AVS2009 Session NS-WeM: Nanoscale Devices and Sensors and Welch Award

Recent years have seen dramatic increases in the ability to synthesize new bulk and nanostructured materials. For many applications, such as sensing and renewable energy, the need for high stability drives a motivation for working with two classes of ‘ultra-stable’ materials: (1) nanoscale carbon, and (2) nanocrystalline metal oxides. While these materials are highly stable they do not provide important properties such as chemical or biological selectivity. Molecular surface chemistry can be used to turn these into “smart” materials by linking molecules that will convey chemical or biological selectivity for sensing or desirable electron-transport properties for applications in renewable energy. We have found that photochemical grafting of alkenes provides a nearly universal method for producing molecular monolayers on a wide range of highly stable materials. Surprisingly, this method works on both wide-bandgap semiconductors such as diamond and TiO₂, as well as metallic materials such as carbon nanofibers. Through a series of studies we have identified the underlying mechanism as an internal photoemission process in which ultraviolet light ejects an electron from the material into an adjacent reactive liquid, leaving behind a reactive (and persistent) hole that serves as a reactive site for molecular grafting. With suitable chemistry, this approach can be used to fabricate (bio)molecular interfaces with a high degree of functionality that can be used (for example) to achieve direct biological-to-electronic signal conversion for sensing. Recent work also shows this to be an excellent approach for novel types of electrocatalytic interfaces of interest for applications in renewable energy. In this talk I will discuss some of our efforts in making and understanding “smart” molecular and biomolecular interfaces to nanocrystalline materials, and some of the resulting applications in sensing and in renewable energy.

Among the emerging devices for the future technology in the nanoscale regime, silicon nanowire (SiNW) devices have received significant attention for applications in logic gates, interconnects, photo detectors and biological and chemical sensors. For biosensing, femtomolar (fM) level detection of protein in solution has been demonstrated using both chemically synthesized nanowires [1] and lithographically defined nanowire field effect transistors (FETs) [2]. This type of sensor offers ultrasensitivity, rapid electronic readout, and does not require bulky sensing apparatus. It has a strong potential to be an ultra-portable and low cost biosensing platform that is badly needed for disease diagnostics and early detection.

In this work, we present our recent work on the similar nanowire FETs defined by e-beam lithography and standard Si processing technologies. This approach provides excellent manufacturability and feasibility of integrating circuitry with the sensor array on a single chip for ultimate system miniaturation and low manufacturing cost. In our process, SiNWs are defined by e-beam lithography in hydrogen silses quioxane resist, followed by a new two-step Si etch process, which is designed to improve the process reliability and controllability. SiNW FETs of 30-100 nm in widths, 10-30 nm in thickness, 5-80 µm in length have been successfully fabricated on Si on insulator (SOI) substrates. Uniform devices with sub threshold slope (SS) of 80 mV/dec and On/off ratio greater than 10⁷ have been made reproducibly. We will present various device design and fabrication considerations for using nanoelectronic FETs as biosensors, e.g. source/drain doping effects, oxidation effects, plasma treatment effects, buffer solution effects and stability. These device issues are quite different from conventional use of nanowires in logic gates. We further integrate these SiNW FETs with SU8 microfluidic channels to detect proteins at low abundance in solution. Preliminary results have indicated a sensitivity or detection limit of 2fM. We expect to present controlled sensing results of protein biomarker under controlled flow conditions.

[1] Patolsky, F., Zheng, G and Lieber, C.M, Nat.Protocols 1, 1711-1724 (2006)

[2] Eric Stern, et al. Nature 445, 519-522 (2007)

Although surfaces or, more precisely, surface atoms determine the way how materials interact with their environment, the influence of surface chemistry on the bulk of the material is generally considered to be small. However, in the case of high surface area materials such as nanoporous gold the influence of surface properties can no longer be neglected. Therefore, actively controlling surface properties such as diffusion barriers and surface stress by surface chemistry should provide an opportunity to manipulate and fine-tune material properties. Specifically, we will show that surface chemistry is an important factor in determining the stability of nanostructured gold surfaces, and that macroscopic strain can be generated by surface-chemistry induced changes of the surface stress. The latter effect can be used to directly convert chemical energy into a mechanical response without generating heat or electricity first and thus opens the door to surface-chemistry driven actuator and sensor technologies.

Prepared by LLNL under Contract DE-AC52-07NA27344.