Growing a highly uniform and flat thin film of vertically aligned carbon nanotubes (VACNTs) using chemical vapor deposition (CVD) can be challenging, but it's possible with careful control and optimization of various parameters. Here's a breakdown of your questions:

Uniformity of VACNT Growth Using CVD:

1. Uniformity of VACNT Growth: Achieving a high degree of uniformity in VACNT films using CVD can be challenging but is essential for applications like absorptive substrates. Uniformity can vary depending on several factors:

- Catalyst Preparation: The uniform dispersion of catalyst nanoparticles on the substrate is crucial for uniform CNT growth.

- Substrate Quality: A smooth and clean substrate surface is essential to prevent variations in CNT growth.

- Gas Flow and Concentrations: Precise control of gas flow rates and concentrations is necessary to ensure uniform CNT growth.

- Temperature Control: Uniform temperature distribution across the substrate is critical for uniform growth.

- Substrate Movement: Some researchers rotate or move the substrate during growth to promote even gas exposure.

- Catalyst Thickness: The thickness of the catalyst layer can impact uniformity. Thinner catalyst layers may promote more uniform growth.

- Post-Growth Treatments: These may be applied to improve uniformity.

2. Alternatives to CVD: While CVD is commonly used for VACNT growth, other methods can be explored for producing flat and uniform substrates:

- Template-Assisted Growth: Using templates or patterned substrates can lead to more controlled and uniform CNT growth.

- Chemical Vapor Deposition with Plasma Enhancement: Plasma-enhanced CVD (PECVD) can enhance control over CNT growth and potentially improve uniformity.

- Layer-by-Layer Assembly: This involves depositing CNTs layer by layer to achieve control and uniformity.

- Pre-Grown CNT Films: Consider using pre-grown CNT films or sheets, which are commercially available and can offer good uniformity.

Visible Regime Absorber with a Flat Substrate:

To achieve a highly absorptive substrate in the visible regime that is also flat, consider these alternative approaches:

1. Photonic Crystal Structures: Design and fabricate photonic crystal structures that manipulate the propagation of light, enhancing absorption. These structures can be engineered for flatness.

2. Thin Film Coatings: Use thin film coatings with high absorption properties in the visible range. Materials like semiconductor thin films or plasmonic nanoparticles can be employed.

3. Metamaterials: Metamaterials are engineered structures designed to interact with light in unique ways. They can be tailored for absorption and flatness.

4. Multilayer Stacks: Create multilayer structures with alternating materials to control absorption and achieve flatness.

5. Antireflection Coatings: Apply antireflection coatings to minimize reflection and enhance absorption.

6. Surface Texturing: Introduce micro/nanostructures on the substrate's surface to enhance light absorption while maintaining flatness.

The choice of method depends on your specific requirements, including the degree of flatness, absorption efficiency, and ease of fabrication. Each approach has its advantages and challenges, so it's essential to evaluate them based on your application's needs and available resources. Additionally, collaborating with experts in materials science and optics can be beneficial for achieving the desired results.