using capacitance variations. electrostatically actuated mems have also been developed to accommodate very low applied forces in the 1-100 nn range. much of current research focuses on developing sample holders that can perform mechanical tests while creating an environmental stimulus such as temperature change, variable strain rates, and different gas environments. in addition, the emergence of high resolution detectors are allowing to monitor dislocation motion and interactions with other defects and pushing the limits of sub-nanometer strain measurements. in situ mechanical tem measurements are routinely coupled with other standard tem measurements such as eels and xeds to reach a comprehensive understanding of the sample structure and properties. aberration corrected tem modern research tems may include aberration correctors, to reduce the amount of distortion in the image. incident beam monochromators may also be used which reduce the energy spread of the incident electron beam to less than 0.15 ev. major aberration corrected tem manufacturers include jeol, hitachi high-technologies, fei company, and nion. ultrafast and dynamic tem it is possible to reach temporal resolution far beyond that of the readout rate of electron detectors with the use of pulsed electrons. pulses can be produced by either modifying the electron source to enable laser-triggered photoemission or by installation of an ultrafast beam blanker. this approach is termed ultrafast transmission electron microscopy when stroboscopic pump-probe illumination is used: an image is formed by the accumulation of many ultrashort electron pulses (typically of hundreds of femtoseconds) with a fixed time delay between the arrival of the electron pulse and the sample excitation. on the other hand, the use of single or a short sequence of electron pulses with a sufficient number of electrons to form an image from each pulse is called dynamic transmission electron microscopy. temporal resolution down to hundreds of femtoseconds and spatial resolution comparable to that available with a schottky field emission source is possible in ultrafast tem, but the technique can only image reversible processes that can be reproducibly triggered millions of times. dynamic tem can resolve irreversible processes down to tens of nanoseconds and tens of nanometers. the technique has been pioneered at the early 2000s in laboratories in germany (technical university berlin) and in the usa (caltech and lawrence livermore national laboratory ). ultrafast tem and dynamic tem have made possible real-time investigation of numerous physical and chemical phenomena at the nanoscale. an interesting variant of the ultrafast transmission electron microscopy technique is the photon-induced near-field electron microscopy (pinem). the latter is based on the inelastic coupling between electrons and photons in presence of a surface or a nanostructure. this method allows one to investigate time-varying nanoscale electromagnetic fields in an electron microscope, as well as dynamically shape the wave properties of the electron beam. limitations there are a number of drawbacks to the tem technique. many materials require extensive sample preparation to produce a sample thin enough to be electron transparent, which makes tem analysis a relatively time-consuming process with a low throughput of samples. the structure of the sample may also be changed during the preparation process. also the field of view is relatively small, raising the possibility that the region analyzed may not be characteristic of the whole sample. there is potential that the sample may be damaged by the electron beam, particularly in the case of biological materials. resolution limits evolution of spatial resolution achieved with optical, transmission (tem) and aberration-corrected electron microscopes (actem). the limit of resolution obtainable in a tem may be described in several ways, and is typically referred to as the information limit of the microscope. one commonly used value is a cut-off value of the contrast transfer function, a function that is usually quoted in the frequency domain to define the reproduction of spatial frequencies of objects in the object plane by the microscope optics. a cut-off frequency, qmax, for the transfer function may be approximated with the following equation, where cs is the spherical aberration coefficient and λ is the electron wavelength: q max = 1 0.67 ( c s λ 3 ) 1 / 4 . =\lambda ^)^.} for a 200 kv microscope, with partly corrected spherical aberrations ("to the third order") and a cs value of 1 µm, a theoretical cut-off value might be 1/qmax = 42 pm. the same microscope without a corrector would have cs = 0.5 mm and thus a 200-pm cut-off. the spherical aberrations are suppressed to the third or fifth order in the "aberration-corrected" microscopes. their resolution is however limited by electron source geometry and brightness and chromatic aberrations in the objective lens system. the frequency domain representation of the contrast transfer function may often have an oscillatory nature, which can be tuned by adjusting the focal value of the objective lens. this oscillatory nature implies that some spatial frequencies are faithfully imaged by the microscope, whilst others are suppressed. by combining multiple images with different spatial frequencies, the use of techniques such as focal series reconstruction can be used to improve the resolution of the tem in a limited manner. the contrast transfer function can, to some extent, be experimentally approximated through techniques such as fourier transforming images of amorphous material, such as amorphous carbon. more recently, advances in aberration corrector design have been able to reduce spherical aberrations and to achieve resolution below 0.5 ångströms (50 pm) at magnifications above 50 million times. improved resolution allows for the imaging of lighter atoms that scatter electrons less efficiently, such as lithium atoms in lithium battery materials. the ability to determine the position of atoms within materials has made the hrtem an indispensable tool for nanotechnology research and development in many fields, including heterogeneous catalysis and the development of semiconductor devices for electronics and photonics.