III. RESULTS AND DISCUSSION

For all the ion-implanted diamonds in this work, surface blistering was not observed because of the deliberate annealing scheme used in this experiment, although there were reports of blistering at higher dose ion implantation.Reference Chaiwong, Yu, Schinarakis and Vilaithong^3, Reference McHargue, Farlow, Lewis and Williams⁴ Figure 1 shows a series of optical micrographs for the diamonds irradiated and annealed at various conditions. The color of transparent diamond changed to pale yellow near ion fluence of 5 × 10¹⁶ ions/cm², then to dark-brown, and finally to black with increasing fluence to 1 × 10¹⁷ ions/cm². Annealing the implanted diamond in vacuum or Ar gas atmosphere at 600 °C for 2 h produced a glossier black with metallic luster but with reduced transparency, compared with the unannealed one [Figs. 1(a) and 1(b)]. The most intense black-metallic luster was seen for the sample irradiated to the highest fluence of 3 × 10¹⁷ ions/cm² and annealed for 6 h in total at 600 °C in vacuum [Fig. 1(c)].

FIG. 1.

Optical micrographs of the black diamonds produced by 70 keV N-ion implantation: (a) 1 × 10¹⁷ ions/cm² N-ion implantation without postirradiation heat treatment, (b) 1 × 10¹⁷ ions/cm² N-ion implantation followed by postirradiation heat treatment at 600 °C for 2 h in vacuum, (c) 3 × 10¹⁷ ions/cm² N-ion implantation followed by postirradiation heat treatment at 600 °C for 6 h in vacuum, and (d) 1 × 10¹⁷ ions/cm² N-ion implantation followed by heat treatment in air showing the disappearance of the black color, suggesting a removal of the irradiated layer as CO₂ by oxidation (the average depth of the removed layers was estimated to be ∼3.5 μm).

The UV–Vis–NIR spectra showed that the light transmittance for the unirradiated diamond was over 70%, but not 100% because of the absorption through the thickness and the scattering from the faceted diamond (Fig. 2). An absorption peak was observed near 380 nm (∼3.2 eV), not at 5.5 eV of pure diamond, possibly due to the presence of impurity in the tested diamond. Consistent with the optical microscopy observation, the largest reduction in light transmittance was observed for the diamond N-implanted to the highest fluence and annealed, with only about 20% transmittance in the wavelength range from 250 to 850 nm covering the entire range of visible light spectrum. There is an increase in the light transmittance near the wavelength of 825 nm (∼1.5 eV) with an extended tail, suggesting that the bandgap of the diamond is greatly reduced, resembling the absorption characteristics of amorphous semiconductors. However, a complete absorption did not occur because the modified layer is very thin.

FIG. 2.

UV–Vis–NIR spectra shows that the light transmittance for the unirradiated diamond was over 70% and the largest reduction in light transmittance was observed for the diamond with 3 × 10¹⁷ N ions/cm² implantation followed by postirradiation heat treatment at 600 °C for 6 h in vacuum, with only about 20% transmittance in the wavelength range from 250 to 850 nm.

About 25% transmittance was seen for the diamond irradiated to a lower fluence and annealed [Fig. 1(b)], and the least reduction in transmittance (30%) was seen for the diamond irradiated to a lower fluence with no annealing [Fig. 1(a)]. However, when the irradiated diamond was annealed for 2 h at 600 °C in air, the black color disappeared restoring to a hazy transparency [Fig. 1(d)], which was accompanied by a weight loss, suggesting a removal of the irradiated layer as CO₂ by oxidation. Taking the density of the diamond, the surface area, and the weight loss into account, the average depth of the removed layers was estimated to be ∼3.5μm.

The binding state of the implanted nitrogen was analyzed by FTIR spectroscopy. Figure 3 shows the FTIR spectra for the irradiated and annealed diamond. The absorbance peaks at 1282 and 1175 cm⁻¹ are known to originate from the N–N and symmetrical 4N-vacancy vibrational modes, respectively.Reference Prelas, Popovici and Bigelow^5, Reference Collins⁶ A strong absorption peak near 1700 cm⁻¹ may be due to the C=O carbonyl group. Absorption by C≡C or C≡N bond stretch near 2199 cm⁻¹ was also evident, indicating that a bond breakage and reformation had occurred in the diamond carbon network. A broad O–H absorption band appeared between 3643 and 2450 cm⁻¹ with rather well-defined peak near 2505 cm⁻¹ because of moisture absorption on the sensitized diamond surface after the specimen was exposed to the atmosphere. The results indicate that a considerable atomic rearrangement had occurred in the N-ion-implanted diamond with sp¹ and sp² conjugated carbon bonds, which were incorporated with the implanted nitrogen atoms in various bonding states. Although profuse interstitials, vacancies, and substitutional nitrogen atoms may have been present right after ion implantation, Raman spectra indicates that a significant portion of them disappears after postimplantation annealing by thermally activated atomic rearrangement at elevated temperature (600 °C), to be described later in detail. Although a light absorption associated with nitrogen inclusions and nitrogen-vacancy color centers is known to exist, the photoluminescence from such defect centers is related to the yellow light spectrum in the wavelength range of 560–600 nm, as seen in the nitrogen-containing natural diamond. Since the absorption for N-ion-implanted diamond had occurred in the entire range of visible light spectrum, the major cause for the darkening is most likely due to the formation of conjugated bonds. The π-electrons in the conjugated bonds are loosely bound than those in the covalent bonds and thus easily excited by absorbing the visible light spectrum.

FIG. 3.

Fourier transform infrared spectroscopic analysis of the black diamond produced by 1 × 10¹⁷ N ions/cm² implantation followed by heat treatment in 5 × 10⁻⁶ Torr vacuum at 600 °C for 2 h. The absorbance peaks are observed at 1282 and 1175 cm⁻¹, which relate to the characteristics of N–N and symmetrical 4N-vacancy bonds, respectively.

XPS spectra suggest that the as-implanted nitrogen atoms form initially nonstoichiometric C–N compounds (C₄N or C₅N) in view of the N_1s XPS peak appearing mostly near ∼400.3 eV [Fig. 4(a)],Reference Jagielski, Moncoffre, Delichere and Marest^7, Reference Ripalda, Diaz, Roman, Galan, Montero, Goldoni, Baraldi, Lizzit, Comelli and Paolucci⁸ while annealing at 600 °C for 2 h produces a near stoichiometric carbon nitride compound C₃N_3.75 as evidenced by the N_1s peak shifted to ∼398.5 eV [Fig. 4(b)].Reference Jagielski, Moncoffre, Delichere and Marest⁷ There is a report that the N_1s peak at 400.2 eV is due to the presence of substitutional nitrogen in graphite,Reference Ripalda, Diaz, Roman, Galan, Montero, Goldoni, Baraldi, Lizzit, Comelli and Paolucci⁸ and the N_1s peaks at 398.4 and 400.2 eV are attributable to the sp³ and sp² bonds, respectively, according to Ref. Reference Raole, Mukherjee and John9. It has been reported that the number of sp² bonds in a diamond increases slightly with a nitrogen doping.Reference Birrell, Gerbi, Auciello, Gibson, Gruen and Carlisle¹⁰ It is also known that irradiation produces disordered carbon structures on the surface of a diamond,Reference Lee, Hembree, Rao and Mansur¹¹ which is confirmed with the Raman data in this work. During subsequent thermal annealing, a significant portion of the amorphous layer recrystallized to diamond and the dangling bonds created by irradiation reorganized to sp² and sp conjugated bonds availing loosely bound π-electrons.

FIG. 4.

X-ray photoelectron spectroscopy analysis of N_1s in diamond: (a) 1 × 10¹⁷ N ions/cm² implanted diamond shows a broad composite N_1s peak near 400.5 eV, indicating that the implanted nitrogen forms mostly nonstoichiometric C–N compounds (C₄N or C₅N). (b) After annealing at 600 °C for 2 h in vacuum, the composite N_1s peak position is shifted to ∼398.5 eV, suggesting that the C–N compound becomes near stoichiometric (C₃N_3.75).

Figure 5 shows Raman spectra of the N-ion-implanted and postimplantation annealed samples in the wave number range of 1200–1600 cm⁻¹, which is taken with an excitation wavelength of 532 nm. Raman spectra indicate that N-ion implantation produced a completely disordered amorphous phase on the surface. This is expected in considering the high dose used in this experiment, a fluence of 1 × 10¹⁷ N ions/cm², in view of the reported data that showed a complete amorphization at much lower dose although the implanted ion specie and energy are different from this experiment.Reference Hickey, Jones and Elliman^12, Reference Prawer¹³ Nonetheless, in this experiment, it is shown that the color and the electrical conductivity changed further with increasing the fluence to 3 × 10¹⁷ ions/cm², suggesting that further change in structure had occurred with increasing the fluence, although a complete amorphitization may have occurred at a much lower fluence.

FIG. 5.

Raman spectra obtained with an excitation wavelength of 532 nm for the N-ion-implanted and postimplantation annealed samples. 1 × 10¹⁷ N ions/cm² implanted diamond shows completely disordered structure in the range of 1200–1600 cm⁻¹. After annealing at 600 °C for 2 h in vacuum, the first-order crystalline diamond peak appears at 1333 cm⁻¹ and a broad composite peak appears between 1400 and 1500 cm⁻¹.

After annealing at 600 °C, Raman spectra show a well-defined sharp peak at 1333 cm⁻¹, characteristic of sp³ diamond bond,Reference Grimsditch and Ramdas¹⁴ and a broad peak centering near 1450 cm⁻¹, a typical composite peak of D (1350 cm⁻¹) and G (1550 cm⁻¹) lines, which was often observed for the diamond-like carbon prepared by chemical-vapor-deposition method.Reference Lee, Hembree, Rao and Mansur¹¹ In spite of the implanted nitrogen, the presence of nitrogen-specific Raman peak is not apparent. However, the range of the composite peak was much wider compared with that observed for a typical disordered carbon, suggesting that the nitrogen inclusion may have modified the vibrational modes. Clearly, further investigation is needed. Surprisingly, the results indicate that once completely disordered structure rearranged to a mixture of good quality diamond and diamond-like carbon, which are connected by sp, sp², and sp³ bonds. It is thus believed again that the dark color of the irradiated diamond is related largely to the absorption in the visible light spectrum as a result of the excitation of the loosely bound π-electrons in the sp² and sp conjugated bonds. The enhanced metallic luster observed in the irradiated diamond can be related to the light scattering by the conduction electrons in the sp² and sp orbitals. The electrical resistivity of pure diamond is known to be in the range of 10¹²–10¹⁶ Ω-cm and that of graphite is in the range of 10⁻⁴–10⁻³ Ω-cm along the basal plane and about 0.2 Ω-cm in the direction perpendicular to the basal plane. A cursory measurement of sheet resistance indicates that the electrical resistivity of the irradiated and annealed diamond is about 5 × 10⁻³ Ω-cm (500 Ω/□ for 1000 Å thick layer), close to a value of highly doped silicon. This result indicates that there is a high concentration of free electrons in the irradiated and annealed diamond, rendering metallic luster.

Colored diamonds are often produced by exposing to ionizing radiation such as gamma rays. However, the color change associated with point defects is temporal and disappears with aging and more rapidly upon thermal exposure. However, the black color produced by ion implantation appears to be permanent in view of the enhanced color by annealing at elevated temperature as seen in this experiment, and the ion-implantation-induced color did not fade away for over 7 years at an ambient temperature in our experiment. It is evident that the introduction of nitrogen into a diamond produces color centers rendering yellow tint, which is overshadowed by the absorption associated with other radiation-induced defects. Nitrogen ion implantation did not produce the color change in cubic zirconia (E _g = 7.8 eV), suggesting that of importance is the modification of carbon network structure with unsaturated conjugated bonds, namely sp and sp² bonds, which is absent in zirconia. There has been a report of color change to black for a diamond subjected to very high temperature above 1000 °C, mainly due to graphitization,Reference Willems, De Corte and Van Tendeloo^1, Reference Chen, Sullivan, Friedmann and Gibson¹⁵ but such phenomena did not occur at 600 °C.