Viscoelastic property mapping with contact resonance force microscopy.

We demonstrate the accurate nanoscale mapping of near-surface loss and storage moduli on a polystyrene-polypropylene blend with contact resonance force microscopy (CR-FM). These viscoelastic properties are extracted from spatially resolved maps of the contact resonance frequency and quality factor of the AFM cantilever. We consider two methods of data acquisition: (i) discrete stepping between mapping points and (ii) continuous scanning. For point mapping and low-speed scanning, the values of the relative loss and storage modulus are in good agreement with the time-temperature superposition of low-frequency dynamic mechanical analysis measurements to the high frequencies probed by CR-FM.

Spring constant calibration of atomic force microscopy cantilevers with a piezosensor transfer standard.

We describe a method to calibrate the spring constants of cantilevers for atomic force microscopy (AFM). The method makes use of a "piezosensor" composed of a piezoresistive cantilever and accompanying electronics. The piezosensor was calibrated before use with an absolute force standard, the NIST electrostatic force balance (EFB). In this way, the piezosensor acts as a force transfer standard traceable to the International System of Units. Seven single-crystal silicon cantilevers with rectangular geometries and nominal spring constants from 0.2 to 40 Nm were measured with the piezosensor method. The values obtained for the spring constant were compared to measurements by four other techniques: the thermal noise method, the Sader method, force loading by a calibrated nanoindentation load cell, and direct calibration by force loading with the EFB. Results from different methods for the same cantilever were generally in agreement, but differed by up to 300% from nominal values. When used properly, the piezosensor approach provides spring-constant values that are accurate to +/-10% or better. Methods such as this will improve the ability to extract quantitative information from AFM methods.

Optical detection of ultrasound with a microchip laser.

A novel optical ultrasonic detector that relies on frequency modulation of a microchip laser is proposed and demonstrated. When the laser is placed in a time-varying acoustic field, the microchip laser cavity length is periodically modulated, creating a frequency-modulated optical output in which the frequency shift is linearly proportional to the acoustic-wave amplitude. With a confocal Fabry-Perot slope filter and a Nd:YAG microchip laser operating at 1.06 microm, a detector response of 7.5 MHz/kPa was measured at an acoustic frequency of 7.75 MHz. A one-dimensional acoustic model is developed to explain the observed detector performance.