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Molecular- and
nano-technology group
Welcome to the molecular- and nano-technology group at
Texas A&M University
We are mostly interested in atomistic level analysis and
design of devices and systems for fields such as
1.2
Vibronics
2
Interpretation and
Proof-of-Concept Experiments
5
Biological and
chemical nanosensing
6
Quantum molecular computing
7
Energetic materials
8
Benchmarks
Please do not hesitate to contact me if you
interested in collaboration or in job opportunities.
Cordially,
Jorge M. Seminario
Professor of Chemical
Engineering
Professor of Electrical and Computer Engineering
Professor of Materials Science and Engineering
Holder of the Lannater and Herb Fox Professorship
Texas A&M University
seminario@tamu.edu
http://che.tamu.edu/seminario
1.
MOLECULAR ELECTRONICS
(MOLETRONICS)
We are developing
new scenarios [1] for the use of molecules and nanoclusters
to mimic semiconductor electronic devices and systems, among those scenarios,
molecular potentials for logic and vibrational signals for communications are
the most important. Our mission is to
provide the theoretical support needed to predict and characterize the behavior
of potential molecular devices and systems, and to interpret the experimental
facts that result from working with systems at the nanometer scale. Our
ultimate goal is to provide the necessary tools for establishing a new paradigm
for the molecular and nano-systems design flow.
Methods of quantum chemistry that have been proven to successfully
determine the characteristics of organo-metallic systems help us to
computationally calculate physical and chemical properties of molecular systems
and to theoretically study their electrical properties, thus it is possible to
characterize a desired molecular device or system. Different aspects need to be addressed as
part of the characterization of the potential molecular devices and systems for
sensors, electronics and catalysis. Our
group is dedicated to the use of precise atomistic methods to study, design,
and simulate new materials for applications to sensing, electronics, and
catalysis. We are interested in the
study of materials at the atomistic level; this includes exotic topics such as
development of new drugs to the design of molecular quantum computers. Especial focus is paid now to the development
of molecular sensors, devices and systems for sensing-logic-communications and the
development of new scenarios whereby molecules can be used.
The
figure below is a demo showing that molecules can be used as molecular logic
gates
A demo
showing that molecules can be used to transmit signals through their
vibrational states is shown below. This
simulation was performed using molecular dynamics in which signals were
injected into one end of a linear molecule and the signal was recovered a few
nanometers away from the insertion point.
as
industry is already in the 30 nm feature size, the use of molecules and
nanoclusters becomes a must in order to take advantage of higher performance
computers needed for the development of practically all fields whereby
computers are the central tools for their research, design, and development.
Molecular electronics, or moletronics for
short, is the ultimate frontier in the use of electromagnetic interactions for
information processing. Organic
molecules, for instance, can be made extremely small, and can transfer
electrons with similar –or even better- electrical characteristics than those
of any working electronic device.
Although technical difficulties associated with the use of smaller molecules
are similar to those encountered with present semiconductor devices, e.g., heat
removal, interconnection, and addressing problems, the use of small organic
molecules allows us to propose other scenarios and architectures where these
problems can be overcame.
Cartoon
showing the need of tools to set a molecule between two electrodes. Adapted from J. Am. Chem. Soc. 2001, 123, 5616-5617.
Moletronics is in its infant years. So far, only few
experiments has been attempted in order to measure the electrical
characteristics of a single molecule. In one of such experiments, named the
break-junction experiment, one gold wire covered by self-assembled 1,4 benzene-dithiol
molecules was broken; then, the two metallic broken ends were approached to
each other until a single molecule was trapped between the broken ends. A
voltage difference was applied between the two terminals and the current
through the single molecule was measured. This experiment has been treated
theoretically with the hope to reach an agreement with the experiment; however,
lack of characterization techniques on the experimental, and computational
approaches on the theoretical fronts have avoided a validation between theory
and experiment as we are used at macroscopic scales. The problem at the
nanoscale is much more complex, as expected, single molecule experiments are
unique in structure because of the immense degrees of freedom available to
attach a molecule between two "huge" electrodes, several possible
configurations are possible depending on the "random" dynamics of
atoms and molecules.
1.1.
ELECTRON TRANSPORT
STUDIES:
1.1.1 Ab initio analysis
of electron currents through benzene, naphthalene, and anthracene nanojunctions
L. Yan, E.
J. Bautista, and J. M. Seminario, Nanotechnology, 18, 485701 (8pp), (2007)
The
current–voltage characteristics of benzene, naphthalene, and anthracene
attached to three types of nanoelectrode conformations are calculated using a
combined density functional theory and Green’s functions approach, whereby the
local chemistry and the extended physics of the problem are properly and fully
incorporated. The selected molecules are important building units for a
scenario of molecular devices to perform analog and logical operations. We find
out that conductances are high and tunneling barriers are low when compared to
saturated alkanes and unsaturated oligophenylene vinylenes. On the other hand,
the conformation of the nanoelectrodes addressing the molecules has a strong
effect on the current–voltage characteristics of the molecules. One of the
studied conformations is able to eliminate the tunneling barrier by withdrawing
electrons from the surface of the nanoelectrodes.
Adapted from L. Yan,
et al. Nanotechnology, 2007
1.1.2. Ab Initio Analysis of Electron Transport in Oligoglycines
E. J.
Bautista, L. Yan, and J. M. Seminario, J. Phys. Chem. C, 111 (39), 14552-14559
(2007)
We study the electrical characteristics of a group of glycine
oligopeptides (1-, 3-, 6-, and 9-mers) molecules attached to gold
nanoelectrodes using a combined density functional theorysGreen’s function
approach. Our procedure considers the applied voltage through the molecule and
contacts, recalculates self-consistently the molecular orbitals, Hamiltonian
and overlap matrices at each applied voltage, and uses these results to estimate
the current-voltage characteristics such that the chemistry of the molecule is fully
considered when including the effect of the nanoelectrodes. Our results show
that oligoglycines can be used to tailor specific properties for the
fabrication of molecular devices and the characteristics may be strongly
affected by the few contact atoms addressing the molecule. Oligoglycines show
transport behaviors that go from ohmic conductance to Schottky barriers as the
length of the oligomers and conformation of the electrode atoms vary.
Adapted from E.
Bautista, et al. JPC. C, 2007
1.1.3 Direct approach for
the electron transport through molecules
D. O. Ortiz and J. M. Seminario, J. Chem. Phys., 127, 111106 1-3, (2007)
A very
simple, straightforward, and easy to implement fully ab initio procedure for
the determination of current-voltage characteristics in molecular junctions is
presented. Application of this procedure predicts reasonably well the
experimental findings for low bias voltages of a break junction experiment and
can help us to characterize its geometry.
Adapted from D. Ortiz and J. M. Seminario. JCP, 2007.
1.1.4. Conductance model
of gold-molecule-silicon and carbon nanotube-molecule-silicon junctions
L. A. Agapito, E. J. Bautista, and J. M.
Seminario, Phys. Rev. B, 76, 115316 1-12, (2007).
We
estimate the conductance of molecular junctions composed of an oligo (phenylene
ethynylene) (OPE) molecule sandwiched between a metallic gold or carbon
nanotube on one end and a semiconducting silicon contact on the other end. Two
very well defined, low and high, states of conductance (logic “0” and “1”) are
obtained through changes in conformation or charge states of the OPE when two
metallic contacts address the molecule. However, when a combination of a
semiconducting and metallic contacts are used, the bistable states are lost at
low bias voltages where a flat region of nearly zero current in the
current-voltage characteristic is predicted regardless of the conformation or
charge state.
Adapted from L.
Agapito, et al. Phys. Rev. B, 2007.
1.1.5. Field-induced
conformational changes in bimetallic oligoaniline junctions
J. A. Sotelo, L. Yan, M. Wang, and J. M.
Seminario, Phys. Rev. A, 022511 (13 pages) (2007)
We report
three types of nonplanar conformations, α, β, and γ, for a
neutral isolated oligoaniline molecule as well as for an oligoaniline with Au
and Pd atoms attached at its ends. Each type of conformation has several
conformers of nearly equal energies. An applied external voltage can be used to
switch between conformations, producing in the process a sharp decrease of
their energies. These bias voltage-induced conformational changes are a
potential switching mechanism for two terminal molecular devices at the
nanoscale domain. They cause the conductivity of the molecule to alternate
between high and low states, compensating for the behavior of typical
three-terminal devices, needed for the development of a gate-less electronics.
Adapted from J. A.
Sotelo, et al. Phys. Rev. A, 2007.
1.1.6. Electron Transport
in Nano-Gold–Silicon Interfaces
L. Yan and
J. M. Seminario, Int. J. Quantum Chem., vol. 107 (2), pp 440-450 (2007)
The
electron transport characteristics of gold–silicon interfaces are studied using
a combined ab initio approach of the Green’s function for electron transfer and
quantum density functional theory (DFT) for finite and extended systems.
The Kohn–Sham Hamiltonian of an extended cluster or molecule and the density of
states (DOS) of bulk Si and Au are used to construct the interface Hamiltonian
to obtain the DOS, electron transmission, and current–voltage characteristics
of the interface. Diode behavior is observed with electron conduction
when the gold side is positively biased with a threshold of 0.8 V. The presence
of molecules trapped at the interface and the geometry of the metal atoms
strongly affect the conductance, implying difficult or even impossible
theory–experiment validations.
Adapted from L. Yan
and J. M. Seminario. Int. J. Quantum Chem., 2007
1.1.7. Barriers for
Electron Transport
J. M. Seminario and L. Yan, Int. J. of Quantum
Chem., Vol. 102, 711-723 (2005).
An study of electron conduction mechanisms and current-voltage
characteristics of thioalkanes has solved and old mismatch between the barriers
for conduction between experimental and theoretical results. Most precise experiments agree on a barrier
of ~1.5 eV of one conduction channel (HOMO) after fitting the data to empirical
models. On the other hand, precise calculations on the alkanes yield a
barrier for the HOMO of ~0.8 eV. The errors with respect to the
experiments of the high quality methods are much less than 0.1 eV. On the
other hand, experiments are performed on SAM with a large number of molecules
where the problems of uncertainty of single molecules have been averaged.
Therefore, there must be and strong error of interpretation of the empirical
models used to predict barriers as neither the most experiments neither most
calculation can be wrong. Thus, we investigated this problem and found
that the mismatch was due to the fact that the conduction mechanism of
thioalkanes or any other molecule is not only through the HOMO (or LUMO) but
several other channels or molecular orbitals have also strong participation
depending on their individual energies and geometrical shapes. The plot
below clarifies that several channels (and each with different barriers)
participate in the conduction through thioalkanes instead of one as generally
accepted. The experimental data were actually yielding a weighted average
barrier of several molecular orbitals.
Adapted from J. M.
Seminario and L. Yan. Int. J. Quantum Chem, 2005
1.1.8. Electronic Structure and Electron Transport Characteristics
of a Cobalt Complex
L. Yan, J. M.
Seminario, J. Phys. Chem. A, Vol. 109, 6628-6633 (2005)
In addition to developing organic materials, we investigated the electron conduction mechanism and I-V characteristics of cobalt complex molecules chemically attached between two gold pins via S-Au thiolate groups with better stable molecule-pin contacts because their dual S-Au bonding. The calculation shows that the cobalt complex has a D2d structure with two ligands perpendicular to each other and the Co atom locates at the center of a distorted octahedron formed by six donor N atoms. At low oxidation state, the bond length between Co2+ and pyrrole nitrogen is much shorter than the bond length between Co2+ and pyridine nitrogen. The oxidation of Co2+ to Co3+ significantly shortens the bond length between Co and pyridine nitrogen. The HOMO energy of the extended molecule is very close to Fermi level of Au, this yields an ease hole-injection to the occupied MOs from Au anode. The cobalt complex is a good electron conductor both at low and high oxidation state.
Adapted from L. Yan
and J. M. Seminario. JPC A, 2005
1.1.9. Analysis of a dinitro based molecular device
A proposed dinitro device, Au-(2’-nitro-4-ethynylphenyl-4’-ethynylphenyl-5’-nitro-1-benzenethiolate)-Au is analyzed using a combination of density functional and Green function theories complemented with information from theoretical and experimental studies of a similar nitroamino device, Au-(2’-amino-4-ethynylphenyl-4’-ethynylphenyl-5’-nitro-1-benzenethiolate)-Au. The dinitro compound might also perform as a molecular memory but with different characteristics than those of the nitroamino, showing well-defined charge states; however, the neutral charge state of the nitroamino presents well-defined resonant tunneling characteristics and a larger intrinsic dipole moment. Density of states, transmission functions and current-voltage characteristics for the neutral, anion, and dianion of the two molecules are compared. The effect of the bias potential is explicitly considered in the calculations as well as the effect of the contacts and the spin states of the open shell systems. The theoretical results for the training molecule are in good agreement with experiment. It is concluded that observed negative differential resistance is due mainly to charge effects combined in less degree with resonant tunneling intrinsic to single molecules.
1.1.10. Electron Transport through Single Molecules: Scattering
Treatment Using Density Functional and Green Function Theories
Green function and density functional theories are used to study electron transport characteristics through single molecules addressed by two metallic contacts. Each contact is modeled with one nanoscopic end connected to the molecule and one macroscopic end connected to an external potential difference. The method can be applied to any molecular system for which ab initio calculations can be performed. It allows us to determine the molecular orbitals participating in the electron-transfer process, the current-voltage characteristics of the junction, the density of states, and the transmission function, among other properties, providing a fundamental tool for the development of molecular electronics.
1.2.1 Detection
of molecules using spectral techniques
1.2.2 Encoding Information using Molecular
Vibronics
Terahertz Signal Generation and Transmission in Molecular Systems
L. Yan, Y. Ma, and J. M. Seminario, J. Nanoscience and
Nanotechnology, 5, 675-684 (2006).
L. Yan, Y. Ma, and J. M. Seminario, Int.
J. High Speed Electronics Syst., vol 16, 669-675 (2006)
Our goal is to develop devices and methods for signal transmission along molecules at terahertz frequencies and to provide a computational proof of the feasibility of using molecular-vibrational modes for molecular circuits. We expect a strong impact in the field of ultrafast computing. The use of molecular vibronics only requires of less than 1 eV of energy to process or transfer one bit of information as opposed to the several thousands of eV needed in current micro-electronics. This research provides the most practical alternative to scale down integrated circuits several orders of magnitude than it is possible allowed with standard bulk devices
Figure 16. adapted from Yan,
et al. J. Nanoscience and Nanotechnology, 2006, and Yan, et al. Int. J. High
Speed Electronics Syst, 2006.
1.2.3 Molecular dynamics
simulations of signal transmission through a glycine peptide chain.
L. Miao and J. M. Seminario, J.
Chem. Phys., 127, 134708 1-5, (2007)
The
injection of finite duration vibrational signals encoding information into a biomolecular
wire of the polypeptide glycine1000 is investigated theoretically using
molecular dynamics simulations and digital signal processing techniques. We
demonstrate that the amplitude modulated signal applied to one of the C–N bonds
of the molecule transmits in the two directions through the long polypeptide
molecule, which is connected to gold clusters at each of its ends. A decay of
the signal propagation speed is observed along with intensity decay. On the
other hand, the molecular dynamics simulations show that signal transmission is
completely achievable at room temperature, thus realistic transmission of
signals through linear molecules can be performed.
Adapted from L. Miao and J. M. Seminario,
JCP, 2007
1.2.4 Dynamics of a
Minimum Unit of a Molecular Random Device
J. M. Seminario, P. A. Derosa, L. E. Cordova, and B. H. Bozard, IEEE Trans. Nanotechnology, 3(1), 215-218 (2004)
In order to obtain feasible information about the stability of the molecular devices that are being proposed as minimum units to build molecular electronic systems, simulations on a minimum square unit (MU4) are performed using molecular dynamics simulations followed by a time-domain data analysis from these simulations using signal-processing techniques based on Fourier theory, which yield valuable frequency-domain information regarding the dynamical nature of the molecular units. Frequency behavior at several temperatures, as well as other dynamical features, are analyzed to complement experimental efforts leading to understand and exploit the intrinsic characteristics of these systems as well as to predict their stability at operating temperatures. (See Flash movie). A MU4 consists of four roughly spherical Au clusters each with 405 atoms and a diameter of 2.3 nm. The four clusters are interconnected by four 2,5-dinitro-1,4-diethynylphenylthiolate-benzene (1) in a square conformation. 1 is a three benzene-ring molecule with two nitro groups (NO2) substituents in the central ring.
Adapted from Seminario et al., IEEE Trans, Nanotechnology, 3(1), 215-218 (2004)
1.3.1 Molecular Potentials
for Processing of Information
The study of alternative scenarios for molecular level signal processing is very important at the nanoscale. Recent work has demonstrated using ab initio density functional theory that molecular electrostatic potential (MEP) gates can perform logical operations. Thus, the possibility to implementing complex logic by combining MEP-based devices is demonstrated. Comparison of the advantages and disadvantages between this and other proposed scenarios and the electron-current method has also been reported. Signal should be processed and transmitted in a different way in molecular devices in order to allow much less power dissipation. The advantage of this approach for post-micro-electronics technologies is the tremendous low power dissipation, the small feature size of molecular devices, and as demonstrated in this work, the compatible nature of input and output signals that allows the implementation of complex logic. The possibility to trigger one MEP digital device by other MEP digital device is one of the most important ones.
1.3.2 Cascade Configuration of Logical
Gates Processing Information Encoded in Molecular Potentials
J. M. Seminario, L. Yan, Int. J. Quantum
Chem., vol. 107 (3), pp 754-761 (2007)
We demonstrate that the outputs of two molecular devices
encoding information as electrostatic potentials are able to trigger a third
one and yield a correct truth table, thus demonstrating the possibility to
successfully interconnect extremely low-power consumption molecular devices.
Adapted from J. M.
Seminario and L. Yan., Int. J. Quantum Chem., 2007
1.3.3. Molecular electrostatic devices on graphite
and silicon surfaces
N. L. Rangel, J. M. Seminario, J. Phys. Chem. A, vol.
110 (44), pp 12298-12302 (2006)
We demonstrate that molecular gates using molecular
electrostatic potentials (MEP) can be used on hydrogenpassivated silicon
substrates without any disturbance of their behavior in vacuum; however, the
use of graphite as a substrate strongly affects such behavior. As expected, the
substrate may become one more design variable. The ability to have several
substrate alternatives is very important for the practical implementation of
this new scenario based on molecular potentials. In general, the effect of the
substrate can be predetermined by calculating the MEP of the surface as this
indicates how strongly its intrinsic potential is.
Adapted from N.
Rangel and J. M. Seminario, JPC A, 2006.
1.3.4 MEP Applications in
Moletronics
The Molecular Electrostatic Potential (MEP) approach is a well-proven tool for the analysis of several aspects in molecular systems, and it has been recently proposed as a medium for information coding in molecular circuits. We recently applied this tool for the interpretation of conductance switching observed in experiments using STM (scanning tunneling microscope) to analyze single molecules (phenylene ethynylene oligomers) isolated in matrices of dodecanethiolate monolayers. We concluded that the switching observed in the experiment is due to conformational changes in the molecule. Being exposed to different matrix environments and charging conditions, the conductance switching of the analyzed family of oligomers is proven to be correlated with the constraints of their surroundings, which affect the ratio of their conformational changes. Both experimental and theoretical studies concur in that high conductance is only possible when all the rings in the molecule are aligned, being this condition necessary but not sufficient for high conductance. Similarly, a change in the charge state does not necessarily yield switching and cannot be observed unambiguously above the top benzene ring. The electron density and the electrostatic potential along the direction of the top CH bond in the upper ring for the three charge states shows no significant differences between themselves when compared to their corresponding charge density and MEP along the NO2 directions, suggesting that charging the molecule does not necessarily change the tip position. The tip remains at the same height regardless of the charge state of the molecule
2.
Interpretation and Proof-of-Concept Experiments
The sharp differences and isolation between theory and
experiment observed in traditional fields of science and engineering tend to
become fuzzy in nanotechnology. Even
more compelling is the fact that the validation processes we were used to
between theory and experiment become complementary processes due to several
types of uncertainties and difficulties appearing at nanometer sizes. Experiments become more and more difficult
approaching nanometer lengths; fortunately, ab initio calculations become more
and more precise in that direction.
Therefore, computational chemistry, especially ab initio, has become a
practical and required tool in the field of nanotechnology. Nanotechnology cannot be done with a
trail-and-error approach due to its intrinsic uncertainties.
2.1.
Rationale
for a Theory-Guided Approach
• Once a project begins, it is difficult to change the approach: the team is already selected and very little can be done to change the approach. Researchers cannot change their field so sharply & infrastructure make any change almost impossible, thus, the project is forced to continue without selecting the best methodology.
• A theory-guided approach (TGA) is one that after a careful study is able to determine what experimental approach a project needs to be successful. A TGA should have the flexibility to change the experimental approach as guided by the theory at any stage: Definitely, for the sake of success, the proper techniques should be incorporated after a thorough analysis and tests among the available procedures using computational simulations. There is no way to know what will be needed even for the next six months for a high-risk high-benefit project. Proposers do their best to present the best they can figure it out using their own labs and experience. As the study starts, findings can be very different to what it was proposed originally.
• Theory, basis of the computational simulations at the nanoscale, is well-established; some equations are still difficult to solve but this is a technicality. Strictly speaking, there is no new physics in whatever is done with nuclei and electrons. Equations of quantum mechanics have been already known for more than 80 years. The problem of new physics simply responds to the need to have simpler equations; however, the problem of interpretation simply reduces to the problem of the ability to express quantum phenomena using classical terms, there is no need to do this. Let’s simply use quantum terms.
2.2.
Challenges for nanotechnology projects:
Find the correct molecules (or clusters) with fix or mobile features
Chemical assembly of nano/molecular circuits
Interface the molecular circuits to semiconductor IC
2.3.
Chemically Assembled Molecular Electronic
Circuits
Computing
is one of the most demanding applications of integrated circuits. It requires
the highest possible speed to process information. Higher speeds imply smaller
circuits, and therefore higher densities of integration. Thus, the most
effective way to make faster circuits is by “scaling down,” i.e., reducing the
device size proportionally. However, under present technology, miniaturization
is mainly constrained by the amount of heat dissipated as the number of devices
increases per unit area and by the ability of lithographic tools to chisel
smaller details on bulk substrates; these are technical constraints. However,
there are physical or natural constraints which are practically material
independent (speed of light, size of atoms, response of the electron, and
Planck constant). Two main scenarios have been proposed for using molecules or
small group of atoms (clusters) to build devices able to perform logical operations,
bypassing, to some extent, problems undermining miniaturization or scaling-down
processes for integrated circuits. One approach is the crossbar; a promising
technology that uses directed self-assembly to make nanoarrays similar in shape
to those already fabricated at larger scales by standard electronics. The other
approach is the nanoCell, which is a complementary design in the sense that
builds up (bottom-up) from single molecules into precise and complex structures
that can be approached by standard lithography. This paper focuses on the
description, advances, and possibilities of the nanoCell approach, which takes
advantage of the great skills developed by chemists to synthesize molecules
with precise arrangements of atoms in a molecule. The nanoCell concept does not
require a deterministic assembly or deposition of molecules and clusters thanks
to the recently discovered programmability feature of molecules. Thus, we show
that the nanoCell is a feasible concept for the development of electronics
beyond deterministic lithographic approaches presently used in the fabrication
of integrated circuits. The great importance and advantage of having molecular
size computing devices is their ability to interact directly with external
agents or molecules producing a sensor device already attached to a
nanoprocessor able to strongly help in the stand-off detection of chemical and
biological agents.
2.4.
Interpretation of Intrinsic Line widths Observed in Inelastic Electron
Tunneling Scattering Experiments
J. M. Seminario, L. E. Cordova, J. Phys. Chem. A, 108 (24), 5142-5144 (2004)
We determine the origin of the intrinsic line width found in the inelastic electron tunneling spectroscopy spectrum of an alkanedithiol molecule using first principles calculation of the second derivative of the energy (Hessian). The reason of the intrinsic width was because the experiment resolution did not allow to distinguish the fine structure of lines lines within the specific peak. As shown in the inset of the Figure below, there are seven lines that the IETS experiment considers as one.
From Seminario & Cordova, J. Phys. Chem. A, 108 (24), 5142-5144 (2004)
2.5.
Interpretation of a Switching Experiment
J. M. Seminario, P. A. Derosa, and J. L. Bastos, J. Am. Chem. Soc. 124, 10266-10267 (2002)
Identifying the factors that trigger current switching behavior of single molecules establishes the means by which it may be possible to tailor these molecules to perform as electronic devices. We provide a theoretical interpretation of switching observed in recent experiments with single molecules using quantum chemistry tools. We conclude that the switching observed in the experiment is mostly due to conformational changes and that some charge changes cannot be observed in STM experiments.
From Seminario, Derosa & Bastos, J. Am. Chem. Soc. 124, 10266-10267 (2002)
2.6.
Nanopore
Experiments using single molecules are very difficult to perform due to the macroscopic nature of the instrumentation used to characterize their electrical behavior. Currently, the measurement and characterization of the electrical behavior of three-terminal devices based on single molecules are still technically impossible. Nevertheless, a group of molecules can be accessed in parallel by a mesoscopic arrangement known as nanopore. Several experiments using a nanopore setting have been performed using a small group of molecules, ca. 1000, suspended in a colloidal solution, which are allowed to self-deposit onto a metal surface at the bottom of the nanopore. The molecules are designed such that an atom or group of atoms at one end of the molecule can form a bond to the metal. The connecting group is called alligator clip. Once the molecules are self-assembled on the metallic surface at the bottom of the nanopore, a second metallic contact is vapor-deposited on top of the self-assembled monolayer, leaving the group of molecules sandwiched between the two metallic terminals. The nanopore arrangement has been used to study several types of molecules.
2.7
Analysis of Metal-Molecule Contacts
A major task in molecular electronics is the precise determination of the molecule-metal interface characteristics. This problem has to be solved before adopting any kind of design approach for molecular circuits. Due to their intrinsic nature, metal and single molecules have unavoidable electrical mismatches for which there is not yet an established way to evaluate them. Even with the intrinsic barrier that the contacts represent, barriers can be strategically used to favor the design of specific devices; however, this requires the precise evaluation of such an interface. We studied the molecular and atomistic nature of the metallic contacts extending them with a Green function approach that considers the "infinite" nature of the contacts. The successfully tested B3PW91/LANL2DZ level of theory as implemented in the Gaussian-98 program is used to obtain the discrete nature of the molecule attached to metal atoms and its electronic characteristics are obtained under the presence of an external electrical field representing the external bias potential applied to the molecule. We compared the I-V characteristics of thio and isonitril alligator clips (defined as the molecular groups attached to the metal atoms) with metals of groups 10 (Ni, Pd, and Pt) and 11 (Cu, Ag, and Au) as depicted in Figure 2. In this way, we presented a straightforward method to compare metal-molecule interfaces. This method can be used for any metal and any molecule. It was predicted that the best metal for the metal-molecule interface corresponds to Pd, followed by Ni and Pt. Cu can be considered intermediate, and the worst case corresponds to Au and Ag. It was demonstrated that the best alligator clip corresponds to S but it is not much better than CN.