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Highly sensitive photodetectors based on hybrid 2D-0D SnS2-copper indium sulfidequantum dotsYun Huang, Xueying Zhan, Kai Xu, Lei Yin, Zhongzhou Cheng, Chao Jiang, Zhenxing Wang, and Jun He Citation: Applied Physics Letters 108, 013101 (2016); doi: 10.1063/1.4939442 View online: http://dx.doi.org/10.1063/1.4939442 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/108/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High-performing visible-blind photodetectors based on SnO2/CuO nanoheterojunctions Appl. Phys. Lett. 107, 241108 (2015); 10.1063/1.4938129 Suppression of dark current through barrier engineer for solution-processed colloidal quantum-dots infraredphotodetectors Appl. Phys. Lett. 107, 091115 (2015); 10.1063/1.4930158 Bulk- and layer-heterojunction phototransistors based on poly [2-methoxy-5-(2′-ethylhexyloxy-p-phenylenevinylene)] and PbS quantum dot hybrids Appl. Phys. Lett. 106, 253501 (2015); 10.1063/1.4922917 Ultraviolet emission from low resistance Cu2SnS3/SnO2 and CuInS2/Sn:In2O3 nanowires APL Mater. 2, 116107 (2014); 10.1063/1.4901295 Quantum dot nanocolumn photodetectors for light detection in the infrared Appl. Phys. Lett. 92, 261113 (2008); 10.1063/1.2949084
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Highly sensitive photodetectors based on hybrid 2D-0D SnS2-copper indiumsulfide quantum dots
Yun Huang,a) Xueying Zhan,a) Kai Xu, Lei Yin, Zhongzhou Cheng, Chao Jiang,Zhenxing Wang,b) and Jun Heb)
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscienceand Technology, Beijing 100190, China
(Received 28 September 2015; accepted 19 December 2015; published online 4 January 2016)
Both high speed and efficiency of photoelectric conversion are essential for photodetectors. As an
emerging layered metal dichalcogenide (LMD), tin disulfide owns intrinsic faster photodetection
ability than most other LMDs but poor light absorption and low photoelectric conversion efficiency.
We develop an efficient method to enhance its performance by constructing a SnS2-copper indium
sulfide hybrid structure. As a result, the responsivity reaches 630 A/W, six times stronger than pristine
SnS2 and much higher than most other LMDs photodetectors. Additionally, the photocurrents are
enhanced by more than 1 order of magnitude. Our work may open up a pathway to improve the
performance of photodetectors based on LMDs. VC 2016 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4939442]
Earth-abundant layered metal dichalcogenides (LMDs)
with general formula MX2 (M¼Mo, W, Sn, Ti, Zr, Hf, Nb,Ta, and X¼S, Se) have attracted a great deal of attentionover the past decade.1–3 The nature of suitable band-gaps and
monolayer-stability enable them ideal candidates for applica-
tion in photodetection, which require high performance in
terms of speed and efficiency of photoelectric conversion.
Motivated by those targets, a great number of photodetectors
based on LMDs, such as MoS2 and WSe2, have been fabri-
cated.4–7 However, a few of them can meet both. As an impor-
tant member of LMDs, tin disulfide (SnS2) has attracted
increasing attention recently. Field effect transistors (FETs)
based on exfoliated SnS2 Nanosheets (NSs) show high on/off
ratio� 106 and mobility up to 230 cm2 V�1 s�1 at room tem-perature.8–10 Monolayer to few-layer SnS2 NSs have been
synthesized by chemical vapor deposition (CVD) on carbon
fiber, mica, and silicon substrates.11–13 SnS2 NSs especially
can behave similar to ultrafast photodetectors with the rising
time� 5 ls,14 which is at least an order of magnitude fasterthan those based on MoS2, WS2, GaTe, and many other lay-
ered materials,6,15,16 indicating its great superiority as ultra-
sensitive photodetectors. However, its responsivity is limited
to 8.8 mA/W. Recently, we synthesized thin SnS2 NSs on car-
bon fibers though a CVD method and verified its potential in
fast and sensitive phototransistors.11 But the indirect bandgap
nature of SnS2 may hinder the photoelectric conversion effi-
ciency from meeting a higher standard. Thus, it is of great
value to explore a strategy that improves the light absorption
of SnS2 photodetectors.
On the other hand, 2D-0D hybrid architectures have
recently received considerable attention for their high per-
formance as photodetectors.17–20 This structure benefits from
strong light absorption, facile bandgap tunability, large
scale manufacturability of quantum dots (QDs),21,22 and the
facilitative separation of photoexcited charges induced by
type II heterojunction formed between the QDs and the lay-
ered materials. Besides, with the single crystal layered mate-
rial as a channel, this structure also avoids the boundary
issues in QDs photodetectors. As a result, the responsivity of
the hybrid graphene-PbS phototransistors could be 107 A/W
with fast response time�10 ms.17 A hybrid MoS2-PbS QDsphotodetector could show high responsivity of 105–106 A/W
as well as fast response time�0.35 s.18 Accordingly, wedesigned a hybrid device combined p-type copper indium sul-
fide (CuInS2, CIS) QDs with the CVD-grown SnS2 NSs. The
schematic of the hybrid device is shown in Fig. 1(a). The CIS
QDs have demonstrated fruitful application in solar cell for its
high light absorption and chemical stability.23–26 Besides,
with its proper energy band structure, CIS-decorated SnS2could form a typical type-II band alignments,27 as shown in
Fig. 1(b), which helps to improve photoresponse of SnS2.
Herein, we report a photodetector based on SnS2-CIS
QDs hybrid architecture. Due to the type II heterojunction
formed between SnS2 and CIS QDs, the device exhibits
enhanced light absorption ability, fast photoresponse, sensitive
and gate-tunable photodetection. It is also notable that the
responsivity reaches as high as 630 A/W at P¼ 2.83 mW/cm2,which is comparable with that of graphene-PbS hybrid photo-
detection at the same power of illumination. Thus, our study
may provide a method to improve the performance of photo-
detectors and expand the building blocks for high performance
optoelectronic devices.
The SnS2 NSs were synthesized by a CVD method as
reported in our previous work.11 The CIS QDs were synthe-
sized through a solvothermal recipe. In a typical process,
0.33 g CuCl2, 0.81 g InCl3, and 1.04 g SC(NH2)2 were added
to 3 ml alcohol and 40 ml deionized water in a 50 ml Teflon
autoclave. Then the reactor was maintained at 240 �C for 3 h.After that, �10 nm CIS QDs were precipitated at the bottomof the reactor. To fabricate device, as-grown thin SnS2 NSs
were drop-casted onto p-doped silicon substrates with
300 nm thick thermal oxidation layer. A standard e-beam
a)Y. Huang and X. Zhan contributed equally to this work.b)Authors to whom correspondence should be addressed. Electronic addresses:
[email protected] and [email protected]
0003-6951/2016/108(1)/013101/5/$30.00 VC 2016 AIP Publishing LLC108, 013101-1
APPLIED PHYSICS LETTERS 108, 013101 (2016)
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lithography (EBL) process was carried to define the electri-
cal metal electrodes. Cr/Au (5 nm/80 nm) metal electrodes
were deposited by thermal evaporation. After the measure-
ment of pristine SnS2 device, CIS QDs were spin-coated
onto its surface, using a method similar with Ref. 17. During
this procedure, 20 mg CIS QDs were dissolved in 5 ml tolu-
ene and centrifuged at 3000 rpm for 30 s. Then two drops
(each drop�0.05 ml) of the supernatant was dropped ontothe rotating substrates (2000 rpm) followed by 3 drops of 2
vol. % 1,2-ethanedithiol-ligands (EDT) in acetonitrile and
another 2 drops of acetonitrile and toluene. This sequence
was repeated for 3 times. Then, the substrate was dried at
30 �C for 10 min.Transmission electron microscope (TEM) and Raman
spectrum were used to characterize our synthesized SnS2 NSs
and CIS QDs. Fig. 1(c) exhibits the HRTEM of the SnS2 NSs
(top panel) and CIS QDs (bottom panel). The high crystallin-
ity of the SnS2 NSs could be concluded from the regular dif-
fraction fringes with the lattice spacing of 3.15 Å indexed to
the (100) plane.28 Meanwhile, the lattice constant�3.18 Å inFig. 1(c) (bottom panel) agrees well with the (112) plane of
CIS.25 Raman spectra of SnS2 NSs, CIS QDs, and the hybrid
structure are shown in Fig. 1(d), respectively. It is clear that
the Raman spectrum of the hybrid structure shows two peaks
assigning to that of SnS2 nanosheets (313.6 cm�1) and CIS
(303.2 cm�1).14,29 The UV-visible-IR absorption spectroscopy
of SnS2 and hybrid SnS2-CIS quantum dots in Fig. 1(e) veri-
fies that the light absorption range of hybrid SnS2-CIS is
enhanced and extended to part of the near infrared (NIR)
region after CIS QDs deposition.
To investigate the performance of SnS2-CIS hybrid pho-
todetector, photoresponse of the pristine and hybrid device
under 473 nm laser were conducted, respectively. Noting
that our previous work has demonstrated oxygen molecules
in ambient could grab electrons from the n-type channel
materials and thus seriously affect the properties of the de-
vice,11,20,30 all the measurements in our experiment were
carried in vacuum �10�5 Torr. The inset of Fig. 2(a) showsan atomic force microscope (AFM) image of the pristine
device, indicating the thickness of the channel SnS2NSs� 13.6 nm. The I-V curves of the same photodetectorbefore and after CIS QDs deposition in the dark and under
laser illumination are shown in Fig. 2(a). It is obvious that
the dark currents of hybrid device slightly increase while the
photocurrents dramatically augment by at least one order.
This increase could also be verified by the photoresponse of
the hybrid and pristine device under different power of inci-
dent illumination, as shown in Fig. 2(b). After decorating
with CIS QDs layer, the sensitivity of the SnS2 device
remained, embodying in the increasing photocurrents as the
power of incident illumination increases. Furthermore, as a
critical parameter for photodetector, the responsivity (Rk) ofthe device was improved. The Rk can be calculated as thefollowing formula:31
Rk ¼ Iph=PS; (1)
where Iph is the photocurrents and P and S are effective illu-mination intensity and area. As plotted in Fig. 2(c), the high-
est responsivity of the hybrid device reaches as high as
630 A/W at P¼ 2.83 mW/cm2, while that of the pristine de-vice is 167 A/W. The time-resolved photoresponse of the de-
vice (Fig. 2(d)) shows that after QDs deposition, the rising
and decay time could remain within 110 ms. Meanwhile, no
obviously slow decay tail was observed,32 indicating the
potential of the hybrid device for sensitive photodetectors.
Here, the enhanced photocurrents and responsivity origin
from the designed type II structure. Band bending induced
build-in field between the interface of CIS and SnS2 drives
the photogenerated electrons and holes to move in opposite
directions, reducing the recombination rate significantly. The
electrons transferred from CIS to SnS2 can increase the
charge density of the channel material, leading to higher
photocurrents. Meanwhile, the holes are trapped in the CIS
FIG. 1. Device schematics and charac-
teristics. (a) Schematic diagram of a
SnS2-CIS QDs hybrid device under
illumination. (b) Band-diagram of
the hybrid device under light illumina-
tion. (c) HRTEM of as-grown SnS2NSs (upper) and CIS QDs (lower). (d)
Raman spectra of the SnS2 NSs, CIS
QDs, and the hybrid device. (e) The
UV-visible-IR absorption spectroscopy
of SnS2 and hybrid SnS2-CIS quantum
dots.
013101-2 Huang et al. Appl. Phys. Lett. 108, 013101 (2016)
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film, which further serve as an effect of photogating and lead
to the transfer threshold voltage shift. It could also explain
the background photocurrents change shown in Fig. 2(b).33
As shown in our previous work, SnS2 displays great
potential as phototransistors with sensitive light response and
gate tunable photoswitch ratio.11 We further verify that its
performance could be improved with hybrid architecture.
Transfer curves with or without light illumination were meas-
ured before and after QDs deposition. Considering possible
gate hysteresis, all the curves are acquired as the gate voltage
was applied from negative to positive values. The hybrid de-
vice shows increased currents both in the dark and under illu-
mination, as presented in Fig. 3(a). Besides, the threshold
voltage shifts from 19.9 V to 8.81 V after QDs deposition,
indicating an obvious electron doping effect. It is notable that
our hybrid device exhibits positive photoconductivity in the
whole range of gate sweeping, significantly different with
Ref. 18, in which the MoS2-PbS QDs photodetectors exhibit
positive photoconductivity only at strong negative Vgs.18 As
plotted in Fig. 3(b), the hybrid device remains sensitive photo-
response compared with the pristine device, showing increas-
ing currents as the power of illumination increases. Even
though the pristine device shows a relatively higher responsiv-
ity than many other LMDs,5,34,35 the responsivity of the
hybrid device is still improved by several to ten times. Figs.
3(c) and 3(d), respectively, depict the responsivity of device
before and after CIS QDs deposition as the power of illumina-
tion increases. We draw these two figures in the same
range (101 mA/W–106 mA/W) to facilitate a comparison. The
highest responsivity measured of hybrid phototransistors is
500 A/W at P¼ 5.94 mW/cm2, Vgs¼ 50 V. Though severalother hybrid devices have shown a responsivity as high as
107 A/W, the results were measured at a very low intensity of
illumination.17–20 Our results are comparable with that of
those hybrid phototransistors at the same intensity of illumina-
tion. Besides, it is obvious that the measured responsivity
displays a tendency of increasing as the intensity of illumina-
tion decreases. At higher illumination intensity, more photo-
generated charges are separated, which induce an increasing
reverse electric field. As a result, the build-in field is lowered,
leading to accelerating charges recombination at the interface
and dropping responsivity. Thus, it is safe to predict that our
hybrid device would show even higher responsivity at an illu-
mination of lower intensity.
Another interesting phenomenon is that the transfer
curves shift as the intensity of illumination increases, as indi-
cated in Fig. 3(b). For one reason, photogenerated electrons
transfer from the CIS layer to SnS2, which results in more
electrons in the channel. For another reason, photogenerated
holes remained in the CIS QDs layer, leading to a photogat-
ing effect, which further changes the conductance of the
channel material through capacitive coupling.17,36 As a
result, the threshold voltage of the hybrid device shifts to
negative gate voltages. To clearly show these collaborative
effects, we plot the threshold voltage shift as a function of
light illumination in Fig. 4(a). Interestingly, the curve can be
fitted very well with the following equation, which has been
used for phototransistors before:19,37
DVth ¼ aPb; (2)
where DVth is the shift of threshold voltage, P is the intensityof light illumination, and a and b are the constant. Besides, itis well known that the channel current could be expressed as38
Ids ¼W
LlCox VG � VT �
VD2
� �VD; VD � VG � VTð Þ;
(3)
where Cox is the capacitance of the gate dielectric per unitarea and L and W are the length and width of the channelmaterial, respectively. Then, the photocurrent of the hybrid
device could be expressed as
FIG. 2. Photodetector performance of
a SnS2 device before and after CIS
QDs deposition. (a) The measured I-V
curves with or without light illumina-
tion. Inset: AFM of the pristine device.
(b) Photoresponse under illumination
with different incident power. (c) The
calculated responsivity as a function of
incident power. (d) The time-resolved
photoresponse showing the rise and
decay time less than 110 ms.
013101-3 Huang et al. Appl. Phys. Lett. 108, 013101 (2016)
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Iph ¼W
LlCoxVDDVT : (4)
Substitute Equation (4) into Equation (1), the responsivity
could be expressed as
Rk ¼ aW
LlCoxVDP
b�1: (5)
Fig. 4(b) shows the responsivity of our hybrid device
as a function of light illumination, which fits very well
with Equation (5). According to our experimental results,
b� 0.48 is higher than that of a PbS-graphene hybriddevice.19 In general, the value of b indicates the efficiency ofcharges transfer from the layer of QDs to the channel mate-
rial. It depends on the quality of interface between the CIS
QDs layer and SnS2 NSs, the surface ligand used to prepare
QDs solution, and the experimental process.39 Therefore, by
carefully choosing the ligand and improving the experimen-
tal procedure, it is possible that the value of b and theresponsivity of the hybrid device could be further improved.
In summary, a hybrid device with sensitive photores-
ponse, fast response time, and improved responsivity was
fabricated by spin-coating CIS QDs layer onto the surface of
SnS2 NSs. These improvements benefit from strong light
absorption of CIS QDs and the type II heterojunction formed
between SnS2 surface and CIS QDs. As a result, the photo-
currents are enhanced by more than one order and the
responsivity of the hybrid device is higher than 600 A/W.
Besides, the experimental data of the responsivity at varied
power of incident illumination agree very well with the theo-
retical equation. Our work may broaden the 2D-0D system
and offer a method to improve the performance of optoelec-
tronic devices based on LMDs.
This work was supported by the National Natural Science
Foundation of China (Nos. 21373065 and 61474033), 973
Program of the Ministry of Science and Technology of China
(No. 2012CB934103), Beijing Natural Science Foundation
(No. 2144059), and CAS Key Laboratory of Nanosystem and
Hierarchical Fabrication. The authors gratefully acknowledge
the support of the K. C. Wong Education Foundation.
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013101-4 Huang et al. Appl. Phys. Lett. 108, 013101 (2016)
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