1. CEMES-CNRS, nMat Group, 29 rue Jeanne Marvig, 31055 Toulouse, France

Correspondence to: Martin Hÿtch¹ Correspondence and requests for materials should be addressed to M.H. (Email: hytch@cemes.fr).

Strained silicon is now an integral feature of the latest generation of transistors and electronic devices^{1, 2, 3} because of the associated enhancement in carrier mobility^{4, 5}. Strain is also expected to have an important role in future devices based on nanowires⁶ and in optoelectronic components⁷. Different strategies have been used to engineer strain in devices, leading to complex strain distributions in two and three dimensions^{8, 9}. Developing methods of strain measurement at the nanoscale has therefore been an important objective in recent years but has proved elusive in practice^{1, 10}: none of the existing techniques combines the necessary spatial resolution, precision and field of view. For example, Raman spectroscopy or X-ray diffraction techniques can map strain at the micrometre scale, whereas transmission electron microscopy allows strain measurement at the nanometre scale but only over small sample areas. Here we present a technique capable of bridging this gap and measuring strain to high precision, with nanometre spatial resolution and for micrometre fields of view¹¹. Our method combines the advantages of moiré techniques¹² with the flexibility of off-axis electron holography¹³ and is also applicable to relatively thick samples, thus reducing the influence of thin-film relaxation effects.