Broadening spectral responses and achieving environmental stability in SnS2/Ag-NPs/HfO2 flexible phototransistors

Layered two-dimensional (2D) materials have gained popularity thanks to their atomically thin physique and strong coupling with light. Here, we investigated a wide band gap ( ≥ 2 eV) 2D material, i.e. , tin disul ﬁ de (SnS 2 ), and decorated it with silver nanoparticles, Ag-NPs, for broadband photodetection. Our results show that the SnS 2 /Ag-NPs devices exhibit broadband photodetection ranging from the ultraviolet to near-infra-red (250 – 1050 nm) spectrum with decreased rise/decay times from 8/20 s to 7/16 s under 250 nm wave-length light compared to the bare SnS 2 device. This is attributed to the localized surface plasmon resonance e ﬀ ect and the wide band gap of SnS 2 crystal. Furthermore, the HfO 2 -passivated SnS 2 /Ag-NPs devices exhibited high photodetection performance in terms of photoresponsivity ( ∼ 12500 AW − 1 ), and external quantum e ﬃ ciency ( ∼ 6 × 10 6 %), which are signi ﬁ cantly higher compared to those of bare SnS 2 . Importantly, after HfO 2 passivation, the SnS 2 /Ag-NPs photodetector maintained the stable performance for several weeks with merely ∼ 5.7% reduction in photoresponsivity. Lastly, we fabricated a ﬂ exible SnS 2 /Ag-NPs photodetector, which shows excellent and stable performance under various bending curvatures (0, 20, and 10 mm), as it retains ∼ 80% of its photoresponsivity up to 500 bending cycles. Thus, our study provides a simple route to realize broadband and stable photoactivity in ﬂ exible 2D material-based devices.


Introduction
Photodetectors are sensors that convert incident photons into electrical signals. 1 The energies of the incident photons may span throughout the electromagnetic spectrum ranging from X-ray, ultraviolet (UV), visible, and infrared (IR) to the terahertz range for different applications. 2,3Commercially, silicon-based photodetectors are dominant in the visible to near-IR spectrum due to their small band gap (1.1 eV) and Earth abundance. 46][7] These techniques pose fabrication complexities and limit the miniaturization of photodetectors to meet the criteria for future scaled devices.[10][11] Semiconducting two-dimensional (2D) materials are advantageous to bulk semiconductors for photodetection because of their atomically thin and flexible nature, dangling bond-free surfaces, and wide range of energy gaps covering the UV to mid-IR spectrum. 12Previously, most efforts have been dedicated towards visible and IR photodetection since majority of well-studied 2D semiconductors such as MoS 2 , WSe 2 , and black phosphorus possess energy gaps < 2 eV. 13 Here, we inves-tigate the broadband UV-visible-near IR photodetection based on wide bandgap (2-2.6 eV) tin disulfide (SnS 2 ). 14The large bandgap of SnS 2 is advantageous to realize a high current on/ off ratio with a suppressed dark current, besides extending the spectral range of photodetection.
2D materials have shown strong light-matter coupling caused by the sharp peaks in their density of states at certain energy levels thanks to the confinement effect. 15,16Despite their strong interaction with incident photons, atomically thin 2D materials absorb a small portion of the incident light due to their optically transparent nature. 17Likewise, the optical and optoelectronic performances of 2D materials based photodetectors are compromised. 18][21] Here, we report the high performance and broadband photodetection of multilayer SnS 2 by decorating it with metal nanoparticles.Our results show that the decoration of silver nanoparticles (Ag-NPs) over SnS 2 channels leads to extended photodetection capabilities from UV to NIR (250-1050 nm).This is attributed to the localization of the electromagnetic field via the surface plasmon-enhanced optical field in the SnS 2 devices.In addition, we passivated the SnS 2 /Ag-NPs devices with HfO 2 , which demonstrated environmental stability with improved performance for more than seven weeks.Lastly, we demonstrated a flexible SnS 2 /Ag-NPs device which retains ∼80% of its photoresponsivity up to 500 bending cycles.Our results open possibilities for investigating wide band gap 2D semiconductors for broadband optoelectronic and photonic applications.

SnS 2 FET
Figs. 1a and b show the device schematic with a circuit diagram and an optical microscopic image of the back-gated SnS 2 field-effect transistor (FET) on a Si/SiO 2 substrate, respectively.Before electrical measurements, we performed atomic force microscopy and Raman spectroscopy on the SnS 2 channel to ascertain its thickness and material quality, and the results are provided in ESI Fig. S1.† Our representative SnS 2 channel is ∼13 nm thick, and it exhibits single Raman mode at ∼316.4 cm −1 (A 1g mode), which is attributed to the relative vibration of Sn and S atoms in the out-of-plane direction. 22,23e performed the basic electrical measurements (transfer and output curves) on the multilayer SnS 2 FET in the dark, which is followed by an optoelectronic response under continuous wave lasers of different wavelengths.Note that all the measurements were performed under ambient conditions throughout this study.Fig. 1c shows the transfer curve obtained by sweeping the gate voltages (V g ) from −60 to +60 V at a constant bias voltage (V ds = 0.5 V) under dark.The I ds −V g trend shows typical n-type (electron dominant) characteristics of the SnS 2 device, indicating that the Fermi level is positioned in the vicinity of the conduction band of SnS 2 .Similar to MoS 2 , the electron-rich characteristics in SnS 2 are attributed to the presence of sulfur vacancies and the strong Fermi level pinning effect close to the conduction band edge in SnS 2 . 24,25he device shows a current ON/OFF ratio of ∼10 3 .From the slope of the transfer curve, i.e., transconductance (g m ), we cal-culated an effective field-effect mobility (µ FE ) of ∼20 cm 2 V −1 s −1 using eqn (1). 26 where 'L' and 'W' are the SnS 2 channel length and width, respectively, and C ox is the gate oxide capacitance per unit area (∼1.15 × 10 −12 F cm −2 for 300 nm thick SiO 2 ).The low-resistive electrical contacts between Cr/Au and the SnS 2 channel are confirmed by realizing the almost linear output characteristics at all the measured V g values of −20 to 20 V with a step of 10 V, as shown in the inset of Fig. 1c.Furthermore, the optoelectronic response of SnS 2 at given laser wavelengths at a fixed laser power (P int = 10 mW cm −2 ) and V ds = 0.5 V is compiled in Fig. 1d.The SnS 2 device exhibits robust photoactive performance under incident wavelengths with a large photocurrent at shorter wavelengths due to the large photon energy.Under larger wavelength (over 540 nm) excitation, the device exhibits a very weak photocurrent as the incident photon energy is smaller than the optical band gap of multilayer SnS 2 (∼2.3 eV).This limits the applicability of large bandgap materials for broadband photodetection.To address this issue, we employed a subtle technique by decorating metallic nanoparticles over SnS 2 flakes.Silver, being relatively inexpensive compared to gold, has a higher refractive index at shorter wavelengths.Similarly, silver nanoparticles, Ag-NPs, exhibit plasmonic resonances at relatively shorter wavelengths.We next focus on optoelectrical characteristics of pristine and Ag-NPs-decorated SnS 2 phototransistors that were investigated at different wavelengths.The pristine SnS 2 device demonstrated the photoresponse across a narrow spectral range (250-540 nm).Interestingly, the photocurrent of the same device increases by two times after depositing Ag-NPs, and the cut-off wavelength is extended to 1050 nm, as shown in Fig. 2c, which is attributed to the enhanced absorption and the photoluminescence effect in 2D materials by Ag-NPs. 28,29he extension in the detection range of the SnS 2 device to near IR is mainly caused by the enhanced and broadband optical absorption in the NPs-decorated SnS 2 devices. 302][33] First is the localized surface plasmon resonance (LSPR); the free electrons in Ag-NPs collectively oscillate at specific frequencies under illumination, leading to enhanced optical absorption and scattering by the NPs, and eventual enhancement in the photocurrent generation in SnS 2 .The resonance frequencies depend on the size, shape, and wavelength of incident light, and can be studied in detail by employing finite element analysis in future.Second is light trapping; the Ag-NPs over the SnS 2 surface scatter the incident light, which is collected by neighboring NPs as plasmons instead of dissipating as free-space light, thus the strong optically interacting NPs trap light and increase the effective optical length and enhance the optical absorption, and hence an enhanced optoelectronic response in NPs-coacted SnS 2 device is observed.Third is the generation of hot electrons; the extinction of plasmons in the Ag-NPs may generate energetic, non-equilibrium charged carriers, also called hot electrons, which can be transferred to the SnS 2 surface, thus improving the photoresponsivity over a broad spectral range.We further compared the photodetector performances by estimating the rise and decay times of photodetectors from time-dependent photocurrent measurements.We obtained the rise and decay time responses by fitting the temporal photocurrent dataset, as shown in ESI Fig. S4, † by using eqn (2) where I ph and I dark are the photocurrent and dark current, respectively, A and B are the constants, and τ r and τ d denote the rise and decay times.For SnS 2 , the average rise (8.1, 8.9, 8.5, and 7.8 s) and decay (19.9, 17.1, 15.8, 14.7 s) times were estimated at 250, 365, 460, and 540 nm wavelengths, respectively.Similarly, in the case of the Ag-NPs-decorated SnS 2 device, the average rise (7.1, 6.9, 7.3, and 6.1 s) and decay (15.8, 14.7, 12.0, 11.4 s) times were estimated at 250, 365, 460, and 540 nm wavelengths, respectively, as shown in Fig. 2d.Interestingly, a decrease in the rise and decay times is observed when SnS 2 is decorated with Ag-NPs.The Ag-NPs sustain the absorption of impurities, which can play a vital role in the reduction of response time. 34The consistency of response times (rise/decay) at each wavelength indicates no significant alteration in the lifetime of the excited carriers. 35o understand the role of traps or defect states in SnS 2 and the Ag-NPs-decorated SnS 2 photodetectors, we evaluated the optical power-dependent photoresponse.The trend of the photocurrent as per the power law has been reported to reveal the existence of trap states in a 2D channel. 36By fitting Fig. 2e against the power law, i.e., I ph ∝ AP ϑ , we estimated ϑ values of 0.72 for SnS 2 and 0.58 for SnS 2 /NPs.Note that the ϑ value closer to 1 indicates the existence of active low energy trap states and the ϑ value far from 1 indicates the active large energy trap states. 37Therefore, these results indicate that the trap states play a vital role in broadening the photodetection range in the SnS 2 /NPs photodetectors.For example, the photogenerated carriers are trapped by the finite trap states, yielding the sub-linear dependence of the power-law formula of photocurrent. 38

Passivated SnS 2 /Ag-NPs/HfO 2 photodetector
Compared to other noble metal-based NPs, Ag-NPs are reported to be oxidized under ambient conditions, and this effect can be exaggerated under illumination conditions. 39As a result, the performance of the Ag-NPs-coated SnS 2 device may degrade over time.To circumvent this, we deposited a ∼15 nm thick high-k dielectric, HfO 2 , by atomic layer deposition, as indicated in Fig. 3a schematic.The stabilities and performances of the pristine SnS 2 , SnS 2 /NPs, and SnS 2 /Ag-NPs/ HfO 2 devices were monitored under ambient conditions for the first seven weeks (Fig. 3b).For more details, see ESI S5 and S6.† Among these three different device configurations, SnS 2 / Ag-NPs/HfO 2 demonstrated the highest photoresponsivity (R ph = I ph /P op × A; where P op is the illuminated laser power and A is the active device area) of ∼12 500 A W −1 at λ = 250 nm.Moreover, from the aging test result, we realized that the R ph is reduced by ∼20%, ∼36%, and 6% for the SnS 2 , SnS 2 /Ag-NPs, and SnS 2 /Ag-NPs/HfO 2 photodetectors after seven weeks under similar storage and operation conditions, as shown in Fig. 3c.This phenomenon is ascribed to the direct impact of oxygen on bare SnS 2 , leading to the deterioration of material quality.
In the case of SnS 2 /Ag-NPs, the degradation is more pronounced due to surface oxidation and the consequent depreciation of the interface between SnS 2 and Ag-NPs.However, the introduction of HfO 2 passivation emerges as a pivotal mitigating factor.This passivation layer effectively impedes the interaction of oxygen with SnS 2 and Ag-NPs, thereby resulting in a mere 6% degradation and ensuring the stability of the devices with consistent performance.In a nutshell, the passivated SnS 2 /Ag-NPs/HfO 2 photodetectors exhibit more stable performance for several weeks.For further comparison among the three device configurations, we computed the external quantum efficiency (EQE = hcR ph /eλ; where h is the Planck constant, c is the speed of light, and e is the elementary charge), which demonstrates the total electron-hole pair generation with the energy of one photon.The EQE was estimated to be ∼8.7 × 10 5 , ∼2.4 × 10 6 , and ∼5.9 × 10 6 % for SnS 2 , SnS 2 /NPs, and SnS 2 /NPs/HfO 2 , respectively.It is pertinent to note that our SnS 2 /NPs/HfO 2 device exhibited superior EQE value compared to previous reports, which are summarized in Table 1.

Mechanical flexibility of the SnS 2 /Ag-NPs photodetector
Among other advantages, 2D materials are mechanically robust, thus they can be incorporated for flexible applications.Therefore, we investigated the mechanical endurance of the Ag-NPs-coated SnS 2 photodetectors by bending polyethylene naphthalate (PEN) substrates with different radii, as shown schematically in Fig. 4a.The photoresponses as a function of time characteristics of the SnS 2 /Ag-NPs photodetectors were investigated at λ = 250 nm while holding the device at the bending position of various radii (0-flat, 20 mm, and 10 mm).Furthermore, we also illuminated at different light wavelengths (λ = 250, 540, and 1050 nm) at three different bending curvatures.
Fig. 4c depicts that the photoresponsivity of the flexible photodetector at 0 (flat), 20, and 10 mm was found to be ∼4778, ∼4575, and ∼4332 A W −1 at λ = 250 nm, respectively.The results show that the photoresponse remains persistent at different curvature states, thereby indicating that the devices exhibit excellent mechanical flexibility and broadband photodetection which are hardly affected by bending conditions.Moreover, the bending stability and reliability are the key factors of flexible photodetectors.Therefore, we bent (20 mm) our devices several times and measured the endurance of the photocurrent.From Fig. 4d, it is observed that the photoresponsivity of the flexible photodetector is ∼4556, ∼4523, ∼4101, and ∼3561 after bending for 0, 50, 100, and 150 cycles, respectively.Importantly, after 150 cycles, the photoresponsivity of the device decreased by ∼21%, which endorsed the photodetection stability of the flexible SnS 2 /Ag-NPs device under multiple bending cycles of 20 mm curvature.Furthermore, to observe the mechanical stability we calculated the response time of flexible the SnS 2 /Ag-NPs device at λ = 250 nm at various bending states.It is found that the response times are slightly reduced from 5.3 to 4.5 s (rise) and 6.7 to 6.1 s (decay) as the bending curvature is increased as shown in Fig. S8.† The reduction of response may be attributed to the  defect states produced by stretching of the channel material SnS 2 .However, the flexible measurements clearly exhibit repeated bending, making it an attractive material for wearable photodetectors.

Conclusion
In summary, we have reported multilayer SnS 2 , SnS 2 /Ag-NP, and SnS 2 /Ag-NPs/HfO 2 transistors to elucidate the light-matter interaction for broadband photodetectors.The light sensing capabilities of SnS 2 are extended from the UV to near NIR (250-1050 nm) spectrum after decorating with Ag-NPs, which endorsed the extension of the cut-off wavelength in SnS 2 .These results suggest the enhanced light absorption with localization of the electromagnetic field via the surface plasmonenhanced optical field and the wide band gap of SnS 2 crystals.Furthermore, the photodetection performance, such as the photocurrent, photoresponsivity, and EQE, is significantly enhanced in the passivated Ag-NP-decorated SnS 2 devices when compared to the bare SnS 2 devices.In addition, we estimated the photo-performance deterioration of each device, where it was found that the SnS 2 /Ag-NPs/HfO 2 device showed excellent stability with a meager reduction in photoresponsivity as compared to other devices.Moreover, we made a flexible SnS 2 /Ag-NPs photodetector, which showed a broadband light response having stability up to 500 bending cycles at a 20 mm curvature.Thus, our results provide an efficient approach to extend the optical and flexible sensing capabilities in wide bandgap 2D materials from UV to NIR for multifunctional photoelectronic devices.

Materials and methods
To make large-scale patterns on Si/SiO 2 substrates, we first performed photolithography.The chips were sonicated in acetone and isopropanol solvents, and later rinsed with piranha solution and washed with de-ionized water for 15 minutes to ensure the proper removal of contamination from the Si/SiO 2 chips.The chips were soft-baked in a furnace at 125 °C to remove moisture.The 2D crystals of SnS 2 were physically peeled off from the bulk crystals using a scotch tape and transferred to pre-patterned Au contact pads on Si/SiO 2 wafers by the dry transfer method using the polydimethylsiloxane (PDMS) stamp technique.We used atomic force microscopy (AFM), XE-100 by Park Systems Inc., for precise thickness measurements of the SnS 2 flakes.Subsequently, we performed electron beam lithography (EBL) to pattern metal electrodes.
After EBL, we evaporated the Cr and Au metals of 5 and 80 nm thick, respectively, for metallization of the electrodes.Finally, the devices were immersed in acetone to lift-off the excess metal from the substrate.In addition, for flexible measurements, we used polyethylene naphthalate (PEN) substrates to fabricate the SnS 2 /Ag-NPs photodetector.Raman spectroscopy (Renishaw, In Via systems) was performed with a laser source of 514 nm under ambient conditions.Furthermore, atomic layer deposition (ALD) was used to deposit HfO 2 (∼15 nm) at 250 °C at a rate of 0.1 nm per cycle.Electrical measurements were performed by employing a voltmeter (Keithley 2400) and a picoammeter (Keithley 6485).

2. 2
Ag-NPs-decorated SnS 2 FETsWe, therefore, decorated Ag-NPs with a density of 40-45 NPs μm −2 over a fresh SnS 2 phototransistor, as shown in the field emission scanning electron microscopy image in the inset of Fig.2a.Note that the following results are obtained from different devices compared to Fig.1; therefore, the maximum current levels are different in Fig.2aas compared to Fig.1cdue to the device-to-device variation.From the comparative plots in Fig.2a, we realized an almost two times increase in the on-state current from 1.39 μA to 2.75 μA after Ag-NPs deposition over SnS 2 .As a result, the SnS 2 device shows a two-times increase in µ FE from ∼15 to ∼32 cm 2 V −1 s −1 , a shift in the threshold voltage (ΔV th ≈ 5 V) from −23 to −28 V and an increase in the electron density concentration [n e = C ox (V g − V th )] from ∼3 × 10 12 to ∼5.5 × 10 12 cm −2 , as shown in Fig.2band ESI S3. † These changes confirm the electron doping of SnS 2 with Ag-NPs deposition induced by the surface charge transfer doping from Ag-NPs to SnS 2 flakes.

Fig. 2
Fig. 2 Optoelectronic performance before and after Ag-NPs deposition.(a) Comparative transfer curves of the SnS 2 device (L = 2.2 μm, W = 4.1 μm, and thickness = 4.8 nm) before and after Ag-NPs deposition.(b) Comparison of the extracted field effect mobility and threshold voltage from (a).(c) Photocurrents of pristine SnS 2 and Ag-NPs-decorated SnS 2 devices at V ds = 0.5 V and V g = 0.The light intensity was kept at 10 mW cm −2 during all the incident wavelengths.(d) Rise (green shaded) and decay (brown shaded) times of SnS 2 and SnS 2 /Ag-NPs photodetectors obtained from temporal photocurrent measurements.(e) The generated photocurrent of SnS 2 and SnS 2 /Ag-NPs devices as a function of illumination power density at V g = 0 V, where the dashed lines represent power-law fitting.

Fig. 4
Fig. 4 Optoelectronic measurements of flexible SnS 2 /Ag-NPs devices.(a) Schematic drawing of the flexible photodetector of SnS 2 /Ag-NPs.(b) Temporal photoresponse of the flexible SnS 2 /Ag-NP photodetector under the given bending curvatures at λ = 250 nm and P = 10 mW cm −2 .(c) Photoresponsivity of the flexible SnS 2 /Ag-NPs photodetector at different wavelengths (λ = 250, 540, and 1050 nm).(d) Time-dependent photoresponse in different numbers of bending cycles.(e) Reduction of photoresponsivity of the SnS 2 /Ag-NPs photodetector after various bending cycles.The bent radius was kept at 20 mm.The inset shows the flexible device in a bent position under light illumination.

Table 1
Summary of the key parameters of previous two-dimensional photodetectors