Effects of High-Quality Carbon Nanowalls Ionization-Assisting Substrates on Surface-Assisted Laser Desorption/Ionization Mass Spectrometry Performance

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is performed using carbon nanowalls (CNWs) for ionization-assisting substrates. The CNWs (referred to as high-quality CNWs) in the present study were grown using a radical-injection plasma-enhanced chemical vapor deposition (RI-PECVD) system with the addition of oxygen in a mixture of CH4 and H2 gases. High-quality CNWs were different with respect to crystallinity and C–OH groups, while showing similar wall-to-wall distances and a wettability comparable to CNWs (referred to as normal CNWs) grown without O2. The efficiency of SALDI was tested with both parameters of ion intensity and fragmental efficiency (survival yield (SY)) using N-benzylpyridinuim chloride (N-BP-CI). At a laser fluence of 4 mJ/cm2, normal CNWs had an SY of 0.97 and an ion intensity of 0.13, while 5-sccm-O2– high-quality CNWs had an SY of 0.89 and an ion intensity of 2.55. As a result, the sensitivity for the detection of low-molecular-weight analytes was improved with the high-quality CNWs compared to the normal CNWs, while an SY of 0.89 was maintained at a low laser fluence of 4 mJ/cm2. SALDI-MS measurements available with the high-quality CNWs ionization-assisting substrate provided high ionization and SY values.

Normal CNWs were grown under the following conditions. During plasma ge tion, 100 sccm of CH4 and 50 sccm of H2 gases were introduced into the CCP and chambers, respectively. The deposition time for CNW growth was 290 s.
High-quality CNWs were grown through the addition of O2 gas and with the growth conditions as those for the normal CNWs. A small amount of oxygen was ad aiming for no change in wall density, because the wall density of CNWs affects SA performance [10]. High-quality CNWs were grown with a deposition time of 320 s f oxygen flow rate of 5 sccm and 380 s for an oxygen flow rate of 10 sccm to achieve a C height of approximately 500 nm. The crystallinity of the CNWs was previously rep to be improved when O2 gas with a flow rate of 5 or 10 sccm was introduced into the source [29,30].
In the present experiment, CNWs that were at least 14 days old after growth used because it has been reported that the CNW edges initially contain many defect the surfaces become highly hydrophobic over time with inert and stable terminations  Normal CNWs were grown under the following conditions. During plasma generation, 100 sccm of CH 4 and 50 sccm of H 2 gases were introduced into the CCP and SWP chambers, respectively. The deposition time for CNW growth was 290 s.
High-quality CNWs were grown through the addition of O 2 gas and with the same growth conditions as those for the normal CNWs. A small amount of oxygen was added, aiming for no change in wall density, because the wall density of CNWs affects SALDI performance [10]. High-quality CNWs were grown with a deposition time of 320 s for an oxygen flow rate of 5 sccm and 380 s for an oxygen flow rate of 10 sccm to achieve a CNW height of approximately 500 nm. The crystallinity of the CNWs was previously reported to be improved when O 2 gas with a flow rate of 5 or 10 sccm was introduced into the CCP source [29,30].
In the present experiment, CNWs that were at least 14 days old after growth were used because it has been reported that the CNW edges initially contain many defects and the surfaces become highly hydrophobic over time with inert and stable terminations [33].
The morphology of the CNWs was observed using scanning electron microscopy (SEM; Hitachi High Technologies Corporation, Tokyo, Japan, SU8230). The average wallto-wall distance was defined as the length of a straight line divided by the number of walls crossing the line in the top view images. Raman spectra of the CNWs were acquired using a laser with a wavelength of 532 nm (Renishaw plc, Wotton-under-Edge, UK, inVia Raman). The chemical bonding states of the CNW surfaces in the C1s and O1s regions were analyzed using X-ray photoelectron spectroscopy (XPS; Vacuum Generator Scientific, Waltham, MA, USA, ESCAlab 250i). The wettability of the CNWs was investigated with contact angle measurements using three different solutions of deionized water (Milli-Q), methylene iodide (Kishida Chemical Co. Ltd., Osaka, Japan, 000-24382), and ethylene glycol (Kishida Chemical Co. Ltd., Osaka, Japan, 000-29332). The volume of the solution droplets was set to 1 mL.

CNWs-SALDI-TOF-MS
Laser light was provided by the fourth harmonic wave with a wavelength of 266 nm from a Nd:YAG laser (repetition rate: 30 Hz, pulse width: 2 ns; Spectra-Physics, Quanta-Ray Pro 250). The laser light was focused on the CNWs/Si sample surface located at the ion collection point of a time-of-flight mass spectrometer system (TOF-MS; Toyama Co., Ltd., Kanagawa, Japan) [9,10]. The background pressure of the ionization chamber was kept below 10 −7 Pa. The ion signals were detected in positive-ion mode using a microchannel plate (MCP) with a high voltage of 2 kV applied. The incident laser fluences were set to 4, 8, and 12 mJ/cm 2 for the detection of N-BP-Cl. Details of CNWs-SALDI-TOF-MS have been previously published [9,10].
N-BP-Cl (C 12 H 12 ClN; Alfa Chemistry, Stony Brook, NY, USA, ACM2876133, 204 Da) was used to evaluate the degree of fragmentation in the desorption/ionization process [7,10,13]. N-BP-Cl was dissolved in methanol (Fuji Film Wako Pure Chemical Corporation, Osaka, Japan, 138-06473). The concentration was adjusted to 0.1 mM with reference to previous literature [7,10]. A total of 5 µL of the N-BP-Cl solution was then dropped onto the CNW ionization-assisting substrate and left to dry in the ambient air. The SY of N-BP-Cl is defined as: where I M and I F are the signal intensities for benzylpyridinium ions ((BP) + ) with m/z of 170, and benzylium ions ((C 7 H 7 ) + ) with m/z of 91, respectively [7,10,13]. When (BP) + is desorbed/ionized, the C-N bond between the benzyl and pyridine is simply cleaved, producing fragment ions. For more details, please refer to the publication on the SY method [19].

Physicochemical Observation of the Normal and High-Quality CNWs
The morphology of the CNWs was observed by acquiring top and cross-sectional SEM images, as shown in Figure 2. Figure 2a shows the morphology of the normal CNWs. The wall-to-wall distance for normal CNWs was 142 ± 3.7 nm. Figure 2b,c show the morphology of the high-quality CNWs, where the wall-to-wall distances for the CNWs were 152 ± 6.2 nm for an oxygen flow rate of 5 sccm and 157 ± 6.6 nm for an oxygen flow rate of 10 sccm. The height of all CNWs was approximately 500 nm, as shown in Figure 2d Figure 3 shows a series of Raman spectra that were normalized with respect to the peak intensity of the 1350 cm − 1 D-band. The observed Raman peaks were identified as being due to graphene materials. The peaks at 1586 cm − 1 were attributed to the six-membered ring structure of graphene, i.e., the G-band. The D-band was due to disorders and defects of the six-membered ring structures. The peak at 1620 cm − 1 was assigned as the D'-band, which is related to the finite size of the graphite crystallites and their edges. If the peak intensities for the D-and D'-bands are large with respect to the G band, then the CNWs are assumed to have defects in the six-membered ring structures and graphene edges. In addition, it has been reported that graphene-based materials with a large amount of graphene edges have a 2D-band at approximately 2690 cm − 1 , a D+G-band at approximately 2940 cm − 1 , and a 2G-band at approximately 3200 cm − 1 . The results indicated that all the CNWs had almost the same characteristics, regardless of the introduction of oxygen [27].  Figure 3 shows a series of Raman spectra that were normalized with respect to the peak intensity of the 1350 cm − 1 D-band. The observed Raman peaks were identified as being due to graphene materials. The peaks at 1586 cm − 1 were attributed to the six-membered ring structure of graphene, i.e., the G-band. The D-band was due to disorders and defects of the six-membered ring structures. The peak at 1620 cm − 1 was assigned as the D'-band, which is related to the finite size of the graphite crystallites and their edges. If the peak intensities for the D-and D'-bands are large with respect to the G band, then the CNWs are assumed to have defects in the six-membered ring structures and graphene edges. In addition, it has been reported that graphene-based materials with a large amount of graphene edges have a 2D-band at approximately 2690 cm − 1 , a D+G-band at approximately 2940 cm − 1 , and a 2G-band at approximately 3200 cm − 1 . The results indicated that all the CNWs had almost the same characteristics, regardless of the introduction of oxygen [27]. Figure 4 shows peak area intensity ratios for the D-band to G-band peaks (I D /I G ) and those for the D'-band to G-band peaks (I D' /I G ). For example, I D /I G were 2.94, 2.75, and 2.69 with O 2 flow rates of 0, 5, and 10 sccm, respectively. I D /I G and I D' /I G decreased when the O 2 flow rate was increased, which indicates that the addition of oxygen allowed the growth of more crystalline CNWs, as previously reported [29,30]. The addition of O 2 gas was assumed to decrease amorphous carbon and nucleation during the initial growth of the CNWs. In addition, CNWs grown with oxygen in this study (high-quality CNWs) may have improved the electrical conductivity and formed monolithic graphene sheets, similar to CNWs grown with oxygen, as previously reported [30]. Figure 5 shows (I) C1s and (II) O1s XPS spectra of the CNWs. The C1s and O1s spectra were normalized with respect to the peak intensities for sp 2 C-C bonds at 284.6 eV. CNWs grown at O 2 flow rates of (a) 0, (b) 5, and (c) 10 sccm are shown in Figure 5. The C1s peak was decomposed into three components centered at 284.6, 285.5, and 286.9 eV ( Figure 5I). The main C1s peak at 284.6 eV corresponded to sp 2 C-C bonds of graphite [27,34]. The peak centered at 285.5 eV originated from sp 3 -bonded carbon atoms (sp 3 C-C), and the peak at 286.9 eV was due to C-O-C bonds [34]. The O1s peak was also observed regardless of the oxygen flow rate ( Figure 5II). It has been reported that O1s is formed by the C-OH Nanomaterials 2023, 13, 63 6 of 14 peak at 532.4 eV and the C=O peak at 530.2 eV [27]. All CNWs exhibited surface-bonding states of typical CNWs [27,34]. The XPS survey spectra was shown in Figure S1 in the Supplementary Materials.  Figure 4 shows peak area intensity ratios for the D-band to G-band peaks (ID/IG) and those for the D'-band to G-band peaks (ID'/IG). For example, ID/IG were 2.94, 2.75, and 2.69 with O2 flow rates of 0, 5, and 10 sccm, respectively. ID/IG and ID'/IG decreased when the O2 flow rate was increased, which indicates that the addition of oxygen allowed the growth of more crystalline CNWs, as previously reported [29,30]. The addition of O2 gas was assumed to decrease amorphous carbon and nucleation during the initial growth of the CNWs. In addition, CNWs grown with oxygen in this study (high-quality CNWs) may have improved the electrical conductivity and formed monolithic graphene sheets, similar to CNWs grown with oxygen, as previously reported [30].   Figure 4 shows peak area intensity ratios for the D-band to G-band peaks (ID/IG) and those for the D'-band to G-band peaks (ID'/IG). For example, ID/IG were 2.94, 2.75, and 2.69 with O2 flow rates of 0, 5, and 10 sccm, respectively. ID/IG and ID'/IG decreased when the O2 flow rate was increased, which indicates that the addition of oxygen allowed the growth of more crystalline CNWs, as previously reported [29,30]. The addition of O2 gas was assumed to decrease amorphous carbon and nucleation during the initial growth of the CNWs. In addition, CNWs grown with oxygen in this study (high-quality CNWs) may have improved the electrical conductivity and formed monolithic graphene sheets, similar to CNWs grown with oxygen, as previously reported [30].  peak centered at 285.5 eV originated from sp 3 -bonded carbon atoms (sp 3 C-C), and the peak at 286.9 eV was due to C-O-C bonds [34]. The O1s peak was also observed regardless of the oxygen flow rate (Figure 5 II). It has been reported that O1s is formed by the C-OH peak at 532.4 eV and the C=O peak at 530.2 eV [27]. All CNWs exhibited surface-bonding states of typical CNWs [27,34]. The XPS survey spectra was shown in Figure S1 in the Supplementary Materials. Figure 5. (I) C1s and (II) O1s XPS spectra of (a) normal CNWs, (b) 5-sccm-O2-, and (c) 10-sccm-O2high-quality CNWs. The C1s spectra and O1s spectra were normalized with respect to the peak intensities for sp 2 C-C bonds at 284.6 eV. Figure 6 shows the peak area ratios of the 284.6 eV peak to C1s, 285.5 eV peak to C1s, 286.9 eV peak to C1s, and O1s to C1s for the CNWs. The ratios of the 284.6 eV XPS peak area to the total peak area of C1s and the total O1s area to the total peak area of C1s increased with the oxygen flow rate. The ratio of the 285.5 eV peak area to the total peak area of C1s decreased with an increase in the oxygen flow rate. The ratio of the 286.9 eV peak area to the total peak area of C1s showed no dependence on the oxygen flow rate. These results indicate that sp 2 C-C bonds at 284.6 eV and C-OH at 532.4 eV increased instead of decreasing the sp 3 C-C bond. Thus, graphene sheets forming the high-quality CNWs were more graphene-like, i.e., the high-quality CNWs had high crystallinity. The C1s spectra and O1s spectra were normalized with respect to the peak intensities for sp 2 C-C bonds at 284.6 eV. Figure 6 shows the peak area ratios of the 284.6 eV peak to C1s, 285.5 eV peak to C1s, 286.9 eV peak to C1s, and O1s to C1s for the CNWs. The ratios of the 284.6 eV XPS peak area to the total peak area of C1s and the total O1s area to the total peak area of C1s increased with the oxygen flow rate. The ratio of the 285.5 eV peak area to the total peak area of C1s decreased with an increase in the oxygen flow rate. The ratio of the 286.9 eV peak area to the total peak area of C1s showed no dependence on the oxygen flow rate. These results indicate that sp 2 C-C bonds at 284.6 eV and C-OH at 532.4 eV increased instead of decreasing the sp 3 C-C bond. Thus, graphene sheets forming the high-quality CNWs were more graphene-like, i.e., the high-quality CNWs had high crystallinity.          Figure 8 shows the contact angles for each droplet on CNW surfaces grown at different oxygen flow rates. Contact angles were similarly obtained for deionized water, ethylene glycol, and methylene iodide on CNW surfaces grown at various oxygen flow rates. The results indicate that the wettability of the CNW surfaces grown with all oxygen flow rates was not significantly affected by the crystallinity. As described in Section 2.1, the surface condition of the CNWs was stable and hydrophobic because the CNWs were more than 14 days old, which was consistent with previous research [33]. It has been reported that differences in wettability affect the sensitivity for detection [9]. This suggests that the addition of oxygen does not affect wettability, i.e., it does not affect the SALDI sensitivity. Figure 9 shows the SALDI mass spectra of N-BP-Cl on high-quality CNWs ionizationassisting substrates. The laser fluences were set at 4, 8, and 12 mJ/cm 2 . All spectra were normalized and shown with respect to the intensity at m/z = 170. Two characteristic signals related to N-BP-Cl were observed. One indicated (BP) + ions at m/z = 170 and the other (C 7 H 7 ) + ions at m/z = 91 [7,10,13]. In addition, relatively small peaks due to pyridine ions ((C 5 H 5 N) + ) also appeared at m/z = 79 in some spectra [10]. For the high-quality CNWs, the (C 7 H 7 ) + ions peak at m/z = 91, which corresponded to fragmentation, became larger. On the other hand, the spectra of high-quality CNWs (Figure 9b,c) at 4 mJ/cm 2 were more clearly observed than in the case of normal CNWs (Figure 9a). ethylene glycol, and methylene iodide on CNW surfaces grown at various oxygen flow rates. The results indicate that the wettability of the CNW surfaces grown with all oxygen flow rates was not significantly affected by the crystallinity. As described in Section 2.1, the surface condition of the CNWs was stable and hydrophobic because the CNWs were more than 14 days old, which was consistent with previous research [33]. It has been reported that differences in wettability affect the sensitivity for detection [9]. This suggests that the addition of oxygen does not affect wettability, i.e., it does not affect the SALDI sensitivity.  Figure 9 shows the SALDI mass spectra of N-BP-Cl on high-quality CNWs ionization-assisting substrates. The laser fluences were set at 4, 8, and 12 mJ/cm 2 . All spectra were normalized and shown with respect to the intensity at m/z = 170. Two characteristic signals related to N-BP-Cl were observed. One indicated (BP) + ions at m/z = 170 and the other (C7H7) + ions at m/z = 91 [7,10,13]. In addition, relatively small peaks due to pyridine ions ((C5H5N) + ) also appeared at m/z = 79 in some spectra [10]. For the high-quality CNWs, the (C7H7) + ions peak at m/z = 91, which corresponded to fragmentation, became larger. On the other hand, the spectra of high-quality CNWs (Figure 9b,c) at 4 mJ/cm 2 were more clearly observed than in the case of normal CNWs (Figure 9a).  Figure 10 shows the ion intensities for N-BP-Cl on the normal and high-quality CNWs ionization-assisting substrates as a function of the laser fluence. The ion intensity was defined as the sum of the peak intensities at m/z = 170 and 91. The ion intensities measured using the CNW ionization-assisting substrates increased with the laser fluence. At a low laser fluence of 4 mJ/cm 2 , the ion intensity for the high-quality CNWs was clearly higher than that for the normal CNWs.

SALDI performance of the high-quality CNWs
The rate of increase in ion intensity as a function of the laser fluence was different for  Figure 10 shows the ion intensities for N-BP-Cl on the normal and high-quality CNWs ionization-assisting substrates as a function of the laser fluence. The ion intensity was defined as the sum of the peak intensities at m/z = 170 and 91. The ion intensities measured using the CNW ionization-assisting substrates increased with the laser fluence. At a low laser fluence of 4 mJ/cm 2 , the ion intensity for the high-quality CNWs was clearly higher than that for the normal CNWs. Figure 11(I) shows SY values for N-BP-Cl on normal and high-quality CN tion-assisting substrates as a function of the ion intensity. For each CNW sub increase in the ion intensity corresponded to the laser fluences of 4, 8, and 12 that order. Here, the SY values for the normal CNWs were retrieved from the p [10]. For example, in the case of a laser fluence of 4 mJ/cm 2 (the lowest ion in each CNW substrate), the average SY values were 0.97, 0.89, and 0.88 for the zation-assisting substrates with normal CNWs, 5-sccm-O2-, and 10-sccm-O2-h CNWs, respectively. The SY values for the high-quality CNWs decreased as t flow increased. Figure 11(II) shows the SY values for N-BP-Cl on high-quality CNW ion sisting substrates as a function of laser fluence. Here, the results of the previous the SY of the present study are compared. The SY value for SiNWs was repo approximately 0.8 under a laser irradiation of 4 mJ/cm 2 [4]. An SY value of 0.97 PtNFs with a citrate buffer was also reported at a laser fluence of approximately [14]. We also reported a high SY value of 0.97 for normal CNWs, which is close lowest laser fluence of 4 mJ/cm 2 [10]. Compared to these reported high SY val values of 0.89 and 0.88 for 5-sccm-O2-and 10-sccm-O2-high-quality CNWs at ence of 4 mJ/cm 2 were comparable. High-quality CNW ionization-assisting grown with oxygen also offered the same two advantages as normal CNW assisting substrates grown without oxygen, as described in Chapter 1. This mea electric field at the graphene edge of CNWs with optimized wall-to-wall dist The rate of increase in ion intensity as a function of the laser fluence was different for normal CNWs grown without O 2 and high-quality CNWs grown with O 2 . As explained in Section 3.1, physicochemical observations of high-quality and normal CNWs showed that the main difference was the crystallinity and the C-OH groups. Therefore, from these results, it can be assumed that the crystallinity and the C-OH bond of the CNWs had an influence.
Why did high-quality CNWs with high crystallinity and C-OH groups enhance the ion intensity? One mechanism could be that the monolithic graphene sheets that form the high-quality CNWs contributed to a further enhancement of the electric field on the CNWs' edge. We have previously indicated that the factor influencing ionization of the measured molecules on CNWs is the electric field concentration at the graphene edges of the CNWs [10]. The O radicals produced during the growth of high-quality CNWs etch the amorphous carbon and suppress branching of the CNWs, which allows for higher crystallinity and electrical conduction [29,30]. The more graphene-like, less branched CNWs, i.e., high-quality CNWs, can enhance the electric field on the edges compared to the normal CNWs, which contributes to the improved ion intensity.
Another mechanism could be the C-OH groups on the surface of high-quality CNWs. It has been reported that the signal was enhanced in oxidized CNTs [6] and GCB [20], mainly due to the increase in C-OH groups. However, it is not sufficiently clear how the structure and electrical properties of the individual graphene sheets of CNWs differ between the normal and high-quality CNWs. More detailed studies, such as using X-ray diffraction, are, thus, required to clarify how these features affect the ionization properties. Figure 11(I) shows SY values for N-BP-Cl on normal and high-quality CNW ionizationassisting substrates as a function of the ion intensity. For each CNW substrate, the increase in the ion intensity corresponded to the laser fluences of 4, 8, and 12 mJ/cm 2 , in that order. Here, the SY values for the normal CNWs were retrieved from the publication [10]. For example, in the case of a laser fluence of 4 mJ/cm 2 (the lowest ion intensity for each CNW substrate), the average SY values were 0.97, 0.89, and 0.88 for the CNW ionization-assisting substrates with normal CNWs, 5-sccm-O 2 -, and 10-sccm-O 2 -high-quality CNWs, respectively. The SY values for the high-quality CNWs decreased as the oxygen flow increased. Nanomaterials 2022, 12, x FOR PEER REVIEW 12 of 14 moderate crystallinity clearly increased the ion intensity while softly ionizing the N-BP-Cl. High-quality CNW ionization-assisting substrates could, thus, be available for massspectrometry analysis of samples, such as biomolecules, while maintaining high soft ionization ability. Figure 11. SY values for N-BP-Cl on high-quality CNWs ionization-assisting substrates as a function of (I) the ion intensity and (II) laser fluence: (a) normal CNWs, (b) 5-sccm-O2-, and (c) 10-sccm-O2high-quality CNWs. For each CNW sample, the increase in the ion intensity corresponds to laser fluences of 4, 8, and 12 mJ/cm 2 , in that order. SY values of (a) normal CNWs [10], SiNWs [4], FDTS-PtNFs with a citrate buffer [14], and CNTs [13] are cited for comparison, respectively.

Conclusions
The laser desorption/ionization properties of high-quality CNWs grown by the introduction of oxygen were investigated in detail. The high-quality CNWs showed high crystallinity and an increase in C-OH groups, while showing wall-to-wall distances and wettability comparable to normal CNWs grown without oxygen. At a laser fluence of 4 mJ/cm 2 , the normal CNWs had an SY value of 0.97 and ion intensity of 0.13, while 5-sccm-O2-high-quality CNWs had an SY value of 0.89 and ion intensity of 2.55. The SY values were decreased for the high-quality CNWs compared to normal CNWs, but the ion intensity was increased. The results presented here indicate that high-quality CNWs had higher ionization efficiency while suppressing the fragmentation in desorption/ionization, i.e., they exhibited high SY values.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.  [10], SiNWs [4], FDTS-PtNFs with a citrate buffer [14], and CNTs [13] are cited for comparison, respectively. Figure 11(II) shows the SY values for N-BP-Cl on high-quality CNW ionizationassisting substrates as a function of laser fluence. Here, the results of the previous study and the SY of the present study are compared. The SY value for SiNWs was reported to be approximately 0.8 under a laser irradiation of 4 mJ/cm 2 [4]. An SY value of 0.97 for FDTS-PtNFs with a citrate buffer was also reported at a laser fluence of approximately 10 mJ/cm 2 [14]. We also reported a high SY value of 0.97 for normal CNWs, which is close to 1, at the lowest laser fluence of 4 mJ/cm 2 [10]. Compared to these reported high SY values, the SY values of 0.89 and 0.88 for 5-sccm-O 2 -and 10-sccm-O 2 -high-quality CNWs at a laser fluence of 4 mJ/cm 2 were comparable. High-quality CNW ionization-assisting substrates grown with oxygen also offered the same two advantages as normal CNW ionization-assisting substrates grown without oxygen, as described in Chapter 1. This means that the electric field at the graphene edge of CNWs with optimized wall-to-wall distances may cause more efficient desorption/ionization, as discussed in the publication [10].
As a result, at a laser fluence of 4 mJ/cm 2 (the lowest value of ion intensity shown in Figure 11(I)), the SY value for the high-quality CNWs decreased compared to that for the normal CNWs, but the ion intensity increased. For example, at a laser fluence of 4 mJ/cm 2 , the normal CNWs had an SY value of 0.97 and an ion intensity of 0.13, while the 5-sccm-O 2 -high-quality CNWs had an SY value of 0.89 and an ion intensity of 2.55. This indicates that the higher crystallinity of CNWs with the same surface morphology can increase the ionization intensity, while having a slightly negative effect on the soft ionization properties, such as the SY. It was assumed that excessive crystallinity results in the formation of a strong electric field that assists ionization. This means that the high-quality CNWs with moderate crystallinity clearly increased the ion intensity while softly ionizing the N-BP-Cl. High-quality CNW ionization-assisting substrates could, thus, be available for mass-spectrometry analysis of samples, such as biomolecules, while maintaining high soft ionization ability.

Conclusions
The laser desorption/ionization properties of high-quality CNWs grown by the introduction of oxygen were investigated in detail. The high-quality CNWs showed high crystallinity and an increase in C-OH groups, while showing wall-to-wall distances and wettability comparable to normal CNWs grown without oxygen. At a laser fluence of 4 mJ/cm 2 , the normal CNWs had an SY value of 0.97 and ion intensity of 0.13, while 5-sccm-O 2 -high-quality CNWs had an SY value of 0.89 and ion intensity of 2.55. The SY values were decreased for the high-quality CNWs compared to normal CNWs, but the ion intensity was increased. The results presented here indicate that high-quality CNWs had higher ionization efficiency while suppressing the fragmentation in desorption/ionization, i.e., they exhibited high SY values.