Charge-Carrier Transport Measurements through Single Molecules

Vorschau

Inhaltsverzeichnis, Datei (95 KB)
Leseprobe, Datei (380 KB)

The goal of this thesis was to establish and to characterize single-molecule junctions by means of the mechanically controllable break-junction (MCBJ) technique. Using this method, an electrode pair with atomic-sized tips can be created. These tips are located exactly opposite to each other and their distance can be adjusted with picometer accuracy. This technique was developed by Moreland et al. in 1985 and further improved by Muller et al. in the subsequent years to study quantum phenomena in superconductors. Mechanically controllable break-junctions are distinguished by an excellent stability of the two electrodes against external vibrations. The stability is achieved by a purely mechanical transaction in a three-point bending mechanism with a very low transmission ratio between the pushing-rod travel distance, and the electrode separation resulting there from (ratio of approximately 1 × 10-5). This lateral stability combined with a sub-atomic spatial electrode positioning accuracy allows single molecules to be contacted.
In the framework of this thesis, an ultra-high vacuum system equipped with a MCBJ bending mechanism was designed, fabricated, characterized, and continuously improved. This experimental system can be used to establish a contact with a single molecule and to perform temperature-dependent investigations of its charge-carrier transport properties. The thermal coupling between the cryostat and the sample holder was improved in several ways to achieve a temperature range of 8 to 300 K. The bending mechanism of the break-junction was also modified considerably. For instance, additional supporting bolts and a proper head opposite to the pushing-rod ensure the back bending of the substrate should the bending force might have exceeded the elastic range of the metal (bending radii larger than 18 mm). As a result, the junction can be opened and closed several hundred times without any sign of fatigue. Moreover, a reliable electrical contact to the sample was implemented by means of spring-loaded contact pins. They provide a stable electrical connection between the measurement instrument and the sample, which is not influenced by the bending procedure.
The MCBJ samples were fabricated using a combination of optical and electron-beam lithography. These fabrication techniques allow 1 μm short, between 75 nm and 120 nm narrow, free-standing bridges to be manufactured. As a result of this geometry, the dynamic range for the opening and closing of the junction is excellent.
In the introductory experimental part, the metal-metal contacts as created via the MCBJ approach were characterized in terms of their mechanical and electrical properties. The attention was focussed especially on those investigations that are important for handling separated electrodes for contacting single molecules. The microscopic geometry of the electrode tips, their positioning accuracy, and the stability were found to be the essential parameters. Gold was used as an electrode material as it is very well suited because of its ductile response to deformation, therefore enabling the formation of atomic-sized tips. A relative distance calibration by means of tunnelling current measurements between the two electrodes revealed that the theoretical ratio between pushing-rod translation and the resulting electrode separation (2.1 × 10-5) corresponds very well to the experimentally measured value (1.8 × 10-5).
The stability of the electrodes at fixed, low temperature (e.g., T = 30 K) is extraordinary. For instance, a constant tunneling resistance of (15 ±3) MΩ can be maintained over many minutes. This means that the distance between the separated electrodes varies only within 5 – 10 picometers. Upon further closing of the junction, it was found that the electrodes cannot be approached arbitrarily close to each other. A “jump-to-contact" to the conductance quantum G0 was observed for tunnelling resistances below 1 MΩ. The excellent stability between the electrodes changes fundamentally at higher temperatures (T > 150 K). Both resolution and the stability worsen at elevated temperatures, which is primarily due to the enhanced mobility of the gold atoms. In slow opening and closing cycles (velocity < 1 picometer/second), conductance quantization in units of G0 was observed, independent of the temperature. These quantum effects become evident at the moment at which all dimensions of the point contact are smaller than the Fermi wavelength of the metal electrons (λF = 0.52 nm).
In the case of gold, a conductance value of G0 signifies, by reason of the transmission which is influenced by the chemical valance of the atom (s-orbital), that a single-atom contact has been created. After the breaking of this point contact, the two electrodes possess atomic-sized tips. Such electrodes enable the contacting of single molecules. Hence, a conditioning procedure is described which allows electrode tips of this type to be fabricated automatically.
Molecules as functional building blocks are a promising concept for the post-CMOS era. Early prognoses about the use of molecular components were euphoric. In the meantime, these forecast had to be adjusted considerably, mainly because of the variances in manufacturing at the nanometer scale. Although molecules can be produced identical thanks to their chemical synthesis, their variable coupling to the electrodes causes large fluctuations in the measurement of the transport properties. To take this into account, special attention was paid to statistics in the experiments. For these reasons, a novel statistical measurement and analysis approach was developed. It facilitates the observation of transport properties, namely, the current-voltage curves, during the controlled manipulation from the formation to the breaking of the molecular junction. Because of the choice of the control parameters made, the measurement approach works independently of the distance of electrodes. Hence temperature ramps can be performed, while the influence of the thermal expansion of the electrode is being compensated. In the subsequent statistical analysis using the software tools developed, it was not only possible to determine the most probable transport characteristics occurring in huge data sets, but also to quantify the variations in the process. For instance, the stochastic fluctuations in the molecule—metal contact, an inherent feature of the thiol-metal bond, can be observed and quantified.
Based on this statistical approach, molecular model systems which allow a independent investigation of the different aspects of molecular transport were studied systematically. In the first part, the molecule—metal contact, which is an eminently important factor for the transport behavior, was studied. Various molecules were investigated, that differ only in their end groups but not in the molecular backbone. The functional end groups chosen, thiol and isocyanide termination, bond to the metal electrode in varying strengths.
In the case of strong coupling, the electron injection rate from the metal to the molecule increases; thus, the dwell time of the electrons on the molecular orbitals shortens. This causes the corresponding energy levels to broaden energetically. By the spectroscopic analysis at low temperatures (T = 50 K), where the influence of the thermal line broadening is negligible, thiol-coupled molecular systems (PDT and BPDT molecules) have been compared with isocyanide-coupled molecular systems (PDI and BPDI molecules). The line broadening caused by the coupling was found to be 50% larger (or 25 mV) in PDT than in PDI. In return, the conductance maxima were found to be slightly higher (95 nS) for PDI than for PDT (70 nS). The isocyanide end group turned out to be a better option, not only concerning the line sharpness, but also concerning the transport properties within the HOMO—LUMO gap, which could be measured in contrast to that in the corresponding thiol-coupled systems. Furthermore, the isocyanide-coupling allows temperature-dependent experiments because the molecule does not lose contact to the electrodes upon cooling. The results indicate that this type of end group possesses an enhanced surface mobility.
In further investigations, the influence of the intramolecular structure on transport at equal coupling was evaluated. Oligophenylenes of increasing length were studied. In the molecular backbone of these oligophenylenes, the electronic overlap of the conjugated subsystems, namely the phenyl rings, is reduced by means of steric hindrance between adjacent methyl groups attached to the phenyl rings. Thus, a selective breaking of the conjugation across the molecules was introduced. According to this idea, molecules with up to four phenyl rings were synthesized (sulphur—sulphur distances of 0.62 nm to 1.97 nm), individually addressed, and electrically characterized. The measurements revealed that the influence of the length of the molecular backbone on the transport properties is significant: For the phenyl-1,4-dithiol, the shortest molecule with one ring and an electrode gap distance of approximately 0.846 nm, only one resonant molecular orbital was lying within the voltage window available. For longer molecules possessing two or more phenyl rings, additional conductance peaks appeared and were found to be characteristic for the particular molecule under investigation. The so-called conductance gap increases from 250 to 580 mV with increasing length of the molecule. This means that the bias for charge-carrier injection has to be increased because the voltage drop across the molecule—metal tunnelling barrier changed according to the increased length of the molecule.
On the basis of the measurements addressing the coupling and the influence of the intramolecular structure on the transport, an attempt to investigate the transport behavior of molecules, which possess an intrinsic functionality, was made. The question of whether single molecules possess an intramolecular function that influences the electrical transport and could eventually be controlled externally was approached by investigating two types of functional molecules: rectifying and voltage-induced switching molecules. A clear diode behavior has been observed for the so-called SB-diode molecule. The behavior turned out to be temperature-independent, and a current rectification ratio of 10 was measured at a voltage of 1.0 V. On the basis of the functional behavior of the current as a function of voltage, even higher ratios could be expected. However, the instability inherent to the electrodes prevented investigations at higher electric-field strengths. Therefore, the maximum rectification ratio was determined to be 15 within the stable voltage window available (-1.8 to +1.8 V).
A voltage-induced switching was examined at a functional molecule, the so-called BPDN-DT molecule, and compared with a reference molecule which did not possess a functional internal structure. The reference molecule did not exhibit a switching behavior at low temperatures; in particular no stochastic contact fluctuations were found at low temperatures. In contrast, it was possible to switch the functional molecule in a controlled and reproducible way from a so-called “off" state to a higher conductive “on" state by applying a voltage pulse. By the comparison with the reference system it was proven that the internal functional nitro groups are truly the origin of the switching behavior. Through time-dependent experiments it was further demonstrated that a bistable energy range of approximately 120 mV exists, in which different switching times between the corresponding states occur. The switching times measured for the single-molecule system of 250 μs were only limited by the electronic instrumentation.
Furthermore, an anomalous hysteresis was found in the transport curves, if also the negative voltage branch was included in the inspections. It turned out that the higher conductive state for the positive voltage branch becomes the lower conductive state for the negative voltage branch. Exploring this knowledge, the molecule could be switched back and forth between the two distinct states in a very controlled and reproducible way. This again enables the single-molecule system to be employed as a real memory cell. The potential to use single molecules as storage devices was demonstrated by repeated write—read—erase—read cycles and by achieving a bit-separation (ratio between “on" and “off" currents) between 7 and 70. The switching pulses used were in the range of milliseconds and during the readout times of several seconds, no change in both states were revealed. Hence, the bit read-out was found to occur non-destructively. Within the measurement time of more than one hour, no degradation of the “on" state was observed, which is an indication for an excellent stability of the molecular system. More than 1500 successive switching cycles were performed without any appearance of an altered switching behavior of the single-molecule switch.