Quantitative electrochemical measurements using in situ ec-S/TEM devices.

Insight into dynamic electrochemical processes can be obtained with in situ electrochemical-scanning/transmission electron microscopy (ec-S/TEM), a technique that utilizes microfluidic electrochemical cells to characterize electrochemical processes with S/TEM imaging, diffraction, or spectroscopy. The microfluidic electrochemical cell is composed of microfabricated devices with glassy carbon and platinum microband electrodes in a three-electrode cell configuration. To establish the validity of this method for quantitative in situ electrochemistry research, cyclic voltammetry (CV), choronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) were performed using a standard one electron transfer redox couple [Fe(CN)6]3-/4--based electrolyte. Established relationships of the electrode geometry and microfluidic conditions were fitted with CV and chronoamperometic measurements of analyte diffusion coefficients and were found to agree with well-accepted values that are on the order of 10-5 cm2/s. Influence of the electron beam on electrochemical measurements was found to be negligible during CV scans where the current profile varied only within a few nA with the electron beam on and off, which is well within the hysteresis between multiple CV scans. The combination of experimental results provides a validation that quantitative electrochemistry experiments can be performed with these small-scale microfluidic electrochemical cells provided that accurate geometrical electrode configurations, diffusion boundary layers, and microfluidic conditions are accounted for.

Novel MEMS-based gas-cell/heating specimen holder provides advanced imaging capabilities for in situ reaction studies.

In prior research, specimen holders that employ a novel MEMS-based heating technology (Aduro™) provided by Protochips Inc. (Raleigh, NC, USA) have been shown to permit sub-Ångström imaging at elevated temperatures up to 1,000°C during in situ heating experiments in modern aberration-corrected electron microscopes. The Aduro heating devices permit precise control of temperature and have the unique feature of providing both heating and cooling rates of 106°C/s. In the present work, we describe the recent development of a new specimen holder that incorporates the Aduro heating device into a "closed-cell" configuration, designed to function within the narrow (2 mm) objective lens pole piece gap of an aberration-corrected JEOL 2200FS STEM/TEM, and capable of exposing specimens to gases at pressures up to 1 atm. We show the early results of tests of this specimen holder demonstrating imaging at elevated temperatures and at pressures up to a full atmosphere, while retaining the atomic resolution performance of the microscope in high-angle annular dark-field and bright-field imaging modes.

Controllable molecular modulation of conductivity in silicon-based devices.

The electronic properties of silicon, such as the conductivity, are largely dependent on the density of the mobile charge carriers, which can be tuned by gating and impurity doping. When the device size scales down to the nanoscale, routine doping becomes problematic due to inhomogeneities. Here we report that a molecular monolayer, covalently grafted atop a silicon channel, can play a role similar to gating and impurity doping. Charge transfer occurs between the silicon and the molecules upon grafting, which can influence the surface band bending, and makes the molecules act as donors or acceptors. The partly charged end-groups of the grafted molecular layer may act as a top gate. The doping- and gating-like effects together lead to the observed controllable modulation of conductivity in pseudometal-oxide-semiconductor field-effect transistors (pseudo-MOSFETs). The molecular effects can even penetrate through a 4.92-mum thick silicon layer. Our results offer a paradigm for controlling electronic characteristics in nanodevices at the future diminutive technology nodes.

A new MEMS-based system for ultra-high-resolution imaging at elevated temperatures.

In recent years, an increasing number of laboratories have been applying in situ heating (and ultimately, gas reaction) techniques in electron microscopy studies of catalysts and other nanophase materials. With the advent of aberration-corrected electron microscopes that provide sub-Angström image resolution, it is of great interest to study the behavior of materials at elevated temperatures while maintaining the resolution capabilities of the microscope. In collaboration with Protochips Inc., our laboratory is developing an advanced capability for in situ heating experiments that overcomes a number of performance problems with standard heating stage technologies. The new heater device allows, for example, temperature cycling from room temperature to greater than 1000 degrees C in 1 ms (a heating rate of 1 million Centigrade degrees per second) and cooling at nearly the same rate. It also exhibits a return to stable operation (drift controlled by the microscope stage, not the heater) in a few seconds after large temperature excursions. With Protochips technology, we were able to demonstrate single atom imaging and the behavior of nanocrystals at high temperatures, using high-angle annular dark-field imaging in an aberration-corrected (S)TEM. The new capability has direct applicability for remote operation and (ultimately) for gas reaction experiments using a specially designed environmental cell.

An improved holey carbon film for cryo-electron microscopy.

Two issues that often impact the cryo-electron microscopy (cryoEM) specimen preparation process are agglomeration of particles near hole edges in holey carbon films and variations in vitreous ice thickness. In many cases, the source of these issues was identified to be the residues and topography often seen in commercially available films. To study and minimize their impact during specimen preparation, an improved holey carbon film has been developed. Rather than using a consumable template based on soft materials that must be removed prior to grid assembly, a method was developed that uses a hard template and a water-soluble release layer to replicate the template pattern into the carbon films. The advantages of this method are the improved purity and flatness of the carbon films, and these attributes are shown to have a dramatic improvement on the distribution of single particles embedded in vitreous ice suspended across the holes. Improving particle distribution is an enabling factor toward increasing the throughput of data collection for cryoEM.

Controlled modulation of conductance in silicon devices by molecular monolayers.

We have controllably modulated the drain current (I(D)) and threshold voltage (V(T)) in pseudo metal-oxide-semiconductor field-effect transistors (MOSFETs) by grafting a monolayer of molecules atop oxide-free H-passivated silicon surfaces. An electronically controlled series of molecules, from strong pi-electron donors to strong pi-electron acceptors, was covalently attached onto the channel region of the transistors. The device conductance was thus systematically tuned in accordance with the electron-donating ability of the grafted molecules, which is attributed to the charge transfer between the device channel and the molecules. This surface grafting protocol might serve as a useful method for controlling electronic characteristics in small silicon devices at future technology nodes.

Scaling constraints in nanoelectronic random-access memories.

Nanoelectronic molecular and magnetic tunnel junction (MTJ) MRAM crossbar memory systems have the potential to present significant area advantages (4 to 6F(2)) compared to CMOS-based systems. The scalability of these conductivity-switched RAM arrays is examined by establishing criteria for correct functionality based on the readout margin. Using a combined circuit theoretical modelling and simulation approach, the impact of both the device and interconnect architecture on the scalability of a conductivity-state memory system is quantified. This establishes criteria showing the conditions and on/off ratios for the large-scale integration of molecular devices, guiding molecular device design. With 10% readout margin on the resistive load, a memory device needs to have an on/off ratio of at least 7 to be integrated into a 64 x 64 array, while an on/off ratio of 43 is necessary to scale the memory to 512 x 512.

Clustering effects on discontinuous gold film NanoCells.

Reproducible negative differential resistance (NDR)-like switching behavior is observed in NanoCells. This behavior is attributed to the formation of filaments and clusters between the discontinuous gold films. Control experiments are performed by self-assembly of insulating molecules between the gold islands and conducting molecules on these islands. Additional control experiments are performed by removing the filaments and clusters between islands using a piranha bath. The results are consistent with theoretical predictions and extend the domain of molecular electronics based in organic molecules to include nanosized clusters as active units. This facilitates a scenario where synthetically accessible organic molecules, with defined characteristics, can be adjusted by metallic nanoclusters as an in situ fine-tuning element, able to compensate for the lack of addressing in the nanosize regime.

NanoCell electronic memories.

NanoCells are disordered arrays of metallic islands that are interlinked with molecules between micrometer-sized metallic input/output leads. In the past, simulations had been conducted showing that the NanoCells may function as both memory and logic devices that are programmable postfabrication. Reported here is the first assembly of a NanoCell with disordered arrays of molecules and Au islands. The assembled NanoCells exhibit reproducible switching behavior and two types of memory effects at room temperature. The switch-type memory is characteristic of a destructive read, while the conductivity-type memory features a nondestructive read. Both types of memory effects are stable for more than a week at room temperature, and bit level ratios (0:1) of the conductivity-type memory have been observed to be as high as 10(4):1 and reaching 10(6):1 upon ozone treatment, which likely destroys extraneous leakage pathways. Both molecular electronic and nanofilamentary metal switching mechanisms have been considered, though the evidence points more strongly toward the latter. The approach here demonstrates the efficacy of a disordered nanoscale array for high-yielding switching and memory while mitigating the arduous task of nanoscale patterning.