# Robust formation of amorphous Sb2S3 on functionalized graphene for high-performance optoelectronic devices in the cyan-gap

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Sep 10, 2020

It can be seen that the surface morphology of graphene before and after UV irradiation is not much different in Figs. 1a and 1b, and the boundary defects of graphene are not increased, resulting in no significant decrease in the transmittance. At a wavelength of 550 nm, the light transmittance is about 97.2% which is close to the single-layer graphene in the literature as shown in Fig. 1c, and surface oxidation of graphene does not affect the light transmittance of graphene after UV irradiation.

Raman spectroscopy is applied to confirm single-layer graphene property after UV irradiation. A low ratio of ID/IG and the high I2D/IG ratio of unmodified graphene (0.05 and 1.63, respectively) corroborate the presence of single-layer graphene with few defects (shown as in Fig. S2). After 5 and 10 min of UV irradiation, the ID/IG peak ratios increased to 0.91 and 1.8, respectively. The UV irradiation for 20 min showed D, G, and 2D band signals without graphene. Therefore, the longer the UV-irradiation time is applied, the more carbon–carbon sp2 bonding structures will be destroyed in graphene, resulting in a larger ID/IG peak ratio and even disappeared eventually, as shown in Fig. 1d.

From I–V sweep with a bias voltage of − 10 to 10 V in Fig. 1e, the resistance of graphene is proportional to UV irradiation time and a current drop from 6.82 to 2.27 mA at a voltage of 10 V after graphene is UV irradiated for 20 min is also observed in Fig. 1f. Hall effect measurement results showed that the sheet resistance of graphene after 10 min irradiation increased from 1,676 to 2,636 Ω/sq, and the carrier mobility was reduced from 710 to 509 ({text{ cm}}^{2} /{text{V}},{text{s }}). The graphene after 20 min of UV irradiation could not measure the sheet resistance and carrier mobility due to its high resistance. We hypothesize that the high-reactive oxygen atom breaks the carbon–carbon sp2 bond and π-bond structure to form the sp3 hybrid orbital, producing oxygen functional groups (COC, C–OH, and CH). The longer the UV irradiation time, the more oxygen functional groups (C–O–C, C–OH, and C–H) will be formed, so the resistance will increase as the UV irradiation time increases24,25.

The UV-irradiated graphene-based plate was vertically fixed and immersed in a beaker of Sb2S3 solution after photolithography, and the beaker was immersed in a low-temperature water bath and the temperature was maintained below 10 °C. The Sb2S3 thin-film deposition time was conducted for 2, 4, 6, 8 h per batch. When the plate was taken out, the color of the film on the plate has become orange. Detail synthesis parameters and reactions of Sb2S3 synthesis are schematically shown in Fig. 2a and described the supplemental information.

We observed that Sb2S3 cannot be effectively deposited on pristine graphene, but with the increase of graphene UV ozone irradiation time, Sb2S3 can be deposited on graphene more effectively in Fig. 2b. A pale yellow color was observed from Sb2S3 film when the UV irradiation time reaches 5 min, but Sb2S3 is amorphous according to XRD pattern. After 10 min and 20 min of UV irradiation time, it was found that the color of the Sb2S3 film deposited on the graphene became dark orange, indicating that the Sb2S3 film was thicker, but the XRD patterns were still amorphous for thicker Sb2S3 film.

After UV irradiation of graphene, C–OH and C–O–C bonds are formed on the surface. These bonds may be negatively charged and can be used to help adsorb Sb2S3 in the CBD reaction. The longer the UV irradiation time, the more C–OH and C–O–C bonds are formed, which causes the graphene to be oxidized and deteriorated, but it can help to adsorb more Sb2S3 molecules and increase Sb2S3 film thickness24.

Photo-responsivity time measurements were examined to find the appropriate UV irradiation time for the highest photocurrent at 4 h CBD Sb2S3 film growth condition. It shows that the components with the modified 5 min and 10 min have a brighter photocurrent increased after UV irradiation, but the photocurrent of the component modified for 20 min has almost no rise as shown in Fig. S3b. Therefore, 10 min UV irradiation for graphene to produce functionalization is the fixed-parameter for different CBD Sb2S3 film growth times.

Not only UV irradiation time affect photocurrent and photo-responsivity time, but the thickness of Sb2S3 film is also another key parameter to improve photoelectronic properties. It can be found that the longer the deposition time, the thicker the Sb2S3 film but the amorphous nature is retained from the XRD measurement in Fig. 3a. Scanning electron microscopy (SEM) was used to observe the Sb2S3 film deposited on the UV irradiated graphene for 2, 4, 6, and 8 h, respectively. The SEM cross-section of the Sb2S3 is shown for different growth times. It can be seen that the film thickness is 141 nm, 485 nm, 559 nm, and 832 nm at deposition time of 2–8 h. It can be seen that the longer the deposition time, the more Sb2S3 particles (Fig. 3f) aggregate together in 2D direction on the graphene and gradually form a film as shown in Fig. 3b–e.

The dimensions of the components are glass (1.5 cm × 1.5 cm), graphene (1 cm × 1 cm) and Sb2S3 (0.5 cm × 0.5 cm). The incident source of the excitation element is a 405 nm laser. The incident light intensity is 10–60 mW/cm2 and the measurement voltage is 1–15 V. The SSG composite components grown for 2–8 h were compared by 405 nm illumination I–V measurement, and it was found that the illumination current was significantly improved at a growth time of 6 h in Fig. 4c, compared to 2, 4, and 8 h CBD deposition time. The I–T curves of SSG at different growth times for atmosphere and vacuum conditions are shown separately in Fig. 4d. At a fixed bias voltage of 15 V, we measured the photo-responsivity of the SSG device at 2, 4, 6 and 8 h in the atmosphere and vacuum environment respectively. Since a higher photocurrent is observed at Sb2S3 film on the UV irradiated graphene due to the oxygen functional group on the graphene surface, the graphene defects are increased, and the graphene high carrier mobility is sacrificed26. Therefore when an electron–hole pair is generated after illumination, the photocarrier is trapped by the modified graphene defect before being injected into the pure graphene layer, compared to no illumination condition, as schematically shown in Fig. 4a,b, resulting in a decrease in carrier mobility, which makes it impossible to obtained photocurrent in a short time, causing the dark current to rise to the illumination current.

A comparison of the signal peak, photocurrent and rise time of the atmosphere and vacuum in Fig. 4e,f. Due to the oxygen atom is reduced in the vacuum environment, the formed depletion region is narrower, and the carrier concentration is increased. Therefore, the photocurrent under vacuum is higher than the atmosphere, and the rise time is also much shortened. The photo-responsivity characteristics of this device are the same as those of photodetection device fabricated by rGO and chemical functionalization, because of some defects in the film cause long rise and fall times27.

When unilluminated, part of the photocarriers are still trapped by the modified graphene defects, making the photocarriers difficult to recombine and prolonging the excitons recombination time, cause additional a few seconds needed for the rise and fall times to complete the on/off action in the SSG.

A large amount of photocurrent can further increase the post-illumination reaction of graphene. But not all of the electrons and holes that are generated are pulled away by the applied electric field because there are many oxygen atoms (O2) in the atmosphere that will attract electrons that jump to high energy states, producing negatively charged oxygen atoms (({text{O}}_{2}^{ – })). The above reasons will cause the rise time to be lower by this effect.

In this experiment, it was found that due to the increase in the thickness of the Sb2S3 film and the unevenness of the surface, the trap inside the film was relatively increased, so that the carrier was not composited before being transferred to the graphene28. Therefore, the growth time of 6 h of illumination I–V and the atmospheric and I–T under vacuum have the best response, so the subsequent experiments are based on the best growth parameters of 6 h as the benchmark.

Different voltages were used to explore the optimal parameters of SSG components. In the experiment, a 405 nm laser light intensity of 60 mW and a switching time of 5 s was used to irradiate the SSG via 6 h CBD Sb2S3 film deposition time. When the applied voltage became smaller, the generated photocurrent decreased, and the electric field inside the material is reduced. Therefore, the internal recombination ability increases for the electron–hole generated in Sb2S3 film under a small electric field, so the photocurrent is reduced. If the applied voltage is too large, it will produce an overshoot phenomenon. When the applied voltage is 15 V, the component has the best photocurrent of 6.18 × 10–4 A and the rise time of 27.05 ms (Fig. S5a).

A vacuum I–T plot of the different light intensity of the SSG component is shown in Fig. S5b. Change the different light intensities to explore the characteristics of the best parameters of SSG components. Using a 405 nm Laser to illuminate the 6 h CBD deposition SSG, the component is fixed at a voltage of 15 V, the switching time is 5 s, and the light intensity is from 10 to 60 mW. It can be found that when the light intensity is weakened, the current also drops. The higher the incident light intensity, the more photons can excite the Sb2S3 to generate more electron–hole pairs, so the photocurrent increases. When the weaker light is applied, since only the surface of the Sb2S3 generates a carrier, the surface carrier is transferred to the graphene, and the photocurrent is reduced by the composite of the material and the graphene defect.

The photocurrent increases as the light intensity increases29,30. The intrinsic defect density of the device can be found through the relationship between photocurrent and light intensity measured by different light intensities, which can be derived from the linear fitting Power Law.

$${text{I}} propto {alpha P}^{beta }$$

(1)

I is the photocurrent measured by the component at 15 V, α is the proportionality constant, P is the optical power intensity of 405 nm, and β represents the defect density of the component itself. When the component defect density is extremely small, the photocurrent is linearly proportional to the incident light intensity, θ should be close to 1, conversely. If the component defect density is extremely large, θ should be much smaller than 1.

The photoelectric response results of a 405 nm laser light illuminating element with a power density ranging from 10 to 60 mW, and we use the fitting I–P to get a beta value of 0.64 (Fig. 5a). Photo-responsivity (R) is a key parameter for evaluating the performance of a photodetector. It is defined as the photocurrent generated by the incident light per unit of power on the effective area of the device. R can be calculated by using the following formula.

$${text{R}} = {text{I}}/{text{P}} cdot {text{S}}$$

(2)

I is the photocurrent (lighting current–dark current), P is the incident light intensity of the xenon lamp, and S is the effective illumination area. Because our growing material area is larger than the illumination spot area, we do not need the area item in this experiment.

When the light of the wavelength of 450 nm is utilized, the optical response is 112 A/W when the optical power intensity is 1 μW/cm2. In the ultraviolet band, when the light of the wavelength of 350 nm is irradiated. The optical response of the component is also 69.76 AW−1. When the wavelength of light absorbed by Sb2S3 exceeds 568 nm, the photo-responsivity value immediately drops to zero and there is no absorption at all. Therefore, it can be found that Sb2S3 is only in the ultraviolet to the visible range for photo-responsivity to occur, and the best photo-responsivity value in the visible light band will also confirm the argument that the Sb2S3 enthalpy gap deposited in this study is about 2.2 eV. The detection rate corresponding to the response value can be calculated by the following formula.

$${text{D}}uplambda* = {text{ R}}S^{frac{1}{2}} /(2e{text{Id}}){1}/{2}$$

(3)

R is the spectral response value, Id is the dark current measured by Xenon lamp, S is the effective illumination area, and e is 1.609 × 10–19 C. After calculation by the above formula, it has the highest response value at 450 nm and the highest detection rate, which is 2.01 × 1012 Jones, shown as Fig. 5b.