For the 1st paragraph take a start by describing what you want to do in your PhD studies. It should be stated in 4 to 5 lines, so the reader may get a clear idea about your intentions.
So, start writing that your objective is to improve the topic of your research, you can write any method which fits your goal. Then write what work you are interested in. Write further 1–2 lines about the objectives you can achieve with this topic.
Try to avoid describing the importance of your idea in the first paragraph. Just be specific to your objective. Write why you want to work on it. Starting lines should cover the what you want to do and then after 5 to 6 lines you should cover why you want to do this.
In 2nd paragraph you need to explain what expertise you have to achieve. For example, write down that you have a chemical degree and how you worked on the aspect of your topic. Write down any internship experience and publication if you have regarding this. Any conference or meeting you attended which can link you to your objective.
3rd paragraph should state why you are intending to achieve your goal by getting this scholarship. This means why you cannot model your system in your country. Describe any technical deficiency you face, availability of labs or any other technical help. Then write about relevant U.S university and any professor you have seen working on your intended area.
In 4th paragraph, write how your work will help your home country and the way you will be working after coming back. Like, mention that you will be joining research institution or any government institution. It will reflect your dedication towards your goal.