Direct Laser Writing of Nanodiamond Films from Graphite under Ambient Conditions
Qiong Nian1, Yuefeng Wang2, Yingling Yang1, Ji Li1, Martin Y. Zhang1, Jiayi Shao3,
Liang Tang3, Gary J. Cheng1,4,5+
1School of Industrial Engineering, Purdue University, West Lafayette, IN, USA.
2School of Materials Engineering, Purdue University, West Lafayette, IN, USA.
3 Department of Physics, Purdue University, West Lafayette, IN, USA.
4School of Mechanical Engineering, Purdue University, West Lafayette, IN, USA.
5 Birck nanotechnology Center, Purdue University, West Lafayette, IN, USA.
+ To whom correspondence should be addressed. E-mail:
SF1
Supplementary figure 1, Sheet resistance of carbon film as a function of distance between the transparent confinement and the backing plate.The error bars show the standard deviations of electrical resistance.The solid line is anexponential fitting of the average sheet resistance. Laser intensity: 2 GW/cm2.
SF2
Supplementary figure 2, Optical transmission spectra of carbon films with differentspace distances between the transparent confinement and thebacking plate. Laser intensity: 2 GW/cm2.
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Supplementary figure 3,Optical imagesof transparent carbon spots with good conductivity (3).
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Supplementary figure 4, Representative Raman spectra of amorphous carbon films (a) and nanodiamond films (b). Excitation wavelength is 568.27 nm.Peak positions were determined by fitting the spectra with Lorentzian and a linear background. The black lines are the experimental results, and the color lines areLorentzian fits for Raman bands.
SF5
Supplementary figure 5, Raman data confirm the transition to diamond phase carbon during CPLD. (a) Raman peakposition vs. Laser intensity in the 1300-1400 cm-1 spectral region (D peak for graphite phase carbon and E peak for diamond phase carbon). Sheet resistances of carbon filmsare also shown here as reference. (b) Intensity ratio of I(D)/I(G), I(E)/I(G)) and peak position of H Raman band as a function of laser intensity. The intensity ratios are defined as peak height ratios. The vertical dashed line indicates the critical laser intensity for phase transition.
SF6
Supplementary figure 6, Representative Raman spectra of carbon film deposited by carbon sputter and processed by CPLD. The blue curve, red curve and cyan curve represent the untreated carbon film, processed by 1.7 GW/cm2 and 5.4 GW/cm2, respectively.
Figure 1 shows the sheet resistance of carbon film as a function of distance between the transparent confinement and the backing plate. To author’s best knowledge, electrical property change has been intensively utilized to characterize the graphite-nanodiamond transformation1-3. Resistivity increase, especially in several orders, plays a significant role to indicate the chemical bond and phase change between graphite/amorphous carbon and nanodiamond. Figure 2 and Figure 3 supply a supplementary proof of optical transmittance change of carbon film after CPLD process. Due to low transmission of graphite layer, optical transmittance is able to distinguish nanodiamond phase showing in Figure 2, which has been mentioned before4-6.
As shown in Figure 4 and Figure 5, the transition to diamond phase carbon during confined laser ablation was further confirmed by Raman spectroscopy measurement. The Raman data and optical microscope images were obtained from a TOBIN YVON T64000 confocal laser micro-Raman spectrometer with a 568.27 nm laser.The Raman spectra obtained in the low laser intensity region exhibit different characteristic peaks compared with those in the high laser intensity region (figure 4). When the laser intensity is below the critical value, the major bands lie at around 1560, 1340, and 1240 cm-1, respectively. Generally, the Raman peak around 1560 cm-1 is labeled as G peak, and the Raman peak around 1340 cm-1 is labeled as D peak7,8. Both G and D peaks are common features of amorphous carbonand ascribed to sp2bonds (graphite phase). When the laser intensity is above 3.7 GW/cm2, the Lorentzian fit of Raman spectra reveals several different bands compared with Raman spectra in low laser intensity region. These additional Raman bands center at around 1332 cm-1, 1286 cm-1 and 1490 cm-1, respectively. The broad and faint Raman band around 1332 cm-1 is due to the stretching of sp3 bonds (diamond phase), and usually used to characterize cubic nanodiamond9-12. Due to size confinement effects at nanoscale12, the position of 1332 cm-1band can be downshifted by severalcm-1, and the bandwidth can be as large as tens of cm-1. For convenience, we label this diamond peak as “E” peak, which is close to, but not, the D peak because of different peak positions, bandwidths and behaviors. The Raman peak around 1286 cm-1 is commonly observed in diamond fabricated by various methods and also regarded as a feature associated with the sp3 bonding9,10. The presence of hexagonal diamond is usually responsible for this Raman band9,13,14. We label this Raman peak as H peak. The Raman peak around 1490 cm-1 has been attributed to diamond structure containing sp2 carbon15; this Raman band has also been observed in CVD diamond11. The sharp Raman band at 1240 cm-1, can be assigned to disordered sp3 carbon or hydrogenated sp3 carbon according to theory16,17. This Raman band is weak in the low laser intensity region and strong in the high laser intensity region. The appearance of these modes associatedwith diamond phase carbon suggests the high transform efficiency of the CPLD method. The diamond modes cannot be observed in weakly transformed diamond films under the visible light excitation,because the Raman cross-section of graphite phase is considerably greater than that of diamond phase8,9,15.
Figure 5a shows the relation between the laser intensity and Raman peak positionin the 1300-1400 cm-1 spectral region (D or E peak). A sudden downshift of peak position was clearly observed in the vicinity of the critical laser intensity. Figure 5b plots the intensity ratios of D and E peaks to G peak as a function of laser intensity.I(D)/I(G) shows different trend compared to I(E)/I(G) as the laser intensity increases. Below the critical laser intensity, I(D)/I(G) decreases monotonically with the laser intensity, indicating the reduction of sp2 fraction and cluster size in amorphous carbon film7,8. However, a monotonicly increasing relationship is observed between I(E)/I(G) and the laser intensity in the high intensity region. The reason for these different behaviors is that the 1332 cm-1Raman peak near D peak is the characteristic peak of the diamond phase, and not the defect mode of the graphite phase. Higher laser intensity generates films with higher fraction of sp3bonding18, and D peak will disappear in carbon films with high fraction of sp3bonding8.The peak position of H Raman band vs. the laser intensity is also plotted in Figure 5b. This faint Raman band only appears when laser intensity is higher than 3.7 GW/cm2. As laser intensity increases, the peak position of this band downshifts and peak intensity increases. All these results confirm that there is a transition to diamond phase carbon in the vicinity of 3.7 GW/cm2.
Bearing in mind that the sp2 carbon mainly determines the electrical conductivity and Raman spectra, the films with high fraction of sp3carbon also can show good conductivity if the sp2 carbon forms a connected net in sp3 carbon3. Even for the films with several percent of sp2 carbon, visible Raman spectra will show a strong sp2 Raman signals compared with sp3 signals, because the absorption of sp2carbon is strong in the visible range15. Actually, sp2 carbon material often exists at surfaces or grain boundaries of diamond phase carbon19. The optical transmission spectra (see Supplementary Figure 2, 3)reveal that the optical band gaps for carbon films obtained by CPLD are bigger than 2.7 eV, even for those with good conductivity.According to previous work, amorphous carbon films are usually characterized by an optical band gap in the range of 0.4-0.7 eV. It has been reported that the optical band gap is 2.3 eV for carbon films with high fraction sp3 carbon (96.6%)20.Therefore, we can conclude that there is a significant content of sp3 carbon in these laser processed films, and the phase transition shows up in visible Raman and electrical measurement only when the fraction of sp2carbon is very low.
In order to avoid the impurities induced by the graphite spray method, a carbon sputter was used to coat carbon film on quartz substrate for clean Raman spectrum collection. Figure 6 shows the Raman spectrum of the as-coated sample, CPLD processed sample with 1.7 GW/cm2 and 5.4 GW/cm2, respectively. It is found the D band shifts to around 1336 cm-1 after laser irradiation with intensity of 5.4 GW/cm2 which beyond the critical value mentioned in the manuscript. However, the laser intensity of 1.7 GW/cm2 cannot supply enough thermal energy to activate the carbon ions and then form the nanodiamonds. Thus its D band does not shift much, which is in a good agreement with Figure 4 and Figure 5.
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