To understand how the nanocar fits into the larger scheme of molecular manufacturing, we will review some instrumental developments in molecular manufacturing that jump-started the field. We will also introduce a couple of molecular components that facilitate the design of mobile molecules—bearings and axles.

Scanning Tunneling Microscopy Introduces a New Frontier

The invention of the Scanning Tunneling Microscope (STM) by IBM’s Gerd Binning and Heinrich Rohrer in 1981 was vital to the development of molecular manufacturing and nanoscale design. The function of the STM is two-fold. First of all, STM imaging allowed scientists to visualize atomic surfaces. Secondly, STM tips are capable of directly manipulating individual atoms and molecules. Both of these functions have been instrumental in nanoscale design, and were both employed by the scientists involved in the design and observation of nanocar 1.

Early advances in nanoscale design came in the form of direct demonstration of the movement of single atoms and molecules with the use of an STM. The first of these demonstrations came in 1989 when IBM fellow Don Eigler spelled out the letters "I B M" on a nickel surface using 35 xenon atoms (figure 2, left). At first glance, the act seemed insignificant, criticized by many as merely a stunt. However, at the heart of the demonstration lies the fact that Eigler was able to move single atoms that could not even be observed less than a decade before. This demonstration was a step forward for the development of nanoscale design, and would be followed by subsequent developments. However, a complication of Eigler’s method was that it required experimental temperatures near absolute zero—an unpractical temperature for the design of useful products.

Figure 2: Direct Manipulation of Single Atoms and Molecules. Left: IBM scientists moved 35 individual xenon atoms on a nickel surface to spell out the letters "I B M". This demonstration represented a vital step in the development of nanoscale design and molecular manufacturing, as Eigler demonstrated that individual atoms could be manipulated using an STM. Right: IBM scientists placed one hundred and ten buckminsterfullerene (C60) molecules in eleven separate wells on a copper surface and manipulated them to serve as a traditional abacus. The demonstration was performed at room temperature, and indicated the capability to move individual molecules using an STM. Images from IBM Zurich Research Laboratory.

The second demonstration also came from IBM scientists and overcame the limitations of the previous experiment. In 1996, IBM’s Zurich laboratory produced a nanoscale abacus that consisted of individual C60 molecules that functioned as beads that could be pushed back and forth along eleven separate rails on a copper surface (figure 2, right). This time around, the components were manipulated at room temperature—a practical temperature for the design and application of nanoscale products. This demonstration represented another vital step in the advancement of nanoscale design. It indicated that molecules could be manipulated at room temperature and constructed into a functional design—also known as ‘bottom-up’ design.

Since the invention of the STM, the field of molecular manufacturing has produced various molecules that serve as molecular building blocks for more complexly designed molecules that are emerging today. The set of building blocks necessary for the development of a design on any scale depends on the targeted function of that design. For example, in manufacturing an automobile, the necessary materials include bearings, axles, and various other components. The same idea applies to nanoscale design. Depending on the target function of a molecule, it is necessary to use various components. Here, we take a look at some of the molecular building blocks required to synthesize mobile molecules. Keep in mind that these components, and various others, will be used to describe the structure and function of nanocar 1.

The detailed structure and chemistry of the various systems used in the design and synthesis of molecular components is beyond the scope of this text. However, we will provide a general overview of a few molecular structures that are instrumental in the design of mobile, functional molecules. With the introduction of each molecular mechanical component, we will provide comparisons with its macroscopic counterpart in order to clarify the functionality of each system.

Bearings

Bearings are structures that function to reduce energy loss to friction during various processes. Bearings are found in almost every rotating part of your car and facilitate smooth rotation of parts from the wheel up to the transmission. Researchers have investigated various systems to replicate this function on the molecular level. Here we take a look at the molecular bearing designed by scientists working in Japan. A monolayer of tightly packed C60 molecules was sandwiched between two single sheets of graphite to form a molecular bearing. The structure resulted in an ultra-lubricated system with zero frictional forces when the graphite sheets were moved along the rotating C60 molecules(figure 3).

Figure 3: Molecular Bearings Left: The C60 molecular bearings consist of a single layer of tightly packed C60 molecules to create a frictionless system of sliding-translational motion of the graphite sheets. The rotating C60 moleculesallow for smooth movement of the graphite sheets. Right: A ball bearing that is used to facilitate reduced friction rotation of wheels. The rotating metallic balls allow for smooth rotation of the outside ring surrounding the balls.

Axles

Axles function to transfer mechanical energy to turn a specific object such as a wheel. An effective axle is characterized by two functions: 1) it must be able to rotate freely and 2) must be in a fixed, linear position. The axle must be able to rotate freely because its function is dependent on its ability to transfer mechanical energy to rotate an attached structure. In the case of an automobile, the axle functions to transfer energy generated by the engine to rotate the wheels of the vehicle. An axle must be in a fixed, linear position because it must provide enough support to withstand forces placed on it, such as the weight of a chassis. On the molecular scale, the two functions of an effective rotor are encompassed in the structure of a triple bond, as opposed to single or double bonds (figure 4). A single bond is able to rotate freely, but is not in a fixed linear position. On the other hand, double bonds are in a fixed position, but are unable to rotate.

Figure 4: Molecular Axles: Here, bonds between benzene molecules are used to illustrate the differences between single, triple, and double bonds. The differences in the characteristics of the three types covalent bonds described above differentiate their functionality as molecular axles. An axle must be fixed in a linear structure and be able to rotate along the axis of the bond. A single bond (left) is able to rotate, but does not provide a fixed angle and position. A double bond (right) is fixed in a 180° angle, but is inhibited from rotational motion. A triple bond (center) is both fixed in a linear position and capable of rotating freely, therefore the most viable option for a molecular axle.

As you have observed, the link between structure and function is vital to designs on both the macro and nanoscale. The structures described here do not, by any means, encompass the countless molecular structures that are required for the synthesis of functional molecules, but serve only to provide a general idea of the types of designs involved in molecular manufacturing. With this brief introduction to molecular manufacturing we are prepared to examine a specific example of molecular manufacturing: nanocar 1.

The structure of nanocars facilitates their function. Therefore, their structure and more importantly the precise, deliberate engineering of their structure through assembly and implementation represents vital progress in nanoscale design and molecular manufacturing. Studying the mechanics of rotors and motors from the bottom up, starting with the simplest molecules possible, the Tour group engineered nanocars as the first in a series of tools to test molecular mechanics and prove the viability of molecular design.

Central to understanding the implications of the Tour Group research is an understanding of the structure of the nanocars—their design and their function. To best address the nanocar structure we will analyze its design and assembly in three components: the wheel, the chassis, and the surface it operates on. As it turns out, each of these aspects is equally important in determining the functionality of the nanocar and is therefore the best way to analyze the structure of the nanocar system.

Wheels

The driving characteristic of the wheel, if you will, is its ability to roll. Apart from this implicit necessity, the wheel must also be of a size that can be imaged with an STM. If a molecule is too big the STM cannot resolve it. Likewise, if a molecule is too small the STM cannot discern it from its neighbors. A wheel must also be large enough to have ‘ground clearance’ whereby the wheel is of sufficient height or radius to elevate the chassis high enough above the operating surface to avoid molecular interactions. Ease of synthesis must also be considered in the selection of a wheel. A molecular wheel must be reactive enough to bond with its chassis in order to synthesize the molecule.

Figure 5: Buckminsterfullerene compared to soccer ball. The alternating pentagonal and hexagonal cyclical bond structures form a spherical shell reminiscent of a soccer ball. The functional properties of the C60 molecule are a result of the ordered bonding of the molecule.

Clearly, there are many considerations that must be taken into account when choosing a wheel. The Tour Group at Rice University selected C60 as their wheel for their first nanocar. C60 is entirely composed of carbon with alternating pentagonal and hexagonal cyclical bond structures that form a spherical shell reminiscent of a soccer ball (figure 5). Ideally, this structure, under the right conditions, could roll with its compact surface and spherical shape. Furthermore, it is of a height that can be imaged with an STM and elevate the chassis to prevent interactions with the operating surface. In a subsequent section, we will learn more about how these concepts were tested and observed by the Tour Group.

Although C60 is an attractive molecule to test rolling versus sliding motion at the molecular level, there are some disadvantages to its structure. The properties of C60 limit the synthesis of the nanocar molecule. In particular, the pi bonds of the molecule react with the palladium catalyst to interfere with chassis synthesis. For this reason, the chassis was synthesized first and the C60 wheels were attached last.

The future of nanocar wheels will include the introduction of more controlled synthesis and variable size. In the short term, the Tour Group is investigating the use of carborane molecules, spheroid molecules comprised of carbon and boron, to have more control over the synthesis. They believe the carborane molecules will exhibit more compliant chemistry for their functional needs (figure 6). In the long term, the Tour Group is investigating large, complex organic molecules that are modeled after bicycle wheels with an outer rim and connecting spokes. This bicycle wheel would be made predominantly of carbon and allow for variable size depending on the length of the spokes and circumference of the rim.

Figure 6: Carborane—Possible Wheel. The Carborane molcule is a spheroid molecules comprised of carbon (white) and boron (black). It is studied for its possibilies of serving as a molecular wheel. Image is from Scientific Glass Engineering website

Chassis

Continuing the analysis of the nanocar structure, we will investigate the chassis of the nanocar, including aspects of its design and functionality. When designing a viable chassis the Tour Group took into consideration three primary goals: one, how to physically connect the wheels, two, how to facilitate rotational motion of the wheels, and three how to develop a structure that allows the orientation of the molecule to be determined by STM. Molecular components of the chassis were also considered in order to maintain ease of synthesis. In addition to developing and understanding nanocar 1, the group also aims to add functionality to the chassis.

Figure 7: Structure of nanocar 1. The various components of the design of nanocar 1 are illustrated in this figure including the four buckminsterfullerene wheels, four alkyne axles, and chassis. Picture courtesy of Rice University Office of Media Relations, reprinted with kind permission from Dr. Kevin Kelly.

The Tour Group addressed these goals in their design of the chassis in several ways (Figure 7). To physically attach the wheels to each other, the Tour Group designed a simple four-axle system reminiscent of a car. There are two axles in the front and two in the back connected to each other by a central shaft. Within the design of the axles, the Tour Group addressed the second goal of how to allow free spinning of the wheels. As discussed earlier, the axles are comprised of triple bonded carbon atoms, which allow for free rotation about the axle, while maintaining linearity with the adjacent axle (figure 4). The spinning of these triple bonds begins at 30 degrees Kelvin and has been shown to have virtually no frictional hindrance on the system. This is optimal for a preliminary research into rolling motion because it constrains free variables of the system. In other words, because the triple bonds do not add meaningful frictional forces to the system above 30 degrees Kelvin, all experimental results can be attributed to the chemistry of the wheel and its rolling or slipping interactions with the surface it operates on. Thirdly, the ease with which the nanocars can be resolved with an STM was in part dictated by the structure of the chassis. Specifically, the chassis was designed to have a central shaft longer than the length of the front or rear axles. The nanocar is wider than it is long. This is important in microscopy because it allows the observer to note the orientation and therefore the direction of translational motion of the nanocar. As far as the synthesis of the molecule, the Tour Group added functional branches to the phenyl groups on the axles and central shaft, which suspend the molecule in solution and allow for better mixing, better reaction, and better yields. These essential goals were addressed to produce a working nanocar; however, the goal of adding functionality was not described.

While the currently published nanocar is devoid of added functionality, the Tour Group is researching additions to the chassis to facilitate transport and motility. Nanotrucks are a popular idea and simply refers to a modified nanocar that can carry objects. A ‘bed’ could be synthesized into the chassis to carry substances ranging from metal ions to oxygen atoms. Of course, the object being carried would most likely be specific to the nanotruck synthesized and the particular chemistry of its ‘bed’. Furthermore, bonding of a metal ion to the chassis of a nanocar would allow for interactions with an electric field, providing a mechanism for controlled motion. Along this vein, the entire chassis of the nanocar could be designed to maintain a dipole moment creating a favored orientation in the presence of an electric field. Another form of motility that is being pursued is the addition of a light-driven single-directional molecular rotor. This type of rotor would ratchet forward through four isomeric states when stimulated by photons. The Tour group is looking to append such a motor to their chassis to create deliberate and controlled motion. Nanomachines that utilize this deliberate and controlled motion will be the next milestone in molecular manufacturing.

The last of the three components of the nanocar to explore is the surface it operates on. Surface chemistry plays an undeniable role in the functionality of the nanocar, and must be taken into account in designing the system. First, a surface must be found where rolling, not sliding, dynamics are likely. Secondly, a surface must be chosen on which observations of the molecule are clear and measurable.

Gold was chosen as the operation surface because it was theorized to be optimal for the aforementioned considerations. As it turns out, gold does accommodate these factors well. It allows for a special type of chemical interaction between itself and C60, known as charge transfer bonding. While we will not go into the details of charge transfer bonding, it will be sufficient for this textbook to consider the molecular interactions between the C60 and the gold surface as a type of van der walls forces or weak molecular interaction. This type of interaction is optimal for two reasons. One, if the bond strengths were any weaker the C60 molecule would simply slide like a bearing, as is the case with the C60 on graphite sheets. If the bond strengths were any stronger, the C60 molecule would not move at all. Therefore, the appropriate strength of molecular interaction between the wheel and the surface must be found to allow for rolling motion. Second, this interaction is temperature dependent and can be adjusted to control motion. Temperature controlled bonding interactions allow for the observers to cool the system down to where the bond strengths are more effective to give the STM time to resolve an image. Likewise, the system can be heated to allow for movement. Of course there is a range of possible temperatures that the system is constrained by, namely the temperature of decomposition of the nanocar. Therefore, the surface must be able to regulate motion while remaining within the available temperature range of the stable molecule. Each of these factors contributes to the ease with which a molecule’s motion can be observed. In the next section, we focus on this subject as we discuss how the Tour Group observed nanocar 1.

Observations beyond the range of the naked eye require more deliberate and cautioned interpretation of what is observed. Specifically, factual and accurate information must be discerned out of various interferences that can lead to unsubstantiated conclusions. The Tour group had to surmount these uncertainties to conclusively show that the nanocars underwent rolling, rather than sliding, translational motion.

Figure 8: Nanocar translational movement across a gold surface at 200°C. The pivoting of the molecule’s motion is due to out-of-sync rotation of wheel movement. The picture indicated path occurred over a minute period (images captured ever minute by STM). Reprinted with kind permission from Dr. Kevin Kelly <picture courtesy of Rice University Office of Media Relations>.

The choice of a gold surface enabled temperature dependent adhesion to the surface, and allowed targeted observation of specific molecules. In the picture to the left, you see several nanocars spread out along the Au(111) surface. As the researchers increased the temperature to 200°C, the cars began to move at such a rate that they noticeably displaced from their original position in a period of one minute (figure 8). This indicates that 200°C is a viable temperature for imaging nanocars on gold with STM. However, imaging of the cars at temperatures above 225°C is currently impossible due to the rapid motion of the molecules at that temperature. The nanocars are moving too quickly to be imaged by the one-minute capture rate of the STM. Once the system was heated to 300-350°C, the car began to decompose. These observations indicated that motion of the nanocars was dependent on temperature.

It was also observed that the cars did not exclusively undergo translational motion, but also rotated. In the image to the right, it can be observed that the cars pivot as they move across the frame, changing direction without moving forward. The Tour Group explained this rotation as an inability to synchronize the rotation of each wheel.

The Tour Group also observed three-wheeled molecules, or trimers, which they found useful in proving rotational motion of the C60 molecules. If the cars’ movement were in fact due to rotation, then the trimer would not be able to move translationally due to the fact that the three wheels cannot rotate in a coordinated manner, that is no two wheels can rotate the same parallel plane. This supports the idea that the C60 molecules are rotating, as opposed to sliding. To strengthen their assertion, the researchers heated a sample of the trimer molecules on the same gold surface to 225°C—a temperature at which the four-wheeled molecules rapidly moved out of the scanning range of the STM. Upon doing so, they observed that the trimers did not undergo significant translational motion, and remained within nanometers of their original position. This showed that both the wheels and axels of the trimer and nanocar allow for rotational motion, therefore substantiating the assertion that translational motion of the nanocar is due to rolling.

The researchers used an STM to pull the nanocar in order to see if there was preferential motion (ie. Due to rolling). When the molecules were pulled perpendicular to the axles (Figure 9; frame a) the molecule moved in the direction of the pull. But, pulling parallel to the axles resulted in no translational motion in the direction of the pull (frame b). Lastly, by pulling perpendicular to the axles, the nanocar resumed its forward path (frame c).

Figure 9: Translational Motion of Pulled Nanocars on Gold Surface at 200°C. Frame (a) depicts the movement of a nanocar pulled perpendicular to its axles. Frame (b) indicates the inhibited motion that results from pulling parallel to the axles. Frame (c) is another instance of a pulling force perpendicular to the axles. Reprinted with kind permission from Dr. Kevin Kelly <picture courtesy of Rice University Office of Media Relations>.

These experiments carried out by the Tour Group combined to substantiate the claim that the nanocars were rolling on the C60 wheels as opposed to sliding. This observation designates the molecule as the first nanocar capable of executing a predetermined, engineered motion. These experiments lay down a foundation of knowledge on the molecular mechanics of motion. Particularly, these experiments conclusively demonstrate rolling motion of the C60 on gold surfaces at a given temperature. This marks the first step in a greater understanding of molecular motion as it applies to molecular manufacturing.

Molecular manipulation as a science has developed in steps. Its early steps involved movement of atoms and molecules, along with the ability to observe those movements. Later came engineered molecular components that carried out predetermined functions, such as bearings and axles. At the present, nanocars are an example of the developments in motility and function of integrated components that serve a unified purpose. But more importantly, nanocars are an indicator of developments to come. They are ushering in an era of deliberate and controlled motion at the molecular level.