Optimizing Buffer Layers via Atomic Layer Deposition for High‑Performance Vertically Aligned Carbon Nanotube Arrays

Background

VACNTs possess extraordinary axial thermal conductivity and are increasingly employed in thermal packaging. High‑quality arrays are typically grown by chemical vapor deposition (CVD) on substrates pre‑coated with a buffer layer that prevents catalyst diffusion. Atomic layer deposition (ALD) delivers conformal, defect‑free oxides on complex geometries, making it an attractive choice for buffer layers. Prior work has demonstrated improved nucleation and uniform tube diameters when ALD‑grown Al₂O₃ or SiO₂ are used as buffers, yet the mechanistic influence of different oxides on VACNT growth remains incompletely understood.

In this investigation, we systematically evaluate ALD‑deposited Al₂O₃, TiO₂, ZnO, and thermally oxidized SiO₂ as buffer layers for VACNT synthesis. We also develop a VACNT/graphene composite film for enhanced heat transfer, with VACNTs providing vertical pathways and graphene supplying transverse conduction.

Methods

Thin films of Al₂O₃, TiO₂, and ZnO were deposited on Si wafers by ALD at 200 °C using TMA, TDMAT, and DEZ precursors with H₂O as the oxidant, yielding 20 nm layers. SiO₂ was grown thermally to the same thickness. A 1 nm Fe catalyst was then evaporated by electron‑beam onto all substrates. VACNT growth proceeded in a commercial CVD system (AIXRON Black Magic II): the catalyst was annealed in H₂ (700 sccm) at 600 °C for 3 min; thereafter, C₂H₂ (100 sccm) and H₂ (700 sccm) were introduced, and growth lasted 30 min at temperatures ranging from 550 to 700 °C.

For the composite film, patterned Fe catalysts (500 µm × 150 µm pitch) were deposited on Al₂O₃, VACNTs were grown at 650 °C for 30 min, then densified with acetone vapor for 20 s. Multilayer graphene (10 wt %) was mixed with epoxy resin, curing agent, and diluent, and the VACNTs were immersed and cured at 120 °C (1 h) followed by 150 °C (1 h). The resulting 300 µm film had protruding VACNT tips on both faces.

Morphology was examined by FESEM (Merlin Compact) and TEM (Tecnai G2 F20 S‑TWIN). Raman spectra (632.8 nm excitation) assessed tube structure. Thermal diffusivity and specific heat were measured by laser flash (Netzach LFA 467) and differential scanning calorimetry (DSC, Mettler Toledo DSC1), enabling calculation of thermal conductivity via λ = α × Cp × ρ.

Results and Discussion

Cross‑sectional SEM images (Fig. 2) reveal that VACNTs are successfully grown on Al₂O₃, TiO₂, and SiO₂, with the thickest, most densely packed arrays on Al₂O₃. In contrast, ZnO yields negligible VACNT growth, likely due to accelerated Ostwald ripening and Fe subsurface diffusion.

Raman analysis (Fig. 4) shows D, G, and G′ bands around 1360, 1580, and 2700 cm⁻¹ respectively. The I_D/I_G ratios are ≥1 across all buffers, confirming multi‑walled tubes; TEM images (Fig. 5) confirm predominantly triple‑walled VACNTs on Al₂O₃ and ≥four‑walled tubes on TiO₂, ZnO, and SiO₂.

Growth rate versus temperature (Fig. 6) shows an initial increase followed by a decline above 650 °C for all buffers, with Al₂O₃ maintaining higher rates beyond 600 °C. This suggests a longer catalyst lifetime on Al₂O₃ and a lower activation energy for nucleation compared to SiO₂.

The VACNT/graphene composite film exhibits enhanced thermal transport: vertical conductivity ≈1.25 W m⁻¹ K⁻¹ and transverse conductivity ≈2.50 W m⁻¹ K⁻¹ (Fig. 8). These values far exceed those of the epoxy matrix alone, confirming that VACNTs provide efficient vertical heat pathways while graphene augments lateral conduction.

Conclusions

ALD‑grown Al₂O₃ emerges as the optimal buffer layer for VACNT synthesis, delivering the thickest, densest, and best‑aligned arrays due to prolonged catalyst stability and lower nucleation barriers. Temperature control is critical, with growth rates peaking near 650 °C. Incorporating VACNTs onto graphene within an epoxy matrix yields a composite with markedly improved vertical and transverse thermal conductivities, underscoring the potential of this architecture for high‑performance thermal management.

Abbreviations

ALD:

Atomic layer deposition

C₂H₂:

Acetylene

CVD:

Chemical vapor deposition

DEZ:

Diethylzinc

DSC:

Differential scanning calorimeter

EB:

Electron‑beam

FESEM:

Field emission scanning electron microscopy

H₂:

Hydrogen

LFA:

Laser flash thermal analyzer

RBMs:

Radial breathing modes

TDMAT:

Tetrakis(dimethylamino)titanium

TEM:

Transmission electron microscopy

TIMs:

Thermal interface materials

TMA:

Trimethylaluminum

VACNTs:

Vertically aligned carbon nanotubes