Cryogenic Variable Temperature Scanning Tunneling Microscope (24108)
An advanced cryogenic variable temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) has been developed at Northwestern University. Incorporation of a new design and materials provides a robust instrument capable of high atomic resolution at 8.2 to 300 K, critical for single molecule surface studies.
ADVANTAGES The cryogenic UHV STM provides excellent thermal and vibrationally stable operation at 8.2 to 300 K. Unit design accommodates direct optical access to the tip-sample interface and direct line-of-sight dosing of the sample in the STM. Feedback-controlled lithography (FCL) enables patterning of individual molecules on surfaces with atomic resolution at cryogenic temperatures.
SUMMARY: Studies of the adsorption of individual molecules on surfaces are enabled by the cryogenic variable temperature UHV STM. In particular, surface diffusion of physisorbed species may be controlled or eliminated, and thermal drift is significantly attenuated at cryogenic temperatures. Although a variety of cryogenic variable temperature UHV STM designs have been presented in the literature, the essential design problem centers between adequate coupling to the cryogen to maintain a stable low temperature while being sufficiently decoupled from any source of environmental vibrations.
This instrument utilizes the ease of use and thermal stability of a dual thermal shield continuous flow cryostat cooling system and the atomic resolution FCL capability of the STM scanner and controls. Present innovations include a Shapal-M® aluminum nitride ceramic used for heat-sinking electrical isolation, a beryllium copper sample holder to improve platform durability and thermal properties, and use of titanium nitride coated platform rails to enhance inertial translation of the sample holder at cryogenic temperatures. By varying the cryogen flow rate and actively heating the thermal shields, the STM may be operated at any temperature from 8.2 to 300 K. The design allows for dosing of samples with an ex situ source through a variable leak valve or with an in situ evaporation/sublimation module. The system affords excellent drift stability (~ 0.008 A/min tip-sample spacing @ 8.2 K), which was unchanged at sustained background acoustic noise levels up to 68 dB. In addition, shuttered sapphire windows allow direct optical access of the tip-sample junction at cryogenic temperatures. Atomic resolution FCL on hydrogen passivated Si(100) surfaces and cryogenic STM studies of individual molecules on degenerately doped silicon surfaces at cryogenic temperatures have been demonstrated with the device.
REFERENCE: Rev. Sci. Instrum., 75, 5280 (2004).
STATUS: The system is fully operational. U.S. Patent 7,414,250
Mark Hersam and Edward Foley
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