1. Introduction

In the ever-evolving world of materials science, two remarkable substances, diamond, and graphene, have captured the attention of researchers and scientists, each boasting an array of exceptional properties [1,2]. Diamond, the epitome of hardness [3], possesses remarkable mechanical strength [4], chemical inertness [3], and electrical insulation [4], while graphene, a single layer of carbon atoms arranged in a honeycomb lattice [5], shines with exceptional electrical conductivity [6], thermal conductivity [7] and flexibility [8,9].

Although extensive individual studies have been conducted on these carbon allotropes, their collective potential remains largely untapped [10]. The synergistic combination of graphene and diamond in a hybrid structure can provide opportunities for pioneering structural materials, advanced sensing capabilities, and efficient therapeutic approaches. In the realm of imaging and sensing, integrating the fluorescent color centers of diamond with graphene's surface functionality could revolutionize biosensing applications. This collaboration can enable high-sensitivity detection of biomolecules, with graphene's large surface area enhancing specificity and sensitivity.

Structures that bind graphene and diamond atoms through covalent bonds [10] can be synthesized using diverse techniques such as annealing, laser irradiation [11], arc-discharge [12], and field emission [13]. For instance, core-shell nanodiamonds without color centers are made by pulsed laser ablation [14,15], and they are used for biosensing exploring only their surface functionalization [16].

Graphitization, a process involving the structural transformation of metastable carbon forms, including cubic and hexagonal diamond, rhombohedral graphite, and disordered structures such as cokes, pyrolytic carbons, carbon blacks, and glassy carbons [17]-from non-graphitic to stable graphite, has been extensively explored due to its versatile applications [12].

Two growth mechanisms of graphitization are recognized: order-order and disorder-order [10]. This study focuses specifically on order-order graphitization [10], which allows for the controlled synthesis of diamond-graphite composites with color centers, addressing limitations observed in previous works [14,15].

Using diamond needles as a template, we meticulously crafted core-shell structures featuring diamond cores enveloped by layers of graphene or sp² carbon materials. The diamond core, embedded with fluorescent color centers (example SiV^-) can serve as an ideal platform for optical reporting, imaging, and sensing biological molecules. For instance, it can function as a nanothermometer [18,19]. The surrounding graphite or sp² carbon layers, with their exceptional optical properties, can absorb light and function as cold photothermal agents like carbon nanotube clusters [20] or drug delivery systems [21]. This innovative composite structure can hold tremendous promise for photothermal therapy, presenting a compelling approach for targeted cancer treatment. By combining the light-absorbing capabilities of graphite with the localized heating properties of diamond, precise targeting of cancerous tissues can be achieved with minimal damage to healthy cells.

2. Materials and methods

The experiment began with the preparation of four 30 mm x 30 mm p-type boron-doped single-side polished Si substrates, each 525 ± 10 µm thick. These substrates underwent mechanical treatment with one-µm diamond powder, resulting in the creation of small diamond particles embedded in the substrate, serving as seeds for subsequent diamond growth [22].

Diamond films were synthesized on the seeded substrates using a Direct Current Plasma Enhanced Chemical Vapor Deposition (DC PECVD) system. The synthesis conditions included a gas mixture of 3% CH₄/H₂, a substrate temperature of approximately 900 °C, a pressure of 98 mbar, and a duration of 1 hour. Post-synthesis, the diamond films predominantly consisted of nano-crystalline diamonds (NCDs) and Single Crystal Diamond Needles (SCDNs) [23].

SiV^- impurities were created using the Chemical Vapor Deposition (CVD) technique. Silicon plates served as sources for Si atom impurities in the diamond films, releasing Si atoms into the gas mixture as SiH_x radicals. These radicals were formed from the etched Si substrates by atomic hydrogen from the plasma, facilitating the formation of SiV^- centers in the growing film [24–26].

Following synthesis, the diamond films with SiV^- centers underwent an isolation process at 590 °C for 24 hours in an air atmosphere. This process selectively oxidized and vaporized the non-SCDN carbon matrix, leaving behind the desired SCDNs [27].

Subsequently, the isolated diamond needles underwent graphitization within a vacuum environment (<2 × 10-1 mbar) at various temperatures and durations to transform them into graphene layers. The graphitization process involved pumping the oven chamber to a pressure below 2 × 10-1 mbar, heating the chamber to the desired temperature over approximately 2 hours, maintaining each sample at the target temperature for the required time (30 minutes or 2 hours), and gradually cooling the sample down to room temperature. The specific graphitization parameters were 1100 °C for 2 hours, 1100 °C for 30 minutes, 1000 °C for 30 minutes, and 900 °C for 30 minutes.

The morphology of the synthesized diamond-graphene composites was characterized using a LEO GEMINI 1550 (Zeiss) microscope with an electron beam acceleration voltage of 10 kV and an In Lens detector. For optical characterization, the RENISHAW inVia Raman spectrometer unit was utilized, operating in both Raman and photoluminescence modes. All presented spectra were acquired using an excitation wavelength of 514.5 nm.

3. Results and discussion

Figure 1 displays SEM images depicting synthesized carbon films and obtained Single Crystal Diamond Needles (SCDNs), along with their corresponding Raman and photoluminescence (PL) spectra. Following Chemical Vapor Deposition (CVD) (Fig. 1(a)), the Raman spectrum exhibited less intense characteristic diamond peaks, and also indicated the presence of non-diamond carbon phases (Fig. 1(b)). At the isolation stage (Fig. 1(b)), the matrix surrounding SCDNs in the carbon film was completely oxidized, leaving behind diamond crystals [27]. Additionally, SCDNs displayed strong PL signal (Fig. 1(d)) attributed to SiV^- centers [28], exhibiting both before and after the isolation process.

 

Fig. 1. SEM images illustrating the synthesized carbon film (a) and the acquired Single Crystal Diamond Needle (SCDN) (b), accompanied by their respective Raman spectra before and after isolation (c) and post-isolation Photoluminescence (PL) spectra (d).

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Our experimental results demonstrated the impact of applied vacuum heating on the structure and optical properties of diamond-graphite composites. Subjecting the samples to graphitization at 1100 °C for 2 hours led to a complete transformation of diamond into an unidentified material, with no visible SiV^- centers (Fig. 2(a)-(d)). The Raman peak of diamond and SiV^- photoluminescence were both eliminated (Fig. 2(c) and (d)), indicating excessive modifications during heating. SEM images revealed a significant reduction in Single Crystal Diamond Needle (SCDN) size (Fig. 1(b) and Fig. 2(a), b), with the smaller SCDNs measuring less than a quarter of their original length (Fig. 2(a), b). Additionally, the edges of the SCDNs became noticeably rounder.

 

Fig. 2. SEM images depicting Single Crystal Diamond Needles (SCDNs) (a, b), along with the characteristic Raman (c) and photoluminescence (PL) (d) spectra after graphitization at 1100°C for 2 hours.

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Graphitization treatments conducted at temperatures of 900°C, 1000°C, and 1100°C for a duration of 30 minutes produced notable outcomes, as depicted in Fig. 3. Observation of the Single Crystal Diamond Needles (SCDNs) surfaces revealed the emergence of thin layered features (Fig. 3(a)-(c)), suggesting the potential formation of diamond-graphite composites during the 30-minute graphitization process.

 

Fig. 3. SEM images of SCDNs graphitized at 1100 °C (a), 1000 °C (b) and 900 °C (c) for 30 minutes and their characteristic Raman spectra after graphitization (d). Insertion in (b) presents characteristic PL spectrum for all obtained samples graphitized for 30 min.

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Subsequent analysis of photoluminescence (PL) within the synthesized composites focused on identifying the presence of SiV^- centers (Fig. 3(b) inset). Each sample exhibited these centers, albeit with varying degrees of PL intensity reduction corresponding to the graphitization temperature.

For instance, at 900°C, the SiV^- PL intensity decreased from 51620 to 29469 arb. units (43% decrease), accompanied by a slight increase in the Full Width at Half Maximum (FWHM) from 5.1 nm to 5.3 nm. Conversely, at 1000°C, the intensity notably decreased from 66259.5 to 1429.87 arb. units (78% decrease), with similar changes in FWHM as observed at 900°C.

At 1100°C, a drastic drop in intensity from 202221 to 13813.3 arb. units (93% decrease) occurred, coupled with an increase in FWHM from 5.6 nm to 6.6 nm. These results suggest a general decrease in resolution across all samples post-graphitization, while the persistent presence of SiV^- centers confirms the survival of a diamond core within the composite structure.

The observed decrease in PL intensity can be attributed to various factors, including fluorescent quenching mechanisms as outlined by previous studies [29]. These mechanisms encompass scenarios where the absorbed energy from the excitation wavelength fails to reach the luminescent ions, encounters nonradiative pathways to the ground state, or is absorbed by the luminescent material itself. In this study, two mechanisms are particularly relevant, contributing to the observed reduction in PL intensity.

Thin carbon films and multilayered graphene exhibit high absorptivity, absorbing both incident laser radiation and luminescent light emitted by the color center. Even a single layer of graphene displays notable absorption characteristics, absorbing 2.3% of light in the visible spectrum [30] and over 10% at longer wavelengths [31]. This absorption phenomenon significantly contributes to the fluorescence quenching of SiV^- centers when situated within a graphitized diamond needle.

During graphitization, elevated temperatures lead to increased conversion of diamond to graphene layers [32,33], resulting in a pronounced intensity reduction observed in the 1100°C sample. Additionally, graphitization processes may introduce new defects or modify existing ones, creating non-radiative pathways that quench the PL signal from SiV^- centers [34]. Furthermore, surface roughening during graphitization can scatter light, further diminishing the measured PL intensity.

Raman spectroscopy (depicted in Fig. 3(d)) corroborated the formation of graphite, evidenced by characteristic D and G peaks at 1350 cm-1 and 1580 cm-1, respectively [26,35,36]. This supports the notion of surface graphitization resulting in a core-shell structure with a preserved diamond core.

It is noteworthy that all diamond peaks remained consistent across the three samples. Particularly, the 900°C treatment exhibited a robust diamond Raman peak at 1330 cm^-1, (Fig. 3(d)) noticeably diminishing at higher temperatures. This suggests that lower temperatures yield core-shell structures with thinner graphitic shells, positioning the color centers closer to the surface. Such configurations hold promise for applications such as imaging and sensing, as demonstrated in prior research [37].

4. Conclusion

In conclusion, our experiment has demonstrated the delicate balance between graphitization and preservation of diamond in diamond-graphene composites. Excessive graphitization poses the risk of transforming diamond entirely, compromising the material's unique properties. However, when graphitization is carefully controlled, it can induce partial graphitization while retaining a significant portion of the diamond. To this end, we have successfully developed a method for creating core-shell diamond-graphite structures, employing a synergistic approach that combines both Chemical Vapor Deposition (CVD) and vacuum graphitization techniques. The resulting core-shell structures exhibit strong photoluminescence (PL) signals from SiV^- color centers, showcasing their potential for imaging and sensing applications. Importantly, the manipulation of sp² carbon content, determined by the thickness of the graphitized layers, can be precisely regulated by adjusting the time and temperature parameters during the graphitization process.