Probing Quantum Transport by switching Anchor Positions (original) (raw)
Related papers
Journal of the American Chemical Society, 2006
We studied the effect of anchoring groups on the conductance of single molecules using alkanes terminated with dithiol, diamine, and dicarboxylic-acid groups as a model system. We created a large number of molecular junctions mechanically and analyzed the statistical distributions of the conductance values of the molecular junctions. Multiple sets of conductance values were found in each case. The I-V characteristics, temperature independence, and exponential decay of the conductance with the molecular length all indicate tunneling as the conduction mechanism for these molecules. The prefactor of the exponential decay function, which reflects the contact resistance, is highly sensitive to the anchoring group, and the decay constant is weakly dependent on the anchoring group. These observations are attributed to different electronic couplings between the molecules and the electrodes and alignments of the molecular energy levels relative to the Fermi energy level of the electrodes introduced by different anchoring groups. For diamine and dicarboxylic-acid groups, the conductance values are sensitive to pH due to protonation and deprotonation of the anchoring groups. Further insight into the binding strengths of these anchoring groups to gold electrodes is obtained by statistically analyzing the stretching length of molecular junctions.
Conductance of a molecular junction
Science, 1997
Molecules of benzene-1,4-dithiol were self-assembled onto the two facing gold electrodes of a mechanically controllable break junction to form a statically stable goldsulfur-aryl-sulfur-gold system, allowing for direct observation of charge transport through the molecules. Current-voltage measurements at room temperature demonstrated a highly reproducible apparent gap at about 0.7 volt, and the conductancevoltage curve showed two steps in both bias directions. This study provides a quantative measure of the conductance of a junction containing a single molecule, which is a fundamental step in the emerging area of molecular-scale electronics.
The Electrical Measurement of Molecular Junctions
Annals of The New York Academy of Sciences, 1998
We present the investigation of the electrical transport of metal/(organic molecule or monolayer)/metal junctions. Utilizing a novel mechanically controllable break junction to form a statically stable system, we have self-assembled molecules of benzene-1,4-dithiol onto two facing gold electrodes allowing for direct observation of charge transport through the molecules. Current-voltage I(V) measurements provides a quantitative measure of the conductance of a junction containing a single molecule. We have also created a technique to form well-defined, stable, and reproducible metallic contacts to a self-assembled monolayer of 4-thioacetylbiphenyl with nanoscale area. Electronic transport measurements show a prominent rectifying behavior arising from the asymmetry of the molecular heterostructure. Variable-temperature measurements reveal the dominant transport mechanisms, such as thermionic emission for the Ti-organic system. These techniques demonstrate the capability of electrically characterizing and engineering conductive molecular systems for future potential device applications.
The conductance of a single molecule transport junction is investigated in the Landauer-Imry regime of coherent tunneling transport. Utilizing aromatic systems with thiol end groups, we have calculated using density functional theory the expected conductance of junctions containing molecules with different levels of conjugation and of different lengths. The calculated variations in transport junction conductance are explained in terms of the continuity of the conjugation path between leads. Molecular conjugation describes this continuity within the molecule, and the interfacial terms (spectral densities or imaginary parts of the self-energy) describe its continuity at the molecule/metal interface. We compare the results from junction conductance calculations with isolated molecule electronic structure calculations These density functional theory calculations suggest that for these dithiol molecules, transport occurs mostly through the occupied orbital manifold. The decay of the transport with length is found to be exponential for poly-Ph dithiol molecules. We compare the calculated conductance of conjugated aromatic molecules with their molecular orbital calculations and with the Green's function formulation and evaluate the relative significance of different factors (such as energetic alignment and spectral density) that control the conductance of molecules.
The Journal of Chemical Physics, 1999
To study the electronic transport of molecular wire circuits, we present a time-independent scattering formalism which includes an ab initio description of the molecular electronic structure. This allows us to obtain the molecule-metal coupling description at the same level of theory. The conductance of junction ␣, ␣Ј xylyl dithiol and benzene-1,4-dithiol between gold electrodes is obtained and compared with available experimental data. The conductance depends dramatically on the relative position of the Fermi energy of the metal with respect to the molecular levels. We obtain an estimate for the injecting energy of the electron onto the molecule by varying the distance between the molecule and the attached gold clusters. Contrary to the standard assumption, we find that the injecting energy lies close to the molecular highest occupied molecular orbital, rather than in the middle of the gap; it is just the work function of the bulk metal. Finally, the adequacy of the widely used extended Hückel method for conductance calculations is discussed.
Physical Review B, 2010
We report a first-principles study of quantum transport in a prototype two-terminal device consisting of a molecular nanowire acting as an interconnect between two gold electrodes. The wire is composed of a series of bicyclo͓1.1.1͔pentane ͑BCP͒ cage-units. The length of the wire ͑L͒ is increased by sequentially increasing the number of BCP cage units in the wire from 1 to 3. A two terminal model device is made out of each of the three wires. A parameter free, nonequilibrium Green's function approach, in which the bias effect is explicitly included within a many body framework, is used to calculate the current-voltage characteristics of each of the devices. In the low bias regime that is considered in our study, the molecular devices are found to exhibit Ohmic behavior with resistances of 0.12, 1.4, and 6.5 ⍀ for the wires containing one, two, and three cages respectively. Thus the conductance value, G c , which is the reciprocal of resistance, decreases as e −L with a decay constant ͑͒ of 0.59 Å −1. This observed variation of conductance with the length of the wire is in excellent agreement with the earlier reported exponential decay feature of the electron transfer rate predicted from the electron transfer coupling matrix values obtained using the two-state Marcus-Hush model and the Koopman's theorem approximation. The downright suppression of the computed electrical current for a bias up to 0.4 V in the longest wire can be exploited in designing a three terminal molecular transistor; this molecular wire could potentially be used as a throttle to avoid leakage gate current.
Measurement of Single Molecule Conductance: Benzenedithiol and Benzenedimethanethiol
Nano Letters - NANO LETT, 2004
We have studied electron transport properties of benzenedithiol and benzenedimethanethiol covalently bonded to gold electrodes by repeatedly creating a large number of molecular junctions. For each molecule, conductance histogram shows peaks at integer multiples of a fundamental conductance value, which is used to identify the conductance of a single molecule. The conductance values of a benzenedithiol and benzenedimethanethiol are 0.011 G 0 and 0.0006 G 0 (G 0) 2e 2 /h), respectively. The conductance peaks are broad, which reflects variations in the microscopic details of different molecular junctions. We have also studied electrochemical gate effect.
Conductance of Single Alkanedithiols: Conduction Mechanism and Effect of Molecule−Electrode Contacts
Journal of the American Chemical Society, 2006
The conductance of single alkanedithiols covalently bound to gold electrodes has been studied by statistical analysis of repeatedly created molecular junctions. For each molecule, the conductance histogram reveals two sets of well-defined peaks, corresponding to two different conductance values. We have found that (1) both conductance values decrease exponentially with the molecular length with an identical decay constant, ≈ 0.84 Å -1 , but with a factor of 5 difference in the prefactor of the exponential function.
Variability of Conductance in Molecular Junctions
The Journal of Physical Chemistry B, 2006
The conductance of molecular junctions, formed by breaking gold point contacts dressed with various thiol functionalized organic molecules, is measured at 293 K and at 30 K. In the presence of molecules, individual conductance traces measured as a function of increasing gold electrode displacement show clear steps below the quantum conductance steps of the gold contact. These steps are distributed over a wide range of moleculedependent conductance values. Histograms constructed from all conductance traces therefore do not show clear peaks either at room or low temperatures. Filtering of the data sets by an objective automated procedure only marginally improves the visibility of such features. We conclude that the geometrical junction to junction variations dominate the conductance measurements.