HRTEM

HRTEM

High-resolution transmission electron microscopy (HRTEM) is an imaging mode of the transmission electron microscope (TEM) that allows for direct imaging of the atomic structure of the sample. HRTEM is a powerful tool to study properties of materials on the atomic scale, such as semiconductors, metals, nanoparticles and sp2-bonded carbon (e.g. graphene, C nanotubes). While HRTEM is often also used to refer to high resolution scanning TEM (STEM, mostly in high angle annular dark field mode), this article describes mainly the imaging of an object by recording the 2D spatial wave amplitude distribution in the image plane, in analogy to a "classic" light microscope. For disambiguation, the technique is also often referred to as phase contrast TEM. At present, the highest point resolution realised in phase contrast TEM is around 0.5 ngstrms (0.050nm). At these small scales, individual atoms of a crystal and its defects can be resolved. For 3-dimensional crystals, it may be necessary to combine several views, taken from different angles, into a 3D map. This technique is called electron crystallography.

One of the difficulties with HRTEM is that image formation relies on phase contrast. In phase-contrast imaging, contrast is not necessarily intuitively interpretable, as the image is influenced by aberrations of the imaging lenses in the microscope. The largest contributions for uncorrected instruments typically come from defocus and astigmatism. The latter can be estimated from the so-called Thon ring pattern appearing in the Fourier transform modulus of an image of a thin amorphous film.

What is HRTEM

HRTEM is an instrument for high-magnification studies of nanomaterials. High resolution makes it perfect for imaging materials on the atomic scale. A main advantage of a TEM over other microscopes is that it can simultaneously give information in real space (in the imaging mode) and reciprocal space (in the diffraction mode). The instrument has a single tilt stage and maximum Tilt Angle of -10 to +10o in Goinometer and the instrument can operate in Bright-Field, Dark-Field, High resolution, SAED and CBED modes. It has a standard probe and a variable temperature probe (100 to 500 K). TEM is coupled with a Gatan digital camera for digital image processing. The instrument can go upto a maximum magnification of 1.5 million. It has an ACD (anti contamination device) working with the aid of liquid nitrogen, which helps the filament from contamination caused by volatile sample. Depending upon the sample compatibility, it can work at three different accelerating voltages i.e., 300, 200 and 100 kV. This has a filament made up of LaB6. The instrument works under a vacuum in the range 10-5 to 10-6 Pa. This gives a lattice resolution of 0.14 nm and a point to point resolution of 0.12 nm.

Theory of operation

Basic principle of TEM is quite similar to their optical counterparts, the optical microscope. The major difference is that in TEM, a focused beam of electrons instead of light is used to "image" and achieve information about the structure and composition of the specimen. An electron source usually named as the Gun produces a stream of electrons which is accelerated towards the specimen using a positive electrical potential. This stream is then focused using metal apertures and magnetic lenses called condenser lenses into a thin, focused, monochromatic beam. Beam strikes the specimen and a part of it gets transmitted through it. This portion of the beam is again focused using a set of lenses called objective lenses into an image. This image is then fed down the column through the intermediate and projector lenses, which enlarges the image, depending upon the set magnification. A phosphor image screen is used to produce the image. The image strikes screen and light is engendered, which enables the user to see the image. The darker areas of the image represent the thicker or denser region of the sample (fewer electrons were transmitted) and the lighter areas of the image represent those areas which are thinner or less dense (more electrons were transmitted)

Image contrast and interpretation

The contrast of a HRTEM image arises from the interference in the image plane of the electron wave with itself. Due to our inability to record the phase of an electron wave, only the amplitude in the image plane is recorded. However, a large part of the structure information of the sample is contained in the phase of the electron wave. In order to detect it, the aberrations of the microscope (like defocus) have to be tuned in a way that converts the phase of the wave at the specimen exit plane into amplitues in the image plane.

The interaction of the electron wave with the crystallographic structure of the sample is complex, but a qualitative idea of the interaction can readily be obtained. Each imaging electron interacts independently with the sample. Above the sample, the wave of an electron can be approximated as a plane wave incident on the sample surface. As it penetrates the sample, it is attracted by the positive atomic potentials of the atom cores, and channels along the atom columns of the crystallographic lattice (s-state model). At the same time, the interaction between the electron wave in different atom columns leads to Bragg diffraction. The exact description of dynamical scattering of electrons in a sample not satisfying the WPOA (almost all real samples) still remains the holy grail of electron microscopy. However, the physics of electron scattering and electron microscope image formation are sufficiently well known to allow accurate simulation of electron microscope images.

Simulated HREM images for GaN[0001]

As a result of the interaction with a crystalline sample, the electron exit wave right below the sample e(x,u) as a function of the spatial coordinate x is a superposition of a plane wave and a multitude of diffracted beams with different in plane spatial frequencies u (spatial frequencies correspond to scattering angles, or distances of rays from the optical axis in a diffraction plane). The phase change e(x,u) relative to the incident wave peaks at the location of the atom columns. The exit wave now passes through the imaging system of the microscope where it undergoes further phase change and interferes as the image wave in the imaging plane (mostly a digital pixel detector like a CCD camera). It is important to realize, that the recorded image is NOT a direct representation of the samples crystallographic structure. For instance, high intensity might or might not indicate the presence of an atom column in that precise location (see simulation). The relationship between the exit wave and the image wave is a highly nonlinear one and is a function of the aberrations of the microscope. It is described by the contrast transfer function.HRTEM can provide structural information at better than 0.2 nm spatial resolution. In most crystalline inorganic materials, including ceramics, semiconductors, and metals, the positions of individual atomic columns can be resolved, at least in low-index zones. When recorded under optimum conditions, electron micrographs can be directly interpreted in terms of the projected crystal potential. In other cases, image simulations are necessary to match proposed structures to image features. Digital image recording and quantification of diffraction pattern intensities is possible with the extreme linearity and high DQE of a CCD camera. Dynamic events induced by the electron beam or indirectly with a heating holder can be followed by video-tape recording from a TV-rate image pick-up system. At lower resolution, amplitude contrast images can be used to observe material features in the 1m-0.5nm range.

Possible Applications distribution and structure of defects, interfaces and grain boundaries

nano-crystalline features in amorphous films

small particle analysis in heterogeneous catalysts

sub-micron morphological and device features

thermodynamic decomposition, diffusion, and phase transformations

Specimen RequirementsFor highest resolution, specimens must be