3.1. Detection of Light Absorption Rate by Nano-Devices
There are two main types of interactions between light and graphene from the perspective of energy band transitions: inter-band transitions and intra-band transitions. The dominant transition mode depends on the energy of the photon. In the near-infrared and visible light bands, the light response is mainly inter-band transitions, and the absorption coefficient is determined by the fine structure constant. In this case, the light absorption of graphene can be regulated by adjusting the position of the Fermi plane due to the vesicle blocking principle. The energy at the K point of the two-dimensional graphene’s Bridgeport region is linearly related to momentum and the effective mass of the carrier is 0, which is a remarkable feature that distinguishes graphene from the electronic structure of conventional materials. This energy band relationship gives graphene unique physical properties, such as the quantum Hall effect and near-ballistic transport of carriers at room temperature. In terms of its optical properties, single-layer graphene has a high light absorption rate due to the linear distribution of Dirac electrons. Graphene absorbs 2.3% of each layer from the visible to the terahertz wide band [15
A constant pressure of 1 V was applied to both ends of the electrode, the incident light wavelength was 950 nm, the measurement time was 20 s, and the measured photocurrent curve is shown in Figure 2
. The photocurrent between the electrodes was measured with a Keithley 4200 semiconductor analyzer. When the current was stable, four sets of photocurrent values were obtained, 128.3 mA, 112.9 mA, 106.3 mA and 95.8 mA, respectively, and their average value was 110.8 mA. Since the power of the incident light source was 5 mW, the actual graphene absorption rate was calculated to be 2.216%. According to the theoretical calculation in previous work [22
], the light absorption rate of graphene was 2.3%. Considering the power loss of light in the experiment, the actual result is not much different from the theoretical calculation. Therefore, this nanodevice can be used to study the absorption of light.
3.2. Photosensitive Detection by Nano-Devices
In this experiment, light-emitting diodes with different wavelengths (650 nm, 750 nm, 850 nm, 940 nm) were used to irradiate the fabricated device at the distance of 10 mm, as shown in Figure 3
a. It can be seen from the figure that at a certain wavelength, the current is 0 when the voltage is 0. When the voltage gradually increases from 0 to 1 V, the current does not change significantly. When the voltage increases from 1 to 2 V, the curvature of the current curve increases greatly. When the voltage is the same and the wavelengths are different, the shorter the wavelength is, the larger the photocurrent is, which means that graphene can better absorb short wavelength light.
The voltage-current diagram when the device is irradiated with 940 nm wavelength at different distances (10 mm, 50 mm, 100 mm, 150 mm, 200 mm) is shown in Figure 3
b. It can be seen from the figure that the current is the maximum when the distance is 10 mm, that is, when the distance is small, the graphene absorbs more photons per unit area. It can also be seen from the figure that when the distance is 50 mm, 100 mm, 150 mm and 200 mm, the curves almost coincide. In other words, when the distance is greater than 50 mm, the distance factor has no effect on the light absorption of graphene.
In order to study the effect of light switching on the light absorption of the graphene device, the graphene device is irradiated intermittently with light at a wavelength of 850 nm and 940 nm. The nano-device was placed on a three-dimensional micrometer operating platform, and the relative position between the sample and the incident laser was adjusted through the change of the current over time. The position with the largest current change of the photoelectric device was taken as the laser irradiation position, that is, the area with the most significant photoelectric response. The left terminal was connected to the SMU1 of the semiconductor tester as a drain, and the right terminal was connected to the SMU3 of the semiconductor tester as a source. In the test, the bias voltage of drain terminal was set to 1 V, and the source terminal was grounded. The laser position is adjusted so that the laser focal plane was on the surface of the test device and the laser was approximately 10 cm away from the surface of the device. The light source was turned off during the first 30 seconds of the experiment, then the light source was turned on for 30–150 s, and so on. The resulting curve is shown in Figure 4
. It can be seen that the current was almost unchanged when there was no light in the first 30 s, and the current dropped sharply at the instant of applying light at 30 s. At 100 s, the current stabilized and reached a minimum value. After 150 s, the light was suddenly turned off and the current increased rapidly. It increased and stabilized after 75 s. Other irradiation cycles showed the same trend as the first one. When comparing light with different wavelengths, it can be seen that the longer the wavelength, the larger the photocurrent.
A constant voltage of 1 V was applied across the gold electrode, and the graphene device was irradiated with light of different wavelengths (365 nm, 450 nm, 550 nm, 650 nm, 750 nm, 850 nm, 940 nm) at a distance of 10 mm. The resulting photocurrent relationship is shown in Figure 5
a, which was measured using a Keithley 4200. The light was off for the first 25 s, and the light source at different wavelengths was on at 25 s until the end of the 50 s. When no light was applied for the first 25 s, the photoelectricity was 0 under constant voltage. When the light source was turned on, the photocurrent increased linearly and stabilized at a certain value. After the light source was turned off at 50 s, the photocurrent rapidly declined to 0. The magnitude of the photocurrent obtained by light irradiation with different wavelengths was 750 nm > 850 nm > 950 nm > 365 nm > 550 nm > 450 nm > 650 nm. It can be seen from the figure that the amplitude of the photocurrent was larger when light wavelengths of 750 nm, 850 nm and 940 nm were irradiated. This also proved that the graphene device has a better absorption of light at infrared wavelengths, and that the photocurrent decreased with increasing wavelengths. The photocurrent was smaller and the amplitude difference was smaller when wavelengths of 365 nm, 450 nm, 550 nm, and 650 nm were irradiated, and the photocurrent changes with wavelength irregularly. Since graphene has an excellent absorption of near-infrared light, cyclic tests were conducted with light of three wavelengths, 750 nm, 850 nm, 940 nm, for five cycles with a period of 60 s, as shown in Figure 5
b. It can be seen from the figure that the photocurrent values of five cycles were very stable when the device was irradiated with different near-infrared wavelengths. The amplitude of the current when the light was irradiated for the first time did not change much compared with the amplitude after the fourth time, which means that this nano-device was sensitive to light and had good fatigue performance. This experiment demonstrates that the graphene device is effective and stable as sensors for detecting near-infrared light.
Transistor devices made of single-layer or few-layer of graphene with zero bandgap can be used as ultrafast photoelectric detectors. The graphene surface absorbs photons to produce electron–hole pairs that can be rapidly compounded, and the speed of the composite is dependent on the temperature and the concentration of electrons and holes. When an external electric field is applied, the electrons and voids can be pulled apart to produce a photocurrent, and the same effect can be achieved when there is a built-in electric field at the interface between the electrode and the graphene. Under the working condition of constant voltage, a change of carrier concentration is reflected as a change of current between source and drain, so the detection of light is realized. Graphene as a carrier transmission channel can improve the response speed of a light detection sensor device. By comparing the response time of the graphene nano-device with that of nanowires made by Hyungwoo [22
], it can be found that the response time of the graphene device is nearly 10 times shorter than that of the nanowire device.
3.3. Thermal Detection by Nano-Devices
Since the maximum detection temperature in the field of electronics and solar cells generally does not exceed 100 °C, the graphene device is heated intermittently with a heating source at different temperatures (50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C) at a distance of 10 mm as shown in Figure 6
The photocurrent measured at a constant voltage of 1 V is shown in Figure 7
a. In the first 20 s of the experiment, there was no heating source, and after 20 s, the heating source was on, and so on, and the data of five cycles were measured. It can be seen on the curve of the same temperature that when the heating source was on at 20 s, the photocurrent decreased. When the heat source was off at 40 s, the photocurrent gradually increased, and every subsequent period showed such a regular change. From the curve of 100 °C, the photocurrent dropped rapidly after the heating source was added. The gap is very large, which indicates that the graphene device has a faster and more sensitive response when detecting high temperature objects. As can be seen from Figure 7
b, the curve of 100 °C has the highest photocurrent compared with other temperatures, and the photocurrent decreases gradually with the increase of irradiation times. When incident light irradiated the nano-device, the energy generated by local surface plasmon resonance between nanogaps and the remaining thermal carriers had a thermal effect on the graphene, in addition to thermal injection of graphene contributing effective carriers. The energy generated by local surface plasmon resonance transforms is converted into the thermal vibration of graphene atoms by means of non-radiative resonance energy transfer, while the remaining thermal carriers are continuously moved by Landau damping. They eventually appeared as heating phenomena in the material. Therefore, the higher is the temperature of the heat source, the more thermal carriers are released, and the greater photocurrent is generated. According to the blackbody radiation formula, it could be calculated that the wavelength of the object radiation at 50–100 °C was 7.77–8.97 μm, so it also proved that this device has a good light response in the mid-infrared wavelength. Since the heating source of this experiment was 10 mm away from the graphene device, it proves that the graphene device can detect the temperature of the object in a non-contact manner.