Photothermoelectric and photovoltaic effects both present in MoS2
Youwei Zhang, Hui Li, Lu Wang, Haomin Wang, Xiaomin Xie, Shi-Li Zhang, Ran Liu and
Zhi-Jun Qiu
Supplementary Information:
1. Electrical instability of MoS2 transistor in ambient environment.
Supplementary Figure S1. Hysteresis behavior and electrical stress instability of MoS2
transistor. (a) Transfer characteristics of a multilayer-MoS2 transistor, measured at Vd = 1 V,
with Vg sweeping from negative to positive and then to negative again, in both DC mode and
AP mode. The black arrows indicate the Vg sweep directions. Inset in panel (a) schematically
shows the DC and AP modes of Vg sweep. (b) Time evolution of drain current at Vd = 1 V and
Vg = 10 V or - 10 V in DC and AP modes.
Supplementary Figure S1a illustrates the representative transfer characteristics measured at
a fixed source-drain voltage (Vd = 1 V) in dark. The red curves show a typical n-type unipolar
conduction with a large current hysteresis when Vg was swept continuously from -20 V to 20
V and then back to -20 V again in DC measurement mode (see inset in Supplementary Fig.
1
S1a). The hysteresis gap varies with the sweeping voltage range and rateS1. This hysteretic
behavior has often been observed in a variety of inorganic and organic semiconductor FETsS2,
S3
and novel low-dimensional devices, such as carbon nanotubeS4 and graphene FETsS5.
Recent studies suggest that a reversible electrochemical charge transfer between
semiconducting channel and the adsorbed H2O/O2 redox couple is the predominant
contributor to the hysteresisS5-S7. Vacuum treatment and heating are found to effectively
suppress hysteresisS1, S4, S5. Apart from hysteresis, this charge transfer effect could also induce
electrical instability under a prolonged bias stressS8. Supplementary Figure S1b shows that the
drain current of the device decreases (increases) over time with positive (negative) gate bias
stress (see the blue and green curves, respectively) when measuring in DC mode.
Measures to reduce the current hysteresis and electrical instability have already been
considered, such as complete encapsulation of the devices with Si3N4S1 and other high-k
layersS9. Nevertheless, these measures would change optical absorption properties of MoS2
due to additional light scattering and absorption in these insulating overlayers. However, the
AP measurement method (inset in Supplementary Fig. S1a) has been reported to distinctively
reduce the hysteresis compared to operating the device in DC modeS7, S10, S11. Referring to the
black curve in Supplementary Fig. S1a, the hysteresis of the MoS2 transistor is almost
completely eliminated using the AP method. In contrast to continuously sweeping in DC
mode, each positive pulse, in the pulsed measurement, is followed by a negative pulse of
equal amplitude, which means that charges transferred during the positive Vg pulse are
immediately recovered by the subsequent negative pulse. As a result, stable electrical
characteristics are obtained when operating the device in AP mode, as exhibited by the black
2
and red curves in Supplementary Fig. S1b. This improvement suggests that the pulsed
measurement allows the current to be monitored without inducing additional bias stress thus
guaranteeing the stability of devices.
2. Optical absorption of multilayer MoS2.
Supplementary Figure S2. Absorption and band structure of MoS2. (a) Optical absorption
spectrum for multilayer-MoS2 flakes. Two main absorption peaks labeled A and B are
observed. The red arrow marks the laser wavelength used in the photoelectrical measurement.
(b) Computed band structure of bulk MoS2 using first-principle calculations. Red and blue
arrows indicate direct band transitions A and B at K point, respectively. The minimum of the
conduction band in the Brillouin zone lies at Q point along the Γ-K line while the valence
band maximum is at Γ point.
As shown in Supplementary Fig. S2a, the optical absorption spectrum of the multilayer
MoS2 flakes shows two main peaks at 667 and 608 nm, corresponding to the A and B exciton
bandsS12. Results of first-principle density functional theory (DFT) calculations (Atomistix
ToolKit
version
13.8.1,
QuantumWise
A/S,
3
www.quantumwise.com)S13
show
in
Supplementary Fig. S2b that the A and B excitons correspond to direct-gap transitions at K
point of the Brillouin zone between the maxima of split valence bands and the minimum of
the conduction band. The valence band splitting is due to spin–orbit coupling and interlayer
interaction in multilayer MoS2S12. Here, only A excitons were excited by using a 633-nm
laser.
3. Photoswitching behavior of MoS2 transistor.
Supplementary Figure S3. Photoswitching behavior of MoS2 transistor at Vg = -15 V and Vd
= 1.0 V in both DC and AP modes.
As discussed in section 1, the DC mode could induce electrical instability of devices, which
can further affect the photoresponse of MoS2 FETs. In Supplementary Fig. S3, the temporal
response of MoS2 transistor measured at Vg = -15 V and Vd = 1.0 V in DC mode (red line) is
displayed after switching on and off the 633-nm laser illuminated in the channel. When the
laser is on, a fast photocurrent increase is observed followed by a slow increase. Similarly,
when the laser is switched off there is a fast decay component followed by a slow decay,
yielding significant persistent photoconductivity. The fast photoresponse is ascribed to the
4
photoelectric effect, such as band-to-band photo-excitation/recombination of carriers. This has
been observed in many reportsS14-S16. The slow photoresponse is attributed to the charge
transfer effect between channel and adsorbed molecules, e.g. H2O/O2. This spatial charge
transfer effect of photo-generated carriers leads to a longer electron-hole recombination time
in contrast to the relatively fast band-to-band recombination. This slow photoresponse can be
eliminated under vacuumS17 or using the AP method in ambient environment as used in this
work (black line in Supplementary Fig. S3).
4. Stability of photoswitching of MoS2 transistor.
Supplementary Figure S4. Stability of photoswitching of MoS2 transistor. (a)
Photoresponse to on-off laser switching at Vd = 1 V and Vg = -15 V in both DC and AP modes.
The time interval for photoswitching is 20 s. (b) Ratio of illumination to dark current (RL/D) as
a function of photoswitching cycle with different time intervals in both DC and AP modes.
As discussed in section 3, the persistent photoconductivity in DC mode strongly affects the
photoswitching performance of the MoS2 transistors and thereby resulting in a large variation
of photoresponse with switching time shown in Supplementary Fig. S4a (red curve). The time
5
intervals are identical for the on and off of the laser. After initial photoswitching cycles, the
ratio of illumination to dark current (RL/D = Iillum/Idark) in Supplementary Fig. S4b rapidly
drops and then saturates at very low values (< 10). The longer switching time interval, the
faster is the decay with RL/D, thus hampering a reliable photo-detection with MoS2 FETs. This
is because more photo-generated carriers are involved in the charge transfer process during
light illumination. In contrast, when using the AP method, no fluctuation in photocurrent with
switching time is observed in Supplementary Fig. S4a (black line). This leads to a high and
stable RL/D (>103, Supplementary Fig. S4b) irrespective of time interval. It has therefore been
demonstrated that the AP method has great potential in generating repeatable and clearly
interpretable photo-sensing signals.
5. Temperature measurement using Raman spectroscopy.
Supplementary Figure S5. Dependence of Raman spectra of multilayer MoS2 on
6
temperature and laser power. (a) Raman spectra of multilayer MoS2 at the contact edge for
temperature from 300 K to 400 K. (b) Raman spectra of multilayer MoS2 at the contact edge
for different laser powers from 0.17 mW to 2.2 mW. Spectra in panels (a) and (b) have been
offset vertically for clarity. (c) Temperature dependence and (d) laser-power dependence of
the Lorentzian E12g peak frequency. The red lines in panels (c) and (d) are linear fit to the
measurement data.
Raman spectroscopy has been widely used for thermal characterization of materials. It has
been shown that the Raman peak position is temperature-sensitive and can be used to quantify
laser-induced local heating effect on a sampleS18. Before characterizing the temperature rise
by laser heating, the shift of Raman frequency with temperature has to be calibrated. During
the calibration experiment, the laser power was minimized to 50 W in order to avoid
appreciable temperature increase by laser illumination and the sample temperature was
controlled by a heating stage (Linkam Scientific Instruments, THMS600) installed on the
sample stage of the optical microscope in our Renishaw inVia Raman spectrometer. In the
subsequent laser-heating experiment, the heating stage was taken off and the incident laser
power was varied accordingly.
Supplementary Figure S5a shows the temperature-dependent Raman spectra of multilayer
MoS2. As seen from the figure, both modes E12g and A1g follow a systematic red shift with
increasing temperature. Since the A1g peak is seriously affected by the movement of the
second-order Raman peak at 420 cm-1 S19, we use the E12g peak to calibrate our temperature
measurement. Supplementary Figure S5c shows the change of Lorentzian-fit peak position of
7
the E12g peak with temperature. A linear fit gives a slope of -0.015 cm-1/K, which is in
agreement with literature valuesS20,
S21
. After the calibration, the multilayer MoS2 at the
contact edge was locally heated with varying laser power. Supplementary Figure S5b shows
the variations of the Raman spectra with increasing incident laser power. Obviously, the
frequencies of both modes E12g and A1g red-shift with increasing laser power, similarly to the
temperature-dependent Raman measurements. This indicates that an incident laser with a
certain power could considerably increase the local temperature of the sample. The peak
frequency of mode E12g as a function of incident laser power is plotted in Supplementary Fig.
S5d. Based on the linear fit of the data, the slope for mode E12g is - 0.74 cm-1/mW. Combining
the temperature-dependent and laser-power-dependent slopes of Raman peak frequency, a
100-W laser illuminating on the MoS2 at the contact edge leads to a temperature rise of
approximately 5 K.
6. Electric field profile along the MoS2 channel.
Supplementary Figure S6. Simulated electric field profiles along the MoS2 channel at Vg =
15 V, 0 V and -15 V. The electric field profiles have been offset vertically for clarity.
8
The electric field profiles along the MoS2 channel were simulated using the TCAD
semiconductor simulation tool SILVACOS22. For simplicity, the Schottky barrier height (B)
is set to 0.15 eV to account for the Fermi level pinning effectS23, S24. The unintentional doping
concentration in MoS2 is assumed to be 11017 cm-3, which has been suggested to be caused
by sulfur vacancies or other structural defectsS25, S26. As shown in Supplementary Fig. S6, the
built-in electric field is maximized at the contact edge for Vg > 0 V. The electric field changes
sign when Vg is altered from positive to negative. Furthermore, both the peak position and the
extension of electric field move into the channel. This simulated peak movement is
qualitatively consistent with the experiment observations in photocurrent shown in Fig. 4b in
main text.
References
S1. Late, D. J., Liu, B., Matte, H. S. S. R., Dravid, V. P. & Rao, C. N. R. Hysteresis in
single-layer MoS2 field effect transistors. ACS Nano 6, 5635-5641 (2012).
S2. Fakhri, M. et al. Facile encapsulation of oxide based thin film transistors by atomic layer
deposition based on ozone. Adv. Mater. 25, 2821-2825 (2013).
S3. Egginger, M., Bauer, S., Schwödiauer, R., Neugebauer, H. & Sariciftci, N. S. Current
versus gate voltage hysteresis in organic field effect transistors. Monatsh. Chem. 140, 735-750
(2009).
S4. Kim, W. et al. Hysteresis caused by water molecules in carbon nanotube field-effect
transistors. Nano Lett. 3, 193-198 (2003).
S5. Levesque, P. L. et al. Probing charge transfer at surfaces using graphene transistors.
9
Nano Lett. 11, 132-137 (2011).
S6. Aguirre, C. M. et al. The role of the oxygen/water redox couple in suppressing electron
conduction in field-effect transistors. Adv. Mater. 21, 3087-3091 (2009).
S7. Qu, M., Li, H., Liu, R., Zhang, S.-L. & Qiu, Z.-J. Interaction of bipolaron with the
H2O/O2 redox couple causes current hysteresis in organic thin-film transistors. Nature
Commun. 5, 3185 (2014).
S8. Cho, K. et al. Electric stress-induced threshold voltage instability of multilayer MoS2 field
effect transistors. ACS Nano 7, 7751-7758 (2013).
S9. Radisavljevic, B. & Kis, A. Mobility engineering and metal-insulator transition in
monolayer MoS2. Nature Mater. 12, 815-820 (2013).
S10. Mattmann, M. et al. Pulsed gate sweep strategies for hysteresis reduction in carbon
nanotube transistors for low concentration NO2 gas detection. Nanotechnology 21, 185501
(2010).
S11. Liu, Z., Qiu, Z.-J., Zhang, Z.-B., Zheng, L.-R. & Zhang, S.-L. Mobility extraction for
nanotube TFTs. IEEE Elec. Dev. Lett. 32, 913-915 (2011).
S12. Alidoust, N., et al. Observation of monolayer valence band spin-orbit effect and induced
quantum well states in MoX2. Nature Commun. 5, 4673 (2014).
S13. Brandbyge, M., Mozos, J. L., Ordejon, P., Taylor, J. & Stokbro, K. Density-functional
method for nonequilibrium electron transport. Phys. Rev. B 65, 165401(2002).
S14. Yin, Z. et al. Single-layer MoS2 phototransistors. ACS Nano 6, 74-80 (2012).
S15. Choi, W. et al. High-detectivity multilayer MoS2 phototransistors with spectral response
from ultraviolet to infrared. Adv. Mater. 24, 5832-5836 (2012).
10
S16. Tsai, D.-S. et al. Few-layer MoS2 with high broadband photogain and fast optical
switching for use in harsh environments. ACS Nano 7, 3905-3911 (2013).
S17. Zhang, W. et al. High-gain phototransistors based on a CVD MoS2 monolayer. Adv.
Mater. 25, 3456-3461 (2013).
S18. DeBorde, T., Aspitarte, L., Sharf, T., Kevek, J. W. & Minot, E. D. Photothermoelectric
effect in suspended semiconducting carbon nanotubes. ACS Nano 8, 216-221 (2014).
S19. Li, H. et al. From bulk to monolayer MoS2: evolution of Raman scattering. Adv. Funct.
Mater. 22, 1385-1390 (2012).
S20. Lanzillo, N.A. et al. Temperature-dependent phonon shifts in monolayer MoS2. Appl.
Phys. Lett. 103, 093102 (2013).
S21. Sahoo, S., Gaur, A. P. S., Ahmadi, M., Guinel, M. J.-F. & Katiyar, R. S.
Temperature-dependent Raman studies and thermal conductivity of few-layer MoS2. J. Phys.
Chem. C 117, 9042-9047 (2013).
S22. SILVACO. ATLAS User’s Manual. SILVACO Inc., Santa Clara, CA 95054, 2008.
S23. Liu, H. et al. Switching mechanism in single-layer molybdenum disulfide transistors: an
insight into current flow across Schottky barriers. ACS Nano 8, 1031-1038 (2014).
S24. Das, S., Chen, H.-Y., Penumatcha, A. V. & Appenzeller, J. High performance multilayer
MoS2 transistors with scandium contacts. Nano Lett. 13, 100-105 (2013).
S25. Qiu, H. et al. Hopping transport through defect-induced localized states in molybdenum
disulphide. Nat. Commun. 4, 2642 (2013).
S26. Zhou, W. et al. Intrinsic structural defects in monolayer molybdenum disulfide. Nano
Lett. 13, 2615-2622 (2013).
11