rust - error[E0412]: このスコープで型 `String` が見つかりません

Question

#![cfg_attr(not(feature = "std"), no_std)]

use ink_lang as ink;

#[ink::contract]

mod XYZ {

    #[ink(storage)]
    pub struct Xyz {
        token_id: u32,
        serial_code: String
    }
    ...
}

エラー：

             serial_code: String
   |                      ^^^^^^ not found in this scope