Morphology engineering of vertically aligned carbon nanotubes for enhanced photoacoustic emission

The vertically aligned carbon nanotube (VACNT) array, characterized by a vertical arrangement of carbon nanotubes at the bottom and a chaotic crown-like structure at the top, is commonly employed in optical ultrasound emitters due to its high light absorptance and anisotropic thermal conductivity. However, the excessively dense carbon nanotubes at the bottom hinder the incorporation of elastic materials, which necessitates regulation of its morphological structure. To validate the proposed hypothesis, a series of VACNT arrays were synthesized using the chemical vapor deposition technique, which enabled precise control over the morphological properties of the arrays by adjusting the growth duration. Experimental results reveal that an 8 µm thick sample achieves the optimal performance balance, yielding a peak sound pressure of approximately 9.6 MPa, a -6 dB bandwidth of 21.2 MHz, and an energy conversion efficiency of 0.39% under 20mJ laser excitation. Microscopic structural analysis reveals that at this thickness, the carbon nanotube array achieves a vertically aligned yet relatively sparse configuration at its base, which effectively facilitates efficient directional heat transfer while ensuring sufficient infiltration of the elastomer. These findings provide critical insights into the design, fabrication, and performance optimization of high-efficiency VACNT-based photoacoustic transmitters.