately
78 K and slowly warmed, while the conductance was measured
at 1 V bias so that the device stayed in its conductive state. The
conductance decreased by approximately 40% as the temperature increased from 78 to 300 K (Figure 3), showing a positive
Figure 3. On-state conductance extracted at different temperatures for
QDs with a diameter of 5.8 nm. Black line is a linear fit of the data.
Figure 4. (a) Room temperature I−V curve of a QD thin film device
showing precipitous change in conductance upon bias sweeping. (b)
Semilog plot of the I−V curve in (a). (QD diameter = 5.8 nm.)
temperature coefficient of resistance. We measured the
temperature dependence of four devices, and all exhibited a
positive temperature coefficient of resistance. This positive
temperature coefficient is the first such observation for
chemically treated QDs. Previously a positive temperature
coefficient has been seen only below 110 K in laser ablated and
ligand free PbSe QDs,18 which has been attributed to an
insulator-to-metal transition at low temperature. On the other
hand, bulk PbSe crystals also exhibit a positive temperature
coefficient, with a resistivity increase by a factor of 10 from 78
to 300 K. The resistivity change of bulk crystals is mainly
caused by the drastic increase in carrier mobility at low
temperature because of reduced carrier-phonon scattering.1 As
mentioned previously, our devices do not show a persistent
gate response, even at low temperature, preventing us from
directly measuring the mobility, but we can deduce the sign of
dμ/dT from the negative sign of dσ/dT. The conductivity, σ, is
determined by the product of the carrier mobility μ and the
carrier density n (σ = neμ). As temperature increases, σ
decreases and n is expected to increase or remain constant if
acceptor states are completely ionized at given temperature.
Thus, μ must decrease with temperature, suggesting bandlike
transport through extended electronic states. A negative dμ/dT
has been previously observed, as a signature of bandlike
transport, in CdSe QDs capped with molecular metal
chalcogenide complexes (MCCs)19 and thiocyanate treated
conductivity increased abruptly from ∼10−11 to 10−1 S/cm
(Figure 4b). This increased conductivity was maintained even
after the bias was reduced below Vth. The film returned to its
insulating state once the voltage was reduced below a holding
voltage Vhold. Vth and Vhold could have the same or opposite
polarity depending on sample, but |Vhold| was always much
smaller than |Vth| and near 0 V. The I−V curves were usually
symmetric and the film switched to the conductive state as the
voltage reached a more negative bias (V−th). The time required
for switching was measured to be less than 1 ms, limited by the
temporal resolution of the preamplifier.
This memory switching was robust, and a device could be
switched on and off many times. For a given device, Vth
remained constant upon repeated switching (Figure 5). While
Vth usually varies from 0.2 to 2 V in the more than 21 devices
measured, a couple of devices showed Vth as high as 10 V. This
variation was observed even when the films were composed of
the same batch of QDs and fabricated with the same procedure.
This Vth variation made it difficult to identify any size
dependence. Out of the 21 devices measured, 16 devices
showed conductance change over 10 orders of magnitude,
while other devices exhibited smaller on/off ratios. Hysteresis
in chemically treated CdSe QD thin films has been seen in the
past,9 but not with such drastic on/off ratios and with such a
rapid transition. The direction of hysteresis observed in CdSe
QDs was clockwise in contrast to our counterclockwise
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redox reaction significantly slows down and the metallic
filaments remain even at zero bias. After exposure to air, the
PbSe QD thin film remains conductive likely because the redox
reaction stops at the oxidized surface.
Though the above filament mechanism appears reasonable,
there are also a couple of difficulties: there has been no report
on lead filament formation though PbSe QDs have been
studied for more than one decade, and it is unclear why the
elements stay in the metallic state at low temperature.
2. Electronic Switching. We propose a possible mechanism
following the three steps labeled in Figure 4b: (1) When Vb is
below Vth, the QD array is insulating, likely because conduction
is prohibited by the Coulomb repulsion of the charges trapped
at the QD surface. EDT treatment will likely leave the surface
poorly passivated, allowing for the trapping of several charges.
As Vb increases, charges injected from the contacts into the
QDs are efficiently trapped at the QD surface. By trapping
multiple charges, the total charging energy can be increased
over kBT and thus prohibit conduction.16 The observed small
bias threshold (Vth ≈ 1 V) indicates that the electric field is
inhomogeneous.2 The inhomogeneous electric field can be
caused by the nonuniform charging of QDs along the film. The
potential drop is mainly distributed between charged and
uncharged QDs and this charge front propagates through the
QD arrays as bias increases.21 (2) When Vb is increased above
Vth, the charge front finally propagates through the entire film
and creates a conductive pathway, leading to an abrupt
transition to the conductive state. (3) After the transition, the
electric field becomes uniformly distributed and the charge
carrier wave function becomes extended in the strongly coupled
QD thin films. The charging energy is greatly reduced, since it
is proportional to the inverse of the localization length of the
charge wave function. Thus, when we reduce Vb below Vth, the
conductive state will persist until the bias is sufficiently lowered
below the reduced charging energy. Then the charges are
detrapped and the QD arrays return to the insulating state.
The above hypothesis is consistent with our experimental
observations, as well as the conclusion that the films are
behaving with bandlike properties. The disappearance of
hysteresis at low temperature can be understood as surface
states are frozen at low temperature. There is likely a potential
barrier between the QD conduction states and the surface
states. At low temperature, the charges do not gain sufficient
thermal energy to overcome this barrier and remain trapped at
the surface states, even when the bias is reduced. Thus, the
carriers will maintain the extended wave function, and in-turn,
high conductance, over the entire bias range. At higher
temperature, the trapped charges can be released at low bias
causing switching to the insulating state. Furthermore, oxidized
samples are conductive and do not show any hysteresis, since
oxidation leads to a high hole density3 and thus no external
electric field is required to turn on conduction. We think the
trap-related electronic switching more likely accounts for the
bistable conduction behavior in QD thin films, as the bistability
is sensitive to the chemical treatment and electronic coupling
between QDs. Further work is necessary to clarify on the origin
of the bistable conduction.
Figure 5. Room temperature I−V curves of a different device in 8
consecutive bias scans, demonstrating reproducibility in threshold
voltage and asymmetry. (QD diameter = 5.8 nm.)
hysteresis. These observations indicate that the mechanism for
the bistable conduction in EDT treated PbSe thin films must be
different from that of CdSe QDs. Upon exposure to air, these
devices no longer showed hysteresis and remained in the
conductive state with a slight increase in conductance,
consistent with previous reports.3,20 Most interestingly, the
bistable conduction exhibited a strong temperature dependence
(Figure 6). After cooling to 175 K, the hysteresis became
Figure 6. I−V curves taken at various temperatures showing the
disappearance of bistability at low temperatures. (QD diameter = 6.2
nm.)
weaker. When the temperature was below 160 K, the hysteresis
completely disappeared and the device remained in the
conductive state over the entire bias range. The hysteresis
was recovered upon warming above 160 K.
Two possible mechanisms may account for the observed
bistability as described below.
1. Filament Formation. under an applied bias of about 1 V,
a reversible redox reaction may happen in the QD thin film.
This would result in a conductive filament bridging the gap
between the electrodes. Given the rapid switching and the
relatively large gap (10 μm), it is more likely and energetically
favorable that metallic filaments are formed at the necking
between neighboring QDs, instead of having a metallic filament
bridging the entire gap. It is most likely that any filament
formed would be composed of lead, as the QDs are terminated
by lead atoms.15 The nearly direct contact between neighboring
QDs could be the cause of such a reaction. Because lead only
acts as short connection between QDs, the conductance is
expected to be dominated by the PbSe QDs and our early
discussion on the positive temperature coefficient of resistivity
still holds. At room temperature, such metallic filaments are not
stable at zero-bias and will likely react with Se, returning the
film to the insulating state. At low temperature (<160 K), the
■
CONCLUSION
In summary, we have observed a positive temperature
coefficient of resistance in strongly coupled PbSe QD thin
films. The large red shift of the absorption peak, high film
conductivity, and SAXS results are all consistent with a strong
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electronic coupling between QDs. This finding demonstrates
semiconductor nanocrystal solids can show bandlike transport
properties when the neighboring QDs are strongly coupled,
which can lead to improved charge extraction and possibly
higher mobilities in QD devices. EDT treated PbSe QD thin
films also exhibit bistable conduction. The conductivity
increases abruptly by up to 10 orders of magnitude when the
bias reaches a threshold, and remains high until the bias is
reduced much lower than the threshold. We attribute the
insulating state to the Coulomb repulsion by the trapped
charges, and the conductive state to the reduced charging
energy due to extended charge carrier wave function.
Alternatively, we acknowledge that the bistability could be
due to the formation of conductive filaments in the QD thin
films. The drastic and fast memory switching may find
applications in memory devices.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the Startup fund by University of
California at Davis and the Hellman fellowship. We thank Prof.
P. Guyot-Sionnest for helpful discussions. We also acknowledge
Prof. Y. Takamura and Peter Greene for their assistance with
the SAXS measurements.
■
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