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A Flexible Solar-blind Ultraviolet Photodetector based on Carbon Nanodots

A flexible solar-blind ultraviolet (UV) photodetector based on the carbon nanodots (CNDs) was fabricated on a polyethylene terephthalate (PET) substrate. The responsivity of 1.2 mA/W is obtained for 10 V applied bias under 254 nm illumination. Further, bending tests were carried out under the 0.2% strain, and the results showed that this flexible photodetector had stable characteristics and no obviously decrease of the photocurrent. The bending performances exhibit excellent potential for the fabrication of smart and flexible photodetectors.


Introduction
Due to the strong absorption by ozone layer, UV radiation with wavelength between 200nm to 280nm has almost no background interference on the earth surface, so it also called solar-blind UV radiation. Based on this characteristic, solar-blind UV photodetectors have attracted more and more researchers with the strong anti-background radiation interference ability and high detection sensitivity [1,2]. Conventional solar-blind UV photodetectors are fabricated on bulk silicon, which hinder their application on the flexible electronic device field. Compared with the photodetectors based on rigid substrates, flexible photodetectors have wide application in biomedical, portable and wearable devices. Sensitive photo-response and high mechanical strength are essential for a high-performance flexible photodetector. From the perspective of detection capability, high detection ability in the target wavelength is very important for the active layer material. In terms of mechanical strength, the flexible device should maintain high sensitivity under repeated bending conditions. In general, in order to meet the requirement that the performance of the flexible photodetector will not be significantly reduced after repeated deformation, the active material also needs to be able to withstand repeated deformation. According to the tensile bending strain equation, the strain of the material is proportional to the thickness of the material [3]. Under a given radius of curvature, the material with thinner thickness has the smaller strain. Therefore, low-dimensional nanomaterial is a perspective material to fabricate the high-performance flexible photodetector. As a kind of low-dimensional nanomaterial, zero-dimensional quantum dots can be synthesized by simple solution method [4,5]. Compared with photodetectors based on traditional bulk materials, photodetectors based on quantum dots generally have higher response and gain due to the high specific surface area and small size. Therefore, using quantum dots as the active layer material can not only strike a balance between the photo-response and flexibility, but also improve the performance of the device and simplify the fabrication process.
Carbon nanodots (CNDs) are quasi-spherical nanoparticles with the diameter below 10 nm, which have excellent properties including outstanding chemical inertness, low toxicity and fascinating optical properties [6]. Based on quantum confinement and edge effects, the band gap of CNDs is tunable [7].  [8], but have almost no absorption in the visible and infrared regions. This means that CNDs are very suitable for preparing solar-blind photodetector. Besides, the solvent of the CNDs is aqueous solution and it can be transferred to the substrate by spin coating. Since most of the common flexible substrates are not resistant to high temperature and organic solvent, it is a good choice to use CNDs as the active layer of flexible photodetectors. Therefore, this work report on a high-performance solar-blind flexible photodetector based on CNDs as an active layer and polyethylene terephthalate (PET) as a flexible substrate. This work also provides a new idea for the use of low-dimensional nanomaterials to construct high-performance, low-cost, and environmental-friendly flexible photodetectors.

CNDs synthesis
The CNDs were prepared by electrochemical method at 60V bias voltage. Two carbon rods (99.99%, length of 10 cm, diameter of 0.6 cm) were vertically fixed at a distance of 2cm as electrodes to react in distilled water. The reaction was carried out with continuously magnetic stirring. Finally, the obtained black-brown liquid was filtered to remove large particles, and then centrifuged at 10000 rpm for 30 minutes. The suspension is as-prepared CNDs solution.

Device fabrication
The Schematic diagram of the photodetector is shown in Figure 1. The PET (thickness of 0.175mm) was ultrasonically cleaned for 30 minutes and UV ozone treated for 30 minutes. Aluminum (Al) electrodes were prepared using thermal evaporation on PET substrate. The as-prepared CNDs solution was mixed with PVA solution (3% in weight in water), and then spin-coated (500 rpm for 10 s and 2000 rpm for 30 s) between the electrodes as the active layer, and the channel width was 50 μm. Finally, the device was annealed at 50 °C for 10 minutes.

Characterization
Transmission electron microscopy (JEOL-02100, Japan) was used for morphology of the CNDs. The UV-visible absorption spectrum was measured by UV-visible spectrophotometer (UV-2600, Shimadzu). Photo-response measurements were performed using a mercury lamp (254nm, OSRAM G11T5/OF) and a semiconductor characterization system (Keithley 4200-SCS).

Results and discussion
The morphology of the as-prepared CNDs is shown in Figure 2a. It can be seen that the CNDs are quasi-spherical and have an average diameter of about 3.3 nm. It is observed that the absorption of CNDs mainly in the ultraviolet region, and gradually weakens with the increase of wavelength ( Figure  2b). Moreover, a main peak at about 230 nm can be observed which corresponds to the π-π* transition of C=C bonds of CNDs [9]. PVA/CNDs has the same absorption peak as CNDs, indicating that introduce PVA into CNDs will not affect the absorption performance. The optical spectra was further analyzed to calculate the energy band gap. The absorption of CNDs follows a power law of the following form [10]: ( ℎ ) 2 = �ℎ − � (1) where α is the absorption coefficient, hν is the energy of the incident photon, B is a constant, and E g is   Figure 3a shows the voltage dependent dark and photocurrent (254 nm) of the photodetector. The power density of 254 nm UV light was kept at 1.5 mW/cm 2 . It can be clearly seen that under dark and illumination, the currents increase linearly with the increase of the applied bias voltage. Such linear relationship indicates that there is a good ohmic contact between Al and PVA/CNDs film. Further, the photocurrent significantly increased compared with the dark current. In order to study the response time of the photodetector, time-dependent photocurrent measurement was also performed at 10 V bias voltage. As shown in Figure 3b, the flexible photodetector has stable switching response. The current increases when the light is irradiated, and recovers after the light is removed. It is worth noting that the dark current of the device is about 1nA, which is lower than many flexible solar-blind photodetectors [11]. For photodetectors, response time is defined as the time interval between 10% and 90% of the response peak. Therefore, the rise time of the device is about 1.74s and the decay time is 2.93s ( Figure  3c). The decay time is little slower than the rise time, which can be attributed to the traps in the active When illuminated by UV light, electron-hole pairs are generated in CNDs. Some of the carriers are captured by the trapping states in PVA/CNDs films. After turning off the light, the trapped carriers are released, which contributes to the slow-response. In addition, responsivity (R λ ) is an important parameter to measure the response performance of photodetectors, which can be calculated by the following formula [11]: = ∆ / (2) where ∆I is the photocurrent of the device, P is the power of the light source, and S is the effective area of device. The S of 5×10 -3 cm 2 was considered for the calculations. The responsivity of about 1.18mA/W was acquired at 10 V bias. In order to study the performance of the device under bending, the photoresponse of the device under different bending strains was tested. The strain (ε) in the device was calculated by the following formula [12]: Where t is the thickness of the device and R is the radius of curvature. The switching characteristics of the device under strains of 0% and 0.2% were tested, as shown in Figure 3d. It can be clearly seen that the photocurrent of the device under 0.2% strain is almost the same as the photocurrent of 0% strain, and the device also has obvious photoresponse and stable switching characteristics. This indicates the flexible photodetector based on CNDs can successfully detect solar-blind UV light under a bending strain.

Conclusion
In conclusion, a flexible solar-blind UV photodetector was successfully prepared based on high-quality CNDs prepared by an electrochemical method. The as-prepared CNDs have good absorption in the solar-blind UV range. Under the 254 nm illumination, the device shows a responsivity of 1.18 mA/W at 10 bias voltage. Under the bending strain, the performance of the device did not deteriorate significantly, demonstrating the potential applicability in fields that cannot be served by conventional rigid photodetectors.