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.

Keywords:

• CVD
• liquid source
• hybrid films
• heat dissipation

1. 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 [1–3]. 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 [4–7]. 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 [8–10], 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 [12–14], and also methods of injecting polymers into diamond foam skeletons, graphene networks, vertical arrays of structured graphene paper, and CNT [15–18]. 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.

2. Experimental

2.1. 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 (C₄H₁₁N) 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, H₂ 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 VDG₆₀₀, VDG₇₀₀, and VDG₈₀₀, respectively. Detailed growth parameters are shown in Table 1.

Table 1. Growth parameters of VDG films.

Sample	Temperature (°C)	Power (kW)	Pressure (kPa)	Time (h)	H2 flux (sccm)	N-butylamine flux
VDG600	600	3.0	11.5	3.0	160	8
VDG700	700	4.0
VDG800	800	5.0

2.2. 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).

3. Results and discussion

3.1. 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(a–c), the growth rate of VDG₆₀₀ 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 VDG₇₀₀ and VDG₈₀₀ 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.

Figure 1. (a)–(c) the SEM images of the films at VDG₆₀₀, VDG₇₀₀, and VDG₈₀₀, respectively (The illustration shows the cross sectional SEM images). (d)–(f) the TEM profile images of VDG₆₀₀, VDG₇₀₀, and VDG₈₀₀ films, respectively.

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 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 VDG₇₀₀ and VDG₈₀₀. Figure 1 (g–i) shows the “heart-shell” 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 sp² 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 VDG₆₀₀ films show a significant drop at 302.0 eV [25,26], indicating the presence of mainly sp³ 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 sp² bonds in the VDG films. The π*-band at 284.5 eV of VDG₈₀₀ sample is significantly more intense than VDG₇₀₀, suggesting that a larger proportion of sp² bonded carbon is induced in the higher temperature films [27], while the small π*-band intensity of the VDG₆₀₀ film is an indirect evidence that its composition is almost free of graphite phases. The nature of sp² bonded carbon was further explored through plasma loss EELS spectroscopy, as shown in Figure 2(b), the graphite phase has a distinct peak at s₃ (27 eV) and the α-C phase at s₁ (22 eV), while the crystalline sp³ bonded carbon (diamond), there is a peak at s₄ (33 eV) corresponding to bulk plasma loss and a shoulder at s₂ (23 eV) corresponding to surface plasma loss [28]. However, the ionization loss spectra of the VDG₆₀₀ film are mainly concentrated in the s₄ band with less intensity in the s₂ and s₃ bands, which demonstrates that this sample contains a large amount of diamond; the loss spectra of VDG₇₀₀ and VDG₈₀₀ are mainly concentrated in the s₃ band, and the VDG₇₀₀ film has more intensity in the s₃ band and less intensity in the s₂ and s₄ bands, based on this fact, the core loss EELS implies the sp²-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.

Figure 2. (a) core-loss EELS spectra and (b) plasmon-loss EELS spectra of VDG films.

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 VDG₆₀₀, 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 sp²-C, sp³-C, and CN/CO bonds [32], respectively. With increasing the temperature, the area of sp²-C and sp³-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.

Figure 3. (a) Raman spectra of VDG films. (b) I_D/I_G and I_2D/I_G ratios of Raman spectra of VDG films. (c) GI-XRD patterns. (d)–(f) XPS C1s spectrum of VDG films.

3.2. 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), C₂ (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 C₂/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 C₂ and CN promoted the graphene nucleation. In addition, a diamond structure coated with graphene is formed with a secondary nucleation process [24,34].

Figure 4. (a) Plasma spectrum of VDG films TIMs. (b) The relationship between the growth temperature, intensity ratio of CH/H_β, CN/H_β, C₂/H_β, and H_α/H_β.

Figure 5. Model of VDG films growth. (a) Under the stimulation of microwaves, the carbon source and hydrogen are transformed into C₂, CH, CN, H_α and H_β. (b) CH and silicon substrate are preferentially combined at the defect to generate SiC. (c) CH promotes the nucleation of diamond based on SiC, and C₂ and CN promote the formation of graphene. (d) Secondary nucleation occurs during growth.

3.3. 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 VDG₆₀₀, VDG₇₀₀, and VDG₈₀₀ 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 VDG₆₀₀ 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 VDG₇₀₀ film have a homogeneous distribution of graphene and diamond morphology, which facilitates heat transfer in a continuous channel. The longitudinal morphology of VDG₈₀₀ 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 VDG₇₀₀ 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.

Figure 6. (a) Thermal conductivity value of VDG films measured by laser flash method. (b) The variation trend of the surface temperature of VDG films tested by a thermocouple (The inset is a schematic diagram of the actual simulation device). (c) LED infrared image integrated with VDG films.

Table 2. Thermal conductivity values of different carbon-based TIMs.

Carbon material type	Thermal conductivity values(W•m^-1•K^-1)	TIMs thickness	References
Carbon/carbon composite blocks	60	15 mm	[35]
Carbon fiber reinforced carbon composites	62.08	65mm	[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]

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 VDG₈₀₀ 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 VDG₈₀₀ film compared to the VDG₇₀₀ 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 VDG₈₀₀ 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/m², 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 VDG₇₀₀ 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.

Figure 7. (a)–(c) Simulation and comparison of heat transfer performance in vertical direction of 600 °C, 700 °C, and 800 °C VDG film TIMs. (d) 2 D and (e) 3 D isotherms of VDG₇₀₀ film TIMs. (f) Comparison of thermal conductivity values of different carbon-based TIMs.

4. Conclusion

In summary, the VDG films with 3 D thermal conductive networks were successfully prepared using the one-step MPCVD method. Combined with the thermal simulation results and the characteristics of diamond 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 VDG₇₀₀ film had the highest through-plane thermal conductivity value (97 W m⁻¹ K⁻¹) among the three samples, and the VDG₈₀₀ 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 VDG₇₀₀ 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.

Authors’ contributions

Guorong Zhang: Conceptualization, Data analysis, Investi­gation, Methodology, Visualization, Writing-original draft. Huiqiang Liu: Data curation, Writing-review & editing, Visualization (equal). Yangxin Xiao: Data curation, Visuali­zation. Bing Wang: Formal analysis. Jian Wang: Writing-review & ­editing. Wen Zhang: Supervision, Data curation. Ying Xiong: Funding acquisition, Investigation, Project administration, Supervision, Writing–review and editing.

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Additional information

Funding

This work was supported by the Key Basic Research of the National Defense Science & Industry (Grant No. JCKY2019404D002), the Natural Science Foundation of Sichuan Province (2022NSFSC0523, 2023NSFSC1990 & 21ZS2232), Natural Science Foundation of Xi’an Modern Chemistry Research Institute (Grant No. 21zh0175) and Natural Science Foundation of Southwest University of Science and Technology (project No. 20zx7103).