Optical microscopy resolution power

The resolution or "resolving power" of a light microscope is usually specified as the minimum distance between two lines or points in the imaged object, at which they will be perceived as separated by the observer. The Rayleigh criterion [42] is extensively used in optical microscopy for determining the resolution of light microscopes. It imposes a resolution limit. The criterion is satisfied, when the centre of the Airy disc for the first object occurs at the first minimum of the Airy disc of the second. This minimum distance r can then be calculated by Equation (3). 

Transmission electron microscopy (TEM) is a powerful and mature microstructural characterization technique. The principles and applications of TEM have been described in many books [16 20]. The image formation in TEM is similar to that in optical microscopy, but the resolution of TEM is far superior to that of an optical microscope due to the enormous differences in the wavelengths of the sources used in these two microscopes. Today, most TEMs can be routinely operated at a resolution better than 0.2 nm, which provides the desired microstructural information about ultrathin layers and their interfaces in OLEDs. Electron beams can be focused to nanometer size, so nanochemical analysis of materials can be performed [21]. These unique abilities to provide structural and chemical information down to atomic-nanometer dimensions make it an indispensable technique in OLED development. However, TEM specimens need to be very thin to make them transparent to electrons. This is one of the most formidable obstacles in using TEM in this field. Current versions of OLEDs are composed of hard glass substrates, soft organic materials, and metal layers. Conventional TEM sample preparation techniques are no longer suitable for these samples [22-24], Recently, these difficulties have been overcome by using the advanced dual beam (DB) microscopy technique, which will be discussed later. 

The resolution of a conventional microcope is limited by the classical phenomena of interference and diffraction. The limit is approximately X/2, X being the wavelength. This limit can be overcome by using a sub-wavelength light source and by placing the sample very close to this source (i.e. in the near field). The relevant domain is near-field optics (as opposed to far-field conventional optics), which has been applied to microscopy, spectroscopy and optical sensors. In particular, nearfield scanning optical microscopy (N SOM) has proved to be a powerful tool in physical, chemical and life sciences (Dunn, 1999). 

When the step separation is wide enough, typical spiral step patterns observable by optical microscopy may appear, but if the separation becomes narrower than the resolution power of the optical microscope, the spirals appear in the forms of polygonal pyramids or conical growth hillocks. Even if spiral patterns are not directly observable, we may assume that these growth hillocks are formed by the spiral growth mechanism. Examples representing the two cases are compared in Fig. 5.8. 

The optical microscope is a sophisticated instrument capable of providing images with a resolution of the order of 1 p,m, molecular information via birefringence, and chemical information via colour changes or through the use of specific dyes. When these factors are combined with relative ease of sample preparation (c.f. electron microscopy) and purchase cost, optical microscopy is a powerful technique for the study of many materials, particularly those that transmit in the visible region of the spectrum. 

Optical near-field microscopy (SNOM and NSOM [24,25]) is a novel technique with a spatial resolution power well beyond the diffraction limit. It was applied to image the fluorescence of single molecules at room temperature [26, 27] (see Chapter 2). Typical spectral linewidths reported were around 20 000 GHz [27], due to the room temperature operation. At liquid helium temperatures the linewidths are reduced to about 10-100 MHz, which allows study of the special effects on single molecular resonators. Moemer et al. [28, 29] combined the SNOM-technique with Single Mol-... 

---