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Reading about Molecular Electronics Devices. Qingling Hang. Contents. Characterization of single molecules Molecules with one metal atom Molecules with two metal atoms Molecular logic gates Molecular memory. Electronic Characterization of Single Molecules with One Metal Atom. - PowerPoint PPT Presentation

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  • Reading about Molecular Electronics DevicesQingling Hang

  • ContentsCharacterization of single moleculesMolecules with one metal atomMolecules with two metal atomsMolecular logic gatesMolecular memory

  • Electronic Characterization of Single Molecules with One Metal AtomFigure 1 The molecules used in this study and their electronic properties. a, Structure of [Co(tpy-(CH2)5-SH)2]2+ (where tpy-(CH2)5-SH is 4'-(5-mercaptopentyl)-2,2':6',2"-terpyridinyl) and [Co(tpy-SH)2]2+ (where tpy-SH is 4'-(mercapto)-2,2':6',2"-terpyridinyl). The scale bars show the lengths of the molecules as calculated by energy minimization. b, Cyclic voltammogram of [Co(tpy-SH)2]2+ in 0.1M tetra-n-butylammonium hexafluorophosphate/acetonitrile showing the Co2+/Co3+ redox peak. c, IV curves of a [Co(tpy-(CH2)5-SH)2]2+ single-electron transistor at different gate voltages (Vg) from -0.4V (red) to -1.0V (black) with Vg -0.15V. Upper inset, a topographic atomic force microscope image of the electrodes with a gap (scale bar, 100nm). Lower inset, a schematic diagram of the device. JIWOONGPARK, ABHAYN.PASUPATHY, JONASI.GOLDSMITH, CONNIECHANG, YUVALYAISH, JASONR.PETTA, MARIERINKOSKI, JAMESP.SETHNA, HCTORD.ABRUA, PAULL.MCEUEN & DANIELC.RALPH, Nature 417, 722 (2002).

  • Differential Conductance ( I/V)Figure 2 Colour-scale plots of differential conductance ( I/V) as a function of the bias voltage (V) and the gate voltage (Vg) for three different [Co(tpy-(CH2)5-SH)2] single-electron transistors at zero magnetic field. Black represents zero conductance and white the maximum conductance. The maxima of the scales are 5nS in a, 10nS in b, and 500nS in c. The I/V values were acquired by numerically differentiating individual IV curves.

  • Magnetic-field Dependence of the Tunnelling SpectrumFigure 3 Magnetic-field dependence of the tunnelling spectrum of a [Co(tpy-(CH2)5-SH)2] single-electron transistor. a, Differential conductance plot of the device shown in Fig. 2a at a magnetic field of 6T. There is an extra level (indicated with the triangle) owing to the Zeeman splitting of the lowest energy level of Co2+. The arrows denote the spin of the tunnelling electron. b, Magnitude of the Zeeman splitting as a function of magnetic field.

  • Kondo effect in MoleculesFigure 4 Devices made using the shorter molecule, [Co(tpy-SH)2]2+, exhibit the Kondo effect. a, Breaking trace of a gold wire with adsorbed [Co(tpy-SH)2]2+ at 1.5K. After the wire is broken the current level suddenly increases (red dot) owing to the incorporation of a molecule in the gap. This is not seen for bare gold wires. b, Differential conductance of a [Co(tpy-SH)2]2+ device at 1.5K showing a Kondo peak. The inset shows I/ V plots for bare gold point contacts for comparison. c, The temperature dependence of the Kondo peak for the device shown in b. The inset shows the V = 0 conductance as a function of temperature. The peak height decreases approximately logarithmically with temperature and vanishes around 20K. d, Magnetic-field dependence of the Kondo peak. The peak splitting varies linearly with magnetic field.

  • Single-molecule Transistors with Two -Metal - Atoms MoleculesFigure 1 Fabrication of single-molecule transistors incorporating individual divanadium molecules. Top left, the structure of [(N,N',N"-trimethyl-1,4,7-triazacyclononane)2V2(CN)4(-C4N4)] (the V2 molecule) as determined by X-ray crystallography; red, grey and blue spheres represent respectively V, C and N atoms. Top right, the schematic representation of this molecule. Main panel, scanning electron microscope image (false colour) of the metallic electrodes fabricated by electron beam lithography and the electromigration-induced break-junction technique. The image shows two gold electrodes separated by 1nm above an aluminium pad, which is covered with an 3-nm-thick layer of aluminium oxide. The whole structure was defined on a silicon wafer. The bright yellow regions correspond to a gold bridge with a thickness of 15nm and a minimum lateral size of 100nm. The paler yellow regions represent portions of the gold electrodes with a thickness of 100nm. Main panel inset, schematic diagram of a single-V2 transistor. WENJIELIANG, MATTHEWP.SHORES, MARCBOCKRATH, JEFFREYR.LONG & HONGKUNPARK, Nature 417, 725 (2002).

  • Differential Conductance (I/V)Figure 2 Plots of differential conductance (I/ V) as a function of bias voltage (V) and gate voltage (Vg) obtained from two different single-V2 transistors D1 (a) and D2 (b). Both measurements were performed at T = 300mK. The I/ V values are represented by the colour scale, which changes in a, from dark red (0) to bright yellow (1.55e2/h) and in b, from dark red (0) to bright yellow (1.3 e2/h). The value of e2/h is 38.8S or (25.8k ) - 1. The labels I and II mark two conductance-gap regions, and the diagrams indicate the charge and spin states of the V2 molecule in each region.

  • Transport Data in an Applied Magnetic FieldFigure 3 Transport data obtained from single-V2 transistors in an applied magnetic field (B). a, A I/V plot as a function of V and B obtained from D1 at Vg = -0.1V and at T = 300mK. The I/V values are represented by a colour scale that varies from dark red (0) to bright yellow (1.3 e2/h). b, A I/V plot as a function of V and Vg obtained from D2 at B = 8T and at T = 300mK. White arrows indicate the two I/V peaks that arise from a Zeeman splitting. To clearly illustrate weak Zeeman-split features, the colour scale has been changed from that in Fig. 2a and varies from dark red (0) to bright yellow (0.55 e2/h).

  • Temperature-dependent Transport DataFigure 4 Temperature-dependent transport data from device D3. a, A plot of conductance (G) versus V with Vg =-2.25V at various temperatures. The temperatures of the measurements (in K) are T = 0.3, 1.0, 2.0, 3.1, 4.2, 6.3, 9.0, 14 and 20, in order of decreasing peak height. Inset, a I/V plot as a function of V and Vg at T = 300mK. The colour scale changes from dark red (0) to bright yellow (0.55 e2/h). b, The Kondo temperature (TK: filled red circles) and the Kondo peak width determined by the full-width at half-maximum (open blue circles) plotted against / in a logarithmic scale. Here - is the energy of the localized electron measured relative to the Fermi level of the metal, and is the level width due to the tunnel coupling to the metallic electrodes. Measurements of the I/V peak widths and slopes that define conductance gaps (inset in a) show that is ~30mV and the gate coupling is a= Cg/Ce = 30meVVg-1 (Cg is the capacitance to the gate, and Ce is the total capacitance). This value of a allows the conversion of Vg to , because = a(Vc - Vg). We estimate that the values of and are accurate to within 20%. Red and blue lines are proportional to exp(-3 / ) and exp(-1.3 / ), respectively. As / exceeds 2, TK and the peak widths approach respective asymptotic values. Inset, plot of the Kondo peak height (GK) as a function of temperature at = 0.43 .

  • Molecular Logic GatesFigure 1. (A) Top view of a linear array of six devices, shown approximately to scale. The wires were a few microns in diameter, and each pad was a few hundred microns across to facilitate making an electrical connection to the device. (B) Side view cross section of a single device junction. Each device consisted of a monolayer of molecules sandwiched between two perpendicularly oriented electrodes and contained several million molecules. (C) The energy level diagram of one of the devices in (A). The Fermi levels (Ef) of the Al electrodes are shown at both ends of the diagram, and discrete molecular redox energy levels [determined by solution-phase voltammetry measurements of the R(1) rotaxane] are shown in the middle. The oxidation states are noted with filled circles. The diagonally striped areas between the electrodes and the rotaxane energy levels are tunneling barriers. The thick barrier is the Al2O3 passivating layer (measured to be 1to 1.5nm), and the thin barrier is the Ti-rotaxane interface (estimated to be 0.5nm). C. P. Collier, E. W. Wong, M. Belohradsk, F. M. Raymo, J. F. Stoddart, P. J. Kuekes, R. S. Williams, J. R. Heath, Science 285, 391 (1999).

  • R(1) Rotaxane MoleculeFigure 2. A drawing of the R(1) rotaxane molecule used here.

  • Operation of the DevicesFigure 3. (A) Current-voltage traces that show the operation of the devices. As prepared, the molecular switches are "closed," and the status of the devices is probed by applying a negative voltage to the bottom electrode. The switches are "opened" by oxidizing the molecules at voltages greater than +0.7 V.Finally, the open switches are again interrogated at negative bias. The current ratio at 2 V between open and closed states is between 50and 70,depending on the specific device. (B) The same data as presented in (A), but plotted as the NDOS. When the closed switch is read, two distinct features are recorded in the NDOS. The first feature corresponds closely to the states shown in Fig. 1C. A single feature is observed in the oxidation NDOS. However, this feature is not a resolved electronic state. Rather, oxidation at around +0.7 to +0.9 V irreversibly changes the molecules, so the NDOS falls to 0as the resonant tunneling process is quenched. When the oxidized devices are interrogated at negative voltage, the electronic states that were observed between 0and 1V for a "switch closed" device are now absent.

  • Experimentally Measured Truth Tables for Logic GatesFigure 4. Experimentally measured truth tables for logic gates configured from linear arrays of molecular switch junctions. For all logic gates, a low input is held at ground, and a high input is held at +2 V.Arbitrary high and low output current levels are assigned on each plot. The inputs are labeled alphabetically, and one device, l