Size control: A one-step alkali metal-assisted electrochemical method for the fabrication of carbon quantum dots (CQDs) with size-dependent photoluminescence is presented (see picture). Their upconversion luminescence may provide a way for the development of novel photocatalysts. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.