Molecular Diodes Research Articles

Electrical and optical studies of gap states in self-assembled molecular aggregates

We fabricated a variety of two-terminal devices using self-assembled monolayers (SAM) of solid-state mixtures comprised of molecular “wires” [1,4-methane-benzenedithiol (Me-BDT)] and molecular insulator “spacers” [1-pentanethiol], which were prepared at various molar concentrations ratio, r of wires/spacers, and sandwiched between two gold electrodes. The devices’ electrical transport was investigated at several r values using the bias voltage (V) dependencies of the conductance and differential conductance at various temperatures. In parallel, we also studied the UV-visible absorption and photoluminescence (PL) emission spectra of the SAM mixtures grown on silica transparent substrates. For r>10−3 we found that two-dimensional (2D) Me-BDT aggregates are formed in the SAM films leading to novel properties compared to SAM films of isolated Me-BDT molecules at concentrations 10−8< r<10−4, which we studied before [V. Burtman, A. S. Ndobe, and Z. V. Vardeny, J. Appl. Phys. 98, 034314 (2005)]. First, an Ohmic response in the current-voltage (I-V) characteristics is obtained up to V∼0.5 V, which results in a new band in the differential conductance spectrum around V=0. Second, a new subgap absorption band is formed at ∼2.4 eV, which is related to a new yellow/red PL emission band. The novel optical and electrical properties of the 2D Me-BDT aggregates are explained by the formation of an electronic continuum band in the Me-BDT energy gap, which is caused by weak in-plane charge delocalization among the molecules forming the aggregates. To verify this model we also studied SAM molecular aggregate diodes using Al electrodes. The 1-eV difference in the electrode work function between Au and Al metals results in a pronounced EF shift with respect to the aggregate-related continuum band in the gap, and consequently, dramatically changes the device I-V characteristics.

Molecular Diodes and Applications

Due to the huge power consumption and expensive fabrication methods required, down scaling silicon devices to sub-100 nm dimensions is becoming very unattractive. On the other hand, it is easier to build electronic circuits using molecules since they are small and their properties can be tuned. In this review, we first discuss the building blocks of molecular electronics. We then describe how these building blocks can be used to build single molecule based digital logic such as AND, OR and XOR gates. The distinction of these molecular electronic building blocks is that for first time, (i) the Tour wires are used as the conductive backbone for the rectifying junctions, (ii) donor/acceptor principles are implemented in the molecular wire itself and (iii) the logic gates are realized using molecular rectifying diodes embedded in the molecular conducting wire itself.

Efficient, single-layer molecular organic light-emitting diodes

The authors demonstrate efficient molecular organic light-emitting diodes that use direct hole injection from poly(3,4-ethylene-dioxythio-phene):poly(styrene-sulfonate) into a single layer of tris(8-hydroxyquinoline) aluminum (III) for carrier transport and electroluminescence. Single-layer devices have a lower operating bias and higher luminous power efficiency than conventional bilayer devices with a 4,4-bis[N-1-napthyl-N-phenyl-amino]biphenyl hole transport layer. The current density-voltage characteristics of single-layer devices follow Schottky-Richardson behavior and are consistent with an Ohmic contact at the anode.

Molecular diodes and ultra-thin organic rectifying junctions: Au–S–CnH2n–Q3CNQ and TCNQ derivatives

Attempts to obtain derivatives of the molecular diode, 2-{4-[1-cyano-2-(1-(omega-acetylsulfanylalkyl)-1H-quinolin-4-ylidene)-ethylidene]-cyclohexa-2,5-dienylidene}-malonitrile [1, CH(3)CO-S-C(n)H(2n)-Q3CNQ], from either 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane (TCNQF(4)) or 2,3,5,6-tetramethyl-7,7,8,8-tetracyano-p-quinodimethane (TMTCNQ) result in ring closure via the cyano group of the pi-bridge and yield di-substituted analogues: 2-{2,3,5,6-tetrafluoro-4-[6-(10-acetylsulfanyldecyl)-3-(1-(10-acetylsulfanyldecyl)-1H-quinolin-4-ylidenemethyl)-6H-benzo[f][1,7]naphthyridin-2-ylidene]-cyclohexa-2,5-dienylidene}-malonitrile (2a) and the 2,3,5,6-tetramethyl derivative (2b). Self-assembled monolayers (SAMs) of these donor-(pi-bridge)-acceptor molecular diodes exhibit asymmetric current-voltage characteristics with electron flow at forward bias from the top contact to surface C(CN)(2) groups. Comparison is made with I-V curves from ultra-thin films of an organic rectifying junction in which TCNQ(-) is electron-donating and a donor-(sigma-bridge)-acceptor diode in which TCNQ degrees is electron-accepting.

Electron transport in molecular junctions

Building an electronic device using individual molecules is one of the ultimate goals in nanotechnology. To achieve this it will be necessary to measure, control and understand electron transport through molecules attached to electrodes. Substantial progress has been made over the past decade and we present here an overview of some of the recent advances. Topics covered include molecular wires, two-terminal switches and diodes, three-terminal transistor-like devices and hybrid devices that use various different signals (light, magnetic fields, and chemical and mechanical signals) to control electron transport in molecules. We also discuss further issues, including molecule-electrode contacts, local heating- and current-induced instabilities, stochastic fluctuations and the development of characterization tools.

Electrochemistry and photoelectrochemistry of photosystem I adsorbed on hydroxyl-terminated monolayers

Direct electrochemistry studies on Photosystem I (PSI) were performed using cyclic voltammetry and square wave voltammetry. PSI centers stabilized in aqueous solution by Triton X-100 surfactant were adsorbed on hydroxyl-terminated hexanethiol modified gold electrodes. We have identified the electron donor, P700, and the electron acceptor sites, F A / F B , based on the previously reported preferred orientation for P700 on hydroxyl-terminated self-assembled monolayers. The reported potential values ( E P700 = +0.51 V vs. NHE; E FA/FB = −0.36 V vs. NHE) correlate very well with the established literature for P700, while the weaker signal for F A / F B lies within previous literature values. We were able to identify both redox centers on the same voltammogram. The P700 center clearly shows reversible electrochemical behavior. The expected F A / F B reduction is small in comparison, reflecting the dominant orientation of PSI with the F A / F B centers farther away and the P700 center nearer the electrode surface. As a molecular diode, PSI does not permit reverse direction conductivity to the F B so the small F A / F B peaks reflect other orientations besides the predominant one. PSI adsorbed on a hydroxyl-terminated hexanethiol modified gold substrate displays a photoenhanced reduction current for the P700 + center in the presence of light and an electron acceptor, methyl viologen. This photoelectrochemical response demonstrates protein functionality after adsorption.

Mid-infrared spectrum of the weakly bound complex CO2–SO2

The mid-infrared spectrum of the CO2–SO2 van der Waals complex was measured near the R(0) transition of the asymmetric stretch of the CO2 monomer at 2349.917 cm−1. The spectra were recorded using a molecular beam tunable diode laser spectrometer, in which the injection nozzle was mounted in a position axial to the Herriott multipass cell. The transitions are c-type, in agreement with spin statistical analysis. Transitions originating from levels with odd Ka in the vibrational ground state are absent from the spectrum. The main spectroscopic constants are ν0 = 2349.6159(1) cm−1, A″ = 5544.98(40) MHz, B″ = 1510.034(37) MHz, C″ = 1489.402(36) MHz, = 4.578(16) kHz, A′ = 5524.58(63) MHz, B′ = 1508.28(14) MHz, C′ = 1488.42(21) MHz, and = 4.07(78) kHz. The spectral assignment was confirmed by microwave-infrared double resonance experiments using several known a-type microwave transitions of the complex.

A MOLECULAR DIODE BASED ON CONJUGATED CO-OLIGOMERS

A molecular diode based on a conjugated co-oligomer composed of p-type and n-type segments is investigated using the first principles method. The co-oligomer is connected to Au electrodes to form an Au –oligomer– Au system. The infinite system is dealt with a finite structure confined in a device region and effects from semi-infinite electrodes. Density functional theory and nonequilibrium Green's function are used to describe the device region self-consistently. The current–voltage (I–V) characteristics of the constructed system are calculated and a rectification behavior is observed. The energy gap and the spatial orientation of molecular orbitals, and the transmission functions are calculated to analyze the I–V characteristics of the molecular diode.

Organic oxide/Al composite cathode in small molecular organic light-emitting diodes

This study addresses the feasibility of using an organic oxide/Al composite cathode to fabricate the small molecular organic light-emitting diodes (OLEDs). A supplementary organic buffer film is placed at the interface between the tris(8-hydroxyquinoline) aluminum (Alq3) and the organic oxide/Al complex layers. Incorporating the rubrene/poly(ethylene glycol) dimethyl ether (PEGDE) buffer layers into the composite cathode structure markedly improves the performance of devices. The luminous efficiencies of Alq3-based OLEDs biased at ∼100mA∕cm2 are 4.8 and 5.1cd∕A for rubrene (50Å)/PEGDE (15Å)∕Al and rubrene (50Å)/PEGDE (15Å)∕LiF (5Å)∕Al cathode devices, and 1.3 and 3.8cd∕A for devices with Al and LiF (5Å)∕Al cathodes, respectively.

Controlled Alignment of Molecular Diodes via Ionic Assembly of Cationic Donor—(π‐Bridged)—Acceptor Molecules on Anionic Surfaces

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Electron Transport through Rectifying Self-Assembled Monolayer Diodes on Silicon: Fermi-Level Pinning at the Molecule−Metal Interface

We report the synthesis and characterization of molecular rectifying diodes on silicon using sequential grafting of self-assembled monolayers of alkyl chains bearing a pi group at their outer end (Si/sigma-pi/metal junctions). We investigate the structure-performance relationships of these molecular devices, and we examine the extent to which the nature of the pi end group (change in the energy position of their molecular orbitals) drives the properties of these molecular diodes. Self-assembled monolayers of alkyl chains (different chain lengths from 6 to 15 methylene groups) functionalized by phenyl, anthracene, pyrene, ethylene dioxythiophene, ethylene dioxyphenyl, thiophene, terthiophene, and quaterthiophene were synthesized and characterized by contact angle measurements, ellipsometry, Fourier transform infrared spectroscopy, and atomic force microscopy. We demonstrate that reasonably well-packed monolayers are obtained in all cases. Their electrical properties were assessed by dc current-voltage characteristics and high-frequency (1-MHz) capacitance measurements. For all of the pi groups investigated here, we observed rectification behavior. These results extend our preliminary work using phenyl and thiophene groups (Lenfant et al., Nano Lett. 2003, 3, 741). The experimental current-voltage curves were analyzed with a simple analytical model, from which we extracted the energy position of the molecular orbital of the pi group in resonance with the Fermi energy of the electrodes. We report experimental studies of the band lineup in these silicon/alkyl pi-conjugated molecule/metal junctions. We conclude that Fermi-level pinning at the pi group/metal interface is mainly responsible for the observed absence of a dependence of the rectification effect on the nature of the pi groups, even though the groups examined were selected to have significant variations in their electronic molecular orbitals.

Effective Manipulation of the Electronic Effects and Its Influence on the Emission of 5‐Substituted Tris(8‐quinolinolate) Aluminum(III) Complexes

The unique electron-transport and emissive properties of tris(8-quinolinolate) aluminum(III) (Alq(3)) have resulted in extensive use of this material for small molecular organic light-emitting diode (OLED) fabrication. So far, efforts to prepare stable and easy-to-process red/green/blue (RGB)-emitting Alq(3) derivatives have met with only a limited success. In this paper, we describe how the electronic nature of various substituents, projected via an arylethynyl or aryl spacer to the position of the highest HOMO density (C5), may be used for effective emission tuning to obtain blue-, green-, and red-emitting materials. The synthetic strategy consists of four different pathways for the attachment of electron-donating and electron-withdrawing aryl or arylethynyl substituents to the 5-position of the quinolinolate ring. Successful tuning of the emission color covering the whole visible spectrum (lambda=450-800 nm) was achieved. In addition, the photophysical properties of the luminophores were found to correlate with the Hammett constant of the respective substituents, providing a powerful strategy with which to predict the optical properties of new materials. We also demonstrate that the electronic nature of the substituent affects the emission properties of the resulting complex through effective modification of the HOMO levels of the quinolinolate ligand.

Towards molecular electronics with large-area molecular junctions

Electronic transport through single molecules has been studied extensively by academic and industrial research groups. Discrete tunnel junctions, or molecular diodes, have been reported using scanning probes, break junctions, metallic crossbars and nanopores. For technological applications, molecular tunnel junctions must be reliable, stable and reproducible. The conductance per molecule, however, typically varies by many orders of magnitude. Self-assembled monolayers (SAMs) may offer a promising route to the fabrication of reliable devices, and charge transport through SAMs of alkanethiols within nanopores is well understood, with non-resonant tunnelling dominating the transport mechanism. Unfortunately, electrical shorts in SAMs are often formed upon vapour deposition of the top electrode, which limits the diameter of the nanopore diodes to about 45 nm. Here we demonstrate a method to manufacture molecular junctions with diameters up to 100 microm with high yields (> 95 per cent). The junctions show excellent stability and reproducibility, and the conductance per unit area is similar to that obtained for benchmark nanopore diodes. Our technique involves processing the molecular junctions in the holes of a lithographically patterned photoresist, and then inserting a conducting polymer interlayer between the SAM and the metal top electrode. This simple approach is potentially low-cost and could pave the way for practical molecular electronics.

Carbon nanotube – molecular resonant tunneling diode

AbstractIn the Rapid Research Letter [1] carbon nanotube (CNT) molecular resonant tunnel diodes (RTD) are proposed to complement bio‐assembled CNT field effect transistors. The cover figure is a schematic representation of a model CNT–pseudopeptide–CNT device in its relaxed structure with hydrogen passivation at the CNT ends. The device has been shown to exhibit the typical current–voltage response of an RTD.The first author, Rajeev Pandey, works as a post‐doctoral researcher in the Laboratory for Terascale and Terahertz Electronics, headed by Roger Lake, at Riverside.The current issue of physica status solidi (a) also contains the Feature Article “On the growth mechanisms of nanocrystalline diamond films” by Wilhelm Kulisch and Cyril Popov [2].

Organometallic molecular rectification

We study the rectification of current through a single molecule with an intrinsic spatial asymmetry. The molecule contains a cobaltocene moiety in order to take advantage of its relatively localized and high-energy d states. A rectifier with large voltage range, high current, and low threshold can be realized. The evolution of molecular orbitals under both forward and reverse biases is captured in a self-consistent nonequilibrium Green function plus density functional theory description. Our calculations demonstrate the plausibility of making excellent molecular diodes by using metallocenes, pointing to a fruitful class of molecules.

Carbon nanotube – molecular resonant tunneling diode

AbstractCarbon nanotube (CNT) molecular resonant tunnel diodes (RTDs) are proposed to complement bio‐assembled CNT field effect transistors (CNTFETs). A model CNT–pseudopeptide–CNT device is shown to exhibit the current–voltage response of an RTD. magnified image (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Controlled alignment of molecular diodes via ionic assembly of cationic donor–(π-bridge)–acceptor molecules on anionic surfaces

Alignment of cationic donor-(pi-bridge)-acceptor molecules via metathesis with a self-assembled monolayer formed from sodium 3-mercapto-1-propanesulfonate results in ultra-thin rectifying Au-S-(CH2)3SO3-/A+-pi-D structures which exhibit optimum current ratios of 450 at +/-1 V for N-methyl-5-(4-dibutylaminobenzylidene)-5,6,7,8-tetrahydroisoquinolinium.

Design-Oriented Introduction of Nanotechnology into the Electrical and Computer Engineering Curriculum

Nanoelectronics has the potential, and is indeed expected, to revolutionize information technology by the use of the impressive characteristics of nanodevices such as carbon nanotube transistors, molecular diodes and transistors, etc. A great effort is being put into creating an introductory course in nanotechnology. However, practically all courses focus on the physics, chemistry, and materials science aspects of this discipline. On the other hand, a more abstract, design-oriented introduction is desirable for electrical and computer engineering majors. In order to teach design-oriented nanotechnology, the teaching curriculum must be extended to include new concepts. In particular, it is necessary to supply the design principles, device models, and software simulation tools. This article describes our approach for introducing nanotechnology system design into the Electrical and Computer Engineering undergraduate curriculum at Stony Brook University. The approach consists of developing a nanodevice library for SPICE-like simulator and a 3-week module on nanotechnology system design utilizing this library. The module will be woven into an existing course on Integrated Electronics.

Improved Molecular Rectification from Self-Assembled Monolayers of a Sterically Hindered Dye

Self-assembled monolayers (SAMs) formed from the reaction of 1-(10-acetylsulfanyldecyl)-4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium iodide (1a) and gold-coated substrates exhibit asymmetric current-voltage (I-V) characteristics with a rectification ratio of 50-150 at +/-1 V. It is the highest to date for a molecular diode, and the improved behavior may be assigned in part to the controlled alignment of the donor-(pi-bridge)-acceptor moieties and in part to steric hindrance, which imposes a nonplanar structure and effectively isolates the molecular orbitals of the donor and acceptor end groups. The molecular origin of the rectification is verified by its suppression upon exposure to HCl vapor, which protonates the dimethylamino group and inhibits the electron-donating properties, with restoration upon exposure to NH3. It is also established by a reduced rectification ratio of ca. 2 at +/-1 V when the cationic D-pi-A+ moieties adopt an antiparallel arrangement in self-assembled films of the derivative, bis-[1-(10-decyl)-4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium]-disulfide diiodide (1b), which adsorbs via one of its terminal donors without rupture of the sulfur-sulfur bond: Au/D-pi-A+-C10H20-S-S-C10H20-+A-pi-D (I-)2.

Inversion of the Rectifying Effect in Diblock Molecular Diodes by Protonation

A new molecular diode based on biphenyl-co-bispyrimidine was synthesized and showed a pronounced rectifying effect. Further studies indicate that protonation on the nitrogen atoms of the diode molecule by strong acids can reversibly alter the rectifying direction. This phenomenon can be envisioned as a starting point for single molecule detection devices.