Making few-layer graphene photoluminescent by UV ozonation

Ultraviolet (UV) ozonation is employed for making graphene photoluminescent. We find that photoluminescence (PL) varies with the ozonation temperature. For roomtemperature ozonized few-layer graphene (FLG), PL is localized at the edges and in the suspended areas of FLG. At an ozonation temperature of 120 °C, PL localized at the edges of FLG disappears, and the surface of trilayer graphene becomes luminescent. These graphene flakes are topographically and chemically characterized to understand the origin of PL. We propose that sp clusters play a key role in making graphene photoluminescent, and that intact carbon layers and charged impurities at the surface of silicon oxide substrate may quench PL. © 2016 Optical Society of America OCIS codes: (160.2540) Fluorescent and luminescent materials; (240.6670) Surface photochemistry; (250.5230)

Nowadays, PL in solid graphene oxide or photochemically reduced graphene oxide is attracting great attention [17][18][19][20].Tsuchiya et al. realized in situ and non-volatile PL and its tunability in graphene oxide through an electrochemical redox reaction achieved by solid electrolyte thin films [20].Their study provides a feasible solution for applications of graphene based PL in nano-optoelectronic devices.
It is also attractive to find a way to etch large-area graphene films on a certain substrate directly into luminescent nanostructures using a dry method.In earlier studies, high temperature oxidation and plasma treatments with different working gases, including nitrogen, hydrogen, and oxygen, were used to create graphene nanostructures [16,[21][22][23][24]. Gokus et al. reported PL in micro-cleaved monolayer graphene flakes after oxygen plasma treatment [16].In their study, when multilayer graphene was treated, only the topmost layer could be selectively converted by oxygen plasma.As a result, such treated multilayer graphene was not photoluminescent due to PL quenching by intact carbon layers [16].
Ultraviolet (UV) ozonation, a type of photochemical oxidation, provides a clean and lowcost route to functionalizing graphene and graphene oxide in a dry environment [25][26][27][28][29][30][31][32][33][34][35][36].In the past few years, this technique has been used to make nanopores in graphene for selective molecular sieving; it has also been used to improve the properties of optoelectronic devices by removing organic residues and lowering the metal-graphene contact resistance [26][27][28][29][30].In our previous work, we studied the topographical morphology of micro-cleaved graphene after UV ozonation and its disorder-induced electronic properties [31][32][33].Later on, Liu observed the newly emerged O-C = O functional group after UV/ozonation treatment that increases with the disposal temperature [34].Unlike layer-by-layer etching in oxygen plasma, UV ozonation can carve multilayer graphene into nanometer thick structures, and it may provide a solution to realizing its PL emission [16,32].
In this report, we employ a specially designed double-chamber UV ozonation vacuum machine to make graphene photoluminescent and explore the underlying PL mechanism.In the following, we study the ozonation of few-layer graphene flakes (FLG), where we define FLG as flakes composed of more than two layers but less than 10 layers.After ozonation at room temperature, we observe PL originating from the edges of supported FLGs and from the surface of suspended FLG (suspended FLG are not in contact with the silicon dioxide substrate).After ozonation at 120 °C, we observe PL originating from the surface of trilayer graphene, while the edges of FLG are not luminescent any more.Based on our analyses of the topography and surface chemical functional groups, we propose that localized sp 2 clusters play a key role in the PL emission of UV ozonized FLG nanostructures.Intact carbon layers and charged impurities at the surface of silicon dioxide (SiO 2 ) substrate may quench PL.

Experimental method
Graphene flakes were transferred onto Si/SiO 2 (300 nm) substrates (patterned with 120 nm deep micro-pits) by exfoliation of Kish graphite and then annealed at 300 °C for 2 hours in an oven in a flow of inert gas.Various monolayer, bilayer and FLG flakes were selected with an optical microscope by comparing their optical contrast [37].Monolayer and bilayer graphene flakes were further identified using a confocal Raman spectroscope (Horiba Jobin Yvon LamRAM HR 800) (Fig. 7 in Appendix A) [38].
Our home-made double-chamber UV ozonation vacuum machine equipped with a lowpressure mercury (Hg) vapor discharge lamp was used for ozonizing graphene.The lamp has a power of 200 W installed in the outer vacuum chamber (~1 Pa).It emits two major UV lines at 184.9 and 253.7 nm.The samples were put in the inner chamber with an oxygen atmosphere.UV ozonation occurs when light penetrates into the inner chamber and dissolves oxygen molecules into radicals.More about the equipment and graphene ozonation process can be found in Appendix B and Visualization 1.During UV ozonation treatment, distance between the UV lamp and graphene was fixed at 50 mm, and the initial oxygen pressure at 96.3 kPa.The UV ozonation can be controlled by tuning the reaction time and substrate temperature.
For the UV ozonized graphene, PL images were taken with a fluorescence microscope (Olympus BX61) under the excitation of 365 nm line from a high-pressure mercury lamp.A confocal laser scanning microscope (CLSM, Zeiss LSM 510 META) was used to obtain highly resolved PL images under the excitation of 364 nm.For the CLSM, the equipped photomultiplier detection system has the capability of detecting PL emission within the visible wavelength range.A false blue color was chosen and designated for the scanned images under excitation of the 364 nm laser.A different fluorescence spectroscope, also used for confocal Raman spectroscope, was used to collect PL spectra under excitation of the 325 nm laser.An atomic force microscope (AFM, Nanonavi E-Sweep, SII) was used for topographically characterizing graphene.In order to overcome instability coming from probe contamination by severely etched graphene clusters, a scanning electron microscope (SEM, Zeiss Ultra Plus) under 3 kV bias was used to get surface topography images.
X-ray photoelectron spectroscopy (XPS) is a surface analytical technique frequently used in determining chemical functional groups in carbon-related materials [39][40][41][42].A freshly cleaved Kish graphite flake (~6 × 6 mm 2 ) was selected for XPS analyses.Before and after different UV ozonation (individually treated at room-temperature and at 120 °C), their ex situ XPS spectra were collected using a Kratos Axis Ultra DLD spectrometer (equipped with a monochromatic Al Kα X-ray source) in an ultrahigh vacuum condition (~5.0 × 10 −9 Torr).The anode power was fixed at 75 W during XPS measurements to decrease the possibility of inducing possible surface chemical transition.

Surface topography and PL characterization
A flake composed of bi-layer graphene and FLGs was chosen for UV ozonation treatment at room temperature.Figure 1(a) shows its optical image before ozonation.Besides the supported graphene, a suspended six-layer graphene with an area of ~14 μm 2 is formed.After ozonation for 12 min, PL can be observed in the CLSM image with a false blue color as shown in Fig. 1(b).Specifically, PL is localized at the edges and suspended region of FLGs.For bilayer graphene or FLG steps, no PL can be observed in the regions as indicated by red arrows.It is notable that similar PL emission also appears at the edges of graphite flakes after the same UV ozonation treatment (Fig. 9 in Appendix C), even though it is weaker than that in FLGs.Relationship between PL emission and the topographical characteristics of FLGs is studied by mapping SEM images as shown in Figs.1(c)-1(f).Firstly, focusing on the edge of FLG in Fig. 1(c), we can see that it contains more isolated nanodots compared with the central area.By contrast, the nonluminescent step has a completely different topography as shown in Fig. 1(d).Namely, it is composed of some nanofilaments as pointed by the red arrow.These nanofilaments look similar to nanostructures in the central area, and they can also be observed along other FLG steps.Secondly, for the suspended FLG, its topography is different from that of the supported one.Topography in the suspended area is nonuniform.Some aggregated clusters are distributed away from the central area as indicated by the red arrow in Fig. 1(e).Further zooming in this region as shown in Fig. 1(f), we can see that the ozonized FLG in the suspended region has smaller nanofilaments than that supported on SiO 2 substrate.
For UV ozonized graphene, morphology transformation from sp 2 crystalline to sp 3 amorphous structures can be identified by Raman characterization of the same treated monolayer.Figure 2(a) shows the AFM image of a monolayer graphene flake (denoted as 1L) connected with FLG after room-temperature ozonation treatment.Most nanodots appear isolated in monolayer graphene with an average size of ~20 nm.Defect modes (D and D' peaks) appear in the Raman spectrum as shown in Fig. 2(b).The crystallite size, or in another word sp 2 cluster, is calculated to be ~2 nm from the intensity ratio of I(D)/I(G) [32].Therefore, the etched nanodots are composed of sp 2 clusters and sp 3 amorphous structures.Figure 2(c) shows PL spectrum of the room-temperature ozonized FLGs.It features a prominent peak at ~590 nm and a shoulder peak at ~730 nm.PL spectrum is collected through a 15X objective with an excitation spot diameter of ~6 μm.In order to enhance signals within the focused spot, PL emission is collected from an area with a wealth of FLG microflakes (Fig. 10 in Appendix D) that are adjacent to the FLGs in Fig. 1.The broad-band emission may stem from a wide distribution of the crystallite size, consistent with the random characteristic of UV ozonized FLG nanostructures in Fig. 1.The PL emission appeared unstable under excitation of the UV laser at 325 nm in the atmosphere environment.In the PL spectrum measurement, it was severely quenched and could not be detected by the detection system within ten-minute UV excitation.
Different PL phenomena are observed in high-temperature (120 °C) ozonized graphene flakes.After 8 min treatment, we can observe PL appearing inside the red circle of the fluorescence image in Fig. 3(a).Compared to its optical image (inset to Fig. 3(a)), the fluorescence comes from the thinnest part of a trilayer graphene.Meanwhile, no PL appears at the edges of the FLG that connects the trilayer, and neither can it be observed in other graphite flakes.We find that PL emission in the trilayer graphene is weaker than that of the room-temperature ozonized FLGs, and it cannot be detected by the PL spectrum measurement system.In spite of the quenching problem, the low PL intensity should be related to the small content of luminescent materials.
SEM images in Figs.3(b), we can observe more topographical details as shown in Fig. 3(c).In particular, nanodots that are present at the edges of FLG ozonized at room temperature cannot be observed at the edges of FLG ozonized at 120 °C.This comparison indicates that forming certain nanostructures is necessary for obtaining PL at the edges of FLGs.When zooming in on the step highlighted by another red frame in the middle of Fig. 3(b), we cannot discern any residues in most of the ozonized trilayer graphene as shown in Fig. 3(d).Different PL phenomena in ozonized graphene flakes are related to their AFM topographic images.As shown in Fig. 4(a), the luminescent FLG (circled in Fig. 1(a)) is composed of dense nano-filaments after room-temperature ozonation.In the adjacent bilayer graphene, a small number of isolated nanodots appear with a size of ~20 nm.Increasing the ozonation temperature to 120 °C, FLG flake completely turns into nanodots with an average size of ~40 nm as shown in Fig. 4(b).Height distribution profiles at the bottom of the AFM images indicate that the nanostructure depth increases from ~1.5 to ~6.0 nm upon this increasing ozonation temperature.Some nanodots can reach a height of 8 nm.Therefore, the etched profile and depth of graphene nanostructures change upon increasing ozonation temperature.Since the FLG is no more than 5 nm thick before etching, we think it may form clusters during UV ozonation at 120 °C.
The topography of FLGs evolves from nanofilaments to nanodots upon rising ozonation temperature by only 100 °C.This trend is quite different from graphene oxidation in an O 2 /Ar gas atmosphere reported by Liu et al. [21].In their study, topographical etching can only occur when the oxidation temperature is higher than 600 °C for trilayer graphene.Meanwhile, the etched structures appear to be one-layer deep pits (with an aperture of ~220 nm at 600 °C) without any aggregation in trilayer graphene.
The photoluminescent FLGs and graphite flakes appear unstable under excitation of UV lasers.However, they remain stable when stored in a vacuum cabinet or excited by the 365 nm line from a high-pressure mercury lamp.One year later, PL signals could still be detected by the fluorescence microscope.

XPS analyses
PL phenomena in FLG nanostructures are further explored by performing XPS analyses of the surface chemical functional groups as shown in Fig. 5. Here, we use graphite flakes instead of the popularly used chemical vapor deposition (CVD) grown graphene film because of the following two reasons [29,42].Firstly, the ozonation depth in FLG exceeds one layer and it increases upon the ozonation temperature.Secondly, PL also appears at the edges of ozonized graphite flakes.The only C1s peak in the survey spectrum (Fig. 11 in Appendix E) shows a high crystalline quality of the mother Kish graphite, and an extra O1s peak appears after UV ozonation treatment.
The change of O1s peak upon ozonation temperature is obtained from the highly resolved narrow XPS spectra as shown in Fig. 5(a).Compared with the fresh graphite, there indeed emerges a rather weak O1s peak in the room-temperature ozonized flake as plotted in the red curve.This result certifies the presence of a minute amount of oxygen groups.The O1s peak intensity increases with the rising ozonation temperature (120 °C) as indicated by the blue curve in Fig. 5(a).This tendency is consistent with the increasing content of O-C = O functional group in Ref. [34].As a consequence, we think the growing nanodots formed at high temperature contain more oxygen functional groups than those nano-filaments formed at room temperature.This result is also consistent with the increasing ozonation depth at 120 °C in Fig. 4(b).
In order to discriminate the composition of oxygen functional groups, highly resolved narrow XPS spectra (0.48 eV) of C1s are scanned for pristine, room-and high-temperature (120 °C) ozonized graphite flakes as plotted in Fig. 5(b).One prominent common feature is the increase of full width at half maximum (FWHM) from 0.48 eV for pristine graphite to 0.73 eV and then to 0.75 eV for room-and high-temperature ozonized graphite, respectively.The C1s widening originates from graphene transition from crystalline to amorphous structures [39][40][41].The C1s peak for room-temperature ozonized graphite can be fitted with four individual mixed Gaussian-Lorentzian curves (Fig. 5(c)).The two peaks at 284.84 and 285.08 eV represent, respectively, the mixed sp 2 and sp 3 hybridizations of carbon atoms due to crystalline transformation.The content of sp 3 amorphous carbon is almost equal to that of sp 2 clusters.The peaks at 286.3 and 287.1 eV represent the major epoxide (C-O) and carbonyl (-C = O) groups, respectively.The oxygen functional group analyses are consistent with the scenario that epoxy groups are formed during ozonation, and then transferred to carbonyl pairs that lead to breakup of carbon bonds [43].For high-temperature ozonized graphite, the ratio of sp 3 to sp 2 increases to 1.8 as the fitted curves in Fig. 5(d).Meanwhile, epoxide also increases relative to carbonyl groups, and this is consistent with the increasing oxygen adsorption.It should be noted that our XPS results deviate from that published on Ref. [34] which did not observe C-O chemical functional group and obvious widening of C1s peak.This deviation may stem from different photochemical disposal and XPS measurement process.The C1s peak intensity appears to be one order of magnitude stronger than the O1s peak component even for that treated at 120 °C.This tendency obviously deviates from the relatively large O1s intensity of ozonized CVD grown graphene by introducing ozone gas from corona discharge [42].The deviation indicates that UV ozonation, in our case, behaves mainly as an etching process, especially at room temperature.Therefore, sp 3 amorphous carbon stems mainly from bond breakage.Apparently, these results are also different from the previous report by Mulyana et al. that abundant oxygen functional groups were adsorbed on CVD grown graphene before lattice breaking via UV ozonation treatment [29].Such deviation originates from the fact that we used high degree Kish graphite flakes and damaged their crystalline morphology by forming nanostructures in the ozonation treatments.

PL mechanism in UV ozonized FLGs
We have observed specific PL phenomena in room-and high-temperature ozonized graphene and studied their topography and surface chemical functional group properties.PL is localized at the edges and suspended area of room-temperature ozonized FLGs.However, we cannot observe PL in ozonized monolayer or bilayer graphene.PL emissions at the edges or in the suspended area of FLGs are related to the specific nanodot or nanofilament structures, respectively.These ozonized FLGs have low oxygen contents based on XPS analyses.For high-temperature (120 °C) ozonized FLGs, PL localized at the edges disappears.Instead, PL appears at the surface of severely etched trilayer graphene.In this case, nanodot structures form in FLGs with increasing oxygen content due to enhanced ozonation process.Based on these experimental results, we now discuss PL emission and quenching mechanisms for UV ozonized graphene nanostructures.
UV ozonation is a photochemical oxidation process as shown schematically in Fig. 6(a).In this process, oxygen molecules dissolve into radicals under UV radiation and then these radicals react with graphene.During the ozonation process, FLGs or graphite flakes can be oxidized not only from the topside but also from sidewalls.As a consequence, the edges appear to be more severely ozonized as shown by SEM images.Such severely ozonized sidewalls have weaker PL quenching capability than graphene, and this contributes to observation of PL emission at the edges [44].As noticed, PL has not been observed at the edge of FLG after oxygen plasma treatment [16].This difference may originate from the intrinsic etching mechanism of plasma, i.e., a combination of physical ion bombardment and chemical vapour reaction.PL observation in the suspended region is related to a dynamic micro photochemical process: air molecules enclosed in the micro-pits decompose into oxygen radicals and then interact locally with the bottom carbon.This means that PL can be observed in the suspended FLG provided that there is no intact carbon layer left.
We think that forming sp 2 clusters within sp 3 matrix is the key factor to make graphene photoluminescent by UV ozonation treatment as shown in Figs.6(b) and 6(c).As discussed, photoluminescent FLGs are specific nanostructures composed of a mixture of sp 2 and sp 3 clusters after either room-or high-temperature ozonation.In our study, the structural rearrangement, including the size, shape and fraction of sp 2 clusters, rather than variation of the chemical groups plays a dominant role in determining the PL emission process.Our conclusion is consistent with previous reports on differently etched carbon materials in Refs.10, 14.
PL observation is a competition between radiative and nonradiative recombination of excitons.For non-photoluminescent ozonized graphene nanostructures, there exist two major PL quenching factors, i.e., intact carbon layers and charged impurities at the surface of SiO 2 substrate as shown in Figs.6(b) and 6(c), respectively [16].Intact carbon is the major factor that can efficiently quench PL in ozonized graphene nanostructures as shown in Fig. 6(b) [16].Graphene is a good conductive material with high carrier mobility.For photo-excited FLG nanostructures, nonradiative recombination channel between excitons and carbon layer forms by charge transfer [44][45][46][47][48][49][50].The transfer rate increases rapidly with decreasing distance between nanostructures and carbon layer [49,50].For in situ ozonized nanostructures in FLGs, severe quenching occurs since these nanostructures are in direct contact with intact carbon layer.As a result, we cannot observe PL in the central area of ozonized FLGs when there are intact carbon layers left.
The excited e-h pairs in sp 2 clusters can be annihilated through nonradiative recombination with charged impurities at the surface of SiO 2 film through charge transfer as shown in the left part of Fig. 6(c) [51].Nonradiative relaxation pathway of excitons leads to PL quenching in the etched-through graphene nanostructures.As a result, PL has not yet been Second, this system provides a vacuum environment before filling in the working gas, such as oxygen.It can eliminate influence from other gases such as nitrogen, vapor, etc.Furthermore, it can provide an adjustable working gas environment (from vacuum to a pressure high up to 3 atm).UV ozonation can stop immediately through vacuuming the inner chamber.
Third, gas inflow can be individually controlled by four gas mass flow meters.The system is supplied with four gases, including oxygen, nitrogen, argon, and hydrogen.
Finally, the sample holder contains both the heating and water-cooling systems.The substrate temperature can be heated as high as 400 °C.The sample holder can also be adjusted in height.In the experiment, distance between sample and the bottom silica window is kept at 22 mm.
To summarize, this machine can provide a more controllable UV ozonation treatment than the commercial ones by venting and pumping gas quickly, and by controlling the critical parameters individually, including substrate temperature, distance between UV lamp and substrate (working distance), and inflowing gas content.
A visual picture of the UV ozonation vacuum machine and its treatment of graphene and graphite flakes can be found in Visualization 1.

Fig. 1 .
Fig. 1.(a) Optical image of a graphene sheet composed of bilayer and FLG flakes, and (b) its CLSM image excited at 364 nm after room temperature ozonation; SEM images of (c) FLG edge, (d) FLG step (close to the fully suspended FLG region), (e) and the fully suspended FLG region with clusters as indicated by the red arrow and (f) its zoomed in image.Note: a false blue colour was preset for CLSM imaging under excitation of the 364 nm laser.

Fig. 2 .
Fig. 2. (a) AFM image of the room-temperature ozonized monolayer graphene, and (b) its confocal Raman spectrum under excitation of 514 nm laser; (c) PL spectrum collected from the edges of room-temperature ozonized FLGs under the excitation of 325 nm laser.
Figure2(c) shows PL spectrum of the room-temperature ozonized FLGs.It features a prominent peak at ~590 nm and a shoulder peak at ~730 nm.PL spectrum is collected through a 15X objective with an excitation spot diameter of ~6 μm.In order to enhance signals within the focused spot, PL emission is collected from an area with a wealth of FLG microflakes (Fig.10in Appendix D) that are adjacent to the FLGs in Fig.1.The broad-band emission may stem from a wide distribution of the crystallite size, consistent with the random characteristic of UV ozonized FLG nanostructures in Fig.1.The PL emission appeared unstable under excitation of the UV laser at 325 nm in the atmosphere environment.In the PL spectrum measurement, it was severely quenched and could not be detected by the detection system within ten-minute UV excitation.Different PL phenomena are observed in high-temperature (120 °C) ozonized graphene flakes.After 8 min treatment, we can observe PL appearing inside the red circle of the fluorescence image in Fig.3(a).Compared to its optical image (inset to Fig.3(a)), the fluorescence comes from the thinnest part of a trilayer graphene.Meanwhile, no PL appears at the edges of the FLG that connects the trilayer, and neither can it be observed in other graphite flakes.We find that PL emission in the trilayer graphene is weaker than that of the room-temperature ozonized FLGs, and it cannot be detected by the PL spectrum measurement system.In spite of the quenching problem, the low PL intensity should be related to the small content of luminescent materials.SEM images in Figs.3(b)-3(d) reveal topographical difference of the flakes showing distinct PL phenomena at high ozonation temperature.Trilayer graphene is severely etched, and only slight residues are left.However, a large amount of carbon materials still exist in the left FLGs.Zooming in the left-top corner of FLGs as highlighted by a red frame in Fig.3(b), we can observe more topographical details as shown in Fig.3(c).In particular, nanodots that are present at the edges of FLG ozonized at room temperature cannot be observed at the edges of FLG ozonized at 120 °C.This comparison indicates that forming certain nanostructures is necessary for obtaining PL at the edges of FLGs.When zooming in on the step highlighted by another red frame in the middle of Fig.3(b), we cannot discern any residues in most of the ozonized trilayer graphene as shown in Fig.3(d).

Fig. 3 .
Fig. 3. PL characteristics of high-temperature (120 °C) ozonized FLGs flake.(a) Optical image (inset) and its fluorescence microscope image under UV excitation at 365 nm from a high pressure Hg lamp.SEM images of (b) the FLGs flake and its zoom-in picture focusing on (c) edges around the top left corner, and (d) step between FLG and trilayer graphene as highlighted by two red frames in (b).

Fig. 4 .
Fig. 4. AFM images of FLGs ozonized at (a) room temperature, and (b) 120 °C with the initial oxygen pressure fixed at 96.3 kPa.

Fig. 5 .
Fig. 5. Comparison of XPS spectra for freshly cleaved Kish graphite, and room-and hightemperature (120 °C) ozonized ones.XPS highly resolved narrow band (a) O1s and (b) C1s spectra of freshly cleaved, room-and high-temperature ozonized Kish graphite (denoted as Fresh, RT Ozonation, and HT Ozonation in turn in the graphs).Highly resolved C1s spectra of (c) room-temperature and (d) high-temperature ozonized Kish graphite flakes fitted with four mixed Gaussian-Lorentzian curves.CPS stands for count per second.

Fig. 6 .
Fig. 6.(a) Schematically illustrating UV ozonation process of graphene; PL mechanism and two major quenching factors, i.e., (b) intact carbon layer and (c) charged impurities at the SiO 2 surface for room-and high-temperature (denoted as RT and HT) ozonized graphene nanostructures.

Fig. 8 .
Fig. 8. Simplified schematic structure of our home-made double-chamber UV ozonation vacuum machine.