Great question! Poe was definitely focused on creating mood and his writing style, which he named "arabesque," was developed specifically to create mood:
Poe believed that a story should create a mood in a reader, or evoke emotions in order to be successful, and that it should not try to teach the reader a lesson. He called his style ‘‘arabesque,’’ and it was notable for its ornate, intricate prose that sought to create a feeling of unsettlement in the reader...In ‘‘The Tell-Tale Heart,’’ an example of arabesque prose is when the narrator describes sneaking into the old man’s room in the middle of the night: ‘‘I heard a slight groan, and I knew it was the groan of mortal terror. It was not a groan of pain or of grief—oh no!—it was the low stifled sound that arises from the bottom of the soul when overcharged with awe.’’ Instead of simply stating that he had heard a groan, the narrator describes the sound in detail, creating in the reader a sense of suspense and foreboding.
You might consider having students work in pairs to identify specific examples of arabesque prose, then they can simplify the words, and finally they can explain how the more ornate wording contributes to the mood. Another activity might be to show a brief clip of a suspenseful horror film, then ask your students to write a description of the scene in the style that Poe might have used.
Good luck with your lesson!