3.1 Optical properties of nitrogen and fluorine doped CDs
Using citric acid as carbon source, water or DMF as solvent, we have synthesized CDs. Ammonium fluoride and urea is added as F and N source for F and N doped CDs. The absorption and PL spectra of pure and doping CDs are showed in Figure 1. In water and DMF, the absorption and PL spectra of CDs have a little difference, which may be caused by solvent effect. The wavelength of the absorption and PL for F doping CDs are similar to those of pure CDs. However, N doping CDs have different optical properties with pure CDs. The absorption spectra of N doping CDs have multiple peaks. The wavelength of PL is shifting to red. N and F co-doping CDs have similar optical properties with N doping CDs. Obviously, impurity of F would not change optical properties of CDs. The absorption spectra of CDs with impurity of N have many peaks. The wavelength of PL spectra for N doping CDs shift to long wavelength. Therefore, using citric acid as carbon source, the wavelength of PL spectra of CDs would be controlled through doping N impurity.
3.2 The influence of location of doping N
CDs have more atoms, which results in the whole CDs being difficult to calculate with Gaussian. In this paper, small molecules have been chosen as unit to obtain the characters of CDs and N or F doping CDs. Configures and electronic density differences maps (EDDM) of pure and N doping molecules are listing in Figure 2. All molecules are stable structures for the positive frequency (Table 1). The cohesive energy per atom (Ep) in Table 1 is calculated using the following formula43:
where EP, ET, EC, EH and EN is cohesive energy per atom, the total energy of CDs, the energies of C, H and N atoms. NC, NH and NN are the numbers of C, H and N atoms.
Molecule without N doping (pure carbon 0) is planar with dipole moment (DM) 0.0018 Debye and D3h symmetry. For N doping molecules, there are two kind of doping types. The first one is doping nitrogen replacing one hydrogen atom of surface. The other one is doping nitrogen atom in the ring of benzene, whose nitrogen atom replaces one carbon atom of benzene ring. N-doping carbon molecules are showed in Figure 2. Nitrogen atoms in N-doping 1 and N-doping 2 molecules take the place of hydrogen atoms. Those in N-doping 3 and N-doping 4 molecules take the place of carbon atoms of benzene ring. In N-doping carbon 1 molecule, except two hydrogen atoms linking to doping N atom, all atoms are in the same plane. This structure has low symmetry C1 and high DM 2.1784 Debye. Benzene rings in N-doping 2 molecule are not in the same plane, which is different from N-doping 1 molecule. It is owing to the change of location of doping N atom, which makes the whole molecule be distorted for large steric resistance. DM of N-doping 2 molecule is 1.5719 Debye. The cohesive energy per atom for N-doping 1 and N-doping 2 is -6.470 and -6.463 eV, respectively. Obviously, N-doping 1 has lower energy and better stability. For N-doping 3 and 4 molecules, doping N atoms are in the ring of benzene. The difference between them is the location of doping N atoms. N-doping 3 is not a planar structure for the hydrogen in nitrogen atom is out of the plane. N-doping 4 is planar. These two molecules have worse symmetry and high DM. The cohesive energy per atom of N-doping 3 (-6.466 eV) is lower than N-doping carbon 4 (-6.057 eV). N-doping 3 molecule has better stability. From the above data, it is confirmed that for two doping types, the steric hindrance would influence the stability molecules.
The aromaticity, optical rotation at 589nm, molecular orbital, and UV-Vis spectra of pure and N-doping molecules have been investigated. Aromaticity is judged with NICS data calculated with NMR. The negative NICS value shows that the molecule is aromatic. And the positive NICS value shows the molecule is anti-aromatic. Investigation of NMR illustrates that NICS(0) value in center benzene is lower (-6ppm) than those in other three benzene rings (-13ppm) for pure carbon 0. Namely, outside benzene rings have better aromaticity, which is because the six delocalized electrons in center benzene ring being share with the outside three benzene rings. Electrons would run to the outside benzene. For N-doping 1 and 2 molecules, NICS(0) value has the same rule with that of pure carbon 0 molecule. NICS(0) value in center benzene is -9ppm and -6ppm for N-doping 1 and 2 molecules. NICS(0) values in three outside benzene rings are -14ppm. Aromaticity of N-doping 1 and 2 molecules is similar to pure carbon 0 molecule. Namely, doping nitrogen atom in surface of pure carbon 0 molecule has little influence on the delocalization of benzene rings. However, for N-doping 3 and N-doping 4 molecules whose nitrogen atoms replace one carbon atom of benzene ring, the NICS values have different rule with pure carbon molecule. NICS(0) value in center benzene is -7ppm and those in other two benzene ring is -13ppm for N-doping carbon 3 molecule. The third benzene ring whose one carbon atom is replaced by nitrogen has NICS(0) value +8, showing that this benzene ring is anti-aromatic. N-doping carbon 4 molecule has similar characters as N-doping 3. NICS(0) value in center benzene is -2ppm and those in other two benzene ring is -13ppm for N-doping 4 molecule. NICS(0) value of the rest benzene ring is +10. In a word, doping nitrogen replacing hydrogen atom has little influence on the delocalization and aromaticity of benzene rings. However, doping nitrogen atom in the ring of benzene would change the delocalization and aromaticity. EDDM (Figure 2b) showed that nitrogen atom has changed the electron distribution.
Table 1 characters of carbon molecule and N-doping carbon molecules.
characters
structure
|
cohesive energy
per atom (eV)
|
Dipole Moment
(Debye)
|
symmetry
|
Frequency
(Hz)
|
589nm
(deg)
|
Band gap
(eV)
|
pure carbon 0
|
-6.557
|
0.0018
|
D3h
|
55
|
-2
|
4.91
|
N-doping 1
|
-6.470
|
2.1784
|
C1
|
47
|
25
|
4.37
|
N-doping 2
|
-6.463
|
1.5719
|
C1
|
45
|
-51
|
4.39
|
N-doping 3
|
-6.466
|
1.5596
|
C1
|
62
|
-208
|
2.20
|
N-doping 4
|
-6.057
|
3.6723
|
Cs
|
29
|
73
|
2.20
|
3 without H
|
-6.632
|
1.7562
|
C1
|
59
|
0
|
4.74
|
4 without H
|
-6.626
|
2.8033
|
C1
|
52
|
2
|
4.74
|
Optical rotation at 589nm for pure carbon 0 and N-doping carbon molecules have been investigated (Table 1). Seen from table, the value of pure carbon 0 is smaller than those of N-doping carbon molecules. Moreover, N-doping 1 and N-doping 4 structures, whose doping N atoms locate in interposition, are right-hand circular. And N-doping 2 and N-doping 3 structures, doping N atoms locating in adjacency, are left-hand circular. Clearly, nitrogen atoms doped into pure carbon structure, the optical rotation would be changed. And the location of doping nitrogen atoms would influence the optical rotation. Therefore, we can use doping nitrogen to change the optical rotation of CDs.
As showing in Figure 3, HOMO and LUMO orbitals of pure carbon and N-doping molecules are different. HOMO and LUMO orbitals have been repeated using B3LYP/6-31G (d, p) and long range corrected exchange correlation functional CAM-B3LYP/6-31G (d, p). The calculating results are the same. The band gap from HOMO to LUMO for pure carbon 0 molecule is 4.91eV, which is in the range of insulator. In fact, carbon dots are semiconductor materials. The band gap of pure carbon 0 is broadening, compared with CDs, which is caused by quantum size effect. Namely, the size of pure carbon 0 structure is smaller than that of CDs. Therefore, the absorption spectra of pure carbon 0 structure shift to short wavelength. Besides, HOMO and LUMO orbitals are calculated with For N doping molecules, the band gap becomes smaller, which results in the wavelength of absorption spectra shifting to red. Moreover, the band gap data of N-doping 3 and 4 molecules with doping nitrogen replacing one carbon atom of benzene ring are 2.20eV, which is in the range of semiconductor. Clearly, doping nitrogen atoms into pure carbon would decrease the band gap, leading to the wavelength of absorption spectra shifting to red, particularly for doping nitrogen replacing one carbon atom of benzene ring structures. Therefore, we can use doping nitrogen to control the luminous wavelength of CDs. This conclusion is similar to the above experimental results. Calculated UV-Vis spectra have been shown in Figure 4. Pure carbon 0, N-doping 1 and 2 molecules have similar absorption spectra. The difference is that the peaks of wavelength of absorption spectra for N-doping 1 and 2 shift to red comparing to pure carbon 0, which is agreement with the above experimental results. It is consistent with the band gap results. Absorption spectra of N-doping 3 and 4 have three absorption peaks, which are assigned to the π-π* transition of the conjugated olefin, and n-π* transitions of a class of unsaturated double bonds containing heteroatoms44. Besides, absorption spectrum of pure carbon 0 has been calculated with ZINDO/S43, with the peak position 267nm. That with b3lyp/6-31g(d,p) is about 250nm which is shorter than that with ZINDO/S, which is similar to reference45.
3.3 Halogen doping and halogen-nitrogen co-doping molecules
For halogen doping molecules, doping halogen atom only can replace the location of hydrogen atom in benzene ring. We have investigated fluorine or chlorine single doping structures and fluorine-nitrogen co-doping structures. Configurations of fluorine and chlorine single doping carbon structures are showing in Figure 5a. Except Cl-doping 1 molecules, other structures are planar. Seen from UV-Vis spectra (Figure 4b), F doping molecules have shorter wavelength than those of Cl doping molecules. F and Cl doping molecules have similar absorption intensity and absorption wavelength with pure carbon molecules, which shows that F and Cl doping have little influence on the optical characters of CDs, which is similar to the above experimental results.
We have investigated two types of F and N co-doping molecules: nitrogen atom in benzene ring and nitrogen atom as amino. There are many isomers for different position of F atoms. All isomers have been optimized. Estimated with the total energy, we have obtained the most stable structures (Figure 6). Absorption spectra of three molecules are different. Structure 1 and 3 have the same absorption spectra, which is similar to pure carbon structure. Structure 2 has different absorption spectra with other molecules. Nitrogen atom in structure 2 can not form delocalized π bonding with the carbon atoms in the ring. Structure 2 have C-N bonding, which is similar to the above N-doping 3 and 4 molecules. Besides, we have calculated molecules similar to N-doping 3 and 4 molecules. The difference is that there is no hydrogen atom in nitrogen atom (Figure 4c). Seen from the data, it is confirmed that without hydrogen atom, 3 and 4 without hydrogen molecules have the same optical properties with pure carbon molecule (Figure 4d). Optical rotation at 589nm and band gap are close to pure carbon 0 molecule. Obviously, the position of nitrogen atom has little influence on the optical properties if the doping nitrogen atom does not destroy the delocalization of benzene ring.
Besides, we have investigated influence of solvent on optical properties of nitrogen doped carbon molecules. We have calculated N doped carbon molecules with water and DMF as solvent32, 34, 46-48. The data are listing in table 2. Obviously, peaks location of UV-Vis spectra in water and DMF are similar, which is shift to longer wavelength than those without solvent. However, theoretical UV-Vis spectra in water and DMF are shift to shorter wavelength that those of experiment, which is maybe caused by quantum size effect. The size of carbon molecules in theory is smaller than prepared CDs in experiment.
Table 2 Peaks location of UV-Vis spectra in water, DMF and without solvent (nm)
|
pure carbon 0
|
N-doping 1
|
N-doping 2
|
N-doping 3
|
N-doping 4
|
in water
|
256
|
263
|
259
|
334/428
|
325/472
|
in DMF
|
256
|
263
|
259
|
334/428
|
320/465
|
no solvent
|
250
|
256
|
255
|
337/416
|
348/449
|