William R. Silveira

A vertical inertial coarse approach for variable temperature scanned probe microscopy

We have developed a variable temperature inertial coarse approach mechanism for use in scanned probe microscopy. This reliable micropositioner has a coarse range of 2.5 mm and can take individual steps of less than 10 nm from 4 to 300 K. This simple, compact device is both nonmagnetic and glueless. It is operational in both horizontal and vertical geometries and is driven by a low voltage sawtooth waveform. We report on the design and performance of the device.

High-Sensitivity Electric Force Microscopy of Organic Electronic Materials and Devices

Conducting and semiconducting organic materials have long been known [1], [2], but recent advances in chemical synthesis [3], [4] have enabled organic materials to begin delivering on the promise of mass-produced economical electronic devices. Organic electronic materials are better suited for constructing high-efficiency light-emitting diodes [5]–[8], solar cells [9], [10], and cheap solution-processable thin-film transistors [6], [11]–[18] than are crystalline inorganic semiconductors such as silicon and gallium arsenide. The electronic/optical properties and solubility of organic materials can be tuned independently by chemical synthesis [4]. Since they can be processed and patterned at ambient temperature, organic electronic materials are compatible with flexible large-area substrates [19].

Non-ideal behavior in a model system: Contact degradation in a molecularly doped polymer revealed by variable-temperature electric force microscopy

We present an electric force microscope and transport study of the degradation of the contact between Au and TPD, a triarylamine widely employed as a hole transporting layer in light emitting diodes. TPD was dispersed into a polystyrene (PS) binder and spin casted onto a quartz substrate with coplanar gold electrodes. Electric force microscopy was used to map the electrostatic potential drop in the device channel while a voltage was applied and the current was measured. Two contact degradation mechanisms were observed. When the TPD-PS film was allowed to age in high vacuum, the TPD crystallized out of solution. We show that the observed loss of current is the result of both a decrease in bulk mobility and a decrease in injection efficiency. The operating temperature of a freshly prepared device was then varied from 296 K to 330 K to simulate heating that might occur during light emitting diode operation. While the current increased in an apparently smooth way as the temperature was raised, electric force microscopy revealed that the underlying injection efficiency had undergone a dramatic change. Above a temperature of 330 K, running current through the device led to a dramatic decrease in injection efficiency which we found was associated with the creation of a dipole layer at the injecting contact. Upon decreasing temperature, we found that a measurable charge remained in the device channel when the applied voltage was switched to zero. The decay of the associated electrostatic potential, which appears to be governed primarily by charge-charge repulsion and not diffusion, provides an estimate the zero-field mobility of the holes in the film.