Youngsang Kim

Observation of molecular orbital gating

The control of charge transport in an active electronic device depends intimately on the modulation of the internal charge density by an external node. For example, a field-effect transistor relies on the gated electrostatic modulation of the channel charge produced by changing the relative position of the conduction and valence bands with respect to the electrodes. In molecular-scale devices, a longstanding challenge has been to create a true three-terminal device that operates in this manner (that is, by modifying orbital energy). Here we report the observation of such a solid-state molecular device, in which transport current is directly modulated by an external gate voltage. Resonance-enhanced coupling to the nearest molecular orbital is revealed by electron tunnelling spectroscopy, demonstrating direct molecular orbital gating in an electronic device. Our findings demonstrate that true molecular transistors can be created, and so enhance the prospects for molecularly engineered electronic devices.

Benzenedithiol: A Broad-Range Single-Channel Molecular Conductor

More than a decade after the first report of single-molecule conductance, it remains a challenging goal to prove the exact nature of the transport through single molecules, including the number of transport channels and the origin of these channels from a molecular orbital point of view. We demonstrate for the archetypical organic molecule, benzenedithiol (BDT), incorporated into a mechanically controllable break junction at low temperature, how this information can be deduced from studies of the elastic and inelastic current contributions. We are able to tune the molecular conformation and thus the transport properties by displacing the nanogap electrodes. We observe stable contacts with low conductance in the order of 10(-3) conductance quanta as well as with high conductance values above ∼0.5 quanta. Our observations show unambiguously that the conductance of BDT is carried by a single transport channel provided by the same molecular level, which is coupled to the metallic electrodes, through the whole conductance range. This makes BDT particularly interesting for applications as a broad range coherent molecular conductor with tunable conductance.

Charge transport characteristics of diarylethene photoswitching single-molecule junctions.

We report on the experimental analysis of the charge transport through single-molecule junctions of the open and closed isomers of photoswitching molecules. Sulfur-free diarylethene molecules are developed and studied via electrical and optical measurements as well as density functional theory calculations. The single-molecule conductance and the current-voltage characteristics are measured in a mechanically controlled break-junction system at low temperatures. Comparing the results with the single-level transport model, we find an unexpected behavior of the current-dominating molecular orbital upon isomerization. We show that both the side chains and end groups of the molecules are crucial to understand the charge transport mechanism of photoswitching molecular junctions.

Characteristics of amine-ended and thiol-ended alkane single-molecule junctions revealed by inelastic electron tunneling spectroscopy.

A combined experimental and theoretical analysis of the charge transport through single-molecule junctions is performed to define the influence of molecular end groups for increasing electrode separation. For both amine-ended and thiol-ended octanes contacted to gold electrodes, we study signatures of chain formation by analyzing kinks in conductance traces, the junction length, and inelastic electron tunneling spectroscopy. The results show that for amine-ended molecular junctions no atomic chains are pulled under stretching, whereas the Au electrodes strongly deform for thiol-ended molecular junctions. This advanced approach hence provides unambiguous evidence that the amine anchors bind only weakly to Au.

Synthesis and Photoswitching Studies of Difurylperfluorocyclopentenes with Extended π‐Systems

In an attempt to design molecular optoelectronic switches functioning in molecular junctions between two metal tips, we synthesized a set of photochromic compounds by extending the π-system of 1,2-bis-(2-methyl-5-formylfuran-3-yl)perfluorocyclopentene through suitable coupling reactions involving the formyl functions, thereby also introducing terminal groups with a binding capacity to gold. Avoiding the presence of gold-binding sulphur atoms in the photoreactive centre, as they are present in the frequently used analogous thienyl compounds, the newly synthesized compounds should be more suitable for the purpose indicated. The kinetics of reversible photoswitching of the new compounds by UV and visible light was quantitatively investigated in solution. The role of conformational flexibility of the π-system for the width of the UV/Vis spectra was clarified by using quantum chemical calculations with time-dependent (TD)-DFT. As a preliminary test of the potential of the new compounds to serve as optoelectronic molecular switches, monolayer formation and photochemical switching on gold surfaces was observed by using surface plasmon resonance.

Vibrational spectra of metal-molecule-metal junctions in electromigrated nanogap electrodes by inelastic electron tunneling

We measure the vibrational signatures of metal-molecule-metal junctions formed from 1,8-octanedithiol and 1,4-benzenedithiol incorporated into electromigrated nanogap electrodes using inelastic electron tunneling spectroscopy (IETS). The junction conductance measured suggests that the IETS spectra have been achieved at the individual molecule level. The IETS spectra provide unambiguous experimental evidence of the existence of the component molecules in the fabricated nanogap electrode testbeds. The intense Au–S stretch peaks elucidate that the thiol anchor group is linked to the broken Au wires during electromigration, thus creating reliable electrical contact to individual molecules.

Current-voltage characteristics of single-molecule diarylethene junctions measured with adjustable gold electrodes in solution.

Summary We report on an experimental analysis of the charge transport through sulfur-free photochromic molecular junctions. The conductance of individual molecules contacted with gold electrodes and the current–voltage characteristics of these junctions are measured in a mechanically controlled break-junction system at room temperature and in liquid environment. We compare the transport properties of a series of molecules, labeled TSC, MN, and 4Py, with the same switching core but varying side-arms and end-groups designed for providing the mechanical and electrical contact to the gold electrodes. We perform a detailed analysis of the transport properties of TSC in its open and closed states. We find rather broad distributions of conductance values in both states. The analysis, based on the assumption that the current is carried by a single dominating molecular orbital, reveals distinct differences between both states. We discuss the appearance of diode-like behavior for the particular species 4Py that features end-groups, which preferentially couple to the metal electrode by physisorption. We show that the energetic position of the molecular orbital varies as a function of the transmission. Finally, we show for the species MN that the use of two cyano end-groups on each side considerably enhances the coupling strength compared to the typical behavior of a single cyano group.

Intrinsic charge transport of conjugated organic molecules in electromigrated nanogap junctions

We present the measurement of charge transport through phenylene conjugated molecules using electromigrated nanogap junctions. To elucidate the intrinsic transport properties of the conjugated molecular junctions, a variety of molecular transport techniques were performed at low temperature, including inelastic electron tunneling spectroscopy, temperature- and length-variable transport measurements, and transition voltage spectroscopy. Such a self-consistent characterization of the molecular junction demonstrates the observation of intrinsic molecular properties in these junctions.

Noise Characteristics of Charge Tunneling via Localized States in Metal−Molecule−Metal Junctions

We report the noise characteristics of charge transport through an alkyl-based metal--molecule--metal junction. Measurements of the 1/f noise, random telegraph noise, and shot noise demonstrated the existence of localized traps in the molecular junctions. These three noise measurements exhibited results consistent with trap-mediated tunneling activated over approximately 0.4 V by trapping and detrapping processes via localized states (or defects). The noise characterizations will be useful in evaluating the influences of localized states on charge transport in molecular or other electronic junctions.

Shot Noise Suppression in SiGe Resonant Interband Tunneling Diodes

We report experimental noise studies of SiGe resonant interband tunneling diodes (RITDs) to probe the tunneling transport properties. The shot noise measurements show the signatures of coherent transport not only in the positive differential resistance (PDR) region but also in the plateau-like region on the negative differential resistance (NDR) side of the current–voltage (I–V) trace. The experimentally extracted Fano factor F < 0.5 may suggest that the coherent transport gradually becomes obvious in the NDR region. The variation of the Fano factor through the resonance process is discussed according to the recent theoretical model of coherent tunneling.