Shaping Carbon into Carbon matrices with Tuneable Diameter and Photo luminescent Properties

doi:10.21203/rs.3.rs-3292971/v2

We report on a new form of nanoscale carbon in the shape of hemispherical carbon matrices and with the bonding characteristics of either sp¹ or sp² carbon. The carbon matrices are formed spontaneously at high temperature from organic molecules adsorbed on the surface of colloidal nanoparticles, with the metal core acting simultaneously as catalyst and template for the carbon matrices growth. This new method of synthesis has demonstrated unprecedented levels of control over the carbon matrices formation, as thermally driven Ostwald ripening of the metal core provides for precise control of the carbon matrices diameter, whilst the propensity of metal nanoparticles to self-assemble into super-lattices enables the formation of highly ordered hexagonal arrays of the carbon matrices on different types of surfaces. Significantly, this new form of carbon exhibits photo luminescent activity in the visible and near-IR ranges, with the wavelength of emitted light determined by the length of sp¹ chains which link sp² domains.

Physical sciences/Materials science/Nanoscale materials/Electronic properties and materials

Physical sciences/Materials science/Materials for devices/Electronic devices

Carbon – the most versatile element in the Periodic Table is able to form a wide variety of stable structures including diamond, graphite, fullerenes, nanotubes, onions and amorphous materials, all of which consist of a network of carbon atoms indifferent hybridisation states, sp³, sp²or sp¹, intermixed in different proportions.^1-7 For instance, carbon nanotubes and graphene consist of primarily sp²-hybridised carbon, with a small proportion of sp³- or sp¹-hybridised carbon atoms at sites of defects or along the edges,^8-11 where as nanodiamonds that primarily comprise sp³-hybridised carbon often contain some sp²-hybrised carbon atoms on the surface.¹² Furthermore, due to a sharp nanoscale curvature of some carbon materials, the atoms may exist in intermediate hybridisation states, such as the case of C₆₀ fullerene where carbon is neither sp² nor sp³-hybridised.¹³

Because the electronic and other physical or chemical properties of carbon nanomaterials are strongly dependent on the bonding between the atoms, the development of preparative methods that can deliver nanocarbons with well-defined and controlled states of hybridisation is an important quest. Significant recent advances have been made in the synthesis of high quality graphene, carbon nanotubes and nanodiamonds, but despite substantial effort, the form of sp¹-hybridised carbon, so called carbyne, still remains elusive. The atomic chain of sp¹-carbon atoms is considered to be the ultimate nanowire, i.e. mechanically robust,¹⁴highly conducting^15-17 and with tuneable electronic properties.^15-17 Isolated carbon chains have been shown to be stable in vacuum inside the column of a TEM or encapsulated within carbon nanotubes.^18-20 However, in bulk, the atomic chains are expected to be highly unstable and undergo cross-linking²¹ with methodologies for the controlled fabrication and stabilisation of carbyne remaining inaccessible. In this respect periodicity in the carbon chains is a possible way to decrease the total energy of the system by kinks waves. Accordingly, the linear fragments with purely sp¹ carbon can be separated by periodic kinks with sp² bonds. ^15,21-22 Alternatively, a new form of carbon, graphyne, proposed to incorporate chains of sp¹-carbon atoms interconnecting the graphene-like planes of sp²-carbon, is expected to be more stable and has been predicted to have unique electronic properties, including both metallic and semiconductor behaviour, and possessing Dirac cones in some specific configurations.^22-27 The coexistence of carbon atoms in two well-defined hybridisation states within the graphyne structure may offer functional features characteristic of carbyne (tuneable electronic band gap) and graphene (high mobility of charge carriers) within the same material, thus opening a wealth of opportunities for practical applications as miniaturized device architectures for optical detection and emission.

In this study, we have demonstrated a new form of nanoscale carbon in the shape of hemispherical carbon matrices and with the bonding characteristics of either sp¹ or sp² carbon. The carbon matrices are formed spontaneously at high temperature from organic molecules adsorbed on the surface of colloidal nanoparticles, with the metal core acting simultaneously as catalyst and template for the carbon matrices growth. This new method of synthesis has demonstrated unprecedented levels of control over the carbon matrices formation, as thermally driven Ostwald ripening of the metal core provides for precise control of the carbon matrices diameter, whilst the propensity of metal nanoparticles to self-assemble into super-lattices enables the formation of highly ordered hexagonal arrays of the carbon matrices on different types of surfaces. Significantly, this new form of carbon exhibits photo luminescent activity in the visible and near-IR ranges, with the wavelength of emitted light determined by the length of sp¹ chains which link sp² domains.

Hexagonal boron nitride sheets (of lateral size 50-200 nm and thickness 1-10 layers) (FLBN) deposited on a gold mesh finder grid of a transmission electron microscope (TEM) were heated in air at 500°C to remove adventitious carbon (Figure 1a; full details are in the methods section). After heating, Raman spectroscopy showed a single band at 1363 cm^-1characteristic of an h-BN phase (Figure S1).²⁸ Silver nanoparticles (AgNPs) of diameter 7.3 ± 0.7 nm were synthesised by a modified Kang and Kim method²⁹ in such a way that a dense monolayer of dodecanethiol molecules were chemisorbed on the silver surface. AgNPs were deposited from solution onto h-BN on a TEM gold grid (for CNC1), on a TEM copper grid (for CNC2) and heated in a TEM holder, in a vacuum of 10^-5 Pa within the TEM column, by rapidly increasing the temperature from 20 ^°C to 650 ^°C (c.a. 120 s to reach maximum temperature, thermal conductivity of copper is 401 W/mK) Sample CNC2; (c.a. 160 s to reach maximum temperature, thermal conductivity of gold is 318 W/mK) Sample CNC1.

TEM imaging clearly indicates that the metallic cores of AgNPs gradually evaporate during the heating process (Figure 2a-b and supporting video file 1), leaving behind carbon ‘casts’ in the form of carbon matrices (CNC) as shown by the video file, arranged in a hexagonal array on the FLBN substrate (Figure 2b). The carbon matrices were also formed inside an ampoule in high vacuum, the AgNPs deposited from solution onto FLBN on a TEM grid were sealed in an ampoule in high vacuum and heated in a tube furnace mimicking the same condition used for the in situ TEM experiments to exclude the effect of the beam in the carbon matrices formation. This experiment undoubtedly excludes the effect of the beam on the carbon matrices formation. The production of carbon matrices was scaled up outside the TEM column in which AgNPs and the support used were inserted in a glass ampoule in high vacuum and heated in a tube furnace to produce carbon matrices (see methods). The resulting powder of CNC can be purified using hydrochloric acid making sure that the residual silver can be unambiguously removed as shown in the EDX of matrices grown on nanofibers (Figure S10).

Electron energy loss spectroscopy (EELS) (Figure 3a) and energy dispersive X-ray spectroscopy (EDX) (Figure 3b and inset) indicate that the structures formed in this process consists of 98.8 wt% of carbon and 1.2 wt% of sulfur. Aberration corrected, high resolution TEM (AC-HRTEM) (Figure 2g) and aberration corrected, scanning TEM (AC-STEM) (Figure 2h) imaging reveal a hemispherical arrangement of 3-4 layers of carbon which is particularly apparent for the side-on orientation of the carbon matrices at the edge of the FLBN substrate (Figure 3a-g) or adsorbed on the surface of carbon nanofibers (Figure S4). The hollow matrices model with open top is consistent with the process of silver metal extrusion and evaporation during the heating process, being particularly apparent in TEM images of the carbon matrices with partial removal of Ag (Figure S2).

Table 1: High-energy measurements ELLS of carbon matrices on FLBN: 9.3, 9.4, 9.5, 10.4, 11.4, 11.5 areas investigated, the figures S17 correspond to areas 9.3, the figure S18 correspond to area 9.4, the figure S19 correspond to area 9.5, the figure S20 correspond to area 10.4, the figure S21 correspond to area 11.4, the figure S22 correspond to area 11.5.

9-3 area	Π* peak	s* peak	sp²ratio
	284.5 eV	292.1 eV
	Area Π* peak	Area s* peak
	2260.9	4878.65	0,31665252
	9390,29	15553.1	0.376464065
	5155,89	8636,01	0,373834642
9-4 area	284,5 eV	291,6 eV
	2527,72	3869,35	0,395137149
	6597,36	11084,3	0,380130256
	9771,81	15296,4	0,389806546
9-5 area	284,5 eV	291,8 eV
	3919,94	5963,05	0,396635026
	4419,94	6073,96	0,421191359
	7972	12805,9	0,383676887
	8666,65	14159	0,3796689078
10-4 area	284,5 eV eV	292,3 eV
	1668,13	2636,77	0,387495644
	3923,85	5475,44	0,417462383
	2041,95	3182,54	0,390641977
11-4 area	284,5 eV	292,3 eV
	4677,73	7271,47	0,391464772
	3247,81	4624,99	0,412535565
	1802,72	2932,48	0,3807062
11-5 area	284,5 eV	292.1 eV
	33,43,96	4468,95	0,428004418
	2343,24	3659,44	0,390365637
	2457,35	3508,12	0,419228985

To determine the nature of inter-atomic bonding within the carbon matrices and to ascertain the hybridisation state of the carbon atoms, detailed STEM-EELS spectral analyses were performed. In the high energy region, the edge of the carbon matrices appears at 284.5 eV, which lies in the region of carbon chains,³⁰(Figures S6)the K-edge of the carbon matrices appears at 292 eV (Figure 3e, Figures S6, and Table 1, Figure S11) when they are grown either on FLBN sheets or carbon supports, which lies in the region of graphitic carbon, differing from diamond at 290 eV and amorphous carbon at 295 eV (Figure S6). However, the broadness of the carbon K-edge above 292 eV (Figure 3e and Figure S6) is unusual for pure sp²-hybridised carbon as shown experimentally (Figure S6) and is comparable to that observed for materials containing sp¹ and or sp² carbon.^30-32 Furthermore, analysis of the low energy region between 0 and 40 eV, which corresponds to both one-particle excitation and collective (plasmon) excitation modes,^15,32 revealed the presence of a one-particle inter-band excitation peak at 5.2 eV, and a peak at 23.6 eV (Figure 3c, the plasmon of BN is at 21,5 eV) corresponding to σ +π plasmon excitation which differs from pure graphitic carbon (27.0 eV), 4 layers graphene (18.0 eV), amorphous carbon (20.3 eV) and diamond (34.0 eV).¹⁵ The high energy EELS measurements suggest that the bonding state within the carbon matrices does not match with well-known forms of carbon, but can be explained by the presence of sp¹-hybridised carbon intermixed within sp²-carbon material,^15,31,32,35 thus indicating a unique nature of the carbon matrices. In addition, the EDX measurements shows the presence of 1,2 % of sulphur which might be present at the edges of the carbon matrices as shown previously for sulphur Nano ribbons Chuvilin A. et al.³³ or the sulphur atoms are between the chains as they stabilize the matrices as previously observed for carbon films.¹⁵

Raman spectroscopy of carbon matrices formed on FLBN showed a D-band and G-band at 1333 (D1) and 1588 cm^-1 (G2), respectively (Figure 3g). D1 is an in-plane breathing (A1g) vibration from the graphitic basal plane and G2 is denoted as sp²clusters with bond angle disorder. The presence of a broad overtone and the strong intensity of the D and G bands imply that the carbon matrices are closer to graphene than amorphous carbon, whilst the high intensity D band is indicative of a high level of disorder which can be in the form of carbon atoms with different hybridisation to sp², dislocations/point-defects in the lattice, or the presence of heteroatoms.^15,16,22 The spectra fitted with the Gaussian distribution shows also bands at 1593 cm^-1 representing the stretching of the phenyl ring and 1165 cm^-1 trans-polycumulene (Figure 3h).^15,16,22Moreover there are some features which lies in the Raman active region 200-700 cm^-1 and they might be related to different collective stretching of sp¹-hybridised C-C bonds, which have been discussed in detail trough theoretical and experimental analysis shown (Figure S8).^15,16,22,34

The functional optical properties of the SAM carbon matrices have been studied using photoluminescence (PL), with the CNC1 and CNC2 structures synthesised on h-BN using a 532 nm laser. Both types of carbon matrices showed photo-activated emission, either in the visible (peak maximum 774 nm, corresponding to a band-gap of 1.6 eV, CNC1) or near-IR range (peak maximum 953 nm, corresponding to a band-gap of 1.3 eV, CNC2), thus demonstrating a strong link between carbon matrices diameter (CNC1 8.7 nm, CNC2 12,7 nm) and their PL wavelength (Figure 3i, S13-16).

The optical functional property of the carbon matrix has been studied using photoluminescence EELS spectra recorded for a (individual) carbon matrix supported on FLBN indicated a peak in the region of 1 eV (Figure 3f). A maximum at energy of 7.8 eV indicative of an inter-band transition was also recorded (Figure 3f, Figure S5).^15,16,35 Using Kramers-Kroning’s theorem, it could be shown that (i) electron excitations with an energy of 1 eV are associated with a transition in the π-band between states separated by a gap of width 1-2 eV, (ii) electron excitations with an energy of 7.8 eV coincide with a transition from the bonding σ-band to the antibonding σ*-band at k = π/a (Figure S5). The optical gap width obtained from the loss function is in good agreement with known experimental and theoretical data. Such a value of the gap is typical of Habbard transitions without any changes of chain periodicity (the n-subband splitting in the middle of the Brillouin zone). Moreover, the P.L. measurements carried on self assembled monolayer of carbon matrices show an intense peak for CNC2 at 1.3 eV (Figure 3i and S16) and a peak at 1.8 eV for CNC1 (Figure 3i, S13-15) which may correspond to the exciton transition of carbynes^15,27,31and matches well the electronic transitions responsible for the PL. All these features are consistent with the notion of a linear chain structure as well as the assumption of semiconducting properties of carbon matrices.^15,16,35

Recent theoretical calculations predicted the existence of sp¹-hybridised carbon in the form of 1D atomic chains, possessing tensile strength twice that of graphene and carbon nanotubes, and three times that of diamond.¹⁴ Stretching or twisting the atomic wire scan significantly alter their band-gap, while a 90-degree end-to-end twist transforms them into magnetic semiconductors.¹⁴ Accordingly, chains of carbon atoms can be considered as the ultimate building blocks for next generation miniature electronic and spintronic devices, and can serve as the smallest interconnects for electro-mechanical devices.^18-19,14 Like graphene, monoatomic carbon chains are just one atom thick, which gives it an extremely large surface area in relation to mass. This property is important in energy storage matrices such as batteries and supercapacitors, where the surface area of the electrode determines energy density and high-density hydrogen storage devices when sp¹ carbon are modified with Ca ions.¹⁴ However, the existence of this phase of carbon has remained the subject of debates until today. It is generally assumed that carbyne is unstable because of the exothermic formation of cross-links neighbouring chains of atoms.

As previously reported by Casari et al., silver nanoparticles have shown a high affinity to form carbon structure containing H-terminated polyyne.³⁴ Okada et. al. have shown the possibility to produce a polymeric composite containing polyyne stabilised by silver nanoparticles.³⁶ Due to their stability, polyyne in liquids can now be synthesized in the form of size-selected samples (short chains) and with well-defined ends groups. Solid-state samples has also been synthesised in powder form. Chalifoux and Tykwinski reported the synthesis of chains up to 44 carbon atoms terminated by bulky group to prevent cross-linking.³⁷ But some chemists would argue that chains with large end caps couldn’t even be considered carbyne.²¹ However, polyyne and cumulune chains, which are the building block of carbyne allotrope, have been observed in a column of a TEM microscope. La Torre et al. have shown that Nobel metals were able to form carbon chains confirmed experimentally as a gold electrode was used to unravel carbon chains (Figure 4).^18-20 Unfortunately, only tiny amounts of sp¹ carbon have been synthesised so far, in the form of monoatomic chains,^38,18-20 free standing in the column of an electron microscope or confined inside narrow nanotubes partially prohibiting exploitation of these predicted functional properties.¹⁴

Our approach utilises the chains of sp³-hybridised carbon, in the form of alkyl groups in dodecanethiol, as precursors for the sp¹-hybridised carbon chains, which can be achieved via the process of de-hydrogenation (Figure 2). The ability of dodecanethiol to assemble spontaneously into well-ordered arrays, so-called self-assembled monolayers SAMs, on metallic surfaces potentially opens a pathway for the scalable mass production of atomic chains and related carbon materials. Silver metallic surfaces are particularly appropriate because AgNPs have shown previously the ability to catalyse reactions of alkynes and diynes^34-36and stabilise highly reactive polyyne structures.^34,36,39 During the temperature increase in our experiments, from 20 ^°C to 650 ^°C in vacuum, the dodecane chains are expected to undergo significant structural transformations, specifically, at around 300 ^°C, C-H bonds begin to dissociate, transforming the alkyl chains to alkene, polyene and polyyne chains, thus gradually converting sp³-hybridised carbon to sp¹-hybridised carbon (Figure 2). However, as carbyne chains are predicted to react with each other,²¹ the close proximity of the molecules within carbon matrices in our experiments, are likely to trigger cross-linking of the carbon chains, leading to the formation of 2D material with aromatic (sp²) regions interconnected by sp¹-hybridised carbons (Figure 2). Such cross-linking immobilises the carbon material around the metal core, preventing desorption of the molecules, whilst de-hydrogenation continues as the temperature rises to 650 ^°C, thus forming carbon shells around the AgNP (Figure S2a-d).

It is important to note that in parallel with the chemical transformations of dodecanethiol molecules, leading to carbon matrices formation, the silver nanoparticles AgNP increase in size due to Ostwald ripening a previously observed for gold nanoparticles grown on carbon fibres step edges, on flat carbon support or within the channels of quasi 1D and 2D carbon supports.^40-42 As the rate of ripening is determined by the thermal conductivity of the TEG grid used, this effect can be harnessed to control the size of the carbon matrices: here, (AgNP) silver nanoparticles grew from an initial 7.3 ± 0.7 nm to 8.7± 1.0 nm in diameter, when heated from 20^°C to 650 ^°C (CNC1); to 12.4 ± 1.3 nm (CNC2). Indeed, the Ostwald ripening of AgNP has been found to be a powerful mechanism for controlling the size of the carbon matrices, with the strong templating effect of the NP defining the matrices diameter (Figures S2a-d and S3).^40-42 Once the maximum temperature of 650 ^°C was reached, TEM imaging clearly demonstrated the fast and efficient evaporation of silver from the matrices (supporting video file 1), and as soon as the metal evaporated, the carbon matrices remained essentially unchanged.

High-resolution TEM imaging indicates that the carbon matrices are different qualitatively to other known forms of carbon nanoparticles, such as fullerenes, carbon onions, carbon black or nano-horns, showing clearly the overall hemispherical morphology of the carbon matrices with shells comprising three or four layers of carbon (Figure 2g-j). The interlayer distances in this graphyne polymorph are between 4-16% longer than in graphite (Figure S2c-d).³⁹ The assignment of the bonding state of carbon atoms within the carbon matrices requires careful correlation of EELS and Raman spectroscopy data. The low energy EELS and Raman spectra both showed the absence of sp³ carbon species confirming the effectiveness of the de-hydrogenation process of dodecanethiol molecules on AgNP, and both characterisation methods are in agreement that the carbon matrices contain a proportion of sp²-hybridised carbon atoms, which nevertheless remain far from the ideal graphitic state, as the carbon matrices appear to contain a high level of defects which may include short chains of sp¹-hybridised carbon interlinking sp²-hybridised domains. The development of such sp¹ and sp² nature would be fully consistent with the de-hydrogenation and cross-linking reactions of dodecanethiol required for the formation of the carbon matrices and would provide for the curvature of their hemispherical surfaces (Figure 1). Most importantly, the development of a carbon form structure containing sp¹ and sp² has important implications for the electronic properties of carbon matrices. A completely graphitised (sp²-hybridised carbon only) carbon matrices of ~ 7.4 ± 1.0 nm diameter would have a vanishingly small electronic band-gap. However, the presence of sp¹-hybridised carbon chains between the graphitic planes would be expected to open the band gap to ~ 1.3 eV, as predicted theoretically for graphyne.²⁷ Indeed, our EELS measurements show a band gap of a carbon matrix CNC at 1 eV (Figure 3i), a band-gap at 1.3 eV of self assembled monolayer CNC2 (Figure 3i and Figure S16) and the PL of CNC1 shows a band gap at 1 eV (Figure 3i, S13-15), the photoluminescence properties of carbon matrices were confirming the theoretical prediction for carbyne species. For the case of small carbon matrices CNC1, the size of the sp²-hybridized domains would be expected to be smaller, whilst the length of the carbon chains interlinking the domains would be expected to be longer than in CNC2, in order to accommodate for the higher carbon matrices curvature as a result the band gap of CNC1 is greater as shown experimentally (Figure 3i and S13-15).

According to our experimental data the shift from 1 eV (carbon matrix) to 1.3 eV (SAM CNC2) in the photoluminescence properties of carbon matrices might be due to the strain induced in SAM CNC2. The symmetric cumulene of the SAMs carbon matrices species when under strain rearranges to broken-symmetry polyyne.^18,44-46 As a 1D electron system, cumulene can be destabilized by Peierls distortion so that polyyne with alternating bond lengths becomes energetically favoured.^18,44-46 The Peierls distortion increases with increasing strain, as it has also been predicted in recent studies^18,44-46,leading to an increasing band gap. In small particles CNC1 either the length or the strain of the chain should be greater yielding a greater band gap as confirmed by our experimental data (Figure 3i, S13-15).

Unlike carbyne chains that are expected to be sensitive to air or heat, these carbon matrices are found to be very stable under ambient conditions in air or at high temperature in vacuum, exhibiting no measurable structural or functional deterioration in our experiments, and thus being as robust as graphene whilst, at the same time, providing a band-gap.

This new, interesting form of nanocarbon – graphyne matrices, discovered in our study, combines the features of graphene and carbyne, and offers significant potential for a variety of applications. The EELS of a carbon matrix shows optical properties in the near-IR range. This opens up avenues for the development of hybrid hetero junctions in the field of photonics,⁴⁷ and hybrid organic and inorganic solar cells.⁴⁸ For example, carbon matrices could be used potentially as injector electrodes, to provide electrons into a dielectric, such as BN or SiO₂, or a semiconductor, as the work function of the sp² and sp¹ domains should be relatively small. The carbon matrices supported onto dielectrics can be exploited in electroluminescent carbon devices, which require electric currents of both electron and holes, recombination of which results in light emission. Indeed, we have found that the carbon matrices can be grown on variety of surfaces, either flat as in the case of FLBN, Si₃N₄ (Figure S12) or cylindrical, as in the case of carbon nanofibers (Figure S4), which could facilitate their integration within functional devices.

Our fabrication process enables not only the reliable synthesis of photoactive carbon structures, but also opens up entirely new possibilities to shape this intrinsically 2D form of carbon^15,27,49 into 3D carbon that possess nanoscale cavities (internal diameters: 7.4 ± 1.0 nm for CNC1 and 12.4 ± 1.3 nm for CNC2). The hollow cavities of these carbon matrices make them structurally related to fullerenes but with available volume greater by a factor of 1,000 as compared to the typical fullerene. Unlike endohedral fullerenes where the internal space is severely restricted, these carbon matrices could serve potentially as nanocontainers for a wide range of optically,^47-48 magnetically⁵⁰ or catalytically^51,52active metals, paving the way to a wealth of applications, from data storage devices^53,54to fuel cells,⁵⁵ all of which rely on structurally robust and electronically active carbon supports.

The fabrication of hollow carbon matrices was achieved either insitu in a transmission electron microscope or in a glass ampoule sealed in high vacuum. A JEOL 2100F equipped with a Gatan Tridiem imaging filter for EELs, with a Gatan 652 double tilt-heating holder was used, at the University of Nottingham U.K. “Nanoscale and Microscale Research Centre”. AC-TEM JEM 2010 was used for the acquisition at room temperature of the CNC supported either on FLBN and CNF (carbon nanofibres).

A commercially available methanolic suspension of few layers boron nitride FLBN was used (Graphene Supermarket). Decanethiol (0.4 mmol) was added to a solution of silver nitrate (0.9 mmol) in ethanol (30 mL) and the mixture was stirred vigorously for 10 mins at room temperature. A saturated solution of sodium borohydride in ethanol (60 mL) was then added to the mixture and stirred vigorously for 2 h at room temperature. The product was precipitated from solution by the further addition of ethanol (350 mL) before storing at -30 ̊C for 24 h. The precipitate was then filtered using a 0.45 μm pore size polytetrafluoroethylene (PTFE) membrane and washed with ethanol (250 mL) and acetone (250mL) before final drying under vacuum to yield a black solid (c.a. 180 mg of AgNPs). A suspension of AgNPs in cyclohexane was prepared using a ratio of 1 mg of AgNPs to 1 ml of cyclohexane. The resulting suspension mixed with 1 mg of FLBN or 1 mg of CNF was then filtered using a 0.45 μm pore size polytetrafluoroethylene (PTFE) membrane and washed with ethanol (250 mL) and acetone (250mL) before final drying under vacuum to yield either a black solid or a white solid (c.a. 2 mg of AgNP@CNF (black solid) AgNPs@FLBN (white/greyish solid). The AgNP@CNF and AgNP@FLBN were inserted in an ampoule under high vacuum and heated at 650 ^°C to yield CNC@CNF and CNC@FLBN. The CNC@CNF and CNC@FLBN could be purified using hydrochloric acid making sure that the residual silver can be unambiguously removed as shown in the EDX of CNC@CNF carbon matrices grown on nanofibers (Figure S10).

The EELS spectra were obtained by using JEOL-ARM microscope equipped with double delta correctors and a mono chromator after the Schottky field emission electron source operated at 60 kV, which enables to achieve a point resolution of 1 Å and energy resolution of 24 meV. The high-resolution (HRTEM) imaging was acquired by a Gatan One View camera with 1−3 fps (4k × 4k resolution) at the AIST in Japan.

Raman spectra were recorded using a Horiba–Jobin–YvonLabRAM Raman microscope, with a laser wavelength of 532 nm operating at low power (ca. 4 mW) and a 600-lines/mm grating. The detector was a Synapse CCD detector. Spectra were collected by recording 64 acquisitions of 5 s duration for each spectral window. Three spectra were recorded for each sample in order to account for sample in homogeneity. The Raman shift was calibrated using the Raleigh peak and the 520.7 cm^-1 silicon line from a Si (100) reference sample.

The PL measurements were performed on FLBN@CNC carbon matrices drop-casted onto a quartz substrate. The excitation was provided by the 532 nm and 633 nm line of a Nd:YAG laser (~ 8 ns pulse) using short- and long-pass interference filters to remove other harmonics. The PL was collected and focused onto the slits of a monochromator using long-pass interference filter (FEL700, Thorlabs). The monochromator grating was a 600 g/mm blazed at 1m. The PL was detected with a liquid N₂-cooled NIR- photomultiplier tube (C5594, Hamamatsu) and the transient decay was recorded using 1GHz oscilloscope (Agilent Infinium). The system response time was measured to be 20 ns. For Raman measurements the carbon matrices samples were on a TEM grids residing on quartz slides. Raman spectra were acquired at T = 300 K using laser excitation at l = 532, 633 nm and power density P = 100W/cm².

Acknowledgements: The author would like to thank Professors Paul Brown for editing the manuscript. The author would like to thank Dr. M. W. Fay,Dr. A. Domergue, Dr R. Mascia, Dr. J. A. Watts,Dr. G. A. Rance, Dr. J. Kerfoot,Dr. L. Turyanska,Dr. Yung-Chang Lin for their technical support. The author would like to thank Professors Paul Brown, Andrei Khlobystov, K. Suenaga, Amalia Patanè and Florian Banhart for discussion. The author acknowledges the support of the TEM and complementary techniques at the NMRC at Nottingham and at the AIST in Japan.

FLBN Few layer boron nitride

AgNP Silver nanoparticles

h-BN Hexagonal boron nitride

EELs Electron energy loss spectroscopy

EDX energy dispersive X-ray

PL Photoluminescence

TEM Transmission electron microscopy

CNF carbon nanofibres

CNC carbon matrices

CNC@CNF carbon matrices on carbon nanofibres

CNC@FLBN carbon matrices on few layer boron nitride

PTFE polytetrafluoroethylene

TEM transmission electron microscope

HRTEM high-resolution transmission electron microscope

HCl Hydrochloric acid

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There is NO Competing Interest.

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Video 1

Shaping Carbon into Carbon matrices with Tuneable Diameter and Photo luminescent Properties

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Abstract

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Introduction

Results

Discussion

Methods

Declarations

Abbreviations

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