Chapter 3: Origin of High Photoluminescence Yield and High SERS Sensitivity of Nitrogen-
3.2. Experimental Details
3.2.1. Sample Preparation
3.2.1.1. Synthesis of Graphene Oxide
In the present work, GO was synthesized by a well-known modified Hummers’ method, as discussed in Chapter 2, Section 2.2.1.1. In brief, 3 g graphite flakes were mixed with 70 mL concentrated H2SO4 under continuous stirring in the present of 1.5 g NaNO3. After 2h stirring, 9 g of KMnO4 was added slowly in the above mixture inside an ice bath. After that, MQ water was slowly added to the prepared mixture at 98 C, and then the solution was quenched with 15 mL of 30% H2O2. Finally, the GO was separated from the unreacted flakes and impurities by centrifugation.
3.2.1.2. Synthesis of undoped GQDs
Undoped GQDs (U-GQDs) were prepared by the top-down method using GO as the precursor, as discussed in Chapter 2, Section 2.2.1.2. Briefly, 600 mg of GO powder was dispersed in 40 mL MQ water and sonicated for 30 min for proper dispersion and then the solution was heated at 220
°C for 12 h into a Teflon lined autoclave. After that, the transparent suspension was collected as U-GQDs.
3.2.1.3. Synthesis of Nitrogen-doped GQDs
Nitrogen-doped GQDs (N-GQDs) were synthesized by a solvothermal route in dimethylformamide (DMF, 99%, Sigma-Aldrich) solvent with the GO as a precursor. Typically, 600 mg of GO powder was dispersed in 40 mL DMF with sonication for 30 min. Then the GO solution was transferred into the Teflon lined autoclave and heated at 220 °C for 7h. After cooling down at room temperature, the solution was centrifuged at 10,000 rpm to collect the yellow suspension of N-GQDs. To optimize the synthesis of N-GQDs with different N-content, different amounts of GO (200, 600, 800 mg) was added in a fixed amount (40 mL) of DMF and reacted at discussed above.
3.2.1.4. Synthesis of Sulfur-doped GQDs
Sulfur-doped GQDs (S-GQDs) was synthesized top-down method in dimethyl sulfoxide (DMSO, Sigma-Aldrich) solvent. In brief, 600 mg of GO powder was mixed with 40 mL of DMSO under ultra-sonication. After that, GO solution was heated at 220 °C for 7h into the Teflon lined autoclave. Finally, the suspension part was collected as S-GQDs.
A schematic of the synthesis process of various types of GQDs in different mediums and the features of the resulting products is shown in Fig. 3.1.
Fig. 3.1. Reaction scheme of different types of GQDs with various solvents and the chemical features of the resulting products.
3.2.2. SERS Detection
The efficiency of undoped and doped GQDs as SERS substrates was monitored with Si (100) substrate coated with 0.2 mg/mL solution of GQDs in MQ water for each sample. The solutions of dye molecules, e.g., RhB and methyl blue (MB), were arranged with different concentrations in methanol. 10 L of GQDs solution was first drop cast on cleaned Si substrate at a constant temperature, 70 °C. After the complete drying of the GQDs solution, 20 L of dye solution was dropped on top of the GQD-coated Si substrate and used for SERS detection.
3.2.3. Fabrication of Liquid Phase White LED
N-GQDs were used as light converting material for the fabrication of liquid phase WLED.
Commercially procured low-cost UV LED (~396 nm) was put into a culture tube containing N- GQDs solution (0.4 mg/mL) and sealed with a glass slip to obtain a liquid phase LED. The same arrangement was implemented with the addition of 50 M RhB solution in N-GQDs solution with a volume ratio of 1:5 for improvement of the purity of the white light.
57 | O r i g i n o f H i g h P L Q Y & S E R S S e n s i t i v i t y o f N - G Q D s
3.2.4. Characterization Techniques
The details of the characterization techniques (TEM, Raman, UV-vis, PL, and TRPL) used to study the systems were described in Chapter 2, Section 2.2.3. Additionally, high-angle annular dark- field (HAADF) images were obtained using a scanning transmission electron microscope (STEM) in aberration-correction mode (JEM 2100F, 200 kV) for high-resolution imaging of few samples.
Atomic force microscopy (AFM) (Cypher, Oxford Instruments) image was acquired in non- contact mode. X-ray photoelectron spectroscopy (XPS) measurement was carried out using a PHI X-tool automated photoelectron spectrometer (ULVAC-PHI, Japan) with an Al Kα X-ray beam (1486.6 eV) at a beam current of 20 mA for the analysis of the chemical compositions and chemical environment. The shift in the binding energy of various material was corrected using the C 1s spectrum at 284.8 eV as the reference value.24 The Raman scattering measurements were performed in a high-resolution Raman spectrometer (LabRam HR800, Jobin Yvon) with different laser excitations: 488, 514 and 633 nm. A 100X objective lens focused the laser beam with a spot size of ~1μm in diameter. The acquisition time was 20 s for collecting the SERS spectrum. Fourier transform infrared spectroscopy (FTIR) measurements were executed in PerkinElmer, Spectrum BX spectrophotometer in the reflectance mode. For XRD, FTIR, and Raman measurements, samples were drop cast on Si (100) wafer, followed by drying at 70 C. UV-vis absorption spectra of the samples were recorded using the PerkinElmer spectrophotometer, Lamda 950. The steady- state PL measurements were carried out in a commercial spectrofluorometer (Horiba, Fluoromax- 4) equipped with a Xe lamp source. Low temperature (80–300 K) PL measurements were carried out using a 405 nm laser excitation and a liquid nitrogen-cooled optical cryostat (Optistat DNV, Oxford Instruments) with the sample kept in the vacuum. Electroluminescence (EL) signal was acquired with the same spectrofluorometer using an external power supply. Time-resolved PL (TRPL) measurements were carried out with the excitation of A 405 nm laser source by a picosecond time-resolved luminescence spectrometer (FSP920, Edinburg Instruments) with an instrument response time of ~50 ps. The vacuum annealing was carried out in a quartz chamber for 2 h at different temperatures (~200, 400, and 600 °C) under 2.510-2 mbar base pressure.