Reading about Molecular Electronics Devices

Qingling Hang

Contents

1. Characterization of single molecules

Molecules with one metal atom

Molecules with two metal atoms

2. Molecular logic gates

3. Molecular memory

Electronic Characterization of Single Molecules with One Metal Atom

Figure 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 0.1 M tetra-n-butylammonium hexafluorophosphate/acetonitrile showing the Co2+/Co3+ redox peak. c, I–V curves of a [Co(tpy-(CH2)5-SH)2]2+ single-electron transistor at different gate voltages (Vg) from -0.4 V (red) to -1.0 V (black) with Vg -0.15 V. Upper inset, a topographic atomic force microscope image of the electrodes with a gap (scale bar, 100 nm). Lower inset, a schematic diagram of the device.

JIWOONG PARK, ABHAY N. PASUPATHY, JONAS I. GOLDSMITH, CONNIE CHANG, YUVAL YAISH, JASON R. PETTA, MARIE RINKOSKI, JAMES P. SETHNA, HÉCTOR D. ABRUÑA, PAUL L. MCEUEN & DANIEL C. RALPH, Nature 417, 722 (2002).

Differential Conductance ( әI/әV)

Figure 2 Colour-scale plots of differential conductance ( әI/әV) 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 5 nS in a, 10 nS in b, and 500 nS in c. The әI/әV values were acquired by numerically differentiating individual I–V curves.

Magnetic-field Dependence of the Tunnelling Spectrum

Figure 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 6 T. 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 Molecules

Figure 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 1.5 K. 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 1.5 K showing a Kondo peak. The inset shows әI/ әV 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 20 K. d, Magnetic-field dependence of the Kondo peak. The peak splitting varies linearly with magnetic field.

Single-molecule Transistors with Two -Metal - Atoms Molecules

Figure 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(µ-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 1 nm 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 15 nm and a minimum lateral size of 100 nm. The paler yellow regions represent portions of the gold electrodes with a thickness of 100 nm. Main panel inset, schematic diagram of a single-V2 transistor.

WENJIE LIANG, MATTHEW P. SHORES, MARC BOCKRATH, JEFFREY R. LONG & HONGKUN PARK, Nature 417, 725 (2002).

Differential Conductance (әI/әV)

Figure 2 Plots of differential conductance (әI/ әV) 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 = 300 mK. The әI/ә 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 38.8 µS or (25.8 kΩ ) - 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 Field

Figure 3 Transport data obtained from single-V2 transistors in an applied magnetic field (B). a, A әI/әV plot as a function of V and B obtained from D1 at Vg = -0.1 V and at T = 300 mK. The әI/әV values are represented by a colour scale that varies from dark red (0) to bright yellow (1.3 e2/h). b, A әI/әV plot as a function of V and Vg obtained from D2 at B = 8 T and at T = 300 mK. White arrows indicate the two әI/әV 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 Data

Figure 4 Temperature-dependent transport data from device D3. a, A plot of conductance (G) versus V with Vg =-2.25 V 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 әI/әV plot as a function of V and Vg at T = 300 mK. 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 ε/Γ in a logarithmic scale. Here - ε is the energy of the localized electron measured relative to the Fermi level of the metal, and Γ is the level width due to the tunnel coupling to the

metallic electrodes. Measurements of the әI/әV peak widths and slopes that define conductance gaps (inset in a) show that Γ is ~30 mV and the gate coupling is = Cg/C = 30 meV Vg-1 (Cg is the capacitance to the gate, and C is the total capacitance). This value of allows the conversion of Vg to ε, because ε = (Vc - Vg). We estimate that the values

of ε and Γ are accurate to within 20%. Red and blue lines

are proportional to exp(-3 ε / Γ) and exp(-1.3 ε / Γ ),

respectively. As ε / Γ 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 Gates

Figure 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 1 to 1.5 nm), and the thin barrier is the Ti-rotaxane interface (estimated to be 0.5 nm).

C. P. Collier, E. W. Wong, M. Belohradský, F. M. Raymo, J. F. Stoddart, P. J. Kuekes, R. S. Williams, J. R. Heath, Science 285, 391 (1999).

R(1) Rotaxane Molecule

Figure 2. A drawing of the R(1) rotaxane molecule used here.

Operation of the Devices

Figure 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 50 and 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 0 as the resonant tunneling process is quenched. When the oxidized devices are interrogated at negative voltage, the electronic states that were observed between 0 and 1 V for a "switch closed" device are now absent.

Experimentally Measured Truth Tables for Logic Gates

Figure 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, labeled L, was configured as a load impedance on the gate. (A) The current output of a two-terminal AND gate as a function of input address, with an accompanying schematic of how the device was configured. (B) The current output (plotted on a logarithmic scale) for a three-input OR gate (solid trace), which was subsequently reconfigured into a two-input OR gate (dotted line) by oxidizing input C. For the two-input gate, the same truth table was measured, but input C was a dummy input. The [001] address state does raise the output current level, but not nearly enough to make the output "high."

Molecular Memory

Fig. 1. (a) Optical micrograph of the nanoelectrode array. Inset: AFM image of four Au nanoelectrodes with a Pd nanowire lying across. (b) Schematic diagram of the Pd/molecular wires/Au junctions on a Si/SiO2 substrate.

Chao Li, Daihua Zhang, Xiaolei Liu, Song Han, Tao Tang, and Chongwu Zhou, Wendy Fan, Jessica Koehne,Jie Han, and Meyya Meyyappan, A. M. Rawlett, D. W. Price, and J. M. Tour, Appl. Phys. Lett. 82, 645 (2003).

Different Molecules

Fig. 2. Molecular wires used. Molecules a, b, and c contain redox centers while molecules d and e do not contain such centers.

Typical I–V Curves of Molecular Devices

Fig. 3. Typical I–V curves of molecular devices. (a), (b), and (c) correspond to molecules a, b, and c shown in Fig. 2, respectively.

Read/Write of Molecules

Fig. 4. (a) I–V curves recorded after the device containing molecule a was written into states 1 and 0. (b) Retention time measurement. (c) Current recorded after the device was repeated written into states 1 and 0.