Positional or Robotic Assembly

It has been a long-standing dream of scientists to manipulate individual atoms and molecules in the same way we manipulate macroscopic-sized objects. This is now achieved through positional (robotic) assembly devices.

In olden days there was the talk of alchemy. Now we have positional assembly in hand. We cannot build atoms through positional assembly, but we can make molecules and various materials made of molecules. According to Ralph C. Merkle [5] if we rearrange the atoms in coal we can make diamond, if we rearrange the atoms in sand (and add a few other trace elements) we can make computer chips, and if we rearrange the atoms in dirt, water and air we can make potatoes. While we have not done so any of these, yet, the prospects of doing them are here with the new interrogating and manipulating devices available in nanotechnology. Positional assembly (also called robotic assembly) refers to several methods that are being developed and can be used to create extremely small devices, machines and systems a millionth of a millimeter in size through manipulation and arrangement of atoms and molecules. In positional assembly, the technician is able to see, control and analyze what is taking place in nanoscale as the construction process is carried out.

In positional or robotic assembly each MBB is positioned in a certain coordinate using a robotic arm like an AFM tip [1,2]. The efficiency of this approach is dependent, to a large extent, on the improvements in the field of atomic force microscopy [6,7]. A familiar example of natural positional assembly is protein synthesis control by the ribosome [2].

The major difficulty in manual-design of positional assembly is overcoming the thermal noise, which can cause positional uncertainty [1]. Such thermal noise could be due to the fact that MBBs may not be stiff enough or the devices used for positional assembly may have vibrations due to thermal stress and strain. This problem can be solved, to some extent, by using stiff and rigid MBBs (like diamondoids) and also lowering the temperature during assembly. Assemblers with robotic arms also need to be developed in order to gain control over steric three-dimensional orientations of a molecule [8].

There are two strategies to any positional assembly [1]: (i). Additive synthesis in which MBBs are arranged to construct the desired nanostructure. (ii). Subtractive synthesis in which small blocks are removed from a large building block or a primitive structure to form an eventual structure (like sculpture). In contrast to self-assembly, which will be discussed in the next chapter, positional or robotic assembly is an approach in which each MBB is positioned in a certain coordinate using a robotic arm like an AFM tip [1,2]. The success of positional (robotic) assembly is dependent, to a large extent, on the availability and improvements in the field of scanning probe microscopy (the family AFMs and STMs) [2,6,9,10] as well as use of appropriate MBBs.

There are numerous alternatives for having MBBs, made from tens to tens of thousands of atoms, which provide a rich set of possibilities for parts. Preliminary investigation of this vast space of possibilities suggests that building blocks that could have multiple links to other building blocks (at least three, and preferably four or more) will make it easier to positionally assemble strong and stiff three-dimensional structures. As it will be discussed in chapter 11, adamantane and heavier diamondoids with the general chemical formula C4n+6H4n +12, are one such group of MBBs. Adamantane is a tetrahedrally symmetric stiff hydrocarbon that provides obvious sites for four, six or more functional groups.

Assemblers, or positional devices, are applied to hold and manipulate MBBs in positional (robotic) assembly. In fact an assembler would hold the MBBs near to their desired positions using a restoring force, F, which is calculated like a spring force from the following equation [11]:

F= s. x, where x stands for distance between a MBB and the desired location (displacement) and s is the spring constant of the restoring force (or stiffness). The relationship between positional uncertainty (mean error in position), e, temperature, T, and stiffness, s, is [11]:

The product (kB T) denotes thermal noise where kB is the Boltzmann constant. A scanning probe microscope (SPM) can provide a s value of 10 (0 [nm])"1 and thus an e value of 0.02 0 [nm] would be obtained at the room temperature which is a small enough mean error in positions for nanostructural assembly [2].

If the e value becomes very large the positional assembly will not work and one has to resort to using more stiff MBBs or other assembly techniques when possible.

K. Eric Drexler suggested that a universal assembler must be designed which would be able to build almost any desired nanostructure

[10]. However, this may seem impossible at the first glance unless the simple assemblers are originally built and these simple assemblers would build more complicated assemblers and so on, to the point that we can have a universal assembler.

An example of simple assemblers [10] is the "Stewart platform" as shown in Figure 1.

Figure 1. The most basic form of an assembler is the Stewart platform consisting of an octahedron connecting the triangular base to the triangular platform by 3 variable-length struts [10].

In this basic form, the Stewart platform is made of an octahedron shape. The choice of this shape is based on the observation that a polyhedron, all of whose faces are triangular, will be rigid and flexible. One of the two triangular faces is designated as the "platform" and the opposing triangular face is designated as the "base". The six struts of the octahedron that connect the base to the platform are designed so as to be varied in length. Then, changing the lengths of the six struts can vary the orientation and position of the platform with respect to the base. Since there are six struts, and if we are able to independently adjust the length of each strut, the Stewart platform could then be used to position the platform in full six degrees of freedom.

Several molecular assembling devices have been designed and proposed inspiring from the basic Stewart platform shown in Figure 1.

The various methods of improving the range of motion of the platform have been introduced.

K. Eric Drexler proposed an advanced version of Stewart platform [10] as shown in Figure 2. In this design the six adjustable-length edges are either in pure compression or pure tension and are never subjected to any bending force.

Figure 2. An advanced form of Stewart platform proposed by K. Eric Drexler [10]. The six adjustable-length edges are either in pure compression or pure tension and are never subjected to any bending force, this positional device is quite stiff.

Ralph C. Merkel [11] has proposed a family of positional devices, which provide high stiffness and strength, as do the Stewart platform, but with an increased range of motion and shortening and lengthening of variable struts (see Figure 3).

Figure 3. Merkle's positional control devices [11]. Left) Crank - A variation of the Stewart platform in which the platform is replaced with a bent shaft. Right) Double-tripod - A stiff and stable device. The main strut connects to the shaft by a screw collar, which can move along the shaft.

The Stewart platform and its modifications can provide quite higher stiffness than a robotic arm of the present day scanning probe microscopes. However, increased stiffness is usually associated with a decreased range of motion. The double tripod design (Figure 3), provides greater stiffness for a given structural mass than a robotic arm, but has a significantly greater range of motion than a Stewart platform.

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