What is the fine-grained topic of the following text: In late 2007, Solar Fields acquired a 7% stake in the company, giving Calyxo GmbH full rights to the technology and the newly established US subsidiary, Calyxo USA , Inc.In February 2011, Solar Fields takes over Q-Cells shares. Only return the topic.
Solar Fields' acquisition and stake in Calyxo GmbH and Q-Cells shares.