9. Nanomachines. Here I will present a few existing kinds of nanomotors and nanomachines, and in the end discuss the wilder visions.

Transcription

1 9. Nanomachines One of the big dreams of nanoscience is manufacturing of machines which work on the nanometer scale. This is not quite as science-fictionish as one might imagine: several kinds of nanomachines already exist, some in nature, some made by man. The man-made ones are, however, still very primitive. Here I will present a few existing kinds of nanomotors and nanomachines, and in the end discuss the wilder visions. But first let s define what is meant by a machine. Here is a definition by Prof. J-P Sauvage (U. Strasbourg): A molecular machine is a molecular object made of at least two components, one of which can be put in motion with respect to the other by an external signal like electricity, light or a chemical reagent. Introduction to Nanoscience,

2 9.1. Electromechanical systems One fully controllable way of making small machines is that utilized for micro-electromechanical (MEMS) or nano-electromechanical (NEMS) systems MEMS Conventional MEMS technology utilizes Si lithography techniques to manufacture mechanical devices on Si wafers. - Using advanced Si etching techniques such as deep reactive ion etching, it is possible to make 3-dimensional structures on Si with small dimensions. Introduction to Nanoscience,

3 [ a) The procedure begins with a heterostructure that contains structural (red) and sacrificial layers (blue) on a substrate (yellow). b) Masks on top of this substrate are patterned by a combination of optical and electron-beam lithography, followed by a thin-film deposition processes. The resulting mask (black) protects the material beneath it during the next stage. c) Unprotected material around the mask is then etched away using a plasma process. Introduction to Nanoscience,

4 d) Finally, a local chemically selective etch step removes the sacrificial layer from specific regions to create freely suspended nanostructures that are both thermally and mechanically isolated. Such a procedure may be repeated numerous times to achieve quite complex final structures. - Here is a couple of examples: - MEMS is in large-scale commercial use e.g. for acceleration sensors ( accelerometers ) in cars, and as strain gauges. - There is also intense research interest into using them in lab-on-a-chip applications to detect very small amounts of molecules or even to act as reactors. Introduction to Nanoscience,

5 NEMS [ - The crucial question from a nanotechnology point of view is of course how small these can be made - Downscaling them is not easy; surface effects of the Si technology tend to limit how small the devices can be made. - Nevertheless, at least in one of the dimensions NEMS systems have already reached the size range of 10 nm. - Here is an example of a nanometer scale mechanical resonator manufactured out of SiC: Introduction to Nanoscience,

6 [ Nanotube motor on chip - But there are already true nanomotors on Si chips, manufactured with a somewhat different approach. Such a thing has been manufactured in the group of Zettl [LBL]. - Here are a few snapshots of the structure in operation Introduction to Nanoscience,

7 i.e. the bar in the middle is actually rotating! (animation at - The axis of the motor is a multi-walled carbon nanotube; here is an artistic image of what the thing looks like: Introduction to Nanoscience,

8 - The entire structure is about 500 nm across, i.e. this truly is a nanoscale device. - The motor has been operated for tens of thousands of cycles with no apparent wear - The rotation frequency can be up to 1 GHz! Introduction to Nanoscience,

9 - It was manufactured as follows: The team deposited MWNTs on the surface of a silicon wafer and selected individual tubes with an atomic force microscope. A gold rotor, nanotube anchors, and opposing stators (stationary parts of the motor) were then simultaneously patterned around the chosen nanotubes using electron beam lithography. A third stator was already buried under the silicon oxide surface. Part of the surface was then etched to provide sufficient clearance for the rotor. - The only obvious downside here is that this manufacturing method is entirely manual and hence not well suited for upscaling. Introduction to Nanoscience,

10 9.2. Nanotube actuators [Poole-Owens ; Baughman, Science 284, 1340 (1999), A device that converts electrical energy to mechanical energy or vice versa is called an actuator. Carbon nanotubes are known to deform when they are electrically charged. A macroscale nanotube actuator based on SW carbon nanotube paper (buckypaper) has been demonstrated. In this two 3 20 mm strips of nanopaper are bonded to each other with ordinary double-sided Scotch tape. Electrodes are connected separately to both pieces of nanotube paper. Introduction to Nanoscience,

11 The whole thing is in an NaCl electrolytic solution. When a positive voltage is applied on the system, the buckypaper-tape strip moves in one direction, and when a negative voltage is applied, in the other. This is because the positive voltage makes the nanopaper shrink, and a negative voltage expand. This device is of course not a nanoscale one, but based on nanotube material... Also actuators based on single nanotubes have been manufactured. Campbell et al have attached a freely hanging MWNT to a source electrode. By applying a voltage to a gate electrode below the nanotubes, the nanotube is induced to bend until it makes contact with a drain electrode. Introduction to Nanoscience,

12 Thus this device makes for an electrically controlled mechanical switch, i.e. a relay! Conventional relays fell out of fashion some 50 years because they were large and slow. Also, since the device involved mechanical motion the metal was subject to fatigue, limiting the lifetime of the device. A nanotube relay can in principle solve all of these problems: they are small and fast, and since there is no dislocation activity in nanotubes (at least at low temperatures) they are not subject to fatigue. Introduction to Nanoscience,

13 9.3. Molecular machine from azobenzene [Poole-Owens 13.2] Also switches based on single organic molecules have been manufactured. This is based on that many molecules are known to exist in two different states, and can be transformed from one to the other e.g. by light or an electrical voltage. An example is azobenzene, two benzene rings joint by two nitrogen atoms. It has trans and cis states, and is known to switch from trans to cis when subject to UV light of wavelength 313 nm. When on the other hand the light has wavelength greater than 380 nm, it switches back to the trans state: Introduction to Nanoscience,

14 This also works for a chain of the molecules. Then the cis transformation takes place at a wavelength of 365 nm, and the back transformation for wavelengths greater than 420 nm. A molecular switch has been manufactured from this by placing a chain of the molecules between a cantilever and a substrate: wavelength 365 gets it to contracted state, greater than 420 or no light back. Thus this device is in essence a molecular machine: it converts light into mechanical work! Introduction to Nanoscience,

15 9.4. Organic machines on surfaces [Phys. Rev. Lett. 86, 456 (2001)] A completely different kind of molecular machine has been manufactured at the Århus university in Denmark. This is an organic molecule, hexa-tert-butyl decacyclene (HtBDC) C 60 H 66 that can move on a Cu(110) metal surface: Introduction to Nanoscience,

16 This molecule has a flat central board of benzene rings and six legs that isolate the flat board from the surface. In an STM image a single molecule looks like follows: The molecules tend to form groups; in this case only the outer legs are visible as bright spots while the legs on the inside are much weaker: These molecules are observed to move on the Cu surface along certain crystallographic directions: Introduction to Nanoscience,

17 MOVIE /home/users/knordlun/ppt/presentation/taydkoul04/htbdc.mpg But the real surprise comes when the HtBDC molecules are pushed away: it turns out that they have removed Cu atoms from the substrate! Typically something like 14 Cu atoms are removed from below one of the rows of HtBDC atoms: Introduction to Nanoscience,

18 A shows molecules and lines where the molecules have been removed from. B shows removed Cu atoms, C relation of molecules (red) to Cu atoms in dark shades The mechanism by which the atom removal takes place is not fully understood, but apparently the presence of the organic molecule weakens the metal-metal bonds making it energetically feasible to Introduction to Nanoscience,

19 pick up the metal atoms. This is a thermally activated mechanism since it does not occur below 260 K. The Cu atoms then become adatoms which diffuse around, until probably they find a step edge and stick there. Thus these molecules act as a something like a harvester for Cu atoms on the surface! Introduction to Nanoscience,

20 9.5. Biological machines Nature is full of machines, in the form of devices which convert chemical energy to mechanical work. It is a common thing to calculate efficiencies of these, comparing the amount of energy released to the amount of work done, and finding that biological machines can have efficiencies of the order of 60 %, while conventional man-made engines even in theory can reach only some 30% and in practice are limited to about half that, 15 % or so. This is often quoted to indicate that nature is much better than man at designing things. But this comparison is really comparing apples to oranges: the man-made heat engines work in a cycle with temperature differences, while the biological ones work in entirely different mechanisms. Also, this comparison does not adress the issue of speed, efficient fuel supply or upscaling at all, which may not be trivial to achieve in molecular motors. This is of course is why our cars still run on heat engines... Gas engines easily something close to rpm i.e. 150 rps and upscaling is no major problem. Also fuel upscaling easy Linear motors Introduction to Nanoscience,

21 [Wolf 3.2.1, A relatively simple example of a biological motor is the spasmoneme spring (first studied in 1676!). This is a structure found in a variety of organisms such as Vorticella convallaria, Carcheslium polypinum, and Zoothamnium geniculatum. These are small animals (protozoans) living in ponds with typical sizes of the order of 50 microns, e.g. infesting tadpoles or attached to leaves. tadpole = grodyngel Introduction to Nanoscience,

22 The spasmoneme is the long extended part of the animal. Normally it is extended with lengths up to 2-3 mm or so. The spasmoneme spring is kept in its extended state by aligned nanometer-size filaments kept apart by negative charges. But if positive charges (like Ca ions) come in the vicinity, they neutralize the Introduction to Nanoscience,

23 negative charges and the filaments collapse into a much denser shape. This occurs over a few ms, at speeds up to 8 cm/s. The Vorticella use this fast contraction as a simple protection mechanism when they are disturbed Real and artificial muscles Real muscles [Wolf 3.2.2, Nature 401 (1999) 505, Muscles generate forces, triggered by a chemical signal (of course initially originating from a nerve in living beings). The muscle motion is driven by myosin molecules. A muscle consists of numerous thick and thin filaments. The thick ones contain the myosin molecules, the thin ones are made of a protein called actin. The myosin molecule move along the actin filament. variety of myosin. Exactly how this happens depends on the The myosin V consists of two identical heads at the end of a shaft which binds to the thick filament. When muscle motion is required, the two heads of the myosin move along the actin filament: Introduction to Nanoscience,

24 Artificial nanomuscles [ A man-made molecular machine has been designed by Prof. J-P. Sauvages research group which is similar to the interaction between myosin and actin in real muscles in that a chemical signal induces relative motion of two molecules with respect to each other. Introduction to Nanoscience,

25 The central idea is to utilize the fact that singly ionized Cu(I) (Cu +1 ) prefers a four-fold coordinated state, while doubley ionized Cu(II) (Cu +2 ) favours a five-fold coordinated state. The molecule in which this is utilized is the following: Introduction to Nanoscience,

26 - In the state shown Cu(II) is 5-fold bonded to 5 nitrogen atoms, 3 of which (marked by red circles) are a so called terpyridine part of a circular rotaxane molecule. - If now the Cu(II) captures an electron, it becomes Cu(I) which prefers a 4-fold bonded state. In this case the rotaxane molecule will rotate so that the Cu finds the two N molecules in a phenantrholine part of the molecule, purple circles. - This also works the opposite way: if the Cu(I) loses an electron, the rotaxane again rotates to the other bonding configuration. Introduction to Nanoscience,

27 - The time constants for the transition are very different, though: the Cu II rotation takes tens of seconds (i.e. slow rate k) while the Cu I rotation needs only tens of millisecond (high rate k). This basic concept can be used to device mechanical movement e.g. in the following way: Introduction to Nanoscience,

28 - Here a rotaxane is linked to a longer about linear molecule - Now the rotaxane has only one phenanthroline (2 N) groups, but the linear extended part has both a phenanthroline (2 N) and a terpyridine (3 N) group. - The nanomuscle is formed by a pair of these molecules. - Now when the ion charge state changes, the metal ion will move along the linear chain! - Hence the pair of the two molecules will change between an extended or contracted situation. - The transition is now actually not between two states of Cu, but is caused by metal ion exchange between Cu I and a metal which prefers +II-valency, e.g. Zn II. What is the energy source of this thing? It could simply be thermal activation ATP synthase motor [Wolf 3.2.3, Introduction to Nanoscience,

29 The above described biological motors are not too close to what we commonly think of as a motor, something with rotating parts. There is, however, one biological rotary motor which exists pretty much in every living cell in higher organisms. This is the ATP synthase motor. - Adenosin triposphate (ATP) is the most import chemical energy source in cells. It is e.g. the energy source for the myosin muscle motion described above. - It is formed in e.g. mitochondria from Adenosin diposphate (ADP) and phosphate via so called ATP synthase. It occurs in a special molecule complex located in a membrane between two parts of a cell. - The chemical (free) energy needed to drive the motor comes from the different concentrations of ions on two sides of the membrane thermodynamically unstable system, wants to equilibrate, energy flow - It has two parts, F 0 and F 1 : Introduction to Nanoscience,

30 F 0 is in the membrane. It takes in H + ions from one side of the membrane. Each ion makes it move one step roundwards. - This round motion generates a torque in F 1. - At the same time the F 1 has taken in ADP and phosphate Introduction to Nanoscience,

31 - The torque generated by ADP moves the different parts of F 1 slightly, and this gives the energy to join ADP with a phosphate to create ATP! ANIMATION fig/atpsynthase.mov [ - Note from the lower part of the movie that positive ions are fed in from below (with higher positive ion concentration), but after one turn of the motor released to above, i.e. the thing really does reduce the concentration gradient. But thanks to the motor, some of the free energy gained from reducing the concentration gradient is transferred to the F 1 part allowing for ATP synthesis! - This process can also be ran in reverse! - I.e. given ATP from somewhere else as an energy source, the motor can be used to pump in positive ions from a region of higher concentration to one of lower. Sort of a man-made molecular motor has been made based on this motor. Sambongi et al. [Science 286 (1999): ] have attached an actin filament to the F 0 part of the motor. Then a supply of ATP can be used to rotate the actin filament! Introduction to Nanoscience,

32 - The application possibilities are not obvious though, and in fact this was done primarily to be able to observe the rotation of the lower part; the actin was thus just a marker. Introduction to Nanoscience,

33 9.6. Moving things atom by atom, self-replicating machines and science fiction [Wolf ch. 8, own judgment] The above has briefly summarized the state-of-the-art in existing nanomachines. They are impressive, but far from the concept of a nanofactory or nanomachines with numerous working parts all working in unison and reliable, illustrated e.g. in the following picture: Introduction to Nanoscience,

34 Whether such structures can be ever built remains an open question. The nanoscience world is awash with atom-level computer simulations of nanomachine part concepts: These are interesting as studies of e.g. the mechanical characteristics of such machine parts if they could be built. But the question is of course, can they? An obvious idea stemming from the use of AFM to move single atoms on surfaces is to do the same in 3D; use a pair of AFM tips as tweezers to build any kind of device atom by atom. - The problem with this idea is that the STM tip radius is huge compared to a single atom, making Introduction to Nanoscience, 2005 JJ J I II 34

35 it completely impossible to move things in confined spaces. On the other hand if e.g. a nanotube is used it is probably too flexible to place atoms very precisely. - Also other problems abound: atoms will always stick to their most preferred bonding configuration, which may not fit the overall design... The idea of atom-by-atom assembly is widely credited to Drexler, but even he himself does not believe in the idea: Quote from Confusion and controversy: Developing assemblers While nano assembly has been described as building things atom by atom, an exp ression that has caught on in the press, this is a misconception. Molecular asse mblers will build with atomic precision by mechanically guiding chemical reactio ns that typically add a few atoms at a time, but some researchers have criticized this misconception as if it were the actual proposal. It is correct that assemblers can t build things by using tiny tweezers to pick up and put down atoms one at a time, but even from the start this was never the idea. The apparent controversy over molecular assemblers is thus an illusion: the critics are talking about something else. The idea of building things by mechanically guiding chemical reactions has withstood scientific scrutiny for over twenty years, and seems sound. It is time to move on, to consider the consequences of molecular assemblers and what they will be able to build. Introduction to Nanoscience,

36 - Thus the concept of using atomic tweezers to move single atoms can well be characterized as science fiction with no realistic basis in present-day science. The only feasible way in which molecular machines could be built is by using techniques of selfassembly or chemical synthesis. As seen on this course, these can already make pretty impressive things. But to actually be able to make a full machine using them would still require huge breakthroughs... Thus the most promising paths to nanomachines are probably those starting from biological devices and modifying them in desired ways. In view of this, the concept of self-replicating machines, gray goo filling the earth and all that, is clearly something to be viewed purely as science fiction. Not theoretically impossible, but so far in the future not sensible to make plans for it... Introduction to Nanoscience,