As I was preparing to sign-off from the blog, I noticed that one of my earlier posts hadn't gone through (blame the spotty connectivity). So, just in case you're interested in hearing about it again, here's a trackback to Sunday's Fred Kavli Distinguished Lecture in Nanoscience...
Just got out of the Fred Kavli Distinguished Lecture in Nanoscience, and distinguished it was! Prof. Charles Lieber of Harvard University presented a dense lecture entitled Semiconductor Nanowires: A Platform for Nanoscience and Nanotechnology in which he discussed a large proportion of his group's recent research. While George Whitesides is definitely established as Harvard's big man on campus when it comes to self-assembly or micro-scale phenomena, Prof. Lieber seems to have made a name for himself on the nanoscale. Despite Prof. Lieber's background in Chemistry, the presentation was pretty devoid of synthetic schemes, focusing instead on the applications of his work to low-dimensional systems. He broke it up into two sections, the first of which covered the properties of nanostructured photovoltaic (PV) nanowires, with the second dealing with interfaces between nanoelectronic and biological materials.
Part I - Nanowires
The first question Lieber looked to answer was: What makes an ideal system? He emphasized the need to have materials with tuneable compositions and properties. Looking at the example of Silicon (Si), he remarked that Si only really becomes useful when it is combined with other compounds. With this in mind he demonstrated how materials with determined and determinable properties can be created, in a fairly ab initio manner, by employing hybrid multicomponent materials as building blocks. He elaborated on this idea by presenting some radial core/shell nanostructures composed of single-crystalline Germanium (Ge) cores over which epitaxial Sillicon shells had been grown (i.e. over which Si had been grown in such a manner that its crystalline orientation is the same as that of the Ge core). Lieber specified that this core/shell structure effectively isolated the core from surface effects/inhomogeneities present on substrates and showed how, in some cases, this resulted in semiconductor mobilities approaching the ballistic limit!
Lieber the presented an extension of this idea, whereby an extra layer was added between the core and shell to yield p-i-n layered nanostructures with photovoltaic properties. Among the more impressive figures mentioned, Lieber presented a nanowire PV device with an open-circuit voltage (Voc) of 0.260 V, and concomitant operational stability of 12+ months. Lieber provided other examples of structures for which the Voc achieved neared 0.5 V: quasi ideal values. He also showed how his team was attempting to understand the fundamental limits to core/shell nanowires by looking into the absolute quantum efficiencies of several of the different organic structures. Interestingly, by slightly varying the composition of the PV nanowires, Lieber was able to tune the nanowire's absorption spectrum to absorb most strongly in the regions of high solar optical density. The take home message of this part, though, I think was that nanowire PVs are sufficiently developed to be able to use them as power output sources for nanoelectronic chips!
Part II - Interface between Biology and Nanoelectronics
There are many examples of technologies which require electronics to interface with biological components (for instance, EKGs). However, electronic components and devices have yet to be merged anywhere near to the scale of natural, biological processes. That is to say, the probes we currently use to interface with biological systems are orders of magnitude larger than the ion channels, substrate synapses and signalling molecules which compose biological systems. Charles Lieber's vision of the future in this regard is can be compared to the history of computing: he is persuaded that we are in the age of vacuum tubes (micro-scale probes) and that once we make the big step towards solid-state transistors (nanoscale probes), sweeping improvements will result.
As device dimensions keep decreasing, a critical parameter (the surface to volume ratio) will keep increasing, resulting in increasing device sensitivity. For example, we can now complete multiplex room temperature detection of disease markers, including single viruses, with 100 billion fold sensitivity! The field is still progressing towards the detection of single molecules and, more importantly, towards the detection of single molecule binding events. Lieber then related a number of examples of his team's work in this regard, including designing nano-scale tips for cellular patch-clamping experiments.