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SEMINARIO et al.: NANOCELL: CHEMICALLY ASSEMBLED MOLECULAR ELECTRONIC CIRCUIT 1615Fig. 1. Cartoon representing a molecular circuit. The molecules inside the cell are chemically deposited. Thus, no predicted pattern is expected; most moleculesare randomly interconnected and oriented.remotely programmed so that they can behave as required,e.g., as logical gates. This almost unthinkable but possibleapproach is the so-called “programmable nanocell” or simplythe nanocell [6]–[12]. Fig. 1 shows a cartoon representingan arbitrarily chosen group of molecules deposited on a twodimensional(2-D) box that can be perfectly approached oraddressed by external tips or contacts needed to read or writeinformation, as in any integrated electronic circuit. These contactsare actually the interface to the standard electronics; thus,they are relatively large with respect to the molecules inside thebox, and these molecules cannot be individually read or written,except for some of them close to the peripheral borders of themolecular circuit. Application of signals (read) to the externaltips does not address a specific molecule but affect a relativelylarge part of them, like using our own fingers to arrange severaltransistors in a 1-cm 2 chip; thus, the figure is not at scale, asthe contacts are supposed to be relatively much larger thanthe molecules. Therefore, having the possibility to build singlemolecules with an arrangement of atoms resembling featuresizes smaller than one Å (10 −10 m), the question is how tomake a nonaddressable arrangement of small devices perform apredetermined function.II. BACKGROUND INFORMATIONPresent microelectronics is basically a lithographic technologyable to pattern and interconnect transistors on a siliconwafer. It is limited by the feature size corresponding to a fractionof the wavelength of the radiation used in the lithographicprocess. Because of advances in the fabrication of masks, thistechnology is able to pattern details even smaller than whatis allowed by the wavelength restrictions. For instance, the193-nm lithography is able to create ∼45-nm channel lengthtransistors [13]. The feature size for a technology is known asthe technology node, which presently has been announced asthe 65-nm node with a smallest feature size or minimum featuresize of ∼35 nm [1].Today’s transistors are used in a complementary metal-oxidesilicon(CMOS) technology, which provides two types of transistors,nMOS and pMOS, whereby silicon is doped with atomsthat are donors or acceptors of electrons, respectively, yieldinga net conducting behavior of negative or positive carriers ofelectricity (“electrons” and “holes,” respectively). Thus, whena “low” voltage is applied to the gates, the nMOS conducts andthe pMOS does not. The reverse takes place when a “high”voltage is applied to their gates. Thus, a switch is made basicallywith two complementary transistors, with their channels“in series” and their gates bridged. For any input at their gates,only one transistor is ON, and the current through both is zero,except during switching when the energy is dissipated. Thisexplains why increasing the clock frequencies also impliesincreasing the heat dissipation in microprocessors, which is oneof the major problems in miniaturization because higher densityof devices (needed for higher speeds of signal transfer) alsoimplies higher energy dissipation per unit area.Moore’s law [1], [14] states empirically that complexityof IC and, therefore, computer performance is doubled every18 months. This is because the size of devices is decreasing by afactor of two every 18 months, and so, the number of transistorsin a chip also doubles every 18 months; such an extraordinaryprogress, without similarities to any other field of technology,has been the case for the last 40 years. Conversely, it is alsoclaimed that the cost of a foundry doubles every three years,reaching a few billion dollars right now; however, this does notseem to affect the exponential progress of electronics at all norwill it determine the end of the progress.
1616 IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006However, there are factors that may have strong influencein the end of the scaling down of semiconductor electronics[15]. These are tunneling of electrons at short distances (leaksand avalanche breakdown), uncontrollable or impractical heatdissipation, and rare or nonuniform doping. One way to solvethis problem would be to find better materials, but this willonly solve problems marginally, as silicon is the most suitablematerial among bulk materials for the lithographic fabricationtechniques. No matter what material is used, tunneling ofelectrons is unstoppable, as any material is equally subjected tothe laws of quantum mechanics; what we need are new devicesoperating under different mechanisms.Even if devices can be scaled down further, addressed, andinterconnected using CMOS or any other material or technology,heat removal problems are going to increase simplybecause current through the devices cannot be scale downequally because signal/noise ratios cannot be decreased withoutjeopardizing signal integrity. As devices are smaller, electroncurrent carrying information creates stronger perturbations,affecting their performance. Therefore, any approach that wewant to develop has to consider the need to change the way weencode information in the computing circuits. There are severalalternatives that might be used alone or in combination to solvethe dissipation problem caused by electrical currents. Possiblealternatives, instead of a charge-current approach, are the molecularelectrostatic potential (MEP) to encode and process information[16], the molecular vibrational states for the transportof signal in molecular circuits [17]–[20], the quantum cellularautomata for transfer and processing of information [21], thespin (spintronics) to encode and process information [22], themechanical bonds to process information [23]–[25], torsionaleffect in molecules to process and amplify signals [26], [27],and the molecular reactivity (breaking-forming bonds, electromigration,etc) to process information [28]. As all of theseprocesses are of atomistic and molecular in nature, the nanocellconcept is suitable in operating under any of the above scenariosfollowing the famous dictum “there is plenty of room at thebottom” by Feynman. Thus, since there is no limitation on thelevel of matter to be used as matter definitely computes, nothingkeeps us from thinking that, in the future, we will be usingquarks or even superstrings or any other exotic kind of waveparticlemoiety for computation.III. AREN’T COMPUTERS FAST ENOUGH?The answer is a straight no. Despite the power of presentcomputers, more is needed. The scientific and technologicalcommunity feels that there are no evident trends to get morecomputational power and there is no clear definition of theproblems that need to be solved to get more computationalpower; however, we plan activities, considering that computerswill be twice faster every ∼18 months. Analysis, design, andsimulation in all areas of science and technology depend on theavailability of faster computers able to provide useful informationabout the outcome of projects or experiments prior to theirreal implementation or construction, thus avoiding high costs oftrial-error experimentation. As a simple example among manyothers, the physics and chemistry of materials would allow theconstruction and design of materials with tailored propertiesusing well-known (for more than 80 years) and precise equationsof nature governing the behavior of these materials. However,because of the lack of sufficient computational resources,the numerical solution of these precise equations needed for theanalysis of realistic systems cannot be done. Solution of theequations demands excessive computational resources, so thatwe are limited to solve those using strongly approximatedmodels.Although impressive advances are constantly achieved inmicroelectronics technology, current computers based on siliconCMOS IC fabricated with precise lithographic techniquesalready show problems of scaling-down and miniaturization aswe approach the 10-nm limit.One alternative parallel to the nanocell is simply substitutingpresent semiconductor devices and systems with molecularones. This alternative is in direct competition with siliconelectronics. Typical applications are already in progress, suchas several crossbar approaches [29]–[46] that will allow us toconstruct molecular memories and logic, hopefully at extremelylower fabrication costs. On the other hand, magnetic storagedevices, in which magnetic states are used to store informationpermanently, are also being developed [47]–[58].IV. PHYSICAL LIMITATIONSAs they become smaller, the sharp properties of digital devicesused to handle logical operations become fuzzy. Deviceslose several of their bulk properties when approaching thesmaller structures of atoms like molecules or nanoclusters.These problems are common to any material used to fabricateelectronic devices, as they depend on the physics governingsmall amounts of matter. For instance the diode, which is thesimplest electronic device, its ideal characteristics are zeroresistance under direct bias (ON state) and infinite resistanceunder reverse bias (OFF state); the ratio of the current at ON andOFF states is infinite. This is almost the case when using vacuumtubes and certainly far from the ideal but still acceptable whenusing semiconductor diodes; practical diodes show a few ohmsunder direct bias and few giga or megohms at reverse bias;typical ON/OFF ratios are between 10 3 −10 6 . With single molecules,a few kilohms would be expected in the direct and severalin the reverse case; ON–OFF ratios bigger than one order ofmagnitude would be very difficult to obtain from single moleculesat the nanometer size. The physics behind this problem is“tunneling.” Basically, electrons cannot be kept at designatedtracks and sites; they tunnel through potential barriers thatwere supposed to maintain sharp ON and OFF changes of thedevices.Even if the fuzziness between ON and OFF states in digitalcircuits can be solved by sophisticated signal processing anderror correction techniques, still there is the problem of fabricationand addressing these small devices. Present fabricationtechniques are photolithography-based, a top-down technique,which practically puts the entire burden in the design andfabrication of a mask able to expose the silicon substrate usingan electromagnetic radiation of a specific wavelength, determiningto some extent the minimum feature. Thus, fabrication
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