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Probing Electronic and Thermoelectric Properties of Single Molecule Junctions
In an effort to further understand electronic and thermoelectric phenomenon at the nanometer scale, we have studied the transport properties of single molecule junctions. To carry out these transport measurements, we use the scanning tunneling microscope-break junction (STM-BJ) technique, which involves the repeated formation and breakage of a metal point contact in an environment of the target molecule. Using this technique, we are able to create gaps that can trap the molecules, allowing us to sequentially and reproducibly create a large number of junctions. By applying a small bias across the junction, we can measure its conductance and learn about the transport mechanisms at the nanoscale. The experimental work presented here directly probes the transmission properties of single molecules through the systematic measurement of junction conductance (at low and high bias) and thermopower. We present measurements on a variety of molecular families and study how conductance depends on the character of the linkage (metal-molecule bond) and the nature of the molecular backbone. We start by describing a novel way to construct single molecule junctions by covalently connecting the molecular backbone to the electrodes. This eliminates the use of linking substituents, and as a result, the junction conductance increases substantially. Then, we compare transport across silicon chains (silanes) and saturated carbon chains (alkanes) while keeping the linkers the same and find a stark difference in their electronic transport properties. We extend our studies of molecular junctions by looking at two additional aspects of quantum transport - molecular thermopower and molecular current-voltage characteristics. Each of these additional parameters gives us further insight into transport properties at the nanoscale. Evaluating the junction thermopower allows us to determine the nature of charge carriers in the system and we demonstrate this by contrasting the measurement of amine-terminated and pyridine-terminated molecules (which exhibit hole transport and electron transport, respectively). We also report the thermopower of the highly conducting, covalently bound molecular junctions that we have recently been able to form, and learn that, because of their unique transport properties, the junction power factors, GS2, are extremely high. Finally, we discuss the measurement of molecular current-voltage curves and consider the electronic and physical effects of applying a large bias to the system. We conclude with a summary of the work discussed and an outlook on related scientific studies
Highly Conducting pi-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes
We measure electronic conductance through single conjugated molecules bonded
to Au metal electrodes with direct Au-C covalent bonds using the scanning
tunneling microscope based break-junction technique. We start with molecules
terminated with trimethyltin end groups that cleave off in situ resulting in
formation of a direct covalent sigma bond between the carbon backbone and the
gold metal electrodes. The molecular carbon backbone used in this study consist
of a conjugated pi-system that has one terminal methylene group on each end,
which bonds to the electrodes, achieving large electronic coupling of the
electrodes to the pi-system. The junctions formed with the prototypical example
of 1,4-dimethylenebenzene show a conductance approaching one conductance
quantum (G0 = 2e2/h). Junctions formed with methylene terminated oligophenyls
with two to four phenyl units show a hundred-fold increase in conductance
compared with junctions formed with amine-linked oligophenyls. The conduction
mechanism for these longer oligophenyls is tunneling as they exhibit an
exponential dependence of conductance with oligomer length. In addition,
density functional theory based calculations for the Au-xylylene-Au junction
show near-resonant transmission with a cross-over to tunneling for the longer
oligomers.Comment: Accepted to the Journal of the American Chemical Society as a
Communication
Simultaneous Determination of Conductance and Thermopower of Single Molecule Junctions
We report the first concurrent determination of conductance
(<i>G</i>) and thermopower (<i>S</i>) of single-molecule
junctions via direct measurement of electrical and thermoelectric
currents using a scanning tunneling microscope-based break-junction
technique. We explore several amine-Au and pyridine-Au linked molecules
that are predicted to conduct through either the highest occupied
molecular orbital (HOMO) or the lowest unoccupied molecular orbital
(LUMO), respectively. We find that the Seebeck coefficient is negative
for pyridine-Au linked LUMO-conducting junctions and positive for
amine-Au linked HOMO-conducting junctions. Within the accessible temperature
gradients (<30 K), we do not observe a strong dependence of the
junction Seebeck coefficient on temperature. From histograms of thousands
of junctions, we use the most probable Seebeck coefficient to determine
a power factor, <i>GS</i><sup>2</sup>, for each junction
studied, and find that <i>GS</i><sup>2</sup> increases with <i>G</i>. Finally, we find that conductance and Seebeck coefficient
values are in good quantitative agreement with our self-energy corrected
density functional theory calculations
Conductance of Single Cobalt Chalcogenide Cluster Junctions
Understanding the electrical properties of semiconducting quantum dot devices have been limited due to the variability of their size/composition and the chemistry of ligand/electrode binding. Furthermore, to probe their electrical conduction properties and its dependence on ligand/electrode binding, measurements must be carried out at the single dot/cluster level. Herein we report scanning tunneling microscope based break junction measurements of cobalt chalcogenide clusters with Te, Se and S to probe the conductance properties. Our measured conductance trends show that the Co-Te based clusters have the highest conductance while the Co-S clusters the lowest. These trends are in very good agreement with cyclic voltammetry measurements of the first oxidation potentials and with density functional theory calculations of their HOMO-LUMO gapsclos
Conductivity measurements in single-molecule junctions: Meta-substituted benzene
The conductance of substituted benzene molecules is measured in metal-molecule-metal junctions formed by breaking gold point contacts. In a previous study, we could see a clear peak in the conductance histogram of 1,4-benzenediamine but not in that of 1,3-benzenediamine. To understand this difference and to obtain a clear peak in meta-substituted molecule, we screen the molecules with different end groups and find methyl selenide and diphenylphosphine are measurable groups. 1,3-Bis(methylseleno)benzene shows a peak in the conductance histogram, whereas tetrahydrobenzo[1,2-b:5,4-b']diselenophene does not. In addition, 1,3-bis(diphenylphosphino)benzene reveals a sharper peak than 1,3-bis(methylseleno)benzene.
We hypothesize the specific conformation of the end groups relative to the phenyl ring enables the favorable alignment of non-pair electrons for the electron tunneling through the molecule. The relative energies of conformers are being calculated
Conductive Molecular Silicon
Bulk silicon, the bedrock of information technology,
consists of
the deceptively simple electronic structure of just SiāSi Ļ
bonds. Diamond has the same lattice structure as silicon, yet the
two materials have dramatically different electronic properties. Here
we report the specific synthesis and electrical characterization of
a class of molecules, oligosilanes, that contain strongly interacting
SiāSi Ļ bonds, the essential components of the bulk semiconductor.
We used the scanning tunneling microscope-based break-junction technique
to compare the single-molecule conductance of these oligosilanes to
those of alkanes. We found that the molecular conductance decreases
exponentially with increasing chain length with a decay constant Ī²
= 0.27 Ā± 0.01 Ć
<sup>ā1</sup>, comparable to that
of a conjugated chain of Cī»C Ļ bonds. This result demonstrates
the profound implications of Ļ conjugation for the conductivity
of silicon
Conductive Molecular Silicon
Bulk silicon, the bedrock of information technology,
consists of
the deceptively simple electronic structure of just SiāSi Ļ
bonds. Diamond has the same lattice structure as silicon, yet the
two materials have dramatically different electronic properties. Here
we report the specific synthesis and electrical characterization of
a class of molecules, oligosilanes, that contain strongly interacting
SiāSi Ļ bonds, the essential components of the bulk semiconductor.
We used the scanning tunneling microscope-based break-junction technique
to compare the single-molecule conductance of these oligosilanes to
those of alkanes. We found that the molecular conductance decreases
exponentially with increasing chain length with a decay constant Ī²
= 0.27 Ā± 0.01 Ć
<sup>ā1</sup>, comparable to that
of a conjugated chain of Cī»C Ļ bonds. This result demonstrates
the profound implications of Ļ conjugation for the conductivity
of silicon
Quantitative CurrentāVoltage Characteristics in Molecular Junctions from First Principles
Using self-energy-corrected density functional theory
(DFT) and
a coherent scattering-state approach, we explain currentāvoltage
(IV) measurements of four pyridine-Au and amine-Au linked molecular
junctions with quantitative accuracy. Parameter-free many-electron
self-energy corrections to DFT KohnāSham eigenvalues are demonstrated
to lead to excellent agreement with experiments at finite bias, improving
upon order-of-magnitude errors in currents obtained with standard
DFT approaches. We further propose an approximate route for prediction
of quantitative IV characteristics for both symmetric and asymmetric
molecular junctions based on linear response theory and knowledge
of the Stark shifts of junction resonance energies. Our work demonstrates
that a quantitative, computationally inexpensive description of coherent
transport in molecular junctions is readily achievable, enabling new
understanding and control of charge transport properties of molecular-scale
interfaces at large bias voltages
Conductive Molecular Silicon
Bulk silicon, the bedrock of information technology,
consists of
the deceptively simple electronic structure of just SiāSi Ļ
bonds. Diamond has the same lattice structure as silicon, yet the
two materials have dramatically different electronic properties. Here
we report the specific synthesis and electrical characterization of
a class of molecules, oligosilanes, that contain strongly interacting
SiāSi Ļ bonds, the essential components of the bulk semiconductor.
We used the scanning tunneling microscope-based break-junction technique
to compare the single-molecule conductance of these oligosilanes to
those of alkanes. We found that the molecular conductance decreases
exponentially with increasing chain length with a decay constant Ī²
= 0.27 Ā± 0.01 Ć
<sup>ā1</sup>, comparable to that
of a conjugated chain of Cī»C Ļ bonds. This result demonstrates
the profound implications of Ļ conjugation for the conductivity
of silicon