Supplementary Material to
Hysteresis and charge trapping in graphene quantum dots
Hemen Kalita,1,a) Harikrishnan V,1,a) Dhanraj B. Shinde,2 Vijayamohanan K. Pillai3 and M.
Aslam1,b)
1
Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076.
2
Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411008,
India.
3
Central Electrochemical Research Institute, Chennai, Karaikudi 630006, India.
A) Field and Time Dependent Stability:
The graphene quantum dot (GQD) samples prepared were found to be stable over a long period
of time. The samples show very little (<5%) depreciation in their resistance over a period of six
months. Furthermore, the prepared samples show no deterioration after repeated cyclic I-V
measurements, as seen from the Fig. S1 below. All measurements are carried out at room
temperature with a scan rate of 500 mV/s.
a)
These authors contributed equally to this work
Author to whom correspondence should be addressed. Electronic mail: m.aslam@iitb.ac.in
b)
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FIG. S1: Current recorded as a function of voltage between -5 V and +5 V for 20 cycles. The
hysteresis loop obtained is found to be stable and did not show any significant deviation during
measurement. Inset shows a magnified image of the hysteresis loop edge highlighting the
repetitive nature of the curve.
B) Physisorption of water molecules - primary cause for inducing charge traps and
hysteresis in GQD
We have performed experiments that provide further evidence supporting our conclusions that
adsorbed water molecules cause charge trapping and hysteresis in GQD. When electrical
measurements are carried out in vacuum, the prepared devices immediately show a drastic
increase in resistance, with a sharp decrease in current (nanoAmpere range), similar to that
obtained after annealing in an Argon atmosphere. (Discussed in page 5 of the manuscript) (Fig
S2). This observation is consistent with the weakened coupling occurring in the quantum dots
upon desorption of water molecules. Moreover, when the devices are purged with oxygen or
nitrogen, no change in the electrical response is observed.
2
FIG. S2: Current voltage measurements carried out in different environments. Identical
responses are obtained in vacuum, oxygen and nitrogen atmospheres.
To further investigate the role of water molecules in charge transport in GQD, electrical
measurements are carried out in environments with different Relative Humidity levels (RH) as
shown in Fig S3. Fig S3(a) shows the variation of resistance with time when the prepared device
is introduced from vacuum to different RH levels. Fig S3(b-d) shows the I-V curves measured at
different RH levels. The sample resistance is clearly found to decrease and hysteresis between
current and voltage is found to increase with increasing RH levels. This is in agreement with the
hypothesis that water molecules adsorbed on GQD result in a lower potential barrier for
electrons, leading to the observed higher conductivity.
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FIG. S3: (a) Evolution of device resistance with time upon introduction from vacuum to an
environment with a fixed RH level. Sample resistance in (a) is plotted in a logarithmic scale for
convenience and visibility. Also shown are I-V curves measured when the device is exposed to
RH levels of (b) 8% (c) 43% and (d) 63%.
As further evidence of humidity induced charge trapping in GQD, the transient current decay
was measured at a fixed DC potential of 3 V in different RH environments. All current decays
obtained could be excellently fit with the bi-exponential decay equation to quantify relaxation
time constants. At low RH levels (Fig. S4), current was found to decay slowly and saturate after
nearly 500 s, with time constants of 6.7 s and 190 s. Upon exposing the samples to higher RH
levels (Fig. S4), the samples show an even faster decay of current with marked decrease of time
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constants, nearly one fortieth of that recorded at a lower RH level. These observations confirm
that higher humidity levels induce more rapid current decay with smaller time constants.
FIG. S4: Transient current decay measured in environments with different RH levels a fixed DC
potential of 3 V. Current decay is found to be quicker and associated time constants are found to
smaller with increasing RH levels.
C) Mechanism of Charge Transport in GQD
Figure S5 shows the variation of the logarithm of conductivity (ln(σ)) against the inverse of
temperature (T-1), measured in vacuum in the range 285 K – 360 K. The resulting plot can be
linearly fitted, revealing an Arrhenius response(𝜎 = σ0 𝑒 −𝐸𝐴/(𝑘𝑇) ), with activation energy (EA)
determined to be 0.140 ± 0.025 eV. The results of the temperature dependent conductivity
measurements and the disorder arising in the system from the variation of quantum dot size and
the dropcasted sample thickness suggest that electron transport in GQD may presumably occur
through thermally assisted hopping. Comparing Fig. S2 to Fig. S3(a), the increase in
conductivity of the device upon increasing relative humidity levels suggests an improvement in
the charge carrier mobility of GQD due to a higher hopping rate arising from better inter-particle
coupling provided by the adsorbed water molecules.
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FIG. S5: Plot of logarithm of conductivity against the inverse of temperature. An Arrhenius
behavior is obtained (𝜎 = σ0 𝑒 −𝐸𝐴/(𝑘𝑇) ). A linear fit reveals the activation energy (EA) to be
0.140 ± 0.025 eV. All measurements were carried out in a vacuum environment.
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