Orientated growth the 3D diamond/graphene hybrid arrays and the application in thermal interface materials

Abstract Diamond and graphene are considered to be one of the most promising thermal interface materials (TIMs) for electronic devices benefited from their highest thermal conductivity in the natural world. However, orientated fabrication of high thermal conductivity diamond and graphene hybrid arrays with three dimensions (3 D) thermal conductive networks are still problematic. Here, we used a unique one-step microwave plasma chemical vapor deposition, n-butylamine, as the liquid source to prepare a novel high thermal conductivity 3 D vertical diamond/graphene (VDG) hybrid arrays films. The orientated 3 D thermal conduction path of the VDG is regulated by the growth temperature, and the through-plane thermal conductivity value of the VDG700 films up to 97 W m−1 K−1. In the actual TIM performance measurement, the system cooling efficiency with our VDG as TIM is higher than the state-of-the-art commercial TIM, demonstrating the superior ability to solve the inter-facial heat transfer issues in electronic systems.


Introduction
"Overheating" is a major problem in the modern electronics industry because of miniaturized and lightened gradually, which can seriously affect the service life and normal operation of the devices [1][2][3]. It is well known that micro gaps are filled with air, which is seriously affected heat dissipation. Thermal interface materials (TIMs), such as phase change materials and thermal pads, hold a dominant position for rapid heat transfer in microelectronics and microsystems [4][5][6][7]. Numerous studies have shown that diamond and graphene composites, by other methods such as annealing, vapour deposition, laser melting and high temperature, and high pressure [8][9][10], exhibit good mechanical properties and high electrical conductivity, show great application potential as TIMs in the natural world [11].
Traditional carbon-based TIMs are made by filling diamond particles, graphene nanosheets, and carbon nanotube (CNT) array into an organic polymer matrix [12][13][14], and also methods of injecting polymers into diamond foam skeletons, graphene networks, vertical arrays of structured graphene paper, and CNT [15][16][17][18]. On comparison, carbon-based TIMs with a vertical alignment of 3 D materials, instead of random orientation, are highly desirable to break through the performance bottleneck of current TIMs. To obtain vertically aligned architecture of TIMs, researchers have done many works, including, graphene paper lateral stacking [18], constructed by in situ growth [19], electric field-assisted [20], and chloroform-assisted methanol-based plasma enhancement [21,22], etc. However, it is still difficult to achieve batch production due to the complexity and the less practicality.
Here, a unique one-step growth of VDG films using microwave plasma chemical vapor deposition (MPCVD) is presented, where a liquid organic compound-n-butylamine acts as the single reactive source. The unique 3 D thermal conduction path of the VDG is regulated by the growth temperature in a single step. The actual heat and finite element simulation methods are performed for the analysis of the heat conductivity mechanism.

Growth of VDG films with MPCVD
3D hybrid VDG films were grown on a silicon substrate by the self-developed stainless steel ring resonator of a 15 kW power MPCVD (Herui Microwave) system (Text S1, Supporting Information). The n-butylamine (C 4 H 11 N) is stored in a stainless steel tank and introduced into the deposition chamber by the pressure difference between the stainless steel tank and the deposition chamber. First, H 2 is fed into the MPCVD equipment. With the temperature and pressure rising slowly until a metastable state is reached (that is, there is a difference of about 150 °C between the growth temperature and the pressure is stable at 6-7 kPa), the liquid source n-butylamine with the valve scale of "6" is fed into the equipment. While the liquid source continues to enter, the equipment starts to heat up and pressure boost. About 15 min later, the liquid source flux with the valve scale of "8" was continued to flow into the valve, and the power was slightly adjusted to the required growth temperature. After 3 h of steady state, the liquid source was closed, and the pressure and temperature were lowered slowly at random. Samples were taken out and the equipment was shut down. During the experiment, the growth temperatures of VDG films were set at 600 °C, 700 °C, and 800 °C, and they were named VDG 600 , VDG 700 , and VDG 800 , respectively. Detailed growth parameters are shown in Table 1.

Characterization
The microstructural of the films was observed using field emission scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM), the chemical bonding states of the VDG films were also analyzed using electron energy loss spectroscopy (EELS). The crystal structure and surface chemical compositions were determined using grazing incidence X-ray diffractometer (GI-XRD) measurements performed with an incident angle of 1.5°, a Raman spectrometer with a laser wavelength of 514.5 nm, and X-ray photoelectron spectroscopy (XPS), respectively. The growth process of gas cleavage was monitored by optical emission spectrometer (OES, PG2000-pro). The thermal conductivity was measured by using a laser flash apparatus (GB/T 22588-2008).

Characterization of VDG films
The content and size structure of diamond and graphene are very important for the construction of high thermal conductivity network structure. As shown in Figure 1(ac), the growth rate of VDG 600 film along each direction does not change much, and equiaxed grains with a thickness of about 18 μm are obtained. The anisotropic growth of the VDG 700 and VDG 800 films is obvious, forming needle-like structures [23], with the height of both being 30 and 32 μm, respectively. Depending on the transverse morphology of the VDG films developing under different power densities, it is possible to observe that the film is completely perpendicular to the substrate.
The surface topographies of VDG films were investigated by TEM in Figure 1(d-i), the selected area electron diffraction (SAED) diagram illustrates that the Table 1. Growth parameters of VDG films. corresponding bright rings are diamond phase (111) D , (220) D , (311) D and graphene phase (002) G crystalline surfaces. The adjacent crystalline planes are 0.20 nm, which is similar to the theoretical spacing value of the (111) D crystalline planes of the diamond phase, indicating that is diamond; and 0.34 nm of adjacent crystalline planes shows the existence of graphene. The Fourier transform the image also verifies the diamond and graphene phase. The difference between the three samples is that the graphene phase only appears in the VDG 700 and VDG 800 . Figure 1 (g-i) shows the "heartshell" structure of the diamond coated with graphene, and this microstructure connects the diamond very well with graphene, which provides favourable conditions for the transfer of heat between phonons [24].
Since it is difficult to distinguish between amorphous and crystallized sp 2 bonded carbon in thin films from TEM microstructure studies, in order to fully understand the bonding structure of the films, we combine EELS ( Figure 2 and Text S2, Supplementary Information) with TEM. As what can be seen from the picture, the VDG films surge up to 289.5 eV (σ*-band), while the VDG 600 films show a significant drop at 302.0 eV [25,26], indicating the presence of mainly sp 3 bonds i.e. diamond in our films. However, the curve at 302.0 eV flattens out as the temperature increases, indicating that the diamond character of the films is diminishing. In addition, the π*-band at 284.5 eV hints the presence of sp 2 bonds in the VDG films. The π*-band at 284.5 eV of VDG 800 sample is significantly more intense than VDG 700 , suggesting that a larger proportion of sp 2 bonded carbon is induced in the higher temperature films [27], while the small π*-band intensity of the VDG 600 film is an indirect evidence that its composition is almost free of graphite phases. The nature of sp 2 bonded carbon was further explored through plasma loss EELS spectroscopy, as shown in Figure 2(b), the graphite phase has a distinct peak at s 3 (27 eV) and the α-C phase at s 1 (22 eV), while the crystalline sp 3 bonded carbon (diamond), there is a peak at s 4 (33 eV) corresponding to bulk plasma loss and a shoulder at s 2 (23 eV) corresponding to surface plasma loss [28]. However, the ionization loss spectra of the VDG 600 film are mainly concentrated in the s 4 band with less intensity in the s 2 and s 3 bands, which demonstrates that this sample contains a large amount of diamond; the loss spectra of VDG 700 and VDG 800 are mainly concentrated in the s 3 band, and the VDG 700 film has more intensity in the s 3 band and less intensity in the s 2 and s 4 bands, based on this fact, the core loss EELS implies the sp 2 -bonded carbon is mainly present in the nano-graphite phase, with a small amount of α-C phase. Due to the higher electronic thermal conductivity of nano-graphite compared to α-C phase [29], this provides direct proof for the subsequent differences in thermal conductivity of VDG films at different temperatures. The EELS observations are consistent with the previously observed FIB-TEM microstructure.
The quality and crystallinity of VDG films were characterized by the Raman test and GI-XRD shown in Figures 3(a-c). For Raman analysis, the two D peaks (1353 cm −1 ) corresponding to amorphous carbon and the G peak (1584 cm −1 ) are characteristic peaks of VDG films. The I D /I G (I stands for strength) value can be used to describe the defect degree of the VDG film, and the I 2D /I G value can be used to judge the number of graphene layers [30,31]. With increasing the growth temperature, the I D /I G ratio (<1) also increased, indicating that VDG films have a good quality with low defect concentration. Moreover, the high I 2D /I G ratio (>1) calculated from the Raman spectrum confirms the good crystallinity and multilayer structure of our samples grown by MPCVD. Figure 3(c) shows the GI-XRD pattern of VDG films, for VDG 600 , three distinct diamond peaks (111) D , (220) D and (311) D can be observed at 2θ of 43.9, 75.4, and 91.4°, respectively; at higher temperatures, in addition to the diamond peaks, two graphene peaks (002) G and (110) G can be observed at 26.3° and 78°, which indicates a change in the diamond and graphene content with increasing temperature and also demonstrates the coexistence characteristic of the diamond and graphene phases. Gauss-Lorentz was used to peak the high-resolution XPS spectrum shown in Figures 3(d-f), three sub-peaks located at 284.6, 285.1, and 286.0 eV can be observed in the C1s spectrum, corresponding to sp 2 -C, sp 3 -C, and CN/CO bonds [32], respectively. With increasing the temperature, the area of sp 2 -C and sp 3 -C peak imply the increase of graphene content and the decrease of diamond, and the content of oxygen and nitrogen-containing groups of VDG films (Text S3, Supporting Information) is extremely low, indicating a very low defect density.

Growth mechanism of VDG films
In the process of VDG films growth, the organic liquid small molecule source was heated and evaporated into the MPCVD cavity in a water bath. The whole process of gas cleavage was monitored by OES spectroscopy (Figure 4). The gas cracking process contains CN (388 nm and 421 nm), C 2 (473 nm, 516 nm, and 563 nm), H α (656.5 nm), H β (486 nm) [33]. Since the content of H β is almost constant during the temperature change, the relative changes of each ion group during the growth of VDG films are calculated by integration (Figure 4(b)). The decrease of CH/H β indicates that the nucleation of diamond mainly depends on CH. The trend of the CH/H β curve demonstrates that diamond nucleation mainly depends on CH. The fact of decreasing of C N /H β and C 2 / H β after increasing indicates that they play a decisive role in promoting the nucleation of graphene. The curve of the H α /H β ratio is due to the dissociation of gas carbide. Therefore, the possible growth models of VDG films is plotted in Figure 5. During the initial stage, the plasma preferentially generates SiC layer on the silicon substrate which contains the defects and CH promotes diamond nucleation, and followed by C 2 and CN promoted the graphene nucleation. In addition, a diamond structure coated with graphene is formed with a secondary nucleation process [24,34].

Characterization of the thermal properties of VDG films as TIMs
The thermal conductivity for TIMs were measured via laser flash method (Text S4, Supplementary Information). In Figure 6(a), at the same test temperature, the thermal    conductivity of VDG 600 , VDG 700 , and VDG 800 are up to 57, 97, 91 W•m −1 •K −1 , respectively, which is higher than those of conventional carbon-based TIMs (Table 2). With increasing the test temperature, the thermal conductivity value of each sample is basically stable and such difference of the thermal properties can be attributed to the different longitudinal structure of VDG films. The VDG 600 film is not conducive to heat transfer in the longitudinal direction because for its vertical array structure is primarily composed of nanoscale clustered carbon structures. The diamond nanowires of VDG 700 film have a homogeneous distribution of graphene and diamond morphology, which facilitates heat transfer in a continuous channel. The longitudinal morphology of VDG 800 film is in the middle, with the diamond in the form of short rods with a small aspect ratio, and although the graphene component is predominant, it is not as continuous as in VDG 700 film. The bonding of individual chemical bonds is entirely responsible for the hybridization of graphene and diamond, which reduces interfacial thermal resistance due to phonon scattering and alleviates barrier to the thermal transport of phonons and electrons.
To evaluate the potential of the VDG films as TIMs in electronic devices, a system was designed to simulate heat dissipation in electronic components as shown in Figures 6(b,c), including a thermocouple temperature test and infrared camera photography. In the actual thermal simulation, the samples were mounted separately between a LED lamp (5 W power) and a commercial heat sink (Metal aluminium), and the thermocouple piece is located above the sample, while the infrared thermal imager is located 50 cm above the LED. According to thermocouple temperature data, each sample tends to be stable after about 3000 s. For the temperature of the hottest spot, the maximum temperature of the ceramic plate with VDG 800 was 68.4 °C, which was lower than that of the best commercial thermal conductive silicone pad and other VDG films. This result seems to contradict the above values, mainly because in practical applications a common heat dissipation mechanism in the horizontal and vertical directions has to be considered. From the previous analysis of the VDG films composition, it is clear that the VDG films are dominated by graphene components at the high temperature, and the graphene layers rely mainly on van der Waals forces to maintain each other, although the van der Waals forces act weakly and are not conducive to heat propagation between the layers. However, monolayer graphene has a high thermal conductivity. Moreover, because our films have a flake structure in the longitudinal direction, we speculate that the superior heat dissipation of the VDG 800 film compared to the VDG 700 film is a result of the higher amount of graphene playing a role in heat dissipation [41]. It has been shown that defects can lead to phonon scattering and affect the propagation of heat [42]. From the I D /I G ratio in the Raman spectrum and the GI-XRD results, what can be seen is that the VDG thin film has fewer defects as the temperature increases, in other words, the crystallinity is better, which supports the conclusion that the VDG 800 film dissipates heat best.
To prove the conductive thermal advantage of these 3 D VDG films, the element simulation methods was used to evaluate the mechanism of VDG films as TIMs (Figure 7, Text S5 Supplementary Information). The heat source on the upper surface of the sample is 500 W/ m 2 , and the lower surface is exposed to air, with symmetric sides. The thermal conductivity values of three different VDG film TIMs were, respectively, set as 57, 97, 91 W m −1 K −1 , and the simulation results were obtained as shown in Figures 7(a-c). As can be seen from the figure, in longitudinal direction, the temperature distribution of VDG 700 film TIMs with larger thermal conductivity coefficient was quite different, and the temperature difference was very obvious. It can be observed from the comparison that the "core-shell" VDG composite film used in this study as the TIMs has very good thermal conductivity, highlighting the importance of the vertical array longer thermal conductivity path (Figure 7(d,e), Text S6 Supplementary Information). In addition, the comparison of the thermal conductivity values of different types of carbon-based TIMs along the preferred direction of heat conduction is shown in Figure 7(f), indicating that our sample has extremely high values compared with the existing literature.

Conclusion
In summary, the VDG films with 3 D thermal conductive networks were successfully prepared using the onestep MPCVD method. Combined with the thermal simulation results and the characteristics of diamond  [36] carbon fiber perpendicular 80 [37] graphite flakes@ graphene nanoplatelets 27.5 [38] Graphene@ carbon fibers 28 [39] Graphene nanoplatelet -carbon nanotube 7.69 2 mm [40] and graphene, it is found that the VDG film TIMs has good thermal conductivity in the transverse and longitudinal direction, which demonstrates that, the VDG film with the "core-shell" structure has a continuous thermal conductivity path, featured with great heat dissipation which is conducive to heat dissipation. The VDG 700 film had the highest through-plane thermal conductivity value (97 W m −1 K −1 ) among the three samples, and the VDG 800 film is lower than the silicone pad with the best thermal conductivity on the market according to practical thermal simulation. From the element simulation methods analysis, the VDG 700 film indeed has the highest thermal conductivity value in the longitudinal direction. This study provides a simple and practical way of constructing vertically aligned architecture and efficiently improves thermal conductivity for the TIMs application.