Directed Self-Assembly and Infrared Reflection Absorption Spectroscopy Analysis of Amphiphilic and Zwitterionic Janus Gold Nanoparticles.

Here, we report an approach to use infrared reflection absorption spectroscopy (IRRAS) for the unambiguous proof of the presence as well as the spatial distribution of organic ligands on the Janus gold nanoparticle (AuNP) surface. For this purpose we synthesized amphiphilic and zwitterionic Janus AuNPs and immobilized these on pretreated gold surfaces by directed self-assembly, exploiting hydrophilic/hydrophobic or electrostatic interactions, respectively. Thus, we obtained macroscopic two-dimensional arrays of Janus AuNPs exhibiting a specific orientation. These arrays were investigated by IRRAS, and the obtained spectra revealed only peaks of the ligands facing the IR beam, while the ligands facing the gold substrate were not detected due to reflection of the IR beam on the AuNP cores. Thus, we describe a straightforward spectroscopic procedure to prove the Janus character of zwitterionic and amphiphilic AuNPs in the size range of 10-15 nm.

Differential adsorption of gold nanoparticles to gold/palladium and platinum surfaces.

Integration of molecule-capped gold nanoparticles (AuNP) into nanoelectronic devices requires detailed knowledge about the AuNP-electrode interface. Here, we report the pH-dependent adsorption of amine or carboxylic acid-terminated gold nanoparticles on platinum or gold/palladium (30% Pd) alloy, respectively. We synthesized amine-terminated AuNP, applying a new solid phase supported approach, as well as AuNP exhibiting carboxylic acid as terminal groups. The pH-induced agglomeration of the synthesized AuNP was investigated by UV-vis, DLS, and ζ-potential measurements. Depending on the pH and the ionic strength of the AuNP solution a preferential adsorption on the different metals occurred. Thereby, we demonstrate that by choosing the appropriate functional group and adjusting the pH as well as the ionic strength a directed binding can be achieved, which is an essential prerequisite for applications of these particles in nanoelectronics. These findings will pave the way for a controlled designing of the interface between molecule-capped AuNP and metallic electrodes for applications in nanoelectronics.

Electronic transport properties of individual 4,4'-bis(mercaptoalkyl)-biphenyl derivatives measured in STM-based break junctions.

Electronic transport measurements of single, systematically varied 4,4'-bis(mercaptoalkyl)-biphenyl derivatives (MABP) are performed in a controlled test-device. The molecules are composed of a central biphenyl unit (BP) carrying two mercaptoalkyl substituents with different chain lengths (m, n = number of CH(2)-units), in the para-position of the BP unit. The total length of both spacers is m + n = 10. The molecular conductance of these individual MABPs deposited on Au (111) substrates is studied using STM-based break junctions. It is shown that the molecular conductance depends on the relative position of the BP unit within the molecule. In the case of the symmetric derivative 5BP5 a value of 0.07 +/- 0.01 nS is obtained, while for 1BP9 the molecular conductance is doubled and a value of 0.17 +/- 0.03 nS results. This relatively high value of conductance for the single Au(tip)-1BP9-Au(substrate) junction is attributed to an increased coupling of the BP unit to the adjacent electrode, i.e. the STM-tip or the Au-substrate. We address the role of the specific contact situation (-S-Au) and of the position of the electrically active molecular moiety and thus come to a deeper understanding of the electronic transport properties of 4,4'-bis(mercaptoalkyl)biphenyl derivatives.

Field-emission resonances at tip/alpha,omega-mercaptoalkyl ferrocene/Au interfaces studied by STM.

The electrical properties of alpha,omega-mercaptoalkyl ferrocenes with different alkyl chain lengths embedded in a self-assembled host matrix of alkanethiols on Au(111) are studied by scanning tunneling microscopy and spectroscopy. Based on current-distance spectroscopy, as well as on the evaluation of Fowler-Nordheim tunneling current oscillations, the apparent barrier height of ferrocene is determined independently by two methods. The electronic coupling of the ferrocene moiety to the Au(111) substrate is shown to depend on the length of the alkane-spacer chain. In a double tunnel junction model our experimental findings are explained, addressing the role of the different molecular moieties of the mercaptoalkyl ferrocenes.

Electrochemical and Electronic Charge Transport Properties of Ni-Doped LiMn2O4 Spinel Obtained from Polyol-Mediated Synthesis.

LiNi0.5Mn1.5O4 (LNMO) spinel has been extensively investigated as one of the most promising high-voltage cathode candidates for lithium-ion batteries. The electrochemical performance of LNMO, especially its rate performance, seems to be governed by its crystallographic structure, which is strongly influenced by the preparation methods. Conventionally, LNMO materials are prepared via solid-state reactions, which typically lead to microscaled particles with only limited control over the particle size and morphology. In this work, we prepared Ni-doped LiMn2O4 (LMO) spinel via the polyol method. The cycling stability and rate capability of the synthesized material are found to be comparable to the ones reported in literature. Furthermore, its electronic charge transport properties were investigated by local electrical transport measurements on individual particles by means of a nanorobotics setup in a scanning electron microscope, as well as by performing DFT calculations. We found that the scarcity of Mn3+ in the LNMO leads to a significant decrease in electronic conductivity as compared to undoped LMO, which had no obvious effect on the rate capability of the two materials. Our results suggest that the rate capability of LNMO and LMO materials is not limited by the electronic conductivity of the fully lithiated materials.

On the application potential of gold nanoparticles in nanoelectronics and biomedicine.

Ligand-stabilized gold nanoparticles (AuNPs) are of high interest to research dedicated to future technologies such as nanoelectronics or biomedical applications. This research interest arises from the unique size-dependent properties such as surface plasmon resonance or Coulomb charging effects. It is shown here how the unique properties of individual AuNPs and AuNP assemblies can be used to create new functional materials for applications in a technical or biological environment. While the term technical environment focuses on the potential use of AuNPs as subunits in nanoelectronic devices, the term biological environment addresses issues of toxicity and novel concepts of controlling biomolecular reactions on the surface of AuNPs.

Striped phase of mercaptoalkylferrocenes on Au(111) with a potential for nanoscale surface patterning.

The molecular structure of submonolayer-coverage phases of 3-(thioacetyl)-propanoylferrocene and 5-ferrocenylpentanethiol in mixed layers with alkanethiols on Au(111) was resolved by scanning tunneling microscopy. The ferrocenes formed a striped surface phase, similar to the lying-down structures of alkanethiols, resulting in equally spaced rows of the ferrocene moieties. The obtained nanoscale lattice of functional groups on the surface offers an interesting potential for the patterning of small, periodic structures with precise distance control via a hydrocarbon spacer.

Scanning tunneling microscopy and spectroscopy studies of 4-methyl- 4'-(n-mercaptoalkyl)biphenyls on Au(111)-(1x1).

4-methyl-4'-(n-mercaptoalkyl)biphenyl (CH3-C6H4-C6H4-(CH2)n-SH, n=3-6, BPn) monolayers assembled on Au(111)-(1x1) in 1,3,5,-trimethylbenzene (TMB) at various temperatures are studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). High resolution STM images reveal that BP3 and BP5 form a (sqrt 3x2sqrt 3) repeating motif superimposed on a temperature-dependent Moire pattern. BP4 and BP6 adlayers are characterized by a coexisting (2sqrt 3x5sqrt 3) majority phase and a temperature-dependent (3xpsqrt 3) minority phase. Assembly at 60 degrees C or 90 degrees C leads to p=5. Compression of the adlayer was found at higher temperatures. Combined with high-resolution structure experiments, the electronic characteristics of BP3 and BP4 self-assembled monolayers (SAMs) were studied by monitoring current-distance (iT-Deltaz) and current-voltage (iT-Ebias) characteristics in TMB employing a gold STM tip|BPn|Au(111)-(1x1) configuration. The semilogarithmic (iT-Deltaz) plots yielded three linear regions in the range 10 pA

Dilithiation of Bis(benzene)molybdenum and subsequent isolation of a molybdenum-containing paracyclophane.

The homoleptic sandwich complex bis(benzene)molybdenum, [Mo(eta6-C6H6)2], was successfully dilithiated by employing an excess of BuLi in the presence of N,N,N',N'-tetramethylethylenediamine (up to 6 equiv each) at slightly elevated temperatures furnishing the highly reactive, ring metalated species [Mo(eta6-C6H5Li)2].tmeda in high yields. Alternatively, this compound was synthesized upon prolonged sonication with 5 equiv of tBuLi/tmeda without heating. An X-ray crystal structure determination revealed a symmetrical, dimeric composition in the solid state, i.e., a formula of [Mo(eta6-C6H5Li)2]2.(thf)6, where the six-membered rings are connected by two pairs of bridging lithium atoms. The synthesis of an elusive ansa-bridged complex failed in the case of a [1]bora and a [1]sila bridge due to the thermal lability of the resulting compounds. Instead, reverse addition of the dilithio precursor to an excess of the appropriate element dihalide facilitated the isolation of several unstrained, 1,1'-disubstituted derivatives, namely, [Mo{eta6-C6H5(BN(SiMe3)2X)}2] (X = Cl, Br) and [Mo{eta6-C6H5(SiiPr2Cl)}2], respectively. However, the incorporation of a less congesting [2]sila bridge was accomplished. In addition to the formation of [Mo{(eta6-C6H5)2Si2Me4}], a molybdenum-containing paracylophane complex was isolated and characterized by means of crystal structure analysis. The ancillary formation of 1 equiv of bis(benzene)molybdenum strongly suggests that this species is generated by deprotonation of the ansa-bridged complex by the dilithiated precursor and subsequent reaction with a second equivalent of the disilane.

STM study of mixed alkanethiol/biphenylthiol self-assembled monolayers on Au(111).

A method is presented for depositing mixed self-assembled monolayers (SAMs) of dodecanethiol (C12) and 4'-methyl-1,1'-biphenyl-4-butane (H3C-C6H4-C6H4-(CH2)4-SH, BP4) by insertion of BP4 into a closely packed SAM of dodecanethiol on Au(111). Insertion takes place at defect sites such as domain boundaries or etch pits in the gold surface that are characteristic of C12 monolayers on gold. With a lower probability, insertion also occurs beside defect sites inside dodecanethiol domains. Insertion at defect sites results in domains of BP4, whereas insertion into C12 domains leads to isolated BP4 molecules. The isolated BP4 molecules are shown not to move at room temperature. By comparing the apparent height of the isolated BP4 molecules and BP4 domains, it is proposed that the isolated molecules have the same conformation as in the full-coverage phase. A simple two-layer model is proposed to characterize the current transport through BP4. The decay constant beta for the phenylene groups is deduced from the apparent STM heights of the inserted BP4 islands compared to the STM heights of the C12 closely packed monolayers.