Information

The diverse tools in the MCF can be sorted by their ability to do one or more of these three basic measurement techniques:

  • Microscopy – magnifying the physical features of a sample up to 10 million times to determine feature sizes, shapes and structure
  • Spectroscopy – sorting the particles or radiation emitted from a sample by energy or mass, usually to determine its composition, and
  • Diffraction – sorting the particles or radiation emitted from a sample by position or direction usually to determine its structure

Below are general overviews and links to more detailed information about each of these techniques.

Microscopy:
Electron Microscopy(SEM, (S)TEM, FIB-SEM):


Electron microscopes are divided into two main types – Scanning Electron Microscopes (SEM’s) and Transmission Electron Microscopes (TEM’s).

In an SEM, an electron beam, formed by electromagnetic and/or electrostatic lenses, is scanned across the surface of the sample and the reflected electrons are collected to form a magnified image.  In principle, SEMs can operate with electrons in a voltage range from ~100V-30kV, although in practice most imaging occurs between  ~1kV – 10kV.

Some good introductions to SEM:

Instead of using reflected electrons, TEMs send a beam of electrons through a sample.   Since the technique requires the electrons to pass through the sample, TEMs typically use voltages from 80kV-300kV to produce electrons with enough energy to penetrate the solid, and yield atomic resolution. Depending on the sample composition, the sample must be ~100 nm thick or thinner to allow electrons to traverse it.  Preparing samples that thin with very flat surfaces is a – often THE – critical step to obtaining high-resolution TEM data.  Some detailed introductions can be found here:

Several Georgia Tech professors write extensively on TEM theory.  The website of Prof. Z.L. Wang’s research group offers a very good introduction to TEM basic principles.

http://www.nanoscience.gatech.edu/zlwang/research/tem.html

Scanning TEM or STEM, is a related technique where the ion beam is rastered – as in an SEM – but the image is formed by transmitted electrons as in traditional TEM.   MCF has a dedicated STEM system, the Hitachi HD-2700.  Its ability to raster the electron beam makes it preferable for many analysis techniques like EDS (described in “Spectroscopy” below) over conventional TEM’s.  Below are introductory explanations of  how STEM systems work:

Electron microscopes can also be combined with  Focused Ion Beam sources to create a hybrid FIB-SEM system.  The ion gun utilizes a Liquid Metal Ion Source (LMIS), which can be used to image the surface of the sample but is more commonly used to remove material (nano-machining/ milling).  Typically the milled area is a vertical and/or angled cut used to expose a cross-section to examine the three-dimensional structure of the sample.  The ion beam can be scanned across the surface of the sample to etch regular shapes (e.g., rectangles, lines, circles, and polygons) or generate arbitrary patterns.  The beam can also catalyze deposition of various materials (e.g., Pt, C, W, etc.) to “write” fine lines (~50 nm minimum) as well as etching them.

Some websites that have further information:

Scanning Probe Microscopy(AFM, STM, nanoindentation):

Another method of imaging samples involves moving a sharp probe across the surface of a sample.  By  measuring and amplifying its deviations as it tracks the surface features, it is possible to build a three-dimensional image detailed enough to measure the size of individual atoms.  This family of techniques is called Scanning Probe Microscopy (SPM).

The most commonly used SPM tool is the Atomic Force Microscope (AFM).  It typically uses a MEMS-fabricated single crystal Si probe with a tip radius of about 10 nm.  Typically, the cantilever is either dragged across the surface (contact mode) or oscillated at its resonance frequency (tapping mode).  As a built-in feedback loop works to hold one parameter of the probe constant, the minute deviations in that parameter from its setpoint value become precise measurements for the micron- to angstrom-sized deviations of the surface.

Because the technique senses the change of probe position due to the tip-sample force, any force mechanism between two solid bodies can be used and measured.  There are numerous variants on basic AFM involving Electrostatic, Magnetic, and Piezoelectric forces (EFM, MFM, and PFM, respectively).  Also, in the modes where the probe contacts the surface; electrical, mechanical, and optical properties can be measured.  All of that additional information is obtained with basically the same precision – and typically at the same time – as the 3D spatial information.  Some brief introductions to AFM:

Nanoscale Tribology:

Material mechanical properties at the microscale or smaller can differ significantly from those in the bulk.  For bulk materials, several testing techniques are available, including scratch tests and indentation.  Nanoindentation systems provide similar – or enhanced – testing capabilities at the micro-scale and below.  Like AFM above, these systems use small probes of specific known geometries to apply a force to a sample.  By directly measuring the force applied and depth of penetration during the indentation, these tools can calculate material properties like Young’s modulus, hardness, and viscoelastic parameters over submicron distances.  Applied forces can range from micronewtons up to tens of newtons, depending on system configuration.  In many cases, nano-indenters can also image using a variation of Atomic Force Microscopy to give a 3D picture of the indented volume and material displacement after the test.  Here are some brief introductions to nanoindentation:

http://www.sciencedirect.com/science/article/pii/S136970210671495X

https://www.hysitron.com/resources-support/education-training/nanoindentation

https://www.bruker.com/fileadmin/user_upload/8-PDF-Docs/SurfaceAnalysis/TMT/Webinars/Quantitative_Nano-Mechanical_Characterization_of_Composites_and_Thin_Films-20120626.pdf

Spectroscopy:
EM-based Spectroscopy(EDS, EELS):

Many electron microscopes – including 7 of the SEM’s and TEM’s in the MCF – are equipped with Energy Dispersive X-ray Spectroscopy (EDS or EDX) detectors.  This technique analyzes X-rays generated when the high-energy electron beam hits the sample.  These X-rays have unique, characteristic energies that allow them to be matched to the element that generated them.  By this technique, it is possible to construct maps of how elements are distributed within a sample, and determine the absolute composition of a sample within ~1%.   EDS data only yields absolute composition ratios, and does not provide chemical state information.

Some good introductions to this technique:

Another EM-based spectroscopy technique is Electron Energy Loss Spectroscopy (EELS) which is available on the FEI Tecnai F30 TEM in the MCF.  This technique sorts the transmitted electrons by the amount of energy lost in inelastic electron scattering while transiting the sample.  The amount of energy lost in a collision is element-specific and is therefore used to map elemental composition and – more importantly – provide information on chemical bonding and valence & conduction band states.

A great introduction to this technique is here:

Surface Science Spectroscopy(XPS, UPS, ToF-SIMS):

Surface-science tools are typically high vacuum or Ultra-High Vacuum (UHV) systems.  The high vacuum level works to keep the sample surfaces pristine.  As the name indicates, these tools analyze only the outermost molecular layers of a material, – typically ~5 nm or less – use photon or particle beams to excite a sample, and generally measure particles ejected by the excitation source.

MCF’s primary x-ray spectroscopy tool is X-ray Photoelectron Spectroscopy (XPS), also known as ESCA (Energy Spectroscopy for Chemical Analysis).  In XPS a beam of monochromatic (single-wavelength) x-rays is directed onto a sample.  By the photoelectric effect, these incoming x-ray photons eject electrons ( hence “photoelectron”) from the the top 1-10 nm of the sample surface. These electrons have characteristic binding energies that provide chemical-state information.  By analyzing the energy distribution of these electrons it is possible to determine both the elements present (from lithium (Z=3) forward) and their chemical bonding state.  The XPS systems in the MCF can perform advanced functions such as line scans and mapping, along with depth profiling and angle-resolved XPS.   XPS can measure absolute elemental composition down to 0.1-0.2% (dependent on specific elements and bonding states).  The lateral resolution of the technique depends on the spot size, which varies from ~20 um to 400 um

Some good introductions to the technique are:

The Kratos XPS system in the MCF has the additional hardware necessary for Ultraviolet Photoelectron Spectroscopy (UPS).  This technique provides significant data for valence band interactions.  Here are a couple of introductions to this technique:

Another spectroscopy technique available in the MCF is Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS).  This technique uses a beam of ions (in this case Bi) to directly knock atoms out of the sample surface.  The ejected – secondary – ions are collected, accelerated to the same kinetic energy, and then allowed to separate based on their differing velocities/masses.  The time that it takes each ejected ion to travel to the detector (hence “time-of-flight”) is related to the mass of the ion.  By measuring the travel time with nanosecond precision, it is possible to determine what compounds are on the surface with very high mass resolution and sensitivity.   ToF-SIMS combines lateral resolution (~500nm) approaching that of an SEM with the depth resolution (~2nm) of XPS.  It is extraordinarily sensitive and can measure composition down to parts per billion.  The main drawback to the technique is  that direct atom counting only gives relative or at best semi-quantitative compositions.  Absolute composition must be calibrated using known standards or obtained from another technique (e.g., EDS or XPS).

Here are a couple of links describing the technique:

Optical Spectroscopy(Raman, FTIR):


Visible light provides most people with the majority of their information about the world.  In spectroscopy there are several techniques that make use of that same general spectral band to measure chemistry and structure.

Anyone who has seen a blue sky has seen the effects of light scattering (in that case elastic Rayleigh scattering).  Raman scattering spectroscopy analyzes light that has been scattered inelastically so that it has changed wavelength.  This wavelength change is a result of energy lost or gained after interaction with the vibrational states of the molecule that scatters the photon. By plotting the intensity of the scattered light versus the wavelength shift, one can obtain a spectral “fingerprint” for the molecules in a sample and their chemical and physical modifications.

Confocal microscopes focus light from a sample through a small pinhole to reject all light not in a very specific height plane – this allows them to create three-dimensional images of samples with high precision. When this type of microscopy is combined with Raman spectroscopy the result is a Confocal Micro-Raman spectrometer (or Micro-Raman) system.  This tool can generate 2D and even 3D chemical maps of samples with micron resolution.

Some good introductions are below:

Diffraction
X-Ray Diffraction(XRD):


X-ray diffraction is a technique that utilizes an incident x-ray beam to identify the structural properties of a sample. When the x-rays interact with the atoms of the sample, the beam is re-radiated into specific directions where there is constructive interference – i.e., diffracted – with characteristic intensities.  This set of diffracted x-rays forms a three-dimensional map of the electron density.  Analyzing the regular relationships in the diffraction pattern gives information to precisely determine the structure and spacing of the atomic lattice of the sample.

Originally, X-ray diffraction was used for to determine the phase – the precise crystalline compound(s) giving rise to a measured chemistry – in crystalline and powder materials.   Many other parameters can be measured, such as the thickness and in-plane stress for thin films.  Certain systems can also analyze molecular spacing and order in amorphous materials by Small-Angle X-ray Scattering (SAXS).   The Empyrean XRD system can perform SAXS measurements, and has a hot stage for performing temperature-controlled experiments up to 1200 °C.

Some introductions to the technique:

Sample prep capabilities

MCF staff can assist users in preparing samples for any of the above techniques.  The following techniques are available:

  • carbon evaporation (Cressington 108A carbon coater, Quorum Q-150 T ES)
  • gold / palladium sputter coating (60:40 Au/Pd alloy; Hummer V sputter coater system)
  • UV / ozone cleaning (SEM: Hitachi ZoneSEM)
  • UV/ ozone cleaning (TEM:  Hitachi ZoneTEM)
  • vacuum oven (drying / driving off solvent)
  • IR heat lamp (sample outgassing / adhesive curing)

MCF provides some types of SEM sample stubs and adhesive materials (e.g. carbon dots, carbon tape, copper tape, colloidal graphite / carbon paint, silver paint, high-temperature nickel paste, etc).