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When and How to Choose A Microscope Cryostat.

Over the past 5 years, people have been finding more applications for compact microscope cryostats. Current research involves trying to resolve ever-smaller signals, and a microscope cryostat permits the use of both a stronger excitation source and a more efficient detector.

Microscope cryostats are divided into two main groups, although the division is far from rigid. Nitrogen cryostats for temperatures to 77 K are used primarily in optical microscopy for pharmaceutical and food-science research in freeze drying, geological research in fluid inclusions, and biological research. Helium cryostats, for temperatures to 4.2 K and lower, are used primarily for applications in Raman, photoluminescence, and infrared (IR) spectroscopy. The cryostats may be used with either a microscope or conventional spectrometer. Low temperatures narrow spectral lines associated with the Raman excitations and sharpen and intensify the spectral features in photoluminescence.

Several parameters are worth considering when choosing a cryostat, and some compromises may be needed. The choice of window materials ranges from Spectrosil B fused quartz for measurements in the ultraviolet (UV), visible, and near IR to Mylar for the far IR.

A large collection angle is important for light collection measurements, such as luminescence and Raman studies. Large, clear access is important for small-signal measurements that benefit from a large illumination area (IR and UV-Visible absorption spectroscopy), and 25-mm window diameters are available.

Some applications, however, require the thinnest possible windows to minimize spherical aberrations. A thinner window limits the aperture size because of the vacuum forces, and a compromise may be necessary. For example, a typical window thickness would be around 1.5 mm, but a 0.5-mm window reduces the sample illumination diameter to 10 mm.

In variable-temperature experiments, if the sample temperature closely tracks any temperature changes set by the controller, the duration of the experiment and cryogen usage are minimized. In some cryostats, the actual temperature lags behind the set temperature for 2 in or less. In other applications, for example photoluminescence-mapping experiments, the sample temperature must remain constant for the entire experiment, which may last many hours. Modern designs maintain temperatures as low as ~3 K for hours at a time with a temperature stability of better than [+ or -] 0.1 K.

High-resolution small-signal measurements and spatial mapping require the mechanical stability of the cryostat to exceed the optical spatial resolution throughout the experiment. One cryostat that does this is the [Microstat.sup.HiRes] from Oxford Instruments, Witney, UK. Such cryostats give a spatial resolution better than 0.7 [micro]m during a 3-hr measurement time. A short distance between the sample and window is also advantageous for high-resolution work as it permits the use of high-magnification objective lenses.

In addition to the factors already mentioned, several others are worth considering:

* Sample holders are available for either transmission or reflection measurements.

* Many experiments subject the sample to optical or electrical excitation. Users should check that cryostats have sufficient cooling power.

* Knowing the temperature difference between the sample position and where the temperature is actually measured is important for some applications. For some cryostats, the temperature difference is insignificant above 6 K.

* Cool-down time from room temperature should be as short as possible for optimal throughput. Cooling times from 300 to 4.2 K in 7 in or less should be possible.

* Vacuum loading means that there is no absorption/ reflection by the cryogenic exchange gas. This enables the use of high-magnification objective lenses. However, this sample environment means that samples have to be thermally well mounted to the sample holder.

Consider some recent applications. A microscope cryostat may be of benefit in an area you had not considered.

A high-resolution capability is particularly valuable in micro-Raman scattering and micro-photoluminescence studies on semiconductor materials. The use of such cryostats has revealed remarkable modulations in the optical emissions of a 2-D electron gas in a semiconductor close to the Fermi edge. High resolution has also revealed that, at low temperatures, an isolated quantum wire behaves as a series of quantum dots. The photoluminescence signal transforms from a broad spectral and spatial band, at 50 K, to a set of spectrally discrete and intense peaks, at 4 K.

Raman spectroscopy at liquid nitrogen temperatures can also be used to reveal diamonds that have been temperature or pressure treated to make them clear and distinguish them from more valuable, naturally clear diamonds.

Microscope-mounted cryostats and optical microscopy are proving invaluable in pharmaceutical research. Controlling sample temperature in situ allows real-time observation of the freeze drying process. With a typical turn-around time of 0.5 hr, many samples can be screened in the time normally required for a single trial, and details of problems, such as skin formation, can be observed during specific stages of the process. Real-time observation of freeze drying has determined the collapse temperature of both pharmaceuticals and food, important because a product processed below the collapse temperature cannot be regenerated by adding water.

Gould is a product manager at Oxford Instruments, Witney, UK.
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Author:Gould, Louise
Publication:R & D
Article Type:Brief Article
Date:Nov 1, 2000
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