The original sample B. braunii strain (BOT-22) was isolated from the Okinawa prefecture, Japan and is cultivated in University of Tsukuba [17] . This strain is classified race B and produces botryococcene (C₃₄H₅₈) as a main component of hydrocarbons [18] . The B. braunii residue samples were prepared as follows. A mass culture system of B. braunii developed in University of Tsukuba provided culture broth of the BOT-22 strain of B. braunii for this work [11] . The broth of the BOT-22 was concentrated by using PSI as flocculants and dried by sunlight [19] . The dried sample of the B. braunii was soaked into n-hexane for extraction of hydrocarbon oils including a small amount of carotenoids and triacylglycerols. After the extraction, the residual sample was recovered by filtration and then dried again. The obtained materials are plate-like shape with some thickness, and show greenish dark brown color in bulk state.

B. braunii residues were set in a quartz dish and placed into an Electric Gold Furnace instrument produced by Massachusetts Institute of Technology (MIT) Lincoln-Lab. equipping with an Ishikawa temperature controller and a handmade furnace assembled by Dr. Shinichi Ito (University of Tsukuba). 200 V of voltage was applied and uniformly heated the interior of furnace. Carbonization was performed with no argon flow from room temperature to 700˚C. Then the sample was naturally cooled to room temperature. The carbonized sample was obtained in bulk state with no fragments.

We have mentioned in our previous report that the syngas such like hydrogen gas or carbon monoxide gas would be the trigger of the reduction reaction for the formation of polygonal crystal which consists of Fe₃O₄ or α-Fe inside [16] . Figure 1(a) shows the very beginning stage of the formation of polygonal crystal, observed in the bulk B. braunii residues carbonized at 700˚C under argon atmosphere. The octahedral shaped crystals imbedded in the sample are clear evidence that reduction reaction occurred. However, the absence of inert gas flow caused a totally different result during the carbonization. In Figure 1(b), hexagonal disk shaped crystals in different sizes were found on the surface and inside of carbonized B. braunii residues. These samples were also carbonized at 700˚C, only the argon flow was stopped initially. A simple diagram summarized as Figure 1(c) shows the difference of the carbonization condition would cause the different crystal shape.

The hexagonal disks were carefully observed under SEM, as shown in Figure 2. The length of one side of the hexagon ranges from several nanometers to one micrometer. Figure 2(a) is a hexagonal disk with bottom part broken. It is interesting to find the crack of this defect is composed of two straight lines; both are almost parallel to the sides of the original hexagonal disk. In Figure 2(b), the hexagonal disk imbedded perpendicular to the surface of the bulk sample, only half of the hexagon could be observed. We assume this figure indicates that the hexagonal crystal is still under growing process. Most of the hexagonal disks were found like in Figure 2(c) and Figure 2(d), which hexagonal disks solitarily attach on the carbonized sample. Figure SI1 shows a SEM image of the bulk sample with some hexagonal disks adhering on the surface. Some of the hexagonal disks have thickness more than 100 nm (See Figure SI2(a), Figure SI2(b)). Magnification shows a part of the hexagonal disk was separated (Figure SI2(c)). Two hexagonal disks attached to each other were also observed under SEM, as shown in Figure SI2(d) Figure SI2(e). In Figure SI2(d), two hexagonal disks with a very small hexagon were found on the edge of bulk sample, indicating that the crystals are on the process of growth. A similar pattern was also found in Figure SI2(e) that two hexagons with one side length of 500 nm attach to each other. Figure SI2(f) shows a zigzag edge resembling the external form of honeycomb structure and that of double hexagon disks observed in Figure SI2(e). In Figure SI2(g), a hexagonal disk with a defect that three sides are parallel to the side of original hexagonal disk was observed. A crystal with a round hole was found (See Figure SI2(h)). In Figure SI2(i), a hexagon shape-like hole was found on the surface of

the bulk sample.

Figure 3 shows the infrared absorption (IR) spectra of B. braunii residue samples carbonized at 700˚C with argon and no argon flow, respectively. The absorption peak appears around 1559 cm⁻¹ and 1573 cm⁻¹ in the sample treated with argon flow and no argon flow, respectively. This characteristic peak implies the partially formation of graphite inside of the bulk sample. Another prominent peak found around 1041 cm⁻¹ and 1573 cm⁻¹ in the sample carbonized with argon flow and no argon flow, respectively, ascribes to the vibration of Si-O stretching. These results suggest that carbonization at 700˚C with no argon flow can also afford to yield graphite structure and the flocculant polysilicato-iron works as a “glue” in the bulk sample.

X-ray photoelectron spectroscopy (XPS) measurement was performed to reveal the components that consists the carbonized samples. As shown in Figure 4(a), both samples mainly consists of carbon and oxygen which peaks appear around 285 eV (C_1s), 531 eV (O_1s) and 747 eV (O_KLL). A slight amount of potassium was detected for the sample treated with argon flow. Significant differences were found in Figure 4(b). Peak at 56 eV, which due to Fe_3p, could only be found in the sample treated with argon flow. Other peaks (Fe_2p1/2 and Fe_2p3/2, 724 eV and 712 eV) of Fe also could only be found in the sample carbonized under argon atmosphere. Peak of magnesium appears at 51 eV in both samples. However, peaks at 153 eV and 102 eV, each representing Si_2s and Si_2p, respectively, only appeared in the sample carbonized with no argon flow.

The hexagonal disks were analyzed by energy dispersive X-ray spectroscopy to detect the chemical components inside. Figure 5 shows the results measured for entire region of two samples treated in different conditions. In the previous study, we revealed that carbonization under argon atmosphere would provide an environment for reduction reaction, that, a large amount of Fe would be generated, as shown in Figure 5(a). We can obviously find the difference in Figure 5(b) that no significant peak due to Fe could be found in the sample treated with no argon flow. Instead, a large ration of potassium and silicon exist in the sample. These results coincide with the IR and XPS measurement results very well.

Mapping was carried out for a hexagonal disk. The results in Figure 6 indicate that components (C, O, Fe, Si, Mg and K) are uniformly dispersed in the sample. Quantitative analysis results of the hexagonal disk are summarized in Table 1. The average calculated ration of atoms number of Fe consisted in hexagonal disk is higher than those in entire region. The ration of carbon and oxygen has no significant change in the hexagonal disk and entire region. We found that in each point analysis for the hexagonal disks, no matter with the place, the ratio of number of atoms for Mg, Si and Fe, are nearly fixed as 2:3:6. The hexagonal disk consists of more than 52% of carbon atoms and 25% of oxygen atoms. The carbon atoms basically come from the B. braunii cells and oxygen atoms come from B. braunii cells, flocculant, and metallic oxide contained in the culture medium.

The hexagonal disks are not “regular” hexagon, if we look carefully. Hexagonal disks were randomly selected to investigate the morphology. We recorded the lengths of each side of the hexagonal disk, as summarized in Table SI1. Six sides of each hexagonal disk are referred as to “a-f”, respectively, where “a” represents the longest side and others are assigned in the counterclockwise direction. It is worth mentioning that the shortest side of each hexagonal disk can be found only in side “b” and “f”. Moreover, the ratio of the length between the longest and shortest side as calculated, is almost close to 1.5. Three sides “c-e” show almost the same length in each hexagonal disk. The ratio between the longest side and the average length of side “c-e” were calculated and summarized in Table SI1. Surprisingly, this ratio is almost fixed to 1.2 and is unrelated to the size or thickness of the hexagonal disks. Combining the knowledge we learned from all the results, we would like to build a hypothesis that this anisotropic crystal,

consisting of large amount of carbon and oxygen, and small amount of Si, Mg, and Fe, is a carbon material with metals and silicon slightly incorporated. Crystal nucleus was formed at the initial stage during the cooling process. EDS line analysis were performed for a hexagonal disk, as shown in Figure SI3. Carbon, magnesium, silicon and potassium are uniformly dispersed. However, the oxygen and iron dramatically increase the intensity in the hexagonal shape area. We assume this indicates the existence of iron oxide. Fu et al. reported as-synthe- sized α-Fe₂O₃ microflakes in microscale with hexagonal shape by a hydrothermal method [20] .

We assume the micro sized hexagonal disk start from a very small size metal oxide resembling the hexagonal disk. The plausible mechanism is as follows. The 700˚C carbonization temperature is high enough to generate a large amount of syngas such as CO, CO₂ and CH₄ during the carbonization process of B. braunii residues. Absence of argon flow then creates an environment that mixed gases stagnate in the gold furnace. Subsequent cooling process affords metal oxides deposition; meanwhile syngas act as a carbon source and grow in the order resembling the hexagonal disk shape of metal oxide. The deposited metal oxides may play the role like a substrate for the graphite growth just like the role metal substrates play in chemical vapor deposition method for graphene or any other carbon materials growth [21] [22] [23] . Since more than half of the components in the hexagonal disk are carbon, we refer to this novel 2D hexagonal disk as “Carbon Hexagon”. It is worth mentioning that absence of argon flow prevents the reduction reaction of Fe₂O₃, thus, instead of polygonal structure, hexagonal disk were obtained.