S. Hassan M. Jafri

Formation and NMR Spectroscopy of ω-Thiol Protected α,ω-Alkanedithiol-Coated Gold Nanoparticles and Their Usage in Molecular Charge Transport Junctions

Gold nanoparticles (AuNPs) coated with stabilizing molecular monolayers are utilized in areas ranging from life sciences to nanoelectronics. Here we present a novel and facile one-pot single phase procedure for the preparation of stable AuNPs with good dispersity, which are coated with α,ω-alkanedithiols whose outer ω-thiol is protected by a triphenylmethyl group. Using dielectrophoresis we were able to trap these AuNPs, coated with ω-thiol protecting groups, in a 20 nm gold electrode nanogap. The ω-thiol group was then deprotected under acidic conditions in situ once the AuNPs were correctly positioned in the device. The subsequent deprotection resulted in an increase of conductance by up to 3 orders of magnitude, indicating that the isolated dithiol-coated AuNPs were fused into a covalently bonded network with AuNP-molecule-AuNP as well as electrode-molecule-AuNP linkages. Furthermore, complete characterization of the AuNP surface-bonded alkanedithiols was achieved using a series of one- and two-dimensional NMR spectroscopy techniques. Our spectra of the molecule-coated AuNPs show well-resolved signals, only slightly broader than for free molecules in solution, which is in contrast to many earlier reported NMR spectral data of molecules attached to AuNPs. Complementary diffusion NMR spectroscopic experiments were performed to prove that the observed alkanedithiols are definitely surface-bonded species and do not exist in free and unattached form.

Cooperative Gold Nanoparticle Stabilization by Acetylenic Phosphaalkenes

Acetylenic phosphaalkenes (APAs) are used as a novel type of ligands for the stabilization of gold nanoparticles (AuNP). As demonstrated by a variety of experimental and analytical methods, both structural features of the APA, that is, the P=C as well as the C≡C units are essential for NP stabilization. The presence of intact APAs on the AuNP is demonstrated by surface-enhanced Raman spectroscopy (SERS), and first principle calculations indicate that bonding occurs most likely at defect sites on the Au surface. AuNP-bound APAs are in chemical equilibrium with free APAs in solution, leading to a dynamic behavior that can be explored for facile place-exchange reactions with other types of anchor groups such as thiols or more weakly binding phosphine ligands.

Nano-fabrication of molecular electronic junctions by targeted modification of metal-molecule bonds

Reproducibility, stability and the coupling between electrical and molecular properties are central challenges in the field of molecular electronics. The field not only needs devices that fulfill these criteria but they also need to be up-scalable to application size. In this work, few-molecule based electronics devices with reproducible electrical characteristics are demonstrated. Our previously reported 5 nm gold nanoparticles (AuNP) coated with ω-triphenylmethyl (trityl) protected 1,8-octanedithiol molecules are trapped in between sub-20 nm gap spacing gold nanoelectrodes forming AuNP-molecule network. When the trityl groups are removed, reproducible devices and stable Au-thiol junctions are established on both ends of the alkane segment. The resistance of more than 50 devices is reduced by orders of magnitude as well as a reduction of the spread in the resistance histogram is observed. By density functional theory calculations the orders of magnitude decrease in resistance can be explained and supported by TEM observations thus indicating that the resistance changes and strongly improved resistance spread are related to the establishment of reproducible and stable metal-molecule bonds. The same experimental sequence is carried out using 1,6-hexanedithiol functionalized AuNPs. The average resistances as a function of molecular length, demonstrated herein, are comparable to the one found in single molecule devices.

Fabrication of reproducible sub-5 nm nanogaps by a focused ion beam and observation of Fowler-Nordheim tunneling

Creating a stable high resistance sub-5 nm nanogap in between conductive electrodes is one of the major challenges in the device fabrication of nano-objects. Gap-sizes of 20 nm and above can be fab ...

Designing sterically demanding thiolate coated AuNPs for electrical characterization of BPDT in a NP–molecule–nanoelectrode platform

Molecular electronics with single or few molecules requires a stable metal–molecule nanojunction platform. Herein, we report the design and synthesis of gold nanoparticles coated with sterically demanding thiol ligands that are essential to fabricate a versatile and stable nanoelectrode–molecule–nanoparticle platform suitable for electrical characterization of small organic molecules. By combining ω-tetraphenylmethane ether functionalized alkyl thioacetate and alkyl thiols, we prepared highly stable gold nanoparticles in a one-phase reaction providing simple and efficient purification. This robust preparation gives highly pure nanoparticles in very high yields (up to 90%) with long-time shelf stability. The synthesis in this work has superior reproducibility compared to previous synthesis processes that are currently being used for such molecular electronics platforms. Electron microscopy confirms the formation of uniform and small nanoparticles in the range of 5 to 7 nm. These nanoparticles with different ligand surface coverages were placed in a 20 nm nanoelectrode setup using dielectrophoretic forces. This setup was utilized to characterize the conductivity of the molecular wire 4,4′-biphenyldithiol introduced via ligand place-exchange under ambient conditions.

An electron energy loss spectrometer based streak camera for time resolved TEM measurements

We propose an experimental setup based on a streak camera approach inside an energy filter to measure time resolved properties of materials in the transmission electron microscope (TEM). In order to put in place the streak camera, a beam sweeper was built inside an energy filter. After exciting the TEM sample, the beam is swept across the CCD camera of the filter. We describe different parts of the setup at the example of a magnetic measurement. This setup is capable to acquire time resolved diffraction patterns, electron energy loss spectra (EELS) and images with total streaking times in the range between 100ns and 10μs.

Enhanced gas sensing performance of graphene/ZnS-CdS hetero-nanowires gas sensor synthesized by Langmuir-Blodgett self-assembly method

Graphene is a promising material in the field of solid-state gas sensors due to the unique two-dimensional structure. Here, we have shown by fabricating graphene/ZnS-CdS hetero-nanowire structure, ...