We report an improved photosensitivity in few-layer tin disulfide (SnS2) field-effect transistors (FETs) following doping with CdSe/ZnS core/shell quantum dots (QDs). The hybrid QD-SnS2 FET devices achieve more than 500% increase in the photocurrent response compared with the starting SnS2-only FET device and a spectral responsivity reaching over 650 A/W at 400 nm wavelength. The negligible electrical conductance in a control QD-only FET device suggests that the energy transfer between QDs and SnS2 is the main mechanism responsible for the sensitization effect, which is consistent with the strong spectral overlap between QD photoluminescence and SnS2 optical absorption as well as the large nominal donor-acceptor interspacing between QD core and SnS2. We also find enhanced charge carrier mobility in hybrid QD-SnS2 FETs which we attribute to a reduced contact Schottky barrier width due to an elevated background charge carrier density.

Two-dimensional (2D) layered nanomaterials have sparked significant research interest for their potential applications in new types of electronic and optoelectronic devices, including field-effect transistors (FETs), sensors, solar cells, photodetectors, and light emitting diodes.1–8 Graphene is probably the most popular example of a 2D nanomaterial that has been extensively explored over the years. However, its application in high-performance optoelectronic devices, such as photodetectors3,9 and photovoltaics,5,10 has been limited due to its zero energy bandgap. Layered metal dichalogenides (LMDs), such as MoS2, WS2, and WSe2, have emerged as alternatives, primarily because of their tunable bandgap energy controlled by thickness (spanning from 1 eV to ∼3 eV) and the transition from an indirect bandgap in the bulk to a direct gap in the monolayer,2,7,11–15 which in turn enhances the 2D material's photoluminescence (PL) quantum yield.15 Combined with a large surface-to-volume ratio, these material characteristics make LMDs appealing for light harvesting, energy conversion, and chemical sensing device applications. Tin disulfide (SnS2) is a less studied type of LMD, in which the group IV element Sn substitutes the transition metals in other, more familiar LMD compounds such as MoS2 and WSe2. We recently demonstrated FET devices based on mechanically exfoliated SnS2 flakes and achieved high performance on par with those based on MoS2.14,16 While bulk SnS2 is known to be an n-type semiconductor with an indirect bandgap of 2.2 eV, we found that the bandgap of SnS2 remained indirect even when the layer thickness was decreased down to a single layer, indicating a potentially inferior light absorption cross-section compared with the direct bandgap LMD materials. Recently, hybrid devices combining 2D LMDs with other semiconductors have been explored for the purpose of improving the device performance and functionality.17–20 Especially for enhancing light absorption and device photosensitivity, the use of colloidal quantum dots (QDs) in a light sensitizing layer in contact with the LMD has been recently proposed.17,21 QDs have large optical absorption cross-section and wide spectral coverage spanning from ultraviolet to near infrared, depending on their core sizes. Combined with the size-dependent tunable bandgap, such characteristics prove highly effective in the utilization of QDs toward enhancing the optical responsivity of 2D material-based photo-FET devices.17 We propose that a similar QD-based sensitization scheme should be able to improve the photosensitivity of 2D SnS2 FETs.

Here we report improved light detection sensitivity in a few-layer SnS2 FET by employing a CdSe/ZnS core-shell QD light sensitization layer. Benefiting from the strong optical absorption of QD as well as the spectral overlap between the PL emission of QDs and SnS2 absorption spectra, which enables an efficient energy transfer from photo-excited QDs to SnS2, QD-SnS2 hybrid FET devices exhibit more than 500% increase in the measured photocurrent response with the corresponding spectral responsivity greater than 650 A/W at 400 nm wavelength. We find that QD sensitization and light illumination induce an increase in the field-effect electron mobility in the layered SnS2, which we explain based on a reduced contact Schottky barrier width via the increased photo-excited charge carrier density.

Few-layer SnS2 flakes were prepared by mechanical exfoliation from a bulk crystal grown by the vertical Bridgman method and dry-transferred onto a silicon wafer with a 300 nm thick SiO2 dielectric layer, which was also used as a back-gated substrate for FET fabrication and characterization.14 After locating target SnS2 flakes by bright-field optical microscopy, source-drain contacts (Ti/Au, 10/50 nm) of FETs with 15 μm channel length were fabricated by optical lithography and electron-beam deposition. For the active FET channels, we utilized few-layer SnS2 flakes (∼10 monolayers, determined by bright-field optical contrast),14 instead of monolayer SnS2, because the thicker flakes provide increased light absorption. This enhanced absorption is beneficial not only for the exciton generation within the 2D material but also for the enhanced energy transfer from QDs to SnS2, as observed in our recent single-particle spectroscopy study, where the energy transfer rate was found to increase with the increasing number of SnS2 layers.22 

Figure 1(a) shows a schematic of back-gated QD-few-layer SnS2 hybrid FET device with CdSe/ZnS QDs deposited on top of the SnS2 channel via drop-casting from mixed solvents (hexane:octane of 9:1 volume ratio with QD solid concentration of 2.5 mg/l). The SnS2 flake itself exhibits an optical absorption spectrum increasing sharply below ∼550 nm (Figure 1(b)), which suggests a bandgap energy of ∼2.3 eV. We utilized octadecylamine-coated core/shell CdSe/ZnS QDs with PL spectrum centered at ∼535 nm (Ocean NanoTech) to impose a strong overlap with the absorption spectrum of SnS2 (Figure 1(b)). The PL emission peak wavelength of 535 nm is associated with a CdSe core with ∼3.4 nm diameter and one monolayer (0.7 nm) thick ZnS shell.22 Assuming a thickness of ∼2.3 nm for the octadecylamine ligand coating on the QD surface, the interspacing between the CdSe core (edge) and the first layer of the SnS2 flake is thus estimated to be ∼3 nm, a distance at which we expect energy transfer to dominate over charge transfer. This large separation between QDs and SnS2, in tandem with the strong spectral overlap between QD PL and SnS2 optical absorption, enables energy transfer from photo-excited QDs to SnS2, as we have recently confirmed by time-resolved single-particle PL studies of QD-SnS2 hybrids.22 

FIG. 1.

(a) Schematic depicting a hybrid CdSe/ZnS QD-SnS2 FET device, with S, D, and G denoting source, drain, and gate, respectively. (b) Optical absorption spectrum of a few-layer SnS2 flake (red, left) and PL spectrum of suspended CdS/ZnS QDs in toluene (blue, right). The inset is an optical image of an actual QD-few-layer SnS2 hybrid FET device. Scale bar: 10 μm.

FIG. 1.

(a) Schematic depicting a hybrid CdSe/ZnS QD-SnS2 FET device, with S, D, and G denoting source, drain, and gate, respectively. (b) Optical absorption spectrum of a few-layer SnS2 flake (red, left) and PL spectrum of suspended CdS/ZnS QDs in toluene (blue, right). The inset is an optical image of an actual QD-few-layer SnS2 hybrid FET device. Scale bar: 10 μm.

Close modal

We next measured FET device characteristics in ambient air, under dark and white-light-illuminated (tungsten lamp ∼3.5 mW) conditions. An optical image of the QD-SnS2 hybrid FET device is shown in the inset of Figure 1(b). The SnS2 FET device without QDs and in the dark displays typical n-type drain-source current-voltage (IDS − VDS) characteristics with increasing IDS toward positive gate voltage, VG. The dark IDS reaches ∼8 nA at VG = 20 V (VDS = 0.5 V), being increased ∼10× compared to IDS at VG = −20 V (Figure 2(a), black curves). Under white light illumination, the photoconductive effect in SnS2 enhances IDS to ∼57 nA (at VG = 20 V, VDS = 0.5 V), which is ∼7× larger than the dark IDS obtained under the same conditions (Figure 2(a), red curves). The absolute photocurrent, ΔIDS,photo = IDS,illuminated − IDS,dark, increases from ∼10 nA at VG = −20 V up to ∼50 nA at VG = 20 V (for VDS = 0.5 V, Figure 2(a) inset). Meanwhile, the FET does not turn off fully under illumination even at VG = −20 V (IDS = ∼14 nA), indicating an elevated background charge carrier density induced by photo-excitation.

FIG. 2.

IDS − VDS − VG characteristics of (a) SnS2 FET device and (b) QD-SnS2 hybrid FET device in the dark (black curves) and under white light illumination (red curves). VG is varied from −20 to 20 V, in steps of 10 V. Insets show absolute photocurrent outputs (ΔIDS,photo) with respect to VDS and VG. (c) ΔIDS,photo − VG characteristics for extended ranges of VG and VDS, for SnS2 FET device (left) and QD-SnS2 hybrid FET device (right). VDS varies from 1 V to 4 V in 1 V increments (indicated by black arrows).

FIG. 2.

IDS − VDS − VG characteristics of (a) SnS2 FET device and (b) QD-SnS2 hybrid FET device in the dark (black curves) and under white light illumination (red curves). VG is varied from −20 to 20 V, in steps of 10 V. Insets show absolute photocurrent outputs (ΔIDS,photo) with respect to VDS and VG. (c) ΔIDS,photo − VG characteristics for extended ranges of VG and VDS, for SnS2 FET device (left) and QD-SnS2 hybrid FET device (right). VDS varies from 1 V to 4 V in 1 V increments (indicated by black arrows).

Close modal

For the hybrid FET device with QDs deposited on top of the SnS2 channel, we observed a significant increase in IDS in the dark as well as under white-light illumination. The dark IDS reached 54 nA at VDS = 0.5 V and VG = 20 V (Figure 2(b), black curves), comparable to the IDS obtained from the bare SnS2 FET (without added QDs) under illumination. At VG = −20 V, IDS still remains as high as 29 nA, indicating only 1.8× modulation of IDS (i.e., on-off ratio) by a VG change of −40 V, much reduced compared with ∼10 on-off ratio observed in the bare SnS2 FET device. This suggests an increased background carrier density in QD-SnS2 hybrid FETs even under dark conditions, induced by the application of QDs. We speculate that QDs may introduce a pseudo-gating effect, particularly via their positively charged octadecylamine ligands, which can exert a positive gate electric field onto the QD-SnS2 hybrid FET device. Similar phenomena have been observed in other hybrid systems, such as graphene/polyvinylidene fluoride (PVDF) and 2D MoS2/PbS QDs.6,17

When the hybrid QD-SnS2 FET was exposed to white light, IDS increased to ∼250 nA (at VDS = 0.5 V and VG = 20 V; Figure 2(b), red curves), compared to the dark IDS of 54 nA measured at the same bias condition. Similar to the SnS2-only FET device, the photocurrent output of hybrid QD-SnS2 FET increases for larger VDS and VG values, with ΔIDS,photo now reaching up to ∼185 nA (VDS = 0.5 V, VG = 20 V; Figure 2(b) inset), representing a 370% enhancement in the photocurrent output compared to the SnS2-only FET. Even at VG = −20 V, ΔIDS,photo for the hybrid FET is as large as ∼100 nA, again indicating a significantly elevated background carrier density. Overall, these enhanced photocurrent outputs demonstrate the positive contribution of QDs to the photosensitivity of SnS2 FET devices. A survey of extended ranges of VG (from −30 V to 30 V) and VDS (from 1 V to 4 V) shows that ΔIDS,photo of the hybrid QD-SnS2 FET can be further increased to over 10 μA by applying larger VG and VDS. At VDS = 4 V (Figure 2(c)), ΔIDS,photo of the hybrid QD-SnS2 FET increases from ∼7 μA to ∼13 μA as VG changes from −30 V to 30 V, in contrast to a smaller increase of ΔIDS,photo observed in the SnS2-only FET, from ∼0.2 μA to 2.5 μA for the same bias variation. This means that the hybrid QD-SnS2 FET device has >500% enhanced ΔIDS,photo at VG = 30 V compared with the SnS2-only FET. For the entire range of explored VDS values, ΔIDS,photo in the hybrid FET is always larger than that in the SnS2-only FET.

The hybrid FET meanwhile cannot be completely turned off even at VG = −30 V. A similar behavior was reported by Kufer et al. for hybrid PbS QD-MoS2 FETs,17 where the incorporation of QDs was found to significantly decrease the FET on-off ratio. It is noteworthy that in their demonstration, the authors explained that the enhanced photodetection of MoS2 by the incorporation of PbS QDs was resulting from the charge transfer from photo-excited PbS QDs to MoS2. In our study, the insulating shell and ligand coating (giving an estimated spacing between CdSe core and SnS2 of ∼3 nm) are believed to prevent such a charge transfer and favor energy transfer (via photons, resulting from PL of the QDs and in turn absorbed in the SnS2). The absence of charge transfer can be further confirmed by measuring the electrical conductance of a CdSe/ZnS QD-only FET device (i.e., QDs drop-cast directly onto SiO2, with contact geometry similar to the device shown in Figure 1(b), but without a SnS2 flake). In such a device, we observed a negligible current under both dark and illuminated conditions,23 proving a negligible charge transfer due to the insulting ZnS shell and ligand coating, and thus confirming that the main interaction mechanism between CdSe/ZnS QDs and SnS2 is the energy transfer.

We also measured the spectral responsivity, R = ΔIDS,photo/Plight with Plight being the incident light power on the device active area, for both the SnS2-only and hybrid QD-SnS2 FET devices in the wavelength (λ) range from 400 nm to 600 nm, at VDS = 1 V and VG = −10 V (Figure 3). Overall, R is enhanced for the hybrid QD-SnS2 FET, reaching ∼650 A/W at λ = 400 nm (cf. dark IDS ∼ 0.1 nA), compared with 400 A/W for the non-sensitized SnS2-only FET. The onset of R starts at ∼550 nm for both FET devices (Figure 3 inset), which is consistent with the absorption band edge of SnS2 (Figure 1(b)). The optical absorption spectrum of CdSe/ZnS QDs in fact also starts at ∼550 nm,24 and thus the measured R after adding QDs mainly features an overall enhanced magnitude with an insignificant change in the spectral shape. It is noted that the onset of R is slow unlike the absorption spectrum (Figure 1(b)), which we suspect to be related to the presence of charge trap states near the band edges of SnS2. Such traps would induce a significant carrier recombination and decreased photocurrent output, consequently lowering the R value near the absorption band edge.

FIG. 3.

Spectral responsivity R of a few-layer SnS2 FET device at VG = −10 V and VDS = 1 V, before (black symbols) and after (red symbols) the application of the CdSe/ZnS QD sensitization. Inset shows a semi-logarithmic plot of the same data.

FIG. 3.

Spectral responsivity R of a few-layer SnS2 FET device at VG = −10 V and VDS = 1 V, before (black symbols) and after (red symbols) the application of the CdSe/ZnS QD sensitization. Inset shows a semi-logarithmic plot of the same data.

Close modal

Apart from the optical sensitization, we also found that the hybrid QD-SnS2 FET under illumination exhibits enhanced field-effect carrier mobility (μ) compared with the few-layer SnS2-only FET. Figure 4(a) shows IDS − VG plot at VDS = 0.5 V, where the FET is not saturated, and thus μ is proportional to the transconductance (dIDS/dVG) via the following relation:25μ = (dIDS/dVG) × L/(W·C·VDS) with L and W representing the length (15 μm) and width ( ∼ 5 μm) of the FET device channel, and C = 11.6 nF/cm2 being the capacitance of the 300 nm thick SiO2 gate dielectric. Figure 4(b) summarizes the obtained μ values. In the dark, the SnS2-only FET (no QDs) displays μ = 0.14 cm2 V−1 s−1, which is comparable to (but at the low end of) field-effect mobilities reported previously for back-gated few-layer SnS2 FETs at the room temperature.14 Interestingly, μ increases to over 0.6 cm2 V−1 s−1 upon white-light illumination. For the hybrid QD-SnS2 FET, μ is already higher than 0.4 cm2 V−1 s−1 even in the dark, and increases to ∼1 cm2 V−1 s−1 under white light illumination, which represents a ∼7× increase compared with the SnS2-only FET in the dark.

FIG. 4.

(a) IDS − VG characteristics of few-layer SnS2 FET at VDS = 0.5 V, before (square) and after (circle) CdSe/ZnS QD sensitization, in the dark (black) and under white-light illumination (red). (b) Histogram summarizing the field-effect mobility, μ, calculated from transconductances obtained in (a).

FIG. 4.

(a) IDS − VG characteristics of few-layer SnS2 FET at VDS = 0.5 V, before (square) and after (circle) CdSe/ZnS QD sensitization, in the dark (black) and under white-light illumination (red). (b) Histogram summarizing the field-effect mobility, μ, calculated from transconductances obtained in (a).

Close modal

In general, the experimentally measured μ can be affected by several extrinsic factors, such as contact barrier height, impurity scattering, and temperature. For example, μ in monolayer MoS2 reported in early studies was lower than 10 cm2 V−1 s−1,26 but it was later shown that the use of graphene contact and h-BN encapsulation could enhance the measured μ in monolayer MoS2 above 1000 cm2 V−1 s−1,27 and even as high as 34 000 cm2 V−1 s−1 for six-layer MoS2.28 This suggests a much higher intrinsic μ in the 2D material. Given our observed FET device characteristics that show an increased background carrier density during illumination and/or after application of QDs on the SnS2 FET, a potential cause for the observed increase in the measured μ might be a reduced contact Schottky barrier width WD, a property which is inversely proportional to the carrier density N (i.e., WD1/N).29 Although the precise WD in the ultrathin SnS2 FET device should be obtained by solving the 2D Poisson equation considering the exact device geometry, the basic qualitative relation between the in-plane Schottky junction depletion width from the electrodes and the back ground carrier density in SnS2 channel should hold true. Since N affects the FET threshold voltage (VT) according to VTN,29 one can then expect that WD decreases with increasing VT (i.e., WD1/VT). From Figure 4(a), we can estimate VT by extrapolating the linear IDS − VG plots to IDS = 0, and find VT of ∼10 V for the SnS2-only FET in the dark, which increases to ∼100 V in the hybrid QD-SnS2 FET under illumination. This 10× increase in VT implies a tenfold decrease in WD and thus an easier carrier tunneling through the Schottky barriers at the source and drain electrodes.

We note that the observed VT shift by the pseudo-gating effect of QDs may in part contribute to the increase in photocurrent response gain. We can estimate the extent of contribution from such a VT shift (approximately 45 V, estimated from the dark IDS − VG curves with/without QDs in Figure 4(a)) by examining the VG-dependent photocurrent output (ΔIDS,photo) data (Figure 2(c)). For instance, ΔIDS,photo of SnS2 only device at VG = 0 V would become ∼2.5 μA by 45 V shift of VT (Figure 2(c) left panel, at VDS = 4 V). On the contrary, ΔIDS,photo of QDs-applied SnS2 device at VG = 0 V is ∼4 μA (Figure 2(c) right panel, at VDS = 4 V); therefore, showing at least ∼38% additional contribution that is not accounted by the simple VT shift. Given the enhanced spectral responsivity starting near the absorption edge of QDs (Figure 3), it is logical to ascribe the origin of this additional photocurrent gain to the contribution from the energy transfer by QDs. We note that the extent of VT shift contribution estimated above is an upper bound value and should be less considering that IDS tends to saturate at high VG.

In summary, we combined core/shell CdSe/ZnS QDs with the few-layer SnS2 to fabricate hybrid QD-SnS2 FETs with improved photo-detection sensitivity via energy transfer from photo-excited QDs to SnS2. Photo-sensitization of SnS2 by the added QDs resulted in an over 5× enhanced photocurrent response in the hybrid FET with the corresponding device spectral responsivity reaching 650 A/W at 400 nm. We also found that the QD sensitization as well as light illumination enhanced the measured carrier mobility in SnS2, which we correlate with an elevated background charge carrier density and consequently decreased contact Schottky barrier width. Our results demonstrate that the energy-transfer-based QD sensitization can be utilized as a new route for enhancing the light harvesting performance of 2D LMD-based opto-electronic devices.

This research was carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory (BNL), which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704.

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