2D/0D graphene hybrids for visible-blind flexible UV photodetectors

Nitrogen-functionalized graphene quantum dots (NGQDs) are attractive building blocks for optoelectronic devices because of their exceptional tunable optical absorption and fluorescence properties. Here, we developed a high-performance flexible NGQD/graphene field-effect transistor (NGQD@GFET) hybrid ultraviolet (UV) photodetector, using dimethylamine-functionalized GQDs (NMe2-GQDs) with a large bandgap of ca. 3.3 eV. The NMe2-GQD@GFET photodetector exhibits high photoresponsivity and detectivity of ca. 1.5 × 104 A W–1 and ca. 5.5 × 1011 Jones, respectively, in the deep-UV region as short as 255 nm without application of a backgate voltage. The feasibility of these flexible UV photodetectors for practical application in flame alarms is also demonstrated.

photon-absorbing layer comprised NMe 2 -GQDs with an energy gap of 3.3 eV, which enables visible-blind UV light detection. HRTEM observations indicated that the NMe 2 -GQDs had diameters of ca. 5 nm (Fig. S1a). AFM observations revealed most of the NMe 2 -GQDs as ca. 1.0 nm thick, which corresponds to 1-2 layers of functionalized GQDs (Fig. S1b and c). The high-resolution N 1 s XPS spectrum of the NMe 2 -GQDs revealed that nitrogen atoms were predominantly engaged in C-N bonding (400.1 eV), which is characteristic of primary dimethyl amines bonded to graphene (Fig. S1d). The band alignment at the graphene/NMe 2 -GQD interface under light irradiation is illustrated schematically in Fig. 1b. The lowest unoccupied molecular orbital (LUMO) level of NMe 2 -GQDs was estimated by subtracting the band gap (Fig. S1e) from the highest occupied molecular orbital (HOMO) level. The HOMO level was measured using ultraviolet photoelectron yield spectroscopy (Fig. S1f). Incident photons excite the ground-state electrons of the NMe 2 -GQDs into excited states. Electron-hole pairs are then generated. The photoexcited carriers are separated at the NMe 2 -GQD/graphene interface because of the internal built-in electric field. Only the electrons are swept into the graphene layer driving efficient charge separation and transfer, whereas the photoexcited holes remain in the NMe 2 -GQD layer.
The spectral response was measured. It corresponded well with typical absorbance behaviour for NMe 2 -GQDs, i.e., a steep rising edge positioned at ca. 420 nm. Figure 1c shows that the photodetector is exclusively sensitive to UV light. It is almost blind to visible light, although the NMe 2 -GQDs have an absorption tail in the visible light range (>450 nm), presumably because the localized edge states or surface states in the NMe 2 -GQDs. The edge states or surface states that contribute to light absorption of NMe 2 -GQDs in the longer wavelength range have little contribution to the photocurrent because the carriers on these states are highly localised. Moreover, they recombine shortly after generation. In contrast, the shorter wavelength light offers sufficient energy to excite the carriers to the LUMO energy level of the NMe 2 -GQDs, generating an efficient photocurrent 18 . A response cut-off wavelength was ca. 430 nm. The UV-to-visible rejection ratio (ΔI370/ΔI450 nm, where ΔI (=I light − I dark ) is the photocurrent) 12, 13 of the photodetector is over two orders of magnitude (>5 × 10 2 ), as shown in the response spectrum, which indicates that the photodetector exhibits a high signal-to-noise ratio. In fact, a negative value of ΔI was detected, as discussed below.
We further characterized the NMe 2 -GQD@GFET hybrid photodetector by measuring the transfer curve upon illumination. Figure 2a and b present the transfer characteristics (V SD = 0.5 V) for the NMe 2 -GQD@GFET hybrid photodetector and a pristine GFET. The Dirac point (charge neutrality point, V D ) from the pristine GFET was observed at ca. 23.5 V, implying unintentional hole doping from contamination, lithography processes, and/or surface oxygen-related adsorbates 19 . After hybridization with the NMe 2 -GQDs, V D shifted to ca. 4.2 V, which suggests that electrons were transferred from the NMe 2 -GQDs to graphene. This transferral is consistent with the higher work function of NMe 2 -GQDs. Upon irradiation (405 nm, 18 μW cm −2 ) of the NMe 2 -GQD@GFET hybrid photodetector, V D shifts to a lower voltage, which suggests that photoexcited electrons are transferred to the graphene. From the transfer curves measured in the absence of light, it is estimated that the field-effect mobility for holes is ca. 985 and 1,575 cm 2 V −1 s −1 , respectively, for the GFET and the NMe 2 -GQD@GFET. The hole-carrier mobility is increased by the incorporation of NMe 2 -GQDs, which is contrary to previously observed trends for graphene hybrid phototransistors, which exhibit decreased carrier mobility because of disorder or defects induced by hybridization 2,10 . The increase in carrier mobility might be attributable to an increase in the density of states near the Fermi level induced by the widely distributed π orbitals of the NMe 2 -GQDs.
When the NMe 2 -GQD@GFET is operated at V G = 0 V, the major carriers in the graphene layer are holes. The holes are compensated by the transferred electrons, which engenders decreased conductance and a negative ΔI (Fig. 1c). The back-gate dependence of the photoresponsivity (Fig. 2c) is also consistent with this trend. A clear carrier type and concentration dependence of the external photoresponsivity R ext (R ext = ΔI/P light , where P light is the incident optical power density) were observed. R ext (R ext = ΔI/(P light • A PD /A light ), where A light is the light spot area) was calculated with a scaling factor (A PD /A light ) that takes into account that only a fraction of the optical power density impinges on the photodetector. In the hole-conduction region (V G < 4.2 V), the holes are compensated by the photoinduced electron carriers, which engenders decreased conductance and a negative R ext . In contrast, the photoinduced electron carriers raise the electron concentration, which engenders increased conductance and a positive R ext in the electron-conduction region (V G > 4.2 V).
We investigated the dynamic performance of the NMe 2 -GQD@GFET hybrid photodetector by measuring its response time. Figure 2d shows the on/off photocurrent of the photodetector at an incident power density of 15 μW cm −2 (V G = 0 V, V SD = 0.1 V, 405 nm. The temporal photoresponse was measured at the on/off cycle of 1 s. The photocurrent level is well retained, demonstrating good reliability and reversibility of NMe 2 -GQD@ GFET hybrid photodetector. Figure 2e gives the transient response dynamics. The rise time was ca. 3.7 s (which corresponds to a rise of ca. 90%). The decay trace can be fitted by a double exponential; 10,20 . Time constants τ 1 and τ 2 are, respectively, 22 s and 77 s. This slow decay is probably associated with the multiplicity of carrier traps in NMe 2 -GQDs from quantum confinement and different localized states in the π-π* gap 21   long as the NMe 2 -GQDs remain positively charged, negative charges in the graphene sheet are highly recirculated because of their high carrier mobility, resulting in high photoconductive gain 2 . Figure 3a presents the optical power density-dependent external photoresponsivity R ext , and the specific detectivity D* of the NMe 2 -GQD@GFET hybrid photodetector at a fixed bias of 0.1 V under light irradiation at wavelengths of 255, 370, and 405 nm. D* (D* = (∆f A PD ) 1/2 R ext /i n , where A PD , ∆f and i n respectively stand for the area of the photodetector channel, electrical bandwidth, and noise current) 23,24 represents the capability of detecting low-level light signals. The noise current of photodetectors is dominated by the shot noise, but the noise current in the shot-noise limit is given as i n = (2eI dark ∆f) 1/2 , where e and I dark respectively denote the electron charge and the dark current 24 . Therefore, D* in the shot-noise limit is calculable by the expression: R ex A PD 1/2 /(2eI dark ) 1/2 23-25 , which has been used to estimate the D* of photodetectors in earlier studies 23,24,26,27 . The NMe 2 -GQD@GFET hybrid photodetector exhibited high photoresponsivity and detectivity in the deep UV region. R ext and D* respectively reached ca. 1.5 × 10 4 A W −1 and ca. 5.5 × 10 11 Jones at 255 nm. The maximum values of R ext and D* are comparable to those reported for other high performance UV photodetectors (Table S1) 6, 28-37 . The increased R ext and D* toward the shorter wavelength is consistent with the fact that the absorption of the NMe 2 -GQDs is enhanced at high-energy wavelengths. The photocurrent increases linearly with the source-drain bias. High R ext and D* is achieved when the bias voltage is increased and the incident optical power density is decreased (Fig. 3b). The decrease in R ext and D* with the incident optical power density is explainable by consideration of the following reason. As more photogenerated electrons are injected into the graphene channel, the corresponding holes left in NMe 2 -GQD layers weaken the original internal field near the NMe 2 -GQD/graphene interface built by the Fermi-level alignment. The ability in charge separation declines with reduced interfacial electric field, leading to decreased photocurrent as the incident optical power density increases. Therefore, a reduced injection of electrons causes R ext and D* to decrease.
Finally, the practical application of the NMe 2 -GQD@GFET hybrid photodetector to flame detection is demonstrated. Figure 4a presents the temporal change in the photocurrent upon illumination by light from a gas match from a distance of ca. 1 m. A clear change of the photocurrent was observed with switching of the gas match on and off, which confirms that the photodetector is useful for flame detection. Figure 4b depicts digital images of the prototype flame alarm mounted on a front panel consisting of "nanoblocks" in conjunction with the open-source Arduino Uno electronics platform.
A movie showing operation of the flame alarm is given in the Supplementary Information (Movie S1). When the photocurrent from the photodetector is increased to the threshold value of the photocurrent set in the program that controls the detectable distance, the green LED is turned on. The flame alarm LED was turned respectively off ( Fig. 4b(i)) and on (Fig. 4b(ii)) in the absence and in the presence of a flame. Our flame alarm based on the NMe 2 -GQD@GFET hybrid photodetector is well operated. These results indicate that the proposed NMe 2 -GQD@GFET hybrid photodetector offers great potential for use in practical devices.
In summary, flexible UV photodetectors based on graphene/nitrogen-functionalized graphene quantum dot hybrid were produced using dimethylamine-functionalized GQDs with a large bandgap of ca. 3.3 eV. The hybrid photodetector exhibited high photoresponsivity and detectivity in the deep ultraviolet region. Its photoresponsivity and detectivity reached values exceeding 10 4 A W −1 and 10 11 Jones, respectively, at 255 nm. Moreover, the potential application for flame alarms was demonstrated. This hybrid photodetector is promising for use in future graphene-based photonic devices. Synthesis of NMe 2 -GQDs. NMe 2 -GQDs were selected for the NGQD layer because their large energy gap (3.3 eV) enables visible-blind UV light detection. The NMe 2 -GQDs were prepared as follows. Oxidized GNPs (oGNPs) were obtained by refluxing GNPs in a 2:1 solution of concentrated H 2 SO 4 and HNO 3 . 5 mg of the oGNPs were dispersed in 5 mL of DMF. The mixture was refluxed at 154.5 °C for 5 h. After cooling to room temperature, the suspension was dried using an evaporation system (Soltramini; Techno Sigma Corp.). The dried powder (NMe 2 -GQDs) was re-dispersed in water and was dialyzed for 3 days. After drying, the NMe 2 -GQD powder was re-dispersed in DMF.
Photodetector device fabrication. Monolayer graphene sheets formed by chemical vapour deposition (CVD) onto a PEN substrate (188 μm) were obtained from Graphene Platform Corp. Gold source and drain electrodes (100 nm) were fabricated directly on the top of the graphene/PEN substrate using sputter deposition through a metal shadow mask. These devices had effective channel length L of 80 μm and a channel width W of 200 μm. NMe 2 -GQD dispersions were subsequently dropped directly onto the top of the graphene layer.
Measurements of the gate-voltage dependence on the photoresponsivity were taken using photodetectors fabricated by replacement of the PEN substrate with a Si substrate. The gate contact was deposited onto the back side of the Si wafer substrate. Monolayer graphene sheets formed by CVD onto a p-doped Si wafer (525 μm, 0.002 cm, 90 nm SiO 2 layer) were obtained from Graphene Platform Corp. The source and drain electrodes were patterned directly on the top of the graphene/Si wafer using a conventional photolithography process with a poly(methyl methacrylate) resist. The contacts were formed with titanium and gold (15 nm/100 nm) using a sputtering deposition method, followed by a lift-off process. These devices were fabricated with L = 10 μm and W = 10 μm. After the gate contact was deposited onto the back side of the Si wafer, the device was diced into 1 × 1 cm substrates. The NMe 2 -GQD dispersions were dropped directly onto the top of the graphene layer.
Photoresponse measurements. A light source was guided into the photodetector through a circular aperture with area A. Light-emitting diodes (LEDs) with different wavelengths were used as light sources to provide light with wavelengths of 255, 370, and 405 nm. A fibre probe, coupled to a monochromator with a deuterium lamp, was used for spectral response measurements. The optical power density through the aperture (P opt ) was measured using a silicon photodetector (S130VC; Thorlabs, Inc.). The incident (P inc ) power density was then calculated using the relation: P inc = (A det × P opt )/A, where A det is the active area of the photodetector. Electrical measurements were taken using a system source meter (2602 A; Keithley Instruments Inc.) in conjunction with LabVIEW software. All measurements were conducted at room temperature in the ambient atmosphere.
Instrumentation. X-ray photoelectron spectroscopy (XPS; Quantera SXM; Ulvac-Phi Inc.) measurements were performed using a monochromated Al Kα radiation source (100 μm spot diameter) of 1486.6 eV under high-vacuum conditions. Scans were acquired in the fixed analyzer transmission mode with pass energy of 26 eV and a surface/detector takeoff angle of 45°. High-resolution transmission electron microscopy (HRTEM; EX-2000; Hitachi Ltd.) was used with accelerating voltage of 200 kV. Optical transmittance and reflectance in the visible/NIR region of the samples was recorded using a spectrophotometer (UV-3600; Shimadzu Corp.). Reflectance was measured at an incident angle of 5° using an additional attachment. Measurements of work function and shallow energy levels were taken using a photoelectron spectrometer (AC-2; Riken Keiki Co. Ltd.). Atomic force microscopy (AFM; NanoscopeV D3100; Veeco Instruments) was used to investigate the height profile.
Data Availability. The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request and with permission of Toyota Central R&D Laboratories, Inc.