Liquid-phase catalysis and characterization of graphene
Fig. 1a shows a schematic diagram of the liquid-phase catalytic growth process of graphene.
Colorless sucrose solution was caramelized at 200℃ to form GQDs, which turned the solution into a yellow viscous liquid. Subsequently, the viscous liquid was diluted with a mixed solvent of alcohol and deionized water. Iodine was then introduced into the GQDs solution and turned the color of the solution from yellow to dark brown, as shown in right inset of Fig. 1a. Details on the preparation process can be found in Methods section. TEM characterization on the yellow solution showed high density and even distribution of GQDs, as shown in Supplementary Fig. 1a and b. The size of these quantum dots was 3.13 nm as shown in Fig. 1k. High-resolution TEM (HRTEM) image in Fig. 1b-d revealed circular shape of the as-grown GQDs. Supplementary Figs. 2a and b show the lattice fringes of the dots with spacing of 0.214 nm and 0.246 nm respectively, which is characteristic spacing of graphene. After the introduction of iodine, the GQDs rapidly grew in size from a few nanometers to hundreds of nanometers and into graphene flakes as shown in HRTEM image of Fig. 1e-g. Interestingly, there were wrinkles at the graphene prepared by the liquid-phase catalysis method as shown in Supplementary Fig. 1d. Lattice fringes from region ① and ② of the HRTEM image in Supplementary Fig. 1d are shown in Supplementary Figs. 2c and d, which has lattice spacing of 0.214 nm in-plane and 0.361 nm in c-axial direction respectively. This revealed good crystallinity of graphene. Selective area electron diffraction (SAED) was performed at the blue dotted circle region in Supplementary Fig. 1c, which showed hexagonal diffraction pattern of graphene (Supplementary Fig. 2e). Furthermore, the FFT patterns of Fig. 1e-g showed perfect hexagonal structures, as shown in Fig. 1h-j respectively. Hence, these results again indicated good crystallinity of graphene prepared by this method. Interestingly, the statistical size of graphene flakes was up to 275.78 nm, as shown in Fig. 1k. There were evidences that the size of some graphene flakes could reach micron scale as shown in Fig. 2a. This implies that the liquid-phase catalysis method is capable of mass producing high quality graphene with size ranging from hundreds of nanometers to micron scale. EDS measurements (Supplementary Fig. 1e) showed high concentration of iodine element on the graphene, suggesting that the iodine remained in the solution and attached to the graphene after completion of the catalytic growth. Fig 2b shows a HRTEM image of region ① in Fig. 2a. It showed that multi-layered graphene was produced in the solution using the liquid-phase catalysis method. Besides, the interlayer spacing of 0.365 nm, as measured in Fig. 2c, indicates that the graphene produced using this method is constructed by layer-by-layer stacking. SAED were performed (as shown in Fig. 2d) to investigate the superposition of the graphene. By measuring the distance (d) between the center spot and the vertices of each hexagonal ring, D (e.g. D =1/d) of each lattice face can be calculated. According to the crystal surface spacing of graphite, the corresponding lattice face of each spot was obtained, as shown in Fig. 2d, where d1=2.946 1/nm, d2=4.950 1/nm, d3=8.620 1/nm and d4=9.481 1/nm corresponding to D1=0.339 nm, D2=0.202 nm, D3=0.116 nm and D4=0.105 nm respectively. Thus, the electron diffraction pattern corresponded to the lattice face53 (002), (101), (112) and (201), which indicate c-axis direction as well as other directions of graphene. This suggests that the graphene flakes were randomly stacked. Also, the TEM images showed that the graphene flakes were not flat but have lots of wrinkles. Therefore, graphene with various morphologies can be grown from GQDs using this method, as illustrated in Fig. 2e.
Modulation of energy bandgap of graphene Intrinsic graphene has zero bandgap, which greatly limits its application, especially in the field of semiconductor. However, the energy bandgap of graphene can be opened up by either doping or controlling its size as demonstrated by others54-56. The graphene solution prepared using the liquid-phase catalysis method was spin-coated on to a substrate due to its good viscosity and then annealed at elevated temperature under vacuum to obtain solid-state graphene films, as shown in Fig. 3a. Using the spin-coating deposition technique, it is possible to produce large area graphene film on any substrate. XPS measurements revealed the absence of iodine at the solid graphene film (see Supplementary Fig. 3), hence suggesting that the catalyst was removed upon annealing and there was no evidence of iodine doping. The graphene film was homogeneous and has a thickness of 609 nm as measured using AFM (as shown in 3b). Optical absorption characterization was performed on both the solution and the film during the preparation process. The absorption peaks of the GQDs solution were located at 285 nm during the nucleation stage. After the liquid-phase catalysis growth of graphene, the absorption peaks of the graphene solution red-shifted and were located at 353 nm. Moreover, its absorption spectrum was widened, which is a notable feature of graphene. After the annealing treatment, the liquid-phase graphene was solidified into graphene thin film, which led to a significant increase in its absorption intensity by at least three orders of magnitudes (e.g. 2015.9%) from that of the GQDs. A further widening of the absorption spectrum was observed (Fig. 3c), which can be attributed to the removal of moisture and carbon dioxide, thus resulting in a higher order of graphene films (see Supplementary Fig. 4). The optical bandgap of graphene can be calculated from its absorption spectrum since it is a direct bandgap material (see Supplementary Fig. 5 and Supplementary Table 1). Detailed calculation is shown in the Supplementary Discussion. The results showed that the bandgap of the solid graphene films varied significantly at different annealing temperatures, for example, the optical bandgap decreases with increasing annealing temperatures. Hence, the bandgap of liquid-phase catalytic graphene can be modulated by annealing treatment. Based on the overall trend, the bandgap can be tuned from wide bandgap semiconductor (2.2 eV) at 500°C to conductor (0 eV) at sufficiently high temperature (see Fig. 3d). This is an exciting discovery as the graphene prepared by the liquid-phase catalysis method will lead to many novel applications in the semiconductor industry.
Fabrication and characterization of photodetector A simple photodetector structure, shown in Fig. 4a, consisting of the graphene film as an absorbent layer was fabricated. The liquid-phase catalyzed graphene was spin-coated on the surface of ITO film. Fig. 4a illustrates the structure of the graphene films, which comprised of randomly stacked graphene flakes as the absorbent layer. The graphene film was then annealed at 600℃, which is limited by the maximum temperature of the ITO film46. An energy band structure of the photodetector with calculated optical bandgap of 1.78 eV is shown in Fig. 4b. Fig. 4c
shows I-V measurements performed on the photodetector under dark and illumination of LEDs with different wavelengths from 365 nm to 940 nm at various power densities (e.g. 365 nm @ 0.18 mWcm-2, 400 nm @ 0.54 mWcm-2, 500 nm @ 1.32 mWcm-2, 555 nm @ 0.95 mWcm-2, 660 nm @ 2.8 mWcm-2, 740 nm @ 2.5 mWcm-2, 850 nm @ 1.4 mWcm-2, 940 nm @ 0.8 mWcm-2). Such measurements demonstrated the capability of the photodetector in detecting wavelength ranging from ultraviolet to infrared. An important figure of merit for a photodetector57 is responsivity (R), which can be calculated using the following formula:
R = Jph/Popt
where Jph is photocurrent, which equals to absolute value of the current density under illumination subtracting that in the dark, and Popt is incident optical power. Another important figure of merit is detectivity, D*, which can be expressed as
where Jdark is dark current density and q is unit charge. Using the I–V characteristics, both R and D* can be calculated. Curves of R and D* vs. applied voltage of the photodetector are shown in Supplementary Fig. 6. When applying 1.85V, the photodetector exhibited peak detectivity ( ) and responsivity as shown in Fig. 4d. Under illumination at 365 nm, the detectivity and responsivity were increased to 1.1x1013 cmHz1/2W-1 and 81.3 mAW-1 respectively. Besides, the trend of D* and R responding to wavelength was similar to that of photocurrent. As shown in Fig. 4e, the photocurrent spectrum revealed a broad response wavelength of the photodetector, which ranged from 400 to 1200 nm. Such broad coverage in the visible and near-infrared range means that the graphene film is invaluable for applications in solar cell, photocatalysis and light-emitting diodes etc. Since the optical bandgap of the graphene film was 1.78 eV, its response wavelength should position at 696.6 nm (e.g. λpeak=1240(nm)/Eg). The broad response wavelength therefore indicates extraordinary phenomena exhibited by the graphene film. Under the excitation of photons with specific energy, electron-hole pairs are excited inside the graphene. These electrons and holes are separated to Al and ITO electrodes respectively, as shown in Fig 4a. When photons with excessive energy (λ>365 nm) incident on to the graphene, heat (and/or other forms of energy) is also released, which contributes to the current of the photodetector.