Top Down

One way of making nanoscale structures and devices is to use traditional methods to guide the synthesis of materials.  All of these processes involve what's known as a 'top down' synthesis approach.  This paradigm dictates that you begin with a bulk material, and then slowly remove bits of it to form things like transistors on a microchip.  Speaking of microchips, let's use that as an example of a top down manufacturing process.

To make a microchip, you start with a thin, polished, silicon wafer.  Silicon is the material of choice for semiconductor wafers.  It's a stable, relatively cheap semiconductor that can be produced in large quantities from sand. 

So anyway, you take the silicon wafer and coat it with a layer of photoresist.  Then you use lithography to shine light on only certain parts of the wafer.  For most photoresists, the light breaks down that portion of the coating and frees it up for etching and doping.  Through a series of such processes, you can remove certain portions of the wafer and then dope them and fill them in with other materials.  This is a top down process because you started with a bulk material and made modifications to it by removing pieces.

Unfortunately, these techniques require the use of lithography, which is limited by what's known as the diffraction limit.  Lithography requires a mask that selectively protects portions of the wafer from light that will break down the photoresist.  The distance from the mask to the wafer, and the size of the slit will define the minimum feature size that you can squeeze out for a given frequency of light.  In general, with higher frequencies (shorter wavelengths) you can achieve finer features.  Current modern techniques use extreme ultraviolet light that is extremely powerful and can yield feature sizes of 90 nm across.

While that is getting close to where we want to be for nanotechnology, it's not enough because the fundamental limit of lithography will soon be reached.

That's okay though, because most lithography in nanotechnology is used for patterning of substrates.  This has proven to be an extremely useful approach at guiding the growth of quantum dots and nanowires.  For instance, you could pattern patches of gold onto a substrate with lithography.  Then you could heat up the gold and let it form small clusters on the surface once it melts.  From there, you could invoke a VLS synthesis method to grow nanowires strictly on the patterns you've created.  So far, this has worked in creating dense clusters of vertical nanowires in distinct patches.  This is one example of a top down approach to creating nanoscale materials.