Electrical characterization of Al/AlOx/molecule/Ti/Al devices

We report experimental electrical characterization of Al/AlO_x/molecule/Ti/Al planar crossbar devices incorporating Langmuir–Blodgett organic monolayers of eicosanoic acid, fast blue, or chlorophyll-B. Current–voltage and capacitance–voltage measurements on all three molecular device structures exhibited controllable switching hysteresis. Control devices containing no molecules showed no evidence of switching. A model of interface trapped charge mediating electronic transport appears consistent with all of the data. This data illustrates the importance of considering the complete device system (consisting of the molecules, the electrodes, and the interfaces) when analyzing its electrical behavior. PACS 85.65.+h; 73.40.Rw; 73.50.Gr     

Polyene chains in molecular electronics

Current ideas of soliton switching as a possible route towards molecular electronics are reviewed. The relation between photoinduced dipole moments in dye molecules and soliton motion is pointed out. Derivates of -carotene are presented as candidates for components of molecular electronic devices.Dedicated to K. Dransfeld on the occasion of his 60th birthday     

Dopant passivation and work function tuning through attachment of heterogeneous organic monolayers on silicon in ultrahigh vacuum

Electronic structures of silicon-organic interfaces were studied by the scanning Kelvin probe technique. These surfaces were fabricated by covalent bonding of a range of phenylacetylene-based molecules (p-X-C₆H₄CCH, where X = CF₃, OCH₃, and H) onto a hydrogen-terminated silicon surface. Organic molecules were bound to the surface under high vacuum conditions by ultraviolet light-induced hydrosilylation. Changes in the electronic structure due to electron-donating ability and dipole moment were analyzed under dark and illuminated conditions. The origin of the silicon band bending was tested to separate the effects of molecular monolayers and unintended dopant passivation. In addition, heterogeneous monolayers were grown by controllably diluting the incoming vapor stream with acetylene during growth. The measured work functions follow a trend linked to dipole moment that can be further tuned by molecular dilution. These results suggest a way to use heterogeneous organic monolayers to tune the electron affinity of silicon and directly alter channel modulation in small semiconductor devices.     

Theory of electron current rectification, switching, and a role of defects in molecular devices

Devices for nano- and molecular-sized electronics may allow for an efficient current rectification and switching. A few molecular scale devices are reviewed here on the basis of first-principles and model approaches. Current rectification by molecular quantum dots can produce the rectification ratio 100. Current switching due to conformational changes in the molecules is slow, on the order of a few kHz. Fast switching (1 THz) may be achieved, at least in principle, in a molecular quantum dot with attractive carrier-carrier interactions due to strong coupling with vibrational excitations. The observed switching in many cases is extrinsic, caused by changes in molecule-electrode geometry or metallic filament formation through the film. PACS 81.07.Nb     

Influence of hydrogen bonds and double bonds on the alkane and alkene derivatives self-assembled monolayers on HOPG surface: STM observation and computer simulation

The adsorption structure and hydrogen-bonded complexes of alkane and alkene derivatives self-assembled on HOPG were studied by scanning tunneling microscopy (STM) and Molecular Mechanics (MM) simulations. The effect of different interior -CHCH- conformations and functional groups in molecules on the structure and stability of self-assembled monolayers was considered. It was found that octadecanol and trans-oleic acid could form stable structure on HOPG and their high-resolution images could be obtained by STM. Octadecanol exhibited two kinds of packing by alkyl-alkyl and adsorbate-substrate interactions. The interior trans-CHCH- group in trans-oleic acid had a slight influence on the self-assembly configuration. However, owing to the cis-double bond in the interior of cis-oleyamine molecule, the ordered monolayer could not be easily imaged by STM at ambient conditions. The adsorption conformations of three kinds of molecules on HOPG surface were optimized by MM simulation, respectively. The analysis of hydrogen-bond interactions by computation stimulation also revealed that the stability of cis-oleyamine on HOPG was the worst. These results demonstrated that molecular self-assembly and its stability could be significantly tailored by changing the molecular structure.     

Molecular conformation and electronic properties of protoporphyrin-IX self-assembled monolayers adsorbed on a Pt(1 1 1) surface

Monolayers of protoporphyrin-IX molecules are prepared on a Pt(1 1 1) surface by a self-assembly process in order to manufacture organic devices with controlled electronic properties. Scanning tunnelling microscopy (STM) and two-colour sum-frequency generation (2C-SFG) are performed ex situ in ambient air, in order to characterize their molecular conformation and electronic properties at the monolayer level, respectively. STM measurements performed with functionalized gold tips reveal a high covering rate of the metal surface. 2C-SFG measurements highlight CH stretching modes of vinyl substituted groups (RCHCH₂) in the 2800-3200 cm⁻¹ infrared spectral range and particular electronic features in the visible spectral range, i.e. a Soret band red shift and band separation compared to the liquid phase. Moreover, similar measurements are performed on Zn(II)Protoporphyrin-IX and 5-[p-(6-mercaptohexoxy)-phenyl]-10,15,20-triphenylporphin films for comparison. These results suggest a film conformation with the molecules having different tilt angles with respect to the substrate normal, depending on the ion metal presence or the chain length bonded to the porphyrin moiety.     

Selfassembly of α,ω‐dithiols on surfaces and metal dithiol heterostructures

α,ω‐Dithiols present an interesting case of molecules with two reactive terminal ‐SH groups (HS‐R‐SH) that allow their use as binders between different metallic entities. They have thus been used in molecular electronics conduction measurements, in “nanogap” electrodes of interest in plasmonics, as building blocks of more complex structures such as metal intercalated superlattices and in the formation of metalized organic thin films, including doped graphene type films. There exist however many problems, because the molecules may end up in undesirable configurations with both thiol terminals bound to the same metal particle/substrate or link with other molecules to produce “multi‐molecule” or “multilayer” structures. This report discusses various key questions on dithiol linking with metal surfaces and the quest of protocols of making problem free dithiol metal structures. It then describes the use of dithiols and their SAMs to produce various metal organic heterostructures useful for molecular electronics and formation of doped metalized organic thin films. We discuss the build up of these structures by self assembly and lithography, their chemical composition and functional properties.

     

Enhancement of Fluorescence and Raman Scattering in Cyanine-Dye Molecules on the Surface of Silicon-Coated Silver Nanoparticles

A method of formation of a composite structure based on silver nanoparticles and a thin protective silicon film (AgNPs/Si) is developed. Enhancement of the fluorescence and Raman scattering in cyaninedye molecules deposited onto the formed nanostructure is observed. The optical properties and morphology stability of particles that are in contact with cyanine-dye solutions in organic solvents are studied. It is shown that the AgNPs/Si composite structure can be multiply used as an SERS-active surface.     

Nuclear polarization of molecular hydrogen recombined on a non-metallic surface

The nuclear polarization of molecules formed by recombination of nuclear polarized H atoms on the surface of a storage cell initially coated with a silicon-based polymer has been measured by using the longitudinal double-spin asymmetry in deep-inelastic positron-proton scattering. The molecules are found to have a substantial nuclear polarization, which is evidence that initially polarized atoms retain their nuclear polarization when absorbed on this type of surface.Received: 15 December 2003, Published online: 24 February 2004PACS: 29.25.Pj Polarized and other targets - 34. Atomic and molecular collision processes and interactions - 39.10. + j Atomic and molecular beam sources and techniques - 13.88. + e Polarization in interactions and scattering     

Self-assembled and electrochemically deposited mono/multilayers for molecular electronics applications

For the development of molecular electronics, it is desirable to investigate characteristics of organic molecules with electronic device functionalities. In near future, such molecular devices could be integrated with silicon to prepare hybrid nanoelectronic devices. In this paper, we review work done in our laboratory on study of characteristics of some functional molecules. For these studies molecular mono and multilayers have been deposited on silicon surface by self-assembly and electrochemical deposition techniques. Both commercially available and specially designed and synthesized molecules have been utilized for these investigations. We demonstrate dielectric layers, memory, switching, rectifier and negative differential resistance devices based on molecular mono and multilayers.     

Modification of single molecule fluorescence using external fields

Controlling and manipulating the fluorescence of single fluorophores is of great interest in recent years for its potential uses in improving the performance of molecular photonics and molecular electronics, such as in organic light-emitting devices, single photon sources, organic field-effect transistors, and probes or sensors based on single molecules. This review shows how the fluorescence emission of single organic molecules can be modified using local electromagnetic fields of metallic nanostructures and electric-field-induced electron transfer. Electric-field-induced fluorescence modulation, hysteresis, and the achievement of fluorescence switch are discussed in detail.     

Formation of a monolayer <Emphasis Type="Italic">h</Emphasis>-BN nanomesh on Rh (111) studied using <Emphasis Type="Italic">in-situ</Emphasis> STM

As a member of the 2D family of materials, h-BN is an intrinsic insulator and could be employed as a dielectric or insulating inter-layer in ultra-thin devices. Monolayer h-BN can be synthesized on Rh (111) surfaces using borazine as a precursor. Using in-situ variable-temperature scanning tunneling microscopy (STM), we directly observed the formation of h in real-time. By analyzing the deposition under variable substrate temperatures and the filling rate of the h-BN overlayer vacant hollows during growth, we studied the growth kinetics of how the borazine molecules construct the h-BN overlayer grown on the Rh surface.     

Ordered arrangement of 9-aminoanthracene on Au(1 1 1) surfaces: A scanning tunneling microscopy study

We present a scanning tunneling microscopy (STM) investigation of 9-aminoanthracene (AA) on the reconstructed Au(1 1 1) surface. The bare Au(1 1 1) surface shows the herringbone reconstruction which is conserved upon deposition of the organic molecules. Most of the AA molecules are found to decorate the regions of fcc-stacking of the gold surface where a periodic linear arrangement is observed. The orientation of the long molecule axis of individual molecules is along the -directions of the Au substrate. In addition, for individual domains of the surface reconstruction, one of the three possible orientations is preferred. On substrate areas which exhibit a high step density, the steps are completely decorated by AA molecules. A detailed analysis of the STM images reveals that the molecules are located on top terrace levels. The fine structure of individual molecules on the terrace shows a clear dependence on the tunneling voltage and resembles the molecular orbitals of the free AA molecule.     

Molecular tuning of quantum Hall edge states

Hybrid organic/inorganic devices may find applications as sensors and in futuristic molecular-electronic devices. Here, we demonstrate molecular control of vertical transport in semiconductor superlattices in strong magnetic fields by adsorption of organic molecules onto the sidewalls of a GaAs/AlGaAs device. The molecules have identical attachment groups functionalized by end groups with different electronegativities. For magnetic fields in quantized Hall states, we find that the adsorbate substantially modifies the network of edge states that carries the electrical current. The data indicate that molecules with appropriately chosen end groups can enhance or decrease the vertical conductivity of the edge state system.     

Self-assembly of bipyridine derivatives at solid/liquid interface: Effects of the number of peripheral alkyl chains and metal coordination on the two-dimensional structures

Self-assembled two-dimensional structures of bipyridine derivatives (bpys) were observed by scanning tunneling microscopy at HOPG/1-phenyloctane interface. Two types of bpy molecules were used in this study. One is bpy 1, which has two alkyl chains on each end, and the other is bpy 2, which holds single alkyl chain on each end. The bpy 1 formed a monolayer, which is composed of a bent molecular structure with interdigitated alkyl chains. In the case of bpy 2, the structure exhibited pm plane group symmetry. A pair of molecules formed a columnar structure, and the alkyl chains aligned in a tail-to-tail orientation. Metal coordination of both bpy samples increased the intermolecular distance at the π-conjugated units, resulting in the formation of lamellar structure. Although both bpys showed straight form, the alkyl chain unit of complexed bpy 1 was not interdigitated, whereas that of complexed bpy 2 was interdigitated. Thus, the metal-metal distances could be tuned by changing the number of alkyl chain unit.     

High‐Resolution Submillimeter Wave Spectroscopy of Hydrogen Sulphide with Heterodyne Spectrometer

A submillimeter heterodyne spectrometer using continuous wave optically pumped molecular laser as local oscillator and Schottky diode as mixer was developed at Physical Research Laboratory, Ahmedabad (India) for quantitative spectroscopy of polyatomic molecules in laboratory. The experimental details of spectrometer and its application to study of molecular line parameters are presented. In particular line strength, collision line width (self and foreign) of H₂S 5_5,05_4,1 transition¹(579.799 GHz) have been measured. Air molecules have been used as foreign (perturbing) molecules taking Earths atmosphere into consideration².     

Organic Magnetic Materials Studied by Positive Muons

Positive muons can be implanted into organic and molecular magnets in order to study their internal magnetic field distribution and any associated dynamics. The muon behaves essentially as a microscopic magnetometer, sensitive to local magnetic order and magnetic fluctuations. We describe some recent experiments using this technique which were performed on a variety of organic systems, including nitronyl nitroxide magnets and materials with spin-Peierls ground states, MEM(TCNQ)₂ and DEM(TCNQ)₂, and demonstrate how the technique can give information concerning their ground states.     

57Fe Mössbauer spectra study of coated α-Fe2O3 nanoparticles

Nano-sized -Fe₂O₃ particles coated with polar organic molecules have been studied using the Mössbauer spectroscopy method. The -Fe₂O₃ nanoparticles were prepared by the microemulsion method. The average particle size of the Fe₂O₃ particles is about 24 Å. Because the particle size is so small that the Mössbauer spectra of the -Fe₂O₃ samples only consist of a quadrupole-split central line. It was proved that the Isomer Shifts (DIS) and the Quadrupole Splitting (DQS) changed as the refluxing time prolongs and the refluxing temperature increases during the preparation of the Fe₂O₃ nanoparticles, which implied an enhancement of the surface electrofield gradient formed by the surface coated polar molecules during the refluxing process.     

Molecular depth profiling and imaging using cluster ion beams with femtosecond laser postionization

The emergence of cluster ion sources as viable SIMS probes has opened new possibilities for detection of neutral molecules by laser postionization. Previous studies have shown that with atomic bombardment multiphoton ionization using high-power femtosecond pulses leads to photofragmentation. The large amount of photofragmentation can be mostly attributed to high amounts of internal energy imparted to the sputtered molecules during the desorption process. Several pieces of preliminary data suggest that molecules subjected to cluster beam bombardment are desorbed with lower internal energies than those subjected to atomic beam bombardment. Lower energy molecules may then be less likely to photodissociate creating less photofragments in the laser postionization spectra. Here we present data taken from coronene films prepared by physical vapor deposition comparing a 40 keV ion source with a 20 keV Au⁺ ion source, which supports this hypothesis. Furthermore, the depth profiling capabilities of cluster beams may be combined with laser postionization to obtain molecular depth profiles by monitoring the neutral flux. In addition, imaging and depth profiling may be combined with atomic force microscopy (AFM) to provide three-dimensional molecular images.