Sample preparation
GO samples were obtained through double-thermal decomposition method in a pyrolysis system under controlled nitrogen flux by employing pyroligneous acid from bamboo (Guadua angustifolia Kunth) as source material. The DTD method involves two-step pyrolysis processes, as shown in Fig. 1; in the first pyrolysis step, from the carbonization process of the Bamboo raw material at 973 K for 1 h, BPA was obtained and collected in a decanting funnel glass; then, BPA was deposited on quartz substrate by roll-coating. In the second pyrolysis step, roll-coated BPA was used as a source to obtain GOF at a fixed carbonization temperature of 973 K with an oxide atomic concentration of 5%, according to previous reports35,37,38,41. Then, GOF were obtained by a mechanical transfer method onto a bakelite as substrate. For more details, see the references36,37,40. The authors confirm that all the methods in experimental research and field studies on plants, as the waste product of the commercial bamboo-guadua angustifolia Kunth, were performed in accordance with the relevant regulations.
Device configuration
Initially, the GOF were easily mechanically transferred from the quartz substrate employing a double sided tape, given that they are attached by weak electrostatic forces. Then, it is cut into a 5x3 mm rectangle shape with a thin blade and, subsequently, each sample is adhered to the bakelite substrate, on which a printed circuit (named the sample holder) is arranged to connect the electrical contacts of the photodetector. The thicknesses of the samples were measured and the average value was around 65 µm. Finally, GOF were electrically contacted by employing high-purity silver paint spots and copper wires with 80 µm approximately of thicknesses, as shown in Figs. 2a and 2b.
Photodetector prototypes were arranged by employing an electrical circuit of voltage divider configuration (Fig. 2a). To allow a greater transfer of electrical power, in this configuration, 1 kΩ of electrical resistance in series to the GOF was used (because this value is close to the electrical resistance measured in each GOF between 1.045 and 1.458 kΩ). To increase the signal-to-noise ratio of the measurements at room temperature, the photodetector prototype was located inside a black box (with an input for the monochromatic light beam), designed to prevent external light from several sources.
Characterization methods
Raman measurements were carried out at room temperature by using a confocal Horiba Jobin Yvon spectrometer, model Labram HR, with an excitation HeCd laser beam at 632.8 nm and 0.25 mW.
Electrical characterization
all GOF photodetector prototypes elaborated here, were electrically characterized at different temperatures, from 20 to 300 K. A closed-cycle helium cooling system was used, incorporating a device inside the cryostat. The pressure inside the cryostat was measured as the vacuum pump worked, employing a 917 Pirani sensor; while temperature was measured by using a Lake Shore 330 Autotuning temperature controller. IV curves were obtained employing a Keithley 6220 precision current source and a Keithley 2182A precision nanovoltmeter, with resolutions of 100 fA and 1 nV, respectively. The current-voltage curves at different temperatures were obtained by connecting the photodetector prototype to a current source and a nanovoltmeter inside the cryostat system without the black box (Fig. 2c).
The electrical current supplied to the photodetector prototype varied from − 1 to 1 mA and the voltage was measured at the two electrical contacts device. The IV curves were taken at different temperatures, varying from 20 to 300 K with a 20 K step. Electrical resistance and econductivity values were determined from the data analysis and the respective curves were plotted as a function of temperature for the range measured. In addition, the temperature dependence of these parameters in the proposed GOF was described by using the theoretical fitting to the experimental data, employing Mott's variable-range hopping model for the three-dimensional case (3D-VRH), given by27,44,45:
$$\sigma ={\sigma }_{0}exp{\left(\frac{-{T}_{0}}{T}\right)}^{\frac{1}{d+1}}.$$
1
Where σ0 is the conductance independent of temperature, 𝑇0 is the characteristic temperature, and d is the dimensionality of the system under investigation, here, the best fit was obtained employing d = 3. At low temperature this model can possibly describe the electrical conductivity response in GO samples. Additionally, in a diffusion hopping conduction process, the relation between the bandgap energy Eg and electrical conductivity it is often written as46:
$$\sigma \left(T\right)={\sigma }_{0}exp\left(-\frac{{E}_{g}}{2{k}_{B}T}\right),$$
2
here kT is the the product of the Boltzmann constant, kB, and the temperature, T.
Optical characterization: the wavelength influence on voltage and resistance was studied at room temperature, employing the following configuration: light from the QTH lamp operating at 120 W was dispersed using a Triax 320 monochromator and the monochromatic light beam illuminated the GOF photodetector located inside the black box. The wavelength of the incident beam is considered in a range between 1,300 nm and 3,000 nm (near infrared). The electrical output is connected to a precision Keithley 197A millivoltmeter with a resolution of 100 µV, with which voltage and electrical resistance are measured with and without the incident beam, and values are stored. The influence of the wavelength on the voltage and electrical resistance were obtained. The photocurrent and the dark current are obtained employing Ohm's law. Then, the responsivity, R, and quantum efficiency, η, were calculated for each GOF from the following equations, respectively47:
$$R\left(\lambda \right)=\frac{{I}_{ph}}{{P}_{o}},$$
3
here Iph is the photocurrent generated and Po is the incident optical power.
$$\eta =R\frac{e\lambda }{hc}g,$$
4
where, h is the Planck constant, λ is the wavelength and g is the gain, defined as the number of carriers detected per generated electron-hole pair, and indicates the performance of the collection system of photogenerated carriers.