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A method to generate an isolated single-cycle attosecond pulse from the interaction of a high-power femtosecond laser pulse with a nano-tube array is demonstrated using a two-dimensional relativistic particle-in-cell simulation. The radiation mechanism is relativistic nonlinear Thomson scattering from the electrons in a target material. Coherent radiation is emitted in the direction of specular reflection for the incident laser pulse while the electrons make a bunch size smaller than a wavelength of the laser pulse. Maintaining the coherence of the radiation from the electrons is essential to get an intense attosecond duration, which is achieved by using a nano-tube array target and a sharply increasing laser pulse. Optimal conditions for attosecond pulse generation are investigated by parameter scanning over plasma density, target thickness and laser pulse duration. 6